Stabilizing Monoatomic Two-Coordinate Bismuth(I) and Bismuth(II) Using a Redox Noninnocent Bis(germylene) Ligand

: The formation of isolable monatomic Bi I complexes and Bi II radical species is challenging due to the pronounced reducing nature of metallic bismuth. Here, we report a convenient strategy to tame Bi I and Bi II atoms by taking advantage of the redox noninnocent character of a new chelating bis(germylene) ligand. The remarkably stable novel Bi I cation complex 4 , supported by the new bis(iminophosphonamido-germylene)xanthene ligand [(P)Ge II (Xant)Ge II (P)] 1 , [(P)Ge II (Xant)Ge II (P) = Ph 2 P-(N t Bu) 2 Ge II (Xant)Ge II (N t Bu) 2 PPh 2 , Xant = 9,9-dimethyl-xan-thene-4,5-diyl], was synthesized by a two-electron reduction of the cationic Bi III I 2 precursor complex 3 with cobaltocene (Cp 2 Co) in a molar ratio of 1:2. Notably, owing to the redox noninnocent character of the germylene moieties, the positive charge of Bi I cation 4 migrates to one of the Ge atoms in the bis(germylene) ligand, giving rise to a germylium(germylene) Bi I complex as suggested by DFT calculations and X-ray photoelectron spectroscopy (XPS). Likewise, migration of the positive charge of the Bi III I 2 cation of 3 results in a bis(germylium)Bi III I 2 complex. The delocalization of the positive charge in the ligand engenders a much higher stability of the Bi I cation 4 in comparison to an isoelectronic two-coordinate Pb 0 analogue (plumbylone; decomposition below − 30 ° C). Interestingly, 4 [BAr F ] undergoes a reversible single-electron transfer (SET) reaction (oxidation) to afford the isolable Bi II radical complex 5 in 5 [BAr F ] 2 . According to electron paramagnetic resonance (EPR) spectroscopy, the unpaired electron predominantly resides at the Bi II atom. Extending the redox reactivity of 4 [OTf] employing AgOTf and MeOTf affords Bi III (OTf) 2 complex 7 and Bi III Me complex 8 , respectively, demonstrating the high nucleophilic character of Bi I cation 4 .


■ INTRODUCTION
−5 However, the synthesis of such isolable species is challenging and requires suitable ligation around the low-valent E atom to prevent E−E bond oligomerization and disproportionation.−15 Utilizing the bis(silylene)xanthene, 16 we achieved the whole series of heavier tetrylones previously. 7,11,13,14The central E 0 atom is two-coordinated by two sufficient donor ligands and obeys the octet rule, retaining its four valence electrons as two lone pairs. 17,18−22 Monovalent group 15 element complex cations of the type L 2 E + (L = donor, E = N, P, As, Sb, Bi) 23−27 are known isoelectronic species but particularly difficult to tame for the heaviest pnictogen, bismuth.−41 Utilizing the same Lewis donor−acceptor stabilization strategy as previously applied for tetrylones, two examples of isolable two-coordinate Bi I cation complexes have been synthesized, namely, cyclic (alkyl)(amino)carbene (cAAC)supported Bi I cation complex A 42 and bis(silylene)-supported Bi I cation complex B 23 (Chart 1a).A is only stable in ethereal solutions at relatively high concentrations under an inert atmosphere; dilution of the solution results in its decomposition.The intrinsic lability of Bi I cation complexes may be attributed to the higher nucleophilicity and electropositive and redox character of the Bi I site caused by notably larger, diffuse, and polarizable valence orbitals of Bi compared to those of its lighter nonmetallic congeners (N, P, As). 43,44These electronic features may enable Bi I cation complexes to act as electron transfer reagents under the concomitant formation of Bi II and Bi III complexes, respectively.However, the redox reactivity of A and B and whether they are suitable precursors for Bi II radical complex are currently unknown.Bi II radical complexes, in turn, are also scarce. 