Oxidation and Oxygenation of Carbonyl Ruthenium(II) Azuliporphyrin

A Janus carbaporphyrin pseudo-dimer

Carbaporphyrin dimers, investigated for their distinctive electronic structures and exceptional properties, have predominantly consisted of systems containing identical subunits. This study addresses the associated knowledge gap by focusing on asymmetric carbaporphyrin dimers with Janus-like characteristics. The synthesis of a Janus-type carbaporphyrin pseudo-dimer 5 is presented. It displays antiaromatic characteristics on the fused side and nonaromatic behavior on the unfused side. A newly synthesized tetraphenylene (TPE) linked bis-dibenzihomoporphyrin 8 and a previously reported dibenzo[g,p]chrysene (DBC) linked bis-dicarbacorrole 9 were prepared as controls. Comprehensive analyses, including 1H NMR spectral studies, single crystal X-ray diffraction analyses, and DFT calculations, validate the mixed character of 5. A further feature of the Janus pseudo-dimer 5 is that it may be transformed into a heterometallic complex, with one side coordinating a Cu(III) center and the other stabilizing a BODIPY complex. This disparate regiochemical reactivity underscores the potential of carbaporphyrin dimers as versatile frameworks, with electronic features and site-specific coordination chemistry controlled through asymmetry. These findings position carbaporphyrin dimers as promising candidates for advances in electronic structure studies, coordination chemistry, materials science, and beyond.

Organometallic chemistry confined within a porphyrin-like framework

The world of modified porphyrins changed forever when an N-confused porphyrin (NCP), a porphyrin isomer, was first published in 1994. The replacement of one inner nitrogen with a carbon atom revolutionised the chemistry that one is able to perform within the coordination cavity. One could explore new pathways in the organometallic chemistry of porphyrins by forcing a carbon fragment from the ring or an inner substituent to sit close to an inserted metal ion. Since the NCP discovery, a series of modifications became available to tune the coordination properties of the cavity, introducing a fascinating realm of carbaporphyrins. The review surveys all possible carbatetraphyrins(1.1.1.1) and their spectacular coordination and organometallic chemistry.

Organometallic Chemistry within the Structured Environment Provided by the Macrocyclic Cores of Carbaporphyrins and Related Systems

The unique environment within the core of carbaporphyrinoid systems provides a platform to explore unusual organometallic chemistry. The ability of these structures to form stable organometallic derivatives was first demonstrated for N-confused porphyrins but many other carbaporphyrin-type systems were subsequently shown to exhibit similar or complementary properties. Metalation commonly occurs with catalytically active transition metal cations and the resulting derivatives exhibit widely different physical, chemical and spectroscopic properties and range from strongly aromatic to nonaromatic and antiaromatic species. Metalation may trigger unusual, highly selective, oxidation reactions. Alkyl group migration has been observed within the cavity of metalated carbaporphyrins, and in some cases ring contraction of the carbocyclic subunit takes place. Over the past thirty years, studies in this area have led to multiple synthetic routes to carbaporphyrinoid ligands and remarkable organometallic chemistry has been reported. An overview of this important area is presented.

Ruthenium(iv) N-confused porphyrin μ-oxo-bridged dimers: acid-responsive molecular rotors

Ruthenium(iv) N-confused porphyrin μ-oxo-bridged complexes were synthesized via oxidative dimerization of a ruthenium(ii) N-confused porphyrin complex using 2,2,6,6-tetramethylpiperidine 1-oxyl. The deformed core planes in the dimers conferred a relatively high ring rotational barrier of ca. 16 kcal mol⁻¹. Rotation of the complexes was controlled by protonating the peripheral nitrogen.

Ring rotation of ruthenium(iv) N-confused porphyrin μ-oxo-dimer was controlled by protonation at the peripheral nitrogen moieties.

Ruthenium(II) and Ruthenium(III) Complexes of p-Benziporphyrin: Merging Equatorial and Axial Organometallic Coordination

A diamagnetic ruthenium(II) complex of 5,10,15,20-tetraphenyl-p-benziporphyrin [Ru^II(p-BzP)(CO)Cl] was obtained via the insertion of ruthenium into p-benziporphyrin using triruthenium(0) dodecacarbonyl [Ru₃(CO)₁₂] as the metal source. The procedure applying dichloro(cycloocta-1,5-diene)ruthenium(II) (polymer, [Ru(COD)Cl₂]_n) afforded the paramagnetic six-coordinate ruthenium(III) p-benziporphyrin [Ru^III(p-BzP)Cl₂]. As shown by X-ray crystallography, the p-phenylene ring in both complexes is sharply tilted out of the N₃ plane, as reflected by the respective N₃ (pyrrole)-C₆ (p-phenylene) dihedral angle [Ru^II(p-BzP)(CO)Cl, 52.5°; Ru^III(p-BzP)Cl₂, 53.7°]. p-Phenylene is bound to the ruthenium cation in an η² fashion, revealing the shortest ever Ru-C distance in the series of p-benziporphyrin complexes [Ru^II(p-BzP)(CO)Cl, 2.275(2) Å; Ru^III(p-BzP)Cl₂, 2.324(5) Å]. The reaction of Ru^II(p-BzP)(CO)Cl with ArMgCl or AlkMgCl results in the formation of diamagnetic six-coordinate ruthenium(II) p-benziporphyrin complexes containing the apically coordinated σ-alkyl or σ-aryl ligands, where the metal ion simultaneously coordinates to three carbon centers respectively accommodating η² (phenylene) and σ (aryl and alkyl) modes. Reactions of σ-aryl (alkyl) carbanions with paramagnetic Ru^III(p-BzP)Cl₂ have been followed by ¹H NMR spectroscopy. The procedure afforded the six-coordinate paramagnetic ruthenium(III) p-benziporphyrin [Ru^III(p-BzP)(Ph)Cl], which binds one σ-aryl ligand, as reflected by the characteristic ¹H NMR spectra spread within the +120 to -120 ppm range. Both paramagnetic complexes Ru^III(p-BzP)(Ph)Cl and Ru^III(p-BzP)(p-Tol)Cl are formed as a mixture of two stereoisomers differentiated by two nonequivalent locations of σ-aryl with respect to the puckered macrocyclic ring. The paramagnetic shifts of σ-aryls are indicative of π-spin delocalization patterns. Analysis of the contact shifts and parallel density functional theory calculations of the spin density distribution in Ru^III(p-BzP)Cl₂, Ru^III(p-BzP)(Ar)Cl, and Ru^III(p-BzP)(Alk)Cl reflect the features of the d_xy²(d_xzd_yz)³ electronic ground state.

