Oxidative additions of E-E bonds (E = chalcogen) to group 10 metals: "Tunable" cleavage of Se-Se bonds by Pt(0) complexes [9]

Full text of DOI:10.1021/ja005870v

4352
J. Am. Chem. Soc. 2001, 123, 4352-4353
Table 1. Thermodynamic Parameters for the Addition of
R′Se-SeR′ Electrophiles to Complexes of Type I at 295 K
in CDCl3
Oxidative Additions of E-E Bonds (E ) Chalcogen)
to Group 10 Metals: “Tunable” Cleavage of Se-Se
Bonds by Pt(0) Complexes
a
b
c
type I complex
electrophile pKadd
∆Hadd
∆Sadd
Vincenzo G. Albano,*,† Magda Monari,† Ida Orabona,‡
Achille Panunzi,*,‡ and Francesco Ruffo‡
[Pt(dmphen)(iPr-fu)]
[Pt(dmphen)(Et-fu)]
[Pt(dmphen)(Me-fu)]
MeSe-SeMe -3.4 -15.0 ( 0.3 -36 ( 1
MeSe-SeMe -3.3 -15.5 ( 0.6 -38 ( 2
MeSe-SeMe -3.0 -13.5 ( 0.6 -31 ( 2
[Pt(dmphen)(MePh-fu)] MeSe-SeMe -2.8
[Pt(dmphen)(Ph-fu)] MeSe-SeMe -2.7
Dipartimento di Chimica “G. Ciamician”
UniVersita` di Bologna, Via Selmi 2, I-40126 Bologna, Italy
Dipartimento di Chimica, UniVersita` di Napoli “Federico II”
Complesso UniVersitario di Monte S. Angelo
[Pt(dmphen)(ClPh-fu)] MeSe-SeMe -2.5
[Pt(dmphen)(Me-fu)]
PhSe-SePh
d
d
[Pt(dmphen)(ClPh-fu)] PhSe-SePh
Via Cintia, I-80126 Napoli, Italy
a The estimated errors are less than (0.2. b In kcal‚mol-1 c In
.
cal‚mol-1‚K-1 d The addition is quantitative.
.
ReceiVed December 12, 2000
Oxidative addition and reductive elimination reactions are of
utmost importance in organometallic chemistry,1 and the knowl-
edge of the factors ruling these processes is extremely useful. A
comprehensive study can be carried out only in the presence of
“tunable” equilibria involving detectable concentrations of both
reagents and products. In fact, in these cases fine variations of
electronic or steric properties have measurable effects on equi-
librium and hence can be rationalized. On the other hand, very
few examples of reversible and tunable addition-elimination
reactions are known,2 since these are in most cases totally shifted
toward either the reagents or the products. Furthermore, equilib-
rium is often prevented by the occurrence of other processes, such
as ligand loss,3 insertion,4 or competing eliminations.5
Scheme 1
Herein, we communicate the first example of tunable oxidative
addition of an E-E bond (E ) Se) to platinum(0) complexes.
When a yellow chloroform solution of a fumaric ester complex
of type I is treated with a 100% molar excess of either MeSe-
SeMe or PhSe-SePh (Scheme 1), the color of the reacting mixture
rapidly turns orange-red. Analytically pure products of type II,
which are the first saturated species containing Pt(II)-SeR
fragments,8 can be crystallized by careful addition of n-pentane.
NMR spectra in CDCl3 of the five-coordinate compounds disclose
that an equilibrium is established in solution because the products
are partially dissociated into the reactants. The corresponding pKadd
constants can be evaluated by integrating suitable separated peaks
in the proton NMR spectrum at 295 K. Six fumaric esters have
been employed, i.e., dimethylfumarate (Me-fu), diethylfumarate
(Et-fu), diisopropylfumarate (iPr-fu), diphenylfumarate (Ph-fu),
di(4-methylphenyl)fumarate (MePh-fu), and di(4-chlorophenyl)-
fumarate (ClPh-fu). A clean relationship between the extent of
the addition and the electronic properties of the olefins has been
found. The equilibrium constants for the addition of MeSe-SeMe
to type I complexes (Table 1) decrease with increasing electron-
withdrawing properties of the coordinated olefins. The razional-
ization is in terms of lowering of the electronic density on the
metal centers and, consequently, of its basicity.
