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Pyramidanes: The Covalent Form of the Ionic Compounds

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† Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305−8571, Japan
‡ Institute of Physical and Organic Chemistry, Southern Federal University, 194/2 Stachki ave., Rostov on Don 344090, Russian Federation
§ Racah Institute of Physics, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel
∥ CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France
⊥ Université de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France
*E-mail for V.Ya.L.: [email protected]
*E-mail for A.S.: [email protected]
*E-mail for V.I.M.: [email protected]
Cite this: Organometallics 2016, 35, 3, 346–356
Publication Date (Web):January 20, 2016
https://doi.org/10.1021/acs.organomet.5b00924
Copyright © 2016 American Chemical Society
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Abstract

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Pyramidane and its derivatives are among the most desirable synthetic chemistry targets, whose appealing square-pyramidal design, fascinating nonclassical structure, and unusual bonding features have attracted the permanently growing interest of organic chemists for decades. Although they have been comprehensively approached on theoretical grounds, no member of the pyramidane family was experimentally realized until very recently, thus remaining one of the biggest synthetic challenges for experimental pursuits. In this paper, we report on a series of stable hybrid pyramidanes of group 14 elements, featuring germanium, tin, or lead at the apex of the square pyramid, capping the four-membered-ring base made of carbon, silicon, or germanium atoms. On the basis of the experimental results (X-ray diffraction and NMR and Mössbauer spectroscopy) and computational studies at the B3LYP/Def2TZVP level of theory (MO, NBO, NRT, and AIM), an extraordinarily high degree of ionicity of the pyramidal apex-to-base bonds was attributed to the overall structure of these nonclassical covalent compounds.

Introduction

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The geometry of a regular square-pyramidal C4v polyhedron is defined as a pyramid having a square base with its apex disposed perpendicular to the center of the base. In organic chemistry, the compound fitting the shape of a square pyramid is best known as pyramidane, the smallest member of the fenestrane family (namely, [3.3.3.3]fenestrane or tetracyclo[2.1.0.01,3.02,5]pentane).(1) One the most remarkable structural feature of pyramidane is its invertedly tetrahedral apical carbon, all four bonds of which lie in the same hemisphere. Such a dramatic departure from the standard tetrahedral regularity causes the principal changes in the hybridization at the apical atom and consequently in the nonclassical bonding interaction mode between the apex and the base within the pyramidane molecule.(1)
The issue of pyramidane stability was first computationally addressed at the semiempirical level as early as 1978,(2a) followed by ab initio calculations by the same authors, to reveal pyramidane as a stable local minimum on the C5H4 potential energy surface (PES).(2b) Other theoretical groups came to the same conclusion on the stability of pyramidane using still higher calculation levels,(3) even the very reliable coupled cluster (CCSD(T)/TZ2P) level of theory:(4) pyramidane represents a stable local minimum with substantial barriers to isomerization.(4, 5) On the basis of these theoretical predictions, Lewars concluded in his recent book on computational chemistry that “Pyramidane is a realistic goal: pyramidane is predicted to be a relative minimum...This presents us with the astonishing possibility that the exotic hydrocarbon may be isolable at room temperature.”(1c) However, since the very beginning of its studies and to date, pyramidane itself and its derivatives have still eluded experimental realization.(6) Some square-pyramidal shapes are known in organometallic chemistry, such as main-group or transition-metal complexes of cyclobutadiene: for example, an Mg salt of the cyclobutadiene dianion(7a) or a (cyclobutadiene)ruthenium tricarbonyl complex.(7b) However, although cyclobutadiene complexes of the s- and d-block elements are known (vide supra), complexes of the p-block elements (which are called pyramidanes) are still without precedent. As the closest approach, one can mention several hybrid pyramidal-shaped systems featuring a C2P2 base capped with a group 14 or 15 element.(8) Considering all that has been mentioned above, it comes as no surprise that pyramidane and its derivatives constitute a synthetic target of paramount importance, especially given the particular role of pyramidal compounds as the bridge between organic and organometallic chemistry.(9)
Apart from pyramidane itself (A), there are other local minima on the C5H4 PES, calculated at the very high CCSD(T) level of theory: they include the highly strained cyclic carbenes bicyclo[2.1.0]pent-2-en-5-ylidene (B) and tricyclo[2.1.0.02,5]pent-3-ylidene (C) (Chart 1).(4) Of these, tricyclic carbene C was predicted to serve as a precursor for pyramidane A after the intramolecular insertion of the carbenic center into one of the C–H bonds.(1d) On the other hand, bicyclic carbene B can also be imagined as the appropriate precursor for pyramidane A, given the very reasonable barrier of ca. 16 kcal/mol for the thermal isomerization of the former to the latter.(4)

Chart 1

Chart 1. Stable Local Minima Found on the C5H4 PES
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Given its intrinsic instability caused by the very high bicyclic ring strain, one should generate B in situ. For this purpose, we proposed the reaction of the cyclobutadiene dianion alkali-metal salt [η4-(Me3Si)4C4]2–·[Li+(thf)]2, recently developed by our group,(10) with the readily available dioxane complexes of dichlorogermylene and dichlorostannylene, finally forming the target bicyclo[2.1.0]pent-2-en-5-ylidene B. This synthetic approach was proven to be successful, and we were able to prepare a series of group 14 element capped pyramidanes, whose syntheses, structures, and particular bonding features are reported in this contribution.(11)

Results and Discussion

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At first, we probed the reaction of the most readily synthetically accessible dilthium salt of the tetrakis(trimethylsilyl)cyclobutadiene dianion [η4-(Me3Si)4C4]2–·[Li+(thf)]2 (12–·[Li+(thf)]2)(10) with the commonly available dioxane complexes GeCl2·diox and SnCl2·diox. Both reactions proceeded smoothly, forming germa- and stannapyramidane derivatives 2 and 3 in practically quantitative yields (Scheme 1).(11) Even more so, such an experimental procedure has been proved successful even with PbCl2, and thus we were able to isolate plumbapyramidane 4 featuring Pb, the heaviest group 14 element (sixth-row element), at the top of the square pyramid (Scheme 1).

Scheme 1

Scheme 1. Synthesis of Germa-, Stanna-, and Plumbapyramidanes 2–4
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The NMR spectral data of all pyramidanes 2–4, particularly those of germa- and stannapyramidanes 2 and 3, were strikingly similar to each other. Moreover, the tin atom in stannapyramidane 3 resonated at the record high field of −2441.4 ppm, thus outperforming exceptionally shielded tin nuclei in stannocenes (from −2100 to −2300 ppm).(12) The observation of only one set of resonances in all NMR spectra (1H, 13C, 29Si) was indicative of the high symmetry of compounds 2–4, as a prerequisite of their regular square-pyramidal composition.
The peculiar structural features of all pyramidanes 2–4 perfectly agree with their true square-pyramidal shapes (because pyramidanes 2–4 are isomorphous, the crystal structure of plumbapyramidane 4 is shown in Figure 1 as a representative example).(13) Thus, all of them uniformly manifest (1) perhapto (η4) coordinated Ge, Sn, and Pb apexes, (2) nearly planar C4 bases with a negligible folding of merely 1° or even less (0.71° for 2, 0.58° for 3, and 1.08° for 4), (3) cyclic C–C bonds of 1.484(2)–1.491(2) Å (for 2), 1.482(2)–1.485(2) Å (for 3), and 1.479(5)–1.487(5) Å (for 4) whose lengths are just between those of standard single and double bonds,(14) and (4) exceptionally stretched apex-to-base bonds that are much longer than the typical single bonds (Ge–C, 2.133(2)–2.143(2) Å (for 2); Sn–C, 2.339(2)–2.343(2) Å (for 3); Pb–C, 2.440(5)–2.453(4) (for 4)).(15)

Figure 1

Figure 1. Crystal structure of plumbapyramidane 4 (ORTEP view). Thermal ellipsoids are given at the 30% probability level; H atoms are not shown. Selected bond lengths (in Å) and bond angles (in deg): Pb1–C1 = 2.440(5), Pb1–C2 = 2.453(4), Pb1–C3 = 2.443(6), C1–C2 = 1.479(5), C2–C3 = 1.487(5), C1–Si1 = 1.870(6), C2–Si2 = 1.871(4), C3–Si3 = 1.867(6); C1–C2–C3 = 89.6(3), C2–C1–C2# = 90.7(4), C2–C3–C2# = 90.0(4), C1–Pb1–C2 = 35.18(12), C2–Pb1–C3 = 35.36(12). Folding angle of the C1–C2–C3–C2# ring: 1.08°.

