Chemoselective "on-site" functionalization of group 4 metal acetamidinates

Article abstract of DOI:10.1021/om020905g

Deprotonation of group 4 monocyclopentadienyl metal acetamidinates can be achieved in high yield using sterically encumbered bases to provide enolate complexes that can subsequently be reacted with electrophiles to produce several new classes of metal ami

Full text of DOI:10.1021/om020905g

Organometallics 2003, 22, 21-23
21
Communications
Ch em oselective “On -Site” F u n ction a liza tion of Gr ou p 4
Meta l Aceta m id in a tes
Yonghui Zhang, Denis A. Kissounko, J ames C. Fettinger, and Lawrence R. Sita*
Department of Chemistry and Biochemistry, University of Maryland,
College Park, Maryland 20742
Received November 1, 2002
Sch em e 1
Summary: Deprotonation of group 4 monocyclopenta-
dienyl metal acetamidinates can be achieved in high
yield using sterically encumbered bases to provide eno-
late complexes that can subsequently be reacted with
electrophiles to produce several new classes of metal
amidinates that are not accessible by conventional
routes.
The amidinate ligand fragment, [N(R1)C(R2)N(R3)]-,
is a structural component found within a diverse range
of mononuclear transition metal and main group metal
complexes.1 Synthetic methods available for the incor-
poration of this group into the coordination sphere of
metals, however, still remain quite limited, and these
include (1) classical salt elimination using metal halides
and alkali metal amidinates, (2) condensation between
metal halides and silyl substituted amidines, and (3)
carbodiimide insertion into metal-carbon bonds.1,2
Recently, we documented that d0 group 4 monocyclo-
pentadienyl metal amidinates of the general formula
(η5-C5R5)MXY[N(R1)C(R2)N(R3)] (1: M ) Zr, R ) H, Me,
X ) Y ) Me, R2 ) Me) are highly active precatalysts
for the stereoselective living polymerization of a wide
variety of R-olefins and R,ω-nonconjugated dienes upon
activation with a borate cocatalyst, such as [Ph3C]-
[B(C6F5)4].3 In the pursuit of new derivatives of 1 that
would permit the further development of structure/
property relationships and that would allow for the
heterogenization of this class of Ziegler-Natta precata-
lyst on solid supports,4 we required amidinates that
could not easily, or ever, be prepared through the
conventional methods enumerated above. Herein, we
now introduce a new synthetic procedure that circum-
vents this problem by providing rapid access to a
number of new amidinate structural classes through the
efficient deprotonation and chemoselective functional-
ization of an acetamidinate (R2 ) Me in 1) fragment
within a metal complex. J ust as the direct deprotonation
of η5-cyclopentadienyl and η6-arene metal complexes
have provided new generations of these ligand types,5
this new synthetic method may prove to be of general
use for extending the scope of amidinate-based metal
complexes.
Ligands with enolizable protons can be functionalized
prior to metal complexation through standard depro-
tonation with a strong base followed by reaction with
an electrophile. On the other hand, to the best of our
knowledge, no reports have appeared in which an
attempt has been made to functionalize a metal-com-
plexed ligand containing enolizable protons through the
same process, and this includes metal complexes con-
taining the ubiquitously occurring acetylacetonato (acac)
ligand.6 For the purposes of the present study, we
selected complexes 1a (M ) Zr)7 and 1b (M ) Hf)8 as
initial test subjects for deprotonation which we felt could
be achieved using a sterically encumbered, non-nucleo-
philic base. As Scheme 1 reveals, this base proved to be
lithium triphenylsilane, LiSiPh3, which can easily be
obtained in crystalline form as its tetrahydrofuran
(THF) adduct.9 More specifically, reaction of 1a with a
(1) For reviews, see: (a) Barker, J .; Kilner, M. Coord. Chem. Rev.
1994, 133, 219-300. (b) Edelmann, F. T. Coord. Chem. Rev. 1994, 137,
403-481.
(2) (a) Coles, M. P.; J ordan, R. F. J . Am. Chem. Soc. 1997, 119,
8125-8126. (b) Sita, L. R.; Babcock, J . R. Organometallics 1998, 17,
5228-5230. (c) Koterwas, L. A.; Fettinger, J . C.; Sita, L. R. Organo-
metallics 1999, 18, 4183-4190 and references therein.
(3) (a) J ayaratne, K. C.; Sita, L. R. J . Am. Chem. Soc. 2000, 122,
958-959. (b) J ayaratne, K. C.; Keaton, R. J .; Henningsen, D. A.; Sita,
L. R. J . Am. Chem. Soc. 2000, 122, 10490-10491. (c) Keaton, R. J .;
J ayaratne, K. C.; Henningsen, D. A.; Koterwas, L. A.; Sita, L. R. J .
Am. Chem. Soc. 2001, 123, 6197-6198.
(5) Davies, S. G. Organotransition Metal Chemistry: Applications
to Organic Synthesis; Pergamon Press: New York, 1982.
(6) For unexpected observations of metal-complexed ligand depro-
tonations by base, see: (a) Okeya, S.; Kusuyama, Y.; Isobe, K.
Nakamura, Y.; Kawaguchi, S. J . Organomet. Chem. 1998, 551, 117-
123. (b) Neculai, A. M.; Roesky, H. W.; Neculai, D.; Magull, J .
Organometallics 2001, 20, 5501-5503.
(7) Keaton, R. J .; Koterwas, L. A.; Fettinger, J . C.; Sita, L. R. J .
Am. Chem. Soc. 2002, 124, 21, 5932-5933.
(4) Hlatky, G. G. Chem. Rev. 2000, 100, 1347-1376.
(8) Kissounko, D. A.; Sita, L. R. To be submitted for publication.
10.1021/om020905g CCC: $25.00 © 2003 American Chemical Society
Publication on Web 12/07/2002

