The first Ni(II) complexes of 5-nitroorotate ligand with the tridentate and bidentate coordination modes. Crystal and molecular structures, vibrational spectra and magnetic properties

Abstract

Two novel Ni(II) complexes of 5-nitroorotate dianion (the nitro-derivative of orotic acid, vitamin B13) have been synthesized and structurally characterized by single crystal X-ray diffraction and vibrational (IR and Raman) spectroscopy. In complex 1, {[Ni(5-NO2HOr)(H2O)3]·H2O}n, crystallizing in the orthorhombic space group Pna2(1), the 5-nitroorotate ligand is tridentate and binds to the nickel(II) ions through the deprotonated N1 pyrimidine nitrogen and two carboxylate oxygen atoms. Among a great number of the reported Ni(II)-orotates, 1 is the first complex where the carboxylate group coordinates to two nickel ions by the anti-syn bridge forming a polymeric structure. Three water molecules complete the octahedral coordination sphere around each Ni atom. Complex 2, [Ni(5-NO2HOr)(H2O)4]·H2O, crystallizes in the monoclinic space group P2(1)/c, and the bidentate 5-nitroorotate ligand coordinates to the Ni(II) ion by only one carboxylate oxygen O1 atom and the deprotonated N1 atom. The distorted octahedral environment of Ni atom is completed by four water molecules. The assignment of the characteristic bands in the IR and Raman spectra has provided further evidence for the nature of the bonding in these compounds. Temperature and field-dependent magnetic studies of 1 reveal the existence of a weak antiferromagnetic exchange interaction transmitted by anti-syn carboxylate bridge with J=−0.89cm−1 (−JS1·S2 convention) superimposed on the zero-field splitting (D=5.4cm−1). The magnetic properties of 2 have been reproduced using a 2D quasi-quadratic lattice model with D=5.5, J1=−1.89 and J2=1.56cm−1 and both magnetic interactions are transmitted through hydrogen bonding.