Stabilization of a dialkylstannylene by unusual B-H...Sn γ-agostic-type interactions. A structural, spectroscopic, and DFT study

Article abstract of DOI:10.1021/om0600036

The reaction between SnCl₂ and the lithium salt [[{nPr ₂P(BH₃)}(Me₃Si)CCH₂]Li(THF) ₂]₂ yields the cyclic dialkylstannylene [{nPr ₂P(BH₃)}(Me₃Si)CCH₂]₂Sn (6) as a 1:1 mixture of rac and meso diastereomers. Fractional crystallization from n-hexane gives samples of pure rac-6 and meso-6 which have been studied by X-ray crystallography, UV/visible, infrared, and multi-element NMR spectroscopy. In the solid state, rac-6 exhibits two short agostic-type B-H...Sn contacts, one to each BH₃ group, whereas meso-6, in which both BH₃ groups lie on the same side of the heterocycle, exhibits a single B-H...Sn contact. Multi-element and variable-temperature NMR spectroscopy suggests that these contacts persist in solution, resulting in unusual ¹¹⁹Sn chemical shifts of 587 and 787 ppm for the rac and meso diastereomers, respectively, significantly upfield in comparison to other dialkylstannylenes. Variable-temperature ¹H, ³¹P{ ¹H}, and ¹¹B{¹H} NMR studies reveal dynamic equilibria for the meso diastereomer involving competitive binding of the two BH₃ groups to the electron-deficient tin center and restricted rotation about the P-C bond of the free phosphine-borane group. DFT studies confirm that the B-H...Sn contacts are an integral part of the structures; the HOMO and LUMO of both stereoisomers are comprised of an orbital of essentially 5p character on tin and an orbital of predominantly 5p character on tin perpendicular to the plane of the heterocycle, respectively. There is significant donation of electron density from the B-H σ-bonds to the vacant 5p orbital on tin; these B-H...Sn interactions stabilize the electron-deficient Sn(II) center by 30 kcal mol^-1 for meso-6 and 40 kcal mol^-1 for rac-6.

Full text of DOI:10.1021/om0600036

Organometallics 2006, 25, 1135-1143
1135
Stabilization of a Dialkylstannylene by Unusual B-H‚‚‚Sn
γ-Agostic-Type Interactions. A Structural, Spectroscopic, and DFT
Study
Keith Izod,* William McFarlane, Brent V. Tyson, Ian Carr, William Clegg, and
Ross W. Harrington
Department of Chemistry, School of Natural Sciences, Bedson Building, UniVersity of Newcastle,
Newcastle upon Tyne, NE1 7RU, U.K.
ReceiVed January 3, 2006
The reaction between SnCl2 and the lithium salt [[{nPr2P(BH3)}(Me3Si)CCH2]Li(THF)2]2 yields the
cyclic dialkylstannylene [{nPr2P(BH3)}(Me3Si)CCH2]2Sn (6) as a 1:1 mixture of rac and meso
diastereomers. Fractional crystallization from n-hexane gives samples of pure rac-6 and meso-6 which
have been studied by X-ray crystallography, UV/visible, infrared, and multi-element NMR spectroscopy.
In the solid state, rac-6 exhibits two short agostic-type B-H‚‚‚Sn contacts, one to each BH3 group,
whereas meso-6, in which both BH3 groups lie on the same side of the heterocycle, exhibits a single
B-H‚‚‚Sn contact. Multi-element and variable-temperature NMR spectroscopy suggests that these contacts
persist in solution, resulting in unusual 119Sn chemical shifts of 587 and 787 ppm for the rac and meso
diastereomers, respectively, significantly upfield in comparison to other dialkylstannylenes. Variable-
temperature 1H, 31P{1H}, and 11B{1H} NMR studies reveal dynamic equilibria for the meso diastereomer
involving competitive binding of the two BH3 groups to the electron-deficient tin center and restricted
rotation about the P-C bond of the free phosphine-borane group. DFT studies confirm that the B-H‚‚‚Sn
contacts are an integral part of the structures; the HOMO and LUMO of both stereoisomers are comprised
of an orbital of essentially 5s character on tin and an orbital of predominantly 5p character on tin
perpendicular to the plane of the heterocycle, respectively. There is significant donation of electron density
from the B-H σ-bonds to the vacant 5p orbital on tin; these B-H‚‚‚Sn interactions stabilize the electron-
deficient Sn(II) center by 30 kcal mol-1 for meso-6 and 40 kcal mol-1 for rac-6.
diamidocarbenes.2-5 In such compounds, the electron-deficient
tetrel center is thermodynamically stabilized by a combination
Introduction
There has been significant progress over the past decade in
the synthesis of heavier group 14 carbene analogues [tetrylenes,
R2E (E ) Si, Ge, Sn, Pb)].1-11 The vast majority of these
compounds are stabilized by the presence of heteroatoms directly
adjacent to the heavier tetrel center; of particular note in this
regard are the diamidotetrylenes (R2N)2E and related com-
pounds, which are analogues of the stable (Arduengo-type)
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Phillips, A. D.; Hino, S.; Power, P. P. J. Am. Chem. Soc. 2003, 125, 7520.
(c) Pu, L.; Olmstead, M. M.; Power, P. P.; Schiemenz, B. Organometallics
1998, 17, 5602. (d) Setaka, W.; Sakamoto, K.; Kira, M.; Power, P. P.
Organometallics 2001, 20, 4460. (e) Eichler, B. E.; Phillips, B. L.; Power,
P. P.; Augustine, M. P. Inorg. Chem. 2000, 39, 5450. (f) Simons, R. S.;
Pu, L.; Olmstead, M. M.; Power, P. P. Organometallics 1997, 16, 1920.
(g) Grutzmacher, H.; Pritzkow, H.; Edelmann, F. T. Organometallics 1991,
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1141. (j) Drost, C.; Hitchcock, P. B.; Lappert, M. F. Organometallics 1998,
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Schnering, H. G.; Marsmann, H. Angew. Chem., Int. Ed. Engl. 1994, 33,
1846. (m) Braunschweig, H.; Drost, C.; Hitchcock, P. B.; Lappert, M. F.;
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W.; Hirai, K.; Sakamoto, K.; Kira, M. J. Am. Chem. Soc. 2004, 126, 2696.
(o) Eichler, B. E.; Pu, L.; Stender, M.; Power, P. P. Polyhedron 2001, 20,
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Trans. 1976, 2275. (b) Davidson, P. J.; Harris, D. H.; Lappert, M. F. J.
Chem. Soc., Dalton Trans. 1977, 2268. (c) Zilm, K. W.; Lawless, G. A.;
Merrill, R. M.; Millar, J. M.; Webb, G. G. J. Am. Chem. Soc. 1987, 109,
7236.
* To whom correspondence should be addressed. E-mail: k.j.izod@
ncl.ac.uk.
(1) For recent reviews of heavier tetrylene chemistry, see: (a) Barrau,
J.; Rima, G. Coord Chem. ReV. 1998, 178-180, 593. (b) Tokitoh, N.;
Okazaki, R. Coord. Chem. ReV. 2000, 210, 251. (c) Kira, M. J. Organomet.
