Mechanistic Aspects of the Copolymerization Reaction of Carbon Dioxide and Epoxides, Using a Chiral Salen Chromium Chloride Catalyst

The air-stable, chiral (salen)CrIIICl complex (3), where H2salen = N,N‘-bis(3,5-di-tert-butyl-salicylidene)-1,2-cyclohexene diamine, has been shown to be an effective catalyst for the coupling of cyclohexene oxide and carbon dioxide to afford poly(cyclohexenylene carbonate), along with a small quantity of its trans-cyclic carbonate. The thus produced polycarbonate contained >99% carbonate linkages and had a Mn value of 8900 g/mol with a polydispersity index of 1.2 as determined by gel permeation chromatography. The turnover number (TON) and turnover frequency (TOF) values of 683 g of polym/g of Cr and 28.5 g of polym/g of Cr/h, respectively for reactions carried out at 80 °C and 58.5 bar pressure increased by over 3-fold upon addition of 5 equiv of the Lewis base cocatalyst, N-methyl imidazole. Although this chiral catalyst is well documented for the asymmetric ring-opening (ARO) of epoxides, in this instance the copolymer produced was completely atactic as illustrated by 13C NMR spectroscopy. Whereas the mechanism for the (salen)CrIII-catalyzed ARO of epoxides displays a squared dependence on [catalyst], which presumably is true for the initiation step of the copolymerization reaction, the rate of carbonate chain growth leading to copolymer or cyclic carbonate formation is linearly dependent on [catalyst]. This was demonstrated herein by way of in situ measurements at 80 °C and 58.5 bar pressure. Hence, an alternative mechanism for copolymer production is operative, which is suggested to involve a concerted attack of epoxide at the axial site of the chromium(III) complex where the growing polymer chain for epoxide ring-opening resides. Preliminary investigations of this (salen)CrIII-catalyzed system for the coupling of propylene oxide and carbon dioxide reveal that although cyclic carbonate is the main product provided at elevated temperatures, at ambient temperature polycarbonate formation is dominant. A common reaction pathway for alicyclic (cyclohexene oxide) and aliphatic (propylene oxide) carbon dioxide coupling is thought to be in effect, where in the latter instance cyclic carbonate production has a greater temperature dependence compared to copolymer formation

Mechanistic Aspects of the Copolymerization Reaction of Carbon Dioxide and Epoxides, Using a Chiral Salen Chromium Chloride Catalyst

The air-stable, chiral (salen)CrIIICl complex (3), where H2salen = N,N‘-bis(3,5-di-tert-butyl-salicylidene)-1,2-cyclohexene diamine, has been shown to be an effective catalyst for the coupling of cyclohexene oxide and carbon dioxide to afford poly(cyclohexenylene carbonate), along with a small quantity of its trans-cyclic carbonate. The thus produced polycarbonate contained >99% carbonate linkages and had a Mn value of 8900 g/mol with a polydispersity index of 1.2 as determined by gel permeation chromatography. The turnover number (TON) and turnover frequency (TOF) values of 683 g of polym/g of Cr and 28.5 g of polym/g of Cr/h, respectively for reactions carried out at 80 °C and 58.5 bar pressure increased by over 3-fold upon addition of 5 equiv of the Lewis base cocatalyst, N-methyl imidazole. Although this chiral catalyst is well documented for the asymmetric ring-opening (ARO) of epoxides, in this instance the copolymer produced was completely atactic as illustrated by 13C NMR spectroscopy. Whereas the mechanism for the (salen)CrIII-catalyzed ARO of epoxides displays a squared dependence on [catalyst], which presumably is true for the initiation step of the copolymerization reaction, the rate of carbonate chain growth leading to copolymer or cyclic carbonate formation is linearly dependent on [catalyst]. This was demonstrated herein by way of in situ measurements at 80 °C and 58.5 bar pressure. Hence, an alternative mechanism for copolymer production is operative, which is suggested to involve a concerted attack of epoxide at the axial site of the chromium(III) complex where the growing polymer chain for epoxide ring-opening resides. Preliminary investigations of this (salen)CrIII-catalyzed system for the coupling of propylene oxide and carbon dioxide reveal that although cyclic carbonate is the main product provided at elevated temperatures, at ambient temperature polycarbonate formation is dominant. A common reaction pathway for alicyclic (cyclohexene oxide) and aliphatic (propylene oxide) carbon dioxide coupling is thought to be in effect, where in the latter instance cyclic carbonate production has a greater temperature dependence compared to copolymer formation

Toward the Design of Double Metal Cyanides for the Copolymerization of CO₂ and Epoxides

Toward the Design of Double Metal Cyanides for the Copolymerization of CO2 and Epoxide

