Unusual alkyl group activation and cationic complex formation from a novel lutetium dialkyl complex supported by a tridentate monoanionic ligand

Article abstract of DOI:10.1039/b306889g

We report herein the synthesis and characterization of a lutetium dialkyl complex supported by a multidentate, anilido-pyridine-imine ligand and its subsequent transformation into an unprecedented cationic monoalkyl derivative.

Full text of DOI:10.1039/b306889g

Unusual alkyl group activation and cationic complex formation from a
novel lutetium dialkyl complex supported by a tridentate monoanionic
ligand†
Thomas M. Cameron,a John C. Gordon,*a Ryszard Michalczykb and Brian L. Scotta
a Chemistry Division, MS J514
b NIH Stable Isotope Resource, Bioscience Division, B-3; Los Alamos National Laboratory, Los Alamos,
NM, USA. E-mail: jgordon@lanl.gov; Fax: 505-667-9905; Tel: 505-667-2376
Received (in West Lafayette, IN) 17th June 2003, Accepted 30th July 2003
First published as an Advance Article on the web 18th August 2003
We report herein the synthesis and characterization of a
lutetium dialkyl complex supported by a multidentate,
anilido-pyridine-imine ligand and its subsequent trans-
formation into an unprecedented cationic monoalkyl de-
rivative.
after filtration and washing with hexanes. Complex 2 is
insoluble in hexanes, slightly soluble in benzene, and soluble in
dichloromethane. X-Ray quality crystals of 2 were grown from
a concentrated benzene solution, and the thermal ellipsoid plot
of 2 is shown in Fig. 1.
The cyclopentadienyl (Cp) family of ligands is ubiquitous in
trivalent organometallic lanthanide (Ln) chemistry, and exam-
ples of bis-and mono-Cp Ln derivatives are known and are the
subject of recent reviews.1 These complexes are synthetically
important and have found use in catalytic hydroamination2 and
polymerization.3 In contrast, non-Cp systems based on multi-
dentate mono and diamido ligands remain a relatively un-
explored area of research.4 Recent developments in this area
involve the synthesis of yttrium complexes stabilized by
pyrrolyl ligand type I,5 chelating anilido-imine donors related to
type II,6 deprotonated aza-18-crown-6,7 and a linked 1,4,7,-tria-
zacyclononane-amide ligand.8 Related reports of organo-
scandium species stabilized by monoanionic b-diketiminato
ligands have been well-documented,9 and the generation of a
variety of alkyl (bis)amido Ln complexes reported by An-
wander et al. has recently appeared.10 We have focused our
initial efforts in this area on [2-{(2,6-Pri2C6H3)NNCMe}-
6-{(2,6-Pri2C6H3)NHCMe2}C5H3N] (1),11 a sterically demand-
ing precursor to an asymmetric, multidentate, monoanionic,
anilido-pyridine-imine ligand (Scheme 1). We herein report the
Complex 2 crystallized with a monoclinic unit cell with one
molecule of benzene per molecule of 2.‡ The geometry about
the metal center in 2 is best described as distorted square
pyramidal. The base of the pyramid is defined by N(1), N(2),
N(3), and C(5), and Lu(1) lies 0.6629 Å above the plane defined
by these atoms. The Lu(1)–C(1) and Lu(1)–C(5) bond lengths
of 2.329(6) Å and 2.349(6) Å, respectively, fall within the
expected range for Lu–C bonds in complexes containing a Lu–
CH2SiMe3 functionality.13 The Lu(1)–N(1) amide bond length
of 2.188(4) Å is within the range expected and consistent with
a monoanionic ligand in 2.12 Of particular interest in 2 are the
short C(1)–H(1A) and H(1B) a-proton–Lu(1) contacts of
2.62(7) Å and 2.52(7) Å respectively. These distances support
Lu–aCH agostic interactions in the solid-state.14
1
The H NMR spectrum of 2 is fluxional at 25 °C but does
approximate the solid-state structure at low temperature (250
°C). For example, at 250 °C two Si(CH3)3 resonances are
observed at 20.75 and 20.45 ppm as well as four a-CH
resonances at 21.81, 21.66, 21.24, and 20.75 ppm. The
synthesis
and
structural
characterization
of
[2-
{(2,6-Pri2C6H3)NNCMe}-6-{(2,6-Pri2 C6H3)NCMe2}C5H3N]-
Lu(CH2SiMe3)2 (2), the first Ln dialkyl complex stabilized by
this ligand class. We also report the synthesis and structure of a
unique cationic complex [2-{(2,6-Pri2C6H3)NNCMe}-
6-{(2,6-Pri2C6H3)NCMe2}C5H3N
Lu(CH2SiMe2CH2Si-
Me3)(THF)][MeB(C6F5)3] (3), generated by an unusual activa-
tion pathway.
12
Treatment of a toluene solution of Lu(CH2SiMe3)3(THF)2
with 1 resulted in the generation of 2 (Scheme 1).† Concentra-
tion of the reaction solution, under reduced pressure, followed
by cooling to 230 °C, gave microcrystalline 2 in 35% yield
Fig. 1 Thermal ellipsoid plot of 2 (35% probability thermal ellipsoids).
Selected bond lengths (Å) and angles (°): Lu(1)–C(1) 2.329(6), Lu(1)–C(5)
2.349(6), Lu(1)–N(1) 2.188(4), Lu(1)–N(2) 2.376(4), Lu(1)–N(3) 2.510(4),
N(3)–C(29) 1.291(6), N(1)–Lu(1)–N(2) 69.03(13), N(2)–Lu(1)–N(3)
65.75(12), N(3)–Lu(1)–C(5) 96.6(2), C(5)–Lu(1)–N(1) 109.3(2), C(1)–
Lu(1)–N(1) 111.09(16), C(1)–Lu(1)–C(5) 107.8(2), C(1)–Lu(1)–N(3)
99.69(16), C(1)–Lu(1)–N(2) 104.73(17).
Scheme 1
† Electronic supplementary information (ESI) available: details of the
synthesis of 2 and 3 and the hydrolysis of 3. See http://www.rsc.org/
suppdata/cc/b3/b306889g/
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This journal is © The Royal Society of Chemistry 2003

