Reaction of rhenium(V)-oxo-halo-triflate complexes (HB(pz)(3))ReO(X)OTf (1, X = Cl, Br, I) with 1 equiv of pyridine N-oxide forms rare d(1) rhenium(VI) cis-dioxo compounds (HB(pz)(3))ReO(2)X (2, X = Cl, Br, I). This reaction likely occurs by initial formation of the d(0) rhenium(VII) dioxo cation (HB(pz)(3))ReO(2)X(+) by oxygen atom transfer, followed by a rapid one electron reduction. The chloride derivative 2a has been characterized by an X-ray crystal structure. The d(1) dioxo compounds are fairly stable, disproportionating slowly to (HB(pz)(3))ReO(3) and (HB(pz)(3))ReOX(2). Electrochemical oxidations of (HB(pz)(3))ReO(2)X to Re(VII) cations are reversible and are at remarkably high potentials (E(1/2) = 0.93 V vs Cp(2)Fe(+/0) in acetonitrile for 2a). When Me(2)SO is used as the oxidant instead of pyridine N-oxide, the Re(V) adducts [(HB(pz)(3))ReO(X)(OSMe(2))][OTf] (5) are formed by triflate displacement. These complexes reversibly lose SMe(2) (for 5a, k = 3.3(4) x 10(-)(6) s(-)(1) at 297 K in CD(2)Cl(2)), as shown by isotope exchange experiments. The intermediate Re(VII) cations (HB(pz)(3))ReO(2)X(+) oxidize Me(2)S much faster than Me(2)SO, indicating that they are highly electrophilic oxygen atom transfer reagents. Complexes 2, however, are relatively unreactive materials. Crystallographic data for 2a: C(9)H(10)BClN(6)O(2)Re; monoclinic, Cc; a = 14.716(3), b = 7.651(2), c = 13.232(3) Å; beta = 110.61(3) degrees; Z = 4.

Rhenium(V) oxo alkyl triflate compounds (HBpz3)ReO(R)OTf [R = Me (4), Et (5), n-Bu (6); HBpz3 = hydrotris(1-pyrazolyl)borate; OTf = OSO2CF3, triflate] are formed on sequential reaction of (HBpz3)ReOCl2 with dialkyl zinc reagents and AgOTf. These triflate compounds are rapidly oxidized at ambient temperatures by oxygen atom donors pyridine N-oxide (pyO) and dimethyl sulfoxide (DMSO) to give (HBpz3)ReO3 (7) and the corresponding aldehyde. In the cases of 5 and 6 this transformation is quantitative. The addition of 2,6-lutidine to a low-temperature oxidation of 5 by DMSO redirects the reaction to form cis-2-butene instead of acetaldehyde. These oxidation reactions do not proceed through alkoxide intermediates, as shown by independent studies of alkoxide oxidations. Reaction of 5 with pyO or DMSO at −47 °C results in the formation of intermediates which are assigned as ylide or “trapped-carbene” complexes [(HBpz3)ReO(OH){CH(L)CH3}]OTf (L = py (8) or SMe2 (9), respectively). Mechanistic studies and analogies with related systems suggest that oxygen atom transfer to 4-6 forms [(HBpz3)ReO2R]+. Transfer of an α-hydrogen from the alkyl group to an oxo ligand then forms an alkylidene complex which is trapped by SMe2 or py to give the observed intermediates. Further oxidation of the ylide complex gives the aldehyde.

The reaction of in situ generated Cp(2)V(OTf)(2) (Cp = cyclopentadienyl; OTf = O(3)SCF(3)) with excess 1,10-phenanthroline and 2,2'-bipyridine yields the d(1) vanadocene coordination compounds [Cp(2)V(phen)][OTf](2) (1) and [Cp(2)V(bpy)][OTf](2) (2), respectively. The compounds have been characterized by UV-vis and EPR spectroscopy and by cyclic voltammetry. The complexes have relatively low vanadium(IV)-vanadium(III) reduction potentials (-0.62 V vs Cp(2)Fe(+/0) in acetonitrile). Structures of 1 and 2 have been determined by X-ray crystallography. Compound 1 crystallized in a monoclinic system, space group P2(1)/n, with a = 10.2763(5) Å, b = 18.1646(9) Å, c = 13.5741(7) Å, beta = 99.4150(10) degrees, and Z = 4. Refinement of its structure by full-matrix least-squares techniques gave final residuals R = 0.040 and R(w) = 0.096. Compound 2 crystallized in a monoclinic system, space group P2(1)/c, with a = 10.6451(6) Å, b = 18.3863(10) Å, c = 12.6993(7) Å, beta = 98.6220(10) degrees, and Z = 4. Refinement of its structure by full-matrix least-squares techniques gave final residuals R = 0.046 and R(w) = 0.101. The two nitrogen atoms and centroids of the two cyclopentadienyl rings for both compounds occupy a distorted tetrahedral geometry around the vanadium(IV) center. The chelated ring plane is inclined closer to one of the neighboring Cp rings with the tilt more evident in 1 ( approximately 8 degrees ) than 2 ( approximately 4 degrees ). The membrane interactions of these compounds and the titanium analogues, [Cp(2)Ti(phen)][OTf](2) (3) and [Cp(2)Ti(bpy)][OTf](2) (4), have been studied with zwitterionic unilamellar liposomes as artificial membranes. We show that the ability of metallocenes to enhance the permeability of a liposomal membrane depends on the hydrophobicity, as well as the size and planarity of the ancillary chelated ligands, but not the nature of the central metal ion. Also provided is evidence that metallocene-induced permeability changes in artificial membranes are not caused by lipid peroxidation.

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Treatment of the d2 rhenium tolylimido complex TpRe(NTol)X2 with RMgX, RLi, or organozinc reagents yields TpRe(NTol)(R)X [R = Ph, Me, Et, i-Pr, n-Bu; X = Cl or I; Tol = p-tolyl; Tp = hydrotris(1-pyrazolyl)borate] or TpRe(NTol)R2 (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)(η2-C2H4)(H)][OTf], which slowly rearranges by ethylene 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}2Ag][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 similarly slow in the oxo-Tp* derivative, Tp*Re(O)Ph(OTf) [Tp* = HB(3,5-Me2pz)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 out dissociation of triflate ion in or before the rate-determining step and are most consistent with an associative pathway. Reaction with 1,10-phenanthroline gives the κ2-Tp complex [κ2-TpRe(NTol)Ph(phen)][OTf], indicating the lability of one arm of the Tp ligand. TpRe(NTol)Ph2, TpRe(NTol)Me2, TpRe(NTol)(Ph)OTf, TpRe(NTol)(OTf)2, [{TpRe(NTol)(Ph)I}2Ag][PF6], and [κ2-TpRe(NTol)Ph(phen)][OTf] have been structurally characterized. The imido ligand is a better electron donor and has a smaller trans influence than an oxo group in this system.