Synthesis and molecular structure of titanium complexes containing a reduced TEMPO radical

Article abstract of DOI:10.1039/b110147a

Two titanium compounds containing monoanionic ligands derived from TEMPO were synthesized and structurally characterized, demonstrating the flexibility of the coordination modes adopted by the ligand.

Full text of DOI:10.1039/b110147a

Synthesis and molecular structure of titanium complexes containing a
reduced TEMPO radical
Mahesh K. Mahanthappa, Kuo-Wei Huang, Adam P. Cole and Robert M. Waymouth*
Department of Chemistry, Stanford University, Stanford, CA, USA. E-mail: waymouth@stanford.edu
Received (in Purdue, IN, USA) 8th November 2001, Accepted 18th January 2002
First published as an Advance Article on the web 11th February 2002
1
Two titanium compounds containing monoanionic ligands
derived from TEMPO were synthesized and structurally
characterized, demonstrating the flexibility of the coordina-
tion modes adopted by the ligand.
TEMPO ligand,29 implicating an h -coordination of TEMPO to
Ti, which was confirmed by X-ray analysis of 1† (Fig. 1). The
short Ti–O bond [1.753(3) Å], long N(1)–O(1) bond [1.412(4)
Å], and pyramidal geometry of the nitrogen atom [C(6)–N(1)–
O(1) 106.8(3)°] indicate that the TEMPO ligand is fully
reduced.9,10,18,30 From this, we formulate 1 as a Ti(IV) complex
TEMPO (2,2,6,6,-tetramethylpiperidine-N-oxyl) enjoys the at-
tention of a variety of chemists due to its stability as a free
radical. Applications of this versatile molecule include its use as
a mild yet selective oxidant of primary and secondary
alcohols,1,2 as a spin-label in the study of complex chemical
environments,3–6 as a mediator of controlled/living free radical
polymerization,7,8 and in the mechanistic study of radical
reactions. Inorganic chemists have extensively studied the
fundamental coordination chemistry of the nitroxide moiety to
transition metals9,10 Mn,11 Co,12 Cu,13 Zn,9 Mo,14,15 Rh,14
Pd,16,17 lanthanides,18 the p-block metalloids such as B19 and
Si,20 and the s-block metals Li, Na, and Mg.21 A number of
these compounds have been investigated in the search for new
molecular magnets.22 TEMPO often serves as an odd-electron
Lewis base towards electrophilic metal centers; surprisingly,
only a limited number of metal complexes with monoanionic
TEMPO ligands have been reported.11,12,15–18,21,23
1
containing an h monoanionic TEMPO ligand, rather than an
antiferromagnetically coupled adduct of a metalloradical and
the organic radical as proposed for TEMPO adducts of Cu31 and
Rh.14 The short Ti(1)–O(1) bond distance of 1.753(3) Å and
large Ti(1)–O(1)–N(1) angle suggest a large pp(O) ? dp(Ti)
contribution, as observed for Ti alkoxides such as CpTi-
Cl2(OC6H11).32 The similarity of this bonding geometry to Ti
alkoxides is also consistent with the formulation as a reduced
TEMPO ligand.
To investigate the influence of ancillary ligation on the
bonding geometry of Ti–TEMPO complexes, we investigated
the solid-state structure of (TEMPO)TiCl3 (2), whose synthesis
was previously reported by Matkovskii and coworkers.24
Reaction of TEMPO with TiCl3 in toluene (generated by the
reduction of TiCl4 with hexamethyldisilane)33 followed by
recrystallization at 230 °C yielded yellow–brown crystals of 2
suitable for X-ray diffraction.† The X-ray structure of 2 (Fig. 2)
The synthesis of TEMPO and di-tert-butylnitroxide com-
plexes of Ti, Zr and Hf was previously reported,24,25 but little is
known about the coordination geometry and oxidation state of
the radical-derived ligand. Evans et al. recently reported a novel
2
reveals an h -O/N chelating coordination mode, analogous to
that reported for other hydroxylamide complexes of Ti.26,27 1
H
NMR studies of this compound in C6D6 demonstrate the
magnetic inequivalence of the TEMPO methyl groups, indicat-
1
2
TEMPO complex of Sm containing both h and h -coordinated
2
2
TEMPO ligands.18 h -coordination is common for TEMPO
ing that the h ligation mode is maintained in non-coordinating
complexes of most transition metals;11–18 for the early metals
solvents at room temperature.34 The N–O bond distance of
1.433(3) Å, pyramidal geometry at nitrogen (C(1)–N(1)–O(1)
110.6(4)°) and short Ti–O(1) bond distance [1.839(3) Å] are
2
h -coordination was observed for the homoleptic Ti(ONR2)4 (R
= Me,26 Et27). In this communication, we report the X-ray
structures of two titanium compounds containing anionic
TEMPO ligands and demonstrate that the binding mode of these
ligands depends sensitively on the ancillary ligation at tita-
nium.
