Elucidation of a Sc(I) Complex by DFT Calculations and Reactivity Studies

Article abstract of DOI:10.1021/ic034947o

Sc(BrMgL)₂Br (L = (R₂NCH₂CH ₂NCMe)₂CH, R = H) was studied by DFT methods leading to the conclusion that this diamagnetic formal scandium(I) system enjoys stabilization of its Sc-based filled d_yz orbital by a δ-acceptor linear combination of BrMgL ring orbitals. Investigation of the reactivity of Sc(BrMgL)₂Br (L = (R₂NCH ₂CH₂NCMe)₂CH, R = Et) with H ₂O·B(C₆F₅)₃ and (HOCH ₂)₂CMe₂, respectively, led to decomposition, with LMgBr being isolated in the latter case.

Full text of DOI:10.1021/ic034947o

Inorg. Chem. 2003, 42, 8803−8810
Elucidation of a Sc(I) Complex by DFT Calculations and Reactivity
Studies†
Ana-Mirela Neculai, Christopher C. Cummins, Dante Neculai, Herbert W. Roesky,* Ga`bor Bunko`czi,
Bernhardt Walfort, and Dietmar Stalke
Institut fu¨r Anorganische Chemie, UniVersita¨t Go¨ttingen, Tammannstrasse 4,
D-37077 Go¨ttingen, Germany, Institut fu¨r Anorganische Chemie, Am Hubland,
D-97074 Wu¨rzburg, Germany, and Department of Chemistry Room 2-227, Massachusetts Institute
of Technology, Cambridge, Massachusetts 02139-4307
Received August 8, 2003
Sc(BrMgL)2Br (L ) (R NCH CH NCMe)2CH, R ) H) was studied by DFT methods leading to the conclusion that
2
2
2
this diamagnetic formal scandium(I) system enjoys stabilization of its Sc-based filled dyz orbital by a δ-acceptor
linear combination of BrMgL ring orbitals. Investigation of the reactivity of Sc(BrMgL)2Br (L ) (R NCH CH NCMe)2CH,
2
2
2
R ) Et) with H O‚B(C F ) and (HOCH ) CMe2, respectively, led to decomposition, with LMgBr being isolated in
2
6
5 3
2 2
the latter case.
Introduction
Incontestably, the principally used technique for accessing
low oxidation states in different ligand environments for these
metals apart from samarium, europium, and ytterbium is
metal vapor synthesis.5 Our recent interest in the chemistry
of â-diketiminato derivatives of scandium6 led us to an
unexpected sandwich-like scandium (+1) complex (Scheme
1 and Figure 1).7
In the organometallic chemistry of the lanthanides, mo-
lecular complexes incorporating low-oxidation-state metals
hold a particular interest. The availability of compounds with
the formal oxidation state +2 for samarium, europium, and
ytterbium due to the stability of the f6, f7, and f14 electronic
configurations is no longer a problem.1 In recent years,
reports on lanthanum and scandium increased the number
of the rare earth +2 compounds.2-4 In most of the metal (0)
or (+1) containing compounds, stabilization is ensured by
using a considerably bulky ligand with π-acceptor properties5
that supports back-donation of electron density from the
metal.
Scheme 1
In comparison with the first reported Sc(I) compound,3
this complex is obtained in a facile route in solution, and it
is diamagnetic. Earlier this year another subvalent scandium
compound with a 1,2,4-triphosphacyclopentadienyl ligand,
[Sc{(P3C2tBu2)2}2], was reported by Cloke and co-workers.4
This compound presented as a mixed valence dimer Sc(I)-
Sc(III) displaying a magnetic behavior similar to that of 1.
A reasonable extension of our work is the investigation
through calculations on the basis of the unusual magnetic
behavior of compound 1 and the investigation of the
reactivity of such a scandium +1 center. In this paper, we
describe the outcome of the theoretical study and the results
of redox reactions undergone by 1 with H2O‚B(C6F5)3 and
(HOCH2)2CMe2. This work is designed to shed light on the
* To whom correspondence should be addressed. E-mail: hroesky@
gwdg.de. Phone: +49-551-393001. Fax: +49-551-393373.
† Dedicated to Professor Reinhard Schmutzler on the occasion of his
70th birthday.
(1) (a) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry:
Principles of Structure and ReactiVity, 4th ed.; Harper Collins College
Publishers: New York, 1993; pp 509-602. (b) Cotton, F. A.;
Wilkinson, E.; Murillo, C. A.; Bochmann, M. AdVanced Inorganic
Chemistry, 6th ed.; John Wiley & Sons: New York, 1999; p 1127.
(c) Avent, A. G.; Khvostov, A. V.; Hitchcock, P. B.; Lappert, M. F.
Chem. Commun. 2002, 1410-1411.
(2) Cassani, M. C.; Duncalf, D. J.; Lappert, F. M. J. Am. Chem. Soc.
1998, 120, 12958-12959.
(3) Arnold, L. P.; Cloke, F. G. N.; Hitchcock, P. B.; Nixon, J. F. J. Am.
Chem. Soc. 1996, 118, 7630-7631.
(4) Clentsmith, G. K. B.; Cloke, F. G. N.; Green, J. C.; Hanks, J.;
Hitchcock, P. B.; Nixon, J. F. Angew. Chem. 2003, 115, 1068-1069;
Angew. Chem., Int. Ed. 2003, 42, 1038-1039.
