Allyl Complexes Research Articles

Mixed Allyl Rare-Earth Borohydride Complexes: Synthesis, Structure, and Application in (Co-)Polymerization Catalysis of Cyclic Esters.

A series of new trivalent rare-earth allyl-borohydride complexes with the formula [RE(BH4 )2 (C3 H5 )(thf)x ] (RE=Sc (1), x=2; RE=Y (2) and La (3), x=3) were synthesized by reaction of the corresponding rare-earth trisborohydrides [RE(BH4 )3 (thf)x ] with half an equivalent of bis(allyl)magnesium. The complexes were fully characterized by determining their X-ray structure. Similar to their previously described Nd (4) and Sm (5) analogues, these complexes display a monomeric structure with two terminal trihapto BH4 groups, one π-η3 allyl ligand, three THF molecules for complexes 2 and 3, and two THF molecules for complex 1. The catalytic behavior of complexes 1-5 toward the ring-opening polymerization (ROP) of l-lactide (l-LA) and ϵ-caprolactone (ϵ-CL) was assessed. The Nd complex featured the best activity toward l-LA (turnover frequency (TOF)=1300 h-1 ) and the order was Nd>La>Sm>Y>Sc. Complexes 1-3 were found very active for the ROP of ϵ-CL (TOF=166 000 h-1 ), which is in line with the already established exceptionnally high performance of complexes 4 and 5. With both monomers, it was shown that the borohydride moiety was the preferentially initiating group, rather than the allyl one. The random copolymerization of l-LA and ϵ-CL was performed with complexes 1-5, in the absence or in the presence of benzyl alcohol as a chain-transfer agent, affording copolymers with ϵ-caprolactone up to 62 % inserted. The copolymers synthesized display a variety of microstructures, that is, blocky, random, or quasi-alternating.

Synthesis and structure elucidation of allyl Pd(II) complexes of NHC ligands derived from substituted imidazo[1,5-a]quinolin-1(2H)-ylidene

Nine Pd(II) complexes involving N-heterocyclic carbenes (NHCs) derived from 2-substituted and 2- and 7-substituted imidazo[1,5-a]quinolin-1(2H)-ylidene with auxiliary allylic ligands were synthesized and characterized. The structure and configuration of the complexes were elucidated on the basis of combination of dynamic NMR and DFT studies. Conformational studies in respect of hindered rotation around Pd-C bond and η3-η1-η3 pseudo allyl rotation were performed. The results from dynamic NMR and DFT studies confirmed the mechanism of selective η3-η1-η3 isomerization, whose energy barriers are affected by steric hindrance of substituents at nitrogen atom. Energy barriers of isomerization (16.7–18.8 kcal/mol) are slightly influenced by the electronic nature of substituents at seventh position in imidazo[1,5-a]quinolin-1(2H)-ylidene moiety. The results from DFT calculations were in good agreement with the experimental energy barriers.

Synthesis of novel allyl palladium complexes bearing purine based NHC and a water soluble phosphine and their catalytic activity in the Suzuki‐Miyaura coupling in water

We have synthesized and fully characterized some novel allyl palladium complexes stabilized by mixed spectator ligands, namely the water soluble sodium 3,3′,3″‐phosphinetriyltribenzenesulfonate, (TPPTS) and three carbenes derived from differently alkylated natural xanthines. In order to explore their potential applications we have tested their catalytic activity in the Suzuki‐Miyaura coupling of para‐bromoacetophenone with phenyl boronic acid, chosen as standard reaction. Only one of the complexes under study displays a remarkable stability in water and catalytic activity. We have therefore undertaken a catalytic investigation. These results are reported in the present paper together with the solid state structural characterization of a silver precursor of the palladium allyl complexes.

