Monomeric platinum(II) hydroxides supported by sterically dominant α-diimine ligands

Article abstract of DOI:10.1021/ic300337a

The use of two new highly sterically bulky α-diimine ligands for the stabilization of neutral, monomeric platinum(II) hydroxo complexes is described. Halide abstraction from LPtCl₂ complexes of these ligands in the presence of water, followed by deprotonation of the cationic aquo complex, leads to LPt(OH)Cl and LPt(OH)₂. The latter can be reprotonated with HNTf₂ to yield a highly fluxional hydroxoaquoplatinum(II) cation.

Full text of DOI:10.1021/ic300337a

Communication
pubs.acs.org/IC
Monomeric Platinum(II) Hydroxides Supported by Sterically
Dominant α-Diimine Ligands
Tracy L. Lohr, Warren E. Piers,* and Masood Parvez
Department of Chemistry, University of Calgary, 2500 University Drive, NW Calgary, Alberta, Canada
S
* Supporting Information
diimine ligand. We have previously employed large 3,5-
terphenyl N-aryl groups in β-diketiminato ligands to stabilize
reactive organoscandium compounds23,24 and have now
extended this strategy to the diimine platform via preparation
of the two new α-diimine ligands La (dimethyl backbone) and
Lb (the diaryl acenaphthenequinonediimine, BIAN) shown in
Scheme 1.
ABSTRACT: The use of two new highly sterically bulky
α-diimine ligands for the stabilization of neutral,
monomeric platinum(II) hydroxo complexes is described.
Halide abstraction from LPtCl2 complexes of these ligands
in the presence of water, followed by deprotonation of the
cationic aquo complex, leads to LPt(OH)Cl and LPt-
(OH)2. The latter can be reprotonated with HNTf2 to
yield a highly fluxional hydroxoaquoplatinum(II) cation.
Scheme 1. Synthesis of Monomeric Platinum(II) Hydroxides
he fundamental chemistry of archetypical oxygen-
T
containing ligands (for example, MO, M−O2, M−
OH, M−OOH, and M−OH2) is of current interest because of
their role in metal-mediated water oxidation catalysis.1,2 An
understanding of the mechanisms by which these ligands
interconvert at a level commensurate with our current
appreciation of the analogous hydrocarbyl ligand trans-
formations is desirable to help with the design of improved
water-splitting catalysts.3,4 One approach to obtaining kinetic
and thermodynamic information on such transformations is to
prepare well-defined, monomeric examples of metal hydroxo5
or dihydroxo6 complexes and to study their reactivity. Despite
the importance of such transformations, and the large number
of metal hydroxo complexes known,7 few studies with this
emphasis have been undertaken.8 To this end, we have become
interested in synthesizing metal hydroxo complexes to assess
their reactivity patterns and relevance to intermediates in water-
splitting cycles. Monomeric neutral platinum(II) hydroxo
complexes were among our first targets.
Ligands La and Lb were assembled via condensation of 3,5-
bis(2,6-diisopropyl)aniline with the corresponding diketone
and fully characterized. Reaction with the hydrate of Zeise’s salt
at room temperature gave the dichlorides 1a and 1b in 98% and
70% yield, respectively, as analytically pure brown/red solids.
The monomeric platinum(II) hydroxo complexes (non-
aqueous solution) that have been reported tend to be found in
pincer ligand environments,9,10 and Milstein has utilized such a
complex to investigate transformations relevant to oxygen−
ligand interconversions.11 We were interested in compounds
with neutral bidentate ligands to allow for a second reactive
ligand site. Phosphine-stabilized monomeric platinum(II)
hydroxo complexes have appeared in the literature,12−14 but
they exhibit a tendency to dimerize.15,16 Similar reactivity
patterns are seen in cationic α-diimine-stabilized platinum(II)
complexes with hydroxo ligands, even when relatively large N-
aryl groups are employed.17−20 However, given the extensive
use of α-diimine ligands to stabilize water-coordinated cations
of platinum(II) relevant to C−H activation chemistry,21,22 their
greater amenability to the installation of significant steric bulk
through modification of the N-aryl group, and their ease of
handling, we sought the preparation of neutral platinum(II)
hydroxide derivatives supported by a stupendously bulky
1
While the aliphatic region of the H NMR spectra of these
complexes offered little in the way of useful information, the
signals for the R groups on the diimine backbone were
diagnostic for each compound. The C2v symmetry of the
dichlorides was reflected in equivalent methyl groups for 1a and
three distinct resonances in the aromatic region for 1b. In the
latter complex, the signal for the proton in the position ortho to
the imine carbon was particularly diagnostic, appearing as a
doublet downfield of the other resonances in the molecule at
8.38 ppm (CD2Cl2).
