137566-69-9Relevant articles and documents
The tungsten-tungsten triple bond. 17.1 mixed amido-phosphido compounds of formula M2(PR2)2(NMe2)4. Comparisons of amido and phosphido ligation and bridged and unbridged isomers
Buhro, William E.,Chisholm, Malcolm H.,Folting, Kirsten,Huffman, John C.,Martin, James D.,Streib, William E.
, p. 557 - 570 (2007/10/02)
The reaction between 1,2-W2Cl2(NMe2)4 and 2 equiv of a lithium organophosphide reagent, LiPR2 (R = t-Bu, Cy, Et, SiMe3, Ph, p-tol, p-C6H4F), in THF (tetrahydrofuran) at or below 0°C yields the mixed amido-phosphido complexes 1,2-W2(PR2)2(NMe2)4 The molybdenum analogue where R = t-Bu has been similarly prepared. With the sterically demanding t-Bu and SiMe3 substituents at phosphorus, the products of a single substitution, 1,2-W2Cl(PR2)(NMe2)4, have been obtained. The latter react with a second equivalent of lithium organophosphide to yield the mixed-phosphido complexes 1,2-W2(PR2)(PR′2)(NMe2) 4. By single-crystal X-ray crystallography five samples have been fully characterized in the solid state. This structural data base contains both anti- and gauche-M2P2N4 ethane-like cores with M-M distances typical of (M≡M)6+ containing compounds with σ2π4 triple bonds: M-M = 2.2 (M = Mo) and 2.3 A? (M = W). The coordination about nitrogen is trigonal planar with short M-N distances, 1.96 A? (av), indicative of M-N double bonds, while the coordination at phosphorus is distinctly more pyramidal and the M-P distances of 2.42 to 2.48 A? are only slightly shorter than those anticipated for M-P single-bond distances. In solution, 31P NMR chemical shifts, as well as the measured barriers to rotation about the M-N and M-P bonds, indicate a competition between metal-amido and metal-phosphido bonding that can be controlled by the substituents at phsophorus, e.g. P(t-Bu)2 is a better σ + π donor than PPh2. The measured rates of gauche:anti isomerization and the equilibrium constants are used to calculate thermodynamic and activation parameters for the rotation about the metal-metal bond. Except for bulky R groups, R = SiMe3 and t-Bu, isomerization to phosphido-bridged dimers occurs in hydrocarbon solutions above 0°C. The solid-state structure of W2(μ-PCy2)2(NMe2)4 deduced from an X-ray study reveals a central puckered W2P2 moiety with a dihedral angle between the W2P planes of 130°. The M-M distance is 2.570 (1) A?, roughly 0.3 A? longer than in the unbridged isomer. The W-P distances, 2.36 (1) to 2.40 (1) A?, are equal or slightly shorter than those of the unbridged isomer while the W-N distances, 1.99 (1) A? (av), are notably longer. A MO analysis employing the Fenske-Hall method indicates that the puckered d3-d3 W2Pv moiety is favored relative to the planar W2(μ-PR2)2 moiety by enhanced M-M bonding. In solution two dynamic processes are observed: (i) rotation about M-N bonds and (ii) an inversion of the W2P2 moiety. The bridged isomers are characterized by lower field 31P chemical shifts, ca. 180-280 ppm, and larger values of J183W-31P of ca. 250-350 Hz, relative to their unbridged isomers. The rate of bridge formation has been studied as a function of the substituents at phosphorus. Bridge formation (ring closure) shows first-order irreversible kinetics for R = Cy, Et and R = t-Bu and R′ = Ph and first-order reversible kinetics for R = Ph, p-tolyl, and p-C6H4F. Bridge formation is favored for the more Lewis basic phosphide ligands but suppressed by steric factors for the most bulky group R = t-Bu.
