Fine-tuning the basicity of metal complexes: Reversible oxidative addition of Se-Se bonds to platinum(II) precursors

Article abstract of DOI:10.1021/om020401i

The oxidative addition of diselenides to novel platinum(II) precursors affording octahedral platinum-(IV) products was investigated. The properties of the substituents on Pt were finely tailored in order to prompt easy reversibility of the reaction. This allowed the achievement of the first example of equilibrium of oxidative addition of an E-E bond (E = chalcogen) to a Pt(II) precursor. The equilibrium constants could be evaluated by NMR spectroscopy, and the results were preliminarily rationalized in terms of steric and electronic factors.

Full text of DOI:10.1021/om020401i

Organometallics 2002, 21, 3503-3505
3503
F in e-Tu n in g th e Ba sicity of Meta l Com p lexes:
Rever sible Oxid a tive Ad d ition of Se-Se Bon d s to
P la tin u m (II) P r ecu r sor s
Achille Panunzi, Giuseppina Roviello, and Francesco Ruffo*
Dipartimento di Chimica, Universita` di Napoli “Federico II”,
Complesso Universitario di Monte S. Angelo, Via Cintia, I-80126, Napoli, Italy
Received May 20, 2002
Summary: The oxidative addition of diselenides to novel
platinum(II) precursors affording octahedral platinum-
(IV) products was investigated. The properties of the
substituents on Pt were finely tailored in order to prompt
easy reversibility of the reaction. This allowed the
achievement of the first example of equilibrium of
oxidative addition of an E-E bond (E ) chalcogen) to a
Pt(II) precursor. The equilibrium constants could be
evaluated by NMR spectroscopy, and the results were
preliminarily rationalized in terms of steric and elec-
tronic factors.
we5 and other researchers6,7 disclosed the occurrence of
tuneable equilibria in the oxidative addition of organotin
or organomercury halides to Pt(0)5 and Pt(II)6,7 precur-
sors of formula [Pt(N,N-chelate)(olefin)] (1) and [PtMe2-
(N,N-chelate)], respectively.
More recently, we have undertaken a study of the
oxidative addition of E-E bonds (E ) chalcogen) to
complexes of type 1 according to eq 2:8
[Pt(N,N-chelate)(olefin)] (1) + RE-ER )
[Pt(ER)2(N,N-chelate)(olefin)] (2) (2)
Oxidative addition and reductive elimination are
pivotal reactions that intervene in several catalytic
cycles and are widely used in synthesis (eq 1).1
The products 2 of reaction 2 are five-coordinate
complexes stabilized by the presence of a nitrogen
chelate with suitable steric hindrance. Within this
study, we have found that the reaction is again a rare
example of tuneable equilibrium, if the olefin is a
fumaric ester8 and the acid is dimethyl diselenide,
MeSe-SeMe. Instead, the reaction is totally shifted
toward the products when alkenes with reduced electron-
withdrawing properties (e.g., acrylic esters9 or ethylene9)
are employed. A rationale of this behavior can be traced
by considering that the metal-to-olefin π-back-donation
decreases as the acceptor features of the alkene de-
crease. Accordingly, the electronic density on the metal
center, and hence its basicity, grows.
On the other hand, as suggested by structural and
spectroscopic properties,10 the extent of π-back-donation
in complexes of type 1 is in all cases substantial. Thus,
despite the formal oxidation state of platinum, these
compounds exhibit a considerable Pt(II) cyclopropam-
etalate character (I in Figure 1), while the other possible
limit form II is less representative.11 As a consequence,
olefin Pt(0) and bis(hydrocarbyl) Pt(II) complexes are
expected to display a parallel chemical behavior.
This consideration has inspired the preparation of Pt-
(II) species [PtR′(R′′)(N,N-chelate)] (3) with chemical
LxM(N) + A-B ) Lx(A)(B)M(N+2)
(1)
Although the general features that favor the oxidation
of a metal according to eq 1 can be reasonably predicted,
the opportunity to assess the experimental validity of
these theoretical assumptions is very rare. This is
mostly because the oxidative additions are generally
totally shifted toward either the reagents or the prod-
ucts. Furthermore, the reactions are often accompanied
by other rearrangements, e.g., ligands loss2 or inser-
tions,3 which prevent the establishment of equilibria.
Direct information on the thermodynamics of the reac-
tion can be achieved when the oxidative addition is
reversible and involves measurable concentrations of
reagents and product at equilibrium (“tuneable equi-
librium”). In this rare case, any fine variation of the
coordination environment can have a quantifiable effect
on the equilibrium position and, hence, can be rational-
ized. Within this area, pioneering studies were per-
formed by using mostly Ir(I) Vaska’s complexes.4 Lately,
* Corresponding author. Fax: +39-081674090. E-mail: ruffo@unina.it.
