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)(2)Ge-II(Xant)Ge-II(NtBu)(2)PPh2, Xant = 9,9-dimethyl-xanthene-4,5-diyl], was synthesized by a two-electron reduction of the cationic (BiI2)-I-III 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 (BiI2)-I-III cation of 3 results in a bis(germylium)(BiI2)-I-III 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 degrees C). Interestingly, 4[BArF] undergoes a reversible single-electron transfer (SET) reaction (oxidation) to afford the isolable Bi-II radical complex 5 in 5[BArF](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 (BiMe)-Me-III complex 8, respectively, demonstrating the high nucleophilic character of Bi-I cation 4.