Synthesis and characterization of the unstable primary amido tin(II) dimer Sn2{N(H)Dipp}4 (Dipp = C6H3-2,6- Pri2) and the first sesqui-amido hemi-chloride derivative Sn2{N(H)Dipp}3Cl: Facile conversion of a primary amide to the imide (SnNDipp)4

Article abstract of DOI:10.1039/b809671f

The primary tin(ii) amido derivatives Sn₂{N(H)Dipp}₄ (1) and Sn₂{N(H)Dipp}₃Cl (2) (Dipp = C₆H ₃-2,6-Prⁱ₂) have been prepared and characterized. Compound 1 was obtained by the transamination of Sn{N(SiMe ₃)₂}₂ with H₂NDipp in a 1:2 ratio or by the reaction of two equivalents of LiN(H)Dipp with SnCl₂. The attempted preparation of Sn(Cl){N(H)Dipp} by reaction of LiN(H)Dipp with SnCl₂ in a 1:1 ratio led to the isolation of the unique species Sn₂{N(H)Dipp}₃Cl, which is the first example of a sesqui-amido derivative of a group 14 element. Both 1 and 2 were characterized by ¹H and ¹¹⁹Sn NMR spectroscopy, X-ray crystallography and Moessbauer spectroscopy. The structures of 1 and 2 feature two tin centers bridged by -N(H)Dipp ligands with the terminal positions being occupied by two N(H)Dipp (1) or -N(H)Dipp and -Cl (2) groups. The compound 1 was found to be unstable under ambient conditions and spontaneously converts to the imide tetramer (SnNDipp)₄ in solution over several days at room temperature, representing a new synthetic route to group 14 element imides.

Full text of DOI:10.1039/b809671f

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PAPER
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Synthesis and characterization of the unstable primary amido tin(II) dimer
Sn2{N(H)Dipp}4 (Dipp = C6H3-2,6-Pri2) and the first sesqui-amido
hemi-chloride derivative Sn2{N(H)Dipp}3Cl: facile conversion of a primary
amide to the imide (SnNDipp)4†
W. Alexander Merrill,a Jochen Steiner,a Audra Betzer,a Israel Nowik,b Rolfe Herber*b and Philip P. Power*a
Received 9th June 2008, Accepted 2nd July 2008
First published as an Advance Article on the web 11th September 2008
DOI: 10.1039/b809671f
The primary tin(II) amido derivatives Sn2{N(H)Dipp}4 (1) and Sn2{N(H)Dipp}3Cl (2) (Dipp =
C6H3-2,6-Pri2) have been prepared and characterized. Compound 1 was obtained by the transamination
of Sn{N(SiMe3)2}2 with H2NDipp in a 1 : 2 ratio or by the reaction of two equivalents of LiN(H)Dipp
with SnCl2. The attempted preparation of Sn(Cl){N(H)Dipp} by reaction of LiN(H)Dipp with SnCl2
in a 1 : 1 ratio led to the isolation of the unique species Sn2{N(H)Dipp}3Cl, which is the first example of
a sesqui-amido derivative of a group 14 element. Both 1 and 2 were characterized by 1H and 119Sn NMR
spectroscopy, X-ray crystallography and Mo¨ssbauer spectroscopy. The structures of 1 and 2 feature two
tin centers bridged by –N(H)Dipp ligands with the terminal positions being occupied by two N(H)Dipp
(1) or –N(H)Dipp and –Cl (2) groups. The compound 1 was found to be unstable under ambient
conditions and spontaneously converts to the imide tetramer (SnNDipp)4 in solution over several days
at room temperature, representing a new synthetic route to group 14 element imides.
of the amide nitrogens from each ring.13 In contrast, the derivative
Introduction
Sn[{N(But)}2SiMe2] crystallizes both as a monomer and as a
Beginning with the synthesis and characterization of
weakly associated dimer as a result of weak tin–tin bonding
Sn{N(SiMe3)2}2 in 1974,1,2 amido (–NR2) ligands have played a
14
˚
(Sn–Sn = 3.68 A).