Both 1a and 1b have been characterized by single-crystal X-
ray diffraction (XRD); details can be found in the Supporting
Information. The central phenyl group of the terphenyl N-aryl
Received: February 17, 2012
Published: April 6, 2012
© 2012 American Chemical Society
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Communication
groups is essentially perpendicular to the plane of the N2C2
diimine atoms; the orthogonality (relative to the central aryl
ring) of the two flanking 2,6-diisopropyl rings allows the ligand
to provide an effective set of flanking “steric brick walls” that
protect the PtCl2 core of the molecule.
A single chloride ion can be abstracted with AgSbF6 in wet
tetrahydrofuran (THF) to provide a mixture of monomeric
THF and aquo-coordinated chloro cations. The addition of an
excess of water (7−10 equiv) results in the formation of the
aquo-coordinated cations 2a and 2b (quantitative by NMR
spectroscopy), which were envisioned as precursors to neutral
hydroxides via deprotonation (vide infra). Attempts to isolate
these compounds were unsuccessful because concentration of
the solutions initiated polymerization of THF. However, they
1
were characterized convincingly by H NMR spectroscopy and
subsequent reactivity. The desymmetrization of the coordina-
tion environment at platinum was indicated by the splitting of
backbone resonances into two singlets for the methyl groups in
2a and two sets of three signals for the naphthalene backbone
protons in the BIAN ligand of 2b. Resonances for both free and
1
bound water were apparent in the H NMR spectra, indicating
that the exchange between free and coordinated water is slow
on the NMR time scale. Broad resonances integrating to two
hydrogens appeared at 7.92−7.96 ppm for 2a and 8.18−8.27
ppm for 2b; the chemical shift is dependent on the
concentration of water in the solution. These signals disappear
upon the addition of deuterated water.
Figure 1. Molecular structure of 3b in the solid state (hydrogens, one
molecule of 18-crown-6, and toluene removed for clarity). Ellipsoids
are at 30% probability. Selected bond lengths (Å) and angles (deg):
Pt−O1, 2.139(3); Pt−Cl1, 2.249(2); Pt−N1, 2.104(3); Pt−N2,
2.026(4); O1−Pt−N1, 175.1(1); O1−Pt−N2, 95.96(1); Cl1−Pt−
O1, 88.1(1); Cl1−Pt−N1, 96.7(1); Cl1−Pt−N2, 174.9(1).
All attempts to deprotonate 2a produced ill-defined product
mixtures, likely resulting from the competing attack of base on
the imine carbon or the removal of acidic α-methyl protons.
The BIAN ligand obviates the latter issue, and when solutions
of 2b are treated with aqueous KOH (2 equiv), 2b is smoothly
deprotonated to produce the new monomeric platinum(II)
hydroxide 3b as a pink, air-sensitive powder (Scheme 1). Use of
Hunig’s base (N,N-diisopropylethylamine) also results in the
facile deprotonation of 2b to yield 3b.
