The 6-amino-6-methyl-1,4-diazepine group as an ancillary ligand framework for neutral and cationic scandium and yttrium alkyls

Article abstract of DOI:10.1039/b606384e

The 6-amino-6-methyl-1,4-diazepine framework is a readily available neutral 6-electron ligand moiety, suitable to support cationic group 3 metal alkyl catalysts; it also provides convenient access to tri- and tetradentate monoanionic ligand derivatives. T

Full text of DOI:10.1039/b606384e

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The 6-amino-6-methyl-1,4-diazepine group as an ancillary ligand
framework for neutral and cationic scandium and yttrium alkyls{
Shaozhong Ge, Se´rgio Bambirra, Auke Meetsma and Bart Hessen*
Received (in Cambridge, UK) 5th May 2006, Accepted 9th June 2006
First published as an Advance Article on the web 27th June 2006
DOI: 10.1039/b606384e
The 6-amino-6-methyl-1,4-diazepine framework is a readily
available neutral 6-electron ligand moiety, suitable to support
cationic group 3 metal alkyl catalysts; it also provides
convenient access to tri- and tetradentate monoanionic ligand
derivatives.
In contrast to their transition-metal analogues (which have long
been known as active catalysts for olefin polymerisation), the
chemistry of cationic rare-earth metal alkyl species has only
recently been developed.1 The use of nitrogen-based facial
tridentate ligand moieties (such as 1,4,7-triazacyclononane,
tris(pyrazolyl)methane and tris(oxazolinyl)methane) has played
an important role in opening up this chemistry.2 A disadvantage of
these ligand systems is that stepwise modification and extension of
these moieties is synthetically quite elaborate. Very recently, the
use of the 6-amino-6-methyl-1,4-diazepine group as a facially
coordinating moiety in biomimetic complexes was described.3 This
framework is readily obtained by reaction of 1,2-diaminoethanes
Scheme 1 Synthesis of complexes 1 and 2.
with nitroethane and formaldehyde, followed by reduction of the
nitro group. Here we show that this group provides an accessible
and versatile basis for neutral and anionic tri- and tetradentate
ligands for use in rare-earth metal organometallic chemistry.
To test the suitability of the 6-amino-6-methyl-1,4-diazepine
group as an ancillary ligand moiety for rare-earth metal alkyl
chemistry, the known permethylated 6-amino-6-methyl-1,4-diaze-
pine (L1)3b was reacted with the group 3 metal trialkyls
M(CH2SiMe3)3(THF)2 (M = Sc, Y). This afforded the complexes
(L1)M(CH2SiMe3)3 (M = Sc 1a or Y 1b, Scheme 1) in high
isolated yields (1a: 94%; 1b: 95%). Solution NMR spectroscopy of
these compounds (C7D8) showed that the three alkyl groups on the
metal centre are equivalent down to 250 uC. The M–CH2
resonances (C6D6, 20 uC) for 1a are found at d 20.14 ppm (1H)
1
and d 40.0 ppm (13C), for 1b at d 20.56 (d, JYH = 2.9 Hz) and
1
d 36.9 ppm (JYC = 35 Hz, JCH = 97.9 Hz) respectively.
A crystal structure determination of 1a was performed, and its
molecular structure is shown in Fig. 1.{ The crystal contains two
independent molecules in the asymmetric unit that do not differ
significantly; only one is explicitly discussed here. The three
nitrogen atoms of L1 are bound to the scandium centre in a fac-
arrangement and the geometry at Sc is approximately octahedral.
Fig. 1 Molecular structure of one of the independent molecules of 1a
˚
The average Sc–N bond length of 2.497 A in 1a is slightly longer
(hydrogen atoms omitted for clarity, thermal ellipsoids drawn at 50%
˚
probability level). Selected bond distances (A) and angles (u): Sc1–N11
Centre for Catalytic Olefin Polymerization, Stratingh Institute of
Chemistry and Chemical Engineering, University of Groningen,
Nijenborgh 4, 9747 AG, Groningen, The Netherlands.
