Synthesis, Reactivity, and DFT Studies of Tantalum Complexes Incorporating Diamido-N-heterocyclic Carbene Ligands. Facile Endocyclic C−H Bond Activation

Reductive Hydrogenation under Single-Site Control: Generation and Reactivity of a Transient NHC-Stabilized Tantalum(III) Alkoxide

One of the most attractive routes for the preparation of reactive tantalum(III) species relies on the efficient salt-free hydrogenolysis of tantalum(V) alkyls or tantalum(V) alkylidenes, a process known as reductive hydrogenation. For silica-crafted tantalum alkyls and alkylidenes, this process necessarily proceeds at well-separated tantalum centers, while related reductive hydrogenations in homogeneous solution commonly involve dimeric complexes. Herein, an NHC scaffold was coordinated to a novel tri(alkoxido)tantalum(V) alkylidene to circumvent the formation of dimers during reductive hydrogenation. Employing this new model system, a key intermediate of the process, namely a hydrido-tantalum alkyl, was isolated for the first time and shown to exhibit a bidirectional reactivity. Upon being heated, the latter complex was found to undergo either an α-elimination or a reductive alkane elimination. In the (overall unproductive) α-elimination step, H₂ and the parent alkylidene were regenerated, while the sought-after transient d²-configured tantalum(III) derivative was produced along the reaction coordinate of the reductive alkane elimination. The reactive low-valence metal center was found to rapidly attack one of the NHC substituents via an oxidative C-H activation, which led to the formation of a cyclometalated tantalum(V) hydride. The proposed elemental steps are in line with kinetic data, deuterium labeling experiments, and density functional theory (DFT) modeling studies. DFT calculations also indicated that the S = 0 spin ground state of the Ta(III) center plays a crucial role in the cyclometalation reaction. The cyclometalated Ta(V) hydride was further investigated and reacted with several alkenes and alkynes. In addition to a rich insertion and isomerization chemistry, these studies also revealed that the former hydride may undergo a formal cycloreversion and thus serve as a tantalum(III) synthon, although the original tantalum(III) intermediate is not involved in this process. The latter reactivity was observed upon reaction with internal alkynes and led to the corresponding η²-alkyne derivatives via vinyl intermediates, which rearrange via a remarkable, hitherto unprecedented, hydrogen shift reaction.

Mercury(II) Complexes of Anionic N-Heterocyclic Carbene Ligands: Steric Effects of the Backbone Substituent

Mercury(II) complexes (Me-maloNHC_Dipp)HgCl (1b), (t-Bu-maloNHC_Dipp)HgCl (2b) and (t-Bu-maloNHC_Dipp)HgMe (2c) supported by anionic N-heterocyclic carbenes have been obtained in good yields from the reaction of the potassium salt of N-heterocyclic carbene ligand precursors and mercury(II) salts, HgCl₂ and MeHgI. These molecules have been characterized by ¹H-NMR, ¹³C-NMR and IR spectroscopy and elemental analysis. X-ray crystal structures of 1b and 2b are also presented. Interestingly, complex 1b is polymeric {(Me-maloNHC_Dipp)HgCl}_n in the solid state, as a result of inter-molecular Hg-O contacts, and features rare three coordinate mercury sites with a T-shaped arrangement, whereas the (t-Bu-maloNHC_Dipp)HgCl (2b) is monomeric and has a linear, two-coordinate mercury center. The formation of T-shaped structure and the aggregation of complex 1b is attributable to the reduced steric demand of the N-heterocyclic carbene ligand backbone substituent.

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.

NHC Core Pincer Ligands Exhibiting Two Anionic Coordinating Extremities

The chemistry of NHCcore pincer ligands of LX₂ type bearing two pending arms, identical or not, whose coordinating center is anionic in nature, is here reviewed. In this family, the negative charge of the coordinating atoms can be brought either by a carbon atom via a phosphonium ylide (R₃P⁺-CR₂^-) or by a heteroatom through amide (R₂N^-), oxide (RO^-), or thio(seleno)oxide (RS^-, RSe^-) donor functionalities. Through selected examples, the synthetic methods, coordination properties, and applications of such tridentate systems are described. Particular emphasis is placed on the role of the donor ends in the chemical behavior of these species.

Lability of Ta–NHC adducts as a synthetic route towards heterobimetallic Ta/Rh complexes

This work highlights the reactivity of Ta–NHC adducts and the aptitude of the NHC motif to transfer from Ta to Rh which is used with profit as an efficient synthetic route to access early/late heterobimetallic complexes.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

Coordination complexes of niobium and tantalum pentahalides with a bulky NHC ligand

The reactivity of niobium and tantalum pentahalides with a bulky NHC ligand is described, including the first crystallographic characterization of a Ta complex with a monodentate NHC ligand.

Synthesis and reactivity of Li and TaMe3 complexes supported by N,N′-bis(2,6-diisopropylphenyl)-o-phenylenediamido ligands

Redox-active diamido ligands sponsor heterodinuclear TaLi complexes and a TaMe₃ species which undergoes photoreduction to yield a Ta(iv) dimer.

