Coupling of cationic olefin complexes of platinum(II) with potential ambident nucleophiles

Article abstract of DOI:10.1016/j.ica.2009.08.035

The reactivity of [PtCl(η²-CH₂{double bond, long}CHR)(tmeda)]⁺ (R = H, 1a, or Me, 1b; tmeda = N,N,N′,N′-tetramethyl-1,2-diaminoethane) towards some ambident nucleophiles like anilines and phenolate anion has been tested. The reaction of 1a with N-methylaniline gives immediately N-addition to the coordinated ethene (3a), but, in the presence of an inorganic carbonate, a partial rearrangement, with the para carbon of the phenyl ring taking the place of nitrogen, is observed (4a and 5a). Reaction with a tertiary aromatic amine, such as N,N-dimethylaniline, leads exclusively to the C-coupled species. The phenolate anion acts initially as an oxygen donor, however the resulting species (6a), in contact with free phenol, rearranges to C-bonded species (7a). For free phenol/6a ratios ≥ 5 the rearranged product has an isomeric ortho/para ratio of ≈3. For lower free phenol/6a ratios (≤ 1) oligomeric complexes, in which two or three platinum ethanide moieties are bound to the same phenol ring, are also formed. In the case of 1b, the above described reactivity has to compete with the base-induced deprotonation of propene, leading to formation of the allyl-bridged platinum dimer [{PtCl(tmeda)}(μ-η¹:η³-CHCHCH₂){Pt(tmeda)}]⁺. The X-ray crystal structure of 1b has also been determined; the structural parameters are very similar to those previously reported for 1a. DFT calculations have shown a similar activation of the two complexes towards nucleophilic addition at the coordinated olefin, although in 1b the electrophilic character of the olefin is masked by the Br?nsted acidity of the propene methyl protons.

Full text of DOI:10.1016/j.ica.2009.08.035

Inorganica Chimica Acta 363 (2010) 205–212
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Coupling of cationic olefin complexes of platinum(II)
with potential ambident nucleophiles
Carmen R. Barone a, Renzo Cini b, Sara de Pinto a, Nicola G. Di Masi a, Luciana Maresca a,
,
*
Giovanni Natile a, Gabriella Tamasi b
a Dipartimento Farmaco-Chimico, Università degli Studi di Bari, Via E. Orabona 4, 70125 Bari, Italy
b Dipartimento di Scienze e Tecnologie Chimiche e dei Biosistemi, Università degli Studi di Siena, Via A. Moro 2, 53100 Siena, Italy
a r t i c l e i n f o
a b s t r a c t
The reactivity of [PtCl(g
2-CH2@CHR)(tmeda)]+ (R = H, 1a, or Me, 1b; tmeda = N,N,N0,N0-tetramethyl-1,2-
Article history:
Received 29 July 2009
Accepted 21 August 2009
Available online 4 September 2009
diaminoethane) towards some ambident nucleophiles like anilines and phenolate anion has been tested.
The reaction of 1a with N-methylaniline gives immediately N-addition to the coordinated ethene (3a),
but, in the presence of an inorganic carbonate, a partial rearrangement, with the para carbon of the phe-
nyl ring taking the place of nitrogen, is observed (4a and 5a). Reaction with a tertiary aromatic amine,
such as N,N-dimethylaniline, leads exclusively to the C-coupled species. The phenolate anion acts initially
as an oxygen donor, however the resulting species (6a), in contact with free phenol, rearranges to C-
bonded species (7a). For free phenol/6a ratios P 5 the rearranged product has an isomeric ortho/para
ratio of ꢀ3. For lower free phenol/6a ratios (6 1) oligomeric complexes, in which two or three platinum
ethanide moieties are bound to the same phenol ring, are also formed. In the case of 1b, the above
described reactivity has to compete with the base-induced deprotonation of propene, leading to forma-
Keywords:
Platinum complexes
Anilines
Phenol
X-ray crystal structure
NMR
DFT calculations
tion of the allyl-bridged platinum dimer [{PtCl(tmeda)}(l- :g
g1 3-CHCHCH2){Pt(tmeda)}]+. The X-ray
crystal structure of 1b has also been determined; the structural parameters are very similar to those pre-
viously reported for 1a. DFT calculations have shown a similar activation of the two complexes towards
nucleophilic addition at the coordinated olefin, although in 1b the electrophilic character of the olefin is
masked by the BrØnsted acidity of the propene methyl protons.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
(R = H, alkyl, or aryl; tmeda = N,N,N0,N0-tetramethyl-1,2-diaminoe-
thane) having highly electrophilic character [5].
Cationic olefin complexes of late transition metals are a topic of
current interest. The coordinated alkene generally becomes very
electrophilic (and though very reactive towards nucleophiles)
and, in addition, can exhibit acidic character or insert into an adja-
cent metal–hydrogen or metal–carbon bond [1]. These complexes,
apart from being proposed as the active species in catalytic cycles
leading to functionalization or polymerization of alkenes, are often
isolable also in the pure state [2,3].
Among them, the most active was that containing ethene, 1a
(R = H), the olefin of which can add inorganic anions [5e,f], tertiary
aliphatic amines [5c,d], and a para ring carbon of N,N,N0,N0-tetra-
methyl-1,8-diaminonaphthalene (widely known as Proton Sponge)
[5j].
