Synthesis, Structure, and Reactivity of Hydroxylaminato Alkyltitanium Complexes

Article abstract of DOI:10.1021/om0305521

Alkyltitanium complexes bearing hydroxylaminato ligands were synthesized by two methods: (i) reaction of the stable nitroxide radical 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) with TiCl₃ to generate (TEMPO)TiCl₃ (1) followed by alkylation with PhCH₂MgCl to furnish (TEMPO)Ti(CH₂Ph)₃ (2), and (ii) protonolysis of Ti(CH₂Ph)₄ (3) with hydroxylamines to yield (R ₂NO)₂Ti(CH₂Ph)₂ (R = CH ₂Ph (4), Et (5)). ¹H NMR studies of these compounds demonstrate that the hydroxylaminato ligands exhibit both η¹- and η²-binding modes with varying degrees of hemilability. X-ray crystallographic analysis of 5 shows that titanium adopts a six-coordinate propeller conformation in which the hydroxylamine anions are η²-bound. The reaction of the complex 2 with B(C ₆F₅)₃ at 20°C forms 1 equiv of toluene and a cationic benzyltitanium complex exhibiting strong η⁶-PhCH ₂B(C₆F₅)₃ anion coordination, in which one of the TEMPO methyl groups has undergone C-H bond activation as evidenced by ¹H, ¹³C NMR, gHSQC, gHMBC, and gROESY experiments. Complexes 2-5 exhibit very low activities for propylene polymerization.

Full text of DOI:10.1021/om0305521

Organometallics 2004, 23, 1405-1410
1405
Syn th esis, Str u ctu r e, a n d Rea ctivity of Hyd r oxyla m in a to
Alk yltita n iu m Com p lexes
Mahesh K. Mahanthappa, Adam P. Cole, and Robert M. Waymouth*
Department of Chemistry, Stanford University, Stanford, California 94305-5080
Received J uly 25, 2003
Alkyltitanium complexes bearing hydroxylaminato ligands were synthesized by two
methods: (i) reaction of the stable nitroxide radical 2,2,6,6-tetramethylpiperidine-N-oxyl
(TEMPO) with TiCl3 to generate (TEMPO)TiCl3 (1) followed by alkylation with PhCH2MgCl
to furnish (TEMPO)Ti(CH2Ph)3 (2), and (ii) protonolysis of Ti(CH2Ph)4 (3) with hydroxyl-
amines to yield (R2NO)2Ti(CH2Ph)2 (R ) CH2Ph (4), Et (5)). 1H NMR studies of these
compounds demonstrate that the hydroxylaminato ligands exhibit both η1- and η2-binding
modes with varying degrees of hemilability. X-ray crystallographic analysis of 5 shows that
titanium adopts a six-coordinate “propeller” conformation in which the hydroxylamine anions
are η2-bound. The reaction of the complex 2 with B(C6F5)3 at 20 °C forms 1 equiv of toluene
and a cationic benzyltitanium complex exhibiting strong η6-PhCH2B(C6F5)3 anion coordina-
tion, in which one of the TEMPO methyl groups has undergone C-H bond activation as
1
evidenced by H, 13C NMR, gHSQC, gHMBC, and gROESY experiments. Complexes 2-5
exhibit very low activities for propylene polymerization.
In tr od u ction
Resu lts
Cyclopentadienyl ligands have figured prominently in
early transition metal chemistry. Recent attention has
focused on other monoanionic ligands to support early
transition metals, motivated in part by a desire to
develop new classes of olefin polymerization catalysts1
and to uncover new modes of reactivity.2 Hemilabile
ligands3-5 have also attracted attention in the develop-
ment of new ligands for catalysis. Recent computational
studies have suggested that hemilability is the source
of a novel mechanism of stereoselectivity for bis-phe-
noxyimine titanium complexes that produce syndiotactic
polypropylene upon activation with MAO.1s,t,6 Homo-
leptic complexes of titanium and zirconium containing
hydroxylamine ligands were previously shown to exhibit
hemilability; however, no catalytic activity has been
reported for these compounds.7
Recently, we reported the synthesis of titanium
compounds containing a monoanionic ligand derived
from the stable nitroxide radical TEMPO (TEMPO )
2,2,6,6-tetramethylpiperidine-N-oxyl).8 Crystallographic
studies of those complexes demonstrate that the TEMPO-
derived ligand adopts either η1- or η2-binding modes
depending on the nature of the ancillary ligation at
titanium. Furthermore, we also demonstrated that the
bond strength of the TEMPO-Ti interaction can be
modulated by the ligation environment, thus permitting
the homolysis of the TEMPO-Ti bond in Cp2TiCl-
(TEMPO) under mild conditions.9 Herein, we describe
the synthesis of three titanium coordination complexes
containing hydroxylamine ligands, the structure of the
first hydroxylaminato-titanium complex with alkyl
ligands, NMR studies of the reactions of two of these
complexes with B(C6F5)3, and preliminary investigations
of their propylene polymerization activity.
Syn th esis of (Hyd r oxyla m in a to)tita n iu m Com -
p lexes. The synthesis of TEMPOTiCl3 (1) was carried
out by reaction of TEMPO with TiCl3 in toluene or by
(2) Examples of monoanionic non-Cp ligands that have found use
in catalysis include aryloxides: (a) Thorn, M. G.; Etheridge, Z. C.;
Fanwick, P. E.; Rothwell, I. P. Organometallics 1998, 17, 3636. (b)
Latesky, S. L.; McMullen, A. K.; Niccolai, G. P.; Rothwell, I. P.;
Huffman, J . C. Organometallics 1985, 4, 902. Ketimides: (c) Zhang,
S.; Piers, W. E.; Gao, X. L.; Parvez, M. J . Am. Chem. Soc. 2000, 122,
5499. Phosphinimides: (d) Kickham, J . E.; Guerin, F.; Stephan, D. W.
J . Am. Chem. Soc. 2002, 124, 11486. (e) LePichon, L.; Stephan, D. W.;
Gao, X. L.; Wang, Q. Y. Organometallics 2002, 21, 1362. Amine
ethers: (f) Boussie, T. R.; Diamond, G. M.; Goh, C.; Hall, K. A.;
LaPointe, A. M.; Leclerc, M.; Lund, C.; Murphy, V.; Shoemaker, J . A.
