Actinide-mediated coupling of 4-fluorobenzonitrile: Synthesis of an eight-membered thorium(IV) tetraazametallacycle

Article abstract of DOI:10.1039/b614050e

An eight-membered thorium(iv) tetraazamacrocycle is produced by the sequential, metal-mediated coupling of four equivalents of 4-fluorobenzonitrile; its formation is consistent with the involvement of an imido intermediate, generated from a thorium ketimide complex. The Royal Society of Chemistry.

Full text of DOI:10.1039/b614050e

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Actinide-mediated coupling of 4-fluorobenzonitrile: synthesis of an
eight-membered thorium(IV) tetraazametallacycle{
Eric J. Schelter, David E. Morris, Brian L. Scott and Jaqueline L. Kiplinger*
Received (in Austin, TX, USA) 29th September 2006, Accepted 20th November 2006
First published as an Advance Article on the web 3rd January 2007
DOI: 10.1039/b614050e
An eight-membered thorium(IV) tetraazamacrocycle is pro-
duced by the sequential, metal-mediated coupling of four
equivalents of 4-fluorobenzonitrile; its formation is consistent
with the involvement of an imido intermediate, generated from
a thorium ketimide complex.
(1)
Over the past decade, there has been a growing interest in actinide
ketimide structures.2c The isolation of complex
2
further
molecular chemistry as 5f systems continue to generate a variety of
novel structural motifs, and demonstrate unusual and fascinating
reactivity patterns, including the activation and coupling of small
molecules.1 Recently, we reported that benzonitrile inserts into
actinide–carbon bonds in complexes of the type (C5Me5)2AnR2
to afford bis(ketimide) complexes (C5Me5)2An[–NLC(Ph)(R)]2
(where An = Th, U; R = CH3, CH2Ph, Ph), which exhibit
interesting physicochemical properties, consistent with enhanced
covalency in the actinide–ketimide linkages.2 Intrigued by the
prospect of tuning the relative energies of the metal 5f- and 6d-
orbitals by varying the electronic profile of the ketimide ligands,
we extended this chemistry to include fluorinated nitriles, and
discovered a new thorium-mediated reaction sequence that
involves the coupling of four equivalents of 4-fluorobenzonitrile
to yield an unusual eight-membered Th(IV) tetraazametallacycle.
Actinide complexes possessing fluorinated ligands are rare. This
is based chiefly upon the perception that the fluorophilic actinide
metal centers will engage in fluoride abstraction, which will shut
down any productive chemistry.3 However, as illustrated in
eqn. (1), treatment of (C5Me5)2Th(CH3)2 (1) with excess 4-fluoro-
benzonitrile yielded the expected bright yellow bis(ketimide)
complex (C5Me5)2Th[–NLC(CH3)(4-F–C6H4)]2 (2) as the major
product (48% isolated), in addition to the novel orange-red Th(IV)
tetraazamacrocycle (3) (3% isolated). Following work-up and
crystallization from cold pentane solution, both complexes were
reproducibly isolated in an analytically pure form, and were
characterized by a combination of 1H/13C/19F NMR and UV-vis/
NIR spectroscopy, electrochemistry, elemental analysis and X-ray
crystallography.{
demonstrates the generality of this nitrile insertion chemistry for
the preparation of actinide ketimide complexes.
Confirmation that the Th(IV) metal center had coupled together
four equivalents of 4-fluorobenzonitrile and formed the eight-
membered tetraazametallacycle 3 was obtained by a single-crystal
X-ray crystallography study (Fig. 2). The molecular structure
reveals that the thorium atom is 9-coordinate, adopting a
characteristic bent-metallocene geometry, with Th–C5Me5 bond
lengths of 2.598(5) and 2.577(5) s and a C5Me5(centroid)–Th–
C5Me5(centroid) bond angle of 133.7(5)u. These values are well
within the range typically found for (C5Me5)2Th(IV) complexes.2c,4
The dianionic tetraaza ligand is contained within the metallocene
wedge and consists of alternating C–N units, originating from the
four 4-fluorobenzonitrile molecules. The ligand is bound to the
thorium metal center in an g3-(N,N9,N0)-fashion through two
Th–N s-bonds (Th(1)–N(2) and Th(1)–N(4)) and one Th–N
dative interaction (Th(1)–N(1)).
The molecular structure of 2 is shown in Fig. 1. The salient
metrical parameters for this complex, namely the Th(1)–N(1)
distance of 2.250(2) s, the N(1)–C(11) distance of 1.260(3) s and
the Th(1)–N(1)–C(11) angle of 175.29(19)u, all compare well to
those presented by the other two previously reported Th(IV)
Fig. 1 Molecular structure of complex 2, with thermal ellipsoids at
the 50% probability level. The molecule has crystallographically imposed
two-fold symmetry, with atom Th(1) lying on the two-fold rotation
axis. Selected bond distances (s) and angles (u): Th(1)–N(1) 2.250(2),
N(1)–C(11) 1.260(3), N(1)–Th(1)–N(1#) 108.67(11), Th(1)–N(1)–C(11)
175.29(19), Th(1)–C5Me5(centroid) 2.543(3), C5Me5(centroid)–Th(1)–
C5Me5(centroid) 143.4(2).{
Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
E-mail: kiplinger@lanl.gov; Fax: +1 505-667-9905;
Tel: +1 505-665-9553
{ Electronic supplementary information (ESI) available: Full experimental
details, crystallographic details, and tables of bond distances and angles
for 2 and 3. See DOI: 10.1039/b614050e
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Chem. Commun., 2007, 1029–1031 | 1029

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Fig. 2 Molecular structure of complex 3, with thermal ellipsoids at the
33% probability level. The C5Me5 methyl groups have been removed for
clarity. Selected bond distances (s) and angles (u): Th(1)–N(1) 2.727(4),
Th(1)–N(2) 2.451(4), Th(1)–N(3) 3.879(4), Th(1)–N(4) 2.234(4), N(1)–
C(21) 1.518(6), N(1)–C(30) 1.313(6), C(30)–N(2) 1.378(6), N(2)–C(37)
1.383(6), C(37)–N(3) 1.285(6), N(3)–C(44) 1.406(6), C(44)–N(4) 1.268(6),
Th(1)–C5Me5(cent) 2.598(5), 2.577(5), C5Me5(cent)–Th(1)–C5Me5(cent)
133.7(5).{
Scheme 1 Proposed reaction pathway for the formation of the thorium
metallamacrocycle 3. ArF = 4-F–C6H4.
The strong interaction between the anionic sp2 N(2) and the Th
metal center subdivides the eight-membered metallamacrocycle
into two smaller ring systems. The six-membered unit is defined by
Th(1)–N(4)–C(44)–N(3)–C(37)–N(2). The Th–N(4) distance in 3 is
2.234(4) s and is in the usual range for a thorium–nitrogen
ketimide bond.2c The next four contiguous bonds in the ring have
distances of 1.268(6) (N(4)–C(44)), 1.406(6) (C(44)–N(3)), 1.285(6)
(N(3)–C(37)) and 1.383(6) s (C(37)–N(2)), showing a pattern of
alternating NLC double and C–N single bonds.5
The four-membered Th(1)–N(1)–C(30)–N(2) moiety in 3 is best
described as a thorium 1,3-diazaallyl. The Th(1)–N(2) bond
distance of 2.451(4) s lies at the high end of the range of Th–N
bond lengths reported for other Th(IV) amide complexes,2c,4b,6 and
the Th(1)–N(1) bond distance of 2.727(4) s is consistent with a
Th–N dative interaction.7 The N(1)–C(30) and C(30)–N(2)
distances, of 1.313(6) and 1.378(6) s, respectively, are intermediate
between N–C single and NLC double bonds, implying some
electronic delocalization over the 1,3-diazallyl N–C–N fragment.5
Additional evidence for this is provided by the observation that
the angles around N(2) sum to 360u, consistent with a planar
sp2-hybridized nitrogen atom. The sum of this structural evidence
allows for the representation of the macrocyclic ligand shown in
Scheme 1.
Fig. 3 Cyclic voltammograms for 2 (black) and 3 (green/red) in y0.1 M
[Bu4N][B(C6F5)4]/THF at a Pt disk electrode. The scan rates are all
200 mV s21
.
the conjugated tetraaza ligand framework to stabilize negative
charge by means of new low-lying p* orbitals.
Several new C–C and C–N bonds are formed in complex 3 at
room temperature. A possible mechanism for the formation of 3 is
given in Scheme 1. Nitrile insertion into the thorium–methyl bond
of complex 1 would lead to the formation of the thorium ketimide
complex 4.2,8 A 1,3-migration9 of the thorium methyl group would
generate thorium imido complex 5, which undergoes a [2 + 2]
cycloaddition with another equivalent of nitrile to give the
diazathoracyclobutene 6.10 Subsequent insertion of two equiva-
lents of nitrile into the thorium ketimide (Th–NLC) functional
group11 would afford the thorium macrocycle product 3.
