A new chelating anilido-imine donor related to β-diketiminato ligands for stabilization of organoyttrium cations

Article abstract of DOI:10.1021/om030157a

A bulky anilido-imine donor that marries the attributes of the β-diketiminato and salicylaldiminato ligand frameworks has been prepared and used to stabilize bis-alkyl yttrium derivatives, which act as precursors to cationic organoyttrium complexes.

Full text of DOI:10.1021/om030157a

Organometallics 2003, 22, 1577-1579
1577
Communications
A New Ch ela tin g An ilid o-Im in e Don or Rela ted to
â-Dik etim in a to Liga n d s for Sta biliza tion of
Or ga n oyttr iu m Ca tion s
Paul G. Hayes, Gregory C. Welch, David J . H. Emslie, Cassandra L. Noack,
Warren E. Piers,*,1 and Masood Parvez
Department of Chemistry, University of Calgary, 2500 University Drive NW,
Calgary, Alberta, Canada T2N 1N4
Received March 4, 2003
Sch em e 1
Summary: A bulky anilido-imine donor that marries the
attributes of the â-diketiminato and salicylaldiminato
ligand frameworks has been prepared and used to
stabilize bis-alkyl yttrium derivatives, which act as
precursors to cationic organoyttrium complexes.
The rise of the â-diketiminato, or “nacnac”, donor
framework (I) to the “A-list” of ancillary ligands has
been dramatic, with recent applications in bioinorganic,
main group, and transition metal chemistry2 Despite
their wide applicability, examples of organoyttrium
compounds stabilized by the nacnac ligands are not
known, and although we have extensively developed
organoscandium nacnac chemistry,3 attempts to prepare
analogous yttrium derivatives have thus far been un-
successful.
3 metals,6 we envisioned a ligand that combines the
steric and electronic features of the nacnac and sali-
cylaldiminato donor frameworks to prepare bis-alkyl
yttrium derivatives. Herein we report the synthesis of
the new monoanionic, bulky anilido-imine ligand family
III7 and its deployment as an ancillary for organoyt-
trium compounds. As a variant on the enormously
popular nacnac ligand family, these new ligands could
find applications in many areas of inorganic chemistry.
The anilido-aldimine ligand 1 was straightforwardly
prepared in two steps from commercially available
starting materials as shown in Scheme 1. Imine forma-
tion occurs smoothly at room temperature using 2,6-di-
isopropylaniline and ortho-fluoro benzaldehyde. The
anilido donor portion of the ligand was installed via a
nucleophilic aromatic displacement of fluoride using
LiN(H)Ar. Although both reagents are reasonably steri-
cally bulky, the reaction is high yielding and no doubt
aided by the electron-withdrawing nature of the neigh-
boring imine group.8 The resulting proteo ligand 1 can
On the other hand, organoyttrium derivatives using
the salicylaldiminato ligand family II can be prepared,
but thermally stable LYR2 complexes were not available
even with bulky examples4 since they are prone to
ligand distribution processes.5 Given the recent interest
in non-cyclopentadienyl ligand environments for group
(1) Phone: 403-220-5746.Fax: 403-289-9488.E-mail: wpiers@ucalgary.ca.
S. Robert Blair Professor of Chemistry 2000-2005. E. W. R. Steacie
Memorial Fellow, 2001-2003.
(2) Bourget-Merle, L.; Lappert, M. F.; Severn, J . R. Chem. Rev. 2002,
102, 3031.
(3) (a) Lee, L. W. M.; Piers, W. E.; Elsegood, M. R. J .; Clegg, W.;
Parvez M. Organometallics 1999, 18, 2947. (b) Knight, L. K.; Piers,
W. E.; McDonald, R. Chem. Eur. J . 2000, 6, 4322. (c) Hayes, P. G.;
Lee, L. W. M.; Knight, L. K.; Piers, W. E.; Parvez, M.; Elsegood, M. R.
J .; Clegg, W.; MacDonald, R. Organometallics 2001, 20, 2533. (d)
Hayes, P. G.; Piers, W. E.; McDonald, R. J . Am. Chem. Soc. 2002, 124,
2132.