W.; Tracht, U.; Turner, H.; Zhang, J .; Uno, T.; Rosen, R. K.; Stevens,
J . C. J . Am. Chem. Soc. 2003, 125, 4306. Amidinates and guanidates:
(g) Keaton, R. J .; J ayaratne, K. C.; Henningsen, D. A.; Koterwas, L.
A.; Sita, L. R. J . Am. Chem. Soc. 2001, 123, 6197. (h) Averbuj, C.; Eisen,
M. S. J . Am. Chem. Soc. 1999, 121, 8755. (i) Bambirra, S.; Meetsma,
A.; Hessen, B.; Teuben, J . H. Organometallics 2001, 20, 782. (j) Duncan,
A. P.; Mullins, S. M.; Arnold, J .; Bergman, R. G. Organometallics 2001,
20, 1808. (k) Mullins, S. M.; Duncan, A. P.; Bergman, R. G.; Arnold, J .
Inorg. Chem. 2001, 40, 6952. Phosphinamides: (l) Kuhl, O.; Koch, T.;
Somoza, F. B.; J unk, P. C.; Hey-Hawkins, E.; Plat, D.; Eisen, M. S. J .
Organomet. Chem. 2000, 604, 116. Tropondiyls: (m) Skoog, S. J .;
Mateo, C.; Lavoie, G. G.; Hollander, F. J .; Bergman, R. G. Organome-
tallics 2000, 19, 1406. (n) Brown, S. J .; Gao, X. L.; Kowalchuk, M. G.;
Spence, R. E. V.; Stephan, D. W.; Swabey, J . Can. J . Chem. 2002, 80,
1618. Sterically bulky silanols: (o) Slaughter, L. M.; Wolczanski, P.
T.; Klinckman, T. R.; Cundari, T. R. J . Am. Chem. Soc. 2000, 122, 7953.
Amides: (p) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861.
(q) Furstner, A.; Mathes, C.; Lehmann, C. W. J . Am. Chem. Soc. 1999,
121, 9453. Aminotroponiminates: (r) Steinhuebel, D. P.; Lippard, S.
J . Organometallics 1999, 18, 3959. Phenoxyimines and other Schiff
bases: (s) Hustad, P. D.; Tian, J .; Coates, G. W. J . Am. Chem. Soc.
2002, 124, 3614. (t) Mitani, M.; Furuyama, R.; Mohri, J .; Saito, J .; Ishii,
S.; Terao, H.; Kashiwa, N.; Fujita, T. J . Am. Chem. Soc. 2002, 124,
7888. (u) Saito, J .; Mitani, M.; Onda, M.; Mohri, J . I.; Ishi, J . I.; Yoshida,
Y.; Nakano, T.; Tanaka, H.; Matsugi, T.; Kojoh, S. I.; Kashiwa, N.;
Fujita, T. Macromol. Rapid Commun. 2001, 22, 1072. (v) Younkin, T.
R.; Conner, E. F.; Henderson, J . I.; Friedrich, S. K.; Grubbs, R. H.;
Bansleben, D. A. Science 2000, 287, 460. (w) Ittel, S. D.; J ohnson, L.
K.; Brookhart, M. Chem. Rev. 2000, 100, 1169. (x) Holland, P. L.;
Cundari, T. R.; Perez, L. L.; Eckert, N. A.; Lachicotte, R. J . J . Am.
Chem. Soc. 2002, 124, 14416. Ovitt, T. M.; Coates, G. W. J . Am. Chem.
Soc. 2002, 124, 1316.
(1) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283.
10.1021/om0305521 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 02/18/2004

1406 Organometallics, Vol. 23, No. 6, 2004
Mahanthappa et al.
Sch em e 1. Syn th esis of TEMP OTiCl3
Sch em e 2. Syn th esis of Bis(h yd r oxyla m in a to)TiR2
temperature 1H NMR studies demonstrate that the
signals associated with the TEMPO ligand sharpen at
60 °C to three broad resonances, consistent with the η1-
bound form. Throughout the temperature range from
22 to 70 °C, the benzylic protons in the 1H NMR
spectrum appear as a sharp singlet, indicating their
chemical equivalence. On the basis of the last observa-
tion, we conclude that broadening of the TEMPO
resonances in complex 2 at room temperature is most
consistent with a η1-coordination with slow ring inver-
sion of the piperidine moiety due to ligation to the
sterically bulky Ti(CH2Ph)3 fragment.
the reaction of TEMPO with TiCl4 in toluene as previ-
ously reported8,10 (Scheme 1). Recrystallization from
toluene gives analytically pure 1 in high yields. The H
1
NMR spectrum of 1 in toluene-d8 at room temperature
reveals that the methyl groups of the TEMPO ligand
are magnetically inequivalent, as expected on the basis
of the previously published X-ray structure, which
demonstrated the η2-binding mode of TEMPO.8 The
proton NMR chemical shifts exhibit a strong tempera-
ture dependence consistent with a fluxional process over
the temperature range from -70 to 80 °C. Upon warm-
ing a toluene-d8 solution of 1 to 27 °C, the diastereotopic
methyl proton resonances are first to coalesce into a
single broad peak due to their relatively small chemical
shift difference (∆δ ) 36 Hz), implying that piperidine-
ring inversion is taking place; heating the sample
further causes the spectrum to simplify to three broad
resonances consistent with (η1-TEMPO)TiCl3. Therefore,
TEMPO is a hemilabile ligand capable of stabilizing an
unusual five-coordinate Ti species at room temperature,
while retaining access to a coordinatively less saturated
species at higher temperatures. Attempts to measure
the activation energy of the η1 T η2 interconversion by
NMR line shape analysis were hampered by tempera-
ture-dependent chemical shifts resulting in overlapping
resonances.
To investigate the influence of hydroxylamine struc-
ture on the resulting coordination geometry,7,11,12 we
attempted to synthesize related complexes of the type
(R2NO)Ti(CH2Ph)3 (R ) alkyl or aryl). Doxsee13 and
Tilley14 previously demonstrated that aryl and alkyl
nitroso compounds insert into titanacyclobutenes and
zirconacyclopentadienes, respectively, to furnish tita-
nium and zirconium complexes having hydroxylaminato
ligands. On the basis of these precedents, we examined
the reaction of Ti(CH2Ph)4 (3) with 2 equiv of PhNO in
Et2O at -60 °C. Warming the solution to room temper-
ature and removal of volatile components yielded a
1
sticky brown solid, whose H NMR spectrum indicated
the complete consumption of 3 to form a mixture of
products. Recrystallization of this mixture from hexanes
yielded only small amounts of an unidentified brown
paramagnetic solid.
