Complex Structural Landscape of Titanium Organophosphonates: Isolation of Structurally Related Ti4, Ti5, and Ti6 Species and Mechanistic Insights

Article abstract of DOI:10.1021/acs.inorgchem.7b01651

[Ti(acac)₂(OⁱPr)₂] reacts with tert-butylphosphonic acid to yield a series of titanium organophosphonates such as tetranuclear [Ti₄(acac)₄(μ-O)₂(μ-^tBuPO₃)₂(μ-^tBuPO₃H)₄]·2CH₃CN (1), pentanuclear [Ti₅(acac)₅(μ-O)₂(OⁱPr)(μ-^tBuPO₃)₄(μ-^tBuPO₃H)₂] (2), hexanuclear [Ti₆(acac)₆(μ-O)₂(OⁱPr)₂(μ-^tBuPO₃)₆] (3), or [Ti₆(acac)₆(μ-O)₃(OⁱPr)(μ-^tBuPO₃)₅(μ-^tBuPO₃H)]·2CH₃CN (4). The isolation of each of these products in pure form depends on the molar ratio of the reactants or the solvent medium. Among these, 3 is obtained as the only product when the reaction is conducted in CH₂Cl₂. The structural analysis reveals that a simple cluster growth route relates the clusters 1-4 to each other and that a reactive cyclic single-4-ring titanophosphonate [Ti(acac)(OⁱPr)₂(^tBuPO₃H)]₂ is the fundamental building block. While the tetranuclear 1 has structural resemblance to the D4R building block of zeolites, the hexanuclear clusters 3 and 4 have the shape of zeolitic D6R building blocks. The presence of adventitious water in the phosphonic acid (arising from small quantities of hydrogen-bonded water) results in the formation of μ-O^2- bridges across an adjacent pair of titanium centers in clusters 1-4. To further verify the stability of the hexanuclear cluster over other structural forms, the reaction of ^tBuPO₃H₂ was performed with [Ti(acac)₂(O)], instead of Ti(acac)₂(OⁱPr)₂, in CH₃CN to yield [Ti₆(acac)₆(μ-O)₄(μ-^tBuPO₃)₄(μ-^tBuPO₃H)₂]·2CH₃CN (5). Compound 5 exhibits a core structure similar to those of 3 and 4 with small variations in the intracluster Ti-O-Ti linkage. Compound 3 is an efficient and selective catalyst for olefin epoxidation under both homogeneous and heterogeneous conditions.

Full text of DOI:10.1021/acs.inorgchem.7b01651

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Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Complex Structural Landscape of Titanium Organophosphonates:
Isolation of Structurally Related Ti4, Ti5, and Ti6 Species and
Mechanistic Insights
Kamna Sharma, Rajendran Antony, Alok Ch. Kalita, Sandeep Kumar Gupta, Paul Davis,
and Ramaswamy Murugavel*
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
S
* Supporting Information
ABSTRACT: [Ti(acac)2(OiPr)2] reacts with tert-butylphosphonic acid to yield a series of titanium organophosphonates such as
tetranuclear [Ti4(acac)4(μ-O)2(μ-tBuPO3)2(μ-tBuPO3H)4]·2CH3CN (1), pentanuclear [Ti5(acac)5(μ-O)2(OiPr)-
(μ-tBuPO3)4(μ-tBuPO3H)2] (2), hexanuclear [Ti6(acac)6(μ-O)2(OiPr)2(μ-tBuPO3)6] (3), or [Ti6(acac)6(μ-O)3(OiPr)-
(μ-tBuPO3)5(μ-tBuPO3H)]·2CH3CN (4). The isolation of each of these products in pure form depends on the molar ratio
of the reactants or the solvent medium. Among these, 3 is obtained as the only product when the reaction is conducted in
CH2Cl2. The structural analysis reveals that a simple cluster growth route relates the clusters 1−4 to each other and that a
reactive cyclic single-4-ring titanophosphonate [Ti(acac)(OiPr)2(tBuPO3H)]2 is the fundamental building block. While the
tetranuclear 1 has structural resemblance to the D4R building block of zeolites, the hexanuclear clusters 3 and 4 have the shape of
zeolitic D6R building blocks. The presence of adventitious water in the phosphonic acid (arising from small quantities of
hydrogen-bonded water) results in the formation of μ-O2− bridges across an adjacent pair of titanium centers in clusters 1−4. To
t
further verify the stability of the hexanuclear cluster over other structural forms, the reaction of BuPO3H2 was performed with
[Ti(acac)2(O)], instead of Ti(acac)2(OiPr)2, in CH3CN to yield [Ti6(acac)6(μ-O)4(μ-tBuPO3)4(μ-tBuPO3H)2]·2CH3CN (5).
