Metalation-resistant β-Diketiminato ligands for thermally robust organoscandium complexes

Article abstract of DOI:10.1021/om900283s

A new ligand design for the widely used β-diketiminato framework implements a "remote steric bulk" strategy for the stabilization of low-coordinate, metalation-resistant organoscandium complexes. In comparison to standard ligands, substantial improvements

Full text of DOI:10.1021/om900283s

Organometallics 2009, 28, 3625–3628 3625
DOI: 10.1021/om900283s
Metalation-Resistant β-Diketiminato Ligands for Thermally Robust
Organoscandium Complexes
Alyson L. Kenward, Jennifer A. Ross, Warren E. Piers,* and Masood Parvez
University of Calgary, 2500 University Drive N.W., Calgary, Canada, T2N 1N4
Received April 14, 2009
Summary: A new ligand design for the widely used β-diketi-
for further transformations requiring the application of heat6
and has also impeded the isolation of base-free yttrium alkyl
cations. Metalation has also been observed to be accelerated in
the presence of Lewis acids.7
minato framework implements a “remote steric bulk” strategy
for the stabilization of low-coordinate, metalation-resistant
organoscandium complexes. In comparison to standard
ligands, substantial improvements in thermal stability for
neutral dialkyl and cationic alkyl organoscandium complexes
are observed.
To circumvent this problem, we have reevaluated the
β-diketiminato ligand design. The need to remove the
offending C-H bonds from the vicinity of the metal’s
coordination sphere while maintaining the steric presence
necessary for mononuclearity and low coordination
numbers led us to consider a ligand with “remote
steric bulk”, using principles articulated and implemented
by Wolczanski8 and Stephan.9 In this instance, relocation
of the N-aryl substitution from the ortho to the meta
positions, in concert with increasing their size, led us to
consider ligands 1 and 2.
Bulky β-diketiminato, or “nacnac”, ligands have emerged
as important ancillaries for metals from across the periodic
table for a variety of applications.1 One area of particularly
high interest has been their use in stabilizing low-coordinate
early transition metal complexes containing metal-alkyl,
amido, and imido moieties.1,2 The most commonly employed
β-diketiminato ligands utilize N-aryl groups with ortho-
substitution (typically isopropyl groups) to provide steric bulk
close to the metal center, with some fine-tuning available via
modulation of the alkyl groups in the ligand backbone, I.
We have successfully deployed these ligands on group
3 metals scandium3 and yttrium,4 preparing low-coordinate
bis-alkyls and well-defined alkyl cations5 of moderate thermal
stability. While workable under ambient conditions, these
compounds are prone to metalative decomposition pathways,
involving alkane elimination via σ-bond metathesis with a
C-H bond of an ortho-isopropyl group from one of the
N-aryl substituents. This has become particularly problematic
(6) (a) Knight, L. K.; Piers, W. E.; Fleurat-Lessard, P.; McDonald,
R.; Parvez, M. Organometallics 2004, 23, 2087. (b) Knight, L. K.; Piers,
W. E.; McDonald, R. Organometallics 2006, 25, 3289. (c) Basuli, F.;
Tomaszewski, J.; Huffman, J. C.; Mindiola, D. J. Organometallics 2003,
22, 4705.
(7) Conroy, K. D.; Hayes, P. G.; Piers, W. E.; Parvez, M. Organo-
metallics 2007, 26, 4464.
(8) Wolczanski, P. T. Polyhedron 1995, 14, 3335.
(9) Stephan, D. W. Organometallics 2005, 24, 2548.
(10) See Supporting Information for detailed preparation of terphe-
nyl aniline H2N-3,5-(2,4,6-iPrC6H2)C6H3 via amination of a known aryl
bromide: Yandulov, D. V.; Schrock, R. R.; Rheingold, A. L.; Ceccarelli,
C.; Davis, W. M. Inorg. Chem. 2003, 42, 796.
