Synthesis and characterization of iron N-confused porphyrins: structural evidences of agostic interaction

Well-defined N3 C1 -anchored Single-Metal-Sites for Oxygen Reduction Reaction

N-, C-, O-, S-coordinated single-metal-sites (SMSs) have garnered significant attention due to the potential for significantly enhanced catalytic capabilities resulting from charge redistribution. However, significant challenges persist in the precise design of well-defined such SMSs, and the fundamental comprehension has long been impeded in case-by-case reports using carbon materials as investigation targets. In this work, the well-defined molecular catalysts with N3 C1 -anchored SMSs, i.e., N-confused metalloporphyrins (NCPor-Ms), are calculated for their catalytic oxygen reduction activity. Then, NCPor-Ms with corresponding N4 -anchored SMSs (metalloporphyrins, Por-Ms), are synthesized for catalytic activity evaluation. Among all, NCPor-Co reaches the top in established volcano plots. NCPor-Co also shows the highest half-wave potential of 0.83 V vs. RHE, which is much better than that of Por-Co (0.77 V vs. RHE). Electron-rich, low band gap and regulated d-band center contribute to the high activity of NCPor-Co. This study delves into the examination of well-defined asymmetric SMS molecular catalysts, encompassing both theoretical and experimental facets. It serves as a pioneering step towards enhancing the fundamental comprehension and facilitating the development of high-performance asymmetric SMS catalysts.

Designing N-Confused Metalloporphyrin-Based Covalent Organic Frameworks for Enhanced Electrocatalytic Carbon Dioxide Reduction

Electrochemical conversion of carbon dioxide (CO2 ) into value-added products is promising to alleviate greenhouse gas emission and energy demands. Metalloporphyrin-based covalent organic frameworks (MN4 -Por-COFs) provide a platform for rational design of electrocatalyst for CO2 reduction reaction (CO2 RR). Herein, through systematic quantum-chemical studies, the N-confused metallo-Por-COFs are reported as novel catalysts for CO2 RR. For MN4 -Por-COFs, among the ten 3d metals, M = Co/Cr stands out in catalyzing CO2 RR to CO or HCOOH; hence, N-confused Por-COFs with Co/CrN3 C1 and Co/CrN2 C2 centers are designed. Calculations indicate CoNx Cy -Por-COFs exhibit lower limiting potential (-0.76 and -0.60 V) for CO2 -to-CO reduction than its parent CoN4 -Por-COFs (-0.89 V) and make it feasible to yield deep-reduction degree C1 products CH3 OH and CH4 . Electronic structure analysis reveals that substituting CoN4 to CoN3 C1 /CoN2 C2 increases the electron density on Co-atom and raises the d-band center, thus stabilizing the key intermediates of the potential determining step and lowering the limiting potential. For similar reason, changing the core from CrN4 to CrN3 C1 /CrN2 C2 lowers the limiting potential for CO2 -to-HCOOH reduction. This work predicts N-confused Co/CrNx Cy -Por-COFs to be high-performance CO2 RR catalyst candidates. Inspiringly, as a proof-of-concept study, it provides an alternative strategy for coordination regulation and theoretical guidelines for rational design of catalysts.

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.

Metal Complexes of Porphyrinoids Containing Nonpyrrolic Heterocycles

The replacement of one or more pyrrolic building block(s) of a porphyrin by a nonpyrrolic heterocycle leads to the formation of so-called pyrrole-modified porphyrins (PMPs), porphyrinoids of broad structural variability. The wide range of coordination environments (type, number, charge, and architecture of the donor atoms) that the pyrrole-modified frameworks provide to the central metal ions, the frequent presence of donor atoms at their periphery, and their often observed nonplanarity or conformational flexibility distinguish the complexes of the PMPs clearly from those of the traditional square-planar, dianionic, N₄-coordinating (hydro)porphyrins. Their different coordination properties suggest their utilization in areas beyond which regular metalloporphyrins are suitable. Following a general introduction to the synthetic methodologies available to generate pyrrole-modified porphyrins, their general structure, history, coordination chemistry, and optical properties, this Review highlights the chemical, electronic (optical), and structural differences of specific classes of metalloporphyrinoids containing nonpyrrolic heterocycles. The focus is on macrocycles with similar "tetrapyrrolic" architectures as porphyrins, thusly excluding the majority of expanded porphyrins. We highlight the relevance and application of these metal complexes in biological and technical fields as chemosensors, catalysts, photochemotherapeutics, or imaging agents. This Review provides an introduction to the field of metallo-PMPs as well as a comprehensive snapshot of the current state of the art of their synthesis, structures, and properties. It also aims to provide encouragement for the further study of these intriguing and structurally versatile metalloporphyrinoids.

