Tin-centered radical and cation: Stable and free

Article abstract of DOI:10.1021/ja030156+

The first stable stannyl radical (^tBu₂MeSi)₃Sn^?(1) has been synthesized by the reaction of ^tBu₂MeSiNa with SnCl₂-dioxane in diethyl ether. The X-ray crystal structure and electron paramagnetic resonance (EPR) data of this radical show that 1 has a planar geometry, being a π-radical in both the solid and the liquid states. One-electron oxidation of 1 with Ph₃C⁺·B(C₆F₅)₄^- in benzene quantitatively produced the corresponding cation (^tBu2MeSi)₃Sn⁺·B(C₆F₅)₄^- (2), representing the stable free stannylium ion that has been fully characterized by X-ray analysis and NMR data. Being free, 2 features a record downfield shifted resonance for stannylium ions: +2653 ppm. Copyright

Full text of DOI:10.1021/ja030156+

Published on Web 07/11/2003
Tin-Centered Radical and Cation: Stable and Free
Akira Sekiguchi,* Tomohide Fukawa, Vladimir Ya. Lee, and Masaaki Nakamoto
Department of Chemistry, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
Received March 10, 2003; E-mail: sekiguch@staff.chem.tsukuba.ac.jp
The chemistry of stable free radicals and free cations of heavier
group 14 elements is one of the most fascinating topics in recent
years.1,2 The story of these Si- and Ge-centered species has
undergone a spectacular evolution from transient intermediates to
stable compounds that can be isolated and even structurally
characterized.3,4 However, a stable stannyl radical and a stable free
tricoordinated stannylium ion were still lacking. The only crystal
structure for a nonsolvated Sn cation was reported for the tri(n-
butyl)stannylium ion,5 but in this compound the cationic Sn-center
is not free, being coordinated with the methyl groups of the
carborane counterion.6 In this paper, we report the first stable
representative of the stannyl radical and its subsequent one-electron
oxidation to a three-coordinated free stannylium ion.
The method we selected for the synthesis of a stable stannyl
radical was based on the one-electron oxidation of the corresponding
stannyl anion (tBu2MeSi)3Sn-Na+ generated in situ by the reaction
of SnCl2-dioxane with tBu2MeSiNa in diethyl ether (Scheme 1).7
Figure 1. ORTEP drawing of 1. Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å): Sn(1)-Si(1) ) 2.6146(5), Sn(1)-Si(2) )
2.6193(5), Sn(1)-Si(3) ) 2.6189(5). Selected bond angles (deg): Si(1)-
Sn(1)-Si(3) ) 119.684(16), Si(1)-Sn(1)-Si(2) ) 120.229(16), Si(3)-
Sn(1)-Si(2) ) 119.988(16).
Scheme 1
The stannyl radical 1 was isolated as extremely air- and moisture-
sensitive orange prisms by recrystallization from hexane in 32%
isolated yield.8 Its crystal structure was unequivocally established
by X-ray crystallography to show a planar structure (359.9° for
the sum of the bond angles around the Sn center) with in-plane
orientation of three methyl groups on the silyl-substituents, which
allows the most effective steric protection of the radical center and
minimizes steric hindrances (Figure 1).8 The Sn atom is sp2-
hybridized, forcing the unpaired electron to occupy a vacant 5pz-
orbital, which implies that 1 is truly a π-radical.9
The radical 1 showed a sharp signal in the electron paramagnetic
resonance (EPR) spectrum measured in hexane at room temperature
with a g-value of 2.0482,8 which is slightly above the typical range
for Sn-centered radicals.10 Surprisingly, we did not observe two
distinct pairs of satellites that one would expect from the coupling
of the unpaired electron with two paramagnetic 117Sn and119Sn
nuclei. Instead, the central signal showed only one pair of satellite
signals with the hyperfine coupling constant (hfcc) a(119,117Sn) of
32.9 mT. Most likely, this value is an average between the a(117Sn)
and a(119Sn) coupling constants, because the difference between
these last two values was estimated to be very small (∼1.5 mT),
which was difficult to detect under the measurement conditions
(line width 1.1 mT). The hfcc value of 32.9 mT is 1 order of
magnitude less than those of all previously known Sn-centered
radicals.11 This fact undoubtedly gives evidence that radical 1
preserves its π-character in solution, being trigonal planar in both
the solid and the liquid states.