We pursued a second synthetic approach toward
mono-hydroxylaminato alkyltitanium complexes based
on protonolysis of 3 with R2NOH (R ) CH2Ph, Et)
(Scheme 2). Stoichiometric reaction of 3 and (PhCH2)2-
NOH in C6D6 yields 0.5 equiv of [(PhCH2)2NO]2Ti(CH2-
Ph)2 and 0.5 equiv of 3, with no evidence for the
formation of a mono-hydroxylaminato product. Treat-
ment of 3 with 2 equiv of R2NOH in Et2O at -60 °C
cleanly generates (R2NO)2Ti(CH2Ph)2 (R ) CH2Ph (4),
Et (5)), which was isolated in moderate yields after
recrystallization from toluene or hexanes. The high-field
Attempts to alkylate 1 with MeLi, MeMgBr, and
AlMe3 in ethereal solvents yielded dark intractable
products; however, treatment of 1 with PhCH2MgCl in
ether followed by extraction with pentane cleanly
yielded orange microcrystals of (TEMPO)Ti(CH2Ph)3 (2).
1
The H NMR spectrum of 2 in C6D6 at 22 °C consists of
broad and poorly defined peaks corresponding to the
TEMPO ligand (δ 0.40-1.50 ppm), a single sharp peak
at δ 3.29 ppm for the benzylic methylene groups, and
aromatic resonances associated with the benzyl ligands
at low field. The equivalence of the benzyl resonances
is consistent with an η1-bound nitroxide or fast inter-
conversion of the benzyl ligands at 22 °C. Variable-
1
region of the H NMR spectrum of 4 consists of a sharp
singlet for the Ti-CH2Ph methylene protons and a pair
of doublets for the diastereotopic benzylic protons
adjacent to nitrogen. These observations are consistent
with the η2-binding of the R2NO moiety to titanium in
1
a high-symmetry environment. H NMR analysis of 5
in C6D6 yields a deceptively simple spectrum consisting
of an apparent triplet (δ 0.814 ppm) corresponding to
the terminal methyl groups of the diethylhydroxylami-
nato ligand, a sharp singlet for the benzylic Ti-CH2Ph
protons, and a set of multiplets for the diastereotopic
methylene protons adjacent to nitrogen. (See Supporting
(3) Braunstein, P.; Naud, F. Angew. Chem., Int. Ed. 2001, 40, 680.
(4) Slone, C. S.; Weinberger, D. A.; Mirkin, C. A. Prog. Inorg. Chem.
1999, 48, 233.
(5) Kuhl, O.; Koch, T.; Somoza, F. B.; J unk, P. C.; Hey-Hawkins,
E.; Plat, D.; Eisen, M. S. J . Organomet. Chem. 2000, 604, 116.
(6) Milano, G.; Cavallo, L.; Guerra, G. J . Am. Chem. Soc. 2002, 124,
13368.
1
Information for the H NMR spectrum of 5.) Evaluation
(7) Wieghardt, K.; Tolksdorf, I.; Weiss, J .; Swiridoff, W. Z. Anorg.
Allg. Chem. 1982, 490, 182.
(8) (a) Mahanthappa, M. K.; Huang, K. W.; Cole, A. P.; Waymouth,
R. M. Chem. Commun. 2002, 502. (b) Mahanthappa, M. K.; Cole, A.
P.; Waymouth, R. M. Organometallics 2004, in press.
(9) Huang, K. W.; Waymouth, R. M. J . Am. Chem. Soc. 2002, 124,
8200.
(10) Golubev, V. A.; Voronina, G. N.; Chernaya, L. I.; Dyachkovskii,
F. S.; Matkovskii, P. E. Zh. Obshch. Khim. 1977, 47, 1825.
(11) Mitzel, N. W.; Parsons, S.; Blake, A. J .; Rankin, D. W. H. Dalton
1996, 2089.
(12) Hughes, D. L.; J imeneztenorio, M.; Leigh, G. J .; Walker, D. G.
Dalton 1989, 2389.
(13) Doxsee, K. M.; J uliette, J . J . J .; Weakley, T. J . R.; Zientara, K.
Inorg. Chim. Acta 1994, 222, 305.
(14) Nakamoto, M.; Tilley, T. D. Organometallics 2001, 20, 5515.

Hydroxylaminato Alkyltitanium Complexes
Organometallics, Vol. 23, No. 6, 2004 1407
found in the homoleptic complexes (η2-R2NO)4Ti (R )
Me, Et). The widening of these angles is accompanied
by the expansion of the O(1)-Ti(1)-N(2) and O(2)-Ti-
(1)-N(1) bond angles, with attendant contraction of the
C(9)-Ti(1)-C(16) bond angle to 93.32(15)°. The Ti-
O-N and Ti-N-O bond angles, however, do not differ
greatly from those previously observed in 1 and in
homoleptic hydroxylaminato-titanium complexes. Com-
parison of the structure of 5 to other known titanium
complexes having two η2-bound monoanionic ligands
demonstrates that the ligation environment is closely
related to that of the homoleptic complex11 in which two
of the η2-hydroxylaminato ligands have been replaced
by benzyl groups.
Rea ction of 2 w ith B(C6F 5)3. At room temperature,
the reaction of 2 and B(C6F5)3 proceeds cleanly in C6D6,
liberating 1 equiv of toluene to generate a red solution.
