Synthesis and thermolytic behavior of tin(IV) formates: In search of recyclable metal-hydride systems

Article abstract of DOI:10.1039/c0dt00812e

The synthesis and characterization of the series of organotin formates together with their thermolytic behavior are described. The diformate Bu ⁿ₂Sn{OC(O)H}₂ (1) was synthesized by the reaction of Buⁿ₂SnH₂ with formic acid. The triorganotin monoformate compounds R₃SnOC(O)H (R = Cy (cyclohexyl)) 3, Mes, (mesityl, 2,4,6-trimethylphenyl) 4, and Dmp (2,6-dimethylphenyl 5) were obtained by the reaction of R₃SnOH with formic acid. Their X-ray crystal structures along with that of the previously reported formate (PhCH ₂)₃SnOC(O)H (2) were determined. The diformate 1 exhibits an extended two-dimensional polymeric structure in which six-coordinate tin centers are linked by formate bridges. The tribenzyltin formate (2) possesses a chain structure in which the five-coordinate Sn(CH₂Ph)₃ units are bridged by formate ions. The cyclohexyl derivative 3 was observed to have a similar structure. In contrast, the Mes and Dmp derivatives 4 and 5 support monomeric structures in which the four-coordinate tin atom is bound to an oxygen of the formate ligand. Heating the compounds in various high boiling solvents produced no decomposition up to 120 °C in the case of 1 and refluxing a solution of 2 or 3 in mesitylene or diglyme left the starting material mostly unchanged, although 3 decomposed to an insoluble orange solid in refluxing decalin. In contrast, the heating of 4 and 5 in refluxing mesitylene led to elimination of CO to give the tin hydroxides. The results are in contrast to the known thermolytic behavior of R₃SnOC(O)H (R = Prⁿ or Buⁿ) complexes, which eliminate CO₂ to generate R ₃SnH. Compounds 3-5 are rare examples of structurally characterized tin formates. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0dt00812e

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Synthesis and thermolytic behavior of tin(IV) formates: in search of recyclable
metal–hydride systems†
Bobby D. Ellis,a Tonya M. Atkins,a Yang Peng,a Andrew D. Sutton,*b John C. Gordon*b and Philip P. Power*a
Received 8th July 2010, Accepted 25th August 2010
DOI: 10.1039/c0dt00812e
The synthesis and characterization of the series of organotin formates together with their thermolytic
behavior are described. The diformate Bun Sn{OC(O)H}2 (1) was synthesized by the reaction of
2
Bun SnH2 with formic acid. The triorganotin monoformate compounds R3SnOC(O)H (R = Cy
2
(cyclohexyl)) 3, Mes, (mesityl, 2,4,6-trimethylphenyl) 4, and Dmp (2,6-dimethylphenyl 5) were obtained
by the reaction of R3SnOH with formic acid. Their X-ray crystal structures along with that of the
previously reported formate (PhCH2)3SnOC(O)H (2) were determined. The diformate 1 exhibits an
extended two-dimensional polymeric structure in which six-coordinate tin centers are linked by formate
bridges. The tribenzyltin formate (2) possesses a chain structure in which the five-coordinate
Sn(CH2Ph)3 units are bridged by formate ions. The cyclohexyl derivative 3 was observed to have a
similar structure. In contrast, the Mes and Dmp derivatives 4 and 5 support monomeric structures in
which the four-coordinate tin atom is bound to an oxygen of the formate ligand. Heating the
compounds in various high boiling solvents produced no decomposition up to 120 ◦C in the case of 1
and refluxing a solution of 2 or 3 in mesitylene or diglyme left the starting material mostly unchanged,
although 3 decomposed to an insoluble orange solid in refluxing decalin. In contrast, the heating of 4
and 5 in refluxing mesitylene led to elimination of CO to give the tin hydroxides. The results are in
contrast to the known thermolytic behavior of R3SnOC(O)H (R = Prn or Bun) complexes, which
eliminate CO2 to generate R3SnH. Compounds 3–5 are rare examples of structurally characterized tin
formates.
costs and (c) reducing the negative impact of heavy metals in the
Introduction
environment.
