Imidazolin-2-iminato complexes of rare earth metals with very short metal-nitrogen bonds: Experimental and theoretical studies

Article abstract of DOI:10.1021/ic900503q

The reactions of 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-imine (Im ^DippNH, 1-H) with trimethylsilylmethyl lithium (LiCH ₂SiMe₃) and anhydrous rare earth metal trichlorides MCI₃ afforded the imidazolin-2-imina

Full text of DOI:10.1021/ic900503q

5462 Inorg. Chem. 2009, 48, 5462–5472
DOI: 10.1021/ic900503q
Imidazolin-2-iminato Complexes of Rare Earth Metals with
Very Short Metal-Nitrogen Bonds: Experimental and
Theoretical Studies
.
Tarun K. Panda, Alexandra G. Trambitas, Thomas Bannenberg, Cristian G. Hrib, Soren Randoll, Peter G. Jones,
and Matthias Tamm*
::
::
Institut fur Anorganische und Analytische Chemie, Technische Universitat Carolo-Wilhelmina, Hagenring 30,
38106 Braunschweig, Germany
Received March 13, 2009
The reactions of 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-imine (ImDippNH, 1-H) with trimethylsilylmethyl lithium
(LiCH2SiMe3) and anhydrous rare earth metal trichlorides MCl3 afforded the imidazolin-2-iminato complexes
[(1)MCl2(THF)3] (2a, M = Sc; 2b, M = Y; 2c, M = Lu) and [(1)GdCl2(THF)2] [LiCl(THF)2] (2d). Treatment of com-
3
plexes 2 with dipotassium cyclooctatetradienide, K2(C8H8) resulted in the formation of two- or three-legged piano-
stool complexes of the type [(η8-C8H8)M(1)(THF)n] (3a, M = Sc, n = 1; 3b, M = Y, n = 2; 3c, M = Lu, n = 2; 3d, M = Gd,
n = 2). X-ray diffraction analyses of all eight complexes 2 and 3 revealed the presence of very short metal-nitrogen
bonds, which are among the shortest ever observed for these elements. [(η8-C8H8)Sc(1)(THF)] (3a) reacted with
2,6-dimethylphenyl isothiocyanate (Xy-NCS) to form the [2 + 2]-cycloaddition product 4, which contains a thioureato-N,
N0 moiety. The related COT-titanium complex [(η8-C8H8)TiCl(1)] (6) could be obtained from [(1)TiCl3] (5) by
reaction with K2(C8H8) and was structurally characterized. As a theoretical analysis of the nature of the
metal-nitrogen bond, density functional theory (DFT) calculations have been carried out for complexes 3a
and 6 and also for the model complexes [(η8-C8H8)Sc(NImMe)] (7), [(η8-C8H8)Ti(NImMe)]+ (8), and [(η8-C8H8)Ti
(NXy)] (9), revealing a marked similarity of the bonding in imidazolin-2-iminato and conventional imido metal
complexes.
Introduction
This interest is primarily stimulated by the ability of the
metal-imido group (M=NR) to undergo a wide range of
metathesis, cycloaddition, C-H bond activation, and hydro-
amination reactions.2 In addition, imido ligands are also
widely used as ancillary ligands in organotransition metal
chemistry, for instance in olefin metathesis and polymeriza-
tion catalysts.3,4 In stark contrast to the large number of
imido complexes containing d-block elements, the imido
chemistry of the f-elements is much less developed,5,6 Never-
theless, a considerable number of actinide, and in particular
uranium, complexes containing terminal or bridging imido
Organoimido complexes of the transition metals have been
extensively studied because of their important role in a
number of biological, industrial, and catalytic processes.1
*To whom correspondence should be addressed. E-mail: m.tamm@tu-bs.
de.
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2009 American Chemical Society
pubs.acs.org/IC
Published on Web 04/30/2009

Article
Inorganic Chemistry, Vol. 48, No. 12, 2009 5463
ligands are known,7 whereas reports on well-defined lantha-
nide imido complexes are scarce.8 More specifically, structu-
rally characterized 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.9
The situation is similar in organogroup 3 metal chemistry10
despite several efforts to isolate terminal scandium imido
complexes.11 Only recently, however, evidence has been
presented for the existence of a transient Sc=NR species.12
Coordination of the formally dianionic imido ligand
(NR)2- as a terminal ligand involves a metal-nitrogen
multiple bond consisting of one σ and either one or two π
interactions.13 This resembles the bonding in transition metal
complexes containing monoanionic imidazolin-2-iminato
ligands such as ImDippN (1), which can be described by the
two limiting resonance structures 1A and 1B (Scheme 1),
indicating that the ability of the imidazolium ring to stabilize
Scheme 1. Mesomeric Structures for the Imidazolin-2-imide
(Dipp = 2,6-Diisopropylphenyl)
1
a positive charge leads to highly basic ligands14 with a strong
electron donating capacity toward early transition metals.15,16
Because of their ability to act as 2σ,4π-electron donors, these
ligands can be regarded as monodentate analogues of cyclo-
pentadienyls, C5R5, and also as monoanionic imido ligands
in a similar fashion to that described for related phosphor-
aneiminato ligands.17 Therefore, lanthanide complexes with
terminal imidazolin-2-iminato ligands, as presented in this
paper, might serve as models for elusive mononuclear
lanthanide imido complexes, and their structural investiga-
tion could lead to a better understanding of lanthanide-
nitrogen multiple bonding.5,18 A preliminary account on
imidazolin-2-iminato lutetium complexes was previously
published,19 and this chemistry has now been extended to
cover the rare earth metals scandium, yttrium, lutetium, and
gadolinium.
ꢀ
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Results and Discussion
Structural Characterization of the Imine 1-H. The pro-
tonated ligand 1,3-bis(diisopropylphenyl)imidazolin-
2-imine (ImDippNH, 1-H) was obtained as a white crystal-
line solid from the reaction of the corresponding
N-heterocyclic carbene 1,3-bis(diisopropylphenyl)imida-
zolin-2-ylidene20 with trimethylsilyl azide (Me3SiN3),
followed by desilylation of the intermediate 1-SiMe3
in methanol.14 Single crystals of 1-H were obtained at
-30 °C from pentane solution, and the molecular struc-
ture of 1-H was determined by X-ray diffraction analysis
(Figure 1a) for comparison with the structures of the
complexes containing its conjugate base 1 (vide infra).
Perhaps surprisingly, the hydrogen H1 is not symmetri-
cally disordered to both sides of the C1-N1 vector,
but instead is well localized in the ring plane. This is
explained by an investigation of the crystal packing,
revealing intermolecular N1 H-C3 hydrogen bonds
3 3 3
˚
(dN 3 3 3 H = 2.36 A), connecting molecules by b translation
(8) (a) Trifonov, A. A.; Bochkarev, M. N.; Schumann, H.; Loebel, J.
Angew. Chem., Int. Ed. Engl. 1991, 30, 1149. (b) Evans, W. J.; Ansari, M. A.;
Ziller, J. W.; Khan, S. I. Inorg. Chem. 1996, 35, 5435. (c) Xie, Z.; Wang, S.;
Yang, Q.; Mak, T. C. W. Organometallics 1999, 18, 1578. (d) Wang, S; Yang,
Q.; Mak, T. C. W.; Xie, Z. Organometallics 1999, 18, 5511. (e) Gordon, J. C.;
Giesbrecht, G. R.; Clark, D. L.; Hay, P. J.; Keogh, D. W.; Poli, R.; Scott, B.
L.; Watkin, J. G. Organometallics 2002, 21, 4726. (f) Cui, D.; Tardif, O.;
Hou, Z. J. Am. Chem. Soc. 2004, 126, 1312.
(9) Chan, H. S.; Li, H. W.; Xie, Z. Chem. Commun. 2002, 652; the
dinuclear ytterbium complex {(dippN)2Yb(μ-Ndipp)}2{[Li2(THF)][Na
(THF)]}2 reported in this reference could be regarded as a lanthanide
complex containing terminal 2,6-diisopropylphenylimido ligands (DippN),
if the brindging lithium and sodium cations are ignored.
(14) Tamm, M.; Petrovic, D.; Randoll, S.; Beer, S.; Bannenberg, T.;
Jones, P. G.; Grunenberg, J. Org. Biomol. Chem. 2007, 5, 523.
(15) (a) Tamm, M.; Randoll, S.; Bannenberg, T.; Herdtweck, E. Chem.
Commun. 2004, 876. (b) Tamm, M.; Beer, S.; Herdtweck, E. Z. Naturforsch.
