Synthesis of titanium(iv) 3,6-di-tert-butylcatecholate complexes

Article abstract of DOI:10.1007/s11172-019-2570-8

Two synthetic approaches were used for the synthesis of 3,6-di-tert-butylcatecholate derivatives of titanium(iv). These approaches are based on the exchange reaction between sodium catecholate and titanium(iv) chloride and the reaction of 3,6-di-tert-buty

Full text of DOI:10.1007/s11172-019-2570-8

Russian Chemical Bulletin, International Edition, Vol. 68, No. 7, pp. 1414—1423, July, 2019
1414
Synthesis of titanium(IV) 3,6-di-tert-butylcatecholate complexes
I. N. Meshcheryakova, A. S. Shavyrin, A. V. Cherkasov, and A. V. Piskunov
G. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences,
49 ul. Tropinina, 603950 Nizhny Novgorod, Russian Federation.
Fax: +7 (831) 462 7497. E-mail: pial@iomc.ras.ru
Two synthetic approaches were used for the synthesis of 3,6-di-tert-butylcatecholate de-
rivatives of titanium(IV). These approaches are based on the exchange reaction between sodium
catecholate and titanium(IV) chloride and the reaction of 3,6-di-tert-butylpyrocatechol with
titanium tetrabutoxide. Depending on the reaction conditions, the former method allows the
synthesis of the titanium monocatecholate complex 3,6-CatTiCl2(THF)2 (3,6-Cat is doubly
deprotonated 3,6-di-tert-butylpyrocatechol) or bimetallic sodium titanium catecholate deriva-
tives. Using the latter method, it is possible to synthesize six-coordinate titanium(IV) bis-cate-
cholate complexes in high yields after the introduction of neutral bidentate donor ligands
(TMEDA, DME, 2,2´-bipyridyl) to the reaction mixture.
Key words: titanium(IV), 3,6-di-tert-butyl-o-benzoquinone, 3,6-di-tert-butylpyrocatechol,
catecholate complexes, redox-active ligands.
In recent literature, there has been a sustained and
A large part of titanium(IV) catecholate complexes char-
acterized by X-ray diffraction are those based on unsub-
stituted pyrocatechol. Complexes with 4-nitro-, 4-methyl-,
4-tert-butyl-, 3-methoxy-, and 3,5-di-tert-butylpyro-
catechols3,4,6,7,10—12 are also known. The majority of
reliably characterized titanium(IV) complexes contain Ti
and Cat (Cat is doubly deprotonated unsubstituted or
substituted pyrocatechol) in a ratio higher than 1. The
complexes, in which the coordination sphere of the tita-
nium atom includes additional bulky neutral or charged
ligands, are rare examples of neutral mononuclear mono-
growing interest in titanium(IV) compounds containing
different diolate, in particular catecholate, ligands. This
type of compounds has attracted great attention due to
prospects of their practical use in different fields, includ-
ing medicine,1—3 the design of ceramic materials,4 coat-
ings,4 photovoltaic cells,5—8 and homogeneous and hetero-
geneous catalysts.9—14 For instance, titanium(IV) mono-
catecholate complexes containing different salen and
salan ligands exhibit anticancer activity.1—3 In particular,
this is due to the presence of the catecholate moiety dis-
playing antibacterial, antitumor, cytotoxic, anti- and
prooxidant activity.15,16 The introduction of the catecho-
late functionality into polyoxotitanium and titanium
alkoxide clusters not only facilitates the study of the clus-
ter structures but also endows these compounds with
important physicochemical properties.4—11,17,18 In par-
ticular, polyoxotitanium catecholate nanoparticles exhibit
an intense absorption band in the visible region of electro-
magnetic radiation assigned to titanium—catechol ligand-
to-metal charge transfer (LMCT), which significantly
increases the efficiency of these compounds as photo-
catalysts. The following two methods are described in the
literature for the synthesis of such clusters: the reactions
of titanium(IV) alkoxides with pyrocatechols under solvo-
thermal conditions and the introduction of the catecholate
moiety into the ready polyoxotitanates. It should be noted
that even slight changes in the reaction conditions (the
ratio of the starting reagents, the solvent, temperature,
and so on) lead to changes in the composition and struc-
ture of the resulting clusters and, as a consequence, to
changes in their chemical and physicochemical properties.
catecholate derivatives of TiIV 1—3,12,13
The only structur-
.
ally characterized compound with the ratio Cat : Ti > 1,
namely, the titanium(IV) bis-catecholate complex, was
synthesized based on unsubstituted pyrocatechol.11 The
use of the close analogs of pyrocatechols, such as substi-
tuted o-aminophenols19 and alpha-diamines,20—26 as li-
gands for the preparation of titanium(IV) complexes allows
the synthesis of various mononuclear monoligand or
bis(ligand) derivatives. As in the case of the known cate-
cholate derivatives, titanium amidophenoxide and diamide
complexes are characterized by reversible intramolecular
ligand-to-metal charge transfer.26 Hence, this class of
compounds holds promise in homogeneous or hetero-
geneous catalysis. The goal of this study is to develop
synthetic approaches for the preparation of TiIV 3,6-di-
tert-butylcatecholate complexes. The presence of bulky
tert-butyl groups adjacent to catecholate oxygen atoms in
the ligand system should decrease the possibility of the
formation of oligomeric structures. Besides, broad op-
portunities for functionalization of the 3,6-di-tert-butyl-
catecholate ligand27—29 and the ability of the latter to
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1414—1423, July, 2019.
