Terminal Titanyl Complexes of Tri- and Tetrametaphosphate: Synthesis, Structures, and Reactivity with Hydrogen Peroxide

Article abstract of DOI:10.1021/acs.inorgchem.6b03149

The synthesis and characterization of tri- and tetrametaphosphate titanium(IV) oxo and peroxo complexes is described. Addition of 0.5 equiv of [OTi(acac)₂]₂ to dihydrogen tetrametaphosphate ([P₄O₁₂H₂]^2-) and monohydrogen trimetaphosphate ([P₃O₉H]^2-) provided a bis(μ₂,κ²,κ²) tetrametaphosphate titanyl dimer, [OTiP₄O₁₂]₂^4- (1; 70% yield), and a trimetaphosphate titanyl acetylacetonate complex, [OTiP₃O₉(acac)]^2- (2; 59% yield). Both 1 and 2 have been structurally characterized, crystallizing in the triclinic P1? and monoclinic P2₁ space groups, respectively. These complexes contain TiO units with distances of 1.624(7) and 1.644(2) ?, respectively, and represent rare examples of structurally characterized terminal titanyls within an all-oxygen coordination environment. Complexes 1 and 2 react with hydrogen peroxide to produce the corresponding peroxotitanium(IV) metaphosphate complexes [O₂TiP₄O₁₂]₂^4-(3; 61% yield) and [O₂TiP₃O₉(acac)]^2- (4; 65% yield), respectively. Both 3 and 4 have been characterized by single-crystal X-ray diffraction studies, and their solid-state structures are presented. Complex 3 functions as an oxygen atom transfer (OAT) reagent capable of oxidizing phosphorus(III) compounds (P(OMe)₃, PPh₃) and SMe₂ at ambient temperature to result in the corresponding organic oxide with regeneration of dimer 1.

Full text of DOI:10.1021/acs.inorgchem.6b03149

Article
pubs.acs.org/IC
Terminal Titanyl Complexes of Tri- and Tetrametaphosphate:
Synthesis, Structures, and Reactivity with Hydrogen Peroxide
Julia M. Stauber and Christopher C. Cummins*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139,
United States
S
* Supporting Information
ABSTRACT: The synthesis and characterization of tri- and
tetrametaphosphate titanium(IV) oxo and peroxo complexes is
described. Addition of 0.5 equiv of [OTi(acac)2]2 to
dihydrogen tetrametaphosphate ([P4O12H2]2−) and monohy-
drogen trimetaphosphate ([P3O9H]2−) provided a bis(μ2,κ2,κ2)
4−
tetrametaphosphate titanyl dimer, [OTiP4O12]2 (1; 70%
yield), and a trimetaphosphate titanyl acetylacetonate complex,
[OTiP3O9(acac)]2− (2; 59% yield). Both 1 and 2 have been
structurally characterized, crystallizing in the triclinic P1 and
̅
monoclinic P21 space groups, respectively. These complexes
contain TiO units with distances of 1.624(7) and 1.644(2)
Å, respectively, and represent rare examples of structurally
characterized terminal titanyls within an all-oxygen coordination environment. Complexes 1 and 2 react with hydrogen peroxide
to produce the corresponding peroxotitanium(IV) metaphosphate complexes [O2TiP4O12]24−(3; 61% yield) and
[O2TiP3O9(acac)]2− (4; 65% yield), respectively. Both 3 and 4 have been characterized by single-crystal X-ray diffraction
studies, and their solid-state structures are presented. Complex 3 functions as an oxygen atom transfer (OAT) reagent capable of
oxidizing phosphorus(III) compounds (P(OMe)3, PPh3) and SMe2 at ambient temperature to result in the corresponding
organic oxide with regeneration of dimer 1.
INTRODUCTION
RESULTS AND DISCUSSION
Synthesis of Titanyl Complexes. The [PPN]+ salts of
■
■
Oxidative degradation of organic ligands is a challenge faced in
many synthetic systems, as most oxidation catalysts require
ligand environments capable of supporting reactive high-valent
metal oxos.1 Furthermore, the deactivation of high-valent metal
oxos through the formation of unreactive dimeric or oligomeric
μ-oxo species is an additional common challenge.2 In
heterogeneous systems, these issues are circumvented through
site isolation of discrete metal oxos within inorganic matrices
that function as thermodynamically stable ligand environ-
ments.3,4 An example of one such system is EniChem’s highly
efficient TS-1 oxidation catalyst,5,6 a titanium-based micro-
porous molecular sieve in which Ti(IV) ions are isolated within
an entirely silicate based framework.7
The success of TS-1 and other industrial oxidation catalysts
has driven synthetic chemists to study molecular models of
heterogeneous systems. Kortz et al. have studied titanium(IV)
oxo-8 and hydroxo-substituted9 polyoxometalates (POMs) as
models of Ti single-site catalysts due to the structural analogy
of POMs and metal oxide surfaces. These oxidatively stable
titanium(IV) POM complexes have served as molecular models
for studying mechanisms of selective oxidation.10 With the
present work, we sought to investigate the suitability of
similarly all-inorganic metaphosphate rings (cyclic oligomers of
the PO3− anion) for stabilization of the titanyl functional group
in an all-oxygen coordination environment.
both dihydrogen tetrametaphosphate ([PPN]2[P4O12H2]) and
monohydrogen trimetaphosphate ([PPN]2[P3O9H]) can be
prepared in multigram quantities under open air conditions by
protonation of their corresponding metaphosphate salts with
strong acid.11,12 We have previously shown that the
[P4O12H2]2− acid can be used to synthesize metal complexes
through protonolysis of basic ligands such as [N(SiMe3)2]−,
leading to the facile preparation of tetrametaphosphate tin(II)
([SnP4O12]2−) and chromium(II) ([CrP4O12]24−) salts.11
Employing the strategy of protolytic ligand replacement, we
chose to investigate the reactivity of both the [P4O12H2]2− and
[P3O9H]2− acids with titanyl bis(acetylacetonate) ([OTi-
(acac)2]2).13 Even though its structure is dimeric, [OTi-
(acac)2]2 provides a commercially available source of the
titanyl (OTi) unit that bears basic ligands potentially
susceptible to protolytic replacement by the metaphosphate
acids. Acetylacetone, the byproduct formed through proto-
nation, is easily separable from the desired metaphosphate salts,
making the preparation of titanyl metaphosphate complexes
convenient and straightforward.
