Redox activation of alkene ligands in platinum complexes with non-innocent ligands

Article abstract of DOI:10.1021/ic8017248

The reactivity of metal olefin complexes with non-innocent ligands (NILs) was examined. Treatment of PtCI₂(diene) with the deprotonated catechol or aminophenol ligands afforded the corresponding Pt(NIL)(diene) complexes. The Pt(^tBA

Full text of DOI:10.1021/ic8017248

Inorg. Chem. 2009, 48, 638-645
Redox Activation of Alkene Ligands in Platinum Complexes with
Non-innocent Ligands
Julie L. Boyer,‡ Thomas R. Cundari,*,† Nathan J. DeYonker,† Thomas B. Rauchfuss,*,‡
and Scott R. Wilson‡
Department of Chemistry and Center for AdVanced Scientific Computing and Modeling
(CASCaM), UniVersity of North Texas, P.O. Box 305070, Denton, Texas 76203, and Department
of Chemistry, UniVersity of Illinois at Urbana-Champaign, 600 South Mathews AVenue,
Urbana, Illinois 61801
Received September 10, 2008
The reactivity of metal olefin complexes with non-innocent ligands (NILs) was examined. Treatment of PtCl2(diene)
with the deprotonated catechol or aminophenol ligands afforded the corresponding Pt(NIL)(diene) complexes. The
Pt(tBAFPh)(COD), Pt(tBAFPh)(nbd), and Pt(O2C6H2tBu2)(COD) (H2tBAFPh ) 2-(2-trifluoromethyl)anilino-4,6-di-tert-
butylphenol, H2O2C6H2tBu2 ) 3,5-di-tert-butylcatechol) complexes were examined by cyclic voltammetry. Treatment
of Pt(tBAFPh)(COD) or Pt(tBAFPh)(nbd) with AgPF6 afforded the imino-semiquinones [Pt(tBAFPh)(COD)]PF6 or
[Pt(tBAFPh)(nbd)]PF6, respectively. The [Pt(tBAFPh)(COD)] complex was unreactive toward nucleophiles, whereas
the oxidized derivative, [Pt(tBAFPh)(COD)]PF6, rapidly and stereospecifically added alkoxides at the carbon trans
to the phenolate. The Pt(tBAFPh)(COD), [Pt(tBAFPh)(COD)]PF6, Pt(tBAFPh)(C8H12OMe), and [Cp2Co][Pt-
(tBAFPh)(C8H12OMe)] complexes were characterized crystallographically.
Introduction
Complementary to metal-centered redox, and more subtle,
are ligand-centered redox reactions.4 Ligand-centered redox
reactions are enabled through the use of non-innocent ligands
(NILs), which are numerous and diverse in structure.
Although NILs undergo redox changes remotely from the
metal, the attendant changes in NIL donor properties affect
the metal center and the other ligands.5 An excellent
comparison of metal-centered versus ligand-centered redox
changes is provided by Vlcek and co-workers. For the
Metal-centered oxidations are ubiquitous in organometallic
transformations. A classic example of this is found in the
Shilov system, where platinum(II) alkyls become susceptible
to nucleophilic attack upon oxidation of the metal center.1
Metal-centered redox changes often lead to dramatic changes
in the ligands. This effect is classically demonstrated by
changes in the acidity of aquo ligands,2 but more recent work
illustrates how redox changes can affect the reactivity of
ligands of more direct relevance to homogeneous catalysis.
For example, Tilset and co-workers estimate that a 1 e-
oxidation increases the acidity of metal hydrides by as much
as 1020.3 Such drastic changes portend the significant
opportunities for exploiting the effects of redox on catalysis.
t
t
[Mn0(CO)3(O2C6H2 Bu2)]2- and [MnI(CO)3(O2C6H2 Bu2)]-
complexes, the average νCO value differs by ∼110 cm-1. In
contrast, the ligand-centered oxidation of [Mn(CO)2(P(OEt)3)2-
6
t
(O2C6H2 Bu2)]- shifts νCO by ∼40 cm-1. Ligand-centered
redox changes may prove more compatible with catalytic
reactions, where gentle structural and energetic changes are
more compatible with rapid turnover rates.7,8 The role of
NILs is well-documented in bioinorganic chemistry. Ex-
* To whom correspondence should be addressed. E-mail: rauchfuz@
uiuc.edu.
†
University of North Texas.
University of Illinois at Urbana-Champaign.
‡
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638 Inorganic Chemistry, Vol. 48, No. 2, 2009
10.1021/ic8017248 CCC: $40.75  2009 American Chemical Society
Published on Web 12/19/2008

Redox ActiWation of Alkene Ligands
amples include modified phenols in galactose oxidase and
in cytochrome c oxidase and the porphyrinate in cytochrome
P450.9
described.11,12,20,21 In this work, we probe the influence
of the amidophenolate NILs on the coordination properties
of alkenes. Metal alkene complexes have broad impor-
tance, including economically significant processes such
as Reppe chemistry and Wacker oxidation, along with
innumerable stoichiometric reactions.22
Although the coordination chemistry of NILs is long-
established,10 many of the prominent M-NIL systems
oxidize at fairly severe potentials.11-13 Furthermore, ligand-
centered redox changes in M-NIL complexes are known to
induce aggregation,14 which quenches the coordinative
unsaturation that is typically required for catalytic activity.
