Characterization of the sterically encumbered terphenyl-substituted species 2,6-Trip2H3C6S n-Sn(Me)2C6H3-2,6 -Trip2, an unsymmetric, group 14 element, methylmethylene, valence isomer of an alkene, its related lithium derivative

Article abstract of DOI:10.1021/ic0005137

The reaction of the recently reported sterically encumbered terphenyl tin(II) halide species Sn(Cl)C₆H₃-2,6-Trip₂ (Trip = C₆H₂-2,4,6-i-Pr₃), 1, with 1 equiv of MeLi or MeMgBr afforded 2,6-Trip₂H₃C₆ Sn-Sn(Me)₂C₆H₃ -2,6-Trip₂, 2, which is the first stable group 14 element methylmethylene (i.e., CH₃CH) analogue of ethylene (H₂CCH₂). Reaction of 1 with 1.5 equiv of MeLi yielded the stannylstannate species 2,6-Trip₂H₃C₆ (Me)₂Sn-Sn(Li)(Me)-C₆H₃ -2,6-Trip₂, 3, whereas reaction of 1 with 1 equiv of t-BuLi gave the heteroleptic stannanediyl monomer Sn(t-Bu)C₆H₃-2,6-Trip₂ (4). The compounds 2-4 were characterized by ¹H, ¹³C (⁷Li, 3 only), and ¹¹⁹Sn NMR spectroscopy in solution and by UV-vis spectroscopy. The X-ray crystal structures of 2-4 were also determined. The formation of the stannylstannanediyl 2 instead of the expected symmetrical, valence isomer distannene form {Sn(Me)C₆H₃-2,6-Trip₂}₂, 6, is explained through the ready formation of LiSn(Me)₂C₆H₃-2,6-Trip₂, 5, which reacts rapidly with 1 to produce 2 which can then react with a further equivalent of MeLi to give 3. The stability of singly bonded 2 in relation to the formally doubly bonded 6 was rationalized on the basis of the difference in the strength of their tin-tin bonds. In contrast to the methyl derivatives, the reaction of 1 with t-BuLi proceeded smoothly to give the monomeric compound 4. Apparently, the formation of a t-Bu analogue of 5 was prevented by the more crowding t-Bu group. Compound 2 is also the first example of a stable molecule with bonding between a two-coordinate, bivalent tin and four-coordinate tetravalent tin. Both compounds 2 and 3 display large J ¹¹⁹Sn-¹¹⁹Sn couplings between their tin nuclei and the tin-tin bond lengths in 2 (2.8909(2) A) and 3 (2.8508-(4) A) are relatively normal despite the presence of the sterically crowding terphenyl substituents.

Full text of DOI:10.1021/ic0005137

5444
Inorg. Chem. 2000, 39, 5444-5449
Characterization of the Sterically Encumbered Terphenyl-Substituted Species
2,6-Trip2H3C6S2n-Sn(Me)2C6H3-2,6-Trip2, an Unsymmetric, Group 14 Element,
Methylmethylene, Valence Isomer of an Alkene, Its Related Lithium Derivative
2,6-Trip2H3C6(Me)2Sn-Sn(Li)(Me)C6H3-2,6-Trip2, and the Monomer Sn(t-Bu)C6H3-2,6-Trip2
(Trip ) C6H2-2,4,6-i-Pr3)
Barrett E. Eichler and Philip P. Power*
Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616
ReceiVed May 12, 2000
The reaction of the recently reported sterically encumbered terphenyl tin(II) halide species Sn(Cl)C6H3-2,6-Trip2
(Trip ) C6H2-2,4,6-i-Pr3), 1, with 1 equiv of MeLi or MeMgBr afforded 2,6-Trip2H3C6S¨n-Sn(Me)2C6H3-2,6-
Trip2, 2, which is the first stable group 14 element methylmethylene (i.e., CH3CH) analogue of ethylene (H2CCH2).
Reaction of 1 with 1.5 equiv of MeLi yielded the stannylstannate species 2,6-Trip2H3C6(Me)2Sn-Sn(Li)(Me)-
C6H3-2,6-Trip2, 3, whereas reaction of 1 with 1 equiv of t-BuLi gave the heteroleptic stannanediyl monomer
1
Sn(t-Bu)C6H3-2,6-Trip2 (4). The compounds 2-4 were characterized by H, 13C (7Li, 3 only), and 119Sn NMR
spectroscopy in solution and by UV-vis spectroscopy. The X-ray crystal structures of 2-4 were also determined.
The formation of the stannylstannanediyl 2 instead of the expected symmetrical, valence isomer “distannene”
form {Sn(Me)C6H3-2,6-Trip2}2, 6, is explained through the ready formation of LiSn(Me)2C6H3-2,6-Trip2, 5, which
reacts rapidly with 1 to produce 2 which can then react with a further equivalent of MeLi to give 3. The stability
of singly bonded 2 in relation to the formally doubly bonded 6 was rationalized on the basis of the difference in
the strength of their tin-tin bonds. In contrast to the methyl derivatives, the reaction of 1 with t-BuLi proceeded
smoothly to give the monomeric compound 4. Apparently, the formation of a t-Bu analogue of 5 was prevented
by the more crowding t-Bu group. Compound 2 is also the first example of a stable molecule with bonding
between a two-coordinate, bivalent tin and four-coordinate tetravalent tin. Both compounds 2 and 3 display large
J 119Sn-119Sn couplings between their tin nuclei and the tin-tin bond lengths in 2 (2.8909(2) Å) and 3 (2.8508-
(4) Å) are relatively normal despite the presence of the sterically crowding terphenyl substituents.
