Zirconocene and molybdenocene compounds : preparation, autoxidation and related studies
Five previously unreported compounds were prepared and characterised; (C[sub]5Me[sub]5)[sub]2Zr(CH[sub]2Ph)[sub]2, (C[sub]5Me[sub]5)[sub]2Zr(CH[sub]2Ph)Cl, (C[sub]5Me[sub]5)(C[sub]5H[sub]5)Zr(CH[sub]2Ph)[sub]2, (C[sub]5Me[sub]5)2Zr(CD[sub]3)[sub]2(c[sub]5H[sub]5)[sub]2Mo(CH[sub]2CMe[sub]3)[sub]2. In addition new routes were found to (C[sub]5Me[sub]5)[sub]2Zr(Ome)[sub]2 and (C[sub]5H[sub]5)[sub]2Mo(CH[sub]2Ph)[sub]2. It was discovered that the latter compound had been prviosuly reported in error. Peaks in the [sup]IH NMR spectra of both dialkyl molybdenocenes were selectively broadened in the presence of oxygen. This phenomenon was acribed to a charge-transfer interatction. The heat of formation, [delta]H[sup]0 of the complex formed between dineopentyl molybdebocene and oxygen was calculated to be -9.1 kJ mol[sup]-1. The dialkyl molybdenocenes also reacted irreversibly with ca. 2.5 mole equivalents of oxygen. It is proposed that initiation of autoxidation occurred by thermal degradation of molybdenum-carbon [sigma]-bonds to give products which were reactive towards molecular oxygen. Intermediates with an empirical formula, CpMo(0)R, (R=Ch[sub]2Ph or CH[sub]2CMe[sub]3), were isolated and the final products of autoxidation were polymeric black solids. The autoxidation of the permethylzirconocene alkyls was investigated and the results were related to the unique properties of the pentamethylcyclopentadienyl ring and compared to previous work on alkyl compounds of zirconium. Rates of reaction, inhibition times, rates of self-initiation and initiation by di-tert-butyl hyponitrite were determined. In contrast to the relatively fast, clean, stoichiometric reactions of dialkyl zirconocenes, those of the permethylzirconocene diakyls were found to be complex. Propagation of autoxidation occurred by facile hydrogen abstraction from the ring methyls and not by S[sub]H2 displacement of alkyl radicals at the zirconium centre. Products of the photolysis of (C[sub]5Me)[sub]2ZrCl[sub]2 and (C[sub]5Me[sub]5)[sub]2ZrMe[sub]2 were identified using ESR spectrometry. Teh pentamethylcyclopentadienyl radical was readily cleaved from both compounds and at low temperatures photolysis of (C[sub]5Me[sub]5)[sub]2ZrMe[sub]2 yielded an ESR spectrum of the methyl radical. Photolysis of (C[sub]5Me[sub]5)[sub]2ZrMe[sub]2 was accompanied by evolution of methane formed by abstraction of hydrigen from ring methyls by methyl radicals, even in the presence of oxygen.