Spin-orbit-induced anomalous pH-dependence in (1)H NMR spectra of Co(III) amine complexes: a diagnostic tool for structure elucidation

The pH-dependent (1)H NMR characteristics of a series of Co(III)-(polyamin)-aqua and Co(III)-(polyamin)-(polyalcohol) complexes, [Co(tach)(ino-kappa(3)-O(1,3,5))](3+) (1(3+)), [Co(tach)(ino-kappa(3)-Omicron(1,2,6))](3+) (2(3+)), [Co(tach)(taci-kappa-Nu(1)-kappa(2)-O(2,6))](3+) (3(3+)), [Co(ditame)(H(2)O)](3+) (4(3+)), and [Co(tren)(H(2)O)(2)](3+) (5(3+)), were studied in D(2)O by means of titration experiments (tach = all-cis-cyclohexane-1,3,5-triamine, ino = cis-inositol, taci = 1,3,5-triamino-1,3,5-trideoxy-cis-inositol, tren = tris(2-aminoethyl)amine, ditame = 2,2,6,6-tetrakis-(aminomethyl)-4-aza-heptane). A characteristic shift was observed for H(-C) hydrogen atoms in the alpha-position of a coordinated amino group upon deprotonation of a coordinated oxygen donor. For a cis-H-C-N-Co-O-H arrangement, deprotonation of the oxygen donor resulted in an additional shielding (shift to lower frequency) of the H(-C) proton, whereas for a trans-H-C-N-Co-O-H arrangement, deprotonation resulted in a deshielding (shift to higher frequency). The effect appears to be of rather general nature: it is observed for primary (1(3+)-5(3+)), secondary (4(3+)), and tertiary (5(3+)) amino groups, and for the deprotonation of an alcohol (1(3+)-3(3+)) or a water (4(3+), 5(3+)) ligand. Spin-orbit-corrected density functional calculations show that the high-frequency deprotonation shift for the trans-position is largely caused by a differential cobalt-centered spin-orbit effect on the hydrogen nuclear shielding. This effect is conformation dependent due to a Karplus-type behavior of the spin-orbit-induced Fermi-contact shift and thus only significant for an approximately antiperiplanar H-C-N-Co arrangement. The differential spin-orbit contribution to the deprotonation shift in the trans-position arises from the much larger spin-orbit shift for the protonated than for the deprotonated state. This is in turn due to a trans-effect of the deprotonated (hydroxo or alkoxo) ligand, which weakens the trans Co-N bond and thereby interrupts the Fermi-contact mechanism for transfer of the spin-orbit-induced spin polarization to the hydrogen nucleus in question. The unexpectedly large long-range spin-orbit effects found here for 3d metal complexes are traced back to small energy denominators in the perturbation theoretical expressions of the spin-orbit shifts.     

Ab initio prediction of the gas- and solution-phase acidities of strong Brønsted acids: the calculation of pKa values less than -10

The intrinsic gas-phase acidities of a series of 21 Br?nsted acids have been predicted with G3(MP2) theory. The G3(MP2) results agree with high level CCSD(T)/CBS acidities for H(2)SO(4), FSO(3)H, CH(3)SO(3)H, and CF(3)SO(3)H to within 1 kcal/mol. The G3(MP2) results are in excellent agreement with experimental gas-phase acidities in the range 342-302 kcal/mol to within <1 kcal/mol for 14 out of 15 acids. Five of the six acids in the range of 302-289 kcal/mol had an average deviation of 5.5 kcal/mol and the strongest acid, (CF(3)SO(2))(3)CH, deviated by 15.0 kcal/mol. These high-level calculations strongly suggest that the experimental acidities in this very acidic part of the scale need to be remeasured. The CCSD(T)/CBS (mixed exponential Gaussian) additive approach for CH(3)CO(2)H, HNO(3), H(2)SO(4), CH(3)SO(3)H, FSO(3)H, and CF(3)SO(3)H gives excellent agreement (+/-1 kcal/mol) with experiment for the DeltaH(f)(0)'s of non-sulfur containing species, and supports the low end of the experimental values for H(2)SO(4) and FSO(3)H. Use of a larger basis set (aug-cc-pV5Z) in the CBS extrapolation improves the agreement with experiment for both H(2)SO(4) and FSO(3)H. The G3(MP2) heats of formation for RSO(3)H molecules tend to be underestimated as compared to the CCSD(T)/CBS approach by 2.5-7.0 kcal/mol. COSMO solvation calculations were used to predict solution free energies and pK(a) values with pK(a)'s up to -17.4. Including the solvation of the proton gives good agreement with experimental pK(a) values in the very acidic regime, whereas it is less reliable for weaker acids. The use of CH(3)CO(2)H and HNO(3) as reference acids in the less acidic and more acidic regions of the scale, respectively, provided improved results to within +/-2 pK(a) units in nearly all cases (+/-3 kcal/mol accuracy).     

