Synthesis and structural characterisation of novel linked bis(β-diketiminato) rare earth metal complexes

Article abstract of DOI:10.1039/b501930c

Rare earth metal complexes based on novel linked bis(β-diketiminato) ligands have been prepared via amine elimination and their structural characterisation revealed that the linker unit has significant influence on the geometry and coordination mode of th

Full text of DOI:10.1039/b501930c

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C O M M U N I C A T I O N
Synthesis and structural characterisation of novel linked
bis(b-diketiminato) rare earth metal complexes†
Daniela V. Vitanova, Frank Hampel and Kai C. Hultzsch*
Institut fu¨r Organische Chemie, Friedrich-Alexander Universita¨t Erlangen-Nu¨rnberg, Henkestr.
42, D-91054, Erlangen, Germany. E-mail: hultzsch@chemie.uni-erlangen.de;
Fax: +49-9131-8526865; Tel: +49-9131-8526866
Received 7th February 2005, Accepted 30th March 2005
First published as an Advance Article on the web 6th April 2005
Rare earth metal complexes based on novel linked bis(b-
diketiminato) ligands have been prepared via amine elim-
ination and their structural characterisation revealed that
the linker unit has significant influence on the geometry
and coordination mode of the ancillary ligand.
The chemistry of rare earth metal complexes has been stimulated
over the last two decades due to their high catalytic activity for
various organic transformations1 and polymer synthesis.2 Non-
cyclopentadienyl ligand sets have attracted considerable interest
as they are easy to modify with respect to steric demand and
electronic properties.3 One promising ancillary ligand set is the
family of b-diketiminato ligands,4 which has been successfully
applied in transition metal catalyst systems.5,6
Catalyst systems of the three-valent rare earth metals com-
monly comprise of two monoanionic ancillary ligands control-
ling the reactivity as well as one reactive amido or alkyl group.
Analogous to the ansa-metallocene ligand set, we envisioned
to accommodate two linked b-diketiminato ligands in the co-
ordination sphere of the metal. Linking the two b-diketiminato
moieties should increase complex stability, with the additional
benefit of an anchor for potential stereoselective control by
Scheme 2 Reagents and conditions: (i) 1 equiv. H2L1 (Ln = La: 60 ◦C,
utilising chiral linkers.9
30 min; Ln = Y: 70 ◦C, 24 h) or H2L2 (Ln = La: 75 ◦C, 4.5 d; Ln = Y:
95 ◦C, 5 d); (ii) Ln = La, 1 equiv. H2L3 (65 ◦C, 24 h) or H2L4 (95 ◦C, 4
Ethylene- and cyclohexyl-linked bis(b-diketiminato) ligands
with different sterically demanding aromatic substituents were
prepared in a two-step standard condensation route (Scheme 1).
d).
X-Ray crystallographic analyses of 2a and 4 confirmed their
monomeric structures (Figs. 1 and 2).‡ In both complexes lan-
thanum is coordinated in a distorted square pyramidal fashion,
in which the amido ligand occupies the apical position and the
two linked b-diketiminato moieties form the basis. However,
the overall geometry of the linked bis(b-diketiminato) ligand
differs significantly when exchanging the ethylene linker unit in
2a for the trans-cyclohexyl unit in 4. Whereas the two aromatic
substituents have a cisoid arrangement in 2a, pointing away
from the apical amido group, they are oriented in a transoid
fashion in 4 with one aromatic substituent pointing away from
the amido group and the other aromatic substituent pointing
towards the amido group. This significant change in ligand
geometry is inflicted by a slight decrease of the dihedral angle
of the linker unit (N2–C–C–N3) from 49.3(4)◦ in 2a (−53.1(5)◦
for the second independent molecule) to 42.25(19) in 4.
Scheme 1 Reagents and conditions: (i) 150 ◦C, 24 h; (ii) [Et3O]+[BF4]−;
In complex 2a the two b-diketiminato moieties display dif-
ferent bonding modes. The b-diketiminato fragment consisting
(iii) Et3N, diamine, CH2Cl2, 25 ◦C.
