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. 2007 Dec 24;46(26):11190-201.
doi: 10.1021/ic7015726. Epub 2007 Dec 1.

Synthesis and characterization of ruthenium bis(beta-diketonato) pyridine-imidazole complexes for hydrogen atom transfer

Adam Wu  1 , Joshua Masland, Rodney D Swartz, Werner Kaminsky, James M Mayer
Affiliations

Synthesis and characterization of ruthenium bis(beta-diketonato) pyridine-imidazole complexes for hydrogen atom transfer

Adam Wu et al. Inorg Chem. .

Abstract

Ruthenium bis(beta-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 DeltaE1/2 = 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 DeltaG degrees for this reaction is -4.5 kcal mol(-1). 4 is not oxidized by TEMPO* (DeltaG degrees = +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 imidazolate 3 and TEMPO-H, with dehydrogenation of the imidazoline ring.

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Figure 1
Figure 1
ORTEP drawings of (a) RuII(acac)2(py-imH) (1), (b) [RuIII(acac)2(py-imH)]+ (2), and (c) RuIII(acac)2(py-im) (3). Hydrogen atoms are omitted for clarity except for the N–H atom.
Figure 2
Figure 2
ORTEP drawings of (a) RuII(hfac)2(py-imH) (4), (b) [RuII(hfac)2(py-im)]− (5), and (c) RuII(hfac)2(py-imnH) (8) [see below], showing one of the two independent molecules in the unit cell for each of 4 and 8. Hydrogen atoms are omitted for clarity except for the N–H atom.
Figure 3
Figure 3
1H NMR spectra of (a) [RuIII(acac)2(py-imH)]OTf (2) and (b) RuIII(acac)2(py-im) (3) in CD3CN. Peaks A to D are assigned as acac-CH3 protons, peaks 1, 2, 3, and 5 as pyridine protons, and 4, 6, 7, and 8 as acac- or imidazole-CH protons. The letters and numbers show the corresponding signals between 2 and 3, as determined by reversible NMR titration by DBU/HOTf. Solvent and impurity peaks are denoted by asterisks (*).
Figure 4
Figure 4
UV-vis spectra of 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) in MeCN.
Scheme 1
Scheme 1
Square Scheme for Hydrogen Atom Transfer
Scheme 2
Scheme 2
Square schemes for (a) the Ru–acac–py-imH (1–3) and (b) the Ru–hfac–py-imH (4–6) systems (in MeCN at 298 K, E1/2 values vs. Cp2Fe+/0).
Scheme 3
Scheme 3
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