Isomeric forms of heavier main group hydrides: Experimental and theoretical studies of the [Sn(Ar)H]2 (Ar = terphenyl) system

Article abstract of DOI:10.1021/ja076453m

A series of symmetric divalent Sn(II) hydrides of the general form [(4-X-Ar′)Sn(μ-H)]₂ (4-X-Ar′ = C₆H ₂-4-X-2,6-(C₆H₃-2,6-ⁱPr ₂)₂; X = H, MeO, ^tBu, and SiMe

Full text of DOI:10.1021/ja076453m

Published on Web 12/04/2007
Isomeric Forms of Heavier Main Group Hydrides:
Experimental and Theoretical Studies of the [Sn(Ar)H]2
(Ar ) Terphenyl) System
Eric Rivard,† Roland C. Fischer,† Robert Wolf,† Yang Peng,† W. Alexander Merrill,†
Nathan D. Schley,† Zhongliang Zhu,† Lihung Pu,‡ James C. Fettinger,†
Simon J. Teat,§ Isreal Nowik,| Rolfe H. Herber,*,| Nozomi Takagi,
Shigeru Nagase,*, and Philip P. Power*,†
Department of Chemistry, UniVersity of California, DaVis, One Shields AVenue, DaVis,
California, 95616, Department of Chemistry, California State UniVersity, Dominguez Hills, 1000
East Victoria Street, Carson, California, 90747, AdVanced Light Source, Lawrence Berkeley
Laboratory, One Cyclotron Road, MS2-400, Berkeley, California, 94720, Racah Institute of
Physics, Hebrew UniVersity of Jerusalem, 91904, Jerusalem, Israel, and Department of
Theoretical and Computational Molecular Science, Institute for Molecular Science,
Okazaki, Aichi 444-8585, Japan
Received August 31, 2007; E-mail: pppower@ucdavis.edu
Abstract: A series of symmetric divalent Sn(II) hydrides of the general form [(4-X-Ar′)Sn(µ-H)]2 (4-X-Ar′
) C6H2-4-X-2,6-(C6H3-2,6-iPr2)2; X ) H, MeO, tBu, and SiMe3; 2, 6, 10, and 14), along with the more hindered
asymmetric tin hydride (3,5-iPr2-Ar*)SnSn(H)2(3,5-iPr2-Ar*) (16) (3,5-iPr2-Ar* ) 3,5-iPr2-C6H-2,6-(C6H2-
2,4,6-iPr3)2), have been isolated and characterized. They were prepared either by direct reduction of the
corresponding aryltin(II) chloride precursors, ArSnCl, with LiBH4 or iBu2AlH (DIBAL), or via a transmetallation
reaction between an aryltin(II) amide, ArSnNMe2, and BH3‚THF. Compounds 2, 6, 10, and 14 were obtained
as orange solids and have centrosymmetric dimeric structures in the solid state with long Sn‚‚‚Sn separations
of 3.05 to 3.13 Å. The more hindered tin(II) hydride 16 crystallized as a deep-blue solid with an unusual,
formally mixed-valent structure wherein a long Sn-Sn bond is present [Sn-Sn ) 2.9157(10) Å] and two
hydrogen atoms are bound to one of the tin atoms. The Sn-H hydrogen atoms in 16 could not be located
by X-ray crystallography, but complementary Mo¨ssbauer studies established the presence of divalent and
tetravalent tin centers in 16. Spectroscopic studies (IR, UV-vis, and NMR) show that, in solution, compounds
2, 6, 10, and 14 are predominantly dimeric with Sn-H-Sn bridges. In contrast, the more hindered hydrides
16 and previously reported (Ar*SnH)2 (17) (Ar* ) C6H3-2,6-(C6H2-2,4,6-iPr3)2) adopt primarily the unsymmetric
structure ArSnSn(H)2Ar in solution. Detailed theoretical calculations have been performed which include
calculated UV-vis and IR spectra of various possible isomers of the reported hydrides and relevant model
species. These showed that increased steric hindrance favors the asymmetric form ArSnSn(H)2Ar relative
to the centrosymmetric isomer [ArSn(µ-H)]2 as a result of the widening of the interligand angles at tin,
which lowers steric repulsion between the terphenyl ligands.
Introduction
have been used as precursors for a variety of novel heteroaro-
matic ring systems including borato- and phosphabenzenes.4
Until recently, examples of stable organotin hydrides were
confined to the +4 oxidation state, where di- and triorganotin
hydrides R2SnH2 and R3SnH are important for a rich variety of
chemical transformations.1 For example, tri-n-butyltin hydride
is a widely used reducing agent in organic and inorganic
chemistry,2 and this versatile species mediates many radical-
driven organic reactions.3 Furthermore, diorganotin dihydrides
Under suitable conditions, dehydrocoupling polymerization of
alkylated stannanes, R2SnH2, can be induced to yield polystan-
nanes (R2Sn)n which exhibit interesting electronic properties.5
In 2000, the synthesis of (Ar*SnH)2, Ar* ) C6H3-2,6-(C6H2-
2,4,6-iPr3)2, the first example of a divalent tin(II) hydride stable
at ambient temperature, was reported.6 This discovery was made
(2) (a) Chen, L.; Cotton, F. A.; Wojtczak, W. A. Inorg. Chim. Acta 1996,
252, 239. (b) Neumann, W. P. Synthesis 1987, 665. (c) Giese, B. Radicals
in Organic Synthesis: Formation of Carbon-Carbon Bonds; Pergamon
Press: Oxford 1986.
† University of California.
‡ California State University.
§ Lawrence Berkeley Laboratory.
(3) (a) Curran, D. P. Synthesis 1988, 417. (b) Kuivila, H. G. Acc. Chem. Res.
1968, 1, 299.
| Hebrew University of Jerusalem.
Institute for Molecular Science.
(4) (a) Fu, G. C. AdV. Organomet. Chem. 2001, 47, 101. (b) Ashe, A. J. Acc.
Chem. Res. 1978, 11, 153. (c) Emslie, D. J. H.; Piers, W. E.; Parvez, M.
Angew. Chem., Int. Ed. 2003, 42, 1252.
(1) (a) Davies, A. G. Organotin Chemistry; VCH: New York, 1997. (b)
Pereyre, M.; Quintand, J.-P.; Rahm, A. Tin in Organic Synthesis; Butter-
worth: London, 1987.
(5) Imori, T.; Tilley, T. D. J. Chem. Soc., Chem. Commun. 1993, 1607.
9
10.1021/ja076453m CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 16197-16208
16197