A necessary requirement for a hydrogen (H2) based economy,
Reagents containing hydridic hydrogen atoms are usually
produced under either harsh or energy intensive conditions,16
neither of which is desirable for large scale applications. One
class of compounds that have the potential to be used in a
recyclable hydride system are metal formates.11 Carbon dioxide
(CO2) can add across metal-hydride bonds to form a metal formate
moiety, e.g. in the case of the ketiminatoaluminium hydride
AlH(OCMeCHCMeNAr)2 (Ar = 2,6-Pri2–C6H3).17 Formate com-
plexes can also be synthesized via metathesis reactions between
metal halides and alkali metal formates.18
especially for use in the transportation sector, is its storage for
controlled delivery, presumably to a fuel cell.1–5 Chemical hydrides
for hydrogen storage are typically composed of lighter elements,
resulting in higher gravimetric storage capacities, making these
attractive as H2 carriers. Due to its high gravimetric capacity of
H2 (19.6 wt%) and low molecular weight (30.7 g mol-1), chemical
hydrogen storage research has been dominated by one material in
particular, ammonia borane (H3N–BH3, AB).6–15
The practical viability of any chemical H2 storage system is
dependent on efficient recyclability of the energy carrier. This
means that once AB is dehydrogenated the resulting “spent fuel”
should ideally be regenerated in order to reform the original B–
H bonds contained within the borane fragment. Depending on
the nature of the H2 carrier material, a variety of regeneration
schemes could be envisioned. We recently described such a cycle
utilizing tin hydrides as reductants towards the regeneration of
AB, starting from polyborazylene (PB).9 In order to increase
overall process efficiencies, recycling of the tin-containing re-
ductant would also be useful.11 In addition, regenerable metal-
hydride systems would be of general practical utility via (a)
reduction in quantities of metal wastes; (b) reducing disposal
Formates have multiple potential decomposition pathways
available to them during thermolysis.19 It has been shown that
under thermolytic conditions, the tripropyl and tributyl tin
formates (R3SnOC(O)H, R = Prn, Bun) release CO2, yielding
tin hydrides.20 This process is reversible for Bun SnOC(O)H.21
3
One common potential side reaction that can occur when metal
formates are subjected to elevated temperatures is the elimination
of CO to yield metal hydroxides. As a possible methodology for
metal-based reductant recycle (so that these could be used in AB
spent fuel regeneration chemistries), we therefore sought to in-
vestigate the conditions for tin hydride generation via thermolysis
reactions of tin(IV) formates. We also sought to synthesize the
formate compounds from the corresponding tin halide and formic
acid. Formic acid itself can be synthesized from H2 and CO2,22
which results in the overall catalytic cycle shown in Scheme 1.
Only a limited number of tin formates are known, and only a
handful of these been structurally characterized. These include
aUniversity of California, Department of Chemistry, Davis, CA, 95616
bLos Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM, 87545
† CCDC reference numbers 783725–783728. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0dt00812e
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(s). 1H NMR (CDCl3) d: 1.35 (m, 3H), 1.68 (m, 5H), 1.94 (m, 3H),
8.31 (s, 1H).
Mes3SnOC(O)H (4)
Benzene (ca. 60 mL) was added to a flask containing Mes3SnOH
(2.672 g, 5.42 mmol). With rapid stirring, HC(O)OH (0.5 mL,
13.25 mmol) was added dropwise to the colorless solution, which
became slightly cloudy. The solution stirred overnight and the
volatiles were removed by evaporation under reduced pressure.
The product was crystallized from benzene by slow evaporation
of the solvent. Yield: 2.513 g (89%). Mp 119–120 ◦C 119Sn{ H}
1
Scheme 1 Proposed catalytic cycle for the regeneration of Sn(IV) hydrides.
