Solid-state synthesis and characterization of σ-alkane complexes, [Rh(L2)(η2,η2-C7H12)]

: The use of solid/gas and single-crystal to single-crystal synthetic routes is reported for the synthesis and characterization of a number of σ -alkane complexes: [Rh(R 2 P-(CH 2 ) n PR 2 )( η 2 , η 2 -C 7 H 12 )][BAr F4 ]; R = Cy, n = 2; R = i Pr, n = 2,3; Ar = 3,5-C 6 H 3 (CF 3 ) 2 . These norbornane adducts are formed by simple hydrogenation of the corresponding norbornadiene precursor in the solid state. For R = Cy ( n = 2), the resulting complex is remarkably stable (months at 298 K), allowing for full characterization using single-crystal X-ray di ﬀ raction. The solid-state structure shows no disorder, and the structural metrics can be accurately determined, while the 1 H chemical shifts of the Rh ··· H − C motif can be determined using solid-state NMR spectroscopy. DFT calculations show that the bonding between the metal fragment and the alkane can be best characterized as a three-center, two-electron interaction, of which σ CH → Rh donation is the major component. The other alkane complexes exhibit solid-state 31 P NMR data consistent with their formation, but they are now much less persistent at 298 K and ultimately give the corresponding zwitterions in which [BAr F4 ] − coordinates and NBA is lost. The solid-state structures, as determined by X-ray crystallography, for all these [BAr F4 ] − adducts are reported. DFT calculations suggest that the molecular zwitterions within these structures are all signi ﬁ cantly more stable than their corresponding σ -alkane cations, suggesting that the solid-state motif has a strong in ﬂ uence on their observed relative stabilities.


INTRODUCTION
The selective and atom-efficient functionalization of the C−H bonds in alkanes continues to be an important area of homogeneous catalysis. In particular, upgrading alkane feedstocks (such as methane and ethane) into commodity chemicals has the potential for enormous economic and societal impact. 1−3 Nevertheless, the controlled functionalization of these fossil-resource-derived hydrocarbons is difficult, and the development of selective transition-metal-catalyzed C− H activation processes that operate at low temperatures is one of the major challenges in the area of homogeneous catalysis. 4−8 A central problem in this endeavor is the fact that alkanes are poor nucleophiles, especially compared with arenes or olefins, where the π-systems encourage coordination to a metal center. In contrast, the C−H σ-bond is strong and non-polar, and steric interactions from the alkyl group also disfavor approach to a metal center. Alkanes are thus poor ligands, coordinating only weakly to metal centers to form σcomplexes with three-center, two-electron (3c-2e) M···H−C interactions, 9,10 typically with bond enthalpies of less than 15 kcal/mol. 11−13 This makes study of the subsequent C−H activation processes (whether by oxidative cleavage, electrophilic activation, or σ-CAM-assisted metathesis 10 ) challenging, as intermediates are difficult to observe. 14 Despite this, extensive investigations on the mechanism of C−H activation of alkanes using Pt(II)-catalyzed processes based upon the [PtCl 4 ] 2− Shilov system have provided detailed insight into these processes. 15−22 These and other 23−26 mechanistic studies have provided compelling indirect evidence for the existence of σ M···H−C intermediates. Such transient intermediates (or closely related transition states) are also central to the catalytic functionalization of alkanes, for example, transition-metal-catalyzed alkane dehydrogenation, 5,27,28 alkane metathesis, 29 and alkane borylation, 30 as well as alkane activation processes catalyzed by post-transition metals, 31 and gas separation and alkane activation processes mediated by framework materials that also have coordinatively unsaturated metal sites. 32−34 Direct evidence for σ-alkane complexes has come from a variety of spectroscopic and crystallographic techniques, slowly building over a period of 40 years. 14 Initial evidence for such species came from IR and UV/vis spectroscopy in COphotodissociation experiments of group 6 and 8 carbonyls in methane matrices at very low temperatures (12 K). 