Synthesis of dialkylscandium complexes supported by β-diketiminato ligands and activation with tris (pentafluorophenyl)borane

Article abstract of DOI:10.1021/om9903801

Treatment of ScCl₃·THF₃ with the lithium salts of the ligands ArNC(R)CHC(R)NAr, where Ar = 2,6-Pr¹-C₆H₃ and R = CH₃ and Bu^t, gives LScCl₂·nTHF derivatives (R = CH₃, n = 1, 1a; R = Bu^t, n = 0, 1b). These compounds can be derivatized by alkylation with methyllithium or benzylpotassium. The dibenzyl compound prepared from 1a, when treated with B(CcFc)S, gives an ion pair, 4, in which the cationic scandium center is stabilized by a η⁶-aryl interaction with the abstracted borate benzyl group.

Full text of DOI:10.1021/om9903801

Organometallics 1999, 18, 2947-2949
2947
Syn th esis of Dia lk ylsca n d iu m Com p lexes Su p p or ted by
â-Dik etim in a to Liga n d s a n d Activa tion w ith
Tr is(p en ta flu or op h en yl)bor a n e
Lawrence W. M. Lee,† Warren E. Piers,*,† Mark R. J . Elsegood,‡
William Clegg,‡ and Masood Parvez†
Department of Chemistry, University of Calgary, 2500 University Drive NW,
Calgary, Alberta T2N 1N4, Canada, and Department of Chemistry, University of Newcastle,
Newcastle upon Tyne NE1 7RU, U.K.
Received May 20, 1999
Summary: Treatment of ScCl3‚THF3 with the lithium
salts of the ligands ArNC(R)CHC(R)NAr, where Ar )
2,6-Pri-C6H3 and R ) CH3 and But, gives LScCl2‚nTHF
derivatives (R ) CH3, n ) 1, 1a ; R ) But, n ) 0, 1b).
These compounds can be derivatized by alkylation with
methyllithium or benzylpotassium. The dibenzyl com-
pound prepared from 1a , when treated with B(C6F5)3,
gives an ion pair, 4, in which the cationic scandium
center is stabilized by a η6-aryl interaction with the
abstracted borate benzyl group.
to a combination of σ and π donation depending on the
steric environment and the electron demand at the
bound metal.6d Our interest9 in the chemistry of dior-
ganoscandium complexes of general formula LScR2,10
coupled with the return to prominence of the â-diketi-
minato ligands, led us to prepare some examples of (â-
diketiminato)ScR2 complexes. In this communication,
we detail those syntheses and present some preliminary
reactivity data, including the generation and charac-
terization of an organoscandium cation.
Lithium salts of the ligands ArNC(R)CHC(R)NAr,
where Ar ) 2,6-Pri-C6H3 and R ) CH3 and But, were
prepared according to literature procedures.6a Reaction
with ScCl3‚THF3 in toluene at high temperatures (85-
100 °C) for extended periods (16-144 h) gave the
dichloride derivatives 1a and 1b in good yields upon
workup (eq 1). X-ray structural analysis11 reveals 1a to
Nonmetallocene complexes of the early-transition-
metal elements are of current interest as potentially
novel olefin polymerization catalysts.1 The flexibility of
ligand design outside of the cyclopentadienyl paradigm
has also allowed for the introduction of catalysts based
on metals other than those of the traditional group 4,
5, and 6 triads, most notably the late-metal-based
systems disclosed recently by Gibson2 and Brookhart.3
These examples make use of a general ligand environ-
ment first devised by McConville4 in which two trigonal
nitrogen donors are held in a chelating arrangement and
the space around the metal center is sculpted by bulky
aryl groups on the nitrogen atoms. A significant advan-
tage to this ligand design is that neutral, monoanionic,
and dianionic versions1b can be prepared and imple-
mented depending on the available oxidation states of
the metal in question.
The monoanionic â-diketiminato ligands, ArNC(R)-
CHC(R)NAr,5 are ideal for use with metals in the +3
oxidation state and have recently been employed for
early6 and late7 transition metals and group 13 main-
group complexes.8 These ligands have several attractive
features, including the tunability possible in both Ar and
R and variable bonding modes ranging from purely σ
be monomeric with one THF molecule retained in the
scandium’s coordination sphere (Figure 1), which has a
five-coordinate, distorted-trigonal-bipyramidal geom-
etry. A symmetrical pattern for the ligand resonances
(6) (a) Qian, B.; Scanlon, W. J .; Smith, M. R., III. Organometallics
1999, 18, 1693. (b) Budzelaar, P. H. M.; van Oort, A. B.; Orpen, A. G.