1
NMR (C6D6) d: -121.3 (s). 13C{ H} NMR (C6D6) d: 21.1 (s), 24.7
1
the triphenyl derivative Ph3SnOC(O)H,23 which has an oligomeric
(s), 129.4 (s), 139.5 (s), 142.1 (s), 144.2 (s), 164.4 (s). H NMR
structure with penta-coordinated tin atoms. KSn(OC(O)H)3,24
a
(C6D6) d: 2.02 (s, 3H), 2.42 (s, 6H), 6.68 (s, 2H), 8.22 (s, 1H).
three dimensional polymer, Me2Sn{OC(O)H},25 which has a two-
dimensional sheet structure, and the monomeric Sn(II) derivative
HC{C(Me)N(C6H-2,6-Pri2)}2SnOC(O)H.26
Dmp3SnOH
2-Bromo-1,3-dimethylbenzene (10.000 g, 54.04 mmol, 3.5 equiv.)
was added dropwise to a flask containing Mg (1.445 g, 59.44
mmol, 3.85 equiv.) and thf (ca. 100 mL). The mixture was heated
and maintained at reflux conditions for 19 h. The resultant
orange/brown solution was cooled to room temperature and was
added dropwise to a cooled (0 ◦C) flask containing SnCl4 (1.8
mL, 15.44 mmol, 1 equiv.) and benzene (ca. 100 mL). A colorless
precipitate formed. The mixture was refluxed for 5 h. The resultant
mixture was filtered through Celite and washed with 10% aq. HCl.
The volatile fractions were removed by evaporation under reduced
pressure. The crude oil that remained was dissolved in toluene
(ca. 100 mL) and poured into 15% aq. NaOH (ca. 150 mL). This
mixture was refluxed for 18 h and cooled to room temperature. The
layers were separated and the aqueous layer was extracted with
three 50 mL portions of diethyl ether. The ethereal extracts were
combined with the original toluene layer and dried over anhydrous
Na2SO4. The volatiles were removed by evaporation under reduced
pressure and the product was crystallized from benzene by slow
evaporation of the solvent. Yield: 5.015 g (72%). Mp 138–140 ◦C.
Calcd for C24H28SnO: C, 63.88; H, 6.25. Found: C, 62.14; H, 6.05.
Experimental
General
Chemicals were purchased from either Aldrich or Acros and used
as received. Bun SnH2,27 (PhCH2)3SnCl,28 (PhCH2)3SnOC(O)H,29
2
Cy3SnOH30 and Mes3SnOH31 were prepared by published meth-
ods. NMR spectra were recorded on INOVA 600 MHz spectrom-
eters and were referenced internally to residual protio-solvent for
1
the H and 13C or externally to SnMe4 for the 119Sn nuclei. The
1
119Sn{ H} chemical shift of (PhCH2)3SnOC(O)H, d -38.8, was
not reported previously.
Bun Sn{OC(O)H}2 (1)
2
Diethyl ether (ca. 15 mL) was added to a flask containing
Bun SnH2 (2.428 g, 10.33 mmol). With rapid stirring, HC(O)OH
2
(0.8 mL, 20.67 mmol) was added dropwise to the colourless
solution. Within 30 min of the addition effervescence of H2 was
observed. The solution stirred overnight and then the volatiles
were removed by evaporation under reduced pressure. The product
was crystallized by sublimation under reduced pressure. Yield:
2.403 g (72%). Mp 86–89 ◦C. Calcd for C10H20O4Sn: C, 37.19; H,
119Sn{ H} NMR (C6D6) d: -114.6 (s). 13C{ H} NMR (C6D6) d:
24.8 (s), 130.0 (s), 144.2 (s), 144.6 (s), 145.6 (s), 164.4 (s). 1H NMR
(C6D6) d: 2.40 (s, 6H), 6.90 (d, 3JHH = 7.5, 2H), 7.06 (t, 3JHH = 7.2
Hz, 1H).