Synthesis and Structures of Nickel and Palladium Salicylaldiminato 1,3,5-Triaza-7-phosphaadamantane (PTA) Complexes

The synthesis of nickel(II) and palladium(II) salicylaldiminato complexes incorporating the water-soluble phosphine 1,3,5-triaza-7-phosphaadamantane(PTA) has been achieved employing two preparative routes. Reaction of the original ethylene polymerization catalyst developed by Grubbs and co-workers (Organometallics 1998, 17, 3149), (salicylaldiminato)Ni(Ph)PPh3, with PTA using a homogeneous methanol/toluene solvent system resulted in the formation of the PTA analogues in good yields. Alternatively, complexes of this type may be synthesized via a direct approach utilizing (tmeda)M(CH3)2 (M = Ni, Pd), the corresponding salicylaldimine, and PTA. Yields by this method were generally near quantitative. The complexes were characterized in solution by 1H/13C/31P NMR spectroscopy and in the solid-state by X-ray crystallography. All derivatives exhibited square-planar geometry with the bulky isopropyl groups on the aniline being perpendicular to the plane formed by the metal center and its four ligands. Such orientation of these sterically encumbering groups is responsible for polymer chain growth during olefin polymerization in favor of chain termination via β-hydride elimination. Polymerization reactions were attempted using the nickel−PTA complexes in a biphasic toluene/water mixture in an effort to initiate ethylene polymerization by trapping the dissociated phosphine ligand in the water layer, thereby eliminating the need for a phosphine scavenger. Unfortunately, because of the strong binding ability of the small, donating phosphine(PTA) as compared to PPh3, phosphine dissociation did not occur at a temperature where the complexes are thermally stable

Synthesis and Structures of Nickel and Palladium Salicylaldiminato 1,3,5-Triaza-7-phosphaadamantane (PTA) Complexes

The synthesis of nickel(II) and palladium(II) salicylaldiminato complexes incorporating the water-soluble phosphine 1,3,5-triaza-7-phosphaadamantane(PTA) has been achieved employing two preparative routes. Reaction of the original ethylene polymerization catalyst developed by Grubbs and co-workers (Organometallics 1998, 17, 3149), (salicylaldiminato)Ni(Ph)PPh3, with PTA using a homogeneous methanol/toluene solvent system resulted in the formation of the PTA analogues in good yields. Alternatively, complexes of this type may be synthesized via a direct approach utilizing (tmeda)M(CH3)2 (M = Ni, Pd), the corresponding salicylaldimine, and PTA. Yields by this method were generally near quantitative. The complexes were characterized in solution by 1H/13C/31P NMR spectroscopy and in the solid-state by X-ray crystallography. All derivatives exhibited square-planar geometry with the bulky isopropyl groups on the aniline being perpendicular to the plane formed by the metal center and its four ligands. Such orientation of these sterically encumbering groups is responsible for polymer chain growth during olefin polymerization in favor of chain termination via β-hydride elimination. Polymerization reactions were attempted using the nickel−PTA complexes in a biphasic toluene/water mixture in an effort to initiate ethylene polymerization by trapping the dissociated phosphine ligand in the water layer, thereby eliminating the need for a phosphine scavenger. Unfortunately, because of the strong binding ability of the small, donating phosphine(PTA) as compared to PPh3, phosphine dissociation did not occur at a temperature where the complexes are thermally stable

Synthesis and Characterization of a Monocyanide-Bridged Bimetallic Iron(II) and Copper(I) Complex

Synthesis and Characterization of a Monocyanide-Bridged Bimetallic Iron(II) and Copper(I) Comple

2-Thia-1,3,5-triaza-7-phosphaadamantane 2,2-Dioxide (PASO₂). Comparative Structural and Reactivity Investigation with the Water-Soluble Phosphine Ligand 1,3,5-triaza-7-phosphaadamantane (PTA)

The derivative 2-thia-1,3,5-triaza-phosphaadamantane 2,2-dioxide (PASO2) has been characterized by spectroscopy and by X-ray crystallography. Unlike its PTA (1,3,5-triaza-7-phosphaadamantane) analogue, replacement of a −CH2− unit in PTA with −SO2− renders PASO2 largely insoluble in water at ambient temperature (<0.70 g/L vs 236 g/L for PTA). This latter property makes it of no use as a water-solubilizing ligand for organometallic complexes. Interestingly, alkylation of PASO2 with methyl iodide occurs quantitatively at the phosphorus center to afford the iodide salt. This derivative has also been characterized by X-ray crystallography and 1H/31P NMR spectroscopy, where the 31P resonance was observed to be shifted downfield relative to PASO2 (−39.2 vs −115.9 ppm). Although PTA has long been thought to react with methyl iodide to provide exclusively a monoalkylated product at one of the nitrogen centers, a close examination of this process via 31P NMR spectroscopy has shown monomethylation to take place at both nitrogen and phosphorus, with the latter being the minor product. Computational studies (DFT and HF) were performed in an effort to explain this difference in reactivity; however, no definitive conclusions could be reached from these calculations. Comparative studies (based on CO stretching frequencies and force constants) of group 6 metal carbonyl derivatives of PTA and PASO2 showed these ligands to be electronically extremely similar. This conclusion was supported by the bonding parameters determined in W(CO)5PTA and W(CO)5PASO2 complexes by X-ray crystallography

