Direct Preparation of a Siroheme Model Compound: Synthesis and Structure of (5,10,15,20-Tetramethylisobacteriochlorinato)nickel(II)

The condensation reaction of pyrrole with acetaldehyde diethyl acetal in a glacial acetic acid solution of nickel acetate under an inert atmosphere yields (5,10,15,20-tetramethylisobacteriochlorinato)nickel(II), Ni(TMiBC), and 5,10,15,20-tetramethylporphyrinogen (TMporphyrinogen) as deduced by spectroscopy and for Ni(TMiBC) by X-ray analysis. Ni(TMiBC) crystallizes in the monoclinic space group C_2h⁵-P2₁/n (a = 23.656 (2) Å, b = 7.644 (1) Å, c = 24.014 (2) Å, β = 118.94 (1)°) with two independent molecules per asymmetric unit. The final agreement indices, based on 523 variables and 5307 unique reflections collected at −69 °C on an automatic diffractometer, are R(F²) = 0.086 and R_W(F²) = 0.124. The conventional R index on F for 4008 reflections having F_°²3σ(F_°;²) is 0.065. The isobacteriochlorin macrocycle exhibits an S₄-ruffled conformation and a bond length pattern indicative of a reduction in aromaticity, both of which are more pronounced than those found in the corresponding porphyrin ((5,10,15,20-tetramethylporphyrinato)nickel(II), Ni(TMP)) and chlorin ((5,10,15,20-tetramethylchlorinato)nickel(II), Ni(TMC)) structures. This reduction in aromaticity is consistent with the ¹H NMR spectra, which point to ring currents in the order Ni(TMiBC) < Ni(TMC) < Ni(TMP).