Meso Configuration Research Articles

Highly Enantioselective Polymerization of β-Butyrolactone by a Bimetallic Magnesium Catalyst: An Interdependent Relationship Between Favored and Unfavored Enantiomers.

Herein, we report that (S,S)-prophenolMg2(μ-OnBu)(THF)2 ((S,S)-1, prophenol = (S,S)-2,6-bis[2-(hydroxydiphenylmethyl)pyrrolidin-1-ylmethyl]-4-methylphenol) is a highly enantioselective (kR/kS = 140) precatalyst for ring-opening polymerization of rac-β-butyrolactone (β-BL) to isotactic poly(3-hydroxybutyrate) (i-PHB), a high performance, biodegradable polyester. Precatalyst (S,S)-1 polymerizes (R)-β-BL with an inversion of stereochemistry to (S)-PHB with a m% (percentage of adjacent linkages with a meso configuration) of 98% at 41% conversion and Tm of 165 °C under a variety of conditions. Complex (S,S)-1 demonstrates unique polymerization kinetics, as it does not polymerize the preferred enantiomer, (R)-β-BL, alone. Mechanistic studies revealed that (S)-β-BL is needed to convert (S,S)-1 into the active enantioselective polymerization catalyst. To the best of our knowledge, (S,S)-1 produces i-PHB with the highest degree of isotacticity observed from a polymerization of rac-β-BL. This study informs the design and understanding of future enantioselective and earth-abundant metal catalysts for ring-opening polymerization of β-lactones.

Micro-CFD modelling of ultrafiltration bio-fouling

ABSTRACT Concentration polarisation on the membrane process is a vast area of interest. The macroscopic characterisation of fouling structure formation occurring during the Ultra-Filtration (UF) of Bovine Serum Albumin (BSA) was studied in this work. A twisted Monte-Carlo (MC)/Computational-Fluid-Dynamic (CFD) approach was developed to calculate macroscopic fluid-dynamic proprieties. Different fluid-dynamic simulations were performed based on the knowledge acquired by an MC analysis that provided boxes of adsorbed molecules (i.e., 3D proteins meso configurations on the UF membranes surface). These represented the deposit layers formed at different distances from the membrane. The 3D meso structure were imported into a bespoke simulation environment, and several meshes were created to perform micro-fluid dynamic calculations (m-FD). From these simulations, a set of macroscopic parameters was calculated. The resistance to the flow of deposit layers accumulated on the membrane surface, , usually estimated by experimental methods, was computed starting from the ab-initio knowledge acquired at sub-nanoscopic scale. Therefore, this work provides a multi-scale description of a complex phenomenon such as that of protein fouling during a UF filtration starting from a sub-nanometric scale.

Crystal structure of (2-acetyl-ferrocen-1-yl)boronic acid.

(2-Acetyl-ferrocen-1-yl)boronic acid, [Fe(C5H5)(C7H8BO3)] or 2-C(O)CH3-1-B(OH)2-Fc [Fc = Fe(η5-C5H3)(η5-C5H5)], crystallizes in the centrosymmetric space group P21/n. The boronic acid functionality inter-acts via intra-molecular hydrogen bonds with the acetyl group and with the -B(OH)2 functionality of an adjacent mol-ecule. The resulting centrosymmetric dimer exhibits an anti-positioning of the ferrocenyl moieties towards the central B2O4 plane. Consequently, an (Rp ,Sp )-, i.e. a meso configuration is present for this dimer. In the crystal, weak C-H⋯O hydrogen bonds consolidate the mol-ecular packing.

Chiral Rhodium(I) Complexes of 1,2‐Bis‐(chloroalkoxyphosphanyl)‐ and 1,2‐Bis‐(amidoalkoxyphosphanyl)‐1,2‐dicarba‐closo‐dodecaboranes(12)

Abstract1,2‐Bis(chloroalkoxyphosphanyl)‐1,2‐dicarba‐closo‐dodecaboranes(12) and 1,2‐bis(amido‐(‐)‐menthyloxyphosphanyl)‐1,2‐dicarba‐closo‐dodecaboranes(12) were synthesised by the reaction of dilithiated 1,2‐dicarba‐closo‐dodecaborane(12) with the corresponding dichloro‐ or chloroamidophosphite. For 1,2‐bis‐(chloro‐1‐adamantyloxyphosphanyl)‐1,2‐dicarba‐closo‐dodecaborane(12) epimerisation in solution was observed and the free activation enthalpy was determined experimentally and theoretically. An indirect determination of the meso configuration was obtained by formation of the corresponding molybdenum tetracarbonyl complex. Chlorido‐bridged dimeric chiral rhodium(I) complexes showing planar or folded Rh2Cl2 rings were prepared. Density functional calculations indicate that steric and packing factors play a much more significant role than electronic effects in influencing the bending of the rhodium complexes.

