Flexible coordination of the carboxylate ligand in tin(II) amides and a 1,3-diaza-2,4-distannacyclobutanediyl

Article abstract of DOI:10.1039/b704588c

A series of tin(ii) amido complexes possessing m-terphenyl carboxylate ligands have been prepared. These complexes, namely [(Me₃Si) ₂NSn(-O₂CC₆H₂Ph₃)] ₂, [(Me₃Si)₂NSn(-O₂CC ₆H₃Mes₂)]₂, and [(Me ₃Si)₂NSn(-O₂CC₆H₂Mes ₂Me)]₂ [Mes = 2,4,6-trimethylphenyl], are the first structurally characterized examples of tin(ii) carboxylate complexes exhibiting discrete Sn₂O₄C₂ heterocyclic cores. Initial reactivity studies led to the isolation of a 1,3-diaza-2,4- distannacyclobutanediyl, [(Mes₂C₆H₃CO ₂)Sn(-NSiMe₃)]₂. This molecule possesses a Sn₂N₂ heterocyclic core and it was crystallised as both the CH₂Cl₂ and Et₂O solvates. Although the tin atoms in this molecule have a formal oxidation state of 3+, preliminary computational studies on this molecule suggest that it is best described as a ground state singlet. Finally, the X-ray crystal structure of (CH ₂Cl)(Cl)Sn[N(SiMe₃)₂]₂, the product of oxidative addition of CH₂Cl₂ to Sn[N(SiMe ₃)₂]₂, is also presented herein. The Royal Society of Chemistry.

Full text of DOI:10.1039/b704588c

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Flexible coordination of the carboxylate ligand in tin(II) amides and a
1,3-diaza-2,4-distannacyclobutanediyl†
Diane A. Dickie,a Peter T. K. Lee,b Ojisamola A. Labeodan,a Gabriele Schatte,c Noham Weinberg,a,d
Andrew R. Lewis,a Guy M. Bernard,e Roderick E. Wasylishene and Jason A. C. Clyburne*b
Received 27th March 2007, Accepted 8th May 2007
First published as an Advance Article on the web 23rd May 2007
DOI: 10.1039/b704588c
A series of tin(II) amido complexes possessing m-terphenyl carboxylate ligands have been prepared.
These complexes, namely [(Me3Si)2NSn(l-O2CC6H2Ph3)]2, [(Me3Si)2NSn(l-O2CC6H3Mes2)]2, and
[(Me3Si)2NSn(l-O2CC6H2Mes2Me)]2 [Mes = 2,4,6-trimethylphenyl], are the first structurally
characterized examples of tin(II) carboxylate complexes exhibiting discrete Sn2O4C2 heterocyclic cores.
Initial reactivity studies led to the isolation of a 1,3-diaza-2,4-distannacyclobutanediyl,
[(Mes2C6H3CO2)Sn(l-NSiMe3)]2. This molecule possesses a Sn2N2 heterocyclic core and it was
crystallised as both the CH2Cl2 and Et2O solvates. Although the tin atoms in this molecule have a
formal oxidation state of 3+, preliminary computational studies on this molecule suggest that it is best
described as a ground state singlet. Finally, the X-ray crystal structure of (CH2Cl)(Cl)Sn[N(SiMe3)2]2,
the product of oxidative addition of CH2Cl2 to Sn[N(SiMe3)2]2, is also presented herein.
Stannylenes (:SnR2) were first isolated8 more than a decade
Introduction
before the analogous carbenes9 (:CR2) and several stannylene
The variety of bonding modes available to carboxylate anions
derivatives are commercially available. We note that some aspects
(Fig. 1), coupled with their ability to readily shift from one
of stannylene chemistry have been less explored than carbene
mode to another, makes them one of the most versatile known
ligand classes for main group and transition metal complexes.1
chemistry, likely due at least in part to the significant commercial
interest in N-heterocyclic carbene chemistry vis-a´-vis catalyst
Another important ligand class is the m-terphenyl substituent,2
which can enforce low coordination numbers at a metal centre
thus often leading to unprecedented bonding3 and/or reactivity4
at that centre. We5,6 and others7 have previously shown that when
the carboxylate functional group is placed within the m-terphenyl
pocket, unusual metal coordination motifs are readily accessible.
Herein we present the preparation of a series of tin(II) carboxylate
complexes and the results of initial reactivity studies of these
complexes.
applications.10 Our previous studies of the reactions of carbenes
with simple reagents11 prompted us to explore similar reactions
with stannylenes. This investigation was further motivated by
observation that although tin(II) carboxylates have long been
studied,12 and are implicated in a variety of catalytic processes,13
commercial resins,14 and even food safety,15 structural data are
relatively rare and composed mainly, though not exclusively,15,16 of
oxalate17 and acetate12b,18 complexes. This is in contrast to Sn(IV)
carboxylate complexes, which are extremely well characterized.19
Results and discussion
Treatment of m-terphenyl carboxylic acids 1–3 with
bis[bis(trimethylsilyl)amido] tin(II), Sn[N(SiMe3)2]2, in anhydrous
CH2Cl2 gives the Sn(II) carboxylate complexes 4–6 with
concomitant loss of hexamethyldisilazane (Scheme 1). 1H and 13
C
NMR studies on the isolated products are consistent with the
presence of two trimethylsilyl groups per m-terphenyl substituent.
