Experimental and theoretical studies of45Sc NMR interactions in solids

Article abstract of DOI:10.1021/ja060477w

Solid-state ⁴⁵Sc NMR spectroscopy, ab initio calculations, and X-ray crystallography are applied to examine the relationships between ⁴⁵Sc NMR interactions and molecular structure and symmetry. Solid-state ⁴⁵Sc (I = 7/2) magic-angle spinning (MAS) and static NMR spectra of powdered samples of Sc-(acac)₃, Sc(TMHD)₃, Sc(NO₃)₃·5H₂O, Sc(OAc)₃, ScCl₃·GH₂O, ScCl₃·3THF, and ScCp₃ have been acquired. These systems provide a variety of scandium coordination environments yielding an array of distinct ⁴⁵Sc chemical shielding (CS) and electric field gradient (EFG) tensor parameters. Acquisition of spectra at two distinct magnetic fields allows for the first observations of scandium chemical shielding anisotropy (CSA). ⁴⁵Sc quadrupolar coupling constants (C_Q) range from 3.9 to 13.1 MHz and correlate directly with the symmetry of the scandium coordination environment. Single-crystal X-ray structures were determined for Sc(TMHD)₃, ScCl₃·6H₂O, and Sc(NO₃) ₃·5H₂O to establish the hitherto unknown scandium coordination environments. A comprehensive series of ab initio calculations of EFG and CS tensor parameters are in excellent agreement with the observed parameters. Theoretically determined orientations of the NMR interaction tensors allow for correlations between NMR tensor characteristics and scandium environments. Solid-state ⁴⁵Sc, ¹³C, and ¹⁹F NMR experiments are also applied to characterize the structures of the microcrystalline Lewis acid catalyst Sc(OTf)₃ (for which the crystal structure is unknown) and a noncrystalline, microencapsulated, polystyrene-supported form of the compound.

Full text of DOI:10.1021/ja060477w

Published on Web 07/25/2006
Experimental and Theoretical Studies of 45Sc NMR
Interactions in Solids
Aaron J. Rossini and Robert W. Schurko*
Contribution from the Department of Chemistry and Biochemistry, UniVersity of Windsor,
Windsor, Ontario, Canada N9B 3P4
Received January 20, 2006; E-mail: rschurko@uwindsor.ca
Abstract: Solid-state 45Sc NMR spectroscopy, ab initio calculations, and X-ray crystallography are applied
to examine the relationships between 45Sc NMR interactions and molecular structure and symmetry. Solid-
state 45Sc (I ) 7/2) magic-angle spinning (MAS) and static NMR spectra of powdered samples of Sc-
(acac)3, Sc(TMHD)3, Sc(NO3)3‚5H2O, Sc(OAc)3, ScCl3‚6H2O, ScCl3‚3THF, and ScCp3 have been acquired.
These systems provide a variety of scandium coordination environments yielding an array of distinct 45Sc
chemical shielding (CS) and electric field gradient (EFG) tensor parameters. Acquisition of spectra at two
distinct magnetic fields allows for the first observations of scandium chemical shielding anisotropy (CSA).
45Sc quadrupolar coupling constants (CQ) range from 3.9 to 13.1 MHz and correlate directly with the
symmetry of the scandium coordination environment. Single-crystal X-ray structures were determined for
Sc(TMHD)3, ScCl3‚6H2O, and Sc(NO3)3‚5H2O to establish the hitherto unknown scandium coordination
environments. A comprehensive series of ab initio calculations of EFG and CS tensor parameters are in
excellent agreement with the observed parameters. Theoretically determined orientations of the NMR
interaction tensors allow for correlations between NMR tensor characteristics and scandium environments.
Solid-state 45Sc, 13C, and 19F NMR experiments are also applied to characterize the structures of the
microcrystalline Lewis acid catalyst Sc(OTf)3 (for which the crystal structure is unknown) and a noncrystalline,
microencapsulated, polystyrene-supported form of the compound.
Introduction
zation reactions. Certain Sc(III) complexes, due to the extreme
electron deficiency at the Sc atom, act as some of the strongest
The chemistry of scandium has not been explored to the extent
of other transition metals due to the relatively high costs of
materials and its limited reactivity arising from restriction to
the +3 oxidation state. The use of scandium in both inorganic
and organometallic chemistry has recently received more
attention, as observed by the increasing number of compounds
that have been synthesized and characterized by single-crystal
X-ray diffraction and other methods.1,2 Scandium is found in
an assortment of materials, such as alloys,3 inorganic materials,4-9
ferroelectric relaxors, and ceramics.10-15 Scandium complexes
are also used in a number of organic syntheses and polymeri-
Lewis acids among transition metal complexes. For instance,
scandium triflate is extensively used in organic synthesis to
catalyze a wide variety of reactions.16,17 There is also increasing
interest in the organometallic chemistry of scandium, since
Sc-C bonds are especially susceptible to insertion reactions
involving unsaturated molecules. Active scandium catalysts for
the copolymerization of ethylene with styrene or norbornene
have been synthesized.18,19 Further advances have also been
made with the incorporation of non-cyclopentadienyl ligand
systems.20,21 Given the increased interest in scandium chemistry,
development of solid-state 45Sc NMR spectroscopy for char-
acterization of molecular structure and dynamics in both
crystalline and disordered materials is of great importance.
(1) Cotton, S. A. Polyhedron 1999, 18, 1691.
(2) Meehan, P. R.; Aris, D. R.; Willey, G. R. Coord. Chem. ReV. 1999, 181,
121.
Scandium has one naturally occurring NMR active isotope,
45Sc, which is 100% naturally abundant, possesses a gyromag-
netic ratio of 6.50880 × 107 rad T-1 s-1 (close to 13C), a nuclear
spin of I ) 7/2, and a moderate nuclear quadrupole moment
(Q) of 0.22 × 10-28 m2. These factors result in a relative
receptivity of 1780 in comparison to 13C, making 45Sc an
excellent nucleus for NMR experimentation. Solid-state 45Sc
(3) Royset, J.; Ryum, N. Int. Mater. ReV. 2005, 50, 19.
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9
10.1021/ja060477w CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 10391-10402
10391

A R T I C L E S
Rossini and Schurko
Chart 1
NMR spectra are normally comprised of relatively broad powder
patterns which result from anisotropic NMR interactions. The
quadrupolar interaction (QI) normally determines the appearance
of such spectra, though the effects of chemical shielding
anisotropy (CSA) also significantly influence the appearance
1
of the spectra. H-45Sc dipolar coupling can be observed in
some instances. While the presence of these interactions
generally complicate solid-state NMR spectra and increase
acquisition times, they also act as a rich source of information
on molecular structure and dynamics.
Several solution 45Sc NMR studies have established a
chemical shift range of approximately 250 ppm.22-35 By
contrast, there are few reported examples of solid-state 45Sc
NMR and even fewer in which chemical shielding (CS) and
electric field gradient (EFG) tensor parameters have been
measured. In a preliminary study by Thompson and Oldfield,
the isotropic scandium chemical shift (δiso), 45Sc quadrupolar
coupling constant (CQ), and the electric field gradient asymmetry
parameter (ηQ) of Sc(OAc)3 (OAc ) acetate) and ScCl3‚6H2O
were measured.36 Solid-state 45Sc NMR was also utilized by
Han et al. to observe hydrogen diffusion in hexagonally close
packed scandium metal,37 by Kataoka et al. to observe temper-
ature-induced phase transitions in Sc(OAc)3,38 and by Koyama
et al., who examined a ternary superconductor, Sc5Co4Si10.39
Several NMR studies on ferroelectric relaxors40-43 and a variety
of Sc-containing alloys and materials of mixed composi-
tions4,9,44,45 have also been reported. In addition, due to the
favorable NMR characteristics, 45Sc NMR has been employed
for the design and optimization of pulse sequences for spin 7/2
nuclei.46-48
In this paper we report a comprehensive solid-state 45Sc NMR
study of scandium coordination complexes, in an effort to gain
an understanding of the relationship between scandium coor-
dination environments and the observed NMR parameters. The
complexes under study are pictured in Chart 1 (hydrogen atoms
and molecules not bound to scandium are omitted for clarity).
