Cyclometalated Iron(II) Thiolate Complexes Featuring Diazine Rings: π-Acceptor Character of the C,N-Chelate Moiety

The combination of electron-donating and π-accepting groups is a compelling strategy in the development of polydentate ligands. A series of thiolate iron(II) complexes featuring N,C,S-tridentate ligands (L_N,C,S), formulated as trans-[Fe(L_N,C,S)(CO)(PMe₃)₂], was synthesized from dibenzothiophene derivatives containing a diazine ring (pyridazine, pyrimidine, or pyrazine) via C-S bond cleavage. The π-acceptor character of the N-donor site in L_N,C,S was demonstrated by comparing the spectroscopic and structural properties of these N,C,S-pincer iron(II) carbonyl complexes with those containing either a diazine or a pyridine ring. Among the diazine variants, the pyridazine-containing L_N,C,S ligands exhibited the strongest π-acceptor ability: the CO stretching frequencies of the trans-[Fe(L_N,C,S)(CO)(PMe₃)₂] complexes increased by about 30 cm^-1 compared with their pyridine-containing counterparts. Natural population analysis revealed that the accepted electron density is delocalized over the C,N-chelate moiety of L_N,C,S, with negative charges increasing in the order: pyridine < pyrimidine < pyrazine < pyridazine. In the pyridazine complexes, the Fe-N bond length is shorter than in the others, and the uncoordinated N atom is oriented away from the central phenyl ring and closer to the iron center. These structural features facilitate electron acceptance from the iron center through π-backbonding interactions.