Dinuclear Oxomolybdate( V ) Species with Oxalato and Pyridine Ligands Revisited: cis/trans Isomerization of [Mo2O4(η2‐C2O4)2(R‐Py)2]2– (R‐Py = Pyridine, Alkyl‐Substituted Pyridine) in Water Evidenced by NMR Spectroscopy

The simple oxohalomolybdate(V) ions [MoOCl4(H2O)]– and [MoOBr4]– were reacted with oxalic acid in mixtures containing an alcohol and a pyridine (R‐Py) to form the dinuclear anions [Mo2O4(η2‐C2O4)2(R‐Py)2]2– based on the {Mo2O4}2+ structural core. The dinuclear anion exists in two isomeric forms — as a trans and as a cis isomer. A trans arrangement of the pyridine ligands relative to the Mo2(μ2‐O)2 bridge was observed for (PyH)2[Mo2O4(η2‐C2O4)2Py2] (1), (PyH)2[Mo2O4(η2‐C2O4)2(3,5‐Lut)2] (2), [MeNC5H3(Me)2]2[Mo2O4(η2‐C2O4)2(3,5‐Lut)2]·H2O (3), (4‐MePyH)2[Mo2O4(η2‐C2O4)2(4‐MePy)2]·1/2(4‐MePy) (5), and [(C6H5)4P]2[Mo2O4(η2‐C2O4)2Py2]·H2O (7) [Py = pyridine; 3,5‐Lut = 3,5‐lutidine; MeNC5H3(Me)2+ = N‐methyl‐3,5‐lutidinium cation and 4‐MePy = 4‐methylpyridine], while a cis arrangement was found in only two compounds, namely (4‐EtPyH)2[Mo2O4(η2‐C2O4)2(4‐EtPy)2] (4) (4‐EtPy = 4‐ethylpyridine) and (4‐MePyH)3[Mo2O4(η2‐C2O4)2(4‐MePy)2]Br (6). The solution chemistry of the trans‐[Mo2O4(η2‐C2O4)2Py2]2– anion was monitored by 1H and 13C variable temperature NMR spectroscopy and showed evidence of two processes: the substitution of a pyridine ligand for water, coupled with cis/trans isomerization. Both reactions were exploited in the preparation of trans‐(PyH)2[Mo2O4(η2‐C2O4)2(H2O)2] (8) from a water/methanol solution of cis‐(4‐EtPyH)2[Mo2O4(η2‐C2O4)2(4‐EtPy)2] (4). trans‐(PyH)2[Mo2O4(η2‐C2O4)2(H2O)2] (8) was seen to react with methanol with the rupture of the molybdenum‐to‐oxalate bonds resulting in the cyclic octanuclear anion which crystallizes as a pyridinium salt, (PyH)2[Mo8O16(OCH3)8(μ8‐C2O4)]·2CH3OH (9). The compounds were fully characterized by X‐ray structural analysis and IR spectroscopy. MO calculations were performed on the isomeric pair of [Mo2O4(η2‐C2O4)2(4‐MePy)2]2– in order to establish their relative energies. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)