Stepwise Preparation of a Molecular Square from NR,NR- and NH,O-Substituted Dicarbene Building Blocks

Unravelling the Roles of Solvophobic Effects and π⋅⋅⋅π Stacking Interactions in the Formation of [2]Catenanes Featuring Di-(N-Heterocyclic Carbene) Building Blocks

A series of [2]catenanes has been prepared from di-NHC building blocks by utilizing solvophobic effects and/or π⋅⋅⋅π stacking interactions. The dinickel naphthobiscarbene complex syn-[1] and the kinked biphenyl-bridged bipyridyl ligand L2 yield the [2]catenane [2-IL](OTf)4 by self-assembly. Solvophobic effects are pivotal for the formation of the interlocked species. Substitution of the biphenyl-linker in L2 for a pyromellitic diimide group gave ligand L3 , which yielded in combination with syn-[1] the [2]catenane [3-IL](OTf)4 . This assembly exhibits enhanced stability in diluted solution, aided by additional π⋅⋅⋅π stacking interactions. The π⋅⋅⋅π stacking was augmented by the introduction of a pyrene bridge between two NHC donors in ligand L4 . Di-NHC precursor H2 -L4 (PF6 )2 reacts with Ag2 O to give the [Ag2 L4 2 ]2 [2]catenane [4-IL](PF6 )4 , which shows strong π⋅⋅⋅π stacking interactions between the pyrene groups. This assembly was readily converted into the [Au2 L4 2 ]2 gold species [5-IL](PF6 )4 , which exhibits exceptional stability based on the strong π⋅⋅⋅π stacking interactions and the enhanced stability of the Au-CNHC bonds.

Organometallic Borromean Rings and [2]Catenanes Featuring Di-NHC Ligands

We report herein a series of organometallic Borromean rings (BRs) and [2]catenanes prepared from benzobiscarbene ligands. The reaction of dinickel complexes of the benzobiscarbenes 1 a-1 c with a thiazolothiazole bridged bipyridyl ligand L² led by self-assembly to a series of organometallic BRs. Solvophobic effects played a crucial role in the formation and stability of the interlocked species. The stability of BRs is related to the N-alkyl substituents at the precursors 1 a-1 c, where longer alkyl substitutes improve stability and inter-ring interactions. Solvophobic effects are also important for the stability of [2]catenanes prepared from 1 a-1 c and a flexible bipyridyl ligand L³ . In solution, an equilibrium between the [2]catenanes and their macrocyclic building blocks was observed. High proportions of [2]catenanes were obtained in concentrated solutions or polar solvents. The proportion of [2]catenanes in solution could be further enhanced by lengthening of the N-alkyl substitutes.

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

Janus di-N-heterocyclic carbene (NHC) ligands are a subclass of poly-NHCs that feature coordination to two transition metals in a facially opposed manner. The combination of the structural features of Janus type ligands, with the properties conferred by the NHC ligands, has conferred Janus-di-NHCs with privileged attributes for their use in diverse areas of research, such as homogeneous catalysis, materials chemistry and supramolecular chemistry. In molecular chemistry, Janus di-NHCs constitute one of the most useful chemical platforms for constructing dimetallic structures, and this includes both homo- and hetero-dimetallic compounds. This review aims to cover the most relevant advances in the use of Janus-di-NHCs during the last 15 years, by classifying them according to their specific structural features.

A 2,2'-diphosphinotolane as a versatile precursor for the synthesis of P-ylidic mesoionic carbenes via reversible C-P bond formation

A metal-templated synthetic route to cyclic (aryl)(ylidic) mesoionic carbenes (CArY-MICs) featuring an endocyclic P-ylide is presented. This approach, which requires metal templates with two cis-positioned open coordination sites, is based on the controlled cyclisation of a P,P'-diisopropyl-substituted 2,2'-diphosphinotolane (1) and leads to chelate complexes coordinated by a phosphine donor and the CArY-MIC carbon atom. The C-P bond formation involved in the former partial cyclisation of 1 proceeds under mild conditions and was shown to be applicable all over the d-block. In the presence of a third fac-positioned open coordination site, the P-C bond formation was found to be reversible, as shown for a series of molybdenum complexes. DFT modelling studies are in line with an interpretation of the target compounds as CArY-MICs.