(6) (a) Piers W. E.; Emslie, D. J . H. Coord. Chem. Rev. 2002, 233-
234, 129. (b) Skinner, M. E. G.; Mountford, P. J . Chem. Soc., Dalton
Trans. 2002, 1694.
(4) Wang, C.; Friedrich, S.; Younkin, T. R.; Li, R. T.; Grubbs, R. H.;
Bansleben, D. A.; Day, M. W. Organometallics 1998, 17, 3149.
(5) (a) Emslie, D. J . H.; Piers, W. E.; McDonald, R. J . Chem. Soc.,
Dalton Trans. 2002, 293. (b) Emslie, D. J . H.; Piers, W. E.; Parvez,
M.; McDonald, R. Organometallics 2002, 21, 4226.
(7) A structurally related, but electronically quite different, indolide-
imine ligand has been reported: Matsugi, T.; Matsui, S.; Kojoh, S.;
Takagi, Y.; Inoue, Y.; Fujita, T.; Kasiwa, N. Chem. Lett. 2001, 566.
(8) Miller, J . Aromatic Nucleophilic Substitution; Elsevier: New
York, 1968.
10.1021/om030157a CCC: $25.00 © 2003 American Chemical Society
Publication on Web 03/21/2003

1578 Organometallics, Vol. 22, No. 8, 2003
Communications
Sch em e 2
F igu r e 1. Molecular structure of 3b. Selected bond
distances (Å): Y(1)-N(1), 2.454(3); Y(1)-N(2), 2.324(3);
Y(1)-O(1), 2.376(3); Y(1)-C(32), 2.381(4); Y(1)-C(33),
2.419(4). Selected bond angles (deg): N(1)-Y(1)-N(2),
75.85(9); N(1)-Y(1)-O(1), 169.47(9); N(2)-Y(1)-O(1),
97.25(9); C(32)-Y(1)-C(33), 108.90(13); N(2)-Y(1)-C(32),
121.86(12); N(2)-Y(1)-C(33), 128.99(12).
be lithiated with n-BuLi to give a lithium salt as a mono
THF adduct. This salt serves as a convenient reagent
for ligand installation upon reaction with YCl3‚THF3.5
;
refluxing these two partners in toluene for 4 days gives
excellent yields of the dichloride 2 as indicated in
Scheme 1. The compound retains 1 equiv of THF, as
determined by 1H NMR spectroscopy; attempts to
remove the THF by heating these solids under vacuum
led only to decomposition. Diagnostic ligand resonances
Lewis acids (vide infra) lead to alkide abstraction, it
appears that the THF ligand is not dissociatively labile.
The yttrium atom lies 0.819(5) Å out of the C3N2
ligand plane, which is consistent with what is observed
in related five-coordinate organoscandium compounds
incorporating a bulky nacnac ligand. Unlike the nacnac
ligands, the donor framework in the anilido-imine ligand
is more localized and the two nitrogens bond to yttrium
with markedly different bond lengths (Y(1)-N(1) )
2.454(3) Å; Y(1)-N(2) ) 2.324(3) Å). The steric effect of
the N-aryl groups is also unbalanced, as demonstrated
by the different values for the angles C(1)-N(1)-C(8)
(111.5(3)°) and C(15)-N(2)-C(20) (115.1(3)°).
1
in the H NMR spectrum include those for the aldimine
proton at 8.2 ppm and four upfield shifted multiplets
between 5.8 and 7.0 ppm for the aromatic protons of
the backbone. The asymmetry of the ligand and re-
stricted rotation about the N-Caryl bonds give rise to
four separate doublets for the isopropyl methyl groups.