2
indicative of reduction of the TEMPO radical to generate an h -
coordinated monoanionic TEMPO ligand.
These results reveal that the coordination geometry of
hydroxyamido ligands to Ti depends on the nature of the
The synthesis of CpTiCl2(TEMPO) (1) was achieved by the
reaction of TEMPO with CpTiCl2(THF), generated in situ from
the Li3N reduction of CpTiCl3 in THF (Scheme 1).28 Toluene
extraction of the reaction product followed by recrystallization
at 245 °C yielded red plates. The 1H and 13C NMR spectra of
1 in C6D6 reveal a single resonance for the methyl groups of the
Fig. 1 X-Ray structure of CpTiCl2(TEMPO). Relevant dimensions (Å and
°): Ti(1)–Cp 2.014, Ti(1)–Cl(1) 2.280(2), Ti(1)–O(1) 1.753(3), N(1)–O(1)
1.412(4), N(1)–C(6) 1.486(5); Cl(1)–Ti(1)–Cl(2) 104.1(1), Cl(1)–Ti(1)–
O(1) 102.6(1), Ti(1)–O(1)–N(1) 155.7(3), O(1)–N(1)–C(6) 106.8(3), C(6)–
N(1)–C(10) 120.3(3).
Scheme 1
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2 L. De Luca, G. Giacomelli and A. Porcheddu, Org. Lett., 2001, 3,
3041–3043.
3 P. P. Borbat, A. J. Costa-Filho, K. A. Earle, J. K. Moscicki and J. H.
Freed, Science, 2001, 291, 266–269.
4 J. F. W. Keana, Chem. Rev., 1978, 78, 37–64.
5 Q. Wang, Y. Zhao, L. X. Song, Z. Q. Fan and L. X. Feng, Macromol.
Chem. Phys., 2001, 202, 448–452.
6 E. P. Talsi, N. V. Semikolenova, V. N. Panchenko, A. P. Sobolev, D. E.
Babushkin, A. A. Shubin and V. A. Zakharov, J. Mol. Catal. A: Chem.,
1999, 139, 131–137.
7 D. Benoit, V. Chaplinski, R. Braslau and C. J. Hawker, J. Am. Chem.
Soc., 1999, 121, 3904–3920.
8 C. J. Hawker, Acc. Chem. Res., 1997, 30, 373–382.
9 G. V. Romanenko, N. V. Podberezskaya and N. V. Pervukhina, J.
Struct. Chem., 1993, 34, 440–468.
10 N. V. Pervukhina, G. V. Romanenko and N. V. Podberezskaya, J.
Struct. Chem., 1994, 35, 367–390.
11 P. Jaitner, W. Huber, G. Huttner and O. Scheidsteger, J. Organomet.
Chem., 1983, 259, C1–C5.
12 P. Jaitner, W. Huber, A. Gieren and H. Betz, J. Organomet. Chem.,
1986, 311, 379–385.
13 A. Caneschi, A. Grand, J. Laugier, P. Rey and R. Subra, J. Am. Chem.
Soc., 1988, 110, 2307–2309.
Fig. 2 X-Ray structure of (TEMPO)TiCl3. Relevant dimensions (Å and °):
Ti(1)–O(1) 1.839(3), Ti(1)–N(1) 2.112(4), N(1)–O(1) 1.433(4), Ti(1)–Cl(1)
2.258(2); Ti(1)–O(1)–N1(1) 79.4(2), Ti(1)–N(1)–O(1) 58.8(2), Cl(1)–
Ti(1)–O(1) 136.6(1), Cl(2)–Ti(1)–O(1) 104.1(1), C(1)–N(1)–O(1)
110.6(4), Cl(1)–Ti(1)–Cl(2) 103.6(1), Cl(1)–Ti(1)–Cl(3) 104.3(1), Cl(2)–
Ti(1)–Cl(3) 99.1(1).
14 T. R. Felthouse, T. Y. Dong, D. N. Hendrickson, H. S. Shieh and M. R.
Thompson, J. Am. Chem. Soc., 1986, 108, 8201–8214.
15 P. Jaitner, W. Huber, A. Gieren and H. Betz, Z. Anorg. Allg. Chem.,
1986, 538, 53–60.
16 M. H. Dickman and R. J. Doedens, Inorg. Chem., 1982, 21, 682–684.
17 M. Okunaka, G. Matsubayashi and T. Tanaka, Bull. Chem. Soc. Jpn.,
1977, 50, 1070–1073.
18 W. J. Evans, J. M. Perotti, R. J. Doedens and J. W. Ziller, Chem.
Commun., 2001, 2326–2327.
19 M. Armbrecht, W. Maringgele, A. Meller, M. Noltemeyer and G. M.
Sheldrick, Z. Naturforsch., Teil B., 1985, 40, 1113–1122.
