Highly selective hydrogenation of amides catalysed by a molybdenum pincer complex: scope and mechanism

Abstract

A series of molybdenum pincer complexes has been shown for the first time to be active in the catalytic hydrogenation of amides.

Synthesis of Molybdenum Pincer Complexes and Their Application in the Catalytic Hydrogenation of Nitriles

“…At the outset of our studies, we investigated whether our previously developed method for the synthesis of molybdenum complexes Mo-1 and Mo-2, could be extended to other pincer ligands (Scheme 3). [13] Thus, PNP ligands (Cy 2 PCH 2 CH 2 ) 2 NH, (Ph 2 PCH 2 CH 2 ) 2 NH, (Et 2 PCH 2 CH 2 ) 2 NH as well as the NNN pincer ligand bis- (2-pyridylmethyl)amine were reacted with Mo (PPh 3 ) 2 (CH 3 CN) 2 (CO) 2 in either dichloromethane (DCM) or tetrahydrofuran (THF). Applying (Cy 2 PCH 2 CH 2 ) 2 NH as ligand in DCM, resulted in the intended formation of Mo(I) complex Mo-6 in 46 % yield.…”

“…Complex Mo-7 proved to be remarkably stable towards chlorination and remained molecularly unchanged even after refluxing for 24 h in DCM and DCE, respectively. All prepared coordination compounds have been characterized by standard techniques including 1 H, 13 C and 31 P{ 1 H} NMR (except Mo-6 and Mo-7, see vide infra) and IR spectroscopy as well as elemental analysis (for NMR and IR spectra, see supporting information). Additionally, we were able to determine solid-state structures of complexes Mo-3a, Mo-3b, Mo-4a, Mo-6 as well as Mo-7 by Xray analysis of suitable single crystals.…”

“…The described characteristics for Mo-6 are in agreement with our previously published solid state structure of Mo-2. [13] The coordination geometry at the Mo atom of the heptacoordinated Mo(II)-complex Mo-4a can be best described as distorted capped octahedral.…”

“…However, under the reported conditions, only modest conversion (42 %) and poor product selectivity (13 %) for the desired primary amine were observed. [13] In order to minimize potential decomposition of the homogeneous molybdenum catalysts, the initial catalyst screening was carried out at 100°C in the presence of 10 mol% NaBHEt 3 . Under these conditions full conversion was observed for Mo-1 and Mo-2, yielding approximately 1 : 1 mixtures of 2a and 3a (Table 1, entries 1-2).…”

“…[12] In 2018, we described the synthesis of a series of molecular defined molybdenum PNP-pincer complexes and subsequently demonstrated their activity in the catalytic hydrogenation of acetophenones, styrenes and formamides ( Figure 1). [13] Based on these works, herein we report the synthesis and structural characterization of a series of previously unknown molybdenum pincer complexes and their behavior in the hydrogenation of nitriles to primary amines.…”