Chemical Reduction of NiII Cyclam and Characterization of Isolated NiI Cyclam with Cryogenic Vibrational Spectroscopy and Inert-Gas-Mediated High-Resolution Mass Spectrometry.

NiII cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) is an efficient catalyst for the selective reduction of CO2 to CO. A crucial elementary step in the proposed catalytic cycle is the coordination of CO2 to a NiI cyclam intermediate. Isolation and spectroscopic characterization of this labile NiI species without solvent has proven to be challenging, however, and only partial IR spectra have previously been reported using multiple photon fragmentation of ions generated by gas-phase electron transfer to the NiII cyclam dication at 300 K. Here, we report a chemical reduction method that efficiently prepares NiI cyclam in solution. This enables the NiI complex to be transferred into a cryogenic photofragmentation mass spectrometer using inert-gas-mediated electrospray ionization. The vibrational spectra of the 30 K ion using both H2 and N2 messenger tagging over the range 800-4000 cm-1 were then measured. The resulting spectra were analyzed with the aid of electronic structure calculations, which show strong method dependence in predicted band positions and small molecule activation. The conformational changes of the cyclam ligand induced by binding of the open shell NiI cation were compared with those caused by the spherical, closed-shell LiI cation, which has a similar ionic radius. We also report the vibrational spectrum of a NiI cyclam complex with a strongly bound O2 ligand. The cyclam ligand supporting this species exhibits a large conformational change compared to the complexes with weakly bound N2 and H2, which is likely due to significant charge transfer from Ni to the coordinated O2.