DALTON J. Chem. Soc., Dalton Trans., 1997, Pages 2167–2169 2167 Formation of a diazadiphosphetidine from the reactions of a bis(aminosilyl) ether with PCl3: crystal structure of cis- [(ButNH)PNBut]2 N. Dastagiri Reddy,a Anil J. Elias *,a and Ashwani Vij b a Department of Chemistry, Indian Institute of Technology, Kanpur, 208 016, UP, India b Single Crystal Diffraction Laboratory, University of Idaho, Moscow, Idaho 83843, USA Reaction of O[SiMe2N(But)H]2 1 with PCl3 in 1 : 1 molar ratio in hexane in presence of NEt3 gave the cyclic fourmembered diazadiphosphetidine cis-[(ButNH)PNBut]2 2 instead of the expected six-membered silazoxyphosphine indicating cleavage of the Si]N bond.A crystal structure analysis of 2 showed that the NH hydrogens of the ButNH groups are in an endo-endo orientation above the (PN)2 ring which is in contrast to the exo-endo orientation for the known structure of its disulfide. Bis(aminosilyl) ethers of the type O[SiMe2N(R)H]2 (R = Me, Et or But) are excellent starting materials for the synthesis of novel inorganic heterocycles as well as metallacycles having Si, N, O and another heteroelement as part of the ring framework.Wannagat and co-workers carried out detailed reactions of O[SiMe2N(R)H]2 ( R = Me or Et ) with a variety of maingroup halides such as PPhCl2, PEtCl2,1 PMeCl2,2 SnCl4, GeCl4, AsCl3, SiMe(CH2]] CH)Cl2, SiCl4, SiBr4,3 and BeCl2 4 in the presence of NEt3 as HCl scavenger or after dilithiation using nbutyllithium.The reactions invariably led to the formation of six-membered heterocycles of the type LnMSi2N2O [LnM = PhP, EtP, MeP, Cl2Sn, Cl2Ge, ClAs, Me(CH2]] CH)Si, Cl2Si, Br2Si or Be]. Reactions with TiCl4 and ZrCl4 also gave similar metallacycles which were spirocyclic in nature.5 Recently Roesky and co-workers carried out reactions of O[SiMe2N(But)H]2 1 after dilithiation with main-group and transition metal halides in low oxidation states to synthesize novel six-membered silazoxy metallacycles with TeII, SnII and GeII 6 as the heteroelement as well as twelve-membered silazoxy metallacycles with ZnII, CoII,7 FeII, MnII, NiII or CrII 6 wherein the metals, were stabilized in low co-ordination and oxidation states. A variety of reactions have also been carried out on PIIISi2N2O ring compounds (Me and Ph substituents on P, Me on N) leading to oxidation of the phosphorus(III) site to PV while retaining the six-membered ring structure.2 Similar silazoxy heterocycles with PV as part of the ring framework were also prepared by reactions of phenoxy thiophosphoryl dihydrazide and phenoxy phosphoryl dihydrazide with tetraalkyl-1,3- dichlorodisiloxanes and structurally characterized.8 Reactions of O[SiMe2N(R)H]2 with PCl3 have been reported briefly as leading to only polymeric products which were not properly identified.1 In our attempts to make silazoxyphosphines with varying ring sizes and substituents on silicon, nitrogen and phosphorus, we observed for the first time that instead of cyclization to form a six-membered silazoxyphosphine, O[SiMe2N(But)H]2 cleaves at the Si]N bonds and forms the diazadiphosphetidine cis-[(ButNH)PNBut]2 2.We report herein the details of this unusual reaction as well as the crystal structure of 2. Experimental All manipulations were carried out using standard Schlenk techniques using a vacuum line in an atmosphere of dry nitrogen. The compound O[SiMe2N(But)H]2 1 was prepared according to the reported procedure,7 PCl3 (Aldrich) was distilled prior to use and hexane and triethylamine were distilled and dried by standard procedures.In a typical reaction 1 (1.22 g, 4.4 mmol) was first dissolved in hexane (30 cm3), the solution cooled to 0 8C and with vigorous stirring, PCl3 (0.62 g, 4.5 mmol) added slowly using a syringe. After adding triethylamine (1.50 cm3), the mixture was brought to room temperature over a period of 15 min and then refluxed for 36 h whereupon a white solid (identified as NEt3?HCl) was observed.This was filtered off using a frit under nitrogen and the filtrate concentrated in vacuo to yield a semisolid mass which was sensitive to air and moisture. On redissolving this in hexane and keeping it at 0 8C for 24 h, colourless crystals of cis-[(ButNH)PNBut]2 2 were obtained (0.43 g, 56%), m.p. 143 8C (from hexane) (Found: C, 55.1; H, 11.2. C16H38N4P2 requires C, 55.2; H, 10.9%); n& max/cm21 3320w, 2915s, 1460s, 1362s, 1220s, 1040m, 1030m, 998s, 915w, 870s, 820m, 790m and 735m (Nujol); dH(C6D6) 1.28 (18 H, s, CH3), 