Six-coordinate titanium complexes of a tripodal aminetris(phenoxide) ligand: Synthesis, structure, and dynamics

Article abstract of DOI:10.1021/ic048403d

The five-coordinate titanium(IV) alkoxide LTi(O^tBu) (LH ₃ = tris(2-hydroxy-3,5-di-tert-butylbenzyl)amine) is protonolyzed readily by the conjugate acids of monoanionic bidentate ligands, both symmetrical (tropolone, acetylacetone, di

Full text of DOI:10.1021/ic048403d

Inorg. Chem. 2005, 44, 2803−2814
Six-Coordinate Titanium Complexes of a Tripodal Aminetris(phenoxide)
Ligand: Synthesis, Structure, and Dynamics
Kevin C. Fortner, Julian P. Bigi, and Seth N. Brown*
Department of Chemistry and Biochemistry, 251 Nieuwland Science Hall,
UniVersity of Notre Dame, Notre Dame, Indiana 46556-5670
Received November 12, 2004
The five-coordinate titanium(IV) alkoxide LTi(OtBu) (LH3
) tris(2-hydroxy-3,5-di-tert-butylbenzyl)amine) is protonolyzed
readily by the conjugate acids of monoanionic bidentate ligands, both symmetrical (tropolone, acetylacetone, di-
p-toluoylmethane) and unsymmetrical (8-hydroxyquinoline, salicylaldehyde, 2,6-diformyl-p-cresol, anthrarufin). The
geometry of these complexes, which is pseudo-octahedral with the tripodal ligand adopting a chiral, propeller-like
conformation, has been confirmed in four cases by X-ray crystallography. Variable-temperature NMR spectroscopy
indicates that the six-coordinate complexes undergo two dynamic processes. First, the ligands undergo a twisting
motion that results in racemization, a process which is over 104 times faster than in five-coordinate complexes. The
rate acceleration upon binding of an equatorial ligand is ascribed to steric repulsions with one of the cis phenoxides;
the dynamics of a binuclear dibenzyl phosphate-bridged compound, which has a unique conformation of the tripodal
ligand, indicates that flexing the cis phenoxide is the rate-limiting step in racemization. Second, the complexes
undergo a process that interchanges the inequivalent arms of the tripodal ligand. This process involves a trigonal
twist that shifts the bidentate ligand between clefts in the tripod. The intermediate geometry in the reaction appears
to be a transition state and not a long-lived intermediate, as judged from the relative rates of interconversion of
tripod arms and chelate ends in the ditoluoylmethane complex. Tripod arm interchange takes place without partial
dissociation of the bidentate chelate, a reaction that has been observed on a slower time scale in one case.
Introduction
are by far the most commonly observed. However, the
structure and dynamics of higher-coordinate species are of
interest as potential models for intermediates in catalytic
reactions of such tripodal complexes.
Ligands derived from deprotonation of derivatives of tris-
(2-hydroxybenzyl)amine are a relatively new class of tri-
Tripodal, trianionic ligands such as trialkoxyamines1 and
triamidoamines2 play important roles in inorganic chemistry
and have been used to stabilize unusual coordination
geometries,3 unusual inorganic functionalities,4 and catalyti-
cally active species.5 While complexes of such tripodal
ligands are known in a variety of coordination numbers and
geometries, five-coordinate trigonal bipyramidal complexes
(4) (a) Cummins, C. C.; Schrock, R. R.; Davis, W. M. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 756-759. (b) Christou, V.; Arnold, J. Angew.
Chem., Int. Ed. Engl. 1993, 32, 1450-1452. (c) Cummins, C. C.;
Schrock, R. R.; Davis, W. M. Inorg. Chem. 1994, 33, 1448-1457.
(d) Zanetti, N. C.; Schrock, R. R.; Davis, W. M. Angew. Chem., Int.
Ed. Engl. 1995, 34, 2044-2046. (e) Mo¨sch-Zanetti, N. C.; Schrock,
R. R.; Davis, W. M.; Wanninger, K.; Seidel, S. W.; O’Donoghue, M.
B. J. Am. Chem. Soc. 1997, 119, 11037-11048.
* Author to whom correspondence should be addressed. E-mail:
seth.n.brown.114@nd.edu.
(1) (a) Verkade, J. G. Coord. Chem. ReV. 1994, 137, 233-295.
(b) Verkade, J. G. Acc. Chem. Res. 1993, 26, 483-489.
(2) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9-16.
(3) (a) Cummins, C. C.; Lee, J.; Schrock, R. R.; Davis, W. M. Angew.
Chem., Int. Ed. Engl. 1992, 31, 1501-1503. (b) Pinkas, J.; Wang, T.
L.; Jacobson, R. A.; Verkade, J. G. Inorg. Chem. 1994, 33, 4202-
4210. (c) Ray, M.; Golombek, A. P.; Hendrich, M. P.; Young, V. G.;
Borovik, A. S. J. Am. Chem. Soc. 1996, 118, 6084-6085.
(d) Rosenberger, C.; Schrock, R. R.; Davis, W. M. Inorg. Chem. 1997,
36, 123-125. (e) Roussel, P.; Alcock, N. W.; Scott, P. Chem.
Commun. 1998, 801-802. (f) Ray, M.; Hammes, B. S.; Yap, G. P.
A.; Rheingold, A. L.; Borovik, A. S. Inorg. Chem. 1998, 37, 1527-
1532. (g) Schneider, S.; Filippou, A. C. Inorg. Chem. 2001, 40, 4674-
4677. (h) Filippou, A. C.; Schneider, S.; Schnakenburg, G. Inorg.
Chem. 2003, 42, 6974-6976.
(5) (a) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76-78.
(b) Ritleng, V.; Yandulov, D. V.; Weare, W. W.; Schrock, R. R.; Hock,
A. S.; Davis, W. M. J. Am. Chem. Soc. 2004, 126, 6150-6163.
(c) DiFuria, F.; Licini, G.; Modena, G.; Motterle, R.; Nugent, W. A.
J. Org. Chem. 1996, 61, 5175-5177. (d) Bonchio, M.; Calloni, S.;
DiFuria, F.; Licini, G.; Modena, G.; Moro, S.; Nugent, W. A. J. Am.
Chem. Soc. 1997, 119, 6935-6936. (e) Nugent, W. A. J. Am. Chem.
Soc. 1992, 114, 2768-2769. (f) McCleland, B. W.; Nugent, W. A.;
Finn, M. G. J. Org. Chem. 1998, 63, 6656-6666. (g) Nugent, W. A.
J. Am. Chem. Soc. 1998, 120, 7139-7140. (h) Kim, Y.; Hong, E.;
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39.
10.1021/ic048403d CCC: $30.25
Published on Web 03/22/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 8, 2005 2803

