Structure and fluxional behavior of cis-bis(stannyl)bis(phosphine)platinum: Oxidative addition of organodistannane to platinum(0) complex

Full text of DOI:10.1021/ja9626392

10922
J. Am. Chem. Soc. 1996, 118, 10922-10923
Structure and Fluxional Behavior of
cis-Bis(stannyl)bis(phosphine)platinum: Oxidative
Addition of Organodistannane to Platinum(0)
Complex
Yasushi Obora, Yasushi Tsuji,* Kayo Nishiyama,
Masahiro Ebihara, and Takashi Kawamura
Department of Chemistry, Faculty of Engineering
Gifu UniVersity, Gifu 501-11, Japan
ReceiVed July 30, 1996
Organodistannanes (1: R3Sn-SnR3) are useful reagents for
double-stannylations of 1,3-dienes,1a,b alkynes,1c and allenes1d
in the presence of a palladium-complex as a catalyst. In the
catalytic cycle, oxidative addition of the Sn-Sn σ-bonds is
believed to be a crucial step. However, there are only two
precedents for it, and the resulting bis(stannyl) complexes have
not been fully characterized. Clark and his co-worker described
the first example of the oxidative addition of organodistannane
(1a: Me3SnSnMe3) to Pt(CH2dCH2)(PPh3)2 or Pt(PPh3)4,2a but
their reported geometry of the adduct is not consistent with our
present study (Vide infra). The second is the oxidative addition
of the Sn-Sn bond of Ph2P(CH2)nMe2SnSnMe2(CH2)nPPh2 (n
) 2, 3) to Pt(PPh3)4 to afford a mixture of cis- and trans-
isomers.2b
Figure 1. Molecular structure of 3a. Hydrogen atoms are omitted
for clarity. The Pt atom is on a crystallographic 2-fold axis. Selected
bond distances (Å) and angles (deg): Pt1-Sn1 2.6289(6), Pt1-P1
2.299(1), Sn1-Pt1-Sn1′ 82.91(2), P1-Pt1-P1 104.23(6).
In this communication, we report structure and fluxional
behavior of cis-bis(stannyl)bis(phosphine)platinum complexes,
which were obtained by the oxidative addition of 1a to platinum-
(0) phosphine complexes (2). The products have a distorted
square-planar structure and show unique fluxional behavior.
These features were investigated by multinuclear (including solid
state 31P) NMR, X-ray crystallographic analysis, and ab initio
molecular orbital calculation.
Hexamethyldistannane (1a) reacts with tetrakis[tri(4-meth-
ylphenyl)phosphine]platinum (2a) at -30 °C to afford the
oxidative addition product (3a) (eq 1). The molecular structure
of 3a was determined by X-ray crystallographic analysis3 as
shown in Figure 1. The complex has a twisted square-planar
structure with cis-orientation of the ligands. The structure is
distinctly distorted from planarity: the dihedral angle between
the PtP2 and the PtSn2 planes is 34.6°.
The solution 31P NMR spectrum of 3a at -90 °C (Figure 2a)
showed resonances centered at 32.2 ppm with satellite peaks
due to 1JP-Pt (2645 Hz, not shown in the Figure), 2JP-Sn(transoid)
(1439 Hz with 117Sn and 1507 Hz with 119Sn),4 2JP-Sn(cisoid) (181
Hz; peaks due to 117Sn and 119Sn were not resolved), and 2JP-P
(13 Hz); only a central part of the spectrum is shown in Figure
Figure 2. Central part of 31P NMR spectra of 3a (161.70 MHz).
Solution NMR spectra measured in CD2Cl2 at (a) -90 °C, (b) -80
°C, (c) -50 °C, (d) -10 °C, (e) 20 °C, and (f) CPMAS solid-state
NMR spectrum measured at room temperature. Chemical shifts are
relative to H3PO4.
1
2 with omitting satellite peaks due to the JP-Pt coupling.
(1) (a) Obora, Y.; Tsuji, Y.; Kakehi, T.; Kobayashi, M.; Shinkai, Y.;
Ebihara, M.; Kawamura, T. J. Chem. Soc., Perkin Trans. 1 1995, 599. (b)
Tsuji, Y.; Kakehi, T. J. Chem. Soc., Chem. Commun. 1992, 1000. (c) Piers,
E.; Skerlj, R. T. J. Org. Chem. 1987, 52, 4421. (d) Mitchell, T. N.;
Schneider, U. J. Organomet. Chem. 1991, 407, 319.