Creation from Confusion and Fusion in the Porphyrin World─The Last Three Decades of N-Confused Porphyrinoid Chemistry

Confusion is a novel concept of isomerism in porphyrin chemistry, delivering a steady stream of new chemistry since the discovery of N-confused porphyrin, a porphyrin mutant, in 1994. These days, the number of confused porphyrinoids is increasing, and confusion and associated fusion are found in various fields such as supramolecular chemistry, materials chemistry, biological chemistry, and catalysts. In this review, the birth and growth of confused porphyrinoids in the last three decades are described.

Reactions of 2-aza-21-carbaporphyrin with aniline derivatives

Post-synthetic modification of N-confused porphyrin (NCP) yields chiral 21-arylazo- or 3,21-bis(arylamino)-NCP in one-step, one-pot reactions of NCP with anilines.

Functional Calixphyrins: Synthetic Strategies and Applications

Calixphyrins are hybrid macrocycles that contain both sp²- and sp³-hybridized carbon atoms and hence bear analogy to both porphyrins and calixpyrroles. Due to the presence of sp³-hybridized carbon atoms, π-conjugation is disrupted in calixphyrins, which leads to conformational flexibility. Hence, these molecules find a use in anion binding, host-guest chemistry and metal-coordination chemistry. During the last decade, studies on calixphyrins have been a topic of wide interest to researchers. Various functionalities have been introduced to the structure of calixphyrins, leading to the development of expanded calixphyrins, core-modified calixphyrins and N-confused calixphyrins with diverse applications. This review outlines the detailed historical origin of the synthesis of calixphyrins with emphasis on current research and development in this area. The modular syntheses of normal calixphyrins, expanded calixphyrins, core-modified calixphyrins and N-confused calixphyrins are also discussed, along with their applicative aspects.

A mechanistic study of nitrite reduction on iron(ii) complexes of methylated N-confused porphyrins

Proton delivery to the prosthetic group is a crucial step to sustain the activity of nitrite reductase. An iron N-confused porphyrin (NCP) complex, which is capable of relaying protons from the outer pyrrolic nitrogen (N_out-H) of the inverted pyrrole ring to the axial coordinated ligand, has been demonstrated to facilitate facile nitrite reduction. Time-dependent FTIR studies on the reaction between [Fe^II(HCTPPMe)Br] (1) and a nitrite anion revealed a two-step process involving conversion of the starting complex 1 to an {Fe(NO)}⁷ intermediate, [Fe(CTPPMe)(NO)] (5), before the detection of [Fe(CTPPCH₂)(NO)] (3), an {Fe(NO)}⁶ end product. Moreover, spectroscopic data confirm that N_out-H on the NCP core is indispensable to the proceeding of the nitrite reduction reaction. Mass spectra have detected the coordination of a nitrite to the iron center while DFT theoretical calculations suggest that subsequent intramolecular proton transfer to a nitro group to form [Fe(CTPPMe)(HNO₂)] (6a) evokes a homolytic N-OH bond fission on axial nitrous acid due to an enhanced π-back-bonding to produce an {Fe(NO)}⁷ intermediate and to release a hydroxyl radical. The subsequent oxidation of an {Fe(NO)}⁷ intermediate by the hydroxyl radical gave the final product, {Fe(NO)}⁶ [Fe(CTPPCH₂)(NO)] (3). This study illustrates a proton assisted small molecule activation on the iron N-confused porphyrin coordination sphere and provides complemental insights into the mechanism of enzymatic nitrite reduction reactions.

N-Confused porphyrins: complexation and 1H NMR studies

The complexation of 2-aza-21-carba-tetraphenylporphyrin and 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-carbaporphyrin with nickel and zinc acetates in organic solvents has been investigated by UV-Vis spectroscopy and ¹H NMR.