Secondary kinetics of methanol decomposition: theoretical rate coefficients for 3CH2 + OH, 3CH2 + 3CH2, and 3CH2 + CH3

Direct variable reaction coordinate transition state theory (VRC-TST) rate coefficients are reported for the (3)CH(2) + OH, (3)CH(2) + (3)CH(2), and (3)CH(2) + CH(3) barrierless association reactions. The predicted rate coefficient for the (3)CH(2) + OH reaction (approximately 1.2 x 10(-10) cm(3) molecule(-1) s(-1) for 300-2500 K) is 4-5 times larger than previous estimates, indicating that this reaction may be an important sink for OH in many combustion systems. The predicted rate coefficients for the (3)CH(2) + CH(3) and (3)CH(2) + (3)CH(2) reactions are found to be in good agreement with the range of available experimental measurements. Product branching in the self-reaction of methylene is discussed, and the C(2)H(2) + 2H and C(2)H(2) + H2 products are predicted in a ratio of 4:1. The effect of the present set of rate coefficients on modeling the secondary kinetics of methanol decomposition is briefly considered. Finally, the present set of rate coefficients, along with previous VRC-TST determinations of the rate coefficients for the self-reactions of CH(3) and OH and for the CH(3) + OH reaction, are used to test the geometric mean rule for the CH(3), (3)CH(2), and OH fragments. The geometric mean rule is found to predict the cross-combination rate coefficients for the (3)CH(2) + OH and (3)CH(2) + CH(3) reactions to better than 20%, with a larger (up to 50%) error for the CH(3) + OH reaction.     

Divergent pathways of C-H bond activation: reactions of (t-Bu(3)PN)(2)TiMe(2) with trimethylaluminum

The reaction of AlMe(3) with (t-Bu(3)PN)(2)TiMe(2) 1 proceeds via competitive reactions of metathesis and C-H activation leading ultimately to two Ti complexes: [(mu(2)-t-Bu(3)PN)Ti(mu-Me)(mu(4)-C)(AlMe(2))(2)](2) 2, [(t-Bu(3)PN)Ti(mu(2)-t-Bu(3)PN)(mu(3)-CH(2))(2)(AlMe(2))(2)(AlMe(3))] 3, and the byproduct (Me(2)Al)(2)(mu-CH(3))(mu-NP(t-Bu(3))) 4. X-ray structural data for 2 and 3 are reported. Compound 3 undergoes thermolysis to generate a new species [Ti(mu(2)-t-Bu(3)PN)(2)(mu(3)-CH(2))(mu(3)-CH)(AlMe(2))(3)] 5. Monitoring of the reaction of 1 with AlMe(3) by (31)P[(1)H] NMR spectroscopy revealed intermediates including (t-Bu(3)PN)TiMe(3) 6. Compound 6 was shown to react with AlMe(3) to give 2 exclusively. Kinetic studies revealed that the sequence of reactions from 6 to 2 involves an initial C-H activation that is a second-order reaction, dependent on the concentration of Ti and Al. The second-order rate constant k(1) was 3.9(5) x 10(-4) M(-1) s(-1) (DeltaH(#) = 63(2) kJ/mol, DeltaS(#) = -80(6) J/mol x K). The rate constants for the subsequent C-H activations leading to 2 were determined to be k(2) = 1.4(2) x 10(-3) s(-1) and k(3) = 7(1) x 10(-3) s(-1). Returning to the more complex reaction of 1, the rate constant for the ligand metathesis affording 4 and 6 was k(met) = 6.1(5) x 10(-5) s(-1) (DeltaH(#) = 37(3) kJ/mol, DeltaS(#) = -203(9) J/mol x K). The concurrent reaction of 1 leading to 3 was found to proceed with a rate constant of k(obs) of 6(1) x 10(-5) s(-1) (DeltaH(#) = 62(5) kJ/mol, DeltaS(#)= -118(17) J/mol x K). Using these kinetic data for these reactions, a stochastic kinetic model was used to compute the concentration profiles of the products and several intermediates with time for reactions using between 10 and 27 equivalents of AlMe(3). These models support the view that equilibrium between 1 x AlMe(3) and 1 x (AlMe(3))(2) accounts for varying product ratios with the concentration of AlMe(3). In a similar vein, similar equilibria account for the transient concentrations of 6 and an intermediate en route to 3. The implications of these reactions and kinetic and thermodynamic data for both C-H bond activation and deactivation pathways for Ti-phosphinimide olefin polymerization catalysts are considered and discussed.     

First-principles study on proton dissociation properties of fluorocarbon- and hydrocarbon-based membranes in low humidity conditions

We present a theoretical study on the proton dissociation properties of the membranes for polymer electrolyte fuel cells. A density functional theory method is used to study the influence of fluorocarbon and hydrocarbon backbones on proton dissociation, the interaction of water molecules with the sulfonic acid group, and the energy barriers for proton dissociation. Better proton dissociation properties of CH(3)SO(3)H compared to CF(3)SO(3)H are observed from statistical analyses of the optimized structures for both systems. However, the calculated energy barriers for proton dissociation are lower for CF(3)SO(3)H than for the CH(3)SO(3)H system. At the same time, the interaction of water molecules is stronger for CH(3)SO(3)H than for CF(3)SO(3)H. Also, the analysis of the hydrogen-bonding network in both systems shows that the number of hydrogen bonds formed around the sulfonic acid group in CH(3)SO(3)H is larger than that in CF(3)SO(3)H. Therefore, the decrease of the energy barrier with increasing number of coordinating water molecules, pronounced in the case of CH(3)SO(3)H, may lower the barrier, which enhances good proton conductivity of a hydrocarbon-based polymer in low humidity conditions. Thus the hydration ability of a sulfonic acid group is an important factor for realizing better proton dissociation in low humidity conditions.     