A tropylium annulated N-heterocyclic carbene

Derivatives of the cationic tropylium annulated imidazolylidene ITrop⁺ are obtained by hydride abstraction from related cycloheptatriene compounds. Spectroscopic, structural and theoretical data indicate that, as a cationic relative of benzimidazolylidenes, ITrop⁺ has highly reduced σ-donor and strong π-acceptor character.

N-Heterocyclic Carbenes: A Door Open to Supramolecular Organometallic Chemistry

The field of metallosupramolecular chemistry is clearly dominated by the use of O-, N-, and P-donor Werner-type polydentate ligands. These molecular architectures are of high interest because of their wide range of applications, which include molecular encapsulation, stabilization of reactive species, supramolecular catalysis, and drug delivery, among others. Only recently, organometallic ligands have allowed the preparation of a variety of supramolecular coordination complexes, and the term supramolecular organometallic complexes (SOCs) is gaining space within the field of metallosupramolecular chemistry. While the early examples of SOCs referred to supramolecular architectures mostly containing bisalkenyl, diphenyl, or bisalkynyl linkers, the development of SOCs during the past decade has been boosted by the parallel development of multidentate N-heterocyclic carbene (NHC) ligands. The first examples of NHC-based SOCs referred to supramolecular assemblies based on polydentate NHC ligands bound to group 11 metals. However, during the last 10 years, several planar poly-NHC ligands containing extended π-conjugated systems have facilitated the formation of a large variety of architectures in which the supramolecular assemblies can contain metals other than Cu, Ag, and Au. Such ligands are Janus di-NHCs and trigonal-planar tris-NHCs-most of them prepared by our research group-which have allowed the preparation of a vast range of NHC-based metallosupramolecular compounds with interesting host-guest chemistry properties. Although the number of SOCs has increased in the past few years, their use for host-guest chemistry purposes is still in its earliest infancy. In this Account, we describe the achievements that we have made during the last 4 years toward broadening the applications of planar extended π-conjugated NHC ligands for the preparation of organometallic-based supramolecular structures, including their use as hosts for some selected organic and inorganic guests, together with the catalytic properties displayed by some selected host-guest inclusion complexes. Our contribution describes the design of several Ni-, Pd-, and Au-based metallorectangles and metalloprisms, which we used for the encapsulation of several organic substrates, such as polycyclic aromatic hydrocarbons (PAHs) and fullerenes. The large binding affinities found are ascribed to the incorporation of two cofacial panels with large π-conjugated systems, which provide the optimum conditions for guest recognition by π-π-stacking interactions. We also describe a series of digold(I) metallotweezers for the recognition of organic and inorganic substrates. These metallotweezers were used for the recognition of "naked" metal cations and polycyclic aromatic hydrocarbons. The recognition properties of these metallotweezers are highly dependent on the nature of the rigid connector and of the ancillary ligands that constitute the arms of the tweezer. A peculiar balance between the self-aggregation properties of the tweezer and its ability to encapsulate organic guests is observed.

A stable Cerberus tris(maloNHC) and its coinage metal complexes

The first stable trisanionic Cerberus-type tris(maloNHC) was synthesized and incorporated into two different zwitterionic coinage metal complexes (Au(i) and Ag(i)).

Anionic Hosts for the Incorporation of Cationic Guests

Pentaphosphaferrocene [Cp*Fe(η5 -P5 )] (1 a) represents an excellent building block for the template-directed synthesis of spherical supramolecules. Here, the self-assembly of 1 a with CuI and CuII halides in the presence of the template complexes [FeCp2 ][PF6 ], [CoCp2 ][PF6 ] and [CoCp2 ] is reported, testifying to the redox behavior of the formed supramolecules. The oxidation or reduction capacity of these reactive complexes does not inhibit their template impact and, for the first time, the cationic metallocene [CoCp2 ]+ is enclosed in unprecedented anionic organometallic hosts. Furthermore, the large variety of structural motifs, as icosahedral, trigonal antiprismatic, cuboidal and tetragonal antiprismatic arrangements of 1 a units are realized in the supramolecules [FeCp2 ]@[{1 a}12 (CuBr)17.3 ] (3), [CoCp2 ]+3 {[CoCp2 ]+ @[{1 a}8 Cu24.25 Br28.25 (CH3 CN)6 ]4- } (4), {[Cp2 Co]+ @[{1 a}8 (CuI)28 (CH3 CN)9.8 ]}{[Cp2 Co]+ @[{1 a)}8 Cu24.4 I26.4 (CH3 CN)8 ]2- } (5), and [{1 a}3 {(1 a)2 NH}3 Cu16 I10 (CH3 CN)7 ] (6), respectively.

