Synthesis, characterization, and acid-base properties of (N-N)PtIV(CH3)2(OH)2-x (OCH3)x (x = 0, 1) complexes

Article abstract of DOI:10.1039/b304475k

(N-N)Pt^IV(CH₃)₂(OH)_2-x (OCH₃)_x (x = 0, 1); N-N = bipy, 4,4′-(CH₃)₂bipy, 4,4′-^tBu₂bipy, ArN=C(CH₃)-C(CH₃)=NAr (Ar = 3,5-(CF₃)₂C₆H₃, 3,5-(CH₃)₂C₆H₃, 2,6-(CH₃)₂C₆H₃) complexes have been prepared by oxidation of the appropriate (N-N)Pt^II(CH₃)₂ complexes using H₂O₂/H₂O. In methanol, the hydroxo-methoxy complexes are formed (x = 1), whereas in acetone, the dihydroxo complexes are formed (x = 0). A single-crystal X-ray structure determination establishes the structure of (4,4′-(CH₃)₂bipy)Pt^IV (CH₃)₂(OH)₂ as a dihydroxo compound with octahedral geometry, with the hydroxo ligands occupying apical coordination sites. The acidities of the protonated bipy hydroxo complexes, (4,4′-R₂bipy)Pt^IV(CH₃)₂(OH) (OH₂)⁺ cations, were determined in methanol. The acidities closely matched that of Cl₂CHCOOH (pK_a = 6.38 in CH₃OH).

Full text of DOI:10.1039/b304475k

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Synthesis, characterization, and acid–base properties of
(N–N)PtIV(CH3)2(OH)2 ؊ x(OCH3)x (x ؍ 0, 1) complexes†
Knut Thorshaug, Irene Fjeldahl, Christian Rømming and Mats Tilset*
Department of Chemistry, University of Oslo, P. O. Box 1033 Blindern, N-0315 Oslo, Norway.
E-mail: mats.tilset@kjemi.uio.no
Received 24th April 2003, Accepted 23rd June 2003
First published as an Advance Article on the web 22nd September 2003
(N–N)PtIV(CH3)2(OH)2 Ϫ x(OCH ) (x = 0, 1; N–N = bipy, 4,4Ј-(CH ) bipy, 4,4Ј-tBu bipy, ArN᎐C(CH )–C(CH )᎐NAr
᎐
᎐
3
3
x
3
2
2
3
(Ar = 3,5-(CF3)2C6H3, 3,5-(CH3)2C6H3, 2,6-(CH3)2C6H3) complexes have been prepared by oxidation of the
appropriate (N–N)PtII(CH3)2 complexes using H2O2/H2O. In methanol, the hydroxo–methoxy complexes are formed
(x = 1), whereas in acetone, the dihydroxo complexes are formed (x = 0). A single-crystal X-ray structure
determination establishes the structure of (4,4Ј-(CH3)2bipy)PtIV(CH3)2(OH)2 as a dihydroxo compound with
octahedral geometry, with the hydroxo ligands occupying apical coordination sites. The acidities of the protonated
bipy hydroxo complexes, (4,4Ј-R2bipy)PtIV(CH3)2(OH)(OH2)ϩ cations, were determined in methanol. The acidities
closely matched that of Cl2CHCOOH (pKa = 6.38 in CH3OH).
solvent. (N–N) symbolizes various diimine and 4,4Ј-substituted
Introduction
bipyridine ligands (4,4Ј-R2bipy). The formation of closely
Selective catalytic functionalization of alkanes is an attractive
related (tmeda)PtIV(CH3)2(OH)(OCH3) complexes arising from
process that continues to pose major challenges to chemists.
the oxidation of (tmeda)PtII(CH3)2 with O2 in methanol has
Progress in the field has been made during the last decades, but
been recently described.4 The oxidation of some (diimine)-
only a few protocols can be employed in order to catalytically
Pt(CH3)2 complexes with O2 was also discussed but details
oxidize methane to value-added products.1 One applicable
concerning the products were not given.4
catalyst is the classical Shilov system.2 It comprises aqueous
platinum salts, and can be used to convert various hydro-
carbons into their corresponding alcohols or alkyl chlorides,
albeit with limited catalytic efficiency. Several studies aimed at a
Results and discussion
The Pt() complexes 1–6 (Scheme 2) were conveniently prepared
deeper understanding of the underlying principles of the Shilov
9
from Pt2(CH3)4(µ2-S(CH3)2)2 and the appropriate 4,4Ј-R2bipy
system have been reported,1 and the catalytic cycle is generally
or diimine.7
described by three steps: (i) C–H activation, (ii) oxidation of
Pt() to Pt(), and (iii) reductive elimination of the functional-
ized hydrocarbon with concomitant regeneration of the active
Oxidation of (N–N)PtII(CH3)2 with H2O2/H2O
Pt() species, as outlined in Scheme 1. In the search for altern-
ative oxidants to the rather impractical Pt() salts used in the
original studies,2 various alternatives have been studied, e.g.
dioxygen,3,4 chlorine,5 and peroxydisulfate.6 For all of these
oxidants, the turnovers are too low for practical applications.
Oxidation of 1–6 can be performed with H2O2/H2O to rapidly
and cleanly afford various Pt() complexes as either the di-
hydroxo complexes 7–11 or the hydroxo–methoxy complexes
12–17. The choice of solvent is the key to a controlled product
formation, as illustrated in Scheme 2. In acetone, only di-
hydroxo complexes are formed, whereas in methanol, hydroxo–
methoxy complexes are exclusively formed. This important
effect of the solvent was independent of the chelating (N–N)
ligands employed. All reactions could be successfully per-
formed at room temperature, except the preparation of 10
and 11 which had to be done at lower temperature (Ϫ30 ЊC to
Ϫ10 ЊC) due to rapid decomposition of the producs under the
reaction conditions at ambient temperature. The bipy-
supported oxidation products 7–9 and 12–14 were stable towards
air and light, whereas the diimine analogues 10 and 11 and 15–
1
17 were air- and light sensitive. A downfield shift of the H
Scheme 1 Schematic presentation of the key steps involved in the
catalytic Shilov cycle.
