Facile Synthetic Access to Rhenium(II) Complexes: Activation of Carbon–Bromine Bonds by Single‐Electron Transfer

Abstract

The five-coordinated Re(I) hydride complexes [Re(Br)(H)(NO)(PR(3))(2)] (R=Cy 1 a, iPr 1 b) were reacted with benzylbromide, thereby affording the 17-electron mononuclear Re(II) hydride complexes [Re(Br)(2)(H)(NO)(PR(3))(2)] (R=Cy 3 a, iPr 3 b), which were characterized by EPR, cyclic voltammetry, and magnetic susceptibility measurements. In the case of dibromomethane or bromoform, the reaction of 1 afforded Re(II) hydrides 3 in addition to Re(I) carbene hydrides [Re(=CHR(1))(Br)(H)(NO)(PR(3))(2)] (R(1)=H 4, Br 5; R=Cy a, iPr b) in which the hydride ligand is positioned cis to the carbene ligand. For comparison, the dihydrogen Re(I) dibromide complexes [Re(Br)(2)(NO)(PR(3))(2)(eta(2)-H(2))] (R=Cy 2 a, iPr 2 b) were reacted with allyl- or benzylbromide, thereby affording the monophosphine Re(II) complex salts [R(3)PCH(2)R'][Re(Br)(4)(NO)(PR(3))] (R'=-CH=CH(2) 6, Ph 7). The reduction of Re(II) complexes has also been examined. Complex 3 a or 3 b can be reduced by zinc to afford 1 a or 1 b in high yield. Under catalytic conditions, this reaction enables homocoupling of benzylbromide (turnover frequency (TOF): 3 a 150, 3 b 134 h(-1)) or allylbromide (TOF: 3 a 575, 3 b 562 h(-1)). The reaction of 6 a and 6 b with zinc in acetonitrile affords in good yields the monophosphine Re(I) complexes [Re(Br)(2)(NO)(MeCN)(2)(PR(3))] (R=Cy 8 a, iPr 8 b), which showed high catalytic activity toward highly selective dehydrogenative silylation of styrenes (maximum TOF of 61 h(-1)). Single-electron transfer (SET) mechanisms were proposed for all these transformations. The molecular structures of 3 a, 6 a, 6 b, 7 a, 7 b, and 8 a were established by single-crystal X-ray diffraction studies.

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]

“…In all of them, the rhenium coordination sphere exhibits an octahedral geometry, with a practically lineal {Re-NO} unit and a -acceptor ligand such as phosphine or aromatic amines, usually coordinating trans-to the -NO group. The found -acceptor ligands include: MeCN (Ciani et al, 1975), EtOH (Ciani et al, 1975, pyrazine (Pacheco et al, 2013(Pacheco et al, , 2014, nitrosyl (Mronga et al, 1982), tricyclohexylphosphine and triisopropylphosphine (Jiang et al, 2010), nicotinic acid and nicotinate anion (Pacheco, Cuevas, Gonzá lez-Platas, Lloret et al, 2015), pyridine, pyrimidine and pyridazine (Pacheco et al, 2013). All Re-Br distances observed in the complex reported herein, as well as the Re-N and N-O distances found, agree with those found for previously reported structures (see Figs.…”

A Highly Stable Porous Viologen Polymer for the Catalysis of Debromination Coupling of Benzyl Bromides with High Recyclability

“…Selective reductive coupling of benzyl bromides to the corresponding 1,2‐diphenylethanes in high yields has been realized by using SmI 2 , vitamin B12, Mn 2 (CO) 10 , Fe(acac) 3 , Cp 2 TiCl, Re(Br)(H)(NO)(PCy 3 ) 2 , NiCl 2 (glyme)/terpyridine, NiCl 2 (PPh 3 ) 2 , RhCl(PPh 3 ) 3 , Co(PMe 3 ) 4 /Me 2 Zn, and NiCl 2 /Sm . However, recyclable heterogeneous catalysis of these two reactions have not been reported in the literature.…”

Rhenium-catalysed reactions in chemical synthesis: selected case studies

“…141,142 Although the formation of unsaturated 17-electron complexes is supported, reports invoking rhenium(II) species are rare and this likely derives from the inaccessibility of versatile d 5 rhenium(II) precursors. 11 Only a handful of rhenium(II) complexes have been synthesised to date, [143][144][145] however, their applications as catalysts in chemical synthesis remain unexplored.…”