ꢀ
(11) Carey, D. T.; Cope-Eatough, E. K.; Vilaplana-Mafe, E.; Mair,
*Corresponding author. E-mail: wpiers@ucalgary.ca.
(1) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. Rev. 2002,
102, 3031.
F. S.; Pritchard, R. G.; Warren, J. E.; Woods, R. J. Dalton Trans. 2003,
1083.
(12) See Supporting Information for details.
(2) For specific examples of Ti, Zr, V, and Cr complexes: (a)
Budzelaar, P. H. M.; van Oort, A. B.; Orpen, A. G. Eur. J. Inorg. Chem.
1998, 1485. (b) Basuli, F.; Huffman, J. C.; Mindiola, D. J. Inorg. Chem.
2003, 42, 8003. (c) Basuli, F.; Bailey, B. C.; Tomaszewski, J.; Huffman, J.
C.; Mindiola, D. J. J. Am. Chem. Soc. 2003, 126, 6052. (d) Basuli, F.;
Kilgore, U. J.; Hu, X.; Meyer, K.; Pink, M.; Huffman, J. C.; Mindiola,
D. J. Angew. Chem., Int. Ed. 2004, 43, 3156. (e) Basuli, F; Bailey, B. C.;
Huffman, J. C.; Mindiola, D. J. Chem. Commun. 2003, 1554. (f) Basuli,
F.; Kilgore, U. J.; Brown, D.; Huffman, J. C.; Mindiola, D. J. Organo-
metallics 2004, 23, 6166. (g) Kakaliou, L.; Scanlon, W. J.; Qian, B.; Baek,
S. W.; Smith, M.; Motry, D. H. Inorg. Chem. 1999, 38, 5964. (h) Rahim,
M.; Taylor, N. J.; Xin, S.; Collins, S. Organometallics 1998, 17, 1315. (i)
Qian, B.; Scanlon, W. J.; Smith, M. R.; Motry, D. H. Organometallics
1999, 18, 1693. (j) MacAdams, L. A.; Buffone, G. P.; Incarvito, C. D.;
Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 2005, 127, 1082.
(k) Kim, W.-K.; Fevola, M. J.; Liable-Sands, L. M.; Rheingold, A. L.;
Theopold, K. H. Organometallics 1998, 17, 4541.
(3) Hayes, P. G.; Lee, L. W. M.; Knight, L. K.; Piers, W. E.; Parvez,
M. Organometallics 2001, 20, 2533.
(4) Kenward, A. L.; Piers, W. E.; Parvez, W. E.; Hayes, P. G.
Organometallics 2009, DOI: 10.1021/om 900082d.
(5) (a) Hayes, P. G.; Piers, W. E.; McDonald, R. J. Am. Chem. Soc.
2002, 124, 2132. (b) Hayes, P. G.; Piers, W. E.; Parvez, M. J. Am. Chem.
Soc. 2003, 125, 5622. (c) Hayes, P. G.; Piers, W. E.; Parvez, M.
Organometallics 2005, 24, 1173. (d) Hayes, P. G.; Piers, W. E.; Parvez,
M. Chem.;Eur. J. 2007, 13, 2632.
(13) Synthesis of [H(3,5-trip-C6H3)NC(Me)CHC(Me)N(3,5-trip-
C6H3)] (2): A 100 mL round-bottom flask, fitted with a Dean-Stark
condenser, was charged with 2,4-pentanedione (137 mg, 1.37 mmol),
p-toluenesulfonic acid (260 mg, 1.37 mmol), and toluene (40 mL). This
mixture was warmed to 60 °C for 30 min, after which time a solution of
3,5-bis(2,4,6-triisopropylphenyl)aniline (1.37 g, 2.76 mmol) in toluene
(25 mL) was added via syringe. The reaction was refluxed overnight,
removing water in the Dean-Stark when necessary, and the solvent
˚
volume in the reaction vessel was kept to 60-70 mL. After 24 h, 5 A
molecular sieves were added, and the reaction was refluxed for another
48 h. After cooling to room temperature, NEt3 (160 mg, 1.6 mmol) was
added and the solution stirred for 20 min. Upon removing the volatiles in
vacuo, the yellow oily crude product was purified by column chromato-
graphy (using a basified silica column) and recrystallized in methanol
to give a fine yellow powder of
Hipt
L
H (1.05 g, 0.986 mmol, 72% yield).