A R T I C L E S
Rivard, et al.
Chart 1. Possible Isomeric Forms for Sn2H4
Contour and degassed twice (freeze-pump-thaw method) prior to use.
[4-H-Ar′]Li (4-H-Ar′ ) Ar′ ) C6H3-2,6-(C6H3-2,6-iPr2)2),15 (Ar′-
SnCl)2,12 and [3,5-iPr2-Ar*]Li (3,5-iPr2-Ar* ) 3,5-iPr2-C6H-2,6-
(C6H2-2,4,6-iPr3)2),16 were prepared according to literature procedures.
1-Iodo-2,6-dibromo-4-tert-butylbenzene17 and 2,6-dichloro-4-trimethylsilyl-
n
benzene18 were also synthesized as described in the literature. BuLi
i
(2.5 M solution in hexanes), anhydrous SnCl2, LiBH4, LiNMe2, Bu2-
AlH (DIBAL), BH3‚THF (1.0 M solution in THF), and 3,5-dichloro-
anisole were purchased from commercial sources and used as received.
The syntheses of 4-MeO-Ar′I (3), 4-tBu-Ar′I (7), 4-Me3Si-Ar′I (11),
and their respective lithium salts (4, 8, and 12) were carried out in an
analogous manner to that for the syntheses of Ar′I and Ar′Li;15 full
details are given in the Supporting Information. 1H and 13C{1H} NMR
spectra were obtained on a Varian Mercury 300 MHz spectrometer
(300.1 and 75.5 MHz, respectively) and referenced internally to residual
protio benzene or chloroform in the C6D6 or CDCl3 solvent. Solution
29Si and 119Sn NMR spectra were recorded on a Varian Inova 600 MHz
spectrometer (119.2 and 224.2 MHz, respectively) and referenced
externally to neat SiMe4 or SnMe4. Infrared spectra were recorded as
Nujol mulls between CsI plates using a Perkin-Elmer 1430 instrument,
while UV-vis data were collected on a Hitachi-1200 instrument.
Melting points were obtained in sealed glass capillaries under nitrogen
using a Mel-Temp II apparatus and are uncorrected.
119Sn Mo1ssbauer Effect Spectroscopy. Temperature-dependent
Mo¨ssbauer experiments were carried out in transmission mode19 using
an ∼5 mCi source of 119Sn in a BaSnO3 matrix. Spectroscopic
calibration was accomplished by using a 20 mg‚cm-2 R-Fe absorber at
room temperature (57Co in Rh source). Isomer shifts are reported with
respect to the centroid of the room-temperature BaSnO3 spectrum. The
samples were transferred in an inert atmosphere glovebox to O-ring-
sealed plastic sample holders and immediately cooled to liquid nitrogen
temperature. They were then transferred to a precooled cryostat and
examined in transmission geometry. All temperature-dependent Mo¨ss-
bauer data were obtained in both warming and cooling modes, and no
evidence of hysteresis was observed. It was noted that 2 is strongly
photochromic. A sealed sample of 2 was exposed to strong sunlight
for ca. 5 min and found to have changed color from orange to green.
A rapidly cooled sample of the green form was examined by Mo¨ssbauer
spectroscopy at 90 K and found to retain the same hyperfine parameters
(IS and QS) as the orange form, as detailed below (Table 3). On
warming the green form to room temperature the orange color slowly
returned and was complete after ca. 30 min.
possible by the introduction of a bulky terphenyl ligand at tin
which sterically protects the reactive Sn-H moiety from
degradation processes. More recently Roesky and co-workers
prepared a terminal Sn(II) hydride that was stabilized by a
â-diketiminate ligand.7
Theoretical studies on lower oxidation state group 14 hydrides
have focused on the relative stability of monomeric EH2 (E )
group 14 element) and the dimeric forms as illustrated by the
tin derivatives in Chart 1.8,9 Trinquier has calculated the energies
of the heavier congeners of ethylene H2EdEH2 (E ) Si, Ge,
Sn and Pb) as well as the isomeric forms shown in Chart 1.8 In
the case of carbon, the planar (D2h) ethylene isomer is by far
the most stable form with the asymmetric structure analogous
to II lying over 70 kcal/mol higher in energy. This picture is
drastically altered in the heavier congeners and for the four tin
isomers I-IV where the energy spread is only about 10 kcal/
mol, with the hydride-bridged isomer IV being the most stable.
Interestingly, Trinquier also showed that the asymmetric, mixed-
valent species HSn-SnH3 (II) was only slightly less stable than
the hydride-bridged dimer IV by 1.3 to 7 kcal/mol (depending
on the calculation method used).8 This work suggested that, for
tin(II) hydrides (RSnH)2 containing a stabilizing bulky group
(R), the isomeric forms analogous to those presented in Chart
1 might have similar relative energies, and hence representative
examples of each structure type could be accessible.10,11
Parallel work on terphenyl stabilized Sn-Sn multiple bonded
complexes has shown that their structures could be altered by
changing the electronic and steric properties of the terphenyl
group.12 We now report the synthesis and characterization of a
range of Sn(II) hydrides and show that modification of the
terphenyl ligand produces different structural types.13 In addition,
a combined theoretical and experimental approach is used to
explain why, in some instances, centrosymmetric structures such
as IV are formed, while, in others, the asymmetric form related
to II is favored.
X-ray Crystallography. Crystals of appropriate quality for X-ray
diffraction studies were removed from a Schlenk tube under a stream
of nitrogen and immediately covered with a thin layer of hydrocarbon
oil (Paratone). A suitable crystal was then selected and attached to a
glass fiber and quickly placed in a low-temperature stream of nitrogen
(90(2) K).20 Data for compound 213 were obtained on a Bruker SMART
1000 instrument using Mo KR radiation (λ ) 0.710 73 Å) in conjunction
with a CCD detector; data for compounds 6, 10, and 14 were similarly
obtained using a Bruker APEX II instrument. The collected reflections
were corrected for Lorentz and polarization effects by using Blessing’s
method as incorporated into the program SADABS.21,22 The structures
Experimental Section
All reactions were performed with the use of modified Schlenk
techniques under an atmosphere of nitrogen or in a Vacuum Atmo-
spheres Nexus Drybox. Solvents were dried and collected using a
Grubbs-type14 solvent purification system manufactured by Glass
(6) Eichler, B. E.; Power, P. P. J. Am. Chem. Soc. 2000, 122, 8785.
(7) Pineda, L. W.; Jancik, V.; Starke, K.; Oswald, R. B.; Roesky, H. W. Angew.
Chem., Int. Ed. 2006, 45, 2602.
(8) (a) Trinquier, G. J. Am. Chem. Soc. 1991, 113, 144. (b) Balasubramanian,
K. J. Chem. Phys. 1988, 89, 5731.
(9) SnH2 has been prepared and characterized in an argon matrix, see: Wang,
X.; Andrews, L.; Chertihin, G. V.; Souter, P. F. J. Phys. Chem. A 2002,
106, 6302.
(10) (a) Lappert, M. F. Main Group Met. Chem. 1994, 17, 114. (b) Stanciu, C.;
Richards, A. F.; Power, P. P. J. Am. Chem. Soc. 2004, 126, 4106. (c) Lee,
V. Ya; Fukawa, T.; Nakamoto, M.; Sekiguchi, A.; Tumanskii, B. L.; Kami,
M.; Apeloig, Y. J. Am. Chem. Soc. 2006, 128, 11643.
(11) Ge(II) hydrides have also been prepared; see: (a) Ding, Y.; Roesky, H.
W.; Noltemeyer, M.; Schmidt, H.-G.; Power, P. P. Organometallics 2001,
20, 1190. (b) Richards, A. F.; Phillips, A. D.; Olmstead, M. M.; Power, P.
P. J. Am. Chem. Soc. 2003, 125, 3204. (c) Spikes, G. H.; Fettinger, J. C.;
Power, P. P. J. Am. Chem. Soc. 2005, 127, 12232. (d) See ref 7.
(12) Pu, L.; Phillips, A. D.; Richards, A. F.; Stender, M.; Simons, R. S.;
Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 2003, 125, 11626.
(13) Rivard, E.; Steiner, J.; Fettinger, J. C.; Giuliani, J. R.; Augustine, M. P.;
Power, P. P. Chem. Commun. 2007, 4919.
(14) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers,
F. J. Organometallics 1996, 15, 1518.
(15) Schiemenz, B.; Power, P. P. Angew. Chem., Int. Ed. Engl. 1996, 35, 2150.
(16) Stanciu, C.; Richards, A. F.; Fettinger, J. C.; Brynda, M.; Power, P. P. J.
Organomet. Chem. 2006, 691, 2540.
(17) (a) Craig, D. J. Am. Chem. Soc. 1935, 57, 195. (b) Drake, N. L.; Eaker, C.
M.; Garman, J. A.; Hamlin, K. E., Jr.; Hayes, R. A.; Haywood, S. T.; Peck,
R. M.; Preston, R. K.; Sterling, J., Jr.; van Hook, J. O.; Walton, E. J. Am.
Chem. Soc. 1946, 68, 1602. (c) Bedard, T. C.; Moore, J. S. J. Am. Chem.
Soc. 1995, 117, 10662.
(18) Wrobel, D.; Wannagat, U. J. Organomet. Chem. 1982, 225, 203.
(19) Adams, R. D.; Captain, B.; Herber, R. H.; Johansson, M.; Nowik, I.; Smith,
J. L., Jr.; Smith, M. D. Inorg. Chem. 2005, 44, 6346.
(20) Hope, H. Prog. Inorg. Chem. 1995, 41, 1.
(21) Blessing, R. H. Acta Crystallogr. 1995, 51A, 33.
9
16198 J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007

Isomeric Forms of Heavier Main Group Hydrides
A R T I C L E S
Table 1. Selected Crystallographic Data for Compounds 2, 6, 10, 14, and 16
2
6
10
14
16
formula
fw
color, habit
cryst syst
space group
a, Å
C60H76Sn2
1034.59
orange, block
triclinic
P1h
9.2996(16)
12.814(2)
12.938(2)
110.141(3)
98.399(3)
110.940(3)
1286.6(4)
1
C74H92O2Sn2
1250.86
yellow, plate
orthorhombic
Pna21
15.8356(8)
15.1379(8)
26.5935(14)
90
90
90
6374.9(6)
4
C80H104Sn2
1303.01
orange, block
monoclinic
P21/n
17.488(3)
11.6926(17)
17.666(3)
90
94.351(3)
90
3602.1(9)
2
C70H102OSi2Sn2
1253.08
orange, block
monoclinic
P21/c
11.4360(7)
25.5484(15)
12.8457(7)
90
114.7570(10)
90
3408.2(3)
2
C87H131Sn2
1414.30
blue, needle
tetragonal
P42/n
33.9416(13)
33.9416(13)
14.7839(12)
90
b, Å
c, Å
R, deg
â, deg
90
90
γ, deg
V, Å3
17031.5(17)
8
Z
cryst dim, mm3
0.12 × 0.11 × 0.03
0.29 × 0.22 × 0.08
0.19 × 0.16 × 0.12
0.39 × 0.31 × 0.20
0.15 × 0.04 × 0.01
d
calc, g cm-3
1.335
1.303
1.201
1.221
1.103
µ, mm-1
1.007
4677
3674
293
0.0554
0.1658
0.828
51658
12337
741
0.0425
0.1058
0.733
6492
4418
386
0.0674
0.1870
0.806
43294
6457
386
0.0327
0.0868
0.775
123148
11248
839
0.0679
no. of reflns
no. of obsd reflns
no. of param
R1 obsd reflns
wR2, all
0.1956
were solved by direct methods and refined with the SHELXTL v.6.1
software package.23 Refinement was by full-matrix least-squares
procedures with all carbon-bound hydrogen atoms included in calculated
positions and treated as riding atoms.
was removed to give a bright yellow solid. Recrystallization of this
product from 15 mL of hexanes (ca. -20 °C) afforded 1 as yellow
blade-shaped crystals (1.40 g, 82%). Mp (°C): 106-108 (dec). UV-
1
vis (hexanes) λmax [nm] (ꢀ in M-1 cm-1): 370 (shoulder). H NMR
(C6D6): δ 7.30 (t, 2H, J ) 7.5 Hz, ArH), 7.15-7.22 (m, 7H, ArH),
3.18 (septet, 4H, JHH ) 6.9 Hz, CH(CH3)2), 3.03 (s, 6H, N(CH3)2),
X-ray crystallographic data for 16 were collected at the ALS
Synchrotron facility at UC Berkeley on a Bruker Platinum 300
diffractometer at 80(2) K using monochromated synchrotron radiation
(λ ) 0.774 90 Å). Absorption corrections were performed using
SADABS.21,22 The structure was solved by the Patterson method, and
all non-hydrogen atoms were refined anisotropically with the exception
3
1.33 (d, 12H, 3JHH ) 6.6 Hz, CH(CH3)2), 1.08 (d, 12H, 3JHH ) 6.6 Hz,
CH(CH3)2). 13C{1H} NMR (C6D6): δ 177.6, 147.6, 146.9, 145.6, 138.8,
130.1, 129.0 and 123. 5 (ArC), 47.7 (br, N(CH3)2), 31.0 (CH(CH3)2),
26.7 (CH(CH3)2), 23.1 (CH(CH3)2). 119Sn{1H} NMR (C6D6): δ 817.
Preparation of [Ar′Sn(µ-H)]2 (2). Route A:13 To a deep yellow
solution of 1 (0.761 g, 1.36 mmol) in 15 mL of Et2O was added
dropwise BH3‚THF (1.40 mL, 1.0 M solution in THF, 1.40 mmol). A
deep blue-green solution was obtained from which an orange micro-
crystalline solid precipitated. After 2 h, the resulting light green solution
was decanted from the precipitate and the solid 2 was dried under
vacuum (0.51 g, 74%). Route B: To a mixture of Ar′SnCl (1.14 g,
2.07 mmol) and LiBH4 (0.060 g, 2.8 mmol) were added 25 mL of cold
(0 °C) diethyl ether. The reaction was warmed to room temperature
and stirred for 2 h to give a green-brown solution over a tan precipitate.
The mixture was then filtered, and the solvent was removed to give an
orange solid that was identified as 2 on the basis of NMR and UV-
vis spectroscopy (0.61 g, 57%). Crystals of 2 that were suitable for an
X-ray diffraction study (orange blocks) were grown from a benzene
solution at ca. 7 °C. Mp (°C): 181-183 (dec). UV-vis (benzene) λmax
i
of some (five out of 15) Pr groups within the 3,5-iPr2-Ar* ligand.
These disordered groups were either refined as a whole with isotropic
atomic displacement parameters, or in two cases, their methyl groups
were refined in split positions with isotropic atomic displacement
parameters. Furthermore, the structure contained an n-hexane solvent
molecule that was strongly disordered over a crystallographic inversion
center. SADI constraints were used to obtain a reasonable geometry.
However the thermal parameters remained quite high, and isotropic
atomic displacement parameters were used. Due to the observed disorder
and the weakly diffracting nature of the selected crystal, the structure
is only of modest quality and, consequently, the two hydrogen atoms
next to Sn(1) could not be located on the Fourier difference map.
Relevant crystallographic and structural refinement parameters are listed
in Table 1.
Theoretical Calculations. All calculations were conducted using
the Gaussian 03 series of electronic structure programs.24 The geom-
etries were optimized with the density functional theory at the B3PW91
level.25 The [4333111/433111/43] basis set augmented by two d
polarization functions (d exponents 0.253 and 0.078) was used for Sn,
while the 6-31G(d) basis set was used for C and H.26
1
[nm] (ꢀ in M-1 cm-1): 595 (70). H NMR (C6D6): δ 9.13 (s, 1H,
1JSn-H ) ca. 89 Hz, Sn-H), 7.30 (t, 2H, 3JHH ) 7.5 Hz, ArH), 7.10 (m,
3H, ArH), 7.03 (d, 4H, 3JHH ) 7.5 Hz, ArH), 3.00 (overlapping septets,
3
4H, CH(CH3)2), 1.11 (d, 6H, JHH ) 6.6 Hz, CH(CH3)2), 1.04 (d, 6H,
3
3JHH ) 6.6 Hz, CH(CH3)2), 1.02 (d, 6H, JHH ) 6.6 Hz, CH(CH3)2),
3
0.93 (d, 6H, JHH ) 6.9 Hz, CH(CH3)2). 119Sn NMR (C6D6): δ 657
(br, ∆ω1/2 ) ca. 270 Hz). IR (Nujol, cm-1): The Sn-H stretching
band is likely obscured by ligand vibrations. 119Sn Mo¨ssbauer (90 K,
mm s-1): Isomer shift (IS) ) 2.805(4); Quadrupolar Splitting (QS) )
2.992(4).
Syntheses. Preparation of Ar′SnNMe2 (1). To a solution of Ar′SnCl
(1.69 g, 3.07 mmol) in 35 mL of Et2O was added dropwise a slurry of
LiNMe2 (0.160 g, 3.11 mmol) in 25 mL of Et2O. The reaction was
stirred for 16 h to give a deep yellow solution along with a white
precipitate. The mixture was then filtered through Celite, and the solvent
Preparation of (4-MeO-Ar′)SnCl (5). A solution of 4-MeO-Ar′Li
(4) (2.74 g, 6.6 mmol) in 25 mL of Et2O was added dropwise to a
slurry of SnCl2 (1.51 g, 8.0 mmol) in 30 mL of Et2O kept at 0 °C. The
reaction mixture was then allowed to warm to room temperature during
(22) Sheldrick, G. M. SADABS, Version 2.10, Siemens Area Detector Absorption
Correction; Universita¨t Go¨ttingen: Go¨ttingen, Germany, 2003.
(23) Sheldrick, G. M. SHELXTL, Version 6.1; Bruker AXS, Inc.: Madison, WI,
2002.
(24) Frisch, M. J., et al. Gaussian 03, revision C.01; Gaussian, Inc.: Wallingford,
CT, 2004.
(25) (a) Becke, A. D. Phys. ReV. 1988, A38, 3098. (b) Becke, A. D. J. Chem.
Phys. 1993, 98, 5648. (c) Perdew, J. P.; Wang, Y. Phys. ReV. 1992, B45,
13244.
(26) (a) Huzinaga, S.; Andzelm, J.; Klobukowski, M.; Radzio-Andzelm, E.;
Sakai, Y.; Tatewaki, H. Gaussian Basis Sets for Molecular Calculations;
Elsevier: Amsterdam, 1984. (b) Francl, M. N.; Pietro, W. J.; Hehre, W.
J.; Binkey, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J. Chem.
Phys. 1982, 77, 3654.
9
J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007 16199