Received: December 23, 2016
© XXXX American Chemical Society
A
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Treatment of 1 equiv of [PPN]2[P4O12H2] with 0.5 equiv of
chromium(II) complex of tetrametaphosphate forms just such a
dimeric cofacial sandwich.11 In 1, the titanium centers reside
0.559 Å above the equatorial four-membered oxygen plane of
the tetrametaphosphate such that two κ2 [P4O12]4− ligands
bridge each Ti center to approximate D2h point group
symmetry. Dimer 1 contains a Ti···Ti distance of 4.233(9) Å,
with a titanyl TiO distance of 1.624(7) Å. While structurally
characterized anionic titanium(IV) oxo complexes are rare,20,21
the observed TiO distance in 1 agrees well with
corresponding metrical parameters reported for other neutral
titanium(IV) oxo complexes with distances spanning 1.61−1.68
Å.22 The TiO distance observed in 1 is consistent with triple-
bond character, as predicted by the titanium and oxygen triple-
bond covalent radii,23 as well as the electronic considerations
associated with d0 transition-metal oxos within tetragonal ligand
fields.24
[OTi(acac)2]2 in an acetone/MeCN mixture at 23 °C afforded
4−
a new species, [OTiP4O12]2 (1), after 12 h of reaction time
(Scheme 1). Analytically pure X-ray-quality crystals of
Scheme 1. Synthetic Protocol for the Preparation of
[PPN]4[OTiP4O12]2 ([PPN]4[1])
As in the case of dihydrogen tetrametaphosphate, the
[PPN]2[P3O9H] acid reacted with 0.5 equiv of [OTi(acac)2]2
in an acetone/MeCN mixture under open air conditions to
[PPN]4[1] were easily isolated from the acetylacetone
byproduct in 70% yield after concentration of the crude
reaction mixture and storage at 23 °C overnight. The identity of
1 was established through an X-ray diffraction study, and the
solid-state structure is shown in Figure 1. The 31P{1H} NMR
g e n e r a t e
a t i t a n y l m e t a p h o s p h a t e c o m p l e x ,
[PPN]2[OTiP3O9(acac)] ([PPN]2[2]; Scheme 2 and Figure
Scheme 2. Synthetic Protocol for the Preparation of
[PPN]2[OTiP3O9(acac)] ([PPN]2[2])
4−
Figure 1. Solid-state structure of [OTiP4O12]2 (1) rendered with
PLATON25 with thermal ellipsoids at the 50% probability level, and
with [PPN]+ cations and solvent molecules of crystallization omitted
for clarity. Selected interatomic distances (Å) and angles (deg): Ti1−
O1 1.6252(13), Ti1−O2 1.9909(12), Ti1−O3 1.9939(13), Ti1−O4a
1.9844(13), Ti1−O5a 2.0014(13), Ti1−Ti1a 4.233(9), ∑(Oeq−Ti−
Oeq) 341.8(9).
Figure 2. Solid-state structure of [OTiP3O9(acac)]2− (2) rendered
with PLATON25 with thermal ellipsoids at the 50% probability level,
and with [PPN]+ cations and solvent molecules of crystallization
omitted for clarity. Selected interatomic distances (Å) and angles
(deg): Ti1−O10 1.644(2), Ti1−O1 2.033(2), Ti1−O2 2.0491(18),
Ti1−O3 2.0145(19), Ti1−O4 2.0113(19), Ti1−O5 2.3211(19),
∑(Oeq−Ti−Oeq) 355.08(2).
spectrum of dimer 1 features a singlet located at δ −27.55 ppm
(Figure S1 in the Supporting Information), which is shifted
slightly upfield from that of [P4O12H2]2−. The TiO
stretching frequency observed by FTIR at 969 cm−1 (Figure
S5 in the Supporting Information) is in line with data reported
for other terminal titanyl complexes reported in the
literature,14−17 and falls within the characteristic range (972−
890 cm−1) associated with the TiO oscillator.18 The formula
of the [OTiP4O12]24− anion was also confirmed by ESI-MS(−)
with an m/z value of 189.88 (Figure S7 in the Supporting
Information, calcd 189.89).
2), with loss of 1 equiv of acetylacetone. Complex 2 was
isolated in 59% yield by crystallization through concentrating
the crude reaction mixture and allowing the resulting solution
to stand at 23 °C. After 12 h, large colorless crystals formed
that were isolated and washed with acetone to afford
analytically pure [PPN]2[2].