On the other hand, when R is other than H, the unsaturated li-
gand behaves differently and can be displaced by inorganic anions
[6] or deprotonated by tertiary amines [5n,o].
The reactivity of the cationic complexes depends upon several
factors such as the nature of the metallic core and of the ancillary
ligands and the overall complex charge. For instance, the electro-
philic character of the unsaturated ligand increases sensibly by
raising the electric charge of the complex from +1 to +2 [4].
Over the years we have isolated and characterized several mon-
In the present study we have extended the investigation to aro-
matic amines and phenolate anion, both types of nucleophiles are
amphiphilic and can act as ethero atom or carbon donors [7–9].
The reactions have been performed on 1a and on the closest ana-
logue, the propene complex 1b.
To fully compare the olefin ground state in the two complexes
(1a and 1b), the X-ray crystal structure of 1b has been determined
(the crystal structure of 1a, obtained by both X-ray and neutron
diffraction analysis, was already known) [5b,g]. Moreover, since
the reactivity of a metal coordinated olefin towards nucleophiles
appears to be ruled by the energy difference between the LUMO
ocationic complexes of formula [PtCl(g
2-CH2@CHR)(tmeda)]+, 1,
* Corresponding author. Tel.: +39 080 5442776; fax: +39 080 5442230.
E-mail address: maresca@farmchim.uniba.it (L. Maresca).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.08.035

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C.R. Barone et al. / Inorganica Chimica Acta 363 (2010) 205–212
*
of the complex and the
p
orbital of the free ligand [10], DFT calcu-
N(CH3) and N(CH2)), 3.37 (t, 2H, Pt–CH2CH2N), 6.68 (d, 2H, 2
CHortho), 7.16 (d, 2H, 2 CHmeta); dPt ꢁ3461 and ꢁ3499.
lations for 1a and 1b have also been performed.
2.2.3. [PtCl{g1-CH2CH2-C6H4-p-N(CH3)2}(tmeda)], (40a)
2. Experimental
The synthetic procedure was completely analogous to that re-
ported for 2.2.1, starting from [PtCl(
(237 mg, 0.5 mmol), K2CO3 (276 mg, 2 mmol), and N,N-dimethyl-
aniline (190 L, 1.5 mmol). The conversion of the starting complex
g
2-C2H4)(tmeda)](ClO4), 1a(ClO4)
2.1. General
l
Solvents and reagents, unless otherwise stated, were commer-
cially available (purchased from Aldrich Chemical Company) and
used as received. Chlorinated solvents were dried over activated
molecular sieves (beads, 4–8 mesh). Elemental analyses were per-
formed with a CHN Eurovector EA 3011. 1H, 13C and 195Pt NMR
spectra were recorded with a 300 MHz Mercury Varian and a
DPX 300 Avance Bruker instruments equipped with probes for in-
verse detection and with z gradient for gradient-accelerated spec-
troscopy. 1H and 13C NMR spectra were referenced to TMS; the
residual proton signal of the solvent was used as internal standard.
1H/13C inversely detected gradient-sensitivity enhanced heterocor-
related 2D NMR spectra for normal coupling (INVIEAGSSI) were ac-
quired using standard Bruker automation programs and pulse
sequences. Each block of data was preceded by eight dummy scans.
The data were processed in the phase-sensitive mode. For 195Pt
NMR spectra, K2PtCl4 was used as external standard (-1643.00
ppm). ESI-MS spectra were recorded with an Agilent 1100 Series
LC-MSD Trap System VL. GC–MS spectra were recorded with an
Agilent 6890N-5973N MSD.
was 70% after 48 h. Anal. Calc. for C16H30ClN3Pt: C, 38.83; H, 6.11;
N, 8.49. Found: C, 39.08; H, 6.02; N, 7.94%. Peak of greatest inten-
sity in ESI-MS: m/z = 459.0 = [MꢁCl]+. NMR (CDCl3, 294 K, ppm): dH
1.59 (t, 2H, 2JPt–H ca. 87 Hz, Pt–CH2CH2C), 2.51 (t, 2H, Pt–CH2CH2C),
2.4–3.0 (m, 16H, tmeda N(CH3) and N(CH2)), 2.88 (s, 6H, N,N-
dimethylaniline N(CH3)2), 6.68 (d, 2H, 2 CHortho), 7.20 (d, 2H, 2
CHmeta).
2.2.4. Acid hydrolysis of amine addition products
In a typical experiment, 0.2 mmol of 3a plus 4a, 5a, or 40a were
treated with 2 mL of concentrated aqueous HCl (37%). The mixture
was kept stirring at room temperature for several hours. The
mother liquor, separated by filtration from the solid [PtCl2(tme-
da)], and divided in two aliquots, was evaporated to dryness in a
vacuum desiccator (in the presence of NaOH and H2SO4, as drying
agents). The two dried samples were dissolved in basic water
(KOH), then one sample was extracted with CDCl3 (for 1H NMR)
and the other with Et2O (for GC–MS). The two organic phases were
kept for a few hours over Na2SO4 and then analyzed.
