Synthesis, characterisation and solution behaviour of fluoroalkyl phosphoryl complexes of tin tetrachloride

Article abstract of DOI:10.1016/j.ica.2009.04.035

The synthesis, characterisation and solution behaviour of a series of octahedral complexes SnCl₄·2L (L = R₂NP(O)(OCH₂CF₃)₂; R = Me (1); Et (2) or L = P(O)(OCH₂R_f)₃; R<

Full text of DOI:10.1016/j.ica.2009.04.035

Inorganica Chimica Acta 362 (2009) 3763–3768
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, characterisation and solution behaviour of fluoroalkyl phosphoryl
complexes of tin tetrachloride
*
Med Abderrahmane K. Sanhoury , Med Taieb Ben Dhia, Med Rachid Khaddar
Laboratory of Coordination Chemistry, Department of Chemistry, Faculty of Sciences of Tunis, University of Tunis El Manar, 2092 Tunis, Tunisia
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis, characterisation and solution behaviour of a series of octahedral complexes SnCl4ꢀ2L
(L = R2NP(O)(OCH2CF3)2; R = Me (1); Et (2) or L = P(O)(OCH2Rf)3; Rf = CF3 (3); C2F5 (4)) are described. Com-
plexes 1–4 were prepared from SnCl4 and 2 equiv. of the ligand, L, in anhydrous CH2Cl2 solution. The
adducts have been characterised by multinuclear (1H, 31P and 119Sn) NMR, IR spectroscopy and elemental
analysis. In dichloromethane solution, the NMR data showed the presence of a mixture of cis and trans
isomers for 1 and 2 and only the cis isomer for 3 and 4. The difference could be interpreted in terms of
the electronic effects of the substituents on the phosphorus atom of the ligand. In addition, the solution
structure of the complexes studied by variable temperature 31P–{1H} and 1H NMR in the presence of
excess ligand indicated that the ligand exchange on the cis isomer dominates the chemistry. The
metal–ligand exchange barriers were estimated to be 13.38 and 11.39 kcal/mol for 1 and 3, respectively.
The results are discussed and compared with those previously reported for the related hexamethylphos-
phoramide adduct, SnCl4ꢀ2HMPA.
Received 5 March 2009
Received in revised form 19 April 2009
Accepted 21 April 2009
Available online 5 May 2009
Keywords:
Fluoroalkyl phosphoryl ligands
Tin tetrachloride
Ligand exchange
119Sn NMR
31P NMR
2J(119Sn–31P) coupling constant
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
the nature of the substituents on the phosphorus atom of the li-
gand prompted us to investigate the solution structure of tin com-
Tin tetrachloride is a strong Lewis acid forming adducts with a
variety of neutral ligands [1–6]. For instance, phosphoramides bind
strongly to this Lewis acid and form hexacoordinate, 2:1, cis- or
trans-complexes, both of which have been characterised in solu-
tion and in the solid state [7,8]. For example, the hexamethylphos-
phoramide complex, SnCl4ꢀ2HMPA, has been found to exist as a
trans-adduct in the solid by X-ray diffraction [9] and in solution
as a mixture of cis and trans isomers by IR and NMR spectroscopy
[10,11], whereas the complex with trimethylphosphate (TMPA) is
predominantly a cis-adduct [10]. The nature of the substituents
on the phosphorus atom of the ligand may be held responsible
for the differences observed [7,12,13]. Recently, we have studied ti-
n(IV) complexes containing the ligands (R2N)2P(O)F and R2NP(O)F2
by multinuclear NMR in solution and showed that whilst the cis
isomer predominates with the former, the latter ligand forms al-
most exclusively the cis-complex [14,15]. In a more recent work,
we have also shown that when the fluorine atom in the ligand
(R2N)2P(O)F was substituted by a fluoroalkoxy (OCH2CF3) group
the isomer distribution in the adduct formed with SnCl4 reversed
and the trans-complex becomes the major isomer observed in
solution [16]. The pronounced dependence of stereochemistry on
plexes with phosphoryl ligands containing more than one
fluoroalkoxy group.
Here we report the synthesis and characterisation of complexes
of SnCl4 with four other ligands R2NP(O)(OCH2CF3)2 (R = Me or Et)
and P(O)(OCH2Rf)3 (Rf = CF3 or C2F5), and the variable temperature
NMR study of dichloromethane solutions of the adducts in the
presence of excess ligand. The phosphoryl ligands used show
rather different behaviour to the related HMPA complex [11] and
to each other.
2. Experimental
2.1. General experimental procedures
All preparations were carried out under a nitrogen atmosphere
in solvents dried by standard techniques [17] and stored over
molecular sieves. NMR spectra were recorded on a Bruker AC-
300 spectrometer in CD2Cl2 solution; 1H at 300 MHz (TMS), 31P
at 121 MHz (85% H3PO4) and 119Sn at 111.8 MHz (SnCl4). 119Sn
NMR spectra were recorded in 10 mm NMR tubes containing 15–
20% deuterated solvent. IR spectra were measured on a Perkin–El-
mer Paragon 1000 PC spectrometer.
Tin tetrachloride (Fluka) was used as received. The ligands
R2NP(O)(OCH2CF3)2 [18] and P(O)(OCH2Rf)3 [19] were prepared
as described in the literature.
* Corresponding author. Tel.: +216 98269495.
E-mail address: senhourry@yahoo.com (M.A.K. Sanhoury).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.04.035

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2.2. [SnCl4ꢀ2(O)P(OCH2CF3)2NR2]
At room temperature, the 31P–{1H} NMR spectra of complexes 3
and 4 showed only one signal, whilst two resonances were ob-
served for 1 and 2; the high frequency resonance was flanked with
Sn satellites (Fig. 1). These signals are shifted to lower frequency
compared with that of the free ligand. Such a behaviour is inter-
preted in terms of inductive effects resulting from a decrease in
the electron density at phosphorus upon coordination of the phos-
phoryl oxygen of the ligand to the tin atom.
A
solution of R2NP(O)(OCH2CF3)2 (4 mmol) in dry CH2Cl2
(10 cm3) was slowly added to SnCl4 (0.52 g, 2 mmol) in CH2Cl2
(20 cm3) and the mixture stirred under N2 for ca. 2 h. The volatiles
were then removed in vacuo and the white solid complex SnCl4ꢀ2L
rinsed with hexane and dried in vacuo. Yields R = Me (1): 1.20 g,
71% R = Et (2): 1.21 g, 68%. Anal. Calc. for C12H20Cl4F12N2O6P2Sn
(1): C, 17.18; H, 2.40; N, 3.34%. Found: C, 17.36; H, 2.85; N,
3.22%. Anal. Calc. for C16H28Cl4F12N2O6P2Sn (2): C, 21.48; H, 3.15;
N, 3.13%. Found: C, 21.41; H, 3.40; N, 3.21%. IR (KBr): mP@O
(1:1223 cmꢁ1, 2:1227 cmꢁ1); mSnAO (1:506 cmꢁ1, 2:512 cmꢁ1).
In the low temperature (228 K) 31P–{1H} NMR spectra, the two
resonances observed for 1 and 2 and the single resonance observed
for 3 and 4 all displayed signals flanked with Sn satellites.
The corresponding 119Sn NMR spectra exhibited at room tem-
perature a broad signal and a lower frequency triplet for 1 and 2
and only a broad signal for 3 and 4, in the region of hexacoordinat-
ed tin species [20]. The broad signals were converted at low tem-
perature into the expected triplets. The spectra showed therefore
at 248 K two triplet signals for each of 1 and 2 (Fig. 2) and at
228 K one triplet for each of 3 and 4. The triplet related to the latter
adduct is relatively broad, even at 208 K, due to its poor solubility
in CH2Cl2 which prevented lower temperature studies.
2.3. [SnCl4ꢀ2(O)P(OCH2Rf)3]
Method as above, but using P(O)(OCH2Rf)3. White solids Rf = CF3
(3) (63%); Rf = C2F5 (4) (61%). Anal. Calc. for C12H12Cl4F18O8P2Sn (3):
C, 15.19; H, 1.27%. Found: C, 14.78; H, 1.88%. Anal. Calc. for
C18H12Cl4F30O8P2Sn (4): C, 17.31; H, 0.97%. Found: C, 16.93; H,
1.40%. IR (KBr): mP@O (3:1246 cmꢁ1
(3:534 cmꢁ1, 4:540 cmꢁ1).
,
4:1258 cmꢁ1); mSnAO
The triplet feature observed in the 119Sn NMR spectra is due to
119Sn–31P coupling. Clearly two species (isomers) were present at
low temperature for 1 and 2 and only one species for 3 and 4,
and in each species the tin atom is coupled to two phosphorus
atoms, showing a stoichiometry of SnCl4ꢀ2L. This is in agreement
with the 31P NMR spectra where signals displaying Sn satellites
with the corresponding coupling constants were observed.
On the basis of the above NMR data and the previously reported
studies [2,10,11], it is possible to assign the complexes in Table 1
that have higher 2J(31P–119Sn) coupling constants to the trans ad-
ducts and those with smaller 2J(31P–119Sn) as the cis isomers,
whilst the only species observed for 3 and 4 could be assigned to
the cis-adducts.
The NMR data suggest therefore that, in solution, complexes 1
and 2 exist as a mixture of cis and trans isomers, whilst adducts
3 and 4 exist only as cis complexes. This, coupled with the fact that
the latter adducts show some ligand dissociation in dichlorometh-
ane solution, indicates that complexes 3 and 4 are less stable in
solution than 1 and 2 mainly due to exchange reactions resulting
from the weaker basicity of ligands in 3 and 4 as compared to that
in 1 and 2. The results are also consistent with the trends observed
for related phosphine and arsine oxide complexes [21].
3. Results and discussion
3.1. Synthesis
The reaction of SnCl4 with the ligands (L = R2NP(O)(OCH2CF3)2
or P(O)(OCH2Rf)3) (R = Me or Et and Rf = CF3 or CF2CF3) in anhy-
drous dichloromethane resulted in the formation of white solids
with the composition SnCl4ꢀ2L. The solids are poorly soluble in
dichloromethane and chloroform, with 4 being less soluble. The
complexes were characterised by elemental analysis and particu-
larly by their NMR data and comparison with the corresponding
data for the free ligands. The possible structures of complexes 1–
4 are shown in Chart 1.
The infrared spectra show strong bands within the range 1220–
1230 cmꢁ1 for 1 and 2, and 1245–1260 cmꢁ1 for 3 and 4, which are
assigned to
m(P@O) stretches. The P@O stretching vibration is
shifted towards lower wave numbers on coordination to the tin
atom compared with its value for the free ligands. The coordination
shift is consistent with phosphoryl coordination to the tin atom.
This shift is 73 cmꢁ1 for 1 against 138 cmꢁ1 for the HMPA complex
[7,10], reflecting a difference in the basicity strength between the
two ligands. This is most probably due to the nature of the substit-
uents on the phosphorus atom in these ligands (i.e. due to differ-
ence in the electronegativities of nitrogen and oxygen atoms
linked directly to the phosphorus atom of the ligand). The absorp-
tion band at 500–540 cmꢁ1 corresponds to a stretching vibration of
the SnAO bond.
It is worth to note that all the NMR spectra were recorded
immediately after dissolution in dichloromethane. However,
recording the spectra after two or three days gave identical results.
This could provide some information as to the stereochemistry of
the complexes in the solid, which could only be confirmed by solid
state analytical techniques such as X-ray, Mossbauer, etc.
3.3. Solution behaviour of the adducts in the presence of excess ligand
3.2. Spectroscopic characterisation
In order to investigate the solution behaviour of the adducts 1–
4 and compare them to the HMPA and (Me2N)2P(O)OCH2CF3 com-
plexes, we have carried out a variable temperature 31P and 1H NMR
study of the three complexes SnCl4ꢀ2(O)P(NMe2)2OCH2CF3, 1 and 3
in the presence of excess ligand in dichloromethane solution.
The room temperature 31P–{1H} NMR spectrum of complex 1 in
the presence of an excess of ligand revealed two resonances, a
well-resolved signal flanked with Sn satellites and a broad signal
shifted towards the free ligand chemical shift region (Fig. 3). The
former well-resolved signal is assigned to the trans adduct.
At low temperature, the 31P–{1H} spectra display separate peaks
for the free ligand, the ligand coordinated in the cis adduct and the
ligand coordinated in the trans adduct. This clearly indicates that
the free ligand signal was exchanging only with that of the cis ad-
duct, leaving the trans signal unaffected.
The NMR spectra of the four complexes were recorded in CD2Cl2
solutions and the data obtained from these spectra are summarised
in Table 1.
L
Cl
Cl
Cl
L
Cl
Cl
L
Sn
Sn
Cl
Cl
L
Cl
Trans
Cis
Chart 1. The possible trans and cis forms of the octahedral complexes SnCl4.2L, 1–4.

