Two geometrical isomers of the five-coordinate Platinum(II) complex [PtBr(SePh)(2,9-dimethyl-1,10-phenanthroline)(dimethylmaleate)]

Article abstract of DOI:10.1016/j.ica.2004.12.022

The oxidative addition of phenylselenium bromide to three-coordinate Pt(0) complex [Pt(2,9-dimethyl-1,10-phenanthroline)(dimethylmaleate)] affords the corresponding five-coordinate Pt(II) complex having trigonal-bipyramidal coordination geometry. The prod

Full text of DOI:10.1016/j.ica.2004.12.022

Inorganica Chimica Acta 358 (2005) 2112–2116
www.elsevier.com/locate/ica
Note
Two geometrical isomers of the five-coordinate Platinum(II)
complex [PtBr(SePh)(2,9-dimethyl-1,10-phenanthroline)
(dimethylmaleate)]
*
Roberto Centore , Giuseppina Roviello, Angela Tuzi
`
Dipartimento di Chimica, Universita di Napoli ‘‘Federico II’’, Complesso Universitario di Monte S. Angelo, via Cinthia, Napoli I-80126, Italy
Received 24 July 2004; accepted 28 December 2004
Available online 29 January 2005
Abstract
The oxidative addition of phenylselenium bromide to three-coordinate Pt(0) complex [Pt(2,9-dimethyl-1,10-phenanthroline)(dim-
ethylmaleate)] affords the corresponding five-coordinate Pt(II) complex having trigonal-bipyramidal coordination geometry. The
product of the reaction exists as two geometrical isomers (rotamers): in the kinetically favoured compound the olefin substituents
are on the same side of the bromide ligand, while the most thermodynamically stable isomer holds the same substituents pointing at
the phenylselenenide ligand. The crystal structure of the two isomers is reported and discussed with respect to the reaction mech-
anism and thermodynamic stability.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Platinum; Selenium; Geometrical isomers; Crystal structure; Hydrogen bonding
1. Introduction
the energy barrier for rotation around the metal to olefin
bond, which is involved in the isomerization process be-
tween the two; the barrier of rotation, in turn, is related
to several features of the complex (nature of the coordi-
nated olefin, of the axial ligands, of the equatorial biden-
tate nitrogen ligand, etc. [1b]).
Actually, the formation of both rotamers has been
effectively recognized in several cases [2], but always
on the basis of NMR analysis (different chemical shifts
of olefin protons); in some cases, also the stereochemis-
try and the molar ratio of the two rotamers were as-
signed by careful analysis and correlation of NMR
spectra within homogeneous classes of five-coordinate
compounds. However, in the absence of full structural
characterization of both rotamers for a given complex,
no explanation of the different thermodynamic stability
can be definitively proposed.
Oxidative addition of electrophylic reagents to three-
coordinate Pt(0) complexes of the type [Pt(N,N-che-
late)(olefin)], with N,N bidentate nitrogen ligand, can
afford five-coordinate Pt(II) complexes having trigonal-
bipyramidal coordination geometry [1a,1b]. With olefin
lacking C2h or D2h symmetry, and the axial ligands being
different, two stereoisomers can form upon this reaction
(two enantiomeric couples if the nitrogen atoms of the
equatorial ligand are not equivalent by symmetry) [1b].
Moreover, these are rotamers which differ by rotation
of 180° around the metal to olefin bond. The favourable
chance of observing and eventually isolating both rota-
mers depends on several factors. Among these, we may
cite the mechanism of the reaction which, as an example,
could favour one of the two isomers over the other, and
Recently, there has been some interest in the oxida-
tive addition of Se–Se and X-Se (X = halogen) to plati-
num(0) and platinum(II) complexes of general formula
*
Corresponding author. Tel.: +39 081 674450; fax: +39 081 674090.
E-mail address: roberto.centore@unina.it (R. Centore).
0020-1693/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.12.022

