(Cyclopentadienylamine)scandium(2,3-dimethyl-1,3-butadiene): A 1,3-diene complex of scandium with Sc(I)- and Sc(III)-like reactivity

Article abstract of DOI:10.1021/om030267h

The scandium 2,3-dimethyl-1,3-butadiene complex [η⁵, η¹-C₅H₄(CH₂)₂NMe ₂]Sc(C₆H₁₀) (2) reacts with PhCN via initial nitrile insertion into the Sc-diene bond to give a dimeric μ²-imido species, but with a 2,2′-bipyridine via the elimination of the free diene. The latter shows that 2 can be used to generate the reactive fragment [η⁵,η¹-C₅H₄(CH₂) ₂NMe₂]Sc^I.

Full text of DOI:10.1021/om030267h

4372
Organometallics 2003, 22, 4372-4374
(Cyclop en ta d ien yla m in e)sca n d iu m (2,3-d im eth yl-1,3-bu ta d ien e):
A 1,3-Dien e Com p lex of Sca n d iu m w ith Sc(I)- a n d
Sc(III)-lik e Rea ctivity
Dirk J . Beetstra, Auke Meetsma, Bart Hessen,* and J an H. Teuben
Dutch Polymer Institute/ Center for Catalytic Olefin Polymerization,
Stratingh Institute for Chemistry and Chemical Engineering, University of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands
Received April 14, 2003
Summary: The scandium 2,3-dimethyl-1,3-butadiene com-
plex [η5,η1-C5H4(CH2)2NMe2]Sc(C6H10) (2) reacts with
PhCN via initial nitrile insertion into the Sc-diene bond
to give a dimeric µ2-imido species, but with a 2,2′-
bipyridine via the elimination of the free diene. The latter
shows that 2 can be used to generate the reactive
fragment [η5,η1-C5H4(CH2)2NMe2]ScI.
types of reactivity behavior, depending on the type of
reagent used. In contrast, only very few 1,3-diene
complexes of the group 3 and lanthanide metals are
known. These are practically limited to 1,4-diphenyl-
1,3-butadiene derivatives, in which the ligand predomi-
nantly has dianionic character and of which little
reactivity has been reported.4 In addition, several
naphthalene-lanthanide complexes show structural
features consistent with 1,3-diene character.5
Transition-metal complexes of the 1,3-butadiene ligand
and its various substituted derivatives display very
interesting features with respect to structure and bond-
ing as well as reactivity. In its complexes, the s-cis-1,3-
butadiene ligand has been observed to adopt a range of
intergraded bonding modes between the η4-diene (A)
and σ2,π-metallacyclopentene (B) extremes.1 These two
structures are related by an oxidative addition operation
in which the diene in structure B is effectively doubly
reduced to a but-2-ene-1,4-diyl dianion. The relative
importance of the diene or metallacyclopentene char-
acter depends on several factors, especially the reducing
power of the low-valent metal fragment. Reactivity
corresponding to this σ2,π-metallacyclopentene charac-
ter includes the insertion of unsaturated substrates into
the metal-methylene bond2 and the attack of Lewis
acids such as B(C6F5)3 on the diene methylene carbon
to yield zwitterionic metal allyl species (which can act
as single-component olefin polymerization catalysts).3
Reactivity associated with η4-diene character includes
the displacement of the neutral diene ligand by various
reagents, which can then undergo oxidative addition or
oxidative coupling reactions on the resultant low-valent
transition-metal species.2c,d Many transition-metal diene
complexes with intermediate bonding modes show both
The group 3 metal scandium has a strong preference
for the trivalent oxidation state, although some com-
pounds with the metal in the monovalent oxidation state
are known, e.g., in solid ScCl.6 Recently a ScIBr unit
sandwiched between two (â-diketiminato)MgBr frag-
ments was reported by Roesky et al.7 We are exploring
the chemistry of Sc diene complexes to see whether “low-
valent” behavior for Sc may be induced by the presence
of a diene ligand. Here we describe the synthesis and
characterization of the first 1,3-butadiene complex of
scandium, [η5,η1-C5H4(CH2)2NMe2]Sc(2,3-dimethyl-1,3-
butadiene), with some aspects of its reactivity. It is
shown that this compound can behave as a source of
the reactive [η5,η1-C5H4(CH2)2NMe2]ScI fragment.
