2,2′-Bipyrimidine (bipym)-bridged dinuclear complexes. Part 1. Preparation, crystal structure, and magnetic properties of [Ni2(H2O)8(bipym)][NO3]4 and [Ni2(H2O)8(bipym)][SO4]2·2H2O

Abstract

Two new dinuclear nickel(II) complexes of formula [Ni₂(H₂O)₅(bipym)][NO₃]₄1 and [Ni₂(H₂O)₈-(bipym)][SO₄]₂·2H₂O 2(bipym = 2,2′-bipyrimidine) have been synthesised and their crystal structures determined by X-ray crystallographic methods. Crystals of 1 and 2 are monoclinic, space group P2₁/n with a= 10.020(2), b= 10.521(2), c= 11.019(2)Å, β= 90.27(1)° and Z= 2 for 1 and space group P2₁/c with a= 8.094(2), b= 11.550(3), c= 11.719(2)Å, β= 91.85(1)° and Z= 2 for 2. Both structures consist of centrosymmetrical dinuclear cations [Ni₂(H₂O)₈(bipym)]⁴⁺ with nitrate counter ions for 1 and water of crystallization and sulfate counter ions for 2. Each nickel atom is six-co-ordinated in a slightly distorted octahedral NiN₂O₄ chromophore: the average Ni^II–N distance is 2.094 Å in 1 and 2.115(2)Å in 2, whereas the mean Ni^II–O distance is 2.094(3)Å in 1 and 2.056(2)Å in 2. The bipyrimidine group joins two adjacent nickel atoms acting as a bis(bidentate) ligand. The C–C bond between the pyrimidine rings of bipym is perpendicular to the Ni ⋯ Ni vector giving two five-membered chelate rings. The intramolecular Ni ⋯ Ni separation is 5.569(1)Å for 1 and 5.660(1)Å for 2. Intramolecular antiferromagnetic spin-exchange coupling between the two nickel(II) ions is observed in both complexes with J=–14 and –12 cm^–1 for 1 and 2, respectively (J is the intradimer exchange parameter in the isotropic Hamiltonian Ĥ=–JŜ_A·Ŝ_B). The efficiency of bipym to transmit electronic effects is compared to that of related bis(chelating) ligands such as oxalate and oxamidate in the parent oxalate– and oxamidate-bridged nickel(II) complexes.