Titanatranes derailed: Static and dynamic triethanolamine slippage induced by polyphenoxide chelation

Article abstract of DOI:10.1021/ic0492901

Aryloxytitanatranes [N(CH₂CH₂O)₃] Ti[OC₆H₂-2,4-^tBu₂-6-CHPh ₃-_n(C₆H₂-3,5-^tBu ₂-2-OH)_n-1] (1, n = 1; 2, n = 2; 3, n = 3) containing aryloxides derived from substituted triphenylmethanes are formed in high yield in the reactions of the corresponding phenols with tert-butoxytitanatrane [N(CH₂CH₂O)₃]Ti(O^tBu). The aryloxytitanatranes adopt monomeric, trigonal bipyramidal structures, as confirmed by X-ray crystallography for 1 and 2. The compounds with pendant phenol groups display fluxional behavior due to interchange of the bound phenoxide with the free phenol groups, most likely via a mechanism involving displacement of one arm of the nitrilotriethoxide by the dangling phenol group. The amidotitanatrane {[N(CH₂CH₂O)₃]Ti(NEt ₂)}₂ reacts with the bisphenol PhCH(C₆H ₂-3,5-^tBu₂-2-OH)₂ to give a crystallographically characterized fully metalated cyclic octatitanium complex of the formula {PhCH(C₆H₂-3,5-^tBu ₂-2-O)₂Ti₂[(OCH₂CH₂) ₃N]₂}₄ (4). The structure of 4 reveals a metallamacrocycle (minimum ring size = 28 members) consisting of four repeating unsymmetrical dititanium units. In each dititanium unit, there is an unprecedented slippage of the nitrilotriethoxide ligand such that both triethanolamine nitrogens coordinate to only one of the two titaniums, while the bisphenoxide is coordinated only to the other titanium. The propensity of the polyphenoxide ligands to chelate is attributed to favorable π-bonding interactions attainable in the eight-membered chelate rings.

Full text of DOI:10.1021/ic0492901

Inorg. Chem. 2004, 43, 6995−7004
Titanatranes Derailed: Static and Dynamic Triethanolamine Slippage
Induced by Polyphenoxide Chelation†
Aaron G. Maestri and Seth N. Brown*
Department of Chemistry and Biochemistry, 251 Nieuwland Science Hall, UniVersity of
Notre Dame, Notre Dame, Indiana 46556-5670
Received May 31, 2004
Aryloxytitanatranes [N(CH2CH2O)3]Ti[OC6H2-2,4-tBu2-6-CHPh3 n(C6H2-3,5-tBu2-2-OH)n 1] (1, n
) 1; 2, n ) 2;
-
-
3, n
) 3) containing aryloxides derived from substituted triphenylmethanes are formed in high yield in the reactions
of the corresponding phenols with tert-butoxytitanatrane [N(CH2CH2O)3]Ti(OtBu). The aryloxytitanatranes adopt
monomeric, trigonal bipyramidal structures, as confirmed by X-ray crystallography for 1 and 2. The compounds
with pendant phenol groups display fluxional behavior due to interchange of the bound phenoxide with the free
phenol groups, most likely via a mechanism involving displacement of one arm of the nitrilotriethoxide by the
dangling phenol group. The amidotitanatrane
tBu2-2-OH)2 to give a crystallographically characterized fully metalated cyclic octatitanium complex of the formula
PhCH(C6H2-3,5-tBu2-2-O)2Ti2[(OCH2CH2)3N]2
(4). The structure of 4 reveals a metallamacrocycle (minimum ring
{[N(CH2CH2O)3]Ti(NEt2)} reacts with the bisphenol PhCH(C6H2-3,5-
2
{
}
4
size
) 28 members) consisting of four repeating unsymmetrical dititanium units. In each dititanium unit, there is
an unprecedented slippage of the nitrilotriethoxide ligand such that both triethanolamine nitrogens coordinate to
only one of the two titaniums, while the bisphenoxide is coordinated only to the other titanium. The propensity of
the polyphenoxide ligands to chelate is attributed to favorable
chelate rings.
π-bonding interactions attainable in the eight-membered
Introduction
One feature is constant across this set of complexes, though:
the aminetriethoxide moiety always serves as a tetradentate
chelating ligand. The high propensity of the aminetriethoxide
group to chelate is reflected in the great stability of atrane
complexes; for example, triethanolamine has even been used
to solubilize titanium dioxide.10
Triply deprotonated triethanolamine, [N(CH2CH2O)3]3-,
forms an extensive series of so-called atrane complexes with
main group and transition metals.1 While these complexes
are most commonly monomeric and five-coordinate, a wide
variety of other structures are known, including examples
of four-,2 six-,3 and seven-coordinate4,5 complexes and
alkoxide-bridged dimers,6 trimers,7 and higher oligomers.8,9
We recently set out to explore the effects of bringing one,
two, or three titanium centers into close proximity using the
* To whom correspondence should be addressed. E-mail:
Seth.N.Brown.114@nd.edu.
(6) (a) Harlow, R. L. Acta Crystallogr. 1983, C39, 1344-1346. (b) Healy,
M. D.; Barron, A. R. J. Am. Chem. Soc. 1989, 111, 398-399. (c)
Naiini, A. A.; Menge, W. M. P. B.; Verkade, J. G. Inorg. Chem. 1991,
30, 5009-5012. (d) Lee, B.; Moise, F.; Pennington, W. T.; Robinson,
G. H. J. Coord. Chem. 1992, 26, 187-197. (e) Kim, Y.; Jnaneshwara,
G. K.; Verkade, J. G. Inorg. Chem. 2003, 42, 1437-1447.
(7) Swisher, R. G.; Day, R. O.; Holmes, R. R. Inorg. Chem. 1983, 22,
3692-3695.
† Dedicated in memory of Ian P. Rothwell, a trailblazer in the study of
early metal aryloxides.
(1) (a) Verkade, J. G. Coord. Chem. ReV. 1994, 137, 233-295. (b)
Verkade, J. G. Acc. Chem. Res. 1993, 26, 483-489.
(2) Korlyukov, A. A.; Lyssenko, K. A.; Antipin, M. Y.; Kirin, V. N.;
Chernyshev, E. A.; Knyazev, S. P. Inorg. Chem. 2002, 41, 5043-
5051.
(8) Kemmitt, T.; Al-Salim, N. I.; Gainsford, G. J. Inorg. Chem. 2000,
39, 6067-6071.
(3) Wieghardt, K.; Kleine-Boymann, M.; Swiridoff, W.; Nuber, B.; Weiss,
J. J. Chem. Soc., Dalton Trans. 1985, 2493-2497.
(4) Boche, G.; Mo¨bus, K.; Harms, K.; Marsch, M. J. Am. Chem. Soc.
1996, 118, 2770-2771.
(9) (a) Saalfrank, R. W.; Bernt, I.; Uller, E.; Hampel, F. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 2482-2485. (b) Waldmann, O.; Koch, R.;
Schromm, S.; Schulein, J.; Muller, P.; Bernt, I.; Saalfrank, R. W.;
Hampel, F.; Balthes, E. Inorg. Chem. 2001, 40, 2986-2995. (c)
Makhankova, V. G.; Vassilyeva, O. Y.; Kokozay, V. N.; Skelton, B.
