[Cr(phen)(ox)2]−: a versatile bis-oxalato building block for the design of heteropolymetallic systems. Crystal structures and magnetic properties of AsPh4[Cr(phen)(ox)2]·H2O, [NaCr(phen)(ox)2(H2O)]·2H2O and {[Cr(phen)(ox)2]2[Mn2(bpy)2(H2O)2(ox)]}·6H2O

Abstract

The new complexes of formula AsPh₄[Cr(phen)(ox)₂]·H₂O (1), [NaCr(phen)(ox)₂(H₂O)]·2H₂O (2) and {[Cr(phen)(ox)₂]₂[Mn₂(bpy)₂(H₂O)₂(ox)]}·6H₂O (3) (AsPh₄=tetraphenylarsonium cation; phen=1,10-phenanthroline; ox=oxalate dianion; bpy=2,2′-bipyridine) have been prepared and characterized by single-crystal X-ray diffraction. The structure of 1 consists of discrete [Cr(phen)(ox)₂]⁻ anions, tetraphenylarsonium cations and uncoordinated water molecules. The chromium environment in 1 is distorted octahedral with Cr–O bond distances between 1.959(3) and 1.947(3) Å and Cr–N bonds of 2.083(4) and 2.072(4) Å. The angles subtended at the chromium atom by the two oxalates are 83.6(2) and 83.3(1)° whereas the N–Cr–N angle is 79.9(2)°. The [Cr(phen)(ox)₂]⁻ unit of 1 is also present in 2 and 3 but it accomplishes different coordination functions, acting as a bridging (2) or terminal (3) ligand. 2 has a layered structure made up of oxalato-bridged bimetallic Cr^III–Na^I helical chains that are interconnected through centrosymmetric Na₂O₂ units. The two oxalates of [Cr(phen)(ox)₂]⁻ in 2 are bis-chelating within the bimetallic chain but one of them is in addition monodentate towards a sodium atom of a neighbouring chain, yielding a sheetlike structure. The sodium atom in 2 has a distorted octahedral geometry with five Na–O(ox) bonds ranging from 2.453(5) to 2.319(4) Å and the sixth position being occupied by an aqua ligand with Na–O(w)=2.384(6) Å. The intralayer chromium–sodium and sodium–sodium separations through bridging oxalate in 2 are 5.560(4) and 3.643(8) Å, respectively. The structure of 3 consists of neutral tetranuclear Cr₂^IIIMn₂^II units in which two terminal [Cr(phen)(ox)₂]⁻ entities act as monodentate ligands towards a central oxalato-bridged manganese(II) dimer. Each manganese atom is six-coordinated as MnN₂O₄: two nitrogen atoms of a chelating bpy, one aqua ligand and three oxalate oxygens build a distorted octahedron around the manganese atom. The Mn–O(ox) and Mn–N(bpy) bond lengths vary in the ranges 2.219(12)–2.160(13) and 2.33(2)–2.14(2) Å, respectively. The intramolecular chromium–manganese separation [5.507(5) and 5.502(5) Å for Cr(2)···Mn(2) and Cr(1)···Mn(1)] is somewhat shorter that the manganese–manganese one [5.703(2) Å]. The magnetic properties of 1–3 have been investigated in the temperature range 1.9–300 K. Very weak antiferromagnetic interactions between the chromium centers are observed in 1 and 2 in agreement with their crystal structures. In the case of 3, significant intramolecular antiferromagnetic interactions between the adjacent chromium(III) and manganese(II) ions ( j=−1.1 cm⁻¹, through the chelating/monodentate oxalato) and between the two manganese(II) ions (J=−2.2 cm⁻¹, through the bis-chelating oxalato) occur, the Hamiltonian being defined as Ĥ=−JŜ_Mn1· Ŝ_Mn2−j[Ŝ_Cr1·Ŝ_Mn1+Ŝ_Cr2·Ŝ_Mn2].