Complexation of molybdenum(V) with glycolic acid: an unusual orientation of glycolato ligand in {Mo2O4}2+ complexes

(PyH)5Mo(V)OCl4(H2O)]3Cl2 and (PyH)nMo(V)OBr4]n reacted with glycolic acid (H2glyc) or its half-neutralized ion (Hglyc(-)) to afford a series of novel glycolato complexes based on the {Mo(V)2O4}2+ structural core: (PyH)3Mo2O4Cl4(Hglyc)]. (1)/ 2CH 3CN (1), (PyH) 3Mo 2O 4Br 4(Hglyc)].Pr(i)OH(2), (PyH)2Mo2O4(glyc) 2Py 2] (3), (PyH) 4Mo 4O 8Cl 4(glyc) 2].2EtOH (4), and Mo 4O 8(glyc) 2Py 4] (5) (Py = pyridine, C 5H 5N; PyH(+) = pyridinium cation, C 5H 5NH (+) and glyc (2-) = a doubly ionized glycolate, (-)OCH 2COO (-)). The compounds were fully characterized by X-ray crystallography and infrared spectroscopy. The Hglyc (-) ion binds to the {Mo 2O 4} (2+) core through a carboxylate end in a bidentate bridging manner, whereas the glyc (2-) ion adopts a chelating bidentate coordination through a deprotonated hydroxyl group and a monodentate carboxylate. The orientations of glyc (2-) ions in 3- 5 are such that the alkoxyl oxygen atoms occupy the sites opposite the multiply bonded oxides. {(C6H5) 4P}Mo(VI)O 2(glyc)(Hglyc)] ( 6), an oxidized complex, features a reversed orientation of the glyc(2-) ion. The theoretical DFT calculations on the Mo(V)2O4(glyc) 2Py 2](2-) and Mo(VI)O2(glyc)2](2-) ions confirm that binding of glycolate with the alkoxyl oxygen to the site opposite the MoO bond is energetically more favorable in {Mo(V)2O4}(2+) species, whereas a reversed orientation of the ligand is preferred in Mo(VI) complexes. An explanation based on the orbital analysis is put forward.