A comparative study of π-arene-bridged lanthanum arylamide and aryloxide dimers. Solution behavior, exchange mechanisms, and x-ray crystal structures of La2(NH-2,6-iPr2C6H3)6, La(NH-2,6-iPr2C6H3)3(THF)3, and La(NH-2,6-iPr2C6H3)3(py)2

Article abstract of DOI:10.1021/ja0398262

Reaction of 3 equiv of 2,6-diisopropylaniline with La[N(SiMe ₃)₂]₃ produces the dimeric species La ₂(NHAr)₆ (1). X-ray crystallography reveals a centrosymmetric structure, where the dimeric unit is bridged by intermolecular η⁶-arene interactions of a unique arylamide ligand attached to an adjacent metal center. Exposure of 1 to THF results in formation of the monomeric tris-THF adduct La(NHAr)₃(THF)₃ (2), which was shown by X-ray crystallography to maintain a fac-octahedral structure in the solid state. ¹H NMR spectroscopy illustrates that the binding of THF to 1 to form 2 is reversible and removal of THF under vacuum regenerates dimeric 1. Addition of pyridine to 1 yields the monomeric bis-yridine adduct La(NHAr)₃(py)₂ (3), which exhibits a distorted trigonal-bipyramidal La metal center. Solution ¹H NMR, IR, and Raman spectroscopy indicate that the π-arene-bridged dimeric structure of 1 is maintained in solution. Variable-temperature ¹H NMR spectroscopic investigations of 1 are consistent with a monomer-dimer equilibrium at elevated temperature. In contrast, variable-temperature ¹H NMR spectroscopic investigations of the aryloxide analogue La₂(OAr)₆ (4) show that the bridging and terminal aryloxide groups exchange by a mechanism in which the dimeric nature of the compound is retained. Density functional theory (DFT) calculations were carried out on model compounds La₂(OC ₆H₅)₆, La₂(NHC₆H ₅)₆, and (C₆H₅R)La(XC ₆H₅)₃, where X = O or NH and R = H, OH, or NH₂. The formation of η⁶-arene interactions is energetically favored over monomeric LaX₃ (X = OPh or NHPh) with the aryloxide π-arene interaction being stronger than the arylamide π-arene interaction. Calculation of vibrational frequencies reveals the origin of the observed IR spectral behavior of both La₂(OC₆H ₅)₆ and La₂(NHC₆H₅) ₆, with the higher energy ν(C=C) stretch due to terminal ligands and the lower energy stretch associated with the bridging ligands.

Full text of DOI:10.1021/ja0398262

Published on Web 04/28/2004
A Comparative Study of π-Arene-Bridged Lanthanum
Arylamide and Aryloxide Dimers. Solution Behavior,
Exchange Mechanisms, and X-ray Crystal Structures of
La2(NH-2,6-iPr2C6H3)6, La(NH-2,6-iPr2C6H3)3(THF)3, and
La(NH-2,6-iPr2C6H3)3(py)2
Garth R. Giesbrecht,1a John C. Gordon,*,1b David L. Clark,*,1c P. Jeffrey Hay,1d
Brian L. Scott,1b and C. Drew Tait1b
Contribution from the Nuclear Materials Technology (NMT) DiVision, Chemistry (C) DiVision,
the Glenn T. Seaborg Institute for Transactinium Science and Theoretical (T) DiVision,
Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Received November 26, 2003; E-mail: john.gordon@science.doe.gov
Abstract: Reaction of 3 equiv of 2,6-diisopropylaniline with La[N(SiMe3)2]3 produces the dimeric species
La2(NHAr)6 (1). X-ray crystallography reveals a centrosymmetric structure, where the dimeric unit is bridged
by intermolecular η6-arene interactions of a unique arylamide ligand attached to an adjacent metal cen-
ter. Exposure of 1 to THF results in formation of the monomeric tris-THF adduct La(NHAr)3(THF)3 (2),
which was shown by X-ray crystallography to maintain a fac-octahedral structure in the solid state. 1H
NMR spectroscopy illustrates that the binding of THF to 1 to form 2 is reversible and removal of THF under
vacuum regenerates dimeric 1. Addition of pyridine to 1 yields the monomeric bis-pyridine adduct
1
La(NHAr)3(py)2 (3), which exhibits a distorted trigonal-bipyramidal La metal center. Solution H NMR, IR,
and Raman spectroscopy indicate that the π-arene-bridged dimeric structure of 1 is maintained in solution.
Variable-temperature 1H NMR spectroscopic investigations of 1 are consistent with a monomer-dimer
equilibrium at elevated temperature. In contrast, variable-temperature 1H NMR spectroscopic investigations
of the aryloxide analogue La2(OAr)6 (4) show that the bridging and terminal aryloxide groups exchange by
a mechanism in which the dimeric nature of the compound is retained. Density functional theory (DFT)
calculations were carried out on model compounds La2(OC6H5)6, La2(NHC6H5)6, and (C6H5R)La(XC6H5)3,
where X ) O or NH and R ) H, OH, or NH2. The formation of η6-arene interactions is energetically favored
over monomeric LaX3 (X ) OPh or NHPh) with the aryloxide π-arene interaction being stronger than the
arylamide π-arene interaction. Calculation of vibrational frequencies reveals the origin of the observed IR
spectral behavior of both La2(OC6H5)6 and La2(NHC6H5)6, with the higher energy ν(CdC) stretch due to
terminal ligands and the lower energy stretch associated with the bridging ligands.
Introduction
and 6-positions of the phenyl ring: for example, use of the
sterically demanding 2,6-di-tert-butylphenoxide group leads to
The identification of alternative ligand systems capable of
stabilizing monomeric lanthanide species while provoking novel
reactivity is a prevalent theme in contemporary 4f-element
chemistry.2-4 To this end, bulky aryloxide anions OAr- have
been extensively investigated as ancillary ligands due to the
ability to modify both the electronic and steric properties of
the resultant complexes by varying the substituents on the phenyl
ring.5,6 This theory is borne out by the observation of varying
structural modes upon changing the size of the group at the 2-
the formation of mononuclear Ln(OAr)3 complexes for the entire
lanthanide series,7 whereas the less sterically encumbered 2,6-
dimethylphenoxide results in formation of dimeric Lewis base
adducts Ln2(O-2,6-Me2C6H3)6L4 (Ln ) Y,8 Nd;9 L ) THF). It
has also been shown that f-element complexes containing less
sterically demanding aliphatic alkoxide ligands such as neo-
pentoxide10 or tert-butoxide11-13 show a strong tendency toward
oligomerization, resulting in the isolation of trinuclear or higher
(7) Hitchcock, P. B.; Lappert, M. F.; Singh, A. Chem. Commun. 1983, 1499.
(8) Evans, W. J.; Olofson, J. M.; Ziller, J. W. Inorg. Chem. 1989, 28, 4308.
(9) Evans, W. J.; Ansari, M. A.; Khan, S. I. Organometallics 1995, 14, 558.
(10) Barnhart, D. M.; Clark, D. L.; Gordon, J. C.; Huffmann, J. C.; Watkin, J.
G.; Zwick, B. D. J. Am. Chem. Soc. 1993, 115, 8461.
(1) (a) NMT-DO, Mail Stop J514. (b) C-SIC, Mail Stop J514. (c) G. T. Seaborg
Institute, Mail Stop E500. (d) T-12, Mail Stop B268.
(2) Edelmann, F. T. Angew. Chem., Int. Ed. Engl. 1995, 34, 2466.
(3) Edelmann, F. T.; Freckmann, D. M. M.; Schumann, H. Chem. ReV. 2002,
102, 1851.
(4) Piers, W. E.; Emslie, D. J. H. Coord. Chem. ReV. 2002, 233, 131.
(5) Mehrotra, R. C.; Singh, A.; Tripathi, U. M. Chem. ReV. 1991, 91, 1287.
(6) Barnhart, D. M.; Clark, D. L.; Gordon, J. C.; Huffman, J. C.; Vincent, R.
L.; Watkin, J. G.; Zwick, B. D. Inorg. Chem. 1994, 33, 3487 and references
therein.
(11) Bradley, D. C.; Chudzynska, H.; Hursthouse, M. B.; Motevalli, M.
Polyhedron 1991, 10, 1049.
(12) Veith, M.; Mathur, S.; Kareiva, A.; Jilavi, M.; Zimmer, M.; Huch, V. J.
Mater. Chem. 1999, 9, 3069.
(13) Gromada, J.; Chenal, T.; Mortreux, A.; Ziller, J. W.; Leising, F.; Carpentier,
J. F. Chem. Commun. 2000, 2183.
9
10.1021/ja0398262 CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 6387-6401
6387

