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Phosphoramidic difluoride complexes of tin(IV) chloride: A multinuclear (119Sn, 31P, 19F and 1H) NMR characterisation in solution
Polyhedron 25 (2006) 3299–3304 www.elsevier.com/locate/poly
Phosphoramidic difluoride complexes of tin(IV) chloride: A multinuclear (119Sn, 31P, 19F and 1H) NMR characterisation in solution M.A.M.K. Sanhoury a
a,*
, M.T. Ben Dhia a, K. Essalah b, M.R. Khaddar
a
Laboratory of Coordination Chemistry, Department of Chemistry, Faculty of Sciences of Tunis, University of Tunis El Manar, 1060 Tunis, Tunisia b Unite´ de recherche de Physico-Chimie Mole´culaire de l’IPEST, BP 51, 2070 la Marsa Tunisie, Tunisia Received 18 March 2006; accepted 4 June 2006 Available online 30 June 2006 Dedicated to Professor Ahmed Baklouti on the occasion of his retirement.
Abstract Two octahedral complexes of the general formula SnCl4 Æ 2(O)PF2NR2 (R = Me (1) or Et (2)) have been synthesised from SnCl4 and the ligand R2NP(O)F2 in anhydrous CHCl3. The new adducts have been characterised by multinuclear (119Sn, 31P, 19F and 1H) NMR, IR spectroscopy and elemental analysis. When compared with previously described hexamethylphosphoramide (HMPA) and trimethylphosphate (TMPA) analogues, the NMR data of the two complexes prepared suggest the presence of only the cis isomer in solution. Steric factors and the Lewis basicity of the ligand may explain the stereochemistry observed. Low temperature 31P and 119Sn NMR spectra show that the compounds partially dissociate in dichloromethane. Additionally, DFT/B3LYP calculations on complex 1 and its ligand have been carried out to support the interpretations of NMR data. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Phosphoryl ligands; Tin tetrachloride;
119
Sn NMR;
31
P NMR;
1. Introduction Studies of tin(IV) adducts continue to provide fundamental information about both the Lewis acid–base model and the reactivity of tin(IV) species [1–3]. In Lewis bases (ligands) of the type R1R2R3P@O, the substituent effects of various R groups (R = alkyl, aryl, halogen, dialkylamino, alkoxy, etc.) on the particular donor character and the interplay between pp–dp bonding of (P@O) and (P–R) components are perhaps the main features in terms of coordination patterns [4]. Tin(IV) chloride forms octahedral complexes with such phosphoryl ligands, having the general formula SnCl4 Æ 2L (L is the phosphoryl ligand) [5,6]. In these compounds two isomers, with the ligand L in cis *
Corresponding author. Tel.: +216 98269495. E-mail address: [email protected] (M.A.M.K. Sanhoury).
0277-5387/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.06.014
19
F NMR;
119
Sn–31P coupling constants; DFT/B3LYP
or trans mutual orientations, are possible. The complex of SnCl4 with hexamethylphosphoramide (HMPA), SnCl4 Æ 2HMPA, has been shown to exist as a trans-adduct in the solid state by X-ray diffraction [7] and in solution as a mixture of both cis and trans isomers by IR and NMR spectroscopies [8,9], whereas the complex with trimethylphosphate (TMPA) is mainly a cis-adduct [8]. Steric factors and the strength of the Lewis basicity (i.e. the donor character of the P@O group of the ligand) as well as other factors (solvent polarity, temperature, etc.) may be held responsible for the differences observed [5,10,11]. In order to explore the effects of substitution of a dimethylamino group by a fluorine atom on the Lewis basicity of HMPA, we have previously studied tin(IV) complexes containing the ligand (R2N)2P(O)F by multinuclear NMR in solution [12] and showed that the predominant species was the cis isomer.
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M.A.M.K. Sanhoury et al. / Polyhedron 25 (2006) 3299–3304
Here we report the synthesis of two new complexes of tin tetrachloride with the ligands R2NP(O)F2 (R = Me or Et) in which two fluorine atoms have substituted two dialkylamino groups in the ligand (R2N)3P(O). The new compounds were characterised by elemental analysis, IR and multinuclear (119Sn, 31P, 19F and 1H) NMR spectroscopy. On the basis of NMR data, we show that these ligands, once freed from the inductive and p-bonding (and/or steric) effects of the two dialkylamino groups, are unique in that they form almost exclusively the cis complex with SnCl4 in solution. We have also carried out DFT/B3LYP calculations on the cis and trans isomers of complex 1 and its ligand in order to support and help the analysis of NMR data.
frequencies of the different stationary points of the PES have been calculated at the same level of theory in order to identify the local minima as well as to estimate the corresponding zero-point vibrational energy (ZPE). 3. Results and discussion 3.1. Synthesis
2. Experimental
Treatment of SnCl4 in a chloroform solution with R2NP(O)F2 (R = Me or Et) gives white solids with the composition SnCl4 Æ 2L (L = R2NP(O)F2). These complexes are soluble in dichloromethane and chloroform. They were characterised by elemental analysis and particularly by their NMR data and comparison with the corresponding data for the free ligands (see Table 1).
2.1. General experimental procedures
3.2. Spectroscopic studies
All preparations were carried out under a nitrogen atmosphere in solvents dried by standard techniques [13] and stored over molecular sieves. NMR spectra were recorded on a Bruker AC-300 instrument in CD2Cl2 as solvent; 31P at 121 MHz (85% H3PO4), 19F at 282 MHz (CFCl3), 1H at 300 MHz (TMS) and 119Sn at 111.8 MHz (SnCl4). IR spectra: Perkin Elmer Paragon 1000 PC. Tin tetrachloride was distilled under vacuum before use. The ligands R2NP(O)F2 were prepared according to methods described in literature (R = Me [14]; R = Et [15]).