2
of N1, N2 and C11–C13 is bonded in a g fashion with the
The new ligands H2L1–H2L4 reacted cleanly with trisamido
complexes [Ln{N(SiMe3)2}3] (Ln = La, Y) to form the desired
linked bis(b-diketiminato) complexes 1–4 (Scheme 2).7 Whereas
reactions of the ethylene-linked ligands H2L1 and H2L2 pro-
ceeded readily at 60–75 ◦C, the more rigid cyclohexyl-linked
ligands H2L3 and H2L4 required higher temperatures.8
˚
lanthanum atom being situated only 1.235 A out of the N2C3
˚
plane and rather long La · · · C distances (3.45–3.73 A). The other
5
b-diketiminato moiety is tilted significantly towards a g bonding
mode. The metal is more displaced from the N2C3 plane (1.883A)
˚
˚
and La · · · C contacts are shorter (3.01–3.18 A). While both
2
La–N bonds in the g bound b-diketiminato moiety are almost
˚
˚
identical (2.536(3) and 2.557(3) A), they vary by almost 0.17 A
for the tilted b-diketiminato ligand (2.559(3) vs. 2.394(3) A).
˚
† Electronic supplementary information (ESI) available: Experimen-
tal procedures and characterising data for all new complexes. See
http://www.rsc.org/suppdata/dt/b5/b501930c/
Contrary to the findings for complex 2a, both b-diketiminato
5
moieties in complex 4 are bound in the g bonding mode with
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 1 5 6 5 – 1 5 6 6
1 5 6 5
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other linker units in order to better understand the factors
governing the binding mode of the b-diketiminato ligands in
these systems.
Generous financial support by the Deutsche Forschungsge-
meinschaft (DFG SPP 1166) and the Fonds der Chemischen
Industrie is gratefully acknowledged. K. C. H. is a DFG Emmy
Noether fellow (2001–2005) and thanks Prof. John A. Gladysz
for his continuous support.
Notes and references
‡ Crystallographic data for 2a: C45H77LaN5Si2, Mr = 883.21, crystal
¯
size 0.30 × 0.30 × 0.30 mm, triclinic, space group P1, a = 11.816(2),
˚
b = 19.◦038(4), c = 22.377(5) A, a = 92.67(3), b = 102.60(3), c =
3
99.47(3) , V = 4827.7(17) A , Z = 4, Dc = 1.215 g cm−3, F(000) = 1868,
˚
−1
˚
Mo-Ka radiation (k = 0.71073 A), T = 173(2) K, l = 0.968 mm
,
The 41058 reflections measured on a Nonius Kappa CCD system
yielded 22127 independent reflections (Rint = 0.0335), R1 (I > 2r(I)) =
0.0383, wR2 (all data) = 0.1174. The asymmetric unit contains two
independent molecules as well as one molecule of isohexane, which
showed strong non-resolvable disorder of all atoms. The isohexane
was refined by using distance restraints; no hydrogen atoms have been
added to the refinement model. CCDC reference number 262110.
Crystallographic data for 4: C46H76LaN5Si2, Mr = 894.21, crystal size
Fig.
1 Molecular structure of one of the two independent
˚
molecules of 2a. Selected bond lengths (A) and bond angles
(◦): La1–N1 2.557(3), La1–N2 2.536(3), La1–N3 2.394(3), La1–N4
2.559(3), La1–N10 2.425(3), La1–C16 3.014(3); N1–La1–N2 72.33(9),
N1–La1–N4 119.95(9), N3–La1–N4 72.06(9), N2–La1–N3 65.47(9),
N4–La1–N10 123.36(9), N3–La1–N10 98.16(10), N2–La1–N10
112.22(10), N1–La1–N10 105.01(10), N2–C14–C15–N3 49.3(4); N2C3
plane–La1 1.235 (ring 1) and 1.883 (ring 2).
¯
0.20 × 0.20 × 0.20 mm, triclinic, space group P1, a = 11.6019(1),
˚
b = 12.3156(1◦), c = 18.3663(2) A, a = 89.618(1), b = 79.329(1),
c = 69.595(1) , V = 2412.28(4) A , Z = 2, Dc = 1.231 g cm−3
,
3
˚
˚
F(000) = 944, Mo-Ka radiation (k = 0.71073 A), T = 173(2) K,
l = 0.970 mm−1, The 21572 reflections measured on a Nonius Kappa
CCD system yielded 11064 independent reflections (Rint = 0.0159),
R1 (I > 2r(I)) = 0.0243, wR2 (all data) = 0.0669. CCDC reference
number 265749. See http://www.rsc.org/suppdata/dt/b5/b501930c/
for crystallographic data in CIF or other electronic format.
1 (a) G. A. Molander and E. D. Dowdy, Top. Organomet. Chem., 1999,
2, 119; (b) H. C. Aspinall, Chem. Rev., 2002, 102, 1807; (c) G. A.
Molander and J. A. C. Romero, Chem. Rev., 2002, 102, 2161; (d) S.
Hong and T. J. Marks, Acc. Chem. Res., 2004, 37, 673; (e) K. C.