leading role in the development of the chemistry of molecular
We now report that the reaction of Sn{N(SiMe3)2}2 with two
tin(II) derivatives.3–9 Currently, about thirty homoleptic, Sn(NR2)2,
equivalents of the primary amine H2NDipp (Dipp = C6H3Pri2-
(R = alkyl, aryl or silyl) and a similar number of heteroleptic,
2,6) under mild conditions afforded the first homoleptic, primary
Sn(X)(NR2), (X = halogen, pseudohalogen, amido, alkoxo,
thiolato, etc.) tin(II) amides have been structurally characterized
and their chemistry explored. The use of large substituents is
crucial for the isolation of the homoleptic amides as monomers.
These have v-shaped coordination and a lone pair of electrons
at the tin center. Such derivatives are generally colored as a
result of an n–p transition from the tin lone pair to the empty
p-orbital. In contrast, primary amido (–N(H)R) molecular tin(II)
derivatives are rare owing to the diminished steric protection
afforded by the ligand which carries only one organic substituent
at nitrogen. At present, they are limited to just one structurally
characterized example, the compound Sn{N(H)Mes*}2 (Mes* =
C6H2-2,4,6-But3).10 In addition, there are few structures
known for tin(II) homoleptic amide dimers. The simplest is
amido, tin dimer {Dipp(H)N}Sn{m-N(H)Dipp}2Sn{N(H)Dipp}
(1) as yellow crystals in moderate yield. The derivative 1 was
also prepared by the reaction of LiN(H)Dipp with SnCl2 in a
2 : 1 ratio. The attempted synthesis of the heteroleptic derivative
[Sn(m-Cl){N(H)Dipp}]2 via the 1 : 1 reaction of LiN(H)Dipp and
SnCl2 led only to the unique sesqui-dimer {Dipp(H)N}Sn{m-
N(H)Dipp}2SnCl (2). Solutions of 1 in toluene were found to be
unstable as they eliminated H2NDipp over several hours to afford
the previously reported Sn4N4 cubane species (SnNDipp)4.15
Experimental
General procedures
11
All manipulations were performed with the use of modified
Schlenk techniques under argon or in a Vacuum Atmospheres
drybox under N2. Solvents were dried and collected using a
Grubbs-type solvent purification system16 (Glass Contour) and
degassed by sparging with dry Ar for 10 min, while ether solvents
were degassed via two freeze–pump–thaw cycles. DippNH2 was
purchased from Aldrich and distilled from a small amount of
the dialkyl amide Sn(NMe2)2 which is associated through
–NMe2 bridging to afford (Me2N)Sn(m-NMe2)2Sn(NMe2)12
with pyramidally coordinated tins. Similarly, the chelated
is dimerized through bridging by one
aDepartment of Chemistry, University of California, Davis, One Shields
Avenue, Davis, California, 95616, USA. E-mail: pppower@ucdavis.edu
˚
CaH2 onto activated 4 A molecular sieves. SnCl2 was obtained
bRacah Institute of Physics, Hebrew University of Jerusalem, 91904,
Jerusalem, Israel. E-mail: herber@vms.huji.ac.il
from Aldrich and used without further purification. LiNHDipp
was prepared by treatment of the corresponding aniline with
nBuLi (1.6 M in hexanes, Acros) in hexanes at 0 ◦C followed
by decanting of the supernatant, washing the precipitated product
† Electronic supplementary information (ESI) available: Fig. S1–S2.
CCDC reference numbers 692606 and 692607. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b809671f
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once with cold hexane and removal of the solvent under reduced
pressure. Sn[N(SiMe3)2]2 was prepared according to the literature
procedure1 from the corresponding lithium amide and SnCl2. All
physical measurements were obtained under strictly anaerobic and
anhydrous conditions. 1H and 13C NMR spectra were acquired on
either a Varian Mercury 300 MHz or Varian Inova 400 MHz
instrument and referenced internally either to residual protio
benzene or trace silicone vacuum grease, d 0.29 ppm in C6D6. 119Sn
NMR spectra were acquired on either a Varian Inova 600 MHz
instrument (223.7 MHz) or a Varian Inova 400 MHz (149.1 MHz)
and were externally referenced to Bu4Sn in C6D6 (-11.7 ppm).
