Carbenes with ferrocenyl substituents

Forever young: the first seventy years of ferrocene

The discovery of ferrocene, [Fe(η5-C5H5)2], seventy years ago has significantly influenced chemical research and provided a key impetus for establishing and rapidly expanding organometallic chemistry, which has continued at a...

Functionalised N-Heterocyclic Carbene Ligands in Bimetallic Architectures

N-Heterocyclic carbenes (NHCs) have become immensely successful ligands in coordination chemistry and homogeneous catalysis due to their strong terminal σ-donor properties. However, by targeting NHC ligands with additional functionalisation, a new area of NHC coordination chemistry has developed that has enabled NHCs to be used to build up bimetallic and multimetallic architectures. This minireview covers the development of functionalised NHC ligands that incorporate additional donor sites in order to coordinate two or more metal atoms. This can be through the N-atom of the NHC ring, through a donor group attached to the N-atom or the carbon backbone, coordination of the π-bond or an annulated π-donor on the backbone, or through direct metalation of the backbone.

Synthesis and characterisation of Pd(ii) and Au(i) complexes with mesoionic carbene ligands bearing phosphinoferrocene substituents and isomeric carbene moieites

While numerous functional ligands combining phosphine and imidazole-2-ylidene (or imidazolin-2-ylidene) donor moieties have already been reported, the chemistry of the corresponding functional mesoionic carbenes (MIC) derived from 1,2,3-triazoles remains nearly untapped. This contribution describes the synthesis of two isomeric series of triazolium salts bearing 1'-(diphenylphosphino)ferrocenyl substituents by [3 + 2] cycloaddition of a P-protected phosphinoferrocene alkyne with azides, or alternatively of a P-protected phosphinoferrocene azide with terminal alkynes, and by subsequent methylation. These salts were used to synthesize structurally unique Pd(ii) complexes featuring a P,C-chelating triazolylidene carbene ligands and Au(i)-MIC complexes with free phosphine groups. The latter were further utilised to prepare Pd(ii)Au(i) heterometallic complexes containing bridging ferrocene phosphino-carbenes as structurally flexible, donor-unsymmetric metalloligands. In addition, the reactivity of the newly prepared, P-protected phosphinoferrocene alkyne Ph₂PfcC[triple bond, length as m-dash]CH·BH₃ (fc = ferrocene-1,1'-diyl) was investigated, and representatives from all reported compound classes were structurally characterised.

Highly Electrophilic, Catalytically Active and Redox-Responsive Cobaltoceniumyl and Ferrocenyl Triazolylidene Coinage Metal Complexes

A convenient access to a triad of triazoles with ferrocenyl and cobaltoceniumyl substituents is reported. N-Alkylation, deprotonation and metalation with CuI /AgI /AuI synthons affords the heteroleptic triazolylidene complexes. Due to the combination of neutral, electron-donating ferrocenyl substituents and cationic, strongly electron-withdrawing cobaltocenium substituents, the mesoionic carbene (MIC) ligands of these complexes are electronically interesting "push-pull", "pull-push" and "pull-pull" metalloligands with further switchable redox states based on their fully reversible FeII /FeIII , (ferrocene/ferrocenium) and CoIII /CoII , (cobaltocenium/cobaltocene) redox couples. These are the first examples of metal complexes of (di)cationic NHC ligands based on cobaltoceniumyl substituents. DFT calculated Tolman electronic parameter (TEP) of the new MIC ligands, show these metalloligands to be extremely electron-poor NHCs with properties unmatched in other carbene chemistry. Utilization of these multimetallic electronically tunable compounds in catalytic oxazoline synthesis and in antitumor studies are presented. Remarkably, 1 mol % of the AuI complex with the dicationic MIC ligand displays full catalytic conversion, without the need for any other additives, in less than 2 hours at ambient temperatures. These results thus firmly establish these new classes of cobaltoceniumyl based (di)cationic MIC ligands as prominent players in several branches of chemistry.