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The resonances of the skeletal carbons in pyramidanes were found at 102.9 ppm (for 2), 106.5 ppm (for 3), and 114.8 ppm (for 4), whose chemical shifts perfectly agree with those of the cyclobutadiene dianion alkali-metal and alkaline-earth-metal salts 12–·[Li+(thf)]2 (104.1 ppm)(10) and 12–·[Mg2+(thf)3] (106.4 ppm).(7a) On the other hand, the former values were distinctly different from those of the cyclobutadiene transition-metal complexes, such as the cyclobutadiene ruthenium tricarbonyl [(Me3Si)4C4]Ru(CO)3 (86.5 ppm).(7b) This points to a substantial amount of the cyclobutadiene dianion character in the pyramidal structures, which in turn implies that in our pyramidanes 2–4 p-block elements (Ge, Sn, and Pb) surprisingly play rather uncommon (for them) roles of the s-block elements (Li and Mg) in 12–·[Li+(thf)]2(7a) and 12–·[Mg2+(thf)3].(7b)
Furthermore, the apical tin in stannapyramidane 3 was extraordinarily shielded (vide supra). This exceptional shielding is highly reminiscent of that in other organotin compounds, featuring high s-electron nonbonding electron density at the tin atom (such as pentastannapropellanes and particularly stannocenes) and exhibiting extremely high-field-shifted tin resonances. Overall, these 119Sn NMR spectral data point to the presence of a high s character lone pair at the tin apex in the stannapyramidane 3.
Such a hypothesis was firmly supported by the 119Sn Mössbauer spectroscopy measurements of the stannapyramidane 3, which allowed us to study metal atom dynamics and hyperfine parameters over the temperature range 90–192 K. The Mössbauer spectrum of 3 features a single resonance, which does not show any observable quadrupole splitting (QS) interactions (Figure 2). As is well-known, the tin(IV) compounds (Sn: 5s0) typically show smaller isomer shift (IS) values, whereas tin(II) compounds (Sn: 5s2) show larger IS values, with the border value between them being ca. 2.40 mm s–1.(16) Our stannapyramidane 3 exhibited an IS at 2.974 ± 0.006 mm s–1 at 90 K that clearly defines the formal oxidation state of the metal atom as Sn(II). Such a low oxidation state is characteristic for stannocenes (IS = 3.75 mm s–1 for (η5-Ph5C5)2Sn:)(17) and tin dication–crown ether complexes (IS = 4.59 mm s–1 for [Sn2+-(15-crown-5)]).(18)

Figure 2

Figure 2. 119Sn Mössbauer spectrum of the stannapyramidane 3 at 128 K. The residuals (bottom trace) indicate that the absorption profiles are not exactly Lorentzian but that there are no other absorptions evident.

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The issue of unexpectedly small QS hyperfine interaction, despite the obvious lack of cubic (i.e. Oh or Td) symmetry around the metal atom, has been previously addressed.(19) The lack of a major QS interaction prevented us from ascertaining the presence of anisotropic motion of the metal atom in 3. Neither the IS nor the QS parameters have a significant temperature dependence over the experimentally accessible temperature range.
In contrast, the temperature dependence of the recoil-free fraction f, which for an optically thin absorber is given by the expression −d ln[A(T)/A(90)]/dT (A is the area under the resonance curve at temperature T), is well fitted by a linear regression and is −(25.77 + 0.17) × 10–3 K–1 with a correlation coefficient of 0.985 for eight data points (Figure 3). These data can be normalized by extrapolation to T = 0 K, assuming that the zero-point motion in the Mössbauer spectroscopy temperature range is negligibly small. This extrapolation leads to an evaluation of the mean square amplitude of vibration (msav) of the metal atom, which is best expressed by the parameter F = k2⟨xav2⟩, where k is the wave vector of the 23.88 keV γ radiation of 119mSn. The F parameter can also be calculated from the Uij value(20) determined by the X-ray diffraction study of 3. At 150 K this leads to Fx,150 = 3.29 + 0.01, in reasonable agreement with the corresponding Mössbauer spectroscopy derived value of FM,150 = 3.59. As was pointed out in previous studies, a major difference between the FX and FM parameters can arise when the resonant atom portion of the molecule is not “well tethered” to the remainder, giving rise to the possibility of vibrational or rotational low-frequency modes, to which X-ray diffraction (but not Mössbauer spectroscopy) is sensitive.

Figure 3

Figure 3. Temperature dependence of the recoil-free fraction f of the stannapyramidane 3.

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It is instructive to compare the root mean square amplitude of vibration (rmsav) of the metal atom in the stannapyramidane 3 with the related dimethyl(diaryl)stannane [o-(Ph2P)C6H4]2SnMe2.(21) In the latter compound, the Sn atom is bound to two methyl and two aryl groups and similarly to 3 shows only a very small QS interaction at 90 K (Figure 4). While the absolute rmsav values for these two compounds are slightly different, their temperature dependences in the range 100–200 K are very similar.

Figure 4

Figure 4. Comparison of the root mean square amplitudes of vibration of the metal atom in stannapyramidane 3 (top straight line) and [o-(Ph2P)C6H4]2}SnMe2 (bottom dashed line).

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Moreover, comparing the Mössbauer spectrum of 3 with those of organotin compounds featuring a high s character lone pair at tin (stannocenes(17) or tin dication–crown ether complexes(18)), which show very small (or zero) QS values, one can suggest that the absence of QS in 3 implies that its lone pair on tin resides in the 5s orbital with almost no 5p orbital admixture. In other words, there is quite negligible hybridization between s and p orbitals on the tin atom in stannapyramidane 3.
To support the experimental observations discussed above (X-ray, NMR, Mössbauer spectroscopy) and to get a deeper insight into the nature of the particular apex-to-base bonding situation in pyramidanes 2–4, we performed extensive computational studies at the B3LYP/Def2TZVP level of theory. First of all, optimized geometries of 2–4, in accord with their experimental crystallographic data, manifested very long apex-to-base bonds of 2.157 Å for the Ge–C bonds in 2, 2.359 Å for the Sn–C bonds in 3, and 2.460 Å for the Pb–C bonds in 4 (for the computed structural and energetic details, see Table S16 and Figure S26 in the Supporting Information). Being neutral as a whole, pyramidanes 2–4 nevertheless exhibited a remarkable charge separation within the molecule, with the positively charged apical atoms (+0.73 for Ge in 2, +0.76 for Sn in 3, and +0.75 for Pb in 4) and negatively charged basal carbons (−0.65 in 2, −0.65 in 3, and −0.64 in 4) (natural population analysis (NPA) charges; see Table S17 in the Supporting Information) (cf. the electronegativities of Ge, Sn, and Pb atoms vs the C atom are 2.01, 1.96, and 2.33 vs 2.55: Pauling scale).
The analysis of the MO diagram of the model stannapyramidane Sn[C4H4] (3′), in comparison with the hypothetical all-carbon pyramidane C[C4H4] (5′), provided a deeper look into the particular bonding situation in these unusual clusters (Figure 5).

Figure 5

Figure 5. Comparison of the cage bonding MOs of the model H-substituted C-based pyramidanes E[C4H4] (5′, E = C; 6′, E = Si; 2′, E = Ge; 3′, E = Sn; 4′, E = Pb).

