Neutral and cationic alkyl and alkynyl complexes of lanthanum: Synthesis, stability, and cis-selective linear alkyne dimerization

Article abstract of DOI:10.1021/om034403u

Neutral triazacyclononane-amide lanthanum dialkyl and dialkynyl complexes were synthesized and structurally characterized. A cationic triazacyclononane-amide lanthanum monoalkyl species was generated and found to be highly active (tof > 100 h^-1) in the rare cis-selective catalytic linear dimerization of phenylacetylene.

Full text of DOI:10.1021/om034403u

936
Organometallics 2004, 23, 936-939
Neu tr a l a n d Ca tion ic Alk yl a n d Alk yn yl Com p lexes of
La n th a n u m : Syn th esis, Sta bility, a n d Cis-Selective
Lin ea r Alk yn e Dim er iza tion
Cornelis G. J . Tazelaar, Sergio Bambirra, Daan van Leusen, Auke Meetsma,
Bart Hessen,* and J an H. Teuben
Center for Catalytic Olefin Polymerization, Stratingh Institute for Chemistry and Chemical
Engineering, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
Received December 24, 2003
Sch em e 1
Summary: Neutral triazacyclononane-amide lantha-
num dialkyl and dialkynyl complexes were synthesized
and structurally characterized. A cationic triazacyclo-
nonane-amide lanthanum monoalkyl species was gen-
erated and found to be highly active (tof > 100 h-1) in
the rare cis-selective catalytic linear dimerization of
phenylacetylene.
The chemistry of cationic group 3 metal and lan-
thanide alkyl complexes is still in its infancy, compared
to that of the transition metals. The latter have been
intensively investigated, especially because of their high
activity in catalytic olefin polymerization.1 The recent
progress made in the synthesis of cationic group 3 metal
and lanthanide alkyl complexes has exclusively focused
on the metals for which the metal trialkyl species
M(CH2SiMe3)3(THF)n (n ) 2, 3) are available as precur-
sors: i.e., for the smaller metals from the series (group
3 metals Sc and Y and lanthanides ranging from Lu
to Tb).2-6 In contrast, only one isolated example, {(η5-
C5Me5)La[CH(SiMe3)2]}[BPh4], was reported for the
largest lanthanide metal, lanthanum.7 Here we describe
the convenient synthesis of neutral lanthanum dialkyl
complexes with tetradentate monoanionic 1,4,7-triaza-
cyclononane-amide (TACN-amide) ancillary ligands
using an in situ peralkylation reaction of LaBr3(THF)4,
followed by addition of the triazacyclononane-amine.
The stability of the resulting dialkyl species depends
strongly on the bridge between the TACN and amide
ligand moieties. A cationic (TACN-amide)lanthanum
monoalkyl species was generated and found to be a
highly active catalyst for the unusual cis-selective linear
dimerization of phenylacetylene.
The ligands N-tert-butyl-2-(4,7-dimethyl[1,4,7]triaza-
non-1-yl)ethylamine (HA) and N-tert-butyl(4,7-dimethyl-
[1,4,7]triazanon-1-yl)dimethylsilylamine (HB) were both
prepared by starting from the known 4,7-dimethyl-1,4,7-
triazacyclononane,8 either by reaction with N-tert-butyl-
chloroacetamide and subsequent reduction with LiAlH4
(for HA)4a or by lithiation of the amine followed by reac-
tion with (tert-butylamino)dimethylchlorosilane (for HB).
Reaction of LaBr3(THF)4 with 3 equiv of Me3SiCH2-
Li in THF at ambient temperature for 3 h, followed by
addition of 1 equiv of HA and subsequent extraction
with and crystallization from pentane, afforded analyti-
cally pure crystalline [η3:η1-Me2TACN(CH2)2NtBu]La-
(CH2SiMe3)2 (1) in 44% isolated yield (Scheme 1).9 The
* To whom correspondence should be addressed. E-mail: hessen@
chem.rug.nl.
(1) (a) Bochmann, M. J . Chem. Soc., Dalton Trans. 1996, 255. (b)
Brinzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth,
R. M.; Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (c) Gibson, V. C.;
Spitzmesser, S. K. Chem. Rev. 2003, 103, 283.
