A thermally robust di-n-butyl thorium complex with an unstable dimethyl analogue

Article abstract of DOI:10.1039/c002122a

Methyl and n-butyl thorium complexes of a rigid 2,6-bis(anilidomethyl) pyridine ligand have been prepared; the n-butyl complex is thermally stable, even at 60 °C, while the methyl complexes exhibit a high tendency to eliminate methane viaσ-bond metathesis. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c002122a

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A thermally robust di-n-butyl thorium complex with an unstable dimethyl
analogue†‡
Carlos A. Cruz,a David J. H. Emslie,*a Hilary A. Jenkinsb and James F. Brittenb
Received 30th November 2009, Accepted 31st January 2010
First published as an Advance Article on the web 16th March 2010
DOI: 10.1039/c002122a
Methyl and n-butyl thorium complexes of a rigid 2,6-
bis(anilidomethyl)pyridine ligand have been prepared; the
n-butyl complex is thermally stable, even at 60 ◦C, while
the methyl complexes exhibit a high tendency to eliminate
methane via r-bond metathesis.
Reaction of [(BDPP)ThCl2(dme)]14 with two equivalents of
nBuLi provided highly soluble [(BDPP)Th(nBu)2] (1) as an off-
white solid in 57% yield (Scheme 1).§ Complex 1 shows remarkable
thermal stability, with no decomposition after days at 60 ◦C in
benzene. The structure of 1 is likely distorted square pyramidal
by analogy with [(BDPP)Th(CH2Ph)2].15 However, NMR spectra
of 1 show one n-butyl environment down to -90 ◦C, with ThCH2
1H and 13C NMR signals located at 0.35 and 87.7 ppm (1JC,H
118 Hz) at room temperature. By contrast, two different alkyl
group environments were observed in the low temperature NMR
spectra of [(BDPP)Th(CH2SiMe3)2] and [(BDPP)Th(CH2Ph)2],
presumably due to increased steric hindrance relative to 1, and
polyhapto benzyl-coordination in the latter complex.14,15
Much of organoactinide chemistry has focused on complexes
supported by carbocyclic ligands, especially C5R5 anions,1 and a
-
broad range of thermally robust [Cp*2AnR2] and [Cp3AnR] (An =
Th or U) complexes have been reported. Hydrocarbyl anions in
these complexes range from bulky and b-hydrogen-free CH2SiMe3
and CH2Ph groups, to sterically undemanding methyl groups, to b-
hydrogen-containing n-butyl,2 isopropyl, and tert-butyl groups.3,4
The stability of the b-hydrogen-containing derivatives is partic-
ularly remarkable and may be attributed, at least in part, to the
high strength of An–C bonds, relative to An–H bonds.5,6 However,
the influence of ancillary ligands on An–C bond strengths has
been noted,5 and b-hydrogen elimination was reported as a facile
decomposition mechanism in early attempts to prepare polyalkyl
derivatives. For example, the reactions of UCl4 with nBuLi or tBuLi
have been reported as a route to colloidal/pyrophoric uranium
metal or soluble uranium(III) hydride species.7
A range of non-cyclopentadienyl alkyl complexes have also
been prepared. However, most employ bulky (e.g. CH2EMe3;
E = Si or C) or potentially polyhapto CH2Ph ligands,4 leav-
ing simple alkyl derivatives less well represented. For exam-
ple, isolated non-cyclopentadienyl thorium methyl complexes
are limited to [ThMe{(m-Me)2Li(tmeda)}3],8 [ThMeR3(dmpe)x]
(R = Me, x = 2; R = CH2Ph, x = 1),9 [(Me3Si)2N}3ThMe]10
and [{(Et8-calix[4]tetrapyrrole)Th}2(m-O)(m-Me){K(dme)}(m-K)-
{K(dme)4}]n,11 and well defined non-cyclopentadienyl actinide
complexes bearing simple b-hydrogen-containing alkyl groups
appear to be absent from the literature.12 Herein we report
the preparation of both methyl and n-butyl derivatives of
a bulky N,N,N-donor ligand, BDPP [BDPP = 2,6-bis(2,6-
diisopropylanilidomethyl)pyridine].13
Scheme 1 Synthesis of complexes 1–3.
