Kinetics, thermodynamics, and effect of BPh3 on competitive C-C and C-H bond activation reactions in the interconversion of allyl cyanide by [Ni(dippe)

Reaction of [(dippe)Ni(micro-H)](2) with allyl cyanide at low temperature quantitatively generates the eta(2)-olefin complex (dippe)Ni(CH(2)=CHCH(2)CN) (1). At ambient temperature or above, the olefin complex is converted to a mixture of C-CN cleavage product (dippe)Ni(eta(3)-allyl)(CN) (3) and the olefin-isomerization products (dippe)Ni(eta(2)-crotonitrile) (cis- and trans-2), which form via C-H activation. The latter are the exclusive products at longer reaction times, indicating that C-CN cleavage is reversible and the crotononitrile complexes 2 are more thermodynamically stable than eta(3)-allyl species 3. The kinetics of this reaction have been followed as a function of temperature, and rate constants have been extracted by modeling of the reaction. The rate constants for C-CN bond formation (the reverse of C-CN cleavage) show a stronger temperature dependence than those for C-CN and C-H activation, making the observed distribution of C-H versus C-CN cleavage products strongly temperature-dependent. The activation parameters for the C-CN formation step are also quite distinct from those of the C-CN and C-H cleavage steps (larger DeltaH(++) and positive DeltaS(++)). Addition of the Lewis acid BPh(3) to 1 at low temperature yields exclusively the C-CN activation product (dippe)Ni(eta(3)-allyl)(CNBPh(3)) (4). Independently prepared (dippe)Ni(crotononitrile-BPh(3)) (cis- and trans-7) does not interconvert with 4, indicating that 4 is the kinetic product of the BPh(3)-mediated reaction. On standing in solution at ambient temperature, 4 decomposes slowly to complex 5, with structure [(dippe)Ni(eta(3)-allyl)(N triple bond C-BPh(3)), while addition of a second equivalent of BPh(3) immediately produces [(dippe)Ni(eta(3)-allyl)](+)[Ph(3)BC triple bond NBPh(3)](-) (6). Comparison of the barriers to pi-sigma allyl interconversion (determined via dynamic (1)H NMR spectroscopy) for all of the eta(3)-allyl complexes reveals that axial cyanide ligands facilitate pi-sigma interconversion by moving into the P(2)Ni square plane when the allyl group is sigma-bound.     

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.     

Pre-catalyst resting states: a kinetic, thermodynamic and quantum mechanical analyses of [PdCl2(2-oxazoline)2] complexes

The treatment of cold ( approximately 3 degrees C) methanolic solutions of Li(2)PdCl(4) with two equivalents of 2-phenyl-2-oxazoline (Phox) results in the isolation of [PdCl(2)(Phox)(2)] (3). This complex undergoes remarkably slow isomerisation (CHCl(3)-d) at room temperature to a corresponding thermodynamic form. In addition to a theoretical treatment (DFT), the isomerisation behaviour has been analysed both kinetically and thermodynamically. These investigations lead to the conclusion that the initially formed (i.e. kinetic) isomer of 3 is the cis-form which undergoes conversion to the corresponding thermodynamic trans-form via a dissociative (D) mechanism involving loss of a Phox ligand. The activation parameters DeltaS(double dagger) and DeltaH(double dagger) are found to be +304 (+/-3) J K(-1) mol(-1) and +176 (+/-1) kJ mol(-1), respectively and indicate a high barrier to Pd-N bond cleavage under these conditions. The thermodynamic parameters show the expected endothermic nature of this process (+140 +/- 17 kJ mol(-1)) and a slight positive overall entropy (DeltaS degrees = +17 +/- 2 J K(-1) mol(-1)); this latter parameter is presumably due to the formation of the lower dipole moment trans-product when compared to the cis-isomer. Calculated (DFT) values of DeltaG(double dagger) and DeltaH(double dagger) are in excellent agreement to those found experimentally. Further theoretical investigation suggests that two 14-electron three-coordinate T-shaped transition states (i.e., [PdCl(2)(Phox)](double dagger)) are involved; the form pre-disposed to yield the thermodynamic trans-product following re-attachment of the released oxazoline is found to be energetically favoured. The analogous alkyloxazoline system [PdCl(2)(Meox)(2)] (4: Meox = 2-methyl-2-oxazoline) has likewise been investigated. This material gives no indication of cis-trans isomerisation behaviour in solution (NMR) and is shown to exist (X-ray) in the trans-form in the solid-state (as do previously reported crystalline samples of 3). A DFT study of 4 reveals similar values of DeltaS(double dagger) and DeltaH(double dagger) if a D type mechanism were operating to rapidly convert cis- to trans-4. However, a significantly higher thermodynamic stability of the trans-isomer relative to the cis-form is revealed versus similar calculations of the Phox derivative 3. This suggests the possibility that (i) reactions of Meox with Li(2)PdCl(4) may lead directly to the trans-form of [PdCl(2)(Meox)(2)] or alternatively (ii) that alkyloxazoline complexes such as 4 may have a different, and presumably much more rapid, mechanism for isomerisation. The results are placed into the context that isomerisation behaviour, or lack thereof, could play a key preliminary role in later substrate modification. This is due to the fact that [PdX(2)(oxazoline)(2)] compounds are well-known (pre-)catalysts for C-C bond forming chemistry.     

