Insights into CO/styrene copolymerization by using Pd(II) catalysts containing modular pyridine-imidazoline ligands

Continuing our studies into the effect that N-N' ligands have on CO/styrene copolymerization, we prepared new C(1)-symmetrical pyridine-imidazoline ligands with 4',5'-cis stereochemistry in the imidazoline ring (5) and 4',5'-trans stereochemistry (6-10) and compared them with our previously reported ligands (1-4). Their coordination to neutral methylpalladium(II) (5 a-10 a) and cationic complexes (5 b-10 b), investigated in solution by NMR spectroscopy, indicates that both the electronic and steric properties of the imidazolines determine the stereochemistry of the palladium complexes. The crystal structures of two neutral palladium precursors [Pd(Me)(2-n)Cl(n)(N-N')] (n=1 for 8 a; n=2 for 9 a') show that the Pd-N coordination distances and the geometrical distortions in the imidazoline ring depend on the electronic nature of the substituents in the imidazoline fragment. Density functional calculations performed on selected neutral and cationic palladium complexes compare well with NMR and X-ray data. The calculations also account for the formation of only one or two stereoisomers of the cationic complexes. The performance of the cationic complexes as catalyst precursors in CO/4-tert-butylstyrene copolymerization under mild pressures and temperatures was analyzed in terms of the productivity and degree of stereoregularity of the polyketones obtained. Insertion of CO into the Pd-Me bond, which was monitored by multinuclear NMR spectroscopy, shows that the N ligand influences the stereochemistry of the acyl species formed.     

Ethene/norbornene copolymerization with palladium(II) alpha-diimine catalysts: from ligand screening to discrete catalyst species

Sixteen palladium(II) alpha-diimine catalysts were investigated in a screening-like procedure for the copolymerization of ethene with norbornene. The resulting copolymers were characterized by (13)C NMR spectroscopy, differential scanning calorimetry, gel permeation chromatography, and viscosimetry. The degree of incorporation of norbornene in the polymer chain is very high for most of the catalysts. To validate the results achieved in the screening, two catalysts, [[ArN=CHCH=NAr]Pd(Me)(CH(3)CN)]BAr(f) (4) (1 b'; Ar=2,6-Me(2)C(6)H(3), BAr(f) (4)=B[3,5-C(6)H(3)(CF(3))(2)](4)) and [[ArN=C(CH(3))C(CH(3))=NAr]Pd(Me)(CH(3)CN)]BAr(f) (4) (2 c'; Ar=2,6-iPr(2)C(6)H(3)), were synthesized as discrete catalytically active species, and their copolymerization behavior was investigated in detail. In agreement with the screening results, 1 b' incorporates norbornene much better in the polymer chain than ethene, a property that has no analogue in metallocene catalysts.     

The mechanism of the hydroalkoxycarbonylation of ethene and alkene-CO copolymerization catalyzed by Pd(II)-diphosphine cations

All the intermediates in the "carboalkoxy" pathway, and their interconversions giving complete catalytic cycles, for palladium-diphosphine-catalyzed hydroalkoxycarbonylation of alkenes, and for alkene-CO copolymerization, have been demonstrated using (31)P{(1)H} and (13)C{(1)H} NMR spectroscopy. The propagation and termination steps of the "hydride" cycles and the crossover between the hydride and carboalkoxy cycles have also been demonstrated, providing the first examples of both cycles, and of chain crossover, being delineated for the same catalyst. Comparison of the propagation and termination steps in the pathways affords new insight into the selectivity-determining steps. Thus, reaction of [Pd(dibpp)(CH(3)CN)(2)](OTf)(2) (dibpp = 1,3-(iBu(2)P)(2)C(3)H(6)) with Et(3)N and CH(3)OH affords [Pd(dibpp)(OCH(3))(CH(3)CN)]OTf, which, on exposure to CO, gives [Pd(dibpp){C(O)OCH(3)}(CH(3)CN)]OTf immediately. Labeling studies show the reaction to be readily reversible. However, the back reaction is strongly inhibited by PPh(3), indicating an insertion/deinsertion pathway. Ethene reacts with [Pd(dibpp){C(O)OCH(3)}(CH(3)CN)]OTf at 243 K to give [Pd(dibpp){CH(2)CH(2)C(O)OCH(3)}]OTf, that is, there is no intrinsic barrier to alkene insertion into the Pd--C(O)OMe bond, as had been proposed. Instead, termination is proposed to be selectivity determining. Methanolysis of the acyl intermediate [Pd(dibpp){C(O)CH(3)}L]X (L = CO, CH(3)OH; X = CF(3)SO(3) (-) (OTf(-)), CH(3)C(6)H(4)SO(3) (-) (OTs(-))) is required in the hydride cycle to give an ester and occurs at 243 K on the timescale of minutes, whereas methanolysis of the beta chelate, required to give an ester from the carbomethoxy cycle, is slow on a timescale of days, at 298 K. These results suggest that slow methanolysis of the beta chelate, rather than slow insertion of an alkene into the Pd--carboalkoxy bond, as had previously been proposed, is responsible for the dominance of the hydride mechanism in hydroalkoxycarbonylation.     

