Studies on a hydrocarbon capped free base tetraphenylporphyrin and its conjugate acids - first observation of a monoprotonated tetrapehylporphyrin &{;CapTPP(H 3+)CF 3CO 2−&};

Abstract

1H-NMR and visible absorbtion spectroscopic titrations of a unique hydrocarbon capped porphyrin 3H 2 with trifluoroacetic acid in chloroform result in the formation of a monoprotonated porphyrin species 3H 3 +&{;CF 3CO 2 −&};. This represents the first observation of a monobasic tetraphenylporphyrin acid. The monocation forms the dibasic 3H 4 2+ &{;CF 3CO 2−&};2 upon further addition of acid. Back-titration of 3H 4 2+&{;CF 3CO 2 −&}; 2 with DMSO demonstrates that the monocation can be generated from the diacid as well as the free base porphyrin. The capping structure on one face of the tetraphenylporphyrin is suggested to diminish the extent of solvation of one proton of 3H 4 2+&{;CF 3CO 2 −&};, thereby rendering the first dissocation constant (represented by C 50(1)) considerably larger than the second dissociation (C 50(2)). 19F NMR chemical shifts of the counterions of −50°C show a similarity to the CF 3CO 2− counterions of TPPH 4 2+&{;CF 3CO 2 −&}; 2 and indicated a strong association with the prophyrin acid species. 2D NMR ROESY spectroscopy with Distance Geometry Refinement afforded a solution structure for 3H 2, from which representative structures of possible inclusion compounds 3H 3 +&{;CF 3CO 2 −&}; and 3H 2&{;CHCl 3&}; were created by molecular modeling. Although the dome of the capping structure is predicted to be large enough to accommodate a trifluoroacetate anion on the inside, 19F NMR spectra and FAB-mass spectra do not support the presence of an acid-base inclusion compound.