Organobismuth radical is a chemical species that has unpaired electrons on bismuth centers within organic frameworks. These radicals are part of the broader family of pnictogen-centered radicals, which include nitrogen, phosphorus, arsenic, antimony, and bismuth. Bismuth radicals, with a +2 oxidation state (Bi(II)), are highly reactive and prone to degradation. They are sensitive to air and moisture, often undergoing disproportionation to form more stable bismuth species with different oxidation state: Bi(III) and Bi(0). This instability makes Bi(II) compounds challenging to isolate and handle. Despite these restrictions, significant progress has been made in recent years with the isolation and characterization of Bi(II) radicals. These species exhibit diverse reactivity, particularly in bond activation, radical polymerization, and cross-coupling reactions.
History The first organopnictogen compound with a +2 oxidation state was the dicacodyl, tetramethyldiarsine ((CH3)2As–As(CH3)2), reported in 1757. However, heavier elements like Sb(II) and Bi(II) compounds were much harder to synthesize due to their high reactivity and tendency to progress via disproportionation reactions into their +3 and 0 oxidation states. As a result, the first Bi(II) compound, tetramethyldibismuthine, was not reported until 1935 by Paneth and Loleit, Following this, a few diorganobismuth(II) compounds were reported after 1982. Despite these developments, long-lived bismuth-centered radicals remained largely unexplored for decades due to the inherent challenges of stabilizing them both electronically and coordinatively. In 2014, the bismuth radical character was observed in the solution phase, generated in situ through the dissociation of diorganobismuthanes. This was followed in 2015 by the Coles group, first solid-state monomeric Bi(II) radical, which was successfully stabilized by bulky aryl substituents. This marked a turning point, enabling continued exploration of various bismuth radicals and their unique reactivity, which remains an active area of research today.
Synthesis
Via reduction of Bi(III) In most cases of bismuth complexes, bismuth has a +3 oxidation state. Therefore, synthesizing bismuth radical species through one electron reduction from Bi(III) complexes would be the most readily accessible route.
The first persistent solution-state bismuth-centered radical character was reported by Iwamoto and coworkers in 2014. The dibismuthine compound could be obtained by reducing chlorobismuthine with KC8 to afford a purple crystalline solid in 47% yield (Scheme 1). Dibismuthine could generate bismuth radicals in the solution phase, by reversible dissociation of the weak bismuth-bismuth bond which results from the steric repulsion of bulky bidentate alkyl groups. The dibismuthine complex exists as a dimer in the solid state, but in solution, bismuth radicals are formed in situ through an equilibrium process. The bismuth radicals were characterized by variable temperature NMR, EPR and UV–Vis spectroscopy. Also, this complex forms a radical coupling product, with the bismuth radical being captured by TEMPO, clearly showing the existence of bismuth radicals in solution. Further research has revealed that these types of transient radical intermediates can also be identified in several forms, including pyridine-dipyrolide ligands (discovered by Turner in 2019) and Bi biradicals with N-terphenyl ligands (discovered by Schulz and coworkers in 2018).
In 2015, the first monomeric Bi(II) radical was reported by Coles and coworkers. A diamido bismuth(III) chloride was synthesized via a metathesis reaction from BiCl3 (Scheme 2). This tricoordinate bismuth chloride was then reduced using excess Mg, yielding a crystalline bismuth(II) radical as a dark red solid. This method produced a thermally stable, crystalline bismuth radical with a yield 86%. These sterically bulky ligands around the bismuth radicals provide effective kinetic protection, preventing the formation of dibismuthane and allowing the isolation of the monomeric Bi(II) radical instead. The crystal structure and magnetic properties were characterized using single-crystal X-ray diffraction (sc-XRD), 1H NMR spectroscopy with Evans’ method, and EPR spectroscopy.
A stable, monomeric Bi(II) radical in the solid state can be isolated in another form that is stabilized by lewis-acidic metal centers. In 2018, Schulz and coworkers reported another stable Bi(II) radical, supported by two electropositive Ga(III) centers. The Bi(II) radical complex was synthesized through the reaction of Cp*BiI2 with 2 equivalents of a Ga(I) complex (Scheme 3). The resulting bismuthinyl radical was fully characterized using sc-XRD, NMR, EPR, SQUID, and DFT studies. The lewis-acidic nature of the Ga ligands facilitates the delocalization of the unpaired electron on the Bi center across the entire ligand, enhancing the radical's stability. In 2020, the same group discovered that this Ga-stabilized Bi(II) radical can also be obtained from BiCl3 by reacting it with two equivalents of Ga(I) complexes, as part of their efforts to synthesize polybismuthane clusters.
Via homolysis of Bi(III)-E bonds Bismuth, a period 6 pnictogen metal, has large and diffuse atomic orbitals. Due to its diffuse orbitals, the overlap with ligand atoms is often inefficient, resulting in weak bonding. Consequently, many Bi(III) compounds are unstable at room temperature and prone to homolytic cleavage, which readily occurs under thermal or photochemical conditions. The homolysis of Bi–O or Bi–N bonds is frequently employed in such cases, generating unstable and transient bismuth radicals, often accompanied by the formation of phenoxy radicals or nitrogen-centered radicals. There are cases, such as stable Bi(III) phenolates, that produce persistent phenoxy radicals in the solution phase. However, the absence of Bi(II) paramagnetic signals suggests challenges in confirming the radical involvement of transient Bi(II) radicals.
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