Speciation of M+3-hydroxypyrone chelation complexes by electrospray ionization ion trap and Fourier transform ion cyclotron resonance mass spectrometry

Kojic acid (5-hydroxy-2-hydroxymethyl-4-pyrone) is known to have a high affinity for transition metals, and it and its derivatized cogeners are used both analytically and clinically. The interactions between kojic acid (KA) and eleven +3 metals (Al(+3), As(+3), Cr(+3), Ga(+3), Fe(+3), In(+3), Yb(+3), Y(+3), Gd(+3), Nd(+3), La(+3)) were examined by electrospray ionization mass spectrometry (ESI-MS) using an ion trap in an aqueous medium. For a subset of five ions, Fourier transform ion cyclotron resonance (FTICR)-MS was conducted to provide accurate mass confirmation of peak assignments for metals having clustering of abundant isotopes. KA readily formed complexes with all the metal ions tested. The most common complexes observed were ML(3)H(+) and M(2)L(5). Different behavior was seen for small and large ionic radius ions, with a relative cut-off between In(+3) ( approximately 80 pm) and Yb(+3) ( approximately 87 pm); a striking trend in % collision energy vs. cluster complexity was revealed. The KA-Cr(+3)complex shows a high affinity for H(2)O molecules in the gas phase, whilst In(+3) shows a preference for dimetal complexes and Y(+3) a deviant behavior when complexed to two neutrals.     

Site-isolated luminescent europium complexes with polyester macroligands: metal-centered heteroarm stars and nanoscale assemblies with labile block junctions

The synthesis of a series of polymeric Eu(III) complexes with polyester ligands, along with supporting emission spectra, luminescence lifetimes, and, for a Eu block copolymer film, atomic force microscopy (AFM) data, is presented. Dibenzoylmethane was derivatized with a hydroxyl initiator site (dbmOH, 1) for tin octoate catalyzed ring opening polymerization of dl-lactide. The resulting poly(lactic acid) macroligand, dbmPLA (2), was combined with EuCl3 to generate Eu(dbmPLA)3 (3). Chelation of both dbmPLA and a polycaprolactone-functionalized bipyridine ligand (bpyPCL2) led to the Eu(III)-centered heteroarm star Eu(dbmPLA)3(bpyPCL2) (4). Unpolarized emission spectra and luminescence lifetimes were recorded for the Eu polymers in CH2Cl2 and for Eu(dbmPLA)3, as a film. Solution data for Eu(dbm)3 and Eu(dbm)3(bpy) were collected for comparison. For Eu tris(dbm) complexes, data were fit to a double exponential decay, indicating the presence of multiple species. Relative amounts of the longer lifetime component increase in the series Eu(dbm)3 solutions to Eu(dbmPLA)3 solutions to Eu(dbmPLA)3 films, perhaps suggesting benefits of the "polymer shell effect" and the diminishment of aquo adducts known to shorten lifetimes. As with the nonpolymeric analogue, data for Eu(dbmPLA)3(bpyPCL2) fit to a single-exponential decay. The sharpness of the feature at 579.7 nm, attributable to the 5D0 --> 7F0 transition in the emission spectrum of 4, lends further support for a homogeneous sample. AFM studies of "as cast" thin films of 4 reveal a lamellar structure with a 17.5 nm repeat. These microstructures, inferred to contain Eu luminophores at the glassy PLA-crystalline PCL domain interfaces, are modified by thermal treatment.     

An experimental and computational study on the effect of Al(OiPr)3 on atom-transfer radical polymerization and on the catalyst-dormant-chain halogen exchange