Upon removal of the volatile components and redisso-
lution of the red oil in C6D6, 1H NMR analysis described
below yields a spectrum consistent with the zwitterionic
structure 6 shown in Figure 2 resulting from abstraction
of a benzyl ligand by B(C6F5)3 and cyclometalation of
one of the TEMPO methyl groups. (See Supporting
Information for the 1H NMR spectrum of 6.) The 1H
NMR spectrum reveals the presence of three diaste-
reotopic pairs of methylene resonances at δ -0.84 and
0.92 ppm (protons 1a,b), δ 1.75 and 2.90 ppm (protons
8a,b), and δ 3.05 and 3.24 ppm (protons 12a,b). (See
Figure 2 for the numbering of the H atoms in 6.) The
last pair of lines is broadened due to coupling of these
benzylic hydrogens (12a,b) to the quadrupolar boron
nucleus in the PhCH2B(C6F5)3 anion. The diastereotopic
doublets at δ -0.84 and 0.92 ppm are assigned to the
cyclometalated TEMPO methylene group (protons 1a,b),
while the remaining three inequivalent methyl groups
(2, 6, and 7) occur as sharp singlets (δ 0.73, 0.99, 1.16
ppm, respectively). The constrained conformation of the
azoxatitanacyclopentane enforces a rigid conformation
on the piperidine ring, evidenced by the well-defined
multiplets in the range δ 0.80-1.80 ppm assigned to
the diastereotopic methylene protons (hydrogens 3-5)
F igu r e 1. ORTEP diagram for (Et2NO)2Ti(CH2Ph)2 (5)
with thermal ellipsoids at the 50% probability level.
of the coupling constants shows that the apparent high-
field “triplet” is an overlapping doublet of doublets with
∆3J H-H ) 0.5 Hz, which explains the appearance of the
diastereotopic hydroxylamine methylene groups as com-
1
plex multiplets (δ 2.85-3.05 ppm). The H NMR spec-
trum of 5 also implies a highly symmetric structure in
which the hydroxylaminato ligands are η2-bound to
1
titanium. Variable-temperature H NMR study of com-
plex 5 in C6D6 shows that the resonances corresponding
to the diastereotopic methylene hydrogens adjacent to
nitrogen broaden but do not coalesce completely upon
heating to 70 °C, indicating that the η2-binding mode
of these ligands is maintained at elevated temperatures.
The strength of the η2-binding interaction in 5 sharply
contrasts the hemilability of the hydroxylaminato ligands
in 1 and (R2NO)4Ti (R ) Et, CH2Ph),7 all of which
exhibit coalescence temperatures below 45 °C. (The
coalescence temperature of 4 was not measured.)
X-r ay Cr ystallogr aph ic An alysis of (Et2NO)2TiBn 2
(5). Slow cooling of a saturated hexanes solution of 5 to
-45 °C yielded crystals suitable for single-crystal X-ray
analysis. The structure of 5 is shown in Figure 1, and
selected metrical parameters for this structure are given
in Table 1, along with analogous dimensions for some
related complexes. (Details of the data acquisition
conditions and structure refinements are given in the
Supporting Information.)
Crystallographic analysis of 5 shows that titanium
adopts a six-coordinate “propeller” conformation, in
which the hydroxylaminato ligands bind in a η2-cisoid
manner with a cis disposition of the Ti-benzyl moieties.
The Ti-O, Ti-N, and N-O bond lengths observed in 5
are comparable to those previously found in 1,8 CpTiCl2-
(η2-ONMe2) (6),12 and(R2NO)4Ti (R ) Me,11 Et7), with
the exception of the Ti-O oxygen bond length in (Et2-
NO)4Ti, which is ∼0.1 Å longer. The Ti-C bond lengths
are in the range of those found in other titanium benzyl
complexes such as Ti(CH2Ph)4 (3)15 and Cp*Ti(CH2-
Ph)3.16 The benzylic hydrogen atoms, whose locations
were inferred from Fourier difference maps, do not
display any interactions with titanium as previously
observed in 3.15 The O(1)-Ti(1)-O(2) and N(1)-Ti-N(2)
bond angles in 5 are substantially wider than those
1
in the ligand backbone. The aromatic region of the H
NMR spectrum displays a distinct set of lines at δ 6.63,
6.59, 6.03, 5.90, and 5.69 ppm as a pair of doublets and
three triplets having equal peak integrals. These five
aromatic resonances, correlated by 1H,1H-gCOSY, occur
at characteristic upfield shifts associated with a PhCH2B-
(C6F5)3 counterion (protons 13-17) η6-bound to titanium
through the phenyl ring.17-20
Two-dimensional proton-carbon correlation NMR
spectroscopy experiments were used to verify the con-
nectivity of 6 and to assign completely the 13C NMR
1
spectrum. 1H,13C-gHSQC and H,13C-gHMBC analyses
of 6 in C6D6 provide support for the η6-bound PhCH2B-
(C6F5)3 anion as demonstrated by the boron quadrupolar
broadened 13C spectral line at δ 34.82 ppm for benzylic
carbon 12 and the downfield ipso carbon resonance at
δ 159.7 ppm (δ 148.5 ppm for the unbound anion).19
Methylene carbon 8 of the Ti-CH2Ph occurs at δ 84.0
(17) Thorn, M. G.; Etheridge, Z. C.; Fanwick, P. E.; Rothwell, I. P.
Organometallics 1998, 17, 3636.
(15) Bassi, I. W.; Allegra, G.; Scordama, R.; Chioccol, G. J . Am.
Chem. Soc. 1971, 93, 3787.
(16) Mena, M.; Pellinghelli, M. A.; Royo, P.; Serrano, R.; Tiripicchio,
A. Chem. Commun. 1986, 1118.
(18) Pellecchia, C.; Grassi, A.; Zambelli, A. J . Mol. Catal. 1993, 82,
57.
(19) Horton, A. D.; deWith, J . Chem. Commun. 1996, 1375.
(20) Deckers, P. J . W.; Hessen, B. Organometallics 2002, 21, 5564.

1408 Organometallics, Vol. 23, No. 6, 2004
Mahanthappa et al.