We have recently shown2c,e,6 that three-coordinate platinum-
(0) complexes of the type [Pt(N,N-chelate)(olefin)] (I) readily
undergo addition by a variety of electrophiles A-B to give five-
coordinate type II products, according to eq 1:
[Pt(N,N-chelate)(olefin)] (I) + A-B )
[Pt(A)(B)(N,N-chelate)(olefin)] (II) (1)
where (N,N-chelate ) 2,9-Me2-1,10-phenanthroline; A-B)
X-SnRnX3-n, X-HgR, Cl-GeRnCl3-n, Cl-PbR2Cl; X ) halides,
R ) alkyls). These reactions have been revealed to be particularly
suitable for the study of addition-elimination processes. In fact,
sterically crowded nitrogen chelates (e.g., dmphen) 2,9-Me2-1,10-
phenanthroline) stabilize the five-coordinate 18 e- products (II)
toward further rearrangements.7 Furthermore, both electronic and
steric features of the complexes can be finely varied by an accurate
choice of the olefin ligand. Thus, several trends concerning the
reversible oxidative addition of organo-tin2c-e and -mercury2c
halides have been rationalized in detail.
The ∆Sadd values, on the other hand, are negative according to
the stoichiometry of the addition and reflect the trend of molecular
crowding within the alkyl fumaric esters (Me-fu, Et-fu, and
iPr-fu). More precisely, the ∆Sadd for the Me-fu precursor is less
negative than that measured for the Et-fu or the iPr-fu analogues
(Table 1). This relates to the conformational freedoms of the
-COOR appendages in the Pt(0) precursors, which undergo
increasing motion restrictions in the five-coordinate derivatives
as the R groups grow bigger. The ∆Hadd values have been used
† University of Bologna.
‡ University of Napoli.
(1) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry; University Science
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1754. (b) Levy, C. J.; Puddephatt, R. J. J. Am. Chem. Soc. 1997, 119, 10127-
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A.; Ruffo, F. Organometallics 1996, 15, 4012-4019. (d) Levy, J.; Puddephatt,
R. J.; Vittal, J. J. Organometallics 1994, 13, 1559-1560. (e) De Felice, V.;
Panunzi, A.; Ruffo, F.; Åkermark, B. Acta Chem. Scand. 1992, 46, 499-
500. (f) Kuyper, J. Inorg. Chem. 1977, 16, 2171-2176.
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A. Eur. J. Inorg. Chem. 2000, 1717-1721. (b) Albano, V. G.; Monari, M.;
Ferrara, M. L.; Panunzi, A.; Ruffo, F. Inorg. Chim. Acta 1999, 285, 70-75.
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for an unprecedented evaluation of the Pt-Se bond energy. By
assuming that ∆Hadd(av)9 ) 2∆HPt-Se - ∆HSe-Se
,
a value of
10
-33 kcal/mol was found for ∆HPt-Se. On changing the hydro-
carbyl group bonded to Se pronounced effects are observed on
the equilibrium. While the addition of MeSe-SeMe to the Pt(0)
precursors used in this work produces tunable equilibria, PhSe-
(8) On the other hand, square-planar complexes containing Pt(II)-Se bonds
are known by far: Kawakami, K.; Ozaki, Y.; Tanaka, T. J. Organomet. Chem.
1974, 69, 151-159.
(9) ∆Hadd(av) is the average value among those reported in Table 1.
(10) A value of -51 kcal/mol was adopted for ∆HSe-Se, which is the average
of several calculated values taken from the following: Maung, N.; Williams,
J. O.; Wright, A. C. THEOCHEM 1998, 453, 181-189.
10.1021/ja005870v CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/12/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 18, 2001 4353
rings are oriented away from the -COOMe groups of the
fumarate ligand and approximately overlap, from opposite sides,
both pyridinic rings of dmphen [average interplanar angle 17.3°].
