The boron-catalyzed polymerization of dimethylsulfoxonium methylide. A living polymethylene synthesis

Trialkyl and aryl organoboranes catalyze the polymerization of dimethylsulfoxonium methylide (1). The product of the polymerization is a tris-polymethylene organoborane. Oxidation affords linear telechelic alpha-hydroxy polymethylene. The polymer molecular weight was found to be directly proportional to the stoichiometric ratio of ylide/borane, and polydispersities as low as 1.01-1.03 have been realized. Although oligomeric polymethylene has been the most frequent synthetic target of this method, polymeric star organoboranes with molecular weights of 1.5 million have been produced. The average turnover frequency at 120 degrees C in 1,2,4,5-tetrachlorobenzene/toluene is estimated at >6 x 10(6) g of polymethylene (mol boron)(-1) h(-1). The mechanism of the polyhomologation reaction involves initial formation of a zwitterionic organoborane.ylide complex which breaks down in a rate-limiting 1,2-alkyl group migration with concomitant expulsion of a molecule of DMSO. The reaction was found to be first order in the borane catalyst and zero order in ylide. DMSO does not interfere with the reaction. The temperature dependence of the reaction rate yielded the following activation energy parameters (toluene, DeltaH(++) = 23.2 kcal/mol, DeltaS(++) = 12.6 cal deg/mol, DeltaG(++) = 19.5 kcal/mol; THF, DeltaH(++) = 26.5 kcal/mol, DeltaS(++) = 21.5 cal deg/mol, DeltaG(++) = 20.1 kcal/mol).     

An unusual exchange between alkylidyne alkyl and bis(alkylidene) tungsten complexes promoted by phosphine coordination: kinetic, thermodynamic, and theoretical studies

d0 Tungsten alkylidyne alkyl complex (Me3SiCH2)3W(CSiMe3)(PMe3) (4a) was found to undergo a rare, PMe3-promoted exchange with its bis(alkylidene) tautomer (Me3SiCH2)2W(=CHSiMe3)2(PMe3) (4b). Thermodynamic studies of the exchange showed that 4b is favored and gave Keq and the enthalpy and entropy of the equilibrium: DeltaH degrees = -1.8(0.5) kcal/mol and DeltaS degrees = -1.5(1.7) eu. Kinetic studies of the alpha-H migration between 4a and 4b by variable-temperature NMR gave rate constants k1 and k-1 for the reversible reactions and activation enthalpies and entropies: DeltaH1 = 16.2(1.2) kcal/mol and DeltaS1 = -22.3(4.0) eu for the forward reaction (4a --> 4b); DeltaH2 = 18.0(1.3) kcal/mol and DeltaS2 = -20.9(4.3) eu for the reverse reaction (4b --> 4a). Ab initio calculations at the B3LYP level revealed that PMe3 binds with the bis(alkylidene) tautomer relatively more strongly than with the alkylidyne tautomer and thus stabilizes the bis(alkylidene) tautomer.     

CASSCF and CASPT2 calculations on the cleavage and ring inversion of bicyclo[2.2.0]hexane find that these reactions involve formation of a common twist-boat diradical intermediate.

(6/6)CASSCF and CASPT2/6-31G calculations have been performed to understand the experimental finding of Goldstein and Benzon (J. Am. Chem. Soc. 1972, 94, 5119) that exo-bicyclo[2.2.0]hexane-d(4) (1b) undergoes ring inversion to form endo-bicyclo[2.2.0]hexane-d(4) (4b) faster than it undergoes cleavage to form cis,trans-1,5-hexadiene-d(4) (3b). Goldstein and Benzon also found that the latter reaction, which must occur via a chairlike transition structure (TS), is much faster than cleavage of 1b to trans,trans-1,5-hexadiene-d(4) (2b) via a boatlike TS. Our calculations reveal that all three of these reactions involve ring opening of 1, through a boat diradical TS (BDTS), to form a twist-boat diradical intermediate (TBDI). TBDI can reclose to 4 via a stereoisomeric boat diradical TS (BDTS'), or TBDI can cleave, either via a half-chair diradical TS (HCDTS) to form 3 or via a boat TS (BTS) to form 2. The calculated values of DeltaH(++) = 34.6 kcal/mol, DeltaS(++) = -1.6 eu, and DeltaH(++) = 35.2 kcal/mol, DeltaS(++) = 2.0 eu for ring inversion of 1 to 4 and cleavage of 1 to 3, respectively, are in excellent agreement with the values measured by Goldstein and Benzon. The higher value of DeltaH(++) = 37.6 kcal/mol, computed for cleavage of TBDI to 2, is consistent with the experimental finding that very little 2b is formed when 1b is pyrolyzed. The relationships between BDTS, HCDTS, and BTS and the chair and boat Cope rearrangement TSs (CCTS and BCTS) are discussed.     