A R T I C L E S
Rivard, et al.
which an orange color developed. After stirring for 12 h, the solvent
was removed under reduced pressure, and the resulting orange powder
was extracted with 40 mL of toluene and filtered. Storage of the solution
at ca. -20 °C for 12 h afforded large orange crystals of (4-MeO-
Ar′)SnCl (1.53 g, 40%). Mp (°C): 187-190 (dec). UV-vis (hexanes)
(4-Me3Si-Ar′)SnCl (13). A solution of (4-Me3Si-Ar′)Li (12) (2.00
g, 4.30 mmol) in 40 mL of Et2O was added dropwise to a precooled
(-78 °C) suspension of SnCl2 (1.20 g, 6.3 mmol) in 30 mL of Et2O.
The reaction mixture was allowed to slowly warm to room temperature
and was stirred for a further 10 h, to give a bright orange reaction
mixture. The volume of the reaction was concentrated to 10 mL, and
50 mL of hexanes were then added. The insoluble solids were separated
by filtration and concentration of the filtrate to 30 mL, followed by
cooling to ca. -20 °C affording 13 as an orange solid (1.65 g, 63%).
Mp (°C): 154-155. UV-vis (hexanes) λmax [nm] (ꢀ in M-1 cm-1):
λ
max [nm] (ꢀ in M-1 cm-1): 402 (730). 1H NMR (C6D6): δ 7.10-7.25
(m, 6H, ArH), 6.99 (s, 2H, ArH), 3.36 (s, 3H, OCH3), 3.18 (septet,
3
3
4H, JHH ) 6.9 Hz, CH(CH3)2), 1.31 (d, 12H, JHH ) 6.6 Hz, CH-
(CH3)2), 1.03 (d, 12H, JHH ) 6.6 Hz, CH(CH3)2). 13C{1H} NMR
3
(C6D6): δ 159.6, 147.5, 146.7, 137.0, 129.2, 125.5, 123.7 and 116.5
(ArC), 54.4 (OCH3), 30.9 (CH(CH3)2), 26.4 (CH(CH3)2), 23.0 (CH-
(CH3)2). Attempts to obtain a 119Sn NMR spectrum have been
unsuccessful to date.
406 (2400). 1H NMR (C6D6): δ 7.54 (s, 2H, ArH), 7.22 (t, 2H, 3JHH
)
6.8 Hz, ArH), 7.11 (d, 4H, 3JHH ) 6.8 Hz, ArH), 3.06 (septet, 4H, 3JHH
3
) 6.9 Hz, CH(CH3)2), 1.27 (d, 12H, JHH ) 6.9 Hz, CH(CH3)2), 1.00
3
(d, 12H, JHH ) 6.9 Hz, CH(CH3)2), 0.24 (s, 9H, Si(CH3)3). 13C{1H}
Synthesis of [(4-MeO-Ar′)Sn(µ-H)]2 (6). Diethyl ether (50 mL)
was added to a rapidly stirring mixture of 5 (1.00 g, 1.72 mmol) and
LiBH4 (0.045 g, 2.1 mmol) at 0 °C. After a few minutes, an orange
precipitate was observed and the initially orange solution became
yellow-green. The reaction was stirred for 1 h, and the precipitate was
allowed to settle. The mother liquor was decanted away, and the
remaining orange precipitate was extracted with 50 mL of benzene and
filtered to afford a pale blue-green solution. Storage of the solution at
ca. 7 °C for 12 h gave orange crystals of 6 (0.35 g, 40%). Mp (°C):
185-187 (dec). UV-vis (benzene) λmax [nm] (ꢀ in M-1 cm-1): 598
(50). 1H NMR (C6D6): δ 9.28 (s, 1H, 1JSn-H ) ca. 95 Hz, Sn-H), 7.30
NMR (C6D6): δ 183.5, 148.3, 144.6, 140.0, 137.7, 135.9, 130.0 and
124.3 (ArC), 31.6 (CH(CH3)2), 26.7 (CH(CH3)2), 23.4 (CH(CH3)2),
-1.37 (Si(CH3)3). 29Si NMR (C6D6): δ -4.3. 119Sn{1H} NMR
(C6D6): δ 904.1.
[(4-Me3Si-Ar′)Sn(µ-H)]2 (14). To a mixture of LiNMe2 (0.160 g,
3.11 mmol) and 13 (2.00 g, 3.11 mmol) were added 40 mL of cold
(ca. -40 °C) diethyl ether. The resulting yellow slurry was warmed to
ambient temperature and stirred overnight. The reaction mixture was
then filtered through Celite to give a yellow filtrate which contained
(4-Me3Si-Ar′)SnNMe2. A solution of BH3‚THF (3.40 mL, 1.0 M
solution in THF, 3.40 mmol) was then added to the amide and yielded
a deep green solution. The reaction was stirred for a further 45 min
and then concentrated to ca. 10 mL, resulting in the precipitation of a
tan colored solid. The mother liquor was decanted away from the
insoluble precipitate (14), and the product was dried under vacuum
(0.95 g, 50%). Crystals of 14 of suitable quality for X-ray diffraction
(orange blocks) were subsequently obtained by cooling a solution of
in situ generated 14 in diethyl ether (-20 °C). Mp (°C): darkens at
65, chars at 115 with gas evolution, melts at 160-161. UV-vis
3
3
(t, 2H, JHH ) 7.5 Hz, ArH), 7.03 (d, 4H, JHH ) 7.5 Hz, ArH), 6.82
3
(s, 2H, ArH), 3.29 (s, 3H, OCH3), 3.08 (septet, 4H, JHH ) 6.9 Hz,
3
CH(CH3)2), 1.04 (d, 12H, JHH ) 6.6 Hz, CH(CH3)2), 0.95 (d, 12H,
3JHH ) 6.9 Hz, CH(CH3)2). 119Sn NMR (C6D6): δ 687 (br, ∆ω1/2
)
ca. 650 Hz). IR (Nujol, cm-1): The Sn-H stretching band is likely
obscured by ligand vibrations.
(4-tBu-Ar′)SnCl (9). A solution of [4-tBu-Ar′]Li (8) (4.00 g, 8.66
mmol) in 50 mL of diethyl ether was added dropwise to a slurry of
SnCl2 (1.93 g, 102 mmol) in 65 mL of cold (-78 °C) Et2O. The
resulting yellow slurry was slowly warmed to room temperature and
stirred for 2 days. The solvent was then removed, and the product was
extracted with 100 mL of warm toluene and filtered. Cooling of the
pale orange filtrate to ca. -20 °C gave a crop of yellow microcrystalline
9 (2.39 g, 45%) after 1 week. Mp (°C): 216-218. UV-vis (hexanes)
1
(benzene) λmax [nm] (ꢀ in M-1 cm-1): 440 (shoulder), 582 (175). H
1
NMR (C6D6): δ 9.12 (s, 1H, JSn-H ) ca. 87 Hz, Sn-H), 7.33 (s, 2H,
3
ArH), 7.31 (m, 2H, ArH), 7.04 (d, 4H, JHH ) 7.5 Hz, ArH), 3.01
3
3
(septet, 4H, JHH ) 6.9 Hz, CH(CH3)2), 1.05 (d, 6H, JHH ) 6.6 Hz,
CH(CH3)2), 1.03 (d, 6H, 3JHH ) 6.6 Hz, CH(CH3)2), 0.92 (d, 6H, 3JHH
) 6.9 Hz, CH(CH3)2), 0.22 (s, 9H, Si(CH3)3). The insolubility of 14
precluded the acquisition of informative 13C{1H} and 119Sn NMR
spectra. IR (Nujol, cm-1): The Sn-H stretching band is likely obscured
by ligand vibrations.
1
λmax [nm] (ꢀ in M-1 cm-1): 402 (1240). H NMR (C6D6): δ 7.38 (s,
3
2H, ArH), 7.18-7.25 (m, 6H, ArH), 3.12 (septet, 4H, JHH ) 6.9 Hz,
3
CH(CH3)2), 1.33 (d, 12H, JHH ) 6.9 Hz, CH(CH3)2), 1.32 (s, 9H,
3
C(CH3)3), 1.05 (d, 12H, JHH ) 6.9 Hz, CH(CH3)2). 13C{1H} NMR
(3,5-iPr2-Ar*)SnCl (15). A solution of 3,5-iPr2-Ar*Li(OEt2)2 (3.0
g, 4.2 mmol) in 40 mL of Et2O was added dropwise to a precooled
(-78 °C) mixture of SnCl2 (1.18 g, 6.2 mmol) in 20 mL of diethyl
ether. The reaction mixture quickly adopted a yellow-orange color that
intensified upon warming to room temperature to produce a dark orange
solution along with a white precipitate. The solvent was removed in
Vacuo, and the remaining residue was extracted with 80 mL of hexanes.
The precipitated material (including excess SnCl2) was removed by
filtration, and the resulting deep-orange filtrate was concentrated to
incipient crystallization. Cooling of the solution to ca. -20 °C yielded
15 (1.67 g, 56%) as orange crystals. Mp (°C): 149-151. UV-vis
(C6D6): δ 150.3, 147.7, 144.4, 129.3, 125.5, 123.7 and 122.8 (ArC),
34.5 (C(CH3)3), 31.2 (CH(CH3)2), 30.9 (C(CH3)3), 26.1 (CH(CH3)2),
24.3 (CH(CH3)2), 22.8 (CH(CH3)2). 119Sn NMR (C6D6): δ 1001 (br).
[(4-tBu-Ar′)Sn(µ-H)]2 (10). To a mixture of 9 (0.64 g, 1.1 mmol)
and LiBH4 (0.031 g, 1.4 mmol) were added dropwise 20 mL of cold
(ca. -20 °C) diethyl ether. The reaction was warmed to ambient
temperature and stirred for 2 h to give a green solution over a white
precipitate. The mixture was then filtered, and the solvent was removed
to give a tan-orange solid (0.48 g, 80%). Crystals of suitable quality
for X-ray diffraction (orange blocks) were grown from a benzene
solution at ca. 7 °C. Mp (°C): darkens at 115, melts at 210. UV-vis
(benzene) λmax [nm] (ꢀ in M-1 cm-1): 594 (100). 1H NMR (C6D6): δ
9.11 (s, 1H, 1JSn-H ) ca. 87 Hz, Sn-H), 7.38 (s, 2H, ArH), 7.05-7.31
(m, 6H, ArH), 3.07 (septet, 2H, 3JHH ) 6.9 Hz, CH(CH3)2), 3.00 (septet,
2H, 3JHH ) 6.6 Hz, CH(CH3)2), 1.32 (d, 6H, 3JHH ) 6.6 Hz, CH(CH3)2),
1
(hexanes) λmax [nm] (ꢀ in M-1 cm-1): 416 (2700). H NMR (C6D6):
3
δ 7.47 (s, 1H, ArH), 7.22 (s, 4H, ArH), 2.98 (septet, 4H, JHH ) 6.6
3
Hz, CH(CH3)2), 2.84 (septet, 2H, JHH ) 6.6 Hz, CH(CH3)2), 2.75
3
3
(septet, 2H, JHH ) 6.9 Hz, CH(CH3)2), 1.43 (d, 12H, JHH ) 6.9 Hz,
3
CH(CH3)2), 1.25 (d, 12H, JHH ) 6.9 Hz, CH(CH3)2), 1.16 (d, 12H,
3JHH ) 6.9 Hz, CH(CH3)2), 1.12 (d, 12H, JHH ) 6.9 Hz, CH(CH3)2).
3
3
1.32 (d, 6H, JHH ) 6.6 Hz, CH(CH3)2), 1.24 (s, 9H, C(CH3)3), 1.03
13C{1H} NMR (C6D6): δ 221.3, 149.9, 148.5, 148.1, 140.5, 134.0, 123.6
and 123.4 (ArC), 35.0 (CH(CH3)2), 31.0 (CH(CH3)2), 30.1 (CH(CH3)2),
26.8 (CH(CH3)2), 25.7 (CH(CH3)2), 23.5 (CH(CH3)2). 119Sn{1H} NMR
(C6D6): δ 961 (br).
3
3
(d, 6H, JHH ) 6.9 Hz, CH(CH3)2), 0.92 (d, 6H, JHH ) 6.9 Hz, CH-
(CH3)2). 13C{1H} NMR (C6D6): δ 147.3, 146.2, 140.9, 137.9, 129.6,
129.0, 127.4 and 123.9 (ArC), 34.4 (C(CH3)3), 31.3 (CH(CH3)2), 31.0
(C(CH3)3), 30.8 (CH(CH3)2), 26.3 (CH(CH3)2), 24.4 (CH(CH3)2), 23.4
(CH(CH3)2), 22.8 (CH(CH3)2). 119Sn{1H} NMR (C6D6): δ 667 (br).
IR (Nujol, cm-1): The Sn-H stretching band is likely obscured by
ligand vibrations.
(3,5-iPr2-Ar*)SnSn(H)2(3,5-iPr2-Ar*) (16). A solution of DIBAL
(0.26 g, 1.8 mmol) in 20 mL of Et2O was added to an orange solution
of 15 (1.27 g, 1.8 mmol) in 30 mL of Et2O at -78 °C. Upon the addition
9
16200 J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007

