The chemistry of transition metal–germanium compounds. Part 2. Synthesis, characterization, structure and reactivity of transition-metal carbonyl-substituted germacyclopent-3-enes

Abstract

A number of transition-metal carbonyl-substituted germacyclopent-3-ene compounds have been prepared by metathesis between 1,1-dibromogermacyclopent-3-enes and sodium salts of transition-metal carbonyl anions. These species have been characterized by a variety of spectroscopic techniques and in the case of 3,4-dimethyl-1,1-bis(tetracarbonylcobaltio)-1-germacyclopent-3-ene 5, 1-bromo-3,4-dimethyl-1-(pentacarbonylmanganio)-1-germacyclopent-3-ene 7 and 1-bromo-3,4-dimethyl-1-(tetracarbonylcobaltio)-1-germacyclopent-3-ene 11 by low-temperature single-crystal X-ray diffraction. Compound 5 crystallizes in the space group P2₁/n with the cell parameters a= 15.556(3), b= 6.691(1), c= 17.726(2)Å, β= 104.23(1)° and Z= 4, 7 in the space group P2₁/c with a= 14.825(2), b= 6.781(1), c= 14.765(2)Å, β= 103.43(0)° and Z= 4, and 11 in the space group P2₁/c with a= 6.441(1), b= 30.040(7), c= 7.472(1)Å, β= 106.38(1)° and Z= 4. All three compounds exhibit a puckered germacyclopent-3-ene ground-state conformation, the origin of which is discussed. In solution the symmetrically substituted compounds exhibit a degenerate equilibrium which involves a planar germacyclopent-3-ene transition state. Reaction of 1-halogeno-1-(transition metal carbonyl)-substituted germacyclopent-3-enes with different transition metal carbonylate anions results in formation of mixed-metal carbonyl derivatives and reduction with LiAlH₄ results in formation of a germanium hydride. Thermolysis of 5 results in loss of a carbonyl ligand and formation of a compound which possesses and intramolecular carbonyl-bridged Co–Co bond. Kinetic data for this decarbonylation reaction, measured by variable-temperature ¹H NMR spectroscopy, reveal activation parameters at an average temperature of 70 °C of ΔG‡=+25.9 kcal mol^–1, ΔH‡=+27.6 kcal mol^–1 and ΔS‡=+5 cal K^–1 mol^–1.