N-confusedmeso-tetraaryl-substituted free-base porphyrins: determination of protonation and deprotonation constants in nonaqueous media

The protonation and deprotonation reactions for a series N-confused meso-tetraaryl-substituted free-base porphyrins containing electron-donating or electron-withdrawing substituents was monitored in CHCl3and DMF by UV-visible spectroscopy during titrations with trifluoroacetic acid or tetra-n-butylammonium hydroxide. The spectroscopic data was also used to calculate equilibrium constants for these reactions. The examined compounds are represented as (XPh)4NCPH2, where "NCP" represents the N-confused porphyrin π-conjugated macrocycle and X is a CH3O , CH3, H or Cl para-substituent on the four meso-phenyl rings (Ph) of the compound. The porphyrins can exist in two tautomeric forms depending upon the solvent and each tautomer undergoes two stepwise protonation reactions leading to formation of the mono- and bis-protonated porphyrins, [(XPh)4NCPH3]+and [(XPh)4NCPH4]2+. A single step deprotonation is observed for the same compounds in DMF and the product is assigned as [(XPh)4NCPH]-. Comparisons are made between UV-visible spectra of the protonated, neutral and deprotonated forms of the porphyrin and the effect of the porphyrin ring substituents and tautomeric form of the neutral porphyrin on the UV-visible spectra and protonation constants is discussed along with data from DFT calculations.

Crystallographic evidence for structural diversity in cationic (tripyrrinato)cobalt(II) complexes

Cobalt(II) complexes of tripyrrin ligands can be transformed into cations by anion exchange using NaBAr F , a salt of a weakly coordinating anion. The complete characterization of these highly sensitive cations (probably solvate complexes) could not be achieved, but the coordination to donor ligands like nitriles or trialkylphosphanes stabilizes the cations sufficiently for the isolation of some single crystals. As the structural analyses of these donor-stabilized cobalt(II) tripyrrins reveal, either four- or five coordinate structures are formed depending on the type and size of ligand used.

N-confused porphyrins as new scaffolds for supramolecular architecture

The peripheral nitrogen of N -confused porphyrins ( NCP ) can serve as a hydrogen bonding donor and acceptor, and a metal coordination site as well. NCP forms versatile dimers with the assistance of metal ions such as Pd II, Pt II, Zn II, Cd II, Hg II, Mn II and Fe II. The square-planar divalent metal complexes of C 6 F 5-substituted NCP with Cu II, Ni II and Pd II act as efficient anion binding receptors. The doubly N -confused porphyrins ( N 2 CP ), possessing both N and NH at the periphery, form self-assembled 1-D hydrogen bonding networks, whose orientations differ in cis (zigzag) and trans (straight) isomers.

[40]Nonaphyrin(1.1.1.1.1.1.1.1.1) and Its Heterometallic Complexes with Palladium–Carbon Bonds

meso-Pentafluorophenyl- substituted [40]nonaphyrin(1.1.1.1.1.1.1.1.1) 3 has been prepared by using a stepwise ring-size-selective synthesis, and has been reduced with NaBH(4) to [42]nonaphyrin(1.1.1.1.1.1.1.1.1) 5. Structurally, 3 is characterized by a figure-of-eight shape, consisting of a porphyrin-like tetrapyrrolic segment and a hexaphyrin-like hexapyrrolic segment, whereas 5 has been found to adopt a distorted nonplanar butterfly-like shape. In the mono-metal complexes 6 and 7, a Zn(II) or Cu(II) ion is bound by the porphyrin-like tetrapyrrolic segment, maintaining the overall structure of 3. Similarly to 3, complexes 6 and 7 are interconvertible with the corresponding complexes 9 and 10 through two-electron reduction with NaBH(4) and oxidation with DDQ. The metal-free hexaphyrin-like segments of 6 and 7 have been shown to serve as a suitable platform for the complexation of two palladium ions, providing hetero-trinuclear metal complexes 11 (Zn(II)-Pd(II)-Pd(II)) and 13 (Cu(II)-Pd(II)-Pd(II)) in high yields, in which the Zn or Cu ion resides at the same porphyrin-like segment, and one Pd ion is bound in an NNCC fashion through double C--H bond activation while the other is bound in an NNC fashion with single C--H bond activation. Multi-metal complexes 11, 12, and 13 exhibit small electrochemical HOMO-LUMO gaps (<0.6 eV), despite their nonplanar conformations.

