Synthesis and Complexation of Multiarmed Cycloveratrylene-Type Ligands: Observation of the "Boat" and "Distorted-Cup" Conformations of a Cyclotetraveratrylene Derivative

Investigations of a previously reported ligand, hexakis(2‐pyridylmethyl)cyclotricatechylene (1), and a new tetrameric bridging ligand, octakis(2‐pyridylmethyl)cyclotetracatechylene (2), the latter constructed on a larger cyclotetraveratrylene (CTTV) scaffold, are described. Variable‐temperature NMR studies support a “sofa” conformation for 2, akin to studies on the parent compound. The coordination chemistry of 2 and its smaller trimeric homologue have also been investigated with silver(I), copper(II) and palladium(II) salts. An unexpected chelating mode was observed for 1 in the structure of DMF⊂[Pd3Cl6(1)]⋅DMF, whereby the palladium cations bridge two veratrole subunits rather than chelating within a single subunit. In the structure of [Ag4(2)][Co(C2B9H11)2]4⋅2.8 CH3CN⋅ H2O, ligand 2 adopts a “boat” conformation, whereas in [Pd4Cl8(2)]⋅4 H2O, 1H NMR spectroscopic studies and calculations indicate that the ligand is present in a previously unobserved “distorted‐cup” conformation. This conformation was calculated to be approximately 90 kJ mol−1 lower in energy than the alternative “sofa” conformation. Thus, coordination‐induced conformational control over CTTV derivatives offers new routes to exploit the host–guest chemistry of these compounds. Cup of glory: Different metal chelating modes in discrete silver(I) and palladium(II) complexes are employed to restrict the conformation of the flexible bridging ligand octakis(2‐pyridylmethyl)cyclotetracatechylene (L). A boat conformation (see picture) and the previously unobserved distorted‐cup conformation, postulated for the cyclotetracatechylene core, are reported for [Ag4(L)] and [Pd4Cl8(L)] complexes, respectively.