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) = 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.