IR spectra were recorded as Nujol mulls between CsI plates
on a Perkin-Elmer 1430 spectrophotometer. UV-vis spectra were
recorded as dilute hexane solutions in 3.5 mL quartz cuvettes
using a HP8452 diode array spectrophotometer. Melting points
were determined on a Meltemp II apparatus using glass capillaries
sealed with vacuum grease, and are uncorrected.
Storage at ca. -13 ◦C for 3 d afforded 2 as pale yellow block-
shaped crystals suitable for X-ray diffraction. Yield 0.06 g (4%),
mp 124–128 ◦C. IR (cm-1) : 3360, 3280 n(N–H); 375br n(Sn–N).
◦
1H NMR (300 MHz, C6D6, 25 C) d 1.14 (d, 12H, J = 6.9 Hz,
DippNH2 decomp. product, CH3), 1.19 (d, 12H, J = 6.9 Hz, CH3),
1.24 (d, 12H, J = 6.9 Hz, CH3), 1.30 (d, 12H, J = 6.9 Hz, CH3),
1.38 (d, 12H, J = 6.9 Hz, CH3), 2.65 (sept, 2H, J = 6.9 Hz,
DippNH2 decomp. product, CH), 3.19 (s br, DippNH2 decomp.
product, NH2), 3.32 (sept, 2H, J = 6.9 Hz, CH), 3.55 (sept br,
2H, J = 6.9 Hz, CH), 5.28 (d br), 6.91 (t, 3H, J = 6.9 Hz, ArH),
1
7.04 (d, 2H, J = 6.9 Hz, ArH), 7.10 (m, ArH).13C{ H} NMR
(75.44 MHz, C6D6, 25 ◦C) d 147.09, 140.68, 139.44, 134.55, 132.33,
124.66, 123.61, 123.29, 123.13, 118◦.97, 117.97, 29.74, 28.16, 22.63.
119Sn NMR (149.1 MHz, C6D6, 25 C) d -29.5 (s, br), -49.7 (s, br).
Preparation of (SnNDipp)4 (3)
Method A. In a manner similar to that reported in ref. 15,
DippNH2 (0.355 g, 2 mmol) and Sn[N(SiMe3)2]2 (0.878 g, 2 mmol)
were combined in a Schlenk flask and subsequently heated to ca.
60 ◦C for 10 min, over which time vapors of HN(SiMe3)2 byproduct
were observed, and intermittent vacuum was applied. Toluene (ca.
20 mL) was added to the resulting yellow solid and the mixture
briefly refluxed. After removal of the solvent ca. 25 mL of hexanes
were added and the mixture filtered via cannula. The resulting
yellow solution was concentrated and stored for 2 d at -13 ◦C,
affording yellow blocks of 3. Yield 0.92 g (78%) IR (cm-1) : no
v(N–H) observed; 385 br v(Sn–N). 1H NMR (C6D6, 25 ◦C) d 1.29
(d, 48H, J = 6.9 Hz, DippN-, CH3), 3.46 (sept, 8H, J = 6.9 Hz,
DippN-, CH), 6.90 (t, 4H, J = 6.9 Hz, C6H3, p-H), 7.13 (d, 8H,
Preparation of Sn2{N(H)Dipp}4 (1)
Method A. To DippNH2 (0.370 g, 2.1 mmol) in 25 mL hexanes
at 0 ◦C was added S◦n[N(SiMe3)2]2 (0.439 g, 1 mmol) in 25 mL
of hexanes, also at 0 C. The resulting clear pale yellow reaction
mixture was stirred for 30 min and kept cold, then stored overnight
at -13 ◦C, after which the solvent was removed under reduced
pressure giving a bright yellow oil. Vacuum was applied at room
temperature for an additional 20 min to remove the HN(SiMe3)2
byproduct. The residue was extracted with cold n-hexane (ca.
