Facile synthetic access to rhenium(II) complexes: activation of carbon-bromine bonds by single-electron transfer

Rhenium-catalysed reactions in chemical synthesis: selected case studies

This Perspective presents and discusses a selection of examples that reinforce the enabling and distinctive reactivity provided by homogeneous rhenium catalysis in chemical synthesis. Specifically, the ability for lower oxidation...

Crystal structure and magnetic study of the complex salt [RuCp(PTA)2-μ-CN-1κC:2κN-RuCp(PTA)2][Re(NO)Br4(EtOH)0.5(MeOH)0.5]

A new RuII-ReII complex salt, μ-cyanido-κ2 C:N-bis-[(η5-cyclo-penta-dien-yl)bis(3,5,7-tri-aza-phosphaadamantane-κP)ruthenium(II)] tetra-bromido-(ethanol/methanol-κO)nitrosylrhenate(II), [Ru(CN)(C5H5)2(C6H12N3P)4][ReBr4(NO)(CH4O)0.5(C2H6O)0.5] or [RuCp(PTA)2-μ-CN-1κC:2κ2 N-RuCp(PTA)2][Re(NO)Br4(EtOH)0.5(MeOH)0.5] (PTA = 3,5,7-tri-aza-phosphaadaman-tane) was obtained and characterized by single-crystal X-ray diffraction, elemental analysis and infrared spectroscopy. The title salt was obtained by liquid-liquid diffusion of methanol/DMSO solutions of (NBu4)[Re(NO)Br4(EtOH)] and [(PTA)2CpRu-μ-CN-1κC:2κ2 N-RuCp(PTA)2](CF3SO3). The RuII and ReII independent moieties correspond to a binuclear and mononuclear complex ion, respectively. A deep geometrical parameter analysis was performed, and no significant differences were found with earlier reports containing similar mol-ecules. The magnetic properties were investigated in the temperature range 2.0-300 K, and the complex behaves as a quasi-magnetically isolated spin doublet with weak anti-ferromagnetic inter-actions.

Hydrosilylation Reactions Catalyzed by Rhenium

Hydrosilylation is an important process, not only in the silicon industry to produce silicon polymers, but also in fine chemistry. In this review, the development of rhenium-based catalysts for the hydrosilylation of unsaturated bonds in carbonyl-, cyano-, nitro-, carboxylic acid derivatives and alkenes is summarized. Mechanisms of rhenium-catalyzed hydrosilylation are discussed.

Electronic Structures of Rhenium(II) β-Diketiminates Probed by EPR Spectroscopy: Direct Comparison of an Acceptor-Free Complex to Its Dinitrogen, Isocyanide, and Carbon Monoxide Adducts

Electron paramagnetic resonance (EPR) studies of the rhenium(II) complex Re(η⁵-Cp)(BDI) (1; BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) have revealed that this species reversibly binds N₂ in solution: flash frozen toluene solutions of 1 disclose entirely different EPR spectra at 10 K when prepared under N₂ versus Ar atmospheres. This observation was additionally verified by the synthesis of stable CO and 2,6-xylylisocyanide (XylNC) adducts of 1, which display EPR features akin to those observed in the putative N₂ complex. While we found that 1 displays an extremely large g_max value of 3.99, the binding of an additional ligand leads to substantial decreases in this value, displaying g_max values of ca. 2.4. Following the generation of isotopically enriched ¹⁵N₂ and ¹³CO adducts of 1, HYSCORE experiments allowed for the measurement of the corresponding hyperfine couplings associated with spin delocalization onto the electron-accepting ligands in these species, which proved to be small. A cumulative assessment of the EPR data, when combined with insights provided by near-infrared (NIR) spectroscopy and time-dependent density functional theory (TDDFT) calculations, indicated that while the binding of electron acceptors to 1 does lead to decreases in g_max in relative accord with the field strength (i.e., π-acidity) of the variable ligand, the magnitude of these decreases is primarily due to the changes in electronic structure at the Re center.