35,36 More recently, fast spectroscopic techniques such as time-resolved infrared spectroscopy have been developed to study the coordination of alkanes with reactive metal carbonyls formed from photodissociation of a CO ligand. 13,37−40 These species are generally short-lived in solution, but the enhanced lifetime of some (hours), especially those of the 5d metals (Re and W), allows for their characterization at low temperatures (typically ca. 193 K) by NMR spectroscopy when in situ photodissociation techniques are used. For example photolysis of Re(η 5 -C 5 H 4 R)(CO) 3 (R = H, i Pr) in alkanes (cyclopentane, pentane) led to the observation of σ-alkane complexes, exemplified by Re(η 5 -C 5 H 5 )(CO) 2 (cyclopentane), A, Chart 1. 41 −43 In a similar manner, photolysis of Mn(η 5 -C 5 H 5 )(CO) 3 in butane or propane forms the corresponding alkane complexes, e.g., B. 44 The supporting ligands can also be changed to other facially capping groups closely related to cyclopentadienyl. 45−47 These complexes are formed as mixtures with the starting materials and decompose on warming to room temperature. At low temperatures they have lifetimes from minutes to hours, and the products of decomposition are often the parent carbonyl, formed from recombination with photoejected CO. One way around this is to photo-eject N 2 from an appropriate precursor, for which a combination of being a significantly poorer nucleophile with a greater efficiency for photo-ejection compared with CO leads to higher in situ yields of the alkane products. 48 Recently, Brookhart and co-workers reported a different methodology for generating σ-alkane complexes at very low temperatures in solution, by protonation of a methyl or ethyl precursor Rh(PONOP)R (PONOP = 2,6-( t Bu 2 PO) 2 C 5 H 3 N; R = Me, Et) using an acid with a non-coordinating counterion at temperatures between 193 and 123 K in CDCl 2 F solvent to form [Rh(PONOP)(alkane)] + in solution, e.g., C. 49−51 These complexes were long-enough lived at these low temperatures to allow full characterization by NMR spectroscopy, as the methane complex has a half-life of about 80 min at 186 K, although the ethane complex is significantly less stable. Both complexes decompose by irreversible coordination of solvent to form spectroscopically characterized solvent complexes, i.e., [Rh(PONOP)(CDCl 2 F)][BAr F 4 ] (Ar F = 3,5-C 6 H 3 (CF 3 ) 2 ). The short lifetimes of all these alkane complexes even at very low temperatures, when coupled with their synthesis at often less than 100% efficiency, is a significant barrier to the generation of single-crystalline material suitable for study by Xray diffraction, the "gold-standard" in coordination chemistry for structural elucidation. Until recently, only two examples had been reported in which a saturated hydrocarbon was located within bonding distance of a transition metal center in the solid state, allowing for an analysis of the molecular structure by single-crystal X-ray diffraction techniques (Chart 2). The first of these, D, shows a molecule of heptane interacting with an iron(II)−porphyrin complex, 52 although crystallographic disorder prevented the accurate analysis of the Fe−alkane interaction. The second example, E, involves cyclic alkane adducts of an unsaturated uranium(III) complex. 53 Both D and E result from incorporation of a solvent molecule within the coordination sphere of the metal, potentially assisted by host− guest effects in addition to any direct M···H−C bond interaction. Neither of these complexes is stable on solvation. Related structural diffraction studies come from the binding of light alkanes to iron centers in an extended metal−organic framework as characterized by powder neutron diffraction experiments, e.g., F. 32,33 Complexes in which there is a close intermolecular approach of an alkane to a group 1 cation (K + ) have also been reported, although these interactions are characterized as being weakly electrostatic and are further stabilized by interactions between the alkane and a hydrophobic ligand pocket. 