87,698 results

  • Crystal, molecular, and electronic structure of [Ta2Cl 4(??-Cl)4(PMe3)4]: A metal-metal single bond between eight-co-ordinate tantalum atoms

    Boyd, Peter; Jones, TC; Nielson, AJ; Rickard, CEF (1984)

    Journal article
    The University of Auckland Library

    The crystal structure of the title complex shows an MCl4M bridge across a weak Ta???Ta interaction and eight-fold co-ordination about each metal centre; an electronic structure calculation supports the existence of a ?? metal???metal bond.

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  • SPIN SPIN INTERACTIONS IN POLYNUCLEAR NICKEL(II) COMPLEXES - A MAGNETIC AND STRUCTURAL STUDY OF BINUCLEAR NICKEL(II) COMPLEXES OF ENOLIC 1,3,5-TRIKETONATES

    Boyd, Peter; LEE, KS; ZVAGULIS, M (1986-01-01)

    Journal article
    The University of Auckland Library

    The bulk magnetic susceptibilities of the binuclear nickel(11) complexes of the 1,3,5- triketonates from diethyl (2-oxocyclopentane-1,3-diy1)bisglyoxylate (H2ecg) (1) and diethyl 2,4,6-trioxoheptanedioate (H2decg) (2) have been measured over the temperature range 6-300 K. Antiferromagnetic spin-spin coupling is observed in each case with Nee1 temperatures in the range 45-65 K. The X-ray crystal structure of [ N i ( e ~ g ) ( p y ) ~ ] ~ . ( p y ) has been solved. The complex crystallize! with one pyridine of solvation in the space group PT [ a 10.507(1), b 13.384(1), c g q 9 9 2 ( 1 ) A, a 103.004(9), /3 115.867(9), y 86.857(10)", Z = 11. The molecule is binuclear with two coplanar triketonate dianions bridging the two six-coordinate nickel atoms through a di-p-0x0 linkage [Ni-Ob-Ni angle 102.0(1)"]. Two pyridine molecules and four triketonate oxygen atoms are bound to each nickel atoms. Magnetostructural correlations are presented in comparison with binuclear copper(11) 1,3,5- triketonates. The marked decrease in spin-spin interaction in the nickel complexes is attributed to increasing metal-bridging oxygen bond lengths that decrease the efficacy of the u antiferromagnetic pathway xyllOb 1 x y and contributions from the ferromagnetic xyllOb / / z2 pathway.

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  • Magnetic and theoretical studies of the electronic structure and spin state of the bis(tetrathiomolybdato)iron trianion, [Fe(MoS)]

    Bowmaker, Graham; Boyd, Peter; Sorrenson, RJ; Reed, CA; McDonald, JW (1987)

    Journal article
    The University of Auckland Library

    The magnetic susceptibility (1.7-300 K) and low-temperature (1.7-30 K) magnetization (1-4 T) of the bis(tetrathiometalato)iron trianion in polycrystalline [EtN][Fe(MoS)] have been measured. The temperature and field dependence of the magnetization have been analyzed in terms of an S = 3/2 orbital singlet ground state with a substantial zero-field splitting, g = 1.985 and D = +4.4 cm. Spin-restricted scattered wave-SCF-X?? calculations of the electronic structure of the [Fe(MoS)] anion (D symmetry) suggest that this spin state arises from occupancy of the iron d orbitals by seven electrons leading to a ground-state electron configuration (z)(xy)(x - y)(xz,yz) with the three unpaired electrons occupying the b and e orbitals of the iron atom. This leads to the formulation of [Fe(MoS)] as an iron(I) complex coordinated by two MoS anions. Even though a d configuration is favored, the resulting charge distribution is close to that of analogous d iron(II) [Fe(MoS)(SH)] complexes. The extra charge in the trianion is principally delocalized over the tetrathiomolybdate anions. Spin-polarized calculations lead to similar conclusions concerning the charge distribution with a change in the nature of the LUMO from an iron (xz,yz)e to a [(x - y)]b orbital. Calculations of the M??ssbauer quadrupole splitting agree well with the experimentally reported value. Contributions to the electric field gradient from both iron p and d electrons are found to be of similar magnitude. Similar calculations on the analogous tungsten complex suggest an explanation for why the isomer shifts of [Fe(WS)] are almost identical, but their quadrupole splittings are markedly different. ?? 1987 American Chemical Society.

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  • An SCF-X?? scattered wave study of the electronic structure and iron quadrupole splitting in the [Fe(MoS4)Cl2]2??? ion

    Boyd, Peter (1987-07)

    Journal article
    The University of Auckland Library

    Spin restricted and spin polarised SCF MS-X?? calculations have been performed to investigate the electronic structure and iron nuclear quadrupole coupling tensor of the [Fe(MoS4)Cl2]2??? dianion. It is found that both the spin restricted and spin polarised models predict a ground state quintet (S = 2) in agreement with that observed experimentally with similar overall charge distributions. However the calculated quadrupole coupling tensor varies in magnitude and sign between the two calculations. The spin polarised calculation gives values of the quadrupole coupling tensor that agree well both in magnitude, sign, and direction with those reported from single crystal M??ssbauer spectroscopy. The principal direction of the electric field gradient tensor is found to be sensitive to the relative occupations of the iron dz2 and dx2-y2 orbitals (which are mixed under C2V symmetry).

