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

Abstract

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) = Ph₂P(NtBu)₂Ge^II(Xant)Ge^II(NtBu)₂PPh₂, Xant = 9,9-dimethyl-xanthene-4,5-diyl], was synthesized by a two-electron reduction of the cationic Bi^IIII₂ precursor complex 3 with cobaltocene (Cp₂Co) 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^IIII₂ cation of 3 results in a bis(germylium)Bi^IIII₂ 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 Pb⁰ 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^IIIMe complex 8, respectively, demonstrating the high nucleophilic character of Bi^I cation 4.

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) Liganda

Scheme 1. Synthesis of Bis(germylene) 1 and Bi^IIII₂ Precursors 2 and 3

Figure 1

Molecular structures of 1 (top) and the cations in 2, 3[BAr^F] and 3[OTf] (bottom). Thermal ellipsoids are drawn at the 50% probability level. H atoms, anionic moieties and solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg): 1: Ge1–C1 2.0607(15), Ge2–C15 2.0458(15), N1–Ge1–C1 99.11(5), N2–Ge1–C1 96.42(5), N3–Ge2–C15 99.21(5), N4–Ge2–C15 92.06(5). 2: Bi1–Ge1 2.7578(6), Bi1–Ge2 2.7497(6), Bi1–I1 3.0921(5), Bi1–I2 2.9991(4), I1–Bi1–I2 169.779(15), Ge1–Bi1–Ge2 109.244(18). 3[BAr^F]: Bi1–Ge1 2.7737(5), Bi1–Ge2 2.7800(5), Bi1–I1 3.0406(3), Bi1–I2 3.0424(3), I1–Bi1–I2 172.326(10) Ge1–Bi1–Ge2 103.895(14). 3[OTf]: Bi1–Ge1 2.8035(5), Bi1–Ge2 2.7813(5), Bi1–I1 3.0404(3), Bi1–I2 3.0679(3), I1–Bi1–I2 174.507(10), Ge1–Bi1–Ge2 106.995(16).

Scheme 2. Synthesis of 4 and Reversible Interconversion of 3 and 4 with I₂

Figure 2

Molecular structures of the cation 4 in 4[BAr^F] and 4[OTf]. Thermal ellipsoids are set at the 50% probability. Hydrogen atoms, counteranions and solvent molecules are omitted for clarity. Selected distances (Å) and angles (deg): 4[BAr^F]: Bi1–Ge1 2.6672(4), Bi1–Ge2 2.6627(4), Ge1–Bi1–Ge2 103.981(12). 4[OTf]: Bi1–Ge1 2.6693(9), Bi1–Ge2 2.6712(9), Ge1–Bi1–Ge2 104.67(3).

Scheme 3. Reversible Interconversion of 4[BAr^F] and 5[BAr^F]₂

Figure 3

Molecular structure of the radical dication 5 in 5[BAr^F]₂. Thermal ellipsoids are set at the 50% probability. Hydrogen atoms, anionic moieties and solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg): Bi1–Ge1 2.7112(3), Bi1–Ge2 2.7147(3), Ge1–Bi1–Ge2 107.583(9).

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

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

Figure 6

Molecular orbitals of cation 4 (a) and radical dication 5 (b).

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

L = Ph₂P(NtBu)₂.

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.

Figure 8

Contour plot of the Laplacian of electron density of cation 4 and radical dication 5 in the Ge–Bi–Ge plane calculated at the BP86-D3(BJ)/def2-TZVP level. Red lines indicate areas of charge concentration [∇²ρ(r) < 0] and blue lines show areas of charge depletion [∇²ρ(r) > 0].

Scheme 5. Reaction of 4[OTf] with AgOTf

Figure 9

Molecular structure of 7. Thermal ellipsoids are set at the 50% probability. Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg): Bi1–Ge1 2.7796(8), Bi1–Ge2 2.7670(8), Bi1–O2 2.410(5), Bi1–O5 2.397(5), Ge1–Bi1–Ge2 107.22(2), O5–Bi1–O2 169.85(18).

Scheme 6. Reaction of 4[OTf] with MeOTf

Figure 10

Molecular structure of the dication in 8. Thermal ellipsoids are set at the 50% probability. Hydrogen atoms, anionic moieties and solvent molecules are omitted for clarity. Selected distances (Å) and angles (deg): Bi1–Ge1 2.7393(7), Bi1–Ge2 2.7426(7), Bi1–C16 2.247(7), Ge1–Bi1–Ge2 108.09(2), C16–Bi1–Ge1 97.6(2), C16–Bi1–Ge2 101.8(2).