Molecular encapsulation via metal-to-ligand coordination in a Cu(I)-folded molecular basket

A molecular basket, composed of a semirigid C3v symmetric tris-norbornadiene framework and three pyridine flaps at the rim, has been shown to coordinate to a Cu(I) cation and thereby fold in a multivalent fashion. The assembly was effective (Ka = 1.73 +/- 0.08 x 10(5) M(-1)) and driven by enthalpy (DeltaH(o) = -7.2 +/- 0.1 kcal/mol, DeltaS(o) = -0.25 eu). Variable temperature (1)H NMR studies, assisted with 2D COSY and ROESY investigations, revealed the existence of Cu(I)-folded basket 10b with a molecule of acetonitrile occupying its interior and coordinated to the metal. Interestingly, 10b is in equilibrium with Cu(I)-folded 10a , whose inner space is solvated by acetone or chloroform. The incorporation of a molecule of acetonitrile inside 10a was found to be driven by enthalpy (DeltaH(o) = -3.3 +/- 0.1 kcal/mol), with an apparent loss in entropy (DeltaS(o) = -9.4 +/- 0.4 eu); this is congruent with a complete immobilization of acetonitrile and release of a "loosely" encapsulated solvent molecule during 10a/b interconversion. From an Eyring plot, the activation enthalpy for incorporating acetonitrile into 10a was found to be positive (DeltaH(double dagger) = 6.5 +/- 0.5 kcal/mol), while the activation entropy was negative (DeltaS(double dagger) = -20 +/- 2 eu). The results are in agreement with an exchange mechanism whereby acetonitrile "slips" into an "empty" basket through its side aperture. In fact, DFT (BP86) calculations are in favor of such a mechanistic scenario; the calculations suggest that opening of the basket's rim to exchange guests is energetically demanding and therefore less feasible.     

Synthesis, basicity, and dynamics of a perfluorocyclohexenyl anion

Electron transfer to perfluoro-1,3-dimethylcyclohexane in moist THF has yielded two quite different products. Tetrabutylammonium iodide irradiated with ultraviolet light gives a tetrabutylammonium enolate, but potassium fluorenone ketyl affords a cyclohexenyl anion. This allylic anion was isolated as its conjugate acid, a rather strong carbon acid. Ring inversion in the anion, measured by (19)F NMR line shape analysis, is characterized by these activation parameter values: DeltaH(++) = 8.84 +/-0.14 kcal/mol and DeltaS(++) = 0.81 +/- 0.6 cal mol(-1) K(-1).     

Influence of terpyridine as pi-acceptor ligand on the kinetics and mechanism of the reaction of NO with ruthenium(III) complexes

The kinetics of the unusually fast reaction of cis- and trans-[Ru(terpy)(NH3)2Cl]2+ (with respect to NH3; terpy=2,2':6',2"-terpyridine) with NO was studied in acidic aqueous solution. The multistep reaction pathway observed for both isomers includes a rapid and reversible formation of an intermediate Ru(III)-NO complex in the first reaction step, for which the rate and activation parameters are in good agreement with an associative substitution behavior of the Ru(III) center (cis isomer, k1=618 +/- 2 M(-1) s(-1), DeltaH(++) = 38 +/- 3 kJ mol(-1), DeltaS(++) = -63 +/- 8 J K(-1) mol(-1), DeltaV(++) = -17.5 +/- 0.8 cm3 mol(-1); k -1 = 0.097 +/- 0.001 s(-1), DeltaH(++) = 27 +/- 8 kJ mol(-1), DeltaS(++) = -173 +/- 28 J K(-1) mol(-1), DeltaV(++) = -17.6 +/- 0.5 cm3 mol(-1); trans isomer, k1 = 1637 +/- 11 M(-1) s(-1), DeltaH(++) = 34 +/- 3 kJ mol(-1), DeltaS(++) = -69 +/-11 J K(-1) mol(-1), DeltaV(++) = -20 +/- 2 cm3 mol(-1); k(-1)=0.47 +/- 0.08 s(-1), DeltaH(++)=39 +/- 5 kJ mol(-1), DeltaS(++) = -121 +/-18 J K(-1) mol(-1), DeltaV(++) = -18.5 +/- 0.4 cm3 mol(-1) at 25 degrees C). The subsequent electron transfer step to form Ru(II)-NO+ occurs spontaneously for the trans isomer, followed by a slow nitrosyl to nitrite conversion, whereas for the cis isomer the reduction of the Ru(III) center is induced by the coordination of an additional NO molecule (cis isomer, k2=51.3 +/- 0.3 M(-1) s(-1), DeltaH(++) = 46 +/- 2 kJ mol(-1), DeltaS(++) = -69 +/- 5 J K(-1) mol(-1), DeltaV(++) = -22.6 +/- 0.2 cm3 mol(-1) at 45 degrees C). The final reaction step involves a slow aquation process for both isomers, which is interpreted in terms of a dissociative substitution mechanism (cis isomer, DeltaV(++) = +23.5 +/- 1.2 cm3 mol(-1); trans isomer, DeltaV(++) = +20.9 +/- 0.4 cm3 mol(-1) at 55 degrees C) that produces two different reaction products, viz. [Ru(terpy)(NH3)(H2O)NO]3+ (product of the cis isomer) and trans-[Ru(terpy)(NH3)2(H2O)]2+. The pi-acceptor properties of the tridentate N-donor chelate (terpy) predominantly control the overall reaction pattern.     