(1) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Book: Mill Valley, CA, 1987.
(2) Stille, J . K.; Law, K. S. Y. J . Am. Chem. Soc. 1976, 98, 5841-
5849.
(3) Semmelhack, M. F. Org. React. 1972, 19, 115-198.
(4) Mondal, J . U.; Blake, D. M. Coord. Chem. Rev. 1982, 82, 205-
238. For other relevant examples on d8 ions, see also: (a) Amatore,
C.; Gamez, S.; J utand, A. J . Organomet. Chem. 2001, 624, 217-222.
(b) Scollard, J . D.; Day, M.; Labinger, J . A.; Bercaw, J . E. Helv. Chim.
Acta 2001, 84, 3247-3268. (c) Osakada, K.; Hataya, K.; Yamamoto,
T. Inorg. Chim. Acta 1997, 259, 203-211. (d) Fanizzi, F. P.; Natile,
G.; Lanfranchi, M.; Tiripicchio, A.; Laschi, F.; Zanello, P. Inorg. Chem.
1996, 35, 3173-3182. (e) Zamachshikov, V. V.; Litvinenko, S. L.;
Mitchenko, S. A.; Pryadko, O. N. Metalloorg. Khim. 1992, 5, 1272-
1279, CAN 119:226154.
(5) (a) Albano, V. G.; Castellari, C.; Monari, M.; De Felice, V.;
Panunzi, A.; Ruffo, F. Organometallics 1996, 15, 4012-4019. (b) De
Felice, V.; Panunzi, A.; Ruffo, F.; Åkermark, B. Acta Chem. Scand.
1992, 46, 499-500.
(6) (a) Puddephatt, R. J .; Rendina, L. M. Chem. Rev. 1997, 97, 1735-
1754. (b) Levy, C. J .; Puddephatt, R. J . J . Am. Chem. Soc. 1997, 119,
10127-10136. (c) Levy, J .; Puddephatt, R. J .; Vittal, J . J . Organome-
tallics 1994, 13, 1559-1560.
(7) Kuyper, J . Inorg. Chem. 1977, 16, 2171-2176.
(8) Albano, V. G.; Monari, M.; Orabona, I.; Panunzi, A.; Ruffo, F. J .
Am. Chem. Soc. 2001, 123, 4352-4353.
(9) Ruffo, F. Unpublished results.
(10) De Castro, C.; Giordano, F.; Molinaro, A.; Orabona, I.; Ruffo,
F. Carbohydr. Res. 2002, 337, 651-655, and references therein.
(11) (a) Dewar, M. J . S. Bull. Soc. Chim. Fr. 1951, 18, C71-C79.
(b) Chatt, J .; Duncanson, L. A. J . Chem. Soc. 1953, 2939-2947.
10.1021/om020401i CCC: $22.00 © 2002 American Chemical Society
Publication on Web 07/19/2002

3504 Organometallics, Vol. 21, No. 17, 2002
Communications
Ta ble 1. Equ ilibr iu m Con sta n ts (Ka d d ) of th e
Ad d ition s of RSe-SeR Electr op h iles to Com p lexes
of Typ e 3 (Sch em e 1)
a
entry
type 3 compound
RSe-SeR
product
Kadd
1
2
3
4
5
6
3a
3a
3b
3c
3d
3d
MeSe-SeMe
PhSe-SePh
PhSe-SePh
PhSe-SePh
MeSe-SeMe
PhSe-SePh
4a Meb
4a P h d
4bP h
4cP h
4d Me
4d P h
c
F igu r e 1.
10 ( 2
8 ( 2
5 ( 1
5 ( 1
e
Sch em e 1
a
At 293 K in CDCl3 solutions. After several hours in solution,
b
signals of decomposition products become detectable. Attempts
to isolate the product were unsuccessful. c No appreciable amounts
d
of product 4a Me at equilibrium. By monitoring the addition at
Sch em e 2
different temperatures (293-314 K), it was also estimated ∆H )
14 ( 2 kcal mol-1 and ∆S ) -44 ( 6 cal mol-1 K-1 e Quantitative
.
addition, i.e., no appreciable amounts of 3d and PhSe-SePh at
equilibrium.
Information). The proton and carbon spectra reveal the
presence of the hydrocarbyl ligands with chemical shifts
and coupling constants to 195Pt in line with the proposed
structures. Complexes of type 3 were reacted with
MeSe-SeMe or PhSe-SePh in dichloromethane (Scheme
1), and the corresponding red products (4) could be
isolated in high yield.18,19 The equivalence of the orga-
noselenide fragments and the chemicals shifts of the
Pt-Me (>1 ppm)20 and -SePh (see below) protons in
the 1H NMR spectra indicated selective formation of the
isomer with the organoselenide ligands mutually trans.
