Comment

S,S-Diphenylsulfimide, Ph₂SNH, has proven to be an extremely useful ligand for later transition metal centres in so far as the complexes produced invariably exhibit strong hydrogen bonding between the sulfimide NH groups and the counter-ions. As a result, the complexes often show unusual structural properties; by way of example, the ostensibly simple reaction of the ligand with Cu(BF₄)₂ results in a multitude of products, including three polymorphs of [Cu(Ph₂SNH)₄](BF₄)₂, the inter-allogon [Cu(Ph₂SNH)₄]₂(BF₄)₄ and the mixed coordination system [Cu(Ph₂SNH)₄][Cu(Ph₂SNH)₅](BF₄)₄ (Holmes et al., 2002).

One of the first examples of a metal-sulfimide complex reported was [Co(Ph₂SNH)₆]Cl₂, in which the ligands are arranged around the metal centre in such a way as to maximize the hydrogen-bonding interactions between the sulfimide NH groups and the chloride counter-ions (Kelly et al., 1998). Hence, each chloride ion acts as a trifurcated acceptor, with three NH bonds pointing upwards and three down. The isostructural nickel(II) complex has since been reported by Sellmann et al. (2001).

We describe here investigations into the effect of the modification of the anion on the hydrogen-bonding interactions between Co-, Fe- and Pt-sulfimide complexes and uncoordinated anions.

In the light of the successful study of [Co(Ph₂SNH)₆]Cl₂, the modification of the anion from the chloride to the larger, less electronegative, iodide was attempted. The reaction of Ph₂SNH with CoI₂ in acetonitrile results in pink crystals of [Co(Ph₂SNH)₆]I₂·Ph₂SNH·2MeCN, (I), which crystallizes in a triclinic space group, indicating that the crystallographically imposed symmetry of the chloride complex has been broken. The crystal structure of (I) is centrosymmetric and the asymmetric unit contains a complete formula unit. The Co^II centre is octahedrally coordinated by six sulfimide ligands, with trans angles lying between 178.44 (8) and 179.23 (8)° (Fig. 1 and Table 1). The ligands are arranged in the same way as in the chloride complex, with three sulfimide NH groups on either side of the metal. Each halide ion therefore acts as a trifurcated acceptor to the [Co(Ph₂SNH)₆]²⁺ cation, with NI distances in the range 3.7302 (19)-3.8461 (19) Å and N-HI angles in the range 157 (2)-165 (2)° (Fig. 2 and Table 2). A mean distance of 3.66 (1) Å has been quoted for NI contact distances for sp²-hybridized NH groups (Desiraju & Steiner, 1999).

However, a seventh sulfimide ligand is included in the structure, this time acting as an outer- rather than an inner-sphere ligand. The ligand is not coordinated to the metal centre but interacts with the [Co(Ph₂SNH)₆]I₂ unit via an N-HI hydrogen-bonding interaction, having an NI distance of 3.987 (2) Å and a more linear N-HI angle of 172 (2)°. Therefore, one of the iodide anions accepts four hydrogen bonds. Two acetonitrile molecules of crystallization are also included in the crystal structure.

The structure appears rather asymmetric, with the second iodide anion only accepting three hydrogen bonds. It was expected that the system might, therefore, take up further units of Ph₂SNH. The addition of excess sulfimide, however, does not result in the incorporation of more of the free ligand. Compound (I) appears to be the only product, even in the presence of a large excess of sulfimide. The system also incorporates a number of weaker C-HI hydrogen bonds using aromatic groups that surround the iodide anions. CI contact distances lie in the range 4.072 (3)-4.423 (2) Å. A mean distance of 4.00 (2) Å has been quoted for the CI contact distance for sp²-hybridized CH groups (Desiraju & Steiner, 1999).

The `capping' of the NH groups within the three-up/three-down arrangement through N-HI hydrogen bonding in (I) clearly limits the extent of strong hydrogen bonding between complexes. N-HI and C-HI hydrogen bonds combine to create one-dimensional stacks of alternating cations and anions, propagating along the [223] direction. C-HI interactions also link adjacent stacks together.