Introduction
ligands, for example, a Trip or a phenyl group, oligomers such
5
as the three- or six-membered ring compounds c-{SnTrip2}3
6
Recent studies of the reactivity of Pb(Br)C6H3-2,6-Trip2 (Trip
) C6H2-2,4,6-i-Pr3) with MeMgBr, Li(t-Bu), and LiPh showed
that the first two-coordinate lead(II) derivatives of simple
organic ligands such as CH3, t-Bu, or Ph could be stabilized by
the use of the sterically encumbering terphenyl group C6H3-
2,6-Trip2 as coligand.1 It was expected that the corresponding
reactions of the related tin halide Sn(Cl)C6H3-2,6-Trip2 (1)2 with
these reagents would lead to products with similar stoichiom-
etries in which tin is bound to these simple groups. In general,
stable, two-coordinate dialkyls or diaryls of tin(II) can be
synthesized by the reaction of an Sn(II) halide with lithium
alkyls or aryls, and, provided that the ligands are of sufficient
size, the diorganotin(II) products are isolated either as mono-
mers3 or as “distannene” dimers4 that feature tin-tin bonds with
varying degrees of multiple character. With less crowded organic
or c-(SnPh2)6 are obtained. All of these species are related in
that they feature two organic groups at each tin and have the
metal in the formal oxidation state +2. Unlike these findings,
it is now shown that the reaction between MeLi or MeMgBr
with Sn(Cl)C6H3-2,6-Trip2 does not lead to the isolation of the
expected alkene-like symmetric product {Sn(Me)C6H3-2,6-
Trip2}2 (6) but to the first instance of a stable group 14 element
valence isomer of an alkene 2,6-Trip2H3C6S¨n-Sn(Me)2C6H3-
2,6-Trip2, 2, and the lithium salt 2,6-Trip2H3C6(Me)2Sn-Sn-
(Li)C6H3-2,6-Trip2, 3 (Scheme 1), in which the tin atoms have
different substituents and different formal oxidation states. In
contrast, the reaction with t-BuLi gives the monomeric stan-
nanediyl Sn(t-Bu)C6H3-2,6-Trip2. The reasons for this unex-
pected result involve the unique steric properties of the bulky
terphenyl group, the small size of the methyl substituents, and
the relative weakness of the putative multiple Sn-Sn bond in
6.
(1) Pu, L.; Twamley, B.; Power, P. P. Organometallics 2000, 19, 2471.
(2) Pu, L.; Senge, M. O.; Olmstead, M. M.; Power, P. P. J. Am. Chem.
Soc. 1998, 120, 12682.
(3) For example: (a) Kira, M.; Yauchibara, R.; Hirano, R.; Kabuto, C.;
Sakurai, H. J. Am. Chem. Soc. 1991, 113, 7785. (b) Weidenbruch,
M.; Schlaefke, J.; Scha¨fer, A.; Peters, K.; Schnering, H. G. v.;
Marsmann, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 1846. (c)
Simons, R. S.; Pu, L.; Olmstead, M. M.; Power, P. P. Organometallics
1997, 16, 1920. (d) Eaborn, C.; Hill, M. S.; Hitchcock, P. B.; Patel,
D.; Smith, J. D.; Zhang, S. Organometallics 2000, 19, 49.
(4) For example: (a) Davidson, P. J.; Lappert, M. F. J. Chem. Soc. Chem.
Commun. 1973, 317. (b) Goldberg, D. E.; Harris, D. H.; Lappert, M.
F.; Thomas, K. M. J. Chem. Soc. Chem. Commun. 1976, 261. (c) Layh,
U.; Pritzkow, H.; Gru¨tzmacher, H. J. Chem. Soc. Chem. Commun.
1992, 260. (d) Weidenbruch, M.; Kilian, H.; Peters, H.; Schnering,
H. G. v.; Marsmann, H. Chem. Ber. 1995, 128, 983.
(5) Masamune, S.; Sita, L. R. J. Am. Chem. Soc. 1985, 107, 6390.
(6) Olson, D. H.; Rundle, R. E. Inorg. Chem. 1963, 2, 1310.
10.1021/ic0005137 CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/05/2000

Sterically Encumbered Terphenyl-Substituted Species
Inorganic Chemistry, Vol. 39, No. 24, 2000 5445
Scheme 1. Illustration of the Possible Relationships between the Compounds 1-3, 5, and 6
1
257.4 (Sn (four-coordinate), J(119Sn-119/117Sn) ) 8332 Hz), 2856.9
Experimental Section
(Sn (two-coordinate), J(119Sn-119/117Sn) ) 8332 Hz).
1
General Procedures. All manipulations were carried out by using
modified Schlenk techniques under an atmosphere of N2 or in a Vacuum
Atmospheres HE-43 drybox. All solvents were distilled from Na/K alloy
and degassed three times before use. The compound Sn(Cl)C6H3-2,6-
Trip2 (1) was prepared according to literature procedures.2 Solutions
of MeLi, MeMgBr, and t-BuLi were purchased commerically and used
as received. 1H, 7Li, 13C, and 119Sn NMR spectroscopic data were
recorded on a Varian INOVA 400 MHz spectrometer. 1H and 13C
spectra were referenced to the deuterated solvent. The 7Li spectrum of
3 was referenced externally to LiCl/D2O and the 119Sn spectra were
referenced externally to SnMe4/C6D6. UV-vis data were recorded on
a Hitachi-1200 spectrometer.
