Synthesis and characterization of 2,6-Dipp2-H3C6SnSnC6 H3-2,6-Dipp2 (Dipp = C6H3-2,6-Pri2): A tin analogue of an alkyne

Article abstract of DOI:10.1021/ja0257164

The reaction of Sn(Cl)C₆H₃-2,6-Dipp₂ (Dipp = C₆H₃-2,6-Prⁱ₂) with a stoichiometric amount of potassium in benzene affords 2,6-Prⁱ₂-H₃C₆SnSnC₆H₃-2,6-Prⁱ₂ (1) as dark blue-green crystals. The compound 1 is a tin analogue of an alkyne. It was characterized by ¹H and ¹³C NMR and UV-vis spectroscopy, cyclic voltammetry, combustion analysis and X-ray crystallography. The structural data show that 1 has a trans-bent, planar C(ipso)SnSnC(ipso) skeleton with a Sn-Sn bond distance of 2.6675(4) A and a Sn-Sn-C angle of 125.24(7)°. The Sn-Sn distance, which is ca. 0.15 A shorter than a conventional Sn-Sn single bond, and the trans-bent structure indicate the presence Sn-Sn multiple bond character unlike the related singly bonded ArPbPbAr species. Copyright

Full text of DOI:10.1021/ja0257164

Published on Web 05/07/2002
Synthesis and Characterization of 2,6-Dipp2-H3C6SnSnC6H3-2,6-Dipp2 (Dipp )
C6H3-2,6-Pri2): A Tin Analogue of an Alkyne
Andrew D. Phillips, Robert J. Wright, Marilyn M. Olmstead, and Philip P. Power*
Department of Chemistry, UniVersity of California, DaVis, One Shields AVenue, DaVis, California 95616
Received January 25, 2002
Table 1. Selected Bond Distances and Angles for Reduced
Ar*SnSnAr* Species and 1
In 1997 it was reported that reduction of the aryltin(II) halide
Sn(Cl)C6H3-2,6-Trip2 (Trip ) C6H2-2,4,6-Pri3) by potassium af-
forded singly reduced valence isomers of distannynes in accordance
with eq 1.1
cmpd
Sn−Sn(Å)
Sn−Sn−C (deg) ref
[K(THF)6][Ar*SnSnAr*]a
[K(18-crown-6)(THF)2][Ar*SnSnAr*] 2.7821(14) 93.6(4),95.0(4)
2.8236(14) 97.3(2)
2.8123(9)
95.20(13)
1
1
K
2:Sn(Cl)Ar*
8 [K(THF)6][Ar*SnSnAr*]
[(THF)3Na{Ar*SnSnAr*}]
[K2Ar*SnSnAr*]
Ar′SnSnAr′ (1)b
2.8107(13) 97.9(3),98.0(4)
3
2
2.7763(9)
2.6675(4)
107.50(14)
125.24(7)
or
(1)
this work
a Ar* ) C6H3-2,6-Trip2. b Ar′ ) C6H3-2,6-Dipp2.
[K(18-crown-6)(THF)2][Ar*SnSnAr*]‚2THF + 2KCl
Ar* ) C6H3-2,6-Trip2
permits the synthesis and structure of a neutral diorganoditin species
of formula Ar′SnSnAr′ (1; Ar′ ) C6H3-2,6-Dipp2; Dipp ) C6H3-
2,6-Pri2), whose structural parameters support the presence of tin-
tin multiple bonding.
The compound 1 was isolated by reaction of Sn(Cl)Ar′ with a
stoichiometric quantity of potassium in benzene at room temper-
ature.10 The product was obtained as dark blue-green crystals which
Subsequently, it was shown that it was possible to induce further
reduction by reaction with alkali metal for extended periods to
obtain the doubly reduced species K2Ar*SnSnAr* as well as its
germanium analogue Na2Ar*GeGeAr*.2 The main structural fea-
tures of the singly and doubly reduced tin products1-3 are a strongly
trans-bent, planar skeleton and tin-tin bonds that are equal to, or
slightly shorter than, the tin-tin distance (2.80 Å) in gray tin4 or
the 2.824 Å calculated for the Sn-Sn single bond in H3SnSnH3.5
Due to the relatively narrow Sn-Sn-C angles in these compounds
(See Table 1.),1-3 they were viewed as singly (IV) or doubly (V)
reduced forms of III which is a singly bonded valence isomer of
1
were spectroscopically and structurally characterized. The H and
13C NMR data were consistent with the presence of the Ar′ ligand.