45Their existence as reactive intermediates 37,46 and transient species 47 has been postulated in previous studies.The unequivocal identification of Bi II radical complexes is challenging, in particular, the electron paramagnetic resonance (EPR) characterization due to the enormous (isotropic and anisotropic) hyperfine interactions and the large nuclear quadrupole moment of the 209 Bi nucleus (I = 9/2; 100% natural abundance).−53  In this work, we present the synthesis and characterization of unprecedented types of remarkably stable monatomic twocoordinate Bi I and Bi II complexes, which are stabilized by the novel chelating bis(iminophosphonamido-germylene)xanthene ligand [(P)Ge II (Xant)Ge II (P)] 1, [(P)Ge II (Xant)Ge II (P) = Ph 2 P(NtBu) 2 Ge II (Xant)Ge II (NtBu) 2 PPh 2 , Xant = 9,9-dimethyl-xanthene-4,5-diyl].The Bi I complex 4 can be regarded as an isoelectronic analog of a two-coordinate Pb 0 complex (plumbylone).In contrast to the thermolabile plumbylone analogue, however, which decomposes above −30 °C, 4 is stable even in boiling benzene most likely due to resonance stabilization: the positive charge of the Bi I cation migrates to one of the germanium atoms in the ligand, giving rise to a (germylium)germylene Bi I situation as suggested by density functional theory (DFT) calculations.Starting from the Bi I complex 4[BAr F ], the single electron oxidation with ferrocenium BAr F (Cp 2 FeBAr F , BAr F = tetrakis[3,5-bis-(trifluoromethyl)phenyl]borate) resulted in the formation of the isolable Bi II radical complex 5[BAr F ] 2 with the unpaired electron mainly located at the bismuth center, and each germanium atom features one positive charge.Oxidative reactions of 4[OTf] with AgOTf and MeOTf (OTf = OSO 2 CF 3 ) afford cationic Bi III complexes, demonstrating the remarkably high nucleophilic character of Bi I complex 4.
Starting from (P)GeCl, chelating ligand 1 is readily accessible in a one-pot synthesis (Scheme 1).Dilithiation of 4,5-dibromo-9,9-dimethylxanthene with 2 M equiv of s-BuLi in Et 2 O, followed by a salt metathesis reaction with (P)GeCl, afforded the desired bis(iminophosphonamido-germylene)xanthene 1 as a yellow powder in 72% yield.Its 31 P{ 1 H} NMR spectrum shows a singlet at δ 26.6 ppm.The molecular structure of 1 established by XRD analysis reveals a Ge•••Ge distance of 4.071 Å, excluding attractive interaction between the two Ge atoms (Figure 1).Compared to bis(amidinato germylene)xanthene, 56 compound 1 is a stronger chelating bis(germylene) ligand due to the P−N bond polarization mentioned above.
Treatment of 1 with 1 M equiv of BiI 3 in THF at room temperature led to the formation of 2 as a brown powder in 85% yield.The iodide counteranion in 2 can be easily displaced by BAr F and OTf anion upon mixing of 2 with 1 M equiv of NaBAr F or AgOTf in dichloromethane (DCM), Journal of the American Chemical Society affording 3[BAr F ] (yield: 92%) and 3[OTf] (yield: 72%) as orange powders, respectively (Scheme 1).As expected, the 1 H NMR spectra of 2, 3[BAr F ], and 3[OTf] are practically identical.The 11 B{ 1 H} NMR spectrum of 3[BAr F ] shows a singlet at δ −6.6 pm, which is attributed to the BAr F anion.The 19 F{ 1 H} NMR signal of 3[OTf] (δ −78.8 ppm) indicates the presence of a "free" OTf anion. 24The 31 P{ 1 H} NMR spectra of 2, 3[BAr F ], and 3[OTf] exhibit a singlet at δ 54.6 ppm, which is significantly downfield shifted compared to that of 1 (δ 26.6 ppm).The molecular structures of 2, 3[BAr F ], and 3[OTf] were determined by XRD analysis (Figure 1; also see the Supporting Information).In each of these compounds, a stereoactive lone pair is present on the four-coordinate bismuth atom with a central Bi III atom adopting a seesaw geometry with two iodine atoms located in axial positions (I1− Bi1−I2:169.779(15)-174.507(10)°).The Ge1−Bi1−Ge2 angles range from 103.895 (14)  With the cationic Bi III I 2 precursors 2, 3[BAr F ], and 3[OTf] in hand, we envisioned that the monatomic Bi I complex could be obtained through their reductive