Ligand Dynamics and Aerobic Allylic Oxidation with Bifunctional Ni(NHC) Complexes

AbstractThe synthesis of three new NiCl(allyl/cinnamyl)(NHC) complexes, containing a bifunctional NHC (N‐heterocyclic carbene) ligand is described. Two different NHCs were utilized that contain a secondary amine (HNR; R=t‐Bu or Mes) moiety on one of the N‐imidazolylidene arms. Complexes with allyl (2 a) and cinnamyl (3 a and 3 b) were each crystallographically characterized and η3‐allyl and κ1‐C NHC ligand coordination modes were found in the solid‐state. In the solution‐state, each of the ligands shows highly fluxional behaviour. Halide substitution of the chloro ligand for iodo effectively slows the ligand dynamics and permits solution‐state characterization that is in agreement with the solid‐state structures. The complexes react with oxygen to ultimately oxidize the allyl or cinnamyl ligands to acrolein (AC) or cinnamaldehyde (CA) and phenyl vinyl ketone (PVK), respectively. The bifunctional nature of the NHC has limited influence on oxidation reactivity, but preliminary evidence suggests it plays a role in the stability of the nickel oxidation products.

Yttrium–Benzyl Complexes Bearing Chiral Iminophosphonamide Ligands: Synthesis and Application in Catalytic Asymmetric Amine‐Silane Dehydrocoupling Reactions

AbstractA series of novel iminophosphonamide ligands with chiral diamine moieties were designed and synthesized. Yttrium–benzyl compounds bearing these chiral iminophosphonamide ligands showed high reactivity and selectivity on the first catalytic asymmetric cross‐dehydrogenative coupling of amines with prochiral silanes under mild conditions. A stable silylamine–boron derivative was synthesized in up to 23% ee, as determined by chiral HPLC analysis. By employing this catalyst system, catalytic kinetic resolution of an axially chiral silane and further in situ allylation were achieved in good yields and enantioselectivities.magnified image

The [3+2] cycloaddition reaction in CpRu(allyl)(acetylene)

The transition metal-mediated coupling of allyl and alkyne ligands to yield cyclopentadienyl and pentadienyl compounds has been studied theoretically using M06 DFT calculations using CpRu(allyl)(acetylene) as a representative example. We find that the first step in the mechanism is the formation of a 16-electron α, π-vinyl olefin intermediate, whereas the alternative, least-motion 3+2 cycloaddition via a concerted process was calculated to lie at a prohibitively high energy and has the characteristics of a symmetry forbidden reaction. The 3+2 concerted path is shown to be unlikely for η3-allyl complexes in general. There are two competing mechanisms, which lead to Cp2Ru + H2 or CpRu(η5-pentadienyl). The pentadienyl product is predicted to form by either the rearrangement of the α, π-vinyl olefin complex or an electrocyclic ring opening of a cyclopentadiene intermediate. The alternative mechanism involves a ring closure of the α, π-vinyl olefin intermediate to yield CpRu(H)(cyclopentadiene). Four mechanistic scenarios were investigated for the reductive elimination of H2 to give Cp2Ru. The lowest energy process occurs via CpRu(H)2(η3-Cp). The other three possibilities are symmetry forbidden. It is shown that the partitioning of pentadienyl versus cyclopentadienyl products depends on the electronic characteristics of the metal. As the transition metal becomes more electron rich, the pathway to the pentadienyl product becomes more favored. Calculations on CpCo(π-allyl)(acetylene)+ and CpTc(π-allyl)(acetylene)− are in agreement with this hypothesis.

Multiple C–H Activations of Linear Alkanes by Various (η5-Cyclopentadienyl)W(NO)(CH2CMe3)2 Complexes

As illustrated in the accompanying diagram, thermolysis of Cp*W(NO)(CH2CMe3)2 (Cp* = η5-C5Me5) at 80 °C in neat linear alkanes effects three successive C–H bond activations of the hydrocarbon substrates and forms Cp*W(NO)(H)(η3-allyl) complexes in which the allyl ligands are derived from the alkanes. These allyl hydrido compounds exist in solutions as mixtures of isomers containing monosubstituted (i.e., terminal) or 1,3-disubstituted (i.e., internal) allyl ligands which can have either an endo or exo orientation with the substituent groups being either proximal or distal to the nitrosyl ligand. Due to steric factors the most abundant isomer in all cases has a monosubstituted allyl ligand in the endo orientation with the alkyl end distal to the nitrosyl ligand. In addition, the relative abundance of Cp*W(NO)(H)(η3-allyl) isomers having monosubstituted allyl ligands decreases with increasing length of the n-alkane chain. Further thermolysis of the Cp*W(NO)(H)(η3-allyl) complexes results in the liberation o...