W.; Reedijk, J.; Van Albada, G. A.; Sorace, L.; Gatteschi, D. New J.
Chem. 2001, 25, 685-689.
(5) A number of (η5-cyclopentadienyl)titanatranes, which are nominally
seven-coordinate if the cyclopentadienyl ring is considered to occupy
three coordination sites, are known. (a) Kim, Y.; Hong, E.; Lee, M.
H.; Kim, J.; Han, Y.; Do, Y. Organometallics 1999, 18, 36-39. (b)
Kim, Y.; Han, Y.; Hwang, J.-W.; Kim, M. W.; Do, Y. Organometallics
2002, 21, 1127-1135.
(10) Kemmitt, T.; Al-Salim, N. I.; Gainsford, G. J.; Henderson, W. Aust.
J. Chem. 1999, 52, 915-919.
10.1021/ic0492901 CCC: $27.50
Published on Web 10/06/2004
© 2004 American Chemical Society
Inorganic Chemistry, Vol. 43, No. 22, 2004 6995

Maestri and Brown
hydroxylated triphenylmethane scaffolds L1OH, L2(OH)2 and
L3(OH)3 illustrated below. Scott and co-workers have
reported that the deprotonated tris(phenoxy)methanes such
as L3(OH)3 form polymetallic complexes with alkali metals
and zinc, where the aryloxides bridge between the bound
metal atoms.11 We find, however, that when bound to
[N(CH2CH2O)3Ti], these deprotonated hydroxytriphenyl-
methanes do not form complexes with bridging aryloxide
ligands. Complexes of the monodeprotonated ligands form
monomeric five-coordinate titanatranes, but polydeprotona-
tion leads, at least in one case, to the formation of a complex
with a chelated bis(phenoxide) and an unprecedented de-
chelated aminetriethoxide ligand.
magnetic stir bar, 2,4-di-tert-butylphenol (1.578 g, 7.65 mmol), and
diphenylbromomethane (1.90 g, 7.60 mmol). The flask was heated,
open to the air, in a 155 °C silicone oil bath; the reaction turned
blood red after 10 min of heating. The progress of the reaction
was monitored by holding moistened pH paper above the neck of
the flask to detect the evolution of HBr, which continued for 4 h.
The reaction was heated for 1 h after the evolution of HBr ceased
and then the red oil was allowed to cool to room temperature,
layered with 50 mL pentane, and stored for 3 days at -20 °C. The
off-white solid was collected on a glass frit, washed with 5 × 40
mL cold pentane, and air-dried to yield 1.86 g of L1OH (55%). 1H
NMR (CDCl3): δ 1.17 (s, 9H; tBu), 1.39 (s, 9H; tBu), 4.55 (s, 1H;
OH), 5.61 (s, 1H; Ph2CH), 6.61 (d, 2.5 Hz, 1H; ArH), 7.16 (d, 7.5
Hz, 4H; ortho-Ph), 7.24 (d, 2.4 Hz, 1H; ArH), 7.30 (t, 7.5 Hz, 4H;
meta-Ph), 7.31 (t, 7.2 Hz, 2H; para-Ph). 13C{1H} NMR (CDCl3):
δ 30.2, 31.7 (C[CH3]3), 34.5, 35.0 (C[CH3]3), 52.3 (ArPh2CH),
122.6, 125.4, 130.2, 136.4, 142.3, 150.3 (ArOH); 127.2, 128.9,
129.6, 142.4 (Ph). Anal. Calcd for C27H32O: C, 87.05; H, 8.66.
Found: C, 86.80; H, 8.63.
2-(Diphenylmethyl)-4,6-tert-butylphenoxytitanatrane, [N(CH2-
CH2O)3]TiOL1 (1). In the drybox, L1OH (0.168 g, 0.432 mmol),
(tert-butoxy)titanatrane (0.116 g, 0.434 mmol), and dichloromethane
(2.5 mL) were added to a 20-mL screw-cap vial. The yellow
solution was stirred for 30 min, then layered with 10 mL of pentane,
and stored at -40 °C. After 48 h, pale yellow crystals suitable for
X-ray analysis appeared. The yellow solid was filtered off onto a
glass frit and the filtrate recrystallized from CH2Cl2/pentane. The
combined yellow solids were collected on a glass frit in the drybox
and washed twice with cold pentane to yield 150.2 mg of 1 (60%).
1H NMR (CDCl3): δ 1.15 (s, 9H; Bu), 1.47 (s, 9H; Bu), 3.16 (t,
6 Hz, 6H; NCH2), 4.37 (t, 6 Hz, 6H; OCH2), 6.60 (s, 1H; Ph2CH),
6.70 (d, 2.5 Hz, 1H; ArH), 7.14 (m, 7H; o-Ph, p-Ph, ArH), 7.21 (t,
7.5 Hz, 4H; m-Ph). 13C{1H} NMR (CD2Cl2): δ 30.6, 31.8
(C[CH3]3), 34.8, 35.5 (C[CH3]3), 49.7 (ArPh2CH), 57.2 (NCH2),
71.9 (OCH2), 121.8, 126.3, 134.2, 137.6, 142.0, 160.3 (ArOTi);
126.1, 128.2, 130.4, 146.2 (Ph). IR: 1558 (s), 1300 (s), 1259 (s),
1101 (s), 1063 (s), 1033 (s), 926 (s), 881 (s), 768 (s), 758 (s), 696
(s), 568 (s). Anal. Calcd for C33H43NO4Ti: C, 70.08; H, 7.66; N,
2.48. Found: C, 69.99; H, 7.46; N, 2.54.
Experimental Section
All reactions were carried out under argon at room temperature
using vacuum-line techniques or in a nitrogen-filled Innovative
Technologies drybox, unless otherwise stated. Dry tetrahydrofuran
was vacuum-transferred from sodium benzophenone ketyl, and
toluene, benzene, and pentane from sodium. Methylene chloride
and chloroform were vacuum-transferred from calcium hydride.
Triethanolamine was distilled under vacuum and stored in the
drybox. HCPh(2-OH-3,5-tBu2C6H2)2 (L2(OH)2),12 [N(CH2CH2O)3]-
t
t
14
TiOtBu,13 and {[N(CH2CH2O)]Ti(NEt2)}2 were prepared using
literature procedures. The trisphenol HC(2-OH-3,5-tBu2C6H2)3 (L3-
(OH)3) was prepared in 55% yield from 2,4-di-tert-butylphenol,
isopropylmagnesium bromide, and triethylorthoformate as described
by Casnati and co-workers;15 a similar procedure has also been
described by Dinger and Scott.16 All other reagents were com-
mercially available and used without further purification.
NMR spectra were measured on a Varian-300 FT-NMR spec-
trometer in CDCl3, CD2Cl2, or C7D8, using the proton impurity of
the solvents as an internal reference for the 1H spectra (300 MHz)
and the 13C resonance of the solvents as an internal reference for
13C{1H} spectra (75.5 MHz). In the assignments, the unsubstituted
phenyl groups are noted as Ph, while the di-tert-butylphenoxy aryl
rings are abbreviated Ar. Unless otherwise noted, spectra were
obtained at 22 °C. IR spectra were recorded on a Perkin-Elmer
PARAGON 1000 FT-IR spectrophotometer as Nujol mulls; fre-
quencies are reported in wavenumbers. Elemental analyses of air-
sensitive compounds were performed by Canadian Microanalytical
Service, Ltd. (Vancouver, BC), and those of air-stable compounds
by M-H-W Laboratories (Phoenix, AZ).