Chem. 2004, 689, 4475. (d) Weidenbruch, M. Eur. J. Inorg. Chem. 1999,
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see: (a) Fjeldberg, T.; Hope, H.; Lappert, M. F.; Power, P. P.; Thorne, A.
J. J. Chem. Soc., Chem. Commun. 1983, 639. (b) Olmstead, M. M.; Power,
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10.1021/om0600036 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 02/01/2006

1136 Organometallics, Vol. 25, No. 5, 2006
Izod et al.
Chart 1
Scheme 1
by Kira and co-workers in the synthesis of 2.8 We thus sought
to use our new dicarbanion complex 5 in the synthesis of a
novel dialkylstannylene. Herein, we describe the synthesis of
this stannylene, its separation into its diastereomers, their solid-
state structures, their behavior in solution, their remarkable
spectroscopic properties, and details of their unusual electronic
structures.
of efficient overlap between the heteroatom lone pairs and the
vacant tetrylene pπ orbital and the increased singlet-triplet
energy gap, due to stabilization of the sp2 lone pair on the tetrel
atom by the electronegative heteroatom centers.
In comparison to diamidotetrylenes, heavier tetrylenes pos-
sessing hydrocarbyl ligands have been less well explored.1
Thermodynamic stabilization of the tetrylene center is absent
in these compounds, which rely solely on kinetic stabilization
by sterically demanding substituents. Of the diorganostannylenes
reported to date homo- and heteroleptic aryl-substituted stan-
nylenes are the most numerous.6
In contrast, dialkylstannylenes are limited to only a very few
examples (Chart 1). The archetypal dialkylstannylene {(Me3-
Si)2CH}2Sn (1),7 reported by Lappert and co-workers in 1976,
exists as a monomer in the gas phase, but as a dimer [a
distannene, R2SndSnR2 (1a)] in the solid state; in solution, 1
is subject to a monomer-dimer equilibrium (1 a 1a). The first
dialkylstannylene shown to exist as a monomer in the solid state,
{(Me3Si)2CCH2}2Sn (2), was reported by Kira and co-workers
in 1991.8 The “helmet-like” bidentate ligand in this stannylene
bears a close resemblance to the two (Me3Si)2CH ligands in 1
but provides sufficient steric bulk to disfavor dimerization. Since
Kira’s original report, only one further dialkylstannylene has
been structurally characterized: Eaborn, Smith, and co-workers
recently reported the seven-membered cyclic stannylene {(Me3-
Si)2C(SiMe2CH2)}2Sn (3).9 In addition, the intramolecularly
base-stabilized dialkylstannylene {(Me3Si)2C(C5H4N)}2Sn (4)10
and a small number of heteroleptic alkylstannylenes of the form
RSnX [X ) e.g., aryl, halide, etc.] have been isolated,6f,o,10,11
including a heteroleptic, intramolecularly base-stabilized di-
alkylstannylene related to 4.11a,b
Results and Discussion
Synthesis and Solid-State Structures. The reaction between
anhydrous tin(II) chloride and the dilithium salt [[{nPr2P(BH3)}-
(Me3Si)CCH2]Li(THF)2]2 (5) in THF cleanly gives the dialkyl-
stannylene [{nPr2P(BH3)}(Me3Si)CCH2]2Sn (6) in excellent
yield as an air-sensitive yellow solid after a straightforward
workup (Scheme 1). Kira and co-workers noted that the closely
related compound 2 is isolated only in low yields via this route
and that its workup is somewhat difficult;8 better yields of 2
are obtained by the reduction of the dichlorostannane {(Me3-
Si)2CCH2}2SnCl2 with KC8.14 In contrast, 6 is isolated in
excellent yield and purity; we attribute this to an increase in
charge delocalization away from the carbanion center in 5
compared to [{(Me3Si)2CCH2}Li(THF)2]2 due to the greater
charge delocalizing ability of R2P(BH3) in comparison to SiMe3.
This makes the former dicarbanion less nucleophilic and, hence,
results in a decreased tendency toward reduction of the tin(II)
center to elemental tin. However, compound 6 is sensitive to
both heat and light, decomposing to give elemental tin and the
free phosphine-borane [{nPr2P(BH3)}(Me3Si)CHCH2]2 on
exposure to light for extended periods (over 1 week) or when
heated to 60°C in hexane solution.
Multi-element [1H, 13C{1H}, 11B{1H}, 31P{1H}, and 119Sn]
NMR spectroscopy establishes that 6 is isolated as a 1:1 mixture
of the two possible diastereomers under these conditions.
Extraction of 6 into n-hexane at room temperature yields
solutions containing mainly rac-6; extraction of the residue into
warm n-hexane gives solutions which contain predominantly
meso-6. Careful crystallization of these fractions from cold
n-hexane gives samples of pure rac- and meso-6 as yellow
blocks and yellow needles, respectively, both of suitable quality
for X-ray crystallography.
The crystal structures of rac- and meso-6 are shown in Figure
1 along with details of selected bond lengths and angles. The
two diastereomers crystallize as discrete dialkylstannylene
species; the closest Sn‚‚‚Sn distance in both cases is ap-
proximately 7.0 Å. Both rac- and meso-6 consist of a five-
membered stannacycle with Sn-C distances of 2.2984(14) and
2.3046(14) Å (rac-6) and 2.272(4) and 2.312(4) Å (meso-6),
respectively. These distances compare with Sn-C distances of
2.218(7) and 2.223(7) Å in the five-membered cyclic stannylene
27 and 2.284(3) and 2.286(3) Å in the seven-membered cyclic
stannylene 3.9 The C-Sn-C angles are 85.32(5)° and 84.53-
(15)° in rac- and meso-6, respectively, and these compare with
C-Sn-C angles of 86.7(2)° and 117.6(1)° in 2 and 3,8,9
respectively; the wide C-Sn-C angle in 3 is a consequence of
We have recently become interested in the chemistry of
phosphine-borane-stabilized carbanions, {R2P(BH3)}CR2-, and
noted that R2P(BH3) and R3Si groups are exactly isoelectronic.
In the course of our studies into these carbanions, we have
synthesized the dicarbanion complex [[{nPr2P(BH3)}(Me3Si)-
CCH2]Li(THF)2]2 (5)12 and observed its similarity to the silicon-
stabilized dicarbanion [{(Me3Si)2CCH2}Li(THF)2]2 originally
reported by Eisch and co-workers13 and subsequently employed
(11) (a) Cardin, C. J.; Cardin, D. J.; Constantine, S. P.; Drew, M. G. B.;
Rashid, H.; Convery, M. A.; Fenske, D. J. Chem. Soc., Dalton Trans. 1998,
2749. (b) Benet, S.; Cardin, C. J.; Cardin, D. J.; Constantine, S. P.; Heath,
P.; Rashid, H.; Teixeira, S.; Thorpe, J. H.; Todd, A. K. Organometallics
1999, 18, 389. (c) Al-Juaid, S. S.; Avent, A. G.; Eaborn, C.; Hill, M. S.;
Hitchcock, P. B.; Patel, D. J.; Smith J. D. Organometallics 2001, 20, 1223.