10 mL), and stored at -13 ◦C for 2 d, affording pale yellow plates
of 2◦suitable for X-ray diffraction. Yield ca. 0.30 g (63%), mp
106 C (darkens). IR (cm-1): 3360, 3280 n(N–H); 350br n(Sn–N).
1H NMR (C6D6, 25 ◦C) d 1.14 (d, 12H, J = 6.9 Hz, DippNH2 co-
product from spontaneous transformation, CH3), 1.17 (d, 12H,
J = 6.9 Hz, CH3), 1.31 (d, 12H, J = 6.9 Hz, compound 3
co-product, CH3), 2.63 (sept, 2H, J = 6.9 Hz, DippNH2 co-
product, CH), 2.99 (sept, 2H, J = 6.9 Hz, DippNH-, CH), 3.19
(s br, DippNH2 co-product, NH2), 3.48 (sept, 2H, J = 6.9 Hz,
compound 3 co-product, CH), 3.83 (s br, DippNH-), 4.17 (s br,
DippNH-), 4.65 (s), 4.75 (s), 5.01 (s), 6.90 (m, ArH), 7.08 (m
◦
1
J = 6.9 Hz, C6H3, m-H).13C{ H} NMR (75.44 MHz, C6D6, 25 C)
d 148.08, 141.95, 125.44, 121.31, 30.59, 29.97. 119Sn NMR (C6D6,
25 ◦C, 223.7 MHz) d 315.0 (s, br).
Method B. [(DippNH)2Sn]2 (2) (0.473 g, 0.5 mmol) was
dissolved in 30 mL toluene and stirred at ca. 25 ◦C for 4 d. After the
solvent was removed under reduced pressure, the oily residue was
heated in a 50 ◦C oil bath for 2 h and then extracted with 10 mL
of hexanes. Storage at -13 ◦C for 5 d afforded yellow crystals of 3.
Yield 0.l5 g (51%).
◦
1
ArH) 13C{ H} NMR (75.44 MHz, C6D6, 25 C). d 146.99, 140.43,
138.41, 124.22, 123.69, 122.75. 119Sn NMR (223.7 MHz, C6D6,
25 ◦C) d -79.3 (s, br).
119Sn Mo¨ssbauer effect spectroscopy
Method B. To SnCl2 (0.379 g, 2.0 mmol) suspended in 20 mL
of Et2O at 0 ◦C was added dropwise a solution of LiNHDipp
(0.732 g, 4.0 mmol) in 30 mL of Et2O. The reaction was stirred for
10 h at room temperature, followed by removal of the solvent under
reduced pressure. The resulting dark yellow solid was extracted
with hexanes (ca. 30 mL), filtered, and concentrated under reduced
pressure. Crystals of 1 suitable for X-ray diffraction appeared after
storage for several days at -13 ◦C. Yield 0.425 g (45%).
Mo¨ssbauer experiments were carried out in transmission mode
using an ~5 mCi source of 119Sn in a BaSnO3 matrix.17 Spectro-
scopic calibration was accomplished by using a 20 mg cm-2 a-
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¨ssbauer
data were obtained in both warming and cooling modes, and
no evidence of hysteresis was observed. The experimental details
of the Mo¨ssbauer data acquisition and reduction have been
described previously17 Temperature monitoring was effected using
the DASWIN software also described previously.17a
Preparation of Sn2{N(H)Dipp}3Cl (2)
LiN(H)Dipp (0.732 g, 4 mmol) was dissolved in 50 mL of cold
diethyl ether (0 ◦C) and added to an Et2O suspension of SnCl2
(0.758 g, 4 mmol), also at 0 ◦C. After stirring at ca. 25 ◦C for
20 h, the solvent was removed under reduced pressure and the
residue was extracted with hexanes (30 mL). The solution was
filtered from its light yellow precipitate using a filter cannula.
