THE TUNGSTEN TUNGSTEN TRIPLE BOND .17. MIXED AMIDO PHOSPHIDO COMPOUNDS OF FORMULA M2(PR2)2(NME2)4 - COMPARISONS OF AMIDO AND PHOSPHIDO LIGATION AND BRIDGED AND UNBRIDGED ISOMERS

被引:31
作者
BUHRO, WE
CHISHOLM, MH
FOLTING, K
HUFFMAN, JC
MARTIN, JD
STREIB, WE
机构
[1] INDIANA UNIV,DEPT CHEM,BLOOMINGTON,IN 47405
[2] INDIANA UNIV,CTR MOLEC STRUCT,BLOOMINGTON,IN 47405
关键词
D O I
10.1021/ja00028a024
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
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-C,H4F), in THF (tetrahydrofuran) at or below 0-degrees-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 rive 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 sigma-2-pi-4 triple bonds: M-M = 2.2 (M = Mo) and 2.3 angstrom (M = W). The coordination about nitrogen is trigonal planar with short M-N distances, 1.96 angstrom (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 angstrom are only slightly shorter than those anticipated for M-P single-bond distances. In solution, P-31 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 phosphorus, e.g. P(t-Bu)2 is a better sigma + pi-donor than PPh2. The measured rates of gauche:antiisomerization 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-degrees-C. The solid-state structure of W2(mu-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-degrees. The M-M distance is 2.570 (1) angstrom, roughly 0.3 angstrom longer than in the unbridged isomer. The W-P distances, 2.36 (1) to 2.40 (1) angstrom, are equal or slightly shorter than those of the unbridged isomer while the W-N distances, 1.99 (1) angstrom (av), are notably longer. A MO analysis employing the Fenske-Hall method indicates that the puckered d3-d3 W2P2 moiety is favored relative to the planar W2(mu-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 P-31 chemical shifts, ca. 180-280 ppm, and larger values of J183W-P-31 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.
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页码:557 / 570
页数:14
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