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  • BIS-BENZIMIDAZOLE-APPENDED BINUCLEATING PORPHYRIN LIGANDS - SYNTHESIS, CHARACTERIZATION, AND X-RAY STRUCTURE

    LARSEN, NG; Boyd, Peter; RODGERS, SJ; WUENSCHELL, GE; KOCH, CA; RASMUSSEN, S; TATE, JR; ERLER, BS; REED, CA (1986-10-29)

    Journal article
    The University of Auckland Library

    The synthesis and characterization of some new binucleating tetraarylporphyrin ligands is reported. The potentially most useful example is ??,??,5,15-bis[N-(2-methylbenzimidazolyl)acetamidophenyl]-??, ??,10,20-bis(pivalamidophenyl)porphine (8). The ligands have two appended benzimidazole arms which are designed to chelate a second metal directly above the porphyrin. The synthetic methodology for obtaining both the 5,15 (i.e., trans) and the 5,10 (i.e., cis) bis-appended ligand from an ??, ??, ??, ??,5,10,15,20 tetra-functionalized starting material is described in detail. A key design feature of the ligand system is amenability to single-crystal X-ray structure determination and this is demonstrated with an X-ray structure of a copper(II) complex. Crystal data for CuC74H66N12O4??1.5(diethyl ether)??toluene are the following: monoclinic, C2/c, a = 31.240 (4) ??, b = 16.769 (5) ??, c = 35.199 (4) ??, ?? = 121.4 (1)??; R = 0.0823, Rw = 0.0826. H bonding between the benzimidazole moieties and the pivalamido pickets is seen to determine the structural disposition of the appendages lying above the porphyrin ring. Iron(III) porphyrin complexes of these new ligands exist in a hydroxo monomer form as well as the familiar ??-oxo dimer form.

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  • TRIMETHYLPHOSPHINE COMPLEXES OF NIOBIUM(IV) AND TANTALUM(IV) - CRYSTAL, MOLECULAR, AND ELECTRONIC-STRUCTURES OF [M2CL4(MU-CL)4(PME3)4] (M = NB OR TA)

    Boyd, Peter; NIELSON, AJ; RICKARD, CEF (1987-02-01)

    Journal article
    The University of Auckland Library

    Reaction of [NbCl4(NCMe)3] or [NbCl4(thf)2](thf = tetrahydrofuran) with three equivalents of PMe3 gives the seven-co-ordinate complex [NbCl4(PMe3)3]. In refluxing benzene this complex converts to [Nb2Cl4(??-Cl)4(PMe3)4] which is obtained in good yield by reacting [NbCl4(NCMe)3] with two equivalents of PMe3. [NbCl4(thf)2] reacts with one equivalent of PMe3 in acetonitrile to give [{NbCl4(PMe3)}n]. [Ta2Cl4(??-Cl)4(PMe3)4] was obtained by reacting dilithium t-butyl-phosphide and PMe3 with TaCl5. Reduction of TaCl5 with Na???Hg amalgam in the presence of [NEt4]Cl gave [NEt4]2[TaCl6]. X-Ray structure determinations show the [M2Cl4(??-Cl)4(PMe3)4] complexes (M = Ta or Nb) are isostructural (space group Im3), containing four symmetrical chlorine bridges [Ta???Cl 2.544(2), Nb???Cl 2.545(1)??] across two metal centres [Ta???Ta 2.830(1), Nb???Nb 2.836(1)??] each of which have co-ordinated two terminal chlorines [Ta???Cl 2.497(2), Nb???Cl 2.506(1)??] and two terminal phosphine ligands [Ta???P 2.677(3), Nb???P 2.675(1)??]. Scattered wave X?? calculations on the electronic structure of the dinuclear complexes [M2Cl4(??-Cl)4(PH3)4](M = Nb or Ta) indicate the formation of a metal???metal single ?? bond between the metal atom dz2 orbitals. The ???????* energy separation suggests greater metal???metal interaction for the tantalum complex.

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  • ELECTRON-SPIN-RESONANCE STUDIES OF ONE-ELECTRON REDUCTION PRODUCTS OF NICKEL(II) AND PALLADIUM(II) COMPLEXES CONTAINING TETRATHIOMOLYBDATE, TETRATHIOTUNGSTATE, AND DIALKYLDITHIOCARBAMATE LIGANDS, "[M(M'S4)N(R2NCS2)2-N)]N-(N=0-2,M=NI OR PD,M'=MO OR W)

    Bowmaker, Graham; Boyd, Peter; CAMPBELL, GK; ZVAGULIS, M (1986-06-01)

