Jack R. Norton

Kinetic and thermodynamic acidity of hydrido transition-metal complexes. 3. Thermodynamic acidity of common mononuclear carbonyl hydrides.

The pK/sub a/ values of the common mononuclear carbonyl hydrides have been determined in acetonitrile by IR measurement of the position of deprotonation equilibria with various nitrogen bases and potassium phenolate. The resulting values cover a range of about 20 pK/sub a/ units, from 8.3 for HCo(CO)/sub 4/ to 26.6 for CpW(CO)/sub 2/(PMe/sub 3/)H. Hydrides with eta/sup 5/-C/sub 5/Me/sub 5/ ligands are appreciably weaker acids than the corresponding hydrides with eta/sup 5/-C/sub 5/H/sub 5/ ligands (e.g., the pK/sub a/ of (eta/sup 5/-C/sub 5/Me/sub 5/)Fe(CO)/sub 2/H is 26.3, while that of (eta/sup 5/-C/sub 5/H/sub 5/)Fe(CO)/sub 2/H is 19.4). The acidities of the group 8 carbonyl hydrides H/sub 2/M(CO)/sub 4/ decrease in the order Fe > Ru > Os.

The Reaction of Cobaloximes with Hydrogen: Products and Thermodynamics

A cobalt hydride has been proposed as an intermediate in many reactions of the Co(dmgBF2)2L2 system, but its observation has proven difficult. We have observed the UV-vis spectra of Co(dmgBF2)2L2 (1) in CH3CN under hydrogen pressures of up to 70 atm. A Co(I) compound (6a) with an exchangeable proton is eventually formed. We have determined the bond dissociation free energy and pK(a) of the new O-H bond in 6a to be 50.5 kcal/mol and 13.4, respectively, in CH3CN, matching previous reports.

Hydrogen exchange between the methyl and hydride ligands of dicyclopentadienylhydridomethyltungsten prior to methane elimination

The elimination of methane from Cp/sub 2/W(H)CH/sub 3/ (Cp-cyclopentadiene) was investigated using thermolysis and acetonitrile-toluene mixtures as the matrix. With the use of deuterium labeled intermediates, an intramolecular methane elimination was found in dilute solutions while the process appears to be intermolecular at higher concentrations. Results for /sup 1/H, /sup 2/H, /sup 13/C NMR were used to calculate the final mole fractions of 16 isotopically labeled species occurring in mixtures of Cp/sub 2/W(H)/sup 13/ CH/sub 3/ Cp/sub 2/W(H)CD/sub 3/ mixtures. Analysis of results shows that the tungsten hydride in Cp/sub 2/W(H)CH/sub 3/ is able to exchange with the C-H bonds in the methyl ligand of another molecule of the same material, at a rate which competes well with intramolecular methane except in a very dilute solution. The compounds discussed are among the first hydrides for which intermolecular exchange with an aliphatic ligand has been reported. 16 references.

Evidence for Formation of a Co–H Bond from (H2O)2Co(dmgBF2)2 under H2: Application to Radical Cyclizations

Under H(2), the radical cyclization of appropriate dienes can be catalyzed by cobaloximes. H• can be abstracted from an intermediate (presumably a cobalt hydride) by trityl radicals (Ar(3)C•) or by TEMPO. The rate-determining step in these reactions is the uptake of H(2), which is second order in cobalt and first order in hydrogen; the third-order rate constant is 106(3) M(-2)·s(-1).

Unusually Weak Metal-Hydrogen Bonds in HV(CO)4(P-P) and Their Effectiveness as H· Donors

The V-H bonds in HV(CO)4(P-P) are unusually weak (55 to 58 kcal/mol). They transfer H* to styrene more rapidly than CpCr(CO)3H does and are effective stoichiometric reagents for the cyclization of appropriate 1,6-dienes.

Electron transfer from hexameric copper hydrides.

The octahedral core of 84-electron LCuH hexamers does not dissociate appreciably in solution, although their hydride ligands undergo rapid intramolecular rearrangement. The single-electron transfer proposed as an initial step in the reaction of these hexamers with certain substrates has been observed by stopped-flow techniques when [(Ph3P)CuH]6 is treated with a pyridinium cation. The same radical cation has been prepared by the oxidation of [(Ph3P)CuH]6 with Cp*2Fe(+) and its reversible formation observed by cyclic voltammetry; its UV-vis spectrum has been confirmed by spectroelectrochemistry. The 48-electron trimer [(dppbz)CuH]3 has been prepared by use of the chelating ligand 1,2-bis(diphenylphosphino)benzene (dppbz).

Zirconium-catalyzed carboalumination of α-olefins and chain growth of aluminum alkyls: kinetics and mechanism.