2
NMR Pt–CH3 resonance and a lowering of the J(PtH) coup-
ling constant was observed for the Pt() to Pt() oxidation, as
expected. Only minor differences in the chemical shifts and
coupling constants were observed between the dihydroxo and
the hydroxo–methoxy complexes, and they do not appear to
reflect any significant differences in the nature of these species.
Recently, hydrocarbon C–H activation was found to occur
under mild conditions at the cationic species obtained by
protonation of (diimine)PtII(CH3)2 with aqueous HBF4 in
trifluoroethanol,7 and the mechanism of this particular C–H
activation process has been studied in detail.8 In an extension
of these studies, and relevant to the oxidation step in the Shilov
cycle, i.e. step (ii) in Scheme 1, we would now like to report on
the oxidation of various neutral (N–N)PtII(CH3)2 complexes.
The oxidation products are described as (N–N)PtIV(CH3)2-
(OH)2 Ϫ x(OCH3)x, where x = 0, 1 and depends on the chosen
1
In the H NMR spectra, the Cs symmetry of all the new
compounds 7–17 is reflected in the single Pt–CH3 1H NMR
resonance with their characteristic 2J(195PtH) satellites. The
most significant differences between the (4,4Ј-R2bipy)Pt() and
(diimine)Pt() complexes are seen in the Pt–CH3 1H NMR
shifts. In the (4,4Ј-R2bipy)Pt() complexes, they are found in
the range 1.68–1.74 ppm whereas in the (diimine)Pt() com-
plexes, they are found further upfield in the range 1.07–1.19
ppm. The 2J(PtH) values are all in the range 69.3–73.5 Hz, with
† Based on the presentation given at Dalton Discussion No. 6, 9–11th
September 2003, University of York, UK.
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 4 0 5 1 – 4 0 5 6
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Scheme 2 Preparation of (N–N)PtIV(CH3)2(OH)2 Ϫ x(OCH3)x (x = 0,1). Reagents and conditions: (i) H2O2/H2O, acetone; (ii) H2O2/H2O, CH3OH.
Table 1 Selected geometric parameters (bond lengths in Å, angles in Њ)
in 8ؒ2CH3OH
Pt(1)–C(13)
Pt(1)–C(14)
Pt(1)–N(1)
2.040(2)
2.040(2)
2.150(2)
Pt(1)–N(2)
Pt(1)–O(1)
Pt(1)–O(2)
2.158(2)
2.018(2)
2.015(2)
O(1)–Pt(1)–O(2)
O(1)–Pt(1)–C(13)
O(1)–Pt(1)–C(14)
O(1)–Pt(1)–N(1)
177.2(1)
91.3(1)
90.9(1)
89.1(1)
O(1)–Pt(1)–N(2)
C(13)–Pt(1)–C(14)
C(13)–Pt(1)–N(1)
C(13)–Pt(1)–N(2)
91.2(1)
86.4(1)
97.9(1)
173.7(1)
no systematic differences between the complexes. 13C{1H}
NMR spectra show single Pt–CH3 signals, together with diag-
nostic 1J(PtC) satellites, in agreement with the symmetry of the
products. The Pt–CH3 13C{1H} NMR resonances are found
slightly upfield from SiMe4, where typical values are in the
range Ϫ3.5 to Ϫ3.8 ppm (1J(PtC) = 644–648 Hz) (7–9), Ϫ1.8 to
Ϫ2.0 ppm (1J(PtC) = 667–671 Hz) (12–14), and Ϫ1.1 ppm (15).
Methanol-d4 was found to be a well-suited NMR solvent for
our studies, and consequently, OH resonances in the complexes
could not be observed due to rapid H/D exchange with the
solvent. On the other hand, in KBr pellets of 7–9 and 12–14, IR
absorptions assigned to an O–H stretch were detected, provid-
ing support for the proposed structures. Furthermore, in 8, the
OH groups were readily located in the X-ray crystal structure
(vide infra).
Fig. 1 ORTEP view of 8ؒ2CH3OH. Hydrogen atoms, except for OH,
have been omitted for clarity. The dotted lines represent hydrogen
bonds.
substitution in the 4,4Ј-position on the bipy ligand and the
simple 1H and 13C{1H} NMR spectra are similar to those of 8,
it seems reasonable to expect 7 and 9 to be symmetrical
dihydroxo compounds with apical hydroxo ligands as well.
Monaghan and Puddephatt have previously reported that the
oxidation of 1 can be achieved simply by stirring the compound
in either acetone or methanol in the presence of air.11 Based on
conductivity measurements in methanol, the oxidation product
in acetone was formulated as a hydroxo–aqua complex, which is
in contrast to the solid state structure reported in Fig. 1. It
should be noted that the 1H NMR data of 7 differ slightly from
those reported for the O2 oxidation product of 1 in acetone.11
The presence of the methoxy group is confirmed by the
Pt–OCH3 1H NMR resonance at 2.59–2.62 ppm (12–14) and
2.78–2.87 ppm (15–17) with characteristic 195Pt satellites with
3J(PtH) values in the range 39.2–39.6 Hz (12–14) and 42.0–43.4
Hz (15–17). Integration of the Pt–CH3 and Pt–OCH3 signals
show that they are present in a 6 : 3 ratio. This supports the view
that there is one methoxy group attached to each platinum
centre. In the 13C{1H} NMR spectra, the OCH3 resonances
in 12–14 appear at 56.8–56.9 ppm, and 56.1 (15). Correlated
1H/13C NMR spectra confirm the assignments of the 1H and 13
resonances of the methoxy groups (12–14).