deiodination.The reaction of 2 with 2 M equivs of KC 8 or {(ArNacnac)Mg I } 2 57 in THF led to decomposition of 2 most likely due to overreduction.To our delight, the reactions of 3[BAr F ] and 3[OTf] with 2 M equivs of cobaltocene (Cp 2 Co) in toluene (3[BAr F ]) and THF (3[OTf]) at room temperature gave red solutions, from which 4[BAr F ] and 4[OTf] were isolated as red crystals in 83 and 45% yields, respectively (Scheme 2).Featuring the same cation moiety, the 1 H NMR spectra of both products reveal a singlet at the same chemical shift (δ 1.08 ppm) for the tert-butyl groups, implying that cation 4 is symmetric and solvent is separated in solution.Accordingly, the 31 P{ 1 H} NMR spectra of 4[BAr F ] and 4[OTf] exhibit a singlet at the same chemical shift of δ 44.5 ppm, significantly upfield shifted compared to those of 2 and 3 (δ 54.6 ppm), but downfield shifted compared to that of 1 (δ 26.6 ppm), respectively.The 11 B{ 1 H} NMR spectrum of 4[BAr F ] shows a sharp singlet at δ −6.6 ppm, corresponding to the weakly coordinating BAr F anion, and the 19 F{ 1 H} NMR spectrum of 4 [OTf] shows a signal at δ −78.9 ppm for the "free" OTf anion. 24Notably, a two-electron oxidation of 4 with I 2 in DCM-d 2 after 5 min at room temperature restored compound 3 in quantitative yield.Single-Electron Transfer Reactions.Owing to the presence of two lone pairs of electrons on the Bi I center in 4, we envisaged that 4 is a suitable precursor for the synthesis of Bi II and Bi 0 radical complexes via single-electron oxidation/ reduction reactions.This is supported by the cyclic voltammetry (CV) analysis of compound 4[BAr F ].The CV exhibits two quasi-reversible oxidation processes at E 1/2 ≈ −0.54 V and −0.09V, followed by a third irreversible oxidation wave at E p,a = 0.66 V vs Fc/Fc + and an irreversible reduction wave at E p,c = −1.53V vs Fc/Fc + (see the Supporting Information, Figure S29).To examine this hypothesis, we carried out single-electron oxidation reactions of Compound 5[BAr F ] 2 is paramagnetic and shows broad resonance peaks in the solution 1 H NMR spectrum at room temperature (see the Supporting Information, Figure S37).The effective magnetic moment (μ eff ) of a microcrystalline solid sample, measured with a superconducting quantum interference device (SQUID) magnetometer, is temperaturedependent (see the Supporting Information, Figure S60).The magnetic moment linearly decreases with decreasing temperature and shows a value of 1.98 μ B at 300 K, which is comparable to reported Bi II radical compounds. 49,50,53The latter value is consistent with a single unpaired electron S = 1/ 2 system.

4[BAr
EPR spectroscopy was applied to explore the electronic nature of the Bi II radical complex 5[BAr F ] 2 in more detail.The  continuous-wave X-band (9 GHz) and pseudomodulated Qband field swept echo (34 GHz) spectra, respectively (see the Supporting Information, Figure S61), exhibit a very broad multiline EPR signal with components spanning the whole experimentally accessible spectral range.As a consequence of the typically very strong hyperfine coupling of 209 Bi (I = 9/2) in the GHz range, yet moderate g-anisotropy, the number of EPR transitions monitored at X-band and Q-band frequencies are limited and much higher magnetic field is required to decouple g-matrix and A-tensor and completely resolve the EPR spectrum.Thus, we used W-band (94 GHz) EPR and obtained a quite well-resolved multiplet of lines with recognizable g-anisotropy, yielding for each g-component 10 lines, which partially overlap with each other (Figure 4).The respectively, and the larger A av = 3799 MHz and A av = 3180 MHz reported for D 49 and G 53 , respectively.The axial part T of the hyperfine coupling (T = ((A 1 + A 3 )/2 − A iso )) found here for 5[BAr F ] 2 with T = −470 MHz is in its absolute value again clearly larger than that found for E and F (−245 MHz and −333 MHz) 58 and very similar to the axial parts given for D 49 and G 53 with T = −498 MHz and T = −462 MHz, respectively.Following the parameters, A av and T can be used to estimate the spin density in the Bi 6s and 6p orbitals according to Morton and Preston. 