2-(r-(2-hydroxy-3,5-di-tert-butylphenyl)benzyl)-4,6-di-tert-bu-
tylphenoxytitanatrane, L2(OH)(OTi[(OCH2CH2)3N]) (2). To a
20-mL vial in the drybox were added L2(OH)2 (0.106 g, 0.217
mmol), {[N(CH2CH2O)3]Ti(NEt2)}2 (0.058 g, 0.109 mmol of
dimer), and CH2Cl2 (2 mL). The yellow solution was swirled for a
few minutes and then allowed to stand for 15 min. The mixture
was layered with pentane (10 mL) and stored at -40 °C for 48 h.
The pale yellow crystals were collected on a glass frit and the filtrate
was recrystallized from CH2Cl2/pentane and then filtered. The
combined solid was washed with 2 × 40 mL cold pentane and
allowed to dry in the glovebox to yield 89.1 mg 2 (58%). 1H NMR
(CDCl3): δ 1.15 (s, 18H; tBu), 1.38 (s, 9H; tBu), 1.46 (s, 9H; tBu),
3.25 (apparent td, 5.5, 1.2 Hz, 6H; NCH2), 4.47 (apparent q, 5.5
Hz, 6H; OCH2), 6.55 (s, 1H; PhCH), 6.65 (sl br, 1H; 3,5-ArH),
6.72 (sl br, 1H; 3,5-ArH), 6.77 (s, 1H; OH), 7.03 (d, 8 Hz, 2H;
o-Ph), 7.18 (m, 5H; m-Ph, p-Ph, 2 ArH). 13C{1H} NMR (CDCl3):
δ 30.2 (ArOH C[CH3]3), 30.6 (ArOTi C[CH3]3), 31.8 (coalesced
ArOH and ArOTi C[CH3]3), 34.5 (coalesced ArOH and ArOTi
C[CH3]3), 35.3 (coalesced ArOH and ArOTi C[CH3]3), 44.2 (Ar2-
PhCH), 56.9 (NCH2), 71.3 (OCH2), 121.4, 125.6, 131.1, 136.2,
141.1, 151.2 (ArOH); 121.6, 126.0, 132.9, 137.3, 142.0, 160.3
(ArOTi); 125.8, 127.8, 129.7, 144.9 (Ph). IR: 3176 (m, νOH), 1298
(s), 1244 (s), 1201 (s), 1159 (m), 1125 (s), 1075 (s), 1062 (s), 923
(s), 906 (s), 886 (s), 765 (s), 722 (s), 702 (s), 623 (s). Anal. Calcd
2-(Diphenylmethyl)-4,6-tert-butylphenol, 2-Ph2CH-4,6-tBu2-
C6H2OH (L1OH). To a 100-mL round-bottom flask were added a
(11) (a) Dinger, M. B.; Scott, M. J. Inorg. Chem. 2000, 39, 1238-1254.
(b) Dinger, M. B.; Scott, M. J. Inorg. Chem. 2001, 40, 1029-1036.
(12) Muller, E.; Schich, A.; Mayer, R.; Scheffler, K. Chem. Ber. 1960, 93,
2649-2662.
(13) Menge, W. M. P. B.; Verkade, J. G. Inorg. Chem. 1991, 30, 4628-
4631.
(14) Naiini, A. A.; Ringrose, S. L.; Su, Y.; Jacobson, R. A.; Verkade, J.
G. Inorg. Chem. 1993, 32, 1290-1296.
(15) Casiraghi, G.; Casnati, G.; Cornia, M.; Pochini, A. Gazz. Chim. Ital.
1978, 108, 79-84.
(16) Dinger, M. B.; Scott, M. J. Eur. J. Org. Chem. 2000, 2467-2478.
6996 Inorganic Chemistry, Vol. 43, No. 22, 2004

Triethanolamine Slippage Induced by Polyphenoxide Chelation
for C41H59NO5Ti: C, 70.78; H, 8.57; N, 2.02. Found: C, 70.39;
H, 8.60; N, 2.06.
placed in calculated positions. Final full-matrix least-squares
refinement on F2 converged at R ) 0.0500 for 5252 reflections
with Fo > 4σ(Fo), R ) 0.0652 for all 6831 unique reflections
(wR2 ) 0.1270, 0.1390, respectively.). All the calculations used
SHELXTL (Bruker Analytical X-ray Systems), with scattering
factors and anomalous dispersion terms taken from the literature.18
Crystals of compound 2 were grown as described above. A
yellow prism (0.33 × 0.37 × 0.43 mm) was 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 ) -103 °C). Data were
analyzed as described for 1, except that the O-H hydrogen was
found on the difference Fourier map and refined isotropically. Final
full-matrix least-squares refinement on F2 converged at R ) 0.0605
for 7349 reflections with Fo > 4σ(Fo), R ) 0.0715 for all 8898
unique reflections (wR2 ) 0.1702, 0.1835, respectively.).
Yellow-orange blocks of octatitanium complex 4 were deposited
after slow diffusion of pentane into a freshly generated solution of
the complex in methylene chloride. A crystal of approximate
dimensions 0.35 × 0.45 × 0.50 mm was rapidly covered in oil
and placed on the tip of a glass fiber in the cold N2 stream of a
Bruker Apex CCD diffractometer (T ) -103 °C). Once removed
from the mother liquor, the crystals showed a marked tendency to
lose solvent and become opaque, so rapid handling was essential.
The structure was solved by direct methods, and remaining
nonhydrogen atoms were found on difference Fourier syntheses.
Hydrogens were placed in calculated positions. Two tert-butyl
groups (with central carbons C250 and C254) were disordered and
were modeled in two sites; the occupancy of the major sites was
refined to 0.700(6) and 0.741(10), respectively. One triethanolamine
group was also disordered over two possible helical twists and was
modeled with two sites occupied by C44, C45, and C46 (major
site occupancy 0.780(6)). All nonhydrogen atoms except those of
two methylene chloride molecules (which showed electron density
characteristic of partial occupancy and were arbitrarily refined at
1/2 occupancy) were refined anisotropically. Hydrogens were placed
in calculated positions. Final full-matrix least-squares refinement
on F2 converged at R ) 0.0759 for 17366 reflections with Fo >
4σ(Fo), R ) 0.1171 for all 28593 unique data (wR2 ) 0.2330,
0.2639, respectively.).