The colorless cyclopentadienylamine scandium dichlo-
ride {[C5H4(CH2)2NMe2]ScCl2}x (1) was obtained in 80%
yield from the reaction of the corresponding lithium
cyclopentadienide with ScCl3(THF)3, followed by vacuum
sublimation.8 Reaction of the dichloride 1 with (2,3-
dimethyl-1,3-butadiene)Mg‚(THF)2 in ether, followed by
workup at or below 0 °C, afforded the complex [C5H4-
(CH2)2NMe2]Sc(C6H10) (2) as red crystals in 48% yield
by crystallization from pentane.9
(4) (a) Mashima, K.; Sugiyama, H.; Nakamura, A. J . Chem. Soc.,
Chem. Commun. 1994, 1581. (b) Kretschmer, W.; Thiele, K.-H. Z.
Anorg. Allg. Chem. 1995, 621, 1093. (c) Kretschmer, W.; Thiele, K.-H.
Z. Anorg. Allg. Chem. 1995, 621, 1304. (d) Emelyanova, N. S.; Trifonov,
A. A.; Zakharov, L. N.; Shestakov, A. F.; Struchkov, Y. T.; Bochkarev,
M. N. J . Organomet. Chem. 1997, 540, 1.
(5) For a recent review, see: Bochkarev, M. N. Chem. Rev. 2002,
102, 2089.
(6) Poeppelmeier, K. R.; Corbett, J . D. Inorg. Chem. 1977, 16, 294-
297.
(7) Neculai, A. M.; Neculai, D.; Roesky, H. W.; Magull, J .; Baldus,
M.; Andronesi, O.; J ansen, M. Organometallics 2002, 21, 2590.
(8) Synthesis of 1: A solution of 1.74 g (12.2 mmol) of [C5H4(CH2)2-
NMe2]Li and 4.20 g (11.4 mmol) of ScCl3(THF)3 in 50 mL of THF was
stirred overnight. Removal of the volatiles followed by sublimation (42
mTorr, 150 °C) of the residual solid afforded 2.31 g (9.14 mmol, 80%)
of 1 as a white crystalline solid. Anal. Calcd C13H22NCl2Sc: C 50.67,
H 7.19, N 4.54, Sc 14.59. Found: C 50.27, H 7.05, N 4.37, Sc 14.40.
The analogous complex [C5Me4(CH2)2NMe2]ScCl2 was reported previ-
ously: Christopher, J . N.; Squire, K. R.; Canich, J . A. M.; Schaffer, T.
D. World Pat. WO 00/18808.
* To whom correspondence should be addressed. E-mail: hessen@
chem.rug.nl.
(1) See for a recent discussion of the bonding of 1,3-diene ligands to
early transition metals: (a) Spencer, M. D.; Wilson, S. R.; Girolami,
G. S. Organometallics 1997, 16, 3055. (b) Del Rio, D.; Galindo, A. J .
Organomet. Chem. 2002, 655, 16, and references therein.
(2) For examples involving mono(cyclopentadienyl) group 4 metal
1,3-diene complexes, see: (a) Hessen, B.; Blenkers, J .; Teuben, J . H.;
Helgesson, G.; J agner, S. Organometallics 1989, 8, 2809. (b) Ausema,
J . B.; Hessen, B.; Teuben, J . H. Recl. Trav. Chim. Pays-Bas 1987, 106,
465. (c) Hessen, B.; Blenkers, J .; Teuben, J . H.; Helgesson, G.; J agner,
S. Organometallics 1989, 8, 830. (d) Hessen, B.; Teuben, J . H. J .
Organomet. Chem. 1988, 358, 135.
