Dinuclear Oxomolybdate( V ) Species with Oxalato and Pyridine Ligands Revisited: cis / trans Isomerization of [Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 (R‐Py) 2 ] 2– (R‐Py = Pyridine, Alkyl‐Substituted Pyridine) in Water Evidenced by NMR Spectroscopy

The simple oxohalomolybdate( V ) ions [MoOCl 4 (H 2 O)] – and [MoOBr 4 ] – were reacted with oxalic acid in mixtures containing an alcohol and a pyridine (R‐Py) to form the dinuclear anions [Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 (R‐Py) 2 ] 2– based on the {Mo 2 O 4 } 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 Mo 2 (μ 2 ‐O) 2 bridge was observed for (PyH) 2 [Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 Py 2 ] ( 1 ), (PyH) 2 [Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 (3,5‐Lut) 2 ] ( 2 ), [MeNC 5 H 3 (Me) 2 ] 2 [Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 (3,5‐Lut) 2 ] · H 2 O ( 3 ), (4‐MePyH) 2 [Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 (4‐MePy) 2 ] · 1/2(4‐MePy) ( 5 ), and [(C 6 H 5 ) 4 P] 2 [Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 Py 2 ] · H 2 O ( 7 ) [Py = pyridine; 3,5‐Lut = 3,5‐lutidine; MeNC 5 H 3 (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 [Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 (4‐EtPy) 2 ] ( 4 ) (4‐EtPy = 4‐ethylpyridine) and (4‐MePyH) 3 [Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 (4‐MePy) 2 ]Br ( 6 ). The solution chemistry of the trans ‐[Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 Py 2 ] 2– anion was monitored by 1 H and 13 C 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 [Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 (H 2 O) 2 ] ( 8 ) from a water/methanol solution of cis ‐(4‐EtPyH) 2 [Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 (4‐EtPy) 2 ] ( 4 ). trans ‐(PyH) 2 [Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 (H 2 O) 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 [Mo 8 O 16 (OCH 3 ) 8 (μ 8 ‐C 2 O 4 )] · 2CH 3 OH ( 9 ). The compounds were fully characterized by X‐ray structural analysis and IR spectroscopy. MO calculations were performed on the isomeric pair of [Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 (4‐MePy) 2 ] 2– in order to establish their relative energies. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)