Synthesis, Structure, and Redox Properties of N-Confused Bis(porphyrinatonickel(II)) Linked by o-Xylene

In a reaction of 5,10,15,20-tetraaryl-2-aza-21-carbaporphyrinato nickel(II) 2 with alpha,alpha'-dibromo-o-xylene, three different complexes containing a xylene moiety were obtained in the presence of a proton scavenger. The products were characterized by mass spectrometry, UV-vis, NMR, and, in the case of the dimeric complex 3, X-ray crystallographic analysis (space group P, a =16.455(3) A, b = 16.776(3) A, c = 18.400(4) A, alpha = 77.43(3) degrees , beta = 75.31(3) degrees , gamma = 66.20(3) degrees , V = 4457.1(19) A3, Z = 2). The monomeric species, diamagnetic 4 and paramagnetic 5, contain one and two bromoxylene residues, respectively, while in 3, the xylene bridge links two macrocyclic subunits, involving their internal carbons (C21) coordinated to diamagnetic nickel(II). Cyclovoltammetric measurements for o-xylene-linked bis(carbaporphyrinoid) 3 indicate cooperative effects resulting from an interaction between the subunits despite the isolation of their aromatic pi-bond systems. An EPR-controlled titration of 3 with bromine allows consecutive detection of the mono- and bis-oxidized species (3Br, 3Br2). The spectral patterns and spin-Hamiltonian parameters indicate metal-centered oxidation in 3Br (gx = 2.358, gy = 2.150, gz = 2.062, A(Br)x = 15, A(Br)y = 35, A(Br)z = 129 G) and interaction of two electron spins of nickel(III) ions in 3Br2 (gx = 2.328, gy = 2.195, gz = 2.065, D = 0.0173 cm(-)1, E = -0.0018 cm(-1), A(Br)z = 63 G). A product of the chemical reduction of a protonated form of the dimer was also detected by means of EPR (g1 = 2.298, g2 = 2.218, g3 = 2.192), although no interaction between the nickel(I) centers can be observed for the reduced species.

Monomeric and polymeric copper and zinc tripyrrins

Neutral transition metal complexes of different alpha,omega-dimethyltripyrrins TrpyMX with M = Cu(II) and Zn(II) have been prepared with a variety of anionic halogeno and pseudohalogeno ligands X, and have been studied with respect to coordination modes and structural distortion. Only four- and five-coordinate species have been observed throughout the series. All four-coordinate species display unstrained, but distorted tetrahedral or strained and distorted square-planar coordination environments for zinc(II) and copper(II) species, respectively, thus following the expectations from simple ligand field arguments. Five-coordinate species do not form easily and were observed either in donor solvents or in the solid as 1D coordination polymers with distorted trigonal-bipyramidal coordination and different topologies.

Cyanide-Promoted Demetalation of TrpyNiNCO: A Route to Sensitive Metallotripyrrins of CoII, FeII, and MnII

Etheral solutions of free base tripyrrins (HTrpy) were prepared by treatment of nickel isocyanate complexes (TrpyNiNCO) with excess cyanide. From these solutions sensitive metallotripyrrins with cobalt(II), iron(II), and manganese(II) ions (TrpyMX) and with a choice of external ligands X could be obtained in pure, crystalline form. Four cobalt and one iron chelate were characterized by X-ray crystallography. Tetracoordinate cobalt(II) species with X = I, NCO, and NCS displayed unstrained tetrahedral coordination geometries, whereas the pentacoordinate TrpyCoNO3 with the O,O-nitrato ligand narrows a trigonal bipyramidal coordination. TrpyFeNCO undergoes a redox-transformation to (TrpyFeNCO)2O upon crystallization and was structurally characterized as this with an almost linear Fe-O-Fe subunit. Donor association was studied by UV-vis spectroscopy employing different solvents and showed that TrpyMnX and TrpyFeX species are very prone to the formation of pentacoordinate species, whereas TrpyCoX compounds have an intermediate tendency to do so. Nevertheless, complex fragments of all three metal ions form 1D coordination polymers with dicyanamido ligands, which were investigated by means of IR and SQUID measurements.

