Lanthanide(III) Di- and tetra-nuclear complexes supported by a chelating tripodal tris(amidate) ligand

Article abstract of DOI:10.1021/acs.inorgchem.5b00299

Syntheses, structural, and spectroscopic characterization of multinuclear tris(amidate) lanthanide complexes is described. Addition of K₃[N(o-PhNC(O)^tBu)₃] to LnX₃ (LnX₃ = LaBr₃, CeI_3<

Full text of DOI:10.1021/acs.inorgchem.5b00299

Article
pubs.acs.org/IC
Lanthanide(III) Di- and Tetra-Nuclear Complexes Supported by a
Chelating Tripodal Tris(Amidate) Ligand
Jessie L. Brown,† Matthew B. Jones,† Andrew J. Gaunt,*,† Brian L. Scott,‡ Cora E. MacBeth,*,§
and John C. Gordon†
‡
†Chemistry Division and Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico
87545, United States
§Department of Chemistry, Emory University, Atlanta, Georgia 30322, United States
S
* Supporting Information
ABSTRACT: Syntheses, structural, and spectroscopic charac-
terization of multinuclear tris(amidate) lanthanide complexes
is described. Addition of K3[N(o-PhNC(O)tBu)3] to LnX3
(LnX3 = LaBr3, CeI3, and NdCl3) in N,N-dimethylformamide
(DMF) results in the generation of dinuclear complexes,
[Ln(N(o-PhNC(O)tBu)3)(DMF)]2(μ-DMF) (Ln = La (1),
Ce (2), Nd(3)), in good yields. Syntheses of tetranuclear
complexes, [Ln(N(o-PhNC(O)tBu)3)]4 (Ln = Ce (4), Nd-
(5)), resulted from protonolysis of Ln[N(SiMe3)2]3 (Ln = Ce,
Nd) with N(o-PhNCH(O)tBu)3. In the solid-state, complexes 1−5 exhibit coordination modes of the tripodal tris(amidate)
ligand that are unique to the 4f elements and have not been previously observed in transition metal systems.
chelating tris(κ2-amidate) coordination mode for the tripodal
INTRODUCTION
■
ligand.31 Similarly, in situ deprotonation of N(o-PhNHC-
There is a research need to elucidate the coordination
chemistry of the f-elements with a wide range of ligand
architectures and donors types. One aspect is to understand
trends and changes in speciation and bonding due to the
(O)tBu)3 with potassium hydride (KH) in DMF, followed by
transmetalation with NiBr2 resulted in formation of [Ni(N(o-
PhNC(O)tBu)3]−. Interestingly, in the solid-state, the Ni(II)
complex exhibits both N-amidate and O-amidate coordination
modes, likely adopted to reduce steric strain afforded by the
bulky tert(butyl) substituents.32 Lastly, the diiron(II) complex,
[Ph4P][KFe2(N(o-PhNC(O)iPr)2], derived from the isopropyl
substituted ligand, N(o-PhNHC(O)iPr)3, is supported via two
μ-1,3-(κN:κO)-amidato ligands, where one pendant arm within
each amidate ligand adopts a bridging coordination mode.33
The versatile ligation modes and reactivity observed with the
tripodal tris(amidate) ligand with the main group elements and
the transition metals lent credence to the expectation that the
4f series of trivalent cations would be capable of supporting
unique coordination modes and molecular assemblies.
relevance that such knowledge can play in controlling and
manipulating f-element behavior in various separation
processes.1−6 Rather than designing separation relevant ligands
directly, one approach is to define the baseline chemical
reactivity, complex structures, and bonding properties of the f-
elements with different functional groups or donor types, such
that should those chemical functionalities be incorporated into
a separation agent or any process in the future then an
understanding of the chemical interactions between metal and
ligand will already exist.7−13 Mixed N,O donor ligands have
attracted recent attention due to their interesting bonding
properties with the f-elements.14−18 In this contribution, we
focus on exploring the chemistry of several early lanthanide
ions in the trivalent oxidation state with mixed N,O donor
ligands. Surprisingly, although there are several examples of
lanthanide complexes with simple amidate ligands,19−30 the
chemistry of the 4f ions with more complex amidate scaffolds
has not been explored. We chose to study the tripodal ligand,
[N(o-PhNC(O)R)3]3− (R = alky, aryl), which features an acyl
substituted amidate moiety capable of wide synthetic
modification and varied coordination chemistry (Chart 1).
MacBeth and co-workers have previously demonstrated
diverse chemistry with a variety of tripodal tris(amidate)
scaffolds with both the transition metals and main-block
elements.31−34 For example, addition of N(o-PhNHC(O)tBu)3
to AlMe3 afforded [Al(N(o-PhNC(O)tBu)3] which exhibited a
Herein we report the syntheses, isolation, and character-
ization of a series of multinuclear Ln(III) (Ln = La, Ce, Nd)
complexes with the multichelating tripodal ligand, [N(o-
t
PhNC(O)R)3]3− (R = Bu). These complexes can be obtained
by either halide-salt metathesis or protonolysis, the latter
utilizing Ln[N(SiMe3)2]3 (Ln = Ce, Nd) as the precursor
material. All reported complexes were structurally characterized
by X-ray crystallography, as well as by 1H NMR and IR
spectroscopies. Preliminary reactivity studies of the Ln(III)
complexes are also discussed.
Received: February 6, 2015
Published: April 6, 2015
© 2015 American Chemical Society
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a
Chart 1. Possible Coordination Modes of Tripodal Amidate Ligands
a
Chart modified from ref 32.
Figure 1. Overlay of the 1H NMR spectra of H3LtBu in DMF-d7 (bottom) and of the reaction between H3LtBu and KH after 5 h (top). The amide
t
protons of H3LtBu are denoted by (+), while the Bu protons are denoted by (#). Residual solvent signals of DMF are denoted by (*).
deprotonation of H3LtBu and develop a cleaner synthetic
RESULTS AND DISCUSSION
■
method, the reaction between H3LtBu and excess KH in DMF-
Initial attempts to isolate the target complexes involved in situ
1
d7 was monitored by H NMR spectroscopy. A dominant
deprotonation of the ligand, N(o-PhNHC(O)tBu)3 (H3LtBu),
product was observed after several hours, tentatively assignable
with a slight excess of KH in DMF, followed by addition of 1
equiv of LnX3 (LnX3 = LaBr3, CeBr3, NdCl3). Unfortunately,
to the potassium salt, K3[N(o-PhNC(O)tBu)3] (K3LtBu).
Notably, the amide NH protons of H3LtBu, observed at 8.99
this synthetic strategy consistently results in complicated
reaction mixtures. Specifically, H3LtBu persists as a major
ppm in DMF-d7, are absent in the 5 h spectrum (Figure 1).
t
1
Furthermore, the Bu protons are shifted upfield from 0.95 to
0.78 ppm upon formation of the putative K3LtBu species. With
these results in hand, and to further avoid the presence of
H3LtBu in the final reaction mixtures, isolation of K3LtBu was
attempted following a procedure similar to Stavropoulos and
co-workers.35 Accordingly, addition of excess KH to a DME
impurity as observed by H NMR spectroscopy even upon
multiple recrystallizations of the isolated materials. Changing
the solvent medium to tetrahydrofuran (THF) or 1,2-
dimethoxyethane (DME) was also not successful, due in large
part to the poor solubility of the ligand and the lanthanide
trihalide starting materials. To attempt to understand the
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Scheme 1
Figure 2. Solid-state molecular structure of [La(N(o-PhNC(O)tBu)3)(DMF)]2(μ-DMF) (1) with 30% probability ellipsoids. Solvent molecules,
disordered components, and hydrogen atoms omitted for clarity.
