Synthesis and characterisation of tin(II) and tin(IV) citrates

Article abstract of DOI:10.1039/a704511e

A series of tin(II) citrates, SnM(C₆H₄O₇) (M = Sn or Zn) and SnMM′(C₆H₄O₇) (M = M′ = Na, K or NH₄; M = NMe₄, M′ = H) and related monotin(II) salts of 1,5-dimethyl and 1,5-di-n-butyl citrate have been synthesized and characterised by spectroscopic methods (IR, ¹H, ¹³C, ¹¹⁹Sn NMR, ¹¹⁹Sn Moessbauer). The crystal structures of dimeric tin(II) 1,5-dimethyl citrate and an oxidation product, Sn(NMe₄)₂(C₆H₅O₇) ₂·3.5H₂O, have also been determined.

Full text of DOI:10.1039/a704511e

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Synthesis and characterisation of tin(II) and tin(IV) citrates
Paul R. Deacon,a Mary F. Mahon,a Kieran C. Molloy*,a and Phillip C. Waterfieldb
a School of Chemistry, University of Bath, Claverton Down, Bath, UK BA2 7AY
b Unilever Research, Port Sunlight Research, Bebington, Wirral, UK L63 3SW
A series of tin() citrates, SnM(C6H4O7) (M = Sn or Zn) and SnMMЈ(C6H4O7) (M = MЈ = Na, K or NH4;
M = NMe4, MЈ = H) and related monotin() salts of 1,5-dimethyl and 1,5-di-n-butyl citrate have been synthesized
and characterised by spectroscopic methods (IR, 1H, 13C, 119Sn NMR, 119Sn Mössbauer). The crystal structures
of dimeric tin() 1,5-dimethyl citrate and an oxidation product, Sn(NMe4)2(C6H5O7)2ؒ3.5H2O, have also been
determined.
The citrate ion (3-carboxy-3-hydroxypentane-1,5-dioate) is ubi-
quitous in nature and is a key constituent of the tricarboxylic
acid (‘citric acid’) cycle.1 It is found in many fruits (oranges,
lemons, currants, beetroots, etc.), fruit drinks and pharm-
aceutical syrups as well as being used to adjust the pH of food
products. Numerous metal complexes of this potentially tetra-
dentate ligand have been prepared.2–6 Of particular importance
is the interaction of tin with citric acid (H4cit), as its use as a
packaging material brings the two materials into intimate con-
tact in many everyday products. The corrosion rates for tin and
tin–iron alloys in the presence of fruit acids have been measured
electrochemically7–9 and the stability constant for [SnMe2]2ϩ
with the citrate ion has been reported.10 Surprisingly, however,
definitive synthetic and structural studies of tin citrates are still
lacking. The only compounds we have been able to find
reported previously are Sn2(cit) and Na2Sn(cit),11 the latter act-
ing as an effective fruit juice stabiliser when storing juice in
cans. pH Titrations have been used to show that SnII forms
stable chelates of unresolved structure with citric acid and ten-
tative claims have been made for the existence of ‘[Sn(H2cit)]2O’
as the precipitate which forms from solutions of the tin(II)
ion and a four-fold excess of citric acid.12 The same report pro-
vides evidence for the existence of 1:1:1 Sn2ϩ :M2ϩ :cit4Ϫ
(M = Cu or Fe) chelates. No structural studies have been made
in this general area of tin chemistry, save for an extended X-ray
absorption fine structure (EXAFS) investigation of [{Sn-
(cit)}4Pt{µ-Sn(cit)}2Pt{Sn(cit)}4]6Ϫ which failed to establish the
connectivity between tin and the citrate ions.13
Sn2(cit)
2 SnCl2
1
CO2H
CO2H
4 NaOH, –4 NaCl
HO2C
SnCl2
HO
4 MOH, –2 MCl, –H2O
SnMM′(cit)
2 M = M′ = Na
3 M = M′ = K
4 M = M′ = NH4
5 M = NMe4, M′ = H
Scheme 1
used to synthesize 4. The origin of the difference is unclear,
though only 5 was crystallised from an organic solvent (meth-
anol), which was also ultimately incorporated into the final
product. A heterobimetallic species, SnZn(cit) 6, has also been
prepared, from 4 and ZnCl2. Compounds 1 and 6 are both
insoluble in all common solvents (6 is sparingly soluble in boil-
ing water) and are both probably polymeric in character.
In order to produce more soluble tin() citrates for spectro-
scopy/crystallisation purposes, we have attempted the synthesis
of a number of tin() salts of selectively esterified citric acid. In
addition to introducing solubilising ester groups, the reduced
number of co-ordination sites on the ligand was also expected
to induce solubility as well as a simplification of the co-
ordination chemistry about the metal. In this way, the various
co-ordinating sites of citric acid could be probed independently.
The syntheses are summarised in Scheme 2.
Our interest in this area stems from the application of tin()
species in dental formulations, where the metal, originally
incorporated as SnF2 simply as a fluoride ion source, is now
known to be orally active (antimicrobial) in its own right.14
Citrate has long been a component of dental formulations and
zinc citrate (ZCT) is still found in many toothpastes. In order to
understand more fully the nature of SnII–H4cit chemistry we
have synthesized several new complexes containing both these
components (including tin in both its oxidation states) and
report the first crystallographic evidence for the bonding in
such species.