Compounds 4–6 crystallise readily, and X-ray crystallographic
studies reveal that the bulky carboxylate anion acts as a bridging
bidentate ligand connecting two tricoordinate tin(II) atoms. This
is in contrast to isoelectronic amidinate complexes, which exhibit
a chelating rather than a bridging coordination mode.20 The tin
atoms in 4–6 exhibit the trigonal pyramidal geometry typical of
divalent tin,21 causing the resultant Sn2O4C2 heterocyclic eight-
membered cores to adopt a chair conformation (Fig. 2). The
core structures of compounds 4–6 have not, to the best of
our knowledge, been previously reported for tin(II) carboxy-
late complexes. The core structure is, however, reminiscent of
Fig. 1 Common bonding modes for carboxylate ligands.
aDepartment of Chemistry, Simon Fraser University, Burnaby, BC, Canada
bDepartment of Chemistry, Saint Mary’s University, Halifax, NS, B3H 3C3,
Canada. E-mail: jason.clyburne@smu.ca; Tel: +1 902 420 5827
cSaskatchewan Structural Sciences Centre, University of Saskatchewan,
Saskatoon, SK, Canada
dDepartment of Chemistry, University College of the Fraser Valley, Abbots-
ford, BC, Canada
eDepartment of Chemistry, University of Alberta, Edmonton, AB, Canada
† CCDC reference numbers 618666–618671. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b704588c
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Scheme 1 Synthesis of tin(II) carboxylate complexes 4–6.
Fig. 2 Structure of [(Me3Si)2NSn(l-O2CC6H3Mes2)]2 5 (left) and its heterocyclic core, which is common to 4–6 (right). Thermal ellipsoids are shown at
50% probability. Hydrogen atoms and CH2Cl2 solvent of crystallisation have been removed for clarity.
2
bis(trimethylsilyl)amido tin(II) triflate [{Sn(NR2)(l-g -OTf)2}]22
in which the central core is a tin-containing eight-membered
heterocyclic ring. The triflate derivative, in the solid-state, is a
coordination polymer with extensive Sn · · · O interactions between
adjacent eight-membered rings. In contrast, compounds 4–6 are
discrete entities, and significant intermolecular interactions are
likely precluded due to the large size of the m-terphenyl carboxylate
ligands.
Although the somewhat greater steric demands of the 2,4,6-
trimethylphenyl substituent in 5 and 6 compared to the phenyl
substituent in 4 do not appear to have much impact on the
solid state structural parameters of the Sn2O4C2 heterocyclic core
(Table 1), this does not hold true for their behaviour in solution.
While routine 1H and 13C NMR spectroscopic studies revealed the
Fig. 3 Structure of (CH2Cl)(Cl)Sn[N(SiMe3)2]2 7. Thermal ellipsoids
expected signals for complexes 4–6, the 119Sn NMR studies were
are shown at 50% probability and hydrogen atoms have been re-
not as straightforward. The two 2,4,6-trimethylphenyl-substituted
˚
moved for clarity. Selected bond lengths (A): Sn(1)–N(1) 2.0318(16);
m-terphenyl derivatives each exhibited a single peak in the 119Sn
NMR spectrum, at d 74 or 76 ppm for 5 and 6 respectively, but
under identical conditions, no signal was observed in the 119Sn
NMR spectrum of 4. Variable temperature studies to temperatures
as low as −85 ◦C were inconclusive.
Sn(1)–N(2) 2.0270(17); Sn(1)–C(1) 2.160(2); Sn(1)–Cl(1) 2.3689(5);
C(1)–Cl(2) 1.782(2). Selected bond angles (◦): N(1)–Sn(1)–N(2) 116.50(7);
N(1)–Sn(1)–Cl(1) 108.49(5); N(1)–Sn(1)–C(1) 109.66(8); N(2)–Sn(1)–C(1)
117.81(8); N(2)–Sn(1)–Cl(1) 102.34(5); Cl(1)–Sn(1)–C(1) 99.94(6).
We note here that care must be taken during the preparation of
4–6 to ensure that a large excess of Sn[N(SiMe3)2]2 is not used
in the reaction. In the absence of other reagents such as the
carboxylic acids 1–3, Sn[N(SiMe3)2]2 will react with the CH2Cl2
solvent to give the insertion product (CH2Cl)(Cl)Sn[N(SiMe3)2]2
7. In a single experiment where an excess of Sn[N(SiMe3)2]2 was
used, 7 was identified spectroscopically23 in the reaction mixture,
and its presence subsequently confirmed by single-crystal X-ray
diffraction studies (Fig. 3). On the other hand, once 4–6 were
formed, they did not appear to react with solvent, and in fact a
molecule of CH2Cl2 was incorporated as a solvent of crystallisation
in each case.