1
suspension of H3LtBu affords K3LtBu as a pink-peach powder in
91% yield (Scheme 1a). Like the protio derivative, this material
is largely insoluble in most organic solvents but is readily
solubilized by DMF. The H NMR spectrum of the isolated
material of K3LtBu in DMF-d7 is spectroscopically identical to
the in situ 1H NMR spectrum at 5 h, exhibiting a singlet at 0.78
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Table 1. Selected Average Bond Distances (Å) for 1·C6H14, 2·C6H14, 3·C6H14, 4·5C6H14, and 5·5C6H14
Article
a
1·C6H14
2·C6H14
3·C6H14
4·C6H14
5·C6H14
b
b
(Ln−O)κ2-amidate
(Ln−O)μ-O:κ2-amidate
(Ln−O)μ-O-amidate
(Ln−N)
2.501(7)
2.475(8)
2.45(1)
2.41(3)
2.38(2)
2.38(1)
c
c
2.397(5)
2.548(9)
2.610(9)
2.437(2)
2.501(6)
b
c
b
c
2.525(2)
2.610(2)
2.62(2)
2.501(2)
2.591(2)
2.60(1)
2.53(1)
2.485(6)
2.56(1)
2.515(1)
2.574(8)
2.404(4)
2.475(9)
b
c
b
c
b
b
2.54(2)
2.51(1)
c
c
2.57(4)
2.54(4)
(Ln−O)DMF/terminal
(Ln−O)DMF/bridging
2.599(2)
2.608(2)
2.583(2)
2.585(2)
2.54(2)
2.541(1)
a
b
c
The error in the average bond lengths is equal to the standard deviation in the experimental values. Denotes 7-coordinate metal center. Denotes
8-coordinate metal center.
Table 2. X-ray Crystallographic Data for Complexes 1·C6H14, 2·C6H14, 3·C6H14, 4·5C6H14, and 5·5C6H14
1·C6H14
2·C6H14
3·C6H14
4·5C6H14
5·5C6H14
empirical formula
crystal habit, color
crystal size (mm)
crystal system
space group
volume (Å3)
a (Å)
C81H113La2N11O9
block, colorless
0.20 × 0.16 × 0.10
monoclinic
C2/c
C81H113Ce2N11O9
block, pale orange
0.3 × 0.18 × 0.12
monoclinic
C2/c
C81H113N11Nd2O9
block, pale purple
0.20 × 0.15 × 0.08
triclinic
C162H226Ce4N16O12
block, pale orange
0.16 × 0.14 × 0.10
monoclinic
C2/c
C162H226N16Nd4O12
block, pale purple
0.20 × 0.20 × 0.20
monoclinic
C2/c
P1
̅
8156(1)
8105.6(18)
30.497(4)
12.2527(16)
23.360(3)
90.00
4010.6(8)
12.2127(14)
16.2838(18)
23.125(3)
99.910(1)
103.228(1)
111.062(1)
2
31410(10)
67.248(12)
15.982(3)
30.730(6)
90.00
30895(2)
67.241(3)
15.9016(6)
30.5419(16)
90.00
30.618(2)
12.2582(9)
23.3809(17)
90.00
b (Å)
c (Å)
α (deg)
β (deg)
111.653(1)
99.00
111.787(1)
90.00
108.002(2)
90.00
108.905(2)
90.00
γ (deg)
Z
4
4
8
8
formula weight (g/mol)
1662.64
1665.06
1673.30
1.386
3150.07
3166.55
density (calculated) (Mg/m3) 1.354
1.364
1.332
1.362
absorption coefficient (mm−1
)
1.094
3448
1.170
1.342
1.200
1.386
F000
3456
1736
13104
13168
total no. reflections
unique reflections
Rint
45308
9708
44668
27527
154417
241436
9457
27527
29777
9708
0.0370
0.0457
0.0000 (twinned
crystal)
0.0996
0.0370
final R indices [I > 2σ(I)]
largest diff peak and hole
R1 = 0.0360,
wR2 = 0.0987
R1 = 0.0370,
wR2 = 0.1018
R1 = 0.0397,
wR2 = 0.0873
R1 = 0.0449,
wR2 = 0.0963
R1 = 0.0675,
wR2 = 0.1396
1.269 and −0.832
1.395 and −0.841
1.124 and −0.824
0.948 and −1.680
2.240 and −1.285
(e− Å−3
)
GOF
1.284
1.262
0.935
0.915
0.895
ppm, assignable to the tBu protons (see Supporting
Information, Figure S8). Lastly, as expected the amide NH
stretching frequency is not observed in its solid-state IR
spectrum (KBr mull) (see Supporting Information, Figure
S1).31
relevant structural parameters and full crystallographic details
for complex 1 can be found in Tables 1 and 2, respectively. In
the solid-state, 1 crystallizes in the monoclinic centrosymmetric
space group C2/c as a hexane solvate. It features a dinuclear
core with two 9-coordinate La(III) centers in distorted
tricapped trigonal prismatic geometries. Each La center in 1·
C6H14 is bound to one tris(amidate) ligand in a chelating
coordination mode and one terminally bound DMF ligand
coordinated through the carbonyl oxygen atom. There is also a
third disordered DMF ligand which bridges both La centers
through its carbonyl oxygen atom. Notably, two pendant arms
of each tris(amidate) ligand bind in a κ2-amidate fashion, while
the third arm bridges the La metal centers via the oxygen atom
(μ-O:κ2-amidate and μ-O-amidate). The average O−C and N−
C bond lengths, 1.29(1) Å and 1.306(6) Å, respectively, are
equivalent and thus indicate significant electron delocalization
through the κ2-amidate and μ-O:κ2-amidate backbone.19,20 The
average κ2-amidate La−O bond distance (2.501(7) Å) appears
With isolated K3LtBu, we attempted to synthesize the target
complexes using DMF as a solvent medium in order to
promote the solubility of both the ligand and the lanthanide-
(III) halide starting materials. The addition of 1 equiv of finely
ground LaBr3 to a DMF solution of K3LtBu results in formation
of [La(N(o-PhNC(O)tBu)3)(DMF)]2(μ-DMF) (1), isolated in
64% yield as a white microcrystalline solid (Scheme 1b).
Complex 1 is soluble in polar solvents such as DMF,
dichloromethane, and chloroform but is only partially soluble
in diethyl ether or toluene and is completely insoluble in
hexanes or n-pentane.
The solid-state molecular structure of 1 was determined by
single-crystal X-ray crystallography (Figure 2). A summary of
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Figure 3. Solid-state molecular structure of [Ce(N(o-PhNC(O)tBu)3)(DMF)]2(μ-DMF)] (2) with 30% probability ellipsoids. Solvent molecules,
disordered components, and hydrogen atoms omitted for clarity.
Figure 4. Solid-state molecular structure of [Nd(N(o-PhNC(O)tBu)3)(DMF)]2(μ-DMF) (3) with 30% probability ellipsoids. Solvent molecules,
disordered components, and hydrogen atoms omitted for clarity.
statistically equivalent relative to the μ-O:κ2-amidate La−O
bond distance (La1−O2 = 2.525(2) Å), and both are similar to
known La−O interactions.36−38 Not surprisingly, the μ-O-
amidate interaction between O2 and the second La center
(La1*−O2 = 2.610(2) Å) is longer relative to both the κ2-
amidate and μ-O:κ2-amidate interactions. The La−O bond
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distance within the terminal DMF ligand (La1−O4 = 2.599(2)
Å) is statistically identical to the La−O bond distance observed
within the bridging DMF ligand (La1−O5 = 2.608(2) Å).