R. Centore et al. / Inorganica Chimica Acta 358 (2005) 2112–2116
2113
[Pt(N,N-chelate)(olefin)] and [PtR(Rꢀ)(N,N-chelate)] (R,
Rꢀ = hydrocarbyl group). These reactions afford, respec-
tively, five coordinate Pt(II) and octahedral Pt(IV) com-
pounds [3,4].
In this communication, we discuss some structural
data concerning the products of the oxidative addition
of phenylselenenium bromide to the three-coordinate
complex [Pt(2,9-dimethyl-1,10-phenanthroline)(dimeth-
ylmaleate)]. This reaction affords two stereoisomers
(rotamers) of the five-coordinate platinum(II) complex
[PtBr(SePh)(2,9-dimethyl-1,10-phenanthroline) (dimeth-
ylmaleate)], shown in Fig. 1.
reported in Table 1. All crystallographic data have been
deposited with the Cambridge Crystallographic Data
Centre (CCDC). Deposition numbers are CCDC
245555 (for I) and CCDC 245556 (for II). These data
can be obtained free of charge at www.ccdc.cam.ac.uk/
conts/retrieving.html.
3. Results and discussion
The oxidative addition of phenylselenenium bromide
to the three-coordinate complex [Pt(2,9-dimethyl-1,10-
phenanthroline)(dimethylmaleate)] gives as the first
product the rotamer in which the olefin substituents
are on the same side of the bromide ligand (rotamer I)
as it can be seen from Fig. 2 in which the X-ray molec-
ular structure is shown.
Both have been isolated and crystallized and fully
characterized by single crystal X-ray analysis.
2. Experimental
If complex I is left in solution at room temperature,
after few days conversion to rotamer II is achieved, in
which the olefin substituents are on the same side of
The complex [PtBr(SePh)(2,9-dimethyl-1,10-phenan-
throline)(dimethylmaleate)] was obtained by reaction
of equimolar amounts of [Pt(2,9-dimethyl-1,10-phenan-
throline)(dimethylmaleate)] and Ph–Se–Br in chloro-
form. A detailed synthetic account is given in [4].
Single crystals were obtained from chloroform solu-
tions, respectively, by cooling at ꢀ20 °C for I and by
slow evaporation at room temperature for II. An En-
raf–Nonius MACH3 diffractometer was used for data
collection (graphite monochromated Mo Ka radiation,
1
the phenylselenenide ligand (Fig. 3). Clearly, rotamer
I is the kinetically favoured product of the reaction
and rotamer II is the thermodynamically most stable
one. The transformation from I to II can be monitored
in solution by NMR spectroscopy [4].
Molecular structures of I and II share several fea-
tures. In both cases, the coordination around platinum
is trigonal bipyramidal, with the olefin and the bidentate
nitrogen ligands occupying the equatorial plane and
bromide and phenylselenide ligands in axial position.
Within the coordination sphere of the metal, bond
lengths and angles are substantially equivalent in the
two structures and fall in the ranges reported in the lit-
erature for typical bipyramidal trigonal platinum(II)
complexes. The orientation of the phenyl ring with re-
spect to the equatorial heteroaromatic ligand is also sim-
ilar in the two rotamers, and is determined by the
valence angle at Se, by the bond C15–Se almost eclipsing
the Pt–N2 bond, and by torsion around the bond C15–
Se (cf. Figs. 2 and 3). For what concerns the coordinated
olefin, the values observed for the bond length C21–C22
are in fair agreement between the two structures and
comparable with those found in complexes of similar
geometry. A significant bending away from Pt of
the four substituents at the double bond is observed.
˚
k = 0.71069 A). Cell parameters were obtained from
least-squares fit [5] of the h angles of 25 reflections in
the range 7.845° 6 h 6 10.865° for I and 10.289° 6
h 6 11.669° for II. Absorption correction was based
on w scans for I and on DIFABS method for II. Struc-
tures were solved by the Patterson method, completed
by cycles of direct Fourier synthesis and refined by the
full matrix least-squares method [6]. Refinement was
on F2 against all independent reflections. C, O, N, Cl,
Pt, Br and Se atoms were anisotropic. H atoms of phe-
nyl and heteroaromatic rings were introduced in calcu-
lated positions; H atoms of methyl and olefin groups
were found in difference Fourier syntheses. Coordinates
of alkene H atoms were refined (Uiso = Ueq of the carrier
atom) without constraints for I and with some restraints
for II; all other H atoms were refined by the riding model.
The largest peaks and holes in the last Fourier difference
were (e Aꢀ3): 0.83, ꢀ0.82 for I and 1.240 and ꢀ1.369 for
˚
II. Some crystal, collection and refinement data are
1
It has not escaped our attention the following unusual feature of
the crystal structure of rotamer II. Most of the electronic density of the
molecule (Pt, Br and Se atoms, the equatorial dinitrogen ligand and
the two olefin carbons) corresponds to a point symmetry quasi C2v. In
the crystal, molecules are placed in the cell in such a way that pseudo
rv planes of the molecules almost coincide with the glide planes of the
P21/c space group. So, limitedly to this part of the molecule which,
however, accounts for a great part of the total electronic density, the
structure can be described in the space group P21 with c-axis halved.
As a consequence, reflections hkl of the diffraction pattern with odd l
have intensities systematically lower than those with even l.
Br
Pt
Br
Pt
CO2Me
CO2Me
N
N
N
N
CO2Me
CO2Me
SePh
I
SePh
II
Fig. 1. Chemical diagrams of the two rotamers