(d) Leung, W.-P.; Kwok, W.-H.; Zhou, Z.-Y.; Mak, T. C. W. Organome-
tallics 2003, 22, 1751. (e) Leung, W.-P.; Weng, L.-H.; Kwok, W.-H.; Zhou,
Z.-Y.; Zhang, Z.-Y.; Mak, T. C. W. Organometallics 1999, 18, 1482. (f)
Cardin, C. J.; Cardin, D. J.; Constantine, S. P.; Todd, A. K.; Teal, S. J.;
Coles, S. Organometallics 1998, 17, 2144. (g) Eaborn, C.; Hitchcock, P.
B.; Smith, J. D.; Sozerli, S. E. Organometallics 1997, 16, 5653.
(12) Izod, K.; McFarlane, W.; Tyson, B. V.; Clegg, W.; Harrington, R.
W. Chem. Commun. 2004, 570.
(13) (a) Eisch, J. J.; Beuhler, R. J. J. Org. Chem. 1963, 28, 2876. (b)
Eisch, J. J.; Gupta, G. J. Organomet. Chem. 1973, 50, C23.
(14) Kira, M.; Ishida, S.; Iwamoto, T.; Yauchibara, R.; Sakurai, H. J.
Organomet. Chem. 2001, 636, 144.

Dialkylstannylene B-H‚‚‚Sn Interactions
Organometallics, Vol. 25, No. 5, 2006 1137
is just 0.35 Å longer than the sum of the covalent radii of Sn
and H (1.68 Å).
To our knowledge, compound 6 represents the first example
of a B-H‚‚‚M agostic-type interaction involving a low-
oxidation-state main group metal center. Only one previous
example has been reported of a short B-H‚‚‚Sn contact: the
Sn‚‚‚H distance in the anion of [10-endo-(SnPh3)-10-µ-H-7,8-
nido-C2B9H10][trans-Ir(CO)(PPh3)2(MeCN)] (7),15 in which the
metal is directly bonded to the boron atom of a carborane cage,
is 2.349 Å.
Although such agostic-type P-B-H‚‚‚M interactions have
not previously been observed for main group metals, there have
been several recent reports of short contacts between the borane
hydrogen atoms of phosphine-borane adducts and transition
metal centers. For example, Shimoi and co-workers have isolated
complexes of the form M(CO)5{H3BPMe3} [M ) Cr, Mo, W],
which are formal analogues of transition metal-alkane σ-com-
plexes;16 Weller and co-workers have recently reported a range
of transition metal complexes of the mixed phosphine/phos-
phine-borane ligand Ph2PCH2P(BH3)Ph2, which provide mod-
els for γ-agostic interactions between transition metals and
hydrocarbyl ligands.17
The B-H distances for the H atoms associated with the tin
centers are similar to the remaining B-H distances in rac-6
[B(1)-H(1C), 1.12(2); B(2)-H(2C), 1.12(2) Å; remaining
B-H, 0.99(3)-1.11(2) Å]; in meso-6, in which there is only one
B-H‚‚‚Sn contact, this B-H distance [B(1)-H(1B), 1.27(5)
Å] appears to be somewhat longer than the remaining B-H
distances [1.00(7)-1.12(8) Å], although this is still within
statistical error. It has been noted that the bridging B-H bonds
in M(CO)5{H3BPMe3} [M ) Cr, Mo, W] and related com-
pounds are typically ca. 10% longer than the remaining, terminal
B-H bonds.16,17
Spectroscopic Characterization. The agostic-type interac-
tions in 6 mitigate the electron deficiency of the Sn(II) centers
and have significant consequences for the electronic structure
of the molecules and, consequently, for their spectroscopic
behavior. The IR spectrum of rac-6 exhibits moderately strong
absorptions at 2385, 2174, and 2107 cm-1 due to the B-H
stretching vibrations; for meso-6, these vibrations give rise to
absorptions at 2439, 2400, 2342, and 2046 cm-1. The absorp-
tions at 2107 and 2046 cm-1 for rac- and meso-6, respectively,
may be assigned to the B-H‚‚‚Sn stretching vibrations. DFT
calculations (see below) confirm this assignment: frequency
calculations indicate that the B-H‚‚‚Sn stretching vibration(s)
lie approximately 270-320 cm-1 lower than the terminal B-H
stretching vibrations in rac-6 and approximately 260-420 cm-1
lower in meso-6.
Figure 1. Molecular structures of (a) rac-6 and (b) meso-6 with
40% probability ellipsoids and with H atoms (except those bonded
to boron atoms) omitted for clarity. Selected bond lengths (Å) and
angles (deg): rac-6: Sn-C(1) 2.2984(14), Sn-C(4) 2.3046(14),
C(1)-P(1) 1.8137(4), C(1)-Si(1) 1.8953(15), C(4)-P(2) 1.8137-
(14), C(4)-Si(2) 1.8996(15), P(1)-B(1) 1.9317(18), P(2)-B(2)
1.9382(18), Sn‚‚‚(1C) 2.32(2), Sn‚‚‚H(1C) 2.32(2), C(1)-Sn-C(4)
85.32(5). meso-6: Sn-C(1) 2.312(4), Sn-C(4) 2.272(4), C(1)-
P(1) 1.802(5), C(1)-Si(1) 1.891(5), C(4)-P(2) 1.817(4), C(4)-
Si(2) 1.904(4), P(1)-B(1) 1.933(6), P(2)-B(2) 1.929(6), Sn‚‚‚H(1B)
2.03(5), C(1)-Sn-C(4) 84.53(15).
the formation of a seven-membered heterocycle and the need
to accommodate two tris(trialkylsilyl)methyl groups adjacent
to the tin atom.9 The C-Sn-C angles in rac- and meso-6 are
somewhat smaller than the corresponding angles in the gas-
phase structure of the acyclic dialkylstannylene 1 [96(1)°]7 and
the C-Sn-C angles in any of the crystallographically charac-
terized diarylstannylenes, which range from 97° to 115°.6
Consistent with their pale yellow colors, the UV/visible
spectra of rac- and meso-6 exhibit absorptions at 338 (ꢀ ) 3080
dm3 mol-1 cm-1) and 360 nm (ꢀ ) 570 dm3 mol-1 cm-1),
respectively. This is in marked contrast to the deep red or purple
monomeric dialkylstannylenes 2 and 3,8,9 which have absorp-
tions at 484 (ꢀ ) 400 dm3 mol-1 cm-1) and 546 nm (ꢀ ) 175
dm3 mol-1 cm-1), respectively; in 2, this absorption has been
Perhaps the most remarkable features of the structures of rac-
and meso-6 are the short, agostic-type B-H‚‚‚Sn contacts; for
both rac- and meso-6, the H atoms of the borane groups were
located in difference maps and were freely refined. Compound
rac-6 has approximate C2 symmetry, with one borane group
situated on either side of the five-membered heterocyclic ring;
one hydrogen atom from each borane group lies in close
proximity to the electron-deficient Sn(II) center [Sn‚‚‚H(1C),
2.31(2); Sn‚‚‚H(2C), 2.37(2) Å]. In meso-6, both borane groups
lie on the same side of the heterocycle and there is only one
short Sn‚‚‚H distance [Sn‚‚‚H(1B), 2.03(5) Å], although this is
significantly shorter than the corresponding distances in rac-6.