1066-5285/19/6807-1414 © 2019 Springer Science+Business Media, Inc.

TiIV 3,6-di-tert-butilcatecholate complexes
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 7, July, 2019
1415
reversibly change the oxidation state in metal complexes
(the redox-active ligand) make it possible to significantly
modify the chemical properties of coordination and
organometallic compounds.30
O(3)
С(6)
С(3)
O(1)
O(2)
С(1)
С(2)
С(5)
С(4)
Results and Discussion
Cl(1)
Cl(2)
Ti(1)
One of the most versatile methods for the synthesis of
various types of organometallic and coordination com-
pounds based on redox-active ligands involves the ex-
change reaction between alkali metal catecholates and
halides of appropriate elements.31 Hence, in the first step,
we employed this synthetic approach in order to prepare
titanium(IV) 3,6-di-tert-butylcatecholate complexes. The
addition of a solution of titanium(IV) chloride in toluene
to a solution of sodium 3,6-di-tert-butylcatecholate
(3,6-CatNa2) in THF at room temperature leads to the
immediate change in the color of the reaction mixture
from pale yellow to deep wine-red. Regardless of the
stoichiometric ratio of the reagents (3,6-CatNa2 : TiCl4 =
= 1 : 1 or 2 : 1), we isolated the monocatecholate complex
3,6-CatTiCl2(THF)2 (1), which contains two chlorine
atoms and two THF molecules coordinated to the titanium
atom, as the major product (Scheme 1).
Therefore, on the experimental time scale, the ex-
change reaction in this medium occurs only in the first
step, and the subsequent reaction of 3,6-CatTiCl2(THF)2
with 3,6-CatNa2, which would afford the TiIV bis-cate-
cholate complex, is not observed (see Scheme 1). Appar-
ently, this behavior of the reaction system is attributed to
the use of the mixed THF—toluene solvent. Toluene can
significantly decrease the rate of exchange reactions of this
type.32 Compound 1 was isolated by recrystallization from
diethyl ether as a red-brown finely crystalline powder. The
composition and structure of complex 1 were determined
by elemental analysis, IR and 1H NMR spectroscopy,
and X-ray diffraction. The highest preparative yields of
3,6-CatTiCl2(THF)2 were achieved using the reagents in
the ratio 3,6-CatNa2 : TiCl4 = 1 : 1.
O(4)
Fig. 1. Molecular structure of complex 1 with thermal ellipsoids
drawn at 50% probability level. Hydrogen atoms are omitted for
clarity.
According to the X-ray diffraction data, the complex
3,6-CatTiCl2(THF)2 (1) is isomorphous with the known
tin catecholate complex 3,6-CatSnCl2(THF)2.33 The
molecular structure of 1 is shown in Fig. 1. Selected bond
lengths and bond angles of complex 1 are given in Table 1.
The titanium atom in complex 1 has a distorted octahedral
environment with the oxygen atoms of the chelating ligand
and two chlorine atoms in the equatorial plane and the
oxygen atoms of THF molecules in apical positions. The
deviation of Ti from the O(1)O(2)Cl(1)Cl(2) plane is
0.010(2) Å; the O(3)—Ti(1)—O(4) angle is 176.38(6)
(Table 1). The tert-butyl groups of the catecholate li-
gand are in the eclipsed conformation. The Ti(1)—O(1)
(1.905(2) Å) and Ti(1)—O(2) (1.901(2) Å) bond lengths
are smaller than the sum of the covalent radii of the cor-
responding elements (2.03 Å)34 and are typical of titani-
um(IV) catecholate complexes.1—13,17,18 The O(1)—C(1)
(1.358(2) Å) and O(2)—C(2) (1.354(2) Å) bond lengths
correspond to O—C single bonds in the known catecholate
complexes.33,35 The C—C distances in the six-mem-
Scheme 1
i. THF, toluene.

1416
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 7, July, 2019
Meshcheryakova et al.
/deg
Table 1. Selected bond lengths (d) and bond angles () in complex 1
Bond
d/Å
Angle
/deg
Angle
Ti(1)—O(1)
Ti(1)—O(2)
Ti(1)—Cl(1)
Ti(1)—Cl(2)
Ti(1)—O(3)
Ti(1)—O(4)
O(1)—C(1)
O(2)—C(2)
1.905(2)
1.901(2)
2.3439(7)
2.3302(6)
2.062(2)
2.073(2)
1.358(2)
1.354(2)
O(2)—Ti(1)—O(1)
O(2)—Ti(1)—O(3)
O(1)—Ti(1)—O(3)
O(2)—Ti(1)—O(4)
O(1)—Ti(1)—O(4)
O(3)—Ti(1)—O(4)
O(2)—Ti(1)—Cl(2)
O(1)—Ti(1)—Cl(2)
78.82(6)
91.25(6)
92.35(6)
91.66(6)
90.32(6)
176.38(6)
91.33(4)
170.01(5)
O(3)—Ti(1)—Cl(2)
O(4)—Ti(1)—Cl(2)
O(2)—Ti(1)—Cl(1)
O(1)—Ti(1)—Cl(1)
O(3)—Ti(1)—Cl(1)
O(4)—Ti(1)—Cl(1)
Cl(2)—Ti(1)—Cl(1)
89.49(4)
88.30(5)
170.47(4)
91.66(4)
88.55(5)
88.92(5)
98.19(2)
bered catecholate carbon ring vary in a narrow range
(1.393(3)—1.408(3) Å). The Ti(1)—O(3) (2.062(2) Å) and
Ti(1)—O(4) (2.073(2) Å) bond lengths are comparable
with the sum of the covalent radii of the titanium and
oxygen atoms (2.03 Å)34 and are significantly smaller than
the sum of their van der Waals radii (3.6 Å).34
temperature-dependent. This indicates that in solution,
the monomeric complex is present in equilibrium with its
oligomeric derivatives. In dilute solutions, this equilibrium
is almost completely shifted to the monomeric complex.
The exchange reaction using titanium tetrachloride
bis(tetrahydrofuranate) TiCl4(THF)2 as the starting re-
agent also did not afford the mononuclear titanium(IV)
bis-catecholate complex (Scheme 2). The mixing of solu-
tions of 3,6-CatNa2 and TiCl4(THF)2 in THF is accom-
panied by the appearance of a deep dark orange color of
the reaction mixture. This reaction afforded bimetallic
sodium titanium catecholate derivatives. Thus, a portion
of red-orange crystals was obtained after the replace-
ment of THF by diethyl ether, the filtration from a so-
dium chloride precipitate, and prolonged crystalliza-
tion. The resulting compound is the bimetallic complex
3,6-Cat4Ti2(OH)2Na2(THF)2 (2), which is apparently
produced by the hydrolysis of one of the reaction products
1
The H NMR spectrum of a dilute solution of com-
pound 1 in C6D6 is in agreement with the expected spec-
tral pattern. The proton signals of THF at  1.22 and 4.35
are broadened and shifted with respect to those of free
THF ( 1.40 and 3.57). This is evidence that the coordina-
tion of tetrahydrofuran molecules to the titanium atom is
retained in solution. The analysis of the 1H NMR spectra
of compound 1 revealed the dependence of the spectral
pattern on the concentration. An increase in the concen-
tration of complex 1 in solution leads to a broadening of
the lines accompanied by the appearance of additional
signals. Besides, the line widths and chemical shifts are
Scheme 2

TiIV 3,6-di-tert-butilcatecholate complexes
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 7, July, 2019
1417
С(4A)
С(5A)
But
С(18A)
But
С(19A)
С(20A)
С(3A)
С(2A)
С(17A)
С(16A)
С(6A)
But
O(2A)
But
С(15A)
С(1A)
O(1A)
O(4A)
O(3A)
Ti(1A)
H(7)
O(7A)
O(5)
Na(1)
Na(1A)
O(5A)
O(7)
H(7A)
Ti(1)
O(3)
O(1)
С(1)
O(4)
But
С(15)
O(2)
But
С(16)
С(6)
С(20)
С(2)
С(3)
С(19)
С(17)
С(5)
But
С(18)
But
С(4)
Fig. 2. Molecular structure of complex 2•2THF with thermal ellipsoids drawn at 50% probability level. Hydrogen atoms (except for
hydrogens of hydroxy groups) and methyl groups of tert-butyl substituents are omitted for clarity.
due to the uncontrolled presence of trace moisture in the
reaction system. Minor compound 2 was characterized by
X-ray diffraction (Fig. 2). The major product of this reac-
tion, the bimetallic complex 3,6-Cat3TiNa2(TMEDA)2
(3), was isolated in 54% yield by the treatment of the
mother liquor in diethyl ether with the bidentate chelating
base tetramethylethylenediamine (TMEDA) (see Scheme 2).
Complex 3 was characterized by elemental analysis, IR
and 1H NMR spectroscopy, and X-ray diffraction.
and 3, respectively. Compound 2 crystallizes in space group
C2/c. The molecule of complex 2 lies in a special position.
There is one THF solvent molecule per asymmetric unit.
Therefore, complex 2 crystallizes as solvate 2•2THF. The
coordination polyhedra of TiIV in complexes 2 and 3 can
be described as distorted octahedra with the O(1), O(3)
(2) and O(1), O(1A) (3) atoms in apical positions and the
oxygen atoms of the catecholate ligands in the equatorial
planes. The O(1)—Ti(1)—O(3) (2) and O(1)—Ti(1)—
O(1A) (3) angles are 163.92(4) and 168.70(6), respec-
tively. Like in complex 1, the bond length distribution in
the catecholate moieties of complexes 2 and 3 (C—O,
The molecular structures of complexes 2 and 3 are
shown in Figs 2 and 3, respectively. Selected bond lengths
and bond angles of these complexes are given in Tables 2
Table 2. Selected bond lengths (d) and bond angles () in complex 2•2THF
Bond
d/Å
Bond
d/Å
Angle
/deg
Angle
/deg
Ti(1)—O(1)
Ti(1)—O(2)
Ti(1)—O(3)
Ti(1)—O(4)
Ti(1)—O(7)
Ti(1)—O(7A) 2.0515(9)
O(1)—C(1) 1.366(2)
1.9673(8)
1.8820(9)
1.9745(8)
1.8852(9)
2.0250(9)
O(2)—C(2)
O(3)—C(15)
O(4)—C(16)
Na(1)—O(1)
1.351(2)
1.366(2)
1.349(2)
2.338(2)
O(2)—Ti(1)—O(4) 105.26(4)
O(2)—Ti(1)—O(1) 80.12(4)
O(4)—Ti(1)—O(1) 87.44(4)
O(2)—Ti(1)—O(3) 94.13(4)
O(4)—Ti(1)—O(3) 79.56(4)
O(1)—Ti(1)—O(3) 163.93(4)
O(2)—Ti(1)—O(7) 155.09(4)
O(4)—Ti(1)—O(7) 95.40(4)
O(1)—Ti(1)—O(7)
O(3)—Ti(1)—O(7)
O(2)—Ti(1)—O(7A)
O(4)—Ti(1)—O(7A) 160.11(4)
O(1)—Ti(1)—O(7A) 108.80(3)
O(3)—Ti(1)—O(7A)
O(7)—Ti(1)—O(7A)
87.18(3)
103.31(4)
89.21(4)
Na(1)—O(3A) 2.343(2)
Na(1)—O(7)
O(7)—H(7)
2.2865(9)
0.873(9)
85.95(3)
74.66(4)

1418
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 7, July, 2019
Meshcheryakova et al.
С(17)
С(16)
С(17A)
С(16A)
But
But
N(2)
N(2A)
С(15)
O(3)
С(15A)
Na(1A)
O(3A)
N(1)
N(1A)
Na(1)
O(1)
Ti(1)
O(1A)
С(1A)
But
С(1)
But
С(6A)
O(2A)
С(6)
O(2)
С(2A)
С(2)
С(5)
С(4)
С(3A)
But
С(3)
But
С(5A)
С(4A)
Fig. 3. Molecular structure of complex 3 with thermal ellipsoids drawn at 50% probability level. Hydrogen atoms and methyl groups
of tert-butyl substituents are omitted for clarity.
1.347(2)—1.366(2) Å; C—C in the six-membered carbon
ring, 1.385(2)—1.411(3) Å) suggests that the redox-active
ligand exists as the dianion.33,35 The Ti—Ocat distances in
2 and 3 vary in a broad range (1.8820(9)—1.976(2) Å) and
are smaller than the sum of the covalent radii of the cor-
responding elements (2.03 Å34). The Ti—O(2) and Ti—O(4)
bond lengths are in the range of 1.8820(9)—1.9251(9) Å
and are comparable with the corresponding distances in
complex 1 (1.901(2)—1.905(2) Å). The Ti—O(1) and
Ti—O(3) bonds (1.9652(9)—1.976(2) Å) are longer than
the Ti—O(2) and Ti—O(4) bonds, which is apparently due
to additional coordination of the oxygen atoms O(1) and
O(3) to the Na atom. The Na—Ocat distances are 2.338(2)—
2.419(2) Å. Besides, each Na+ cation in complexes 2 and
3 is coordinated by one THF (in 2; Na—O, 2.253(3) Å)
or TMEDA (in 3; Na—N, 2.448(2)—2.500(4) Å) molecule.
The hydroxyl oxygen atoms in complex 2 are coordinated
in a μ3 mode and are linked to two Ti4+ cations (Ti—O(7),
2.0250(9), 2.0515(9) Å) and one Na+ cation (Na—O(7),
2.2865(9) Å). The Ti(1) and Ti(1A) atoms in complex 2
are at a distance of 3.2415(5) Å from each other.
Therefore, the use of a solution of TiCl4 in toluene or
a solution of TiCl4(THF)2 in THF in the exchange reac-
tion with sodium catecholate resulted in the formation of
different products. The presence of toluene in the reaction
mixture apparently leads to a decrease in the reaction
rate of nucleophilic substitution of chloride anions32 in
3,6-CatTiCl2(THF)2 that formed in the first step. In pure
THF, the exchange reaction proceeds much faster and to
a greater extent but affords heterometallic sodium titanium
catecholate derivatives.