However, despite numerous attempts, a 119Sn NMR signal could
not be detected. It is probable that the signal is broadened to a
sufficient extent to be undetectable directly owing to the large
chemical shift anisotropies caused by the tin environment.11 The
UV-vis spectrum affords two moderately intense absorptions at
410 and 597 nm which may be due to πfπ* and n-π* transitions.
An X-ray structure determination revealed a centrosymmetric
molecule (Figure 1) that has trans-bent skeleton, as well as a planar
C(1)Sn(1)Sn(1A)C(1A) array as required by symmetry.12 The
Sn-Sn distance is 2.6675(4) Å, and the Sn(1)-Sn(1A)-C(1A) angle
is 125.24(7)°. The central aryl ring of the ligand is almost coplanar
(torsion angle 3.2°) with the C(1)Sn(1)Sn(1A)C(1A) array. Fur-
thermore, there is an angle of 4.9° between the Sn(1)-C(1) bond
and the C(1)‚‚‚C(4) vector. The flanking aryl rings are oriented at
82.8° with respect to the central aryl ring. A cyclic voltammogram
of 1 in THF solution displayed a quasi-reversible reduction at ca.
-1.22 V vs SCE.12b An irreversible oxidation was observed at ca.
0.0 V.
the hypothetical triply bonded distannyne I. The double bonding
in the twice reduced V is supported by analogy with the isoelec-
tronic neutral group 15 species RS˙ b˙ dS˙ b˙ R.6,7 Furthermore, EPR
spectral data for IV supported the location of the unpaired electron
spin density in a π-orbital which results from the overlap of a 5p
orbital from each tin. The multiple Sn-Sn bonds in IV and V are
not particularly short in comparison to a conventional Sn-Sn single
bond,4,5 but it could be argued that the multiple Sn-Sn bonding is
partly masked by lengthening of the σ-bond which could have been
weakened by the high p-character of the σ-bonding orbitals, and
by Coulombic repulsion in the case of the dianion V. It is probable
that IV and V are obtained through the reduction of neutral
Ar*SnSnAr*; however, the structure and the degree of multiple
bonding in this species has remained undefined experimentally.
Such a molecule is of key importance in heavier group 14 element
chemistry, where the only precedent is the compound Ar*PbPbAr*,8
which has a long lead-lead bond of 3.188(1) Å and a Pb-Pb-C
angle of 94.26(4)° consistent with metal-metal single bonding.8,9
It is now reported that the use of a modified terphenyl substituent
The compound 1 is a stable ditin analogue of an alkyne. The
Sn-Sn distance is shorter, and the Sn-Sn-C angle is considerably
wider, than those observed for the reduced compounds in Table 1.
The Sn-Sn distance is shortened in comparison to that of a normal
single bond4,5 and close to the 2.659 Å calculated for the
hypothetical compound TbtSnSnTbt (Tbt ) C6H2-2,4,6-{CH-
(SiMe3)2}3) which has a Sn-Sn-C angle of 122° and a torsion
angle of 10.7° between the C-Sn-Sn planes.5 Interestingly,
calculations for the structurally uncharacterized compound
Ar*SnSnAr* also yield a trans-bent structure (Sn-Sn-C )
111.0°).5 However, the torsion angle of 54.7° is quite high, and
there is a long Sn-Sn distance of 2.900 Å.5,13 The calculated
structure of Ar*SnSnAr* and the experimentally determined
* To whom correspondence should be addressed. E-mail: pppower@ucdavis.edu.
9
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J. AM. CHEM. SOC. 2002, 124, 5930-5931
10.1021/ja0257164 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Bruker SMART 1000 diffractometer was funded in part by NSF
Instrumentation Grant CHE-9808259. We thank Professor Frank
Osterloh for his essential help in recording the electrochemical data.
Supporting Information Available: Tables of data collection
parameters, atom coordinates, bond distances, angles, anisotropic
thermal parameters, and hydrogen coordinates (PDF). An X-ray
crystallographic file (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
References
(1) Olmstead, M. M.; Simons, R. S.; Power, P. P. J. Am. Chem. Soc. 1997,
119, 11705-11706.
(2) Pu, L.; Senge, M. O.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc.
1998, 120, 12682-12683.
(3) Pu, L.; Haubrich, S. T.; Power, P. P. J. Organomet. Chem. 1999, 582,
100.
(4) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Clarendon: Oxford,
1984; p 1279.
(5) Takagi, N.; Nagase, S. Organometallics 2001, 20, 5498-5500.