59The A av = 1980 MHz of 5[BAr F ] 2 yields with the A = 77,530 MHz 59 for a 100% occupied Bi 6s (obtained assuming g = 2.0023) a population P 6s (Bi) ≅ 0.03.The T = −470 MHz of 5[BAr F ] 2 on the other hand yields with the P = 1659 MHz 59 for a 100% occupied Bi 6p orbital and the angular factor −2/5 a population P 6p (Bi) ≅ 0.71.Thus, the SOMO orbital of 5[BAr F ] 2 has predominant 6p character with only a small admixture of 6s in agreement with the DFT results (see below) and the findings for the other Bi radicals D, E, F and G. [49][50][51][52][53]58 A total Bi spin population of about 3/4 is deduced from the EPR spectrum, suggesting substantial spin delocalization into the ligand as reported also for E and F. 58 Significantly less spin delocalization is reported for D 49 and G 53 .This might at first sight contradict the small difference between the T-values of 5[BAr F ] 2 and G 53 with −470 and −462 MHz, but can immediately be explained by the significantly different g av -values (1.99 and 1.79, respectively) of the two radicals. Th larger g av of 5[BAr F ] 2 induces a larger magnetic moment, which partially compensates for the higher Bi spin density in G for the resulting hyperfine couplings.This effect explains the significantly different P 6p (Bi) values deduced for 5[BAr F ] 2 and G at rather similar T values.The interpretation of the different total P(Bi) spin densities for the complexes in terms of spin delocalization into the ligands has to be taken with care since the analysis following Morton and Preston 59 is based on the assumption of atomic Bi 6s and 6p orbitals and can only give estimates.Quantitative comparison with DFT data requires information about ligand hyperfine couplings.Unfortunately, the direct neighbor Ge of Bi in 5[BAr F ] 2 has only a relatively low abundance with 73 Ge (7.76%) and a high spin stable isotope with I = 9/2, which is a rather bad reporter for spin delocalization into the ligand, and produces no discernible splitting in the EPR spectrum, making hyperfine selective methods necessary for obtaining further information.
DFT Calculations.DFT calculations at the BP86-D3(BJ)/ def2-TZVP level were performed to shed light on the electronic structure, stability, and chemical bonding of monocation 4, and radical dication 5.For completeness, we also calculated the hypothetical neutral analogue Bi 0 compound 6. Figure S71 shows the calculated geometries of the three compounds and the most important bond lengths and angles.It is interesting to note that the calculated Bi−Ge distances in neutral 6 (2.654 Å) stay nearly the same in cation 4 (2.655Å) but are slightly longer in radical dication 5 (2.715 Å).The cation 4 has an electronic singlet ground state with a triplet state of 29.6 kcal mol −1 higher in energy, while 5 and 6 have doublet ground states where the quartet states are higher in energy by 47.7 and 29.3 kcal mol −1 , respectively.The theoretical values for 4 and 5 are in very good agreement with the experimental data.
The energy minimum structure of neutral 6 has C s symmetry.The geometry optimizations of 4 and 5 gave structures which are slightly distorted from C s symmetry.Calculations with enforced C s symmetry gave structures which have very small imaginary frequencies (2.4i for 4 and 9.4i for 5) even after using a superfine grid.The energy difference between the C s structures and the energy minima are >0.1 kcal mol −1 for 4 and 0.4 kcal mol −1 for 5 and the differences in the bond lengths and angles are negligible.We decided to use the C s structures shown in Figure S71 (see the Supporting Information) for the bonding analysis for simplicity, which does not affect our conclusion.
