SYNTHETIC, STRUCTURAL, AND MECHANISTIC STUDIES OF THE C-H BOND ACTIVATION OF PHENOLS BY W(PME3)6 AND W(PME3)4(ETA-2-CH2PME2)H

Hexakis(trimethylphosphine)tungsten(0), W(PMe3)6, has been synthesized by the reduction of WCl6 with Na(K) alloy using PMe3 as a reactive solvent and structurally characterized by X-ray diffraction techniques. Facile dissociation of one of the PMe3 ligands of W(PMe3)6 produces the metalated complex W(PMe3)4(eta-2-CH2PMe2)H, with which it is in equilibrium. W(PMe3)6 and W(PMe3)4(eta-2-CH2PMe2)H react with phenols to give 4- and 5-membered oxametallacycle derivatives as a result of competitive sp2 vs sp3 C-H bond activation. 2-Alkylphenols 2-RC6H4OH (R = H, Et, Pr(i), Bu(t), Ph) give specifically the 4-membered oxametallacycle derivatives W(PMe3)4H2(eta-2-OC6H3R), whereas 2-methylphenol gives specifically the 5-membered oxametallacycle derivative W(PMe3)4H2{eta-2-OC6H4(CH2)}. 2,6-Dimethylphenol and 2,4,6-trimethylphenol also react to give 5-membered oxametallacycles, W(PMe3)4H2{eta-2-OC6H3Me(CH2)} and W(PMe3)4H2{eta-2-OC6H2Me2(CH2)}, respectively. Mechanistic studies suggest that the formation of a 5-membered rather than a 4-membered oxametallacycle in the reaction with 2-methylphenol is a result of thermodynamic control. All the oxametallacycles react rapidly with hydrogen to result in an equilibrium with the aryloxide derivatives W(PMe3)4H3(OAr). For most of the derivatives, the equilibrium is shifted strongly in favor of the alkoxide derivative; however, for the complexes W(PMe3)4H2{eta-2-OC6H3Me(CH2)} and W-(PMe3)4H2{eta-2-OC6H2Me2(CH2)}, the oxametallacycles are the most stable form. W(PMe3)6 is cubic, Imm3mBAR (No. 229), a = 11.303 (2) angstrom, v = 1444.1 (6) angstrom3, Z = 2. W(PMe3)4H2(eta-2-OC6H4) is monoclinic, P2, In (No. 14), a = 9.713 (2) angstrom, b = 16.008 (5) angstrom, c = 16.283 (2) angstrom, beta = 93.51 (1)-degrees, V = 2527.1 (9) angstrom3, Z = 4. W(PMe3)4H2 {eta-2-OC6H2Me2(CH2)} is monoclinic, P2(1)/n (No. 14), a = 9.898 (3) angstrom, b = 28.065 (9) angstrom, c = 10.663 (3) angstrom, beta = 104.33 (2)-degrees, V = 2870 (1) angstrom3, Z = 4.