Diversification of the Carbodicarbene Class by Embedding an Anionic Component in its Scaffold.

Carbodicarbene (CDC) has become an emerging ligand in many fields due to its strong σ-donating ability.

Binuclear Macrocyclic Silver(I) Complex of a Bis(carbone) Pincer Ligand: Synthesis and Application as a Carbone-Transfer Agent.

A facile synthesis of a binuclear AgI complex 2 of a bis(carbone) ligand L and its application as a carbone-transfer agent for the generation of other transition-metal complexes of AuI (3), NiII (4), and PdII (5) is presented. Complex 2 was synthesized through multiple synthetic routes under mild reaction conditions using the tetracationic [LH4][OTf·Cl]2 precursor salt, the dicationic [LH2][OTf]2 ylide salt, and the free ligand L. The first two synthesis routes require no prior isolation of the air-, moisture-, and temperature-sensitive free ligand L, thus affording complex 2 with high yield and purity. Multinuclear NMR techniques, high-resolution mass spectrometry, and single-crystal X-ray diffraction analysis confirmed the identity of complex 2 as a binuclear AgI complex of L with a molecular formula of [L2Ag2][OTf]2 and a 16-membered-ring metallomacrocyclic structure. During the transmetalation reaction with AuI, the binuclear nature of complex 2 remains intact to give analogous complex 3 ([L2Au2][OTf]2). However, the dimeric structure was disrupted upon the carbone-transfer reaction with NiII and PdII, yielding mononuclear C-N-C pincer-type complexes 4 ([LNiCl][OTf]) and 5 ([LPdCl][OTf]), respectively. These results demonstrated the versatile use of complex 2 as a carbone-transfer agent to other transition metals regardless of the type or size of the metals or the geometry they prefer.

Reactive Dicarbon as a Flexible Ligand for Transition-Metal Coordination and Catalysis.

Dicarbon is a reactive carbon allotrope that naturally exists only in the high-temperature medium of stellar space. We report the successful preparation of a series of bottleable phosphine-stabilized dicarbon (PDC) molecules. We explore the use of these molecules as a new complementary class of carbene-like ligands featuring strong σ-donor (>NHCs and CAAcs) but weak π-acceptor properties. Steric map analysis of PDC based on Cavallo's SambVca program reveals comparable steric volume bulk of 32.5%, similar to the conventional IMes carbene. However, our PDCs exhibit dynamic steric flexibility modulated by the nature of the metal complexes and catalytic reaction environment. We demonstrate the catalytic utility of the PDC framework by its successful implementation for Suzuki-Miyaura cross-coupling and the reductive coupling reaction of an aldehyde and alkyne. Detailed investigations of the reductive coupling reaction reveal an important secondary interaction between PDC and metal complexes, which plays a critical role in the catalytic system.

Mirror-plane disorder in a nickel chloride Schiff base complex: a suitable case study for crystallographic instruction.

The nickel chloride complex of the Schiff base N2,N2'-propanediylbis(2,3-butanedione-2-imine-3-oxime), namely, chlorido(3,9-dimethylundeca-3,8-diene-2,10-dione 10-oxime 2-oximato-κ4N,N',N'',N''')nickel(II), [NiCl(C11H19N4O2)], at 100 K crystallizes in the orthorhombic space group Cmce. The structure exhibits mirror disorder of the main molecule that is not present in the bromide analogue. The relatively small number of unique reflections in the data set and the disorder imposed by the crystallographic mirror plane present a challenging educational case study.

A Bis-(carbone) Pincer Ligand and Its Coordinative Behavior toward Multi-Metallic Configurations.

Carbones are divalent carbon(0) species that contain two lone pairs of electrons. Herein, we have prepared the first known stable and isolable free bis-(carbone) pincer framework with a well-defined solid-state structure. This bis-(carbone) ligand is an effective scaffold for forming monometallic (Ni and Pd) and trinuclear heterometallic complexes with Au-Pd-Au, Au-Ni-Au, and Cu-Ni-Cu configurations. Sophisticated quantum-theoretical analyses found that the metal-metal interactions are too weak to play a significant role in upholding these multi-metallic configurations; rather, the four lone pairs of electrons within the bis-(carbone) framework are the main contributors to the stability of the complexes.

Synergistic Catalysis by Brønsted Acid/Carbodicarbene Mimicking Frustrated Lewis Pair-Like Reactivity.

Carbodicarbene (CDC), unique carbenic entities bearing two lone pairs of electrons are well-known for their strong Lewis basicity. We demonstrate herein, upon introducing a weak Brønsted acid benzyl alcohol (BnOH) as a co-modulator, CDC is remolded into a Frustrated Lewis Pair (FLP)-like reactivity. DFT calculation and experimental evidence show BnOH loosely interacting with the binding pocket of CDC via H-bonding and π-π stacking. Four distinct reactions in nature were deployed to demonstrate the viability of proof-of-concept as synergistic FLP/Modulator (CDC/BnOH), demonstrating enhanced catalytic reactivity in cyclotrimerization of isocyanate, polymerization process for L-lactide (LA), methyl methacrylate (MMA) and dehydrosilylation of alcohols. Importantly, the catalytic reactivity of carbodicarbene is uniquely distinct from conventional NHC which relies on only single chemical feature of nucleophilicity. This finding also provides a new spin in diversifying FLP reactivity with co-modulator or co-catalyst.

