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

Three heterometallic oxo-bridged compounds, [Cr(phen)(μ-O)Nb(CO)]·2HO (1; phen = 1,10-phenanthroline), [Cr(terpy)(HO)(μ-O)Nb(CO)]·4HO (2; terpy = 2,2';6',2''-terpyridine) and [Cr(terpy)(CO)(HO)][Cr(terpy)(CO)(μ-O)Nb(CO)]·3HO (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 P422 and P2/n space groups, respectively, contain a square-shaped {Cr(μ-O)Nb} unit, while that of complex salt 3 (P1[combining macron] space group) consists of a mononuclear cation containing Cr and trinuclear anions in which two Cr ions are bridged by a -O-Nb-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 for 1, 2 and 3, respectively) and confirmed by DFT calculations, originates from two Cr ions with spin 3/2 interacting through diamagnetic -O-Nb-O- bridge(s).