[PPN] [1] crystallized in the triclinic space group P1 with
̅
4
two [PPN]+ cations and half of the [OTiP4O12]24− dimer in the
asymmetric unit. As shown in Figure 1, the complex adopts a
[MP4O12]2 coordination geometry in which two tetrameta-
phosphate rings form a cofacial sandwich around the pair of
The 31P{1H} NMR spectrum of 2, recorded in acetonitrile,
features a singlet located at δ −18.43 ppm that is assigned to
the three phosphorus atoms of the [P3O9]3− ligand (Figure S8
in the Supporting Information). The presence of a single broad
2−
titanyl ions, resembling the structure of [Cp*TiP4O12]2
reported by Ishii et al.19 We reported previously that the
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signal can be ascribed to rapid rotation of the [P3O9]3− unit on
the NMR time scale in solution and is therefore represented as
a weighted average. Collection of the 31P{1H} NMR spectrum
at −35 °C resolved the broad singlet into a doublet and a triplet
with an AX2 splitting pattern located at δ −19.13 (2JPP = 17.2
Hz) and −18.20 ppm (2JPP = 17.2 Hz) in a 2:1 integral ratio
(Figure S29 in the Supporting Information). The doublet
corresponds to the phosphorus atoms flanking the two
equatorially bound oxygen atoms, and the triplet is assigned
to the phosphorus bound to the axially coordinated oxygen
atom of the [P3O9]3− ligand.
oxo (1000−1100 ppm)15,28,29 and hydroxo ligands (700−1000
ppm),29 respectively, and are in line with our assignment of this
species as the mixed OTi/Ti(OH)2 bis-tetrametaphosphate
complex. When a sample was permitted to age for 12 h, these
resonances were observed to decrease in intensity with new
signals appearing at δ 758 and 566 ppm (Figure S42 in the
Supporting Information). These new signals are found within
the region expected for Ti(IV) μ2-O (650−850 ppm) and μ3-O
species (450−600 ppm),30 suggesting that, after an extended
period of time in the presence of a large excess of H2O, dimer 1
is converted to μ-oxo oligomers.
Complex 2 exhibits behavior similar to that of dimer 1 in
being air stable in both the solid state and in solution. In
contrast to 1, however, 2 is stable to excess water, as judged by
the unchanged 31P NMR spectrum when a sample was left to
stand in the presence of water (ca. 200 equiv) over the course
of 24 h in acetonitrile (Figure S30 in the Supporting
Information). The water stability of 2 afforded the opportunity
to study the titanyl oxygen exchange with 17OH2 and 18OH2 by
ESI-MS(−) and 17O NMR spectroscopy. Comba and Merbach
have reported that the oxygen exchange rate for terminal oxo
ligands in systems of hydrolyzed Ti(IV) is extremely fast, as
measured by 17O NMR fast-injection and line-broadening
techniques.29 Consistent with these studies, the titanyl oxygen
of complex 2 exchanged readily with both 18OH2 and 17OH2 as
evaluated by ESI-MS(−) studies. As a result of this facile
exchange, a 17O NMR spectrum of 2 (Figure 3) was obtained
An X-ray diffraction study confirmed the solid-state structure
of 2 as a terminal titanyl complex with one [P3O9]3− and one
acetylacetonate ligand bound to the titanium(IV) center to
complete its entirely oxygen based octahedral coordination
environment. Figure 2 displays a thermal ellipsoid plot of the
anion from the [PPN]2[2] salt. As a result of the strong trans
influence imparted by the oxo ligand,26 the Ti1−O5 distance of
2.3211(19) Å is significantly longer than that expected for a
Ti−O single bond (1.99 Å), as predicted by the titanium and
oxygen single-bond covalent radii,23 suggesting this interaction
is at best a weak one and that O5 may be simply in proximity of
the Ti center by virtue of being part of the trimetaphosphate
ring. Consistent with multiple bonding to a terminal oxo ligand,
the IR spectrum of 2 displays the TiO oscillator at ν 953
cm−1 (Figure S11 in the Supporting Information). This
stretching frequency is 16 cm−1 lower in energy than that of
1 and is consistent with the slightly longer TiO distance of 2
(1.644(2) Å) in comparison with that of 1 (1.6252(13) Å). In
contrast to 2, the position trans to the oxo ligand is vacant in
dimer 1, and thus 1 should be able to maximize the TiO
multiple bonding in accord with both its shorter bond distance
and higher energy stretching frequency. The formula of 2 was
also confirmed by ESI-MS(−) with an m/z value of 300.91
(Figure S14 in the Supporting Information, calcd 300.82),
which corresponds to the monoanionic complex [OTiP3O9]−,
with loss of the acac− ligand under ESI-MS(−) conditions.
Dimer 1 was prepared under open-atmosphere conditions
and is air stable both in solution and as a solid. Dimer 1,
however, reacted with excess water (ca. 50−100 equiv) in
acetonitrile at 23 °C over the course of 5 min to form a new
species exhibiting a higher order splitting pattern in its 31P{1H}
NMR spectrum consistent with an A2A′2B2B′2 system (Figure
S38 in the Supporting Information).27 Although the identity of
this new species has not yet been confirmed, we propose that
this product is a bimetallic titanium(IV) complex in which one
of the TiO units has reacted with H2O to form two hydroxo
ligands, while the other titanyl unit of dimer 1 is preserved
(OTi/Ti(OH)2). When an acetonitrile solution containing
this species was heated to 80 °C under vacuum, the 31P{1H}
NMR spectrum of the resulting residue showed resonances
corresponding to dimer 1, revealing the reversibility of the
aquation reaction (Figure S43 in the Supporting Information).
When a solution of 1 with excess water was allowed to stand at
23 °C for 12 h, complete conversion of the initial species to a
new product was observed (Figure S40 in the Supporting
Information).
Figure 3. 17O NMR spectrum of 2 after treatment with excess (40
equiv) 17OH2 (MeCN, 68 MHz, 20 °C, externally referenced to D2O).
