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.

Rare-earth metal ethylene and ethyne complexes

Guanidinate homometallic rare-earth ethyl complexes [LLn(μ2-η1:η2-Et)(Et)]2 (Ln = Y(1-Y), Lu(1-Lu)) and heterobimetallic rare-earth ethyl complexes LLn(Et)(μ2-η1:η2-Et)(μ2-η1-Et)(AlEt2) (Ln = Y(2-Y), Lu(2-Lu)) have been synthesized by the treatment of LLn(CH2C6H4NMe2-o)2 (L = (PhCH2)2NC(NC6H3iPr2-2,6)2) with different equivalents of AlEt3 in toluene at ambient temperature. Interestingly, the unprecedented rare-earth ethyne complex [LY(μ2-η1-Et)2(AlEt)]2(μ4-η1:η1:η2:η2-C2H2) (3-Y) containing a [C2H2]4- unit was afforded from 2-Y. The formation mechanism study on 3-Y was carried out by DFT calculations. Furthermore, the nature of the bonding of 3-Y was also revealed by NBO analysis. The reactions of LLn(CH2 C6H4NMe2-o)2 (Ln = Y, Lu) with AlEt3 (4 equiv.) in toluene at 50 °C produced firstly the non-Cp rare-earth ethylene complex LY(μ3-η1:η1:η2-C2H4)[(μ2-η1-Et)(AlEt2)(μ2-η1-Et)2(AlEt)] (4-Y), and the Y/Al ethyl complex LY[(μ2-η1-Et)2(AlEt2)]2 (5-Y) as an intermediate of 4-Y was isolated from the reaction of LY(CH2C6H4NMe2-o)2 with AlEt3 (4 equiv.) in toluene at -10 °C.

A binuclear guanidinate yttrium carbyne complex: unique reactivity toward unsaturated C-N, C-O and C-S bonds

A guanidinato-stabilized binuclear yttrium carbyne complex [(PhCH2)2NC(NC6H3iPr2-2,6)2]2Y2(μ2-Me)(AlMe3)2(μ4-CH) (1) was synthesized via C-H bond activation and its versatile reactivities were investigated. Complex 1 underwent σ-bond metathesis with PhSSPh and nucleophilic addition with PhCN to form the corresponding yttrium thiolate complex 3 and aza-allyl complex 4 respectively. Additionally, the rare yttrium carbide complex 5 was also prepared by treatment of complex 1 with S8. Interestingly, in the reaction with PhNCS, the C[double bond, length as m-dash]S double bond was cleaved, followed by C-H bond activation to give the yttrium sulfide complex 7 with a ketenimine dianion ligand. Unexpectedly, the reaction of complex 1 with CO (1 atm) resulted in deoxygenative coupling of CO, to afford mono- or dioxo-yttrium complexes at different temperatures. The mechanism of the possible formation processes of complexes 3 and 9 was elucidated by DFT calculations.

Rare-earth-metal half-sandwich complexes incorporating methyl, methylidene, and hydrido ligands

Complexes Cp*₃Ln₃(μ₂-CH₃)₃(μ₃-CH₂)(μ₃-H)(thf)₃ (Ln = Y, Dy; Cp* = C₅Me₅) are formed via elimination of CH₄ and C₂H₄ from the respective trinuclear polymethyl complexes [Cp*Ln(μ₂-CH₃)₂]₃, involving the methyl/methylidene complex Cp*₃Ln₃(μ₂-CH₃)₃(μ₃-CH₂)(μ₃-CH₃)(thf)₂ as an intermediate. Compound Cp*₃Y₃(μ₂-CH₃)₃(μ₃-CH₂)(μ₃-H)(thf)₃ displays "Schrock-type" nucleophilic carbene reactivity, converting ketones into the corresponding terminal alkenes. Treatment of Cp*₃Y₃(μ₂-CH₃)₃(μ₃-CH₂)(μ₃-H)(thf)₃ with [D₄]methanol proved accessibility of all small-group functionalities as indicated by the simultaneous formation of deuterated HD and deuterated methanes CH₃D and CH₂D₂.