The first example of a nitrile hydratase model complex that reversibly binds nitriles

Nitrile hydratase (NHase) is an iron-containing metalloenzyme that converts nitriles to amides. The mechanism by which this biochemical reaction occurs is unknown. One mechanism that has been proposed involves nucleophilic attack of an Fe-bound nitrile by water (or hydroxide). Reported herein is a five-coordinate model compound ([Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+)) containing Fe(III) in an environment resembling that of NHase, which reversibly binds a variety of nitriles, alcohols, amines, and thiocyanate. XAS shows that five-coordinate [Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+) reacts with both methanol and acetonitrile to afford a six-coordinate solvent-bound complex. Competitive binding studies demonstrate that MeCN preferentially binds over ROH, suggesting that nitriles would be capable of displacing the H(2)O coordinated to the iron site of NHase. Thermodynamic parameters were determined for acetonitrile (DeltaH = -6.2(+/-0.2) kcal/mol, DeltaS = -29.4(+/-0.8) eu), benzonitrile (-4.2(+/-0.6) kcal/mol, DeltaS = -18(+/-3) eu), and pyridine (DeltaH = -8(+/-1) kcal/mol, DeltaS = -41(+/-6) eu) binding to [Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+) using variable-temperature electronic absorption spectroscopy. Ligand exchange kinetics were examined for acetonitrile, iso-propylnitrile, benzonitrile, and 4-tert-butylpyridine using (13)C NMR line-broadening analysis, at a variety of temperatures. Activation parameters for ligand exchange were determined to be DeltaH(+ +) = 7.1(+/-0.8) kcal/mol, DeltaS(+ +) = -10(+/-1) eu (acetonitrile), DeltaH(+ +) = 5.4(+/-0.6) kcal/mol, DeltaS(+ +) = -17(+/-2) eu (iso-propionitrile), DeltaH(+ +) = 4.9(+/-0.8) kcal/mol, DeltaS(+ +) = -20(+/-3) eu (benzonitrile), and DeltaH(+ +) = 4.7(+/-1.4) kcal/mol DeltaS(+ +) = -18(+/-2) eu (4-tert-butylpyridine). The thermodynamic parameters for pyridine binding to a related complex, [Fe(III)(S(2)(Me2)N(3)(Pr,Pr))](+) (DeltaH = -5.9(+/-0.8) kcal/mol, DeltaS = -24(+/-3) eu), are also reported, as well as kinetic parameters for 4-tert-butylpyridine exchange (DeltaH(+ +) = 3.1(+/-0.8) kcal/mol, DeltaS(+ +) = -25(+/-3) eu). These data show for the first time that, when it is contained in a ligand environment similar to that of NHase, Fe(III) is capable of forming a stable complex with nitriles. Also, the rates of ligand exchange demonstrate that low-spin Fe(III) in this ligand environment is more labile than expected. Furthermore, comparison of [Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+) and [Fe(III)(S(2)(Me2)N(3)(Pr,Pr))](+) demonstrates how minor distortions induced by ligand constraints can dramatically alter the reactivity of a metal complex.     

Mechanistic investigations of the palladium-catalyzed aerobic oxidative kinetic resolution of secondary alcohols using (-)-sparteine

The mechanistic details of the Pd(II)/(-)-sparteine-catalyzed aerobic oxidative kinetic resolution of secondary alcohols were elucidated, and the origin of asymmetric induction was determined. Saturation kinetics were observed for rate dependence on [(-)-sparteine]. First-order rate dependencies were observed for both the Pd((-)-sparteine)Cl(2) concentration and the alcohol concentration at high and low [(-)-sparteine]. The oxidation rate was inhibited by addition of (-)-sparteine HCl. At low [(-)-sparteine], Pd-alkoxide formation is proposed to be rate limiting, while at high [(-)-sparteine], beta-hydride elimination is proposed to be rate determining. These conclusions are consistent with the measured kinetic isotope effect of k(H)/k(D) = 1.31 +/- 0.04 and a Hammett rho value of -1.41 +/- 0.15 at high [(-)-sparteine]. Calculated activation parameters agree with the change in the rate-limiting step by increasing [(-)-sparteine] with DeltaH(++) = 11.55 +/- 0.65 kcal/mol, DeltaS(++) = -24.5 +/- 2.0 eu at low [(-)-sparteine], and DeltaH(++) = 20.25 +/- 0.89 kcal/mol, DeltaS() = -5.4 +/- 2.7 eu at high [(-)-sparteine]. At high [(-)-sparteine], the selectivity is influenced by both a thermodynamic difference in the stability of the diastereomeric Pd-alkoxides formed and a kinetic beta-hydride elimination to maximize asymmetric induction. At low [(-)-sparteine], the selectivity is influenced by kinetic deprotonation, resulting in lower k(rel) values. A key, nonintuitive discovery is that (-)-sparteine plays a dual role in this oxidative kinetic resolution of secondary alcohols as a chiral ligand on palladium and as an exogenous chiral base.     