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Stabilization of the organic all-carbon pyramidane C[C4H4] (5′) is mostly due to the formation of the two doubly degenerate HOMO-1 and HOMO-2 (1e), whereas the HOMO (2a1) clearly revealed the presence of a lone pair at the carbon apex. All-bonding apex-to-base interaction is represented by the HOMO-8 (1a1). In heteronuclear organometallic pyramidanes 6′ and 2′–4′, the order of the highest occupied MOs is reversed (in comparison to 5′): the doubly degenerate 1e orbital now represents HOMO and HOMO-1, whereas the lone pair 2a1 orbital is HOMO-2. This is caused by the steady stabilization of the 2a1 orbital on going from sila- to germa- to stanna- to plumbapyramidanes and simultaneous destabilization of the 1e orbitals occurring in the same direction. Overall, such a trend results in the square-planar pyramidal system destabilization due to the rise of the highest occupied four-electron 1e-orbital energy level, which in turn causes progressive weakening of the apex-to-base E–C interaction (E = Si–Pb) in pyramidanes E[C4H4] 6′ and 2′–4′ (as compared to that in all-carbon pyramidane 5′). This agrees well with the exceptionally low values of the apex-to-base bond orders (Wiberg bond indices,(22) WBI), 0.47 for the Ge–C bonds in 2′, 0.45 for the Sn–C bonds in 3′, and 0.45 for the Pb–C bonds in 4′, values that are much lower than those for the basal C–C bonds, 1.16 in 2′, 1.17 in 3′, and 1.17 in 4′ (in contrast, the Capical–Cbasal bond order in the all-carbon pyramidane 5′ is notably larger at 0.70 (1.10 for the Cbasal–Cbasal bond)).
Natural resonance theory (NRT)(23) revealed strong electron delocalization for the most part of C-based pyramidanes. There are four most important degenerate resonance contributors (see Figure S27 in the Supporting Information) for each model compound 5′, 6′, and 2′ with the NRT weight (%) for one structure of 21.69, 19.68, 22.87, respectively. Stannapyramidane 3′ has a similar selection of the four resonance extremes, forming two doubly degenerate pairs of them with the different NRT weights of 19.41 and 17.63. In the plumbapyramidane 4′, electron delocalization is remarkably diminished: there is only one dominant Lewis structure (bicyclic plumbylene) with an NRT weight of 36.75. The NRT analysis also clearly showed the presence of a lone pair (LP) at the apical atom for all C-based pyramidanes,(24) as well as weak pyramidal E–C bonding with a high degree of ionicity for hybrid (E ≠ C) systems. Thus, in the three delocalized hybrid structures 6′, 2′, and 3′, the natural bond orders of the E–C bonds are 0.7310, 0.7279, and 0.7340/0.6105 with the percentages of ionic character of the bonds being 54.5, 55.9 and 57.8/51.7, respectively (compare with the same values for all-carbon pyramidane 5′ of 0.7360 and 28.8; see Table S18 in the Supporting Information). The NRT bond orders b for the basal C–C bonds of hybrid pyramidanes 6′, 2′, and 3′ manifest some degree of double bonding (bCC = 1.0099, 1.0111 and 1.1093/1.0259), whereas bCC in 5′ is 0.9915, implying single C–C bonds.
For the sake of comparison with WBI and NRT bond orders, for pyramidal E–C bonds we also calculated effective bond orders (EBO),(25) which better describe the bonding interactions involving heavy elements. Thus, the EBO values are 0.83 (for 5′), 0.79 (for 2′, 3′, and 6′), and 0.86 (for 4′) (cf. EBO values for the basal C–C bonds are 1.00 (for 5′), 0.99 (for 2′–4′ and 6′)).
Some degree of aromaticity of the four-membered-ring base is also confirmed in all C-based pyramidanes by their negative nucleus-independent chemical shifts (NICS).(26) The NICS(1) values for 5′, 6′, 2′, 3′, and 4′ are −9.5, −8.6, −8.8, −8.9, and −9.3, respectively. The more reliable NICS(1)zz(27) provided much greater values, −17.1, −22.6, −24.2, −27.3, and −28.9, respectively, which gave evidence for the strong p interaction in the C4 base.
The topological analysis method atoms in molecules (AIM)(28) agreed well with the conclusion made above on the exceptional polarization of the apex-to-base bonds in pyramidanes. Thus, Bader’s AIM analysis of the model stannapyramidane 3′ revealed an extraordinarily high degree of ionicity for the apex-to-base Sn–C bonds (based on the small electron density ρ(r) and positive Laplacian of the electron density ∇2ρ(r) at the bond critical points (BCP)) on the one hand and classical covalent bond character for the C–C bonds within the C4 base (large ρ(r) and negative ∇2ρ(r)) on the other hand (Table 1). This is in sharp contrast to the case of the homonuclear all-carbon pyramidane 5′, in which all C–C bonds (both apex-to-base and basal) are typical covalent bonds (large ρ(r) and negative ∇2ρ(r)) (Table 1).
Table 1. Topological Analysis of AIM Theory (B3LYP/Def2TZVP):a
 pyramidal bondbasal bond
structure (C4v)ρ(r)∇2ρ(r)he(r)bond typeρ(r)∇2ρ(r)he(r)bond type
5′, C[C4H4]0.172–0.047–0.115C0.279–0.689–0.274C
6′, Si[C4H4]0.0770.129–0.038II0.277–0.690–0.267C
2′, Ge[C4H4]0.0750.129–0.027II0.278–0.706–0.268C
3′, Sn[C4H4]0.0590.147–0.012II0.278–0.710–0.268C
4′, Pb[C4H4]0.0550.145–0.009II0.279–0.718–0.269C
7′, C[Si4H4]0.0950.171–0.052II0.090–0.109–0.050C
8′, Si[Si4H4]0.059–0.014–0.021C0.093–0.137–0.051C
9′, Ge[Si4H4]0.0540.008–0.017II0.093–0.140–0.051C
10′, Sn[Si4H4]0.0430.027–0.010II0.093–0.140–0.051C
11′, Pb[Si4H4]0.0400.036–0.008II0.093–0.142–0.051C
12′, C[Ge4H4]0.0830.053–0.033II0.082–0.039–0.040C
13′, Si[Ge4H4]0.053–0.006–0.017C0.083–0.051–0.040C
14′, Ge[Ge4H4]0.0490.012–0.014II0.083–0.053–0.040C
15′, Sn[Ge4H4]0.0400.026–0.008II0.084–0.055–0.040C
16′, Pb[Ge4H4]0.0370.035–0.006II0.084–0.056–0.040C
a

Definitions: ρ(r), electron density at the bond critical point (e/au3); ∇2ρ(r), Laplacian of the electron density (e/au5); he(r), local electron energy (H/au3); II, intermediate interaction; C, covalent.

Finally, on the basis of the available experimental (X-ray, NMR, Mössbauer spectroscopy) and computational (NPA/NRT, AIM) data, one can reliably ascribe a very important extent of ionicity for the apex-to-base E–C bonds (E = Si–Pb) in sila-, germa-, stanna-, and plumbapyramidanes as the signature of their nonclassical bonding nature. This in turn leads to the striking conclusion of the crucial contribution of the ionic resonance form ((Me3Si)4C4]2– → E2+) B to the overall composition of the neutral pyramidane covalent resonance form A (Scheme 2).

Scheme 2

Scheme 2. Covalent A vs Hypothetical Ionic B Resonance Extremes for the Hybrid Pyramidanes 2–4 (2, E = Ge; 3, E = Sn; 4, E = Pb)
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Following the successful preparation of the first C-based pyramidanes 2–4, we then attempted synthesis of their heavier analogues featuring Si4 and Ge4 bases. Before starting experimental work, we approached the problem from the computational direction to clarify the combination of which factors would best favor stabilization (and isolation) of the target compounds.
Accordingly, we systematically studied the general trends in the effect of substituents and apical and basal atoms on the overall stability of the pyramidal systems E[E′4R4] (E = C–Pb; E′ = Si–Ge; R = H, SiH3, SiMe3, SiMetBu2) (Tables 2 and 3 and Table S16 in the Supporting Information).
Table 2. Calculated Data (B3LYP/Def2TZVP) for the Model Compounds E[E′4H4]a
  E′ = SiE′ = Ge
EE′ = C, λλΔEλΔE
C01+10.91+25.6
Si03+4.93+16.0
Ge03+4.83+15.0
Sn03+4.53+14.9
Pb03+4.43+14.6
a

Definitions: λ, number of imaginary frequencies of the square-planar pyramidal C4v structures; ΔE, pyramidane’s relative energy toward the distorted C2v form (with zero-point correction (ZPE), kcal mol–1). For more information, see Table S16 in the Supporting Information.

Table 3. Calculated Data (B3LYP/Def2TZVP) for the Model Compounds E[E′4R4]a
 E′ = C, R = SiMe3E′ = Si, R = SiMetBu2E′ = Ge, R = SiMetBu2
EλΔEλΔEλΔE
C1+2.41+5.21+12.6
Si1+0.70 1+2.1
Ge0 0 1+2.3
Sn0 0 1+1.8
Pb1+0.31+0.84+1.4
a

Definitions: λ, number of imaginary frequencies of the square-planar pyramidal C4 structures; ΔE, pyramidane’s relative energy toward the distorted C2 form (with ZPE, kcal mol–1). For more information, see Table S16 in the Supporting Information.

For the simplest H-substituted models, the energy minima can be found only for the pyramidanes with the carbon base (E′ = C, E = C–Pb). Other pyramidanes (E′ = Si, Ge; E = C–Pb) represent less stable structures: either transition states (TS) or third-order saddle points (Table 2 and Table S16 in the Supporting Information).
As for the influence of substituents, we found that the successive increase in their size (from H to H3Si to Me3Si to tBu2MeSi) results in the overall stabilization of the square-planar pyramidal form (Table 3 and Table S16 in the Supporting Information). Accordingly, most pyramidal structures E[Si4R4] (those with E = Si, Ge, Sn; R = SiMetBu2) correspond to the energy minima on the PES. For other structures E[Si4R4] and E[Ge4R4] (R = SiMetBu2) stationary points can be found as energy minima C/C′ (λ = 0) or TS D (λ = 1) accompanied by a decrease in the inversion barriers (Scheme 3). An increase in the size of substituents results also in the notable planarization of the folded isomers C/C′ (see parameters BD and FA in Table S16).