Iron Complexes of C- and N-Methylated 2-Aza-21-carbaporphyrin: NMR Studies

Insertion of iron(II) into methylated derivatives of N-confused porphyrins 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-carbaporphyrin (MeCTPPH)H, 2-aza-5,10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (CTPPMe)H2, and 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (MeCTPPMe)H yielded N- or C-methylated high-spin iron(II) complexes (MeCTPPH)Fe(II)Br, (HCTPPMe)Fe(II)Br, and (MeCTPPMe)Fe(II)Br. One electron oxidation of (Me-CTPPH)Fe(II)Br using Br2, accompanied by deprotonation of a C(21)-H(21) fragment and formation of an Fe-C(21) bond, produces an intermediate-spin, five-coordinate iron(III) complex (MeCTPP)Fe(III)Br. Simultaneously, a high-spin complex [(MeCTPPH)Fe(III)Br]+ was formed which preserved the side-on interaction between the metal ion and the inverted pyrrole ring. &[(MeCTPPH)Fe(III)Br]+ was also obtained by titration of (MeCTPP)FeIIIBr with TFA due to the C(21) protonation. A titration of (HCTPPMe)Fe(II)Br and (MeCTPPMe)Fe(II)Br with Br2 yielded solely corresponding high-spin iron(III) species [(HCTPPMe)Fe(III)Br+ and [(MeCTPPMe)Fe(III)Br+. Dioxygen reacts cleanly with (MeCTPPH)Fe(II)Br carbaporphyrin to form solely (MeCTPP)Fe(III)Br. The 1H NMR spectra of paramagnetic iron(II) and iron(III) complexes were examined. The characteristic patterns of pyrrole, C-methyl, and N-methyl resonances were found diagnostic of the ground electronic state of iron and the coordinating nature of the N-confused pyrrole. The characteristic C-Me resonances occur in a unique window (520-420 ppm) for iron(III) C-methylated N-confused porphyrins which remains in contrast with relatively small values found for iron(II) C-methylated derivatives (50-80 ppm).

Iron Complexes of N-Confused Pyriporphyrin: NMR Studies

Insertion of iron(II) into 6,11,16,21-tetraaryl-3-aza-m-benziporphyrin (N-confused pyriporphyrin, (PyPH)H) yielded the high-spin iron(II) complex, (PyPH)Fe(II)Br. The coordination of iron(II) to the perimeter nitrogen atom of (PyPH)Fe(II)Br resulted in the formation of the diiron species. Oxidation and oxygenation of (PyPH)Fe(II)Br were followed by 1H NMR spectroscopy. The addition of Br2 to the solution of (PyPH)Fe(II)Br in the absence of dioxygen results in a one-electron oxidation yielding the high-spin iron(III) N-confused pyriporphyrin [(PyPH)Fe(III)Br]+ which preserves the side-on interaction between the inverted pyridine ring and metal ion. The reaction of (PyPH)Fe(II)Br with dioxygen ends up with the formation of a five-coordinate species (PyPO)Fe(III)Br] ((PyPOH)H = 3-aza-22-hydroxy-m-benziporphyrin, PyPO = the corresponding dianion) which is formed by oxygenation at the C(22) position. Coordination of a metal ion by 3-aza-22-hydroxy-benziporphyrin imposes a steric constraint on the geometry of the ligand. The halide ligand of (PyPO)Fe(III)Br coordinates on one of the two inequivalent faces of the macrocycle, leading to two distinct species: syn and anti. The (1)H NMR spectra of paramagnetic iron(II) and iron(III) N-confused pyriporphyrin complexes have been examined. The characteristic patterns of pyrrole and pyridine resonances have been found to be diagnostic of the ground electronic state of iron and the donor nature of the C(22)H and N(3) centers. The enormous downfield H(22) paramagnetic shift, determined for the iron(II) N-confused pyriporphyrin, provides a distinct resonance in a peculiar spectroscopic window (350-800 ppm) for a series of axial ligands which can be considered as a diagnostic sign of an agostic Fe(II)...{C(22)-H} interaction. Coordination of the pyridine moiety via the perimeter N(3) atom is reflected unambiguously by the H(2/4) resonance at 201 ppm.