2,6-Trip2H3C6S2n-Sn(Me)2C6H3-2,6-Trip2 (2). Method A: MeMg-
Br (0.77 mL, 2.30 mmol, 1.0 equiv) was added dropwise to a pale
yellow solution of 1 (1.47 g, 2.31 mmol) in pentane (80 mL) at ca.
-78 °C. Slow warming to ca. 0 °C caused the solution to turn bright
green with a concomitant formation of a colorless precipitate (mag-
nesium halide). The solution was further stirred at ca. 25 °C for 1 h
and the solvents were removed under reduced pressure. The green solid
was extracted with hexanes (100 mL) and the solution was filtered
through a Celite pad. The volume of the solution was then reduced to
ca. 10 mL and the flask was stored in a ca. -20 °C freezer overnight
to afford green crystals of 2 (1.25 g, 0.98 mmol, 85%).
2,6-Trip2H3C6(Me)2Sn-Sn(Li)(Me)C6H3-2,6-Trip2 (3). MeLi (2.40
mL, 3.84 mmol, 1.5 equiv) was added dropwise to an orange solution
of 1 (1.63 g, 2.56 mmol) in diethyl ether (40 mL) at ca. -10 °C. The
solution initially became a green color, but it had changed to yellow-
orange after the addition of the MeLi was completed. The solution
was warmed to ca. 10 °C and stirred for 5 min, after which the solvent
was removed under reduced pressure. The orange solid was extracted
with hexanes (40 mL) and filtered through a Celite pad. The volume
of the solution was reduced to ca. 3 mL under reduced pressure, and
the flask was stored at ca. -20 °C for 2 days to afford orange-yellow
crystals of 3 (1.51 g, 1.17 mmol, 91%). Mp ) 166 °C (dec to brown).
Anal. Calcd for C75H107LiSn: C, 71.89; H, 8.61. Found: C, 72.10; H,
9.05. UV-vis (hexanes) λmax, ꢀ(mol-1cm-1): 305 nm, 16050 (tails to
528 nm). 1H NMR (C6D6, 399.77 MHz, 25 °C): δ -0.05 (s, 3 H, Sn-
(CH3)2), -0.01 (s, 3 H, Sn(CH3)2), 1.04-1.37 (m, 36 H, o- and p-CH-
3
(CH3)2), 2.77-3.00 (m, 8 H, o-CH(CH3)2), 3.10 (sept., 2 H, J ) 6.8
3
Hz, p-CH(CH3)2), 3.54 (sept, 2 H, J ) 7.0 Hz, p-CH(CH3)2), 6.92-
7.40 (br m, 14 H, aromatic H’s). 7Li {1H} NMR (C6D6, 155.36 MHz):
1
2
δ -3.35 (1J(7Li-119Sn) ) 736 Hz, J(7Li-117Sn) ) 702 Hz, J(7Li-
119Sn) ) 47 Hz). 13C {1H} NMR (C6D6, 100.53 MHz): δ -3.82 (Sn-
(CH3)2), -1.61 (Sn-(CH3)2), 23.01-24.12 (multiple peaks, o-CH-
(CH3)2), 25.58-26.18 (multiple peaks, p-CH(CH3)2), 29.95 (o-
CH(CH3)2), 30.29 (o-CH(CH3)2), 30.94 (o-CH(CH3)2), 31.93 (o-
CH(CH3)2), 34.27 (p-CH(CH3)2), 34.45 (p-CH(CH3)2), 120.08 (m-Trip),
120.20 (m-Trip), 120.65 (m-Trip), 120.83 (m-Trip), 121.01 (m-Trip),
121.19 (m-Trip), 121.25 (m-Trip), 121.32 (m-Trip), 130.00 (m-C6H3),
130.24 (m-C6H3), 125.47 (p-C6H3), 142.65 (i-Trip), 146.65 (p-Trip),
147.86 (o-Trip), 147.90 (o-Trip), 147.92 (o-Trip), 148.10 (o-Trip),
148.58 (o-C6H3), 148.72 (o-C6H3), 161.20 (i-C6H3). 119Sn {1H} NMR
(C6D6, 149.24 MHz): δ -431 (q, 1J(119Sn-7Li) ) 736 Hz, 1J (119Sn-
Method B: MeLi (1.45 mL, 2.32 mmol, 1.1 equiv) was added
dropwise to a pale yellow solution of 1 (1.34 g, 2.11 mmol) in hexanes
(60 mL) at -78 °C, to afford a bright green solution. The solution was
stirred at -78 °C for 30 min and was allowed to warm to ca. 25 °C.
