Comment

In an attempt to extend the chemistry of subvalent indium compounds, we have been investigating the feasibility of producing solutions in which an indium(I) halide is acceptably stable to disproportionation at temperatures between 190 and 300 K. One strategy has involved co-condensation of the vapours of the halide and an excess of a potential solvent (e.g. toluene or toluene/ether), followed by warming of the mixture to see whether a solution of practical use in synthesis can be formed. Although ready dissolution without disproportionation is rare, indium(I) iodide has proved to be the halide most prone to dissolve in organic solvents, of which pyridine or pyridine-containing mixtures have been among the most promising. Solutions of indium(I) iodide in a pyridine/m-xylene (2:1) mixture are lastingly stable below 243 K but undergo slow disproportionation at room temperature, giving a mixture of indium metal and triiodotripyridineindium(III), (I). Crystallographic studies of (I) at 150 K have afforded a rare structural characterization of a neutral six-coordinate indium(III) iodide complex.

The orthorhombic crystals of (I) (Fig. 1) consist of neutral molecules with indium in a pseudo-octahedral environment and with the meridional stereochemistry suggested in a preliminary report (Small & Worrall, 1982) but different from that deduced on the basis of IR measurements (Adams et al., 1968). Unlike the corresponding chloride (Jeffs et al., 1984) and bromide (Small & Worrall, 1982) adducts, the iodo compound takes up no additional pyridine molecules of solvation, presumably as a result of the extra bulk of the iodide ligands.

The In-I bond lengths [2.8390 (6) and 2.8676 (3) Å] are comparable to those found in the related complex [InI₃(4-MeC₅H₄N)₃], (II) [2.803 (4), 2.848 (4) and 2.893 (4) Å; Brown & Tuck, 1996], there being a less pronounced but still distinct shortening of the unique In-I bond trans to a pyridine ring. For these two neutral six-coordinate InI₃ complexes, the In-I distances occur in the range 2.80-2.90 Å. As expected, the corresponding distances are shorter in similar five-coordinate complexes (2.66-2.80 Å), and shorter still in four-coordinate complexes (2.50-2.74 Å), as revealed by the 19 hits in a search of the Cambridge Structural Database (Version 5.26; Allen, 2002). The terminal and bridging In-I distances of the In₂I₆ molecules of solid indium(III) iodide are 2.644 (2) and 2.842 (2) Å, respectively (Kniep et al., 1982).

The In-N bond lengths in (I), with an average of 2.316 Å, are not significantly different from those in (II) (2.31 Å), [InCl₃(C₅H₅N)₃]·C₅H₅N (2.327 Å) and [InBr₃(C₅H₅N)₃]·C₅H₅N (2.30 Å). In every case, the longest In-N bond is that trans to a halogen, viz. 2.323 (5) versus 2.309 (4) Å in (I), 2.34 (2) versus 2.28 (3)/2.30 (3) Å in (II), 2.377 (21) versus 2.302 (7) Å in [InCl₃(C₅H₅N)₃]·C₅H₅N and 2.32 (2) versus 2.28 (3)/2.31 (2) Å in [InBr₃(C₅H₅N)₃]·C₅H₅N, even if the difference is not always statistically significant. Any systematic dependence on the nature of the halogen is, at best, small, although no strict comparison can be made in view of the diversity of the data (most of which relate to crystals at room temperature).

The molecules in (I) each lie on a twofold rotation axis running through the I2-In-N2 group and parallel to the crystallographic b axis. As in (II), the I1-In-I1ⁱ and N1-In-N1ⁱ angles (symmetry code as in Table 1) depart from linearity, and each pyridine ring is tilted out of the appropriate plane by an average of 40 (2)°, in keeping with both the stronger non-bonded repulsion exerted by atom I2, as compared with N2, and minimization of the interaction between the iodide and pyridine functions. Analysis of the dimensions of the coordinated pyridine molecules indicate no unusual features. The packing of the molecules of (I) (Fig. 2) gives little evidence of specific interactions. At 3.17 Å, for example, the shortest HI distance is consistent with a normal van der Waals contact, although the existence of weak hydrogen bonding is not precluded (Table 2). Likewise, short II contacts are evidently disfavoured (there are none below 5 Å), which could imply that the I atoms carry a substantial negative charge.

