A crystallizable dinuclear tuck-in-tuck-over tuck-over dialkyl tren uranium complex and double dearylation of BPh4- to give the BPh2-functionalized metallocycle [U{N(CH2CH 2NSiMe3)2</s

Article abstract of DOI:10.1021/ja904459q

(Chemical Equation Presented) The unprecedented formation of a dinuclear tuck-in-tuck-over tuck-over dialkyl Tren-uranium (IV) complex and the first example of double dearylation of BPh₄^- in a molecular context to give a BPh₂

Full text of DOI:10.1021/ja904459q

Published on Web 07/08/2009
A Crystallizable Dinuclear Tuck-In-Tuck-Over Tuck-Over Dialkyl Tren Uranium
-
Complex and Double Dearylation of BPh4 To Give the BPh2-Functionalized
Metallocycle [U{N(CH2CH2NSiMe3)2(CH2CH2NSiMe2CHBPh2)}(THF)]
Benedict M. Gardner, Jonathan McMaster, William Lewis, Alexander J. Blake, and Stephen T. Liddle*
School of Chemistry, UniVersity of Nottingham, UniVersity Park, Nottingham, NG7 2RD, U.K.
Received June 2, 2009; E-mail: stephen.liddle@nottingham.ac.uk
Cyclopentadienyl-based ligands, and the pentamethylcyclopentadi-
enyl ligand in particular, have been enormously successful at supporting
novel reactivity patterns in actinide chemistry.1 In recent years a
number of groups have investigated the use of nonmetallocene ligands
with uranium(III/IV) chemistry and novel and diverse reactivity profiles
have emerged.2
within minutes. Intermediates were not observed, suggesting the decom-
position of the putative benzyl derivative of 2 to 4 and the H+ transfer/
dimerization to form 5 are rapid.9
Recently, we have investigated the capacity of the TrenTMS ligand
{N(CH2CH2NSiMe3)3} to support novel uranium(IV)-metal bonds.
We have reported the syntheses of [(TrenTMS)U(X)(THF)] (X ) Cl,
1;3 X ) I, 24) and demonstrated their utility in the preparation of the
first structurally authenticated uranium-gallium3 and -rhenium
bonds.4 However, in preliminary reactions with some transition metal
anions, we have noted that KX elimination is not straightforward, and
thermolysis is required.
We targeted [(TrenTMS)U(THF)2][BPh4] (3) as a precursor since we
reasoned the BPh4- anion would be more labile than coordinated
halides, and KBPh4 elimination is a proven synthetic method in
f-element chemistry.5 Since KBPh4 does not react with 1 or 2 in THF,
we anticipated that treatment of 2 with KCH2C6H5 would give the
metallocycle [U{N(CH2CH2NSiMe3)2(CH2CH2NSiMe2CH2)}(THF)]
(4)6 which would undergo protonolysis with Et3NHBPh4 to afford 3.
Herein, we show that this superficially straightforward chemistry is
far more complex, as evidenced by the unprecedented formation of a
dinuclear tuck-in-tuck-over tuck-over dialkyl Tren-uranium(IV) com-
plex, and the first example of double dearylation of BPh4- in a
molecular context to give a BPh2-functionalized metallocycle.
Figure 1. Molecular structure of 5. Thermal ellipsoids set at 30% probability;
hydrogen atoms omitted for clarity. Selected bond lengths (Å): U(1)-N(1) 2.239(3),
U(1)-N(2) 2.257(3), U(1)-N(3) 2.292(3), U(1)-N(4) 2.677(3), U(1)-N(6)
2.738(3), U(1)-C(1) 2.667(5), U(2)-N(5) 2.296(4), U(2)-N(6) 2.381(3), U(2)-N(7)
2.267(3), U(2)-N(8) 2.616(3), U(2)-C(1) 2.669(4), U(2)-C(6) 2.493(5).