A workup procedure similar to method A afforded green crystals of 2
(0.25 g, 0.20 mmol, 9%). Mp ) 210-213 °C (dec to red). Anal. Calcd
for C37H52Sn: C, 72.20; H, 8.52. Found: C, 72.88; H, 8.96. UV-vis
(hexanes) λmax, ꢀ(mol-1 cm-1): 689 nm, 271. 1H NMR (C6D6, 399.77
1
1
119Sn) ) 4437 Hz, J (119Sn-117Sn) ) 4046 Hz)), +151 (s, J(119Sn-
3
MHz, 25 °C): δ -0.34 (s, 6H, Sn(CH3)2), 1.07 (br d, 24 H, J ) 6.6
1
119Sn ) 4464 Hz, J(119Sn-117Sn) ) 4088 Hz) (2J(119Sn-7Li) ) 47
Hz, o-CH(CH3)2), 1.31 (d, 12 H, 3J ) 7.0 Hz, p-CH(CH3)2), 2.85 (sept,
2 H, 3J ) 7.0 Hz, p-CH(CH3)2), 2.99 (br s, 2H, o-CH(CH3)2), 3.21 (br
s, 2 H, o-CH(CH3)2), 7.10 (s, 4 H, m-Trip), 7.07-7.22 (br m, 3 H, p-
and m-C6H3). 13C {1H} NMR (C6D6, 100.53 MHz): δ 0.34 (Sn(CH3)2),
24.12 (o-CH(CH3)2), 26.22 (p-CH(CH3)2), 30.85 (o-CH(CH3)2), 34.59
(p-CH(CH3)2), 121.90(m-Trip), 126.10 (m-Trip), 127.70 (i-Trip) 131.11
(br, o-Trip), 146.45 (p-Trip), 146.94 (o-C6H3), 147.19 (m-C6H3), 148.58
(p-C6H3), 165.23 (i-C6H3). 119Sn {1H} NMR (C6D6, 149.24 MHz): δ
Hz)).
Sn(t-Bu)(C6H3-2,6-Trip2) (4). t-BuLi (1.53 mL, 2.30 mmol, 1.1
equiv) was added dropwise to a pale yellow solution of 1 (1.33 g, 2.09
mmol) in hexanes (50 mL) at -78 °C. Slow warming to ca. -40 °C
caused the solution to turn deep red, and by ca. -5 °C, the solution
had turned dark purple. The solution was stirred at 25 °C for 1 h, and
the solution was filtered through a Celite pad. The volume of the

5446 Inorganic Chemistry, Vol. 39, No. 24, 2000
Eichler and Power
Table 1. Selected Crystallographic Data for Compounds 2‚0.5
Hexane, 3‚0.5 Hexane, and 4
affords 2 in up to 85% yield, it is likely that Sn(Me)C6H3-2,6-
Trip2 is generated initially. However, this molecule apparently
reacts rapidly with another 1 equiv of MeLi or MeMgBr to give
5 or its equivalent magnesium halide derivative. This species
may then react further with 1 to afford the product 2. In addition,
compound 2 may be converted to 3 by its reaction with a further
1 equiv of LiMe. Alternatively, 3 can be generated directly from
the reaction of 1 with 1.5 equiv of MeLi. In contrast to this
reaction sequence, treatment of 1 with t-BuLi generates neutral
Sn(t-Bu)C6H3-2,6-Trip2, 4, directly which, probably for steric
reasons, does not react further with t-BuLi to generate a t-Bu-
substituted analogue of 5. Attempts to convert 2 into its
symmetrical isomer 6 by a thermal rearrangement resulted in
decomposition.
2‚0.5 hexane 3‚0.5 hexane
4
formula
fw
C77H111Sn2
1274.16
C78H114LiSn2 C40H58Sn
1296.13 657.55
yellow block purple-red shard
crystal color/habit green block
crystal system
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
monoclinic
P21/n (No. 14) P1h (No. 2)
16.8704(5)
19.2696(6)
21.4850(6)
triclinic
monoclinic
C2/c (No. 15)
32.1821(13)
9.5744(4)
14.4163(6)
15.9717(7)
19.0328(8)
67.083(1)
74.633(1)
72.654(1)
3797.2(3)
2
1.134
0.695
0.0509
0.1337
24.7577(10)
99.600(1)
106.900(1)
6886.7(4)
4
1.229
0.765
0.0383
0.0957
7299.0(5)
8
1.197
0.724
0.0385
0.0865
Z
F
calcd (mg/m3)
Since the overall steric congestion in 2 and the putative 6
should be similar, the preference for the symmetric structure 2
over its unsymmetric isomer 6 is unlikely to be due to steric
effects. It is more probable that the instability of 6 is a result of
the difference in the Sn-Sn bond strengths of the two
compounds. Tetraorganoditin species such as 6 are often referred
to as “distannenes” owing to their stoichiometric resemblance
to their carbon analogues, the alkenes. But this name can be a
misleading10 one since the currently known examples dissociate
in solution owing to the weakness of the Sn-Sn bond. For
example the Sn-Sn bond enthalpy11 of {(Me3Si)2CH}2SnSn-
{CH(SiMe3)2}2, which has the shortest Sn-Sn bond, 2.768(1)
Å,4b in “distannenes”, is 12.6 kcal mol-1. This is much less
than typical12 Sn-Sn single bond enthalpies which are ca. 40
kcal mol-1. Thus, it can be argued that, if 6 has a comparable
(ca. 10 kcal mol-1) Sn-Sn bond energy and it is to be
energetically preferred over 2, ca. 30 kcal mol-1 would have to
be found from the differences in tin carbon bond strengths in 2
and 6 to compensate for this difference. It seems doubtful that
such differences in energy between tetravalent and divalent tin-
carbon bonds would be sufficient to overcome this obstacle.13
In addition, it is notable that calculations14 on model hydrogen
compounds of formula Sn2H4 indicate that the doubly bridged
trans-HSn(µ-H)2SnH is more stable than the unsymmetric
stannylstannanediyl H3Sn-S¨nH but that the latter is more stable
than the “distannene” form H2SnSnH2. The lower bridging
tendency of the methyl group suggests that the bridging structure
in MeSn(µ-Me)2SnMe may not be the most stable, and that the
Me3S¨n-SnMe isomer may be the preferred one for the hypo-
thetical species Sn2Me4 although this has not been substantiated
by calculations. The high sensitivity10 of the “soft double
bond”15 in distannenes to steric effects also suggests that
dissociation to monomers would be preferred over rearrange-
ment to the unsymmetrical isomer. In the previously known
“distannenes” such a rearrangement would have resulted in three
bulky groups at one tin atom which would be disfavored
sterically. In 2, however, such a configuration is feasible since
two of these substituents are relatively small methyl groups.
µ (mm-1
)
R1a (obsd)
wR2 (all data)
a R1 ) Σ||Fo| - |Fc||/|Fo|. wR2 ) [Σw(Fo - Fc2)2/Σ[w(Fo)2]]1/2
.
2
solution was reduced to ca. 2 mL, and the flask was stored at ca. -20
°C overnight, providing large, red-purple crystals of 4 (0.45 g, 0.68
mmol, 33%). Mp ) 112-115 °C. Anal. Calcd for C40H58Sn2: C, 73.06;
H, 8.87. Found: C, 73.31; H, 8.73. UV-vis (hexanes) λmax, ꢀ(mol-1
1
cm-1): 485 nm, 655. H NMR (C6D6, 399.77 MHz, 25 °C): 1.13 (d,
3
3
12 H, J ) 7.2 Hz, o-CH(CH3)2), 1.17 (d, 12 H, J ) 7.2 Hz, o-CH-
(CH3)2), 1.41 (d, 12 H, 3J ) 7.2 Hz, p-CH(CH3)2), 135 (C(CH3)3), 2.75
3
3
(sept., 2 H, J ) 7.2 Hz, p-CH(CH3)2), 3.29 (sept, 4 H, J ) 7.2 Hz,
o-CH(CH3)2), 7.05-7.25 (br m, 3 H, p- and m-C6H3), 7.34 (s, 4 H,
m-Trip). 13C {1H} NMR (C6D6, 100.53 MHz): δ 23.08 (o-CH(CH3)2),
24.21 (p-CH(CH3)2), 26.76 (o-CH(CH3)2), 27.55 (C(CH3)3), 31.02 (o-
CH(CH3)2), 34.74 (p-CH(CH3)2), 70.07 (C(CH3)3, 1J(13C-119/117Sn) )
160 Hz), 121. 51 (m-Trip), 126.72 (p-C6H3), 129.60 (m-C6H3), 135.18
(i-Trip), 144.62 (p-Trip), 147.02 (o-Trip), 149.08 (o-C6H3), 179.94 (i-
C6H3), 1J(13C-119/117Sn) ) 286/274 Hz. 119Sn {1H} NMR (C6D6, 149.20
MHz): δ 1904.
X-ray Crystallographic Studies. The crystals were removed from
the Schlenk tube under a stream of N2 and immediately covered with
a layer of hydrocarbon oil. A suitable crystal was selected, attached to
a glass fiber, and immediately placed in a low-temperature nitrogen
stream.7 All data were collected near 130 K using Bruker SMART 1000
(Mo KR radiation and a CCD area detector). The SHELXTL version
5.03 program package was used for the structure solutions and
refinements.8 Absorption corrections were applied using the SADABS
program.9 The crystal structures were solved by direct methods and
refined by full-matrix least-squares procedures. All non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were included in the
refinement at calculated positions using a riding model included in the
SHELXTL program. Compounds 2 and 3 were crystallized from
hexanes as the solvates 2‚0.5 hexane and 3‚0.5 hexane. The structure
of 4 displays a partial occupancy by two species, the stannanediyl Sn-
(t-Bu)C6H3-2,6-Trip2 (4, 83% occupancy) and the isomeric stannanediyl
Sn(i-Bu)C6H3-2,6-Trip2 (17% occupancy) as a result of contamination
of commercial t-BuLi solutions with i-BuLi. An anomalous electron
density of 2.86 electrons/Å3 was observed within the covalent radius
of Sn(2) in which is attributed to uncorrected absorption effects. Some
details of the data collection and refinement are given in Table 1. Further
details are provided in the Supporting Information.
(10) Power, P. P. Dalton Trans. 1998, 2939.
(11) Zilm, K. W.; Lawless, G. A.; Merrill, R. M.; Millar, J. M.; Webb, G.
G. J. Am. Chem. Soc. 1987, 109, 7236.
(12) Simoes, J. A. M.; Liebman, J. F.; Slayden, S. W. Thermochemistry
of Organometallic Compounds of Germanium, Tin and Lead. In The
Chemistry of Organic Germanium, Tin and Lead Compounds; Patai,
S., Ed.; Wiley: Chichester, 1995; Chapter 4.
(13) Indeed there is evidence to suggest that some bonds to divalent tin
(i.e., Sn(II)-N) are stonger than their Sn(IV)-N counterparts. See:
Lappert, M. F.; Power, P. P.; Sanger, A. R.; Srivastava, R. C. Metal
and Metalloid Amides; Ellis Horwood-Wiley: Chichester, 1979; p 265.
(14) Trinquier, G. J. Am. Chem. Soc. 1991, 113, 144.
(15) Driess, M.; Gru¨tzmacher, H. Angew. Chem., Int. Ed. Engl. 1996, 35,
828.
Results and Discussion
Synthesis. The generation of compounds 2 and 3 can be
accounted for in accordance with Scheme 1. Although the
reaction of 1 with 1 equiv of MeLi, or preferably MeMgBr,
(7) Hope, H. Prog. Inorg. Chem. 1995, 41, 1.
(8) SHELXTL version 5.1: Bruker AXS, Madison, WI, 1998.
