Synthesis and Electrochemical Characterization of Acetylacetone (acac) and Ethyl Acetate (EA) Appended β-Trisubstituted Push–Pull Porphyrins: Formation of Electronically Communicating Porphyrin Dimers

Abstract

Two new families of “push–pull” tetraphenylporphyrins with one acetylacetone (acac) or ethyl acetate (EA) moiety at a β-pyrrole position of the macrocycle and two Br or Ph substituents at the antipodal β-positions were synthesized and structurally, spectroscopically, and electrochemically characterized. The examined porphyrins are represented as MTPP­(R)2acac and MTPP­(R)2EA (where R = Br or Ph and M = H2, Co, Ni, Cu, or Zn). NiTPP­(Br)2acac exhibits an extremely nonplanar conformation (Δ24 = 0.44 Å, ΔCβ = 0.82 Å), while H2TPP­(Br)2EA and ZnTPP­(Ph)2EA exhibit a quasi-planar conformation. All of the synthesized acac-appended porphyrins show a keto–enol tautomerism in solution, which results in formation of hydrogen bonded dimers as evidenced by 1H NMR and mass spectrometry. Dimers were also detected under the electrochemical conditions for the dibromo derivatives but not the diphenyl substituted porphyrins. A facile stepwise and reversible electrogeneration of the electronically communicating porphyrin dimers is observed for MTPP­(Br)2acac where M = CuII, NiII, or ZnII.

Electrochemistry of Tri‐substituted Porphyrins with β‐Appended Ethyl Acetoacetate and Acetylacetone in Neutral and Basic Nonaqueous Solvents

“…Our own laboratories have long been interested in the electrochemistry of synthetic porphyrins and recently characterized two families of tri‐substituted push‐pull tetraphenylporphyins (TPP) containing Ni(II), Cu(II), Zn(II) or H 2 centers in nonaqueous solvents containing 0.1 M TBAP . These compounds possessed a β ‐acetylacetone (acac) or β ‐ethyl acetate (EA) moiety on one pyrrole ring of the macrocycle and two β ‐Br or β ‐phenyl (Ph) substituents on the antipodal pyrrole .…”

“…Our own laboratories have long been interested in the electrochemistry of synthetic porphyrins and recently characterized two families of tri‐substituted push‐pull tetraphenylporphyins (TPP) containing Ni(II), Cu(II), Zn(II) or H 2 centers in nonaqueous solvents containing 0.1 M TBAP . These compounds possessed a β ‐acetylacetone (acac) or β ‐ethyl acetate (EA) moiety on one pyrrole ring of the macrocycle and two β ‐Br or β ‐phenyl (Ph) substituents on the antipodal pyrrole . Although the EA‐ and acac‐appended porphyrins are structurally similar to each other (see Scheme ), the dibromo acac derivatives, represented as MTPP(acac)Br 2 , differed substantially in their electrochemical behavior, giving rise to electrochemically active dimers upon reduction in CH 2 Cl 2 solutions.…”

“…Although the EA‐ and acac‐appended porphyrins are structurally similar to each other (see Scheme ), the dibromo acac derivatives, represented as MTPP(acac)Br 2 , differed substantially in their electrochemical behavior, giving rise to electrochemically active dimers upon reduction in CH 2 Cl 2 solutions. Dimers were also electrochemically observed for MTPP(acac)Br 2 in pyridine and the redox active species in both solvents was assigned as a hydrogen bonded dimer which was also detected by 1 H NMR and mass spectrometry …”

“…As expected, hydrogen bonded porphyrin dimers were not observed for the structurally related EA appended porphyrins in Scheme which lacked an enolic OH group in the structure. Surprisingly, however, a dimerization was not electrochemically observed for the structurally related β ‐acac appended porphyrin derivatives containing two antipodal β ‐Ph substituents . This difference between the two series of β ‐Br and β ‐Ph compounds in the appended acac porphyrins of Scheme might be related to subtle differences between the electron‐withdrawing effects of the antipodal β ‐substituents on the hydroxyl group of the appended acac group, which could affect not only the equilibrium between the keto and enol forms of the MTPP(acac)X 2 compounds but also the observed electrochemical dimerization upon reduction in CH 2 Cl 2 or pyridine containing 0.1 M TBAP .…”

“…Surprisingly, however, a dimerization was not electrochemically observed for the structurally related β ‐acac appended porphyrin derivatives containing two antipodal β ‐Ph substituents . This difference between the two series of β ‐Br and β ‐Ph compounds in the appended acac porphyrins of Scheme might be related to subtle differences between the electron‐withdrawing effects of the antipodal β ‐substituents on the hydroxyl group of the appended acac group, which could affect not only the equilibrium between the keto and enol forms of the MTPP(acac)X 2 compounds but also the observed electrochemical dimerization upon reduction in CH 2 Cl 2 or pyridine containing 0.1 M TBAP . These observations are further investigated in the current manuscript which examines the chemical reactions following electroreduction of MTPP(acac)X 2 as well as the redox properties and coupled chemical reactions of a structurally related family of tri‐substituted porphyrins containing a single β ‐ethyl acetoacetate (EAA) moiety and two H, Br or Ph β ‐substituents.…”