Compound 5 exhibits a core structure similar to those of 3 and 4 with small variations in the intracluster Ti−O−Ti linkage.
Compound 3 is an efficient and selective catalyst for olefin epoxidation under both homogeneous and heterogeneous conditions.
examples of organic-soluble molecular titanium phosphonate
cages [(Cp*TiO3PR)4(μ-O)2] (R = Me, Ph) and
[(Cp*Ti)3(tBuPO3)2{tBuPO2(OH)}(μ-O)2] were reported by
Roesky and co-workers from the reaction of Cp*TiMe3 with
phosphonic acids.5 These molecules can be considered as
secondary building units (SBUs) for the eventual preparation of
solid-state titanophosphate materials.6 They also reported on
the synthesis of eight-membered dititanophosphonate rings
[(OiPr)3Ti(μ-O)2PPh2]2 and [(OiPr)3Ti(μ-O)2P(tBu)-
(OSiMe3)]2.7 Around the same time Mehring et al. reported
on the reactions of titanium isopropoxide with RP(O)(OH)2
INTRODUCTION
■
Among the catalytically active metals, titanium is of special
interest due to high abundance and low toxicity. Titanium-
doped zeolites and silicates are efficient catalysts in many
organic transformations, notably olefin oxidation and hydrox-
ylation of phenols.1 Several organic-soluble molecular titano-
silicates have been synthesized using different titanium
precursors and organosilanols (RSi(OH)3 or R2Si(OH)2),
exhibiting different nuclearities with different coordination
geometries around the titanium metal ion.1h,i,2 Selections of
these discrete titanosiloxanes have been transformed into high-
titanium-content titanosilicate materials and used in several
organic oxidation reactions.2b Similar to the case for organo-
silanols, phosphonic acids (RPO3H2) and phosphinic acids
(R2PO2H) have also been used to prepare structurally diverse
discrete titanium clusters due to both structural and functional
similarities between silicate and phosphonate clusters in many
instances.3 Different phosphonic acids have been reported to
form structurally diverse titanium clusters with different
nuclearities under different reaction conditions.4 The first
t
(R = Ph, Me, Bu, 4-NCC6H4) in DMSO, resulting in the
isolation of [Ti4(μ3-O)(OiPr)5(μ-OiPr)3(RPO3)3]·DMSO.8
These compounds are the first examples of Ti oxo-alkoxides
modified by tridentate phosphonates (RPO32−). The reaction
of titanium isopropoxide with tert-butylphosphonic acid under
Received: June 28, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01651
Inorg. Chem. XXXX, XXX, XXX−XXX

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rigorous exclusion of water (in toluene) gave the titanium
phosphonate tetramer [Ti(OiPr)2(tBuPO3)]4.8a
butylphosphonates, most of which contain titanium in a
pentacoordinate state.