Compound Al(OiPr)3 is shown to catalyze the halide-exchange process leading from [Mo(Cp)Cl2(iPrN=CH-CH=NiPr)] and CH3CH(X)COOEt (X=Br, I) to the mixed-halide complexes [Mo(Cp)ClX(iPrN=CH-CH=NiPr)]. On the other hand, no significant acceleration is observed for the related exchange between [MoX3(PMe3)3] (X=Cl, I) and PhCH(Br)CH3, by analogy to a previous report dealing with the Ru(II) complex [RuCl2(PPh3)3]. A DFT computation study, carried out on the model complexes [Mo(Cp)Cl2(PH3)2], [MoCl3(PH3)3], and [RuCl2(PH3)3], and on the model initiators CH3CH(Cl)COOCH3, CH3Cl, and CH3Br, reveals that the 16-electron Ru(II) complex is able to coordinate the organic halide RX in a slightly exothermic process to yield saturated, diamagnetic [RuCl2(PH3)3(RX)] adducts. The 15-electron [MoCl3(PH3)3] complex is equally capable of forming an adduct, that is, the 17-electron [MoCl3(PH3)3(CH3Cl)] complex with a spin doublet configuration, although the process is endothermic, because it requires an energetically costly electron-pairing process. The interaction between the 17-electron [Mo(Cp)Cl2(PH3)2] complex and CH3Cl, on the other hand, is repulsive and does not lead to a stable 19-electron adduct. The [RuCl2(PH3)3(CH3X)] system leads to an isomeric complex [RuClX(PH3)3(CH3Cl)] by internal nucleophilic substitution at the carbon atom. The transition state of this process for X=Cl (degenerate exchange) is located at lower energy than the transition state required for halogen-atom transfer leading to [RuCl3(PH3)3] and the free radical CH3. On the basis of these results, the uncatalyzed halide exchange is interpreted as the result of a competitive S(N)i process, whose feasibility depends on the electronic configuration of the transition-metal complex. The catalytic action of Al(OiPr)3 on atom-transfer radical polymerization (and on halide exchange for the 17-electron half-sandwich Mo(III) complex) results from a more favorable Lewis acid-base interaction with the oxidized metal complex, in which the transferred halogen atom is bound to a more electropositive element. This conclusion derives from DFT studies of the model [Al(OCH3)3]n (n=1,2,3,4) compounds, and on the interaction of Al(OCH3)3 with CH3Cl and with the [Mo(Cp)Cl3(PH3)2] and [RuCl3(PH3)3] complexes.     

Kinetic, DFT and TD-DFT studies on the mechanism of stabilization of pyramidal H3PO3 at the [Mo3M'S4(H2O)10]4+ clusters (M' = Pd, Ni)

The kinetics of reaction between the [Mo(3)M'S(4)(H(2)O)(10)](4+) clusters (M' = Pd, Ni) and H(3)PO(3) has been studied in 4.0 M Hpts/Lipts (pts(-) = p-toluenesulfonate). For both complexes there is an initial kinetic step with small absorbance changes that corresponds to substitution of the water coordinated to Pd by a molecule of tetrahedral H(3)PO(3). For the Pd complex, tautomerization of H(3)PO(3) occurs in a slower kinetic step with much larger absorbance changes; it leads to formation of [Mo(3)Pd(pyr-H(3)PO(3))S(4)(H(2)O)(9)](4+) in which H(3)PO(3) adopts a pyramidal structure, but the process is not as favored as for H(3)PO(2). The kinetics of this second step is independent of the concentration of H(3)PO(3) but dependent on the concentration of Hpts on the supporting electrolyte. For the Ni complex, the second step is severely hindered and its kinetics could not be studied. DFT calculations indicate that tautomerization of H(3)PO(3) is expected to be less favoured than that of H(3)PO(2), both processes being less favored at the Ni cluster than at its Pd analogue. With regard to the tautomerization mechanism, the calculations indicate that the mechanism previously proposed for H(3)PO(2) can be the same for H(3)PO(3), although the initial H-shift can also occur through a protonation-deprotonation sequence with participation of external protons instead of a second molecule of the phosphorus acid. TD-DFT studies have been also carried out to understand the similarity between the spectra of the starting complex and the reaction intermediate formed in the first kinetic step as well as the large spectral changes associated to the tautomerization process. Although it contains contributions from several transitions, the intense band observed for clusters containing coordinated pyr-H(3)PO(3) involves essentially ligand-to-metal charge-transfer (LMCT) transitions.     

Pulsed-field ionization electron spectroscopy and conformation of copper-diammonia

Copper-diammonia, Cu(NH3)2, and its deuterated species, Cu(ND3)2, are produced in supersonic molecular beams and studied by pulsed-field ionization zero electron kinetic energy photoelectron spectroscopy and ab initio calculations. Structural isomers with a copper atom binding to an ammonia dimer or two ammonia molecules are obtained by the calculations. By comparing the experimental measurements to the theoretical calculations, the neutral and ionic forms of copper-diammonia are determined to be in a doubly bound linear conformation in their ground electronic states. The adiabatic ionization potentials of Cu(NH3)2 and Cu(ND3)2 are measured as 29,532 (5) and 29,313 (5) cm(-1), respectively. The metal-ligand symmetric stretching frequencies are measured to be 436 cm(-1) for Cu+-(NH3)2 and 398 cm(-1) for Cu+-(ND3)2, and the metal-ligand bending frequencies 75,139 cm(-1) for CuCu+-(NH3)2 and 70125 cm(-1) for CuCu+-(ND3)2. Moreover, the dissociation energy of Cu(NH3)2-->CuNH3+NH3 is determined to be 11(3) kcal mol(-1) through a thermodynamic relationship.     