Ta ble 1. Selected Bon d Len gth s (Å) a n d Bon d An gles (d eg) for 5 a n d Rela ted Hyd r oxyla m in a to-tita n iu m
Com p lexes
5
TEMPOTiCl3
Ti(ONMe2)4
Ti(ONEt2)4
CpTiCl2(ONMe2)
Bond Lengths
Ti-O
1.889(3)
1.891(2)
2.104(3)
2.134(3)
1.436(3)
1.427(4)
2.145(4)
2.136(3)
1.839(3)
2.112(4)
1.433(4)
1.918(1)
1.976(1)
2.230(1)
2.095(1)
1.432(1)
1.424(1)
1.980(3)
2.108(5)
1.402(7)
1.866(1)
2.128(2)
1.418(2)
Ti-N
N-O
Ti-CH2
Bond Angles
Ti-O-N
O-Ti-N
Ti-N-O
77.18(16)
78.69(16)
41.73(10)
40.96(11)
61.09(14)
60.35(14)
110.77(11)
115.64(11)
120.09(12)
150.99(12)
110.2(3)
79.4(2)
41.8(1)
58.8(2)
82.1(1)
74.1(1)
39.5(1)
40.8(1)
58.4(1)
65.1(1)
101.2(1)
89.8(1)
91.9(1)
104.8(1)
110.5(1)
107.7(1)
75.0(2)
40.0(2)
65.1(2)
79.5(1)
40.9
59.6(1)
O-Ti-O′
O-Ti-N′
N-Ti-O′
N-Ti-N′
O-N-C
90.1(1)
87.0(2)
90.7(2)
103.9(3)
110.6(4)
108.9(4)
110.6(4)
110.3(4)
110.6(2)
109.2(2)
110.0(3)
C-Ti-C
reference
93.32(15)
8
11
7
1
12
Analysis of 6 by H,1H-ROESY at -15 °C in toluene-
d8 is also consistent with the proposed structure shown
in Figure 2, in that proton 1a of the cyclometalated
TEMPO correlates with both the piperidine backbone
(protons 3 and 4) and benzyl protons 8a and 9. Proton
1b correlates with benzylic protons 12a,b of the coordi-
nated η6-PhCH2B(C6F5)3 anion and aromatic protons 13,
16, and 17. Hydrogen 8a exhibits complementary ROE
interactions with TEMPO methyl group 6, along with
weaker correlations to protons 13 and 14 of the ben-
zylborate counterion. Diastereotopic partner hydrogen
8b also interacts with the protons 1a,b. Finally, the
aromatic protons 9-11 of Ti-CH2Ph (δ 6.37, 6.96, and
7.13 ppm, respectively) interact with the aromatic
protons 13-17 of the borate counterion.
F igu r e 2. Proposed structure of zwitterion 6 based on one-
and two-dimensional NMR experiments.
ppm with a corresponding ipso carbon resonance at δ
149.0 ppm in the 13C NMR spectrum. The downfield
occurrence of the resonance for benzylic carbon 8 along
with the upfield shift of ortho protons 9 (δ 6.37 ppm)
could be consistent with partial η2-benzyl coordination
to titanium; however, the observed symmetry of the
aromatic ring (proton 9-11) rules out this possibil-
ity.20,21 Evidence for TEMPO cyclometalation derives
from gHMBC signals correlating methyl group 2 with
a quaternary carbon (δ 79.7 ppm) and cyclometalated
carbon 1 (δ 90.4 ppm), as well as a correlation between
carbon 1 and TEMPO backbone carbon 3 (δ 38.4 ppm).
Horton previously reported analogous NMR evidence for
a similar C-H activation process upon reaction of [(Me3-
Si)2N]2Zr(CH2Ph)2 with B(C6F5)3 to yield a cationic
cyclometalated zirconium compound strongly complexed
by a η6-PhCH2B(C6F5)3 anion.19,22 Rothwell also ob-
served that bis(2,6-di-tert-butylphenoxy)Zr(CH2Ph)2 un-
dergoes cyclometalation of one of the tert-butyl groups
upon reaction with B(C6F5)3 to yield [η1-(2,6-tBu2-
C6H3O)][κ2-O-C6H3-6-tBu-CMe2CH2]Zr[η6-PhCH2B-
(C6F5)3] as verified by X-ray crystallography.23
Rea ction of 5 w ith B(C6F 5)3. Reaction of 5 with
B(C6F5)3 in C6D6 yields a red species 7, which exhibits
1
poor hydrocarbon solubility. The H NMR spectrum of
this complex reveals the presence of two types of
terminal methyl groups (apparent triplets at δ 0.80 and
0.68 ppm) and four pairs of diastereotopic methylene
protons (multiplet δ 2.69-2.92 ppm) from the diethyl-
hydroxylaminato ligands along with some minor impu-
rities. (See Supporting Information for the 1H NMR
spectrum of 7.) In contrast to zwitterion 6, 7 shows no
evidence for a η6-bound PhCH2B(C6F5)3 counterion or
η2-benzyl binding to titanium; instead, a single sharp
1H NMR resonance for the boron-bound benzylic meth-
ylene group (δ 3.24 ppm) is observed and no unusual
upfield aromatic resonances are found. Complex 7
appears less prone to C-H activation of the hydroxy-
laminato ligands, although some toluene is observed in
the spectrum along with unassigned multiplets (δ 1.15-
1.35, 0.87 ppm). From these observations, we conclude
that ionic species 7 is present as a loosely bound ion
pair in which fluxionality of the five-coordinate titanium
center yields a symmetric 1H NMR spectrum. Attempts
to characterize this complex further were hampered by
(21) Latesky, S. L.; McMullen, A. K.; Niccolai, G. P.; Rothwell, I.
P.; Huffman, J . C. Organometallics 1985, 4, 902.
(22) Wright, J . M.; Landis, C. R.; Ros, M.; Horton, A. D. Organo-
metallics 1998, 17, 5031.
(23) Thorn, M. G.; Etheridge, Z. C.; Fanwick, P. E.; Rothwell, I. P.
J . Organomet. Chem. 1999, 591, 1.

Hydroxylaminato Alkyltitanium Complexes
Organometallics, Vol. 23, No. 6, 2004 1409
plied by Scott Specialty Gases and were dried by passage
through alumina and deoxygenated by passage through Q-5
(Engelhard).
its low solubility in aromatic hydrocarbon solvents and
decomposition upon exposure to CD2Cl2.
Complexes 4 and 5 both exhibited very low activity
(5-8 kg PP/mol‚Ti‚h) for propylene polymerization in
the presence of methylaluminoxane (MAO). The activi-
ties of these complexes were lower than that of Ti(CH2-
Ph)4 but yielded low-tacticity polypropylenes ([mmmm]
) 18.5-22.1%) with a microstructure similar to that
obtained with Ti(CH2Ph)4, implying that the hydrox-
lamine ligands may not be stable to the MAO activator.
Attempted activation of 2, 4, or 5 with B(C6F5)3 and
triisobutylaluminum as a scavenging agent did not yield
an active polymerization catalyst at room temperature;
the zwitterion 6 was similarly inactive.
