2,2′-Bipyrimidine (bipym)-bridged dinuclear complexes. Part 2. Synthesis, crystal structure and magnetic properties of [Fe2(H2O)8(bipym)][SO4]2·2H2O and [Fe2(H2O)6(bipym)(SO4)2]

Abstract

Two new dinuclear iron(II) complexes of formulae [Fe₂(H₂O)₈(bipym)][SO₄]₂·2H₂O 1 and [Fe₂(H₂O)₆-(bipym)(SO₄)₂]2(bipym = 2,2′-bipyrimidine) have been synthesised and their crystal structures determined by single-crystal X-ray diffraction. Crystals of 1 and 2 are monoclinic, space group P2₁/c with a= 8.138(1), b= 11.661(2), c= 11.886(2)Å, β= 91.85(1)° and Z= 2 for 1 and space group P2₁/n with a= 6.275(2), b= 13.550(4), c= 10.937(2)Å, β= 96.43(2)° and Z= 2 for 2. The structure of 1 consists of centrosymmetric dinuclear cations [Fe₂(H₂O)₈(bipym)]⁴⁺, unco-ordinated sulfate anions and water of crystallization whereas that of 2 is made up of neutral dinuclear [Fe₂(H₂O)₆(bipym)(SO₄)₂] units. The co-ordination geometry around each iron atom is that of a highly distorted octahedron: the Fe^II–N distances are longer (average value 2.22 Å) than the Fe^II–O ones (average values 2.11 and 2.09 in 1 and 2, respectively). In both complexes, the bipyrimidine group joins two adjacent iron atoms acting in a bis(chelating) faschio. The C–C bond between the pyrimidine rings of bipym is perpendicular to the Fe ⋯ Fe vector giving two five-membered chelate rings, the bite angle of bipym being 74.9(1)° in 1 and 74.1(1)° in 2. The intramolecular metal–metal separation is 5.836(1) and 5.909(1)Å in 1 and 2, respectively. Magnetic susceptibility versus temperature data for both compounds were fitted to the Heisenberg–Dirac–Van-Vleck S₁=S₂= 2 spin exchange model with J=–3.4 cm^–1, g= 2.28 and θ=–0.7 cm^–1 for 1 and J=–3.1 cm^–1, g= 2.23 and θ=–1.3 cm^–1 for 2. The efficiency of bipym to transmit electronic effects between iron(II) ions is compared to that of related oxygen-donor potentially bis(chelating) ligands.