Electrophilic Activation of Osmium-Nitrido Corroles: The OsN Triple Bond as a π-Acceptor Metallaligand in a Heterobimetallic OsVIN-PtII Complex.

Presented herein is a first investigation of the chemical reactivity of osmium-nitrido corroles, which are known for their unusual thermal, chemical, and photochemical stability. Elemental chlorine perchlorinates the ß-positions of the triarylcorrole but leaves the OsN unit untouched. The OsN unit is also unaffected by a variety of other electrophilic and nucleophilic reagents. Upon photolysis, however, the anion of Zeise's salt associates with the nitrido ligand to generate an OsVI≡N-PtII complex. The very short OsN-Pt linkage [1.895(9)-1.917(8) Å] and the downfield 195Pt NMR resonance (-2702 ppm) suggest that the OsN corrole acts as a π-accepting ligand toward the Pt(II) center. This finding represents a rare example of the successful photochemical activation of a metal-ligand multiple bond that is too kinetically inert to exhibit any appreciable reactivity under thermal conditions.

Electronic Structure of Manganese Corroles Revisited: X-ray Structures, Optical and X-ray Absorption Spectroscopies, and Electrochemistry as Probes of Ligand Noninnocence.

Presented herein is a detailed multitechnique investigation of ligand noninnocence in S = 3/2 manganese corrole derivatives at the formal MnIV oxidation state. The Soret maxima of Mn[T pXPC]Cl (T pXPC = meso-tris( p-X-phenyl)corrole, where X = CF3, H, Me, and OMe) were found to red-shift over a range of 37 nm with increasing electron-donating character of X. For Mn[T pXPC]Ph, in contrast, the complex Soret envelopes were found to be largely independent of X. These observations suggested a noninnocent corrole•2--like ligand for the MnCl complexes and an innocent corrole3- ligand for the MnPh complexes. Single-crystal X-ray structures of three Mn[T pXPC]Cl complexes revealed skeletal bond-length alternations indicative of a noninnocent corrole, while no such alternation was observed for Mn[T pOMePC]Ph. B3LYP density functional theory (DFT) calculations on Mn[TPC]Cl yielded strong spatial separation of the α and ß spin densities, consistent with an antiferromagnetically coupled MnIII-corrole•2- description. By comparison, relatively little spatial separation of the α and ß spin densities was found for Mn[TPC]Ph, consistent with an essentially MnIV-corrole3- description. X-ray absorption of near-edge spectroscopy (XANES) revealed a moderate blue shift of 0.6 eV for the Mn K-pre-edge of Mn[T pCF3PC]Ph and a striking enhancement of the pre-edge intensity, relative to Mn[T pCF3PC]Cl, consistent with a more oxidized, i.e., MnIV, center in Mn[T pCF3PC]Ph. Time-dependent DFT calculations indicated that the enhanced intensity of the Mn K-pre-edge of Mn[T pCF3PC]Ph results from the extra 3d z2 hole, which mixes strongly with the Mn 4p z orbital. Combined with similar results on Fe[TPC]Cl and Fe[TPC]Ph, the present study underscores the considerable potential of metal K-edge XANES in probing ligand noninnocence in first-row transition-metal corroles. Cyclic voltammetry measurements revealed highly negative first reduction potentials for the Mn[T pXPC]Ph series (∼-0.95 V) as well as large electrochemical HOMO-LUMO gaps of ∼1.7 V. The first reductions, however, are irreversible, suggesting cleavage of the Mn-Ph bond.

A New Family of 3d-4f Bis-Calix[4]arene-Supported Clusters.

Calix[4]arenes are versatile ligands that, whilst also serving other purposes, can act as platforms for the synthesis of a wide range of 3d, 4f, and 3d-4f polymetallic clusters. The empirical metal ion binding rules established for calix[4]arene are closely mirrored by bis-calix[4]arene, a relatively new ligand in which two equivalents of the former are directly tethered at a methylene bridge position. The direct tethering within bis-calix[4]arene gives rise to some structural features that are related to calix[4]arene coordination chemistry, but the prevailing clusters have fascinating new topologies and coordination behaviors. Here, we present the synthesis of a family of new bis-calix[4]arene-supported 3d-4f clusters, as well as their structural characterization and magnetic properties. Comparison is drawn with calix[4]arene coordination chemistry, showing logical extension of common structural fragments and cluster capping behaviors upon moving to bis-calix[4]arene. This approach therefore holds great potential for tuning cluster formation and composition at a high level through subsequent ligand alteration.