The metal complexes of N-confused porphyrin as heme model compounds

Recently, metal complexes of the isomers and analogs of porphyrin have become important model compounds for heme enzymes and proteins. While the chemistry of metalloporphyrins as heme models still attracts attention, the isomers and analogs of porphyrins provide insight into the biological choice of porphine as the macrocycle of choice and also help model reactive intermediates, such as high valent oxidation states. In this mini-review, we discuss the heme-relevant chemistry of N-confused porphyrin, an isomer of porphyrin with an inverted pyrrole ring, and focus on the chemistry of manganese, iron, and cobalt. The metallation chemistry of this macrocycle is more diverse than normal porphyrin, and involves tautomerization, C-H bond activation, the Lewis basicity of the external nitrogen, and issues with nucleophilic sensitivity. Despite the challenges posed by N-confused porphyrin, significant progress has been made toward generating heme-model complexes with this macrocycle.

Molecular assembling using axial phenolate on an iron N-confused porphyrin complex

The reaction of sodium phenolate and Fe(HCTPPH)Br assembles a dimeric iron(III) complex with a channel-like solid state packing; the intramolecular iron-to-iron distance of 8.864 A is the longest among the available N-confused porphyrin dimeric complexes.

The synthesis of isostructural Mo2+ porphyrin and N-confused porphyrin complexes

We have synthesized the first early transition metal N-confused porphyrin complex Mo(NCTPP)(pip)2; this species is isostructural to its normal porphyrin analog Mo(TPP)(pip)2 but exhibits significant electronic differences arising from the inversion of a single pyrrolic group.

Synthesis and crystal structure of the first lanthanide complex of N-confused porphyrin with an η2agostic C–H interaction

The reaction of Ln[N(SiMe3)2]3.[LiCl(THF)3](x)(Ln = Yb and Er) with N-confused tetraphenylporphyrin (H2NCTPP) followed by Na(L(OMe))(L(OMe)=(eta5-C5H5)Co[P(=O)(OMe)2]) gives (NCTPP)Ln(L(OMe)), whose X-ray structures exhibit an eta2 agostic interaction between the metal centre and the inner C-H bond of the NCTPP ligand.

Reactivity of Iron(II) 5,10,15,20-Tetraaryl-21-oxaporphyrin with Arylmagnesium Bromide: Formation of Paramagnetic Six-Coordinate Complexes with Two Axial Aryl Groups

Coordination of sigma-aryl carbanions by chloroiron(II) 5,20-ditolyl-10,15-diphenyl-21-oxaporphyrin (ODTDPP)Fe(II)Cl has been followed by (1)H NMR spectroscopy. Addition of pentafluorophenyl Grignard reagent (C(6)F(5))MgBr to the toluene solution of (ODTDPP)Fe(II)Cl in the absence of dioxygen at 205 K resulted in the formation of the high-spin (ODTDPP)Fe(II)(C(6)F(5)). The titration of (ODTDPP)Fe(II)Cl with a solution of (C(6)H(5))MgBr carried at 205 K yields a rare six-coordinate species which binds two sigma-aryl ligands [(ODTDPP)Fe(II)(C(6)H(5))(2)](-). Warming of the [(ODTDPP)Fe(II)(C(6)H(5))(2)](-) solution above 270 K results in the decomposition to mono-sigma-phenyliron species (ODTDPP)Fe(II)(C(6)H(5)). Controlled oxidation of [(ODTDPP)Fe(II)(C(6)H(5))(2)](-) with Br(2) affords (ODTDPP)Fe(III)(C(6)H(5))Br, which demonstrates a typical (1)H NMR pattern of low-spin sigma-aryl iron(III) porphyrin. The considered oxidation mechanism involves the (ODTDPP)Fe(III)(C(6)H(5))(2) species, which is readily reduced to the iron(I) 21-oxaporphyrin, followed by oxidation with Br(2) and replacement of one bromide anion by aryl substituent. The (1)H NMR spectra of paramagnetic iron complexes have been examined in detail. Functional group assignments have been made with the use of selective deuteration. The peculiar (1)H NMR spectral features of [(ODTDPP)Fe(II)(p-CH(3)C(6)H(4))(2)](-) (sigma-p-tolyl: ortho, 30.8; meta, 53.6; para-CH(3), 42.1; furan: -16.0; beta-H pyrrole: -27.5, -34.3, -41.8 ppm, at 205 K) are without a parallel to any iron(II) porphyrin or heteroporphyrin and indicate a profound alteration of the electronic structure of iron(II) porphyrin upon the coordination of two sigma-aryls.

