Synthesis, isolation and structural characterisation of alkoxytitanium triflate complexes

Article abstract of DOI:10.1002/ejic.201100830

Treatment of [Ti(OiPr)₄] with trimethylsilyl triflate results in the formation of [Ti(OiPr)₃(OTf)] (2) in high yield. Subsequent treatment of the triflate derivative 2 with a series of facially coordinating N₃-donor ligands results in the production of a series of charge-separated metal alkoxide salts of the general formula [{L}Ti(OiPr) ₃][OTf] {L = tris(pyrazolyl)methane (3a), 1,3,5-triethyl-1,3,5- triazacyclohexane (3b), 1,3,5-tribenzyl-1,3,5-triazacyclohexane (3c), 1,3,5-tris(p-fluorobenzyl)-1,3,5-triazacyclohexane (3d), and 1,3,5-tris[(1S)-1-phenylethyl]-1,3,5-triazacyclohexane (3e)}. The products were characterized by ¹H and ¹³C NMR spectroscopy and in the case of 3a-c by single-crystal X-ray diffraction. Reaction of 2 with 1,3,5-triphenyl-1,3,5-triazacyclohexane results in the formation of complex 4 [{L′}₂Ti(OiPr)₂(OTf)₂], which contains two 3,4-dihydroquinazoline ligands (L′), a result of catalytic activation of the triazacyclohexane ligands by [Ti(OiPr)₃(OTf)] towards electrophilic aromatic substitution. Copyright

Full text of DOI:10.1002/ejic.201100830

FULL PAPER
DOI: 10.1002/ejic.201100830
Synthesis, Isolation and Structural Characterisation of Alkoxytitanium Triflate
Complexes
Matthew G. Davidson[a][‡] and Andrew L. Johnson*[a]
Keywords: Titanium / Alkoxides / Triflates / Lewis acids
Treatment of [Ti(OiPr)4] with trimethylsilyl triflate results in
the formation of [Ti(OiPr)3(OTf)] (2) in high yield. Subse-
quent treatment of the triflate derivative 2 with a series of
facially coordinating N3-donor ligands results in the pro-
duction of a series of charge-separated metal alkoxide salts
of the general formula [{L}Ti(OiPr)3][OTf] {L = tris(pyrazolyl)-
methane (3a), 1,3,5-triethyl-1,3,5-triazacyclohexane (3b),
1,3,5-tribenzyl-1,3,5-triazacyclohexane (3c), 1,3,5-tris(p-fluo-
robenzyl)-1,3,5-triazacyclohexane (3d), and 1,3,5-tris[(1S)-1-
phenylethyl]-1,3,5-triazacyclohexane (3e)}. The products
1
were characterized by H and 13C NMR spectroscopy and in
the case of 3a–c by single-crystal X-ray diffraction. Reaction
of 2 with 1,3,5-triphenyl-1,3,5-triazacyclohexane results in
the formation of complex 4 [{LЈ}2Ti(OiPr)2(OTf)2], which con-
tains two 3,4-dihydroquinazoline ligands (LЈ), a result of cata-
lytic activation of the triazacyclohexane ligands by [Ti(OiPr)3-
(OTf)] towards electrophilic aromatic substitution.
(OTf) into complexes has been the utility of Me3SiOTf,[8]
which reacts with M–OR and M–Cl functionalities to form
the corresponding M–OTf complex and Me3SiOR or Me3-
SiCl, respectively.[2b,9] As part of a continuing study of the
chemistry of titanium alkoxide and aryloxide chemistry and
their activities in Lewis acid mediated organic transforma-
tions, we have previously described the synthesis of air- and
moisture-stable TiIV triflate complexes of C3-symmetric
aminetris(phenolato) ligands and their activity as catalysts
for aza-Diels–Alder reactions.[2b,10] More recently, we have
described the interaction of strong organic acids such as
bis(trifluoromethylsulfonyl)amide, with titanium alkoxide
complexes and the structural chemistry of the products.[11]
Herein we report an initial study of the coordination
chemistry of the titanium triflate complex [Ti(OiPr)3(OTf)]
(2) with a range of neutral facially coordinating N3-donor
ligands, synthesised by using a salt-free process, with the
aim of investigating the coordination chemistry of these sys-
tems.
Introduction
Titanium(IV) compounds of the general form [TiX4]
have been employed extensively as Lewis acid catalysts in
a wide range of synthetic organic transformations.[1] Fine-
tuning of the Lewis acidity of these systems can be achieved
by the careful selection of X from alkoxide, halide, phenol-
ate or triflate ligands.[2] Of these systems, it is the Lewis
acid complexes based on weakly coordinating anions[3] such
as triflate (trifluromethanesulfonate, [OTf]–) that have re-
ceived the most attention.[4] In the vast majority of cases,
preparation of the metal triflate complex involves a salt me-
tathesis and elimination, e.g. reaction of [Ti(OiPr)3Cl] with
[Ag{OSO2CF3}] ([Ag{OTf}]) to form [Ti(OiPr)3OTf].[5]
Such compounds are also subject to spontaneous/solvent-
induced ligand redistribution; e.g., reaction of [Ti(OtBu)3-
Cl] with [Ag{OTf}] in tetrahydrofuran (thf) results in the
formation of [Ti(OtBu)2(OTf)2(thf)2] rather than the ex-
pected [Ti(OtBu)3(OTf)].[6] A major disadvantage of such
systems is, as the scale of reaction increases, the difficulty
and disadvantage of being air- and moisture-sensitive li-
quids requiring storage and manipulation under inert con-
ditions. Furthermore, in significant quantities, compounds
such as TiCl4 can present a considerable caustic and corros-
ive hazard.[7] Therefore, the design of new titanium-based
reagents that are convenient to use yet retain high catalytic
activity is of considerable importance to the synthetic com-
munity. One alternative synthetic methodology for the in-
corporation of weakly coordinating ligands such as triflate
Results and Discussion
Our interest in the development of the stoichiometric and
catalytic chemistry of C3-symmetric[12] titanium triflate
compounds[2b,10] prompted us to explore the reactions of
representative fac-N3 donor ligands with compounds
[Ti(OiPr)3OTf] to establish a framework for future studies.