Isomeric Forms of Heavier Main Group Hydrides
A R T I C L E S
Chart 2. Terphenyl Ligands Used to Construct the Aryltin(II)
Hydrides
of DIBAL, a deep blue solution was observed. The reaction was allowed
to warm to room temperature overnight, and the solvent was then
removed under vacuum. The product was dissolved in 50 mL of hexanes
and filtered, and the filtrate was concentrated to ca. 2 mL which yielded
dark-blue crystals of 16 upon storage of the solution at ca. -20 °C for
2 weeks (0.90 g, 71%). Mp (°C): 153-154 (dec). UV-vis (hexanes)
1
λmax [nm] (ꢀ in M-1 cm-1): 422 (540), 480 (shoulder), 622 (220). H
NMR (C6D6): δ 7.92 (s, 2H, 1JSn-H ) ca. 528 Hz, Sn-H), 7.43 (s, 2H,
ArH), 7.11 (s, 8H, ArH), 2.93 (overlapping septets, 12H, CH(CH3)2),
2.54 (septet, 4H, 3JHH ) 6.6 Hz, CH(CH3)2), 1.36 (d, 48H, 3JHH ) 6.9
Hz, CH(CH3)2), 1.19 (d, 48H, 3JHH ) 6.9 Hz, CH(CH3)2). 13C{1H} NMR
(C6D6): δ 148.2, 147.8, 147.7, 142.4, 136.4, 124.0 and 122.7 (ArC),
34.4 (CH(CH3)2), 30.9 (CH(CH3)2), 30.1 (CH(CH3)2), 27.6 (CH(CH3)2),
26.1 (CH(CH3)2), 25.9 (CH(CH3)2), 24.6 (CH(CH3)2) (ipso carbon not
observed). 119Sn{1H} NMR (C6D7, 25 °C): δ 1727. 119Sn{1H} NMR
(C6D7, -70 °C): δ 34. IR (Nujol, cm-1): 1810 (m) and 1783 (m) [Sn-
H stretches]. 119Sn Mo¨ssbauer (90 K, mm s-1): Site A: IS ) 1.370(4)
[QS ) 0.967(4)]; Site B: IS ) 3.252(7) [QS ) 3.713(7)].
1
feature we were able to extract an averaged JSn-H coupling
constant of 89 Hz, which was considerably smaller than the
Sn-H coupling (592 Hz) observed for 17 in solution.6 For
comparison, typical values of 1JSn-H for organotin(IV) hydrides
are much larger and are typically near ∼1900 Hz.28 Presumably,
the weaker Sn-H bonds in the bridged structure account for
Results and Discussion
1
the unusually low value of the JSn-H coupling. The spectral
Synthesis and Spectroscopy of the Terphenyl Tin(II)
Hydrides [Ar*Sn(µ-H)]2 (Ar* ) C6H3-2,6-(C6H2-2,4,6-iPr3)2)
and [Ar′Sn(µ-H)]2 (Ar′ ) C6H3-2,6-(C6H3-2,6-iPr2)2). The first
stable tin(II) hydride [Ar*Sn(µ-H)]2 (17) was synthesized in
2000 via reduction of the corresponding chloride [Ar*Sn(µ-
Cl)]2 with DIBAL.6 Stability was conferred by use of the
sterically crowding terphenyl ligand Ar* (Ar* ) C6H3-2,6-
(C6H2-2,4,6-iPr3)2). It was isolated as orange crystals that
dissolved in hydrocarbons to produce an intense blue solution.
The color change was originally interpreted in terms of its
dissociation into Ar*SnH monomers which had a blue color as
a result of an n-p transition. 1H NMR spectroscopy afforded a
signal at 7.87 ppm attributable to the Sn-H moiety that
data for 2 are therefore consistent with the retention of a bridged
structure in solution while it appears that 17 adopts a different
structure from 2 in solution (see below). The IR spectra of 2
and 17 also display major differences. In contrast to 17 which
features two well-defined, moderately intense Sn-H stretching
vibrations at 1828 and 1771 cm-1 in the IR spectrum, the less-
hindered hydride, 2, displayed no IR bands in the region 1500
to 2800 cm-1 29
.
In summary the experimental data for [Ar*Sn(µ-H)]2 and
[Ar′Sn(µ-H)]2 show that, although their solid-state structures
are similar, their solution behavior differs markedly as a result
of the para substituents on the flanking aryl rings. However the
identity of the species present in solutions of 17 was only
established by further variation of the electronic and steric
properties of the terphenyl ligands discussed below.
Use of Electronically and Sterically Modified Terphenyl
Ligands. We prepared an analogous series of Sn(II) hydrides
with the use of modified terphenyl ligand derivatives designated
4-X-Ar′ and 3,5-iPr2-Ar* (Chart 2) to investigate the effects
of the substituents on the hydride structure.
Beginning with the readily synthesized precursor, 4-MeO-
Ar′Li, featuring an electron-withdrawing methoxy group, we
were able to prepare the requisite tin(II) chloride [(4-MeO-
Ar′)Sn(µ-Cl)]2 (5) by reaction with anhydrous SnCl2 in diethyl
ether. Compound 5 was obtained as a highly crystalline orange
solid and was shown by X-ray crystallography to possess a
dimeric structure held together by Sn-Cl bridging interactions.30
Conversion of 5 into the tin(II) hydride, [4-MeO-Ar′)Sn(µ-
H)]2 (6), was possible using DIBAL; however we obtained
significantly purer material when LiBH4 was used as the
reducing agent (Scheme 1). A terphenyltin(II) hydride containing
an electron-releasing tert-butyl group in the 4-position of the
central aryl ring was also prepared. The precursor terphenylio-
dide, 4-tBu-Ar′I (7) was synthesized in a multistep procedure
(see Supporting Information for details) and converted into [(4-
1
displayed a JSn-H of 592 Hz. We decided to build upon these
studies by employing the slightly less hindered Ar′ ligand (Ar′
) C6H3-2,6-(C6H3-2,6-iPr2)2) with the object of isolating higher
oligomers (i.e., trimers) in the solid state.
The corresponding reduction of [Ar′Sn(µ-Cl)]2 with DIBAL
was explored as a method to prepare Ar′SnH; however this route
did not yield clean products. Noting that diborane was used
previously to generate organotin(IV) hydrides, R3SnH, from
organotin(IV) amide precursors,27 we decided first to prepare
the aryltin amide, Ar′SnNMe2 (1), and then explore its reactivity
toward BH3‚THF to obtain the desired tin(II) hydride. Upon
treating a bright yellow solution of 1 in Et2O with a slight excess
of BH3‚THF, a deep-blue solution was immediately seen which
later (ca. 30 min) became pale green as copious amounts of a
microcrystalline orange solid precipitated. This orange solid was
later identified as the dimeric hydride species [Ar′Sn(µ-H)]2 (2)
by X-ray crystallography.13 In contrast to 17, the less-hindered
2 was only sparingly soluble in typical organic solvents
(hexanes, THF, ether, and benzene) and showed a tendency to
decompose over time in solution. Dissolution of this product
in C6D6 afforded a pale-blue solution, and examination of the
1H NMR spectrum revealed the presence of a number of ligand-
based resonances along with a significantly deshielded signal
at 9.13 ppm. The latter resonance contained two broad flanking
satellites in the correct intensity for coupling of a proton to
117/119Sn nuclei (I ) 1/2; abundance ) ca. 8% each). From this
(28) (a) Gil, V. M. S.; Philipsborn, W. v. Magn. Reson. Chem. 1989, 27, 409.
(b) Curran, D. P.; Hadida, S.; Kim, S.-Y.; Luo, Z. J. Am. Chem. Soc. 1999,
121, 6607.
(29) (a) Harrison, P. G. Chemistry of Tin; Chapman and Hall: New York, 1989.
(b) Saito, M.; Hashimoto, H.; Tajima, T.; Ikeda, M. J. Organomet. Chem.
2007, 692, 2729.
(30) The structures of the substituted (4-X-Ar′)SnCl derivatives will be discussed
in a forthcoming publication.
(27) (a) Kula, M.-R.; Lorberth, J.; Amberger, E. Chem. Ber. 1964, 97, 2087.
(b) For a similar transformation using silanes, see: Hays, D. S.; Fu, G. C.
J. Org. Chem. 1997, 62, 7070.
9
J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007 16201