(9) SADABS, an empirical absorption correction program part of the
SAINTPlus NT version 5.0 package, BRUKER AXS, Madison, WI,
1998.

Sterically Encumbered Terphenyl-Substituted Species
Inorganic Chemistry, Vol. 39, No. 24, 2000 5447
the Sn-C and Sn-Sn bond lengths in 2 with those observed in
other sterically crowded molecules shows that they display some,
but not an excessive, elongation. For instance, comparison of
the Sn-C distances for the tin methyl groups in Me3SnSnPh3
(Sn-C(Me) ) 2.138(5) Å),17 shows that they are ca. 0.03 Å
shorter than the Sn-C(Me) bonds (ca. 2.17 Å) in 2. However,
in the more crowded molecule (t-Bu)2TripSnSnTrip(t-Bu)218 the
Sn-C(t-Bu) bonds are significantly longer at ca. 2.26 Å.
Similarly, the Sn(1)-C(1) distance, 2.201(2) Å, to the bulky
terphenyl group is 0.06 Å longer than the Sn-C(Ph) distances
in Ph3SnSnPh3,19 but is shorter than the Sn-C(Trip) bond of
ca. 2.22 Å in (t-Bu)2TripSnSnTrip(t-Bu)2. The two-coordinate
Sn(II) features a slightly longer Sn-C bond distance of 2.227-
(2) Å, but this is within the previously known range (2.21-
2.23 Å) for two-coordinate Sn(II)-aryl derivatives.3
The most interesting structural parameters in 2 are the Sn-
(1)-Sn(2) bond distance, 2.8909(2) Å, and the interligand angle
of 101.17(5) ° at Sn(2). The only known structure with a bond
between a divalent and tetravalent tin concerns Sn(2-{(Me3-
Si)2CH}C5H4N){Sn(SiMe3)3},20 which has an Sn-Sn bond
2.8689(5) Å long between four-coordinate and three-coordinate
tin centers. The Sn-Sn distance in 2 is slightly longer despite
the lower (two) coordination at one of its tin atoms. The slightly
increased Sn-Sn distance is probably due to the large size of
the terphenyl substituents. Nonetheless, the tin-tin bond cannot
be regarded as being unduly lengthened since Sn-Sn bonds as
long as 3.034(1) Å have been observed in previously mentioned
(t-Bu)2TripSnSnTrip(t-Bu)2 compound.18 The interligand angle
at Sn(2) is in the middle of the known range (87-118°)4,21 for
divalent diorganotin(II) compounds, which also suggests a
moderate degree of crowding in 2.
Figure 1. Thermal ellipsoid (30%) plot of 2. H atoms are not shown.
Selected bond distances and angles are given in Table 2.
The structure of 3 (Figure 2) has many similarities to that of
2. In 3 both tins are four-coordinate, but Sn(1) has the same
formal oxidation state (i.e., Sn(I)) as the two-coordinate tin
center (i.e., Sn(2)) in 2. Accordingly, the Sn-Sn bond length
in 3, 2.8508(4) Å, is similar to that in 2 although it displays
some shortening. Possibly, the four-coordination in both tins
leads to an increase s-p orbital mixing in the Sn-Sn bond
which leads to the shorter Sn-Sn distance. However, the Sn-
(1)-C bond lengths, 2.202(4) Å (Me) and 2.259(4) Å (terphe-
nyl), are slightly longer than those to Sn(2), perhaps as a result
of the negative charge density at Sn(1) which reduces the ionic
contribution to the Sn-C bond strengths. The Li-Sn(1)
distance, 2.685(8) Å, is extremely short in comparison to the
2.871(7) Å observed in the complex [Li(PMDETA)SnPh3],22
or the 2.89(4) Å in (THF)3LiSn{N(SiMe3)CH2}3CCH3.23 It is
possible that the simultaneous binding of the Li+ ion by the
ortho-Trip groups of the terphenyls plays a role in the shortening
of the distance. The Li-Sn bond is maintained in solution as
shown by the observation of large 7Li-119/117Sn couplings. The
Figure 2. Thermal ellipsoid (30%) plot of 3. H atoms are not shown.
Selected bond distances and angles are given in Table 2.
Structural Data and Bonding. The structure of 2 is
illustrated in Figure 1. It consists of well-separated molecules
that crystallize along with 2.0 equiv of hexane per unit cell or
0.5 hexane per asymmetric unit. No strong interactions are
apparent between the molecules themselves or with the co-
crystallized hexane. It can be seen that the two tin centers are
quite distinct since Sn(1) is bound to three carbons as well as
Sn(2), whereas Sn(2) is two coordinate with bonds to Sn(1)
and C(37). The geometry at Sn(1), whose interligand angles
vary from 93.56(9) to 119.30(6)°, is grossly distorted from
idealized tetrahedral. This can be attributed to the large variation
in size and electronic properties of the substituent groups. The
widest angle Sn(2)-Sn(1)-C(1) ) 119.30(6)° involves the
bulky aryl substituent. The Sn(1)-C bond lengths are in the
range 2.164(2) to 2.201(2) Å with the latter value associated
with the bulky aryl group. The two coordinate Sn(2) center has
an interligand angle of 101.17(5) ° as well as Sn(2)-C(37) and
Sn(2)-Sn(1) bond lengths of 2.227(2) Å and 2.8909(2) Å.
Previous results16 have shown that tin-carbon and tin-tin
distances in catenated tin compounds are significantly affected
by the steric properties of the organo groups. Comparison of
(17) Parkanyi, L.; Kalman, A.; Pannell, K. H.; Cervantes-Lee, F.; Kapoor,
R. N. Inorg. Chem. 1996, 35, 6622.
(18) Weidenbruch, M.; Schlaefke, J.; Peters, K.; Schnering, H. G. v. J.
