Four-and Eight-membered Cyclic Phosphazene Derivatives of Zirconium , Titanium and Vanadium . Crystal Structures of the Complexes [ ZrCI , ( Me , SiNPPh , NSiMe , ) ]

Results and Discussion In 1989 we reported on the reaction of TiCl, with (Me,Si),NPPh,NSiMe, 1, which under different reaction conditions yields three different products 2-4 all of which possess a planar, cyclic, PN,Ti unit as the common f e a t ~ r e . ~ Analogous reactions of silylated benzamidinates with other halides of Group 4 lead, depending on the stoichiometry, to the formation of four-membered cyclic (molar ratio 1:l) or spirocyclic compounds (molar ratio 1 : 2).' In the aminoiminophosphorane case, a 1 : 2 molar ratio led to double addition, 3, but only one ring was closed. Since the steric requirement of the aminoiminophosphorane chelate is greater than of the benzamidine ligand, a second ring closure to form the spirocyclic isomer of 3 might be unfavourable. We have investigated the analogous reaction with the slightly bigger zirconium [rcov z 136 (Ti), z 148 pm (Zr) "1. Independent of the molar ratio, a 1 : 1 reaction product 5 is formed in up to 80% yield, which after recrystallisation from MeCN has been isolated and characterised by means of X-ray analysis as the MeCN adduct 5a [equation (l)]. Compounds of type 5 are of interest as potential catalysts in organic chemistry." During the course of our investigations we found a straightforward two-step synthesis of compound 1 [equation (2)], which so far was only accessible via two independent multistep procedures; Since the primary reactions to form compounds 5 and 3 involve only the cleavage of one Me,Si unit per molecule, it was interesting to replace the amino-function by a trimethylsilyloxy group. Surprisingly, in the solid residue of the reaction (3) of TiC1, with Me,SiOPPh,NSiMe, 6,13 no Me,Si groups could a total yield up to 80% was achieved.

The X-ray structures of [hCI,( Me,SiNPPh,hlSiMe,)]*MeCN and [{TiCI,-(OPPhZN)},]*4MeCN have been determined.While the cyclic Zr-N distances in the former have to be considered as single bonds, the short Ti-N of the latter (almost planar) suggest double-bond character.
The incorporation of transition metals as building blocks of and a ~y c l i c ~-~ phosphazene systems has found increasing interest over the past few years.For example, phosphazene polymers with metals in the skeleton have been synthesised.'In the course of our investigations we were interested in including other transition metals and other building units, e.g.oxygen instead of nitrogen.

Results and Discussion
In 1989 we reported on the reaction of TiCl, with (Me,Si),NPPh,NSiMe, 1, which under different reaction conditions yields three different products 2-4 all of which possess a planar, cyclic, PN,Ti unit as the common f e a t ~r e .~ Analogous reactions of silylated benzamidinates with other halides of Group 4 lead, depending on the stoichiometry, to the formation of four-membered cyclic (molar ratio 1:l) or spirocyclic compounds (molar ratio 1 : 2 ) .' In the aminoiminophosphorane case, a 1 : 2 molar ratio led to double addition, 3, but only one ring was closed.Since the steric requirement of the aminoiminophosphorane chelate is greater than of the benzamidine ligand, a second ring closure to form the spirocyclic isomer of 3 might be unfavourable.We have investigated the analogous reaction with the slightly bigger zirconium [rcov z 136 (Ti), z 148 pm (Zr) "1.Independent of the molar ratio, a 1 : 1 reaction product 5 is formed in up to 80% yield, which after recrystallisation from MeCN has been isolated and characterised by means of X-ray analysis as the MeCN adduct 5a [equation (l)].Compounds of type 5 are of interest as potential catalysts in organic chemistry." During the course of our investigations we found a straightforward two-step synthesis of compound 1 [equation (2)], which so far was only accessible via two independent multistep procedures; Since the primary reactions to form compounds 5 and 3 involve only the cleavage of one Me,Si unit per molecule, it was interesting to replace the amino-function by a trimethylsilyloxy group.Surprisingly, in the solid residue of the reaction (3) of TiC1, with Me,SiOPPh,NSiMe, 6,13 no Me,Si groups could a total yield up to 80% was achieved.
t Supplementary data available: see Instructions for Authors, J. Chem.be detected spectroscopically.The structure of compound 7 has been determined by means of a low-temperature single-crystal X-ray analysis of the MeCN adduct 7a, which forms yellow blocks after recrystallisation from MeCN.The structure analysis reveals the presence of an almost planar, eightmembered heterocycle with very short Ti-N bonds and P-N-Ti units close to linearity.Thus, 7 is valence isoelectronic with a 'OSiMe, Ti -N-P' CI;? Ph2 Ph2 CI,0SiMe3