1H NMR spectra were recorded on Unity Inova 300, Varian
XL-400, Gemini 400, and Unity Inova 500 spectrometers and
were referenced relative to the residual protiated solvent peaks
in the samples. 13C NMR spectra were obtained at 125 MHz
using a Varian Unity Inova 500 spectrometer or at 100 MHz
using a Varian Gemini 400 spectrometer. gCOSY, gHSQC,
gHMBC, and gROESY were performed on a Unity Inova 500
spectrometer using pulse sequences from the Varian Chemp-
ack without modification. gCOSY spectra were measured at
22 °C in C6D6 with a proton spectral width of -3 to 13 ppm,
256 real points in the t1 dimension, and 3248 points in the t2
dimension; the data were processed using 90° weighted sine-
bell squared functions in both dimensions with 128 points of
linear prediction and were zero-filled to a two-dimensional
matrix of 2048 × 2048. gHSQC experiments were performed
at 20 °C in C6D6 with a proton spectral width of -2 to 12 ppm
and a carbon spectral width of -30 to 170 ppm, 256 complex
points in t1, 1792 points in t2; data were processed using 90°
shifted sine-bell squared function weighting in both dimen-
sions with 128 points of linear prediction used in the t1
dimension and zero-filled to a two-dimensional matrix of 2048
× 2048. gHMBC experiments were performed at 20 °C in C6D6
with a proton spectral width of -8 to 8 ppm and a carbon
spectral width of -15 to 220 ppm, 256 real points in t1, 2048
points in t2; data were processed using 90° shifted sine-bell
squared function weighting in both dimensions with 128 points
of linear prediction used in the t1 dimension and zero-filled
to a two-dimensional matrix of 2048 × 1024. gROESY experi-
ments were performed at -15 °C in toluene-d8 with a proton
spectral width of -1.8 to 8.2 ppm, 150 real points in t1, 2048
points in t2, with a mixing time of 0.3 s; data were processed
using 90° shifted sine-bell squared function weighting in both
dimensions with 128 points of linear prediction used in the t1
dimension and zero-filled to a two-dimensional matrix of 2048
× 1024.
Con clu sion
We have demonstrated the synthesis of a series of
complexes of the structural type (R2NO)nTi(CH2Ph)4-n
(n ) 1, 2) by benzylation of the corresponding metal
halides or by protonolysis of Ti(CH2Ph)4 with stable
N,N-dialkylhydroxylamines. Structural characterization
of (Et2NO)2Ti(CH2Ph)2 shows that it adopts a propeller
conformation in the solid state due to the η2-binding
mode of the hydroxylaminato ligands. In conjunction
with our previous structural study of (TEMPO)TiCl3,
this work demonstrates that the binding mode of the
N,N-dialkylhydroxylaminato ligands depends on both
the steric requirements of the ligand and ancillary
1
ligation at the metal center. Variable-temperature H
NMR studies show that all of these anionic ligands
exhibit varying degrees of hemilabile binding. Reaction
of (TEMPO)Ti(CH2Ph)3 with B(C6F5)3 generates a novel
zwitterionic complex in which the TEMPO moiety
undergoes C-H activation of one of the methyl groups.
Preliminary investigations of the polymerization behav-
ior of these complexes upon activation with MAO and
B(C6F5)3 demonstrate that they generate low-activity
propylene polymerization catalysts.
Elemental analyses were carried out at Desert Analytics
Laboratory (Tuscon, AZ).
TEMP OTiCl3: Meth od 1. At room temperature, TEMPO
(0.612 g, 3.91 mmol) was quickly added to a slurry of TiCl3
(1.000 g, 6.49 mmol) in 25 mL of toluene to generate an orange-
brown solution. Heating the reaction to reflux for 5 min yielded
a yellow solution and a dark precipitate, which was allowed
to cool with stirring over 12 h. The contents of the flask were
allowed to settle, the brilliant yellow supernatant solution was
decanted to a clean Schlenk tube, and the solvent was removed
under reduced pressure to yield a yellow solid, which was
washed with 30 mL of pentane and dried in vacuo. Yield: 0.547
g (45% based on TEMPO).
Exp er im en ta l Section
Ma ter ia ls. Standard Schlenk techniques and a MBraun
Labmaster 100 drybox were used to handle all oxygen- and
moisture-sensitive compounds. Toluene and pentane were
dried and deoxygenated by passage through columns contain-
ing alumina Q-5 (Engelhard).24 Hexanes, diethyl ether, benzene-
d6, and toluene-d8 were vacuum transferred from Na/K alloy,
and CD2Cl2 was dried over CaH2. TEMPO (Aldrich) was doubly
sublimed at 1 × 10-5 Torr prior to use to remove oily residues,
and N,N-diethylhydroxylamine (Aldrich) was vacuum trans-
ferred from oil-free KH to remove traces of water. C6H5CH2-
MgCl (1.0 M) in diethyl ether and N,N-dibenzylhydroxylamine
were also purchased from Aldrich. TiCl4 (99.8%) from Strem
was used without further purification. B(C6F5)3 from Albemarle
Corporation was purified by recrystallization from pentane to
remove hydrated impurities. Methylaluminoxane (unmodified),
supplied by Albermale as a 10 wt % solution in toluene, was
dried under vacuum (5 × 10-6 Torr) at 45 °C to remove solvent
TEMP OTiCl3: Meth od 2. TEMPO (2.47 g, 15.8 mmol) in
40 mL of toluene was quickly added over 90 s to a solution of
TiCl4 (1.30 mL, 11.8 mmol) in 25 mL of toluene to generate a
yellow solution and an orange-yellow precipitate in an exo-
thermic reaction. After 2 h of stirring at room temperature,
the reaction was Schlenk filtered through Celite to give a
yellow solution, which was concentrated in a vacuum to 25
mL and cooled to -45 °C to induce crystallization. Decanting
the supernatant solution, washing the resulting solid with
pentane, and drying it in a vacuum yielded a product identical
1
to the one produced by method 1 above as verified by H and
1
13C NMR. Yield: 1.219 g (50%). H NMR (C6D6, 400 MHz, 18
25
and residual trimethylaluminum prior to use. TiCl3 and Ti-
°C): δ 0.80 (s, CH3, 6H), 0.89 (s, CH3, 6H), 0.95-1.80 (mult,
-CH2-CH2-CH2-, 6H). 1H NMR (C6D6, 400 MHz, 72 °C): δ
0.966 (s, CH3, 12H), 1.24 (br s, 4H, -N-C(CH3)2-CH2-, 4H),
1.50 (br s, -CH2-CH2-CH2-). 13C{1H} NMR (C6D6, 400 MHz,
18 °C): δ 67.1, 37.4, 30.8, 24.0, 16.0. 13C{1H} NMR (C6D6, 100
MHz, 69 °C): δ 67.38, 37.79, 27.07 (br), 16.20. Anal. Calc for
C9H18NOTiCl3: C 34.82, H 5.84, N 4.51. Found: C 34.42, H
6.18, N 4.51.
