Reaction of iron dicarbonyl phosphine complexes of 2-(phenylazo)pyridines with dimethyl acetylenedicarboxylate. Synthesis and crystal structure of a 2,3,1-diazaferrole complex

Article abstract of DOI:10.1016/0022-328X(96)06243-2

The complexes Fe(CO)₂(PⁿBu₃)L (L=2-(phenylazo)pyridine, 4-methyl-2-(phenylazo)pyridine, and 4,6-(dimethyl)-2-(phenylazo)pyridine) undergo a [3+2] cycloaddition reaction with dimethyl acetylenedicarboxylate (DMAD), RC≡CR' (R=R'= COOCH₃), to produce the complexes Fe(CO)₂(PⁿBu₃)(C(COOCH₃)=C(COOCH₃)N(X-py)N(Ph)) (X=H, 4-CH₃, 4,6-(CH₃)₂), which contain a 2,3,1-diazaferrole ring. This reaction is very sensitive to the acetylene used and to the electron density at the iron center. No reaction occurs with an acetylene unless R and R' are both ester groups, and DMAD does not react with Fe(CO)₂(PⁿBu₃) (5-(trifluoromethyl)-2-(phenylazo)pyridine) or with the tricarbonyls Fe(CO)₃L. The complexes Fe(CO)₂(PPh₃)L do not react with DMAD, presumably due to steric effects, but Fe(CO)₂(P(OCH₃)₃)(4-methyl-2-phenylazopyridine) reacts to give Fe(CO)₂(P(OCH₃)₃)-(C(COOCH₃)=C(COOCH₃)N(4-CH₃-2-py)N(Ph)). This complex crystallizes in the space group P2₁/c with a=15.027(4) A, b=8.244(1) A, c=22.560(5) A, β=108.03(2)°, V=2651.1(10) A³ and Z=4. The iron atom is in an approximate square pyramidal environment with P(OCH₃)₃ in the axial position. The 2,3,1-diazaferrole ring is essentially planar with sp₂ hybridization for all C and N ring atoms, and bond length data are consistent with a possible delocalized π system. The results obtained in this study are compared with similar studies on analogous α-diimine complexes.