High-valent Rhenium Research Articles

N,N,O-Coordinated tricarbonylrhenium precatalysts for the aerobic deoxydehydration of diols and polyols

The bench-top stable and synthetically easily accessible, low-valent NNO–rhenium complex L4Re(CO)3 provides an alternative to high-valent rhenium catalysts in DODH chemistry.

Rhenium-catalysed hydroboration of aldehydes and aldimines.

The first examples for the rhenium-catalysed hydroboration of aldehydes, ketones and aldimines, including heteroaromatic quinoline, are reported herein. Reactions are remarkably chemoselective and tolerant of several functional groups. A wide array of rhenium complexes were efficient pre-catalysts for these hydroborations, including new low-valent complexes of the formula [Re(N-N)(CO)3(L)]X (N-N = bipy derivative, L = labile ligand/solvent, and X = [BArF4]- and [B(3,5-di-tBu-cat)2]-), which have been characterized fully including an X-ray diffraction study for [Re(bipy)(CO)3(quin)][BArF4] (2). A new silver spiroboronate ester Ag[B(3,5-di-tBu-cat)2](NCCH3)3 (3) was prepared and characterized fully, including an X-ray diffraction study, and used to make one of the new rhenium complexes.

Oxygen Atom Transfer and Intramolecular Nitrene Transfer in a Rhenium β-Diketiminate Complex.

We present two routes to the oxo rhenium complex OReCl2(BDI) (1) (BDI = N,N'-bis(2,6-diisopropylphenyl)-β-diketiminate) and discuss the properties and reactivity of this material. Several adducts of 1 with DMAP (1-DMAP; DMAP = 4-dimethylaminopyridine), isonitriles (1-XylNC; XylNC = 2,6-dimethylphenyl isocyanide), and phosphines (1-PEt3; PEt3 = triethylphosphine) were isolated and characterized. Additionally, to probe the ancillary limitations of the BDI framework in high-valent rhenium complexes, oxygen atom transfer (OAT) reactivity with 1 was pursued. It was found that under thermolysis conditions OAT between 1 and PEt3 was observed by NMR spectroscopy, which indicated the formation of a new species, (ArN)ReCl2(MAD)(PEt3) (2; Ar = 2,6-diisopropylphenyl, MAD = 4-((2,6-diisopropylphenyl)imino)pent-2-enide). A mechanism for the generation of 2 involving nitrene transfer to rhenium from the BDI ligand is proposed. X-ray crystal structures of complexes 1, 1-PEt3, 1-DMAP, and 2 were determined and are discussed in detail.

Complexes of High-Valent Rhenium Supported by the PCP Pincer

The PCP ligand can be introduced into the coordination sphere of Re by metalation in a reaction with L2Re(O)Cl3 precursors, leading to an octahedral (PCP)Re(O)Cl2 (2), in which the aryl donor of PCP is cis to the oxo ligand. Halide exchange with Me3SiBr or Me3SiI furnished analogous (PCP)Re(O)Br2 (3) and (PCP)Re(O)I2 (4). Treatment of 2 with LiAlH4 resulted in the isolation of (PCP)ReH6 (5) upon workup. NMR data suggest a classical hexahydride nature for 5. 5 reacted with PMe3 and 4-dimethylaminopyridine (DMAP) by losing H2 and forming adducts (PCPiPr)ReH4(PMe3) (6) and (PCP)ReH4(DMAP) (7). On the other hand, reaction of 5 with CO resulted in loss of all the hydrides and formation of the Re(I) compound (PCPiPr)Re(CO)3 (8). Finally, thermolysis of the hexahydride 5 led to loss of half the hydrides as H2 and formation of the dimeric species formulated as (PCPiPr)2Re2H6 (9).

N-H cleavage as a route to new pincer complexes of high-valent rhenium.