Further, different phosphonic acids can change or modulate
the nuclearity of the resulting cluster, depending on the nature
of the phosphonic acid substituent. For example, the reaction of
allylphosphonic acid with titanium isopropoxide in DMSO
results in a tetrameric cluster, but bis(trimethylsilyl)-
allylphosphonate reacts with titanium isopropoxide in isopropyl
alcohol to yield a octanuclear cluster.9 Similarly, bis-
(trimethylsilyl)phosphonates RP(O)(OSiMe3)2 (R = ethyl, 3-
chloropropyl, benzyl, 3,5-dimethylphenyl, 2-naphthylmethyl)
react with titanium isopropoxide to give octa-, hepta-, or
tetranuclear clusters.9 Use of similar phosphonates in reactions
with titanium isopropoxide in the presence of acetic acid
resulted in hexanuclear oxo clusters.10 In case of R = 2-
bromopropyl, xylyl, the same reaction yields the pentanuclear
cluster [Ti5O(OiPr)11(OAc)(O3PR)3].11 The use of hydro-
thermal methods for the reaction of phenylphosphonic acid
with titanium ethoxide in ethanol resulted in a tetrameric
cluster which has been used to synthesize polyoxotitanate
nanoclusters Ti25 and Ti26 in the presence of an ammonium
salt.12 Similarly, tert-butylphosphonic acid forms dimeric,
trimeric, and tetrameric titanium clusters on reacting with
titanium methoxide, Cp*TiMe3, and titanium isopropoxide,
respectively.5,8a,13
RESULTS AND DISCUSSION
■
Reactivity of [Ti(acac)2(OiPr)2] with tBuPO3H2. A
detailed investigation of the reactivity of [Ti(acac)2(OiPr)2]
with tert-butylphosphonic acid has been carried out in two
different solvents (acetonitrile and dichloromethane), resulting
in the isolation of a series of titanium phosphonate clusters.
While the reactions carried out in acetonitrile seemed to be
highly dependent on the ratio of the reactants and amount of
the solvent used, yielding tetranuclear, pentanuclear, and
hexanuclear titanophosphonate clusters 1, 2, and 4, the runs
carried out in dichloromethane invariably produced the
hexanuclear cluster 3 (Scheme 1). The isolation of multiple
Scheme 1. Complex Reaction of [Ti(acac)2(OiPr)2] with
tBuPO3H2 in CH3CN or CH2Cl2
Organophosphate ligands can replace phosphonates to yield
even more structurally diverse titanium clusters.14−16 For
example, the reaction of Ti(OSiMe3)4 with di-tert-butylphos-
phate in hexane resulted in the tetrameric titanium complex
[TiO(SiMe3)(O2P(OtBu)2]4 similar to D4R SBUs observed in
zeolites.14 The same phosphate ligand reacts with titanium
alkoxide to give dimeric complexes [Ti(OR)3(O2P(OtBu)2)]2
(R = Et, Pr]). The addition of KOEt to a solution of
[Ti(OEt)3(O2P(OtBu)2)]2 leads to the formation of a
potassium-containing titanium phosphate, [Ti2K(OEt)8(O2P-
(OtBu)2)]2,8a which is a centrosymmetric dimer in the solid
state.15
Hence, it is evident from the structural diversity that the
structure of the clusters has a profound influence on the choice
of the titanium precursor as well as the steric requirements or
the nature of the organic substituent on phosphorus.17 Even
when the substituent on the phosphorus is kept the same (e.g.,
tBuPO3H2), just by changing the source of titanium (e.g.,
Ti(OiPr)4 or Cp*TiMe3) and controlling the presence or
absence of adventitious water in the reaction system, one can
isolate different structural types.5,8a
products in the reactions carried out in acetonitrile is also the
result of variable amounts of adventitious water present in the
reaction medium. Hence, these reactions were carefully
optimized to isolate any one of the three products in a given
run. Further, since all the four isolated clusters seem to have
formed through a common building block (a single-4-ring), a
detailed mechanism for the growth of titanium phosphonates
clusters is also presented.