Scheme 3

Scheme 3. Interconversion between the Two Degenerate Distorted-Pyramidal Structures C and C′ (Energy Minima) and Square-Planar Pyramidal Structure D (TS)
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Considering the effect of the skeletal atoms, one can notice that on descending group 14, that is on going from C to Si to Ge as the basal atoms, the destabilization of pyramidanes takes place: thus, for example, all model C-based pyramidanes are stable energy minima, whereas almost all Si- and Ge-based pyramidanes are third-order saddle points (Table 2 and Table S16 in the Supporting Information). This phenomenon comes as no surprise due to the well-known “inert pair effect”: that is, the reluctance of heavier group 14 elements to hybridize, delocalize their π electrons, and form multiple bonds. On the other hand, for the apical atoms the tendency is opposite: making the apex heavier, that is on going from C to Si to Ge to Sn to Pb, causes a notable stabilization of the square-planar pyramidal structures (Tables 2 and 3 and Table S16) that may be driven by the electronegativity difference between the apical and basal atoms. Thus, for example, carbon-capped hybrid pyramidanes C[E′4R4] (E′ = Si, Ge; R = SiMetBu2) are all destabilized. Thus, the general trends in the influence of the basal and apical atoms are as follows: (1) for the heavier basal atoms E′, one can expect the progressive destabilization of the square-planar-pyramidal form D (Scheme 3) (compared with the folded pyramidal forms C and C′ (Scheme 3)); (2) for the heavier apical atoms E, the square-planar-pyramidal form D becomes more and more stable.
Overall, synthetically feasible pyramidanes E[C4R4], E[Si4R4], and E[Ge4R4] (E = Ge, Sn, Pb) are all stabilized, being either energy minima or TSs with the energy difference for the folded minimum structure of no more than 2.3 kcal/mol (Table 3). For these systems the general trend is operative: the square-planar-pyramidal form becomes progressively more stable when the basal atoms are more electronegative than the apical atom (Tables 2 and 3). The same holds true for the homonuclear pyramidanes C[C4R4], Si[Si4R4], and Ge[Ge4R4]: thus, for example, for all-carbon pyramidane C[C4H4] (5′), its basal C atoms have NPA charges more negative than that of the apical C atom (Table S17 in the Supporting Information). This is in line with the earlier observation (the rule of topological charge stabilization) that the more electronegative atoms in pyramidanes (other than C[C4H4]) should occupy positions with negative charges greater than that in the parent C[C4H4].(29) In accord with this rule, TS-like systems C[E′4R4] (E′ = Si, Ge) are highly destabilized (in comparison with their folded counterparts), because Gimarc’s rule in such case is not obeyed.
In general, MO diagrams of the heavy pyramidanes show the same trends as those for the C4-based systems, i.e. reversed 1e and 2a1 orbitals in the case of C[E′4H4] (E′ = Si, Ge) and other heavy systems (Figure 6 and Figure S28 in the Supporting Information). The only notable differences are the MO energies: thus, the lowest orbitals 1a1 have energies higher than those in the C4-pyramidanes, due to the higher energies of Si4-ring MOs. Energy differences (in eV) between 1e and 2a1 orbitals in 7′–11′ are smaller: 1.29 (for 7′), 0.15 (for 8′), 0.55 (for 9′), 0.92 (for 10′), 1.36 (for 11′) (compare with those for C4 systems: 0.93 (for 5′), 0.90 (for 6′), 1.80 (for 2′), 2.31 (for 3′) and 3.29 (for 4′)). The lower energy level of the 1e orbitals of the pyramidal systems with an apical C atom C[E′4H4] can be explained by the shorter C–E′ bonds and thus better fragment orbital overlap. In contrast, with the heavier apical atom E, the apex-to-base distances E–E′ in pyramidanes E[E′4H4] become greater, which results in poorer orbital overlap and therefore in an increase in the orbital energy.

Figure 6

Figure 6. Comparison of the cage bonding MO of the model H-substituted Si-based pyramidanes E[Si4H4] (7′, E = C; 8′, E = Si; 9′, E = Ge; 10′, E = Sn; 11′, E = Pb).

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Topological AIM analysis disclosed that the basal bonds in all structures are covalent because of the negative values of the Laplacian of electron density at the corresponding BCP (Table 1; see in Figure S7 in the Supporting Information an example of the Sn[Si4H4] molecular graph). The assignment of the chemical bonds between the apical and basal atoms is not that straightforward, and thus can be best classified as intermediate interactions, representing the exceptionally polar covalent bonds. Such a conclusion was drawn from the values of the local electron energy, which are negative for the covalent interactions, even despite the positive values of the Laplacian of electron density. Small values he(r) are indicative of the high ionicity of these bonds, which rises upon an increase in the difference in electronegativity between the basal and apical atoms.
AIM conclusions are in line with the NBO data discussed above, according to which the bonds E–E′ are highly polarized toward the basal atoms. That is, again, the overall stabilization of the pyramidal systems requires polarization of pyramidal bonds from the apex to the base, whereas in the case of opposite polarization, that is, from the base to the apex, the pyramidal system is highly destabilized (see Tables 2 and 3).
The NRT predicts strong electron delocalization for practically all heavy pyramidanes (Figure S27 in the Supporting Information). For each of them, there are four degenerate resonance structures with the lone pair at the apical atom with an NRT weight of 12.48–20.65% for one structure. For the bond ionicity of such highly delocalized structures, these values for the pyramidal bonds are in the range of 30–39%, as the manifestation of the less pronounced ionic character in comparison to that of hybrid pyramidanes E[C4H4] (Table S18 in the Supporting Information).
The strength of the apex-to-base interaction in the heavy pyramidanes 7′–16′ is quite comparable with that of carbon-based systems 2′–6′. Thus, the EBO values are 0.90 (for 7′), 0.82 (for 8′, 9′, 13′, and 14′), 0.81 (for 10′, 11′, and 16′), 0.89 (for 12′), and 0.84 (for 15′) (for comparison, see WBI values in Table S17 in the Supporting Information).
Both isotropic NICS(1) and out-of-plane NICS(1)zz for the heavy pyramidanes 7′–16′ are negative, indicating aromaticity of the four-membered-ring base. Thus, NICS(1)/NICS(1)zz values are −17.7/–21.0 (for 7′), −18.8/–22.1 (for 8′), −18.6/–22.4 (for 9′), −17.8/–22.7 (for 10′), −17.5/–22.9 (for 11′), −17.1/–17.3 (for 12′), −18.0/–20.1 (for 13′), −17.9/–20.6 (for 14′), −17.6/–21.4 (for 15′), and −17.5/–21.7 (for 16′). Moreover, NICS(1)zz values for 7′–16′ are similar to those of carbon pyramidanes 2′–6′.
Inspired by these theoretical predictions, we finally decided to attempt synthesis of the hybrid pyramidal combinations E[Si4R4] and E[Ge4R4] (E = Ge, Sn, Pb).
The dioxane complexes of dichlorogermylene and dichlorostannylene (or free dichlorostannylene) readily reacted with both tetrasila-(30a) and tetragermacyclobutadiene(30b) dianion derivatives, forming the corresponding “heavy” pyramidanes 10, 14, and 15, isolated as the yellow or orange crystalline compounds (Scheme 4).

Scheme 4

Scheme 4. Synthesis of the “Heavy” Pyramidanes 10, 14, and 15, Featuring Si4 or Ge4 Bases
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Similarly to the above case of the C4-based pyramidanes 2–4, NMR spectra of the Ge4-based pyramidanes 14 and 15 were strikingly similar to each other. Likewise for stannapyramidane 3, the Sn-capped “heavy” pyramidanes 10 and 15 manifested extreme shielding of their apical tins: −2104 ppm (for 10) and −1900 ppm (for 15). This observation can again be attributed to the aformentioned high s character lone pair residing at the tin centers.
Crystal structures of all “heavy” pyramidanes 10, 14, and 15 were unequivocally confirmed by X-ray crystallographic analysis.(31) Analogously to the above case of the carbon-based pyramidanes 2–4, stannatetrasilapyramidane 10 features (1) an η4-coordinated Sn apex, (2) a nearly planar Si4 ring with a negligible folding of 0.70°, (3) cyclic Si–Si bonds of 2.273(2)–2.280(2) Å that are intermediate between typical single and double bonds,(32) and (4) remarkably long apex-to-base Sn–Si bonds of 2.763(4)–2.801(3) Å (Figure 7).