2004, 59b, 1497. (c) Tamm, M.; Randoll, S.; Herdtweck, E.; Kleigrewe, N.;
Kehr, G.; Erker, G.; Rieger, B. Dalton Trans. 2006, 459. (d) Beer, S.; Hrib, C.
G.; Jones, P. G.; Brandhorst, K.; Grunenberg, J.; Tamm, M. Angew. Chem.,
Int. Ed. 2007, 46, 8890. (e) Beer, S.; Brandhorst, K.; Grunenberg, J.; Hrib, C.
G.; Jones, P. G.; Tamm, M. Org. Lett. 2008, 10, 981. (f) Stelzig, S. H.; Tamm,
M.; Waymouth, R. M. J. Polym. Sci. Part A., Polym. Chem. 2008, 46, 6064.
(g) Beer, S.; Brandhorst, K.; Hrib, C. G.; Wu, X.; Haberlag, B.; Grunenberg,
J.; Jones, P. G.; Tamm, M. Organometallics 2009, 28, 1534.
::
(16) Kuhn, N.; Gohner, M.; Grathwohl, M.; Wiethoff, J.; Frenking, G.;
(10) Emslie, D. J. H.; Piers, W. E. Coord. Chem. Rev. 2002, 233-234, 131.
(11) (a) Beetstra, D. J.; Meetsma, A.; Hessen, B.; Teuben, J. H. Organo-
metallics 2003, 22, 4372. (b) Knight, L. K.; Piers, W. E.; Fleurat-Lessard, P.;
Parvez, M.; McDonald, R. Organometallics 2004, 23, 2087. (c) Knight, L. K.;
Piers, W. E.; McDonald, R. Organometallics 2006, 25, 3289.
Chen, Y. Z. Anorg. Allg. Chem. 2003, 629, 793.
(17) Dehnicke, K.; Greiner, A. Angew. Chem., Int. Ed. 2003, 42, 1340.
(18) Clark, D. L.; Gordon, J. C.; Hay, P. J.; Poli, R. Organometallics 2005,
24, 5747.
(19) Panda, T. K.; Randoll, S.; Hrib, C. G.; Jones, P. G.; Bannenberg, T.;
(12) Scott, J.; Basuli, F.; Fout, A. R.; Huffman, J. C.; Mindiola, D. J.
Tamm, M. Chem. Commun. 2007, 5007.
(20) Arduengo, A. J.; Krafczyk, R.; Schmutzler, R.; Craig, H. A.;
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Angew. Chem., Int. Ed. 2008, 47, 8502.
(13) Cundari, T. R. Chem. Rev. 2000, 100, 807.

5464 Inorganic Chemistry, Vol. 48, No. 12, 2009
Panda et al.
˚
Figure 1. (a) ORTEPdiagramof1-Hwith thermal displacementparametersdrawnat50% probability. Selectedbondlengths [A] and angles [deg]: N1-C1
1.2888(17), C1-N2 1.3815(15), C1-N3 1.4004(16); N1-C1-N2 124.05(11), N1-C1-N3 131.75(11), N2-C1-N3 104.19(10). (b) C-H N hydrogen
bonds in 1-H.
3 3 3
(Figure 1b); the chains thus formed lie in layers parallel to
the xy plane and are further linked by additional
Scheme 2. Preparation of Imidazolin-2-iminato Rare Earth Metal
Complexes
C-H π and π-stacking interactions. The structural
3 3 3
parameters are in good agreement with those reported
for other imidazolin-2-imines;14,21 and the slight short-
˚
ening of the C1-N1 bond distance [1.2888(17) A] with
respect to the corresponding silyl derivative 1-SiMe3
˚
[1.263(3)/1.265(3) A for two independent molecules] is
also in agreement with the trend observed for related
imines.14
Preparation and Structural Characterization of Imida-
zolin-2-iminato Rare Earth Metal Dichlorides. The reac-
tions of the imine 1-H with trimethylsilylmethyl lithium
(Me3SiCH2Li) and the anhydrous rare earth metal ha-
lides MCl3 (M = Sc, Y, Lu) in THF solution afforded
the imidazolin-2-iminato complexes [(1)MCl2(THF)3]
(2a, M = Sc; 2b, M = Y; 2c, M = Lu) M as colorless
crystalline solids in good yield after extraction with
pentane and crystallization from THF/pentane solution
(1:2) (Scheme 2). 2a-2c could be fully characterized by
1H and 13C NMR spectroscopy and elemental analysis. In
general, coordination to the rare earth metals does not
have a significant impact on the resonances observed for
the hydrogen and carbon atoms of the heterocyclic
1
imidazoline moiety; the H NMR spectra at room tem-
perature exhibit two doublet resonances as expected for
one set of diastereotopic isopropyl CH3 groups, indicat-
ing the presence of equivalent 2,6-diisopropylphenyl
groups. The molecular structures of 2a-2c were also
established by X-ray diffraction analyses; the three com-
plexes are isostructural and crystallize as THF solvates in
the space group P21/n. Their overall structural para-
meters are very similar, and pertinent data are summar-
ized in Table 1. The representative molecular structure of
the scandium complex 2a is shown in Figure 2.
In all cases, the metal (M) is hexacoordinated with the
two chlorine atoms in trans-position, and the three THF
ligands adopt a meridional arrangement. The Cl1-M-Cl2
and O2-M-O3 angles deviate significantly from 180°,
whereas the N1-M-O1 axes are close to linearity. Thus,
the coordination geometries about the metal atoms can be
described as distorted octahedra or, since the M-O1
bond in each complex is considerably longer than the
other two M-O bonds, this THF ligand can be regarded
as being trans-coordinated to a square-pyramid with the
imido N1 atom at the apex. Presumably, the elongation of
the M-O1 bonds is a result of the strong N1-M inter-
actions, revealed by very short M-N bond lengths of
::
(21) Kuhn, N.; Fawzi, R.; Steimann, M.; Wiethoff, J.; Blaser, D.; Boese,
R. Z. Naturforsch. 1995, 50b, 1779.

Article
Inorganic Chemistry, Vol. 48, No. 12, 2009 5465
˚
Table 1. Selected Bond Lengths [A] and Angles [deg] in Complexes 2a-2d
2a THF
2b THF
2c THF
2d
3
3
3
(M = Sc)
(M = Y)
(M = Lu)
(M = Gd)
M-N1
1.963(2)
2.1278(18)
2.4937(16)
2.3440(15)
2.3328(14)
2.6120(7)
2.6176(7)
2.091(3)
2.459(2)
2.303(2)
2.287(2)
2.5589(8)
2.5668(9)
2.153(3)
2.396(3)
2.384(3)
M-O1
2.3776(19)
2.2053(18)
2.1937(18)
2.4676(10)
2.4758(10)
M-O2
M-O3
M-Cl1
2.7299(9)
2.8430(10)
2.6229(10)
1.266(5)
175.5(3)
98.94(11)
80.50(3)
M-Cl2
M-Cl3
N1-C1
1.264(3)
174.5(2)
178.38(8)
163.65(3)
157.14(7)
101.7(2)
1.254(3)
1.251(4)
174.2(2)
177.86(9)
161.66(3)
157.54(8)
101.5(2)
M-N1-C1
N1-M-O1
Cl1-M-Cl2
O2-M-O3
N2-C1-N3
N1-M-Cl1
N1-M-Cl2
O1-M-O2
Cl1-M-Cl3
174.35(16)
177.63(6)
160.47(2)
157.43(5)
101.59(16)
Figure 3. ORTEP diagram of 2d with thermal displacement parameters
drawn at 50% probability, the two THF ligands coordinated to lithium
are disordered; the respective carbon atoms have been omitted for clarity.
102.0(3)
91.29(8)
171.41(8)
163.38(9)
166.39(3)
found in benzamidinate and amido complexes, respec-
tively.22,24
Following the conditions for the preparation of com-
plexes 2a-2c, the use of GdCl3 resulted in the formation
of the gadolinium complex [(1)GdCl2(THF)2] [LiCl
3
(THF)2] (2d), which was isolated as a colorless crystalline
solid after extraction with pentane and crystallization
from THF/pentane (1:2) solution (Scheme 2). The mole-
cular structure of 2d was determined by an X-ray diffrac-
tion analysis, and its presentation in Figure 3 reveals that
the trans-coordinated THF ligand in 2a-2c has been
formally replaced by a chlorine atom (Cl2). The resulting
negative charge is neutralized by a [Li(THF) 2]+ moiety,
capping the chlorine atoms Cl1 and Cl2. The trans-
influence of the imidazolin-2-iminato results in a weak-
˚
ening of the Gd-Cl2 bond [2.8430(10) A], which is
significantly longer than the other two Gd-Cl distances
Figure 2. ORTEP diagram of 2a in 2a THF with thermal displacement
parameters drawn at 50% probability.
