Titanium complexes supported by a sterically encumbering N-anchored tris-arylphenoxide ligand

Article abstract of DOI:10.1016/j.inoche.2005.06.028

A family of neutral and cationic Ti(IV) complexes supported by the sterically demanding and trianionic ligand (O₃N)^3- (O ₃N^3- = tris(2-O,3-Ad,5-tBubenzyl)amine, Ad = 1-adamantyl) have been prepared from the hydrolysis of TiCl₄(THF)₂ with the free base (O₃N)H₃ and excess NEt₃, followed by anion exchange reactions. These (O₃N)Ti⁺ cores are resistant to moisture and air, are mononuclear, and possess C₃ symmetry on the NMR timescale.

Full text of DOI:10.1016/j.inoche.2005.06.028

Inorganic Chemistry Communications 8 (2005) 903–907
www.elsevier.com/locate/inoche
Titanium complexes supported by a sterically encumbering
N-anchored tris-arylphenoxide ligand
a
a
b
´
Sara A. Cortes , Miguel A. Munoz Hernandez , Hidetaka Nakai ,
˜
b
b
c
c
Ingrid Castro-Rodriguez , Karsten Meyer , Alison R. Fout , Deanna L. Miller ,
c
c,
*
John C. Huffman , Daniel J. Mindiola
a
Centro de Investigaciones Quimicas, Universidad Autonoma del Estado de Morelos, Cuernavaca, Morelos, Mexico
Department of Chemistry and Biochemistry, Mail Code 0358, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0358, USA
b
c
Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington, IN 47405, USA
Received 28 May 2005; accepted 20 June 2005
Available online 15 August 2005
Abstract
A family of neutral and cationic Ti(IV) complexes supported by the sterically demanding and trianionic ligand (O3N)3ꢀ
(O3N3ꢀ = tris(2-O,3-Ad,5-tBubenzyl)amine, Ad = 1-adamantyl) have been prepared from the hydrolysis of TiCl4(THF)2 with the
free base (O3N)H3 and excess NEt3, followed by anion exchange reactions. These (O3N)Ti+ cores are resistant to moisture and
air, are mononuclear, and possess C3 symmetry on the NMR timescale.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Tripodal ligands; N-anchored; Titanium complexes; Trianionic-amines; X-ray structure
Unlike the popularized triamido-amine ligands
(N3N)3ꢀ [1], N-anchored tris-arylphenoxide ligands of
the type (O3N)3ꢀ have received far less attention [2–5]
despite these ancillary ligands being inherently more
resistant to hydrolysis than the amide analogues. One
major drawback of the (O3N)3ꢀ is their ability to poly-
merize via bridging of the polyphenolic motifs [1a]. An
obvious strategy to avoid this problem is to incorporate
steric groups proximal to the hydroxyl motif, thereby
disfavoring intermolecular interactions [2–4]. However,
several examples of mononuclear systems supported by
non-encumbering N-anchored tris-arylphenoxide li-
gands have been scarcely reported, thus hinting that
electronic factors might also play a role in formation
of mono versus polynuclear compounds [6]. Recent re-
ports by Kol [2] Brown [4] and others [3,5,6] have dem-
onstrated that the (O3N)3ꢀ framework is highly
compatible with Ti(IV), and depending on the steric nat-
ure of the tripodal ligand, these systems can have con-
siderable resistance to hydrolysis. Given the high Ti–O
bond enthalpy (ꢁ673 kJ/mol) [7] and stability of the
N-anchored tris-arylphenoxide core, we decided to
investigate the chemistry of Ti(IV) complexes supported
with the sterically demanding ligand (O3N)3ꢀ
(O3N3ꢀ = tris(2-O,3-Ad,5-tBubenzyl)amine,
Ad = 1-
adamantyl, Scheme 1). The choice of 1-adamantyl
(Ad) and tBu substituents in the aryl motifs are specifi-
cally designed for steric protection of the (O3N)Ti scaf-
fold but allow the entry of only small molecules into the
deep pocket provided by the sterically encumbering Ad
groups.
*
Corresponding author. Tel.: +1 812 855 2399; fax: +1 812 855
In order to prepare the free base (O3N)H3 we modi-
fied the Mannich-type procedure described by Kol [2]
8300.
E-mail address: mindiola@indiana.edu (D.J. Mindiola).
1387-7003/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2005.06.028

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S.A. Cortes et al. / Inorganic Chemistry Communications 8 (2005) 903–907
Scheme 2. Synthesis of complexes 1-Cl, 1-OTf, [1][B(C6F5)4].
Scheme 1.
ing C3 symmetry in solution. As a result, salient spectro-
scopic features for 1-Cl include the observation of one
Ad and tBu environment. Most notably, the methylene
protons are diastereotopic thus rendering the complex
chiral. Similar observations have been reported for the
2,4-tBu or 2,4-dimethyl phenoxide derivatives
(O3N)Ti(R) (Rꢀ = OiPr [2], OtBu and O2CCF3 [4]).
Single crystals of 1-Cl suitable for X-ray diffraction
analysis were obtained from toluene at ꢀ35 °C (Fig. 1).3
Upon inspection, the structure of 1-Cl reveals a tita-
nium(IV) center disposed within a C3 symmetric N-
anchored tris-arylphenoxide scaffold. The Ti–O distances
using instead the sterically encumbering 2-tBu,4-Ad
phenol, [8] hexamethylenetetramine, and a catalytic
amount of p-toluenesulfonic acid in 1-propanol.1
Recrystallization of the crude product from ethanol af-
fords the free ligand in 45–55% yield as a white powder.
Treatment of the (O3N)H3 with 1 equiv of TiCl4(THF)2
and 4 equiv of NEt3 for 12 h, generates the chloro tita-
nium precursor (O3N)Ti(Cl) 1-Cl in 90% yield as an yel-
low powder (Scheme 2).2 As expected, complex 1-Cl is
remarkably stable in air and prolonged exposure to ex-
cess O2 or H2O does not lead to degradation as sug-
gested by its 1H NMR spectrum. 1H and 13C NMR
spectra of 1-Cl are in accord with the molecule possess-
˚
are similar (ꢁ1.80 A) and the Ti center lies in a trigonal
bipyramidal environment with the N and Cl occupying
the axial positions and with all Ad groups syn with respect
to the Clꢀ group. The Ti atom sits ꢁ0.22 A above the
˚
mean plane defined by the O-atoms, a value comparable
with that reported by Kol and co-workers [2]. In order
to avoid collisions of the sterically demanding adamantyl
groups, the aryloxide fragments in 1-Cl are skewed in a
propeller-like fashion (Fig. 2). The depth of the cavity,
which is defined as the distance from the titanium atom
to the mean plane of the furthest adamantyl hydrogens
1
(O3N)H3 was synthesized as follows: Hexamethylenetetramine
(0.84 g, 6.02 mmol), 2-Ad-5-tBuphenol (11.7 g, 41.2 mmol), 4 mL of
1-propanol, and 0.4 g p-toluenesulfonic acid (catalyst) are heated in a
reaction vessel to 140 °C for 6 days. The melt gradually changes color
to bright yellow. Upon cooling, 50 mL of Et2O and 50 mL EtOH are
added and the mixture stirred, filtered, and washed with 10 mL of
EtOH to afford a white powder (Yield: 5.6 g, 45%). 1H NMR (25 °C,
399.8 MHz, C6D6): d 7.45 (d, JH–H = 1.5 Hz, 3H, aryl-H), 7.03 (d, JH–
H = 1.5 Hz, 3H, aryl-H), 6.50 (s, 3H, aryl-OH), 3.40 (s, 6H, NCH2-
aryl), 2.38 (s, 18H, Ad-H), 2.15 (s, 9H, Ad-H), 1.87 (m, 18H, Ad-H),
1.37 (s, 27H, aryl-tBu).
˚
blocking the cavity, was estimated to be ꢁ4.28 A.
2
1-Cl was prepared according to the following procedure: Trieth-
ylamine (0.20 g, 2.0 mmol) was added to a solution of TiCl4(THF)2
(0.17 g, 0.5 mmol) in toluene (30 mL). This solution was added to a
solution of (O3N)H3 (0.45 g, 0.50 mmol) in toluene (30 mL) and stirred
for 12 h to yield a deep orange solution with a white solid (NHEt3Cl).
