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 Å) 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 NaBArF4 in CH2Cl2, 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). LCuBArF4 and a triflate analogue LCuI(OTf) readily react with oxygen
in dichloromethane to produce, in the latter case, a hydroxo-bridged dimer [LCuII(μ-OH)]2(OTf)2, of the intact
(unoxidized) ligand L. Since the analogous LCuCl does not react as fast with O2 in CH2Cl2, outer-sphere electron
transfer is concluded to be ineffective for oxidation of cuprous ion here.