g
1-CH2CH2-OPh)(tmeda)], (6a)
2.2. Syntheses
2.2.5. [PtCl(
[PtCl(
2-C2H4)(tmeda)](ClO4), 1a(ClO4) (237 mg, 0.5 mmol),
g
2.2.1. [PtCl{
CH2CH2-C6H4-p-NH(CH3)}(tmeda)], (4a)
[PtCl(
2-C2H4)(tmeda)](ClO4), 1a(ClO4) [5a] (237 mg, 0.5 mmol),
was suspended in CH2Cl2 (5 mL) in the presence of K2CO3 (276 mg,
2 mmol). N-methylaniline (163 L, 1.5 mmol) was then added and
g -
1-CH2CH2-N(CH3)C6H5}(tmeda)], (3a), and [PtCl{g1
was suspended in CH2Cl2 (5 mL). Sodium phenolate (58 mg, 0.5
mmol) was then added and the mixture was stirred for a few min-
utes at room temperature. The mother liquor was then filtered and
evaporated in vacuo. The obtained oily product, after trituration
with diethyl ether, left a yellowish powder, which was recovered
by filtration and pumped in vacuo. Alternatively, 6a could be pre-
pared from 1a(ClO4) (237 mg, 0.5 mmol) and phenol (94 mg,
1 mmol), in the presence of K2CO3 (276 mg, 2 mmol). The mixture
was kept stirring for 24 h at room temperature, and the subsequent
working up was performed as previously described. The isolated
yield, referred to platinum, was ca. 95% in both cases. Anal. Calc.
for C14H25ClN2OPt: C, 35.94; H, 5.39; N, 5.99. Found: C, 35.61;
H, 5.36; N, 5.90%. Peak of greatest intensity in ESI-MS: m/z =
g
l
the mixture was stirred in the dark for 48 h at room temperature.
Themotherliquorwasthenfilteredand evaporatedin vacuo. Theob-
tained oily product, after trituration with diethyl ether, gave a solid
white powder, which was recovered by filtration and pumped in va-
cuo. The solid was a mixture of 3a (major) and 4a (minor). Pure 3a
could be obtained, as colorless needles, by crystallization of the reac-
tion product from CH2Cl2/diethyl ether. Anal. Calc. for C15H28ClN3Pt:
C, 37.46; H, 5.87; N, 8.74. Found: C, 37.62; H, 5.87; N, 8.58%. Peak of
greatest intensity in ESI-MS (both for 3a and its mixture with 4a): m/
z = 445.0 = [MꢁCl]+. NMR (CDCl3, 294 K, ppm) for 3a: dH 1.55 (t, 2H,
2JPt–H ca. 88 Hz, Pt–CH2CH2–N), 2.96 (s, 3H, N-methylaniline N(CH3)),
2.4–3.0 (m, 16H, tmeda N(CH3) and N(CH2)), 3.42 (t, 2H, Pt–
CH2CH2N), 6.55 (t, 1H, CHpara), 6.75 (d, 2H, 2 CHortho), 7.18 (t, 2H, 2
CHmeta); dPt ꢁ3456. NMR (CDCl3, 294 K, ppm) for 4a: dH 1.55 (t, 2H,
2JPt–H ca. 88 Hz, Pt–CH2CH2C), 2.59 (t, 2H, Pt–CH2CH2C); dPt ꢁ3493.
2
490.8 = [M+Na]+. NMR (CDCl3, 294 K, ppm): dH 1.8 (t, 2H, JPt–H
ca. 104 Hz, Pt–CH2CH2O), 2.4–3.0 (m, 16H, tmeda N(CH3) and N(CH2)),
4.1 (m, 2H, Pt–CH2CH2O), 6.87 (t, 1H, CHpara), 7.04 (d, 2H, 2 CHorto),
7.24 (t, 2H, 2 CHmeta).
2.2.6. Rearrangement of 6a in the presence of added phenol
In a typical experiment 6a (117 mg, 0.25 mmol) was dissolved
in CH2Cl2 and mixed with a solution of phenol in the same solvent
(phenol quantity ranging from 0.05 to 1.25 mmol). The chlorinated
solvent was evaporated in vacuo, to leave a sticky solid, which was
left standing at 278–298 K. After 1–4 days the sticky solid was trit-
urated with diethyl ether to give a yellowish powder. The conver-
sion of 6a into 7a was quantitative.
2.2.2. [(tmeda)ClPt{CH2CH2-N(CH3)-C6H4-p-CH2CH2}PtCl(tmeda)],
(5a)
[PtCl(g
2-C2H4)(tmeda)](ClO4), 1a(ClO4) (237 mg, 0.5 mmol),
was suspended in CH2Cl2 (5 mL) in the presence of K2CO3
(276 mg, 2 mmol). A stoichiometric amount, in terms of platinum,
of the reaction mixture 3a plus 4a (240 mg, 0.5 mmol) was then
added and the mixture was stirred in the dark for 24 h at room
temperature. After filtration, the mother liquor was evaporated
in vacuo and the oily residue triturated with diethyl ether and
dried. The obtained white solid was the dinuclear species 5a. Anal.