    Journal article
    The University of Auckland Library

    The redox properties of nickel(II) and palladium(II) complexes of the type [M(M???S4)n(R2NCS2)2 ???n]n???(n= 0???2, M = Ni or Pd, M???= Mo or W) have been studied using d.c. and a.c. cyclic voltammetry at a platinum electrode in dichloromethane solution. The series of complexes show initial reversible or quasi-reversible one-electron reduction to give species containing a single unpaired electron. E.s.r. spectra have been used to identify the species produced after one-electron reduction. In the case of palladium, the reduction potential increases smoothly with increasing n whilst the unpaired electron is increasingly delocalised from the central metal ion with decreasing g anisotropy. In contrast, the reduction potential of the nickel complexes increases sharply from n= 0 to 1 with a corresponding increase in the g anisotropy. The reduction potential increases again for n= 2, however the e.s.r. spectrum shows an unusual ???reversal??? of g anisotropy (g??? < g???) compared to that expected for a d9 planar complex with the unpaired electron in a nickel dxy orbital (g??? > g???). It is suggested that in the case of n= 1 or 2 the unpaired electron is now occupying a molecular orbital composed of the low-lying molybdenum (or tungsten)d orbitals. This is supported by scattered wave X?? calculations of the electronic structure of the model complexes [Ni(H2NCS2)2], [Ni(MoS4)(H2NCS2)]??? and [Ni(MoS4)2]2??? and their oneelectron reduction products.

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  • AN SCF-MS-X-ALPHA STUDY OF THE BONDING AND NUCLEAR-QUADRUPOLE COUPLING IN 1-1 COMPLEXES OF AMINES WITH DIATOMIC HALOGENS AND INTERHALOGENS

    Bowmaker, Graham; Boyd, Peter (1987-12-01)

    Journal article
    The University of Auckland Library

    SCF-MS-X?? calculations of the electronic structure of the 1 : 1 complexes of pyridine with I2, IBr and ICl, and of the 1 : 1 trimethylamine???I2 complex have been carried out to investigate the bonding and nuclear quadrupole coupling in these molecules. Good agreement (to within 10% in most cases) is obtained between the calculated and observed halogen nuclear quadrupole coupling constants for these molecules. The agreement in the case of the nitrogen quadrupole coupling parameters is not as good, but the calculations do support some of the previously reported conclusions concerning the changes in the nitrogen orbital populations which occur upon complex formation. The results essentially support previous conclusions, based on Townes and Dailey analyses of the experimental quadrupole coupling data, concerning the extent of charge transfer and the distribution of the transferred charge in these molecules. They are, however, at variance with some of the results obtained from chemical shifts in the photoelectron spectra. The electric dipole moments of the molecules have also been calculated. These are ca. 30% higher than the highest reported experimental values, but show the expected trends from one compound to another.

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  • AN SCF-MS-X-ALPHA STUDY OF THE BONDING AND NUCLEAR-QUADRUPOLE COUPLING IN DIATOMIC HALOGENS AND INTERHALOGENS IN THE GROUND X1-SIGMA AND B3-PI-0 EXCITED-STATES

    Bowmaker, Graham; Boyd, Peter (1987-04-01)

    Journal article
    The University of Auckland Library

    SCF-MS-X?? calculations of the electronic structure of diatomic halogens and interhalogens XY (X = I, Br, Cl; Y = I, Br, Cl, F) have been used to investigate the bonding and nuclear quadrupole coupling in these molecules. Calculations have been carried out for the ground X1 ?? electronic state, and for the excited B3 ??0 state in the case of I2, Br2, ICl and IBr. Good agreement (to within 10% in most cases) is obtained between the calculated and observed nuclear quadrupole coupling constants for the molecules in the ground state. For the excited state the agreement is not as good, but the calculation does reproduce the observed decrease in the coupling constants to less than one quarter of their ground state values, and analysis of the contributions to the field gradients clearly shows the reasons for this. The electric dipole moments and electric quadrupole moments of the molecules have also been calculated. However, these prove to be much more strongly dependent on the variables used in the calculation (atomic sphere radii, inclusion of d orbitals). The results of the calculations have also been used to test some of the assumptions made in the Townes and Dailey method of analysis of nuclear quadrupole coupling data.

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  • SYNTHETIC, STRUCTURAL, AND THEORETICAL-STUDIES ON THE ELECTRON-DEFICIENT CUBANES (RC5H4)4TI4S4, (RC5H4)4V4S4, AND [(RC5H4)4V4S4]+

    DARKWA, J; LOCKEMEYER, JR; Boyd, Peter; RAUCHFUSS, TB; RHEINGOLD, AL (1988-01-06)

