Time-resolved infra-red spectroscopy reveals competitive water and dinitrogen coordination to a manganese(i) carbonyl complex

Abstract

Time-resolved infra-red (TRIR) spectroscopy has been used to demonstrate that photolysis of [Mn(C^N)(CO)₄] (C^N = bis-(4-methoxyphenyl)methanimine) in heptane solution results in ultra-fast CO dissociation and ultimate formation of a rare Mn-containing dinitrogen complex fac-[Mn(C^N)(CO)₃(N₂)] with a diagnostic stretching mode for a terminal-bound NN ligand at 2249 cm⁻¹. An isotopic shift to 2174 cm⁻¹ was observed when the reaction was performed under ¹⁵N₂ and the band was not present when the experiment was undertaken under an atmosphere of argon, reinforcing this assignment. An intermediate solvent complex fac-[Mn(C^N)(CO)₃(heptane)] was identified which is formed in less than 2 ps, indicating that CO-release occurs on an ultra-fast timescale. The heptane ligand is labile and is readily displaced by both N₂ and water to give fac-[Mn(C^N)(CO)₃(N₂)] and fac-[Mn(C^N)(CO)₃(OH₂)] respectively. The fac-[Mn(C^N)(CO)₃(heptane)] framework showed a significant affinity for N₂, as performing the reaction under air produced significant amounts of fac-[Mn(C^N)(CO)₃(N₂)]. Kinetic analysis reveals that the substitution of heptane by N₂ (k = (1.028 ± 0.004) × 10⁹ mol⁻¹ dm³ s⁻¹), and H₂O is competitive on fast (<1 μs) time scales. The binding of water is reversible and, under an atmosphere of N₂, some fac-[Mn(C^N)(CO)₃(OH₂)] converts to fac-[Mn(C^N)(CO)₃(N₂)].