Insights into the past and future of Janus-di-N-heterocyclic carbenes

Synthesis of P-bridged, planar bis(NHC) BCl3 adducts

Planar PV- or PIII-bridged bis(NHCs), which have only been employed in transition metal complex chemistry so far, were subjected to BCl3-containing solutions targeting the corresponding bis(NHC) BCl3 adducts. While the P(O)NEt2-bridged bis(NHC) showed the expected adduct formation, the PNEt2-bridged bis(NHC) reacted not only at the carbene moiety but also at the P-NEt2 functional group. The latter enabled access to the first 1,4-diphosphinine bis(NHC) main group adduct; its formation and properties were investigated by DFT calculations. Through the same route, a 1,4-diphosphinine bis(imidazolium) scaffold was generated and explored theoretically and experimentally. The new 1,4-diphosphinines are shown to possess high global aromaticity and a unique P-centred reactivity, allowing the formation of hitherto inaccessible [4 + 2]-cycloaddition products, thus suggesting potential new applications compared to previously known 1,4-diphosphinine derivatives.

Mesoionic Janus-Type Dicarbene: Complexes, Adducts, and Catalytic Studies

The preparations of homo- and hetero-bimetallic complexes as well as thiourea and selenourea derivatives of a mesoionic Janus-type N-heterocyclic dicarbene (diNHC) are reported. Analogues of its monocationic intermediate NHC have also been obtained for comparison. Using the main group adducts, the π-acceptor properties of both NHCs were determined using low temperature 77Se NMR spectroscopy completing their stereoelectronic profiling. Moreover, catalytic investigations reveal that the mesoionic dipalladium Janus-diNHC complex can be used in the sequential C2- and C5-arylation of 1-methylpyrrole for the preparation of non-symmetrical 2,5-diarylpyrroles.

Discrete Quadruple Stacks Formed in a Nanosized Metallorectangle

An iridium-cornered nanosized metallorectangle was obtained by combining a quinoxalinophenanthrophenazine-connected Janus-di-imidazolylidene ligand and 4,4'-bipyridine. This metallorectangle was used as host for a series of planar molecules, including pyrene, triphenylene, perylene, coronene, and N,N'-dimethyl-naphthalenetetracarboxy-diimide (NTCDI). The binding of coronene and NTCDI followed a strongly positive cooperative 1:2 stoichiometric binding model, as the inclusion of the first guest generates the geometrical requirements for the optimum encapsulation of the second planar molecule. The simultaneous encapsulation of coronene and NTCDI produces a heteroguest inclusion system, whose exchange dynamics was studied by means of variable temperature 1H NMR spectroscopy.

A corannulene-based metallobox for the encapsulation of fullerenes

A corannulene-bis-N-imidazolium salt was used for the synthesis of two corannulene-bis-N-heterocyclic carbenes of dirhodium(i) complexes of formula (corannulene-di-NHC)[RhCl(COD)]2 and (corannulene-di-NHC)[RhCl(CO)2]2. Both complexes were characterized by spectroscopic techniques, and the electron-donating properties of the corannulene-di-NHC ligand were studied by means of infrared spectroscopy and cyclic voltammetry. The complex (corannulene-di-NHC)[RhCl(COD)]2 was used for the encapsulation of fullerenes C60 and C70, generating host-guest complexes with 2 : 1 stoichiometry, as evidenced by 1H NMR and ITC titrations. Then, a tetra-rhodium(i) metallo-rectangle supported by two corannulene-bis-imidazolylidene ligands and two cofacial 4,4'-bipyridine ligands was prepared and characterized. This metallobox is capable of quantitatively encapsulating fullerenes C60 and C70, forming complexes that are highly stable even at high temperatures. The molecular structure of the metallobox with encapsulated C60 reveals a perfect size and shape complementarity that benefits from the concave-convex π-π interaction between the polyaromatic surfaces of the host and the guest.

Tapping into the coordinative potential of a C-functional 1,4-diphosphabarrelene using two sets of complementary ligand centres

7,8-Dihydro-1,4-diphosphabarrelene diselones and bis(NHCs) were synthesised and employed as multitopic P,Se and P,C ligands in coordination chemistry, benefitting from a unique bent, P-bridged topology, thus being promising new building blocks.

Application of phosphorus-bridged rigid, bent bis(NHCs) as dipodal ligands in main group and transition metal chemistry

Phosphorus-bridged rigid, bent bis(N-heterocyclic) carbenes have not been reported, so far, despite having structural features that could make them interesting ligands in coordination and main group element chemistry. In previous reports, we had demonstrated that tuning of σ3- and σ4-phosphorus environments in planarised bis(NHCs) affects electronic properties and can provide additional coordination sites. Herein, we report on first examples of synthesis and conversion of 1,4-diphosphabarrelene-related compounds into rigid bent, doubly P-bridged bis(NHCs). The formation of main group element adducts with substrates from group 13, 14 and 15 illustrates opportunities to access novel scaffolds and to create nonplanar branching points. DFT calculations reveal the new bis(NHCs) to be good candidates as novel soft/hard ligands with up to four coordination sites. The synthesis of a dinuclear Fe(CO)4 complex is demonstrated. The thermal retro-[4 + 2] cycloaddition was theoretically and experimentally explored for a variety of ionic and zwitterionic 1,4-diphosphabarrelenes, and the generation and trapping of a dinuclear Fe(0) bis(NHC) complex with a tricyclic 1σ2,4 σ2-diphosphinine scaffold is presented.

