Dynamic behavior and alkyne insertion reactions of alkenyl- and thiolate-bridged diiron carbonyl complexes

Abstract

The alkenyl- and thiolate-bridged diiron carbonyl complexes [Fe2(μ-SC6H4CR1CH)(CO)6] (R1 = H or Me), which are derived from [Fe3(CO)12] and benzothiophenes, reacted with alkynes (R2CCR3) under photoirradiation conditions to form the acyl complexes [Fe2{μ-SC6H4CR1CHCR2C(R3)CO}(CO)5]. In this reaction, an alkyne and CO are inserted into the Fe–alkenyl bond, and the bulky substituents of alkynes, including a redox active ferrocenyl group, occupy the R3 position. To elucidate the reaction pathway, the reaction of the in situ generated acetonitrile complex [Fe2(μ-SC6H4CHCH)(CO)5(NCCH3)] with ethynylbenzene was monitored by 1H NMR spectroscopy. The results demonstrated that the acetonitrile complex undergoes facile insertion of the alkyne, which is followed by CO insertion to give the corresponding acyl complex [Fe2{μ-SC6H4(CH)3C(Ph)CO}(CO)5]. The reactivity of the bridging alkenyl moiety is related to the dynamic behavior of the alkenyl-thiolate ligand (SC6H4CHCH)2- as well as the entering alkyne. Temperature dependent 31P NMR measurements of the dmpm-bridged complex [Fe2(μ-SC6H4CHCH)(μ-dmpm)(CO)4] (dmpm = bis(dimethylphosphino)methane) revealed that the activation parameters for the flip-flop motion of the alkenyl-thiolate ligand are ΔH‡ = 46.7(7) kJ mol-1 and ΔS‡ = −46(2) J mol-1 K-1.