To probe the reactivity of the m-terphenyl tin(II) carboxylate
complexes, we attempted to remove an amido ligand from
5 through a protonolysis reaction. Studies have shown that
B(C6F5)3·H2O is an excellent Brønsted acid, with a pKa of 8.4
in acetonitrile.24 This strong acid exhibits novel reactivity in main
group chemistry, and was successfully used in the preparation
of the first monoalumoxane complex.25 We expected similar
reactivity of B(C6F5)3·H2O with 5. However, the reaction afforded
compound 8. On isolation, the crystalline product 8 exhibited
1
a H NMR spectrum consisting of a singlet with an integration
corresponding to nine hydrogens at d 0.11 ppm and the anticipated
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◦
˚
Table 1 Selected structural data (bond lengths/A, bond angles/ ) for tin(II) carboxylate complexes 4–6
4 (Ar = R = Ph)
Sn(1)–O(1)
5 (Ar = 2,4,6-trimethylphenyl, R = H)
6 (Ar = 2,4,6-trimethylphenyl, R = Me)
2.243(2)
2.199(2)
1.262(3)
1.259(4)
2.072(2)
1.734(3)
1.727(3)
4.397
Sn(1)–O(11)
Sn(1)–O(51)
Sn(2)–O(12)
Sn(2)–O(52)
O(11)–C(17)
O(12)–C(17)
O(51)–C(57)
O(52)–C(57)
Sn(1)–N(41)
Sn(2)–N(81)
Si(41)–N(41)
Si(42)–N(41)
Si(81)–N(81)
Si(82)–N(81)
Sn(1) · · · Sn(2)
2.2175(16)
2.2194(18)
2.2222(17)
2.2407(16)
1.267(3)
1.264(3)
1.258(3)
1.263(3)
2.090(2)
2.087(2)
1.738(2)
1.733(2)
1.735(2)
1.742(2)
4.411
Sn(1)–O(1)
2.240(4)
Sn(1)–O(2)
Sn(1)–O(2)
2.208(4)
1.257(7)
1.259(7)
2.093(5)
1.729(5)
1.737(5)
4.557
O(1)–C(1)
O(1)–C(1)
O(2)–C(1)
O(2)–C(1)
Sn(1)–N(1)
Sn(1)–N(1)
Si(1)–N(1)
Si(1)–N(1)
Si(2)–N(2)
Si(2)–N(2)
Sn(1) · · · Sn(1)
O(1)–Sn(1)–O(2)
N(1)–Sn(1)–O(1)
N(1)–Sn(1)–O(2)
Sn(1) · · · Sn(1)
O(1)–Sn(1)–O(2)
N(1)–Sn(1)–O(1)
N(1)–Sn(1)–O(2)
96.63(8)
90.97(9)
90.08(9)
O(11)–Sn(1)–O(51)
O(12)–Sn(2)–O(52)
N(41)–Sn(1)–O(11)
N(41)–Sn(1)–O(51)
N(81)–Sn(2)–O(12)
N(81)–Sn(2)–O(52)
Si(41)–N(41)–Si(42)
Si(41)–N(41)–Sn(1)
Si(42)–N(41)–Sn(1)
Si(81)–N(81)–Si(82)
Si(81)–N(81)–Sn(2)
Si(82)–N(81)–Sn(2)
100.44(7)
99.91(6)
90.63(7)
89.79(8)
91.90(7)
98.15(15)
91.53(17)
89.53(17)
90.42(7)
Si(1)–N(1)–Si(2)
Si(1)–N(1)–Sn(1)
Si(2)–N(1)–Sn(1)
122.61(15)
113.18(13)
123.93(14)
121.48(12)
123.94(11)
114.58(12)
120.14(12)
115.14(12)
124.72(11)
Si(1)–N(1)–Si(2)
Si(1)–N(1)–Sn(1)
Si(2)–N(1)–Sn(1)
120.3(3)
116.1(3)
123.3(3)
peaks for an m-terphenyl ligand at d 2.01 and 2.27 ppm. The upfield
shift was assigned to a single trimethylsilyl group.
Curiously, as with compound 4, no signal was observed in the
solution state 119Sn NMR studies on 8, nor were variable temper-
ature experiments conclusive. Solid state 119Sn NMR studies were
performed on samples of 8 and gave broad isotropic peaks at d
−108 and −121 ppm. Unfortunately these studies were hampered
by the extreme air and moisture sensitivity of the compound.
In order to obtain the atom connectivity of the molecule we
performed crystallographic studies and found that the isolated
compound was a 1,3-diaza-2,4-distannacyclobutanediyl 8·CH2Cl2
(Fig. 4), formed through an unknown mechanism. This molecule
is structurally similar to that described in a recent report by
Lappert, specifically {[ClSn(l-NSiMe3)]2}∞ 9.26 Lappert’s group
also reported a rational, although nonetheless curious, synthetic
procedure for the preparation of 9 in high yield, which we followed
for subsequent preparations of 8. Specifically, compound 5 was
treated with two equivalents of AgOCN in Et2O, to give 8
Fig. 4 Structure of [(Mes2C6H3CO2)Sn(l-NSiMe3)]2 8·CH2Cl2. Thermal
ellipsoids are shown at 50% probability. Hydrogen atoms and solvent of
crystallisation have been removed for clarity.
= =
(Scheme 2). The carbodiimide by-product Me3SiN C NSiMe3
was identified in the IR spectrum of the crude reaction mixture,
Scheme 2 Rational synthesis of [(Mes2C6H3CO2)Sn(l-NSiMe3)]2 8.
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and metallic silver precipitated. In the course of these experiments
we also crystallographically characterised 8·Et2O. Elemental anal-
yses on the bulk materials are in accord with the results obtained
by crystallography.
The structural parameters of the two solvates (Table 2) are
comparable. Neither 8·CH2Cl2 nor 8·Et2O showed any significant
contact between the tin atoms and the solvent of crystallisation.
The carboxylate coordination was found to be terminal monoden-
8, the bulky m-terphenyl substituent prevents both inter- and
intramolecular interactions at tin.
Only one other heavy (i.e., Si, Ge, Sn, Pb) Group 14/15 hetero-
cycle analogous to 8 and 9 has been reported to date, namely [(2,6-
Dipp2C6H3)C6H2Ge(l-NSiMe3)]2 (Dipp = 2,6-diisopropylphenyl)
10.27 Molecule 10, in our opinion, is dramatically different from
either 8 or 9, and this is likely because of the following two reasons.
First, 10 possesses a strong and somewhat inflexible Ge–C bond
whereas the tin derivatives have flexible carboxylate and chloride
counterions, i.e., good leaving groups. Secondly, both the size and
electronegativity of tin are significantly different from those of
the germanium analogue. For these reasons, marked differences in
bonding should be anticipated between that observed for 8 and 9
and that observed for 10.