Figure 2. ORTEP drawing of 2. Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å): Sn(1)-Si(1) ) 2.6930(7), Sn(1)-Si(2) )
2.6868(8), Sn(1)-Si(3) ) 2.6792(8). Selected bond angles (deg): Si(3)-
Sn(1)-Si(2) ) 118.03(3), Si(3)-Sn(1)-Si(1) ) 120.40(3), Si(2)-Sn(1)-
Si(1) ) 121.56(2).
-
Ph3C+‚B(C6F5)4 in benzene resulted in the immediate formation
of a two-layer reaction mixture, in which the dark-red lower layer
was found to contain the target stannylium ion 2 (Scheme 2).8 The
Scheme 2
cation 2 was nearly quantitatively isolated by recrystallization from
CH2Cl2 as dark-brown cubic-shaped crystals. X-ray analysis showed
that the cationic part of 2 has a perfectly planar geometry around
the central sp2-hybridized Sn atom (360.0° for the sum of the bond
angles around the Sn center) with the same in-plane arrangement
of methyl groups on the silyl substituents (Figure 2).8 The closest
distance between the cationic Sn atom and the fluorine atom of
The successful preparation of stannyl radical 1 prompted us to
generate the corresponding stannylium ion by one-electron oxidation
of radical 1. The reaction of stoichiometric amounts of 1 and
9
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J. AM. CHEM. SOC. 2003, 125, 9250-9251
10.1021/ja030156+ CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Maerker, C.; Kapp, J.; Schleyer, P. v. R. In Organosilicon Chemistry:
from Molecules to Materials; Auner, N., Weis, J., Eds.; VCH: Weinheim,
1996; Vol. II. (c) Reed, C. A. Acc. Chem. Res. 1998, 31, 325. (d) Lickiss,
P. D. In The Chemistry of Organic Silicon Compounds; Rappoport, Z.,
Apeloig, Y., Eds.; John Wiley & Sons Ltd.: New York, 1998; Vol. 2,
Part I, Chapter 11. (e) Zharov, I.; Michl, J. In The Chemistry of Organic
Germanium, Tin and Lead Compounds; Rappoport, Z., Ed.; John Wiley
& Sons Ltd.: New York, 2002; Vol. 2, Part I, Chapter 10.
the counteranion is greater than 5 Å, which is outside of the range
of van der Waals interactions. The average Si-Sn bond length in
2 was 2.6863(8) Å, which is quite normal for the Si-Sn bond
length.12 On the other hand, the average Si-Sn bond length in 1
of 2.6176(5) Å is shorter than that in 2, which can be explained by
the hyperconjugation between the 5pz-orbital on the Sn atom and
the σ*-orbitals of the Si-C(tBu) bonds in 1.
(3) For isolable silyl and germyl radicals, see: (a) Olmstead, M. M.; Pu, L.;
Simons, R. S.; Power, P. P. Chem. Commun. 1997, 1595. (b) Sekiguchi,
A.; Matsuno, T.; Ichinohe, M. J. Am. Chem. Soc. 2001, 123, 12436. (c)
Sekiguchi, A.; Fukawa, T.; Nakamoto, M.; Lee, V. Ya.; Ichinohe, M. J.
Am. Chem. Soc. 2002, 124, 9865.
(4) For free silyl and germyl cations, see: (a) Sekiguchi, A.; Tsukamoto,
M.; Ichinohe, M. Science 1997, 275, 60. (b) Sekiguchi, A.; Matsuno, T.;
Ichinohe, M. J. Am. Chem. Soc. 2000, 122, 11250. (c) Kim, K.-C.; Reed,
C. A.; Elliott, D. W.; Mueller, L. J.; Tham, F.; Lin, L.; Lambert, J. B.
Science 2002, 297, 825. (d) Sekiguchi, A.; Fukawa, T.; Lee, V. Ya.;
Nakamoto, M.; Ichinohe, M. Angew. Chem., Int. Ed. 2003, 42, 1143.
(5) Zharov, I.; King, B. T.; Havlas, Z.; Pardi, A.; Michl, J. J. Am. Chem.
Soc. 2000, 122, 10253.
(6) During the reviewing of this paper, the crystal structure of a free Tip3Sn+
(Tip ) 2,4,6-triisopropylphenyl) appeared, see: Lambert, J. B.; Lin, L.;
Keinan, S.; Mu¨ller, T. J. Am. Chem. Soc. 2003, 125, 6022.
