Single and double alkyl abstraction from a bis(anilido)xanthene thorium(IV) dibenzyl complex: Isolation of an organothorium cation and a thorium dication

Article abstract of DOI:10.1021/om7011503

Reaction of [(XA₂)ThCl₂(dme)] [XA₂ = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene] with 2 equiv of PhCH₂MgCl resulted in the formation of solvent-free [[XA₂)Th(CH₂Ph)₂] (1). Subsequent reaction of 1 with either 1 or 2 equiv of B(C₆F₅)₃ allowed isolation of the first non-cyclopentadienyl actiniae alkyl cation, [(XA ₂)Th(CH₂Ph)]-[PhCH₂B(C₆F ₅)₃] (2), and a rare example of an actiniae dication, [(XA₂)Th][PhCH₂B(C₆F₅) ₃]₂ (3). The X-ray crystal structure of the dication reveals π-coordination of both PhCH₂B(C₆F ₅)₃ anions, and solution NMR spectroscopy shows that a similar mode of contact ion pairing is operative in the monocation.

Full text of DOI:10.1021/om7011503

Organometallics 2008, 27, 15–17
15
Communications
Single and Double Alkyl Abstraction from a Bis(anilido)xanthene
Thorium(IV) Dibenzyl Complex: Isolation of an Organothorium
Cation and a Thorium Dication
Carlos A. Cruz, David J. H. Emslie,* Laura E. Harrington, and James F. Britten
Department of Chemistry, McMaster UniVersity, 1280 Main Street West, Hamilton, ON, L8S 4M1, Canada
ReceiVed NoVember 15, 2007
Such complexes are anticipated to be particularly reactive as a
result of tetravalency (in contrast with predominantly trivalent
group 3 and lanthanide metals) combined with the electropos-
itive nature and large ionic radii of early actinides. The potential
for covalency and f-orbital participation in early actinide
metal–ligand bonding can also lead to reactivity that deviates
substantially from that of related transition metal and lanthanide
complexes.6
Summary: Reaction of [(XA2)ThCl2(dme)] [XA2 ) 4,5-bis(2,6-
diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene] with
2 equiV of PhCH2MgCl resulted in the formation of solVent-
free [(XA2)Th(CH2Ph)2] (1). Subsequent reaction of 1 with either
1 or 2 equiV of B(C6F5)3 allowed isolation of the first
non-cyclopentadienyl actinide alkyl cation, [(XA2)Th(CH2Ph)]-
[PhCH2B(C6F5)3] (2), and a rare example of an actinide
dication, [(XA2)Th][PhCH2B(C6F5)3]2 (3). The X-ray crystal
structure of the dication reVeals π-coordination of both
PhCH2B(C6F5)3 anions, and solution NMR spectroscopy shows
that a similar mode of contact ion pairing is operatiVe in the
monocation.
Herein we report the synthesis of a dibenzyl thorium
complex,[(XA2)Th(CH2Ph)2](1;XA2 )4,5-bis(2,6-diisopropyl-
anilido)-2,7-di-tert-butyl-9,9-dimethylxanthene),4,7 followed
by reaction of 1 with either 1 or 2 equiv of B(C6F5)3 to give
the first non-cyclopentadienyl actinide alkyl cation and the
first example of double alkyl abstraction to form an actinide
dication.
Organometallic complexes of the early transition metals and
lanthanides have received considerable recent attention, in no
small part due to their proficiency as catalysts for transforma-
tions such as olefin polymerization, hydrogenation, and hy-
droamination.1 Over the course of these studies, complexes
supported by non-carbocyclic ancillary ligands have played an
increasingly prominent role, providing a wide variety of metal-
binding environments that contrast those provided by cyclo-
pentadienyl anions, and allowing facile tuning of both steric
and electronic properties.2 By contrast, the vast majority of
organoactinide(IV) chemistry has involved neutral complexes
Reaction of [(XA2)ThCl2(dme)]4 with 2 equiv of benzyl
magnesium chloride at -78 °C resulted in the formation of base-
free [(XA2)Th(CH2Ph)2] (1)7 as a hexane-soluble light yellow
solid in 56% yield. At 50 °C, the 1H NMR of 1 is suggestive of
C2V symmetry. However, at below -40 °C, a spectrum
consistent with a Cs-symmetric complex lacking top-bottom
symmetry and containing two distinct benzyl groups is ob-
1
served.8 The JC,H coupling constants of 120 and 139 Hz for
-
bearing carbocyclic (e.g. C5R5 or C8R82-) ancillaries.3
CH2Ph indicate that one benzyl group is η1-coordinated, while
the other adopts an η2- or η3-coordination mode.9
A major current focus of our research is investigation of the
potential for rigid non-carbocyclic ligands to stabilize neutral
and cationic organothorium(IV) and uranium(IV) complexes.4,5
X-ray quality crystals of 1 were grown from hexanes at -30
°C. The unit cell contains two distinct molecules of 1 [Th-CH2
) 2.503(3)–2.545(3) Å], and in both cases, one benzyl group
is located in the ligand plane while the other occupies an apical
site. For both molecules, the in-plane benzyl group adopts a
bonding mode intermediate between η2- and η3-coordination
[Th-CH2-Cipso ) 85.6(2)° and 87.5(2)°; Th-Cipso ) 2.826(3)
