Diverse reactions of PhI(OTf)2 with common 2-electron ligands: Complex formation, oxidation, and oxidative coupling

Article abstract of DOI:10.1021/ic302176f

The crystal structures of bis-pyridine stabilized iodine dications [PhI(pyr)₂]²⁺ are reported as triflate salts, representing the first ligand supported iodine dications to be structurally characterized. The pyridine complexes are susceptible to ligand exchange in reaction with stronger N-based donors such as 4-dimethylaminopyridine. Attempts to extend this reactivity to N-heterocyclic carbene and phosphine ligands, as has been accomplished in the earlier p-block groups, resulted in redox chemistry, with oxidation of the ligands rather than coordination.

Full text of DOI:10.1021/ic302176f

Article
pubs.acs.org/IC
Diverse Reactions of PhI(OTf)2 with Common 2‑Electron Ligands:
Complex Formation, Oxidation, and Oxidative Coupling
Thomas P. Pell, Shannon A. Couchman, Sara Ibrahim, David J. D. Wilson, Brian J. Smith, Peter J. Barnard,
and Jason L. Dutton*
Department of Chemistry, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, Victoria, Australia 3086
S
* Supporting Information
ABSTRACT: The crystal structures of bis-pyridine stabilized
iodine dications [PhI(pyr)2]2+ are reported as triflate salts,
representing the first ligand supported iodine dications to be
structurally characterized. The pyridine complexes are
susceptible to ligand exchange in reaction with stronger N-
based donors such as 4-dimethylaminopyridine. Attempts to
extend this reactivity to N-heterocyclic carbene and phosphine
ligands, as has been accomplished in the earlier p-block groups,
resulted in redox chemistry, with oxidation of the ligands rather than coordination.
pseudo-halides). There is a single system comprising ligand
stabilized iodine dications, with a [PhI(pyr)2]2+ architecture
generated from the iodine(III) pseudo-halide PhI(OTf)2 (8-R;
R = -H, -NMe2). This class of compound was first reported in
1994 and subsequently reinvestigated by Zhdankin in 2002, but
has not been structurally verified, nor described in terms of
electronic structure.17,18 Nevertheless, these compounds have
found some recent use in organic synthesis,19−21 and the
related derivative with R = CN was recently used to access
high-valent palladium compounds.22 Our interest in this class of
compound, and the precursor PhI(OTf)2, lies in using them as
an entry point to study the coordination chemistry of
polycationic iodine species.
INTRODUCTION
■
The study of ligand stabilized polycationic p-block centers has
emerged as a major area of study in synthetic chemistry.1 In
recent years there have been notable successes in the isolation
of compounds containing B2+,2 B3+,3 Ge2+,4−6 Sn2+,7 P2+,8 P3+,9
S2+,10,11 Se2+,12,13 and Te2+.14−16 The accepted bonding model
for these compounds is a coordinative interaction between the
ligands and the main group element center, and the propensity
for these compounds to engage in ligand exchange reactions
has been harnessed to form compounds with unique bonding
arrangements for a variety of elements (e.g., 1−7).
In this study, we have confirmed the structural form of these
iodine-centered dications and analyzed the electronic structure
and bonding using density functional theory (DFT)
calculations. Attempts at extending the chemistry to utilize
phosphine and N-heterocyclic carbene ligands resulted in
The concept of using ligands to stabilize main group
centered polycations has only very rarely been extended to
the halogens. The primary reason for this is that potential
starting materials are unavailable, especially so for the lighter
elements. Iodine, however, has a variety of compounds which
could be used as Lewis acidic starting materials for the
formation of precursors to dications (e.g., I(III) halides and
Received: October 5, 2012
Published: November 13, 2012
© 2012 American Chemical Society
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Updated 1H NMR data for 8-H and 8-NMe2 (500 MHz, CD3CN, δ
significantly different reactivity, with oxidation of the ligands
rather than coordination being observed.
ppm).