Eur. J . Inorg. Chem. 1998, 1485. (c) Kim, W.-K.; Fevola, M. J .; Liable-
Sands, L. M.; Rheingold, A. L.; Theopold, K. H. Organometallics 1998,
17, 4541. (d) Rahim, M.; Taylor, N. J .; Xin. S.; Collins, S. Organome-
tallics 1998, 17, 1315. (e) Lappert, M. F.; Liu, D. S. J . Organomet.
Chem. 1995, 500, 203. (f) Hitchcock, P. B.; Lappert, M. F.; Liu, D.-S.
J . Chem. Soc., Chem. Commun. 1994, 2637. (g) Richeson, D. S.;
Mitchell, J . F.; Theopold, K. H. Organometallics 1989, 8, 2570. (h)
Bercaw, J . E.; Davies, D. L.; Wolczanski, P. T. Organometallics 1986,
5, 443.
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1998, 17, 4121. (b) Feldman, J .; McLain, S. J .; Parthasarathy, A.;
Marshall, W. J .; Calabrese, J . C.; Arthur, S. D. Organometallics 1997,
16, 1514.
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1999, 705. (b) Radzewich, C. E.; Coles, M. P.; J ordan, R. F. J . Am.
Chem. Soc. 1998, 120, 9384. (c) Qian, B.; Ward, D. L.; Smith, M. R.,
III. Organometallics 1998, 17, 3070. (d) Cheng, M.; Lobkovsky, E. B.;
Coates, G. W. J . Am. Chem. Soc. 1998, 120, 11018.
* To whom correspondence should be addressed. Phone: 403-220-
5746. FAX: 403-289-9488. Email: wpiers@ucalgary.ca.
† University of Calgary.
‡ University of Newcastle.
(1) (a) McKnight, A. L.; Waymouth, R. M. Chem. Rev. 1998, 98, 2587.
(b) Britovsek, G. J . P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int.
Ed. Engl. 1999, 38, 428.
(2) Britovsek, G. J . P.; Gibson, V. C.; Kimberley, B. S.; Maddox, P.
J .; McTavish, S. J .; Solan, G. A.; White, A. J . P.; Williams, D. J . Chem.
Commun. 1998, 849.
(3) (a) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J . Am. Chem.
Soc. 1998, 120, 4049. (b) Small, B. L.; Brookhart, M. J . Am. Chem.
Soc. 1998, 120, 7143.
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Macromolecules 1996, 29, 5241.
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10.1021/om9903801 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 07/17/1999

2948 Organometallics, Vol. 18, No. 16, 1999
Communications
Sch em e 1
free dibenzyl compound (3). Although 3 is formally an
eight-electron compound, normal values for the 13C
1
chemical shift of the benzyl Cipso atom13 and the J CH
coupling constant (121(1) Hz) suggest there is no mul-
tihapto bonding within the benzyl groups,14 although a
fluxional system cannot be ruled out in the absence of
VT NMR studies.
F igu r e 1. Molecular structure of 1a . Selected bond lengths
(Å): Sc(1)-N(1), 2.107(4); Sc(1)-N(2), 2.175(4); Sc(1)-O(1),
2.203(4); Sc(1)-Cl(1), 2.380(2); Sc(1)-Cl(2), 2.356(2); N(1)-
C(2), 1.363(6); N(2)-C(4), 1.312(7); C(2)-C(3), 1.387(7);
C(3)-C(4), 1.399(8). Selected bond angles (deg): N(1)-
Sc(1)-N(2), 86.77(17); N(1)-Sc(1)-O(1), 95.82(16); N(2)-
Sc(1)-O(1), 175.31(16); N(1)-Sc(1)-Cl(1), 123.79(13); N(1)-
Sc(1)-Cl(2), 104.53(13); N(2)-Sc(1)-Cl(1), 92.33(12); N(2)-
Sc(1)-Cl(2), 94.85(13); Sc(1)-N(1)-C(2), 122.4(3); Sc(1)-
N(2)-C(4), 121.1(4).