For example, the unsaturated dithiolene derivatives of CpCoIII
and (arene)RuII dimerize upon oxidation.15 The N-substituted
amidophenolate ligands popularized by Wieghardt et al.
resolve the deficiencies with traditional NILs, offering both
mild redox potentials and steric bulk that inhibits redox-
induced aggregation.13,16-18
Studies of the role of NILs in other areas of organo-
metallic chemistry are accelerating.19,20 Despite these
advances, the influence of redox poise of an NIL on
alkenes remains underdeveloped. Metal alkene complexes
with NILs have been reported, but well-defined systems
with fully characterized redox partners have not been
For this work, we selected the non-innocent ligand 2-(2-
t
trifluoromethyl)anilino-4,6-di-tert-butylphenol (H2 BAFPh).
Since amidophenolates are derived from t-Bu-substituted
catecholates, the resulting complexes exhibit enhanced
solubility in noncoordinating solvents that facilitates the
isolation of reactive radicals. In this ligand, the amine is
functionalized with a 2-trifluoromethylphenyl group, and the
plane of this aryl ring is perpendicular to the MON plane of
the chelate ring. This ligand has been shown to form
homoleptic complexes of the type [M(tBAFPh)2]z (M ) Co,
Ni, and Pd).18,23 Recently, we have isolated the mixed-ligand,
organometallic complex, Cp*IrtBAFPh, and its redox-related
partner.8
Results and Discussion
Synthesis of Pt-Diene-Amidophenolate Complexes.
Our targeted case was the 1,5-cyclooctadiene (COD) com-
plex, Pt(tBAFPh)(COD) (1). The complex forms in excellent
yield by treating PtCl2(COD) with the sodium salt of the
ligand. Unusual for platinum olefin complexes, the compound
is intensely orange. Otherwise, it exhibits conventional
spectroscopic properties. The NMR spectrum confirms the
structural rigidity of the N-aryl substituent: the slowed
rotation of the trifluoromethylphenyl group is manifested in
diastereotopic vinylic signals for the protons cis to the
N-substituted amido at δ 4.10 and 4.36. Mass spectrometry
and X-ray crystallography also confirmed the formation of
the Pt(tBAFPh)(COD).
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Related to 1, the norbornadiene complex, Pt(tBAFPh)-
(nbd) (2), was also prepared. We were, however, unable to
obtain analogous Pd complexes: treatment of PdCl2(COD)
t
with the freshly prepared Na2 BAFPh afforded a metallic
precipitate, as well as the previously reported Pd(tBAFPh)2.23
t
The catecholate analogues, Pt(O2C6H2 Bu2)(COD) (3) and
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Boyer et al.
Table 1. Electrochemical Dataa
complex
E1
E2
1/2
1/2
Pt(tBAFPh)(COD) (1)
0.31
0.40
0.67
1.18
1.13
1.21
Pt(tBAFPh)(nbd) (2)
t
Pt(O2C6H2 Bu2)(COD) (3)
Pt(O2C6H4)(COD) (4)
0.84(irrev)
-0.45
1.17(irrev)
0.55
Pt(tBAFPh)(C8H12OMe) (1OMe)
a Electrochemical data for scan rates of 50 mV/s in 0.1 M NBu4PF6 in
CH2Cl2 solutions reported as V versus Ag/AgCl.
Figure 2. Molecular structure of 1 (left) and [1]PF6 (right) with thermal
ellipsoids drawn at 50%. Color scheme: Pt (pink), P (purple), F (green), O
(red), N (blue), and C (gray). Hydrogen atoms were omitted for clarity.
Table 2. Selected Bond Lengths (Å) for 1, [1]PF6, 1OMe, and
Cp2Co[1OMe]
Figure 1. Cyclic voltammogram of 1 (left) and 1OMe (right) recorded at
scan rates of 50 mV/s in 0.1 M NBu4PF6 solutions in CH2Cl2.