5906 | Dalton Trans., 2008, 5905–5910
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X-Ray crystallography
The transamination reaction (eqn (1)) proceeds smoothly in
hexanes below room temperature to afford 1 as pale yellow crystals
in good (63%) yield. The salt elimination approach (eqn (2))
proceeds in diethyl ether between 0 ◦C and room temperature to
produce 1 in 45% yield (not optimized). Crystals of 1 are very
air and moisture sensitive pale yellow plates which melt with
decomposition at 106 ◦C. Freshly prepared crystalline samples
afford relatively “clean” 1H NMR and 119Sn NMR spectra initially,
although some transformation into 3 is immediately apparent even
at ambient temperature in C6D6 as evidenced by the development
of a septet signal at 3.46 ppm (3) and at 2.63 ppm (DippNH2
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).18 Data for all compounds were obtained on
a Bruker SMART 1000 instrument using Mo Kx radiation (l =
˚
0.71073 A) in conjunction with a CCD detector. The collected
reflections were corrected for Lorentz and polarization effects
by using Blessing’s method as incorporated into the program
SADABS.19,20 The structures were solved by direct methods and
refined with the SHELXTL v.6.1 software package.21 Refinement
was by full-matrix least-squares procedures with all carbon-bound
hydrogen atoms included in calculated positions and treated as
riding atoms. N-bound hydrogens were located directly from the
Fourier difference map. A summary of crystallographic and data
collection parameters is given in Table 1.
1
co-product). The H NMR spectrum of 1 displays two sets of
signals in a 1 : 1 ratio for the bridging and terminal –N(H)Dipp
groups, whereas the 119Sn NMR spectrum displays just one signal
at d = -79.3, which is well upfield of the values reported for
[n = 3 (d = 148) or 4 (d = 165)],13 or
for the three coordinate tin in
(d = 158),22 and is far upfield of the two three-coordinate tins in
[Sn(NDipp)2{Sn(m-NMe2)}2] (d = 1000.8, 706.8).23
Results and discussion
Crystals of 3 could be readily isolated from toluene solutions
of 1 upon standing for 4 days at ambient temperature. Moreover
Synthesis
1
the conversion of 1 into 3 was immediately apparent from H
NMR spectroscopy. A sample of 1 in C6D6 in a sealed NMR
The bis(amido) tin(II) dimer 1 was synthesized by two different
methods in accordance with eqn (1) and (2).
◦
1
tube afforded several H signals at 25 C that corresponded not
only to the bridging and terminal DippN(H)– moieties in 1 but
also to small amounts of 3 as well as free DippNH2 evolved
during conversion. There were also additional DippN(H)– methyl,
methine, and amino-H signals likely resulting from a monomer–
dimer equilibrium involving 1. The 119Sn NMR spectrum of 1
showed the characteristic singlet at -79.3 ppm along with a signal
at 315.0 ppm due to 3. Additional minor signals were also observed
at d = -74.2, 155.1, and 301.2 ppm. After the NMR tube was
(1)
(2)
Table 1 Selected crystallographic data for compounds 1–3
Compound
1·hexane
2
3
Formula
FW
C54H86N4Sn2
1028.65
Yellow plate
Monoclinic
P21/c
11.5303(8)
26.4308(17)
17.8070(11)
90
98.771(2)
90
5363.3(6)
4
0.40 ¥ 0.40 ¥ 0.20
90(2)
1.274
0.968
1.39 to 27.50
61636
12312
0.0716
8693
C36H54ClN3Sn2
801.65
Yellow block
Monoclinic
P21/n
12.9777(5)
14.6393(5)
19.3383(7)
90
98.017(2)
90
3638.1(2)
4
0.50 ¥ 0.30 ¥ 0.30
90(2)
1.464
1.474
1.75 to 27.50
48673
8365
C48H68N4Sn4
1175.82
Yellow block
Triclinic
¯
P1
12.1960(7)
19.1210(11)
21.4771(13)
84.2250(10)
76.2090(10)
86.7710(10)
4836.7(5)
4
0.40 ¥ 0.25 ¥ 0.25
90(2)
1.615
2.077
0.98 to 27.53
64111
22200
0.0927
17000
22000/0/1009
0.0344
0.0893
Color, habit
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
V/A
Z
3
˚
Crystal dimension/mm
T/K
d
calc/g cm-3
Abs. coefficient m/mm-1
q Range/◦
Reflections collected
Unique reflections
R(int)
0.0200
7728
8365/0/403
0.0177
0.0464
Obs. reflections [I > 2s(I)]
Data/restraints/parameters
R1, observed reflections
wR2, all
12312/0/576
0.0425
0.1118
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gently heated for ca. 30 s, the 1H spectrum displayed only signals
corresponding to DippNH2 and 3 in a roughly 1 : 1 ratio. The
same sample, recorded at 70 ◦C, also gave a much cleaner 119Sn
spectrum which consisted of a major singlet at 321.8 ppm and
another minor signal at 315.1 ppm, the former corresponding to
3, which was shifted downfield due to the well-known temperature
dependence of 119Sn chemical shift.24
The sesqui complex 2 was synthesized by the reaction of a
1 : 1 ratio of SnCl2 and in situ generated LiNHDipp in diethyl
ether solution. The intention was to synthesize the complex
Sn{N(H)Dipp}(Cl) (as in eqn (3)) which was expected to be
dimeric with either chloride or amido bridging.
(3)
Instead the unique species Sn{N(H)Dipp}3Cl (2) was obtained
in low yield as yellow crystals. The mechanism whereby 2 is
formed is currently unknown. One possibility is that a trimeric
species [Sn(Cl){NHDipp}]3 is formed in solution. Trimers of
this type are precedented by the structurally characterized [Sn(m-
Fig. 1 Molecular structure of 1, C-bound hydrogen atoms omitted for
clarity.
25
Cl){N(C6H2Me3-2,4,6)SiMe3)]3 and they may exist in equilib-
rium with complexes of type 2 as described in eqn (4).
(4)
The integrity of 2 is maintained in solution as evidenced by
1
its H, 13C and 119Sn NMR spectra. The 119Sn NMR spectrum
of 2 displays two signals of equal intensity at d = -29.5 and
-49.7, upfield of that of the trimeric {Sn(m-Cl)[N(Mes)SiMe3]}3
(d = 67),25 and of {Sn(m-Cl)NR2}2
(d =
240),26 but well downfield of that reported for {Sn(m-Cl)NMe2}2
(d = -333).27
The previously reported cubane species 3, synthesized by the
1 : 1 reaction of Sn{N(SiMe3)2}2 and H2NDipp2,15 was obtained
by a spontaneous elimination of H2NDipp from 2 under ambient
conditions to afford 3, the overall reaction being described by
eqn (5).
(5)
A detailed mechanism for this reaction is not known although
the structures of intermediate stages of the process probably
resemble those described for the transamination reactions of
{Sn(NMe2)2}2 with H2NDipp. A119Sn NMR spectrum of 3 dis-
played a signal d = 315 which is upfield of those previously
observed for (SnNSiButMe2)4 (d = 742),28 or for (SnNEMe3)4
[E = Si (d = 782),29 Ge (d = 954),30 or Sn (d = 797)30].
Fig. 2 Molecular structure of 2, C-bound hydrogen atoms omitted for
clarity.
of the bulky Dipp substituents. The bridging Sn–N distances
˚
˚
average 2.28(2) A and span the range 2.248(3)–2.329(3) A; the
˚
terminal Sn–N bond lengths average 2.119(2) A. These may be
˚
compared to the 2.266(5) A average bridging Sn–N distance
12
˚
in {Sn(NMe2)2}2 and the 2.256(6) and 2.397(6) A bridging
Structures
13
distances in
,
as well as the terminal
The structures of 1 and 2 are illustrated in Fig. 1 and 2.