These distances are well within the sum of the van der Waals
radii of Sn and H (3.37 Å), and the Sn‚‚‚H distance in meso-6
(15) Kim. J.; Kim, S.; Do, Y. J. Chem. Soc., Chem. Commun. 1992,
938.
(16) (a) Shimoi, M.; Nagai. S.-I.; Ichikawa, M.; Kawano, Y.; Katoh,
K.; Uruichi, M.; Ogino, H. J. Am. Chem. Soc. 1999, 121, 11704. (b)
Kakizawa, T.; Kawano, Y.; Shimoi, M. Organometallics 2001, 20, 3211.
(c) Shimoi, M.; KAtoh, K.; Kawano, Y.; Kodama, G.; Ogino, H. J.
Organomet. Chem. 2002, 659, 102.
(17) (a) Ingleson, M.; Patmore, N. J.; Ruggiero, G. D.; Frost, C. G.;
Mahon, M. F.; Willis, M. C.; Weller, A. S. Organometallics 2001, 20, 4434.
(b) Merle, M.; Koicok-Kohn, G, Mahon, M. F.; Frost, C. G.; Ruggiero, G.
D.; Weller, A. S.; Willis, M. C. Dalton Trans. 2004, 3883.

1138 Organometallics, Vol. 25, No. 5, 2006
Izod et al.
attributed to the transition between the lone pair and the vacant
5p orbital on tin. Clearly, the observed B-H‚‚‚Sn interactions
cause a significant perturbation of these orbitals and a conse-
quent shift in this absorption; for comparison, the compound
{2,4,6-(CF3)3C6H2}2Sn (7),6g in which the electron deficiency
of the Sn(II) center is mitigated by short C-F‚‚‚Sn contacts,
has λmax ) 345 nm (ꢀ ) 1534 dm3 mol-1 cm-1).
from thermal or photolytic decomposition of the samples, which
is in rapid exchange with diamagnetic 6. Unfortunately, attempts
to detect such a radical species by EPR spectroscopy were
unsuccessful.
The 1H, 11B{1H}, and 31P{1H} spectra of rac-6 in d8-toluene
are invariant over the temperature range 303-183 K. In contrast,
the corresponding spectra of meso-6 reveal that this compound
is subject to dynamic processes in solution. As the temperature
Multi-element and variable-temperature NMR spectroscopy
also indicate that the agostic-type B-H‚‚‚Sn interactions
observed in the solid-state persist in solution. At room temper-
ature, the 1H NMR spectra of rac- and meso-6 are as expected,
although in the latter compound, only a single resonance is
observed for the BH3 and SiMe3 groups, suggesting that meso-6
is subject to a dynamic process in solution by which the two
distinct environments observed for each of these groups in the
solid state are averaged on the NMR time scale (see below).
The SiMe3 groups give rise to sharp singlets at 0.25 and 0.14
1
is reduced below ambient, the signals in the H{11B} NMR
spectrum in the region associated with the propyl groups and
the CH2CH2 backbone protons become significantly broader and
more complex and increasingly difficult to interpret. However,
the signal due to the SiMe3 groups broadens as the temperature
is reduced until, at approximately 208 K, it decoalesces into
two very broad, equal-intensity signals at 0.00 and 0.31 ppm;
these signals sharpen as the temperature is reduced further, and
at 188 K, two relatively sharp singlets are observed. The BH3
resonances broaden and become obscured by the propyl
resonances as the temperature decreases; this is partly a
consequence of increased broadening by the 11B nuclei at lower
temperatures and to difficulties in effectively decoupling the
1H and 11B nuclei.
1
ppm in the H NMR spectra of rac- and meso-6, respectively.
The BH3 resonances of both rac- and meso-6 are so broad that
they are virtually indistinguishable from the baseline; these
signals collapse on decoupling the quadrupolar 11B nuclei to
give broad doublets at approximately 0.90 and 1.07 ppm for
rac- and meso-6, respectively (2JPH ) 7.6 Hz in both cases).
The propyl substituents give rise to a sequence of complex
multiplets between 0.77 and 2.03 ppm (rac-6) and between 0.84
and 1.88 ppm (meso-6), whereas the backbone CH2CH2 protons
give rise to complex multiplets at 2.30 and 2.87 ppm (rac-6)
and 2.36 and 2.69 ppm (meso-6). The 13C{1H} spectra of rac-
and meso-6 are essentially as expected: the quaternary carbon
centers give rise to broad signals at 38.14 and 42.00 ppm,
respectively; satellites due to coupling to the tin nuclei were
not resolved in either case.
A clearer picture of the solution behavior of meso-6 is
provided by its variable-temperature 31P{1H} and 11B{1H}
spectra. The 31P{1H} spectrum of meso-6 at 291 K consists of
a poorly resolved, broad quartet (Figure 2a). This signal
broadens as the temperature is lowered and decoalesces below
232 K into two signals at 18.3 and 25.1 ppm, the higher-field
signal of which is considerably sharper than the lower-field
signal. As the temperature is decreased, these signals broaden
and decoalesce further until, at 183 K, the spectrum consists of
two large, equal-intensity, poorly resolved quartets at 16.7 (A)
and 22.2 ppm (B) along with two smaller signals (∼16%
intensity of the major signals), one (C) partially obscured by
signal A, at approximately 18 ppm, and the other (D) at 34.0
ppm. The variable-temperature 11B{1H} spectra of meso-6 also
exhibit significant broadening at lower temperatures (Figure 3).
Below 213 K, the broad doublet observed at room temperature
decoalesces into two very broad signals of approximately equal
intensity. As the temperature decreases, these signals decoalesce
further until, at 183 K, the spectrum consists of two very broad
signals at -31 (E) and -34 ppm (F), the high-field signal of
which partially obscures a new moderate-intensity signal at
approximately -36 ppm (G).
The 31P{1H} spectra consist of broad, poorly resolved quartets
centered at 22.3 and 23.4 ppm for the rac- and meso-
diastereomers, respectively [1JPB ) 82 Hz, ∆ν1/2 ) 240 Hz (rac-
1
6); JPB ) 93 Hz, ∆ν1/2 ) 240 Hz (meso-6)], and the 11B{1H}
spectra consist of broad doublets at -34.5 [∆ν1/2 ) 180 Hz]
and -33.2 ppm [∆ν1/2 ) 180 Hz], respectively.
