Calixsmaragdyrin: A Versatile Ligand for Coordination Complexes

The Ru­(II) and BF2 complexes of calixsmaragdyrin were prepared under simple reaction conditions and characterized by HR-MS, 1D and 2D NMR spectroscopy, optical spectroscopy, and electrochemistry, and the structure of the Ru­(II) complex of calixsmaragdyrin was elucidated by X-ray crystallography. The crystal structure of the Ru­(II) complex revealed that the Ru­(II) ion is hexacoordinate with the three pyrrole nitrogen ligands from the tripyrrin unit of the calixsmaragdyrin macrocycle, and the remaining coordination sites of Ru­(II) ion were occupied by two carbonyl groups and one hydroxyl (−OH) group. The calixsmaragdyrin macrocycle in the Ru­(II) complex was distorted with a dome-like structure. In the BF2 complex of calixsmaragdyrin, the BF2 unit was bound to two pyrrolic nitrogens of the dipyrrin moiety of calixsmaragdyrin as deduced by detailed 1- and 2-dimensional NMR spectroscopy studies. The Ru­(II) complex displayed a strong Soret-like absorption band at 449 nm with the absence of Q-bands, whereas the BF2 complex showed a Soret-like band at 475 nm with two well-defined Q-bands at 787 and 883 nm, respectively. Quantum mechanical DFT calculations yielded relaxed equilibrium structures that were similar to the X-ray crystal structures, and the related charge density distributions indicated that the d orbital of the Ru­(II) ion was contributing to the HOMO and LUMO states. In addition, TD-DFT calculations successfully reproduced the large bathochromic shifts, oscillator strengths, and electronic transitions that were observed in the experimental absorption spectra of all three complexes. Both the Ru­(II) and the BF2 complexes of calixsmaragdyrin were stable under redox conditions.