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.

Rare-Earth-Metal Methyl and Methylidene Complexes Stabilized by TpR,R'-Scorpionato Ligands─Size Matters.

Homoleptic tetramethylaluminates Ln(AlMe4)3 react with KTptBu,Me (TptBu,Me = tris(3-tBu-5-Me-pyrazolyl)borato) to yield rare-earth-metal methylidene complexes (TptBu,Me)Ln(µ3-CH2)[(µ-Me)AlMe2]2 (Ln = La, Ce, Nd). The lanthanum reaction is prone to additional C-H- and B-N-bond activation, affording coproducts La[HB(pzMe,tBu)(pzCMe2,Me)2][(µ-CH2)(µ-Me)AlMe2]2 and [La(µ-pztBu,Me)(AlMe4)2]2 (pztBu,Me = 3-tBu-5-Me-pyrazolato). The protonolysis reaction of Ln(AlMe4)3 and HpztBu,Me provides more efficient access to [Ln(µ-pztBu,Me)(AlMe4)2]2 (Ln = La, Nd). Treatment of Ln(AlMe4)3 with KTpMe,Me led to methylidene complexes (TpMe,Me)Ln(µ3-CH2)[(µ-Me)AlMe2]2 (Ln = Nd, Sm) or bis(tetramethylaluminate) complexes (TpMe,Me)Ln(AlMe4)2 (Ln = Y, Lu). The neodymium reaction generated methine derivative (TpMe,Me)Nd[(µ4-CH)(AlMe2)2(µ-pz,Me,Me)][(µ-Me)AlMe2] as a minor coproduct. The reaction of Ln(GaMe4)3 (Ln = Y, La, Ce, Nd, Sm, Ho) with HTptBu,Me gave methylidene complexes (TptBu,Me)Ln(µ3-CH2)[(µ-Me)GaMe2]2 (Ln = La, Ce, Nd, Sm) and alkyl complexes (TptBu,Me)LnMe[(µ-Me)GaMe3] (Ln = Y, Ho), while competing B-N bond activation reactions produced GaMe2[BH(Me)(µ-pztBu,Me)2] and (TptBu,Me)Ln(η2-pztBu,Me)[(µ-Me)GaMe3] (Ln = Y, Ho). The steric impact of the TpR,Me ligands was examined by cone angle calculations. Rare-earth-metal methylidene complexes (TptBu,Me)Ln(µ3-CH2)[(µ-Me)EMe2]2 (E = Al, Ga) successfully promote carbonyl methylenation reactions upon addition of ketone.

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 [TptBu,MeLn(µ3-CH2){(µ2-Me)MMe2}2] (Ln = Ce, Nd, Sm; M = Al, Ga) react with 2,6-diisopropylaniline (H2NAriPr) 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 (pKa) of the aromatic amine. Crucially, terminal arylimides [TptBu,MeLn(═NAriPr)(THF)2] (Ln = Ce, Nd, Sm) are formed only for M = Ga, and for M = Al, the Lewis-acid-stabilized imides [TptBu,MeLn(NAriPr)(AlMe3)] (Ln = Ce, Nd, Sm) are persistent. In stark contrast, the [GaMe3]-stabilized imide [TptBu,MeLn(NAriPr)(GaMe3)] (Ln = Nd, Sm) is reversibly formed in noncoordinating solvents.

The Alkylaluminate/Gallate Trap: Metalation of Benzene by Heterobimetallic Yttrocene Complexes [Cp*2Y(MMe3R)] (M = Al, Ga).