One of the most significant effects of the agostic-type
B-H‚‚‚Sn interactions in rac- and meso-6 is observed in their
119Sn spectra: rac- and meso-6 give rise to very broad,
featureless resonances at 587 and 787 ppm, respectively, in
which coupling to 11B and 31P is not resolved [∆ν1/2 ) 430 and
330 Hz, respectively]. These chemical shifts compare with a
shift of 2323 ppm for the structurally analogous five-membered
cyclic dialkylstannylene 2,8 a difference of more than 1500 ppm;
the closely related acyclic dialkylstannylene 1 and the seven-
membered cyclic stannylene 3 have 119Sn chemical shifts of
2315 and 2299 ppm, respectively.7,9 Since 6 and 2 differ only
in the substitution of two R2P(BH3) groups for two exactly
isoelectronic SiMe3 groups, this difference in chemical shift may
be attributed to the difference in electron density at the Sn(II)
centers caused by the agostic-type B-H‚‚‚Sn interactions. The
lower chemical shift of rac-6, in which there are two agostic-
type B-H‚‚‚Sn contacts, compared to meso-6, in which there
is only one, appears to further corroborate this premise. While
the 119Sn chemical shifts of 6 are substantially lower than those
of closely related dialkylstannylenes, they are similar to those
of the diarylstannylenes {2,6-(2,4,6-Me3C6H2)2C6H3}2Sn (635
ppm)6f and 7 (723 ppm).6g
These spectra are consistent with the operation of two distinct
dynamic equilibria in solution. We attribute the higher-energy
process to competitive binding of the two BH3 groups to the
Sn(II) center (Scheme 2), which is rapid on the NMR time scale
at room temperature but which may be frozen out below 232
K. Thus, at low temperatures, the solution-state structure
resembles that observed in the solid state, and so there are two
distinct SiMe3 and PPr2(BH3) environments.
Below 213 K, the operation of a second dynamic process
becomes apparent. We initially considered that this might be
due to a monomer-dimer (i.e., stannylene-distannene) equi-
librium, related to that observed for the stannylene 1 in solution.
Unfortunately, for meso-6, a variable-temperature 119Sn NMR
experiment was uninformative in this regard: the 119Sn signal
broadens significantly as the temperature is reduced until, below
213 K, the signal is indistinguishable from the baseline; rac-6
exhibits similar behavior. In contrast, the 119Sn spectrum of 1
at 165 K exhibits signals at 2315 ppm and at 740 and 725 ppm
due to the stannylene and distannene forms, respectively.7
The extremely broad nature of the 119Sn signals and the lack
of observable tin satellites in the 13C{1H} and 31P{1H} spectra
of rac- and meso-6 are consistent with the presence of a low-
concentration, possibly tin-centered, radical, most likely arising

Dialkylstannylene B-H‚‚‚Sn Interactions
Organometallics, Vol. 25, No. 5, 2006 1139
Figure 2. (a) Observed and (b) simulated variable-temperature 31P{1H} NMR spectra of meso-6 in d8-toluene. Rate A, exchange between
the free and bound PPr2(BH3) groups; rate B, exchange between rotamers of the free PPr2(BH3) group.
Scheme 3a
a Si ) SiMe3, P ) P(BH3)nPr2
concentration experiments suggest that formation of distannene
species at low temperature is unlikely under the conditions
used: 31P{1H} spectra of samples of meso-6 at concentrations
of 0.25 and 0.05 M recorded at temperatures between 273 and
188 K are essentially indistinguishable. Simulation of the
variable-temperature 31P{1H} spectra of meso-6 (Figure 2(b))
and a subsequent line shape analysis yields values of ∆H‡ )
34 ( 1 kJ mol-1 and ∆S‡ ) -35 ( 4 J K-1 mol-1 for the
phosphine-borane exchange process, and ∆H‡ ) 41 ( 2 kJ
mol-1 and ∆S‡ ) 10 ( 10 J K-1 mol-1 for the dynamic process
observed at lower temperatures; this is consistent with an
intramolecular process in both cases. We note that the closely
related cyclic stannylene 2 is monomeric in both the solid state
and in solution and that rac-6 does not exhibit dynamic behavior
in its NMR spectra. We note also that poorly resolved splitting
by 11B is apparent in the 31P spectrum of meso-6 at 291 K, thus
confirming, as expected, that this dynamic process does not
involve P-B bond cleavage.
Figure 3. Variable-temperature 11B{1H} NMR spectra of meso-6
in d8-toluene.
Scheme 2
We therefore propose that the dynamic process observed
below 213 K is associated with a restricted rotation about the
highly sterically hindered P-C bond of the free phosphine-
borane group (Scheme 3b). The low-temperature NMR spectra
are thus consistent with the presence of two rotamers, one of
which is significantly favored over the other; Newman projec-
tions of the three principal rotamers of meso-6 are illustrated in
Chart 2. It is to be expected that the two observed dynamic
A stannylene-distannene equilibrium for meso-6 should
generate two distinct distannene isomers (Scheme 3a) and so
should result in a much more complex 31P spectrum at low
temperatures than is observed. Although it is possible that only
the symmetric isomer of the distannene is accessible, variable-

1140 Organometallics, Vol. 25, No. 5, 2006
Izod et al.
Figure 4. Calculated structures for (a) rac-6a and (b) meso-6a.
Table 1. Comparison of Selected Bond Lengths (Å) and
Angles (deg) for rac-6 and 6a and for meso-6 and 6a
Chart 2. Newman Projections of the Three Principal
Rotamers of meso-6 Viewed along the P-C Bond of the Free
Phosphine-Borane Group
rac-6
rac-6a
meso-6
meso-6a
Sn-C(1)
2.2984(14)
2.3046(14)
2.31(2)
2.324
2.342
2.341
2.348
1.847
1.928
1.843
1.929
1.240
1.240
85.25
2.312(4)
2.272(4)
2.03(5)
2.370
2.301
2.155
Sn-C(4)
Sn‚‚‚H(1X)
Sn‚‚‚H(2X)
C(1)-P(1)
C(1)-Si(1)
C(4)-P(2)
C(4)-Si(2)
B(1)-H(1X)a
B(2)-H(2)
C(1)-Sn-C(4)
2.37(2)
1.8137(14)
1.8953(15)
1.8137(14)
1.8996(15)
1.12(2)
1.802(5)
1.891(5)
1.817(4)
1.904(4)
1.27(5)
1.836
1.921
1.854
1.939
1.255
1.12(2)
85.32(5)
processes (exchange of coordinated and free phosphine-borane
groups and rotation about the P-C bond of the free phosphine-
borane group) will be somewhat interdependent and thus that
the ability of a particular rotamer to participate in the phos-
phine-borane exchange process will influence its abundance.
In rotamers i and ii, the BH3 groups are positioned such that
they are able to participate readily in the process involving
exchange of the free and bound BH3 groups. The ease of this
exchange will probably be different in rotamers i and ii, so that
they will have different energies and therefore different abun-
dances; for rotamer iii, the borane exchange process would be
disfavored. Thus, rotamers i and ii have a significant abundance,
whereas rotamer iii is not observed. On purely steric grounds,
rotamers i and ii are likely to be more stable than rotamer iii
since in the latter rotamer the two propyl groups lie close to
the bulky SiMe3 group and the sterically occluded tin center.
