Characterization of Ar′Sn(μ-Br)Sn(Ar′)CH2C 6H4-4-Pri (Ar′ = C6H3-2,6-Dipp2; Dipp = C6H3-2,6-Pr i2

Article abstract of DOI:10.1021/ja0316871

The title compound is the first stable structural analogue for the monobridged isomer of a heavier group 14 alkene analogue. Copyright

Full text of DOI:10.1021/ja0316871

Published on Web 03/16/2004
Characterization of Ar′Sn(µ-Br)Sn(Ar′)CH2C6H4-4-Pri (Ar′ ) C6H3-2,6-Dipp2;
Dipp ) C6H3-2,6-Pri2): A Stable Structural Analogue for a Heavier Group 14
Element Monobridged Alkene Isomer HM(µ-H)MH2 (M ) Sn or Pb)
Corneliu Stanciu, Anne F. Richards, and Philip P. Power*
Department of Chemistry, UniVersity of California, DaVis, One Shields AVenue, DaVis, California 95616
Received December 12, 2003; E-mail: pppower@ucdavis.edu
Since the synthesis of R2SnSnR2 (R ) CH(SiMe3)2) in 1973,1
numerous examples of heavier group 14 element (tetrel) alkene
analogues have been isolated and characterized.2 The great majority
of these possess trans pyramidal, dimeric structures (I) that display
a preference for dissociation to monomers with increasing atomic
number as shown by eq 1.
Nonetheless, in a series of computational studies3,4 on the simple
hydrogen derivatives M2H4 in the early 1990s, Trinquier showed
that a number of other structural minima (II-V) could exist as
illustrated by the structures below.
Figure 1. Selected bond lengths (Å) and angles (deg) for 1. H atoms are
not shown. Sn(1)-Sn(2) ) 2.9407(4), Sn(1)-Br(1) ) 2.7044(5), Sn(2)-
Br(1) ) 2.7961(5), Sn(1)-C(1) ) 2.201(4), Sn(1)-C(61) ) 2.196(3), Sn-
(2)-C(31) ) 2.212(4), Sn(1)-Br(1)-Sn(2) ) 64.61(1), Br(1)-Sn(1)-Sn(2)
) 59.20(1), Br(1)-Sn(2)-Sn(1) ) 56.18(1), Sn(1)-Sn(2)-C(31) )
107.83(9), Br(1)-Sn(2)-C(31) ) 96.891), Sn(2)-Sn(1)-C(1) ) 117.66(9),
C(1)-Sn(1)-Br(1) ) 103.8(1), C(1)-Sn(1)-C(61) ) 104.6(1), Sn(2)-
Indeed, for tin and lead, the trans, doubly bridged isomer II was
found to be the absolute minimum, with the cis isomer III lying
Sn(1)-C(61) ) 133.96(9).
slightly higher (ca. 2 kcal mol-1) in energy.5 Furthermore, the un-
symmetric isomer IV was only 5.0 kcal mol-1 less stable. Through
the use of these guidelines and a combination of substituents we
have shown that organosubstituted tin derivatives of the doubly
bridged and unsymmetric structures II6 and IV7 could be isolated
and characterized. It was also shown that, although a trans pyramidal
dimer (I, shown in eq 1) is preferred for germanium, it could be
transformed into the unsymmetric form (IV) by reaction with a
Lewis base.8 Despite this, a stable analogue of the singly bridged
isomer V has proven to be elusive.5 Reference to the Trinquier
calculations, however, shows that V lies just 0.9 kcal above IV, or
7.9 kcal mol-1 above the absolute minimum II, which suggests that,
energetically at least, a derivative of such a species should also be
obtainable. We now show that by suitable manipulation of
substituents the singly bridged derivative Ar′Sn(µ-Br)Sn(Ar′)-
(CH2C6H4-4-Pri), 1, (Ar′ ) C6H3-2,6-Dipp2; Dipp ) C6H3-2,6-Pri2)
can be isolated and characterized structurally and spectroscopically.
