J. CHEM. SOC. DALTON TRANS. 1984 1787 Adducts of Silicon Tetrafluoride with Aminocyclophosphazenes : Synthesis and Characterization Bettadapura S. Suresh, Vadapalli Chandrasekhar, and Doddaballapur K. Padma * Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 01 2, India Stable solid adducts of SiF4 with the following aminocyclophosphazenes have been synthesized : N3P3(NHCH2CH2NH)(NMe2)4, (1) ; N3P3(NHCH2CH2NH)C14, (2) ; N3P3(NMe2)4C12, (3) ; N3P3(NHMe),, (4) ; N3P3(NMe2),, (5) ; N,P,(NHMe),, (6) ; N4P4(NMe2),, (7) ; and N,P,(NHBu'),, (8). They have been characterized by elemental analysis, i.r., and n.m.r. ( H, 31 P, and 19F) spectroscopy. The composition of the adducts varies depending on the ring size and also on the nature of the substituents on the phosphorus.The number of SiF4 molecules accommodated by the ligands is larger when the ring size is large, while it is less when the ligands contain chlorine. Except in the cases of ligands (1) and (2), bonding is through the ring nitrogens. With ( I ) , both exocyclic nitrogen and ring nitrogen atoms, and with (2) only exocyclic nitrogen atoms, participate in co-ordination. In these two cases the silicon is six-co-ordinated, while in the other cases it is five-co-ordinated. Silicon tetrafluoride, a good Lewis acid, is known to form stable adducts with a variety of donor groups such as amines and a few with oxygen donor^.^-^ There is not much inform- ation about adducts with ligands containing phosphorus and also with multidentate ligands containing more than two donor sites.The only relevant work is that of Beattie and Ozin who obtained addition compounds, SiF4*PMe3 and SiF4-2PMe3, at low temperature. However, no adduct form- ation has been observed between silicon tetrafluoride and triphenylphosphine even when the reactants are cooled to -96 oC.6 Therefore, it was of interest to see whether amino- cyclophosphazenes, which behave as potential ligands to- wards suitable electron acceptors, having donor sites at ring nitr~gen,~ exocyclic nitr~gen,~ and phosphorus,8 would bond with silicon tetrafluoride. Preliminary studies indicated reaction with the eight cyclophosphazenes studied. Experimental Materials.-The following ligands were prepared by standard methods: 4,4,6,6-tetrakis(dimethylamino)-2,2-spiro- CH2NH)(NMe2)4 (l);9*10 4,4,6,6-tetrachloro-2,2-spiro- CH2NH)C14 (2); l1 2-cis-4-dichloro-2,4,6,6-tetrakis(dimethyl- amino)cyclotriphosphazene, N3P3(NMe2)4C12 (3) ; l2 hexakis- (methylamino)cyclotriphosphazene, N3P3(NHMe)6 (4) ; l3 hexakis(dimethylamino)cyclotriphosphazene, N3P3(NMe2), ( 5 ) ; l2 octakis(methylamino)cyclotetraphosphazene, N4P4- (NHMe)8 (6) ; l4 octakis(dimethy1amino)cyclotetraphosph- azene, N4P4(NMe2)8 (7) ; l4 and octakis(t-buty1amino)cyclo- tetraphosphazene, N4P4(NHBut)8 @).I5 Silicon tetrafluoride was prepared by fluorination of silicon tetrachloride with lead fluoride in acetonitrile.16 Its purity (99.8%) was checked by i.r.spectroscopy and chemical analysis. Diethyl ether was washed with cold water, dried over CaCl,, and distilled over lithium aluminium hydride.Chloro- form was washed with water, dried by refluxing over P4O10, and then distilled. (et hylenediamino)cyclotriphosphazene, NSPS(NHCH2- (et hylenediamino)cyclotriphosphazene, NjPj(NHCH2- Preparation of the Adducts.-The reaction vessel consisted of a round-bottomed flask (250 cm3) fitted with standard joints and vacuum stopcocks. A known amount of the ligand was dissolved in a suitable solvent (100 cm3) and placed in the reaction vessel. The latter was connected to a glass globe containing a known amount of SiF,. The solution was frozen in liquid nitrogen and the air was pumped out of the system. After bringing the contents of the flask to room temperature, a four-fold excess of SiF4 was added by opening the inter- connecting stopcocks.In all cases immediate formation of a precipitate was observed. The reaction mixture was stirred with a magnetic pellet for a few hours. The excess of unreacted SiF4 remaining after the reaction was condensed back into the glass globe along