Syntheses and structures of mononuclear lutetium imido complexes with very short Lu-N bonds

Article abstract of DOI:10.1039/b711669a

The reaction of 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-imine (Im ^DippNH) with trimethylsilylmethyllithium and anhydrous lutetium trichloride affords the imido complex [LuCl₂(ImN^Dipp)(THF) ₃] (1), which, on f

Full text of DOI:10.1039/b711669a

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COMMUNICATION
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Syntheses and structures of mononuclear lutetium imido complexes with
very short Lu–N bonds{
Tarun K. Panda, So¨ren Randoll, Cristian G. Hrib, Peter G. Jones, Thomas Bannenberg and Matthias Tamm*
Received (in Cambridge, UK) 1st August 2007, Accepted 13th September 2007
First published as an Advance Article on the web 27th September 2007
DOI: 10.1039/b711669a
donating capacity towards early transition metals.9,10 Due to their
The reaction of 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-
imine (ImDippNH) with trimethylsilylmethyllithium and
ability to act as 2s,4p-electron donors, these ligands can be
regarded as monodentate analogues of cyclopentadienyls, C5R5,
and also as monoanionic imido ligands in a similar fashion to that
described for related phosphoraneiminato ligands.11 Therefore,
lanthanide complexes with terminal imidazolin-2-iminato ligands,
as presented in this communication, can serve as accurate models
for elusive mononuclear lanthanide imido complexes, and their
structural investigation could lead to a better understanding of
lanthanide–nitrogen multiple bonding.5,12
anhydrous lutetium trichloride affords the imido complex
[LuCl2(ImNDipp)(THF)3] (1), which, on further reaction with
dipotassium cyclooctatetraenide, K2(C8H8), leads to the half-
sandwich cyclooctatetraenyl complex [(g8-C8H8)Lu(ImNDipp)-
(THF)2]; both complexes contain very short Lu–N bond
lengths, which are shorter than any previously reported Lu–N
distances.
The starting material 1,3-bis(2,6-diisopropylphenyl)imidazolin-
2-imine (ImDippNH) was synthesized, according to the published
procedure, from the corresponding Arduengo-carbene 1,3-bis(2,6-
diisopropylphenyl)imidazolin-2-ylidene13 by reaction with tri-
methylsilyl azide, followed by desilylation of the ImDippN–SiMe3
intermediate in methanol.8 Reaction of the resulting imine
ImDippNH with trimethylsilylmethyllithium and anhydrous lute-
tium trichloride in THF solution affords the lutetium complex
[LuCl2(NImDipp)(THF)3] (1) as a colourless crystalline solid in
good yield after extraction with pentane (Scheme 2). Treatment of
1 with a THF solution of the dipotassium cyclooctatetraenide salt
K2(C8H8) leads to the formation of the cyclooctatetraenyl lutetium
complex [(g8-C8H8)Lu(NImDipp)(THF)2] (2). Both compounds 1
and 2 could be fully characterized by 1H and 13C NMR
spectroscopy, elemental analysis and X-ray crystal structure
determination.{ It should be noted that coordination to the
lutetium atom does not have a significant impact on the resonances
observed for the hydrogen and carbon atoms of the heterocyclic
Organoimido complexes of the transition metals have been
extensively studied in the past because of their important role in
a number of biological, industrial and catalytic processes.1 On the
one hand, this interest is stimulated by the ability of the metal–
imido group (MLNR) to undergo a wide range of metathesis,
cycloaddition, C–H bond activation and hydroamination reac-
tions.2 On the other hand, imido ligands are also widely used as
ancillary ligands in organotransition metal chemistry, e.g. in
important catalysts for olefin metathesis and polymerization.3,4 In
stark contrast to the rich chemistry of imido complexes containing
d-block elements, lanthanide imido complexes are largely unex-
plored,5 and reports on well-defined imido complexes containing
4f-elements are scarce.6 More specifically, structurally character-
ized lanthanide complexes containing terminal imido groups are
unknown to date, since the imido group is generally found to bind
in a capping or bridging fashion.
