A novel and simple route for bromide replacement in pyrazolyl-pyridazine ReI tricarbonyl complexes leads to a zwitterion stabilized by hydrogen bonding

Abstract

The reaction of 3-pyrazolyl-pyridazine-6-carboxylic (LCOOH) and 3-3,5-dimethylpyrazolyl-pyridazine-6-carboxylic (Me2LCOOH) acids with bromotricarbonyl-tetrahydrofuran-rhenium(I) dimer in DMF solution followed by water addition, leads simple and cleanly to the rhenium(I) de-halided fac-aquo-tricarbonyl-complexes [(LCOO)(H2O)Re(CO)3] and [(Me2LCOO)(H2O)Re(CO)3] respectively, without the requirement of using a silver salt. It was possible to isolate each compound with the pyrazolyl-pyridazine carboxylate ligand showing the chelating N,O-mode ([(N,O-LCOO)(H2O)Re(CO)3] and [(N,O-Me2LCOO)(H2O)Re(CO)3]). For the [(Me2LCOO)(H2O)Re(CO)3] compound, it was also possible to isolate the ligand as a chelating diimine ([(N,N-Me2LCOO)(H2O)Re(CO)3]). This last coordination mode leads to a zwitterionic but neutral molecule, with a naked carboxylate as negative extreme and the metal with positive charge in the other side. The negative formal charge in the carboxylate is stabilized within the crystal structure of [(N,N-Me2LCOO)(H2O)Re(CO)3] through hydrogen bonds to water molecules from neighboring molecules dimers, with donor acceptor distances (D···A) of 2.566(10)Å and 2.645(6)Å. DFT optimization of the zwitterion in the gas phase suggests that its geometry would twist to allow the formation of an intramolecular hydrogen bond between carboxylate and the coordinated water molecule. This shows the key stabilizing effect of the hydrogen bond for the negative charge of the carboxylate into the zwitterion crystal structure.