The mixture was filtered, and the solvent was evaporated. The yellow
solid was washed with n-hexane and dried in vacuum. Crystals suitable
for structure determination were grown from toluene at ꢀ35 °C
(0.44 g, 90%). 1H NMR (25 °C, 399.8 MHz, C6D6): d 7.42 (d, JH–
H = 2.0 Hz, 3H, aryl-H), 6.76 (d, JH–H = 2.0 Hz, 3H, aryl-H), 3.91
(broad s, 3H, NCH 2-aryl), 2.63 (broad s, 3H, NCH2-aryl), 2.43 (s,
18H, Ad-H), 2.17 (s, 9H, Ad-H), 2.13 (d, JH–H = 11.2, 9H, Ad-H), 1.80
(d, JH–H = 11.2, 9H, Ad-H), 1.35 (s, 27H, aryl-tBu). 13C{H} NMR
(25 °C, 100.6 MHz, C6D6): d 162.21–124.14 (aryl), 59.34 (NCH2),
41.39, 38.10, 37.61 (Ad), 35.08 (C (CH3)3), 32.24 (C(CH3)3), 30.02
(Ad). IR (cmꢀ1) 3024 (s), 2962 (m), 2905 (m), 2851 (m), 2324 (w), 2210
(w), 1960 (m), 1815 (m), 1598 (w), 1529 (w), 1484 (m), 1412 (w), 1318
(w), 1250 (m), 1213 (m), 1127 (w). Anal. Calcd. (found) for
3
Crystal data for C84H108ClNO3Ti, 1-Cl Æ 3C7H8: M = 1263.06,
orthorhombic, space group Pna2(1), a = 17.685(6), b = 25.045(9),
3
˚
˚
c = 16.175(7) A, a = b = c = 90°, U = 7164(5) A , Z = 4,
Dc = 1.171 g cmꢀ3, l(Mo Ka) = 0.206 mmꢀ1, T = 119(2) K, Bruker
SMART 6000, total reflcns 85067, unique reflcns F > 4r(F) 16545,
observed reflcns 10935 (Rint = 0.0848). The structure was solved using
SHELXS-97 and refined with SHELXL-97. A direct-methods solution
was calculated which provided most non-hydrogen atoms from the
E-map. Full-matrix least squares/difference Fourier cycles were
performed which located the remaining non-hydrogen atoms. All
non-hydrogen atoms were refined with anisotropic displacement
parameters. All hydrogen atoms were located in subsequent Fourier
maps and included as isotropic contributors in the final cycles of
refinement. GoF = 0.898 and the final refinement converged at
R1 = 0.0468 and wR2 = 0.1084 (F2 all data). The structure was found
as proposed with two independent molecules per asymmetric unit and
the remaining electron density is located on bonds.
C
63H84NO3TiCl: C, 76.69 (76.82); H, 8.58 (8.69); N, 1.42 (1.32).

S.A. Cortes et al. / Inorganic Chemistry Communications 8 (2005) 903–907
905
Fig. 2. A perspective view of 1-OTf with thermal ellipsoids at the 50%
probability level (bottom) with hydrogen atoms and the SO2CF3 group
omitted for clarity. Independent molecules as well as solvent molecules
˚
have been also excluded. Selected bond lengths (A): Ti(1)–O(69),
2.031(2); Ti(1)–O(32), 1.808(2); Ti(1)–O(54), 1.791(2); Ti(1)–O(10),
1.828(2); Ti(1)–N(2), 2.172(3). Selected angles (°): O(69)–Ti(1)–N(2),
173.5(1); O(32)–Ti(1)–O(10), 130.1(1); O(32)–Ti(1)–O(54), 110.0(1);
O(10)–Ti(1)–O(54), 116.9(1); O(32)–Ti(1)–O(69), 90.2(1); O(32)–Ti(1)–
N(2), 84.3(1); O(10)–Ti(1)–O(69), 96.9(1); O(10)–Ti(1)–N(2), 84.0(1);
O(54)–Ti(1)–O(69), 100.7(1); O(54)–Ti(1)–N(2), 84.6(1).