1
1
6.24. Found: C, 38.19, H, 6.80. 119Sn{ H} NMR (C6D6) d: -14.1.
1
Dmp3SnOC(O)H (5)
1
13C{ H} NMR (C6D6) d: 12.2 (s), 24.0 (s), 25.1 (s), 25.5 (s) 169.5
(s). 1H NMR (C6D6) d: 0.78 (t, 3JHH = 7.2 Hz, 6H), 0.89 (t, 3JHH
=
Benzene (ca. 20 mL) was added to a flask containing Dmp3SnOH
(0.933 g, 2.07 mmol). With rapid stirring, HC(O)OH (0.5 mL,
13.25 mmol) was added dropwise to the colorless solution, which
became slightly cloudy. The solution was stirred overnight and
the volatiles were removed by evaporation under reduced pressure.
The product was crystallized from benzene by slow evaporation of
the solvent. Yield: 0.927 g (94%). Calcd for C25H28O2Sn: C, 62.66;
7.2 Hz, 4H), 1.22 (m, 4H), 1.50 (m, 4H), 8.13 (s, 2H).
Cy3SnOC(O)H (3)
Toluene (ca. 20 mL) was added to a flask containing Cy3SnOH
(0.520 g, 1.35 mmol). The slurry was gently heated until all of the
Cy3SnOH had dissolved. With rapid stirring, HC(O)OH (0.5 mL,
13.25 mmol) was added dropwise to the colorless solution. A col-
orless precipitate immediately formed which then redissolved. The
solution was stirred overnight and the volatiles were removed by
evaporation under reduced pressure. The product was crystallized
from chloroform by slow evaporation of the solvent. Yield: 0.489
g (88%). Mp 159–165 ◦C. Calcd for C19H34O2Sn: C, 51.27; H, 7.70.
◦
1
H, 5.89. Found: C, 62.15; H, 6.01. Mp 159–161 C 119Sn{ H} NMR
(C6D6) d: -131.4 (s). 13C{ H} NMR (C6D6) d: 25.0 (s), 130.3 (s),
1
144.3 (s), 144.8 (s), 146.0 (s). 1H NMR (C6D6) d: 2.37 (s, 6H), 6.83
(d, 3JHH = 7.2 Hz, 2H), 6.99 (t, 3JHH = 7.2 Hz, 1H), 8.13 (s, 1H).
X-Ray crystallography
Found: C, 50.59; H, 7.79. 119Sn{ H} NMR (CDCl3) d: 30.8 (s).
Suitable crystals were selected and covered with a layer of
hydrocarbon oil under a flow of dry nitrogen. They were mounted
1
1
13C{ H} NMR (CDCl3) d: 27.0 (s), 29.1 (s), 31.2 (s), 34.2 (s), 166.4
10660 | Dalton Trans., 2010, 39, 10659–10663
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Table 1 Selected crystallographic data and collection parameters for 1, 2, 4 and 5
Compound
1
2
4·C6H6
5
Formula
FW
[C10H20O4Sn]6
1937.70
C22H22O2Sn
437.09
C31H37O2Sn
560.30
C25H28O2Sn
479.16
Crystal system
Triclinic
P1
Monoclinic
P21/c
Triclinic
P1
Monoclinic
C2/c
¯
¯
Space group
˚
a/A
11.0023(10)
15.0429(14)
25.678(2)
101.3680(10)
102.0150(10)
100.9650(10)
3955.0(6)
2
11.5721(6)
15.8683(9)
10.6892(6)
90
103.4760(10)
90
1908.81(18)
4
1.521
1.350
1.81 to 27.50
1.056
R1 = 0.0168
wR2 = 0.0402
0.488 and -0.234
8.6013(7)
16.5717(13)
18.8491(15)
80.2110(10)
84.3680(10)
89.5200(10)
2634.7(4)
4
32.550(5)
7.7354(13)
23.036(4)
90
132.147(2)
90
4300.4(12)
8
1.480
1.205
1.69 to 27.49
1.352
R1 = 0.0377
wR2 = 0.1061
1.684 and -1.175
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
Dc/g cm-3
m/mm-1
1.627
1.932
1.413
0.995
q range/◦
GOF on F2
Final R indices
[I > 2s(I)]
0.84 to 25.25
1.080
R1 = 0.0413
wR2 = 0.1021
2.303 and -1.466
1.10 to 27.50
1.104
R1 = 0.0439
wR2 = 0.1084
3.664 and -1.022
-3
˚
Maximum peak and hole/e A
24
on a glass fiber attached to a copper pin and placed in the
cold N2 stream on the diffractometer. X-ray data were collected
on a Bruker SMART 1000 diffractometer at 90(2) K using
sheet structure bears a resemblance to that of Me2Sn{OC(O)H}2
which is a sheet polymer with linear Me2Sn moieties bridged by
formate anions that produces an octahedral geometry at tin. The
Sn–O bonds in 1 display a wider range of Sn–O bonds lengths
˚
Mo-Ka radiation (l = 0.71073 A) see Table 1. Absorption
corrections were applied using SADABS.32 The structures were
solved using direct methods and refined by the full-matrix least-
squared procedures in SHELX.33 All of the non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were placed in
calculated positions and included in the refinement using a riding
model.