Fortner et al.
1412 (w), 1361 (m), 1306 (w), 1260 (s), 1240 (m), 1204 (w),
1171 (w), 1128 (w), 915 (w), 873 (w), 852 (m), 811 (w), 758 (m).
Only unique peaks are reported for the individual compounds.
anionic, tripodal ligands that have recently been explored
as supporting ligands for early6-8 and later9 transition metals
as well as main group elements.10 The ligands are attractive
in that they are sterically and electronically tunable by
judicious choice of substituents on the aryl rings and appear
to lend good stability to their complexes. For example,
titanium(IV) complexes [N(CH2C6H2-3,5-tBu2-2-O)3]Ti(OR)
(LTi[OR]) have been observed to be highly resistant to
hydrolysis.11 Here, we describe the preparation of a variety
of six-coordinate complexes of the LTiIV fragment containing
bidentate, monoanionic chelates. Six-coordination is readily
achieved in these complexes, and their geometries are
relatively undistorted octahedra. The complexes do exhibit
a rich variety of fluxional behaviors, including ligand
conformational changes, configurational changes about the
metal, and partial dissociation of the bidentate ligand.
[Nitrilotris(3,5-di-tert-butyl-2-cresolato)](tropolonato)titanium-
(IV), LTi(trop). To a solution of 300 mg of LTi(OtBu)
(0.380 mmol) in 20 mL of benzene was added tropolone (48 mg,
0.392 mmol, 1.03 equiv), causing an immediate color change from
yellow to orange. The solution was stirred for 1 h, and the solvent
t
and BuOH were removed on a rotary evaporator, leaving behind
an orange oil. The oil was dissolved in a minimum of benzene
(∼3 mL), and 30 mL of CH3CN was added to the solution. The
CH3CN/benzene solution was mixed thoroughly and was placed
in a refrigerator. Crystallization was complete within 24 h. The
orange crystals were suction-filtered on a glass frit, washed with
CH3CN (3 × 5 mL), and air-dried. Yield: 306 mg (96%). 1H NMR
t
t
(C6D6): δ 1.37 (s, 27H, Bu), 1.55 (s, 27H, Bu), 3.82 (br s, 6H,
NCH2Ar), 6.09 (t, 9.8 Hz, 1H, tropolone 5-H), 6.41 (dd, 11.1,
9.8 Hz, 2H, tropolone 4,6-H), 6.62 (d, 11.1 Hz, 2H, tropolone 3,
7-H), 6.97 (d, 2.5 Hz, 3H, ArH), 7.47 (d, 2.5 Hz, 3H, ArH). 13C-
{1H} NMR (CDCl3): δ 30.24, 31.93, 34.40, 35.08, 62.08, 123.44,
123.94, 124.56, 125.69, 128.33, 135.00, 139.06, 141.98, 159.36,
182.16. IR: 1596 (m), 1574 (w), 1522 (s), 1435 (s), 1332 (m),
1073 (w), 968 (w), 714 (w). FAB-MS: 837 (M+). Anal. Calcd for
C52H71NO5Ti: C, 74.53; H, 8.54; N, 1.67. Found: C, 74.74; H,
8.38; N, 1.76.
Experimental Section
General Procedures. Unless otherwise noted, all procedures
were carried out on the benchtop. CD2Cl2 used in variable-
temperature NMR experiments was dried over molecular sieves,
followed by CaH2, and stored in an inert-atmosphere drybox before
use. Tetrahydrofuran was dried over sodium benzophenone ketyl.
2,6-Diformyl-p-cresol was prepared by the MnO2 oxidation of
commercially available 2,6-bis(hydroxymethyl)-p-cresol as de-
scribed by Xie et al.12 LTi(OtBu) (LH3 ) tris(2-hydroxy-3,
5-di-tert-butylbenzyl)amine) was prepared as previously described.11
All other reagents and solvents were commercially available and
used as received. 1H and 13C{1H} NMR spectra (referenced to tetra-
methylsilane) were obtained on a Varian VXR-300 or VXR-500
spectrometer and were recorded at room temperature (20 °C) unless
otherwise indicated. 31P NMR spectra were referenced to external
85% H3PO4. IR spectra were measured as evaporated films on a
Perkin-Elmer PARAGON 1000 FT-IR spectrometer and are
reported in wavenumbers (cm-1). Fast atom bombardment (FAB)
mass spectra were obtained on a JEOL LMS-AX505HA mass
spectrometer using 3-nitrobenzyl alcohol as a matrix. Peaks reported
are the mass number of the most intense peak of isotope envelopes;
in all cases, isotope patterns were in agreement with values
calculated from the molecular formulas. For EI mass spectral
analysis, a JEOL GCmate spectrometer using EI ionization mode
was employed. Elemental analyses were performed by M-H-W
Laboratories (Phoenix, AZ).
[Nitrilotris(3,5-di-tert-butyl-2-cresolato)](acetylacetonato)ti-
tanium(IV), LTi(acac), was prepared by the same method as the
tropolonate, using 300 mg of LTi(OtBu) (0.380 mmol) and
0.100 mL (0.974 mmol, 2.6 equiv) of 2,4-pentanedione. The yield
of orange LTi(acac) was 291 mg (94%). 1H NMR (CDCl3): δ 1.25
t
t
(s, 27H, Bu), 1.31 (s, 27H, Bu), 1.84 (s, 6H, acac CH3), 3.84
(br s, 6H, NCH2Ar), 5.51 (s, 1H, acac CH), 6.91 (d, 2.1 Hz, 3H,
ArH), 7.13 (d, 2.1 Hz, 3H, ArH). 13C{1H} NMR (CDCl3): δ 26.16,
30.35, 31.99, 34.42, 35.10, 62.40, 104.80, 123.40, 123.85, 125.02,
134.86, 141.00, 159.45, 190.36. IR: 1594 (m), 1520 (m), 1026 (w).
FAB-MS: 815 (M+). Anal. Calcd for C50H73NO5Ti: C, 73.60; H,
9.02; N, 1.72. Found: C, 73.73; H, 8.99; N, 1.73.
Di-p-toluoylmethane (Hdtm). In the drybox, to a solution of
0.465 g of 4′-methylacetophenone (Aldrich, 3.46 mmol) in 2 mL
of dry THF in a 20-mL screw-cap vial was added a solution of
1.215 g of lithium bis(trimethylsilyl)amide (Aldrich, 7.26 mmol,
2.1 equiv) in 10 mL of THF. To this solution was added, dropwise
over 5 min with occasional swirling, a solution of 0.499 g of
p-toluoyl chloride (Aldrich, 3.22 mmol, 0.93 equiv) in 2 mL of
THF. The reaction mixture gets very hot, and near the end of the
addition Li(dtm) begins to precipitate as an off-white solid. The
vial was capped securely and allowed to stand overnight at room
temperature. The reaction mixture was then opened to the air and
suction filtered on a glass frit, and the precipitate was washed with
2 × 5 mL Et2O. The filtrate was reserved, and the precipitate was
partitioned between 1 M HCl (30 mL) and Et2O (40 mL). The
aqueous layer was discarded, and the organic layer (including a
small amount of suspended solids) was treated with an additional
20 mL each of aqueous HCl and Et2O. The organic layer was
washed with 20 mL of water, dried over MgSO4, and evaporated
All of the LTi complexes have the following IR peaks, within
3 cm-1: 2954 (s), 2904 (m), 2868 (m), 1474 (m), 1444 (m),
(6) Kol, M.; Shamis, M.; Goldberg, I.; Goldschmidt, Z.; Alfi, S.; Hayut-
Salant, E. Inorg. Chem. Commun. 2001, 4, 177-179.
(7) (a) Michalczyk, L.; de Gala, S.; Bruno, J. W. Organometallics 2001,
20, 5547-5556. (b) Kim, Y.; Verkade, J. G. Organometallics 2002,
21, 2395-2399. (c) Bull, S. D.; Davidson, M. G.; Johnson, A. L.;
Robinson, D. E. J. E.; Mahon, M. F. Chem. Commun. 2003, 1750-
1751. (d) Davidson, M. G.; Doherty, C. L.; Johnson, A. L.; Mahon,
M. F. Chem. Commun. 2003, 1832-1833.
(8) (a) Groysman, S.; Segal, S.; Shamis, M.; Goldberg, I.; Kol, M.;
Goldschmidt, Z.; Hayut-Salant, E. J. Chem. Soc., Dalton Trans. 2002,
3425-3426. (b) Kim, Y.; Kapoor, P. N.; Verkade, J. G. Inorg. Chem.
2002, 41, 4834-4838. (c) Groysman, S.; Goldberg, I.; Kol, M.;
Goldschmidt, Z. Organometallics 2003, 22, 3793-3795.
(9) Hwang, J.; Govindaswamy, K.; Koch, S. A. Chem. Commun. 1998,
1667-1668.
to give 0.391
g of di-p-toluoylmethane, mp 125-126.5°
(lit.13 123.5-125°). A second crop of 0.189 g was obtained by
evaporating the filtrate to dryness and working up in the same
manner; combined yield, 0.579 g, 71% based on toluoyl chloride.
The 1H NMR spectrum in CDCl3 indicates that the compound exists
(10) Chandrasekaran, A.; Day, R. O.; Holmes, R. R. J. Am. Chem. Soc.
2000, 122, 1066-1072.
(11) Ugrinova, V.; Ellis, G. A.; Brown, S. N. Chem. Commun. 2004, 468-
469.
(12) Xie, R. G.; Zhang, Z. J.; Yan, J. M.; Yuan, D. Q. Synth. Commun.
1994, 24, 53-58.
(13) Choshi, T.; Horimoto, S.; Wang, C. Y.; Nagase, H.; Ichikawa, M.;
Sugino, E.; Hibino, S. Chem. Pharm. Bull. 1992, 40, 1047-1049.
2804 Inorganic Chemistry, Vol. 44, No. 8, 2005