The infrared spectra show strong bands within the range 1285–1310 cm 1 attributed to mP@O. 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 about 35 cm 1 and is consistent with phosphoryl coordination to the tin atom. This shift is 30 cm 1 for Me2NP(O)F2 against 138 cm 1 for HMPA [5], explaining the difference in the basicity strength between these two ligands which is most probably due to the substitution of two dimethylamino groups by two fluorine atoms. The absorption band at 560–570 cm 1 corresponds to a stretching vibration of the Sn–O group. The NMR spectra of the two complexes prepared are quite similar and the data obtained from these spectra are summarised in Table 1. The 31P NMR resonances of the bound ligands are shifted to lower frequency compared with those of the free ligand, whereas the 19F and 1H NMR resonances show a higher frequency shift on complexation. The difference in the 31P chemical shift between free and bound ligands is more important than that observed in 19F and 1H NMR spectra, confirming coordination of the fluorophosphoramide ligand through the oxygen atom. The coupling constant 1JP–F is larger for the complexed ligand than for the free one (Table 1). The 31P NMR spectra of each of the complexes prepared showed a triplet pattern (the triplet being due to phosphorus–fluorine coupling) with splitting due to 31P–Sn
2.2. Preparation of complexes In a typical reaction R2NP(O)F2 (8 mmol) in dry CHCl3 (10 cm3) was slowly added to SnCl4 (1.04 g, 4 mmol) in CHCl3 (20 cm3) over a 30 min period. The resulting solution, on continual stirring, began to precipitate the white solid complex SnCl4 Æ 2(O)PF2NR2. The precipitation was complete when the product was left 24 h at 15 °C. This was collected, washed with n-hexane/CCl4 and obtained as a white powder (Yields: R = Me: 1.24 g, 60%; R = Et: 1.21 g, 53%). Anal. Calc. for C4H12N2O2P2F4SnCl4: C, 9.26; H, 2.33; N, 5.40. Found: C, 9.00; H, 2.62; N, 5.68%. Anal. Calcd. for C8H20N2O2P2F4SnCl4: C, 16.72; H, 3.51; N, 4.87. Found: C, 16.66; H, 3.52; N, 5.04%. IR (KBr): mP@O (1280–1310 cm 1); mSn–O (560–570 cm 1). 2.3. Computational details Density functional theory (DFT) calculations were carried out on the cis and trans isomers of complex 1 and its ligand using the suite of programs Gaussian 98W [16], with the non-local hybrid functional denoted as B3LYP [17]. Basis sets used were 6-31G* [18] for C, H, N, F, P, O and Cl and Steven, Bach and Krauss (SBK) [19] with ECP with polarisation for Sn. The geometries of both the cis and trans isomers of complex 1 and its ligand were optimised using analytical gradient. The harmonic vibrational
Table 1 NMR data (d/ppm and J/Hz) for the complexes SnCl4 Æ 2L and the ligands R2NP(O)F2 in CD2Cl2 at 75 °C Compound Me2NP(O)F2 Et2NP(O)F2 SnCl4 Æ 2(O)PF2NMe2 (1) SnCl4 Æ 2(O)PF2NEt2 (2)
d31P 2.50 5.00 7.63 10.40
d19F
d1H
d119Sn
1
2
81.50 83.50 83.16 84.87
2.78 – 2.93 –
– –
997 1003 1015 1023
– – 145 150
507 510
JP–F
JP–Sn
M.A.M.K. Sanhoury et al. / Polyhedron 25 (2006) 3299–3304
coupling. The latter splitting can be clearly seen only at low temperature (Fig. 1). In addition, the spectra also contain a triplet of low intensity indicating the existence of a small amount of another species. This minor triplet also showed 31P–Sn splitting at low temperature. The 119Sn NMR spectrum of each complex displayed at room temperature one broad signal in the region of hexacoordinated species [20]. This broad signal was converted at low temperature into the expected triplet. At 75 °C, the spectrum showed a triplet due to 119Sn–31P coupling. Clearly one species was present at this temperature and in which the tin atom is coupled to two phosphorus atoms, showing a stoichiometry of SnCl4 Æ 2L (Fig. 2). This is in agreement with the 31P NMR spectra where a triplet displaying Sn satellites with the corresponding coupling constants was present (Fig. 1). At lower temperatures, the 119Sn spectrum (Fig. 3) showed, in addition to the major triplet, two doublets in the region of pentacoordinated tin [20]. This indicates that these different tin species are presumably in rapid equilibrium with each other and with the free ligand, consistent with 31P and 19F NMR spectra where ligand exchange reactions are fast at room temperature. The rates of these reactions were decreased by lowering the temperature and separate signals corresponding to the free and bound ligands present in solution were observed. Cooling to
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Fig. 2. 119Sn NMR spectrum of the complex SnCl4 Æ 2(O)PF2NMe2 in CD2Cl2 at 75 °C.
85 °C revealed the presence in the 31P NMR spectrum (and also in the 19F NMR spectrum) of a new signal at 9.41 ppm (Fig. 1), in addition to SnCl4 Æ 2L and to the
Fig. 1. 31P-{1H} NMR spectrum of the complex SnCl4 Æ 2(O)PF2NEt2 in CD2Cl2 at pentacoordinated tin complex, L: free ligand).
85 °C (*: Sn satellites, Lo: octahedral tin complex, Lp:
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Fig. 3.