Guest-Shuttling in a Nanosized Metallobox

An iridium-conjoined long and narrow metallorectangle was obtained by combining a quinoxalinophenanthrophenazine-connected Janus-di-imidazolylidene ligand and pyrazine. The size and shape of this assembly together with the fused polyaromatic nature of its panels provides it with properties that are uncommon for other metallosupramolecular assemblies. For example, this nanosized 'slit-like' metallobox is able show very large binding affinities with planar organic molecules in such a way, that the cavity is asymmetrically occupied by the guest molecule. This unsymmetrical conformation leads to the existence of a large amplitude motion of these guests, which slide between the two sides of the cavity of the host, thus constituting rare examples of molecular shuttles.

Design of a Mesoionic Janus-type Dicarbene

A versatile synthetic strategy for the preparation of homo- and heterobimetallic complexes bearing an unprecedented mesoionic Janus-type diNHC is presented. Moreover, its electronic property is evaluated, and a preliminary catalytic application in the direct diarylation of 1-methylpyrrole is demonstrated.

Nickelacarborane-Supported Bis-N-heterocyclic Carbenes

Metallacarboranes have attracted significant attention due to their unique properties. Considerable efforts have been made on the reactions around the metal centers or the metal ion itself, while transformations of functional groups of the metallacarboranes have been much less explored. We presented here the formation of imidazolium-functionalized nickelacarboranes (2), their subsequent conversion to nickelacarborane-supported N-heterocyclic carbenoids (NHCs, 3), and the reactivities of 3 toward Au(PPh₃)Cl and Se powder, which resulted in the formation of bis-gold carbene complexes (4) and NHC selenium adducts (5). Cyclic voltammetry of 4 shows two reversible peaks, corresponding to the interconversion transformations Ni^II ↔ Ni^III and Ni^III ↔ Ni^IV. Theoretical calculations demonstrated relatively high-lying lone-pair orbitals, weak B-H···H-C interactions between the BH units and the methyl group, and weak B-H···π interactions between the BH groups and the vacant p-orbital of the carbene.

Synthesis and a combined experimental/theoretical structural study of a comprehensive set of Pd/NHC complexes with o-, m-, and p-halogen-substituted aryl groups (X = F, Cl, Br, CF3)

Pd/NHC complexes (NHCs - N-heterocyclic carbenes) with electron-withdrawing halogen groups were prepared by developing an optimized synthetic procedure to access imidazolium salts and the corresponding metal complexes. Structural X-ray analysis and computational studies have been carried out to evaluate the effect of halogen and CF₃ substituents on the Pd-NHC bond and have provided insight into the possible electronic effects on the molecular structure. The introduction of electron-withdrawing substituents changes the ratio of σ-/π-contributions to the Pd-NHC bond but does not affect the Pd-NHC bond energy. Here, we report the first optimized synthetic approach to access a comprehensive range of o-, m-, and p-XC₆H₄-substituted NHC ligands, including incorporation into Pd complexes (X = F, Cl, Br, CF₃). The catalytic activity of the obtained Pd/NHC complexes was compared in the Mizoroki-Heck reaction. For substitution with halogen atoms, the following relative trend was observed: X = Br > F > Cl, and for all halogen atoms, the catalytic activity changed in the following order: m-X, p-X > o-X. Evaluation of the relative catalytic activity showed a significant increase in the catalyst performance in the case of Br and CF₃ substituents compared to the unsubstituted Pd/NHC complex.

Bis[cyclic (alkyl)(amino)carbene] isomers: Stable trans-bis(CAAC) versus facile olefin formation for cis-bis(CAAC)

Isomeric bis(aldiminium) salts with a 1,4-cyclohexylene framework were synthesized. The first isolable bis(CAAC) was prepared from the trans-stereoisomer and its ditopic ligand competency was proven by conversion to iridium(I) and rhodium(I) complexes. Upon deprotonation, the cis-isomer yielded an electron rich olefin via a classic, proton-catalyzed pathway. The CC bond formation from the desired cis-bis(CAAC) was shown to be thermodynamically very favorable and to involve a small activation barrier. Compounds that can be described as insertion products of the cis-bis(CAAC) into the E-H bonds of NH₃, CH₃CN and H₂O were also identified.

Synthesis and disassembly of an organometallic polymer comprising redox-active Co4S4 clusters and Janus biscarbene linkers

Here, we show for the first time that main-chain organometallic polymers (MCOPs) can be prepared from Janus N-heterocyclic carbene (NHC) linkers and polynuclear cluster nodes. The crosslinked framework Co₄S₄-MCOP is synthesized via ligand displacement reactions and undergoes reversible electron transfer in the solid state. Discrete molecular cluster species can be excised from the framework by digesting the solid in solutions of excess monocarbene. Finally, we demonstrate a synthetic route to monodisperse framework particles via coordination modulation.

Janus-type homo-, hetero- and mixed valence-bimetallic complexes with one metal encapsulated in a cyclodextrin

Bis-azolium salts with one azolium capping a perbenzylated α-cyclodextrin have been designed to generate Janus-type bimetallic complexes with various combinations of copper, silver, gold or palladium salts. Encapsulation of one metal center inside the cavity allowed (trans)metalation and oxidation reactions to be controlled at selected positions. In particular, it was possible to oxidize Au^I into Au^III selectively on the position outside the cavity of the cyclodextrin on the bis-Au^I Janus complex.

Bi- and trimetallic complexes with macrocyclic xanthene-4,5-diNHC ligands

Three different types of bimetallic NHC-metal complexes were synthesized, whose NHC units are attached at the 4,5-positions of xanthene. The NHC units are in close proximity and are designed such that each carbene coordinates one ML unit, while the chelation of one metal by two NHC is not possible. Several xanthene-((NHC)ML)₂ complexes with ML = RhCl(cod), IrCl(cod), RhCl(CO)₂, IrCl(CO)₂, AuCl, AgCl, CuCl and Pd(allyl)Cl were synthesized and investigated.