Study of Insertion of Olefins and/or Carbon Monoxide into Phosphine−Imine Palladium Methyl Complexes

Kinetics, Mechanism and Theoretical Studies of Norbornene-Ethylene Alternating Copolymerization Catalyzed by Organopalladium(II) Complexes Bearing Hemilabile α-Amino-pyridine

Cationic methylpalladium complexes bearing hemilabile bidentate α-amino-pyridines can serve as effective precursors for catalytic alternating copolymerization of norbornene (N) and ethylene (E), under mild conditions. The norbornyl palladium complexes in the formula of {[RHNCH₂(o-C₆H₄N)]Pd(C₇H10Me)(NCMe)}(BF₄) (R = iPr (2a), tBu (2b), Ph (2c), 2,6-Me₂C₆H₃ (2d), 2,6-iPr₂C₆H₃ (2e)) were synthesized via single insertion of norbornene into the corresponding methylpalladium complexes 1a-1e, respectively. Both square planar methyl and norbornyl palladium complexes exhibit facile equilibria of geometrical isomerization, via sterically-controlled amino decoordination-recoordination of amino-pyridine. Kinetic studies of E-insertion, N-insertion of complexes 1 and 2, and the geometric isomerization reactions have been examined by means of VT-NMR, and found in excellent agreement with the results estimated by DFT calculations. The more facile N-insertion in the cis-isomers, and ready geometric isomerization, cooperatively lead to a new mechanism that accounts for the novel catalytic formation of alternating COC.

Methyl Complexes of the Transition Metals

Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition-metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition-metal complexes containing M-CH3 fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M-CH3 compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl-bridged complexes and reactivity.

(Pyridyl)benzoazole palladium(ii) complexes as homogeneous catalysts in hydrogenation of alkenes and alkynes

Complexes 1–6 form “cocktail” catalysts in the hydrogenation and isomerization of alkenes and alkynes where the active species are homogeneous.

Effect of chalcogens on CO insertion into the palladium-methyl bond of [(N^N^X)Pd(CH3)](+) (X = O, S, Se) and on CO/ethylene copolymerisation

Neutral chloromethylpalladium(II) complexes, [Pd(Cl)(CH3)(L)] (1a-5a) with ligands κ(2)-N^S-2-((3,5-di-tert-butyl-1H-pyrazol-1-yl)methyl)-6-(phenylthiomethyl)pyridine (L1), κ(2)-N^S-2-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-6-(phenylthiomethyl)pyridine (L2), κ(2)-N^Se-2-((3,5-di-tert-butyl-1H-pyrazol-1-yl)methyl)-6-(phenylselanylmethyl)pyridine (L3), κ(2)-N^Se-2-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-6-(phenylselanylmethyl)pyridine (L4), and κ(2)-N^N-2-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-6-(phenoxymethyl)pyridine (L5) have been synthesised and characterised by various spectroscopic techniques. Ligands L1-L4 exhibit Npy^S/Se bidentate coordination whereas L5 shows an Npy^Npz bidentate coordination mode in their corresponding neutral palladium complexes. Abstraction of chloride in neutral palladium complexes with NaBAr4 (BAr4 = tetrakis[3,5-bis(trifluoromethyl)-phenyl]borate) resulted in the formation of the cationic palladium complexes 1b-5b, in which L1-L4 adopt a tridentate Npz^Npy^X (X = S or Se) coordination mode in their respective cationic palladium complexes (1b-4b) whilst L5 in complex 5b adopts a Npy^Npz bidentate coordination mode and the palladium centre is stabilized by the weakly coordinating acetonitrile. Compounds 1b-5b readily undergo CO insertion into the Pd-CH3 bond to form Pd-acyl that determines their ability to catalyse CO/ethylene copolymerisation to polyketones.

Structurally characterized intermediates in the stepwise insertion of CO–ethylene or CO–methyl acrylate into the metal–carbon bond of Pd(ii) complexes stabilized by (phosphinomethyl)oxazoline ligands

The initial CO-ethylene or CO-methyl acrylate insertion steps into the Pd-Me bond of methylpalladium(II) complexes with (phosphinomethyl)oxazoline ligands, leading to metallacycles, have been fully characterized, including by X-ray diffraction.