E-mail: B.Hessen@rug.nl
{ Electronic supplementary information (ESI) available: Full experimental
and characterisation data of the compounds. See DOI: 10.1039/b606384e
2.504(3), Sc1–N12 2.465(3), Sc1–N13 2.521(3), Sc1–C111 2.267(3), Sc1–
C115 2.256(4), Sc1–C119 2.309(4); N11–Sc1–N12 73.91(8), N11–Sc1–N13
69.63(8), N12–Sc1–N13 66.45(8), C111–Sc1–C115 100.87(13), C111–Sc1–
C119 96.68(13), C115–Sc1–C119 102.31(13), N11–Sc1–C119 159.47(11),
N12–Sc1–C111 163.19(11), N13–Sc1–C115 153.24(11).
3320 | Chem. Commun., 2006, 3320–3322
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than that reported for the triazacyclononane complex
4
Sc(Me3[9]aneN3)(CH2SiMe3)3 (average 2.463 A), while the
˚
average Sc–CH2 distances are very similar. There is no notable
asymmetry in the bonding of the three amine donors in 1a: the
Sc–N distance for the NMe2-group is intermediate between the
other two Sc–N distances in the complex. The smallest N–Sc–N
angle involves the amine nitrogens linked by the (CH2)2-bridge,
N(12)–Sc–N(13) of 66.45(8)u.
The neutral trialkyl complexes 1 can be converted in THF
solvent to the dialkyl cations [(L1)M(CH2SiMe3)2(THF)]+ (M =
Sc, 2a; Y, 2b) by reaction with [PhMe2NH][BAr4] (Ar = Ph, C6F5).
This was seen by NMR spectroscopy when performing these
reactions in THF-d8, and the BPh4-salt of the Sc cation 2a was
isolated in 75% yield from THF–cyclohexane.§ The 13C NMR
resonances of the M–CH2 groups in 2 relative to those in 1 show
1
the typical downfield shift and (for Y) increase in JYC
associated with conversion to the cationic species (for 2b:
d 42.3 ppm, JYC = 41 Hz).
Ethene polymerisation experiments with 1a and 1b activated by
[PhMe2NH][B(C6F5)4] were performed in toluene, and the results
are listed in Table 1. For both metals active polymerisation
catalysts are obtained. This shows that the 6-amino-6-methyl-1,4-
diazepine group is suitable as an ancillary ligand moiety for
cationic rare-earth metal alkyl catalysts. Remarkably, the activity
of the Sc system increases substantially when the temperature is
increased from 50 uC to 70 uC, but this is accompanied by a strong
broadening of the polymer molecular weight distribution. This
might be due to the transformation of the initially formed cation
into another species that is also active, and of which the nature is
presently unclear.
Scheme 2 Synthesis of ligand L2 and L3H and complex 3 and 4.
substituent. The THF molecule is located in a trans position
relative to the amide nitrogen. The Y–N(amide) distance of
˚
2.215(3) A is substantially shorter than the Y–N distances to the
remaining ligand amine nitrogens. Low-temperature solution
NMR studies on 3 show a fully asymmetric structure, with two
resonances (d 2.44, 2.12 ppm) for the diastereotopic methylene
protons of the alkyl group transferred to the ligand, and four
Two new ligand derivatives were prepared by the acid-catalysed
condensation of the 1,4-dimethylated 6-amino-6-methyl-1,4-diaze-
pine with benzaldehyde and with o-hydroxybenzaldehyde. This
produced the 6-imino-6-methyl-1,4-diazepines L2 and L3H
(Scheme 2).