These complexes are Sc(acac)3, Sc(TMHD)3, Sc(OAc)3, Sc-
(NO3)3‚5H2O, ScCl3‚6H2O, ScCl3‚3THF, and ScCp3 (acac )
acetylacetonate, TMHD ) 2,2,6,6-tetramethyl-3,5-heptanedi-
onato, THF ) tetrahydrofuran, and Cp ) cyclopentadienyl).
All of these complexes have crystal structures which were
previously reported or are reported for the first time herein.
These complexes were chosen because they afford a range of
coordination environments about the scandium nucleus leading
to the observation of an assortment of distinct CS and EFG
tensor parameters. Quantum mechanical calculations of NMR
interaction tensors are utilized to examine the orientation of
NMR tensors within molecular frames and to help rationalize
the origin of scandium NMR interactions. We also demonstrate
the application of solid-state 45Sc NMR for probing unknown
molecular structures, including the Lewis acid catalyst Sc(OTf)3
and a polystyrene microencapsulated (ME) form of Sc(OTf)3
(OTf ) SO3CF3). Application of solid-state NMR to the ME-
Sc(OTf)3 is of particular interest, since structural changes
imparted by microencapsulation increase the catalytic activity
of Sc(OTf)3 in carbon-carbon bond-forming reactions.
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1989, 27, 103.
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100, 9579.
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2002, 21, 1579.
Experimental Section
Samples of tris(cyclopentadienyl) scandium (ScCp3) and scandium
chloride hexahydrate (ScCl3‚6H2O) were purchased from Sigma-Aldrich
Canada, Ltd., and used without further purification. A sample of
scandium acetate hydrate (Sc(OAc)3‚xH2O) was acquired from Sigma-
Aldrich Canada, Ltd., and was recrystallized from a 5.0 M aqueous
solution of acetic acid and dried in vaccuo to produce anhydrous
scandium acetate (Sc(OAc)3). Samples of tris(2,2,6,6-tetramethyl-3,5-
heptanedionato)scandium (Sc(TMHD)3), scandium nitrate pentahydrate
(Sc(NO3)3‚5H2O), scandium trifluoromethanesulfonate (Sc(OTf)3), and
scandium trifluoromethanesulfonate microencapsulated in a styrene
polymer (ME-Sc(OTf)3) were purchased from Strem Chemicals, Inc.,
and used without further purification. Samples of scandium tris-
(acetylacetonate) (Sc(acac)3) and scandium chloride tris(tetrahydrofuran)
(ScCl3‚3THF) were synthesized in the research laboratories of Prof.
Warren Piers at the University of Calgary using standard procedures.49,50
(34) Petrosyants, S. P.; Ilyukhin, A. B. Russ. J. Coord. Chem. 2004, 30, 194.
(35) Gierezyk, B.; Schroeder, G. Pol. J. Chem. 2003, 77, 1741.
(36) Thompson, A. R.; Oldfield, E. J. Chem. Soc., Chem. Commun. 1987, 27.
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G. Phys. ReV. B: Condens. Matter 1987, 36, 615.
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(39) Koyama, T.; Sugita, H.; Wada, S.; Tsutsumi, K. J. Phys. Soc. Jpn. 1999,
68, 2326.
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1997, 199, 173.
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2000, 240, 1473.
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Gregorovic, A.; Zalar, B. Phys. ReV. B: Condens. Matter 2003, 67.
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ReV. B: Condens. Matter 2004, 69, 054103.
(44) Sato, K.; Takeda, S.; Fukuda, S.; Minamisono, T.; Tanigaki, M.; Miyake,
T.; Maruyama, Y.; Matsuta, K.; Fukuda, M.; Nojiri, Y. Z. Naturforsch., A:
Phys. Sci. 1998, 53, 549.
(45) Tien, C.; Charnaya, E. V.; Sun, S. Y.; Wu, R. R.; Ivanov, S. N.; Khazanov,
E. N. Phys. Status Solidi B 2002, 233, 222.
(46) Madhu, P. K.; Johannessen, O. G.; Pike, K. J.; Dupree, R.; Smith, M. E.;
Levitt, M. H. J. Magn. Reson. 2003, 163, 310.
(48) Brauniger, T.; Ramaswamy, K.; Madhu, P. K. Chem. Phys. Lett. 2004,
383, 403.
(49) Atwood, J. L.; Smith, K. D. J. Chem. Soc., Dalton Trans. 1974, 921.
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2003, 41, 679.
9
10392 J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006

45Sc NMR Interactions in Solids
A R T I C L E S
Table 1. Summary of Observed 45Sc EFG and CS Tensor Parametersa
c
compound
C
Q (MHz)b
ηQ
δ
iso (ppm)d
Ω
(ppm)e
κf
R
(deg)
â
(deg)
γ
(deg)
Sc(acac)3
13.0(3)g
13.1(2)
6.2(2)
4.6(2)
3.9(2)
8.4(2)
8.3(2)
0.22(3)
0.93(2)
0.75(5)
0.18(6)
0.77(9)
0.30(5)
0.88(4)
82(1)
70(10)
110(20)
60(10)
-0.7(2)
-0.7(3)
-0.8(1)
0.65(10)
0.9(1)
90(15)
0(10)
80(15)
0(45)
45(10)
85(35)
-24(10)
83(4)
15(15)
10(10)
7(3)
30(5)
4(4)
0(5)
Sc(TMHD)3
Sc(NO3)3‚5H2O
Sc(OAc)3
ScCl3‚6H2O
ScCl3‚3THF
Cp3Sc
89.5(10)
-18.5(10)
-6.2(8)
125.4(5)
202(1)
90(5)
65(15)
90(45)
40(10)
12(25)
130(15)
73(5)
180(10)
200(20)
135(15)
-0.1(3)
0.0(3)
62.8(8)
73(8)
a The CS tensor is described by three principal components ordered such that σ11 e σ22 e σ33. The EFG tensor is described by three principal components
d
ordered such that |V11| e |V22| e |V33|. b CQ ) eQV33/h. c ηQ ) (V11 - V22)/V33. δjj ) (σiso,ref - σjj)(106)/(1 - σiso,ref) ≈ σiso,ref - σjj, where jj ) 11, 22,
or 33, δiso ) (δ11 + δ22 + δ33)/3. e Ω ) δ11 - δ33. f κ ) 3(δ22 - δiso)/Ω. g The uncertainty in the last digits of each value is denoted in parentheses.
A second sample of ScCl3‚3THF was prepared by adding anhydrous
ScCl3 to an excess of THF. The sample was then dried in vaccuo and
used without further purification. All samples were finely ground,
packed into 4 mm outer diameter zirconia rotors and sealed with airtight
caps under a nitrogen or argon atmosphere. The Sc(acac)3 and
Sc(TMHD)3 samples were prepared in a similar manner outside of the
glovebox.
Solid-state 45Sc, 13C, and 19F NMR spectra were acquired on a Varian
Infinity Plus spectrometer with an Oxford 9.4 T (ν0(1H) ) 400 MHz)
wide-bore magnet. Additional 45Sc static spectra were acquired on a
Bruker Avance 500 spectrometer with an 11.75 T (ν0(1H) ) 500 MHz)
magnet. 45Sc chemical shifts were reported with respect to an external
standard solution of 0.11 m (mol of solute/kg of solvent) ScCl3 in 0.05
M HCl (δiso ) 0.0 ppm). As there is currently no universally applied
scandium NMR standard, this sample was chosen due to the sharp peak
observed in its solution NMR spectrum, and the insensitivity of the
scandium chemical shift to changes in hydrogen chloride concentrations
from 0.05 to 1.0 M.23 13C chemical shifts were referenced to tetram-
ethylsilane (δiso ) 0.0 ppm) by using the high-frequency peak of
adamantane as a secondary reference (δiso ) 38.57 ppm). 19F chemical
shifts were referenced to fluorotrichloromethane (δiso ) 0.0 ppm) by
setting the 19F resonance of Teflon (δiso ) -122.0 ppm) as a secondary
reference.
using restricted Hartree-Fock (RHF) and hybrid density functional
theory (DFT) employing the B3LYP functional.58 The basis sets
6-31G**, 6-31+G**, 6-311G**, and 6-311+G** were used for all
calculations. All-electron basis sets on scandium were also employed
for calculations on Sc(acac)3.59 CS tensors were calculated using the
gauge-including atomic orbitals (GIAO) method.60,61 [Sc(H2O)6]3+ was
chosen to model the chemical shielding of the solution standard,62 and
absolute shieldings were converted to shifts as described in Tables 1
and 2.