Li[B(C6F5)4]. However, exchange of chloride for ꢀOTf
in THF at ꢀ35 °C via treatment of 1-Cl with AgOTf re-
sulted in clean formation of the triflato analogue
(O3N)Ti(OTf) 1-OTf, as a dark red microcrystalline
compound in 82% yield (Scheme 1).4 Complex 1-OTf dis-
plays 1H and 13C NMR spectra similar to 1-Cl, which is
also in accord with a C3 symmetric system (vide supra).
The 19F NMR spectrum of 1-OTf reveals incorporation
of the triflato group, and as a result, the reduction poten-
tial of 1-OTf is considerably more positive (ꢀ1.62 V)
Fig. 1. Complete space filling model of 1-Cl looking down the Cl–Ti–
N vector (top). A perspective view of 1-Cl with thermal ellipsoids at the
50% probability level (bottom) with hydrogen atoms omitted for
clarity. Independent molecules as well as solvent molecules have been
4
1-OTf was prepared according to the following procedure: A
˚
also omitted. Selected bond lengths (A): Ti(1)–O(11), 1.816(7);
solution of AgOTf (0.11 g, 0.41 mmol) in THF was added to a solution
of 1-Cl (0.4 g, 0.40 mmol) in THF at ꢀ35 °C. After being stirred for 1 h
at room temperature, the volatile components were removed under
reduced pressure. The resultant red solid was extracted with toluene
(20 mL), washed with hexane (15 mL) and dried in vacuum (0.37 g,
82%). Complex 1H NMR (25 °C, 399.8 MHz, C6D6): d 7.43 (d, JH–
H = 2.4 Hz, 3H, aryl-H), 6.75 (d, JH–H = 2.4 Hz, 3H, aryl-H), 3.49
(broad s, 6H, NCH2-aryl), 2.26 (s, 18H, Ad-H), 2.17 (s, 9H, Ad-H),
2.01 (d, JH–H = 11.8, 9H, Ad-H), 1.79 (d, JH–H = 11.8, 9H, Ad-H),
Ti(1)–O(33), 1.798(9); Ti(1)–O(55), 1.804(2); Ti(1)–N(3), 2.251(9); Ti(1)–
Cl(2), 2.310(1). Selected angles (°): Cl(2)–Ti(1)–N(3), 178.45(6);
O(33)–Ti(1)–O(11), 123.13(9); O(33)–Ti(1)–O(55), 112.04(9); O(11)–
Ti(1)–O(55), 120.61(9); O(33)–Ti(1)–Cl(2), 97.56(6); O(33)–Ti(1)–N(3),
83.98(8); O(11)–Ti(1)–Cl(2), 96.23(5); O(11)–Ti(1)–N(3), 82.74(7);
O(55)–Ti(1)–Cl(2), 96.77(6); O(55)–Ti(1)–N(3), 82.80(9).
t
1.30 (s, 27H, aryl- Bu). 13C{H} NMR (25 °C, 100.6 MHz, C6D6): d
Given the sterically congested environment around
the titanium center in complex 1-Cl we sought exchange
with a labile or weakly coordinating anion in lieu of Clꢀ,
in order to open a coordination site. Unfortunately,
compound 1-Cl failed to react with nucleophiles such
as LiMe or salts such as Na[B(3,5-(CF3)2C6H3)4] or
161.65-124.22 (aryl), 59.43 NCH2, 42.06, 38.92, 37.36 (Ad), 35.05 (C
(CH3)3), 32.07 (C(CH3)3), 29.78 (Ad). 19F NMR (25 °C, 376.4 MHz,
C6D6) d:ꢀ75.10. IR (cmꢀ1) 3066 (s), 3024 (s), 2966 (m), 2907 (m), 2854
(m), 2325 (w), 2210 (w), 1960 (m), 1815 (m), 1514 (m), 1465 (s), 1193
(w), 1085 (w), 1035 (m), 980 (m). Anal. Calcd. (found) for
C
64H84NO6TiF3S: C, 69.86 (70.19); H, 7.69 (7.87); N, 1.27 (1.41).