1.53 (18 H, s, CH3) and 2.60 (2 H, br s, NH); dP(C6D6) 89.1 (s).These data were found to agree with the reported values for 2.9–11 Crystallography Single crystals of cis-[(ButNH)PNBut]2 2 suitable for X-ray studies were obtained by slow crystallization under nitrogen from hexane at 0 8C. Crystal data and data collection parameters. C16H38N4P2, M = 348.44, monoclinic, space group Pc, a = 9.6654(5), b = 5.9212(3), c = 18.9757(9) Å, b = 100.68(10)8, U = 1067.18(9) Å3, T = 213 K, graphite-monochromated Mo-Ka radiation, l = 0.710 73 Å, Z = 2, Dc = 1.084 Mg m23, F(000) = 384, colourless crystals with dimensions 0.35 × 0.20 × 0.15 mm, m(Mo-Ka) = 0.207 mm21, SADABS absorption correction,12 maximum and minimum transmission 0.962 and 0.783, Siemens SMART diffractometer with a CCD detector at 254 8C, q range for data collection 2.14–25.008, limiting indices 212 < h < 12, 26 < k < 7, 225 < l < 24, reflections collected 10 309, independent reflections 3102 (Rint = 0.0297). The data were acquired using Siemens SMART software and processed on a SGI-Indy/Indigo 2 workstation by using the SAINT software.13 Structure solution and refinement.The structure was solved by direct methods using the SHELXS 90 14 program and refined by full-matrix least squares on F 2 using SHELXL 93, incorporated in SHELXTL-PC V 5.03.15 All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were located from the difference electron-density maps and included in the refinement process in an isotropic manner.The final R indices were2168 J. Chem. Soc., Dalton Trans., 1997, Pages 2167–2169 [I>2s(I)]; R(F) = 0.047 and wR(F 2) = 0.113, parameters refined = 188, goodness of fit = 1.06. Atomic coordinates, thermal parameters, and bond lengths and angles have been deposited at the Cambridge Crystallographic Data Centre (CCDC).See Instructions for Authors, J. Chem. Soc., Dalton Trans., 1997, Issue 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 186/494. Results and Discussion In our attempts to make PIII-containing silazoxy heterocycles by the reactions of compound 1 with PPhCl2 in presence of a tertiary amine or after lithiation we observed a general hesitancy for the reaction to proceed.A similar trend in reactivity was observed when transamination was attempted using P- (NR2)3 (R = Me or Et) with 1. However, a reaction of PCl3 with 1 in the presence of NEt3 was found to proceed slowly on refluxing in hexane. Instead of the expected silazoxy phosphine, the reaction gave exclusively a diazadiphosphetidine 2 (Scheme 1). All reactions reported so far of O[SiMe2N(R)H]2 and MeN- [SiMe2N(R)H]2 as such or after metallation, with main-group and transition-metal halides, have resulted in the formation of six- or twelve-membered heterocycles indicating the stability of the Si]N bond during such reactions.Reactions of phosphorus( III) dihalides like PRCl2 (R = Me, Et or Ph) with O[Si- Me2N(R)H]2 and MeN[SiMe2N(R)H]2 (R = Me or Et) are reported to give silazoxy and silaza phosphines which have been characterized by spectral and analytical techniques.1,2 The fact that O[SiMe2N(But)H]2 1 behaves differently may be related to a variety of factors. The bulkiness of the tertiary butyl group possibly prevents attack of the PCl2 moiety of the HN(But)- SiMe2OMe2Si(But)NPCl2 unit formed in the first step of the reaction on the other amino hydrogen.This may lead to the formation of ClSiMe2OMe2Si(But)NP(Cl)N(But)H which may further cleave at the Si]N bond leading to ButN]] PNHBut. Dimerization of the latter can lead to the diazadiphosphetidine 2. It is noteworthy that isolation of mono- and di-chloro analogues of the diazadiphosphetidines16–18 were not observed in this reaction.Reactions leading to cleavage of Si]N bonds with phosphorus chlorides are well documented.19 This being the first step followed by N]H cleavage to precipitate amine hydrochloride may also bring about the formation of 2. This is further assisted by the fact that the P]Cl bonds in PCl3 are comparatively weaker (326 kJ mol21) than a standard Si]Cl bond (381 kJ mol21).20 In addition, the inherent stability of the diazadiphosphetidine 2 over the sterically crowded silazoxy phosphine also might contribute to the reaction proceeding in this way, similar to the observation of Markovskii et al.10 where 2 is also formed in the reaction of (2,2,6,6-tetramethylpiperidino)- phosphorus dichloride with tert-butylamine. Structure of cis-[(ButNH)PNBut]2 The compound cis-[(ButNH)PNBut]2 2 was first prepared in 1963 by Holmes and Forstner 11 by the reaction of tertbutylamine with PCl3.Although initially the molecule was Scheme 1 P N P N But NBut But ButN H H N Si O Si N P But Me Me Me MeBut Cl 1/2 O[SiMe2N(But)H]2 + PCl3 Et3N thought to be ButN]] PNHBut, subsequent reports on the compound with a molecular weight determination and a single signal in the 31P NMR spectrum confirmed the molecule as a diazadiphosphetidine existing as a pure configurational isomer.9,10 While three different structural isomers are possible with respect to the orientation of the ButNH groups on the (PN)2 ring, namely the NH hydrogens in the exo-exo (a), exo-endo (b) and endo-endo (c) orientations, the crystal structure shows that the orientation (c) is preferred.It is noteworthy that this was the structure predicted by Norman and co-workers 9 in the solution phase based on 2JPNH values from 31P NMR data measured at various temperatures. Fig. 1 shows the molecular structure of compound 2 with the atom numbering scheme. Selected bond distances and angles are given in Table 1.In contrast, the crystal structure of the disulfide of the diazadiphosphetidine cis-[(ButNH)P(S)NBut]2 9 shows the exo-endo orientation (b). A similar orientation was observed for the phosphorus(III) diazadiphosphetidine [(PhNH)P2(NPh)2]2- NPh.21 The endo-endo orientation is similar to the orientation of the N(Me) groups observed in the case of cis- [(Ph2P)N(Me)PNBut]2.22 The crystal structure of 2 also provides data for an interesting comparison of the P]N ring bond distances of phosphorus-(III) and -(V) 1,3,2,4-diazadiphosphetidines.It is generally observed that these distances in phosphorous( III) diazadiphosphetidines are comparatively longer than those of phosphorous(V) diazadiphosphetidines.23–25 Muir16 Fig. 1 Molecular structure of cis-[(ButNH)PNBut]2 2 showing the atom numbering scheme P N P N N N But H But H P N P N N N H But But H P N P N N N H But H But ( a) (b) (c) Table 1 Selected bond lengths (Å) and angles (8) for compound 2 P(1)]N(3) P(1)]N(2) P(1)]N(1) P(1)]P(2) P(2)]N(2) P(2)]N(4) N(3)]P(1)]N(2) N(3)]P(1)]N(1) N(2)]P(1)]N(1) N(3)]P(1)]P(2) N(2)]P(1)]P(2) N(1)]P(1)]P(2) N(2)]P(2)]N(4) N(2)]P(2)]N(1) N(4)]P(2)]N(1) N(2)]P(2)]P(1) 1.619(6) 1.743(5) 1.763(6) 2.616(7) 1.702(6) 1.710(5) 105.3(3) 104.7(3) 79.6(2) 117.5(2) 40.0(2) 40.8(2) 105.0(3) 81.8(2) 105.0(3) 41.2(2) P(2)]N(1) N(1)]C(1) N(2)]C(5) N(3)]C(9) N(4)]C(13) N(4)]P(2)]P(1) N(1)]P(2)]P(1) C(1)]N(1)]P(2) C(1)]N(1)]P(1) P(2)]N(1)]P(1) C(5)]N(2)]P(2) C(5)]N(2)]P(1) P(2)]N(2)]P(1) C(9)]N(3)]P(1) C(13)]N(4)]P(2) 1.725(5) 1.463(9) 1.495(8) 1.493(8) 1.489(8) 118.2(2) 42.0(2) 126.9(5) 124.1(4) 97.2(3) 125.8(5) 122.4(4) 98.8(3) 129.7(5) 131.1(5)J.Chem. Soc., Dalton Trans., 1997, Pages 2167–2169 2169 while comparing the structures of (ButNPCl)2 (average ring P]N distance 1.689 Å) and [ButNP(O)Cl]2 (average ring P]N distance 1.661 Å) have proposed that a possible reason for this can be due to a lesser delocalization of the nitrogen lone pairs on to the phosphorus atoms in the phosphorous(III) heterocycles.On comparing the structure of 2 with that of cis- [(ButNH)P(S)NBut]2 9 we observe that the average ring P]N distance in the former is 1.733 Å while that of latter is 1.685 Å. A similar variation is observed in the cases of [(PhNH)PNPh]3 26 (average ring P]N distance 1.722 Å) and [(PhNH)P(S)NPh]2 27 (average ring P]N distance 1.698 Å). In conclusion, cleavage of O[SiMe2N(But)H]2 at the Si]N bond on reaction with PCl3 is observed instead of substitution of the NH hydrogen.The diazadiphosphetidine 2 formed is characterized by X-ray structural analysis to have the NH groups of the ButNH moiety in an endo-endo orientation above the (PN)2 ring as predicted from solution studies. The method offers a new synthetic route to a variety of diazadiphosphetidines and indicates the need for a relook into the reactions of silazoxy and silaza diamines with transition- and main-group metal halides. Further work in this regard is currently underway.Acknowledgements A. J. E. thanks the Department of Science and Technology, India, (DST) for financial assistance for this work under the SERC young scientist scheme (SR/OY/C-03/94 ). N. D. R. thanks University Grants Commission (UGC), India for a research fellowship. References 1 U. Wannagat, K. Giesen and F. Rabet, Z. Anorg. Allg. Chem., 1971, 382, 195. 2 U. Wannagat, K.-P. 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