Smart N-Heterocyclic Carbene Ligands in Catalysis

It is well-recognized that N-heterocyclic carbene (NHC) ligands have provided a new dimension to the design of homogeneous catalysts. Part of the success of this type of ligands resides in the limitless access to a variety of topologies with tuned electronic properties, but also in the ability of a family of NHCs that are able to adapt their properties to the specific requirements of individual catalytic transformations. The term "smart" is used here to refer to switchable, multifunctional, adaptable, or tunable ligands and, in general, to all those ligands that are able to modify their steric or electronic properties to fulfill the requirements of a defined catalytic reaction. The purpose of this review is to comprehensively describe all types of smart NHC ligands by focusing attention on the catalytically relevant ligand-based reactivity.

A Redox-Switchable Germylene and its Ligating Properties in Selected Transition Metal Complexes

The synthesis, structure, and full characterization of a redox-switchable germylene based on a [3]ferrocenophane ligand arrangement, [Fc(NMes)₂ Ge] (4), is presented. The mesityl (Mes)-substituted title compound is readily available from Fc(NHMes)₂ (2) and Ge{N(SiMe₃ )₂ }₂ , or from the dilithiated, highly air- and moisture-sensitive compound Fc(NLiMes)₂ ⋅3 Et₂ O (3) and GeCl₂ . Cyclic voltammetry studies are provided for 4, confirming the above-mentioned view of a redox-switchable germylene metalloligand. Although several 1:1 Rh^I and Ir^I complexes of 4 (5-7) are cleanly formed in solution, all attempts to isolate them in pure form failed due to stability problems. However, crystalline solids of [Mo(κ¹ Ge-4)₂ (CO)₄ ] (8) and [W(κ¹ Ge-4)₂ (CO)₄ ] (9) were isolated and fully characterized by common spectroscopic techniques (8 by X-ray diffraction). DFT calculations were performed on a series of model compounds to elucidate a conceivable interplay between the metal atoms in neutral and cationic bimetallic complexes of the type [Rh(κ¹ E-qE)(CO)₂ Cl]^0/+ (qE=[Fc(NPh)₂ E] with E=C, Si, Ge). The bonding characteristics of the coordinated Fc-based metalloligands (qE/qE⁺ ) are strongly affected upon in silico oxidation of the calculated complexes. The calculated Tolman electronic parameter (TEP) significantly increases by approximately 20 cm^-1 (E=C) to 25 cm^-1 (E=Si, Ge) upon oxidation. The change in the ligand-donating abilities upon oxidation can mainly be attributed to Coulombic effects, whereas an orbital-based interaction appears to have only a minor influence.

Diferrocenyl tosyl hydrazone with an ultrastrong NH⋯Fe hydrogen bond as double click switch

The intramolecular NH⋯Fe hydrogen bond in diferrocenyl hydrazone 2 raises the barrier for intramolecular electron transfer in its mixed-valent cation 2+ and is only disrupted by double oxidation to 22+.

1-Ferrocenylmethyl-3-(2,4,6-trimethylbenzyl)-1H-imidazolidin-3-ium iodide and trans-bis(3-benzyl-1-ferrocenylmethyl-1H-imidazolidin-2-ylidene)diiodidopalladium(II)

Owing to increasing interest in the use of N-heterocyclic carbenes (NHCs) based on imidazolidinium ions as ligands in the design of highly efficient transition-metal-based homogeneous catalysts, the characterizations of the 1-ferrocenylmethyl-3-(2,4,6-trimethylbenzyl)imidazolidin-3-ium iodide salt, [Fe(C(5)H(5))(C(19)H(24)N(2))]I, (I), and the palladium complex trans-bis(3-benzyl-1-ferrocenylmethyl-1H-imidazolidin-2-ylidene)diiodidopalladium(II), [Fe(2)Pd(C(5)H(5))(2)(C(16)H(17)N(2))(2)I(2)], (II), are reported. Compound (I) has two iodide anions and two imidazolidinium cations within the asymmetric unit (Z' = 2). The two cations have distinctly different conformations, with the ferrocene groups orientated exo and endo with respect to the N-heterocyclic carbene. Weak C-H donor hydrogen bonds to both the iodide anions and the π system of the mesitylene group combine to form two-dimensional layers perpendicular to the crystallographic c direction. Only one of the formally charged imidazolidinium rings forms a near-linear hydrogen bond with an iodide anion. Complex (II) shows square-planar coordination around the Pd(II) metal, which is located on an inversion centre (Z' = 0.5). The ferrocene and benzyl substituents are in a trans-anti arrangement. The Pd-C bond distance between the N-heterocyclic carbene ligands and the metal atom is 2.036 (7) Å. A survey of related structures shows that the lengthening of the N-C bonds and the closure of the N-C-N angle seen here on metal complexation is typical of similar NHCs and their complexes.

