Photoinduced behaviour of N,N-bidentate manganese(I) and rhenium(I) tricarbonyl complexes for singlet oxygen generation and CO release

Abstract

The combined action of singlet oxygen (¹O₂) and photoinduced carbon monoxide (CO) released by tricarbonyl metal complexes is a promising synergic treatment against multi-resistant bacterial infections. In this work, we explore the use of a polydentate ligand (bpm = 2,2-bipyrimidine) that offers the opportunity to accommodate two metal centers exhibiting both singlet oxygen generation and carbon monoxide releasing properties in a single molecule. A series of monometallic ([(bpm)M(CO)₃Br]; M = Mn, Re) and homo or hetero bimetallic ([Br(CO)₃M(bpm)M'(CO)₃Br]; M = Mn, Re) compounds were synthesized in moderate to good yields by modulating the metal precursor or the stoichiometry, also the syn:anti isomers ratio for the bimetallic complexes was dependent on the experimental conditions used. DFT modelling shows the anti-isomer is more stable than the syn-isomer by less than 8 kJ mol^-1, which is consistent with those experimentally observed in terms of majority product and the effect of experimental conditions over the anti-syn ratio. The HOMO-LUMO gap is lower for the mono and bimetallic rhenium(I) compounds compared to the values for the manganese(I) analogues, while the heterometallic complex shows intermediate values for the anti-isomer. The photophysical characterization shows typical absorption and emission bands with MLCT character. In addition, CO-release and ¹O₂ generation quantum yields were evaluated for the monometallic Mnbpm and Rebpm homologues and compared with values obtained for the homo- and hetero-bimetallic complexes. Interestingly the replacement of a Mn(CO)₃Br moiety in MnbpmMn by a Re(CO)₃Br one makes the heterometallic MnbpmRe molecule a molecular oxygen sensitizer and partially retaining its carbon monoxide releasing ability.

Keywords: Heterobimetallic Mn-Re Complexes; PhotoCORMs; Singlet Oxygen; Tricarbonyl Metal Complexes.