Mechanisms of water oxidation catalyzed by the cis,cis-[(bpy)2Ru(OH2)]2O4+ ion

The cis,cis-[(bpy)(2)Ru(III)(OH(2))](2)O(4+) micro-oxo dimeric coordination complex is an efficient catalyst for water oxidation by strong oxidants that proceeds via intermediary formation of cis,cis-[(bpy)(2)Ru(V)(O)](2)O(4+) (hereafter, [5,5]). Repetitive mass spectrometric measurement of the isotopic distribution of O(2) formed in reactions catalyzed by (18)O-labeled catalyst established the existence of two reaction pathways characterized by products containing either one atom each from a ruthenyl O and solvent H(2)O or both O atoms from solvent molecules. The apparent activation parameters for micro-oxo ion-catalyzed water oxidation by Ce(4+) and for [5,5] decay were nearly identical, with DeltaH(++) = 7.6 (+/-1.2) kcal/mol, DeltaS() = -43 (+/-4) cal/deg mol (23 degrees C) and DeltaH(++) = 7.9 (+/-1.1) kcal/mol, DeltaS(++) = -44 (+/-4) cal/deg mol, respectively, in 0.5 M CF(3)SO(3)H. An apparent solvent deuterium kinetic isotope effect (KIE) of 1.7 was measured for O(2) evolution at 23 degrees C; the corresponding KIE for [5,5] decay was 1.6. The (32)O(2)/(34)O(2) isotope distribution was also insensitive to solvent deuteration. On the basis of these results and previously established chemical properties of this class of compounds, mechanisms are proposed that feature as critical reaction steps H(2)O addition to the complex to form covalent hydrates. For the first pathway, the elements of H(2)O are added as OH and H to the adjacent terminal ruthenyl O atoms, and for the second pathway, OH is added to a bipyridine ring and H is added to one of the ruthenyl O atoms.     

Hypohalite ion catalysis of the disproportionation of chlorine dioxide

The disproportionation of chlorine dioxide in basic solution to give ClO2- and ClO3- is catalyzed by OBr- and OCl-. The reactions have a first-order dependence in both [ClO2] and [OX-] (X = Br, Cl) when the ClO2- concentrations are low. However, the reactions become second-order in [ClO2] with the addition of excess ClO2-, and the observed rates become inversely proportional to [ClO2-]. In the proposed mechanisms, electron transfer from OX- to ClO2(k1OBr- = 2.05 +/- 0.03 M(-1) x s(-1) for OBr(-)/ClO2 and k1OCl-= 0.91 +/- 0.04 M(-1) x s(-1) for OCl-/ClO2) occurs in the first step to give OX and ClO2-. This reversible step (k1OBr-/k(-1)OBr = 1.3 x 10(-7) for OBr-/ClO2, / = 5.1 x 10(-10) for OCl-/ClO2) accounts for the observed suppression by ClO2-. The second step is the reaction between two free radicals (XO and ClO2) to form XOClO2. These rate constants are = 1.0 x 10(8) M(-1) x s(-1) for OBr/ClO2 and = 7 x 10(9) M(-1) x s(-1) for OCl/ClO2. The XOClO2 adduct hydrolyzes rapidly in the basic solution to give ClO3- and to regenerate OX-. The activation parameters for the first step are DeltaH1(++) = 55 +/- 1 kJ x mol(-1), DeltaS1(++) = - 49 +/- 2 J x mol(-1) x K(-1) for the OBr-/ClO2 reaction and DeltaH1(++) = 61 +/- 3 kJ x mol(-1), DeltaS1(++) = - 43 +/- 2 J x mol(-1) x K(-1) for the OCl-/ClO2 reaction.     

The ozonolysis of acetylene--a quantum chemical investigation.