Calc. for C23H47Cl2N5Pt2: C, 32.32; H, 5.54; N, 8.19. Found: C, 32.04;
H, 5.27; N, 8.33%. Peak of greatest mass in ESI-MS: m/z =
2.2.7. [PtCl(
Scheme 2)
g
1-CH2CH2-C6H4-OH)(tmeda)], (mononuclear 7a) (see
The complex was prepared according to the general procedure
given above, using a phenol/6a ratio close to 5; the reaction tem-
perature was kept in the range 293–298 K and the reaction time
was ca. 24 h. Anal. Calc. for C14H25N2ClOPt: C, 35.94; H, 5.39; N,
5.99. Found: C, 35.73; H, 5.33; N, 5.88%. Peak of greatest intensity
in ESI-MS: m/z = 490.8 = [M+Na]+. NMR (CDCl3, 294 K, ppm): dH
817.8 = [MꢁCl]+. NMR (CDCl3, 294 K, ppm): dH 1.55 (t, 4H, JPt–H
2
ca. 88 Hz, Pt–CH2CH2N and Pt–CH2CH2C), 2.60 (t, 2H, Pt-CH2CH2C),
2.89 (s, 3H, N-methylaniline N(CH3)), 2.4–3.0 (m, 32H, tmeda
2
1.58 (t, 2H, JPt–H ca. 91 Hz, Pt–CH2CH2C), 2.45 (m, 2H, Pt–

C.R. Barone et al. / Inorganica Chimica Acta 363 (2010) 205–212
207
CH2CH2C), 2.4–3.0 (m, 16H, tmeda N(CH3) and N(CH2)), 6.73 (t, 1H,
CHpara), 6.88 (d, 1H, CHortho), 7.04 (m, 2H, 2 CHmeta).
formed through PARST-97 [15] and the molecular graphics was
carried out through ORTEP-32 [16]. All the calculations were per-
formed by using Pentium IV personal computers operating through
the Windows-XP system. CCDC-665826 (1b) contains the Supple-
mentary crystallographic data for this paper.
2.2.8. Acid hydrolysis of phenolate addition products
The acid hydrolysis was performed as previously described for
the amine addition products (see Section 2.2.4). In each experi-
ment, the resulting acid solution was divided in two aliquots and
extractions with CDCl3 (for 1H NMR) and with Et2O (for GC–MS)
were performed. The two organic phases were kept for a few hours
over a mixture of Na2CO3 and Na2SO4 and then analyzed.
2.4. Computational details
The calculations of complexes 1a and 1b were performed in the
gas phase by using GAUSSIAN-98/RevA.11.3 [17], implemented on
IBM SP5 machines of CINECA (Consorzio Interuniversitario del
Nord-Est per i Calcoli Avanzati, Casalecchio di Reno, Bologna) at
the B3LYP level [18,19], using as basis set: Lanl2dz for Pt and 6-
31G(d,p) for all other atoms (H, C, N, and Cl) [20]. The geometry
optimizations were performed without any symmetry constraint.
Cartesian coordinates for computed structures 1a and 1b are re-
ported in the Supporting information Section (Tables S2 and S3).
2.3. X-ray crystallography
Data collection for 1b(ClO4) was performed through a Siemens
P4 four circle diffractometer, located at CIADS (Centro per Analisi
e Determinazioni Strutturali, University of Siena, Italy) at room
temperature (293 2 K) (Table S1). Cell constant determination
was performed through a least-squares analysis from the values
of 40 high intensity randomly selected reflections in the range
3. Results and discussion
10 6 2h 6 41°. The original data sets (2113 reflections, Rint
=
3.1. Reactions with aromatic amines
0.0375, R(sigma) = 0.0217) were corrected for Lorentz-polariza-
tion effects through XSCANS computer package [11]. The absorption
The overall reaction pattern of 1a with N-methylaniline is de-
picted in Scheme 1. The aminic function adds to the coordinated
ethene to give the addition product 2a, which is the dominant spe-
cies in the presence of excess amine (amine/Pt ratio P 3). When
the reaction is carried out in the presence of solid K2CO3, apart
from deprotonation of 2a to give 3a, another species, 4a, is also
formed. 4a, which amounts to ca. 20% of the reaction products in
the used experimental conditions (r.t. and 48 h), has elemental
composition and ESI-MS molecular peak coincident with that of
3a. Its NMR spectra, however, unambigously showed how the pla-
timum–ethanide moiety was bound to the para carbon of the phe-
nyl ring.
effects were corrected through the
the XEMP computer software [12].
w-scan techniques by using
The structure was solved through direct methods implemented
in the SHELXS-97 software [13a,b] and refined via SHELXL-97 program
[13b,c], both implemented under WINGX package [14]. All the atoms
of the coordination shell and most of the other non-hydrogen
atoms were located from the structure solution, whereas the
remaining atoms were located from subsequent cycles of Fourier-
difference synthesis and least-squares calculations. The hydrogen
atoms were located via the HFIX options of SHELXL-97 [13c] and left
riding on the atoms to which they are attached. The non-hydrogen
atoms were refined as anisotropic, and the hydrogen atoms were
treated as isotropic with thermal factors equal to 1.2 or 1.5 times
those of the corresponding heavier atoms. Twenty-four cycles of
least-squares calculations gave a nice convergence for the model.