Hence, in order to prepare the mononuclear titani-
um(IV) bis-catecholate complex, we studied the reaction
of 3,6-di-tert-butylpyrocatechol (3,6-CatH2) with titani-
um(IV) tetrabutoxide (Scheme 3). The appropriate pyro-
catechol was prepared by the reduction of 3,6-di-tert-
Table 3. Selected bond lengths (d) and bond angles () in complex 3
Bond
d/Å
Bond
d/Å
Angle
/deg
Angle
/deg
Ti(1)—O(1) 1.9652(9)
Ti(1)—O(2) 1.9251(9)
Ti(1)—O(3) 1.976(2)
O(1)—C(1) 1.362(2)
O(2)—C(2)
1.347(2)
1.351(2)
O(2A)—Ti(1)—O(2)
O(2A)—Ti(1)—O(1A)
O(2)—Ti(1)—O(1A)
O(1A)—Ti(1)—O(1)
O(1)—Ti(1)—O(3A)
101.40(6)
79.65(4)
93.16(4)
168.71(6)
100.55(4)
O(2)—Ti(1)—O(3A)
O(2)—Ti(1)—O(3)
O(1)—Ti(1)—O(3)
O(3)—Ti(1)—O(3A)
91.62(4)
162.61(4)
88.25(4)
78.28(5)
O(3)—C(15)
Na(1)—O(1A) 2.419(2)
Na(1)—O(3)
2.419(2)

TiIV 3,6-di-tert-butilcatecholate complexes
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 7, July, 2019
1419
Scheme 3
L = TMEDA (4), DME (5), bipy (6)
butyl-o-benzoquinone (3,6-Q) with hydrazine hydrate in
methanol and was characterized by IR and 1H NMR
spectroscopy. The spectroscopic characteristics of this
compound were not reported in the previous study.36 The
reaction of titanium tetraalkoxides with diols is widely
used for the synthesis of titanium oxo clusters, the reaction
generally being performed under solvothermal condi-
tions.4—9,18 The reaction at ambient temperature favors
the formation of low-molecular-weight products.1—3,11,13
Upon the mixing of colorless solutions of Ti(OBu)4 and
3,6-CatH2 in Et2O at room temperature, the reaction
mixture immediately turned deep brown. The addition of
TMEDA to the reaction mixture is accompanied by
a change in the color of the solution to orange and the
formation of the crystalline complex 3,6-Cat2Ti(TMEDA)
(4) (see Scheme 3). By analogy with complex 4, we also
synthesized titanium bis-catecholate derivatives contain-
ing coordinated 1,2-dimethoxyethane (3,6-Cat2Ti(DME)
(5)) and 2,2´-bipyridyl (3,6-Cat2Ti(bipy) (6)) molecules
(see Scheme 3). Compounds 5 and 6 were isolated in high
yields as brown and maroon-red finely crystalline powders,
respectively. The compositions and structures of com-
pounds 4—6 were confirmed by IR and 1H NMR spectro-
scopy and elemental analysis. The molecular structure of
complex 4 is shown in Fig. 4. Selected bond lengths and
bond angles of complex 4 are given in Table 4.
Like in complexes 1—3, the coordination environment
of the titanium atom in compound 4 can be described as
a distorted octahedron. The equatorial base of the octa-
hedron is formed by the oxygen atoms O(1) and O(1A) of
two catecholate ligands and nitrogen atoms of the TMEDA
molecule. The oxygen atoms O(2) and O(2A) are in apical
positions. The titanium atom lies in the O(1)O(1A)N(1)
N(1A) plane; the O(2)—Ti(1)—O(2A) angle is 161.84(5).
The Ti—O bond lengths (1.9054(8)—1.9216(8) Å) are
comparable with the Ti—Ocat distances in complexes 1—3
(1.8820(9)—1.9251(9) Å). The bond length distribution
in the catecholate moieties (C—O, 1.355(2)—1.357(2) Å;
C—C, 1.389(2)—1.410(2) Å) is also similar to that in
complexes 1—3 (C—O, 1.347(2)—1.366(2) Å; C—C,
But
С(5A)
С(5)
С(6A)
С(4)
С(6)
С(4A)
С(1A)
But
С(3A)
O(1A)
С(1)
С(3)
But
O(1)
С(2)
С(2A)
But
O(2)
Ti(1)
O(2A)
N(1A)
N(1)
Fig. 4. Molecular structure of complex 4 with thermal ellipsoids drawn at 50% probability level. Hydrogen atoms and methyl groups
of tert-butyl substituents are omitted for clarity.

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Meshcheryakova et al.
/deg
Table 4. Selected bond lengths (d) and bond angles () in complex 4
Bond
d/Å
Angle
/deg
Angle
Ti(1)—O(1)
Ti(1)—O(2)
Ti(1)—N(1)
O(1)—C(1)
O(2)—C(2)
1.9216(8)
1.9054(8)
2.255(2)
1.355(2)
1.357(2)
O(2A)—Ti(1)—O(2)
O(2A)—Ti(1)—O(1)
O(2)—Ti(1)—O(1)
O(1)—Ti(1)—O(1A)
O(2A)—Ti(1)—N(1)
161.84(5)
90.31(3)
79.93(3)
115.08(5)
90.00(3)
O(2)—Ti(1)—N(1)
O(1)—Ti(1)—N(1)
O(1A)—Ti(1)—N(1)
N(1)—Ti(1)—N(1A)
104.30(4)
85.42(4)
156.94(4)
76.97(6)
1.385(2)—1.411(3) Å) and corresponds to the dianionic
form of coordination of the redox-active ligand. The Ti—N
bond lengths (2.255(2) Å) in complex 4 are larger than the
sum of the covalent radii of the corresponding elements
(2.04 Å34) but are significantly smaller than the sum of
their van der Waals radii (3.7 Å34).
the formation of monomeric structures. The synthesized
titanium diolate compounds are rare examples of com-
plexes with the ratio Cat : Ti  1.
