A Dearomatized Anionic PNP Pincer Rhodium Complex: C–H and H–H Bond Activation by Metal–Ligand Cooperation and Inhibition by Dinitrogen

Abstract

The anionic dearomatized complex [(PNP*)Rh I Cl]K (2; PNP = 2,6-bis((di-tert-butylphosphino)methyl)pyridine, PNP* = deprotonated PNP) was prepared by reaction of the aromatic (PNP)Rh I Cl complex 1 with KN(SiMe 3 ) 2 in dry benzene. Spectroscopic characterization and DFT calculations confirm a nonaromatic square-planar structure of complex 2. Under an atmosphere of dry argon, 2 undergoes facile CÀH activation of benzene by cooperation between the metal center and the pincer ligand, with aromatization of the ligand, to form the complex (PNP)Rh I (C 6 H 5 ) (3a). This reaction is inhibited by dinitrogen, which reacts with 2 to form the complex (PNP*)Rh I (N 2 ) (4), indicating higher stabilization of the 14-electron (PNP*)Rh I species 5 by dinitrogen as compared with chloride. Similarly, treatment of 2 with CO results in KCl liberation to form the dearomatized (PNP*)Rh I CO (8). In a protic environment, the dearomatized complex 2 is quickly reprotonated to regenerate the aromatic starting complex 1. Upon treatment with MeI, 2 undergoes oxidative addition to form the nonaromatic (PNP*)Rh III (CH 3 )Cl (10), while the dearomatized ligand remains intact. Complex 2 undergoes facile activation of H 2 to form the monohydride (PNP)Rh I (H) (11a) and with D 2 to form (PNP)Rh I (D) (11b) with benzylic-D incorporation, via metalÀligand cooperation by aromatization of the ligand. The reactivity of 2 with H 2 is significantly higher than that of 4. ' INTRODUCTIONMetalÀligand cooperation plays an important role in homogeneous catalysis by metal complexes, 1À3 such as in the hydrogenation of polar bonds by ruthenium amido complexes. 1b,c Recently, we discovered a new mode of metalÀligand cooperation involving reversible dearomatization of pyridine-and acridine-based pincer complexes. 3À9 MetalÀligand cooperation via reversible dearomatization can play a key role in the catalytic OÀH bond activation of alcohols, 3 exemplified by the direct coupling of alcohols to form esters with the liberation of H 2 , 4a,b the hydrogenation of esters to alcohols under mild pressure, 4c the dehydrogenative coupling of alcohols with amines to produce amides 4d and polyamides, 4e,f the dehydrogenative amidation of esters with amines, 4g and the acylation of alcohols with esters. 4h These reactions are catalyzed by the dearomatized (PNN*)Ru II (H)(CO) complex (PNN = 2-((ditert-butylphosphino)methyl)-6-(diethylaminomethyl)pyridine; PNN* = deprotonated PNN). 4 Furthermore, this powerful complex promotes splitting of water to H 2 and O 2 in consecutive heatand light-induced steps. 5 Very recently, a bipyridine-based PNN analogue of this complex was shown to be effective in the direct hydrogenation of amides to form amines and alcohols. 4i The dearomatized (PNP*)Ru II (H)(CO) analogue (PNP = 2,6-bis-((di-tert-butylphosphino)methyl)pyridine; PNP* = deprotonated PNP) is effective in the NÀH bond activation of amines and ammonia, 6a the coupling of alcohols and amines to form imines and H 2 , 6b and the dehydrogenation of secondary alcohols to ketones....

C–O Bond Activation by a Tantalum‐Bonded Pincer Ligand – Ligand Modification Effects on the Selectivity of C–H Bond Cleavage Processes

“…In A , the formal oxidation state of the tantalum centre is V, whereas B is formally a Ta III derivative. The proposed bonding mode for the activated moiety can be compared to those of noninnocent ligands such as o ‐iminobenzoquinones6 or N , N ‐bis(phenoxide)amides7 and it is also closely related to that of dearomatized pyridine‐based pincer ligands, which display a metal–ligand cooperative reactivity 11,17…”

Sequential Double Dearomatization of the Pyrazolate‐Based “Two‐in‐One” Pincer Ligand in a Dinuclear Rhodium(I) Complex

“…[2] A prominent class of pincer ligands is based on a central pyridine that is equipped with two donor substituents in the 2-and 6-positions of the heterocycle to form two five-membered chelate rings; [3,4] examples are the tridentate PNP and PNN ligands in rhodium(I) complexes I [5] and II [6] (Scheme 1). These complexes can be deprotonated at one of the methylene groups of the chelate arms to give dearomatized species (such as III in Scheme 1) [6,7] that are capable Scheme 1. Rh I CO complexes of classical pincer-type PNP (I) and PNN (II) ligands and ligand dearomatization via deprotonation of II.…”

“…Rh I CO complexes of classical pincer-type PNP (I) and PNN (II) ligands and ligand dearomatization via deprotonation of II. [5][6][7] L*{Rh(CO)} 2 (2). X-ray crystallographic characterization of 1 and the K[2,2,2] + salt 3′ ([2,2,2] is a cryptand) evidences pyridine dearomatization upon deprotonation, and all spectroscopic and structural signatures are in good agreement with those of related mononuclear rhodium complexes based on a common PNN pincer ligand, which corroborates that pyrazolate-bridged Lis best described as a scaffold with two pincer-type subunits.…”

“…A prominent class of pincer ligands is based on a central pyridine that is equipped with two donor substituents in the 2‐ and 6‐positions of the heterocycle to form two five‐membered chelate rings;, examples are the tridentate PNP and PNN ligands in rhodium(I) complexes I and II (Scheme ). These complexes can be deprotonated at one of the methylene groups of the chelate arms to give dearomatized species (such as III in Scheme ), that are capable of heterolytically cleaving substrates X–H driven by re‐aromatization . Twofold pincer deprotonation is challenging but has been reported for some PNP complexes of group 10 metals, and double dearomatization has recently been demonstrated for a manganese(I) complex with a {N 2 S 2 } donor pyridinophane ligand …”

“…Rh I CO complexes of classical pincer‐type PNP ( I ) and PNN ( II ) ligands and ligand dearomatization via deprotonation of II …”