(3) (a) Temme, B.; Erker, G.; Karl, J .; Luftmann, R.; Fro¨hlich, R.;
Kotila, S. Angew. Chem., Int. Ed. Engl. 1995, 34, 1755. (b) Temme,
B.; Karl, J .; Erker, G. Chem. Eur. J . 1996, 2, 919. (c) J ime´nez Pintado,
G.; Thornton-Pett, M.; Bouwkamp, M.; Meetsma, A.; Hessen, B.;
Bochmann, M. Angew. Chem., Int. Ed. Engl. 1997, 36, 2358. (d) Devore,
D. D.; Timmers, F. J .; Hasha, D. L.; Rosen, R. K.; Marks, T. J .; Deck,
P. A.; Stern, C. L. Organometallics 1995, 14, 3132.
10.1021/om030267h CCC: $25.00 © 2003 American Chemical Society
Publication on Web 10/02/2003

Communications
Organometallics, Vol. 22, No. 22, 2003 4373
F igu r e 1. Molecular structure of 2. Selected interatomic
distances (Å) and angles (deg): Sc-N ) 2.358(2), Sc-C(10)
) 2.249(3), Sc-C(11) ) 2.401(2), Sc-C(12) ) 2.400(2), Sc-
C(13) ) 2.251(2), C(10)-C(11) ) 1.455(3), C(11)-C(12) )
1.386(3), C(12)-C(13) ) 1.464(3), Sc-C(10)-C(11) ) 77.6(2),
Sc-C(13)-C(12) ) 77.3(1).
F igu r e 2. Molecular structure of 3. Selected interatomic
distances (Å) and angles (deg): Sc(1)-N(1) ) 2.375(3),
Sc(1)-N(2) ) 2.017(2), Sc(1)-N(2a) ) 2.056(2), N(2)-C(10)
) 1.465(3), N(2)-Sc(1)-N(2a) ) 87.14(9), Sc(1)-N(2)-
Sc(1a) ) 92.86(9), Sc(1)-N(2)-C(10) ) 138.2(2), Sc(1a)-
N(2)-C(10) ) 127.6(3).
A crystal structure determination of 2 (Figure 1)10
showed that the prone-oriented butadiene fragment has
considerable 2-ene-1,4-diyl character, as indicated by
the relatively short central C-C bond of 1.386(3) Å and
relatively long C-CH2 bonds of 1.455(3) and 1.464(3)
Sch em e 1
1
Å. In the H and 13C NMR spectra of 2, the resonances
for the diene methylene groups are observed at δ 1.76
2
and 2.58 ppm (d, J HH ) 6.8 Hz) and δ 59.0 ppm (∆ν1/2
) 183 Hz, broadened by the quadrupolar 45Sc nucleus,
I ) 7/2), respectively.
In its structural features, the diene complex 2 thus
predominantly displays Sc(III) σ2,π-metallacyclopentene
character. In line with this, 2 reacts readily with polar
unsaturated substrates to give products that derive from
insertion into the Sc-methylene bonds. With 1 equiv
of benzonitrile, 2 reacts to give the colorless µ2-imido
complex {[C5H4(CH2)2NMe2]Sc[µ2-NC(Ph)C6H10]}2 (3),
which was characterized by single-crystal X-ray diffrac-
tion of crystals of its benzene solvate (Figure 2).11 The
Cp-amine ligands adopt a trans geometry around the
central Sc2N2 core, which is practically equilateral.
Although common for many transition metals, M2(µ2-
NR)2 species of group 3 metals to our knowledge have
not yet been reported and have only very recently been
observed for lanthanide metals.12
Sch em e 2
The formation of this product can be explained by the
sequence depicted in Scheme 2, in which the nitrile
initially inserts into one of the Sc-CH2 bonds, followed
(9) Synthesis of 2: A suspension of 1 (220 mg, 0.87 mmol) and
(C6H10)Mg(THF)2 (295 mg, 1.17 mmol) in 30 mL ether was stirred at
0 °C for 1.5 h. At 0 °C the volatiles were removed in vacuo, and the
residue was extracted with pentane. Concentrating the solution and
cooling to -80 °C afforded 2 (110 mg, 0.42 mmol, 42%) as red crystals.