For comparison, the characterization of the related symmetric trans
pyramidal species {Sn(Ar′)(CH2C6H4-4-But)}2, 2, is also reported.9
Compound 1 can be isolated10 via the direct reaction of Ar′SnCl11
with 0.5 equiv of BrMgCH2C6H4-4-Pri, which yielded 1 as large
orange crystals. 119Sn NMR spectroscopy of 1 in C6D6 solution
afforded two signals of equal intensity at 1399.8 and 2274.3 ppm.
X-ray crystal data12 showed that the solid-state structure (Figure
1) features bromine bridging two tins, one of which (Sn(1)) is
bonded to Ar′ and CH2C6H4-4-Pri groups and the other to a single
Ar′. The Sn-Sn distance is 2.9407(4) Å, and the bridged Sn-Br
distances are unequal, having the values 2.7044(5) (Sn(1)) and
2.7961(5) Å. The three Sn-C bond lengths are in the narrow range
2.196(3)-2.212(4) Å. The interligand angles surrounding the tins
display extreme variation and are provided in the caption of Figure
1. The reaction of Ar′SnCl with 1 equiv of BrMgCH2C6H4-4-But
afforded 2, which has a trans pyramidal structure (Figure 2) found
for most (SnR2)2 dimers.2 The most prominent features of the
structure are the Sn-Sn bond length of 2.7705(8) Å and an out of
plane angle of 42°. 119Sn NMR spectroscopy of 2 in C6D6 afforded
a signal at 1205.7 ppm.
The singly bridged structure of 1 is unique for low-coordinate
tin(II) organometallic derivatives. The details of the structure and
119Sn NMR spectroscopy establish that it is bromide bridged rather
than an unsymmetric species of the type Ar′SnSn(Br)(Ar′)-
(CH2C6H4-4-Pri). The chemical shifts obtained for Sn(1), 1399.8
ppm, and Sn(2), 2274.3 ppm, both lie in the Sn(II) rather than the
Sn(IV) range and suggest that the structure in the solid state is
retained in solution.9,13 The bridging Sn-Br distances differ by ca.
0.09 Å and are similar to the 2.81(2) Å bridging Sn-Br distance
in [Fe(Sn{N(But)}2SiMe2)2Br2]2.14 In addition, the disposition of
the C(1) and C(61) ligands is toward the bromine side of the
molecule. This is the same structural distortion as that predicted
for V and, as further observed by Trinquier,4 is consistent with the
9
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J. AM. CHEM. SOC. 2004, 126, 4106-4107
10.1021/ja0316871 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
(6) Eichler, B. E.; Power, P. P. J. Am. Chem. Soc. 2000, 122, 8785.
(7) (a) Eichler, B. E.; Power, P. P. Inorg. Chem. 2000, 39, 5450. (b) Phillips,
A. D.; Hino, S.; Power, P. P. J. Am. Chem. Soc. 2003, 125, 7520.
(8) Richards, A. F.; Phillips, A. D.; Olmstead, M. M.; Power, P. P. J. Am.
Chem. Soc. 2003, 125, 3204.
(9) Reaction of BrMgCH2C6H4-4-Pri with Ar′SnCl afforded [Sn(Ar′)CH2C6H4-
4-Pri]2 (119Sn NMR δ ) 1209 ppm), which did not crystallize. The benzyl
group CH2C6H4-4-But was used instead to obtain 2 with the assumption
that substitution of But for Pri at the para position would not substantially
alter its steric or electronic properties. X-ray-quality crystals of 2 could
be grown with this substituent, which showed that it is dimeric with no
bridging present. Its chemical shift, which differs significantly from those
of 1, indicates that 1 does not dissociate and supports the retention of the
bridged structure in solution.