C
X-Ray quality crystals were grown for 8, and the ORTEP26
view is given in Fig. 1. Selected bond distances and angles are
given in Table 1. To the best of our knowledge, the crystal
structure presented herein is the first to be reported for any
complex generally described as (4,4Ј-R2bipy)PtIV(CH3)2(OH)2.10
The X-ray structure confirms the formulation of 8 as a di-
hydroxo complex with the OH groups occupying the apical
positions with respect to the main coordination plane. Complex
8 cocrystallized with methanol as a 1 : 2 adduct. The X-ray
crystal structure shows quite clearly that hydrogen bonding
exists and is quite strong between Pt–OH and the cocrystallized
HOCH3. This is reflected in the O(2)–O(22) distance of
only 2.59 Å. Since 7, 8, and 9 only differ in the H vs. alkyl
Particularly noteworthy is the H NMR resonance, referenced
1
to residual solvent signals in dichloromethane-d2, for Pt–CH3
(2J(PtH)) observed at 1.59 ppm (72.3 Hz) in 7 vs. 1.84 ppm
(70 Hz) reported for the hydroxo–aqua complex.11
Details concerning the kinetics and mechanisms of the oxid-
ation process outlined in Scheme 2 are beyond the scope of this
contribution, so we limit ourselves to mentioning the following.
(i) When H2O2/H2O was added to the dihydroxo compound 7 in
methanol, no reaction was observed. Thus, 7 is not an inter-
mediate in the reaction mechanism in the formation of 12
from 1. (ii) Only a single isomer is formed in either of the two
solvents. (iii) The hydroxo–methoxy complex is formed in
methanol, whereas the dihydroxo species are formed in acetone
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(containing water). These findings appear to agree with a pre-
viously proposed mechanism of these and related oxidations.
The mechanism involves axial electrophilic attack by the oxid-
ant at the square planar Pt() complex, accompanied by hetero-
lytic cleavage of the oxidant. All of this is possibly assisted by
the coordination of a sixth ligand (water or methanol) prior to
or in concert with the electrophilic attack, thus completing the
octahedral Pt() ML6 structure.4,12
The fact that 7 is not converted to 12 in methanol suggests
that dissociation of hydroxide to give pentacoordinate Pt()
species does not occur under these conditions. Pentacoordinate
Pt() species that are of interest in the context of C–H activ-
ation have been recently described.13 We are currently pursuing
the possibility that activation of 7–17 with Lewis acids might
promote ligand dissociation and formation of related penta-
coordinate species.
a weighted average of the chemical shifts of the protonated
and unprotonated Pt species. Details of the experimental
procedures and data analysis are given in the Experimental
section.
If we assume the equilibrium between the neutral and the
protonated complexes to be described by Scheme 3, then an
analytical expression for the equilibrium constant Keq may be
derived.17 Fig. 2 illustrates the experimental data and the least-
squares-fit of the analytical expression for Keq. We found no
significant differences between the pKa’s determined for the
three different (4,4Ј-R2bipy)Pt() species. According to our
estimates, the pKa values of the protonated hydroxo species, 7Ј–
9Ј, in methanol-d4 are in the range 6–7, i.e. essentially the same
as that of the reference acid Cl2CHCOOH. For comparison, the
aqueous pKa for cis-Pt(NH3)2(OH)(H2O)ϩ has been reported to
be 7.87.19 The reason that we provide a rather large uncertainty
for the pKa data is the following. The proton-transfer equi-
librium constant with the reference acid was estimated both in
Oxidation of (4,4Ј-R2bipy)PtII(CH3)2 with meta-chloro-
perbenzoic acid (R ؍ H, CH3)
Ϫ
the presence and absence of added salts, including PPNϩBF4
and Cl2CHCOOX/Cl2CHCOOH (X = Na, K) buffers. With-
The dimethyl complexes 1 and 2 were oxidized with meta-
chloroperbenzoic acid (mCPBA) in acetone. The reaction
products (18, 19) have the overall composition (4,4Ј-R2bipy)-
Pt(CH3)2(OH)2ؒ(m-C6H4Cl(COOH)). These products both
showed single Pt–CH3 1H NMR resonances in methanol-d4 at
out the buffer, Keq was estimated as ca. 0.2, independent of
Ϫ
whether PPNϩBF4 was added or not. On the other hand, the
presence of the buffer resulted in values of Keq that were up to
10 times greater, depending on the concentration of the buffer,
Ϫ
but again independent of whether PPNϩBF4 was added or
2
1.81 ppm (18) and 1.77 ppm (19), with J(PtH) = 69.6 Hz (18)
not. It appears that the protonated form gains some extra
stabilization in the buffered medium, possibly through some
specific hydrogen bonding interaction. Alternatively, the differ-
ences in the pKa estimates in the absence and presence of buffer
may be caused by the chemical shifts of protonated and/or
unprotonated species being slightly dependent on the buffer
concentration. Either way, this again may complicate the pKa
determinations. Due to this uncertainty, we choose to report the
pKa values of 7Ј–9Ј in methanol to be in the range 6–7.
and 69.3 Hz (19), respectively. All of these values and the struc-
tural implications of the simple 1H NMR spectra compare well
to those observed for 7 and 8. The composition and the
expected coordination geometry was unambiguously confirmed
by an X-ray diffraction crystal structure investigation for 18.
Unfortunately, the quality of the structure determination was
inadequate for an assessment of the finer structural details
including bond distances and angles. There were, however, indi-
cations of short O ؒ ؒ ؒ O distances that would suggest14 hydro-
gen bonding interactions between the carboxylic acid group in
m-C6H4Cl(COOH) and the Pt–OH groups. The slight upfield
2
shift of the Pt–CH3 resonances and lowering of J(PtH) in 18
compared to 7, may also be readily explained by protonation at,
or hydrogen bonding interaction with, a Pt–OH group.
It is in line with earlier studies of RO–ORЈ oxidative addition
to Pt() complexes15 when we find that only the OH groups
oxidatively add to the Pt() centre, and not the m-C6H4Cl-
(COOH) fragment. This may be readily understood in terms of
the mechanism that has already been outlined for the oxidation
reactions, i.e. a coordinated solvent molecule (H2O) assists the
heterolytic cleavage of the O–O bond in mCPBA. It is the better
leaving group, m-C6H4Cl(COOϪ) rather than OHϪ, that departs
during the O–O cleavage.
