Lechoslaw Latos-Grazynski

Figure Eights, Möbius Bands, and More: Conformation and Aromaticity of Porphyrinoids

The aromatic character of porphyrins, which has significant chemical and biological consequences, can be substantially altered by judicious modifications of the parent ring system. Expansion of the macrocycle, which is achieved by introducing additional subunits, usually increases the so-called free curvature of the ring, leading to larger angular strain. This strain is reduced by a variety of conformational changes, most notably by subunit inversion and p surface twisting. The latter effect creates a particularly convenient access to Möbius aromatic molecules, whose properties, predicted over 40 years ago, are of considerable theoretical importance. The conformational processes occurring in porphyrin analogues are often coupled to other chemical phenomena, and can thus be exploited as a means of constructing functional molecular devices. In this Review, the structural chemistry of porphyrinoids is discussed in the context of their conformational dynamics and p-electron conjugation.

Poly- and oligometalloporphyrins associated through coordination

Abstract Significant developments in the construction of oligoporphyrins and oligometalloporphyrins supply a large variety of new interesting compounds with potential use in electron and energy transfer. Their physicochemical properties depend primarily on the nature of the linkage between porphyrin building blocks. In this review we have focused on the expanding role of the coordination bond in creation of polyporphyrin architecture.

Three-level topology switching in a molecular Möbius band.

Möbius pi-conjugation in cyclic molecules leads to the reversal of Huckel aromaticity rules and affects the electronic and magnetic properties of these systems. We found the first example of a medium-sized macrocyclic structure that is sufficiently flexible to switch between three distinct pi-conjugation topologies, planar (T0), Möbius (T1), and twisted Huckel (T2), without changing its oxidation level. The switching is under thermodynamic and kinetic control and can be realized in a three- or four-step cycle. On titration with trifluoroacetic acid (TFAH) or dichloroacetic acid (DCAH), the Möbius free base (T1-H(2)), which is the preferred structure in dichlorofluoromethane at 150 K, undergoes a series of acid-base reactions involving changes of the pi-conjugation topology. The forms observed in the course of titration involve a Möbius aromatic monocation ([T1-H(3)](+)), an antiaromatic twisted Huckel species ([T2-H(4)(A)](+)) containing a coordinated carboxylate anion (A = TFA, DCA), and two additional Möbius forms ([T1-H(4)(A)(HA)(n)](+) (n = 1, 2)), containing complex carboxylate anions. The protonated forms undergo a thermally activated ring planarization to yield an antiaromatic quasi-planar dication [T0-H(4)](2+), characterized in the solid state as a TFA salt. The corresponding free base (T0-H(2)) is metastable but can be trapped by addition of triethylamine at low temperatures.

Tetraphenylbenziporphyrin—A Ligand for Organometallic Chemistry

6,11,16,21-Tetraphenylbenziporphyrin (TPBPH)H, an analogue of tetraphenylporphyrin with one of the pyrrole groups replaced by a benzene ring, is formed in good yield in the condensation of the appropriate precursor with pyrrole and benzaldehyde. (TPBPH)H gives organometallic complexes with palladium(II) and platinum(II), [(TPBP)PdII] and [(TPBP)PtII], in which the metal ion is bound in the macrocyclic cavity by three pyrrolic nitrogen atoms and a carbon atom of the benzene ring. In the reaction with silver(I) acetate benziporphyrin does not yield a stable complex but undergoes selective acetoxylation at the internal carbon atom. (TPBPH)H is reversibly reduced to 6-benziphlorin and reacts with a water or methanol molecule to give 6-hydroxy- or 6-methoxy-6-benziphlorin, respectively.

Palladium Vacataporphyrin Reveals Conformational Rearrangements Involving Hückel and Möbius Macrocyclic Topologies

5,10,15,20-Tetraaryl-21-vacataporphyrin (butadieneporphyrin, an annulene-porphyrin hybrid) which contains a vacant space instead of heteroatomic bridge acts as a ligand toward palladium(II). The metal ion of square-planar coordination geometry is firmly held via three pyrrolic nitrogen atoms where the fourth coordination place is occupied by a monodentate ligand or by an annulene part of vacataporphyrin. The macrocycle reveals the unique structural flexibility triggered by coordination of palladium. The structural rearrangements engage the C(20)C(1)C(2)C(3)C(4)C(5) annulene fragment which serves as a linker between two pyrrolic rings of vacataporphyrin albeit the significant ruffling of the tripyrrolic block is also of importance. Two fundamental modes of interactions between the palladium ion and annulene moiety have been recognized. The first one resembles an eta(2)-type interaction and involves the C(2)C(3) unit of the butadiene part. Alternatively the profound conformational adjustments allowed an in-plane coordination through the deprotonated trigonally hybridized C(2) center of butadiene. The coordinated vacataporphyrin acquires Hückel or extremely rare Möbius topologies readily reflected by spectroscopic properties. The palladium vacataporphyrin complexes reveal Hückel aromaticity or Möbius antiaromaticity of [18]annulene applying the butadiene fragment of vacataporphyrin as a topology selector. The properties of specific conformers were determined using (1)H NMR and density functional theory calculations.