M.A.M.K. Sanhoury et al. / Polyhedron 25 (2006) 3299–3304
119
Sn NMR spectrum of the complex SnCl4 Æ 2(O)PF2NEt2in CD2Cl2 at
free ligand, which we assign to SnCl4 Æ L. This assignment is supported by the presence of a doublet pattern in the 119 Sn NMR spectrum (10% relative intensity) (see Fig. 3). This result suggests that the 1:1 complex is in equilibrium with the 1:2 complex and is detectable at 85 °C in significant concentration. Such an equilibrium was observed by Denmark et al. [21], sometimes referred to as partial dissociation of the type:
85 °C (La and Le: ligands in axial and equatorial positions respectively).
Table 2 119 Sn NMR dataa of the complexes SnCl4 Æ 2L in dichloromethane Ligand (L)
Cis (Me2N)3P(O)b (Me2N)2P(O)Fc Me2NP(O)F2d
569 548 507
% Cise
2
JP–Sn
d119Sn Trans
Cis
Trans
580 567
120 126 145
191 192 –
–
50 80 100
a
Measured at slow exchange where triplets are observed. At 25 °C. c 5 °C. d 75 °C. e The approximate percentage of the cis isomer measured from NMR signals. b
SnCl4 2L SnCl4 L + L In solution, complexes of the general formula SnCl4 Æ 2L exist as a mixture of both cis and trans isomers [22]. By correlating the IR and Raman data of the complex SnCl4 Æ 2(HMPA) with 1H and 31P NMR studies, Ruzicka and Merbach [8] have shown that the coupling constants 2 J31P–119Sn were different for cis and trans isomers in the complexes of SnCl4 with (MeO)3PO (TMPA) and Me2N(MeO)2PO. At 203 K, the cis isomers of the two complexes had 2J(31P–119Sn) of 146 and 141 Hz respectively, and those for the trans isomers were 195 and 194 Hz respectively, whereas coupling constants of 102 and 168 Hz at 183 K were observed for the cis and trans complexes of SnCl4 Æ 2HMPA [9], respectively. These results are in agreement with the coupling constants of 126 and 192 Hz observed for SnCl4 Æ 2(O)PF(NMe2)2 at 268 K in our previous work [12] (see Table 2). By comparing the coupling constants, the isomers of the complexes in Table 2 could be assigned by analogy, i.e., all of the complexes which have smaller 2J(31P–119Sn) coupling
119
Sn
constants could be assigned to the cis isomers and those with larger 2J(31P–119Sn) coupling constants could be assigned to the trans ones. It is therefore possible to assign the only triplet observed in the 119Sn NMR spectra of the complexes SnCl4 Æ 2(O)PF2NR2 at 198 K to the cis isomer (2J(31P–119Sn) = 145 Hz for R = Me and 150 Hz for R = Et). The two doublets observed in the five coordinate adduct region of the 119Sn NMR spectrum at 85 °C (Fig. 3) presumably correspond to the two possible trigonal bipyramidal isomers. The chemical shifts and the magnitude of the couplings suggest that the downfield doublet at 479 ppm (2J(31P–119Sn) = 148 Hz) is due to the equatorial isomer while the upfield doublet at 489 ppm (2J(31P– 119 Sn) = 140 Hz) is due to axial phosphorus [23,24].
M.A.M.K. Sanhoury et al. / Polyhedron 25 (2006) 3299–3304
3.3. Theoretical calculations Due to difficulties in obtaining crystals suitable for X-ray analysis, the geometries of the cis and trans isomers of 1 were optimised by means of the DFT/B3LYP method in order to confirm the NMR data obtained for the two complexes prepared. The optimised structure of the cis isomer is shown in Fig. 4. The cis and trans isomers of complex 1 and its ligand have been identified as local minima on the singlet potential energy surfaces (PES). Optimised values of selected geometrical parameters are listed in Table 3. The potential energy difference between the two isomers of 1 was 2 kcal mol 1. This difference indicates that the Boltzmann population of the cis adduct corresponds to practically 100%. This is in good agreement with the experimental NMR data. ˚ , as well as the The value of the Sn–O distance, 2.228 A other bond distances and angles are in good agreement with the values reported for similar molecules using X-ray diffraction [7,9,25]. Interestingly, the P–O distance ˚ is identical to that of 1.49 obtained by the crysof 1.491 A tal structure for the HMPA complex [7]. In complex 1, the coordination mode is confirmed by a lengthening of the P@O bond compared to that of the free ˚ ) calculated at the same level of theory. This ligand (1.467 A lengthening is consistent with the low stretching frequency observed in the IR spectra and may also explain the differences observed in the 31P NMR chemical shifts between the free and bound ligands in solution. On forming complex 1 the ligand also shows a shortening of both the P–F and ˚ . This is accompanied P–N bond distances by about 0.02 A by an increase in the bond angles at the phosphorus atom, i.e., the F–P–F and F–P–N angles are increased by 1.3° and 2.6° respectively compared to those of the free ligand (see Table 3). The changes in bond distances are supported, in solution, by an observed increase in the 1JP–F coupling constant
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Table 3 Selected B3LYP-optimized geometrical parametersa for complex 1 and its ligand Ligand Sn–Cl1 Sn–Cl2 Sn–Cl3 Sn–Cl4 Sn–O1 Sn–O2 P1–O1 P2–O2 P1–F1 P1–F2 P2–F3 P2–F4 P1–N1 P2–N2 Sn–O1–P1 Sn–O2–P2 O1–Sn–O2 Cl1–Sn–Cl2 Cl3–Sn–Cl4 F1–P1–F2 F3–P2–F4 F1–P1–N1 F2–P1–N1 a
– – – – – – 1.467 – 1.580 1.585 – – 1.640 – – – – – – 96.5 – 103.7 106.2
Cis 2.421 2.402 2.410 2.410 2.254 2.254 1.491 1.491 1.570 1.565 1.569 1.565 1.619 1.619 138.7 138.6 79.6 162.5 103.3 097.8 97.9 106.3 108.9
Trans 2.424 2.429 2.424 2.429 2.204 2.204 1.494 1.494 1.567 1.564 1.564 1.567 1.616 1.616 139.0 139.0 180.0 89.9 89.9 097.9 97.9 107.0 109.2
˚ and angles in °. See Fig. 4 for labelling of the atoms. Distances are in A
value found in the complex compared with that in the free ligand. Comparable changes in geometry, i.e., shortened bonds and increased angles at the phosphorus atom, have been observed in related complexes [26,27]. It is worth noting that the longer Sn–O distance (2.25 ˚ in the HMPA complex) and sharper Sn–O– versus 2.13 A P bond angles (139 versus 165° in the HMPA complex) in complex 1 probably reflect the effect of the substitution of two dimethylamino groups in HMPA by two fluorine atoms. The substantially longer Sn–O distance in complex 1 strongly suggests a weaker coordination of Me2NP(O)F2
Fig. 4. DFT/B3LYP optimised structures and relative energies of cis and trans isomers of complex 1.