The conventional R1 and wR2 factors converged to 0.0387 and
0.0957. The analysis of molecular geometry parameters was per-
3a and 4a have almost coincident signals for C H2, but distinctly
a
different ones for CbH2 (at 3.42 and 2.59 ppm for 3a and 4a, respec-
tively), reflecting the different nature of the nucleophile donor
atom, the aminic nitrogen in the former case and a ring carbon
in the latter one (Fig. S1). Moreover, the aromatic proton reso-
nances were in accordance with a monosubstituted phenyl ring
+
NHMe
Cl
N
N
Cl
N
N
K2CO3
+
Pt
Pt
Cl
N
N
1a
4a
NHMe
Pt
1a, K2CO3
NMe
+
N
N
Cl
N
N
Pt
Cl
N
N
K2CO3
5a
Pt
Cl
Pt
H
Me
Me
N
N
2a
3a
Scheme 1. Reaction pattern of complex [PtCl(g
2-CH2@CH2)(tmeda)]+, 1a, with N-methylaniline.

208
C.R. Barone et al. / Inorganica Chimica Acta 363 (2010) 205–212
in the case of the major component 3a and with a 1,4-disubstituted
phenyl ring in the case of the minor component 4a. The structure
of 4a was also confirmed by analysis of the corresponding acid
demolition products (see later on in this section). Two 195Pt NMR
signals were present at ꢁ3456 and ꢁ3493 ppm, the higher field
signal was the less intense and therefore was assigned to 4a. The
mixture of 3a and 4a was further reacted with an equimolar
amount of complex 1a, always in the presence of solid K2CO3, to
give quantitative formation of dinuclear 5a. 5a was characterized
via elemental analysis, ESI-MS and NMR. In the 2D (1H, 1H) COSY
spectrum (CDCl3, 300 MHz, 294 K) the N- and C-coupled Pt–
Moving to the case of 1b, we found that neither N-methyl- nor
N,N-dimethylaniline were able to give C-coupled addition products,
but the inorganic carbonate, present in the reaction scene, promoted
olefindeprotonation with formation of thealreadyreporteddinucle-
ar cation [PtCl(tmeda)(l- :g
g1 3-CHCHCH2)Pt(tmeda)]+ [5n].
As shown in the years by Buncel [7] and Terrier [8] the reaction
with an aniline is a well established procedure for testing the
strength of an electrophile. Aniline binds exclusively through the
aminic function when reacted with electron-deficient organic mol-
ecules, such as 1,3,5-trinitrobenzene [7] or 4-nitrobenzofuroxane
(NBF) [21]. In contrast, the reaction with 4,6-dinitrobenzofuroxane
(DNBF) and 4,6-dinitrobenzofurazane (DNBZ) leads, besides to the
N-coupling, also to C-coupling with the para carbon of the aromatic
amine [7] (Fig. S3).
The nucleophilic attack of an electron-rich phenyl ring carbon
onto an electron-deficient carbon (C7 of DNBF or DNBZ) produces
the so called Wheland intermediate [22], which can exhibit acidic
properties and give rise to spontaneous deprotonation of the phe-
nyl ring carbon. For instance, products of coupling of DNBF and
DNBZ with 1,3,5-trimethoxybenzene have been isolated by Terrier
as stable salts of alkali metals [8a]. An aminic functionality present
on the electron-rich phenyl ring can capture the outgoing proton;
this situation is featured by the behavior of Proton Sponge (pKa =
12) towards both polynitroaromatics (DNBF and DNBZ) [8a] and
complex 1a [5j]. The aromatic amines used in the present investi-
gation have lower basicity (pKa of anilines is in the range 5–6),
therefore coupling with 1a via a phenyl ring carbon can only be fa-
voured by the presence of a stronger base like carbonate. In this
context we ought to mention a recent study by Vitagliano et al.,
in which the nucleophilic attack of a phenyl carbon of 1,3-dime-
thoxybenzene onto a dicationic platinum ethene complex (a stron-
ger electrophile than our compounds 1a and 1b) generates a
Wheland intermediate with a remarkable acidic character. The
released proton can cleave the platinum–carbon bond, affording
directly the ethylated 1,3-dimethoxybenzene [23].
C H2CbH2– moieties gave rise to two spin systems, of equal inten-
a
sity, similar to those formed in 3a and 4a, respectively (Fig. S2).
Moreover, the aromatic proton signals are consistent with a 1,4-
disubstituted phenyl ring. Two signals of equal intensity are pres-
ent in the 195Pt NMR spectrum, their frequencies (at ꢁ3461 and
ꢁ3499 ppm) are close to those of the mononuclear species 3a
and 4a.
The mixture of 3a and 4a, as obtained from the reaction, was
hydrolyzed with concentrated aqueous HCl (37%). The GC–MS
analysis of the organic fraction showed a major peak at m/z =
107, corresponding to N-methylaniline, and a minor one at m/z =
135, corresponding to p-ethyl-N-methylaniline (the presence of
the ethyl group on the para carbon of the aromatic ring was re-
vealed by a 1H NMR spectrum). Hydrolysis of dinuclear 5a pro-
duced essentially p-ethyl-N-methylaniline. Thus, in the used
experimental conditions, the acid (HCl) cleaved the Pt–C
r bond
only when the nucleophilic attack to the olefin was promoted by
a carbon donor and reversed the reaction in the case of amine addi-
tion. In all cases the platinum was recovered as [PtCl2(tmeda)]
complex.