2-(Bis-(2-hydroxy-3,5-di-tert-butylphenyl)methyl)-4,6-di-tert-
butylphenoxytitanatrane, L3(OH)2(OTi[(OCH2CH2)3N]) (3). To
a 100-mL round-bottom flask in the drybox were added L3(OH)3
(0.091 g, 0.145 mmol), tert-butoxytitanatrane (0.038 g, 0.144
mmol), and a magnetic stir bar. The flask was attached to a swivel
frit and taken to the vacuum line. Ten milliliters of CH2Cl2 was
vacuum-transferred to the flask and the yellow solution stirred for
30 min. The volume was reduced to 2 mL and then pentane (10
mL) was vacuum-transferred to the flask. Pale yellow crystals grew
overnight, were isolated on the swivel frit, and were dried in vacuo
to yield 62 mg 3 (52%). 1H NMR (CD2Cl2): δ 1.09 (s, 18H; tBu),
t
t
t
1.13 (s, 9H; Bu), 1.38 (s, 18H; Bu), 1.45 (s, 9H; Bu), 3.33 (t, 6
Hz, 6H; NCH2), 4.57 (t, 6 Hz, 6H; OCH2), 6.30 (s, 1H; Ar3CH),
6.35 (d, 2.5 Hz, 2H; ArH), 6.57 (s, 2H; OH), 6.64 (d, 2.5 Hz, 1H;
ArH), 7.18 (d, 2.5 Hz, 2H; ArH), 7.21 (d, 2.5 Hz, 1H; ArH). 13C-
{1H} NMR (CD2Cl2): δ 30.4 (ArOH C[CH3]3), 30.7 (ArOTi
C[CH3]3), 31.8 (coalesced ArOH and ArOTi C[CH3]3), 34.7 (ArOH
C[CH3]3), 34.8 (ArOTi C[CH3]3), 35.5 (coalesced ArOH and ArOTi
C[CH3]3), 40.1 (Ar3CH), 57.3 (NCH2), 72.0 (OCH2), 122.2, 125.4,
130.8, 137.2, 142.3, 151.2 (ArOH); 122.3, 126.0, 131.6, 137.4,
143.0, 160.7 (ArOTi). IR: 3179 (m, νOH), 1298 (s), 1244 (s), 1201
(s), 1159 (m), 1125 (s), 1075 (s), 1062 (s), 923 (s), 906 (s), 886
(s), 765 (s), 722 (s), 702 (s), 623 (s). Anal. Calcd for C49H75NO6-
Ti: C, 71.59; H, 9.20; N, 1.70. Found: C, 71.60; H, 9.25; N, 1.63.
{L2(O)2Ti2[(OCH2CH2)3N]2}4 (4). To a 20-mL vial in the
drybox were added {[N(CH2CH2O)3]TiNEt2}2 (0.102 g, 0.192
mmol), L2(OH)2 (0.096 g, 0.192 mmol), and CH2Cl2 (2 mL). The
solution was swirled for a few minutes and then allowed to stand
for 15 min. Pentane was carefully layered on the solution and
allowed to mix over 60 h into the CH2Cl2 at -40 °C, depositing
orange blocks of X-ray quality. The solid was then isolated by
gravity filtration onto a glass frit, washed with 3 × 30 mL pentane,
and allowed to dry. Yield of the orange powdery solid was 45 mg
(42%). IR: 1259 (m), 1233 (w), 1156 (w), 1125 (m), 1098 (m),
1074 (m), 1062 (s), 1026 (w), 1014 (w), 922 (m), 906 (w), 881
(m), 839 (w), 764 (w), 730 (s), 621 (s), 600 (s).
Variable-Temperature NMR Studies of 2 and 3. In a typical
procedure, a 5-mm NMR tube sealed to a ground glass joint was
charged with 0.012 g (0.017 mmol) of 2 and toluene-d8 (0.7 mL)
in a drybox under an N2 atmosphere. The tube was flame-sealed
while cooled in liquid nitrogen under dynamic vacuum and the seal
checked for integrity. The tube was transferred to the NMR probe,
Results
Preparation and Characterization of Monometalated
Hydroxylated Triphenylmethanes. To compare the bonding
of mono-, di-, and trititanium complexes, we sought a series
of analogous hydroxylated triphenylmethanes, HCPh3-n(2-
OH-3,5-tBu2C6H2)n (n ) 1, L1OH; n ) 2, L2(OH)2; n ) 3,
L3(OH)3). The triphenol L3(OH)3 was prepared from the
bromomagnesium salt of 2,4-di-tert-butylphenol and trieth-
ylorthoformate;15,16 it has also been prepared by an alternate
route16 and studied as a ligand11 by Scott and co-workers.
The bisphenol ligand L2(OH)2 was also available, by acid-
catalyzed condensation of 2,4-di-tert-butylphenol with benz-
aldehyde,12 but L1OH had not been prepared previously. It
was synthesized in 55% yield by heating 2,4-di-tert-bu-
tylphenol with neat benzhydryl bromide (eq 1).
1
and H NMR spectra were recorded between 25 and 100 °C. At
25 °C the o-tert-butyl resonances are observed at δ 1.75 and 1.65.
Upon warming, the tert-butyl resonances broaden, coalescing at
70 °C. The dynamic process was modeled as an AB exchange using
the program gNMR17 to obtain rate constants. The NMR spectra
of 3 were analyzed analogously, except that they were modeled as
an AB2 exchange.
X-ray Crystal Structure Determinations of 1, 2, and 4‚
2C5H12‚8.5CH2Cl2. Crystals of compound 1 were grown as
described above. A small yellow plate (0.08 × 0.19 × 0.28 mm)
was 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 )
-103 °C). Data were reduced, correcting for absorption and decay,
using the program SADABS. The crystal was triclinic. The titanium
atom was located on a Patterson map, and remaining nonhydrogen
atoms were found on difference Fourier syntheses. Hydrogens were
Complexes in which one phenoxide of a polyphenol is
ligated to titanium can be readily prepared via protonolysis
of the polyphenols. Thus, tert-butoxytitanatrane [N(CH2-
(17) Budzelaar, P. H. M. gNMR v.3.6.5. Cherwell Scientific Publishing:
(18) International Tables of Crystallography; Kluwer Academic Publish-
Oxford, 1996.
ers: Dordrecht, The Netherlands, 1992; Vol. C.
Inorganic Chemistry, Vol. 43, No. 22, 2004 6997

Maestri and Brown
Table 1. Crystallographic Details for [N(CH2CH2O)3]TiOL1 (1), L2(OH)(OTi[N(CH2CH2O)3]) (2), and {L2(OTi[N(CH2CH2O)3])2}4‚2C5H12‚8.5CH2Cl2
(4‚2C5H12‚8.5CH2Cl2)
1
2
4‚2C5H12‚8.5CH2Cl2
empirical formula
fw
C33H43NO4Ti
565.58
C41H59NO5Ti
693.79
C206.5H321Cl17N8O32Ti8
4413.56
temp (K)
λ, Å
space group
total data collected
no. of indep. reflns
Rint
170(2)
170(2)
170(2)
0.71073 (Mo KR)
I2/a
77317
28593
0.0381
0.71073 (Mo KR)
P1h
9696
6831
0.0162
0.71073 (Mo KR)
P1h
17541
8898
0.0364
obsd reflns [I > 2σ(I)]
5252
7349
17366
a (Å)
10.4044(7)
10.5516(7)
14.9791(10)
90.9430(10)
102.2840(10)
104.3430(10)
1552.6(2)
2
10.9755(11)
11.3514(11)
16.730(2)
80.299(2)
74.070(2)
79.102(2)
1953.1(3)
2
27.581(2)
30.981(2)
28.825(2)
90
95.7410(10)
90
24507.3(29)
4
1.196
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
calcd F (g/cm3)
cryst size (mm)
1.210
1.180
0.08 × 0.19 × 0.28
0.33 × 0.37 × 0.43
0.35 × 0.45 × 0.50
µ (mm-1
)
0.311
0.261
0.493
no. refined params
488
633
1270
R indices [I > 2σ(I)]a
R1 ) 0.0500
wR2 ) 0.1270
R1 ) 0.0652
wR2 ) 0.1390
0.984
R1 ) 0.0605
wR2 ) 0.1702
R1 ) 0.0715
wR2 ) 0.1835
0.998
R1 ) 0.0759
wR2 ) 0.2330
R1 ) 0.1171
wR2 ) 0.2639
1.000
R indices (all data)a
GOF
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) (∑[w(Fo - Fc )2]/∑w(Fo )2)1/2
.