Interestingly, the La−ODMF bond lengths are longer relative to
other La−DMF solvento complexes,39−41 likely a function of
the steric constraints imposed by the bulky tris(amidate) ligand.
Lastly, as expected the average La−N bond length (2.62(2) Å)
is longer relative to the κ2-amidate and μ-O:κ2-amidate La−O
bond lengths and is typical of La−N interactions.42−44
Following the structural determination of 1, we attempted to
extend this chemistry to Ce and Nd with the intention of
isolating a series of lanthanide tris(amidate) complexes for
comparative purposes. Accordingly, addition of either CeI3 or
NdCl3 to a DMF solution of K3LtBu results in formation of
[Ce(N(o-PhNC(O)tBu)3)(DMF)]2(μ-DMF) (2) or [Nd(N(o-
PhNC(O)tBu)3)(DMF)]2(μ-DMF) (3), respectively (Scheme
1b). Complex 2 can be isolated as an orange microcrystalline
powder in 50% yield, while complex 3 can be isolated as a pale
purple microcrystalline powder in 51% yield; both have
identical solubility properties as complex 1. Complexes 2 and
3 were structurally characterized by single-crystal X-ray
crystallography, and their solid-state molecular structures can
be found in Figures 3 and 4. A summary of their relevant
structural parameters and full crystallographic details are
tabulated in Tables 1 and 2, respectively.
In the solid-state, 2 is isostructural with 1·C6H14 crystallizing
as a hexane solvate in the monoclinic centrosymmetric space
group C2/c. As observed in 1·C6H14, the average κ2-amidate
Ce−O bond distance (2.475(8) Å) in 2·C6H14 is statistically
equivalent relative to the μ-O:κ2-amidate Ce−O bond distance
(Ce1−O2 = 2.501(2) Å), and both are similar to known Ce−O
interactions.45−47 Further, the μ-O-amidate interaction between
O2 and the second Ce center (Ce1*−O2 = 2.591(2) Å) is
significantly longer relative to both the κ2-amidate and μ-O:κ2-
amidate interactions. Following the expected trends of the
lanthanide contraction, the Ce−O bond distances are shorter
relative to the La−O bond distances observed in 1·C6H14 with
the exception of the κ2-amidate bonding interactions which are
statistically identical.48 The Ce−O bond length within the
terminal DMF ligand (Ce1−O4 = 2.583(2) Å) is equivalent to
the Ce−O bond length observed within the bridging DMF
ligand (Ce1−O4 = 2.585(2) Å) and as observed in 1·C6H14 are
longer relative to other Ce−DMF solvento complexes.49−51
Lastly, the average Ce−N bond distance (2.60(1) Å) is longer
relative to both the average κ2-amidate and μ-O:κ2-amidate
Ce−O bond distances but interestingly is equivalent to the
average La−N bond distance (2.62(2) Å) observed in 1·C6H14.
The solid-state molecular structure of complex 3 (Figure 4)
exhibits identical geometry and connectivity as 1·C6H14 and 2·
C6H14 but crystallizes in the triclinic centrosymmetric space
and/or the increased steric congestion about the smaller
Nd(III) ion,48 both of which may influence the crystal packing
and structural metrics. However, the relatively large error values
associated with 3·C6H14 may also mean that the longer average
μ-O:κ2-amidate Nd−O bond length is not particularly
significant in a chemical bonding sense that requires ration-
alization. The average Nd−O bond distance within the
terminally bound DMF ligands (2.54(2) Å) and the average
Nd−O bond distance within the bridging DMF ligand
(2.541(1) Å) are statistically identical and are slightly shorter
relative to both 1·C6H14 and 2·C6H14. Lastly, the average Nd−
N bond length (2.56(1) Å) is equivalent to the average Ln−N
bond distances observed in 1·C6H14 and 2·C6H14 and is
comparable to other reported Nd−N bonding interac-
tions.45,52,53
There are several lanthanide complexes containing non-
multipodal amidate ligands (only one N,O group per ligand)
reported in the literature, many containing ancillary cyclo-
pentadienyl groups as supporting coligands,21−24,26,28−30,54,55
for example, the dinuclear complex, [Cp′2SmOC(nBu)NPh]2.29
However, more relevant non-organolanthanide examples
include the Y dinuclear tris(amidate) complex, [Y(tBu[O,N]-
(CH3)2Ph)3]2, reported by Schafer and co-workers.19 Each Y
center is bound to three discrete amidate ligands. Similar to 1·
C6H14 - 3·C6H14, two are in a κ2-amidate coordination mode,
and one acts as a bridging ligand to another Y atom through the
amidate oxygen atom via a μ-O:κ2-amidate interaction. Finally,
Yao and co-workers recently reported the anionic, monomeric
lanthanide-lithium tetra(amidate) complexes, Li(THF)Ln-
(C6H5C(O)NC6H3(iPr)2)4(THF) (Ln = La, Nd).20 These
complexes feature four amidate ligands: one group is chelating
(κ2-amidate), two groups are chelating and bridge the metal
centers (μ-O:κ2-amidate), and another group is nonchelating
but still bridges the metal centers (μ-O:μ-N-amidate). The
structural determination of 1·C6H14 - 3·C6H14 reveals that
dinuclear species can persist even when a bulky tripodal
tris(amidate) scaffold is employed. Moreover, the isolation of 2·
C6H14 is the first example of a Ce containing amidate complex;
La and Nd amidates are very rare, with most amidate complexes
pertaining to the middle to late 4f ions, highlighting the scarcity
of coordination chemistry knowledge for early lanthanide
amidate complexes.
Following structural determination, complexes 1−3 were
1
characterized by H NMR spectroscopy. The room-temper-
ature 1H NMR spectrum of 1 in C6D6 features two overlapping
singlets at 0.99 and 1.00 ppm in a 1:2 ratio, respectively,
t
assignable to the Bu protons (see Supporting Information,
Figure S9). On the basis of the solid-state molecular structure
of 1 (vide supra) the observed splitting is likely a consequence
of the κ2-amidate and μ-O:κ2-amidate interactions of the
t
amidate pendant arms causing the Bu protons to be in two
group P1 as a hexane solvate. In contrast to what is observed in
̅
1·C6H14 and 2·C6H14, the average κ2-amidate Nd−O bond
distance (2.45(1) Å) and the average μ-O-amidate Nd−O bond
distance (2.485(6) Å) are equivalent and significantly shorter
relative to the average μ-O:κ2-amidate Nd−O bond distance
(2.53(1) Å). The Nd−O bond distances in 3·C6H14 are
comparable to other reported Nd−O bond interactions.47,52,53
Interestingly, the average μ-O:κ2-amidate Nd−O bond length is
slightly longer than that same bond in 2·C6H14 (2.501(2) Å),
and the average μ-O-amidate Nd−O bond length is much
shorter than the average μ-O-amidate Ln−O bond distances in
1·C6H14 and 2·C6H14. These bonding differences may be due to
several factors including the different crystallization space group
slightly different environments. Three resonances are also
observed for the DMF ligands, observed at 2.09, 2.57, and 7.93
ppm in a 3:3:1 ratio, respectively. The signals attributed to the
DMF ligands differ from noncoordinated DMF in C6D6, thus
supporting the notion that they remain coordinated to the
Ln(III) centers while in solution.56 As expected of an f1 ion, the
1H NMR spectrum of complex 2 in C6D6 is paramagnetically
broadened and exhibits complicated splitting patterns in the
range 0 to +9 ppm (see Supporting Information, Figure S11).
