Rhenium(I) tricarbonyl complexes of 5,10,15,20-tetraphenyl-21-thia and 21-oxaporphyrins

Abstract

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.