(6) Neculai, A. M.; Roesky, H. W.; Neculai, D.; Magull, J. Organome-
tallics 2001, 26, 5501-5503.
(7) Neculai, A. M.; Neculai, D.; Roesky, H. W.; Magull, J.; Baldus, M.;
Andronesi, O.; Jansen, M. Organometallics 2002, 21, 2590-2592.
(5) (a) Cloke, F. G. N. Chem. Soc. ReV. 1993, 17-24 and references
therein. (b) Arnold, P. L.; Cloke, F. G. N. Chem. Commun. 1997,
481-482.
10.1021/ic034947o CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/03/2003
Inorganic Chemistry, Vol. 42, No. 26, 2003 8803

Neculai et al.
300 K): δ 244.11. EI-MS: m/z (rel int %) 1062 [C41H35N4B2F20O2-
Sc+, 5], 976 [C41H35N4B2F20O2Sc+ - C5H12N, 50], 86 [C5H12N,
100].
Reaction of 1 with (HOCH2)2CMe2. A solution of 0.125 g of
(HOCH2)2CMe2 (2 mmol) in toluene (5 mL) was added to a solution
of 1 (obtained from reaction of 0.5 g (1 mmol) of LScBr2 with 2
mL of (C3H5)MgBr (1 M in ether, 2 mmol) of a yield of 25%) in
toluene (15 mL). The reaction takes place instantaneously with
formation of a brown solution that was stirred for 6 h which was
then concentrated to approx 10 mL, and 1 mL THF was added.
Crystals of 3 were obtained after several days at -26 °C.
The reaction was repeated in a NMR tube with 0.020 g of 1
(0.02 mmol) in C6D6 (0.5 mL) and 0.002 g of (HOCH2)2CMe2 in
1
C6D6 (0.5 mL). The H NMR spectrum was recorded after 5 min
1
and 24 h. The second H NMR spectrum showed no changes in
comparison with the first one. Spectral data for 3 follow. 1H NMR
(500 MHz, C6D6, 300 K): δ 4.74 (s, 2 H, C(Me)CHC(Me)), 3.62
and 2.63 br signals assigned to 16H, CH2, 1.80 (s, 6 H, CCH3),
0.72 (t, 24 H, NCH2CH3).
Figure 1. Molecular structure of 1. Solvent and protons are omitted for
clarity.
X-ray Crystallography. Data for the crystal structure of
1‚toluene were collected on a Stoe Image Plate IPDS II-System,
for 2 on a Stoe-Siemens-Huber four circle diffractometer, and for
3 on Bruker Smart Apex CCD diffractometer. For refinement of 2
and 3 as nonmerohedric twins, the two matrices of the two domains
were determined, and every domain was integrated on its own. Then
a new file with the reflections of the dominant domain was written,
without reflections that were strongly overlapped with reflections
of the other domain. With these data, the structures were solved
and refined.
question of the best formal oxidation state assignment for
the scandium center in complex 1.
Experimental Section
General Comments. All manipulations were performed on a
high-vacuum line or in a glovebox under a purified N2 atmosphere,
using Schlenk techniques with rigorous exclusion of moisture and
air. All the necessary glassware was oven-dried at 150 °C for a
minimum period of 12 h, assembled hot, and cooled under high
vacuum with intermittent flushing of nitrogen or argon. The samples
for spectral measurements were prepared inside a MBraun MB 150-
GI glovebox where the O2 and H2O levels were normally maintained
below 1 ppm. Toluene was distilled from Na/benzophenone, THF
from Na prior to use. The melting points of 1, 2, and 3 were
measured in sealed capillaries on a Bu¨hler SPA-1 instrument.
1H, 13C, and 45Sc NMR spectra (benzene-d6, THF-d8, toluene-
d8) were recorded on Bruker MSL-400, AM-250, and AM-200
instruments. Solid-state 45Sc and 13C NMR spectra were recorded
on Bruker A-600. The solvents for NMR measurements were dried
over K or CaH2 and trap-to-trap distilled prior to use. Mass spectra
were obtained on a Finnigan MAT 8230 by EI technique. EPR
spectra were recorded on a Varian Century-Line 9 GHz. UV-vis
spectrum was recorded on a Perkin-Elmer 320. Magnetic measure-
ments were performed on a Squid-Magnetometer (Quantum Design,
California) at different magnetic fields in the range of temperatures
between 2 and 300 K.
Reaction of 1 with H2O‚B(C6F5)3. A 0.1 g (1.8 mmol) portion
of H2O‚B(C6F5)3 was added to a solution of 0.075 g (0.07 mmol)
of 1 (obtained from reaction of 0.3 g (0.6 mmol) of LScBr2 with
1.2 mL of (C3H5)MgBr (1 M in ether, 1.2 mmol) in a yield of
25%) in toluene (10 mL). The reaction mixture immediately turned
to brown. After stirring for a further hour, the solution was
concentrated to approx 5 mL until it became turbid and 5 mL THF
was added to dissolve the precipitate. Colorless crystals of 2 suitable
for X-ray analyses appeared after one week at -26 °C (approx
0.050 g). Mp 167-178 °C. Anal. Calcd for C41H35N4B2F20O2Sc:
C, 46.36; H, 3.32; N, 5.27. Found: C, 46.86; H, 3.98; N, 5.06. 1H
NMR (500 MHz, C6D6, 300 K): δ ) 3.51 (s, 1 H, C(Me)CHC-
(Me), 2.96 (t, 4 H; Et2NCH2CH2), 2.75 (m, 8 H, NCH2CH3), 2.58
(t, 4 H, Et2NCH2CH2), 1.40 (s, 6H, CCH3), 0.72 (t, NCH2CH3).
