Journal of the American Chemical Society
Article
2,6),29a [N(SiMe2CH2PiPr2)2]ScEt2,29b and (DADMB)YEt-
(THF)2 (DADMB = 2,2′-bis-[(tert-butyl-dimethylsilyl)-
amido]-6,6′-dimethylbiphenyl)30 have been crystallographically
characterized as well as many rare earth complexes containing
bridging ethyl groups, [μ-Et2Si(C5H4)(C5Me4)]2Lu2(μ-Et)(μ-
H),31 La[(μ-Et)2AlEt2]3,32 {Ln[(μ-Et)3AlEt]2}n,33 Ln[(μ-Et)2-
AlEt2]2(THF)2,34 (C5Me5)2Sm(μ-Et)2AlEt2,35 (C5Me5)2Sm-
(THF)(μ-η2-Et)AlEt3,36 [Me2Si(MeC9H5)2]Y(μ-Et)(μ-Me)-
AlEt2,37 and (iPr3C6H2CO2AlEt3)2La(μ-Et)2AlEt2.38
obtained on a Varian 1000 FT-IR spectrophotometer. Elemental
analyses were performed with a PerkinElmer 2400 Series II CHNS
analyzer.
(C5Me5)2YEt, 2. In a nitrogen-filled glovebox, finely divided LiEt
(22 mg, 0.61 mmol), (C5Me5)2Y(μ-Ph)2BPh2, 1, (406 mg, 0.598
mmol), and a magnetic stir bar were added to a 100 mL Schlenk flask
equipped with a custom-made cold-filtration apparatus (Supporting
Information, Figure S1) containing a medium/fine frit, which was
capped with a second 100 mL Schlenk flask. This apparatus was sealed
and brought out of the glovebox. The apparatus was placed under high
vacuum (10−5 Torr) for 30 min, and then pentane (20 mL) was
vacuum transferred onto the solids at −78 °C. The reaction mixture
was allowed to warm to −15 °C and was stirred at this temperature for
12 h. The resulting light yellow slurry was cooled to −45 °C and
filtered at this temperature to remove the white insoluble material,
presumably LiBPh4. The yellow filtrate was placed under vacuum at
−45 °C to remove the solvent. The resulting yellow solid residue was
placed under dynamic high vacuum (10−5 Torr) and slowly warmed to
room temperature over 1 h. Once the yellow solids were completely
dry, the apparatus was brought back into the glovebox, where the
solids were collected to afford 2 as a yellow powder (92 mg, 40%).
Crystals of 2 suitable for X-ray diffraction were obtained via slow
The Ti4+ ethyl complex, Ti(Me2PCH2CH2PMe2)EtCl3,26,27
constituted one of the first examples of agostic systems.39,40
Initially, it was argued that d0 complexes could not β-H
eliminate because they did not have d electrons to back-
donate.26 Nevertheless, β-H elimination has been shown to be a
viable decomposition pathway for rare earth alkyl complexes
containing β-hydrogen atoms.15,16,41−44 However, direct
observation of this pathway for isolable complexes with the
simplest of these ligands, (CH2CH3)1−, has been elusive.
Given the apparent difference in stability between
(C5Me5)2ScEt and “(C5Me5)2LuEt”, it was desirable to
synthesize and isolate reactive ethyl complexes of rare earth
metals larger than scandium to investigate their relative thermal
stability, their ability to perform C−H bond activation and/or
β-H elimination, and possibly their structure. One approach
involves the tetraphenylborate salt of a metallocene cation,
(C5Me5)2Ln(μ-Ph)2BPh2, previously shown to provide facile
access to unsolvated alkyl complexes, (C5Me5)2LnR, eq 3, that
1
evaporation of a concentrated pentane solution at −78 °C. H NMR
(C7D14, −70 °C): δ 1.88 (s, C5Me5, 30H), 0.19 (m, CH2CH3, 5H).
1
13C{1H} NMR (C7D14, 5 °C): δ 116.0 (C5Me5), 29.5 (d, JCY = 34.6
Hz, CH2CH3), 20.1 (CH2CH3), 10.1 (C5Me5). 13C NMR (C7D14, −30
1
1
°C): δ 116.0 (C5Me5), 29.5 (t of d, JCH = 131 Hz, JCY = 35 Hz,
CH2CH3), 20.1 (q, 1JCH = 131 Hz, CH2CH3), 10.1 (q, 1JCH = 125 Hz,
C5Me5). 89Y NMR (C7D14, − 30 °C): 53 ppm. IR: 2963s, 2906s,
2857s, 2726m, 2589m, 2482m, 2426m, 2308w, 2032w, 1961w, 1778w,
1554w, 1489m, 1438s, 1417m, 1379m, 1246w, 1176w, 1061w, 1022m,
965m, 819m, 801m, 660w, 621w, 590m, 552m cm−1. Anal. Calcd for
C22H35Y: C, 68.03; H, 9.08. Found: C, 67.72; H, 9.20.
(C5Me5)2Y[iPrNC(Et)NiPr-κ2N,N′], 4. In a nitrogen-filled glovebox,
a solution of LiEt in methylcyclohexane (1.3 mL, 0.067 M, 0.087
mmol) was quickly combined with a mixture of (C5Me5)2Y(μ-
Ph)2BPh2, 1 (62 mg, 0.091 mmol), in methylcyclohexane (15 mL) at
−35 °C. The reaction mixture was kept at −35 °C in the glovebox
freezer for 2 d, during which time it was occasionally stirred. The
resulting yellow solution was decanted from the white insoluble
are highly reactive in C−H bond activation.45−47 We report the
synthesis of an ethyl complex by this route using yttrium as the
metal to enhance the amount of data obtainable via NMR
spectroscopy due to the diamagnetic nature of the Y3+ ion and
its 100% naturally abundant nuclear spin of I = 1/2.
i
material and added to a cold stirred solution of PrNCNiPr (14
μL, 0.089 mmol) in methylcyclohexane (1 mL). The yellow reaction
mixture was allowed to warm to room temperature and was stirred for
18 h, during which time the solution became colorless. The solvent
was removed under vacuum, and the resulting oily residues were
extracted with hexane (10 mL). Removal of solvent under vacuum
yielded 4 as a white powder (25 mg, 55%). Crystals of 4 suitable for X-
ray diffraction were grown from a concentrated hexane solution at −35
°C. 1H NMR (C6D12): δ 3.62 (sept, 3JHH = 6.0 Hz, CHMe2, 2H), 2.38
EXPERIMENTAL SECTION
■
The syntheses and manipulations described below were conducted
under nitrogen with rigorous exclusion of air and water using Schlenk,
vacuum line, and glovebox techniques. Solvents were sparged with
argon and dried over columns containing Q-5 and molecular sieves.
NMR solvents (Cambridge Isotope Laboratories) were dried over
NaK alloy, degassed by three freeze−pump−thaw cycles, and vacuum
transferred before use (with the exception of D2O, which was used as
received). LiEt (Aldrich) was purchased as a 0.5 M solution in 9:1
benzene/cyclohexane and was placed under vacuum to remove the
solvent before use. iPrNCNiPr (Aldrich) was dried over
molecular sieves and degassed by three freeze−pump−thaw cycles
before use. Me3SiCl (Alfa Aesar) was dried over CaH2 and vacuum
transferred before use. Ultrahigh purity CO2 (Airgas), CH2CH2
(Airgas), H2 (Praxair), CH4 (Airgas), 13CH4 (Cambridge Isotope
Laboratories), and CD4 (Sigma-Aldrich) were used as received. The
unsolvated metallocene complexes (C5Me5)2Y(η3-C3H5)48 and
(C5Me5)2Y(μ-Ph)2BPh2,49 1, were prepared as previously described.
1H and 13C{1H} NMR spectra were obtained on a Bruker GN500
MHz spectrometer at 25 °C or a CRYO500 MHz spectrometer at 5
°C, while 13C and 89Y NMR spectra were obtained on an
AVANCE600 MHz spectrometer at −30 °C, unless otherwise
specified, and were calibrated against the residual protio-solvent
signal. IR samples were prepared as KBr pellets, and the spectra were
(q, 3JHH = 7.5 Hz, CH2CH3, 2H), 1.94 (s, C5Me5, 30H), 1.25 (t, 3JHH
=
7.5 Hz, CH2CH3, 3H), 1.12 (d, 3JHH = 6.0 Hz, CHMe2, 12H); (C6D6):
3
3
δ 3.49 (sept, JHH = 6.0 Hz CHMe2, 2H), 2.19 (q, JHH = 7.5 Hz,
CH2CH3, 2H), 2.04 (s, C5Me5, 30H), 1.09 (t, 3JHH = 7.5 Hz, CH2CH3,
3H), 1.10 (d, 3JHH = 6.0 Hz, CHMe2, 12H). 13C{1H} NMR (C6D12): δ
173.4 (NCN), 117.5 (C5Me5), 46.5 (CHMe2), 36.5 (CH2CH3), 13.7
(CH2CH3), 12.2 (C5Me5). IR: 2970s, 2854s, 2720m, 2597w, 1648w,
1470s, 1377s, 1325s, 1242s, 1202s, 1166s, 1113s, 1063s, 1040s, 978m,
933w, 795w, 775m, 702w, 610m, 548w cm−1. Anal. Calcd for
C29H49N2Y: C, 67.68; H, 9.60; N, 5.44. Found: C, 67.35; H, 9.69; N,
5.30.
Alternative Synthesis of 4. In a nitrogen-filled glovebox, freshly
isolated (C5Me5)2YEt, 2 (60 mg, 0.15 mmol), was dissolved in cold
i
hexane (10 mL, −35 °C). To this stirred yellow solution, PrNC
NiPr (27 μL, 0.17 mmol) was added via syringe. The reaction solution
was allowed to warm to room temperature and was stirred for 3 h,
during which time it became colorless. The solution was filtered, and
the solvent was removed under vacuum to yield 4 as a white solid (76
mg, 98%).
[(C5Me5)2Y(μ-O2CEt)]2, 5. As described above for the synthesis of
4, LiEt (7.6 mg, 0.21 mmol) and (C5Me5)2Y(μ-Ph)2BPh2, 1 (161 mg,
B
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX