Synthesis and reactivity of homoleptic -metalated n,n -dimethylbenzylamine rare-earth-metal complexes

Article abstract of DOI:10.1021/om200363z

A series of new homoleptic rare-earth-metal complexes was synthesized using salt metathesis reactions between LnCl₃ and ±-K(DMBA) (Ln = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Gd (6), Y (7); DMBA = N,N-dimethylbenzylamine). All seven complexes were found to be free of coordinating solvent, and no formation of any "-ate" species was observed. The ligands in complexes 1-7 displayed η⁴ coordination to the metal center. Subsequently, the reactivity of 1 and 7 was investigated via various protonolysis reactions using silylamines, anilines, and phenols to generate a diverse array of other homoleptic lanthanide complexes.

Full text of DOI:10.1021/om200363z

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pubs.acs.org/Organometallics
Synthesis and Reactivity of Homoleptic r-Metalated
N,N-Dimethylbenzylamine Rare-Earth-Metal Complexes
Andrew C. Behrle and Joseph A. R. Schmidt*
Department of Chemistry, University of Toledo, 2801 W. Bancroft Street, MS 602, Toledo, Ohio 43606, United States
S Supporting Information
b
ABSTRACT: A series of new homoleptic rare-earth-metal
complexes was synthesized using salt metathesis reactions
between LnCl3 and R-K(DMBA) (Ln = La (1), Ce (2),
Pr (3), Nd (4), Sm (5), Gd (6), Y (7); DMBA = N,N-
dimethylbenzylamine). All seven complexes were found to
be free of coordinating solvent, and no formation of any “-ate”
species was observed. The ligands in complexes 1ꢀ7 displayed
η4 coordination to the metal center. Subsequently, the reactivity
of 1 and 7 was investigated via various protonolysis reactions using silylamines, anilines, and phenols to generate a diverse array of
other homoleptic lanthanide complexes.
he effective use of f-element complexes as catalysts for
R-metalated transition-metal complex, a zirconium species by
Norton and co-workers.32 For the rare-earth metals, only ortho-
metalated N,N-dimethylbenzylamine complexes exist and, of
these, only the middle and late lanthanides proved to be stable
enough for isolation.33,34 Herein, we report our findings of a new
class of homoleptic trialkyl rare-earth-metal complexes using
R-metalated N,N-dimethylbenzylamine ligands as stable benzyl
ligand derivatives to form lanthanide complexes free of coordi-
nating solvent.
T
reactions such as hydroamination,1ꢀ4 ring-opening polym-
erization of lactide,5ꢀ8 and hydrosilylation,9ꢀ12 as well as pre-
cursors for the generation of thin films,13ꢀ16 has been demon-
strated in many recent reports. Unfortunately, the homoleptic
starting materials underpinning this chemistry are generally
derived from only a handful of ligands. That is, of the homoleptic
lanthanide complexes that exist, the majority make use of
alkylsilane (ꢀCH2SiMe3), silylamide (ꢀN(SiMe3)2), or benzyl
(ꢀCH2C6H5) derivatives.17ꢀ25 The preparation of these homo-
leptic starting materials via salt metathesis reactions between
rare-earth-metal chlorides and lithium salts often leads to the
generation of lanthanate ions ([LnR4]ꢀ).26 Furthermore, alkyl-
silane complexes require careful sublimation to purify and have
been shown to exhibit only moderate thermal stability.27 In
contrast, while the lanthanide tribenzyl complexes do not often
form the lanthanate ion, they instead retain coordinated solvent,
making them not truly homoleptic complexes and limiting their
potential as starting materials in some cases.28 Thus, the dis-
covery of new homoleptic lanthanide complexes with easy isola-
tion protocols and effective protonolysis reactivity would repre-
sent a significant advance in the field of lanthanide chemistry.
The paucity of trialkyl lanthanide complexes can be attributed
to the required extended coordination sphere and high Lewis
acidity or electrophilicity of these metals. The ionic radii of Ln3+
ions range from 1.00 to 1.17 Å.29 While these properties make
lanthanum alkyl complexes very reactive, in turn they also make
their synthesis and manipulation quite challenging. Our efforts
have focused on the development of unexplored homoleptic
trialkyl rare-earth-metal complexes utilizing simple ligands that
fulfill both the electronic and steric requirements of these metal
centers. The N,N-dimethylbenzylamine ligand scaffold is not
unknown in the literature; however, nearly all of its previous uses
have been limited to ortho-metalated complexes.30,31 There is
only a single example of a crystallographically characterized
A series of homoleptic R-metalated N,N-dimethylbenzyla-
mine (DMBA) Ln complexes (Ln = La, Ce, Pr, Nd, Sm, Gd,
Y) was synthesized by following a simple procedure35 involving
salt metathesis of rare-earth-metal chlorides with R-potassiated
dimethylbenzylamine at ꢀ50 °C in THF (Scheme 1). R-La-
(DMBA)3 (1) was isolated by recrystallization from THF/
pentane at ꢀ25 °C, affording orange crystals in 77% yield.
R-Y(DMBA)3 (7) was synthesized in a manner similar to that
for 1, except the THF solvent was removed under reduced pres-
sure at 0 °C in order to prevent product decomposition. Subse-
quently, 7 was recrystallized from a concentrated solution of
pentane at room temperature to give yellow crystals in 79% yield.
The La complex 1 showed no evidence of decomposition in THF
at room temperature, while 7 decomposed readily in this solvent.
Spectroscopically, 1 exhibited fluxional behavior at room tem-
perature, but upon cooling to ꢀ78 °C, it displayed much sharper
resolution with resonances indicative of a C3-symmetric species.
At low temperature, 1 adopted an ABCDX splitting pattern for
the five protons of the phenyl rings, with a resonance shifted
1
significantly upfield to 3.10 ppm in its H NMR spectrum,
assigned to the ortho proton closest to the metal center. This
upfield chemical shift is indicative of a disruption of the aromatic
system and can be attributed to η2 coordination of the phenyl
Received: May 2, 2011
Published: July 14, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of r-Ln(DMBA)3 (Ln = La (1), Ce (2),
Pr (3), Nd (4), Sm (5), Gd (6), Y (7; C3 Isomer))
Figure 2. ORTEP diagram of 7, lacking the approximate C3 symmetry
observed in 1ꢀ6. Thermal ellipsoids are given at the50% probability level.
X-ray-quality crystals were achieved for both 1 and 7, and their
crystal structures were solved. Both 1 and 7 crystallized in
monoclinic space groups. The structure of compound 1 con-
firmed its approximate C3 symmetry (Figure 1). The average
LaꢀCbenzyl bond distance in 1 is 2.648(3) Å (Table 1), which is
significantly shorter than the average LaꢀCbenzyl bond distance
of 2.755(2) Å in La(CH2Ph)3(THF)3.28 Similarly, the average
LaꢀCipso bond distance is 2.834(3) Å, somewhat shorter than
the average LaꢀCipso bond distance of 2.873(6) Å for (o-Me2N-
C6H4-CH2)3La.35,36 The shorter bond distances observed for the
R-DMBA ligands in 1 reflect the allylic nature of the three-carbon
anionic fragment in these ligands, causing the entire unit to be
closer to the metal center to properly bind in this allylic motif.
The average bite angle for 1 is 63.98(7)° for NꢀLaꢀCortho. The
Figure 1. ORTEP diagram of 1, showing the pseudo-3-fold rotation axis
in these lanthanide complexes. Thermal ellipsoids are given at the 50%
probability level.
Table 1. Selected Bond Distances (Å) and Angles (deg) for
1ꢀ7a
tilt of the phenyl group can be measured as the LnꢀCbenzylꢀ
Cipso angle, with an average phenyl tilt angle for 1 of 82.7(2)°. In
contrast, we were fortunate to crystallize the low-symmetry
isomer of complex 7, in which one of the three DMBA ligands
is oriented in the opposite direction from the other two (Figure 2).
The formation of this lower symmetry isomer is due to the
smaller ionic radius of Y3+, with reorientation necessary to relieve
steric stress resulting from the decreased YꢀCbenzyl bond
distances, which are more than 0.1 Å shorter than those observed
in 1. Specifically, the YꢀCbenzyl bond distances fall in the range of
2.488(4)ꢀ2.552(4) Å and are slightly elongated compared to the
YꢀCbenzyl bond distances of Y(CH2Ph)3(THF)3 (2.452(3)ꢀ
2.463(3) Å).24 The average YꢀCipso bond distance is 2.758(4) Å
and is somewhat shorter than the average YꢀCipso bond distance
of 2.801(3) Å for (o-Me2N-C6H4-CH2)3Y.35 The closer ap-
proach to the metal center also results in larger bite angles for 7
(66.9(1)ꢀ72.4(1)°), while the phenyl tilt angles span a wide
range (81.9(2)ꢀ87.5(2)°).