Scheme 2. Reactivity of Platinum(II) Hydroxides
The 1H NMR spectrum of 3b shows the expected
asymmetrical BIAN ligand aromatic proton resonances and a
singlet at 1.63 ppm in THF-d8 (2.75 ppm in C6D6), integrating
to one hydrogen. This resonance is only observable in
rigorously dried solvents and broadens into the baseline
2
when excess water is added. The signal appears in the H
NMR spectrum at the same chemical shift when 3b-d1 is
prepared using KOD in deuterated water. The O−H stretch
was detected in the IR spectrum at 3567 cm−1; this band shifted
to 2605 cm−1 in 3b-d1.25,26 Single crystals could be grown from
a toluene/THF solution with 18-crown-6 present as a
crystallization aid.27
The molecular structure of 3b is shown in Figure 1, along
with selected metrical data. Orientation of the N-terphenyl
ligands is more canted out of the diimineplatinum chelate plane
than in 1b (see Figure S2 in the Supporting Information)
because of packing interactions with the (unseen) 18-crown-6
guest. The Pt−O bond length is 2.139(3) Å, which compares
well with the few other terminal PtII−OH bond lengths in the
literature (2.025−2.194 Å).2,8 The geometry at the platinum
center is similar to that of the starting material 1b, with the
main difference being the lengthening of the Pt−N1 bond
(2.104 Å) in comparison to that of Pt−N2 (2.026 Å). This
latter value is closer to the Pt−N distances found in dichloride
1b. The longer Pt−N1 length is a result of the larger trans
influence of the hydroxo ligand in comparison to chloride.12
Resubjecting isolated 3b to the conditions used to generate
cations 2 led to the hydroxy aqua cation 4b (Scheme 2). A
larger excess of water was required to cleanly produce 4b, and
in contrast to 2b, the 1H NMR spectrum of 4b indicated that it
has a symmetrical coordination environment, suggesting an
averaged structure, as shown in Scheme 2, in which the three
hydrogens undergo rapid exchange. Attempted isolation again
resulted in polymerization of the THF solvent, so 4b was
treated in situ with aqueous KOH, resulting in rapid
deprotonation to yield the neutral cis-dihydroxo complex 5b,
which was more rigorously characterized because of its ready
isolation.
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Inorganic Chemistry
Communication
Dihydroxo complex 5b was also accessible directly from 3b
by stirring with a THF/water suspension of Ag2O overnight;
this allowed for isolation of 5b as a dark-purple solid in 80%
yield (Scheme 2). The cis-hydroxide 5b also shows symmetrical
ACKNOWLEDGMENTS
■
W.E.P. acknowledges the NSERC of Canada (Discovery Grant)
and the Canada Council of the Arts (Killam Research
Fellowship, 2012-14). T.L.L. thanks the NSERC for a PGS-D
Award and the Alberta Innovates-Technology Futures for a
graduate student scholarship.
1
BIAN ligand resonances in the H NMR but distinct from
those observed for its protonated derivative 4b. In dry solvents,
a singlet integrating to two protons was observed at 1.02 ppm
in THF-d8 (1.52 ppm in C6D5Br) for the Pt−OH protons. This
assignment was confirmed through the synthesis of 5b-d2 and
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2
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Scheme 2.
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ASSOCIATED CONTENT
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■
S
* Supporting Information
Full experimental procedures, spectral data for 3b, 4b, and 5b,
X-ray structures of 1a, 1b, and 5b, and X-ray crystallographic
data in CIF format. This material is available free of charge via
the Internet at http://pubs.acs.org.
(27) Crystals of complexes with this extremely large ligand invariably
include an aromatic solvent molecule that π-stacks with the
naphthalene portion of the BIAN ligand or a molecule that fills
space in this region of the molecule in motifs reminiscent of host−
guest interactions. The best crystals of 3b that we obtained included a
molecule of 18-crown-6 that was used in early experiments to
solubilize the KOH reagent.
AUTHOR INFORMATION
■
(28) Pavia, D. L., Lampman, G. M.; Kriz, G. S. Introduction to
Spectroscopy, 3rd ed.; Thomson Learning: Tampa, FL, 2001.
Corresponding Author
*E-mail: wpiers@ucalgary.ca.
Notes
The authors declare no competing financial interest.
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