Two [1,2,4-(Me3C)3C5H2]2CeH molecules are involved in hydrogenation of pyridine to piperidine as shown by experiments and computations.

Hydrogenation of pyridine to piperidine catalyzed by [1,2,4-(Me3C)3C5H2]2CeH, abbreviated as Cp'2CeH or [Ce]'-H, is reported. The reaction proceeds from Cp'2Ce(2-pyridyl), isolated from the reaction of pyridine with Cp'2CeH, to Cp'2Ce(4,5,6-trihydropyridyl), and then to Cp'2Ce(piperidyl). The cycle is completed by the addition of pyridine, which generates Cp'2Ce(2-pyridyl) and piperidine. The net reaction depends on the partial pressure of H2 and temperature. The dependence of the rate on the H2 pressure is associated with the formation of Cp'2CeH, which increases the rate of the first and/or second additions of H2 but does not influence the rate of the third addition. Density functional theory calculations of several possible pathways are consistent with three steps, each of which are composed of two elementary reactions, (i) heterolytic activation of H2 with a reasonably high energy, ΔG(⧧) = 20.5 kcal mol(-1), on Cp'2Ce(2-pyridyl), leading to Cp'2CeH(6-hydropyridyl), followed by an intramolecular hydride transfer with a lower activation energy, (ii) intermolecular addition of Cp'2CeH to the C(4)═C(5) bond, followed by hydrogenolysis, giving Cp'2Ce(4,5,6-trihydropyridyl) and regenerating Cp'2CeH, and (iii) a similar hydrogenation/hydrogenolysis sequence, yielding Cp'2Ce(piperidyl). The calculations reveal that step ii can only occur in the presence of Cp'2CeH and that alternative intramolecular steps have considerably higher activation energies. The key point that emerges from these experimental and computational studies is that step ii involves two Cp'2Ce fragments, one to bind the 6-hydropyridyl ligand and the other to add to the C(4)═C(5) double bond. In the presence of H2, this second step is intermolecular and catalytic. The cycle is completed by reaction with pyridine to yield Cp'2Ce(2-pyridyl) and piperidine. The structures of Cp'2CeX, where X = 2-pyridyl, 4,5,6-trihydropyridyl, and piperidyl, are fluxional, as shown by variable-temperature (1)H NMR spectroscopy.

Intermediate-valence tautomerism in decamethylytterbocene complexes of methyl-substituted bipyridines.

Multiconfigurational, intermediate valent ground states are established in several methyl-substituted bipyridine complexes of bis(pentamethylcyclopentadienyl)ytterbium, Cp2*Yb (Me(x)-bipy). In contrast to Cp2*Yb(bipy) and other substituted-bipy complexes, the nature of both the ground state and the first excited state are altered by changing the position of the methyl or dimethyl substitutions on the bipyridine rings. In particular, certain substitutions result in multiconfigurational, intermediate valent open-shell singlet states in both the ground state and the first excited state. These conclusions are reached after consideration of single-crystal X-ray diffraction (XRD), the temperature dependence of X-ray absorption near-edge structure (XANES), extended X-ray absorption fine-structure (EXAFS), and magnetic susceptibility data, and are supported by CASSCF-MP2 calculations. These results place the various Cp2*Yb(bipy) complexes in a new tautomeric class, that is, intermediate-valence tautomers.

The reaction of bis(1,2,4-tri-t-butylcyclopentadienyl)ceriumbenzyl, Cp'2CeCH2Ph, with methylhalides: a metathesis reaction that does not proceed by a metathesis transition state.