Organomet. Chem. 1991, 414, 319.
(19) (a) Preut, H.; Haupt, H.; Huber, H. Z. Anorg. Allg. Chem. 1973, 396,
81. (b) Piana, H.; Kirchgassner, W. S.; Schubert, U. Chem. Ber. 1991,
124, 743. (c) Eckardt, K.; Fuess, H.; Hattori, M.; Ikeda, R.; Ohki, H.;
Weiss, A. Z. Naturforsch A 1995, 50, 758.
(20) Cardin, C. J.; Cardin, D. J.; Constantine, S. P.; Todd, A. K.; Teat, S.
T.; Coles, S. Organometallics 1998, 17, 2144.
(21) (a) Weidenbruch, M. Eur. J. Inorg. Chem. 1999, 373. (b) Power, P.
P. Chem. ReV. 1999, 99, 3463.
(16) (a) Puff, H.; Breuer, B.; Gehrke-Brinkmann, G.; Kind, P.; Reuter, H.;
Schuh, W.; Wald, W.; Weidenbruck, G. J. Organomet. Chem. 1989,
363, 265. (b) Schneider-Koglin, C.; Behrends, K.; Dra¨ger, M. J.
Organomet. Chem. 1993, 448, 29. (c) Sita, L. AdV. Organomet. Chem.
1995, 38, 189.
(22) Reed, D.; Stalke, D.; Wright, D. S. Angew. Chem., Int. Ed. Engl. 1991,
30, 1495.
(23) Hillmann, K. W.; Gade, L. H.; Gevert, O.; Steinert, P.; Lauher, J.
Inorg. Chem. 1995, 34, 4069.

5448 Inorganic Chemistry, Vol. 39, No. 24, 2000
Eichler and Power
Table 2. Selected Bond Lengths (Å) and Angles (°) for 2-4
2
Sn(1)-Sn(2)
2.8909(2)
2.201(2)
2.164(2)
2.182(3)
2.227(2)
Sn(1)-C(1)
Sn(1)-C(73)
Sn(1)-C(74)
Sn(2)-C(37)
Sn(2)-Sn(1)-C(1)
Sn(1)-Sn(2)-C(37)
C(1)-Sn(1)-C(73)
C(1)-Sn(1)-C(74)
C(73)-Sn(1)-C(74)
C(73)-Sn(1)-Sn(2)
C(74)-Sn(1)-Sn(2)
C(6)-C(1)-Sn(1)
C(2)-C(1)-Sn(1)
C(38)-C(37)-Sn(2)
C(42)-C(37)-Sn(2)
119.30(6)
101.17(5)
112.74(9)
93.56(9)
109.37(10)
115.67(7)
102.58(6)
118.3(2)
122.02(15)
117.06(15)
123.32(15)
3
Sn(1)-Sn(2)
Sn(1)-C(1)
Sn(1)-C(73)
Sn(1)-Li
Sn(2)-C(37)
Sn(2)-C(74)
Sn(2)-C(75)
2.8508(4) Li-C(17)
2.538(9)
2.601(9)
2.707(9)
2.367(9)
2.513(9)
Figure 3. Thermal ellipsoid (30%) plot of 4. H atoms are not shown.
Selected bond distances and angles are given in Table 2.
2.259(4)
2.202(4)
2.685(8)
2.229(4)
2.159(4)
2.172(4)
Li-C(18)
Li-C(44)
Li-C(45)
Li-C(46)
be discussed elsewhere.25 In general, the 119Sn NMR chemical
shifts of divalent tin species (>+200 ppm) lie considerably
downfield of their tetravalent (<+200 ppm) counterparts.26
The 119Sn NMR chemical shift of compound 4 (δ 1904) is
consistent with this generalization and is indicative of the
deshielded environment at the tin atom as a result of its lower
coordination number. The chemical shift of the divalent tin in
2 is 2856.9 ppm which is further downfield than the shifts
observed for 4, Sn[C6H2-2,4,6-{CH(SiMe3)2}3] (δ ) 2208),3b
Li-centroid (C13-C18) 2.746
Li-centroid (C43-C48) 2.424
C(1)-Sn(1)-Sn(2) 120.06(9)
C(37)-Sn(2)-Sn(1) 110.81(9)
C(1)-Sn(1)-C(73) 95.02(14)
C(1)-Sn(1)-Li
C(73)-Sn(1)-Sn(2) 90.05(12)
C(73)-Sn(1)-Li 121.1(2)
C(37)-Sn(2)-C(74) 108.14(14)
C(37)-Sn(2)-C(75) 96.71(15)
C(74)-Sn(2)-C(75) 102.78(18)
110.9(2)
SnC(SiMe3)2(CH2)2C(SiMe3)2 (δ ) 2323)3a or Sn{CH(SiMe3)2}2
(δ ) 2315).10 The downfield shift is probably due to an increase
in paramagnetic effects as a result of the substitution of an
organic group by the more electropositive -Sn(Me)2C6H3-2,6-
Trip2 moiety. The 119Sn NMR chemical shift of the tetravalent
site in 2 is +257.4 ppm, which is at the lower end of the range
normally found for four-coordinate tin compounds.26 The value
of the coupling constant (119/117Sn-119/117Sn) between the tin
atoms in compound 2 is 8330 Hz, which is larger than is
normally observed for hexaorganoditin species such as Ph3-
SnSnPh3 (1J(117Sn-119Sn) ) 4470 Hz).26 It is also greater than
the coupling in the four-coordinate tin/three-coordinate tin
species reported by Cardin and co-workers16 by ca. 2000 Hz.