Experimental
1
The H NMR spectrum of a dilute solution of com-
All operations associated with the synthesis of titanium(IV)
catecholate complexes were performed under conditions exclud-
ing exposure to atmospheric oxygen and moisture using
a Schlenk vacuum line. All solvents were purified and dried
according to standard procedures.37 The following commercial
reagents were used: TiCl4, Ti(OBu)4, TMEDA, and 2,2´-bi-
pyridyl (all from Aldrich). The reagents 3,6-Q and TiCl4(THF)2
were synthesized by the procedures described in the literature
(see Refs 38 and 39, respectively).
pound 4 in CDCl3 shows proton signals of tert-butyl groups
at  1.24, catecholate moieties at  6.77, and the TMEDA
molecule. The chemical shifts of diamine protons (3.40,
3.55 ppm) substantially differ from those of the free ligand
(2.24, 2.38 ppm), which is indicative of the retention of
Ti—TMEDA coordination in solution. Noteworthy is that
the 1H NMR spectra of complex 4 are also characterized
by the dependence of the chemical shifts and the line width
of the signals on the concentration. An increase in the
concentration of compound 4 in a solution in CDCl3 or
C6D5CD3 leads to a broadening of the proton signals of
tert-butyl groups, the catecholate moiety, and TMEDA.
As in the case of compound 1, this is most probably attri-
buted to the existence of the monomer—oligomer equi-
librium. This equilibrium is evidenced by the temperature
dependence of the line width of the signals in the 1H NMR
spectrum. A decrease in the temperature of the sample
leads to a narrowing of the signals corresponding to protons
of the catecholate ligand and to an increase in the number
of these signals. This attests to dynamic processes, result-
ing in the magnetic nonequivalence of these hydrogen
atoms. Currently, we failed to identify compounds exist-
ing in equilibrium. The investigation of this process is
underway.
To summarize, we synthesized and characterized for
the first time titanium(IV) complexes based on the 3,6-di-
tert-butylpyrocatecholate dianion. The exchange reactions
between titanium tetrachloride and sodium catecholate
or between titanium(IV) tetrabutoxide and 3,6-di-tert-
butylpyrocatechol allows the targeted synthesis of mono-
and bis-diolate derivatives of titanium, respectively. The
structures of four new compounds were established by
X-ray diffraction. The geometric parameters of the syn-
thesized complexes are generally similar to those of the
known titanium(IV) catecholate derivatives. The presence
of bulky tert-butyl substituents, which shield the oxygen
atoms of the catecholate ligand, was shown to facilitate
The 1H NMR spectra were recorded on Bruker DPX-200
(200 MHz) and Bruker Avance III (400 MHz) spectrometers
with trimethylsilane as the internal standard. The IR spectra were
measured on a FSM 1201 Fourier-transform infrared spectro-
meter as Nujol mulls in KBr cells.
Synthesis of (3,6-di-tert-butylcatecholato)dichlorobis(tetra-
hydrofuran)titanium(IV), 3,6-CatTiCl2(THF)2 (1). A. Reaction of
3,6-CatNa2 with TiCl4 (2 : 1). A solution of 3,6-Q (0.2 g,
0.9 mmol) in THF (15 mL) was added to sodium metal (2.3 g,
0.1 mol). The mixture was stirred until the initial color of quinone
completely disappeared and a pale yellow solution of 3,6-CatNa2
was obtained. The solution was separated from excess metal, and
then a solution of TiCl4 (0.05 mL, 0.45 mmol) in toluene (3 mL)
was slowly added with continuous stirring to the resulting solution
at room temperature. The reaction mixture immediately turned
deep wine-red. The mixture was stirred for 30 min, THF was
removed, and the dry residue was dissolved in Et2O. The ethe-
real solution was filtered off from the NaCl precipitate on a Schott
glass filter no. 4. The filtrate was concentrated and kept overnight
at room temperature resulting in the formation of a maroon-
brown finely crystalline product. The preparative yield of ana-
lytically pure complex 1 was 0.13 g (30% with respect to the
starting 3,6-Q). Found (%): C, 54.93; H, 7.84. C22H36Cl2O4Ti.
Calculated (%): C, 54.68; H, 7.51. IR, /cm–1: 1588 m, 1488 m,
1389 m, 1383 s, 1358 s, 1344 w, 1299 w, 1285 m, 1269 w, 1248 w,
1238 w, 1203 m, 1174 m, 1045 m, 1032 m, 1011 s, 990 s, 945 m,
922 m, 856 v.s, 827 m, 815 m, 692 s, 652 m, 605 s, 560 w, 475 v.s,
467 v.s. 1H NMR (200 MHz, C6D6, 20 C), : 1.22 (m, 8 H,
H2CTHF); 1.41 (s, 18 H, But); 4.35 (m, 8 H, CH2OTHF); 6.64
(s, 2 H, CHcatechol).
B. Reaction of 3,6-CatNa2 with TiCl4 (1 : 1). The reaction
was performed as described in A. The starting reagents were used
in the following amounts: 0.2 g (0.9 mmol) of 3,6-Q and 0.1 mL

TiIV 3,6-di-tert-butilcatecholate complexes
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 7, July, 2019
1421
(0.9 mmol) of TiCl4. The yield of the analytically pure product
was 0.354 g (81.3%).
(400 MHz, CDCl3, 20 C), : 1.36 (s, 54 H, But); 2.23 (s, 24 H,
MеTMEDA); 2.39 (s, 8 H, H2CTMEDA); 6.47 (s, 6 H, CHcatechol).
