New cationic group 4 metal alkyl complexes

Cationic group 4 metal alkyl complexes containing tetradentate Schiff base ligands, (acen) Zr(R)⁺ and (F6-acen) Zr(R)⁺, are prepared by protonolysis of suitable neutral dialkyl precursors. These complexes display electrophilic behavior and are moderately active ethylene polymerization catalysts in the presence of 1 molar equivalent of AlR₃.     

Catalysis of CC-coupling reactions by cyclopropenylidene palladium complexes

Several mixed palladium(II) complexes bearing 2,3-diarylcyclopropenylidene ligands (aryl = phenyl, mesityl, naphthyl) and triaryl- and trialkylphosphines have been prepared. Single crystal structure details of one of the dimeric chloro-bridged complexes as well as of two monomeric phosphine substituted complexes are presented and compared with appropriate structural features of similar 2,3-diaminocyclopropenylidene- and cycloheptatrienylidene complexes. The new complexes were tested as catalysts in Suzuki-Mijaura coupling reactions with bromo- and chloroarenes and their catalytic activity compared with that of analogous NHC- and cycloheptatrienylidene complexes.     

Insertion reactions of SO2 into Pd-OR bonds: preparation of alkyl sulfito complexes of palladium(II)

Mononuclear palladium-hydroxo complexes of the type [Pd(N-N)(C6F5)(OH)][(N-N = 2,2'-bipyridine (bipy), 4,4'-dimethyl-2,2'-bipyridine (Me2bipy), or N,N,N',N'-tetramethylethylenediamine (tmeda) react with SO2(1 atm) at room temperature in alcohol (methanol, ethanol, propanol or isopropanol) to yield alkyl sulfito palladium complexes [Pd(N-N)(C6F5)(SO2OR)](R = Me, Et, Pr or iPr). Similar alkyl sulfito complexes [Pd(N-N)(C6F5)(SO2OR)](N-N = bis(3,5-dimethylpyrazol-1-yl)methane); R = Me or Et) are obtained when [Pd(N-N)(C6F5)Cl] is treated with KOH in the corresponding alcohol ROH and SO2 is bubbled through the solution. The reaction of [Pd(bipy)(C6F5)(OH)] with SO2 in tetrahydrofuran gives [Pd(N-N)(C6F5)(SO2OH)]. The X-ray diffraction study of [Pd(tmeda)(C6F5)(SO2OPr)] has established the sulfur coordination of the propyl sulfito ligand.     

Aryl palladium carbene complexes and carbene-aryl coupling reactions

Transmetalation of an aminocarbene moiety from [W(CO)5{C(NEt2)R}] to palladium leads to isolable monoaminocarbene palladium aryl complexes [{Pd(mu-Br)Pf[C(NEt2)R]}2] (R = Me, Ph; Pf = C6F5). When [W(CO)5{C(OMe)R}] is used, the corresponding palladium carbenes cannot be isolated since these putative, more electrophilic carbenes undergo a fast migratory insertion process to give alkyl palladium complexes. These complexes could be stabilized in the eta3-allylic form for R = 2-phenylethenyl or in the less stable eta3-benzylic fashion for R = Ph. Hydrolysis products and a pentafluorophenylvinylic methyl ether (when R = Me) were also observed. The monoaminocarbenes slowly decompose through carbene-aryl coupling to produce the corresponding iminium salts and, depending on the reaction conditions, the corresponding hydrolysis products. The electrophilicity of the carbene carbon, which is mainly determined by the nature of the heteroatom group, controls the ease of evolution by carbene-aryl coupling. Accordingly, no carbene-aryl coupling was observed for a diaminocarbene palladium aryl complex.     

Low-valence palladium complexes: Stoichiometric reactions and catalysis

New data on the structure and reactivity of palladium clusters are surveyed. The mechanisms of stoichiometric and catalytic reactions of the palladium cluster complexes with alkenes, alcohols, aldehydes, formic acid, CO, and phenol are discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 807–815, March, 1998.     

Synthesis of neutral and cationic monocyclopentadienyl alkyl niobium and tantalum complexes

Compound [NbCp′Me₄] (Cp′ = η⁵-C₅H₄SiMe₃, 1) reacted with several ROH compounds (R = tBu, SiiPr₃, 2,6-Me₂C₆H₃) to give the derivatives [NbCp′Me₃(OR)] (R = tBu 2a, SiiPr₃2b, 2,6-Me₂C₆H₃2c). The diaryloxo tantalum compound [TaCp^∗Me₂(OR)₂] (Cp^∗ = η⁵-C₅Me₅, R = 2,6-Me₂C₆H₃3) was obtained by reaction of [TaCp^∗Cl₂Me₂] with 2 equiv of LiOR (R = 2,6-Me₂C₆H₃). Abstraction of one methyl group from these neutral compounds 1-3 with the Lewis acids E(C₆F₅)₃ (E = B, Al) gave the ionic derivatives [NbCp′Me₂X][MeE(C₆F₅)₃] (X = Me 4-E. X = OR; R = SiiPr₃5b-E, 2,6-Me₂C₆H₃5c-E. E = B, Al) and [TaCp^∗Me(OR)₂][MeE(C₆F₅)₃] (R = 2,6-Me₂C₆H₃6-E; E = B, Al). Polymerization of MMA with the aryloxoniobium compound 2c and Al(C₆F₅)₃ gave syndiotactic PMMA in a low yield, whereas the tetramethylniobium compound 1 and the diaryloxotantalum derivative 3 were inactive.     

A study on ligand-coupling reactions of imidoyl palladium complexes

A new ligand-coupling reaction of imidoylpalladium(II) complexes is described. Heating a toluene solution of imidoylpalladium complexes gave rise to the α-diimines (1,4-diazabutadienes).     

Synthesis and reactions of some sulfur ylide complexes of palladium

The iodo-bridged sulfur ylide complex [Pd(μ-I)((CH₂)₂(SO)(CH₃))]₂ (1) when treated with dithiolates, acetylacetone and various Lewis bases gave [Pd((CH₂)₂(SO)(CH₃))(S ∼ S)] (S ∼ S = S₂CN(C₂H₅)₂, S₂COC₂H₅ and S₂P(OC₂H₅)₂), [Pd((CH₂)₂(SO)(CH₃))(acac)] (acac = acetylacetonate) and [PdI((CH₂)₂(SO)(CH₃))(base)]a (base = PPh₃, (P(OMe)₃, P(OPh)₃ and C₅H₅N). In the presence of a phase transfer catalyst (PTC). The reactions rates and yields were greatly increased. Reaction of several related sulfur ylide complexes with I₂, HI or aqueous NaOH gave 1. The single crystal structure of [Pd((CH₂)₂(SO)(CH₃))₂] was determined (orthorhombic, Pbcn, a 13.379(2), b 8.081(1), c 9.048(2) Å, V 978.2 ų, Z = 4). The compound has a rather long PdCH₂ bond (2.096(1) Å, mean).     

Latent cationic palladium(II) phosphine carboxylate complexes for norbornene polymerization

The catalytic efficacy of trans‐[(R₃P)₂Pd(O₂CR′)(LB)][B(C₆F₅)₄] ( 1 ) (LB = Lewis base) and [(R₃P)₂Pd(κ²‐O,O‐O₂CR′)][B(C₆F₅)₄] ( 2 ) for mass polymerization of 5‐n‐butyl‐2‐norbornene (Butyl‐NB) was investigated. The nature of PR₃ and LB in 1 and 2 are the most critical components influencing catalytic activity/latency for the mass polymerization of Butyl‐NB. Further, it was shown that 1 is in general more latent than 2 in mass polymerization of Butyl‐NB. 5‐n‐Decyl‐2‐norbornene (Decyl‐NB) was subjected to solution polymerization in toluene at 63(±3) °C in the presence of several of the aforementioned palladium complexes as catalysts and the polymers obtained were characterized by gel permeation chromatography. Cationic trans‐[(R₃P)₂PdMe(MeCN)][B(C₆F₅)₄] [R = Cy ( 3a ), and ⁱPr ( 3b )] and trans‐[(R₃P)₂PdH (MeCN)][B(C₆F₅)₄] [R = Cy ( 4a ), and ⁱPr ( 4b )], possible products from thermolysis of trans‐[(R₃P)₂Pd(O₂CMe)(MeCN)][B(C₆F₅)₄] [R = Cy ( 1a ) and ⁱPr ( 1g )], as well as trans‐[(R₃P)₂Pd(η³‐C₃H₅)][B(C₆F₅)₄] [R = Cy ( 5a ), and ⁱPr ( 5b )], were also examined as catalysts for solution polymerization of Decyl‐NB. A maximum activity of 5360 kg/(molPd h) of 2a was achieved at a Decyl‐NB/Pd: 26,700 ratio which is slightly better than that achieved with 5a [activity: 5030 kg/(molPd h)] but far less compared with 4a [activity: 6110 kg/(molPd h)]. Polydispersity values indicate a single highly homogeneous character of the active catalyst species. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 103–110, 2009     

Intramolecular alkyl phosphine dehydrogenation in cationic rhodium complexes of tris(cyclopentylphosphine)

[Rh(nbd)(PCyp(3))(2)][BAr(F) (4)] (1) [nbd = norbornadiene, Ar(F) = C(6)H(3)(CF(3))(2), PCyp(3) = tris(cyclopentylphosphine)] spontaneously undergoes dehydrogenation of each PCyp(3) ligand in CH(2)Cl(2) solution to form an equilibrium mixture of cis-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(2)][BAr(F) (4)] (2 a) and trans-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(2)][BAr(F) (4)] (2 b), which have hybrid phosphine-alkene ligands. In this reaction nbd acts as a sequential acceptor of hydrogen to eventually give norbornane. Complex 2 b is distorted in the solid-state away from square planar. DFT calculations have been used to rationalise this distortion. Addition of H(2) to 2 a/b hydrogenates the phosphine-alkene ligand and forms the bisdihydrogen/dihydride complex [Rh(PCyp(3))(2)(H)(2)(eta(2)-H(2))(2)][BAr(F) (4)] (5) which has been identified spectroscopically. Addition of the hydrogen acceptor tert-butylethene (tbe) to 5 eventually regenerates 2 a/b, passing through an intermediate which has undergone dehydrogenation of only one PCyp(3) ligand, which can be trapped by addition of MeCN to form trans-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(PCyp(3))(NCMe)][BAr(F) (4)] (6). Dehydrogenation of a PCyp(3) ligand also occurs on addition of Na[BAr(F) (4)] to [RhCl(nbd)(PCyp(3))] in presence of arene (benzene, fluorobenzene) to give [Rh(eta(6)-C(6)H(5)X){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (7: X = F, 8: X = H). The related complex [Rh(nbd){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] 9 is also reported. Rapid ( approximately 5 minutes) acceptorless dehydrogenation occurs on treatment of [RhCl(dppe)(PCyp(3))] with Na[BAr(F) (4)] to give [Rh(dppe){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (10), which reacts with H(2) to afford the dihydride/dihydrogen complex [Rh(dppe)(PCyp(3))(H)(2)(eta(2)-H(2))][BAr(F) (4)] (11). Competition experiments using the new mixed alkyl phosphine ligand PCy(2)(Cyp) show that [RhCl(nbd){PCy(2)(Cyp)}] undergoes dehydrogenation exclusively at the cyclopentyl group to give [Rh(eta(6)-C(6)H(5)X){PCy(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (17: X = F, 18: X = H). The underlying reasons behind this preference have been probed using DFT calculations. All the complexes have been characterised by multinuclear NMR spectroscopy, and for 2 a/b, 4, 6, 7, 8, 9 and 17 also by single crystal X-ray diffraction.     

Pd-H elimination reactions in palladium(II) allylic complexes

The ease of H elimination from the 4- (beta-) position in a series of allylic complexes [Pd(5-C(6)F(5)-1-3-eta(3)-cyclohexenyl)XL](n+) (X, L = Br, N-, P-donor, C(6)Cl(2)F(3); n = -1, 0, +1) was compared by analyzing their decomposition products at 50 degrees C. Pd-H elimination does not occur for cationic complexes, whereas it is the main decomposition pathway for neutral and anionic complexes. In addition to the charge of the complex, the ease of this Pd-H elimination is determined by the trans influence of the ligands (aryl > PMe(3) > Br, N-donor).     

Ring-opening reactions of aromatic N-heterocycles by scandium and yttrium alkyl complexes

The C-N bond in aromatic N-heterocycles is a strong bond, its cleaving involving mostly examples of metal-element multiple bonds. We report on the C-C coupling of two molecules of an aromatic N-heterocycle mediated by scandium and yttrium benzyl complexes supported by a ferrocene 1,1'-diamide ligand. The reaction with 1-methylimidazole leads, ultimately, to the formation of a ring-opened imidazole coupled to a 1-methylimidazole fragment, a structure showing extended conjugation of double bonds. The experimental evidence agrees with involvement of only sigma bonds in these transformations.     

Bis-phosphine monoxide platinum(II) and palladium(II) cationic complexes as Lewis acid catalysts in Diels-Alder and sulfoxidation reactions

A series of bis-phosphine monoxide (BPMO) palladium(II) and platinum(II) cationic complexes of the type [M(BPMO-κ²-P,O)₂][X]₂ (M = Pd, Pt; BPMO = Ph₂P-(CH₂)_n-P(O)Ph₂ with n = 1 (dppmO), 2 (dppeO), 3 (dpppO); X = BF₄, TfO) were prepared from the corresponding chlorides [MCl₂(BPMO-κ¹-P)₂] upon treatment with 2 equiv. of AgX in wet acetone/CH₂Cl₂ or MeOH solutions. They were characterized by ¹H and ³¹P{¹H} NMR spectroscopies and, in the case of the complex [Pt(dppeO-κ²-P,O)₂][BF₄]₂, also by X-ray crystallography. These complexes were tested as catalysts in some Diels-Alder and oxidation reactions with different substrates. In the latter reaction Pt(II) complexes showed moderate activity, while for the former one, both classes of complexes were active in the C-C coupling, in particular the Pt(II) species showed interesting high endo/exo diasteroselectivity depending on the counteranion.     

Novel phosphite palladium complexes and their application in C-P cross-coupling reactions

A mono- and a 1,3-bis-phosphite arene ligand based on 2,2′-biphenol have been synthesized in order to study the synthesis of the corresponding palladium(II) complexes starting from different Pd precursors. Novel bis-phosphite palladium complex 1 [PdCl₂(L)₂] (L = dibenzo[d,f][1,3,2]dioxaphosphepin, 6-phenoxy), C,P-chelate bonded monophosphite palladium complex 2 [Pd(κ₂-L)(μ-Cl)]₂, and PCP-pincer palladium complex 3 have been prepared from these ligands in promising to excellent yields (50-95%). Additionally, complexes 1 and 3 have been characterized by X-ray crystal structure determinations. The application of 2,6-bis-phosphite pincer palladium(II) complex 3 in C-P cross-coupling between diphenylphosphine-borane and a wide range of various aryl iodides under very mild conditions is reported. Kinetic investigations indicate that 3 merely acts as a pre-catalyst and that Pd nanoparticles are the actual catalytically active species.     

Synthesis and characterization of palladium amido complexes containing pincer CNO ligands through nitrene insertion

Catellani reactions of aryl iodides allow the double functionalization at the ortho and ipso positions in one pot. Catellani reactions involving a nitrene intermediate are not yet known. In this paper, a few palladium amino complexes were prepared from PdCl₂, anthranil and iodoarenes, and their structures were determined by X‐ray single‐crystal diffraction. These complexes are supported by a pincer C,N,O tridentate ligand forming fused six‐membered palladacycles. The high stability of the palladium complexes inhibits their reductive elimination.     

Migratory insertion in N-heterocyclic carbene complexes of palladium; an experimental and DFT study

The first authenticated example of migration of a methyl group from palladium(II) to a coordinated N-heterocyclic carbene is described.     

Thermal studies on palladium alkyl xanthates

Eleven palladium(II) alkylxanthates: Pd(ROCSS)₂ [RMe, Et, nPr, iPr, nBu, iBu, tBu, nAm, iAm, nHex and cyclohex], have been prepared and their thermal properties investigated by thermogravimetric analysis. The complexes, although volatile under vacuum (10^?2 mm Hg), decompose without volatilization at normal atmospheric pressure leaving a residue of palladium metal at 950°C. The intermediate decomposition products were identified mass spectrometrically and a thermal decomposition mechanism is proposed.     

Synthesis of cationic hydride and related complexes of palladium and nickel with tricyclohexylphosphine or triisopropylphosphine

The preparation and characterization of a series of cationic hydride complexes of palladium and nickel of the type [MH(L)(PCy₃)₂]BPh₄ (M = Pd, Ni; L = pyridines, pyrazole, imidazole; Cy = cyclohexyl; Ph = phenyl) from new hydride complexes, trans-MH(NO)₃(PCy₃)₂ are described. Trans-PdH(NO₃)(PCy₃)₂ is prepared conveniently by treatment of Pd(NO₃)₂(PCy₃)₂ (yellow form) with NaBH₄ (yield 93%). The relative stability of the nickel triad hydride complexes is discussed.Preparation of PdHCl(PR₃)₂ (R = Cy or i-Pr = isopropyl) by a new method and their derivatives are also reported.