Metal Complexes with a Hexadentate Macrocyclic Diamine-Tetracarbene Ligand.

A hexadentate macrocyclic N-heterocyclic carbene (NHC) ligand precursor (H4 L)(PF6 )4 containing four benzimidazolium and two secondary amine groups, has been synthesized and characterized. Coordination chemistry of this new macrocyclic diamine-tetracarbene ligand has been studied by the synthesis of its Ag(I), Au(I), Ni(II), and Pd(II) complexes. Reactions of (H4 L)(PF6 )4 with different equiv of Ag2 O result in Ag(I) complexes [Ag(H2 L)](PF6 )3 (1) and [Ag2 (H2 L)](PF6 )4 (2). A mononuclear Au(I) complex [Au(H2 L)](PF6 )3 (3) and a trinuclear Au(I) complex [Au3 (H2 L)(Cl)2 ](PF6 ) (4) are obtained by transmetalation of 1 and 2 with AuCl(SMe2 ), respectively. Reactions of (H4 L)(PF6 )4 with Ni(OAc)2 and Pd(OAc)2 in the presence of NaOAc yield [Ni(L)](PF6 )2 (5) and [Pd(L)](PF6 )2 (6), respectively, containing one Ni(II) and Pd(II) ion with distorted square-planar geometry. Using more NaOAc results in the formation of unusual dinuclear complexes [Ni2 (L-2H)](PF6 )2 (7) and [Pd2 (L-2H)](PF6 )2 (8) (L-2H = deprotonated ligand after removing two H+ ions from two secondary amine groups in L), respectively, featuring a rare M2 N2 core formed by two bridging amides. 7 is also formed by the reaction of 5 with 1.0 equiv of Ni(OAc)2 ·4H2 O in the presence of NaOAc. Transmetalation of 2 with 2.0 equiv of Ni(PPh3 )2 Cl2 gives [Ni2 (L)(μ-O)](PF6 )2 (9), the first example of a dinuclear Ni(II) complex with a singly bridging oxo group. 9 is converted to 7 in good yield through the treatment with NaOAc.