The ozonolysis of acetylene was investigated using CCSD(T), CASPT2, and B3LYP-DFT in connection with a 6-311+G(2d,2p) basis set. The reaction is initiated by the formation of a van der Waals complex followed by a [4pi + 2pi] cycloaddition between ozone and acetylene (activation enthalpy DeltaH(a)(298) = 9.6 kcal/mol; experiment, 10.2 kcal/mol), yielding 1,2,3-trioxolene, which rapidly opens to alpha-ketocarbonyl oxide 5. Alternatively, an O atom can be transferred from ozone to acetylene (DeltaH(a)(298) = 15.6 kcal/mol), thus leading to formyl carbene, which can rearrange to oxirene or ketene. The key compound in the ozonolysis of acetylene is 5 because it is the starting point for the isomerization to the corresponding dioxirane 19 (DeltaH(a)(298) = 16.9 kcal/mol), for the cyclization to trioxabicyclo[2.1.0]pentane 10 (DeltaH(a)(298) = 19.5 kcal/mol), for the formation of hydroperoxy ketene 15 (DeltaH(a)(298) = 20.6 kcal/mol), and for the rearrangement to dioxetanone 9 (DeltaH(a)(298) = 23.6 kcal/mol). Compounds 19, 10, 15, and 9 rearrange or decompose with barriers between 13 and 16 kcal/mol to yield as major products formanhydride, glyoxal, formaldehyde, formic acid, and (to a minor extent) glyoxylic acid. Hence, the ozonolysis of acetylene possesses a very complicated reaction mechanism that deserves intensive experimental studies.     

The solution structures and dynamics and the solid-state structures of substituted cyclopentadienyltitanium(IV) trifluorides

Organotitanium fluorides (C5Me4R)TiF3 (R = H, Me, Et) sublimate with formation of crystalline dimers. From solution, we obtained crystals of dimers and tetramers. The tetramer [{(C5Me5)TiF3}4] irreversibly dissociates in the solid state to dimers (DeltaH = 8.33 kcal mol(-1)). The variable-temperature (1)H and (19)F NMR spectroscopy measurements of the toluene-d(8) solution of [{(C5Me5)TiF3}2] revealed at 202 K one monomeric, two dimeric (with C2h and Cs symmetry), two tetrameric (with D2 and C2v symmetry), and two trimeric (both C2 symmetry) molecules. With the increase in temperature and dilution of the solution, the composition of the solution shifts to the smaller molecules. The thermodynamic and activation parameters for the reversible dissociation of dimers to monomers in the solution are DeltaH = 9.2 kcal mol(-1), DeltaS = 24.2 cal mol(-1) K(-1), DeltaH(double dagger) = 12.2 kcal mol(-1), DeltaS(double dagger) = 9.7 cal mol(-1) K(-1). The dissociation path with a weakly double-bridged transition-state dimer was proposed. The thermodynamic parameters for the reversible dissociation of the C2v tetramer to the dimers in solution are DeltaH = 7.9 kcal mol(-1) and DeltaS = 26.8 cal mol(-1) K(-1). From both tetramers, the D2 molecule is 0.34(5) kcal mol(-1) lower in enthalpy and 6.5(5) cal mol(-1) K(-1) lower in entropy than the C2v molecule. The structures of both trimers were proposed. The low-temperature 19F NMR spectra of the CDCl3 solution of [{(C5Me5)TiF3}2] are consistent with equilibria of a monomer, two dimers (with C2h and Cs symmetry), and a trimer. The vapor pressure osmometric molecular mass determination of CDCl3 solution of [{(C5Me5)TiF3}2] at 302 K is consistent with the equilibrium of the dimer and the monomer.     

Pyridine ligand rotation in self-assembled trigonal prisms. Evidence for intracage solvent vapor bubbles

The rate of interconversion of the two inequivalent edges of the pyridine rings in the trigonal prism 3c, self-assembled from 3 equiv of the star connector, tetrakis[4-(4-pyridylethynyl)phenyl]cyclobutadienecyclopentadienylcobalt, and 6 equiv of a platinum linker, cis-(Me3P)2Pt(2+) 2 TfO(-), was determined by DNMR in nitromethane. It exhibits a highly unusual bilinear Eyring plot. In the low temperature regime, the activation enthalpy DeltaH(double dagger) is approximately 12 kcal/mol and an activation entropy DeltaS(double dagger) ranges from approximately -15 to approximately 0 cal/mol x K as a function of the nature and concentration of the anions present. The reaction is attributed to hindered rotation of the pyridine rings about the Pt-N bond, facilitated by a tight pairing with a counterion. Above a counterion-dependent limiting temperature, DeltaH(double dagger) and DeltaS(double dagger) change abruptly to approximately 35 kcal/mol and approximately 60 cal/mol x K, respectively. The changes largely compensate, such that the reactions have comparable rates in the two regimes, both amenable to DNMR measurement, but their mechanisms clearly differ. Several kinetic models for the involvement of ion pairing equilibria fit the observed data nearly equally well, and they all contain a reaction step with high DeltaH(double dagger) and DeltaS(double dagger) values in the high-temperature regime. Its mechanism is proposed to involve a counterion-assisted reversible dissociation of one or two adjacent Pt-N bonds, followed by nearly free rotation of the terminal pyridine ring or rings and subsequent bond reclosure, which is similar to the last presumed step in the initial prism assembly. An interpretation of the very high DeltaS(double dagger) value is suggested by molecular dynamics calculations: at equilibrium, there is a bubble of gaseous nitromethane solvent inside the prism, and it collapses when the prism opens as the transition state is reached. A simple calculation of the entropy of cavitation provides quantitative support for this tentative proposal. The presence of such voids might be generally important for the formation and properties of self-assembled cages.     