    Journal article
    The University of Auckland Library

    Treatment of (MeCp),V,S4 with PBu, gives the electron-deficient (56e) cubane (MeCp),V4S4 (1). Desulfurization of a 1:l mixture of (MeCp),V,S, and Cp2V2S4 gave a mixture of ring-substituted cubanes (MeCp),,Cp,V4S4 ( x = 1-4) while (MeCp),V,S, and Cp2V react to give exclusively (MeCp),Cp2V4S4. Compound 1 has a triplet ( S = 1) ground state, exhibits Curie-Weiss magnetic behavior, and has a well-resolved isotropically shifted 'H NMR spectrum. Cyclic voltammetry (vs Ag/AgCl) established the following redox series: 1+/1 (772 mV), 1/1- (176 mV), and 1-/1*- (-1281 mV). The salt [l](BF,) was prepared by the reaction of 1 and Ph3CBF4. The diamagnetic 52e cubanes (RCp),Ti4S4 (R = Me (2), i-Pr) were prepared from (RCp)TiCI,(THF), and (Me,Si),S. The compounds 1, [1](BF4), and 2 were characterized by single-crystal X-ray diffraction. Compound 1 crystallized in the cubic space group P43n with a = 16.551 ( 3 ) A with Z = 6; 671 unique reflecti_ons were processed to a final R ( F ) = 5.31 (R,(F) = 5.80). Compound [1](BF4) crystallized in the tetragonal space group I4 with a = 10.600 (3) A and c = 12.600 ( 3 ) A with Z = 2; 463 unique reflections were processed to a final R = 6.40 (R,(F) = 6.55). Compound 2 crystallized i n the orthorhombic space group Cmca with a = 1.412 (5), b = 17.293 (6), c = 24.503 ( 9 ) %, with Z = 8; 558 unique reflections were processed to give R = 10.5 (R,(F) = 12.6). 1 and [1]+ are extremely similar structurally; no bond distances differ by more than 0.03 A. The structure of 2 revealed two sets of Ti-Ti distances, four distances of 3.00 %, and two of 2.93 A. Electronic structure calculations of Cp4V4S4 and [Cp4V4S4]+ were performed by the scattered wave-Xa method. The ground-state configuration for the Cp4V4S4 metal bonding orbitals is a,2e4t22. The t2 orbital is predicted to be nonbonding, consistent with the structural results. The bonding i n 1, 1+, and 2 is discussed i n light of recent results from other laboratories

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  • Valence trapping in mixed-valence MnIIMnIII complexes of a macrocyclic binucleating ligand

    Chang, HR; Larsen, SK; Boyd, Peter; Pierpont, CG; Hendrickson, DN (1988)

    Journal article
    The University of Auckland Library

    The electronic structures of two mixed-valence MnMn complexes, [LMnClBr]??HO (2) and [LMnBr]??1/2CHCl (3), and the Mn complex LMnCl (4) are probed. The L ligand is the dianion of the Schiff base condensation of 2 mol of 1,3-diaminopropane and 2 mol of 2,6-diformyl-4-tert-butylphenol. The X-ray structure of complex 4 has been determined by using direct methods to give discrepancy factors of R = 0.0501 and R = 0.0697 for 1884 observed (|F| > 6.0??|F|) reflections. Complex 4 crystallizes in the triclinic space group P1 with one molecule in a unit cell with dimensions a = 7.270 (1) ??, b = 9.457 (2) ??, c = 12.316 (2) ??, ?? = 108.36 (1)??, ?? = 94.92 (1)??, and ?? = 99.58 (1)??. The observed and calculated densities are 1.39 (2) and 1.41 g cm, respectively. The L ligand in LMnCl is essentially planar, with the Mn ions displaced by 0.69 ?? on either side of the ligand plane. The Mn???Mn distance is 3.168 (3) ??. The X-ray structure of complex 2 has been determined by using direct methods to give R = 0.0711 and R = 0.0974 for 1732 observed (|F| > 6.0??|F|) reflections. Complex 2 crystallizes in the monoclinic space group C2/c with 8 molecules in a unit cell with dimensions a = 21.064 (7) ??, b = 13.559 (4) ??, c = 27.652 (6) ??, and ?? = 105.51 (2)??. The observed and calculated densities are 1.38 (2) and 1.34 g cm. The structural results for [LMnClBr]??HO show that this complex is valence trapped. The Mn ion has an average Mn-O length of 1.936 (10) ??, while the Mn ion exhibits Mn-O lengths of 2.129 (10) and 2.386 (11) ??. The Mn ion is coordinated axially to two halide ions and is 0.15 ?? out of the ligand-atom plane. The Mn ion is displaced 1.25 A from the ligand-atom plane on the opposite side and interacts with one of the halide ions coordinated to the Mn ion. Electrochemical results are presented for LMnX (X = Cl or Br) to show that these complexes have two one-electron oxidation waves at ca. +0.6 and ca. +1.2 V vs NHE. Variable-temperature magnetic susceptibility data are fit to give magnetic exchange parameters of +0.24, -2, and -1 cm for complexes 4, 2, and 3, respectively. Similar EPR data are found for the two mixed-valence complexes. At 7.5 K a glass of 3 gives an X-band spectrum with a broad g = 2 signal and apparent fine structure signals at g = 29.0, 7.4, 5.4, and 4.1. A combination of Orbach relaxation processes involving low-lying spin states and modulation of the g, g values for the ground doublet by strain effects on the magnitude of the isotropic exchange and/or single-ion zero-field splitting are suggested as the origin of the absence of resolution of manganese hyperfine structure in the g = 2 signal. Relatively weak antiferromagnetic interactions are seen for the five known binuclear MnMn complexes, whereas, strong antiferromagnetic interactions are found for the five known binuclear MnMn complexes. The possible causes of this difference in magnitude of exchange interactions are discussed.

