Cationic PCP and PCN NHC Core Pincer-Type Mn(I) Complexes: From Synthesis to Catalysis

Abstract

The coordination chemistry of NHC core pincer-type PCP and PCN ligands with Mn(I) center was systematically investigated. The reaction of [Mn(CO)5Br] with free carbenes, conveniently prepared from the corresponding imidazolium salts and a strong base, lead to the initial formation of bidentate fac-[MnBr(CO)3(κ 2 P,C,L)] complexes bearing a pending phosphine or pyridine donor extremity (L), which upon bromide abstraction, afford cationic [Mn(CO)3(κ 3 P,C,L)](OTf) derivatives exhibiting a meridional or a facial coordinating geometry of tridentate NHC core ligand depending on its relative flexibility. All bi-and tridentate Mn(I) complexes were characterized by X-ray diffraction analysis. The impact of the tridentate ligand structure on catalytic performance was illustrated in the Mn-catalyzed hydrogenation of acetophenone revealing fac-[Mn(CO)3(κ 3 P,C,N)](OTf) based on a 5,6-membered metallacyclic architecture to be the most active, thus evidencing the role of the pyridine arm lability in the catalytic cycle.

Redox-Switchable Behavior of Transition-Metal Complexes Supported by Amino-Decorated N-Heterocyclic Carbenes

“…Important changes are worth mentioning in the molecular structure of + , is consistent with an enhanced π-backbonding from the Mn center into the more π-acidic, oxidized ligand 1 •+ . It is of note that this example represents the shortest Mn−NHC bond reported in the literature, being comparable only with two Mn(I) and Mn(IV) complexes bearing pincer (1.9360( 16)) Å [60] and tripodal (1.938(2)) Å [61] imidazol-2-ylidene ligands, respectively. From a magnetic point of view, both the triplet state (S = 1) and broken symmetry state (Ms = 0) are quasi-isoenergetic (i.e., within DFT error, a few tens of cm −1 ).…”

Manganese-N-heterocyclic carbene (NHC) complexes – An overview

“…The same group also very recently reported the synthesis of complexes with tridentate PCP and PCN ligands (Fig. 11) [55] which were obtained through halide abstraction from the corresponding NHC-P complex. These complexes were then used in the catalytic hydrogenation of acetophenone (see section 6.5).…”

“…More recently, Valyaev, Sortais, Canac and co-workers investigated the hydrogenation of acetophenone using cationic PCP and PCN complexes, showing that the fac-[Mn(CO) 3 (j 3 -P,C,N)(OTf)] precatalyst was the most active of the congeners studied (Scheme 20) [55]. The superior catalyst activity was attributed to the presence of the 5,6-fused membered architecture promoting the dissociation of the pyridine arm, which results in a vacant site for the catalytic process.…”

N‐Heterocyclic and Mesoionic Carbenes of Manganese and Rhenium in Catalysis

“…The highest performance of 15 was explained by the presence of a 5,6‐fused membered architecture, which promotes the dissociation of the pyridine arm creating a vacant site. Notably, the performance of 15 remained lower than that of the parent complex 10 , which bears a simple bidentate R 2 P‐NHC ligand framework [17] …”

“…The impact of changing the “spectator” N‐mesityl substituent in 10 by an additional donor arm has been investigated by Canac, Sortais, Valyaev and co‐workers [17] . The evaluation of the pincer‐type complexes 13–15 in the catalytic hydrogenation of acetophenone revealed that 15 was the most efficient catalyst (Scheme 9).…”