Synthesis, structure, and substitution mechanism of new Ru(II) complexes containing 1,4,7-trithiacyclononane and 1,10-phenanthroline ligands

Two new Ru complexes containing the 1,10-phenanthroline (phen) and 1,4,7-trithiacyclononane ([9]aneS3, SCH2CH2SCH2CH2SCH2CH2) ligands of general formula [Ru(phen)(L)([9]aneS3)]2+ (L = MeCN, 3; L = pyridine (py), 4) have been prepared and thoroughly characterized. Structural characterization in the solid state has been performed by means of X-ray diffraction analyses, which show a distorted octahedral environment for a diamagnetic d6 Ru(II), as expected. 1H NMR spectroscopy provides evidence that the same structural arrangement is maintained in solution. Further spectroscopic characterization has been carried out by UV-vis spectroscopy where the higher acceptor capability of MeCN versus the py ligand is manifested in a 9-15-nm blue shift in its MLCT bands. The E1/2 redox potential of the Ru(III)/Ru(II) couple for 3 is anodically shifted with respect to its Ru-py analogue, 4, by 60 mV, which is also in agreement with a higher electron-withdrawing capacity of the former. The mechanism for the reaction Ru-py + MeCN--> Ru-MeCN + py has also been investigated at different temperatures with and without irradiation. In the absence of irradiation at 326 K, the thermal process gives kinetic constants of k2 = 1.4 x 10(-5) s(-1) (DeltaH(++) = 108 +/- 3 kJ mol(-1), DeltaS(++) = -8 +/- 9 J K(-1) mol(-1)) and k-2 = 2.9 x 10(-6) s(-1) (DeltaH(++) = 121 +/- 1 kJ mol(-1), DeltaS(++) = 18 +/- 3 J K(-1) mol(-1)). The phototriggered process is faster and consists of preequilibrium formation of an intermediate that thermally decays to the final Ru-MeCN complex with an apparent rate constant of (k1Khnu)app = 1.8 x 10(-4) s(-1) at 304 K, under the continuous irradiation experimental conditions used.     

The amide rotational barriers in picolinamide and nicotinamide: NMR and ab initio studies

Pyridine carboxamides are a class of medicinal agents with activity that includes the reduction of iron-induced renal damage, the regulation of nicotinamidase activity, and radio- and chemosensitization. Such pharmacological activities, and the prevalence of the carboxamide moiety and the importance of amide rotations in biology, motivate detailed investigation of energetics in these systems. In this study, we report the use of dynamic nuclear magnetic resonance to measure the amide rotational barriers in the pyridine carboxamides picolinamide and nicotinamide. The activation enthalpies and entropies of DeltaH++ = 12.9 +/- 0.3 kcal/mol and DeltaS++ = -7.7 +/- 0.9 cal/mol K for nicotinamide and DeltaH++ = 18.3 +/- 0.4 kcal/mol and DeltaS++ = +1.3 +/- 1.0 cal/mol K for picolinamide report a substantial energetic difference for these regioisomers. Ab initio calculations of the rotational barriers are in good agreement with the experimentally determined values and help partition the 5.4 kcal/mol enthalpy difference into its major contributions. Of principal importance are the variations in steric interactions in the ground states of picolinamide and nicotinamide, superior pi electron donation from the pyridine ring in the transition state of nicotinamide, and an intramolecular hydrogen bond in the ground state of picolinamide.     

Thermodynamic and kinetic studies of H atom transfer from HMo(CO)3(eta(5)-C5H5) to Mo(N[t-Bu]Ar)3 and (PhCN)Mo(N[t-Bu]Ar)3: direct insertion of benzonitrile into the Mo-H bond of HMo(N[t-Bu]Ar)3 forming (Ph(H)C=N)Mo(N[t-Bu]Ar)3

