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.