N-Heterocyclic imino-catalyzed 1,4-regioselective azide-alkyne cycloaddition (AAC): a metal-free approach.

An unprecedented synthetic approach has been devised to efficiently synthesize regioselective 1,4-disubstituted 1,2,3-triazoles. This technique relies on the use of innovative metal-free highly basic N-heterocyclic imino catalysts. The experimental observations have been supported further by TD-DFT computational studies.

N-Heterocyclic Imine-Supported Bimetallic Cu(II) Catalyst for Azide-Alkyne Cycloaddition: Solvent-free, Reductant-free, ppm-level Catalysis to Access 1,4-Disubstituted Triazoles.

In this article, we present a unique bimetallic paddle wheel copper(II) complex with the molecular formula [Cu2 C42 H54 N6 O8 ]. Several characterization tools have been employed to analyze this complex including single-crystal X-ray diffraction, HRMS, FTIR, and UV-vis spectroscopy. This copper(II) complex excels admirably as a catalytic system in parts-per-million level (ppm) loading for the azide-alkyne 'click' reaction under solvent-free conditions, allowing for the quantitative conversion of numerous 1,4-disubstituted 1,2,3-triazole. The specially designed coordinated ligand (perimidin-2-imine) in the Cu(II) complex accelerates the reaction rate significantly during the oxidative homocoupling reaction (OHC) and acts as a base for Cu-coordinated alkyne deprotonation. It has been demonstrated that the catalyst loading of 2.5 ppm is adequate to catalyze the cycloaddition of benzyl azide to phenylacetylene, leading to the extremely high turnover number of 120000 and a turnover frequency of 5000 h-1 . Synergistic evidence from stoichiometric reactions and experimental results provides insights into the plausible mechanism for the reaction. Each copper atom contributes to the outcome of the proposed reaction, one by bonding to the acetylide and the other by activating the azide as part of a bimetallic synergistic pathway.

Remodeling of N-Heterocyclic Iminato Ligand Frameworks for the Facile Synthesis of Isoureas from Alcohols and Carbodiimides Promoted by Organoactinide (Th, U) Complexes.

A new class of actinide complexes [(L)An(N{SiMe3}2)3] (An = Th or U) (Th1-Th3 and U1-U3) supported by highly nucleophilic seven-membered N-heterocyclic iminato ligands were synthesized and fully characterized by single-crystal X-ray diffraction. These complexes were successfully exploited as powerful catalysts for the addition of alcohols to carbodiimides to yield the corresponding desirable isourea products at room temperature with short reaction times and excellent yields. Thorough stoichiometric, thermodynamic, and kinetic studies were carried out, allowing us to propose a plausible mechanism for the catalytic reaction.

Recent developments in highly basic N-heterocyclic iminato ligands in actinide chemistry.

In the last decade, major conceptual advances in the chemistry of actinide molecules and materials have been made to demonstrate their distinct reactivity profiles as compared to lanthanide and transition metal compounds, but some difficult questions remain concerning the intriguing stability of low-valent actinide complexes, and the importance of the 5f-orbitals in reactivity and bonding. The imidazolin-2-iminato moiety has been extensively used in ligands for the advancement of actinide chemistry owing to its unique capability of stabilizing the reactive and highly electrophilic metal ions by virtue of its strong electron donation and steric tunability. The current review article describes recent developments in the chemistry of light actinide metal ions (thorium and uranium) bearing these N-heterocyclic iminato moieties as supporting ligands. In addition, the effect of ring expansion of the N-heterocycle on the catalytic aptitude of the organoactinides is also described herein. The synthesis and reactivity of actinide complexes bearing N-heterocyclic iminato ligands are presented, and promising apposite applications are also presented. The current review focuses on addressing the catalytic behavior of actinide complexes with oxygen-containing substrates such as in the Tishchenko reaction, hydroelementation processes, and polymerization reactions. Actinide complexes have also found new catalytic applications, as demonstrated by the potent chemoselective carbonyl hydroboration and tandem proton-transfer esterification (TPTE) reaction, featuring coupling between an aldehyde and alcohol.