Symmetric Assembly of a Sterically Encumbered Allyl Complex: Mechanochemical and Solution Synthesis of the Tris(allyl)beryllate, K[BeA′3] (A′ = 1,3-(SiMe3)2C3H3)

The ball milling of beryllium chloride with two equivalents of the potassium salt of bis(1,3-trimethylsilyl)allyl anion, K[A′] (A′ = [1,3-(SiMe3)2C3H3]), produces the tris(allyl)beryllate K[BeA’3] (1) rather than the expected neutral BeA’2. The same product is obtained from reaction in hexanes; in contrast, although a similar reaction conducted in Et2O was previously shown to produce the solvated species BeA’2(OEt2), it can produce 1 if the reaction time is extended (16 h). The tris(allyl)beryllate is fluxional in solution, and displays the strongly downfield 9Be NMR shift expected for a three-coordinate Be center (δ22.8 ppm). A single crystal X-ray structure reveals that the three allyl ligands are bound to beryllium in an arrangement with approximate C3 symmetry (Be–C (avg) = 1.805(10) Å), with the potassium cation engaging in cation–π interactions with the double bonds of the allyl ligands. Similar structures have previously been found in complexes of zinc and tin, i.e., M[M′A′3L] (M′ = Zn, M = Li, Na, K; M′ = Sn, M = K; L = thf). Density functional theory (DFT) calculations indicate that the observed C3-symmetric framework of the isolated anion ([BeA′3]−) is 20 kJ·mol−1 higher in energy than a C1 arrangement; the K+ counterion evidently plays a critical role in templating the final conformation.

Synthesis and reductive chemistry of bimetallic and trimetallic rare-earth metallocene hydrides with (C5H4SiMe3)1− ligands

The reductive chemistry of [Cp'2Ln(μ–H)(THF)x]y [Ln = Y, Dy, Tb; Cp' = (C5H4SiMe3)1−; x = 2, 0 and y = 2, 3] was examined to determine if these hydrides would be viable precursors for 4fn5d1 Ln2+ ions that could form 5d1-5d1 metal–metal bonded complexes. The hydrides were prepared by reaction of the chlorides, [Cp'2Ln(μ–Cl)]2, 1-Ln, with allylmagnesium chloride to form the allyl complexes, [Cp'2Y(η3–C3H5)(THF)], 2-Ln, which were hydrogenolyzed. The solvent-free reaction of solid 2-Ln with 60 psi of H2 gas in a Fischer-Porter apparatus produced, in the Y case, the trimetallic species, [Cp'2Y(μ–H)]3, 3-Y, and in the Dy and Tb cases, the bimetallic complexes [Cp'2Ln(μ–H)(THF)]2, 4-Ln (Ln = Dy, Tb). The latter complexes could be converted to 3-Dy and 3-Tb by heating under vacuum. Isopiestic data indicate that 3-Y solvates to 4-Y in THF. Reductions of 4-Y, 4-Dy, and 4-Tb with KC8 in the presence of a chelate such as 2.2.2-cryptand or 18-crown-6 all gave reaction products with intense dark colors characteristic of Ln2+ ions. In the yttrium case, with either chelating agent, the dark green product gives a rhombic EPR spectrum (g1 = 2.01, g2 = 1.99, g3 = 1.98, A = 24.1 G) at 77 K. However, the only crystallographically-characterizable products obtainable from these solutions were Ln3+ polyhydride anion complexes of composition, [K(chelate)]{[Cp'2Ln(μ–H)]3(μ–H)}. Reduction of 1-Y with KC8 in the presence of 2.2.2-cryptand also yields an intensely colored product with an axial EPR spectrum (gx = gy = 2.05, Ax = Ay = 35.5 G; gz = 2.07, Az = 34.5) similar to that of (Cp'3Y)1− ion, but crystals were not obtained from this system.

