X-ray crystallographic, morphological and thermal decomposition studies of 2:1 adducts of diisopropyl methylphosphonate with stannous and stannic halides

J. inorg, nut1 Chem. VoL 41, pp. 1261-1268 Pergamon Press Ltd., 1979 Printed in Great Britain

X-RAY CRYSTALLOGRAPHIC, MORPHOLOGICAL AND THERMAL DECOMPOSITION STUDIES OF 2:1 ADDUCTS OF DIISOPROPYL METHYLPHOSPHONATE WITH STANNOUS AND STANNIC HALIDESt C. OWENS Department of Chemistry,Rutgers University,Camden, NJ 08102, U.S.A. L. L. PYTLEWSKI Department of Chemistry, Drexel University, Philadelphia,PA 19104, U.S.A. C. M. MIKULSKI Department of Chemistry & Physics, Beaver College,Glenside,PA 19038, U,S.A. and N. M. KARAYANNIS Amoco ChemicalsCorporation,Naperville, IL 60540, U.S.A. (Received 5 December 1978; received[or publication 2 February 1979)

Abstract--X-ray and morphologicalstudies of monomeric, trans-square planar Sn(dimp)2X2(X = CI, Br; dimp = diisopropyl methylphosphonate) and trans-octahedral Sn(dimp)2X4 (X = CI, Br, I) adducts revealed that the crystals of these compounds are triclinic, space group P 1. The Sn(II) and SnflV) chloride and bromide adducts have four molecules per unit cell, while the SnI4 adduct has two moleculesper unit cell. The two stannous halide adducts are almost isostructural. The X-ray powder patterns of the SnCh and SnBr4 complexes exhibit some similarities, but that of the SnI4 adduct is significantlydifferent. Studies of the thermal behavior of the Sn(IV) adducts at 140-210°C,indicatedthat Sn(dimp)2X4decomposesin two steps, i.e. (i) Formationof a linear oligomeric species of the type {[(i-C3H70)CH3POOI2SnX2}n (n = 4 for X = C1), involvingdouble Sn--(O-P-O)z-Snbridges, with simultaneouseliminationof propyleneand HX. (ii) Decompositionof the precedingoligomericintermediateto yield highly cross-linkedpolymeric [(CH3PO3)2Sn]n and isopropyl halide. Poor reproducibilityin the amounts of volatile materials evolved from experimentto experimentdid not allow the collectionof consistent kinetic data for the thermal decompositionof Sn(dimp)2X4.The Sn(dimp)2X2adducts also decompose at elevated temperatures, yieldinga linear, high polymer of the [(CH3(OH)POO)2Sn], type, via formation of the {[(i-C3H70)CH3POO]2Sn}, intermediate,which is subsequentlyattacked by the hydrogenhalide present in the system. INTRODUCTION Monomeric 2: 1 adducts of diisopropyl methylphosphonate (dimp; (i-C3H70)2CH3P=O) with SnX2(X = CI, Br) and SnX4(X = CI, Br, I) have been synthesized and characterized by means of IR and proton NMR studies at these laboratories[l,2]. On the basis of this previous characterization work, the Sn(II) adducts were formulated as trans-square planar (I) and the Sn(IV) adducts as trans-octahedral (1I), as far as the arrangement of the dimp ligands is concerned[l]. Metal halide adducts with dimp decompose at elevated temperatures with elimination of a mixture of isopropyl halide, hydrogen halide and propylene, and formation of polynuclear metal complexes with the isopropyl methylphosphonato (imp; (i-C3H7)CH3POO-), methylphosphonato (mp; CH3PO32-), methylhydrogenphosphonato (mpH; CH3(OH)POO-) or methyl-

dimp

(0

pyrophosphonato (mpp; OO(CH3)P-O--P(CH3)OO ~-) anionic ligands[3-7]. The final product of the thermal decomposition of Sn(dimp)2X4 is polymeric Sn(mp)2, as reported previously[3,4], while Sn(dimp)2X2 was found to yield Sn(mpH)2 at elevated temperatures, during the present study. This paper deals with X-ray crystallographic, morphological and related studies of the adducts of types (I) and (II), the thermal decomposition of Sn(dimp)2X2, and detailed studies of the thermal decomposition of Sn(dimp)2X,. The latter studies provided convincing evidence supporting the gradual elimination of isopropyl and halo groups from the adduct and the gradual increase [8] of the molecular weight of the polymerizing residue. This evidence is of particular interest, as there appears to be some disagreement between various laboratories regarding the nature of metal complexes with anionic organo-phosphonato or -phosphato ligands. Thus, whereas these[3-7,9] and most other[8,10--15] laboratories have characterized metal complexes with ligands of these types as polymeric, involving mainly bridging organophosphoryl ligands, a number of research groups have proposed monomeric structures for the same compounds, with the anionic phosphoryl ligand acting as a bidentate chelating agent ( i . e . ¢ \ p / O ~ . M [16--18]. These differing views are now