1H, 13C, and 73Ge NMR spectral analysis of substituted aryltrimethylgermanes

NMR chemical shifts of 1H, 13C, and 73Ge are reported for a series of monosubstituted aromatic trimethylgermanes of the type XC6H4Ge(CH3)3; X = p-N(CH3)2, p-OCH3, p-OC2H5, p-C(CH3)3, p-Si(CH3)3, p-Ge(CH3)3, p-Sn(CH3)3, p-CH3, m-CH3, -H, m-OCH3, p-Cl, p-Br, m-F, m-CF3, p-CF3, o-OCH3, and o-CH3. The relatively narrow 73Ge resonances show a strong correlation with Hammett sigma constants, with a correlation coefficient of 0.976 and 0.876 for 73Ge chemical shifts in meta- and para-substituted derivatives, respectively. The 13C chemical shifts of the methyl carbons bonded to germanium also display a relationship, with correlation coefficients of 0.904, 0.993, and 0.911 for para-, meta- and all derivatives, respectively. Comparisons of the Hammett plots for the homologous series XC6H4M(CH3)3; M = C, Si, Ge, Sn, show that, in general, correlation coefficients decrease while slopes increase significantly down the group, presumably reflecting the corresponding increase in chemical shift range of the group 14 atom. The Hammett constant derived for the p-Ge(CH3)3 group of +0.13 compares with the NMR-derived constants of -0.12 for p-C(CH3)3, +0.14 for p-Si(CH3)3, and -0.14 for p-Sn(CH3)3. The indication of electron release by carbon and tin can be rationalized through traditional hyperconjugative arguments for carbon and by the low electronegativity and consequent inductive effect of tin. The small electron attraction suggested by the positive constants for silicon and germanium can be simply, and perhaps naively, attributed to pi-acceptor interactions with the benzene ring.     

Eu(III) and Tb(III) luminescence sensitized by thiophenyl-derivatized nitrobenzoato antennas

Thiophenyl-derivatized nitrobenzoic acid ligands have been evaluated as possible sensitizers of Eu(III) and Tb(III) luminescence. The resulting solution and solid-state species were isolated and characterized by luminescence spectroscopy and X-ray crystallography. The Eu(III) complex with 2-nitro-3-thiophen-3-yl-benzoic acid, 1, crystallizes in the monoclinic space group C2/c with a = 28.569(3) A, b = 17.7726(18) A, c = 17.7073(18) A, beta= 126.849(2) degrees, and V = 7194.6(13) A3. The Tb(III) complex with this ligand, 2, is isostructural, and its cell parameters are a = 29.755(6) A, b = 18.123(4) A, c = 19.519(4) A, beta= 130.35(3) degrees, and V = 8021(3) A3. Eu(III) crystallizes with 3-nitro-2-thiophen-3-yl-benzoic acid as a triclinic complex, 3, in the space group P1 with a = 11.045(2) A, b = 12.547(3) A, c = 15.500(3) A, alpha = 109.06(3)degrees, beta = 94.79(3) degrees, gamma = 107.72(3) degrees. and V = 1893.5(7) A3. With the ligand 5-nitro-2-thiophen-3-yl-benzoic acid, Eu(III) yields another molecular compound, 4, triclinic P1, with a = 10.649(2) A, b = 14.009(3) A, c = 15.205(3) A, alpha= 112.15(3) degrees, beta = 100.25(3) degrees, gamma = 106.96(3) degrees, and V = 1900.5(7) A3. All compounds dissolve in water and methanol, and the methanolic solutions are luminescent. The solution species have a metal ion-to-ligand ratio of 1:1. The quantum yields have been determined to be in the range of 0.9-3.1% for Eu(III) and 4.7-9.8% for Tb(III). The highest values of these correspond to the most intense luminescence reported for Ln(III) solutions with this type of sensitizer. The lifetimes of luminescence are in the range of 248.3-338.9 micros for Eu(III) and 208.6-724.9 micros for Tb(III). The stability constants are in the range of log 11 = 2.73-4.30 for Eu(III) and 3.34-4.18 for Tb(III) and, along with the energy migration pathways, are responsible for the reported efficiency of sensitization.     

Theoretical investigation of carbon-sulfur triple bonds

By means of first principle calculations we have investigated a set of molecules that are presumed to contain carbon-sulfur triple bonds, namely HCSOH, H(3)SCH, cis-FCSF, F(3)CCSF(3), and F(5)SCSF(3). For HCSOH, FCSF, and H(3)SCH we used the CCSD(T) methodology and the correlation-consistent basis sets. On the other hand, F(3)CCSF(3) and F(5)SCSF(3) were studied at the B3LYP, M06-2X, MP2, and G3 levels of theory. We found that none of these molecules display a carbon-sulfur adiabatic bond dissociation energy (ABDE) as strong as diatomic CS (170.5 kcal mol(-1)), or a diabatic bond dissociation energy (DBDE) larger than the one found in SCO (212.0 kcal mol(-1)), although the DBDE of FCSF comes quite close at 208.3 kcal mol(-1). The CS ABDEs of F(3)CCSF(3), F(5)SCSF(3), and H(3)SCH are comparable to that of a single C-S bond. In contrast with the experimental results, F(3)CCSF(3) and F(5)SCSF(3) are predicted to be linear with C(3v) and C(s) symmetry, respectively, at the B3LYP/6-311+G(3df,2p) level. MP2/6-311+G(2df,2p) calculations support the C(3v) symmetry for F(3)CCSF(3), despite F(5)SCSF(3) not having a perfect linear structure; the CSC angle is 174.6°, which is nearly 20° larger than the experimental value. The analysis of the carbene structures of HCSOH and H(3)SCH revealed that they are not significant, because the triplet state is dissociative in these cases. However, for F(3)CCSF(3) and F(5)SCSF(3) , the carbene triplet states lie 0.81 and 0.77 eV above the singlet state, respectively. In the same vein, our investigation supports the presence of a strong double bond for HCSOH. The conflicting evidence available for F(3)CCSF(3) and F(5)SCSF(3) makes it very difficult to determine the nature of the CS bonds. However, the bond dissociation energies and the singlet-triplet splittings clearly suggest that these compounds should be considered as masked sulfinylcarbenes. The analysis of the bond dissociation energies challenges the existence of a triple bond in these five molecules, but from a strictly thermodynamic standpoint, cis-FCSF is found to be the candidate most likely to exhibit triple-bond character.     

