Rotonation occurs in the nitrogen not bound to the redox-active metal

The BDFEs in (acac)2RuII(py-imH) and in TpOsIII(NH2Ph)Cl2, 62.1 and 61.five kcal mol-1 EPZ004777 msds respectively, are unusually low for N bonds. In such systems there's a formal separation in between the redox as well as the acid/base web sites (see also Section six): you'll find three chemical bonds and four ?separation amongst the metal center plus the acidic/basic nitrogen (Figure 12). Even with this separation the redox and acid/base chemistry continues to be coupled. In general, protonation/deprotonation of metal-imidazole complexes outcomes inside a adjust of 0.three to 0.5 V in reduction possible (Table 21 scan/nsw074 and reference 425). Although this `thermochemical communication' is substantially significantly less than in compounds exactly where the proton is bound to an atom straight bonded for the metal, as discussed above, imidazole complexes are still able to mediate concerted H-transfer reactions. Of these systems possibly essentially the most effectively explored would be the iron(II)tris(2-2-biimidazoline)2+ (FeIIH2bip) and iron(II)tris(two,2-(tetrahydro)pyrimidine)2+ (FeIIH2bip).84,426?27428 The FeII(H2bim) and FeII(H2bip) systems have comparable acid/base properties in MeCN, with pKa = 17.5. The systems have slightly diverse redox potentials in MeCN, E1/2(FeIII/II(H2bim)) = -0.31 V84 and E1/2(FeIII/II(H2bip)) = -0.55 V428 (each vs. Fc+/0). Application of eq 7 provides BDFE(FeIIH2bim) = 71.7 kcal mol-1 and BDFE(FeIIH2bip) = 66.2 kcal mol-1. The FeIIH2bip and FeIIIH2bip compounds are each mixtures of high-spin and low-spin types at ambient temperatures in MeCN, which indirectly impacts their hydrogen atom self-exchange rate.427 The connected cobalt-H2bim complexes have comparable thermochemistry, with a BDFE of 70.5 kcal mol-1.429 Within this method, CoIIH2bim is higher spin even though CoIIH2bim is low spin, and HAT reactions that interconvert these two are very slow.428,429 These iron and cobalt H2bim and H2bip systems all have substantial ground-state entropy alterations (S? connected with their 1e-/1H+ redox couples, so the initial analyses of those systems making use of BDEs has been revised.39,40 Associated ruthenium systems have already been developed utilizing a bidentate 2-(2-pyridyl)-imidazole ligand (py-imH) and with either acac (two,4-pentanedionato) or 1,1,1,5,five,5-hexafluoro-acac (hfacac) as supporting ligands.430 In both sets of compounds, deprotonation on the imidazole reduces the redox prospective by 0.36 jir.2013.0113 V (and, equivalently, oxidation from RuII to RuIII tends to make the imidazole proton far more acidic by 6.1 pKa units). The BDFEs in (acac)2RuII(py-imH) and in TpOsIII(NH2Ph)Cl2, 62.1 and 61.five kcal mol-1 respectively, are unusually low for N bonds. HAT reactions of (acac)2RuII(py-imH) show substantial H/D kinetic isotope effects and involve substantial hydrogen tunneling.75 Replacing acac with hfacac increases the BDFE by a remarkable 17.six kcal mol-1.430 This is principally the result of differences within the reduction potentials; the pKa with the imidazole ligand is just not strongly affected. This illustrates that the effect off ligands and substituents onNIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptChem Rev.