22 Organometallics, Vol. 22, No. 1, 2003
Communications
Sch em e 3
these protons is smaller than the line width of the
resonances is consistent with the 1H NMR data reported
for other enolates.12
In aprotic polar coordinating solvents, such as diethyl
ether (Et2O), evidence exists to suggest that 2a can
undergo loss of LiCl to produce neutral, solvent-
stabilized species such as 3, shown in Scheme 2. Indeed,
upon attempts to grow crystals of 2a from a Et2O/
pentane solvent mixture, continual precipitation of LiCl
was observed to occur and subsequent single-crystal
X-ray analysis of the dark orange crystalline material
that was obtained revealed it to be the dinuclear species
4 shown in Figure 1, which is presumed to arise from
dimerization of 3 upon displacement of the weakly
coordinated solvent molecule (see Scheme 2).13 In 4, the
methylene carbon of the enolized amidinate, C12, is
trigonal coplanar, as revealed by the sum of the bond
angles about this atom, ∑θC12, which is 360°, and the
C11-C12 bond length of 1.332(8) Å supports full double-
bond character for the interaction between these two
carbon centers.
F igu r e 1. Molecular structure (30% thermal ellipsoids)
of 4. Hydrogen atoms have been removed for the sake of
clarity.
Sch em e 2
Regarding the reactivity of 2a and 2b toward elec-
trophiles, a variety of transformations have been carried
out which successfully place a substituent at the eno-
lized carbon of the acetamidinate ligand according to
Scheme 3.10 The products of these reactions that are of
most interest to us, however, are those that provide new
types of amidinate ligands that are not accessible by
conventional synthetic procedures, such as compounds
slight excess of LiSiPh3 in THF at 25 °C yielded, upon
removal of the solvent after 2 h, a near-quantitative
yield of complex 2a which can be isolated as a purple
powder that is analytically pure on the basis of chemical
analysis.10 Importantly, use of the less hindered base
LiSiMe2Ph led to preferential nucleophilic attack at the
metal center, rather than deprotonation, to provide a
zirconium silyl complex.11 Finally, in terms of reaction
time and yield of product, deprotonation of the hafnium
analogue 1b to provide 2b was best achieved using
KN(SiMe3)2 rather than LiSiPh3.10
(12) See for instance: Abiko, A. Inoue, T.; Masamune, S. J . Am.
Chem. Soc. 2002, 124, 10759-10764.
(13) Crystal data for 4: C36H62Cl2N4Zr2, Mr ) 804.24, monoclinic,
C2/c, a ) 12.6867(13) Å, b ) 17.2867(18) Å, c ) 18.6795(19) Å, â )
108.344(2)°, V ) 3888.4(7) Å3, Z ) 4, Dcalcd ) 1.374 mg/m3, µ ) 0.702
mm-1, F(000) ) 1680, Mo KR radiation (λ ) 0.710 73 Å), T ) 293(2)
K, 2θmax ) 25°, 9423 independent reflections collected, 6392 reflections
in refinement, final R indices (I > 2σ(I)) R1 ) 0.0763 and wR2 ) 0.1795.
Crystal data for 5a ‚(tol): C32H46Cl2N2Hf, Mr ) 708.10, monoclinic, P21/
c, a ) 14.9125(4) Å, b ) 8.1333(2) Å, c ) 26.7677(8) Å, â ) 104.6570-
(10)°, V ) 3140.94(15) Å3, Z ) 4, Dcalcd ) 1.497 mg/m3, µ ) 3.514 mm-1
,
F(000) ) 1432, Mo KR radiation (λ ) 0.710 73 Å), T ) 293(2) K,
2θmax ) 27.5°, 7226 independent reflections collected, 5555 reflections
in refinement, final R indices (I > 2σ(I)) R1 ) 0.0250 and wR2 ) 0.0535.
Crystal data for 5b: C19H33Cl3N2Hf, Mr ) 574.31, orthorhombic, Pbca,
a ) 13.8548(5) Å, b ) 15.9428(6) Å, c ) 20.4171(7) Å, R ) â ) γ )
1H and 13C NMR spectra taken in THF-d8 support the
enolate structure for 2 shown in Scheme 1, in which the
deprotonated acetamidinate is now formally serving as
90°, V ) 4509.8(3) Å3, Z ) 8, Dcalcd ) 1.692 mg/m3, µ ) 4.987 mm-1
,
1
a dianionic ligand. Thus, a H NMR (400 MHz, 25 °C)
F(000) ) 2272, Mo KR radiation (λ ) 0.710 73 Å), T ) 193(2) K,
2θmax ) 27.5°, 5181 independent reflections collected, 4523 reflections
in refinement, final R indices (I > 2σ(I)) R1 ) 0.0200 and wR2 ) 0.0459.
Crystal data for 5c: C20H37Cl3N2HfSi, M ) 618.45, monoclinic, P21/n,
a ) 11.7097(10) Å, b ) 14.6441(12) Å, c ) 15.1842(13) Å, â ) 101.8770-
spectrum of 2a in this solvent displays a set of two
resonances at δ 2.91 and 2.89 ppm for the diastereotopic
methylene protons of the enolized acetamidinate frag-
ment, and both of these were found to correlate, via a
2D 13C-1H HSQC NMR spectrum, with a 13C NMR
resonance at 55.6 ppm (1J (13C-1H) ) 157 Hz).10 Obser-
(10)°, V ) 2548.0(4) Å3, Z ) 4, Dcalcd ) 1.612 mg/m3, µ ) 4.464 mm-1
,
F(000) ) 1232, Mo KR radiation (λ ) 0.710 73 Å), T ) 293(2) K,
2θmax ) 27.5°, 39 099 independent reflections collected, 5123 reflections
in refinement, final R indices (I > 2σ(I)) R1 ) 0.0220, wR2 ) 0.0536.
Crystal data for 6‚OEt2: C44H51BClF15N2O2Zr, Mr ) 1062.35, mono-
clinic, P21/n, a ) 12.7912(4) Å, b ) 16.2876(5) Å, c ) 22.7788(6) Å,
â ) 92.3540(10)°, V ) 4741.7(2) Å3, Z ) 4, Dcalcd ) 1.488 mg/m3, µ )
0.385 mm-1, F(000) ) 2168, Mo KR radiation (λ ) 0.710 73 Å), T )
293(2) K, 2θmax ) 30.0°, 13 661 independent reflections collected, 10 772
reflections in refinement, final R indices (I > 2σ(I)) R1 ) 0.0358 and
wR2 ) 0.0959.
2
vation that the J (1H-1H) coupling constant between
(9) Woo, H. G.; Freeman, W. P.; Tilley, T. D. Organometallics 1992,
11, 2198-2205.
(10) Detailed information is provided in the Supporting Information.
(11) Zhang, Y.; Sita, L. R. Unpublished results.

