Noriaki Ochi

New Palladium(II) Complex of P,S-Containing Hybrid Calixphyrin. Theoretical Study of Electronic Structure and Reactivity for Oxidative Addition

The palladium complex of P,S-containing hybrid calixphyrin 1 was investigated with the DFT method. There are two kinds of valence tautomer in 1: one is a Pd(II) form in which the calixphyrin moiety possesses -2 charges and the Pd center takes +2 oxidation state, and the other is a Pd(0) form in which the calixphyrin is neutral and the Pd center takes zero oxidation state. Complex 1 takes the Pd(II) form in the ground state. Though the Pd center takes +2 oxidation state, DFT computations clearly show that the oxidative addition of phenyl bromide (PhBr) to 1 occurs with moderate activation enthalpy, as experimentally proposed. The first step of the oxidative addition is the coordination of PhBr with the Pd center to form intermediate 1INTa, in which the Pd center and the calixphyrin moiety are neutral; in other words, the valence tautomerization from the Pd(II) form to the Pd(0) form occurs in the palladium calixphyrin moiety. The activation enthalpy is 22.5 kcal/mol, and the enthalpy change of reaction is 20.3 kcal/mol. The next step is the C-Br sigma-bond cleavage of PhBr, which occurs with activation enthalpy of 2.0 kcal/mol relative to 1INTa. On the other hand, the oxidative additions of PhBr to palladium complex of P,S-containing hybrid porphyrin 2 and that of conventional porphyrin 3 need much larger activation enthalpies of 49.1 and 74.4 kcal/mol, respectively. The differences in the reactivity among 1, 2, and 3 were theoretically investigated; in 1, the valence tautomerization occurs with moderate activation enthalpy to afford the Pd(0) form which is reactive for the oxidative addition. In 2, the tautomerization from the Pd(II) form to the Pd(0) form needs very large activation enthalpy (43.3 kcal/mol). In 3, such valence tautomerization does not occur at all, indicating that the Pd(II) must change to the Pd(IV) in the oxidative addition of PhBr to 3, which is a very difficult process. These differences are interpreted in terms of the pi* orbital energies of P,S-containing hybrid calixphyrin, hybrid porphyrin, and conventional porphyrin and the flexibility of their frameworks.

Synthesis of thiophene-containing hybrid calixphyrins of the 5,10-porphodimethene type

The synthesis, structure, and optical and electrochemical properties of thiophene-containing hybrid calixphyrins are reported. The 5,10-porphodimethene type 14pi- and 16pi-S,N2,X-hybrid calixphyrins (X = NH, O, S) were prepared by acid-promoted dehydrative condensation between a thiatripyrrane and the corresponding 2,5-bis[hydroxy(phenyl)methyl]heteroles followed by DDQ oxidation. Both crystallographic and spectroscopic analyses of the newly prepared hybrid calixphyrins have revealed that the combination of heteroles explicitly influences the electronic structures of the pi-conjugated framework. The 14pi-S,N2,X-hybrid calixphyrins have proven to be fluorescent in solution.

{2 + 2} Cycloaddition of Alkyne with Titanium−Imido Complex: Theoretical Study of Determining Factor of Reactivity and Regioselectivity