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.     

C-H vs C-C bond activation of acetonitrile and benzonitrile via oxidative addition: rhodium vs nickel and Cp* vs Tp' (Tp' = hydrotris(3,5-dimethylpyrazol-1-yl)borate, Cp* = η(5)-pentamethylcyclopentadienyl)

The photochemical reaction of (C(5)Me(5))Rh(PMe(3))H(2) (1) in neat acetonitrile leads to formation of the C-H activation product, (C(5)Me(5))Rh(PMe(3))(CH(2)CN)H (2). Thermolysis of this product in acetonitrile or benzene leads to thermal rearrangement to the C-C activation product, (C(5)Me(5))Rh(PMe(3))(CH(3))(CN) (4). Similar results were observed for the reaction of 1 with benzonitrile. The photolysis of 1 in neat benzonitrile results in C-H activation at the ortho, meta, and para positions. Thermolysis of the mixture in neat benzonitrile results in clean conversion to the C-C activation product, (C(5)Me(5))Rh(PMe(3))(C(6)H(5))(CN) (5). DFT calculations on the acetonitrile system show the barrier to C-H activation to be 4.3 kcal mol(-1) lower than the barrier to C-C activation. A high-energy intermediate was also located and found to connect the transition states leading to C-H and C-C activation. This intermediate has an agostic hydrogen interaction with the rhodium center. Reactions of acetonitrile and benzonitrile with the fragment [Tp'Rh(CNneopentyl)] show only C-H and no C-C activation. These reactions with rhodium are compared and contrasted to related reactions with [Ni(dippe)H](2), which show only C-CN bond cleavage.     

Triggering water exchange mechanisms via chelate architecture. Shielding of transition metal centers by aminopolycarboxylate spectator ligands