Inset: ESI-MS(−) of 2 after treatment with excess 17OH2 (MeCN,
3200 V).
after spiking a saturated acetonitrile solution of the compound
with 17OH2 (ca. 40 equiv). The solution 17O NMR spectrum
displays one sharp signal located at δ 1086 ppm that is assigned
as the -yl oxygen of 2, in accord with data from 17O NMR
studies of related terminal titanyl complexes.15,28
The formation of 1 and 2 from [OTi(acac)2]2 is noteworthy
in illustrating that the tri- and tetrametaphosphates are capable
of breaking up the titanium(IV) bis-μ-oxo core of [OTi-
(acac)2]2 to result in the formation of terminal TiO units.
Titanium(IV) oxo species are typically susceptible to hydrolysis
in the presence of moist air, as they have the propensity to
dimerize and oligomerize to form unreactive species containing
Ti−O−Ti linkages;3,29 therefore, the formation of a well-
defined, air-stable terminal titanyl complex is unusual.
Structurally characterized examples of complexes containing
terminal titanyl units are known and typically contain nitrogen-
based macrocyclic ligands31,32 such as porphyrins,16 phthalo-
cyanines,33 and tacn derivatives (tacn = 1,4,7-triazacyclono-
To gain more insight into the identity of these products, a
reaction was carried out using 17O-enriched water. The 17O
NMR spectrum of an acetonitrile solution of 1 treated with
excess (ca. 40 equiv) 17OH2 displays signals located at δ 1060
and 930 ppm (Figure S41 in the Supporting Information),
which fall within the range expected for terminal titanium(IV)
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nane);17,34 however, examples containing an oxygen-only
An X-ray diffraction study confirmed the identity of this
product as the peroxotitanium(IV) κ2 tetrametaphosphate
dimer [PPN]4[O2TiP4O12]2 (3), with the peroxo ligands
bound η2 to each titanium center. Figure 4 shows a thermal
coordination environment at titanium are extremely rare.35
36
Such examples include [OTi(OSMe2)5]2+
and [OTi-
37
(CO3)3]4−
complexes and a sandwich-type 8-tungsto-2-
arsenate(III) POM system ([(OTi)2(α-AsW9O33)2]4−).8 Addi-
tionally, direct evidence for the formation of terminal [O
Ti(aq)]2+ units in dilute acidic aqueous solutions of Ti(IV) has
been observed.29 The IR spectrum of such a solution contains a
band at 975 cm−1 attributed to the TiO vibration, while the
17O NMR spectrum features a resonance located at δ 1028 ppm
that is assigned to the -yl oxygen. These data are in line with the
IR and 17O NMR spectroscopic signatures of complexes 1 and
2, suggesting that the ligand field exerted by metaphosphates
may be similar to that of water.
Reactivity of Titanyl Complexes with Hydrogen
Peroxide. The conversion of titanium(IV) oxo to titanium(IV)
η2-peroxo complexes through reaction with H2O2 is a well-
understood transformation38 that has been thoroughly studied
in both molecular39,40 and heterogeneous41 systems. This
reactivity is of particular interest, as hydrogen peroxide is used
with many titanium silicate systems in catalytic oxidation
reactions of organic substrates.6,41−43 Not only is this
transformation relevant to oxidation processes but also, because
these reactions are typically accompanied by the appearance of
an intense orange color, titanyl reactivity with hydrogen
peroxide has been the basis for a colorimetric H2O2 sensing
technology.44
4−
Figure 4. Solid-state structure of [O2TiP4O12]2 (3) rendered with
PLATON25 with thermal ellipsoids at the 50% probability level, and
with [PPN]+ cations and solvent molecules of crystallization omitted
for clarity. Selected interatomic distances (Å) and angles (deg): Ti1−
O10 1.815(8), Ti1−O20 1.795(8), Ti1−O1 1.896(13), Ti1−O2
1.935(12), Ti1−O3b 1.986(13), Ti1−O4a 1.963(12), O20−O10
1.486(6), Ti1−Ti1b 4.23(1), ∑(Oeq−Ti−Oeq) 352.0(7).
Treatment of dimer 1 with 2 equiv of urea·H2O2 (UHP) or
Me3SiOOSiMe3 in acetonitrile resulted in a color change from
colorless to orange, concomitant with the clean formation of
the corresponding tetrametaphosphate peroxotitanium(IV)
dimer ([O2TiP4O12]24−, 3) with loss of 2 equiv of H2O or
Me3SiOSiMe3, respectively. When UHP was used as the
peroxide source (Scheme 3), the reaction was complete within
ellipsoid plot of the anion from this salt. The O10−O20 peroxo
ligand is disordered over two positions in which the major part
lies in the O2−Ti1−O4a plane. A crystallographic mirror plane
relates the halves of the molecule such that the major part of
the symmetry-generated O10b−O20b peroxo ligand lies in the
O4−Ti1b−O2a plane. The complex contains Ti−Operoxo and
O−O distances of 1.795(8), 1.815(8), and 1.486(6) Å,
respectively. These distances are in accord with data for related
peroxotitanium(V) complexes reported in the literature.21,40
The identity of 3 was also confirmed by ESI-MS(−), with an
m/z value of 197.95 (Figure S21 in the Supporting Information,
calcd 197.89). The TiO vibration at 969 cm−1 in the IR
spectrum of 1 is absent in the spectrum of 3, and a new stretch
attributed to the peroxo O−O vibration is located at 856 cm−1
(Figure S18 in the Supporting Information), consistent with a
side-on η2 coordination mode.45 The strong absorption at λmax
379 nm in the UV−vis spectrum of 3 is assigned to the pπ(O2)
→ dπ(Ti) ligand to metal charge-transfer (LMCT) transition46
and compares well with other LMCT bands that have been
observed for similar peroxotitanium(IV) complexes.45−47 Like
1, dimer 3 is air stable both in the solid state and in solution.