Yttrocene derivatives [Cp*2Y(MMe4)] (Cp* = C5Me5; M = Al, Ga) and Cp*2Y[Me3Al{B(NDippCH)2}] (Dipp = C6H3iPr2-2,6) deprotonate benzene at elevated temperatures via the release of methane. The formation of [Cp*2Y(Me2MPh2)] (M = Al, Ga), Cp*2Y(MPh4) (M = Al, Ga), Cp*2Y[Me2AlPh{B(NDippCH)2}], and Cp*2Y[AlPh3{B(NDippCH)2}] can be controlled via the temperature applied. The activation temperature and formation of the coordinatively unsaturated "reactive" [Cp*2YMe] strongly depend on the coordination strength of the displaceable Lewis acids [AlMe3]2, GaMe3, and [Me2Al{B(NDippCH)2}]2. Hence, [Cp*2Y(AlMe4)] requires temperatures above 100 °C to metalate benzene, while Cp*2Y[AlMe3{B(NDippCH)2}] undergoes C-H-bond activation even at ambient temperatures. A kinetic deuterium isotope effect was observed for the reactions in C6D6 solutions. Distinct differences in the stabilities of the bulky Group 13 anions ([Me2MPh2]-, [MPh4]-, [Me3Al{B(NDippCH)2}]-, [Me2AlPh{B(NDippCH)2}]-, and [AlPh3{B(NDippCH)2}]-) are assessed by detailed studies of the coordination chemistry with tetrahydrofuran (THF) and by variable-temperature 1H NMR spectroscopy. Thus, increased steric bulk or a reduced Lewis acidity of the Group 13 metal center promote temperature-sensitive dissociation of trivalent Group 13 alkyl entities. Consequently, compound Cp*2Y[AlPh3{B(NDippCH)2}] was found to engage in a dissociation equilibrium with [Cp*2YPh] and AlPh2{B(NDippCH)2} in a C6D6 solution at ambient temperature. The reaction of Cp*2Y[AlPh3{B(NDippCH)2}] with THF results in the concomitant formation of monometallic Cp*2YPh(THF) and the solvent-separated ion pair [Cp*2Y(THF)2][AlPh3{B(NDippCH)2}].

Emergence of a New [NNN] Pincer Ligand via Si-H Bond Activation and ß-Hydride Abstraction at Tetravalent Cerium.

The cerium(IV) pyrazolate complexes [Ce(Me2 pz)4 ]2 and [Ce(Me2 pz)4 (thf)] initiate ß-hydride abstraction of the bis(dimethylsilyl)amido moiety, to afford a heteroleptic cerium(IV) species containing a dimethylpyrazolyl-substituted silylamido ligand, namely [Ce(Me2 pz)3 (bpsa)] (bpsa=bis((3,5-dimethylpyrazol-1-yl)dimethylsilyl)amido; Me2 pz =3,5-dimethylpyrazolato), along with some cerium(III) species. Remarkably, the nucleophilic attack of the pyrazolyl at the silicon atom and concomitant Si-H-bond cleavage is restricted to the tetravalent cerium oxidation state and appears to proceed via the formation of a transient cerium(IV) hydride, which engages in immediate redox chemistry. When [Ce(Me2 pz)4 ]2 is treated with [Li{N(SiMe3 )2 }], that is, in the absence of the SiH functionality, any redox chemistry did not occur. Instead, the ceric ate complex [LiCe2 (Me2 pz)9 ] and the stable mixed-ligand ceric species [Ce(Me2 pz)2 {N(SiMe3 )2 }2 ] were obtained.

Rare-earth metal and actinide organoimide chemistry.

The chemistry of actinide (An) and rare-earth metal (Ln and group 3) complexes featuring multiple bonding interactions with main-group fragments has witnessed an enormous growth since the first mentioning in the mid-eighties and apparent stagnation in the nineties. The recent surge of interest is particularly owing to our eagerness to acquire a fundamental understanding of the chemical bonding properties of such long-time elusive compounds but also the potential emergence of unprecedented reactivity in organic or inorganic transformations. Contrary to uranium imide chemistry, traditional and routine synthesis protocols seem less viable for rare-earth metal imide complexes. The present review puts its main emphasis on identifying reaction pathways currently available/elaborated for the generation of [An[double bond, length as m-dash]NR] and [Ln[double bond, length as m-dash]NR] moieties. We also address the intriguing structural and reactivity features of such organoimide derivatives as highlighted by small-molecule activation, group-transfer capability, and the redox chemistry of uranium, cerium, ytterbium, samarium and europium.

Rare-Earth Metal Diimide Complexes via Alkylaluminate Templating, Including a Ceric Derivative.

Protonolysis of lanthanide tris(tetramethylaluminate)s with two equivalents of 2,6-diisopropylaniline affords LaIII and CeIII diimide compounds Ln[(µ-NC6 H3 iPr2 -2,6)2 AlMe2 ](thf)4 featuring a bidentate AlMe2 -linked diimido ligand. As revealed for the corresponding Ce(GaMe4 )3 -reaction, formation of the diimide complexes proceeds via tetrametallic complexes of the type [Ce{(µ-NC6 H3 iPr2 -2,6)(HNC6 H3 iPr2 -2,6)(MMe3 )}]2 (Me=Al, Ga). Oxidation of the cerium(III) complex with hexachloroethane leads to a neutral CeIV diimide species. Partial protonolysis with phenylacetylene and hydrogenolysis via H3 SiPh give conclusive insights into the reactive coordination sites of such diimide complexes.