A R T I C L E S
Rivard, et al.
Scheme 1. Preparative Routes to Terphenyl Tin(II) Hydrides
(II) amide, (4-Me3Si-Ar′)SnNMe2, with a slight excess of BH3‚
THF gave the hydride [(4-Me3Si-Ar′)Sn(µ-H)]2 (14) as orange
crystals. This compound, once crystallized, was almost insoluble
in benzene which precluded the observation of solution 13C-
{1H} and 119Sn NMR data. However the 1H NMR spectrum of
14 in C6D6 yielded a characteristic singlet at 9.12 ppm, which
was flanked by diagnostic Sn-H satellites (1JSn-H ) ca. 87
Hz), suggesting that this species was structurally similar to the
other [(4-X-Ar′)Sn(µ-H)]2 derivatives discussed above. Nota-
1
bly, the ligand H NMR signals and UV-vis spectrum of 14
differed substantially from those reported for the dark green
distannyne 18. Moreover, while the distannyne melts without
decomposition at 181 °C, the hydride analogue 14 shows marked
signs of decomposition at 65 °C with eventual melting occurring
at 160 °C. These observations, along with the results from X-ray
crystallography (see below), eliminate the possibility that the
previously reported species 18 was the hydride bridged product
[(4-Me3Si-Ar′)Sn(µ-H)]2.
In addition to the above ligands, we decided to investigate
the use of the extremely hindered terphenyl ligand 3,5-iPr2-
Ar* (3,5-iPr2-Ar* ) 3,5-iPr2-C6H-2,6-(C6H2-2,4,6-iPr3)2; Chart
2) which had already been shown to yield species whose
structures differ considerably from those with the less bulky
Ar′ ligand. For example, the unusual chromium(I) complex
[(3,5-iPr2-Ar*)Cr‚THF]33 was isolated and can be formally
regarded as a base-stabilized monomer surrogate of the known
multiply bonded chromium(I) dimer Ar′CrCrAr′.34 In a similar
manner as that reported for the synthesis of 17, the reaction of
(3,5-iPr2-Ar*)SnCl (15) with DIBAL gave a deep-blue solution
from which small, dark-blue, platelike crystals were obtained
and identified as the asymmetric tin(II) hydride (3,5-iPr2-Ar*)-
SnSn(H)2(3,5-iPr2-Ar*) (16).
tBu-Ar′)Sn(µ-H)]2 (10) according to Scheme 1 (via LiBH4).
As with 2, compounds 6 and 10 were obtained as orange crystals
that were only sparingly soluble in organic solvents. Well-
separated singlet resonances were observed at 9.28 and 9.11
1
ppm in the H NMR spectra (C6D6) of 6 and 10. Each signal
had broad tin satellites from which an average 1JSn-H coupling
constant of ca. 90-95 Hz could be obtained. Furthermore
compounds 2, 6, and 10 each gave similar UV-vis spectra with
weak absorptions positioned at 595, 598, and 594 nm, respec-
tively. Attachment of a lipophilic tBu group onto the Ar′ ligand
in 10 made the hydride sufficiently soluble in C6D6 to quickly
record 13C{1H} and 119Sn NMR spectra. Consequently a broad
resonance located at 667 ppm was observed in the 119Sn NMR
spectrum of 10; however the width of this signal (ca. 450 Hz)
was too large to allow for the determination of the Sn-H
coupling constant in the proton-coupled 119Sn NMR spectrum.
Using significantly longer acquisition times (up to 24 h) the
119Sn NMR spectra of 2 and 6 were also recorded and gave
similarly broad resonances at 657 and 687 ppm, respectively.
These results showed that compounds 2, 6, and 10 all have
similar structures in solution. Each of the tin(II) hydrides showed
signs of decomposition (charring) upon heating to ca. 100 °C
in the solid state; however samples of these hydrides decompose
in solution over time at much lower temperatures (40 °C) to
yield unknown species that we are currently trying to identify.
A further hydride [(4-Me3Si-Ar′)Sn(µ-H)]2 (14), containing
a para-trimethylsilyl group as part of the terphenyl ligand, was
prepared because of an unusual prior result in which the
distannyne (4-Me3Si-Ar′)SnSn(4-Me3Si-Ar′) (18) was found
to have a strongly bent structure and a long Sn-Sn bond in
comparison to the correspondingly wide C-Sn-Sn angle and
multiple Sn-Sn bonding in the Ar′SnSnAr′ analogue.31 To
investigate this phenomenon further, and to rule out the
possibility that 18 was actually a tin(II) hydride,32 14 was
synthesized using a similar amide metathesis route as that
employed for 2. Specifically, treatment of in situ generated tin-
It is noteworthy that both 16 and 17 have similar deep blue
colors in solution, suggesting that these species have similar
structures when dissolved. The UV-vis spectrum of 16 in
hexanes consists of three relatively weak absorptions at 422,
480, and 622 nm, respectively, while 17 displays a single broad
absorption at 608 nm of moderate intensity (ꢀ ) 160 M-1 cm-1).
The IR spectrum of 16 revealed two sharp bands of medium
intensity located at 1810 and 1783 cm-1 which are close to the
range of 1900 ( 50 cm-1 observed for Sn(IV) hydrides.29 This
is consistent with the retention of an asymmetric mixed-valent
structure for 16 in solution. Significantly, very similar Sn-H
stretches were also observed in the IR spectrum of 17 [1828
and 1771 cm-1, respectively], and a color change from orange
to blue was noted upon sample dissolution.6 Thus, it seems likely
that the centrosymmetric structure of 17 found in the solid state
is mostly converted into the mixed-valent form Ar*SnSn(H)2Ar*
and not the originally proposed monomer Ar*SnH, upon
dissolution in Nujol.
1
The H NMR spectrum of 16 in deuterated toluene at room
temperature showed only one set of signals for the aryl ligands.
In addition, a broad singlet at 7.92 ppm was present and was
assigned as a tin hydride resonance because of the observation
1
(31) Fischer, R. C.; Pu, L.; Fettinger, J. C.; Brynda, M. A.; Power, P. P. J. Am.
Chem. Soc. 2006, 128, 11366.
of accompanying tin satellites (average JSn-H ) ca. 528 Hz).
For comparison, the hydride resonance in 17 (7.87 ppm) is very
(32) It is often difficult to discount the presence of hydrogen atoms using X-ray
crystallography alone; see: (a) Schneider, J. J.; Goddard, R.; Werner, S.;
Kru¨ger, C. Angew. Chem., Int. Ed. Engl. 1991, 30, 1124. (b) Abrahamson,
A. B.; Niccolai, G. P.; Heinekey, D. M.; Casey, C. P.; Bursten, B. E. Angew.
Chem., Int. Ed. Engl. 1992, 31, 471. (c) Kersten, J. L.; Rheingold, A. L.;
Theopold, K. H.; Casey, C. P.; Widenhoefer, R. A.; Hop, C. E. C. A. Angew.
Chem., Int. Ed. Engl. 1992, 31, 1341. (d) Lutz, F.; Bau, R.; Wu, P.; Koetzle,
T. F.; Kru¨ger, C.; Schneider, J. J. Inorg. Chem. 1996, 35, 2698.
1
similar with a Sn-H coupling constant of 592 Hz.6 The H
(33) Wolf, R.; Brynda, M.; Ni, C.; Long, G. J.; Power, P. P. J. Am. Chem. Soc.
2007, 129, 6076.
(34) Nguyen, T.; Sutton, A. D.; Brynda, M.; Fettinger, J. C.; Long, G. J.; Power,
P. P. Science 2005, 310, 844.
9
16202 J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007