t
Specifically, the Bu protons are tentatively assignable to two
broad resonances at 0.57 and 2.05 ppm in a 1:2 ratio,
respectively. Furthermore, the Me protons of the DMF ligands
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Scheme 2
Figure 5. Solid-state molecular structure of [Ce(N(o-PhNC(O)tBu)3)]4 (4) with 30% probability ellipsoids. Solvent molecules, disordered
components, and hydrogen atoms omitted for clarity. Complex 5 has identical molecular connectivity (see Supporting Information, Figure S20).
are either partially overlapping with other resonances or are
completely obscured. However, a well-isolated but significantly
broadened resonance at 8.55 ppm is assignable to the aldehyde
complexes 2 and 3 are typical of Ln(III) complexes, in which
the electronic transitions are largely energy and line-width
independent of the local ligand coordination environment
about the Ln(III) center (see Supporting Information, Figures
S4 and S5).50,57−60
The use of DMF as a reaction medium to prepare complexes
1−3 results in disordered solvento-bridged dinuclear com-
plexes, thus inhibiting simple comparisons of structural metrics
in pursuit of defining bonding trends across the 4f series.
Therefore, we sought an alternative synthetic approach by
employing a noncoordinating solvent with the goal of isolating
1
proton of the DMF ligands. The H NMR spectrum of 3 is
similar to 2, featuring significant paramagnetic broadening and
complicated splitting patterns in the range −1 to +11 ppm (see
Supporting Information, Figure S12). Complexes 1−3 were
further characterized by solid-state IR spectroscopy (KBr mull)
which exhibit spectra similar to one another and are dominated
by the stretches of the tris(amidate) ligands (see Supporting
Information, Figure S2). Lastly, the UV−vis spectra of
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monomeric Ln(III) tris(amidate) species via protonolysis, a
method that has proven successful in similar systems.19,20
Accordingly, addition of H3LtBu to a tol-d8 solution of
Ce[N(SiMe3)2]3 resulted in the appearance of several new
paramagnetically shifted and broadened resonances at −25.2,
−13.7, 3.36, 4.93, 8.16, and 17.4 ppm over the course of several
centers. For example, both the average κ2-amidate Ce−O bond
distance (2.397(5) Å) and the average μ-O-amidate Ce−O
bond distance (2.501(6) Å) are significantly shorter relative to
the average μ-O:κ2-amidate Ce−O bond distance (2.610(9) Å).
The average Ce−N bond length (2.57(4) Å) of the 8-
coordinate Ce centers in 4·5C6H14 is equivalent to the average
Ce−N bond length of the 7-coordinate Ce centers and to the
Ln−N bonding interactions observed in 1·C6H14-3·C6H14.
Notably, both the average κ2-amidate and μ-O-amidate Ce−O
bond distances in 4·5C6H14 are shorter in comparison to those
same bonds in dinuclear 2·C6H14, while the average μ-O:κ2-
amidate Ce−O bond distance in 4·5C6H14 is significantly
longer. These observations are likely due to the lower
coordination number of the Ce(III) ions and the tetranuclear
configuration of the Ce(III) nuclei in 4·5C6H14, thus resulting
in a less sterically congested ligand coordination environment.8
It should also be noted that both the average μ-O:κ2-amidate
and μ-O-amidate bond distances of the 8-coordinate centers are
statistically longer relative to those same bonds of the 7-
coordinate centers. This may be a consequence of the larger
coordination number and greater steric congestion about the 8-
coordinate metal ion.
1
hours as observed by H NMR spectroscopy (see Supporting
Information, Figure S16). The slow reactivity is likely due to
the poor solubility of H3LtBu in tol-d8; however, upon mild
heating, H3LtBu readily dissolves, and the reaction is complete
within 1 h (see Supporting Information, Figure S17). With
these results in hand, we sought to isolate the paramagnetic
1
product observed in the in situ H NMR spectra. Interestingly,
addition of 1 equiv of H3LtBu to a toluene solution of either
Ce[N(SiMe3)2]3 or Nd[N(SiMe3)2]3, followed by gentle
heating, affords the tetranuclear complexes, [Ln(N(o-PhNC-
(O)tBu)3)]4 (Ln = Ce (4), Nd (5)), respectively, rather than
the anticipated monomeric product (Scheme 2).
Both complexes 4 and 5 were structurally characterized by
single-crystal X-ray crystallography. The solid-state molecular
structure of 4 can be found in Figure 5, while the solid-state
molecular structure of 5 can be found in the Supporting
Information. A summary of relevant structural parameters for
complexes 4 and 5 are tabulated in Table 1, while full
crystallographic details are tabulated in Table 2. Complexes 4
and 5 crystallize in the monoclinic centrosymmetric space
group C2/c as hexane solvates and are isostructural. In the
solid-state, 4·5C6H14 and 5·5C6H14 feature tetranuclear cores
with two 7-coordinate Ln(III) centers in distorted trigonal
prismatic geometries and two 8-coordinate Ln(III) centers in,
according to Haigh’s criteria, distorted dodecahedron geo-
metries.61 Each 7-coordinate Ln center is bound to one tripodal
tris(amidate) ligand with two pendant arms in a κ2-amidate
chelating coordination mode and one arm in a μ-O:κ2-amidate
coordination mode. Each 8-coordinate Ln center is also bound
to one tripodal tris(amidate) ligand but with only one pendant
arm exhibiting κ2-amidate chelation, while the other two arms
are in a μ-O:κ2-amidate coordination modes. To complete the
tetranuclear structure the 7-coordinate Ln center forms an μ-O-
amidate interaction with an adjacent 8-coordinate Ln ion, while
each 8-coordinate Ln center has two pendant arms involved in
μ-O-amidate bridging interactions: one arm forms a bridging
interaction with the other 8-coordinate Ln center, and another
arm forms a bridging interaction with a nearby 7-coordinate Ln
center. As a consequence of this complicated multinuclear
structure, some of the bond types are not equivalent between
the 7- and 8-coordinate centers, and thus the Ln(III) centers
will be discussed separately regarding their metrical parameters.
Lastly, it should be noted, as observed in 1·C6H14-3·C6H14, the
average O−C and N−C bond lengths (4·5C6H14: C−O =
1.31(1) Å, C−N: 1.31(1) Å; 5·5C6H14: C−O: 1.30(1) Å, C−N:
1.30(1) Å) indicate electron delocalization through the κ2-
amidate and μ-O:κ2-amidate backbone.
Complex 5·5C6H14 exhibits similar bonding trends to the
isostructural Ce tetranuclear complex. Specifically, the 7-
coordinate Nd(III) centers exhibit an average κ2-amidate
Nd−O bond distance (2.38(2) Å) and an average μ-O-amidate
Nd−O bond distance (2.404(4) Å) which are shorter relative
to the average μ-O:κ2-amidate Nd−O bond distance (2.515(1)
Å). Furthermore, the average Nd−N bond length (2.51(1) Å)
is statistically identical to the average Nd−N bond length
(2.56(1) Å) in 3·C6H14. The 8-coordinate Nd(III) centers in 5·
5C6H14 exhibit an average κ2-amidate Nd−O bond distance
(2.38(1) Å) and an average μ-O-amidate Nd−O bond distance
(2.475(9) Å) which are also significantly shorter relative to the
average μ-O:κ2-amidate Nd−O bond distance (2.574(8) Å). As
observed in 4·5C6H14, both the average μ-O:κ2-amidate and μ-
O-amidate bond distances of the 8-coordinate centers are
statistically longer relative to those same bonds of the 7-
coordinate centers. Lastly, the average Nd−N bond length
(2.54(4) Å) is statistically identical to both the average Nd−N
bond length of the 7-cordinate metal centers and to 3·C6H14.
