Palladium(II), Ruthenium(II), and Ruthenium(III) Complexes of 23-Thiaazuliporphyrin: The Case of Coordination-Induced Contraction

Abstract

5,10,15,20-Tetraaryl-23-thiaazuliporphyrin (SAz) was synthesized starting from nonfunctionalized azulene using a "1 + 3" method to be applied as a monoanionic macrocyclic ligand that provides a peculiar [CNSN] coordination cavity. An insertion of palladium(II) afforded the cationic [Pd(II)(SAz)](+), which readily undergoes the seven-membered ring contraction to form palladium(II) 23-thiabenzocarbaporphyrin [Pd(SBzC)] providing the first example of metal azuliporphyrinoid contraction. A reaction of SAz and a ruthenium source ([RuCl2(CO)3]2, [RuCl2(p-cymene)]2, or [RuCl2(cod)]) yielded ruthenium(II) 23-thiaazuliporphyrin [Ru(II)(SAz)Cl(CO)]. As shown by X-ray crystallography the thiophene ring in [Ru(SAz)Cl(CO)] is sharply tilted out of the plane of the two pyrrole nitrogen and carbon atoms being bound to the ruthenium through the pyramidal sulfur in the η(1) fashion. In solution, as demonstrated by variable-temperature (1)H NMR investigations, [Ru(SAz)Cl(CO)] exists as an equilibrium mixture of two isomers that are differentiated by the direction of thiophene folding (toward or outward of the axial chloride ligand). Apart of [Ru(II)(SAz)Cl(CO)], ruthenium(III) 23-thiaazuliporphyrin [Ru(III)(SAz)Cl2] was obtained when [RuCl2(p-cymene)]2 or [RuCl2(cod)]n were used for insertion. The most characteristic (1)H NMR features of paramagnetic [Ru(SAz)Cl2] are negative isotropic shifts of resonances assigned to meso-aryl, azulene, and pyrrolic hydrogen atoms. The analysis of contact shifts and the parallel density functional theory calculations of spin density distribution documented that in [Ru(SAz)Cl2] the metal ion acquires the d(xy)(2)(d(xz)d(yz))(3) ground electronic state. This C(s) symmetry complex has singly occupied d(xz) or d(yz) orbitals that are symmetrically unequivalent. The resulting two different spin density distributions, when merged, reflect the spectroscopic image with the very specific π-spin delocalization, also including the azulene moiety.

C−H and C−M Activation, Aromaticity Tuning, and Co⋅⋅⋅Ru Interactions Confined in the Azuliporphyrin Framework

“…They adopt the favorable localization of positive charges on a seven‐membered ring of azulene . Alternative routes, affording aromatization of azuliporphyrin, involved a regioselective nucleophilic substitution of the seven‐membered ring to form the appropriate adduct with the 1 H NMR features of aromatic fully conjugated carbaporphyrins . In fact, the addition of monoanionic [Co(CO) n ] − formed from [Co 2 (CO) 8 ] affords the electronic structure related to the azuliporphyrin product of seven‐membered ring substitution.…”

“…Positive charge tends to accumulate over the seven‐membered ring, increasing the macrocyclic aromaticity of the form in question (resonance structure 1‐Co III ‐1 ). Stabilization of a cationic species by metal coordination and aromaticity enhancement were clearly observed for the cationic square‐planar palladium(II) complex of monoanionic 23‐thiaazuliporphyrin . In the case of 1‐Co III , the changes are noticeably less pronounced as axial pyridine ligands can also be platforms for partial delocalization of the charge, diminishing the importance of the resonance structures with positive charge located over azulene ( 1‐Co III ‐1 , Scheme ).…”

“…meso ‐Tetraphenylazuliporphyrin, a synthetically readily available carbaporphyrinoid, has been proven to be a suitable macrocyclic ligand, which forms series of diamagnetic complexes and rare examples of ruthenium porphyrin π‐cation radicals . On the other hand, the ruthenium 23‐thiaazuliporphyrin afforded paramagnetic ruthenium(III) species …”

Expanded Porphyrin Contraction: From [22]Triphyrin(6.6.0) to [22]Triphyrin(6.5.0)

“…Skeletal rearrangements are encountered rather rarely; they may be exemplified by a core heteroatom replacement or extrusion, macrocyclic fusion, and ring expansion or contraction. For porphyrinoid aromatic macrocycles a contraction may take place at both—a meso ‐link or at a cyclic subunit, like a pyrrole in a regular porphyrin, a phenylene ring in benziporphyrin or naphthiporphyrin, or at the outer heptacyclic ring of azuliporphyrinoids as well . The meso ‐link extrusion promoted by rhenium coordination was observed in an electron‐deficient tetrakis(trifluoromethyl)porphyrin .…”

“…Again, basic conditions induced a reversible Achmatowicz rearrangement of a dioxaporphyrin, yielding a contracted macrocycle . Examples of cyclic unit contractions include a modified pyrrole ring contraction to azacyclobutene, azulene contraction to benzocyclopentadiene, and finally benzene ring contractions into cyclopentadiene. The latter case constitutes a systematically studied collection of contractions triggered by different metal cations, like palladium(II), gold(III), or rhodium(III), and refers to benzene in different arrangements, like meta ‐ and para ‐phenylene, as well as 1,4‐naphthalene.…”