Phosphinito- and phosphonito-oxazoline Pd(ii) complexes as CO/ethylene insertion intermediates: synthesis and structural characterization

The phosphinito-oxazoline ligand 4,4-dimethyl-2-[methoxy(diphenylphosphine)]-4,5-dihydrooxazole (2a) and the phosphonite-oxazoline ligand 4,4-dimethyl-2-[methoxy(6H-dibenz[c,e][1,2]oxaphosphorin)]-4,5-dihydrooxazole (8a) were prepared by deprotonation of (4,5-dihydro-4,4-dimethyloxazol-2-yl)methanol (1a) and reaction with the corresponding P-Cl function, similar to the ligands 2b (4,4-dimethyl-2-[1-oxy(diphenylphosphine)-1-methylethyl]-4,5-dihydrooxazole) and 8b (4,4-dimethyl-2-[1-oxy(6H-dibenz[c,e][1,2]oxaphosphorin)-1-methylethyl]-4,5-dihydrooxazole) reported previously. These ligands react with [PdClX(COD)] to give complexes of the type [PdClX(P,N)] (3a P,N = 2a, X = Cl; 4a P,N = 2a, X = Me; 4b P,N = 2b , X = Me; 9a P,N = 8a, X = Cl; 9b P,N = 8b, X = Cl; 10a P,N = 8a, X = Me; 10b P,N = 8b, X = Me). Complexes 4a,b and 10a,b reacted with AgCF(3)SO(3) to yield [PdMe(P,N)OSO(2)CF(3)] 5a,b and 11a,b, respectively. From the stepwise insertion reaction of CO and ethylene into the Pd-C bond of 5a and 11a,b, the alkyl ketone chelate complexes [Pd{CH(2)CH(2)C(O)Me}(P,N)]CF(3)SO(3) 7a and 14a,b respectively, have been isolated and spectroscopically characterized. Complexes 3a.CH(2)Cl(2), 5a, 9b, 10a,b, [PdMe(H(2)O)(P,N)]CF(3)SO(3) 12b, (P,N = 8b) and 14a,b have also been characterized by X-ray crystallography and the structures of 14a,b represent still rare examples of structurally characterized CO/ethylene coupling products.

Stereocontrol in Alkyne Cyclocarbonylation Reactions Promoted by a Bioxazoline Palladium(ii) Complex

Insertion of 1,2-disubstituted alkynes into [Pd(CH3)(CO)(BIOX)]+[B{3,5-(CF3)2C6H3}4]- (1), where BIOX=(4S,4'S)-(-)-4,4',5,5'-tetrahydro-4,4'-bis(1-methylethyl)-2,2'-bioxazole, leads to the formation of five-membered palladacycles, which, by reaction with carbon monoxide, produce a mixture of two diastereomeric forms of a palladium complex containing an eta3-allylic gamma-lactone ligand. On leaving the mixture in solution, one isomer was converted into the other, reaching a diastereomeric excess of up to 94 %. The steric and electronic factors responsible for the epimerization process were investigated by theoretical methods. Cleavage of the eta3-allyl--palladium bond by nucleophiles allowed highly substituted chiral butenolides to be synthesized in good enantiomeric excess.

Reactions of Vinyl Acetate and Vinyl Trifluoroacetate with Cationic Diimine Pd(II) and Ni(II) Alkyl Complexes: Identification of Problems Connected with Copolymerizations of These Monomers with Ethylene