Restricted rotation about the P-C bond can be expected to
affect the chemical shift of the free phosphine-borane group
more than that of the bound group, and so we tentatively assign
the low-field signals B and D in the low-temperature 31P{1H}
NMR spectra to the free phosphine-borane group and the high-
field signals A and C to the phosphine-borane group associated
with the B-H‚‚‚Sn agostic-type interaction.
Unfortunately, we were unable to distinguish the unique
B-H‚‚‚Sn proton signals from the remaining B-H resonances
in the low-temperature 1H or 1H{11B} spectra of either rac- or
meso-6 over the temperature range available to us, and so we
were unable to observe directly the B-H‚‚‚Sn contact by this
means. However, it is likely that exchange between individual
hydrogen atoms within a single BH3 group is a very low-energy
process and that this will be extremely fast on the NMR time
scale. Shimoi and co-workers have estimated that the free energy
of activation ∆G‡ for exchange between the terminal and
bridging H atoms in M(CO)5{η1-H3BPMe3} [M ) Cr, Mo, W]
is less than 30 kJ mol-1 at 193 K.16
84.53(15)
83.99
a X ) C (rac-6), B (meso-6).
electronic effects were taken into consideration, we have
modeled the geometries of the complete molecules; while this
is computationally expensive, it allows an exact comparison
between the solid-state structures determined by X-ray crystal-
lography and the calculated gas-phase structures of rac- and
meso-6. Geometries were optimized using the B3LYP hybrid
functional18 with an effective core potential Lanl2dz basis set19
on the tin atoms and an all-electron 6-31G(d,p) basis set on the
remaining atoms.20
The calculated structures of the two diastereomers (rac-6a
and meso-6a, respectively) are remarkably similar to those
determined crystallographically; the structures of rac- and meso-
6a are shown in Figure 4, and a comparison of selected bond
lengths and angles for both stereoisomers of 6 and 6a is given
in Table 1. In general, bond lengths in the calculated structures
are slightly longer, by approximately 0.03-0.07 Å, than those
determined crystallographically; for example, the Sn-C dis-
tances in rac-6 are 2.3046(14) and 2.2984(14) Å and in rac-6a
these are 2.342 and 2.324 Å, while the Sn-C distances in
meso-6 are 2.272(4) and 2.312(4) Å and in meso-6a these are
2.301 and 2.370 Å. However, the calculated angles are very
close to those observed in the solid state; the C-Sn-C angles
for rac-6 and rac-6a are 85.32(5)° and 85.25°, respectively,
whereas the corresponding angles for meso-6 and meso-6a are
84.53(15)° and 83.99°, respectively.
The calculated gas-phase structures of rac- and meso-6a
replicate the close H‚‚‚Sn contacts observed in the solid state
(18) (a) Becke A. D. J. Chem. Phys. 1993, 98, 5648. (b) Stephens, P. J.;
Devlin, F. J.; Chablowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98,
11623. (c) Hertwig, R. H.; Koch, W. Chem. Phys. Lett. 1997, 268, 345.
(19) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt,
W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay P. J.; Wadt, W. R.
J. Chem. Phys. 1985, 82, 299.
(20) (a) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
(b) Francl, M. M.; Petro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M.
S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654.
DFT Calculations. To gain further insight into the nature of
the Sn‚‚‚H interactions in these molecules, we have undertaken
a DFT study of both diastereomers. So that all steric and

Dialkylstannylene B-H‚‚‚Sn Interactions
Organometallics, Vol. 25, No. 5, 2006 1141
for rac- and meso-6 extremely well: there are two short Sn‚‚‚H
contacts in rac-6a, in which the two borane groups are on either
side of the heterocycle, and one short Sn‚‚‚H contact in meso-
6a, in which both borane groups are on the same side of the
heterocycle. The Sn‚‚‚H distances in rac-6a are 2.34 and 2.35
Å, while the Sn‚‚‚H distance in meso-6a is 2.14 Å; these
compare with Sn‚‚‚H distances of 2.31(2) and 2.37(2) Å in rac-6
and 2.03(5) Å in meso-6. Clearly, the shorter Sn‚‚‚H distances,
and thus the stronger interactions, in meso-6/6a appear to be a
direct consequence of the availability of only one borane group
with which to form an agostic-type interaction in this diaste-
reomer; in rac-6/6a the electron deficiency of the Sn(II) center
is mitigated by the formation of two weaker, agostic-type
interactions (see below).
The B-H bonds associated with the B-H‚‚‚Sn contact in
both rac-6a and meso-6a are slightly elongated (1.24/1.24 and
1.26 Å, respectively) in comparison to the remaining B-H
bonds, which span the range 1.20-1.22 Å. In accord with an
agostic-type interaction, population analysis reveals an ac-
cumulation of charge at the hydrogen atoms associated with
these interactions. Thus, in rac-6a, these hydrogen atoms have
Mulliken charges of -0.18 (for each of the two hydrogen atoms
close to the tin center) compared to charges ranging from -0.08
to -0.09 on the remaining borane H atoms; in meso-6a, the
charge on the H atom associated with the tin center is calculated
to be -0.19 compared with charges ranging from -0.07 to
-0.10 on the remaining borane H atoms. For comparison, Durig
and Shen have calculated that the borane hydrogen atoms in
the neutral adduct H3BPH3 have a Mulliken charge of -0.01
but that this increases to -0.035 in H3BPF3.21
Figure 5. HOMO (upper) and LUMO (lower) of (a) rac-6a and
(b) meso-6a.
constituent diastereomers. The electron deficiency at the Sn(II)
centers in 6 is mitigated by unique, agostic-type B-H‚‚‚Sn
interactions: one such interaction in the case of meso-6 and
two in the case of rac-6. Spectroscopic studies clearly indicate
that the short B-H‚‚‚Sn contacts observed by X-ray crystal-
lography persist in solution; these contacts have a marked effect
on the 119Sn chemical shifts of rac- and meso-6, which are more
than 1500 ppm lower than the chemical shift observed for the
corresponding silicon-substituted stannylene 2. Variable-tem-
perature NMR spectroscopy suggests that meso-6 is subject to
two independent dynamic equilibria involving (i) exchange
between the coordinated and free BH3 groups and (ii) restricted
rotation about the P-C bond of the free phosphine-borane
group. DFT calculations indicate that the B-H‚‚‚Sn agostic-
type interactions stabilize rac- and meso-6 by ca. 40 and 30
kcal mol-1, respectively, via substantial donation of the B-H
bonding electron density into the vacant 5p orbital on tin (the
LUMO in each case).
Consistent with the increased stabilization by two agostic-
type B-H‚‚‚Sn interactions in rac-6a compared to one such
interaction in meso-6a, the calculated energy of rac-6a is lower
than that of the meso diastereomer, although by just 5.7 kcal
mol-1
.
