Carbomolybdate clusters formed by carbonyl insertion into molybdenum-oxygen bonds. Structural investigation of the [RMo 4O 15X] 3− core (R  C 9H 4O, X  OCH 3; R  C 14H 10, X  C 7H 5O 2; R  C 14H 8, X  OH)

Abstract

[C 4H 9) 4N] 2[Mo 2O 7] reacts with a variety of organic species containing α-diketone groups to give tetranuclear complexes of general composition [RMo 4O 15X] 3−. The complexes [(C 4H 9) 4N] 3[(C 9H 4O)Mo 4O 15(OCH 3)] ( I), [(C 4H 9) 4N] 3[(C 14H 10)Mo 4O 15(C 6H 5CO 2)] ( 11) and [(C 4H 9) 4N] 3[(C 14H 8)Mo 4O 15(OH)] ( III) were synthesized from the reactions of dimolybdate with ninhydrin, benzil and phenanthraquinone, respectively. Complex II may also be prepared from dimolybdate and benzoin in acetonitrile-methanol solution, from which it co-crystallizes with the binuclear species [(C 4H 9) 4N] 2[Mo 2O 5(C 6H 5C(O)C(O)C 6H 5) 2] · CH 3CN · CH 3OH ( IV). Complexes I–III exhibit the tetranuclear core, previously described for the α-glyoxal derivatives [(C 4H 9) 4N] 3[(HCCH)Mo 4O 15X], where X  F − or HCO 2 −. The ligands may be formally described as diketals, formed by insertion of ligand carbonyl subunits into molybdenum-oxygen bonds. The structures I–III differ most dramatically in the identity and coordination mode of the anionic ligand X − which occupies a position opposite the diketal moiety relative to the [Mo 4O 11] 2+ central cage. Thus, I exhibits a doubly bridging methoxy group in this position, while II possesses a benzoate ligand with an unusual μ 3-O,O′coordination mode. Complex III presents a hydroxy-group unsymmetrically bonded to three of the molybdenum centres. The stereochemical consequences of the various coordination modes are discussed. Crystal data: Compound I, monoclinic space group Pc, a = 24.888(2), b = 12.897(3), c = 24.900(3) Å, β = 101.94(2)°, D calc = 1.28 g cm −1 for Z = 4. Structure solution and refinement based on 8695 reflections with F o ⩾ 6σ(F o) (Mo- K α , λ = 0.71073 Å) converged at a conventional discrepancy factor of 0.060. Compound II, orthorhombic space group Pbca, a = 20.426(6), b = 26.916(6), c = 32.147(7) Å, V = 17673.2(20) Å 3, D calc = 1.33 g cm −3 for Z = 8; 5224 reflections, R = 0.076. Compound III, tetragonal space group I4 1/ a, a = b = 48.129(6), c = 13.057(2) Å, V = 30246.2(12) Å 3, D calc = 1.35 g cm −3 for Z = 16; 5554 reflections, R = 0.053. Compound IV, orthorhombic space group Pnca, a = 16.097(4), b = 16.755(4), c = 25.986(7) Å, V = 7008.1(13) Å 3, Z = 4, D calc = 1.18 g cm −3 ; 2944 reflections, R = 0.061.