20 U. Losehand, N. W. Mitzel and D. W. H. Rankin, J. Chem. Soc., Dalton
Trans., 1999, 4291–4297.
hydroxylamine and the ancillary ligation at Ti. The TEMPO
1
2
ligand of 1 binds h to CpTiCl2 whereas Me2NO binds h in
CpTiCl2(ONMe2).35 For the less-sterically hindered TiCl3
2
fragment, the TEMPO ligand binds h , but appears to cause
some distortion as the Cl(1)–Ti(1)–Cl(2) and Cl(1)–Ti(1)–Cl(3)
bond angles are expanded at the expense of the the Cl(3)–Ti(1)-
Cl(2) angle (Fig. 2). These results, along with those of the
groups of Evans18 and Mulvey21 imply that TEMPO can be
reduced by group 4, lanthanide or s-block elements to form
stable and sterically demanding anionic ligands. Investigations
of the reactivity of these compounds in catalytic reactions are
ongoing and will be reported elsewhere.
21 G. C. Forbes, A. R. Kennedy, R. E. Mulvey and P. J. A. Rodger, Chem.
Commun., 2001, 1400–1401.
22 D. Gatteschi, Adv. Mater., 1994, 6, 635–645.
23 P. Jaitner and W. Huber, Inorg. Chim. Acta, 1987, 129, L45–L46.
24 V. A. Golubev, G. N. Voronina, L. I. Chernaya, F. S. Dyachkovskii and
P. E. Matkovskii, Zh. Obsch. Khim., 1977, 47, 1825–1832.
25 P. B. Brindley and M. J. Scotton, J. Organomet. Chem., 1981, 222,
89–96.
We acknowledge the National Science Foundation for
financial support (NSF-CHE 9910240). M. K. M. acknowl-
edges graduate fellowship support from the Fannie and John
Hertz Foundation.
26 N. W. Mitzel, S. Parsons, A. J. Blake and D. W. H. Rankin, J. Chem.
Soc., Dalton Trans., 1996, 2089–2093.
27 K. Wieghardt, I. Tolksdorf, J. Weiss and W. Swiridoff, Z. Anorg. Allg.
Chem., 1982, 490, 182–190.
28 M. Kilner and G. Parkin, J. Organomet. Chem., 1986, 302, 181–191.
29 1H NMR (C6D6, 400 MHz), d 6.16(s, Cp-H, 5H), 1.16 (s, CH3, 12 H),
1.00–1.25 (m, –CH2CH2CH2–, 6H). 13C NMR (C6D6, 400 MHz), d
119.8, 63.2, 39.4, 26.7, 16.6.
Notes and references
†
Crystal data for CpTiCl2(TEMPO) (1): C14H23Cl2NOTi, M = 340.15,
monoclinic, a = 7.835(1), b = 12.019(2), c = 17.930(3) Å, U = 1661.3(7)
Å3, T = 180 K, space group P21/n (no. 14), Z = 4, m(Mo-Ka) = 8.29 cm21
,
4836 total reflections, 2403 unique reflections (Rint = 0.059) used in all
calculations. The final wR (F2) = 0.088 (all data).
Crystal data for (TEMPO)TiCl3 (2): C9H18Cl3NOTi, M = 310.51,
orthorhombic, a = 8.796(1), b = 12.266(1), c = 12.563(1) Å, U =
1355.4(4) Å3, T = 135 K, space group P212121 (no. 19), Z = 4, m(Mo-Ka)
= 11.98 cm21, 6067 total reflections, 1328 unique reflections used in all
calculations. The final wR (F2) = 0.080 (all data).
30 Z. Ciunik, J. Mol. Struct., 1997, 412, 27–37.
31 J. Laugier, J. M. Latour, A. Caneschi and P. Rey, Inorg. Chem., 1991,
30, 4474–4477.
32 M. Frauenkron, N. Tzavellas, N. Klouras and C. P. Raptopoulou,
Monatsh. Chem., 1996, 127, 1137–1143.
CCDC reference numbers 178085 and 178086. See http://www.rsc.org/
suppdata/cc/b1/b110147a/ for crystallographic data in CIF or other
electronic format.
33 A. R. Hermes and G. S. Girolami, Inorg. Synth., 1998, 32, 309–310;
CAUTION: this reaction can lead to violent exotherms!.
34 1H NMR (C6D6, 400 MHz, 18 °C), d 0.80 (s, CH3, 6H), 0.89 (s, CH3,
6H), 0.95–1.80 (m, –CH2CH2CH2–, 6H). 13C NMR (C6D6, 400 MHz,
18 °C), d 67.1, 37.4, 30.8, 24.0, 16.0.
1 A. E. J. deNooy, A. C. Besemer and H. vanBekkum, Synthesis
(Stuttgart), 1996, 1153–1174.
35 D. L. Hughes, M. Jimeneztenorio, G. J. Leigh and D. G. Walker, J.
Chem. Soc., Dalton Trans., 1989, 2389–2395.
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