(2) (a) Akhtar, M.; Clark, H. C. J. Organomet. Chem. 1970, 22, 233.
(b) Weichmann, H. J. Organomet. Chem. 1982, 238, C49.
(3) Crystal data of 3a. Monoclinic, space group C2/c, yellow, a )
19.369(7) Å, b ) 13.645(5) Å, c ) 18.056(3) Å, â ) 98.83(2)°, V ) 4715-
(2) Å3, Z ) 4, T ) -136 °C, dcalcd ) 1.594 g cm-3, µ(Mo KR) ) 40.96
cm-1, R ) 0.029, Rw ) 0.027, GOF ) 1.730.
(4) The ratio of the coupling constants coincides with that of the
gyromagnetic ratio4a of 117Sn and 119Sn, Viz., 0.956. (a) Brevard, C.;
Granger, P. Handbook of High Resolution Multinuclear NMR; John Wiley
& Sons: New York, 1981; p 168.
CPMAS solid-state 31P NMR measured at room temperature
(Figure 2f) shows the similar 2JP-Sn(transoid) (1500 Hz) and 1JP-Pt
(2700 Hz, not shown in the figure) values, although 2JP-Sn(cisoid)
coupling which is estimated to be ca. 180 Hz seems to be buried
in the line width (300 Hz) of the main peak. The solution NMR
spectra indicate that the complex shows reversible fluxional
behavior. On warming, satellite peaks due to the two kinds of
2JP-Sn coalesced at -50 °C (Figure 2c) with maintaining the
1JP-Pt coupling (2644 Hz, not shown) and then sharpened at 20
2
°C with the averaged JP-Sn (629 Hz with 117Sn and 663 Hz
S0002-7863(96)02639-X CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 44, 1996 10923
Table 1. 31P, 119Sn, and 195Pt NMR Data of 3a-ca
3c
3a
3b
c-3c
t-3c
31P
32.17 ppm
2629 Hz
32.28 ppm
2618 Hz
625 Hz
9.78 ppm
2417 Hz
653 Hzb
687 Hzb
11.00 ppm
2745 Hz
169 Hz
1JP-Pt
2JP-117Sn 629 Hz
2JP-119Sn 663 Hz
653 Hz
176 Hz
Figure 3. MP2-optimized geometries and Mulliken atomic charges
on Pt, Sn, and P for ground (GS) and transition (TS) states.
119Sn
1JSn-Pt
2JSn-P
195Pt
-15.24 ppm -12.54 ppm -29.90 ppm 12.50 ppm
8559 Hz
663 Hz
-5252 ppm -5218 ppm -5247 ppm -5307 ppm
2629 Hz 2618 Hz 2417 Hz 2745 Hz
8532 Hz
653 Hz
8389 Hz
687 Hz
7269 Hz
176 Hz
Scheme 1
1JPt-P
a 31P: 161.70 MHz, 119Sn: 148.95 MHz, 195Pt: 85.32 MHz.
Chemical shifts are from H3PO4, Me4Sn, and H2PtCl6, respectively.
The spectra were measured at room temperature in CD2Cl2 (3a and
3b) and in C6D5CD3 (3c). b Measured at 70 °C.
with 119Sn)4,5 (Figure 2e). The thermodynamic parameters were
obtained by analyzing the line width alternation: ∆Hq ) 42 (
1 kJ mol-1 and ∆Sq ) 7.6 ( 5.8 J mol-1 K-1. The rate of the
fluxional process depended on neither concentration of 3a (from
8.8 mol dm-3 to 22 mol dm-3) nor the solvent employed, CD2-
Cl2 or C6D5CD3. Furthermore, 1 molar excess of P(4-MeC6H4)3
added to 3a did not affect the rate.