Suitable single crystals of ScCl3‚6H2O and Sc(NO3)3‚5H2O were
selected for X-ray diffraction experiments from the purchased samples
and used without recrystallization. Sc(TMHD)3 was recrystallized in
dichloromethane. All crystals were coated in Nujol oil to prevent the
absorption of water. Diffraction experiments were performed on a
Siemens SMART System CCD diffractometer. The data were processed
using the SAINT+ software package,63 and an absorption correction
was applied to the data using SADABS. The structure was solved by
direct methods and refined by full-matrix least-squares against F2 using
the SHELXTL software package.64 For experimental parameters refer
to Table S2.
Powdered ScCl3‚3THF, Sc(OAc)3, and Sc(OTf)3 were packed into
1.0 mm glass capillary tubes under nitrogen and flame sealed. Powder
X-ray diffraction patterns were collected using a Bruker AXS HI-STAR
system using a general area detector diffractions system. The X-ray
source employed was Cu KR radiation (1.540598 Å) with an area
detector using a 2θ range between 4.0 and 65.0°. Powder X-ray
diffraction patterns were simulated with the Powder Cell software
package.65
Central-transition selective π/2 pulse widths were calculated by
scaling the nonselective pulses by a factor of a quarter, i.e., (I + 1/2)-1
.
Unless otherwise noted, the Hahn-echo pulse sequence of the form
{π/2-τ-π-τ-acquire} was used for the acquisition of 45Sc NMR
spectra. Static 45Sc NMR spectra of Sc(acac)3, Sc(OAc)3, and ScCl3‚
3THF were also acquired with an echo pulse sequence of the form
{π/2-τ-π/2-τ-acquire} (90-90 echo). Rotary-assisted polarization
transfer (RAPT)51 and amplitude-modulated double frequency sweep
(AM-DFS)52 pulse sequences were employed for the acquisition of
selected spectra. 45Sc NMR experiments were conducted at ν0(45Sc) )
97.4 MHz (9.4 T) and 122.0 MHz (11.75 T). The rf fields used for
acquisition of the static 45Sc NMR spectra were generally set to half
of the pattern width to avoid line shape distortions.53 All experimental
parameters can be found in Table S1 (Supporting Information).
Analytical simulations of 45Sc solid-state NMR spectra were performed
using WSolids,54 and numerical simulations were performed using
SIMPSON.55
Results and Discussion
In the first part of this section, a detailed discussion of 45Sc
NMR spectroscopy of scandium coordination compounds is
presented. In all of the cases discussed herein, it is possible to
utilize MAS to almost completely average the scandium CSA,
(50) Morgan, G. T.; Moss, H. W. J. Chem. Soc., Trans. 1914, 189.
(51) Yao, Z.; Kwak, H. T.; Sakellariou, D.; Emsley, L.; Grandinetti, P. J. Chem.
Phys. Lett. 2000, 327, 85.
(52) Kentgens, A. P. M.; Verhagen, R. Chem. Phys. Lett. 1999, 300, 435.
(53) Bodart, P. R.; Amoureux, J. P.; Dumazy, Y.; Lefort, R. Mol. Phys. 2000,
98, 1545.
13C NMR spectra were acquired using Hahn-echo and variable-
(54) Eichele, K.; Wasylishen, R. E. WSolids1: Solid-State NMR Spectrum
Simulation, v. 1.17.30, 2001.
amplitude cross- polarization MAS (VACP/MAS) pulse sequences. 1H
(55) Bak, M.; Rasmussen, J. T.; Nielsen, N. C. J. Magn. Reson. 2000, 147,
296.
f
13C CP was performed on all samples for the acquisition of 13C
spectra, except for the Sc(OTf)3 samples, for which 19F f 13C CP was
applied. The TPPM56 decoupling sequence was employed in all cases.
Solid-state 19F NMR spectra were acquired with a rotor-synchronized
Hahn-echo pulse sequence. All 13C and 19F NMR data are presented in
the Supporting Information.
(56) Bennett, A. E.; Rienstra, C. M.; Auger, M.; Lakshmi, K. V.; Griffin, R. G.
J. Chem. Phys. 1995, 103, 6951.
(57) Frisch, M. J. Gaussian 03, rev. B.03; Gaussian, Inc.: Pittsburgh, PA, 2003.
(58) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B: Condens. Matter 1988, 37,
785.
(59) Huzinaga, S., Ed. Gaussian Basis Sets for Molecular Calculations;
Elsevier: New York, 1984.
(60) Wolinski, K.; Hinton, J. F.; Pulay, P. J. Am. Chem. Soc. 1990, 112, 8251.
(61) Ditchfield, R. Mol. Phys. 1974, 27, 789.
(62) Rudolph, W. W.; Pye, C. C. J. Phys. Chem. A 2000, 104, 1627.
(63) Bruker SAINT+, v. 6.45; Bruker AXS Inc.: Madison, WI, 2003.
(64) Sheldrick, G. M. SHELXTL, v. 6.10; Bruker AXS Inc.: Madison, WI, 2000.
(65) Kraus, W.; Nolze, G. PowderCell for Windows, v. 2.4; Federal Institute
for Materials Research and Testing: Berlin, Germany, 2000.
Calculations of CS and EFG tensors were performed using Gaussian
0357 running on two Dell Precision workstation running Red Hat Linux
9.0. Molecular coordinates were input from structures determined by
single-crystal X-ray diffraction experiments, and proton positions were
geometry-optimized for most structures. Calculations were carried out
9
J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006 10393

A R T I C L E S
Rossini and Schurko
Table 2. Calculated 45Sc NMR Parameters Showing Best Agreement with Experimenta
compound/method
CQ (MHz)b
ηQ
δ
iso (ppm)c
Ω
(ppm)
κ
R
(deg)
â
(deg)
γ
(deg)
d
Sc(acac)3
13.0(3)
-11.6
-12.3
13.1(2)
14.4
0.22(3)
0.44
0.40
0.93(2)
0.67
0.71
0.75(5)
0.72
0.96
0.18(6)
0.18
82(1)
77.0
101.8
89.5(10)
76.1
70(10)
61.3
95.1
110(20)
82.5
118.8
60(10)
58.9
-0.7(2)
-0.69
-0.67
-0.7(3)
0.57
90(15)
89
85
0(10)
90
90
80(15)
51
25
0(45)
1
83(4)
85
85
15(15)
68
5
10(10)
39
33
7(3)
6
0(5)
1
2
90(5)
0
0
65(15)
75
89
90(45)
90
RHF/6-311G** on Sce
B3LYP/6-311G**
Sc(TMHD)3
RHF/6-311+G** on Sc
B3LYP/6-311G** on Sc
Sc(NO3)3‚5H2O
RHF/6-311G**
B3LYP/6-311+G**
Sc(OAc)3
15.3
98.4
0.36
6.2(2)
-5.6
-5.8
4.6(2)
3.3
-18.5(10)
7.3
46.4
-6.2(8)
15.8
-0.8(1)
-0.74
0.20
0.65(10)
0.86
70.9
73(5)
24.9
RHF/6-311G**
B3LYP/6-311G**
ScCl3‚6H2O
RHF/6-311G** on Sc
B3LYP/6-311+G** on Sc
ScCl3‚3THF
RHF/6-311G** on Sc
B3LYP/6-311G**
ScCp3
RHF/6-311G** on Sc
B3LYP/6-311+G** on Sc
4.6
3.9(2)
5.3
5.3
8.4(2)
9.7
0.00
0.77(9)
0.78
0.74
0.30(5)
0.26
0.35
0.88(4)
0.90
-19.4
125.4(5)
154.4
231.3
202(1)
176.8
303.0
62.8(8)
-74.6
115.1
102.6
180(10)
228.9
486.2
200(20)
157.8
316.7
135(15)
86.1
0.99
0.9(1)
0.96
0
0
90
40(10)
40
38
12(25)
12
18
130(15)
20
45(10)
51
52
85(35)
80
73
-24(10)
60
-47
30(5)
26
25
4(4)
5
4
73(8)
43
36
0.98
-0.1(3)
-0.11
-0.26
0.0(3)
-0.02
-0.34
10.0
8.3(2)
-8.5
-7.9
0.74
300.5
36
a All computational results can be found in Table S4. b Only the magnitude of CQ can be measured experimentally. For definitions of all NMR parameters
refer to Table 1. c The theoretical isotropic shifts were calculated by comparison of the calculated shielding to that calculated from a geometry-optimized
model of [Sc(H2O)6]3+ at the appropriate level of theory. d Experimental results from this paper. e In cases where the basis set used for scandium is explicitly
stated the 6-31G** basis set was employed on all other atoms.
