Photochemistry of tricarbonyl(η5-cyclopentadienyl)hydrido complexes of molybdenum and tungsten and of dicarbonyl(η5-cyclopentadienyl)-(ethylene)hydridotungsten in solution and in frozen gas matrices at 12 K
Abstract
The photochemical reactions of [MH(CO)3(η5-C5H5)] complexes (M = Mo or W) and of trans-[WH(CO)2(C2H4)(η5-C5H5)] have been studied by a combination of conventional solution and low-temperature (12 K) matrix isolation techniques. In pentane solution the photolysis of [MH(CO)3(η5-C5H5)] complexes proceeds with the formation of the dimeric complexes [{M(CO)3(η5-C5H5)}2], [{M(CO)2(η5-C5H5)}2], and [{WH(CO)2(η5-C5H5)}2] together with evolution of H2 and CO. The photo-induced reactions with N2 and C2H4 lead to monosubstitution products but only [WH(CO)2(C2H4)(η5-C5H5)] is stable enough at room temperature for it to be fully characterised. The olefin–hydrido-complex [WH(CO)2(C2H4)(η5-C5H5)] exists only as the trans isomer at room temperature. Under thermal and especially photochemical reaction conditions the olefin ligand in [WH(CO)2(C2H4)(η5-C5H5)] can be replaced by CO, N2, PMe3, and tetrahydrofuran (thf) ligands leading to substitution products with different thermal stabilities. The photolysis of [WH(CO)2(C2H4)(η5-C5H5)] in pentane solution in the absence of potential ligands leads to the loss of the olefin and the formation of [{WH(CO)2(η5-C5H5)2}2]. In Ar and CH4 matrices at 12 K u.v. irradiation results in dissociative loss of one of the CO ligands to give the co-ordinately unsaturated 16-electron species[MH(CO)2(η5-C5H5)](M = Mo or W). This process was reversed using visible light, i.e. there is an equilibrium (i). The reactivity of the [MH(CO)2(η5-C5H5)] species was apparent in their reactions with N2 and C2H4[MH(CO)3(η5-C5H5)] [graphic omitted] [MH(CO)2(η5-C5H5]+ CO (i) at 12 K to produce trans-[MH(CO)2(N2)(η5-C5H5)] complexes and both cis and trans isomers of [WH(CO)2(C2H4)(η5-C5H5)] but only the trans isomer of [MoH(CO)2(C2H4)(η5-C5H5)]. In CO matrices the formation of the radicals [M(CO)3(η5-C5H5)]˙ and HCO˙ is indicative of photo-induced metal–hydrogen bond cleavage. The photolysis in 13CO matrices results initially in the formation of all possible [MH(13CO)3–n(CO)n(η5-C5H5)](n= 0–3) derivatives indicating easy CO exchange. Ultimately bands of H13CO˙ and 13CO-enriched [M(CO)3(η5-C5H5)]˙ species were observed. The identities of [MH(CO)2(η5-C5H5)], [MH(CO)2{N2)(η5-C5H5)], and [M(CO)3(η5-C5H5)]˙ species were confirmed by energy-factored force-field fitting procedures for the 13CO-enriched molecules. Upon u.v. irradiation of trans-[WH(CO)2(C2H4)(η5-C5H5)] in CH4 matrices the primary product is the cis isomer and this is followed by insertion of C2H4 into the W–H bond to generate the 16-electron species [W(CO)2(C2H5)(η5-C5H5)]. A monocarbonyl complex, possibly [WH(CO)(C2H4)(η5-C5H5)], is the final product. In CO matrices, however, the olefin is substituted by CO, and the corresponding hydrido-carbonyl complex [WH(CO)3(η5-C5H5)] is formed. The cis⇄trans isomerisation and the insertion of C2H4 into a M–H bond are relevant to the understanding of hydroformylation reactions catalysed by transition metal hydrides.
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