Catalytic regeneration of a Th-H bond from a Th-O bond through a mild and chemoselective carbonyl hydroboration.

Here we present an unprecedented chemoselective hydroboration for aldehydes and ketones catalysed by actinides. The reaction features a very low catalyst loading (0.1-0.004 mol%) and quantitative product formation in less than 15 minutes, at room temperature. Thermodynamic and kinetic studies including stoichiometric and labeling studies with deuterated pinacolborane allow us to propose a plausible mechanism for this remarkable catalytic regeneration of a Th-H bond via carbonyl hydroboration.

Organoactinides in catalytic transformations: scope, mechanisms and Quo Vadis.

The last decade has witnessed wonderful and remarkable advances in the chemistry of early actinides, providing information on not only the reactivity of these types of compounds in stoichiometric reactions but also their utilization in challenging catalytic processes. This canvas of knowledge allows the design of chemical reactivities to reach a high level of sophistication. As compared to early/late transition metal and lanthanide complexes, the actinides display complementary and in some cases real unique performances for similar organic transformations. The study of organoactinides allows us to design new chemical transformations due to their distinctive electronic structures, which feature a large coordination number exhibiting steric interactions. This review highlights the latest results obtained since 2008 on the catalytic activities of organoactinides. This review presents a brief introduction, and two main parts: (i) organoactinide mediated catalytic transformations of small molecules, including hydroelementation, coupling reactions, etc.; (ii) organoactinide catalysed polymerization and oligomerization reactions, including olefin, diene, cyclic ester and epoxide substrates. At the end, we present our Quo Vadis opinion and pose some challenging questions and our personal opinion regarding where this field should continue to develop.

Thorium complexes possessing expanded ring N-heterocyclic iminato ligands: synthesis and applications.

Six and seven membered N-heterocyclic iminato ligands (L) are introduced allowing access a new class of Th(iv) complexes of the type Cp*2Th(L)(CH3). These complexes were studied in the Tishchenko reaction. Stoichiometric reactions together with kinetic and thermodynamic studies permit us to propose a plausible mechanism.

Olefin oxygenation by water on an iridium center.

Oxygenation of 1,5-cyclooctadiene (COD) is achieved on an iridium center using water as a reagent. A hydrogen-bonding interaction with an unbound nitrogen atom of the naphthyridine-based ligand architecture promotes nucleophilic attack of water to the metal-bound COD. Irida-oxetane and oxo-irida-allyl compounds are isolated, products which are normally accessed from reactions with H2O2 or O2. DFT studies support a ligand-assisted water activation mechanism.

Amide-functionalized naphthyridines on a Rh(II) -Rh(II) platform: effect of steric crowding, hemilability, and hydrogen-bonding interactions on the structural diversity and catalytic activity of dirhodium(II) complexes.

Ferrocene-amide-functionalized 1,8-naphthyridine (NP) based ligands {[(5,7-dimethyl-1,8-naphthyridin-2-yl)amino]carbonyl}ferrocene (L(1) H) and {[(3-phenyl-1,8-naphthyridin-2-yl)amino]carbonyl}ferrocene (L(2) H) have been synthesized. Room-temperature treatment of both the ligands with Rh2 (CH3 COO)4 produced [Rh2 (CH3 COO)3 (L(1) )] (1) and [Rh2 (CH3 COO)3 (L(2) )] (2) as neutral complexes in which the ligands were deprotonated and bound in a tridentate fashion. The steric effect of the ortho-methyl group in L(1) H and the inertness of the bridging carboxylate groups prevented the incorporation of the second ligand on the {Rh(II) -Rh(II) } unit. The use of the more labile Rh2 (CF3 COO)4 salt with L(1) H produced a cis bis-adduct [Rh2 (CF3 COO)4 (L(1) H)(2) ] (3), whereas L(2) H resulted in a trans bis-adduct [Rh2 (CF3 COO)3 (L(2) )(L(2) H)] (4). Ligand L(1) H exhibits chelate binding in 3 and L(2) H forms a bridge-chelate mode in 4. Hydrogen-bonding interactions between the amide hydrogen and carboxylate oxygen atoms play an important role in the formation of these complexes. In the absence of this hydrogen-bonding interaction, both ligands bind axially as evident from the X-ray structure of [Rh2 (CH3 COO)2 (CH3 CN)4 (L(2) H)2 ](BF4 )2 (6). However, the axial ligands reorganize at reflux into a bridge-chelate coordination mode and produce [Rh2 (CH3 COO)2 (CH3 CN)2 (L(1) H)](BF4 )2 (5) and [Rh2 (CH3 COO)2 (L(2) H)2 ](BF4 )2 (7). Judicious selection of the dirhodium(II) precursors, choice of ligand, and adaptation of the correct reaction conditions affords 7, which features hemilabile amide side arms that occupy sites trans to the Rh-Rh bond. Consequently, this compound exhibits higher catalytic activity for carbene insertion to the CH bond of substituted indoles by using appropriate diazo compounds, whereas other compounds are far less reactive. Thus, this work demonstrates the utility of steric crowding, hemilability, and hydrogen-bonding functionalities to govern the structure and catalytic efficacyof dirhodium(II,II) compounds.