Complexes 1-4 were examined by cyclic voltammetry
(CV) as a prelude to chemical oxidations and to benchmark
the influence of BAFPh2- relative to standard catecholate
t
bond
1a
[1]PF6
1OMea
Cp2Co[1OMe]a
ligands (Table 1). Complex 1 oxidizes reversibly at 0.31 and
1.18 V versus Ag/AgCl (Figure 1). The norbornadiene
complex 2 also undergoes two reversible oxidations at the
similar potentials of 0.40 and 1.13 V. These data provide
rare evidence for the increased donor ability of COD versus
the other common chelating diene, nbd. Complex 3, contain-
ing the substituted catecholate, oxidizes reversibly, as seen
for 1, but at a potential that is 350 mV more positive. The
complex of the unsubstituted catecholate 4 oxidizes irreVers-
ibly and at still more positive potentials.12 The complex
Pt(mnt)(COD) (mnt ) maleonitriledithiolate) displays no
observable oxidation events within the same potential
range.11
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C6-C1
C2-N
1.398(17)
1.391(16)
1.386(17)
1.387(18)
1.383(16)
1.399(17)
1.376(16)
1.335(15)
1.973(10)
1.961(8)
2.128(16)
2.155(14)
1.380(2)
2.144(15)
2.160(12)
1.402(19)
1.448(3)
1.415(4)
1.352(4)
1.437(4)
1.370(4)
1.417(3)
1.341(3)
1.316(3)
2.020(2)
2.0115(19)
2.162(3)
2.145(3)
1.388(6)
2.181(3)
2.192(3)
1.390(5)
1.445(14)
1.416(14)
1.357(15)
1.435(15)
1.366(15)
1.433(15)
1.370(13)
1.283(12)
2.024(9)
1.413(15)
1.399(14)
1.395(15)
1.383(15)
1.393(15)
1.389(16)
1.415(14)
1.392(13)
2.003(9)
C1-O
Pt-N
Pt-O
2.117(7)
2.089(8)
Pt-C7
Pt-C8
C8-C7
Pt-C9
Pt-C10
C10-C9
2.098(12)
1.503(16)
2.110(11)
2.131(11)
1.410(15)
2.067(13)
1.467(19)
2.085(11)
2.092(11)
1.396(17)
a Data for only one crystallographically independent molecule are given;
the two molecules are structurally similar.
range from 1.377 to 1.402 Å, consistent with the unperturbed
aromaticity of the phenylene group. For the oxidized complex
[1]PF6, these C-C distances were more disparate, ranging
from 1.352(4) to 1.437(4) Å. Relative to 1, [1]PF6 exhibits
elongated Pt-O (1.961(8) to 2.0115(19) Å) and Pt-N
distances (1.973(10) to 2.020(2) Å). The Pt-alkene distances
barely changed (less than 0.04 Å) upon oxidation of 1. In
contrast, Bennett showed the Ru-alkene distances in [Ru-
(acac)2(o-CH2dCHC6H4-2-NMe2)]0/+ were 0.06-0.08 Å
longer in the Ru(III) derivative.24 The observed lengthening
of Pt-ligand bonds in 1 versus [1]PF6 were reproduced in
the DFT-calculated structures of 1 and 1+: ∆PtO ∼ 0.03 Å,
Chemical Oxidation of Diolefin Complexes. Oxidation
of a THF solution of 1 with AgPF6 gave the dark orange-
colored product, [Pt(tBAFPh)(COD)]PF6 ([1]PF6). This robust
species exhibited a well-resolved isotropic EPR spectrum
centered at giso ) 1.9984, a value consistent with a ligand-
centered radical, indicative of an imino-semiquinone co-
ligand.17 Reduction of a solution of [1]PF6 with NEt3
regenerated 1. The nbd derivative, [Pt(tBAFPh)(nbd)]PF6
([2]PF6), was prepared similarly. Complexes 1+/0 and 2+/0
represent rare examples of alkene M-NIL complexes
isolated in two redox states.
∆
PtN ∼ 0.05 Å, and ∆PtC ∼ 0.02-0.04 Å. Plotting the HOMO
of 1 ( Figure 3) shows it to be Pt-O/N/C bonding, with a
smaller contribution from the olefin carbons of the COD than
from the O and N atoms of tBAFPh. Additionally, for 1, the
C-C bond lengths of the amidophenolate ring are calculated
The salt [1]PF6 and its precursor were further characterized
by X-ray crystallography (Figure 2 and Table 2). In 1, the
carbon-carbon distances within the amidophenolate ring
(24) Bennett, M. A.; Heath, G. A.; Hockless, D. C. R.; Kovacik, I.; Willis,
A. C. J. Am. Chem. Soc. 1998, 120, 932–941.
640 Inorganic Chemistry, Vol. 48, No. 2, 2009

Redox ActiWation of Alkene Ligands
Figure 3. Calculated spin density for 1+ (isovalue ) 0.0008), left. Plot of the calculated HOMO of 1 (isovalue ) 0.02), right. Hydrogen atoms were omitted
for clarity.
Figure 4. Molecular structure of 1OMe (left) and Cp2Co[1OMe] (right) with thermal ellipsoids drawn at 50%. Color scheme: Pt (pink), Co (purple), F
(green), O (red), N (blue), and C (gray). Hydrogen atoms were omitted for clarity.
Scheme 1
to be in a narrow range, 1.39-1.41 Å, suggestive of
aromaticity, whereas for 1+, the C-C range of 1.38-1.45
Å supports an imino-semiquinone description. Again, this
computed result mimics the crystallographic analysis of 1
and [1]PF6. Finally, plotting the spin density (Figure 3) of
1+ shows the majority of the electron density to be on the
imino-semiquinone ring, consistent with the EPR spectral
assignment (vide supra). The experimental and calculated
data, taken together, support the formulation of [1]PF6 as a
ligand-centered radical.