906
S.A. Cortes et al. / Inorganic Chemistry Communications 8 (2005) 903–907
than that observed for the 1-Cl analogue (ꢀ1.70 V).5 To
ascertain whether complex 1-OTf was a discrete salt or
not we conducted single crystal X-ray diffraction studies.
The molecular structure of 1-OTf is depicted in Fig. 2
and exposes a neutral Ti(IV) center confined in a similar
environment to the structure of 1-Cl.6 It is clear from the
molecular structure that the OTfꢀ group is coordinated
formation of the salt [(O3N)Ti][B(C6F5)4] [1][B(C6F5)4]
in 89% isolated yield.7 Unfortunately, treatment of 1-
Cl with Lambertꢀs [Et3Si][B(C6F5)4] reagent, [10] does
not afford [1][B(C6F5)4] in one step (Scheme 2).7 Com-
plex [1][B(C6F5)4] was characterized by a combination
of 1H, 13C, 19F, 11B NMR spectra all of which lend sup-
port to a C3 symmetric molecule in solution. As ex-
pected, the salt [1][B(C6F5)4] is insoluble in benzene
and hexane but readily dissolves in THF, Et2O, and
CH2Cl2. Despite formally being a 12eꢀ complex,
[1][B(C6F5)4] appears to be remarkably stable and expo-
sure of the salt to water or O2 fail to give new products.
Likewise, complex [1][B(C6F5)4] does not appear to
coordinate polar molecules such as Et2O, THF, CO,
CO2, N2O, or O2CPh2, regardless of this system having
an open coordination site. Thus it appears that the
(O3N)Ti+ scaffold is sterically protected in spite of this
system having an open coordination site in the axial po-
sition. However, heating a solution of [1][B(C6F5)4]
gradually results in decomposition.
˚
to the metal center (Ti–OOTf, 2.031(2) A), and that the
metal center is in the mean plane defined by the O-atoms
˚
of the N-anchored ligand (ꢁ0.18 A). As with 1-Cl, the
depth of the cavity provided by the adamantyl groups
˚
was estimated at ꢁ4.14 A.
Complexes 1-Cl and 1-OTf are chiral systems, but dis-
play inversion barriers of DGzð340Þ ¼ 15.3 and DGzð268Þ
¼
12.0 kcal=mol, respectively, which are estimated from
variable temperature NMR spectra in toluene-d8 [2,9].
Unlike 1-Cl we speculate that complex 1-OTf might be
undergoing dissociation in solution, thus giving a lower
barrier to racemization.
The AgOTf step is needed in order to promote anion
exchange of 1-OTf with a salt such as Li[B(C6F5)4].
Brown and co-workers have previously demonstrated
that the trifluoroacetate group can be readily substituted
in the (O3N)Ti+ scaffold [4]. Accordingly, treatment of
1-OTf with Li[B(C6F5)4] in benzene at room tempera-
ture leads to clean anion substitution concomitant with
In conclusion, we have demonstrated that the steri-
cally encumbering ligand (O3N)3ꢀ can stabilize neutral
and cationic titanium(IV) systems. All these compounds
appear resistant to hydrolysis and coordination of polar
substrates, which could be the result of the sterically
protecting Ad groups.
Acknowledgements
5
Cyclic voltammetry was performed in pre-dried solutions of THF
(0.3 M of pre-dried and recrystallized TBAH, Aldrich). A platinum
disk (2.0 mm diameter, Bioanalytical Systems), a platinum wire, and
silver wire were employed as the working electrode, the auxiliary, and
the reference electrode, respectively. A one compartment cell was used
in the CV measurement. The electrochemical response was collected
with the assistance of an E2 Epsilon (BAS) autolab potentiostat/
galvanostat with a BAS software. The IR drop correction was applied
when significant resistance was noted. All the potentials were reported
against ferrocenium/ferrocene couple (0 V) measured as an internal
standard. All spectra were recorded under an N2 atmosphere. In a
typical experiment 15–20 mg of crystalline 1-Cl and 1-OTf was
dissolved in 4 mL of a TBAH solution in THF at 26 °C.