(6) Tokitoh, N.; Arai, Y.; Sasamori, T.; Okazaki, R.; Nagase, S.; Uekusa,
Figure 1. Selected bond lengths (Å) and angles (deg) for 1. H atoms are
not shown. Sn(1)-Sn(1A) ) 2.6675(4), Sn(1)-C(1) ) 2.191(3), C(1)-
H.; Oshashi, Y. J. Am. Chem. Soc. 1998, 120, 433-434.
(7) Twamley, B.; Sofield, C. D.; Olmstead, M. M.; Power, P. P. J. Am. Chem.
C(2)
) 1.403(4), C(1)-C(6) ) 1.405(4), Sn(1A)-Sn(1)-C(1) )
Soc. 1999, 121, 3357-3367.
125.24(7), Sn(1)-C(1)-C(2) ) 124.9(2), Sn(1)-C(1)-C(6) ) 115.8(2),
C(1)-C(2)-C(19) ) 119.8(2), C(1)-C(6)-C(7) ) 118.6(2), C(2)-C(1)-
C(6) ) 119.3(3).
(8) Pu, L.; Twamley, B.; Power, P. P. J. Am. Chem. Soc. 2000, 122, 3524.
(9) The structure of Ar*PbPbAr*8 and calculations on a model species indicate
that the Pb-Pb bonding is single. See: Chen, Y.; Hartmann, M.;
Diedenhofen, M.; Frenking, G. Angew. Chem., Int. Ed. 2001, 40, 2052-
2055.
structure of 1, which differ only in the presence or absence of p-Pri
groups on the flanking rings, illustrate the importance of these
groups to the stability of the two configurations. It is becoming
clear that the p-Pri groups play a key role in determining the overall
structure of these compounds as well as other terphenyl derivatives.
Previous calculations14 have shown that they are important in
stabilizing the controversial compound Na2Ar*GaGaAr*.15 In
addition, the structures of the lithium derivatives of these ligands,
C6H6‚LiC6H3-2,6-Trip2 (i.e., C6H6‚LiAr*) and (LiC6H3-2,6-Dipp2)2
(i.e., (LiAr′)2), show that the absence of the p-Pri groups decreases
steric congestion sufficiently to allow dimerization to occur.10c The
Sn-C(1) distance, 2.191(3) Å in 1, is marginally shorter than the
divalent tin carbon distance (2.227(2) Å) in Ar*(Me)2SnSnAr*.16
This, together with the near coplanarity of the central aryl ring and
the C(1)-Sn(1)-Sn(1A) array, suggests the possibility of conjuga-
tion. However, the different C-Sn-Sn angles at tin, which may
indicate changes in σ-bonding, makes it difficult to draw conclusions
from the structural data.
(10) (a) Under strictly anaerobic and anhydrous conditions, a benzene solution
(50 mL) of 2,6-Dipp2-C6H3SnCl (1.10 g, 2 mmol, prepared by a method
10c
identical to that used for (2,6-Dipp2-H3C6)SnI10b from LiC6H3-2,6-Dipp2
and SnCl2), was added dropwise to finely divided potassium (0.086 g,
2.2 mmol) in 10 mL of benzene at room temperature. The reaction mixture
was stirred for 2 days after which the precipitate was allowed to settle
for 4 h. The intensely dark blue-green solution was decanted from the
precipitated solid. The volume of the solution was reduced to incipient
crystallization and stored in a ca. 6 °C refrigerator to give the product 1
as dark blue-green crystals. Yield: 0.31 g, 0.30 mmol, 32.2%; mp dec
208-210 °C. Anal. Calcd for C60H74Sn2, 1: C 70.31, H 7.22. Found: C
71.02, H 7.54. UV-vis (hexanes) λmax ꢀ (L mol-1 cm-1) 410 nm, 4300;
597 nm, 1700. 1H NMR (C6D6, 399.77 MHz, 25 °C) δ 1.13 (d, 24 H, 3J
3
) 6.0 Hz, o-CH(CH3)2), 1.39 (d, 24 H, J ) 6.0 Hz, o-CH(CH3)2), 2.87
(sept, 8 H, 3J ) 6.0 Hz, o-CH(CH3)2), 6.22 (t, 2 H, 3J ) 7.2 Hz, p-C6H3),
3
7.05 (d, 8 H, J ) 7.2 Hz, m-Dipp), 7.19 (t, 4 H, 3J ) 7.2 Hz, p-Dipp),
7.51 (d, 4 H, 3J ) 7.2 Hz, m-C6H3). 13C {1H} NMR (C6D6, 100.53 MHz,
25 °C) δ 27.44 (o-CH(CH3)2), 32.67 (o-CH(CH3)2), 34.98 (o-CH(CH3)2),
124.55 (p-C6H3), 125.94 (m-Dipp), 130.65 (m-C6H3), 131.68 (i-Dipp),
141.72 (p-Dipp), 150.84 (o-Dipp), 153.98 (i-C6H3), 159.02 (o-C6H3).