Isomeric Forms of Heavier Main Group Hydrides
A R T I C L E S
Figure 1. Molecular structures of tin hydrides 2, 6, 10, 14, and 16 at the 30% probability level with carbon-bound hydrogen atoms removed for clarity. For
i
16 the Me groups of the Pr moieties have also been removed for clarity.
NMR spectrum of 16 shows dynamic behavior and the Sn-H
resonances become sharper as the temperature is lowered.
Furthermore at -70 °C, the signals for the 3,5-iPr2-Ar* ligands
become more numbered, indicating a lowering of symmetry that
is possibly due to restricted rotation of the terphenyl ligands.
Despite the apparent lowering in symmetry, only a single Sn-H
environment could be found as evidenced by a shifted resonance
at 7.42 ppm which displayed well-resolved Sn-H coupling
[1J119Sn-H ) 1288 Hz; 1J117Sn-H ) 1234 Hz). These values are
more than twice as large as those observed for 17 (and 16 at
room temperature), although they are smaller than typical Sn-H
coupling constants found in tetravalent tin(IV) hydrides (ca.
1900 Hz).28 A possible explanation for these observations will
be detailed below.
X-ray Crystal Structures of the Tin(II) Hydrides. The
structures of the terphenyltin(II) hydrides are found in Figure
1. The 4-X-Ar′ substituted tin hydrides 2, 6, 10, and 14 adopt
centrosymmetric structures with highly pyramidalized Sn co-
ordination and planar Sn2H2 cores; a similar structural arrange-
ment was also observed for the more hindered Ar* derivative
17.6 In addition, the terphenyl ligands occupy relative positions
that are trans to each other, thus minimizing interligand
repulsion. The Sn-Sn separations in 2, 6, and 10 (Table 2) were
in the narrow range 3.1150(10) to 3.1260(11) Å, while the
9
J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007 16203

A R T I C L E S
Rivard, et al.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for the
Reported Tin(II) Hydrides with Standard Deviations in Parentheses
Table 3. Selected Mo¨ssbauer Parameters (in mm s-1) at 90 K for
Compounds 2, 16, and 20
compound
2
6
10
14
16
16
2
(3,5-iPr2
−
Ar*)SnSn(H)2
(3,5-iPr2
20
Sn-Sn
Sn-C(ipso) 2.221(6)
3.1260(11) 3.1176(4) 3.1150(10) 3.0532(8) 2.9157(10)
parameter
[Ar
′
Sn(
µ
-H)]2
−Ar*)
Ar*SnSnMe2Ar*
2.216(4) 2.208(6)
2.219(4)
1.94(2)
1.94(2)
2.216(3) 2.228(5)
2.200(3) 2.247(6)
1.95(3)
isomer
shift (IS)
2.805(4) orangea 1.370(4) [site A] 1.29(1)
2.800(3) greena
3.252(7) [site B]
2.70(1)
Sn-H
1.82(8)
1.99(8)
1.93(2)
3.00(1) [site C]b
1.90(3)
C(ipso)-
Sn-Sn
C(ipso)-
Sn-H
98.11(5)
94.32(9) 95.29(14) 96.18(7) 126.44(15)
quadrupolar
splitting (QS) 2.997(5) green
2.992(4) orange
0.967(4) [site A] 0.32(1)
95.18(10)
91(2)
101.3(7) 106.37(17)
100.0(19) 95.1(11)
98.5(11)
3.713(7) [site B]
4.50(1)
2.05(1) [site C]b
99(2)
91.9(15)
a Compound 2 exhibits photochromism (see Experimental Section for
details). b For compound 20 site C has been assigned to monomeric
Ar*SnMe. The three Mo¨ssbauer environments in 20 have essentially equal
absorption areas.
silylated derivative 14 had a Sn-Sn distance that was ca. 0.06
Å shorter [3.0532(8) Å]. For comparison the Ar* derivative 17
had a Sn-Sn distance of 3.1192(3) Å. The average bridging
Sn-H distances in each of the dimers are 1.91-1.94(2) Å and
are significantly longer than the terminal Sn-H bond [1.74(3)
Å] in the monomeric species [{HC(CMeNDipp)2}SnH] reported
by Roesky et al.7 It is interesting to note that the presence of
bridging hydrides in 2 leads to a dramatic change in the overall
structure when compared to the hydride-free distannyne ana-
logue Ar′SnSnAr′ (19).35 For example, the deep green distan-
nyne 19 has a much wider C(ipso)-Sn-Sn angle [126.24(7)°]
and a Sn-Sn distance [2.6675(4) Å] that is ca. 0.5 Å shorter
than that in 2. In addition, the relative orientation of the central
aryl rings of the terphenyl ligands are coplanar with the C-Sn-
Sn-C unit in 19, while in 2 these rings are canted to an
approximately orthogonal arrangement. The preparation and
structural characterization of the hydride analogue 2 is valuable
in reinforcing our initial assignment of the structure Ar′SnSnAr′,
as it is often difficult to rule out the presence of hydrogen atoms
bound to heavy atoms by X-ray crystallography alone.32
For reasons stated above, we also obtained the X-ray structure
of the silylated derivative 14. The overall structure of hydride
[(4-Me3Si-Ar′)Sn(µ-H)]2 (14) is similar to that of the distannyne
(4-Me3Si-Ar′)SnSn(4-Me3Si-Ar′) (18).31 However in the case
of 18 we were not able to observe any hydrogen atoms bound
to tin. We have subsequently repeated the synthesis of 18 by
the reduction of the tin(II) chloride 13 with potassium metal
and have obtained an extremely high quality structure of 18 (R
value <2.5%). As before, the overall structure of 18 was similar
to that of 14, and we could not locate any tin-bound hydrogen
atoms (however all of the ligand bound hydrogen atoms could
be located). The Sn-Sn distance in 18 [3.0660(10) Å; C(ipso)-
Sn-Sn angle: 99.25(14)°]31 is close to the value found in 14;
however it is still different within experimental error. It is
important to mention that crystals of 14 and 18 have vastly
different physical and spectroscopic properties (Vide supra).
These observations show that compounds 14 and 18 are indeed
distinct species but that their similar geometries are likely due
to packing effects that are largely controlled by the bulky 4-Me3-
Si-Ar′ ligands.
single bond as typical Sn-Sn single bonds within distannanes
R3SnSnR3 are in the range 2.76 to 2.88 Å.36 The terphenyl
ligands in 16 display two different orientations, with one of the
two central aryl rings being twisted nearly perpendicular to the
C(ipso)-Sn-Sn-C(ipso) unit [C(22)-C(21)-Sn(2)-Sn(1)
dihedral angle: -76.8°], whereas the other is arranged in a
coplanar fashion. Two different C(ipso)-Sn-Sn angles [126.44-
(15)° and 106.37(17)°] are observed which are much wider than
the SnsSn-C(ipso) angles in the corresponding centrosym-
metric terphenyltin(II) hydrides where these angles are in the
range: 94.32(9)° to 101.03(7)°. Unfortunately the tin-bound
hydrogen atoms in 16 could not be located by X-ray crystal-
lography due to the modest quality of the data. However the
different C(ipso)-Sn-Sn angles, the twisting of the aryl ligands,
and the short Sn-Sn distance strongly support the presence of
a mixed-valent tin(I/III) hydride in which two hydrogen atoms
are bound to Sn(1) (corresponding to form II in Chart 1). The
assignment of the Sn-H groups to Sn(1) is also supported by
the structures of the mixed-valent species Ar*SnSn(R)2Ar* (R
) Me and Ph; 20 and 21) which have C(ipso)-Sn-Sn angles
of 119.30(6)° and 101.17(5)° for 20 and 113.01(6)° and 108.46-
(8)° for the phenylated analogue 21; of note, both 20 and 21
have wider intraligand angles at tin when R groups are
bound.37,38 Moreover, compounds 20 and 21 have similar Sn-
Sn bond lengths [2.8909(2) and 2.9688(2) Å, respectively] to
that in 16 [2.9157(10) Å], consistent with a similar bonding
environment within each of these mixed valent species.
Mo1ssbauer Spectroscopy of Compounds 2 and 16. To
further establish the mixed-valent structure of 16 beyond any
doubt, Mo¨ssbauer data were recorded. For comparison, a
spectrum of the symmetric tin(II) hydride 2 was also obtained
(Table 3). The Mo¨ssbauer spectrum of 2 (Figure 2) is in
agreement with the X-ray data and reveals the presence of a
single tin(II) environment, with a doublet at 2.805(4) mm s-1
.
The quadrupolar splitting (QS) associated with 2 was quite large
[2.992(4) mm s-1] and indicates a highly asymmetric electronic
environment is present at the tin center. In the case of 16, a
more complicated spectrum was obtained which contained two
distinct tin sites in an approximately equal intensity ratio. The
first tin resonance [site A; IS ) 1.370(4) mm s-1] corresponds
The structure of the most hindered tin(II) hydride analogue
16 differed greatly from those of compounds 2, 6, 10, and 14
(Figure 1). The most striking feature of the dinuclear complex
16 is the significantly shorter Sn-Sn distance [2.9157(10) Å]
which is 0.15-0.2 Å shorter than that in any of the other tin-
(II) hydrides. This Sn-Sn distance is indicative of a lengthened
(36) (a) Pa´rka´nyi, L.; Ka´lma´n, A.; Pannell, K. H.; Cervantes-Lee, F.; Kapoor,
R. N. Inorg. Chem. 1996, 35, 6622. (b) Puff, H.; Breuer, B.; Gehrke-
Brinkmann, Kind, P.; Reuter, H.; Schuh, W.; Wald, W.; Weidenbru¨ck, G.
J. Organomet. Chem. 1989, 363, 265. (c) Kleiner, N.; Dra¨ger, M. J.
Organomet. Chem. 1984, 270, 151.
(37) Eichler, B. E.; Power, P. P. Inorg. Chem. 2000, 39, 5444.
(38) Phillips, A. D.; Hino, S.; Power, P. P. J. Am. Chem. Soc. 2003, 125, 7520.
(35) (a) Phillips, A. D.; Wright, R. J.; Olmstead, M. M.; Power, P. P. J. Am.
Chem. Soc. 2002, 124, 5930. (b) Power, P. P. Chem. Commun. 2003, 2091.
9
16204 J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007

