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

Abstract

The macrocycle 1,4,7-trithiacyclononane (ttcn) reacts with [(η⁵-Ind)Mo(CO)₂(NCMe)₂]⁺ (or [(η⁵-Ind)Mo(CO)₂(κ²-dme)]⁺) to give [(η³-Ind)Mo(CO)₂(κ³-ttcn)]⁺ as the BF₄⁻ salt (1), but its reaction with [(η⁵-Ind)Mo(CO)₂(C₃H₆)(FBF₃)] affords the C–S bond cleavage product [(η⁵-Ind)Mo(CO)(κ³-1,4,7-trithiaheptanate)]BF₄ (6), which has been characterised by X-ray crystallography (Ind = C₉H₇, 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 [(η⁵-Ind)Mo(CO)₂(NCMe)₂]⁺ (or [(η⁵-Ind)Mo(CO)₂(κ²-dme)]⁺) to give [(η⁵-Ind)Mo(CO)₂(κ²-tt)]⁺ (2), characterised by X-ray crystallography, and [(η⁵-Ind)Mo(CO)₂(κ²-ttcd)]⁺ (3), respectively. The cyclopentadienyl (Cp = C₅H₅) analogues [(η⁵-CpMo(CO)₂(κ²-tt)]⁺ (4) and [(η⁵-CpMo(CO)₂(κ²-ttcn)]⁺ (5) have also been synthesised and 5 characterised by X-ray crystallography. DFT calculations showed that the η⁵-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 κ³ coordination. As indenyl complexes undergo slippage with a small barrier (<10 kcal mol⁻¹), the kinetically preferred species [(η³-Ind)Mo(CO)₂(κ³-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.