Heterotetrametallic Re-Zn-Zn-Re Complex Generated by an Anionic Rhenium(I) β-Diketiminate

We report the synthesis of an anionic rhenium(I) compound, Na[Re(η⁵-Cp)(BDI)] (1; Cp = cyclopentadienide, BDI = N, N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) and initial investigations of its use as a strong chemical reductant and metalloligand. Chemical oxidation of 1 gives a rare example of a rhenium(II) compound Re(η⁵-Cp)(BDI) (2), while protonation of 1 yields the rhenium(III) hydride complex Re(H)(η⁵-Cp)(BDI) (3). The reaction of 1 with ZnCl₂ generated both 2 and the zinc(I) compound [ZnRe(η⁵-Cp)(BDI)]₂ (4), which features a linear, tetrametallic Re(I)-Zn(I)-Zn(I)-Re(I) core. Computational studies of 4 were performed to characterize the metal-metal bonding interactions; the results indicate a dative interaction from rhenium to zinc and covalent bonding between the two zinc centers. One-electron oxidation of 4 yielded both 2 and the triflate-bridged zinc(II) complex [(μ-OTf)ZnRe(η⁵-Cp)(BDI)]₂ (5, OTf = trifluoromethanesulfonate).

High oxidation state bromocarbyne complexes

Bromination of the carbyne complexes [W(CR)Br(CO)₂(dcpe)] (R = Ph, SiPh₃; dcpe = 1,2-bis(dicyclohexylphosphino)ethane) provides high oxidation state derivatives [W(CPh)Br₃(dcpe)] and [W(CBr)Br₃(dcpe)], the latter via an unprecedented bromodesilylation process.

A well-defined low-valent cobalt catalyst Co(PMe3)4 with dimethylzinc: a simple catalytic approach for the reductive dimerization of benzyl halides

We report a simple catalytic version of a cobalt-catalysed reductive homocoupling of benzyl halides by combining Co(PMe₃)₄ and Me₂Zn.

Probing the catalytic potential of chloro nitrosyl rhenium(I) complexes

The reduction of the mononitrosyl Re(II) salt [NMe(4)](2)[ReCl(5)(NO)] (1) with zinc in acetonitrile afforded the Re(i) dichloride complex [ReCl(2)(NO)(CH(3)CN)(3)] (2). Subsequent ligand substitution reactions with PCy(3), PiPr(3) and P(p-tolyl)(3) afforded the bisphosphine Re(i) complexes [ReCl(2)(NO)(PR(3))(2)(CH(3)CN)] (3, R = Cy a, iPr b, p-tolyl c) in good yields. The acetonitrile ligand in 3 is labile, permitting its replacement with H(2) (1 bar) to afford the dihydrogen Re(I) complexes [ReCl(2)(NO)(PR(3))(2)(η(2)-H(2))] (4, R = Cy a, iPr b). The catalytic activity of 2, 3 and 4 in hydrogen-related catalyses including dehydrocoupling of Me(2)NH·BH(3), dehydrogenative silylation of styrenes, and hydrosilylation of ketones and aryl aldehydes were investigated, with the main focus on phosphine and halide effects. In the dehydrocoupling of Me(2)NH·BH(3), the phosphine-free complex 2 exhibits the same activity as the bisphosphine-substituted systems. In the dehydrogenative silylation of styrenes, 3a and 4a bearing PCy(3) ligands exhibit high catalytic activities. Monochloro Re(I) hydrides [Re(Cl)(H)(NO)(PR(3))(2)(CH(3)CN)] (5, R = Cy a, iPr b) were proven to be formed in the initiation pathway. The phosphine-free complex 2 showed in dehydrogenative silylations even higher activity than the bisphosphine derivatives, which further emphasizes the importance of a facile phosphine dissociation in the catalytic process. In the hydrosilylation of ketones and aryl aldehydes, at least one rhenium-bound phosphine is required to ensure high catalytic activity.