Stepwise Deoxygenation of Nitrite as a Route to Two Families of Ruthenium Corroles: Group 8 Periodic Trends and Relativistic Effects.

Given the many applications of ruthenium porphyrins, the rarity of ruthenium corroles and the underdeveloped state of their chemistry are clearly indicative of an area ripe for significant breakthroughs. The tendency of ruthenium corroles to form unreactive metal-metal-bonded dimers has been recognized as a key impediment in this area. Herein, by exposing free-base meso-tris(p-X-phenyl)corroles, H3[TpXPC] (X = CF3, H, Me, and OMe), and [Ru(COD)Cl2]x in refluxing 2-methoxyethanol to nitrite, we have been able to reliably intercept the series Ru[TpXPC](NO) in a matter of seconds to minutes and subsequently RuVI[TpXPC](N), the products of a second deoxygenation, over some 16 h. Two of the RuVIN complexes and one ruthenium corrole dimer could be crystallographically analyzed; the Ru-Nnitrido distance was found to be ∼1.61 Å, consistent with the triple-bonded character of the RuVIN units and essentially identical with the Os-Nnitrido distance in analogous osmium corroles. Spectroscopic and density functional theory (DFT) calculations suggest that the RuNO corroles are best viewed as innocent {RuNO}6 complexes, whereas the analogous FeNO corroles are noninnocent, i.e., best viewed as {FeNO}7-corrole•2-. Both RuVIN and OsVIN corroles exhibit sharp Soret bands, suggestive of an innocent macrocycle. A key difference between the two metals is that the Soret maxima of the OsVIN corroles are red-shifted some 25 nm relative to those of the RuVIN complexes. Careful time-dependent DFT studies indicate that this difference is largely attributable to relativistic effects in OsVIN corroles. The availability of two new classes of mononuclear ruthenium corroles potentially opens the door to new applications, in such areas as catalysis and cancer therapy.

Cobalt- and Rhodium-Corrole-Triphenylphosphine Complexes Revisited: The Question of a Noninnocent Corrole.

A reinvestigation of cobalt-corrole-triphenylphosphine complexes has yielded an unexpectedly subtle picture of their electronic structures. UV-vis absorption spectroscopy, skeletal bond length alternations observed in X-ray structures, and broken-symmetry DFT (B3LYP) calculations suggest partial CoII-corrole•2- character for these complexes. The same probes applied to the analogous rhodium corroles evince no evidence of a noninnocent corrole. X-ray absorption spectroscopic studies showed that the Co K rising edge of Co[TPC](PPh3) (TPC = triphenylcorrole) is red-shifted by ∼1.8 eV relative to the bona fide Co(III) complexes Co[TPC](py)2 and Co[TPP](py)Cl (TPP = tetraphenylporphyrin, py = pyridine), consistent with a partial CoII-corrole•2- description for Co[TPC](PPh3). Electrochemical measurements have shown that both the Co and Rh complexes undergo two reversible oxidations and one to two irreversible reductions. In particular, the first reduction of the Rh corroles occurs at significantly more negative potentials than that of the Co corroles, reflecting significantly higher stability of the Rh(III) state relative to Co(III). Together, the results presented herein suggest that cobalt-corrole-triphenylphosphine complexes are significantly noninnocent with moderate CoII-corrole•2- character, underscoring-yet again-the ubiquity of ligand noninnocence among first-row transition metal corroles.

Electronic Structure of Cobalt-Corrole-Pyridine Complexes: Noninnocent Five-Coordinate Co(II) Corrole-Radical States.

Two sets of complexes of Co-triarylcorrole-bispyridine complexes, Co[TpXPC](py)2 and Co[Br8TpXPC](py)2 have been synthesized, where TpXPC refers to a meso-tris(para-X-phenyl)corrole ligand with X = CF3, H, Me, and OMe and Br8TpXPC to the corresponding ß-octabrominated ligand. The axial pyridines in these complexes were found to be labile and, in dilute solutions in dichloromethane, the complexes dissociate almost completely to the five-coordinate monopyridine complexes. Upon addition of a small quantity of pyridine, the complexes revert back to the six-coordinate forms. These transformations are accompanied by dramatic changes in color and optical spectra. 1H NMR spectroscopy and X-ray crystallography have confirmed that the bispyridine complexes are authentic low-spin Co(III) species. Strong substituent effects on the Soret maxima and broken-symmetry DFT calculations, however, indicate a CoII-corrole•2- formulation for the five-coordinate Co[TpXPC](py) series. The calculations implicate a Co(dz2)-corrole("a2u") orbital interaction as responsible for the metal-ligand antiferromagnetic coupling that leads to the open-shell singlet ground state of these species. Furthermore, the calculations predict two low-energy S = 1 intermediate-spin Co(III) states, a scenario that we have been able to experimentally corroborate with temperature-dependent EPR studies. Our findings add to the growing body of evidence for noninnocent electronic structures among first-row transition metal corrole derivatives.