with some vapours of the solvent. The flask was detached from the assembly and filled with dry nitrogen. The precipitate was filtered off under suction in a dry nitrogen atmosphere, washed with the solvent as quickly as possible, and dried under vacuum. The amount of unreacted SiF4 was determined after dissolving the solvent vapours in benzene and freezing out the benzene at -80 OC.16 By measuring the pressure and volume of SiF4 at constant temperature before and after the experiment the stoicheiometry of the reaction could be determined.The experimental procedure adopted for the synthesis of adducts of ligands (4) and ( 6 ) was slightly different. These ligands are not soluble in solvents such as diethyl ether and benzene, but slightly soluble in chloroform at room tem- perature, and completely soluble in boiling chloroform. Suspensions of these ligands were treated with SiF4 for 12 h with continuous stirring at room temperature. The solids were then filtered off, washed with hot chloroform, and dried under vacuum. Details of the preparation of the individual adducts are given in Table 1. Characterization of' the Adducts.-The adducts were characterized by ix., n.m.r., and analytical data. The i.r. spectra in the range 400-4 000 cm-I of the pure ligands and the adducts were recorded using Nujol mulls and a Perkin- Elmer 599 spectrophotometer.The important absorption bands are assigned in Table 2. Proton n.m.r. spectra were recorded for CDC13 solutions with SiMel as internal standard using a Varian T-60 spectro- meter. Phosphorus-31 and I9F n.m.r. spectra were recorded for CD3CN solutions with 85% H3P04 as external standard and CFC13 as internal standard respectively. Bruker WH 90, operating at 36.431 MHz, and WM250, operating at 233 MHz, instruments were used for recording 31P and I9F spectra respectively. The chemical shifts are expressed in terms of the 6 scale, with upfield shifts negative (Table 3). The analytical data are given in Table 4.1788 J. CHEM. SOC. DALTON TRANS. 1984 Table 1. Preparation of adducts of silicon tetrafluoride with aminocyclophosphazenes Amount (mmol) Combining Reaction r h 3 ratio, time Reacted ligand : Ligand Solvent (h) Ligand SIF, SiF, SiF, Adduct (1) OEtz 6 7.89 32.55 12.23 2 : 3 (14 (2) OEtt 6 6.75 29.50 6.81 1 : 1 (2a) (3) OEtz 6 3.45 16.24 3.39 1 : l (3a) (4) CHClj 10 6.56 28.45 10.04 2 : 3 (44 ( 5 ) OEtz 6 2.95 13.35 4.57 2 : 3 (5a) (6) CHClj 10 4.85 19.65 15.04 1 : 3 ( W (7) OEtz 6 3.47 16.24 10.59 1 : 3 (7a) (8) OEtz 6 4.25 18.85 8.63 1 : 2 @a) Solubility of adduct in CHC13 Soluble White, powdery hygroscopic solid Insoluble Slightly yellowish hygroscopic sticky solid Soluble White, powdery hygroscopic solid Insoluble White, powdery hygroscopic solid Soluble White, powdery non-hygroscopic solid Insoluble Slight yellowish sticky, hygroscopic solid Soluble White, powdery, non-hygroscopic solid Soluble White, powdery non-hygroscopic solid and CH3CN Nature of adduct Table 2.Infrared spectral data (cm-') of aminocyclophosphazene adducts of silicon tetrafluoride and the free ligands v(N-H) 3 360m, br 3 230m, br 3 250m 3 180m 3 430m, br 3 350m, br 3 350m, br 3 250m, br 3 200m, br 3 330m, br 3 200m, br 3 290m, br 3 250m, br 3 l a m , br 3 400m, br 3 270m, br 3 350m 3 l5Om 3 430m 3 350m 3 380m 3 320m 3 250s, br v( F N ) 1200s 1 180s 1260s 1 205s 1 220s 1250s 1200s 1 230s 1190s 1230s 1260s 1 175s 1275s 1200s 1 180s 1215s, br 1 300s, br 1 265s 1310s 1340s 1 23Os, br 1200s 1 240s 1320s v(Si-F)/ v(Si-F) v( PN=P) S(Si-F) + 730s 900s 480m 7 50s 760s 7 30s 890w 470m 76qsh) 750s 900m 480m 730s 900m 475m 750s 770s 900s 470m 7 30s 760(sh) 900m 475w 770s 900s 480m 790(sh) 7 30s 920s 480m 7 50s Results and Discussion It is evident from the experimental results that SiF4 forms stable adducts of various compositions depending on the nature of the aminocyclophosphazenes.The possible sites of co-ordination are (i) the endocyclic nitrogen, (ii) the exocyclic nitrogen, and (iii) the ring phosphorus. In several studies reported on similar adducts, the effect of complex formation is reflected in the i.r. spectra of the complexes. The major effect of CF donation of a nitrogen lone pair of electrons will be on the in-plane bonding system." In general, the i.r. spectra are more complex than those of the free ligands, and v(P=N) undergoes a positive shift of 40-200 nitrogen can also be seen in the 'H n.m.r.spectrum of the substituents. The NH proton in N3P3[NH(CH2)2CH3]s under- goes a downfield shift of about 6 1.4 upon adduct formation, cm-I . 