A R T I C L E S
Giesbrecht et al.
order oligomers, illustrating the potential degree of control over
nuclearity in these systems. The coordination and reaction
chemistry of such species has been extensively documented and
has been the subject of much discussion.14,15
In contrast to the considerable literature available on lan-
thanide aryloxide (OAr) systems, much less is known about the
analogous arylamide (NHAr) derivatives. Although the chem-
istry of polydentate nitrogen-based systems is an increasingly
popular avenue of study,16,17 the chemistry of lanthanides
supported by simple amido ligands is generally dominated by
the bistrimethylsilyl derivative N(SiMe3)2 .
- 18-21 Considering the
ubiquitous presence of aryloxide ligands in lanthanide chem-
istry,22,23 it is curious that the corresponding arylamido chemistry
is so underdeveloped. Evans and co-workers were the first to
report the utility of such ligands with yttrium and the lan-
thanides,24 and we and others have since employed the 2,6-
diisopropylanilido ligand to advance the chemistry of lanthanide-
imido functionalities.25,26 Despite these reports, little is known
about the comparative behavior of aryloxide and arylamide
ligands when complexed to the lanthanides. Here we report
the synthesis and solution behavior of La2(NHAr)6 (Ar ) 2,6-
iPr2C6H3), and its complexation with Lewis bases. We compare
the observed chemistry of this arylamide system with the
structurally related aryloxide La2(OAr)6 (Ar ) 2,6-iPr2C6H3)27
and offer insights into the nature and relative strengths of
La-OAr versus La-NHAr bonding.
Figure 1. Thermal ellipsoid view of La2(NH-2,6-iPr2C6H3)6 (1) drawn with
30% probability ellipsoids and emphasizing the η6-arene and the local three-
legged piano stool geometry about the La metal center. Isopropyl methyl
groups have been omitted for clarity.
La2(NHAr)6 (1) in THF. Slow evaporation of the solvent yields
2 as large colorless crystals in good yield (eq 2). Compound 2
La2(NHAr)6 y+THF
z
(2)
La(NHAr) (THF)
3
3
-THF
2
has increased solubility compared to that of the unsolvated dimer
1, readily dissolving in aromatic solvents such as benzene or
toluene as well as in coordinating solvents such as THF. The
reaction outlined in eq 2 is found to be chemically reversible.
Removal of THF under dynamic vacuum over a toluene solution
regenerates dimeric 1 in near quantitative yield. Addition of
pyridine to a toluene solution of 1 yields the monomeric bis-
pyridine adduct La(NHAr)3(py)2 (3) in good yield (eq 3).
Results and Discussion
I. Synthesis and Reactivity. Reaction of monomeric
La[N(SiMe3)2]3 with 3 equiv of 2,6-diisopropylaniline in re-
fluxing toluene solution produces the bright yellow dimeric
complex La2(NHAr)6 (1) (Ar ) 2,6-iPr2C6H3) in moderate yield
(eq 1). Compound 1 is only sparingly soluble in pentane or
toluene, ∆
La (NHAr)
2La[N(SiMe3)2]3 + 6H2NAr
8
(1)
6
2
xs pyridine
-6HN(SiMe3)2
2La(NHAr) (py)
La2(NHAr)6
8
(3)
1
3
2
toluene
3
Ar ) 2,6-iPr2C6H3
Compound 3 is isolated as a bright yellow solid with similar
solubility characteristics to those of the tris-THF adduct 2. The
formation of the pyridine adduct 3 is not chemically reversible.
Compounds 1-3 are moisture sensitive but are indefinitely
stable when stored under an inert atmosphere, showing no signs
of decomposition after months at ambient temperature.
hexane but has increased solubility in aromatic solvents. This
is in contrast to the previously reported Y, Yb, and Sm ana-
logues, which were found to be virtually insoluble in nonco-
ordinating solvents.24,25 The higher solubility of the lanthanum
derivative may be attributed to the larger size of the La metal
center.28 The monomeric Lewis base adduct La(NHAr)3-
(THF)3 (2) is conveniently prepared by simple dissolution of
II. Solid-State and Molecular Structures. A. La2(NH-2,6-
iPr2C6H3)6 (1). Slow evaporation of a saturated toluene solution
of 1 yielded yellow crystalline blocks that were suitable for
X-ray diffraction. The solid-state molecular structure of 1 is
presented in Figure 1. A listing of relevant bond lengths and
angles is available in Table 1; complete details of the structural
analyses of compounds 1-3 are listed in Table 4. Compound 1
possesses a centrosymmetric π-arene-bridged dimeric structure
analogous to that reported for the Y and Sm analogues,24,25 as
well as the f-element diisopropylphenoxide dimers M2(OAr)6
(M ) La, Nd, Sm, U).6,27,29 In the solid-state structure of 1,
each La atom is ligated by three N atoms of arylamido ligands,
and each metal center engages in an η6-arene interaction with
the aryl ring of an arylamido ligand attached to an adjacent La
center. The three N atoms and the aryl ring centroid support a
(14) van der Sluys, W. G.; Sattelberger, A. P. Chem. ReV. 1990, 90, 1027.
(15) Barnhart, D. M.; Clark, D. L.; Gordon, J. C.; Huffman, J. C.; Watkin, J.
G.; Zwick, B. D. Inorg. Chem. 1995, 34, 5416 and references therein.
(16) Gade, L. H. Chem. Commun. 2000, 173.
(17) Kempe, R. Angew. Chem., Int. Ed. 2000, 39, 468.
(18) Alyea, E. C.; Bradley, D. C.; Copperwaite, R. G. Dalton Trans. 1972, 1580.
(19) Bradley, D. C.; Ghotra, J. G.; Hart, F. A. Dalton Trans. 1973, 1021.
(20) Ghotra, J. S.; Hursthouse, M. B.; Welch, A. J. Chem. Commun. 1973, 669.
(21) Evans, W. J.; Golden, R. E.; Ziller, J. W. Inorg. Chem. 1991, 30, 4963.
(22) Bradley, D. C.; Mehrotra, R. C.; Gaur, D. P. Metal Alkoxides; Academic
Press: London, 1978.
(23) Chisholm, M. H.; Rothwell, I. P. In ComprehensiVe Coordination Chemistry;
Wilkinson, G., Gillard, R., McCleverty, J. A., Eds.; Pergamon Press:
London, 1987; Vol. 2.
(24) Evans, W. J.; Ansari, M. A.; Ziller, J. W.; Khan, S. I. Inorg. Chem. 1996,
35, 5435.
(25) 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.
(26) Chan, H. S.; Li, H. W.; Xie, Z. Chem. Commun. 2002, 652.
(27) Butcher, R. J.; Clark, D. L.; Grumbine, S. K.; Vincent-Hollis, R. L.; Scott,
B. L.; Watkin, J. G. Inorg. Chem. 1995, 34, 5468.
(29) van der Sluys, W. G.; Burns, C. J.; Huffman, J. C.; Sattelburger, A. P. J.
Am. Chem. Soc. 1988, 110, 5924.
(28) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751.
9
6388 J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004