The {2 + 2} cycloaddition of alkyne across the Ti=N bond of [(H(3)SiO)(2)Ti(=NSiH(3))] 1 was theoretically investigated. Though this cycloaddition is symmetry forbidden in a formal sense by the Woodward-Hoffmann rule, the cycloaddition of 2-butyne (MeC[triple bond]CMe) easily occurs with moderate activation barrier (7.6 kcal/mol) and considerably large exothermicity (41.0 kcal/mol), where the CCSD(T)-calculated energies are presented hereafter. The moderate activation barrier is interpreted in terms of the considerably polarized Ti=N bond; Because the d(pi)-p(pi) bonding orbital largely consists of the p(pi) orbital of the N and moderately of the d(pi) orbital of the Ti, the pi* orbital of 2-butyne interacts with the d(pi)-p(pi) bonding orbital so as to form a bonding overlap with the p(pi) orbital of the N, into which the pi orbital of 2-butyne mixes in an antibonding way with the p(pi) orbital of N. As a result, the C[triple bond]C bond of 2-butyne is polarized in the transition state and the symmetry forbidden character becomes very weak, which is the reason of the moderate activation barrier. The {2 + 2} cycloaddition of 1-methoxy-1-propyne (MeC(alpha)[triple bond]C(beta)OMe) occurs with smaller activation barrier (3.2 kcal/mol) than that of 2-butyne, when the C(alpha) and C(beta) approach the Ti and N, respectively. The higher reactivity of this alkyne is interpreted in terms of its polarized C[triple bond]C bond. In the reverse regioselective {2 + 2} cycloaddition in which the C(alpha) and C(beta) approach the N and Ti, respectively, the activation barrier becomes larger. From these results, it is concluded that the regioselective {2 + 2} cycloaddition can be performed by introducing such pi-electron donating group as methoxy on one C atom of alkyne. The major product contains the Ti-C(alpha) and N-C(beta) bonds, where the methoxy group is introduced on the C(beta). The ratio of the major to minor products is theoretically estimated to be very large.

Heterolytic activation of dihydrogen molecule by hydroxo-/sulfido-bridged ruthenium-germanium dinuclear complex. Theoretical insights.

Heterolytic activation of dihydrogen molecule (H2) by hydroxo-/sulfido-bridged ruthenium-germanium dinuclear complex [Dmp(Dep)Ge(μ-S)(μ-OH)Ru(PPh3)](+) (1) (Dmp = 2,6-dimesitylphenyl, Dep = 2,6-diethylphenyl) is theoretically investigated with the ONIOM(DFT:MM) method. H2 approaches 1 to afford an intermediate [Dmp(Dep)(HO)Ge(μ-S)Ru(PPh3)](+)-(H2) (2). In 2, the Ru-OH coordinate bond is broken but H2 does not yet coordinate with the Ru center. Then, the H2 further approaches the Ru center through a transition state TS2-3 to afford a dihydrogen σ-complex [Dmp(Dep)(HO)Ge(μ-S)Ru(η(2)-H2)(PPh3)](+) (3). Starting from 3, the H-H σ-bond is cleaved by the Ru and Ge-OH moieties to form [Dmp(Dep)(H2O)Ge(μ-S)Ru(H)(PPh3)](+) (4). In 4, hydride and H2O coordinate with the Ru and Ge centers, respectively. Electron population changes clearly indicate that this H-H σ-bond cleavage occurs in a heterolytic manner like H2 activation by hydrogenase. Finally, the H2O dissociates from the Ge center to afford [Dmp(Dep)Ge(μ-S)Ru(H)(PPh3)](+) (PRD). This step is rate-determining. The activation energy of the backward reaction is moderately smaller than that of the forward reaction, which is consistent with the experimental result that PRD reacts with H2O to form 1 and H2. In the Si analogue [Dmp(Dep)Si(μ-S)(μ-OH)Ru(PPh3)](+) (1Si), the isomerization of 1Si to 2Si easily occurs with a small activation energy, while the dissociation of H2O from the Si center needs a considerably large activation energy. Based on these computational findings, it is emphasized that the reaction of 1 resembles well that of hydrogenase and the use of Ge in 1 is crucial for this heterolytic H-H σ-bond activation.

Theoretical prediction of O–H, Si–H, and Si–C σ-bond activation reactions by titanium(IV)–imido complex

The O–H σ-bond activation of methanol, the Si–H σ-bond activation of silane, and the Si–C σ-bond activation of methylsilane by titanium(IV)–imido complex (Me3SiO)2Ti(NSiMe3) were theoretically inve...