H N), 7.01 (s, 8H, m-C6H2(trip)),
4
Me
1H NMR (CD2Cl2): 12.90 (s, 1H, N
6.70 (d, JH-H = 1.4 Hz, 4H, o-C6H3), 6.60 (t, JH-H = 1.4 Hz, 2H,
3 3 3 3
4
p-C6H3), 4.89 (s, 1H, CH), 2.90 (septet, JH-H = 6.9 Hz, 4H, p-iPrCH
3
(trip)), 2.77 (septet, JH-H = 6.9 Hz, 8H, o-iPrCH(trip)), 2.05 (s, 6H,
3
CH3), 1.28 (d, 3JH-H = 6.9 Hz, 24H, p-iPrCH3(trip)), 1.06 (d, 3JH-H
=
3
6.9 Hz, 24H, o-iPrCH3(trip)), 1.03 (d, JH-H = 6.9 Hz, 24H, o-iPr-
CH3(trip)). 13C{1H} NMR (CD2Cl2): 159.98, 148.54, 147.12, 145.93,
141.76, 137.49 (Cipso), 126.83 (p-C6H3), 122.05 (o-C6H3), 120.97 (m-
C6H2(trip)), 98.54 (CH), 34.91(p-iPrCH(trip)), 30.94 (o-iPrC (iPr-
CH3(trip)), 21.46 (CH3). Anal. Calcd for C77H106N2: C, 87.27; H,
10.08; N, 2.64. Found: C, 87.12; H, 10.14; N, 2.32.
r
2009 American Chemical Society
Published on Web 05/19/2009
pubs.acs.org/Organometallics

3626 Organometallics, Vol. 28, No. 13, 2009
Communication
Ligands 1 and 2 were prepared from 2,4-pentanedione and
the appropriate anilines10 via standard condensation meth-
odology.2a,11,12 Of particular interest, the terphenyl aniline
utilized in the preparation of 213 was synthesized by first
preparing the hexaisopropyl terphenyl bromide according to
an established procedure14 and then by direct amination with
NH315 (Scheme S1). Because of the more remote steric bulk,
1 and 2 can conveniently be affixed to scandium utilizing an
alkane elimination protocol, whereas salt elimination/alky-
lation strategies were necessary with ligands I. Thus, dialkyl
scandium complexes were accessible without having to resort
to lithium salts of the ligands.
Scandium tris-alkyls Sc(CH2SiMe3)3(THF)2 and Sc-
(CH2SiMe2Ph)3(THF)2 react with proteo-ligands 1 and 2
at room temperature, generating bis-alkyl complexes 3 and 4
(Scheme 1) with isolated yields ranging from 51% to 80%.
The NMR spectra of isolated 3 and 4 reveal that no THF
remains coordinated, and, unlike the scandium and yttrium
complexes of ligand I, the 1H NMR spectra are temperature
invariant to -100 °C.
Figure 1. Thermal ellipsoid diagram (50%) of 3a. Selected bond
˚
distances (A): Sc(1)-N(1), 2.1277(18); Sc(1)-N(2), 2.1279(17);
Sc(1)-C(1), 2.215(2); Sc(1)-C(2), 2.210(2). Selected bond an-
gles (deg): N(1)-Sc(1)-N(2), 83.06; C(1)-Sc(1)-C(2), 111.65
(8); Sc(1)-C(1)-Si(1), 123.33(10); Sc(1)-C(2)-Si(2), 121.11
(11). Selected dihedral angles (deg): C(4)-N(1)-C(8)-C(9),
82.6(3); C(6)-N(2)-C(22)-C(23), -56.5(3).