Lnꢀ
Lnꢀ
Ln-
Cipso
ꢀ
Cbenzyl
Cipso
Cortho
LnꢀN CbenzylꢀLn bite angle
La 2.648(3) 2.834(3) 2.856(2) 2.652(2) 82.65(15) 63.98(7)
Ce 2.617(3) 2.814(3) 2.840(3) 2.624(3) 82.99(19) 64.57(9)
Pr 2.597(4) 2.797(3) 2.830(3) 2.605(3) 82.93(19) 64.91(11)
Nd 2.588(3) 2.785(3) 2.818(3) 2.603(3) 82.73(18) 65.24(9)
Sm 2.555(4) 2.760(4) 2.803(4) 2.568(4) 83.0(2)
Gd 2.545(3) 2.790(3) 2.743(3) 2.518(3) 84.49(17) 69.82(8)
2.516(4) 2.758(4) 2.751(4) 2.471(3) 84.2(2) 69.53(12)
a All values are averages of those observed in the R-Ln(DMBA)3
complex.
65.79(12)
Y
ring to the lanthanum, giving rise to an overall η4 binding mode
and resulting in a pseudoallylic arrangement for the three carbon
atoms interacting with the metal center. In contrast, the yttrium
complex 7 did not exhibit fluxional behavior at room tempera-
The remaining R-Ln(DMBA)3 complexes (Ln = Ce, Pr, Nd,
Sm, Gd) were synthesized using procedures analogous to those
used for 7, and each was recrystallized from toluene/pentane at
ꢀ25 °C.37 As expected, complexes 2ꢀ6 were paramagnetic, and
their 1H NMR spectra revealed only broad and uninterpretable
signals. The magnetic moment of each complex was determined
using the Evans NMR method38 and showed no anomalous values
(see the Supporting Information).39 The solid-state structures of
2ꢀ6 were all very similar to that of R-La(DMBA)3 (1), containing
1
ture; however, unlike complex 1, its H NMR displayed two
independent isomers in a nearly 1:1 ratio, with one having C3
symmetry and the other Cs symmetry. The 1H NMR spectrum of
each isomer of 7 showed AA0BB0C coupling patterns for each
inequivalent phenyl group. This indicates rapid rotation of these
phenyl groups in solution, implying η2 coordination from the
DMBA ligands to the yttrium metal center, unlike the η4
coordination observed in 1.
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COMMUNICATION
a
Scheme 2. Protonolysis Reactions of r-Ln(DMBA)3
This decrease in hapticity is likely a result of minimization of the
steric interactions caused by the very bulky aryloxides. The
LaꢀCbenzyl and LaꢀN bond distances are 2.602(2) and
2.664(2) Å, respectively, which are quite similar to those in the
homoleptic R-La(DMBA)3 precursor. While the generation of 8
was initially unanticipated, it provided insight into the donor
capabilities and Lewis basicity of N,N-dimethylbenzylamine. The
lanthanum clearly preferred the more basic aryloxide and neutral
tetrahydrofuran ligands over neutral N,N-dimethylbenzylamine,
which is present in solution as a reaction byproduct. This result
stems from the fact that the aryloxide and tetrahydrofuran are
both harder bases than the tertiary amine, and THF has less steric
bulk than the neutral N,N-dimethylbenzylamine.
a Legend: Ln = La, Y; Ar = 2,6-di-tert-butylphenyl, 4-tert-butylphenyl;
Ar’ = 2,6-diisopropylphenyl, 4-tert-butylphenyl.