A novel PNN-type pincer ligand has been accessed via imine reduction with LiAlH4 to provide the phosphine diamino proligand, PNHNH (1). The ligand, 1, as well as PNHP can be metallated directly, via N-H cleavage, with L2ReOX2(OEt) precursors to access six-coordinate (PNP)ReOCl2 (2) and (PNNH)ReOX2 (3-Cl, X = Cl; 3-Br, X = Br) in good yield. 3-Cl and 3-Br undergo dehydrohalogenation upon treatment with NEt3, furnishing the five-coordinate phosphine/diamido PNN-type compounds (PNN)ReOX (4-Cl, X = Cl; 4-Br, X = Br) in excellent yield, presenting as a mixture of rotameric diasteromers. The reversibility of this deprotonation, and the coordinative unsaturation of 4-Cl is shown in reactions with HCl(aq) and PMe3 providing 3-Cl and 4-PMe3, respectively. Treatment of 4-Cl with AgOAc, AgOTf, or NaHBEt3 lead to formation of (PNN)ReO(OAc) (4-OAc), (PNN)ReO(OTf) (4-OTf), and (PNN)ReO(H) (4-H), all isolated in excellent yields in varying diasteromeric ratios. The nature of the isomerism was analyzed based on solid-state structural studies and solution NMR data.

New insights into hydrosilylation of unsaturated carbon-heteroatom (C═O, C═N) bonds by rhenium(V)-dioxo complexes.

The hydrosilylation of unsaturated carbon-heteroatom (C═O, C═N) bonds catalyzed by high-valent rhenium(V)-dioxo complex ReO2I(PPh3)2 (1) were studied computationally to determine the underlying mechanism. Our calculations revealed that the ionic outer-sphere pathway in which the organic substrate attacks the Si center in an η(1)-silane rhenium adduct to prompt the heterolytic cleavage of the Si-H bond is the most energetically favorable process for rhenium(V)-dioxo complex 1 catalyzed hydrosilylation of imines. The activation energy of the turnover-limiting step was calculated to be 22.8 kcal/mol with phenylmethanimine. This value is energetically more favorable than the [2 + 2] addition pathway by as much as 10.0 kcal/mol. Moreover, the ionic outer-sphere pathway competes with the [2 + 2] addition mechanism for rhenium(V)-dioxo complex 1 catalyzing the hydrosilylation of carbonyl compounds. Furthermore, the electron-donating group on the organic substrates would induce a better activity favoring the ionic outer-sphere mechanistic pathway. These findings highlight the unique features of high-valent transition-metal complexes as Lewis acids in activating the Si-H bond and catalyzing the reduction reactions.

Performance of Density Functionals for Activation Energies of Re-Catalyzed Organic Reactions

By employing high-level coupled cluster CCSD(T)-F12 calculations as reference, we herein systematically assessed the performance of 16 popular density functional theory (DFT) approximations for typical rhenium-catalyzed reactions. The reactions under study cover those catalyzed by low-valent rhenium(I)/(III) carbonyl complexes as well as high-valent organorhenium(VII) bisperoxo complex. Without DFT dispersion correction, the four best-performing functionals for the barrier heights are B2GP-PLYP, TPSSh, B3LYP, and PBE0 with the mean unsigned deviations (MUDs) under 1.6 kcal/mol. Among these four functionals, B2GP-PLYP generates more accurate barrier heights, while B3LYP and TPSSh behave more reliably in the barrier trend description for these Re-catalyzed reactions. In general, herein the hybrid functionals are better choices than pure GGA or pure meta-GGA functionals. DFT empirical dispersion corrections were found to have beneficial effects on MUDs only for four tested functionals of BMK, CAM-B3LYP, LC-ωPBE, and ωB97X. Often associated with very large errors up to about 15 kcal/mol in barrier height for many tested functionals, the reaction catalyzed by high-valent rhenium(VII) bisperoxo is apparently different from the ones catalyzed by low-valent rhenium(I)/(III) carbonyl complexes. For reactions catalyzed by Re(I)/(III) carbonyl complexes, ωB97XD with dispersion correction performs excellently (MUD = 0.63 kcal/mol) and hence is highly recommended for these Re(I)/Re(III)-mediated reactions.