1,5-Dimethyl,15 3-tert-butyl 1,5-dimethyl16 and 3-tert-butyl
citrates16 I–III have been prepared by well documented pro-
cedures. Reaction of 1,5-dimethyl citrate with Sn(O2CMe)2 in
methanol yields the monotin() complex 7 as its methanol
adduct. Attempts to prepare 7 from soluble Sn(OBun)2,17 gener-
ated in situ from Sn(OMe)2 in an excess of butanol, yielded
the transesterification product 8, although 7 is reformed on
recrystallisation of 8 from methanol. Syntheses involving the
triester II as a bulky alcohol were not successful, in as much as
reaction with Sn(OMe)2 in toluene yielded a crystalline material
9 with very low organic content. Microanalytical data (C, 3.70;
H, 1.04%) lie between the theoretical values for Sn6O4(OMe)2-
(OH)2 (C, 2.75; H, 0.90%) and Sn6O4(OMe)3(OH) (C, 4.06; H,
1.14%). The species Sn6O4(OMe)4 has been reported previously
as the controlled hydrolysis product of Sn(OMe)2 and shown to
contain an [Sn6O4]4ϩ adamantane core18 and the product we
have reproducibly isolated seems consistent with this class of
compound, though unfortunately the crystals obtained were
unsuitable for diffraction purposes. However, 9 and Sn6O4-
(OMe)4 exhibit ν(Sn᎐O) at 547 and 570 cmϪ1, respectively and
Results and Discussion
Synthesis
Ditin() citrate 1 has been prepared from citric acid and tin()
chloride (1:2) in the presence of 4 equivalents of base (Scheme
1). Similarly, salts of formula SnMMЈ(C6H4O7) have been pre-
pared in the same way using equimolar quantities of the two
principal reagents. The formation of compound 5 is somewhat
surprising as the method of preparation followed exactly that
J. Chem. Soc., Dalton Trans., 1997, Pages 3705–3712
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CO2Me
CO2Me
CO2H
CO2H
Table 1 Selected geometric data (bond lengths in Å, angles in Њ) for
O
compound 7
HO2C
Sn
O
HO
O
O(1Ј)᎐Sn(1)
O(8)᎐Sn(1)
C(1)᎐O(1)
C(3)᎐O(3)
C(4)᎐O(4)
C(6)᎐O(6)
C(8)᎐O(7)
H(8)᎐O(8)
2.393(5)
2.272(6)
1.416(6)
1.198(7)
1.457(7)
1.339(7)
1.237(7)
0.960(2)
O(2)᎐Sn(1)
Sn(1)᎐O(1)
C(8)᎐O(2)
C(3)᎐O(4)
C(6)᎐O(5)
C(7)᎐O(6)
C(9)᎐O(8)
2.266(5)
2.094(5)
1.265(7)
1.337(7)
1.194(7)
1.450(8)
1.416(7)
7
Sn(O2CMe)2,
MeOH
MeOH, H+
–2 MeCO2H
CO2Me
CO2H
MeO2C
HO2C
CO2H
HO
CO2But
HO
III
ButO2CMe, H+
NaOH
O(2)᎐Sn(1)᎐O(1Ј)
O(8)᎐Sn(1)᎐O(2)
O(1)᎐Sn(1)᎐O(1Ј)
C(1)᎐O(1)᎐Sn(1Ј)
137.4(1)
84.4(2)
67.3(1)
129.4(4)
O(8)᎐Sn(1)᎐O(1)
O(2)᎐Sn(1)᎐O(1)
O(1Ј)᎐Sn(1)᎐O(8)
95.5(2)
73.3(2)
83.7(2)
CO2Me
CO2But
I
MeO2C
Sn(OMe)2, BunOH
Sn(OMe)2, BunOH
HO
CO2Bun
II
1
CO2Bun
to exclude the terminal CH2CO2R groups from co-ordinating to
the tin, chelation involving only the hydroxy and central carb-
oxylate group has been observed elsewhere in non-esterified
citrate compounds. For example, the antimony compounds
Sn(OMe)2, toluene
O
Sn
O
'Sn6O4(OMe)x(OH)y'
O
8
9
19
Sb2Ag2(H2cit)4 and SbM(H2cit)2(H2O)2ؒH2O (M = Li, Na or
Scheme 2
K)3,19 have been found to contain divalent citrate anions which
co-ordinate exclusively through the central carboxylate and the
α-hydroxyl group. The carbonyl group incorporating O(3Ј),
which approaches Sn(1) from above the Sn2O2 plane at a dis-
tance of only 2.787 Å, could be construed as expanding the co-
ordination sphere about tin to SnO5E, but the strength of the
C(3)᎐O(3) bond [1.198(7) Å], which is comparable to that of
unco-ordinated C(6)᎐O(5) [1.194(7) Å] and shorter than the
hydrogen-bonded C(8)᎐O(7) [1.237(7) Å] (see below), suggests
otherwise.
The strongest of the Sn᎐O bonds involves the alkoxide O(1)
[Sn(1)᎐O(1) 2.094(5) Å], while the intermolecular Sn(1)᎐O(1Ј)
[2.393(5) Å] is somewhat weaker, though both lengths are com-
parable with the bonds in dimeric Sn(OBut)2 [1.97(2)–2.16(1)
Å].20 The bond to the monodentate carboxylate group
[Sn(1)᎐O(2) 2.266(5) Å] is intermediate between the extremes
found in compounds such as Sn(O2CH)2 which contain biden-
tate ligands (2.14–2.36 Å).21 The Sn᎐O(H)CH3 interaction
[2.272(6) Å] is in the middle of the range for such solvates
(2.117–2.404 Å).22,23
The lattice structure of compound 7 is built around hydrogen
bonds linking H(8) [attached to O(8) of the co-ordinated
methanol] and the non-bonded oxygen from the central carb-
oxylate O(7), as shown in Fig. 2. Atom H(8) was located
[O(8)᎐H(8) 0.960(2) Å] and the length and linearity of the
hydrogen bond [H(8) ؒ ؒ ؒ O(7) 1.63 Å, O(8)᎐H(8)᎐O(7) 166.8Њ]
reflects its strength. The non-bonded distance O(7)᎐O(8) is
2.577 Å.
Fig. 1 The asymmetric unit of compound 7 showing the labelling
scheme used in the text and tables. Primed atoms are related to their
unprimed analogues by an inversion centre at the centre of the Sn2O2
ring. Thermal ellipsoids are at 30% probability
have similar Mössbauer isomer shift (i.s.) values (2.66 and 2.78
mm sϪ1). Our intended target Sn[OC(CH2CO2Me)2(O2CBut)]2,
may well have been an intermediate in this reaction. The reac-
tion of III with Sn(OBun)2 (generated in situ as described
above) 17 yielded an insoluble material which analysed for
Sn2(cit) 1, rather than Sn(O2CCH2)2C(OH)CO2But as hoped.
When an aqueous solution of compound 5 was allowed to
evaporate over several weeks under aerobic conditions a viscous
oil formed which partially crystallised on standing. Several
crystals were hand-picked, washed with diethyl ether and found
by X-ray crystallography to be the oxidation product [NMe4ϩ]2-
[Sn(Hcit)22Ϫ]ؒ3.5H2O 10. The quality of this structure determin-
ation is low, but is of sufficient quality to unequivocally identify
the basic structural units and their interactions. The asym-
metric unit (Fig. 3) consists of two molecules which are essen-
tially identical within the limitations of the data set. Selected
metric data are given in Table 2. Only the co-ordination about
Sn(1) will be discussed. This consists of a distorted octahedral
SnO6 arrangement from the bonding of two [Hcit3Ϫ] ligands to
the tin, the charge balance being achieved by the two quater-
nary ammonium cations. The dianion adopts a fac rather than a
mer isomeric form, in which the deprotonated α-hydroxy
groups are trans to each other [O(3)᎐Sn(1)᎐O(1) 178.4(5)Њ]. The
X-Ray crystallography
The asymmetric unit of tin() 1,5-dimethyl citrate 7 as its mono-
methanolate is shown in Fig. 1. Selected metric data are given in
Table 1. The molecule adopts a dimeric structure in which each
tin atom is located in a distorted trigonal bipyramidal pseudo-
four-co-ordinate SnO4E (E = lone pair) environment. Three of
the four oxygens around the tin are due to the central carboxyl-
ate O(2) and the α-hydroxyl group O(1) from one dimethyl
citrate ligand, together with oxygen from the co-ordinated
methanol molecule O(8). The fourth oxygen is due to the α-
hydroxyl oxygen of an adjacent molecule O(1Ј). The two halves
of the dimeric unit are linked by an inversion centre (i) such
that central Sn2O2 ring is constrained to be planar. Although
the 1,5-dimethyl citrate ester was deliberately prepared in order
Ϫ
Ϫ
remaining CO2 and CH2CO2 groups are mutually cis, while
one methylene carboxylate group remains protonated and
uninvolved in metal binding. Other compounds which contain
3706
J. Chem. Soc., Dalton Trans., 1997, Pages 3705–3712

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Table 2 Selected geometric data (bond lengths in Å, angles in Њ) for
compound 10
O(1)᎐Sn(1)
O(6)᎐Sn(1)
O(10)᎐Sn(1)
C(1)᎐O(1)
C(1)᎐O(2)
C(5)᎐O(4)
2.10(1)
2.13(1)
1.96(2)
1.11(2)
1.21(2)
1.20(3)
1.19(3)
1.29(2)
1.41(3)
1.19(2)
2.04(2)
1.89(2)
2.06(3)
1.56(3)
1.59(4)
1.30(4)
1.19(3)
1.32(4)
1.45(3)
1.15(4)
O(3)᎐Sn(1)
O(8)᎐Sn(1)
O(11)᎐Sn(1)
C(12)᎐O(14)
C(3)᎐O(3)
C(5)᎐O(5)
C(6)᎐O(7)
2.07(2)
2.07(1)
2.12(2)
1.20(3)
1.32(3)
1.35(3)
1.28(2)
1.20(2)
1.38(2)
1.23(3)
2.11(2)
2.04(2)
2.03(3)
1.23(2)
1.27(4)
1.46(4)
1.41(5)
1.46(4)
1.27(3)
1.33(3)
C(6)᎐O(6)
C(7)᎐O(8)
C(7)᎐O(9)
C(9)᎐O(10)
C(10)᎐O(12)
O(15)᎐Sn(2)
O(17)᎐Sn(2)
O(22)᎐Sn(2)
C(13)᎐O(15)
C(15)᎐O(17)
C(17)᎐O(19)
C(18)᎐O(21)
C(19)᎐O(23)
C(23)᎐O(25)
C(24)᎐O(27)
C(10)᎐O(11)
C(12)᎐O(13)
O(17)᎐Sn(2)
O(20)᎐Sn(2)
O(24)᎐Sn(2)
C(13)᎐O(16)
C(17)᎐O(18)
C(18)᎐O(20)
C(19)᎐O(22)
C(21)᎐O(24)
C(23)᎐O(26)
C(24)᎐O(28)
Fig. 3 The anionic part of the asymmetric unit (two molecules) of
compound 10 showing the labelling scheme used in the text and tables.