Figure 7

Figure 7. Crystal structure of stannatetrasilapyramidane 10 (ORTEP view). Thermal ellipsoids are given at the 30% probability level. H atoms are not shown. Only the main position (86%) of the rotationally disordered structure is depicted for clarity. Selected bond lengths (in Å) and bond angles (in deg): Si1–Si2 = 2.280(2), Si2–Si3 = 2.279(2), Si3–Si4 = 2.273(2), Si1–Si4 = 2.275(2), Sn1–Si1 = 2.786(3), Sn1–Si2 = 2.801(3), Sn1–Si3 = 2.782(3), Sn1–Si4 = 2.763(4); Si1–Si2–Si3 = 89.5(1), Si2–Si3–Si4 = 90.4(1), Si3–Si4–Si1 = 89.8(1), Si2–Si1–Si4 = 90.3(1), Si1–Sn1–Si2 = 48.2(1), Si2–Sn1–Si3 = 48.2(1), Si3–Sn1–Si4 = 48.4(1), Si1–Sn1–Si4 = 48.4(1). Folding angle of the Si1–Si2–Si3–Si4 ring: 0.70°.

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A comparison of the geometries of the stannapyramidane 3, stannatetrasilapyramidane 10, and stannatetragermapyramidane 15 is instructive in terms of the regular changes in the geometry of the differently based pyramidanes descending group 14. Thus, if both C4-based 3 and Si4-based 10 feature practically planar bases (folding angles are 0.6 and 0.7°, respectively), the “heaviest” Ge4-based pyramidane 15 has a severely distorted base (folding angle 26.5°) (Figure 8).

Figure 8

Figure 8. Square-planar pyramidal Sn[C4]- and Sn[Si4]-pyramidanes 3 and 10 vs folded pyramidal Sn[Ge4]-pyramidane 15.

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The departure of 15 from square-planar regularity is so great that it is better viewed as the bicyclo[1.1.1]pentane rather than the true tetracyclic pyramidane. However, in solution 15 manifests a highly symmetrical structure, as was evidenced by the observation of only one set of resonances in its NMR spectra, which comes as no surprise given the exceptionally low energy barrier calculated for the inversion of its Ge4 base (Scheme 3, E1–E4 = Ge, E5 = Sn, R = SiMetBu2, ΔE = 1.8 kcal/mol).
Moreover, pentagermapyramidane Ge[Ge4(SiMetBu2)4] (14), representing a unique example of the first homonuclear pyramidane, also showed only one set of NMR resonances, despite the theoretically predicted folded geometry of its Ge4 base (folding angle 22.6°).(33) Again, the reason behind such a contradiction is the exceptionally low inversion barrier on the way between the two folded pyramidal minima structures C and C′ through the square-planar pyramidal transition state structure D (Scheme 3, E1–E5 = Ge, R = SiMetBu2, ΔE = 2.3 kcal/mol). Amazingly, both theoretically predicted structures (folded C/C′ and planar D) were experimentally realized with the isolation and crystallographic characterization of the two structurally distinct forms of 14: folded pyramidal 14a (folding angles 17.59 and 17.69°) with the η2-coordinated Ge apex and square-planar pyramidal 14b (folding angle 0.09°) with the η4-coordinated Ge apex.(33) For the related examples of the Ge5R4 clusters (R = CH(SiMe3)2, 2,6-(2,4,6-Me3-C6H2)2-C6H3), closely resembling our folded pyramidal form 14a, a notable amount of the singlet biradicaloid form was proposed on the basis of the X-ray structural and computational studies.(34)

Conclusions

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A series of the first representatives of the square-pyramidal structures E[E′(SiR3)4] (E = Ge, Sn, Pb; E′ = C, Si, Ge; R = SiMe3, SiMetBu2), the so-called pyramidanes, entirely consisting of group 14 elements, has been synthesized and fully characterized. Combined experimental and computational studies revealed the nonclassical nature of the apex-to-base bonding interactions in pyramidanes, which therefore alternatively can be represented by the charge-separated cyclobutadiene dianion–apical atom dication resonance form. Apart from the remarkable ionicity of the pyramidal bonds, experimental (X-ray crystallography, NMR and Mössbauer spectroscopy) and theoretical (MO, NBO, NRT, and AIM) data disclosed the presence of the very high s character lone electron pair at the apex of the square pyramid. The detailed computational studies revealed the general trends in the influence of the nature of substituents, as well as apical and basal atoms, on the overall structure, stability, and bonding situation in pyramidal systems.

Experimental Section

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General Considerations

All experimental manipulations were performed using high-vacuum-line techniques or under an argon atmosphere of an MBRAUN MB 150B-G glovebox. All solvents were predried by conventional methods and finally dried and degassed over a potassium mirror under vacuum immediately prior to use. NMR spectra were recorded on Bruker AC-300FT NMR (1H NMR at 300.1 MHz; 13C NMR at 75.5 MHz; 29Si NMR at 59.6 MHz), Bruker AV-400FT NMR (1H NMR at 400.1 MHz; 13C NMR at 100.6 MHz; 29Si NMR at 79.5 MHz; 119Sn NMR at 149.2 MHz), and Bruker AVANCE 500T (207Pb NMR at 104.6 MHz) spectrometers. UV–vis spectra were recorded on a Shimadzu UV-3150 UV–vis spectrophotometer in hexane. High-resolution mass spectra were measured on a JEOL AccuTOF CS (JMS-T100CS) mass spectrometer with the atmospheric pressure chemical ionization (APCI) method. Tetrakis(trimethylsilyl)cyclobutadiene dianion dilithium salt (12–·[Li+(thf)]2),(10) tetrakis(di-tert-butylmethylsilyl)tetrasilacyclobutadiene dianion dipotassium salt ([(tBu2MeSi)4Si4]2–·[K+(thf)2]2),(30a) tetrakis(di-tert-butylmethylsilyl)tetragermacyclobutadiene dianion dipotassium salt ([(tBu2MeSi)4Ge4]2–·[K+(thf)2]2),(30b) dichlorogermylene dioxane complex,(35) dichlorostannylene dioxane complex,(36) and 1,2,3,4-tetrabromo-1,2,3,4-tetrakis(di-tert-butylmethylsilyl)cyclotetrasilane ([(tBu2MeSi)4Si4]Br4)(37) were prepared according to the published procedures.

Experimental Procedure and Spectral and Crystallographic Data for the Germapyramidane Ge[C4(SiMe3)4] (2)

A mixture of tetrakis(trimethylsilyl)cyclobutadiene dianion dilithium salt (12–·[Li+(thf)]2; 100 mg, 0.186 mmol) and GeCl2·diox (45 mg, 0.191 mmol) was placed in a reaction tube with a magnetic stirring bar. Then dry, oxygen-free THF (1.5 mL) was introduced into the reaction tube by vacuum transfer, and the reaction mixture was stirred at room temperature for 30 min to form a yellow-orange solution. Then the solvent was removed under vacuum, and dry hexane was introduced. After the inorganic salt was filtered off, the residue was recrystallized from pentane to give 2 as pale yellow crystals (74 mg, 96%). Mp: 106–108 °C. 1H NMR (C6D6, δ, ppm): 0.25 (s, 36 H, 12 CH3). 13C NMR (C6D6, δ, ppm): 3.08 (CH3), 102.86 (skeletal C). 29Si NMR (C6D6, δ, ppm): −12.60 (Si substituents). UV–vis (hexane): λmax/nm (ε) 226 (6500), 245 sh (3900). HRMS (APCI): m/z calcd for C16H36GeSi4 [M]+ 414.1106, found 414.1121. Anal. Calcd for C16H36GeSi4: C, 46.48; H, 8.78. Found: C, 46.14; H, 8.67.
Single crystals of 2 for X-ray diffraction analysis were grown from a hexane solution. Diffraction data were collected at 150 K on a Bruker AXS APEX II CCD X-ray diffractometer (Mo Kα radiation, λ = 0.71073 Å, 50 kV/30 mA). The structure was solved by direct methods with the SHELXS-97 program(38) and refined by full-matrix least-squares methods with the SHELXL-97 program.(39) Crystal data for 2: molecular formula C16H36GeS4, MW = 413.40, orthorhombic, Pnma, a = 11.6135(6) Å, b = 17.5625(9) Å, c = 11.4704(6) Å, V = 2339.5(2) Å3, Z = 4, Dcalcd = 1.174 g cm–3. The final R factor was 0.0242 for 2357 reflections with Io > 2σ(Io) (Rw = 0.0716 for all data, 2487 reflections); GOF = 1.082.
X-ray crystallographic data for 2 have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition no. CCDC 1433858. These data can be obtained free of charge from the CCDC (www.ccdc.cam.ac.uk/data_request/cif).