The N3 donor ligands, selected on the basis of their syn-
thetic utility and availability, are shown in Figure 1. In both
cases the triazacyclohexane (TAC) and the tris(pyrazolyl)-
methane [HC(Pz)3] ligand systems are known to coordinate
to metal centres through a facial coordination arrangement,
with titanium systems containing triazacyclohexane,[13] tri-
[a] Department of Chemistry, University of Bath,
Claverton Down, Bath BA2 7AY, United Kingdom
Fax: +44-1225-386231
E-mail: A.L.Johnson@Bath.ac.uk
[‡] Senior author
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azacyclononane,[13a] tris(pyrazolyl)methane[14] and tris(pyr- bridged apically by a disordered triflate group. The second
azolyl)borate[15] ligands having previously been exploited in triflate group and one of the terminal alkoxide units are
the synthesis of complexes for use in α-olefin polymeriza- disordered over the two titanium centres, but the second
tion, styrene polymerization, and as models for bioinor- triflate’s coordination environment is such that this group
ganic systems.
serves to bridge adjacent dimetallic units to form an ex-
tended polymeric system. The 1H NMR spectrum of 2
shows all three isopropoxide units to be equivalent on the
NMR time scale, with the presence of a single set of broad
resonances for both the CH(Me)2 and CH(Me)2 moieties
on the isopropoxide groups. Low-temperature 1H NMR
studies (–60 °C) on 2 showed only a sharpening of the sig-
nals to well-defined doublet and septet resonances. Com-
plex 2 has previously been synthesized by the reaction of
[ClTi(OiPr)3] with [Ag(OTf)] and reported by Jørgensen et
al., although no characterization data was provided.[5] In
the absence of donor solvents, e.g. thf, complex 2 appears
to be stable to spontaneous ligand rearrangement and for-
mation of the bis(triflate) complex [Ti(OiPr)2(OTf)2], al-
though the effect of solvent on such spontaneous rearrange-
ments has not been investigated by us.
The reactions of 2 with the fac-N3-donor ligands HC-
(Pz)3 [tris(pyrazolyl)methane, 1a], Et3-TAC (1,3,5-triethyl-
1,3,5-triazacyclohexane, 1b), Bz3-TAC (1,3,5-tribenzyl-
1,3,5-triazacyclohexane, 1c), (p-F-Bz)3-TAC [1,3,5-tris(p-
fluorobenzyl)-1,3,5-triazacyclohexane, 1d], [(S)-Ph(Me)-
CH]3-TAC {1,3,5-tris[(S)-1-phenylethyl]-1,3,5-triazacyclo-
hexane, 1e} and Ph3-TAC (1,3,5-triphenyl-1,3,5-triazacyclo-
hexane, 1f) are summarized in Scheme 2. The reaction with
1 equiv. of the appropriate fac-N3 donor proceeded
smoothly in a 24–84% yield in toluene to give the products
3a–e and 4 as analytically and spectroscopically pure solids.
Figure 1. Ligands used in the course of this study.
Synthesis and Characterisation
The reaction of [Ti(OiPr)4] with 1 equiv. of [Me3SiOSO2-
CF3] (TMS-OTf) in toluene under an inert gas proceed
rapidly at room temperature to afford Me3SiOiPr and the
highly air- and moisture-sensitive titanium triflate complex
[Ti(OiPr)3(OTf)]n (2) in high yield (Scheme 1).
1
The H NMR spectra of 3a and 3b–e at room tempera-
ture all feature characteristic resonances in the region of δ
= 1.1–1.4 ppm and δ = 4.4–4.9 ppm, respectively, corre-
sponding to the isopropoxide ligands. Despite the difference
1
in ligand systems used in complexes 3a–e, H NMR spec-
troscopy reveals little change in the characteristic reso-
nances for the methine hydrogen atom, suggesting that any
trans effect experienced by the isopropoxide group, originat-
ing from the N atom in the Pz3CH or TAC ligands is not
significant. The resonances for the coordinated fac-N3-do-
nor ligands were as expected on the basis of previous stud-
ies and were consistent with the Cs symmetry proposed in
Scheme 3 (see Exp. Section for further details). For the TAC
complexes 3b–e coordination of the TAC ligand to the tita-
Scheme 1. Proposed equation for the synthesis of 2, indicating the
polymeric nature of 2 observed in the solid state.
