Photochemistry of Binuclear Ferraazetines: CO vs Alkyne Insertion

An investigation of the low-temperature matrix isolation photochemistry of ferraazetines, Fe₂(μ-CRCHNBu^t)(CO)₆ (1a, R = H; 1b, R = CH=CHPh) in rigid matrices is reported. Near-UV irradiation of 1a and 1b in CO-saturated methylcyclohexane (MCH) at 298 K yields ferrapyrrolinones 2a and 2b, Fe₂{μ-CRCHNBu^tC(O)}(CO)₆, and in the presence of tert-butylacetylene, la and lb form ferrapyridines 3a and 3b, Fe₂(μ-Bu_tCCHCRCH=NBu^t)(CO)₆. The photoinduced insertion of CO and tert-butylacetylene occur at different positions in the ferraazetine metallacycle. CO insertion occurs between the Fe-N bond of the metallacycle whereas tert-butylacetylene insertion occurs between the Fe-C bond. At 15 or 8 K in MCH, 1:1 tert-butyl acetylene/MCH, or CO matrices, the primary photoprocess for la and lb involves Fe-N bond cleavage to yield coordinatively unsaturated compounds 4a and 4b, respectively. Irradiation of 4a in a MCH matrix gives bridging CO compound 5, Fe₂(μ-CHCHNBu^t)(μ-CO)(CO)₅, which is formed by a CO ligand swinging from the adjacent Fe center to occupy the vacant coordination site created by the Fe-N cleavage. In a 1:1 tert-butylacetylene/MCH matrix, irradiation of 4a at 98 K gives a tert-r-butylacetylene adduct 6, Fe₂(μ-CHCHNBu^t)(CO)₆(Bu^tC≡CH). When warmed to 298 K, 6 inerts the tert-butylacetylene ligand to form 3a. Irradiation of 4a in a ¹³CO matrix at 8 K gives CO insertion product 2a with ¹³CO incorporation at a terminal position with no evidence of incorporation at the carbonyl of the acyl ligand. When 4b is irradiated in MCH at 15 K, compound 7, Fe₂{µ-C(CH=NBu^t)CHCHPh}(CO)₆, is formed. 7 derives from a ligand exchange of the C=NBu^t group of the ferraazetine ring in 4b for its pendant styryl group, CH=CHPh. When irradiated at 98 K or 298 K in MCH, lb forms 7 with no detectable intermediates and the photoconversion is much faster at these temperatures than at 15 K. At 298 K 7 quantitatively re-forms lb after 2 h in the dark.