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M.A.M.K. Sanhoury et al. / Polyhedron 25 (2006) 3299–3304
compared to HMPA with SnCl4, consistent with the experimental NMR data (see Table 2). 4. Conclusion On the basis of the above results, the NMR data suggest the presence of only the cis geometry for the complexes SnCl4 Æ 2(O)PF2NR2 in dichloromethane solution within the temperature range studied. This could be explained in terms of a decrease both in the Lewis base character of the P@O group of the ligand (O)PF2NR2 and in its bulkiness compared with HMPA as a result of the substitution of two dialkylamino groups in the ligand (R2N)3P(O) by two fluorine atoms. Our results are in good agreement with those reported by Smith and Wilkins [28] and Drago [29] who showed that in SnX4 Æ 2L octahedral adducts the overlap of tin(IV) 5s, 5p and 5d orbitals depends on the positive charge on tin(IV). In our case, the presence of two fluorine atoms in the ligands renders the electronegativity of the donor atom of the ligand comparable to that of each of the four chlorine atoms on tin. This leads to a possible sp3d2 hybridisation and the molecule adopts the configuration which minimises the splitting between the dx2 y 2 and dz2 orbitals, namely the cis one. The results further confirm the fact that the less bulky the ligand the more important the ratio of the cis isomer [30] (see Table 2). The NMR data also indicate that steric effects and/or the Lewis basicity of the ligand contribute to the equilibrium between hexacoordinate and pentacoordinate complexes. We have also shown that DFT calculations are a useful supporting tool for such a study since a combination of empirical and theoretical studies can, in favourable cases, overcome the limitations of each. Acknowledgement We are indebted to Professor A. Baklouti of the Department of Chemistry, Faculty of Sciences of Tunis, University of Tunis El Manar for his valuable help and discussions of this work. References [1] D. Fa`rsaiu, R. Leu, P.J. Ream, J. Chem. Soc., Perkin Trans. 2 (2001) 427. [2] S.E. Dann, A.R.J. Genge, W. Levason, G. Reid, J. Chem. Soc., Dalton Trans. (1997) 2207. [3] C.H. Yoder, L.A. Margolis, J.M. Horne, J. Organomet. Chem. 633 (2001) 33, and references therein. [4] H.R. Hays, D.J. Peterson, in: G.M. Kosolopoff, L. Mair (Eds.), Organic Phosphorus Compounds, Vol. 3, Wiley, New York, 1972. [5] E. Le Coz, J.E. Guerchais, Bull. Soc. Chim. Fr. 1 (1971) 80. [6] S.J. Ruzicka, A.E. Merbach, Inorg. Chim. Acta 20 (1976) 223.
[7] L.A. Aslanov, V.M. Ionov, V.M. Attiya, A.B. Permin, V.S. Petrosyan, Zhur. Strukt. Khim. 18 (1977) 1103. [8] S.J. Ruzicka, A.E. Merbach, Inorg. Chim. Acta 22 (1977) 191. [9] S.E. Denmark, X. Su, Tetrahedron 55 (1999) 8727. [10] D. Tudela, V. Ferna`ndez, J.D. Tornero, J. Chem. Soc., Dalton Trans. (1985) 1281. [11] J. Rupp-Bensadon, E.A.C. Lucken, J. Chem. Soc., Dalton Trans. (1983) 495. [12] M.A.M. Khouna, M.T. Ben Dhia, M.R. Khaddar, Phosphorus, Sulfur and Silicon 180 (2005) 1673. [13] D.D. Perrin, W.L.F. Armarego, D.R. Perrin, Purification of Laboratory Chemicals, Pergamon Press, Oxford, 1966. [14] G. Schrader, B.I.O.S. Report No. 1103, London 1945. [15] A. Kovache, H. Jean, G. Garnier, Chim et Industr. 64 (1950) 287. [16] M.J. Frisch, Trucks, G.W., Schlegel, G.E. Scuseria, Robb, M.A., Cheeseman, J.R., Zakrzewski, V.G., Montgomery, J.R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Menucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, B.B.J.Stefanov, G. liu, A. Liashenko, P. Piskorz, I. Koaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challcombe, P.M.W. Gill, B.G. Johnson, W. Chen, M.W. Wong, J.L. Andres, M. HeadGordon, E.S. Replogle, J.A. Pople, GAUSSIAN 98 Revision A.7, Gaussian Inc., Pittsburgh PA. 1998. [17] (a) A.D. Becke, J. Chem. Phys. 98 (1993) 5648; (b) C. Lee, W. Yang, R.G. Parr, Phys. Rev. B. 37 (1988) 785. [18] (a) W.J. Hehre, R. Ditchfield, J.A. Pople, J. Chem. Phys. 56 (1972) 2257; (b) G.A. Petersson, A. Bennett, T.G. Tensfeldt, M.A. Al-Laham, W.A. Shirley, J. Mantzaris, J. Chem. Phys. 89 (1988) 2193; (c) G.A. Petersson, M.A. Al-Laham, J. Chem. Phys. 94 (1991) 6081; (d) P.C. Hariharan, J.A. Pople, Theoret. Chimica Acta 28 (1973) 213; (e) V.A. Rassolov, M.A. Ratner, J.A. Pople, P.C. Redfern, L.A. Curtiss, J. Comp. Chem. 22 (2001) 976. [19] (a) W.J. Stevens, H. Basch, M. Krauss, J. Chem. Phys. 81 (1984) 6026; (b) W.J. Stevens, H. Basch, M. Krauss, P. Jasien, Can. J. Chem. 70 (1992) 612; (c) T.R. Cundari, W.J. Stevens, J. Chem. Phys. 98 (1993) 5555. [20] M.S. Holt, W.L. Wilson, J.H. Nelson, Chem. Rev. 89 (1989) 11. [21] S.E. Denmark, B.R. Henke, E. Weber, J. Am. Chem. Soc. 109 (1987) 2512. [22] R. Selvaraju, K. Panchanatheswaran, Polyhedron 16 (1997) 2621. [23] J. Holecek, M. Nadvornik, K. Handlir, A. Lycka, J. Organomet. Chem. 241 (1983) 177. [24] C.J. Jameson, in: J. Mason (Ed.), Multinuclear NMR, Plenum Press, New York, 1987, p. 89. [25] C.I. Bra`nden, Acta Chem. Scand. 