Remarkably, signals of the Pt(IV) products were
accompanied by those of the parent compounds; that is,
complexes of type 4 were partially dissociated into the
reactants. Spectra were recorded at regular intervals
of time until an equilibrium composition was reached.21
Thus, according to our preliminary assumption, tune-
able equilibria were established in solution. The corre-
sponding equilibrium constants Kadd (Table 1) could be
evaluated by integrating suitable separated peaks. An
analysis of the results discloses the following significant
features.
properties tailored to match those of type 1 Pt(0)
complexes containing fumaric esters, with the expecta-
tion that an appropriate choice of R′ and R′′ may allow
the establishment of an unprecedented tuneable equi-
librium between the oxidation states II and IV according
to Scheme 1.
In this preliminary report we wish to communicate
the successful achievement of the target, accomplished
through the “ad hoc” synthesis of new platinum(II)
compounds. The tuneable oxidative addition of diphenyl
diselenide12 and dimethyl diselenide has been investi-
gated, and the results have been usefully compared with
those previously obtained with the couple Pt(0)/Pt(II).8
In principle, Pt(II) complexes of formula [Pt(CH2-
CO2R′′′)2(N,N-chelate)] would be the most effective
candidates for mimicking fumaric ester Pt(0) precursors
of type 1. However, the lack of a convenient synthesis
prompted us to use the alternative preparation of mixed
R′,R′′ platinum(II) complexes (3 in Scheme 2).
In these species, the electron-withdrawing substitu-
ents (CO2R′′′) are placed on the same carbon atom. The
choice of R′-R′′′ is aimed to disclose significant elec-
tronic and steric effects on the reaction.
The chelate was 4,4′-(t-Bu)2-6,6′-bipyridine, which has
been elegantly adopted by Puddephatt14 for assisting
the solubility of Pt(II) derivatives.
Complexes of type 3 could be prepared in high yield
by treating the halo precursors14 with the appropriate
malonic derivative under basic conditions (Scheme
2).15-17 Characterization was carried out through NMR
spectroscopy and elemental analysis (see Supporting
(16) A compound related to 3d has been prepared some years ago
in our laboratory for a different purpose: Albano, V. G.; Monari, M.;
Orabona, I.; Ruffo, F.; Vitagliano, A. Inorg. Chim. Acta 1997, 265, 35-
44.
(17) Synthesis of 3d : a stirred solution of [PtClMe{4,4′-(t-Bu)2-6,6′-
bipy}]14 (0.20 g, 0.39 mmol) and KI (0.19 g, 1.2 mmol) in dimethylfor-
mamide (2.5 mL) was heated at 373 K for 5 min. After the orange
solution was cooled at 313 K, dimethylmalonate (0.97 mmol) and
anhydrous K2CO3 (0.11 g, 0.78 mmol) were added, and the mixture
was stirred for 24 h at 313 K. The solvent and excess malonate were
then removed in vacuo, and the resulting solid was extracted with
chloroform. Removal of chloroform in vacuo and washing with hexane
gave the product as a yellow solid (yield: 75%). Selected 1H [13C] NMR
data (δ, 295 K): 10.03 (d, 1H, NCH), 9.01 (d, 1H, 3J Pt-H ) 38 Hz, NCH),
4.20 (s, 1H, 2J Pt-H ) 133 Hz, [20.4 (1J Pt-C ) 586 Hz)], Pt-CH), 3.59 (s,
2
3H, OMe), 1.09 (s, 3H, J Pt-H) 83 Hz, Pt-Me).
(18) The octahedral products of type 4 are indicated by a letter
(recalling the parent type 3 compound), followed by Me or Ph according
to the substituent on Se.
(19) Synthesis of 4d P h : a solution of diphenyl diselenide (0.20 mmol)
in dichloromethane (0.5 mL) was added with stirring to a solution of
3d (0.20 mmol) in the minimum amount of dichloromethane. After 5
min the solvent was removed in vacuo, and the oily residue grinded
with ether. The orange microcrystalline solid was washed with diethyl
ether and dried in vacuo (yield ) 80%). Selected 1H NMR data (δ, 295
(12) Previous literature reports that diphenyl diselenide quantita-
tively adds to the electron-rich platinum(II) complexes [PtMe2(N,N-
chelate)] (N,N-chelate
)
1,10-phen,13a 2,2′-bipy13b), affording the
corresponding octahedral products [Pt(SePh)2Me2(N,N-chelate)]. The
oxidation is reasonably favored by the presence of a substantial charge
density on the metal precursors.