M.A.K. Sanhoury et al. / Inorganica Chimica Acta 362 (2009) 3763–3768
3765
Table 1
NMR data (d/ppm and J/Hz) for complexes 1–4 in CD2Cl2 at 228 K.
a
Compound
d31P (2JPASn
)
d119Sn (2JPASn
)
cis
trans
cis
trans
SnCl4ꢀ2(O)P(OCH2CF3)2NMe2 (1)
SnCl4ꢀ2(O)P(OCH2CF3)2NEt2 (2)
SnCl4ꢀ2(O)P(OCH2CF3)3 (3)
1.15 (s,139)
0.04 (s,147)
1.63 (s,196)
0.34 (s,205)
–
–
ꢁ537 (t,140)
ꢁ539 (t,145)
ꢁ516 (t,154)
ꢁ514 (t,157)
ꢁ562 (t,197)
ꢁ564 (t,202)
ꢁ13.78 (s,152)
–
–
SnCl4ꢀ2(O)P(OCH2CF2CF3)3 (4)
ꢁ12.62 (s,158)
a
in a CD2Cl2/CH2Cl2 mixture.
*
Fig. 1. 31P–{1H} NMR spectrum of 1 at 298 K in CH2Cl2 ( : Sn satellites of Ltrans).
Fig. 2. 119Sn NMR spectra of 2 in dichloromethane at 248 K.
The differences in 31P chemical shift between the free ligand sig-
lows the same order and the highest magnitude of the 2J(P–Sn)cis
being observed for the ligand P(O)(OCH2CF3)3 (see Table 2).
nal and that of the ligand coordinated in the cis adduct (the cis coor-
dination chemical shifts) are given in Table 2. The
D
d(31P) values
As shown in Fig. 4, the room temperature 1H NMR spectrum of
complex 1 in the presence of excess ligand exhibited a broad mul-
tiplet for the protons of the dimethylamino group. When the solu-
tion was cooled to 268 K, a well-resolved doublet separated from
the broad resonance. In comparison with the 31P results, we assign
the doublet to the trans adduct and the remaining broad peak to a
coalescence of the signals of free and cis-coordinated ligands. At
lower temperatures, the broad peak was split into two separate
doublets. At 248 K and in addition to the trans doublet, two sharp-
ened doublets corresponding to well-resolved free and bound li-
gand signals appeared (Fig. 4). This clearly indicated that the
ligand exchange slowed when the temperature was decreased.
show, in contrast to the expectations, that the smallest difference
is observed with HMPA, the strongest Lewis base amongst the li-
gands studied. As illustrated in Table 2, the difference
D
d(31P) in-
creases with decreasing number of dimethylamino groups. One
can explain this as due to a sizeable N ? P inductive effect by means
p donation from the nitrogen centres of NMe2 to compensate the
loss of electron density around phosphorus following complexation.
The lack of NMe2 groups in P(O)(OCH2CF3)3 leads to the absence of
such a compensation and as a result this ligand gives the largest
coordination chemical shift (ꢂ10 ppm). Similarly, the increase in
the magnitude of the two bond P–Sn coupling in the cis-adducts fol-
of