Using a simple fragment analysis, compound 8 can be viewed as
a dimer of the tin iminyl radical, [(Mes2C6H3CO2)Sn(NSiMe3)]•,
for which persistent examples of dicoordinate tin aminal radicals
are, to our knowledge, unknown. This model offers several
attractive features, and also, perhaps, provides insight into new
synthetic directions. First of all, the tin–nitrogen bonds in both
complexes 8 and 9 are quite long, and indeed these are longer than
in the starting materials. This again is quite surprising since one
would anticipate that an imide–tin bond would be shorter than
a tin–amide bond. Secondly, this model avoids the invocation of
the peculiar oxidation state three for tin. Indeed, the Mo¨ssbauer
spectra obtained by Lappert clearly suggests that there is nothing
particularly unusual for the tin center in 9.26 In our opinion, and
indeed this is reflected by an assessment of the products formed
in the reaction, the nitrogen fragment, which enters the ring to
help satisfy its valency, is the primary reaction centre. In the initial
report of 9, the authors concluded that the Sn2N2 ring behaved
like a typical diamagnetic material with a 6-p electron 4-centre
core (pseudo-aromatic).26 This is in contrast to the germanium
analogue 1027,28 and other isoelectronic heterocycles including
P2C2,29 B2P2,30 and E2N2 (E = S, Se, Te)31 which have been shown
through extensive experimental and theoretical studies to have
varying degrees of biradical character.32 As we mentioned above,
we are reluctant to directly compare our results, and those for
compound 9, with the germanium analogue 10. We note that the
experimental data for 8 (sharp NMR resonances, the lack of an
unusual absorption in the visible spectrum, and absence of an EPR
signal) is more consistent with the ground state singlet 9 than the
apparent biradical 10.
˚
tate, with an Sn–O bond length of 2.149(3) A in 8·CH2Cl2 or
˚
2.169(2) A in 8·Et2O. The somewhat longer formally non-bonded
˚
Sn · · · O distances of 2.628(3) and 2.573(2) A, respectively, are close
enough to suggest possible intramolecular carboxylate shifting in
solution. The nitrogen atoms in 8 are almost trigonal planar, with
ꢀ
N = 355.73◦ (8·CH2Cl2) or 354.11◦ (8·Et2O). As in 4–6, the
tin atoms show trigonal pyramidal geometry, however the N–
Sn–N and N–Sn–O angles in 8, which range from 75.94(9) to
93.17(8)◦, are much more acute than they are in 5. The Sn · · · Sn
˚
˚
separation of 3.3896(5) A in 8·CH2Cl2 or 3.404(4) A in 8·Et2O,
suggests no transannular bonding. The N–Si bond lengths of
˚
˚
1.677(3) A (8·CH2Cl2) or 1.674(2) A (8·Et2O) are significantly
shorter than those in 4–6, while the N–Sn bond lengths are slightly
longer, ranging from 2.116(3) to 2.198(2) A. These structural
˚
parameters are similar to those in the related compound {[ClSn(l-
NSiMe3)]2}∞ 9.26
Extended intermolecular Sn · · · Cl coordination in 9 gives rise
to infinite one-dimensional chains in the solid state, however
these interactions are disrupted in solution, leading to a dramatic
solvent dependence in the reported solution state 119Sn NMR data,
with chemical shifts ranging from d −83 [PhMe/C6D6] to −285
[(Me2N)3PO/PhMe/C6D6] ppm.26 These values are all significanly
upfield from the isotropic solid state chemical shift of 9 at d
−17 ppm. This effect is exacerbated by the ability of the more
polar solvents to act as donors to the electron-deficient tin centre.
The isotropic solid state chemical shifts of 8 (−108 and −121 ppm)
are most similar to the solution state chemical shift of 9 in CD2Cl2.
This is not surprising because in both cases the predominant
species should be disctrete Sn2N2 heterocycles. In the case of
{[ClSn(l-NSiMe3)]2}∞ 9, the CD2Cl2 breaks the intermolecular
Sn–Cl bonds but does not form tin–solvent interactions. For
◦
˚
Table 2 Selected structural data (bond lengths/A, bond angles/ ) for
8·CH2Cl2 and 8·Et2Oa
8·CH2Cl2
8·Et2O
In order to shed light on the bonding observed in the un-
usual compound 8, we performed DFT calculations (B3LYP/
LANL2DZ) using Gaussian 9833 on a model compound of 8,
namely [HCO2Sn(l-NSiMe3)]2. The results of this study, although
not comprehensive, have provided some insight into the bonding
(Fig. 5) and broadly parallel those obtained for 9.26 Both the singlet
and the triplet states of [HCO2Sn(l-NSiMe3)]2 were optimized and
the singlet was found to be by 54 kJ mol−1 more stable than the
triplet, which is consistent with the value of 76 kJ mol−1 reported
by Lappert for compound 9.26 The calculated singlet–triplet gap
is small and alters its sign with the deformation of molecular
structure: from 68 kJ mol−1 for the singlet-optimized geometry to
−10 kJ mol−1 for the triplet-optimized geometry.