Reactions of Pt-Imino-semiquinone-Alkene Com-
plexes. In [1]PF6, the diolefin is activated toward nucleophilic
attack. Treatment of solutions of [1]PF6 with NaOR (R )
Et or Me) efficiently afforded the alkoxide adducts (Scheme
1). The nucleophilic addition proved reversible: treatment
of 1OMe with HPF6 regenerated [1]PF6. The isotropic EPR
spectrum of 1OMe was simulated for a single species with
giso ) 2.009, indicative of regioselective addition of the
methoxide. The cyclic voltammogram of 1OMe in CH2Cl2
0.55 and -0.45 V, respectively (Figure 1). The DFT-
optimized C-C bond lengths of the aromatic ring of 1OMe
range from 1.38 to 1.46 Å, consistent with the ligand-
centered, imino-semiquinone formulation.
In 1OMe, the nucleophile is situated at an exo position,
trans to the phenolate (Figure 4). As in [1]PF6, 1OMe features
an imino-semiquinone ligand, as indicated by the pattern of
bond distances within the chelating N,O ligand. Compared
to [1]PF6, the C-N distances are elongated (1.341(3) to
1.370(13) Å) and the C-O distances are shortened (1.316(3)
to 1.283(12) Å) in 1OMe. The Pt-O distance is most
strongly affected by the addition of the methoxide, with an
increase from 2.0115(19) to 2.117(7) Å. These changes in
bond length between [1]PF6 and 1OMe are reflected both in
direction and in magnitude upon comparison of the DFT-
optimized structures of 1+ and 1OMe, although the elonga-
tion of the C-N bond is modest, δCN < 0.01 Å, in the
calculations.
As indicated by the voltammetry measurements, 1OMe
can be readily reduced. Treatment of 1OMe with Cp2Co gave
solution displays one oxidation and one reduction, at E1/2
)
Inorganic Chemistry, Vol. 48, No. 2, 2009 641

Boyer et al.
Figure 5. Left: Kohn-Sham LUMO of 1+, orthogonal to the square plane of Pt, CF3 coming out toward the reader (hydrogen atoms omitted for clarity).
Middle: KS LUMO of 1+ viewed along the Pt-O bond. Note only a small polarization of the CdC π portion of the orbital (right-hand side of figure, BAFPh
ligand shown in wireframe for clarity). Right: KS LUMO of 1+ viewed along the Pt-N bond. Note only a small polarization of the CdC π portion of the
orbital (right-hand side of figure, BAFPh ligand shown in wireframe for clarity).
the anionic derivative that was characterized crystallographi-
cally as its Cp2Co+ salt. The anion in Cp2Co[1OMe] retains
the stereochemistry of its precursor. Bond distances indicate
that the complex contains an amidophenolate, not an imino-
semiquinone ligand. Thus, reduction results in an increase
in both the C-N (1.370(13) to 1.415(14) Å) and C-O bonds
(1.283(12) to 1.392(13) Å). Correspondingly, the Pt-N and
Pt-O distance decreased. Distances within the phenolate ring
are again consistent with aromaticity. Of the four structures,
this anionic complex displays the shortest Pt-Calkene distances
(2.085(11) and 2.092(11) Å). The NMR spectrum of
Cp2Co[1OMe] is broadened due to paramagnetic impurities,
but sufficiently well resolved to demonstrate that a single
Figure 6. Depiction of the four possible regiochemistries for methoxide
isomer is present, again confirming that the methoxide adds
stereospecifically. In view of the isolability of Cp2Co[1OMe],
we attempted the reaction of 1 with NaOMe, but no reaction
occurred.
addition of 1+ to yield 1OMe. COD is coming out toward the reader,
whereas tBAFPh goes into the plane. Different isomers arising from different
olefin carbons, to which methoxide is added, are indicated in parentheses.
Observed product 1OMe corresponds to addition product A.
Table 3. Relative Calculated Free Energies (in kcal mol-1) of the
Various Isomers of 1OMe
DFT Calculations. The combination of spectroscopy,
electrochemistry, and reactivity displayed by Pt(tBAFPh)
complexes of diolefins reveals a rich chemistry that is
potentially tunable via the use of appropriate NILs. Of
particular interest, in the context of catalysis, is the re-
giospecificity of nucleophilic addition that is observed upon
oxidation of the starting materials. Simulations, first sought
to assess if kinetic or thermodynamic discrimination led to
the observed product, 1OMe. On the basis of a natural
population analysis (NPA), little difference in calculated
atomic charges is seen among the alkene carbon atoms in 1
and 1+ (q(Col) ) -0.29 to -0.31 e- for the four olefin
carbons in 1, and q(Col) ) -0.24 to -0.26 e- for the four
olefin carbons in 1+). Furthermore, the frontier orbitals of
1+, in particular, low-energy unoccupied orbitals with orbital
character on the COD double bonds in 1+, were analyzed to
assess whether the regiochemistry of methoxide addition to
1+ is orbitally controlled. Only small polarization of the CdC
π orbitals of the COD orbitals could be discerned (Figure
5). Hence, we focused on the possibility that the regiochem-
istry of methoxide addition to 1+ was dictated by thermo-
dynamic considerations.
