A potential novel and general route for bromide replacement in diimine Re^I tricarbonyl complexes leading to carboxylates: The effect in luminescence

The reaction of the 3,5-dimethylpyrazolyl-pyridazine-6-carboxylic acid (Me₂LCOOH) or pyrazolyl-pyridazine-6-carboxylic acid (LCOOH) with [(THF)(CO)₃Re(µ-Br)₂Re(CO)₃(THF)] in DMF followed by the addition of traces of water leads to a series of dehalided rhenium(I) tricarbonyl carboxylates: [(Me₂LCOO)(H₂O)Re(CO)₃] and [(LCOO)(H₂O)Re(CO)₃] instead of the normal rhenium(I) tricarbonyl bromide obtained by reaction in chloroform, [(Me₂LCOOH)Re(CO)₃Br]. The reaction is believed to occur through adventitious amine in the reaction media which leads to the carboxylic acid deprotonation and reaction. To test this idea the diimine-rhenium(I) tricarbonyl bromide [(phen)Re(CO)₃Br] and [(bppdz)Re(CO)₃Br] (phen: 1,10-phenanthroline; bppdz: 3,6-bis(1H-pyrazol-1-yl)pyridazine) were reacted with glacial acetic acid and triethylamine in dry DMF at room temperature. These reaction lead to the corresponding rhenium(I) acetates [(phen)Re(CO)₃(OOCCH₃)] and [(bppdz)Re(CO)₃(OOCCH₃)] in good to high yields using soft reaction conditions and without the use of a silver salt. Thiethylammonium bromide is also obtained from the reaction. Moreover, the fact that this unexpected rhenium(I) tricarbonyl carboxylates were obtained in as N,N and N,O coordination isomers for [(Me₂LCOO)(H₂O)Re(CO)₃] and the comparison of the newly synthetized [(phen)Re(CO)₃(OOCCH₃)] and [(bppdz)Re(CO)₃(OOCCH₃)] compounds with their corresponding bromides allow to stablish that the replacement of the halide by a carboxylate hypsochromically shift the energy of the MLCT band while the emission experiences the opposite, it is bathochromically shifted. The emission quantum yields are slightly diminished. Interestingly the [(phen)Re(CO)₃(OOCCH₃)] and [(bppdz)Re(CO)₃(OOCCH₃)] acetates are dual emitters.