Tungsten Biscorroles: New Chiral Sandwich Compounds.

The oxidative metalation method, involving the interaction of free-base meso-triarylcorroles and W(CO)6 in refluxing decalin, led to a set of three tungsten(VI) biscorroles, the first homoleptic sandwich compounds involving corroles. Single-crystal X-ray structures of two of the complexes revealed square-antiprismatic coordination and strongly domed corroles with long W-N distances of 2.15-2.22 Šand a substantial displacement of ∼1.17 Šof the metal relative to the mean N4 planes of the ligands. The structures correspond to approximate C2 symmetry and are thus chiral. DFT calculations strongly indicate that the enantiomers are configurationally stable and hence amenable to chiral resolution. Their other notable properties include a strongly blueshifted Soret band at (357±2) nm, a relatively intense π→W(dz2 ) near-IR feature at (781±3) nm, and a low electrochemical HOMO-LUMO gap of approximately 1.3 V. The results obtained herein suggest that metallobiscorroles may emerge as a new class of inherently chiral chromophores with novel optical and electrochemical properties.

Metal-Ligand Misfits: Facile Access to Rhenium-Oxo Corroles by Oxidative Metalation.

With the exception of a single accidental synthesis, rhenium corroles are unknown, but of great interest as catalysts and potential radiopharmaceuticals. Oxidative metalation of meso-triarylcorroles with [Re2 (CO)10 ] in refluxing decalin has provided a facile and relatively high-yielding route to rhenium(V)-oxo corroles. The complexes synthesized could all be fully characterized by single-crystal X-ray structure analyses.

Bis-Calix[4]arenes: From Ligand Design to the Directed Assembly of a Metal-Organic Trigonal Antiprism.

Calix[4]arenes (C[4]s) are versatile platforms for the construction of polymetallic clusters containing paramagnetic metal ions. Synthetic modification at the C[4] methylene bridge allows for the design of bis-C[4]s that, depending on the linker employed, can be used to either dictate which clusters can be formed or direct the assembly of a new metal-organic polyhedron (MOP). The assembly resulting from the latter approach displays thermal stability and uptake of N2 or H2 gas, confirming that this is a viable route to the synthesis of new, functional supramolecular architectures.

Complexation of Lanthanides with Glutaroimide-dioxime: Binding Strength and Coordination Modes.

The complexation of lanthanides (Nd(3+) and Eu(3+)) with glutaroimide-dioxime (H2L), a cyclic imide dioxime ligand that has been found to form stable complexes with actinides (UO2(2+) and NpO2(+)) and transition metal ions (Fe(3+), Cu(2+), etc.), was studied by potentiometry, absorption spectrophotometry, luminescence spectroscopy, and microcalorimetry. Lanthanides form three successive complexes, M(HL)(2+), M(HL)L, and M(HL)2(+) (where M stands for Nd(3+)/Eu(3+) and HL(-) stands for the singly deprotonated ligand). The enthalpies of complexation, determined by microcalorimetry, show that the formation of these complexes is exothermic. The stability constants of Ln(3+)/H2L complexes are several orders of magnitude lower than that of the corresponding Fe(3+)/H2L complexes but are comparable with that of UO2(2+)/H2L complexes. A structure of Eu(3+)/H2L complex, identified by single-crystal X-ray diffractometry, shows that the ligand coordinates to Eu(3+) in a tridentate mode, via the two oxygen atoms of the oxime group and the nitrogen atom of the imide group. The relocation of protons of the oxime groups (-CH═N-OH) from the oxygen to the nitrogen atom, and the deprotonation of the imide group (-CH-NH-CH-) result in a conjugated system with delocalized electron density on the ligand (-O-N-C-N-C-N-O-) that forms strong complexes with the lanthanide ions.

"Ligands-with-Benefits": Naphthalene-Substituted Schiff Bases Yielding New Ni(II) Metal Clusters with Ferromagnetic and Emissive Properties and Undergoing Exciting Transformations.

