Abstract
Complexes [Co(CTPPMe)(NO)] (1) and [Co(HCTPP)(NO)] (2) with an {Co(NO)}8, the Enemark−Feltham notation of the total number of electrons in the metal d and π*(NO) orbitals, electronic configuration were prepared and thoroughly characterized by IR, UV-Vis, 1H and 15N NMR spectra as well as single-crystal X-ray diffraction. Cyclic voltammetry (CV) and the combined infrared or UV-Vis spectroelectrochemistry (IR or UV-Vis SEC) were also applied to understand the redox chemistry of complexes 1 and 2. IR and UV-Vis SEC studies revealed a porphyrin-based and a CoNO-centered oxidations for 1^0/+ and 2^0/+, respectively. The investigations also indicated a porphyrin-based reduction for 1^0/− and a CoNO-centered reduction for 2^0/−. IR and UV-Vis SEC spectra also demonstrated such good CoNO-centered redox reversibility of 1^0/−/0 or 2^0/+/0 observed in the cyclic voltammogram. It is noteworthy that the reduction of complex 1 or 2 resulted in no facile denitrosylation that commonly occurs in the reduction of {Co(NO)}8 cobalt−nitrosyl porphyrins. Moreover, CV and IR SEC studies on the known {Co(NO)}7 [Co(CTPPO)(NO)] (3) showed a porphyin-based 1st oxidation and a CoNO-centered 1st reduction. Concluded IR SEC results of the cobalt−nitrosyl N-confused porphyrins suggested Δν(NO) ≈ 100 and 35 cm−1 for the CoNO-centered and the porphyrin-based redox processes, respectively. Significantly, the reduction of 1 caused the shift of ν(NO) as large as 95 cm−1 that was first observed in the reduction of {Co(NO)}8 cobalt−nitrosyl porphyrinoid compounds. Suggestive evidences of IR SEC intrigued us to isolate the products reduced by Co(Cp*)2 and examine the subsequent reactivity of the reduced products. Only the product [Co(CTPPMe)(NO)][Co(Cp*)2] (4) of the reduction reaction of 1 to was quantitatively (89.7%) isolated and successfully characterized. Complex 4, to our knowledge, is the first isolated {Co(NO)}9 cobalt−nitrosyl porphyrinoid complex. In the presence of a proton source (NH4PF6), H2(g) evolution occurred with the oxidation of complex 4 to 1. The parallel reaction to evolve H2(g) was also active as conducting NH4+ into the reduction reaction of complex 2 by Co(Cp*)2. We then serendipitously observed that the addition of methanol, ethanol, or water to the THF solution of 4 (protic solvent/THF, 1:1, v/v) stimulated the NO-to-N2O conversion. The strategy of using MeOH to trigger the conversion of NO-to-N2O, however, was not effective as using the reduced product of complex 2. Furthermore, reducing complex 3 to its {Co(NO)}8 state generated neither H2(g) nor N2O(g) in the presence of NH4+ and MeOH, suggesting not only the reduction site but also the electron-richness were demanded for converting the coordinating NO to N2O. Mechanistic insight into the N2O formation was provided by monitoring the whole reaction process of complex 4 and MeOH using IR spectroscopy and ESI(−) mass spectrometry. Complex 4 was NO-reduction-silent in neat THF, but was partially activated to a hydrogen-bonded species 4···MeOH in THF/MeOH (1:1, v/v). This species coupling with 4 transformed NO into N2O, which was fragmented from an [N2O2]-bridging intermediate. An intensified IR peak at 1622 cm−1 was ascribed to ν(NO) due to the formation of an [N2O2]-containing intermediate. Time-course ESI(−) mass spectra supported the presence of the dimeric [Co(NCP)]2(N2O2) intermediate. The observation that the mass signals of the corresponding dimeric fragments exhibited maximum intensity after the reaction had proceeded for approximately 30 min is consistent with the result that the intensity of the peak at 1622 cm−1 in the IR spectrum reached its plateau at 30 min. Five complete NO-to-N2O conversion cycles have been achieved without significant decreasing on the amount of N2O produced.