Similar to complexes 1−3, the solid-state molecular
structures of 4 and 5 feature a tripodal tris(amidate) ligand
with a combination of κ2-amidate, μ-O:κ2-amidate, and μ-O-
amidate ligation modes not observed before with this particular
scaffold and the transition metals. Further, the isolation and
structural characterization of 4 and 5 demonstrate how a
tripodal amidate ligand platform can support unprecedented
tetranuclear cluster formation with the lanthanides, that are not
formed with non-multipodal amidate ligands.19−30 The absence
of a coordinating solvent does not overcome the preference of
large Ln(III) ions for higher coordination numbers, and it
appears that despite the steric bulk of the tripodal amidate
ligands mononuclear complexes are not accessible. In fact, and
unpredictably, novel tetranuclear cores are formed instead in 4
and 5 containing two types of Ln(III) coordination environ-
ments with different coordination numbers (seven and eight).
In complex 4·5C6H14, the 7-coordinate Ce centers exhibit an
average κ2-amidate Ce−O bond distance (2.41(3) Å) and an
average μ-O-amidate Ce−O bond distance (2.437(2) Å), which
are significantly shorter in comparison to the average μ-O:κ2-
amidate Ce−O bond distance (2.548(9) Å). The average Ce−
N bond length (2.54(2) Å) is statistically equivalent relative to
the average Ce−N bond length (2.60(1) Å) in 2·C6H14 and is
comparable to previously reported Ce−N bonding interac-
tions.45,46,62 The 8-coordinate Ce(III) centers in 4·5C6H14
exhibit similar metrical trends as the 7-coordinate Ce(III)
1
Complexes 4 and 5 exhibit complicated H NMR spectra.
1
For example, the H NMR spectrum of 5 in C6D6 features six
broad and paramagnetically shifted resonances at −18.6, −11.9,
2.67, 3.77, 9.56, and 11.5 ppm in a 1:1:1:1:1:1 ratio, assignable
t
to the Bu protons of the amidate ligands (see Supporting
Information, Figure S15). Unfortunately, the high solubility of
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4 in both nonpolar and ethereal solvents impedes its
crystallization in meaningful yields, and beyond the isolation
of a few single crystals suitable for X-ray crystallography,
characterization studies of 4 were performed on the crude
material. Following extraction into hexanes, the 1H NMR
spectrum of the crude reaction mixture containing complex 4 in
C6D6 exhibits similar features as 5 (see Supporting Information,
Figure S13). Lastly, complexes 4 and 5 were characterized by
IR spectroscopy (KBr mull), featuring spectra similar to 1−3,
and by UV−vis spectroscopy, in which the reported spectra are
typical of Ln(III) ions (see Supporting Information).50,57−60
Since mononuclear complexes were not formed under the
reaction conditions studied, disruption of the di- and tetra-
nuclear complexes was explored, specifically with Lewis bases,
in an attempt to break up the multinuclear cores. Monitoring
the addition of excess triphenylphosphine oxide (TPPO) to a
DMF-d7 solution of complex 1 by multinuclear NMR
spectroscopies revealed no observable reactivity. Addition of
more basic ligands such as excess acetonitrile (MeCN),
pyridine or dimethylaminopyridine (DMAP) also did not
tetranuclear complexes 4−5 demonstrate how switching from
simple nonpodal amidate ligands to a multipodal amidate
framework provides access to novel Ln(III) amidate complexes
within the 4f series. Finally, complexes 2 and 4 are the first
examples of cerium coordinated to amidate ligands, helping to
fill a knowledge gap regarding the coordination chemistry of
the early trivalent lanthanide ions with mixed N,O donors.
EXPERIMENTAL SECTION
■
General. All reactions and subsequent manipulations were
performed under anaerobic and anhydrous conditions either under
high vacuum or in an atmosphere of high purity argon gas.
Dichloromethane (DCM), diethyl ether (Et2O), 1,2-dimethoxyethane
(DME), N,N-dimethylformamide (DMF), hexanes, n-pentanes,
tetrahydrofuran (THF), and toluene were purchased anhydrous
from Sigma-Aldrich and stored over a mixture of activated 3 and 4
Å molecular sieves for at least 48−72 h before use. Benzene-d6, N,N-
dimethylformamide-d7, and toluene-d8 were dried over a mixture of
activated 3 and 4 Å molecular sieves for at least 48 h before use. The
lanthanide tris(amides), Ln[N(SiMe3)2]3 (Ln = Ce, Nd), were
prepared according to modified literature procedures (vide
infra).65−68 K[N(SiMe3)2] was recrystallized from toluene before
use. 2,2′,2″-tris(pivalamidotriphenyl)amine (N(o-PhNHC(O)tBu)3,
H3LtBu) was prepared as previously described.31 All other reagents
were purchased from commercial suppliers and used as received.
NMR spectra were recorded on a Bruker Avance 400 MHz
spectrometer. 1H and 13C{1H} NMR spectra were referenced to
external SiMe4 using the residual protio solvent peaks as internal
standards (1H NMR experiments) or the characteristic resonances of
the solvent nuclei (13C NMR experiments). IR spectra were recorded
on a PerkinElmer FTIR spectrometer as KBr mulls. UV−vis
experiments were performed either on an Agilent 8453 UV−vis
spectrometer (complexes 2 and 4) or a Cary 5E UV−vis
spectrophotometer (complexes 3 and 5). Elemental analyses for
complexes 1−3 were performed by the Micro-Elemental Laboratory at
ALS Environmental (Columbia Analytical Services).
1
reveal any observable reactivity as monitored by H NMR
spectroscopy, even over prolonged reaction times. The lack of
reactivity may be a consequence of the oxophilic nature of the
Ln(III) ions and/or the steric protection afforded by the bulky
tert(butyl)-substituted amidate ligands. More likely though,
especially in the case of MeCN, is the notion that certain Lewis
bases weakly coordinate to the lanthanides; thus, failure to
substitute the strongly coordinating DMF solvent molecules
may not be unexpected.63,64 In contrast, addition of excess
DMF to a C6D6 solution of tetranuclear complex 5 resulted in
rapid in situ conversion to dinuclear complex 3 as observed by
1H NMR spectroscopy. Following removal of all volatiles in
vacuo, the 1H NMR spectrum of the crude reaction material in
C6D6 revealed the absence of any resonances associated with
complex 5 and is consistent with the formation of complex 3
(see Supporting Information, Figure S19). The observation that
complex 5 is structurally disrupted by DMF but not by, if only
slightly, more basic ethereal solvents (e.g., THF) is likely a
consequence of DMF’s high polarity and its large partial
negative charge on the donating O atom, resulting in an ability
to bind strongly to hard Ln(III) ions.