18.19 The effect of co-ordination through the endocyclicJ. CHEM. SOC. DALTON TRANS. 1984 1789 while the CH3 and CH2 protons do not undergo any change in chemical shift.20 It has also been observed that the 31P n.m.r. chemical shifts of the adducts move upfield compared to those of the ligand~.'~.~' adducts (1 : 1 and 2 : 1, ligand: SiF4) with unidentate ligands indicating that the ring nitrogens are involved in co-ordin- ation. The v(N-H) frequencies have suffered a negative shift and the n.m.r. resonances of the NH protons have shifted substantially downfield (6 5.0), suggesting that the exocyclic Silicon tetrafluoride forms five- and six-co-ordinated nitrogens of the ethylenediamine group also participate in co-ordination.The strong band at 900 cm-' in the i.r. spectrum depending on the experimental conditions, the latter being more common than the former.18 In six-co-ordinated SiF4 adducts, which have octahedral structures, the v(Si-F) frequency of SiF4 at 1 031 cm-I undergoes a negative shift of about 300 ~ m - ~ . ~ * ~ ~ - ~ ~ The five-co-ordinate, 1 : 1 adducts of SiF4 can have either a trigonal bipyramidal or tetragonal pyramidal structures. The v(Si-F) in these cases are found to lie in the region 900-1 000 cm-'. The 19F n.m.r. spectra are characteristic of five- or six-co-ordination in silicon fluorides ; chemical shifts for the series SiF4, SiFS-, and SiFs2- are 6 160.3, 136.0, and 128.2 respectively against CC1,F as internal ~tandard.~' This was made use of in concluding that the silicon in SiF4*Pt(PPh3)2 is five-co-ordinate, the 19F spectrum at room temperature in acetone consisting of a singlet at 6 137.3.6 In the light of the above observations, the bonding in the present adducts will now be discussed.Adducts with Ligands (I) and (2).-2N,P,(NHCH2CHzNH)- (NMe2)4.3SiF4, (la). The v(P=N) absorption band has under- gone a positive shift accompanied by splitting, Table 2, Table 3. N.m.r. data * for the adducts of silicon tetrafluoride and for the pure ligands Compound 'H 31P 19F NH 2.2, NCH, 2.6, NCH2 3.34 NH 7.2, NCHj 2.63, NCH2 3.4 NCH, 2.78 NCHS 2.80 NCHj 2.55 NCH, 2.70 NCH, 2.63 NCH3 2.73 NH 2.20, NBu' 1.28 NH 3.63, NBu' 1.40 -131.42, - 131.33 - 131.29, - 131.25 24.6 9.6 - 3.1 19.58 - 137.04 7.71 - 137.0 - 1.98 - 136.87 * Chemical shifts are expressed in terms of the 6 scale, with upfield shifts negative.+ could be assigned to a P-N=P linkage,20 or to a five-co- ordinated SiF4 adduct. The I9F n.m.r. spectrum shows a broad signal centred around 6 -131, suggesting that the silicon is six-co-ordinate. Thus, the band at 900 cm-' could be assigned to a ligand vibration and those at 700-800 cm-' to v(Si-F). Based on the above observations, adduct (la) contains six-co-ordinated silicon and both exocyclic and endo- cyclic nitrogens participate in this co-ordination. This mode has been noted in a few other complexes of cyclophospha- z e n e ~ . ~ ~ , ~ ' The composition 2 : 3 suggests that the adduct is an equimolar mixture of 1 : 1 and 1 : 2 six-co-ordinated silicon adduct s.N3P3(NHCH2CH2NH)Cl4*SiF4, (2a). The non-particip- ation of ring nitrogens can be seen in the moderate increase in the v(P=N) frequency of about 30 cm-' on adduct formation, compared to the other adducts. However, the v(N-H) absorptions have undergone a substantial negative shift (100 cm-') indicating that the ethylenediamino-nitrogens are involved in co-ordination with SiF4. The single strong band of v(Si-F) at 730 cm-' indicates that silicon is six-co-ordinate. The non-participation of the ring nitrogens could be due to the presence of four chlorine atoms in the ring which makes the endocyclic nitrogens more basic. Adducts with Ligands (3)-(8).