Bonding Nature of "Ionic Carbenes" in [M3(μ3-CH2)]-Containing Compounds: The Covalent Interaction

While a series of trinuclear rare-earth metal methylene (divalent >CH₂) complexes with the so-called "ionic carbene" have been known for decades, the nature of metal-carbene interactions in this class of compounds remains elusive. Herein, a quantum chemical investigation has been performed to reveal the bonding nature in typical trimetallic "ionic carbene" species with the [M₃(μ₃-CH₂)] (M = Sc, Y, La, and Ac) cluster core. Through various chemical bonding analyses, we have demonstrated that there exists a non-negligible covalent interaction between μ₃-CH₂ and M₃ moieties, and the chemical bonding can be accounted for with two three-center two-electron (3c-2e) bonds. The chemical bonding analyses reveal that the metal d-electron configuration plays an important role in stabilizing various μ₃-coordinated carbene complexes. The late transition metals do not favor such a μ₃-coordination geometry, thus explaining why ionic carbene complexes are usually found for rare-earth and early transition metals. A series of ionic carbene complexes with early transition metals, lanthanides, and actinides are predicted to be stable as well. These reactive ionic carbene complexes may have characteristic properties for organic synthesis and catalysis.

Rare Earth Starting Materials and Methodologies for Synthetic Chemistry

The number of rare earth (RE) starting materials used in synthesis is staggering, ranging from simple binary metal-halide salts to borohydrides and "designer reagents" such as alkyl and organoaluminate complexes. This review collates the most important starting materials used in RE synthetic chemistry, including essential information on their preparations and uses in modern synthetic methodologies. The review is divided by starting material category and supporting ligands (i.e., metals as synthetic precursors, halides, borohydrides, nitrogen donors, oxygen donors, triflates, and organometallic reagents), and in each section relevant synthetic methodologies and applications are discussed.

Trinuclear scandium methylidyne complexes stabilized by pentamethylcyclopentadienyl ligands

The first examples of scandium methylidyne complexes [(Cp*)Sc(μ2-X)]3(μ3-CH) (Cp* = C5Me5; X = Br, Me, OMe), free of Lewis acids, can be achieved in high yields from [(Cp*)ScMe2]2 through a facile route. The chemical and geometrical flexibility to incorporate organic substrates indicates a rich chemistry of complex [(Cp*)Sc(μ2-OMe)]3(μ3-CH).

Potential Precursors for Terminal Methylidene Rare-Earth-Metal Complexes Supported by a Superbulky Tris(pyrazolyl)borato Ligand

A series of solvent-free heteroleptic terminal rare-earth-metal alkyl complexes stabilized by a superbulky tris(pyrazolyl)borato ligand with the general formula [TptBu,Me LnMeR] have been synthesized and fully characterized. Treatment of the heterobimetallic mixed methyl/tetramethylaluminate compounds [TptBu,Me LnMe(AlMe4 )] (Ln=Y, Lu) with two equivalents of the mild halogenido transfer reagents SiMe3 X (X=Cl, I) gave [TptBu,Me LnX2 ] in high yields. The addition of only one equivalent of SiMe3 Cl to [TptBu,Me LuMe(AlMe4 )] selectively afforded the desired mixed methyl/chloride complex [TptBu,Me LuMeCl]. Further reactivity studies of [TptBu,Me LuMeCl] with LiR or KR (R=CH2 Ph, CH2 SiMe3 ) through salt metathesis led to the monomeric mixed-alkyl derivatives [TptBu,Me LuMe(CH2 SiMe3 )] and [TptBu,Me LuMe(CH2 Ph)], respectively, in good yields. The SiMe4 elimination protocols were also applicable when using SiMe3 X featuring more weakly coordinating moieties (here X=OTf, NTf2 ). X-ray structure analyses of this diverse set of new [TptBu,Me LnMeR/X] compounds were performed to reveal any electronic and steric effects of the varying monoanionic ligands R and X, including exact cone-angle calculations of the tridentate tris(pyrazolyl)borato ligand. Deeper insights into the reactivity of these potential precursors for terminal alkylidene rare-earth-metal complexes were gained through NMR spectroscopic studies.

Trimethylscandium

The hitherto unknown homoleptic tetramethylaluminate complex [Sc(AlMe₄)₃] could be obtained by reacting the ate complex [Li₃ScMe₆(thf)_1.2] with AlMe₃ in the cold. It cocrystallizes with AlMe₃ as [Sc(AlMe₄)₃(Al₂Me₆)_0.5] and decomposes at ambient temperature in n-pentane via multiple C-H bond activations to the mixed methyl/methylidene complex [Sc₃(μ₃-CH₂)₂(μ₂-CH₃)₃(AlMe₄)₂(AlMe₃)₂]. Donor-induced methylaluminate cleavage of [Sc(AlMe₄)₃(Al₂Me₆)_0.5] produced [ScMe₃]_n in good yield, which could be derivatized with trimethyltriazacyclononane (Me₃TACN) to form the structurally characterizable [(Me₃TACN)ScMe₃]. Additionally, half-sandwich complex [Cp*Sc(AlMe₄)₂] and sandwich complex [Cp*₂Sc(AlMe₄)] were accessible by salt metathesis reactions from [Sc(AlMe₄)₃(Al₂Me₆)_0.5] and KCp* (Cp* = C₅Me₅). ⁴⁵Sc NMR spectroscopy was applied as a significant probe to evidence the existence of [ScMe₃]_n. Compounds [(Me₃TACN)ScMe₃] (+624.6 ppm) and [ScMe₃(thf)_x] (+601.7 ppm) gave large ⁴⁵Sc NMR shifts, revealing the strong deshielding effect of the σ-bonded alkyl ligands on the scandium nuclei. Ultimately, cationized [Sc(AlMe₄)₃(Al₂Me₆)_0.5] was employed in isoprene polymerization, leading to polymers in high yields (>95%) and with high (>90%) cis-1,4-polyisoprene content.