3
˚
˚
[Gd-Cl1 = 2.7299(9) A, Gd-Cl3 = 2.6229(10) A]; as
expected, the shortest bond is observed between gadoli-
nium and the terminal chlorine atom. Again the Gd-N1
˚
˚
˚
1.963(2) A (2a), 2.1278(18) A (2b), and 2.091(3) A (2c)
together with almost linear M-N1-C1 angles. For scan-
dium, there is only one complex featuring a shorter Sc-N
˚
distance is very short, with 2.153(3) A being the shortest
value ever reported for gadolinium-nitrogen systems.
The previous shortest Gd-N distances ranging from
2.190(3) to 2.240(3) A were observed for five-membered
˚
bond length of 1.9366(14) A, and that is the AlMe3 adduct
of the recently reported transient scandium-imido complex
[(PNP)Sc(N-Dipp)] containing the PNP pincer ligand bis
(2-diisopropylphosphino-4-methylphenyl)amide.12 The
˚
amidolanthanide heterocycles.25
Preparation and Structural Characterization of Cyclo-
octatetraenyl Rare Earth Metal Complexes. For most of
its history, organolanthanide chemistry has been domi-
nated by cyclopentadienyl (Cp) ligands or by ligands that
can be regarded as Cp analogues.26 As mentioned above,
the imidazolin-2-iminato ligand 1 represents such a
monodentate Cp analogue because of its ability to act
as a 2σ,4π-electron donor. The second most important
˚
only other Sc-N bond lengths that also lie below 2 A
have been observed in the β-diketiminato complexes
˚
[(nacnac)ScCl(NHtBu)] [1.986(2) A] and [(nacnac)Sc
˚
(CH3)(NHtBu)] B(C6F5)3 [1.969(3) A] with nacnac =
3
[Dipp-NC(tBu)CHC(tBu)N-Dipp].11b,11c The metal-ni-
trogen bond length in 2b is also the second shortest
reported for yttrium complexes to date; and the shortest
˚
value [2.116(6) A] was observed in a tetranuclear cyclo-
carbocyclic ligand in organo-f-element chemistry is the
pentadienyl-yttrium complex containing μ3-ethylimido
ligands.22 Short Y-N distances have also been reported
for yttrium-phosphoraneiminato complexes.23 In con-
trast, the Lu-N bond length in 2c is still the shortest ever
observed for lutetium complexes,19 and the previous
2-
10-π-electron aromatic cyclooctatetraene dianion C8H8
which is well suited to forming stable mononuclear
,
(24) Nishiura, M.; Hou, Z.; Wakatsuki, Y.; Yamaki, T.; Miyamoto, T. J.
Am. Chem. Soc. 2003, 125, 1184.
(25) Shah, S. A. A.; Dorn, H.; Roesky, H. W.; Lubini, P.; Schmidt, H.-G.
˚
shortest Lu-N distances of 2.145(2) and 2.149(4) A were
Inorg. Chem. 1997, 36, 1102.
(26) (a) Schumann, H.; Meese-Marktscheffel, J. A.; Esser, L. Chem. Rev.
1995, 95, 865. (b) Edelmann, F. T. Angew. Chem., Int. Ed. Engl. 1995, 34,
2466. (c) Edelmann, F. T.; Freckmann, D. M. M.; Schumann, H. Chem. Rev.
2002, 102, 1851. (d) Edelmann, F. T. Complexes of Group 3 and Lanthanide
Elements. In Comprehensive Organometallic Chemistry III; Crabtree, R. H.,
Mingos, D. M. P., Eds.; Elsevier: Oxford, 2006; Vol 4, p 1.
(22) Cui, D.; Nishiura, M.; Hou, Z. Angew. Chem., Int. Ed. 2005, 44, 959.
::
(23) (a) Grob, T.; Seybert, G.; Massa, W.; Harms, K; Dehnicke, K. Z.
::
Anorg. Allg. Chem. 2000, 626, 1361. (b) Anfang, S.; Grob, T.; Harms, K.;
Seybert, G.; Massa, W.; Greiner, A.; Dehnicke, K. Z. Anorg. Allg. Chem.
1999, 625, 1853. (c) Anfang, S.; Harms, K.; Weller, F.; Borgmeier, O.;
Lueken, H.; Schilder, H.; Dehnicke, K. Z. Anorg. Allg. Chem. 1998, 624, 159.

5466 Inorganic Chemistry, Vol. 48, No. 12, 2009
Panda et al.
˚
Table 2. Selected Bond Lengths [A] and Angles [deg] in the COT Complexes 3a-3d, and 6
3a THFa
3b THF
3c THF
3d THF
6 THF
3
3
3
3
3
(M = Sc)
(M = Y)
(M = Lu)
(M = Gd)
(M = Ti)
M-N1
1.982(2)/1.965(2)
2.241(2)/2.271(2)
2.163(2)
2.4329(18)
2.407(2)
2.577(3)-2.700(3)
2.122(2)
2.380(2)
2.380(2)
2.544(3)-2.643(3)
2.191(2)
2.4501(19)
2.4718(18)
2.619(3)-2.735(3)
1.7905(15)
M-O1
M-O2
M-C(COT)
M-Cl
2.377(2)-2.507(3)/2.380(3)-2.507(3)
2.292(2)-2.540(2)
2.4305(5)
1.303(2)
N1-C1
1.254(4)/1.263(4)
166.6(2)/168.1(2)
87.20(9)/86.44(9)
1.255(3)
163.51(19)
86.92(7)
87.57(7)
76.93(7)
1.244(4)
166.7(2)
89.76(8)
84.90(8)
74.47(8)
1.258(3)
162.89(19)
87.17(7)
86.75(7)
77.04(6)
M-N1-C1
N1-M-O1
N1-M-O2
O1-M-O2
N1-M-Cl
N2-C1-N3
149.48(13)
86.17(5)
105.06(14)
102.3(2)/102.4(2)
101.4(2)
101.5(2)
101.7(2)
a Two independent molecules in the asymmetric unit.
Figure 4. ORTEP diagram of one of the two independent molecules 3a
in 3a THF with thermal displacement parameters drawn at 50% prob-
ability.
3
Figure 5. ORTEP diagram of 3d in 3d THF with thermal displacement
parameters drawn at 50% probability.
3
complexes with the lanthanide elements.26,27 Accord-
ingly, the substitution of the two chloride ligands in
unit; the structure of one molecule is shown in Figure 4. It
should be noted that reports on COT-scandium com-
plexes are extremely scarce,29-31 and we are only aware of
one other structurally authenticated scandium complex
containing a η8-C8H8 ligand, namely [(η8-C8H8)Sc{μ-
(η4:η4-C8H8)Li(THF)2].31 The COT ligand in 3a is essen-
tially η8-bound but in a slightly distorted fashion with the
2-
complexes 2a-2d by C8H8 could afford cyclooctate-
traenyl (COT) complexes of the type [(η8-C8H8)M(1)
(THF)n] that are related to mixed sandwich COT-Cp rare
earth metal complexes.27 Indeed, the COT complexes 3a-
3d were isolated in satisfactory yield by treatment of the
dichlorides 2a-2d with the dipotassium cyclooctatetrae-
nide salt K2(C8H8), followed by extraction with pentane
and crystallization from THF/pentane mixtures
˚
Sc-C distances ranging from 2.377(2) to 2.507(3) A
(molecule 1) and from 2.380(3) to 2.507(3) A (molecule
˚
2). The coordination spheres at scandium are both
completed by the imido ligand 1 and by one THF ligand
to afford a two-legged piano-stool geometry. The
1
(Scheme 2).28 The H NMR spectra (in C6D6) of the
diamagnetic complexes 3a, 3b, and 3c exhibit a singlet
resonance for the C8H8 hydrogen atoms at 6.49, 6.29,
and 6.28 ppm, respectively, and, similarly to complexes
2a-2c, two doublet resonances are observed for the
diasterotopic isopropyl CH3 groups, indicating that rota-
tion around the C-N-metal axes is fast on the NMR time
scale. In contrast, the paramagnetic nature of the gado-
linium complex 3d does not allow NMR spectroscopic
characterization.
The molecular structures of 3a-3d could be established
by X-ray diffraction analyses; all four COT complexes
crystallize as THF solvates. The scandium complex 3a
crystallizes in the triclinic space group P1 with two
structurally closely similar molecules in the asymmetric
˚
scandium-nitrogen bond lengths of 1.982(2) A (Sc1-
˚
N1) and 1.965(2) A (Sc2-N4) are very short and only
slightly longer than in 2a (see Tables 2 and 3 for com-
parison).
