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.

No abstract

The abrB gene of Bacillus subtilis is believed to encode a repressor that controls the expression of genes involved in starvation-induced processes such as sporulation and the production of antibiotics and degradative enzymes. Two such genes, spoVG, a sporulation gene of B. subtilis, and tycA, which encodes tyrocidine synthetase I of the tyrocidine biosynthetic pathway in Bacilus brevis, are negatively regulated by abrB in B. subtilis. To examine the role of abrB in the repression of gene transcription, the AbrB protein was purified and then tested for its ability to bind to spoVG and tycA promoter DNA. In a gel mobility shift experiment, AbrB was found to bind to a DNA fragment containing the sequence from -95 to +61 of spoVG. AbrB protein exhibited reduced affinity for DNA of two mutant forms of the spoVG promoter that had been shown to be insensitive to abrB-dependent repression in vivo. These studies showed that an upstream A+T-rich sequence from -37 to -95 was required for optimal AbrB binding. AbrB protein was also observed to bind to the tycA gene within a region between the transcription start site and the tycA coding sequence as well as to a region containing the putative tycA promoter. These fridings reinforce the hypothesis that AbrB represses gene expression through its direct interaction with the transcription initiation regions of genes under its control.When cells of the spore-forming bacterium Bacillus subtilis encounter a nutritionally poor environment, characteristic of a culture that has entered stationary phase of growth, genes that function in such diverse processes as the production of degradative enzymes and antibiotics, competence development, and sporulation are induced through mechanisms that operate at the transcriptional level (1-5). The transcription of many stationary phase-induced genes requires the product of the spoOA gene (1-5). The amino acid sequence of the spoOA gene product shows homology with the activator class of the two-component regulatory proteins, which supports the hypothesis that spoOA is part of a pathway that senses the nutritional environment (6, 7). One of the primary functions of SpoOA is to repress the transcription of the abrB gene, whose product is believed to be a negative regulator of stationary phase-induced genes (8, 9). The amino acid sequence of the abrB product shares homology with the DNAbinding regions of known transcriptional regulatory proteins, suggesting that AbrB negatively affects transcription through a direct interaction with promoter DNA (9). The current model of spoOA and abrB function is in keeping with earlier studies that showed that abrB is the site of mutations that partially suppress the pleiotropy of spoOA mutations (10-13). To repress abrB transcription is not the sole function of spoOA since a spoOA abrB double mutant is unable to sporulate.Two genes that are subject to abrB-dependent repression are spoVG (14, 15), a sporulation gene, and tycA, a Bacillus brevis gene that encodes tyrocidine synthetase I, an enzyme of the b...

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.

The syntheses of Ni(PTA)4 (5), Pd(PTA)4 (6), and Pt(PTA)4 (8) are accomplished through the reduction of the MIICl2 salts in water with excess 1,3,5-triaza-7-phosphaadamantane (PTA). The products are obtained as partially protonated derivatives with protonation occurring at the nitrogen atoms of the bound PTA ligands. Crystal structures illustrating the monoprotonated and bis-protonated derivatives of 8, [Pt(PTA)3(PTAH)][Cl] (1S) and [Pt(PTA)2(PTAH)2][BF4]2 (2S), are reported. The crystal structure of an intermediate along the reduction pathway, [PdCl(PTA)3][Cl] (7), is also presented, showing a trans influence whereby the Pd−P bond length trans to the Cl- ligand is shorter, 2.238(3) Å, than the average Pd−P bond length cis to the Cl- ligand, 2.334(2) Å. Complex 8 is also protonated at the metal center to form [(H)Pt(PTA)4][X], where X = Cl-, BF4 -, HCO3 -, and C3H5CO2 -, through the addition of weak acids such as CO2/H2O, NH4Cl, PTAH+Cl-, HPy+BF4 -, and crotonic acid. Addition of strong acids such as HCl or HBF4 results in protonation at the PTA ligands. Thereby, the Pt metal center is shown to have a pK a between 7.44 and 9.25. The bound PTA ligands on complex 8 have a pK a below 4.69 but above 2.12. Ni(CO)4 - n (PTA) n derivatives are also reported for n = 3 (9), 2 (10), and 1 (11). Using IR data of 11, PTA is determined to have an electronic parameter (χ) as defined by Tolman to be 15.3 cm-1, indicating PTA to be slightly less donating than PPh3, where χ = 12.8 cm-1. Complex 10 is crystallized out of a MeOH/ether solution maintained at −15 °C and characterized by X-ray crystallography. It is a distorted tetrahedron and has a crystallographically determined Tolman cone angle of PTA of 103°. Monitored reactions of Ni(PTA)4 with CO in water via in situ IR techniques show that the rate observed for the dissociation of PTA to provide Ni(CO)(PTA)3 is 7.92 × 10-4 s-1 at 20 °C. This rate, along with 31P NMR results, indicates that the M0(PTA)4 complexes exhibit little dissociation and slow exchange of the bound PTA ligands.