8-H: 8.62 (d, 3JH−H 4.5 Hz, 4H, o-py), 8.37 (bs, 2H, o-Ph), 8.05 (bs,
2H, p-py), 7.83 (bs, 4H, m-py), 7.67 (t, 3JH−H 7.0 Hz, 1H, p-Ph), 7.45
EXPERIMENTAL PROCEDURES
3
■
(t, JH−H 7.0 Hz, 2H, m-Ph).
All manipulations were carried out in a Saffron Beta model glovebox
under an N2 atmosphere. All solvents were dried over CaH2, vacuum
distilled, and stored in the glovebox over 3 Å molecular sieves. All
reagents were obtained from Alfa Aesar. Triphenylphosphine,
PhI(OAc)2, and 4-dimethylaminopyridine were dried under vacuum
prior to transfer into the glovebox. TMS-OTf was used as received.
Pyridine was dried over CaH2, distilled, and stored in the glovebox
over 3 Å molecular sieves prior to use. The N-heterocyclic carbene
(NHC) iPr2IM(Me)2 was synthesized according to Kuhn’s protocol.23
Compounds 8-H and 8-NMe2 were synthesized according to the
reported procedure;17 however, our 1H NMR data differed
significantly from the original report, and is provided below. Single
crystals of compounds 8-H and 8-NMe2 were grown from the CD3CN
solutions at −35 °C in the glovebox. Single crystals were very rapidly
(<30 s) selected from samples of crystals transferred into paratone-n
oil and immediately into a cold stream of N2 gas on the diffractometer.
The solution and refinement of 8-H was nontrivial. The metrical
parameters from the data collection suggested an orthorhombic or
tetragonal crystal system, but no suitable space group for solution
could be found (Table 1). A solution was possible in the triclinic space
8-NMe2: 7.95 (d, 3JH−H 7.5 Hz, 4H, o-DMAP), 7.78 (bs, 2H, o-Ph),
7.54 (t, 3JH−H 8.0 Hz, 1H, p-Ph), 7.35 (t, 3JH−H 8.0 Hz, 2H, p-Ph), 6.78
3
(d, JH−H 7.5 Hz, 4H, m-DMAP), 3.16 (s, 12 H, -NMe2 DMAP).
Generation of 9. A solution of iPr2IM(Me)2 (0.2 mmol; CD3CN 1
mL) was added to solutions of PhI(OTf)2 or PhI(OAc)2 or 8-NMe2
(0.1 mmol; CD3CN 1 mL) giving colorless solutions. Aliquots were
removed for 1H and 13C{1H} NMR and mass spectral studies,
confirming quantitative generation of 9 and PhI by comparison to
Clyburne’s data.31
Generation of 10. A solution of Ph3P (0.05 g, 0.1908 mmol;
CH2Cl2, 2 mL) was added to a slurry of 8-NMe2 (0.143 g, 0.1908
mmol; CH2Cl2 3 mL) and stirred for 30 min, giving a colorless
solution. N-hexane (15 mL) was added resulting in the deposition of
an oil. The solution was stored at −35 °C overnight giving 10 as a
1
colorless solid. Yield 0.088 g, 86%. H NMR (CDCl3, 500 MHz, δ
ppm) 8.10−7.75 (overlapping multiplets, 17H) 7.20 (d, 3JH−H 6.5 Hz,
2H), 3.42 ppm (s, 6H) ; 31P{1H} NMR (CDCl3, 202.5 MHz, δ ppm)
58.
RESULTS AND DISCUSSION
■
Weiss’ original methods were used to prepare compounds 8-
NMe2 and 8-H, via PhI(OTf)2, formed in situ from PhI(OAc)2
and TMS-OTf (Scheme 1).