Sn(1)–N(1)
Sn(1)–N(1)*
Sn(1)–O(1)
Sn(1) · · · O(2)
N(1)–Si(1)
O(1)–C(7)
O(2)–C(7)
2.194(3)
2.116(3)
2.149(3)
2.628(3)
1.677(3)
1.286(5)
1.231(5)
3.3896(5)
Sn(1)–N(1)
Sn(1)–N(1)*
Sn(1)–O(1)
Sn(1) · · · O(2)
N(1)–Si(1)
O(1)–C(7)
O(2)–C(7)
2.198(2)
2.119(2)
2.169(2)
2.573(2)
1.674(2)
1.286(4)
1.234(4)
3.404(4)
Sn(1) · · · Sn(1)*
Sn(1) · · · Sn(1)*
N(1)–Sn(1)–N(1)*
O(1)–Sn(1)–N(1)*
O(1)–Sn(1)–N(1)
Sn(1)–N(1)–Sn(1)*
Sn(1)–N(1)–Si(1)
Sn(1)*–N(1)–Si(1)
76.28(11)
92.72(11)
81.98(10)
103.72(11)
123.99(15)
128.02(16)
N(1)–Sn(1)–N(1)*
O(1)–Sn(1)–N(1)*
O(1)–Sn(1)–N(1)
Sn(1)–N(1)–Sn(1)*
Sn(1)–N(1)–Si(1)
Sn(1)*–N(1)–Si(1)
75.94(9)
93.17(8)
81.77(8)
104.06(9)
123.05(12)
127.00(12)
It is tempting to suggest that a standard four-centred six-p
electron system is at play in 8, but this clearly oversimplifies
the situation given the well-recognized poor 5p–p 2p–p overlap
a Symmetry transformations used to generate equivalent atoms:
* −x,−y,−z + 1.
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Fig. 5 Frontier molecular orbitals of [HCO2Sn(l-NSiMe3)]2. calculated for its crystallographic geometry.
that would be required between the tin and nitrogen atoms.34
The planarity of the Sn2N2 core could be taken as evidence for
p-electron delocalisation within the ring, but the low inversion
barrier for nitrogen as well as the steric strain imparted by the bulky
ligands can easily enforce planarity. The presence of a silyl group
on the nitrogen further complicates matters, as this substituent is
known to engage in negative hyperconjugation with four-centre six
p-electron systems (p*–r* overlap), thereby lowering the energy of
the LUMO.35 This effect is likely to be at least partially responsible
for the remarkably short N–Si bond observed in both 8 and 9. We
note that the HOMO is located primarily on the nitrogen atoms,
and this is reminiscent of the bonding structure suggested for
the Nieke-type biradicals, where the HOMO is primarily based
on the most electronegative element, specifically carbon versus
phosphorus. In our case, the HOMO is primarily based on the
two nitrogen sites, that is to say the HOMO is based primarily
on the most electronegative element within the ring. In light of
these results, further theoretical and experimental work is required
before a definitive statement can be made about the nature of the
bonding in compound 8, and indeed in many other main group
biradicaloid complexes as well.32
Solution-state NMR spectra were recorded in 5 mm tubes at Simon
Fraser University on Bruker AMX 400 or 600 MHz spectrometers
or Varian AS 400 or 500 MHz spectrometers, or at Dalhousie
University by Dr Mike Lumsden on a Bruker Avance 500 MHz
spectrometer. Chemical shifts are reported in parts per million
1
(ppm) downfield from SiMe4 (1H and 13C), or SnMe4 (119Sn). H
and 13C spectra are calibrated to the residual signal of the solvent.
119Sn spectra are calibrated to the external SnMe4 standard. Solid-
state 119Sn NMR spectra were recorded at the University of
Alberta on a Bruker Avance 500 NMR spectrometer, operating
at a frequency of 186.6 MHz for 119Sn with ramped CP/MAS
(cross-polarization, with magic angle spinning) and TPPM (two-
pulse phase modulated) decoupling. The MAS frequencies were 6
and 11 kHz, the contact time was 7 ms, the recycle delay was 3 s
and up to 29168 transients were co-added.
Infrared spectra were obtained using a Bomem MB spectrom-
eter with the % transmittance values reported in cm−1. Melting
points were measured using a Mel-Temp apparatus and are
uncorrected. Elemental analyses were obtained at Simon Fraser
University by Mr M. K. Yang on a Carlo Erba Model 1106 CHN
analyzer. Anhydrous solvents were obtained from an MBraun
Solvent Purification system, or were purchased from Aldrich and
used without further purification. Sn[N(SiMe3)2]2 was purchased
from Strem and used as received. All other reagents and solvents
were purchased from commercial sources and used without further
purification, except deuterated solvents for NMR experiments,
which were dried over P2O5 and distilled prior to use.
Conclusions
In conclusion, we have presented a series of tin complexes that
illustrate the versatility of the m-terphenyl carboxylate ligand in
low-valent main group chemistry. In one case, the carboxylate acts
as a bridging bidentate ligand and in the other case is an extra-
annular ligand stabilising an unusual tin–nitrogen heterocycle.
The complexes 4–6 exhibit a chair-like Sn2O4C2 core which is
a previously unknown structural motif for tin(II) compounds.
Loss of [SiMe3] and rearrangement of the carboxylate-bridged
eight-membered dimer gives an amido-bridged four-membered
heterocyclic molecule 8 possessing a formally trivalent tin centre.
The experimental and calculated properties of this Sn2N2 com-
pound agree with those reported for {[ClSn(l-NSiMe3)]2}∞ 926
and also suggest that AgOCN oxidation is a general route to such
molecules. By incorporating a m-terphenyl ligand, we were able
to isolate the Sn2N2 core free from inter-molecular interactions, a
feature that should be very useful for future studies of this unusual
system.