A macrocyclic silver polycarbene complex based on 1,2,4-triazole units: synthesis and postsynthetic modification

A method for the synthesis of a new rectangular 1,2,4-triazole-containing macrocycle via photochemical [2 + 2] cycloaddition with a metal–carbene template was developed.

Metallosupramolecular Architectures Obtained from Poly-N-heterocyclic Carbene Ligands

Over the past two decades, self-assembly of supramolecular architectures has become a field of intensive research due to the wide range of applications for the resulting assemblies in various fields such as molecular encapsulation, supramolecular catalysis, drug delivery, metallopharmaceuticals, chemical and photochemical sensing, and light-emitting materials. For these purposes, a large number of coordination-driven metallacycles and metallacages featuring different sizes and shapes have been prepared and investigated. Almost all of these are Werner-type coordination compounds where metal centers are coordinated by nitrogen and/or oxygen donors of polydentate ligands. With the evolving interest in the coordination chemistry of N-heterocyclic carbenes (NHCs), discrete supramolecular complexes held together by M-C_NHC bonds have recently become of interest. The construction of such metallosupramolecular assemblies requires the synthesis of suitable poly-NHC ligands where the NHC donors form labile bonds with metal centers thus enabling the formation of the thermodynamically most stable reaction product. In organometallic chemistry, these conditions are uniquely met by the combination of poly-NHCs and silver(I) ions where the resulting assemblies also offer the possibility to generate new structures by transmetalation of the poly-NHC ligands to additional metal centers forming more stable C_NHC-M bonds. Stable metallosupramolecular assemblies obtained from poly-NHC ligands feature special properties such as good solubility in many less polar organic solvents and the presence of the often catalyticlly active {M(NHC)_n} moiety as building block. In this Account, we review recent developments in organometallic supramolecular architectures derived from poly-NHC ligands. We describe dinuclear (M = Ag^I, Au^I, Cu^I) tetracarbene complexes obtained from bis-NHC ligands with an internal olefin or two external coumarin pendants and their postsynthetic modification via a photochemically induced single or double [2 + 2] cycloaddition to form dinuclear tetracarbene complexes featuring cyclobutane units. Even three-dimensional cage-like structures can be prepared by this postsynthetic strategy. Cylinder-like trinuclear, tetranuclear, and hexanuclear (M = Ag^I, Au^I, Cu^I, Hg^II, Pd^II) complexes have been obtained from benzene-bridged tris-, tetrakis-, or hexakis-NHC ligands. These complexes resemble polynuclear assemblies obtained from related polydentate Werner-type ligands. Contrary to the Werner-type complexes, cylinder-like assemblies with three, four, or six silver(I) ions sandwiched in between two tris-, tetrakis-, or hexakis-NHC ligands undergo a facile transmetalation reaction to give the complexes featuring more stable M-C_NHC bonds, normally with retention of the metallosupramolecular structure. This unique behavior of NHC-Ag⁺ complexes allows the prepration of assemblies containing various metals from the poly-NHC silver(I) assemblies. Narcissistic self-sorting phenomena have also been observed for mixtures of selected poly-NHC ligands and silver(I) ions. Even a very early type of metallosupramolecular assembly, the tetranuclear molecular square, can be prepared from four bridging dicarbene ligands and four transition metal ions either by a stepwise assembly or by a single-step protocol. At this point, it appears that procedures for the synthesis of metallosupramolecular assemblies using polydentate Werner-type ligands and metal ions can be transferred to organometallic chemistry by using suitable poly-NHC ligands. The resulting structures feature stable M-C_NHC bonds (with the exception of the labile C_NHC-Ag⁺ bond) when compared to M-N/M-O bonds in classical Werner-type complexes. The generally good solubility of the compounds and the presence of the often catalytically active {M(NHC)_n} moiety make organometallic supramolecular complexes a promising new class of molecular hosts for catalytic transformations and encapsulation of selected substrates.