1JCa-H of 99 Hz supports the above discussed agostic interaction
in solution at 250 °C.15,16
well-characterized example of a cationic, Ln dialkyl complex
has appeared in the literature.13b To the best of our knowledge
3 is the first reported example of a stable, non-Cp-based,
cationic, monoalkyl complex of an 4f-element.19 We are
currently in the process of studying the mechanism concerned
with the transformation of 2 to 3 and exploring the reactivity of
both complexes with other small molecules.
Treatment of a CD2Cl2 solution of 2 with B(C6F5)3 in the
presence of THF resulted in the formation of
[2-{(2,6-Pri2C6H3)NNCMe}-6-{(2,6-Pri2C6H3)NCMe2}C5H3N
Lu(CH2SiMe2CH2SiMe3)(THF)][MeB(C6F5)3] (3) (Scheme
1).† Cationic 3 is stable for weeks in solution and has been fully
characterized by multinuclear NMR spectroscopy (including
two-dimensional experiments).16,17 A graphical representation
of 3 including selected chemical shifts is shown in Fig. 2.
Complex 3 possesses an asymmetric environment around the
metal center, resulting in diastereotopic protons Ha through Hh
as well as the inequivalent (non-isopropyl) methyl groups. At
room temperature four isopropyl methine resonances and eight
isopropyl methyl group resonances are observed by NMR
techniques.16 There are therefore four inequivalent isopropyl
groups, implying restricted rotation about the N–Ar bonds on
the NMR time-scale and a sterically congested metal center.
The 1JCa-H of 96 Hz in 3 supports a Lu–aCH agostic interaction
in solution15 and the Ddm,p value of 2.6 in the 19F NMR
spectrum of 3 supports a non-contacting cation–anion pair.6
Alkyl group abstraction from Y, Sc, and Lu species,
generating cationic complexes is a known process.6,7,9,13b The
generation of 3 from treatment of 2 with B(C6F5)3 in the
presence of THF in lieu of alkyl group (CH2SiMe3) abstraction
was thus unexpected. A possible mechanism for the transforma-
tion of 2 to 3 may involve concerted silyl methyl group
extraction by B(C6F5)3, accompanied by alkyl migration
(Scheme 2). To our knowledge this is the first reported instance
of a lanthanide mediated rearrangement of this kind.18 Steric
crowding around the lutetium center in 2 may render the alkyl
group a-carbons inaccessible to the boron reagent, redirecting
reactivity to the silyl methyl groups.
Notes and references
‡ Crystal data: for 2: C42H68N3Si2Lu·C6H6, M = 924.25, a = 18.450(5), b
= 14.220(4), c = 19.167(5) Å, V = 4863(2) Å3, monoclinic, space group
P21/c, Z = 4, m(Mo–Ka) = 2.112 mm21, T = 203 K, final R1 (I > 2s) =
0.0533, wR2 (I > 2s) = 0.0998, GOF (on F2) = 2.115. Hydrogen atom
positions H(1A) and H(1B) were found on the difference map and refined
with isotropic temperature factors set to 0.08 Å2. CCDC 213144. See http:
//www.rsc.org/suppdata/cc/b3/b306889g/ for crystallographic data in .cif
format.
1 (a) S. Arndt and J. Okuda, Chem. Rev., 2002, 102, 1953; (b) C. J.
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U. Kilimann and F. T. Edelmann, Coord. Chem. Rev., 1995, 141, 1; (e)