19F NMR (188 MHz, ext C6F6, C6D6, 300 K): -136.7 (q, 8F, BC6F5
ortho), -155.8 (t, 4F, BC6F5 para), -162.7 (m, 8F, BC6F5 meta).
45Sc NMR (121 MHz, referenced to [Sc(H2O)6]3+ in D2O, C6D6,
The structures were solved by direct methods (SHELX-97) and
refined against F2 using SHELXL-97.8 R values were defined as
2
2 2 0.5
R1 ) ∑||Fo| - |Fc||/∑|Fo|, wR2 ) [∑w(Fo - Fc2)2/∑w(Fo ) ]
,
w ) [σ2(Fo ) + (g1P)2 + g2P]-1, P ) 1/3[max(Fo , 0) + 2Fc2].
Heavy atoms were refined anisotropically. Hydrogen atoms were
included using the riding model with Uiso tied to Uiso of the parent
atoms. Crystal data collection details, and the solution and refine-
ment procedures, are summarized in Table 1.
2
2
Theoretical Calculations. NMR shielding calculations for
complexes 1 and Cp2Sc(BH4)9 were carried out using density
functional theory (DFT) and gauge-including atomic orbitals
(GIAOs) using the Amsterdam Density Functional package (version
ADF2000.02).10 The method involved has been described in detail
by Schreckenbach in his description of the 57Fe NMR shieldings
in ferrocene.11 For the purpose of minimizing the calculation time,
the model system Sc(BrMgL)2Br (L ) (R2NCH2CH2NCMe)2CH,
R ) H) was used where ethyl groups remote from scandium in the
real system are replaced by hydrogen atoms. The remainder of the
molecule was left intact. Since the real system is known to
crystallize with C2V molecular point group symmetry, geometry
(8) (a) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467-473.
(b) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure
Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(9) (a) Schreckenbach, G.; Ziegler, T. J. Phys. Chem. 1995, 99, 606-
611. (b) Schreckenbach, G.; Ziegler, T. Int. J. Quantum Chem. 1996,
60, 753-766. (c) Schreckenbach, G.; Ziegler, T. Int. J. Quantum Chem.
1997, 61, 899-918. (d) Wolff, S. K.; Ziegler, T. J. Chem. Phys. 1998,
109, 895-905. (e) Wolff, S.; Ziegler, K. T.; van Lenthe, E.; Baerends,
E. J. J. Chem. Phys. 1999, 110, 7689-7698. (f) Gilbert, T. M.; Ziegler,
T. J. Phys. Chem. A 1999, 103, 7535-7543.
(10) (a) Baerends, E. J.; Ros, P. Chem. Phys. 1975, 8, 412-418. (b)
Versluis, L.; Ziegler, T. J. Chem. Phys. 1988, 88, 322-328. (c) Velde,
G. T.; Baerends, E. J. J. Comput. Phys. 1992, 99, 84-98. (d) Fonseca
Guerra, C.; Snijders, J. G.; Velde, G. T.; Baerends, E. J. Theor. Chem.
Acc. 1998, 99, 391-403.
(11) Schreckenbach, G. J. Chem. Phys. 1999, 110, 11936-11949.
8804 Inorganic Chemistry, Vol. 42, No. 26, 2003

Elucidation of a Sc(I) Complex by DFT
Table 1. Summary of Crystallographic Data
1‚toluene
2‚toluene
3‚0.5toluene
formula
fw
C41H78Br3Mg2N8Sc
1016.42
C48H43N4B2F20O2Sc
1154.44
C20.50H39BrMgN4
445.78
space group
cryst syst
a, Å
b, Å
c, Å
Cmcm
P1h
P21/c
orthorhombic
21.5760(12)
12.9704(7)
17.7340(8)
triclinic
monoclinic
17.508(4) Å
7.2970(14)
19.674(4)
10.865(2)
13.518(3)
19.172(4)
71.83(3)
85.12(3)
69.11(3)
2498.6(9)
2
1.534
R, deg
â, deg
γ, deg
V, Å3
Z
110.493(3)
2148.51(17)
4
1.360
2354.4(8)
4
1.258
D
calcd, g/cm3
temp, K
133(2)
133(2)
173(2)
2θ range, deg
1.83-24.71
-25 e h e 25
-15 e k e 14
-20 e l e 18
26177/2256
[R(int) ) 0.0720]
99.9
1.12-23.21
-11 e h e 12
-13 e k e 14
0 e l e 21
12159/12159
[R(int) ) 0.0000]
98.3
2.21-27.10
-22 e h e 21
0 e k e 9
0 e l e 25
23318/5067
[R(int) ) 0.0768]
97.5
index ranges
reflns collected/unique
completeness to θ (%)
data/restraints/params
GOF
2256/0/199
1.107
12159/856/767
1.080
5067/0/266
1.052
final R indices [I > 2σ(I)]
R1 ) 0.0315,
wR2 ) 0.0849
R1 ) 0.0346,
wR2 ) 0.0867
0.946 and -0.608
R1 ) 0.0454,
wR2 ) 0.1364
R1 ) 0.0486,
wR2 ) 0.1403
0.329 and -0.435
R1 ) 0.0567,
wR2 ) 0.1372
R1 ) 0.0874,
wR2 ) 0.1570
1.966 and -0.596
R indices (all data)
largest diff peak and hole (e/A3)
optimization on the model system was carried out with imposition
of this symmetry constraint. Since no structural data are available
for Cp2Sc(BH4), we carried out geometry optimization taking the
η3-BH4 bonding mode as a starting point, employing no symmetry
constraints. Since the Cp2Sc(BH4) system exhibited a tendency to
Cs symmetry, final optimization was carried out with incorporation
of this constraint. Single-point calculations, preceding the NMR
calculations, were run using the Zora(V) basis set for all atoms, as
implemented in the ADF suite. Full electronic configuration was
used for all atoms. Relativistic effects were included by virtue of
