Indenyl ring slippage in crown thioether complexes [IndMo(CO)2L]+ and C-S activation of trithiacyclononane: experimental and theoretical studies

The macrocycle 1,4,7-trithiacyclononane (ttcn) reacts with (η(5)-Ind)Mo(CO)(2)(NCMe)(2)](+) (or (η(5)-Ind)Mo(CO)(2)(κ(2)-dme)](+)) to give (η(3)-Ind)Mo(CO)(2)(κ(3)-ttcn)](+) as the BF(4)(-) salt (1), but its reaction with (η(5)-Ind)Mo(CO)(2)(C(3)H(6))(FBF(3))] affords the C-S bond cleavage product (η(5)-Ind)Mo(CO)(κ(3)-1,4,7-trithiaheptanate)]BF(4) (6), which has been characterised by X-ray crystallography (Ind = C(9)H(7), indenyl). In contrast to ttcn, the macrocycles 1,3,5-trithiane (tt) and 1,4,7,10-tetrathiacyclododecane (ttcd) fail to induce changes in the coordination mode of indenyl: tt and ttcd react with (η(5)-Ind)Mo(CO)(2)(NCMe)(2)](+) (or (η(5)-Ind)Mo(CO)(2)(κ(2)-dme)](+)) to give (η(5)-Ind)Mo(CO)(2)(κ(2)-tt)](+) (2), characterised by X-ray crystallography, and (η(5)-Ind)Mo(CO)(2)(κ(2)-ttcd)](+) (3), respectively. The cyclopentadienyl (Cp = C(5)H(5)) analogues (η(5)-CpMo(CO)(2)(κ(2)-tt)](+) (4) and (η(5)-CpMo(CO)(2)(κ(2)-ttcn)](+) (5) have also been synthesised and 5 characterised by X-ray crystallography. DFT calculations showed that the η(5)-Ind/Cp coordination mode is always the most stable. However, a molecular dynamics study of the macrocycles conformations revealed that the major conformer of ttcn was a chair, which favoured κ(3) coordination. As indenyl complexes undergo slippage with a small barrier (<10 kcal mol(-1)), the kinetically preferred species (η(3)-Ind)Mo(CO)(2)(κ(3)-ttcn)](+) (1) is the observed one. The conversion to 6 proceeds stepwise, with loss of ethylene followed by loss of CO, as calculated by DFT, with a barrier of 38.7 kcal mol(-1), consistent with the slow uncatalysed reaction.