˚
(from 2.137(4) to 2.413(4) A) than that in Me2Sn{OC(O)H}2
˚
but the average Sn–O distance of 2.258 A in 1 is very similar
24
˚
to the 2.246(2) A in Me2Sn{OC(O)H}2. These correspond to
a delocalized p system across the C–O bonds in each bridging
formate unit. The formate groups are bound to the tin atoms
in a distorted square planar SnO4 arrangement. The metrical
parameters of the butyl groups are in themselves unremarkable.
Further attempts at heating 1 under nitrogen or in a toluene
solution resulted in recovery of the tin diformate with no evidence
Results and discussion
of any decomposition product. The generation of Bun SnH2 from
The reaction between dibutyltin dihydride and formic acid yielded
2
dibutyltin diformate Bun Sn{OC(O)H}2, 1, with concomitant
1 requires breaking four Sn–O bonds, which may account for the
difficulty in effecting thermal decomposition products at moderate
temperatures. In order to obtain more facile elimination of CO2
we focused on formates that possessed monomeric structures and
lower coordination numbers at tin.
The first compound investigated was tribenzyltin formate,
(PhCH2)3SnOC(O)H, 2, which was prepared via a dehydration re-
action between bis(tribenyltin)oxide and formic acid, as previously
described.23,29 This compound crystallizes with one molecule in the
asymmetric unit; however, the solid-state structure is polymeric
and is composed of chains in which the tribenzyl tin moieties are
linked by formate ions that symmetrically bridge the tin atoms.
The structural motif is similar to the solid-state structure of the
phenyl derivative.23 The molecular structure of 2 is shown in Fig. 2
and the extended polymeric structure is shown in Fig. 3. It can be
seen that the tin atoms are five-coordinate with almost equal Sn–
2
release of H2 gas. The colourless solid was heated to 120 ◦C under
dynamic vacuum in an attempt to effect CO2 release; however this
resulted in sublimation of the tin diformate, whose structure was
confirmed by X-ray crystallography. The molecular structure of 1
is shown in Fig. 1. There are six dibutyltin diformate molecules
in the asymmetric unit, which exhibits an extended 2-D polymeric
structure with each formate moiety bridging two tin atoms. The
˚
O bonds of 2.2596(11) and 2.2700(11) A. The symmetric bridging
may be compared with that in the triphenyl tin formate analogue
˚
which has Sn–O distances of 2.219(6), 2.317(6) and 3.372(6) A.
The three atoms form an almost linear array with an O–Sn–O
angle of 171.08(4) ◦, similar to the triphenyltin formate structure.23
The bridging formate moieties have indistinguishable C–O bond
lengths implying complete p-electron delocalization within the
formate CO2 array.
Fig. 1 Molecular structure of 1 with thermal ellipsoids presented at a
30% probability level. Hydrogen atoms are not shown.