Titanium Complexes of a Tripodal Aminetris(phenoxide) Ligand
exclusively as the cis enol: δ 2.43 (s, 6H, CH3), 6.81 (s, 1H, d
C-H), 7.29, 7.89 (d, 9 Hz, 4H each, Ar-H), 16.95 (v br s, 1H,
O-H).
123.78, 124.68, 125.36, 125.43, 135.57, 136.55, 140.50, 141.89,
159.42, 167.75, 194.20. IR: 1631 (s), 1604 (w), 1543 (m), 1402
(w), 1146 (w), 906 (w), 790 (w). FAB-MS: 837 (M+). Anal. Calcd
for C52H71NO5Ti: C, 74.53; H, 8.54; N, 1.67. Found: C, 74.75;
H, 8.38; N, 1.70.
[Nitrilotris(3,5-di-tert-butyl-2-cresolato)](di-p-toluoylmeth-
anato)titanium(IV), LTi(dtm), was prepared by the same method
as LTi(trop) and LTi(acac), using 103 mg of LTi(OtBu) and
38.9 mg of Hdtm (0.154 mmol, 1.2 equiv). The yield of orange
crystals was 84.5 mg (67%). 1H NMR (CD2Cl2, 258 K; the molecule
has ∼Cs symmetry on the NMR time scale at this temperature):
[Nitrilotris(3,5-di-tert-butyl-2-cresolato)](2,6-diformyl-p-creso-
lato)titanium (IV), LTi(dfc). To a solution of 200 mg of
LTi(OtBu) (0.253 mmol) in 20 mL of dichloromethane was added
2,6-diformyl-p-cresol (45 mg, 0.274 mmol, 1.1 equiv). The solution
was refluxed for 4 h, with the color of the solution changing from
yellow to orange after a few minutes at reflux. The solvent and
tBuOH were removed on a rotary evaporator to leave behind an
orange solid. The solid was recrystallized by dissolving it in a
minimum of CH2Cl2 and layering the solution with 20 mL of
CH3CN. Reddish-orange crystals of LTi(dfc), which were suitable
for X-ray analysis, formed within 24 h. The crystals were filtered,
washed with CH3CN (3 × 5 mL), and air-dried on a glass frit.
Yield: 210 mg (94%). 1H NMR (CD2Cl2, 273 K; the molecule has
∼Cs symmetry on the NMR time scale at this temperature): δ 1.01
t
t
t
δ 1.08 (s, 18H, Bu), 1.26 (s, 9H, Bu), 1.27 (s, 18H, Bu), 1.64
t
(s, 9H, Bu), 2.28 (s, 3H, CH3), 2.46 (s, 3H, CH3), 3.60-4.20
(br, 6H, NCH2), 6.86 (s, 1H, CH[COTol]2), 7.01 (d, 8 Hz, 2H, tolyl
ArH), 7.02 (d, 2 Hz, 2H, L ArH), 7.05 (d, 8 Hz, 2H, tolyl ArH),
7.07 (d, 2 Hz, 3H, L ArH), 7.29 (d, 2 Hz, 1H, L ArH), 7.35
(d, 8 Hz, 2H, tolyl ArH), 8.03 (d, 8 Hz, 2H, tolyl ArH). 13C{1H}
NMR (CD2Cl2, 258 K): δ 21.69, 21.84, 29.52, 29.88, 31.71, 31.80,
34.38, 34.51, 34.80, 35.43, 60.92, 61.77, 97.76, 123.43, 123.65,
124.02, 124.30, 125.18, 126.52, 127.80, 128.03, 129.48, 129.73,
134.05, 134.47, 134.73, 135.63, 140.77, 141.97, 143.43, 143.69,
158.66 (br), 160.59, 181.33, 186.43. IR: 1609 (w), 1589 (m), 1548
(s), 1520 (s), 1489 (s), 1318 (m), 1184 (m), 1062 (m), 1018 (w),
784 (m), 694 (w), 634 (m), 603 (s). FAB-MS: 967 (M+). Anal.
Calcd for C62H81NO5Ti: C, 76.91; H, 8.43; N, 1.45. Found: C,
77.12; H, 8.27; N, 1.52.
t
t
t
(s, 18H, Bu), 1.25 (s, 27H, Bu), 1.57 (s, 9H, Bu), 3.72 (broad s,
2H, NCH2Ar trans to CHO), 3.75 (br d, 13.4 Hz, 2H, NCHH′Ar
cis to CHO), 4.08 (br d, 13.4 Hz, 2H, NCHH′Ar cis to CHO), 7.02
(sl br s, 3H, ArH), 7.09 (sl br s, 2H, ArH), 7.24 (br s, 1H, ArH),
7.49 (d, 1.9 Hz, 1H, cresol ArH), 8.01 (d, 1.9 Hz, 1H, cresol ArH),
9.15 (s, 1H, bound CHO), 10.82 (s, 1H, free CHO). 13C{1H} NMR
(CD2Cl2, 273 K): δ 20.27, 29.76, 30.08, 31.86, 34.58, 34.65, 35.01,
35.54, 61.56, 123.71, 123.79, 124.44, 124.98, 125.61, 126.44,
126.54, 128.70, 135.05, 135.62, 139.13, 141.57, 142.98, 143.17,
158.32, 160.83, 167.33, 189.92, 194.05. IR: 1691 (m), 1638 (s),
1552 (m), 969 (w), 840 (m). EI-MS: 879 (M+).
[Nitrilotris(3,5-di-tert-butyl-2-cresolato)](8-oxyquinolinato)-
titanium(IV), LTi(hq). To a solution of 200 mg of LTi(OtBu)
(0.253 mmol) in 20 mL of benzene was added solid 8-hydroxy-
quinoline (50 mg, 0.342 mmol, 1.35 equiv). The solution was
refluxed for 2 h, with the color changing from yellow to orange
after a few minutes at reflux. The volatiles were removed on a
rotary evaporator. The orange oil was redissolved in benzene
(7 mL) and allowed to reflux for an additional hour to ensure that
all of the LTi(OtBu) had reacted. The solvent was again evaporated,
and the orange oil was dissolved in 50 mL of hexane and was
washed with CH3CN (3 × 20 mL) to remove unreacted 8-hydroxy-
quinoline. The hexane layer was placed on a rotary evaporator,
and the solvent was removed to leave an orange solid. The solid
was recrystallized by dissolving it in a minimum of benzene, and
layering the solution with 20 mL of CH3CN. Orange crystals, which
were suitable for X-ray analysis, formed within 24 h. The crystals
were filtered, washed with CH3CN (3 × 5 mL), and air-dried on a
glass frit to give 179 mg (82%) LTi(hq). 1H NMR (CD2Cl2):
δ 1.08 (s, 27H, tBu), 1.28 (s, 27H, tBu), 3.78 (br s, 6H, NCH2Ar),
6.82 (dd, 4.8, 1.4 Hz, 1H, hq H-2), 6.92 (dd, 8.2, 4.8 Hz, 1H, hq
H-3), 7.00 (dd, 7.8, 1.2 Hz, 1H, hq H-4), 7.06 (d, 2.5 Hz, 3H, ArH),
7.13 (d, 2.5 Hz, 3H, ArH), 7.16 (dd, 8.2, 1.2 Hz, 1H, hq H-5),
7.48 (dd, 8.2, 7.8 Hz, 1H, hq H-6), 8.02 (dd, 8.2, 1.4 Hz, 1H, hq
H-7). 13C{1H} NMR (C6D6): δ 30.50, 32.35, 34.82, 35.61, 61.53,
113.24, 116.27, 120.84, 124.32, 124.66, 125.32, 129.97, 130.32,
136.61, 137.54, 141.96, 144.43, 144.51, 160.45, 162.35. IR: 1601
(w), 1580 (w), 1499 (m), 1470 (s), 1320 (w), 1106 (m), 822 (w),
784 (w), 744 (m), 635 (w). FAB-MS: 860 (M+).
1,5-Dioxyanthraquinonebis{[nitrilotris(3,5-di-tert-butyl-2-
cresolato)]titanium(IV)}, (LTi)2(anruf). 200 mg of LTi(OtBu)
(0.253 mmol) and 32 mg of anthrarufin (0.133 mmol, 0.53 equiv)
were dissolved in 20 mL of benzene. The solution was refluxed
for 2 h, changing color from yellow to orange to red in a few
minutes. The volatiles were removed on a rotary evaporator to leave
behind a red solid. The solid was redissolved in hexane (10 mL),
and 50 mL of acetone was added to the solution. The solution was
placed in a -20 °C freezer for 2 days after which small red crystals
had precipitated. The solid was recrystallized by dissolving it in a
minimum of benzene and layering with 20 mL of CH3CN. After
24 h, the red crystals of (LTi)2(anruf) were filtered on a glass frit,
washed with CH3CN (3 × 5 mL), and air-dried. Yield: 102 mg
(48%). 1H NMR (CD2Cl2, 273 K; the molecule has ∼C2h symmetry
on the NMR time scale at this temperature): δ 0.86 (s, 36H, tBu),
t
1.25 (s, 54H, Bu), 1.56 (s, 18H, t-Bu), 3.70 (br s, 4H, NCH2Ar
trans to quinone), 3.76 (br s, 4H, NCHH′Ar cis to quinone), 4.03
(br s, 4H, NCHH′Ar cis to quinone), 6.31 (dd, 7.4, 1.3 Hz, 2H,
anruf 4,8-H), 6.98 (sl br s, 4H, ArH), 7.03 (sl br s, 6H, ArH), 7.12
(dd, 8.6, 1.3 Hz, 2H, anruf 2,7-H), 7.26 (br s, 2H, ArH), 7.32
(dd, 8.6, 7.4 Hz, 2H, anruf 3,6-H). 13C{1H} NMR (C6D6, 323 K):
δ 30.56, 32.40, 34.85, 35.60, 61.94, 120.43, 120.98, 124.08, 124.54,
125 (very broad), 129.19, 134.49, 136.09 (broad), 137.96, 141.99
(broad), 159.9 (very broad), 168.56, 184.16. IR: 1611 (s), 1585
(s), 1532 (w), 1469 (m), 1330 (w), 1094 (w), 1060 (w). FAB-MS:
1672 (M + H). Anal. Calcd for C104H138N2O10Ti2: C, 74.71; H,
8.32; N, 1.68. Found: C, 75.09; H, 8.45; N, 1.62.
[Nitrilotris(3,5-di-tert-butyl-2-cresolato)](2-formylphenoxo)-
titanium(IV), LTi(sal), was prepared by the same method as the
hydroxyquinolinate complex, using 200 mg of LTi(OtBu)
(0.253 mmol) and 0.5 mL of salicylaldehyde (4.7 mmol, 19 equiv),
1
yielding 184 mg (87%) of LTi(sal) as orange crystals. H NMR
t
t
(C6D6): δ 1.38 (s, 27H, Bu), 1.48 (br, 27H, Bu), 3.79 (br s, 6H,
NCH2Ar), 6.30 (ddd, 7.8, 7.0, 0.9 Hz, 1H, sal 4-H), 6.48 (dd, 7.8,
1.8 Hz, 1H, sal 3-H), 6.89 (dd, J ) 8.5, 0.9 Hz, 1H, sal 6-H), 7.00
(d, 2.4 Hz, 3H, ArH), 7.04 (ddd, J ) 8.5, 7.0, 1.8 Hz, 1H, sal
5-H), 7.46 (d, 2.4 Hz, 3H, ArH), 8.01 (s, 1H, CHO). 13C{1H} NMR
(CD2Cl2): δ 30.12, 32.04, 34.73, 35.32, 61.86, 119.54, 120.28,
Di-µ-(dibenzylphosphato)bis[nitrilotris(3,5-di-tert-butyl-2-
cresolato)]dititanium(IV), {LTi[O2P(OCH2Ph)2]}2. To a solution
of 200 mg of LTi(OtBu) (0.253 mmol) dissolved in 20 mL of
dichloromethane was added dibenzyl phosphate (75 mg,
0.270 mmol, 1.07 equiv), causing an immediate color change from
yellow to red. The solution was stirred for 1 h at room temperature,
Inorganic Chemistry, Vol. 44, No. 8, 2005 2805