Reaction of [(BDPP)ThCl2(dme)] with three equivalents
of
MeLi
provided
the
trimethyl
“ate”
complex,
[(BDPP)ThMe3{Li(dme)}] (2) in 74% isolated yield (Scheme 1).§
1
At room temperature, the H and 13C NMR spectra of 2 show a
single Th-Me peak (1H NMR d 0.02 ppm; 13C NMR d 55.9 ppm),
and only one CHMe2 peak was observed for the BDPP ligand.
At -70 ◦C in d8-toluene, several BDPP ligand signals began to
de-coalesce but were not resolved even at -90 ◦C. These data
require rapid exchange of the in-plane and axial methyl groups in
2, as well as rapid transfer of the Li(dme)+ moiety between methyl
ligands. The involvement of MeLi dissociation in this process is
unlikely given that the thorium-methyl groups in 2 do not undergo
exchange with LiCD3/LiI in toluene, even after several hours at
room temperature. Similar dynamic behaviour was observed for
[ThMe{(m-Me)2Li(tmeda)}3], in which all thorium methyl groups
are equivalent (1H NMR d 0.02 ppm) on the NMR timescale,
even at -90 ◦C.8
aDepartment of Chemistry, McMaster University, 1280 Main Street
West, Hamilton, ON, L8S 4M1, Canada. E-mail: emslied@mcmaster.ca;
Fax: (905) 522-2509; Tel: (905) 525-9140
bMcMaster Analytical X-Ray Diffraction Facility, McMaster University,
1280 Main Street West, Hamilton, ON, L8S 4M1, Canada
† D. J. H. E. thanks NSERC of Canada for a Discovery Grant, and Canada
Foundation for Innovation (CFI) and Ontario Innovation Trust (OIT) for
New Opportunities Grants. C. A. C. thanks the Government of Ontario
for an Ontario Graduate Scholarship (OGS) and NSERC of Canada for
a PGS-D Scholarship.
‡ Electronic supplementary information (ESI) available: Crystallographic
information, experimental details, and the synthesis and characterisation
of all new compounds. CCDC reference number 756442. For ESI and
crystallographic data in CIF format see DOI: 10.1039/c002122a
6626 | Dalton Trans., 2010, 39, 6626–6628
This journal is
The Royal Society of Chemistry 2010
©

8
As a solid, compound 2 is stable for extended periods of time
at 20 ◦C. However, in solution, 2 decomposes over several days to
form orange-brown [(BDPP*)Th(m-Me)2Li(dme)] [3; BDPP* =
˚
in 7-coordinate [ThMe{(m-Me)2Li(tmeda)}3] (2.65–2.77 A). The
˚
Th–N(1) distance [2.298(9) A] in 3 is slightly shorter than Th–
N(3) [2.350(9) A], presumably due to metallation of the aryl
substituent on N(1). For comparison, the Th–Namido distances in
˚
i
2,6-NC5H3(CH2NAr)(CH2N{C6H3 Pr(CMe2)-2,6}); Ar = 2,6-
diisopropylphenyl; donor atoms in BDPP* are in italics]§ as the
˚
˚
[(BDPP)Th(CH2Ph)2] are 2.27 and 2.30 A. At 2.614(9) A, the
1
major product (> 80% yield by H NMR spectroscopy). Highly
Th–Npy distance in 3 is longer than that in [(BDPP)Th(CH2Ph)2]
[2.545(4) A], but is comparable to Th–Npy in sterically encumbered
soluble 3 was isolated from hexamethyldisiloxane in low yield,
and shows no sign of decomposition after days in benzene at
80 ◦C, despite the presence of a b-hydrogen-containing tertiary
alkyl group in the BDPP* ligand. The two methyl groups in 3 are
observed at -0.09 and -0.32 ppm in the 1H NMR spectrum, and
at 53.0 and 41.4 ppm in the 13C NMR spectrum. The metallated
carbon atom (ThCMe2Ar) lies at 73.6 ppm in the 13C NMR
spectrum, and the attached methyl groups (ThCMe2Ar) give rise
to singlets at 2.39 and 1.22 ppm in the 1H NMR spectrum.
