Preparation and Interconversion of Binuclear 2-Ferrazetine and Isomeric Ferrapyrrolinone Complexes

The complex Fe₂(μ-CH₂)(CO)₈ reacts with the phosphine imides R′₃P=NR (R, R′ = Ph; R = Bu^t, R′ = Buⁿ) to form a mixture of the 3-ferra-4-pyrrolin-2-one complexes Fe₂(μ-CH=CHN(R)C{O})(CO)₆ and the 2-ferrazetine complexes Fe₂(μ-CH=CHNR)(CO)₆. the latter complexes derive from the former by loss of CO, which is accelerated by heating (50 °C, 3 h) or by adding CO-labilizing agents (halides, [HB(s-Bu)₃]⁻). This reaction is reversed by photolysis of the 2-ferrazetine complexes under 1 atm of CO. However, when the 2-ferrazetine complexes are allowed to thermally react with CO, the isomeric 2-ferra-4-pyrrolin-3-one complexes Fe₂(μ-C{O}CH=CHNR)(CO)₆ are produced instead. These latter complexes have been found to add electrophiles (HBF₄ and [Me₃O]BF₄) to the acyl oxygen to form the cationic complexes [Fe₂{μ-C(OR)= (CHCH=NBu^t)}(C0)₈]BF₄ (R = H, Me). the 3-ferra-4-pyrrolin-2-one complexes also react with acid (HBF₄) under 1 atm of CO to form the complexes [Fe₂{μ-C(H)(CH=NHR)}(CO)₈]BF₄, which possess protonated μ₂,η¹-azallylidene ligands. the dimethylcarbene complex Fe₂(μ-CMe₂)(CO)₈ also reacts with (Buⁿ)₃P=NBu^t, but this reaction stops at the acyl complex Fe₂(μ-CMe₂)(CO)₇(C{0}N(Bu^t)P(Buⁿ)₃) since this species cannot undergo the hydrogen migration needed to give entry to the ferrapyrrolinone and ferrazetine complexes. Mechanisms are proposed for these various transformations, and the following complexes have been crystallographically characterized: Fe₂(μ-CH=CHNPhC{O})(CO)₆, Fe₂(μ-CH=CHNBu^t)(CO)₆, Fe₂(μ-C{O}CH=CHNPh)(CO)₆, [Fe₂(μ-CHCH=NHBu^t)(CO)₈][CF₃S0₃], and [Fe₂(μ-C{OMe}=CHCH=NBu^t)(CO)₈][BF₄].