In agreement with the previously reported structure of
the lutetium complex 3c, the yttrium and gadolinium
complexes 3b and 3d contain two coordinated THF
ligands resulting in three-legged piano-stool geometries
around the metal atoms. The complexes are isostructural
and crystallize in the monoclinic space group P21/n; the
(29) (a) Westerhof, A.; de Liefde Meijer, H. J. J. Organomet. Chem. 1976,
116, 319. (b) Westerhof, A.; de Liefde Meijer, H. J. J. Organomet. Chem.
1978, 144, 61. (c) Bruin, P.; Booij, M.; Teuben, J. H.; Oskam, A. J.
Organomet. Chem. 1988, 350, 17.
(30) Burton, N. C.; Cloke, F. G. N.; Hitchcock, P. B.; de Lemos, H. C.;
Sameh, A. A. J. Chem. Soc., Chem. Commun. 1989, 1463.
(31) Rabe, G. W.; Zhang-Presse, M.; Yap, G. P. A. Inorg. Chim. Acta
2003, 348, 245.
(27) Edelmann, F. T. New J. Chem. 1995, 19, 535.
(28) For a related recent protocol for the preparation of scorpionate-COT
complexes, see: Amberger, H.-D.; Edelmann, F. T.; Gottfriedsen, J.; Herbst-
::
Irmer, R.; Jank, S.; Kilimann, U.; Noltemeyer, M.; Reddmann, H.; Schafer,
M. Inorg. Chem. 2009, 48, 760.

Article
Inorganic Chemistry, Vol. 48, No. 12, 2009 5467
4 2THF 0.5
Table 3. Crystallographic Data
3
3
1-H
2a THF
2b THF
2d
3a THF
3b THF
3d THF
hexane
6 THF
3
3
3
3
3
3
Empirical
formula
C
27H37N3
C43H68Cl2
N3O4Sc
C43H68Cl2
N3O4Y
18.1725(16) 10.7373(8)
C
43H68Cl3
C43H60
GdLiN3O4 N3O2Sc
C
47H68
N3O3Y
C
47H68
GdN3O3
C55H75
C
39H52
N4O2SSc
11.5604(14)
12.4633(14)
ClN3OTi
28.1598(14)
14.3409(6)
22.3609(10)
90
125.210(3)
90
7378.0(6)
8
662.19
C2/c
-173
0.71073
1.192
0.337
0.0421
0.1068
˚
a (A)
19.7270(14) 17.975(2)
6.5185(6) 12.2576(16) 12.3217(12) 12.3656(9)
20.1008(16) 19.817(2) 19.9922(18) 35.321(3)
90
106.755(4)
90
2475.0(3)
4
10.4088(14) 12.2941(11) 12.2615(6)
10.7170(14) 12.1767(12) 12.1517(6)
˚
b (A)
˚
c (A)
37.471(2)
92.590(3)
92.633(3)
107.290(3)
3979.2(8)
4
695.90
P1
-140
0.71073
1.162
0.223
0.0600
0.1658
29.395(3)
90
94.233(3)
90
4388.5(7)
4
811.95
P21/n
-140
0.71073
1.229
1.371
0.0457
0.1307
29.4262(14) 18.806(2)
R (deg)
β (deg)
γ (deg)
90
96.139(6)
90
4341.3(9)
4
806.86
P21/n
-173
0.71073
1.234
0.336
0.0496
0.1107
90
96.103(3)
90
4451.2(7)
4
850.81
P21/n
-140
0.71073
1.270
1.472
0.0359
0.0908
90
90
90
4689.7(6)
4
961.54
P212121
-173
0.71073
1.362
1.626
0.0443
0.0947
90
94.096(3)
90
4373.2(4)
4
880.29
P21/n
-173
0.71073
1.337
1.558
0.0306
0.0721
104.876(3)
105.057(3)
96.412(3)
2482.3(5)
2
877.02
P1
-140
0.71073
0.957
0.218
0.0465
0.1286
3
˚
V (A )
Z
formula weight 403.60
space group
T (°C)
P21/n
-173
0.71073
1.083
0.063
˚
λ (A)
Dcalcd (g cm-3
)
μ (mm-1
R(Fo or Fo
Rw(Fo or Fo
)
2
)
0.0513
0.1340
2
)
Scheme 3. Reaction of the Scandium-COT Complex 3a with 2,6-
Dimethylphenyl Isothiocyanate (XyNCS)
molecular structure of 3d is shown in Figure 5, and
pertinent structural data are summarized in Table 3. In
all cases, the metal-nitrogen bond lengths are marginally
elongated in comparison with complexes 2. This is ac-
companied by a greater deviation from linearity at N1,
and the M-N1-C1 angles range from 162.89(19)° (3d) to
166.7(2)° (3c). The C8H8 ligands in 3b-3d are η8-bound,
albeit in slightly distorted fashions. On average the me-
tal-carbon distances are relatively long in comparison
with other COT complexes containing Y, Lu, and Gd
metal atoms,32 which might be a consequence of the
sterically demanding Dipp-substituents of the imidazo-
lin-2-imide 1.
Reactivity of the Imidazolin-2-iminato Scandium Com-
plex 3a. In view of the isolobal relationship between
mononegative imidazolin-2-imides such as 1 and dinega-
tive imido ligands,17,19 the [(η8-C8H8)Sc(1)] fragment in
3a can be regarded as isolobal and isoelectronic with 16-
electron titanium imido complexes of the type [(η8-C8H8)
Ti(NR)] (R = tBu, Dipp).33-35 The reactivity of these
unique one-legged piano-stool (“pogo stick”) imido
complexes toward a variety of organic substrates such
as CO2, CS2, isocyanates, and isothiocyanates was stu-
died, and the Ti(NR) unit was shown to be reasonably
reactive, undergoing cycloaddition or cycloaddition-
elimination reactions.34 Therefore, the reaction of 3a
with similar substrates was studied, since the 16-electron
[(η8-C8H8)Sc(1)] fragment, provided by elimination of the
THF ligand, could be expected to exhibit similar reactiv-
ity. Whereas the reaction of 3a with 2,6-dimethylphenyl
isocyanate (Xy-NCO) did not result in the formation of a
clean product, addition of the corresponding isothiocya-
nate Xy-NCS afforded a white powder in high yield,
which could be recrystallized from THF/pentane solution
at -35 °C (Scheme 3).
X-ray diffraction analysis revealed the formation of the
[2 + 2] cycloaddition product 4, in which the NdC bond
has added across the scandium-nitrogen bond in [(η8-
C8H8)Sc(1)] to give a thioureato-N,N0 moiety (Figure 6).
This result is in contrast to the observation made for the
reaction of [(η8-C8H8)Ti(NDipp)] with Xy-NCS, which
was reported to result in the formation of an N,S-bound
cycloaddition product, in agreement with its decomposi-
tion to afford the carbodiimide Dipp-NdCdNdXy.34
Similar behavior has been reported for [Cp2ZrdNtBu
(THF)],36 whereas the reaction of Ph-NCS with the
diimido complex [(DippN)2Mo(OtBu)2] afforded the
structurally characterized complex [(DippN)MoL(OtBu)2]
(L = Dipp-N-C(S)-N-Ph) as the only example in which
a dianionic N,N0-bound thioureato ligand is bound to a
(32) (a) Panda, T. K.; Benndorf, P.; Roesky, P. W. Z. Anorg. Allg. Chem.
::
2005, 631, 81. (b) Schumann, H.; Winterfeld, J.; Kohn, R. D.; Esser, L.; Sun,
J.; Dietrich, A. Chem. Ber. 1993, 126, 907. (c) Roesky, P. W. J. Organomet.
Chem. 2001, 621, 277. (d) Schumann, H.; Sun, J.; Dietrich, A. Monatsh.
Chem. 1990, 121, 747. (e) Panda, T. K.; Zulys, A.; Gamer, M. T.; Roesky, P.
W. Organometallics 2005, 24, 2197. (f) Panda, T. K.; Gamer, M. T.; Roesky,
P. W. Inorg. Chem. 2006, 45, 910. (g) Schumann, H.; Winterfeld, J.; Hemling,
H.; Hahn, F. E.; Reich, P.; Brzezinka, K.-W.; Edelmann, F. T.; Kilimann,
U.; Herbst-Irmer, R. Chem. Ber. 1995, 128, 395. (h) Schumann, H.;
::
Rosenthal, E. C. E.; Demtschuk, J.; Muhle, S. H. Z. Anorg. Allg. Chem.
