Preparation, Properties, and Structures of Pentanuclear [{Ni2L}2(μ-csalen)M]2+ Complexes (L = Macrocyclic N6S2 Donor Ligand)

The dinuclear nickel complex [Ni2L(mu-Cl)](+) (1), where L2- is a 24-membered macrocyclic N6S2 ligand, reacts readily with 3-form-yl-4-hydroxy-benzoic acid (Hfhba) to form the carboxylato-bridged complex [Ni2L(mu-fhba)](+) (2). Complex 2 undergoes a condensation reaction with ethylene diamine to produce a tetranuclear complex [{Ni2L}(2)(mu-H(2)csalen)](2+) (3), in which two dinuclear {Ni2L} units are bridged via the deprotonated carboxylate functions of the csalen ligand N,N'-bis(4-carboxysalicylidene)-1,2-diaminoethane. The same compound can also be prepared directly from 1 and H(2)csalen. The complexation of 3 with NiCl2 center dot 6H(2)O, Cu(OAc)(2)center dot H2O or Pd(OAc)(2) provides pentanuclear complexes of the type [{Ni2L}(2)(mu-csalen) M](2+) [M = Ni (4a), Cu (4b), Pd (4c)]. All complexes were isolated as perchlorate salts and studied by ESI-MS, infrared, and UV/Vis spectroscopy. The tetraphenylborate salt of 4c was also characterized by X-ray crystallography. The [(csalen) M] complex units act in all cases as quadridentate bridging ligands linking two bioctahedral {Ni2L} units via mu-(1,3)-bridging carboxylate functions. The palladium complex 4c was found to catalyze Heck-coupling reactions of various iodobenzenes with methyl acrylate and styrene.