Alkylation of dichloride 2 using LiCH2SiMe2R gives
the bis-alkyl compounds 3 (R ) Me, 3a ; R ) Ph,5b 3b),
in which the THF ligand is retained. The X-ray struc-
ture of 3b has been determined, and an ORTEP diagram
is shown in Figure 1.9 The yttrium center exhibits
distorted trigonal bipyramidal geometry, in which the
imine nitrogen N(1) and the THF ligand occupy the axial
sites. This geometry implies that the two alkyl groups
are equivalent, although upon cooling, the uncharac-
teristically broad signal for the Y-CH2 groups splits into
an AB quartet when restricted mobility of the alkyl
groups renders the C-H moieties diastereotopic, while
the 13C resonance remains a broad singlet with unre-
solved Y-C coupling. Since all attempts to remove THF
failed, and reactions of these compounds with strong
While the presence of THF is not ultimately desirable,
it does appear to fortify bis-alkyl compounds 3 against
a metalation process involving the C-H bonds of the
aryl isopropyl methyl groups, which is a factor in nacnac
organoscandium chemistry.3c Furthermore, upon reac-
tion of 3b with B(C6F5)3, abstraction of one CH2SiMe2-
Ph group occurs exclusively, forming ion pair 4b with
no observation of THF‚B(C6F5)3 (Scheme 2). At low
1
temperature, H and 19F NMR spectroscopy indicates
that 4b exists as three isomers, as indicated by three
separate aldimine hydrogen resonances, and three sets
of ortho, meta, and para fluorine signals. The ∆δm,p
values in the 19F NMR spectrum10 are 2.6, 2.6, and 3.4
ppm, indicating that the anion is not contacting the
cation through the abstracted carbon atom. Due to the
instability of 4b toward -C6F5 back-transfer (see below)
and the complexity of the low-temperature NMR spec-
tra, we are not sure if the isomers are geometric in
nature or are distinguished by different modes of ion-
ion contact.11
(9) Crystal data for 3b: C53H73N2OSi2Y, MW ) 899.22, monoclinic,
P21/ n, a ) 10.8537(3) Å, b ) 18.7713(7) Å, c ) 24.9649(9) Å, â )
91.444(2)°, V ) 5084.7(3) Å3, Z ) 4, Fcalc ) 1.175 g cm-3, Mo KR
radiation, λ ) 0.71073 Å, T ) 173(2) K, 14 631 measured reflections,
8495 unique, 5919 reflections with Inet > 2.0σ(Inet), µ ) 1.23 mm-1
,
min./max. transmission ) 0.748 and 0.818, R1 (I>2σ) ) 0.053, wR2
0.098, GoF ) 1.06, no. of parameters ) 535, final difference map within
+0.28 and -0.33 e Å3. Crystallographic data (excluding structure
factors) for the structure reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC-201288. Copies of the data can be obtained free
of charge on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (fax: (+44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk.
(10) Horton A. D.; de With, J . Organometallics 1997, 16, 5424.

Communications
Organometallics, Vol. 22, No. 8, 2003 1579
While prone to C6F5 back-transfer as described above,
if 4b is generated at low temperature and immediately
treated with an excess of THF, the resulting ion pair is
stable indefinitely even at temperatures as high as 60
°C. The 19F and 1H NMR spectra simplify dramati-
cally,13 indicating that the metal is stabilized by two
THF donors, symmetrizing the structure. Compound
4b‚THF can be isolated from these solutions in 87%
yield as an analytically pure yellow solid and stored
indefinitely under an inert atmosphere.
Cationic organoyttrium compounds are rare species
due to their high reactivity, and few are stable enough
to characterize even in solution;14 the remarkable
stability of 4b‚THF offers the opportunity to explore the
chemistry of these elusive species. This has been made
possible by the development of a new ligand family,
related structurally to the important nacnac family of
ligands. The anilido-imine is sterically similar to the
most efficacious nacnac ligands, but the more localized
donor structure provides the opportunity to probe
electronic modifications which have been shown to have
profound effects on the reactivity of nacnac complexes.15
The potential of this new ligand environment for use in
other areas is high, given the wide applicability of the
nacnac ligand framework.2
F igu r e 2. Molecular structure of 5. Selected bond dis-
tances (Å): Y(1)-N(1), 2.421(2); Y(1)-N(2), 2.262(2);
Y(1)-O(1), 2.337(2); Y(1)-C(32), 2.492(3); Y(1)-C(38),
2.460(3); Y(1)-F(1), 2.786(2). Selected bond angles (deg):
N(1)-Y(1)-N(2), 77.52(8); N(1)-Y(1)-O(1), 166.54(8);
N(2)-Y(1)-O(1), 103.61(8); C(32)-Y(1)-C(38), 118.18(10);
C(33)-C(32)-Y(1), 102.8(2); C(37)-C(32)-Y(1), 144.7(2).
Ack n ow led gm en t. Funding for this work was pro-
vided by NSERC of Canada and the University of
Calgary. NSERC is also acknowledged for an E. W. R.