Studies of the decomposition of the ethylene hydrophenylation catalyst TpRu(CO)(NCMe)Ph

TpRu(CO)(NCMe)Ph is a catalyst for the conversion of benzene and ethylene to ethylbenzene. Previously, the formation of ethylbenzene has been shown to occur through a pathway that involves ethylene coordination to Ru, insertion of ethylene into the Ru–phenyl bond and Ru–mediated benzene C–H activation. The effect of ethylene pressure and catalyst concentration (between 0.2 and 0.01 mol % based on benzene) on the decomposition of TpRu(CO)(NCMe)Ph was examined. Studies have shown that there are two competing catalyst deactivation pathways. At higher concentrations of TpRu(CO)(NCMe)Ph, the dominant deactivation pathway is likely initiated by a binuclear reaction of two Ru complexes that leads to formation of unidentified paramagnetic species. Kinetic studies reveal that this pathway for catalyst decomposition occurs with a second-order rate of 0.007 (1) M−1 s−1. At lower Ru concentrations, ethylene C–H activation to form the allyl complex TpRu(CO)(η3-C4H7) is the predominant deactivation pathway. The effect of ethylene pressure on catalyst decomposition was also examined. At higher ethylene pressure nearly quantitative formation of TpRu(CO)(η3-C4H7) was observed.

Rich Coordination Chemistry of π-Acceptor Dibenzoarsole Ligands.

A series of dibenzoarsole (also known as 9-arsafluorene) derivatives have been prepared, and their coordination chemistry has been investigated. The different ligand topology and the arsenic substituents govern the reactivity of the ligands. We report various crystal structures of palladium and platinum complexes derived from this family of ligands. The biphenyl backbone of the bridged bidentate ligands allows very flexible coordination. We have also studied the application of an allylic Pd complex in nucleophilic substitution reactions, revealing that the benzoarsole substituent is susceptible to metal insertion.

A Cationic Unsaturated Platinum(II) Complex that Promotes the Tautomerization of Acetylene to Vinylidene

AbstractComplex [PtMe2(PMe2Ar )] (1), which contains a tethered terphenyl phosphine (Ar =2,6‐(2,6‐iPr2C6H3)2C6H3), reacts with [H(Et2O)2]BArF (BArF−=B[3,5‐(CF3)2C6H3]4−) to give the solvent (S) complex [PtMe(S)(PMe2Ar )]+ (2⋅S). Although the solvent molecule is easily displaced by a Lewis base (e.g., CO or C2H4) to afford the corresponding adducts, treatment of 2⋅S with C2H2 yielded instead the allyl complex [Pt(η3‐C3H5)(PMe2Ar )]+ (6) via the alkyne intermediate [PtMe(η2‐C2H2)(PMe2Ar )]+ (5). Deuteration experiments with C2D2, and kinetic and theoretical investigations demonstrated that the conversion of 5 into 6 involves a PtII‐promoted HC≡CH to :C=CH2 tautomerization in preference over acetylene migratory insertion into the Pt−Me bond.

A Cationic Unsaturated Platinum(II) Complex that Promotes the Tautomerization of Acetylene to Vinylidene.

Complex [PtMe2 (PMe2 ArDipp2 )] (1), which contains a tethered terphenyl phosphine (ArDipp2 =2,6-(2,6-i Pr2 C6 H3 )2 C6 H3 ), reacts with [H(Et2 O)2 ]BArF (BArF- =B[3,5-(CF3 )2 C6 H3 ]4- ) to give the solvent (S) complex [PtMe(S)(PMe2 ArDipp2 )]+ (2⋅S). Although the solvent molecule is easily displaced by a Lewis base (e.g., CO or C2 H4 ) to afford the corresponding adducts, treatment of 2⋅S with C2 H2 yielded instead the allyl complex [Pt(η3 -C3 H5 )(PMe2 ArDipp2 )]+ (6) via the alkyne intermediate [PtMe(η2 -C2 H2 )(PMe2 ArDipp2 )]+ (5). Deuteration experiments with C2 D2 , and kinetic and theoretical investigations demonstrated that the conversion of 5 into 6 involves a PtII -promoted HC≡CH to :C=CH2 tautomerization in preference over acetylene migratory insertion into the Pt-Me bond.

