Alessandro Bismuto

Galactose oxidase (GO) has been widely investigated in terms of biological and model complex studies, due to the unique oxidation state of the Cu^II–phenoxyl radical. Although properties and reactivities of the Cu^II–phenoxyl radical have been clarified, details of the roles of the other structural factors in the active site of GO are still unclear. Recent progress of model studies on GO is reviewed, focusing on some new aspects of structural factors.

Abstract

The phenoxyl radical plays important roles in biological systems as cofactors in some metalloenzymes, such as galactose oxidase (GO) catalyzing oxidation of primary alcohols to give the corresponding aldehydes. Many metal(II)–phenoxyl radical complexes have hitherto been studied for understanding the detailed properties and reactivities of GO, and thus the nature of GO has gradually become clearer. However, the effects of the subtle geometric and electronic structural changes at the active site of GO, especially the structural change in the catalytic cycle and the effect of the second coordination sphere, have not been fully discussed yet. In this Review, we focus on further details of the model studies of GO and discuss the importance of the structural change at the active site of GO.

The power of F : A high‐valent nickel–fluoride complex oxidizes hydrocarbons via a hydrogen atom transfer mechanism while also facilitating oxidative fluorination. The fluoride complex demonstrates a 10³‐fold enhanced rate constant for hydrocarbon oxidation when compared to its chloride analogue, demonstrating that fluoride imbues a high driving force for oxidation.

Abstract

In the search for highly reactive oxidants we have identified high‐valent metal–fluorides as a potential potent oxidant. The high‐valent Ni–F complex [Ni^III(F)(L)] (2 , L=N ,N′‐(2,6‐dimethylphenyl)‐2,6‐pyridinedicarboxamidate) was prepared from [Ni^II(F)(L)]⁻ (1 ) by oxidation with selectfluor. Complexes 1 and 2 were characterized by using ¹H/¹⁹F NMR, UV‐vis, and EPR spectroscopies, mass spectrometry, and X‐ray crystallography. Complex 2 was found to be a highly reactive oxidant in the oxidation of hydrocarbons. Kinetic data and products analysis demonstrate a hydrogen atom transfer mechanism of oxidation. The rate constant determined for the oxidation of 9,10‐dihydroanthracene (k ₂=29 m ⁻¹ s⁻¹) compared favorably with the most reactive high‐valent metallo‐oxidants. Complex 2 displayed reaction rates 2000–4500‐fold enhanced with respect to [Ni^III(Cl)(L)] and also displayed high kinetic isotope effect values. Oxidative hydrocarbon and phosphine fluorination was achieved. Our results provide an interesting direction in designing catalysts for hydrocarbon oxidation and fluorination

The isolation and characterization of the zwitterionic intermediate in 1,1‐carboboration reactions is reported. The highly reactive zwitterionic intermediates are generated from tris(pentafluorophenyl)borane and alkynes under reaction conditions of 1,1‐carboboration.

Abstract

The reaction of a Lewis acidic borane with an alkyne is a key step in a diverse range of main group transformations. Alkyne 1,1‐carboboration, the Wrackmeyer reaction, is an archetypal transformation of this kind. 1,1‐Carboboration has been proposed to proceed through a zwitterionic intermediate. We report the isolation and spectroscopic, structural and computational characterization of the zwitterionic intermediates generated by reaction of B(C₆F₅)₃ with alkynes. The stepwise reactivity of the zwitterion provides new mechanistic insight for 1,1‐carboboration and wider B(C₆F₅)₃ catalysis. Making use of intramolecular stabilization by a ferrocene substituent, we have characterized the zwitterionic intermediate in the solid state and diverted reactivity towards alkyne cyclotrimerization.

The reaction of the cyclometalated five‐coordinate 16 VE iridium(III) compound [IrCl(H)(P(t Bu)₂C₆H₄‐κ² P ,C )(P(t Bu)₂Ph)] (1 ) with the strong π‐acceptor ligand trifluorophosphane resulted quickly in the quantitative formation of the new iridium(I) complex trans‐[IrCl(PF₃)(P(t Bu)₂Ph)₂] (2 ). This unexpected spontaneous reductive elimination was already observed in reactions of 1 with the very strong π‐acceptor ligands CO and NO⁺. First indications during reactions of 1 with lesser strong π‐acceptor ligands like alkyl or arylphosphanes did not show this inversion behavior of the cyclometalation. The title species 2 was characterized by spectroscopic methods and its molecular structure in the crystal was confirmed by X‐ray crystallography.

Pt complexes made easy: Platinum(II) complexes bearing biaryl‐2,2′‐diyl ligands were synthesized without any air‐ or moisture‐sensitive reagents, such as n‐butyllithium. The prepared complexes exhibited intense green emission with high quantum efficiency of up to 0.80, even at 298 K, because of their tightly packed structure and because of the strong σ‐donating ability of the biaryl ligand.