Synthesis and Structures of (Dialkylamino)ethylcyclopentadienyl Derivatives of Zinc

Lithium salts of (dialkylamino)ethylcyclopentadienyl react with either ZnCl2 or Zn(OAc)2 to afford dimeric derivatives containing bridging chlorides or acetates, respectively. X-ray structural determinations of three such derivatives show that the cyclopentadienyl ligands are bound to the zinc centers in an η1 fashion and via their amine groups, thereby leading to a stable six-membered metallacycle

2-Thia-1,3,5-triaza-7-phosphaadamantane 2,2-Dioxide (PASO₂). Comparative Structural and Reactivity Investigation with the Water-Soluble Phosphine Ligand 1,3,5-triaza-7-phosphaadamantane (PTA)

The derivative 2-thia-1,3,5-triaza-phosphaadamantane 2,2-dioxide (PASO2) has been characterized by spectroscopy and by X-ray crystallography. Unlike its PTA (1,3,5-triaza-7-phosphaadamantane) analogue, replacement of a −CH2− unit in PTA with −SO2− renders PASO2 largely insoluble in water at ambient temperature (<0.70 g/L vs 236 g/L for PTA). This latter property makes it of no use as a water-solubilizing ligand for organometallic complexes. Interestingly, alkylation of PASO2 with methyl iodide occurs quantitatively at the phosphorus center to afford the iodide salt. This derivative has also been characterized by X-ray crystallography and 1H/31P NMR spectroscopy, where the 31P resonance was observed to be shifted downfield relative to PASO2 (−39.2 vs −115.9 ppm). Although PTA has long been thought to react with methyl iodide to provide exclusively a monoalkylated product at one of the nitrogen centers, a close examination of this process via 31P NMR spectroscopy has shown monomethylation to take place at both nitrogen and phosphorus, with the latter being the minor product. Computational studies (DFT and HF) were performed in an effort to explain this difference in reactivity; however, no definitive conclusions could be reached from these calculations. Comparative studies (based on CO stretching frequencies and force constants) of group 6 metal carbonyl derivatives of PTA and PASO2 showed these ligands to be electronically extremely similar. This conclusion was supported by the bonding parameters determined in W(CO)5PTA and W(CO)5PASO2 complexes by X-ray crystallography

Synthesis and Structural Characterization of Double Metal Cyanides of Iron and Zinc: Catalyst Precursors for the Copolymerization of Carbon Dioxide and Epoxides

Several synthetic approaches for the preparation of double metal cyanide (DMC) derivatives of iron(II) and zinc(II) are described. These include (1) metathesis reactions of ZnCl2 or ZnI2 with KCpFe(CN)2CO in aqueous solution, (2) reactions of KCpFe(CN)2CO and its phosphine-substituted analogues with Zn(CH3CN)4(BF4)2 and subsequent displacement of acetonitrile at the zinc centers by the addition of a neutral (phosphine) or anionic (phenoxide) ligand, and (3) reactions of the protonated HCpFe(CN)2(phosphine) complexes with Zn(N(SiMe3)2)2, followed by the addition of phenols. All structures are based on a diamond-shaped planar arrangement of the Fe2(CN)4Zn2 core with various appended ligands at the metal sites. Although attempts to replace the iodide ligands in [CpFe(μ-CN)2PPh3ZnI(THF)]2 with acetate using silver acetate failed, two novel cationic mixed-metal cyanide salts based on the [CpFe(PPh3)(μ-CN)2Zn(NC5H5)]22+ framework were isolated from pyridine solution and their structures were defined by X-ray crystallography. The anionic ligand bound to zinc in these derivatives, which serve as an anionic polymerization initiator, was shown to be central to the catalytic copolymerization reaction of CO2/epoxide to provide polycarbonates and cyclic carbonates. The structurally stabilized phosphine-strapped complexes [CpFe(μ-CN)2Zn(X)THF]2(μ-dppp), where X = I or phenolate, were shown to be thermally stable under the conditions (80 °C) of the copolymerization reaction by in situ infrared spectroscopy. Both of these derivatives were proposed to serve as mimics for the heterogeneous DMC catalysts in the patent literature, with the derivative where the initiator is a phenolate being more active for the production of polycarbonates