Dimethylmagnesium revisited

A compilation of solvent-free homometallic methyl compounds of the type MMex (x = 1-6) is provided and categorised according to their method of characterisation (powder or single crystal X-ray diffraction, gas electron diffraction (GED), reactivity, unconfirmed). Recrystallisation of polymeric [MgMe2]n from excess GaMe3 led to the formation of highly pure [MgMe2]n suitable for single crystal X-ray crystallographic studies. Transient Mg(GaMe4)2 could be detected in excess GaMe3 by NMR spectroscopy, but its isolation as Mg(GaMe4)2 failed. On one occasion tetrameric [Mg(GaMe4)(OMe)]4 could be isolated as a minor co-product. The formation of single-crystalline [MgMe2]n from a saturated ethereal solution could be reproduced as reported earlier by Coates et al. Assessing the reactivity of potassium methoxide methanol adduct toward Mg(AlMe4)2, the protonolysis reaction with MeOH gave unprecedented [Mg(AlMe4){Al(OMe)2Me2}]2 featuring one 8-membered [MgOAlO]2 metalloxane ring and two 4-membered metallacycles.

Rare-earth metal-mediated PhCN insertion into N,N-bis(trimethylsilyl)naphthalene-1,8-diamido dianion – a synthetic approach to complexes coordinated by ansa-bridged amido-amidinato ligand

We report the first example of rare earth metal-mediated insertion of PhCN into Si–N bonds with the formation of an amidinato moiety.

Cleavage of Dinitrogen from Forming Gas by a Titanium Molecular System under Ambient Conditions

Simple exposure of a hexane solution of [TiCp*Me₃ ] (Cp*=η⁵ -C₅ Me₅ ) to an atmosphere of commercially available and inexpensive forming gas (H₂ /N₂ mixture, 13.5-16.5 % of H₂ ) at room temperature leads to the methylidene-methylidyne-nitrido cube-type complex [(TiCp*)₄ (μ₃ -CH)(μ₃ -CH₂ )(μ₃ -N)₂ ] via dinitrogen cleavage. This paramagnetic compound reacts with [D₁ ]chloroform to give the titanium(IV) methylidyne-nitrido species [(TiCp*)₄ (μ₃ -CH)₂ (μ₃ -N)₂ ], whereas its one-electron oxidation with AgOTf or [Fe(η⁵ -C₅ H₅ )₂ ](OTf) (OTf=O₃ SCF₃ ) yields the diamagnetic ionic derivative [(TiCp*)₄ (μ₃ -CH)(μ₃ -CH₂ )(μ₃ -N)₂ ](OTf). The μ₃ -nitrido ligands of the methylidyne-nitrido cubane complex can be protonated with [LutH](OTf) (Lut=2,6-lutidine) or hydrogenated with NH₃ ⋅BH₃ to afford μ₃ -NH imido moieties.

Small molecule activation by mixed methyl/methylidene rare earth metal complexes

Diverse reactivity patterns of mixed tetramethyl/methylidene rare-earth complexes bearing bulky benzamidinate coligands with PhCN, alkynes, and CS₂ have been established and fully characterized.

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

Although protected from intermolecular deactivation by a picket-fence-type arrangement of bulky amido ligands the CH₂²⁻ moiety is readily converted into oxo species.

