- Email: [email protected]
Synthesis, characterisation and solution behaviour of fluoroalkyl phosphoryl complexes of tin tetrachloride
Inorganica Chimica Acta 362 (2009) 3763–3768
Contents lists available at ScienceDirect
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
Article history: 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 119 Sn NMR 31 P NMR 2 119 J( Sn–31P) coupling constant
a b s t r a c t The synthesis, characterisation and solution behaviour of a series of octahedral complexes SnCl42L (L = R2NP(O)(OCH2CF3)2; R = Me (1); Et (2) or L = P(O)(OCH2Rf)3; Rf = CF3 (3); C2F5 (4)) are described. Complexes 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 hexamethylphosphoramide adduct, SnCl42HMPA. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction 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 solution and in the solid state [7,8]. For example, the hexamethylphosphoramide complex, SnCl42HMPA, 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 tin(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 almost 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
* Corresponding author. Tel.: +216 98269495. E-mail address: [email protected] (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
the nature of the substituents on the phosphorus atom of the ligand prompted us to investigate the solution structure of tin complexes 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 AC300 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–Elmer 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.
3764
M.A.K. Sanhoury et al. / Inorganica Chimica Acta 362 (2009) 3763–3768
2.2. [SnCl42(O)P(OCH2CF3)2NR2] 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 SnCl42L 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 cm1, 2:1227 cm1); mSnAO (1:506 cm1, 2:512 cm1). 2.3. [SnCl42(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 cm1, 4:1258 cm1); mSnAO (3:534 cm1, 4:540 cm1). 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 anhydrous dichloromethane resulted in the formation of white solids with the composition SnCl42L. The solids are poorly soluble in dichloromethane and chloroform, with 4 being less soluble. The complexes were characterised by elemental analysis and particularly 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 cm1 for 1 and 2, and 1245–1260 cm1 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 cm1 for 1 against 138 cm1 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 substituents on the phosphorus atom in these ligands (i.e. due to difference in the electronegativities of nitrogen and oxygen atoms linked directly to the phosphorus atom of the ligand). The absorption band at 500–540 cm1 corresponds to a stretching vibration of the SnAO bond.
At room temperature, the 31P–{1H} NMR spectra of complexes 3 and 4 showed only one signal, whilst two resonances were observed 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 interpreted in terms of inductive effects resulting from a decrease in the electron density at phosphorus upon coordination of the phosphoryl oxygen of the ligand to the tin atom. 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 temperature 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 hexacoordinated tin species [20]. The broad signals were converted at low temperature 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. The triplet feature observed in the 119Sn NMR spectra is due to 119 Sn–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 SnCl42L. 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 adducts 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 dichloromethane 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]. 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 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 Sn
Cl Cl
L Sn
Cl
Cl
L
Cl
Cl
L
Cl
Trans
Cis
Chart 1. The possible trans and cis forms of the octahedral complexes SnCl4.2L, 1–4.
In order to investigate the solution behaviour of the adducts 1– 4 and compare them to the HMPA and (Me2N)2P(O)OCH2CF3 complexes, we have carried out a variable temperature 31P and 1H NMR study of the three complexes SnCl42(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 adduct, leaving the trans signal unaffected.
3765
M.A.K. Sanhoury et al. / Inorganica Chimica Acta 362 (2009) 3763–3768 Table 1 NMR data (d/ppm and J/Hz) for complexes 1–4 in CD2Cl2 at 228 K. Compound
d31P (2JPASn) cis
trans
cis
SnCl42(O)P(OCH2CF3)2NMe2 (1) SnCl42(O)P(OCH2CF3)2NEt2 (2) SnCl42(O)P(OCH2CF3)3 (3) SnCl42(O)P(OCH2CF2CF3)3 (4)
1.15 (s,139) 0.04 (s,147) 13.78 (s,152) 12.62 (s,158)
1.63 (s,196) 0.34 (s,205) – –
537 539 516 514
a
d119Sn
a
(2JPASn) trans
(t,140) (t,145) (t,154) (t,157)
562 (t,197) 564 (t,202) – –
in a CD2Cl2/CH2Cl2 mixture.
Fig. 1.
P–{1H} NMR spectrum of 1 at 298 K in CH2Cl2 (*: Sn satellites of Ltrans).
31
Fig. 2.
119
Sn NMR spectra of 2 in dichloromethane at 248 K.