3
K): 10.44 (d, 1H, NCH), 8.69 (d, 1H, J Pt-H ) 22 Hz, NCH), 6.58 (t,
2H, 4H-Ph), 6.45 (d, 4H, 2,6H-Ph), 6.35 (t, 4H, 3,5H-Ph), 4.10 (s,
2
2
(13) (a) Canty, A. J .; J in, H.; Skelton, B. W.; White, A. H. Inorg.
Chem. 1998, 37, 3975-3981. (b) Aye, K. T.; Vittal, J . J .; Puddephatt,
R. J . J . Chem. Soc., Dalton Trans. 1993, 1835-1839.
(14) Rendina, L. M.; Vittal, J . J .; Puddephatt, R. J . Organometallics
1995, 14, 1030-1038.
(15) Newkome, G. R.; Theriot, K. J .; Fronczek, F. R.; Villar, B.
Organometallics 1989, 8, 2513-2523.
1H, J Pt-H ) 93 Hz, Pt-CH), 3.81 (s, 6H, OMe), 1.85 (s, 3H, J Pt-H
69 Hz, Pt-Me).
)
(20) De Felice, V.; De Renzi, A.; Giovannitti, B.; Panunzi, A.;
Tesauro, D. J . Organomet. Chem. 2000, 593-594, 445-453.
(21) This took a few minutes when the samples were exposed to
light, or several hours in the dark, thus suggesting the free radical
nature of the reaction.

Communications
Organometallics, Vol. 21, No. 17, 2002 3505
(i) Diphenyl diselenide is a stronger oxidant than
dimethyl diselenide (cf. entries 1 and 2 or 5 and 6). Two
effects cooperate to stabilize the octahedral products
bearing phenylselenide fragments. First, phenyl is a
better electron acceptor than methyl, and this facilitates
the formal reduction of selenium, which accompanies
the reaction. Furthermore, an interaction between the
aromatic rings of the chelate and the phenyl group on
Se is established in solution, as indicated by the
former ones are more easily oxidized, as expected due
to their low formal charge. For instance, it was previ-
ously found8 that dimethyl diselenide adds to [Pt(N,N-
chelate)(E-MeO2CCHdCHCO2Me)] with Kadd ) 1.0 ×
103, while the reaction is much more shifted toward the
reactants when 3d (entry 5) is used as Pt precursor (Kadd
) 5). The same Pt(0) complex reacts quantitatively with
diphenyl diselenide,8 while a “tuneable” equilibrium is
established when the electrophile is added to 3a (entry
2). The contribution, although little, of the limit form
II (Figure 1) is plausibly responsible for the higher
basicity of the Pt(0) complexes, because in this structure
the metal center is not depleted of electrons and retains
the low oxidation state.
In conclusion, this preliminary investigation has
disclosed the first reversible cleavage of an E-E bond
(E ) chalcogen) by a Pt(II) precursor,12 which is
prompted by a careful choice of the electronic and steric
properties of the ligands. This result can be considered
as a substantial upgrade of the knowledge4-8 of the
thermodynamics of oxidative addition to platinum group
metals. Further investigations by using modified type
3 compounds are currently in progress.
1
significant high-field shift of the Ph protons in the H
NMR spectrum. This type of interaction, which had been
previously disclosed within related works,8,13,22 may play
an important role in stabilizing the Pt(IV) product.
(ii) An electronic effect is also observed when the
methyl on platinum is substituted for an aryl (e.g., cf.
entries 2 and 6). Diphenyl diselenide quantitatively adds
to the methyl precursor 3d . On the other hand, the more
effective electron-withdrawing ligand 4-MeO-C6H4 sta-
bilizes the lower oxidation state, and, hence, the addi-
tion of diphenyl diselenide to 3a is not complete (Kadd
) 10, entry 2).
(iii) A more subtle effect is observed when a fine
variation of ligands is carried out (cf. entries 2, 3, and
4); that is, the addition becomes slightly less favored
on changing the ester substituents R′′′ from methyl to
isopropyl. This fine influence can be explained by
assuming that a moderate increase of the steric hin-
drance perturbs the axial positions of the octahedron
and, hence, disfavors the reaction.
Ack n ow led gm en t. The authors thank the MURST
(Programmi di Ricerca Scientifica di Rilevante Interesse
Nazionale, Cofinanziamento 2000-2001) and the Centro
Interdipartimentale di Metodologie Chimico-Fisiche,
Universita` di Napoli ”Federico II”, for NMR facilities.
(iv) A comparison between the behavior of related Pt-
(0)8 and Pt(II) complexes discloses a great similitude,
provided a careful choice of the ligands. However, the
Su p p or tin g In for m a tion Ava ila ble: Text giving syn-
thetic details and characterization data for the new complexes.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(22) J anzen, M. C.; J ennings, M. C.; Puddephatt, R. J . Can. J . Chem.
2002, 80, 41-45.
OM020401I