17 (1963) 759. [26] M.D. La Prade, C.E. Nordman, Inorg. Chem. 8 (1969) 1669. [27] L.E. Sutton (Ed.), Tables of Interatomic Distances and Configuration in Molecules and Ions, The Chemical Society, London, 1958, supplement, 1965. [28] J.A.S. Smith, E.J. Wilkins, Chem. Commun. (1965) 381. [29] J.C. Hill, R.S. Drago, R.H. Herber, J. Am. Chem. Soc. 91 (1969) 1644. [30] D.S. Dyer, R.O. Ragsdale, Inorg. Chem. 8 (1969) 1116.
Phosphoramidic difluoride complexes of tin(IV) chloride: A multinuclear (119Sn, 31P, 19F and 1H) NMR characterisation in solution M.A.M.K. Sanhoury a
a,*
, M.T. Ben Dhia a, K. Essalah b, M.R. Khaddar
a
Laboratory of Coordination Chemistry, Department of Chemistry, Faculty of Sciences of Tunis, University of Tunis El Manar, 1060 Tunis, Tunisia b Unite´ de recherche de Physico-Chimie Mole´culaire de l’IPEST, BP 51, 2070 la Marsa Tunisie, Tunisia Received 18 March 2006; accepted 4 June 2006 Available online 30 June 2006 Dedicated to Professor Ahmed Baklouti on the occasion of his retirement.
Abstract Two octahedral complexes of the general formula SnCl4 Æ 2(O)PF2NR2 (R = Me (1) or Et (2)) have been synthesised from SnCl4 and the ligand R2NP(O)F2 in anhydrous CHCl3. The new adducts have been characterised by multinuclear (119Sn, 31P, 19F and 1H) NMR, IR spectroscopy and elemental analysis. When compared with previously described hexamethylphosphoramide (HMPA) and trimethylphosphate (TMPA) analogues, the NMR data of the two complexes prepared suggest the presence of only the cis isomer in solution. Steric factors and the Lewis basicity of the ligand may explain the stereochemistry observed. Low temperature 31P and 119Sn NMR spectra show that the compounds partially dissociate in dichloromethane. Additionally, DFT/B3LYP calculations on complex 1 and its ligand have been carried out to support the interpretations of NMR data. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Phosphoryl ligands; Tin tetrachloride;
119
Sn NMR;
31
P NMR;
1. Introduction Studies of tin(IV) adducts continue to provide fundamental information about both the Lewis acid–base model and the reactivity of tin(IV) species [1–3]. In Lewis bases (ligands) of the type R1R2R3P@O, the substituent effects of various R groups (R = alkyl, aryl, halogen, dialkylamino, alkoxy, etc.) on the particular donor character and the interplay between pp–dp bonding of (P@O) and (P–R) components are perhaps the main features in terms of coordination patterns [4]. Tin(IV) chloride forms octahedral complexes with such phosphoryl ligands, having the general formula SnCl4 Æ 2L (L is the phosphoryl ligand) [5,6]. In these compounds two isomers, with the ligand L in cis *
Corresponding author. Tel.: +216 98269495. E-mail address: [email protected] (M.A.M.K. Sanhoury).
0277-5387/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.06.014
19
F NMR;
119
Sn–31P coupling constants; DFT/B3LYP
or trans mutual orientations, are possible. The complex of SnCl4 with hexamethylphosphoramide (HMPA), SnCl4 Æ 2HMPA, has been shown to exist as a trans-adduct in the solid state by X-ray diffraction [7] and in solution as a mixture of both cis and trans isomers by IR and NMR spectroscopies [8,9], whereas the complex with trimethylphosphate (TMPA) is mainly a cis-adduct [8]. Steric factors and the strength of the Lewis basicity (i.e. the donor character of the P@O group of the ligand) as well as other factors (solvent polarity, temperature, etc.) may be held responsible for the differences observed [5,10,11]. In order to explore the effects of substitution of a dimethylamino group by a fluorine atom on the Lewis basicity of HMPA, we have previously studied tin(IV) complexes containing the ligand (R2N)2P(O)F by multinuclear NMR in solution [12] and showed that the predominant species was the cis isomer.