O0

tA part of this paper was taken from the Ph.D. Thesis of C. Owens, Drexel Institute of Technology(1969).

\/\o4n

discussed, in light of the new evidence presented herein.

1261

C. OWENSet al.

1262 EXPERIMENTAL

SnX2 and SnX4 adducts with dimp. These were prepared by methods described elsewhere[l,2]. Purification and recrystallization of these compounds were accomplished from CS2, by using a H-shaped recrystallization cell, functioning by means of a temperature gradient, as previously described[19]. Whenever the formation of large amounts of crystals of the adduct was sought, no seed crystals were employed and the crystal growth occurred on the cell walls. On the other hand, in order to grow large crystals, as soon as a few crystals large enough for manipulation were formed, they were removed from the cell, and a single crystal was suspended in the same side of the H-shaped cell, on a long human hair, as a seed crystal. It was established that small temperature gradients (2-5°C) favor the formation of large and almost perfectly shaped crystals; 2-4 weeks were required for the crystals to grow up to 3-10ram in edge. The appearance of twinning was not observed in any of the recrystallized adducts. Analyses, melting points, molecular weight determinations, IR and ~H NMR spectra of the Sn(dimp)2X2 and Sn(dimp)2X4 adducts have been already reported[l]. Density determinations (Table 1) were performed by drying recrystallized samples of the adducts in an evacuated desiccator over CaCI2, before determining the volume of a selected mass of the complex (weighed to the nearest 0.1 mg) in a gas pycnometer, similar to that described by Schumb and Rittner [20], by using dry nitrogen as the pycnometer fluid. For X-ray powder diffraction studies, recrystallized samples of the adducts were g~ound as gently as possible, in order to avoid structural deformation, and their patterns were obtained on a Norelco X-ray diffractometer, using CuK~ radiation and a scintillation counter as detector. Intensities of the diffracted radiation were recorded on a Brown (Model Y-153) strip-chart recorder, as a function of time, as the 20 angles were scanned at a rate of one degree/rain from 4 to 86°. The number of molecules per unit cell was' calculated on the basis of the experimental density, theoretical molecular weight and the maximum d-spacing. Table 1 gives the results of these X-ray studies, along with the densities and molecular weights of the adducts. Laue photographs of single crystals of the adducts (about I mm in edge) were obtained by using a Polaroid camera, equipped with a zinc sulfide fluorescent screen and a Picker X-ray generator, having a copper target source for inhomogeneous X-rays. The tube was operated at 30 kV and 15 mA, and exposure times were ca. 15 min. Photographs were taken with the incident X-ray beam parallel to each crystallographic axis, in order to determine the crystal symmetry about each axis. Morphological measurements of crystal geometry were performed on single crystals of the two Sn(II) halide adducts, which formed the best defined crystals by recrystallization. A clean, sharp corner of the crystal was selected, and the angles between faces were measured by using a Nikkon microscope with a three-dimensional adjustable stage, able to rotate 360° in the x-y plane, and an eye-piece equipped with cross hairs; measurements of the lengths of the edges were effected by using a pair of precision calipers on a millimeter rule. Results of the morphological measurements are shown below. Thermal decomposition studies. The final products of the thermal decomposition of the Sn(II) and Sn(IV) halide adducts with dimp can be obtained in sufficiently pure form only by 4 . 0 mm

