Synthesis and reactivity of tantalum corrole complexes

One Carbon Ring Expansion of Bipyrrole to Bipyridine Enables Access to a π-Extended, Non-innocent, Corrole-like Ligand

A π-extended, diaza-triphenylene embedded, mono-anionic corrole analogue and its Ni^II complex were synthesized from a diaza-triphenylene precursor, which was obtained from a double one-carbon insertion into a naphthobipyrrole diester. Following conversion to the corresponding activated diol and acid-catalyzed condensation with pyrrole, subsequent reaction with pentafluorobenzaldehyde afforded mono-anionic, π-extended bipyricorrole-like macrocycle. Attempted Ni^II insertion with Ni(OAc)₂  ⋅ 4H₂ O resulted an ESR active, Ni^II bipyricorrole radical complex, which was converted to a stable cationic Ni^II complex upon treatment with [(Et₃ O)⁺ (SbCl₆ )^- ]. Both complexes were characterized by ¹ H and ¹³ C NMR, UV/Vis spectroscopy and single crystal X-ray diffraction analysis. The Ni^II bipyricorrole radical complex is converted to a cationic Ni^II complex by single-electron reduction using cobaltocene. Both the cationic Ni^II complex and the radical Ni^II complex exhibited ligand-centered redox behavior, whereas the Ni^II remains in the +2 oxidation state.

Applications of Low-Valent Transition Metalates: Development of a Reactive Noncarbonyl Rhenium(I) Anion

Low-valent transition metalates─anionic, electronic-rich organometallic complexes─comprise a class of highly reactive chemical reagents that find integral applications in organic synthesis, small-molecule activation, transient species stabilization, and M-E bond formation, among others. The inherent reactivity of such electron-rich metal centers has necessitated the widespread use of strong backbonding ligands, particularly carbonyls, to aid in the isolation and handling of metalate reagents, albeit sometimes at the expense of partially masking their full reactivity. However, recent synthetic explorations into transition-metalate complexes devoid of archetypic back-bonding ligands have led to the discovery of highly reactive metalates capable of performing a variety of novel chemical transformations.Building on our group's long-standing interest in reactive organometallic species, a series of rational progressions in early-to-middle transition-metal chemistry ultimately led to our isolation of a rhenium(I) β-diketiminate cyclopentadienide metalate that displays exceptional reactivity. We have found this Re(I) metalate to be capable of small-molecule activation; notably, the complex reversibly binds dinitrogen in solution and can be utilized to trap N₂ for the synthesis of functionalized diazenido species. By employing isolobal analogues to N₂ (CO and RNC), we were able to thoroughly monitor the mechanism of activation and conclude that the metalate's sodium counterion plays an integral role in promoting dinitrogen activation through a novel side-on interaction. The Re(I) metalate is also used in forming a variety of M-E bonds, including a series of uncommon rhenium-tetrylene (Si, Ge, and Sn) complexes that display varying degrees of multiple bonding. These metal tetrylenes act to highlight deviations in chemical properties within the group 14 elements. Our metalate's utility also applies to metal-metal bond formation, as demonstrated through the synthesis of a heterotetrametallic rhenium-zinc dimer. In this reaction, the Re(I) metalate performs a dual role as a reductant and metalloligand to stabilize a transient Zn₂²⁺ core fragment. Finally, the metalate displays unique reactivity with uranium(III) to yield the first transition metal-actinide inverse-sandwich bonds, in this case with three rhenium fragments bound through their Cp moieties surrounding the uranium center. Notably, throughout these endeavors we demonstrate that the metalate displays reactivity at multiple locations, including directly at the rhenium metal center, at a Cp carbon, through a Cp-sandwich mode, or through reversibly bound dinitrogen.Overall, the rhenium(I) metalate described herein demonstrates utility in diverse applications: small-molecule activation, the stabilization of reduced and/or unstable species, and the formation of unconventional M-E/M-M bonds or heterometallic complexes. Moving forward, we suggest that the continued discovery of noncarbonyl, electron-rich transition-metal anions featuring new or unconventional ligands should produce additional reactive organometallic species capable of stabilizing unique structural motifs and performing novel and unusual chemical transformations.