Table 1. X-ray Refinement Details for 8-H and 8-NMe2
8-H
8-NMe2
empirical formula
FW (g/mol)
crystal system
space group
a (Å)
C18H15F6I1N2O6S2
660.34
orthorhombic
Pbcn
C22H25F6I1N4O6S2
746.48
orthorhombic
Pnma
Scheme 1. Synthesis of Base Stabilized Iodine Polycations
Following Weiss’ Protocol
10.879(2)
20.300(4)
10.876(2)
90
21.866(4)
9.752(2)
13.102(3)
90
b (Å)
c (Å)
α (deg)
β (deg)
90
90
γ (deg)
90
90
V (Å3)
2401.9(8)
4
2793(1)
4
Z
Dc (mg m−3
)
1.826
1.775
radiation, λ (Å)
temp (K)
0.71073
173(2)
0.0300
0.0633
1.162
0.71073
173(2)
0.0584
0.1387
1.355
R1[I > 2σI]
wR2(F2)
GOF (S)
group P1, which allowed for identification of the iodine atom and
̅
The syntheses proceeded smoothly in both CH2Cl2 and
CD3CN. The original synthetic report gave low-field proton
NMR data for PhI(OTf)2,32 but a recent study collected higher
resolution proton NMR data, which our findings matched.33 In
addition we collected 19F NMR data, which sheds some light on
the bonding. The 19F chemical shift in CH2Cl2 was found to be
−78.5 ppm. Fluorine chemical shifts of triflate groups have
been used in several cases to establish the ionic/covalent nature
of the triflate, by comparison with known model systems.7,34,35
In this case the chemical shift of the triflate is much closer to
that of “free” triflate (c.f. [NOct4][OTf], −79.0 ppm), than
bound triflate (c.f. TMS-OTf −77.0 ppm, Me-OTf −75.0
ppm), indicating that PhI(OTf)2 could be considered as a
synthetic equivalent of [PhI]2+ in solution. The stability of
PhI(OTf)2 has also not been reported. A sample of PhI(OTf)2
in CDCl3 (0.15 M), kept in the glovebox at room temperature
triflate anions. Use of the ADDSYM routine in the PLATON software
suite24 suggested P2/c as an alternate space group, which allowed for
location of the rings about the iodine center. At this stage many of the
atoms gave nonpositive definite thermal parameters upon refinement.
Rerunning the ADDSYM routine suggested an orthorhombic crystal
system with a Pbcn space group. This allowed for suitable refinement
of the thermal parameters, but the refinement values and residual
electron density were unacceptably high. Finally, the TWINROTMAT
routine in PLATON suggested the presence of a twin with the matrix
0 0 1 0 −1 0 1 0 0 and a BASF of approximately 0.25. Application of
this twin law finally gave a satisfactory refinement.
Density functional theory (DFT) calculations were performed using
the GAUSSIAN-09 program25 at the ωB97XD/6-311++G(d,p) level
of theory.26−29 Geometries were optimized at this level, and second
derivative calculations determined to ensure correct identification of
stationary points and to determine harmonic vibrational frequencies
for zero-point, thermal and entropic corrections to the electronic
energy. Solvent contributions to the free energies were calculated
employing the polarizable continuum model (PCM) approach.30
1
under normal laboratory light, was monitored using H NMR
spectroscopy. No decomposition was apparent after 3 days;
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after 5 days some dark material precipitated from solution and
other phenyl containing compounds could be observed in the
1H NMR spectrum. A sample left undisturbed for 14 days
resulted in the growth of a large number of needle-like crystals
in addition to the dark material. Unit cell analysis of the crystals
by X-ray diffraction and subsequent full data collection revealed
the crystals to be the known salt [para−I-C6H4−I-C6H5][OTf],
containing iodine in both the +1 and +3 formal oxidation
states.36
the clean syntheses of polycations 3, 6, and 7,4,13,14 and more
recently displacement by phosphines from Ch(OTf)2 synthons
(Ch = Se, Te) has been successfully undertaken.15
Triphenylphosphine (Ph3P) was found not to react with
PhI(OAc)2, as monitored by 31P{1H} NMR spectroscopy. The
reaction of PhI(OTf)2 with 2 stoichiometric equivalents of
triphenylphosphine (Ph3P) in CDCl3 gave a clear, colorless
solution. A sample of the reaction mixture was examined by
31P{1H} NMR spectroscopy which revealed 2 resonances; one
at 51 ppm and one at −6 ppm, in an approximate 1:1 ratio by
integration. The signal at −6 ppm is consistent with free