The nature of the HOMO and LUMO and of the interaction
between the tin centers and the borane groups in both rac- and
meso-6a was probed via natural bond orbital (NBO) analyses;22
the HOMO and LUMO of rac- and meso-6a are illustrated in
Figure 5. The calculated HOMO in both cases is largely
comprised of a lone pair on tin of essentially 5s character,
whereas the LUMO in each case is comprised predominantly
of the vacant 5px orbital on tin which lies perpendicular to the
plane of the heterocycle. There are significant interactions
between the vacant Sn 5px orbitals and the B-H σ-orbitals in
both diastereomers. The NBO analyses reveal that there is
substantial delocalization of the B-H σ-bonding electron density
into the Sn 5px orbital, from two B-H bonds in rac-6a and
one B-H bond in meso-6a. Analysis of the donor-acceptor
interactions reveals that these agostic-type interactions stabilize
Thus, the incorporation of phosphine-borane functionality
provides a novel method for the stabilization of electron-
deficient dialkyltetrylenes. We are currently investigating
whether agostic-type B-H‚‚‚Sn and related interactions may
be used to provide sufficient stabilization to enable the isolation
of less sterically hindered diorganotetrylenes than 6.
Experimental Section
All manipulations were carried out using standard Schlenk
techniques under an atmosphere of dry nitrogen or argon. Ether,
n-hexane, and light petroleum (bp ) 40-60 °C) were distilled under
nitrogen from sodium/potassium alloy; all solvents were stored over
a potassium film. Deuterated toluene was distilled from potassium
and deoxygenated by three freeze-pump-thaw cycles and was
stored over activated 4 Å molecular sieves. All compounds were
used as supplied by the manufacturer except [[{nPr2P(BH3)}(Me3-
Si)CCH2]Li(THF)2]2 (5),12 which was prepared by a previously
published method.
rac- and meso-6a by 40 (i.e., 2 × 20) and 30 kcal mol-1
,
respectively.
Conclusions
The novel diastereomeric dialkylstannylene 6 may be syn-
thesized in excellent yield and readily separated into its
(21) Durig, J. R.; Shen, Z. J. Mol. Struct. (THEOCHEM) 1997, 397,
179.
NMR spectra were recorded on a JEOL Lambda500 spectrometer
operating at 500.16 (1H), 125.65 (13C), 160.35 (11B), 99.25 (29Si),
202.47 (31P), and 186.50 (119Sn) MHz; chemical shifts are quoted
in ppm relative to tetramethylsilane (1H, 13C, and 29Si), external
BF3(OEt2) (11B), external 85% H3PO4 (31P), and external Me4Sn
(22) (a) Carpenter, J. E.; Weinhold, F. J. Mol. Struct. (THEOCHEM)
1988, 169, 41. (b) Carpenter, J. E. PhD thesis, University of Wisconsin,
Madison, WI, 1987. (c) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980,
102, 7211. (d) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 78, 4066.
(e) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 1736. (f) Reed, A. E.;
Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735. (g) Reed,
A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899.
(
119Sn), as appropriate. The positions of the BH3 signals in the 1H
NMR spectra of rac- and meso-6 and JPH for these signals were

1142 Organometallics, Vol. 25, No. 5, 2006
Izod et al.
Table 2. Crystallographic Data for rac-6 and meso-6a
1
determined using a selective H{11B} experiment. X-band EPR
spectra were recorded at 295 and 77 K as solutions/frozen solutions
in toluene on a Bruker EMX spectrometer. UV/vis spectra were
recorded in matched quartz cells as 1.0 mM solutions in methyl-
cyclohexane on a Hitachi U-3010 spectrometer; infrared spectra
were recorded as powders on a Nicolet Avatar 370DTGS spec-
trometer. Elemental analyses were obtained by the Elemental
Analysis Service of London Metropolitan University.
rac-6
meso-6
formula
fw
cryst size, mm
cryst syst
space group
a, Å
C22H56B2P2Si2Sn
579.1
C22H56B2P2Si2Sn
579.1
0.15 × 0.15 × 0.15 0.20 × 0.10 × 0.10
monoclinic
P21/n
monoclinic
P21/n
11.3845(8)
17.4859(12)
16.5863(11)
106.129(1)
3171.8(4)
4
7.0626(4)
11.9388(6)
37.699(2)
93.165(2)
3173.9(3)
4
b, Å
c, Å
Preparation of [{nPr2P(BH3)}(Me3Si)CCH2]2Sn (6). To a cold
(-78 °C) slurry of SnCl2 (0.52 g, 2.74 mmol) in diethyl ether (20
mL) was added, dropwise, a solution of [{nPr2P(BH3)}(Me3Si)-
CCH2]2Li2(THF)4 (1.85 g, 2.42 mmol) in diethyl ether (30 mL),
excluding light as much as possible. This mixture was allowed to
attain room temperature and was stirred for 18 h. Solvent was
removed in vacuo, and the sticky solid was extracted into warm
n-hexane (50 mL) and filtered. The solution was concentrated to
ca. 10 mL and was cooled to -30 °C for 24 h. The yellow crystals
of rac-/meso-6 (1:1) were isolated by filtration and were washed
with a little cold (-78 °C) light petroleum. Yield 1.43 g, 78%.
Anal. Calcd for C22H56P2Si2B2Sn: C, 45.87; H, 9.27%. Found: C,
45.63; H, 9.75%.
â, deg
V, Å3
Z
F
calcd, g cm-3
1.213
0.99
19862
1.212
0.99
µ, mm-1
no. reflns measd.
no. unique reflns, Rint
22094
5587, 0.041
4872
0.827-0.908
0.049, 0.099
0.057, 0.103
1.196
5970, 0.028
no. reflns with F2 > 2σ(F2) 5733
trans coeff range
R, Rwa (F2 > 2σ)
R, Rwa (all data)
Sa
0.790-0.866
0.021, 0.054
0.022, 0.055
1.041
refined params
296
296
max, min diff map, e Å-3
0.37, -0.52
1.42, -1.37
2
2
a Conventional R ) ∑||Fo| - |Fc||/∑|Fo|; Rw ) [∑w(Fo - Fc )2/
The rac and meso isomers of 6 were separated by the following
procedure. The 1:1 mixture of crystalline rac- and meso-6 was
extracted into n-hexane (10 mL) at room temperature, leaving a
yellow residue, and this extract was cooled to 5 °C for 18 h to
give a crop of predominantly cube-shaped crystals of rac-6. The
mother liquor was warmed slightly until the small amount of needle-
shaped crystals of meso-6 had dissolved, and the crystals of pure
rac-6 were isolated by filtration. The yellow residue from the initial
extraction was extracted into hot (50 °C) n-hexane, and the yellow
solution was filtered, concentrated to 5 mL, and cooled to 5 °C for
18 h. The pale yellow, needle-shaped crystals of meso-6 were
isolated by filtration and washed with a little cold (-78 °C) light
petroleum.
∑w(Fo )2]1/2; S ) [∑w(Fo2 - Fc )2/(no. data - no. params)]1/2 for all data.
