METAL ALKOXIDES - MODELS FOR METAL-OXIDES .18. STRUCTURE, BONDING, AND DYNAMIC BEHAVIOR OF BIS(ETA-2-ETHYLENE)HEXAKIS(NEOPENTOXY)DITUNGSTEN - STUDIES OF THE REVERSIBLE ADDITION OF C-C DOUBLE-BONDS TO A W-W TRIPLE-BOND

Hydrocarbon solutions of W2(OCH2-t-Bu)6 and ethylene (greater-than-or-equal-to 2 equiv) react at 0-degrees-C to yield W2(OCH2-t-Bu)6(eta2-C2H4)2, 1, which has been characterized by NMR studies and a single-crystal X-ray diffraction study. Crystal data for 1 at -155-degrees-C: a = 11.754 (4) angstrom, b = 31.405 (14) angstrom, c = 11.274 (4) angstrom, beta = 104.96 (2)-degrees, Z = 4, d(cald) = 1.564 g cm-3, and space group P2(1)/c. The molecular structure of 1 has virtual C2 symmetry. There are four bridging OR ligands that span a W-W bond of distance 2.533 (1) angstrom in an unsymmetrical manner; W-O (bridge) = 2.00 (1) and 2.31 (1) angstrom (av) for the four short and four long distances, respectively. The long W-O distances are trans to terminal W-OR or W-(eta2-C2H4) ligands. The dinuclear molecule may be viewed as a dimer of two pseudotetrahedral W(OR)3(eta2-C2H4) moieties and the W(eta2-C2H4) fragments are metallacyclopropanes: C-C = 1.45 (2) angstrom and W-C = 2.14 (2) angstrom (av). The alignment of the two olefinic C2 axes are mutually orthogonal and perpendicular to the W-W axis. This arises from the mutual competition of W d(pi)-olefin pi* bonding and W-W bonding. In solution, compound 1 is in equilibrium with W2(OCH2-t-Bu)6 and C2H4 and above 0-degrees-C reacts further with ethylene to yield W2(OCH2-t-Bu)6(mu-CCH2CH2CH2) and ethane by way of an intermediate metallacyclopentane-ethylene complex W2(OCH2-t-Bu)6(CH2)4(eta2-C2H4) [J. Am. Chem. Soc. 1989, 111, 5284]. At -20-degrees-C the H-1 and C-13{H-1} NMR spectra are consistent with those expected on the basis of the solid-state structure. By a combination of C-H/C-D labeling and nuclear Overhauser enhancement (NOE) difference spectroscopy, an assignment of the four ethylenic and six methylenic protons (of the OR ligands) has been made. Spin saturation transfer difference spectroscopic studies of 1 in toluene-d8 in the temperature range 5-25-degrees-C reveal a dynamic process that is distinct from the equilibrium involving 1 and W2(OCH2-t-Bu)6 and C2H4. The selective site exchange amongst the C-H protons is consistent with the pairwise opening and closing of OR bridges about an intermediate that has C2v symmetry (W2(mu-OR)2(OR)4(eta2-C2H4)2) and is related to the previously characterized compound W2(CH2Ph)2(mu-O-i-Pr)2(O-i-Pr)2(eta2-C2Me2)2. The activation parameters for the fluxional behavior of 1 are DELTAH double dagger = 13.4 +/- 0.9 kcal mol-1 and DELTAS double dagger = -13 +/- 3 cal K-1 mol-1. The bonding in 1 was investigated by MO calculations on the hypothetical molecule W2(OH)6(eta2-C2H4)2 with structural data taken from the X-ray study of 1. The calculations employed the method of Fenske and Hall and extended Huckel theory. Calculations were also carried out on the doubly bridged species W2(mu-OH)2(OH)4(eta2-C2H4)2, Which has a structure based on the fusing of two square-based pyramids. In these geometries, rotation about the W-C2 olefin axis is not restricted in the manner seen in the ground-state geometry. Attempts to determine the role of 1 in the formation of the metallacyclopentane-ethylene complex W2(OCH2-t-Bu)6(CH2)4(eta2-C2H4) were partially obscured by the competitive reversible formation of 1 from W2(OCH2-t-Bu)6 and ethylene. In the presence of a large excess of ethylene (10, 20, and 30 equiv), the disappearance of 1 and conversion to W2(OCH2-t-Bu)6(CH2)4(eta2-C2H4) and W2(OCH2-t-Bu)6-(mu-CCH2CH2CH2) is independent of the concentration of C2H4 and first order in 1.