the zero order regular approximation (ZORA).12 However, no spin-
orbit coupling effects were taken into account in the derivation of
the isotropic shielding for the scandium atoms. The local density
approximation (LDA) by Vosko, Wilk, and Nusair (VWN)13 was
used together with the exchange and correlation corrections
published by Perdew in 1991 (PW91).14
Figure 2. (a) A contour plot of the molecule’s highest occupied molecular
orbital (HOMO) in the yz plane, with Sc located at the origin. The Sc-Br
vector defines the z axis (vertical) and the HOMO in the C2V point group
has b2 symmetry. (b) This shows the same 10 × 10 Å2 area in the yz plane,
but it displays contours corresponding to the total SCF electron density.
vealed that the HOMO (Figure 2a) is comprised largely of
the scandium dyz orbital, which is antibonding with respect
to the filled bromine py orbital. Nodal surfaces cutting
through the yz plane are indicated in Figure 2a by dash-
dot-dash (-‚-) lines, while solid (s) lines and dashes (- - -)
denote positive and negative contours, respectively.
From the standpoint of energy considerations, the HOMO
is located nearly equidistant between the HOMO - 1 and
LUMO orbitals, a characteristic typically identified with a
nonbonding (lone pair) orbital (Figure 5). This is consistent
with assigning a formal +1 oxidation state to the scandium
center. Examination of the HOMO in three dimensions
suggests a substantial stabilization by virtue of δ-back-
bonding to the two heterocyclic rings sandwiching the
scandium atom (Figure 3).
The bromine atom (top) and the scandium atom (bottom)
are the only atoms that have significant electron density in
the yz plane within this area.
Schreckenbach has elucidated the electronic mechanism
of the 57Fe NMR shielding in ferrocene and in iron
pentacarbonyl.11 He makes note that the diamagnetic con-
tribution to the shielding, σd, does not contribute to relative
chemical shifts because it is due to contributions from core
Results and Discussion
Our aforementioned work7 formulated, accordingly to the
X-ray and NMR studies, compound 1 as Sc(BrMgL)2Br
(L ) (R2NCH2CH2NCMe)2CH, R ) Et). But magnetic and
EPR measurements exposed an interesting and unanticipated
magnetic behavior (diamagnetic) for the compound, a
detailed understanding of which warranted a computational
study as follows.
A DFT study performed on the model system Sc-
(BrMgL)2Br (L ) (R2NCH2CH2NCMe)2CH, R ) H) re-
(12) (a) Snijders, J. G.; Baerends, E. J.; Ros, P. Mol. Phys. 1979, 38, 1909-
1929. (b) Ziegler, T.; Tschinke, V.; Baerends, E. J.; Snijders, J. G.;
Ravenek, W. J. Phys. Chem. 1989, 93, 3050-3056. (c) van Lenthe,
E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993, 99, 4597-
4610.
(13) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200-
1211.
(14) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson,
M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671-6687.
Inorganic Chemistry, Vol. 42, No. 26, 2003 8805

Neculai et al.
electronic structure. Herein we use Schreckenbach’s method
to analyze via density functional theory (DFT) calculations
the electronic structure of 1. The major MO mixings
contributing to σp,oc-vir are elucidated and are compared with
those found for a reference scandium(III) sandwich system,
Cp2Sc(BH4).16 Both systems have been studied by solution
45Sc NMR methods,15,16 and the present work suggests that
it would be of interest to probe their predicted anisotropies
by solid-state 45Sc NMR techniques. A priori expectations
for any scandium(I) system include a small HOMO-LUMO
gap and therefore a strong downfield shift relative to
scandium(III) systems. This is because the MO mixings that
comprise σp,oc-vir become stronger as the gap between the
involved orbitals diminishes. A small HOMO-LUMO gap
is expected for scandium(I) because this oxidation state
assignment implies a lone pair of electrons localized on
scandium, i.e., a high-lying HOMO. To the extent that the
HOMO becomes delocalized onto the ligands, the effect on
the 45Sc NMR downfield shift may be expected to diminish;
Schreckenbach found that the most substantive MO mixings
comprising σp,oc-vir for ferrocene are between MOs having
substantial iron 3d character.11 Calculations were carried out
on the same model system Sc(BrMgL)2Br (L ) (R2NCH2-
CH2NCMe)2CH, R ) H) where ethyl groups remote from
scandium in the real system are replaced by hydrogen atoms
(Figure 4).