Sn(dimp)~_CI2

gradually increasing the temperature of a solution or suspension of the metal salt or its dimp adduct in a large excess of dimp (decomposition is complete at 170-210°C.). Under these conditions, practically complete elimination of the isopropyl groups of the ligand and the halogen of the tin salt takes place. Thus, when Sn(dimp)2X4 (X =CI, Br, I) is treated in this manner, amorphous Sn(mp)2, contaminated with minor amounts of halogen (less than 1%), is obtained as the final product[l, 3, 4]. Study of the corresponding decomposition of Sn(dimp)2X2 (X = CI, Br) revealed that halogen-free Sn(mpH)2 (Anal.: Found(Calc.)%: C, 7.67(7.78); H, 2.67(2.61); P, 19.84(20.07); Sn, 38.13(38.45)), a white solid of low crystallinity, is obtained. When thermal decomposition studies were performed on melts of Sn(dimp)2X,, only partial elimination of the halogen and isopropyl groups present was observed. A typical example of a series of experiments of this type is illustrated by the analyses of the residues obtained by heat treatment of molten Sn(dimp)2Cl4 at various temperatures in the 140--190°C region (Table 2). In each case 5 g of the adduct were introduced in a flask, which was submerged in a constant temperature bath. The flask was mechanically stirred and connected to a set-up of trapping devices for the volatile liquid and the gaseous decomposition products. Numerous attempts at the collection of consistent kinetic data for the decomposition reaction were made. However, agreement between and within each set of experiments was poor. It was, nevertheless, established that the reaction proceeds in two steps, as follows. First step. As soon as the adduct melts, a gaseous mixture of propylene (identified by its IR spectrum and by the tH NMR spectrum of its solution in CC14) and hydrogen chloride (which was dissolved in water and determined quantitatively with silver nitrate) starts evolving. The production of this propylene-HCl mixture was previously attributed[3-7] to the catalytic dehydrochlorination of the isopropyl chloride initially formed[4, 16], in the presence of the decomposing stannic chloride complex[21, 22]. Between 1 and 2 mol of each component (C3H6 and HCI) per tool of Sn(dimphC14 used were collected; however, their yield was not accurately reproducible at any given decomposition temperature. It should be also noted that small amounts of isopropyl chloride were eliminated without decomposition, during the first step. Second step. When the production of C3H6+ HCI started subsiding, the molten residue solidified rather abruptly. At this point, isopropyl chloride (identified by means of its IR and ~H NMR spectra, and boiling point (35.6°C)) is mainly evolved (admixed with minor amounts of C3H6 and HCI). Again, the net yield of isopropyl chloride was inconsistent from experiment to experiment, and was usually less than 1 tool per tool of the Sn(dimp)2Cl4 adduct used. Illustrative of the deviations of our experimental results from run to run are the data presented in Tables 3 and 4. Table 3 shows the amounts of volatile materials collected at regular intervals, during three successive attempts at the decomposition of neat Sn(dimp)2Cl4 at 16&C, and Table 4 gives the rate constants k~ and k2, for the first and second step of the decomposition reaction of this adduct (respectively). Plots of log kt or log k2 vs time are suggestive of a first order dependence of the reaction, with respect to the evolution of propylene or isopropyl chloride, respectively[23]. We later attempted to collect better kinetic data, by studying the thermal decomposition of mixtures of SnC14 with excess dimp; however, as no significant improvement in the reproducibilities of the amounts of C3H6+HCI or

3 . 0 mm

Sn(dimp)2Br2

619.5(639)

620(620.9)

810(798.7)

865(987)

Sn(dimp) 2Br2

Sa(dimp) 2C14

Sn(dimp) 2Br 4

Sa(dimp) 2I 4

2.169~0.032

1. 910~0.042

1.636~0.029

1.783~0.007

I.490e0.008

Deaeity, K/ml,20°C.

2

4

4

4

4

+ The ¢ r y a t a l e

of all

adduct la eolutiom. the adducte belomg to the tricliaic

ayetem,el~ace

group P l .

M.W. valuee f o r m o a ~ e r i c tim c h l o r i d e o r bromide a d d u c t a w i t h dimp; im t h e c a a e o f S m ( d ~ P ) 2 1 4 , t h e eamewhat l o w e r due t o ~ dieeociatiom of this experimeatml M . W . ( r e l a t i v e t o t h e t h e o r e t i c a l f o r a l o a o m e r i c ~ c i e e ) ia prem~bly

lJ

Approximate No. of Mol/Uait Cell

In bemzeme) were la good agreement with theoretical

8.84(100),7.62(67),7.19(39),5.79(10),4.92(27),4.79 ( 10), 4.72(8) ,4.31(6) ,4.19(16) ,3.93(20), 3.72( 14), 3.59(10) ,3-36(10) ,3.28(15) ,3.16(24) ,3.04(13) ,2.98 (14),2.88(14),2.83(17),2.70(6),2.63(11),2-51(10), 2.32(10) ,2.29(13) ,2.21 (8) ,2.20(9) ,2.10( 7), 1.86(7)