Long-lived palladium catalysts for CO/vinyl arene polyketones synthesis: a solution to deactivation problems

A series of cationic palladium complexes of general formula [Pd(Me)(MeCN)(N-N)][PF(6)] (N-N = (phen) 1 a, 4,7-dichloro-1,10-phenanthroline (4,7-Cl(2)-phen) 2 a, 4,7-diphenyl-1,10-phenanthroline (4,7-Ph(2)-phen) 3 a, 4-methyl-1,10-phenanthroline (4-Me-phen) 4 a, 4,7-dimethyl-1,10-phenanthroline (4,7-Me(2)-phen) 5 a, 5,5,6,6-tetrafluoro-5,6-dihydro-1,10-phenanthroline (F(4)-phen) 6 a, containing different substituted phenanthroline ligands, have been prepared from the corresponding neutral chloro derivatives [Pd(Me)(Cl)(N-N)], (1 b-6 b). The X-ray crystal structure of [Pd(Cl)(2)(4,7-Cl(2)-phen)] (2 b') was determined. DFT calculations show that the electron density on the metal is tuned by the substituents on the ligands. The catalytic behavior of complexes 1 a-6 a in the CO/styrene and CO/p-Me-styrene copolymerizations was studied in detail, showing that the generated catalysts are active for at least 90 h, yielding copolymers of high molecular weight. A firm correlation between the electron density on palladium on the one hand and the catalytic activity of the complexes and the molecular weight and the stereochemistry of the polyketones synthesized on the other hand has been established: the catalyst containing the F(4)-phen is thus far the most active among those tested, yielding the syndiotactic CO/styrene copolymer with a stereoregularity of 96 % (uu triad) and with an M(w) value of 1 000 000.     

Mono- and dinuclear bioxazoline-palladium complexes for the stereocontrolled synthesis of CO/styrene polyketones

The coordination chemistry of the chiral bioxazoline ligand (4S,4'S)-2,2'-bis(4-isopropyl-4,5-dihydrooxazole) to Pd(II) provides evidence that the ligand bonding can occur either through chelation of one Pd(II) ion leading to a mononuclear species with the expected cis geometry, or by double bridging of two Pd(II) ions giving a dinuclear complex with trans geometry. The species in solution are identified by 1H NMR spectroscopy. Both the mononuclear and the dinuclear complexes promote the CO/styrene copolymerization, yielding the corresponding polyketone with a fully or a predominantly isotactic microstructure, depending on the reaction medium. The nature of the anion present in the palladium precatalysts affects the polyketone stereochemistry. MALDI-TOF analysis of the copolymers synthesized reveals the presence of p-hydroxyphenolic end-groups, thus confirming and explaining the role of 1,4-hydroquinone as a molecular weight regulator.     

N,N′-bis(substituted-phenyl)oxamides and their dinuclear pentacoordinate nickel(II) complexes

The preparation, spectroscopic characterization and magnetic study of N,N′-bis(substituted-phenyl)oxamidate-bridged nickel(II) dinuclear complexes of formula {[Ni(N₃-mc)]₂(μ-CONC₆H₄-X)}(PF₆)₂ (N₃-mc = 2,4,4-trimethyl-1,5,9-triazacyclo-dodec-1-ene (Me₃-N₃-mc) or 2,4,4,9-tetramethyl-1,5,9-triazacyclododec-1-ene (Me₄-N₃-mc), X = 2-Cl, 4-Cl, 2-OCH₃, 4-OCH₃) are reported. These paramagnetic nickel(II) complexes have been characterized by both one- and two-dimensional (COSY) ¹H NMR techniques. The COSY spectrum of 5 has allowed to achieve the assignment of the phenyl protons of the N,N′-diphenyloxamidate. The crystal structures of [Ni(Me₃-N₃-mc)(μ-CONC₆H₄-4-Cl)]₂(PF₆)₂ (6), [Ni(Me₃-N₃-mc)(μ-CONC₆H₄-4-OMe)]₂(PF₆)₂ (8) and [Ni(Me₄-N₃-mc)(μ-CONC₆H₄-2-Cl)]₂(PF₆)₂ (9) have been determined and their magnetic properties have been studied. The value of magnetic coupling between the two nickel(II) ions across the oxamidate bridge [J = − 37.6 (6), −39.9 (8) and −39.7 cm⁻¹ (9)] is sensitive to the distortion of the coordination sphere of the metal ions and the topology of the molecular bridge.     