2
2
2
1
exposed to air. H NMR spectra clearly indicate that the
complexes contain only one [N(CH2CH2O)3]Ti unit per
polyphenol. For example, spectra of 2 and 3 at room
temperature show distinct aryl and tert-butyl resonances for
bound phenoxide and free phenol groups, and IR spectra of
2 and 3 (but not 1) show the retention of free O-H groups
(e.g., νOH ) 3176 cm-1 in 2).
CH2O)3]Ti(OtBu)13 reacts with the ligands L1OH, L2(OH)2,
and L3(OH)3 to produce the monometalated complexes
[N(CH2CH2O)3]Ti(OL1) (1), [N(CH2CH2O)3]Ti[OL2(OH)]
(2), and [N(CH2CH2O)3]Ti[OL3(OH)2] (3), respectively (eq
2). In all cases yields are quantitative by NMR, though some
The structures of 1 and 2 were confirmed by single-crystal
X-ray structure determinations (Table 1). Both complexes
have generally similar overall structures (Figures 1 and 2)
and metrical data (Table 2). The aryl groups of the phenoxide
ligands are arranged in a propeller-like fashion around the
central methine carbon. The titanium centers are trigonal
bipyramidal, with Ti-Oeq distances averaging 1.836 Å in 1
and 1.839 Å in 2 and Ti-N bond distances of 2.263(2) Å in
1 and 2.280(2) Å in 2. These distances are typical of those
seen in other five-coordinate titanatranes such as [N(CH2-
13
CH2O)3]TiOSiPh3 and [N(CH2CH2O)3]TiOCMe2CMe2-
OTi[(OCH2CH2)3N].14 The Ti-Oaryloxide distances of 1.8231-
(14) in 1 and 1.8183(14) in 2 are similar to those observed
in recently reported five-coordinate complexes with 2,6-
diisopropylphenoxide, [N(CH2C6H2Me2O)3]Ti(OAr) and [N-
(CH2C6H2Me2O)2(CH2CH2O)]Ti(OAr) (1.834(2) and 1.828(3)
Å, respectively).19 Presumably 1 and 2 adopt monomeric,
rather than alkoxide-bridged dimeric structures, due to the
steric bulk of the 2,6-disubstituted aryloxides. For example,
(triphenylsiloxy)titanatrane is monomeric in the crystal,13
while the isopropoxide complex is dimeric.6a
material is lost on isolation. These protonolysis reactions can
also be carried out using the diethylamido complex {[N(CH2-
CH2O)3]Ti(NEt2)}2 in a titanium:polyphenol ratio of 1:1, and
this method produced somewhat higher isolated yields in the
preparation of 2 than did the use of the tert-butoxide. The
complexes are yellow crystalline materials that are stable
indefinitely in the solid or in solution in the absence of air
and moisture, but decompose within hours in solution when
There are significant differences between the two struc-
tures, which stem from the formation of a hydrogen bond
between the unmetalated phenol O-H group and the
(19) Kim, Y.; Verkade, J. G. Organometallics 2002, 21, 2395-2399.
6998 Inorganic Chemistry, Vol. 43, No. 22, 2004

Triethanolamine Slippage Induced by Polyphenoxide Chelation
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
[N(CH2CH2O)3]TiOL1 (1) and L2(OH)(OTi[N(CH2CH2O)3]) (2)
[N(CH2CH2O)3]TiOL1 L2(OH)(OTi[N(CH2CH2O)3])
(1)
(2)
Ti-O1
Ti-O2
Ti-O3
Ti-O4
Ti-N
1.8406(15)
1.8308(15)
1.837(2)
1.8231(14)
2.263(2)
1.823(2)
1.8754(15)
1.818(2)
1.8183(14)
2.280(2)
O1-Ti-O2
O1-Ti-O3
O2-Ti-O3
O1-Ti-O4
O2-Ti-O4
O3-Ti-O4
O1-Ti-N
O2-Ti-N
O3-Ti-N
O4-Ti-N
Ti-O4-C10
118.10(7)
115.79(7)
113.08(7)
100.96(7)
102.95(7)
102.72(7)
77.73(6)
77.69(7)
78.00(7)
178.68(7)
165.36(14)
112.90(7)
115.86(8)
118.46(7)
101.32(7)
103.47(6)
101.42(7)
78.00(7)
78.01(6)
77.78(7)
178.52(6)
166.47(13)
Figure 1. SHELXTL plot (30% thermal ellipsoids) of [N(CH2CH2O)3]-
TiOL1 (1).
Figure 3. Eyring plot of the rate constant for degenerate phenoxide-
phenol interchange in 2 (toluene-d8).
1
The H NMR spectrum of 2 indicates that the hydrogen
bond is probably maintained in solution, as judged by the
downfield shift of the OH resonance (δ 6.77 vs 4.55 for free
L1OH), but that it is rapidly breaking and reforming, since
the three ethylene arms of the nitrilotriethoxide ligand are
equivalent on the NMR time scale. At room temperature,
the bound and free phenoxide groups of 2 are distinct, but
the resonances due to the aryl groups are slightly broadened.
As the temperature is raised, the resonances reversibly
broaden and eventually coalesce, indicating that the titanium
must be hopping from the bound to the free phenol at a
reasonable rate (eq 3). The rate of this reaction was measured
Figure 2. SHELXTL plot (30% thermal ellipsoids) of L2(OH)(OTi[N-
(CH2CH2O)3]) (2).
equatorial oxygen O2 (O2-O5 distance of 2.751 Å). This
interaction results in elongation of the bond from O2 to
titanium (1.875(2) Å compared with Ti-O1 and Ti-O3
bonds of 1.823(2) and 1.818(2) Å, respectively), consistent
with a decrease in electron density on O2 upon formation
of a hydrogen bond. The hydrogen bond is presumably also
responsible for the difference in conformation of the [N(CH2-
CH2O)3]Ti moiety in the two structures. In 1, the bond to
the methine group from the aryloxide bisects an O-Ti-O
group, while in 2 it approximately eclipses the Ti-O2 bond
to allow hydrogen bonding to the phenol.