Rhodium-103 NMR of [Rh(X)(PPh3)3] [X = Cl, N3, NCO, NCS, N(CN)2, NCBPh3, CNBPh3, CN] and derivatives containing CO, isocyanide, pyridine, H2 and O2. Ligand, solvent and temperature effects

Rhodium-103 chemical shifts are reported for 62 compounds, namely [Rh(X)(PPh3)3] [X = Cl, N3, NCO, NCS, N(CN)2, NCBPh3, CNBPh3, CN] and derivatives formed by replacement of a phosphine by CO, xylyl isocyanide (XNC) and pyridine and/or by oxidative addition of H2 or O2 to give trans-[Rh(X)(PPh3)2(CO)] (delta in the range -816 to -368 ppm) trans-[Rh(X)(PPh3)2(XNC)] (delta -817 to -250 ppm), cis-[Rh(X)(PPh3)2(py)] (the trans isomer is formed with X = CN, CNBPh3) (delta -233 to 170 ppm), [Rh(X)(H)2(PPh3)3] (delta -611 to 119), trans-[Rh(X)(H)2(PPh3)2(py)] (delta -30 to 566 ppm), [Rh(X)(O2)(PPh3)3] (delta 1393 to 3273 ppm) and cis-[Rh(X)(O2)(PPh3)2(py)] (delta 1949 to 3374 ppm). For the majority of these compounds data were obtained from solutions in chloroform and in toluene at temperatures of 247 and 300 K; for [Rh(X)(PPh3)3] (delta -562 to -4 ppm) data are reported at a number of temperatures in the range 195-300 K for solutions in chloroform, toluene and dichloromethane and at 300 K for solutions in DMSO. The expected trend to lower delta(103Rh) with decreasing temperature (vibrational shielding) is observed for the dichloromethane data, but data from solutions {of [Rh(X)(PPh3)3]} in chloroform and toluene show a number of features which diverge from this pattern, i.e. shifts to higher delta are found to accompany a decrease in temperature, most noticeably where X = CN and Cl [on changing the solvent from dichloromethane to chloroform changes in delta(103Rh) of up to 172 ppm are observed]. These results are interpreted in terms of a hydrogen-bonded interaction with the solvent that is enhanced by the presence of a polarizable ligand (CN, Cl). With a ligand (O2CCF3) that is only weakly polarizable the solvent dependence of delta(103Rh) is minimal.     

Synthesis, structure, and spectroscopic properties of Am(IO3)3 and the photoluminescence behavior of Cm(IO3)3

We have prepared Am(IO(3))(3) as a part of our continuing investigations into the chemistry of the 4f- and 5f-elements' iodates. Single crystals were obtained from the reaction of Am(3+) and H(5)IO(6) under mild hydrothermal conditions. Crystallographic data on an eight-day-old crystal are (21 degrees C, Mo Kalpha, lambda = 0.71073 Angstroms): monoclinic, space group P2(1)/c, a = 7.2300(5) Angstroms, b = 8.5511(6) Angstroms, c = 13.5361(10) Angstroms, beta = 100.035(1) degrees, V = 824.06(18), Z = 4. The structure consists of Am(3+) cations bound by iodate anions to form [Am(IO(3))(8)] units, where the local coordination environment around the americium centers is a distorted dodecahedron. There are three crystallographically unique iodate anions within the structure that bridge in both bidentate and tridentate fashions to form the overall three-dimensional structure. Repeated collection of X-ray diffraction data with time for a crystal of (243)Am(IO(3))(3) revealed an anisotropic expansion of the unit cell, presumably from self-irradiation damage, to generate values of a = 7.2159(7) Angstroms, b = 8.5847(8) Angstroms, c = 13.5715(13) Angstroms, beta = 99.492(4) degrees, V = 829.18(23) after approximately five months. The Am(IO(3))(3) crystals have also been characterized by Raman spectroscopy and the spectral results compared to those for Cm(IO(3))(3). Three strong Raman bands were observed for both compounds and correspond to the I-O symmetric stretching of the three crystallographically distinct iodate anions. The Raman profile suggests a lack of interionic vibrational coupling of the I-O stretching, while intraionic coupling provides symmetric and asymmetric components that correspond to each iodate site. Photoluminescence data for both Am(IO(3))(3) and Cm(IO(3))(3) are reported here for the first time. Assignments for the electronic levels of the actinide cations were based on these photoluminescence measurements and indicate the presence of vibronic coupling between electronic transitions and IO(3)(-) vibrational modes in both compounds.     

