UCL Discovery

Abstract

α-Substituted alkenyl complexes [Fe2(CO)6(µ-RCCH2)(µ-PPh2)] 1a–1e (R = Ph, Me, Prn, Bun or But) have been prepared via regioselective hydrodimetallation of primary alkynes by [Fe2H(CO)7(µ-PPh2)]. Thermolysis in toluene results in isomerisation to the -substituted complexes [Fe2(CO)6(µ-HCCHR)(µ-PPh2)] 2a–2e. Isomerisation is accelerated in the presence of a range of tertiary phosphines yielding [Fe2(CO)5(PR3)(µ-HCCHR)(µ-PPh2)] 3a–3e in which the phosphine is coordinated to the -bound metal centre and lies trans to the phosphido-bridge. Addition of trimethyl phosphite to [Fe2(CO)6(µ-PhCCH2)(µ-PPh2)] 1a at 60–70 °C affords mono- and di-substituted -alkenyl complexes [Fe2(CO)5{P(OMe)3}(µ-PhCCH2)(µ-PPh2)] 4 and [Fe2(CO)4{P(OMe)3}2(µ-PhCCH2)(µ-PPh2)] 5 respectively. Above 100 °C, conversion into the -substituted isomers [Fe2(CO)5{P(OMe)3}(µ-HCCHPh)(µ-PPh2)] 6 and [Fe2(CO)4{P(OMe)3}2(µ-HCCHPh)(µ-PPh2)] 7 occurs cleanly. Crystallographic studies have been carried out on 1a, [Fe2(CO)6(µ-PrnCCH2)(µ-PPh2)] 1c, [Fe2(CO)6(µ-HCCHPh)(µ-PPh2)] 2a, [Fe2(CO)5(PPh3)(µ-HCCHPh)(µ-PPh2)] 3a, 6 and 7 and compared with related structures. Different structural characteristics are seen between isomers, isomers being characterised by a longer Fe–C interaction and a more obtuse Fe–C–C angle. The mechanisms of alkyne addition to [Fe2H(CO)7(µ-PPh2)] and alkenyl isomerisation have been probed using PhC2D. Hydrodimetallation results in an equal distribution of the deuterium over both sites in 1a, and since they do not in exchange on the NMR timescale suggests that a radical process is operating. Thermolysis of this mixture leads to a 31 mixture of [Fe2(CO)6(µ-DCCHPh)(µ-PPh2)] 2a-d1 and [Fe2(CO)6(µ-HCCDPh)(µ-PPh2)] 2a-d1. A mechanism for – alkenyl isomerisation is proposed in which oxidative additions of the trans (exo) and cis (endo) -protons to terminal sites on the diiron centre, giving a parallel alkyne intermediate, are in competition.