119Sn NMR (C6D6, 149.00 MHz, 25 °C) δ no signal observed. (b) Pu, L.;
Olmstead, M. M.; Power, P. P.; Schiemenz, B. Organometallics 1998,
17, 5602-5606. (c) Schiemenz, B.; Power, P. P. Angew. Chem., Int. Ed.
Engl. 1996, 35, 2150-2152.
(11) Eichler, B. E.; Phillips, B. L.; Power, P. P.; Augustine, M. P. Inorg. Chem.
2000, 39, 3444-5449.
Although the hypothetical species Ar*SnSnAr* and TbtSnSnTbt
have been described as distannynes,5 this name is misleading in
respect of the bond order. The Sn-Sn distances calculated for
TbtSnSnTbt, and observed in 1, are clearly shorter than single
bonds, but they are not as short17 as the Sn-Sn double bond
(12) (a) Crystal data for 1 at 90 K with Mo KR (λ ) 0.71073 Å) radiation: a
) 20.4324(11) Å, b ) 15.7584(9) Å, c ) 16.2418(9) Å, orthorhombic,
space group Pccn, Z ) 4, R1 ) 0.0317 for 4127 (I > 2σ(I)) data, wR2
) 0.0845 for all (7028) data. (b) Cyclic voltammetric data for 1 were
obtained under anaerobic conditions using a PAR model 263 potentiostat/
galvanostat with a Pt working electrode (against a SCE reference) scanning
at 100 mV sec-1 in THF solution with 0.1 M [NBu4]PF6 as the electrolyte.
(13) This result suggests that a tin-tin single bond in this compound class is
ca. 2.9 Å and is the distance to which the bond lengths in Table 1 should
be compared. The Sn-Sn distance of 2.8909(2) Å in Ar*(Me)2SnSnAr*11
supports this argument.
t
t
(2.59(1) Å) in the cyclotristannene (Bu3 Si)2SnSn(SiBut3)Sn(SiBu3 )
where, possibly, the geometric constraints of the three-membered
ring favor alignment of the tin p-orbitals to afford more efficient
π-overlap.18 They are similar to the Sn-Sn multiple bonds in the
tristannaallene Sn{Sn(SiBut3)2}2 (Sn-Sn ) 2.68(1) Å)17 and ca.
0.1 Å shorter than the Sn-Sn distance 2.768(1) Å in the compound
R2SnSnR2 (R ) CH(SiMe3)2) which is the shortest, currently known
Sn-Sn bond in a “distannene”.19 Furthermore, the trans-bent
geometry is indicative of lone pair character at each tin. The bonding
in 1 thus approximates to II and lies between the extremes of the
hypothetical linear triply bonded I and the singly bonded III.
(14) Takagi, N.; Schmidt, M. W.; Nagase, S. Organometallics 2001, 20, 1646-
1651.
(15) Su, J.; Li, X.-W.; Crittendon, C.; Robinson, G. H. J. Am. Chem. Soc.
1997, 119, 5471-5472.
(16) Eichler, B. E.; Power, P. P. Inorg. Chem. 2000, 39, 5450-5453.
(17) Sn-Sn distances as short as 2.479(4) Å have been reported in Sn(IV)-
Sn(II) compounds. See: Nardelli, M.; Pelizzi, C.; Pelizzi, G.; Tarasconi,
P. Z. Anorg. Allg. Chem. 1977, 431, 250-260.
(18) Wiberg, N.; Lerner, H.-W.; Vasisht, S.-K.; Wagner, S.; Karaghiosoff, K.;
No¨th, H.; Ponikwar, W. Eur. J. Inorg. Chem. 1999, 1211-1218.
(19) Goldberg, D. E.; Hitchcock, P. B.; Lappert, M. F.; Thomas, K. M.;
Fjelberg, T.; Haaland, A.; Schilling, B. E. R. J. Chem. Soc., Dalton Trans.
1986, 2387-2394.
Acknowledgment. We are grateful to the National Science
Foundation (CHE-0094913) for generous financial support. The
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