Development of Peripheral Functionalization Chemistry of meso-Free Corroles

In contrast to the extensive development of meso functionalization of porphyrins, that of corroles has been only rarely explored until the development of practical synthetic methods of meso -free corroles in 2015. Ready availability of meso -free corroles opened up meso -functionalization chemistry of corroles, giving rise to successful synthesis of various meso-substituted corroles such as meso -halogen, meso -nitro, meso -amino, meso -oxo, and meso iminocorroles as well as meso-meso linked corrole dimers and corrole tapes. In some cases, 2NH corroles existed as stable or transient radical species. The impacts of meso -functionalization on the structures, electronic properties, optical characteristics, and aromaticity are highlighted in this Minireview.

Synthesis and molecular structure of perhalogenated rhenium-oxo corroles

As part of our efforts to develop rhenium-oxo corroles as photosensitizers for oxygen sensing and photodynamic therapy, we investigated the potential β-perhalogenation of five ReO meso-tris(para-X-phenyl)corroles, Re[TpXPC](O) (X = CF3, H, F, CH3, and OCH3), with elemental chlorine and bromine. With Cl2, β-octachlorinated products Re[Cl8TpXPC](O) were rapidly obtained for X = CF3, H, and CH3, but X = OCH3 resulted in overchlorination on the meso-aryl groups. Full β-octabromination proved slower relative to Cu and Ir corroles, but the desired Re[Br8TpXPC](O) products were finally obtained for X = H and F after a week at room temperature. For X = CH3 and OCH3, these conditions led to undecabrominated products Re[Br11TpXPC](O). Compared to the β-unsubstituted starting materials, the β-octahalogenated products were found to exhibit sharp 1H NMR signals at room temperature, indicating that the aryl groups are locked in place by the β-halogens, and substantially redshifted Soret and Q bands. Single-crystal X-ray structures of Re[Cl8TpCF3PC](O), Re[Cl8TpCH3PC](O), and Re[Br8TpFPC](O) revealed mild saddling for one Cl8 structure and the Br8 structure. These structural variations, however, appear too insignificant to explain the slowness of the β-octabromination protocols, which seems best attributed to the deactivating influence of the high-valent Re center.

Rhenium-Imido Corroles

Metallocorroles involving 5d transition metals are currently of interest as near-IR phosphors and as photosensitizers for oxygen sensing and photodynamic therapy. Their syntheses, however, are often bedeviled by capricious and low-yielding protocols. Against this backdrop, we describe rhenium-imido corroles, a new class of 5d metallocorroles, synthesized simply and in respectable (∼30%) yields via the interaction of a free-base corrole, Re₂(CO)₁₀, K₂CO₃, and aniline in 1,2,4-trichlorobenzene at ∼190 °C in a sealed vial under strict anaerobic conditions. The generality of the method was shown by the synthesis of six derivatives, including those derived from meso-tris(pentafluorophenyl)corrole, H₃[TPFPC], and five different meso-tris(p-X-phenyl)corroles, H₃[TpXPC], where X = CF₃, F, H, CH₃, OCH₃. Single-crystal X-ray structures obtained for two of the complexes, Re[TpFPC](NPh) and Re[TpCF₃PC](NPh), revealed relatively unstrained equatorial Re-N distances of ∼2.00 Å, a ∼ 0.7-Å displacement of the Re from the mean plane of the corrole nitrogens, and an Re-N_imido distance of ∼1.72 Å. Details of the corrole skeletal bond distances, diamagnetic ¹H NMR spectra, relatively substituent-independent Soret maxima, and electrochemical HOMO-LUMO gaps of ∼2.2 V all indicated an innocent corrole macrocycle. Surprisingly, unlike several other classes of 5d metallocorroles, the Re-imido complexes proved nonemissive in solution at room temperature and also failed to sensitize singlet oxygen formation, indicating rapid radiationless deactivation of the triplet state, presumably via the rapidly rotating axial phenyl group. By analogy with other metal-oxo and -imido corroles, we remain hopeful that the Re-imido group will prove amenable to further elaboration and thereby contribute to the development of a somewhat challenging area of coordination chemistry.

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 β-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 Os^VI≡N-Pt^II 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.