Selected bond distances and angles are given in Table 2. In
the structure of 1, two N(H)Dipp groups bridge the tin centers
in a symmetric fashion. The Dipp groups are syn with respect
to Sn2N2 core which is folded (fold angle = 142.1◦) along the
Sn–Sn vector. The coordination at each tin is completed by a
terminally bound N(H)Dipp group. These terminal amide groups
are also disposed in a syn fashion with respect to each other,
but are anti with respect to the bridging amides. The overall
configuration of the core is presumably a result of steric effects
˚
˚
Sn–N distances of 2.067(4) A in {Sn(NMe2)2}2 and 2.083(6) A
in
, in which the terminal Sn–N bond
lengths are statistically longer than those previously observed.
The structure of 2 resembles that of 1 in its dimeric nature and
in the orientation of the bridging and terminal amido ligands. The
major difference is that one of the terminal amides is replaced by
a chloride. Oddly, this does not introduce any major changes to
the Sn–N distances within the Sn4N4 core and an almost identical
˚
average Sn–N bond length of 2.284(4) A is observed. The fold angle
5908 | Dalton Trans., 2008, 5905–5910
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◦
˚
Table 2 Important bond distances (A) and angles ( ) for compounds 1
and 2
{(DippNH)2Sn}2 (1)
Sn(1)–N(1)
Sn(1)–N(2)
Sn(1)–N(3)
Sn(2)–N(1)
Sn(2)–N(2)
Sn(2)–N(4)
Sn(1)–Sn(2)
N(1)–C(10)
N(2)–C(20)
N(3)–C(30)
N(4)–C(40)
2.304(3)
2.248(3)
2.117(3)
2.233(3)
2.329(3)
2.120(4)
3.484
1.441(5)
1.433(5)
1.408(4)
1.397(5)
N(1)–Sn(1)–N(3)
N(2)–Sn(1)–N(3)
N(1)–Sn(1)–N(2)
N(1)–Sn(2)–N(4)
N(2)–Sn(2)–N(4)
N(1)–Sn(2)–N(2)
Sn(1)–N(3)–C(30)
Sn(1)–N(2)–C(20)
Sn(1)–N(1)–C(10)
Sn(2)–N(1)–C(10)
Sn(2)–N(2)–C(20)
Sn(2)–N(4)–C(40)
87.58(12)
93.37(12)
75.24(11)
89.86(13)
85.89(13)
75.02(11)
121.4(2)
118.7(2)
119.6(2)
122.9(2)
124.5(2)
137.1(3)
Sn2(DippNH)3Cl (2)
Sn(1)–N(1)
Sn(1)–N(2)
Sn(1)–N(3)
Sn(2)–N(1)
Sn(2)–N(2)
Sn(2)–Cl(1)
Sn(1)–Sn(2)
N(1)–C(10)
N(2)–C(20)
N(3)–C(30)
2.2937(13) N(1)–Sn(1)–N(2)
2.2850(13) N(1)–Sn(2)–N(2)
2.1019(14) N(1)–Sn(1)–N(3)
2.2770(13) N(2)–Sn(1)–N(3)
2.2815(12) N(1)–Sn(2)–Cl(1)
76.42(4)
76.82(5)
90.14(5)
86.37(5)
86.23(3)
Fig. 3 119Sn Mo¨ssbauer spectrum of 1 at 88.7 K. The isomer shift scale is
with respect to a BaSn03 absorber spectrum at room temperature.
0.067 0.006 and 0.66 0.01 mm s-1 respectively, strongly suggest
that the decomposition product is a form of stannic oxide. The
minority site is undecomposed 1 remaining in the sample.
The Mo¨ssbauer spectrum of 3 resembles that of 1. The associ-
ated parameters are included in Table 3. Again, the temperature-
dependence of IS is not sufficiently sensitive over the accessible
temperature range to permit an evaluation of Meff. The QS
parameter is slightly more temperature dependent and shows the
normal decrease in QS with increasing temperature, associated
with normal thermal expansion of covalent solids.