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*
Fig. 3. 31P–{1H} NMR spectrum of 1 in the presence of an excess of ligand in dichloromethane at 298 K ( : Sn satellites).
Table 2
NMR dataa of cis isomers of the complexes SnCl4ꢀ2L in dichloromethane and their metal–ligand exchange barriers.
*
D )
G (kcal molꢁ1
g
Ligand (L)
D
d(31P)f
d(119Sn)cis
2J(31P–119Sn)cis
(Me2N)3P(O)b
(Me2N)2P(O)OCH2CF3
Me2NP(O)(OCH2CF3)2
3.2
5.8
7.8
10.1
ꢁ569
ꢁ555
ꢁ537
ꢁ516
126
107
140
154
16.87h
15.15
13.38
11.39
c
d
e
P(O)(OCH2CF3)3
a
Measured at slow exchange where well-resolved signals are observed.
b
c
d
e
f
At 298 K.
268 K.
248 K.
228 K.
D
d(31P) = d(31P)L ꢁ d(31P)Lcis
.
g
h
Calculated from Eyring equation at Tc.
Value taken from Ref. [22].
As in the 31P NMR spectra, the bound ligand signal which ex-
changed with that of the free ligand is attributed to the cis adduct.
The same dichotomy was simultaneously observed for the fluoro-
alkyl protons in complexes 1–4.
Kc ¼ ðkT=hÞ expðꢁ
D
G=RTÞ
ð1Þ
D
G– ¼ 19:14Tcð10:32 þ log Tc=kcÞ ꢃ 10ꢁ3
ð2Þ
the peak
where kc is the exchange rate ((
separation in the absence of exchange. The calculated metal–ligand
exchange barriers are gathered in Table 2.
p
ꢃ
D
t
)/21/2) at Tc and
D
t
These variable temperature NMR studies show that, in the pres-
ence of excess ligand, two distinct types of processes over two
different well-separated temperature ranges can be observed; a
cis–trans isomerisation (298–268 K) and a faster ligand exchange
on the cis adduct (298–248 K). This behaviour is qualitatively sim-
ilar to that observed in analogous tin tetrachloride adducts [22,23].
Despite that we have not carried out detailed kinetic studies, we
believe that the exchange rate of the free ligand on the trans isomer
is much slower than that in the two exchange processes mentioned
above, in good agreement with results obtained for related systems
by Knight and Merbach [24].
The exchange rate on the cis adduct was found to be indepen-
dent of the concentration of the free ligand. Furthermore, traces
of the pentacoordinated tin species with partially dissociated li-
gand could be observed in solution in the absence of added free li-
gand. All these observations together allow concluding to a
limiting dissociative D mechanism proceeding via five-coordinated
intermediates, consistent with trends generally observed for tin
tetrahalide adducts [25].
Examination of Table 2 shows that the decrease in the metal–li-
gand exchange barriers is accompanied by a higher frequency shift
of 119Sn NMR resonances and a decrease of the complex stability in
the order: (Me2N)3P(O) > (Me2N)2P(O)OCH2CF3 > Me2NP(O)(OCH2-
CF3)2 > P(O)(OCH2CF3)3 > P(O)(OCH2CF2CF3)3, consistent with re-
duced Lewis basicity of the ligand as the substituents on the
phosphorus atom become more electronegative (see Table 2). In
this case and since the tin tetrahalide is kept constant (i.e. only
SnCl4 was used), the decrease in the exchange barrier of 5.5 kcal/
mol as well as the difference in 119Sn chemical shift of 50 ppm be-
tween HMPA and P(O)(OCH2CF3)3 complexes may indicate the
difference in the strength of the bonding interaction with SnCl4.
Such a frequency/interaction relationship has even been reported
for the often difficult to observe germanium resonance [27].
4. Conclusion
For comparison purposes, the coalescence temperatures for li-
gand exchange on the cis adducts were estimated for the three
complexes SnCl4ꢀ2(O)P(NMe2)2OCH2CF3, 1 and 3, and the exchange
barriers have been determined using the Eyring Eq. (1) together
with Eq. (2) [26]:
New fluoroalkyl phosphoryl complexes with SnCl4 have been
synthesised and studied in solution by NMR spectroscopy. It was
shown that subsequent substitution of dialkylamino groups in
(R2N)3P(O) for fluoroalkoxy groups leads to increasing rates of

