Zinc-based Cyclens Containing Pyridine and Cross-Bridges: X-ray and DFT Structures, Lewis Acidity, Gas-Phase Acidity, and Pka Values

It is well known that catalytic centers containing the zinc(II) ion can act as both Lewis and Bronsted-Lowry acids. In addition to coordination number, the Zn coordination geometry can also strongly impact the acidity of the active site, no matter what measure of acidity is considered. Herein, we report the first pentacoordinate, zinc-ammonia complex containing a pyridine based tetraazamacrocycle, [(pyclen)Zn(NH3)](PF6)2, that has applications in Lewis acid catalysis. From this structure, we obtain binding energies and acidities for a series of related pyclen and cross-bridged cyclen type tetraazamacrocycles comprising the pentacoordinate N4Zn(II)–OH2 entity, collectively referred to as [(R-cyclen)Zn–OH2]2+. Results from gas- and aqueous-phase density functional theory (M05-2X) and ab initio (MP2) calculations reported herein demonstrate that molecular geometry has a substantial influence on both the Zn–OHx binding strengths and the deprotonation energy of coordinated H2O, but not necessarily in a predicable way. While generally more constrained N–Zn–N coordination leads to greater Zn–OH2 binding energies (Lewis acidities), the corresponding Lewis acidities of the complexes don’t always correlate with the Zn–OH2 bond lengths nor the (Bronsted) acidity of the coordinated H2O. Additionally, the order of the Lewis acid strength of the [(R-cyclen)Zn(II)] complexes changes as the basic –OH2 ligand is replaced with its –OH counterpart.