Ruthenium bis(β-diketonato) complexes have been prepared at both the Ru II and Ru III oxidation levels and with protonated and deprotonated pyridine-imidazole ligands. Ru II (acac) 2 (py-imH) (1), [Ru III (acac) 2 (py-imH)]OTf (2), Ru III (acac) 2 (py-im) (3), Ru II (hfac) 2 (py-imH) (4), and [DBU-H] [Ru II (hfac) 2 (py-im)] (5) have been fully characterized, including X-ray crystal structures (acac = 2,4-pentanedionato, hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato, py-imH = 2-(2′-pyridyl) imidazole, DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene). For the acac-imidazole complexes 1 and 2, cyclic voltammetry in MeCN shows the Ru III/II reduction potential (E 1/2 ) to be −0.64 V vs. Cp 2 Fe +/0 . E 1/2 for the deprotonated imidazolate complex 3 (−1.00 V) is 0.36 V more negative. The Ru II bis-hfac analogs 4 and 5 show the same ΔE 1/2 = 0.36 V but are 0.93 V harder to oxidize than the acac derivatives (0.29 V and −0.07 V). The difference in acidity between the acac and hfac derivatives is much smaller, with pK a values of 22.1 and 19.3 in MeCN for 1 and 4. From the E 1/2 and pK a values, the bond dissociation free energies (BDFEs) of the N-H bonds in 1 and 4 are calculated to be 62.0 and 79.6 kcal mol −1 in MeCN -a remarkable difference of 17.6 kcal mol −1 for such structurally similar compounds. Consistent with these values, there is facile net hydrogen atom transfer from 1 to TEMPO • (2,2,6,6-tetramethylpiperidine-1-oxyl radical) to give 3 and TEMPO-H. The ΔG° for this reaction is −4.5 kcal mol −1 . Complex 4 is not oxidized by TEMPO • (ΔG° = +13.1 kcal mol −1 ), but in the reverse direction TEMPO-H readily reduces in situ generated Ru III (hfac) 2 (py-im) (6). A Ru II -imidazoline analog of 1, Ru II (acac) 2 (py-imnH) (7), reacts with 3 equiv of TEMPO • to give the imidazolate complex 3 and TEMPO-H, with dehydrogenation of the imidazoline ring.

Reported herein are thermochemical studies of hydrogen atom transfer (HAT) 2+ , is surprisingly well predicted by the trends for electron transfer half-reaction entropies, ΔS o ET , in aprotic solvents. This is because both ΔS o ET and ΔS o HAT have substantial contributions from vibrational entropy, which varies significantly with the metal center involved. The close connection between ΔS o HAT and ΔS o ET provides an important link between these two fields and provides a starting point from which to predict which HAT systems will have important ground-state entropy effects.

Bimolecular rate constants have been measured for reactions that involve hydrogen atom transfer (HAT) from hydroxylamines to nitroxyl radicals, using the stable radicals TEMPO • (2,2,6,6-tetramethylpiperidine-1-oxyl radical), 4-oxo-TEMPO • (2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl radical), di-tert-butylnitroxyl ( t Bu 2 NO • ), and the hydroxylamines TEMPO-H, 4-oxo-TEMPO-H, 4-MeO-TEMPO-H (2,2,6,6-tetramethyl-N-hydroxy-4-methoxy-piperidine), and t Bu 2 NOH. The reactions have been monitored by UV-vis stopped-flow methods, using the different optical spectra of the nitroxyl radicals. The HAT reactions all have |∆G°| e 1.4 kcal mol -1 and therefore are close to self-exchange reactions. The reaction of 4-oxo-. Surprisingly, the rate constant for the analogous deuterium atom transfer reaction is much slower: k 2D,MeCN ) 0.44 ( 0.05 M -1 s -1 with k 2H,MeCN /k 2D,MeCN ) 23 ( 3 at 298 K. The same large kinetic isotope effect (KIE) is found in CH 2 Cl 2 , 23 ( 4, suggesting that the large KIE is not caused by solvent dynamics or hydrogen bonding to solvent. The related reaction of 4-oxo-TEMPO • with 4-MeO-TEMPO-H(D) also has a large KIE, k 3H /k 3D ) 21 ( 3 in MeCN. For these three reactions, the E aD -E aH values, between 0.3 ( 0.6 and 1.3 ( 0.6 kcal mol -1 , and the log(A H /A D ) values, between 0.5 ( 0.7 and 1.1 ( 0.6, indicate that hydrogen tunneling plays an important role. The related reaction of t Bu 2 NO • + TEMPO-H(D) in MeCN has a large KIE, 16 ( 3 in MeCN, and very unusual isotopic activation parameters, E aD -E aH ) -2.6 ( 0.4 and log(A H /A D ) ) 3.1 ( 0.6. Computational studies, using POLYRATE, also indicate substantial tunneling in the (CH 3 ) 2 NO • + (CH 3 ) 2 NOH model reaction for the experimental self-exchange processes. Additional calculations on TEMPO( • /H), t Bu 2 NO( • /H), and Ph 2 NO( • /H) self-exchange reactions reveal why the phenyl groups make the last of these reactions several orders of magnitude faster than the first two. By inference, the calculations also suggest why tunneling appears to be more important in the self-exchange reactions of dialkylhydroxylamines than of arylhydroxylamines.