The tin-tin coupling constants observed in 3 (1J(117/119Sn-
117/119Sn) ) ca. 4050 Hz) is very close to the value in 2. In 3
the coordination number of the Sn(I) center is increased from
2 to 4, and this results in a dramatic upfield change in the 119Sn
chemical shift by ca. 3300 ppm to -431 ppm. The shift of the
four-coordinate tin atom is moved by only ca. 100 ppm
downfield. The assignment of the two resonances in the 119Sn
NMR spectrum of 3 is also facilitated by the observation of
C(2)-C(1)-Sn(1)
C(6)-C(1)-Sn(1)
112.8(2)
130.5(3)
C(38)-C(37)-Sn(2) 121.3(3)
C(42)-C(37)-Sn(2) 121.8(2)
Li-Sn(1)-Sn(2)
116.7(2)
C(74)-Sn(2)-Sn(1) 111.33(12)
C(75)-Sn(2)-Sn(1) 125.33(12)
4
Sn(1)-C(1)
2.2114(18)
2.227(2)
101.61(8)
121.03(13)
118.76(13)
Sn(1)-C(37)
C(1)-Sn-C(37)
C(2)-C(1)-Sn
C(6)-C(1)-Sn
Sn(2)-C bond lengths are slightly longer than the corresponding
distances in 2.
The monomeric, two-coordinate tin(II) alkyl/aryl 4 (Figure
3) has the expected V-shaped coordination at tin and is
isomorphous with its lead analogue.1 The Sn-C(t-Bu) distance
2.227(2) Å is marginally longer than the 2.211(2) Å Sn-
C(terphenyl) bond length. The latter is virtually identical to the
2.213(13) Å Sn-C bond previously reported for the iodide Sn-
(I)C6H3-2,6-Trip2.24 The C-Sn-C angle, 101.61(6)°, is slightly
wider than the 100.5(5) ° in its lead analogue, and it is very
similar to the 102.6(3)° observed for the C-Sn-I angle in Sn-
(I)C6H3-2,6-Trip2 and the 101.17(5)° angle at Sn(2) in 2. As
already discussed for 2, these angles are near the middle of the
range for two-coordinate diorganotin species.
7
coupling to Li (I ) 3/2). The resonance at -431 ppm is split
into a quartet with large 1J(119Sn-7Li) coupling of ca. 740 Hz.
The other tin resonance at +151 ppm has a much smaller
coupling of 47 Hz (2J ) 119Sn-7Li). The coupling of 740 Hz
is the largest one bond 119Sn-7Li coupling observed to date,27
which is consistent with the short Sn-Li distance observed in
the X-ray crystal structure.
NMR Spectroscopy. The solution 119Sn NMR spectra for
compounds 2-4 provide considerable information on the
structural and electronic environment at tin. The 119Sn NMR
spectra of compounds 2 and 4 were also studied in more detail
by CP MAS 119Sn NMR spectroscopy, but this information will
1
The H and 13C NMR spectra of 2 indicate that the Trip
groups are rotationally hindered. Broad singlets in the 1H NMR
(25) Eichler, B. E.; Phillips, B. L.; Power, P. P., Augustine, M. P. Inorg.
Chem. 2000, 39, 5450.
(24) Pu, L.; Olmstead, M. M.; Power, P. P.; Schiemenz, B. Organometallics
1998, 17, 5602.
(26) Wrackmeyer, B. Annu. Rep. NMR Spectrosc. 1999, 38, 203.
(27) Wrackmeyer, B. Annu. Rep. NMR Spectrosc. 1985, 16, 73.

Sterically Encumbered Terphenyl-Substituted Species
Inorganic Chemistry, Vol. 39, No. 24, 2000 5449
spectrum at ca. 3 ppm appear in the region of the methine
hydrogens (CH(CH3)2) of the isopropyl groups. The 13C NMR
spectrum of the o-isopropyl groups also displays two sets of
resonances. The two tin methyl groups are assignable to a singlet
at -0.34 ppm in the 1H spectrum and a broad singlet at +0.34
Conclusion. The unsymmetric stannanestannanediyl 2 is
preferred over the symmetric isomer 6 owing to the weakness
of the Sn-Sn polar dative bonds in 6 relative to the covalent
Sn-Sn single bond in 2. The isolation of 2 is made possible by
the unique steric combination of the large terphenyl ligands and
the small methyl groups which, unlike other large substituents,
stabilize the bonding between two and four coordinate tins.
1
ppm in the 13C spectrum. The H and 13C NMR spectra of 3
are more complex than those of 2. Not only are the two 2,6-
Trip2C6H3- groups inequivalent owing to the different ligand
sets at the tins (i.e., -Me2Sn-SnMeLi-), in addition, one Trip
group on each terphenyl ligand is coordinated to a lithium ion
(see Figure 2). Thus, all four Trip groups in the molecule are
inequivalent. Two singlets are observable for the methyl groups
Acknowledgment. We are grateful to the National Science
Foundation for financial support.
Supporting Information Available: Three X-ray files, in CIF
format, are available.. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
on tin in both the H and 13C NMR spectra of 3. The methyl
group bound to the same tin as the lithium atom may couple to
7Li, thereby reducing the height of the peaks and possibly
rendering it unobservable above the baseline.
IC0005137