Reactions of some dinuclear transition metal complexes

Wilson, F. I. C. (1988) Reactions of some dinuclear transition metal complexes. (PhD thesis), Kingston Polytechnic, .


The work in this thesis consists of three related studies. (i) Further investigations have been made into the mechanism of the photochemical reaction between (Mn[sub]2(CO)[sub]8{PBu[sub]3}[sub]2) and benzyl halides, equation 1. A previous study in these laboratories, which included an (Mn[sub]2(CO)[sub]8{PBu[sub]3}[sub]2) + PhCH[sub]2Cl -->(MnCl(CO)[sub]4PBu[sub]3) + (Mn(PhCH[sub]2)(CO)[sub]4PBu[sub]3) 1 examination of reaction kinetics, led to the suggestion that the mechanism involved CO dissociation from the manganese dimer, followed by metal-metal bond homolysis and subsequent reaction of the 15-electron intermediate, (Mn(CO)[sub]3PBu[sub]3, with benzyl chloride. The present work begins with the realisation that the previous data is also compatible with a radical chain process. In an attempt to distinguish between these two mechanisms, studies have been make of the variation in the quantum yield for the formation of (MnCl(CO)[sub]4PBu[sub]3) with benzyl chloride concentration. An examination of the reaction between (Mn[sub]2(CO)[sub]8DPPM) and benzyl chloride has also been carried out. Unfortunately, these studies still do not provide an unambiguous distinction between the mechanisms. (ii) A study has been made of the thermal reaction between (Mn[sub]2(CO)[sub]8a{PBu[sub]3}[sub]2) and benzyl and other halides. At 40 C the reaction is first order in both manganese dimer and benzyl choride and is not inhibited by added CO. This contrasts with that previously reported for the photochemical reaction and indicates that the thermal reaction follows a different pathway. Reaction of the metal dimer with 6-bromo-hex-1-ene suggests a free radical process and reactions with substituted benzyl bromides indicate an electron transfer step. Thus a mechanism is proposed involving electron transfer from the dimer to the organic halide, as the first step. Subsequent fission of the radical anion so formed into a halide ion and an organic radical, such as PhCH2, is followed by reaction of the dinuclear radical cation with the halide ion generating (MnCI(CO)[sub]4PBu[sub]3) and (Mn(CO)[sub]4PBu[sub]3). The manganese-centered radical then combines with the benzyl radical to yield (MnBz(CO)[sub]4PBu[sub]3). This reaction is thus a non-chain process. (iii) In view of the findings detailed above that (Mn[sub]2(CO)[sub]8{PBu[sub]3}[sub]2) reacts with benzyl chloride according to equation 1, it was deemed appropriate to examine the reactions between benzyl halides and a range of dinuclear metal complexes. The complexes studied were (M[sub]2(CO)[sub]10) (M = Mn, Re), (Mo[sub]2(CO)[sub]6Cp[sub]2), (Fe(CO)[sub]2Cp)[sub]2, (Fe[sub]2(CO)[sub]3PBu[sub]3Cp[sub]2), (Fe(CO){P(OMe)[sub]3}Cp)[sub]2 and (Co[sub]2(CO)[sub]6{PBu[sub]3}[sub]2), (Cp = D[sup]5-C[sub]5H[sub]5). Only (Fe[sub]2(CO)[sub]3PBu[sub]3Cp[sub]2) reacted with benzyl chloride under thermal conditions. All the dimers, except (M[sub]2(CO)[sub]10) (M = Mn, Re), reacted with benzyl bromide giving the corresponding bromo-complexes. Photochemical reactions with benzyl chloride were observed for all dimers except (Co[sub]2(CO)[sub]6{PBu[sub]3}[sub]2) and all, including the cobalt complex, reacted with benzyl bromide. In all cases, halo-complexes were formed, however, the observation of the associated benzyl complexes depended on both the benzyl halide and the metal dimer.

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