Fiber-optic infrared reflectance spectroelectrochemical studies of osmium and ruthenium nitrosyl porphyrins containing alkoxide and thiolate ligands

Abstract

We have examined the redox behavior of the osmium and ruthenium compounds (OEP)M(NO)(OEt) and (OEP)M(NO)(SEt) (OEP = octaethylporphyrinato dianion; M = Os, Ru) by cyclic voltammetry and infrared spectroelectrochemistry. The compound (OEP)Os(NO)(OEt) undergoes a single reversible oxidation process in dichloromethane. In contrast, the thiolate compound (OEP)Os(NO)(SEt) undergoes a net irreversible oxidation resulting in formal loss of the SEt ligand. Extended Hückel calculations on crystal structures of these two compounds provide insight into the nature of their HOMOs. In the case of the alkoxide compound, the HOMO is largely metal centered, with 70% of the charge located in the metal's orbital and approximately 25% on the porphyrin ring. However, the HOMO of the thiolate compound consists of a pi bonding interaction between the metal dxz orbital and the px orbital on the sulfur, and a pi antibonding interaction between the metal d orbital and a pi* orbital on NO. The redox behavior of the Ru analogues have been determined, and are compared with those of the Os compounds.

Preparation, Characterization, Electrochemistry, and Infrared Spectro­electrochemistry of Ruthenium ­Nitrosyl Porphyrins Containing η1‐O‐Bonded Axial Carboxylates

“…A Bruker Vector 22 and/or a Tensor 27 FTIR spectrometer equipped with a mid-IR fiber-optic dip probe and liquid nitrogen cooled MCT detector (RemSpec Corporation, Sturbridge, MA, USA) were used for the infrared spectroelectrochemistry. [12,41,42] The electrochemical experiments were performed in triplicate to determine reproducibilities. X-ray diffraction data were collected using a diffractometer with a Bruker APEX ccd area detector and graphite-monochromated Mo-K α radiation (λ = 0.71073 Å).…”

“…In all the electrochemical experiments, a three-electrode cell was utilized and consisted of a 3.0-mm diameter Pt disk working electrode, a Pt wire counter electrode, and a Ag/AgCl reference electrode. [12,41,42] Solutions were deaerated before use by passing a stream of N 2 gas through the solution for a minimum of 10 min. A blanket of N 2 was maintained over the solution while performing the experiment; care was taken to minimize evaporative cooling due to N 2 flow in the headspace, as this could cause rolling baselines in the resulting IR difference spectra.…”

“…In all cases, the observed small magnitudes of Δυ NO upon oxidation). We note that the related porphyrin oxidations in T(p-OMe)Ru(NO)Me [27] and (OEP)Ru(NO)(OMe) [12] results in larger observed Δυ NO s of +52 cm -1 and +50 cm The difference spectra obtained for the oxidations of the aryloxide (non-carboxylate) compound 7, [26] the trifluoroacetate compound 8, [26] and the p-nitroarylcarboxylate compound 4 are shown in Figure 9 for comparison. In addition to the observed Δυ NO s, changes are observed in the 1650-1750 cm -1 region, specifically a new band at 1723 cm -1 for 8 and 1674 cm -1 for 4, that are not observed in the non-carboxylate compound 7.…”

“…[12,41,42] In each of these experiments, the Pt working electrode was held at a potential slightly more positive (for oxidations) than the respective peak potential E pa , or slightly more negative (for reductions) than the respective peak potential E pc . The IR spectra of the neutral compounds in support electrolyte were used as backgrounds for the difference spectra.…”

“…In contrast, the low-spin {RuNO} 6 (por)Ru(NO)(O-alkoxide) complexes are relatively stable. [11][12][13][14] We have been interested in determining the effects of axial Oligand identity on the redox behavior of such (por)Ru(NO)-(O-ligand) species. We have prepared a representative set of (por)Ru(NO)(carboxylate) complexes (Figure 1) to determine their redox behavior, and have expanded this study to include ferrocenylcarboxylates which allow for the redox behavior to be probed on the potentially four redox sites por/Ru/NO/Fc frag- Figure 1.…”