Communications
Organometallics, Vol. 22, No. 1, 2003 23
F igu r e 3. Molecular structure (30% thermal ellipsoids)
of 6. Hydrogen atoms have been removed for the sake of
clarity.
group of 5c should now provide a convenient handle by
which to anchor this species to solid inorganic oxide
supports, we have also carried out the direct function-
alization of chloromethylated polystyrene beads (1.3
meq/g) with 2a in which 70% of the chloromethyl sites
were found to have reacted.
As a final initial survey of reactivity, we were
interested in determining whether the enolates 2a and
2b could be “remotely activated” through reaction with
a group 13 Lewis acid to produce charge-neutral zwit-
terionic complexes that could then function as Ziegler-
Natta polymerization initiators.14 To our great satis-
faction, treatment of a Et2O solution of 2a with 1 equiv
of B(C6F5)3 provided, in 66% yield, the Et2O-coordinated
zwitterionic complex 6 upon crystallization from solution
at -30 °C. Figure 3 displays the molecular structure of
6 as obtained from crystallographic analysis, and it is
interesting to note that the structural environment
around the zirconium atom is very similar to that
observed for the solid-state structure of an Et2O complex
of a cationic zirconium acetamidinate that is a known
active initiator for olefin polymerization.10,13,15
In conclusion, we have demonstrated that depro-
tonation of group 4 metal acetamidinates can be achieved
in high yield to provide enolate complexes that can
subsequently be used to produce several new classes of
metal amidinates that are not accessible by conventional
routes. Further studies of the scope and utility of this
new procedure are currently in progress.
Ack n ow led gm en t. Funding for this work was pro-
vided by the NSF (Grant No. CHE-0092493), for which
we are grateful.
Su p p or tin g In for m a tion Ava ila ble: Text and tables
giving details of the synthesis and characterization of 2-6,
including crystallographic analysis of 4-6. This material is
available free of charge via the Internet at http://pubs.acs.org.
F igu r e 2. Molecular structure (30% thermal ellipsoids)
of (a) 5a , (b) 5b, and (c) 5c. Hydrogen atoms have been
removed for the sake of clarity.
OM020905G
(14) Kim, Y. H.; Kim, T. H.; Lee, B. Y.; Woodmansee, D.; Bu, X.;
Bazan, G. C. Organometallics 2002, 21, 3082-3084.
(15) Keaton, R. J .; J ayaratne, K. C.; Fettinger, J . C.; Sita, L. R. J .
Am. Chem. Soc. 2000, 122, 12909-12910.
5a -c (M ) Hf), the molecular structures of which are
shown in Figure 2.10,13 While the chlorodimethylsilyl