triphenylphosphine. The signal at 51 ppm is downfield of the
expected resonance of Ph3P acting as a ligand, and is consistent
with an oxidation to P(V). The proton NMR spectrum of the
same sample gave resonances consistent with PhI. Examination
of the literature for potential oxidation products revealed that
the [Ph3P-OTf] cation has been reported to give an identical
31P{1H} NMR resonance to our observation.37 The reaction
was repeated using 1 equivalent of triphenylphosphine in
CDCl3 (Scheme 2), which showed complete conversion of
Compounds 8-H and 8-NMe2 were synthesized from freshly
prepared samples of PhI(OTf)2 using Weiss’ reported
1
procedure and isolation. For both compounds the H NMR
data we obtained was in disagreement with the data in the
original report in the same solvent (CD3CN). The resonances
in the literature for 8-H in particular appear to be more
consistent with protonated pyridine.17 Indeed, we have found
these compounds to be particularly sensitive to adventitious
water, readily forming protonated pyridines if solvents and
other solid reagents are not rigorously dried. Single crystals
were grown from the NMR samples by holding the solutions
overnight at −35 °C. Compound 8-H is substantially less stable
than 8-NMe2, decomposing within a few hours if held in
solution at room temperature. The crystals of both compounds
were also found to be highly moisture sensitive, rapidly
degrading in paratone-n oil while being manipulated under the
microscope. However, samples of sufficient quality could be
mounted in both cases. X-ray diffraction studies conclusively
identified the compounds as the dicationic pyridine-iodine
complexes as OTf salts (Figures 1 and 2).
Scheme 2. Oxidation of Ph3P Using PhI(OTf)2
PhI(OTf)2 to PhI, and only the signal at 51 ppm in the 31P{1H}
NMR spectrum. Addition of n-hexane to the solution resulted
in the deposition of a colorless oil, which turned into a
crystalline material upon standing at −35 °C overnight. Rather
than the expected phosphonium salt, single crystal X-ray
diffraction studies revealed the crystals to be the known, but
not structurally characterized salt [Ph3PO-SiMe3][OTf]. This
compound has approximately the same reported chemical shift
as [Ph3P-OTf], at 51 ppm.38 A control experiment found that
addition of Ph3P to a CH2Cl2 solution of TMS-OTf resulted
only in a slight broadening of the single resonance at −6 ppm
in the 31P{1H} NMR spectrum, and no evidence of a peak at 51
ppm, allowing the conclusion that PhI(OTf)2 is integral to the
process. The production of this compound in the reaction can
be rationalized in terms of reported behavior of [Ph3P-
OTf][OTf] and related compounds, where an equilibrium
between [Ph3P-OTf][OTf] and Ph3PO + triflic anhydride has
been described.39
Figure 1. Solid-state structure of 8-NMe2. Hydrogen atoms and triflate
counterions are omitted for clarity. Selected bond distances (Å) and
angles (deg): I(1)−N(1) 2.185(8), I(1)−N(2) 2.217(8), I(1)−C(10)
2.104(10), N(1)−I(1)−N(2) 172.4(3), C(10)−I(1)−N(1) 85.7(3).
We next turned to NHC ligands. The NHC ligand chosen
i
was Pr2NHC(Me)2, selected for its known ability to stabilize
highly charged and/or low valent main group centers.4,13,14
Dichloromethane is not compatible with NHC chemistry, so
the transformations were carried out in CD3CN. Reaction of
two stoichiometric equivalents of NHC with PhI(OTf)2
resulted in the immediate production of a dark orange solution.
The 1H NMR spectrum of the mixture was clean, revealing only
signals arising from the iodine −Ph group and the NHC. The
NHC signals were shifted from the free NHC in CD3CN, in
particular the methine proton of the isopropyl groups was
shifted to 4.51 ppm, from 4.25 ppm in the free carbene.
However, the −Ph had identical resonances as a sample of PhI
dissolved in CD3CN, indicating reduction of the iodine center,
rather than formation of the target complex. The NHC
containing product was isolated as a sticky resin by addition of
Figure 2. Solid-state structure of 8-H. Hydrogen atoms and triflate
counterions are omitted for clarity. Selected bond distances (Å) and
angles (deg): I(1)−N(1) 2.220(3), I(1)−C(21) 2.089(4), N(1)−
I(1)−N(1A) 167.6(2), C(21)−I(1)−N(1) 83.80(8).