Scheme 3 Equilibrium between (4,4Ј-R2bipy)Pt(IV)(CH3)2(OH)2 (7–9)
and their protonated forms (7Ј-9Ј).
Brønsted base properties of Pt-bonded hydroxo ligands
The hints at hydrogen bonding interactions may be inferred
1
from the H NMR data and in the crystal structure (albeit
lacking in quality, as mentioned above) of 18 suggests that the
Pt–OH group must have a basicity that is comparable to that of
m-chlorobenzoate. The pKa of m-chlorobenzoic acid has been
determined to be 8.83 in methanol.16 Prompted by this, we
decided to undertake an investigation of the Brønsted base
properties of the dihydroxo compounds 7–9. Quantitative
studies were performed by 1H NMR in methanol-d4.17 Acidities
of Pt() diammine and related complexes have been extensively
studied by NMR methods,18,19 but data for Pt() systems are
scarce. (We note that a relevant neutral Pt() dimethyl mono-
hydroxy complex was reportedly14 protonated by dilute HNO3).
We found 2,2-dichloroacetic acid (the pKa of Cl2CHCOOH
in methanol is reported to be 6.3816) to be well-suited for
these studies. A proton-transfer equilibrium was immediately
established. The relative acidities allowed for successive addi-
tions of controlled amounts of acid with measurable changes
in the position of the Pt–CH3 1H NMR resonance, observed as
Fig. 2 Pt–CH3 1H NMR chemical shifts at various initial ratios
between the acid and the (4,4Ј-R2bipy)PtIV(CH3)2(OH)2 complex. Fully
drawn lines are least-square-fits of the derived analytical expression for
Keq.17
In the pKa determination that is based on Scheme 3, we
assume that the protonated species and their counter-ions are
separate ions pairs. The data in Fig. 2 may in principle be
described by an equilibrium between neutral species and
associated ion pairs, giving slightly different estimated values
for Keq and δiЈ (iЈ = 7Ј, 8Ј, 9Ј), but the goodness-of-fits are
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significantly lower compared to those obtained for the equi-
librium given in Scheme 3.20 In addition, the estimated δiЈ values
of 2.02 (7Ј–9Ј) associated ion pairs deviate more from the values
obtained by full protonation with HOTf and CF3COOH than
those estimated by treating the right hand side as separate ions
(vide infra). Thus, if we consider the right hand side of Scheme
3, separate ions are in better agreement with the experimental
data than are contact ion pairs.21
Synthesis
(bipy)PtIV(CH3)2(OH)2 (7). To a yellow suspension of 1
(0.336 g, 0.881 mmol) in acetone was added H2O2/H2O
(0.180 ml) while stirring. The solids quickly dissolved and the
solution changed colour to light yellow. After 10 minutes, most
of the volatiles were removed by evaporation (attention:
peroxides). The product was precipitated and washed with pen-
tane before drying under vacuum to give the product as a light-
yellow powder in 98% yield. Anal. calc. for C12H16N2O2Pt: C,
34.70; H, 3.88; N, 6.74. Found: C, 32.60; H, 3.99; N, 6.80%. 1H
NMR δH(CD3OD) 1.73 (6H, s, J(PtH) 70.7 Hz, Pt–CH3)), 7.83
(2H, ddd, J = 7.9, 5.4, 1.1 Hz, Ar–H ), 8.26 (2H, ddd, J = 8.2,
7.9, 1.6 Hz, Ar–H ), 8.64 (2H, ddd, J = 8.2, 1.1, 0.6 Hz (poorly
resolved), Ar–H ), 9.02 ppm (2H, ddd, J = 5.4, 1.6, 0.6 Hz,
Ar–H ). 13C{1H} NMR δC{H}(CD3OD) Ϫ3.5 (J(PtC) = 648 Hz),
125.2, 128.2, 141.3, 148.6, 156.8. ν(OH) = 3392 cmϪ1 (very
broad).
The protonated species 7Ј–9Ј were not isolated, but by extra-
polation of the analytical expression for Keq, we were able to
extract the Pt–CH3 1H NMR chemical shifts of these species in
methanol-d4. The coupling constants 2J(PtH) in the protonated
2
species were also estimated by extrapolation of a J(PtH) vs.
initial [acid]/[Pt()] ratio plot similar to Fig. 2. The estimated
1H NMR shift values in methanol-d4 are: 2.08 ppm (7Ј), 2.04
ppm (8Ј), and 2.05 ppm (9Ј), with 2J(PtH) approximately equal
to 67 Hz. These values are in excellent agreement with the Pt–
CH3 1H NMR values δH 2.09 ppm (2J(PtH) = 66 Hz) observed
by protonation of 7 with 1.6 equivalents of HOTf and δH 2.07
ppm (2J(PtH) = 66 Hz) observed by protonation with 15 equiv-
alents of CF3COOH. Under these conditions, essentially
complete protonation of 7–9 was achieved, evidenced by the
fact that a further increase of the acid concentrations to
8 equivalents for HOTf or 25 equivalents for CF3COOH
did not change the position of the observed Pt–CH3 resonances.
(A much larger excess of HOTf eventually resulted in decom-
position of the complex, possibly by intermediacy of the
unobserved diprotonated species, i.e. the dicationic Pt()
bis(aqua) complexes).
(4,4Ј-(CH3)2bipy)PtIV(CH3)2(OH)2 (8). Prepared analogous to
7 as a light-yellow product in 82% yield. Anal. calc. for
C14H20N2O2Pt: C, 37.72; H, 4.55; N, 6.32. Found: C, 37.65; H,
4.96; N, 6.38%. 1H NMR δH(CD3OD) 1.68 (6H, s, J(PtH) = 70.8
Hz, Pt–CH3), 2.62 (6H, s, Ar–CH3), 7.62 (d, 2H, J = 5.7 Hz,
Ar–H ), 8.48 (2H, s, Ar–H ), 8.81 (d, 2H, J = 5.7 Hz, Ar–H ).