2000, 626, 2161. (i) Xia, J.; Jin, S.; Chen, W. Polyhedron 1996, 15, 809. (j)
::
Schumann, H.; Janiak, C.; Kohn, R. D.; Loebel, J.; Dietrich, A. J.
::
Organomet. Chem. 1989, 365, 137. (k) Schumann, H.; Kohn, R. D.; Reier,
F.-W.; Dietrich, A.; Pickardt, J. Organometallics 1989, 8, 1388. (l)
::
Schumann, H.; Winterfeld, J.; Esser, L.; Kociok-Kohn, G. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 1208. (m) Wang, J.; Smith, J.; Zheng, C.; Maguire, J.
A.; Hosmane, N. S. Organometallics 2006, 25, 4118. (n) Schumann, H.;
::
Winterfeld, J.; Glanz, M.; Kohn, R. D.; Hemling, H. J. Organomet. Chem.
1994, 481, 275. (o) Jin, J.; Zhuang, X.; Jin, Z.; Chen, W. J. Organomet. Chem.
1995, 490, C8. (p) Wayda, A. L.; Rogers, R. D. Organometallics 1985, 4,
1440.
(33) Dunn, S. C.; Hazari, N.; Jones, N. M.; Moody, A. G.; Blake, A. J.;
Cowley, A. R.; Green, J. C.; Mountford, P. Chem.-Eur. J. 2005, 11, 2111.
(34) Dunn, S. C.; Hazari, N.; Cowley, A. R.; Green, J. C.; Mountford, P.
Organometallics 2006, 25, 1755.
(35) Cloke, F. G. N.; Green, J. C.; Hazari, N.; Hitchcock, P. B.;
Mountford, P.; Nixon, J. F.; Wilson, D. J. Organometallics 2006, 25, 3688.
(36) Zuckerman, R. L.; Bergman, R. G. Organometallics 2000, 19, 4795.

5468 Inorganic Chemistry, Vol. 48, No. 12, 2009
Panda et al.
Figure 6. ORTEP diagram of 4 with thermal displacement parameters
drawn at 50% probability. The Dipp-isopropyl groups have been omitted
˚
for clarity. Selected bond lengths [A] and angles [deg]: Sc-N1 2.2505(10),
Sc-N4 2.1413(10), Sc-C(COT) 2.2638(15)-2.4101(14), C1-N1 1.3384
(15), N1-C28 1.4184(15), N4-C28 1.3360(15), C28-S 1.6753(12); N1-
Sc-N4 60.90(4), Sc-N1-C28 91.51(7), Sc-N4-C28 98.79(7), N1-
C28-N4 107.91(10), Sc-N1-C1 147.57(8), Sc-N4-C29 137.05(8),
N4-C28-S 127.48(9), N1-C28-S 124.46(9).
Figure 7. ORTEP diagram of 6 in 6 THF with thermal displacement
parameters drawn at 50% probability.
3
transition metal.37 The N-(imidazolin-2-ylidene)-N0-(2,6-
dimethylphenyl)thioureato(-1) ligand in 4 is unsymme-
trically coordinated to the scandium atom, with the
Scheme 4. Preparation of Imidazolin-2-iminato Titanium Complexes
˚
Sc-N4 bond length of 2.1413(10) A being shorter than
˚
the Sc-N1 bond lengths of 2.2505(10) A. This might be a
result of the stronger donating properties of the amido N4
nitrogen atom in comparison with those of the iminoimi-
dazole-nitrogen atom N1; however, the steric impact of
the large Dipp-substituents must also be taken into
account. Steric overcrowding can also be held responsible
for the observation that the thioureato plane containing
the atoms N1, N4, C28, and S is tilted with respect to the
plane containing Sc, N1, and N4 (dihedral angle = 12.2°).
˚
The C1-N1 distance of 1.3384(15) A is significantly
longer than the corresponding exocyclic C-N bonds in
the imidazolin-2-iminato complexes 2 and 3 (see Tables 2
and 3 for comparison), which indicates an increase of
charge separation and pronounced ylidic imidazolium-
amido-type characteristics of the ligand.
Complex 6 crystallizes as a THF solvate in the mono-
clinic space group C2/c, and the solid-state structure of
6 THF was established by X-ray diffraction (Figure 7);
Preparation and Structural Characterization of a Cyclo-
octatetraenyl Titanium Complex. For comparison, we
aimed toward the synthesis of the COT-titanium complex
[(η8-C8H8)TiCl(1)] (6), which can be regarded as being
isoelectronic with [(η8-C8H8)Sc(1)(THF)] or, after ab-
straction of the chloride anion, could form the cation
[(η8-C8H8)Ti(1)]+ as an isoelectronic and isostructural
analogue of [(η8-C8H8)Sc(1)]. Dropwise addition of TiCl4
to a solution of the silylated imine 1-SiMe3 in hexane at
room temperature afforded [(1)TiCl3] (5) as an orange
precipitate, which can be isolated in good yield by filtra-
tion. The COT ligand can be conveniently introduced by
reaction of 5 with K2(C8H8) in THF, and dark red crystals
of complex 6 were obtained by extraction with toluene
and crystallization from THF/pentane mixtures
(Scheme 4). The NMR spectroscopic data of 6 are very
similar to those of the COT-scandium complex 3a; for
instance, the 1H and 13C NMR resonances (in C6D6) for
the C8H8 hydrogen and carbon atoms in 6 are observed at
6.41 and 101.4 ppm, respectively, as compared to 6.49 and
96.0 ppm in 3a. Unfortunately, all attempts to remove the
chloride atom from the Ti atom in 6, for example, by
treatment with sodium or silver triflates, have hitherto
proved unsuccessful and lead to decomposition.
3
selected bond lengths and angles are presented in Table 2.
As expected from the trend of the metal radii, all metal-
element distances in 6 are shorter than those found for the
COT complex 3a, which contains the neighboring group 3
metal scandium.38 As also observed for complexes
3a-3d, the geometry of the COT ligand in 6 is appreciably
distorted; the Ti-C bond lengths vary from 2.292(2)-
˚
2.540(2) A, with the longest distances being those trans to
the imidazolin-2-iminato ligand. The same puckering of
the COT ligand has been observed for the pyridine (py)
adduct of the imido complex [(η8-C8H8)Ti(NtBu)] and
has been ascribed to the strong trans influence of the
imido ligand.34 Furthermore, the coordination sphere in
this adduct, [(η8-C8H8)Ti(NtBu)(py)], is very similar to
that in 6, and both complexes adopt two-legged piano-
stool geometries with acute N-Ti-N and N1-Ti-Cl
angles, respectively; the average N-Ti-N value for the
four independent molecules in the imido-pyridine com-
plex is 86.4°,34 which is almost identical with the corre-
sponding N1-Ti-Cl angle of 86.17(5)°. As expected, the
˚
Ti-N1 bond length in 6 [1.7905(15) A] is longer than the
Ti-N distances observed for the COT-imido complexes
(37) Gibson, V. C.; Redshaw, C.; Clegg, W.; Elsegood, M. R. J. J. Chem.
Soc., Chem. Commun. 1994, 2635.
(38) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751.

Article
Inorganic Chemistry, Vol. 48, No. 12, 2009 5469
Figure 9. PLUTO drawing of the calculated structure 6. Selected bond
˚
lengths [A] and angles [deg]; for comparison, experimental values are
given in parentheses: Ti-N1 1.795 [1.7905(15)], Ti-Cl 2.415 [2.4305(5)],
C1-N1 1.287 [1.303(2)]; Ti-N1-C1 157.3 [149.48(13)], N1-Ti-Cl 87.5
[86.17(5)].
Figure 8. PLUTO drawing of the calculated structure 3a. Selected bond
˚
lengths [A] and angles [deg]; for comparison, experimental values are
given in parentheses: Sc-N1 1.975 [1.982(2)/1.965(2)], Sc-O 2.355 [2.241
(2)/2.271(2)], C1-N1 1.262 [1.254(4)/1.263(4)]; Sc-N1-C1 168.4 [166.6
(2)/168.1(2)], N1-Sc-O 85.3 [87.20(9)/86.44(9)].