The molecular structure of 5 is illustrated in Figure 1 with selected
bond lengths. The U(2)-Tren ligand is coordinated normally, except
for the bridging N(6) center. Bridging Tren amides are known but
usually result from alkali metal occlusion.10 However, the coordination
of the U(1)-Tren ligand is unprecedented. In addition to the three
anionic amides, two trimethylsilyl groups are metalated. The C(1)
center bridges U(1) and U(2) in a tuck-in-tuck-over coordination mode,
with essentially identical U-C bond distances of 2.667(5) and 2.669(5)
Å. In contrast, terminal C(6) binds in a tuck-over manner with a
significantly shorter C(6)-U(2) bond length of 2.493(5) Å. The U-C
bond distances compare well to the small number of related metallocyclic
uranium(IV)-alkyls.6b,c The U-Namide and -Namine bond distances are
typical of U(IV)-N bond lengths11 and are commensurate with their
binding modes.
Scheme 1. Synthesis of 5 and 6
Treatment of 5 with 2 equiv of Et3NHBPh4 does not give 3. Instead,
the tuck-in metallocycle 6 was isolated as pale green crystals in 52%
crystalline yield, Scheme 1, and the characterization data support its
formulation.8
The molecular structure of 6 is depicted in Figure 2 with selected
bond lengths. The U(1)-C(1) bond distance of 2.644(9) Å compares
well to 5 and related metallocyclic uranium(IV)-alkyls.6 The
U-Namido and U-Namine bond lengths of 2.244(6) (av.) and 2.573(6)
Å are typical of U(IV)-N bond distances.11 The U(1)-N(1) bond
is short at 2.193(6) Å, and the bite angle of the tuck-in arm is acute
at 68.7(2)° [cf. 86.41(17)° in 5]. The boron center is trigonal planar
[Σ∠ ) 360°] and the B(1)-C(1) distance of 1.493(11) Å is short,
suggesting B-C multiple bond character. For comparison, average
B-CH2 and B-CPh bond lengths of 1.444 and 1.576 Å were
reported for [Mes2BCH2][Li(12-crown-4)2]12 and BPh3.13 To further
Reaction of 2 with KCH2C6H5 proceeds cleanly in toluene to repro-
ducibly give complex 5, Scheme 1, isolated as yellow crystals from hexane
in 54% crystalline yield. A variable-temperature 1H NMR study and X-ray
crystallography enabled us to conclusively identify 5 as a dinuclear tuck-
in-tuck-over tuck-over dialkyl,7 which is further supported by FTIR and
CHN data.8 Monitoring the reaction by 1H NMR spectroscopy showed
the smooth conversion of 2 to 5 with concomitant formation of toluene
9
10388 J. AM. CHEM. SOC. 2009, 131, 10388–10389
10.1021/ja904459q CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
To conclude, the unprecedented dinuclear tuck-in-tuck-over tuck-
over dialkyl Tren-uranium(IV) complex 5 extends the palate of
novel chemistry which may be achieved with uranium and
nonmetallocene ligands, and the BPh2-functionalized complex 6
-
reveals a new double dearylation reaction for the BPh4 anion.
Acknowledgment. We thank the Royal Society, the EPSRC,
the University of Nottingham, and the NSCCS for support and Dr.
R. Bourne (Nottingham) for obtaining GC-MS data.
Supporting Information Available: Experimental, X-ray and
computational data for 5 and 6. This material is free of charge via the
Internet at http://pubs.acs.org.
Figure 2. Molecular structure of 6. Thermal ellipsoids set at 30%
probability; hydrogen atoms omitted for clarity. Selected bond lengths (Å):
U(1)-N(1) 2.193(6), U(1)-N(2) 2.262(6), U(1)-N(3) 2.277(6), U(1)-N(4)
2.573(6), U(1)-C(1) 2.644(9), U(1)-O(1) 2.565(5), B(1)-C(1) 1.493(11),
B(1)-C(16) 1.596(11), B(1)-C(22) 1.591(12).
References
(1) (a) Marks, T J.; Streitwieser, A. In The Chemistry of the Actinide Elements;
Katz J. J., Seaborg, G. T., Morss, L. R., Eds.; Chapman and Hall: New
York, 1986; Vol. 2, pp 1547-1587. (b) Burns, C. J.; Eisen, M. S. In The
Chemistry of the Actinide and Transactinide Elements; Morss, L. R.,
Edelstein, N. M., Fuger, J., Eds.; Springer: Dordrecht, 2006; Vol. 5, pp
2799-2910.
