Chemical Reduction of Ni II Cyclam and Characterization of Isolated Ni I Cyclam with Cryogenic Vibrational Spectroscopy and Inert-Gas-Mediated High-Resolution Mass Spectrometry

Ni cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) is an efficient catalyst for the selective reduction of CO to CO. A crucial elementary step in the proposed catalytic cycle is the coordination of CO to a Ni cyclam intermediate. Isolation and spectroscopic characterization of this labile Ni 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 Ni cyclam dication at 300 K. Here, we report a chemical reduction method that efficiently prepares Ni cyclam in solution. This enables the Ni 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 H and N messenger tagging over the range 800-4000 cm 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 Ni cation were compared with those caused by the spherical, closed-shell Li cation, which has a similar ionic radius. We also report the vibrational spectrum of a Ni cyclam complex with a strongly bound O ligand. The cyclam ligand supporting this species exhibits a large conformational change compared to the complexes with weakly bound N and H , which is likely due to significant charge transfer from Ni to the coordinated O .