π-Arene-Bridged Lanthanum Arylamide and Aryloxide Dimers
A R T I C L E S
Table 1. Selected Bond Distancesa (Å) and Angles (deg) for
La2(NH-2,6-iPr2C6H3)6 (1)
La1-N1
La1-N3
La1-C1
2.434(3)
2.360(3)
3.209(4)
3.028(3)
2.995(4)
La1-N2
La1-arene(cent)
La1-C2
La1-C4
La1-C6
2.347(4)
2.730
3.129(4)
2.972(3)
3.080(4)
93.12(11)
122.37(12)
La1-C3
La1-C5
N1-La1-N2
N1-La1-arene(cent) 98.47
N2-La1-arene(cent) 122.11
La1-N1-C1
La1-N3-C25
111.27(11) N1-La1-N3
N2-La1-N3
N3-La1-arene(cent) 103.32
La1-N2-C13 149.3(2)
151.0(3)
136.4(3)
a cent ) centroid of C6H5.
pseudo-tetrahedral geometry around the central metal atom, with
the angles anchored by the La atom ranging from 93.12(11)°
to 122.37(12)°. The coordination geometry of each La atom
approximates that of a three-legged piano stool. The distances
between the La metal center and the six C atoms of the arene
ring range from 2.972(3) to 3.209(4) Å with an average distance
of 3.069(4) Å (La(1)-arene(cent) ) 2.730 Å). This average
La-C bond distance is similar to those reported for the
structurally related aryloxide species La2(O-2,6-iPr2C6H3)6
(La-C ave ) 3.062(10) Å; range ) 2.978(10)-3.164(9) Å)27
and are somewhat longer than the Sm-Carene bonds found in
the Sm analogue Sm2(NHAr)6 (Sm-C ave ) 2.952(10) Å; range
) 2.838(10)-3.094(10) Å),25 in accord with the larger ionic
radius of La. The La-N distances for terminal arylamido ligands
in 1 (2.347(4) and 2.360(3) Å) are slightly shorter than the
bridging La-N distance (2.434(3) Å) and are in the expected
range reported for other lanthanum-amido bonds. Again, the
La-N bond lengths are slightly longer than those observed in
Sm2(NHAr)6. The La-N-C bond angle for the bridging ligand
is 151.0(3)° and the La-N-C bond angles for the terminal
ligands are 136.4(3)° and 149.3(2)°.
Figure 2. Thermal ellipsoid view of La(NH-2,6-iPr2C6H3)3(THF)3 (2) drawn
with 30% probability ellipsoids and illustrating the facial arrangement of
NHAr and THF ligands. Isopropyl methyl groups have been omitted for
clarity.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
La(NH-2,6-iPr2C6H3)3(THF)3 (2)
La1-N1
2.379(6)
2.395(4)
2.656(3)
107.7(2)
160.0(2)
85.6(2)
La1-N2
2.401(5)
2.688(4)
2.671(4)
107.9(2)
87.1(2)
119.9(2)
76.7(1)
81.0(2)
81.8(2)
77.9(1)
143.1(4)
158.6(6)
La1-N3
La1-O1
La1-O2
La1-O3
N1-La1-N2
N1-La1-O1
N1-La1-O3
N2-La1-O1
N2-La1-O3
N3-La1-O2
O1-La1-O2
O2-La1-O3
La1-N2-C7
N1-La1-N3
N1-La1-O2
N2-La1-N3
N2-La1-O2
N3-La1-O1
N3-La1-O3
O1-La1-O3
La1-N1-C1
La1-N3-C13
81.6(2)
147.3(1)
150.7(2)
77.8(1)
74.2(1)
149.4(4)
B. La(NH-2,6-iPr2C6H3)3(THF)3 (2). Slow evaporation of
a saturated THF solution of 2 yielded large colorless blocks
that were amenable to X-ray diffraction. The molecular structure
of 2 is shown in Figure 2, with selected bond lengths and angles
given in Table 2. The complex crystallizes as discrete molecules
with no unusual intermolecular contacts. The three arylamido
ligands and the three THF molecules in 2 define a distorted
octahedron with the NHAr groups arranged in a facial geometry
about the La center. The angles involving the arylamido ligands
are all larger than 90° (N1-La1-N2 ) 107.7(2)°, N1-La1-
N3 ) 107.9(2)°, N2-La1-N3 ) 119.9(2)°) whereas those
involving THF groups are all smaller than 90° (O1-La1-O2
) 77.8(1)°, O1-La1-O3 ) 77.9(1)°, O2-La1-O3 ) 74.2-
(1)°), reflecting the steric demands of the diisopropylanilido
groups. The overall geometric features are similar to those
Nd. The La-O(THF) distances (2.672(3) Å ave) are signifi-
cantly longer than those observed in La(O-2,6-iPr2C6H3)3(THF)2
(2.52(1) Å ave) as well as other THF adducts of La such as
La(η5-C5Me5)[CH(SiMe3)2]2(THF) (2.547(6) Å),30 La(η5-C5H5)3-
(THF) (2.57(1) Å),31 (η5:η1:η1-C5H3(CH2CH2NMe2)2-1,2)LaI2-
(THF) (2.558(3) Å) and (η5:η1:η1-C5H3(CH2CH2NMe2)2-1,3)-
LaI2(THF) (2.593(3) Å),32 although the axial THF La-O(THF)
bond length found in La(O-2,6-Ph2C6H3)3(THF)2 is similar to
that reported here (2.67(2) Å).33
C. La(NH-2,6-iPr2C6H3)3(py)2 (3). Cooling a saturated
toluene solution of 3 to -30 °C yielded the bis-pyridine adduct
as large yellow plates. The molecular structure of 3 is presented
in Figure 3 with selected bond lengths and angles given in Table
3. Complex 3 maintains a similar geometry to that previously
reported for Ln(OAr)3(THF)2 (Ln ) La, Pr, Gd, Er, Lu; Ar )
2,6-iPr2C6H3)6,27 and consists of a distorted trigonal-bipyramidal
determined for the Nd analogue Nd(NH-2,6-iPr2C6H3)3(THF)3
24
and the aryloxide complex Y(O-2,6-Me2C6H3)3(THF)3.8 In
contrast, the La diisopropylphenoxide compound La(O-2,6-
iPr2C6H3)3(THF)2 crystallizes as a bis-THF adduct, resulting in
a trigonal bipyramidal metal center.27 The La-N distances in
2 (2.379(6) - 2.401(5) Å) are slightly longer than the La-O
i
distances reported for La(OC6H3 Pr2-2,6)3(THF)2 (2.169(7) -
(30) van der Heijden, H.; Schaverien, C. J.; Orpen, A. G. Organometallics 1989,
2.233(8) Å), resulting in a less crowded coordination sphere,
allowing for the presence of an additional THF ligand. The
La-N distances are also longer than the analogous metal-
arylamido distances in Nd(NH-2,6-iPr2C6H3)3(THF)3 (2.304(8)
- 2.320(8) Å) as a result of the larger ionic radius of La versus
8, 255.
(31) Rogers, R. D.; Atwood, J. L.; Emad, A.; Skidora, D. J.; Rausch, M. D. J.
Organomet. Chem. 1981, 216, 383.
(32) Fedushkin, I. L.; Dechert, S.; Schumann, H. Organometallics 2000, 19,
4066.
(33) Deacon, G. B.; Feng, T.; Skelton, B. W.; White, A. H. Aust. J. Chem.
1995, 48, 741.
9
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A R T I C L E S
Giesbrecht et al.
La-N distance to arylamide ligands is 2.374(3) Å and well
within the range observed in 1 and 2. The axial pyridine ligands
bend away from this unique arylamido ligand, resulting in an
axial N-La-N angle of 150.23(11)°, which is significantly
smaller than the expected 180° for a trigonal-bipyramidal
molecule. A slight distortion also exists between the arylamido
ligands in the equatorial plane, with one smaller N-La-N angle
(N1-La1-N2 ) 91.87(13)°) and two larger N-La-N angles
(N1-La1-N3 ) 133.58(12)°; N2-La1-N3 ) 134.53(13)°).
The La-Narylamido-Cipso bond angles are all approximately 150°
and similar to those reported for the tris-THF adduct 2. The
bonds between the La atom and the N atoms of the two pyridines
are longer than those to the arylamido ligands (La1-N4 )
2.707(3) Å; La1-N5 ) 2.729(3) Å). A comparison to other
similar compounds is unavailable as solid-state structures of
species of the general type Ln(XAr)3(py)x (Ln ) lanthanide; X
) O, NH; x ) 2, 3) have not been reported; however, the related
thorium aryloxide complex Th(O-2,6-Me2C6H3)4(py)2 exhibits
Th-N distances of 2.662(8) and 2.696(8) Å.34
Figure 3. Thermal ellipsoid view of La(NH-2,6-iPr2C6H3)3(py)2 (3) drawn
with 30% probability ellipsoids and illustrating the trigonal-bipyramidal
coordination between two trans py and three equatorial NHAr ligands.
Isopropyl methyl groups have been omitted for clarity.
III. Spectroscopic Characterization. A. Vibrational Spec-
troscopy. Infrared (IR) spectroscopy has been shown to be
diagnostic in the identification of η-arene-bridged dimeric
structures for lanthanide and actinide compounds.6,27 Infrared
spectroscopy shows two distinct aromatic ν(CdC) vibrational
modes consistent with two different arene environments for
π-bound bridging and terminal aryloxide ligands in M2(OAr)6
compounds both in solution and in the solid state. For example,
the IR spectra of Ln2(µ-η6-OAr)2(OAr)4 (Ln ) Sm, Nd, Er, and
La) reveal a higher frequency band near 1590 cm-1 in Nujol
and at 1588 cm-1 in benzene solution. The second band appears
at 1572 cm-1 in both the solid state and in solution. The higher
energy band is twice the intensity of the lower energy band
and corresponds to the terminal aryloxide ligands. The lower
energy frequency indicates a weakening of the CdC bond and
is consistent with a π-arene bridging interaction. This lower
energy feature is absent in all monomeric Lewis base adducts
(Table 5). Similar frequencies were also identified for η6-arene
coordination of Ln(SAr*)2 (Ln ) Eu, Yb; Ar* ) 2,6-Trip2C6H3;
Trip ) 2,4,6-iPr3C6H2).35
Table 3. Selected Bond Distances (Å) and Angles (deg) for
La(NH-2,6-iPr2C6H3)3(py)2 (3)
La1-N1
2.370(4)
2.397(4)
2.729(3)
91.87(13)
94.35(12)
134.53(13)
83.83(11)
79.58(11)
143.8(3)
149.8(3)
La1-N2
La1-N4
2.354(3)
2.707(3)
La1-N3
La1-N5
N1-La1-N2
N1-La1-N4
N2-La1-N3
N2-La1-N5
N3-La1-N5
La1-N1-C1
La1-N3-C13
N1-La1-N3
N1-La1-N5
N2-La1-N4
N3-La1-N4
N4-La1-N5
La1-N2-C7
133.58(12)
112.94(11)
83.63(10)
90.32(11)
150.23(11)
151.7(3)
Table 4. Crystallographic Dataa
1
2
3
formula
mol wt
temp, K
C72H108N6La2
1335.46
203(2)
C52H86LaN3O4
956.15
203(2)
C46H64LaN5
825.93
203(2)
crystal system
space group
crystal size, mm
monoclinic
P21/n
monoclinic
P21
monoclinic
P21
0.25 × 0.21 ×
0.30 × 0.20 ×
0.29 × 0.21 ×
0.12
0.14
0.12
In the present study we employ both IR and Raman spec-
troscopy and therefore report the Raman of the aryloxide ana-
logue for comparison purposes. The solid-state (Nujol) vibra-
tional spectra of La2(NH-2,6-iPr2C6H3)6 (1) exhibit two distinct
ν(CdC) stretching modes in the aromatic region at 1587 and
1575 cm-1 in the IR and 1589 and 1576 cm-1 in the Raman.
This is consistent with maintenance of two different arene
environments in solid state, as expected on the basis of the
solid-state molecular structure. These two sets of vibrational
modes are also evident in both the IR and Raman spectra of
La2(O-2,6-iPr2C6H3)6 (4) (Figure 4), with Nujol IR frequencies
at 1588 and 1571 cm-1 and with the corresponding Raman
frequencies at 1591 and 1572 cm-1. This is fully consistent with
the presence of the η-arene-bridged dimeric structure observed
in the solid state. In comparison, La(NHAr)3(THF)3, with only
terminal arylamido ligands, exhibits only one peak in the region
at 1588 (IR) and 1589 (Raman) cm-1 in the solid state. Vibra-
tional spectra of monomeric La(NHAr)3(py)2 consist of two
a, Å
b, Å
c, Å
â, deg
V, Å3
Z
14.066(3)
15.480(3)
16.147(3)
92.652(4)
3512.2(11)
4
1.263
1.242
1392
1.8-25.4
22592
6185
1.223
0.0452
0.0758
13.137(4)
16.922(4)
13.442(3)
119.253(3)
2607.3(11)
2
1.218
0.862
1016
1.74-25.36
16872
9402
1.274
0.0480
0.0956
16.013(6)
14.159(5)
21.202(6)
112.122(11)
4453(3)
4
1.232
0.994
1728
1.04-25.52
28486
14478
1.164
0.0279
0.0719
D
calc, g/cm3
abs coeff, mm-1
F(000)
θ range, deg
total reflcns
indep reflcns
GOF
R1
wR2
a R1) Σ||Fo|-|Fc||/Σ|Fo| and R2w) [Σ[ω(Fo2 - Fc )2]/Σ[ω(Fo )2]]1/2; ω
2
2
2
) 1/[σ2(Fo ) + (aP)2], where a ) 0.0326, 0.0419, and 0.0454.
metal center with three equatorial arylamide ligands and two
axial py ligands. A feature common to 3 as well as Ln(O-2,6-
iPr2C6H3)3(THF)2 species is that two arylamido ligands lie with
their phenyl rings almost in the plane of the trigonal bipyramid,
while the remaining group (containing N1) is twisted, such that
the aryl ring is almost perpendicular to the plane. The average
(34) Berg, J. M.; Clark, D. L.; Huffman, J. C.; Morris, D. E.; Sattelberger, A.
P.; Streib, W. E.; van der Sluys, W. G.; Watkin, J. G. J. Am. Chem. Soc.
1992, 114, 10811.
(35) Niemeyer, M. Eur. J. Inorg. Chem. 2001, 1969.
9
6390 J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004