* Corresponding author. E-mail: emslied@mcmaster.ca. Fax: (905)-522-
2509. Tel: (905)-525-9140.
(1) (a) Hou, Z.; Wakatsuki, Y. Coord. Chem. ReV. 2002, 231, 1. (b)
Molander, G. A.; Romero, J. A. C. Chem. ReV. 2002, 102, 2161. (c) Hong,
S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673.
(2) (a) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283.
(b) Edelmann, F. T.; Freckmann, D. M. M.; Schumann, H. Chem. ReV.
2002, 102, 1851. (c) Piers, W. E.; Emslie, D. J. H. Coord. Chem. ReV.
2002, 233, 131. (d) Gromada, J.; Carpentier, J.-F.; Mortreux, A. Coord.
Chem. ReV. 2004, 248, 397. (e) Arndt, S.; Okuda, J. AdV. Synth. Catal.
2005, 347, 339. (f) Zeimentz, P. M.; Arndt, S.; Elvidge, B. R.; Okuda, J.
Chem. ReV. 2006, 106, 2404.
(6) Keogh, D. W. Actinides: Inorganic & Coordination Chemistry. In
Encyclopedia of Inorganic Chemistry; King, R. B., Crabtree, R. H., Eds.;
John Wiley & Sons: Chichester, England, 2005; Vol. 1, p 2.
(7) For the synthesis of N-mesityl- and N-cyclohexyl-substituted 4,5-
bis(amido)xanthene ligands, and the preparation of titanium(IV) bis(amido)
and dibenzyl complexes, see: Porter, R. M.; Danopoulos, A. A. Polyhedron
2006, 25, 859.
(3) (a) Burns, C. J.; Clark, D. L.; Sattelberger, A. P. Actinides:
Organometallic Chemistry. In Encyclopedia of Inorganic Chemistry; King,
R. B., Lukehart, C. M., Eds.; John Wiley & Sons: Chichester, England,
2005; Vol. 1, p 33. (b) Barnea, E.; Eisen, M. S. Coord. Chem. ReV. 2006,
250, 855.
(8) Selected 1H and 13C NMR chemical shifts (ppm): Complex 1 (C7D8,
-50 °C): ThCH2 (1.34, 1.26), ipso-CH2C6H5 (149.3, 144.2), ThCH2 (93.4,
90.9). Complex 2 (C6D6, 20 °C): BCH2C6H5 (6.52 [o], 5.98 [m], 6.56 [p]),
BCH2 (3.25), ThCH2 (2.40), ipso-BCH2C6H5 (162.9), ipso-ThCH2C6H5
(144.5), ThCH2 (93.1), BCH2 (35.9).
(4) Cruz, C. A.; Emslie, D. J. H.; Harrington, L. E.; Britten, J. F.;
Robertson, C. M. Organometallics 2007, 26, 692.
(5) (a) Jantunen, K. C.; Batchelor, R. J.; Leznoff, D. B. Organometallics
2004, 23, 2186. (b) Jantunen, K. C.; Haftbaradaran, F.; Katz, M. J.;
Batchelor, R. J.; Schatte, G.; Leznoff, D. B. Dalton Trans. 2005, 3083.
(9) (a) Chen, Y.-X.; Marks, T. J. Organometallics 1997, 16, 3649. (b)
Horton, A. D.; de With, J. Organometallics 1997, 16, 5424. (c) Pellecchia,
C.; Immirzi, A.; Grassi, A.; Zambelli, A. Organometallics 1993, 12, 4473.