Paramagnetic effects on the relaxation rate and shift difference of the (17)O nucleus of bulk water enable the study of water exchange mechanisms on transition metal complexes by variable temperature and variable pressure NMR. The water exchange kinetics of [Mn(II)(edta)(H2O)](2-) (CN 7, hexacoordinated edta) was reinvestigated and complemented by variable pressure NMR data. The results revealed a rapid water exchange reaction for the [Mn(II)(edta)(H2O)](2-) complex with a rate constant of k(ex) = (4.1 +/- 0.4) x 10(8) s(-1) at 298.2 K and ambient pressure. The activation parameters DeltaH(double dagger), DeltaS(double dagger), and DeltaV(double dagger) are 36.6 +/- 0.8 kJ mol(-1), +43 +/- 3 J K(-1) mol(-1), and +3.4 +/- 0.2 cm(3) mol(-1), which are in line with a dissociatively activated interchange (I(d)) mechanism. To analyze the structural influence of the chelate, the investigation was complemented by studies on complexes of the edta-related tmdta (trimethylenediaminetetraacetate) chelate. The kinetic parameters for [Fe(II)(tmdta)(H2O)](2-) are k(ex) = (5.5 +/- 0.5) x 10(6) s(-1) at 298.2 K, DeltaH(double dagger) = 43 +/- 3 kJ mol(-1), DeltaS(double dagger) = +30 +/- 13 J K(-1) mol(-1), and DeltaV(double dagger) = +15.7 +/- 1.5 cm(3) mol(-1), and those for [Mn(II)(tmdta)(H2O)](2-) are k(ex) = (1.3 +/- 0.1) x 10(8) s(-1) at 298.2 K, DeltaH(double dagger) = 37.2 +/- 0.8 kJ mol(-1), DeltaS(double dagger) = +35 +/- 3 J K(-1) mol(-1), and DeltaV(double dagger) = +8.7 +/- 0.6 cm(3) mol(-1). The water containing species, [Fe(III)(tmdta)(H2O)](-) with a fraction of 0.2, is in equilibrium with the water-free hexa-coordinate form, [Fe(III)(tmdta)](-). The kinetic parameters for [Fe(III)(tmdta)(H2O)](-) are k(ex) = (1.9 +/- 0.8) x 10(7) s(-1) at 298.2 K, DeltaH(double dagger) = 42 +/- 3 kJ mol(-1), DeltaS(double dagger) = +36 +/- 10 J K(-1) mol(-1), and DeltaV(double dagger) = +7.2 +/- 2.7 cm(3) mol(-1). The data for the mentioned tmdta complexes indicate a dissociatively activated exchange mechanism in all cases with a clear relationship between the sterical hindrance that arises from the ligand architecture and mechanistic details of the exchange process for seven-coordinate complexes. The unexpected kinetic and mechanistic behavior of [Ni(II)(edta')(H2O)](2-) and [Ni(II)(tmdta')(H2O)](2-) is accounted for in terms of the different coordination number due to the strong preference for an octahedral coordination environment and thus a coordination equilibrium between the water-free, hexadentate [M(L)](n+) and the aqua-pentadentate forms [M(L')(H2O)](n+) of the Ni(II)-edta complex, which was studied in detail by variable temperature and pressure UV-vis experiments. For [Ni(II)(edta')(H2O)](2-) (CN 6, pentacoordinated edta) a water substitution rate constant of (2.6 +/- 0.2) x 10(5) s(-1) at 298.2 K and ambient pressure was measured, and the activation parameters DeltaH(double dagger), DeltaS(double dagger), and DeltaV(double dagger) were found to be 34 +/- 1 kJ mol(-1), -27 +/- 2 J K(-1) mol(-1), and +1.8 +/- 0.1 cm(3) mol(-1), respectively. For [Ni(II)(tmdta')(H2O)](2-), we found k = (6.4 +/- 1.4) x 10(5) s(-1) at 298.2 K, DeltaH(double dagger) = 22 +/- 4 kJ mol(-1), and DeltaS(double dagger) = -59 +/- 5 J K(-1) mol(-1). The process is referred to as a water substitution instead of a water exchange reaction, since these observations refer to the intramolecular displacement of coordinated water by the carboxylate moiety in a ring-closure reaction.     

Oxidative reactivity difference among the metal oxo and metal hydroxo moieties: pH dependent hydrogen abstraction by a manganese(IV) complex having two hydroxide ligands

Clarifying the difference in redox reactivity between the metal oxo and metal hydroxo moieties for the same redox active metal ion in identical structures and oxidation states, that is, M(n+)O and M(n+)-OH, contributes to the understanding of nature's choice between them (M(n+)O or M(n+)-OH) as key active intermediates in redox enzymes and electron transfer enzymes, and provides a basis for the design of synthetic oxidation catalysts. The newly synthesized manganese(IV) complex having two hydroxide ligands, [Mn(Me(2)EBC)(2)(OH)(2)](PF(6))(2), serves as the prototypic example to address this issue, by investigating the difference in the hydrogen abstracting abilities of the Mn(IV)O and Mn(IV)-OH functional groups. Independent thermodynamic evaluations of the O-H bond dissociation energies (BDE(OH)) for the corresponding reduction products, Mn(III)-OH and Mn(III)-OH(2), reveal very similar oxidizing power for Mn(IV)O and Mn(IV)-OH (83 vs 84.3 kcal/mol). Experimental tests showed that hydrogen abstraction proceeds at reasonable rates for substrates having BDE(CH) values less than 82 kcal/mol. That is, no detectable reaction occurred with diphenyl methane (BDE(CH) = 82 kcal/mol) for both manganese(IV) species. However, kinetic measurements for hydrogen abstraction showed that at pH 13.4, the dominant species Mn(Me(2)EBC)(2)(O)(2), having only Mn(IV)O groups, reacts more than 40 times faster than the Mn(IV)-OH unit in Mn(Me(2)EBC)(2)(OH)(2)(2+), the dominant reactant at pH 4.0. The activation parameters for hydrogen abstraction from 9,10-dihydroanthracene were determined for both manganese(IV) moieties: over the temperature range 288-318 K for Mn(IV)(OH)(2)(2+), DeltaH(double dagger) = 13.1 +/- 0.7 kcal/mol, and DeltaS(double dagger) = -35.0 +/- 2.2 cal K(-1) mol(-1); and the temperature range 288-308 K for for Mn(IV)(O)(2), DeltaH(double dagger) = 12.1 +/- 1.8 kcal/mol, and DeltaS(double dagger) = -30.3 +/- 5.9 cal K(-1) mol(-1).     

