[Cr(phen)(ox)2]-: A versatile Bis-Oxalato Building for the Design of Heteropolymetallic System. Crystal Structures and Magnetic Properties of AsPh4[Cr(phen)(ox)2].H2O, [NaCr(phen)(ox)2(H2O)].H2O abd {[Cr(phen)(ox)2]2[Mn2(bipy)2(H2O)2(ox)]}.6H2O

Abstract

[EN] The new complexes of formula AsPh4[Cr(phen)(ox)(2)] . H2O (1), [NaCr(phen)(ox)(2)(H2O)] . 2H(2)O (2) and {[Cr(phen)(ox)(2)](2)[Mn-2(bpy)(2)(H2O)(2)(ox)]} . 6H(2)O (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) Angstrom and Cr-N bonds of 2.083(4) and 2.072(4) Angstrom. The angles subtended at the chromium atom by the two oxalates are 83.6(2) and 83.3(1)degrees whereas the N-Cr-N angle is 79.9(2)degrees. 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) Angstrom and the sixth position being occupied by an aqua ligand with Na-O(w) = 2.384(6) Angstrom. The intralayer chromium-sodium and sodium-sodium separations through bridging oxalate in 2 are 5.560(4) and 3.643(8) Angstrom, respectively. The structure of 3 consists of neutral tetranuclear (Cr2Mn2II)-Mn-III 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) Angstrom, respectively. The intramolecular chromium-manganese separation [5.507(5) and 5.502(5) A for Cr(2) Mn(2) and Cr(1)...Mn(1)] is somewhat shorter that the manganese-manganese one [5.703(2) Angstrom]. 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 (H) over cap = -J (S) over cap(Mn1) . (S) over cap(Mn2) - j[(S) over cap(Cr1) . (S) over cap(Mn1) + (S) over cap(Cr2) . (S) over cap(Mn2)].