Low-coordinate iron(II) amido complexes of beta-diketiminates: synthesis, structure, and reactivity

The synthesis, structure, and reactivity of a series of low-coordinate Fe(II) diketiminate amido complexes are presented. Complexes L(R)FeNHAr (R = methyl, tert-butyl; Ar = para-tolyl, 2,6-xylyl, and 2,6-diisopropylphenyl) bind Lewis bases to give trigonal pyramidal and trigonal bipyramidal adducts. In the adducts, crystallographic and (1)H NMR evidence supports the existence of agostic interactions in solid and solution states. Complexes L(R)FeNHAr may be oxidized using AgOTf, and the products L(R)Fe(NHAr)(OTf) are characterized with (19)F NMR spectroscopy, UV/vis spectrophotometry, solution magnetic measurements, elemental analysis, and, in one case, X-ray crystallography. In the structures of the iron(III) complexes L(R)Fe(NHAr)(OTf) and L(R)Fe(OtBu)(OTf), the angles at nitrogen and oxygen result from steric effects and not pi-bonding. The reactions of the amido group of L(R)FeNHAr with weak acids (HCCPh and HOtBu) are consistent with a basic nitrogen atom, because the amido group is protonated by terminal alkynes and alcohols to give free H(2)NAr and three-coordinate acetylide and alkoxide complexes. The trends in complex stability give insight into the relative strength of bonds from three-coordinate iron to anionic C-, N-, and O-donor ligands.

Iron and Copper Complexes of Tetraphenyl-m-benziporphyrin: Reactivity of the Internal C−H Bond

Iron and copper complexes of tetraphenyl-m-benziporphyrin (TPmBPH)H have been prepared and structurally characterized. The iron system, (TPmBPH)Fe(II)Br, contains a high-spin Fe(II) center. In the solid state the complex forms dimeric units linked by weak CH.Br hydrogen bonds. The Cu complex contains a tetrameric copper cluster with a Cu(2)Cl(4)(2)(-) unit bridging two [(TPmBPCl)Cu(II)](+) fragments. The formation of (TPmBPCl)H represents an example of copper-catalyzed chlorination on the internal carbon atom of (TPmBPH)H.

Synthesis, structure, theoretical studies, and Ligand exchange reactions of monomeric, T-shaped arylpalladium(II) halide complexes with an additional, weak agostic interaction

A series of monomeric arylpalladium(II) complexes LPd(Ph)X (L = 1-AdPtBu2, PtBu3, or Ph5FcPtBu2 (Q-phos); X = Br, I, OTf) containing a single phosphine ligand have been prepared. Oxidative addition of aryl bromide or aryl iodide to bis-ligated palladium(0) complexes of bulky, trialkylphosphines or to Pd(dba)2 (dba = dibenzylidene acetone) in the presence of 1 equiv of phosphine produced the corresponding arylpalladium(II) complexes in good yields. In contrast, oxidative addition of phenyl chloride to the bis-ligated palladium(0) complexes did not produce arylpalladium(II) complexes. The oxidative addition of phenyl triflate to PdL2 (L = 1-AdPtBu2, PtBu3, or Q-phos) also did not form arylpalladium(II) complexes. The reaction of silver triflate with (1-AdPtBu2)Pd(Ph)Br furnished the corresponding arylpalladium(II) triflate in good yield. The oxidative addition of phenyl bromide and iodide to Pd(Q-phos)2 was faster than oxidative addition to Pd(1-AdPtBu2)2 or Pd(PtBu3)2. Several of the arylpalladium complexes were characterized by X-ray diffraction. All of the arylpalladium(II) complexes are T-shaped monomers. The phenyl ligand, which has the largest trans influence, is located trans to the open coordination site. The complexes appear to be stabilized by a weak agostic interaction of the metal with a ligand C-H bond positioned at the fourth-coordination site of the palladium center. The strength of the Pd.H bond, as assessed by tools of density functional theory, depended upon the donating properties of the ancillary ligands on palladium.