Redox-Active Ligands Facilitate Bimetallic O2Homolysis at Five-Coordinate Oxorhenium(V) Centers

Five-coordinate oxorhenium(V) anions with redox-active catecholate and amidophenolate ligands are shown to effect clean bimetallic cleavage of O(2) to give dioxorhenium(VII) products. A structural homologue with redox-inert oxalate ligands does not react with O(2). Redox-active ligands lower the kinetic barrier to bimetallic O(2) homolysis at five-coordinate oxorhenium(V) by facilitating formation and stabilization of intermediate O(2) adducts. O(2) activation occurs by two sequential Re-O bond forming reactions, which generate mononuclear eta(1)-superoxo species, and then binuclear trans-mu-1,2-peroxo-bridged complexes. Formation of both Re-O bonds requires trapping of a triplet radical dioxygen species by a cis-[Re(V)(O)(cat)(2)](-) anion. In each reaction the dioxygen fragment is reduced by 1e(-), so generation of each new Re-O bond requires that an oxometal fragment is oxidized by 1e(-). Complexes containing a redox-active ligand access a lower energy reaction pathway for the 1e(-) Re-O bond forming reaction because the metal fragment can be oxidized without a change in formal rhenium oxidation state. It is also likely that redox-active ligands facilitate O(2) homolysis by lowering the barrier to the formally spin-forbidden reactions of triplet dioxygen with the closed shell oxorhenium(V) anions. By orthogonalizing 1e(-) and 2e(-) redox at oxorhenium(V), the redox-active ligand allows high-valent rhenium to utilize a mechanism for O(2) activation that is atypical of oxorhenium(V) but more typical for oxygenase enzymes and models based on 3d transition metal ions: O(2) cleavage occurs by a net 2e(-) process through a series of 1e(-) steps. The implications for design of new multielectron catalysts for oxygenase-type O(2) activation, as well as the microscopic reverse reaction, O-O bond formation from coupling of two M=O fragments for catalytic water oxidation, are discussed.

Computational Study on the Reaction Mechanism of Hydrosilylation of Carbonyls Catalyzed by High-Valent Rhenium(V)−Di-oxo Complexes

Density functional theory (DFT) calculations have been performed to elucidate the reaction mechanism of hydrosilylation of carbonyl compounds catalyzed by high-valent rhenium(V)-di-oxo complexes ReO2I(PR3)2 (R = Me, Ph). The calculations suggest that the most favorable mechanism involves the rate-determining dissociative [2 + 2] addition of the Si-H bond across a Re=O bond to form a Re(V) hydrido siloxy intermediate; this is followed by carbonyl coordination, reduction of the carbonyl, rearrangement, and final intramolecular nucleophilic attack from the alkoxy group to the silyl center (dissociative retro-[2 + 2] addition). It was also found that the additional oxo ligand in the ReO2I(PR3)2 complexes promotes the [2 + 2] addition across the rhenium-oxo bond both kinetically and thermodynamically, as compared to the neutral rhenium(V)-mono-oxo complex ReOCl3(PMe3)2. The effect of different silanes on the [2 + 2] addition barriers is also studied.

Reduction of carbonyl groups by high-valent rhenium oxides

The oxo-rhenium complexes [Re 2O 7] ( 1), [ReMeO 3] ( 2), [Re(η 5-C 5H 5)O 3] ( 3), [ReO 2Cl(DMSO) 2] ( 4), [ReO 2Me(PhC CPh)] ( 5), and [ReOCl 3(PPh 3) 2] ( 6) catalyze the hydrosilylation of aliphatic and aromatic aldehydes with dimethylphenylsilane in C 6D 6 solutions. Compound 1 catalyzes the hydrosilylation of aldehydes at room temperature, affording the corresponding silyl ethers in good yield, but is ineffective as ketone hydrosilylation catalyst. The reactions of aldehydes and ketones with dimethylphenylsilane using compounds 2– 6 as catalysts, require heating at 80 °C. The scope of rhenium oxides-mediated hydrosilylation with a variety of aldehydes and ketones has been studied. Methyltrioxorhenium(VII) ( 2) and oxotrichlorobis(triphenylphosphine)rhenium(V) ( 6) shown to be the most active and versatile catalysts for the hydrosilylation of aliphatic and aromatic aldehydes and also ketones. The dioxo-rhenium derivatives [ReO 2Me(PhC CPh)] and [ReO 2Cl(DMSO) 2] also catalyze the reaction of dimethylphenylsilane and aldehydes, although longer reaction times were required.