CRYSTAL STRUCTURE OF (1R,2S)-1,2-BIS(4-CHLOROPHENYL)-3,8-DIMETHOXYACENAPHTHENE-1,2-DIOL: TETRAMERIC STRING OF FOUR CONFORMERS CONNECTED BY CLASSICAL HYDROGEN BONDS AND MOLECULAR ACCUMULATION ALIGNMENT BY LINKING OF THE TETRAMERS WITH THE AID OF NON-CLASSI

Crystal structure of (1 R ,2 S )-1,2-bis(4-chlorophenyl)-3,8-dimethoxyacenaphthene-1,2-diol, C 26 H 20 O 4 Cl 2 , is reported and discussed from the viewpoints of characteristics in spatial organization, i.e. , single molecular structures of four conformers, tetrameric aggregates, and higher ordered structures, with clarification of a classical and two non-classical hydrogen bonding interactions as the structure determining interactions. The title compound crystallizes with four independent molecules (conformers G, B, R, and Y) in the asymmetric unit. Furthermore, each independent molecule displays a meso configuration, with one 4-chlorophenyl group R and the other S . The four molecules are related by inversion center in the asymmetric unit of P -1 space group, exhibiting the number of molecules is eight, Z = 8. Single molecular structure of each conformer shows that the two benzene rings are bonded with large dihedral angles against the naphthalene plane and the two phenyl rings are oriented in the same direction with respect to the naphthalene ring plane ( syn- orientation). The four conformers are classified into two groups according to overlapping feature of phenyl rings, i.e. , conformers G and Y have larger slippage of the phenyl rings than conformers B and R. In the molecular packing, four conformers Y, B, R, and G are connected by classical O–H…O(H) hydrogen bonds in head-to-head fashion forming S-shaped tetramer. Tetramers composed of four conformers are stacked into columnar structure along a -axis through non-classical C–H…Cl hydrogen bonds between conformers G. The columns are linked into a sheet structure by non-classical C–H…Cl hydrogen bonds between conformers G and Y along ab -diagonal. The waved sheets are interlocked by two types of non-classical C–H…p hydrogen bonds forming the stripe structure along c -axis, i.e. , non-classical C–H…pi hydrogen bonds between conformers R and Y, and those between conformers B and G.

A heteropentanuclear metal string complex [Mo2NiMo2(tpda)4(NCS)2] with two linearly aligned quadruply bonded Mo2 units connected by a Ni ion and a meso configuration of the complex

A dimeric molybdenum precursor and nickel ions are used to synthesize a symmetric heteropentanuclear complex, [Mo2NiMo2(tpda)4(NCS)2]. This complex possesses unique structural features, as the four ligands are coordinated to the metal framework in a meso configuration. Furthermore, the central Ni2+ ion is in a high spin state.

Configurational Isomerization of Dinuclear Iridium and Rhodium Complexes with a Series of NPPN Ligands, 2-PyCH2(Ph)P(CH2)nP(Ph)CH2-2-Py (Py = Pyridyl, n = 2–4)