Hultzsch, Adv. Synth. Catal., 2005, 347, 367.
2 H. Yasuda, Top. Organomet. Chem., 1999, 2, 255.
3 (a) W. E. Piers and D. J. H. Emslie, Coord. Chem. Rev., 2002, 233–234,
131; (b) F. T. Edelmann, D. M. M. Freckmann and H. Schumann,
Chem. Rev., 2002, 102, 1851.
4 L. Bourget-Merle, M. F. Lappert and J. R. Severn, Chem. Rev., 2002,
102, 3031.
5 (a) B. J. O’Keefe, M. A. Hillmyer and W. B. Tolman, J. Chem. Soc.,
Dalton Trans., 2001, 2215; (b) G. W. Coates and D. R. Moore, Angew.
Chem., Int. Ed, 2004, 43, 6618.
6 For b-diketiminato rare earth metal polymerisation catalysts, see:
(a) Y. Yao, Y. Zhang, Q. Shen and K. Yu, Organometallics, 2002, 21,
819; (b) P. G. Hayes, W. E. Piers and R. McDonald, J. Am. Chem.
Soc., 2002, 124, 2132; (c) Y. Yao, Y. Zhang, Z. Zhang, Q. Shen and
K. Yu, Organometallics, 2003, 22, 2876.
7 b-Diketiminato complexes are commonly prepared via salt metathe-
sis, see: (a) P. B. Hitchcock, M. F. Lappert and S. Tian, J. Chem.
Soc., Dalton Trans., 1997, 1945; (b) P. G. Hayes, W. E. Piers, L. W. M.
Lee, L. K. Knight, M. Parvez, M. R. J. Elsegood and W. Clegg,
Organometallics, 2001, 20, 2533; (c) G. B. Nikiforov, H. W. Roesky,
T. Labahn, D. Vidovic and D. Neculai, Eur. J. Inorg. Chem., 2003,
433; (d) A. G. Avent, P. B. Hitchcock, A. V. Khostov, M. F. Lappert
and A. V. Protchenko, Dalton Trans., 2003, 1070; see also ref. 6.
8 The complexes were generally prepared in toluene solution, except
for complex 1a, where the reaction temperature and solubility of the
ethylene-linked ligand permitted the use of hexanes.
9 For related linked aminotroponiminate complexes of the rare earth
metals, see: M. R. Bu¨rgstein and P. W. Roesky, Organometallics, 2003,
22, 1372, and references cited therein.
10 Preliminary experiments indicate that complex 3 copolymerises
cyclohexene oxide and CO2 with low activity (60 ◦C, 41 atm
CO2 TOF = 5 h−1, 87% carbonate linkages, overall TON = 90,
Mn = 3940, Mw/Mn = 7.3). We are indebted to Prof. J. Okuda
and Dr. K. Beckerle (RWTH Aachen) for obtaining the GPC
data.
˚
Fig. 2 Molecular structure of 4. Selected bond lengths (A) and bond
angles (◦): La1–N1 2.4977(15), La1–N2 2.4340(14), La1–N3 2.5063(15),
La1–N4 2.5939(15), La1–N10 2.4543(15), La1–C11 3.0554(18),
La1–C12 3.0469(18), La1–C13 2.9690(17), La1–C16 3.036(2);
N1–La1–N2 72.15(5), N1–La1–N4 132.85(5), N3–La1–N4 73.45(5),
N2–La1–N3 63.72(5), N4–La1–N10 97.34(5), N3–La1–N10 133.96(5),
N2–La1–N10 100.05(5), N1–La1–N10 118.20(5), N2–C21–C26–N3
42.25(19); N2C3 plane–La1 1.941 (ring 1) and 1.940 (ring 2).
˚
˚
the lanthanum atom being displaced by 1.94 A out of the N2C3
plane and short La · · · C contacts (2.97–3.06 A for ring 1, 3.04–
˚
3.19 A for ring 2). The La–N bonds within the b-diketiminato
˚
moiety vary less dramatically by only 0.06–0.09 A (2.4977(15)
˚
˚
vs. 2.4340(14) A and 2.5939(15) vs. 2.5063(15) A).
We are currently investigating the catalytic properties
of these novel linked bis(b-diketiminato) complexes for
hydroamination1d,e and copolymerisation of epoxides and
CO2.5b,10 The drastic difference in ligand geometry of ethylene-
linked vs. cyclohexyl-linked ligands should be reflected in their
reactivity and stereoselectivity. Furthermore, we are exploring
1 5 6 6
D a l t o n T r a n s . , 2 0 0 5 , 1 5 6 5 – 1 5 6 6