The experimental reaction between [1,2,4-(Me(3)C)(3)C(5)H(2)](2)CeCH(2)Ph and CH(3)X, X = F, Cl, Br, and I, yields the metathetical exchange products, [1,2,4-(Me(3)C)(3)C(5)H(2)](2)CeX and CH(3)CH(2)Ph. The reaction is complicated by the equilibrium between the benzyl derivative and the metallacycle [1,2,4-(Me(3)C)(3)C(5)H(2)][(Me(3)C)(2)C(5)H(2)C(CH(3))(2)CH(2)]Ce, plus toluene since the metallacycle reacts with CH(3)X. Labelling studies show that the methyl group of the methylhalide is transferred intact to the benzyl group. The mechanism, as revealed by DFT calculations on (C(5)H(5))(2)CeCH(2)Ph and CH(3)F, does not proceed by way of a four-center mechanism, a sigma-bond metathesis, but by a lower barrier process involving a haptotropic shift of the Cp(2)Ce fragment so that at the transition state the para-carbon of the benzene ring is attached to the Cp(2)Ce fragment while the CH(2) fragment of the benzyl group attacks CH(3)F that is activated by coordination to the metal ion. As a result the mechanism is classified as an associative interchange process.

Fluorine for hydrogen exchange in the hydrofluorobenzene derivatives C6H(x)F(6-x), where x = 2, 3, 4 and 5 by monomeric [1,2,4-(Me3C)3C5H2]2CeH: the solid state isomerization of [1,2,4-(Me3C)3C5H2]2Ce(2,3,4,5-C6HF4) to [1,2,4-(Me3C)3C5H2]2Ce(2,3,4,6-C6HF4).

The reaction between monomeric bis(1,2,4-tri-tert-butylcyclopentadienyl)cerium hydride, Cp'2CeH, and several hydrofluorobenzene derivatives is described. The aryl derivatives that are the primary products, Cp'2Ce(C6H(5-x)F(x)) where x = 1,2,3,4, are thermally stable enough to be isolated in only two cases, since all of them decompose at different rates to Cp'2CeF and a fluorobenzyne; the latter is trapped by either solvent when C6D6 is used or by a Cp'H ring when C6D12 is the solvent. The trapped products are identified by GC/MS analysis after hydrolysis. The aryl derivatives are generated cleanly by reaction of the metallacycle, Cp'((Me3C)2C5H2C(Me2)CH2)Ce, with a hydrofluorobenzene, and the resulting arylcerium products, in each case, are identified by their (1)H and (19)F NMR spectra at 20 degrees C. The stereochemical principle that evolves from these studies is that the thermodynamic isomer is the one in which the CeC bond is flanked by two ortho-CF bonds. This orientation is suggested to arise from the negative charge that is localized on the ipso-carbon atom due to C(o)(delta+)F(o)(delta-) polarization. The preferred regioisomer is determined by thermodynamic rather than kinetic effects; this is illustrated by the quantitative, irreversible solid-state conversion at 25 degrees C over two months of Cp'2Ce(2,3,4,5-C6HF4) to Cp'2Ce(2,3,4,6-C6HF4), an isomerization that involves a CeC(ipso) for C(ortho)F site exchange.

Reactions of monomeric [1,2,4-(Me3C)3C5H2]2CeH and CO with or without H2: an experimental and computational study.

Addition of CO to [1,2,4-(Me3C)3C5H2]2CeH,Cp'2 CeH, in toluene yields the cis-(Cp'2Ce)2(mu-OCHCHO), in which the cis-enediolate group bridges the two metallocene fragments. The cis-enediolate quantitatively isomerizes intramolecularly to the trans-enediolate in C6D6 at 100 degrees C over 7 months. When the solvent is pentane, Cp'2Ce(OCH2)CeCp'2 forms, in which the oxomethylene group or the formaldehyde dianion bridges the two metallocene fragments. The cis-enediolate is suggested to form by insertion of CO into the Ce-C bond of Cp'2Ce(OCH2)CeCp'2, generating Cp'2CeOCH2COCeCp'2. The stereochemistry of the cis-enediolate is determined by a 1,2-hydrogen shift in the OCH2CO fragment that has the OC(H2) bond anti-periplanar relative to the carbene lone pair. The bridging oxomethylene complex reacts with H2, but not with CH4, to give Cp'2CeOMe, which is also the product of the reaction between Cp'2CeH and a mixture of CO and H2. The oxomethylene complex reacts with CO to give the cis-enediolate complex. DFT calculations on C5H5 model metallocenes show that the reaction of Cp2CeH with CO and H2 to give Cp2CeOMe is exoergic by 50 kcal mol-1. The net reaction proceeds by a series of elementary reactions that occur after the formyl complex, Cp2Ce(eta2-CHO), is formed by further reaction with H2. The key point that emerges from the calculated potential energy surface is the bifunctional nature of the metal formyl in which the carbon atom behaves as a donor and acceptor. Replacing H2 by CH4 increases the activation energy by 17 kcal mol-1.

