Selective synthesis and redox sequence of a heterobimetallic nickel/copper complex of the noninnocent Siamese-twin porphyrin

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.

H2 and carbon-heteroatom bond activation mediated by polarized heterobimetallic complexes

The field of heterobimetallic chemistry has rapidly expanded over the last decade. In addition to their interesting structural features, heterobimetallic structures have been found to facilitate a range of stoichiometric bond activations and catalytic processes. The accompanying review summarizes advances in this area since January of 2010. The review encompasses well-characterized heterobimetallic complexes, with a particular focus on mechanistic details surrounding their reactivity applications.

Stepwise assembly of heterobimetallic complexes: synthesis, structure, and physical properties

Bimetallic active sites are ubiquitous in metalloenzymes and have sparked investigations of synthetic models to aid in the establishment of structure-function relationship. We previously reported a series of discrete bimetallic complexes with [FeIII-(μ-OH)-MII] cores in which the ligand framework provides distinct binding sites for two metal centers. The formation of these complexes relied on a stepwise synthetic approach in which an FeIII-OH complex containing a sulfonamido tripodal ligand served as a synthon that promoted assembly. We have utilized this approach in the present study to produce a new series of bimetallic complexes with [FeIII-(μ-OH)-MII] cores (M = Ni, Cu, Zn) by using an ancillary ligand to the FeIII center that contains phosphinic amido groups. Assembly began with formation of an FeIII-OH that was subsequently used to bind the MII fragment that contained a triazacyclononane ligand. The series of bimetallic complexes were charactered structurally by X-ray diffraction methods, spectroscopically by absorption, vibrational, electron paramagnetic resonance spectroscopies, and electrochemically by cyclic voltammetry. A notable finding is that these new [FeIII-(μ-OH)-MII] complexes displayed significantly lower reduction potentials than their sulfonamido counterparts, which paves way for future studies on high valent bimetallic complexes in this scaffold.

Siamese-Twin Porphyrin Goes Platinum: Group 10 Monometallic, Homobimetallic, and Heterobimetallic Complexes

A series of Pt^II-based monometallic (H₂PtL), homobimetallic (Pt₂L), and heterobimetallic (NiPtL and PdPtL) group 10 complexes of the previously established expanded twin porphyrin (H₄L) were prepared. Structural characterization of the bimetallic Pt^II series (Pt₂L, NiPtL, and PdPtL) revealed their similar general structures, with slight differences correlated to the ion size. An improvement of the metal-ion insertion process also allowed efficient preparation of the known Pd₂L complex, and the novel heterobimetallic NiPdL complex was also structurally characterized. UV-vis spectroscopy, NMR spectroscopy, magnetic circular dichroism (MCD), and (spectro)electrochemistry were used to characterize the complexes; the electronic properties followed largely established lines for metal complexes of the twin porphyrin, except that the Pt^II-based systems exhibited more complex UV-vis spectral signatures. MCD spectra accompanied by density functional theory (DFT)/time-dependent DFT computations (TDDFT) rationalize the origins of the optical features of the twin porphyrin. The presence of the nonplanar, nonaromatic macrocyclic π system with conjugation pathways confined to each half of the molecule could be visualized. Significant pyrazole(π) → pyrrole(π*) charge-transfer character was predicted for several transitions in the visible region. This study adds to our fundamental understanding of the formation, structure, and electronic structure of bimetallic complexes of this class of expanded metalloporphyrins containing nonpyrrolic moieties.

meso-Alkylidenyl dibenzihexaphyrins: synthesis and protonation studies

The synthesis and characterization of the alkylidenyl-dibenzihexaphyrins bearing four indanedionyl groups at themeso-positions linkedviafourmeso-exocyclic double bonds is reported.

Core-modified 48π and 42π decaphyrins: syntheses, properties and structures

Protonation triggered transition of a 48π nonaromatic decaphyrin to a 48π Hückel antiaromatic decaphyrin is reported: the flexibility of the macrocycle due to the presence of twelve meso carbon bridges facilitates such a transition through conformational change.

Modular bimetallic complexes with a sulfonamido-based ligand

A series of bimetallic complexes prepared with the ligands N,N,N',N'-tetramethylethane-1,2-diamine (TMEDA) and N,N',N''-[2,2',2''-nitrilotris(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido) ([MST]3-) is described. Four diiron compounds of the formulation (TMEDA)FeII(X)-(μ-OH)-FeIIIMST were prepared, in which the X- ligands are the anions OTf-, Br-, SCN-, or N3-. Additionally, two heterobimetallic compounds of the formulation (TMEDA)MII(OTf)-(μ-OH)-FeIIIMST (MII = CoII or NiII) were synthesized. All these compounds have similar spectroscopic and structural properties. The diiron compounds exhibit perpendicular-mode electron paramagnetic resonance spectra consistent with S = 1/2 spin ground states, which is expected for high-spin FeII and FeIII centres that are antiferromagnetically coupled. The heterobimetallic (TMEDA)NiII(OTf)-(μ-OH)-FeIIIMST complex had a spin state of S = 3/2 that also resulted from antiferromagnetic coupling between the high-spin NiII and FeIII centres. The modularity of this system is further demonstrated by the substitution of the TMEDA ligand with ethylenediamine (en); for this species two equivalents of en coordinate to the FeII centre to form [(en)2FeII-(μ-OH)-FeIIIMST]OTf. These results demonstrate that a modular bimetallic system has been developed in which the key components can be modified.

