Contrasting crystal supramolecularity for [Fe(phen)₃]I₈ and [Mn(phen)₃]I₈: complementary orthogonality and complementary helicity

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Abstract

The crystallisation, crystal structures, and analyses of crystal supramolecularity for [Fe(phen)₃]I₈ and [Mn(phen)₃]I₈ are reported. Crystalline [Fe(phen)₃]I₈ contains the first example of one-dimensionally extended sixfold aryl embraces (6AE) between [M(phen)₃]²⁺ complexes. The interchain domains contain twisted I₈^2– ions, which are translated along the axis of the domain to form polyiodide helices. Sections of the polyiodide helix slot into the grooves between the phen ligands in each [Fe(phen)₃]²⁺ complex: these grooves are canted in helical fashion in each complex, even though the extended 6AE chain is not helical, and there is good local registry and geometrical complementarity between the polyiodide helices and the cation chains. The polyiodide helices are buttressed further by concerted C–H⋯I interactions. The crystal approaches trigonal symmetry. In contrast, crystalline [Mn(phen)₃] (I₃)(I₅) contains neither 6AE nor the parallel fourfold aryl embraces (P4AE) characteristic of [M(phen)₃]^z complexes, but does display a relatively rare instance of a homo-chiral P4AE. The association of [Mn(phen)₃]²⁺ complexes and the I₃^– and I₅^– ions is effective and close, and manifests well the complementary orthogonality of [M(phen)₃]^z complexes and polyiodides. There are two formula units in the asymmetric unit of [Mn(phen)₃](I₃)(I₅), and the high-spin [Mn(phen)₃]²⁺ cations are distorted from the octahedral stereochemistry of low-spin [Fe(phen)₃]²⁺. The issues of whether these two structures could be regarded as substitutional polymorphs, and which is the more stable, and whether each could crystallise with the structure of the other, are discussed.

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Introduction

Since molecular crystals are excellent supramolecular assemblies,¹ we use crystallisation and crystal packing analysis to investigate crystal supramolecular motifs. The bidentate ligands 2,2′-bipyridyl (bipy) and 1,10-phenathroline (phen), in their complexes, are able to engage in offset-face-to-face (OFF) and edge-to-face (EF) motifs, similar to those well known for phenyl groups.² Concerted action of these EF and OFF motifs leads to multiple aryl embraces, as principal supramolecular motifs. Complexes [M(bipy)₃]^z form the sixfold aryl embrace (6AE) shown in Fig. 1(a),³ while bis- and tris-phen complexes more commonly form the parallel fourfold aryl embrace (P4AE), shown in Fig. 1(b).^4,5 The 6AE is comprised of six ligands engaged in a concerted cycle of six EF interactions, while the P4AE is comprised of one OFF and two EF motifs.

Fig. 1 The sixfold aryl embrace (6AE) commonly formed between two [M(bipy)₃] complexes, and the parallel fourfold aryl embrace (P4AE) commonly formed between two [M(phen)₃] complexes.

Another phase of our investigations involves inorganic functionalities containing atoms of high atomic number, and high dispersion attractions, with a focus on iodine and polyiodides. In previous papers we have described the crystallisation and crystal packing analysis of dimorphs of [CuI(phen)₂]I₃,⁶ of trimorphs of [Fe(phen)₃]I₁₂,⁷ of [M(phen)₃]I₇ (M = Mn, Fe),⁸ and of [Fe(phen)₃]I₁₄ and [Fe(phen)₃]I₁₈.⁹ These systems combine the supramolecularity of M–phen complexes, and of polyiodides, and the occurrence of polymorphism in two systems indicates a balance of the energies of (a) the embraces between the complex cations, (b) the polyiodide associations, and (c) the associations of the complex cations and the polyiodide ions. The [Fe(phen)₃]²⁺–polyiodide crystal system is particularly rich, yielding a total of 13 crystalline compounds, ranging from [Fe(phen)₃]I₄ to [Fe(phen)₃]I₁₈.¹⁰ In all of these a strong feature is the complementary orthogonality of the ligand planes in the complexes and of segments in the polyiodide ions, with I_x chains lying facial and edge-peripheral to phen ligands.

In this paper we report the crystallisation and crystal supramolecularity of [Fe(phen)₃]I₈, and of [Mn(phen)₃]I₈. The crystal structures are very different, and this pair of compounds is another candidate for substitutional polymorphism.^11,12

Crystallisation of [Fe(phen)₃]I₈

[Fe(phen)₃](BF₄)₂ was precipitated by addition of NaBF₄ to FeSO₄ + 3phen in water, and [Fe(phen)₃]I₂ was crystallised by addition of NaI in acetone to [Fe(phen)₃](BF₄)₂ in acetone. Crystallisation mixtures were prepared as mixtures of solutions of [Fe(phen)₃]I₂ and I₂, with solvent composition, molar ratio, and concentration as the three main variables. Table 1 reports the main experiments and their products, which included some other [Fe(phen)₃]I_x crystals.

Table 1 Relevant crystallisation experiments in the [Fe(phen)₃]I_x system

Expt	Solvent	Mole ratio I2∶[Fe(phen)3]I2	Conc. of [Fe(phen)3]^2+/mM	Product
a From this solution crystals of [Fe(phen)3]I7, [Fe(phen)3]I8 and [Fe(phen)3]I12 grew concurrently.
1	CH3CN∶H2O 5∶2	5∶1	3.6	[Fe(phen)3]I8
2	CH3CN∶MeOH 5∶2	5∶1	3.6	[Fe(phen)3]I8
3	CH3CN∶EtOAc 5∶2	5∶1	3.6	[Fe(phen)3]I8
4	CH3CN∶H2O 5∶1	5∶1	4.2	[Fe(phen)3]I8
5	CH3CN∶H2O 10∶3	5∶1	3.9	[Fe(phen)3]I7
6	CH3CN∶H2O 5∶2	5∶1	3.6	[Fe(phen)3]I7
7	CH3CN∶H2O 2∶1	5∶1	3.3	[Fe(phen)3]I8
8^a	CH3CN∶H2O 5∶3	5∶1	3.1	[Fe(phen)3]I7 and [Fe(phen)3]I8
9	CH3CN∶H2O 10∶7	5∶1	2.9	[Fe(phen)3]I8
10	CH3CN∶H2O 5∶4	5∶1	2.8	[Fe(phen)3]I8

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In view of some difficulties in reproduction of crystalline products in the [Fe(phen)₃]I_x system, we provide additional information about the experiments in Table 1.

Experiments 1–3. A standard solution that was 5 mM in [Fe(phen)₃]I₂ and 25 mM in I₂ in CH₃CN was prepared by dissolving [Fe(phen)₃]I₂ (0.425 g, 0.50 mmol) in 100 mL CH₃CN, then dissolving I₂ (0.635 g, 2.50 mmol) with stirring aided by warming to ca. 60 °C. From this standard solution 5 mL aliquots were treated with 2 mL aliquots of either H₂O, MeOH or EtOAc. These solutions were kept in sealed containers at 25 °C for 20 d, after which time crystals were present in all solutions.

Experiments 4–10. As in experiments 1–3, a standard solution that was 5 mM in [Fe(phen)₃]I₂ and 25 mM in I₂ in CH₃CN was used. 10 mL aliquots of this were treated with volumes of water, according to Table 1, and kept in sealed containers at room temperature overnight. Crystals were collected after 24 h, and washed with Et₂O.

Note that experiments 1 and 6 had the same composition, but yielded [Fe(phen)₃]I₈ when crystallised over a longer period, and [Fe(phen)₃]I₇ over a shorter period.

Other similar experiments using CH₃CN alone as solvent, and using DMF mixed with H₂O or with Et₂O, yielded crystals of [Fe(phen)₃]I₇.

Crystals of [Fe(phen)₃]I₈ are stable in air, and to loss of I₂ under ambient conditions.

Crystallisation of [Mn(phen)₃]I₈

The precursor [Mn(phen)₂I₂] was prepared by an adaptation of a literature method.¹³ A solution of NaI (0.899 g, 6.0 mmol) in 5 mL warm EtOH was added to a solution of MnCl₂·4H₂O (0.594 g, 3.0 mmol) in 5 mL warm EtOH. A white precipitate of NaCl appeared immediately and was removed by filtration. To the filtrate was added a solution of 1,10-phenanthroline hydrate (1.189 g, 6 mmol) in 10 mL warm EtOH. A bright lime green–yellow solid immediately precipitated, and was collected after 5 min by suction filtration. The solid was rinsed with EtOH and air dried. It was used without further purification.