As expected for Th(IV) systems, the electrochemistry observed
for complexes 2 and 3 are strictly due to ligand-based processes
(Fig. 3).2b For the metallacycle 3, two reversible reduction waves
are observed at E1/2 = 22.22 and 22.56 V, and an irreversible
reduction step is found at Ep,c = 23.05 V (all vs. [(C5H5)2Fe]+/0).
This contrasts with the electrochemistry seen for complex 2, which
is dominated by an irreversible reduction process (Ep,c = 22.53 V),
and with previously characterized non-fluorinated thorium
bis(ketimide) complexes, having comparable reductions at more
negative potentials (y22.8 to 23.0 V).2 The additional reversible
reductions observed for complex 3 are attributed to the ability of
Clearly, the key step in this sequence is the ketimide to imido
transformation, resulting from a 1,3-migration of the methyl group
from the thorium metal center to the imino carbon on the ketimide
ligand. Although this transformation has no precedent, it is
reminiscent of the intramolecular 1,3-migrations of alkyl groups
from Ti and Zr to the electrophilic imino carbon (M–NLC) in
1030 | Chem. Commun., 2007, 1029–1031
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c = 20.7781(11) s, b = 95.014(1)u, V = 9658.1(9) s3, T = 141(2) K, Z = 8,
m(Mo-Ka) = 3.144 mm21, l = 0.71073 s, 40160 reflections measured, 9059
tetradentate Schiff base and dibenzotetramethyltetraazaannulene
complexes.12
unique (Rint = 0.0631), which were used in all calculations. Final R1
=
0.0505, wR(F2) = 0.0985 (all data). Disordered solvent molecules were
treated using PLATON/SQUEEZE.14 CCDC 622495. For crystallographic
data in CIF or other electronic format, see DOI: 10.1039/b614050e
Unlike their uranium counterparts, thorium imido complexes
remain exceedingly scarce but have been implicated as reactive
intermediates in numerous catalytic cycles for the oligomerization
and hydroamination of alkynes.13 To date, efforts to stabilize the
putative imido complex 5 have proven unsuccessful, since the
insertion of two equivalents of nitrile to give the thermodynami-
cally preferred bis(ketimide) complex 2 takes place, even with a
deficiency of nitrile. Furthermore, attempts to increase the yield of
3 by using deficient or excess nitrile and a large dilution of reagents
have not changed the observed ratio of products. We are currently
exploring avenues to overcome the low yield of this interesting
reaction, and are examining the synthetic versatility of this
chemistry in the context of using actinide ketimide and imido
complexes for the synthesis of new C–N bonds.
1 (a) I. Castro-Rodriguez and K. Meyer, Chem. Commun., 2006, 1353; (b)
M. Ephritikhine, Dalton Trans., 2006, 2501; (c) O. T. Summerscales,
F. G. N. Cloke, P. B. Hitchcock, J. C. Green and N. Hazari, Science,
2006, 311, 829.
2 (a) J. L. Kiplinger, D. E. Morris, B. L. Scott and C. J. Burns,
Organometallics, 2002, 21, 3073; (b) D. E. Morris, R. E. Da Re,
K. C. Jantunen, I. Castro-Rodriguez and J. L. Kiplinger,
Organometallics, 2004, 23, 5142; (c) K. C. Jantunen, C. J. Burns,
I. Castro-Rodriguez, R. E. Da Re, J. T. Golden, D. E. Morris,
B. L. Scott, F. L. Taw and J. L. Kiplinger, Organometallics, 2004, 23,
4682; (d) R. E. Da Re, K. C. Jantunen, J. T. Golden, J. L. Kiplinger and
D. E. Morris, J. Am. Chem. Soc., 2005, 127, 682.
3 (a) E. A. Cueller and T. J. Marks, Inorg. Chem., 1981, 20, 2129; (b)
M. Weydert, R. A. Andersen and R. G. Bergman, J. Am. Chem. Soc.,
1993, 115, 8837; (c) J. Dorn, E. F. Murphy, S. A. A. Shah and
H. W. Roesky, J. Fluorine Chem., 1997, 86, 121.
4 (a) Z. Lin, J.-F. Le Merechal, M. Sabat and T. J. Marks, J. Am. Chem.
Soc., 1987, 109, 4127; (b) R. S. Sternal, M. Sabat and T. J. Marks, J. Am.
Chem. Soc., 1987, 109, 7920; (c) J. A. Pool, B. L. Scott and
J. L. Kiplinger, J. Am. Chem. Soc., 2005, 127, 1338; (d) J. A. Pool,
B. L. Scott and J. L. Kiplinger, Chem. Commun., 2005, 2591.
5 (a) F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen
and R. Taylor, J. Chem. Soc., Perkin Trans. 2, 1987, S1; (b)
P. B. Hitchcock, M. F. Lappert and D.-S. Liu, J. Organomet. Chem.,
1995, 488, 241.
For their financial support of the work, we acknowledge the
G. T. Seaborg Institute for Transactinium Science (Postdoctoral
Fellowship to E. J. S.), LANL (Director’s Postdoctoral Fellowship
to E. J. S.), the LANL LDRD Program and the Division of
Chemical Sciences, Office of Basic Energy Sciences. We thank
Kimberly C. Jantunen for executing preliminary reactions and
Dr. Jeffery T. Golden for helpful discussions.
Notes and references
6 (a) H. W. Turner, R. A. Andersen, A. Zalkin and D. H. Templeton,
Inorg. Chem., 1979, 18, 1221; (b) T. M. Gilbert, R. R. Ryan and
A. P. Sattelberger, Organometallics, 1988, 7, 2514; (c) D. M. Barnhart,
D. L. Clark, S. K. Grumbine and J. G. Watkin, Inorg. Chem., 1995, 34,
1695.
{ Reactions and manipulations were performed at 21 uC in a recirculating
Vacuum Atmospheres Model HE-553-2 inert atmosphere (N2) dry box
with a MO-40-2 Dri-Train, or using standard Schlenk and high vacuum
line techniques.
Synthesis of (C5Me5)2Th[–NLC(4-F–C6H4)–NLC(4-F–C6H4)–N–C(4-F–
C6H4)LN–C(4-F–C6H4)(CH3)2] (3): A toluene solution (10 mL) of
4-fluorobenzonitrile (0.54 g, 4.75 mmol) was slowly added to a toluene
solution (30 mL) containing (C5Me5)2Th(CH3)2 (1) (0.54 g, 1.02 mmol).
The reaction mixture immediately changed color from clear and colorless
to bright yellow. The solution was stirred for 14 h and the volatiles removed
under reduced pressure. The resulting yellow residue was extracted with
pentane (40 mL) and filtered through a Celite-packed coarse-porosity frit.
The yellow filtrate was stored at 230 uC for 3 weeks, during which time the
product deposited as orange-red crystals, accompanied by a yellow powder
of (2). The macrocycle product (3) was isolated by removing the crystals by
pipette, collecting them by vacuum filtration, washing with pentane (3 6
2 mL), and drying them under reduced pressure. Yield 0.030 g, 3%. 1H
NMR (toluene-d8, 300 MHz): d 8.63 (dd, JHH = 8.4 Hz, JHF = 6.0 Hz, 2 H,
7 (a) M. G. B. Drew and G. R. Willey, J. Chem. Soc., Dalton Trans.,
1984, 727; (b) D. L. Clark and J. G. Watkin, Inorg. Chem., 1993, 32,
1766.
8 For examples, see: (a) W. J. Evans, J. H. Meadows, W. E. Hunter and
J. L. Atwood, J. Am. Chem. Soc., 1984, 106, 1291; (b) G. Erker,
W. Fro¨mberg, J. L. Atwood and W. E. Hunter, Angew. Chem., Int. Ed.
Engl., 1984, 23, 68; (c) J. E. Bercaw, D. L. Davies and P. T. Wolczanski,
Organometallics, 1986, 5, 443; A. Dormond, A. Aaliti, A. El Bouadili
and C. Moise, Inorg. Chim. Acta, 1987, 139, 171.
9 Rare, examples of metal-mediated 1,3-hydride and -alkyl shifts have
been observed for transition metal and lanthanide systems. For
examples, see: (a) C. Bianchini, M. Peruzzini, A. Vacca and
F. Zanobini, Organometallics, 1991, 10, 3697; (b) L. Giannini,
E. Solari, S. De Angelis, T. R. Ward, C. Floriani, A. Chiesi-Villa and
C. Rizzoli, J. Am. Chem. Soc., 1995, 117, 5801; (c) M. Scha¨fer, J. Wolf
and H. Werner, Organometallics, 2004, 23, 5713; (d) K. C. Jantunen,
B. L. Scott, P. J. Hay, J. C. Gordon and J. L. Kiplinger, J. Am. Chem.
Soc., 2006, 128, 6322.
10 (a) K. M. Doxee, J. B. Farahi and H. Hope, J. Am. Chem. Soc., 1991,
113, 8889; (b) M. J. Carney, P. J. Walsh, F. J. Hollander and
R. G. Bergman, Organometallics, 1992, 11, 761; (c) Z. Hou, T. L. Breen
and D. W. Stephan, Organometallics, 1993, 12, 3158.
