Rotonation occurs at the nitrogen not bound towards the redox-active metal
This illustrates that the effect off ligands and Aprotinin supplier substituents onNIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptChem Rev. Author manuscript; available in PMC 2011 December 8.Warren et al.PageBDFEs will not be normally straightforward. Electron-withdrawing groups, for instance, will make a complex extra oxidizing but additionally more acidic, and these two effects are in opposite AprotininMedChemExpress Aprotinin directions when it comes to the BDFE.Rotonation occurs at the nitrogen not bound towards the redox-active metal center. In such systems there's a formal separation among the redox and the acid/base web sites (see also Section six): you can find 3 chemical bonds and four ?separation among the metal center and the acidic/basic nitrogen (Figure 12). Even with this separation the redox and acid/base chemistry continues to be coupled. Normally, protonation/deprotonation of metal-imidazole complexes outcomes within a adjust of 0.3 to 0.five V in reduction prospective (Table 21 scan/nsw074 and reference 425). Although this `thermochemical communication' is substantially much less than in compounds where the proton is bound to an atom directly bonded towards the metal, as discussed above, imidazole complexes are still able to mediate concerted H-transfer reactions. Of those systems perhaps essentially the most nicely 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 equivalent acid/base properties in MeCN, with pKa = 17.five. The systems have slightly distinct 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 offers 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 forms at ambient temperatures in MeCN, which indirectly affects their hydrogen atom self-exchange rate.427 The associated cobalt-H2bim complexes have comparable thermochemistry, using a BDFE of 70.5 kcal mol-1.429 In this method, CoIIH2bim is higher spin although CoIIH2bim is low spin, and HAT reactions that interconvert these two are extremely slow.428,429 These iron and cobalt H2bim and H2bip systems all have big ground-state entropy adjustments (S? associated with their 1e-/1H+ redox couples, so the initial analyses of these systems utilizing BDEs has been revised.39,40 Related ruthenium systems have already been created applying a bidentate 2-(2-pyridyl)-imidazole ligand (py-imH) and with either acac (two,4-pentanedionato) or 1,1,1,five,five,5-hexafluoro-acac (hfacac) as supporting ligands.430 In each sets of compounds, deprotonation on the imidazole reduces the redox potential by 0.36 jir.2013.0113 V (and, equivalently, oxidation from RuII to RuIII makes the imidazole proton a lot more acidic by six.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 significant H/D kinetic isotope effects and involve substantial hydrogen tunneling.75 Replacing acac with hfacac increases the BDFE by a exceptional 17.6 kcal mol-1.430 This can be principally the result of differences inside the reduction potentials; the pKa with the imidazole ligand is not strongly impacted.