54 Recognizing that a significant barrier to characterizing σalkane complexes in the solid state by X-ray diffraction is their instability in solution on the time scales and at the temperatures required for the production of single crystals, we recently reported the use of solventless conditions, 55 i.e., gas/solid reactivity, to synthesize a σ-alkane complex directly in the solid state, Scheme 1. In this crystal-to-crystal transformation, 56−62 a crystalline norbornadiene (NBD) Rh precursor, [1-NBD]-[BAr F 4 ], was treated with dihydrogen, resulting in addition of H 2 to the diene to form directly a saturated norbornane (NBA) fragment bound to the Rh(I) center through two 3c-2e Rh··· H−C interactions, [BAr F 4 ]. 59 Hydrogenation of ringstrained NBD in [Rh(L) 2 (NBD)] + (L = phosphine) was reported by Schrock and Osborn in 1976 using solution methods, 63−65 and has since been studied in some detail, in particular for the asymmetric hydrogenation of alkenes. 66,67 Of course, when the reaction is performed in solvent, the alkane generated is simply lost from the metal's coordination sphere, either as the desired product of the reaction or in generating In this contribution we now report, using similar methodology, an exploration into the effects of changing the chelating phosphine backbone length (ethylene and propylene), the identity of the phosphine substituent (Cy, i Pr, O i Pr), and the anion on the resulting stability of the alkane complex. As part of this we report the remarkable example of an indefinitely (months) room-temperature-stable transition metal σ-alkane complex, alongside its characterization by solid-state NMR spectroscopy and an assessment of its electronic structure by computational methods.  F 4 ] could be prepared analytically pure and recrystallized from CH 2 Cl 2 /pentane solvent mixture to give reasonably sized (e.g., 0.5 × 0.5 × 0.1 mm) blood-red single crystals in 59% yield. The resulting single-crystal X-ray structure of [BAr F 4 ] is shown in Figure 1.  Figure 1B and Supporting Information, Figure S1). The cross-cage B···B distances are 17.26(2), 19.23(1), and 19.832(8) Å, while the unit cell volume is 3287.04(13) Å 3 (Z = 2). One of the [BAr F 4 ] − anions is orientated so that two of its aryl groups enfold the NBD fragment ( Figure 1C). These metrics and the gross motif are very similar to those reported for [ (Figure 2A), in line with crystallographic non-equivalence of the phosphine groups. In the 13 C{ 1 H} NMR spectrum ( Figure 3A), in addition to signals assigned to [BAr F 4 ] − aryl groups (δ 164−116) and alkyl phosphine groups (δ 40−20), four signals are observed at δ 91, 82, 70 (br), and 55. 71 Aided by DFT chemical-shift calculations, these signals are assigned to CC (C1, C4), CC (C2, C5), C(7), and C(3,6), respectively (see Supporting Information, Figures S2 and S11). The slight broadening of the bridge methylene group (C7) in comparison with the other signals of the NBD ligand is consistent with strong homonuclear coupling between the geminal protons. 72 2  Figure 2B). Such a downfield shift and increase in the 103 Rh− 31 P coupling constant is consistent with the formation of a σ-alkane complex, as reported for [1-NBA][BAr F 4 ]. 59 The 13 C{ 1 H} SSNMR spectrum demonstrates the disappearance of signals due to NBD between δ 100 and 55 ( Figure 3B), while DFT chemical shift calculations suggest that signals due to the NBA ligand are located between δ 21 and 45, in the region also associated with the cyclohexyl groups on the phosphine ligand. No signals that could be attributed to a coordinated [BAr F 4 ] − aryl group were observed (i.e., δ 90− 100), 59 59 This means that for 2 hydrogenation of the NBD precursor to form the σ-alkane complex as the only product has no real temporal constraint; unlike for [1-NBD][BAr F 4 ] in which rapid transfer at low temperature of freshly hydrogenated single crystals to the X-ray diffractomer is required. It should be noted that both previously reported neutral complexes D 52 and E, 53 that show evidence for an alkane ligand in close approach to the metal center, have not been reported to proceed with further reactivity in the solid state.

RESULTS
The solid-state structure, as determined by single-crystal Xray diffraction at 150 K, of [2-NBA][BAr F 4 ], is shown in Figure  4, which clearly shows a saturated NBA fragment interacting with the Rh center through two 3c-2e Rh···H−C interactions. Figure 5 shows that the relationship in the lattice between the [BAr F 4 ] − anions and the cation is very similar to [2-NBD][BAr F 4 ]. Interestingly this single-crystal to single-crystal transformation involves a change in space group upon hydrogenation, from P1̅ (Z = 2) in the NBD adduct to P2 1 / n (Z = 4) in the alkane adduct. Similar changes in space group during a single-crystal to single-crystal transformation have recently been reported by Balch and co-workers for the αand β-forms of Au 2 (μ-dppe) 2 I 2 ·2OCMe 2 (P2 1 /c and P1̅ , respectively). 74 Complex [BAr F 4 ] shows no disorder in the solid state for the cation, and the high quality of the diffraction data resulted in a very good structural refinement that even allowed for the location and free refinement of the hydrogen atoms. This is the first time this has been achieved in such a motif by single-crystal X-ray diffraction, as disorder (i.e., D 52 or    have precluded such analysis. In framework materials such as F (Chart 2) D-atoms have been located by powder neutron diffraction experiments. 32 In Although H atoms were located in the difference map, the limited accuracy possible with with X-ray crystallography means any metrical differences involving H atoms are not statistically significant, with C−H distances ranging from 0.97(4) to 1.02(4) Å. However, in combination with DFT calculations, some important features can be highlighted. In particular a small lengthening of the C−H bonds when bound to the Rh center is seen, with the calculated C1−H11 and C2−H21 distances averaging 1.157 Å compared to 1.101 Å for the spectator C1−H12 and C2−H22 bonds. A widening of the C− C−H angles when the C−H bond is interacting with the Rh center is also apparent [X-ray: C1−C2−H21 120 (2) is also related to crystallographically characterized σ-bis-silane, 85 σ-dihydrogen, 86 adjacent CH/BH agostic, 87 and σ-diborane(4) complexes. 88,89 There is also a structural similarity with the intramolecular agostic norbornyl complex [Pt(C 7 H 11 )-( t Bu 2 PCH 2 CH 2 P t Bu 2 )][BPh 4 ] reported by Spencer, which is formed by protonation of a neutral NBE complex. 90 Figure 5A, B), and the same relationship  between a single anion and cation is also retained ( Figure 5C), with the NBA ligand sitting in a cleft between two of the anion aryl rings. Similar to [BAr F 4 ] there is also a close approach of one of the bridge NBA methylene protons on C7 to the center of two of the anion aryl groups [2.98 and 3.18 Å]. The B···B cross cage distances have also not changed significantly between the two structures, while the unit cell volume has changed by only +1.8% (normalized Z): 6691.62(9) (Z = 4) versus 3287.04(13) Å 3 (Z = 2), respectively. This cavity described by the anions allows for the movement of the organic fragment in going from NBD to NBA in the final product in which a transformation from two bound alkenes to one bidentate σ-bis-alkane fragment occurs. We suggest that the hydrogenation proceeds via an NBE intermediate, 92 possibly related to that reported for complex  94 We have previously reported that [1-NBA][BAr F 4 ] produced via the solid-state route, when dissolved in CDCl 2 F, affords a major product at low temperature (−110°C) that is not the bound alkane complex (free NBA was observed), which we tentatively described as a solvent adduct of [Rh-( i Bu 2 PCH 2 CH 2 P i Bu 2 )] + or a complex with agostic interactions. Warming to −20°C resulted in the formation of [1-BAr F 4 ]. 59 Thus, the alkane is only weakly bound at best in solution, being rapidly displaced by solvent.
Dissolving Warming to 173 K results in a much simpler set of spectra that shows a single 31 P environment, only two environments in the 19 F NMR spectrum in a ratio of 6:18, and a 1 H NMR spectrum in the aromatic region that shows only four environments in the relative ratio 3:6:2:1. These data are very much like those for independently prepared  in CD 2 Cl 2 solution at 193 K (vide infra). We thus suggest that, on warming, libration of the metal fragment and rotation around the B−C bond occur. Minor decomposition products, indicated by free [BAr F 4 ] − , also grow in with increasing time and temperature.