Mono- and bis-N-heterocyclic carbene complexes of tantalum and niobium with high oxidation states

Reactions of two free NHCs 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and 1,3-dimesitylimidazol-2-yliden (IMes) with group VB metals MCl₅ (M = Ta, Nb) gave a mono-(NHC) complex 1 and two bis-(NHC) complexes 2 and 3 with the high oxidation states of Nb(v) and Ta(v).

Synthesis and reactivity of cyclometalated triamidophosphine complexes of niobium and tantalum

The triamidophosphine protioligand 1 reacts with the homoleptic pentakis(dimethylamido) precursors of niobium and tantalum [M(NMe2)5, where M = Nb, Ta] to form cyclometalated complexes of the type [N2PCN-κ(5)-N,N,P,C,N]M(NMe2) (2-M). Apart from the three amido donors, one benzylic position of the ligand backbone is deprotonated over the course of this reaction, resulting in the formation of a new M-C bond. As a consequence, a metallaziridine substructure is formed, and the triamidophosphine moiety thus serves as a tetraanionic pentadentate ligand. The dimethylamido complexes 2-M can be converted into the corresponding triflates [N2PCN-κ(5)-N,N,P,C,N]M(OTf) (3-M) and alkyl complexes [N2PCN-κ(5)-N,N,P,C,N]M(CH2SiMe3) (4-M) by treatment with triethylsilyl triflate (Et3SiO3SCF3) followed by (trimethylsilyl)methyllithium (LiCH2SiMe3). The alkyl complexes exhibit interesting reactivities, including a second cyclometalative backbone activation affording the trimethylphosphine-stabilized complexes [NP(CN)2-κ(6)-N,P,C,N,C,N]M(PMe3) (5-M). In the case of tantalum, the formation of a dinuclear hydrido complex (6) is observed upon hydrogenation of 4-Ta. In the case of niobium, the metallaziridine substructure in 4-Nb is prone to ring opening via protonation with triphenylsilylamine (Ph3SiNH2), resulting in formation of the corresponding imido complex [PN3-κ(4)-P,N,N,N]Nb=NSiPh3 (7).

Terminal phosphinidene formation via tantalaziridine complexes

Terminal, 4-coordinate phosphinidenes of Ta supported by bulky anilide ligands are prepared by an apparent reaction sequence involving metallaziridine phosphanide complexes.

Tantallaaziridines: from synthesis to catalytic applications

Tantallaaziridines are a class of organometallic compounds containing three-membered rings that include a tantalum, carbon and nitrogen atom. Closely related complexes containing the related iminoacyl ligand can be used as starting materials for tantallaaziridine synthesis. Tantallaaziridines can also be synthesized by reduction of imines or β-hydrogen abstraction of amides to give stable, isolable complexes. Tantallaaziridines possess a reactive Ta-C bond that can undergo stoichiometric transformations with unsaturated organic molecules to give new organometallic products. Upon quenching of such reactions with aqueous workup, selectively substituted small molecule amines can be prepared. Tantallaaziridines have also been proposed as catalytic intermediates in the direct α-alkylation of amines, hydroaminoalkylation, by exploiting the β-hydrogen abstraction route to regenerate the catalytically active species. Recent progress and remaining challenges in the field will be discussed.

Synthesis and reactivity of niobium complexes having a tripodal triaryloxide ligand in bidentate, tridentate, and tetradentate coordination modes

The synthesis and reactivity of niobium complexes incorporating a tripodal triphenol (tris(3,5-tert-butyl-2-hydroxylphenyl)methane = H(3)[O(3)]) have been investigated. Addition of one equivalent of NbCl(5) in CH(3)CN to H(3)[O(3)] in toluene led to partial HCl elimination, giving [H(O(3))]NbCl(3)(CH(3)CN) (1) with a bidendtate bis(aryloxide) ligand and a pendant phenol arm. Treatment of 1 with THF afforded [H(O(3))]NbCl(3)(THF) (2). Deprotonation of 1 with NEt(3) in toluene promoted coordination of the pendant phenol group to generate (Et(3)NH)[(syn-O(3))NbCl(3)] (3-syn). Prolonged heating of 3-syn resulted in clean conversion to the anti isomer (3-anti). Attempted deprotonation of 2 with PhCH(2)MgCl provided [H(O(3))]Nb(CH(2)Ph)(3) (4), in which alkylation took place at the metal center but the pendant phenol arm remained intact. When 3-syn was treated with PhCH(2)MgCl, [O(3)C]Nb(CH(2)Ph) (5) was produced via C-H activation of the methine C-H bond. The analogous reaction with 3-anti provided a benzylidene complex [anti-O(3)]Nb(CHPh)(THF) (6). During the course of the reaction, the anti ligand conformation is retained. Upon heating, 4 underwent methine C-H and phenol O-H activation, yielding the metalatrane 5. Complexes 1, 3-syn, 3-anti, 4, and 5 were characterized by X-ray diffraction.