Fig. 1 atoms labelled
The molecule of compound 5a in the crystal, with non-hydrogen   Compound 9, a brick-red microcrystalline solid, is moderately stable in air, but suffers immediate decomposition in solution.An analogous eight-membered heterocycle bearing CZF5 groups on phosphorus has recently been synthesised by the same route and characterised by an X-ray structure analysis.16 Unfortunately we were not able to grow single crystals of 9, but the mass spectrum (m/z = 670, M , 23%) clearly supports the proposed structure.The formation of 9 sharply contrasts with the reactions of [VCl,(NSiMe,)] with C1P(CF,),NSiMe3 and of nitride halides of Group 6 with C1PPh,NSiMe3 10 which lead to six-' and eight-membered rings with only one metal atom in the skeleton.So far the eightmembered rings have only been characterised by 31P NMR and mass spectroscopy.
The molecular structure of compound 5a (Fig. 1, Table 1) is composed of an almost planar four-membered ring (standard deviation 2.32 pm) with both silicon atoms lying also close to this plane [Si(l) and Si(2) deviate 2.8 and 9.0 pm from this plane respectively].The phosphorus atom has an almost tetrahedral environment, while the geometry at zirconium consists of a distorted octahedron due to the small cyclic N-Zr-N angle of 69.4(1)".The Zr-N-C angle of the co-ordinated MeCN is about 13" from linearity, the N&-C unit being tilted towards the ring.Similar deviations from 180" have also been found in the titanium compounds 3a, 4 and 7a. 4 While the Zr-N(3) bond has dative character, the Zr-N bonds in the ring have to be considered single bonds."With the P-N bonds resembling those of cyclic phosphazenes,' and sp2-hybridised nitrogen atoms, the resonance forms in Scheme 1 can be drawn.
Compound 5a is isomorphous with its titanium homologue 3,4 a comparison of the cell constants being given in Table 2.
The unit-cell volume of 5a is somewhat bigger due to the size of the metal atoms.The single-crystal X-ray structure of compound 7a (Fig. 2, Table 3) exhibits an almost planar eight-membered ring with two alternating Ti-N-P-0 sequences.Compound 7a is valence isoelectronic with its vanadium analogues 9 and 8, the structure of the latter having been described earlier.,The molecule contains an inversion centre.The most striking feature in the structure of 7a is the very short Ti-N bonds of 174.1(2) pm and the large P-N-Ti angles of 175.9(1)", so that sp hybridisation can be attributed to the nitrogen atoms.The resonance forms in Scheme 2 can be assigned.From these it is clear that the Ti-N bonds have appreciable double-bond character.Thus, resonance form IV can be regarded as dominant.The Ti-N bond in [TiCl2(NPPh2S)].3C5H5N,172.0(2) pm [Ti-N-P 172.5( 2)"], is probably best described as a double bond."Similar bonding features have been observed l 9 in some phosphoraneiminato Table 4 Crystallographic data for compounds 5a and 7a * Fig. 2 The molecule of compound 7a in the crystal, with independent non-hydrogen atoms labelled Table 3 Selected bond lengths (pm) and angles (") for compound 7a Ti( I)-N( 1 where one set of Ti-N bonds was found to be 169 pm.,'The P-N distance in compound 7a [160.5(2)pm] is longer than in most other cyclic pho~phazenes,'~ and the P-0 separation [ 15 1.7(2) pm] is close to that calculated (1 50 pm) for a double bond.The Ti-0 bond length of 197.9(2) pm is in the range found for titanium bonded to bridging sp2-hybridised oxygen atoms.21Since low-temperature data had been collected, a clear distinction between oxygen and nitrogen atoms could be made by comparison of the isotropic thermal parameters with those obtained by exchanging 0 and N. A strong trans effect is shown by the co-ordinated MeCN ligands, the Ti-N bond trans to the ring nitrogen being longer by 12.5(3) pm compared to the one in the cis position.

Experimental
All manipulations were performed in vessels predried and heated under dry nitrogen or argon or in uacuo.Solvents  and SiMe,N, distilled prior to use.The compound ZrC1, was used as obtained, while Ph2PSiMe3,,,* [VCl,(NSiMe,)],14 Me,SiOPPh,NSiMe, 6,13 and ClPPh,NSiMe, lo', were prepared according to the literature.The 31P NMR spectra were recorded on a Bruker AM 250 spectrometer; elemental analyses were carried out by Mikroanalytisches Labor Beller, Gottingen.The mass spectrum of compound 9 was recorded on a Finnigan Mat 8230 mass spectrometer.Only the highest peak of any isotope distribution is given.The boiling and melting points are listed without thermometer corrections.

Table 1
Selected bond lengths (pm) and angles (") for compound 5a previously synthesised tetraazadiphosphadivanadocine 8,, accessible by treating VOCl, with 1. Attempts to exchange the Me,SiO groups in 8 with chlorine by means of SOCl,, PCl,, etc. led to a total cleavage of the ring skeleton.Nevertheless, the tetrachlorinated derivative 9 can be synthesised by a double

Table 2
Comparison of the cell constants of compounds 5a and 3 were dried and degassed by standard methods, TiCl, purified by condensation in uacuo, and commercially available SiMe,Cl * A s a minor change, LiPh was not quenched with Bu'C1 and the resulting SiMe,Ph separated by fractional distillation.Details in common; crystal system monoclinic, space group P2,ln. *