Figure 1 The molecular structure of (I), showing the atom-labelling scheme (ORTEPIII; Burnett & Johnson, 1996). Displacement ellipsoids are drawn at the 40% probability level. [Symmetry code: (i) -x+1, y, .]

Figure 2 A packing diagram of (I), viewed along the a axis, with the b axis aligned vertically. Dotted lines indicate relatively short HI distances, which may be indicative of weak hydrogen bonding. Key: In atoms are vertically hatched, I atoms are horizontally hatched, and N, C and H atoms are unfilled white circles.

Experimental

Crystals of (I) were grown from a solution initially containing InI in pyridine/m-xylene (2:1) over a period of one month at room temperature.

Crystal data

[InI3(C5H5N)3] Mr = 732.84 Orthorhombic, P b c n a = 9.6622 (2) Å b = 14.9139 (4) Å c = 13.9850 (4) Å V = 2015.25 (9) Å3 Z = 4 Dx = 2.415 Mg m-3 Mo K radiation Cell parameters from 13 983 reflections = 5-28° = 5.77 mm-1 T = 150 K Block, colourless 0.11 × 0.09 × 0.06 mm

Data collection

Nonius KappaCCD diffractometer scans Absorption correction: multi-scan(DENZO/SCALEPACK; Otwinowski & Minor, 1997)Tmin = 0.53, Tmax = 0.71 13 983 measured reflections 2293 independent reflections 1670 reflections with I > 3(I) Rint = 0.038 max = 27.5° h = -12 12 k = -19 19 l = -18 18

Refinement

Refinement on F R[F2 > 2(F2)] = 0.025 wR(F2) = 0.029 S = 1.01 1670 reflections 102 parameters H-atom parameters constrained Weighting scheme: part 1, Chebychev polynomial (Watkin, 1994; Prince, 1982), [weight] = 1/[A0T0(x) + A1T1(x) + + An-1Tn-1(x)], where Ai are the Chebychev coefficients 0.403, 0.231 and 0.196, and x = F/Fmax; robust weighting (Prince, 1982), W = [weight][1 - (F/6F)2]2 (/)max = 0.006 max = 0.60 e Å-3 min = -0.86 e Å-3

Table 1 Selected geometric parameters (Å, °) In1-I1 2.8676 (3)   In1-I2 2.8390 (6) In1-N1 2.309 (4) In1-N2 2.323 (5) I1-In1-I1i 173.079 (18) I1-In1-I2 93.460 (9) I1-In1-N1 89.64 (9) I2-In1-N1 93.74 (9) I1-In1-N1i 89.91 (9) N1-In1-N1i 172.52 (18) I1-In1-N2 86.540 (9) I2-In1-N2 179.995 N1-In1-N2 86.26 (9) Symmetry code: (i) .

Table 2 Hydrogen-bond geometry (Å, °) D-HA D-H HA DA D-HA C2-H21I1ii 1.00 3.17 3.904 (5) 131 C6-H61I1iii 1.00 3.18 3.946 (4) 135 Symmetry codes: (ii) ; (iii) -x+1, -y+1, -z+1.

H atoms were positioned geometrically and treated using a riding model, with C-H distances assumed to be 1.00 Å. The U_iso(H) values were taken to be 1.2U_eq(C).

Data collection: COLLECT (Nonius, 2000); cell refinement: DENZO (Otwinowski & Minor, 1997); data reduction: DENZO; program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: CRYSTALS (Betteridge et al., 2003); molecular graphics: CRYSTALS; software used to prepare material for publication: CRYSTALS.

Supplementary data for this paper are available from the IUCr electronic archives (Reference: JZ1705 ). Services for accessing these data are described at the back of the journal.