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R. Centore et al. / Inorganica Chimica Acta 358 (2005) 2112–2116
Table 1
Crystal, collection and refinement data for I and II
conformation is s-trans around the bond C21–C23 and
s-skew around C22–C25, in order to release the close
contact between alkoxyl oxygen atoms (O2ꢁ ꢁ ꢁO4 =
I
II
˚
2.94 A). In II the conformation is s-cis around the bond
Temperature (K)
Chemical formula
Crystal system
Space group
298
298
C26H25BrN2O4SePt Æ CHCl3
C22–C25 and s-skew around C21–C23 so as to release
the contact between carbonyl oxygen atoms (O1ꢁ ꢁ ꢁO3 =
triclinic
ꢀ
monoclinic
P21/c
P1
9.72(2)
˚
3.07 A).
˚
a (A)
11.14(1)
11.363(8)
23.97(3)
90
The first noteworthy feature is the selectivity of the
reaction of oxidative addition. Actually, rotamer I is
formed selectively during the reaction, no trace of II
being observed in the ‘‘fresh’’ NMR spectrum of the reac-
tion product as obtained, in substantially quantitative
yield, after few minutes of reaction. That feature could
be related to the reaction mechanism. In particular, in
the starting three-coordinate Pt(0) complex the regions
above and below the coordination plane of platinum have
different sterical encumbrance, the region containing ole-
fin protons being less sterically hindered; so, the selective
formation of rotamer I could point to a stereocontrol of
the reaction driven by the phenylselenenide ligand in-
stead of the bromide ligand. Radical chain mechanism
[9], as well as non radical SN2 type mechanism [10], is
compatible with the observed experimental behaviour.
The second feature is concerned with the different
thermodynamical stability of the two rotamers, and in
particular with the reasons why rotamer II is more sta-
ble than I. In order to get inside this question, we ob-
serve preliminarily that since the interconversion of the
rotamers takes place in solution, the different stability
of the twoꢀs is not related to crystal packing effects,
but it must come from intramolecular effects. Looking
˚
b (A)
12.593(3)
13.978(2)
63.34(2)
86.31(4)
78.93(6)
1500(3)
˚
c (A)
a (°)
b (°)
c (°)
101.4(1)
90
3
˚
V (A )
2959(2)
4, 2.027
7.630
Z, Dx (g/cm3)
2, 1.999
7.528
l (mmꢀ1
h Range
)
1.63–27.99°
7232/385
0.0439
1.74–27.98°
6862/358
0.0471
Data/parameters
R1, on F(I > 2r(I))
R1, wR2 (all data) on F2
P
0.1027,0.0821
ꢁ
0.1245, 0.1263
ꢀ
ꢂ
ꢃ
P
1=2
2
kF ojꢀjF c
k
w
F 2oꢀF 2c
ð
Þ
P
ꢀ
ꢁ
P
R1 ¼
;
wR2 ¼
.
2
jF o
j
w
ð
F 2
oÞ
A measure of this is given by the two torsion angles
H21–C21–C22–C25 (ꢀ145° for I and 136° for II) and
H22–C22–C21–C23 (151° for I and ꢀ128° for II), which
indicate a significant sp3 character for the olefin carbons
[1b,7]. This feature and the significant elongation of the
olefinic carbon–carbon bond with respect to the length
˚
of the same bond in the free olefin (1.331 A [8]) can be
considered a measure of the p-backdonation from the
metal to the olefin [1b]. The conformation of the dimeth-
ylmaleate ligand is different in the two rotamers. In I the
Fig. 2. X-ray molecular structure of I. Thermal ellipsoids are drawn at 30% probability level. The chloroform solvent molecule is not shown for
˚
clarity. Selected bond lengths (A), bond angles (°) and torsion angles (°): Pt–N1 2.185(7), Pt–N2 2.177(6), Pt–Se 2.450(2), Pt–Br 2.526(2), C21–C22
1.45(1), C21–C23 1.49(1), C22–C25 1.51(1), N1–Pt–N2 76.2(2), Se–Pt–N1 88.7(2), Se–Pt–N2 93.3(2), Se–Pt–Br 171.54(3), Pt–Se–C15 105.4(2), C20–
C15–Se–Pt -79.7(7) N2–Pt–Se–C15 ꢀ6.8(3), C22–C21–C23–O1 169.7(7), C21–C22–C25–O3 ꢀ102(1).