while partially averaging the second-order QI. Therefore, the
values of CQ, ηQ, and δiso can be readily obtained by analytical
simulations of the 45Sc MAS NMR central transition spectra
(Table 1). Static 45Sc NMR spectra allow for the measurement
of both quadrupolar (CQ and ηQ) and CS tensor parameters (δiso,
as well as the span, Ω, and the skew, κ), as well as the Euler
angles (R, â, and γ) which describe the relative orientation of
EFG and CS tensors (see Table 1 for definitions of these
parameters). Static spectra acquired at a second field are used
to confirm the observed CS tensor parameters and Euler
angles.66,67 The shapes of the static powder patterns are sensitive
to the pulse sequences and rf fields used in their acquisition
(Figure S1); fortunately, the locations of discontinuities, which
are paramount for determination of spectral parameters, are
largely unaffected by these factors. To the best of our
knowledge, there are no previous reports of scandium CSA in
the literature. Spectral simulations presented throughout the
paper account for scandium CSA; for comparison, spectral
simulations that neglect the presence of CSA are found in the
Supporting Information (Figure S2). In the second part of this
section, theoretically calculated CS and EFG tensors and their
orientations and origins are investigated in depth. In the final
part of this section, we examine the application of 45Sc NMR
to structurally characterize systems for which crystallographic
and/or other structural data are unavailable. Details on newly
determined crystal structures are found in the Supporting
Information.
frequency side of the MAS spectrum is not predicted by the
analytical simulation; however, a numerical simulation using
SIMPSON reveals that this shoulder is due to the presence of
an underlying satellite transition (Figure 1a, inset).
The nature of the EFG and CS tensors and their relation to
one another as well as the molecular frame can be rationalized
by considering the molecular structure. The scandium atom is
coordinated by six oxygen atoms from the three bidentate
acetylacetonato ligands.68 Inspection of the crystal structure
reveals that the coordination sphere formed by the oxygen atoms
more closely resembles a trigonal antiprism and deviates
considerably from octahedral geometry (and therefore spherical
symmetry).2 There are only minor variations in the Sc-O bond
lengths (∼0.02 Å); however, deviations of up to 10° from ideal
octahedral angles are observed in the O-Sc-O bond angles.
In this respect, the observation of a relatively large CQ is not
surprising (this is the second largest value of CQ observed in
this series of complexes). The value of ηQ is closer to zero than
one, indicating that V33 is the pseudo-unique component of the
EFG tensor. The appreciable span indicates that the scandium
chemical shielding is anisotropic, and the skew indicates that
σ
σ
11 is the pseudo-unique component (i.e., the values of σ22 and
33 are closer to one another than σ11 and σ22). The Euler angles
indicate that the largest component of the EFG tensor which
defines the magnitude of the quadrupolar interaction, V33, is
nearly coincident with σ11, the principal component of the CS
tensor which describes the direction of least magnetic shielding.
Given the near-axial symmetry of each tensor, one would expect
that V33 and σ11 are aligned near the pseudo-threefold axis of
the molecule. Proposed tensor orientations are investigated
further in the discussion of ab initio calculations for this
molecule and all remaining systems.
Solid-State 45Sc NMR Experiments. Sc(acac)3. The solid-
state 45Sc MAS NMR spectrum of the central transition (Figure
1a) is simulated with CQ ) 13.0 MHz, ηQ ) 0.22, and δiso
)
82 ppm. Experimental and simulated static 45Sc NMR spectra
of Sc(acac)3 acquired at two different magnetic field strengths
(Figure 1b) reveal that Ω ) 70 ppm and κ ) -0.7 (all Euler
angles are tabulated in Table 1). The “shoulder” on the low-
Sc(TMHD)3. Simulation of the 45Sc MAS NMR spectrum
of Sc(TMHD)3 (Figure 1c) reveals CQ ) 13.1 MHz and δiso
)
89.5 ppm. Simulation of the static 45Sc NMR spectra (Figure
(66) Cheng, J. T.; Edwards, J. C.; Ellis, P. D. J. Phys. Chem. 1990, 94, 553.
(67) Power, W. P.; Wasylishen, R. E.; Mooibroek, S.; Pettitt, B. A.; Danchura,
W. J. Phys. Chem. 1990, 94, 591.
(68) Anderson, T. J.; Neuman, M. A.; Gordon, A. M. Inorg. Chem. 1973, 12,
927.
9
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45Sc NMR Interactions in Solids
A R T I C L E S
Figure 1. Solid-state 45Sc NMR spectra and analytical simulations of Sc(acac)3 and Sc(TMHD)3. (a) MAS spectrum of Sc(acac)3, νrot ) 15 kHz. Inset:
SIMPSON simulation utilizing ideal pulses and processed with 100 Hz of exponential line broadening. (b) Static spectra of Sc(acac)3 at 9.4 and 11.75 T. The
static spectrum at 9.4 T was acquired with a 90-90 echo. (c) MAS RAPT-echo spectrum of Sc(TMHD)3, νrot ) 15 kHz. (d) Static spectra of Sc(TMHD)3
at 9.4 and 11.75 T.
1d) yield parameters similar to those observed in Sc(acac)3, with
Ω ) 110 ppm and κ ) -0.7; however, the Euler angles are
different. The ligands and their coordination to Sc in Sc(acac)3
and Sc(TMHD)3 are analogous, so that the observation of similar
values of δiso and CQ are not surprising. However, ηQ ) 0.93,
which indicates that V11 is the most distinct component of the
EFG tensor and is oriented along or near the 2-fold molecular
axis, in contrast to Sc(acac)3. The difference in relative tensor
orientations between the two complexes can be explained by
comparison of the crystal structures. In Sc(acac)3, each of the
ligands forms an approximately planar six-membered ring with
the scandium center; however, in Sc(TMHD)3, there is a 22°
deviation away from planarity in two of the three six-membered
rings. The molecule is now more properly described as
possessing C2 symmetry, and this difference in symmetry is the
major cause of the distinct EFG tensor orientation. The low
value of â also means that V33 and σ33 are closely oriented,
unlike in Sc(acac)3. Thus, changes in the geometry of the ligand
at a distance from the scandium center have dramatically
affected the relative orientation of the EFG and CS tensors.
Sc(NO3)3‚5H2O. The 45Sc MAS NMR spectrum of Sc(NO3)3‚
5H2O (Figure 2a) reveals a much narrower powder pattern than
those observed for the diketonato compounds. From the MAS
and static spectra (Figure 2b) the measured parameters are CQ
) 6.2 MHz, ηQ ) 0.75, δiso ) -18.5 ppm, Ω ) 60 ppm, and
κ ) -0.8. The 45Sc nucleus is highly shielded in comparison
to the distorted octahedral environments discussed above, and
a relatively small scandium CSA is also observed. The observa-
tion of a relatively small CQ is unexpected, given the consider-
ably nonspherical coordination environment about scandium that
is revealed in the single-crystal X-ray structure (details in Table
S2). However, inspection of the Sc-O bond lengths reveals
that they are longer than those observed in Sc(acac)3 and
Sc(OAc)3 (the Sc-O bonds of water and the nitrate ligands are
on average 0.14 and 0.21 Å longer, respectively). The decreased
EFG must therefore result from the increased Sc-O bond
distances. Similar observations have been made in numerous
systems where increased bond lengths and augmented ionic
character tend to result in lowering of CQ, since the magnitude
of V33 is proportional to 1/r3 in a simple point charge model.69,70
Sc(OAc)3. MAS and static 45Sc NMR spectra of Sc(OAc)3
reveal a relatively narrow central transition (Figure 2c,d), and
simulations yield CQ ) 4.6 MHz, ηQ ) 0.18, δiso ) -6.2 ppm,
Ω ) 73 ppm, and κ ) 0.65. Solid-state 45Sc NMR spectra of
Sc(OAc)3 have been reported previously by Thompson and
Oldfield, as well as Kataoka et al. Both groups reported CQ )
5 MHz and ηQ ) 0 from simulations of static central transition
spectra.36,38 Kataoka reported a refined CQ ) 4.45 MHz on the
basis of a simulation of satellite transitions, which is closer to
our result. In both papers, the effects of CSA were neglected in
the simulations and ηQ was assumed to be zero. From the
appearance of our static spectra, it is apparent that scandium
CSA is present and the EFG tensor is nonaxial. These factors
must be accounted for to properly simulate the static spectra.