A 4.54 mM solution of [Mn(phen)₂I₂] (0.335 g, 0.50 mmol) was prepared in 100 mL of CH₃CN plus 10 mL water. This was treated with a 0.1 M solution of I₂ in CH₃CN, such that the mole ratio of I₂ to [Mn(phen)₂I₂] was 5, and the concentration of Mn was 3.7 mM. Crystals of [Mn(phen)₃]I₇ and [Mn(phen)₃]I₈ grew concurrently, and were collected after 1 d and washed with Et₂O. These crystals showed some loss of I₂ after several days under ambient conditions.

All crystalline products were identified by single crystal diffraction and microscopic examination of crystal habit.

Crystal structure determination

Diffraction data for both structures were collected at ambient temperature using a Nonius CAD4 diffractometer with graphite monochromated MoKα radiation (λ = 0.7107 Å). Metal and iodine atoms were refined anisotropically, while each phen ligand was refined with thermal motion described by a 12 parameter TL group (where T is the translation tensor, L is the libration tensor). Hydrogen atoms of the cations were included in calculated positions and were included in the group thermal motion for that ligand. Details of data collection and refinement are given in Table 2.

Table 2 Crystal data for [Fe(phen)₃]I₈ and [Mn(phen)₃]I₈^a

	[Fe(phen)3]I8	[Mn(phen)3]I8
a Click b008381j.txt for full crystallographic data (CCDC no. 1350/41).
Formula	C36H24FeI8N6	C36H24MnI8N6
M	1611.7	1610.8
Crystal system	monoclinic	triclinic
Space group	P21/c	P1̄
a/Å	11.624(5)	13.819(7)
b/Å	23.535(4)	15.322(8)
c/Å	16.179(7)	20.732(11)
α/°	90	87.56(3)
β/°	107.93(2)	87.42(3)
γ/°	90	85.69(3)
V/Å^3	4211(3)	4369(4)
D c/g cm^–3	2.54	2.45
Z	4	4
µ Mo/mm^–1	6.20	5.92
2θmax	50	46
Crystal decay(%)	6	23
Min trans. factor	0.18	0.35
Max trans. factor	0.42	0.57
Unique reflections	7391	8115
Observed reflections	4793	4789
R merge	0.014	0.023
R	0.036	0.057
R w	0.055	0.070

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The low-spin [Fe(phen)₃]²⁺ cation in [Fe(phen)₃]I₈ has regular octahedral coordination, but in the high-spin [Mn(phen)₃]²⁺ cation in [Mn(phen)₃]I₈ there is some distortion of the octahedral geometry, which will be discussed after the crystal packing has been described.

Crystal structure of [Fe(phen)₃]I₈

This crystal has a distinctive packing feature, which is a pseudo-hexagonal array of chains of [Fe(phen)₃]²⁺ complexes, surrounded by polyiodide ions, shown in Fig. 2. The metal complexes are aligned along their threefold axes, and the chains they form are not interpenetrated by any polyiodide atoms.

Fig. 2 Axial view of the pseudo-hexagonal array of chains of [Fe(phen)₃]²⁺ complexes and surrounding polyiodide atoms (purple) in [Fe(phen)₃]I₈. The space group is P2₁/c, with centres of inversion located between chains. I–I bonds within but not between I₈^2– ions are drawn. Click image or fig2.htm to access a 3D representation.

Each chain of [Fe(phen)₃]²⁺ complexes is an infinite chain of 6AE motifs, illustrated in Fig. 3. The complexes are aligned along their (non-crystallographic) threefold axes, and each 6AE has virtual threefold symmetry. Within the 6AE there are six concerted EF interactions involving six phen ligands, and within each complex each phen ligand functions as EF donor at both ends, and EF acceptor at both ends. This is the phen homologue of the (6AE)_∞ chain already described for [M(bipy)₃] complexes.³ In [Fe(phen)₃]I₈ the 6AE chain is not exactly linear, with an Fe⋯Fe⋯Fe angle of 177°. The 6AE is almost centrosymmetric, and therefore enantiomeric [Fe(phen)₃]²⁺ complexes alternate along the chain.

Fig. 3 The infinite 6AE chain formed by [Fe(phen)₃]²⁺ complexes in [Fe(phen)₃]I₈. There are six EF interactions between six different phen ligands in each centrosymmetric embrace domain. The Fe⋯Fe⋯Fe angle along the chain is 177°, and the Fe⋯Fe separation is 8.09 Å.