We thank Indiana University-Bloomington, the Ca-
mille and Henry Dreyfus Foundation (New Faculty
Award to D.J.M.), the Alfred P. Sloan Foundation (Fel-
lowship award to D.J.M., 2005–2007), and the National
Science Foundation (CHE-0348941) for financial sup-
port of this research. A.R.F. and D.L.M. acknowledge
7
[1][B(C6F5)4] was prepared according to the following procedure: A
6
Crystal data for C67.32H88.50Cl2.94F3NO6.33STi, 1-OTf Æ 2CH2Cl2
solution of Li[B(C6F5)4] (0.20 g, 0.23 mmol) in benzene was added to a
solution of 1-OTf (0.25 g, 0.23 mmol) in benzene at room temperature.
A white precipitate (LiOTf) was formed immediately. After being
stirred for 1 h, the volatile components were removed under reduced
pressure. The resultant red solid was washed with hexane (15 mL) and
dried in vacuum (0.33 g, 89%). Alternatively, complex [1][B(C6F5)4]
and 1/3Et2O: M = 1254.36, monoclinic, space group C2/c,
˚
a = 47.842(7), b = 27.983(4), c = 30.294(5) A, b = 106.013(4),
3
˚
U = 3 8 9 8 3 ( 1 0 ) A
,
Z = 2 4 , D c = 1 . 2 8 2 g c m ꢀ3
,
l ( M o
Ka) = 0.345 mmꢀ1, T = 125(2) K, Bruker SMART 6000, total reflcns
274803, unique reflcns F > 4r(F) 44861, observed reflcns 19636
(Rint = 0.1591). The structure was solved using SHELXS-97 and
refined with SHELXL-97.2 A direct-methods solution was calculated
which provided most non-hydrogen atoms from the E-map. Full-
matrix least squares/difference Fourier cycles were performed which
located the remaining non-hydrogen atoms. All non-hydrogen atoms
were refined with anisotropic displacement parameters. Three inde-
pendent molecules were present, as well as dichloromethane and
diethyl ether solvent molecules. Because of the number of atoms
present, no attempt was made to locate and refine hydrogen atoms,
although many were present in the difference Fourier map phased on
the non-hydrogen atoms. All hydrogen atoms were placed in ideal
positions and refined as riding atoms with relative isotropic displace-
ment parameters. GoF = 0.823, and the final refinement converged at
R1 = 0.0606 and wR2 = 0.1251 (F2, all data).
can be prepared quantitatively by addition of
1 equiv of
[Et3Si][B(C6F5)4] in toluene at ꢀ35 °C. 1H NMR (25 °C, 399.8 MHz,
CD2Cl2): d 7.36 (s, 6H, aryl-H), 3.82 (broad s, 6H, NCH2-aryl), 2.10 (s,
27H, Ad-H), 1.86 (d, JH–H = 13.6, 9H, Ad-H), 1.74 (d, JH–H = 13.6,
9H, Ad-H), 1.29 (s, 27H, aryl-tBu). 13C{H} NMR (25 °C, 100.6 MHz,
CD2Cl2): d 161.64–123.84 (aryl), 59.09 NCH2, 41.69, 37.88, 37.12 (Ad),
34.97 (C (CH3)3), 31.44 (C(CH3)3), 29.36 (Ad). 19F NMR (25 °C,
CD2Cl2, 376.43 MHz) d:ꢀ133.44 (B(C6F5)4),ꢀ164.18 (B(C6
F5)4),ꢀ167.99 (B(C6F5)4). 11B (25 °C, 128.4, CD2Cl2):d-
16.91(B(C6F5)4). IR (cmꢀ1) 3083 (s), 3023 (s), 2966 (w), 2907 (m),
2854 (w), 2325 (w), 2210 (w), 1959 (w), 1814 (m), 1643 (w), 1513 (m),
1476 (s), 1366 (w), 1276 (w), 1192 (w), 1085 (w), 980 (m). Anal. Calcd.
(found) for C87H84NO3TiBF20: C, 64.10 (62.74); H, 5.19 (5.30); N, 0.86
(0.71).

S.A. Cortes et al. / Inorganic Chemistry Communications 8 (2005) 903–907
907
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j.inoche.2005.06.028.
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