Synthetic procedures
2,4,6-Triphenylbenzoic acid (1). Prepared according to a varia-
tion of literature procedures.36 Under an inert atmosphere, n-BuLi
(20 mL, 1.6 M in hexanes) was added dropwise to a suspension of
2,4,6-triphenylbromobenzene (10.0 g, 26.0 mmol) in ca. 100 mL
anhydrous Et2O. The mixture was allowed to stir for 3.5 h and then
CO2 was bubbled through the solution for 1.5 h. The reaction was
quenched with H2O and dilute (ca. 1 M) HCl, then extracted with
Et2O. The combined organic fractions were dried over MgSO4
and the solvent was removed under vacuum to give a yellow
powder that was recrystallized from CH2Cl2. Slow evaporation
of the CH2Cl2 solution resulted in a white powder. Yield: 6.07 g
(67%), mp = 259–260 ◦C (lit.,36 253–255 ◦C). 1H NMR (CD2Cl2,
499.768 MHz) d 7.70 (d, J = 7 Hz, 2H), 7.66 (s, 2H), 7.40–7.51
(m, 13H). IR (Nujol mull) m 1696 (vs), 1597 (m), 1575 (w), 1561
(w), 1493 (m), 1445 (m), 1399 (w), 1293 (s), 1133 (w), 1076 (w),
886 (w), 777 (s), 765 (m), 699 (vs).
Experimental
General experimental
A nitrogen-atmosphere MBraun UL-99–245 dry box and standard
Schlenk techniques on a double manifold vacuum line were used
in the manipulation of air and moisture sensitive compounds.
2,6-Bis(2,4,6-trimethylphenyl)benzoic acid37 (2). Prepared ac-
cording to the same procedure as 1, beginning with 2,6-bis(2,4,
2866 | Dalton Trans., 2007, 2862–2869
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6-trimethylphenyl)iodobenzene (5.0 g, 11.4 mmol). The crude
product was recrystallized from Et2O to give a white powder. Yield:
3.17 g (78%), mp = 292–295 ◦C. 1H NMR (CDCl3, 400.136 MHz)
d 7.47 (t, J = 7.5 Hz, 1H), 7.08 (d, J = 7.5 Hz, 2 H), 6.97 (s, 4H),
[(Me3Si)2NSn(l-O2CC6H2Mes2Me)]2 (6). Prepared according
to the same procedure as 7, beginning with 4-methyl-2,6-bis(2,4,6-
trimethylphenyl)benzoic acid 3 (0.45 g, 1.21 mmol). Yield = 0.47 g
(60%), mp = 151–155 ◦C. 1H NMR (C6D6, 499.767 MHz) d 6.85
(s, 4H), 6.69 (s, 2H), 2.21 (s, 12H), 2.20 (s, 6H), 1.98 (s, 3H),
1
2.36 (s, 6 H), 1.99 (s, 12 H). 13C{ H} (CDCl3, 125.680 MHz) d
1
136.7, 128.5, 128.0, 31.2, 20.6 (quaternary carbons not observed).
IR (Nujol mull) m 3208 (br), 1733 (vs), 1705 (s), 1611 (m), 1577
(m), 1365 (s), 1216 (vs), 1127 (m), 844 (m), 820 (m), 799 (m), 791
(m), 688 (m). Anal. calcd for C25H26O2: C, 83.76; H, 7.31. Found:
C, 83.41; H, 7.57%.
0.00 (s, 18H). 13C{ H} NMR (CD2Cl2, 125.678 MHz) d 137.8,
137.6, 136.6, 136.4, 136.2, 129.2, 128.8, 127.9, 21.3, 21.0, 20.8, 5.2.
119Sn (C6D6, 223.867 MHz) d 75.86. IR (Nujol mull) m 1715 (w),
1612 (m), 1597 (m), 1573 (m), 1536 (vs), 1486 (m), 1246 (s), 1164
(w), 1121 (w), 1102 (w), 1033 (w), 921 (s), 865 (vs), 849 (vs), 834
(s), 790 (m), 757 (w), 709 (w), 669 (m), 608 (s). Anal. calcd for
C64H90N2O4Si4Sn2: C, 59.08; H, 6.97; N, 2.15. Found: C, 58.78; H,
6.68; N, 2.01%.
4-Methyl-2,6-bis(2,4,6-trimethylphenyl)benzoic acid (3). Pre-
pared according to the same procedure as 1, beginning
with 4-methyl-2,6-bis(2,4,6-trimethylphenyl) iodobenzene (3.00 g,
6.61 mmol). The crude product was recrystallized from hexanes to
give a white powder. Yield = 0.90 g (37%), mp = 257–259 ◦C. 1H
NMR (C6D6, 499.767 MHz) d 6.86 (s, 4H), 6.73 (s, 2H), 2.20 (s,
[(Mes2C6H3CO2)Sn(l-NSiMe3)]2 (8). Under an inert atmo-
sphere, silver cyanate (0.12 g, 0.80 mmol) was added to a solution
of [(Me3Si)2NSn(l-O2CC6H3Mes2)]2 5 (0.46 g, 0.36 mmol) in
15 mL anhydrous Et2O. The beige suspension was stirred overnight
in the dark, then filtered through glass wool and Celite and cooled
to −30 ◦C to give colourless crystals. Yield = 0.23 g (57%), mp =
170 ◦C (decomp.). 1H NMR (CD2Cl2, 500.132 MHz) d 7.48 (t, J =
7.5 Hz, 1H), 7.08 (d, J = 7.5 Hz, 2H), 6.83 (s, 4H), 2.27 (s, 6 H), 2.01
1
12H), 2.19 (s, 6H), 2.03 (s, 3H). 13C{ H} (CDCl3, 125.679 MHz) d
169.5, 140.4, 139.6, 137.0, 136.9, 136.5, 130.0, 129.2, 127.9, 21.4,
20.7, 20.6. IR (Nujol mull) m 3208 (br), 1731 (vs), 1699 (m), 1613
(m), 1597 (w), 1573 (w), 1224 (vs), 1186 (m), 1160 (m), 1110 (s),
864 (m), 844 (s), 831 (m), 768 (m). Anal. calcd for C26H28O2: C,
83.83; H, 7.58. Found: C, 83.67; H, 7.49%.
