Quadruply bonded bis[bis(diphenylphosphino)ethane]tetraiododimolybdenum: twisted and eclipsed rotational conformations and their significance
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The structural characterizations of two crystallographic forms of β-Mo2I4(dppe)2 are reported, and the.relationship between the degree of torsional twist that the molecules exhibit in the solid state and the energy of the δ → δ* transition is discussed. The title compound can be prepared by reaction of the quadruply bonded Mo2(μ-O2CCH3)4 with Me3SiI in toluene in the presence of 2 equiv of dppe and can be crystallized from many solvents. A solution of the compound in dichloromethane-methanol produces green monoclinic crystals of the dichloromethane solvate β-Mo2I4(dppe)2.2/3CH2Cl2(1), in which there are twisted molecules, la, located on a general position, and eclipsed molecules, lb, which reside on crystallographic inversion centers. The crystal belongs to the space group P21/n with unit cell dimensions a = 13.600 (4) A, b = 17.755 (5) A, c = 30.30 (1) A, β = 98.74 (2)°, V = 8426 (4) A3, and Z = 6. The asymmetric unit is defined by 1.5 molecular formulas of Mo2I4(dppe)2 and one CH2C12 molecule. The structure is unusual in that la possesses a mean torsional twist angle of x = 27.9° and lb assumes the fully eclipsed orientation with an average x = 0°, where x is the angle of internal rotation away from the eclipsed conformation. A significant difference exists in the degree of Mo-Mo interaction for la and lb as is evident from the Mo-Mo bond distances of 2.180 (4) and 2.129 (5) A for the cocrystallized molecules la and lb, respectively. A second crystal of Mo2I4(dppe)2 was grown from a dichloro-methane-toluene mixture and is designated as β'-Mo2I4(dppe)2.C7H8(2). The β' form crystallizes in the monoclinic P21/n space group with the following unit cell dimensions: a = 12.015 (2) A, b = 25.048 (4) A, c = 19.407 (4) A, β = 95.3 (2)°, V = 5814 (2) A 3, and Z = 4. The midpoints of the Mo-Mo bonds reside on a set of general positions. The molecule is internally rotated by an average value of x = 25.7°,.and the Mo-Mo distance is 2.179 (3) A. The variation in δ overlap in the three different types of molecules la, lb, and 2 was effectively probed by recording the visible spectra of the samples in the solid state. Two distinct bands are observed at 670 nm (14 900 cm-1) and at 920 nm (10900 cm-1) in the spectrum of β-Mo2I4(dppe)2•2/3CH2Cl2 and are assigned to theδ — δ* (Alg -> Alu) transitions of molecules lb (x = 0°) and la (x = 27.9°), respectively; the intensity ratio is in approximate accord with the ratio of the δ/δ overlap integrals. The solid-state visible spectrum of β'-Mo2I4(dppe)2•C7Hg contains only one band at λmax= 878 nm (11 400 cm-1), in agreement with the results of the X-ray structure, which shows that only one type of molecule is present. Dichloromethane solution spectra of both β and β' forms are identical, with the δ —δ* transition being located at 860 nm (11 600 cm”1)- The electronic absorption properties of 1 and 2 as well as the results of 31P(1H) NMR spectroscopy and cyclic voltammetry studies are discussed. © 1986, American Chemical Society. All rights reserved.
author list (cited authors)
Cotton, F. A., Dunbar, K. R., & Matusz, M.