The energetically most favorable reaction pathways for rupture of the Ge−Bi bonds in 4, 5 and 6 suggest that bismuth dissociates always as neutral Bi atom even from radical dication 5.This is in line with the calculated charge distribution.The NBO atomic charges on Bi are −0.23e in 4 and 0.26e in 5. Notably, the partial charge on Bi in neutral 6 (−0.25e) is nearly the same as that in cation 4. The calculated values of the bond dissociation energy (BDE) are D e = 62.2 kcal mol −1 for neutral 6, D e = 87 kcal mol −1 for cation 4 and D e = 64.1 kcal mol −1 for the radical dication 5.The zero-point energy (ZPE) and thermal corrections (see Supporting Information, Figure S71) suggest that all three compounds are thermodynamically stable with respect to liberation of Bi 0 .It is amazing that the BDE for the dissociation of neutral 6 and radical dication 5 has nearly the same value.The difficulty in isolating neutral 6 can be explained by its very low ionization potential (IP).Notably, the calculated adiabatic IP for 6 is only 3.55 eV at the BP86-D3(BJ)/def2-TZVP level, which is even much less than for the cesium atom (4.04 eV at the BP86-D3(BJ)/def2-TZVP), 3.89 eV experimentally. 60e analyzed the bonding situation in 4 and 5 in more detail using a variety of methods.Figure 5 shows the natural orbitals that are related to the valence state of Bi in the two compounds.In 4, there is a σ lone pair orbital at Bi with 90% s character and a strongly polarized Bi−Ge π orbital, which is 90% localized at Bi that mimics a π lone-pair orbital.There are two Bi−Ge σ-bonding orbitals which are slightly polarized toward Ge.The Bi atom in 4 has two lone-pair orbitals.−64 The orbitals of the radical dication 5 are very similar but the π lonepair orbital is occupied by only one electron, which may better be named as "lone single orbital" (Figure 6b).The NBO spin density of 0.87e on Bi also supports this assignment (see the Supporting Information, Figure S74 for the spin density plot).The unpaired electron in a π-type orbital is in agreement with the EPR data, showing a small s character and a predominant p orbital character of the unpaired electron.
The NBO orbitals of 4 and 5 support the Lewis structures shown in Scheme 4. A more detailed analysis using the EDA-NOCV method reveals a bonding situation that is more complex than that described by the Lewis structure.We carried out EDA-NOCV calculations of 4 and 5 using Bi and the bis(germylene) ligand L with different charges and different  electron configurations (see the Supporting Information, Tables S25 and S26).−70 It turned out that the best description of the L(Ge)-Bi bonds in 4 and 5 has neutral Bi and singly or doubly charged ligand in the given electron configurations, which agrees with the calculated partial charges given by the NBO method.The electronic reference states of Bi are the 2 D excited state for 4 and the 4 S ground state for 5 whereas the ligands L + and L 2+ have doublet and triplet states, respectively.
The breakdown of the total orbital interactions ΔE orb in pairwise contributions shows that in 4 and 5 there are two large σ terms ΔE orb(1) and ΔE orb(2) and one weaker π term ΔE orb(3) , which can be identified by inspecting the associated deformation densities displayed in Figure 7.The radical dication 5 has two electron-sharing σ Ge−Bi−Ge interactions and one very weak π Ge←Bi→Ge back-donation of the singly occupied π orbital at Bi.This supports the bonding, as suggested by the NBO method and depicted by the Lewis structure shown in Scheme 4. The EDA-NOCV results for cation 4 suggest that the two Ge−Bi−Ge σ bonds actually arise from one dative interaction into the vacant 6p π∥ AO of Bi Ge→ Bi(6p π∥ )←Ge (+,−) but the second σ bond comes from electron-sharing interactions of the singly occupied 6p σ AO of bismuth, Ge−Bi(6p σ )�Ge.Such a detail of the interatomic interactions cannot adequately be sketched by a Lewis structure.It does not invalidate the description given by the Lewis structure shown in Scheme 4 but underlines the fact that the actual interatomic interactions are more complex than those suggested by Lewis formulas.We thus propose the relevance of the main resonance structures 4′, 4″, 5′ and 5″ (Scheme 4).The increase in the oxidation states of the Ge atoms in 4 and 5 is in line with the Ge(3d) binding energies obtained by X-ray photoelectron spectroscopy (XPS) (see  Energy values are given in kcal mol −1 .b The percentage contribution with respect to total attraction is given in parentheses.c The percentage contribution in parentheses is given with respect to total orbital interaction.Supporting Information, Figure S64).In fact, the values of 4 (32.38 eV) and 5 (32.48 eV) are significantly blue-shifted in comparison with that of the Ge II atoms in the "free" bisgermylene 1 (30.88 eV).
