The Planar Cyclooctatetraene Bridge in Bis-Metallic Macrocycles: Isolating or Conjugating?

Abstract

A minor modification of the reported procedure for the synthesis of a corrole dimer that is fused by the cyclooctatetraene (COT) unit, (Htpfc)COT, allowed for its isolation in 18% yield. Of the two redox isomers that this interesting macrocycle does form, the current focus is on the reduced form, in which each subunit resembles that of monomeric corroles with a trianionic N coordination core. The corresponding bis-gallium(III) complex was prepared as an entry into the potentially rich coordination chemistry of (Htpfc)COT. Both X-ray crystallography and DFT calculations disclosed that the COT moiety is essentially planar with very unusual nonalternating C-C bonds. The same holds true for the bis-gallium(III) complexes [(Ga-tpfc)]COT(py) and [(Ga-tpfc)]COT(py), obtained with one and two pyridine molecules coordinated to each metal ion, respectively. The electronic spectra of both the free base and the gallium(III) complexes display an extremely low energy band (λ at 720-724 nm), which points toward extensive π delocalization through the COT bridge. This aspect was fully addressed by examining the interactions between the two corrole subunits in terms of electrochemistry and DFT calculations of the oxidized and reduced macrocycle. The new near-IR bands that appear upon both oxidation (λ 1250 nm) and reduction (λ 1780 nm) serve as additional supporting evidence for this conclusion.

Singly, Doubly, and Triply Linked Corrole Oligomers: Synthesis, Structures, and Linking Position Dependent Properties

“…Dimer 7 was probably formed via 2–2′ linked dimer, which has been later confirmed by preparing 7 from the isolated 2–2′ linked dimer via oxidation with DDQ (vide infra). Later, Gross and co‐workers reported a modified synthesis of 7 by heating 1 a in the same solvent at 200 °C in the air …”

“…Recently Gross et al. reported the gallium(III) metalation of 7 with GaCl 3 in pyridine to afford gallium(III) complex 7GaPy 2 and 7GaPy 4 , whose electronic conjugation through the cyclooctatetraene bridge was modulated by the axially coordinated pyridines and their redox state …”

“…Later, Gross and co-workers reported a modified synthesis of 7 by heating 1 a in the same solvent at 200°C in the air. [20] It is also known that corroles are photosensitive and not very stable in solution under light and air. Cavaleiro et al reported the photo-dimerization of 1 a.…”

“…Recently Gross et al reported the gallium(III) metalation of 7 with GaCl 3 in pyridine to afford gallium(III) complex 7GaPy 2 and 7GaPy 4 , whose electronic conjugation through the cyclooctatetraene bridge was modulated by the axially coordinated pyridines and their redox state. [20] The synthesis of 3-3',5-5' doubly linked corrole dimer 26 was reported in 2015. [33] 5-5' Linked corrole dimer 12 was synthesized in two ways; i) an oxidative coupling of mesounsubstituted corrole 11 as shown in Scheme 6, and ii) an acidcatalyzed [2 + 2]-type condensation reaction of 1,1,2,2-tetrapyrrolylethane with pentafluorophenyldipyrromethane monocarbinol followed by DDQ oxidation (Scheme 12).…”

Graphene‐induced planarization of cyclooctatetraene derivatives

“…8,9,10 It has been demonstrated both experimentally and computationally that COT can be planarized through annulation with strained rings or by incorporation into a rigid planar π-system, forcing the central COT moiety to adopt a planar geometry with alternating single and double bonds. 1,4,8,9,[11][12][13][14][15][16][17][18][19][20][21][22][23][24] Merino and coworkers showed that an alternative approach for planarizing COT to adopt a D 4h symmetric structure is substitution of four alternating carbons with heavier group 14 elements (Si, Ge, and Sn). 25 Based on statistical analysis of X-ray data of 47 COT rings, Dominikowska and Palusiak further found that COT planarizes when acting as an effective π-type ligand in various organometallic complexes.…”