Interestingly, we have no evidence for the formation of simple [Co(Ph₂SNH)₆]I₂, even when exactly six molar equivalents (or slightly less) of sulfimide are used. It is not clear why the observed structure should be such a favourable arrangement, and this result serves to highlight the unpredictability and structural diversity of complexes of Ph₂SNH.

In the light of the fact that previous results indicate that the sulfimide ligands can readily adjust the relative orientations of the NH groups in order to facilitate the maximum degree of interaction with anions (e.g. multiple acceptors such as BF₄^-, etc.), the question of the effect of introducing a third halide into the basic [ML₆]ⁿ⁺ unit becomes germane.

Attempts to prepare [Co(Ph₂SNH)₆]Cl₃ have proved unsuccessful; however, direct reaction of FeCl₃ with six equivalents of the ligand does provide a route to [Fe(Ph₂SNH)₆]Cl₃, (II), the first iron-sulfimide complex. Unlike [Co(Ph₂SNH)₆]Cl₂, which, though it has a low solubility in MeCN, may be recrystallized from the hot solvent, this product does not even show appreciable solubility upon heating. To circumvent this recrystallization problem, the reaction was performed without stirring after the initial mixing of reagents. This approach promoted the growth of small but usable crystals of (II). The product crystallizes in a cubic space group with one-sixth of an Fe^III centre on a site of symmetry, one sulfimide ligand and a total of one-half of a chloride anion in the asymmetric unit (Fig. 3). Atom Cl1 lies on a site of threefold symmetry and has an occupancy of , while atom Cl2 lies on a site of symmetry and has an occupancy of . The angle at the metal centre is distorted from 90 to 86.21 (9)° (Table 3). The complete structure contains three chloride anions, one of which is inequivalent to the other two. The arrangement of the sulfimide ligands is identical to that in the structure of [Co(Ph₂SNH)₆]Cl₂, with two Cl atoms, Cl1 and a symmetry-equivalent, placed above and below the metal centre, each accepting three hydrogen bonds (Fig. 4 and Table 4). The third Cl atom, Cl2, is isolated in the structure and is not involved in any strong hydrogen-bonding interactions. Atom Cl2 is instead surrounded by phenyl groups of the Ph₂SNH ligands and is involved in six symmetry-equivalent C-HCl hydrogen bonds, having a CCl distance of 3.607 (3) Å (C-H = 0.95 Å, HCl = 2.86 Å and C-HCl = 136°). In the light of the previously observed tendency of the sulfimide ligands to maximize the hydrogen bonding in the system, this is a surprising observation, especially as the chloride anion is a strong acceptor. This result clearly indicates that the hydrogen-bonding arrangement within the [M(Ph₂SNH)₆]Cl₂ unit is extremely stable.

In the course of our investigations of metal-sulfimide complexes, we have observed two quite different types of reaction with platinum centres. When reacted with (PPh₄)₂[PtCl₄] in CH₂Cl₂, a simple substitution to give [Pt(Ph₂SNH)₄]Cl₂ takes place (Kelly et al., 2000); in this product, the NH groups of the ligands are seen to orientate themselves in such a way as to maximize the hydrogen bonding to the anions. In reactions involving starting materials bearing nitrile ligands, the metal actually mediates addition reactions of the sulfimide to the nitriles; this phenomenon has been observed for Pt^II (Kelly & Slawin, 1999) and, most effectively, for Pt^IV starting materials (Makarycheva-Mikhailova et al., 2003).

We present here the results of an investigation into the reaction of [Pt(Ph₂SNH)₄]Cl₂ with iodine. This reaction was undertaken for two reasons: firstly, it would ascertain whether or not a stable Pt^IV-sulfimide complex (as opposed to a sulfimide/nitrile hybrid) could be isolated. Note that some previous examples of iodo-Pt^IV species containing S,N-donor ligands have proven to be unstable; for example, the Pt^II species Pt(S₂N₂H)I(PMe₂Ph) is the only isolable product from the reaction of Pt(S₂N₂H)(PMe₂Ph)₂ with iodine (Jones et al., 1988). In addition, the aim was to observe the effect that introduction of the iodo ligands would have on the hydrogen-bonding arrangements of the sulfimide ligands.