10.00(11),8.93(100),8.50(81),7.89(5),7.13(23),6.86 (4), 6.55(25) ,5.90(4) ,9.01( 1), 4.67(5), 4.93(8) ,4.02 (4), 3.81(10) ,3.%(3) ,3.26( 10), 2.98(32) ,2.90(4), 2.85(3) ,2.57(8) ,2.52(4) ,2.24(16) ,2.14(37,1.87(3), 1.78(9), 1.71(2)

8.~(86) ,8.18(21) ,7.43(227,6.75(100) ,6.37(19) ,6.10 (39) ,5.57(8) ,4.74( 12), 4.44(10) ,4.39(7) ,4.31( 10), 3.56(57) ,3.52(31) ,2.99(8) ,2.77(16) ,2.65(8), 2.14( 12), 2.10(9),2.04(9) ,2.01(29)

9.02(86),8.58(15),7.82(19),7.19(26),6.86(14),6.55 (17), 5.94(6) ,5.06(6) ,4.69(10) ,4.95(7) ,4.44(7) ,3.81 (16) ,3.77(23) ,3.4~(6) ,3.25(8) ,3.22(5) ,3-16( 4), 2.98 (8), 2.89(6) ,2.84(13) ,2.78(5), 2.56(100)

8.84(32), 7.75(28), 6.96(47), 6.65(15), 6.46(26), 5.79(100) ,4.60(18) ,4.48(20) ,3.74(67) ,3.63(19), 2.93(19) ,2.84(17) ,2.79(18)

X-ray pattera, e d spacings A(relative Inte~itiee)

• Data fr~ ref. 1. Molecular weight determlaatiome(cryoacopically

551.5(550)

Molecular weight, g/mol,fouad(theor.)

Sn(dimp) 2CI 2

Adduct

Table 1. Densities, molecular weights,* X-ray powder diffraction patterns, Bravais lattice assignments* and approximate number of molecules per unit cell for Sn(dimp)2Xo ( X = C 1 , Br, 1; n = 2 or 4) adducts

B

B =__

<<

X

C. OWENSet al.

1264

Table 2. Elemental analyses of the residues of the thermal decomposition of neat Sn(dimp)2CI4 at various temperatures ............. Analysis .........................

Decomposition o temperature ! C.

c~

140

15.51

150

m6

cz~

~

s,~

3.82

12.94

25.36

13.74

13.15

3.49

14.74

27.11

12.92

160

13.67

3.68

15.95

29.37

12.18

190

9.94

3.14

16.87

32.96

10.98

an(dimp) 2C14

25.61 (27.O8)

5.55 (5.52)

10.17 (9.98)

18.60 (19,12)

23.03 (22.84)

Sa(mP) 2

8.60 (7.83)

2.32 (1.97)

19.86 (20.19)

37.54 (38.69)

0.76 (0.00)

Comparative Data*,

_to_.=_d_!~.~c__d:).. . . . . . .

" Previously reported analytical data (1,3,4).

Table 3. Thermal decomposition of Sn(dimp)2Cl4at 160°C Heating

-~-~Z~-~-~-m~-~

~imelmin.

Run 1+

~

2+

°

Run ~+

i-C~HoCI liquid collected,mmol* ---,-t ........................ lh~ 1+ Rl1~ ~÷ Run ~+

6 9 12

0.24 2.58 4.33

0.28 2.46 4.25

0.20 2.46 4.41

16 21

5.76 7.15

5.56 7.27

5.68 7.54

0.23 2.15

26 36 51 66

7.45 7.63 7.87 8.22

7.63 7.7~ 8.14 8.86

7.79 8.03 8.26 8.66

81

8.66

9.81

8.98

96 126

8.94 9.24

10.30 11.10

9.30 9-53

" Cumulative data. + Sample sizos,g(mmol):

O.OO 0.23

0.00 1.46

4.52

1.69

2.48

5.08 5.99 6.66 7.00 7.34 7.57

2.37 2.60 2.71 3.28 3.61 3.61

3-39 4.52 4.97 5.76 6.44 6.66

Run 1 5.O595(8.14); Run 2 4.6172(7.43);

Run 3 5.2651(8.48). Amount of final residue,g: Run 1 2.9686; Run 2 2.6986; Run 3 ~o853.