Choice of a spin singlet or triplet: electronic properties of Bis-Co(II), Bis-Ni(II), Bis-Cu(II) and Bis-Zn(II) oxygen doubly N-confused hexaphyrin (1.1.1.1.1.1)

Fused hexaphyrins have many physical and chemical properties and can coordinate transition metal ions. In this study, we investigated the geometric structure, charge decomposition analysis (CDA), spin density, frontier molecular orbital (FMO) compositions and absorption spectra of four oxygen doubly N-confused hexaphyrin (1.1.1.1.1.1) (ONCP) complexes with the metal ions Co(II), Ni(II), Cu(II) and Zn(II) (designated ONCP-d-Co, ONCP-d-Ni, ONCP-d-Cu and ONCP-d-Zn). Based on their energies, geometric structures, FMO characteristics and comparison to experiments, ONCP-d-Co and ONCP-d-Cu have the mix-states of the triplet state and broken-symmetry state (antiferromagnetic state) rather than the spin singlet of a closed shell as previously reported. Moreover, based on analyses of the spin density and spin population of the spin triplet ONCP-d-Co and ONCP-d-Cu complexes, the charge transfer in ONCP-d-Cu is greater than that in ONCP-d-Co because the spin density in ONCP-d-Cu is concentrated not only on the Cu ion but also on the ONCP ligand. Thus, the CDA value for ONCP-d-Cu is larger. Finally, through comparative analysis of the FMO compositions and absorption spectra, the complexes and ligand are shown to have very similar absorption spectra with characteristics that arise mainly from π → π* transitions both in the B-band and the Q-band, which is due to the FMO compositions being dominated by the highly delocalized conjugated system, rather than by the metal ions. The absorption maxima of the Q-band are ONCP-d-Co (1020 nm) > ONCP-d-Zn (1012 nm) > ONCP-d-Ni (997 nm) > ONCP-d-Cu (988 nm), which is inversely proportional to the energy gap in their FMOs. Graphical Abstract The present work investigates the geometric structure, charge decomposition analysis (CDA), spin density, frontier molecular orbital (FMO) compositions and absorption spectra of four oxygen doubly N-confused hexaphyrin (1.1.1.1.1.1) (ONCP) complexes with the metal ions Co(II), Ni(II), Cu(II) and Zn(II) (designated ONCP-d-Co, ONCP-d-Ni, ONCP-d-Cu and ONCP-d-Zn). Based on their energies, geometric structures, FMO characteristics and comparison to experiments, ONCP-d-Co and ONCP-d-Cu have the mix-state of the triplet state and broken-symmetry state (antiferromagnetic state) rather than the spin singlet of a close shell as were previously reported. Meanwhile, ONCP-d-Ni and ONCP-d-Zn show spin singlet structure. The calculated CDA shows the following order: ONCP-d-Cu (1.487) > ONCP-d-Ni (1.255) > ONCP-d-Co (1.211) > ONCP-d-Zn (1.201). Through comparisons of spin density and spin populations of ONCP-d-Co and ONCP-d-Cu, charge transfer between Cu and ligand ONCP is greater than that of Co and ONCP, which makes the CDA value of ONCP-d-Cu obviously larger than that of the other complexes.

Selective Synthesis of a Series of Isostructural MII CuI Heterobimetallic Complexes Spontaneously Assembled by an Unsymmetrical Naphthyridine-Based Ligand

Metal-metal cooperation is integral to the function of many enzymes and materials, and model complexes hold enormous potential for providing insights into the capabilities of analogous multimetallic cores. However, the selective synthesis of heterobimetallic complexes still presents a significant challenge, especially for systems that hold the metals in close proximity and feature open or reactive coordination sites for both metals. To address this issue, a rigid, naphthyridine-based dinucleating ligand featuring distinct binding environments was synthesized. This ligand enables the selective synthesis of a series of M^II Cu^I bimetallic complexes (M=Mn, Fe, Co, Ni, Cu, Zn), in which each metal center exclusively occupies its preferred binding pocket, from simple chloride salts. The precision of this selectivity is evident from cyclic voltammetry, ESI-MS and anomalous X-ray diffraction measurements.