The previously determined X-ray crystal structure71 reveals
a coordination polymer structure, with a unique Sc atom in a
six-coordinate environment that results from bridging acetate
ligands (our own powder XRD pattern (Figure S3) confirms
that recrystallization successfully produced anhydrous Sc-
(OAc)3). The Sc-O bond lengths are similar to those in Sc-
(acac)3; however, the Sc(acac)3 metal environment is nearly
octahedral, with all trans-O-Sc-O bond angles equal to 180°
(69) Lucken, E. A. C. Nuclear Quadrupole Coupling Constants; Academic: New
York, 1969.
(70) Cohen, M. H.; Reif, F. Solid State Phys. 1957, 5, 321.
(71) Fuchs, R.; Strahle, J. Z. Naturforsch., B: Chem. Sci. 1984, 39, 1662.
9
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A R T I C L E S
Rossini and Schurko
Figure 2. Solid-state 45Sc NMR spectra and analytical simulations of Sc(NO3)3‚5H2O and Sc(OAc)3: (a) MAS spectrum of Sc(NO3)3‚5H2O, νrot ) 8 kHz;
(b) static spectra of Sc(NO3)3‚5H2O at 9.4 and 11.75 T; (c) MAS spectrum of Sc(OAc)3, νrot ) 8 kHz; (d) static spectra of Sc(OAc)3 at 9.4 and 11.75 T
acquired with a 90-90 echo.
Figure 3. Solid-state 45Sc NMR spectra and analytical simulations of ScCl3‚6H2O and ScCl3‚3THF: (a) MAS spectrum of ScCl3‚6H2O, νrot ) 10 kHz; (b)
static spectra of ScCl3‚6H2O at 9.4 and 11.75 T; (c) MAS spectrum of recrystallized ScCl3‚3THF, νrot ) 12.5 kHz; (d) static spectra of recrystallized
ScCl3‚3THF at 9.4 and 11.75 T acquired with a 90-90 echo.
and cis-O-Sc-O bond angles deviating only slightly from 90°
((3.5°). The relatively small CQ can be accounted for by the
nearly spherically symmetric arrangement of oxygen atoms
about the scandium. The δiso and Ω values are similar to those
of Sc(NO3)3‚5H2O, but the 45Sc nucleus is relatively shielded
from the external magnetic field compared to those in Sc(acac)3
and Sc(TMHD)3, which have distorted octahedral oxygen
coordination environments. The symmetry of both the CS and
EFG tensors is quite distinct from that of Sc(NO3)3‚5H2O, which
is to be expected given their structural differences. The value
of ηQ for Sc(OAc)3 indicates that V33 is the distinct component
of the EFG tensor, and κ indicates that σ33 is the unique
component of the CS tensor. The low value of â indicates that
V33 and σ33 are nearly collinear and are likely directed along
the C3 axis of the molecule, in the direction of propagation of
the polymer structure.
ScCl3‚6H2O. The 45Sc MAS NMR spectrum of ScCl3‚6H2O
(Figure 3a) reveals a very narrow powder pattern with a breadth
of ca. 1 kHz, with CQ ) 3.9 MHz, ηQ ) 0.77, and δiso ) 125.4
ppm. Static spectra (Figure 3b) reveal a large span, Ω ) 180
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45Sc NMR Interactions in Solids
A R T I C L E S
Figure 4. Solid-state 45Sc NMR spectra of ScCl3‚3THF (first sample): (a) MQMAS contour plot, νrot ) 8 kHz; (b) two resolved sites from cross sections
of the indirect dimension and analytical simulations; (c) MAS spectrum (νrot ) 15 kHz) and the simulated pattern that results from addition of the individually
simulated patterns; (d) static spectra.
ppm, and a nearly axial CS tensor, κ ) 0.9. The static spectra
contain a distortion that is unaccounted for in the line shape of
the static simulations at both fields, which likely arises from a
small amount of impurity, as opposed to incomplete excitation
or intramolecular dynamics. The EFG parameters for ScCl3‚
6H2O are different from those reported by Thompson and
Oldfield,36 who found that CQ ) 14 MHz and ηQ ) 0.60, on
the basis of the acquisition and simulation of a static 45Sc NMR
spectrum. The differences in their parameters from ours could
arise from their neglect of scandium CSA and the absence of
45Sc MAS NMR data. It is also unclear whether proton
decoupling was applied in their NMR experiments, which has
adverse effects on the appearance of the 45Sc static NMR powder
pattern in this case (Figure S4).
MAS NMR spectra acquired from this compound revealed the
presence of two distinct scandium sites. A 45Sc multiple-
quantum MAS (MQMAS) NMR experiment72 was performed
to resolve the overlapping sites (Figure 4a), with simulations
(Figure 4b) yielding CQ ) 8.5 MHz, ηQ ) 0.32, and δiso
201.5 ppm (site 1) and CQ ) 8.5 MHz, ηQ ) 0.50, and δiso
)
)
211.0 ppm (site 2). Simulated spectra are coadded to simulate
the observed MAS spectra (Figure 4c). The overlapping static
powder patterns render determination of the CS tensor param-
eters and Euler angles difficult (Figure 4d).
The observation of two scandium sites in the NMR spectra
was surprising because the previously reported crystal structure
indicates that only one crystallographically and magnetically
distinct Sc site should be present.49 The powder XRD pattern
(Figure S3) indicates that this sample is highly crystalline,
though additional high intensity peaks indicate that there is a
second species or crystalline phase present. The presence of
impurities in this sample is unlikely, given the roughly equal
NMR signal intensities and the similar NMR parameters of the
two scandium sites. Recrystallization of anhydrous ScCl3 in an
excess of THF was performed. NMR spectra acquired from this
second sample yielded one scandium resonance as expected
(Figure 3c,d), and spectra are simulated with CQ ) 8.4 MHz,
ηQ ) 0.30, δiso ) 202 ppm, Ω ) 200 ppm, and κ ) -0.1. The
powder XRD pattern of the recrystallized sample closely
matches the predicted pattern. Clearly, the combination of 45Sc
MQMAS NMR and powder XRD is extremely useful for
differentiating Sc sites in similar coordination environments.
ScCl3‚3THF consists of a scandium atom coordinated by three
chlorine atoms and three THF molecules in a meridional fashion.
The large positive chemical shift (the highest in this series) and
large span are consistent with the notion that replacement of
an oxygen with chlorine in the first coordination sphere results
This complex possesses the smallest observed CQ and largest
Ω in the series of compounds studied herein. These two
observations can be rationalized in terms of molecular structure.
The crystal structure for ScCl3‚6H2O was not previously
reported, and our initial assumption was that the scandium atom
would be coordinated by six oxygen atoms. While this structural
model is consistent with the quadrupolar data, it is inconsistent
with the high isotropic shift and large span. Refinement of the
crystal structure of ScCl3‚6H2O reveals a highly symmetrical
coordination environment about scandium, involving two Cl and
four O atoms which form a [ScCl2(H2O)4]+ unit of ap-
proximately D4h symmetry. The two Sc-Cl distances are
equivalent and the angle separating them is 180.0°, while there
is less than 1.0° deviation from ideal octahedral O-Sc-O
angles. In this respect, the observations of a relatively small
CQ and large scandium CSA is not surprising. The large span
and axially symmetric skew indicate that σ33 is likely oriented
near the Sc-Cl bond axis, and that there are large paramagnetic
deshielding contributions perpendicular to this axis.
ScCl3‚3THF. 45Sc NMR spectra were acquired from two
different samples of ScCl3‚3THF (Figures 3 and 4). Initial 45Sc
(72) Medek, A.; Harwood: J. S.; Frydman, L. J. Am. Chem. Soc. 1995, 117,
12779.