Complex 3 is the product of metallation at the methyne carbon
in an isopropyl group of the BDPP ligand (Scheme 1). Metallation
of N-2,6-diisopropylphenyl groups is common in early transition
metal and f-element chemistry, but typically occurs at a methyl
˚
14,15
˚
[Th(BDPP)2] [2.615(6) A].
To probe the mechanism for the formation of complex 3,
[(BDPP)Th(13CD3)3{Li(dme)}] (2-13C3,d9) was prepared by re-
action of [(BDPP)ThCl2(dme)] with Li13CD3/LiI. Thermal de-
composition of 2-13C3,d9 yielded only 13CHD3 (not 13CD4) and
[(BDPP*)Th(m-13CD3)2Li(dme)] (rather than the 13CD3/13CHD2
complex) by 13C and 13C{ H} NMR spectroscopy (Fig. 2). These
1
products are consistent with a simple s-bond metathesis pathway,
rather than a mechanism involving initial a-deuterium abstraction
to release CD4.
group; rare examples of isopropyl methyne metallation have been
t
reported for [(BDPP)Lu(AlMe4)]16 and [(nacnac)(X)Ti CH Bu]
=
(X = Cl, Br, OTf, BH4, CH2SiMe3).17
Crystals of 3·hexane were obtained from toluene–hexane at
◦
-30 C,18,19 and reveal distorted octahedral geometry at thorium
and approximately tetrahedral geometry at lithium (Fig. 1). Tho-
rium lies in a plane with N(1), N(2), N(3) and C(32), while C(33)
and C(7) are located above and below the plane of the BDPP*
ligand backbone. The acute C(32)–Th–C(33) angle [80.9(4)◦] is
primarily a consequence of Li(dme)+ bridging between the two
methyl groups, while constraints imposed by the BDPP* ligand
backbone are responsible for the acute N(1)–Th–N(2), N(2)–Th–
N(3) and N(1)–Th–C(7) angles of 63.5(3)◦, 63.9(3)◦ and 70.7(3)◦,
respectively.
1
Fig. 2 Thorium methyl and methane signals in 13C and/or 13C{ H} NMR
spectra of the reaction to form 3-13C2,d6 from 2-13C3,d9 in C6D6.
Reaction of two equivalents of complex
2
with
[(BDPP)ThCl2(dme)] provided [(BDPP)ThMe2] (4, Scheme 2) in
1
81% yield.§ The H and 13C NMR spectra of 4 are indicative of
a highly symmetric molecule containing a single set of CHMe2,
CH2 and Th–CH3 (1H NMR d 0.01 ppm; 13C NMR d 82.5 ppm)
environments, even at -80 ◦C. This behaviour is analogous to that
observed for dibutyl complex 1. However, while organothorium
BDPP complexes such as 1, [(BDPP)Th(CH2SiMe3)2],14 and
[(BDPP)Th(CH2Ph)2]15 rival the thermal stability of their bis-
pentamethylcyclopentadienyl counterparts,20 dimethyl complex
Fig. 1 X-Ray crystal structure of [(BDPP*)Th(m-Me)2Li(dme)]·hexane
(3·hexane) with thermal ellipsoids at 50%. Hydrogen atoms and hexane
are omitted for clarity. In addition, only one of two orientations of the
Li(dme) unit is shown.
The Th–C(32) and Th–C(33) distances of 2.679(13) and
˚
˚
2.635(12) A are longer than Th–C(7) [2.566(12) A], likely due
to Li(dme)+ coordination, but lie at the shorter end of the
range of Th–C distances observed for bridging methyl groups
Scheme 2 Synthesis of complex 4. L = BDPP.