Neutral dialkyl compounds require activation via
alkide abstraction with a strong electrophile in order
to attain competency as an olefin polymerization media-
tor. Dibenzyl compound 3 reacts rapidly and cleanly
with 1 equiv of B(C6F5)3 to form an ion pair (4) in which
the abstracted benzyl group remains closely associated
with the scandium center through strong η6-bonding15
1
(eq 2). Four doublets in the H NMR spectrum of 4 for
1
in the H NMR spectrum of 1a is observed, suggesting
the compound is fluxional at room temperature. At-
tempts to thermally remove the THF under vacuum
resulted primarily in decomposition.12 In 1a , the scan-
dium center lies 0.694 Å out of the NC3N plane, but the
Sc-C(2,3,4) distances are quite long (3.062(6), 3.327-
(6), 3.066(6) Å, respectively) suggesting a negligible π
contribution to the L-Sc bonding in this compound.
Unlike 1a , the dichloride derivative obtained from the
But-substituted ancillary is devoid of THF. The bulkier
But groups on the ligand backbone push the Ar moieties
forward, relative to the methyl-substituted ancillary,
forcing them to assume a more rigid configuration
perpendicular to the N-Sc-N plane.6b This perturba-
tion makes for a more sterically demanding ligand
environment than in 1a , accounting for the lack of THF
coordination. Although the nuclearity of 1b is unknown,
its ready solubility in benzene is suggestive of a mon-
omeric structure in solution.
the ligand Pri methyl groups indicate that the plane of
symmetry in the molecule has been broken by a strong
(13) Three signals are observed in the Cipso region at 149.3, 143.2,
and 142.2 ppm.
(14) (a) J ordan, R. F.; Lapointe, R. E.; Baenziger, N.; Hinch, G. D.
Organometallics 1990, 9, 1539. (b) Bei, X.; Swenson, D. C.; J ordan, R.
F. Organometallics 1997, 16, 3282. (c) Latesky, S. L.; McMullen, A.
K.; Niccolai, G. P.; Rothwell, I. P.; Huffmann, J . C. Organometallics
1985, 4, 902.
(15) (a) Green, M. L. H.; Sassmannshausen, J . Chem. Commun.
1999, 115. (b) Bochmann, M.; Green, M. L. H.; Powell, A. K.;
Sassmannshausen, J .; Triller, M. U.; Wocadlo, S. J . Chem. Soc., Dalton
Trans. 1999, 43. (c) Gillis, D. J .; Quyoum, R.; Tudoret, M.-J .; Wang,
Q.; J eremic, D.; Roszak, A. W.; Baird, M. C. Organometallics 1996,
15, 3600. (d) Bochmann, M.; Lancaster, S. J .; Hursthouse, M. B.; Abdul
Malik, K. M. Organometallics 1994, 13, 2235. (e) Lancaster, S. J .;
Robinson, O. B.; Bochmann, M.; Coles, S. J .; Hursthouse, M. B.
Organometallics 1995, 14, 2456. (f) Gillis, D. J .; Tudoret, M.-J .; Baird,
M. C. J . Am. Chem. Soc. 1993, 115, 2543. (g) Pellecchia, C.; Immirzi,
A.; Grassi, A.; Zambelli, Z. Organometallics 1993, 12, 4473. (h)
Pellecchia, C.; Grassi, A.; Immirzi, A. J . Am. Chem. Soc. 1993, 115,
1160.
Dichloride 1a can be smoothly alkylated with con-
ventional organolithium or potassium reagents (Scheme
1). Use of CH3Li gives a THF-ligated dimethyl deriva-
tive (2), but incorporation of the more sterically de-
manding benzyl group using PhCH2K results in a THF-
(10) Fryzuk, M. D.; Giesbrecht, G.; Rettig, S. J . Organometallics
1996, 15, 3329.
(11) Crystals of 1a were grown from toluene/hexanes containing a
few drops of THF:
C33H49N2OCl2Sc, tetragonal, I1/a (No. 88), a )
27.598(5) Å, c ) 17.567(5) Å, V ) 13380(3) Å3, Z ) 16, Dcalcd ) 1.202
g cm-3, 2θmax ) 50.1°, Mo KR radiation, λ ) 0.710 69 Å, T ) -103 °C,
6399 measured reflections, 5792 unique, 2631 reflections with Inet
>
2.0σ(Inet), µ ) 4.05 cm-1, R ) 0.0652, Rw ) 0.1777, GOF ) 7.37, 363
parameters.
(12) Protonated ligand was the major product isolated from this
reaction.

Communications
Organometallics, Vol. 18, No. 16, 1999 2949
ion-ion interaction. A doublet, triplet, triplet pattern
(6.71, 6.34, and 6.15 ppm) for the ortho, meta, and para
protons of the η6-bound benzylic ring is characteristic
of this moiety.15
The solution data are substantiated by an X-ray
structural analysis16 (Figure 2), which shows clearly the
η6-bonding of the abstracted benzyl group. Within this
moiety, the average Sc(1)-carbon bond distance is
2.679(2) Å within a narrow range of 2.619-2.774(2) Å.