Other platinum(0) phosphine complexes (2b and 2c) also react
with 1a to afford the oxidative addition products (3b and 3c)
(eq 1). Although 3a and 3b consists of a single isomer, 3c is
a mixture of two isomers (c- and t-3c in 9:1 ratio). All the
products including even 3c showed good elementary analysis
data. 31P, 119Sn, and 195Pt NMR spectral data of 3a, 3b,6 and
3c measured at room temperature are listed in Table 1. The
2JP-Sn values of 3a, 3b, and c-3c are similar (625-687 Hz),
but that of t-3c is quite smaller (169 and 176 Hz). Importantly,
c-3c showed analogous fluxional behavior (∆Hq ) 60 ( 1 kJ
mol-1 and ∆Sq ) 32 ( 5 J mol-1 K-1), in which satellite peaks
due to 2JP-Sn coalesced at 2 °C, and 2JP-Sn(transoid) (1482 Hz with
117Sn and 1551 Hz with 119Sn),4 2JP-Sn(cisoid) (177 Hz), and 2JP-P
(21 Hz) appeared at -50 °C. These J values of c-3c are very
close to those of 3a. Accordingly, c-3c was assigned to cis-
[(Me3Sn)2(MePh2P)2Pt], while t-3c to the corresponding trans-
isomer. The trans-isomer t-3c did not show any fluxional
behavior at all. The ratio between c- and t-3c (9:1) did not
change after 1 week at room temperature, indicating no
interconversion between c- and t-3c.
The intramolecular conservation of the P and Sn nuclear spin
states, which is shown by the sharp satellite peaks due to the
1JP-Pt and the line width alternation of the satellite peaks due
to the 2JP-Sn in 31P NMR (Figure 2a-e), rules out dissociative
mechanisms for the fluxional process. Associative processes
are also unlikely, since the concentration of the complex, the
coexistence of excess phosphine, and the nature of the solvent
did not affect the fluxional process (Vide supra). We attribute
the fluxional process to the intramolecular twist-rotational
motion Via pseudo tetrahedral transition state (Scheme 1).7
Ab initio calculation8 was carried out for Pt(SnH3)2(PtH3)2
as a model complex. The most stable calculated geometry (GS)
is planar (Figure 3). Thus, the distortion from planarity of 3a
may be attributed to steric effect associated with the substituents
on the P and the Sn atoms. The activation energy of the
fluxional process was calculated to be 80.0 kJ mol-1, which is
consistent with the experimental values. In the calculated
transition state (TS), the P-Pt-P angle increases dramatically,
and electron density was considerably transferred from the Sn
onto the Pt and the P, suggesting decrease of the activation
energy by introduction of electron withdrawing substituents on
the P atoms and increase of it by substitution of the Sn with a
more electronegative element such as C.
Addition of a chelate phosphine (Ph2P(CH2)nPPh2, n ) 2-4)
to a solution of 3a displaced the P(4-MeC6H4)3 completely, and
the corresponding Pt(SnMe3)2[Ph2P(CH2)nPPh2] (3d: n ) 2,
3e: n ) 3, and 3f: n ) 4) was obtained quantitatively (31P
NMR). In contrast to the 31P NMR spectra of 3a-c, those of
3d-f showed 2JP-Sn(transoid) and 2JP-Sn(cisoid) couplings separately
even at room temperature (see Supporting Information). The
rotational motion of 3d-f is slow on the NMR time scale; the
satellite peaks due to the 2JP-Sn(transoid) and 2JP-Sn(cisoid) coalesced
at 80 °C for 3d and 3e and at 50 °C for 3f. With these chelate
phosphines, the large P-Pt-P angle at the transition state, which
would facilitate the rotation as suggested by the ab initio
calculation, would not be attained easily.
Acknowledgment. This work was supported by the Grants-
in-Aid for Scientific Research on Priority Area of Reactive
Organometallics from the Ministry of Education, Science and
Culture, Japan.
Supporting Information Available: Analytical data of 3a-c, 31
P
NMR data of 3d-f at room temperature, details of ab initio calculation,
tables of crystal data and refinement details, atomic coordinates, thermal
parameters, bond distances, bond angles and torsion angles of 3a (17
pages). See any current masthead page for ordering and Internet access
instructions.
2
2
(5) The JP-Sn values observed at 20 °C are equal to (| JP-Sn(transoid)| -
| JP-Sn(cisoid)|)/2. Sign of the 2JP-Sn(transoid) and 2JP-Sn(cisoid) should be opposite.
(6) Clark et al. proposed that 3b has the trans-structure. However, 3b
should have the same structure as 3a, the cis-structure, since the both
complexes show essentially the same NMR spectral data.
(7) For NiX2(PR3)2 (X ) Cl, Br, I) complexes, structural interconversion
between square planar (diamagnetic) and tetrahedral (paramagnetic) isomers
was reported.7a (a) La Mar, G. N.; Sherman, E. O. J. Chem. Soc., Chem.
Commun. 1969, 809, and references cited therein.
2
JA9626392
(8) Gausian 94 package. For details, see Supporting Information.