π-Arene-Bridged Lanthanum Arylamide and Aryloxide Dimers
A R T I C L E S
Table 5. Infrared Vibrational Frequencies (cm-1) for the ν(CdC)
Stretch in 2,6-Diisopropylphenoxide and 2,6-Diisopropylanilide
Complexes
to be so similar that they would not be resolved as two dis-
tinct bands. This is the case for the oxygen-bridged complex
La2(µ-O-2,6-iPr2OC6H3)2(O-2,6-iPr2OC6H3)4(NH3)2,27 which shows
only a single set of ν(CdC) vibrational modes at 1583 cm-1
by IR spectroscopy. Thus the combination of IR and Raman
spectroscopy clearly indicate that the π-arene bridge is main-
tained in both the solid state and in solution. This understanding
gleaned from the vibrational spectra is important for our
interpretation of solution behavior monitored by 1H NMR
spectroscopy.
B. Room-Temperature H NMR Spectroscopy. The H
NMR spectrum of 1 recorded under ambient conditions reveals
arylamido groups in a 2:1 ratio, consistent with the presence of
one bridging and two terminal -NHAr groups per metal center.
The NH protons appear as two broad singlets of 1:2 ratio at
5.37 and 5.12 ppm, consistent with our interpretation (vibrational
spectroscopy) that the π-arene-bridged dimeric structure is
maintained in solution. Despite the presence of diastereotopic
methyl groups in 1, we were only able to observe a single
resonance arising from the isopropyl methyl groups of the
terminal arylamido groups. The results of variable-temperature
1H NMR studies of this compound and its aryloxide congener
will be presented later.
compound
ν(CdC)
medium
ref
Arylamides
La2(NHAr)6
La2(NHAr)6
Sm2(NHAr)6
Sm2(NHAr)6
La(NHAr)3(THF)3
La(NHAr)3(py)2
KNHAr
1587
1575
1572
1575
1572
Nujol
benzene
Nujol
benzene
Nujol
Nujol
Nujol
neat
this work
this work
25
1588
1590
1590
1588
1588
1581
1588
25
this work
this work
this work
this work
1
1
H2NAr
Aryloxides
1571
La2(OAr)6
La2(OAr)6
Nd2(OAr)6
Nd2(OAr)6
Sm2(OAr)6
Sm2(OAr)6
Er2(OAr)6
Er2(OAr)6
1588
1588
1588
1590
1586
1589
1590
1589
1588
1587
1584
1585
1585
1585
1586
1586
1586
1583
1581
1584
1585
Nujol
benzene
Nujol
benzene
Nujol
benzene
Nujol
benzene
Nujol
Nujol
Nujol
Nujol
Nujol
Nujol
Nujol
Nujol
Nujol
Nujol
Nujol
Nujol
neat
27
27
6
6
6
6
6
6
29
27
6
6
6
6
6
6
6
1572
1571
1572
1572
1572
1571
1572
1553
U2(OAr)6
La(OAr)3(THF)2
Pr(OAr)3(THF)2
Nd(OAr)3(THF)2
Sm(OAr)3(THF)2
Gd(OAr)3(THF)2
Er(OAr)3(THF)2
Yb(OAr)3(THF)2
Lu(OAr)3(THF)2
La2(OAr)6(NH3)2
KLa(OAr)4
The room-temperature 1H NMR spectrum of 2 in d8-THF is
consistent with a monomeric Lewis base adduct in solution. The
spectrum shows one type of arylamido ligand and one type of
THF ligand environment in a 1:1 ratio, indicative of the
formation of a tris-THF adduct which adopts either a static fac-
octahedral geometry in solution or possesses a mer geometry
with ligands rapidly equilibrating on the NMR time scale.
Monitoring the stepwise addition of THF to La2(NHAr)6 (1)
27
79
27
27
LiOAr
HOAr
pyridine ν(CdC) modes, one at 1568 (IR) and 1569 (Raman)
and another at 1594 cm-1 (IR and Raman)36-38 and one
ν(CdC) mode predominantly from the arylamido ligand (IR )
1588 cm-1; Raman ) 1589 cm-1). This latter peak may be
broadened by the pyridine mode, but the single-peak structure
from the arylamido ligand is consistent with the terminal nature
of the ligand. These arylamido ν(CdC) modes for the mono-
meric THF and pyridine adducts of the arylamide compare
favorably with that determined for the monomeric aryloxide
species, La(OAr)3(THF)2 (1587 cm-1).27
1
by H NMR reveals the disappearance of resonances arising
from dimeric 1 and the growth of peaks due to the tris-THF
complex 2. Figure 5a shows the results of the addition of
between 1 and 6 equiv of THF to La2(NHAr)6 (1) in C6D6. For
the sake of clarity, only the arylamido NH region of the spectra
is shown. The initial spectrum consists of two peaks in a 2:1
ratio at 5.37 (A1) and 5.12 ppm (A2), consistent with the
presence of La2(NHAr)6 (1) only. As THF is added to the
sample, these signals due to 1 decrease in intensity, while the
resonance at 5.08 ppm (B), corresponding to the tris-THF adduct
La(NHAr)3(THF)3 (2), grows in. Additionally, a small peak at
5.28 ppm (C) is also observed. We suggest that this peak
arises from the presence of a small quantity of the more
traditional nitrogen-bridged dimer La2(µ-NHAr)2(NHAr)4(THF)2
(III) illustrated in Scheme 1. We have previously shown that
the addition of NH3 to the π-arene-bridged aryloxide dimer
La2(OAr)6 to form the tetra-ammonia adduct La(OAr)3(NH3)4
passes through the oxygen-bridged aryloxide dimer La2(µ-OAr)2-
(OAr)4(NH3)2.27 It may be that a similar interconversion is
present in this case, although we note that we were unable to
isolate La2(µ-NHAr)2(NHAr)4(THF)2 (III) free from other
arylamido-containing species, and thus the assignment of the
Of significance is that the IR spectrum of 1 recorded in
benzene solution also shows two ν(CdC) frequencies at 1588
and 1572 cm-1, suggesting the solid-state η6-arene-bridged
structure (I) is maintained in solution. Although a dimeric
1
peak at 5.28 ppm in the H NMR spectrum to this complex
remains speculative at this time (Scheme 1).
Interestingly, the addition of THF to the arylamido-bridged
dimer 1 is reversible; if a solid sample of the tris-THF adduct
structure involving arylamido ligands bridging through nitrogen
would also possess two types of arylamido ligands (II), the
ν(CdC) stretching frequencies in this case would be expected
(36) Kline, C. H.; Turkevich, J. J. Chem. Phys. 1944, 12, 300.
(37) Wilmshurst, J. K.; Bernstein, H. J. Can. J. Chem. 1957, 35, 1183.
(38) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination
Compounds, 4th ed.; Wiley-Interscience: New York, 1986.
9
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A R T I C L E S
Giesbrecht et al.
Figure 4. V(CdC) stretching region of the solid-state infrared (left) and Raman (right) spectra of La2(O-2,6-iPr2C6H3)6 (4), La2(NH-2,6-iPr2C6H3)6 (1),
La(NH-2,6-iPr2C6H3)3(THF)3 (2), and La(NH-2,6-iPr2C6H3)3(py)2 (3) (top to bottom). Asterisk denotes peaks arising from pyridine solvent.
2 is dissolved in toluene and subjected to prolonged exposure
to vacuum, compound 1 is obtained. The results of this H
ranging from 0 to 60 °C. In general, the isopropyl methyl groups
appear as doublets due to coupling to a single methine proton.
At high temperature, all of the isopropyl methyl groups are
averaged to give a single doublet at 1.25 ppm, indicating a rapid
bridge-terminal ligand exchange process. Cooling the sample
results in decoalescence and a slowing of the exchange process
until two doublets of 2:1 intensity at 1.29 and 1.17 ppm are
frozen out at 0 °C, suggesting that a dimeric structure is retained
in solution. Line-shape analysis allowed for the extraction of
exchange rate constants at various temperatures; the subsequent
Arrhenius plot yielded activation parameters of ∆Gq ) 14.4
1
NMR experiment are shown in Figure 5b. The initial sample
of La(NHAr)3(THF)3 (2) reveals a single peak at 5.08 ppm (B);
exposure of this sample to vacuum for 12 h results in the
regeneration of peaks at 5.37 and 5.12 ppm (A1 and A2),
consistent with the re-formation of the π-arene-bridged dimer
1. This type of behavior has not been reported for the aryloxide
derivative, for which adduct formation with THF or pyridine is
irreversible.27 Interestingly, after 3 h under dynamic vacuum,
the dimer 1 and the tris-THF adduct 2 are visible, as well as
the purported nitrogen-bridged dimer La2(µ-NHAr)2(NHAr)4-
(THF)2 (III), lending further credence to the interconversion
postulated in Scheme 1.
The room-temperature 1H NMR spectrum of 3 in d5-pyridine
is consistent with expectations for a monomeric complex; the
spectrum reveals a single type of arylamido group in solution.
The addition of pyridine to compound 1 does not appear to be
reversible, as exposure of a d8-toluene solution of 3 to vacuum
does not result in any change in the 1H NMR spectrum. In this
sense, the bis-pyridine adduct 3 behaves more like the aryloxide
species La(OAr)3(THF)2 and La(OAr)3(py)3, for which solvent
loss was not observed.
C. Variable-Temperature NMR Spectroscopy. 1. Arylox-
ide System. In view of the structural similarities between the
arylamide dimer La2(NH-2,6-iPr2C6H3)6 reported here and the
related aryloxide La2(O-2,6-iPr2C6H3)6 (4) reported previously,
we were interested in exploring the relative effect that arylamide
versus aryloxide substituents would have on the solution be-
havior. Thus, we undertook a more detailed study of compound
4 in order to determine the kinetic parameters associated with
the fluxional process operative in solution.
kcal mol-1, ∆Sq ) 5 cal K-1 mol-1, ∆Hq ) 16.0 kcal mol-1
,
and Ea ) 16.5 kcal mol-1. The slightly positive value of ∆Sq
implies that the aryloxide ligands are averaged via a dissociative
transition state, although the small value of ∼5 cal K-1 mol-1
suggests only a minimal rearrangement occurs. This allows us
to propose a possible mechanism by which the bridging and
terminal ligands are exchanged, as well as discount other
possibilities. One possibility is that the dimer breaks apart into
two monomeric units and rapidly re-forms in solution; this
would account for the averaging of all three aryloxide ligands.
However, there is no evidence by 1H NMR spectroscopy for a
monomer-dimer equilibrium. A second possible means of
exchange is outlined in Scheme 2, whereby the π-arene-bridged
dimer converts to a dimeric species which is bridged via the
aryloxide oxygen atoms; such a structure has been observed
for the Lewis base stabilized complex La2(µ-O-2,6-iPr2C6H3)2-
(O-2,6-iPr2C6H3)4(NH3)227 as well as other f-element complexes
such as Th2(OCH-i-Pr2)8 and U2(NEt2)8.39 However, the
15
mechanism presented in Scheme 2 does not exchange the bridge
and terminal aryloxide groups, but only the metal center with
which the bridging aryloxide group is interacting.
Figure 6 shows a stacked plot of the isopropyl methyl region
of the H NMR spectra of 4 in C7D8 solution at temperatures
(39) Reynolds, J. G.; Zalkin, A.; Templeton, D. H.; Edelstein, N. M.; Templeton,
1
L. K. Inorg. Chem. 1976, 15, 2498.
9
6392 J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004