9
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A R T I C L E S
Rossini and Schurko
Figure 5. Solid-state 45Sc NMR spectra of ScCp3 and simulations: (a) MAS spectrum, νrot ) 12 kHz; (b) static spectra at 9.4 and 11.75 T.
in magnetic deshielding of the scandium nucleus. The Sc-Cl
bonds are considerably more covalent than Sc-O bonds; as
such, there are increased paramagnetic shielding contributions
perpendicular to the Sc-Cl bond axes. Given the meridional
arrangement of Cl and O atoms, σ11 should be oriented in a
direction perpendicular to all three Sc-Cl bonds. The nonaxial
κ indicates that the CS tensor does not have a pseudo-unique
component. The skew, the moderate value of CQ, and the
nonaxial ηQ are all consistent with the meridional arrangement
of Cl and O atoms in the in the first coordination sphere.
ScCp3. This compound represents a departure from all other
systems studied herein and is mentioned as an aside. Given
current interest in organometallic scandium compounds,18-21
solid-state 45Sc NMR could be a useful method to gain insight
into their molecular structure and dynamics. In this regard,
Figure 6. 45Sc chemical shift and shielding scales. The chemical shielding
scale was generated from the RHF/6-311G** on Sc series of calculations.
The theoretical result for ScCp3 has been omitted.
ScCp3 is a good entry point for studies upon π-coordinated
systems. Herein we present some preliminary room-temperature
NMR data. Analytical simulation of 45Sc MAS (Figure 5a) and
constructed from all of the complexes studied herein (Figure
6). Magnetic deshielding is observed upon replacement of
oxygen with chlorine; for instance, the isotropic chemical shift
of scandium in ScCl3‚6H2O is 125.4 ppm (coordination by two
chlorine atoms), while the observed shift of ScCl3‚3THF is 202
ppm (coordination by three chlorine atoms). Sc-Cl bonding
must play a significant role in making paramagnetic shielding
contributions and increasing the span of the scandium CS
tensors. A trend in the observed values of Ω is also observed:
coordination of scandium by more than one type of atom or by
ligands engaging in multiple binding modes (e.g. ScCp3) is seen
to result in sizable Ω values.
A correlation between molecular symmetry and size of CQ
is also observed. A CQ of 2.02 MHz in hexagonally closed
packed Sc metal has been previously observed,37 which is
reflective of the spherically symmetric scandium environment.
Compounds that possess coordination environments with bond
angles close to ideal octahedral angles, such as Sc(OAc)3 and
ScCl3‚6H2O, are seen to possess relatively small values of CQ.
Distortions away from octahedral symmetry result in an increase
in CQ as observed in Sc(acac)3 and Sc(TMHD)3. Values of ηQ
near zero or one are observed when symmetry elements such
as rotational axes are present.
static (Figure 5b) NMR spectra of ScCp3 yield δiso ) 62.8 ppm,
CQ ) 8.3 MHz, ηQ ) 0.88, Ω ) 135 ppm, and κ ) 0.0. The
discontinuities in static spectra are difficult to resolve at both
9.4 and 11.75 T, increasing the errors in the CS tensor
parameters and Euler angles. The appearance of the spectra are
suggestive of some intramolecular dynamics, though this is
beyond the scope of the current paper (see 13C NMR data in
Figure S6).
The structure of ScCp3 consists of two η5-Cp rings and two
η1-Cp rings. The η1-Cp rings bridge two adjacent scandium
atoms leading to the formation of polymeric chains. In the
majority of NMR studies of metal nuclei in metallocene
systems,73-77 the values of δiso typically occur far to the low-
frequency end of the chemical shift range, consistent with nuclei
which are highly shielded from the magnetic field. In this
respect, the “midrange” δiso for this metallocene system is
unusual. The η5-Cp rings are likely responsible for significant
magnetic shielding of the 45Sc nucleus, while η1-Cp must result
in some degree of deshielding. With the absence of other
organometallic species for comparison, it is difficult to assess
the origin of the moderate value of CQ, though this will be the
subject of a future study.
Summary of Observed 45Sc NMR Parameters. To aid in
illustrating the observed trends, a chemical shift scale has been
The aforementioned trends illustrate the utility of solid-state
45Sc NMR for the analysis of molecular structure. Values of
CQ can be used to gauge the spherical symmetry of the ground-
state electronic structure about a scandium nucleus, while the
values of δiso and Ω can be used to provide information about
the nature of the ligands that are bound to scandium and their
(73) Schurko, R. W.; Hung, I.; Macdonald, C. L. B.; Cowley, A. H. J. Am.
Chem. Soc. 2002, 124, 13204.
(74) Hung, I.; Schurko, R. W. Solid State Nucl. Magn. Reson. 2003, 24, 78.
(75) Hung, I.; Schurko, R. W. J. Phys. Chem. B 2004, 108, 9060.
(76) Willans, M. J.; Schurko, R. W. J. Phys. Chem. B 2003, 107, 5144.
(77) Widdifield, C. M.; Schurko, R. W. J. Phys. Chem. A 2005, 109, 6865.
9
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45Sc NMR Interactions in Solids
A R T I C L E S
Sc(NO3)3‚5H2O and Sc(OAc)3. Calculations performed upon
Sc(NO3)3‚5H2O yield several results which are close to the
experimental values. There are no discernible symmetry ele-
ments present, with the exception of a pseudo-mirror plane, that
could account for the observation of such a small value of CQ.
As mentioned earlier, the most reasonable explanations for the
small CQ, despite the lack of spherical symmetry, are the
increased Sc-O distances. It is notable that V33 is oriented in
the direction of one of the nitrogen atoms in a nitrate group
(Figure 7c), though it is uncertain why this is the case. The
calculated orientation of the CS tensor conforms to the symmetry
of the atomic arrangement and is oriented such that the σ11 and
σ33 are contained in the pseudo-mirror plane formed by the
nitrate ligands.
In Sc(OAc)3, calculations successfully predict that V33 and
σ33 are the pseudo-distinct components (Figure 7d) and reveal
that they are nearly collinear with the C3 axis of the molecule.
Thus, the tensors are aligned in the direction of chain propaga-
tion of this coordination polymer and are clearly constrained
by the symmetry of the molecule. The predicted Euler angles
are in excellent agreement with the experimental values.
ScCl3‚6H2O and ScCl3‚3THF. The experimental and theo-
retical parameters obtained for ScCl3‚6H2O indicate that V11
and σ33 are the pseudo-unique components, and the value of R
indicates they are not closely aligned (Figure 7e). The values
of ηQ indicate that V11 is the pseudo-unique component and
should be aligned along or near the C4 axis of the molecule;
however, this is not the case. The theoretical prediction aligns
V33 at ca. 27° from the C4 axis, with V11 and V22 oriented
approximately 27 and 18° away from Sc-O bonds (C2 axes),
respectively. The theoretical CS tensor places σ33 along the Sc-
Cl bond, approximately 26° from V33, which is consistent with
σ33 being the unique component of the CS tensor, while σ11
and σ22 are in the ScO4 plane, oriented near the Sc-O bonds.
Close agreement is observed between the experimental and
predicted NMR tensor parameters and tensor orientations. It is
worth noting that poor agreement is observed between the
experimental values and those obtained from calculations upon
a [ScCl2(H2O)4]+ unit if proton positions from the crystal
structure are utilized (Table S5). Accordingly, EFG calculations
employing optimized proton positions yield much better agree-
ment with experimental values (Table S4).
Figure 7. Proposed EFG and CS tensor orientations: (a) Sc(acac)3; (b)
Sc(TMHD)3; (c) Sc(NO3)3‚5H2O; (d) Sc(OAc)3; (e) ScCl3‚6H2O; (f) ScCl3‚
3THF; (g) ScCp3.
modes of bonding. The presence of symmetry elements, such
as rotational axes and mirror planes, are reflected in the values
of ηQ and κ, as well as in the Euler angles.
First Principles Calculations of 45Sc Interaction Tensors.
Ab initio calculations are performed to predict the orientations
of the CS and EFG tensors in the molecular frames (Figure 7).
This allows for relationships between the observed 45Sc NMR
parameters and molecular structure and symmetry to be
confirmed and/or established. Calculated EFG and CS tensor
parameters showing the best agreement with experimental values
are presented for Sc(acac)3, Sc(TMHD)3, Sc(NO3)3‚5H2O,
Sc(OAc)3, ScCl3‚6H2O, ScCl3‚3THF, and ScCp3 in Table 2.
