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 [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 O4) 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 ].½(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-Me-PyH) 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 1 3 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 1 6 (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.