Figure 3. HOMO orbital of 1 in three dimensions.
Depicted in Figure 5a is a set of three energy level
diagrams (corresponding to Rx, Ry, and Rz) for Sc-
(BrMgL)2Br indicating via vertical dashed lines the five
strongest occupied-virtual MO couplings as a function,
respectively, of the three Cartesian axis directions. Separation
of the occupied-virtual MO couplings in this manner comes
about because the magnetic operator has the same symmetry
properties as the Rx, Ry, and Rz rotations about the Cartesian
axes. Note that the HOMO is, as anticipated for scandium-
(I), energetically high-lying.
In the point group C2V, for example, Rx has B2 symmetry,
and thus, the magnetic field can induce mixing about the x
axis between occupied and virtual orbitals whose direct
product is also of B2 symmetry. The strongest such MO
mixing associated with Rx is accordingly seen to be between
37B2 (the HOMO) and 59A1 (the LUMO + 1). Associated
with this pair of orbitals is a small energy gap and a B2
symmetric direct product. In contrast to the delocalization
of the HOMO, the LUMO + 1 with which the HOMO may
Figure 4. Calculated structure of the model compound Sc(MgBrL)2Br,
where L is (R2NCH2CH2NCMe)2CH with R ) H. Shown also are the
Cartesian axis directions used for the NMR calculation. The molecule is
oriented with Sc at the origin, the Sc-Br2 vector defining the z axis, and
the xz plane being a mirror plane containing Sc, the three bromine atoms,
and the two Mg atoms. Selected bond distances (Å): Sc1-Br2, 2.799; Sc-
N13, 2.319; Sc-C15, 2.494; Sc-C8, 2.510; Mg4-N13, 2.163; Mg4-Br6,
2.525; Sc1-Mg4, 3.276.
electrons and is thus essentially invariant from one compound
to another. Further, Schreckenbach points out that this truism
has been noted previously for a variety of nuclei. By far the
greatest contributor to the total shielding, in the absence of
large spin-orbit effects,15 typically is the occupied-virtual
2
2
mix strongly via Rx is a nearly pure dx -y orbital. From a
physical point of view, in the presence of an applied magnetic
field a current is induced about the x axis involving the lobes
2
2
of scandium’s dyz and dx -y orbitals that reside in the yz plane.
Even though the LUMO + 12 (64A1) is energetically well-
separated from the HOMO, it is this orbital (scandium
component of the paramagnetic shielding term, σp,oc-vir
.
Additionally, it may turn out, as for the ferrocene 57Fe case,
that only a few occupied-virtual molecular orbital (MO)
couplings determine the paramagnetic shielding almost
completely. In such a case, the NMR shift and its associated
anisotropy represent a sensitive probe of frontier MO
2
localized and of dz symmetry) with which the HOMO
couples second-strongest. It is worthwhile at this point to
make mention of the calculated chemical shielding anisotropy
associated with Sc(BrMgL)2Br. The Cartesian axes indicated
(16) Mancini, M.; Bougeard, P.; Burns, R. C.; Mlekuz, M.; Sayer, B. G.;
Thompson, J. I. A.; McGlinchey, M. J. Inorg. Chem. 1984, 23, 1072-
1078.
(15) Rodriguez-Fortea, A.; Alemany, P.; Ziegler, T. J. Phys. Chem. A 1999,
103, 8288-8294.
8806 Inorganic Chemistry, Vol. 42, No. 26, 2003

Elucidation of a Sc(I) Complex by DFT
Figure 5. (a) Plot of the 5 strongest field-induced mixings of occupied and virtual energy levels of Sc(MgBrL)2Br for each of the three rotations Rx, Ry,
and Rz, from left to right (left side). (b) Plot of the 5 strongest field-induced mixings of occupied and virtual energy levels of Cp2ScBH4 for each of the three
rotations Rx, Ry, and Rz, from left to right (right side).
in Figure 4 map onto the magnetic shielding in the following
manner: x, δ11; y, δ33; z, δ22. What this means is that the
strongest σp,oc-vir and largest downfield chemical shifts are
associated with the Cartesian x axis direction. The system is
found to be rhombic; i.e., δ11 * δ22 * δ33. The orbitals that
are most strongly mixed by the Ry magnetic operator are
37B2 (HOMO) with 32A2 (LUMO), and the second
being 34B2 (HOMO - 13) with 32A2 (LUMO), and by
the Rz magnetic operator are 37B2 (HOMO) with 51B1
(LUMO + 3) and 37B2 (HOMO) with 52B1 (LUMO + 5),
respectively. Note that although HOMO-LUMO mixing is
facilitated by Ry, it is associated with δ33 and the direction
of weakest paramagnetic shielding. This is because the
LUMO is mostly ligand-based with only a very small
contribution from the scandium dxy orbital. LUMO + 3 and
LUMO + 5 both enjoy strong contributions from scandium’s
dxz orbital, leading to the intermediate coupling and downfield
shift associated with δ22 and Rz (see Supporting Information).
Now it is appropriate to ask if the calculated shieldings
for Sc(BrMgL)2Br really do signify a downfield shift relative
to a typical scandium(III) reference compound. For this
purpose we choose Cp2Sc(BH4), a sandwich complex geo-
metrically similar to Sc(BrMgL)2Br but with what most
would agree to be an unambiguous example of scandium in
the +3 oxidation state.
symmetry about Sc, we find instead that δ22 ≈ δ33, such
that the system is (accidentally) nearly axial about z. The
magnetic operator of Rz symmetry permits strong mixing
between HOMO - 3 and LUMO + 2, on one hand, and
between HOMO - 2 and LUMO + 1 on the other.