Isolable dicarbon stabilized by a single phosphine ligand.

In contrast to naturally occurring F2, O2 and N2, diatomic C2 is an intriguing species that has only been observed indirectly in the gas phase, and because of its high reactivity has eluded isolation in the condensed phase. It has previously been stabilized in L→C2←L compounds but the bonding situation of the central C2 in this motif differs remarkably from that of free C2. Here we have prepared and structurally characterized diatomic C2 as a monoligated complex L→C2 using a bulky phosphine ligand bearing two imidazolidin-2-iminato groups (L is (NHCR=N)2(CH3)P, where NHCR is an N-heterocyclic carbene). The compound is stable in solution at ambient temperature and has also been isolated in the solid state. Reactivity studies, in combination with quantum chemical analysis, suggest that the two carbon atoms of the L→C2 complex both have carbene character. The complex underwent intermolecular C-H bond activation upon thermolysis and exhibited hydroalkoxylation-like reactivity with methanol.

Directing Group-Promoted Inert C-O Bond Activation Using Versatile Boronic Acid as a Coupling Agent.

A simple Ni(cod)2 and carbene mediated strategy facilitates the efficient catalytic cross-coupling of methoxyarenes with a variety of organoboron reagents. Directing groups facilitate the activation of inert C-O bonds in under-utilized aryl methyl ethers enabling their adaptation for C-C cross-coupling reactions as less toxic surrogates to the ubiquitous haloarenes. The method reported enables C-C cross-coupling with readily available and economical arylboronic acid reagents, which is unprecedented, and compares well with other organoboron reagents with similarly high reactivity. Extension to directing group assisted chemo-selective C-O bond cleavage, and further application towards the synthesis of novel bifunctionalized biaryls is reported. Key to the success of this protocol is the use of directing groups proximal to the reaction center to facilitate the activation of the inert C-OMe bond.

Carbodicarbene: geminal-Bimetallic Coordination in Selective Manner.

The reaction of Pd(OAc)2 with free carbodicarbene (CDC) generates a Pd acetate trinuclear complex 1 via intramolecular C(sp3 )-H bond activation at one of the CDC methyl side arms. The solid structure of 1 reveals the capability of CDC to facilitate a double dative bond with two palladium centers in geminal fashion. This is attributed to the chelating mode of CDC, which can frustrate π-conjugation within the CDC framework. Such effect maybe also amplified by ligand-ligand interaction. The formation of other gem-bimetallic Pd-Pd, Pd-Au, and Ni-Au provides further structural evidence for this proof-of-concept in selective installation. Structural analysis is supported by computational calculations based on state-of-the-art energy decomposition analysis (EDA) in conjunction with natural orbitals for chemical valence (NOCV) method.

Interface-Controlled Synthesis of Au-BINOL Hybrid Nanostructures and Mechanism Study.

The combined functionality of components in organic-inorganic hybrid nanomaterials render them efficient nanoreactors. However, the development in this field is limited due to a lack of synthetic avenues and systematic control of the growth kinetics of hybrid structures. In this work, we take advantage of an ionic switch for regio-control of Au-BINOL(1,1'-Bi-2-naphthol) hybrid nanostructures. Aromatic BINOL molecules assemble into nanospheres, concomitant with the growth of the Au nanocrystals. The morphological evolution of Au nanocrystals is solely controlled by the presence of halides in the synthetic system. Here we show that quaternary ammonium surfactants (CTAB or CTAC), not only bridging Au and BINOL, but also contributing to the formation of concentric or eccentric structures when their concentrations are tuned to the range of 10-5 to 10-3 M. This facile strategy offers the potential advantage of scalable production, with diverse functional organic-inorganic hybrid nanocomposites being produced based on the specific archetype of Au-BINOL hybrid nanocomposites.

One-Pot Tandem Photoredox and Cross-Coupling Catalysis with a Single Palladium Carbodicarbene Complex.

The combination of conventional transition-metal-catalyzed coupling (2 e- process) and photoredox catalysis (1 e- process) has emerged as a powerful approach to catalyze difficult cross-coupling reactions under mild reaction conditions. Reported is a palladium carbodicarbene (CDC) complex that mediates both a Suzuki-Miyaura coupling and photoredox catalysis for C-N bond formation upon visible-light irradiation. These two catalytic pathways can be combined to promote both conventional transition-metal-catalyzed coupling and photoredox catalysis to mediate C-H arylation under ambient conditions with a single catalyst in an efficient one-pot process.

Carbodicarbenes: Unexpected π-Accepting Ability during Reactivity with Small Molecules.