The initial employment of the fluorescent bridging ligand N-naphthalidene-2-amino-5-chlorobenzoic acid (nacbH2) in metal cluster chemistry has led to new Ni12 (1) and Ni5 (2) clusters with wheel-like and molecular-chain topologies, respectively. The doubly-deprotonated nacb(2-) ligands were found to adopt four different coordination modes within 1 and 2. The nature of the ligand has also allowed unexpected organic transformations to occur and ferromagnetic and emission behaviors to emerge. The combined work demonstrates the ability of some "ligands-with-benefits" to yield beautiful structures with exciting topologies and interesting physicochemical properties.

Iron Complexes of Square Planar Tetradentate Polypyridyl-Type Ligands as Catalysts for Water Oxidation.

The tetradentate ligand, 2-(pyrid-2'-yl)-8-(1″,10″-phenanthrolin-2″-yl)-quinoline (ppq) embodies a quaterpyridine backbone but with the quinoline C8 providing an additional sp(2) center separating the two bipyridine-like subunits. Thus, the four pyridine rings of ppq present a neutral, square planar host that is well suited to first-row transition metals. When reacted with FeCl3, a µ-oxo-bridged dimer is formed having a water bound to an axial metal site. A similar metal-binding environment is presented by a bis-phenanthroline amine (dpa) which forms a 1:1 complex with FeCl3. Both structures are verified by X-ray analysis. While the Fe(III)(dpa) complex shows two reversible one-electron oxidation waves, the Fe(III)(ppq) complex shows a clear two-electron oxidation associated with the process H2O-Fe(III)Fe(III) → H2O-Fe(IV)Fe(IV) → O═Fe(V)Fe(III). Subsequent disproportionation to an Fe═O species is suggested. When the Fe(III)(ppq) complex is exposed to a large excess of the sacrificial electron-acceptor ceric ammonium nitrate at pH 1, copious amounts of oxygen are evolved immediately with a turnover frequency (TOF) = 7920 h(-1). Under the same conditions the mononuclear Fe(III)(dpa) complex also evolves oxygen with TOF = 842 h(-1).

Ligand Noninnocence in Coinage Metal Corroles: A Silver Knife-Edge.

A silver ß-octabromo-meso-triarylcorrole has been found to exhibit a strongly saddled geometry, providing the first instance of a strongly saddled corrole complex involving a metal other than copper. The Soret maxima of the Ag octabromocorroles also redshift markedly in response to increasingly electron-donating para substituents on the meso-aryl groups. In both these respects, the Ag octabromocorroles differ from simple Ag triarylcorrole derivatives, which exhibit only mild saddling and substituent-insensitive Soret maxima. These results have been rationalized in terms of an innocent M(III)-corrole(3-) description for the simple Ag corroles and a noninnocent M(II)-corrole(·2-) description for the Ag octabromocorroles. In contrast, all copper corroles are thought to be noninnocent, while all gold corroles are innocent. Uniquely among metallocorroles, silver corroles thus seem poised on a knife-edge, so to speak, between innocent and noninnocent electronic structures and may tip either way, depending on the exact nature of the corrole ligand.

Octaiodoporphyrin.

Interaction of 3,4-diiodopyrrole with 4-trifluoromethylbenzaldeyde under carefully optimized, Lindsey-type conditions at -10 °C has led to the synthesis of the first ß-octaiodoporphyrin, H2[I8TpCF3PP]. The free ligand readily yielded Ni, Cu, and Zn complexes, which all proved amenable to single-crystal X-ray structure analyses. The zinc complex Zn[I8TpCF3PP] exhibits the most saddled porphyrin core for any simple porphyrin known to date and a dramatically red-shifted optical spectrum with a Soret maximum at 495 nm.

Light-Driven Proton Reduction in Aqueous Medium Catalyzed by a Family of Cobalt Complexes with Tetradentate Polypyridine-Type Ligands.

A series of tetradentate 2,2':6',2″:6″,2‴-quaterpyridine-type ligands related to ppq (ppq = 8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2'-yl)quinoline) have been synthesized. One ligand replaces the 1,10-phenanthroline (phen) moiety of ppq with 2,2'-bipyridine and the other two ligands have a 3,3'-polymethylene subunit bridging the quinoline and pyridine. The structural result is that both the planarity and flexibility of the ligand are modified. Co(II) complexes are prepared and characterized by ultraviolet-visible light (UV-vis) and mass spectroscopy, cyclic voltammetry, and X-ray analysis. The light-driven H2-evolving activity of these Co complexes was evaluated under homogeneous aqueous conditions using [Ru(bpy)3](2+) as the photosensitizer, ascorbic acid as a sacrificial electron donor, and a blue light-emitting diode (LED) as the light source. At pH 4.5, all three complexes plus [Co(ppq)Cl2] showed the fastest rate, with the dimethylene-bridged system giving the highest turnover frequency (2125 h(-1)). Cyclic voltammograms showed a significant catalytic current for H2 production in both aqueous buffer and H2O/DMF medium. Combined experimental and theoretical study suggest a formal Co(II)-hydride species as a key intermediate that triggers H2 generation. Spin density analysis shows involvement of the tetradentate ligand in the redox sequence from the initial Co(II) state to the Co(II)-hydride intermediate. How the ligand scaffold influences the catalytic activity and stability of catalysts is discussed, in terms of the rigidity and differences in conjugation for this series of ligands.

