4 6 DESCRIPTION AND DISCUSSION OF THE STRUCTURE

1-Bromonaphthalene forms a 1:1 complex with antimony trichloride, * the crystal structure of which is built up of double layers of SbCl. _j 3_"i

JL molecules alternating with double layers of 1-Bromonaphthalene tilted -’I about 25° to [010] (see Fig 4.2). By halogen bridging pairs of SbCl_

molecules form dimers which are themselves part of an infinite chain of molecules by further halogen bridging and chese chains in turn are cross

linked to others to produce the double sheet. (Fig 4.2a). Finally the structure contains plane-to-plane molecular stacks of I-G^qHt^Bï’ molecules in a similar arrangement to that found in the crystal structure of

naphthalene. The layers of antimony trichloride and organo component are parallel to the ab plane and alternate in the

(*) The transformation matrix from triclinic xyz(X) to orthogonal XYZ(%),

is given by: 1 cos# cos^

0 sinV -cosoc sinX 0 0 sinp since*

C l C l 71.07

68.90

3

(a)

Pig.4.5 A view of the equatorial arrangement around the Sh atoms, view down CI3- Sbi and C16- Sb2 respectively.

c direction. Fig (4.2).

Bond distances and angles in the SbCl^ and l-Og^HyBr molecules are quoted in Table 4.5a,b. There are two pairs of non-equivalent molecules. Both the non-equivalent antimony atoms are bonded to three chlorine atoms, (average 2.31 and 2.37 a,nd are close to a further three chlorine atoms - two from a non-equivalent molecule (3.^19» 3.68 and 3.39, 3.65 ?) and one from a symmetrically related molecule (3.77 and 3*77 %) - and are situated near a bromo-naphthalene molecule (3.34, 3*30 %).

The usual criterion used to decide if a neighbouring atom is coordinated or not is the shortness of the interaction distance relative to the usually expected sum of the van der Waals radii.

Generally 8b. ..Br, 3b...01, and 8b...0 contacts are 4.05, 3*93 and 3.9'"' respectively (Pauling, I96O; Bondi A., 1964). In the present structure all the above mentioned distances are appreciably shorter the sum

of the van der Waals radii so that the coordination number of the

two atoms becomes seven. Geometrically this corresponds to a distorted pentagonal bipyramidal arrangement around the 8b atoms. The Sb(l), Cl(l), 01(2), 01(4), 01(5), 01(3!^) and Sb(2), Cl(4), Cl(5), Cl(p-^), 01(2 ), 01(6^) are approximately in the equatorial plane (see T^hle 4.5° •and Fig. with average out of plane distances 0.12 and

0.10

%.

The crystal structure of SbCl^, has been described in terms of an eight fold cocoxlination ('Lipka, 1979) i.e. a 3*.2:3 coordination

(see diagram 1.6.1 p I7). The change of coordination number to 1:5:1 in the present case could be due to the interaction with a relatively large

organic molecule which blocks further approach. According to the proposed geometry it would seem that the lone electron-pair is stereochemically inactive.

Shj. C

12

right angle. , Plotted using Comp,Prog. STEREO.

A view of SbCl^• 1-Bromonaphthalene down Cl^-Sh.^ and at

C l Sbg ClA, Sb Bri Bri

A view of SbCl^• 1-Bromonaphthalene doi.m Cl^-Sb^ and at right angle. Plotted using Comp, Prog, STEREO,

99

In each ShGl^ molecule, one of the Sh-Gl bonds is significantly shorter (2.270 and 2,288 2 respectively) than those found

(2.3/1 and 2.37 2) in the SbClo molecule alone (Lipka, 1979)» and a long Sb-Gl bond (2.422 2) is also observed in the second molecule. These Ihsnomena seem to be related in some way to the donor-acceptor

nature of the interaction of SbCl^ and 1-bromonaphthalene. "îi In both molecules of SbGl^, two bonds lie in a plane nearly

parallel to that of the organic molecule, which the third bond is

approximately perpendicular to that plane (see Fig 4.2,4,3,4,4) , 'n^ore § exactly, the Gl(3)~Sb(l) bond directs towards the mid-point of the