Reaction of L2 with the yttrium trialkyl Y(CH2SiMe3)3(THF)2
is rapid and quantitative (NMR), and resulted in a product in
which the imine carbon atom of the ligand has been alkylated to
give the tridentate monoanionic ligand {(Me3SiCH2)
PhC(H)NCMe[(CH2NMeCH2)2]}2. The coordination site on the
metal that is vacated by the alkyl group that has migrated to the
ligand is filled by one molecule of THF. The structure of this
complex (3) was established by single-crystal X-ray diffraction
(Fig. 2).{ The compound contains a monoanionic fac-tridentate
ligand in which the nitrogen on the 6-position of the 1,4-diazepine
skeleton is an amide with a phenyl(trimethylsilylmethyl)methyl
Table 1 Catalytic ethene polymerization with [(L1)M(CH2SiMe3)3]
(M = Sc 1a, Y 1b) activated with [PhMe2NH][B(C6F5)4]a
Productivity/
102 kg(PE)
PE
Trialkyl T/uC yield/g mol(M)21 h21 bar21 1025Mw Mw/Mn
Fig. 2 Molecular structure of one of the independent molecules of 3
(hydrogen atoms omitted for clarity, thermal ellipsoids drawn at 50%
˚
1a
1a
1b
1b
a
50
70
50
70
4.43
9.66
3.24
4.57
5.32
11.60
3.89
9.32
2.35
4.95
1.43
3.0
9.4
3.1
2.6
probability level). Selected bond distances (A) and angles (u): Y1–N11
2.695(3), Y1–N12 2.589(3), Y1–N13 2.215(3), Y1–C120 2.469(3), Y1–
C124 2.446(4), Y1–O1 2.487(3); N11–Y1–N12 62.94(10), N11–Y1–N13
74.85(10), N12–Y1–N13 71.39(11), C120–Y1–C124 110.10(14), O1–Y1–
C120 87.37(11), O1–Y1–C124 85.74(13), N11–Y1–C120 157.45(10), N12–
Y1–C124 151.54(14), O1–Y1–N13 152.12(10).
5.48
Conditions: 1 l steel autoclave (stirring rate 600 rpm), 250 ml
toluene, 10 mmol trialkyl, 10 mmol [PhMe2NH][B(C6F5)4], 5 bar
ethene, 10 min run time.
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Chem. Commun., 2006, 3320–3322 | 3321

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observability. CCDC 606108 and 606109. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/b606384e
resonances (d 0.18, 20.50, 20.74, 20.88 ppm) for the diaster-
eotopic YCH2Si methylene protons. Intermolecular alkylation of
ligand imino functionalities by metal alkyl species has been
observed previously for early transition metals.5
§ Synthesis of 2a. THF (2 ml) was added to a mixture of 73.8 mg (150 mmol)
of (L1)Sc(CH2SiMe3)3 and 66.2 mg (150 mmol) of [PhMe2NH][B(C6H5)4].
The solution was homogenised by agitation and allowed to stand for about
20 min. The clear, slightly yellow solution was carefully layered with
cyclohexane (3 ml). After standing overnight a white solid had precipitated.
