PCP Pincer Complexes Research Articles

Reactivity of Iridium PCP Pincer Complexes toward CO and CO2. Crystal Structures of IrH(κ2-O2COH){C6H3-2,6-(CH2PBut2)2} and IrH(C(O)OH){C6H3-2,6-(CH2PBut2)2}·H2O

The reactivity of a series of PCP pincer complexes with carbon monoxide and carbon dioxide has been studied. The reactions of CO2 with PCP pincer iridium complexes provide the 16-electron complex Ir(η2-CO2){C6H3-2,6-(CH2PBut2)2} (2). Analogously, reactions with CO yield the 16-electron monocarbonyl compound Ir(CO){C6H3-2,6-(CH2PBut2)2} (4). When IrH(OH){C6H3-2,6-(CH2PBut2)2} (1b) is reacted with CO2, the hydrido bicarbonate complex IrH(κ2-O2COH){C6H3-2,6-(CH2PBut2)2} (3) is obtained. Analogously, when IrH2{C6H3-2,6-(CH2PBut2)2} (1a) is reacted first with CO2 and then with H2O, the reaction affords the same species (3). Thus, compound 3 can be obtained irrespective of the order of addition of the substrates CO2 and H2O to 1a. Similarly, the reaction of 1b with CO affords the insertion product IrH(C(O)OH){C6H3-2,6-(CH2PBut2)2} (5). The identities of 3 and 5 have been confirmed by single-crystal X-ray structure determinations.

Dehydrogenation of secondary amines to imines catalyzed by an iridium PCP pincer complex: initial aliphatic or direct amino dehydrogenation?

The PCP pincer complex, IrH 2(C 6H 3-2,6-(CH 2PBu t 2) 2) catalyzes the transfer dehydrogenation of secondary amines to imines. The catalytic system is highly sensitive to both steric and electronic factors and greater than 99% regioselectivity are observed in the dehydrogenation of the asymmetric substrates, cyclohexyethylamine and benzylpropyl amine. Good to excellent yields are obtained when the reactions are carried out in toluene solution. The dehydrogenation of 2,2,2′,2′-tetramethyldibutylamine leads exclusively to the production of the corresponding imine indicating that the catalytic reaction pathway involves direct amino rather than initial aliphatic dehydrogenation.

ChemInform Abstract: Selective Dehydrogenation of Alcohols and Diols Catalyzed by a Dihydrido Iridium PCP Pincer Complex.

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Selective dehydrogenation of alcohols and diols catalyzed by a dihydrido iridium PCP pincer complex

The PCP pincer complex, IrH2{C6H3-2,6-(CH2P-t-Bu2)2} (1) catalyzes the transfer dehydrogenation of primary and secondary alcohols. Dehydrogenation occurs across the C—O bond rather than the C—C bonds and the corresponding aldehydes or ketones are obtained as the sole products arising from the dehydrogenation reactions. Methanol is an exception to this pattern of reactivity and undergoes only stoichiometric dehydrogenation with 1 to give the carbonyl complex, Ir(CO){C6H3-2,6-(CH2P-t-Bu2)2} (2). The products are obtained in nearly quantitative yields when the reactions are carried out in toluene solutions. Under the same conditions, 2,5-hexanediol is converted to the annulated product, 3-methyl-2-cyclopenten-1-one which has been isolated in 91% yield in a preparative scale reaction.Key words: alcohol, dehydrogenation, ketones, iridium pincer complex, annulation.

Selective dehydrogenation of alcohols and diols catalyzed by a dihydrido iridium PCP pincer complex

Selective dehydrogenation of alcohols and diols catalyzed by a dihydrido iridium PCP pincer complex

Selective C−C vs C−H Bond Activation by Rhodium(I) PCP Pincer Complexes. A Computational Study

A theoretical study of the oxidative addition of C−C vs C−H bonds to a rhodium(I) complex with PCP-type ligands has been carried out. Special attention has been paid to the effect of different bulky substituents at the phosphorus atoms of the chelate ligand. Therefore, B3LYP/lanl2dz+p//B3LYP/lanl2dz and ONIOM(B3LYP/lanl2dz+p:B3LYP/lanl2dz)//ONIOM(B3LYP/lanl2dz:HF/lanl1mb) methods have been utilized. According to the calculations, C−H activation is always the kinetically favored process (ΔΔE⧧ 20 kJ·mol-1), though the C−C activation product is more stable (ΔΔE 20 kJ·mol-1). C−H addition is a reversible process; the product of the C−H activation can interconvert to the C−C activation product via an intermediate structure. Bulky substituents are found to increase the barrier for C−H activation relative to that for C−C activation. With additional ligands (e.g., phosphines), hexacoordinate complexes are formed. This is more favorable for the C−C activation products. Our calculations show that the activation reaction proceeds via complexes with a pentacoordinated rhodium atom. Thus, in the presence of donor ligands, the activation reaction is inhibited.

Highly efficient and regioselective production of trisubstituted alkenes through heck couplings catalyzed by a palladium phosphinito PCP pincer complex

The PCP ligand precursor 1,3-bis(diisopropylphosphinito)benzene is conveniently prepared from the reaction of chlorodiisopropylphosphine and resorcinol in the presence of 4-dimethylaminopyridine. Refluxing the bis(phosphinito)benzene with Pd(COD)Cl2 in toluene for 5 h yields the palladium complex PdCl{C6H3-2,6-(OPPri2)2} (1). The molecular structure of the complex was determined through a single-crystal X-ray diffraction study. The complex is a highly efficient catalyst for the olefinic coupling of aryl bromo and iodo compounds under aerobic conditions. Disubstituted alkenes were obtained in good yields from the catalytic coupling of styrene and aryl iodides, whereas styrene reactions with aryl bromides yield exclusively trisubstituted alkenes. Trisubstituted alkenes were also obtained from the reactions of 1,1 disubstituted alkenes (α-methylstyrene, n-butylacrylate) and bromobenzene. Conditions have been found which result in the regioselective product ion of one isomer of the trisubstituted alkene products in≥90% yield.

High yield olefination of a wide scope of aryl chlorides catalyzed by the phosphinito palladium PCP pincer complex: [PdCl{C6H3(OPPri2)2-2,6}

[PdCl{C6H3(OPPri2 )2-2,6}] efficiently catalyzes the olefination of a broad scope of aryl chlorides; as a result of the high thermal stability of the catalyst, the coupled products are obtained in excellent and in many cases nearly quantitative yields.

Iridium PCP pincer complexes: highly active and robust catalysts for novel homogeneous aliphatic dehydrogenations

The PCP pincer complexes, IrH2{C6H3(CH2PR2)2-2,6} (R = But, Pri) are extraordinarily active and robust catalysts for aliphatic dehydrogenation reactions. Their application to alkane dehydrogenation has resulted in the first efficient catalytic systems for homogeneous thermochemical alkane dehydrogenation without the use of a sacrificial hydrogen acceptor and for dehydrogenation of n-alkanes to α-olefins. The pincer catalysts also effect other aliphatic dehydrogenations which had not previously been accomplished via homogeneous catalysis including the conversion of cycloalkanes to arenes, ethylbenzene to styrene and tetrahydrofuran to furan. All of these catalytic reactions are inhibited by even traces of nitrogen. This is apparently due to the formation of the remarkably stable dinitrogen complexes: [Ir{C6H3(CH2PR2)2-2,6}]2(µ-N2). Mechanistic studies have indicated that the high activity of the pincer catalysts may be in part due to ‘agostic promotion’ by phosphino aliphatic groups.