The thermodynamic hydride donor abilities of 1-benzyl-1,4-dihydronicotinamide (BzNADH, 59 +/- 2 kcal/mol), C(5)H(5)Mo(PMe(3))(CO)(2)H (55 +/- 3 kcal/mol), and C(5)Me(5)Mo(PMe(3))(CO)(2)H (58 +/- 2 kcal/mol) have been measured in acetonitrile by calorimetric and/or equilibrium methods. The hydride donor abilities of BzNADH and C(5)H(5)Mo(PMe(3))(CO)(2)H differ by 13 and 24 kcal/mol, respectively, from those reported previously for these compounds in acetonitrile. These results require significant revisions of the hydricities reported for related NADH analogues and metal hydrides. These compounds are moderate hydride donors as compared to previously determined compounds.

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The free energies for hydride donation (ΔG M + ) have been determined in acetonitrile solution for a series of seven molybdenum and tungsten compounds (1−7) of general formula (C5R5)M(CO)2(L)H, which yield the salts [(C5R5)M(CO)2(L)(NCMe)][BF4] in these reactions. These data constitute the first thermodynamic data for hydride transfer by transition metal hydrides, and were gathered from equilibrium studies with carbenium ion salts of known hydride ion affinities in acetonitrile. The metal hydride ΔG M + values range from ca. 79 to 89 kcal/mol, and these values may be compared with pK as for related compounds to demonstrate that proton-transfer processes are somewhat more sensitive to changes in co-ligands than are hydride transfer processes. Additionally, kinetic studies of hydride transfer reactions with hydride acceptor [(p-MeOPh)2CPh][BF4] exhibit second-order rate constants ranging from ca. 200 to 7500 M-1 s-1. These rates show a correlation with thermodynamic driving force, and a Brönsted plot yields a slope of 0.20. The thermodynamic data may be used in conjuction with the appropriate thermodynamic cycles to calculate energies for various processes in which compounds such as 1−7 are known to function as hydride donors.

Gibbs free energies for scission of C−H bonds leading to carbanion and proton (mode a), radical pair (mode b), and carbenium ion and hydride (mode c) have been determined for a series of acidic C−H bonds in ca. 45 weak acids. This involved the use of the equilibrium acidities (or homolytic bond dissociation enthalpies), redox potentials in DMSO or MeCN solution, and the appropriate thermodynamic cycles in the two solvents. The introduction of electron-donating groups generally results in small-to-negligible effects on ΔG R - values (mode a scission) but in a relatively large stabilizing influence on the ΔG R + values for heterolytic cleavage to hydride and carbenium ion (mode c). Electron-withdrawing groups exert large stabilizing effects on ΔG R − , but their effects on ΔG R + are dependent on the nature of the substituents. The heterolytic processes a are usually favored in solution due to the strong solvation of the proton. Homolytic scission (mode b) is usually more favorable than the corresponding heterolytic process c, but they can be comparable when strong electron-donating groups such as dialkylamino are present. Indeed, heterolytic cleavage to hydride and carbenium ion was ca. 10 kcal/mol more favorable than homolytic cleavage for a series of highly stabilized 2-benzoyl-N,N‘-dialkylperhydropyrimidines.

The chelating trisphenol ligands tris(2-hydroxyphenyl)amine (1H 3 ) and tris(2-hydroxy-4,6-dimethylbenzyl)amine (2H 3 ) proved to be excellent precursors for the chelating phenoxides, and the latter has been used to prepare a series of cyclopentadienylmetal derivatives of early transition metals. For niobium and tantalum, reactions with CpMCl 4 lead to the compounds CpMCl(1) and CpMCl( 2). An X-ray diffraction study of CpNbCl(1) establishes a pseudo-octahedral structure with a trans disposition of the η 5 -cyclopentadienyl ring and the nitrogen atom of the chelating ligand. Similar reactions of CpTiCl 3 lead to the CpTi(1) and CpTi(2) analogues. Electrochemical experiments provide useful information on the reduction potentials of the compounds, from which it is clear that ligand 2 is a stronger donor than is 1. At the same time, it appears that chelate ring size is important; while the reduction of complexes containing 1 are largely reversible, those of complexes containing 2 are irreversible. This is interpreted to mean that the six-membered rings in the latter are opening during reduction, a process involving formal loss of an aryloxide from the metal center. In an attempt to correlate this solution reactivity with catalytic efficiency in a bond-forming process, the compounds were screened for activity as styrene polymerization catalysts in the presence of methylaluminoxane cocatalyst. While the niobium and tantalum analogues were inactive, the titanium compounds of 1 showed high activity and appreciable selectivity for the preparation of syndiotactic polystyrene.(1) (a) Wesleyan University. (b) Yale University.(2) (a) Bradley, D. C.

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