Synthetic studies are reported that show that the reaction of either H2SnR2 (R = Ph, n-Bu) or HMo(CO)3(Cp) (1-H, Cp = eta(5)-C5H5) with Mo(N[t-Bu]Ar)3 (2, Ar = 3,5-C6H3Me2) produce HMo(N[t-Bu]Ar)3 (2-H). The benzonitrile adduct (PhCN)Mo(N[t-Bu]Ar)3 (2-NCPh) reacts rapidly with H2SnR2 or 1-H to produce the ketimide complex (Ph(H)C=N)Mo(N[t-Bu]Ar)3 (2-NC(H)Ph). The X-ray crystal structures of both 2-H and 2-NC(H)Ph are reported. The enthalpy of reaction of 1-H and 2 in toluene solution has been measured by solution calorimetry (DeltaH = -13.1 +/- 0.7 kcal mol(-1)) and used to estimate the Mo-H bond dissociation enthalpy (BDE) in 2-H as 62 kcal mol(-1). The enthalpy of reaction of 1-H and 2-NCPh in toluene solution was determined calorimetrically as DeltaH = -35.1 +/- 2.1 kcal mol(-1). This value combined with the enthalpy of hydrogenation of [Mo(CO)3(Cp)]2 (1(2)) gives an estimated value of 90 kcal mol(-1) for the BDE of the ketimide C-H of 2-NC(H)Ph. These data led to the prediction that formation of 2-NC(H)Ph via nitrile insertion into 2-H would be exothermic by approximately 36 kcal mol(-1), and this reaction was observed experimentally. Stopped flow kinetic studies of the rapid reaction of 1-H with 2-NCPh yielded DeltaH(double dagger) = 11.9 +/- 0.4 kcal mol(-1), DeltaS(double dagger) = -2.7 +/- 1.2 cal K(-1) mol(-1). Corresponding studies with DMo(CO)3(Cp) (1-D) showed a normal kinetic isotope effect with kH/kD approximately 1.6, DeltaH(double dagger) = 13.1 +/- 0.4 kcal mol(-1) and DeltaS(double dagger) = 1.1 +/- 1.6 cal K(-1) mol(-1). Spectroscopic studies of the much slower reaction of 1-H and 2 yielding 2-H and 1/2 1(2) showed generation of variable amounts of a complex proposed to be (Ar[t-Bu]N)3Mo-Mo(CO)3(Cp) (1-2). Complex 1-2 can also be formed in small equilibrium amounts by direct reaction of excess 2 and 1(2). The presence of 1-2 complicates the kinetic picture; however, in the presence of excess 2, the second-order rate constant for H atom transfer from 1-H has been measured: 0.09 +/- 0.01 M(-1) s(-1) at 1.3 degrees C and 0.26 +/- 0.04 M(-1) s(-1) at 17 degrees C. Study of the rate of reaction of 1-D yielded kH/kD = 1.00 +/- 0.05 consistent with an early transition state in which formation of the adduct (Ar[t-Bu]N)3Mo...HMo(CO)3(Cp) is rate limiting.     

Stepwise oxidation of anilines by cis-[RuIV(bpy)2(py)(O)]2+

The net six-electron oxidation of aniline to nitrobenzene or azoxybenzene by cis-[Ru(IV)(bpy)(2)(py)(O)](2+) (bpy is 2,2'-bipyridine; py is pyridine) occurs in a series of discrete stages. In the first, initial two-electron oxidation is followed by competition between oxidative coupling with aniline to give 1,2-diphenylhydrazine and capture by H(2)O to give N-phenylhydroxylamine. The kinetics are first order in aniline and first order in Ru(IV) with k(25.1 degrees C, CH(3)CN) = (2.05 +/- 0.18) x 10(2) M(-1) s(-1) (DeltaH(++) = 5.0 +/- 0.7 kcal/mol; DeltaS(++) = -31 +/- 2 eu). On the basis of competition experiments, k(H)2(O)/k(D)2(O) kinetic isotope effects, and the results of an (18)O labeling study, it is concluded that the initial redox step probably involves proton-coupled two-electron transfer from aniline to cis-[Ru(IV)(bpy)(2)(py)(O)](2+) (Ru(IV)=O(2+)). The product is an intermediate nitrene (PhN) or a protonated nitrene (PhNH(+)) which is captured by water to give PhNHOH or aniline to give PhNHNHPh. In the following stages, PhNHOH, once formed, is rapidly oxidized by Ru(IV)=O(2+) to PhNO and PhNHNHPh to PhN=NPh. The rate laws for these reactions are first order in Ru(IV)=O(2+) and first order in reductant with k(14.4 degrees C, H(2)O/(CH(3))(2)CO) = (4.35 +/- 0.24) x 10(6) M(-1) s(-1) for PhNHOH and k(25.1 degrees C, CH(3)CN) = (1.79 +/- 0.14) x 10(4) M(-1) s(-1) for PhNHNHPh. In the final stages of the six-electron reactions, PhNO is oxidized to PhNO(2) and PhN=NPh to PhN(O)=NPh. The oxidation of PhNO is first order in PhNO and in Ru(IV)=O(2+) with k(25.1 degrees C, CH(3)CN) = 6.32 +/- 0.33 M(-1) s(-1) (DeltaH(++) = 4.6 +/- 0.8 kcal/mol; DeltaS(++) = -39 +/- 3 eu). The reaction occurs by O-atom transfer, as shown by an (18)O labeling study and by the appearance of a nitrobenzene-bound intermediate at low temperature.     