13C{1H} NMR δC{H}(CD3OD) Ϫ3.8 (J(PtC) = 646), 125.8,
128.7, 147.8, 153.8, 156.7. ν(OH) = 3370 cmϪ1 (very broad).
(4,4Ј-t-Bu2bipy)PtIV(CH3)2(OH)2 (9). Prepared analogous to 7
as a light-yellow product in 86% yield. Anal. calc. for C20H32-
N2O2Pt: C, 45.53; H, 6.11; N, 5.31. Found: C, 43.21; H, 5.85; N,
6.26%. 1H NMR δH(CD3OD) 1.49 (s, 18H, Ar–tBu), 1.68 (s, 6H,
J(PtH) = 70.5 Hz, Pt–CH3), 7.84 (dd, 2H, J = 5.7, 1.9 Hz,
Ar–H ), 8.59 (d, 2H, J = 1.9 Hz, Ar–H ), 8.89 (d, 2H, 5.7 Hz,
Ar–H ). 13C{1H} NMR δC{H}(CD3OD) Ϫ3.7 (J(PtC) = 644 Hz),
30.7, 36.7, 122.1, 125.2, 148.1, 156.8, 166.2). ν(OH) = 3060 cmϪ1
(very broad).
Conclusions
We have demonstrated that several bipy and diimine-based
(N–N)Pt(CH3)2 complexes undergo smooth oxidation with
H2O2/H2O and mCPBA to give interesting Pt() hydroxo com-
plexes. The identity of the oxidation product depends on the
choice of solvent, where (N–N)Pt(CH3)2(OH)2 and (N–N)–
Pt(CH3)2(OH)(OCH3) are obtained in acetone and methanol,
respectively. The formation of these products may be rational-
ized in terms of previously proposed mechanisms. The mech-
anistic insight will allow for designed syntheses of novel
complexes by variation of the two axial ligands. The chemistry
of the Pt() complexes is currently being further investigated,
and the results will be reported in due time.
(N–N)PtIV(CH ) (OH) (10) [N–N ؍ Ar–N᎐C(CH )–C(CH )᎐
᎐
᎐
3
3
2
2
3
N–Ar; Ar ؍ 3,5-(CH3)2C6H3]. To a deeply red solution of 4
(36.9 mg, 71.3 µmol) in acetone (5 ml) at Ϫ30 ЊC was added
H2O2/H2O (42.6 µl) in portions over several hours while raising
the temperature slowly to Ϫ10 ЊC. Excess H2O2 was quenched
with Na2S2O3 in H2O, the volatiles were evaporated, and the
residue was washed with pentane before drying under vacuum.
The product was isolated as a light brown solid in 54% yield. 1H
NMR δH(CD3OD) 1.19 (s, 6H, J(PtH) = 69.3 Hz, Pt–CH3), 2.38
(s, 12H, Ar–CH3), 2.47 (s, 6H, N–C(CH3)), 6.73 (s, 4H, Ar–H ),
7.05 (s, 2H, Ar–H ). The compound, pure by NMR, failed to
give satisfactory elemental analysis data.
Experimental
General considerations
(4,4Ј-R2bipy)PtII(CH3)2 (1–3) and (diimine)PtII(CH3)2 (4–6)
9
were synthesized in toluene from Pt2(CH3)4(µ2-S(CH3)2)2 and
the appropriate 4,4Ј-substituted bipyridine or diimine.7 1H and
13C{1H} NMR spectra were recorded on a Bruker Avance DXP
300 instrument. The 1H NMR spectra were obtained in
CD3OD, DMSO, or CD2Cl2, and the chemical shifts reported
are relative to SiMe4, using the residual solvent resonances at
δH 3.30 ppm, 2.49 ppm, 5.32 ppm as internal references. The
13C NMR spectra were obtained in CD3OD or CDCl3, the
chemical shifts are given relative to SiMe4 and the solvent
resonances at δC 49.0 ppm and 77.2 ppm were used as internal
references. IR spectroscopy was performed on KBr tablets
using a Perkin Elmer One Spectrometer. Elemental analyses
were performed by Ilse Beetz Microanalytisches Laboratorium,
Kronach, Germany. 4,4Ј-R2bipy (R = H, CH3, t-Bu) was used as
received from Sigma-Aldrich, H2O2/H2O (30%), acetone, and
methanol were all used as received from Kebo Lab, and
mCPBA was used as received from Fluka. 2,2-Dichloroacetic
acid was degassed and Cl2CHCOOX (X = K, Na) were
dried under vacuum before use, all were purchased from
Sigma-Aldrich.
(N–N)PtIV(CH ) (OH) (11) [N–N ؍ Ar–N᎐C(CH )–C(CH )᎐
᎐
᎐
3
3
2
2
3
N–Ar; Ar ؍ 2,6-(CH3)2C6H3]. To a red solution of 5 (0.1088 g,
0.210 mmol) in acetone at Ϫ10 ЊC was added H2O2/H2O (30%,
64 µl). After two hours, the solvent was evaporated while keep-
ing the reaction mixture ice-cold. The residue was washed with
pentane before drying under vacuum. The product was isolated
as a brown-yellow solid in 61% yield. Anal. calc. for C22H32-
N2O2Pt: C, 47.91; H, 5.85; N, 5.08. Found: C, 47.34; H, 6.05; N,
1
5.15%. H NMR δH(CD3OD) 1.10 (s, 6H, J(PtH) = 72.2 Hz,
Pt–CH ), 2.29 (s, 12H, Ar–CH ), 2.37 (s, 6H, N᎐CCH ),
᎐
3
3
3
6.95–7,20 (m, 6H, Ar–H ).