10.1021/om7011503 CCC: $40.75
2008 American Chemical Society
Publication on Web 12/20/2007

16 Organometallics, Vol. 27, No. 1, 2008
Communications
Figure 1. Molecular structure of [(XA2)Th(CH2Ph)2] (1) with thermal
ellipsoids at the 50% probability level. Hydrogen atoms and isopropyl
and tert-butyl groups are omitted for clarity.
and 2.851(3) Å; Th(1)-Cortho ) 3.191(3) and 3.510(3) Å;
Th(2)-Cortho ) 3.126(4) and 3.647(4) Å].10 By contrast, the apical
benzyl group is η1-coordinated in one molecule [Th(1)-CH2-
Cipso ) 115.1(2)°; Th(1)-Cipso ) 3.402(3) Å], but approaches η2-
coordination in the other [Th(2)-CH2-Cipso ) 96.1(2)°;
Th(2)-Cipso ) 3.058(3) Å].9
Slow addition of dibenzyl complex 1 to 1 equiv of B(C6F5)3
in hexanes resulted in the immediate precipitation of
[(XA2)Th(CH2Ph)][PhCH2B(C6F5)3] (2) as a bright yellow solid
in 95% yield. Despite numerous attempts, X-ray quality crystals
of 2 were not obtained. However, the nature of 2 was established
by NMR spectroscopy, elemental analysis, and reactivity (Vide
infra). The only other well characterized molecular actinide alkyl
cations are [(C5Me5)2Th(CH2SiMe3)][C0(B9C2H11)2] and
[(C5Me5)2ThMe(THF)n]X (n ) 0, 1 or 2; X ) various weakly-
coordinating anions) reported by Marks et al.,11 and
[(C5Me5)2UMe(THF)][BPh4] reported by Evans et al.12
Solution NMR data for 2 between 50 and -90 °C are
commensurate with a Cs-symmetric complex lacking top-bottom
symmetry.8 These data, as well as the high solubility of 2,
indicate tight ion pairing, presumably through π-coordination
of the anion. Evidence for this coordination mode is provided
by a ∆δm-p value of 3.93 ppm in the 19F NMR spectrum, the
upfield position of the m-BCH2Ph protons (5.98 ppm), and the
downfield shift of the ipso-BCH2Ph carbon (162.9 ppm) relative
to that of the “free” anion (148.2 ppm). Through-space coupling
between the o- and m-BCH2Ph protons and one of the CHMe2
groups was also observed by 2D-ROESY NMR spectroscopy.
The benzyl group on thorium is η1-coordinated, based on a 1JC,H
value of 119 Hz for ThCH2Ph.9
Figure 2. Molecular structure of [(XA2)Th][PhCH2B(C6F5)3]2 ·
3toluene · 2hexane with thermal ellipsoids at the 50% probability level:
(a) side view, (b) front end view. Hydrogen atoms and solvent are
omitted for clarity. Isopropyl and tert-butyl groups are also omitted in
(a). Pentafluorophenyl groups are omitted in (b).
Scheme 1. Synthesis of Complexes 1-3
are π-coordinated to thorium both in the ligand plane and in
the apical site, with Th-C(X) bond distances increasing in the
order C(X) ) Cpara < Cmeta < Cortho < Cipso [from 2.900(7) to
3.280(7) Å and 2.937(6) to 3.257(7) Å for the borate anions in
the ligand plane and the apical site, respectively]. To minimize
unfavorable steric interactions between benzylborate anions and
ligand isopropyl groups, the thorium atom resides 0.908(6) Å
above the plane defined by the NON-donor atoms, and the 2,6-
diisopropylphenyl groups are rotated to give C(30) · · · C(45) and
C(33) · · · C(42) distances of 4.139(12) and 8.531(11) Å,
respectively.
Slow diffusion of a hexane solution of B(C6F5)3 into a toluene
solution of 2 at -30 °C resulted in precipitation of a bright
orange crystalline solid. Solution NMR spectroscopic studies
on this product were not possible due to insolubility in solvents
with which it did not react. However, the complex was identified
as dicationic [(XA2)Th][PhCH2B(C6F5)3]2 (3; Figure 1) by X-ray
crystallography and elemental analysis.13
The solid-state structure of 3 is shown in Figure 2. The XA2
backbone is approximately planar and the benzyl borate anions
(10) Bond alternation, as found in the ring of the η3-coordinated benzyl
group in [Cp*Zr(η3-CH2Ph)(η7-CH2Ph)]+, was not observed: Pellecchia,
C.; Immirzi, A.; Pappalardo, D.; Peluso, A. Organometallics 1994, 13, 3773.
(11) (a) Lin, Z.; Le Marechal, J.-F.; Sabat, M.; Marks, T. J. J. Am. Chem.
Soc. 1987, 109, 4127. (b) Yang, X.; Stern, C. L.; Marks, T. J. Organome-
tallics 1991, 10, 840. (c) Yang, X.; King, W. A.; Sabat, M.; Marks, T. J.