The solid was washed with pentane and dried under vacuum to give 90.4 mg
(114 mmol, 76%) of pure 2a as an off-white powder. 1H NMR (400 MHz,
Reaction of L3H with Y(CH2SiMe3)3(THF)2 resulted in a
product (4, Scheme 2) in which the phenolic –OH group of the
ligand has been deprotonated, and where the imino ligand moiety
remains intact (as evidenced by the 1H and 13C NMR resonances
at d 7.64 ppm and d 161.1 ppm for the aldimine –CHLN group
and the nCLN IR band at 1622 cm21)." Although suitable crystals
of 4 for a single-crystal structure determination have not yet been
obtained, the solution 1H and 13C spectra indicate a Cs symmetric
structure with a tetradentate iminophenolatediazepine ligand and
two alkyl groups attached to the metal centre. The yttrium is again
6-coordinate, as no additional THF is bound. The NMR
resonances for the YCH2Si groups are found at d 20.51 and
THF-d8, 298 K, d): 7.33 (br, 4 6 2H, 2-PhB), 6.92 (t, 4 6 2H, JHH
=
7.48 Hz, 3-PhB), 6.78 (t, 4 6 1H, JHH = 7.10 Hz, 4-PhB), 3.04 (d, 2H,
JHH = 14.0 Hz, CCHH), 2.95 (m, 2H, NCH2), 2.42 (s, 6H, NCH3), 2.34 (m,
2H, NCH2), 2.30 (s, 6H, N(CH3)2), 2.21 (d, 2H, JHH = 14.0 Hz, CCHH),
0.64 (s, 3H, CCH3), 20.01 (s, 18H, Si(CH3)3), 20.26 (s, 6H, ScCH2). 13C
NMR (100.6 MHz, THF-d8, d): 166.4 (q, JBC = 46.5 Hz, 1-Ph), 138.4 (d,
JCH = 153.1 Hz, 2-Ph), 127.1 (d, JCH = 152.9 Hz, 3-Ph), 123.3 (d, JCH
=
155.6 Hz, 4-Ph), 69.7 (t, JCH = 142.7 Hz, CCH2), 63.5 (s, MeC), 60.6 (t,
JCH = 142.7 Hz, CH2CH2), 52.1 (q, JCH = 136.3 Hz, NMe2), 48.4 (br,
ScCH2), 43.2 (q, JCH = 137.6 Hz, NMe), 12.5 (q, JCH = 128.9 Hz, CMe),
4.8 (q, JCH = 119.3 Hz, SiMe3). Anal. calcd for C46H73BN3OSi2Sc: C,
69.41%; H, 9.24%; N, 5.28%. Found: C, 68.60%; H, 9.09%; N, 5.24%.
" Synthesis of 4. (Me3SiCH2)3Y(THF)2 (0.495 g, 1.0 mmol) was dissolved
in 20 ml of cold toluene (230 uC). A solution of L3H (0.261 g, 1.0 mmol) in
6 ml of cold toluene (230 uC) was added dropwise while stirring. The
mixture was allowed to warm to room temperature and stirred for 30 min.
The volatiles were removed under reduced pressure and the residue was
washed with pentane yielding pure 4 (0.41 g, 0.78 mmol, 78%) as a slightly
yellow solid. 1H NMR (500 MHz, C6D6, d): 7.64 (s, 1H, NLCH), 7.29 (t,
1H, JHH = 7.26 Hz, Ph), 7.23 (d, 1H, JHH = 7.75 Hz, Ph), 7.12 (d, 1H,
JHH = 7.75 Hz, Ph), 6.67 (t, 1H, JHH = 7.26 Hz, Ph), 2.87 (m, 2H,
NCH2CH2), 2.02 (s, 6H, NMe), 1.78 (d, 2H, JHH = 13.3 Hz), 1.55 (m, 2H,
NCH2CH2), 1.53 (d, 2H, JHH = 13.3 Hz), 0.42 (s, 18H, SiMe3), 0.33 (s, 3H,
CMe), 20.51 (dd, 2H, JYH = 2.9 Hz, JHH = 11.3 Hz), 20.55 (dd, 2H,
JYH = 2.9 Hz, JHH = 11.3 Hz). 13C{1H} NMR (125.7 MHz, C6D6, d):
166.6 (O–Ph), 161.1 (NLCH), 135.5 (Ph), 134.7 (Ph), 122.6 (Ph), 122.6 (Ph),
115.7 (Ph), 71.5 (CCH2), 62.1 (CMe), 57.2 (NCH2CH2), 49.4 (NMe2), 30.3
(d, YCH2, JYC = 38.2 Hz), 17.1 (CMe), 4.7 (SiMe3). Anal. calcd for
C23H44N3OSi2Y: C, 52.75%; H, 8.47%; N, 8.02%. Found: C, 52.80%; H,
8.43%; N, 8.03%.