J. Richter and F. T. Edelmann, Coord. Chem. Rev., 1996, 147, 373; (f)
Y. K. Gun’ko and F. T. Edelmann, Coord. Chem. Rev., 1996, 156, 1; (g)
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2 S. Hong and T. J. Marks, J. Am. Chem. Soc., 2002, 124, 7886 and
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7 L. Lee, D. J. Berg, F. W. Einstein and R. J. Batchelor, Organometallics,
1997, 16, 1819.
In summary complexes 2 and 3 represent the first well-
characterized Ln complexes employing ligand 1. While exam-
ples of Sc- and Y-alkyl cations have been reported,6–9 only one
8 S. Bambirra, D. van Leusen, A. Meetsma, B. Hessen and J. H. Teuben,
Chem. Commun., 2001, 637.
9 L. M. W. Lee, W. E. Piers, M. R. J. Elsegood, W. Clegg and M. Parvez,
Organometallics, 1999, 18, 2947.
10 F. Estler, G. Eickerling, E. Herdtweck and R. Anwander, Organome-
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11 (a) M. Bruce, V. C. Gibson, C. Redshaw, G. A. Solan, A. J. P. White and
D. J. Williams, Chem. Commun., 1998, 2523; (b) V. C. Gibson, S.
McTavish, C. Redshaw, G. A. Solan, A. J. P. White and D. J. Williams,
Dalton Trans., 2003, 221.
12 S. Arndt, P. Voth, T. P. Spaniol and J. Okuda, Organometallics, 2000,
19, 4690.
13 (a) H. Schumann, D. M. M. Freckmann and S. Dechert, Z. Anorg. Allg.
Chem., 2002, 628, 2422; (b) S. Arndt, T. P. Spaniol and J. Okuda, Chem.
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Boersma, J. T. B. H. Jastrzebski, H. Kooijman, A. L. Spek and G. van
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Fig. 2 Graphical representation of 3 including selected chemical shifts
assigned by multinuclear NMR spectroscopy. The MeB(C6F5)3 anion has
been omitted for clarity (ov
underlined, Ar = 2,6-Pri2C6H3).
= overlapping, carbon resonances are
14 All other Lu–alkyl ligand carbon contacts are > 4 Å making bSiC or gCH
interactions unlikely. W. T. Klooster, L. Brammer, C. J. Schaverien and
P. H. M. Budzelaar, J. Am. Chem. Soc., 1999, 121, 1381 and references
therein.
15 X. Bei, D. C. Swenson and R. F. Jordan, Organometallics, 1997, 16,
3282 and references therein. Note that low 1JCa–H values may be a result
of steric interactions which increase the Lu–C–Si angle leading to a
reduced H–C–H angle and therefore reduced 1JCaH values.
16 See the ESI for full details on the NMR characterization of 2 and 3.
17 The atomic connectivity of the alkyl ligand was confirmed indirectly by
alkyl group protonolysis. Treating a sample of 3 with water afforded the
alkane Me3SiCH2SiMe3 as determined by 1H NMR spectroscopy.
18 A related rearrangement has been observed for a Ti system: E. E. C. G.
Gielens, J. Y. Tiesnitsch, B. Hessen and J. H. Teuben, Organometallics,
1998, 17, 1652. Formation of this alkyl group was also observed upon
thermolysis of Pt(CH2SiMe3)2L2: S. Katherine and G. B. Young,
Organometallics, 1989, 8, 2068.
19 (a) C. J. Schaverien, Organometallics, 1992, 11, 3476; (b) S. Arndt, T.
P. Spaniol and J. Okuda, Organometallics, 2003, 22, 775.
Scheme 2
CHEM. COMMUN., 2003, 2282–2283
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