Characterization of the Rh···H−C σ
Interaction by SSNMR Spectroscopy. The ready dissociation of the NBA alkane ligand when dissolved in solution (even at very low temperatures) prevents the observation of the Rh···HC interaction by solution NMR techniques. Although 1 H SSNMR spectroscopy might thus then appear to be the ideal technique, the investigation of hydrogen interacting with metal centers by 1 H solid-state NMR spectroscopy can be problematic due to the small chemical shift range (∼20 ppm) and large line widths that arise from strong proton background signals from supporting ligands and strong homonuclear proton− proton dipolar interactions. 95 Perhaps for this reason σ complexes (e.g., dihydrogen complexes) have not been extensively studied using SSNMR techniques, although isotopic substitution of H for D avoids many of the issues associated with 1 H SSNMR. 92 Figure 6, and analysis is aided by DFT-calculated 13 C and 1 H chemical shifts. It is apparent that there are three distinct regions in the projected 1 H NMR spectrum: a set of signals grouped around δ 8 assigned to the [BAr F 4 ] − anion, one centered around δ 1 assigned to the Cy/ NBA groups and a broad signal ca. δ −2, in the chemical shift region associated with σ interactions. The first two regions also correlate as expected to the appropriate chemical shift regions of the 13 C NMR spectrum. For the high field region of the 1 H NMR projection the broad signal centered at ca. δ −2 correlates to a signal at δ 25 in the 13 C{ 1 H} NMR spectrum; and calculations predict the C1 and C2 contact carbons ( Figure  6B)  (i.e., δ 30.0 and 24.8, respectively). A cross peak between a signal in the 1 H NMR projection at approximately δ −1 and δ( 13 C) 43.5 is assigned to the bridge CH 2 groups on the NBA that sit in a cleft between two [BAr F 4 ] − aryl groups (i.e., C7, Figure 5C). Such an orientation might be expected to promote ring-current shielding, accounting for the relatively high field chemical shift of the CH 2 protons. This analysis is given further weight by the observation of a similar cross peak in the 1 H− 13 C HECTOR SSNMR spectrum of [2-NBD][BAr F 4 ] (Figure S2), between a high field signal in the 1 H NMR projection (δ −0.52) and the signal assigned to the bridge CH 2 group (δ 70), which also has a similar relative orientation of aryl and methylene groups in the solid state to that found in [2-NBA][BAr F 4 ] ( Figure 1C). Similar high-field, ring-current effect, chemical shifts have been observed by 1 H SSNMR techniques in host−guest complexes where the distances between the shifted proton and the centroid of an arene is ∼2.6 Å, similar to those found here. 102 , and equally importantly these are only reproduced when the anion is present ( Figure 6B and Supporting Information, Figure S11). In the 1 H NMR projection of [2-NBD][BAr F 4 ] there are also no high-field signals observed at ca. δ −2. Taken collectively these data, alongside the fact that the same sample batch was used for both X-ray diffraction analysis, solid-state NMR studies and microanalysis, suggests that the highest-field signals in the 1 H NMR projection of the 1 H− 13 C HECTOR SSNMR spectrum can be assigned to the Rh···H−C environments. This identification of intermolecular σ M···H−C interaction involving an alkane complex using solid-state NMR spectroscopy complements recently reported M···H−Si examples. 98 2.  (Table S1) with five of Rh−C(aryl) distances spanning the range 2.270(2)− 2.357(2) Å. There is one longer Rh−C(aryl) distance, that to the ipso carbon C27 [Rh−C27 2.472(2) Å] that suggests an η 5 coordination motif. 104 These are reproduced in the structures computed by DFT for  ] (see Supporting Information, Figure S9, Table S3). Solution NMR spectroscopic characterization was performed by dissolving [BAr F 4 ] in cold CD 2 Cl 2 (197 K) and transfer to a pre-cooled spectrometer. However, even at this temperature decomposition was observed to unidentified species that also liberated free anion. Nevertheless the low temperature 1 H and 19  (1) signals due to a bound arene are observed in the 1 H NMR spectrum at δ 7.24 (1 H) and δ 7.08 (2 H), while the nonbound rings are equivalent (due to rotation around the B1− C27 bond) and show as two environments in a 3:6 ratio. Two resonances in a relative ratio of 6:18 are observed in the 19 F NMR spectrum. Thus,  ] is an accessible species in solution (albeit being very reactive in CD 2 Cl 2 ), perhaps suggesting that its lack of formation from [2-NBA][BAr F 4 ] is unlikely to be thermodynamic in origin, but more likely kinetic factors in the solid state are important. However, there is the caveat that the solution and solid-state thermodynamics might differ due the local environment provided by the lattice. Whatever the reasons behind this stability, clearly the Cy groups in the phosphine have an influence in this, as this is the only structural feature that has changed between 1 and 2.
