Mechanistic Studies into the Sn/Hg Exchange Reaction of 1,2-Fc(PPh₂)(SnMe₃) with HgCl₂: Competitive Sn–Me over Sn–Fc Cleavage in Noncoordinating Solvents

Tin–mercury exchange represents one of the most versatile and cleanest routes to arylmercuric halides. We found that reaction of the ferrocenylstannane 1,2-Fc­(PPh₂)­(SnMe₃) (1) with HgCl₂ in acetone results in the unexpected spontaneous formation of 2·HgCl₂, a diferrocenylmercury (Fc₂Hg)-supported diphosphine chelate ligand as its HgCl₂ complex. Mechanistic investigations into the generation of 2·HgCl₂ reveal initial formation of an adduct of 1 with HgCl₂, followed by competitive Sn–Me and Sn–Fc bond cleavage with formation of chloromercury and chlorodimethylstannyl-substituted ferrocene species. When the reaction is performed in chloroform as a noncoordinating solvent, formation of 2·HgCl₂ is not observed, but instead 1,2-Fc­(PPh₂)­(SnMe₂Cl) (5) is generated as the major product. 5 is initially isolated as a complex with MeHgCl (generated as a byproduct), but the latter can be easily released by heating under high vacuum. When 5 is further reacted with 2 equiv of HgCl₂ in acetone, the adduct 1,2-Fc­(PPh₂·HgCl₂)­(HgCl) (6·HgCl₂) forms. An X-ray crystal structure of 6·HgCl₂ shows two individual molecules that form Hg···Cl-bridged dimers, which in turn are linked by intermolecular Hg···Cl contacts to give a polymeric structure. In contrast, the equimolar reaction of 5 and HgCl₂ results in initial complexation to give 5·HgCl₂, which slowly transforms into the diferrocenylmercury species 2·HgCl₂. These results confirm that both 1 and the byproduct 5 obtained by Sn–Me bond cleavage are competent intermediates in the formation of complex 2 in acetone. The preferential cleavage of the Sn–Me over the Sn–Fc bond in noncoordinating solvents is attributed to the presence of the diphenylphosphino group in an ortho position. These observations may have broader implications due to the formation of MeHgCl as a highly toxic and volatile byproduct and suggest that noncoordinating solvents are better avoided and extreme caution is necessary when performing Sn/Hg exchange reactions on donor-substituted substrates.