New heterodonor NPPN tetradentate ligands, 2-PyCH2(Ph)P(CH2)nP(Ph)CH2-2-Py (meso- and rac-Ln; n = 2–4, Py = pyridyl), were prepared and reacted with [Cp*MCl2]2 (M = Ir, Rh; Cp* is pentamethylcyclopentadienyl) in the presence of NH4BF4 to afford a series of dinuclear complexes [(Cp*MCl)2(meso-Ln)](BF4)2 (M = Ir, n = 2 (2a), 3 (3a), 4 (4a); M = Rh, n = 2 (2c), 3 (3c), 4 (4c)) and [(Cp*MCl)2(rac-Ln)](BF4)2 (M = Ir, n = 2 (2b), 3 (3b), 4 (4b); M = Rh, n = 2 (2d), 3 (3d), 4 (4d)), which were characterized by IR, 1H and 31P{1H} NMR, and ESI mass spectroscopic techniques and X-ray crystallography. The configurations around the two metal centers were controlled by the configuration of the coordinated P atoms so as to avoid repulsive interaction between the phenyl group on P and the chloride ligand, resulting in the formation of stereospecific isomers; a meso configuration of the metal centers is induced from meso-Ln (abbreviated as meso-P2/meso-M2), and in contrast, a rac configuration is induced from rac-Ln (rac-P2/rac-M2). Furthermore, inversion of metal centers for the Ir2 complexes occurred in DMSO at higher temperatures (60–100 °C), generating equilibrium mixtures of minor diastereomers (meso-P2/rac-M2 or rac-P2/meso-M2) in low ratios together with the major isomers (meso-P2/meso-M2 or rac-P2/rac-M2). The equilibrium constants, K = [minor isomer]/[major isomer], varied appreciably depending on the lengths of the methylene chains as well as configurations of the NPPN ligands; the overall propensity for the K values was observed to be L2 < L3 < L4 and meso-Ln < rac-Ln, while rac-L3, rac-L4, and meso-L4 showed almost identical equilibrium constants, presumably resulting from no steric influence between the two metal centers.

Molecular Approach To Understand the Tacticity Effects on the Hydrophilicity of Poly(N-isopropylacrylamide): Solubility of Dimer Model Compounds in Water

Although it has been suggested that the tacticity affects the hydrophilicity of poly(N-isopropylacrylamide) (PNiPA), little is known about the physical background of this phenomenon. In this study, we investigated the solubility of the dimer model compounds (DNiPA). The partition coefficient of DNiPA in the two phases of a water/chloroform mixture has indicated that DNiPA with the racemo configuration (r-DNiPA) is more soluble in water than DNiPA with the meso configuration (m-DNiPA). The difference of the hydration free energy between m- and r-DNiPA is estimated to be 1.2 kJ mol(-1). The molecular mechanics (MM) calculations with the GB/SA model have revealed that r-DNiPA in water is more stable by ca. 1 kJ mol(-1) than m-DNiPA, which is in excellent agreement with the experimental result. The MM calculations have also indicated that the intramolecular interaction of m-DNiPA is stronger than that of r-DNiPA, while r-DNiPA is advantageous in terms of the hydration free energy and conformational entropy.

Thermal Cracking Meso-Characteristic of LuHui Granite

Rock macro mechanic character is decided by rock composite and meso configuration. LuHui granite minerals composite, micro configuration and the rock thermal cracking in differ temperature are observed by micro-photometer in meso. LuHui granite is composited by kinds of minerals, and its minerals’ inhomogeneity is visible, the change of cementation among rock crystal grains, dislocation and micro cracking in crystal is produced in crystal grains under temperature. By means of experiment, the development of granite micro cracks and interior configuration, and change laws of micro cracks were observed. The quantitative analysis of the micro mechanism and laws of granite thermal crack was made, granite cracks number increases acutely in 240-260 degree C after a narrow range fluctuation in 80 degree C. Finally, the thermal cracks threshold of granite was determined primarily.

Polymeric Modification of Graphene through Esterification of Graphite Oxide and Poly(vinyl alcohol)

Polymeric Modification of Graphene through Esterification of Graphite Oxide and Poly(vinyl alcohol)

Configuration determination of flexible tetracarboxylate ligands in two supramolecular structures

Two supramolecular structures have been prepared using flexible (2S,3S,4R,5R)-tetrahydrofurantetracarboxylic acid (H4Laa) and cis,cis,cis-1,2,3,4-cyclopentanetetracarboxylic acid (H4Lbb) under similar conditions; Laa retains its original meso configuration, while Lbb undergoes configuration conversion to the racemic form.