Table 4. Rate constants of the decomposition of Sn(dimphCl4 at various temperatures Decomposition temperature I°C"

kl ,min-1 "~,

k2,min-1

14o

o.o3to.Ol

0.o29~o.oo3

15o

o.08±o.o2

0.034*0.009

160

0,12±0.01

0.07~0.03

170

0.22±0.04

0.13-1;0.06

180

0.56~-0.10

0.~8t-0.09

19o

o.76±o.16

o.60~o.22

X-Ray crystallographic, morphological and thermal decomposition studies i-C3H7C1 evolved (relative to those observed during the decomposition of neat Sn(dimp)2Cl4) was brought about, further work in this direction was abandoned. The residues of the decomposition reactions described in Tables 2-4 are generally amorphous solids, insoluble in water and all common organic solvents, and presumably highly polymeric intermediates between the initial monomeric Sn(dimp)2Cl4 adduct and the final polymeric Sn(mp)2 complex. In order to further substantiate the conclusion that these intermediate products in particular (and metal complexes with anionic RPO3 2-, R(OH)POO , (RO)R'POO-, (RO)~.POO-, and the corresponding pyrophosphonato or pyrophosphato ligands in general[3-7, %15, 24] are polymeric the following additional experiments were conducted: (i) To demonstrate the effects of the gradual polymerization of the molten residue of the decomposing Sn(dimp)2Cl4 adduct[8, 25], the increases of the viscosity of the melt with time of heating were determined at 150-+ I°C, by using a Brookfield viscometer, Model LVT, at 60 rpm, with the No. 2 spindle. The results of these measurements, given in Table 5, show that the viscosity of the melt decreased initially, owing to the increase in temperature, and then remained almost constant for a period of time, during which viscosity increases arising from polymerization were offset by thermal effects. Eventually, the increases in molecular weight became the dominant factor and caused very rapid viscosity increases. (it) The thermal decomposition of molten neat Sn(dimp)~C14 at 160°C was interrupted when about two tool equivalents of propylene gas were collected; this was effected by removing the sample container from the thermostatted oil bath and immersing

CH3--CH--CH~

l"o

I265

it in a slurry of dry ice-isopropanol(ca. -78°C), to stop further reaction. The solid residue analyzed as Sn(imp),C12 (Found(Calc.)%: C, 21:16(20.72); H, 4,63(4.35); P, 13.41(13.36); Sn, 25.92(25.59); CI, 14.86(15.29)). This intermediate is soluble in many organic solvents; its molecular weight, determined cryoscopically in benzene, was found to be 1831 g/tool, corresponding to a degree of polymerization of four (theoretical vahte for a tetramer: 1855 g/molL The overall evidence indicates that the first step of the thermal decomposition reaction involves formation of a linear tetramer of type (III), while during the second step the organophosphoryl ligands are completely depropylated, and a highly cross-.linked. polymeric product (Sn(mp)2), presumably of type (IV), is formed. The dramatic increases in viscosity, observed after heating molten Sn(dimp):Cl4 at 150°C for about 40 rain (Table 5). correspond to the initial stages of the second step of the decomposition reaction. Differential scanning calorimetry. A Perkin-Elmer differential scanning calorimeter (Model DSC lB) was used for further investigation of the thermal lability of gently ground samples of the crystalline Sn(II) and Sn(IV) halide adducts with dimp. The instrument was calibrated to within 1° of the melting points of indium and lead, and the measurements were performed at a temperature rate of increase of 10°/rain, under a flush of dry nitrogen. The results of these studies (heats of fusion (Hs) and thermal decomposition temperatures) are shown in Table 6, together with the melting points of the adducts. As would be expected, Hs decreases along the series chloride > bromide > iodide; the lattice energies of salts involving these halo groups also decrease according to the same order.

7

t o"1

/1

°' / °7\P/ \ o. 1/

o/Ix

:\!/i\ CH. 0 / I

\0

<,,,)

o

CH,

\I 0 /

I

0

I

I

(IV)

Table 5. Changes in viscosity of decomposing Sn(dimp)2Ch melt with time of heating at 150-+ I°C Heating tiaelmin,

Vigcosity, cpiae

Heating tJlelain,

Viscosity, cpiee

Eeatimg tiaeluin,

Vimcomity, cpoiee

30

22

18

53

46

1

25

27

23

54

52

2 3

20 18

30 41

24 25

55 56

55 61

4

18

~5

a8

58

83

5

18

47

30

59

100

9 12

18 18

49 51

36 40

60 61

127 177

18

18

52

~5

62

5o0

melt*

• I n i t i a l melt at ~O°C.

