Comparison of H-H versus Si-H sigma-Bond Coordination and Activation on 16e Metal Fragments. Organosilane, N(2), and Ethylene Addition to the Agostic Complex W(CO)(3)(PR(3))(2) and Dynamic NMR Behavior of the Latter.
Variable-temperature (31)P{(1)H} NMR spectroscopy of the agostic complexes M(CO)(3)(PCy(3))(2) (M = Mo, W) indicates dynamic behavior as evidenced by collapse below -20 degrees C of a singlet to an AB signal plus a shifted singlet. The inequivalency of the phosphines is possibly due to the presence of conformational isomers resulting from hindered rotation of the M-P bond or, less likely, a geometric isomer with pseudo-cis PCy(3) ligands. Further studies on the coordination chemistry of W(CO)(3)(PR(3))(2) (R = iPr, Cy) were performed. The bridging dinitrogen complex [W(CO)(3)(PiPr(3))(2)](2)(&mgr;-N(2)) (1) was cleanly formed in the reaction of W(CO)(3)(PiPr(3))(2) with N(2). Complex 1 was structurally characterized and compared with other bridging dinitrogen compounds of tungsten. The ethylene complex W(CO)(3)(PCy(3))(2)(eta(2)-C(2)H(4)) (2) was synthesized and characterized by X-ray crystallography in order to compare the binding mode of ethylene with that of H(2). Phenylsilane reacted with W(CO)(3)(PR(3))(2) (R = iPr, Cy) to form the thermally unstable oxidative addition (OA) products WH(SiH(2)Ph)(CO)(3)(PR(3))(2) (3, R = Cy; 4, R = iPr). Diphenylsilane reacted with W(CO)(3)(PiPr(3))(2) at 60 degrees C to form the bridging silyl species [W(CO)(3)(PiPr(3))(&mgr;-SiHPh(2))](2) (5), which was confirmed by spectroscopic techniques and X-ray crystallography to have two 3-center 2-electron W.H.Si interactions. Detailed comparisons of the binding and activation of silanes versus H(2) on various 16e metal centers suggest a high degree of similarity, but relative ease of OA depends on the electrophilicity of the metal-ligand fragment and other factors such as bond energetics. Increasing the electrophilicity of the metal center (e.g., adding positive charge) may aid in stabilizing alkane coordination.
Butts
Matthew D.
MD
Chemical Science and Technology Division, MS J514, Los Alamos National Laboratory, Los Alamos, New Mexico 87545.
Bryan
Jeffrey C.
JC
Luo
Xiao-Liang
XL
Kubas
Gregory J.
GJ