Magnetic Circular Dichroism of Transition-Metal Complexes of Perfluorophenyl-N-Confused Porphyrins: Inverting Electronic Structure through a Proton.

Neutral and deprotonated anionic Ni(II), Pd(II), Cu(II), and Cu(III) complexes of tetrakis(perfluorophenyl)-N-confused porphyrin (PF-NCP) were prepared and investigated by UV-visible and magnetic circular dichroism (MCD) spectroscopies. As in the previously reported Ni(II) adduct of tetraphenyl N-confused porphyrin, we observe sign reverse (positive to negative intensities with increasing energy) features in the MCD spectra of the neutral Ni(II), Pd(II), and Cu(II) complexes of PF-NCP, which is indicative of rare ΔHOMO < ΔLUMO relationships. Upon deprotonation of Ni(II), Pd(II), and Cu(II) complexes, these features revert to those of more typical porphyrin MCD spectra consistent with a ΔHOMO > ΔLUMO condition. The Cu(III) PF-NCP complex shows features similar to those of the deprotonated divalent metal systems. Spectroscopic features in all target complexes as well as previously published metal-free and Ni(II) NCP systems were correlated with the density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. Calculation data are consistent with the tautomeric rearrangement of the electronic structures of NCP cores playing dominant roles, with smaller contribution from the central metal ions in the observed optical and magneto-optical properties. This is true for all described NCP systems to date, as they affect the stabilization/destabilization of the N-confused porphyrin-centered Gouterman orbitals.

Two-Step, One-Flask Synthesis of an N-Confused Porphyrin Bearing Pentafluorophenyl Substituents.

A two-step, one-flask reaction of pyrrole and pentafluorobenzaldehyde was investigated as a streamlined synthetic route to an N-confused porphyrin bearing pentafluorophenyl substituents previously prepared by a stepwise route. A survey of acid catalysts, acid catalyst concentration, DDQ quantity, and reaction time was performed with monitoring by HPLC. The targeted N-confused porphyrin was observed from many reaction conditions. The best condition afforded the N-confused porphyrin in an isolated yield of 10-12% (245-281 mg), providing improved access to this interesting porphyrinoid.

Two-Step, One-Flask Synthesis of a Meso-Substituted Phlorin.

A two-step, one-flask reaction of pyrrole with pentafluorobenzaldehyde and acetone was investigated to determine the potential for a streamlined synthesis of a phlorin and/or 5-isocorrole as an alternative to stepwise, dipyrromethanecarbinol routes. Analytical-scale reactions were performed examining the effect of reactant concentration, reactant ratio, acid catalyst (TFA or BF3·OEt2), concentration of acid catalyst, oxidant quantity, and reaction time on the distribution of phlorin and 5-isocorrole as well as three additional porphyrinoids (porphodimethene, porphyrin, and corrole). Phlorin was observed ubiquitously in yields up to 20-26%, whereas 5-isocorrole was not detected. Promising reaction conditions for the one-flask synthesis of the phlorin were performed on a preparative scale. The best reaction condition afforded the phlorin in an isolated yield of 20-21% (249-268 mg). Preliminary attempts to extend the methodology to the preparation of phlorins derived from other ketones resulted in a low yield of phlorin from acetophenone (5%) and no detectable phlorin from benzophenone. The discovery of reaction conditions for the two-step, one-flask synthesis of a phlorin provides easier access to this interesting compound, and provides encouragement for the further study of reactions of pyrrole with an aldehyde and a ketone.

Combined MCD/DFT/TDDFT Study of the Electronic Structure of Axially Pyridine Coordinated Metallocorroles.

A series of metallocorroles were investigated by UV-vis and magnetic circular dichroism spectroscopies. The diamagnetic distorted square-pyramidal main-group corrole Ga(tpfc)py (2), the diamagnetic distorted octahedral transition-metal adduct Co(tpfc)(py)2 (3), and paramagnetic distorted octahedral transition-metal complex Fe(tpfc)(py)2 (4) [H3tpfc = tris(perfluorophenyl)corrole] were studied to investigate similarities and differences in the electronic structure and spectroscopy of the closed- and open-shell metallocorroles. Similar to the free-base H3tpfc (1), inspection of the MCD Faraday B-terms for all of the macrocycles presented in this report revealed that a ΔHOMO < ΔLUMO [ΔHOMO is the energy difference between two highest energy corrole-centered π-orbitals and ΔLUMO is the energy difference between two lowest energy corrole-centered π*-orbitals originating from ML ± 4 and ML ± 5 pairs of perimeter] condition is present for each complex, which results in an unusual sign-reversed sequence for π-π* transitions in their MCD spectra. In addition, the MCD spectra of the cobalt and the iron complexes were also complicated by a number of charge-transfer states in the visible region. Iron complex 4 also exhibits a low-energy absorption in the NIR region (1023 nm). DFT and TDDFT calculations were used to elaborate the electronic structures and provide band assignments in UV-vis and MCD spectra of the metallocorroles. DFT and TDDFT calculations predict that the orientation of the axial pyridine ligand(s) has a very minor influence on the calculated electronic structures and absorption spectra in the target systems.