Dalton Trans., 2010, 39, 6626–6628 | 6627
This journal is
The Royal Society of Chemistry 2010
©

4 is entirely decomposed (to yield several unidentified products)
after two hours at room temperature.¶ This behaviour lies in
clear contrast to that of [Cp*2ThMe2], which is more stable
than the CH2SiMe3 and CH2Ph ◦analogues, being only 50%
decomposed after 1 week at 100 C.21 The thermal instability
of b-hydrogen-free 4 is also unusual in light of the stability of
the b-hydrogen-containing n-butyl analogue (1; vide supra).ꢀ
However, monitoring the decomposition of in situ generated
3 T. J. Marks and V. W. Day, in Actinide Hydrocarbyl and Hydride
Chemistry - Fundamental and Technological Aspects of Organo-f-
Element Chemistry, ed. T. J. Marks, I. L. Fragala` and D. Reidel,
Publishing Company, Dordrecht, 1985, pp. 115.
4 C. J. Burns and M. S. Eisen, in Organoactinide Chemistry: Synthesis
and Characterization - The Chemistry of the Transactinide Elements, ed.
L. R. Morss, N. M. Edelstein and J. Fuger, Springer, Dordrecht, The
Netherlands, 3 edn, 2006, vol. 5, pp. 2799.
5 J. W. Bruno, H. A. Stecher, L. R. Morss, D. C. Sonnenberger and T. J.
Marks, J. Am. Chem. Soc., 1986, 108, 7275.
6 D. C. Sonnenberger, L. R. Morss and T. J. Marks, Organometallics,
1985, 4, 352; J. W. Bruno, T. J. Marks and L. R. Morss, J. Am. Chem.
Soc., 1983, 105, 6824.
1
[(BDPP)Th(13CD3)2] by 13C and 13C{ H} NMR spectroscopy
again showed the formation of only H13CD3 (rather than 13CD4),
consistent with a straightforward s-bond metathesis mechanism
for methane elimination.
7 W. J. Evans, D. J. Wink and D. R. Stanley, Inorg. Chem., 1982, 21, 2565
and references therein.
8 H. Lauke, P. J. Swepston and T. J. Marks, J. Am. Chem. Soc., 1984, 106,
6841.
In conclusion, a highly soluble 2,6-bis(anilidomethyl)-pyridine
di-n-butyl thorium complex (1) has been prepared, and is resistant
to b-hydrogen elimination even at 60 ◦C. To the best of our
knowledge, 1 is the first isolated non-cyclopentadienyl thorium
complex bearing simple b-hydrogen-containing alkyl groups. Rare
examples of non-cyclopentadienyl thorium methyl complexes
(2–4) were also prepared, several of which show an unexpectedly
high tendency to engage in methane elimination via s-bond
metathesis. Dialkyl complexes such as those reported here are
of particular interest as precatalysts for olefin polymerization.22
This work highlights the extent to which longer chain primary
alkyl groups may in some situations provide access to f-element
dialkyl complexes with greatly enhanced thermal stability and
solubility relative to methyl derivatives, and decreased steric
hindrance and/or coordinative saturation relative to analogues
bearing common CH2EMe3 (E = C or Si) or CH2Ph anions.
9 P. G. Edwards, R. A. Andersen and A. Zalkin, J. Am. Chem. Soc.,
1981, 103, 7792; P. G. Edwards, R. A. Andersen and A. Zalkin,
Organometallics, 1984, 3, 293.
10 H. W. Turner, R. A. Andersen, A. Zalkin and D. H. Templeton, Inorg.
Chem., 1979, 18, 1221.
11 I. Korobkov, S. Gambarotta and G. P. A. Yap, Angew. Chem., Int. Ed.,
2003, 42, 814.
12 Compounds formed by metallation of a ligand 2,6-diisopropylphenyl
group at a methyl (CHMe2) position have been reported, and contain
a M–CH2CHMeAr linkage. For an example in actinide chemistry, see:
A. Athimoolam, S. Gambarotta and I. Korobkov, Organometallics,
2005, 24, 1996.