Hydrogen for fluorine exchange in CH4-xFx by monomeric [1,2,4-(Me3C)3C5H2]2CeH: experimental and computational studies.

The monomeric metallocenecerium hydride, Cp'(2)CeH (Cp' = 1,2,4-tri-tert-butylcyclopentadienyl), reacts instantaneously with CH(3)F, but slower with CH(2)F(2), to give Cp'(2)CeF and CH(4) in each case, a net H for F exchange reaction. The hydride reacts very slowly with CHF(3), and not at all with CF(4), to give Cp'(2)CeF, H(2), and 1,2,4- and 1,3,5-tri-tert-butylbenzene. The substituted benzenes are postulated to result from trapping of a fluorocarbene fragment derived by alpha-fluoride abstraction from Cp'(2)CeCF(3). The fluoroalkyl, Cp'(2)CeCF(3), is generated by reaction of Cp'(2)CeH and Me(3)SiCF(3) or by reaction of the metallacycle, [(Cp')(Me(3)C)(2)C(5)H(2)C(Me(2))CH(2)]Ce, with CHF(3), and its existence is inferred from the products of decomposition, which are Cp'(2)CeF, the isomeric tri-tert-butylbenzenes and in the case of Me(3)SiCF(3), Me(3)SiH. The fluoroalkyls, Cp'(2)CeCH(2)F and Cp'(2)CeCHF(2), generated from the metallacycle and CH(3)F and CH(2)F(2), respectively, are also inferred by their decomposition products, which are Cp'(2)CeF, CH(2), and CHF, respectively, which are trapped. DFT(B3PW91) calculations have been carried out to examine several reaction paths that involve CH and CF bond activation. The calculations show that the CH activation by Cp(2)CeH proceeds with a low barrier. The carbene ejection and trapping by H(2) is the rate-determining step, and the barrier parallels that found for reaction of H(2) with CH(2), CHF, and CF(2). The barrier of the rate-determining step is raised as the number of fluorines increases, while that of the CH activation path is lowered as the number of fluorines increases, which parallels the acidity.

Hydrogen for fluorine exchange in C6F6 and C6F5H by monomeric [1,3,4-(Me3C)3C5H2]2CeH: experimental and computational studies.

The net reaction of monomeric Cp'(2)CeH [Cp' = 1,3,4-(Me(3)C)(3)(C(5)H(2))] in C(6)D(6) with C(6)F(6) is Cp'(2)CeF, H(2), and tetrafluorobenzyne. The pentafluorophenylmetallocene, Cp'(2)Ce(C(6)F(5)), is formed as an intermediate that decomposes slowly to Cp'(2)CeF and C(6)F(4) (tetrafluorobenzyne), and the latter is trapped by the solvent C(6)D(6) as a [2+4] cycloadduct. In C(6)F(5)H, the final products are also Cp'(2)CeF and H(2), which are formed from the intermediates Cp'(2)Ce(C(6)F(5)) and Cp'(2)Ce(2,3,5,6-C(6)F(4)H) and from an unidentified metallocene of cerium and the [2+4] cycloadducts of tetra- and trifluorobenzyne with C(6)D(6). The hydride, fluoride, and pentafluorophenylmetallocenes are isolated and characterized by X-ray crystallography. DFT(B3PW91) calculations have been used to explore the pathways leading to the observed products of the exergonic reactions. A key step is a H/F exchange reaction which transforms C(6)F(6) and the cerium hydride into C(6)F(5)H and Cp'(2)CeF. This reaction starts by an eta(1)-F-C(6)F(5) interaction, which serves as a hook. The reaction proceeds via a sigma bond metathesis where the fluorine ortho to the hook migrates toward H with a relatively low activation energy. All products observed experimentally are accommodated by pathways that involve C-F and C-H bond cleavages.