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  • SPIN SPIN INTERACTIONS IN POLYNUCLEAR NICKEL(II) COMPLEXES - A LOW-TEMPERATURE SUSCEPTIBILITY AND MAGNETIZATION STUDY OF THE CUBANE TETRANUCLEAR COMPLEX NI4(II)(CYCLOHEXANE-R-1,C-3,C-5-TRIAMINE)4(MU-OH)4(NO3)4.7H2O

    Boyd, Peter; MARTIN, RL; SCHWARZENBACH, G (1988-01-01)

    Journal article
    The University of Auckland Library

    The bulk magnetic susceptibility of the complex Ni4(chta)4(??-OH)4(NO3)4.7H2O ( chta = cyclohexane-r-1,c-3,c-5-triamine) has been measured over the temperature range from 1.9 to 300 K at a field of 1 T. The observed data were fitted by a Heisenberg exchange model for the susceptibility of a cubane [Ni4II(??-OH)4]4+ cluster with a single exchange interaction J between each pair of NiII ions for a single ion g value 2.20 and an antiferromagnetic spin-spin coupling parameter 2J = -1.14 cm-1. The bulk magnetization measured at fields of 1 to 5 T in the temperature range from 5 to 1.9 K differed from that calculated from the simple Heisenberg model above. Several possibilities were examined for this deviation including intercluster interactions, single ion zero-field splitting and differing exchange coupling constants between nickel(II) ions in the cluster. This last model was successful in describing the complete set of magnetic data. A comparison with other known oxygen-bridged nickel(II) cubane clusters shows a change from antiferromagnetic to ferromagnetic coupling as the Ni-O-Ni bond angle decreases. It appears that the cubane cluster studied here is in a region very close to the crossover from antiferromagnetic to ferromagnetic coupling between the nickel ions.

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  • THEORETICAL-STUDIES ON THE STABILITY OF TL-C SIGMA-BONDS IN ALIPHATIC ORGANOTHALLIUM COMPOUNDS

    SCHWERDTFEGER, P; Boyd, Peter; Bowmaker, Graham; MACK, HG; OBERHAMMER, H (1989-01-04)

    Journal article
    The University of Auckland Library

    Calculations of the electronic structure and bonding in organothallium compounds (TIR, TIR???, T1R2, TIR2+, TIR3, TIR4-, T1R2Br, and TIR??? with R = C H 3 , R??? = C2H3, or C 2 H ) , in the hydrides ( R = H ) , their dissociation products (CH,, C 2 H 3 , C 2 H , C 2H6 , and TIBr), and TI2 have been carried out. A modified version of Pople???s S C F program GAUS S IANB Z has been used, in which the core electrons on the TI atom are described by a quasi-relativistic [Ptl-core pseudopotential. All bond distances and angles have been optimized with a Fletcher-Powell procedure. SCF-SW-Xa calculations have been carried out to determine the extent of the TI 5d core orbital participation in the T1-C bond. Vibrational frequencies have been calculated f rom the harmonic diagonal S C F valence force field. As a result, the previously undetected molecule T1CH3 is found to be stable with respect to dissociation into TI and CH,. The isolated T1(CH3), molecule is found to possess the expected planar-trigonal (TIC,) structure. It is explained why organothallium chemistry is mainly the chemistry of Tl(II1) in contrast to inorganic thallium chemistry and t h a t TI-TI bonds a r e very weak or do not exist is a relativistic effect.

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  • An SCV-MS-X?? study of the bonding and nuclear quadrupole coupling in oxygen compounds of the halogens

    Bowmaker, Graham; Boyd, Peter (1989)

    Journal article
    The University of Auckland Library

    SCF-MS-X?? calculations of the electronic structure of the oxy-halogen compounds Cl2O, ClO, ClO2, ClO2???, ClO3???, ClO4???, BrO, BrO2, BrO3???, IO3???, and IO4??? have been used to investigate the bonding and nuclear quadrupole coupling in these molecules and ions. The halogen d orbital participation in the bonding increases with increasing oxidation number of the halogen atom in the molecule or ion. The inclusion of the halogen d orbitais results in an increase in the principal component of the halogen electric field gradient tensor, but only in some cases this result in improved agreement between the calculated and observed halogen nuclear quadrupole coupling constants. In other cases better agreement is obtained for calculations involving the halogen p orbitais only in the valence shell. Thus the results do not provide unambiguous evidence for the involvement of halogen d orbitais in the bonding. In the case of the ClO3??? and BrO3??? ions for which 17O quadrupole coupling constants have been reported, the X?? values have been calculated. Agreement between the calculated and observed values is much poorer than for the halogen coupling constants. The results of the calculations have also been used to investigate some of the approximations made in the Townes and Dailey method of analysis of nuclear quadrupole coupling data.

