A series of zinc phenoxides of the general formula (2,6-R2C6H3O)2Zn(base)2 [R = Ph, tBu, iPr, base = Et2O, THF, or propylene carbonate] and (2,4,6-Me3C6H2O)2Zn(pyridine)2 have been synthesized and characterized in the solid state by X-ray crystallography. All complexes crystallized as four-coordinate monomers with highly distorted tetrahedral geometry about the zinc center. The angles between the two sterically encumbering phenoxide ligands were found to be significantly more obtuse than the corresponding angles between the two smaller neutral base ligands, having average values of 140° and 95°, respectively. In a noninteracting solvent such as benzene or methylene chloride at ambient temperature, the ancillary base ligands are extensively dissociated from the zinc center, with the degree of dissociation being dependent on the base as well as the substituents on the phenolate ligands. That is, stronger ligand binding was found in zinc centers containing electron-donating tert-butyl substituents as opposed to electron-withdrawing phenyl substituents. In all instances, the order of ligand binding was pyridine > THF > epoxides. These bis(phenoxide) derivatives of zinc were shown to be very effective catalysts for the copolymerization of cyclohexene oxide and CO2 in the absence of strongly coordinating solvents, to afford high-molecular-weight polycarbonate (M w ranging from 45 × 103 to 173 × 103 Da) with low levels of polyether linkages. However, under similar conditions, these zinc complexes only coupled propylene oxide and CO2 to produce cyclic propylene carbonate. Nevertheless, these bis (phenoxide) derivatives of zinc were competent at terpolymerization of cyclohexene oxide/propylene oxide/CO2 with little cyclic propylene carbonate formation at low propylene oxide loadings. While CO2 showed no reactivity with the sterically encumbered zinc bis(phenoxides), e.g., (2,6-di-tert-butylphenoxide)2Zn(pyridine)2, it rapidly inserted into one of the Zn−O bonds of the less crowded (2,4,6-trimethylphenoxide)2Zn(pyridine)2 to provide the corresponding aryl carbonate zinc derivative. At the same time, both sterically hindered and sterically nonhindered phenoxide derivatives of zinc served to ring-open epoxide, i.e., were effective catalysts for the homopolymerization of epoxide to polyethers. The relevance of these reactivity patterns to the initiation step of the copolymerization process involving these monomeric zinc complexes is discussed.

The syntheses of a variety of group 10 metal complexes of the water-soluble phosphine triazaphosphaadamantane (PTA) are described. Treatment of Ni(NO3)2 with NaNO2 and PTA provides the nitrosyl complex [Ni(NO)(PTA)3]NO3 (1). Complex 1 is soluble in water, DMSO, and CH3CN but insoluble in THF, acetone, or hydrocarbons. X-ray crystallography shows the nitrosyl ligand to be coordinated in a near linear mode (∠Ni−N−O = 171.5(4)°) with a Ni−N bond length of 1.653(4) Å. Concordantly, the υ(NO) vibration in H2O occurs at 1830 cm-1. The series of zerovalent M(PTA)4 (M = Ni, Pd, Pt) complexes, 2, 3, and 6 have been prepared in good yields by several procedures:  (i) the ligand exchange reaction of Ni(cod)2 with PTA; (ii) the reduction of PdCl2 or PtCl2 with hydrazine in the presence of PTA; and (iii) the ligand exchange reaction of Pt(PPh3)4 with PTA. All three derivatives are very water soluble (0.30 M) and resistant to PTA dissociation in solution at ambient temperature. Complexes 2, 3, and 6 can be crystallized from 0.10 M HCl to afford the nitrogen-protonated derivatives, [M(PTAH)4]Cl4. These salts were characterized by X-ray crystallography and shown to exist as slightly distorted tetrahedra with one nitrogen atom of each PTA ligand protonated. The M−P bond lengths are shorter than those found in related derivatives containing poorer electron-donating and/or sterically more encumbering phosphine ligands. The cis-MCl2(PTA)2 (M = Pd and Pt) derivatives, 4 and 7, were obtained by the metathesis reaction of (NH4)2PdCl4 or K2PtCl4 with PTA in refluxing ethanol. When the palladium reaction was carried out in a large excess of PTA, formation of the zerovalent Pd(PTA)4 complex occurred via the intermediacy of the [Pd(PTA)3Cl]+ cation as indicated by 31P NMR and mass spectrometry. The X-ray structures of the Pd(II) and Pt(II) derivatives, cis-PdCl2(PTA)2 and [cis-PtCl2(PTAH)2]Cl2, revealed these to exist as slightly distorted square planar complexes where the P−M−P angles are expanded to 94.4°. The platinum derivative, which contains the nitrogen protonated PTA ligands, displays an extensive array of hydrogen bonding and electrostatic interactions involving water, PTA, and HCl.