The differences in 31P chemical shift between the free ligand signal and that of the ligand coordinated in the cis adduct (the cis coordination chemical shifts) are given in Table 2. The Dd(31P) values show, in contrast to the expectations, that the smallest difference is observed with HMPA, the strongest Lewis base amongst the ligands studied. As illustrated in Table 2, the difference Dd(31P) increases with decreasing number of dimethylamino groups. One can explain this as due to a sizeable N ? P inductive effect by means of 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 (10 ppm). Similarly, the increase in the magnitude of the two bond P–Sn coupling in the cis-adducts fol-
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). As shown in Fig. 4, the room temperature 1H NMR spectrum of complex 1 in the presence of excess ligand exhibited a broad multiplet for the protons of the dimethylamino group. When the solution 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 sharpened doublets corresponding to well-resolved free and bound ligand signals appeared (Fig. 4). This clearly indicated that the ligand exchange slowed when the temperature was decreased.
3766
M.A.K. Sanhoury et al. / Inorganica Chimica Acta 362 (2009) 3763–3768
Fig. 3.
P–{1H} NMR spectrum of 1 in the presence of an excess of ligand in dichloromethane at 298 K (*: Sn satellites).
31
Table 2 NMR dataa of cis isomers of the complexes SnCl42L in dichloromethane and their metal–ligand exchange barriers. Ligand (L) b
(Me2N)3P(O) (Me2N)2P(O)OCH2CF3c Me2NP(O)(OCH2CF3)2d P(O)(OCH2CF3)3e a b c d e f g h
Dd(31P)f
d(119Sn)cis
2
3.2 5.8 7.8 10.1
569 555 537 516
126 107 140 154
DG*(kcal mol1)g
J(31P–119Sn)cis
16.87h 15.15 13.38 11.39
Measured at slow exchange where well-resolved signals are observed. At 298 K. 268 K. 248 K. 228 K. Dd(31P) = d(31P)L d(31P)Lcis. Calculated from Eyring equation at Tc. Value taken from Ref. [22].
As in the 31P NMR spectra, the bound ligand signal which exchanged with that of the free ligand is attributed to the cis adduct. The same dichotomy was simultaneously observed for the fluoroalkyl protons in complexes 1–4. These variable temperature NMR studies show that, in the presence 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 similar 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 independent of the concentration of the free ligand. Furthermore, traces of the pentacoordinated tin species with partially dissociated ligand could be observed in solution in the absence of added free ligand. 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]. For comparison purposes, the coalescence temperatures for ligand exchange on the cis adducts were estimated for the three complexes SnCl42(O)P(NMe2)2OCH2CF3, 1 and 3, and the exchange barriers have been determined using the Eyring Eq. (1) together with Eq. (2) [26]:
K c ¼ ðkT=hÞ expðDG=RTÞ
ð1Þ
DG– ¼ 19:14T c ð10:32 þ log T c =kc Þ 103
ð2Þ 1/2
where kc is the exchange rate ((p Dt)/2 ) at Tc and Dt the peak separation in the absence of exchange. The calculated metal–ligand exchange barriers are gathered in Table 2. Examination of Table 2 shows that the decrease in the metal–ligand 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)(OCH2CF3)2 > P(O)(OCH2CF3)3 > P(O)(OCH2CF2CF3)3, consistent with reduced 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 between 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 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 ligand (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
3768
M.A.K. Sanhoury et al. / Inorganica Chimica Acta 362 (2009) 3763–3768
exchange on the cis adduct is observed. On the basis of the low-frequency shifts generally associated with hypercoordination in most nuclei, our results have also shown that the chemical shifts of the 119 Sn resonance for adducts of the type SnCl42L could provide a qualitative indication of the extent of the donation of electron density from the ligands to the central tin atom. Acknowledgement 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 Department of Chemistry, Faculty of Sciences of Tunis, University of Tunis El Manar for his valuable help and continuous support. References [1] T. Munguia, M. Lopez-Cardoso, F. Cervantes-Lee, K.H. Pannell, Inorg. Chem. 46 (2007) 1305. [2] S.E. Denmark, J. Fu, J. Am. Chem. Soc. 125 (2003) 2208. [3] D. Fàrsaiu, R. Leu, P.J. Ream, J. Chem. Soc., Perkin Trans. 2 (2001) 427. [4] S.E. Dann, A.R.J. Genge, W. Levason, G. Reid, J. Chem. Soc., Dalton Trans. (1997) 2207. [5] C.H. Yoder, L.A. Margolis, J.M. Horne, J. Organomet. Chem. 633 (2001) 33 (and refs. therein). [6] See for example A.R.J. Genge, W. Levason, G. Reid, J. Chem. Soc., Dalton Trans. (1997) 4549 (and refs. therein).