Abstract

Transition‐metal complexes bearing biaryl‐2,2′‐diyl ligands tend to show intense luminescence. However, difficulties in synthesis have prevented their further functionalization and practical applications. Herein, a series of platinum(II) complexes bearing biaryl‐2,2′‐diyl ligands, which have never been prepared in air, were synthesized through transmetalation and successive cyclometalation of biarylboronic acids. This approach does not require any air‐ or moisture‐sensitive reagents and features a simple synthesis even in air. The resulting (Et₄N)₂[Pt(m,n‐F₂bph)(CN)₂] (m,n‐F₂bph=m,n‐difluorobiphenyl‐2,2′‐diyl) complexes exhibit intense green emissions with high quantum efficiencies of up to 0.80 at 298 K. The emission spectral fitting and variable‐temperature emission lifetime measurements indicate that the high quantum efficiency was achieved because of the tight packing structure and strong σ‐donating ability of bph.

Blue skies over green fields : Sky‐blue to green emission has been realized in a new class of carbazolylgold(III) complexes with the judicious choice of N‐heterocycles in the C^C^N ligand. The complexes have been used to fabricate both vacuum‐deposited and solution‐processed organic light‐emitting devices, in which the green‐emitting devices realize a high maximum external quantum efficiency of 12.3 %, with an operational half‐life of over 5300 h at 100 cd m⁻².

Abstract

A new class of sky‐blue‐ to green‐emitting carbazolylgold(III) C^C^N complexes containing pyrazole or benzimidazole moieties has been successfully designed and synthesized. Through the judicious choice of the N‐heterocycles in the cyclometalating ligand and the tailor‐made carbazole moieties, maximum photoluminescence quantum yields of 0.52 and 0.39 have been realized in the green‐ and sky‐blue‐emitting complexes, respectively. Solution‐processed and vacuum‐deposited organic light‐emitting devices (OLEDs) based on the benzimidazole‐containing complexes have been prepared. The sky‐blue‐emitting device shows an emission peaking at 484 nm with a narrow full‐width at half‐maximum of 57 nm (2244 cm⁻¹), demonstrating the potential of this class of complexes in the application of OLEDs with high color purity. In addition, high maximum external quantum efficiencies of 12.3 % and a long operational half‐lifetime of over 5300 h at 100 cd m⁻² have been achieved in the vacuum‐deposited green‐emitting devices.

Where radicals come from: Molecule‐induced radical formation (MIRF) processes, where one closed‐shell molecule helps another one dissociate, are viable alternatives for simple unimolecular homolytic bond breaking events.

Abstract

Radical chain reactions are commonly initiated through the thermal or photochemical activation of purpose‐built initiators, through photochemical activation of substrates, or through well‐designed redox processes. Where radicals come from in the absence of these initiation strategies is much less obvious and are often assumed to derive from unknown impurities. In this situation, molecule‐induced radical formation (MIRF) reactions should be considered as well‐defined alternative initiation modes. In the most general definition of MIRF reactions, two closed‐shell molecules react to give a radical pair or biradical. The exact nature of this transformation depends on the σ‐ or π‐bonds involved in the MIRF process, and this Minireview specifically focuses on reactions that transform two σ‐bonds into two radicals and a closed‐shell product molecule.

A study on the reactivity of the N‐heterocyclic silylene (NHSi) Dipp₂NHSi (1,3‐bis(diisopropylphenyl)‐1,3‐diaza‐2‐silacyclopent‐4‐en‐2‐yliden) with different transition metal carbonyl complexes and calculations on the electronic features of these NHSi ligands are presented and compared to NHC (N‐Heterocyclic Carbene) ligands. The reaction of Dipp₂NHSi with the [Ni(CO)₄], [M(CO)₆] (M=Cr, Mo, W), [Mn(CO)₅(Br)] and [Fe(η ⁵‐C₅H₅)(CO)₂(I)] leads to the mono‐ and bis‐silylene complexes [{Ni(CO)₂(μ‐Dipp₂NHSi)}₂] (2), [M(CO)₅(Dipp₂NHSi)] (M=Cr 3, Mo 4, W 5), [Mn(CO)₃(NHSi)₂(Br)] (9) and [(η ⁵‐C₅H₅)Fe(CO)₂(Dipp₂NHSi‐I)] (10).