When the reaction of 1 with UHP was monitored by 31P
NMR spectroscopy at 5 °C in acetonitrile, an intermediate
evincing an A2A′2B2B′2 splitting pattern (Figure S35 in the
Supporting Information) was observed before complete
conversion to 3 occurred. This intermediate is assigned as the
OTi/Ti(O−O) bistetrametaphosphate complex, which
completely converts to complex 3 after 80 min at 5 °C and
can only be observed at low temperature (≤5 °C).
4−
Scheme 3. Treatment of [OTiP4O12]2 (1) with 2 equiv of
4−
H2O2 to Yield [O2TiP4O12]2 (3) with Loss of 2 equiv of
H2O
10 min, while the corresponding reaction with Me3SiOOSiMe3
was much more sluggish and required 12 h for completion.
Both reactions were monitored with 31P NMR spectroscopy,
wherein progress was associated with growth of a singlet at δ
−28.13 ppm (Figure S15 in the Supporting Information).
Additionally, the reaction was followed by UV−vis spectrosco-
py by noting the appearance and growth of the absorption
located at λmax 379 nm (Figure S20 in the Supporting
Information). The product was isolated (61% yield) as orange
blocks obtained by layering diethyl ether onto an acetonitrile
solution of the complex.
Complex 2 also reacted cleanly with H2O2(aq) or
Me3SiOOSiMe3 to generate a peroxotitanium(IV) trimetaphos-
phate acetylacetonate complex, [PPN]2[O2TiP3O9(acac)]
([PPN]2[4]; Scheme 4 and Figure 5), which was isolated in
65% yield after crystallization. Complex 4 is characterized by a
sharp singlet in its 31P{1H} NMR spectrum located at δ −17.73
ppm (Figure S22 in the Supporting Information), a strong
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Scheme 4. Reaction of [OTiP3O9(acac)]2− (2) with H2O2 to
Yield [O2TiP3O9(acac)]2− (4) with Loss of H2O
oxidation processes such as diphenylmethanol, phenol, and
cyclooctene (see section 14 in the Supporting Information for
experimental details).7,43 These results are consistent with the
lack of catalytic activity observed for 1 and 2 in the presence of
H2O2.
We did, however, test the ability of 3 and 4 to behave as
stoichiometric oxidants toward phosphorus(III) compounds
and sulfides. Treatment of 3 with P(OMe)3, PPh3, and SMe2 at
ambient temperature resulted in the formation of the
corresponding organic oxide as well as clean regeneration of
1
1, as determined by 31P and H NMR analyses (Scheme 5).
Scheme 5. Oxygen Atom Transfer (OAT) Reactions from
4−
[O2TiP4O12]2 (3) to P(OMe)3, PPh3, and SMe2 to
Regenerate the Corresponding Organic Oxide and
a
4−
[OTiP4O12]2 (1)
a
Conditions: (a) 23 °C, 10 min, MeCN; (b) 23 °C, 30 min, MeCN;
(c) 23 °C, 2 h.
Treatment of 4 with PPh3, however, required heating at 70 °C
to generate OPPh3 and 3. Related vanadium(V) peroxo systems
are thermodynamically powerful but kinetically sluggish OAT
reagents, and in this respect they are reminiscent of nitrous
oxide.51
Figure 5. Solid-state structure of [O2TiP3O9(acac)]2− (4) rendered
with PLATON25 with thermal ellipsoids at the 50% probability level,
and with [PPN]+ cations and solvent molecules of crystallization
omitted for clarity. Selected interatomic distances (Å) and angles
(deg): Ti1−O20 1.850(4), Ti1−O10 1.823(5), Ti1−O1 2.063(3),
Ti1−O2 2.048(2), Ti1−O3 2.040(2), Ti1−O4 1.997(2), Ti1−O5
2.166(2), O10−O20 1.443(6), ∑(Oeq−Ti−Oeq) 353.9(8).
CONCLUSIONS
■
The present metaphosphate complexes represent rare examples
of air-stable Ti(IV) oxo/peroxo systems containing entirely
oxygen based coordination environments at titanium. The
metaphosphate ligands support the formation of terminal
titanyls as opposed to the more commonly encountered μ-oxo
oligomeric structures, making complexes 1−4 molecular
models of site isolated titanyl and peroxo units found within
heterogeneous oxidation catalysts.
stretch at νO−O 903 cm−1 in its IR spectrum (Figure S25 in the
Supporting Information), and an absorption located at λmax 403
nm (Figure S27 in the Supporting Information, ϵ = 674 M−1
cm−1) in the visible region of the electromagnetic spectrum.
Crystals of [PPN]2[4] suitable for an X-ray diffraction study
were grown as orange blocks obtained by layering Et2O onto an
acetonitrile solution of the complex. [PPN]2[4] crystallized in
the monoclinic space group P21, and Figure 5 shows a thermal
ellipsoid plot of the anion that displays the peroxo ligand
oriented in an η2 binding mode in which the O−O moiety is
coincident with the O2−Ti1−O3 plane of the [P3O9]3− and
acac− ligands. The structure determination revealed unremark-
able Ti−Operoxo and O−O distances of 1.850(4), 1.823(5), and
1.443(6) Å, respectively.
Examples of molecular peroxotitanium(IV) complexes that
serve as oxidants toward organic substrates46 are rare due to the
well-documented stability of the titanium−peroxo structural
unit and, therefore, its corresponding lack of oxidizing
capacity.6,48 It is instead believed that titanium(IV) hydro-
peroxo species play the active role in substrate oxidation within
heterogeneous systems,10,43,49 and therefore the preparation of
well-defined titanium hydroperoxo complexes has been
targeted by synthetic chemists.50
EXPERIMENTAL SECTION
■
General Methods. Unless otherwise stated, all manipulations were
performed under open-air conditions in the fume hood.