Synthesis of N(o-PhNKC(O)tBu)3 (K3LtBu). To a stirring
suspension of N(o-PhNHC(O)tBu)3 (H3LtBu, 102 mg, 0.188 mmol)
in DME (7 mL) was added excess potassium hydride (KH, 28 mg,
0.70 mmol) as an off-white/gray powder. Over the course of several
hours, the solution gradually changed from a white suspension to a
slightly particulated orange solution. Mild heat and vigorous bubbling
was observed, presumed to be the expected hydrogen reaction
byproduct. After ∼18 h, the solution turned to a heavily particulated,
pink-peach solution. The pink-peach material was isolated on a
medium porosity filter frit and washed with excess DME (5 mL) and
DCM (6 mL) to remove unreacted H3LtBu. The material was then
dried in vacuo for 2 h before use (112 mg, 91%). 1H NMR (DMF-d7,
25 °C, 400 MHz): δ 0.78 (s, 27 H, tBu), 6.26 (t, 3 H, JHH = 8.0 Hz,
aryl CH), 6.53 (m, 6 H, JHH = 7.7 Hz, aryl CH), 7.55 (d, 3 H, JHH = 7.0
Hz, aryl CH). Note: The proteo derivative of the triamidoamine
ligand, N(o-PhNHC(O)tBu)3 (H3LtBu), exhibits a resonance at 8.99
ppm in its 1H NMR spectrum in DMF-d7 assignable to the NH
protons. This resonance is not observed in the 1H NMR spectra of the
in situ synthesis of K3LtBu or the isolated material of K3LtBu (see the
Supporting Information for corresponding spectra). IR (KBr pellet,
cm−1): 2967(s), 2922(2), 2867(s), 1660(w), 1650(w sh), 1589(m),
1514(s), 1443(s), 1399(s), 1360(s), 1302(sh), 1278(s), 1259(s),
1229(s), 1185(m), 1171(w), 1155(m), 1109(s), 1083(sh), 1045(m),
1025(w), 997(w), 937(s), 933(s), 904(s), 855(w sh), 844(m), 811(w),
799(w), 771(s), 752(s), 737(sh), 699(w), 678(w), 629(m), 619(sh),
553(m), 511(m), 478(w), 459(m).
SUMMARY
■
The tripodal tris(amidate) ligand, [N(o-PhNC(O)tBu)3]3−,
readily coordinates to Ln(III) ions (Ln = La, Ce, Nd) with a
combination of κ2-amidate, μ-O:κ2-amidate, and μ-O-amidate
ligation modes not observed in previously reported transition
metal-amidate complexes. Specifically, synthesis and isolation of
the dinuclear solvento complexes, [Ln(N(o-PhNC(O)tBu)3)-
(DMF)]2(μ-DMF) (Ln = La (1), Ce(2), Nd(3)), were
achieved in good yields by employing the lanthanide trihalide
precursors in the presence of the isolated amidate salt, K3[N(o-
PhNC(O)tBu)3], in DMF. In contrast, in noncoordinating
solvents, the tetranuclear complexes, [Ln(N(o-PhNC-
(O)tBu)3)]4 (Ln = Ce (4), Nd(5)), were isolated with no
evidence for generation of mononuclear species under the
conditions examined. The bridged multinuclear complexes
synthesized, isolated and structurally characterized likely occur
as a consequence of the preference of the large early Ln(III)
ions to achieve higher coordination numbers compared to
smaller transition metals. While the dinuclear complexes 1−3
highlight differences between transition metal and lanthanide
amidate bonding trends with multipodal amidate scaffolds, the
Synthesis of Ce[N(SiMe3)2]3. Finely ground CeBr3 (57 mg, 0.15
mmol) was suspended in THF (1 mL) to which 2.9 equiv of
K[N(SiMe3)2] (87 mg, 0.44 mmol) was added dropwise as a THF
solution (2 mL). No color change was observed with the addition. The
solution was heated gently for 10 min resulting in a color change from
colorless to bright yellow concomitant with the deposition of a fine
white particulate. The solution was removed from heating, allowed to
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cool to room temperature, and filtered through a Celite column (0.5
cm × 2 cm) supported on glass wool. All volatiles were removed in
vacuo, and the material was extracted into hexanes (3 mL) and filtered
through a new Celite column (0.5 cm × 2 cm) supported on glass
wool. All volatiles were removed in vacuo to give a bright yellow solid
(75 mg, 83%). 1H NMR (C6D6, 25 °C, 400 MHz): δ −3.46 ppm (s, 54
H, SiMe3). By 1H NMR spectroscopy, the material is spectroscopically
identical to previous reports.67,68
suitable for X-ray analysis were grown from a toluene/hexanes vapor
diffusion solution. Anal. Calcd for C75H99Ce2N11O9·(0.5C6H14): C,
1
57.76; H, 6.59; N, 9.50 Found: C, 57.95; H, 6.85; N, 8.77. H NMR
(C6D6, 25 °C, 400 MHz): δ 0.57 (br s, 18 H, Me), 2.05 (br s, 45 H,
Me, 2 and DMF), 2.50 (br s, 9 H, Me, DMF), 5.91 (br s, 4 H, aryl
CH), 6.09 (br s, 3 H, aryl CH), 6.38 (br s, 5 H, aryl CH), 7.07 (br s, 6
H, aryl CH), 7.57 (br s, 4H, aryl CH), 7.76 (br s, 2 H, aryl CH), 8.55
(br s, 3 H, CH, DMF). UV−vis (toluene, 2.6 × 10−5 M): λmax = 295
nm. IR (KBr pellet, cm−1): 1685(sh), 1678(sh), 1664(sh), 1655(s),
1651(sh), 1632(sh), 1623(w), 1594(m), 1560(m), 1542(m), 1524(s),
1494(m), 1481(s), 1460(sh), 1444(s), 1400(m), 1386(w), 1358(m),
1344(w), 1306(m), 1274(m), 1250(w), 1222(m), 1196(w), 1183(w),
1173(w), 1161(m), 1116(w), 1100(m), 1066(w), 1053(w), 1037(w),
945(w), 933(m), 910(w), 868(w), 839(w), 800(w), 768(sh), 754(s),
746(m), 693(w), 669(m), 651(w), 626(m), 598(w), 596(w), 577(w),
567(w), 545(w), 500(w), 482(w).