Computational Study of Low-Coordinate Rhenium Diolates, Metallaoxetanes, Oxo Complexes, and Carbenes

CCSD energies with a triple-ζ quality basis set at DFT geometries show that low coordination numbers favor metallaoxetane over diolate isomers in high-valent rhenium complexes. Minima and transition states are located and used to rationalize recently published experimental studies of the gas-phase chemistry of the title complexes. The experimental and theoretical studies shed light on the related chemistry of isoelectronic osmium complexes involved in catalytic asymmetric dihydroxylation.

Mass Spectrometric Study of the Conversion of Rhenium Diolates to Metallaoxetanes and Carbenes. Coordination Number, Polar, and Steric Effects

A combination of mass spectrometric gas-phase ion−molecule reactions and density functional theory electronic structure calculations indicates that, at least in high-valent rhenium oxo complexes, a diolate complex can dissociate preferentially to form a carbene complex, which most likely proceeds via a metallaoxetane intermediate. The experimental study finds effects on the efficiency of carbene production due to coordination number, polar substituents, and steric hindrance. Both qualitative theory and DFT calculations confirm the central role played by coordination number and furthermore define the structural conditions under which the reaction sequence, diolate → metallaoxetane → carbene complex, should become favorable. The results are relevant, not only to oxidation chemistry but also for the initiation mechanism in some olefin metathesis catalysts.

Metallaoxetanes and carbenes from diolates in high-valent rhenium oxo chemistry: the importance of the coordination number.

ReV diolate complexes cleave to form either the ReVII trioxo complex or a rhenium carbene, depending on the coordination number of the original diolate complex (see picture). The carbene complexes, detected by using collision induced mass spectrometry, are the best experimental indication to date for intermediate “metallaoxetanes”, which would be the first-formed intermediate in [2+2] reactions of olefins with a d0 oxo complex. This sharp dichotomy in reactivity can be explained by a qualitative π-strain model.

Induced Internal Electron Transfer Chemistry in Rhenium Sulfide Systems

This paper demonstrates the proclivity with which high-valent rhenium sulfur complexes undergo internal electron transfer. Specifically, reaction of [Et{sub 4}N][ReS{sub 4}] with 1.5 molar equiv of tetraalkylthiuram disulfide in acetonitrile gives the dinuclear Re(IV) complexes, Re{sub 2}({mu}-S){sub 2}(S{sub 2}CNR{sub 2}), 1, in very high yield. This dimer reacts with an additional equivalent of tetraalkylthiuram disulfide in the presence of excess Lewis acids, or with 0.5 molar equiv of tetraalkylthiuram disulfide and 1 molar equiv of [Cp{sub 2}Fe][PF{sub 6}], to give the dinuclear Re(III) species [Re{sub 2}({mu}-S-S{sub 2}CNR{sub 2}){sub 2}(S{sub 2}CNR{sub 2}){sub 3}]{sup +}, 2, in high yield. The reaction of [ReS{sub 4}]{sup {minus}} with 3 molar equiv of tetraalkylthiuram disulfide in a mixture of dichloromethane and acetonitrile gives the mononuclear Re(V) species [Re(S{sub 2}CN(R){sub 2}){sub 4}]-[Cl], 3, in high yield. Each of these reactions involves induced internal electron transfer in which the formal oxidation state of the metal center is reduced by the addition of an oxidant (i.e., tetraalkylthiuram disulfide). The bound sulfide is the reductant both for the metal and the external oxidant. The reformation of 1 from 2, in which the metal is oxidized, can be effected using reductants such as H{sub 2}. Electrochemical properties and chemical reactivitiesmore » of the complexes are presented.« less

Polyoxotungstate anions containing high-valent rhenium. 1. Keggin anion derivatives

Polyoxotungstate anions containing high-valent rhenium. 1. Keggin anion derivatives