Synthesis of 3,6-di-tert-butylpyrocatechol, 3,6-CatH2. Hydr-
azine hydrate was added dropwise with continuous stirring to
a solution of 3,6-Q (2 g, 9.08 mmol) in methanol (30 mL) until
the solution was completely colorless. Then water was added
dropwise to the methanolic solution resulting in the formation
of a white finely crystalline precipitate of 3,6-CatH2. The pre-
cipitate was separated by filtration, dried in air, and recrystallized
from hexane. The yield of the product was 1.88 g (93%). Found (%):
C, 75.79; H, 10.05. C14H22O2. Calculated (%): C, 75.63; H, 9.97.
IR, /cm–1: 3532 m, 3502 m, 3469 s, 1611 m, 1501 m, 1483 s,
1429 s, 1418 s, 1387 s, 1361 s, 1277 s, 1217 s, 1202 s, 1165 s,
1143 s, 1027 m, 964 s, 933 m, 904 m, 812 s, 770 m, 668 w, 652 s,
580 w, 533 w, 520 w. 1H NMR (200 MHz, C6D6, 20 C), : 1.36
(s, 18 H, But); 4.78 (s, 2 H, OH); 6.82 (s, 2 H, CHcatechol).
Reaction of 3,6-CatH2 with Ti(OBu)4. A. Synthesis of bis(3,6-
di-tert-butylcatecholato)(N,N,N´,N´-tetramethylethylenediamine)-
titanium(IV), 3,6-Cat2Ti(TMEDA) (4). A solution of 3,6-CatH2
(0.2 g, 0.9 mmol) in Et2O (10 mL) was gradually added with
continuous stirring to a solution of Ti(OBu)4 (0.15 mL, 0.45 mmol)
in the same solvent (5 mL). The solution immediately turned
brown. The reaction mixture was stirred for 30 min at room
temperature. Then Et2O was removed, the dry residue was dis-
solved in hexane (15 mL), and TMEDA (0.5 mL) was added.
The color of the solution changed to orange-brown followed by
Synthesis of (3-3,6-di-tert-butylcatecholato)-bis(2-3,6-di-
tert-butylcatecholato)-bis(N,N,N´,N´-tetramethylethylenediamine)-
disodium titanium(IV), 3,6-Cat3TiNa2(TMEDA)2 (3). A solution
of TiCl4(THF)2 (0.15 g, 0.45 mmol) in THF (3 mL) was slowly
added with continuous stirring to a solution of 3,6-CatNa2, which
was synthesized by the above-described procedure from 3,6-Q
(0.2 g, 0.9 mmol), in the same solvent (15 mL) at room tem-
perature. The color of the solution immediately changed from
pale yellow to orange-brown. The reaction mixture was stirred
for 30 min, THF was removed, and the dry residue was dissolved
in Et2O. The solution was separated from the white NaCl pre-
cipitate on a Schott glass filter no. 4. The filtrate was concen-
trated to 30% of the initial volume and kept overnight at room
temperature. A small amount of red-orange crystals of complex
2•2THF that precipitated were separated and studied by X-ray
diffraction. Tetramethylethylenediamine (0.5 mL) was added to
the residual solution. An orange crystalline product precipitated
within 1—2 h. The yield of complex 3 was 0.16 g (54%). Found (%):
C, 66.17; H, 9.18. C54H88N4Na2O6Ti. Calculated (%): C, 65.97;
H, 9.02. IR, /cm–1: 1590 w, 1573 w, 1534 w, 1481 s, 1400 v.s,
1357 s, 1309 m, 1291 s, 1283 s, 1261 s, 1239 v.s, 1202 m, 1181 m,
1158 m, 1141 s, 1129 m, 1101 m, 1074 m, 1034 m, 1025 s, 980 s,
945 s, 939 s, 927 m, 922 s, 828 w, 810 s, 789 s, 776 m, 691 v.s,
650 s, 608 w, 598 w, 572 v.s, 501 s, 468 s, 423 w. 1H NMR
Table 5. Crystallographic data and the X-ray diffraction data collection and structure refinement statistics for complexes 1—4
Parameter
1
2•2THF
3
4
Formula
Molecular weight
Crystal system
Space group
a/Å
C
22H36Cl2O4Ti
483.31
Monoclinic
P21/n
10.940(2)
15.9464(13)
14.226(4)
90
C72H114Na2O14Ti2
1345.41
C54H92N4Na2O6Ti
987.19
C34H56N2O4Ti
604.70
Tetragonal
P42/n
15.1310(3)
15.1310(3)
14.8527(4)
90
Monoclinic
C2/c
22.7672(17)
13.8148(10)
24.0782(18)
90
Monoclinic
C2/c
13.5016(4)
22.0513(6)
20.1625(6)
90
b/Å
c/Å
/deg
/deg
94.11(3)
90
102.8690(10)
90
107.8390(10)
90
90
90
/deg
V/Å3
2475.4(8)
4
7383.0(9)
4
5714.3(3)
4
3400.48(16)
4
Z
F000
1024
2896
2144
1312
dcalc/Mg m–3
/mm–1
Crystal size/mm
Scan range, /deg
hkl ranges
1.297
1.210
1.147
1.181
0.585
0.287
0.213
0.289
0.200.200.05
2.93—26.02
–13  h  13
–19  k  19
–17  l  17
0.380.360.16
2.31—28.68
–30  h  30,
–18  k  18,
–32  l  32
0.400.270.11
2.13—28.79
–18  h  18,
–29  k  29,
–27  l  27
0.370.110.10
2.35—28.71
–20  h  16,
–20  k  20,
–20  l  20
Number of reflections
observed
35676
4871
0.0509
1.013
40941
9465
0.0264
1.021
36174
7426
0.0461
1.036
30122
4400
0.0389
1.002
unique
Rint
S(F2)
R1, wR2 (I > 2(I))
R1, wR2 (based on all reflections)
Residual electron density,
0.0367, 0.0735
0.0538, 0.0788
0.0354, 0.0896
0.0439, 0.0947
0.0428, 0.1023
0.0592, 0.1097
0.0330, 0.0856
0.0430, 0.0917
max/min)/e Å–3
)
0.37/–0.29
0.36/–0.37
0.30/–0.49
0.33/–0.42

1422
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 7, July, 2019
Meshcheryakova et al.