8
34
[(η -C8H8)Ti(NtBu)(py)] (1.72 A on average), [(η8-
˚
(see Supporting Information); however, comparison be-
tween the two systems is complicated by the low symme-
try and by the presence of the THF ligand in 3a or the
chlorine ligand in 6, respectively. Therefore, we per-
formed computations on the isoelectronic model com-
plexes [(η8-C8H8)Sc(NImMe)] (7) and [(η8-C8H8)Ti
(NImMe)]+ (8) (ImMeN = 1,3-dimethylimidazolin-2-
imide), assuming C2v symmetry (Figure 10). For compar-
ison, the “true” titanium imido complex [(η8-C8H8)Ti
(NXy)] (9) (Xy = 2,6-dimethylphenyl) was calculated,
which also allows us to make reference to the 16-electron
titanium imido complexes of the type [(η8-C8H8)Ti(NR)]
(R = tBu, Dipp).33-35
8
˚
C8H8)Ti(NtBu)] [1.699(6) A], and [(η -C8H8)Ti(NDipp)]
[1.7217(18) A];33 it is also slightly longer than the Ti-N
˚
5
15c
˚
distance reported for [(η -C5H5)TiCl2(1)] [1.778(2) A].
It should be noted that the shorter Ti-N distance in the
latter complex is accompanied by an almost linear Ti-
N1-C1 angle of 175.8(1)°, whereas the corresponding
angle in 6 [149.48(13)°] indicates a strong deviation from
linearity; this bending is probably due to steric factors,
since it allows a decrease in the interaction between the
Dipp-substituents and the large C8H8 ring. In addi-
tion, one should refer to previous studies, which have
shown that the potential energy surface of imido species is
rather shallow with respect to changes of the M-N-C
angle.15b,34,39
˚
The metal-nitrogen distances in 7 (1.925 A) and 8
(1.745 A) are shorter than those established for 3a and 6,
˚
Theoretical Investigation of the Bonding in Cyclooctate-
traenyl Scandium and Titanium Imido Complexes. For
comparison of the metal-nitrogen bonding in COT
scandium and titanium complexes, we performed density
functional theory (DFT) calculations on the Sc and Ti
complexes 3a and 6. Both structures were freely optimized
with no geometric constraints, and the resulting calcu-
lated geometries (Figures 8 and 9) are in very good
agreement with those established by X-ray diffraction
(vide supra). In 3a, the calculated Sc-N1 distance of
which is in agreement with the expected increase in bond
strength upon dissociation of the THF or chloride ligand,
respectively. In contrast, the Ti-N distance in 9 (1.691 A)
is almost identical with the bond lengths that have been
˚
˚
established experimentally [1.7217(18) A] and theoreti-
cally (1.70 A) for [(η -C8H8)Ti(NDipp)].33 An analysis of
8
˚
the canonical orbitals in 7, 8, and 9 reveals the presence of
two orthogonal orbitals in each complex that are clearly
associated with metal-ligand π-bonding. Figure 10
shows the isosurfaces of these orbitals: HOMO and
HOMO-3 in the imidazolin-2-iminato complexes 7 and
8, HOMO and HOMO-4 in the 2,6-dimethylphenyl imido
complex 9, respectively (see also Supporting Information
for the presentation of an extended set of frontier orbi-
tals). Visual inspection of these orbitals clearly indicates
that the orbital shapes are very similar and that the three
complexes are in an isolobal relationship. The energies of
the π-orbitals in the neutral complexes 7 and 9 are very
similar, whereas the cationic charge of complex 7 natu-
rally leads to a significantly higher stabilization of these
frontier orbitals.
A bonding analysis of the π-orbitals in the scandium
complex 7 reveals 5% and 9% orbital contribution from
the metal center (Figure 10), which is in agreement with
the expected predominantly ionic character of the Sc-N
bond; for similar considerations concerning the Lu-N
bond in the corresponding lutetium complex, see ref 19.
Replacing trivalent scandium in 7 by tetravalent titanium
affords cationic 8 and an increase of the metal orbital
contribution (11%, 15%), in line with a decrease of the
˚
1.975 A perfectly matches the average of the Sc-N1 bond
˚
lengths [1.965(2) and 1.982(2) A], which have been ex-
perimentally determined for two crystallographically in-
dependent molecules of 3a. The two-legged piano-stool
geometry with a comparatively small N1-Sc-O angle of
85.5° has also been well reproduced. The same is true for
6, which shows excellent correlation between theoreti-
cally and experimentally derived values, for example, Ti-
˚
N1 = 1.795 versus 1.7905(15) A; Ti-Cl = 2.415 versus
˚
2.4305(5) A; N1-Ti-Cl = 87.5° versus 86.17(5)°. The
marked deviation of the Ti-N1-C1 angle from linearity
(157.3°) was confirmed, albeit to a smaller extent than
found by X-ray diffraction analysis of 6 [149.48(13)°].
Visual inspection of the canonical orbitals in 3a and
6 reveals the existence of ligand-metal π-bonding
~
(39) (a) Jorgensen, K. A. Inorg. Chem. 1993, 32, 1521. (b) Antinolo, A.;
e
ꢀ
ꢀ
Espinosa, P.; Fajardo, M.; Gomez-Sal, P.; Lopez-Mardomingo, C.; Martin-
Alonso, A.; Otero, A. J. Chem. Soc., Dalton Trans. 1995, 1007. (c) Ciszewski,
J. T.; Harrison, J. F.; Odom, A. L. Inorg. Chem. 2004, 43, 3605.

5470 Inorganic Chemistry, Vol. 48, No. 12, 2009
Panda et al.
complexes. In view of the imidazolin-2-iminato complexes 2
and 3 presented in this paper, this implies that mononuclear
rare earth metal imido complexes should be isolable under
the right conditions, although a higher tendency to form
clusters must certainly be conceded because of the higher
charge of the imido species. Future studies will continue to
exploit the reactivity along the metal-nitrogen bond in these
imidazolin-2-iminato complexes, and preliminary studies
reveal that these systems are highly active in the catalytic
ring-opening polymerization of lactones.
Experimental Section
General Information. All manipulations of air-sensitive
materials were performed with the rigorous exclusion of oxygen
and moisture in flame-dried Schlenk-type glassware either on a
dual manifold Schlenk line, interfaced to a high vacuum (10-4
torr) line, or in an argon-filled glovebox (MBraun 200B). All
solvents were purified by a solvent purification system from
˚
MBraun and stored over molecular sieve (4 A) prior to use.
Deuterated solvents were obtained from Sigma Aldrich (all g99
atom % D) and were degassed, dried, and stored in the argon-
filled glovebox. NMR spectra were recorded on Bruker DPX
200, Bruker DRX 400, and Bruker DPX 600 devices. The
chemical shifts are expressed in parts per million (ppm) using
tetramethylsilane (TMS) as internal standard (1H, 13C). Ele-
mental analysis (C, H, N) succeeded by combustion and gas
chromatographical analysis with an Elementar vario MICRO.
1,3-Bis (2,6-diisopropylphenyl)imidazolin-2-imine (ImDippNH,
1-H),14 the corresponding silylated imine ImDippNSiMe3
(1-SiMe3),14 [LiCH2SiMe3],41 and K2(C8H8)42 were prepared
according to published procedures. The anhydrous trichlorides
LnCl3 (Ln = Sc, Y, Lu, Gd) were purchased from Sigma-
Aldrich and used as received.
Figure 10. Isosurfaces and eigenvalues of the metal-ligand π-bonding
orbitals in complexes 7, 8, and 9.
General Procedure for the Preparation of Complexes [(1)
MCl2(THF)3] (2a, M = Sc; 2b, M = Y; 2c, M = Lu) and [(1)
orbital energies as the valence state of the metal in-
creases.40 An additional increase of the metal orbital
contribution (15%, 21%) is observed for [(η8-C8H8)Ti
(NXy)] (9), in which the mononegative imidazolin-2-
iminato ligand in 8 has been replaced by a “true” dine-
gative imido ligand with a stronger π-donating ability
toward the metal center. Despite these differences, com-
parison of the bonding in complexes 7, 8, and 9 clearly
indicates that both imidazolin-2-iminato and imido
ligands can be regarded as 2σ,4π-electron donors, capable
of forming highly polarized triple bonds with the early
transition metals. For imidazolin-2-iminato ligands, this
behavior can be mainly attributed to the ability of the
imidazolium ring to stabilize a positive charge as indi-
cated by resonance structure 1B in Scheme 1, and the
contribution from this structure to the overall electronic
ground state can be expected to increase significantly
upon metal-nitrogen interaction.
GdCl2(THF)2] [LiCl(THF)2] (2d). A mixture of anhydrous
3
MCl3 (0.5 mmol, M = Sc, Y, Lu, or Gd) and LiCH2SiMe3 (47
mg, 0.5 mmol) was treated with THF (10 mL). After stirring for
2 h, a solution of 1-H (202 mg, 0.5 mmol) in THF (5 mL) was
added. The reaction mixture was allowed to stir for another 12 h.