[PPN]2[P4O12H2]11 and [PPN]2[P3O9H]12 were prepared as
previously reported. All reagents were purchased from Sigma-Aldrich
(urea·H2O2, [OTi(acac)2]2, 30% H2O2), or Gelest (Me3SiOOSiMe3)
and used as received. Acetone (H2O content <0.5 w/w %) was
purchased from Macron Fine Chemicals, and acetonitrile was
purchased from Acros Organics; both were used without further
purification. 17OH2 (70% enriched) and 18OH2 (97% enriched) were
purchased from Cambridge Isotope Laboratories and used as received.
IR spectra were recorded on a Bruker Tensor 37 Fourier transform IR
(FTIR) spectrometer. Deuterated solvents (D2O, CD3CN) were
obtained from Cambridge Isotope Laboratories and used as received.
NMR spectra were obtained on a Varian Inova-500 NMR
spectrometer with an Oxford Instruments Ltd. superconducting
magnet, on a Bruker Avance 400 instrument equipped with Magnex
Scientific superconducting magnets, or on a Varian Mercury 300
spectrometer with an Oxford Instruments Ltd. superconducting
magnet. 1H and 13C{1H} NMR spectra are referenced to residual
protio solvent signals. 31P{1H} NMR chemical shifts are reported with
respect to an external reference (85% H3PO4, δ 0.0 ppm), and 17O
NMR chemical shifts are externally referenced to D2O (δ 0.0 ppm).
NMR spectra were simulated using the program gNMR version 5.1.
ESI-MS(−) data were obtained on a Waters Q-TOF micro mass
spectrometer using a source temperature of 100 °C and a desolvation
temperature of 150 °C. Mass spectrometry samples were run in neat
We were unable to observe a stable titanium(IV) hydro-
peroxo complex, and 1 and 2 did not show any signs of catalytic
activity toward organic substrates in the presence of H2O2 (see
section 13 in the Supporting Information for experimental
details). Both results are likely due to the rapid conversion of 1
and 2 to 3 and 4, respectively, upon addition of H2O2 without a
long-lived hydroperoxo intermediate for interception by a
substrate. No reaction was observed when 3 and 4 were
independently treated with substrates relevant to TS-1
E
DOI: 10.1021/acs.inorgchem.6b03149
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
acetonitrile at concentrations <1 μM, and the data were processed
using the program mMass Version 5.4.1. UV−vis spectra were
collected on an Ocean Optics USB4000 spectrophotometer with a
DT-Mini-2GS UV−vis−NIR light source. Elemental analyses were
performed by Robertson Microlit Laboratories, Inc. (http://www.
robertson-microlit.com). X-ray crystallographic details are provided in
section 15.1 in the Supporting Information.
[PPN]4[OTiP4O12]2 ([PPN]4[1]). To a stirred suspension of
[PPN]2[P4O12H2] (350 mg, 0.251 mmol, 1.00 equiv) in acetone (7
mL) was added [OTi(acac)2]2 (72 mg, 0.14 mmol, 0.55 equiv) as a
solid. MeCN (10 mL) was added dropwise until a homogeneous
solution was obtained. The pale yellow reaction mixture was stirred at
room temperature (23 °C) for a total of 12 h, at which point the
mixture was filtered through a pad of Celite to remove unreacted
[OTi(acac)2]2. The filtrate was concentrated to ca. 3 mL under
vacuum, during which time colorless crystals formed on the bottom
and sides of the vial. The pale yellow supernatant was removed, and
the colorless crystals were washed with acetone (2 mL) and dried
under reduced pressure. The supernatant was allowed to stand at room
temperature for an additional 24 h to allow a second crop of crystals to
grow. These crystals were isolated, washed with acetone (2 mL), and
dried under reduced pressure. Total yield: 256 mg, 0.176 mmol, 70%.
The solid-state structure determination revealed the presence of one
molecule of MeCN and one molecule of acetone per
[PPN]4[O2TiP4O12]2 ([PPN]4[3]). To a stirred colorless solution of
[PPN]4[1] (125 mg, 0.0429 mmol, 1.00 equiv) in MeCN (5 mL) was
added urea hydrogen peroxide (10 mg, 0.11 mmol, 2.4 equiv) as a
solid. The solution turned orange immediately upon addition. The
reaction mixture was stirred at ambient temperature for a total of 10
min, at which point the orange solution was filtered through a pad of
Celite to remove the urea byproduct. The orange solution was then
concentrated to ca. 0.5 mL. Et2O (5 mL) was then added until the
solution became cloudy. The resulting orange solution was then
filtered through a piece of microfiber filter paper, and the filtrate was
allowed to stand at 23 °C for 12 h, during which time large diffraction-
quality orange crystals grew. The pale orange supernatant was
removed, and the crystals were dried under reduced pressure to
afford analytically pure [PPN]4[O2TiP4O12]2. Yield: 77 mg, 0.026
mmol, 61%. The solid-state structure determination revealed six
MeCN molecules of crystallization per [PPN]4[O2TiP4O12]2 complex.
Anal. Found (calcd) for C156H138N10O28P16Ti2 ([PPN]4[O2TiP4O12]2·
6MeCN): C, 57.98 (58.70); H, 4.41 (4.36); N, 3.76 (4.39). The most
likely explanation for the discrepancies in the C and N analyses is
variable acetonitrile content within the voids of the crystal lattice
(section 15.1 in the Supporting Information). ESI-MS(−) of
[O2TiP4O12]2− (CH3CN, m/z): 197.95 (calcd 197.89). IR (ATR,
cm−1): ν 1262 (s, PO), 884 (s, O−O). 31P{1H} NMR (CD3CN, 25
°C, 161.9 MHz): δ 22.20 (s, 8 P, PPN+), −28.09 (s, 8 P, P4O12) ppm.