Configurational and Conformational Properties of 1,3,7,9-Tetraphospha-Cyclododeca-1,2,7,8-tetraene: An Ab Initio Study and NBO Analysis

An investigation employing the ab initio molecular orbital (MO) and density functional theory (DFT) methods to calculate structural optimization and conformational interconversion pathways for the two diastereoisomeric forms, (±) and meso configurations of 1,3,7,9-tetraphospha-cyclododeca-1,2,7,8-tetraene (1) was undertaken. Two axial symmetrical conformations are found for (±)-1 configuration. (±)-1-TB axial symmetrical form is found to be about 0.35 and 0.99 kcal mol−1 more stable than (±)-1-Crown axial symmetrical conformation, as calculated by HF/6-31G*//HF/6-31G* and B3LYP/6-31G*//HF/6-31G* levels of theory, respectively. The unsymmetrical meso-1-TBCC form is found to be the most stable geometry, among the various conformations of meso-1 configuration. HF/6-31G*//HF/6-31G* and B3LYP/6-31G*//HF/6-31G* results showed that between the two most stable conformations of (±) and meso configurations, (±)-1-TB is more stable than meso-1-TBCC by about 3.35 and 2.43 kcal mol−1, respectively. In addition, MP2/6-31G* and B3LYP/6-311+G** results showed that the (±)-1-TB form is about 1.10 and 2.36 kcal mol−1 more stable than the meso-1-TBCC form. Further, NBO results revealed that in the most stable form of meso configuration (meso-1-TBCC), the sum of the π* allenic antibonding orbital occupancies (Σ π *occupancy) is greater than dl configuration ((±)-1-TB). Also, NBO results indicated that in the (±)-1-TB conformer, the sum of σ and π allenic moieties bonding orbital deviations (Σ σ dev+Σ π dev) from their normal values, is lower than in the meso-1-TBCC form.

Stereoselective Synthesis of the First Chatt‐Type Bis(dinitrogen)‐Molybdenum(0) Complex with a Tetraphosphane Ligand

AbstractThe first Chatt‐type Mo0 dinitrogen complex with a tetraphosphane ligand has been prepared and characterized by NMR as well as infrared and Raman spectroscopy. Importantly, the employed reaction route allows the stereospecific synthesis of this complex as trans‐[Mo(N2)2(meso‐prP4)] (prP4 = a tetraphos ligand with a central propylene bridge). The stereoselectivity in the reaction course is induced by the oxido‐iodido‐molybdenum(IV) precursor [Mo(O)I(prP4)]+ which directs both phenyl groups of the bridging P atoms of prP4 into a meso configuration. The paper establishes a general strategy to synthesize mononuclear Mo0 dinitrogen and related molybdenum complexes with multidentate phosphane ligands which has not been possible to date. Moreover, the obtained molybdenum tetraphos N2 complex should exhibit a higher thermodynamic stability in the reactions of the Chatt cycle of synthetic nitrogen fixation than the conventional bis(diphos) complexes, due to the linkage of the two diphosphane units by an alkyl bridge. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)

Isomerization in bent phosphametallocenes: Combining rotational barriers and the intramolecular slip-inversion-slip mechanism to control stereo-conformation

Four new phosphazirconocenes, [(4,5,6-trihydro-1-phosphapentalenyl) 2ZrCl 2] ( 8), [(4,5,6,7-tetrahydro-1-phosphindolyl) 2ZrCl 2] ( 9), [(4,5,6,7,8-pentahydro-1-phosphazulenyl) 2ZrCl 2] ( 10), (2,4-dimethylphospholyl) 2ZrCl 2 ( 11), were prepared and characterized. A crystal structure of [(4,5,6,7-tetrahydro-1-phosphindolyl) 2ZrCl 2] ( 9), was obtained that revealed disorder ( rac and meso isomers in the same crystal) and demonstrates unambiguously the meso configuration in the bent early transition metal phosphametallocenes. Lacking a spectroscopic probe of the rotational dynamics in phosphametallocenes, propylene polymerization was chosen to provide a record of the stereoselective insertions and by inference insight into the phosphametallocene dynamics. Phosphazirconocenes 8– 11 were evaluated, and the rac/ meso mixture of [(4,5,6,7-tetrahydro-1-phosphindolyl) 2ZrCl 2] ( 9) was found to produce a mixture of isotactic (iPP) and atactic (aPP) polypropylenes. 13C NMR spectral analysis of the stereo-errors contained in the iPP fraction indicated enantiomorphic site control as the source of the stereoselectivity. The rotational dynamics of the η 5-phospholyl ligand of the parent phosphazirconocene, [(C 4H 4P) 2ZrCl 2], was examined as a first order surrogate of reasonable computing complexity for the catalytically active species. The phosphazirconocene was examined using a restricted Hartree–Fock method with a split basis set of 6-311+G(d,p) for C, H, Cl, P and of LANL2DZ for Zr. The rotational potential energy surface was found to be unsymmetrical with a maximum barrier of 8.8 kcal mol −1, which is about an order of magnitude greater than that reported for Cp 2ZrCl 2. The projection of the calculated rotational dynamics onto a 1-phosphatetrahydroindenyl system is illustrated. This model leads to an interpretation of the polymerization results and the rotational dynamics in phosphametallocenes.