Rare-earth metal methylidene complexes with Ln3(µ3-CH2)(µ3-Me)(µ2-Me)3 core structure.

Trinuclear rare-earth metal methylidene complexes with a Ln3(µ3-CH2)(µ3-Me)(µ2-Me)3 structural motif were synthesized by applying three protocols. Polymeric [LuMe3]n (1-Lu) reacts with the sterically demanding amine H[NSiMe3(Ar)] (Ar = C6H3iPr2-2,6) in tetrahydrofuran via methane elimination to afford isolable monomeric [NSiMe3(Ar)]LuMe2(thf)2 (4-Lu). The formation of trinuclear rare-earth metal tetramethyl methylidene complexes [NSiMe3(Ar)]3Ln3(µ3-CH2)(µ3-Me)(µ2-Me)3(thf)3 (7-Ln; Ln = Y, Ho, Lu) via reaction of [LnMe3]n (1-Ln; Ln = Y, Ho, Lu) with H[NSiMe3(Ar)] is proposed to occur via an "intermediate" species of the type [NSiMe3(Ar)]LnMe2(thf)x and subsequent C-H bond activation. Applying Lappert's concept of Lewis base-induced methylaluminate cleavage, compounds [NSiMe3(Ar)]Ln(AlMe4)2 (5-Ln; Ln = Y, La, Nd, Ho) were converted into methylidene complexes 7-Ln (Ln = Y, Nd, Ho) in the presence of tetrahydrofuran. Similarly, tetramethylgallate complex [NSiMe3(Ar)]Y(GaMe4)2 (6-Y) could be employed as a synthesis precursor for 7-Y. The molecular composition of complexes 4-Ln, 5-Ln, 6-Y and 7-Ln was confirmed by elemental analyses, FTIR spectroscopy, (1)H and (13)C NMR spectroscopy (except for holmium derivatives) and single-crystal X-ray diffraction. The Tebbe-like reactivity of methylidene complex 7-Nd with 9-fluorenone was assessed affording oxo complex [NSiMe3(Ar)]3Nd3(µ3-O)(µ2-Me)4(thf)3 (8-Nd). The synthesis of 5-Ln yielded [NSiMe3(Ar)]2Ln(AlMe4) (9-Ln; Ln = La, Nd) as minor side-products, which could be obtained in moderate yields when homoleptic Ln(AlMe4)3 were treated with two equivalents of K[NSiMe3(Ar)].

Rare-earth-metal methyl, amide, and imide complexes supported by a superbulky scorpionate ligand.

The reaction of monomeric [(Tp(tBu,Me) )LuMe2 ] (Tp(tBu,Me) =tris(3-Me-5-tBu-pyrazolyl)borate) with primary aliphatic amines H2 NR (R=tBu, Ad=adamantyl) led to lutetium methyl primary amide complexes [(Tp(tBu,Me) )LuMe(NHR)], the solid-state structures of which were determined by XRD analyses. The mixed methyl/tetramethylaluminate compounds [(Tp(tBu,Me) )LnMe({µ2 -Me}AlMe3 )] (Ln=Y, Ho) reacted selectively and in high yield with H2 NR, according to methane elimination, to afford heterobimetallic complexes: [(Tp(tBu,Me) )Ln({µ2 -Me}AlMe2 )(µ2 -NR)] (Ln=Y, Ho). X-ray structure analyses revealed that the monomeric alkylaluminum-supported imide complexes were isostructural, featuring bridging methyl and imido ligands. Deeper insight into the fluxional behavior in solution was gained by (1) H and (13) C NMR spectroscopic studies at variable temperatures and (1) H-(89) Y HSQC NMR spectroscopy. Treatment of [(Tp(tBu,Me) )LnMe(AlMe4 )] with H2 NtBu gave dimethyl compounds [(Tp(tBu,Me) )LnMe2 ] as minor side products for the mid-sized metals yttrium and holmium and in high yield for the smaller lutetium. Preparative-scale amounts of complexes [(Tp(tBu,Me) )LnMe2 ] (Ln=Y, Ho, Lu) were made accessible through aluminate cleavage of [(Tp(tBu,Me) )LnMe(AlMe4 )] with N,N,N',N'-tetramethylethylenediamine (tmeda). The solid-state structures of [(Tp(tBu,Me) )HoMe(AlMe4 )] and [(Tp(tBu,Me) )HoMe2 ] were analyzed by XRD.