Complex 2 can be isolated as an analytically pure crystal- nium centre results in a resolution of the (NCH2)3 ring
line solid by recrystallization from concentrated toluene methylene signals into two well-separated doublets (δ = 3.48
solutions.
and 4.21 ppm for 3c; δ = 3.49 and 4.20 ppm for 3d; δ = 2.56
Although crystals of 2 suitable for single-crystal X-ray and 4.54 ppm for 3e) for the axial and equatorial hydrogen
diffraction were grown from both toluene and benzene, it atoms. In the case of complex 3b the signals are coinciden-
1
has not been possible to find a suitable model for the con- tal, and appear in the H NMR spectrum as overlapping
siderable disorder that is present in the solid-state structure doublets (δ ≈ 4.03 ppm). This signal resolution is probably
of 2. After collecting and partially solving several data sets, the best evidence for κ3-coordination of the fac-N3 ligand
we are confident the geometry of the complex obtained is to the metal atom and indicates that the titanium centres
that shown in Scheme 1. The asymmetric unit cell (ASU) appears to remain coordinated to the N-donor ligand in
consists of a complex with a dimetallic {Ti(μ-OiPr)2- solution.
(OiPr)4} core, in which the titanium centres are bridged by
two alkoxido ligands. The dimetallic fragment is also periments on 3a–e provide little information on the coordi-
As with closely related triflate complexes, 19F NMR ex-
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Alkoxytitanium Triflate Complexes
Scheme 2. Schematic illustration of the synthesis of complexes 3a–e and 4.
3,4-Dihydroquinazoline systems are typically prepared
by condensation reactions between aniline derivatives and
formaldehyde and have previously been isolated as interme-
diates in the synthesis of Tröger’s base.[16] The precise
mechanism by which complex 4 is formed is unknown, but
it is possible to suggest that the formation of the 3,4-dihy-
droquinazoline system is a result of activation of the Ph3-
TAC ligand by a strong Lewis acid {i.e. [Ti(OiPr)3(OTf)]}
followed by electrophilic aromatic substitution. The reac-
tion of triazacyclohexane systems with strong Lewis acids
(such as TiCl4 and SnCl4) has previously been shown to
produce N-methyleneamine derivatives in situ.[17] In the
case of 1,3,5-triphenyl-1,3,5-triazacyclohexane, reaction
with strong Lewis acids are known to produce N-methyl-
ideneaniline (Ph–N=CH2), the initial product in the reac-
tion of aniline and formaldehyde,[17a] and activate the meth-
yleneamide derivative towards electrophilic aromatic substi-
tution. It is therefore possible to hypothesise that the 3,4-
dihydroquinazoline ligand results from such activation, and
that ligand scrambling within the reaction mixture results
in the formation of 4 along with half an equivalent of
[Ti(OiPr)4] as shown in Scheme 3. Similar ligand rearrange-
ment reactions have previously been reported for related tri-
s(alkoxy)metal triflate systems,[11,18] but we believe the
monodentate nature of the dihydroquinazoline ligand pro-
motes a ligand redistribution, just as the fac-N3 ligands ap-
pear to stabilise the [Ti(OiPr)3]+ cation against ligand redis-
tribution.
Scheme 3. Proposed equation for the synthesis of complex 4.
nation state of the triflate anion, [OTf]–, in the solution
state.[13e]
In an attempt to produce an analogous cationic com-
plexes with the 1,3,5-triphenyl-1,3,5-triazocyclohexane li-
gand 1f, [Ti(OiPr)3(OTF)] (2) was treated with Ph3-TAC in
a manner analogous to that of previous reactions. Addition
resulted in an immediate colour change (from colourless to
orange), and standing overnight at room temperature re-
sulted in the formation of a crop of orange crystals.
1
Analysis of the crystalline material by H NMR spec-
troscopy showed the presence of a doublet and a septet at
1
δ = 1.82 and 5.31 ppm, respectively, in the H NMR spec-
trum, corresponding to isopropoxide ligands, which appear
at significantly higher ppm values than those found for
1
complexes 2a–e. The H NMR spectrum also showed the
presence of several other resonances, but specifically a
sharp singlet and doublet at δ = 8.62 and 8.46 ppm, respec-
tively. The isolated crystalline solid of 4 was found to show
only limited solubility in chlorinated and inert organic sol-
vents. As a result 13C{1H} NMR spectroscopic data for 4
could not be obtained. Analysis by single-crystal X-ray dif-
fraction was therefore relied upon to identify the nature of
complex 4. The molecular structure will be discussed at a
later point, but studies showed the product to be a titanium
bis(alkoxide) bis(triflate) system coordinated by two 3-
phenyl-3,4-dihydroquinazoline ligands.
Attempts to monitor the reaction by 1H NMR spec-
troscopy were not wholly successful, but the 1H NMR spec-
trum of the reaction mixture does show the presence of
both free 3-phenyl-3,4-dihydroquinazoline and resonances
that correspond to a second isopropoxide-containing spe-
cies, although the relative integrals of these species do not
provide conclusive support for the proposed reaction
scheme.
Importantly, throughout this study a second, in situ,
method for the synthesis of compounds 3a–3e and 4 has
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M. G. Davidson, A. L. Johnson
FULL PAPER
also been evaluated. Stoichiometric amounts of Me3SiOTf and three isopropoxide groups, which are orientated in a
were added to a toluene solution of [Ti(OiPr)4] in the pres- transoid fashion to the nitrogen atoms of the respective do-
ence of the fac-N3 nitrogen ligands 1a–f in order to investi- nor ligands.
gate any possible effect on the complex formation and to
Complex 3a crystallizes in the monoclinic space group
reveal any differences between the products formed by the P21/c, with one independent molecule per asymmetric unit.
two methods, i.e. stoichiometries, different ligand coordina- The tris(pyrazolyl)methane ligand is coordinated in a κ3-
tion modes, different polymorphs, etc. Our investigation has fashion, with the lone pairs of the tris(pyrazolyl)methane
shown that for the compounds discussed here, both meth- pointing towards the Ti centre in approximately the same
odologies were equally effective, and for comparable experi- direction as the Ti–N bond. The Ti–N distances lying in the
ments, the products were found to be identical by multinu- range 2.27–2.89 Å, which are comparable to those found in
clear NMR spectroscopy, elemental analysis and unit cell [Ti(NtBu){C(Me2Pz)3}Cl(thf)][14b]
[2.358(3)–2.16(9) Å],
determination. This observation could have a significant ef- which contains the anionic ligand [C(Me2Pz)3]–, (Me2Pz =
fect on any future utility of these complexes, as there will 3,5-dimethylpyrazole), and related Ti–tris(pyrazolyl)borate
be no need to preform the catalytic species, thereby saving complex, such as [Ti{HB(Mes2Pz)3}Cl2(OtBu)], (2.17–
time.