A persistent (amino)(ferrocenyl)carbene()

Deprotonation of N,N-diisopropyl-C-ferrocenylaldiminium triflate 2 cleanly leads to the corresponding 1,2-diamino-1,2-diferrocenylethene 3, the dimer of the desired (amino)(ferrocenyl)carbene. Fulvene 6, obtained by addition of the lithium salt of tetramethylcyclopentadiene to methoxyformamidinium methylsulfate 5, reacts with dicarbonylcyclopentadienylbromoiron(II), and with a mixture of FeCl(2) and Cp* lithium salt, affording the corresponding tetramethylferrocenylaldiminium salt 7, and nonamethylferrocenylaldiminium salt 8, respectively. Although the deprotonation of 7 gives a complex mixture of products, the treatment of 8 at -78 °C with sodium hexamethyldisilazide allowed for the isolation of the corresponding (amino)(ferrocenyl)carbene 9 as a yellow powder. However, even in the solid state, it is stable for less than 48 h at -20 °C. In addition to NMR spectroscopy, evidence for the carbene nature of 9 was found by a trapping experiment with sulfur that leads to the corresponding adduct 10, which was characterized by a single crystal X-ray diffraction study.

1,1'-Bis(N-benzimidazolylidene)ferrocene: synthesis and study of a novel ditopic ligand and its transition metal complexes

Diiridum complexes containing 1,1'-bis(N-benzimidazolylidene)ferrocene, a novel ditopic ligand comprised of two N-heterocyclic carbenes (NHCs) linked directly via their N-substituents to each cyclopentadienyl ring of a ferrocene moiety, were synthesized. Crystallographic analyses of these C(2)-symmetric bimetallic complexes revealed the benzimidazolylidene moieties were intramolecularly stacked in nearly opposing orientations, effectively forming Janus-type bis(NHC) structures in the solid state. Using a variety of electrochemical techniques, the oxidation potentials of the ferrocenyl groups in these complexes were found to depend on the auxillary ligands coordinated to the Ir centers (i.e., 1,5-cyclooctadiene vs. carbonyl). Similarly, the nu(CO) of carbonyls ligated to the Ir centers varied in accord with the oxidation state of the ferrocene contained with the bis(NHC) ligand. These results suggest that the Ir and Fe centers in these complexes are electronically coupled and that the electron donating ability of the bis(NHC) ligand reported herein can be tuned electrochemically.

Carbene complexes of gold: preparation, medical application and bonding

New preparative methods for gold carbene complexes have been developed and older ones modified to prepare compounds with specific inherent properties for targeted applications. One of the important areas of application that has grown rapidly and wherein carbene complexes are increasingly significant falls within the field of medicine. Sophisticated theoretical calculations have accompanied many synthetic studies. These aspects are covered in this critical review (65 references).

The 1,4-Diphosphabuta-1,3-diene Ligand for Coordination of Divalent Group 13 and 14 Elements: A Density Functional Study

The 1,4-diphosphabuta-1,3-diene (DPB) ligand as a tool for stabilizing anionic group 13 (B, Al, Ga) and neutral group 14 (C, Si, Ge) cyclic Arduengo-type carbenes is studied by quantum chemical calculations at density functional level. Accordingly, for the former group this ligand is better suited than the corresponding 1,4-diazabuta-1,3-diene (DAB) ligand. It results in larger electron affinities for the corresponding doublet states. For the latter group the DPB ligand yields essentially smaller singlet-triplet separations than the DAB ligand. An exception is the anionic boron compound with relative low singlet stability for both the DAB and DPB ligands.