113794-50-6Relevant articles and documents
Tetranuclear halide-alkoxide clusters of molybdenum formed by the coupling of M-M triple-bonded dinuclear compounds. Synthesis, characterization, and molecular structures of Mo4F2(O-i-Pr)10, Mo4X3(O-i-Pr)9, and Mo4X4(O-i-Pr)8 (X = Cl, Br, I)
Chisholm, Malcolm H.,Clark, David L.,Errington, R. John,Folting, Kirsten,Huffman, John C.
, p. 2071 - 2084 (2008/10/08)
Hydrocarbon solutions of Mo2(O-i-Pr)6(M≡M) react with MeCOX or Me3SiX (GX, 1.5 equiv) to give Mo4X3(O-i-Pr)9 (I; X = Cl, Br, I) and GO-i-Pr. Complexes I may be viewed as the coupled products of Mo2X2(O-i-Pr)4 and Mo2X(O-i-Pr)5. A further reaction between I and GX (1 equiv) or the direct reaction between Mo2(O-i-Pr)6 and GX (2 equiv) carried out in toluene leads to Mo4X4(O-i-Pr)8 compounds (II). The compounds II are unstable in solution at elevated temperatures (>60°C) and react further to give Mo4O2X2(O-i-Pr)6 compounds (III) with elimination of i-PrX (2 equiv). The ease of the conversion of II to III follows the order X = I > Br > Cl and occurs more readily in CH2Cl2 than in hydrocarbon solvents. Attempts to prepare fluoro analogues of II and III by related reaction procedures have failed to give an isolable single product, as did the addition of i-PrOH (>8 equiv) to the known compound Mo4F4(O-t-Bu)8. The reaction between Mo2(O-i-Pr)6(M≡Af) and 1 equiv of either MeCOF or PF3 in toluene at -78°C yielded the thermally unstable, red, crystalline compound Mo4F2(O-i-Pr)10 (IV). Compounds I have been characterized by a variety of spectroscopic techniques and, for X = Br, by a single-crystal X-ray study. Collectively the data present a uniform structural picture involving butterfly Mo4 units [five Mo-Mo distances of 2.50 A? (averaged) and one long Mo-Mo distance of 3.29 A?] supported by alkoxide bridges, of which two cap the external triangular faces of the butterfly and four edge-bridge backbone and wingtip molybdenum atoms. The wingtip molybdenum atoms have two terminal ligands (X and O-i-Pr) while the backbone molybdenum atoms have only one terminal ligand (X or O-i-Pr). The local geometry about each molybdenum atom is square pyramidal, MO4X, where X (Cl or O-i-Pr) occupies the apical site. The compounds II show low-temperature limiting 1H NMR spectra indicative of a butterfly C21, Mo4 structure related to I by the substitution of the apical O-i-Pr ligand on the backbone molybdenum atom by X. The 1H NMR spectra for II recorded at room temperature indicate O-i-Pr ligand scrambling on the NMR time scale. X-ray studies for X = Br confirmed the anticipated butterfly Mo4 geometry [five Mo-Mo distances of 2.50 A? (averaged) and one Mo-to-Mo distance of 3.2 A?] but, for X = Cl, revealed a square Mo4 unit (Mo-Mo = 2.38 A?) supported by eight μ-O-i-Pr ligands, four above and four below the M4 plane. The Mo-Cl bonds are terminal and radiate from the center of the square. The square and butterfly Mo4 units can be viewed as arachno fragments of the well-known closo-Mo6(μ3-X)84+ moiety. The arachno Mo4 subunits all have abnormally long radial Mo-L bonds. This phenomenon arises from what is termed the radial cluster influence. The compounds III are formulated as derivatives of II in which the two μ3-O-i-Pr ligands are replaced by capping (μ3) oxo ligands and the backbone molybdenum atoms lack terminal Mo-X bonds. Compound IV contains a rectangular Mo4 unit having two short, 2.23 A?, and two long, 3.41 A?, distances corresponding to M≡M and nonbonding M-to-M distances, respectively. The M≡M bonds are unbridged, and the tetranuclear compound is viewed as a dimer, [Mo2(μ-F)(μ-O-i-Pr)(O-i-Pr)4]2. In solution the molecules undergo facile disproportionation to give Mo2(O-i-Pr)6 and other products presumably derived from the reactive species Mo2F2(O-i-Pr)4. The latter has been trapped, by the addition of PMe3 (2 equiv) to hydrocarbon solutions of IV, as the adduct Mo2F2(O-i-Pr)4(PMe3)2, for which spectroscopic data indicate a structural analogy to the previously characterized compound Mo2F2(O-t-Bu)4(PMe3) 2(M≡M). Collectively these studies provide the first examples of the coupling of M-M triple bonds to give 12-electron M4 clusters. These results are compared to the previously reported condensation of Mo-Mo quadruple bonds. Crystal data: (i) for Mo4Br3(O-i-Pr)9 at -164°C, a = 18.645 (7) A?, b = 11.825 (3) A?, c = 19.046 (7) A?, β = 94.64 (2)°, Z = 4, dcalcd = 1.833 g cm-3, and space group P21/c; (ii) for Mo4Br4(O-i-Pr)8 at -160°C1 a = 20.042 (5) A?, b = 10.980 (2) A?, c = 18.602 (4) A?, β = 112.60 (1)°, Z = 4, dcalcd = 2.067 g cm-3, and space group A2/a; (iii) for Mo4Cl4(O-i-Pr)8 at -162°C, a = 9.971 (10) A?, b = 9.971 (10) A?, c = 18.272 (19) A?, Z = 2, dcalcd = 1.825 g cm-3, and space group I4/m; (iv) for Mo4F2(O-i-Pr)10 at -160°C, a = 13.143 (2) A?, b = 10.339 (2) A?, c = 9.088 (1) A?, α = 108.87 (1)°, β = 77.16 (1)°, γ = 113.03 (1)°, Z = 1, and space group P1.