Abstract

A study on the reactivity of the N‐heterocyclic silylene Dipp₂NHSi (1,3‐bis(diisopropylphenyl)‐1,3‐diaza‐2‐silacyclopent‐4‐en‐2‐yliden) with the transition metal complexes [Ni(CO)₄], [M(CO)₆] (M=Cr, Mo, W), [Mn(CO)₅(Br)] and [(η ⁵‐C₅H₅)Fe(CO)₂(I)] is reported. We demonstrate that N‐heterocyclic silylenes, the higher homologues of the now ubiquitous NHC ligands, show a remarkably different behavior in coordination chemistry compared to NHC ligands. Calculations on the electronic features of these ligands revealed significant differences in the frontier orbital region which lead to some peculiarities of the coordination chemistry of silylenes, as demonstrated by the synthesis of the dinuclear, NHSi‐bridged complex [{Ni(CO)₂(μ‐Dipp₂NHSi)}₂] (2), complexes [M(CO)₅(Dipp₂NHSi)] (M=Cr 3, Mo 4, W 5), [Mn(CO)₃(Dipp₂NHSi)₂(Br)] (9) and [(η ⁵‐C₅H₅)Fe(CO)₂(Dipp₂NHSi‐I)] (10). DFT calculations on several model systems [Ni(L)], [Ni(CO)₃(L)], and [W(CO)₅(L)] (L=NHC, NHSi) reveal that carbenes are typically the much better donor ligands with a larger intrinsic strength of the metal–ligand bond. The decrease going from the carbene to the silylene ligand is mainly caused by favorable electrostatic contributions for the NHC ligand to the total bond strength, whereas the orbital interactions were often found to be higher for the silylene complexes. Furthermore, we have demonstrated that the contribution of σ‐ and π‐interaction depends significantly on the system under investigation. The σ‐interaction is often much weaker for the NHSi ligand compared to NHC but, interestingly, the π‐interaction prevails for many NHSi complexes. For the carbonyl complexes, the NHSi ligand is the better σ‐donor ligand, and contributions of π‐symmetry play only a minor role for the NHC and NHSi co‐ligands.

Iridium(III) catalysts that bear an amide‐pendant cyclopentadienyl ligand ([Cp^AIrI₂]₂) were synthesized and characterized. Double aromatic homologation reactions of benzamides with alkynes by fourfold C−H activation proceeded in good to high yields when these [Cp^AIrI₂]₂ catalysts were used, demonstrating their superior catalytic performance in this challenging transformation.

Abstract

The synthesis, characterization, and catalytic performance of iridium(III) catalysts that bear an amide‐pendant cyclopentadienyl ligand ([Cp^AIrI₂]₂) is reported. These [Cp^AIrI₂]₂ catalysts were obtained from the complexation of a Cp^A ligand precursor with [Ir(cod)OAc]₂ followed by oxidation. Double aromatic homologation reactions of benzamides with alkynes by fourfold C−H activation proceeded in good to high yield with these [Cp^AIrI₂]₂ catalysts, demonstrating their superior catalytic performance in this challenging transformation.

A meso ‐Free B^III 5,10‐bis(p‐ dimethylaminophenyl)subporphyrin was synthesized, which serves as a nice precursor of various meso ‐substituted B^III subporphyrins. Interestingly, reactions of meso ‐free B^III subporphyrins with brominating reagents gave meso‐meso′ linked subporphyrin dimers often as a major product along with meso ‐bromosubporphyrins.

Abstract

meso‐ Free B^III 5,10‐bis(p ‐dimethylaminophenyl)subporphyrins were synthesized. They display red‐shifted absorption and fluorescence spectra, bathochromic behaviors in polar solvents, a high fluorescence quantum yield (Φ _F=0.57), and a small HOMO–LUMO gap mainly due to destabilized HOMO as compared with meso ‐free B^III 5,10‐diphenylsubporphyrin. This subporphyrin serves as a nice precursor of various meso ‐substituted B^III subporphyrins such as B^III meso ‐nitrosubporphyrin, B^III meso ‐aminosubporphyrin, and meso‐meso’ linked B^III azosubporphyrin dimer. Reactions of meso ‐free B^III subporphyrins with NBS or bis(2,4,6‐trimethylpyridine)bromonium hexafluorophosphate gave meso‐meso′ linked subporphyrin dimers, often as a major product along with meso ‐bromosubporphyrins.

Crystalline 1,4‐distannabarrelene compounds [(ADC Ar ) 3 Sn 2 ]SnCl 3 ( 3‐Ar ) (ADC Ar = {CC(NDipp)CAr}; Dipp = 2,6‐iPr 2 C 6 H 3 , Ar = Ph or DMP; DMP = 4‐Me 2 NC 6 H 4 ) derived from anionic dicarbenes Li(ADC Ar ) ( 2‐Ar ) (Ar = Ph or DMP) have been reported. The cationic moiety of 3‐Ar features a barrelene framework with three coordinated Sn(II) atoms at the 1,4‐positions, whereas the anionic unit SnCl 3 is formally derived from SnCl 2 and chloride ion. The all carbon substituted bis‐stannylenes 3‐Ar have been characterized by NMR spectroscopy and X‐ray diffraction. DFT calculations reveal that the HOMO of 3‐Ph (ε = ‒6.40 eV) is mainly the lone‐pair orbital at the Sn(II) atoms of the barrelene unit. 3 ‐ Ar readily react with sulfur and selenium to afford the mixed‐valence Sn(II)/Sn(IV) compounds [(ADC Ar ) 3 SnSn(E)](SnCl 6 ) 0.5 (E = S 4‐Ar , Ar = Ph or DMP; E = Se 5‐Ph ).