Visible light sensitized CO₂ activation by the tetraaza [Co^IIN₄H(MeCN)]²⁺ complex investigated by FT-IR spectroscopy and DFT calculations

In situ FT-IR measurements and electronic structure calculations are reported for the reduction of CO₂ catalyzed by the macrocyclic complex [Co^IIN₄H]²⁺ (N₄H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(17),2,11,13,15-pentaene). Beginning from the [Co^IIN₄H]²⁺ resting state of the complex in wet acetonitrile solution, two different visible light sensitizers with substantially different reducing power are employed to access reduced states. Accessing reduced states of the complex with a [Ru(bpy)₃]²⁺ sensitizer yields an infrared band at 1670 cm^-1 attributed to carboxylate, which is also observed for an authentic sample of the one-electron reduced complex [CoN₄H(MeCN)]⁺ in CO₂ saturated acetonitrile solution. The results are interpreted based on calculations using the pure BP86 functional that correctly reproduces experimental geometries. Continuum solvation effects are also included. The calculations show that Co is reduced to Co^I in the first reduction, which is consistent with experimental d-d spectra of square Co(I) macrocycle complexes. The energy of the CO₂ adduct of the one-electron reduced catalyst complex is essentially the same as for [CoN₄H(MeCN)]⁺, which implies that only a fraction of the latter forms an adduct with CO₂. By contrast, the calculations indicate a crucial role for redox noninnocence of the macrocyclic ligand in the doubly reduced state, [Co^I(N₄H) ^-•], and show that [Co^I(N₄H) ^-•] binds partially reduced CO₂ fairly strongly. Experimentally accessing [Co^I(N₄H) ^-•] with an Ir(bpy)₃ sensitizer with greater reducing power closes the catalytic cycle as FT-IR spectroscopy shows CO production. Use of isotopically substituted C¹⁸O₂ also shows clear evidence for ¹⁸O-substituted byproducts from CO₂ reduction to CO.