(2) Lee, L.; Berg, D. J ., Einstein, F. W.; Batchelor, R. J . Organo-
metallics 1997, 16, 1819.
(3) (a) Lee, L. W. M.; Piers, W. E.; Elsegood, M. R. J .; Clegg, W.;
Parvez, M. Organometallics 1999, 18, 2947. (b) Hayes, P. G.; Piers,
W. E.; McDonald, R. J . Am. Chem. Soc. 2002, 124, 2132. (c) Hayes, P.
G.; Welch, G. C.; Emslie, D. J . H.; Noack, C. L.; Piers, W. E.; Parvez,
M. Organometallics 2003, 22, 1577.
(4) (a) Bambirra, S.; Van Leusen, D.; Meetsma, A.; Hessen, B.;
Teuben, J . H. Chem. Commun. 2001, 637. (b) Bambirra, S.; Van
Leusen, D.; Meetsma, A.; Hessen, B.; Teuben, J . H. Chem. Commun.
2003, 522.
(5) (a) Arndt, S.; Spaniol, T. P.; Okuda, J . Chem. Commun. 2002,
896. (b) Arndt, S.; Zeimentz, P. M.; Spaniol, T. P.; Okuda, J .; Honda,
M.; Tatsumi, K. J . Chem. Soc., Dalton Trans. 2003, 3622. (c) Arndt,
S.; Spaniol, T. P.; Okuda, J . Organometallics 2003, 22, 775. (d) Arndt,
S.; Spaniol, T. P.; Okuda, J . Angew. Chem., Int. Ed. 2003, 42, 5075.
(6) Cameron, T. M.; Gordon, J . C.; Michalczyk, R.; Scott, B. L. Chem.
Commun. 2003, 2282.
(8) Flassbeck, C.; Wieghardt, K. Z. Anorg. Allg. Chem. 1992, 608, 60.
(9) Synthesis of 1: at ambient temperature, solid LiCH2SiMe3 (0.28
g, 3.0 mmol) was added to a suspension of LaBr3(THF)4 (0.67 g, 1.0
mmol) in THF (60 mL). The solution was stirred for 3 h, after which
Me2-TACN-(CH2)2NHBut (HA, 0.25 g, 1.0 mmol) was added. After the
mixture was stirred for 3 h more, the volatiles were removed in vacuo.
The mixture was extracted with pentane (2 × 50 mL), and the obtained
extract was concentrated to 20 mL and cooled (-30 °C), yielding
crystalline 1 (0.25 g, 0.44 mmol, 44%). 1H NMR (500 MHz, 25 °C,
C6D6): δ 3.08 (m, 2 H, NCH2), 2.82 (m, 2H, NCH2), 2.40-2.35 (m, 2 H,
NCH2), 2.27 (s, 6 H, NMe2), 2.17-2.13 (m, 4 H, NCH2), 1.79-1.74 (m,
2 H, NCH2), 1.71-1.66 (m, 3 H, NCH2), 1.44 (s, 9 H, But), 0.46 (s, 18
H, SiMe3), -0.66 (d, J ) 10.5 Hz, 2 H, LaCH2Si), -0.80 (d, J ) 10.5
Hz, 2 H, LaCH2Si). 13C NMR (125.7 MHz, 25 °C, C6D6): δ 59.7 (t, J )
132 Hz, NCH2), 55.7 (t, J ) 135 Hz, NCH2), 54.9 (s, But C), 54.3 (t, J
) 133 Hz, NCH2), 52.1 (t, J ) 130 Hz, NCH2), 48.1 (t, J ) 104 Hz,
LaCH2), 47.5 (t, J ) 128 Hz, NCH2), 47.0 (q, J ) 135 Hz, NMe), 30.2
(q, J ) 123, But Me), 5.2 (q, J CH ) 116 Hz, SiMe3). Anal. Calcd for
C
22H53N4LaSi2: C, 46.46; H, 9.39; N, 9.85; Found: C, 45.90; H, 9.21;
(7) Schaverien, C. J . Organometallics 1992, 11, 3476.