A
B
C
D
∆G (kcal mol-1
)
0.00
19.62
9.59
5.35
of the HOMO of 1 (Figure 3), which has a π orbital located
t
on the BAFPh ligand, more N than O character, and less
CdC character. Also, we note the asymmetry in the spin
density (left-hand side of Figure 3) of 1+. Hence, we cannot
discount a subtle trans effect (i.e., a transition-state versus a
t
ground-state effect) mediated by the oxidation of BAFPh,
which serves to enhance reactivity toward nucleophiles and
to discriminate between the two double bonds of COD.
DFT computations were utilized to evaluate the various
possible isomers of 1OMe in order to assess their relative
thermodynamic stabilities (Figure 6 and Tables 3 and 4).
Four isomeric minima were obtained with the oxygen on
the OMe group in various locations, as defined by its position
relative to the Pt-O/N bonds (cis and trans) and the CF3
group (syn and anti). It was assumed, and test calculations
supported this assumption, that methoxide addition would
occur from the exo face of the COD. Table 3 summarizes
the calculated relative free energies of the isomers, in kcal
mol-1, for full QM/MM chemical models. Calculated
electronic energies and enthalpies of formation show similar
trends.
One further point of potential relevance to the regiochem-
istry of methoxide addition concerns the NPA-calculated
atomic charges that change very little for both Pt and O (of
tBAFPh) (by <0.05 e- upon oxidation of 1 to 1+). The NPA
atomic charge on the nitrogen of tBAFPh, however, changes
the most substantially upon oxidation of 1 (-0.68 e-) to 1+
(-0.55 e-). This change is consistent with the orbital makeup
The isomer predicted to be most stable matches the
experimentally observed structure; it is ∼5 kcal mol-1 lower
in energy than the next lowest energy isomer. This result
642 Inorganic Chemistry, Vol. 48, No. 2, 2009

Redox ActiWation of Alkene Ligands
Table 4. Crystallographic Data for [Pt(tBAFPh)(COD)], [Pt(tBAFPh)(COD)](PF6)·Acetone, [Pt(tBAFPh)(C8H12OMe)], and
Cp2Co[Pt(tBAFPh)(C8H12OMe)]
empirical formula
C29H36F3NOPt
666.68
C32H42F9NO2PPt
869.73
C30H39F3NO2Pt
697.71
C40H49CoF3O2Pt
886.82
formula weight
temperature (K)
wavelength (Å)
crystal system
space group
a (Å)
297(2)
0.71073
297(2)
193(2)
0.71073
193(2)
0.71073
0.71073
triclinic
monoclinic
P21/n
12.6949(13)
20.1797(17)
21.209(2)
90
95.355(4)
90
5409.7(9)
8
monoclinic
P21/c
11.6855(6)
20.0618(10)
24.0812(13)
90
93.006(3)
90
5637.6(5)
8
monoclinic
P21/n
19.758(2)
11.9094(13)
32.082(3)
90
99.856(6)
90
7437.8(14)
8
j
P1
10.0548(3)
11.6944(3)
15.6582(4)
91.681(2)
92.4950(10)
107.526(2)
1752.29(8)
2
b (Å)
c (Å)
R (°)
ꢀ (°)
γ (°)
volume (Å3)
Z
Fcalcd (Mg/m3)
µ (Mo KR) (mm-1
F(000)
1.637
5.229
2640
1.648
4.126
862
1.644
5.025
2776
1.584
4.253
3552
)
crystal size (mm)
θ range (°)
reflections collected
0.26 × 0.15 × 0.02
1.40 to 25.90
44589
0.36 × 0.25 × 0.25
1.83 to 30.18
32239
0.31 × 0.12 × 0.02
1.69 to 25.69
45355
0.10 × 0.06 × 0.04
1.83 to 25.44
87655
independent reflections
absorption correction
max and min transmission
data/restraints/parameters
goodness of fit on F2
final R indices (obsd data)
10083 [R(int) ) 0.0847]
integration
0.9016 and 0.4299
10083/618/742
1.079
R1 ) 0.0573
wR2 ) 0.1073
R1 ) 0.1494
wR2 ) 0.1439
2.911 and -1.320
10277 [R(int) ) 0.0232]
10577 [R(int) ) 0.1297]
integration
0.9062 and 0.3049
10577/0/681
0.998
R1 ) 0.0541
wR2 ) 0.0952
R1 ) 0.1547
wR2 ) 0.1317
1.664 and -1.813
13517 [R(int) ) 0.2870]
integration
0.8832 and 0.7863
13517/907/899
0.984
R1 ) 0.0673
wR2 ) 0.1028
R1 ) 0.1948
wR2 ) 0.1386
1.317 and -1.564
integration
0.5135 and 0.3326
10277/1163/592
1.026
R1 ) 0.0247
wR2 ) 0.0604
R1 ) 0.0322
wR2 ) 0.0634
0.792 and -0.470
R indices (all data)
largest diff peak and hole (e ·Å-3
)
supports the inference that the observed methoxide addition
product, 1OMe, is thermodynamically controlled. Whereas
deconvolution of the overall QM/MM energy into QM
(electronic) and MM (steric) components is not without
ambiguity, analysis of the separate QM and MM energies
implies that the preference for isomer A over the other
isomers is both steric (A versus B and C) and electronic (A
versus D) in origin.