Effect of 5-Me substituent(s) on the catalytic activity of palladium(II) 2,2-bipyridine complexes in CO/4-tert-butylstyrene copolymerization

The coordination of two 5-substituted-2,2^′-bipyridines L (L¹=5-methyl-2,2^′-bipyridine, L²=5,5^′-dimethyl-2,2^′-bipyridine) to palladium was studied. The neutral complexes [Pd(L)Cl₂] and [Pd(L)(Me)Cl], and the cationic complexes obtained after chlorine abstraction [Pd(L)₂][BAr^′₄]₂ and [Pd(L)(Me)(NCMe)][BAr^′₄] (Ar^′=3,5-(CF₃)₂-C₆H₃), respectively, were isolated and characterized by NMR and FAB mass spectroscopy. The complex [Pd(L²)(L³)][BAr^′₄]₂ (L³=2,2^′-bipyridine) bearing different ligands, was prepared for comparison purposes. The activity of the monocationic and dicationic complexes as catalytic precursors in the CO/4-tert-butylstyrene copolymerization was compared with that of related well-known catalysts containing the unsubstituted 2,2^′-bipyridine as nitrogen ligand, to evaluate the influence of the substituents in 5- and 5,5^′-position. The presence of one or two substituents on the nitrogen ligand has a positive effect on productivity using both types of precursors. No influence was observed on the polymer properties in terms of molecular weight and tacticity. Analysis of the reactivity of the methyl-palladium complexes towards carbon monoxide shows further differences depending on the nitrogen ligand.     

Origin of the regio- and stereoselectivity in palladium-catalyzed electrophilic substitution via bis(allyl)palladium complexes

Palladium-catalyzed allylic substitution of aryl allyl chlorides with aromatic and heteroaromatic aldehydes was performed in the presence of hexamethylditin. This procedure involves palladium-catalyzed formation of transient allylstannanes followed by generation of a bis(allyl)palladium intermediate, which subsequently reacts with the aldehyde electrophile. The catalytic substitution reaction proceeds with high regio- and stereoselectivity. The stereoselectivity is affected by the steric and electronic properties of the allylic substituents. Various functionalities including NO(2), COCH(3), Br, and F groups are tolerated under the applied catalytic conditions. Density functional calculations at the B3PW91/DZ+P level of theory were applied to study the steric and electronic effects controlling the regio- and stereoselectivity of the electrophilic addition. The development of the selectivity was studied by modeling the various bis(allyl)palladium species occurring in the palladium-catalyzed substitution of cinnamyl chloride with benzaldehyde. It was found that the electrophilic attack proceeds via a six-membered cyclic transition state, which has a pronounced chair conformation. The regioselectivity of the reaction is controlled by the location of the phenyl group on the eta(1)-allyl moiety of the complex. The stereoselectivity of the addition process is determined by the relative configuration of the phenyl substituents across the developing carbon-carbon bond. The lowest energy path corresponds to the formation of the branched allylic isomer with the phenyl groups in anti configuration, which is in excellent agreement with the experimental findings.     

Study of electronic effect in bifunctional catalysts for the copolymerization of CO2 and PO/CHO

Carbon dioxide (CO₂) is an easily available renewable carbon source that can be used as a comonomer in the catalytic ring-opening polymerization of epoxides to form aliphatic polycarbonates. Herein, a series of new Salen-Co(III) bifunctional catalysts were synthesized for the first time, and they were studied to catalyze the copolymerization of CO₂ and propylene oxide (PO)/cyclohexene oxide (CHO). At the same time, the effects of reaction conditions (electronic effect, temperature, time) on catalytic activity and selectivity were investigated. The results show that the Salen-Co(III) complexes with electron-withdrawing groups have higher selectivity and activity for propylene carbonate (PPC)/cyclohexylene carbonate (PCHC). At the same time, the Salen-Co(III) complexes can better catalyze the copolymerization of CHO and CO₂ than that of PO and CO₂. The catalytic efficiency of the four complexes increased with increasing temperature, and the best reaction condition is 80°C, 30 min and 2 MPa of CO₂.     