Fortner et al.
and the volatiles were removed on a rotary evaporator to give a
reddish-orange solid, which was mixture of {LTi[O2P-
aromatic resonances. It was assumed that this process also
contributes to the rate of arm interchange, and so this rate
(multiplied by a statistical factor of /3) was subtracted from the
measured overall rate of arm interchange to give the rate constant
for direct (nondissociative) arm interchange.
a
(OCH2Ph)2]}2 and unreacted LTiOtBu as determined by NMR. The
solid was dissolved in a minimum of hexane and loaded onto a
silica gel column. The column was eluted with hexane, and the
fast-moving reddish-orange band was collected. The compound was
then recrystallized by dissolving it in a minimum of dichlo-
romethane and layering with CH3CN (20 mL). Red crystals, which
were suitable for X-ray analysis, formed within 24 h. The crystals
were filtered, washed with CH3CN (3 × 5 mL), and dried on a
glass frit to yield 91 mg (36%) of {LTi[O2P(OCH2Ph)2]}2. 1H NMR
4
X-ray Crystallography of LTi(trop), LTi(acac), LTi(hq)‚
CH3CN‚0.5C6H6, and LTi(dfc)‚CH2Cl2. Crystals of LTi(trop),
LTi(acac), and LTi(hq) were grown through slow diffusion of
CH3CN into benzene solutions of the complexes; crystals of
LTi(dfc) were grown by diffusion of CH3CN into CH2Cl2. Suitable
crystals were placed in inert oil and transferred to the tip of a glass
fiber in the cold N2 stream of a Bruker Apex CCD diffractometer
(T ) 100 K). Data were reduced, correcting for absorption and
decay, using the program SADABS. The space groups of the
monoclinic crystals were determined by their systematic absences;
the hydroxyquinolinate crystal was triclinic and was solved and
refined successfully in the space group P1h. The titanium atoms in
LTi(trop) and LTi(dfc)‚CH2Cl2 were found on Patterson maps;
LTi(acac) and LTi(hq)‚CH3CN‚0.5C6H6 were solved using direct
methods. Remaining nonhydrogen atoms were found on difference
Fourier maps. All heavy atoms were refined anisotropically, and
hydrogens were placed in calculated positions. All of the calcula-
tions used SHELXTL (Bruker Analytical X-ray Systems), with
scattering factors and anomalous dispersion terms taken from the
literature.15 Further details about the individual structures may be
found in Table 1.
t
t
(C6D6): δ 1.19 (s, 18H, Bu), 1.41 (s, 36H, Bu), 1.45 (s, 36H,
tBu), 1.50 (s, 18H, Bu), 3.10 (d, 13.5 Hz, 4H, NCHH′Ar cis to
t
phosphate), 3.15 (s, 4H, NCH2Ar trans to phosphate), 4.91
(d, 13.5 Hz, 4H, NCHH′Ar cis to phosphate), 5.20, 5.27 (dd, 12.3,
6.5 Hz, 4H ea., POCHH′Ph), 6.22 (d, 2.5 Hz, 2H, ArH),
6.83-7.13 (m, 26 H, ArH (6H), POCH2Ph (20 H)), 7.33
(d, 2.5 Hz, 4H, ArH). 13C{1H} NMR (CD2Cl2): δ 30.94, 31.24,
31.84, 32.12, 34.20, 34.62, 35.22, 35.26, 60.47, 66.35, 68.82
(d, 5.5 Hz), 123.14, 123.65, 124.01, 126.08, 126.38, 127.36, 128.06,
128.66, 131.26, 134.52, 137.43, 137.55, 141.01, 141.23, 160.25,
161.80. 31P NMR (CD2Cl2): δ -11.93 (quintet, JPH ) 6.5 Hz).
IR: 1456 (m), 1391 (w), 1200 (s), 1053 (m), 1026 (m), 843 (m),
695 (w). FAB-MS: 1986 (M - H), 994 (LTi(O2P[OCH2Ph]2) +
H). The analytical sample retained half a mole of dichloromethane.
Anal. Calcd for C118.5H161ClN2O14P2Ti2: C, 70.09; H, 7.99; N, 1.38.
Found: C, 69.76; H, 7.76; N, 1.47.
X-ray Crystallography of {LTi[O2P(OCH2Ph)2]}2‚4CH2Cl2.
Crystals of {LTi[O2P(OCH2Ph)2]}2 were grown by diffusion of
CH3CN into CH2Cl2. Data acquisition and reduction were as
described above. The systematic absences exhibited by the mono-
clinic crystal were consistent with the space group C2/c, except
that a significant number of the reflections expected to be absent
due to the c glide were observed with I > 3σ(I), although these
reflections were generally weak (avg I/σ(I) ) 3.3). The structure
was successfully solved and refined in C2/c, with the dimeric
titanium complex on the crystallographic inversion center. Solution
of the structure ignoring the presence of the glide plane (space group
C2) was possible, but refinement was poorly behaved, with many
atoms unable to be refined anisotropically. There was substantial
disorder in the structure. One of the tert-butyl groups (methyls
bonded to C120) was disordered over two different orientations
(major orientation ) 74.7(7)%). One of the benzyloxy groups was
also found in two different orientations, with the major orientation
refining to 58.3(3)% occupancy. The phenyl groups in this
disordered benzyloxy group were modeled as rigid hexagons. One
dichloromethane molecule was found in two orientations, one of
which was unreasonably close to one of the benzyloxy conformers,
so the occupancies of the two solvent orientations were fixed to
the complementary benzyloxy occupancies. Thermal parameters for
one of the remaining dichloromethanes indicated partial occupancy,
and it was arbitrarily refined at half occupancy. All non-hydrogen
atoms were refined anisotropically, and hydrogen atoms were placed
in calculated positions.
Variable-Temperature NMR Studies. Samples were prepared
by dissolving 10-20 mg of the desired compound in 0.5 mL of
CD2Cl2 in a drybox in a 5-mm NMR tube with a screw cap. Spectra
were acquired on a Varian VXR-500 NMR spectrometer at
temperatures between -100 and +40 °C. Rate constants for the
dynamic processes were determined by full line-shape analysis using
the program gNMR.14 The rate of the twisting (racemization)
process was generally analyzed using the exchange of the two most
upfield tert-butyl resonances (ortho to the phenoxides on the
mutually trans arms of the tripod ligand). For LTi(dtm), the
exchange of the resonances due to the tert-butyl groups para to the
phenoxides was also simulated, and the rate constants were
averaged. For LTi(trop), arm interchange was rapid at all temper-
atures studied, making this procedure impossible, so for this
compound the racemization rate constant was obtained by simulat-
ing the exchange of axial and equatorial hydrogens on the CH2
groups. The rate of arm interchange was generally measured using
the exchange of the resonances of the tert-butyl groups ortho to
the phenoxides on the unique arm (trans to the bidentate chelate)
with those on the two other arms. (The chemical shift dispersion
of the para tert-butyl groups was invariably too small for these
resonances to be useful.) For LTi(dtm), the arm interchange rate
was determined to be the same, within experimental error, as the
rate of interchange of the tolyl methyl groups at all temperatures
at which they were measured, and both values were averaged in
the determination of the activation parameters for the reaction. For
LTi(acac), it was not possible to measure the arm interchange rate
by analysis of the tert-butyl region, so the value measured by
interchange of the acac methyl groups was used. In LTi(hq), arm
interchange and twisting occur at comparable rates, and the tert-
butyl region was successfully simulated with simultaneous deter-
mination of both rate constants. The rate of exchange of the bound
and free aldehydes in LTi(dfc) was measured by simulating the
exchange of the two formyl CHO and the two diformylcresolate
Results
Synthesis and Structure of Bidentate Chelates of
Nitrilotris(3,5-di-tert-butyl-2-cresolato)titanium(IV). A five-
coordinate titanium complex of triply deprotonated tris-
(2-hydroxy-3,5-di-tert-butylbenzyl)amine), LTi(OiPr), was
reported by Kol and co-workers.6 The analogous light yellow
(14) Budzelaar, P. H. M. gNMR v.3.6.5; Cherwell Scientific Publishing:
(15) International Tables of Crystallography; Kluwer Academic Publish-
Oxford, 1996.
ers: Dordrecht, The Netherlands, 1992; Vol. C.
2806 Inorganic Chemistry, Vol. 44, No. 8, 2005