Previous calculations indicated that the susceptibility of
coordinated alkenes toward nucleophilic addition is gov-
erned by charge, concluding that cationic complexes are
required to promote reactions. Senn calculated a larger
activation barrier for the nucleophilic attack of ammonia on
the neutral complex, [Rh(PH3)2Cl(CH2dCH2)] (∆Eq ) 147
kJ/mol), than the cationic complex, [Pd(PH3)2Cl(CH2d
CH2)]+ (∆Eq ) 17 kJ/mol).25 Sakaki studied the effect of
charge on the addition of ammonia to Pd-ethylene com-
plexes. The cationic complex, [PdF(NH3)2(CH2dCH2)]+, had
a low calculated energy of activation (∆Eq ) 33 kJ/mol),
whereas the neutral and anionic derivatives, [PdF3(CH2d
CH2)]- and [PdF2(NH3)(CH2dCH2)], respectively, were
strongly destabilized toward the addition.26 Hoffmann pro-
posed that nucleophilic attack of metal alkene complexes
proceeds through an olefin-slipped (η2 to η1) transition state.27
Calculations show an increase of 15-20 cm-1 in νCdC upon
oxidation of 1 to 1+, resulting from a decrease in backbond-
ing in 1+ versus 1, which would facilitate olefin slippage.
Hahn has reported a ∆νCdC of 12 cm-1 between the cationic
[Rh(PNP)(CH2dCHPh)]+ and the dicationic [Pd(PNP)-
(CH2dCHPh]2+ (PNP ) 2,6-bis(diphenylphosphanylmeth-
yl)pyridine).28
Conclusions
The mild and reversible oxidation potentials of the
amidophenolate complexes versus the related catecholate
complexes clearly demonstrate the advantages of the ami-
dophenolate ligand for organometallic chemistry and its
potential for tunable organometallic catalysis. The N-
substituted amidophenolate ligand was utilized in the isola-
tion of a pair of metal alkene complexes in which the redox-
active ligand is in two different oxidation states. Complexes
1 and 1+ were amenable to full spectroscopic and crystal-
lographic characterization.
The susceptibility of coordinated olefins to nucleophilic
attack is the basis for a variety of transformations, some of
which are useful in organic synthesis.22 The electrophilic
activation of alkenes is of increasing interest.26,28,29 The
greater electrophilicity of cationic versus neutral platinum
olefin complexes has been well-documented, especially by
Natile and co-workers.28-30 The redox-active character of
the amidophenolate allowed us to “turn on” the electrophi-
(28) Hahn, C. Chem.-Eur. J. 2004, 10, 5888–5899.
(29) Chianese, A. R.; Lee, S. J.; Gagne´, M. R. Angew. Chem., Int. Ed.
2007, 46, 4042–4059.
(30) (a) Fanizzi, F. P.; Intini, F. P.; Maresca, L.; Natile, G.; Gasparrini, F.
J. Chem. Soc., Dalton Trans. 1990, 1019–1022. (b) Maresca, L.; Natile,
G. J. Chem. Soc., Chem. Commun. 1983, 40–41. (c) Pietropaolo, R.;
Cusmano, F.; Rotondo, E.; Spadaro, A. J. Organomet. Chem. 1978,
155, 117–122. (d) Betts, M. S. J.; Harris, A.; Haszeldine, R. N.; Parish,
R. V. J. Chem. Soc. A 1971, 3699–3705. (e) Maresca, L.; Natile, G.
Comments Inorg. Chem. 1994, 16, 95–112.
(25) Senn, H. M.; Bloechl, P. E.; Togni, A. J. Am. Chem. Soc. 2000, 122,
4098–4107.
(26) Sakaki, S.; Maruta, K.; Ohkubo, K. Inorg. Chem. 1987, 26, 2499–
2505.
(27) Eisenstein, O.; Hoffmann, R. J. Am. Chem. Soc. 1981, 103, 4308–
4320.
Inorganic Chemistry, Vol. 48, No. 2, 2009 643

Boyer et al.
Pt(tBAFPh)(C8H12OMe), 1OMe. A solution of 116 mg (0.14
mmol) of [1]PF6 and 8 mg (0.14 mmol) of NaOMe in 20 mL of
MeOH was allowed to stand for 1 h. The solvent was removed in
vacuo. The dark red material was extracted into ∼15 mL of hexanes.