Initiator selectivity during free radical copolymerizations of styrene and methyl methacrylate initiated by anaerobic copper(II) oxidation of carbon-13 labeled acetophenone

Nuclear magnetic resonance (NMR) analysis of the 13C-labeled chain ends of polystyrene, polyMMA, and styrene-MMA copolymers prepared by polymerizations initiated using 13C-labeled-phenacyl radicals were investigated. The phenacyl radicals were generated by anaerobic oxidation of acetophenone-methyl-13C using a Cu(II) octanoate-pyridine complex in the presence of triethylamine and triphenylphosphine. NMR analysis of the 13C-labeled chain ends of these polymers afforded insight into the initiation mechanism. In copolymerization experiments using 13C-labeled acetophenone initiator, the NMR spectra provided evidence that the phenacyl radical reacts 2.7 times faster with styrene than with MMA. The resonances of the labeled phenacyl carbons also showed that the sequence and stereosequence distributions of monomer units at the chain ends are nearly the same as those that prevail along the polymer chains. Styrene–styrene, styrene–MMA, and MMA–styrene enchainments at the chain ends are equally likely to have meso (erythro) or racemic(threo) configurations but the ratio of meso to racemic MMA-MMA enchainments is ∼ 3/7. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2347–2356, 2008

Luminescent Lanthanide Complexes with 1,2-Bis(ethylsulfinyl)ethane: Structural Dependences on Reacting Solvent, Ratio, and Co-Ligand

The present paper reports our sequential efforts to investigate the structures and properties of flexible disulfoxide–lanthanide complexes. Combining our reported results, on the basis of changing synthesis conditions and using a co-ligand, six new complexes, [Ln(meso-L)4](ClO4)3(H2O) n (n = 0.5, Ln = Eu (1), Gd (2); n = 0, Ln = Tb (3)) and [Ln(meso-L)2(phen)2](ClO4)3(CHCl3)2 (Ln = Eu (4), Gd (5), and Tb (6)) (herein, L = 1,2-bis(ethylsulfinyl)ethane, phen = 1,10-phenanthroline) were synthesized, and their structures were characterized by IR spectroscopy, elemental analysis, and X-ray single-crystal diffraction. The dependence of the formation of 1–3 on the solvents used and reaction ratios in the preparation was confirmed. Complexes 1–3 are isomorphous ionic complexes, and the coordination cation is chiral and has a mononuclear structure, in which the LnIII atom is eight-coordinated by four L ligands. Complexes 4–6 are also isomorphous mononuclear ionic complexes, and in each complex cation, the central LnIII atom is eight-coordinated by two L ligands and two phen molecules. In 1–6, all the L ligands exhibit the meso configuration and bis-monodentate chelating coordination mode. Further, the absorption and luminescent spectra of L, phen and complexes 4–6 in solution were investigated at room temperature. The absorption spectra show that only a very slight shift of the absorption band occurred for the ligands L and phen when coordinating to metal ions, being consistent with their origin. The photoluminescence of 4 and 6 exhibits the usual metal-centred emission spectra arising from ligand-to-metal energy transfer and the emission of π → π* transitions from the phen ligand; the latter is also seen in the emission spectrum of 5.

The structure of N- meso-(6,8,8,14,16,16-hexamethyl-1,5,9,13-tetraazacyclohexadeca-5,13-diene)nickel(II) chloro(isothiocyanato)zincate(II)

