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Studies on some Ligand-Bridged Rhenium and Manganese Carbonyl complexes. Hou, Frank Liang

Abstract

A number of rhenium and manganese carbonyl complexes have been prepared from the reactions of the ligands f₄fars, f₆fars and .f₄fos with Re₂ (CO)₁₀, Mn₂(CO)₁₀ and ReMn(CO)₁₀. They can be classified into two types. The first type has the general formula (L-L)M₂(CO)₈ in which the ligand L-L bridges the two metal carbonyl moieties. The second type of complexes, i- f₄farsM₂(CO)₈, are isomers of the first type and have the structure (CO)₄M-(CH₃)₂As-M(CO)₄-(CH₃)₂AsC = CCF₂CF₂ which is the result of a ligand rearrangement reaction. The new complexes (L-L)M₂(CO)₈ are the first examples of 1igand bridged derivatives containing metal carbonyl fragments otherwise held together only by metal-metal bonds. They readily react with iodine with cleavage of the M-M bond to yield (L-L)[M(CO)₄I] ₂. The kinetics of these fast cleavage reactions were studied by the stopped-flow technique. The enthalpies of activation for f₄farsRe₂(CO)₈, f₆farsRe₂(CO)₈, and f₄fars(CO)₄,ReMn(CO)₄ are 6.9 ± 0.3, 9.3 ± 0.9, 14.2 + 0.9 kcal mole⁻¹ respectively while the approximate entropy values are -13.8±1.0, -11.1 ± 3.0 and 1.4 ± 3.2 e.u. respectively. No activation parameters were obtainable for the reaction of f₄farsMn₂(CO)₈, with iodine because it has a half-life time of less than 2 milliseconds at 25°C. Prolonged heating of f₄farsMn₂(CO)₈, (M = Re, Mn) results in the thermal rearrangement of these complexes to give their isomers i-(L-L)Mn₂(CO)₈,. The kinetics of these thermal rearrangement reactions were studied by ¹H n.m.r. spectroscopy. The enthalpies of activation for f₄farsMn₂(CO)₈, f₄farsRe₂(CO)₈, and f₄fars(CO)₄,ReMn(CO)₄ are 36.8 ± 2.0, 46.8 ± 0.5 and 39.2 ± 5.2 respectively while the entropy values are 15.0 ± 4.7, 23.2 x 1.1 and 20.9 ± 2.8 e.u. respectively. The mechanisms of these reactions and the iodine cleavage reactions are discussed and possible intermediates are suggested in the light of the kinetic and related chemical and spectroscopic studies. The iodine complexes f₄fars[M(CO)₄I]₂ (M = Re, Mn) are unstable and, on heating, give metal carbonyl iodides and the chelate complexes f4farsM(CO)₃I. The latter are related to f₄farsM(CO)₃Cl (M = Re, Mn) which were obtained as by-products in the reaction of the ligands with the parent carbonyls. As an appendix to this thesis, reactions of some fluorinated acetylenes with tetrakis(triphenylphosphine)-platinum(O) and the reaction of bis (trifluoromethyl)diazomethane with bis(trimethylsilyl)acetylene are described.

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