Isomeric Forms of Heavier Main Group Hydrides
A R T I C L E S
Figure 3 shows the results of calculations on the least hindered
member of the terphenyltin(II) hydride series, (Ar′SnH)2. It is
noteworthy that each of the calculated structures are close in
energy (within 7 kcal mol-1). When the Ar′ ligand is present,
the most stable gas-phase structure is actually the asymmetric
form related to II, while the formally double-bonded isomer I
is only less stable by 2.2 kcal mol-1. Interestingly, the hydride-
bridged form IV is the least stable isomer by 7.0 kcal mol-1
.
Nonetheless, this is the form that (Ar′SnH)2 adopts in the solid
state. From these data it is anticipated that, due to the low energy
difference between each of the isomers, factors such as crystal
packing forces could become very important in dictating which
structure is favored upon crystallization. For example the known,
multiply bonded, trans-bent stannyne Ar′SnSnAr′ (19) was
calculated to be only 5.3 kcal mol-1 more stable than the more
strongly bent, formally singly bonded isomer shown at the top
right of Figure 3.41,42 However by adding a remote trimethylsilyl
group at the 4-position of the central ring of the terphenyl ligand,
we have been able to crystallize a single-bonded form in the
solid state, (4-Me3Si-Ar′)SnSn(4-Me3Si-Ar′) (18),31 that has
overall metrical parameters which match closely those listed in
Figure 3. The calculated structure for the hydride-bridged isomer
[Ar′Sn(µ-H)]2 corresponds reasonably well to the structure of
2 determined by X-ray crystallography. The calculated Sn-Sn
separation (3.191 Å) is ca. 0.06 Å longer than that in 2 [3.1260-
(11) Å] while the C(ipso)-Sn-Sn angle was nearly identical
[98.0° vs 98.11(15)° in 2]. Furthermore, the structure of 2
contains central terphenyl rings that are oriented orthogonally
to the Sn2H2 core, which is also reproduced by the theoretical
data (see Supporting Information for more details).
Various isomers of (Ar*SnH)2 were calculated and shown
to have relative energies within ca. 7 kcal mol-1 of each other
(Figure 4). The hydride-bridged form [Ar*Sn(µ-H)]2 was
calculated to be the least stable entity, and the asymmetric form,
Ar*SnSn(H)2Ar*, the most stable. However, as in the case of
the Ar′ derivative, the hydride-bridged isomer is the form
obtained upon crystallization of (Ar*SnH)2. In other words
crystallization does not yield the most stable structures calculated
for the gas phase.
Figure 2. Mo¨ssbauer spectra for [Ar′Sn(µ-H)]2 (2) (a) and (3,5-iPr2-Ar*)-
SnSn(H)2(3,5-iPr2-Ar*) (16) (b).
to the tetravalent, formally Sn(III) site in 16, while the remaining
resonance at IS ) 3.252(7) mm s-1 is assigned to the lower
oxidation state, divalent Sn(I) center (site B). Both of the
resonances for 16 were split into doublets with the more
asymmetrically coordinated divalent Sn(I) site B having a larger
quadrupolar splitting [3.713(7)] than the tetravalent site A which
likely contains two bound hydrogen atoms [QS of site A )
0.967(4) mm s-1]. The temperature dependence of the recoil-
free fraction over the range 92 e T e 170 K is identical for the
two sites within experimental error. These data are consistent
with our prior assignment of an asymmetric structure for 16
with two hydrogen atoms located on a single tin site. Interest-
ingly, the Mo¨ssbauer spectrum of the related species Ar*SnSnMe2-
Ar* 20 has quite similar parameters (Table 3).39
Solid-State Structures and Theoretical Analysis. Although
the calculations of Trinquier on the isomers I-IV of Sn2H4
showed that they do not differ greatly in energy,8,40 the large
size discrepancy between hydrogen and the terphenyl ligands
made detailed calculations on the relative stabilities of forms I,
II, and IV in the tin(II) hydride analogues (ArSnH)2 featuring
the same ligand series as employed in our experimental studies
(namely Ar′, Ar*, and 3,5-iPr2-Ar*) desirable.
When the relative energies of the possible isomers of the
bulky tin(II) hydride [(3,5-iPr2-Ar*)SnH]2 were calculated
(Figure 4), the most stable isomer was also predicted to be the
i
asymmetric form II. However the presence of the added Pr2
ligands in 3,5-iPr2-Ar* destabilizes both the Sn-Sn bonded
form I and the hydride-bridge structure IV to the extent that
the latter isomer now lies higher in energy (relative to the
asymmetric form II) by 14.3 kcal mol-1. Apparently the added
bulk within the 3,5-iPr2-Ar* ligand has a significant effect on
the relative stability of the various tin(II) hydride forms in the
gas phase, and contrary to what was observed with the Ar′ and
Ar* ligands, the energy difference is sufficiently large to ensure
that the asymmetric form is the one that is obtained on
crystallization (cf. structure of 16). The calculated structure of
16 reproduces well the experimentally determined Sn-Sn and
Sn-C bond lengths [Sn-Sn: 2.959 Å vs 2.9157(10) Å in 16;
Sn-C(ipso): 2.230 and 2.248 Å vs 2.228(5) and 2.247(6) Å in
16]. However each of the calculated Sn-Sn-C(ipso) angles is
overestimated by 6°-18° [111.9° and 144.7° vs 106.37(17)°
(39) Power, P. P.; Stanciu, C.; Nowik, I.; Herber, R. H. Inorg. Chem. 2005, 44,
9461.
(40) Our calculations show that the asymmetric species HSn-SnH3 (form II)
is the most stable isomer of the Sn2H4 isomer series. This is in contrast to
work by Trinquier (ref 8) which showed that the hydride-bridged isomer
[HSn(µ-H)]2 was the most stable form. These differences are likely a
reflection of the higher level of calculations used in this present study.
(41) Takagi, N.; Nagase, S. Organometallics 2007, 26, 469.
(42) Takagi, N.; Nagase, S. Organometallics 2007, 26, 3627.
9
J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007 16205

A R T I C L E S
Rivard, et al.
Figure 3. Calculated energies and selected geometric parameters for various isomer forms of (Ar′SnH)2 at the B3PW91 level. Values in parentheses correspond
to energies relative to Ar′SnSnAr′ (set at 0.0 kcal/mol).
Figure 4. Calculated relative energies and selected geometric parameters for various isomer forms of (Ar*SnH)2 and [(3,5-iPr2-Ar*)SnH]2 at the B3PW91
level.
and 126.44(15)°]. The location of two hydrogen atoms at the
tin center with the widest Sn-Sn-C(ipso) angle in the
theoretical structure matches the assignment of two asymmetri-
cally disposed hydrogen atoms at Sn(1) in 16. Importantly,
Mo¨ssbauer studies on 16 clearly show the presence of a mixed-
valent structure in the solid state, thus corroborating the presence
of asymmetrically bound Sn-H groups. The additional crowding
posed by the 3,5-iPr2-Ar* ligand leads to the formation of an
asymmetric form in order to widen the interligand angles at
the tin centers and alleviate steric repulsion. The behavior of
the various terphenyl tin(II) hydrides in solution is by no means
trivial, and we will discuss this topic below.
Scheme 2. Monomer-dimer Equilibria in Ar*SnSnPh2Ar* (21)38
Me or Ph with larger, stabilizing coligands introduces new types
of equilibria as shown in Scheme 2. In solution, the asymmetric
Ar*SnSn(Ph)2Ar* (21) is the predominant form at low temper-
atures, while warming to room temperature leads to significant
quantities of monomeric Ar*SnPh.38 For organotin(II) hydrides
RSnH, additional structural equilibria need to be considered
because of the stronger bridging tendency of hydrogen (Scheme
3).
Solution Behavior of Tin(II) Hydrides. Although distan-
nylenes (SnR2)2 containing four bulky organic substituents (e.g.,
[(Sn{CH(SiMe3)2}2)2])43 display only simple monomer-dimer
equilibria in solution, the use of smaller substituents such as
(43) (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.
9
16206 J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007

