Nickel macrocyclic complexes spectra

The prototypical photochemical system for CO2 reduction contains a photosensitizer (or photocatalyst) to capture the photon energy, an electron relay catalyst (that might be the same species as the photosensitizer) to couple the photon energy to the chemical reduction, an oxidizable species to complete the redox cycle and CO2 as the substrate. Figure 1 shows a cartoon of the photochemical CO2 reduction system. An effective photocatalyst must absorb a significant part of the solar spectrum, have a long-lived excited state and promote the activation of small molecules. Both organic dyes and transition metal complexes have been used as photocatalysts for CO2 reduction. In this chapter, CO2 reduction systems mediated by cobalt and nickel macrocycles and rhenium complexes will be discussed. 

The complex [Ni(2,3-Me2[14]-l,3-diene-l,4,8,lI-N4)] [ZnCU] is square planar and low-spin. The visible spectra show bands near 21.3 kK (characteristic of square planar nickel(II)), near 26.1 kK (due to the imine functions), and near 35.1 kK. The infrared spectra of all of the nickel complexes prepared show absorptions near 3195 and 1595 cm assignable to the N—H stretching vibration and to the symmetric imine vibration, respectively. A strong sharp band also occurs near 1210 cm and is characteristic of the a-diimine function. The NMR spectrum of the perchlorate complex in nitromethane shows a methyl singlet at 2.33 ppm. The ligand can be hydrogenated on nickel(II) with Raney nickel and hydrogen to produce the fully saturated macrocyclic complex [Ni(2,3-Me2[14]-ane-1,4,8,1 1-N4] ... 

The macrocyclic product [Ni(L54)]I is isolated through refluxing an aqueous solution of L52 and nickel(II) acetate with the subsequent addition of an aqueous solution of Nal (Eq. 2,15) [23], The diamagnetism and electronic absorption spectrum of [Ni(L54)]+ indicate that it has a square-planar structure. The peak with the maximal m/z in the mass spectrum of the iodide corresponds to the [Ni(L54)]+ cation. It should be noted that, in the mass spectra of a number of other macro-cyclic coordination compounds of the [ML]X type, the most intense peak in the high-mass number range is [ML]+ as, for example, in case of the nickel( II) complex with the 13-membered macrocycle derived from 3,7-diaza-l,9-diaminononane [24]. 

Subjecting 2 or 3 to strongly acidic conditions (e.g. 1 M CF3CO2H or concentrated HQ) leads to rapid demetalation and the production of two isomeric free-base macrocycles 10 and 11 [11, 12]. The structure of 10 was conflrmed by its spectral properties and its remetalation to form the nickel complex 5. On the other hand, the structure of 11 was determined solely by its spectral properties. For instance, the NMR spectrum of 11 demonstrated the presence of a fully unsaturated system. Typical also was the presence of pyrrolic proton signals in the 6-7 ppm range of the NMR spectrum [12]. Compound 11 was very unstable... 

The thermodynamics and kinetics of H+ binding to cobalt(I) and nickel(I) macrocycles have been determined. The pAia of Ni(cyclam)(H), / / 5 5 -NiHTIM(H) + and A-rac-CoHMD(H) + are 1.8, 1.9 and 11.7, respectively [14, 24, 27]. As seen from Table 3, protonation rate constants for A-rac-CoHMD depend on acid strength. The results are consistent with an associative reaction of the square-planar complex with an acid, HA. Whereas the spectrum of 7V-rac-CoHMD(H) + suggests the formation of a [Co (H )] + species with an absorption band at 440 nm (520 M cm ), Ni(cyclam)(H) + shows no significant absorbance in the 300-700 nm region [14, 24]. 

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