Reaction of [Pt(Ph₂SNH)₄]Cl₂ with iodine in CH₂Cl₂ takes place rapidly, and the product [Pt(Ph₂SNH)₄I₂]Cl₂, (III), appears stable both in solution and in the solid state. A structure determination confirms that oxidation to Pt^IV has taken place, giving an elongated octahedral coordination with the Pt^IV ion situated on an inversion centre (Fig. 5 and Table 5); the asymmetric unit contains one-half of a formula unit and the two I atoms adopt mutually trans positions. It is likely that this conformation is favoured because it allows the NH units of the sulfimide to become involved in significant hydrogen-bonding interactions with the chloride counter-ions (Table 6). As in the Pt^II analogue (and indeed other homoleptic complexes of Ph₂SNH), the ligands arrange themselves in such a way that the two pairs of cis S atoms extend from opposite sides of the MN₄ plane; as a result, each pair of NH groups can co-operate in hydrogen bonding to a different chloride ion. The average HCl distance is thus 2.46 Å (somewhat longer than in the Pt^II analogue, where the HCl distance is 2.25 Å; Kelly et al., 2000), with average N-HCl angles of 166°. Although strong hydrogen bonds are limited to within the [Pt(Ph₂SNH)₄I₂]Cl₂ unit, weaker C-HCl hydrogen bonds (Table 6) are apparent between aryl CH groups and atom Cl3.

The structure of (III) contains two molecules of dichloromethane solvent per cationic complex; thus, the asymmetric unit contains one molecule of dichloromethane, which was found to be disordered. The solvent molecule was modelled over two sets of positions with a refined major occupancy of 80.6 (11)%. The metal-bound iodide has a close contact with one of the Cl atoms of the solvent molecule [I1Cl2(x, y + 1, z) = 3.815 (4) Å].

Comparison of the metal-nitrogen bond lengths in the three title complexes (Tables 1, 3 and 5) shows that the Co-N bond lengths [mean 2.143 (6) Å] are significantly longer than the Pt-N [2.042 (3) Å] and Fe-N [2.078 (2) Å] bond lengths.

From the investigations presented here, it is clear that the three-up/three-down conformation of the [M(Ph₂SNH)₆]ⁿ⁺ ion is extremely stable. Attempts to alter this conformation through the introduction of less electronegative anions, or even extra anions, still leads to the same conformation being observed in (I) and (II), while the introduction of iodide ligands to give an elongated octahedron in (III) still allows two sulfimide NH groups on each side of the complex to hydrogen bond to the chloride anions. In all three compounds, there are weaker hydrogen-bonding interactions between relatively acidic CH groups and the halide anions, resulting from the halide anions being positioned in the gaps between close-packed Ph₂SNH ligands.

Experimental

For the preparation of compound (I), CoI₂ (51 mg, 0.16 mmol) was dissolved in MeCN (5 ml), and a solution of Ph₂SNH·H₂O (250 mg, 1.14 mmol) in MeCN (5 ml) was added with stirring to give an intense blue-coloured solution containing a pink precipitate. This solid was redissolved by heating to give a clear solution from which pink crystals of (I) were grown by slow cooling. For the preparation of compound (II), FeCl₃·6H₂O (44 mg, 0.16 mmol) was dissolved in MeCN (10 ml) and a solution of Ph₂SNH·H₂O (250 mg, 1.14 mmol) in MeCN (10 ml) was added. The solution was stirred only to mix the two solutions and was then left to stand. The product has very limited solubility in MeCN, and precipitation of an orange solid was observed, but, on standing, small yellow crystals of (II) formed overnight. For the preparation of compound (III), [PtI₂(Ph₂SNH)₄]Cl₂ was first prepared by the equimolar reaction of [Pt(Ph₂SNH)₄]Cl₂ with iodine in CH₂Cl₂, followed by removal of the solvent in vacuo. Crystals of (III) were grown by slow diffusion of ether into a solution of the product in CH₂Cl₂.

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Supplementary data for this paper are available from the IUCr electronic archives (Reference: BM1594 ). Services for accessing these data are described at the back of the journal.

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