Calculations on Sc(acac)3 employing the basis sets of Huzinaga59
are shown in Table S3, and the results of all other calculations
are shown in Table S4. Cartesian coordinate files containing
the orientation of the EFG and CS tensors for each compound
are provided in the Supporting Information.
Sc(acac)3 and Sc(TMHD)3. Calculations performed upon Sc-
(acac)3 generally produce results which are in good agreement
with the experimentally observed parameters. The B3LYP/6-
311G** calculation on Sc(acac)3 orients the largest component
of the EFG tensor, V33, and least shielded component of the CS
tensor, σ11, coincident with the pseudo-C3 axis, in agreement
with our proposed orientation on the basis of the experimental
values of κ and ηQ. The theoretical Euler angles are found to
be in excellent agreement with experimentally determined
angles. Similar tensor orientations have been observed in
Co(acac)3 and Al(acac)3, both of which have analogous mo-
lecular geometries.78,79
The orientation of the CS tensor in ScCl3‚6H2O, which gives
rise to high magnetic shielding along the Cl-Sc-Cl axis and
deshielding in the ScO4 plane, can be explained using Ramsey’s
description of contributions to paramagnetic shielding. The
paramagnetic shielding tensor at a nucleus, A, is given as80
All calculations performed upon Sc(TMHD)3 predict inter-
mediate positive values of κ, distinct from the experimentally
determined value of -0.7. V11 is the pseudo-unique component
of the EFG tensor (Figure 7b) and is oriented near the pseudo-
C2 axis of the molecule and σ22, while V33 and σ33 are nearly
coincident and directed toward the bent ligands. The theoretical
Euler angles are close to the observed angles (if â is near zero,
R and γ are approximately interchangeable). The calculations
are correctly predicting the orientation of the EFG and CS
tensors within the molecular frame but consistently underesti-
mate σ22 and overestimate σ33.
e2
µ0
σpARâ ) -
(E - E )-1
×
∑
k
0
2
4π
k*0
2m
L
L
[ 0| L |k k|
iAâ|0 + 0|
iAR|k k| L |0 ]
∑
∑
∑
∑
iR
iâ
3
3
i
i
i
i
riA
riA
where Ek - E0 is the energy difference between occupied ( 0 )
and virtual ( k ) MOs and Li and LiA are the orbital angular
momentum operators of the ith electron with respect to the gauge
(78) Reynhard, E. J. Phys. C: Solid State Phys. 1974, 7, 4135.
(79) Schurko, R. W.; Wasylishen, R. E.; Foerster, H. J. Phys. Chem. A 1998,
102, 9750.
(80) Ramsey, N. F. Phys. ReV. 1950, 78, 699.
9
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A R T I C L E S
Rossini and Schurko
Figure 8. Solid-state 45Sc NMR spectra and analytical simulations of Sc(OTf)3 and ME-Sc(OTf)3: (a) MAS spectrum of Sc(OTf)3, νrot ) 4 kHz; (b) static
spectra of Sc(OTf)3 at 9.4 and 11.75 T; (c) MAS spectrum of ME-Sc(OTf)3, νrot ) 12 kHz; (d) static spectra of ME-Sc(OTf)3 at 9.4 T (AMDFS-Hahn echo)
and 11.75 T.
origin and nucleus, respectively. The angular momentum
operators describe the mixing of occupied and virtual MOs via
rotation about an axis perpendicular to a plane containing the
MOs, which can be visualized as physical rotation of charge
about an applied magnetic field.
polymer. The observation of a high value of ηQ, which indicates
that V11 is the unique component of the EFG tensor, is not
surprising in this context. It is difficult to comment on the
observed CS tensor orientation; nonetheless, it is of interest to
note that the calculations do predict a relatively large span. The
predicted isotropic shifts fluctuate widely between the B3LYP
and RHF calculations, perhaps suggesting that use of an isolated
cluster in this instance is insufficient to properly model the CS
tensor.
Magnetic shielding along the Cl-Sc-Cl axis is high since
the occupied and virtual MOs in the ScO4 plane are far apart in
energy and do not have the appropriate symmetry for magnetic-
dipole allowed mixing; as a result, the paramagnetic contribution
to deshielding perpendicular to the ScO4 plane is minimal.
Conversely, deshielding arising from paramagnetic contributions
in the plane are high, since there are energetically similar MOs
of appropriate symmetry in the ScO2Cl2 planes which are
available for mixing. The large Ω and high δiso are consistent
with this qualitative model and are not observed in any of the
complexes which do not have Sc-Cl bonds.
For ScCl3‚3THF, the calculated value of ηQ is in good
agreement with the observed value and indicates that V33 is the
pseudo-unique component of the tensor. V33 is nearly coincident
with the Cl-Sc-Cl pseudo-axis ( 174.5°), whereas V11
coincides with the C2 axis of the molecule formed by O-Sc-
Cl axis ( 179.4°) (Figure 7f). It is important to note that σ33 is
oriented near the Cl-Sc-Cl axis ( to the ClScO3 plane), σ22
near the Cl-Sc-O axis ( to the Cl2ScO2 plane), and σ11 near
the O-Sc-O axis ( to the Cl3ScO plane). This is consistent
with arguments above, where the large paramagnetic deshielding
contributions are generated perpendicular to the plane with the
most coordinated Cl atoms. A detailed MO analysis is beyond
the scope of the current study.
Summary of ab Initio Calculations. Inspection of all the
calculations reveals that the RHF method is generally superior
to the B3LYP method. The former typically result in both CS
and EFG tensor parameters that are close to the experimentally
observed values, while the latter do not correctly predict CS
tensor parameters in most cases. In the case of extended systems
(i.e., coordination polymers such as Sc(OAc)3 and ScCl3‚6H2O)
calculations upon isolated small charged clusters are sufficient
to accurately calculate the observed 45Sc EFG and CS tensor
parameters, indicating that the interactions are mainly intramo-
lecular in origin. Examination of the calculated isotropic shifts
reveals that the RHF calculations typically result in errors of
less than (25 ppm with larger basis sets (with the exception of
ScCp3), while the B3LYP calculations generally produce poorer
agreement and greatly overestimate the effects of coordination
by chlorine on chemical shifts.
Solid-State 45Sc NMR Experiments on Sc(OTf)3 and ME
Sc(OTf)3. Solid-State NMR of Microcrystalline (MC) Sc-
(OTf)3. 45Sc NMR spectra of MC-Sc(OTf)3 and corresponding
analytical simulations are shown in Figure 8. Simulations of
ScCp3. Calculated and observed EFG parameters of ScCp3
are in good agreement. Calculations were carried out using a
[ScCp4]- cluster (Figure 7g) and reveal that V11 is nearly
coincident with one of the η1-Cp-Sc bonds and that V33 is
oriented in the direction of propagation of the metallocene
MAS (Figure 8a) and static (Figure 8b) patterns yield CQ )
4.7 MHz, ηQ ) 0.15, δiso ) -28 ppm, Ω ) 32 ppm, and κ )
0.3. The static spectra possess rounded edges and do not
terminate sharply into the baseline. This is most likely due to
minor long-range disorder in the sample.
9
10400 J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006

45Sc NMR Interactions in Solids
A R T I C L E S
Table 3. Observed 45Sc NMR Parameters of Sc(OTf)3, ME Sc(OTf)3, and Sc(OAc)3
compound
CQ (MHz)a
ηQ
δ
iso (ppm)
Ω
(ppm)
κ
R
(deg)
â
(deg)
γ (deg)
Sc(OTf)3
ME-Sc(OTf)3
Sc(OAc)3
4.7(2)
4.2(2)
4.6(2)
0.15(5)
0.1(1)
0.18(6)
-28(1)
1.8(3)
-6.2(8)
32(3)
26(8)
73(5)
0.3(1)
-0.1(2)
0.65(10)
75(30)
90(30)
0(45)
18(4)
15(5)
7(3)
0(25)
0(25)
90(45)
a See Table 1 for definitions of parameters.
illustrate this, Kobayashi et al. acquired the solution 45Sc NMR
spectrum of Sc(OTf)3 dissolved in 1,3,5-triphenylpentane and
observed a positive chemical shift of 24 ppm with respect to
the resonance of Sc(OTf)3 dissolved in CD3CN. Solid-state 45Sc
NMR experiments reveal that the chemical shift of the ME-
Sc(OTf)3 is +29.8 ppm with respect to that of the pure MC
form, which is consistent with the solution NMR data of
Kobayashi. The observation of similar 45Sc EFG and CS tensor
parameters, with the exception of the δiso, suggests that the
structure of Sc(OTf)3 remains fundamentally the same upon
microencapsulation within polystyrene; most notably, the EFG
parameters strongly suggest coordination of Sc by six bridging
triflate ligands as well as retention of its coordination polymer
structure. This is supported by the observation of similar IR
spectra for both MC- and ME-Sc(OTf)3.17 The absence of a
chemical shift close to that of MC-Sc(OTf)3 also suggests that
Sc(OTf)3 is encapsulated at the molecular level; there is no large
chemical shift distribution indicating the presence of nano- or
microcrystalline domains.