1H NMR (C6D6, 25 °C): δ 6.20 (ps.t, J ) 2.8 Hz, 2H, C5H4), 6.01 (ps.t,
J ) 2.8 Hz, 2H, C5H4); 2.58 (d, J ) 6.8 Hz, 2H, syn-CH2); 2.11 (t, J )
6.2 Hz, 2H, CH2), 1.87 (t, J ) 6.2 Hz, 2H, CH2), 1.82 (s, 6H, Me), 1.76
by an attack of the other diene methylene group on the
carbon atom of the imide intermediate. This yields an
electronically unsaturated metal imido species that will
readily dimerize. A similar reaction sequence was
earlier observed in our group in the reaction of (C5Me5)-
Hf(2,3-dimethyl-1,3-butadiene)Cl with acetylene, in which
case a µ-alkylidene complex was formed.13
(d,
J ) 6.8, 2H, anti-CH2), 1.71 (s, 6H, Me). Anal. Calcd for
C
15H24NSc: C 68.66, H 9.22, N 5.34, Sc 16.79. Found: C 68.00, H 9.27,
N 5.23, Sc 16.68.
In contrast with the insertion chemistry described
above for the reaction of 2 with benzonitrile, the reaction
of 2 with 2 equiv of 4,4′-dimethyl-2,2′-bipyridine leads
to liberation of free 2,3-dimethyl-1,3-butadiene (as seen
by NMR spectroscopy). From a reaction in toluene,
followed by crystallization from hexane, the resulting
bis(4,4′-dimethyl-2,2′-bipyridine) adduct [C5H4(CH2)2-
(10) Crystal data for 2: C15H24NSc, space group C2/c, monoclinic, a
) 17.707(1) Å, b ) 8.4651(6) Å, c ) 19.903(1) Å, â ) 105.984(1)°, V )
2868.0(3) Å3; T ) 125(1) K, Z ) 8, µ(Mo KR) ) 4.93 cm-1, 13 775
reflections measured (3544 unique), final refinement converged at
Rw(F2) ) 0.0975 with 250 parameters.
(11) Crystal data for 3‚(C6H6): (C22H29N2Sc)2‚C6H6, space group P1h,
triclinic, a ) 8.7222(8) Å, b ) 10.5565(9) Å, c ) 12.580(1) Å, R )
70.455(2)°, â ) 88.427(2)°, γ ) 88.719(2)°, V ) 1091.05(16) Å3; T )
110(2) K, Z ) 2, µ(Mo KR) ) 3.50 cm-1, 5899 reflections measured
(4441 unique), final refinement converged at Rw(F2) ) 0.1232 with 381
parameters.
NMe2]Sc(η2-N2C12H12 2
(4) was isolated as a black
)
(12) (a) Chan, H.-S.; Li, H.-W.; Xie, Z. Chem. Commun. 2002, 652.
(b) Gordon, J . C.; Giesbrecht, G. R.; Clark, D. L.; Hay, P. J .; Keogh, D.
W.; Poli, R.; Scott, B. L.; Watkin, J . G. Organometallics 2002, 21, 4726.
(13) Hessen, B.; Van Bolhuis, F.; Teuben, J . H. Organometallics
1987, 6, 1352.

4374 Organometallics, Vol. 22, No. 22, 2003
Communications
Sch em e 3
crystalline material (Scheme 3), which nevertheless was
persistently contaminated with free bipyridine.
A crystal structure determination14 of the compound
revealed that the metal center is pseudo-six-coordinate,
with one η5-cyclopentadienyl group, two η2-bipyridine
ligands, and the coordinated pendant amine (Figure 3).