Hydrogen bonds are shown as dotted lines
O(3)᎐Sn(1)᎐O(1)
O(6)᎐Sn(1)᎐O(3)
O(8)᎐Sn(1)᎐O(3)
O(10)᎐Sn(1)᎐O(1)
O(10)᎐Sn(1)᎐O(6)
O(11)᎐Sn(1)᎐O(1)
O(11)᎐Sn(1)᎐O(6)
O(11)᎐Sn(1)᎐O(10)
O(20)᎐Sn(2)᎐O(15)
O(22)᎐Sn(2)᎐O(15)
O(22)᎐Sn(2)᎐O(20)
O(24)᎐Sn(2)᎐O(17)
O(24)᎐Sn(2)᎐O(22)
O(27)᎐Sn(2)᎐O(17)
O(27)᎐Sn(2)᎐O(22)
87.1(6)
79.4(6)
91.6(6)
92.5(7)
99.0(7)
173.8(5)
91.9(6)
81.9(7)
89.5(8)
92.1(10)
171.9(10)
175.7(8)
91.7(11)
94.2(9)
83.6(10)
O(6)᎐Sn(1)᎐O(1)
O(8)᎐Sn(1)᎐O(1)
O(8)᎐Sn(1)᎐O(6)
O(10)᎐Sn(1)᎐O(3)
O(10)᎐Sn(1)᎐O(8)
O(11)᎐Sn(1)᎐O(3)
O(11)᎐Sn(1)᎐O(8)
O(17)᎐Sn(2)᎐O(15)
O(20)᎐Sn(2)᎐O(17)
O(22)᎐Sn(2)᎐O(17)
O(24)᎐Sn(2)᎐O(15)
O(24)᎐Sn(2)᎐O(20)
86.3(5)
93.8(5)
171.0(6)
178.4(5)
90.0(7)
98.4(6)
89.0(6)
92.8(9)
84.9(9)
87.1(11)
91.3(10)
96.3(9)
O(27)᎐Sn(2)᎐O(15) 171.5(9)
O(27)᎐Sn(2)᎐O(20)
O(27)᎐Sn(2)᎐O(24)
95.8(9)
81.6(10)
Fig. 4 The unit cell of compound 10 viewed along a showing hydrogen
bonds between [Sn(Hcit)2]2Ϫ units (dotted lines) only. The [NMe4]ϩ
units and lattice water have been omitted for clarity
general all such bonds are shorter than in the tin() derivative 7
described earlier.
The lattice structure of compound 10 is a consequence of
two hydrogen-bonded networks. The simplest of these involves
the C᎐O and C᎐OH groups of the citrate ligands, with the non-
᎐
co-ordinating oxygens of the central carboxylic acid groups
[O(7), O(12) on Sn(1); O(21), O(28) on Sn(2)] interacting with
the presumably protonated oxygens of the non-chelating
methylene carboxylic acids [O(5), O(13) on Sn(1); O(19), O(25)
on Sn(2)]. Thus, O(7) and O(13) hydrogen bond with O(25) and
O(21) respectively (2.66, 2.61 Å), while O(12) is linked to O(5)
generated by the operator 1 Ϫ x, 1 Ϫ y, Ϫz (2.57 Å). Atoms
O(28) and O(19) (symmetry operator 1 Ϫ x, Ϫy, 1 Ϫ z) behave
similarly (2.67 Å). These features are illustrated in Fig. 4.
In addition, seven water molecules O(1Ј)–O(8Ј) [two included
Fig. 2 The lattice structure of compound 7 viewed along a. Hydrogen
bonds are shown as dotted lines
the same tridentate citrate bonding to the central metal are the
anions [M(Hcit)(H2O)Ϫ] (M = Mn2ϩ or Mg2ϩ).24,25
Of the Sn᎐O bonds, the ones to the α-hydroxy groups
[Sn(1)᎐O(3) 2.07(2), Sn(1)᎐O(10) 1.96(2) Å] are shorter than
those to the carboxylic acid groups [2.07(1)–2.13(1) Å], and in
J. Chem. Soc., Dalton Trans., 1997, Pages 3705–3712
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Fig. 5 The unit cell of compound 10 viewed along a showing hydrogen bonds involving water (dotted lines)
Table 3 Selected intermolecular interactions (Å) for compound 10
Table 4 Spectroscopic data for tin citrates
Compound
i.s./mm sϪ1 q.s./mm sϪ1
δ (119Sn)
νasym(CO2)
Ϫ724.9a
Ϫ579.9
Ϫ578.3
Ϫ579.6
Ϫ582.9
1553
1593
1599
1599
1616
1567
1561, 1734
1543, 1740
Involving carboxylate groups only
1
2
3.07
3.06
2.97
2.99
2.80
3.17
3.11
3.13
0.16
3.17
3.29
3.25
1.90
1.85
1.81
1.84
1.91
1.92
2.01
1.98
1.36
1.93
1.90
1.90
O(7) ؒ ؒ ؒ O(25)
O(13) ؒ ؒ ؒ O(21)
2.66
2.61
O(12) ؒ ؒ ؒ O(5a)
O(28) ؒ ؒ ؒ O(19b)
2.57
2.67
3
4
Involving water molecules
5
b
6
—
O(1) ؒ ؒ ؒ O(3Ј)
O(7) ؒ ؒ ؒ O(1Јc)
O(4Ј) ؒ ؒ ؒ O(2Ј)
2.55
2.75
2.93
O(1Ј) ؒ ؒ ؒ O(9Ј)
O(3Ј) ؒ ؒ ؒ O(3Јc)
O(4Ј) ؒ ؒ ؒ O(17b)
O(4Ј) ؒ ؒ ؒ O(4Јd)
O(5Ј) ؒ ؒ ؒ O(6Ј)
O(8Ј) ؒ ؒ ؒ O(24b)
O(6Ј) ؒ ؒ ؒ O(2)
3.05
2.72
2.78
2.70
2.68
2.80
3.40
7
Ϫ482.0
Ϫ497.0
Ϫ54.2, Ϫ55.3 1644, 1727
Ϫ494.1
Ϫ489.8
Ϫ480.6
8
10
11
12
13
O(2Ј) ؒ ؒ ؒ O(23b) 2.68
1560
1555
1553
O(2Ј) ؒ ؒ ؒ O(6Ј)
O(5Ј) ؒ ؒ ؒ O(10)
O(8Ј) ؒ ؒ ؒ O(7e)
O(3) ؒ ؒ ؒ O(3Ј)
2.87
2.88
2.65
2.78
a Solid-state CP MAS data. b Insoluble in all common solvents.
