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 <b>1</b> and 5,10,15,20-tetraphenyl-21-oxaporphyrin <b>2</b> have been synthesized by thermal reaction of corresponding free base porphyrin with Re₂(CO)₁₀. The compounds <b>1</b> and <b>2</b> 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 <b>1</b> whereas it coordinates to three inner nitrogens but not with furan oxygen to form compound <b>2</b>. The 21-thiaporphyrin ring is more distorted in compound <b>1</b> compared to 21-oxaporphyrin ring in compound <b>2</b> on complexation with Re­(CO)₃. The presence of three carbonyl groups in compounds <b>1</b> and <b>2</b> are verified by ¹³C NMR and IR spectroscopy. The absorption spectra of compounds <b>1</b> and <b>2</b> 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 <b>1</b> and <b>2</b> showed two reversible porphyrin ring based reductions supporting their electron deficient nature. The compound <b>1</b> is very stable under protonation conditions, and the protonation occurs at the uncoordinated pyrrole ring whereas the compound <b>2</b> undergoes decomplexation under same conditions. Furthermore, compound <b>1</b> showed the fluxional behavior in coordination mode of binding in solution