C O M M U N I C A T I O N S
agreement with that observed in solution. AIM calculations indicate
no bonding between the methinyl hydrogens and tin. The common
structure in solid, liquid, and gas phases fulfills all criteria for a
free, tricoordinate stannylium cation and provides a new coordina-
tion number for diamagnetic Sn.
Acknowledgment. This work was supported by the National
Science Foundation (Grant CHE-0091162). We thank C. L. Stern
for solving the X-ray structure.
Supporting Information Available: Atom coordinates from the
X-ray structure, NMR characterization and CIF file of the cation, and
details of the DFT, AIM, and tin chemical shift calculations. This
information is available free of charge via the Internet at http://
pubs.acs.org.
Figure 2. The immediate structure around Sn in Tip3Sn+, showing Sn
(pink), ipso aryl carbon (green), and the six isopropyl methinyl hydrogens
(blue).
the average Sn/CH2 distance in the tributylstannyl cation (2.81 Å).8
The only candidates for agostic interactions with Sn are the
isopropyl methinyl hydrogens, which were refined and located
during the solution of the crystal structure. Figure 2 shows the
location of these hydrogens, in a prismatic geometry identical to
that of the closest ortho hydrogens in the trimesitylsilylium cation.2
The Sn/H distances average 2.64 Å, well beyond the covalent Sn-H
bond length of 1.70 Å but less than the sum of the van der Waals
radii (3.37 Å). The van der Waals radius of cationic Sn, however,
is unknown and is almost certainly less than that of neutral Sn, so
these comparisons are somewhat misleading. There is no angular
References
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401.
(2) Kim, K.-C.; Reed, C. A.; Elliott, D. W.; Mueller, L. J.; Tham, F.; Lin, L.;
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1279-1280.
distortion within the isopropyl groups of Tip3Sn+. The Cipso-Cortho
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(7) Lambert, J. B.; Lin, L. J. Org. Chem. 2001, 66, 8537-8539.
(8) Zharov, I.; King, B. T.; Havlas, Z.; Pardi, A.; Michl, J. J. Am. Chem.
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CH angles average to a very normal sp2 121.5° (122.2° in
trimesitylsilylium2), and the Cortho-C-H angles average to a normal
sp3 106.5°.
(9) Henderson, L. D.; Piers, W. E.; Irvine, G. J.; McDonald, R. Organo-
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(10) Davies, A. G. Organotin Chemistry; Weinheim, 1997.
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Soc. 1998, 120, 11096-11105.
It is worthwhile to compare the X-ray structure with the results
of calculation, which refer to the gas phase.13 The DFT/B3LYP
geometry optimization showed that the crystal and gas phase
structures are essentially identical. An atoms in molecule (AIM)
calculation14 showed that there are no bond critical points between
Sn and the six methinyl hydrogens and that there are bonds only
between Sn and the three aryl carbons. The 119Sn chemical shift
was calculated to be δ 763 with Gaussian 98 using the GIAO
method and the MPW1PW91 density functional.18 Calculations at
this level are systematically higher frequencies than are observation
values.19 The plots of a more extensive study in the Supporting
Information demonstrate that the calculated value of δ 763 is
entirely consistent with the observed value of δ 714.
In summary, the crystal structure of tris(2,4,6-triisopropylphenyl)-
stannylium tetrakis(pentafluorophenyl)borate reveals a geometry
around Sn that is planar and tricoordinate. There is no solvent
present, and the distance between the cation and anion is quite long.
The isopropyl methyl groups are positioned away from Sn, and
the methinyl hydrogens are well beyond the sum of the covalent
radii. There are no angle distortions within the isopropyl groups.
There are no atoms along the central axis perpendicular to the plane
through Sn and its attached carbon atoms, indicating no trigonal
bipyramidality. The calculated structure is very similar to the crystal
structure, and the calculated 119Sn chemical shift is in good
(12) Crystal data for Tip3Sn+ TPFPB-: C69H69BF20Sn, M ) 1407.74, space
group C2/c, a ) 29.336(7), b ) 13.324(3), and c ) 34.701(10) Å, R )
90°, â ) 95.07(2)°, γ ) 90°, V ) 13510(6) Å3, Z ) 8, Dc ) 1.384 g
cm-3, µ(Mo KR) ) 0.473 mm-1, 859 variables refined with 16,615 unique
reflections to R1 ) 0.0499. Mass spectrum: M+ (nominal) 727; M+ (obs)
727-730. Anal. Calcd: C, 58.87; H, 4.94. Found: C, 58.71; H, 4.66.
(13) Geometry optimization from the X-ray structure was carried out at the
DFT/B3LYP level with the effective core potential basis set of LACVP**
for Sn and 6-31G for all other atoms, as implemented in the program
Jaguar 4.2 (Schro¨dinger, Inc., Portland, OR). There are only minor differ-
ences from the X-ray structure. Bonding was studied by the AIM method.14
Huzinaga’s full valence basis set15 as implemented by GamessUS16 was
used to calculate the electron density of the geometry-optimized complex
for input into Morphy.17 The only bond (3, -1) critical points involving
Sn were between Sn and the ipso aryl carbons.
(14) Bader, R. F. W. Chem. ReV. 1991, 91, 893-928.
(15) Huzinaga, S.; Andzelm, J.; Klobukowski, M.; Radzio-Andzelm, E.; Sakai,
Y.; Tatewaki, H. Basis Sets for Molecular Calculations; Elsevier:
Amsterdam, 1984.
(16) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon,
M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S. J.;
Windnus, T. L.; Depuis, M.; Montgomery, J. A. J. Comput. Chem. 1993,
14, 1347-1363.
(17) Popelier, P. L. A. Comput. Phys. Commun. 1996, 93, 212-240.
(18) Chemical shift calculations were carried out by Gaussian 98 using the
GIAO method and the MPW1PW91 density functional. Revisions A3-
A9, Gaussian, Inc. Pittsburgh, PA, 1999.
(19) Avalle, P.; Harris, R. K.; Karadakov, P. B.; Wilson, P. J. Phys. Chem.
Chem. Phys. 2002, 4, 5925-5932. Divas-Reyes, R.; De Proft, F.;
Biesemans, M.; Willem, R.; Geerlings, P. J. Phys. Chem. A 2002, 106,
2753-2759.
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