2 4 ] − anions is not octahedral around the cation, but instead eight anions adopt a gyrobifastigium arrangement (formed from two face-sharing trigonal prisms, Supporting Information, Figure S1). There is no crystallographically imposed symmetry in this cation. Again, as for [BAr F 4 ], weak C−H···F−C hydrogen bonding is apparent between the cation and the CF 3 groups. The 31 P{ 1 H} SSNMR of [BAr F 4 ] shows an ABX pattern, consistent with crystallographically independent phosphorus environments. Addition of H 2 to [BAr F 4 ] in a solid/ gas reaction and following the temporal evolution of products by 31 Figure S5). A similar phase change (at 298 K) was reported for [1-BAr F 4 ]. 59 Despite repeated attempts we could not obtain single crystals of [3-NBA]-[BAr F 4 ] suitable for an X-ray diffraction study, due to it being short-lived. However, DFT calculations on the [3-NBA] + cation suggest its structure will be closely related to that of [2-NBA] + (Supporting Information,  F 4 ]. Yellow/orange crystals suitable for a single-crystal X-ray diffraction study of zwitterion  were obtained by recrystallization of the material produced via the solid/gas synthesis, using a C 6 F 6 /pentane solvent system. The structure is unremarkable, and very similar to those described for [1-BAr F 4 ] 59 and [2-BAr F 4 ] adopting what might be best described as an η 5 -aryl binding motif in the solid state (Supporting Information, Figure S6).
2.4. [Rh( i Pr 2 PCH 2 CH 2 CH 2 P i Pr 2 )] + . Interested in keeping the P-alkyl groups the same, but varying the bite-angle 105 of the c h e l a t i n g p h o s p h i n e , w e p r e p a r e d [ R h -( i Pr 2 PCH 2 CH 2 CH 2 P i Pr 2 )(NBD)][BAr F 4 ], [BAr F 4 ], for which orange crystals suitable for single-crystal X-ray diffraction could be grown from CH 2 Cl 2 /pentane solution. Although the extended solid-state structure shows the familiar octahedral arrangement of [BAr F 4 ] − anions around each of the [Rh( i Pr 2 PCH 2 CH 2 CH 2 P i Pr 2 )(NBD)] + cations, in this case there are two independent cations in the unit cell, one of which lies on a crystallographically imposed mirror plane. The structure could be accurately solved using a unit cell that contained 12 cations (space group C2/c), see Figure 9A, and leads to three crystallographically different phosphorus environments in the solid state. Other structural metrics are unremarkable, and there are also a number of weak C−H··· F−C hydrogen bonds (  [BAr F 4 ] shows at least two overlapping signals (Figure 8), one much broader than the other, at δ 21 and 19, respectively. This difference in line width between these two signals is marked, perhaps reflecting the symmetry attributes of the extended unit cell in [BAr F 4 ]. Addition of H 2 to crystalline [BAr F 4 ] in a solid/gas reaction resulted in a immediate change in color from orange to claret-red (Scheme 5). Following this process using 31

Journal of the American Chemical Society
Article over 12 h to form orange-yellow  ] at 298 K, as identified by its independent synthesis (eq 2); however, it is likely that crystal size influences the rate of this change, as commented upon for [1-NBA][BAr F 4 ]. 59 At 233 K complex [4-NBA][BAr F 4 ] appears to be stable by 31 P{ 1 H} SSNMR spectrometry, with the transformation to  halted. There also appears to be no loss of single-crystallinity on addition of H 2 in the solid state. However, despite repeated attempts, crystalline material that produced a reliable crystallographic solution for the structure of [4-NBA][BAr F 4 ] was not forthcoming, 106 although a partial structure showed the approximate heavy-atom positions of the cation.
