Alkali metal cation–cyclen complexes: Effects of alkali metal cation size on the structure and binding energy

Threshold collision-induced dissociation of alkali metal cation–cyclen complexes, M+(cyclen), with xenon is studied using guided ion beam tandem mass spectrometry techniques. The alkali metal cations examined here include: Na+, K+, Rb+ and Cs+. In all cases, M+ is the only ionic product observed, corresponding to endothermic loss of the intact cyclen ligand. The cross section thresholds are analyzed to extract zero and 298 K M+–cyclen bond dissociation energies (BDEs) after properly accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energy distributions of the reactants, and the lifetimes for dissociation. Density functional theory calculations at the B3LYP/def2-TZVPPD and B3LYP/6-31+G* levels of theory are used to determine the structures of cyclen and the M+(cyclen) complexes. Theoretical BDEs are determined from single point energy calculations at the B3LYP/def2-TZVPPD and MP2(full)/def2-TZVPPD levels of theory using the B3LYP/def2-TZVPPD optimized geometries, and also at the B3LYP/6-311+G(2d,2p) and MP2(full)/6-311+G(2d,2p) levels of theory using the B3LYP/6-31+G*optimized geometries. The agreement between theory and experiment is reasonably good for all levels of theory examined, but is best for the calculations performed at the B3LYP/def2-TZVPPD level of theory. The M+–cyclen BDEs decrease as the size of the alkali metal cation increases, consistent with the electrostatic nature of the binding in these complexes. The M+(cyclen) structures and BDEs are compared to those previously reported for the analogous M+(12-crown-4) complexes to examine the effects of the donor atom (N versus O) on the structures and strength of binding.

Figure optionsDownload high-quality image (129 K)Download as PowerPoint slideHighlights
► Alkali metal cation binding affinities of cyclen are determined using threshold CID methods.
► Ground-state C4(++++) conformers of M+(cyclen) are produced by ESI for Na+, K+, Rb+, and Cs+.
► M+–cyclen BDEs obtained by TCID and B3LYP/def2-TZVPPD agree very well.
► Cyclen exhibits a strong preference over 12-crown-4 for binding to hard metal cations, e.g. Na+.
► Cyclen is competitive with, but binds the larger alkali metal cations, K+, Rb+, and Cs+, slightly less strongly than 12-crown-4.