Key to symmetry operations relating designated atoms to reference
atoms at x, y, z: a 1 Ϫ x, 1 Ϫ y, Ϫz; b 1 Ϫ x, Ϫy, 1 Ϫ z; c Ϫx, 1 Ϫ y, Ϫz;
d Ϫx, Ϫy, 1 Ϫ z; e x, y, 1 ϩ z.
scopic data (Table 4) are the measurements relating to the two
crystal structures. The Mössbauer and 119Sn NMR data for
compound 10 are unique in being the only data referring to a
at half occupancies, O(5Ј) and O(8Ј)] were refined and are
linked in a complex fashion to each other and to the citrate
ligands. The short contacts for each of these oxygens are given
in Table 3, and shown in Fig. 5. The water molecules form (i)
puckered sheets running through the centre of the unit cell
parallel to a [O(2Ј), O(4Ј), O(5Ј), O(6Ј)], (ii) a symmetry-
generated group of eight molecules based on O(1Ј) and O(3Ј) at
the top and bottom centres of the cell and (iii) a group based
solely on O(8Ј) in the cell corners. Effectively, water molecules
fill the void space generated by the earlier hydrogen-bonded
network.
tin() species. This is clearly reflected in the i.s. of 0.16 mm sϪ1
,
while the quadrupole splitting (q.s.) of 1.36 mm sϪ1 arises from
the asymmetric co-ordination (differing Sn᎐O bonds) about the
tin. This environment is clearly more distorted than most SnO6
octahedra, which have small or unresolved q.s. {e.g. tin() site
in [Sn2O(O2CCF3)4O(OCF3)2]2, q.s. 0.52 mm sϪ1}. A more
reasonable parallel can be drawn with Sn(O2CCF3)4 (q.s. 1.56
mm sϪ1), which is believed to contain two bi- and two mono-
dentate carboxylate groups.26 In addition, two resonances in the
119Sn NMR spectrum (δ Ϫ54.2, Ϫ55.3) suggest that the two
non-equivalent tin sites present in the solid state are retained in
solution, presumably through the C᎐O ؒ ؒ ؒ HO hydrogen bonds.
᎐
Spectroscopy
The Mössbauer data for compound 7 (i.s. 3.11, q.s. 2.01 mm
The starting point for any discussion of the available spectro-
s
Ϫ1), particularly the i.s., are intermediate between values for the
3708
J. Chem. Soc., Dalton Trans., 1997, Pages 3705–3712

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homoleptic Sn(OR)2 (R = Me 2.80, 2.02; Bun 2.87, 1.97 mm
infrared spectra were recorded as Nujol mulls on KBr plates
17
27
sϪ1
)
and Sn(O2CR)2 (R = Me 3.31, 1.77 mm sϪ1
)
in accord-
and all NMR data were recorded on saturated solutions in
D2O at ambient temperature unless indicated otherwise.
Tin() analyses were determined iodometrically.31 Sodium
and potassium analyses were determined by flame photometry.
Zinc analysis was performed by atomic absorption spec-
trophotometry.
ance with the heteroleptic nature of the co-ordination in this
species. The co-ordination number of the simple tin() alk-
oxides cited above is, however, uncertain, but an SnO4E co-
ordination as in the carboxylates is likely. In solution, soluble
oligomeric tin() alkoxides (R = Bun or But) display pseudo-
three-co-ordinate SnO3E stereochemistry and thus exhibit tin
NMR chemical shifts significantly different to those of 7, e.g.
[Sn(OBut)2]2, δ(119Sn) Ϫ93.6.20 The 119Sn NMR signal for 7 (δ
Ϫ482.0) is, however, in line with data for simple soluble pseudo-
four-co-ordinate carboxylates (SnO4E) such as tin() laurate
(11), palmitate (12) and stearate (13) (δ Ϫ494.1, Ϫ489.8,
Ϫ480.6) which have been spectroscopically characterised for the
first time as part of this study. Clearly, 8 is structurally similar
to 7 given the overall similarity of the spectral data [δ(119Sn)
Ϫ497.0; i.s. 3.13, q.s. 1.98 mm sϪ1], though both the analytical
and spectroscopic data rule out co-ordinated alcohol. The
microanalysis suggests the presence of water (probably from the
butanol used in the reaction) which is more likely to generate
a hydrogen-bonded lattice, similar to that of 10, than is the
bulkier butanol.
The IR data for compounds 7 and 10 are of interest since
they provide reference points against which the bonding of the
carboxylate groups in the remaining species can be judged.
Both compounds contain unidentate carboxylate groups,
though in both cases the carbonyl groups are involved in hydro-
gen bonding. Thus, for 7, νasym(CO2) at 1561 cmϪ1 represents a
strongly hydrogen-bonded carboxylate, while for 10 νasym(CO2)
at 1644 cmϪ1, though at higher wavenumber than might have
been expected, is in accordance with the weak hydrogen-
bonding present for at least some of the carbonyl groups [e.g.
O(2), O(9), O(23), O(16)].
Syntheses
Ditin(II) citrate, Sn2(C6H4O7) 1.11 Anhydrous citric acid (2.11
g, 11.0 mmol) was dispersed in deoxygenated water (40 cm3)
with rapid stirring until fully dissolved. To this clear solution
solid SnCl2 (4.46 g, 23.5 mmol) was added and the mixture
allowed to stir for 10 min until a clear solution was formed. 2 
Sodium hydroxide (20 cm3) was added dropwise to the reaction
mixture with stirring whilst also rapidly purging the flask with
N2. As the pH rose to 5.0 and above a dense white precipitate
was formed. The solid product was rapidly filtered off and
washed with successive aliquots of water, ethanol and diethyl
ether. In order to remove the water held within the solid it was
necessary to dry the solid in vacuo whilst warming to 70–80 ЊC.
The product was a fine, white powder. Yield: 4.3 g (92%) [Found
(Calc. for C6H4O7Sn2): C, 16.6 (16.9); H, 1.27 (1.65); SnII, 53.8
(55.8)%]. Selected infrared data (cmϪ1): 1553s (br), νasym
-
(CO2Sn); 489m (br), ν(Sn᎐O). 13C CP MAS NMR data: δ 48.8
(CH2), 52.9 (CH2), 79.9 (tertiary C), 178.4 (CH2CO2Sn) and
181.3 (R2CO2Sn).