1H NMR (CD3CN, 25 °C, 400.1 MHz): δ 7.48−7.71 (m, 120 H,
PPN+) ppm. 13C{1H} NMR (CD3CN, 25 °C, 100.6 MHz): δ 134.6 (s,
[PPN]4[OTiP4O12
]
complex. Anal. Found (calcd) for
2
C149H129N5O27P16Ti2 ([PPN]4[OTiP4O12]2·MeCN·acetone): C,
59.19 (59.40); H, 4.61 (4.32); N, 2.29 (2.32). ESI-MS(−) (CH3CN,
m/z): 189.88 (calcd 189.89). IR (ATR, cm−1): ν 1262 (s, PO), 969
(s, TiO). 31P{1H} NMR (CD3CN, 25 °C, 161.9 MHz): δ 22.20 (s,
1
PPN+), 133.2 (m, PPN+), 130.4 (m, PPN+), 128.1 (d, JPC = 108 Hz,
PPN+) ppm. UV−vis: λmax (ε) 379 nm (1104 M−1 cm−1).
Alternative Preparation Using Me3SiOOSiMe3. In the fumehood,
to a stirred solution of [PPN]4[1] (100 mg, 0.0340 mmol, 1.00 equiv)
in MeCN (5 mL) was added Me3SiOOSiMe3 (18 μL, 0.086 mmol, 2.5
equiv) via 50 μL microsyringe. The solution gradually changed from
colorless to bright orange over the course of 12 h, at which point the
reaction mixture was concentrated to ca. 1 mL. To the concentrated
solution was added Et2O (15 mL) in order to precipitate orange solids.
The solids were isolated by filtration, washed with Et2O (15 mL), and
dried under reduced pressure, affording [PPN]4[3] as an orange solid.
Yield: 85 mg, 0.029 mmol, 86%.
1
8 P, PPN+), −27.55 (s, 8 P, P4O12) ppm. H NMR (CD3CN, 25 °C,
400.1 MHz): δ 7.48−7.71 (m, 120 H, PPN+) ppm. 13C{1H} NMR
(CD3CN, 25 °C, 100.6 MHz): δ 134.6 (s, PPN+), 133.2 (m, PPN+),
1
130.4 (m, PPN+), 128.1 (d, JPC = 108 Hz, PPN+) ppm.
[PPN]2[OTiP3O9(acac)] ([PPN]2[2]). To a stirred suspension of
[PPN]2[P3O9H] (300 mg, 0.228 mmol, 1.00 equiv) in acetone (7 mL)
was added [OTi(acac)2]2 (66 mg, 0.13 mmol, 0.55 equiv) as a solid.
MeCN (10 mL) was added dropwise until the reaction mixture
became homogeneous. The pale yellow reaction mixture was stirred at
ambient temperature (23 °C) for a total of 6 h, at which point the pale
yellow solution was filtered through a pad of Celite to remove
unreacted [OTi(acac)2]2. All volatiles were then removed under
reduced pressure, affording a pale yellow residue. The yellow residue
was triturated with Et2O (10 mL), and the Et2O was subsequently
decanted from the resulting solids. The resulting solids were
suspended in acetone (5 mL) and filtered away from the pale yellow
supernatant. The solids were washed with acetone (3 mL) and dried
under reduced pressure, affording [PPN]2[OTiP3O9(acac)] as a
cream-colored powder (310 mg, 0.210 mmol, 92%). X-ray diffraction
quality crystals of [PPN]2[OTiP3O9(acac)] were grown by concen-
tration of the crude reaction mixture by ca. 90% and allowing the
resulting mixture to stand at 23 °C. After 12 h, large colorless crystals
formed that were isolated and washed with acetone to afford
analytically pure [PPN]2[2]. Overall yield of crystals: 199 mg, 0.134
mmol, 59%. The solid-state structure determination revealed the
presence of one MeCN molecule of crystallization per
[PPN]2[OTiP3O9(acac)] complex. Anal. Found (calcd) for
C79H70N3O12P7Ti ([PPN]2[OTiP3O9(acac)]·MeCN): C, 62.28
(62.61); H, 4.76 (4.65); N, 2.72 (2.77). ESI-MS(−) of [OTiP3O9]−
(CH3CN, m/z): 300.91 (calcd 300.82). The complete dianionic
complex including the acac ligand is not detectable under ESI-MS(−)
conditions, and only the [OTiP3O9]− ion is observed. IR (ATR,
cm−1): ν 1261 (s, PO), 953 (s, TiO), 933 (s, Ti−Oacac). 31P{1H}
NMR (CD3CN, 25 °C, 161.9 MHz): δ 22.20 (s, 4 P, PPN+), −18.52
(bs, 3 P, P3O9) ppm. 1H NMR (CD3CN, 25 °C, 400.1 MHz): δ 7.48−
7.71 (m, 60 H, PPN+), 5.67 (s, 1 H, CH), 2.06 (s, 6 H, CH3) ppm.