Synthesis of Nd[N(SiMe3)2]3. Finely ground NdCl3 (72 mg, 0.29
mmol) was suspended in THF (2 mL) to which 2.9 equiv of
K[N(SiMe3)2] (160 mg, 0.802 mmol) was added dropwise as a THF
solution (2 mL). No color change was observed with the addition. The
solution was heated gently for 10 min resulting in a color change from
colorless to pale blue concomitant with the deposition of a fine white
particulate. The solution was removed from heating, allowed to cool to
room temperature, and filtered through a Celite column (0.5 cm × 2
cm) supported on glass wool. All volatiles were removed in vacuo, and
the material extracted into hexanes (3 mL) and filtered through a new
Celite column (0.5 cm × 2 cm) supported on glass wool. This step was
repeated a second time. All volatiles were removed in vacuo to give a
Synthesis of [Nd(N(o-PhNC(O)tBu)3)(DMF)]2(μ-DMF) (3). To a
pale orange solution of N(o-PhNKC(O)tBu)3 (K3LtBu, 114 mg, 0.174
mmol) in DMF (4 mL) was added finely ground NdCl3 (47 mg, 0.188
mmol) as a pale purple crystalline powder. Upon the addition, the
solution became less intense in color. Once the NdCl3 was observed to
be fully dissolved (∼10 min), all volatiles were removed in vacuo for
∼3 h until the crude material was completely dry. The material was
then extracted into toluene (4 mL) and filtered through a Celite
column (0.5 cm × 2 cm) supported on glass wool to give a pale purple
filtrate. All volatiles were removed in vacuo, and the material was
extracted into toluene (2 mL) for a second time. The solution was
filtered through a new Celite column (0.5 cm × 2 cm) supported on
glass wool to give a pale purple filtrate. This filtrate was concentrated
to less than 1 mL and subsequently layered with excess hexanes (∼6
mL). Storage of this solution at −35 °C for 24 h resulted in the
deposition of pale purple crystals (70 mg, 51%). Crystals suitable for
X-ray analysis were grown from a toluene/hexanes vapor diffusion
solution. Anal. Calcd for C75H99N11Nd2O9·(0.5C6H14)(C7H8): C,
1
pale blue solid (113 mg, 68%). H NMR (C6D6, 25 °C, 400 MHz): δ
−6.25 ppm (s, 54 H, SiMe3). By 1H NMR spectroscopy, the material is
spectroscopically identical to previous reports.66,67
Synthesis of [La(N(o-PhNC(O)tBu)3)(DMF)]2(μ-DMF) (1). To a
pale orange solution of N(o-PhNKC(O)tBu)3 (K3LtBu, 61 mg, 0.093
mmol) in DMF (2 mL) was added finely ground LaBr3 (37 mg, 0.098
mmol) as a white crystalline powder. Upon the addition, the solution
became less intense in color. Once the LaBr3 was observed to be fully
dissolved (∼15 min), all volatiles were removed in vacuo for ∼1.5 h
until the crude material was completely dry. The material was then
extracted into toluene (3 mL) and filtered through a Celite column
(0.5 cm × 2 cm) supported on glass wool to give a pale yellow, almost
colorless, filtrate. This filtrate was concentrated to less than 1 mL and
subsequently layered with excess hexanes (∼5 mL). Storage of this
solution at −35 °C for 24 h resulted in the deposition of a white
microcrystalline powder (47 mg, 64%). Crystals suitable for X-ray
analysis were grown from a toluene/hexanes vapor diffusion solution.
Anal. Calcd for C75H99La2N11O9·(C6H14): C, 58.25; H, 6.45; N, 9.06
Found: C, 57.99; H, 6.56; N, 8.81. 1H NMR (C6D6, 25 °C, 400 MHz):
δ 0.99−1.00 (m, 54 H total, Me), 2.09 (s, 9 H, Me, DMF), 2.57 (s, 9
H, Me, DMF), 6.84 (q, 8 H, JHH = 7.9 Hz, aryl CH), 6.89 (q, 10 H, JHH
= 7.9 Hz, aryl CH), 7.21 (t, 6 H, JHH = 7.8 Hz, aryl CH), 7.93 (s, 3 H,
CH, DMF). 13C{1H} NMR (C6D6, 25 °C, 100 MHz): δ 29.6 (Me),
32.3 (Me, DMF), 36.7 (Me, DMF), 41.7 (CMe), 121.8, 123.9, 125.5,
129.8, 140.6, 147.2, 165.9 (CH, DMF), 181.7 (CO). IR (KBr pellet,
cm−1): 1685(w), 1676(sh), 1670(sh), 1664(sh), 1655(s), 1652(sh),
1640(sh), 1634(sh), 1619(sh), 1593(m), 1578(sh), 1571(sh),
1560(s), 1543(s), 1535(s), 1511(sh), 1507(s), 1499(s), 1480(s),
1465(m), 1444(s), 1427(w), 1400(s), 1377(m), 1356(s), 1334(s),
1311(sh), 1274(m), 1218(m), 1193(m), 1190(m), 1166(sh),
1119(sh), 1101(m), 1063(sh), 1041(w), 1037(w), 944(sh), 934(m),
924(w), 908(w), 868(w), 837(w), 804(w), 753(s), 740(sh), 694(w),
669(m), 651(w), 622(m), 598(w), 585(w), 579(w), 545(w), 502(w),
481(w).
1
59.27; H, 6.67; N, 8.95 Found: C, 59.22; H, 6.87; N, 8.58. H NMR
(C6D6, 25 °C, 400 MHz): δ 0.10 (br s, 18 H, Me), 1.57−1.74 (br m,
45 H, Me, 3 and DMF), 2.96 (s, 9 H, Me, DMF), 5.78 (br s, 4 H, aryl
CH), 5.87−5.95 (br m, 6 H, aryl CH), 6.30 (br s, 3 H, aryl CH), 6.61
(br s, 4 H, aryl CH), 7.35−7.42 (br m, 7 H, aryl CH), 9.99 (br s, 3 H,
CH, DMF). UV−vis (toluene, 5.1 × 10−3 M, nm): λmax = 806, 802,
750, 744, 741, 738, 679, 598, 588, 586, 583, 578, 529, 514, 475, 462,
431. IR (KBr pellet, cm−1): 1686(sh), 1659(s), 1651(sh), 1633(sh),
1622(sh), 1592(m), 1562(sh), 1549(s), 1542(s), 1496(sh), 1480(s),
1448(m), 1401(s), 1378(m), 1356(s), 1340(s), 1313(sh), 1273(m),
1219(s), 1195(s), 1191(s), 1161(sh), 1142(w), 1101(m), 1062(m),
1039(m), 946(m), 934(m), 909(m), 865(w), 836(w), 806(w),
766(sh), 753(s), 742(sh), 693(m), 669(m), 661(w), 621(m),
598(w), 585(w), 579(w), 544(w), 511(sh), 501(m), 481(w).
Synthesis of [Ce(N(o-PhNC(O)tBu)3)]4 (4). To a yellow solution
of Ce(NR2)3 (R = SiMe3, 28 mg, 0.045 mmol) in toluene (2 mL) was
added N(o-PhNHC(O)tBu)3 (H3LtBu, 27 mg, 0.050 mmol) as a white
powder. No color change was observed, and H3LtBu was largely
insoluble. The solution was then gently heated for ∼30 min during
which H3LtBu slowly solubilized, and the solution turned to an orange
color. The solution was then removed from heat, allowed to cool to
room temperature, and filtered through a Celite column (0.5 cm × 2
cm) supported on glass wool to give an orange filtrate. All volatiles
were removed in vacuo, and the material was extracted into hexanes (2
mL) and filtered through a new Celite column (0.5 cm × 2 cm)
supported on glass wool to give a slightly darker orange filtrate.
Extraction into hexanes (1 mL) was repeated for a second time. This
material crystallizes poorly from hexanes, toluene, and ethereal
solvents due to its extremely high solubility resulting in intractable
yields. However, a small amount of pale orange crystals suitable for X-
ray analysis were grown at −35 °C from a toluene/hexanes vapor
diffusion solution. Except X-ray crystallography, all characterization on
complex 4 was performed on the crude reaction material, isolated by
removal of volatiles from the crude reaction mixture, extraction into
hexanes, and filtration through a Celite column (0.5 cm × 2 cm)
supported on glass wool. After exposure to reverse pressure for several
Synthesis of [Ce(N(o-PhNC(O)tBu)3)(DMF)]2(μ-DMF) (2). To a
pale orange solution of N(o-PhNKC(O)tBu)3 (K3LtBu, 57 mg, 0.088
mmol) in DMF (3 mL) was added finely ground CeI3 (48 mg, 0.092
mmol) as a green-yellow crystalline powder. Upon the addition, the
solution became less intense in color. Once the CeI3 was observed to
be fully dissolved (∼5 min), all volatiles were removed in vacuo for ∼2
h until the crude material was completely dry. The material was then
extracted into toluene (4 mL) and filtered through a Celite column
(0.5 cm × 2 cm) supported on glass wool to give a pale orange filtrate.