When the reaction was carried out with an amine/Pt ratio of 1:1
and always in the presence of K2CO3, dinuclear species with two
platinum ethanide residues bound to the phenyl ring of N-methy-
laniline did form. The GC–MS analysis of the organic fraction, after
hydrolysis of the crude reaction products with concentrated HCl,
showed an intense peak at m/z = 163, corresponding to a N-methy-
laniline having two ethyl substituents on the aromatic ring. After
3.2. Reactions with phenolate anion
the link of the first Pt–C H2CbH2– moiety, the phenyl ring is acti-
vated towards further electrophilic attack.
1a reacted rapidly and quantitatively with phenolate anion to
a
give the O-addition product [PtCl(g
1-CH2-CH2-OPh)(tmeda)], 6a.
In the case of N,N-dimethylaniline, the tertiary aminic nitrogen
did not add to the coordinated ethene, but, in anhydrous condi-
tions and in the presence of inorganic carbonate, a carbon addition
The same result was obtained reacting 1a with phenol in the pres-
ence of solid K2CO3 (Scheme 2), but a longer reaction time was re-
quired. The structure of 6a was unambiguously established on the
basis of the 1H NMR spectrum, which clearly showed the presence
product analogous to 4a, namely [PtCl(g
1–CH2–CH2–C6H4–p-
NMe2)(tmeda)] (40a), formed in nearly quantitative yield. 40a was
characterized by elemental analysis, ESI-MS, and NMR. Hydrolysis
with concentrated aqueous HCl produced, almost exsclusively, p-
ethyl-N-methylaniline (GC–MS and 1H NMR).
The reactivity of complex 1a towards N-methyl-p-anisidine was
also tested; the N-addition product was the exclusive form and
there was no evidence of C-coupling taking place. The results just
described clearly indicate that is the carbon in para position of
the aromatic ring the preferred site of attack by 1a.
of the Pt–C H2CbH2–OPh moiety [the a and b methylenes resonate
a
at 1.8 (2JPt–H = 104 Hz) and 4.1 ppm, respectively] and an aromatic
portion consistent with a preserved monosubstituted phenyl ring
(Fig. S4). 6a is rather moisture sensitive and, if not kept in anhy-
drous conditions, it undergoes some hydrolysis with an OHꢁ [5h]
replacing the phenolate anion which is released as free phenol.
If 6a is kept in contact with phenol, it undergoes a rearrange-
ment from O-bonded phenolate to C-bonded phenol. This transfor-
mation occurs and it is even faster if performed in the solid phase
OH
+
OH
Cl
N
N
Cl
N
N
Cl
N
N
K2CO3
OH
+
Pt
Pt
Pt
O
n
1a
6a
7a
n = 1, 2, 3
Scheme 2. Reaction pattern of complex [PtCl(
g
2-CH2@CH2)(tmeda)]+, 1a, with phenolate anion.

C.R. Barone et al. / Inorganica Chimica Acta 363 (2010) 205–212
209
(see Section 2.2.6). We have carried on several experiments, using
different amounts of phenol. Using a phenol/6a ratio P 5, the only
observed transformation was the complete isomerization of 6a
into mononuclear 7a (Scheme 2), which was characterized via
emental analysis, ESI-MS, and NMR. In the 1H NMR spectrum the
b methylene resonates at 2.45 ppm as compared to 4.1 ppm in 6a
and the pattern of aromatic protons is consistent with a dominant
1,2-disubstituted phenyl ring (Fig. S5). The acid hydrolysis (carried
out in similar conditions to those used for the amine coupled prod-
ucts) performed on 7a produced ortho- and para-ethylphenol, the
ortho isomer being ꢂ70% of the total mixture (GC–MS analysis).
It is to be noted that the same acid hydrolysis performed on 6a re-
leased unalkylated phenol.
Using a phenol/6a ratio 6 1, polinuclear 7a species were also
formed (Scheme 2). The GC–MS analysis of the organic fraction,
produced by acid hydrolysis of the crude reaction products,
showed, besides the presence of ortho- and para-ethylphenol
(in the same ratio found in the previous experiment), also the
presence of some di- and tri-ethylphenols. In particular, while a
phenol/6a ratio close to 1 afforded only trace amounts of polyalky-
lated phenols, using a phenol/6a ratio of 0.2, the polyalkylated
phenols became a relevant percentage of the total mixture.
[25]. The activated complex, formed by the ether and the catalyst
[25b,c], can evolve intramolecularly, by shifting the alkyl group
on a phenyl carbon of the same ether molecule, or intermolecu-
larly, via detachment of the alkyl group and its subsequent electro-
philic attack on a phenyl ring belonging to another molecule
(Fig. S6) [25c]. In the formation of ortho-alkyl phenols, the intramo-
lecular reaction pattern seems to be dominant, while para isomers
are essentially formed via the intermolecular path. This dual mech-
anism can lead to an ortho/para ratio higher than expected for a
purely aromatic electrophilic substitution [26].
