Phosphine derivatives of the monomeric zinc phenoxide complexes, (phenoxide)2ZnLn, where phenoxide equals 2,6-di-tert-butylphenoxide, 2,4,6-tri-tert-butylphenoxide, and 2,6-diphenylphenoxide and n = 1 or 2, have been synthesized from the reaction of Zn[N(SiMe3)2]2 and the corresponding phenol followed by the addition of phosphine. The complexes have been characterized in solution by 31P NMR spectroscopy and in selected instances in the solid-state by X-ray crystallography. The small, basic phosphine, PMe3, provided the only case of an isolated complex possessing two phosphine ligands (i.e., n = 2). For all other larger phosphines only the monophosphine adducts were obtained. Furthermore, only fairly basic phosphines were found to bind to zinc, e.g., whereas PPh3 (pKa = 2.73) was ineffective, PPh2Me (pKa = 4.57) did form a strong bond to zinc. The solid-state structures of the monophosphine adducts consist of a near-trigonal planar geometry about the zinc center, where the average P-Zn-O angles are larger than the O-Zn-O angles. On the other hand, the bisphosphine adduct, Zn(O-2,4,6-tBu3C6H2)(2).2PMe3, is a distorted tetrahedral structure with O-Zn-O and P-Zn-P bond angles of 108.8(2) degrees and 107.1(9) degrees, respectively. Competitive phosphine binding studies monitored by 31P NMR spectroscopy provided a relative binding order of PPh3 approximately PtBu3 << PPh2Me < PCy3 < PMe2Ph < PnBu3 < PEt3 < PMe3. Hence, the relative binding of basic phosphine ligands at these congested zinc sites is largely determined by their steric requirements. All phosphine adducts, with the exception of PMe2Ph and PMe3, were found to undergo slow self-exchange (< 600 s-1) with free phosphine by 31P NMR spectroscopy. However, the two small phosphines, PMe2Ph (cone angle = 122 degrees) and PMe3 (cone angle = 118 degrees), were shown to undergo rapid exchange presumably via an associative mechanism. Although there was no kinetic preferences for PCy3 binding to cadmium vs zinc, cadmium was thermodynamically favored by about a factor of 2.5. The addition of up to 3 equiv of PCy3 to the Zn(O-2,6-tBu2C6H3)2 or Zn(O-2,4,6-tBu3C6H2)2 derivatives did not significantly alter the reactivity of these catalysts for the copolymerization of cyclohexene oxide (CHO) and CO2 to high-molecular weight poly(cyclohexene carbonate). However, the presence of PCy3 greatly retarded their ability to homopolymerize CHO to polyether or to afford polyether linkages during the copolymerization of CHO/CO2.

Recently, we have demonstrated that monomeric Zn(II) phenoxides which possess sterically encumbering substituents in the 2-and 6-positions of the phenolate ligands along with two ether donors, typified by (2,6-di-tert-butylphenoxide) 2 Zn II (THF) 2 , are the most active catalysts known for the copolymerization of cyclohexene oxide and carbon dioxide (eq 1). 1 Solution 1 H NMR

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.

Novel complexes of orotic acid and dihydroorotic acid with Cu(I) bis(triphenylphosphine) have been synthesized. These complexes are the first examples of orotic acid derivatives bound to copper in the +1 oxidation state. Both complexes have been characterized in the solid state by FTIR(KBr) and X-ray crystallography. The orotate derivatives display a rare monodentate carboxylate coordination mode.

The pyridine bound 2-aminopyridine (2APH) derivative of tungsten pentacarbonyl has been prepared from photogenerated W(CO)5THF and 2APH. Deprotonation of the distal amine group by sodium hydride has provided two complexes, [Na][W(CO)5(2AP)] and [Na]2[W(CO)4(2AP)]2. Both complexes have been characterized by X-ray crystallography with the monomeric derivative being crystallized as its [Na2(18-crown-6)][W(CO)5(2AP)]2 salt which exhibits strong Na+...-NH interactions. Photolysis of W(CO)6 in the presence of excess 2-aminopyridine in THF has led to an efficient synthesis of the chelated neutral derivative, W(CO)4(2APH).2APH, where the extra equivalent of 2APH is hydrogen bonded to its bound counterpart. The 2-aminopyridine molecule of solvation was almost quantitatively removed via aqueous washings. Deprotonation of W(CO)4(2APH) with NaH afforded the amidopyridine derivative which was shown to rapidly undergo reaction with CO2 to yield the chelated carbamate complex, W(CO)4(OC(O)2AP)-. Nevertheless, because of the presence of small quantities of free 2-aminopyridine during the reactions with CO2, we have not been able to conclusively rule out participation by a ligand substitution process involving NC5H4NHCOOH. Ab initio computations were found to substantiate many of these experimental observations. That is, in the monodentate bound W(CO)5(2APH) derivative, binding through the pyridine nitrogen atom is favored by about 29 kJ/mol over the amine nitrogen atom, whereas the opposite site for binding is preferred for the deprotonated amido analogue, W(CO)5(2AP)-. Furthermore, both forms of W(CO)5(2AP)- were found to be more stable than the chelated tungsten tetracarbonyl anion plus CO. On the other hand, CO2 insertion into the W(CO)4(2AP)- anion to provide the chelated carbamate, W(CO)4(OC(O)2AP)-, was thermodynamically favored by >110 kJ/mol. Finally, both experimental and theoretical studies were inconclusive with regard to identifying reaction intermediates during the CO2 insertion pathway which involve prior interactions of CO2 at the amido nitrogen center.