2.4901(4)
3.495
N(2)–Sn(2)–Cl(1)
Sn(1)–N(1)–C(10)
86.13(3)
129.06(10)
114.26(9)
138.61(11)
113.59(9)
129.55(10)
1.4414(18) Sn(1)–N(2)–C(20)
1.4367(19) Sn(1)–N(3)–C(30)
1.395(2)
Sn(2)–N(1)–C(10)
Sn(2)–N(2)–C(20)
along the Sn–Sn vector is 148.5◦, a little wider than that seen in 1,
and the terminal Sn(1)–N(3) distance is 2.102(1) A which is very
close to that seen in 1.
˚
The Mo¨ssbauer spectra of 2 are both qualitatively and quantita-
tively different from those of 1 and 3 and clearly show the presence
of two distinct tin sites in the structure. A representative spectrum
is shown in Fig. 4 and the hyperfine and derived parameters are
included in Table 3. Assuming equal recoil-free fractions at 89 K,
the ratio of the two sites is ~1 : 1.13 The isomer shift of both
sites 3.042 0.011 and 3.222 0.012 mm s-1 at 89 K clearly
identify the metal atom as Sn(II). As in the case of 1 and 3
there is no evidence for resonant absorbance at ~0 mm s-1 and
thus decomposition of the sample (to a Sn(IV) oxide) prior to
spectroscopic analysis can be ruled out. The assignment of which
pair of hyperfine parameters belong to the tin atom ligated to three
nitrogens, and which to the tin atom ligated to two nitrogens and
a chlorine clearly cannot be made on the basis of the presently
available data since contributions to the QS parameter come not
only from the nearest neighbor ligand atoms, but also from the
lone pair of electrons associated with each Sn(II) site. A selective
isotopic labeling procedure, using either enriched or depleted 119Sn
in one of the two molecular sites would be required to settle this
point definitively.
Crystals of 3 proved to have identical crystallographic param-
eters to those reported earlier15 for (SnNDipp)4 which has a
distorted cubane structure with an average Sn–N distance near
◦
˚
2.2 A and N–Sn–N and Sn–N–Sn angles near 81 and 98 .
Mo¨ssbauer spectroscopy
Compound 1. The Mo¨ssbauer spectra of 1 consist of a single
doublet and a typical spectrum is shown in Fig 3. The hyperfine
parameters at 90 K, as well as the temperature dependence of the
recoil-free fraction, f are summarized in Table 3. The isomer shift
(IS) of 2.884 0.0 13 mm s-1 at 90 K clearly indicates the tin site
is Sn(II). The temperature dependence of the IS parameter is too
small to be statistically significant, and a meaningful value of the
effective vibrating mass (Meff) could not be obtained. Similarly,
the quadrupole splitting (QS) parameter temperature dependence
is very small, and an average value of 2.355 0.009 mm s-1 is
included in Table 3. A 5 minute exposure of 1 to ambient laboratory
conditions is sufficient to turn the virgin pale yellow compound
to a white decomposition product. Mo¨ssbauer analysis of this
product at room temperature showed the presence of Sn(IV) to
Sn(II) in the ratio of ~3 : 1. The IS and QS of the Sn(IV) site,
Mo¨ssbauer data for tin amides are relatively scarce. Lappert’s
monomeric amide Sn{N(SiMe3)2}2 affords an isomer shift of
Table 3 Mo¨ssbauer parameters for 1, 2 and 3
Parameter
1
2 Sn(Cl)
2 Sn{N(H)Dipp}
3
IS(90)/mm s-1
2.884(13)
2.356(13)
15.29(2)
3.222(10)
1.654(10)
19.1(10)
3.042(10)
2.414(10)
17.39(3)
2.905(13)
2.167(13)
15.49(19)
QS(90)/mm s-1
-[dlnA/dT]/K-1 ¥ 103
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The Royal Society of Chemistry 2008
Dalton Trans., 2008, 5905–5910 | 5909
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2.88 mm s-1 and a quadrupole splitting of 3.52 mm s-1, while
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, both of which have
WI, 2002.
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Acknowledgements
We are grateful to the National Science Foundation (CHE-
0641020) for financial support.
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5910 | Dalton Trans., 2008, 5905–5910
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