13C{1H} NMR (CD3CN, 25 °C, 100.6 MHz): δ 191.6 (s, CO),
[PPN]2[O2TiP3O9(acac)] ([PPN]2[4]). To a stirred colorless
solution of [PPN]2[2] (100 mg, 0.0677 mmol, 1.00 equiv) was
added a solution of 30% H2O2 (10 μL, 0.098 mmol, 1.4 equiv) via 10
μL syringe. The solution immediately changed from colorless to bright
orange upon complete addition. The orange solution was stirred at
ambient temperature for a total of 10 min, at which point all volatiles
were removed under reduced pressure. The resulting orange residue
was triturated with THF (5 mL), and then the THF was subsequently
decanted away from the orange solids. The solids were dissolved in
MeCN (1 mL), and then Et2O (3 mL) was added until the solution
became cloudy. The orange solution was then filtered through a piece
of microfiber filter paper and allowed to stand at ambient temperature
(23 °C). Large diffraction-quality orange crystals grew over the course
of 12 h. The pale orange mother liquor was removed, and the crystals
were dried under reduced pressure to afford analytically pure
[PPN]2[O2TiP3O9(acac)]. Yield: 66 mg, 0.044 mmol, 65%. The
solid-state structure shows one Et2O molecule of crystallization per
[PPN]2[O2TiP3O9(acac)] complex. Anal. Found (calcd) for
C81H77N2O14P7Ti ([PPN]2[O2TiP3O9(acac)]·Et2O): C, 61.69
(61.95); H, 4.37 (4.52); N, 2.07 (1.88). ESI-MS(−) of [O2TiP3O9]−
(CH3CN, m/z): 316.82 (calcd 316.81). The complete dianionic
complex including the acac ligand is not detectable under ESI-MS(−)
conditions, and only the [O2TiP3O9]− ion is observed. IR (ATR,
cm−1): ν 1251 (s, PO), 934 (s, Ti−Oacac), 903 (O−O). 31P{1H}
NMR (CD3CN, 25 °C, 161.9 MHz): δ 22.20 (s, 4 P, PPN+), −17.73
(s, 3 P, P3O9) ppm. 1H NMR (CD3CN, 25 °C, 400.1 MHz): δ 7.48−
7.71 (m, 60 H, PPN+), 5.52 (s, 1 H, CH) ppm. The CH3 resonance
overlaps with CD3CN solvent. 13C{1H} NMR (CD3CN, 25 °C, 100.6
MHz): δ 190.2 (s, CO) 134.6 (s, PPN+), 133.2 (m, PPN+), 130.4
134.6 (s, PPN+), 133.2 (m, PPN+), 130.4 (m, PPN+), 128.1 (d,1JPC
=
108 Hz, PPN+), 102.6 (s, CH), 26.7 (s, CH3) ppm. 17O NMR (MeCN,
(m, PPN+), 128.1 (d, JPC = 108 Hz, PPN+), 102.3 (s, CH), 26.6 (s,
1
25 °C, 68 MHz): δ 1086 (s, OTi) ppm.
CH3) ppm. UV−vis: λmax (ε) 403 nm (674 M−1 cm−1).
F
DOI: 10.1021/acs.inorgchem.6b03149
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Alternative Preparation Using Me3SiOOSiMe3. In the fumehood,
to a stirred solution of [PPN]2[3] (100 mg, 0.0677 mmol, 1.00 equiv)
in MeCN (5 mL) was added Me3SiOOSiMe3 (22 μL, 0.10 mmol, 1.5
equiv) via 50 μL microsyringe. The solution gradually changed from
colorless to bright orange over the course of 12 h, at which point the
reaction mixture was concentrated to ca. 1 mL. To the concentrated
solution was added Et2O (15 mL) in order to precipitate orange solids.
The solids were isolated by filtration, washed with Et2O (15 mL), and
dried under reduced pressure, affording [PPN]2[4] as an orange solid.
Yield: 90 mg, 0.060 mmol, 90%.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b03149.
■
S
́
(10) Antonova, N. S.; Carbo, J. J.; Kortz, U.; Kholdeeva, O. A.;
Poblet, J. M. Mechanistic Insights into Alkene Epoxidation with H2O2
by Ti- and Other TM-Containing Polyoxometalates: Role of the Metal
Nature and Coordination Environment. J. Am. Chem. Soc. 2010, 132,
7488−7497.
Experimental details and characterization data for all
compounds, including crystallographic information for
[PPN]4[1], [PPN]2[2], [PPN]4[3], and [PPN]2[4]
(PDF)
Crystallographic data (CIF)
Crystallographic data (CIF)
(11) Jiang, Y.; Chakarawet, K.; Kohout, A. L.; Nava, M.; Marino, N.;
Cummins, C. C. Dihydrogen Tetrametaphosphate, [P4O12H2]2−:
Synthesis, Solubilization in Organic Media, Preparation of its
Anhydride [P4O11]2− and Acidic Methyl Ester, and Conversion to
Tetrametaphosphate Metal Complexes via Protonolysis. J. Am. Chem.
Soc. 2014, 136, 11894−11897.
(12) Chakarawet, K.; Knopf, I.; Nava, M.; Jiang, Y.; Stauber, J. M.;
Cummins, C. C. Crystalline Metaphosphate Acid Salts: Synthesis in
Organic Media, Structures, Hydrogen-Bonding Capability, and
Implication of Superacidity. Inorg. Chem. 2016, 55, 6178−6185.
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(TiO(C5H7O2)2)2. Inorg. Chem. 1972, 11, 2989−2993.
Crystallographic data (CIF)
Crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for C.C.C.: ccummins@mit.edu.
■
ORCID
Christopher C. Cummins: 0000-0003-2568-3269
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the NSF under the NSF Center
CHE-1305124. Yanfeng Jiang is thanked for assistance with X-
ray crystallography, and Wesley Transue and Shiyu Zhang are
thanked for assistance with VT-NMR experiments.
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DOI: 10.1021/acs.inorgchem.6b03149
Inorg. Chem. XXXX, XXX, XXX−XXX