M.A.K. Sanhoury et al. / Inorganica Chimica Acta 362 (2009) 3763–3768
3767
Fig. 4. Variable temperature 1H NMR spectra of 1 in the presence of excess ligand in CD2Cl2 at a: 298; b: 268; c: 258 and d: 248 K.
formation of the cis complex with SnCl4. Even with the bulkier li-
gand (CF3CF2CH2O)3P(O), only the cis isomer was found to be
formed. Thus, the preferential streochemistry in the complexes
studied is likely due to electronic rather than steric effects from
the fluoroalkoxy groups. Our variable temperature NMR studies
indicate that, in the presence of excess ligand, a potential ligand

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exchange on the cis adduct is observed. On the basis of the low-fre-
quency shifts generally associated with hypercoordination in most
nuclei, our results have also shown that the chemical shifts of the
119Sn resonance for adducts of the type SnCl4ꢀ2L could provide a
qualitative indication of the extent of the donation of electron den-
sity from the ligands to the central tin atom.
[13] J. Rupp-Bensadon, E.A.C. Lucken, J. Chem. Soc., Dalton Trans. (1983) 495.
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Acknowledgement
[15] M.A.M. Khouna Sanhoury, M.T. Ben Dhia, K. Essalah, M.R. Khaddar, Polyhedron
25 (2006) 3299.
[16] M.A.M.K. Sanhoury, M.T. Ben Dhia, K. Essalah, A. Guesmi, M.R. Khaddar,
Polyhedron 27 (2008) 1754.
[17] W.L.F. Armarego, D.D. Perrin, Purification of Laboratory Chemicals, 14th ed.,
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[18] C.M. Timperley, M. Bird, J.F. Broderick, I. Holden, I.J. Morton, M.J. Waters, J.
Fluorine Chem. 104 (2000) 215.
We are grateful to the Tunisian Ministry of High Education and
Scientific Research and Technology for financial support (LR99ES14)
of this research and to Professor Emeritus A. Baklouti of the Depart-
ment of Chemistry, Faculty of Sciences of Tunis, University of Tunis
El Manar for his valuable help and continuous support.
[19] C.M. Timperley, I. Holden, I.J. Morton, M.J. Waters, J. Fluorine Chem. 106 (2000)
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