A New Ligand System Based on a Bipyridine-Functionalized Calix[4]arene Backbone Leading to Mono- and Bimetallic Complexes

The synthesis of a new ligand system for mono- and bimetallic complexes based on a calixarene is described.
Ligand BBPC (3, bis(bipyridine)-calix[4]arene) is obtained in three steps in 40% overall yield by first brominating
one of the methyl groups of the 4,4‘-dimethyl-2,2‘-bipyridine in two steps and subsequently reacting it with p-tert-butylcalix[4]arene under basic conditions. Reaction of BBPC (3) with 2 equiv of [Rh(NBD)2]BF4 or [Rh(NBD)(CH3CN)2]BF4 (NBD = norbornadiene) produces the bimetallic compound BBPC[Rh(NBD)BF4]2 (4). Treatment of the
ligand with PdCl2(CH3CN)2 leads to the isolation of the bimetallic complex BBPC[PdCl2]2 (5). When the nickel
precursor NiBr2(DME) (DME = dimethoxyethane) is reacted with BBPC, the bimetallic complex BBPC[NiBr2]2 (6)
is isolated which, upon crystallization from methanol, gives the mononuclear bis(bipyridine) complex BBPC[NiBr(OMe)] (7). Full characterization includes X-ray structural studies of complexes 4, 5, and 7. The bimetallic compounds
4 and 5 show metal to metal distances of 4.334 Å (for 4) and 3.224 Å (for 5). For all three complexes, unique
molecular packing arrangements were found, based on hydrophobic/hydrophilic interactions.