π-Arene-Bridged Lanthanum Arylamide and Aryloxide Dimers
A R T I C L E S
1
1
Figure 5. (a) 500.13 MHz H NMR spectra of the arylamido NH region of La2(NH-2,6-iPr2C6H3)6 (1) upon addition of THF. (b) 500.13 MHz H NMR
spectra of the arylamido NH region of La(NH-2,6-iPr2C6H3)3(THF)3 (2) upon exposure to vacuum.
Scheme 1
It is also possible that an intermediate involving more than
two oxygen-bridged aryloxide groups is involved (Scheme 3).
This would allow for bridging and terminal groups to be
exchanged; however, this may be discounted on the basis of
the large negative value of ∆Sq that would be expected for such
an ordered transition state.
Scheme 4 reveals a mechanism that is consistent with all of
the experimental data. In this mechanism, (a) one of the π-arene
interactions between a lanthanum metal center (La1) and a
bridging aryloxide group (O1) is cleaved, while the other
metal-π-arene bond within the dimer remains intact, (b) the
La(OAr)3 unit rotates 120° about the La2-arene bond, and
(c) an aryloxide group which was originally in a terminal
position (O3) re-coordinates to La1, with a new π-arene
interaction becoming a bridging aryloxide unit. The transition
state for this exchange is only minimally more disordered than
the starting dimer, consistent with the small positive value of
∆Sq. Although we have only shown the exchange involving one
lanthanum center, the same dissociation-rotation-reassociation
can also occur at the second metal site. If this mechanism is
operative in this complex, the ∆Hq value of 16.0 kcal mol-1
corresponds to the energy required to break the π-arene bond.
This number correlates well with theoretical calculations of the
strength of such interactions in model compounds containing a
benzene ring bound in an η6-manner to a lanthanide center (see
later).
9
J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004 6393

A R T I C L E S
Giesbrecht et al.
1
2. Arylamide System. The variable-temperature H NMR
spectra of La2(NH-2,6-iPr2C6H3)6 (1) also show evidence for
fluxional behavior. The isopropyl methyl region of the 1H NMR
spectra of 1 show the same basic features as those found for
the aryloxide derivative 4, although the coalescence of two
doublets (1.31 and 1.26 ppm) to one doublet (1.29 ppm) occurs
between 25 and 90 °C, approximately 30° higher than that for
compound 4. As for compound 4, line-shape analysis and the
subsequent Arrhenius plot yielded activation parameters: ∆Gq
) 15.1 kcal mol-1, ∆Sq ) 24 cal K-1 mol-1, ∆Hq ) 22.5 kcal
mol-1, and Ea ) 23.1 kcal mol-1. The positive value of ∆Sq
suggests that the bridging and terminal arylamido ligands are
averaged by a dissociative mechanism. However, the value
determined for the arylamide system is much larger than that
observed for the aryloxide species (24 vs 5 cal K-1 mol-1).
Thus, it is unlikely that the same mechanism is operative in
the two systems. In the arylamido case, it is probable that the
dimer breaks apart into two monomers (eq 4) and randomly
re-forms, which would average the arylamido groups be-
tween the bridging and terminal positions. This postulate is
supported by the observation of a monomer-dimer equilibrium
at elevated temperatures, which was not found for 4 (see later).
One must be careful when comparing ∆Sq values, since these
numbers are small and subject to considerable error. We
determined the error in this parameter to be approximately 5
cal K-1 mol-1; thus we can say with some certainty that both
systems involve a mechanism with a transition state that is more
disordered than the reactants. Also, despite the error associated
with ∆Sq, it appears that the fluxionality occurring in the
arylamido complex 1 involves a more disordered transition state
than the aryloxide complex 4 and therefore a different mech-
anism.
Figure 6. Variable-temperature 500.13 MHz 1H NMR spectra of the
isopropyl methyl region of La2(O-2,6-iPr2C6H3)6 (4).
amido groups, respectively. As the temperature is increased, a
third peak at 4.73 ppm (D) grows in. We suggest that this
peak arises from the monomeric species La(NH-2,6-iPr2C6H)3,
which contains only one type of arylamido group. As the sample
is heated, resonances assigned to the dimer 1 are seen to
decrease, while those resulting from the monomer are observed
to increase, although even at 90 °C the dimer 1 is still the
predominant species in solution. When cooled back to room
temperature, the resonances of the monomer quickly disappear
until only the dimer is visible, implying a low barrier to
dimerization.
We note that as the temperature is increased to 50 °C, the
NH peak at 5.35 ppm (A1) appears to separate into a broad
resonance at 5.40 ppm and a smaller, sharper peak at 5.46 ppm.
This suggests that a further equilibrium and/or dynamic process
may be occurring in addition to the monomer-dimer equilib-
rium proposed in eq 4. Although the process that gives rise to
this splitting was not investigated, we believe the presence of
this minor peak has a minimal effect on the calculation of
thermodynamic parameters for this system, and the equilibrium
described in eq 4 is the major mode by which the bridging and
terminal groups are exchanged.
In addition to the fluxional behavior described above, at
elevated temperatures there is evidence for an equilibrium
between dimeric La2(µ-NH-2,6-iPr2C6H)6 (1) and a monomeric
species such as La(NH-2,6-iPr2C6H)3 (eq 4). Figure 7 shows a
To confirm that peak D in Figure 7 corresponds to a
i
monomeric species such as La(NHC6H3 Pr2-2,6)3, we also
1
obtained an H NOESY NMR spectrum of the sample (Figure
8). To observe any possible exchange between the dimer 1
(peaks A1 and A2 in Figure 7) and a monomeric lanthanum
arylamido species (peak D in Figure 7), we performed the
experiment at 45 °C, where all three resonances were visible in
1
the 1D H NMR spectrum. The presence of large cross-peaks
between the resonances at 5.45 (A1) and 5.19 (A2) ppm
confirms that rapid bridge-terminal exchange occurs within the
dimer at this temperature. Also, small cross-peaks are visible
between the resonance at 4.90 ppm (D, monomer) and those at
5.45 and 5.19 ppm, indicating that the arylamido groups of
monomeric La(NH-2,6-iPr2C6H)3 exchange with both the bridg-
ing and terminal positions of the arylamido groups in La2(NH-
2,6-iPr2C6H)6 (1), supporting the existence of a monomer-dimer
equilibrium in solution.
stacked plot of the 1H NMR spectrum of 1 in C7D8 solution at
temperatures from 25 to 90 °C, in which the arylamido NH
region of the spectra have been expanded. At room temper-
ature, two broad singlets of ratio 1:2 are visible at 5.35 (A1)
and 5.08 ppm (A2), due to the bridging and terminal aryl-
9
6394 J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004

π-Arene-Bridged Lanthanum Arylamide and Aryloxide Dimers
A R T I C L E S
Scheme 2
Scheme 3
Scheme 4
Thermodynamic parameters for the equilibrium were deter-
mined by measuring the relative concentrations of monomer
and dimer over a range of temperatures. A plot of ln(Keq) vs
alkoxide complex shown in eq 5, in which ∆H° ) 17 kcal
mol-1, ∆G°298 ) 5 kcal mol-1, and ∆S° ) 40 cal K-1 mol-1 15
.
1/T yielded the following parameters: ∆H° ) 20 kcal mol-1
,
Th2(OCHiPr2)8 h 2 Th(OCHiPr2)4
(5)
∆G°298 ) 4 kcal mol-1, ∆S° ) 55 cal K-1 mol-1, and Keq
)
298
1.19 × 10-3. These values may be compared with those found
for the monomer-dimer equilibrium process for the thorium
In comparing the two systems, both the entropy and enthalpy
terms are found to be similar for the two equilibria. The close
9
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A R T I C L E S
Giesbrecht et al.
1
Figure 7. Variable-temperature 500.13 MHz H NMR spectra of the arylamido NH region of La2(NH-2,6-iPr2C6H3)6 (1).
rium, i.e., a mechanism such as that outlined in Scheme 4 is
not valid for compound 1. The observation of similar values of
∆H° (20 kcal mol-1) and ∆Hq (22.5 kcal mol-1) for the
formation of a dative bond with a neutral donor implies that
the exchange of bridging and terminal arylamido groups occurs
through a single pathway.
IV. Theoretical Calculations. A. Structural Results. The
experimental studies indicate that the relative strength of the
La-η-arene bonds follow the order OAr > NHAr. To gain a
better understanding of the nature of the π-arene interactions
in compounds 1 and 4, DFT calculations were carried out on
simplified model complexes La2(OC6H5)6, La2(NHC6H5)6, and
(C6H5R)La(XC6H5)3, where X ) O or NH and R ) H, OH or
NH2. Remarkably, the geometry optimization of the model
dimers using the B3YLP functional produced π-arene-bridged
geometries for both systems. The calculated structures of the
model dimer complexes La2(µ-NHPh)2(NHPh)4 and La(µ-OPh)2-
(OPh)4 are shown in Figure 9, and the relevant calculated
structural parameters are given in Table 6. While the calcula-
tions reproduce the overall experimental structures, the bond
lengths are all slightly overestimated. Larger differences are
noted in the La-C contacts of the π-arene groups, where the
average La-C distance is overestimated by ∼0.13 Å in the
arylamide complex and ∼0.15 Å in the aryloxide complex. The
interactions involving the π-arene moieties will be discussed
below.
B. Electronic Structure. The principal metal-ligand bonding
interaction in three-coordinate lanthanide LnX3 complexes arises
from a σ bonding interaction between occupied ligand lone pair
orbitals and empty metal 5d, 6s, and 6p acceptor orbitals. As
discussed in previous papers on tris-amide and tris-alkyl
complexes,25,40-44 the resultant metal-ligand bond is variously
described as ranging from highly polar to ionic with the
lanthanide ion having significant positive charge. In addition,
for both tris-amide and tris-arylamide complexes, there is also
a π lone pair orbital on the nitrogen ligand. The σ and π
symmetry frontier orbitals of the NHPh ligand are shown
Figure 8. 500.13 MHz 1H NOESY spectrum of the arylamido NH region
of La2(NH-2,6-iPr2C6H3)6 (1) in C7D8 at 318 K indicating bridge-terminal
exchange and monomer-dimer equilibrium.
correlation of the entropy change may be explained by the
similarity of the structures of the thorium and lanthanum
complexes. The enthalpy values for the two systems are also
within the same range (20 kcal mol-1 for La; 17 kcal mol-1 for
Th), implying that the π-arene bridging mode is energetic-
ally similar to the alkoxide-bridged structure observed for
Th2(OCHiPr2)8. Thus, it is interesting that no comparable mono-
mer-dimer equilibrium is observed for the aryloxide complex
La2(µ-O-2,6-iPr2C6H)6 (4), which remains dimeric (I) at all
temperatures under study. Additionally, there is never evidence
that complex 4 bridges through the aryloxide oxygen atoms (II)
by variable-temperature 1H NMR spectroscopy. Thus, the
π-arene interaction in La2(O-2,6-iPr2C6H)6 (4) must be stronger
than the analogous interaction observed in the arylamido
derivative La2(NH-2,6-iPr2C6H3)6 (1).
(40) Maron, L.; Eisenstein, O. J. Phys. Chem. A 2000, 104, 7140.
(41) Maron, L.; Eisenstein, O. New J. Chem. 2001, 25, 255.
(42) Perrin, L.; Maron, L.; Eisenstein, O.; Lappert, M. F. New J. Chem. 2003,
27, 121.
(43) Clark, D. L.; Gordon, J. C.; Hay, P. J.; Martin, R. L.; Poli, R. Organo-
metallics 2002, 21, 5000.
(44) Brady, E. D.; Clark, D. L.; Gordon, J. C.; Hay, P. J.; Keogh, D. W.; Poli,
R.; Scott, B. L.; Watkin, J. G. Inorg. Chem. 2003, 42, 6682.
The thermodynamic parameters presented for compound 1
also suggest that the bridging and terminal arylamido groups
are only exchanged as a result of a monomer-dimer equilib-
9
6396 J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004