An investigation of carbodicarbenes, the less explored member of the carbenic complex/ligand family has yielded unexpected electronic features and concomitant reactivity. Observed 1,2-addition of E-H bonds (E = B, C, Si) across the carbone central carbon and that of the flanking N-heterocyclic carbene (NHC) fragment, combined with single-crystal X-ray studies of a model Pd complex strongly suggests a significant level of π-accepting ability at the central carbon of the NHC moiety. This feature is atypical of classic NHCs, which are strong σ-donors, with only nominal π-accepting ability. The unanticipated π-acidity is critical for engendering carbodicarbenes with reactivity more commonly observed with frustrated Lewis pairs (FLPs) rather than the more closely related NHCs and cyclic (alkyl)(amino)carbenes (CAACs).

Structural and electronic trends for five coordinate 1(st) row transition metal complexes: Mn(ii) to Zn(ii) captured in a bis(iminopyridine) framework.

The preparation and characterization of a series of divalent 3d transition metal complexes supported by a tridentate planar bis(iminopyridine) ligand are reported. The complexes {2,6-[PhC[double bond, length as m-dash]N(tBu2C6H3)]2C5H3N}MBr2 (M = Mn, Fe, Co, Ni, Cu, Zn), 1-6, were characterized by single crystal X-ray structural studies revealing complexes with pentacoordinate distorted square pyramidal coordination environments. This assembly of complexes provided a unique array for examining the relationship between experimental structure and computed electronic structure. While experimental structural features basically correlated with the Irving-Williams series, some clear deviations were rationalized through the computational analysis. A balance of bis(imino)pyridine/metal with bonding/antibonding π interactions was used to explain the divergent directions of Fe(ii)-N and Co(ii)-N bond lengths. Similarly, orbital details were used to justify the opposing change in Cu-Brap and Cu-Brbas bond lengths. Furthermore, computational analysis provided a unique method to document a surprising low bond order for the M-N bonds of bis(imino)pyridine ligand in this series.

Expanding the Ligand Framework Diversity of Carbodicarbenes and Direct Detection of Boron Activation in the Methylation of Amines with CO2.

A simple and convergent synthetic strategy used to increase the diversity of the carbodicarbene ligand framework through incorporation of unsymmetrical pendant groups is reported. Structural analysis and spectroscopic studies of ligands and their Rh complexes are reported. Reactivity studies reveal carbodicarbenes as competent organocatalysts for amine methylation using CO2 as a synthon. A unique BH-activated boron-carbodicarbene complex was isolated as a reaction intermediate, providing mechanistic insight into the CO2 functionalization process.

Nickel-catalysed para-CH activation of pyridine with switchable regioselective hydroheteroarylation of allylarenes.

para-CH activation of pyridine with allylbenzene is described by Ni/Al cooperative catalysis in combination with a bulkier NHC ligand and a Lewis acid, leading to linear hydroheteroarylation products. Interestingly, the branch selectivity can be achieved by using the combination of a less sterically hindered amino-NHC ligand and AlMe3 through tandem reaction of facile alkene isomerization followed by a slow CH bond activation process.

Synthesis and isolation of an acyclic tridentate bis(pyridine)carbodicarbene and studies on its structural implications and reactivities.

The simple synthetic development of acyclic pincer bis(pyridine)carbodicarbene is depicted herein. Presented is the first isolated structural pincer carbodicarbene with a C-C-C angle of 143°, larger than the monodentate framework. More importantly, theoretical analysis showed that this carbodicarbene embodies a more allene-like character. Palladium complexes supported by this pincer ligand are active catalysts for Heck-Mizoroki and Suzuki-Miyaura coupling reactions.

Controlled regiodivergent C-H bond activation of imidazo[1,5-a]pyridine via synergistic cooperation between aluminum and nickel.

The catalytic method features a cooperative interaction between Ni and Al imparting remote C-H alkenylation at the C5 position of imidazo[1,5-a]pyridine with alkynes at mild conditions. Exclusion of AlMe3 switches the selectivity to the C3 position. Reactions with styrene and other olefinic substrates affording C5-adducts by Ni/Al catalysis are also included.

Mechanistic study of a switch in the regioselectivity of hydroheteroarylation of styrene catalyzed by bimetallic Ni-Al through C-H activation.

We previously reported a highly efficient protocol for bimetallic Ni-Al-catalyzed hydroheteroarylation of styrene with benzimidazole based on C-H bond activation. We have now delineated the mechanism of this process, providing a rationale for an observed switch in regioselectivity in the presence of the Lewis acid, AlMe3. The present mechanistic study gives insights for the rational development of catalysts that exhibit required linear/branched selectivity.

Synthesis of a guanidine NHC complex and its application in borylation reactions.

Synthesis of guanidine-linked NHC can be achieved easily through reacting amino-NHC with carbodiimide. Subsequently, guanidine-NHC Ag and Cu complexes were isolated and fully characterized. These Cu complexes are found to be versatile catalysts for hydroboration, semihydrogenation and carboboration of alkynes in a highly stereo- and regioselective fashion.

Ligand promoted Pd-catalyzed dehydrogenative alkenylation of hetereoarenes.

An efficient Pd-catalyzed cross-dehydrogenative coupling of heteroarenes with styrenes and other olefinic substrates has been developed. This alkenylation paradigm encompasses a wide range of substrates and provides a straightforward approach toward C2-E-alkenylated azole motifs.