A Convenient Synthetic Route to Partial-Cone p-Carboxylatocalix[4]arenes.

p-Carboxylatocalix[n]arenes have emerged as useful building blocks for the construction of a diverse range of supramolecular assemblies. A convenient route to a p-carboxylatocalix[4]arene that is locked in a partial-cone conformation is presented. The conformation gives the molecule markedly different topological directionality relative to those previously used in self- and metal-directed assembly studies.

Definitive molecular level characterization of defects in UiO-66 crystals.

The identification and characterization of defects, on the molecular level, in metal-organic frameworks (MOFs) remain a challenge. With the extensive use of single-crystal X-ray diffraction (SXRD), the missing linker defects in the zirconium-based MOF UiO-66, Zr6 O4 (OH)4 (C8 H4 O4 )6 , have been identified as water molecules coordinated directly to the zirconium centers. Charge balancing is achieved by hydroxide anions, which are hydrogen bonded within the pores of the framework. Furthermore, the precise nature of the defects and their concentration can be manipulated by altering the starting materials, synthesis conditions, and post-synthetic modifications.

Remarkable pressure responses of metal-organic frameworks: proton transfer and linker coiling in zinc alkyl gates.

Metal-organic frameworks demonstrate a wide variety of behavior in their response to pressure, which can be classified in a rather limited list of categories, including anomalous elastic behavior (e.g., negative linear compressibility, NLC), transitions between crystalline phases, and amorphization. Very few of these mechanisms involve bond rearrangement. Here, we report two novel piezo-mechanical responses of metal-organic frameworks, observed under moderate pressure in two materials of the zinc alkyl gate (ZAG) family. Both materials exhibit NLC at high pressure, due to a structural transition involving a reversible proton transfer between an included water molecule and the linker's phosphonate group. In addition, the 6-carbon alkyl chain of ZAG-6 exhibits a coiling transition under pressure. These phenomena are revealed by combining high-pressure single-crystal X-ray crystallography and quantum mechanical calculations. They represent novel pressure responses for metal-organic frameworks, and pressure-induced proton transfer is a very rare phenomenon in materials in general.

Oxidative metalation as a route to size-mismatched macrocyclic complexes: osmium corroles.

Heavy-element corroles are of great interest as optical sensors, near-IR dyes, phosphors, organic light-emitting diodes, and anticancer compounds. Insertion of 5d metals into corroles, however, is often a difficult and unpredictable process. Against this backdrop, oxidative metalation of meso triarylcorroles with [Os3 (CO)12 ]/NaN3 in refluxing 1:2 diethylene glycol monomethyl ether/glycol has provided a convenient and relatively high-yielding route to nitridoosmium(VI) corroles, three of which could be characterized with single-crystal X-ray structure analysis.

MOFs under pressure: the reversible compression of a single crystal.

The structural change and resilience of a single crystal of a metal-organic framework (MOF), Zn(HO(3)PC(4)H(8)PO(3)H)·2H(2)O (ZAG-4), was investigated under high pressures (0-9.9 GPa) using in situ single crystal X-ray diffraction. Although the unit cell volume decreases over 27%, the quality of the single crystal is retained and the unit cell parameters revert to their original values after pressure has been removed. This framework is considerably compressible with a bulk modulus calculated at ∼11.7 GPa. The b-axis also exhibits both positive and negative linear compressibility. Within the applied pressures investigated, there was no discernible failure or amorphization point for this compound. The alkyl chains in the structure provide a spring-like cushion to stabilize the compression of the system allowing for large distortions in the metal coordination environment, without destruction of the material. This intriguing observation only adds to the current speculation as to whether or not MOFs may find a role as a new class of piezofunctional solid-state materials for application as highly sensitive pressure sensors, shock absorbing materials, pressure switches, or smart body armor.