1
(s, 12 H), 0.11 (s, 9H). 13C{ H} NMR (CD2Cl2, 125.772 MHz) d
138.0, 137.2, 136.9, 136.6, 128.9, 127.7, 127.5, 20.9, 20.1, 1.6. 119Sn
(186.6 MHz, CP/MAS) d −100. IR (Nujol mull) m 2729 (w), 1715
(w), 1610 (m), 1588 (m), 1574 (s), 1556 (s), 1519 (s), 1484 (m), 1461
(vs), 1359 (vs), 1309 (w), 1259 (s), 1249 (s), 1180 (w), 1140 (m),
1101 (m), 1088 (m), 1060 (m), 1032 (m), 938 (w), 847 (vs), 810 (m),
785 (s), 771 (m), 752 (m), 737 (m), 709 (s), 667 (w). Anal. calcd for
C56H68N2O4Si2Sn2: C, 59.69; H, 6.08; N, 2.49. Found: C, 59.90; H,
5.97; N, 2.67%.
[(Me3Si)2NSn(l-O2CC6H2Ph3)]2 (4). Under an inert atmo-
sphere, a solution of Sn[N(SiMe3)2]2 (1.38 g, 3.14 mmol) in 5 mL
anhydrous CH2Cl2 was added dropwise to a stirring suspension of
2,4,6-triphenylbenzoic acid 1 (1.00 g, 2.85 mmol). After 1.5 h,
the solution was decanted from a brown precipitate that was
recrystallized from warm hexanes to give colourless crystals.
◦
1
Yield = 0.59 g (32%), mp = 242–247 C (decomp.). H NMR
(CD2Cl2, 499.767 MHz) d 7.69 (d, J = 7.6 Hz, 2H), 7.57 (s,
2H), 7.39–7.49 (m, 13H), 0.00 (s, 18H). 13C NMR{ H} (CD2Cl2,
1
Crystallographic studies
125.678 MHz) d 141.0, 139.9, 129.2, 128.9, 128.8, 127.9, 127.7,
127.2, 5.3 (quaternary carbons not observed). IR (Nujol mull) m
3083 (w), 3058 (w), 3034 (w), 1595 (s), 1583 (m), 1567 (vs), 1544
(vs), 1503 (s), 1496 (s), 1439 (s), 1346 (m), 1262 (m), 1252 (m), 1241
(s), 1194 (w), 1150 (m), 1073 (w), 1056 (w), 1030 (w), 936 (vs), 869
(vs), 849 (s), 833 (s), 790 (m), 778 (m), 759 (s), 740 (w), 698 (s), 687
(m), 670 (m). Anal. calcd for C64H90N2O4Si4Sn2: C, 59.08; H, 6.97;
N, 2.15. Found: C, 58.91; H, 5.43; N, 2.41%.
Data collection (compounds 4 and 6). Single crystals of 4
or 6 were mounted on a glass fibre. Data were collected at
the temperature indicated in Table 3 on a Nonius Kappa-CCD
diffractometer with COLLECT.38 The unit cell parameters were
calculated and refined from the full data set. Crystal cell refinement
and data reduction were carried out using DENZO.39 The data
were scaled using SCALEPACK.39 The SHELXTL-NT V6.1
suite of programs40 was used to solve the structure by direct
methods. Subsequent difference Fourier transformations allowed
the remaining atoms to be located. All of the non-hydrogen atoms
were refined with anisotropic thermal parameters. The hydrogen
atom positions were calculated geometrically and were included
as riding on their respective heavy atoms.
[(Me3Si)2NSn(l-O2CC6H3Mes2)]2 (5). Under an inert atmo-
sphere, a solution of Sn[N(SiMe3)2]2 (1.35 g, 3.07 mmol) in 5 mL
anhydrous CH2Cl2 was added dropwise to a stirring suspension of
2,6-bis(2,4,6-trimethylphenyl)benzoic acid 2 (1.00 g, 2.79 mmol)
in 15 mL anhydrous CH2Cl2. Over the course of 5.5 h, the
solution faded from bright orange to very pale yellow. It was
then concentrated under vacuum and cooled to −30 ◦C to give
Data collection (compounds 5, 7, and 8·Et2O). Single crystals
of 5, 7, or 8·Et2O were coated with oil (Paratone 8277, Exxon),
collected on top of the nylon fibre of a CryoLoopTM (diameter of
the nylon fibre: 10 microns; loop diameter 0.2–0.3 mm; Hampton
Research, USA) that had previously been attached using epoxy
resin to a metallic pin. All measurements were made on a
Nonius Kappa CCD 4-Circle Kappa FR540C diffractometer
◦
1
colourless crystals. Yield = 1.47 g (83%) mp = 168–169 C. H
NMR (C6D6, 499.767 MHz) d 6.92 (d, J = 7 Hz, 2H), 6.89 (s,
4H), 6.87 (br, 1H), 2.27 (s, 6H), 2.17 (s, 12H), 0.09 (s, 18H).
1
13C{ H} (CD2Cl2, 125.678 MHz) d 137.2, 136.8, 128.6, 128.0,
21.0, 20.8, 2.32 (quaternary carbons not observed). 119Sn (C6D6,
223.867 MHz) d 74.43. IR (Nujol mull) m 1613 (m), 1580 (m),
1538 (vs), 1442 (s), 1257 (m), 1245 (s), 1183 (w), 1146 (w), 1065
(w), 1033 (w), 926 (vs), 866 (vs), 849 (vs), 833 (s), 790 (m), 780
(m), 755 (m), 740 (m), 710 (w), 699 (w), 670 (m). Anal. calcd for
C62H86N2O4Si4Sn2: C, 58.49; H, 6.81; N, 2.20. Found C, 57.99; H,
6.63; N, 2.27%.
