Facial vs. meridional coordination in gaseous Ni(II)-hexacyclen complexes revealed with infrared ion spectroscopy

Infrared Ion Spectroscopic Characterization of the Gaseous [Co(15-crown-5)(H2O)]2+ Complex

We report fingerprint infrared multiple-photon dissociation spectra of the gaseous monohydrated coordination complex of cobalt(II) and the macrocycle 1,4,7,10,13-pentaoxacyclopentadecane (or 15-crown-5), [Co(15-crown-5)(H2O)]2+. The metal-ligand complexes are generated using electrospray ionization, and their IR action spectra are recorded in a quadrupole ion trap mass spectrometer using the free-electron laser FELIX. The electronic structure and chelation motif are derived from spectral comparison with computed vibrational spectra obtained at the density functional theory level. We focus here on the gas-phase structure, addressing the question of doublet versus quartet spin multiplicity and the chelation geometry. We conclude that the gas-phase complex adopts a quartet spin state, excluding contributions of doublet species, and that the chelation geometry is pseudo-octahedral with the six oxygen centers of 15-crown-5 and H2O coordinated to the metal ion. We also address the possible presence of higher-energy conformers based on the IR spectral evidence and calculated thermodynamics.

Computational insight towards the binding affinity and participation of aliphatic unsaturated sidearms of aza-18-crown-6 extractants for Sr2+ encapsulation in different solvent medium

CONTEXT

Macrocyclic extractants capture cations in solution phase; therefore, their structures and energetics in different solvents are worth investigating. In this computational research work, the optimized geometry of three aza-18-crown-6 extractants (1-3) has been obtained in suitable solvent mediums to work out the influence of the solvation on their binding affinity with Sr2+. The designed macrocyclic extractants are remarkable as the N-substituted side chain bears the rarely examined simple aliphatic unsaturated double and triple bond. All significant structural perturbations of the macro ring wherein Sr2+ binds are examined. Prediction of the auxiliary effect of the simple aliphatic unsaturated side arm on the binding of Sr2+ through solvent competition or cation-pi interaction is undertaken. The binding affinity of the complex formed in the solvent and gas phases is calculated. Results obtained in this study favor the utilization of solvents of low dielectric constant (CHCl3 and DCM) for effective binding of Sr2+ ions by the extractants.

METHOD

All DFT calculations were performed using the Gaussian 09 program. The optimized geometries and their structural features were visualized by GaussView. Density functional calculation involving B3LYP functional and LANL2DZ basis set was employed to obtain optimized geometry and energy of the extractants and their Sr2+ complex. The solvation effects were considered by employing the calculations with the polarized continuum model (PCM). The computational method's reliability was assured by comparing the optimized structure with that of X-ray reported structure of a similar type of complex.