Titanium Complexes of a Tripodal Aminetris(phenoxide) Ligand
Table 1. X-ray Crystallographic Details for LTi(trop), LTi(acac), LTi(hq)‚CH3CN‚0.5C6H6, LTi(dfc)‚CH2Cl2, and {LTi[O2P(OCH2Ph)2}2‚4CH2Cl2
LTi(hq)‚
CH3CN‚0.5C6H6
LTi(dfc)‚
CH2Cl2
{LTi[O2P(OCH2Ph)2]}2‚
LTi(trop)
LTi(acac)
4CH2Cl2
empirical formula
fw
C52H71NO5Ti
838.00
C50H73NO5Ti
815.99
C59H78N3O4Ti
941.14
C55H75Cl2NO6Ti
964.96
C122H168Cl8N2O14P2Ti2
2327.92
temp (K)
λ, Å
100(2)
100(2)
100(2)
0.71073
100(2)
100(2)
0.71073
(Mo KR)
C2/c
64 921
15 322
0.0503
0.71073
(Mo KR)
P21/c
51 287
12 059
0.71073
(Mo KR)
P21/c
51 108
12 054
0.71073
(Mo KR)
P21/n
47 640
13 009
(Mo KR)
space group
total data collected
no. of indep reflns
Rint
P1h
19 331
9283
0.0468
0.0317
0.0279
0.0548
obsd reflns [I > 2σ(I)]
10 329
10 401
6657
9711
12 120
a (Å)
23.5527(12)
11.1270(5)
19.8941(10)
90
111.2940(10)
90
4857.7(4)
4
1.146
14.4659(7)
11.9192(6)
28.9215(15)
90
103.0750(10)
90
4857.4(4)
4
1.116
12.6377(8)
14.5709(9)
16.8667(10)
81.1100(10)
69.2120(10)
65.4030(10)
2640.1(3)
2
20.6770(9)
11.7837(5)
22.4263(10)
90
106.3570(10)
90
5243.0(4)
4
1.222
27.0249(18)
13.5399(9)
34.491(2)
90
101.6600(10)
90
12 360.2(14)
4
1.251
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
calcd F (g/cm3)
cryst size (mm)
1.184
0.48 × 0.18 × 0.09
0.43 × 0.28 × 0.18
0.30 × 0.15 × 0.15
0.27 × 0.10 × 0.08
0.40 × 0.30 × 0.30
µ (mm-1
)
0.221
0.219
0.210
0.314
0.387
no. refined params
532
518
605
586
725
R indices [I > 2σ(I)]a
R1 ) 0.0426
wR2 ) 0.1096
R1 ) 0.0507
wR2 ) 0.1155
1.018
R1 ) 0.0429
wR2 ) 0.1167
R1 ) 0.0500
wR2 ) 0.1234
1.019
R1 ) 0.0475
wR2 ) 0.1054
R1 ) 0.0754
wR2 ) 0.1189
1.005
R1 ) 0.0817
wR2 ) 0.2183
R1 ) 0.1070
wR2 ) 0.2438
1.012
R1 ) 0.0966
wR2 ) 0.2617
R1 ) 0.1145
wR2 ) 0.2758
1.076
R indices (all data)a
GOF
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) (∑[w(Fo - Fc )2]/∑w(Fo )2)1/2
.
2
2
2
titanium(IV) tert-butoxide LTi(OtBu)11 reacts rapidly with
the symmetrical, bidentate enolic ligands tropolonate (trop),
acetylacetonate (acac), and di-p-toluoylmethanide (dtm) in
their protonated forms. An equivalent of tert-butyl alcohol
is liberated and chelated, six-coordinate complexes of the
LTi(IV) moiety are produced in good yield (eq 1).
LTi moiety is bonded to each phenoxy group. A 1:1 complex,
LTi(Hanruf), can be observed by NMR in reactions using a
1:1 mixture of LTi(OtBu) and H2anruf, but it is always mixed
with starting material and (LTi)2(anruf) and was not isolated
in pure form.
The structures of four of these complexes, LTi(trop)
(Figure 1), LTi(acac) (Figure 2), LTi(hq) (Figure 3), and
LTi(dfc) (Figure 4), have been determined by X-ray crystal-
lography (Tables 1 and 2). The structures of all four
compounds are quite similar and have several noteworthy
features. The complexes are roughly octahedral with the
bidentate ligand occupying both an axial position (trans to
the central amine nitrogen of the nitrilotricresolate ligand)
Phenols with pendant donors, such as 8-hydroxyquinoline
(Hhq), salicylaldehyde (Hsal), and 2,6-diformyl-p-cresol
(Hdfc), react similarly, but more slowly, with reactions going
to completion within several hours in refluxing benzene or
dichloromethane (eq 2). Anthrarufin (1,5-dihydroxyan-
thraquinone, H2anruf) reacts analogously with 2 equiv of
LTi(OtBu) to give the complex (LTi)2(anruf) where an
Inorganic Chemistry, Vol. 44, No. 8, 2005 2807

Fortner et al.
Figure 1. SHELXTL plot (40% thermal ellipsoids) of LTi(trop).
Figure 3. SHELXTL plot (40% thermal ellipsoids) of LTi(hq).
Figure 4. SHELXTL plot (40% thermal ellipsoids) of LTi(dfc).
Figure 2. SHELXTL plot (40% thermal ellipsoids) of LTi(acac).
chelate pointed at it and the other pointed away. This
generates a significant steric clash between the bidentate
chelate and the aryloxide, which is partially relieved by tilting
of the plane of the bidentate ligand out of the Ti-N-O2
plane, away from the intruding aryloxide (by 6.6°, 18.5°,
22.5°, and 17.1° in the four structures).
The inequivalence of the two groups bound trans to the
bidentate chelate (neutral nitrogen N and phenoxide oxygen
O2) generates asymmetry in the binding of the nominally
symmetric tropolonate and acetylacetonate ligands. As
expected, the greater trans influence of the phenoxide
significantly elongates the equatorial Ti-O5 bond relative
to the axial Ti-O4 bond (by 0.1330(14) Å in the tropolonate
and 0.1355(14) Å in the acetylacetonate). The asymmetry
in Ti-O distances is reflected in perceptible bond alternation
and an equatorial position (trans to one of the tripodal
phenoxide ligands). While these are the first examples of
octahedral titanium complexes with the nitrilotris(cresolate)
ligand,6-7,11 the ability of such ligands to support octahedral
geometries is well-established in tantalum complexes of the
related N(CH2C6H2Me2O)3 ligand.8 That the tripodal ligand
experiences some strain in achieving an octahedral geometry
is indicated by the relatively acute “trans” angle O1-Ti-
O3 (159° avg in the four complexes), similar to the angles
seen in the tantalum complexes.8 However, in contrast to
the tantalum complexes, the tripodal ligand retains the
pseudo-C3 propeller-like conformation it adopts in the
trigonal bipyramidal LTiX complexes in these octahedral
complexes. The complexes thus have C1 symmetry, with the
phenyl group of one of the aryloxides cis to the bidentate
2808 Inorganic Chemistry, Vol. 44, No. 8, 2005