Experimental Procedure and Spectral and Crystallographic Data for the Stannapyramidane Sn[C4(SiMe3)4] (3)

A mixture of tetrakis(trimethylsilyl)cyclobutadiene dianion dilithium salt (12–·[Li+(thf)]2; 100 mg, 0.186 mmol) and SnCl2·diox (53 mg, 0.191 mmol) was placed in a reaction tube with a magnetic stirring bar. Then dry, oxygen-free THF (1.5 mL) was introduced into the reaction tube by vacuum transfer, and the reaction mixture was stirred at room temperature for 30 min to form a yellow-orange solution. Then the solvent was removed under vacuum, and dry hexane was introduced. After the inorganic salt was filtered off, the residue was recrystallized from pentane to give 3 as light yellow crystals (83 mg, 97%). Mp: 130–132 °C. 1H NMR (C6D6, δ, ppm): 0.27 (s, 36 H, 12 CH3). 13C NMR (C6D6, δ, ppm): 4.09 (CH3), 106.49 (skeletal C). 29Si NMR (C6D6, δ, ppm): −14.24 (Si substituents). 119Sn NMR (C6D6, δ, ppm): −2441.48. UV–vis (hexane): λmax/nm (ε) 245 sh (14800), 256 (22600), 318 (700). HRMS (APCI): m/z calcd for C16H36Si4Sn [M]+ 460.0916, found 460.0923. Anal. Calcd for C16H36Si4Sn: C, 41.82; H, 7.90. Found: C, 41.73; H, 7.66.
Single crystals of 3 for X-ray diffraction analysis were grown from a hexane solution. Diffraction data were collected at 150 K on a Bruker AXS APEX II CCD X-ray diffractometer (Mo Kα radiation, λ = 0.71073 Å, 50 kV/30 mA). The structure was solved by direct methods with the SHELXS-97 program(38) and refined by full-matrix least-squares methods with the SHELXL-97 program.(39) Crystal data for 3: molecular formula C16H36Si4Sn, MW = 459.50, monoclinic, Cm, a = 11.742(2) Å, b = 17.046(3) Å, c = 6.3160(13) Å, β = 112.50(3)°, V = 1167.9(4) Å3, Z = 2, Dcalcd = 1.307 g cm–3. The final R factor was 0.0131 for 2422 reflections with Io > 2σ(Io) (Rw = 0.0330 for all data, 2422 reflections); GOF = 1.190.
X-ray crystallographic data for 3 have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition no. CCDC 926174. These data can be obtained free of charge from the CCDC (www.ccdc.cam.ac.uk/data_request/cif).

Experimental Procedure and Spectral and Crystallographic Data for the Plumbapyramidane Pb[C4(SiMe3)4] (4)

A mixture of tetrakis(trimethylsilyl)cyclobutadiene dianion dilithium salt (12–·[Li+(thf)]2; 50 mg, 0.10 mmol) and PbCl2 (45 mg, 0.16 mmol) was placed in a reaction tube with a magnetic stirring bar. Then dry, oxygen-free THF (1.5 mL) was introduced into the reaction tube by vacuum transfer, and the reaction mixture was stirred at room temperature for 30 min to form a yellow-orange solution. Then the solvent was removed under vacuum, and dry hexane was introduced. After the inorganic salt was filtered off, the residue was recrystallized from pentane to give 4 as bright orange crystals (51 mg, 93%). Mp: 158–160 °C (subl). 1H NMR (C6D6, δ, ppm): 0.30 (s, 36 H, 12 CH3). 13C NMR (C6D6, δ, ppm): 5.12 (CH3), 114.18 (skeletal C). 29Si NMR (C6D6, δ, ppm): −18.00 (Si substituents). 207Pb NMR (C6D6, δ, ppm): −5358.83. UV–vis (hexane): λmax/nm (ε) 220 (16000), 255 (18000), 265 (17000). Anal. Calcd for C16H36PbSi4: C, 35.07; H, 6.62. Found: C, 34.50; H, 6.45.
Single crystals of 4 for X-ray diffraction analysis were grown from a pentane solution. Diffraction data were collected at 150 K on a Bruker AXS APEX II CCD X-ray diffractometer (Mo Kα radiation, λ = 0.71073 Å, 50 kV/30 mA). The structure was solved by direct methods with the SHELXS-97 program(38) and refined by full-matrix least-squares methods with the SHELXL-97 program.(39) Crystal data for 4: molecular formula C16H36PbSi4, MW = 548.00, monoclinic, Cm, a = 11.7157(19) Å, b = 17.054(3) Å, c = 6.2809(10) Å, β = 112.4050(10)°, V = 1160.2(3) Å3, Z = 2, Dcalcd = 1.569 g cm–3. A total of 10818 reflections were collected with 2315 independent reflections (Rint = 0.0280). The final R factor was 0.0189 for reflections with Io > 2σ(Io) and for all data; GOF = 1.065.
X-ray crystallographic data for 4 have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition no. CCDC 1433859. These data can be obtained free of charge from the CCDC (www.ccdc.cam.ac.uk/data_request/cif).

Experimental Procedure and Spectral and Crystallographic Data for the Stannatetrasilapyramidane Sn[Si4(SiMetBu2)4] (10)

Tetrakis(di-tert-butylmethylsilyl)tetrasilacyclobutadiene dianion dipotassium salt (prepared by the reduction of [(tBu2MeSi)4Si4]Br4 (80 mg, 0.075 mmol) with KC8 (72 mg, 0.533 mmol)) was mixed with SnCl2·diox (25 mg, 0.090 mmol) in dry, oxygen-free THF (2 mL). Then the reaction mixture was stirred at room temperature for 30 min. Then the solvent was removed under vacuum, and dry hexane was introduced. After the inorganic salt was filtered off, the residue was recrystallized from hexane to give 10 as bright yellow crystals (5 mg, 8% (based on the amount of the starting tetrabromocyclotetrasilane used)). Mp: 238 °C (dec). 1H NMR (C6D6, δ, ppm): 0.35 (s, 12 H, 4 CH3), 1.25 (s, 72 H, 8 (C(CH3)3)). 13C NMR (C6D6, δ, ppm): −3.5 (CH3), 21.0 (C(CH3)3), 30.0 (C(CH3)3). 29Si NMR (C6D6, δ, ppm): −33.5 (skeletal Si), 24.8 (substituent Si). 119Sn NMR (tol-d8, δ, ppm): −2104.2. UV–vis (hexane): λmax/nm (ε) 399 (5600). Anal. Calcd for C36H84Si8Sn: C, 50.25; H, 9.84. Found: C, 50.02; H, 9.49.
Single crystals of 10 for X-ray diffraction analysis were grown from a hexane solution. Diffraction data were collected at 150 K on a Bruker AXS APEX II CCD X-ray diffractometer (Mo Kα radiation, λ = 0.71073 Å, 50 kV/30 mA). The structure was solved by direct methods with the SHELXS-97 program(38) and refined by full-matrix least-squares methods with the SHELXL-97 program.(39) Crystal data for 9: molecular formula C36H84Si8Sn, MW = 860.44, triclinic, P1̅, a = 12.7913(8) Å, b = 12.8192(8) Å, c = 16.8594(11) Å, α = 68.348(1)°, β = 87.935(1)°, γ = 76.584(1)°, V = 2495.6(3) Å3, Z = 2, Dcalcd = 1.145 g cm–3. A total of 34840 reflections have been collected with 10150 independent reflections (Rint = 0.0893). The final R factor was 0.0332 for reflections with Io > 2σ(Io) and 0.0341 for all data; GOF = 1.085.
X-ray crystallographic data for 10 have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition no. CCDC 1433860. These data can be obtained free of charge from the CCDC (www.ccdc.cam.ac.uk/data_request/cif).

Experimental Procedure and Spectral and Crystallographic Data for the Pentagermapyramidane Ge[Ge4(SiMetBu2)4] (14)

Tetrakis(di-tert-butylmethylsilyl)tetragermacyclobutadiene dianion dipotassium salt (150 mg, 0.12 mmol) was mixed with GeCl2·diox (30 mg, 0.13 mmol) in dry, oxygen-free THF (1.5 mL). The reaction mixture was stirred at room temperature for 30 min. Then the solvent was removed under vacuum, and dry hexane was introduced. After the inorganic salt was filtered off, the residue was recrystallized from toluene to give 14 as orange crystals (31 mg, 26%). Mp: 240 °C (dec). 1H NMR (C6D6, δ, ppm): 0.44 (s, 12 H, 4 CH3), 1.26 (s, 72 H, 8 (C(CH3)3)). 13C NMR (C6D6, δ, ppm): −2.4 (CH3), 22.5 (C(CH3)3), 30.3 (C(CH3)3). 29Si NMR (C6D6, δ, ppm): 37.7 (substituent Si). UV–vis (hexane): λmax/nm (ε) 311 (1700), 403 (650), 490 (1300). Anal. Calcd for C36H84Ge5Si4: C, 43.56; H, 8.53. Found: C, 43.44; H, 8.53.
Single crystals of both 14a and 14b for X-ray diffraction analysis were grown from a benzene solution. Diffraction data were collected at 150 K on a Bruker AXS APEX II CCD X-ray diffractometer (Mo Kα radiation, λ = 0.71073 Å, 50 kV/30 mA). The structures were solved by direct methods with the SHELXS-97 program(38) and refined by full-matrix least-squares methods with the SHELXL-97 program.(39) Crystal data for 14a (folded pyramidane): molecular formula C36H84Ge5Si4, MW = 992.34, triclinic, P1̅, a = 12.223(1) Å, b = 14.770(1) Å, c = 14.911(1) Å, α = 102.763(1)°, β = 100.479(1)°, γ = 104.787(1)°, V = 2456.1(3) Å3, Z = 2, Dcalcd = 1.342 g cm–3. The final R factor was 0.0412 for 8460 reflections with Io > 2σ(Io) (Rw = 0.1216 for all data, 9957 reflections); GOF = 1.053. Crystal data for 14b (square-planar pyramidane): molecular formula C36H84Ge5Si4, MW = 992.34, triclinic, P1̅, a = 12.261(2) Å, b = 14.742(2) Å, c = 14.878(2) Å, α = 103.005(3)°, β = 100.259(3)°, γ = 105.015(5)°, V = 2449.5(6) Å3, Z = 2, Dcalcd = 1.345 g cm–3. The final R factor was 0.0586 for 9115 reflections with Io> 2σ(Io) (Rw = 0.1735 for all data, 9995 reflections); GOF = 1.069.
X-ray crystallographic data for 14a and 14b have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition nos. CCDC 1044815 and CCDC 1044816. These data can be obtained free of charge from the CCDC (www.ccdc.cam.ac.uk/data_request/cif).