We then investigated if this coordination chemistry could be
extended to other ligand sets. In groups 15 and 16, both N-
heterocyclic carbene and trisubstituted phosphine ligands have
been found to be compatible with Lewis acids that also bind
pyridines. Displacement of triflates by NHCs has been used in
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Et2O to the solution. Single crystals could not be obtained, but
an examination of the literature for possible oxidation products
revealed that the coupled [NHC-NHC]2+ dication has been
reported by Clyburne from oxidation of the same NHC by the
spectroscopy. The use of Ph3P in attempted ligand exchange
reactions gave a different result than the reaction of Ph3P with
PhI(OTf)2. As the reaction of Ph3P with PhI(OTf)2/TMS-OAc
required only 1 equiv of Ph3P to bring the reaction to
completion, and with the observation that the reaction of the
NHC with PhI(OTf)2 or 8-NMe2 gave the same result, one
stoichiometric equivalent of Ph3P was added to a CD3CN
solution of 8-NMe2. Over the course of 5 min the solution
turned from yellow to colorless. An aliquot was removed for 1H
and 31P{1H} NMR spectroscopy. The 31P{1H} NMR spectrum
showed a single peak at 59 ppm, slightly downfield from the 51
ppm observed for [Ph3PO-SiMe3]+ in the reaction with
ferrocenium cation.31 Our H and 13C{1H} NMR data was
1
virtually identical to that in Clyburne’s report (Clyburne’s
NMR data was obtained in THF-d8, ours in CD3CN), allowing
for assignment of 9 as the NHC containing product in the
reaction (Scheme 3). An experiment reacting PhI(OAc)2 with 2
equiv of the NHC resulted in an identical oxidative coupling
reaction.
1
i
PhI(OTf)2/TMS-OAc. The H NMR spectrum showed two
Scheme 3. Oxidative Coupling of Pr2NHC(Me)2 Using
4-DMAP containing compounds. Iodobenzene could also be
identified, indicating that the iodine had been reduced and the
target complex was not formed. A small amount of Ph3P was
added to the NMR sample, and this simply resulted in the
appearance of a signal for free Ph3P appearing in the 31P{1H}
NMR spectrum, confirming that only 1 equiv of Ph3P was
required. The reaction was repeated on a larger scale using
CH3CN as the solvent. Upon discharge of the yellow color,
Et2O was added which resulted in the formation of an oil,
which solidified into a colorless powder upon standing at −35
°C for 1 h. The solution was decanted, the solids washed with
Et2O and were then dried in vacuo. A sample was redissolved in
CD3CN, the 31P{1H} NMR spectrum gave the same singlet at
59 ppm. The 1H NMR spectrum was consistent with a
compound containing Ph3P and 4-DMAP in a 1:1 ratio by
integration. Single crystals suitable for X-ray analysis have not
yet been obtained; however, based on reports of related
compounds by Burford,8 we tentatively assign the complex as
salt 10 (Scheme 4). Within the scope of this report, the
important point to note is that a redox process rather than a
ligand exchange reaction occurred.
