A Comparison of the Influences of Alkoxide and Thiolate Ligands on the Electronic Structure and Reactivity of Molybdenum(3+) and Tungsten(3+) Complexes. Preparation and Structures of M₂(O^tBu)₂(S^tBu)₄, [Mo(S^tBu)₃((NO)]₂, and W(S^tBu)₃(NO)(py)

Abstract

M(2)(O(t)Bu)(6) compounds (M = Mo, W) react in hydrocarbon solvents with an excess of (t)BuSH to give M(2)(O(t)Bu)(2)(S(t)Bu)(4), red, air- and temperature-sensitive compounds. (1)H NMR studies reveal the equilibrium M(2)(O(t)Bu)(6) + 4(t)BuSH <==> M(2)(O(t)Bu)(2)(S(t)Bu)(4) + 4(t)BuOH proceeds to the right slowly at 22 degrees C. The intermediates M(2)(O(t)Bu)(4)(S(t)Bu)(2), M(2)(O(t)Bu)(3)(S(t)Bu)(3), and M(2)(O(t)Bu)(5)(S(t)Bu) have been detected. The equilibrium constants show the M-O(t)Bu bonds to be enthalpically favored over the M-S(t)Bu bonds. In contrast to the M(2)(O(t)Bu)(6) compounds, M(2)(O(t)Bu)(2)(S(t)Bu)(4) compounds are inert with respect to the addition of CO, CO(2), ethyne, (t)BuC triple bond CH, MeC triple bond N, and PhC triple bond N. Addition of an excess of (t)BuSH to a hydrocarbon solution of W(2)(O(t)Bu)(6)(mu-CO) leads to the rapid expulsion of CO and subsequent formation of W(2)(O(t)Bu)(2)(S(t)Bu)(4). Addition of an excess of (t)BuSH to hydrocarbon solutions of [Mo(O(t)Bu)(3)(NO)](2) and W(O(t)Bu)(3)(NO)(py) gives the structurally related compounds [Mo(S(t)Bu)(3)(NO)](2) and W(S(t)Bu)(3)(NO)(py), with linear M-N-O moieties and five-coordinate metal atoms. The values of nu(NO) are higher in the related thiolate compounds than in their alkoxide counterparts. The bonding in the model compounds M(2)(EH)(6), M(2)(OH)(2)(EH)(4), (HE)(3)M triple bond CMe, and W(EH)(3)(NO)(NH(3)) and the fragments M(EH)(3), where M = Mo or W and E = O or S, has been examined by DFT B3LYP calculations employing various basis sets including polarization functions for O and S and two different core potentials, LANL2 and relativistic CEP. BLYP calculations were done with ZORA relativistic terms using ADF 2000. The calculations, irrespective of the method used, indicate that the M-O bonds are more ionic than the M-S bonds and that E ppi to M dpi bonding is more important for E = O. The latter raises the M-M pi orbital energies by ca. 1 eV for M(2)(OH)(6) relative to M(2)(SH)(6). For M(EH)(3) fragments, the metal d(xz)(),d(yz)() orbitals are destabilized by OH ppi bonding, and in W(EH)(3)(NO)(NH(3)) the O ppi to M dpi donation enhances W dpi to NO pi* back-bonding. Estimates of the bond strengths for the M triple bond M in M(2)(EH)(6) compounds and M triple bond C in (EH)(3)M triple bond CMe have been obtained. The stronger pi donation of the alkoxide ligands is proposed to enhance back-bonding to the pi* orbitals of alkynes and nitriles and facilitate their reductive cleavage, a reaction that is not observed for their thiolate counterpart.

Insights into the Metathesis Reaction Involving M−M, C−C, and M−C Triple Bonds from Computations Employing Density Functional Theory on Model Compounds M2(OH)6 and M2(SH)6, Where M = Mo and W

“…For the second step, rearrangement to the alkylidyne species, for M ) W and E ) O, the reaction is thermodynamically favored; ∆G°) -6 kcal/mol, while, for M ) Mo and E ) O, ∆G°is calculated to be +5 kcal/mol. Experimentally, 6 is known to readily react with alkynes in a quantitative manner to give alkylidyne complexes, while Mo 2 (O t Bu) 6 only reacts with terminal alkynes under more forcing conditions to give low yields of ( t BuO) 3 MotCR.…”

“…The observed reduced activity of Mo 2 (O t Bu) 6 complexes to alkyne cleavage can be seen to arise from the significantly higher barrier to the reaction. 19 -0.36 -0.36 Moreover for the model compounds with hydroxide ligands, the metathesis reaction leading to the methylidyne ligands is thermodynamically unfavorable.…”

“…The forward reaction involving the mutual scission of M-M and C-C triple bonds, the "chop-chop reaction", was first noted by Schrock 20 years ago in the reactions between certain alkynes and W 2 (O t Bu) 6 , eq 3. A variation on the reaction shown in eq 3 involving terminal alkynes was subsequently developed as a simple route to ( t BuO) 3 WtCR compounds which are commonly employed as alkyne metathesis catalysts (e.g., R ) t Bu).…”

“…The introduction of trialkylsiloxide ligands as in W 2 (OSiMe t Bu 2 ) 6 5 or thiolates in W 2 (O t Bu) 2 (S t Bu) 4 6 completely shuts down the metathesis reaction. Moreover, the related molybdenum compound Mo 2 (O t Bu) 6 (1) (a) Schrock, R. R. Polyhedron 1995, 14, 3177. (b) Schrock, R. R. Acc.…”

“…7 In work in our laboratory, we had observed the formation of alkyne adducts in a variety of reactions employing M 2 (OR) 6 compounds, where M ) Mo and W and R ) t Bu, i Pr, and CH 2 t Bu. In certain cases, we also found evidence for an equilibrium involving the alkyne adducts and the alkylidyne complexes, eq 4, but only when the metal was tungsten.…”