N, 9.76.
10.1021/om034403u CCC: $27.50 © 2004 American Chemical Society
Publication on Web 02/05/2004

Communications
Organometallics, Vol. 23, No. 5, 2004 937
F igu r e 2. Molecular structure of {[Me(µ-CH2)TACN-
(SiMe2)NtBu]La(CH2SiMe3)}2 (2), with 50% probability
ellipsoids. Only one of the two independent molecules in
the unit cell is shown. Selected interatomic distances (Å)
and angles (deg): La(1)-C(114) ) 2.627(2), La(1)-C(118) )
2.747(2), La(1)-C(118a) ) 2.803(2), La(1)-N(11) ) 2.425(2),
La(1)-N(12) ) 2.788(2), La(1)-N(13) ) 2.518(2), La(1)-
N(14) ) 2.890(2), N(13)-C(118) ) 1.488(3); N(11)-La(1)-
N(12) ) 60.16(5), C(118)-La(1)-C(118a) ) 83.95(6), La(1)-
C(118)-La(1a) ) 96.05(6), La(1)-C(114)-Si(12) ) 135.1(1).
F igu r e 1. Molecular structure of [Me2TACN(CH2)2NtBu]-
La(CH2SiMe3)2 (1), with 50% probability ellipsoids. Selected
interatomic distances (Å) and angles (deg): La-C(15) )
2.632(2), La-C(19) ) 2.618(2), La-N(1) ) 2.348(2), La-
N(2) ) 2.720(2), La-N(3) ) 2.783(2), La-N(4) ) 2.754(2);
N(1)-La-N(2) ) 68.60(7), C(15)-La-C(19) ) 99.41(8),
N(3)-La-C(19) ) 147.74(7), N(4)-La-C(15) ) 95.42(8),
La-C(15)-Si(1) ) 142.4(1), La-C(19)-Si(2) ) 126.2(1).
when the reaction is performed at -10 °C. In contrast,
compound 1 with the (CH2)2 bridge is stable at ambient
temperature in C6D6 solution for at least 1 day, as seen
by NMR spectroscopy. This indicates that the nature
of the bridging moiety in the TACN-amide ligand has a
profound effect on the thermal stability of the (TACN-
amide)La(CH2SiMe3)2 derivatives. In 2, the two La
centers are bridged by the methylene group resulting
from the NMe deprotonation. The La2C2 diamond is not
quite equilateral (with the shortest La-C distance to
the metal that carries the NCH2 functionality), and the
La-N distance to the nitrogen with the CH2 group is
significantly shorter than the other La-N(amine) dis-
tances. The N-CH2 distance is normal for a N-C single
bond. The N(amide)-La-N(bridgehead) angle in 2 of
60.16(5)° is noticeably smaller than the 68.60(7)° angle
in 1, and this reduction of the ligand bite angle is the
likely source of the instability of the dialkyl complex
with the SiMe2 bridge. NMR spectroscopy indicates the
presence in solution of two isomers of 2, in a 3:1 ratio.
These isomers possibly derive from cis-trans isomerism
relative to the central La2C2 diamond. The methylene
protons of the SiCH2 and NCH2 groups are diastereo-
topic; the resonances of the former are located at δ -0.62
and -0.94 ppm (2J HH ) 11.0 Hz) and the latter at δ 2.77
and 1.49 ppm (2J HH ) 13.5 Hz) in the major isomer.
The dialkyl complex 1 reacts with 2 equiv of phenyl-
acetylene to generate the moderately soluble dinuclear
dialkynyl complex {[Me2TACN(CH2)2NtBu]La(CCPh)-
(µ-CCPh)}2 (3) and 2 equiv of SiMe4. A crystal structure
determination12 of 3 (Figure 3) revealed a twisted cis-
type arrangement of the TACN-amide ligands around
a central La(µ-CCPh)2La core that is somewhat puck-
ered, as seen from the dihedral angle C(53)-La(1)-
C(23)-La(2) of -20.07(6)°. The µ-alkynyl groups are
bridging the metal centers in an asymmetric fashion,
complex was characterized by single-crystal X-ray dif-
fraction,10 and its molecular structure is shown in
Figure 1. The metal center has a distorted-octahedral
coordination geometry, with the three amine nitrogens
of the TACN moiety capping a trigonal face. There is
considerable asymmetry in the N(1)-La-C(alkyl)
angles: 127.12(8)° for C(15) and 104.68(7)° for C(19).