(bipy)PtIV(CH3)2(OH)(OCH3) (12). To a yellow suspension of
1 (65.0 mg, 0.17 mmol) in methanol (5 ml) was added H2O2/
H2O (16 µl) while stirring. The solution slowly changed colour
to light yellow and after 1 h, the volatiles were removed by
evaporation. The product was washed with pentane and dried
under vacuum to give the product as a light-yellow powder in
85% yield. Anal. calc. for C13H18N2O2Pt: C, 36.36; H, 4.23; N,
D a l t o n T r a n s . , 2 0 0 3 , 4 0 5 1 – 4 0 5 6
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1
6.52. Found: C, 35.50; H, 4.25; N, 10.27%. H NMR δH(CD3-
mCPBA (12.9 mg, 56.2 mmol). After solvent evaporation and
OD) 1.74 (s, 6H, J(PtH) = 71.2 Hz, Pt–CH3), 2.62 (s, 3H, J(PtH)
= 39.6 Hz, Pt–OCH3), 7.85 (dd, 2H, J = 7.5, 4.9 Hz, Ar–H ),
8.29 (ddd, 2H, J = 8.3, 7.5, 1.1 Hz, Ar–H ), 8.66 (d, 2H, J =
8.3 Hz, Ar–H ), 9.04 (d, 2H, J = 4.9 Hz, Ar–H ). 13C{1H} NMR
δC{H}(CD3OD) Ϫ1.8 (J(PtC) = 671 Hz), 56.8, 125.3, 128.3,
141.5, 148.5, 156.8. ν(OH) = 3350 cmϪ1 (very broad).
washing with diethyl ether, the product was isolated as a white
solid in 35% unoptimized yield. H NMR δH(CD3OD) 1.77 (s,
6H, J(PtH) = 69.3 Hz, Pt–CH3), 6.38 (s, 4H, Ar–H ), 7.00 (s, 2H,
Ar–H ). 13C{1H} NMR δC{H}(CD3OD) Ϫ1.1, 21.4, 56.1, 120.0,
129,5, 140.4, 146.4, 177.6.
1
Acid–base equilibrium studies
(4,4Ј-(CH3)2bipy)PtIV(CH3)2(OH)(OCH3) (13). Prepared
analogous to 12 as a light-yellow product in 97% yield. Anal.
calc. for C15H22N2O2Pt: C, 39.39; H, 4.85; N, 6.12. Found: C,
Experimental considerations. All experiments were performed
in CD3OD, which was dried and distilled under N2 atmosphere
and stored over 3Å molecular sieves.22 A controlled amount of
selected complex 7–9 was added to a J. Young NMR tube, and
CD3OD was vacuum transferred into the NMR tube. Inside the
glovebox, controlled amounts of Cl2CHCOOH were added
1
38.29; H, 4.56; N, 5.64%. H NMR δH(CD3OD) 1.69 (s, 6H,
J(PtH) = 70.8 Hz, Pt–CH3), 2.59 (s, 3H, J(PtH) = 39.2 Hz, Pt–
OCH3), 2.63 (s, 6H, Ar–CH3), 7.66 (d, 2H, J = 5.7 Hz, Ar–H ),
8.52 (s, 2H, Ar–H ), 8.84 (d, 2H, J = 5.7 Hz, Ar–H ). 13C{1H}
NMR δC{H}(CD3OD) Ϫ2.0 (J(PtC) = 668 Hz), 21.4, 56.8, 125.9,
128.8, 147.8, 154.0, 156.6. ν(OH) = 3372 cmϪ1 (very broad).
1
using a syringe and the H NMR spectra were measured after
each addition. The [Cl2CHCOOH]o/[(4,4Ј-R2bipy)PtIV(CH3)2-
(OH)2]o ratio was found from the Cl2CHCOOH and Pt–CH3 1H
NMR integrals.
(4,4Ј-t-Bu2bipy)PtIV(CH3)2(OH)(OCH3)
(14).
Prepared
Low-temperature 1H NMR were conducted down to Ϫ80 ЊC
in an attempt to observe separate Pt–CH3 signals for the two
species in equilibrium, but to no avail. Due to the still rapid
exchange processes, only one averaged set of signals could be
observed.
analogous to compound 12 as a light yellow powder in 97%
yield. Anal. calc. for C21H34N2O2Pt: C, 46.57; H, 6.33; N, 5.17.
Found: C, 44.38; H, 5.85; N, 5.06%. 1H NMR δH(CD3OD) 1.52
(s, 18H, Ar–tBu), 1.70 (s, 6H, J(PtH) = 70.0 Hz, Pt–CH3), 2.61
(s, 3H, J(PtH) = 39.2 Hz, Pt–OCH3), 7.88 (dd, 2H, J = 5.7 Hz,
1.9 Hz, Ar–H ), 8.64 (d, 2H, J = 1.9 Hz, Ar–H ), 8.93 (d, 2H, J =
5.7 Hz, Ar–H ). 13C{1H} NMR δC{H}(CD3OD) Ϫ2.0 (J(PtC) =
667 Hz), 30.6, 36.7, 56.9, 122.3, 125.4 148.1, 156.8 166.4. ν(OH)
= 3400 cmϪ1 (very broad).
Mathematical considerations. Assume the observed Pt–CH3
1H NMR chemical shift to be the weighted average between the
neutral and the protonated species, i.e. δobs = δi ϩ (δiЈ Ϫ δi)xiЈ,
where δobs is the observed Pt–CH3 1H NMR resonance, xiЈ is the
molar fraction of the protonated species in the equilibrium
mixture (iЈ = 7Ј–9Ј), and δi and δiЈ are the 1H NMR resonances
of the neutral (7–9) and the protonated (7Ј–9Ј) species respect-
ively. δi is directly observable whereas δiЈ is estimated by the
curve fitting. We further consider the equilibrium equation
together with the required mass- and charge-balances. The
molar fraction xiЈ can then be expressed as:
(N–N)PtIV(CH ) (OH)(OCH ) (15) [N–N ؍ Ar–N᎐C(CH )–
᎐
3
2
3
3
C(CH )᎐N–Ar; Ar ؍ 3,5-(CH ) C H ]. To a solution of 4 in
᎐
3
3
2
6
3
methanol (5 ml) was added H2O2/H2O (62.5 µl). The volatiles
were removed by evaporation, before the product was washed
with pentane and isolated as a yellow solid in 30% unoptimized
yield. Anal. calc. for C23H34N2O2Pt: C, 48.84; H, 6.06; N, 4.95.