Detailed inspection of the deformation densities reveals further interesting electronic information.The charge flow red → blue associated with ΔE orb (1) shows an area charge accumulation at Bi for both compounds 4 and 5 that stems from the hybridization of the 6s/6p σ AOs.
We also analyzed the electronic charge in 4 and 5 with the quantum theory of atoms in molecules (QTAIM) method developed by Bader. 71Figure 8 shows the Laplacian distribution of electron density ∇ 2 ρ(r) in the Ge−Bi−Ge planes of the two compounds.As expected, there are bond critical points (BCPs) for the Bi−Ge bonds where the areas of charge accumulation (∇ 2 ρ(r) < 0, indicated by red dotted lines) are closer to the Ge atoms, signaling a polarization of the bonds toward Ge.The negative values of the energy density H(r c ) at the BCPs are characteristic of covalent interactions. 72ote that there is no area of charge accumulation for the σ lone-pair electrons at Bi, because the 6s AO of the latter is very diffuse.
We carried out further calculations on the precursor 3 precursor.The computed distances and angles are in good agreement with the experimental data (Supporting Information, Figure S67).Remarkably, the NBO analysis revealed that the BiI 2 moiety carries an overall negative charge of −0.45e (Bi: 0.46e; I1: −0.48e; I2: −0.43e) and the ligand has a positive charge of +0.55e.EDA-NOCV calculations using the fragments BiI 2 and the ligand L with charges and electronic state showed that the best description is given by BiI 2 − (ate-type) and L 2+ (T) in the electronic triple state, which give two electronsharing Bi-L single bonds (see the Supporting Information, Table S24, Figure S68).This agrees with the shape of the natural orbitals of 3 suggested by the NBO analysis (see the Supporting Information, Figure S69).A lone-pair orbital at Bi is found in the NBO analysis, and it also appears as dominant part of the HOMO of 3 (see the Supporting Information, Figure S70).The EDA-NOCV and NBO methods suggest that the best description of the bonding situation of 3 is sketched as in Scheme 2.

Reactivity of Bi I Complex 4[OTf] toward AgOTf.
Interestingly, the CV of 4[OTf] exhibits two quasi-reversible oxidation processes at E 1/2 ≈ −0.46 V and −0.1 V and an irreversible oxidation wave at E p,a = 0.44 V vs Fc/Fc + (see the Supporting Information, Figure S36).Upon adding 1 M equiv of AgOTf to 4[OTf] in DCM, only half of 4[OTf] was consumed.NMR monitoring showed that a new compound was formed in the solution along with a black metallic precipitate.We thus speculated that a two-electron oxidation reaction occurred and compound 7 was furnished (Scheme 5).
When two molar equivalents of AgOTf were added, nearly all of 4[OTf] was converted to 7. After workup, 7 was isolated as a yellow powder in 80% yield.The 31 P{ 1 H} NMR spectrum of 7 displays a singlet at δ = 58.9ppm.The 19 F{ 1 H} NMR spectrum shows two signals at δ = −76.9and δ = −78.6 ppm, indicating that the OTf anions have two distinct chemical environments.Notably, a lighter congener Sb I cation undergoing Sb I →Ag I coordination with AgOTf, instead of the twoelectron oxidation, was recently reported. 26he molecular structure of compound 7 was confirmed by XRD analysis (Figure 9).Compound 7 crystallizes in monoclinic space group P12 1 /c1.Similar to compounds 2 and 3, the central Bi III site also adopts a seesaw geometry with the two OTf units located in the axial positions (O5−Bi1− O2:169.85(18)°).There is a strong interaction between these two OTf units and the Bi III atom [Bi−O lengths: 2.410(5) and 2.397(5) Å].In contrast, one OTf anion is noncoordinated to   As expected, the red color of 4[OTf] faded upon the addition of MeOTf.Multinuclear NMR analysis confirmed the formation of new species 8 (Scheme 6).After workup, compound 8 was isolated as a pale-yellow powder in 65% yield.Its 1 H NMR spectrum shows two singlets at δ 1.04 and 1.09 ppm, respectively, for the tert-butyl groups, implying an asymmetric structure in solution.In addition, the 1 H NMR resonance at δ 2.43 ppm corresponds to the methyl group on the bismuth center, which is downfield shifted compared to that of BiMe 3 (δ 1.11 ppm). 73The 31 P{ 1 H} NMR spectrum of 8 displays a singlet at δ 53.4 ppm, downfield shifted compared to that of 4[OTf] (δ 44.5 ppm).The 19 F{ 1 H} NMR spectrum exhibits a singlet at δ −78.6 ppm, indicating a weak coordinating interaction between the OTf anions and Bi III atom in solution.