The structure of the final product [4-BAr F 4 ] ( Figure 9B) reveals that the coordinated [BAr F 4 ] − anion has slipped with the aryl ring binding with the metal center in an η 4 -motif, with two longer Rh−C distances: Rh−C20, 2.552(2) Å and Rh− C  Figure 9C shows the solid-state structure, which demonstrates chelation of one DCE to the Rh(I) center through two Rh−Cl interactions. Isolated DCE complexes are surprisingly rare, 70,110,111 even though this is a common solvent used in synthesis and catalysis. In DCE solution the room temperature 31 P{ 1 H} NMR spectrum shows a single environment at δ 50.2 [J(RhP) 190 Hz] that is very similar to that observed for the product formed from solvation of [BAr F 4 ] at low temperature in CD 2 Cl 2 , perhaps suggesting a solvent adduct is also formed under these conditions. Solvent (CDF 2 Cl) adducts have also been suggested for complex C (on warming and loss of alkane) 49,51 and when [BAr F 4 ] is dissolved in this solvent. 59 2.5. [Rh{( i PrO) 2 PCH 2 CH 2 P(O i Pr) 2 }] + . As well as changing the steric environment of the phosphine, it was of interest to vary the electronics of the chelating ligand, in the anticipation that this would lead to a more persistent alkane complex. Changing to a more electron-withdrawing phosphite was attractive as calculations on [M(pincer)(methane)] + (M = Rh, Ir) systems have indicated that electron-withdrawing groups enhance the binding strength of the alkane. 50 The isopropoxide ligand ( i PrO) 2 PCH 2 CH 2 P(O i Pr) 2 was chosen, 112 as this maintains steric parameters similar to those of the isobutyl ligand in 1 but is likely to have significantly different electronic properties associated with the π-acidic phosphites.
Addition  Figure S8). Recrystallization from pentane yields single crystals suitable  to shed light on the factors determining the relative stabilities of the σ-alkane complexes. Initial geometries for optimization were derived from the relevant crystallographically determined structures or, when not available (as is the case for [Y-NBA] + , Y = 3−5), were adapted from the NBD precursor. The above discussion of the crystallographic structures highlighted the good agreement between experiment and computation for a range of key structural parameters and a full comparison is provided in the Supporting Information ( Figure S9 and Tables S2 and S3).