Found: C, 47.07; H, 6.05; N, 5.15%. 1H NMR δH(CD3OD) 1.08
(s, 6H, J(PtH) = 72.1 Hz, Pt–CH ), 2.37 (s, 18H, Ar–CH , N᎐
᎐
3
3
CCH3), 2.85 (s, 3H, J(PtH) = 42.0 Hz, Pt–OCH3), 6.38 (s, 4H,
Ar–H ), 7.00 (s, 2H, Ar–H ). 13C{1H} NMR δC{H}(CD3OD)
Ϫ1.1, 21.4, 56.1, 120.0, 129,5, 140.4, 146.4, 177.6.
(N–N)PtIV(CH ) (OH)(OCH ) (16) [N–N ؍ Ar–N᎐C(CH )–
᎐
3
2
3
3
where α = [Cl2CHCOOH]o/[(4,4Ј-R2bipy)Pt(IV)(CH3)2(OH)2]o.
Thus, by plotting the observed Pt–CH3 1H NMR resonance as a
function of the experimental variable α, we can estimate Keq and
the 1H NMR resonances of the protonated species 7Ј–9Ј. α was
found from the Cl2CHCOOH and Pt–CH3 integrals.
C(CH )᎐N–Ar; Ar ؍ 2,6-(CH ) C H ]. Prepared analogous to
᎐
3
3
2
6
3
15 as a yellow solid in 36% yield. Anal. calc. for C23H34N2O2Pt:
C, 48.84; H, 6.06; N, 4.95. Found: C, 36.15/36.19; H, 5.16/4.76;
N, 4.45/3.10%. The reasons for this poor elemental analysis are
1
unclear. H NMR δH(CD3OD) 1.12 (s, 6H, J(PtH) = 73.0 Hz,
Pt–CH ), 2.26 (s, 6H, N᎐CCH , 2.33 (s, 12H, Ar–CH ), 2.78
(s, 3H, J(PtH) = 43.4 Hz, Pt–OCH3), 7.05–7.21 (m, 6H, Ar–H ).
᎐
3
3
3
X-Ray crystallographic analysis of compound 8
X-Ray data were collected on a Siemens SMART CCD dif-
fractometer using graphite-monochromated Mo-Kα radiation
(λ = 0.710 73 Å). Data-collection method: ω-scan, range 0.3Њ,
crystal to detector distance 5 cm. Data reduction and cell
determination were carried out with the SAINT and XPREP
programs.23 Absorption corrections were applied by the use of
the SADABS program.24 The structure was determined and
refined using the SHELXTL program package.25 The non-
hydrogen atoms were refined with anisotropic thermal param-
eters; all hydrogen atoms were allowed for as riding atoms.
(N–N)PtIV(CH ) (OH)(OCH ) (17) {N–N ؍ Ar–N᎐C(CH )–
᎐
3
2
3
3
C(CH )᎐N–Ar; Ar ؍ 3,5-(CF ) C H ]. Prepared analogous to
᎐
3
3
2
6
3
15 as a yellow solid in 67% yield. Anal. calc. for C23H22F12-
N2O2Pt: C, 35.35; H, 2.84; N, 3.58. Found: C, 34.29; H, 2.77; N,
1
3.63%. H NMR δH(CD3OD) 1.07 (s, 6H, J(PtH) = 73.5 Hz,
Pt–CH ), 2.47 (s, 3H, N᎐CCH ), 2.87 (s, 3H, J(PtH) = 43.4 Hz,
᎐
3
3
Pt–OCH3), 7.83 (s, 4H, Ar–H ), 8.05 (s, 2 H, Ar–H ).
Reaction between 1 and mCPBA to give 18. To a solution of 1
(9.5 mg, 24.9 mmol) in acetone was added mCPBA (5.6 mg,
24.4 mmol). After solvent evaporation and washing with diethyl
ether, the product was isolated as a white powder in 48% yield.
1H NMR δH(DMSO) 1.62 (s, 6H, J(PtH = 71.9 Hz), 7.84 (dd,
2H, J = 7.8, 5.4 Hz, Ar–H ), 8.29 (dd, 2H, J = 8.3, 7.8 Hz, Ar–
H ), 8.74 (d, 2H, J = 8.3 Hz), 8.91 (d, 2H, J = 5.4 Hz, Ar–H ),
7.20–7.40 (m, 2H, Ar–H (mCPBA). The compound, pure by
NMR, failed to give satisfactory elemental analysis data.
Crystal data for C14H20N2O2Ptؒ2CH3OH (8ؒ2CH3OH). M =
¯
507.49, T = 105(2) K, triclinic, space group P1, a = 7.2537(5) Å,
b = 10.9870(9) Å c = 12.0195(9) Å, α = 81.881(3)Њ, β = 87.298(3)Њ,
γ = 74.897(4)Њ, V = 915.50(12) Å3, Z = 2, Dx = 1.841 Mg mϪ3
,
µ = 7.683 mmϪ1, collected 19278 reflections, 10219 unique
(Rint = 0.0246), final R indices (I > 2σ(I )) R1 = 0.0273, wR2 =
0.0648, R indices (all data) R1 = 0.0339, wR2 = 0.0669.
CCDC reference number 209032.
Reaction between 2 and mCPBA to give 19. To an orange
solution of 2 (23.0 mg, 56.2 mmol) in acetone (5 ml) was added
See http://www.rsc.org/suppdata/dt/b3/b304475k/ for crystal-
lographic data in CIF or other electronic format.
D a l t o n T r a n s . , 2 0 0 3 , 4 0 5 1 – 4 0 5 6
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G. Bandoli, A. Dolmella, S. Antonaroli and B. Crociani, J. Chem.
Soc., Dalton Trans., 2002, 212.
