Laser induced pyrolysis of transition metal organometallic complexes.

Pyrolysis mechanisms of a number of transition metal organometallic compounds of iron, manganese, cobalt, and chromium have been studied by employing carbon dioxide infrared laser powered homogeneous pyrolysis (CO2 IR LPHP). This was coupled with a variety of analytical techniques, particularly NMR and IR spectroscopies and gas chromatography, to identify reaction products. The pyrolysis of butadiene iron tricarbonyl, BdFeTC, produced a wide range of organic reaction products, especially butadiene, cis- and trans-butene-2, ethyne, benzene, and polymers of butadiene. The formation of benzene was thought to be due to trimerisation of ethyne. The laser pyrolysis of CH3Mn(CO)5 and CH3COMn(CO)5 yielded methane and carbon monoxide as observed pyrolysis products. In the case of CH3COMn(CO)5, the reaction takes place in two steps, beginning with the loss of one CO group to form methyl manganese pentacarbonyl which is found to be a reversible reaction. Both CH3Mn(CO)5 and CH3COMn(CO)5 afforded trimethyl silane manganese pentacarbonyl and methane as reaction products when co-pyrolysed with trimethyl silane. The pyrolyses of C5H5Mn(CO)3, and CH3C5H4Mn(CO)3 were investigated and were found to begin with the successive loss of CO followed by the homolytic breakage of the C5H5-Mn or CH3C5H5-Mn bond. The observed reaction products of CH3C5H4Mn(CO)3 were cyclopentadiene, benzene, ethyne and methylcyclopentadiene. The source of benzene was thought to be the isomerisation of the methyl cyclopentadiene radical. In the case of LPHP of C5H5Mn(CO)3, detected organic reaction products were cyclopentadiene and ethyne. Metallocene compounds of iron, cobalt and chromium investigated by the same technique yielded cyclopentadiene, ethyne, benzene and naphthalene (except naphthalene was not observed in the pyrolysis of chromocene). The mechanism of formation of ethyne and benzene is similar to cyclopentadiene manganese tricarbonyl compounds but the occurrence of naphthalene is thought to be due to the dimerisation of cyclopentadiene and cyclopentadienyl radical while both are coordinated to the metal atom to yield a dicyclopentadienyl radical, followed by isomerisation to azulene radical then on to form naphthalene radical and finally naphthalene.