N-METHYLTETRAPHENYLPORPHYRIN WITH AN INVERTED N-METHYLPYRROLE RING: THE FIRST ISOMER OF N-METHYLTETRAPHENYLPORPHYRIN

In the course of the mild methylation of 5,10,15,20-tetraphenyl-21-carbaporphyrin (CTPPH2) a novel isomer of N-methyl-5,10,15,20-tetraphenylporphyrin with an inverted N-methylated pyrrole ring, i.e. 2-N-methyl-5,10,15,20-tetraphenyl-21-carbaporphyrin (2-NCH3TPPH) has been synthesised. The protonation of 2-NCH3CTPPH proceeds stepwise including mono- and two di-cationic species. 2-NCH3CTPPH can be considered as a new anion-specific binding agent. The lability of the 21-CH proton afforded the easy insertion of nickel(II) to form a stable organometallic macrocyclic complex.

A Direct Link between Annulene and Porphyrin Chemistry-21-Vacataporphyrin

A novel molecule, aza-deficient porphyrin 5,10,15,20-tetraaryl-21-vacataporphyrin has been synthesised by a substraction of a tellurium atom from 5,10,15,20-tetraaryl-21-telluraporphyrin under treatment of HCl. The new macrocycle is an annulene-porphyrin hybrid and at the same time is directly related to 21-heteroporphyrins but has a vacant space instead of heteroatomic bridge. The molecule preserves the fundamental structural and spectroscopic features of the parental 5,10,15,20-tetraarylporphyrin with three nitrogen atoms and two CH groups favorably prearranged for coordination.

NICKEL COMPLEXES OF 21-OXAPORPHYRIN AND 21,23-DIOXAPORPHYRIN

The nickel(I) and nickel(II) complexes of 5,20-bis(p-tolyl)-10, 15-diphenyl-21-oxaporphyrin (ODTDPPH) and 5,10,15,20-tetraphenyl-21, 23-dioxaporphyrin (O2 TPP) have been investigated. These oxa analogues of 5,10,15,20-tetraarylporphyrin, where one or two pyrrole rings are replaced by a furan moiety, have been synthesized by condensation of the respective precursors, namely 2,5-bis(arylhydroxymethyl)furan, pyrrole, and arylaldehyde. Insertion of nickel(II) into ODTDPPH or O2 TPP yielded high-spin five- and six-coordinate ([(ODTDPP)Ni(II) Cl] and [(O2 TPP)NiIICl2 ]) complexes, which can be reduced with moderate reducing reagents. The EPR spectra of [(ODTDPP)Ni(I) ] and [(O2 TPP)Ni(I) Cl] revealed the Ni(I) oxa(dioxa)porphyrin rather than a Ni(I) anion radical electronic structure. In the structures of [(ODTDPP)Ni(II) Cl], [(O2 TPP)Ni(IICl) 2 ], and [(ODTDPP)Ni(I) ], determined by X-ray diffraction, the furan ring is planar and coordinates in the η(1) fashion through the trigonal oxygen atom; the nickel ion lies in the furan plane for the latter two complexes, but slightly outside it in [(ODTDPP)Ni(II) Cl]. The Ni-N and Ni-O bond lengths decrease upon reduction of high-spin five-coordinate [(ODTDPP)Ni(II) Cl] to four-coordinate [(ODTDPP)Ni(I) ]. The pattern of downfield pyrrole resonances in (1) H NMR spectra of [(ODTDPP)Ni(IICl) ] and [(O2 TPP)-Ni(II) Cl2 ] has been established. The downfield positions of furan resonances are unusual for Ni(II) heteroporphyrins; they have been accounted for by the nearly in-plane coordination of the furan moiety as opposed to the side-on coordination found for thiophene- or selenophene-containing heteroporphyrins. An example of ion-pair formation, [(O2 TPPH)2 ][Ni(II) Cl4 ], was produced from [(O2 TPP)Ni(II) Cl2 ] by acidification with HCl.