Acknowledgements
14 A. J. Canty, S. D. Fritsche, H. Jin, R. T. Honeyman, B. W. Skelton
and A. H. White, J. Organomet. Chem., 1996, 510, 281.
15 K. T. Aye, J. J. Vittal and R. J. Puddephatt, J. Chem. Soc.,
Dalton. Trans., 1993, 1835.
Financial support from the Norwegian Research Council
(NFR) is gratefully acknowledged (stipends to K. T. and I. F.).
16 F. Rived, M. Rosés and E. Bosch, Anal. Chim. Acta, 1998, 374, 309.
17 Details are outlined in the Experimental section.
References and notes
1 J. A. Labinger and J. E. Bercaw, Nature (London), 2002, 417, 507;
R. H. Crabtree, J. Chem. Soc., Dalton Trans., 2002, 2437; A. E.
Shilov and G. B. Shul’pin, Chem. Rev., 1997, 97, 2879; B. A.
Arndtsen, R. G. Bergman, T. A. Mobley and T. H. Peterson,
Acc. Chem. Res., 1995, 28, 154.
2 N. F. Goldshlegger, V. V. Eskova, A. E. Shilov and A. A. Shteinman,
Zh. Fiz. Khim., 1972, 46, 1353.
18 M. S. Davies, J. W. Cox, S. J. Berners-Price, W. Barklage, Y. Qu and
N. Farrell, Inorg. Chem., 2000, 39, 1710; M. S. Davies, D. S. Thomas,
A. Hegmans, S. J. Berners-Price and N. Farrell, Inorg. Chem., 2002,
41, 1101; R. Griesser, G. Kampf, L. E. Kapinos, S. Komeda,
B. Lippert, J. Reedijk and H. Sigel, Inorg. Chem., 2003, 42, 32.
19 T. G. Appleton, J. R. Hall, S. F. Ralph and C. S. M. Thompson,
Inorg. Chem., 1989, 28, 1989.
3 Y. V. Geletii and A. E. Shilov, Kinet. Katal., 1983, 24, 486.
4 V. V. Rostovtsev, L. M. Henling, J. A. Labinger and J. E. Bercaw,
Inorg. Chem., 2002, 41, 3608.
5 I. T. Horvath, R. A. Cook, J. M. Millar and G. Kiss,
Organometallics, 1993, 12, 8.
6 N. Basickes, T. E. Hogan and A. Sen, J. Am. Chem. Soc., 1996, 118,
12111.
7 L. Johansson, O. B. Ryan and M. Tilset, J. Am. Chem. Soc., 1999,
121, 1974.
8 H. Heiberg, L. Johansson, O. Gropen, O. B. Ryan, O. Swang and
M. Tilset, J. Am. Chem. Soc., 2000, 122, 10831; L. Johansson,
M. Tilset, J. A. Labinger and J. E. Bercaw, J. Am. Chem. Soc., 2000,
122, 10846; L. Johansson and M. Tilset, J. Am. Chem. Soc., 2001,
123, 739; L. Johansson, O. B. Ryan, C. Rømming and M. Tilset,
J. Am. Chem. Soc., 2001, 123, 6579; B. J. Wik, M. Lersch and
M. Tilset, J. Am. Chem. Soc., 2002, 124, 12116; H. A. Zhong,
J. A. Labinger and J. E. Bercaw, J. Am. Chem. Soc., 2002, 124, 1378.
9 G. S. Hill, M. J. Irwin, C. J. Levy, L. M. Rendina and R. J.
Puddephatt, Inorg. Synth., 1998, 32, 149.
10 Based on a search in the SciFinder Scholar 2002 database.
11 P. K. Monaghan and R. J. Puddephatt, Organometallics, 1984, 3,
444.
12 R. A. Gossage, A. D. Ryabov, A. L. Spek, D. J. Stufkens, J. A. M.
van Beek, R. van Eldik and G. van Koten, J. Am. Chem. Soc., 1999,
121, 2488.
20 By visual inspection, we see that the equilibrium as given in Scheme
3 describes the experimental data much better than a model with
associated ions. Furthermore, if we take r2 as a quantitative measure
of the goodness-of-fit, we see that the equilibrium as given in
Scheme 3 (r2 = 0.977, 0.996, 0.997 for 7, 8, and 9, respectively) gives a
better fit to the observed data than if the ions on the right hand side
in Scheme 3 are described as contact-ion pairs (r2 = 0.934, 0.983,
0.983 for 7, 8, and 9, respectively).
21 A scheme incorporating neutral species, contact ion pairs, and
separate ions, may of course be constructed. Although this may lead
to better curve fits, the improved fit is most likely a consequence of
an increased number of adjustable parameters in the model, offering
no further physical insight.
22 J. Leonard; B. Lygo and G. Procter, Advanced Practical Organic
Chemistry, Stanley Thornes, Cheltenham, 2nd edn., 1998.
23 SMART and SAINT, Area Detector Control and Integration
Software Version 5.054, Bruker Analytical X-Ray Instruments Inc.,
Madison, WI, 1996.
24 G. M. Sheldrick, SADABS Version 2.01, Program for Empirical
Correction of Area Detector Data, University of Göttingen,
Germany, 2000.
25 G. M. Sheldrick, SHELXTL, Strucure Determination Programs
Version 5.10, Bruker Analytical X-Ray Instruments Inc., Madison.
WI, 1997.
26 M. N. Burnett and C. K. Johnson, ORTEP-III, Oak Ridge Thermal
Ellipsoid Plot Program for Crystal Structure Illustrations, Report
ORNL-6895, Oak Ridge National Laboratory, Oak Ridge, TN,
1996.
13 U. Fekl, W. Kaminsky and K. I. Goldberg, J. Am. Chem. Soc., 2001,
123, 6423; S. Reinartz, P. S. White, M. Brookhart and J. L.
Templeton, J. Am. Chem. Soc., 2001, 123, 6425; F. Di Bianca,
D a l t o n T r a n s . , 2 0 0 3 , 4 0 5 1 – 4 0 5 6
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