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

Abstract 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,MeLnMeR] have been synthesized and fully characterized. Treatment of the heterobimetallic mixed methyl/tetramethylaluminate compounds [TptBu,MeLnMe(AlMe4)] (Ln=Y, Lu) with two equivalents of the mild halogenido transfer reagents SiMe3X (X=Cl, I) gave [TptBu,MeLnX2] in high yields. The addition of only one equivalent of SiMe3Cl to [TptBu,MeLuMe(AlMe4)] selectively afforded the desired mixed methyl/chloride complex [TptBu,MeLuMeCl]. Further reactivity studies of [TptBu,MeLuMeCl] with LiR or KR (R=CH2Ph, CH2SiMe3) through salt metathesis led to the monomeric mixed‐alkyl derivatives [TptBu,MeLuMe(CH2SiMe3)] and [TptBu,MeLuMe(CH2Ph)], respectively, in good yields. The SiMe4 elimination protocols were also applicable when using SiMe3X featuring more weakly coordinating moieties (here X=OTf, NTf2). X‐ray structure analyses of this diverse set of new [TptBu,MeLnMeR/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.

In particular, given that the yttrium derivatives are extremely temperature sensitive, careful adjustment of the reaction conditions was required to afford complexes [Tp tBu,Me LnMe(OTf)] , and [Tp tBu,Me Ln(NTf 2 ) 2 ] (4-Lu). The ambient-temperature 1 H NMR spectra of the diamagnetic compounds 1-4 showed only one set of signals for the pyrazolyl groups of the Tp tBu,Me ligand with chemical shifts similar to those of the starting compounds (spectral data are presented in the Supporting Information). This indicates a highly fluxional behavior, which is in accordance with previous studies on complex [Tp tBu,Me LuMe(AlMe 4 )]. [17] However, these previous studies also reported that similar complexes behave differently at lower temperatures, with the pyrazolyl rings revealing a 2:1 splitting in the 1 H NMR spectra in accordance with the C s symmetry of these complexes in the solid state. [17] For 1-Lu and 3-Lu, the Lu-bound Me groups gave sharp singlets at d = 0.39 and 0.14 ppm, respectively.
The ambient-temperature 1 H NMR spectrum of 1-Y in C 6 D 6 showed a broadened signal at d = 0.26 ppm for the terminal methyl moiety, not indicative of any YÀH coupling. To further investigate this behavior, a low-temperature 1 H NMR spectroscopy study was carried out ( Figure S2 in the Supporting Infor-  . Due to solubility issues in toluene at temperatures below 20 8C and rapid decomposition of complex 1-Y in THF, a few drops of [D 8 ]THF were added to a precooled solution of 1-Y in [D 8 ]toluene. Remarkably, the chosen NMR solvent "mixture" showed a strong influence on the chemical shift of the Y-Me moiety at low temperature, revealing a doublet at d = À0.13 ppm ( 2 J(YÀH) = 1.5 Hz) markedly shifted to higher fields compared with 1-Y in [D 6 ]benzene (d = 0.26 ppm, Figure S1, Supporting Information). The 1 H-89 Y HSQC NMR spectrum of 1-Y at 0 8C shows a cross peak at d = 515 ppm on the 89 Y NMR scale (Figure 1), which is shifted to higher field in comparison to precursor [Tp tBu,Me YMe(AlMe 4 )] (d = 798 ppm). [18] The 13 C NMR spectra of the fluorine-containing complexes 1-Ln, 2-Lu, 3-Lu, and 4-Lu showed one set of signals for the Tp tBu,Me ligand but 13 C resonances of the CF 3 groups could not be detected, which is consistent with already reported compounds. [23] Notwith-standing, the presence of OTf and NTf 2 moieties was unambiguously evidenced by 19 F NMR spectroscopy revealing one sharp resonance at d = À78.0, À78.1, À77.5, À77.9, and À76.9 ppm each for complexes 1-Y, 1-Lu, 2-Lu, 3-Lu, and 4-Lu, respectively.
Generation of di(halogenido) and mixed methyl/halogenido and methyl/alkyl complexes Further efforts to generate Ln III alkylidenes led to the idea of targeting mixed methyl/alkyl (Me/R) complexes [Tp tBu,Me LuMeR]. The latter might be convertible to the envisaged alkylidene species following a thermal or donor-induced intramolecular elimination of either methane or the respective HR analog to Petasis (see Scheme 1/path B). Note that halfsandwich complexes of the type [(C 5 Me 4 SiMe 3 )LnMe 2 ] 3 were previously shown to undergo such reactions affording tetrametallic cuboid clusters [(C 5 Me 4 SiMe 3 )Ln(m 3 -CH 2 )] 4 (Ln = Tm, Lu). [6] Preliminary NMR-scale reactivity studies probing the olefination capability of [Tp tBu,Me LuMe 2 ] toward 9-fluorenone at 50 8C (according to Petasis) indicated the exclusive formation of the respective alkoxide species. Therefore, to evade such preferential nucleophilic attack of the methyl moiety at the carbonyl functionality, the initial formation of an alkylidene species was envisaged. To provide a more versatile platform for further derivatization reactions, the above-mentioned precursors [Tp tBu,Me LnMe(AlMe 4 )] and [Tp tBu,Me LuMe 2 ] were treated with one equivalent of SiMe 3 X (here X = Cl, I) in toluene for the generation of mixed alkyl/halogenido compounds as depicted in Scheme 3.

1-Lu
The mixed alkyl complex [Tp tBu,Me LuMe(CH 2 SiMe 3 )] (8-Lu) was obtained by reacting 7-Lu with LiCH 2 SiMe 3 . Due to the thermal lability of 8-Lu, the reaction was performed at temperatures below 0 8C. Such low temperatures are also beneficial to the use of Li salts because conducting the involved metathesis reactions at ambient temperature favors the formation of LiTp tBu,Me . [20] In contrast, the mixed methyl/benzyl complex [Tp tBu,Me LuMe(CH 2 Ph)] (9-Lu) is thermally stable, but a prolonged reaction time is crucial when reacting 7-Lu with potassium benzyl. For both mixed bis(alkyl) complexes 8-Lu and 9-Lu, the 1 H and 13 C NMR spectra show only one set of signals for the pyrazolyl groups. The Ln-bound methyl groups appeared as narrow singlets at d = 0.19 (8-Lu) and 0.39 ppm (9-Lu). In agreement with literature reports, the methylene moieties of the neosilyl and benzyl ligand feature distinctly shifted signals at d = À0.71 and 1.63 ppm, respectively, attributable to a strong electronic influence of the SiMe 3 /Ph groups.
Complexes 8-Lu and 9-Lu were crystallized from saturated toluene solutions at À35 8C and their solid-state structures analyzed by X-ray crystallography ( Figure 4). As commonly observed for Ln III ÀTp tBu,Me complexes with coordination number 5, both complexes adopt a distorted trigonal-bipyramidal geometry. The pyrazolyl nitrogen atoms N2 and N4 and the methyl carbon C25 reside in the equatorial plane, whereas the methylene carbon atom C26 and the pyrazolyl nitrogen atom N6 occupy the axial positions. In comparison with complexes 1-Lu and 3-Lu the LuÀN(pz) bond lengths are slightly elongated for the mixed alkyl compounds 8-Lu (2.352(2)-2.487(2) ) and 9-Lu (2.310(2)-2.466(2) ).
In spite of these sobering findings, the successful isolation of mixed alkyl complexes 8-Lu and 9-Lu spurred our interest in the evaluation of the steric effects on the ancillary Tp tBu,Me ligand caused by the distinct triflato, halogenido, or alkyl co-ligands. According to a method recently reported by our group, we calculated the exact ligand cone angles V8 (the procedure is given in the Supporting Information). [39] According to Allen and co-workers, the term "exact" refers to the acute mathematical solution and does not reflect the accuracy of the input structure itself. [39b] As a prerequisite for meaningful interpretations, the metal centers should have the same coordination number (CN, here 5) and the same overall charge. A general overview of the determined cone angles is summarized in Table 2.
The Tp tBu,Me ligand engages in an exclusive trigonal-bipyramidal coordination geometry at the Lu complexes under study, and hence, very similar cone angles (V8 = 277.1 to 280.98 for CN = 5) were calculated. For 1-Lu, two different cone angles are displayed due to the respective disorder in one tert-butyl group. Nonetheless, the noticeable trend makes complexes with mixed alkyl co-ligands the least sterically demanding, followed by the di(halide) complexes, whereas the weakly coordinating triflato or triflimido moieties allow for the largest cone angles. Another important finding is that the mathematically exact method determines cone angles distinctly higher than those reported for Tp tBu,Me complexes in the literature (V8 = 2448). [40] Therefore, further efforts should be expended to build up a library for better comparison.

Conclusions
Aiming at new synthesis protocols for terminal rare-earthmetal alkylidene complexes, we gained access to unprecedented mono-tris(pyrazolyl)borate complexes. Following TMS-elimination protocols by applying complexes [Tp tBu,Me YMe(AlMe 4 )] Hydrogen atoms except for BH and CH 2 are omitted for clarity. For 9-Lu the disorder in one tBu group and toluene are omitted for clarity. Selected bond lengths are given in Table 1.  [41] [Tp tBu,Me YMe(AlMe 4 )], [17] [Tp tBu,Me LuMe(AlMe 4 )], [17] and [Tp tBu,Me LuMe 2 ] [18] were synthesized according to literature procedures. The NMR spectra of air-and moisture-sensitive compounds were recorded by using J. Young valve NMR tubes on a Bruker AVII + 400 spectrometer ( 1 H, 400.13; 13 C, 100.61; 19   X-ray data for compounds of 1-Lu, 3-Lu, 4-Lu, 5-Lu, 6-Lu, 7-Lu, 8-Lu, and 9-Lu were collected on a Bruker APEX II DUO instrument equipped with an ImS microfocus sealed tube and QUAZAR optics for MoK a (l = 0.71073 ) and CuK a (l = 1.54184 ) radiation. The data collection strategy was determined using COSMO [42] employing w-scans. Raw data were processed using APEX [43] and SAINT, [44] corrections for absorption effects were applied using SADABS. [45] The structures were solved by direct methods and refined against all data by full-matrix least-squares methods on F 2 using SHELXTL [46] and ShelXle. [47] Disorder models were calculated using DSR, a program for refining structures in ShelXl. [48] All graphics were produced employing ORTEP-3 [49] and POV-Ray. [50] Further details of the refinement and crystallographic data are listed in Table S1 (Supporting Information) and in the CIF files. CCDC 1945695, 1945696, 1945697, 1945698, 1945699, 1945700, 1945701, 1945702 contain the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Data Centre.