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.     

Transition states for the dimerization of 1,3-cyclohexadiene: a DFT, CASPT2, and CBS-QB3 quantum mechanical investigation

Quantum mechanical calculations using restricted and unrestricted B3LYP density functional theory, CASPT2, and CBS-QB3 methods for the dimerization of 1,3-cyclohexadiene (1) reveal several highly competitive concerted and stepwise reaction pathways leading to [4 + 2] and [2 + 2] cycloadducts, as well as a novel [6 + 4] ene product. The transition state for endo-[4 + 2] cycloaddition (endo-2TS, DeltaH(double dagger)(B3LYP(0K)) = 28.7 kcal/mol and DeltaH(double dagger)(CBS-QB3(0K)) = 19.0 kcal/mol) is not bis-pericyclic, leading to nondegenerate primary and secondary orbital interactions. However, the C(s) symmetric second-order saddle point on the B3LYP energy surface is only 0.3 kcal/mol above endo-2TS. The activation enthalpy for the concerted exo-[4 + 2] cycloaddition (exo-2TS, DeltaH(double dagger)(B3LYP(0K)) = 30.1 kcal/mol and DeltaH(double dagger)(CBS-QB3(0K)) = 21.1 kcal/mol) is 1.4 kcal/mol higher than that of the endo transition state. Stepwise pathways involving diallyl radicals are formed via two different C-C forming transition states (rac-5TS and meso-5TS) and are predicted to be competitive with the concerted cycloaddition. Transition states were located for cyclization from intermediate rac-5 leading to the endo-[4 + 2] (endo-2) and exo-[2 + 2] (anti-3) cycloadducts. Only the endo-[2 + 2] (syn-3) transition state was located for cyclization of intermediate meso-5. The novel [6 + 4] "concerted" ene transition state (threo-4TS, DeltaH(double dagger)(UB3LYP(0K)) = 28.3 kcal/mol) is found to be unstable with respect to an unrestricted calculation. This diradicaloid transition state closely resembles the cyclohexadiallyl radical rather than the linked cyclohexadienyl radical. Several [3,3] sigmatropic rearrangement transition states were also located and have activation enthalpies between 27 and 31 kcal/mol.     

A mechanistic study of the reaction between a diiron(II) complex [FeII(2)(mu-OH)2(6-Me3-TPA)2](2+) and O2 to form a diiron(III) peroxo complex

A kinetic study of the reaction between a diiron(II) complex [Fe(II)(2)(mu-OH)(2)(6-Me(3)-TPA)(2)](2+) 1, where 6-Me(3)-TPA = tris(6-methyl-2-pyridylmethyl)amine, and dioxygen is presented. A diiron(III) peroxo complex [Fe(III)(2)(mu-O)(mu-O(2))(6-Me(3)-TPA)(2)](2+) 2 forms quantitatively in dichloromethane at temperatures from -80 to -40 degrees C. The reaction is first order in [Fe(II)(2)] and [O(2)], with the activation parameters DeltaH(double dagger) = 17 +/- 2 kJ mol(-1) and DeltaS(double dagger) = -175 +/- 20 J mol(-1) K(-1). The reaction rate is not significantly influenced by the addition of H(2)O or D(2)O. The reaction proceeds faster in more polar solvents (acetone and acetonitrile), but the yield of 2 is not quantitative in these solvents. Complex 1 reacts with NO at a rate about 10(3) faster than with O(2). The mechanistic analysis suggests an associative rate-limiting step for the oxygenation of 1, similar to that for stearoyl-ACP Delta(9)-desaturase, but distinct from the probable dissociative pathway of methane monoxygenase. An eta(1)-superoxo Fe(II)Fe(III) species is a likely steady-state intermediate during the oxygenation of complex 1.     

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.     

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.     