2
2
904 (s), 852 (s), 826 (s), 779 (m), 752 (m), 734 (m), 679 (m), 640
(s), 623 (m), 590 (s), 463 (m), 434 (m) cm-1
.
Crystal Structure Determinations of rac- and meso-6. For rac-
6, measurements were made at 120 K on a Bruker Nonius APEX2
CCD diffractometer using a synchrotron X-ray source (λ ) 0.6898
Å); for meso-6, measurements were made at 150 K on a Bruker
AXS SMART CCD diffractometer using graphite-monochromated
Mo KR radiation (λ ) 0.71073 Å). For both compounds, cell
parameters were refined from the observed positions of all strong
reflections in each data set. Intensities were corrected semi-
empirically for absorption on the basis of symmetry-equivalent and
repeated reflections. The structures were solved by direct methods
and refined on F2 values for all unique data. Table 2 gives further
details. The hydrogen atoms of the BH3 groups were located in
difference maps and were freely refined. Non-hydrogen atoms were
refined anisotropically, and all remaining H atoms were constrained
with a riding model; U(H) was set at 1.2 (1.5 for methyl groups)
times Ueq for the parent atom. Programs were Bruker AXS SMART
(control) and SAINT (integration), and SHELXTL for structure
solution, refinement, and molecular graphics.23
DFT Calculations. Geometry optimizations on the gas-phase
molecules were performed with the Gaussian03 suite of programs
(revision C.01)24 on a cluster of Compaq ES40 Alphaservers, via
the EPSRC National Service for Computational Chemistry Software
(http://www.nsccs.ac.uk). Optimizations were performed using the
B3LYP hybrid functional18 with an Lanl2dz effective core potential
basis set19 for Sn and a 6-31G(d,p) all-electron basis set on the
remaining atoms20 (default parameters were used throughout).
Minima were confirmed by the absence of imaginary vibrational
frequencies. NBO analyses were performed using the NBO 3.0
module of Gaussian03.22
Spectroscopic data for rac-6: 1H NMR (d8-toluene, 20 °C): δ
0.25 (s, 18H, SiMe3), 0.77 (t, 6H, CH2CH2CH3), 0.83 (t, 6H, CH2-
CH2CH3), 0.90 (br d, JPH ) 7.6 Hz, 6H, BH3), 1.31-2.03 (m, 16H,
CH2CH2CH3), 2.30 (m, 2H, CH2CH2), 2.87 (m, 2H, CH2CH2). 13C-
{1H}NMR (d8-toluene, 20 °C): δ 3.11(d, JPC ) 2.0 Hz, SiMe3),
16.21 (d, JPC ) 6.4 Hz, CH2CH3), 16.32 (d, JPC ) 6.2 Hz, CH2CH3),
17.62 (CH2CH3), 18.73 (CH2CH3), 31.01 (d, JPC ) 10.2 Hz, CH2P),
31.25 (d, JPC ) 3.2 Hz, CH2P), 38.14 (d, JPC ) 5.3 Hz, CSn), 39.73
(d, JPC ) 15.6 Hz, CH2CH2). 11B{1H} NMR (d8-toluene, 20 °C):
δ -34.5 (br d, JPB ) 82 Hz). 29Si{1H} NMR (d8-toluene, 20 °C):
δ ) 2.57 (d, JPSi ) 9.1 Hz). 31P{1H} NMR (d8-toluene, 20 °C): δ
22.3 (br q, JPB ) 82 Hz). 119Sn NMR (d8-toluene, 20 °C) δ 578
(br). UV/vis: λmax 338 nm (ꢀ ) 3080 dm3 mol-1 cm-1). IR: 2963
(m), 2931 (m), 2872 (m), 2832 (m), 2817 (m), 2385 (m), 2174
(w), 2107 (m), 1455 (m), 1405 (w), 1738 (w), 1247 (s), 1078 (s),
1050 (m), 1029 (m), 976 (w), 900 (s), 833 (s), 757 (s), 686 (m),
675 (m), 646 (s), 594 (s), 459 (s), 444 (s), 416 (m) cm-1
.
Spectroscopic data for meso-6: 1H NMR (d8-toluene, 20 °C): δ
0.14 (s, 18H, SiMe3), 0.84 (m, 12H, CH2CH2CH3), 1.07 (br d,
JPH ) 7.6 Hz, 6H, BH3), 1.16-1.88 (m, 16H, CH2CH2CH3), 2.36
(m, 2H, CH2CH2), 2.69 (m, 2H, CH2CH2). 13C{1H}NMR (d8-
toluene, 20 °C): δ 2.35 (d, JPC ) 2.1 Hz, SiMe3), 16.26 (d, JPC
)
Acknowledgment. The authors are grateful to the Royal
Society and the EPSRC for support and to the EPSRC-funded
National Crystallography Service and CCLRC for access to
synchrotron facilities. We thank Dr. C. Dennison and Dr. K.
Sato for undertaking the EPR spectroscopy.
13.8 Hz, CH2CH3), 16.38 (d, JPC ) 13.6 Hz, CH2CH3), 17.57 (CH2-
CH3), 18.45 (CH2CH3), 29.81 (d, JPC ) 26.5 Hz, CH2P), 31.25 (d,
JPC ) 33.4 Hz, CH2P), 39.86 (CH2CH2), 42.00 (CSn). 11B{1H}
NMR (d8-toluene, 20 °C): δ -33.2 (br, d, JPB ) 93 Hz). 29Si{1H}
NMR (d8-toluene, 20°C): δ ) 0.98 (d, JPSi ) 3.7 Hz). 31P{1H}
NMR (d8-toluene, 20 °C): δ 23.4 (br, d, JPB ) 93 Hz). 119Sn NMR
(d8-toluene, 20°C): δ 787 (br). UV/vis: λmax 360 nm (ꢀ ) 570
dm3 mol-1 cm-1). IR: 2962 (m), 2930 (m), 2871 (m), 2439 (w),
2400 (m), 2342 (m), 2046 (m), 1456 (w), 1405 (w), 1380 (w), 1261
(m), 1248 (s), 1218 (w), 1081 (m), 1067 (s), 1040 (m), 979 (m),
(23) (a) APEX2, SMART and SAINT software for CCD diffractometers;
Bruker AXS, Inc.: Madison, WI, 2004 and 1997. (b) Sheldrick, G. M.
SHELXTL user manual, version 6; Bruker AXS, Inc.: Madison, WI, 2001.
(24) Frisch, M. J. et al. Gaussian 03, rev C.01; Gaussian, Inc.:
Wallingford CT, 2004.

Dialkylstannylene B-H‚‚‚Sn Interactions
Organometallics, Vol. 25, No. 5, 2006 1143
Supporting Information Available: For rac- and meso-6,
details of structure determination, atomic coordinates, bond lengths
and angles, and displacement parameters in CIF format. For rac-
and meso-6a, details of DFT calculations, final atomic coordinates,
and energies. Complete details of ref 24. This material is available
free of charge via the Internet at http://pubs.acs.org. Observed and
calculated structure factor details are available from the authors
upon request.
OM0600036