[7] E. Le Coz, J.E. Guerchais, Bull. Soc. Chim. Fr. 1 (1971) 80. [8] S.J. Ruzicka, A.E. Merbach, Inorg. Chim. Acta 20 (1976) 221. [9] L.A. Aslanov, V.M. Ionov, V.M. Attiya, A.B. Permin, V.S. Petrosyan, Zhur. Strukt. Khim. 18 (1977) 1103. [10] S.J. Ruzicka, A.E. Merbach, Inorg. Chim. Acta 22 (1977) 191. [11] S.E. Denmark, X. Su, Tetrahedron 55 (1999) 8727. [12] D. Tudela, V. Fernàndez, J.D. Tornero, J. Chem. Soc., Dalton Trans. (1985) 1281. [13] J. Rupp-Bensadon, E.A.C. Lucken, J. Chem. Soc., Dalton Trans. (1983) 495. [14] M.A.M. Khouna, M.T. Ben Dhia, M.R. Khaddar, Phosphorus, Sulfur, and Silicon 180 (2005) 1673. [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., Butterworth-Heinemann, Oxford, 1996. [18] C.M. Timperley, M. Bird, J.F. Broderick, I. Holden, I.J. Morton, M.J. Waters, J. Fluorine Chem. 104 (2000) 215. [19] C.M. Timperley, I. Holden, I.J. Morton, M.J. Waters, J. Fluorine Chem. 106 (2000) 153. [20] M.S. Holt, W.L. Wilson, J.H. Nelson, Chem. Rev. 89 (1989) 11. [21] M.F. Davis, W. Levason, G. Reid, M. Webster, Polyhedron 25 (2006) 930 (and refs. therein). [22] S.J. Ruzicka, C.M.P. Favez, A.E. Merbach, Inorg. Chim. Acta 23 (1977) 239. [23] C.T.G. Knight, A.E. Merbach, Inorg. Chem. 24 (1985) 576. [24] C.T.G. Knight, A.E. Merbach, J. Am. Chem. Soc. 106 (1984) 804. [25] L. Helm, A.E. Merbach, Chem. Rev. 105 (2005) 1923. [26] H. Freibolin, Basic One- and Two-dimentional NMR Spectroscopy, Wiley-VCH, Weinheim, Germany, 1991. P. 263. [27] C.H. Yoder, T.M. Agee, C.D. Schaeffer Jr., M.J. Carroll, A.J. Fleisher, A.S. DeToma, Inorg. Chem. 47 (2008) 10765.
Contents lists available at ScienceDirect
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
Article history: 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 119 Sn NMR 31 P NMR 2 119 J( Sn–31P) coupling constant
a b s t r a c t The synthesis, characterisation and solution behaviour of a series of octahedral complexes SnCl42L (L = R2NP(O)(OCH2CF3)2; R = Me (1); Et (2) or L = P(O)(OCH2Rf)3; Rf = CF3 (3); C2F5 (4)) are described. Complexes 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 hexamethylphosphoramide adduct, SnCl42HMPA. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction 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 solution and in the solid state [7,8]. For example, the hexamethylphosphoramide complex, SnCl42HMPA, 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 tin(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 almost 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
* Corresponding author. Tel.: +216 98269495. E-mail address: [email protected] (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
the nature of the substituents on the phosphorus atom of the ligand prompted us to investigate the solution structure of tin complexes 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 AC300 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–Elmer 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.
3764
M.A.K. Sanhoury et al. / Inorganica Chimica Acta 362 (2009) 3763–3768
2.2. [SnCl42(O)P(OCH2CF3)2NR2] 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 SnCl42L 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 cm1, 2:1227 cm1); mSnAO (1:506 cm1, 2:512 cm1). 2.3. [SnCl42(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 cm1, 4:1258 cm1); mSnAO (3:534 cm1, 4:540 cm1). 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 anhydrous dichloromethane resulted in the formation of white solids with the composition SnCl42L. The solids are poorly soluble in dichloromethane and chloroform, with 4 being less soluble. The complexes were characterised by elemental analysis and particularly 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 cm1 for 1 and 2, and 1245–1260 cm1 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 cm1 for 1 against 138 cm1 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 substituents on the phosphorus atom in these ligands (i.e. due to difference in the electronegativities of nitrogen and oxygen atoms linked directly to the phosphorus atom of the ligand). The absorption band at 500–540 cm1 corresponds to a stretching vibration of the SnAO bond.