Experimental Procedure and Spectral and Crystallographic Data for the Stannatetragermapyramidane Sn[Ge4(SiMetBu2)4] (15)

Tetrakis(di-tert-butylmethylsilyl)tetragermacyclobutadiene dianion dipotassium salt (230 mg, 0.18 mmol) was mixed with SnCl2·diox (52 mg, 0.19 mmol) in dry oxygen-free THF (2 mL). Then the reaction mixture was stirred at room temperature for 30 min. Then the solvent was removed under vacuum, and dry hexane was introduced. After the inorganic salt was filtered off, the residue was recrystallized from toluene to give 15 as orange crystals (20 mg, 11%). Mp: 240 °C (dec). 1H NMR (C6D6, δ, ppm): 0.45 (s, 12 H, 4 CH3), 1.27 (s, 72 H, 8 (C(CH3)3)). 13C NMR (C6D6, δ, ppm): −1.91 (CH3), 22.4 (C(CH3)3), 30.5 (C(CH3)3). 29Si NMR (C6D6, δ, ppm): 38.3 (substituent Si). 119Sn NMR (C6D6, δ, ppm): –1899.5. UV–vis (hexane): λmax/nm (ε) 323 (8900), 459 (4300). Anal. Calcd for C36H84Ge4Si4Sn: C, 41.63; H, 8.15. Found: C, 42.04; H, 8.00.

119Sn Mössbauer Spectroscopy

Temperature-dependent Mössbauer effect spectroscopy was carried out in transmission geometry using a 10 mCi CaSn*O3 source as described previously.(40) Spectrometer calibration was effected using a Fe(0) foil as an absorber at room temperature, and the spectrometer zero point was obtained from a room-temperature BaSnO3 absorption spectrum, which is also the reference point for the isomer shift values reported. The temperature range of the Mössbauer effect measurements was 90–192 K.

Computational Details

Geometry optimization, frequency analysis, wave function stability, and all related calculations were performed using the Gaussian 09 program(41) based on the B3LYP/Def2TZVP level.(42) The WBI, NPA charge distribution, and NRT analyses were performed using the NBO6.0 program.(43) The AIM analysis was carried out using the program AIMAll.(44) The views of the optimized geometries and molecular orbitals were generated using the Chemcraft 1.8 program.(45)

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00924.

  • Crystallographic data for 2 (CIF)

  • Crystallographic data for 4 (CIF)

  • Crystallographic data for 10 (CIF)

  • Details of X-ray crystallography for the pyramidanes 2, 4, and 10 (tables of the crystallographic data including atomic positional and thermal parameters) and computational details (bond lengths and other structural parameters, resonance structures) (PDF)

  • Cartesian coordinates for the calculated structures (XYZ)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Authors
    • Vladimir Ya. Lee - Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305−8571, Japan;  Email: [email protected]
    • Akira Sekiguchi - Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305−8571, Japan;  Email: [email protected]
    • Vladimir I. Minkin - Institute of Physical and Organic Chemistry, Southern Federal University, 194/2 Stachki ave., Rostov on Don 344090, Russian Federation;  Email: [email protected]
  • Authors
    • Olga A. Gapurenko - Institute of Physical and Organic Chemistry, Southern Federal University, 194/2 Stachki ave., Rostov on Don 344090, Russian Federation
    • Yuki Ito - Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305−8571, Japan
    • Takahiko Meguro - Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305−8571, Japan
    • Haruka Sugasawa - Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305−8571, Japan
    • Ruslan M. Minyaev - Institute of Physical and Organic Chemistry, Southern Federal University, 194/2 Stachki ave., Rostov on Don 344090, Russian Federation
    • Rolfe H. Herber - Racah Institute of Physics, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel
    • Heinz Gornitzka - CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France;  Université de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France
  • Notes

    The authors declare no competing financial interest.

Acknowledgment

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This work was financially supported by Grant-in-Aid for Scientific Research program (Nos. 15K05413, 23655027, 24245007, and 90143164) from the Ministry of Education, Science, Sports, and Culture of Japan, the Basic Part of Internal SFedU Grant (Project No. 213.01-2014/005), and the JSPS–RFBR Joint Japan–Russia Research Project. The authors thank Prof. Masaichi Saito (University of Saitama, Saitama, Japan) for the measurement of the 207Pb NMR spectrum.

References

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    The [Ph5C5]- anion reacts with GeI2, SnCl2, or Pb(OAc)2 in ethereal solvents to form the sandwich compds. [η5-Ph5C5]2E (I, E = Ge, Sn, Pb) in 60, 73, and 54% yields, resp. The infusible air- and water-stable products (off-white, bright-yellow, and deep-red colored, resp.) exhibit a weak parent ion in the mass spectrum plus the [Ph5C5E]+ cation (100%) and [Ph5C5H]+ and [Ph5C5]+. The E = Pb deriv. is thermochromic. The A1u νasym ring-metal stretch is obsd. in the IR at 164, 149, and 135 cm-1 and the corresponding A1g νsym stretch at 153, 138 and 134 cm-1 in the Raman for I (E = Ge, Sn, Pb), resp. Pentaphenylstannocene is formed in 23% yield from η5-cyclopentadienyltin(II) chloride as light-yellow, monoclinic platelets which are moderately air-stable, decompg. at 150°. The 119Sn Moessbauer spectra at 78 K of deca- and pentaphenylstannocene (IS = 3.75, 3.67; QS = 0.583, 0.868 mm s-1, resp.) are similar to other stannocenes. The crystal structure of pentaphenylstannocene contg. 1 mol. of PhMe was detd. The mol. is bent through 151.1(2)° between the tin vectors to the pentaphenylcyclopentadienyl (2.487(5) Å) and cyclopentadienyl (2.391(5) Å) ring centroids. The Ph groups are not antiparallel to their cyclopentadienyl ring.
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    Despite the presence of the apical LPs, computations revealed low Lewis basicity of pyramidanes, presumably because of the exceptionally high s character of these LPs (for example, NBO analysis shows 93% s contribution in stannapyramidane 3). Accordingly, 3 did not show any notable signs of complexation with the Lewis acids, such as BF3 and B(C6F5)3. Thus, the Sn–B/Sn–F bond distances are ca. 4.50/4.66 Å in 3[BF3], and the Sn–B distance is 6.02 Å and the shortest Sn–F distance is 4.75 Å in 3[B(C6F5)3], whereas the complexation energies are almost 0 in both cases.