Scheme 1
These variable-temperature NMR spectroscopic observa-
tions suggest that one of the consequences of the change in
ligand structure is to allow the metal to lie in the plane of the
ligand, favoring a C2v, rather than Cs, symmetric ground-
statestructure. Thisisin line withthe hypothesisthatit issteric
conflict between the ortho isopropyl groups of ligand I and the
alkyl groups of a tetracoordinate metal complex that force the
metal to adopt an out-of-plane stance in such complexes.2a,3
Thus, in dialkyl scandium complexes of ligand I, the alkyl
groups inhabit diastereotopic environments to alleviate steric
interactions with the ortho isopropyl groups that are averaged
on the NMR time scale. The assignment of C2v symmetry
to compounds 3 and 4 was confirmed by the X-ray
Figure 2. Thermal ellipsoid diagram (30%) of 4b. Selected bond
˚
distances (A): Sc(1)-N(1), 2.127(4); Sc(1)-N(2), 2.122(4);
Sc(1)-C(78), 2.201(4); Sc(1)-C(87), 2.202(5). Selected bond
angles (deg): N(1)-Sc(1)-N(2), 84.96(14); C(78)-Sc(1)-C
(87), 116.40(19). Selected dihedral angles (deg): C(2)-N(1)-
C(6)-C(7), 77.0(6); C(4)-N(2)-C(42)-C(43), -82.0(6).
structural analysis of two of these complexes. Thermal ellip-
soid diagrams of 3a and 4b are shown in Figures 1 and 2
respectively. In contrast to complexes of ligand I, in 3a and 4b
(14) Yandulov, D. V.; Schrock, R. R. J. Am. Chem. Soc. 2002, 124,
6252.
(15) Surry, D. S.; Buchwald, S. L. J. Am. Chem. Soc. 2007, 129, 10354.

Communication
Organometallics, Vol. 28, No. 13, 2009 3627
Scheme 2
Scheme 3
the MR2 group is nearly coplanar with the C3N2 atoms of
theligand. In 3a, the deviation of the scandium center fromthe
˚
C3N2Sc plane is 0.4896(3) A, which is manifest in reduction
of the N(1)-Sc(1)-N(2) angle by about 10° from those found
in complexes of ligand I. In complex 4b, the metal deviates
˚
only 0.224(3) A from the C3N2Sc plane, providing further
observed in early metal alkyl cations.16 Interestingly, the
chemical shifts of the coordinated aniline (7.24, 6.51, 6.49,
2.53 ppm in 5b) are representative of σ-donation from
the nitrogen lone pair. This N-based coordination con-
trasts recently characterized arene-based π-donation of
coordinated NMe2Ph in β-diketiminato yttrium alkyl
cations.4,17
indication that redirection of the ligand bulk has reduced
the steric confinements at the metal center.
In an NMR-scale reaction, treatment of bis-alkyl 4b with
[CPh3][B(C6F5)4] cleanly afforded a new cationic product, 6
Further inspection of the solid-state structures of 3 and
4 reveals that the “uprightness” of the N-aryl groups relative
to the ligand plane differs from those found in complexes of
ligands I, as exemplified by the dihedral angle C(2)-N(1)-
C(6)-C(7) of 77.0(6) in 4b. The N-aryl groups in I are much
closer to perpendicular (89-99°). In spite of this, overall,
ligands 1 and 2 provide impressive steric coverage about
the scandium center, which, in concert with the bulky alkyl
groups, renders these new complexes both monomeric and
base free.