2
2
Respectively, these orbitals are strong in scandium dx -y
,
dxy and dxz, dyz character (see Supporting Information). The
HOMO for Cp2Sc(BH4) is quite delocalized, involving
metal-ring π bonding as well as terminal B-H bonding.
2
The LUMO is nearly a pure scandium dz orbital consistent
with assignment of this molecule as a Lewis acidic 16e
complex. This orbital plays a role in the strongest occupied-
virtual mixings associated with δ22 and δ33 (see Supporting
Information). Having discussed the magnetic anisotropy
associated with the two scandium systems considered here,
as well as the underlying molecular orbital basis for the
anisotropy, we turn attention now to the total isotropic
shielding for these systems. The compound Cp2Sc(BH4) has
been reported to have an isotropic solution 45Sc NMR shift
in C6D6 of +67.5 ppm relative to saturated ScCl3 in D2O.
Taking Cp2Sc(BH4) as our reference compound, we can
convert a given calculated shielding value σ(calc) into a
chemical shift δ via the equation: δ(calc) ) δ(ref, calc) -
δ(calc) + 67 ppm. The expectation is that Sc(BrMgL)2Br
should exhibit its 45Sc NMR signal ca. 40 ppm downfield
relative to Cp2Sc(BH4). On the other hand, the compound
Sc(BrMgL)2Br (L ) (R2NCH2CH2NCMe)2CH, R ) Et) has
been reported to resonate at δ ) 167.5 ppm relative to
McGlinchey and co-workers assigned to the borohydride
ligand of Cp2Sc(BH4) an η3 bonding mode, despite the fact
-
that their orbital analysis suggested BH4 to be only a 4e
donor to the Cp2Sc+ fragment; i.e., they call the complex a
16e system.16 What we found was that the system converged
nicely to an η2-BH4 (see Supporting Information). Interest-
ingly, as expected for scandium(III) relative to scandium(I),
the energy level diagrams (Figure 5b) are reflective of a large
HOMO-LUMO gap, essentially double the value obtained
for Sc(BrMgL)2Br. Also, while the symmetry of Cp2Sc(BH4)
is low consistent with an expectation of rhombic magnetic
+ 7
[Sc(H2O)6]3 , presumably the same reference used in the
case of Cp2Sc(BH4). The discrepancy between the calculated
and observed δiso is rather substantial, given that the 45Sc
chemical shift range has been estimated to be only ca. 340
ppm.17 One possible explanation is that the experimental
(17) Kirakosyan, G. A.; Tarasov, V. P.; Buslaev, Yu. A. Magn. Reson.
Chem. 1989, 27, 103-111.
Inorganic Chemistry, Vol. 42, No. 26, 2003 8807

Neculai et al.
Scheme 2
reference samples used might possibly differ in their chemical
shifts; the pH of the aqueous scandium solutions was not
stated in either case. A recent 45Sc NMR study reported the
use of Sc(NO3)3 in water at pH 1 as the reference standard.18
On the other hand, Schreckenbach has pointed out that
calculated metal NMR shifts are very sensitive to small
changes in structure (i.e., bond lengths) and to the absolute
calculated MO energies, both of which can vary according
to the specific choice of computational method.14 Regardless
of our ability at present to accurately calculate absolute
isotropic NMR shieldings, we assert that the type of study
carried out here has great value in being able to relate the
anisotropy of the magnetic shielding to the molecular
electronic structure. We expect that the gross features of the
anisotropy should be relatively independent of the compu-
tational method employed, and accordingly propose that
solid-state 45Sc NMR may be the best method firmly to
connect the molecular orbitals discussed herein to the NMR
observable.19
The foregoing discussion substantiates the formal oxidation
state of +1 for Sc in complex 1, while recognizing that the
phenomenon of δ back-bonding to the LMgBr ring LUMOs
draws some of electron density away from Sc. It is difficult
to conceive a reaction complex 1 might undergo in which
scandium maintains its unusual oxidation state(I).19 This is
because the disposition and the electronic configuration of
LMgBr have a decisive contribution to the stability of
complex 1. Attempts to exchange LMgBr for other ligands
were not successful. Accordingly, investigation of the
reactivity of 1 has proceeded in the direction of redox
reactions with substrates possessing acidic hydrogens, in-
cluding alcohols and water.
Figure 6. Molecular structure of 2. Solvent and protons are omitted for
clarity.