A mechanism based on Michaelis-Menten kinetics with competitive inhibition is proposed for both the Zr-catalyzed carboalumination of α-olefins and the Zr-catalyzed chain growth of aluminum alkyls from ethylene. AlMe(3) binds to the active catalyst in a rapidly maintained equilibrium to form a Zr/Al heterobimetallic, which inhibits polymerization and transfers chains from Zr to Al. The kinetics of both carboalumination and chain growth have been studied when catalyzed by [(EBI)Zr(μ-Me)(2)AlMe(2)][B(C(6)F(5))(4)]. In accord with the proposed mechanism, both reactions are first-order in [olefin] and [catalyst] and inverse first-order in [AlR(3)]. The position of the equilibria between various Zr/Al heterobimetallics and the corresponding zirconium methyl cations has been quantified by use of a Dixon plot, yielding K = 1.1(3) × 10(-4) M, 4.7(5) × 10(-4) M, and 7.6(7) × 10(-4) M at 40 °C in benzene for the catalyst species [rac-(EBI)Zr(μ-Me)(2)AlMe(2)][B(C(6)F(5))(4)], [Cp(2)Zr(μ-Me)(2)AlMe(2)][B(C(6)F(5))(4)], and [Me(2)C(Cp)(2)Zr(μ-Me)(2)AlMe(2)][B(C(6)F(5))(4)] respectively. These equilibrium constants are consistent with the solution behavior observed for the [Cp(2)Zr(μ-Me)(2)AlMe(2)][B(C(6)F(5))(4)] system, where all relevant species are observable by (1)H NMR. Alternative mechanisms for the Zr-catalyzed carboalumination of olefins involving singly bridged Zr/Al adducts have been discounted on the basis of kinetics and/or (1)H NMR EXSY experiments.

Kinetics and Thermodynamics of H–/H•/H+ Transfer from a Rhodium(III) Hydride

The thermodynamics and kinetics of all three cleavage modes for Rh-H, the transfer of H(-), H(+), or H•, have been studied for the Rh(III) hydride complex Cp*Rh(2-(2-pyridyl)phenyl)H (1a). The thermodynamic hydricity, ΔG°H(-), for 1a has been measured (49.5(1) kcal/mol) by heterolytic cleavage of H2 with Et3N in CH3CN. The transfer of H(-) from 1a to 1-(1-phenylethylidene)pyrrolidinium is remarkably fast (kH(-) = 3.5(1) × 10(5) M(-1) s(-1)), making 1a a very efficient catalyst for the ionic hydrogenation of iminium cations. The pKa of 1a in CH3CN has been measured as 30.3(2) with (tert-butylimino)tris(pyrrolidino)phosphorane (12), and the rate constant for H(+) transfer from 1a to 12 has been estimated (kH(+) = 5(1) × 10(-4) M(-1) s(-1)) from the half-life of the equilibration. Thus, 1a is a poor H(+) donor both thermodynamically and kinetically. However, 1a transfers H• to TEMPO smoothly, forming a stable Rh(II) radical Cp*Rh(2-(2-pyridyl)phenyl)• (14a) that can activate H2 at room temperature and 1 atm. The metalloradical 14a has a g value of 2.0704 and undergoes reversible one-electron reduction at -1.85 V vs Fc(+)/Fc in benzonitrile, implying a bond-dissociation enthalpy for the Rh-H bond of 1a of 58.2(3) kcal/mol--among the weakest Rh(III)-H bonds reported. The transfer of H• from 1a to Ar3C• (Ar = p-(t)BuC6H4) is fast, with kH• = 1.17(3) × 10(3) M(-1) s(-1). Thus, 1a is a good H(-) and H• donor but a poor H(+) donor, a combination that reflects the high energy of the Rh(I) anion [Cp*Rh(2-(2-pyridyl)phenyl)](-).

Transition-Metal Hydride Radical Cations.

Transition-metal hydride radical cations (TMHRCs) are involved in a variety of chemical and biochemical reactions, making a more thorough understanding of their properties essential for explaining observed reactivity and for the eventual development of new applications. Generally, these species may be treated as the ones formed by one-electron oxidation of diamagnetic analogues that are neutral or cationic. Despite the importance of TMHRCs, the generally sensitive nature of these complexes has hindered their development. However, over the last four decades, many more TMHRCs have been synthesized, characterized, isolated, or hypothesized as reaction intermediates. This comprehensive review focuses on experimental studies of TMHRCs reported through the year 2014, with an emphasis on isolated and observed species. The methods used for the generation or synthesis of TMHRCs are surveyed, followed by a discussion about the stability of these complexes. The fundamental properties of TMHRCs, especially those pertaining to the M-H bond, are described, followed by a detailed treatment of decomposition pathways. Finally, reactions involving TMHRCs as intermediates are described.

[(η2-C2H4)Os(CO)4] as a vibrational model for type I′ ethene chemisorbed as a metallacyclopropane on metal surfaces

A comprehensive vibrational assignment of the hydrocarbon features in the infrared and Raman spectra of (η2-C2H4)Os(CO)4 has been achieved with the help of data from the (13C2H4) and (C2D4) isotopologues. Close analogies with the vibrational spectra of thiirane (ethene sulfide) assist in the assignment and emphasize the metallacyclopropane nature of the (C2H4Os) group. Comparisons are made with analogous spectra from (η2-C2H4)Fe(CO)4 and from Zeises salt, K[(η2-C2H4)PtCl3].Taking into account the metal–surface selection rule associated with on-specular vibrational electron energy loss or reflection–absorption infrared spectra, good agreement is found between the wavenumber/intensity pattern from the a1 modes of the complex and from ethene chemisorbed on metal surfaces previously designated Type I′.