(10) All manipulations were carried out under anaerobic and anhydrous
conditions. 1: To a solution of Ar′SnCl (1.1 g, ca. 2 mmol) in THF (20
mL), cooled to ca. -78 °C, was slowly added a freshly prepared THF
solution (5 mL) containing isopropylbenzylmagnesium bromide (1.0
mmol), via syringe. The initial light yellow solution became orange and
red upon warming to room temperature. The reaction was stirred at room
temperature for 12 h, whereupon the solvent was removed and the solid
residue was extracted with hexane (50 mL); filtration through Celite
afforded a deep-red solution, which was concentrated to ca. 20 mL. After
24 h at ca. -20 °C, large dark orange crystals were formed, which were
suitable for X-ray analysis. Yield: 0.45 g (0.36 mmol, 36.1%). UV-vis
Figure 2. Selection bond lengths (Å) and angles (deg) for 2. Sn(1)-Sn-
(1A) ) 2.7705(8), Sn(1)-C(1) ) 2.180(5), Sn(1)-C(31) ) 2.175(6), C(1)-
Sn(1)-C(31) ) 101.4(2), Sn(1A)-C(1) ) 112.1(1), Sn(1)-Sn(1)-C(31)
) 116.2(2).
1
(hexane) λmax, ꢀ (M-1 cm-1): 405, 1900. H NMR (399.77 MHz, C6D6,
3
25 °C): δ 0.95 [dd, JHH ) 6.4 Hz, 24H, o-CH(CH3)2(Dipp)], 1.09 [d,
3JHH ) 6.8 Hz, 6H, CH2C6H4-4-CH(CH3)2], 1.15 [d, 3JHH ) 6.8 Hz, 6H,
3
-C6H4-4-CH(CH3)2, 1.20 [d, JHH ) 6.8 Hz, 18H, CH(CH3)2C(7)+C-
(19)Dipp], 2.16 (s, broad, 2H, -CH2C6H4-4-CH(CH3)2-), 2.61 [sept, 3JHH
3
view that it represents the midpoint in the transformation of the
trans pyramidal geometry in I to the unsymmetric structure IV, that
is, from Ar′(Br)SnSn(Ar′)CH2C6H4-4-Pri to Ar′SnSn(Br)(Ar′)-
CH2C6H4-4-Pri. The Sn-Sn distance, 2.9407(4) Å, in 1 is longer
than that calculated for V, 2.74 Å. However, similarly long distances
have been observed for the unsymmetric Ar*SnSnMe2Ar*6a (Sn-
) 7.0 Hz, 1H, CH2C6H4-4-CH(CH3)2], 2.88 [sept, JHH ) 6.8 Hz, 3H,
o-CH(CH3)2(C(7)+C(19)Dipp), 3.05 [sept, 3JHH ) 6.8 Hz, 5H, CH(CH3)2-
3
(C(7), C(37), and C(45)Dipp)], 6.67 [dd, JHH ) 8.8 Hz, 4H, -CH2-
3
C6H4-4-CH(CH3)3], 7.03 [d, JHH ) 8.8 Hz, 6H, (m-C6H3(C(1), C(31),
and C(19)Dipp))], 7.13-7.19 (br, m, 12H, Ar, H). 13C NMR (100.53 MHz,
C6D6, 25 °C): δ 23.20 (CH(CH3)2, Dipp), 23.99 (p-CH(CH3)2, CH2C6H4-
4-CH(CH3)3), 24.45 (-CH2-), 25.82 (CH(CH3)2, C(7) Dipp), 25.86 (CH-
(CH3)2, C(19) Dipp), 26.50 (CH(CH3)2, C(19) Dipp), 26.54 (CH(CH3)2,
C(19) Dipp), 30.87 (CH(CH3)2, C(19 and C(7) Dipp), 30.96 (CH(CH3)2,
C(7) Dipp), 31.07 (CH(CH3)2, C(31) Dipp), 31.98 (CH(CH3)2, CH2C6H4-
4-CH(CH3)2, C(7) and C(19) Dipp), 124.14 (m-C(31) Dipp), 125.78 (p-
C6H3), 125.84 (m-C(7) Dipp), 127.35 (p-CH2C6H4-4-CH(CH3)2), 127.51
(p-C(1) Dipp), 127.58 (p-C(31) Dipp), 127.62 (p-C(7) Dipp), 129.14 (o-
p-C(7) Dipp), 129.57 (m-C(31) Dipp), 130.02 (p-Dipp), 130.43 (m-Bn),
130.75 (m-Ph), 136.94 (o-Ph), 137.02 (ipso-Bn), CH2C6H4-4-CH(CH3)2,
137.45 (o-C(31) Dipp), 138.96 (i-C(31) Dipp), 145.54 (o-C(7) Dipp),
147.21 (o-(C(1) and C(7) Dipp)), 147.64 (o-C(31) Dipp), 184.82 (i-C(37)
and C(48)), 191.85 (i-C6H3(C(1))). 119Sn NMR (C6D6, 149.24 MHz):
1399.8 (tetracoordinated Sn), 2274.3 (tricoordinated tin). 2: To a solution
of Ar′SnCl (0.55 g, ca. 1 mmol) in THF (20 mL), pre-cooled to ca. -78
°C, was slowly added 4 mL of a freshly prepared THF solution (2.5 M)
of p-tert-butylbenzyl-magnesium bromide (2.5 M) via syringe. The solution
became amber immediately and deep red upon warming to room
temperature. The reaction was stirred for a further 12 h, and the solvent
was pumped off. The residue was extracted with hexane (40 mL), and
the solution was allowed to settle for ca. 3 h. It was then decanted,
concentrated to about half of the initial volume, and let stand in a ca.