3.1. Electronic Structure of [2-NBA] + . The presence of two C−H → Rh σ-interactions in [2-NBA] + is confirmed by an analysis of the topology of the total electron density using the quantum theory of atoms-in-molecules (QTAIM) approach (see Figure 10). Curved bond paths between Rh and both H11 and H21 are seen, and these, in combination with the properties of the associated bond critical points (BCPs) are consistent with the presence of two equivalent σ-interactions (electron density, ρ(r) = 0.060 au; Laplacian, ∇ 2 ρ(r) = 0.208, and total energy density, H(r) = −0.013 au; data are for the Rh···H11 BCP). 115 . The presence of a ring critical point (RCP) approximately midway between Rh and the center of the C1−C2 bond is consistent with the chelating binding mode of the bidentate NBA ligand; no evidence for any C−C → Rh agostic interaction is seen. 120 Key molecular orbitals of the [2-NBA] + cation are shown in Figure 11. The high lying occupied orbitals are consistent with a Rh(I) complex exhibiting a square-planar coordination geometry. Thus, four high-lying occupied d-orbitals are apparent, with the HOMO (E = −6.98 eV) being dominated by Rh(d z 2 ) character. Below this lie two essentially non-bonding d xz and d yz orbitals, while the d xy orbital (E = −8.63 eV) is stabilized via π-back-donation interactions with the σ* ABMOs of the C1−H11 and C2−H21 bonds. The LUMO is  Table S5. predominantly d x 2 −y 2 in character and is M−L σ-antibonding with both the phosphorus centers and the C1−H11 and C2− H21 σ-BMOs. A significant HOMO−LUMO gap of 1.77 eV is computed. Evidence for Rh−alkane σ-bonding interactions is seen in two low-energy σ-BMOs (E = −12.09, −12.75 eV). A fragment analysis suggests that a large number of NBA orbitals contribute to these, making further interpretation on the basis of these delocalized MOs alone problematic.
In order to place the bonding interactions between the NBA ligand and the cationic {Rh(L 2 )} + fragment on a more quantitative footing NBO analyses were performed on [2-NBA] + . Important donor−acceptor pairs within [2-NBA] + are shown in Figure 12 along with the associated interaction energies, ΔE (2) , quantified using second order perturbation theory. NBA coordination arises via σ-donation from the σ CH orbital into the vacant trans-σ* RhP orbital (ΔE (2) = 20.0 kcal/ mol). This is reinforced by back-donation from a Rh lone pair (corresponding to the d xy orbital) into the unoccupied σ* CH orbital (ΔE (2) = 3.6 kcal/mol), further enhanced by donation from both the cis-and trans-σ RhP orbitals (ΔE (2) = 3.9 and 1.3 kcal/mol, respectively, giving a total back-donation of 8.7 kcal/ mol). σ CH → Rh donation is therefore the major component of the Rh−alkane interaction, this being more than twice that of σ* CH ← Rh back-donation. Overall, these σ-interactions in [2-NBA] + are relatively weak, as shown by comparing with the πinteractions of the CC double bond of the NBD ligand in [2-NBD] + (π CC → trans-σ* RhP = 56.3 kcal/mol; LP Rh (π) → π* CC = 36.1 kcal/mol). We have also assessed the possibility of a C1−C2···[Rh] σ-interaction in [2-NBA] + , but the small stabilization energy (ΔE (2) ≈ 0.5 kcal/mol) arising from two separate σ CC → σ* RhP donations indicates this interaction is negligible.
These five cations, as their [BAr F 4 ] − salts, show widely different stabilities in the solid state, from being unobservable (Y = 5) to being observable for minutes (Y = 3), hours (Y = 1, 4) or even months (Y = 2). The computed structures of the [Y-NBA] + cations, however, show relatively little variation in the key computed distances across the series (Rh···H11, 1.903−1.932 Å; C1−H11, 1.153−1.158 Å). This is also reflected in a narrow range computed for the BCP parameters and ΔE (2) values derived from the QTAIM and NBO studies, respectively, as well as the computed C−H stretching frequencies associated with the Rh···C−H bonds (see Supporting Information, Tables S4−S6). No correlation is therefore evident between the geometric or electronic properties of the free cations and their relative stabilities in the solid state.

CONCLUSIONS
Through manipulation of the identity of the supporting bidentate phosphine ligand (i.e., Cy 2 PCH 2 CH 2 PCy 2 ) we have been able to synthesize a remarkably stable example of a σalkane complex [BAr F 4 ] by a solid−gas crystal to crystal synthetic route. The stability of this complex has allowed for both accurate determination of the solid-state structure, including hydrogen positions for the Rh···H−C σ interactions, as well as solid-state NMR spectroscopic data to be collected in  which these interactions can also be observed in the 1 H NMR