Hydride, hydrogen atom, proton, and electron transfer driving forces of various five-membered heterocyclic organic hydrides and their reaction intermediates in acetonitrile.

The enthalpy changes of 47 five-membered heterocyclic compounds (ZH) [33 substituted 2,3-dihydro-2-phenylbenzo[d]imidazoles (1H-5H), 9 substituted 2,3-dihydro-2-phenylbenzo[d]thiazoles (6H), and 5 substituted 2,3-dihydro-2-phenylbenzo[d]oxazoles (7H)] as a class of very important organic hydride donors to release hydride anion were determined by using titration calorimetry. The result shows that the enthalpy change scale of the 47 ZH in acetonitrile ranges from 49.0 to 93.4 kcal/mol. Such a long energy scale evidently shows that the 47 ZH can construct a large and useful library of organic hydride donors, which can provide various organic hydride donors that the hydride-releasing enthalpies are known. The enthalpy changes of the 47 ZH to release hydrogen atom and the 47 ZH+* to release proton and hydrogen atom were also evaluated by using relative thermodynamic cycles according to Hess' law. The results show: (1) the enthalpy change scale of the 47 ZH to release hydrogen atom covers a range from 71.8 to 91.4 kcal/mol, indicating that the 47 ZH all should be weak hydrogen atom donors. (2) The enthalpy change scales of the 47 ZH+* to release proton and to release hydrogen atom range from 17.5 to 25.7 and from 27.2 to 52.4 kcal/mol, respectively, implying that the proton-donating abilities of ZH+* are generally quite larger than the corresponding hydrogen atom-donating abilities. The standard redox potentials of the 47 ZH and the 47 corresponding salts (Z+) were measured by using cyclic voltammetry (CV) and Osteryoung square wave voltammetry (OSWV), the results display that the standard oxidation potential scale of ZH ranges from -0.254 to -0.002 V for 1H-5H and from 0.310 to 0.638 V for 6H-7H, implying that 1H-5H should be strong one-electron reducing agents and 6H-7H should be weak one-electron reducing agents; the standard reduction potential scale of Z+ ranges from -1.832 to -2.200 V for 1+-5+ and from -1.052 to -1.483V for 6+-7+, meaning that 1+-5+ belong to very weak one-electron oxidation agents. The energies of the intramolecular hydrogen bond in 3H, 3H+*, and 3* with a hydroxyl group at ortho-position on the 2-phenyl ring were estimated by using experimental method, the results disclose that the hydrogen bond energy is 3.2, 2.8-3.0, and 3.9-4.0 kcal/mol for 3H, 3H+*, and 3* in acetonitrile, respectively, which is favorable for hydrogen atom transfer but unfavorable for hydride transfer from 3H. The relative effective charges on the active center in ZH, ZH+*, Z*, and Z+, which is an efficient measurement of electrophilicity or nucleophilicity as well as dimerizing ability of a chemical species, were estimated by using experimental method; the results indicate that 1*-5* belong to electron-sufficient carbon-radicals, 6*-7* belong to electron-deficient carbon radicals, they are all difficult to dimerize, and that 1+-5+ belong to weak electrophilic agents, 6+-7+ belong to strong electrophilic agents. All these information disclosed in this work could not only supply a gap of the chemical thermodynamics of the five-membered heterocyclic compounds as organic hydride donors, but also strongly promote the fast development of the chemistry and applications of the five-membered heterocyclic organic hydrides.