As a means to more readily demonstrate the breadth of ligands
amenable to protonolysis reactions with R-Ln(DMBA)3, we
turned our attention to full protonolysis of 1 and 7, in which
all three DMBA ligands were displaced by other anionic ligands,
such as bulky aryloxides, anilides, and silylamides. Overall, 1 and
7 showed excellent reactivity in full protonolysis reactions to very
simply generate numerous other homoleptic lanthanide com-
plexes (Scheme 2). As expected for the less sterically encumbered
aryloxides, La(OAr)3 (9) and Y(OAr)3 (11) exhibited dimeric
behavior in solution for the less bulky 4-tert-butylphenyl deriva-
tives. In each case, to achieve full protonolysis, 1 was treated with
3 equiv of protonating agent in THF at room temperature, while
the analogous reactions of 7 were conducted at ꢀ78 °C in order
to prevent decomposition of 7 in THF. Product isolation
consisted of either simply washing the solid with pentane or
extracting the product into pentane after the THF had been
removed under reduced pressure, followed by crystallization at
ꢀ25 °C. No traces of liberated neutral N,N-dimethylbenzyla-
mine could be found in any of the products after isolation,
confirming that full protonolysis had occurred and that no
tertiary amine remained coordinated to the metal. Thus, proto-
nolysis of R-Ln(DMBA)3 proved to be a very effective and simple
means of generating homoleptic aryloxide and amide complexes.
In conclusion, we have disclosed the synthesis of a new series
of solvent-free homoleptic rare-earth-metal complexes [R-Ln-
(DMBA)3]. These complexes were readily produced via salt
metathesis with no formation of the corresponding “-ate” species.
The size of the metal’s ionic radius determined whether a 3-fold
axis of symmetry was produced at the metal center, while the
R-DMBA ligand showed η4 coordination in virtually every case.
The reactivity of 1 and 7 was probed via protonolysis reactions. A
heteroleptic lanthanide complex (8) was isolated, revealing that
lanthanum preferred coordination of a THF molecule to the
neutral N,N-dimethylbenzylamine ligand. Full protonolysis reac-
tions with several phenols and amines resulted in easily isolated
homoleptic complexes, demonstrating the utility and versatility of
the R-Ln(DMBA)3 compounds as precursors to other Ln com-
plexes. Further investigation of the organometallic reactivity of
R-Ln(DMBA)3 complexes and their use in catalysis is underway.
Figure 3. ORTEP diagram of 8 with tert-butyl methyl groups removed
for clarity. Thermal ellipsoids are given at the 50% probability level.
a 3-fold axis of rotation and η4 coordination to the metal centers
(see the Supporting Information). As anticipated, the LnꢀCbenzyl
bond distance decreased as the metal’s ionic radius became
smaller, due to the lanthanide contraction (Table 1). One notice-
able feature of complexes 2ꢀ6 is the increase in the phenyl tilt
angle as the LnꢀCbenzyl bond length decreased. This phenyl tilt
angle implies that, as the ionic radius of the metal shrinks, the
phenyl group becomes more orthogonal to the metal. Also, as the
metal’s radius decreases, the ligands move closer to the metal,
resulting in larger ligand bite angles. On the basis of a comparison
of the LnꢀCbenzyl bond distances and the phenyl tilt angles of 6
and 7, R-Gd(DMBA)3 (6) may represent the absolute minimum
radius necessary to fully preserve the 3-fold symmetry.
In order to demonstrate the utility of R-Ln(DMBA)3 com-
plexes as effective precursors in lanthanide chemistry, the re-
activity of 1 and 7 was investigated through a series of
protonolysis reactions. Initial attempts consisted of treatment
of these complexes with 1 equiv of 2,6-di-tert-butylphenol. The
expected monoaryloxide product was not formed; rather, a
disubstituted product, La(DMBA)(OAr)2(THF) (8), was gen-
erated in low yield. As a means to increase the yield of 8, the
reaction was repeated with nearly 2 equiv of 2,6-di-tert-butyl-
phenol to more appropriately reflect the product stoichiometry
(Scheme 2). 8 was recrystallized from a THF/pentane solution at
’ ASSOCIATED CONTENT
S
Supporting Information. Text, figures, tables, and CIF
b
files giving full experimental procedures and characterization
data for R-K(DMBA) and 1ꢀ17, crystallographic procedures
and crystal data for 1ꢀ8, ORTEP diagrams of 2ꢀ6, and
additional crystallographic data, including bond lengths and
angles, of compounds 1ꢀ8. This material is available free of
charge via the Internet at http://pubs.acs.org.
1
room temperature. Its H NMR spectrum and X-ray crystal
structure (Figure 3) revealed a decrease in hapticity of the N,N-
dimethylbenzylamine ligand from η4 to η2. The 1H NMR
spectrum of 8 displayed a classical AA0BB0C splitting pattern
for the phenyl group of the N,N-dimethylbenzylamine ligand.
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’ AUTHOR INFORMATION
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Corresponding Author
*E-mail: Joseph.Schmidt@utoledo.edu. Tel: 419-530-1512.
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