Facile Preparation of a Scandium Terminal Imido Complex Supported by a Phosphazene Ligand

Synthesis and Reactivity of the Rare-Earth Metal Complexes Bearing the Indol-2-yl-Based NCN Pincer Ligand

The pincer rare-earth dialkyl complexes [κ3-LRE(CH2SiMe3)2 (RE = Lu(1a), Yb(1b), Er(1c), Y(1d), Dy(1e))] with the indol-2-yl-based NCN pincer ligand were synthesized by the reactions of the proligand HL (L = 1-Me2NCH2CH2-3-(2-iPrC6H5N═CH)C8H4N) with RE(CH2SiMe3)3(THF)2. These complexes exhibited a variety of reactivities toward organic compounds such as amines, triphenylphosphine ylide, N-phenylimidazole, pyridine derivatives, and o-carborane leading to σ-bond metathesis, migration insertion, and redox reaction products. The reactions of the dialkyl rare-earth metal complexes with o-carborane afforded the novel NCN pincer-ligated carboryne-based metallacyclopropanes which reacted with diphenyl ketone to give insertion products of the RE-C2-ind and one of the RE-Ccage bonds, while the reaction of the carboryne-based metallacyclopropanes with diphenyldiazomethane produced the di-aza-metallacyclopentanes via the insertions of the N═N bond of the diphenyldiazomethane into two RE-Ccage bonds and the RE-C2-ind bond. The reactions of the dialkyl complexes with 2 equiv of 2,2'-bipyridine afforded the pincer-ligated bis(2,2'-bipyridyl monoanionic radical) complexes via the homolytic redox reaction.

Terminal dysprosium and holmium organoimides

Terminal rare-earth-metal imide complexes TptBu,MeLn(NC6H3iPr2-2,6)(dmap) of the mid-late rare-earth elements dysprosium and holmium were synthesized via double methane elimination of Lewis acid stabilized dialkyl precursors TptBu,MeLnMe(GaMe4) with primary aniline derivative H2NC6H3iPr2-2,6 (H2NAriPr). Exploiting the weaker Ln-CH3⋯[GaMe3] interaction compared to the aluminium congener, addition of the aniline derivative leads to the mixed methyl/anilido species TptBu,MeLnMe(HNAriPr) which readily eliminate methane after being exposed to the Lewis base DMAP ([double bond, length as m-dash]N,N-dimethyl-4-aminopyridine). Under the same conditions, [AlMe3]-stabilized dimethyl rare-earth-metal complexes transform immediately to Lewis acid bridged imides TptBu,MeLn(μ2-NC6H3Me2-2,6)(μ2-Me)AlMe2 (Ln = Dy, Ho). DMAP/THF donor exchange is accomplished by treatment of TptBu,MeLn(NC6H3iPr2-2,6)(dmap) with 9-BBN in THF while the terminal imides readily insert carbon dioxide to afford carbamate complexes.

A Terminal Yttrium Phosphinidene

Terminal, nondirectional ionic "multiple" bond interactions between group 15 elements and rare-earth metals (Ln) have remained a challenging target until present. Although reports on terminal imide species have accumulated in the meantime, examples of terminal congeners with the higher homologue phosphorus are yet elusive. Herein, we present the synthesis of the first terminal yttrium organophosphinidene complex, TptBu,MeY(═PC6H3iPr2-2,6)(DMAP)2, according to a double-deprotonation sequence previously established for organoimides of the smaller rare-earth metals. Subsequent deprotonation of the primary phosphane H2PC6H3iPr2-2,6 (H2PAriPr) with discrete dimethyl compound TptBu,MeYMe2 in the presence of DMAP under simultaneous methane elimination generated a terminal multiply bonded phosphorus. The primary phosphide intermediates TptBu,MeYMe(HPAriPr) and TptBu,MeYMe(HNPAriPr)(DMAP) are isolable species and were also obtained and fully characterized for holmium and dysprosium. The Lewis acid-stabilized yttrium phosphinidene TptBu,MeY[(μ2-PAriPr)(μ2-Me)AlMe2] was obtained by treatment of H2PAriPr with TptBu,MeYMe(AlMe4) but could not be converted into a terminal phosphinidene via cleavage of trimethylaluminum. The corresponding reaction of H2PAriPr with TptBu,MeYMe(GaMe4) led to adduct [GaMe3(PH2AriPr)] rather than to the formation of a yttrium phosphinidene. The yttrium-phosphorus interaction in the obtained organophosphide and phosphinidene complexes was scrutinized by 31P/89Y NMR spectroscopy and DFT calculations, unambiguously supporting the existence of multiple bonding.