As a terminal ligand, the formally dianionic imido ligand
(NR)22 coordinates with a metal–nitrogen multiple bond consist-
ing of one s and either one or two p interactions.7 This resembles
the bonding in transition metal complexes containing monoanionic
imidazolin-2-iminato ligands of type I, which can be described by
the two limiting resonance structures IA and IB (Scheme 1),
indicating that the ability of the imidazolium ring to stabilize a
positive charge leads to highly basic ligands8 with a strong electron
1
imidazoline moiety.8 The H NMR spectra of both complexes at
room temperature exhibit two doublet resonances as expected for
one set of diastereotopic isopropyl CH3 groups, indicating the
Scheme 1
Institut fu¨r Anorganische und Analytische Chemie, Technische
Universita¨t Carolo-Wilhelmina, Hagenring 30, D-38106, Braunschweig,
Germany. E-mail: matthias.tamm@tu-bs.de; Fax: +49 (531) 391-5387;
Tel: +49 (531) 391-5309
{ Electronic supplementary information (ESI) available: Details of the
electronic structure calculation. See DOI: 10.1039/b711669a
Scheme 2
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Chem. Commun., 2007, 5007–5009 | 5007

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presence of magnetically equivalent 2,6-diisopropylphenyl groups.
The complexes 1 and 2 crystallize as THF solvates, and their
molecular structures are shown in Fig. 1 and Fig. 2.
of 174.2(2)u.14 The previous shortest Lu–N distances of 2.145(2)
and 2.149(4) s were found in amido and benzamidinate
complexes, respectively.15,16
The Lu–N1 bond length of 2.122(2) s in 2 also falls below these
values, although it is slightly longer than in 1. This is accompanied
by a greater deviation from linearity at N1, and the Lu–N1–C1
angle is 166.7(2)u. The cyclooctatetraenyl ligand is essentially
g8-bound, but in a slightly distorted fashion with the Lu–C and
C–C distances ranging from 2.544(3) to 2.643(3) s and 1.395(5) to
1.422(5) s, respectively. The coordination sphere at lutetium is
completed by two THF ligands to give a three-legged piano-stool
geometry. The imidazole ring subtends an angle of 55.3u with the
plane containing the COT centroid, Lu and N1, indicating an
intermediate position between a horizontal (90u) and perpendicular
(0u) orientation.
In compound 1, the metal is hexacoordinated in a meridional
fashion with the chlorine atoms and two of the three THF ligands
adopting a trans-arrangement. The Cl1–Lu–Cl2 and O2–Lu–O3
angles of 161.66(3)u and 157.54(8)u deviate significantly from 180u,
whereas the N1–Lu–O1 axis is almost linear [177.86(9)u]. Thus, the
coordination geometry about lutetium can be described as a
distorted octahedron. Since the Lu–O1 bond is considerably longer
than the other two Lu–O bonds [2.459(2) s vs. 2.303(2) and
2.287(2) s], this THF ligand can also be regarded as being trans-
coordinated to a square-pyramid with the imido N1 atom at the
apex. Presumably, the elongation of the Lu–O1 bond is a result of
the strong N1–Lu interaction, revealed by a very short Lu–N bond
length of 2.091(3) s, the shortest ever observed for lutetium–
nitrogen systems, together with an almost linear Lu–N1–C1 angle
To elucidate the nature of the metal–nitrogen bond in
imidazolin-2-iminato lutetium complexes and to address the
question of lanthanide–nitrogen multiple bonding,3,12 we per-
formed a DFT (density functional theory) calculation on the
model complex [(g8-C8H8)Lu(NImMe)] (3) (ImMe
N = 1,3-
dimethylimidazolin-2-imide), assuming C2v symmetry. This
pseudo-two coordinate system was chosen since it allows an
estimate of the full potential of this interaction without lowering of
the symmetry by THF coordination. All computations were
performed using the hybrid density functional method B3PW91
implemented in the Gaussian03 program.17 For all main-group
elements (C, H, N) the all-electron triple-f basis set (6-311G**) was
used, whereas for the Lu atom the Stuttgart relativistic, small core
ANO/ECP basis set was employed.18 The Lu–N bond in the
model complex 3 is 2.040 s, indicating an increase in the bond
strength in comparison with the THF complex 2.
An analysis of the canonical molecular orbitals in 3 reveals
several low lying orbitals, which contribute to Lu–N s-bonding.