The Sc-N(amine) distance of 2.564(4) Å is 0.2 Å longer
than that in 2 or 3 in response to the high coordination
number of the metal center in 4. Inspection of the C-C
and C-N distances within the 4,4′-dimethyl-2,2′-bi-
pyridine ligands reveals that both ligands appear to be
reduced to their radical monoanions (e.g., as seen from
the central bipyridine C-C bonds in 4 of 1.41-1.42 Å
as compared to 1.48 Å in neutral bipyridine15). This
indicates that the effective oxidation state of the Sc
metal center in 4 is Sc(III), a tribute to the strong
reducing power of lower oxidation states of Sc.16 1H
NMR spectra of solutions of 4 in C6D6 show only
resonances of some free 4,4′-dimethyl-2,2′-bipyridine
(which always seems to be present in samples of 4),
suggesting that the compound is paramagnetic. An
Evans method17 determination of the magnetic moment
of 4 on a benzene solution of 4 generated in situ from 2
and 2 equiv of 4,4′-dimethyl-2,2′-bipyridine yielded µeff
F igu r e 3. Molecular structure of 4. Selected interatomic
distances (Å) and angles (deg): Sc(1)-N(1) ) 2.564(4),
Sc(1)-N(2) ) 2.272(3), Sc(1)-N(3) ) 2.252(3), Sc(1)-N(4)
) 2.219(3), Sc(1)-N(5) ) 2.228(3), N(2)-C(14) ) 1.385(5),
N(3)-C(16) ) 1.374(5), C(14)-C(16) ) 1.410(5), N(4)-C(26)
) 1.364(5), N(5)-C(28) ) 1.368(5), C(26)-C(28) ) 1.422(5),
N(1)-Sc(1)-N(4) ) 154.3(1), N(2)-Sc(1)-N(5) ) 147.5(1),
N(2)-Sc-N(4) ) 85.6(1).
or partial decomposition of 4 (e.g., by partial loss of free
ligand, a possible source of the persistent presence of
4,4′-dimethyl-2,2′-bipyridine in solid samples). Attempts
to drive such a reaction by removal of 4,4′-dimethyl-
2,2′-bipyridine from 4 by, for example, vacuum sublima-
tion did not yield well-defined organometallic products.
The scandium diene complex 2 is thus found to be able
to eliminate the neutral diene ligand and can act in this
way as a precursor for the [C5H4(CH2)2NMe2]ScI frag-
ment. Preliminary experiments in which 2 was reacted
with reagents that are known to give facile oxidative
addition to low-valent transition-metal centers (I2,
PhSSPh) showed that also in these cases the diene
ligand is readily eliminated, yielding species that, by
1H NMR spectroscopy, appear to be Cs symmetric
[C5H4(CH2)2NMe2]ScX2 compounds (X ) I, SPh). Pres-
ently we are investigating the scope of this approach
with other group 3 and lanthanide metals.
) 1.54 (expected spin-only for an S ) 1 system: µeff
)
2.83). This indicates either a degree of magnetic cou-
pling between the bipyridyl radical anion ligands in 4
(14) Crystal data for 4: C33H38N5Sc, space group P21/a, monoclinic,
a ) 9.277(2) Å, b ) 17.315(3) Å, c ) 17.786(3) Å, â ) 100.951(3)°, V )
2805.0(9) Å3; T ) 100(1) K, Z ) 4, µ(Mo KR) ) 2.94 cm-1, 16 598
reflections measured (3885 unique), final refinement converged at
Rw(F2) ) 0.1316 with 504 parameters.
(15) See for example: Echegoyen, L.; DeCain, A.; Fischer, J .; Lehn,
J .-M. Angew. Chem., Int. Ed. Engl. 1991, 30, 838.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails of the preparation and characterization of compounds
1-4, NMR data of preliminary reactivity studies on 2 and
X-ray structural data for compounds 2-4. This material is
available free of charge via the Internet at http://pubs.acs.org.
(16) Several lanthanide complexes with multiple bipyridyl radical
anion ligands have been reported; see for example: Fedushkin, I. L.;
Petrovskaya, T. V.; Girgsdies, F.; Nevodchikov, V. I.; Weimann, R.;
Schumann, H.; Bochkarev, M. N. Russ. Chem. Bull. 2000, 49, 1869.
(17) (a) Evans, D. F. J . Chem. Soc. 1959, 2003. (b) Schubert, E. M.
J . Chem. Educ. 1992, 69, 62.
OM030267H