Open-Shell Early Lanthanide Terminal Imides

Group 3- and 4f-element organometallic chemistry and reactivity are decisively driven by the rare-earth-metal/lanthanide (Ln) ion size and associated electronegativity/ionicity/Lewis acidity criteria. For these reasons, the synthesis of terminal "unsupported" imides [Ln═NR] of the smaller, closed-shell Sc(III), Lu(III), Y(III), and increasingly covalent Ce(IV) has involved distinct reaction protocols while derivatives of the "early" large Ln(III) have remained elusive. Herein, we report such terminal imides of open-shell lanthanide cations Ce(III), Nd(III), and Sm(III) according to a new reaction protocol. Lewis-acid-stabilized methylidene complexes [Tp^tBu,MeLn(μ₃-CH₂){(μ₂-Me)MMe₂}₂] (Ln = Ce, Nd, Sm; M = Al, Ga) react with 2,6-diisopropylaniline (H₂NAr^iPr) via methane elimination. The formation of arylimide complexes is governed by the Ln(III) size, the Lewis acidity of the group 13 metal alkyl, steric factors, the presence of a donor solvent, and the sterics and acidity (pK_a) of the aromatic amine. Crucially, terminal arylimides [Tp^tBu,MeLn(═NAr^iPr)(THF)₂] (Ln = Ce, Nd, Sm) are formed only for M = Ga, and for M = Al, the Lewis-acid-stabilized imides [Tp^tBu,MeLn(NAr^iPr)(AlMe₃)] (Ln = Ce, Nd, Sm) are persistent. In stark contrast, the [GaMe₃]-stabilized imide [Tp^tBu,MeLn(NAr^iPr)(GaMe₃)] (Ln = Nd, Sm) is reversibly formed in noncoordinating solvents.

A monoanionic pentadentate ligand platform for scandium-pnictogen multiple bonds

A new monoanionic pentadentate ligand is designed to accommodate Sc = E bonds (E = N, P). The imido complex is stable enough to isolate and characterize, and reacts rapidly with CO₂. The phosphinidene, on the other hand, is highly reactive and induces C-C bond cleavage to yield a phosphido-pyridyl complex which also undergoes rapid reacton with CO₂.

Cerium-carbon dative interactions supported by carbodiphosphorane

A set of complexes containing dative interactions between a rare-earth metal and carbon are reported. Complex 2, Br₃Ce(CDP)(THF), with a Ce←C bond was synthesized by the reaction of CeBr₃ with a carbon(0) ligand, carbodiphosphorane (CDP). More significantly, a trivalent cerium complex 3, [BrCe(CDP)₂](BPh₄)₂, with two σ dative interactions C→Ce←C was also isolated, which represents an unusual example of two dative interactions formed with the same atom in a molecule. Furthermore, π donation by the second lone-pair electrons of the CDP ligand is rather weak. Single-crystal X-ray diffraction shows that the Ce-C bond lengths in these complexes are comparable with those in cerium(iii)-carbene species. Density functional theory calculations support the dative interaction formation in these complexes and the strength of σ-donation in 3 is stronger than that in 2.

Rare-earth metal and actinide organoimide chemistry

Elaborate synthesis schemes pave the way to f-element and group 3 complexes with multiply bonded imido ligands displaying intriguing reactivity.