Selective hydrosilylation of alkynes and ketones: contrasting reactivity between cationic 3-iminophosphine palladium and nickel complexes.

The catalytic hydrosilylation of alkynes and ketones has been explored utilizing palladium- and nickel(allyl) complexes supported by 3-iminophosphine ligands. Palladium and nickel demonstrated distinctly different reactivity profiles, with palladium proving very effective for the hydrosilylation of electron-deficient alkynes, while nickel excelled with ketones and internal alkynes. Additionally, in many cases, regioselective hydrosilylation was observed.

Palladium allylic complexes with enantiopure bis(diamidophosphite) ligands bearing a cyclohexane-1,2-diamine skeleton as catalysts in the allylic substitution reaction

A series of cationic allyl palladium complexes [Pd(η3-CH3-C3H5)(P-P)]X (X = PF6, 2a-c, 2e; and X = BPh4, 3a, 3b, 3d, 3e) and [Pd(η3-1,3-Ph2-C3H3)(P-P)]X (X = PF6, 6b; and X = BPh4, 7a) have been prepared. The bis(diamidophosphite) ligands (P-P) contain a diazaphospholidine terminal fragment derived from (R,R)- and (S,S)-N,N’-dibenzyl- and (R,R)-N,N’-dimethyl-cyclohexane-1,2-diamines and dialcoxy bridging fragment derived from (R,R)- and (S,S)-butanediol, (R,R)-cyclohexanediol, (4R,5R)- and (4S,5S)-4,5-di(hydroxymethyl)-2,2-dimethyl-1,3-dioxolane and (R)- and (S)-binaphthol. Complexes [Pd(η3-CH3-C3H5l)P2]X (X = PF6, 4f, 4g; and X = BPh4, 5f), where P are monodentate diamidophosphite ligands with diazaphospholidine heterocyclic backbone obtained from (R,R)- and (S,S)-N,N’-dibenzylcyclohexane-1,2-diamine and alcoxy groups coming from (R)-phenyl-ethanol and (S)-borneol have been also prepared. Neutral palladium complexes [PdCl2(P-P)] (1a, 1c) were synthesized to prove the C2 symmetry of the P-P ligand. The new compounds were fully characterized in solution by NMR spectroscopy. The X-ray crystal structure determination for 2e-(R,R,Ral,Ral;R,R) and 1a-(S,S;Sal,Sal;S,S) has been achieved.The new allyl-palladium complexes were applied in the asymmetric allylic substitution reaction of the benchmark substrate rac-3-acetoxy-1,3-diphenyl-1-propene with dimethyl malonate and benzylamine as nucleophiles in order to test their catalytic potential. The best results were obtained with the 3a-(R,R;Ral,Ral;R,R) precursor (up to 84% ee) while complexes with the e ligand derived from the (R,R)-N,N’-dimethylcyclohexane-1,2-diamine terminal fragment resulted inactive in the process. The influence of the nature and the absolute configuration of both the bridging and the terminal fragments of the bis(diamidophosphite) ligand on the asymmetric induction is discussed. A preliminary study of the anion effect (PF6− vs. BPh4-) on the activity and the enantioselectivity of the Pd-catalysed allylic substitution has also been performed.