This may be associated with minimizing the steric
interference between the alkyl groups and the methyl
substituents on the TACN moiety. Nevertheless, room-
temperature NMR spectra of 1 are consistent with an
averaged Cs symmetry, and only at significantly lower
temperatures is a broadening of the resonances ob-
served, associated with the interconversion between the
two asymmetric conformers. Thus, for La the TACN-
amide dialkyl complex is much more flexible than for
the smaller metal Y, where the slow-exchange limit is
already reached at around 0 °C at the same spectrom-
eter frequency.4a
Performing the same reaction sequence described
above, but now using 1 equiv of the TACN-amine HB
(Scheme 1), resulted in the isolation in moderate
yield (62% of crude material, 17% after recrystallization)
of a product that was characterized by single-crystal
X-ray diffraction11 as the dinuclear {[Me(µ-CH2)TACN-
(SiMe2)NtBu]La(CH2SiMe3)}2 (2; Figure 2). This product
is the result of the metalation of one of the methyl
substituents on the TACN ligand moiety, with concomi-
tant formation of SiMe4 and subsequent dimerization.
It is isolated as the sole organometallic product even
(10) Crystal data for 1: C22H53LaN5Si2, space group P21/n, mono-
clinic, a ) 8.5787(3) Å, b ) 17.6777(7) Å, c ) 19.8289(9) Å, â )
99.498(1)°, V ) 2965.9(2) Å3; T ) 100(1) K, Z ) 4, µ(Mo KR) ) 15.35
cm-1, 20 516 reflections measured (6852 unique), final refinement
converged at Rw(F2) ) 0.0676 with 474 parameters.
(11) Crystal data for 2‚C7H8: (C18H43LaN4Si2)2‚C7H8, space group
P1h, triclinic, a ) 10.8289(4) Å, b ) 12.8117(5) Å, c ) 20.3165(8) Å, R
) 91.104(1)°, â ) 93.765(1)°, γ ) 97.498(1)°, V ) 2787.44(19) Å3; T )
100(1) K, Z ) 4, µ(Mo KR) ) 16.32 cm-1, 26 514 reflections measured
(13 308 unique), final refinement converged at Rw(F 2) ) 0.0570 with
868 parameters.
(12) Crystal data for 3‚1.5C7H8: (C30H41LaN4)2.1.5C7H8, space group
P1h, triclinic, a ) 14.3298(7) Å, b ) 16.0756(8) Å, c ) 16.8773(8) Å, R
) 99.229(1)°, â ) 111.439(1)°, γ ) 94.851(2)°, V ) 3528.4(3) Å3; T )
100(1) K, Z ) 2, µ(Mo KR) ) 12.37 cm-1, 32 332 reflections measured
(16 750 unique), final refinement converged at Rw(F2) ) 0.0842 with
1054 parameters.

938 Organometallics, Vol. 23, No. 5, 2004
Communications
Sch em e 2
Neutral lanthanide metallocene species are known to
catalyze the dimerization of terminal alkynes to give
predominantly the linear head-to-head enyne dimer
with a trans configuration around the enyne double
bond.17 Formation of trans-enyne is readily explained
by 2,1-insertion of the alkyne into the metal-alkynyl
bond, followed by product release via alkyne deproto-
nation. Recently, Hou et al. demonstrated that neutral
cyclopentadienyl-amide lanthanide species catalyze the
linear dimerization of terminal alkynes with high selec-
tivity for the cis-enyne product.18 The cationic {[η3:η1-
Me2TACN(CH2)2NtBu]La(CH2SiMe3)}+ species is also
able to catalyze this rare reaction with high selectivity
(Scheme 2), but at a much faster rate. In reactions
performed at 50 °C on an NMR tube scale with
1/[PhNMe2H][B(C6F5)4] and 45 or 450 equiv of phenyl-
acetylene in 0.4 mL of C6D5Br solvent, full conversion
was reached (in 15 and 240 min, respectively; the
relatively slow conversion in the latter experiment may
be due to the low polarity of the medium at high
substrate concentration). Combined NMR and GC/MS
characterization of the products revealed a high selec-
tivity for cis-1,4-diphenylbut-1-en-3-yne (99% of the
dimer fraction), with a small amount of trimers of the
alkyne as side product (e1%; multiple isomers, as seen
by GC/MS).
