Reversible CO-induced chloride shuttling in Rh^I tweezer complexes containing urea-functionalized hemilabile ligands

The urea moiety, which acts as a good hydrogen-bond donor, has been incorporated into a hemilabile phosphinoalkyl thioether ligand. Upon reaction of the ligand with a Rh^I precursor, a tweezer complex with near-parallel planar urea moieties 2 forms. The host-guest interaction of 2 with Cl^- has been characterized in solution and in the solid state. Cl^- binding with the urea groups in 2 is retained under CO in nonpolar solvents to give a five-coordinate CO adduct 3. In polar solvents, CO binding to Rh^I results in a Cl^- shift from the urea host site to the Rh^I metal center with a concomitant breaking of the Rh-S bonds. This is an unusual example of how two types of different interactions important in molecular recognition (ligand coordination to a metal and hydrogen bonding) can be regulated within one molecule through small-molecule coordination chemistry.