Phlorins bearing different substituents at the sp3-hybridized meso-position.

Phlorins bearing different substituents at the sp(3)-hybridized meso-position were investigated. The extent to which different substituents at this unique position can influence phlorin spectroscopic properties, structure, and stability is of interest given that such substituents are not in direct conjugation with the phlorin macrocycle. While the effect of various substituents at the sp(2)-hybridized positions has been the subject of prior investigations, the impact of different substituents at the saturated carbon atom has not been systematically examined. In this study, phlorins with different combinations of geminal methyl and phenyl substituents were prepared in yields of 24-49% via dipyrromethane + dipyrromethanedicarbinol routes, and their NMR spectra, UV-vis spectra, X-ray crystal structures, and stability toward light and air were compared. The nature of the substituents at the sp(3)-hybridized position was found to impact spectroscopic properties, structure, and stability to varying degrees. Thus, the choice of substituents at the sp(3)-hybridized meso-position provides a further option for altering phlorin properties.

The first TDDFT and MCD studies of free base triarylcorroles: a closer look into solvent-dependent UV-visible absorption.

Absorption spectra of several free base triarylcorroles were investigated by MCD spectroscopy. The MCD spectra exhibit unusual sign-reverse (positive-to-negative intensities in ascending energy) features in the Soret- and Q-type band regions, suggesting a rare ΔHOMO < ΔLUMO relationship between π and π* MOs in the corrole core.

Structure and spectroscopic characterization of free base and metal complexes of 5,5-dimethyl-10,15-bis(pentafluorophenyl)isocorrole.

Copper and zinc complexes of 5,5-dimethyl-10,15-bis(pentafluorophenyl)isocorrole are presented, including the first crystal structures of the free base and 5-isocorrole metal complexes.

Investigation of complementary reactions of a dipyrromethane with a dipyrromethanemonocarbinol leading to a 5-isocorrole.

Two complementary dipyrromethane + dipyrromethanemonocarbinol routes to a meso-substituted 5-isocorrole were investigated. While both routes could afford the identical 5-isocorrole, self-condensation of the different dipyrromethanemonocarbinol precursors could potentially lead to a second porphyrinoid of different structure (a porphyrin or a porphodimethene). The two reaction routes were examined to compare the distribution of porphyrinoid products, probe the effect of key reaction parameters on the product distribution, and identify conditions for the efficient preparation of the 5-isocorrole so that its spectral properties and stability toward light and air could be assessed. For each route, a systematic survey of reaction parameters was performed via analytical-scale reactions monitored for the yields of the 5-isocorrole and self-condensation product by HPLC. The two reaction routes were found to afford very different product distributions in accordance with the anticipated nucleophilicity of the dipyrromethane and dipyrromethanemonocarbinol precursors. The most promising reaction condition (InCl(3), 0.32 mM) was performed on a preparative-scale providing the 5-isocorrole in an isolated yield of 31% (102 mg). Spectroscopic analysis was consistent with the 5-isocorrole structure. The stability of the 5-isocorrole in dilute solution upon exposure to light and air was assessed by UV-vis spectroscopy and HPLC and was found to be excellent.

Dipyrromethane + dipyrromethanedicarbinol routes to an electron deficient meso-substituted phlorin with enhanced stability.

Two dipyrromethane + dipyrromethanedicarbinol routes to a meso-substituted phlorin bearing electron-withdrawing pentafluorophenyl substituents (TpFPPhl) were investigated in an attempt to obtain a phlorin with enhanced stability toward light and air and to explore the application of dipyrromethanecarbinol chemistry to the preparation of phlorins. For each route, a systematic survey of reaction parameters for the two-step, one-flask reaction leading to TpFPPhl was performed. The analytical-scale reactions were monitored for yield of TpFPPhl by HPLC. Sharp differences were observed in the yield of TpFPPhl afforded by the two synthetic routes. The most promising reaction condition (TFA catalysis, 100 mM) was performed on a preparative scale providing TpFPPhl in a yield of 45% (189 mg). The stability of the electron-deficient phlorin in dilute solution upon exposure to light and air was probed in a number of solvents, and decomposition was monitored by UV-vis spectroscopy and HPLC. Many of the solutions of TpFPPhl were found to be quite stable for periods of approximately 8 h, with decomposition requiring exposure periods of several days. Taken together, this work contributes an efficient synthesis of a meso-substituted phlorin of practical stability and provides further insights toward the adaptation of dipyrromethanecarbinol chemistry to the preparation of diverse porphyrinoids.