An investigation of C-S bond activation in transition metal crown thioether complexes using extended Huckel theory and electrospray mass spectrometry

摘要

Complexes of Re and Tc with 1,4,7-trithiacyclononane (9S3) differ from their later transition metal analogues in that their d~6 form ([M(9S3)_2]~+) undergoes instantaneous C-S bond cleavage yielding ethene and [M(9S3)L]~+ (L=SCH_2CH_2SCH_2CH_2S), a stable metal(III) thilate complex, cleanly in aqueous solution. This contrast is interpreted as signifying increased #pi#-back donation by Re and Tc, compared to later metals, into ligand C-S δ orbitals. In order to validate this hypothesis within an established theoretical framework, and to compare the predicted relative C-S bond lability with relative experimental lability in a series of d~6 analogues, extended Huckel theory (EHT) was used to investigate the bonding (M=Mo, Tc, Ru, Rh or Pd) while electrospray mass spectrometry (ES-MS) was used to compare ethene loss, in a series of analogous complexes (M = Tc, Re, Ru or Os). The C-S overlap populations were smaller for M = Tc~(II) and Tc~(I) than for later metal(II) analogues, and were smaller for Tc~I than for Tc~(II). Fragment molecular orbitals corresponding to C-S δ were more highly populated for M = Tc(II) and Tc~I than for later analogues, and also more highly populated for Tc~I than for TC~(II). ES-MS showed that ethene loss from Tc/Re~I and Tc/Re~(II) complexes occurred at much lower energies than from the Ru/Os~(II) analogues. EHT supports the hypothesis that C-S activation is caused by #pi#-back donation into C-S δ orbitals, and correctly that ethene loss occurs more readily from thenium and technetium d~5 and especially d~6 complexes than from later transition metal analogues.