The crucial point is the chemical shift of the free cationic Sn
atom, which was anticipated to be greatly deshielded.2e According
to an extrapolation based on the empirical correlation of 29Si and
119Sn NMR chemical shifts, one can expect the resonance of the
free stannylium ion to be in the range of +1500-2000 ppm.13 In
fact, the real free stannylium ion is even more deshielded: careful
measurement of the 119Sn NMR spectrum of 2 in CD2Cl2 allowed
us to detect the signal corresponding to the cationic Sn atom at
+2653 ppm.8 It is worth mentioning that this value is much more
downfield shifted than any of the very few other examples reported
for stannylium ions: counteranion-coordinated nBu3Sn+ (+454
ppm),5 Mes3Sn+ (+806 ppm),14 and Tip3Sn+ (+714 ppm).6
Moreover, the chemical shift of +2653 ppm greatly exceeds the
calculated value for a free triorganostannylium ion of ca. +1000
ppm calculated at the HF/DZ+P and MP2/DZ+P levels using
IGLO/DZ and IGLO/DZ+P methods.15 The 119Sn NMR chemical
shift of +2841 ppm calculated for the model stannylium ion (H3-
Si)3Sn+ at the GIAO-B3LYP/[7s6p5d](Sn):6-311G(d)(Si,C,H)//
B3LYP/[6s5p4d](Sn):6-31G(d)(Si,C,H) level using the Gaussian 98
program agrees reasonably with the experimental value of +2653
ppm.
(7) In this reaction, SnCl2-dioxane plays two roles: as a substrate for the
generation of the stannyl anion, and as a one-electron oxidizing reagent.
The right choice of the oxidizing reagent was crucial. Thus, when we
used SnCl2 itself, we were not able to isolate the stannyl radical because
t
of unavoidable direct oxidation of Bu2MeSiNa by SnCl2 (E1/2(red) )
-0.21 and -1.20 V) to form tBu2MeSiSiMetBu2 and metallic tin.
However, when we applied SnCl2-dioxane complex (E1/2(red) ) -0.78
V), in which the oxidizing ability of the stannylene unit is decreased by
complexation with dioxane, we have achieved the exact matching of redox
properties of SnCl2-dioxane and tBu2MeSiNa to prevent the undesirable
direct oxidation of silylsodium. For electrochemical studies of SnCl2 and
SnCl2-dioxane, see: Lee, V. Ya.; Basova, A.; Matchkarovskaya, I. A.;
Faustov, V. I.; Egorov, M. P.; Nefedov, O. M.; Rakhimov, R. D.; Butin,
K. P. J. Organomet. Chem. 1995, 499, 27.
(8) For the experimental procedures, spectral data, crystal data of 1 and 2,
EPR chart of 1, and NMR charts of 2, see the Supporting Information.
(9) All other experimentally known to date stannyl radicals were found to be
pyramidal σ-radicals: Iley, J. In The Chemistry of Organic Germanium,
Tin and Lead Compounds; Patai, S., Ed.; John Wiley & Sons Ltd.: New
York, 1995; Chapter 5.
Acknowledgment. We thank Mr. Kazuya Ishimura and Prof.
Shigeru Nagase at IMS for theoretical calculations on the stannylium
ion.
(10) Typical g-values for Sn-centered radicals range from 1.988 to 2.027 (see
ref 9).
Supporting Information Available: Experimental procedures,
spectral data of 1 and 2, tables of crystallographic data including atomic
positional and thermal parameters for 1 and 2, EPR chart of 1, NMR
charts of 2 (PDF and CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
(11) Typical hfcc values for Sn-centered radicals a(119Sn) and a(117Sn) lie in
the region 132.5-342.6 mT (see ref 9).
(12) Normal Si-Sn bond lengths lie in the region 2.561-2.789 Å: Mackay,
K. M. In The Chemistry of Organic Germanium, Tin and Lead Com-
pounds; Patai, S., Ed.; John Wiley & Sons Ltd.: New York, 1995; Chapter
2.
(13) Arshadi, M.; Johnels, D.; Edlund, U. Chem. Commun. 1996, 1279.
(14) Lambert, J. B.; Zhao, Y.; Wu, H.; Tse, W. C.; Kuhlmann, B. J. Am. Chem.
Soc. 1999, 121, 5001.
References
(1) For recent reviews on radicals of heavier group 14 elements, see: (a)
Chatgilialoglu, C. Chem. ReV. 1995, 95, 1229. (b) Power, P. P. Chem.
ReV. 2003, 103, 789.
(2) For recent reviews on cations of heavier group 14 elements, see: (a)
Lambert, J. B.; Kania, L.; Zhang, S. Chem. ReV. 1995, 95, 1191. (b)
(15) Cremer, D.; Olsson, L.; Reichel, F.; Kraka, E. Isr. J. Chem. 1994, 33,
369.
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