-20 °C freezer overnight. Orange crystals were formed, which were
suitable for single-crystal X-ray analysis. Yield: 0.28 g, 0.42 mol, 42.1%.
UV-vis (hexane) λmax, ꢀ (M-1 cm-1): 320 nm (sh). 1H NMR (C6D6,
Sn ) 2.8909(2) Å, Ar* ) C6H3-2,6-Trip2; Trip ) C6H2-2,4,6-Pri )
3
or Ar*SnSnPh2Ar* (Sn-Sn ) 2.9688(5) Å.7b Presumably, the large
substituent size, the strained geometries at the tins, as well as the
fact that bromine (and not hydrogen) is the bridging atom, contribute
to Sn-Sn bond lengthening in 1.15 The dimeric structure of 2 is
also significant in that the Sn-Sn distance, 2.7705(8) Å, is among
the shortest observed for R2SnSnR2 compounds1,2 and lends support
to the idea that a species analogous to V is intermediate between
R2SnSnR2 and RSnSnR3. The 119Sn NMR chemical shift of 2
(1205.7 ppm) indicates that the dimeric structure is retained in
solution and does not coincide with either of the two 119Sn NMR
signals seen for 1.13
In summary, the isolation of 1 increases the number of structur-
ally modelled isomers of a distannene to four. Only the cis, doubly
bridged isomer III remains unmodelled.
3
399.77 MHz, 25 °C): δ 1.01 (d, JHH ) 6.4 Hz, 12H, CH(CH3)2), 1.14
(d, 3JHH ) 6.4 Hz, 12H, CH(CH3)2), 1.23 (s, 9H, t-C(CH3)3), 2.08 (s, 2H,
CH2C6H4-4-But), 6.97-7.07 (m, 11H, aromatic Hs). 13C NMR (100.53
MHz, C6D6, 25 °C): δ 23.05 (CH(CH3)2), 26.57 (C(CH3)3), 31.98
(C(CH3)3), 34.11 (-CH2-), 123.99 (m-Dipp), 125.72 (m-C6H3), 126.78
(p-C6H4), 126.99 (m-C6H4), 129.99 (m-Dipp), 125.72 (m-C6H3), 126.78
(p-C6H4), 126.99 (m-C6H4), 129.19 (p-Dipp), 129.79 (o-C6H4), 137.54
(i-C6H4), 140.49 (p-C6H3), 144.56 (o-C6H3), 145.81 (i-Dipp), 147.21
(o-Dipp), 199.20 (i-C6H3). 119Sn NMR (C6D6, 149.20 MHz, 25 °C): δ
1205.7.
Acknowledgment. We are grateful to the National Science
Foundation (CHE-0096913) for financial support.
Supporting Information Available: X-ray data (CIF) for 1 and 2.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(11) Phillips, A. D.; Wright, R. J.; Olmstead, M. M.; Power, P. P. J. Am. Chem.
Soc. 2002, 124, 5930.
References
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)
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See: Eichler, B. E.; Pu, L.; Stender, M.; Power, P. P. Polyhedron 2001,
20, 551 and refs therein.
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have also shown that the monohydrogen-bridged structure is also a true
minimum for M ) Si, and has been detected by microwave spectroscopy.
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