26
(CH2C6H5)4 were synthesized using known procedures. Gas-
eous and liquid propylene (polymer grade, 99.5%) were sup-
(24) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J . Organometallics 1996, 15, 1518.
(25) Hermes, A. R.; Girolami, G. S. Inorg. Synth. 1998, 32, 309.
(26) Zucchini, U.; Albizzati, E.; Giannini, U. J . Organomet. Chem.
1971, 26, 357.

1410 Organometallics, Vol. 23, No. 6, 2004
Mahanthappa et al.
TEMP OTi(CH2C6H5)3 (2). A slurry of TEMPOTiCl3 (0.617
g, 2.00 mmol) in 25 mL of diethyl ether was cooled to -10 °C,
and 6.0 mL of 1.0 M C6H5CH2MgCl was added over 6 min to
provide an orange solution with a lightly colored precipitate.
The cold bath was removed and the reaction allowed to stir at
room temperature for 10 h. Removal of the solvent under
reduced pressure yielded an oily solid, which was extracted
with pentane (3 × 40 mL). The combined pentane extracts
were filtered through Celite and concentrated in a vacuum to
10 mL; cooling to -45 °C produced orange crystals. This crude
product was further recrystallized from ∼30 mL of hexanes
Ph), 6.89 (app d, 4H, TiCH2-o-Ph), 6.80 (app d, 4H, TiCH2-p-
Ph), 3.08-3.26 (m, 8H, (CH3CH2)2NO-Ti), 2.50 (s, 4H, Ti-
CH2Ph), 1.07 (app t, 12H, Ti-ON(CH2CH3)). 13C{1H} NMR (125
MHz, CDCl3, 20 °C): δ (ppm) 149.22, 127.90, 126.72, 121.51,
75.76, 52.06, 9.77. Anal. Calc for C22H34N2O2Ti: C 65.02, H
8.43, N 6.89. Found: C 64.75, H 8.47, N 6.82.
Rea ction of 2 w ith B(C6F 5)3 (6). A solution of 2 (0.0072
g, 15 µmol) in 1.5 mL of C7D8 was added to a solution of
B(C6F5)3 (0.0082 g, 16 µmol) in 1.5 mL of C7D8 to obtain a dark
red solution of zwitterionic complex 6. The solvent was
removed in vacuo to yield a ruddy oil, which was redissolved
in C6D6. 1H NMR (500 MHz, C6D6, 22 °C): δ (ppm) 7.13 (t,
1
at -45 °C. Yield: 0.317 g (33%). H NMR (500 MHz, CDCl3,
3
50 °C): δ (ppm) 7.12-7.16 (t, 4H, m-C6H5), 6.80-6.90 (m, 6H,
o,p-C6H5), 3.29 (s, 6H, Ti-CH2Ph), 1.55 (s, 12H, -CH3), 1.20-
1.40 (br, 4H, -N-CMe2-CH2-), 0.50-0.70 (br, 2H, CH2-CH2-
CH2-); 1H NMR (300 MHz, C6D6, 70 °C): δ (ppm) 7.20 (t, 4H,
m-C6H5), 7.04 (d, 4H, o-C6H5), 6.90 (t, 2H, p-C6H5), 3.47 (s, 6H,
Ti-CH2Ph), 1.27 (br s, 6H, -CH2-CH2-CH2-), 0.81 (s, 12H, -N-
C(CH3)2-). 13C{1H} NMR (125 MHz, CDCl3, 50 °C): δ (ppm)
149.15, 128.23, 126.57, 122.38, 93.57, 63.96, 39.00, 29.79,
16.77. 13C{1H} NMR (75 MHz, C6D6, 70 °C): δ (ppm) 149.52,
128.59, 127.16, 122.87, 93.98, 63.94, 39.08, 26.89, 16.91. Anal.
Calc for C30H39NOTi: C 75.46, H 8.23, N 2.93. Found: C 75.76,
H 8.01, N 2.98.
2H, J H-H ) 7.5 Hz, overlap with solvent, TiCH2-m-Ph), 6.96
3
3
(t, 1H, J H-H ) 7.0 Hz, TiCH2-p-Ph), 6.63 (d, 1H, J H-H ) 8.0
Hz, BCH2-o-Ph), 6.59 (d, 1H, 3J H-H ) 7.5 Hz, BCH2-o-Ph), 6.37
3
3
(d, 2H, J H-H ) 8.0 Hz, TiCH2-o-Ph), 6.03 (app t, 1H, J H-H
)
3
7.0 Hz, BCH2-m-Ph), 5.90 (app t, 1H, J H-H ) 7.0 Hz, BCH2-
3
m-Ph), 5.69 (t, 1H, J H-H ) 8.0 Hz, BCH2-p-Ph), 3.24 (m, 1H,
2
BCH2Ph), 3.05 (m, 1H, BCH2Ph), 2.90 (d, 1H, J H-H ) 11.5
2
Hz, TiCH2Ph), 1.75 (d, 1H, J H-H ) 11.5 Hz, TiCH2Ph), 1.53
(dt, 1H, 3J H-H ) 13.0 Hz, 2J H-H ) 3.65 Hz, ONC(CH3)2CH2-),
3
2
1.45 (dt, 1H, J H-H ) 13.0 Hz, J H-H ) 3.65 Hz, ONC-
(CH3)2CH2-),1.18-1.36 (m, 3H, CH2CH2-CH2), 1.16 (s, 3H,
ONC(CH3)2-), 1.00-1.08 (m, 1H, CH2CH2-CH2), 0.99 (s, 3H,
((P h CH2)2NO)2Ti(CH2P h )2 (4). A solution of (PhCH2)2NOH
(0.209 g, 0.980 mmol) in 20 mL of ether was added slowly to
Ti(CH2Ph)4 (0.205 g, 0.497 mmol) in 20 mL of diethyl ether at
-30 °C, causing the solution to immediately lighten in color.