At room temperature, the 31P–{1H} NMR spectra of complexes 3 and 4 showed only one signal, whilst two resonances were observed 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 interpreted in terms of inductive effects resulting from a decrease in the electron density at phosphorus upon coordination of the phosphoryl oxygen of the ligand to the tin atom. 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 temperature 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 hexacoordinated tin species [20]. The broad signals were converted at low temperature 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. The triplet feature observed in the 119Sn NMR spectra is due to 119 Sn–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 SnCl42L. 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 adducts 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 dichloromethane 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]. 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 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 Sn
Cl Cl
L Sn
Cl
Cl
L
Cl
Cl
L
Cl
Trans
Cis
Chart 1. The possible trans and cis forms of the octahedral complexes SnCl4.2L, 1–4.
In order to investigate the solution behaviour of the adducts 1– 4 and compare them to the HMPA and (Me2N)2P(O)OCH2CF3 complexes, we have carried out a variable temperature 31P and 1H NMR study of the three complexes SnCl42(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 adduct, leaving the trans signal unaffected.
3765
M.A.K. Sanhoury et al. / Inorganica Chimica Acta 362 (2009) 3763–3768 Table 1 NMR data (d/ppm and J/Hz) for complexes 1–4 in CD2Cl2 at 228 K. Compound
d31P (2JPASn) cis
trans
cis
SnCl42(O)P(OCH2CF3)2NMe2 (1) SnCl42(O)P(OCH2CF3)2NEt2 (2) SnCl42(O)P(OCH2CF3)3 (3) SnCl42(O)P(OCH2CF2CF3)3 (4)
1.15 (s,139) 0.04 (s,147) 13.78 (s,152) 12.62 (s,158)
1.63 (s,196) 0.34 (s,205) – –
537 539 516 514
a
d119Sn
a
(2JPASn) trans
(t,140) (t,145) (t,154) (t,157)
562 (t,197) 564 (t,202) – –
in a CD2Cl2/CH2Cl2 mixture.
Fig. 1.
P–{1H} NMR spectrum of 1 at 298 K in CH2Cl2 (*: Sn satellites of Ltrans).
31
Fig. 2.
119
Sn NMR spectra of 2 in dichloromethane at 248 K.
The differences in 31P chemical shift between the free ligand signal and that of the ligand coordinated in the cis adduct (the cis coordination chemical shifts) are given in Table 2. The Dd(31P) values show, in contrast to the expectations, that the smallest difference is observed with HMPA, the strongest Lewis base amongst the ligands studied. As illustrated in Table 2, the difference Dd(31P) increases with decreasing number of dimethylamino groups. One can explain this as due to a sizeable N ? P inductive effect by means of 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 (10 ppm). Similarly, the increase in the magnitude of the two bond P–Sn coupling in the cis-adducts fol-
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). As shown in Fig. 4, the room temperature 1H NMR spectrum of complex 1 in the presence of excess ligand exhibited a broad multiplet for the protons of the dimethylamino group. When the solution 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 sharpened doublets corresponding to well-resolved free and bound ligand signals appeared (Fig. 4). This clearly indicated that the ligand exchange slowed when the temperature was decreased.
3766
M.A.K. Sanhoury et al. / Inorganica Chimica Acta 362 (2009) 3763–3768
Fig. 3.
P–{1H} NMR spectrum of 1 in the presence of an excess of ligand in dichloromethane at 298 K (*: Sn satellites).
31
Table 2 NMR dataa of cis isomers of the complexes SnCl42L in dichloromethane and their metal–ligand exchange barriers. Ligand (L) b
(Me2N)3P(O) (Me2N)2P(O)OCH2CF3c Me2NP(O)(OCH2CF3)2d P(O)(OCH2CF3)3e a b c d e f g h
Dd(31P)f
d(119Sn)cis
2
3.2 5.8 7.8 10.1
569 555 537 516
126 107 140 154
DG*(kcal mol1)g
J(31P–119Sn)cis
16.87h 15.15 13.38 11.39
Measured at slow exchange where well-resolved signals are observed. At 298 K. 268 K. 248 K. 228 K. Dd(31P) = d(31P)L d(31P)Lcis. Calculated from Eyring equation at Tc. Value taken from Ref. [22].