A Facile Palladium‐Mediated Contraction of Benzene to Cyclopentadiene: Transformations of Palladium(II) p‐Benziporphyrin

The contraction of benzene and its derivatives to form a cyclopentadiene ring has rarely been reported. Pioneering studies on the photooxidation of benzene led to the conclusion that cyclopentadienecarboxyaldehyde was formed in this reaction. Since then, research on photoinduced reactions of hydroxyand dihydroxybenzene revealed interesting mechanistic features, including ring contraction from benzene to cyclopentadiene. A similar structural motif was detected in the course of thermal decomposition of anisole or dihydroxybenzene. The oxidation of phenol with dioxygen in the presence of metallic copper resulted in the aromatic ring contraction to afford substituted cyclopentenes. Carbocycle contraction to benzvalene followed by opening of the ring to form benzene was postulated in theoretical studies on the high-temperature intramolecular topomerization of [1,2C2]benzene to [1,3C2]and [1,4C2]benzene. [5] In more general terms, the benzene contraction belongs to an exclusive group of reactions where the cleavage of aromatic structures is of fundamental importance. Significantly, oxidative ring cleavage is a key metabolic step in the biodegradation of aromatic compounds by bacteria. The common metabolic pathway is a ring fission by catechol dioxygenases that contain a nonheme iron(II) center in the active site. The representative examples where such a challenge has been chemically addressed include cleavage of the aromatic rings with formation of metallacyclopentadiene complexes according to a retro-alkyne cyclotrimerization mechanism, a reductive silylation of silylsubstituted arenes, or insertion of tungsten into unstrained aromatic rings. Recently, an impressive room-temperature C C bond fission of an arene by a metallacarborane was reported. Porphyrinoids (including carbaporphyrinoids) provide a unique macrocyclic platform that is suitable for exploring organometallic chemistry confined to a particular macrocyclic environment. Often C H or C C bonds are held close to the metal center, thus enforcing an unusual coordination geometry and unique reactivity. Herein we report the contraction of the benzene ring embedded in palladium(II) p-benziporphyrin 1. This process affords palladium(II) 21formyl-21-carbaporphyrin 4 and palladium(II) 21-carbaporphyrin 5, and proceeds via palladium(II) 22-hydroxycyclohexadieneporphyrin 3 as a spectroscopically detectable intermediate. Reaction of palladium(II) chloride with p-benziporphyrin 1 in acetonitrile results in the formation of the four-coordinate palladium(II) p-benziporphyrin 2 (Scheme 1). The geometry

Phosphorus Complexes of N-Fused Porphyrin and Its Reduced Derivatives : New Isomers of Porphyrin Stabilized via Coordination

N-fused isophlorin 3 and its tautomeric phlorin forms 4 and 5, the new constitutional isomers of porphyrin which preserve the basic skeleton of their maternal N-fused porphyrin, have been identified in the course of investigation of phosphorus insertion into N-fused porphyrin 2. N-fused porphyrin reacts with PCl3 in toluene yielding phosphorus(V) N-fused isophlorin 3-P wherein the macrocycle acts as a trianionic tridentate ligand. The identical product has been formed in the reaction of N-confused porphyrin 1 and POCl3 or PCl3. The coordinating environment of phosphorus(V) in 3-P as determined by X-ray crystallography resembles a distorted trigonal pyramid with the nitrogen atoms occupying equatorial positions with the oxygen atom lying at the unique apex. Phosphorus(V) is significantly displaced by 0.732(1) A from the N3 plane. The P-N distances are as follows P-N(22) 1.664(2), P-N(23) 1.645(2), and P-N(24) 1.672(2). All P-N(pyrrolic) bond lengths are markedly shorter than the P-N distances in phosphorus porphyrins. 3-P is susceptible to proton addition at the inner C(9) carbon atom, yielding aromatic 4-P. The modified macrocycle acts as a dianionic ligand and allows the efficient 18 pi-electron delocalization pathway. Two stereoisomers affording the syn (4-P syn) and anti (4-P anti) location of the H(9) atom with respect to the oxygen atom of the PO unit have been identified by (1)H NMR. A regioselective reduction of free base N-fused porphyrin 2 with NaBH4 yielded a nonaromatic isomer of 4, that is, N-fused phlorin 5 due to an addition of a hydride to the C(15) carbon and a proton to one of the pyrrolic nitrogens. The isomer 5 reacts with PCl 3 yielding phosphorus(V) fused isophlorin 3-P. Density functional theory has been applied to model the molecular and electronic structure of porphyrin isomers 3, 4, and 5 and their phosphorus(V) complexes.