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insoluble orange compound and multiple tin-containing products
(based on 119Sn NMR spectroscopy). None of the products could
be shown to contain a Sn–H bond.
A further increase in steric hindrance is afforded by use of
the mesityl or the related 2,6-dimethylphenyl (Dmp) substituents
in Mes3SnOC(O)H, 4, and Dmp3SnOC(O)H, 5. Trimesityltin
formate 4 was synthesized from Mes3SnOH and formic acid. The
isolated crystals were of suitable quality for a single crystal X-
ray diffraction study. This compound crystallized as a monomer
with two nearly identical independent molecules in the asymmetric
unit. The molecular structure of one of the independent molecules
of 4 is shown in Fig. 4. This represents the first structurally
characterized monomeric organotin formate complex. The coor-
dination geometry at the tin centers is distorted tetrahedral and
there are no close contacts between neighboring molecules. The
˚
Sn–O distances of 2.066(3) and 2.070(3) A are shorter than in
Fig. 2 Molecular structure of 2 with thermal ellipsoids presented
the bridging tin formates and the C–O distances of 1.319(6) and
1.314(6) A of the coordinated oxygen is longer as expected with
the localization of the p-electrons in the C–Oterminal bonds in which
the C–O distances are 1.204(6) and 1.216(6) A.
at a 30% probability level. All hydrogen atoms (except those on the
˚
˚
formate group) are not shown. Selected bond lengths (A) and angles
(◦): Sn1–O1 2.2700(11), Sn1–O2A 2.2596(11), O1–C1 1.256(2), O2–C1A
1.256(2), Sn1–O1–C1 122.40(11), Sn1–O2A–C1A 122.54(1), O2–C1–O1A
125.30(15), O1–Sn1–O2A 171.08(4).
˚
Fig. 3 Schematic representation of the extended polymeric structure of
2.
Compound 2 was heated in refluxing mesitylene (bp 163–
166 ◦C) or diglyme (bp 162 ◦C) for 24 h. The solvents were
evaporated under reduced pressure and the resulting material was
analyzed by multi-nuclear NMR spectroscopy for the formation
of either the tin hydride or tin hydroxide. NMR spectroscopy
of the material, however, only contained signals attributable to
tribenzyltin formate. This prompted us to investigate tin formates
with bulkier substituents that would favor monomers and help
facilitate tin hydride formation.
Fig. 4 Molecular structure of one of the molecules in the asymmetric
unit of 4 with thermal ellipsoids presented at a 30% probability level. All
hydrogen atoms (except those on the formate group) and the co-crystalliz◦ed
˚
benzene molecule are not shown. Selected bond lengths (A) and angles ( ),
values in brackets correspond to the second independent molecule: Sn1–O1
2.066(3) [2.070(3)], O1–C1 1.319(6) [1.314(6)], C1–O2 1.204(6) [1.216(6)],
Sn1–O1–C1 125.9(3) [124.7(3)], O1–C1–O2 123.5(5) [124.4(4)].
The more crowded tricyclohexyltin formate derivative
Cy3SnOC(O)H, 3, was synthesized in a dehydration reaction via
treatment of the tin hydroxide with formic acid. The crystals
obtained from crystallization from chloroform were of poor
quality and only a crude structure could be determined from the
X-ray diffraction data. Attempts to grow crystals from benzene,
hexanes and toluene were unsuccessful. The compound possesses a
solid-state structure similar to the benzyl derivative with polymeric
chains, as was previously predicted.23 The tin center is nearly co-
linear with the oxygen atoms of the bridging formate moieties (O–
Sn–O 172.9(7)◦) and the formate groups are again symmetrically
Compound 4 was heated in refluxing mesitylene for 24 h;
however the species isolated was identified as starting material.
4 was then heated in decalin at reflux for 8 h and a mixture
of two compounds was obtained which were identified by 119Sn
NMR spectroscopy as the starting material (d = -121 ppm) and
trimesityltin hydroxide (d = 108 ppm). We concluded that the
decomposition pathway where CO is eliminated is preferred to
the CO2 elimination route under these particular experimental
conditions.
