Synthesis and Characterization of Ruthenium Bis(β-diketonato) Pyridine-Imidazole Complexes for Hydrogen Atom Transfer

Publisher Website
Google Scholar
Abstract
Ruthenium bis(β-diketonato) complexes have been prepared at both the RuII and RuIII oxidation levels and with protonated and deprotonated pyridine−imidazole ligands. RuII(acac)2(py-imH) (1), [RuIII(acac)2(py-imH)]OTf (2), RuIII(acac)2(py-im) (3), RuII(hfac)2(py-imH) (4), and [DBU−H][RuII(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 RuIII/II reduction potential (E1/2) to be −0.64 V versus Cp2Fe+/0. E1/2 for the deprotonated imidazolate complex 3 (−1.00 V) is 0.36 V more negative. The RuII bis-hfac analogues 4 and 5 show the same ΔE1/2 = 0.36 V but are 0.93 V harder to oxidize than the acac derivatives (0.29 and −0.07 V). The difference in acidity between the acac and hfac derivatives is much smaller, with pKa values of 22.1 and 19.3 in MeCN for 1 and 4, respectively. From the E1/2 and pKa 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 a 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. 4 is not oxidized by TEMPO• (ΔG° = +13.1 kcal mol-1), but in the reverse direction TEMPO-H readily reduces in situ generated RuIII(hfac)2(py-im) (6). A RuII-imidazoline analogue of 1, RuII(acac)2(py-imnH) (7), reacts with 3 equiv of TEMPO• to give the imidazolate3 and TEMPO-H, with dehydrogenation of the imidazoline ring.