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    Nucleus-independent chem. shifts (NICS), the neg. of the abs. magnetic shieldings (in ppm) computed at the ab initio GIAO-HF/6-31 + G* level at ring centers (non-weighted means of the heavy atom coordinates), are proposed as a simple and efficient magnetic probe for characterizing aromaticity and antiaromaticity. For a series of five membered heterocycles, NICS correlate with arom. stabilization energies, magnetic susceptibility exaltations, and geometric criteria of aromaticity. Arom. compds. have neg. NICS (e.g., -9.7 for benzene and -15.1 for pyrrole), whereas antiarom. systems, in contrast, exhibit pos. NICS values (18.1 for pentalene and 27.6 for cyclobutadiene). In addn., NICS can characterize the individual rings in polycyclic arom. (e.g., -19.7 and -7.0 for the five- and seven-membered rings in azulene) and antiarom. (e.g., -2.5 and 22.5 for the six- and four-membered rings in benzocyclobutadiene) systems as well as the spherical aromaticity of cage compds., e.g., closo-B12H122- (-34.4) and the 1,3-dehydro-5,7-adamantanediyl dication (-50.1). The C60 NICS confirm that the 5-rings are paramagnetic and the 6-rings are diamagnetic, but the magnitudes are not large.
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    From Tetragermacyclobutene to Tetragermacyclobutadiene Dianion to Tetragermacyclobutadiene Transition Metal Complexes
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    The reaction of 3,4-dichlorotetragermetene deriv. 2 with Na2[Fe(CO)4] in THF produced a (tetragermacyclobutadiene)tricarbonyliron complex, [{η4-(tBu2MeSi)4Ge4}]Fe(CO)3 4, which has a slightly folded Ge4 ring perhapto coordinated to the Fe center. Structural and spectral characteristics of 4 show a remarkable π-donating ability of the tetragermacyclobutadiene ligand toward the transition metal, surpassing that of tetrasilacyclobutadiene and cyclobutadiene ligands. Redn. of 2 with KC8 resulted in exclusive formation of the dipotassium salt of the tetragermacyclobutadiene dianion deriv. 32-·[K+(thf)2]2, representing a rare example of a 6π-electron compd. that, on the basis of its structural and magnetic properties, was recognized as a nonarom. species. Reaction of 32-·[K+(thf)2]2 with CpCoI2(PPh3) produced a (cyclopentadienyl)(tetragermacyclobutadiene)cobalt complex, [{η4-(tBu2MeSi)4Ge4}]CoCp 7, as the first example of a sandwich compd. featuring an all-germanium-contg. cyclic polyene ligand.
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    Although the crystal structures of all “heavy” pyramidanes 10, 14, and 15 were undoubtedly confirmed by X-ray crystallography, all of them showed positional (or rotational) disorder in their structures. Therefore, the crystallographically reliable metric parameters of only stannatetrasilapyramidane 10 are discussed in this paper.

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    Heavy analogues of the 6π-electron anionic ring systems: Cyclopentadienide ion and cyclobutadiene dianion
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    An account is given of the development of the SHELX system of computer programs from SHELX-76 to the present day. In addn. to identifying useful innovations that have come into general use through their implementation in SHELX, a crit. anal. is presented of the less-successful features, missed opportunities and desirable improvements for future releases of the software. An attempt is made to understand how a program originally designed for photog. intensity data, punched cards and computers over 10000 times slower than an av. modern personal computer has managed to survive for so long. SHELXL is the most widely used program for small-mol. refinement and SHELXS and SHELXD are often employed for structure soln. despite the availability of objectively superior programs. SHELXL also finds a niche for the refinement of macromols. against high-resoln. or twinned data; SHELXPRO acts as an interface for macromol. applications. SHELXC, SHELXD and SHELXE are proving useful for the exptl. phasing of macromols., esp. because they are fast and robust and so are often employed in pipelines for high-throughput phasing. This paper could serve as a general literature citation when one or more of the open-source SHELX programs (and the Bruker AXS version SHELXTL) are employed in the course of a crystal-structure detn.
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  • Abstract

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    Chart 1

    Chart 1. Stable Local Minima Found on the C5H4 PES
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    Scheme 1

    Scheme 1. Synthesis of Germa-, Stanna-, and Plumbapyramidanes 2–4
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    Figure 1

    Figure 1. Crystal structure of plumbapyramidane 4 (ORTEP view). Thermal ellipsoids are given at the 30% probability level; H atoms are not shown. Selected bond lengths (in Å) and bond angles (in deg): Pb1–C1 = 2.440(5), Pb1–C2 = 2.453(4), Pb1–C3 = 2.443(6), C1–C2 = 1.479(5), C2–C3 = 1.487(5), C1–Si1 = 1.870(6), C2–Si2 = 1.871(4), C3–Si3 = 1.867(6); C1–C2–C3 = 89.6(3), C2–C1–C2# = 90.7(4), C2–C3–C2# = 90.0(4), C1–Pb1–C2 = 35.18(12), C2–Pb1–C3 = 35.36(12). Folding angle of the C1–C2–C3–C2# ring: 1.08°.

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    Figure 2

    Figure 2. 119Sn Mössbauer spectrum of the stannapyramidane 3 at 128 K. The residuals (bottom trace) indicate that the absorption profiles are not exactly Lorentzian but that there are no other absorptions evident.

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    Figure 3

    Figure 3. Temperature dependence of the recoil-free fraction f of the stannapyramidane 3.

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    Figure 4

    Figure 4. Comparison of the root mean square amplitudes of vibration of the metal atom in stannapyramidane 3 (top straight line) and [o-(Ph2P)C6H4]2}SnMe2 (bottom dashed line).

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    Figure 5

    Figure 5. Comparison of the cage bonding MOs of the model H-substituted C-based pyramidanes E[C4H4] (5′, E = C; 6′, E = Si; 2′, E = Ge; 3′, E = Sn; 4′, E = Pb).

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    Scheme 2

    Scheme 2. Covalent A vs Hypothetical Ionic B Resonance Extremes for the Hybrid Pyramidanes 2–4 (2, E = Ge; 3, E = Sn; 4, E = Pb)
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    Scheme 3

    Scheme 3. Interconversion between the Two Degenerate Distorted-Pyramidal Structures C and C′ (Energy Minima) and Square-Planar Pyramidal Structure D (TS)
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    Figure 6

    Figure 6. Comparison of the cage bonding MO of the model H-substituted Si-based pyramidanes E[Si4H4] (7′, E = C; 8′, E = Si; 9′, E = Ge; 10′, E = Sn; 11′, E = Pb).

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    Scheme 4

    Scheme 4. Synthesis of the “Heavy” Pyramidanes 10, 14, and 15, Featuring Si4 or Ge4 Bases
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    Figure 7

    Figure 7. Crystal structure of stannatetrasilapyramidane 10 (ORTEP view). Thermal ellipsoids are given at the 30% probability level. H atoms are not shown. Only the main position (86%) of the rotationally disordered structure is depicted for clarity. Selected bond lengths (in Å) and bond angles (in deg): Si1–Si2 = 2.280(2), Si2–Si3 = 2.279(2), Si3–Si4 = 2.273(2), Si1–Si4 = 2.275(2), Sn1–Si1 = 2.786(3), Sn1–Si2 = 2.801(3), Sn1–Si3 = 2.782(3), Sn1–Si4 = 2.763(4); Si1–Si2–Si3 = 89.5(1), Si2–Si3–Si4 = 90.4(1), Si3–Si4–Si1 = 89.8(1), Si2–Si1–Si4 = 90.3(1), Si1–Sn1–Si2 = 48.2(1), Si2–Sn1–Si3 = 48.2(1), Si3–Sn1–Si4 = 48.4(1), Si1–Sn1–Si4 = 48.4(1). Folding angle of the Si1–Si2–Si3–Si4 ring: 0.70°.

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    Figure 8

    Figure 8. Square-planar pyramidal Sn[C4]- and Sn[Si4]-pyramidanes 3 and 10 vs folded pyramidal Sn[Ge4]-pyramidane 15.

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  • References

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      The crystal structure of germapyramidane 2 is given in the Supporting Information, whereas the crystal structure of the stannapyramidane 3 was discussed in a preliminary communication.(11)

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      Typical lengths for C–C single and C═C double bonds are 1.54 and 1.34 Å, respectively.

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      The sum of the single bond covalent radii are as follows: (1) for the Ge and C atoms, 1.96 Å; (2) for the Sn and C atoms, 2.15 Å; (3) for the Pb and C atoms, 2.19 Å.

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      Despite the presence of the apical LPs, computations revealed low Lewis basicity of pyramidanes, presumably because of the exceptionally high s character of these LPs (for example, NBO analysis shows 93% s contribution in stannapyramidane 3). Accordingly, 3 did not show any notable signs of complexation with the Lewis acids, such as BF3 and B(C6F5)3. Thus, the Sn–B/Sn–F bond distances are ca. 4.50/4.66 Å in 3[BF3], and the Sn–B distance is 6.02 Å and the shortest Sn–F distance is 4.75 Å in 3[B(C6F5)3], whereas the complexation energies are almost 0 in both cases.

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      Although the crystal structures of all “heavy” pyramidanes 10, 14, and 15 were undoubtedly confirmed by X-ray crystallography, all of them showed positional (or rotational) disorder in their structures. Therefore, the crystallographically reliable metric parameters of only stannatetrasilapyramidane 10 are discussed in this paper.

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      The typical range for the Si–Si single bonds is 2.31–2.41 Å (average value 2.34 Å): Kaftory, M.; Kapon, M.; Botoshansky, M. In The Chemistry of Organic Silicon Compounds; Rappoport, Z.; Apeloig, Y., Eds.; Wiley: New York, 1998; Vol. 2, Part 1, Chapter 5.
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