2.25 Å).[19] The N–Ti–N “bite angles” in 3b (N–Ti–Nav.
Attempts to synthesize the bis- and tris(triflate) com- 75.61°) are slightly smaller than “bite angle” observed in
plexes, i.e. [Ti(OiPr)2(OTf)2] and [Ti(OiPr)(OTf)3], respec- [Ti(NtBu){C(Me2Pz)3}Cl(thf)], (N–Ti–Nav. 80.52°). This
tively, by reaction of stoichiometric amounts of Me3SiOTf compression is accompanied by a small lengthening of the
with [Ti(OiPr)4] in toluene, have to date been unsuccessful, Ti···CH distance to 3.406(2) Å {cf. 3.332(3)° in
forming only intractable, insoluble materials. The addition [Ti(NtBu){C(Me2Pz)3}Cl(thf)]}, but not at the expense of
of donor solvents such as thf or pyridine to these systems Ti–N contact. The steric demands of the three OiPr groups
results in the formation of a series of soluble complexes, about the Ti atom are presumably the cause of this displace-
which have yet to be fully and properly characterised.
ment of the {HC(Pz)3} ligand, an observation that is sup-
ported by the large O–Ti–O angles shown in Table 1. As a
result the cis- and trans-N–Ti–O angles deviate considerably
from the ideal.
Solid State Molecular Structures
Crystal data for complexes 3a–c are shown in Table 3. In
all cases, the complexes are ionic, comprising of well sepa-
rated titanium-based cations, and triflate anions. The mo-
lecular structures of the cationic component of 3a, 3b and
3c are shown in Figures 2, 3 and 4 respectively. Within all
three cations the titanium centres are situated in a pseudo
octahedral environment coordinated by the tridentate tris-
(pyrazol)methane [HC(Pz)3] (3a), or a tridentate trialkyl-
triazacyclohexane (R3-TAC) (3b; R = Et, 3c; R = CH2Ph)
Table 1. Selected geometric data for the complexes 3a, 3b and 3c.
3a
3b
3c
Bond lengths [Å]
Ti(1)–O(1)
Ti(1)–O(2)
Ti(1)–O(3)
Ti(1)–N(1)
Ti(1)–N(2)
Ti(1)–N(3)
1.765(2)
1.7848(10)
1.787(3)
1.766(3)
1.780(3)
2.350(3)
2.319(3)
2.339(3)
1.8066(19)
1.7643(19)
2.271(2)
2.2877(18)
2.2798(18)
2.3006(11)
Bond angles [°]
O(1)–Ti(1)–O(2)
O(1)–Ti(1)–O(3)
O(2)–Ti(1)–O(3)
N(1)–Ti(1)–N(2)
N(1)–Ti(1)–N(3)
N(2)–Ti(1)–N(3)
O(1)–Ti(1)–N(1)
O(2)–Ti(1)–N(2)
O(3)–Ti(1)–N(3)
102.02(10)
103.03(10)
102.63(10)
75.04(6)
76.46(6)
75.32(6)
161.69(8)
157.97(8)
161.49(8)
106.09(4)
60.16(5)
147.89(4)
109.23(14)
105.76(14)
105.30(13)
59.61(10)
59.66(10)
59.41(9)
146.55(12)
148.37(11)
145.86(11)
Additional distances [Å]
Δ values
–
0.145
[C(101)]
0.070 [C(101)]
0.057 [C(201)]
0.203 [C(301)]
Complex 3b, which crystallises in the cubic space group
P213 with a third of a molecule per asymmetric unit (the Ti
atom of the cation and the carbon and sulfur of the triflate
anion residing on centre of threefold symmetry, the rest of
the molecule is generated by symmetry operations) is re-
vealed to possess an all syn-conformation in the solid state
(Figure 3).
Figure 2. Diagram showing the molecular structures of the cationic
component of [{H(Pz)3}Ti(OiPr)3][OTf] (3a) (30% probability el-
lipsoids). Disordered based on O(1) and O(3) have been omitted
and the major component shown only.
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Alkoxytitanium Triflate Complexes
Figure 3. Diagram showing the molecular structures of the cationic
component of [{Et3-TAC}Ti(OiPr)3][OTf] (3b) (30% probability el-
lipsoids). Hydrogen atoms have been omitted for clarity. Disor-
dered based on O(1) has been omitted and the major component
shown only. Symmetry transformations used to generate equivalent
atoms: #A = y,z,x #B = z,x,y.
Figure 4. Diagram showing the molecular structures of the cationic
component of [{Bz3-TAC}Ti(OiPr)3][OTf] (3c) (30% probability el-
lipsoids). Disordered based on O(1) and O(3) have been omitted
and the major component shown only.