Double bonds in motion: bis(oxazolinylmethyl)pyrroles and their metal-induced planarization to a new class of rigid chiral C2-symmetric complexes

The synthesis of a new class of chiral C(2)-symmetric tridentate N-donor ligands, a series of 2,5-bis(2-oxazolinylmethyl)pyrroles, was achieved in four steps starting from the known 2,5-bis(trimethylammoniomethyl)pyrrole diiodide (1). Reaction of 1 with NaCN in dimethyl sulfoxide gave 2,5-bis(cyanomethyl)pyrrole (2) cleanly, which was then cyclized with amino alcohols to give the 2,5-bis(2-oxazolinylmethyl)pyrroles 3 a-c (3 a: bis[2-(4,4'-dimethyl-5-hydrooxazolyl)methyl]pyrrole; 3 b: (S,S)-bis[2-(4-isopropyl-4,5-dihydrooxazolyl)methyl]pyrrole; 3 c: (S,S)-bis[2-(4-tertiobutyl-4,5-dihydrooxazolyl)methyl]pyrrole). Metallation of 3 a-c with one molar equivalent of tBuLi and their subsequent reaction with a stoichiometric amount of [PdCl(2)(cod)] (cod=cyclooctadiene) gave the palladium(II) complexes 4 a-c. Whereas the arrangement of the N-donor atoms in the crystallographically characterized complex 4 a is almost ideally square planar, all three heterocycles in the ligand are twisted out of the coordination plane, leading to a chiral conformation of the complex. Attempts to freeze out these two conformers in solution at 200 K (NMR) failed, and this suggests that the activation barrier for conformational racemization is significantly below 10 kcal mol(-1). The palladium-induced shift of two double bonds as well as the porphyrinogen/porphyrin-type oxidation of the complexes 4 a-c led to the planarization of the 2,5-bis(oxazolinylmethyl)pyrrolide ligands in the palladium(II) complexes 5 a-c, 6 b, and 6 c, and to the formation of rigid chiral C(2)-symmetric systems as shown by X-ray diffraction studies. The formation of the conjugated system of double bonds in this transformation is accompanied by the emergence of an intra-ligand chromophore. This is evident in the absorption spectrum of 6 c which displays an intense band with a maximum at 485 nm attributable to an intra-ligand pi*<--pi transition and a characteristic vibrational progression of nu approximately 1350 cm(-1). Complexes 4 b and 4 c were tested in the catalytic asymmetric Michael addition of ethyl 2-cyanopropionate to methyl vinylketone (catalyst loading: 1 mol %) and were found to give maximum ee values of 43 % (4 b) and 21 % (4 c) at low conversions.     

Novel Oligopyrrole Carboxamide based Nickel(II) and Palladium(II) Salens,Their Targeting of Human G‐Quadruplex DNA,and Selective Cancer Cell Toxicity

DNA targeting by various metal complexes is a key strategy toward the restriction of cancer cell proliferation. Toward this end, we designed and synthesized novel salen‐based Ni^II and Pd^II metal complexes with positively charged flanking side chains comprising N‐methylpyrrole carboxamides of varying lengths. The compounds showed high specificity toward G‐quadruplex DNA over duplex DNA. Sufficient inhibition of the telomerase activity was observed, which was ascertained by the prominent restriction of cancer cell proliferation in the long‐term cell viability and telomerase inhibition assays. The compounds exhibited selective cancer cell death following an apoptotic pathway. Analysis of the binding mode showed partial stacking of the salen moiety over the G‐tetrads and association of the pendant oligopyrrole carboxamide units with the grooves. The conjugation of the tetrad‐binding metal salen core with groove‐oriented flexible oligopyrrole moieties resulted in the high selectivity and stabilization of the human G‐quadruplex DNA structures.     

(1)H and (19)F PGSE diffusion and HOESY NMR studies on cationic palladium (II) 1,3-diphenylallyl complexes in THF solution

THF solutions of the cationic chiral 1,3-diphenylallyl bidentate phosphine complexes [Pd(eta(3)-PhCHCHCHPh)(Duphos)](CF(3)SO(3)), Duphos = 1,2-Bis-((2R,5R)-2,5-dimethylphospholano)benzene), 2, and [Pd(eta(3)-PhCHCHCHPh)(P,S)]BF(4), 4, P,S = [8-((o-(diphenylphosphino)benzyl) thiomethyl]-(7,7'-dimethyl)-exo-norborneol, have been studied via pulsed gradient spin-echo (PGSE) diffusion, (1)H, (19)F HOESY and a variety of other multi-dimensional NMR methods. On the basis of the (1)H, (19)F HOESY data, the anions show a preference for a specific structural position with respect to the eta(3)-PhCHCHCHPh allyl ligand, i.e. the anion does not move evenly around the periphery of the cation. THF is shown to promote significant ion pairing, although neither 2 nor 4 shows 100% ion pairing.     