The solvent was evaporated, and the compound was repeatedly
extracted with 30 mL (3 ꢀ 10 mL) of pentane. The complexes
were recrystallized from THF/pentane (1:2) at -30 °C to obtain
colorless crystals.
Analytical Data for [(1)ScCl2(THF)3] (2a). Yield: 301 mg
(75%). Anal. Calcd for C43H68Cl2N3O4Sc (2a THF): C, 64.00;
3
1
H, 8.49; N, 5.20. Found: C, 63.80; H, 7.96; N, 5.35. H NMR
(C6D6, 200 MHz, 25 °C): δ 7.36-7.24 (m, 6 H, Ph), 5.93 (s, 2 H,
NCH), 3.57 (br, 4 H, THF), 3.31 (sept., 4 H, CHMe2), 1.55 (d, 12
H, CH3), 1.25 (br, 4 H, THF), 1.20 (d, 12 H, CH3). 13C NMR
(C6D6, 50.3 MHz, 25 °C): δ 159.2 (NCN), 149.2 (ipso-C), 137.1
(o-C), 129.5 (p-C), 124.4 (m-C), 113.8 (NCH), 67.5 (THF), 29.5
(CHMe2), 25.5 (THF), 24.9 (CHCH3), 24.6 (CHCH3) ppm.
Analytical Data for [(1)YCl2(THF)3] (2b). Yield: 255 mg
(60%). Anal. Calcd for C39H60Cl2N3O3Y (2b): C, 60.15; H, 7.76;
N, 5.39. Found: C, 59.54; H, 7.59; N, 5.17. 1H NMR (C6D6, 200
MHz, 25 °C): δ = 7.17 (br, 6 H, Ph), 5.95 (s, 2 H, NCH), 3.57 (br,
4 H, THF), 3.42 (sept., 4H, CHMe), 1.55 (d, 12 H, CH3), 1.4 (br,
4 H, THF), 1.34 (d, 12 H, CH3). 13C NMR (C6D6, 50.3 MHz,
25 °C): δ 159.3 (NCN), 154.1 (ipso-C), 148.3 (o-C), 128.2 (p-C),
123.8 (m-C), 113.7 (NCH), 67.4 (THF), 28.6 (CHMe2), 25.4
(THF), 23.8 (CHCH3), 23.7 (CHCH3) ppm.
Conclusions
The present study gives further evidence for the suitability
of imidazolin-2-iminato ligands to form particularly strong
metal-nitrogen bonds. The reactivity of the polarized Sc-N
bond in complex 3a toward 2,6-dimethylphenyl isothiocya-
nate and also the theoretical study reveal a strong analogy
between imidazolin-2-iminato and conventional imido
(41) Vaughn, G. D.; Krein, K. A.; Gladysz, J. A. Organometallics 1986, 5,
(40) Parkin, G. Classification of Organtransition Metal Compounds. In
Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, D.
M. P., Eds.; Elsevier: Oxford, 2006; Vol. 1, p 1, and references cited therein.
936.
(42) Gilbert, T. M.; Ryan, R. R.; Sattelberger, A. P. Organometallics
1988, 7, 2514.

Article
Analytical Data for [(1)LuCl2(THF)3] (2c). Yield: 300 mg
Inorganic Chemistry, Vol. 48, No. 12, 2009 5471
pressure in >95% yield (56 mg, 0.08 mmol). Colorless crystals
of complex 4 were isolated from THF/pentane solution at
-35 °C. Anal. Calcd for C44H53N4SSc: C, 73.92; H, 7.47; N,
7.84. Found: C, 74.45; H, 7.48; N, 8.70. 1H NMR (toluene d8,
400 MHz, 25 °C): δ 7.24-7.28 (2H, m, p-Dipp-CH), 7.09-7.14
(4H, m, m-Dipp-CH), 6.89-6.91 (2H, m, m-Xy-CH), 6.79-6.83
(1H, m, p-Xy-CH), 6.42 (8H, s, C8H8), 6.26 (2H, s, NCH), 3.48
(2H, sept., CHMe2), 2.73 (2H, sept., CHMe2), 1.83 (6H, s, Xy-
CH3), 1.50 (6H, d, Dipp-CH3), 1.29 (6H, d, Dipp-CH3), 0.92
(6H, d, Dipp-CH3), 0.90 (6H, d, Dipp-CH3). 13C NMR (toluene
d8, 400 MHz, 25 °C): δ 190.5 (CS), 151.8 (i-Xy-CN), 148.1
(NCN), 146.8 (o-Dipp-C), 133.2 (i-Dipp), 132.2 (o-Xy-CH),
130.6 (p-Dipp-CH), 128.1 (m-Xy-CH), 127.7 (m-Dipp-CH),
123.6 (p-Xy-CH), 118.8 (NCH), 97.3 (C8H8), 28.8 (CHMe2),
28.6 (CHMe2), 26.1 (Dipp-CH3), 25.6 (Dipp-CH3), 23.3 (Dipp-
CH3), 23.1 (Dipp-CH3), 20.1 (Xy-CH3) ppm.
(69%). Anal. Calcd for C39H60Cl2N3O3Lu (2c): C, 54.16; H,
6.99; N, 4.85. Found: C, 54.81; H, 6.95; N, 4.70. 1H NMR
(C6D6, 200 MHz, 25 °C): δ = 7.18 (br, 6H, Ph), 5.91 (s, 2 H,
NCH), 3.70 (br., 4 H, CHMe), 3.52 (br, 4 H, THF), 1.53 (d, 12H,
CH3), 1.37 (br, 4 H, THF), 1.26 (d, 12H, CH3) ppm; 13C NMR
(C6D6, 50.3 MHz, 25 °C): δ 159.2 (NCN), 154.2 (ipso-C),
148.1 (o-C), 128.4 (p-C), 123.6 (m-C), 113.5 (NCH), 67.4
(THF), 28.7 (CHMe2), 25.1 (THF), 23.8 (CHCH3), 23.7
(CHCH3) ppm.
Analytical Data for [(1)GdCl2(THF)2] [LiCl(THF)2]
3
(2d). Yield: 350 mg (73%). Anal. Calcd for C43H68Cl3GdLi-
N3O4 (2d): C, 53.71; H, 7.12; N, 4.37. Found: C, 53.58; H, 6.85;
N, 4.20. Because of the highly paramagnetic nature of the
compound, no NMR spectra were recorded.
General Procedure for the Preparation of Complexes
[(η8-C8H8)M(1)(THF)n] (3a, M = Sc, n = 1; 3b, M = Y,
n = 2; 3c, M = Lu, n = 2; 3d, M = Gd, n = 2). A solution of the
respective complex 2 (0.5 mmol) in THF (5 mL) was treated with
freshly prepared K2(C8H8) (from 39 mg of potassium and 0.055
mL of cyclooctatetraene). After stirring for 12 h, the solvent was
evaporated, and the residue was extracted with 30 mL (3 ꢀ 10
mL) of pentane. After filtration and evaporation, the complexes
were recrystallized from THF/pentane (1:2) at -30 °C.
Synthesis and Characterization of [(1)TiCl3] (5). TiCl4
(199.3 mg, 1.05 mmol) was added dropwise to a solution of 1-
SiMe3 (500 mg, 1.05 mmol) in hexane (20 mL) at ambient
temperature, and the resulting reaction mixture was stirred for
12 h. The orange precipitate was isolated by filtration, washed
with hexane (3 ꢀ 5 mL) and dried in vacuo. The product was
obtained as an orange solid in 95% yield (526 mg). Anal. Calcd
for C27H36Cl3N3Ti: C, 58.24; H, 6.52; N, 7.55. Found: C, 58.63;
H, 6.61; N, 7.08. 1H NMR (CDCl3, 200 MHz, 25 °C): δ 7.73-
7.65 (m, 4 H, m-CH), 7.53-7.49 (m, 2 H, p-CH), 6.85 (s, 2 H,
NCH), 2.89 (sept., 4 H, CHMe), 1.59 (d, 12 H, CH3), 1.30 (d, 12
H, CH3) ppm. 13C NMR (CDCl3, 50.3 MHz, 25 °C): δ 146.2
(ipso-C), 131.3 (o-C), 130.3 (p-C), 124.5 (m-C), 116.2 (NCH),
29.2 (CHMe2), 24.4 (CHCH3), 23.4 (CHCH3) ppm; the NCN
resonance was not observed.