Complex 3c, which crystallises in the orthorhombic
space group Pc21b, with one molecule per asymmetric unit,
shows an anti,anti,syn configuration of the benzyl groups
of the TAC ligand (Figure 3). An examination of the Ti–
N bond lengths with in the two complexes shows the two
complexes to be comparable, within experimental error; the
only significant differences being the relative conformations
of the ethyl and benzyl substituents of the TAC ligands.
Structural parameters for the coordination environment
around titanium in the cations [(R3-TAC)Ti(OiPr)3]+ (Fig-
ures 3 and 4) are summarised in Table 1. In both complexes
the Ti–N bond length of [3b: 2.3006(11) Å] and {3c:
2.350(3) Å [N(1)], 2.319(3) Å [N(2)] & 2.339(3) Å [N(3)]}
and bite angle {3b: 60.16(5)° [N(1)–Ti(1)–N(1*)] and 3c:
59.56(10)° (av. N–Ti–N)} are comparable those found in the
related complexes [{TAC}TiCl3]+ and [{TAC}Ti(NtBu)-
Cl2].[13e,14b] As in other triazocyclohexanes complexes, the
facial κ3-coordination of the ligand results in a mis-direc-
tion of the nitrogen lone pairs, and results in a reduced
bonding situation. However, bonding can be improved by
bending of the N-substituent towards the {N3}-plane and
therefore the lone pair towards the metal.[13e]
Table 2. Selected geometric data for the complexes 4.
Bond lengths [Å]
Ti(1)–O(1)
Ti(1)–O(2)
Ti(1)–O(3)
Ti(1)–O(6)
1.757(3)
1.757(3)
2.142(3)
2.105(3)
Ti(1)–N(1)
Ti(1)–N(2)
2.172(3)
2.178(3)
Bond angles [°]
O(1)–Ti(1)–O(2) 101.32(13)
O(1)–Ti(1)–O(6) 91.64(12)
O(2)–Ti(1)–O(3) 91.08(12)
O(3)–Ti(1)–O(6) 76.11(11)
O(1)–Ti(1)–O(3) 167.31(12)
O(2)–Ti(1)–O(6) 166.76(12)
N(1)–Ti(1)–N(3)
This bending can be expressed by the distance Δ of the
α-C atom of the nitrogen substituent above the {N3} plane.
For a perfect tetrahedral angle a value of Δ = 0.49 Å is
expected. Any reduction of this value indicates a bending
of the N-lone pair towards the metal centre. The observed
Δ values for 3b and 3c are shown in Table 2 and fall into
Scheme 4. A diagrammatic representation of the effect of metal co-
ordination to the TAC ligands 1b–e.
two groups. For substituents in the syn-orientation Δ values the asymmetric unit cell (ASU) differ only in the relative
range between 0.14–0.20 Å, and for anti-oriented substitu- orientations of the dihydroquinazoline ligands and the iso-
ents the bending is significantly greater with values between propoxide ligands and are equivalent, within experimental
0.05–0.07 Å (Scheme 4).
error, to each other. Figure 5 shows the molecular structure
¯
Complex 4 crystallises in the triclinic space group P1 of one of the two molecules in the ASU, and is taken as
with two molecules of complex and two molecules of disor- representative of both molecules that are present. Selected
dered toluene per asymmetric unit. The two molecules in bond lengths and angles for 4 are reported in Table 2.
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M. G. Davidson, A. L. Johnson
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within the {NCN} section of the ligand highlights a signifi-
cant degree of delocalisation over this section of the ligand.
An observation that is further supported by examining the
angles about the amine nitrogen [average ΣN = 359.4(3)°]
which suggest sp2-hybridisation at this atom.
Figure 6. Diagram highlighting the bond localisation in the
{C3NCN} portion of the dihydroquinazoline ligand. The bond
lengths shown are for the ligand based on [N(1),N(2),C(31)–C(44)].
Conclusions
In this paper we have described effective synthetic routes
to the first-reported cationic titanium alkoxide compounds
containing tris(pyrazolyl)methane and triazacyclohexane li-
gands. These compounds form the basis for a comprehen-
sive study of the stoichiometric chemistry of cationic tita-
nium complexes derived from face-capping ligands. In ad-
dition, we have found that the mono-triflate complex 2 is
sufficiently Lewis-acidic to be capable of catalyzing electro-
philic aromatic substitution reactions and synthesising di-
hydroquinazoline systems in a Mannich-type reaction. Re-
ports on the further chemistry of these new complexes and
their selected activity in Lewis acid catalysed reactions, such
as the catalytic formation of dihydroquinazoline systems
will be described at a later date.
Figure 5. Diagram showing the molecular structures of the complex
[Ti(OiPr)2(OTf)2(L)2] (4) (L = N-phenyl-3,4-dihyroquinazoline)
(30% probability ellipsoids). Solvent of crystallisation has been
omitted for clarity.
For both molecules within the asymmetric unit cell the
titanium centre adopts a pseudo octahedral geometry with
a, cis-alkoxide, cis-triflate trans-dihydroquinazoline ar-
rangement. The gross structural arrangement is comparable
to the related titanium-bis(isopropoxide) bis(triflamide)
complex previously reported by Johnson et al.,[11] with the
alkoxide ligands arranged in a transoidal configuration with
respect to the “weakly coordinated” triflate groups.
The Ti–OiPr bond lengths in 4 are only marginally
smaller [Ti–Oav. 1.757(3) Å] than those found in complexes
3a–c, indicating the absence of any significant trans-influ-
ence. Not unsurprisingly the Ti–OTf bonds [Ti–Oav.