1
(Scheme 3). The H NMR spectrum of 6 exhibits features
representative of a C2v symmetric complex, along with
liberated Ph3CCH2SiMe2Ph. The 11B NMR spectrum con-
tains only one resonance at -16.5 ppm, typical for weakly
coordinating perfluoroarylborate anions. The resonance
at -7.9 ppm in the 29Si NMR spectrum for the coordinated
alkylsilane moiety is upfield of those observed in early metal
complexes containing β-Si-C agostic interactions.18
Furthermore, when this alkide abstraction is performed in
nondeuterated solvent, there is no evidence of coordinated
solvent, and this observation, along with the overall symmetric
spectroscopic features, are in accord with 6 being a three-
coordinate scandium alkyl cation. Despite several attempts,
neither X-ray quality crystals nor a solid sample of 6 could be
isolated. Similar to the neutral bis-alkyls, the scandium cations
5 and 6 supported by these redesigned β-diketiminate ligands
Scandium compounds 3 and 4 are significantly more
thermally robust than the scandium bis-alkyls supported
by ligands I. For example, while the complexes I-Sc-
(CH2SiMe3)2 and I-Sc(CH2SiMe2Ph)2 begin to undergo
1
metalation at 60 °C within 15 min,3 the H NMR spectra
of 3 and 4 show no evidence of decomposition when heated
to over 100 °C (see Figure S1, for example). When heated at
120 °C for over 5 h, signs of decomposition slowly begin to
appear, although no well-defined metalation product could
be identified by 1H NMR spectroscopy.
(16) (a) Horton, A. D.; de With, J. Organometallics 1997, 16, 5424. (b)
Horton, A. D.; de With, J.; van der Linden, A.; van de Weg, H.
Organometallics 1996, 15, 2672. (c) Tjaden, E. B.; Swenson, D. C.;
Jordan, R. F.; Peterson, J. L. Organometallics 1995, 14, 371. (d) Bei, Z.;
Swenson, D. C.; Jordan, R. F. Organometallics 1997, 16, 3282. (e)
Hollink, E.; Wei, P.; Stephan, D. W. Organometallics 2004, 23, 1562.
(17) Yu, N.; Nishiura, M.; Li, X.; Xi, Z.; Hou, Z. Chem. Asian J. 2008,
3, 1406.
(18) (a) Bolton, P. D.; Clot, E.; Adams, N.; Dubberley, S. R.; Cowley,
A. R.; Mountford, P. Organometallics 2006, 25, 2806. (b) Clot, E.;
Eisenstein, O. Struct. Bond. 2004, 113, 1. (c) Tredget, C. S.; Clot, E.;
Mountford, P. Organometallics 2008, 27.
Encouraged by the remarkable thermal stability of these
complexes, we further explored their reactivity toward typical
activating reagents, including [HNMe2Ph][B(C6F5)4] and
[CPh3][B(C6F5)4]. The protonolysis of the bis-alkyls 4 with
Bronstead acid [HNMe2Ph][B(C6F5)4] proceeded cleanly with
e
the formation of new cationic complexes 5 and alkane libera-
tion (Scheme 2). The product cations are subject to retention of
the aniline produced in the reaction, as has commonly been

3628 Organometallics, Vol. 28, No. 13, 2009
Communication
exhibit remarkable thermal stability. Over several days in
room-temperature solution, there are no observable signs of
decomposition, and heating to 80 °C overnight results in only
slight decomposition (to protonated ligand and unidentifiable
scandium-containing products).
The unparalleled thermal stability of these neutral and
cationic scandium complexes verifies that in relocating the
front-lying steric bulk in the β-diketiminato framework, we
have implemented an effective ligand design strategy for
preparing a metalation-resistant framework. These new
complexes can now be subject to a previously unattainable
thermal regime, and our ongoing studies are investigating
reactions under these high-temperature conditions. Given
the widespread use of β-diketiminato ligands I, these new
designs with remote steric bulk have substantial promise for
stabilizing low-coordinate compounds of the early transition
and main group metals.
Acknowledgment. Funding for this work was provided
by NSERC of Canada in the form of a Discovery Grant to
W.E.P., a PGSD toA.L.K., and a PGSM toJ.A.R. A.L.K.
and J.A.R. also thank the Alberta Ingenuity Fund for
Studentships.
Supporting Information Available: Additional schemes and
figures, experimental details, and crystallographic information
files. This material is available free of charge via the Internet at
http://pubs.acs.org.