Zinc-based cyclens containing pyridine and cross-bridges: X-ray and DFT structures, Lewis acidity, gas-phase acidity,...

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-OH_x 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.