13 F. Gue´rin, D. H. McConville and J. J. Vittal, Organometallics, 1996, 15,
5586.
14 C. A. Cruz, D. J. H. Emslie, L. E. Harrington, J. F. Britten and C. M.
Robertson, Organometallics, 2007, 26, 692.
15 C. A. Cruz, D. J. H. Emslie, C. M. Robertson, L. E. Harrington, H. A.
Jenkins and J. F. Britten, Organometallics, 2009, 28, 1891.
16 M. Zimmermann, F. Estler, E. Herdtweck, K. W. To¨rnroos and R.
Anwander, Organometallics, 2007, 26, 6029.
17 F. Basuli, B. C. Bailey, L. A. Watson, J. Tomaszewski, J. C. Huffman
and D. J. Mindiola, Organometallics, 2005, 24, 1886; F. Basuli, B. C.
Bailey, J. C. Huffman and D. J. Mindiola, Organometallics, 2005, 24,
3321.
Notes and references
§ A qualitative flame test was used to test for lithium in complexes 1–4.
Positive results, indicating the presence of lithium, were obtained for 2 and
3, but not for 1 and 4. The presence or absence of lithium in 1–4 was also
verified by elemental analysis (for 1–3), an X-ray crystal structure of 3, and
the incorporation of dme in complexes 2 and 3, but not 1 and 4.
¶ Addition of MeLi to the reaction mixture did not result in simplification
of the 1H NMR spectrum.
18 Structure 3·hexane: C43H70LiN3O2Th, M = 900.02, monoclinic, a =
˚
34.258(7), b = 14.286(3), c = 17.419(4) A, b = 102.922(4), U = 8309(3)
3
˚
A , T = 173(2) K, space group C2/c, Z = 8, 41676 reflections, 7306
unique (Rint = 0.1567), R1 = 0.0645 and wR2 = 0.1268 [I > 2s(I)].
19 Complex 3 is the first crystallographically characterized tert-alkyl
actinide complex.
ꢀ UV-irradiation (medium pressure mercury lamp) of 1 in benzene lead
20 J. W. Bruno, G. M. Smith, T. J. Marks, C. K. Fair, A. J. Schultz and
J. M. Williams, J. Am. Chem. Soc., 1986, 108, 40; P. J. Fagan, J. M.
Manriquez, E. A. Maatta, A. M. Seyam and T. J. Marks, J. Am. Chem.
Soc., 1981, 103, 6650.
21 J. M. Manriquez, P. J. Fagan and T. J. Marks, J. Am. Chem. Soc., 1978,
100, 3939.
22 Z. Hou and Y. Wakatsuki, Coord. Chem. Rev., 2002, 231, 1; V. C. Gibson
and S. K. Spitzmesser, Chem. Rev., 2003, 103, 283; Y. X. Chen, M. V.
Metz, L. T. Li, C. L. Stern and T. J. Marks, J. Am. Chem. Soc., 1998,
120, 6287.
only to extensive decomposition.
1 C. J. Burns, D. L. Clark and A. P. Sattelberger, in Actinides:
Organometallic Chemistry - Encyclopedia of Inorganic Chemistry, ed.
R. B. King and C. M. Lukehart, John Wiley & Sons, Chichester,
England, 2005, vol. 1, pp. 33.
2 Di-n-butyl cyclopentadienyl thorium complexes include [Cp*2ThnBu2]
and [{Me2Si(C5Me4)2}ThnBu2]: J. W. Bruno, T. J. Marks and L. R.
Morss, J. Am. Chem. Soc., 1983, 105, 6824; C. M. Fendrick, L. D.
Schertz, V. W. Day and T. J. Marks, Organometallics, 1988, 7, 1828.
6628 | Dalton Trans., 2010, 39, 6626–6628
This journal is
The Royal Society of Chemistry 2010
©