A yellow compound which was crystallised from a solution of (6,8,8,14,16,16-hexamethyl-1,5,9,13-tetraazacyclohexadeca-5,13-diene)bis(isothiocyanato)nickel(II) in aqueous zinc(II) chloride has cations with singlet ground state nickel(II) in square-planar coordination by the nitrogen atoms of the macrocycle. The asymmetric unit has two similar cations. The N 4 group of one cation is near coplanar (r.m.s. displacements ±0.009(1) Å, with Ni displaced by 0.048(1) Å from this plane) while the other cation has significant tetrahedral twisting of the N 4 group (r.m.s. displacements of N atoms ±0.126(2) Å, with Ni displaced by 0.027(2) Å from this plane). The mean Ni–N distances are Ni–N amine = 1.950(6) and Ni–N imine = 1.897(6) Å. Both cations have N- meso configurations with saddle conformations, with the substituted chelate rings in boat conformations tilted to one side of the NiN 4 ‘plane’ and the unsubstituted chelate rings tilted to the other side, one in a boat conformation and the other with the central methylene group disordered, the components forming boat {s.o.f. 0.70(1) and 0.74(1) for the two cations} and chair conformation chelate rings. The counter-ions have tetrahedrally coordinated zinc(II) ions, one as [ZnCl 2(NCS) 2] 2− ions and the other with one ligand site with disordered Cl − {s.o.f. 0.78(1)} and NCS − ligands, i.e. with disordered [ZnCl 2(NCS) 2] 2− and [ZnCl(NCS) 3] 2− ions, with an overall composition of [Ni( trans-Me 6[16]diene)][ZnCl 1.9(NCS) 2.1].

Ab initio study and NBO analysis of configurational and conformational properties of cyclododeca-1,2,7,8-tetraene

The results concerning an investigation employing the ab initio molecular orbital (MO) and density functional theory (DFT) methods to calculate structural optimization and conformational interconversion pathways for the two diastereoisomeric forms, (±) and meso configurations of cyclododeca-1,2,7,8-tetraene ( 1) are reported in this work. Two axial symmetrical conformations are found for (±)- 1 configuration. (±)- 1-TB axial symmetrical form is found to be about 4.97 and 4.48 kcal mol −1 more stable than (±)- 1-crown axial symmetrical conformation, as calculated by HF/6-31G ∗//HF/6-31G ∗ and B3LYP/6-31G ∗//HF/6-31G ∗ levels of theory, respectively. The results show that the interconversion of (±)- 1-TB and (±)- 1-crown conformations can take place via (±)- 1-T unsymmetrical twist minimum geometry. Conformational interconversion barrier height between (±)- 1-TB and (±)- 1-T forms is found to be 11.48 kcal mol −1, as calculated by B3LYP/6-31G ∗//HF/6-31G ∗ method. Also, based on the B3LYP/6-31G ∗//HF/6-31G ∗ results, the conformational interconversion barrier height between (±)- 1-T and (±)- 1-crown forms is found to be 12.23 kcal mol −1. The unsymmetrical meso- 1-TBBC form is found to be the most stable geometry, among the various conformations of meso- 1 configuration. Conformational racemization of meso- 1-TBBC form can take place via another energy minimum geometry, namely meso- 1-TBCC. Conformational interconversion barrier height between meso- 1-TBBC and meso- 1-TBCC forms is 10.56 kcal mol −1, as calculated by B3LYP/6-31G ∗//HF/6-31G ∗ method. The results show also that conformational racemization of meso- 1 -TBCC can take place via the plane symmetrical meso- 1 -BCC geometry, and requires an energy about 5.35 kcal mol −1, as calculated by B3LYP/6-31G ∗//HF/6-31G ∗ method. In addition, HF/6-31G ∗//HF/6-31G ∗ and B3LYP/6-31G ∗//HF/6-31G ∗ results showed that between the two most stable conformations of (±) and meso configurations, (±)- 1-TB is more stable than meso- 1-TBBC by about 4.55 and 4.04 kcal mol −1, respectively. Also, MP2/6-31G ∗ and B3LYP/6-311+G ∗∗ methods were used to evaluate the HF/6-31G ∗//HF/6-31G ∗ and B3LYP/6-31G ∗//HF/6-31G ∗ results for the more stable conformations of (±) and meso configurations of compound 1 (namely: (±)- 1-TB, meso- 1-TBBC, meso- 1-TCCC and meso- 1-TBCC). Accordingly, MP2/6-31G ∗ and B3LYP/6-311+G ∗ ∗ results showed that the (±)- 1-TB form is about 3.55 and 3.54 kcal mol −1 more stable than the meso- 1-TBBC form. MP2/6-31G ∗ and B3LYP/6-311+G ∗ ∗ calculated energy differences between the most stable conformations of (±) and meso-configurations of compound 1, are found to be in good agreement with HF/6-31G ∗//HF/6-31G ∗ and B3LYP/6-31G ∗//HF/6-31G data. Further, using NBO (Natural Bond Orbital) analysis, π and π ∗ allenic bonding and antibonding orbital occupancies and also the deviations of σ and π bonding orbitals of allenic moieties were investigated. NBO results revealed that in the most stable form of meso configuration ( meso- 1-TBBC), the sum of the π ∗ allenic antibonding orbital occupancies ( Σ π occupancy ∗ ) is greater than dl configuration ((±)- 1-TB). In addition, NBO results indicated that in the (±)- 1-TB conformer, the sum of σ and π allenic moieties bonding orbital deviations ( Σ σ dev+Σπ dev), from their normal values, is lower than in the meso- 1-TBBC form. All these facts could explain the relative more stability of (±)- 1-TB conformer, as compared to the meso- 1-TBBC form.

