From a square core to square opening: structural diversity and magnetic properties of the oxo-bridged [CrIIINbV] complexes

Abstract

Three heterometallic oxo-bridged compounds, [Cr₂(phen)₄(μ-O)₄Nb₂(C₂O₄)₄]·2H₂O (1; phen = 1,10-phenanthroline), [Cr₂(terpy)₂(H₂O)₂(μ-O)₄Nb₂(C₂O₄)₄]·4H₂O (2; terpy = 2,2′;6′,2′′-terpyridine) and [Cr(terpy)(C₂O₄)(H₂O)][Cr₂(terpy)₂(C₂O₄)₂(μ-O)₂Nb(C₂O₄)₂]·3H₂O (3), have been synthesized using a building block approach and characterized by IR spectroscopy, single-crystal and powder X-ray diffraction, magnetization measurements, and DFT calculations. The molecular structures of 1 and 2, crystallizing in P4₂2₁2 and P2₁/n space groups, respectively, contain a square-shaped {Cr^III₂(μ-O)₄Nb^V₂} unit, while that of complex salt 3 (P space group) consists of a mononuclear cation containing Cr^III and trinuclear anions in which two Cr^III ions are bridged by a –O–Nb^V–O– fragment. Besides hydrogen-bonding patterns resulting in a 1D- or 3D-supramolecular arrangement in 1–3, an unusual intermolecular contact has been noticed between parallel oxalate moieties occurring due to the electrostatic attraction of electron-rich carbonyl oxygen and severely electron-depleted carbon atoms in the crystal packing of 2. The antiferromagnetic coupling observed in all three compounds, determined from magnetization measurements (J = −13.51(2), −8.41(1) and −7.44(4) cm⁻¹ for 1, 2 and 3, respectively) and confirmed by DFT calculations, originates from two Cr^III ions with spin 3/2 interacting through diamagnetic –O–Nb^V–O– bridge(s).