the gradual formation of crystalline complex 4. The reaction
mixture was kept at room temperature for 2 h to achieve more
complete precipitation of the product. The product was sepa-
rated from the mother liquor by decantation and dried under
reduced pressure. The yield of analytically pure complex 4 was
0.196 g (72%). Found (%): C, 67.62; H, 9.38. C34H56N2O4Ti.
Calculated (%): C, 67.53; H, 9.33. IR, /cm–1: 1477 s, 1440 s,
1397 s, 1351 s, 1313 m, 1281 s, 1258 s, 1248 s, 1232 m, 1203 m,
1169 w, 1155 w, 1144 m, 1121 m, 1103 w, 1065 m, 1049 w, 1019 m,
1013 w, 1001 w, 986 m, 953 m, 938 m, 925 m, 827 s, 811 s, 794 m,
778 v.s, 729 v.s, 691 v.s, 674 v.s, 648 s, 604 w, 586 w, 567 s, 551 m,
531 m, 501 m. 1H NMR (400 MHz, CDCl3, 20 C), : 1.24
(s, 36 H, But); 3.40 (s, H, H2CTMEDA); 3.55 (s, H, MeTMEDA);
6.77 (s, 4 H, CHcatechol).
ment parameters for nonhydrogen atoms. The hydroxyl hydrogen
atom in complex 2 was found in a difference Fourier map and
refined isotropically. The other hydrogen atoms of complexes
1—4 were positioned geometrically and refined isotropically with
fixed thermal parameters U(H)iso = 1.2U(C)eq (U(H)iso = 1.5U(C)eq
for the methyl groups).
Selected geometric parameters of complexes 1—4 are given
in Tables 1—4, respectively. The crystallographic data and the
X-ray diffraction data collection and structure refinement statis-
tics are given in Table 5. The structures were deposited with the
Cambridge Crystallographic Data Centre (CCDC 1879922 (1),
1879923 (2), 1879924 (3), 1879925 (4)) and are available at ccdc.
cam.ac.uk/structures.
This study was financially supported by the Russian
Science Foundation (Project No. 17-13-01428). The
spectroscopic and structural studies of the compounds
were performed using equipment of the Analytical Center
of the G. A. Razuvaev Institute of Organometallic Chem-
istry of the Russian Academy of Sciences.
B. Synthesis of bis(3,6-di-tert-butylcatecholato)(1,2-dimeth-
oxyethane)titanium(IV), 3,6-Cat2Ti(DME) (5). Compound 5 was
synthesized by the procedure described above for complex 4. The
starting reagents were used in the following amounts: 0.2 g
(0.9 mmol) of 3,6-CatH2 and 0.15 mL (0.45 mmol) of Ti(OBu)4.
Then DME (0.5 mL) was added to the resulting solution in
hexane (15 mL). The brown crystalline precipitate that formed
was isolated as described above. The yield of the analytically pure
product was 0.164 g (63%). Found (%): C, 66.64; H, 8.77.
References
C32H50O6Ti. Calculated (%): C, 66.43; H, 8.71. IR, /cm–1
:
1552 m, 1482 m, 1422 m, 1365 s, 1310 w, 1283 m, 1262 w, 1242 m,
1202 m, 1181 w, 1141 w, 1101 m, 1036 m, 1029 m, 981 s, 954 m,
939 m, 924 w, 869 m, 831 m, 812 m, 800 m, 715 s, 704 s, 682 m,
651 s, 619 w, 589 m, 552 s, 526 w, 501 s, 493 s, 474 w. 1H NMR
(200 MHz, (CD3)2CO, 20 C), : 1.25 (s, 36 H, But); 3.27 (s, 6 H,
CH3O (DME)); 3.45 (s, 4 H, CH2 (DME)); 6.39 (s, 4 H,
CHcatechol).
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/cm–1: 1601 m, 1574 w, 1567 w, 1485 m, 1447 s, 1394 s, 1357 m,
1315 m, 1283 m, 1259 m, 1245 s, 1232 w, 1204 w, 1174 w, 1143 w,
1062 w, 1045 w, 1028 m, 982 s, 958 w, 940 m, 923 w, 853 w,
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Received February 19, 2019;
in revised form March 26, 2019;
accepted March 30, 2019