Catalytic propene polymerization: determination of propagation,termination, and epimerization kinetics by direct NMR observation of the (EBI)Zr(MeB(C6F5)3)propenyl catalyst species

Treatment of (EBI)ZrMe2 with B(C6F5)3 and 100 equiv of 1-hexene in toluene-d8 at -40 degrees C for 1 h, followed by addition of >5 equiv of propene, generates a living polypropene catalyst (3) with a polyhexene "tail". Upon further addition of propene, speciation of the catalyst and propagation kinetics are measured directly by NMR, yielding a propagation rate law that is first order in [propene] and [living catalyst] with a rate constant of 2.6 M-1 s-1 at -40 degrees C and the activation parameters DeltaH = 3.4(0.2) kcal/mol and DeltaS = -42(2) cal/(mol K). In the absence of propene, elimination of vinylidene-terminated polymer is observed with a first-order rate constant of 3.3 (0.5) x 10-5 s-1 at -40 degrees C and DeltaH = 12.9(1.4) kcal/mol and DeltaS = -23(5) cal/(mol K). Use of 1-13C-propene and 1,1-2H-2,3-13C-propene and 1H, 19F, and 13C NMR spectroscopy provide rigorous characterization of the living catalyst and enable the first direct observation of both 1,3-isomerization and chain-end epimerization processes at a metallocene catalyst. Isomerization and epimerization occur with rates that are similar to that of vinylidene termination and are consistent with previous mechanistic hypotheses featuring isomerization via a tertiary alkyl intermediate and switching between enantiofaces of the coordinated alkene.     

Coordination-driven face-directed self-assembly of trigonal prisms. Face-based conformational chirality

The coordination-driven self-assembly of four different trigonal prisms from 3 equiv of one of four different tetrapyridyl star connectors and 6 equiv of a platinum linker dication in nitromethane is presented. This face-directed approach affords high yields without template assistance. The prisms have been characterized by multinuclear and DOSY NMR and dual ESI-FT-ICR mass spectrometry. The use of a conformationally chiral star connector leads to a conformationally chiral prism when connector arm ends attached to a vertex have a strongly correlated twist sense and chirality is communicated across polyhedral faces, edges, and vertices. Molecular mechanics results suggest that in the smallest prism 3d collective effects dominate and the all-P and all-M conformers are strongly favored. NMR data prove that the two edges of the pyridine rings in the triflate salts of 3a-3d are distinct. An Eyring plot of rates obtained from line-shape analysis and 1-D EXCHSY NMR yields an activation enthalpy DeltaH(double dagger) of approximately 12 kcal/mol and activation entropy DeltaS(double dagger) of approximately -15 cal/mol x K for the edge interconversion process, compatible with pyridine rotation around the Pt-N bond. For 3c, this behavior is observed only up to approximately 318 K. At higher temperatures, the Eyring plot is again linear but follows a very different straight line, with a DeltaH(double dagger) of approximately 35 kcal/mol and DeltaS(double dagger) of approximately 60 cal/mol x K. This highly unusual result is further investigated and discussed in the following companion paper.     

A theoretical study of the primary oxo transfer reaction of a dioxo molybdenum(VI) compound with imine thiolate chelating ligands: a molybdenum oxotransferase analogue.

The reaction mechanism of an analogue system of the molybdenum oxotransferases was investigated at the density functional (B3P86) level of theory. Kinetic measurements by Schultz and Holm suggest that the reaction MoO(2)(t-BuL-NS)(2) + X --> MoO(t-BuL-NS)(2) + OX (t-BuL-NS = bis(4-tert-butylphenyl)-2-pyridylmethanethiolate(1-)) occurs through an associative transition state. Our results on the model reaction, MoO(2)(SCH(2)CHNH)(2) + P(CH(3))(3) --> MoO(SCH(2)CHNH)(2) + OP(CH(3))(3), support this hypothesis, and indicate that this reaction proceeds through a two-step mechanism via an associative intermediate. The DeltaH(++) for the first, and rate-determining, step was predicted to be 9.4 kcal/mol, and DeltaH(++) for the second step (release of the OP(CH(3))(3) product) was predicted to be 3.3 kcal/mol. These results are in good agreement with the experimental system, for which the rate determining DeltaH(++) = 9.6(6) kcal/mol. Shultz and Holm's experimental model undergoes a significant ligand rearrangement in the oxo transfer reaction: the reactant, MoO(2)(t-BuL-NS)(2), has a trans-S arrangement of the ligands, while the product, MoO(t-BuL-NS)(2), has a trans-N arrangement. To investigate the driving force behind the ligand rearrangement, four model compounds, that systematically removed the unsaturation at the N and the chelate character of the ligands, were modeled at the B3P86 level of theory. For all models of the dioxo species, the trans-N isomer was higher in energy than the trans-S isomer. The analysis of these results indicated that a trans influence accounts for approximately 16% of the energy difference, the unsaturation at the nitrogens accounts for approximately 26%, and the ring strain from the chelator accounts for approximately 58% of the energy difference between the two isomers (trans-N and trans-S). For all models of the monooxo species, only the trans-N species was a stable geometry. Therefore, for the reverse oxo transfer reaction, ligand rearrangement must occur after or during the attack of the OX substrate.