Inorganic Chemistry, Vol.56, No.19, 11917-11928, 2017
Metal Complexes with a Hexadentate Macrocyclic Diamine-Tetracarbene Ligand
A hexadentate macrocyclic N-heterocyclic carbene (NHC) ligand precursor (H4L)(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 (H4L)(PF6)(4) with different equiv of Ag2O result in Ag(I) complexes [Ag(H2L)](PF6)(3) (1) and [Ag-2(H2L)](PF6)(4) (2). A mononuclear Au(I) complex [Au(H2L)](PF6)(3) (3) and a trinuclear Au(I) complex [Au-3(H2L)(Cl)(2)](PF6) (4) are obtained by transmetalation of 1 and 2 with AuCl(SMe2), respectively. Reactions of (H4L)(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 [Ni-2(L-2H)](PF6)(2) (7) and [Pd-2(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 M2N2 core formed by two bridging amides. 7 is also formed by the reaction of 5 with 1.0 equiv of Ni(OAc)(2).4H(2)O in the presence of NaOAc. Transmetalation of 2 with 2.0 equiv of Ni(PPh3)(2)Cl-2 gives [Ni-2(L)(mu-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.