14
i
[Ti4(acac)4(μ-O)2(μ-tBuPO3)2(μ-tBuPO3H)4]·2CH3CN (1). The
reaction of [Ti(acac)2(OiPr)2] with tert-butylphosphonic acid
in a 1:1.5 ratio using acetonitrile as solvent gives the
tetranuclear titanium phosphonate 1, as shown in Scheme 2.
Scheme 2. Synthesis of [Ti4(acac)4(μ-
O)2(μ-tBuPO3)2(μ-tBuPO3H)4]·2CH3CN (1)
The present study hence is aimed at probing the structural
landscape of titanium phosphonates in detail and establishing a
reaction pathway and, if possible, even a mechanism for the
formation and growth of these clusters in a given reaction
medium under identical reaction conditions. Although there is a
clear advantage in either using an organometallic precursor
such as Cp*TiMe3 (eliminating alkane as byproduct) or using
an alkoxide such as titanium isopropoxide (the byproduct is an
alcohol), we have used in the present study [Ti(acac)2(OiPr)2],
which contains both chelating diketonate and terminal alkoxide
ligands. Such attempts to modify the titanium centers using
similar bidentate ligands without seriously impairing the
reactivity at the metal center has precedence in the
literature.9−11,18 A further likely advantage of using [Ti-
(acac)2(OiPr)2] may stem from the possible stabilization of
six-coordinate titanium centers in the resultant clusters, which
is in stark contrast to the earlier reported titanium tert-
The titanium precursor has been added to the suspension of
phosphonic acid in acetonitrile at room temperature under an
inert atmosphere and filtered to obtain a clear solution after
stirring for 12 h. Pale yellow single crystals of 1 suitable for X-
ray diffraction have been obtained over a period of 20 days
from this solution and characterized by spectroscopic and
analytical techniques, in addition to a single-crystal X-ray
diffraction study.
B
DOI: 10.1021/acs.inorgchem.7b01651
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The FT-IR spectrum exhibits carbonyl stretching vibrations
of the acetylacetone at 1605 cm−1. The strong characteristic
Ti−O−P stretching has been observed at 1139 and 1075 cm−1
(Figure S1 in the Supporting Information). The TGA trace of 1
(Figure S6 in the Supporting Information) shows that
compound is stable up to 100 °C. Compound 1 crystallizes
in the monoclinic P21/c space group. A perspective view of the
full molecule and the molecular core of 1 is shown in Figure 1.
Scheme 3. Synthesis of [Ti5(acac)5(μ-
O)2(OiPr)(μ-tBuPO3)4(μ-tBuPO3H)2] (2)
disorder can be better understood by invoking a simple model
depicted in Figure 2. Thus, the structure of 2 can be described
Figure 1. (left) Molecular structure of [Ti4(acac)4(μ-
O)2(μ-tBuPO3)2(μ-tBuPO3H)4]·2CH3CN (1). Hydrogen atoms are
omitted for clarity. (right) Core structure of 1.
Titanophosphonate 1 is a tetranuclear cluster incorporating
two fully deprotonated phosphonate, four monodeprotonated
phosphonate, four acetylacetonate (acac), and two μ2-O
functionalities. Two solvent acetonitrile molecules are present
in the lattice. Molecule 1 exists as a discrete entity with no
interaction with the neighboring molecules. Each titanium
metal ion in 1 exists in a distorted-octahedral geometry (bond
angles at the metal range between 81.8 and 176.8°). All six
phosphonate ligands in the cluster behave as bidentate ligands
(Harris notation: [2.110]). The titanium metal ion is bonded to
six oxygen atoms, two of which are from acac (equatorial plane)
and three are from phosphonic acid (one at the equatorial plane
and two at axial positions). The sixth coordination site is taken
up by a μ2-oxide anion that forms a bridge with the neighboring
titanium center (equatorial plane).
Figure 2. Schematic representation of the positional disorder of the
Ti(acac)(OiPr) unit in the crystal of 2. In the bottom molecule, the
atoms within the encircled portion (shown in red and blue) exhibit a
SOF of 0.5, while the rest of the molecule shown in black has a SOF of
1.0.