˚
using monochromated Mo-Ka radiation (k = 0.71073 A) at the
temperature indicated in Tables 3 (5) and 4 (7, and 8·Et2O).
Cell parameters were initially retrieved using the COLLECT38
software, and refined with the HKL DENZO and SCALEPACK
software,39 that was also used for data reduction. The structure
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Dalton Trans., 2007, 2862–2869 | 2867
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Table 3 Crystallographic data for compounds 4–6
Compound name (no.)
[(Me3Si)2NSn(l-O2CC6H2Ph3)]2 (4)
[(Me3Si)2NSn(l-O2CC6H3Mes2)]2 (5)
[(Me3Si)2NSn(l-O2 CC6H2Mes2Me)]2 (6)
Formula
M
C64H74Cl4N2O4Si4Sn2
1426.79
150(2)
C63H88Cl2N2O4Si4Sn2
1357.99
173(2)
C66H94Cl4N2O4Si4Sn2
1470.97
150(2)
T/K
Colour
Colourless
Colourless
0.25 × 0.25 × 0.25
Monoclinic
P2/c
17.2290(2)
19.5480(2)
20.4440(2)
90
95.4530(6)
90
6854.23(13)
4
Colourless
Size/mm
Crystal system
Space group
0.125 × 0.20 × 0.225
0.125 × 0.30 × 0.40
Triclinic
Triclinic
¯
P1
¯
P1
˚
a/A
10.429(2)
13.276(3)
14.405(3)
104.92(3)
110.62(3)
101.70(3)
1706.1(6)
1
1.389
1.004
0.0348
0.0737
11.8023(6)
12.5695(9)
14.2170(10)
67.446(3)
65.690(2)
76.473(3)
1767.8(2)
1
1.382
0.971
0.0635
0.1723
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
D
calc/Mg m−3
1.316
0.920
0.0368
0.0716
l/m−1
R1
wR2
a
b
CCDC no.
618667
618668
618669
ꢀ
ꢀ
ꢀ
ꢀ
a R1 = [ ꢀFo| − |Fcꢀ]/[ |Fo|] for [Fo > 2r(Fo2)]. b wR2 = {[ w(Fo − Fc2)2]/[ w(Fo2)2]}
.
2
2
1/2
Table 4 Crystallographic data for 7, 8·CH2Cl2 and 8·Et2O
Compound name (no.) (CH2Cl)(Cl)Sn[N(Si Me3)2]2 (7)
[(Mes2C6H3CO2)Sn (l-NSiMe3)]2 (8·CH2Cl2) [(Mes2C6H3CO2)Sn(l-NSiMe3)]2 (8·Et2O)
Formula
M
C13H38Cl2N2Si4Sn
524.40
173(2)
Colourless
0.15 × 0.20 × 0.20
Triclinic
¯
P1
8.55900(10)
11.3010(2)
14.9150(3)
105.9710(12)
96.8970(13)
108.9690(14)
1276.35(4)
2
1.365
1.397
0.0247
0.0562
C29H36Cl2NO2SiSn
648.29
223(2)
Colourless
0.2 × 0.45 × 0.45
Triclinic
¯
P1
10.5733(6)
11.8229(6)
13.8362(7)
67.2930(10)
79.9680(10)
89.7190(10)
1567.46(14)
2
1.374
1.048
0.0433
0.1173
C60H78N2O5Si2Sn2
1200.80
173(2)
Colourless
0.20 × 0.20 × 0.25
Triclinic
¯
P1
10.8922(3)
11.7957(3)
12.3240(3)
77.2409(16)
75.5426(16)
87.3334(17)
1495.34(7)
1
1.333
0.922
0.0427
0.0920
T/K
Colour
Size/mm
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
D
calc/Mg m−3
l/mm−1
R1
wR2
a
b
CCDC no.
618666
618670
618671
ꢀ
ꢀ
ꢀ
ꢀ
a R1 = [ ꢀFo| − |Fcꢀ]/[ |Fo|] for [Fo > 2r(Fo2)]. b wR2 = {[ w(Fo − Fc2)2]/[ w(Fo2)2]}
.
2
2
1/2
was solved using direct methods using SIR-9741 and refined by
full-matrix least-squares method on F2 with SHELXL97-2.42 The
non-hydrogen atoms were refined anisotropically. Hydrogen atoms
were included at geometrically idealized positions and were not
refined. The isotropic thermal parameters of the hydrogen atoms
were fixed at 1.2 times that of the preceding carbon atom.
solve the crystal structures. Refinement was conducted using full-
matrix least-squares calculations and SHELX-TL PC V 5.03. All
non-hydrogen atoms were refined with anisotropic displacement
parameters. Hydrogen atoms were treated as riding models and
were updated after each refinement.
Data collection (compound 8·CH2Cl2). A single crystal of
8·CH2Cl2 was mounted on a glass fibre and centred on a
Siemens 1 K SMART/CCD diffractometer. Data were collected
at the temperature indicated in Table 4 using Mo (Ka) radiation.
Lorentz and polarization corrections were applied and data were
corrected for absorption using redundant data and the SADABS
program. Direct methods and Fourier techniques were used to
Acknowledgements
The authors thank Dr Hilary Jenkins (SMU) and Dr Michael
Jennings (UWO) for collection of X-ray crystallographic data.
Thanks also to Michelle Hauser and Sarah Khoo for their
synthetic contributions. JACC and REW acknowledge support
from the Canada Research Chairs Program and the Canadian
2868 | Dalton Trans., 2007, 2862–2869
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View Article Online
Foundation for Innovation, and the Natural Sciences and Engi-
neeering Research Council of Canada.
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