Steacie Memorial Fellowship to W.E.P. (2001-2003)
and a PGS B Fellowship to P.G.H. P.G.H. also thanks
the Killam Foundation for a Fellowship and the Alberta
Heritage Fund for a Steinhauer Award, and D.J .H.E.
thanks the Alberta Ingenuity Fund for a Postdoctoral
Associateship.
The stability of 4b is strongly dependent on the
temperature and the presence or absence of excess THF.
For example, if 3b is activated in C6D5Br at -30 °C,
ion pair 4b is only persistent at temperatures lower
than -20 °C; above these temperatures, facile -C6F5
transfer processes occur, ultimately yielding bis-pen-
tafluorophenyl derivative 5 and unidentified boron-
containing products. Compound 5 was isolated in low
yield and identified by X-ray crystallography12 and
features a distinct Y-F (Y(1)-F(1) ) 2.786(2) Å)
interaction involving one of the ortho F atoms in the
solid state as shown in Figure 2. Apart from this feature,
neutral compound 5 is in other ways structurally similar
to the dialkyl starting complex 3b.
Su p p or tin g In for m a tion Ava ila ble: Full experimental
details for the synthesis of all new compounds, plus spectro-
scopic and other characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM030157A
(11) For example, it is possible that the phenyl substituent from
the abstracted CH2SiMe2Ph group might be interacting in an η6
bonding mode with the cationic center (see ref 3a).
(13) Selected NMR data for 4b‚THF (C7D8, 270 K). 1H NMR: δ 7.92
(s, CHdNAr), 1.48 (br s, BCH2SiMe2Ph), 0.46 (s, BCH2SiMe2Ph), 0.15,
2
0.13 (s, Y-CH2SiMe2Ph), -0.13 (d, J H-Y ) 3.6 Hz, Y-CH2SiMe2Ph).
(12) Crystal data for 5: C47H47F10N2OY, MW ) 934.78, triclinic, P1h,
a ) 10.272(2) Å, b ) 11.673(3) Å, c ) 19.348(4) Å, R ) 84.118(14)°, â
) 82.302(14)°, γ ) 74.364(15)°, V ) 2208.6(8) Å3, Z ) 2, Fcalc ) 1.406
g cm-3, Mo KR radiation, λ ) 0.71073 Å, T ) 173(2) K, 35 807 measured
reflections, 9949 unique, µ ) 1.400 mm-1, min./max. transmission )
0.7211 and 0.7867, R1(I>2σ) ) 0.0496, wR2 0.0898, GoF ) 1.112, no.
of parameters ) 550, final difference map within +0.543 and -0.506
13C{1H} NMR: δ 174.03 (CHdNAr), 44.56 (d, 1J C-Y ) 53.6 Hz, Y-CH2-
SiMe2Ph), 8.83 (br, BCH2SiMe2Ph), 2.53, 2.49 (Y-CH2SiMe2Ph), -0.35
(BCH2SiMe2Ph). 19F NMR: -130.7 (d, 3J F-F ) 21 Hz, ortho-F), -163.9
3
3
(t, J F-F ) 21 Hz, para-F), -166.4 (t, J F-F ) 21 Hz, meta-F). 11B
NMR: δ -14.2. Anal. Calcd for C75H81N2O2Si2BF20Y: C, 57.50; H, 5.50;
N, 1.89. Found: C, 57.01; H, 5.60; N, 1.84. Full spectroscopic details
can be found in the Supporting Information.
e
Å3. Crystallographic data (excluding structure factors) for the
(14) (a) Lee, L. W. M.; Berg, D. J .; Einstein, F. W.; Batchelor, R. J .
Organometallics 1997, 16, 1819. (b) Bambirra, S.; van Leusen, D.;
Meetsma, A.; Hessen, B.; Teuben, J . H. Chem. Commun. 2001, 637.
(c) Arndt, S.; Spaniol, T. P.; Okuda, J . Chem. Commun. 2002, 896.
(15) Allen, S. D.; Moore, D. R.; Lobkovsky, E. B.; Coates, G. W. J .
Am. Chem. Soc. 2002, 124, 14284.
structure reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publication
no. CCDC-206037. Copies of the data can be obtained free of charge
on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(fax: (+44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk.