Facile Heterogeneous and Homogeneous Anion Induced Electrosynthesis: An Efficient Method for Obtaining π-Extended Porphyrins

Abstract

Two closely related electrosynthetic approaches are applied for the preparation of novel π-extended tetraphenylporphyrins from malononitrile-appended meso-β di-fused porphyrins, represented as MTPP(MN) 2 , where TPP = the dianion of tetraphenylporphyrin and MN = malononitrile. The first method involves application of a controlled reducing potential at a platinum electrode in CH 2 Cl 2 , while the second proceeds via cyanide anion induced electron transfer. Both methods produced the same decyanated, π-extended di-fused porphyrins represented as MTPP(VCN) 2 where VCN = vinyl cyanide and M = H 2 , Ni II , Cu II , or Zn II in almost quantitative yields. The final isolated and purified porphyrin products are characterized by a split Soret band ranging from 411−497 nm and two broad intense Q bands. The new π-extended porphyrins are easier to reduce than the parent MTPP or MTPP(MN) 2 compounds by 760−800 mV and 180−190 mV, respectively, and possess an electrochemical HOMO− LUMO gap ranging from 1.48 to 1.66 V. They are also characterized by two reversible one-electron ring-centered reductions in CH 2 Cl 2 and three reversible one-electron ring-centered reductions in THF. A fourth irreversible reduction is seen in THF at more negative potentials and is assigned to one or two of the fused cyanobenzene rings of the macrocycle.

Synthesis, Spectral, Redox, and Sensing Studies of β-Dicyanovinyl-Appended Corroles and Their Metal Complexes

“…The most notable spectral difference in the series is the increased intensity for the Q band of [ 3Cu ] − , which is almost equal to that of the apparent split Soret band, resulting in an unusual spectral envelope. This observation can be attributed to the intact unsymmetrical and extended π-system following the one-electron reduction, which causes a splitting of the frontier orbitals resulting in altered electronic and photophysical properties as compared to their symmetrical counterparts. − Moreover, the spectrally monitored one-electron reduction of AgTPC­(MN) ( 3Ag ) shows similar changes (Figure S26), and the spectrum of [ 3Ag ] − resembles that of the copper analogue, further confirming that both spectra correspond to a complex consisting of a divalent metal bound to a trianionic corrole with an unsymmetrical and extended π-system.…”

Synthesis and Electrochemistry of Free‐Base Porphyrins Bearing Trifluoromethyl meso‐Substituents

“…As a matter of fact, more than two reduction waves can be observed on the CV curves of porphyrins i) when electroactive substituents and/or metal centers are involved ii) with porphyrins incorporating very strong EWG substituents like NO 2 or CN or iii) when studying the electrochemistry of porphyrins in solvents featuring unusually large accessible potential windows. [60][61][62][63] As shown in table 2, the second reduction of 4H 2 , 5H 2 and 6H 2 was systematically observed as a fully irreversible wave cathodically shifted by about 500 mV after the first one. Those reduction waves involving formation of the dianions PH 2 2À remained irreversible at all investigated scan rates (0.1-1 V/s) and their relative intensity was found to remain the same regardless of the number of CF 3 substituents on the porphyrin ring (see Figure ESI 7).…”

“…Such standard signature was for instance observed with the C 6 F 5 ‐substituted reference compounds 7 H 2 , 8 H 2 and 9 H 2 , which contrasts with the three reduction waves observed with the CF 3 ‐subsituted compounds 4 H 2 , 5 H 2 and 6 H 2 (see Figure 4A). As a matter of fact, more than two reduction waves can be observed on the CV curves of porphyrins i ) when electroactive substituents and/or metal centers are involved ii ) with porphyrins incorporating very strong EWG substituents like NO 2 or CN or iii ) when studying the electrochemistry of porphyrins in solvents featuring unusually large accessible potential windows [60–63] …”

Recent Advances in Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds

“…In 2020, Sankar and Kadish electrochemical and chemical syntheses of cyano-substituted naphtho-fused π-extended tetraphenylporphyrins from the corresponding tetracyano precursors ( Scheme 315 ). 592 The electrosynthetic method involved an application of a controlled reducing potential of −1.6 V or −1.7 V at a platinum electrode, followed by an oxidizing potential of 0.0 V in DCM solution, while the chemical synthesis proceeded via a cyanide anion induced electron transfer. Both approaches were found to produce the same decyanated products 315.2a – d in 86–94% yields ( Scheme 315 ).…”

“… a Reagents and conditions: (a) TBAP, DCM, glassy carbon or Pt cathode, controlled applied potential; (b) TBACN, CHCl 3 , stir, 30 min. …”