Molybdenum(V) complexes with formate: Geometric isomerism of the [Mo2O4Cl2(Py)2(HCOO)]− ion

Abstract Reactions of (PyH) 5 [MoOCl 4 (H 2 O)] 3 Cl 2 with formate resulted in trans -{(Py) 2 H}[Mo 2 O 4 Cl 2 (Py) 2 (HCOO)] ( 1 ) and cis -(PyH)[Mo 2 O 4 Cl 2 (Py) 2 (HCOO)] ( 2 ), whereas the bromide starting material (PyH)[MoOBr 4 ], yielded (PyH) 3 [Mo 2 O 4 Br 4 (HCOO)]·2CH 3 CN ( 3 ) and cis -(PyH)[Mo 2 O 4 Br 2 (Py) 2 (HCOO)] ( 4 ) (where Py stands for pyridine, PyH + for pyridinium cation and (Py) 2 H + for a hydrogen-bonded PyH + ⋯Py ion). In all, the dinuclear metal–metal bonded { Mo 2 O 4 } 2 + core may be recognized with its six coordination sites distributed among halides, pyridine ligands and formate. The latter is coordinated via both oxygen atoms, with each to a different metal ion. The [Mo 2 O 4 Cl 2 (Py) 2 (HCOO)] − ion exhibits geometric isomerism: the pyridine ligands, on each metal ion one, are either trans or cis to each other. The trans isomer crystallized with (Py) 2 H + countercations, whereas the cis isomer as a PyH + salt. In the crystal lattice of cis -(PyH)[Mo 2 O 4 Cl 2 (Py) 2 (HCOO)] ( 2 ), as confirmed by the X-ray structure analysis, pyridinium cation forms a hydrogen bond with the doubly-bridging oxide of the cis -[Mo 2 O 4 Cl 2 (Py) 2 (HCOO)] − ion. The countercations of trans -{(Py) 2 H}[Mo 2 O 4 Cl 2 (Py) 2 (HCOO)] ( 1 ) cannot participate in hydrogen-bonding. The DFT calculations on the isomers of the [Mo 2 O 4 Cl 2 (Py) 2 (HCOO)] − ion show the trans isomer to be by ca . 15 kJ/mol more stable than the cis isomer. The calculations on the hydrogen-bonded PyH + ⋯[Mo 2 O 4 Cl 2 (Py) 2 (HCOO)] − ion-pairs show a reversed order of stability. Hydrogen-bonding and weak C H⋯Cl interactions between PyH + cations and the cis -[Mo 2 O 4 Cl 2 (Py) 2 (HCOO)] − ion increase the stability of the cis compound.