The structural and stereogenic properties of pentaerythritoxy-bridged cyclotriphosphazene derivatives: spiro–spiro, spiro–ansa and ansa–ansa isomers

Reactions of pentaerythritol with hexachlorocyclotriphosphazene, N3P3Cl6, and gem-disubstituted cyclotriphosphazene derivatives, N3P3Cl4R2 [R = Ph, NHBu(t) or (OCH2CF2CF2CH2O)0.5] gave a series of pentaerythritol-bridged derivatives linked spiro-spiro, spiro-ansa and ansa-ansa. The structures and stereogenic properties of the products were characterised by X-ray crystallography and 31P NMR spectroscopy on addition of the chiral solvating agent, (S)-(+)-2,2,2-trifluoro-1-(9-anthryl)ethanol. Molecules with spiro-spiro and spiro-ansa bridged gem-disubstituted cyclophosphazenes [R = Ph, NHBu(t) or (OCH2CF2CF2CH2O)0.5] are found to be chiral and exist as racemates. Molecules with ansa-ansa bridged cyclophosphazenes [R = Cl or (OCH2CF2CF2CH2O)0.5] have been characterised for the first time and are shown to have meso configurations. Analysis of crystal structure data shows that the six-membered chair form of the spiro rings and the eight-membered boat-chair form of the ansa rings in the bridged compounds are similar to analogous spiro and ansa exocyclic ring conformations of 1,3-propanedioxy-derivatives of cyclophosphazenes.

Synthesis, Structure, and Kinetics and Stereochemistry of Base-Catalyzed Hydrolysis of meso- and rac-[Co2(tmpdtne)Cl2]4+, Bis(pentaamine) Complexes Devoid of Deprotonatable NH Centers

The bis(pentadentate) ligand tmpdtne binds two Co(II) centers, and the entity is readily oxidized to the dicobalt(III) derivative [Co(2)(tmpdtne)Cl(2)](4+) which has been separated into two isomeric forms. NMR studies establish these as meso and rac isomers arising from the different or same absolute configurations for the asym configuration about each Co(III) center. Each dinuclear ion base hydrolyses to the dihydroxo derivative [Co(2)(tmpdtne)(OH)(2)](4+) with retained asym configurations about each metal ion and also retained rac or meso configurations. The kinetics for the stepwise loss of the two Cl(-) ligands is uniphasic, and data are presented to show that the loss of the first chloride is rate determining and is followed by very rapid intramolecular and loss of the second Cl(-) via a hydroxo-bridged species to yield the observed dihydroxo derivative. Meso and rac forms of the latter have been crystallized. The X-ray crystal structure of the rac-dihydroxo complex is reported, and it establishes the configurations of all the complexes reported. The (1)H NMR spectra for the hydroxo ions show very high field Co-OH resonances (ca. delta-0.5 ppm) not observed previously for such ions, and this result is discussed in the context of published (1)H NMR data for bridged Co-OH-Co species. The base hydrolysis kinetics for the dichloro ions are first order in [OH(-)], and deprotonation at an alpha-CH(2) center (alpha to a pyridyl) is identified as the source of the catalysis, since there is no NH center available for deprotonation on the ligand. These data further support the new pseudoaminate base hydrolysis mechanism first reported in 2003. The values of k(OH) for the second-order base-catalyzed reaction are ca. 4.0 M(-1) s(-1) for both the rac and meso isomers, and these results are discussed in terms of the increased acidities of these 4+ cations compared to their 2+ ion counterparts.