Titanium Complexes of a Tripodal Aminetris(phenoxide) Ligand
Table 2. Selected Bond Distances (Å) and Angles (deg) for LTi(trop), LTi(acac), LTi(hq)‚CH3CN‚0.5C6H6, LTi(dfc)‚CH2Cl2, and
{LTi[O2P(OCH2Ph)2}2‚4CH2Cl2
LTi(trop)
LTi(acac)
LTi(hq)‚CH3CN‚0.5C6H6
LTi(dfc)‚CH2Cl2
{LTi[O2P(OBn)2]}2‚4CH2Cl2
Ti-O1
Ti-O2
Ti-O3
Ti-N
Ti-O4
Ti-O5
Ti-N1
1.8974(10)
1.8259(9)
1.8746(10)
2.2386(11)
1.9643(9)
2.0973(10)
1.8811(10)
1.8382(10)
1.8714(10)
2.2518(11)
1.9452(10)
2.0807(10)
1.911(2)
1.829(2)
1.864(2)
2.272(2)
1.913(2)
1.867(2)
1.828(2)
1.882(2)
2.245(3)
1.928(2)
2.161(2)
1.877(2)
1.870(2)
1.875(2)
2.249(3)
1.964(3)
2.055(3)
2.346(2)
O1-Ti-O2
O1-Ti-O3
O2-Ti-O3
N-Ti-O1
N-Ti-O2
N-Ti-O3
N-Ti-O4
N-Ti-O5
N-Ti-N1
O1-Ti-O4
O2-Ti-O4
O3-Ti-O4
O1-Ti-O5
O2-Ti-O5
O3-Ti-O5
O1-Ti-N1
O2-Ti-N1
O3-Ti-N1
O4-Ti-O5
O4-Ti-N1
Ti-O4-P
94.39(4)
161.03(4)
96.42(4)
84.21(4)
87.93(4)
80.66(4)
170.82(4)
94.70(4)
94.70(4)
160.35(4)
97.95(4)
84.53(4)
87.33(4)
81.12(4)
173.12(4)
89.71(4)
94.38(7)
158.28(7)
99.82(7)
84.31(7)
87.60(7)
79.99(7)
170.07(7)
98.31(10)
157.50(10)
97.30(10)
84.22(9)
86.82(10)
80.57(9)
88.63(10)
166.52(11)
93.69(11)
84.99(10)
86.89(11)
81.88(11)
175.90(15)
87.90(15)
173.33(10)
89.76(9)
98.19(7)
102.44(7)
99.01(7)
91.53(7)
98.10(4)
100.71(4)
95.15(4)
85.68(4)
177.37(4)
84.22(4)
99.03(4)
98.19(4)
93.99(4)
83.42(4)
176.64(4)
83.15(4)
98.47(10)
98.78(10)
95.04(10)
82.91(9)
176.22(9)
80.55(9)
94.73(12)
97.19(15)
98.15(12)
87.94(11)
174.00(14)
88.53(12)
82.16(7)
172.89(7)
85.32(7)
76.68(4)
84.88(4)
84.54(9)
88.00(18)
75.81(7)
165.1(2)
165.1(3)
115.8(3)
Ti-O5-P0A
O4-P-O5A
in the bidentate chelate, which is larger in the acetylacetonate
(0.032 Å avg) than in the tropolonate (0.016 Å avg). The
unsymmetrical chelates 8-hydroxyquinolinate and diformyl-
cresolate reflect the trans influence of the tripod in their
observed regioisomers, with the phenoxide group apical and
the neutral donor equatorial. In solution, only one regioisomer
of all of the unsymmetrical chelates is observed, and by
comparison to these two structures it is presumed to be the
one with the phenoxide donor apical.
Variable-Temperature NMR Spectroscopy of Bidentate
Chelates of Nitrilotris(3,5-di-tert-butyl-2-cresolato)ti-
tanium(IV). 1H NMR spectra of the bidentate chelate
complexes are consistent with the retention in solution of
the chiral, C1-symmetric structures exhibited in the solid state,
as illustrated for LTi(dtm) (Figure 5) and LTi(dfc) (Figure
6). At low temperatures, the spectra generally exhibit six
2 Hz doublets due to the aryl protons of the tripod, six
doublets with coupling constants of 13-15 Hz for the
diastereotopic methylene protons, and six singlets for the tert-
butyl resonances, as well as the resonances due to the
bidentate chelate. As the temperature is raised, two fluxional
processes are generally observed. At temperatures between
about -90 and -60 °C, there is a process that interconverts
two of the aryl environments of the tripod, but leaves the
third set of aryl environments unaffected. (This process also
interconverts the axial and equatorial hydrogens of the NCH2
groups, although the line shape in the methylene region is
usually complex and was not analyzed in detail.) This process
corresponds to a twisting of the tripodal ligand’s benzyl
groups, which inverts the helical sense of the tripod but does
not affect the first coordination sphere of titanium. This
motion establishes an effective time-averaged mirror plane
containing the bidentate ligand, the central tertiary amine,
and the tripod phenoxide trans to the bidentate chelate; it
renders the two tripod phenoxides trans to each other
equivalent.
A second fluxional process, usually seen at higher tem-
peratures (above ∼0 °C for both LTi[dtm] and LTi[dfc]),
renders all of the phenoxide groups of the tripod equivalent.
At the same time, in LTi(dtm), the resonances for the two
tolyl groups of the ditoluoylmethane ligand also broaden and
coalesce as the two ends of the bidentate ligand exchange.
The other complexes of symmetrical bidentate ligands show
the same qualitative behavior. Although the rate of arm
exchange in LTi(acac) could not be measured quantitatively,
it occurs in the same temperature range as acac methyl group
exchange, and both arm exchange and ligand end exchange
are too fast to measure in LTi(trop). (This kind of exchange
is of course impossible for the unsymmetrical chelates such
as hydroxyquinoline or salicylaldehyde.)
Quantitatively, the rates of exchange in LTi(dtm) measured
from the tolyl methyl groups and from the tert-butyl groups
are identical within experimental error (Figure 7). This is
consistent with a motion that slides the bidentate ligand from
one cleft in the tripod to another with the bidentate ligand
moving like a windshield wiper blade, such that each time
the ligand switches clefts in the tripod it also interchanges
ligand ends (Scheme 1a). One could also imagine a motion
where the bidentate ligand ends and its position in the tripod
clefts were interchanged statistically, for example by moving
the bidentate ligand so that its two-fold axis coincides with
the tripod’s three-fold axis, followed by mutual rotation of
Inorganic Chemistry, Vol. 44, No. 8, 2005 2809

Fortner et al.
1
Figure 6. Variable-temperature H NMR spectra (CD2Cl2, 500 MHz) of
LTi(dfc), with temperatures in °C. The region from 2.5 to 7.5 ppm (which
includes the CH2 resonances) is omitted. The vertical scale of the upfield
(tert-butyl and methyl) region is reduced relative to the downfield region,
but the horizontal scale is constant in the plot.
1
Figure 5. Variable-temperature H NMR spectra (CD2Cl2, 500 MHz) of
LTi(dtm). Temperatures are given in °C.
the two groups around their common axis before the
bidentate ligand collapses to its equilibrium position (Scheme
1b). However, because each such “sit-and-spin” motion
would result in a change in tripod orientation two-thirds of
the time but a change in bidentate ligand only one-half of
the time, such a motion would predict a kMe/karm ratio of 3/4,
which is clearly excluded by the data.
The activation parameters for ligand flexing and for tripod
arm exchange for the seven complexes, derived from Eyring
plots of the rate data (Figures S2 and S3), are compiled in
Table 3. While the pattern described above is typical, there
are a few noteworthy deviations. Most significantly, the
tropolonate ligand in LTi(trop) is symmetrical by NMR, and
all of the aryl groups of the tripod are equivalent, even at
-100 °C, suggesting that the arm interchange process is
extremely fast. However, the methylene protons decoalesce
at low temperatures into a pair of mutually coupled doublets
(Figure S1), allowing the rate of ligand racemization to be
measured. The fact that arm interchange is much faster than
ligand twisting in this complex, but comparable in rate in
LTi(hq), and much slower in the other complexes, indicates
that the two mechanisms operate independently of one
another. In the anthrarufin complex (LTi)2(anruf), the ligand
twisting motion is complicated by the presence of two
diastereomers (a meso and dl pair, depending on the relative
configuration of the two tripodal ligands), which can be
discerned at low temperature by the presence of two distinct
Figure 7. Ratio of rate constants measured in CD2Cl2 for tolyl methyl
group exchange (kMe) and for tripod arm tert-butyl group exchange (karm
)
in LTi(dtm) as a function of temperature. The solid line indicates the ratio
predicted by synchronous rearrangement (Scheme 1a), and the dashed line
indicates that predicted by statistical rearrangement (Scheme 1b).
peaks for the 2,6-H resonances of the anthrarufin (δ 6.04
and 5.83, in a 2:1 ratio, at -100 °C). Qualitatively, the
twisting motion leads to spectral changes, ultimately averag-
ing the two diastereomers and the mutually trans phenoxides
within each diastereomer, between about -90 and -60 °C,
but the rates were not measured quantitatively due to the
complexity of the spectra. The rate of the twisting motion
was not determined for LTi(acac) due to poor chemical shift
dispersion of the tert-butyl resonances.
Finally, the latent symmetry of the 2,6-diformyl-p-cresolate
ligand allows one to address the possibility of dissociation
of the neutral aldehyde donor, which would allow exchange
2810 Inorganic Chemistry, Vol. 44, No. 8, 2005