Diversity of reactivity modes upon interplay between Au(iii)-bound isocyanides and cyclic nitrones: a theoretical consideration

Metal-bound isocyanides demonstrate a very rich chemistry towards 1,3-dipoles of the allyl anion type such as nitrones. Depending on the metal centre, dipole nature and substituent in isocyanides, cycloadducts, imine + isocyanates or metallacycles may be formed. In this work, the reasons for such diversity, intimate details of the reaction mechanism and main factors determining the chemoselectivity in the reaction between Au(iii)-bound isocyanides ([AuCl₃(C[triple bond, length as m-dash]NR)]) and cyclic nitrones ^-O-N[combining low line]⁺[double bond, length as m-dash]C(H)XXX[combining low line] (XXX = CH₂CH₂CMe₂, CH₂CH₂CH₂, OCH₂CMe₂, CH₂OCMe₂ and CH₂CH₂O) are analysed in detail by theoretical (DFT) methods. The formation of cycloadducts is controlled by the LUMO_{π*(C[triple bond, length as m-dash]N)} of isocyanides and it occurs in a stepwise manner via the initial nucleophilic addition of nitrone at the C atom of C[triple bond, length as m-dash]NR (except for nitronate ^-O-N[combining low line]⁺[double bond, length as m-dash]C(H)CH₂CH₂O[combining low line]). The imine + isocyanate products are formed upon oxygen transfer from nitrones to isocyanides, and N-O bond cleavage is the main factor determining this process. A metallacycle is formed upon decomposition of the cycloadduct, and this process includes deprotonation of the oxadiazoline CH group/N-O bond cleavage and Cl^- elimination/cyclization. The main factor controlling the metallacycle formation is the acidity of the endocyclic CH group in the cycloadduct. Effects of the substituent R in C[triple bond, length as m-dash]NR and the nitrone nature on the reactivity are analysed.

Addition of N-nucleophiles to gold(iii)-bound isocyanides leading to short-lived gold(iii) acyclic diaminocarbene complexes

Addition of hydrazone to gold(iii)–isocyanides led to the generation of rare short-lived gold(iii) acyclic diaminocarbene complexes.

Synthesis and structural characterization of metal complexes with macrocyclic tetracarbene ligands

Fourteen Ag(i), Au(i), Ni(ii), Pd(ii) and Pt(ii) complexes were prepared with macrocyclic tetradentate N-heterocyclic carbene (NHC) ligands.

Palladium(ii)-Acetylacetonato Complexes with Mesoionic Carbenes: Synthesis, Structures and Their Application in the Suzuki-Miyaura Cross Coupling Reaction

A series of novel palladium(ii) acetylacetonato complexes bearing mesoionic carbenes (MICs) have been synthesized and characterized. The synthesis of the complexes of type (MIC)Pd(acac)I (MIC = 1-mesityl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene (1), 1,4-(2,4,6-methyl)-phenyl-3-methyl-1,2,3-triazol-5-ylidene (2), 1,4-(2,6-diisopropyl)-phenyl-3-methyl-1,2,3-triazol-5-ylidene (3); acac = acetylacetonato) via direct metalation starting from the corresponding triazolium iodides and palladium(ii) acetylacetonate is described herein. All complexes were characterized by ¹H- and ¹³C-NMR spectroscopy and high resolution mass spectrometry. Additionally, two of the complexes were characterized by single crystal X-ray crystallography confirming a square-planar coordination geometry of the palladium(ii) center. A delocalized bonding situation was observed within the triazolylidene rings as well as for the acac ligand respectively. Complex 2 was found to be an efficient pre-catalyst for the Suzuki-Miyaura cross coupling reaction between aryl-bromides or -chlorides with phenylboronic acid.

Oxidative addition of N-ether-functionalized 2-chlorobenzimidazole to d10 metals

Reaction of 2-chloro-N-(methoxymethyl)benzimidazole 1 with zerovalent group 10 metal complexes in the presence of an additional proton source yielded, via an oxidative addition of the C2–Cl bond, complexes with a protic NH,NR-substituted (R=methoxymethyl) benzimidazolin-2-ylidene ligand. The oxidative addition of 1 to Ni0 and Pd0 complexes proceeded with the exclusive formation of the trans-configured complexes trans-[2]BF4 and trans-[3]BF4, respectively. Contrary to this observation, the reaction of 1 with a more substitution inert Pt0 complex leads, depending on the reaction temperature, to a mixture of cis-/trans-[4]BF4 or exclusively to trans-[4]BF4. The molecular structures of all three trans-configured complexes were determined.