Disodium tin(II) citrate monohydrate, Na2Sn(C6H4O7)ؒH2O
2.11 Solid SnCl2 (9.50 g, 50.1 mmol) was dissolved in deoxygen-
ated water (25 cm3). To this solution was slowly added 2 
NaOH (50 cm3) whilst purging the flask with N2. A dense white
precipitate of hydrous tin() oxide was formed. This solid was
rapidly filtered off, washed with water and immediately resus-
pended in deoxygenated water (25 cm3). Anhydrous citric acid
(9.60 g, 50.0 mmol) was added to this suspension and the mix-
ture allowed to stir at room temperature for 10 min. Further 2 
NaOH (50 cm3) was added dropwise to the reaction mixture
until the slurry dispersed and a clear solution was formed. This
solution was evaporated to dryness on a rotary evaporator
yielding a colourless glass. The resulting solid was dispersed in
PriOH (20 cm3), filtered and dried in vacuo. Yield: 11.9 g (96%)
[Found (Calc. for C6H6Na2O8Sn): C, 19.9 (19.4); H, 1.90 (1.62);
Na, 13.2 (12.4); SnII, 27.3 (32.0)%]. Selected infrared data
(cmϪ1): 3393s (br), ν(H᎐O᎐H); 1635m (sh), δ(H᎐O᎐H); 1593s
(br), νasym(CO2Sn); 500–575 (br), ν(Sn᎐O). 1H NMR: δ 2.74 (q,
4 H, J = 15.3 Hz, CH2). 13C NMR: δ 46.7 (CH2), 76.1 (tertiary
C), 177.2 (CH2CO2Sn) and 183.7 (R2CO2Sn).
On the basis of the above, it seems reasonable that the
monotin() species 2–5 all contain tin bound via the α-hydroxy
and central CO2 groups [i.s. 2.80–3.06, q.s. 1.81–1.91 mm sϪ1; δ
(
119Sn) Ϫ578.3 to Ϫ582.9] in which the tin-bound carboxylate
group is essentially monodentate [νasym(CO2Sn) 1593–1616
cmϪ1]. The reaction of salicyclic acid and Sn(OMe)2 is known to
generate a product containing a related six-membered SnO2C2O
heterocycle.28 Thus, by analogy, Sn2(cit) 1 also contains one tin
in this environment, while the second tin is bound to the two
bidentate methylene carboxylate groups [νasym(CO2Sn) 1553 (br)
cmϪ1], presumably intermolecularly given the low solubility of
the species. The two CH2CO2 moieties appear non-equivalent in
the 13C cross polarisation magic angle spinning (CP MAS)
NMR spectrum of 1 (δ 48.8, 52.9) though the two tin environ-
ments must, however, be very similar as they only give rise to
line broadening in both the Mössbauer and 119Sn NMR spectra,
rather than resolvable signals for each metal environment. It
may be that the bonding in 1 is not as homogeneous as we have
so far implied.
The heterobimetallic citrate 6, which is formed in a stepwise
manner from 4, should contain tin in a similar environment to
that present in 7 (q.s. 1.92 mm sϪ1), while parallel comments can
be made about the zinc co-ordination as for the second tin site
in 1 [νasym(CO2M): 1567v (br) cmϪ1]. No structural data are
available for Zn2(cit) (a component of many contemporary
toothpastes) due to its insolubility, but the structure of
Zn(H2O)3(Hicit) (Hicit = 1-isopropyl citrate) has been deter-
mined.29 In this species an octahedral ZnO6 core is made up of
ligation to the metal to three water molecules, the α-OH, the
central and one terminal CO2 groups.
Hydrated dipotassium tin(II) citrate, K2Sn(C6H4O7)ؒ1.5H2O 3.
The preparation of compound 3 followed exactly the same pro-
cedure as for 2 with the exception that 2  KOH solution was
used in place of the 2  NaOH. Yield: 91% [Found (Calc. for
C6H7K2O8.5Sn): C, 17.6 (17.5); H, 1.73 (1.70); K, 17.9 (19.0);
SnII, 26.3 (28.8)%]. Selected infrared data (cmϪ1): 3373s (br),
ν(H᎐O᎐H); 1650m (br), δ(H᎐O᎐H); 1599s (br), νasym(CO2Sn);
480–500m (br), ν(Sn᎐O). 1H NMR data: δ 2.78 (q, 4 H, J = 15.3
Hz, CH2). 13C NMR data: δ 47.2 (CH2), 76.6 (tertiary C), 177.2
(CH2CO2Sn) and 184.4 (R2CO2Sn).
Hydrated diammonium tin(II) citrate, (NH4)2Sn(C6H4O7)ؒ
0.5H2O 4. The preparation of compound 4 followed exactly the
same procedure as for 2 with the exception that 2  NH4OH
solution was used in place of the 2  NaOH. Yield: 90% [Found
(Calc. for C6H13N2O7.5Sn): C, 21.3 (20.5); H, 4.28 (3.73); N, 8.17
(7.95)%]. Selected infrared data (cmϪ1): 3192w (br), ν(H᎐O᎐H);
1684w (br), δ(H᎐O᎐H); 1599s (br), νasym(CO2Sn); 500–570 (br),
ν(Sn᎐O). 1H NMR data: δ 2.72 (q, 4 H, J = 15.4 Hz, CH2) and
7.13 (br s, 8 H, NH4). 13C NMR: δ 46.3 (CH2), 75.6 (tertiary C),
176.7 (CH2CO2Sn) and 183.7 (R2CO2Sn).
Experimental
Spectra were recorded on the following instruments: JEOL
GX270 (1H, 13C NMR), GX400 (119Sn NMR), Perkin-Elmer
599B (IR). Details of our Mössbauer spectrometer and
related procedures are given elsewhere.30 For all compounds,
J. Chem. Soc., Dalton Trans., 1997, Pages 3705–3712
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Tetramethylammonium tin(II) citrate methanol (1/2), (NMe4)-
HSn(C6H4O7)ؒ2MeOH 5. Solid SnCl2 (4.75 g, 25.1 mmol) was
dissolved in deoxygenated water (15 cm3). To this clear solution
2  NaOH (25 cm3) was added dropwise whilst continuously
purging the reaction flask with N2. A dense white precipitate of
hydrous tin() oxide was formed. This solid was rapidly filtered
on a Schlenk-stick, washed with water and immediately resus-
pended in deoxygenated water (15 cm3). Anhydrous citric acid
(4.80 g, 25.0 mmol) was added and the mixture allowed to stir at
room temperature for 10 min. Tetramethylammonium hydrox-
ide pentahydrate (9.15 g, 51.0 mmol) was dissolved in
deoxygenated water (25 cm3) and the fresh solution was added
dropwise to the reaction mixture until the slurry had dispersed
and a completely clear solution had formed. This solution was
evaporated to dryness on a rotary evaporator yielding a colour-
less, hygroscopic oil which was dissolved in methanol (20 cm3)
and allowed to precipitate overnight in a refrigerator. The
product was filtered on a Schlenk-stick, washed with cold ether
and dried in vacuo. Yield: 7.4 g (66%) [Found (Calc. for C12-
H26NO9Sn: C, 32.5 (32.3); H, 5.61 (5.61); N, 3.57 (3.10); SnII,
25.4 (26.7)%]. Selected infrared data (cmϪ1): 1616s (br),
room temperature for 72 h. After this time the reaction mixture
was slowly poured into saturated sodium hydrogencarbonate
solution (100 cm3) and extracted with ether. This procedure was
repeated twice (2 × 150 cm3) and the combined ether extracts
evaporated to dryness on a rotary evaporator. Light petroleum
(b.p. 60–80 ЊC) (150 cm3) was added to the residue and the oil
dissolved with rapid stirring. The by-product of the reaction
(trimethyl citrate) readily crystallises from this solution and can
be efficiently removed by filtration. The clear filtrate was sub-
sequently evaporated to yield the desired product as a colour-
less viscous oil. Yield: 5.4 g (78%) [Found (Calc. for C12H20O7):
C, 53.8 (52.2); H, 7.92 (7.30)%]. Selected infrared data (liquid
film; cmϪ1): 3489s (sh), ν(C᎐O᎐H); 1757s (sh), νasym(CO2But),
1
1728s (sh), νasym(CO2Me). H NMR (CDCl3): δ 1.51 [s, 9 H,
(CH3)3C], 2.82 (q, 4 H, J = 15.4 Hz, CH2) and 3.69 (s, 6 H,
OCH3). 13C NMR: δ 27.5 [(CH3)3C], 43.2 (CH2), 51.6 (OCH3),
72.9 (tertiary C), 83.1 [(CH3)3C], 169.9 (CH2CO2CH3) and
172.1 (R2CO2But).