-

C. OWEN'Set al.

1266

Table 6. Heats of fusion, thermal decomposition temperatures and m.p. of Sn(II) and Sn(IV) halide adducts with dimp

Adduet

Hf 'kcal'm°l'l

Decomposition ~perature I °C.

Meltin$~oint1°C.*

Sn(dimP)2Cl2

14.71

176

Sn(dlmP)2Br2

12.23

171

110-110.5

Sn(dimP)2Cl4

11.23

190

89-89.5

Sn(dimP)2Br4

10.56

193

116-116.5

Sn(dimP)2I4

8.87

210

100.8-101.4

88-89

* Melting ~oint data from r e f . 1.

IR spectra. The IR spectra of the Sn(dimp)2X, (X==C1,Br, I; n=2 or 4) adducts and Sn(mph were reported elsewhere ll, 3, 4]. The new Sn(mpHh complex shows an IR spectrum very similar to that of the previously reported Cu(mpHh[7], i.e. cm-~ (band assignments) [1,7,26-30]: 3140s, 2720s, 2660s, 2350m, b, 2120mw, b((P)-OH bands), 1730mw, 1590m(PO3 combination modes), 1418ms, 1305s((P)-CH3 modes), ll60vs, l125vs, 1080vs, b, 1048vs, b, 1015vs, b, 972 vs, b(all six bands primarily due to PO3 stretching modes), 888 vs((P)--CH3),770 vs, 720 s(vp_ c), 551 ms, 502 ms, 480 sh, 425 w, 392 vw(PO3 bending modes+ /C ), 352 ms(vs,_o), 340 m, sh, 331 m, sh(PO3 rocking modes). P\O The spectrum of this complex was obtained on a Nujol mull between IRTRAN 2(4000-700cm-~) and high-density polyethylene (700-200cm-j) windows, in conjunction with a PerkinElmer 621 spectrophotometer. DISCUSSION

X-ray and morphological studies of the Sn(dimp)2X, adducts. Laue photographs of the tin halide adducts with dimp revealed a complete absence of any elements of symmetry in all cases; it was, therefore, concluded that all these crystalline adducts belong to the triclinic system, space group P1 (Table I). The morphological measurements on the two stannous halide adducts support this lattice assignment, since they establish the absence of any right angles between planar faces. Among the X-ray powder patterns obtained (Table 1), those of the two stannous halide adducts (X-------CI,Br) exhibit considerable similarities, which are suggestive of closely resembling (although not isomorphous) structures. There are also some similarities between the patterns of the SnCL and SnBr4 adducts, but not as close as those observed in the patterns of the corresponding Sn(II) halide adducts; the X-ray pattern of Sn(dimp)214 shows significant differences from those of the stannic chloride and bromide analogs, especially in the 5-8 ,~ region. The Sn(II) and Sn(IV) chloride and bromide complexes have four molecules per unit cell, as indicated by calculations based on their maximum d-spacing, experimental density and theoretical molecular weight for a monomeric

i-CsH7% / C H 3

o//P \ o i ,- C H,

x_>(

CH3

o/ \O-I-C3H z (v)

configuration. In contrast the SnI4 adduct has two molecules per unit cell.