Fig. 3 shows the isosurfaces for the four highest occupied
molecular orbitals. The HOMO and HOMO-3 point towards
the presence of two additional p-bonds between the imido ligand
and the lutetium atom, whereas the degenerate set of HOMO-1
Fig. 1 ORTEP diagram of 1 in 1?THF with thermal displacement
parameters drawn at 50% probability. Selected bond lengths [s] and
angles [u]: Lu–N1 2.091(3), Lu–O1 2.459(2), Lu–O2 2.303(2), Lu–O3
2.287(2), Lu–Cl1 2.5589(8), Lu–Cl2 2.5668(9), N1–C1 1.251(4); Lu–N1–
C1 174.2(2), N1–Lu–O1 177.86(9), Cl1–Lu–Cl2 161.66(3), O2–Lu–O3
157.54(8), N2–C1–N3 101.5(2).
Fig. 2 ORTEP diagram of 2 in 2?THF with thermal displacement
parameters drawn at 50% probability. Selected bond lengths [s] and
angles [u]: Lu–N1 2.122(2), Lu–O1 2.380(2), Lu–O2 2.380(2), Lu–C(COT)
2.544(3)–2.643(3); Lu–N1–C1 166.7(2), N1–Lu–O1 89.76(8), N1–Lu–O2
84.90(8), O1–Lu–O2 74.47(8), N2–C1–N3 101.5(2).
Fig. 3 Isosurfaces for the four highest occupied molecular orbitals of the
model complex [(g8-C8H8)Lu(NImMe)] (3) (ImMeN = 1,3-dimethylimida-
zolin-2-imide).
5008 | Chem. Commun., 2007, 5007–5009
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Eur. J. Inorg. Chem., 2003, 583; A. Antin˜olo, M. Fajardo, A. Otero and
S. Prashar, Eur. J. Inorg. Chem., 2003, 17; J. Sundermeyer, Z. Anorg.
Allg. Chem., 2003, 629, 744; K. Mun˜iz, Chem. Soc. Rev., 2004, 33, 166;
U. Radius, Z. Anorg. Allg. Chem., 2004, 630, 957; N. Hazari and
P. Mountford, Acc. Chem. Res., 2005, 38, 839; D. N. Zarubin and
N. A. Ustynyuk, Russ. Chem. Rev. (Engl. Transl.), 2006, 75, 671.
3 R. R. Schrock and C. Czekelius, Adv. Synth. Catal., 2007, 349, 55;
R. R. Schrock and A. H. Hoveyda, Angew. Chem., Int. Ed., 2003, 42,
4592.
4 P. D. Bolton and P. Mountford, Adv. Synth. Catal., 2005, 347, 355;
V. C. Gibson and S. K. Spitzmesser, Chem. Rev., 2003, 103, 283.
5 G. R. Giesbrecht and J. C. Gordon, Dalton Trans., 2004, 2387.
6 A. A. Trifonov, M. N. Bochkarev, H. Schumann and J. Loebel, Angew.
Chem., Int. Ed. Engl., 1991, 30, 1149; W. J. Evans, M. A. Ansari,
J. W. Ziller and S. I. Khan, Inorg. Chem., 1996, 35, 5435; Z. Xie,
S. Wang, Q. Yang and T. C. W. Mak, Organometallics, 1999, 18, 1578;
S. Wang, Q. Yang, T. C. W. Mak and Z. Xie, Organometallics, 1999, 18,
5511; H. S. Chan, H. W. Li and Z. Xie, Chem. Commun., 2002, 652;
J. C. Gordon, G. R. Giesbrecht, D. L. Clark, P. J. Hay, D. W. Keogh,
R. Poli, B. L. Scott and J. G. Watkin, Organometallics, 2002, 21, 4726;
D. J. Beetstra, A. Meetsma, B. Hessen and J. H. Teuben,
Organometallics, 2003, 22, 4372; D. Cui, O. Tardif and Z. Hou, J. Am.
Chem. Soc., 2004, 126, 1312.
and HOMO-2 represent COT–metal d-bonds.
A bonding
analysis19 of the p-orbitals HOMO and HOMO-3 reveals 6 and
11% orbital contribution from the metal center, respectively.