Titanium Complexes of a Tripodal Aminetris(phenoxide) Ligand
Scheme 1. Possible Intramolecular Mechanisms for Tripod Arm Exchange in Six-Coordinate Complexes: (a) Concerted, or “Ballistic”, Mechanism;
(b) Statistical, or “Sit-and-Spin”, Mechanism, Characterized by a Long-Lived Nonoctahedral Intermediate
Table 3. Activation Parameters for Fluxional Processes in LTiX Complexes (Except As Noted, All Data Were Obtained in CD2Cl2, and All ∆Gq
Values Are Reported at 300 K; na ) Not Applicable)
apical ligand
OiPra
racemization
arm interchange
dissociation
∆Gq ) 17.8 kcal/molb
na
na
na
tropolonate
∆Hq ) 10.7(6) kcal/mol
∆Sq ) +7(3) eu
too fast to measure at -100 °C
∆Gq ) 8.6(11) kcal/mol
8-hydroxyquinolinate
acetylacetonate
salicylaldehyde
anthrarufin
∆Hq ) 8.46(17) kcal/mol
∆Sq ) -10.2(8) eu
∆Hq ) 11.58(7) kcal/mol
∆Sq ) +4.4(3) eu
na
na
na
na
∆Gq ) 11.5(3) kcal/mol
∆Gq ) 10.26(11) kcal/mol
not measured
∆Hq ) 7.65(20) kcal/mol
∆Sq ) -11.9(9) eu
∆Gq ) 11.2(3) kcal/mol
∆Hq ) 7.14(18) kcal/mol
∆Sq ) -11.3(9) eu
∆Hq ) 12.46(10) kcal/mol
∆Sq ) +2.7(4) eu
∆Gq ) 10.5(3) kcal/mol
∆Gq ) 11.66(16) kcal/mol
not measured
∆Hq ) 9.57(13) kcal/mol
∆Sq ) -15.1(5) eu
∆Gq ) 14.10(20) kcal/mol
diformylcresolate
di-p-toluoylmethane
∆Hq ) 7.29(23) kcal/mol
∆Sq ) -9.4(12) eu
∆Hq ) 13.6(4) kcal/molc,d
∆Sq ) -2.4(12) euc,d
∆Hq ) 17.0(6) kcal/molc
∆Sq ) +4.8(18) euc
∆Gq ) 10.1(4) kcal/mol
∆Gq ) 14.3(5) kcal/molc,d
∆Gq ) 15.6(8) kcal/molc
∆Hq ) 7.1(3) kcal/mol
∆Sq ) -12.4(16) eu
∆Hq ) 11.52(12) kcal/mol
∆Sq ) -10.3(4) eu
na
∆Gq ) 10.9(6) kcal/mol
∆Gq ) 14.61(17) kcal/mol
a Reference 6. b 386 K, C6D5NO2. c CDCl3. d These values are for the overall rate of arm interchange, which includes a contribution from the dissociative
mechanism. If this contribution is subtracted, the activation parameters for the nondissociative interchange are: ∆Hq
∆Sq ) -6.2(11) eu, ∆Gq ) 14.5(4) kcal/mol.
) 12.6(3) kcal/mol,
between the bound and free aldehydes (as well as the two
aryl protons on the cresolate). At temperatures below about
10 °C, this exchange does not take place at a sufficiently
fast rate to be observable by NMR, but exchange does begin
to set in at higher temperatures (Figure 6). This exchange
takes place at similar temperatures to the arm interchange
process, but close examination of the spectra clearly shows
that the aldehyde dissociation is noticeably slower than arm
interchange. This is most evident in the 0 and 15 °C spectra
in Figure 6, where the tripod aryl (and tert-butyl) resonances
have already begun to show significant broadening, while
the cresolate aryl and aldehyde resonances remain rather
sharp. The activation parameters of the two processes are
also quite different, with the dissociative pathway exhibiting
Inorganic Chemistry, Vol. 44, No. 8, 2005 2811

Fortner et al.
influence of the aryloxide and amine again differentiating
the axial and equatorial titanium-phosphate bonds
(1.964(3) vs 2.055(3) Å). The Ti-O-P angles are rather
obtuse (165°), which may contribute to the preference for a
bridged rather than a chelated structure, as may the need to
form a four-membered ring to chelate. The acetate complex
LTi(OAc) is monomeric and nonchelating in the solid state,16
and the trifluoroacetate complex LTi(O2CCF3) has also been
inferred to be monodentate on the basis of IR spectroscopy.11
The central eight-membered ring appears rather flat
(rms deviation from the plane of 0.070 Å), but this may be
due to unresolved disorder of the phosphate oxygen atoms
between alternative puckered conformations, as suggested
by the elongated thermal ellipsoids of O4 and O5. Early
metal di-µ-phosphato complexes that are not further bridged
by other ligands are generally puckered,17 although the
observation of widely varying conformations of the eight-
membered ring in similar compounds suggests that the
structure is rather flexible.18
One important difference between the phosphate-bridged
dimer and the four monomeric complexes that have been
characterized crystallographically is the conformation of the
tripodal ligand. While the monomeric complexes all have a
pseudo-C3-symmetric conformation of the tripod, in the
dibenzyl phosphate complex both of the aryloxides cis to
the phosphates are folded back away from phosphate ligands,
thus resembling the conformation shown by the octahedral
tantalum complexes of nitrilotris(cresolate) ligands.8 This
difference in conformation is reflected in the NMR behavior
of {LTi[O2P(OCH2Ph)2]}2, which shows time-average C2h
symmetry at all temperatures from -100 °C to room
temperature. Thus, while the racemization of the ligand
conformation can be frozen out in all of the bidentate
chelates, it is rapid in {LTi[O2P(OCH2Ph)2]}2 at all temper-
atures examined, and while arm exchange is discernible in
all of the bidentate complexes at or below room temperature,
it is not observed in {LTi[O2P(OCH2Ph)2]}2.
Figure 8. SHELXTL plot (40% thermal ellipsoids) of the titanium complex
in {LTi(µ-O2P[OCH2Ph]2)}2‚4CH2Cl2. Phenyl groups of the dibenzyl
phosphates and tert-butyl methyl groups have been omitted for clarity.
a higher enthalpy of activation and more positive entropy
of activation, as expected (Table 3, Figure S4). Dissociation
of the neutral aldehyde presumably leads to a five-coordinate
complex in which the arms of the tripod are equivalent.
However, even when this contribution to the overall rate of
tripod arm interchange is subtracted, the rate of nondis-
sociative arm interchange exceeds the rate of aldehyde
dissociation by a factor of 3-10 in the temperature range
where both can be measured. Thus, aldehyde dissociation is
only a minor contributor to arm interchange in LTi(dfc).
Preparation, Structure, and Dynamics of the Dimeric
Dibenzyl Phosphate Complex {LTi[O2P(OCH2Ph)2]}2.
The titanium tert-butoxide complex LTi(OtBu) reacts readily
with dibenzyl phosphate to form a complex of the empirical
formula LTi[O2P(OCH2Ph)2] (eq 3). In contrast to the
complexes with apical ligands that form five- or six-
membered chelate rings discussed above, the dibenzyl
phosphate group does not chelate to the titanium. Instead,
the dibenzyl phosphate groups bridge two LTi centers,
forming a dimeric complex. The dimeric nature of the
product is suggested by observation of a prominent peak at
m/z ) 1986 in the complex’s FAB mass spectrum and is
confirmed by X-ray crystallography (Figure 8).
The bond distances and angles in the complex are similar
to the monomeric complexes, with the differential trans
(16) Fortner, K. C.; Brown, S. N., unpublished results.
(17) (a) Lugmair, C. G.; Tilley, T. D. Inorg. Chem. 1998, 37, 1821-1826.
(b) Huang, C.-H.; Huang, L.-H.; Lii, K.-H. Inorg. Chem. 2001, 40,
2625-2627. (c) Dean, N. S.; Mokry, L. M.; Bond, M. R.; O’Connor,
C. J.; Carrano, C. J. Inorg. Chem. 1996, 35, 2818-2825. (d) Chang,
Y.-D.; Salta, J.; Zubieta, J. Angew. Chem., Int. Ed. Engl. 1994, 33,
325-327. (e) Chen, Q.; Salta, J.; Zubieta, J. Inorg. Chem. 1993, 32,
4485-4486.
(18) (a) Bond, M. R.; Mokry, L. M.; Otieno, T.; Thompson, J.; Carrano,
C. J. Inorg. Chem. 1995, 34, 1894-1905. (b) Dean, N. S.; Bond, M.
R.; O’Connor, C. J.; Carrano, C. J. Inorg. Chem. 1996, 35, 7643-
7648.
Discussion
Mechanism of Ligand Racemization. The tripodal tita-
nium alkoxide LTi(OtBu) reacts readily with tropolone,
2812 Inorganic Chemistry, Vol. 44, No. 8, 2005

