Coordination Properties of Bridging Diazene Ligands in Unusual Diiron Complexes

Saouma, Müller, and Peters recently ( J. Am. Chem. Soc., 2009) reported the synthesis of several diiron complexes with two types of bridging diazene HN═NH ligands, designated μ-η 1:η 1 (bridging end-on ) and μ-η 2:η 2 (bridging side-on ), with tridentate phosphine {XBP R 3 } ligands ({XBP R 3 } = XB(CH 2 PR 2 ) 3 −; X = Ph; R = Ph, CH 2 Cy). In the present study the energies, optimized geometries, and vibrational frequencies of these complexes with X = H and R = H, CH 3, CF 3 were obtained theoretically, and the electronic structures were analyzed in terms of iron−diazene donor−acceptor interactions. Natural bond orbital analysis indicates that both bridging end-on and side-on diazenes possess strong π-acceptor properties, which cause significant occupation of their antibonding π*(N−N) LUMOs and weakening of the N−N bonds as the result of coordination. The most pronounced weakening was obtained in complexes with bridging side-on diazene, where interaction of iron d-AOs with π*(N−N) orbitals of the diazene ligands results in the occupation of the π*-orbital by more than one electron. Modification of the structures of the phosphine ligands affects the strength of the diazene N−N bond: phosphines with donor groups R = H, CH 3 facilitate the weakening of the bond, while the acceptor group R = CF 3 strengthens the N−N bond.