R. Centore et al. / Inorganica Chimica Acta 358 (2005) 2112–2116
2115
˚
H22ꢁ ꢁ ꢁBr2 = 3.03 A) and in the range reported in the lit-
erature [12c,13], so it is reasonable to think that this
interaction could play a role in stabilizing II with respect
to I. A further experimental finding, not in contrast with
this picture, consists in the observation that in all five-
coordinated Pt(II) olefin complexes reported in the liter-
ature, which give this type of stereoisomerism and have
one halogen as axial ligand, and for which both rota-
mers have been observed in solution, the most abundant
rotamer is the one having more olefin hydrogens from
the side of the halogen ligand [2d,2e,2f].
Finally, we think that the possible implications that
the interaction Calkene–Hꢁ ꢁ ꢁX (X = halogen) could play
in some important catalytic processes, whose key step
involves coordination of an olefin molecule to alkyl-hal-
ogen complexes of low valence transition elements, are
worth of being investigated.
Fig. 3. X-ray molecular structure of II. Thermal ellipsoids are drawn
at 30% probability level. The chloroform solvent molecule is not
˚
shown for clarity. Selected bond lengths (A), bond angles (°) and
torsion angles (°): Pt–N1 2.190(7), Pt–N2 2.189(7), Pt–Se 2.461(3), Pt–
Br 2.518(3), C21–C22 1.48(1), C21–C23 1.49(1), C22–C25 1.46(1), N1–
Pt–N2 75.5(3), Se–Pt–N1 88.4(2), Se–Pt–N2 91.0(2), Se–Pt–Br
177.17(4), Pt–Se–C15 106.8(3), C20–C15–Se–Pt ꢀ99.9(9), N2–Pt–Se–
C15 ꢀ9.6(4), C22–C21–C23–O1 86(1), C21–C22–C25–O3 2(2).
Acknowledgements
at the X-ray molecular structure of the rotamers (Figs. 2
and 3), no relevant difference is found in the pattern of
bond lengths and angles, as well as in the coordination
sphere of the metal, as already noted, so the different
stability should probably derive from secondary, non-
bonded interactions. In particular, we have noted above
that the relative position of the p conjugated systems of
the phenyl and of the heteroaromatic chelate ligand is
almost the same in the two rotamers and no feature of
steric encumbrance is evident in the two compounds
so as to allow for their different stability. In this respect,
we observe that any different effect of steric crowding be-
tween I and II should necessarily involve contacts
among olefin substituents and the axial ligands; how-
ever, van der Waals radii of Br and Se are very close
Financial support of MIUR of Italy is acknowledged.
We thank the CIMCF of the University of Naples
‘‘Federico II’’ for the Nonius X-ray equipment.
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