Scandium Terminal Imido Chemistry

Research into transition metal complexes bearing multiply bonded main-group ligands has developed into a thriving and fruitful field over the past half century. These complexes, featuring terminal M═E/M≡E (M = transition metal; E = main-group element) multiple bonds, exhibit unique structural properties as well as rich reactivity, which render them attractive targets for inorganic/organometallic chemists as well as indispensable tools for organic/catalytic chemists. This fact has been highlighted by their widespread applications in organic synthesis, for example, as olefin metathesis catalysts. In the ongoing renaissance of transition metal-ligand multiple-bonding chemistry, there have been reports of M═E/M≡E interactions for the majority of the metallic elements of the periodic table, even some actinide metals. In stark contrast, the largest subgroup of the periodic table, rare-earth metals (Ln = Sc, Y, and lanthanides), have been excluded from this upsurge. Indeed, the synthesis of terminal Ln═E/Ln≡E multiple-bonding species lagged behind that of the transition metal and actinide congeners for decades. Although these species had been pursued since the discovery of a rare-earth metal bridging imide in 1991, such a terminal (nonpincer/bridging hapticities) Ln═E/Ln≡E bond species was not obtained until 2010. The scarcity is mainly attributed to the energy mismatch between the frontier orbitals of the metal and the ligand atoms. This renders the putative terminal Ln═E/Ln≡E bonds extremely reactive, thus resulting in the formation of aggregates and/or reaction with the ligand/environment, quenching the multiple-bond character. In 2010, the stalemate was broken by the isolation and structural characterization of the first rare-earth metal terminal imide-a scandium terminal imide-by our group. The double-bond character of the Sc═N bond was unequivocally confirmed by single-crystal X-ray diffraction. Theoretical investigations revealed the presence of two p-d π bonds between the scandium ion and the nitrogen atom of the imido ligand and showed that the dianionic [NR]^2- imido ligand acts as a 2σ,4π electron donor. Subsequent studies of the scandium terminal imides revealed highly versatile and intriguing reactivity of the Sc═N bond. This included cycloaddition toward various unsaturated bonds, C-H/Si-H/B-H bond activations and catalytic hydrosilylation, dehydrofluorination of fluoro-substituted benzenes/alkanes, CO₂ and H₂ activations, activation of elemental selenium, coordination with other transition metal halides, etc. Since our initial success in 2010, and with contributions from us and across the community, this young, vibrant research field has rapidly flourished into one of the most active frontiers of rare-earth metal chemistry. The prospect of extending Ln═N chemistry to other rare-earth metals and/or different metal oxidation states, as well as exploiting their stoichiometric and catalytic reactivities, continues to attract research effort. Herein we present an account of our investigations into scandium terminal imido chemistry as a timely summary, in the hope that our studies will be of interest to this readership.

Insertion/Rearrangement Reactivity of a Lutetacyclopentadiene towards N,N'-Diphenylcarbodiimide: Cooperative Effect of the Metal Center, Concentration of LiCl, and Solvent

The reactivity of lutetacyclopentadiene towards N,N'-diphenylcarbodiimide (DPC) was systematically investigated to efficiently construct three types of new N-containing fused cyclic complexes. The outcome of these reactions significantly depended on the the metal center, the concentration of LiCl, the number of equivalents of DPC, and the solvent. Thus, two unexpected reaction modes of amidinate were discovered for the first time, which were ascribed to an unusual κ(1) coordination mode of amidinate driven by a rigid seven-membered ring. These results are in striking contrast with the previously well-investigated zirconacyclopentadiene, which reacts with DPC to give azazirconacyclopentene and alkyne through β,β' CC bond cleavage. The difference in reactivity between lutetacyclopentadiene and zirconacyclopentadiene can be attributed to the highly ionic character of the LuC(sp(2) ) bonds. DFT calculations agreed well with the experimental results.

Reactivity of halfsandwich rare-earth metal methylaluminates toward potassium (2,4,6-tri-tert-butylphenyl)amide and 1-adamantylamine

Donor functionalities drastically affect salt-metathetical methylaluminato–amido exchange reactions in Cp^RLn(AlMe₄)₂ by triggering methylaluminate cleavage and C–H bond activation of proximal aliphatic groups.

Differences in the cyclometalation reactivity of bisphosphinimine-supported organo-rare earth complexes

A novel yttrium complex [L_nY(CH₂SiMe₃)₂] is resistant to cyclometalation, while samarium variants undergo C–H activation, forming unique cyclometalated motifs.