20.55 ppm (1H; 2JHH = 11.3 Hz, 1JYH = 2.9 Hz) and d 30.0 ppm
1
(13C; JYC
= 38 Hz). The compound is related to the
triazacyclononanephenolate complexes of scandium reported by
Mountford et al.4
In conclusion, the 6-amino-6-methyl-1,4-diazepine ligand frame-
work proves to be a highly versatile and readily accessible ligand
moiety for the synthesis of a range of neutral and monoanionic
ancillary ligands that can be used in organo rare-earth metal
chemistry. We also expect these ligands to be useful for the early
transition metals. The synthesis of derivatives with larger rare-
earth metals (especially La) and the study of the reactive and
catalytic properties of these compounds and their cationic
derivatives is in progress.
This investigation was financially supported by the Chemical
Sciences division of the Netherlands Organisation for Scientific
Research (NWO-CW). The authors thank A. Jekel for polymer
GPC analyses.
1 For an overview, see: W. E. Piers and D. J. H. Emslie, Coord. Chem.
Rev., 2002, 233, 131; S. Arndt and J. Okuda, Adv. Synth. Catal., 2005,
347, 339.
2 (a) S. Hajela, W. P. Schaefer and J. E. Bercaw, J. Organomet. Chem.,
1997, 532, 45; (b) S. Bambirra, D. van Leusen, A. Meetsma, B. Hessen
and J. H. Teuben, Chem. Commun., 2001, 637; (c) S. C. Lawrence,
B. D. Ward, S. R. Dubberley, C. M. Kozak and P. Mountford, Chem.
Commun., 2003, 2880; (d) B. D. Ward, S. Bellemin-Laponnaz and
L. H. Gade, Angew. Chem., Int. Ed., 2005, 44, 1668.
3 (a) R. A. Peralta, A. Neves, A. J. Bortoluzzi, A. Casellato, A. dos Anjos,
A. Greatti, F. R. Xavier and B. Szpoganicz, Inorg. Chem., 2005, 44, 7690;
(b) A. C. M. Appel, R. Hage, S. W. Russell and D. Tetard, WO 01/85717
A1.
4 C. S. Tredget, S. C. Lawrence, B. D. Ward, R. G. Howe, A. R. Cowley
and P. Mountford, Organometallics, 2005, 24, 3136.
Notes and references
{ Crystallographic data 1a: C22H56N3Si3Sc, M = 491.92, monoclinic,
˚
space group P21/c, a = 18.418(1), b = 17.990(1), c = 18.694(1) A, b =
95.799(1)u, V = 6162.4(6) A , Z = 8, Dx = 1.060 g cm23, F(000) = 2176, m =
3
˚
3.68 cm21, l(MoKa) = 0.71073 A, T = 100(1) K, 41674 reflections
˚
measured, GooF = 0.964, wR(F2) = 0.1084 for 12024 unique reflections
and 971 parameters and R(F) = 0.0544 for 7135 reflections obeying the
F0 ¢ 4.0s(F0) criterion of observability; 3: 2(C31H64N3OSi3Y)?0.5(C7H8),
¯
M = 1382.13, triclinic, space group P1, a = 10.653(1), b = 17.454(2), c =
3
˚
˚
21.730(2) A, a = 80.864(2)u, b = 87.771(2)u, c = 85.449(2)u, V = 3975.2(7) A ,
Z = 2, Dx = 1.155 g cm23, F(000) = 1490, m = 15.84 cm21, l(MoKa) =
˚
0.71073 A, T = 100(1) K, 30900 reflections measured, GooF = 0.996,
wR(F2) = 0.1154 for 15266 unique reflections and 729 parameters and
5 H. Tsurugi, Y. Matsuo, T. Yamagata and K. Mashima, Organometallics,
2004, 23, 2797.
R(F) = 0.0534 for 10248 reflections obeying the F0 ¢ 4.0s(F0) criterion of
3322 | Chem. Commun., 2006, 3320–3322
This journal is ß The Royal Society of Chemistry 2006