Deoxygenation of a Cluster-Coordinated Nitric Oxide

The reaction of [Fe(CO)₃(NO)]⁻ with Ru₃(CO)₁₂ yields the nitrosyl carbonyl cluster [FeRu₃(CO)₁₂(NO)]⁻. The rate of this reaction shows a first-order dependence on the concentration of Ru₃(CO)₁₂ and is inhibited by addition of excess CO. A single-crystal X-ray crystallographic analysis of PPN[FeRu₃(CO)₁₂(NO)] [P2₁/c space group, a = 19.690 (4) Å, b = 16.262 (3) Å, c = 15.911 (5) Å, β = 107.24 (2)^°, Z = 4] revealed that the FeRu₃ tetrahedral core contained a terminal, linear nitrosyl ligand bound to the iron. Three carbonyls bridge the three iron-ruthenium bonds such that the overall symmetry is C₃. [FeRu₃(CO)₁₂(NO)]⁻ cleanly reacts to form CO₂ and the nitrido cluster [FeRu₃N(CO)₁₂]⁻. A structural analysis of the Et₄N⁺ salt of this cluster [.P] space group, a = 12.464 (1) Å, b = 12.697 (1) Å, c = 9.437 (2) Å, α = 94.40 (2)^°, β = 100.17 (2)^°, γ = 107.75 (1)^°, Z = 2] revealed that it has a butterfly framework of metal atoms with the nitrogen coordinated to all four metals. Three terminal carbonyl ligands are also coordinated to each metal. The iron is disordered over all four sites in the structure which means that two isomeric forms exist and cocrystallize. Isomer I contains Fe in the wing-tip position while the other (II) contains Fe in the hinge position. Solution spectroscopic evidence (infrared, ¹³C NMR and ¹⁵N NMR) shows that both isomers persist in solution. Measurement of the equilibrium constant for the isomerization, I⇌II, at several temperatures between 25 and 68^°C yielded ΔH = −3.5 + 1.0 kcal/mol and ΔS = −13 ± 2 eu. The rate of conversion of I →II at 25^°C is (4.2 ± 0.2) × 10⁻⁷ s⁻¹. Possible mechanisms for this unique isomerization are presented. A kinetic analysis of the deoxygenation of [FeRu₃(CO)₁₂(NO)]⁻ to give [FeRu₃N(CO)₁₂]⁻ over the temperature range from 25 to 65^°C revealed that the deoxygenation is first order in cluster concentration. Possible mechanisms of the NO deoxygenation consistent with this and other observations are discussed.