Bonding and Theoretical Studies. Compounds 8-H and
8-NMe2 are the first ligand stabilized iodine-centered
polycations to be structurally characterized. An examination
of the solid-state structures reveals the expected T-shaped
geometry about the iodine(III) center based on an AX3E2
electron pair geometry, with the pyridine ligands orientated
trans with respect to each other. The I−N bond distances are
2.19−2.22 Å which can be compared to the slightly longer 2.26
Å I−N bond in the linear bispyridine iodine(I) cation.42 The I−
N bonds in 8-H are slightly longer than 8-NMe2. The I−C
bond distances are 2.10(1) and 2.089(4) Å for 8-H and 8-
NMe2, respectively. The N−I−N bond angles were found to be
167.6(2)° for 8-H and 172.4(3)° for 8-NMe2. DFT
optimization of 8-H yielded a geometry with an I−C bond
length of 2.115 and I−N bond lengths of 2.270 Å, slightly
PhI(OTf)2
Ligand exchange reactions have been used to generate p-
block coordination complexes that are otherwise inaccessible
from simple electrophilic starting materials.15 This is partic-
ularly the case in group 16, where use of binary or tetrahalides
with R3P or NHC ligands results in reduction of the group 16
centers and oxidation of the ligands.40,41 We surmised that
reaction of R3P and NHC with the dicationic pyridine
coordination complexes might furnish the corresponding
complexes with new ligands. To confirm that the class of
complexes is susceptible to ligand exchange reactions, 8-H was
reacted with two stoichiometric equivalents of 4-DMAP in
CD3CN, which resulted in a color change from colorless to
yellow. An NMR spectrum of the sample revealed a mixture of
broad peaks in the aryl region. Addition of Et2O to the solution
resulted in the formation of a light yellow precipitate. The
precipitate was washed with Et2O and dried in vacuo. A sample
of the solid redissolved in CD3CN revealed resonances in the
1H NMR spectrum consistent with compound 8-NMe2, and no
pyridine or pyridine-containing complexes was evident. The
ligand exchange product 8-NMe2 was isolated in 70% yield. To
our knowledge, this is the first demonstration of ligand
exchange reactions at a Lewis acidic iodine center using neutral
two-electron donors.
Ligand exchange reactions were then attempted on
compound 8-NMe2 using Ph3P and the NHC (Scheme 4).
Reaction of 8-NMe2 with Pr2NHC(Me)2 resulted in an
identical oxidative coupling of the NHC ligand with free 4-
DMAP as an additional product as monitored by H NMR
i
1
i
Scheme 4. Reactions of Pr2NHC(Me)2 and Ph3P with 8-NMe2
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longer than those found in the crystal structures. The N−I−N
bond angle was calculated to be 176.6°. The Mulliken atomic
charge on the iodine atom in 8-H was calculated to be +1.14,
highly positive for a halogen atom, and reflective of the “dative”
nature of the N−I bonds.
In MO terms, the geometry about the iodine(III) centers in
8-H and 8-NMe2 can be understood in terms of donation of a
pyridine lone pair into the p-based Lewis acidic lowest
unoccupied molecular orbital (LUMO) of the esoteric
[PhI]2+ fragment, orientated orthogonal to the I−C bond
axis. The LUMO for the resulting [PhI-Py]2+ fragment is also a
σ symmetric orbital, orientated along the I−N bond axis,
allowing for formation of the second N−I bond. The highest
occupied frontier orbitals in 8-H are all ligand based π-type
orbitals (Figure 3). The p-orbital based lone pair on the iodine
Figure 4. Optimized geometry of PhI(OTf)2.
atom in PhI(OTf)2 is likely to be slightly more electrophilic.
This is also consistent with the dissociation energies (ΔH0)
calculated for the removal of a single OTf− (509 kJ/mol) or
OAc− (590 kJ/mol), which indicates that while the OTf is
rather strongly bound, the binding is weaker than that of the
OAc fragment.
Figure 3. Frontier molecular orbitals of 8-H.
Thermochemical Studies of Coordination vs Oxida-
tion. For the reactions between PhI(OTf)2, PhI(OAc)2, 8-H
and the NHC, thermochemical calculations were carried out to
gain an understanding of how favorable the oxidation reactions
are compared to the unobserved simple coordination reactions.
In the gas phase, all reactions were found to be strongly
endothermic, including those reactions observed experimen-
tally. The role of solvent on these reactions was modeled using
the continuum PCM approach. All of the reactions involving
the NHC were found to be exothermic in CH3CN; the values
discussed are those calculated using CH3CN as this was the
solvent in which the experiments were conducted. The free
energy (ΔG298) for the simple coordination reaction of the
NHC with PhI(OTf)2 was calculated to be −276 kJ/mol. This
compares to −587 kJ/mol for the observed oxidative coupling
reaction. If the starting material is PhI(OAc)2, ΔG298 for the
oxidative coupling was calculated to be −378 kJ/mol, and
finally if [PhI(pyr)][OTf]2 is used as the oxidant, ΔG298 was
calculated to be −481 kJ/mol. Therefore, in this system we can
order the oxidative capacity of the iodine(III) compounds
PhI(OTf)2 > [PhI(pyr)2]2+ > PhI(OAc)2. This is also
consistent with the observation of Ph3P reacting with
PhI(OTf)2 and the dications, but not PhI(OAc)2.
atom is buried, with its largest contribution in the HOMO-7.