For a better understanding of the mechanisms operating in the
case of 6a, the rearrangement was investigated in the presence of
perdeuterated phenol. Using an excess of phenol-d6 (condition
allowing the exclusive formation of mononuclear 7a) and then
performing the hydrolysis with HCl, the GC–MS analysis of the
recovered organic fraction revealed the presence of ortho- and
para-ethylphenol in the usual ratio (close to 3:1), both containing
two deuterium atoms on the aromatic ring (m/z = 124). This feature
implies that, in the rearrangement, the platinum–ethanide moiety
has moved to a perdeuterated phenol molecule. It is to be noted
that, in strong acidic conditions, the phenol undergoes H/D
exchange at the ortho and para positions and, as a consequence,
only the deuterium atoms in positions 3 and 5 are retained [27].
When 6a was treated with stoichiometric or substoichiometric
quantities of phenol-d6, the GC–MS spectrum of the organic frac-
tion, after hydrolysis of the crude reaction products, did show a
mixture of mono-, di-, and tri-ethylphenols. For the mono-ethyl-
phenols (usual ortho/para ratio close to 3:1) two peaks at m/
z = 122 and 124 were found (the former peak corresponded to
alkylation of undeuterated phenol and the second peak to alkyl-
ation of perdeuterated phenol). The intensity of the higher mass
peak decreased by decreasing the quantity of free phenol used
(the free phenol was always perdeuterated); using a phenol/6a
These findings give further evidence that a Pt–C H2CbH2 moiety
a
on a phenyl ring acts as an activating substituent. The presence of
excess phenol disfavors, on a statistical basis, formation of poly-
alkylated species.
The transformation 6a ? 7a is reminiscent of the rearrange-
ment of alkyl aryl ethers, which has been known for over a century
[24]. This rearrangement is catalyzed by Lewis acids. Reaction
yields and possible mechanisms depend upon several factors, such
as nature and quantity of catalyst, as well as the reaction condi-
tions (homogeneous or heterogeneous). In the case of homoge-
neous conditions a relevant role is also played by the solvent
D
D
D
D
α
O
D
N
N
N
β
O
D
Pt
Pt
+
D
O
Cl
N
Cl
O
D
D
D
D
D
6a
D
D
D
D
N
N
.
.
.
.
O
Pt
D
Cl
7a
ortho isomer
Scheme 3. Inter-molecular rearrangement of complex [PtCl(g
1-CH2CH2-OPh)(tmeda)], 6a, in the presence of free phenol-d6.

210
C.R. Barone et al. / Inorganica Chimica Acta 363 (2010) 205–212
Table 1
ratio of 0.2, the peak at m/z = 124 almost disappeared. In the latter
situation the obtained di- and tri-ethylphenols were also deprived
of deuterium (only peaks at m/z = 150 and 178, respectively).
Therefore, the obtainment of deuterated mono-ethylphenols
in the reaction performed in the presence of an excess of phenol-
d6 is indicative of a transposition reaction taking place mainly
through an intermolecular concerted mechanism as shown in
Scheme 3. This requires that an external perdeuterated phenol
molecule while donating the hydroxyl proton to the ether oxygen
of 6a uses the electron-rich ortho carbons (and to a lesser extent
the para carbon) to perform nucleophilic attack on the b carbon
of 6a. If the used phenol-d6 is only equimolar with respect to 6a,
while the transposition reaction is progressing, undeuterated phe-
nol (initially present in 6a) is liberated in the reaction mixture with
the consequence that the transposed molecules are a mixture of
non deuterated and perdeuterated species. A further decrease of
free perdeuterated phenol (phenol/6a ratio of 0.2) not only practi-
cally abolishes the formation of deuterated alkylation products, but
also favors multiple alkylation of the same aromatic nucleus.
A system, closely related to the one presently described, has
been recently investigated by Benedetti et al. [28]. The cationic
platinum complex, considered in that study, is an analogous of
1a, having 1,10-phenanthroline (1,10-phen) instead of tmeda in
the platinum coordination sphere. The addition product corre-
Selected bond lengths (Å) and angles (°) for [PtCl(
1b(ClO4).
g
2–CH2@CH–CH3)(tmeda)](ClO4),
Vector
Length
Vector
Length
Pt–Cl(1)
Pt–N(1)
Pt–N(2)
Pt–C(1o)
Pt–C(2o)
2.289(3)
2.074(9)
2.105(9)
2.195(10)
2.221(11)
N(1)–C(1)
N(2)–C(2)
C(1)–C(2)
1.510(14)
1.491(15)
1.48(2)
C(1o)–C(2o)
C(2o)–C(3o)
1.39(2)
1.48(2)
Vector
Angle
Vector
Angle
N(1)–Pt–N(2)
N(1)–Pt–C(1o)
N(1)–Pt–C(2o)
N(2)–Pt–C(1o)
N(2)–Pt–C(2o)
84.5(4)
99.1(4)
90.6(4)
161.9(4)
161.4(5)
N(1)–Pt–Cl(1)
N(2)–Pt–Cl(1)
C(1o)–Pt–Cl(1)
C(2o)–Pt–Cl(1)
174.6(3)
90.3(3)
85.5(4)
94.8(3)
Table 2
Bond distances (Å) and 13C chemical shifts (acetone-d6, 294 K, ppm) for free and
coordinated olefins (1JPt–C in Hz are reported in brackets).