Organometallics 1993, 12, 4254. (d) Jia, L.; Yang, X.; Stern, C.; Marks,
T. J. Organometallics 1994, 13, 3755. (e) Chen, Y.-X.; Stern, C. L.; Yang,
S.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 12451. (f) Jia, L.; Yang, X.;
Stern, C. L.; Marks, T. J. Organometallics 1997, 16, 842.
Double alkyl, aryl, or hydride abstraction has been
observed only in a handful of cases,14–17 and the resulting
dications are typically stabilized by additional Lewis base
(O-, N-, or P-donor) coordination. To the best of our
knowledge, the only other “Lewis base-free” dications formed
(13) Other structurally characterized organoactinide dications are
[(COT)U(hmpa)3][BPh4]2 and [Cp*2U(NCMe)5]X2 (X ) I, OTF or BPh4):
(a) ref 14b. (b) Maynadié, J.; Berthet, J.-C.; Thuéry, P.; Ephritikhine, M.
J. Am. Chem. Soc. 2006, 128, 1082. (c) Maynadié, J.; Berthet, J.-C.; Thuéry,
P.; Ephritikhine, M. Organometallics 2006, 25, 5603.
(12) (a) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Organometallics
2005, 24, 3407. (b) The uranium alkynyl cation, [(Et2N)2U(C2tBu)(HC2tBu)]-
[BPh4], has also been reported: Dash, A. K.; Wang, J. X.; Berthet, J.-C.;
Ephritikhine, M.; Eisen, M. S. J. Organomet. Chem. 2000, 604, 83.

Communications
Organometallics, Vol. 27, No. 1, 2008 17
by this method are [(tBu3PN)2Ti]X2,15 [(nacnac)Sc]X2,16 and
[(C5H4CMe2Ph)2Zr]X2,17 where X is the MeB(C6F5)3 anion,
and [LSc(CH2SiMe3)][B(C6F5)4]2 generated in situ and character-
ized by NMR spectroscopy (L ) 1,1,1-tris[(S)-4-isopropyloxazoli-
nyl]ethane)18 or suggested on the basis of extremely high 1-hexene
polymerization activity (L ) 1,4,7-trithiacyclononane).19
°C. However, polymer formation was not observed, even after
24 h, presumably due to the ability of thorium to engage in
strong π-arene coordination, despite flanking of the metal with
bulky N-(2,6-diisopropylphenyl) groups. Future work will focus
on the reactivity of 2 with various unsaturated substrates,
including alkenes under more forcing conditions. Bis(anilido)
xanthene thorium alkyl cations with a reduced tendency toward
arene coordination will also be pursued.
Dibenzyl complex 1 and monocation 2 were tested as catalysts
for ethylene polymerization under 1 atm of C2H4 at 25 and 60
Acknowledgment. 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 Op-
portunities Grants. D.J.H.E. also thanks McMaster University
for support in the form of a start-up fund, and C.A.C. thanks
the Government of Ontario for an Ontario Graduate Scholar-
ship (OGS).
(14) (a) See for example: Bosch, B. E.; Erker, G.; Fröhlich, R.; Meyer,
O. Organometallics 1997, 16, 5449. (b) Cendrowski-Guillaume, S. M.;
Lance, M.; Nierlich, M.; Ephritikhine, M. Organometallics 2000, 19, 3257.
(c) Arndt, S.; Spaniol, T. P.; Okuda, J. Angew. Chem., Int. Ed. 2003, 42,
5075. (d) Elvidge, B. R.; Arndt, S.; Zeimentz, P. M.; Spaniol, T. P.; Okuda,
J. Inorg. Chem. 2005, 44, 6777. (e) Bambirra, S.; Meetsma, A.; Hessen, B.
Organometallics 2006, 25, 3454. (f) reference 2e.
(15) Guérin, F.; Stephan, D. W. Angew. Chem., Int. Ed. 2000, 39, 1298.
(16) Hayes, P. G.; Piers, W. E.; McDonald, R. J. Am. Chem. Soc. 2002,
124, 2132. nacnac ) CH{CtBuN(C6H3iPr2-2,6)}2.
Supporting Information Available: Experimental and X-ray
crystallographic data in PDF format and CIF files are available free
of charge via the Internet at http://pubs.acs.org.
(17) Green, M. L. H.; Sassmannshausen, J. Chem. Commun. 1999, 115.
(18) Ward, B. D.; Bellemin-Laponnaz, S.; Gade, L. H. Angew. Chem.,
Int. Ed. 2005, 44, 1668.
(19) Tredget, C. S.; Bonnet, F.; Cowley, A. R.; Mountford, P. Chem.
Commun. 2005, 3301.
OM7011503