Journal of the
Compound 8 was isolated as pale-yellow crystals and its molecular structure was established by XRD analysis (Figure 10).It crystallizes in the orthorhombic space group P2 1 2 1 2 1 as an ion triple with a separated dication and two OTf counteranions.The Bi III center adopts a trigonal pyramidal geometry due to methyl coordination.The Bi−C distance of 2.247 (7) Å is shorter than that of [(TBDSi 2 )BiMe][BAr F ] 2 (2.300 Å). 23 The Ge−Bi distances in 8 of 2.7393(7) and 2.7426 (7) Å are longer than those in the two-coordinate Bi I complex 4 and radical dication 5 (2.6627(4)−2.7147(3)Å), but shorter than those in the four-coordinate 2, 3, and 7 (2.7497 (6)  We are currently applying these redox-ligand-active Bi I and Bi II complexes in photoredox single-electron transfer catalysis in our laboratory.We believe that the stabilization of Bi I cations and Bi II radical dication complexes by using redox noninnocent ligands is a promising strategy to gain access to related low-valent p-block metal complexes that are otherwise very difficult to tame.

■ ASSOCIATED CONTENT
* sı Supporting Information

Chart 1 .
(a) Reported Examples of Bi I Cation Complexes (A,B); (b) Neutral Bi II Radical Complexes (C−E); (c) Cationic Bi II Radical Complexes (F,G); (d) This Work: Bi I and Bi II Complexes with a Redox Non-innocent Bis(germylene) Ligand a a Ar = 2,6-Diisopropylphenyl.

Scheme 2 .
Scheme 2. Synthesis of 4 and Reversible Interconversion of 3 and 4 with I 2 4[BAr F ].The latter was reacted with 1 M equiv of Cp 2 Fe[BAr F ] in Et 2 O, furnishing the desired Bi II radical complex 5[BAr F ] 2 in 77% yield (Scheme 3).Complex 5[BAr F ] 2 is a brownish powder in the solid state and highly air-sensitive, but it can be stored under a N 2 atmosphere at −30 °C for several weeks.In addition, treatment of 5[BAr F ] 2 with 1 M equiv of Cp 2 Co in THF for 5 min at room temperature restored compound 4[BAr F ] in quantitative yields.We also conducted the single-electron reduction of 4[BAr F ] in an attempt to synthesize neutral Bi 0 compound 6.When one molar equiv of Cp 2 Co or KC 8 was reacted with 4[BAr F ] in THF at −78 °C, the reaction ultimately resulted at ambient temperature in formation of elemental bismuth as a precipitate due to decomposition.XRD analysis revealed that compound 5[BAr F ] 2 is an ion triple that crystallizes in the triclinic space group P1̅ and contains the monatomic Bi II moiety 5 as shown in Figure 3. Similar to 4[BAr F ], 5[BAr F ] 2 features a two-coordinate Bi II center, but there are two separated BAr F counteranions present in the molecular structure, indicating that 5[BAr F ] 2 is a Bi II radical complex.The Ge1−Bi1−Ge2 angle of 107.583(9)°in 5[BAr F ] 2 is slightly larger than that of 4 in 4[BAr F ] (103.981(12)°) and 4[OTf] (104.67(3)°).Additionally, the Ge−Bi distances (2.7112(3) and 2.7147(3) Å) of 5[BAr F ] 2 are slightly longer than those of 4 (2.6627(4)−2.6712(9)Å) but shorter than those of 3 (2.7737(5)−2.8035(5)Å), presumably due to the weaker π-back-donation from the Bi II center to both Ge atoms in 5[BAr F ] 2 .

