title: W(2)Cl(4)(NR(2))(2)(PR'(3))(2) Molecules. 7. Preparation, Characterization, and Structures of W(2)Cl(4)(NHR)(2)(NH(2)R)(2) and W(2)Cl(4)(NHR)(2)(PMe(3))(2) (R = sec-Butyl and Cyclohexyl) and (31)P{(1)H} NMR Studies of Trans-to-Cis Isomerizations of W(2)Cl(4)(NHR)(2)(PMe(3))(2).
abstract: Treatment of W(2)Cl(6)(THF)(4) with alkylamine NH(2)R (R = Bu(s), Cy) affords a dinuclear species having the stoichiometry W(2)Cl(4)(NHR)(2)(NH(2)R)(2) (R = Bu(s) (1), Cy (2)). This has been confirmed by single-crystal X-ray diffraction studies for 2 with the following crystal data: tetragonal space group P&fourmacr;2(1)c, a = 12.774(2) Å, c = 9.934(2) Å, Z = 2. The molecule possesses an eclipsed structure with strong N-H.Cl intramolecular hydrogen bonding, with disordering of the whole set of ligands containing the amide, amine, and chlorine ligands. Attempts have been made to treat and refine both ligand sets separately for this molecule, and the final refinement converges with reasonable bond distances and angles to R = 0.028 and wR2 = 0.066. In both orientations, the ligand arrangements are the same. Each W atom is surrounded by a trans set of two Cl and two N atoms with a W-W separation of 2.2884(9) Å. Substitution of the amine ligands in 1 and 2 by the monodentate phosphine PMe(3) proceeds smoothly to produce trans-W(2)Cl(4)(NHR)(2)(PMe(3))(2) (R = Bu(s) (3), Cy (4)) in high yields. In solution, both 3 and 4 readily undergo isomerization to the corresponding cis-W(2)Cl(4)(NHR)(2)(PMe(3))(2) (R = Bu(s) (5), Cy (6)). The characterization of 3-6 has been accomplished by IR, (1)H NMR, and (31)P{(1)H}NMR spectroscopy and mass spectrometry. The crystal data for 5 and 6 are as follows: for 5, monoclinic space group P2(1)/a, a = 13.339(3) Å, b = 13.446(3) Å, c = 15.179(3) Å, beta = 99.33(2) degrees, Z = 4; for 6, P2(1)/n, a = 8.455(1) Å, b = 25.714(3) Å, c = 13.454(1) Å, beta = 104.839(8) degrees, Z = 4. Each of these phosphine-containing complexes is characterized by a W(2)(6+) metal core unit and has an eclipsed W(2)Cl(4)N(2)P(2) conformation. The W-W bond distances for 5 and 6 are 2.321(1) and 2.3229(5) Å, respectively, and these compounds are shown to have PMe(3) ligands cis to the amides. On the other hand, kinetic studies by (31)P{(1)H}NMR spectroscopy show that the trans-to-cis transformation in solution is an irreversible process of the first order, which is different from the reversible process observed in the case of tert-butyl analog. The initial rate constant was 10(1) x 10(-)(3) min(-)(1), and the rate constants in the presence of excess PMe(3) were shown to be roughly constant (average 4.5 x 10(-)(3) min(-)(1)) over a range of [PMe(3)]. These observations could be understood if, in addition to a dissociative mechanism, internal flip steps operate as a second mechanism for the process, and the flip barrier is 25-29 kcal/mol.
authors.affiliation: Department of Chemistry and Laboratory for Molecular Structure and Bonding, Texas A&M University, College Station, Texas 77843-3255.
authors.forename: F. Albert
authors.initials: FA
authors.lastname: Cotton
authors.forename: Evgeny V.
authors.initials: EV
authors.lastname: Dikarev
authors.forename: Wai-Yeung
authors.initials: WY
authors.lastname: Wong