Kinetics and mechanism of ligand substitution in iron tricarbonyl 1,4-dimethyltetraazabutadiene

Article abstract of DOI:10.1021/ic50224a008

Thermal carbon monoxide substitution in Fe(CO)₃(N₄Me₂)² proceeds readily to form monosubstituted products Fe(CO)₂L(N₄Me₂) with L = PMe₃, PMe₂Ph, PBu₃, PEt₂Ph, P(OEt)₃, P(OMe)₃, PH₂Ph, P(c-Hx)₃, PPh₃, AsMe₃, AsEt₃, AsMe₂Ph, 4-CNpy, and Me₃CNC. Bis- and trissubstituted products are also observed in the case of tert-butyl isocyanide. The substitution proceeds solely by a second-order process with a rate law that is first order in both Fe(CO)₃(N₄Me₂) and entering ligand. In addition, the rate is strongly dependent on the nature of the ligand, particularly its size and basicity. Activation parameters further support the associative nature of the reaction in toluene: PMe₃, ΔH? = 6.9 ± 0.2 kcal/mol, ΔS? = -31.4 ± 0.7 eu; PBu₃, ΔH? = 7.3 ± 0.3 kcal/mol, ΔS? = -38.6 ± 1.1 eu; PEt₂Ph, ΔH? = 7.3 ± 0.1 kcal/mol, ΔH? = -41.1 ± 0.2 eu; P(OMe)₃, ΔH? = 11.0 ± 0.2 kcal/mol, ΔS? = -35.0 ± 0.6 eu; Me₃CNC, ΔH? = 11.6 ± 0.5 kcal/mol, ΔS? = -34.9 ± 1.4 eu; AsMe₃, ΔH? = 13.3 ± 0.3 kcal/mol, ΔS? = -34.3 ± 0.9 eu. Likewise in methanol: AsMe₃, ΔH? = 9.8 ± 0.1 kcal/mol, ΔS? = -34.2 ± 0.2 eu; PPh₃, ΔH? = 11.9 ± 0.8 kcal/mol, ΔS? = -38.8 ± 1.6 eu. The rate of reaction is increased in polar solvents and to a greater extent in alcohol solvents, which are capable of hydrogen bonding with the tetraazabutadiene nitrogens. In the presence of excess BF₃, the rate of substitution is increased by a factor of 10⁶. Factors which facilitate nucleophilic attack in Fe(CO)₃(N₄Me₂) are discussed and contrasted with the dissociative mechanisms found for other iron carbonyls.