Vinyl acetate (VA) and vinyl trifluoroacetate (VA(f)) react with [(NwedgeN)Pd(Me)(L)][X] (M = Pd, Ni, (NwedgeN) = N,N'-1,2-acenaphthylenediylidene bis(2,6-dimethyl aniline), Ar(f) = 3,5-trifluoromethyl phenyl, L = Ar(f)CN, Et2O; X = B(Ar(f))4-, SbF6-) to form pi-adducts 3 and 5 at -40 degrees C. Binding affinities relative to ethylene have been determined. Migratory insertion occurs in a 2,1 fashion (DeltaG++ = 19.4 kcal/mol, 0 degrees C for VA, and 17.4 kcal/mol, -40 degrees C for VA(f)) to yield five-membered chelate complexes [(NwedgeN)Pd(kappa2-CH(Et)(OC(O)-CH3))]+, 4, and [(NwedgeN)Pd(kappa2-CH(Et)(OC(O)CF3))]+, 6. When VA is added to [(NwedgeN)Ni(CH3)]+, an equilibrium mixture of an eta2 olefin complex, 8c, and a kappa-oxygen complex, 8o, results. Insertion occurs from the eta2 olefin complex, 8c (DeltaG++ = 15.5 kcal/mol, -51 degrees C), in both a 2,1 and a 1,2 fashion to generate a mixture of five- and six-membered chelates, 9(2,1) and 9(1,2). VA(f) inserts into the Ni-CH3 bond (-80 degrees C) to form a five-membered chelate with no detectable intermediate. Thermolysis of the Pd chelates results in beta-acetate elimination from 4 (DeltaG++ = 25.5 kcal/mol, 60 degrees C) and beta-trifluoroacetate elimination from 6 (DeltaG = 20.5 kcal/mol, 10 degrees C). The five-membered Ni chelate, 9(2,1), is quite stable at room temperature, but the six-membered chelate, 9(1,2), undergoes beta-elimination at -34 degrees C. Treatment of the OAc(f) containing Pd chelate 6 with ethylene results in complete opening to the pi-complex [(NwedgeN)Pd(kappa2-CH(Et)(OAc(f)))(CH2CH2)]+ (OAc(f) = OC(O)CF3), 18, while reaction of the OAc containing Pd chelate 4 with ethylene establishes an equilibrium between 4 and the open form 16, strongly favoring the closed chelate 4 (DeltaH = -4.1 kcal/mol, DeltaS = -23 eu, K = 0.009 M(-1) at 25 degrees C). The open chelates undergo migratory insertion at much slower rates relative to those of the simple (NwedgeN)Pd(CH3)(CH2CH2)+ analogue. These quantitative studies provide an explanation for the behavior of VA and VA(f) in attempted copolymerizations with ethylene.

Nickel and iron complexes with oxazoline- or pyridine-phosphonite ligands; synthesis, structure and application for the catalytic oligomerisation of ethylene

The bis(oxazolinyl)phenylphosphonite ligand (bis(4,4-dimethyl-2-(1-hydroxy-1-methylethyl)-4,5-dihydrooxazole)phenylphosphonite, NOPONMe2)) and the new pyridine-phosphonite ligand (2-ethyl(1'-methyl-1-hydroxy)pyridine-6H-dibenz[c,e][1,2]oxaphosphorin) have been used for the preparation of the mononuclear complexes [NiCl2(NOPONMe2)] 18 and [NiCl2(6)2] 19, respectively, which catalyze the oligomerisation of ethylene with activities up to 57300 mol C2H4 mol Ni(-1) h(-1) (19 in the presence of only 6 equivalents of AlEtCl2). The selectivities for C4 dimers were as high as 90% (18 in the presence of only 2 equivalents of AlEtCl2) with selectivities for 1-butene of 21-22% of the C4 fraction. In the presence of 400 or 800 equivalents of MAO as cocatalyst, complex 19 yielded turnover frequencies of 7400 mol C2H4 mol Ni(-1) h(-1) and 13200 mol C2H4 mol Ni(-1) h(-1), respectively. The selectivities for 1-butene and ethylene dimers were similar to those obtained with AlEtCl2. The fact that 19 with a cyclic phosphonite moiety leads to higher activities and selectivities than 18 which contains an acyclic phosphonite group underlines the importance of the ligand on the catalytic properties of its metal complex. An unprecedented dinuclear iron complex [FeCl2(4,4-dimethyl-2-[(1-hydroxy-1-methyl)ethyl]-4,5-dihydrooxazolate)]2 20 was also obtained which contains two pentacoordinated metal centers coordinated by a bridging-chelating oxazoline-alcoholate. Complexes 18-20 are paramagnetic in solution, as determined by the Evans method.