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M.A.M.K. Sanhoury et al. / Polyhedron 25 (2006) 3299–3304
Here we report the synthesis of two new complexes of tin tetrachloride with the ligands R2NP(O)F2 (R = Me or Et) in which two fluorine atoms have substituted two dialkylamino groups in the ligand (R2N)3P(O). The new compounds were characterised by elemental analysis, IR and multinuclear (119Sn, 31P, 19F and 1H) NMR spectroscopy. On the basis of NMR data, we show that these ligands, once freed from the inductive and p-bonding (and/or steric) effects of the two dialkylamino groups, are unique in that they form almost exclusively the cis complex with SnCl4 in solution. We have also carried out DFT/B3LYP calculations on the cis and trans isomers of complex 1 and its ligand in order to support and help the analysis of NMR data.
frequencies of the different stationary points of the PES have been calculated at the same level of theory in order to identify the local minima as well as to estimate the corresponding zero-point vibrational energy (ZPE). 3. Results and discussion 3.1. Synthesis
2. Experimental
Treatment of SnCl4 in a chloroform solution with R2NP(O)F2 (R = Me or Et) gives white solids with the composition SnCl4 Æ 2L (L = R2NP(O)F2). These complexes are soluble in dichloromethane and chloroform. They were characterised by elemental analysis and particularly by their NMR data and comparison with the corresponding data for the free ligands (see Table 1).
2.1. General experimental procedures
3.2. Spectroscopic studies
All preparations were carried out under a nitrogen atmosphere in solvents dried by standard techniques [13] and stored over molecular sieves. NMR spectra were recorded on a Bruker AC-300 instrument in CD2Cl2 as solvent; 31P at 121 MHz (85% H3PO4), 19F at 282 MHz (CFCl3), 1H at 300 MHz (TMS) and 119Sn at 111.8 MHz (SnCl4). IR spectra: Perkin Elmer Paragon 1000 PC. Tin tetrachloride was distilled under vacuum before use. The ligands R2NP(O)F2 were prepared according to methods described in literature (R = Me [14]; R = Et [15]).
The infrared spectra show strong bands within the range 1285–1310 cm 1 attributed to mP@O. 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 about 35 cm 1 and is consistent with phosphoryl coordination to the tin atom. This shift is 30 cm 1 for Me2NP(O)F2 against 138 cm 1 for HMPA [5], explaining the difference in the basicity strength between these two ligands which is most probably due to the substitution of two dimethylamino groups by two fluorine atoms. The absorption band at 560–570 cm 1 corresponds to a stretching vibration of the Sn–O group. The NMR spectra of the two complexes prepared are quite similar and the data obtained from these spectra are summarised in Table 1. The 31P NMR resonances of the bound ligands are shifted to lower frequency compared with those of the free ligand, whereas the 19F and 1H NMR resonances show a higher frequency shift on complexation. The difference in the 31P chemical shift between free and bound ligands is more important than that observed in 19F and 1H NMR spectra, confirming coordination of the fluorophosphoramide ligand through the oxygen atom. The coupling constant 1JP–F is larger for the complexed ligand than for the free one (Table 1). The 31P NMR spectra of each of the complexes prepared showed a triplet pattern (the triplet being due to phosphorus–fluorine coupling) with splitting due to 31P–Sn
2.2. Preparation of complexes In a typical reaction R2NP(O)F2 (8 mmol) in dry CHCl3 (10 cm3) was slowly added to SnCl4 (1.04 g, 4 mmol) in CHCl3 (20 cm3) over a 30 min period. The resulting solution, on continual stirring, began to precipitate the white solid complex SnCl4 Æ 2(O)PF2NR2. The precipitation was complete when the product was left 24 h at 15 °C. This was collected, washed with n-hexane/CCl4 and obtained as a white powder (Yields: R = Me: 1.24 g, 60%; R = Et: 1.21 g, 53%). Anal. Calc. for C4H12N2O2P2F4SnCl4: C, 9.26; H, 2.33; N, 5.40. Found: C, 9.00; H, 2.62; N, 5.68%. Anal. Calcd. for C8H20N2O2P2F4SnCl4: C, 16.72; H, 3.51; N, 4.87. Found: C, 16.66; H, 3.52; N, 5.04%. IR (KBr): mP@O (1280–1310 cm 1); mSn–O (560–570 cm 1). 2.3. Computational details Density functional theory (DFT) calculations were carried out on the cis and trans isomers of complex 1 and its ligand using the suite of programs Gaussian 98W [16], with the non-local hybrid functional denoted as B3LYP [17]. Basis sets used were 6-31G* [18] for C, H, N, F, P, O and Cl and Steven, Bach and Krauss (SBK) [19] with ECP with polarisation for Sn. The geometries of both the cis and trans isomers of complex 1 and its ligand were optimised using analytical gradient. The harmonic vibrational
Table 1 NMR data (d/ppm and J/Hz) for the complexes SnCl4 Æ 2L and the ligands R2NP(O)F2 in CD2Cl2 at 75 °C Compound Me2NP(O)F2 Et2NP(O)F2 SnCl4 Æ 2(O)PF2NMe2 (1) SnCl4 Æ 2(O)PF2NEt2 (2)
d31P 2.50 5.00 7.63 10.40
d19F
d1H
d119Sn
1
2
81.50 83.50 83.16 84.87
2.78 – 2.93 –
– –
997 1003 1015 1023
– – 145 150
507 510
JP–F
JP–Sn
M.A.M.K. Sanhoury et al. / Polyhedron 25 (2006) 3299–3304
coupling. The latter splitting can be clearly seen only at low temperature (Fig. 1). In addition, the spectra also contain a triplet of low intensity indicating the existence of a small amount of another species. This minor triplet also showed 31P–Sn splitting at low temperature. The 119Sn NMR spectrum of each complex displayed at room temperature one broad signal in the region of hexacoordinated species [20]. This broad signal was converted at low temperature into the expected triplet. At 75 °C, the spectrum showed a triplet due to 119Sn–31P coupling. Clearly one species was present at this temperature and in which the tin atom is coupled to two phosphorus atoms, showing a stoichiometry of SnCl4 Æ 2L (Fig. 2). This is in agreement with the 31P NMR spectra where a triplet displaying Sn satellites with the corresponding coupling constants was present (Fig. 1). At lower temperatures, the 119Sn spectrum (Fig. 3) showed, in addition to the major triplet, two doublets in the region of pentacoordinated tin [20]. This indicates that these different tin species are presumably in rapid equilibrium with each other and with the free ligand, consistent with 31P and 19F NMR spectra where ligand exchange reactions are fast at room temperature. The rates of these reactions were decreased by lowering the temperature and separate signals corresponding to the free and bound ligands present in solution were observed. Cooling to
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Fig. 2. 119Sn NMR spectrum of the complex SnCl4 Æ 2(O)PF2NMe2 in CD2Cl2 at 75 °C.