Reactions of acids with naphthyridine-functionalized ferrocenes: protonation and metal extrusion.

Reaction of 1,8-naphthyrid-2-yl-ferrocene (FcNP) with a variety of acids affords protonated salts at first, whereas longer reaction time leads to partial demetalation of FcNP resulting in a series of Fe complexes. The corresponding salts [FcNP·H][X] (X = BF(4) or CF(3)SO(3) (1)) are isolated for HBF(4) and CF(3)SO(3)H. Reaction of FcNP with equimolar amount of CF(3)CO(2)H for 12 h affords a neutral complex [Fe(FcNP)(2)(O(2)CCF(3))(2)(OH(2))(2)] (2). Use of excess acid gave a trinuclear Fe(II) complex [Fe(3)(H(2)O)(2)(O(2)CCF(3))(8)(FcNP·H)(2)] (3). Three linear iron atoms are held together by four bridging trifluoroacetates and two aqua ligands in a symmetric fashion. Reaction with ethereal solution of HCl afforded [(FcNP·H)(3)(Cl)][FeCl(4)](2) (4) irrespective of the amount of the acid used. Even the picric acid (HPic) led to metal extrusion giving rise to [Fe(2)(Cl)(2)(FcNP)(2)(Pic)(2)] (5) when crystallized from dichloromethane. Metal extrusion was also observed for CF(3)SO(3)H, but an analytically pure compound could not be isolated. The demetalation reaction proceeds with an initial proton attack to the distal nitrogen of the NP unit. Subsequently, coordination of the conjugate base to the electrophilic Fe facilitates the release of Cp rings from metal. The conjugate base plays an important role in the demetalation process and favors the isolation of the Fe complex as well. The 1,1'-bis(1,8-naphthyrid-2-yl)ferrocene (FcNP(2)) does not undergo demetalation under identical conditions. Two NP units share one positive charge causing the Fe-Cp bonds weakened to an extent that is not sufficient for demetalation. X-ray structure of the monoprotonated FcNP(2) reveals a discrete dimer [(FcNP(2)·H)](2)[OTf](2) (6) supported by two N-H···N hydrogen bonds. Crystal packing and dispersive forces associated with intra- and intermolecular π-π stacking interactions (NP···NP and Cp···NP) allow the formation of the dimer in the solid-state. The protonation and demetalation reactions of FcNP and FcNP(2) with a variety of acids are reported.

A trinuclear bright red luminophore containing cyclometallated Ir(III) motifs.

The trinuclear complex [{Ir(ppy)(2)}(3)(L(1))(2)](OTf)(3) (1) is a bright red luminophore whereas the monomer [Ir(ppy)(2)L(2)](OTf) (2) exhibits emission in the green region.

Hydroxycarbonyl complexes as key intermediates in the base-assisted reduction of ruthenium carbonyls.