The solvent was removed under vacuum to give a dark red solid.
Yield: 51 mg (52%). FD-MS (m/z): 697 {[PtC8H12OMe(tBAFPh)]}.
Anal. Calcd for C30H39F3NO2Pt (Found): C 51.62 (51.55), H 5.64
(5.68), N 2.01 (2.06). Crystals were grown at room temperature
by slow evaporation of solvent from an acetone/methanol solution
under N2. A stock solution of aqueous HPF6 (0.35 mL, 0.1 M) was
added to 24 mg (0.035 mmol) of 1OMe in 10 mL of MeCN.
Immediately, the solution changed from red to orange. Mass
spectrometry and UV-vis spectroscopy supported the assignment
of the product as 1PF6.
licity of the coordinated alkene. Whereas 1 is unreactive
toward nucleophiles, the oxidized derivatives rapidly and
stereospecifically add alkoxides at carbon. Although
[1OMe]- proved sufficiently stable to characterize, 1 exhib-
ited no reactivity toward NaOMe. Thus, the electrophilic
enhancement by ligand-centered oxidation was required.
Summarizing, the present work demonstrates the influence
of redox poise of non-innocent ligands is sufficient to induce
the reactivity of alkene ligands. Further studies involving
monoalkenes would be of interest.
Experimental Section
Reactions and manipulations were performed under a nitrogen
atmosphere using standard Schlenk line techniques. Solvents were
Pt(tBAFPh)(nbd), 2. A flask was charged with 0.133 g (0.37
18
dried and deoxygenated prior to use. The compounds H2 BAFPh,
t
mmol) of PtCl2(nbd), 0.136 g (0.37 mmol) of H2 BAFPh, and 50
t
PtCl2(COD), and PtCl2(nbd)31 were prepared according to standard
methods. Elemental analyses were conducted by the School of
Chemical Sciences Microanalytical Laboratory. H NMR spectra
mL of CH2Cl2. To the colorless solution was added 0.4 mL of NEt3,
affording a vibrant orange solution. After the solution was stirred
2 h, the solvent was removed under vacuum. The product was
extracted into 100 mL of hexanes. The solvent was removed under
1
were acquired on a Varian Unity 500 instrument. Cyclic voltam-
metry was performed on a BAS CV-50W voltammetric analyzer.
The following conditions were employed: 0.1 M NBu4PF6 as the
supporting electrolyte, glassy carbon as the working electrode, Ag/
AgCl in saturated KCl as the reference electrode, and Pt wire as
the counter electrode. X-band EPR spectra were collected on a
Varian E-122 spectrometer. Samples were prepared as 1 mM
solutions in 1:1 CH2Cl2/toluene. Variable-temperature spectra were
recorded using an E-257 variable-temperature accessory with liquid
nitrogen as a coolant. The magnetic fields were calibrated with a
Varian NMR Gauss meter, and the microwave frequency was
measured with an EIP frequency meter. EPR spectra were simulated
using SIMPOW software.32
1
vacuum to give a vibrant orange solid. Yield: 0.153 g (64%). H
NMR ((CD3)2CO): δ 1.077 (s, 9H, CH3), 1.391 (s, 9H, CH3), 2.834
(m, 2H, CH2), 3.881 (m, 2H, CH), 4.1470 (m, 2H, CH), 4.749 (m,
2H, CH), 5.837 (d, 1H, CH), 6.506 (d, 1H, CH), 7.183 (d, 1H,
CH), 7.406 (t, 1H, CH), 7.664 (t, 1H, CH), 7.749 (d, 1H, CH).
FD-MS (m/z): 650 {[Pt(tBAFPh)(nbd)]+}. Anal. Calcd for
C28H32F3NOPt (Found): C 51.67 (51.52), H 4.96 (5.02), N 2.15
(2.36).
[Pt(tBAFPh)(nbd)]PF6, [2]PF6. This complex was prepared
analogously to [Pt(tBAFPh)(COD)]PF6. Yield: (84%). Anal. Calcd
for C28H32F9NOPPt (Found): C 42.25 (41.99), H 4.06 (4.28), N
1.76 (1.97).
Pt(tBAFPh)(COD), 1. A solution of 93 mg (1.7 mmol) of
NaOMe in 8 mL of MeOH was added dropwise to a stirring solution
t
of 315 mg (0.86 mmol) of H2 BAFPh in 8 mL of MeOH. The
t
Pt(O2C6H2 Bu2)(COD), 3. A flask was charged with 91 mg (2
mmol) of NaOH, 250 mg (1 mmol) of tBu2C6H2(OH)2, and 30 mL
of MeOH. This solution was then added to a solution of 356 mg (1
mmol) of Pt(COD)Cl2 in 60 mL of CH2Cl2. The resulting yellow
solution was stirred for 3 h, and the solvent was removed under
vacuum. The orange product was extracted into ∼30 mL of CH2Cl2.