85 °C revealed the presence in the 31P NMR spectrum (and also in the 19F NMR spectrum) of a new signal at 9.41 ppm (Fig. 1), in addition to SnCl4 Æ 2L and to the
Fig. 1. 31P-{1H} NMR spectrum of the complex SnCl4 Æ 2(O)PF2NEt2 in CD2Cl2 at pentacoordinated tin complex, L: free ligand).
85 °C (*: Sn satellites, Lo: octahedral tin complex, Lp:
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Fig. 3.
M.A.M.K. Sanhoury et al. / Polyhedron 25 (2006) 3299–3304
119
Sn NMR spectrum of the complex SnCl4 Æ 2(O)PF2NEt2in CD2Cl2 at
free ligand, which we assign to SnCl4 Æ L. This assignment is supported by the presence of a doublet pattern in the 119 Sn NMR spectrum (10% relative intensity) (see Fig. 3). This result suggests that the 1:1 complex is in equilibrium with the 1:2 complex and is detectable at 85 °C in significant concentration. Such an equilibrium was observed by Denmark et al. [21], sometimes referred to as partial dissociation of the type:
85 °C (La and Le: ligands in axial and equatorial positions respectively).
Table 2 119 Sn NMR dataa of the complexes SnCl4 Æ 2L in dichloromethane Ligand (L)
Cis (Me2N)3P(O)b (Me2N)2P(O)Fc Me2NP(O)F2d
569 548 507
% Cise
2
JP–Sn
d119Sn Trans
Cis
Trans
580 567
120 126 145
191 192 –
–
50 80 100
a
Measured at slow exchange where triplets are observed. At 25 °C. c 5 °C. d 75 °C. e The approximate percentage of the cis isomer measured from NMR signals. b
SnCl4 2L SnCl4 L + L In solution, complexes of the general formula SnCl4 Æ 2L exist as a mixture of both cis and trans isomers [22]. By correlating the IR and Raman data of the complex SnCl4 Æ 2(HMPA) with 1H and 31P NMR studies, Ruzicka and Merbach [8] have shown that the coupling constants 2 J31P–119Sn were different for cis and trans isomers in the complexes of SnCl4 with (MeO)3PO (TMPA) and Me2N(MeO)2PO. At 203 K, the cis isomers of the two complexes had 2J(31P–119Sn) of 146 and 141 Hz respectively, and those for the trans isomers were 195 and 194 Hz respectively, whereas coupling constants of 102 and 168 Hz at 183 K were observed for the cis and trans complexes of SnCl4 Æ 2HMPA [9], respectively. These results are in agreement with the coupling constants of 126 and 192 Hz observed for SnCl4 Æ 2(O)PF(NMe2)2 at 268 K in our previous work [12] (see Table 2). By comparing the coupling constants, the isomers of the complexes in Table 2 could be assigned by analogy, i.e., all of the complexes which have smaller 2J(31P–119Sn) coupling
119
Sn
constants could be assigned to the cis isomers and those with larger 2J(31P–119Sn) coupling constants could be assigned to the trans ones. It is therefore possible to assign the only triplet observed in the 119Sn NMR spectra of the complexes SnCl4 Æ 2(O)PF2NR2 at 198 K to the cis isomer (2J(31P–119Sn) = 145 Hz for R = Me and 150 Hz for R = Et). The two doublets observed in the five coordinate adduct region of the 119Sn NMR spectrum at 85 °C (Fig. 3) presumably correspond to the two possible trigonal bipyramidal isomers. The chemical shifts and the magnitude of the couplings suggest that the downfield doublet at 479 ppm (2J(31P–119Sn) = 148 Hz) is due to the equatorial isomer while the upfield doublet at 489 ppm (2J(31P– 119 Sn) = 140 Hz) is due to axial phosphorus [23,24].