Since the recognition of metal-hydroxycarbonyl (or, metallacarboxylic acid) as a key intermediate in water gas shift (WGS) catalysis, numerous metal complexes incorporating hydroxycarbonyl ligands have been synthesized. In this Perspective, a brief account on the chemistry of metallacarboxylic acid and metallacarboxylate is presented. Special emphasis is placed on the hydroxycarbonyl-bridged dinuclear compounds. The relevance of hydroxycarbonyl complexes in the base-assisted reduction of ruthenium carbonyls is investigated. In particular, Lavigne's synthesis of Roper's catalyst Ru(CO)(2)(PPh(3))(3), triruthenium dodecacarbonyl Ru(3)(CO)(12), and Bera's mechanistic investigation on the preparation of the metal-metal singly bonded diruthenim(I) complex [Ru(2)(CO)(4)(CH(3)CN)(6)](2+) are described in detail. These reactions involve the decarboxylation of a "[Ru(m)(COOH)(n)]" species, followed by the reductive elimination of HX (X = H, Cl or OH), steps akin to WGS catalysis. The prospect of hydroxycarbonyl chemistry in the development of bimetallic catalysis is examined.

Mapping the transformation [{Ru(II)(CO)(3)Cl(2)}(2)]-->[Ru(I) (2)(CO)(4)](2+): implications in binuclear water-gas shift chemistry.

The complete sequence of reactions in the base-promoted reduction of [{Ru(II)(CO)(3)Cl(2)}(2)] to [Ru(I) (2)(CO)(4)](2+) has been unraveled. Several mu-OH, mu:kappa(2)-CO(2)H-bridged diruthenium(II) complexes have been synthesized; they are the direct results of the nucleophilic activation of metal-coordinated carbonyls by hydroxides. The isolated compounds are [Ru(2)(CO)(4)(mu:kappa(2)-C,O-CO(2)H)(2)(mu-OH)(NP(F)-Am)(2)][PF(6)] (1; NP(F)-Am=2-amino-5,7-trifluoromethyl-1,8-naphthyridine) and [Ru(2)(CO)(4)(mu:kappa(2)-C,O-CO(2)H)(mu-OH)(NP-Me(2))(2)][BF(4)](2) (2), secured by the applications of naphthyridine derivatives. In the absence of any capping ligand, a tetranuclear complex [Ru(4)(CO)(8)(H(2)O)(2)(mu(3)-OH)(2)(mu:kappa(2)-C,O-CO(2)H)(4)][CF(3)SO(3)](2) (3) is isolated. The bridging hydroxido ligand in 1 is readily replaced by a pi-donor chlorido ligand, which results in [Ru(2)(CO)(4)(mu:kappa(2)-C,O-CO(2)H)(2)(mu-Cl)(NP-PhOMe)(2)][BF(4)] (4). The production of [Ru(2)(CO)(4)](2+) has been attributed to the thermally induced decarboxylation of a bis(hydroxycarbonyl)-diruthenium(II) complex to a dihydrido-diruthenium(II) species, followed by dinuclear reductive elimination of molecular hydrogen with the concomitant formation of the Ru(I)--Ru(I) single bond. This work was originally instituted to find a reliable synthetic protocol for the [Ru(2)(CO)(4)(CH(3)CN)(6)](2+) precursor. It is herein prescribed that at least four equivalents of base, complete removal of chlorido ligands by Tl(I) salts, and heating at reflux in acetonitrile for a period of four hours are the conditions for the optimal conversion. Premature quenching of the reaction resulted in the isolation of a trinuclear Ru(I) (2)Ru(II) complex [{Ru(NP-Am)(2)(CO)}{Ru(2)(NP-Am)(2)(CO)(2)(mu-CO)(2)}(mu(3):kappa(3)-C,O,O'-CO(2))][BF(4)](2) (6). These unprecedented diruthenium compounds are the dinuclear congeners of the water-gas shift (WGS) intermediates. The possibility of a dinuclear pathway eliminates the inherent contradiction of pH demands in the WGS catalytic cycle in an alkaline medium. A cooperative binuclear elimination could be a viable route for hydrogen production in WGS chemistry.