Comparison of the Catalytic Activity of [(η⁵-C₅H₅)Ru(2,2′-bipyridine)(L)]OTf versus [(η⁵-C₅H₅)Ru(6,6′-diamino-2,2′-bipyridine)(L)]OTf (L = labile ligand) in the Hydrogenation of Cyclohexanone. Evidence for the Presence of a Metal–Ligand Bifunctional Mechanism under Acidic Conditions

Abstract

The two title complexes as well as the dimeric complex [Ru(II)(η 5 -C 5 H 5 )(6,6′-diamino-2,2′-bipyridine)] 2 (OTf) 2 have been synthesized and characterized by NMR and single-crystal X-ray crystallography. The direct structural comparison of the 2,2′-bipyridine and 6,6′-diamino-2,2′-bipyridine complexes suggests that the electronic and steric environments of the ruthenium centers in both complexes are essentially equivalent, providing for a unique opportunity to probe the influence of the noncoordinated amine substituent on the relative reactivity and catalytic activity of the complexes. Opposite to what would be anticipated on the basis of steric effects, the bulkier amine-substituted ligand results in a catalyst showing substantially higher activity in the hydrogenation of cyclohexanone in acidic medium, which is attributed to the operation of a metal−ligand bifunctional hydrogenation mechanism mediated by the amine substituents in their protonated form acting as proton shuttles.

Preparation and Structural Characterization of [CpRu(1,10‐phenanthroline)(CH3CN)][X] and Precursor Complexes (X=PF6, BArF, TRISPHAT‐N)

“…Similarly, the RuÀ NCÀ Me bond length average of 2.074 Å is in line with other RuÀ NCÀ Me complexes (2.071 Å). [35,36,66,[80][81][82] The RuÀ C Cp distances are in the range of 2.137-2.178 Å and their average (2.153 Å) agrees with those reported for [CpRu(bipy)(CH 3 CN)][OTf] cationic complex. [35] The RuÀ Cp distance, in which Cp is the Cp centroid, for complex [PF 6 ] is 1.793 Å, and hence comparable to that of 1.791 Å observed for CpRu(bipy) complex.…”

“…These values highlight the σ‐donor character of the 1,10‐phenanthroline ligand. Similarly, the Ru−NC−Me bond length average of 2.074 Å is in line with other Ru−NC−Me complexes (2.071 Å) [35,36,66,80–82] …”

“…[14,33,34] CpRu(bipyridine) complexes were shown to be active catalysts for both deoxygenation of 1,2-hexanediol and glycerol [35] and hydrogenation of ketone in acidic conditions. [36] Previously, Herzon et al described the efficient anti-Markovnikov hydration of alkynes catalyzed by [CpRu(N N)] (N N = diimine ligand) derivatives. [37][38][39] Of interest in our laboratory, the [CpRu] fragment is also an attractive alternative to transition metal salts and complexes for the decomposition of diazo derivatives providing metal carbene or carbenoid intermediates upon dinitrogen extrusion.…”

“…Kitamura successfully applied enantiopure dipyrroloimidazole ligands and [CpRu] in enantioselective dehydrative C‐, N‐, and O‐allylation [14,33,34] . CpRu(bipyridine) complexes were shown to be active catalysts for both deoxygenation of 1,2‐hexanediol and glycerol [35] and hydrogenation of ketone in acidic conditions [36] . Previously, Herzon et al .…”

The Reactivity of Multidentate Schiff Base Ligands Derived from Bi‐ and Terphenyl Polyamines towards M(II) (M=Ni, Cu, Zn, Cd) and M(III) (M=Co, Y, Lu)

“…The differences observed in the 1 H NMR spectra of 1b-ZnÀ NH 2 as a function of the donating ability of the solvents used, are consistent with those observed for a Ru(II) complex bearing a 6,6'-diamino-2,2'bipyridine ligand in the literature, where the amino groups were observed to dissociate in CD 3 CN but remaining ligated in CD 2 Cl 2 . [35] Figure 13. 1 H NMR (600 MHz, CDCl 3 ) spectrum of 1b-ZnÀ NH 2 (2.6 • 10 À 3 M) showing the aromatic region.…”