As in the 31P NMR spectra, the bound ligand signal which exchanged with that of the free ligand is attributed to the cis adduct. The same dichotomy was simultaneously observed for the fluoroalkyl protons in complexes 1–4. These variable temperature NMR studies show that, in the presence 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 similar 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 independent of the concentration of the free ligand. Furthermore, traces of the pentacoordinated tin species with partially dissociated ligand could be observed in solution in the absence of added free ligand. 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]. For comparison purposes, the coalescence temperatures for ligand exchange on the cis adducts were estimated for the three complexes SnCl42(O)P(NMe2)2OCH2CF3, 1 and 3, and the exchange barriers have been determined using the Eyring Eq. (1) together with Eq. (2) [26]:
K c ¼ ðkT=hÞ expðDG=RTÞ
ð1Þ
DG– ¼ 19:14T c ð10:32 þ log T c =kc Þ 103
ð2Þ 1/2
where kc is the exchange rate ((p Dt)/2 ) at Tc and Dt the peak separation in the absence of exchange. The calculated metal–ligand exchange barriers are gathered in Table 2. Examination of Table 2 shows that the decrease in the metal–ligand 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)(OCH2CF3)2 > P(O)(OCH2CF3)3 > P(O)(OCH2CF2CF3)3, consistent with reduced 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 between 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 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 ligand (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
3768
M.A.K. Sanhoury et al. / Inorganica Chimica Acta 362 (2009) 3763–3768
exchange on the cis adduct is observed. On the basis of the low-frequency shifts generally associated with hypercoordination in most nuclei, our results have also shown that the chemical shifts of the 119 Sn resonance for adducts of the type SnCl42L could provide a qualitative indication of the extent of the donation of electron density from the ligands to the central tin atom. Acknowledgement 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 Department of Chemistry, Faculty of Sciences of Tunis, University of Tunis El Manar for his valuable help and continuous support. References [1] T. Munguia, M. Lopez-Cardoso, F. Cervantes-Lee, K.H. Pannell, Inorg. Chem. 46 (2007) 1305. [2] S.E. Denmark, J. Fu, J. Am. Chem. Soc. 125 (2003) 2208. [3] D. Fàrsaiu, R. Leu, P.J. Ream, J. Chem. Soc., Perkin Trans. 2 (2001) 427. [4] S.E. Dann, A.R.J. Genge, W. Levason, G. Reid, J. Chem. Soc., Dalton Trans. (1997) 2207. [5] C.H. Yoder, L.A. Margolis, J.M. Horne, J. Organomet. Chem. 633 (2001) 33 (and refs. therein). [6] See for example A.R.J. Genge, W. Levason, G. Reid, J. Chem. Soc., Dalton Trans. (1997) 4549 (and refs. therein).
[7] E. Le Coz, J.E. Guerchais, Bull. Soc. Chim. Fr. 1 (1971) 80. [8] S.J. Ruzicka, A.E. Merbach, Inorg. Chim. Acta 20 (1976) 221. [9] L.A. Aslanov, V.M. Ionov, V.M. Attiya, A.B. Permin, V.S. Petrosyan, Zhur. Strukt. Khim. 18 (1977) 1103. [10] S.J. Ruzicka, A.E. Merbach, Inorg. Chim. Acta 22 (1977) 191. [11] S.E. Denmark, X. Su, Tetrahedron 55 (1999) 8727. [12] D. Tudela, V. Fernàndez, J.D. Tornero, J. Chem. Soc., Dalton Trans. (1985) 1281. [13] J. Rupp-Bensadon, E.A.C. Lucken, J. Chem. Soc., Dalton Trans. (1983) 495. [14] M.A.M. Khouna, M.T. Ben Dhia, M.R. Khaddar, Phosphorus, Sulfur, and Silicon 180 (2005) 1673. [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., Butterworth-Heinemann, Oxford, 1996. [18] C.M. Timperley, M. Bird, J.F. Broderick, I. Holden, I.J. Morton, M.J. Waters, J. Fluorine Chem. 104 (2000) 215. [19] C.M. Timperley, I. Holden, I.J. Morton, M.J. Waters, J. Fluorine Chem. 106 (2000) 153. [20] M.S. Holt, W.L. Wilson, J.H. Nelson, Chem. Rev. 89 (1989) 11. [21] M.F. Davis, W. Levason, G. Reid, M. Webster, Polyhedron 25 (2006) 930 (and refs. therein). [22] S.J. Ruzicka, C.M.P. Favez, A.E. Merbach, Inorg. Chim. Acta 23 (1977) 239. [23] C.T.G. Knight, A.E. Merbach, Inorg. Chem. 24 (1985) 576. [24] C.T.G. Knight, A.E. Merbach, J. Am. Chem. Soc. 106 (1984) 804. [25] L. Helm, A.E. Merbach, Chem. Rev. 105 (2005) 1923. [26] H. Freibolin, Basic One- and Two-dimentional NMR Spectroscopy, Wiley-VCH, Weinheim, Germany, 1991. P. 263. [27] C.H. Yoder, T.M. Agee, C.D. Schaeffer Jr., M.J. Carroll, A.J. Fleisher, A.S. DeToma, Inorg. Chem. 47 (2008) 10765.