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  • Modeling the photosynthetic water oxidation center. Preparation and properties of tetranuclear manganese complexes containing [Mn4O2]6+,7+,8+ cores, and the crystal structures of Mn4O2(O2CMe)6(bipy)2 and [Mn4O2(O2CMe)7(bipy)2](ClO4)

    Vincent, JB; Christmas, C; Chang, HR; Li, Q; Boyd, Peter; Huffman, JC; Hendrickson, DN; Christou, G (1989)

    Journal article
    The University of Auckland Library

    An inorganic model approach to the photosynthetic water oxidation enzyme has been initiated, and synthetic entry into tetranuclear Mn complexes containing [Mn4O2]6+,7+,8+ cores has been achieved. They have been obtained by bipyridine (bipy)-mediated conversion of trinuclear [Mn3O]-containing species, with the product oxidation level governed by the exact identity of the [Mn3O] reagent employed. Treatment of Mn3O(O2CMe)6(py)3 with ???3 equiv of bipy in MeCN yields Mn4O2(O2CMe)6(bipy)2 (1) in 91% yield. Complex 1??2CHCl3 crystallizes in triclinic space group P1 with (at -160??C) a = 13.883 (3) ??, b = 10.592 (2) ??, c = 8.848 (1) ??, ?? = 91.18 (1)??, ?? = 72.14 (1)??, ?? = 71.44 (1)??, V = 1163.84 ??3, and Z = 1. A total of 3064 unique data with F > 3??(F) were refined to values of R and Rw of 3.23 and 3.75%, respectively. The molecule lies on an inversion center and contains a planar Mn4 rhombus with two ??3-O atoms, one above and one below the Mn4 plane. The resulting [Mn4O2]6+ core is mixed valence (2MnII, 2MnIII) and can be considered as fusion of two Mn3O units by edge-sharing. Peripheral ligation is by six ??2-O2CMe and two terminal bipy groups to yield a complex with imposed Ci symmetry. Treatment of Mn3O(O2CR)6(py)2(H2O) (R = Ph, 3-Me-Ph) with ???3 equiv of bipy in MeCN yields Mn4O2(O2CR)7(bipy)2 (R = Ph (2) or 3-Me-Ph (3)) containing MnII, 3MnIII. Similarly, treatment of [Mn3O(O2CR)6(py)3](ClO4) (R = Me, Et, Ph) with ???3 equiv of bipy in MeCN yields [Mn4O2(O2CR)7(bipy) 2](ClO4) (R = Me (4), Et (8), Ph (9)) containing 4MnIII. Use of 4,4???-Me2-bipy instead of bipy results in the corresponding complex [Mn4O2(O2CMe)7(4,4???-Me 2-bipy)2] (ClO4) (5). Complex 4 has been found to undergo facile carboxylate substitution when more acidic carboxylic acids are added; addition of PhCOOH or HCOOH to CH2Cl2 solutions of 4 yields 9 and [Mn4O2(O2CH)7(bipy)2] (ClO4) (6), respectively. Complex 9 can also be synthesized directly by reaction of NBun4MnO4 with Mn(O2CMe)2??4H2O in pyridine solution in the presence of PhCOOH, bipy, and NBun4ClO4. Complex 4 crystallizes in triclinic space group P1 with (at -155??C) a = 21.133 (11) ??, b = 11.428 (5) ??, c = 11.839 (6) ??, ?? = 102.12 (2)??, ?? = 119.72 (2)??, ?? = 78.20 (2)??, V = 2410.61 ??3, and Z = 2. A total of 6294 unique data were refined to values of R and Rw of 9.05 and 8.94%, respectively. Complex 4 contains an [Mn4O2]8+ core which is not planar as found in 1 but exhibits an Mn4 butterfly arrangement with both ??3-O atoms on the same side of the molecule. Peripheral ligation is again by ??2-O2CMe and terminal bipy groups, but now there is an additional MeCO2- ligand bridging the two "hinge" Mn atoms of the butterfly to yield C2 symmetry. Complexes 1 and 4 display both "short" and "long" Mn???Mn separations, 2.779 (1), 3.288-3.481 (1) ?? and 2.848 (5), 3.312-3.385 (5) ??, respectively. Variable-temperature, solid-state, magnetic-susceptibility studies have been performed on representative complexes 1 and 4 in the temperature range 5-300 K. The observed susceptibility data have been fitted to models involving isotropic exchange interactions between the high-spin manganese ions in the clusters. In the case of MnII2MnIII2 complex 1, fitting the data gave J13 = -3.12 cm-1 for the interaction between the two di-??-oxo-bridged MnIII ions and J(MnII-MnIII) = -1.97 cm-1. With these parameters, complex 1 has an S = 2 ground state with six other spin states within 15 cm-1. Fitting the data for MnIII4 complex 4 gave for the di-??-oxo-bridged MnIII pair J13 = -23.5 cm-1, and for the other MnIII-MnIII interaction J = -7.8 cm-1. The ground state for 4 has S = 3 with two lowest lying excited states being two energetically degenerate S = 2 states at ???15 cm-1 above the S = 3 ground state. The nature of the ground and low-lying spin states for 1 and 4 were confirmed by using magnetization measurements at fields up to 48 kG and temperatures down to 1.8 K. The change in the magnitude of spin-spin interaction between the two central di-??-oxo-bridged MnIII ions in 1 and 4 can be related to the differing single-ion coordination in these two complexes. The electronic difference spectrum of the [Mn4O2(O2CPh)7(bipy) 2]0,+ pair has been found to be extremely similar to the S0 ??? S1 difference spectrum of the water oxidation site, suggesting the latter involves a MnII ??? MnIII transition also. Complex 4 in CDCl3 has been found to display a well-resolved 1H NMR spectrum in which all expected proton resonances have been located and assigned. Both ??and ?? spin-delocalization mechanisms appear to be operative. The combined results of this work are discussed with respect to their biological relevance, and it is proposed that complexes containing the [Mn4O2]6+,7+,8+ cores represent potential synthetic analogues of the S-1, S0, and S1 states, respectively, of the water oxidation enzyme.