quantitatively by line shape simulation of the exchanging
o-tert-butyl resonances in toluene-d8 (Tc ) 343 K) and Eyring
analysis of the rate data (Figure 3) gives activation parameters
for the exchange (∆Hq ) 7.12 ( 0.16 kcal/mol, ∆Sq )
-28.8 ( 0.5 cal/mol‚K). Observed exchange rates are
Inorganic Chemistry, Vol. 43, No. 22, 2004 6999

Maestri and Brown
unaffected by increasing the concentration of 2 by a factor
of 10 in toluene-d8 and are similar in CD2Cl2. Complex 3
undergoes analogous exchange of the two free phenol groups
with the bound phenoxide, and activation parameters for that
reaction are the same, within experimental error, as in the
The structure of 4 as determined by X-ray crystallography
is illustrated in Figure 4. The complex consists of a cyclic
tetramer of fully metalated L2(O)2Ti2[(OCH2CH2)3N]2 units,
creating a metallamacrocycle with a minimum ring size of
28 members. Each dititanium unit is connected to the
succeeding one by a dangling CH2CH2O arm from one of
the nitrilotriethoxide units. This architecture is thus quite
different from previously characterized high-nuclearity hexati-
reaction of 2 (∆Hq ) 7.03 ( 0.13 kcal/mol, ∆Sq
-30.9 ( 1.6 cal/mol‚K).
)
Preparation and Characterization of a Polymetalated
Hydroxylated Triphenylmethane. While one phenol group
per molecule in L1OH, L2(OH)2, or L3(OH)3 can be readily
metalated, the presence of the first titanium center substan-
tially disfavors metalation of the remaining phenols. Treat-
ment of the polyphenols L2(OH)2 or L3(OH)3 with an excess
of the tert-butoxide complex [N(CH2CH2O)3]Ti(OtBu) results
in clean formation of 2 and 3, respectively, but further
metalation does not take place. To achieve polymetalation,
the more reactive titanium amide complex {[N(CH2CH2O)3]-
Ti(NEt2)}2 must be used. For example, treatment of the
bisphenol ligand L2(OH)2 with a stoichiometric quantity of
the diethylamidotitanatrane dimer results in the formation
of orange solutions. NMR spectra of these solutions indicate
that all of the phenol groups have been consumed, with
liberation of diethylamine, but resonances due to the phenolic
ligands are very broad, possibly indicating the formation of
a variety of different oligomers. This mixture can be
crystallized to give complex 4 in 42% yield as orange
crystals, which lose solvent rapidly when removed from the
mother liquor (eq 4). Reactions of L3(OH)3 with {[N(CH2-
3-
tanium8 and octairon9a,b clusters containing N(CH2CH2O)3
ligands, where adjoining metal centers are linked by bridging
alkoxides. Complex 4 has crystallographic C2 symmetry, but
in fact the overall symmetry is very close to S4, with the
Ti1/Ti3 unit showing extremely similar bond distances and
angles to its crystallographically inequivalent but chemically
equivalent Ti2/Ti4 counterpart (Table 3).
While this repeating dititanium unit has the formula L2(O)2-
Ti2[(OCH2CH2)3N]2, the two titanium atoms experience very
different coordination environments (Figure 5). Most surpris-
ingly, one of the titanium atoms in the repeating unit (Ti3
or Ti4) coordinates both of the triethanolamine nitrogens.
Such an aminetriethoxide shift has never been observed
before in polynuclear metallatranes. The nitrogen-coordinated
titanium atoms are seven-coordinate, with one triethanola-
mine unit supplying two terminal alkoxides and one bridging
alkoxide and the other triethanolamine supplying two bridg-
ing alkoxides. The other titanium atom in the repeating unit
(Ti1 or Ti2) is six-coordinate and is chelated by the bis-
phenoxide ligand L2(O)22-, with both aryloxides terminal.
The average O-Ti-O bite angle of 99.8° is typical of
titanium chelates of methylenebis(aryloxides), with the
average Ti-OAr distance of 1.87 Å on the long end of the
range of observed values.20 The coordination geometry of
Ti1 and Ti2 is roughly octahedral, but with a severe trigonal
distortion, such that the octahedral face occupied by the
terminal alkoxides shows much shorter distances (1.85(3)
Å avg) and wider angles (99.1(8)° avg) than does the
bridging alkoxide face (2.06(3) Å, 73(3)° respectively). The
seven-coordinate geometry of Ti3 and Ti4 can be described
most simply as a capped octahedron, with O13 for example
capping the N1-O21-O22 face on Ti3 (Figure 5). Because
this bridging alkoxide has, effectively, no trans ligand on
the seven-coordinate titanium, it has a much shorter bond
distance than any of the other bridging alkoxides (1.965(2)
Å vs 2.07(3) Å). The overall coordination spheres of the
two titaniums in each repeating unit share a (very ap-
proximate) mirror plane which contains both nitrogens and
(20) (a) Floriani, C.; Corazza, F.; Lesueur, W.; Chiesi-Villa, A.; Guastini,
C. Angew. Chem. Ind. Ed. 1989, 28, 66-67. (b) Corazza, F.; Floriani,
C.; Chiesi-Villa, A.; Guastini, C. Inorg. Chem. 1991, 30, 145-148.
(c) Okuda, J.; Fokken, S.; Kang, H. C.; Massa, W. Chem. Ber. 1995,
128, 221-227. (d) Chisholm, M. H.; Huang, J.-H.; Huffman, J. C.;
Streib, W. E.; Tiedtke, D. Polyhedron 1997, 16, 2941-2949. (e) Rao,
P. V.; Rao, C. P.; Wegelius, E. K.; Kolehmainen, E.; Rissanen, K. J.
Chem. Soc., Dalton Trans. 1999, 4469-4474. (f) Gielens, E. E. C.
G.; Dijkstra, T. W.; Berno, P.; Meetsma, A.; Hessen, B.; Teuben, J.
H. J. Organomet. Chem. 1999, 591, 88-95. (g) Ozerov, O. V.; Parkin,
S.; Carr, S. D.; Ladipo, F. T. Organometallics 2000, 19, 4187-4190.
(h) Ozerov, O. V.; Brock, C. P.; Carr, S. D.; Ladipo, F. T.
Organometallics 2000, 19, 5016-5025. (i) Kingston, J. V.; Sarveswa-
ran, V.; Parkin, S.; Ladipo, F. T. Organometallics 2003, 22, 136-
144. (j) Gonza´lez-Maupoey, M.; Cuenca, T.; Frutos, L. M.; Castao,
O.; Herdtweck, E. Organometallics 2003, 22, 2694-2704.
CH2O)3]Ti(NEt2)}2 gave similar broad NMR spectra in situ,
but no crystalline material was obtained.