Lutetium-methanediide-alkyl complexes: synthesis and chemistry

The first four-coordinate methanediide/alkyl lutetium complex (BODDI)Lu2 (CH2 SiMe3 )2 (μ2 -CHSiMe3 )(THF)2 (BODDI=ArNC(Me)CHCOCHC(Me)NAr, Ar=2,6-iPr2 C6 H3 ) (1) was synthesized by a thermolysis methodology through α-H abstraction from a Lu-CH2 SiMe3 group. Complex 1 reacted with equimolar 2,6-iPrC6 H3 NH2 and Ph2 C+O to give the corresponding lutetium bridging imido and oxo complexes (BODDI)Lu2 (CH2 SiMe3 )2 (μ2 -N-2,6-iPr2 C6 H3 )(THF)2 (2) and (BODDI)Lu2 (CH2 SiMe3 )2 (μ2 -O)(THF)2 (3). Treatment of 3 with Ph2 C=O (4 equiv) caused a rare insertion of Lu-μ2 -O bond into theC=O group to afford a diphenylmethyl diolate complex 4. Reaction of 1 with PhN=C=O (2 equiv) led to the migration of SiMe3 to the amido nitrogen atom to give complex (BODDI)Lu2 (CH2 SiMe3 )2 -μ-{PhNC(O)CHC(O)NPh(SiMe3 )-κ(3) N,O,O}(THF) (5). Reaction of 1 withtBuN=C formed an unprecedented product (BODDI)Lu2 (CH2 SiMe3 ){μ2 -[η(2) :η(2) -tBuN=C(=CH2 )SiMe2 CHC=NtBu-κ(1) N]}(tBuN=C)2 (6) through a cascade reaction of N=C bond insertion, sequential cyclometalative γ-(sp(3) )-H activation, C=C bond formation, and rearrangement of the newly formed carbene intermediate. The possible mechanistic pathways between 1, PhN=C=O, and tBuN=C were elucidated by DFT calculations.

M4(CH2)4 cubane-type rare-earth methylidene complexes: unique reactivity toward unsaturated C-O, C-N, and C-S bonds

The reactivity of the cubane-type rare-earth methylidene complex [Cp'Lu(μ(3)-CH(2))](4) (1, Cp' = C(5)Me(4)SiMe(3)) with various unsaturated electrophiles was investigated. The reaction of 1 with CO (1 atm) at room temperature gave the bis(ketene dianion)/dimethylidene complex [Cp'(4)Lu(4)(μ(3)-CH(2))(2)(μ(3),η(2)-O-C=CH(2))(2)] (2) in 86 % yield through the insertion of two molecules of CO into two of the four lutetium-methylidene units. In the reaction with the sterically demanding N,N-diisopropylcarbodiimide at 60 °C, only one of the four methylidene units in 1 reacted with one molecule of the carbodiimide substrate to give the mono(ethylene diamido)/trimethylidene complex [Cp'(4)Lu(4)(μ(3)-CH(2))(3) {iPrNC(=CH(2))NiPr}] (3) in 83% yield. Similarly, the reaction of 1 with phenyl isothiocyanate gave the ethylene amido thiolate/trimethylidene complex [Cp'(4)Lu(4)(μ(3)-CH(2))(3) {PhNC(S)=CH(2)}] (4). In the case of phenyl isocyanate, two of the four methylidene units in 1 reacted with four molecules of the substrate at ambient temperature to give the malonodiimidate/dimethylidene complex [Cp'(4)Lu(4)(μ(3)-CH(2))(2) {PhN=C(O)CH(2)(O)C=NPh}(2)] (5) in 87% yield. In this reaction, each of the two lutetium-methylidene bonds per methylidene unit inserted one molecule of phenyl isocyanate. All the products have been fully characterized by NMR spectroscopy, X-ray diffraction, and microelemental analyses.

Ln4(CH2)4 cubane-type rare-earth methylidene complexes consisting of "(C5Me4SiMe3)LnCH2" units (Ln = Tm, Lu)

Tetranuclear cubane-type rare-earth methylidene complexes consisting of four "Cp'LnCH(2)" units, [Cp'Ln(μ(3)-CH(2))](4) (4-Ln; Ln = Tm, Lu; Cp' = C(5)Me(4)SiMe(3)), have been obtained for the first time through CH(4) elimination from the well-defined polymethyl complexes [Cp'Ln(μ(2)-CH(3))(2)](3) (2-Ln) or mixed methyl/methylidene precursors such as [Cp'(3)Ln(3)(μ(2)-Me)(3)(μ(3)-Me)(μ(3)-CH(2))] (3-Ln). The reaction of the methylidene complex 4-Lu with benzophenone leads to C═O bond cleavage and C═C bond formation to give the cubane-type oxo complex [Cp'Lu(μ(3)-O)](4) and CH(2)═CPh(2), while the methyl/methylidene complex 3-Tm undergoes sequential methylidene addition to the C═O group and ortho C-H activation of the two phenyl groups of benzophenone to afford the bis(benzo-1,2-diyl)ethoxy-chelated trinuclear complex [Cp'(3)Tm(3)(μ(2)-Me)(3){(C(6)H(4))(2)C(O)Me}] (6-Tm).