π-Arene-Bridged Lanthanum Arylamide and Aryloxide Dimers
A R T I C L E S
Table 6. Comparison of Selected Calculated and Experimental
Structural Parameters for La2(XAr)6 (X ) O, NH; Ar ) 2,6-iPr2C6H3
(exp), Ph (calc))
experimental
calculated
La2(µ-NHAr)2(NHAr)4 La2(µ-NHPh)2(NHPh)4
La-Nterminal
La-Nbridging
La-Carene
2.347, 2.360 Å
2.434 Å
2.367, 2.375 Å
2.472 Å
2.972-3.209 Å
3.186-3.248 Å
Nterminal-La-Nterminal 122.3
112.4
Nterminal-La-Nbridging 93.2, 111.2
102.5, 104.9
La2(µ-OPh)2(OPh)4
2.219, 2.221 Å
2.312 Å
La2(µ-OAr)2(OAr)4
2.187, 2.197 Å
2.273 Å
La-Oterminal
La-Obridging
La-Carene
2.978-3.164 Å
3.133-3.310 Å
Oterminal-La-Oterminal 106.4
110.5
Oterminal-La-Obridging 112.7, 115.2
108.2, 112.7
Table 7. Calculated Lanthanum Metal Charge and Mulliken
Orbital Populations
charge (La)
6s,6p (e-)
5d(e-)
La(NHPh)3
+1.47
+1.32
+1.33
+1.44
0.24
0.44
0.38
0.40
1.29
1.24
1.29
1.16
La(NHPh)3(η6-C6H5R)
La2(µ-NHPh)2(NHPh)4
La2(µ-OPh)2(OPh)4
accept π-donation from the benzene ring in the series of model
complexes (η6-C6H5R)La(NHPh)3 (R ) H, OH, NH2) discussed
below or from one of the bridging NHPh ligands in the dimeric
complex La2(µ-η6-NHPh)2(NHPh)4. The overall charge on the
La has been reduced slightly to +1.32 and +1.33 for (η6-
C6H5R)La(NHPh)3 and La2(µ-η6-NHPh)2(NHPh)4, respectively.
The 5d population remains relatively unchanged with slight
increases of 0.20 and 0.14 e- in the 6s,6p populations
(respectively) as a result of interaction with an η6-arene unit.
Some of the key molecular orbitals are shown in Figure 10 for
the case of (η6-C6H6)La(NHPh)3. Relatively little change in
energy is evident for the highest occupied molecular orbitals
arising from the π lone pair orbitals of NHPh, for example.
The major changes are the stabilization of the occupied benzene
π-bonding orbitals upon complexation to La(NHPh)3, by 0.06
au for the highest occupied benzene π-bonding orbital (e1g) and
0.104 au for the totally symmetric benzene π-bonding orbital
(a2u) upon complexation. However, there is very little metal
admixture into these orbitals in the (η6-C6H6)La(NHPh)3 mo-
lecular orbitals. Comparing La2(µ-η6-OPh)2(OPh)4 to La2(µ-η6-
NHPh)2(NHPh)4, the La atoms are slightly more electropositive
in the aryloxide derivative (+1.44 vs +1.33) with somewhat
smaller 5d populations (1.16 e- vs 1.24 e-). The DFT
calculations suggest that the metal-ligand σ-bonding in the
LnX3 unit takes place primarily through interaction with La 5d
orbitals, while the metal-π-arene interaction takes place
primarily through the 6s/6p manifold.
Figure 9. Views of the fully optimized (a) La2(µ-OPh)2(OPh)4 and (b)
La2(µ-NHPh)2(NHPh)4 structures from B3LYP calculations.
schematically in IV, where conjugation with the π electrons of
the benzene ring is depicted.
C. Thermochemistry. The experimentally observed differ-
ences in solution behavior and coordination chemistry between
the arylamide and aryloxide dimers led us to examine the relative
magnitude of π-bonding interactions with La(XPh)3 units (X
) O, NH). We recall that the arylamide dimer La2(NHAr)6 (1)
reacts reVersibly with THF to form La(NHAr)3(THF)3 (2), while
the analogous reactivity for the aryloxide complex La2(OAr)6
(4) is irreVersible. The two π-arene-bridged dimers also exhibit
different NMR behavior (vide infra) in which the former
complex appears to break apart into monomeric units in solution
In the DFT calculations on the La(NHPh)3 monomer, the
overall charge on the La atoms is calculated as +1.47 (i.e., much
less ionic than the formal oxidation state of +3) that gives orbital
populations of (6s,6p)0.245d1.29 for La from the standard Mulliken
analysis (Table 7). The coordinatively unsaturated monomer can
9
J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004 6397

A R T I C L E S
Giesbrecht et al.
Table 8. Calculated Interaction Energies of π-Bonded Complexes
calculated binding energy (kcal mol-1
)
interaction
X ) NH
X ) O
La(XPh)3 + La(XPh)3
La(XPh)3 + benzene
La(XPh)3 + HOPh
La(XPh)3 + H2NPh
-45.1
-15.4
-17.3
-19.3
-48.1
-17.1
-19.7
-21.9
experimental value of ∆Hq of 16.0 kcal mol-1 obtained for the
aryloxide-bridged dimer 4, in which only one of the π-arene
interactions was thought to be cleaved in solution. This number
is similar to the value of 19.7 kcal mol-1 calculated for breaking
the interaction between La(OPh)3 and C6H5OH (the values in
Table 8 are calculated for bond forming, not bond breaking).
For each monomer V, the π-bonding interaction increases in
the order benzene < phenol < aniline, and the π-arene
interaction is stronger to La(OPh)3 than La(NPh)3. The slightly
weaker affinity of arylamide complexes for π-arene ligands is
consistent with the experimental results, but the relatively small,
calculated energy changes are surprising in view of the
drastically different solution chemistry.
The experimental enthalpy of activation, ∆Hq, for ligand
exchange in dimer 1 is 22.5 kcal mol-1 determined in toluene
solution, which is low by a factor of 2 compared to the
calculated value (45 kcal mol-1, Table 8) and corresponds to
each metal-arene bond being worth only 11 kcal mol-1. In view
of the calculated stabilization afforded to monomeric La(NPh)3
by coordination of simple π-arenes such as benzene, we
reevaluated our interpretation of the solution behavior of the
arylamide dimer La2(NH-2,6-iPr2C6H3)6 (1). Indeed, the calcula-
tions suggest that we should consider the interaction of
monomeric La(NH-2,6-iPr2C6H3)3 with toluene solvent to form
(η6-C6H5Me)La(NH-2,6-iPr2C6H3)3 of structure type V as a
distinct possibility during the monomer-dimer equilibrium. The
chemical equilibrium would take the form of eq 6.
Figure 10. Orbital energies of selected molecular orbitals in C6H6 (left),
(η6-C6H6)La(NHAr)3 (center), and La(NHAr)3 (right) from DFT calculations.
while the latter loses only one π-bonding interaction under
similar conditions.
The calculated energy of forming the dimeric species was
determined from the calculated structures of both monomeric
La(XPh)3 and dimeric La2(XPh)6 for each species (X ) O, NH)
as given in Table 8. While the dimer formation is energetically
favored in both cases (-48.1 kcal mol-1 for La2(OPh)6, -45.1
kcal mol-1 for La2(NHPh)6), the aryloxide forms the slightly
stronger π-arene interaction by about 3 kcal mol-1. To further
isolate the nature of the π-arene interaction, the binding of a
sequence of benzene, phenol, and aniline to La(XPh)3 monomers
(X ) O, NH) was examined to form the hypothetical monomer
(η6-C6H5R)La(XPh)3 (V) (X ) O, NH; R ) H, OH, NH2). As
La2(NHAr)6 + 2C6H5Me h 2(η6-C6H5Me)La(NHAr)3 (6)
Using the data in Table 8, the reaction between La2(NHPh)6
and 2 equiv of benzene to give (η6-C6H6)La(NHPh)3 yields a
reaction energy of 14.3 kcal mol-1, in much better agreement
with the experimental enthalpy of activation of 22.5 kcal mol-1
for ligand exchange in 1 determined in toluene solution. A
structure containing a π-bound solvent molecule such as that
described in eq 6 has been recently reported for the related
complex (η6-C6H5Me)Nd[N(C6F5)2]3,45 although we were unable
to conclusively determine if such an interaction was present in
our system. For La2(O-2,6-iPr2C6H3)6 (4), we believe that only
a single π-arene bond is cleaved (Scheme 4) and a dimeric
structure is maintained in solution at all times; thus the
interaction of the “uncomplexed” La center of the dimeric
intermediate with a molecule of the toluene solvent is likely
sterically disfavored.
F. Vibrational Frequencies. Vibrational frequencies were
computed at the optimum geometries from the DFT calculations
to aid in the interpretation of the experimental IR spectra of
the various species present in solution. Since the IR peaks of
most interest have been assigned to ring breathing modes, initial
calculations on benzene were used to calibrate the DFT results.
shown in Table 8, all of the binding energies for the formation
of a π-arene interaction lie in the vicinity of 15-22 kcal mol-1
,
suggesting that the interactions in the dimer should be ∼30-
45 kcal mol-1. The dimers themselves are calculated to be
somewhat higher at 45-48 kcal mol-1. The binding energies
calculated for monomers V are qualitatively similar to the
(45) Click, D. R.; Scott, B. L.; Watkin, J. G. Chem. Commun. 1999, 633.
9
6398 J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004