[Ti5(acac)5(μ-O)2(OiPr)(μ-tBuPO3)4(μ-tBuPO3H)2] (2). Com-
plex 2 was formed when only a slight excess of [Ti-
as a pentanuclear titanophosphonate cluster incorporating six
phosphonate (four fully deprotonated and two monodeproto-
nated), five acetylacetonate, one alkoxide, and two μ2-O
functionalities. All five titanium centers in the molecule exist
in a distorted-octahedral geometry. Two pairs of titanium ions
are bridged by μ2-O, while the phosphonate group binds in a
tridentate fashion. Other bond lengths and angles around
titanium are similar to those found for 1.
t
(acac)2(OiPr)2] was used for the reaction with BuPO3H2
(5:6 molar ratio) in acetonitrile with constant stirring at
room temperature. From the resulting reaction mixture, pale
yellow crystals of 2 were isolated over a period of 7 days.
Although this reaction has been repeated several times under
almost identical conditions, cluster 2 was obtained as the
product only in a couple of instances, while cluster 4 has been
obtained in other instances in view of the close structural
relationship between them (vide infra).
Due to the insolubility of cluster 2, it has been characterized
only in the solid state by elemental analysis and IR
spectroscopy (see the Experimental Section), apart from its
molecular structure determination by single-crystal X-ray
diffraction studies. The quality of the crystals obtained for
compound 2 was rather poor (flakelike), and hence the
structure refinement converged at a slightly higher R factor of
10.58%.
[Ti6(acac)6(μ-O)3(OiPr)(μ-tBuPO3)5(μ-tBuPO3H)]·2CH3CN
(4). The reaction of [Ti(acac)2(OiPr)2] with tert-butylphos-
phonic acid in acetonitrile in a 1:1 stoichiometry gives the
hexanuclear complex [Ti6 (acac)6 (μ-O)3 (Oi Pr)-
(μ-tBuPO3)5(μ-tBuPO3H)]·2CH3CN (4) as pale yellow
crystals (Scheme 4). Compound 4 has been fully characterized
by analytical and spectroscopic techniques and a single-crystal
X-ray diffraction study. Compound 4 is the most stable form of
the three different clusters (1, 2, or 4) obtained from the
reactions conducted in acetonitrile.
The molecular structure of 2 can be viewed as an addition
product of 1 with 1 equiv more of Ti(acac)2(OiPr)2 (Scheme
3). A positional disorder with 0.5 site occupancy for the fifth
Ti(acac)(OiPr) unit is observed in its molecular structure. This
A single-crystal X-ray diffraction study reveals that compound
4 crystallizes in the monoclinic P21/c space group. A
perspective view of the molecular structure and core of
[Ti6(acac)6(μ-O)3(OiPr)(μ-tBuPO3)5(μ-tBuPO3H)]·2CH3CN
C
DOI: 10.1021/acs.inorgchem.7b01651
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methane to yield the missing symmetric hexamer 3, which is
described below.
Scheme 4. Synthesis of [Ti6(acac)6(μ-
O)3(OiPr)(μ-tBuPO3)5(μ-tBuPO3H)]·2CH3CN (4)
[Ti6(acac)6(μ-O)2(OiPr)2(μ-tBuPO3)6] (3). Since the reaction
of tBuPO3H2 with [Ti(acac)2(OiPr)2] yielded as many as three
different clusters, 1, 2, and 4 (see below), in acetonitrile with
different nuclearities, the same reaction was repeated in
dichloromethane with the aim of controlling the adventitious
water in the reaction mixture (vide supra). Thus, the reaction of
[Ti(acac)2(OiPr)2] with tert-butylphosphonic acid in dichloro-
methane gives the hexanuclear titanium complex [Ti6(acac)6(μ-
O)2(OiPr)2(μ-tBuPO3)6] (3) as pale yellow crystals, as shown
in Scheme 5. Compound 3 has been fully characterized by
using analytical and spectroscopic techniques, and structure
determination was done by a single-crystal X-ray diffraction
study.
