Isolation and Crystal Structure of a Water-Soluble Iridium Hydride:  A Robust and Highly Active Catalyst for Acid-Catalyzed Transfer Hydrogenations of Carbonyl Compounds in Acidic Media

Abstract

This paper reports the isolation and structural determination of a water-soluble hydride complex [Cp*Ir(III)(bpy)H](+) (1, Cp* = eta(5)-C(5)Me(5), bpy = 2,2'-bipyridine) that serves as a robust and highly active catalyst for acid-catalyzed transfer hydrogenations of carbonyl compounds at pH 2.0-3.0 at 70 degrees C. The catalyst 1 was synthesized from the reaction of a precatalyst [Cp*Ir(III)(bpy)(OH(2))](2+) (2) with hydrogen donors HCOOX (X = H or Na) in H(2)O under controlled conditions (2.0 < pH < 6.0, 25 degrees C) which avoid protonation of the hydrido ligand of 1 below pH ca. 1.0 and deprotonation of the aqua ligand of 2 above pH ca. 6.0 (pK(a) value of 2 = 6.6). X-ray analysis shows that complex 1 adopts a distorted octahedral geometry with the Ir atom coordinated by one eta(5)-Cp*, one bidentate bpy, and one terminal hydrido ligand that occupies a bond position. The isolation of 1 allowed us to investigate the robust ability of 1 in acidic media and reducing ability of 1 in the reaction with carbonyl compounds under both stoichiometric and catalytic conditions. The rate of the acid-catalyzed transfer hydrogenation is drastically dependent on pH of the solution, reaction temperature, and concentration of HCOOH. The effect of pH on the rate of the transfer hydrogenation is rationalized by the pH-dependent formation of 1 and activation process of the carbonyl compounds by protons. High turnover frequencies of the acid-catalyzed transfer hydrogenations at pH 2.0-3.0 are ascribed not only to nucleophilicity of 1 toward the carbonyl groups activated by protons but also to a protonic character of the hydrido ligand of 1 that inhibits the protonation of the hydrido ligand.

Water‐Soluble Phenanthroline Complexes of Rhodium, Iridium and Ruthenium for the Regeneration of NADH in the Enzymatic Reduction of Ketones

“…[1] The transition-metal-based enantioselective hydrogenation or transfer hydrogenation of ketones in organic solvents [2][3][4][5][6][7][8][9][10] as well as in aqueous solutions [11][12][13][14][15][16][17][18][19][20][21] is one of the most commonly used synthetic approaches. Biotransformations of organic substrates by various enzymes have also found widespread applications, particularly because of their capability to efficiently perform enantioselective transformations.…”

Catalytic Disproportionation of Formic Acid to Generate Methanol

“…Similar acid-promoted pathways have been invoked for ketone reduction catalyzed by 1. [10] The improved selectivity would also be consistent with the diminished water content in concentrated HCO 2 H solutions, as a higher ratio of HC(OH) 2 + to H 3 O + would disfavor reaction (1). Scheme 1 lays out a plausible pathway for methanol formation supported by the available data.…”

“…1 H NMR spectroscopic studies revealed that dissolving precatalyst 1 in 3 m HCO 2 H/ D 2 O causes rapid conversion at ambient temperature to a mixture of deuteride [Cp*Ir(bpy)D]OTf (2) [10] and another species in an approximately 9:1 ratio. Electrospray ionization mass spectrometry under the same conditions featured ion peaks for deuteride 2 (m/z 486.2, Figure S22 in the Supporting Information) and identified the minor species as the formate complex [Cp*Ir(bpy)(O 2 CH)]OTf (3, m/z 529.1).…”

“…While dehydrogenation of HCO 2 H occurs simultaneously, we have shown that the catalyst activity and the selectivity to methanol can be tuned by changing reaction conditions. Aqueous HCO 2 H solutions containing [Cp*Ir(bpy)-(H 2 O)](OTf) 2 (1, Cp* = pentamethylcyclopentadienyl, bpy = 2,2'-bipyridine) [10] produced methanol upon heating. This result was surprising, because HCO 2 H is commonly used as a source of H 2 in transfer hydrogenation reactions, and 1 is a well-known transfer hydrogenation catalyst, with no prior reports of methanol-producing side reactions.…”

Unexpectedly fast catalytic transfer hydrogenation of aldehydes by formate in 2-propanol–water mixtures under mild conditions

“…Both aldehydes and ketones were successfully hydrogenated by H-transfer from aqueous formate using Ru(II)-complexes as catalysts [42][43][44][45][46][47]. Rh(I)-and Ir(I)-complexes are also known to act as catalysts in such reactions [48,49], in fact, Ir(I)-complexes with monotosylated ethylenediamine ligands [9,10] showed outstanding catalytic activities up to turnover frequencies, TOF = 3.0 × 10 5 h −1 (TOF = mol reacted aldehyde × (mol catalyst) −1 × h −1 ) [10].…”