Thermal decomposition of the adducts and gradual polymerization of the residues. The features of the thermal decomposition of the Sn(II) and Sn(IV) halide adducts with dimp were described, in some detail, in the experimental section. Initially, elimination of one isopropyl halide molecule per dimp molecule present occurs; this is most probably effected by a mechanism involving oligomerization of the original monomeric adduct (which contains exclusively P = O oxygen bonded, unidentate dimp ligands), by coordination of isopropoxy oxygens of the dimp ligands[31] to tin ions of an adjacent monomeric unit (V); subsequently, isopropyl halide is eliminated[4, 16], and a linear oligomer or polymer, involving eight-membered Sn-(O-P-O)2-Sn rings (VI), is formed[4]. The decomposition of the isopropyl halide formed to HX (X=C1, Br, I) and propylene, during the first step of the reaction (vide supra), is most probably catalyzed by the molten tin halide complex residue [2t, 22], as already mentioned. The fact that a, presumably linear, tetrameric Sn(imp)~Cl2 intermediate was isolated by interrupting the progress of the decomposition of Sn(dimp)2C14, favors the proposed mechanism for the first step of this reaction. The second step of the decomposition is characterized by an abrupt increase in molecular weight of the decomposing complex, as suggested by the progressively increasing viscosity of the melt (Table 5) and the subsequent solidification of the residue (owing to an abrupt increase in melting point; both Sn(mpH)2 and Sn(mp)2 do not melt or decompose at temperatures up to 350°C). The solidified residue does not seem to contribute to the catalytic dehydrohalogenation of isopropyl halide, since i-C3H7C1 is the main volatile product, during the second step of the decomposition of Sn(dimp)2Cl4. The Sn(mp)2 polymer is presumably formed by a reaction similar to (1); in this case, the isopropoxy oxygens of a linear polymeric intermediate Sn(imphX2 molecule (III or VI) coordinate to Sn4+ ions of neighboring linear polymeric units (and vice versa), forming a

I-CsHrO\ / C H s

\L/

0/P

~0

~ I/ + 2(i CaHTX)

/I°\ ~o p/O CHf ~O-i-C~H 2 (vo

X-Ray crystallographic, morphologicaland thermal decomposition studies second intermediate, which eventually yields the highly cross-linked polymeric Sn(mp)2 complex(IV) and isopropyl halide. On the other hand, the formation of Sn(mpH): may be also considered as proceeding in two steps, i.e. formation of Sn(imph[3-7, 9] and attack of the coordinated imp ligands in this intermediate species by hydrogen halide [24, 32-34], which is present in the system, viz.: ](i-C~H70)CH~POOI2Sn + 2HX

,[CH3(OH)POO]2Sn

+ 2(i-C3HvX).

(2)

Polymeric character of metal complexes with imp, mpH, mp and related ligands. The evidence presented in this work may be considered as conclusive as far as the tetrameric nature of Sn(imp)2Cl2 and the higher polymeric character of Sn(mpH)2 and Sn(mp)2 are concerned. During previous studies by these laboratories, a whole series of obviously polynuclear metal complexes with a variety of (RO)R'POO , R'(OH)POO , R'PO32-, and (RO)2POO- (R,R'= alkyl or aryl) have been prepared by reaction of metal halides with neutral phosphonate or phosphate esters, at elevated temperatures [3-7, 9]; all these complexes are insoluble in organic solvents and water, do not melt at temperatures up to 300-350°C[3-7,9], and have, in many cases, textures characteristic of polymeric species (membrane- or rubber-like materials)[3]. Polymeric structures have been also proposed by other research groups for metal complexes with ligands of the above types[10-15,35-38], as well as diorganophosphinato ligands ([(R2POO)],M) [8, 25, 39], on the basis of similar properties. The magnetic properties of transition metal complexes with these ligands have been also found to point to polynuclear ligand-bridged structures (antiferromagnetic exchange interactions, occurring via the - O - P - O - bridges)[37,9,40--42]. Finally, and most importantly, numerous crystal structure determinations established that metal complexes with the ligands under discussion are indeed polymeric[42--49]. It is, therefore, somewhat surprising that some [(RO)2POO],M (R=alkyl) complexes were recently characterized as monomeric metal chelates[18]. The new Sn(mpH)2 complex apparently involves a square-planar environment for the Sn 2÷ ion (SnO4 moieties), as suggested by the occurrence of the ~,s,-o mode at 352cm-~[I]. A linear, double-bridged, polynuclear structure (VII), with terminal bidentate chelating mpH ligands at the two ends of the chain[3-7, 9, 42-50], is considered as most likely for this compound. As far as Sn(mp)2 is concerned, the highly cross-linked structure (IV), involving hexacoordinated Sn4+ (SnO6 moieties) was previously proposed[l, 3, 4]. The far-IR spectrum of this compound (not reported so far) shows a band attributable to the us,-o mode at 326 cm ', as would be expected for hexacoordinated stannic complexes with organophosphoryl ligands [1].

Acknowledgement--The support of the earlier stages of the present study by the U.S. Army Edgewood Arsenal (contract No. DAAA-15-C-67-0644)is gratefully acknowledged.

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HO~p/CH3

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HO~ p.,...~Hs

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~OH

1268

26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

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