In conclusion, we have shown that lanthanum dialkyl
complexes with triazacyclononane-amide ancillary
ligands can conveniently be synthesized directly from
LaBr3(THF)4 via an in situ alkylation procedure fol-
lowed by addition of the ligand. These species, and their
cationic monoalkyl derivatives, should thus readily be
available for all the group 3 and lanthanide metals,
allowing a comparison of their behavior as a function
of the metal ionic radius. The cationic TACN-amide
lanthanum hydrocarbyl system provides a highly active
and selective catalyst for the dimerization of phenyl-
acetylene to cis-1,4-diphenylbut-1-en-3-yne. The source
of the selectivity for the cis-enyne product is as yet
unclear. The mechanism proposed by Yi et al. for (C5-
Me5)Ru(PPh3)H3-catalyzed formation of this product16
involves a vinylidene intermediate and is unlikely to
play a role here, whereas Eisen et al. proposed an
isomerization of an intermediate insertion product to
explain cis-enyne formation in [(Et2N)3U][BPh4]-cata-
lyzed alkyne conversions.19 In contrast, for the high
F igu r e 3. Molecular structure of {[Me2TACN(CH2)2NtBu]-
La(CCPh)(µ-CCPh)}2 (3), with 30% probability ellipsoids.
Selected interatomic distances (Å) and angles (deg): La(1)-
C(15) ) 2.674(2), La(2)-C(45) ) 2.655(2), La(1)-C(23) )
2.698(2), La(2)-C(23) ) 2.876(2), La(1)-C(53) ) 2.855(2),
La(2)-C(53) ) 2.705(2), La(1)-N(1) ) 2.702(2), La(1)-N(2)
) 2.867(2), La(1)-N(3) ) 2.756(2), La(1)-N(4) ) 2.365(2),
C(15)-C(16) ) 1.226(3), C(23)-C(24) ) 1.221(3); La(1)-
C(15)-C(16) ) 174.3(2), La(1)-C(23)-C(24) ) 160.9(2),
La(2)-C(23)-C(24) ) 95.5(2).
with La(1)-C(23) ) 2.698(2) Å and La(1)-C(23)-C(24) )
160.8(2)° vs La(2)-C(23) ) 2.876(2) Å and La(2)-C(23)-
C(24) ) 95.5(2)°. This type of bridging mode is similar
to that found in lanthanidocene µ-alkynyl dimers.13
There is no significant difference in CtC bond length
between the bridging and terminal alkynyl ligands. The
13C NMR spectrum of 3 shows two resonances (at δ
163.9 and 161.2 ppm) for the La-bound alkynyl carbons.
Reaction of the dialkyl complex 1 with the Brønsted
acid [PhNMe2H][B(C6F5)4] in both C6D5Br/d8-THF and
1
C6D6/d8-THF solvent mixtures was seen by H and 13C
NMR to generate SiMe4, free PhNMe2, and an alkyl
species, likely to be {[η3:η1-Me2TACN(CH2)2NtBu]La-
(CH2SiMe3)(d8-THF)x}[B(C6F5)4] (4).14 As was observed
for the analogous Y species,4a the 13C NMR Ln-CH2
resonance in cationic 4 (δ 55.4 ppm in C6D6/d8-THF) is
found downfield from that in the neutral dialkyl precur-
sor 1 (δ 47.9 ppm in the same solvent). The reaction of
the dialkynyl species 3 with [PhNMe2H][B(C6F5)4] and
the reaction of the ionic La alkyl 4 with 1 equiv of
phenylacetylene (in C6D5Br/d8-THF solvent) both lead
to the formation of a poorly soluble product, visible as
an orange oily precipitate in the NMR tube. In the
reaction of the dialkynyl compound, it was observed by
1H NMR that the phenylacetylene liberated in the initial
reaction of 3 with the Brønsted acid is rapidly converted
into cis-1,4-diphenylbut-1-en-3-yne.15
(16) (a) Yi, C. S.; Liu, N. Organometallics 1996, 15, 3968. (b) Yi, C.