π-Arene-Bridged Lanthanum Arylamide and Aryloxide Dimers
Table 9. Calculated and Experimental Vibrational Frequencies Involving Ring-Breathing Modes of π-Arene Bridged Dimeric Complexesa
La2(µ-OPh)2(OPh)4 La2(µ-NHPh)2(NHPh)4
A R T I C L E S
-1
-1
calcd, cm-1 (rel intensity)
exptl, cm
calcd, cm-1 (rel intensity)
exptl, cm
1596, (0.21), t
1574, (0.40), br
1588 (solid), 1588 (soln)
1571 (solid), 1572 (soln)
1597, (0.34), t
1579, (0.94), br
1587 (solid), 1588 (soln)
1575 (solid), 1572 (soln)
a Calculated relative intensities and assignment of bridge (br) and terminal (t) rings are also shown. Calculated frequencies have been corrected as described
in the text.
The two relevant modes in the 1400-1600 cm-1 spectral region
Based on the remarkable similarity in the solid-state structures,
with average La-C distances ) 3.069(4) and 3.062(10) Å for
arylamido and aryloxido ligands, one might not anticipate much
difference in solution behavior. In a similar fashion, the
vibrational spectroscopy reveals a shift to lower energy in the
diagnostic ν(CdC) stretch upon coordination of the arene ring
to the metal center. Here again, the observed frequency is 1572
cm-1 for both systems, suggesting little difference in the nature
of metal-arene bonding interaction.
The observed solution chemistry, however, is markedly dif-
ferent between the two systems. La2(NHAr)6 reveals a monomer-
dimer equilibrium in solution and reversible Lewis base adduct
formation with THF. In contrast, La2(OAr)6 does not reveal an
accessible equilibrium (by 1H NMR), and once the Lewis base
adducts are formed with THF, the ligands are not readily
removed. These differences imply subtle differences in the
nature of the La-arene bond, brought about by differences in
electronic structure between OAr and NHAr ligands.
are the IR-allowed e1u band observed at 1484 cm-1 and the
Raman allowed e2g band at 1601 cm-1. In the 6-31G basis set
the B3LYP calculations predict these transitions to occur at 1541
and 1654 cm-1, which are 57 and 53 cm-1 higher than
experiment, respectively. Accordingly, in the La complexes the
calculated harmonic frequencies were corrected by 55 cm-1
when compared with the observed bands.
With six phenyl rings in the dimeric complexes there will be
one family of 12 vibrational transitions corresponding roughly
to the six original e1u doubly degenerate benzene modes and
another family of 12 vibrational transitions arising from the e2g
modes. Of the former set, there are two with appreciable IR
intensity in the region 1480-1500 cm-1 in both the aryloxide
and arylamide complexes. The observed IR spectra in the region
1570-1590 cm-1 correspond to transitions in the second family,
as shown in Table 9. In the arylamide complexes the solution
bands at 1572 and 1588 cm-1 correspond to the calculated
transitions at 1579 and 1597 cm-1. In the aryloxide dimer there
are calculated bands at 1574 and 1596 cm-1 that agree well
Examples of η6-coordination of a neutral arene ring to a
lanthanide metal center are increasingly common and generally
fall into three structural types: (i) zero-valent lanthanides
stabilized by two neutral 1,3,5-tri-tert-butylbenzene groups VI;46
(ii) trivalent lanthanide centers with highly electron-withdrawing
substituents such as halogenoaluminate [AlX4]- VII47-59 or
dodecafluorodiphenyl amido ligands VIII (Ln ) Nd);45 and (iii)
dimeric trivalent lanthanide compounds containing intermo-
lecular π-arene interactions IX6,24,25,27 or monomeric species
containing intramolecular π-arene interactions X,33,60,61 although
a number of other less common structural motifs have been
with experimentally observed bands at 1572 and 1588 cm-1
.
In each case the lower energy band in the calculations
corresponds to the vibrations of the bridging π-arene ligands.
Thus the calculation of vibrational frequencies reveals the origin
of the observed IR spectral behavior of both La2(OAr)6 and
La2(NHAr)6, with the higher energy ν(CdC) stretch due to
terminal ligands and the lower energy stretch associated with
the bridging ligands. The shift to lower energy is due to electron
donation to the La center upon coordination.
The effect of π-bonding on the bridging ligands can also be
seen from the results of calculations carried out on the analogous
O-bridged dimer (II) where there are no π-bonding interactions
involving the metals. While this particular species has not
been isolated, the analogous ammonia adduct La2(µ-O-2,6-
iPr2C6H3)2(O-2,6-iPr2C6H3)4(NH3)2 has been reported.27 In the
case of the theoretical O-bridged dimer II, the calculated IR
frequencies for the bridging (1591 cm-1) and terminal (1595,
1598 cm-1) aryloxide ligands are much more similar than in
the π-bridging case in Table 9, thus giving rise to a single strong
band, as observed in the IR spectrum of La2(µ-O-2,6-
iPr2C6H3)2(O-2,6-iPr2C6H3)4(NH3)2 (1583 cm-1).
62
reported.
Concluding Remarks
The structural similarities between La2(OAr)6 and La2(NHAr)6
complexes afford the opportunity to compare the solution
chemical reactivity and nature of metal-ligand bonding in an
essentially isostructural series of 4f element compounds. Both
compounds exist in the solid state as the rather unusual π-arene-
bridged dimeric unit. In both cases, we find the application of
vibrational spectroscopy to be a very powerful tool for evaluat-
ing the π-arene interaction through changes in the ν(CdC)
stretch upon coordination of an arene ring to the metal center.
The nature of the lanthanide-arene chemical bond in the
general class of zero-valent lanthanide complexes VI is very
(46) Brennan, J. G.; Cloke, F. G. N.; Sameh, A. A.; Zalkin, A. Chem. Commun.
1987, 1668.
9
J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004 6399

A R T I C L E S
Giesbrecht et al.
similar to the traditional description in d-block chemistry.63-65
Computational studies indicate that the bonding interaction is
best described as back-donation of a filled lanthanide 5d orbital
into an unoccupied arene π* acceptor orbital. The lanthanide-
arene bonding in the other two classes of compounds exempli-
fied by VII-X are more likely to originate from donation of
filled π orbitals on the arene into unoccupied orbitals on the
lanthanide metal center. The comparative study of OAr and
NHAr ligands of structure type IX sheds additional light on
this proposal. The calculations indicate that formation of
η6-arene interactions is energetically favored over monomeric
LaX3 (X) OPh or NHPh) and shows 6s/6p orbital contribution
to bonding with the aryloxide π-arene interaction being stronger
than the arylamide π-arene interaction. This order of stability
is consistent with the observed chemical behavior and indicates
that the aryloxide ligand has more electron density on the arene
ring than the arylamide.
solvent to produce monomeric (η6-C6H5Me)La(NHAr)3. The
calculations reproduce relative trends and give more insights
into nature of bonding.
Experimental Section
General Considerations. All manipulations were carried out under
an inert atmosphere of oxygen-free UHP grade argon using standard
Schlenk techniques or under oxygen-free helium in a Vacuum
Atmospheres glovebox. 2,6-Diisopropylaniline was purchased from
Aldrich, dried over molecular sieves, and distilled over sodium prior
to use. Pyridine was purchased from Aldrich and distilled over sodium
benzophenone prior to use. La[N(SiMe3)2]367 and [La(OAr)3]2 (4)27 were
prepared according to literature procedures. Hexane, toluene, and
tetrahydrofuran were deoxygenated by passage through a column of
supported copper redox catalyst (Cu-0226 S) and dried by passing
through a second column of activated alumina.68 C6D6, C7D8, d8-THF,
and d5-pyridine were distilled over sodium benzophenone and degassed
prior to use. 1H and 13C{1H} NMR spectra were recorded on a Bruker
AMX 300 spectrometer at ambient temperature unless otherwise noted.
1H chemical shifts are given relative to residual C6D5H (δ ) 7.15 ppm),
C7D7H (δ ) 2.09 ppm), C4D7HO (δ ) 3.58 ppm), or C5D4HN (δ )
8.74 ppm). 13C chemical shifts are given relative to C6D6 (δ ) 128.39
ppm), C4D8O (δ ) 67.57 ppm), or C5D5N (δ ) 150.35 ppm). The 1H
NOESY spectrum was obtained on a Bruker DRX-500 spectrometer
operating at 500.13 MHz in C7D8 at 318 K; t1 was incremented in 512
steps, and the data were zero-filled to 1024 words before Fourier
transformation. A total of 32 scans were recorded for each increment
with tmix ) 250 ms with a relaxation time of 800 ms. Simulations of
the variable-temperature 1H NMR spectra in order to extract exchange
constants were performed using DNMR71. Infrared spectra were
recorded on a Nicolet Avatar 360 FT-IR spectrometer as Nujol mulls
between KBr plates or as benzene solutions in a solution IR cell. Raman
spectra were collected with a Nicolet Model 960 FT-Raman spectrom-
eter attached to a Nicolet Model 560 Magna-IR with an extended
XT-KBr beam splitter and 180° sampling geometry. The excitation
source consisted of 0.24 W of 1064 nm light from a CW Nd:VO4 laser.
The interferogram was detected with an InGaAs detector operated at
room temperature. An average of 128 and 1500 scans at 4 cm-1
resolution were taken for the solid compounds. Elemental analyses were
performed by the Micro-Mass Facility at the University of California,
Berkeley.
One simple means to rationalize which arene moiety will have
the greatest electron density is to compare the acidity of the
parent acids. It is well established that phenol is a stronger acid
that aniline, due in part to the greater ability of the phenoxide
anion to stabilize the negative charge on the arene ring compared
to the corresponding anilide anion.66 Thus the order OAr-
>
NHAr- suggests greater electron density on the OAr arene
moiety than in the NHAr arene moiety, indicating that the
aryloxide ligand would form the strongest metal-arene bonding
interaction.
The metal-arene bonding is still admittedly weak, and
electronic structure calculations were unable to identify a single
molecular orbital interaction to account for the bonding.
However, a Mulliken population analysis did reveal excess
electron density in the otherwise virtual 5d La orbitals, consistent
with the qualitative bonding picture. Of significance is that the
calculations were able to predict two vibrational modes in the
ν(CdC) stretching region of both the IR and Raman spectra
and give close agreement between theory and experiment,
identifying the origin of the two vibrational modes. The
calculation of thermochemical data reproduce the relative
strengths of La-π-arene interactions, with OAr > NHAr, and
suggest an alternative description of the monomer-dimer
equilibrium in 1, to propose the involvement of the toluene
La2(NH-2,6-iPr2C6H3)6 (1). 2,6-Diisopropylaniline (2.57 g, 14.5
mmol) in 10 mL of toluene was added to a pale yellow toluene solution
(75 mL) of La[N(SiMe3)2]3 (3.00 g, 4.84 mmol). This caused the
formation of a bright yellow slurry. The reaction mixture was stirred
at room temperature for 15 min and then heated to reflux for 15 min,
during which time a golden yellow solution formed. Upon cooling, a
bright yellow solid precipitated. The solvent was then removed under
vacuum and the precipitate washed with hexane to remove HN(SiMe3)2.
The resulting solid was collected by filtration and pumped to dryness
(2.63 g, 80% yield). X-ray quality crystals were grown by evaporation
of saturated toluene solutions of 1. 1H NMR (C6D6): δ 7.19 (d, 3JH-H
) 7.2 Hz, 4H, m-H), 7.09 (d, 3JH-H ) 7.2 Hz, 8H, m-H), 6.82 (t, 3JH-H
(47) Cotton, F. A.; Schwotzer, W. J. Am. Chem. Soc. 1986, 108, 4657.
(48) Fan, B.; Shen, Q.; Lin, Y. J. Organomet. Chem. 1989, 376, 61.
(49) Fan, B.; Shen, Q.; Lin, Y. J. Organomet. Chem. 1989, 377, 51.
(50) Fan, B.; Shen, Q.; Lin, Y. Youji Huaxue 1989, 9, 414.
(51) Liang, H.; Shen, Q.; Jin, S.; Lin, Y. Chem. Commun. 1992, 480.
(52) Liang, H.; Shen, Q.; Guan, J.; Lin, Y. J. Organomet. Chem. 1994, 474,
113.
(53) Biagini, P.; Lugli, G.; Millini, R. Gazz. Chim. Ital. 1994, 124, 217.
(54) Biagini, P.; Lugi, G.; Abis, L.; Millini, R. New J. Chem. 1995, 19, 713.
(55) Liu, Q.; Lin, Y.; Shen, Q. Acta Crystallogr., Sect. C 1997, 53, 1579.
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194.
3
) 7.2 Hz, 4H, p-H), 6.68 (t, JH-H ) 7.2 Hz, 2H, p-H), 5.37 (s, 2H,
(57) Troyanov, S. I. Koord. Khim. 1998, 24, 381.
NH), 5.12 (s, 4H, NH), 3.05 (sept, 3JH-H ) 6.6 Hz, 8H, CHMe2), 2.72
(58) Troyanov, S. I. Koord. Khim. 1998, 24, 373.
3
3
(sept, JH-H ) 6.6 Hz, 4H, CHMe2), 1.32 (d, JH-H ) 6.6 Hz, 48H,
(59) Troyanov, S. I. Koord. Khim. 1998, 24, 632.
CHMe2), 1.24 (d, JH-H ) 6.6 Hz, 24H, CHMe2). 13C{1H} NMR
3
(60) Deacon, G. B.; Nickel, S.; MacKinnon, P.; Tiekink, E. R. T. Aust. J. Chem.
1990, 43, 1245.
(C6D6): δ 157.90, 152.82, 137.22, 133.12, 126.03, 123.57, 116.94,
111.95, 31.76 (CHMe2), 24.00 (CHMe2), 22.27 (CHMe2), 15.92
(CHMe2). IR (Nujol, cm-1): 1587 (m), 1575 (m), 1324 (s), 1251 (m),
1168 (m), 1149 (m), 1104 (m), 1055 (m), 1037 (m), 882 (w), 840 (s),
796 (s), 767 (s), 738 (s). Anal. Calcd for C72H108La2N6: C, 64.75; H,
8.15; N, 6.29. Found: C, 62.81; H, 8.50; N, 6.20.
(61) Deacon, G. B.; Feng, T.; Forsyth, C. M.; Gitlits, A.; Hockless, D. C. R.;
Shen, Q.; Skelton, B. W.; White, A. H. Dalton Trans. 2000, 961.
(62) For a comprehensive review of Ln-arene complexes, see: Bochkarev, M.
N. Chem. ReV. 2002, 102, 2089.
(63) King, W. A.; Marks, T. J.; Anderson, D. A.; Duncalk, D. J.; Cloke, F. G.
N. J. Am. Chem. Soc. 1992, 114, 9221.
(64) King, W. A.; Di Bella, S.; Lanza, G.; Khan, K.; Duncalf, D. J.; Cloke, F.
G. N.; Fragala, I. L.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 627 and
references therein.
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9
6400 J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004