2.124(3) Å] are also comparable to complexes with a trans-
triflate, oxygen ligand arrangement.[20]
The two dihydroquinazoline ligands occupy the axial po-
sitions about the titanium centre [Ti–Nav. 2.175(3) Å] but
the N–Ti–N axis is bent [N–Ti–N 167.57(12)°] at the metal
centre with the corresponding vectors resulting in a dis-
placement of the ligands away from the alkoxide ligands,
presumably to reduce steric interaction between the iso-
propoxide ligands and the C–H groups at C(37) and C(57).
This is accompanied by a significant widening of the iPrO–
Ti–OiPr angle [O(1)–Ti(1)–O(2) 101.32(13)°] and a re-
duction in the TfO–Ti–OTf angle [O(3)–Ti(1)–O(6)
76.11(11)°].
Experimental Section
General Remarks: All manipulations were carried out under an
atmosphere of dry argon using standard Schlenk and glove-box
techniques. Solvents were purified by conventional procedures and
distilled prior to use. Ti(OiPr)4 was purchased from Aldrich and
used as received, without further purification. The fac-N3 ligands
HC(Pz)3 (tris(pyrazolyl)methane, 1a),[22] (tris(p-fluorobenzyl)-
1,3,5-triazacyclohexane, 1d)[13e] and (tris-s-phenylethyl-1,3,5-triaza-
cyclohexane, 1e)[23] were synthesised according to literature tech-
niques. Et3-TAC (triethyl-1,3,5-triazacyclohexane, 1b), Bz3-TAC
(tribenzyl-1,3,5-triazacyclohexane, 1c), and Ph3-TAC (triphenyl-
1,3,5-triazacyclohexane, 1f) were purchased from Aldrich and used
as received.
Solution 1H, and 13C{1H} NMR experiments were performed at
1
ambient temperature using a Bruker Advance-300. H NMR spec-
The bond lengths and angles within the dihydroquinazo-
line ligands are directly comparable with previously de-
scribed dihydroquinazoline derivatives;[16a,21] Figure 6
shows a diagram highlighting the localisation of bonding
within the {C3NCN} section of the dihydroquinazoline li-
gand found in 4. The bond lengths are taken as exemplars
for the other ligands found within the ASU which show
troscopic data are referenced to residual non-deuterated solvent.
Elemental analyses were performed externally by Elemental Micro-
analysis Ltd., Okehampton, UK.
[Ti(OiPr)3(OTf)] (2): Neat trimethylsilyl triflate (1.8 mL, 2.22 g,
10 mmol) was added to a solution of [Ti(OiPr)4] (3.1 mL, 10 mmol)
in 50 mL of toluene. The mixture was stirred for 3 h at room tem-
perature, after which the solvent was removed in vacuo. 20 mL of
identical trends. An examination of the N–C bond lengths fresh toluene was added to the residue which was dissolved by
5156 Eur. J. Inorg. Chem. 2011, 5151–5159
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Alkoxytitanium Triflate Complexes
warming. The clear solution was then filtered, by cannula, in to a
clean Schlenk and the solution left to cool to room temperature,
yielding a crop of colourless crystals. The product was isolated by
filtration, and washed with 5 mL of cold hexane and dried in
vacuo; yield 3.14 g 84%. C10H21F3O6S1Ti1 (374.23): calcd. C 32.10,
H 5.66; found C 32.51, H 8.86. 1H NMR (300 MHz, 23 °C,
CDCl3): δ = 1.34 [br. m, 18 H, OCH(CH3)2], 4.91 [br. m, 3 H,
[{p-FBz3-TAC}Ti(OiPr)3][OTf] (3d): In-situ method; yield 0.64 g
(81%) [(direct) Yield 0.57 g (72%)]; yield 0.44 g 72%.
C34H45F6N3O6S1Ti1 (785.69): calcd. C 51.98, H 5.77, N 5.33; found
1
C 52.04, H 5.81, N 5.29. H NMR (300 MHz, 23 °C), CDCl3 : δ =
1.18 [d (3J = 7 Hz), 18 H, OCH(CH3)2], 3.49 [d (3J = 9 Hz), 3 H,
(CHaHbN)], 3.86 (s, 6 H, TAC-CH2-C6H4F), 4.21 [d (3J = 9 Hz), 3
H, (CHaHbN)], 4.57 [sept (3J = 7 Hz), 3 H, OCH(CH3)2] 6.91–
OCH(CH3)2] ppm. 13C{1H} NMR (75.5 MHz, 23 °C, CDCl3): δ = 7.04 (m, 6 H, CH), 7.21–7.32 (m, 6 H, CH) ppm. 13C{1H} NMR
26.4, 76.3 ppm. 19F NMR (376 MHz, 23 °C): δ = –77.4 ppm.
(75.5 MHz, 23 °C CDCl3 ): δ = 26.9, 57.3, 73.3, 80.7, 116.4 (d, J
2
4
= 21.9 Hz), 127.4 (d, J = 3.0 Hz), 132.8 (3J = 8.3 Hz), 163.5 (1J =
The titanium complexes 3a–e and 4 were prepared using standard
procedures for both the direct and in-situ synthesis, examples of
both procedures are given below.
249.2 Hz) ppm. 19F NMR (376 MHz, 23 °C): δ = –74.8 ppm.
[{s-(PhC(Me)H)3-TAC}Ti(OiPr)3][OTf] (3e): In-situ method; yield
0.46 g (60%) [(direct) yield 0.48 g (62%)]. C37H54F3N3O6S1Ti1
(773.80): calcd. C 57.43, H 7.03, N 5.43; found C 57.28, H 7.16, N
5.40. 1H NMR (300 MHz, 23 °C), CDCl3 : δ = 1.34 [d (3J = 8 Hz),
18 H, OCH(CH3)2], 1.74 [d (3J = 6 Hz), 9 H, TAC-CH(Me)Ph],
2.56 [d (3J = 9 Hz), 3 H, (CHaHbN)], 4.09 [q (3J = 6 Hz), 3 H,
TAC-CH(Me)Ph], 4.87 [sept (3J = 8 Hz), 3 H, OCH(CH3)2], 7.09–
7.29 (m, 15 H, CH) ppm. 13C{1H} NMR (75.5 MHz, 23 °C
CDCl3): δ = 17.9, 26.9, 63.0, 71.1, 80.9, 126.2, 128.4, 129.4, 135.2
ppm. 19F NMR (376 MHz, 23 °C): δ = –76.6 ppm.
[{HC(Pz)3}Ti(OiPr)3][OTf] (3a): In-situ method; neat trimethylsilyl
triflate (0.22 g, 1 mmol) was added to a solution of tris(pyrazolyl)-
methane (0.21 g, 1 mmol) and [Ti(OiPr)4] (0.31 mL, 1 mmol) in
10 mL of toluene. The mixture was stirred for 20 min at room tem-
perature, after which the solvent was removed in vacuo. Fresh tolu-
ene (15 mL) was then added to the residue. Warming resulted in
dissolution of the residue and formation of a clear solution, which
was filtered by cannula into a clean Schlenk. Cooling the solution
at –15 °C resulted in the formation of a crop of colourless crystals,
which were isolated by filtration and washed with 5 mL of cold
hexane and dried in vacuo; yield 0.41 g, 70%. C20H31F3N6O3STi
(540.46): calcd. C 40.82, H 5.31, N 14.28; found C 40.32, H 5.29,
N 14.13. 1H NMR (300 MHz, 23 °C, CDCl3): δ = 1.20 [d (3J =
12 Hz), 6 H, OCH(CH3)2], 4.73 [sept (3J = 6 Hz), 3 H, OCH-
(CH3)2], 6.34 [dd, (3J = 1.8 Hz, 3J = 2.6 Hz), 3 H, CH], 7.82, [d (3J
= 1.6 Hz), 3 H, CH], 8.38 [d (3J = 2.6 Hz), 3 H, CH], 9.97 (s, 1 H,
N3CH) ppm. 13C{1H} NMR (75.5 MHz, 23 °C, CDCl3): δ = 26.1,
74.7, 79.8, 107.48, 132.8, 143.4 ppm. 19F NMR (376 MHz, 23 °C):
δ = –76.1 ppm.
[Ti(OiPr)2(OTf)2{κ1-(C6H4)NC(H)N(Ph)CH2}2]
(4):
In-situ
method; yield 0.26 g (30% based on Ti) [(direct) yield 0.21 g (24%
based on Ti)]. C36H38F6N4O8S2Ti1·C7H8: calcd. C 53.09, H 4.77,
1
N 5.76; found C 52.9, H 4.70, N 5.81. H NMR (400 MHz, 23 °C,
CDCl3): δ = 1.82 [d (3J = 7 Hz), 12 H, OCH(CH3)2], 4.46 (br. m,
4 H, CH2N), 5.34 [sept (3J = 7 Hz), 2 H, OCH(CH3)2], 6.7–7.2 (m,
16 H, CH), 8.42 [d (3J = 12 Hz), 2 H, CH], 8.62 (s, 2 H, N–CH–
N) ppm.
X-ray Crystallographic Study: Crystallographic data for com-
pounds 3a, 3b, 3c, and 4 are summarised in Table 3. All data collec-
tions were implemented on a Nonius–Kappa CCD diffractometer.
Structure solution and refinement was performed using
SHELX86[24] and SHELX97[25] software, respectively. Full matrix
anisotropic refinement was implemented in the final least-squares
cycles throughout. All data were corrected for Lorentz and polari-
sation and, with for extinction. Hydrogen atoms were included at
calculated positions throughout.
Direct Method: To a solution of 2 (0.37 g, 1 mmol) in 10 mL of
toluene,
a toluene solution (2 mL) of tris(pyrazolyl)methane
(0.21 g, 1 mmol) was added. The reaction was stirred for 20 min.
The solvent was then removed in vacuo and fresh toluene added to
the solid residue. The solid was dissolved with warning followed by
cannula filtration into a clean Schlenk. Cooling of the clear solu-
tion to –15 °C resulted in the formation of a crop of colourless
crystals, which were isolated by filtration and washed with 5 mL of
cold hexane and dried in vacuo; yield 0.42 g (71%). Analysis
showed the product to be identical to that prepared by the in-situ
method.
In 3a two of the three isopropyl groups within the cationic fragment
based on O(1) and O(3), are disordered over two positions. For
both isopropyl groups the disordered atoms were modelled aniso-
tropically in 70:30 occupancy ratios. The ASU of 3b consist of 1/3
of the complex, with the Ti atom of the cation and the carbon and
sulfur atoms of the triflate anion residing on special 3-forl rota-
tional axis.
[{Et3-TAC}Ti(OiPr)3][OTf] (3b): In-situ method; yield 0.33 g (60%)
[(direct); yield 0.40 g (73%)]. C19H42F3N3O6S1Ti1 (545.51): calcd.
