No abstract

Dimeric phenoxide derivatives of zinc and cadmium have been synthesized from the reaction of the corresponding metal bistrimethylsilylamide and two equivalents of 2,6-F2C6H3OH in tetrahydrofuran. The zinc analogue, [Zn(O-2,6-F2C6H3)2·THF]2 (1), has been characterized in the solid state via X-ray crystallography, where the zinc centers are shown to possess distorted tetrahedral geometry containing two bridging phenoxides and a terminal phenoxide and THF ligand. The distance between the metal centers (Zn···Zn) was found to be 3.059 Å, and the THF ligands lie on opposite sides of the plane formed by the two zinc atoms and two bridging phenoxide ligands' oxygen atoms. There are several Zn···F nonbonding distances involving the bridging phenoxide ligands that are less than the van der Waals internuclear distance. In addition, both the zinc and cadmium dimeric derivatives have been prepared such that the labile THF ligands are replaced by the sterically encumbering basic phosphine, PCy3. The solid-state structures of [Zn(O-2,6-F2C6H3)2·PCy3]2 (2) and [Cd(O-2,6-F2C6H3)2·PCy3]2 (5) are similar to that of complex 1, where the tricyclohexylphosphine ligands, like the THF ligands, are accommodated in a trans configuration. The 31P NMR spectrum of complex 2 in C6D6 upon addition of free PCy3 exhibits sharp resonances assigned to both the complex (9.58 ppm) and free PCy3 (10.6 ppm), which is indicative of slow exchange of the phosphine ligands. On the other hand, the phosphine ligands on the cadmium derivative (5) are involved in an exchange process with free PCy3 via a rapid equilibrium between 5 and two equivalents of Cd(O-2,6-F2C6H3)2(PCy3)2. The equilibrium reaction strongly favors the monomer cadmium bisphosphine complex at low temperature (−80 °C). As expected, the 113Cd and 31P NMR spectra of complex 5 in solution in the absence of excess PCy3 is quite similar to that determined in the solid state by CP/MAS. Complex 1 and its chloro- and bromophenoxide analogues were shown to be effective catalysts for the copolymerization of cyclohexene oxide and CO2, the terpolymerization of cyclohexene oxide/propylene oxide/CO2, and the homopolymerization of cyclohexene oxide. In the case of the copolymerization process (80 °C and 55 bar), the polycarbonate copolymer that was produced is completely alternating, with no polyether linkages. At the same time, the homopolymerization of cyclohexene oxide to afford polyether in the presence of 1 as catalyst is much more facile than the copolymerization process. Importantly, for both copolymerization and homopolymerization processes catalyzed by complex 1, the initiator of the polymer chain growth is a difluorophenoxide unit, as revealed by 19F NMR, with both CO2 insertion and epoxide ring-opening being involved in the initiation step. At 80 °C and 55 bar, the coupling of propylene oxide and CO2 led almost exclusively to propylene carbonate. On the other hand, at lower temperatures (i.e., 40 °C), copolymer formation was favored over cyclic carbonate production. Because of the relative rate...

Zinc complexes derived from benzoic acids containing electron-withdrawing substituents have been synthesized from Zn(II)(bis-trimethylsilyl amide)(2) and the corresponding carboxylic acid (2,6-X(2)C(6)H(3)COOH, where X = F, Cl, or OMe) in THF and structurally characterized via X-ray crystallography. The 2,6-difluorobenzoate complex crystallizes from THF or CH(3)CN as a seven membered zinc aggregate, where the metal atoms are interconnected by a combination of 10 mu-benzoates and mu(4)-oxo ligands, that is, [(2,6-difluorobenzoate)(10)O(2)Zn(7)](solvent)(2), solvent = THF (1) and CH(3)CN (1a). On the other hand, the 2,6-dichlorobenzoate zinc derivative crystallizes from THF as a dimer, [(2,6-dichlorobenzoate)(4)Zn(2)](THF)(3) (2), where the two zinc centers are bridged by three benzoate ligand. One of the zinc centers possesses a tetrahedral ligand environment where the fourth ligand is a unidentate benzoate, and the other zinc center has an octahedral arrangement of ligands which is accomplished by the additional binding of three THF molecules. Upon dissolution of complex 1 or 2 in the strongly binding pyridine solvent, disruption of these zinc carboxylates occurs with concomitant formation of mononuclear zinc bis-benzoates with three pyridine ligands in the metal coordination sphere. Complexes 1 and 2 were found to be effective catalysts for the copolymerization of cyclohexene oxide and carbon dioxide to afford polycarbonates devoid of polyether linkages, that is, completely alternating copolymers. Although these catalysts or catalyst precursors in the presence of CO(2)/propylene oxide afforded mostly propylene carbonate, they did serve as efficient catalysts for the terpolymerization of carbon dioxide/cyclohexene oxide/propylene oxide. The reactivities of these zinc carboxylates were very similar to those previously reported analogous complexes which have not been structurally characterized. Hence, it is suggested here that all of these zinc carboxylates provide similar catalytic sites for CO(2)/epoxide coupling processes.

The reaction of Cd[N(SiMe(3))(2)](2) with 2 equiv of the corresponding phenol in toluene has led to the isolation of [Cd(O-2,6-R(2)C(6)H(3))(2)](2) derivatives, where R represents the sterically bulky (t)Bu and Ph substituents. The dimeric nature of these complexes in the solid state has been established via X-ray crystallography, i.e., trigonal geometry around cadmium is observed in 1 (R = (t)Bu) where the two cadmium centers are bridged by two phenoxides with each metal containing a terminal phenoxide. Complex 2 (R = Ph) contains an additional interaction of the metal centers with carbon atoms of the aromatic substituents on the phenoxide ligands. These dimeric structures are maintained in weakly coordinating solvents as revealed by (113)Cd NMR in d(2)-methylene chloride, which displays (111)Cd-(113)Cd coupling. Nevertheless, because of the excessive steric requirements of these phenoxide ligands, these dimers are easily disrupted in solution by weak donor ligands such as epoxides. Three bisepoxide adducts have been isolated as crystalline solids and characterized by X-ray crystallography. As previously observed in other Cd(O-2,6-(t)Bu(2)C(6)H(3))(2) x L(2) complexes, these epoxide adducts adopt a crystallographically imposed square-planar geometry about the cadmium centers, with the exception of the exo-2,3-epoxynorbornane derivative, which displays a distorted tetrahedral geometry. Temperature-dependent (113)Cd NMR studies have established that there is little difference in the binding abilities of these epoxides with either complex 1 or complex 2. Importantly, it is concluded from these studies that the lack of reactivity of alpha-pinene oxide and exo-2,3-epoxynorbornane toward copolymerization reactions with carbon dioxide, in the presence of zinc bisphenoxide catalysts, is not due to differences in epoxide metal binding. This is further affirmed by the isolation and crystallographic characterization of the very stable Zn(O-2,6-(t)Bu(2)C(6)H(3))(2) x (exo-2,3-epoxynorbornane)(2) derivative.

The synthesis of Cd(O-2,6-F2C6H3)2(PCy3)2 has been achieved by the reaction of Cd[N(SiMe3)2]2 and 2,6-difluorophenol in THF in the presence of 2 equiv of tricyclohexylphosphine, and its distorted tetrahedral structure has been established by X-ray crystallography. Comparison of the solution and solid-state 113Cd NMR spectra of this derivative suggest that a facile solution equilibrium exists between it and its dimeric monophosphine analogue and free PCy3. At ambient temperature, the equilibrium lies strongly in favor of the dimeric complexes, whereas at −80 °C, the monomeric complex is dominant.