Analytical Data for [(η8-C8H8)Sc(1)(THF)] (3a). Yield:
250 mg (72%). Anal. Calcd for C43H60N3O2Sc (3a THF): C,
74.21; H, 8.69; N, 6.03. Found: C, 73.99; H, 8.20; N, 6.28. H
3
1
NMR (C6D6, 200 MHz, 25 °C): δ 7.28-7.20 (m, 4 H, m-CH),
7.12-7.08 (m, 2 H, p-CH), 6.49 (s, 8 H, C8H8), 5.67 (s, 2 H,
NCH), 3.49-3.43 (m, THF), 2.75 (sept., 4 H, CHMe), 1.4-1.3
(m, THF), 1.25 (d, 12 H, CH3), 1.09 (d, 12 H, CH3) ppm; 13C
NMR (C6D6, 50.3 MHz, 25 °C): δ 148.2 (ipso-C), 137.2 (o-C),
129.6 (p-C), 124.3 (m-C), 112.6 (NCH), 96.5 (C8H8), 67.2 (THF),
29.1 (CHMe2), 26.2 (THF), 25.1 (CHCH3), 24.9 (CHCH3) ppm;
the NCN resonance was not observed.
Synthesis and Characterization of [(η8-C8H8)TiCl(1)]
(6). A solution of [(1)TiCl3] (6) (278.4 mg, 0.5 mmol) in THF
(20 mL) was treated with 10 mL of a THF solution of freshly
prepared K2(C8H8) (from 39 mg of potassium and 0.055 mL of
cyclooctatetraene). After stirring for 12 h, the solvent was
evaporated, and the residue was extracted with 25 mL of
toluene. After filtration and evaporation, ruby-colored crys-
tals of complex 6 were isolated from THF/pentane solution
(1:2) at -30 °C. Yield: 190 mg (64%). Anal. Calcd for
Analytical Data for [(η8-C8H8)Y(1)(THF)2] (3b). Yield:
210 mg (57%). Anal. Calcd for C43H60N3O2Y (3b): C, 69.80; H,
8.17; N, 5.67. Found: C, 69.47; H, 7.91; N, 5.42. 1H NMR
(C6D6, 200 MHz, 25 °C): δ 7.28-7.21 (m, 4 H, m-CH), 7.12-
7.11 (m, 2 H, p-CH), 6.29 (s, 8 H, C8H8), 5.85 (s, 2 H, NCH), 3.12
C
35H44ClN3Ti: C, 71.24; H, 7.52; N, 7.12. Found: C, 70.29;
(sept., 4 H, CHMe), 1.31 (d, 12 H, CH3), 1.17 (d, 12 H, CH3). 13
C
H, 8.28; N, 6.56; low %C analyses were also consistently found
for related COT-titanium complexes.33,34 1H NMR (C6D6, 200
MHz, 25 °C): δ 7.30-7.23 (m, 4 H, m-CH), 7.12-7.11 (m, 2 H,
p-CH), 6.41 (s, 8 H, C8H8), 5.72 (s, 2 H, NCH), 2.82 (sept., 4 H,
CHMe), 1.41 (d, 12 H, CH3), 1.00 (d, 12 H, CH3). 13C NMR
(C6D6, 50.3 MHz, 25 °C): δ 147.6 (o-C), 133.1 (ipso-C), 131.4
(p-C), 124.0 (m-C), 115.0 (NCH), 101.4 (C8H8), 28.8 (CHMe2),
25.1 (CHCH3), 23.6 (CHCH3) ppm; the NCN resonance was
not observed.
NMR (C6D6, 50.3 MHz, 25 °C): δ 148.3 (ipso-C), 137.3 (o-C),
128.5 (p-C), 123.7 (m-C), 113.1 (NCH), 93.4 (C8H8), 67.1 (THF),
28.6 (CHMe2), 25.1 (THF), 24.5 (CHCH3), 23.7 (CHCH3) ppm;
the NCN resonance was not observed.
Analytical Data for [(η8-C8H8)Lu(1)(THF)2] (3c). Yield:
300 mg (62%). Anal. Calcd for C47H68N3O3Lu (3c THF): C,
3
62.86, H 7.63, N 4.67; Found: C 62.15, H 7.04, N 4.12. 1H NMR
(C6D6, 200 MHz, 25 °C): δ 7.27-7.20 (m, 4 H, m-CH), 7.11-
7.10 (m, 2 H, p-CH), 6.28 (s, 8 H, C8H8), 5.86 (s, 2 H, NCH),
3.25-3.18 (m, THF), 3.09 (sept., 4 H, CHMe), 1.30 (d, 12H,
CH3), 1.24-1.19 (m, THF), 1.16 (d, 12H, CH3). 13C NMR
(C6D6, 50.3 MHz, 25 °C): δ 148.3 (ipso-C), 137.2 (o-C), 128.5 (p-
C), 123.7 (m-C), 113.2 (NCH), 92.8 (C8H8), 68.1 (THF), 28.6
(CHMe2), 25.2 (THF), 24.4 (CHCH3), 23.7 (CHCH3) ppm; the
NCN resonance was not observed.
Electronic Structure Calculations. The calculations were
performed using the Gaussian03 package.43 All structures were
(43) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A. Gaussian 03,
revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
Analytical Data for [(η8-C8H8)Gd(1)(THF)2] (3d).
Yield: 350 mg (79%). Anal. Calcd for C47H68GdN3O3 (3d
3
THF): C, 64.12; H, 7.78; N, 4.77. Found: C, 63.92; H, 7.51; N,
4.47. Because of the highly paramagnetic nature of the com-
pound, no NMR spectra were recorded.
Reaction of 3a with 2,6-Dimethylphenyl Isothiocyanate.
A solution of XyNCS (13.34 mg, 0.08 mmol) in toluene (2 mL)
was added to a solution of 3a (51 mg, 0.08 mmol) in toluene
(2 mL), and the reaction mixture was stirred at room tempera-
ture for 1 h. After the solvent was removed under reduced
pressure, the residue was washed with 2 mL of pentane. A white
powder was obtained after removing the solvent under reduced

5472 Inorganic Chemistry, Vol. 48, No. 12, 2009
Panda et al.
fully optimized at the DFT level employing the B3PW91 hybrid
functional44 which has been frequently employed for calcula-
tions on rare earth metal complexes.45 For all main-group
elements (C, H, N, O, and Cl) and for the 3d transition metals
titanium (Ti) and scandium (Sc) the all-electron triple-ζ basis set
(6-311G**) was used as implemented in the Gaussian03 pro-
gram package. The bonding analysis was performed employing
the program GaussSum 2.1.46 Further details together with
presentations and coordinates in x,y,z format of all optimized
structures and isosurfaces of selected frontier orbitals can be
found in the Supporting Information.
data are summarized in Table 3 (data for compound 2c were
reported earlier19). Special features and exceptions: Compound
1-H: The NH hydrogen was refined freely. Compound 2d: The
THF molecules at lithium are each disordered over two posi-
tions. Compound 3a: One isopropyl group is disordered over
two positions. Compound 3d: The uncoordinated THF is poorly
defined. For compound 4, a poorly defined region of residual
electron density was tentatively identified as two THF molecules
and half a hexane, but could not be refined satisfactorily. The
program SQUEEZE (A.L. Spek, University of Utrecht, Nether-
lands) was therefore used to remove mathematically the effects
of the solvent.
X-ray Crystal Structure Determinations. Data were re-
corded on various area detectors (Bruker, Oxford Diffraction)
at low temperature. Structures were refined anisotropically
using the program SHELXL-97.47 Hydrogen atoms were in-
cluded using rigid methyl groups or a riding model. Pertinent
Acknowledgment. This work was supported by the
Deutsche Forschungsgemeinschaft (DFG) through the pro-
::
::
gram “Lanthanoidspezifische Funktionalitat in Molekul und
Material” (SPP 1166). We thank Priv.-Doz. Jorg Grunenberg
and Dipl.-Chem. Kai Brandhorst for helpful discussion.
::
(44) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Perdew, J. P.;
Wang, Y. Phys. Rev. B 1992, 45, 13244.
(45) (a) Barros, N.; Eisenstein, O.; Maron, L. Dalton Trans. 2006, 3052.
(b) Maron, L.; Bourissou, D. Organometallics 2007, 26, 1100.
(46) (a) O’Boyle, N. M. GaussSum 2.1; 2007; available at http://gausssum.
sf.net.(b) O’Boyle, N. M.; Tenderholt, A. L.; Langner, K. M. J. Comput.
Chem. 2008, 29, 839.
Supporting Information Available: Details of the electronic
structure calculations together with presentations and coordi-
nates in x,y,z format of all optimized structures; isosurfaces of
selected frontier orbitals. This material is available free of charge
via the Internet at http://pubs.acs.org.
(47) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.