Table 2. Selected Bond Distances (Å) and Angles (deg) for 2
bond lengths
angles
C(1)-C(2)
1.508(4)
1.521(4)
1.402(4)
1.331(3)
1.296(3)
1.290(3)
2.0449(17)
2.0481(17)
2.163(2)
2.171(2)
2.402(2)
2.392(2)
O(1)-Sc-O(2)
166.1(7)
129.5(2)
145.84(17)
149.90(17)
85.33(8)
123.9(2)
124.1(2)
79.15(8)
79.15(8)
C(4)-C(5)
C(2)-C(3)
N(1)-C(2)
B(1)-O(1)
B(2)-O(2)
Sc(1)-O(1)
Sc(1)-O(2)
Sc(1)-N(1)
Sc(1)-N(2)
Sc(1)-N(3)
Sc(1)-N(4)
C(2)-C(3)-C(4)
Sc(1)-O(1)-B(1)
Sc(1)-O(2)-B(2)
N(1)-Sc(1)-N(2)
N(1)-C(2)-C(3)
N(2)-C(4)-C(3)
N(1)-Sc(1)-N(3)
N(2)-Sc(1)-N(4)
N(3)-Sc(1)-N(4)
C(2)-N(1)-Sc(1)
C(4)-N(2)-Sc(1)
116.59(7)
128.53(17)
128.51(18)
six-membered ring with the â-diketiminato backbone (Figure
6). The differences in bond lengths within the â-diketiminato
moiety among 1, 2, and LScBr221 are negligible (C(1)-C(2)
1.411 Å in 1, 1.401 Å in 2, and 1.394 Å in LScBr2; N(1)-
C(1) 1.385 Å in 1, 1.330 Å in 2, and 1.340 Å in LScBr2),
and the B-O bond length in 2 (av 1.292 Å) is also very
close to a covalent B-O bond (1.311 Å),20 although much
shorter than the coordinative B-O bond found in H2O‚
B(C6F5)3 (1.597 Å)22 (Table 2). An interesting feature of this
structure is the arrangement of two C6F5 groups almost
parallel contiguous to the ScNCCCN ring in 1. Obviously
this has a direct influence on the electron density on the
proton from the C(Me)CHC(Me) position whose resonance
in the 1H NMR spectrum is significantly upfield shifted from
4.82 ppm in LScBr2 to 3.51 ppm in 2. As expected, the
crystal packing reveals the incidence of π-π stacking
interactions between the C6F5 groups of different molecules.
Reaction of 1 with the Water Adduct H2O‚B(C6F5)3.
For a controlled hydrolysis, H2O‚B(C6F5)3 was used, and
product 2 was isolated (Scheme 2). The volatiles of this
reaction were analyzed by GC-MS, and C6F5H was identified
as a byproduct (Scheme 2).
With these findings, we can propose a path involving the
reduction of water by scandium, which in turn is oxidized
to the (3+) state. In compensation to the fragmentation of
molecule 1, the ligand (L) migrates for stabilizing the
oxidized scandium, and the B(C6F5)3 molecule is cleaved.20
In compound 2, scandium adopts a pseudo-octahedral
geometry (O(1)-Sc-O(2) 166.1°) forming a perfect planar
(18) Hill, N. J.; Levason, W.; Popham, M. C.; Reid, G.; Webster, M.
Polyhedron 2002, 21, 1579-1588.
(19) See ring displacement reactions with participation of Ln(0) compounds
in ref 5a.
(20) Splitting of the B(C6F5)3 molecule is not new. See: Neculai, D.;
Roesky, H. W.; Neculai, A. M.; Magull, J.; Walfort, B.; Stalke, D.
Angew. Chem. 2002, 114, 4470-4471; Angew. Chem., Int. Ed. 2002,
41, 4294-4295.
(21) Neculai, A. M.; Neculai, D.; Nikiforov, G.; Roesky, H. W.; Magull,
J.; Schlicker, C.; Herbst-Irmer, R.; Noltemeyer, M. Eur. J. Inorg.
Chem. 2003, 3120-3126.
(22) Bergquist, C.; Bridgewater, B. M.; Harlan, C. J.; Norton, K. R.;
Friesner, R. A.; Parkin, G. J. Am. Chem. Soc. 2000, 122, 10581-
10590.
8808 Inorganic Chemistry, Vol. 42, No. 26, 2003

Elucidation of a Sc(I) Complex by DFT
Scheme 3
Table 3. Selected Bond Distances (Å) and Angles (deg) for 3 and 1a
compound 3
compound 1
C(1)-C(2)-C(4)
Mg-N(1)
Mg-N(2)
Mg-N(3)
Mg-N(4)
C(1)-C(2)
C(2)-C(3)
C(2)-N(1)
C(4)-N(2)
2.087(3)
2.094(3)
2.309(3)
2.335(3)
1.532(5)
1.411(5)
1.341(5)
1.334(5)
C(2)-C(3)-C(4)
N(2)-Mg-N(1)
N(4)-Mg-N(3)
N(3)-Mg-N(1)
N(2)-Mg-N(4)
128.0(3)
Mg-N(1)
Mg-N(2)
2.142(2)
2.345(2)
129.1(4)
88.22(12)
99.29(11)
79.32(11)
78.87(11)
N(1)-Mg-N(1)#2
N(2)-Mg-N(2)#2
N(2)-Mg-N(1)
82.14(12)
110.62(12)
79.76(8)
C(1)-C(3)
C(1)-C(2)
C(1)-N(1)
1.511(4)
1.411(3)
1.385(3)
3
3
a Symmetry transformations used to generate equivalent atoms for 1: #1 -x, y, z; #2 x, y, -z + /2; #3 -x, y, -z + /2.
transformation L has not changed (preservation of its
monoanionic character) (Scheme 1). The pentacoordinated
magnesium atom is in a pyramidal environment having the
Mg-N bond distances slightly shorter compared to those in
1 (Mg-N(â-diketiminato) 2.087 Å; 2.094 Å in 3 and 2.142
Å in 1; Mg-N(coordinated arms) 2.309 Å, 2.335 Å in 3
and 2.345 Å in 1) attributable most likely to the higher
coordination number of Mg in 1 versus 3. These values
accommodate with the literature data for â-diketiminato
magnesium complexes.23 Furthermore, we were interested
in a comparison of distances and angles within the â-dike-
timinato backbone (C(1)-C(2) in 1 1.411 Å, C(2)-C(3)
1.411 Å, C(4)-C(3) 1.415 Å in 3; C(1)-N(1) in 1 1.385 Å,
C(2)-N(1) in 3 1.341 Å, the internal angle being 129.1° in
1 and 128.03° in 3). A closer look shows that in fact the
dimensions do not change so much to conclude that the
LMgBr in 1 is negatively charged (anion or radical-anion).
