Rhenium(I) Tricarbonyl Complexes of 5,10,15,20-Tetraphenyl-21-thia and 21-Oxaporphyrins

The hexa-coordinated rhenium­(I) tricarbonyl complexes
of 5,10,15,20-tetraphenyl-21-thiaporphyrin 1 and 5,10,15,20-tetraphenyl-21-oxaporphyrin 2 have been synthesized by thermal reaction of corresponding
free
base porphyrin with Re2(CO)10. The compounds 1 and 2 are characterized by HR-MS mass, 1H, and 13C NMR, FTIR, UV–vis, and electrochemical
techniques and the structures are determined by X-ray single crystal
analysis. The X-ray analysis revealed different coordination behavior
of Re­(CO)3 with 21-thiaporphyrin and 21-oxaporphyrin. Interestingly,
the Re­(CO)3 coordinates to two of the three inner nitrogens
and one sulfur instead of three inner nitrogens as generally expected
to produce unique compound 1 whereas it coordinates to
three inner nitrogens but not with furan oxygen to form compound 2. The 21-thiaporphyrin ring is more distorted in compound 1 compared to 21-oxaporphyrin ring in compound 2 on complexation with Re­(CO)3. The presence of three carbonyl
groups in compounds 1 and 2 are verified
by 13C NMR and IR spectroscopy. The absorption spectra
of compounds 1 and 2 showed ill-defined
Q-bands along with broad Soret band and the extinction coefficients
are much lower than their corresponding free base porphyrins. The
compounds 1 and 2 showed two reversible
porphyrin ring based reductions supporting their electron deficient
nature. The compound 1 is very stable under protonation
conditions, and the protonation occurs at the uncoordinated pyrrole
ring whereas the compound 2 undergoes decomplexation
under same conditions. Furthermore, compound 1 showed
the fluxional behavior in coordination mode of binding in solution.