Compound
d(C@C)
d(PtꢁCH2)
d(PtꢁCHR)
d (CH2)
d (CHR)
Ethene
1.33
1.390(2)
ꢁ
ꢁ
ꢁ
123.3
80.2
[166]
115.7
77.6
ꢁ
ꢁ
1a5g
2.184(5)
2.166(5)
ꢁ
Propene
1b
1.33
1.39(2)
ꢁ
133.7
104.6
[153]
sponding to 6a, namely [PtCl(g
1-CH2-CH2-OPh)(1,10-phen)], in
2.195(10)
2.221(11)
[149]
the presence of excess phenol not only rearranged, shifting the al-
kyl platinum moiety to an ortho carbon of the aromatic ring, but
the phenolate oxygen also gave a ring closing reaction by substitu-
tion of the cis chlorido ligand. The authors proposed for the reac-
tion a concerted mechanism similar to that reported in Scheme 3.
The ortho isomer of mononuclear 7a could, in principle, give the
ring closing reaction, with elimination of HCl and formation of a
Pt–O bond; differently from the system investigated by Benedetti
et al., in our case this step requires the presence of a strong base
(KOH).
The reaction with phenolate anion was performed also on com-
pound 1b. The formation of the O-coupled species, 6b, was rapid
and quantitative, but its reaction with phenol, performed as
already described in the case of 6a, was rather complex. With
time, a number of products, including those derived from deproto-
nation of 1b [5n], were formed and the system was not further
investigated.
Cl(1)
C(5)
N(2)
C(3o)
C(1o)
Pt
C(6)
C(2)
C(2o)
C(4)
N(1)
C(3)
C(1)
3.3. The coordinated olefin in the cationic complexes
Fig. 1. ORTEP drawing for [PtCl(g
2-CH2@CH–CH3)(tmeda)]+, 1b. Ellipsoids enclose
20% probability.
Relevant structural and NMR parameters for free and coordi-
nated ethene and propene are collected in Tables 1 and 2. The
structure of 1a(ClO4) had already been reported [5g], whereas
the X-ray crystal structure of 1b(ClO4) is part of the present inves-
tigation (Table S1). The crystal structure is drawn in Fig. 1 and se-
lected geometrical parameters are reported in Table 1. The
structures of the cations 1a and 1b are very similar and share a
number of features, such as the elongation of the Pt–N bond trans
to the olefin with respect to that cis to it (1a, 2.119(2) and
2.084(2) Å; 1b, 2.105(9) and 2.074(9) Å, respectively) and the sym-
metrical coordination to the metal core with the two Pt–C dis-
tances almost identical in both complexes (1a, 2.184(5) and
2.166(5) Å; 1b, 2.195(10) and 2.221(11) Å). The main difference
lies in the orientation of the alkene, which is perpendicular to
the coordination plane in 1a (deviation of only 2° with respect to
the ideal value of 90°) and slightly canted in 1b (deviation of ca.
14°). The just quoted difference could be due to crystal packing ef-
fects and is not relevant to the solution behavior of the two com-
plexes, since the olefin is free to rotate about the axis connecting
the metal core to the centroid of the carbon–carbon double bond
[29].
13C NMR parameters (Table 2) have been acquired for both com-
pounds (acetone-d6, 294 K). Upon coordination, the olefinic carbon
resonances undergo an upfield shift; moreover, the 1JPt–C values for
the two carbons of 1b are very similar and close to that of 1a, indi-
cating that the symmetrical coordination of the olefin is retained
also in solution.
3.4. DFT calculations
The geometries of 1a and 1b have been optimized at the B3LYP
level using, as basis set, Lanl2dz for Pt and 6-31G(d,p) for all other
atoms and the atom coordinates of the crystal structures as start-
ing point [5g]. The calculated geometric values (Table S4) compare
well to the experimental data [30].
DFT calculations have shown how for both complexes (1a and
1b) the LUMO is lower in energy than the
*
p
of the free olefin (en-
ergy values: 10.9 kcal/mol for ethene and ꢁ132.76 kcal/mol for 1a,
17.17 kcal/mol for propene and ꢁ130.32 kcal/mol for 1b) (Fig. 2).

C.R. Barone et al. / Inorganica Chimica Acta 363 (2010) 205–212
211
Fig. 2. Lowest Unoccupied Molecular Orbital (LUMO, isodensity value = 0.04) (left) and calculated molecular structure (right) for 1b (viewed from the same observation
point).
This property, shared also by closely related styrene complexes
[5r], is in full agreement with the observed reactivity. Low lying
LUMO levels favor the interaction with an incoming nucleophile
and a positive charge on the metal fragment is an important aiding
factor for the reaction to take place. Although, in principle, such an
activation towards nucleophilic addition may apply only to transi-
tion states, in the present case there is an intrinsic activation al-
ready in the ground state. Moreover, in the case of 1b, the LUMO
appears to be more localized on the substituted carbon, fully sup-
porting the experimentally observed prevalence of the Markovni-
kov type of attack in the nucleophilic addition [5l,p,q].
cam.ac.uk/data_request/cif. Supplementary data associated with
this article can be found, in the online version, at doi:10.1016/
j.ica.2009.08.035.
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