-N3P3(NMe2)4C12*SiF4, (3a), 2N3P3(NHMe)6*3SiF4, (4a), 2N3P3(NMe2)6*3SiF4, (5a), N4P4(NHMe),.3SiF4, (6a), N4P4(NMe2)8*3SiF4, (7a), and N4P4(NHBu')8*2SiF4, (8a).In the above six adducts a general pattern is observed. There is an increase in the v(P=N) frequency accompanied by splitting in the i.r. spectrum indicating that only endocyclic nitrogens are involved in co- ordination. This is supported by the observation that the NCH, protons have undergone only a very small downfield shift of about 6 0.02 upon adduct formation, which indicates that the exocyclic nitrogens are not involved in bonding with SiF4. The appearance of v(Si-F) absorptions in the region 700-800 cm-' and also at 900 cm-I suggests that silicon is five-co-ordinated. This is supported by the I9F n.m.r. spectra Table 4.Analytical data for the adducts of silicon tetrafluoride with aminocyclophosphazenes Elemental analysis (%) Calc. Found C H N C H N 22.85 5.70 24.0 23.05 6.3 23.85 7 A c r A \ r * \ 5.45 1.35 15.95 5.60 1.65 15.45 19.75 4.95 20.15 19.75 5.35 20.20 15.3 5.10 26.75 15.7 5.80 26.7 25.95 6.50 22.7 26.3 6.80 22.6 13.1 4.35 22.95 13.1 5.00 22.6 22.75 5.70 19.9 23.35 6.35 20.45 40.15 8.30 17.55 40.55 8.80 17.55 Composition of adduct ligand: SiF, 2 : 3 1 : l 1 : l 2 : 3 2 : 3 1 : 3 1 : 3 1 : 2 M.p. ("C) r 7 Pure ligand Adduct 138 149-1 52 (decomp.) (decomp.) 198 225-230 104 145-150 257 200 (decomp.) 104 160-170 (sublim.) 206 260-268 (charg.) 230-237 160 (decomp.) 180-200 220-235 (decomp.) (decomp.)1790 J. CHEM. SOC. DALTON TRANS. 1984 which show a single sharp signal at 6 - 137 characteristic of five-co-ordinated silicon fluorides.The jlP n.m.r. spectra also show an upfield shift which is characteristic of cyclophosph- azene complexes, except in the case of (8a), having a com- position 1 : 2, which shows a downfield shift on complexa- tion.18*21 A downfield shift on complexation occurs only in the case of adduct (8a). The composition of the adducts, 2 : 3 or 1 : 3, indicates that they are equimolar mixtures of 1 : 1 and 1 : 2 and 1 : 2 and 1 : 4 adducts respectively. This has been observed with other SiF4 a d d ~ c t s . ~ * ~ ~ - ~ ~ It seems that steric factors determine the co-ordination number of silicon, the bulky ligands favouring five-co-ordinati~n.~~*~~ Among the adducts (1 a)- (8a) we see a regular trend in composition which can be attributed to steric as well as electronic factors.For instance, (i) the combining ratio (ligand: SiF4) varies from 1 : 1 and 1 : 2 in the case of trimeric ligands to 1 : 2 and 1 : 4 in the case of tetrameric ligands. This can be explained on the basis of the ring size of the ligands. (ii) Even when the ring size is the same the composition varies. This can be attributed to the nature of the substituents present in the ligand molecules. Con- sidering the adducts (la)-(5a), the trimeric phosphazenes, it is clear that the ligands which do not contain any chlorine co-ordinate a larger number of SiFI molecules than those which contain chlorine. This might be due to electronic factors ; chlorine being highly electronegative draws the electron density towards itself making the ring nitrogens less basic.It was observed in the present investigation that SiF4 does not form any adduct with hexachlorocyclotriphosphazene (N3P3Cla). On the other hand, the variation in composition of adducts (6a)-(8a) of tetrameric phosphazenes can be explained on the basis of steric factors. Here the substituents on the phosphazene rings are of similar electronegativity and none of the ligands contains chlorine; t-butyl being more bulky than methyl, ligand (8) can accommodate fewer SiF4 molecules than ligands (6) and (7). Acknowledgements Two of us (B. S. S. and V. C.) are grateful to the Department of Atomic Energy, India and the Council of Scientific and Industrial Research, India respectively for financial assistance.We also thank Dr. M. Woods, Professor S. S. Krishnamurthy, and Professor A. R. 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