Synthesis and reactivity of aryl- and alkyl-rhenium(V) imido-triflate compounds: An unusual mechanism for triflate substitution

Treatment of the d(2) rhenium tolylimido complex TpRe(NTol)X-2 with RMgX, R Li, or organozinc reagents yields TpRe(NTol)(R)X [R = Ph, Me,-Et, i-Pr, n-B u; X = Cl or I; Tol = p-tolyl; Tp = hydrotris(1-pyrazolyl)borate] or TpRe(N Tol)R-2 (R = Ph, Me). Iodide for triflate metathesis with AgOTf yields TpRe (NTol)(X)OTf [X = Ph, Et, Cl, OTf (=OSO2CF3)]. Reaction of TpRe(NTol)(Et)I with excess rather than stoichiometric AgOTf generates the ethylene hydride cation [TpRe(NTol)(eta(2)-C2H4)(H)][OTf], which slowly rearranges by ethyl ene insertion to form TpRe(NTol)(Et)OTf. Treatment of TpRe(NTol)(Ph)I with AgPF6 gives not iodide abstraction but rather [{TpRe(NTol)(Ph)I}(2)Ag][PF6] , with two Re-I-Ag linkages. Excess pyridine (py) slowly displaces triflate in TpRe(NTol)Ph(OTf) (11) to give [TpRe(NTol)Ph(py)][OTf]. The reaction is first-order in 11 and first-order in py. Triflate substitution is similarl y slow in the oxo-Tp* derivative, Tp*Re(O)Ph(OTf) [Tp* = HB(3,5-Me(2)pz)(3) ]. These reactions are many orders of magnitude slower than substitution in the oxo-Tp derivative TpRe(O)Ph(OTf). Kinetic and mechanistic data rule ou t dissociation of triflate ion in or before the rate-determining step and a re most consistent with an associative pathway, Reaction with 1,10-phenanth roline gives the k(2)-Tp complex [K-2-TpRe(NTOl)Ph(phen)][OTf], indicating the lability of one arm of the Tp ligand. TpRe(NTol)Ph-2, TpRe(NTol)Me-2, T pRe(NTol)(Ph)OTf, TpRe(NTol)(OTf)(2), [{TpRe(NTol)(Ph)I}(2)Ag][PF6], and [K -2-TpRe(NTol)Ph(phen)] [OTf] have been structurally characterized. The imid o ligand is a better electron donor and has a smaller trans influence than an oxo group in this system.