π-Arene-Bridged Lanthanum Arylamide and Aryloxide Dimers
A R T I C L E S
La(NH-2,6-iPr2C6H3)3(THF)3 (2). 1 (300 mg, 0.220 mmol) was
dissolved in 5 mL of THF, forming a pale yellow solution. Slow
evaporation resulted in large colorless crystals of 2 (330 mg, 84% yield).
Details of Theoretical Calculations. All calculations were carried
out using the B3LYP functional74 and employing relativistic effective
core potentials (RECP) on the La atoms.75,76 All calculations used the
“large core” RECP in which the 5s2 5p6 6s2 5d1 electrons were explicitly
treated as “valence” electrons with the remaining electrons replaced
by the RECP. The starting basis set for the -OAr and -NHAr ligands
was the 6-31G basis. Additional calculations were carried out using
the 6-31G* basis including d functions on all first-row atoms. The
effects of diffuse functions on the heteroatoms were examined using
the 6-31+G* basis, but these additional functions did not affect any
calculated energy differences significantly. The calculations employed
Gaussian 9877 and Gaussian 03.78 All structures were optimized in the
6-31G basis in B3LYP calculations. Vibrational frequencies were
calculated at the resulting geometry for selected complexes discussed
in the paper.
3
1H NMR (d8-THF): δ 6.85 (d, JH-H ) 8.0 Hz, 6H, m-H), 6.54 (t,
3JH-H ) 8.0 Hz, 3H, p-H), 4.10 (s, 3H, NH), 3.55 (br t, 12H, R-CH2),
3
2.95 (sept, JH-H ) 7.2 Hz, 6H, CHMe2), 1.70 (br t, 12H, â-CH2),
3
1.16 (d, JH-H ) 7.2 Hz, 36H, CHMe2). 13C{1H} NMR (d8-THF): δ
154.60, 132.93, 122.82, 114.18, 68.38 (R-THF), 30.56 (CHMe2), 26.53
(â-THF), 23.99 (CHMe2). IR (Nujol, cm-1): 1588 (m), 1420 (s), 1330
(s), 1299 (m), 1255 (s), 1221 (m), 1151 (m), 1138 (m), 1111 (m), 1083
(m), 1018 (m), 881 (m), 865 (m), 838 (s), 737 (s), 684 (m). Anal. Calcd
for C48H78LaN3O3: C, 65.21; H, 8.89; N, 4.75. Found: C, 65.19; H,
8.82; N, 4.80.
La(NH-2,6-iPr2C6H3)3(py)2 (3). Toluene (5 mL) was added to 1
(300 mg, 0.220 mmol), forming a pale yellow slurry. Pyridine was
added (1/2 mL), causing a pale yellow solution to form. Cooling to
-30 °C resulted in large colorless crystals of 3 (140 mg, 75% yield).
1H NMR (d5-pyridine): δ 8.74 (s, 4H, o-py-H), 7.59 (s, 2H, p-py-H),
7.22 (s, 4H, m-py-H), 7.14 (d, 3JH-H ) 7.5 Hz, 6H, m-H), 6.73 (t, 3JH-H
) 8.0 Hz, 3H, p-H), 5.18 (s, 3H, NH), 3.33 (sept, 3JH-H ) 6.6 Hz, 6H,
Acknowledgment. We thank Dr. Ryszard Michalczyk for
help in the acquisition of the NOESY spectrum of compound
1. This work was performed under the auspices of the Labora-
tory Directed Research and Development Program and the
Office of Basic Energy Sciences, Division of Chemical Sciences,
U.S. Department of Energy. Los Alamos National Laboratory
is operated by the University of California for the U.S.
Department of Energy under Contract W-7405-ENG-36.
3
CHMe2), 1.18 (d, JH-H ) 6.6 Hz, 36H, CHMe2). 13C{1H} NMR (d5-
pyridine): δ 155.14, 136.14, 133.47, 124.49, 123.32, 114.08, 30.19
(CHMe2), 24.13 (CHMe2) (one pyridine peak obscured by solvent
resonance). IR (Nujol, cm-1): 1623 (w), 1594 (s), 1588 (s), 1568 (sh),
1418 (s), 1331 (w), 1256 (s), 1215 (w), 1170 (w), 1150 (w), 1113 (w),
1055 (m), 1000 (m), 882 (w), 840 (m), 746 (m), 700 (m), 682 (m),
620 (m). Anal. Calcd for C46H64LaN5: C, 66.89; H, 7.81; N, 8.48.
Found: C, 65.86; H, 7.90; N, 8.34.
Supporting Information Available: Crystallographic data in
CIF format for 1-3, coordinates for the optimized model
structures La2(µ-OPh)2(OPh)4 and La2(µ-NHPh)2(NHPh)4, and
full details of the kinetic and thermodynamic analyses. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Crystallographic Studies. Crystals of 1, 2, and 3 were mounted on
glass fibers using a spot of silicone grease. Due to air sensitivity, the
crystals were mounted from a pool of mineral oil under argon gas flow.
The crystals were placed on a Bruker P4/CCD diffractometer and cooled
to 203 K using a Bruker LT-2 temperature device. The instrument was
equipped with a sealed, graphite monochromatized Mo KR X-ray source
(λ ) 0.71073 Å). A hemisphere of data was collected using æ scans,
with 30 s frame exposures and 0.3° frame widths. Data collection and
initial indexing and cell refinement were handled using SMART69
software. Frame integration, including Lorentz-polarization corrections,
and final cell parameter calculations were carried out using SAINT70
software. The data were corrected for absorption using the SADABS71
program. Decay of reflection intensity was monitored via analysis of
redundant frames. The structures were solved using Direct methods
and difference Fourier techniques. For 2 and 3, the amido hydrogen
atom positions were located on the difference map and refined with
isotropic temperature factor set to 0.08 Å2. For 1 - 3, all other hydrogen
atom positions were idealized and rode on the atom they were attached
to. The final refinement included anisotropic temperature factors on
all non-hydrogen atoms. For 2, the electron density of a disordered
THF molecule was removed from the unit cell using PLATON/
SQUEEZE.72 This resulted in two THF molecules per cell being
removed (74 e-/cell and 326 Å3). For 3, the structure was refined as a
racemic twin, with the minor component batch-scale-factor converging
to 0.423(8). Structure solution, refinement, graphics, and creation of
publication materials were performed using SHELXTL NT.73 Addi-
tional details of data collection and structure refinement are listed in
Table 4.
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