Coordination-Induced Molecular Tweezing: Ruthenium Clusters Docked at Azuliporphyrinogens

The reaction of [Ru₃(CO)₁₂] with a series of conformationally flexible thiaazuliporphyrinogens, namely, dithiadiazuli-, thiatriazuli- and tetraazuliporphyrinogen, yielded a series of complexes with two azulene rings coordinated by either Ru₂(CO)₅ and Ru₄(CO)₉ or two Ru₄(CO)₉ clusters. For dithiadiazuliporphyrinogen, three fundamental arrangements were detected, with two clusters being located at the same side or different sides of the meso plane. Coordination altered the electron density on the rings, allowing for dispersion-promoted interactions between coordinated azulenes. To create this face-to-face arrangement, the ligand framework performs a tweezing movement with coordinated azulenes approaching each other. The tweezer-like arrangement of the coordinated azulene rings assists the formation of azulene inclusion complexes as determined by ¹H NMR titration.

Regioselective oxidation and metalation of meso-unsubstituted azuliporphyrins

Azuliporphyrins are intriguing porphyrin analogues that incorporate an azulene ring in place of a pyrrolic unit. This system undergoes regioselective oxidation reactions and favors the formation of stable organometallic derivatives. Reaction of meso-unsubstituted azuliporphyrins with Co₂(CO)₈ or CoCl₂·6H₂O gave 21-oxyazuliporphyrins, while Cu(OAc)₂ produced the corresponding copper(ii) complexes. Treatment of an oxyazuliporphyrin with Ni(OAc)₂ or Pd(OAc)₂ afforded analogous nickel(ii) and palladium(ii) derivatives. Silver(i) acetate in pyridine reacted with azuliporphyrins to give moderate yields of silver(iii) benzocarbaporphyrins, and the prevalence of structures with a formyl moiety at the sterically crowded 2¹-position suggested that the ring contraction reactions were triggered in part by intramolecular attack from an axial peroxide ligand. Related thiaazuliporphyrins reacted with palladium(ii) acetate to give palladium(ii) benzothiacarbaporphyrins but this chemistry did not give rise to structures with 2¹-formyl groups, suggesting that the ring contraction reactions occurred by a different mechanistic pathway. These results demonstrate the existence of a rich tapestry of oxidation and metalation reactions for azuliporphyrin systems.

Out of the Blue! Azuliporphyrins and Related Carbaporphyrinoid Systems

First reported in 1997, azuliporphyrins have proven to be a truly remarkable family of porphyrin analogues. In this system, although the porphyrin framework is retained, one of the pyrrolic moieties has been replaced by an azulene unit. Azulene favors electrophilic substitution at the 1,3-positions, which are structurally analogous to the α-positions in pyrrole, and this property facilitates the construction of azulene-containing porphyrinoid systems. Azuliporphyrins were first prepared from tripyrranes and 1,3-azulenedicarbaldehyde using a "3 + 1" variant on the MacDonald reaction. Subsequently, azulenes were shown to react with acetoxymethylpyrroles under acidic conditions to generate azulitripyrranes that could be utilized in a back-to-front "3 + 1" methodology to form azuliporphyrins and related heteroporphyrinoids. In addition, the favorability of azulenes toward 1,3-substitution was applied to one-pot syntheses of tetraarylazuliporphyrins and calix[4]azulenes. Azuliporphyrins have significant diatropic character that is greatly enhanced upon protonation. They have been shown to form organometallic complexes with Ni(II), Pd(II), Pt(II), Ir(III), Rh(III), and Ru(II) and undergo selective oxidations at the internal carbon with copper(II) or silver(I) salts to afford 21-oxyazuliporphyrins. In addition, oxidative ring contractions readily occur under basic conditions in the presence of peroxides to give benzocarbaporphyrins, and this reactivity provides access to tetraarylbenzocarbaporphyrins and their organometallic derivatives. A diazulenylmethane dialdehyde has been shown to react with dipyrrylmethanes in the presence of HCl or HBr to give diazuliporphyrins that were isolated in a monoprotonated form, and metalation with palladium(II) acetate afforded a stable zwitterionic palladium(II) complex. Equally intriguing dicarbaporphyrinoids incorporating indene and azulene rings have been reported, and these systems exhibit significant aromatic character. Recent studies have demonstrated that calixazulenes form supramolecular complexes with quaternary ammonium salts and afford a 1:1 complex with C60. In addition, conjugated structures have been prepared from calixazulenes that are structurally related to quatyrin, the theoretically important hydrocarbon analogue of the porphyrins. Examples of expanded azuliporphyrinoids have also been described. These azulene-containing porphyrinoids exhibit unique and complex reactivity that compares favorably with better studied porphyrin analogue systems such as the N-confused porphyrins, and azuliporphyrin derivatives show promise in the development of new catalytic systems.