Titanium Complexes of a Tripodal Aminetris(phenoxide) Ligand
8-hydroxyquinolinate (3.8 and 2.0 × 102 s-1 at -50 °C,
respectively) > salicylaldehyde (3.4 × 102 s-1 at -20 °C)
. anthrarufin, diformylcresolate, di-p-toluoylmethanide
(3.9, 2.5, and 1.8 × 102 s-1 at 30 °C, respectively). This
ordering shows no correlation with the electronic properties
of the chelates, but does parallel their sizes. The five-
membered chelates are both on the fast end of the scale,
consistent with their lower steric profile as compared to the
six-membered rings. The diformylcresolate complex and the
anthrarufin complex are each more substituted on one end
of the chelate as compared to the salicylaldehyde complex
and show slowed arm interchange, while di-p-toluoyl-
methanide, which is extended on both ends as compared to
acetylacetonate, imparts a markedly slower rate of arm
exchange.
One plausible mechanism for arm interchange would
involve dissociation of the neutral, equatorial donor of the
bidentate chelate to form a C3-symmetric, five-coordinate
titanium complex. Three lines of evidence disfavor this
mechanism. First, increasing the size of the bidentate ligand
slows arm interchange rather than accelerating it as would
be expected for a dissociative mechanism. Second, the
observation that tripod arm exchange takes place concurrently
with end-to-end exchange of the symmetrical bidentate
ligands (tropolonate and the â-diketonates) is difficult to
reconcile with this mechanism. If dissociation takes place
equatorially, as expected given the pronounced trans influ-
ence observed in these complexes, then reassociation would
(by microscopic reversibility) also take place equatorially,
and tripod arm exchange would be expected to take place
without scrambling the ends of the bidentate ligand. (Partial
dissociation of â-diketonatotitanium(IV) complexes has been
estimated to require ∼25 kcal/mol19 and would thus be
expected to be energetically inaccessible in any case.) Finally,
ligand dissociation can be monitored independently in the
diformylcresolate complex, and it takes place too slowly to
account for the tripod arm exchange. While this is definitive
only for the dfc complex, because this complex has one of
the slowest rates of tripod arm exchange and is expected to
have one of the more weakly bound equatorial ligands, the
conclusion can reasonably be applied to most of the other
complexes.
Scheme 2. Schematic Mechanism of Ligand Racemization in Six-
Coordinate Complexesa
a Only the equatorial plane is shown.
â-diketones, 8-hydroxyquinoline, salicylaldehydes, and an-
thrarufin, with loss of tert-butyl alcohol and formation of
six-coordinate complexes (eqs 1 and 2). In all cases, low-
temperature NMR spectroscopy indicates that the tripodal
ligand maintains the same pseudo-C3-symmetric, propeller-
like conformation observed in the trigonal bipyramidal
complexes.6-7,11 In five-coordinate LTi(OiPr), this conforma-
tion is rather resistant to racemization, with a barrier to
twisting of 17.8 kcal/mol at 113 °C.6 The six-coordinate
complexes are markedly more mobile, with racemization
taking place on the NMR time scale at temperatures below
-60 °C in all cases. Even if the rates are extrapolated to
386 K, the six-coordinate complexes racemize from 103 to
105 times faster than the isopropoxide.
In contrast to the marked difference between the five- and
six-coordinate complexes, the differences among the six-
coordinate species are relatively slight. At -70 °C, where
the rates can all be measured, all of the oxygen-ligated
complexes racemize at rates clustered within a factor of
3 (between 170 and 540 s-1). Only the hydroxyquinolinate
complex falls out of this cluster, and even it racemizes only
an order of magnitude more slowly (krac ) 19 s-1 at
-70 °C). Thus, while the presence of an equatorial ligand
dramatically accelerates the flexing of the ligand backbone,
its nature is of little importance.
One can conclude that the equatorial ligand binds much
more tightly to the transition state for ligand flexing than to
the ground state. In the ground state, one of the mutually
trans aryloxide groups of the tripodal ligand points toward
the equatorial ligand (O1 and its attendant aryl ring in the
structures of the chelated complexes, Figures 1-4). Enhanced
binding of the equatorial ligand might plausibly be due to
an opening of the cleft where it binds when that arm flexes
away (Scheme 2). This explanation requires that twisting the
first arm is rate-determining and that the second arm flips
back and forth rapidly in this high-energy conformation. The
behavior of the µ-phosphato complex {LTi[O2P(OCH2Ph)2]}2
is consistent with this: in this species, the ligand adopts such
an unsymmetrical geometry as its low-energy conformation,
and even at -100 °C the tripodal ligand exhibits local Cs
symmetry in the NMR, indicating that the central phenoxide
arm is indeed rapidly flexing back and forth.
More compatible with the data is an intramolecular, twist-
type mechanism (Scheme 1). Such interconversion of
octahedral structures via trigonal prismatic geometry is very
well precedented, having been documented in titanium(IV)
bis(diketonate) complexes,19,20 in zirconium(IV) diketim-
inates,21 and in a zirconium(IV) complex of a dianionic
tripod.22 The geometric constraints of the tripod limit the
possible trigonal prisms to two essentially enantiomorphous
structures,23 corresponding to whether the bidentate group
(19) Baggett, N.; Poolton, D. S. P.; Jennings, W. B. J. Chem. Soc., Dalton
Trans. 1979, 1128-1134.
(20) Fay, R. C.; Lindmark, A. F. J. Am. Chem. Soc. 1983, 105, 2118-
2127.
(21) Rahim, M.; Taylor, N. J.; Xin, S.; Collins, S. Organometallics 1998,
17, 1315-1323.
(22) Toupance, T.; Dubberley, S. R.; Rees, N. H.; Tyrrell, B. R.; Mountford,
P. Organometallics 2002, 21, 1367-1382.
Mechanism of Ligand Arm Exchange. Unlike race-
mization, arm exchange is extremely sensitive to the nature
of the chelating ligand, with the rates decreasing in the order
tropolonate (too fast to measure at -100 °C) . acac,
Inorganic Chemistry, Vol. 44, No. 8, 2005 2813

Fortner et al.
migrates to the cleft on its right or the one on its left. Passage
through such a structure explains why tripod arm exchange
and exchange of the ends of symmetrical bidentate ligands
occur at the same time. It also explains why increasing bulk
on either end of the bidentate ligand has a similar decelerating
effect on the arm exchange (compare [LTi]2[anruf] or
LTi[dfc] to LTi[sal]). In the octahedral complex, the apical
end of the chelate is in the center of the flanking tert-butyl
groups of the tripod and the equatorial end is below them,
while in the intermediate geometry both ends are in similar
contact with the bulky periphery of the tripod.
The symmetry of this system also allows one to address a
subtle point about this mechanism that has not previously
been amenable to study. In the intermediate geometry, the
tripod has approximately regained its three-fold axis, which
is now more or less coincident with the two-fold (or pseudo-
two-fold) axis of the bidentate ligand. If this structure were
a true intermediate, with a significant lifetime, then one
would expect facile rotation of the two fragments about the
common axis, leading to interconversion of the structures
with the bidentate ligand intercalated in the three possible
clefts, and with either end of the bidentate ligand facing the
clefts (Scheme 1b). Conversely, if the nonoctahedral structure
were a transition state, then the motion of the bidentate ligand
would be “ballistic”, with the bidentate ligand doing an
uninterrupted flip between clefts (Scheme 1a). The experi-
mental data (Figure 7) clearly support the latter mechanism.
These results indicate that the trigonal prismatic geometry
in this system most likely represents the energy maximum
of a transition state, although a lifetime for this geometry
that is finite but too short to permit swiveling of the bidentate
ligand relative to the tripod cannot be ruled out. While
trigonal prismatic titanium(IV) complexes are known,24 they
are rare. The facile geometric interconversions of many
octahedral Ti(IV) complexes imply that prismatic geometries
are energetically accessible. The present study provides the
first evidence that these structures are likely to be maxima,
rather than long-lived alternative structures, on the potential
energy surfaces of many six-coordinate titanium(IV) com-
plexes.
Conclusions
The aminetris(phenoxide)titanium(IV) tert-butoxide com-
plex [N(CH2C6H2-3,5-tBu2-2-O)3]Ti(OtBu) (LTiOtBu) reacts
readily with the protonated forms of bidentate, monoanionic
ligands to form octahedral, six-coordinate complexes. Che-
lates that form five- or six-membered rings form monomeric
complexes, while dibenzyl phosphate forms a phosphate-
bridged dimer. Six-coordination is readily achieved in the
presence of the tripodal ligand, with a substantial barrier to
dissociation of the sixth ligand in the one case in which it
has been observed (∆Hq ) 17.0 kcal/mol for dissociation of
the bound aldehyde in the 2,6-diformyl-p-cresolate complex).
The tripodal ligand in the monomeric complexes displays a
propeller-like conformation, with inversion of the helical
sense of the conformation taking place much more rapidly
than in the five-coordinate LTi(OiPr). This twisting process
is relatively insensitive to the sterics or electronics of the
bidentate ligand and occurs in a stepwise fashion rather than
involving concurrent twisting of all three arms. Motion of
the bidentate ligand between clefts in the tripod, which
exchanges the phenoxide arms cis and trans to the equatorial
donor of the bidentate ligand, also takes place readily, with
rates that increase as the steric profile of the bidentate ligand
decreases. Arm exchange takes place predominantly by a
nondissociative twist mechanism. The nonoctahedral geom-
etry that mediates the exchange appears to have too short a
lifetime to allow the bidentate ligand to reorient itself during
its passage.
Acknowledgment. We thank Dr. Alicia Beatty for
assistance with the X-ray crystallography. Financial support
of this work by the University of Notre Dame and BLURP
is gratefully acknowledged.
(23) Technically, because the tripodal arms are folded to the right or to
the left, the two possible trigonal prisms are diastereomers and the
two directions of bidentate ligand migration are inequivalent. However,
because counterclockwise and clockwise rotation are the reverse of
each other, and because the equilibrium constant for this degenerate
reaction is unity, microscopic reversibility requires that rotation in
the two directions must take place at equal rates. For discussion of an
analogous situation in an organic rotor, see: Kelly, T. R.; Sestelo, J.
P.; Tellitu, I. J. Org. Chem. 1998, 63, 3655-3665.
(24) (a) Steinhuebel, D. P.; Lippard, S. J. Inorg. Chem. 1999, 38, 6225-
6233. (b) Goedken, V. L.; Ladd, J. A. J. Chem. Soc., Chem. Commun.
1982, 142-144. (c) Blake, A. J.; McInnes, J. M.; Mountford, P.;
Nikonov, G. I.; Swallow, D.; Watkin, D. J. J. Chem. Soc., Dalton
Trans. 1999, 379-391. (d) Housemekerides, C. E.; Ramage, D. L.;
Kretz, C. M.; Shontz, J. T.; Pilato, R. S.; Geoffroy, G. L.; Rheingold,
A. L.; Haggerty, B. S. Inorg. Chem. 1992, 31, 4453-4468.
1
Supporting Information Available: Variable-temperature H
NMR spectra of LTi(trop), and Eyring plots for ligand racemization
rates, arm exchange rates, and aldehyde exchange rate constants;
crystallographic data for LTi(trop), LTi(acac), LTi(hq)‚CH3CN‚
0.5C6H6, LTi(dfc)‚CH2Cl2, and {LTi[O2P(OCH2Ph)2}2‚4CH2Cl2 in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC048403D
2814 Inorganic Chemistry, Vol. 44, No. 8, 2005