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  • Preparation, structure, and magnetochemistry of hexanuclear manganese oxide complexes: Chemically and thermally induced aggregation of MnO(OCPh)(py)(HO) forming products containing the [MnO] core

    Schake, AR; Vincent, JB; Christou, G; Folting, K; Huffman, JC; Li, Q; Boyd, Peter; Hendrickson, DN (1989)

    Journal article
    The University of Auckland Library

    A variety of synthetic procedures are described that convert the trinuclear complex MnO(OCPh)(py)(HO) (1) into the hexanuclear complexes [MnO(OCPh)(py) (MeCN)]??2MeCN (2) and [MnO(OCPh)(py) ]??EtO (3). With two exceptions, the procedures involve treatment of 1 with phenolic molecules (phenol, p-cresol, tyrosine, 2,2???-biphenol, 8-hydroxyquinoline) or the mononuclear Mn complexes [Mn(biphen)(biphenH)] and [Mn(Brbiphen)(OCPh)] in MeCN and lead to high (50-75%) yields of complex 2. It is proposed that the mechanism involves reduction of the [MnO] unit of 1 to a [MnO] species, which spontaneously aggregates to 2 containing the [MnO] core. This proposal is supported by the formation of 2 when 1 is reduced with the outer-sphere reductant sodium acenaphthylenide. Complex 3 is obtained in 34% yield when a PhCN solution of 1 is refluxed for 10 min. Complex 2 crystallizes in triclinic space group P1 with, at -140??C, a = 15.389 (4) ??, b = 19.513 (6) ??, c = 14.433 (4) ??, ?? = 91.84 (1)??, ?? = 94.08 (1)??, ?? = 87.85 (1)??, and Z = 2. The structure was solved and refined by using 5999 unique reflections with F > 3.0??(F). Complex 3 crystallizes in triclinic space group P1 with, at -160??C, a = 24.394 (16) ??, ?? = 19.876 (11) ??, c = 19.245 (12) ??, ?? = 89.71 (3)??, ?? = 105.43 (3)??, ?? = 79.89 (3)??, and Z = 2. The structure was solved and refined by using 6000 unique reflections with F > 3.0??(F). Final values of discrepancy indices R (R) for 2 and 3 are 7.82 (7.17) and 7.51% (7.63%), respectively. Complex 2 contains a [MnO] core that can be conveniently described as two edge-sharing Mn tetrahedra at the center of each of which is a ??-O ion. Peripheral ligation to the octahedrally coordinated Mn centers is provided by ten bridging benzoate, two terminal py, and two terminal MeCN groups. The complex is mixed valence (Mn Mn ), and the Mn centers are assigned as the two central metal ions bridged by two O ions. Complex 3 contains two independent [MnO] complexes in the asymmetric unit, each of which is essentially identical with that in complex 2, except that the terminal ligands are all py. The results of solid-state magnetic susceptibility studies on complex 2 in the temperature range 2.95-300 K are described. With use of the idealized symmetry of two edge-sharing tetrahedra (D), the derivation by the Kambe vector-coupling method of a theoretical model to account for the intramolecular exchange interactions is described. Least-squares fitting of the susceptibility versus temperature data to the model yields the parameters J = -42.0 cm, J = -0.8 cm, J = -2.4 cm, and g = 1.90, where the J values refer to the Mn-Mn, Mn-Mn, and Mn-Mn interactions, respectively. These exchange parameters give an S = 0 ground state with two degenerate S = 1 states at 4 cm and degenerate S = 0, 1, 2 states at 9 cm higher in energy. The magnitudes and signs of the exchange parameters are compared with those reported for other oxo-bridged Mn complexes. Complexes 2 and 3 join a small but growing number of high-nuclearity Mn aggregates, and the prospective and potential procedures that could be employed for the synthesis of higher nuclearity Mn aggregates that might display molecular ferromagnetism are discussed. ?? 1989 American Chemical Society.