(2) Selected examples: (a) Roussel, P.; Scott, P. J. Am. Chem. Soc. 1998, 120,
1070. (b) Diaconescu, P. L.; Arnold, P. L.; Baker, T. A.; Mindiola, D. J.;
Cummins, C. C. J. Am. Chem. Soc. 2000, 122, 6108. (c) Korobkov, I.;
Gambarotta, S.; Yap, G. P. A. Angew. Chem., Int. Ed. 2002, 41, 3433. (d)
Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Science 2005, 309, 1835. (e)
Hayton, T. W.; Boncella, J. M.; Scott, B. L.; Palmer, P. D.; Batista, E. R.;
Hay, P. J. Science 2005, 310, 1941. (f) Ephritikhine, M. Dalton Trans.
2006, 2501. (g) Castro-Rodr´ıguez, I.; Meyer, K. Chem. Commun. 2006,
1353. (h) Summerscales, O. T.; Cloke, F. G. N.; Hitchcock, P. B.; Green,
J. C.; Hazari, N. Science 2006, 311, 829. (i) Fox, A. R.; Bart, S. C.; Meyer,
K.; Cummins, C. C. Nature 2008, 455, 341. (j) Cantat, T.; Graves, C. R.;
Scott, B. L.; Kiplinger, J. L. Angew. Chem., Int. Ed. 2009, 48, 3681. (k)
Fox, A. R.; Cummins, C. C. J. Am. Chem. Soc. 2009, 131, 5716.
(3) Liddle, S. T.; McMaster, J.; Mills, D. P.; Blake, A. J.; Jones, C.; Woodul,
W. D. Angew. Chem., Int. Ed. 2009, 48, 1077.
validate 6 and probe the B(1)-C(1) bond we carried out DFT
calculations on a full model of 6.8 The calculation reproduced the
metrical parameters and inspection of the Kohn-Sham orbitals,
and Mayer bond orders (B-C ) 1.33) confirm the manifestation
of a B(1)-C(1) π-bond perturbed by the polarizing uranium center.
2RCH2 + 2Et3NH+ + 2BPh4-f
-
(1)
-
2RC(H)BPh2 + 2Et3N + Ph2 + 2PhH + H2
-
BPh4 f BPh+2 + Ph2 + 2e-
(2)
(3)
(4)
(5)
-
BPh4 f BPh3 + 1/2Ph2 + e-
(4) Gardner, B. M.; McMaster, J.; Lewis, W.; Liddle, S. T. Chem. Commun.
2009, 2851.
-
BPh4 + H+ f BPh3 + PhH
(5) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. Angew. Chem., Int. Ed. 1997,
36, 774.
(6) For reactions forming uranium tuck-in metallocycles with silyl amides, see:
(a) Simpson, S. J.; Turner, H. W.; Andersen, R. A. Inorg. Chem. 1981, 20,
2991. (b) Boaretto, R.; Roussel, P.; Kingsley, A. J.; Munslow, I. J.; Sanders,
C. J.; Alcock, N. W.; Scott, P. Chem. Commun. 1999, 1701. (c) Boaretto,
R.; Roussel, P.; Alcock, N. W.; Kingsley, A. J.; Munslow, I. J.; Sanders,
C. J.; Scott, P. J. Organomet. Chem. 1999, 591, 174.
2S-BPh3 f S-BPh+2 + BPh4 + S
-
To shed light on the formation of 6, we analyzed the reaction mother
liquor using GC-MS, which revealed the presence of benzene and
biphenyl.8 Thus, the overall reaction can be represented by eq 1.
Monitoring the reaction by variable temperature 1H NMR spectroscopy
showed conversion of 5 to 6, and no intermediates were observed.8,14
The stoichiometry of eq 1 suggests that eqs 2-515 should be considered
(S ) solvent): (i) formation of BPh2+, eq 2, appears unlikely but could
be facilitated by a redox active uranium center, and this would account
for the generation of Ph2 and BPh2; (ii) eq 3 is known for BPh4- and
accounts for the formation of Ph2;16 (iii) attack of BPh3 by a carbanion
center with extrusion of Ph- (or PhH) seems unlikely on steric grounds,
but this cannot be ruled out;17 (iv) formation of C6H6 may be accounted
for with eq 4, point (iii), or direct extrusion of Ph- from BPh4- which
then abstracts H+ from Et3NH+ or the cyclometalated arm in an
acid-base reaction;18 (v) previous electrochemical studies have dem-
onstrated that eq 5 is viable,15 which would sustain eqs 3 and 4, generate
a BPh2+ of sufficient reactivity to allow nucleophilic attack by a carbanion
center, and regenerate BPh4- which is a potential source of Ph-.
(7) For a tuck-in tuck-over uranium metallocene, see: (a) Evans, W. J.; Miller,
K. A.; DiPasquale, A. G.; Rheingold, A. L.; Stewart, T. J.; Bau, R. Angew.
Chem., Int. Ed. 2008, 47, 5075.
(8) For full details, see the Supporting Information.
(9) Rapid metallocycle formation and H-D migration/scrambling has been
noted for U-Camidosilyl bonds previously; see refs 6a-c.
(10) For example, see: (a) Rousel, P.; Hitchcock, P. B.; Scott, P. Inorg. Chem.
1997, 36, 5716.
(11) Berthet, J. C.; Ephritikhine, M. Coord. Chem. ReV. 1998, 178-180, 83.
(12) Olmstead, M. M.; Power, P. P.; Weese, K. J. J. Am. Chem. Soc. 1987,
109, 2541.
(13) Zettler, F.; Hausen, H. D.; Hess, H. J. Organomet. Chem. 1974, 72, 157.
(14) Attempts to study the reaction in the presence of radical traps were thwarted
by decomposition reactions (see ref 8). Radical traps can also generate
false-positive results; see: (a) Albe´niz, A. C.; Espinet, P.; Lo´pez-Ferna´ndez,
R.; Sen, A. J. Am. Chem. Soc. 2002, 124, 11278.
-
(15) Photochemical and electrochemical studies of BPh4 have showed a two-
-
electron oxidation of BPh4 to form borenium cations is feasible: (a)
Williams, J. L. R.; Doty, J. C.; Grisdale, P. J.; Searle, R.; Regan, T. H.;
Happ, G. P.; Maier, D. P. J. Am. Chem. Soc. 1967, 89, 5153. (b) Crawford,
C. L.; Barnes, M. J.; Peterson, R. A.; Wilmarth, W. R.; Hyder, M. L. J.
Organomet. Chem. 1999, 581, 194. (c) Pal, P. K.; Chowdhury, S.; Drew,
M. G. B.; Datta, D. New J. Chem. 2002, 26, 367.
(16) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Chem. Commun. 2005, 4681.
(17) Reaction of an iron bromo-boryl complex with NaBPh4 has been shown to
generate BPh3 and Ph-: Coombs, D. L.; Aldridge, S.; Rossin, A.; Jones,
C.; Willock, D. J. Organometallics 2004, 23, 2911.
(18) For a migratory insertion of B(C6F5)2 boryl into a C-H bond giving
RB(H)(C6F5)2, see: Aldridge, S.; Kays (ne´e Coombs), D. L.; Al-Fawaz,
A.; Jones, K. M.; Horton, P. N.; Hursthouse, M. B.; Harrington, R. W.;
Clegg, W. Chem. Commun. 2006, 2578.
The formation of 6 is remarkable and is, as far as we are aware,
-
the first example of double dearylation of BPh4 in a molecular
context.15 The reason why the use of BPh4 as a counteranion is
-
avoided in homogeneous catalysis is open to debate. It is usually
-
assumed that BPh4 can block incoming substrates by weak
coordination.19 The BPh4 anion can also become metalated.20
-
(19) (a) Metz, M. V.; Schwartz, D. J.; Stern, C. L.; Marks, T. J. Organometallics
2002, 21, 4159. (b) Gladysz, J. A. Chem. ReV. 2000, 100 (4), 1167
(Editorial).
-
Monodearylation of BPh4 (eq 3) has been recognized as another
16
-
potentially detrimental role for BPh4
.
The double dearylation
(20) Hlatky, G. G.; Turner, H. W.; Eckman, R. R. J. Am. Chem. Soc. 1989,
-
111, 2728.
reactivity of BPh4 described here adds to the growing list of
possible reactions that should be contemplated when using BPh4
-
.
JA904459Q
9
J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10389