Pyrrole-appended derivatives of O-confused oxaporphyrins and their complexes with nickel(II), palladium(II), and silver(III)

Condensation of 2,4-bis(phenylhydroxymethyl)furan with pyrrole and p-toluylaldehyde formed, instead of the expected 5,20-diphenyl-10,15-di(p-tolyl)-2-oxa-21-carbaporphyrin, a pyrrole addition product [(H,pyr)OCPH]H(2); this product can formally be considered as an effect of hydrogenation of 3-(2'-pyrrolyl)-5,20-diphenyl-10,15-di(p-tolyl)-2-oxa-21-carbaporphyrin ([(pyr)OCPH]H). The new oxacarbaporphyrinoid presents the (1)H NMR spectroscopy features of an aromatic molecule, including the upfield shift of the inner H21 atom. Insertion of NiCl(2) or PdCl(2) into [(H,pyr)OCPH]H(2) gave two structurally related organometallic complexes, [(pyr)OCP]Ni(II)] and [(pyr)OCP]Pd(II)], in which the metal ions are bound by three pyrrolic nitrogens and the trigonally hybridized C21 atom of the inverted furan. The reaction of [(H,pyr)OCPH]H(2) with silver(I) acetate yields a stable Ag(III) complex [(C(2)H(5)O,pyr)OCP]Ag(III)] substituted at the C3 position by the ethoxy and pyrrole moieties. The macrocyclic frame of [(H,pyr)OCPH]H(2) is conserved. Addition of trifluoroacetic acid to [(C(2)H(5)O,pyr)OCP]Ag(III)] yielded a new aromatic complex [(pyr)OCP]Ag(III)](+). The structures of [(pyr)OCP]Ni(II)] and [(C(2)H(5)O,pyr)OCP]Ag(III)] have been determined by X-ray crystallography. In both molecules the macrocycles are only slightly distorted from planarity and the nickel(II) and silver(III) are located in the NNNC plane. The dihedral angle between the macrocyclic and appended-pyrrole planes of [(pyr)OCP]Ni(II)] reflects the biphenyl-like arrangement with the NH group pointing out toward the adjacent phenyl ring on the C5 position. Tetrahedral geometry around the C3 atom was detected for [(C(2)H(5)O,pyr)OCP]Ag(III)]. The Ni[bond]C and Ag[bond]C bond lengths are similar to other nickel(II) or silver(III) carbaporphyrinoids where the trigonal carbon atom coordinates the metal ion. The trend detected in the (13)C chemical shifts for the appended-pyrrole resonances has been rationalized by the extent of effective conjugation between the macrocycle and the appended pyrrole moiety controlled by the hybridization of the C3 atom and the metal ion oxidation state. The dianionic or trianionic macrocyclic core of the pyrrole-appended derivatives is favored to match the oxidation state of nickel(II), palladium(II), or silver(III), respectively.     

NMR studies on a palladium(II) complex containing an incompletely coordinated facultative pentadentate Schiff base ligand

Palladium(II) dichloride reacts with 1,10‐bis(2‐pyrrolyl)‐2,5,9‐triaza‐1,9‐decadiene to give a [Pd(C₁₅H₂₀N₅)]Cl complex in which the ligand is four‐coordinated, leaving one pyrrole group dangling. By using COSY, gHSQC, gHMBC connectivities and NOE experiments it has been concluded that one linkage isomer exists in DMSO solution, in spite of the fact that different sets of N atoms of potentially pentadentate ligand might be involved in coordination, and that the three chelate rings in the complex cation are arranged in a sequence: five‐membered, six‐membered, five‐membered which is different from that (5–5–6) found by x‐ray studies on the related [Ni(C₁₅H₂₀N₅)]Cl compound. NMR studies allowed an unambiguous assignment of all ¹H and ¹³C NMR resonances for the complex. Results of x‐ray structural analysis of [Pd(C₁₅H₂₀N₅)](CH₃COO)H₂O supported the five‐membered, six‐membered, five‐membered ring sequence in the [Pd(C₁₅H₂₀N₅)]⁺ complex cation and show an E (trans) orientation of the dangling pyrrole group with respect to the metal center. Copyright © 2002 John Wiley & Sons, Ltd.