1
C 41.84, H 7.76, N 7.70; found C 41.76, H 7.99, N 7.59. H NMR
(300 MHz, 23 °C, CDCl3): δ = 1.19 [t (3J = 9 Hz), 9 H, TAC-
CH2CH3] 1.25 [d (3J = 7 Hz), 18 H, OCH(CH3)2], 2.84 [q (3J =
9 Hz), 6 H, TAC-CH2CH3], 4.03 [m, 6 H, (NCH2)3], 4.52 [sept (3J
= 7 Hz), 3 H, OCH(CH3)2] ppm. 13C{1H} NMR (75.5 MHz, 23 °C,
CDCl3): δ = 10.6, 26.7, 48.1, 73.2, 79.9 ppm. 19F NMR (376 MHz,
23 °C): δ = –76.5 ppm.
In addition the isopropyl group within the ASU exhibits disorder
centred about the CH group of the isopropyl unit, C(11). Accord-
ingly the methyl groups are modelled anisotropically of over two
positions, C(12) & C(13) vs. C(12A) & C(13A) in a 50:50 ratio.
In complex 3c the two isopropyl groups based on O(1) and O(2)
exhibit significant disorder, and as such have been modelled iso-
tropically over 2 positions, in a 70:30 occupancy ratio. Additional
disorder is present in the anion component of 3c with the fluorine
and oxygen atoms of the triflate unit showing rotational disorder
about the C–S axis. The disorder could not be well defined and
as such the disordered atoms are modelled isotropically in single
positions.
[{Bz3-TAC}Ti(OiPr)3][OTf] (3c): In-situ method; yield 0.48 g (66%)
[(direct); yield 0.45 g (61%)]. C34H48F3NN3O6STi (745.73): calcd.
1
C 55.81, H 6.61, N 5.74; found C 55.98, H 6.74, N 5.72. H NMR
(300 MHz, 23 °C), CDCl3 : δ = 1.15 [d (3J = 6 Hz), 18 H,
OCH(CH3)2], 3.48 [d (3J = 9 Hz), 3 H (CHaHbN)] 3.85 (s, 6 H,
TAC-CH2-Ph), 4.21 [d (3J = 9 Hz), 3 H (CHaHbN)], 4.54 [sept (3J
= 6 Hz), 3 H, OCH(CH3)2], 7.18–7.27 (m, 15 H, CH) ppm.
13C{1H} NMR (75.5 MHz, 23 °C, CDCl3): δ = 26.9, 58.2, 73.5,
80.6, 128.8, 129.4, 130.9, 131.5 ppm. 19F NMR (376 MHz, 23 °C):
δ = –76.2 ppm.
The ASU of 4 consists of two molecules of the titanium complex
and two half-molecules of disordered toluene [C(1s)–C(7s) and
C(11s)–C(17s), respectively]. Both half molecules of toluene reside
on special positions between adjacent asymmetric unit cells and
Eur. J. Inorg. Chem. 2011, 5151–5159
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5157

M. G. Davidson, A. L. Johnson
FULL PAPER
Table 3. Crystal data and structure refinement for compounds 3a, 3b, 3c, and 4.
3a
3b
3e
4
Formula
Formula weight
T /K
Crystal system
Space group
a /Å
b /Å
c /Å
α /°
β /°
C20H31F3N6O6S1Ti1
588.47
150(2)
monoclinic
P21/c
11.9370(2)
14.9290(2)
16.4330(3)
C19H42F3N3O6S1Ti1
C34H48F3N3O6S1Ti1 C36H38F6N4O8S2Ti1·0.5C7H8
545.52
150(2)
cubic
P213
731.71
962.78
150(2)
triclinic
150(2)
orthorhombic
Pc21b
¯
P1
9.41900(10)
16.4870(2)
24.1100(3)
10.5520(2)
20.1980(3)
20.3100(4)
88.2430(10)
89.8210(10)
80.7170(10)
4269.95(13)
4
13.9840(2)
103.6660(10)
γ /°
U /Å3
2845.58(8)
4
1.374
0.437
2734.60(7)
4
1.325
0.445
3744.07(8)
4
Z
Dc /gcm–3
1.298
0.344
1.442
0.380
μ /mm–1
F(000)
1224
1160
1544
1916
Crystal size /mm
Theta range /°
Reflections collected
Independent reflections
Reflections [IϾ2σ(I)]
Data/restraints/parameters
Goodness-of-fit on F2
Final R1, wR2 [IϾ2σ(I)]
Final R1, wR2 (all data)
Max., min. diff./ eÅ–3
Absolute structure parameter
0.32ϫ0.25ϫ0.13
3.69 to 27.52
36116
0.32ϫ0.25ϫ0.22
4.37 to 27.45
53932
0.15ϫ0.12ϫ0.08
3.55 to 26.38
72363
0.30ϫ0.28ϫ0.17
8.52 to 25.93
39104
6499 [R(int) = 0.0430]
2094 [R(int) = 0.0547]
7647 [R(int) = 0.0626] 15810 [R(int) = 0.0386]
6499/0/399
1.032
0.0481, 0.1254
0.0649, 0.1383
0.427, –0.404
–
2094/0/124
1.112
0.0261, 0.0622
0.0318, 0.0652
0.178, –0.169
–0.02(3)
7647/1/410
1.045
0.0820, 0.2257
0.0949, 0.2405
1.404, –0.746
0.00(5)
15810/6/1069
1.118
0.0704, 0.1589
0.0842, 0.1659
1.292, –0.594
–
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Received: August 5, 2011
Published Online: October 19, 2011
Eur. J. Inorg. Chem. 2011, 5151–5159
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5159