Although this composition is in agreement with the expected
predominantly ionic character of the Lu–N bond, it demonstrates
that the imidazolin-2-iminato ligand can be regarded as a
2s,4p-electron donor and forms a highly polarized triple bond
with the metal atom.20 In that respect, the bonding situation is
similar to that calculated for isostructural ‘‘pogo-stick’’ titanium
imido complexes of the type [(g8-C8H8)Ti(NR)],21 although, as
expected for a 3d-metal, a significantly higher degree of covalency
is found in these systems.
Future work on imidazolin-2-iminato lanthanide complexes
will exploit the reactivity along the highly polarized Ln–N
bond, and preliminary studies reveal that complexes of the type
[(g8-C8H8)Ln(ImN)(THF)x] (Ln = Sc, Y, La, Lu) are highly active
in the catalytic ring-opening polymerization of lactones.22
This work was supported by the Deutsche Forschungs-
gemeinschaft (DFG) through the programme ‘‘Lanthanoid-
spezifische Funktionalita¨ten in Moleku¨l und Material’’ (SPP
1166). We thank Priv.-Doz. Dr. J. Grunenberg for helpful
discussion.
7 T. R. Cundari, Chem. Rev., 2000, 100, 807.
8 M. Tamm, D. Petrovic, S. Randoll, S. Beer, T. Bannenberg, P. G. Jones
and J. Grunenberg, Org. Biomol. Chem., 2007, 5, 523.
9 M. Tamm, S. Randoll, T. Bannenberg and E. Herdtweck, Chem.
Commun., 2004, 876; M. Tamm, S. Beer and E. Herdtweck,
Z. Naturforsch., B: Chem. Sci., 2004, 59b, 1497; M. Tamm,
S. Randoll, E. Herdtweck, N. Kleigrewe, G. Kehr, G. Erker and
B. Rieger, Dalton Trans., 2006, 459.
Notes and references
10 N. Kuhn, M. Go¨hner, M. Grathwohl, J. Wiethoff, G. Frenking and
Y. Chen, Z. Anorg. Allg. Chem., 2003, 629, 793.
{ Preparation of 1 and 2; 1: to a mixture of LuCl3 (140.7 mg, 0.5 mmol)
and LiCH2SiMe3 (47 mg, 0.5 mmol), THF (10 mL) was added. After 2 h,
the imidazolin-2-imine ImDippNH (201.6 mg, 0.5 mmol) was added. The
solvent was removed, and the residue was subsequently extracted with
pentane. Complex 1 was crystallised from THF–pentane. Yield: 300 mg,
69%. [C39H60Cl2LuN3O3]: calcd C 54.16, H 6.99, N 4.85; found C 54.81, H
6.95, N 4.70%. dH (C6D6, 25 uC, 200 MHz): 7.18 (br, 6H, Ph), 5.91 (s, 2H,
NCH), 3.70 (br, 4H, THF), 3.52 (sept., 4H, CHMe), 1.53 (d, 12H, CH3),
1.37 (br, 4H, THF), 1.26 (d, 12H, CH3) ppm; dC (C6D6, 25 uC, 50.3 MHz):
159, 154, 148, 128, 123, 113, 28, 25, 23 ppm. 2: K2C8H8 (0.5 mmol), freshly
prepared from potassium (39 mg) and C8H8 (0.055 mL), was added to a
solution of 1 (432.4 mg, 0.5 mmol) in THF (10 mL), and the reaction
mixture was stirred for 12 h. The solvent was removed, and the residue was
subsequently extracted with pentane. Complex 2 was crystallised from
THF–pentane. Yield: 210 mg, 51%. [C47H68LuN3O3]: calcd C 62.86, H
7.63, N 4.67; found C 62.15, H 7.04, N 4.12%. dH (C6D6, 25 uC, 200 MHz):
7.27–7.20 (m, 4H, m-H), 7.11–7.10 (m, 2H, p-H), 6.28 (s, 8H, C8H8), 5.86
(s, 2H, NCH), 3.25–3.18 (m, 4H, THF), 3.09 (sept., 4H, CHMe), 1.30 (d,
12H, CH3), 1.24–1.19 (m, 4H, THF), 1.16 (d, 12H, CH3) ppm; dC (C6D6,
25 uC, 50.3 MHz): 158, 153, 148, 128, 123, 113, 93, 69, 28, 24, 23 ppm.
