The donor chemistry o f the cyclosiloxanes (Me2SiO) 3 and (Me2SiO)4 was investigated with a number of covalent metal h a lid e s, but in each case the siloxane ring proved to be in s u f f ic ie n t ly basic to y ie ld coordination compounds. With MCI3.2NMe3 (M = T i , Cr) and C rC l3.3MeCN,
(Me2SiO) 3 ^ fa ile d to r e a c t , and the ligand was recovered unchanged. Reactions with MCl^ (M = T i , Sn) although not providing adducts did give some in terestin g r e s u lt s . Dropwise addition of MC14 to a two-fold excess of (Me2SiO)3 4 in CgHg gave co lo u rle ss, fuming o ils on removal of solvent (10 days). The IR spectra of the products in the 4000-400 cm- ^ region were v ir t u a lly id e n tic a l. Strong broad bands a t *\»2964 cm \ v ( C H ) ; ^1264 cm~\ 6(CH3) ; <\»1070 c m , v (S i-O ); and 850 cm”^, v(Si-C ) a re typ ical of those found in a wide range of both lin e a r and c y c lic silo x a n es (321), and are, therefore, of l i t t l e diagnostic value. S k e le ta l ring modes found in the free ligands in the 1000-300 cm-1 region are absent from the spectra of the products however, suggesting a ring opened product. Further evidence fo r ring opening comes from the NMR spectra (60 MHz) of the products, which were id en tica l for both the (Me2SiO)3 and (Me2SiO)4 based products. A se rie s of sharp resonances associated w ith Si-Me protons were observed at 6 0.2-0 .7 ; on the b asis of each u n it in the (Me2SiO)x chain giving a separate resonance, a chain length of at le a s t 12 u n its is in dicated . By analogy with
Cl(Me2SiO )^TiCl3 (358), a stru ctu re of the type Cl(Me2S iO )12T iC l3 is proposed in the present in stan ce. A strong Ti-C l mode a t 385 cm-1
supports t h is form ulation. Polymerisation of the cyclosiloxanes has been achieved using a v a rie ty o f Lewis acid c a t a ly s t s , but always employing high temperatures (359). For example, T iC l4 fa ile d to polymerise (Me2SiO)4 at le s s than 543 K. Reaction times for these experiments were, however, r e la t iv e ly short («'»6 ho u rs). The present re su lt merely suggests that the
polym erisation is r e la t iv e ly slow.
One point to emerge i s that under s im ila r reactio n conditions rea ctio n s of T iC l4 with (Me2SiO)3 or (Me2SiO)4 gave id e n tica l products. Although there was no observable reactio n between (Me2SiO)4 and SnCl4, work up of the reactio n products of (Me2SiO) 3 and SnCl4 found only the tetram eric rin g . I t would appear th at the isom erisation reaction
4(Me2SiO) 3 --- > 3(Me2SiO)4
i s promoted by both T iC l4 and SnCl4 . Ring expansion and consequently the r e l i e f of rin g s t r a in has previously been noted fo r (Me2S iS ) 2 , which spontaneously isom erises to (Me2S iS) 2 a t room temperature (321), and for the Lewis acid catalysed expansion o f cy clo sila za n e s (321).
5 .5 Complexes of Hexamethylborazine with Covalent Metal Halides Reaction of hexamethylborazine (HMB) with an equimolar amount of T i C l4 in CgHg so lu tio n led to the iso la tio n of 2 TiC l4 .HMB as an orange s o lid . No evidence was found fo r the 1:1 adduct reported by Nflth and coworkers (6 0 ). Anderson and Lagowski found no reaction between SnCl4 and HMB even a fte r 22 hours at 373 K (361). Extended reaction times
(1 year) gave a s im ila r r e s u lt , with HMB being recovered in b etter than 95% y ie ld . No reactio n was observed between HMB and e ith e r VCl3.3MeCN or T i C l3.3THF, while the C rC l3.2NMe3/HMB system yielded only (C rC l3)2.2NMe3 .
2 TiC l4 .HMB was found to be highly air-m oisture se n s itiv e and
in so lu b le in non-polar solvents (e .g . CgHg, CHC13 and CH2C12) . In donor so lv en ts, so lu tio n was accompanied by ligand replacement. This was investig ated in d e ta il for the 2 T iC l4.HMB/MeCN and THF systems.
Removal o f solven t in vacuo and sublimation of the r e s u lt a n t pale yellow s o lid in vacuo (363 K) gave a white sublimate of HMB. The yellow
sublim ation residues were id e n tifie d as T1Cl^. 2L (L = MeCN, THF), i . e .
2 T iC l4 .HMB --- »- T i C l4.2L + HMB (L = MeCN, THF)
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In the 1200-500 cm-1 region the IR spectrum of 2 T iC l4 .HMB clo se ly resembles that of the fre e ligand (337, 362, 363), stro n g ly suggesting that the HMB ring re ta in s i t s in te g rity on complexation. The most marked d ifferen ce s between the spectra of the free and complexed ring occur in the 1450-1350 cm-1 region, where the B-N stre tc h in g band a t 1415 cm-1 in HMB appears as a doublet (1426, 1350 cm-1 ) in the Ti complex. A s h if t to lower wavenumbers i s co n sista n t with a reduction in the B-N bond order and the lo c a lis a t io n of the N lone pair in the formation of
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