The slightly shorter C-N bonds in 3 compared to 1 can be
attributed to the different coordination numbers around the
â-diketiminato nitrogen atoms, together with removal of the
δ-back-bonding understood to be operative in the case of 1
(Table 3).
Figure 7. Molecular structure of 3. Solvent and protons are omitted for
clarity.
Reaction of 1 with (HOCH2)2CMe2; Formation of
LMgBr (3). The reduction using alcohols led as expected
to the cleavage of the sandwich molecule 1 with isolation
only of the protective groups LMgBr (Scheme 3).
1
The reaction was monitored by H NMR spectroscopy.
Because of the complexity of the reaction mixture, only
representative resonances were followed. The observations
are consistent with a redox reaction; namely, the HOCH2
resonances fully disappeared from the initial position (2.04
ppm) in the spectrum of the alcohol in the deuterated solvent.
The resonance corresponding to the proton from C(Me)CHC-
(Me) position was shifted from 2.82 ppm in 1 to 4.74 ppm
in the reaction mixture, analogous with the resonance found
for the same proton from the final isolated product LMgBr.
The characteristic resonances of LMgBr can be found in the
final spectrum. A spectral subtraction of the resonances of
3 gave no useful information about the scandium product(s).
The only useful information extracted from the 45Sc NMR
spectrum (due to the multiple peaks) was related to the
disappearance of the characteristic resonance for complex 1
(167 ppm).
Note that the deviation of the Mg from the N2C3 plane is
significantly lower in 3 (0.22 Å) compared to 1 (1.09 Å). In
general, the magnesium ion coordination of L may be
understood to be flexible.24 In compound 1, the conformation
is presumably forced by sandwiching Sc together with
repulsive interactions involving the Sc and Mg-bound
bromine atoms.
Conclusion
Herein we have provided computational support for the
statement that in compound 1 the formal oxidation state of
(23) (a) Dove, A. P.; Gibson, V. C.; Hormnirum, P.; Marshall, E. L.; Segal,
J. A.; White, A. J. P.; Williams, D. J. J. Chem. Soc., Dalton Trans.
2003, 3088-3097. (b) Chisholm, M. H.; Gallucci, J.; Phomphrai, K.
Inorg. Chem. 2002, 41, 2785-2794. (c) Chamberlain, B. M.; Cheng,
M.; Moore, D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates, G. W. J.
Am. Chem. Soc. 2001, 123, 3229-3238. (d) Cheng, M.; Attygalle, A.
B.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 1999, 121,
11583-11584.
(24) (a) Randall, D. W.; DeBeer George, S.; Holland, P. L.; Hedman, B.;
Hodgson, K. O.; Tolman, W. B.; Solomon, E. L. J. Am. Chem. Soc.
2000, 122, 11632-11648. (b) Deng, L.; Schmid, R.; Ziegler, T.
Organometallics 2000, 19, 3069-3076.
Examination of the bond lengths and angles revealed by
the X-ray structure of 3 (Figure 7) is important because it
facilitates the understanding of the influence of the coordina-
tion of ScBr to the LMgBr molecule.
Moreover, the structure of 3 indicates that during the
Inorganic Chemistry, Vol. 42, No. 26, 2003 8809

Neculai et al.
scandium is +1. The DFT calculations are in good agreement
with the experimental data and disclosed that HOMO has
substantial Sc dyz character, but it is additionally delocalized
onto the ligands via a δ back-bonding mechanism. This
delocalization represents stabilization of the unusual +1
oxidation state for scandium. Moreover, although the skeletal
arrangement of the molecule was not maintained, redox
processes in which 1 was involved helped to understand the
chemistry of such a system and are in concert with the initial
formulation of 1. Also the fact that the L ligand was not
activated during these reactions lends a bias toward the
formulation of the ligand as having consistent monoanionic
character.25 Importantly, the synthetic method for obtaining
compound 1 relies on solution chemistry, thus illustrating
an alternative to metal atom vapor methods for preparing
low-valent scandium compounds.
Acknowledgment. Financial support of the Deutsche
Forschungsgemeinschaft is gratefully acknowledged.
Supporting Information Available: The CIF files for com-
pounds 2 and 3 and details of the computational study. This material
is available free of charge via the Internet at http://pubs.acs.org.
IC034947O
(25) Eisenstein, O.; Hitchcock, P. B.; Khvostov, A. V.; Lappert, M. F.;
Maron, L.; Perrin, L.; Protchenko, A. V. J. Am. Chem. Soc. 2003,
125, 10790-10791.
8810 Inorganic Chemistry, Vol. 42, No. 26, 2003