Influence of the [2.1.1]-(2,6)-pyridinophane macrocycle ring size constraint on the structure and reactivity of copper complexes.

The macrocycle [2.1.1]-(2,6)-pyridinophane (L) binds to CuCl to give a monomeric molecule with tridentate binding of the ligand but in a distorted tetrahedral "3 + 1" geometry, where one nitrogen forms a longer (by 0.12 A) bond to Cu. In dichloromethane solvent this pyridine donor undergoes facile site exchange with a second pyridine in the macrocycle, to give time-averaged mirror symmetry. Both experimental and density functional theory studies of the product of chloride abstraction, using NaBAr(F)(4) in CH(2)Cl(2), show that the Cu(+) binds in a trigonal pyramidal, not planar, arrangement in LCu(+). This illustrates the ability of macrocyclic ligand constraint to impose an electronically unfavorable geometry on 3-coordinate Cu(I). LCuBAr(F)(4) and a triflate analogue LCu(I)(OTf) readily react with oxygen in dichloromethane to produce, in the latter case, a hydroxo-bridged dimer [LCu(II)(micro-OH)](2)(OTf)(2), of the intact (unoxidized) ligand L. Since the analogous LCuCl does not react as fast with O(2) in CH(2)Cl(2), outer-sphere electron transfer is concluded to be ineffective for oxidation of cuprous ion here.