Functionalization of Methane Initiated by Cp*W(NO)(CH2CMe3)(η3-CH2CHCMe2)

Cp*W(NO)(CH2CMe3)(η3-CH2CHCMe2) (1) initiates a stepwise conversion of methane into unsymmetrical unsaturated ketones. The first step involves the activation of methane by 1 in cyclohexane to form Cp*W(NO)(CH3)(η3-CH2CHCMe2) (2) in good yield as a mixture of two isomers that differ with respect to the exo/endo orientation of their allyl ligands. Subsequent exposure of a cyclohexane solution of 2 to 500 psig of CO pressure and 75 °C for 3 h results in the 1,1-insertion of CO into the W–CH3 linkage of 2 and formation of the yellow acyl complex Cp*W(NO)(C(═O)CH3)(η3-CH2CHCMe2) (5). Additional carbonylation of 2 can be effected by maintaining the cyclohexane solution at 750 psig of CO pressure and 75 °C for 3 days, a process that results in the formation of the isomeric η2-unsaturated-ketone complexes Cp*W(NO)(CO)(η2-Me2C═CHCH2C(O)CH3) (6a) and Cp*W(NO)(CO)(η2-H2C═CHCMe2C(O)CH3) (6b) in a 60:40 ratio. Finally, exposure of 2 to CO under more forcing conditions (1000 psig at 170 °C for 3 days) produces Cp*W(NO)...

Investigating reduced metal species via sequential ion/ion and ion/molecule reactions: The reactions of transition metal phenanthrolines with allyl iodide

Sequential ion/ion and ion/molecule reactions are demonstrated in a linear ion trap mass spectrometer and are used to probe the reactivity of metal complexes in unusual ionization states. Taking advantage of the instrument’s electron transfer dissociation (ETD) capabilities, the mono- and bis-phenanthroline complexes of Fe(I), Co(I), Ni(I), Cu(I), and Zn(I) were formed by reduction of the corresponding M(II) species in an ion/ion reaction with the fluoranthene radical anion. The chemistry of the M(I) species was probed in ion/molecule reactions with allyl iodide. The bis-phenanthroline complexes generally give slow reactions and the metals were oxidized to M(II) iodide complexes with presumably the release of allyl radicals. The mono-phenanthroline complexes are much more reactive and give M(II) iodide complexes, M(II) allyl complexes, and adducts. The overall reactivity is in accord with density functional theory calculations and mirrors that of proposed intermediates in condensed-phase catalytic cycles.

ChemInform Abstract: Development of Nickel Hydrosilylation Catalysts

AbstractReview: [27 refs.

Fluxional allyl Pd(II) and Pt(II) complexes of NHC ligands derived from substituted 1,8-naphthalimides - Synthesis and structure elucidation

Six catalytically relevant Pd(II) and Pt(II) complexes involving N-heterocyclic carbenes (NHCs) derived from substituted 1,8-naphthalimides and allylic ligands were synthesized and characterized. The structures and conformations of the complexes were elucidated on the basis of combination of dynamic NMR spectroscopy and DFT studies. Conformational studies in respect to hindered rotation around Pd-C (Pt-C) bonds were performed. The results from dynamic NMR studies and DFT calculations confirmed hindered rotation around Pd-C (Pt-C) bonds.

Synthesis, characterization and a reactivity study of some allyl palladium complexes bearing bidentate hemi-labile carbene or mixed carbene/PPh3 ligands

With the aim at synthesizing novel allyl complexes that can potentially act as catalysts in the Tsuji–Trost catalyzed reaction, we have synthesized and characterized some allyl and 2-Meallyl palladium derivatives with one hemilabile bidentate or two strong mono-coordinating spectator ligands. The hemilabile ligands are constituted by one nitrogen heterocyclic carbene (NHC) fragment acting as the pivot bearing a labile wing with a pyridine nitrogen or sulfur atom as the second stabilizing atom. One of two monodentate ligands is in all cases PPh3 whereas the other is a mono- or partially coordinated hemilabile carbene. The complexes were characterized by standard spectroscopic methods and elemental analysis and in two cases by SC-XRD technique.The reactivity of two selected complexes toward the Tsuji–Trost reaction was tested by stoichiometric allyl amination carried out with piperidine and the results of such a mechanistic investigation integrated by a computational study are also reported in this paper.