3-tert-Butyl citrate, (HO)C[CO2But](CH2CO2H)2 III.16 3-
tert-Butyl-1,5-dimethyl citrate II (5.00 g, 18.1 mmol) was dis-
solved in methanol (20 cm
1
νasym(CO2Sn); 567w (br), ν(Sn᎐O). H NMR data: δ 2.73 (q, 4
3). 2  Sodium hydroxide (20 cm3)
H, J = 15.1 Hz, CH2), 3.10 (s, 12 H, NCH3) and 3.26 (s, 6 H,
CH3OH). 13C NMR: δ 46.1 (CH2), 48.4 (CH3OH), 54.5, 54.7,
54.8 (NCH3), 76.1 (tertiary C), 176.0 (CH2CO2Sn) and 183.4
(R2CO2Sn).
was slowly added to this solution and the mixture stirred at
room temperature for 3 h. The solvent volume was then reduced
by 50% on a rotary evaporator and the resulting aqueous solu-
tion acidified to pH 3 with concentrated hydrochloric acid. The
product was extracted with ethyl acetate (3 × 100 cm3), the
extracts combined and the solvent removed in vacuo yielding a
white solid product. The crude material was recrystallised from
boiling ethyl acetate–hexane yielding a white, crystalline solid.
Hydrated tin(II) zinc citrate, ZnSn(C6H4O7)ؒH2O 6. Diam-
monium tin() citrate 4 (2.50 g, 7.3 mmol) was dissolved in
deoxygenated water (25 cm3), solid ZnCl2 (0.99 g, 7.3 mmol)
added and the mixture refluxed under N2 for 12 h. After this
time a dense white precipitate had formed around the inner
surface of the flask. This solid was carefully suspended in the
supernatant liquid, the resulting suspension allowed to cool to
room temperature and methanol (5 cm3) added to assist the pre-
cipitation. The product was filtered on a Schlenk-stick, washed
with successive aliquots of water, ethanol and ether and thor-
oughly dried in vacuo. Yield: 2.1 g (77%) [Found (Calc. for
C6H4O7SnZn): C, 19.9 (19.4); H, 1.61 (1.08); SnII, 28.3 (31.8);
Zn, 14.2 (17.6)%]. Selected infrared data (cmϪ1): 3420w (br),
ν(H᎐O᎐H); 1556–1579s, νasym(CO2M); 500–557m (br), ν(M᎐O)
(M = Sn or Zn). The NMR spectra were not recorded due to the
poor solubility of the compound in common solvents.
Yield: 1.8 g (41%) [Found (Calc. for C10
H16O7): C, 48.2 (48.4);
H, 6.50 (6.57)%]. M.p. 124 ЊC (lit.,16
124 ЊC). Selected infrared
data (cmϪ1): 3485m (sh), ν(O᎐H); 1755s (sh), νasym(CO2But);
1705s (sh), νasym(CO2H). 1H NMR (CDCl3): δ 1.41 (s, 9 H, CH3)
and 2.84 (q, 4 H, J = 15.1 Hz, CH2). 13C NMR: δ 26.5
[(CH3
) C], 43.0 (CH ), 73.0 (tertiary C), 84.0 [(CH ) C], 172.9
3
2
3 3
(R2CO2But) and 173.1 (CH2CO2H).
Tin(II) 1,5-dimethyl citrate methanol (1/1), C[(O)CO2Sn](C-
H2CO2Me)2ؒMeOH 7. 1,5-Dimethyl citrate I (250.0 mg, 1.1
mmol) was dissolved in deoxygenated, dry distilled methanol
(25 cm3). Tin() acetate (249.0 mg, 1.1 mmol) was carefully
added with the exclusion of atmospheric oxygen. The reaction
mixture was refluxed for 2 h under a nitrogen atmosphere. After
this time a turbid off-white suspension remained which was
filtered hot on a Schlenk-stick. The clear filtrate was allowed to
cool slowly to room temperature where, upon standing, a mass
of small colourless crystals precipitated which were filtered on a
Schlenk-stick and washed with a little ice-cold ether. Yield: 0.36
g (89%) [Found (Calc. for C9H14O8Sn): C, 27.6 (29.3); H, 3.53
(3.83)%]. Selected infrared data (cmϪ1): 1734s (sh), νasym(CO2-
1,5-Dimethyl citrate monohydrate, (HO)C(CO2H)(CH2CO2-
Me)2ؒH2O I.15 Anhydrous citric acid (50.0 g, 260.4 mmol) was
dissolved in methanol (250 cm3) together with 98% sulfuric acid
(2.0 g). The mixture was refluxed for 1 h then allowed to cool to
room temperature and cold water (250 cm3) added with stirring.
The solution was neutralised with solid calcium carbonate to
remove the excess of citric and sulfuric acids as their insoluble
calcium salts. The suspension was filtered and the filtrate evap-
orated to dryness in vacuo. This yielded a colourless glass which
was redissolved in cold water (100 cm3), filtered to remove
traces of insoluble salts and the filtrate acidified to pH 4.5 with
concentrated hydrochloric acid. The resulting white precipitate
was filtered off, recrystallised twice from water and air-dried.
Yield: 18.0 g (29%) [Found (Calc. for C8H14O8): C, 40.6 (40.3);
H, 5.97 (5.88)%]. M.p. 122 ЊC (lit.,15 125 ЊC). Selected infrared
data (cmϪ1): 3420s (sh), ν(H᎐O᎐H); 1742s (sh), νasym(CO2H);
1717s (sh), νasym(Co2Me); 1638m (br), δ(H᎐O᎐H); 1232m (sh),
1
Me); 1561s (br), νasym(CO2Sn); 569m (sh), ν(Sn᎐O. H NMR
[(CD3)2SO]: δ 2.74 (q, 4 H, J = 15.3 Hz, CH2), 3.38 (s, 3 H,
CH3OH) and 3.55 (s, 6 H, OCH3). 13C NMR: δ 45.2 (CH2), 48.9
(CH3OH), 51.3 (OCH3), 75.7 (tertiary C), 172.0 (CH2CO2Me)
and 182.0 (R2CO2Sn).
Attempted synthesis of tin(II) bis(3-tert-butyl 1,5-dimethyl cit-
rate). 3-tert-Butyl 1,5-dimethyl citrate II (0.71 g, 2.6 mmol) was
dissolved in freshly distilled toluene (10 cm3) and added to
tin() bis(methoxide) (0.23 g, 1.3 mmol). The resulting suspen-
sion was gently refluxed for 2 h yielding a slightly turbid solu-
tion. This was filtered hot on a Schlenk-stick and the clear
filtrate allowed to stand overnight at room temperature. A
mass of small, colourless crystals was formed which were fil-
tered off and washed with a little ice-cold ethanol and ether.
Yield: 0.11 g (12.7%). The product was tentatively identified as
Sn6O4(OMe)3OH [Found (Calc. for C3H10O9Sn6): C, 3.7 (4.1);
H, 1.04 (1.14)%]. Selected infrared data (cmϪ1): 547s (br),
ν(Sn᎐O).
