Catalytic N–H Bond Formation Promoted by a Ruthenium Hydride Complex Bearing a Redox-Active Pyrimidine-Imine Ligand

Abstract

The synthesis of a piano-stool ruthenium hydride, [(η 5 -C 5 Me 5 )Ru(PmIm)H] (PmIm = (N-(1,3,5-trimethylphenyl)-1-(pyrimidin-2-yl)ethan-1-imine), for the dual purpose of catalytic dihydrogen activation and subsequent hydrogen atom transfer for the formation of weak chemical bonds is described. The introduction of a neutral, potentially redox-active PmIm supporting ligand was designed to eliminate the possibility of deleterious C(sp 2 )−H reductive coupling and elimination that has been identified as a deactivation pathway with related rhodium and iridium catalysts. Treatment of [(η 5 -C 5 Me 5 )RuCl 2 ] n with one equivalent PmIm ligand in the presence of zinc and sodium methoxide resulted in the isolation of the diruthenium complex, [(η 5 -C 5 Me 5 )Ru(PmIm)] 2 , arising from the C−C bond formation between two PmIm chelates. Addition of H 2 to the ruthenium dimer under both thermal and blue light irradiation conditions furnished the targeted hydride, [(η 5 -C 5 Me 5 )Ru(PmIm)H], which has a relatively weak DFT-calculated Ru−H bond dissociation free energy (BDFE) of 47.9 kcal/mol. Addition of TEMPO to [(η 5 -C 5 Me 5 )Ru(PmIm)H] generated the 17-electron metalloradical, [(η 5 -C 5 Me 5 )Ru(PmIm)], which was characterized by EPR spectroscopy. The C−C bond forming process was reversible as the irradiation of [(η 5 -C 5 Me 5 )Ru(PmIm)] 2 generated [(η 5 -C 5 Me 5 )Ru(PmIm)H] and a piano-stool ruthenium complex containing an enamide ligand derived from H-atom abstraction from the PmIm chelate. Equilibration studies were used to establish an experimental estimate of the effective Ru−H BDFE, and a value of 50.8 kcal/mol was obtained, in agreement with the observed loss of H 2 and the DFT-computed value. The ruthenium hydride was an effective catalyst for the thermal catalytic hydrogenation of TEMPO, acridine, and a cobalt-imido complex and for the selective reduction of azobenzene to diphenylhydrazine, highlighting the role of this complex in catalytic weak bond formation using H 2 as the stoichiometric reductant.

Pentamethylcyclopentadienyl Metalloradical Iron Complexes Containing Redox Noninnocent α-Diimine-Type Ligands: Synthesis, Molecular, and Electronic Structures

“…Of note, a related ruthenium-based metalloradical exhibited significant delocalization of spin density on the metal center and on the supporting ligand. 19 As shown in Figure 4, cyclic voltammograms (CV) of Fe1 and Fe2 were recorded in a THF solution and exhibited quasireversible anodic and cathodic waves with half-wave potentials (E 1/2 ) of −0.7 and −2.5 V (vs the ferrocenium/ferrocene couple), respectively. Systemically varying scan rates and plotting peak current versus square root of scan rate (the Randles−Sevcik plot) revealed a linear relationship with variable slope, suggesting the quasi-reversible nature of the oxidation processes (Figures S5 and S6).…”

“…To avoid catalyst deactivation that occurs from C–H reductive elimination from the L,X-type ligand in the rhodium compounds as well as associated iridium congeners, , our laboratory has reported the synthesis of piano-stool ruthenium hydrides bearing neutral, bidentate pyrimidine-imine ligands . This complex was catalytically active for the synthesis of weak chemical bonds using H 2 as the terminal reductant .…”

“…To avoid catalyst deactivation that occurs from C–H reductive elimination from the L,X-type ligand in the rhodium compounds as well as associated iridium congeners, , our laboratory has reported the synthesis of piano-stool ruthenium hydrides bearing neutral, bidentate pyrimidine-imine ligands . This complex was catalytically active for the synthesis of weak chemical bonds using H 2 as the terminal reductant . While the putative 17-electron metalloradical was observed by EPR spectroscopy, this complex underwent reversible C–C bond formation at the redox-active chelate, preventing isolation and full characterization.…”

“…19 This complex was catalytically active for the synthesis of weak chemical bonds using H 2 as the terminal reductant. 19 While the putative 17-electron metalloradical was observed by EPR spectroscopy, this complex underwent reversible C−C bond formation at the redox-active chelate, preventing isolation and full characterization.…”

Prospect of Ru(edta) complexes in nitrogen cycle electrocatalysis: a mini review

“…nitrogen-to-ammonia conversion, nitrite reduction, conversion of ammonia, etc. [18][19][20][21][22][23] While reports on the application of Ru(edta) complexes (edta 4− = ethylenediaminetetraacetate) to mediate N-cycle electrochemical reactions are available in the literature, we for the first time systematically review the Ru(edta)-catalyzed N-cycle-related electrochemical transformations, highlighting the potential role of Ru(edta) complexes to function as efficient 'molecular catalysts' in a homogeneous solution involving an electrode as a heterogeneous outer-sphere electron donor or acceptor. The [Ru III (edta)(H 2 O)] − complex, due to its intrinsic lability, could easily bind to N-cycle molecules (via rapid substitution reactions), thus providing a lower energy pathway (rendering a substantial drop in the over-potential required to initiate the electrochemical transformation of N-cycle molecules directly).…”