The related tris(2,6-dimethylphenyl)tin formate 5 was synthe-
sized similarly by treating Dmp3SnOH with formic acid. This
compound also crystallizes as a monomer; however there is only
one molecule in the asymmetric unit. The molecular structure
of 5 is shown in Fig. 5. The coordination geometry at the tin
center is also distorted tetrahedral similar to 4. The pertinent
bond lengths are indistinguishable from the mesityl-substituted
derivative, although there are small differences in the angles. The
only notable structural difference is that the torsion angle for
˚
bound to the tin centers (2.30(2) and 2.34(2) A).
Compound 3 was heated under reflux in either mesitylene,
diglyme or decalin (bp 189–91 ◦C) for 8 h. The products obtained
after evaporation of the volatile materials were analyzed by multi-
nuclear NMR spectroscopy. Whereas the compounds that were
heated in mesitylene and diglyme did not exhibit signals in their
NMR spectra that were not attributable to the tin formate, the
sample of 3 that was heated in decalin had decomposed into an
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Sn1O1C1O2 moiety is -11.7(9)◦, while in the mesityl structure
4 the corresponding angles are -169.1(4) and -169.5(4)◦.
Acknowledgements
This work was funded by the U.S. Department of Energy Center
of Excellence for Chemical Hydrogen Storage.
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Fig. 5 Molecular structure of 5 with thermal ellipsoids presented at a 30%
probability level. All hydrogen atoms (except o◦n the formate group) are not
˚
shown. Selected bond lengths (A) and angles ( ): Sn1–O1 2.063(3), O1–C1
1.310(5), C1–O2 1.201(6), Sn1–O1–C1 121.2(3), O1–C1–O2 126.7(4).
Compound 5 was heated in mesitylene under reflux for 24
h. Analysis of the product after evaporation of the volatile
components indicated that only unreacted tin formate was present.
A sample of 5 was heated for 24 h in refluxing decalin. Multi-
nuclear NMR spectroscopy of the resulting isolated product
indicated complete conversion to the tin hydroxide Dmp3SnOH
1
(
119Sn{ H}NMR; d-114.6) with CO2 elimination.
The 119Sn{ H} NMR spectra of the compounds 4 and 5 and
1
Dmp3SnOH display upfield chemical shifts in the range -114.6
to -131.4 ppm. These are similar to shifts previously reported
for compounds with triaryl tin(IV) moieties bound to oxygen
containing ligands and are consistent with four coordinate tins.34,35
Compounds 1 and 3 have chemical shifts that are ca. 100 ppm
further downfield however. This is consistent with previously noted
chemical shift trends when the organic substituents are changed
from aryls to alkyls.35
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Conclusions
Bun Sn{OC(O)H}2, (PhCH2)3SnOC(O)H and Cy3SnOC(O)H ex-
2
hibit polymeric structures in the solid-state and undergo neither
CO nor CO2 elimination decomposition pathways when exposed
to prolonged heating at elevated temperatures. Mes3SnOC(O)H
and Dmp3SnOC(O)H are the first structurally characterized
examples of monomeric tin formates. Both compounds undergo
CO elimination to yield tin hydroxides, contrary to the behavior
reported for trialkyltin formates which eliminate CO2 resulting in
tin hydride formation. Further studies with other tin formates are
currently underway to probe the factors that influence the thermal
decomposition pathways of these species.
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32 SADABS: Area-Detection Absorption Connection, I. Bruker AXS,
Madison, WI, 1995.
33 SHELX and SHELXL PC: Version 5.03, I. Bruker AXS, Madison, WI,
1995.
34 K. C. Molloy, S. J. Blunden and R. Hill, J. Chem. Soc., Dalton Trans.,
1988, 1259.
35 B. Wrackmeyer, Annu. Rep. NMR Spectrosc., 1999, 38, 203.
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