1
νsym(CO2Me). H NMR (CDCl3): δ 2.87 (q, 4 H, J = 15.4 Hz,
CH2), 3.66 (s, 6 H, OCH3) and 5.00 (br s, 2 H, H2O). 13C NMR:
δ 43.9, 44.2 (CH2), 52.2, 53.2 (OCH3), 74.2 (tertiary C), 171.7,
171.8 (CH2CO2CH3) and 176.4 (R2CO2H).
3-tert-Butyl 1,5-dimethyl citrate, (HO)C[CO2But](CH2CO2-
Me)2 II.16 1,5-Dimethyl citrate I (6.00 g, 25.2 mmol) was sus-
pended in tert-butyl acetate (50 cm3) and 60% perchloric acid
(1.0 cm3) solution carefully added. The mixture was stirred at
3710
J. Chem. Soc., Dalton Trans., 1997, Pages 3705–3712

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Attempted synthesis of tin(II) 3-tert-butyl citrate. Tin()
methoxide (0.15 g, 0.83 mmol) was dissolved in deoxygenated n-
butanol (25 cm3) with gentle warming. Solid 3-tert-butyl citrate
III (0.21 g, 0.83 mmol) was added with stirring and the reaction
mixture refluxed for 30 min. The suspension was allowed to
cool to room temperature, the solid collected on a Schlenk-
stick, washed with successive aliquots of ethanol and ether
dried in vacuo. Yield: 0.09 g (44%). The product was identified
as Sn2(cit) 1 (Found: C, 16.9; H, 1.24%). Selected infrared data
(cmϪ1): 1553s (br), νasym(CO2Sn), 494m (br), ν(Sn᎐O).
X-Ray crystallography
Compound 7. A crystal of approximate dimensions 0.2 ×
0.2 × 0.2 mm was mounted in a Lindemann capillary with some
methanol and used for data collection.
Crystal data. C9H14O8Sn, M = 368.9, monoclinic, space
group P21/c, a = 9.793(6), b = 10.416(5), c = 12.849(6) Å, β =
104.21(4)Њ, U = 1270.6 Å3, Z = 4, Dc = 1.92 g cmϪ3, µ(Mo-
Kα) = 20.4 cmϪ1, F(000) = 728.
Data were measured at room temperature on a CAD4 auto-
matic four-circle diffractometer in the range 2 р θ р 22Њ. 1776
Reflections were collected of which 1261 were unique with
I у 3σ(I). Data were corrected for Lorentz-polarization effects
and also for absorption32 (maximum and minimum absorption
corrections 1.148 and 0.850). The structure was solved by Pat-
terson methods and refined using the SHELX33,34 suite of pro-
grams. In the final least-squares cycles all atoms were refined
anisotropically. Hydrogen atoms were included at calculated
positions (C᎐H 0.96 Å) where relevant, except in the case of the
methanolic proton [H(8)] which was located in the penultimate
Fourier-difference map and refined. All hydrogen atoms were
given a common isotropic thermal parameter (0.093 Å2).
Final residuals after 10 cycles of least squares were R = 0.0245,
RЈ = 0.0269, for a weighting scheme of w = 3.8581/[σ2(F) ϩ
0.000 270(F)2]. Maximum final shift/e.s.d. was less than 0.0005.
The maximum and minimum residual electron densities were
0.32 and Ϫ0.34 e ÅϪ3, respectively, in the region of the tin atom
and as such have no chemical significance.
Tin(II) 1,5-di-n-butyl citrate C[(O)CO2Sn](CH2CO2Bun)2ؒ
H2O 8. Tin() methoxide (0.38 g, 2.1 mmol) was dissolved in
deoxygenated n-butanol (25 cm3) with gentle warming. 1,5-
Dimethyl citrate I (0.50 g, 2.1 mmol) was carefully added to the
solution of soluble tin() n-butoxide and the reaction mixture
gently refluxed for 30 min under an atmosphere of N2. The
resulting colourless solution was allowed to cool slowly to room
temperature whereupon a fine, white precipitate was formed.
The product was filtered on a Schlenk-stick and washed with
ice-cold ether. Yield: 0.64
g (72%) [Found (Calc. for
C14H24O8Sn): C, 38.8 (38.3); H, 5.16 (5.52)%]. Selected infrared
data (cmϪ1): 3433, ν(H2O); 1740s (sh), νasym(CO2Bun); 1543s
(br), νasym(CO2Sn), 1365–1377m, νsym(CO); 523m (br),
ν(Sn᎐O). 1H NMR (CDCl3): δ 0.91 (t, 6 H, J = 6.7, CH3), 1.36–
1.58 [m, 8 H, CH3(CH2)2], 2.88 (q, 4 H, J = 15.3, CH2) and 4.10
(t, 4 H, J = 6.7 Hz, OCH2). 13C NMR: δ 13.1 (CH3), 19.0 (CH2),
30.5 (CH2), 43.6 (CH2CO2), 65.5 (OCH2), 7.57 (tertiary C),
173.0 (CH2CO2Bun) and 180.6 (R2CO2Sn).
Compound 10. A crystal of approximate dimensions 0.5 ×
0.5 × 0.2 mm was used for data collection.
Crystal data. C40H82N4O35Sn2, M = 706.2, triclinic, space
Hydrated bis(tetramethylammonium) bis(citrato)stannate(IV),
[NMe4]2[Sn{C(O)(CO2)(CH2CO2)(CH2CO2H)}2]ؒ3.5H2O 10. A
dilute aqueous solution of tetramethylammonium tin()
citrate-methanol(1/2) 5 was allowed to stand in an open conical
flask. After several weeks the clear solution had evaporated to
a colourless, viscous oil from which a crop of crystals formed
on prolonged standing. These crystals could not be separated
from the supernatant oil by filtration. A small number of crys-
tals were removed by hand and carefully washed with a little
ether [Found (Calc. for C20H41N2O17.5Sn): C, 33.8 (33.9); H,
6.02 (5.85); N, 4.0 (4.0)%]. Selected infrared data (cmϪ1):
¯
group P1 (no. 2), a = 12.610(2), b = 15.571(5), c = 15.967(9) Å,
α = 83.24(19), β = 76.70(5), γ = 75.91(4)Њ, U = 2952.9 Å3, Z = 4,
Dc = 1.59 g cmϪ3, µ(Mo-Kα) = 9.4 cmϪ1, F(000) = 1460.
Data were measured on the FAST system at the EPSRC
X-ray crystallographic service (University of Wales, Cardiff).
119 88 Reflections were collected over the accessible reciprocal
sphere of which 4536 were unique with I у 2σ(I). Data were
corrected for Lorentz-polarisation but not for absorption. The
structure was solved and refined as above. In the final least-
squares cycles the tin atoms were refined anisotropically. All
other atoms were treated isotropically. Refinement was con-
ducted in three blocks: one block for each of the cations, the
anions and the water molecules, respectively. Hydrogen atoms
were not included. Of the oxygen atoms, O(5Ј) and O(8Ј) were
included at half occupancies. Final residuals after 44 cycles of
blocked-matrix least squares were R = RЈ = 0.0513. The max-
imum and minimum residual electron densities were 0.43 and
1
1727m (sh), νasym(CO2H); 1644s (br), νasym(CO2Sn). H NMR
(CD3OD): δ 2.80 (m, 4 H, CH2) and 3.19 [s, 12 H, N(CH3)4].