Crystal data: 1: C43H68Cl2LuN3O4, M = 936.87, monoclinic, a =
18.1126(10), b = 12.3042(7), c = 19.9567(11) s, b = 96.113(10)u,
V = 4422.3(4) s3, T = 133 K, space group P21/n (no. 14), Z = 4, Dc =
1.407 g cm23, m(Mo-Ka) = 2.395 mm21. 92 021 reflections measured,
13 523 unique (Rint = 0.0768) which were used in all calculations. Final
R1 = 0.0699 and wR2 = 0.0840 (all data). CCDC 656208. 2:
11 K. Dehnicke and A. Greiner, Angew. Chem., Int. Ed., 2003, 42, 1340.
12 D. L. Clark, J. C. Gordon, P. J. Hay and R. Poli, Organometallics, 2005,
24, 5747.
13 A. J. Arduengo, R. Krafczyk, R. Schmutzler, H. A. Craig, J. R. Goerlich,
W. J. Marshall and M. Unverzagt, Tetrahedron, 1999, 55, 14523.
14 The Cambridge Structural Database (CSD version 5.28, November
2006) contains 155 structures with 595 Lu–N distances; the distances
range from 2.146 to 2.775 s with a mean value of 2.421 s.
15 M. Nishiura, Z. Hou, Y. Wakatsuki, T. Yamaki and T. Miyamoto,
J. Am. Chem. Soc., 2003, 125, 1184.
16 D. Cui, M. Nishiura and Z. Hou, Angew. Chem., Int. Ed., 2005, 44, 959.
17 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin,
J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone,
B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson,
H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene,
X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. Ochterski, P. Y. Ayala, K. Morokuma,
G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski,
S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui,
A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu,
A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox,
T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen,
M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN 03
(Revision C.02), Gaussian, Inc., Wallingford, CT, 2004.
C47H68LuN3O3, M = 898.01, orthorhombic, a = 12.2567(10), b =
3
˚
˚
12.5050(10), c = 31.988(3) A, V = 4902.8(7) A , T = 133 K, space group
P212121, Z = 4, Dc = 1.217 g cm23, m(Mo-Ka) = 2.051 mm21. 101 213
reflections measured, 14 992 unique (Rint = 0.0515) which were used in all
calculations. Final R1 = 0.0354 and wR2 = 0.0748 (all data). CCDC
656209. For crystallographic data in CIF or other electronic format, see
DOI: 10.1039/b711669a
18 X. Cao and M. Dolg, J. Chem. Phys., 2001, 115, 7348.
19 (a) N. M. O’Boyle, GaussSum 2.1, 2007; available at http://
gausssum.sf.net; (b) N. M. O’Boyle, A. L. Tenderholt and
K. M. Langner, J. Comput. Chem., 2007, DOI: 10.1002/jcc.20823.
20 It should be emphasized that this assignment does not suggest a ‘‘true’’
covalent triple bond, and the polarity of the Lu–N bond is in line with
the computed atomic charges for lutetium of +1.69 (NBO) and +0.72
(Mulliken). In addition, Lu–N bond orders of 0.74 (Wiberg bond index)
and 0.71 (NAO bond order) have been calculated.
1 (a) D. E. Wigley, Prog. Inorg. Chem., 1994, 42, 239.
2 P. Mountford, Chem. Commun., 1997, 2127; Carlos C. Roma˜o,
F. E. Ku¨hn and W. A. Herrmann, Chem. Rev., 1997, 97, 3197;
A. A. Danopoulos, J. C. Green and M. B. Hursthouse, J. Organomet.
Chem., 1999, 591, 36; P. R. Sharp, J. Chem. Soc., Dalton Trans., 2000,
2647; L. H. Gade and P. Mountford, Coord. Chem. Rev., 2001, 216–217,
65; J. K. Brask and T. Chivers, Angew. Chem., Int. Ed., 2001, 40, 3960;
A. P. Duncan and R. G. Bergman, Chem. Rec., 2002, 2, 431; R. A. Eikey
and M. M. Abu-Omar, Coord. Chem. Rev., 2003, 243, 83; W.-H. Leung,
21 S. C. Dunn, N. Hazari, N. M. Jones, A. G. Moody, A. J. Blake, A.
R. Cowley, J. C. Green and P. Mountford, Chem.–Eur. J., 2005, 11,
2111.
22 T. K. Panda and M. Tamm, unpublished results.
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Chem. Commun., 2007, 5007–5009 | 5009