Oxidations of NADH analogues by cis-[RuIV(bpy)2(py)(O)]2+ occur by hydrogen-atom transfer rather than by hydride transfer

Oxidations of the NADH analogues 10-methyl-9,10-dihydroacridine (AcrH2) and N-benzyl 1,4-dihydronicotinamide (BNAH) by cis-[RuIV(bpy)2(py)(O)]2+ (RuIVO2+) have been studied to probe the preferences for hydrogen-atom transfer vs hydride transfer mechanisms for the C-H bond oxidation. 1H NMR spectra of completed reactions of AcrH2 and RuIVO2+, after more than approximately 20 min, reveal the predominant products to be 10-methylacridone (AcrO) and cis-[RuII(bpy)2(py)(MeCN)]2+. Over the first few seconds of the reaction, however, as monitored by stopped-flow optical spectroscopy, the 10-methylacridinium cation (AcrH+) is observed. AcrH+ is the product of net hydride removal from AcrH2, but hydride transfer cannot be the dominant pathway because AcrH+ is formed in only 40-50% yield and its subsequent oxidation to AcrO is relatively slow. Kinetic studies show that the reaction is first order in both RuIVO2+ and AcrH2, with k = (5.7 +/- 0.3) x 10(3) M(-1) s(-1) at 25 degrees C, DeltaH(double dagger) = 5.3 +/- 0.3 kcal mol(-1) and DeltaS(double dagger) = -23 +/- 1 cal mol(-1) K(-1). A large kinetic isotope effect is observed, kAcrH2/kAcrD2 = 12 +/- 1. The kinetics of this reaction are significantly affected by O2. The rate constants for the oxidations of AcrH2 and BNAH correlate well with those for a series of hydrocarbon C-H bond oxidations by RuIVO2+. The data indicate a mechanism of initial hydrogen-atom abstraction. The acridinyl radical, AcrH*, then rapidly reacts by electron transfer (to give AcrH+) or by C-O bond formation (leading to AcrO). Thermochemical analyses show that H* and H- transfer from AcrH2 to RuIVO2+ are comparably exoergic: DeltaG degrees = -10 +/- 2 kcal mol(-1) (H*) and -6 +/- 5 kcal mol(-1) (H-). That a hydrogen-atom transfer is preferred kinetically suggests that this mechanism has an equal or lower intrinsic barrier than a hydride transfer pathway.     

P-S Bond scission by bis(cyclopentadienyl)molybdenum(IV) dichloride, Cp(2)MoCl(2)(aq): first documented example of an organometallic complex hydrolyzing thiophosphinates

Thiophosphinate hydrolysis involving P-S bond scission is desirable for the degradation of organophosphate neurotoxins, and we report the first case for such a hydrolytic process by an organometallic compound. The metallocene, bis(cyclopentadienyl)molybdenum(IV) dichloride, Cp(2)MoCl(2) (Cp = eta(5)-C(5)H(5)), hydrolyzes a variety of thioaryl diphenylphosphinates in an aqueous THF solution. P-S scission of p-methoxythiophenyl diphenylphosphinate has a 500-fold rate of acceleration in the presence of Cp(2)MoCl(2)(aq) with activation parameters of 20(3) kcal mol(-)(1) and -15(3) cal mol(-)(1) K(-)(1) for DeltaH(double dagger) and DeltaS(double dagger), respectively. These activation parameters and the rate acceleration are consistent with an intermolecular hydrolytic process in which the Cp(2)Mo serves as a Lewis acid to activate the phosphinate for nucleophilic attack. Furthermore, rho = 2.3 (25 degrees C) which indicates a single nonconcerted mechanism in which the rate determining step is the nucleophilic attack on the activated phosphinate.     

Bimetallo-radical carbon-hydrogen bond activation of methanol and methane

Carbon-hydrogen bond cleavage reactions of CH3OH and CH4 by a dirhodium(II) diporphyrin complex with a m-xylyl tether (.Rh(m-xylyl)Rh.(1)) are reported. Kinetic-mechanistic studies show that the substrate reactions are bimolecular and occur through the use of two Rh(II) centers in the molecular unit of 1. Second-order rate constants (T = 296 K) for the reactions of 1 with methanol (k(CH3OH) = 1.45 x 10-2 M-1 s-1) and methane (k(CH4) = 0.105 M-1 s-1) show a clear kinetic preference for the methane activation process. The methanol and methane reactions with 1 have large kinetic isotope effects (k(CH3OH)/k(CD3OD) = 9.7 +/- 0.8, k(CH4)/k(CD4) = 10.8 +/- 1.0, T = 296 K), consistent with a rate-limiting step of C-H bond homolysis through a linear transition state. Activation parameters for reaction of 1 with methanol (DeltaH = 15.6 +/- 1.0 kcal mol-1; DeltaS = -14 +/- 5 cal K-1 mol-1) and methane (DeltaH = 9.8 +/- 0.5 kcal mol-1; DeltaS = -30 +/- 3 cal K-1 mol-1) are reported.     