For the benzyl group remaining on scandium, the Sc-
1
(1)-C(55)-C(56) angle of 117.1(4)° and the low J CH of
108(2) Hz17 is indicative of normal η1-bonding. The
diketiminato ligand appears to be relatively more
π-bound in this complex, compared to 1a , as evidenced
by the greater degree to which scandium lies out of the
ligand plane (1.114 Å) and the shorter Sc(1)-C(1) and
Sc(1)-C(3) distances of 2.835(2) and 2.787(2) Å, respec-
tively. A stronger π component is to be expected for a
more electrophilic, cationic scandium center. The N(1)-
Sc(1)-N(2) angle of 94.79(7)° is significantly larger than
the corresponding angle in 1a .
The structure of 4 in solution is remarkably static.
Heating samples to 90 °C resulted in no substantial
1
change in the H NMR spectrum. Furthermore, 4 was
F igu r e 2. Molecular structure of 4. Only one orientation
of the disordered benzyl group16 is shown for clarity.
Selected bond lengths (Å): Sc(1)-N(1), 2.1143(11); Sc(1)-
N(2), 2.0864(19); Sc(1)-C(1), 2.835(2); Sc(1)-C(2), 2.955-
(2); Sc(1)-C(3), 2.787(2); Sc(1)-C(55), 2.229(2); Sc(1)-
C(30), 2.713(2); Sc(1)-(C31), 2.676(2); Sc(1)-C(32), 2.644(2);
Sc(1)-C(33), 2.619(2), Sc(1)-C(34), 2.649(2); Sc(1)-C(35),
2.774(2); C(55)-C(56), 1.474(6); C(55)-C(56X), 1.508(11);
C(35)-C(36), 1.482(3); C(36)-B(1), 1.679(3). Selected bond
angles (deg): N(1)-Sc(1)-N(2), 94.79(7); N(1)-Sc(1)-
C(55), 109.13(8); N(2)-Sc(1)-C(55), 110.33(8); Sc(1)-C(55)-
C(56), 117.1(4); Sc(1)-C(55)-C(56X), 114.7(9); C(35)-
C(36)-B(1), 115.71(19).
totally unreactive toward H2 or ethylene, even at
elevated temperatures. Evidently, the η6-bound benzyl
group is quite strongly attached in this system. It has
been suggested by McConville that toluene binding to
titanium cations in a similar ligand environment is
responsible for suppressed activity of those systems in
that solvent.18 To this end, it is encouraging that a
dimethylscandium complex free of THF can be obtained
from 1b and methyllithium. The reactivity of this base-
free dimethyl complex toward standard and nonstand-
ard activators is currently being explored in detail.
tion of Calgary, Alberta, Canada, and the NSERC of
Canada’s Research Partnerships office (CRD program)
and the EPSRC (U.K.). W.E.P. thanks the Alfred P.
Sloan Foundation for a Research Fellowship (1996-
2000).
Ack n ow led gm en t. This work was generously sup-
ported by the NOVA Research & Technology Corpora-
(16) Crystals of 4 were grown from benzene. A 2-fold disorder of
the benzyl ligand attached to scandium in 4 was revealed in refine-
ment. There are two orientations of the phenyl group with a common
position for the methylene carbon. Crystal data: C61H55N2BF15Sc‚C6D6,
monoclinic, P21/c, a ) 14.854(2) Å, b ) 10.9486(17) Å, c ) 37.357(6) Å,
â ) 96.889(3)°, V ) 6031.4(17) Å3, Z ) 4, Dcalcd ) 1.367 g cm-3, 2θmax
) 55.38°, Mo KR radiation, λ ) 0.710 73 Å, T ) 160(2) K, 10 575
measured reflections, 6442 with Inet > 2.0σ(Inet), µ ) 2.11 cm-1, R )
0.0461, Rw ) 0.0919, GOF ) 0.875, 841 parameters.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details and characterization data for the compounds
prepared in this paper and tables of crystal data, atomic
coordinates and isotropic thermal parameters, and bond
lengths and angles for compounds 1a and 4. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
(17) This value for J CH may point to an R-agostic interaction,
although this question has not been probed further at this point.
(18) Scollard, J . D.; McConville, D. H. J . Am. Chem. Soc. 1996, 118,
10008.
OM9903801