S.; Liu, N.; Rheingold, A. L.; Liable-Sands, L. M. Organometallics 1997,
16, 3910.
(17) (a) Heeres, H. J .; Teuben, J . H. Organometallics 1991, 10, 1980.
(b) den Haan, K. H.; Wielstra, Y.; Teuben, J . H. Organometallics 1987,
6, 2053.
(13) See e.g.: Atwood, J . L.; Hunter, W. E.; Wayda, A. L.; Evans,
W. J . Inorg. Chem. 1981, 20, 4115.
(14) Data for 4 are as follows. 1H NMR (300 MHz, 20 °C, C6D6/ THF-
d8): δ 2.91 (m, 2 H, NCH2), 2.61 (m, 2 H, NCH2), 2.20 (m, 4 H, NCH2),
2.11 (s, 6 H, NMe), 1.75 (m, 4 H, NCH2), 1.28 (s, 9 H, But), 0.42 (s, 9
H, SiMe3), -0.82 (br, 2 H, LaCH2Si). 13C{1H} NMR (75.4 MHz, 20 °C,
C6D6/THF-d8): δ 60.0 (NCH2), 55.4 (NCH2), 55.4 (LaCH2Si), 54.7 (s,
But C), 53.7 (NCH2), 52.6 (NCH2), 47.0 (NMe), 45.2 (NCH2), 29.7
(NCMe3), 4.3 (LaCH2SiMe3). Data for the anion are omitted.
(15) As indicated by the characteristic 1H NMR doublet resonances
(18) Nishiura, M.; Hou, Z.; Wakatsuki, Y.; Yamaki, T.; Miyamoto,
T. J . Am. Chem. Soc. 2003, 125, 1184.
(19) (a) Wang, J . Q.; Dash, A. K.; Berthet, J . C.; Ephritikhine, M.;
Eisen, M. S. Organometallics 1999, 18, 2407. (b) Dash, A. K.;
Wang, J . Q.; Berthet, J . C.; Ephritikhine, M.; Eisen, M. S. J .
Organomet. Chem. 2000, 604, 83. (c) Wang, J . Q.; Dash, A. K.;
Kapon, M.; Berthet, J . C.; Ephritikhine, M.; Eisen, M. S. Chem. Eur.
J . 2002, 8, 5384.
3
for cis-PhCHdCHCtCPh at δ 6.52 and δ 5.89 with J HH ) 11.7 Hz
(C6D5Br/d8-THF solvent).16

Communications
Organometallics, Vol. 23, No. 5, 2004 939
selectivity of the catalysis by neutral cyclopentadienyl-
amide lanthanide systems, Hou et al. invoked a mech-
anism involving a bimetallic active species.18 The ob-
servation that the generation of cationic TACN-amide
lanthanum acetylide species, either from 2 or from 4,
results in the formation of a poorly soluble organo-
metallic product (even in bromobenzene/THF mixtures)
might be an indication that a dinuclear species is
formed. In this case, this is likely to be dicationic, and
such a compound is expected to be relatively poorly
soluble. The nature of this species and its behavior are
the subjects of current investigations.
Ack n ow led gm en t. We thank A. J ekel for the GC/
MS analysis and ExxonMobil Chemicals for financial
support.
Su p p or tin g In for m a tion Ava ila ble: Text giving full
experimental characterization data for HB and 1-4 and tables
of crystal data, structure solution and refinement details,
atomic coordinates, bond lengths and angles, and anisotropic
thermal parameters for 1-3 as well as details of catalytic
phenylacetylene dimerization experiments. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM034403U