A density functional study of phosphorus nitride P3N5: refined geometries,properties, and relative stability of alpha-P3N5 and gamma-P3N5 and a further possible high-pressure phase delta-P3N5 with kyanite-type structure

The crystal structures and the enthalpy-pressure phase diagram of P(3)N(5) were investigated by using density functional methods. Applying both approximations to the electron exchange and correlation gives a consistent picture for the two known polymorphs, alpha-P(3)N(5) and gamma-P(3)N(5). The calculated zone-center phonon modes compare very well with the experimental results. They indicate low-frequency bending modes for two-coordinate N atoms of alpha-P(3)N(5), which are responsible for a C2/c --> C c structural modulation of alpha-P(3)N(5) at moderate pressures. alpha-P(3)N(5) transforms into gamma-P(3)N(5) at pressures of about 6 GPa. We propose gamma-P(3)N(5) transforms into a delta-P(3)N(5) with Kyanite-type structure at pressures exceeding 43 GPa. Upon quenching, this triclinic modification of P(3)N(5) is likely to distort into a more symmetric monoclinic structure.     

Iron-catalyzed oxidative coupling of alkylamides with arenes through oxidation of alkylamides followed by Friedel-Crafts alkylation

FeCl(3) in combination with t-BuOOt-Bu as an oxidant was found to be an efficient catalyst for oxidation of alkylamides to α-(tert-butoxy)alkylamides. FeCl(2) and CuCl showed, respectively, almost the same and slightly lower activities compared with FeCl(3) in the tert-butoxylation of N-phenylpyrrolidone (1a), whereas no tert-butoxylated product was obtained by use of Fe(OTf)(3), RuCl(3), or Zr(OTf)(4). FeCl(3) was found to be effective also as a catalyst for the Friedel-Crafts alkylation with thus obtained α-(tert-butoxy)alkylamides. The Friedel-Crafts alkylation proceeded smoothly also in the presence of a catalytic amount of Fe(OTf)(3), RuCl(3), or Zr(OTf)(4). In contrast, FeCl(2) and CuCl, which showed certain activity toward the tert-butoxylation, failed to promote the Friedel-Crafts alkylation. Among the transition metal complexes thus far examined, only FeCl(3) showed high catalytic activities for both the oxidation and the Friedel-Crafts alkylation. The bifunctionality of FeCl(3) was utilized for the oxidative coupling of alkylamides with arenes through a tandem reaction consisting of oxidation of alkylamides to α-(tert-butoxy)alkylamides and the following Friedel-Crafts alkylation. The FeCl(3)-catalyzed oxidative coupling is applicable to a wide variety of alkylamides and arenes, though a combination of FeCl(3) with Fe(OTf)(3) was found to be effective for the reaction of arenes with low nucleophilicity. A Fe(II)-Fe(III) catalytic cycle is concerned with the tert-butoxylation, whereas a Fe(III) complex as a Lewis acid catalyzes the Friedel-Crafts alkylation.     

C-X···π halogen and C-H···π hydrogen bonding: interactions of CF3X (X = Cl, Br, I or H) with ethene and propene

Using FTIR and Raman spectroscopy, the formation of halogen bonded complexes of the trifluorohalomethanes CF(3)Cl, CF(3)Br and CF(3)I with ethene and propene dissolved in liquid argon has been investigated. For CF(3)Br and CF(3)I, evidence was found for the formation of C-X···π halogen bonded 1:1 complexes. At a higher ratio of CF(3)I/propene, weak absorptions due to a 2:1 complex were also observed. Using spectra recorded at different temperatures, the complexation enthalpies for the complexes were determined to be -5.3(2) kJ mol(-1) for CF(3)Br·ethene, -7.5(2) kJ mol(-1) for CF(3)I·ethene, -5.6(1) kJ mol(-1) for CF(3)Br·propene, -8.8(1) kJ mol(-1) for CF(3)I·propene and -16.5(6) kJ mol(-1) for (CF(3)I·)(2)propene. The complexation enthalpies of the hydrogen bonded counterparts, with CF(3)H as the Lewis acid, were determined to be -4.6(4) kJ mol(-1) for CF(3)H·ethene and -5.1(2) kJ mol(-1) for CF(3)H·propene. For both hydrogen bonded complexes, a blue shift, by +4.8 and +4.0 cm(-1), respectively, was observed for the C-H stretching mode. The results from the cryospectroscopic study are compared with ab initio calculations at the MP2/aug-cc-pVDZ(-PP) level.     

An investigation of Staudinger reactions involving cis-1,3,5-triazidocyclohexane and tri(alkylamino)phosphines

The reaction of 1,3,5-cis-triazidocyclohexane with the electron-rich tris(dialkylamino)phosphines P(NMe(2))(3) (1) and N(CH(2)CH(2)NMe)(3)P (2b) in acetonitrile for 3 h furnished the corresponding tris-phosphazides 1,3,5-cis-(R(3)PN(3))(3)C(6)H(9), 3a (R(3)P = 1) and 3b (R(3)P = 2b), in 90% and 92% yields, respectively. The same reaction with the relatively electron-poor tris(dialkylamino)phosphine MeC(CH(2)NMe)(3)P (4) for 2 days gave the tris-iminophosphorane, 1,3,5-cis-(R(3)PN)(3)C(6)H(9), 5a (R(3)P = 4), in 60% yield. Compound 3b is a thermally stable solid that did not lose dinitrogen when refluxed in toluene for 24 h or when heated as a neat sample at 100 degrees C /0.5 Torr for 10 h. By contrast, tris-phosphazide 3a decomposed to the tris-iminophosphorane 1,3,5-cis-(R(3)PN)(3)C(6)H(9), 5b (R(3)P = 1), in 3 h in quantitative yield upon heating to 100 degrees C in toluene. Factors influencing the formation of the phosphazides or the iminophosphoranes in these reactions are discussed. The reaction of 3b with 4 equiv of benzoic acid gave [N(CH(2)CH(2)NMe)(3)P=NH(2)]PhCO(2) ([6bH]PhCO(2)) in quantitative yield along with benzene (56% yield) and dinitrogen. The same reaction with 3a gave [(Me(2)N)(3)P=NH(2)]PhCO(2) ([7aH]PhCO(2)) (quantitative yield), benzene (15% yield), and dinitrogen(.) Treatment of [6bH]PhCO(2) with KO(t)Bu afforded N(CH(2)CH(2)NMe)(3)P=NH (6b) in 40% overall yield. Compound 6b upon treatment with PhCH(2)CH(2)Br produced [6bH]Br in 90% yield along with styrene. The new compounds were characterized by analytical and spectroscopic methods, and selected compounds (3b, 5a, and [6bH]Br) were structured by X-ray crystallography. A special feature of 3b is its capability to function as a starting material for 6b, which was not accessible by other synthetic routes.     

Facially coordinating triamine ligands with a cyclic backbone: some structure-stability correlations

Metal complex formation of the two cyclic triamines 6-methyl-1,4-diazepan-6-amine (MeL(a)) and all-cis-2,4,6-trimethylcyclohexane-1,3,5-triamine (Me(3)tach) was studied. The structure of the free ligands (H(x)MeL(a))(x+) and H(x)Me(3)tach(x+) (0 ≤ x ≤ 3) was investigated by pH-dependent NMR spectroscopy and X-ray diffraction experiments. The crystal structure of (H(2)Me(3)tach)(p-O(3)S-C(6)H(4)-CH(3))(2) showed a chair conformation with axial nitrogen atoms for the doubly protonated species. In contrast to a previous report, Me(3)tach was found to be a stronger base than the parent cis-cyclohexane-1,3,5-triamine (tach); pK(a)-values of H(3)Me(3)tach(3+) (25 °C, 0.1 M KCl): 5.2, 7.4, 11.2. The crystal structures of (H(3)MeL(a))(BiCl(6))·2H(2)O and (H(3)MeL(a))(ClO(4))Cl(2) exhibited two distinct twisted chair conformations of the seven membered diazepane ring. [Co(MeL(a))(2)](3+) (cis: 1(3+), trans: 2(3+)), trans-[Fe(MeL(a))(2)](3+) (3(3+)), [(MeL(a))ClCd(μ(2)-Cl)](2) (4), trans-[Cu(MeL(a))(2)](2+) (5(2+)), and [Cu(HMeL(a))Br(3)] (6) were characterized by single crystal X-ray analysis of 1(ClO(4))(3)·H(2)O, 2Br(3)·H(2)O, 3(ClO(4))(3)·0.8MeCN·0.2MeOH, 4, 5Br(2)·0.5MeOH, and 6·H(2)O. Formation constants and redox potentials of MeL(a) complexes were determined by potentiometric, spectrophotometric, and cyclovoltammetric measurements. The stability of [M(II)(MeL(a))](2+)-complexes is low. In comparison to the parent 1,4-diazepan-6-amine (L(a)), it is only slightly enhanced. In analogy to L(a), MeL(a) exhibited a pronounced tendency for forming protonated species such as [M(II)(HMeL(a))](3+) or [M(II)(MeL(a))(HMeL(a))](3+) (see 6 as an example). In contrast to MeL(a), Me(3)tach forms [M(II)L](2+) complexes (M = Cu, Zn) of very high stability, and the coordination behavior corresponds mainly to an "all-or-nothing" process. Molecular mechanics calculations showed that the low stability of L(a) and MeL(a) complexes is mainly due to a large amount of torsional strain within the pure chair conformation of the diazepane ring, required for tridentate coordination. This behavior is quite contrary to Me(3)tach and tacn (tacn =1,4,7-triazacyclononane), where the main portion of strain is already preformed in the free ligand, and the amount, generated upon complex formation, is comparably low.