C(8)“C(9) bond, (see Fig 4.3,1), while the Gl(6)-Sb(2) bond points almost exactly to G(7) (Fig 4.3,2), The perpendicular distances from the two independent antimony atoms to the plane of the corresponding bromonaphthalene molecule are 3*34 and 3«30 2 respectively. A similar situation is found in the 2SbCl^. naphthalene (Hulme and Sz^manski,

1969), but in this compound two SbGl^ molecules related by a centre of if: symmetry are situated on opposite sides of the plane of the aroma^tic

molecule, giving an alternating distribution of organic and inorganic layers. In addition to this, in 2SbCl^.naphthalene the distances between Sb atoms and the plane of the organic component (3.20 2) is of the same order of magnitude as that found in the present work.

Therefore, in the SbCl^. l-G^gHyBr compound, it seems there may also be electron donation from the aromatic 7T-system to the Sb atom. The Sb-Cl bonds apposite to the interaction with the ^-systems are not of

% the same length. For the longer Sb-aromatic ring contact, the Sb(l)-Cl(3) < distance is essentially the same as the other Sb-Cl bonds. On the other

hand, the shorter Sb-aromatic ring contact is opposite a much longer 8b(2)- Cl(6) bond. Such an extension of an Sb-halogen bond has been already

observed in other compounds. For example, (2.37 2) in 28bCl^'naphtha-" if lena (Hulme and Szymanski, I969); (2.37 2) in 23bCl^-p-xylene 1 (Hulme and Mullen, I976); (2.44 2) in 2SbClo.l,4-dithiane (Kiel and Engler, % 1974); (2.51 2) in SbCl^.aniline (Hulme and Scruton, I968) and

(2.39 2) in 28bGl^. phenanthrene (Demaldeet.al. 1972). This t lengthening of the 3b-Gl bonds give some support to the view that

antimony in the SbCl^ molecule may act as an acceptor towards 7f-donors. % The large values of some carbon atom temperature factors

(especially suggests some departure from

ideality, but could also be due to residual errors (e.g. absorption (/t-6l.), and extinction effects) uncorrected in the experimental data.

There is no significant shortening of G-G bond in the region to | which the Sb atom is pointing in contrast with the situation found in

the 2SbCl^.naphthalene complex. Instead slight alterations of some other G-C bonds as well as some angles were observed (see Table 4,5b).

i

5.0 STUDY OF (2:11 81)01^.1-GHLORONAPHTHALSNB COMPLEX

5*1 PHASE DIAGRAM FOR SbClo-l-a^^QH^Cl SYSTEM

Determination ce-the ptese diagram for this system was carried out according to the method described in Chapter 2. During the experiment a considerable amount of supercooling occurred in spite of continuous agitation.

The results in Table 5*1 permit the construction of a phase diagram as shovm in Pig (5«l) and suggest (2:1) SbCl^.l-C^^HyCl compound

formation with melting point ca.42°C. This is in good agreement with earlier results mentioned in the literature (Menschutkin, I9I2).

TABLE 5.1 80 <0 -20 100 50 Mol % S6C/3 % _{Mol SbCl} T°C Fig.5.1 SbCl3--1-Ch.loronaph.th.a- lene system. J- 0 -2-0•5 30 16^1.0 45 23Ï1.0 50 28-1.0 60 32^2.: 70 33:^2.0 80 27-1.0 90 3o:^i!5 100 73Ï1.0

a

5.2 PREPARATION AND ANALYSIS OF THE CRYSTALS

On the basis of the results summarised in Fig (5.I), SSbCl^.l-C^gHyCl was prepared following a procedure described below.

Antimony trichloride was dissolved in 10-15/^ excess of 1-chloronaph- thalene at about 45°C under anhydrous conditions away from the atmosphere. When the solution was allowed to cool to room temperature, yellowish

I

crystals started to grow. A variety of crystals were obtained after several hours.

Analyses for Sb(lll) and 1-chloronaphthalene were carried out by f titration vs. 0.025 M KIO^ and by NMR respectively as described in

Chapter 2. Results which are shown in Table 5.2, confirm the mole ratio