The cold bath was removed, and the reaction was allowed to
stir for 15.5 h at room temperature. The reaction solvent was
concentrated to 10 mL, causing a yellow solid to settle out of
solution. The yellow supernatant solution was decanted, and
the resulting yellow solid was dried in vacuo. Analytically pure
yellow-orange crystals were obtained after two recrystalliza-
tions from toluene. Yield: 0.105 g (32%). Higher yields (∼75%)
of this material can be obtained by removal of the ethereal
solvent from the reaction mixture followed by two recrystal-
ONC(CH3)2-), 0.92 (d, 1H, 2J H-H ) 11.5 Hz, O-NC(CH3)(CH2-
2
Ti)), 0.73 (s, 3H, ONC(CH3)(CH2-Ti)), -0.84 (d, 1H, J H-H
)
11.5 Hz, O-NC(CH3)(CH2Ti)). 13C{1H} NMR (125 MHz, C6D6,
20 °C): δ (ppm) 159.74 (ipso B-CH2Ph),149.58 (ipso B-C6F5),
148.99 (ipso Ti-CH2Ph), 147.64 (B-o-C6F5), 138.55 (B-m-C6F5),
136.48 (B-p-C6F5), 132.31, 132.02 (BCH2-o-Ph), 130.75 (BCH2-
m-Ph), 129.60 (BCH2-p-Ph), 129.29 (TiCH2-o-Ph), 126.21 (TiCH2-
m-Ph), 125.91 (BCH2-m-Ph), 124.22 (TiCH2-p-Ph), 90.38 (NC-
1
(CH3)(CH2Ti)-), 84.03 (TiCH2Ph, J C-H ) 124 Hz), 79.72
(NC(CH3)(CH2Ti)-), 64.58 (NC(CH3)2-), 39.20, 38.42 (-CH2-
CH2CH2-), 34.82 (br, BCH2Ph), 28.81, 26.83, 19.37 (-CH3),
16.48 (-CH2CH2CH2-).
Gen er a tion of [(η2-Et2NO)2Ti(CH2P h )][P h CH2B(C6F 5)3].
A solution of 5 (0.0127 g, 31.2 µmol) in 1.5 mL of C6D6 was
added to a solution of B(C6F5)3 (0.0153 g, 29.2 µmol) at room
temperature. 1H NMR (500 MHz, C6D6, 22 °C): δ (ppm) 6.88-
7.14 ppm (m, 10H, BCH2Ph, TiCH2Ph), 3.24 (s, 2H, BCH2Ph),
2.68-2.92 (m, 8H, ON(CH2CH3)2), 2.68 (s, 2H, TiCH2Ph), 0.797
1
lizations from toluene at -45 °C. H NMR (500 MHz, CDCl3,
20 °C): δ (ppm) 7.30-7.50 (m, 20H, ON(CH2Ph)2), 7.06 (d, 4H,
Ti(CH2-o-Ph)), 7.16 (t, 4H, Ti(CH2-m-Ph)), 6.83 (t, 2H, Ti(CH2-
p-Ph)), 4.21 (d, 4H, TiON(CH2Ph)2), 4.07 (d, 4H, TiON(CH2-
Ph)2), 2.70 (s, 4H, TiCH2Ph). 13C{1H} NMR (125 MHz, CDCl3,
19 °C): δ (ppm) 149.10, 133.24, 131.01, 128.44, 128.40, 128.23,
126.97, 121.58, 78.16, 61.64. Anal. Calc for C42H42N2O2Ti: C
77.05, H 6.47, N 4.28. Found: C 77.14, H 6.50, N 4.43.
(Et2NO)2Ti(CH2P h )2 (5). Et2NOH (0.30 mL, 2.91 mmol)
was added dropwise over 5 min to Ti(CH2Ph)4 (0.608 g,1.47
mmol) in 40 mL of hexanes at -30 °C. The cold bath was
removed upon finishing the addition, and the reaction was
allowed to stir for 6 h at room temperature. The reaction
solvent was concentrated to 12 mL and cooled to -45 °C to
produce a yellow precipitate, which was isolated by decanting
the supernatant solution. The crude product was recrystallized
from hexanes at -45 °C to produce crystals suitable for single-
crystal X-ray analysis. Yield: 383 mg (64%). 1H NMR (500
3
3
(dd, J H-H ) 7.5 and 7.0 Hz, ON(CH2CH3)2), 0.675 (dd, J H-H
) 7.5 and 7.0 Hz, ON(CH2CH3)2).
Ack n ow led gm en t. We gratefully acknowledge fi-
nancial support from the NSF (NSF-CHE 9910240).
M.K.M. acknowledges graduate fellowship support from
the Fannie and J ohn Hertz Foundation. We thank Dr.
Stephen R. Lynch for assistance with two-dimensional
NMR experiments, and the Albemarle Corporation for
the generous gift of MAO and organoboron cocatalysts.
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra of
complexes 5-7, text giving the experimental details associated
with data collection and refinement, and tables of crystal data,
positional parameters, anisotropic thermal factors, bond dis-
tances, bond angles, and torsional angles for 5. This material
is available free of charge via the Internet at http://pubs.acs.org.
3
MHz, C6D6, 20 °C): δ (ppm) 7.19 (app t, J H-H ) 7 Hz, 4H,
3
TiCH2-m-Ph), 7.15 (app d, J H-H ) 7.0 Hz, 4H, TiCH2-o-Ph),
6.89 (tt, 3J H-H ) 7 Hz, 5J H-H ) 1.5 Hz, 4H, TiCH2-p-Ph), 2.85-
3.05 (m, 8H, (CH3CH2)2NO-Ti), 2.71 (s, 4H, Ti-CH2Ph), 0.814
3
1
(dd, J H-H ) 7.0 and 7.5 Hz, 12H, Ti-ON(CH2CH3)). H NMR
(400 MHz, CDCl3, 19 °C): δ (ppm) 7.10 (app t, 2H, TiCH2-m-
OM0305521