11 J. L. Kiplinger, J. A. Pool, E. J. Schelter, J. D. Thompson, B. L. Scott
and D. E. Morris, Angew. Chem., Int. Ed., 2006, 45, 2036.
12 (a) C. Floriani, S. Ciurli, A. Chiesi-Villa and C. Guastini, Angew. Chem.,
Int. Ed. Engl., 1987, 26, 70; (b) C. Floriani, E. Solari, F. Corazza,
A. Chiesi-Villa and C. Guastini, Angew. Chem., Int. Ed. Engl., 1989, 28,
64; (c) E. Solari, C. Floriani, A. Chiesi-Villa and C. Rizzoli, J. Chem.
Soc., Dalton Trans., 1992, 367; (d) L. Giannini, E. Solari, S. De Angelis,
T. R. Ward, C. Floriani, A. Chiesi-Villa and C. Rizzoli, J. Am. Chem.
Soc., 1995, 117, 5801.
13 (a) A. Haskel, T. Straub and M. S. Eisen, Organometallics, 1996, 15,
3773; (b) A. Haskel, J. Q. Wang, T. Straub, T. G. Neyroud and
M. S. Eisen, J. Am. Chem. Soc., 1999, 121, 3025; (c) T. Straub,
A. Haskel, T. G. Neyroud, M. Kapon, M. Botoshansky and M. S. Eisen,
Organometallics, 2001, 20, 5017.
meta), 7.28 (dd, JHH = 8.4 Hz, JHF = 5.4 Hz, 2 H, meta), 7.18 (dd, JHH
=
8.1 Hz, JHF = 5.7 Hz, 2 H, meta), 7.04 (m, 2 H, meta), 6.90 (t, JHH = 8.4 Hz,
2 H, ortho), 6.49 (t, JHH = 8.4, 2 H, ortho), 6.16 (t, JHH = 7.5 Hz, 2H,
ortho), 6.06 (t, JHH = 8.4 Hz, 2 H, ortho), 2.13 (s, 30 H, C5(CH3)5) and 1.33
(s, 6 H, CH3). 13C NMR (toluene-d8, 75 MHz): d 176.6 (s, NLC), 164.7 (d,
Ar–C, JCF = 248.3 Hz), 163.2 (d, Ar–C, JCF = 248.3 Hz), 162.8 (d, Ar–C,
JCF = 251.3 Hz), 162.4 (d, Ar–C, JCF = 246.8 Hz), 158.3 (s, NLC), 155.5 (s,
NLC), 149.1 (d, Ar–C, JCF = 3.8 Hz), 140.5 (d, Ar–C, JCF = 3.8 Hz), 135.5
(d, Ar–C, JCF = 3.8 Hz), 132.2 (d, Ar–C, JCF = 8.3 Hz), 130.9 (d, Ar–C,
JCF = 8.3 Hz), 130.6 (d, Ar–C, JCF = 7.5 Hz), 128.2 (s, Ar–C), 125.1 (s,
C5(CH3)5), 115.8 (s, Ar–C), 115.5 (s, Ar–C), 115.2 (s, Ar–C), 114.9 (s, Ar–
C), 114.7 (s, Ar–C), 14.6 (s, CH3), 23.2 (s, C(CH3)2) and 12.9 (s, C5(CH3)5).
19F NMR (toluene-d8, 282 MHz): d 2111.61 (m, Ar–F), 2113.79 (m, Ar–
F), 2114.22 (m, Ar–F) and 2116.37 (m, Ar–F). Anal. calc. for
C55H64N4F4Th: C, 60.65; H, 5.92; N, 5.14. Found: C, 60.46; H, 5.95; N,
5.07%.
Crystal structure data for 2: C36H44F2N2Th, M = 774.77, monoclinic,
space group C2/c (no. 15), a = 12.9201(6), b = 14.0054(7), c = 18.2113(9) s,
b = 98.677(1)u, V = 3257.6(3) s3, T = 141(2) K, Z = 4, m(Mo-Ka) =
4.615 mm21, l = 0.71073 s, 17967 reflections measured, 4014 unique
(Rint = 0.0222), which were used in all calculations. Final R1 = 0.0197,
wR(F2) = 0.0488 (all data). CCDC 622496.
Crystal structure data for 3: C50H52F4N4Th?C5H12, M = 1089.14,
monoclinic, space group C2/c (no. 15), a = 24.6845(13), b = 18.9028(10),
14 A. L. Spek, Acta Crystallogr., Sect. A: Found. Crystallogr., 1990, 46,
C34.
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Chem. Commun., 2007, 1029–1031 | 1031