Mechanistic Studies into the Sn/Hg Exchange Reaction of 1,2-Fc(PPh 2 )(SnMe 3 ) with HgCl 2: Competitive Sn-Me over Sn-Fc Cleavage in Noncoordinating Solvents

Alain C. Tagne Kuate, Roger A. Lalancette, Frieder Jäkle

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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(PPh 2 )(SnMe 3 ) (1) with HgCl 2 in acetone results in the unexpected spontaneous formation of 2·HgCl 2 , a diferrocenylmercury (Fc 2 Hg)-supported diphosphine chelate ligand as its HgCl 2 complex. Mechanistic investigations into the generation of 2·HgCl 2 reveal initial formation of an adduct of 1 with HgCl 2 , 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·HgCl 2 is not observed, but instead 1,2-Fc(PPh 2 )(SnMe 2 Cl) (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 HgCl 2 in acetone, the adduct 1,2-Fc(PPh 2 ·HgCl 2 )(HgCl) (6·HgCl 2 ) forms. An X-ray crystal structure of 6·HgCl 2 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 HgCl 2 results in initial complexation to give 5·HgCl 2 , which slowly transforms into the diferrocenylmercury species 2·HgCl 2 . 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.

Original languageEnglish (US)
Pages (from-to)677-687
Number of pages11
Issue number3
StatePublished - Feb 11 2019

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry

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