Molecular Rectangles Based on Rhenium(I) Coordination Chemistry

Abstract

There has been substantial recent interest in the assembly of rigid inorganic and organometallic host species based on transition metal corners and difunctional organic ligand edges.1-5 Most of these compounds are tetrametallic with a square geometry.6 Several have now been shown to bind small molecules,1a-c,2,3a and some have been used as solution phase sensors.3a,7 Because the cavities of the squares tend to align to form channels in the solid state,2,3b the compounds also are potentially useful in sieving and separations applications. Indeed, in initial electrochemical sieving studies involving redox active permeants, thin films of neutral rhenium-derived molecular squares have displayed sizeselective permeant transport capabilities.8 A compelling extension of the size-selective tetrametallic host chemistry would be the induction of significant guest shape selectivity. Toward this goal, we report the synthesis, characterization, and preliminary binding properties of two members of a new class of tetrametallic hosts featuring rectangular cavities. Our earlier attempts to make a mixed-ligand tetrametallic cyclophane, of similar dimensions to the rectangular cyclophanes reported by Odell and co-workers9 and by Geuder and coworkers10 and derived from the original tetrarhenium squares (i.e., alternating 4,4′-bipyridyl and pyrazyl edges), gave only the square molecules, not rectangles. Some success in rectangle synthesis was achieved by using a two-step process to synthesize first a relatively robust rhenium-thiolate dimeric edge and then the molecular rectangle.11 The cavity height (∼3.8 A) is insufficient, however, to allow the molecule to function as a host. Nevertheless, by building upon that strategy we have now been able to synthesize molecular rectangles by first creating a stabilized bimetallic edge by using a difunctional chelating bridge, 2,2′bipyrimidine (bpym), and then adding difunctional pyridyl-based bridges (Scheme 1).