7000 Inorganic Chemistry, Vol. 43, No. 22, 2004

Triethanolamine Slippage Induced by Polyphenoxide Chelation
Figure 4. SHELXTL plot (30% thermal ellipsoids) of {L2(OTi[N(CH2CH2O)3])2}4 (4). Tert-butyl groups are omitted for clarity.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for {L2(OTi[N(CH2CH2O)3])2}4‚2C5H12‚8.5CH2Cl2 (4‚2C5H12‚8.5CH2Cl2)
Ti1-O1
Ti1-O2
Ti1-O32
Ti1-O13
Ti1-O21
Ti1-O22
1.842(2)
1.884(2)
1.825(2)
2.099(2)
2.007(2)
2.078(2)
Ti2-O3
Ti2-O4
Ti2-O23
Ti2-O42
Ti2-O31
Ti2-O33
1.850(2)
1.895(3)
1.806(3)
2.085(3)
2.032(2)
2.076(2)
Ti3-O11
Ti3-O12
Ti3-O13
Ti3-O21
Ti3-O22
Ti3-N1
1.864(3)
1.865(3)
1.963(2)
2.093(2)
2.086(2)
2.323(3)
2.322(3)
Ti4-O41
Ti4-O43
Ti4-O42
Ti4-O31
Ti4-O33
Ti4-N4
1.852(3)
1.869(3)
1.967(2)
2.106(3)
2.070(3)
2.329(3)
2.334(3)
Ti3-N2
Ti4-N3
O1-Ti1-O2
99.38(11)
O3-Ti2-O4
100.31(11)
O1-Ti1-O32
O2-Ti1-O32
O13-Ti1-O21
O13-Ti1-O22
O21-Ti1-O22
N1-Ti3-N2
98.09(12)
99.25(11)
72.56(10)
69.90(9)
77.15(9)
147.89(11)
161.3(2)
140.0(2)
139.0(2)
93.95(10)
92.86(9)
91.04(9)
O3-Ti2-O23
O4-Ti2-O23
O31-Ti2-O42
O33-Ti2-O42
O31-Ti2-O33
N3-Ti4-N4
99.77(12)
97.96(12)
71.39(10)
71.14(10)
76.86(10)
149.13(11)
163.0(3)
132.6(2)
142.7(2)
94.44(10)
91.92(10)
91.68(10)
Ti1-O1-C112
Ti1-O2-C122
Ti1-O32-C32
Ti1-O13-Ti3
Ti1-O21-Ti3
Ti1-O22-Ti3
Ti2-O3-C212
Ti2-O4-C222
Ti2-O23-C23
Ti2-O42-Ti4
Ti2-O31-Ti4
Ti2-O33-Ti4
both titaniums, and which bisects the bis(aryloxide) ligand.
However, this mirror plane is violated by the dangling
alkoxide arms that link the dititanium units. The orientations
of these links are enantiomorphous in the Ti1/Ti3 and Ti2/
Ti4 subunits.
The isolated octatitanium complex 4 is insoluble in all
organic solvents, which precludes its characterization by
NMR spectroscopy, and we were unable to obtain satisfac-
tory elemental analysis of the complex, possibly because of
variable entrapment of solvent. However, in the presence of
excess tert-butyl alcohol in toluene-d8, solid 4 dissolves on
heating to form equimolar amounts of [N(CH2CH2O)3]Ti-
[OL2(OH)] (2) and [N(CH2CH2O)3]TiOtBu (eq 5). This
confirms the bulk composition of 4, as well as the thermo-
dynamic instability of the fully metalated aryloxide with
respect to alcoholysis.
Inorganic Chemistry, Vol. 43, No. 22, 2004 7001

Maestri and Brown
Scheme 1. Proposed Mechanism of Degenerate Phenol/Phenoxide
Interchange in L2(OH)(OTi[N(CH2CH2O)3]) (2)
Figure 5. Repeating unit in the structure of {L2(OTi[N(CH2CH2O)3])2}4
(4). The Ti2-Ti4 unit (linked to the Ti1-Ti3 unit by O23) is chemically
equivalent to the Ti1-Ti3 unit but is crystallographically inequivalent.
the only thorough study one of exchange of rhenium
alkoxides L2Re(CO)3(OR) with alcohols or phenols reported
by Simpson and Bergman.21 The principal mechanism
inferred in this system, involving protonation of the alkoxide
and dissociation to a [L2Re(CO)3]+[RO- -H- -OR′]- ion pair,
seems unlikely to be relevant to the highly oxophilic and
electronically unsaturated titanium complexes studied here.
The lack of a significant solvent effect (between toluene and
methylene chloride) also argues against the formation of ionic
intermediates in the reaction.
Instead, we propose that exchange involves coordination
of the free phenolic oxygen to titanium, with proton transfer
to one arm of the aminetriethoxide ligand taking place
concurrently or in a subsequent step (Scheme 1). Dissociation
of the protonated arm of the triethanolamine and pseudo-
rotation of the five-coordinate, “arm-off” species would
position the protonated arm appropriately to exchange with
the originally apical phenoxide group, completing the
interchange. Direct exchange between phenoxide and phenol
by an analogous pathway would also effect an interchange
between the groups. However, the constraints imposed by
the chelate ring would require extremely acute C-O-H
angles during the proton transfer, if both phenoxide oxygens
are coordinated to titanium. In contrast, the crystal structure
of 2 demonstrates the feasibility of forming a hydrogen bond
between a phenol and the deprotonated triethanolamine,
though this is only indirectly relevant to proton transfer in a
Discussion
The hydroxylated triphenylmethanes Ph3-nCH(C6H2-3,5-
tBu2-2-OH)n (n ) 1, L1OH; n ) 2, L2(OH)2; n ) 3, L3(OH)3)
react readily with tert-butoxytitanatrane [N(CH2CH2O)3]Ti-
(OtBu), undergoing metathesis of the phenol with the tert-
butoxide to give the aryloxytitanatranes 1, 2, and 3,
respectively, and free tert-butyl alcohol. Regardless of the
stoichiometry used, only one phenol group per triphenyl-
methane is metalated by tert-butoxytitanatrane. In all cases,
the complexes form monomeric, trigonal bipyramidal com-
plexes with fully chelated and deprotonated triethanolamine
ligands and monodentate phenoxides, as confirmed by X-ray
crystallography for 1 and 2. Complex 2 thus contains one
free OH group and compound 3 two free OH groups.
NMR spectra of 2 and 3 confirm that free and bound
aryloxide groups are distinct on the NMR time scale near
room temperature, but begin to exchange as the temperature
is raised. Thus, the titanium can “hop” from one oxygen of
L2(OH)2 or L3(OH)3 to another, with the proton of the newly
metalated oxygen taking the place vacated by the titanium.
The reactions are first-order in titanium, but have surprisingly
negative entropies of activation (∆Sq ) -28.8 ( 0.5 cal/
mol‚K for 2, -30.9 ( 1.6 cal/mol‚K for 3). This degenerate
phenol metathesis is analogous to the tert-butoxide/phenol
interchange used in the synthesis of compounds 1-3, and
metathesis of alcohols or phenols with metal alkoxides is
ubiquitous in the synthetic chemistry of titanium. Surpris-
ingly, few mechanistic studies of this reaction have been
reported for titanium or indeed for any transition metal, with
(21) Simpson, R. D.; Bergman, R. G. Organometallics 1993, 12, 781-
796.
7002 Inorganic Chemistry, Vol. 43, No. 22, 2004

Triethanolamine Slippage Induced by Polyphenoxide Chelation
species with both groups coordinated to titanium. Initial
exchange with a triethanolamine arm, rather than with the
phenoxide, is also favored by the greater basicity of the
alkoxide. In general, metal alkoxides react favorably with
phenols,22 though there are rare exceptions.23 Finally, the
strongly negative entropies of activation and low enthalpies
of activation suggest that phenol binding to titanium is taking
place and are consistent with a highly ordered four-center
transition state.
a structure analogous to dimeric {[N(CH2CH2O)3Ti(OiPr)}2,
except with the axial groups syn rather than anti, seems
reasonable.