Three monomeric Cd(II) phenoxide complexes have been prepared by reacting Cd[N(SiMe(3))(2)](2) with 2 equiv of 2,6-disubstituted phenols bearing sterically bulky tert-butyl or phenyl groups. The strongly coordinating solvents THF, tetrahydrothiophene (THT), and pyridine used for these reactions were incorporated into the metal's coordination sphere, leading to complexes with a general formulation of Cd(O-2,6-R(2)C(6)H(3))(2)(solv)(2)(-)(3). The Cd complexes obtained have been characterized crystallographically and have been found to adopt differing solid-state geometries. The X-ray crystal structure of Cd(O-2,6-(t)BuC(6)H(3))(2)(THF)(2), 1, previously reported by Buhro, displayed unusual square-planar coordination of the metal center. Complex 2, Cd(O(t)BuC(6)H(3))(2)(THT)(2), has been found to take on the same square-planar geometry, even with the more strongly donating thioether ligand. Alternatively, complex 3, Cd(O-2,6-Ph(2)C(6)H(3))(2)(THF)(2), has been found to adopt distorted-tetrahedral geometry, quite similar to its zinc congener. The O(1)-Cd-O(2) angle between the phenoxide ligands in 3 is 150.1(2) degrees, and the angle between the ether ligands is 83.1(3) degrees. When the strongly basic solvent pyridine was used, a five-coordinate complex 4, Cd(O-2,6-(t)BuC(6)H(3))(2)(py)(3), was isolated. This complex 4 is best described as having trigonal bipyramidal geometry with the phenoxide ligands and one pyridine defining the equatorial plane and two axial pyridine ligands having an angle of 169.7(2) degrees. The angle between the phenoxide ligands in 4 is 156.1(2) degrees. These complexes 1-4 have also been characterized in noncoordinating solvent solutions by (1)H, (13)C, and (113)Cd NMR spectroscopy and have been found to contain labile donor ligands. Preliminary studies indicate that, in a noncoordinating solvent, a rapid equilibrium exists between species with and without coordinated donor solvent ligands.

Novel orotic acid and uracil derivatives of tungsten and chromium(0), [Et4N]2[Cr(CO)4(orotate)] (1), [Et4N]2[W(CO)4(orotate)] (2), [Et4N]2[W(CO)4(dihydroorotate)] (3), and [Et4N][W(CO)5(uracilate)] (4) [where orotate = (C5H2O4N2)2-; dihydroorotate = (C5H4O4N2)2-; uracilate = (C4H3O2N2)-], have been synthesized via reaction of M(CO)5THF with the tetraethylammonium salt of the corresponding acid or uracil. These complexes have been characterized in solution by IR and 13C NMR spectroscopy and in the solid state by X-ray crystallography. The geometry of the metal dianions in 1 and 2 is that of a distorted octahedron consisting of four carbonyl ligands and a nearly planar five-membered orotate chelate ring, bound through the N1 and one of its carboxylate oxygen atoms. The uracil ring, including the exocyclic oxygens, itself deviates from planarity by only 0.009 Å. However, the structure of complex 3, which closely resembles that of complexes 1 and 2, has a puckered uracil ring. The structure of complex 4 consists of the uracilate ligand bound through the deprotonated N1 to a tungsten pentacarbonyl fragment. Although the orotate complexes are resistant to thermal decarboxylation, they readily undergo decarbonylation reactions. In this regard, quantitative investigations of the lability of the carbonyl ligands on complexes 1−4 have been carried out. All complexes exhibited a low energy barrier for CO dissociation as demonstrated by 13CO exchange studies. For example, the first-order rate constants for intermolecular CO exchange in complexes 2 and 3 were measured to be 6.05 × 10-4 and 3.17 × 10-3 s-1 at 0 °C, respectively. This facile CO dissociation is attributed to competition of the metal center with the uracil ring for the π donation of electron density from the deprotonated N1 atom of the orotate ligand. As expected, this interaction is enhanced when the pseudoaromaticity of the uracil ring is disrupted in complex 3. The activation parameters for the intermolecular exchange of CO in complex 2 were determined to be ΔH ⧧ = 63.2 ± 3.8 kJ/mol and ΔS ⧧ = − 82.8 ± 13.0 J/mol·K, values consistent with a bond-making/bond-breaking (M···CO/M N) mechanistic pathway. The rate of intermolecular CO exchange was similarly examined in complex 4. The uracilate ligand displayed a π donating capability comparable to that seen for chloride in the W(CO)5Cl - anion but much less π donor character than the phenoxide ligand in W(CO)5OPh -. The activation parameters of the CO exchange process in complex 4 were found to be ΔH ‡ = 106.9 ± 4.3 kJ/mol and ΔS ⧧ = 16.3 ± 13.7 J/mol·K.

A synthetic methodology for the preparation of a large variety of eta(3)-HB(3-Phpz)(3)Cd(acetate) adducts is presented which involves replacement of toluene in the eta(3)-HB(3-Phpz)(3)Cd(acetate) solvate complex by the appropriate cyclic ether or cyclic thioether. In this manner, adducts of THF, dioxane, propylene oxide, cyclohexene oxide, and propylene sulfide were isolated. The solid-state structures of several of these complexes were determined by X-ray crystallography, revealing a six-coordinate complex where the acetate ligand is shown to be fairly symmetrically bonded to the cadmium center. In methylene chloride solution, the cyclic ether or thioether readily dissociates to afford the five-coordinate complex, as demonstrated by (113)Cd NMR. A quantitative assessment of the binding of these base adducts of eta(3)-HB(3-Phpz)(3)Cd(acetate) was determined by measuring the temperature dependence of the equilibrium constants for the five- and six-coordinate derivatives. The presence of one sharp (113)Cd resonance in this equilibrium mixture is indicative of rapid intermolecular exchange between the five- and six-coordinate complexes when compared to the chemical shift differences in these two species ( approximately 6600 Hz at 89 MHz). The order established for ether binding is THF > dioxane > propylene sulfide > cyclohexene oxide >/= propylene oxide, with DeltaH degrees and DeltaS degrees spanning the ranges -27.7 to 24.3 kJ/mol and -89.7 to -94.1 J/(mol K). The epoxide and thioepoxide adducts were shown to serve as models for the initiation step in the copolymerization of epoxides with carbon dioxide catalyzed by metal carboxylates. That is, the carboxylate ligand was shown to ring-open the epoxide or thioepoxide, subsequently affording polyethers or polythioethers with ester end groups. By way of contrast, in the presence of CO(2) and epoxides, this system led to cyclic carbonate production.