The presence of 1H-45Sc dipolar coupling (likely from
multiple proton sites) also indicates that there are protons close
to the scandium atom (less than 4 Å). This observation is
compatible with the proposal that there is an interaction between
the benzene rings of the polystyrene and scandium; however,
the nature of this interaction is not well defined. For most
transition metals, interactions with aromatic π-systems typically
result in magnetic shielding of the nucleus (i.e., a negative
chemical shift), opposite to both solution and solid-state NMR
observations. Interestingly, the 45Sc nucleus might be unique
in this respect, since large magnetic shielding effects are not
observed in the π-coordinated ScCp3 molecules.
Figure 9. Model of Sc(OTf)3 (left) and comparison to the structure of
Sc(OAc)3 (right). Fluorine and hydrogen atoms have been omitted for clarity.
There is currently no crystal structure of Sc(OTf)3; however,
the 45Sc NMR data can be used to make structural inferences.
The small CQ suggests that the coordination environment of
scandium must be of high symmetry, while the low Ω and
negative isotropic chemical shift suggest that there is symmetric
coordination by oxygen atoms from one type of ligand. The
oxygen atoms should form a nearly idealized octahedron about
the scandium center. Given the similarity of these parameters
with those of Sc(OAc)3 (Table 3), Sc(OTf)3 should have a
coordination polymer structure akin to that of Sc(OAc)3. This
is reasonable because triflate and acetate anions are similar in
structure and can therefore coordinate to Sc in an equivalent
manner. This is in accordance with the previously reported IR
studies which suggests that the triflate ligands are bridging.81
The structure of Sc(OAc)3 is shown in Figure 9 along with a
proposed model of Sc3(OTf)3. In addition, the solid-state 13C
and 19F NMR spectra only contain one signal apiece (Figure
S7), which is also consistent with the proposed structure (one
type of carbon and fluorine atom). Thus, 45Sc NMR spectros-
copy can be applied as a rapid probe of structure and bonding
in the context of the other systems examined within this work.
Solid-State NMR of Microencapsulated (ME) Sc(OTf)3.
The 45Sc NMR spectra of ME-Sc(OTf)3 reveal spectra similar
to those observed from the microcrystalline sample (Figure 8c
and 8d), with δiso ) 1.8 ppm, CQ ) 4.2 MHz, ηQ ) 0.1, Ω )
26.0 ppm, and κ ) -0.1. A strong 1H-45Sc dipolar interaction
(Figure S8) could not be completely decoupled in the static
experiments at 11.75 T (Figure 8d), as evidenced by the lack
of definition in the pattern. However, the visible discontinuities
in the pattern are useful for the confirmation of the parameters
obtained from the simulation of the spectrum acquired at 9.4
T. Comparison of the 45Sc NMR parameters of Sc(OTf)3 and
ME-Sc(OTf)3 (Table 3), the latter has a significantly higher
chemical shift (scandium is less shielded) and a slightly smaller
CQ.
The 19F and 13C NMR spectra of MC-Sc(OTf)3 and
ME-Sc(OTf)3 support the 45Sc NMR data, suggesting that the
ligands are not significantly changed and the fundamental
coordination polymer structure is retained upon microencapsu-
lation. The 19F NMR spectra of MC-Sc(OTf)3 and ME-Sc(OTf)3
(Figure S7a) have identical chemical shifts at ca. -77(1) ppm.
As well, in the 13C Hahn-echo and 19F-13C VACP/MAS NMR
spectra of each system, the CF3 resonances are the same at 118
ppm (Figure S7b,c). The CF3 carbon resonance is not visible
1
in the H-13C VACP/MAS spectra of ME-Sc(OTf)3, which
indicates that CP is inefficient at this high spinning speed;
however, at lower spinning speeds where 1H-13C VACP would
be more efficient, the CF3 resonance is broadened beyond ready
detection by large 19F-13C dipolar couplings. Correspondingly,
the polystyrene peaks are only observed in the 13C Hahn-echo
Kobayashi et al. have previously reported solution 45Sc NMR
data for Sc(OTf)3 and ME-Sc(OTf)3.82 A positive chemical shift
of ca. 30 ppm was observed in the solution 45Sc NMR resonance
of ME-Sc(OTf)3 relative to that of Sc(OTf)3, which was
suggested to originate from an interaction between the benzene
rings of the polystyrene and the scandium center. To further
1
and H-13C VACP/MAS NMR spectra, for similar reasons.
Conclusions
The 45Sc NMR interaction tensor parameters have been
measured for a number of well-characterized scandium com-
pounds possessing a broad range of metal coordination environ-
ments. For the first time, clear relationships are drawn between
(81) Hamidi, M. M.; Pascal, J. Polyhedron 1993, 13, 1787.
(82) Kobayashi, K. Chem. Commun. 2003, 2003, 449.
9
J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006 10401

A R T I C L E S
Rossini and Schurko
the scandium coordination environments and the nature of the
45Sc NMR interaction tensors. EFG tensor parameters are
primarily affected by the geometry of the scandium coordination
environment, while CS tensor parameters are affected by both
the symmetry of the coordination environment and nature of
bonding in the first coordination sphere. Anisotropic chemical
shielding of the scandium nucleus is observed in all of the
systems and plays a significant role in defining the scandium
chemical shift range. Ab initio calculations of the CS and EFG
tensor parameters are in very good agreement with experimental
data. In addition, the predicted tensor orientations within the
molecular frames aid in rationalizing the origin of the observed
CS and EFG tensor parameters. Solid-state 45Sc NMR spec-
troscopy has also been successfully applied to structurally
characterize two systems for which single-crystal X-ray dif-
fraction data is unavailable and/or unobtainable. It has been
demonstrated that Sc(OTf)3 is a coordination polymer that
possesses a structure similar to Sc(OAc)3 and that this structure
is fundamentally the same upon microencapsulation in poly-
styrene. However, the distinct chemical shift and significant 1H-
45Sc dipolar couplings observed in the 45Sc NMR spectra of
ME-Sc(OTf)3 is consistent with the proposal that immobilization
is due to an interaction between the scandium center and the
phenyl groups of the polystyrene.
and Imperial Oil. A.J.R. thanks NSERC for an Undergraduate
Summer Research Award and the Ontario Ministry of Training,
Colleges, and Universities for an Ontario Graduate Scholarship.
R.W.S. is also grateful to the Canadian Foundation for Innova-
tion (CFI), the Ontario Innovation Trust (OIT), and the
University of Windsor for funding the Solid-State NMR Facility
at the University of Windsor. We thank Dr. Warren Piers and
Korey Conroy (University of Calgary) for synthesis of Sc(acac)3
and ScCl3·3THF. Mr. Mike Fuerth is thanked for his assistance
with experiments at 11.75 T. Dr. Charles L. B. Macdonald is
thanked for his assistance with single-crystal X-ray diffraction
experiments and the refinement of crystal structures.
Supporting Information Available: Cartesian coordinate files
of EFG and CS tensor orientations within molecular frames,
CIF files of the single-crystal X-ray diffraction structures of
Sc(TMHD)3, Sc(NO3)3‚5H2O, and ScCl3‚6H2O, tables of the
experimental NMR parameters employed, tables of X-ray
crystallographic data, tables of ab initio calculation results, 45Sc
NMR spectra without 1H decoupling, powder X-ray diffraction
patterns, 13C and 19F NMR spectra, and the complete ref 57.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. This research was funded by the Natural
Sciences and Engineering Research Council (NSERC, Canada)
JA060477W
9
10402 J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006