Yield: 423 mg (85%). 1H NMR (CDCl3): δ 1.25 (s, 9H, CH3), 1.36
(s, 9H, CH3), 2.28 (m, 4H, CH2), 2.62 (m, 4H, CH2), 5.27 (m, 4H,
CH), 6.61 (d, 1H, CH), 6.76 (d, 1H, CH). FD-MS (m/z): 523
sodium salt of the ligand was then added to a solution of 323 mg
(0.86 mmol) of Pt(COD)Cl2 in 35 mL of CH2Cl2. The resulting
orange solution was stirred for 2 h, and the solvent was removed
under vacuum. The orange product was extracted into ∼30 mL of
CH2Cl2. The solvent was removed under vacuum to afford a bright
orange solid. Yield: 545 mg (95%). 1H NMR ((CD3)2CO): δ 1.067
(s, 9H, CH3), 1.413 (s, 9H, CH3), 2.224-2.700 (m, 8H, CH2), 4.104
(m, 1H, CH), 4.370 (m, 1H, CH), 5.293 (m, 2H, CH), 5.774 (d,
1H, CH), 6.506 (d, 1H, CH), 7.258 (d, 1H, CH), 7.435 (t, 1H, CH),
7.72 (t, 1H, CH), 7.828 (d, 1H, CH). FD-MS (m/z): 666 {[Pt(tBAF-
Ph)(COD)]+}. Anal. Calcd for C29H36F3NOPt (Found): C 52.22
(51.83), H 5.45 (5.59), N 2.17 (2.10). Crystals were grown at room
temperature by slow evaporation of solvent from an acetone/
methanol solution under an atmosphere of N2.
{[Pt(O2C6H2 Bu2)(COD)]+}. Anal. Calcd for C22H32O2Pt (Found):
t
C 50.45 (50.25), H 6.16 (6.49), N 0.0 (0.21).
Cp2Co[Pt(tBAFPh)(C8H12OMe)], Cp2Co[1OMe]. A solution of
6.9 mg (0.01 mmol) of 1OMe and 1.9 mg (0.01 mmol) of Cp2Co
in 1 mL of MeCN was prepared. The resulting solution appeared
yellow in color. When the solution was allowed to stand, single
crystals appeared in the NMR tube.
[Pt(tBAFPh)(COD)]PF6, [1]PF6. A solution of 69 mg (0.27
mmol) of AgPF6 in 10 mL of THF was added dropwise to a stirring
solution of 182 mg (0.27 mmol) of 1 in 10 mL of THF. The
resulting dark orange solution and metallic precipitate were stirred
for 2 h. The mixture was filtered, and the solvent was removed
under vacuum. The orange solids were washed with ∼15 mL of
hexanes. The dark orange product was extracted into MeOH, and,
again, the solvent was removed under vacuum. Yield: 182 mg
(83%). Anal. Calcd for C29H36F9NOPPt (Found): C 42.89 (42.78),
H 4.47 (4.50), N 1.73 (1.79). Crystals were grown at room
temperature by vapor diffusion of Et2O into an acetone solution of
the compound.
Computational Methods. All computations were carried out
with the Gaussian 03 software package.33 Equilibrium geometries
and harmonic vibrational frequencies were determined using a
QM/MM approach.34 The high level of theory was density
functional theory (DFT) using the B3LYP hybrid density
functional. The Stevens relativistic effective core potentials
(ECPs)35 and attendant triple-ꢁ valence basis sets were utilized
to model the platinum atoms, and main group elements were
modeled with the 6-31G(d) all-electron basis set. The low level
of theory used the universal force field (UFF).36 All stationary
points are confirmed minima on the potential energy surface.
The QM partitioning included almost the entire complex, except
(31) Drew, D.; Doyle, J. R. Inorg. Synth. 1990, 28, 346–349.
(32) Nilges, M. J.; Matteson, K.; Belford, R. L. SIMPOW6.
644 Inorganic Chemistry, Vol. 48, No. 2, 2009

Redox ActiWation of Alkene Ligands
t
for the t-butyl groups on BAFPh. Thermodynamic quantities
The research at UNT was supported in part by a grant from
the Offices of Basic Energy Sciences, U.S. Department of
Energy (Grant No. DEFG02-03ER15387). Calculations
employed the UNT computational chemistry resource, which
is supported by the NSF through Grant No. CHE-0342824.
Additional support was also provided by the University of
North Texas Academic Computing Services for the use of
the UNT Research Cluster.
were calculated at 298.15 K and 1 atm.
Acknowledgment. This research was supported by the
U.S. Department of Energy (DEFG02-90ER 14146). We
thank Dr. Mark Nilges for assistance with EPR spectroscopy.
(33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene,
M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala,
P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; AlLaham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision
C.02; Gaussian, Inc.: Wallingford, CT, 2004.
Supporting Information Available: The CIF files for crystal
structures and additional spectroscopic evidence. This material is
available free of charge via the Internet at http://pubs.acs.org.
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Inorganic Chemistry, Vol. 48, No. 2, 2009 645