Site preferences in hetero-metallic [Fe9-xNix] clusters: a combined crystallographic, spectroscopic and theoretical analysis

The reaction of mixtures of Fe(O₂CMe)₂·2H₂O and Ni(O₂CMe)₂·4H₂O of various compositions with di-2-pyridyl ketone (py₂CO, dpk) in MeCN under an inert atmosphere afforded a family of hetero-metallic enneanuclear clusters with general formula [Fe_9-xNi_x(μ₄-OH)₂(O₂CMe)₈(py₂CO₂)₄] (2, x = 1.00; 3: x = 6.02; 4, x = 7.46; 5, x = 7.81). Clusters 2-5 are isomorphous to the homo-metallic [Fe₉] cluster (1) previously reported by some of us, and also isostructural to the known homo-metallic [Ni₉] cluster. All four clusters contain a central M^II ion in an unusual 8-coordinate site and eight peripheral M^II ions in distorted octahedral environments. The distribution of Fe^II and Ni^II ions over these two distinct coordination sites in 2-5 can be established through a combination of X-ray fluorescence and Mössbauer spectroscopies, which show that Fe^II preferentially occupies the unique 8-coordinate metal site while Ni^II accumulates in the octahedral holes. Density functional theory indicates that the distribution of ions across the two sites arises not from any intrinsic preference of the Fe^II ions for the 8-coordinate sites, but rather because of the large ligand field stabilization energy available to Ni^II in octahedral coordination.

Reaching across the Divide: How Monometalation of One Binding Pocket Affects the Empty Binding Pocket in a Siamese-Twin Porphyrin Palladium Complex

Siamese-twin porphyrin is a pyrazole-containing expanded porphyrin incorporating two porphyrin-like binding pockets. The macrocycle, however, does not possess an aromatic π system but rather two separated conjugation pathways that are isolated by the pyrazole junctions. Mono- and bimetallic complexes of the Siamese-twin porphyrin are known. This work addresses in detail the electronic consequences that monometalation (with Pd^II) has on the electronic properties of the nonmetalated binding pocket by studying the solid-state structure, acid/base, and electrochemical properties of the monopalladium twin-porphyrin complex. Specifically, metalation leads to a switch of the protonation sites of the free-base pocket. The unusual location of the protons at adjacent pyrrolic nitrogen atoms was revealed using X-ray diffraction and 1D/2D NMR spectroscopy. The one-electron oxidation and reduction events are both ligand-centered, as derived by spectroelectrochemical and electron paramagnetic resonance measurements, but are located on different halves of the molecule. Single-electron oxidation (-0.32 V vs Fc/Fc⁺) generated an organic radical centered on the metal-coordinating side of the ligand, while single-electron reduction (-1.59 V vs Fc/Fc⁺) led to the formation of an organic radical on the free-base side of the macrocycle. Density functional theory calculations corroborated the redox chemistry observed. The possibility of selectively preparing the monometallic complexes carrying two distinct redox sites-a metal-containing oxidation site and a metal-free reduction site-further expands the potential of Siamese-twin porphyrins to serve as an adjustable platform for multielectron redox processes in chemical catalysis or molecular electronics applications.

Rapid Hydrogen and Oxygen Atom Transfer by a High-Valent Nickel-Oxygen Species

Terminal high-valent metal-oxygen species are key reaction intermediates in the catalytic cycle of both enzymes (e.g., oxygenases) and synthetic oxidation catalysts. While tremendous efforts have been directed toward the characterization of the biologically relevant terminal manganese-oxygen and iron-oxygen species, the corresponding analogues based on late-transition metals such as cobalt, nickel or copper are relatively scarce. This scarcity is in part related to the "Oxo Wall" concept, which predicts that late transition metals cannot support a terminal oxido ligand in a tetragonal environment. Here, the nickel(II) complex (1) of the tetradentate macrocyclic ligand bearing a 2,6-pyridinedicarboxamidate unit is shown to be an effective catalyst in the chlorination and oxidation of C-H bonds with sodium hypochlorite as terminal oxidant in the presence of acetic acid (AcOH). Insight into the active species responsible for the observed reactivity was gained through the study of the reaction of 1 with ClO^- at low temperature by UV-vis absorption, resonance Raman, EPR, ESI-MS, and XAS analyses. DFT calculations aided the assignment of the trapped chromophoric species (3) as a nickel-hypochlorite species. Despite the fact that the formal oxidation state of the nickel in 3 is +4, experimental and computational analysis indicate that 3 is best formulated as a Ni^III complex with one unpaired electron delocalized in the ligands surrounding the metal center. Most remarkably, 3 reacts rapidly with a range of substrates including those with strong aliphatic C-H bonds, indicating the direct involvement of 3 in the oxidation/chlorination reactions observed in the 1/ClO^-/AcOH catalytic system.