Rapid atmospheric carbon dioxide fixation by nickel(ii) complexes: meridionally coordinated diazepane-based 3N ligands facilitate fixation

Abstract

Octahedral complexes of the type [Ni(L)(H₂O)₃](ClO₄)₂ (1 and 2), where L is the tridentate 3N ligand 4-methyl-1-(pyrid-2-ylmethyl)-1,4-diazacycloheptane (L1, 1), or 4-methyl-1-(N-methylimidazolyl)-1,4-diazacycloheptane (L2, 2), have been isolated and characterized using elemental analysis, ESI-MS and electronic absorption spectroscopy. The DFT optimized structures of 1 and 2 reveal that the tridentate 3N ligands are coordinated meridionally constituting a distorted octahedral coordination geometry around nickel(II). In methanol solution, the complexes, upon treatment with triethylamine, generate the reactive red colored low-spin square planar Ni–OH intermediate [Ni(L1/L2)(OH)]⁺ (1a and 2a), as characterized by ESI-MS and electronic absorption spectroscopy, and energy minimized structures. The latter when exposed to the atmosphere rapidly absorbs atmospheric CO₂ to produce the carbonate bridged dinickel(II) complexes [Ni₂(L1/L2)₂(μ-CO₃)(H₂O)₂](ClO₄)₂ (3 and 4), as characterized by elemental analysis and the IR spectral feature (∼1608 cm⁻¹) characteristic of bridging carbonate. The single crystal X-ray structure of 3 reveals the presence of a dinickel(II) core bridged by a carbonate anion in a symmetric mode. Both the Ni(II) centers are identical to each other with each Ni(II) possessing a distorted octahedral coordination geometry constituted by a meridionally coordinated 3N ligand, a carbonate ion and a water molecule. The decay kinetics of the red intermediates generated by 1 (k_obs, 7.7 ± 0.1 × 10⁻⁵ s⁻¹) and 2 (k_obs, 5.8 ± 0.3 × 10⁻⁴ s⁻¹) in basic methanol solution with atmospheric CO₂ has been determined by absorption spectroscopy. DFT studies illustrate that meridional coordination of the 3N ligand and the electron-releasing imidazole ring as in 2 facilitate fixation of CO₂. The carbonate complex 3 efficiently catalyzes the conversion of styrene oxide into cyclic carbonate by absorbing atmospheric and pure CO₂ with excellent selectivity.