Conformational analysis of indole alkaloids corynantheine and dihydrocorynantheine by dynamic 1H NMR spectroscopy and computational methods: steric effects of ethyl vs vinyl group

1H NMR (400 MHz) spectra of the indole alkaloid dihydrocorynantheine recorded at room temperature show the presence of two conformers near coalescence. Low temperature 1H NMR allowed characterization of the conformational equilibrium, which involves rotation of the 3-methoxypropenoate side chain. Line-shape analysis yielded enthalpy of activation DeltaH(double dagger) = 71 +/- 6 kJ/mol, and entropy of activation DeltaS(double dagger) = 33 +/- 6 J/mol.K. The major and minor conformation contains the methyl ether group above and below the plane of the ring, respectively, as determined by low-temperature NOESY spectra, with free energy difference DeltaG degrees = 1.1 kJ/mol at -40 degrees C. In contrast to dihydrocorynantheine, the corresponding rotamers of corynantheine are in the fast exchange limit at room temperature. The activation parameters determined for corynantheine were DeltaH(double dagger) = 60 +/- 6 kJ/mol and DeltaS(double dagger) = 24 +/- 6 J/mol.K, with DeltaG degrees = 1.3 kJ/mol at -45 degrees C. The difference in the exchange rates of the rotamers of corynantheine and dihydrocorynantheine (respectively, 350 s(-1) and 9 s(-1) at 0 degrees C) reflects the difference in the steric bulk of the vinyl and the ethyl group. The conformational equilibria involving the side chain rotation as well as inversion of the bridgehead nitrogen in corynantheine and dihydrocorynantheine was studied by force-field (Amber and MMFF) and ab initio (density-functional theory at the B3LYP/6-31G level) computational methods, the results of which were in good agreement with the 1H NMR data. However, the calculations identified the rotamers as essentially isoenergetic, the experimental energy differences being to small to be reproduced exactly by the theory. Comparison of density-functional and force-field calculations with experimental results identified Amber as giving the most accurate results in the present case.     

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.     

Cleavage of carbon-carbon bonds in aromatic nitriles using nickel(0)

The nickel(0) fragment [(dippe)Ni] has been found to react with a variety of aromatic nitriles. Initial pi-coordination to the C=C and Ctbd1;N bonds of 2-cyanoquinoline is found to lead ultimately to C-CN oxidative addition. 3-Cyanoquinoline reacts similarly, although no eta(2)-CN complex is observed. 2-, 3-, And 4-cyanopyridines react initially to give eta(2)-nitrile complexes that then lead to quantitative formation of C-CN oxidative addition products. Benzonitrile reacts similarly but undergoes reversible insertion into the Ph-CN bond to give an equilibrium mixture of Ni(II) and Ni(0) adducts. A series of para-substituted benzonitriles has been studied in terms of both the position of the equilibrium between (dippe)Ni(eta(2)-arylnitrile) right harpoon over left harpoon (dippe)Ni(CN)(aryl) and the rate of approach to equilibrium, and the Hammett plots indicate a buildup of negative charge at the ipso carbon both in the transition state and the Ni(II) product. Terephthalonitrile gives both eta(2)-nitrile and oxidative addition adducts, as well as dimetalated products. No C-C or C-N cleavage of the aromatic ring is seen with quinoline or acridine; only eta(2)-arene complexes are formed. The structures of many of these compounds are supported by X-ray data.     

Introduction of clutch function into a molecular gear system by silane-silicate interconversion

Introduction of the clutch-declutch mechanism into a new gear system, bis(4-methyl-9-triptycyl)difluorosilane 1, is achieved by the reversible attachment of fluoride ion giving the corresponding fluorosilicate 2. Although the phase isomers of 1 (1(dl) and 1(meso)) cannot be separated because of the equilibrium via a slow gear slippage process (DeltaH(double dagger) = 17.2 +/- 0.2 kcal x mol(-1) and DeltaS(double dagger) = 0.9 +/- 0.9 cal x mol(-1) x K(-1)), 1 works as meshed molecular gears in solution at room temperature. On the other hand, silicate 2 in the solid state has quite an unusual TBP structure having two organic triptycyl groups at the apical positions and three electronegative fluorine atoms at the equatorial positions against the Muetterties rule. Rotation of the two triptycyl groups around Si-C bonds in 2 is facile and independent to each other in solution. Silicate 2 is reverted to the corresponding silane mixture by treating with excess water.     

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.