This can be compared to the p-type lone pair orbitals in PhI,
found in the highest occupied molecular orbital (HOMO) and
HOMO−1.43 The LUMO of 8-H is a σ symmetric antibonding
orbital associated with the I−N bonds, which is consistent with
a 2-electron reduction of the compound rupturing these bonds
giving PhI, and with their ability to engage in ligand exchange
reactions. The I−N bond dissociation energy (ΔH0) for 8-H
was calculated to be 244 kJ/mol in the gas phase, as compared
to 154−164 kJ/mol for the bis-pyridine iodine(I) cation.44
The electronic structures of the PhI(OTf)2 and PhI(OAc)2
precursors were also investigated. The 19F{1H} NMR chemical
shift of PhI(OTf)2 suggested that the triflates were only weakly
bound to the iodine center, with a chemical shift closer to that
of ionic OTf− than known bound systems. This weakly bound
system is not reproduced by DFT calculations. The optimized
geometry of PhI(OTf)2 (Figure 4) gives a structure consistent
with relatively strongly bound triflate substituents, with large
asymmetry in the S−O bond distance. The S−O bond distance
for the oxygen bound to I is 1.563 Å, while the S−O distances
for the unbound oxygens are much shorter at 1.434 Å. The I−
O bond distance is calculated to be 2.154 Å, which is essentially
identical to the bond distance calculated (2.178 Å), and
observed (2.172 Å) in PhI(OAc).45 A possible explanation for
the anomalous NMR chemical shift of the fluorine atoms lies in
the minimum energy conformation for the triflates with respect
to the phenyl ring. They are found orientated with the −CF3
groups positioned above and below the plane of the ring, 3.201
Å from the centroid. In this region, the NMR shifts are likely to
be shielded by phenyl ring current, accounting for the upfield
shift of the −CF3 groups versus that expected for a strongly
bound triflate.
CONCLUSIONS
■
The first structural verification of ligand stabilized dicationic
iodine compounds, using pyridine ligands, has been carried out.
With respect to triphenylphosphine, the PhI(OTf)2 precursor
was found to be a stronger oxidizing agent than PhI(OAc)2.
PhI(OAc)2 has found use as an oxidizing agent in many
important systems; the observation that PhI(OTf)2 is a more
reactive agent, coupled with the ease of its generation from
commercially available PhI(OAc)2, should mean that it finds
increasing use in this respect. For NHCs, iodine(III) may
The Mulliken charge on the iodine atom for PhI(OTf)2 is
1.10, while for PhI(OAc)2 it is 1.03, indicating that the iodine
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(16) Dutton, J. L.; Ragogna, P. J. Chem.Eur. J. 2010, 16, 12454−
12461.
represent a limit to the oxidative capacity of NHCs, which may
have implications for the use of NHCs as spectator ligands, in
for example, high oxidation state transition metal chemistry.
The iodine dications, which can be isolated away from the
TMS-OAc byproduct, were also found to be strong oxidizing
agents in their own right. The work of Sanford, Yates, and
Ritter has shown that iodine(III) reagents with an PhIR2
framework are excellent oxidants for the clean generation of
high-oxidation state transition metal compounds,22,46−48 and
we are currently investigating the redox chemistry of these
dicationic species toward late transition metals.
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ASSOCIATED CONTENT
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(23) Kuhn, N.; Kratz, T. Synthesis 1993, 561−562.
S
* Supporting Information
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lated geometries/reactions. X-ray data for compounds 8-H, 8-
NMe2, and [Ph3P−O−SiMe3][OTf] in .cif format. CCDC
reference numbers 908359−908361. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: j.dutton@latrobe.edu.au.
Notes
The authors declare no competing financial interest.
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; O. Farkas, Foresman,
ACKNOWLEDGMENTS
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