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  • SCALED HARTREE-FOCK FORCE-FIELD CALCULATIONS FOR ORGANOTHALLIUM COMPOUNDS - NORMAL-MODE ANALYSIS FOR TLCH3, TL(CH3)2+, TL(CH3)3, TL(CH3)2BR, AND TL(CH3)4-

    SCHWERDTFEGER, P; Bowmaker, Graham; Boyd, Peter; Ware, David; Brothers, Penelope; NIELSON, A (1990-02)

    Journal article
    The University of Auckland Library

    In a recent paper we presented Hartree-Fock (HF) calculations for aliphatic organothallium compounds. The diagonal HF force constants obtained from a Fletcher-Powell geometry optimization are now used for a normal-mode analysis of TlCH,, Tl(CH,),+, T1(CH3),, T1(CHJ2Br, and Tl(CH3)c. In order to calculate frequencies comparable to experimental values, the HF force field has been scaled by using scaling factors obtained from experimental infrared and Raman measurements on T1(CH3)2+ and TlBr. The vibrational spectra of Tl(CH3)2+ were remeasured (infrared and Raman) in order to obtain an accurate force field. Predictions are made for the vibrational spectrum of the as yet undetected TlCH, molecule. Experimental infrared and Raman results for T1(CH3), compare reasonably well with our calculated frequencies (ex- perimental v,,(Tl-C) = 450 cm-', v,,(Tl-C) = 465 cm-l, v,,(C-H) = 2917 cm-l, v,,(C-H) = 3020 cm-l; scaled HF u,,(Tl-C) = 488 cm-', v,,(Tl-C) = 514 cm-', vSp(C-H) = 2892 cm-l, Y, (C-H) = 2999 cm-l). Relativistic and correlation effects are analyzed for the vibrational frequencies ~ f i l ( C H , ) ~ +

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  • Potential antitumor agents. 62. Structure-activity relationships for tricyclic compounds related to the colon tumor active drug 9-oxo-9H-xanthene-4-acetic acid

    Rewcastle, Gordon; Atwell, Graham; Palmer, Brian; Boyd, Peter; Baguley, Bruce; Denny, William (1991)

    Journal article
    The University of Auckland Library

    A series of tricyclic analogues of 9-oxo-9H-xanthene-4-acetic acid have been prepared and evaluated for their ability to cause hemorrhagic necrosis in subcutaneously implanted colon 38 tumors in mice, in an effort to extend the structure - activity relationships for this series. As was found previously with analogues of flavone-8-acetic acid (FAA) (Atwell et al. Anti-Cancer Drug Des. 1989, 4, 161), all electronic modifications of the XAA nucleus led to severe decreases or complete abolition of activity, suggesting narrow structure - activity relationships. Dipole moments for many of the compounds were computed, and the degree to which the molecular dipole moment lay out of the plane of the aromatic part of these molecules was found to be determined largely by the contributions from the acetic acid moiety relative to that from the tricyclic ring system. There did not appear to be any general relationship between the magnitude of the dipole moment and activity. However, for compounds containing the 9-carbonyl functionality, the orientation of the dipole vector may be of significance. In all compounds possessing an ether group peri to the acetic acid side chain, there was a close approach (ca. 2.4 ??) between this and the side chain OH.

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  • Theoretical studies on CuF+, CuF, CuF???, CuF2, and CuF2 ???

    Schwerdtfeger, P; Boyd, Peter; Bowmaker, Graham; Aldridge, LP (1990-09)

    Journal article
    The University of Auckland Library

    Ab initio SCF studies were performed with Cu and F basis sets of near-Hartree-Fock (HF) limit quality to obtain accurate SCF results for the molecular ground state properties of CuF, CuF, CuF, CuF, and CuF , as well as for the first two low-lying excited states of CuF. A study on the effects of electron correlation was carried out by M??ller-Plesset (MP) and configuration interaction (Cl) calculations. The effect of relativity on theCu nuclear quadrupole coupling in CuF was determined by use of a coupled HF procedure for a first-order spin-orbital-averaged Pauli operator. At the HF level theCu coupling constant was found to be 35.8 MHz (in eqQ h), while allowing for relativity the value was reduced to 29.1 MHz, which is in better agreement with the experimental value of 22.0 MHz. The calculated molecular properties for CuF [r = 1.737 ??, D=4.38 eV, ?? = 562 cm (MP4);r= 1.796 ??, D = 3.91 eV, ??=585 cm (CISD)] were in good agreement with experiment (r = 1.745 ??, D = 4.43 eV, ??=623 cm). The adiabatic ground-state potential curve of CuF avoids crossing near the equilibrium distance between the two ionic potential curves Cu-F and Cu-F. At the crossing point the Cu and F electric field gradients show a sharp discontinuity.

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  • HALOGEN EXCHANGE IN BORON TRIHALIDES - THE BCL3-BI3 REACTION

    Boyd, Peter; TAYLOR, MJ (1992-03-02)

    Journal article
    The University of Auckland Library

    ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a ???Full Text??? option. The original article is trackable via the ???References??? option.

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