Modern applications of low-valent early transition metals in synthesis and catalysis

Low-valent early transition metals are often intrinsically highly reactive as a result of their strong propensity toward oxidation to more stable high-valent states. Harnessing these highly reducing complexes for productive reactivity is potentially powerful for C-C bond construction, organic reductions, small-molecule activation and many other reactions that offer orthogonal chemoselectivity and/or regioselectivity patterns to processes promoted by late transition metals. Recent years have seen many exciting new applications of low-valent metals through building new catalytic and/or multicomponent reaction manifolds out of classical reactivity patterns. In this Review, we survey new methods that employ early transition metals and invoke low-valent precursors or intermediates in order to identify common themes and strategies in synthesis and catalysis.

Insertion, protonolysis and photolysis reactivity of a thorium monoalkyl amidinate complex

The reactivity of the thorium monoalkyl complex Th(CH2SiMe3)(BIMA)3 [1, BIMA = MeC(NiPr)2] with various small molecules is described. While steric congestion prohibits the insertion of N,N'-diisopropylcarbodiimide into the Th-C bond in 1, the first thorium tetrakis(amidinate) complex, Th(BIMA)4 (2), is synthesized via an alternative salt metathesis route. Insertion of p-tolyl azide leads to the triazenido complex Th[(p-tolyl)NNN(CH2SiMe3)-κ2N1,2](BIMA)3 (3), which then undergoes thermal decomposition to the amido species Th[(p-tolyl)N(SiMe3)](BIMA)3 (4). The reaction of 1 with 2,6-dimethylphenylisocyanide results in the thorium iminoacyl complex Th[η2-(C[double bond, length as m-dash]N)-2,6-Me2-C6H3(CH2SiMe3)](BIMA)3 (5), while the reaction with isoelectronic CO leads to the products Th[OC([double bond, length as m-dash]CH2)SiMe3](BIMA)3 (6) and Th[OC(NiPr)C(CH2SiMe3)(C(Me)N(iPr))O-κ2O,O'](BIMA)2 (7), the latter being the result of CO coupling and insertion into an amidinate ligand. Protonolysis is achieved with several substrates, producing amido (9), aryloxide (10), phosphido (11a,b), acetylide (12), and cationic (13) complexes. Ligand exchange with 9-borabicyclo[3.3.1]nonane (9-BBN) results in formation of the thorium borohydride complex (BIMA)3Th(μ-H)2[B(C8H14)] (14). Complex 1 also reacts under photolytic conditions to eliminate SiMe4 and produce Th(BIMA)2(BIMA*) [15, BIMA* = (iPr)NC(CH2)N(iPr)], featuring a rare example of a dianionic amidinate ligand. Complexes 2, 3, 5, 6, 11a, and 12-15 were characterized by 1H and 13C{1H} NMR spectroscopy, FTIR, EA, melting point and X-ray crystallography. All other complexes were identified by one or more of these spectroscopic techniques.

Relativistic effects in metallocorroles: comparison of molybdenum and tungsten biscorroles

The homoleptic sandwich compounds - Mo and W biscorroles - have afforded a novel platform for experimental studies of relativistic effects. A 200 mV difference in reduction potential and a remarkable 130 nm shift of a near-IR spectral feature have been identified as manifestations of relativistic effects on the properties of these complexes.

Viking Helmet Corroles: Activating Inert Oxidometal Corroles

Chemically inert oxidometal(V) corrols of molybdenum and rhenium undergo clean ligand-exchange reactions upon the action of SiCl₄ . The resulting dichlorido complexes show trigonal prismatic coordination of the metal ion with the chlorine atoms residing in a cis configuration, and were studied by optical and resonance spectroscopy as well as DFT calculations. In situ reactivity studies with carbon nucleophiles indicate high reactivity for chlorine replacement. Treatment with sodium cyclopentadienide paves the way to robust molybdenum corrolocene half-sandwich complexes. These organometallic compounds are the first corrole species that stabilize an air-stable and diamagnetic low spin d² -Mo^IV center. Structural, spectroelectrochemical, and chemical investigations prove a reversible Mo^IV /Mo^V redox couple close to the Fc/Fc⁺ potential for these systems. The high stability of the compounds in both redox states calls for future applications in catalysis and as redox switch.