M.A.M.K. Sanhoury et al. / Polyhedron 25 (2006) 3299–3304
3.3. Theoretical calculations Due to difficulties in obtaining crystals suitable for X-ray analysis, the geometries of the cis and trans isomers of 1 were optimised by means of the DFT/B3LYP method in order to confirm the NMR data obtained for the two complexes prepared. The optimised structure of the cis isomer is shown in Fig. 4. The cis and trans isomers of complex 1 and its ligand have been identified as local minima on the singlet potential energy surfaces (PES). Optimised values of selected geometrical parameters are listed in Table 3. The potential energy difference between the two isomers of 1 was 2 kcal mol 1. This difference indicates that the Boltzmann population of the cis adduct corresponds to practically 100%. This is in good agreement with the experimental NMR data. ˚ , as well as the The value of the Sn–O distance, 2.228 A other bond distances and angles are in good agreement with the values reported for similar molecules using X-ray diffraction [7,9,25]. Interestingly, the P–O distance ˚ is identical to that of 1.49 obtained by the crysof 1.491 A tal structure for the HMPA complex [7]. In complex 1, the coordination mode is confirmed by a lengthening of the P@O bond compared to that of the free ˚ ) calculated at the same level of theory. This ligand (1.467 A lengthening is consistent with the low stretching frequency observed in the IR spectra and may also explain the differences observed in the 31P NMR chemical shifts between the free and bound ligands in solution. On forming complex 1 the ligand also shows a shortening of both the P–F and ˚ . This is accompanied P–N bond distances by about 0.02 A by an increase in the bond angles at the phosphorus atom, i.e., the F–P–F and F–P–N angles are increased by 1.3° and 2.6° respectively compared to those of the free ligand (see Table 3). The changes in bond distances are supported, in solution, by an observed increase in the 1JP–F coupling constant
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Table 3 Selected B3LYP-optimized geometrical parametersa for complex 1 and its ligand Ligand Sn–Cl1 Sn–Cl2 Sn–Cl3 Sn–Cl4 Sn–O1 Sn–O2 P1–O1 P2–O2 P1–F1 P1–F2 P2–F3 P2–F4 P1–N1 P2–N2 Sn–O1–P1 Sn–O2–P2 O1–Sn–O2 Cl1–Sn–Cl2 Cl3–Sn–Cl4 F1–P1–F2 F3–P2–F4 F1–P1–N1 F2–P1–N1 a
– – – – – – 1.467 – 1.580 1.585 – – 1.640 – – – – – – 96.5 – 103.7 106.2
Cis 2.421 2.402 2.410 2.410 2.254 2.254 1.491 1.491 1.570 1.565 1.569 1.565 1.619 1.619 138.7 138.6 79.6 162.5 103.3 097.8 97.9 106.3 108.9
Trans 2.424 2.429 2.424 2.429 2.204 2.204 1.494 1.494 1.567 1.564 1.564 1.567 1.616 1.616 139.0 139.0 180.0 89.9 89.9 097.9 97.9 107.0 109.2
˚ and angles in °. See Fig. 4 for labelling of the atoms. Distances are in A
value found in the complex compared with that in the free ligand. Comparable changes in geometry, i.e., shortened bonds and increased angles at the phosphorus atom, have been observed in related complexes [26,27]. It is worth noting that the longer Sn–O distance (2.25 ˚ in the HMPA complex) and sharper Sn–O– versus 2.13 A P bond angles (139 versus 165° in the HMPA complex) in complex 1 probably reflect the effect of the substitution of two dimethylamino groups in HMPA by two fluorine atoms. The substantially longer Sn–O distance in complex 1 strongly suggests a weaker coordination of Me2NP(O)F2
Fig. 4. DFT/B3LYP optimised structures and relative energies of cis and trans isomers of complex 1.
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compared to HMPA with SnCl4, consistent with the experimental NMR data (see Table 2). 4. Conclusion On the basis of the above results, the NMR data suggest the presence of only the cis geometry for the complexes SnCl4 Æ 2(O)PF2NR2 in dichloromethane solution within the temperature range studied. This could be explained in terms of a decrease both in the Lewis base character of the P@O group of the ligand (O)PF2NR2 and in its bulkiness compared with HMPA as a result of the substitution of two dialkylamino groups in the ligand (R2N)3P(O) by two fluorine atoms. Our results are in good agreement with those reported by Smith and Wilkins [28] and Drago [29] who showed that in SnX4 Æ 2L octahedral adducts the overlap of tin(IV) 5s, 5p and 5d orbitals depends on the positive charge on tin(IV). In our case, the presence of two fluorine atoms in the ligands renders the electronegativity of the donor atom of the ligand comparable to that of each of the four chlorine atoms on tin. This leads to a possible sp3d2 hybridisation and the molecule adopts the configuration which minimises the splitting between the dx2 y 2 and dz2 orbitals, namely the cis one. The results further confirm the fact that the less bulky the ligand the more important the ratio of the cis isomer [30] (see Table 2). The NMR data also indicate that steric effects and/or the Lewis basicity of the ligand contribute to the equilibrium between hexacoordinate and pentacoordinate complexes. We have also shown that DFT calculations are a useful supporting tool for such a study since a combination of empirical and theoretical studies can, in favourable cases, overcome the limitations of each. Acknowledgement We are indebted to Professor A. Baklouti of the Department of Chemistry, Faculty of Sciences of Tunis, University of Tunis El Manar for his valuable help and discussions of this work. References [1] D. Fa`rsaiu, R. Leu, P.J. Ream, J. Chem. Soc., Perkin Trans. 2 (2001) 427. [2] S.E. Dann, A.R.J. Genge, W. Levason, G. Reid, J. Chem. Soc., Dalton Trans. (1997) 2207. [3] C.H. Yoder, L.A. Margolis, J.M. Horne, J. Organomet. Chem. 633 (2001) 33, and references therein. [4] H.R. Hays, D.J. Peterson, in: G.M. Kosolopoff, L. Mair (Eds.), Organic Phosphorus Compounds, Vol. 3, Wiley, New York, 1972. [5] E. Le Coz, J.E. Guerchais, Bull. Soc. Chim. Fr. 1 (1971) 80. [6] S.J. Ruzicka, A.E. Merbach, Inorg. Chim. Acta 20 (1976) 223.
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