Isomeric Forms of Heavier Main Group Hydrides
A R T I C L E S
Scheme 3. Possible Equilibria Available to Organotin(II) Hydrides
Table 4. Calculated UV-vis and IR Spectral Data for Tin(II)
Hydrides
R
)
Ar
′
Ar*
3,5-iPr2
−Ar*
UV-vis [nm (intensity)]
form I
482.8 (f ) 0.341)
492.7 (0.371)
348.8 (0.059)
678.8 (0.006)
349.9 (0.025)
335.0 (0.025)
346.7 (0.121)
335.0 (0.130)
478.2 (0.032)
332.4 (0.060)
608 (100)
510.6 (0.377)
331.9 (0.061)
726.2 (0.005)
366.7 (0.027)
365.1 (0.166)
404.8 (0.136)
364.7 (0.045)
460.7 (0.037)
293.6 (0.046)
622 (220)
334.0 (0.033)
664.5 (f ) 0.006)
339.9 (0.072)
339.3 (0.044)
345.2 (f ) 0.159)
339.9 (0.072)
479.3 (f ) 0.024)
317.4 (0.100)
595 (70)
form II
form IV
monomer
experimental
The solution behavior of 2 and its para-substituted derivatives
differs from those of the more hindered species 16 and 17. As
mentioned, [Ar′Sn(µ-H)]2 (2) crystallizes as an orange solid with
a centrosymmetric, hydride-bridged structure (Figure 1). Com-
pound 2 dissolves sparingly in organic media to give very pale
blue solutions, while 16 and 17 dissolve readily to produce deep
blue solutions. Analysis of 2 by solution 1H NMR spectroscopy
revealed typical ligand based signals while a Sn-H resonance
could be located at 9.13 ppm. The Sn-H resonance was also
flanked by broad tin satellites which afforded an average 1JSn-H
coupling constant of 89 Hz. Typically Sn-H coupling constants
are an order of magnitude larger; thus it appeared that 2 retained
its dimeric structure with Sn-H-Sn bridges in solution. We
were unable to locate any IR bands for 2 in the region of 1550
to 2800 cm-1 further supporting the lack of any terminally bound
Sn-H groups in this species.44
480 (shoulder)
422 (540)
R
) Me
IR [cm-1 (intensity, assignment)]
form I
1836.0 (476.4, terminal Sn-H stretch)
807.0 (166.3, Sn-H bending)
form II
form III
form IV
1843.6 (209.1, terminal Sn-H stretch)
1836.6 (209.1, terminal Sn-H stretch)
1820.7 (313.3, terminal Sn-H stretch)
813.1 (355.3, bridging Sn-H stretch)
1190.7 (1450.7, bridging Sn-H stretch)
1007.4 (112.3, Sn-H bending)
Ar′SnH monomer
1734 (terminal Sn-H stretch)
the acquisition of an informative 119Sn NMR spectrum. Thus it
appears that the 4-X-Ar′SnH derivatives featured in this study
all have predominantly dimeric structures in solution and that
small changes in the electronic nature of the Ar′ ligands have
little effect on the structural type adopted.
In order to provide information on this phenomenon, the UV-
vis and IR spectra of the various dimeric and monomeric forms
of Ar′SnH, Ar*SnH, (3,5-iPr2-Ar*SnH), and, in the case of
the IR spectra, the model species (MeSnH)2 were calculated at
the B3PW91 level; pertinent spectral parameters are listed in
Table 4. Significantly, each of the forms of (MeSnH)2 containing
terminal Sn-H groups were found to have Sn-H stretching
modes in the narrow range of 1820 to 1843 cm-1. However in
the hydride-bridged isomer [MeSn(µ-H)]2, the Sn-H stretching
mode was shifted considerably to a position of 1191 cm-1 with
In the original 2000 report the orange species 17 was shown
to crystallize as a hydrogen-bridged dimer with a similar
structure to that of 2 in the solid state. However it was noted
that 17 dissolved in Nujol to give deep blue-colored mulls, and
a similar deep blue color was seen when 17 was dissolved in
organic solvents. These observations were interpreted in terms
of the existence of a blue colored monomeric form, Ar*SnH,
as the major species in solution. However the data obtained for
16 strongly suggest that the solution form of 17 is not the
monomer but the asymmetric Ar*SnSn(H)2Ar*. The IR and
NMR spectra of 17 are very similar to the corresponding spectra
for 16. The calculations indicate that the asymmetric species
ArSnSn(H)2Ar are also expected to have two Sn-H stretches
around 1800 cm-1 (Table 4). Moreover, only a single Sn-H
stretching vibration is expected for the monomeric species
Ar*SnH and also for the Sn-Sn bonded species Ar*(H)Snd
Sn(H)Ar* related to form I in Chart 1. The magnitude of the
Sn-H coupling constant in 17 indicates that the Sn-H moiety
is terminal and not bridging as it is in the solid state. The
moderately intense UV-vis absorption found for 17 at 608 nm
is also consistent with the asymmetric form because all other
isomers, including monomeric Ar*SnH, are calculated to have
no UV-vis absorptions above 500 nm (Table 4). The 119Sn
NMR spectral data for 17 are more complex. A broad resonance
at 695 ppm was observed which is very close to the value of
657 ppm seen for 2. This chemical shift difference is quite small
considering that the known chemical shift window for Sn(II)
species currently spans over ca. 3000 ppm.45 This observation
is consistent with the existence of detectable quantities of the
no vibrational modes predicted between 1480 and 3050 cm-1
.
The IR spectrum of 2 is therefore consistent with the retention
of a dimeric form with Sn-H-Sn bridges.44 Calculations on
the Ar′SnH monomer afforded a Sn-H stretching band at 1734
cm-1 which rules out a monomer-dimer equilibrium process.
The calculations predict that 2 should have no UV-vis
absorption above 488 nm. Nonetheless, we observed a very
weak and broad absorption centered at 595 nm (ꢀ ) 70 M-1
cm-1). It is unclear at this moment which species is responsible
for this absorption; however the only species calculated to have
an absorption above 500 nm was the asymmetric form Ar′SnSn-
(H)2Ar′ (at 665 nm). Hence it is possible that a minor amount
of Ar′SnSnH2Ar′ is present in solution when 2 is dissolved,
thus providing a possible explanation for the faint blue color
observed in solution; however we have not be able to detect
this species by IR or NMR spectroscopy. The spectroscopic data
for the para-substituted analogues 4-X-Ar′SnH (X ) MeO,
tBu, and SiMe3) are similar to those of 2 with the exception
that the greater insolubility of the SiMe3 derivative prevented
(44) Sn-H-Sn vibration has been calculated to be at 1191 cm-1 in the model
species [MeSn(µ-H)]2; therefore the Sn-H-Sn vibration in 2 is likely
obscured by terphenyl ligand vibrations that occur in this region. Our
attempts to prepare the deuterated analogue [Ar′Sn(µ-D)]2 by the reaction
of Ar′SnCl with Li[AlD4] or NaD have been unsuccessful to date.
(45) Stanciu, C.; Richards, A. F.; Stender, M.; Olmstead, M. M.; Power, P. P.
Polyhedron 2006, 25, 477 and references therein.
9
J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007 16207

A R T I C L E S
Rivard, et al.
symmetric dimer [Ar*Sn(µ-H)]2 in solution. Closer inspection
of the 1H NMR spectrum of 17 also revealed the presence of a
minor quantity of the symmetric tin hydride species (ca. 10%)
as evidenced by a singlet at 9.82 ppm along with accompanying
tin satellites [1JSn-H ) ca. 90 Hz]. Thus the data show that a
minor quantity of the hydride-bridged dimer [Ar*Sn(µ-H)]2 is
present when 17 is dissolved, while the asymmetric form
analogous to 16 which can be detected on the IR time scale is
not observed by 119Sn NMR spectroscopy. This is may due to
equilibria involving other hydride forms and/or line broadening
of the expected resonances in Ar*SnSn(H)2Ar* as a result of
the anticipated large chemical shift anisotropy of this species.46
Attempts to observe other structural forms of 17 in the
temperature range -70 to 60 °C gave 1H and 119Sn NMR spectra
similar to those observed at room temperature.
it appears that when 16 is cooled there is a decreasing
contribution from the monobridged isomer III to the equilibrium
structure on the NMR time scale.
Conclusions
A series examples of tin(II) hydrides stabilized by terphenyl
ligands have been synthesized and characterized by using a
combination of theoretical and experimental methods. In the
crystalline state, centrosymmetric dimers of the general form
[ArSn(µ-H)]2 are observed when either Ar* or the electronically
modified ligands 4-X-Ar′ (X ) H, MeO, tBu, and SiMe3) are
used. However with the more encumbered ligand 3,5-iPr2-Ar*,
an asymmetric structure, (3,5-iPr2-Ar*)SnSn(H)2(3,5-iPr2-Ar*),
is obtained which is also predicted to be the most stable form
in the gas phase by detailed theoretical calculations. The
formation of the asymmetric form leads to a widening of the
interligand angles at tin, thus alleviating steric repulsion between
the terphenyl ligands and providing impetus for the observed
structural difference in this system. In solution, it appears that
the less-hindered tin(II) hydrides adopt mostly symmetric
dimeric structures with Sn-H-Sn bridges in solution, while
the more hindered analogues (Ar*SnH)2 and [(3,5-iPr2-Ar*)-
SnH]2 exist predominantly as asymmetric forms with possible
equilibria involving hydride-bridged (III and IV) and non-
bridged forms (I and II). Future work will involve further
exploration of the nature of the various possible equilibria
between forms I-IV of the tin(II) hydrides via ligand modifica-
tion and the investigation of the reaction chemistry of this new
class of main group hydrides.
In contrast, for 16 we do not see any evidence for the
symmetric dimer [(3,5-iPr2-Ar*)Sn(µ-H)]2 in solution as the
1H NMR spectrum afforded only a single hydride resonance at
7.92 ppm (1JSn-H ) 532 Hz); only a single broad resonance
was found at 1723 ppm in the 119Sn NMR spectrum. As with
17, two distinct Sn-H stretching modes were observed in the
terminal Sn-H stretching region which strongly suggests that
the major species present is asymmetric (3,5-iPr2-Ar*)SnSn-
(H)2(3,5-iPr2-Ar*). For comparison, the known mixed-valent
species Ar*SnSn(Ph)2Ar* (21) yields a broad downfield posi-
tioned, room-temperature 119Sn NMR resonance at 1517 ppm.
Upon cooling 21 to -70 °C, the signal at 1517 ppm gradually
disappears and a new set of signals emerge at 2857 and 257
ppm, consistent with the existence of an asymmetric form at
low temperature. Therefore for 21, it was reasoned that the
monomeric species Ar*SnPh was predominant at room tem-
perature. In the case of 16, we cannot rule out the presence of
the monobridged isomer III, (3,5-iPr2-Ar*)SnSn(H)(µ-H)(3,5-
iPr2-Ar*), due to the greater propensity of the hydride group
to migrate and undergo bridging interactions (relative to a phenyl
group). Upon cooling a sample of 16 in toluene to -70 °C, the
signal at 1723 ppm disappears and is replaced by a new, broad,
resonance at +34 ppm. It is possible that this latter resonance
belongs to the tetravalent tin atom in the asymmetric structure
(3,5-iPr2-Ar*)SnSn(H)2(3,5-iPr2-Ar*), while the remaining
two-coordinate, divalent tin center is not observed due its greater
chemical shift anisotropy.46 Furthermore, we observe an increase
Acknowledgment. The authors are grateful to the National
Science Foundation and the U.S. Department of Energy for
financial support. The Advanced Light Source is supported by
the Director of the Office of Science and Basic Energy Services,
of the U.S. Department of Energy under Contract No. DE-AC02-
05CH11231. E.R. thanks the Natural Sciences and Engineering
Research Council (NSERC) of Canada for a Postdoctoral
Fellowship. R.C.F. thanks the Max Kade Foundation for
financial support. R.W. thanks the Alexander von Humboldt
Foundation for a Feodor Lynen Research Fellowship. L.P.
thanks the American Chemical Society for a PRF award. N.T.
and S.N. thank the Grant-in-Aid for Creative Scientific Research,
Scientific Research on Priority Area and Next Generation Super
Computing Project (Nanoscience Program) from the MEXT of
Japan. We also would like to thank Prof. Marilyn Olmstead,
Christine Beavers, Allen Ng, and Assaf Aharoni for their
assistance.
1
in the value of the JSn-H associated with the tin hydride
resonance in the 1H NMR spectrum of 16 from a value of 532
Hz at room temperature to 1234 (117Sn-H) and 1288 Hz
(
119Sn-H) at -70 °C; the location of the Sn-H resonance
gradually shifts from 7.87 to 7.48 ppm upon cooling. This
increase in the value of the Sn-H coupling constant is consistent
with an increase of s-character in the Sn-H bond, which is
what one would expect as the average structure of 16 in solution
approaches that of the non-hydrogen-bridged form II. Therefore
Supporting Information Available: Complete experimental
details and crystallographic data for 6, 10, 14, and 16 (CIF);
full citation for ref 24. Single-point energy calculations for
(MeSnH)2 isomers. Calculated geometries for the terphenyl tin
hydride isomers (RSnH)2 (R ) Ar′, Ar*, and 3,5-iPr2). This
material is available free of charge via the Internet at
http://pubs.acs.org.
(46) Line broadening caused by the larger anisotropy of its chemical shift in
Sn-Sn bonded species is a phenomenon observed previously; see: (a)
Eichler, B. E.; Phillips, B. L.; Power, P. P.; Augustine, M. P. Inorg. Chem.
2000, 39, 5450. (b) Spikes, G. H.; Giuliani, J. R.; Augustine, M. P.; Nowik,
I.; Herber, R. H.; Power, P. P. Inorg. Chem. 2006, 45, 9132.
JA076453M
9
16208 J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007