All volatiles were removed in vacuo, and the material was extracted
into toluene (2 mL) for a second time. The solution was filtered
through a new Celite column (0.5 cm × 2 cm) supported on glass
wool to give a slightly darker orange filtrate. This filtrate was
concentrated to less than 1 mL and subsequently layered with excess
hexanes (∼5 mL). Storage of this solution at −35 °C for 24 h resulted
in the deposition of pale orange crystals (34 mg, 50%). Crystals
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Inorg. Chem. 2015, 54, 4064−4075

Inorganic Chemistry
Article
hours, an orange powder was afforded (26 mg, 85% crude). Elemental
analysis of 4 was not performed due to extremely low crystallization
yields despite modifying solvent conditions, solution concentrations
and temperature of crystallization. 1H NMR (C6D6, 25 °C, 400 MHz):
δ −25.56 (br s, 18 H, Me), −13.80 (br s, 18 H, Me), 3.44 (br s, 18 H,
Me), 5.04 (br s, 18 H, Me), 6.62 (d, 4 H, JHH = 3.0 Hz, aryl CH), 7.06
(d, 4 H, JHH = 4.0 Hz, aryl CH), 7.41 (t, 4 H, JHH = 7.0 Hz, aryl CH),
7.52 (t, 4 H, JHH = 7.0 Hz, aryl CH), 7.62 (t, 4 H, JHH = 7.0 Hz, aryl
CH), 7.81 (d, 4 H, JHH = 7.0 Hz, aryl CH), 8.05 (t, 4 H, JHH = 6.0 Hz,
aryl CH), 8.35 (br s, 18 H, Me), 8.42 (t, 4 H, JHH = 7.0 Hz, aryl CH),
8.77 (d, 4 H, JHH = 6.0 Hz, aryl CH), 8.92 (m, 4 H, aryl CH), 9.59 (m,
4 H, aryl CH), 10.18 (t, 4 H, JHH = 6.0 Hz, aryl CH), 17.62 (br s, 18
H, Me). UV−vis (toluene, 1.2 × 10−5 M): λmax = 298 nm. IR (KBr
pellet, cm−1): 1686(sh), 1655(m), 1660(sh), 1645(sh), 1634(w),
1594(m), 1560(sh), 1542(sh), 1555(s), 1498(w), 1481(s), 1445(s),
1400(s), 1376(w), 1355(m), 1334(sh), 1318(m), 1274(m), 1247(sh),
1218(m), 1195(sh), 1184(m), 1158(sh), 1118(sh), 1101(w),
1040(w), 1032(w), 934(s), 913(m), 840(m), 803(w), 766(sh),
755(s), 742(sh), 699(w), 669(w), 656(w), 623(w), 587(w), 544(w),
505(w), 493(w).
was monitored via analysis of redundant frames. The structure was
solved using direct methods and difference Fourier techniques. All
hydrogen atom positions were idealized and rode on the atom they
were attached to. Structure solution, refinement, graphics, and creation
of publication materials were performed using SHELXTL.72 The
program PLATON-SQUEEZE was used to remove disordered solvent
molecules from the unit cell where appropriate, and details are in the
crystallographic files.73 A summary of relevant crystallographic data for
1·C6H14, 2·C6H14, 3·C6H14, 4·5C6H14, and 5·5C6H14 is presented in
Table 2.
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic details (as CIF files) for 1·C6H14, 2·C6H14, 3·
C6H14, 4·5C6H14, and 5·5C6H14 and spectral data for 1−5. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: gaunt@lanl.gov.
■
Synthesis of [Nd(N(o-PhNC(O)tBu)3)]4 (5). A toluene suspension
of N(o-PhNHC(O)tBu)3 (H3LtBu, 28.2 mg, 0.052 mmol) (2 mL) was
heated until it formed a colorless solution (∼45 min). Upon cooling to
room temperature, 0.9 equiv of Nd(NR2)3 (R = SiMe3, 28.6 mg, 0.046
mmol) was added as a pale blue powder. A slight color change to pale
purple was observed after an additional 30 min of stirring at room
temperature. The solution was then filtered through a Celite column
(0.5 cm × 2 cm) supported on glass wool to give a pale purple filtrate.
All volatiles were removed in vacuo, and the material was extracted
into hexanes (4 mL) and filtered over a new Celite column (0.5 cm ×
2 cm) supported on glass wool. All volatiles were removed in vacuo,
and the material was extracted into Et2O (1 mL) and filtered a third
time. The filtrate was concentrated to less than 1 mL and set up as a
reverse vapor diffusion with hexanes (∼2 mL). Storage of this solution
at −35 °C for 24 h resulted in the deposition of pale purple crystals
suitable for X-ray analysis (20 mg, 64%). Even with multiple
crystallizations, a persistent impurity of unknown formulation was
observed at ∼0.3 ppm in the 1H NMR spectra of the crystals inhibiting
*E-mail: cora.macbeth@emory.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Lanthanide chemistry and characterization was performed
under a U.S. Department of Energy, Office of Science, Basic
Energy Sciences, Early Career Program award (contract DE-
AC52-06NA25396). J.L.B. thanks the G. T. Seaborg Institute at
Los Alamos National Laboratory for a Postdoctoral Fellowship.
We also thank Timothy J. Boyle from Sandia National
Laboratories for obtaining the crystallographic data for complex
5·5C6H14 and the grateful use of the Bruker X-ray
diffractometer via the National Science Foundation CRIF:MU
award to Prof. Kemp of the University of New Mexico
(CHE04-43580).
1
characterization by elemental analysis. H NMR (C6D6, 25 °C, 400
MHz): δ −18.61 (br s, 18 H, Me), −11.94 (br s, 18 H, Me), 2.67 (br s,
18 H, Me), 3.77 (br s, 18 H, Me), 4.76 (m, 4 H, aryl CH), 5.15 (s, 2
H, aryl CH), 5.55 (s, 2 H, aryl CH), 6.14−6.37 (m, 12 H, aryl CH),
6.88 (m, 4 H, aryl CH), 7.04 (t, 2 H, JHH = 6.0 Hz, aryl CH), 7.51−
7.60 (m, 16 H, aryl), 8.58 (s, 2 H, aryl CH), 8.77 (s, 2 H, aryl CH),
8.84 (s, 2 H, aryl CH), 9.56 (br s, 18 H, Me), 11.48 (br s, 18 H, Me).
UV−vis (toluene, 2.5 × 10−3 M, nm): λmax = 808, 804, 752, 747, 742,
738, 682, 601, 590, 588, 582, 580, 529, 516, 478, 465, 434. IR (KBr
pellet, cm−1): 1687(sh), 1655(w), 1642(w), 1589(sh), 1544(s),
1494(sh), 1480(s), 1447(s), 1400(s), 1378(m), 1356(s), 1334(sh),
1320(s), 1272(s), 1218(s), 1193(sh), 1183(s), 1155(sh), 1120(sh),
1101(m), 1040(m), 1033(sh), 945(sh), 933(s), 908(s), 860(w),
835(w), 799(w), 767(sh), 753(s), 738(sh), 693(m), 669(w),
647(w), 621(m), 597(w), 584(m), 541(w), 500(m), 479(w).
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