13C NMR: δ 56.5 [N(CH3)4], 47.1, 48.8 (CH2), 76.4 (tertiary
C), 176.0 (CH2CO2H), 178.2 (CH2CO2Sn) and 183.8
(R2CO2Sn).
Tin(II) laurate, Sn[O2C(CH2)10Me]2 11. The compound
Sn(OMe)2 (1.0 g, 5.53 mmol) was suspended in freshly distilled
tetrahydrofuran (thf) (50 cm3) under an atmosphere of dinitro-
gen. A solution of lauric acid (2.21 g, 11.06 mmol) also in thf
(20 cm3) was added dropwise and the mixture refluxed for 30
min to yield a colourless solution. The solvent was evaporated
in vacuo to afford the required compound in quantitative yield
and analytically pure form [Found (Calc. for C24H46O4Sn): C,
Ϫ0.62 e ÅϪ3
.
Despite a reasonably acceptable R factor, this structure did
not converge well. Attempted anisotropic refinement resulted in
unsatisfactory thermal parameters for many atoms. Similarly,
efforts to apply a weighting scheme either before or after an
empirical absorption correction proved fruitless and shift/e.s.d.
values remained high throughout refinement. Sample quality
may account for some of the above but the very large error of
0.19Њ in α may also have introduced some systematic error into
the data set.
1
55.7 (55.5); H, 8.90 (9.21)%]. H NMR (C6D6): δ 0.91 (s, 6 H,
CH3), 1.28 [s, 32 H, (CH2)8], 1.64 (s, 4 H, CH2CH3) and 2.27
(s, 4 H, CH2CO2). 13C NMR: δ 14.2 (CH3), 23.1, 25.6, 29.6,
29.7, 30.0, 30.1, 32.3, 36.4 (CH2) and 182.2 (CO2Sn).
CCDC reference number 186/676.
Also prepared by the same method were: tin() palmitate,
Sn[O2C(CH2)14Me]2 12 [Found (Calc. for C32H62O4Sn): C, 61.1
(61.0); H, 9.86 (10.20)%]; 1H NMR (C6D6) δ 0.91 (s, 6 H, CH3),
1.32 [s, 48 H, (CH2)12], 1.65 (s, 4 H, CH2CH3) and 2.27 (s, 4 H,
CH2CO2); 13C NMR: δ 14.2 (CH3), 23.0, 25.6, 29.6, 29.7, 29.9,
30.1, 32.3, 36.3 (CH2) and 182.0 (Co2Sn); tin() sterate,
Sn[O2C(CH2)16Me]2 13 [Found (Calc. for C36H70O4Sn): C, 63.1
Acknowledgements
We thank Unilever plc for financial support and for supplying
CP MAS data for Sn2(cit), and Professor M. B. Hursthouse
(Cardiff) and the EPSRC X-ray diffraction service for collec-
tion of the diffraction data for compound 10.
1
(63.0); H, 10.22 (10.50)%]; H NMR (C6D6): δ 0.90 (s, 6 H,
References
CH3), 1.34 [s, 56 H, (CH2)14], 1.63 (s, 4 H, CH2CH3) and 2.25
(s, 4 H, CH2CO2); 13C NMR: δ 14.2 (CH3), 22.9, 25.5, 29.6,
29.7, 29.9, 30.1, 32.3, 35.9 (CH2) and 182.1 (CO2Sn).
1 L. Stryer, Biochemistry, W. H. Freeman and Co., New York, 4th
edn., 1995, p. 509.
J. Chem. Soc., Dalton Trans., 1997, Pages 3705–3712
3711

View Article Online
2 P. J. Sadler and H. Sun, J. Chem. Soc., Dalton Trans., 1995,1395.
3 G. Smith, D. S. Sagatys, R. C. Bott and D. E. Lynch, Polyhedron,
1993, 12, 1491.
4 D. E. Zacharias and J. P. Glusker, Acta Crystallogr., Sect. C, 1993,
49, 1727.
5 A. A. Sheikh-Osman, R. Bertani, A. Tapparo, G. G. Bombi and
B. Corain, J. Chem. Soc., Dalton Trans., 1993, 3229.
6 D. S. Sagatys, G. Smith, R. C. Bott and D. E. Lynch, Polyhedron,
1993, 12, 709.
7 J. C. Sherlock and S. C. Britton, Br. Corros. J., 1972, 7, 180.
8 A. R. Willey, Br. Corros. J., 1972, 7, 29.
9 D. W. Gruenwedel and H.-S. Hao, J. Agr. Food Chem., 1973, 21, 246.
10 G. Arena, A. Contino, S. Musumeci and R. Purrello, J. Chem. Soc.,
Dalton Trans., 1990, 3383.
11 M. M. Besso, US Pat., 3 213 120, 1965.
12 T. D. Smith, J. Chem. Soc., 1965, 2145.
20 T. Fjeldberg, P. B. Hitchcock, M. F. Lappert, S. J. Smith and
A. J. Thorne, J. Chem. Soc., Chem. Commun., 1985, 939.
21 P. G. Harrison and E. W. Thornton, J. Chem. Soc., Dalton Trans.,
1978, 1274.
22 N. W. Alcock and S. M. Roe, J. Chem. Soc., Dalton Trans., 1989,
1589.
23 H. Reuter and D. Schroder, Acta Crystallogr., Sect. C, 1992, 48,
1112.
24 H. L. Carrell and J. P. Glusker, Acta Crystallogr., Sect. B, 1973, 29,
638.
25 C. K. Johnson, Acta Crystallogr., 1965, 18, 1004.
26 T. Birchall and J. P. Johnson, Inorg. Chem., 1982, 21, 3724.
27 J. D. Donaldson and A. Jelen, J. Chem. Soc. A, 1968, 1448.
28 W. D. Honnick and J. J. Zuckerman, Inorg. Chem., 1978, 17, 501.
29 S. Tsuji, T. Shibata, Y. Ito, S. Fujii and K. Tomita, Acta Crystallogr.,
Sect. C, 1991, 47, 528.
13 V. I. Korsunsky, P. G. Antonov and T. P. Lutsko, Polyhedron, 1992,
11, 1403.
14 N. Tinanoff, J. Clin. Dentistry, 1990, 2, 22.
15 G. Schroeter and L. Shultz, Ber. Deutsch. Chem. Ges., 1902, 35,
2085.
16 M. J. Milewska, A. Chimiak and Z. Glowacki, J. Prakt. Chem.,
1987, 329, 447.
30 K. C. Molloy, T. G. Purcell, K. Quill and I. W. Nowell,
J. Organomet. Chem., 1984, 267, 237.
31 U. S. Pharmacopeia, The US Pharmacopeial Convention, Inc.,
Rockville, MD, Assay for Stanous Ion, 1990, USP XXII, 1274.
32 N. Walker and D. Stewart, Acta Crystallogr., Sect. A, 1983, 39, 158.
33 G. M. Sheldrick, SHELX 76, A program for crystal structure
determination, University of Cambridge, 1976.
17 R. Gsell and M. Zeldin, J. Inorg. Nucl. Chem., 1975, 37, 1133.
18 P. G. Harrison, B. J. Haylett and T. J. King, J. Chem. Soc., Chem.
Commun., 1978, 112.
34 G. M. Sheldrick, SHELXS 86, A program for crystal structure
determination, University of Göttingen, 1986.
19 D. W. Hartley, G. Smith, D. S. Sagatys and C. H. L. Kennard,
J. Chem. Soc., Dalton Trans., 1991, 2735.
Received 26th June 1997; Paper 7/04511E
3712
J. Chem. Soc., Dalton Trans., 1997, Pages 3705–3712