Scheme 3 . 2 Figure 3 .
Scheme 3. Reversible Interconversion of 4[BAr F ] and 5[BAr F ] 2 simulation of the experimental data resulted in the following gmatrix principal values of g = [2.39,1.92, 1.66] for an S = 1/2 system and the hyperfine tensor principal values A = [1370, 2920, 1650] MHz, with the largest A-component along the intermediate g-component.The average g value (g av = (g 1 + g 2 + g 3 )/3) of 5[BAr F ] 2 is g av = 1.99 and rather close to the free electron g, as observed for the gallium-stabilized Bi radicals E and F 58 , but distinctly different from the nitrogen-coordinated Bi radicals D 49 and G 53 , which show a g av significantly smaller than 2. All EPR values of G refer only to the protonated compound (R = H); the methylated variant (R = Me) has almost identical values.The symmetry of the hyperfine tensor of 5[BAr F ] 2 corresponds to that of E and F, 58 which is also aligned with its largest component along the intermediate gcomponent.In contrast, the hyperfine tensors reported for D 49 and G 53 are aligned with their largest component along the smallest g-component.The magnitude of the hyperfine coupling of 5[BAr F ] 2 with an average value (A av = (A 1 + A 2 + A 3 )/3) A av = 1980 MHz is intermediate between the smaller A av = 1383 MHz and A av = 1650 MHz of E and F, 58

Figure 4 .
Figure 4. Pseudomodulated W-band field swept echo EPR-spectrum of 5[BAr F ] 2 recorded at 10 K.The experimental spectrum is displayed as black and the corresponding simulation as red line.The g-matrix derived from the simulation is g = [2.39,1.92, 1.66] and the hyperfine coupling A = [1370, 2920, 1650] MHz.The signal labeled by an asterisk is related to an impurity from manganese.

Figure 5 .
Figure 5. Natural orbitals and their AO compositions of cation 4 (left) and the radical dication 5 (right).

Scheme 4 .
Scheme 4. Main Resonance Structures of Cation 4 and Radical Dication 5, Respectively a

Figure 7 .
Figure 7. Plot of the deformation densities Δρ (1) −Δρ (3) which are associated with the pairwise orbital interactions ΔE orb(1) −ΔE orb(3) of cation 4 and radical dication 5.The eigenvalues ν are a measure for the relative amount of charge transfer.The direction of the charge flow is red → blue.
to 2.8035(5) Å), respectively.■ CONCLUSIONS In summary, two remarkably resonance-stabilized Bi I complexes, 4[BAr F ] and 4[OTf], supported by an electron-rich chelating bis(iminophosphonamido-germylene) ligand 1, have been synthesized by reduction of the bis(germylium)Bi III I 2 cation precursors 3[BAr F ] and 3[OTf] with Cp 2 Co, respectively.Featuring two lone pairs at the Bi I atom center, Bi I cation complex 4 can be considered as an isoelectronic analogue of a Pb 0 complex.Notably, due to the redox noninnocent character of bis(germylene) ligand 1, the positive charge of Bi III I 2 cation in 3 and Bi I complex 4 migrates to the germanium atoms in 1 which increases the stability of the Bi III and Bi I centers substantially.This is also corroborated experimentally by the XPS data.The single-electron oxidation of 4[BAr F ] with Cp 2 Fe[BAr F ] furnishes the Bi II radical complex 5[BAr F ] 2 with an unpaired electron located at the Bi II center and both Ge atoms each featuring one positive charge.Oxidation of 4[OTf] with 2 M equivs AgOTf and addition of MeOTf affords the Bi III complexes 7 and 8, respectively, demonstrating the nucleophilic character of Bi I cation 4.
48ong these, C48is considered a redox radical Bi II /Bi III couple and lacks observable EPR signals.Compounds D 49 and E 50,51 are isolable neutral Bi II radicals with the unpaired electron predominantly residing at the Bi atom.In addition, F 52 and G 53 represent known cationic Bi II complexes.

of Bi I Complex 4[OTf] toward MeOTf.
To investigate the nucleophilic character of the Bi I complexes, we conducted the reaction of 4[OTf] with electrophilic MeOTf.