Influence of the counterion on the coordinating properties of (2,2′-bipyridyl)bis(oxalato)chromate(III) anion: crystal structures and magnetic properties of AsPh4[Cr(bipy)(ox)2]·H2O and [NaCr(bipy)(ox)2(H2O)]·2H2O

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Abstract
Two new chromium(III)-containing complexes of formula AsPh4[Cr(bipy)(ox)2]·H2O 1 and [NaCr(bipy)(ox)2(H2O)]· 2H2O 2 (bipy = 2,2′-bipyridine and ox = oxalate dianion) have been synthesized and characterized by single-crystal X-ray diffraction. The structure of 1 consists of discrete [Cr(bipy)(ox)2]– mononuclear anions, tetraphenylarsonium cations and uncoordinated water molecules. The structure of 2 reveals a novel two-dimensional framework which is made up of oxalato-bridged bimetallic CrIII–NaI helical chains which are interconnected through centrosymmetric Na2O2 units. Within the chain, a regular alternation of the metal ions is observed, the oxalate group acting as a bis(chelating) ligand. In addition to this coordination mode, two oxalates each act as monodentate ligands towards a sodium atom of a neighbouring chain thus leading to a sheetlike polymeric structure. The chromium environment is distorted octahedral in both complexes: two nitrogen atoms from a bidentate bipy ligand and four oxygen atoms from either two chelating (1) or two bis(chelating) (2) oxalate groups build the coordination polyhedron around the chromium atom. The Cr–N bond lengths [values in the ranges 2.077(3)–2.057(3) (1) and 2.067(4)–2.058(4) Å (2)] are somewhat longer than the Cr–O ones [values in the ranges 1.960(2)–1.946(2) (1) and 1.968(3)–1.949(3) Å (2)]. The sodium atom in 2 is also six-coordinated: a coordinated water molecule [2.371(5) and 2.325(4) Å for Na(1)–O(17) and Na(2)–O(18), respectively] and five oxygens from three oxalate groups [values of the Na–O (ox) bonds in the ranges 2.511(4)–2.331(4) and 2.481(4)–2.364(4) Å around Na(1) and Na(2), respectively] build a distorted octahedral NaO6 environment. The intralayer chromium–sodium and sodium–sodium distances through bridging oxalate in 2 vary in the ranges 5.657(4)–5.579(2) and 3.534(3)–3.497(4) Å, respectively. Variable-temperature magnetic susceptibility measurements of 1 and 2 reveal the occurrence of very weak antiferromagnetic interactions together with zero-field splitting effects in both compounds. The use of the [Cr(bipy)(ox)2]– unit as a ligand towards different univalent and divalent metal ions aimed at designing new heterobimetallic systems is analysed and discussed in the light of available structural data.

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