(4) is shown in Figure 3. In a sense, compound 4 can be
considered as a further grown version of 2 through the
Scheme 5. Synthesis of [Ti6(acac)6(μ-
O)2(OiPr)2(μ-tBuPO3)6] (3)
Figure 3. (left) Molecular structure of [Ti6(acac)6(μ-O)3(OiPr)-
(μ-tBuPO3)5(μ-tBuPO3H)]·2CH3CN (4). Hydrogen atoms are
omitted for clarity. (right) Core structure of 4.
X-ray diffraction studies reveal that the compound
[Ti6(acac)6(μ-O)2(OiPr)2(μ-tBuPO3)6] (3) is a novel Ti−O−
P cluster, which crystallizes in the monoclinic P21/n space
group. A perspective view of the molecular structure and the
core structure of 3 are shown in Figure 4.
attachment of a second [Ti(acac)(OiPr)] unit to the cluster 2.
In other words, 4 is formed by double titanium capping of the
original tetrameric cluster 1. Thus, the hexanuclear 4 consists of
six phosphonate, six acetylacetonate (acac), one OiPr, and three
μ2-O functionalities. All six titanium centers exist in a distorted-
octahedral geometry, albeit with different coordination environ-
ments.
Ti1 is coordinated by two oxygen atoms of acac, one oxygen
of a partially deprotonated phosphonate ligand, two oxygens of
fully deprotonated phosphonate, and one μ2-O which bridges
Ti1 to Ti2. The other five coordination sites of Ti2 are
occupied by one acac chelate and three fully deprotonated
phosphonate ligands. Ti3 is bound to one acac, three
deprotonated phosphonates, and one isoproproxide. Ti4 is
found to be coordinated to one acac ligand, three deprotonated
phosphonates, and one μ2-O which bridges Ti4 to Ti6. The six-
coordinate Ti5 metal center is coordinated to one acac and one
partially deprotonated and two fully deprotonated phosphonate
ligands apart from one μ2-O which bridges it to Ti6. Thus, Ti6
is the only titanium center in the molecule that is surrounded
by two μ2-O bridges (Ti4 and Ti5) apart from being
surrounded by one acac ligand and two deprotonated
phosphonate ligands.
Although the structure of 4 may appear very symmetric at
first sight, the possible loss of isopropoxide from Ti5
(diagonally opposite to Ti3), though hydrolysis and subsequent
formation of an additional Ti−O−Ti bridge, render this cluster
less symmetric. Hence, as the hydrolysis of an isopropoxide
seems to be the essential cause of the symmetry lowering, the
above reaction was repeated in the less hygroscopic dichloro-
Figure 4. (left) Molecular structure of [Ti6(acac)6(μ-
O)2(OiPr)2(μ-tBuPO3)6] (3). Hydrogen atoms are omitted for clarity.
(right) Core structure of 3.
Compound 3 is a drum-shaped symmetric hexatitanophosph-
onate cluster incorporating six phosphonate, six acetylaceto-
nate, two μ2-O, and two isopropoxide functionalities. The
central core, which is Ti6O20P6, consists of six titanium and six
phosphorus atoms that occupy the alternate vertices of a drum.
The Ti···P edges of this hexagonal drum are bridged by a μ2-O
atom. In the centrosymmetric hexanuclear cluster, all titanium
centers exist in a distorted-octahedral geometry. Ti1 and Ti2
centers display an almost identical coordination geometry
where the metal ions are surrounded by three phosphonate
oxygen atoms, one acac ligand, and a bridging oxide that
connects these two metal ions. The third metal center lacks the
D
DOI: 10.1021/acs.inorgchem.7b01651
Inorg. Chem. XXXX, XXX, XXX−XXX