The molecule would conform to an idealized C2 symmetry was
it not for the conformations of the -COOMe groups. The
orientations of the phenyl rings with respect to the dmphen ligand
are similar to those found in the two reported polymorphs of
[Pt(Me)2(SePh)2(1,10-phen)]11,13 and indicate specific interactions
between the ring dipoles. It is worth noting the stereochemical
resemblance of this molecule to that of the just cited dimethyl-
platinum(IV) derivative,11,13 despite the different oxidation states
of Pt. The similarity seems to be more than formal and points to
a description of the platinum-fumarate bond to the limit of a
metallacyclopropane ring.
In conclusion, an unprecedented addition-elimination equi-
librium has been discovered. This finding is of interest because
organic chalcogenides -ER are attracting a great deal of attention
as ligand fragments,14 and the close similarity between products
of type II and the intermediates invoked in the Pd-catalyzed
addition of E-E bonds (E ) chalcogen) to unsaturated hydro-
carbons.15 The equilibrium can be tuned by varying electronic
and steric properties of the ligands, according to trends which
have found reasonable explanation. Extension of this study to
other dichalcogenides is in progress, although to date, no O-O,
S-S, and Te-Te bond has been found to establish tunable
equilibria with precursors of type I. The same held true when
the additions were performed11,13 on Pt(II) species yielding Pt-
(IV) products, thus confirming the rarity of tunable equilibria in
addition-elimination reactions.
Figure 1. (a) ORTEP drawing of the molecule [Pt(SePh)2(dmphen)-
(Me-fu)]. Selected bond lengths (Å) and angles (deg): Pt-C(1) 2.090,
Pt-C(2) 2.092(4), C(1)-C(2) 1.438(6), Pt-N(1) 2.181, Pt-N(2) 2.192-
(3), Pt-Se(1) 2.5197, Pt-Se(2) 2.5142(5); Pt-Se-C 105.3 (av). (b) View
down the SePtSe axis showing the ring overlaps.
SePh completely adds even to the less basic [Pt(dmphen)(ClPh-
fu)]. This result can be attributed only in part to the electronic
properties of the SeMe and SePh ligands. A determining contribu-
tion to the stability of the SePh derivatives, as pointed out by the
solid-state structure of [Pt(SePh)2(dmphen)(Me-fu)] (see hereaf-
ter), derives from stabilizing interactions between phenyl groups
and pyridine rings. According to NMR spectra, these interactions
are strong enough to be retained in solution, as demonstrated by
the shift of the PhSe- protons of type II products at higher field
(δ 6.35-5.95) with respect to those of the PhSe-SePh molecule.
The X-ray diffraction study of the representative species
[Pt(SePh)2(dmphen)(Me-fu)]12 has also been prompted by its close
similarity to the platinum(IV) derivatives [PtMe2(SePh)2(bipy)]11
and [PtMe2(SePh)2(1,10-phen)],13 whose structures were previ-
ously described. The molecular geometry is depicted in Figure
1a,b and exhibits the expected coordination of the SePh ligands
in the axial sites, while the equatorial plane is defined by the
olefin double bond and the chelate dmphen ligand. The phenyl
Acknowledgment. We thank the CNR, the MURST (Cofinanziamento
1998-2000, PRIN 9803243241), and the University of Bologna (project
“Selected research topics”) for financial support and the Centro Interdi-
partimentale di Metodologie Chimico-Fisiche, Universita` di Napoli
“Federico II” for NMR facilities.
Supporting Information Available: Preparation and spectroscopic
data for the products and tables of crystallographic data, atomic
coordinates, isotropic and anisotropic thermal parameters, and bond
lengths and angles for [Pt(SePh)2(dmphen)(Me-fu)] (PDF) as well as CIF
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
JA005870V
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(12) Crystal data: C32H30N2O4PtSe2, space group P21/n (No. 14), Mw
)
859.59, a ) 13.296(1) Å, b ) 11.8738(9) Å, c ) 19.282(2) Å, â ) 100.581-
(2)°, V ) 2992.4(4) Å3, Z ) 4, T )293(2) K, Dcalc ) 1.908 Mg/m3, µ )
7.162 mm-1, Mo KR with graphite monochromator, Bruker CCD diffrac-
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