In light of these other, apparently less exotic, possibilities,
why does complex 4 adopt its observed “slipped” structure?
One possible factor that may be important is a preference
for forming a chelated eight-membered ring with terminal
aryloxides. Normally, one thinks of five- and six-membered
chelate rings as being most favorable.26 However, d0 metal
alkoxides generally have rather obtuse M-O-C angles,
presumably to maximize π donation from the oxygen lone
pairs to the electron-poor metal center.27 Such large angles
are incompatible with five- or six-membered rings, yet the
eight-membered rings here allow relatively large angles of
149° on average, only modestly more acute than the 165°
angles seen in the unconstrained phenoxides 1 and 2. Indeed,
even larger chelate rings containing alkoxides are known for
titanium and show little evidence of strain.28 Complex 4
provides a dramatic example of how this preference for
chelation can even outweigh the well-known predilection of
the nitrilotriethoxide ligand to adopt a tetradentate binding
mode.
The facility with which partial dechelation of the trietha-
nolamine ligand takes place in the phenol interchange
reactions of 2 or 3 is surprising, given the well-known
stability of the metallatrane structure. Nevertheless, this
dynamic dechelation is pushed even farther in fully metalated
complex 4, where triethanolamine slippage is observed even
in the static structure. Although metalation of the free phenols
in 2 or 3 is relatively difficult, requiring titanium amides
rather than alkoxides, once the second titanium does bind in
4, there is a net ligand shift. The bisphenoxide chelates with
terminal aryloxides to one titanium, which effectively loses
its aminetriethoxide group to a neighboring titanium, which
binds the two nitrogens and five of the six oxygens of two
aminetriethoxide ligands. The sixth CH2CH2O group extends
to ligate the phenoxide-bound titanium of a neighboring
dititanium unit in the cyclic complex 4. It thus appears that
the relative proclivities of the bisphenoxide and the amine-
triethoxide to chelate are closely balanced, with aminetri-
ethoxide chelation preferred (although only modestly) in 2
and 3, and the bis(phenoxide) ligand chelating preferentially
in the fully metalated 4.
Conclusions
Mono-, di-, and trihydroxylated triphenylmethanes L1OH,
L2(OH)2, and L3(OH)3 are readily monometalated by [N(CH2-
CH2O)3]TiOtBu to give complexes 1-3. In these monomeric
complexes, the nitrilotriethoxide is fully chelated, and the
aryloxide is monodentate. However, complexes 2 and 3,
which contain free phenols, readily undergo intramolecular
migration of titanium between the phenol units, apparently
via an intermediate which contains a chelated bis(phenoxide)
and a monoprotonated nitrilotriethoxide. Further metalation
of the free phenols in 2 and 3 is difficult, requiring metathesis
with titanium amides, and mixtures of products are formed.
In the crystallographcially characterized fully metalated
complex 4, the bisphenoxide ligand L2(O)22- is chelated, even
though this requires an unprecedented displacement of the
nitrilotriethoxide ligand from the chelated titanium onto a
neighboring titanium in the cyclic octatitanium complex.
Thus, the proclivity of methylenebis(phenoxides) to chelate
to titanium appears to rival or even exceed the propensity
of nitrilotriethoxide to form the usual chelated atrane
structure.
Metallatranes where two nitrilotriethanol ligands are bound
to a single metal center are known.8,24 In these cases, though,
the second triethanolamine is bound as a simple terminal
alkoxide, with the amine uncoordinated. There is no prece-
dent for a “double atrane” structure similar to 4. Conversely,
methylenebis(phenoxide) ligands20d,25 and methylidynetris-
(phenoxide)11 ligands are well-known to be able to bridge
metal centers. Even if the phenoxides adopt a terminal
bonding mode, an alternative structure for L2(OTi[(OCH2-
CH2)3N])2 where each aryloxide is bonded to a different
titanium appears reasonable. A simple bis(complex) similar
to 1 or 2 might be possible, and if this brought the two
titanium centers too close to each other, the nitrilotriethoxide
oxygens could presumably bridge the titaniums. For example,
(22) (a) Erikson, T. K. G.; Bryan, J. C.; Mayer, J. M. Organometallics
1988, 7, 1930-1938. (b) Uno, T.; Hatano, K.; Nishimura, Y.; Arata,
Y. Inorg. Chem. 1990, 29, 2803-2807. (c) Holland, P. L.; Andersen,
R. A.; Bergman, R. G.; Huang, J.; Nolan, S. P. J. Am. Chem. Soc.
1997, 119, 12800-12814. (d) Bergquist, C.; Storrie, H.; Koutcher,
L.; Bridgewater, B. M.; Friesner, R. A.; Parkin, G. J. Am. Chem. Soc.
2000, 122, 12651-12658.
Acknowledgment. We thank Dr. Alicia Beatty for
assistance with the X-ray crystallography. Financial support
of this work by the National Science Foundation (Grant
CHE-97-33321-CAREER) is gratefully acknowledged. S.N.B.
(23) Thorn, D. L.; Harlow, R. L.; Herron, N. Inorg. Chem. 1996, 35, 547-
548.
(26) Hancock, R. D. J. Chem. Educ. 1992, 69, 615-621.
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W. A. J. Am. Chem. Soc. 1999, 121, 6258-6268.
(25) (a) Clegg, W.; Elsegood, M. R. J.; Teat, S. J.; Redshaw, C.; Gibson,
V. C. J. Chem. Soc., Dalton Trans. 1998, 3037-3039. (b) Hagenau,
U.; Heck, J.; Kaminsky, W.; Schauwienold, A.-M. Z. Anorg. Allg.
Chem. 2000, 626, 1814-1821. (c) Matsuo, T.; Kawaguchi, H.; Sakai,
M. J. Chem. Soc., Dalton Trans. 2002, 2536-2540. (d) Matsuo, T.;
Kawaguchi, H. Inorg. Chem. 2002, 41, 6090-6098.
(28) (a) Fandos, R.; Teuben, J. H.; Helgesson, G.; Jagner, S. Organome-
tallics 1991, 10, 1637-1639. (b) Fokken, S.; Spaniol, T. P.; Okuda,
J.; Sernetz, F. G.; Mu¨lhaupt, R. Organometallics 1997, 16, 4240-
4242. (c) Hampel, F.; van Eikema Hommes, N.; Hoops, S.; Maaref,
F.; Schobert, R. Eur. J. Inorg. Chem. 1998, 1253-1262. (d) Anthis,
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Inorganic Chemistry, Vol. 43, No. 22, 2004 7003

Maestri and Brown
Supporting Information Available: Crystallographic data for
1, 2, and 4‚2C5H12‚8.5CH2Cl2 in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
also thanks the Camille and Henry Dreyfus Foundation (New
Professor Award), DuPont (Young Professor Award), and
the Dow Chemical Company (Innovation Recognition Pro-
gram) for support. A.G.M. acknowledges support by a J.
Peter Grace Fellowship.
IC0492901
7004 Inorganic Chemistry, Vol. 43, No. 22, 2004