The structure of the polyiodide in [Fe(phen)₃]I₈ is a twisted I₈^2–, shown in Fig. 4. The orthogonality of this ion, with angles close to 90 and 180°, is notable.

Fig. 4 The structure and dimensions of the I₈^2– ion in [Fe(phen)₃]I₈. The torsion angle I8–I6–I3–I1 is 100.7°, and the unmarked I–I–I angles are 177.9, 172.2, 174.5 and 174.0° at I2, I4, I5 and I7, respectively.

The shortest distance between I atoms in different I₈^2– ions is 3.92 Å, which occurs between I₈^2– translated along the z axis. This generates a helix of iodine atoms, illustrated in Fig. 5. At the longer connection between I₈^2– ions the I–I–I angles are 130 and 148°, contrasting the 90 or 180° angles at all other atoms, and consequently the helix is trapezoidal rather than triangular in projection along its axis.

Fig. 5 The polyiodide helix formed by translation of I₈^2– ions in [Fe(phen)₃]I₈. The longer inter-I₈^2– connection is I1···I8 = 3.92 Å. At this linkage the angles I2–I1–I8 and I1–I8–I7 are 148.4 and 130.4° respectively.

This crystal is comprised of apparently segregated elements: the 6AE chain of complex cations, and the helix of polyiodide anions. The next question is about the association of these two elements. We observe that only the polyiodide strand is helical: the 6AE chain involves alternating enantiomers of [Fe(phen)₃]²⁺. There is one repeat of the helix per unit cell, but two complex cations per repeat (along z). Close examination of Fig. 2 shows that there are six polyiodide helices around each cation chain, and three cation chains around each polyiodide helix.

Segments of the polyiodide slot into the grooves between phen ligands in each complex. It is evident from Fig. 3 that the grooves between phen ligands in [Fe(phen)₃]²⁺, occurring around the threefold equator of the complex, are not obstructed by the 6AE which occur at the threefold poles of the complex. Further, the grooves are canted in helical fashion, even though all of the grooves on the 6AE chain are not canted with the same helicity. Nevertheless, there is effective engagement of local helicity in the polyiodides with local helicity in the [Fe(phen)₃]²⁺ complexes, which allows the polyiodide segments to nestle nicely in the grooves. The three linear segments of I₈^2– occupy the three grooves of [Fe(phen)₃]²⁺, as shown in Fig. 6. All grooves are occupied, and all polyiodide segments occupy a groove.

Fig. 6 The occupancy of the three grooves of a [Fe(phen)₃]²⁺ complex by three different linear segments of the I₈^2– ion in crystalline [Fe(phen)₃]I₈, shown with atoms labelled (left) and with space-filled (right).

The association of polyiodide segments with the faces of phen ligands in the grooves of the metal complexes is a distinctive feature of this crystal (and of many other [M(phen)₃]I_x crystals). It is buttressed further here by C–H⋯I motifs on the side of the polyiodide segment not in the groove. These interactions are illustrated in Figs. 7, 8 and 9 for the three segments of the I₈^2– ion. The I1–I2–I3 segment is very effectively surrounded by the groove on one side and the peripheral H atoms of two phen ligands in two different 6AE chains on the other side. In addition there are C–H⋯I interactions with adjacent complexes in the groove 6AE chain, and the contiguous I atoms of the helix are also involved.

Fig. 7 Illustration of the surroundings of the I1–I2–I3 segment of the I₈^2– ion in [Fe(phen)₃]I₈. Located in the groove of the central complex on the right, the I₃ segment forms I⋯H–C interactions (black and white candystripes, drawn for H⋯I < 3.4 Å) with two complexes in different 6AE chains on the left (red and white candystripe is Fe⋯Fe for a 6AE). The two adjacent complexes of the 6AE chain on the right also form C–H⋯I interactions with the terminal I atoms of the I₃ segment. The adjacent I atoms (I4, I8) in the polyiodide helix are included to show that they also form C–H⋯I interactions with these complexes.

Fig. 8 Surroundings of the I3–I4–I5–I6 segment of the I₈^2– ion in [Fe(phen)₃]I₈. Note the well developed C–H⋯I interactions (black and white candystripes, drawn for H⋯I  < 3.4 Å) from phen ligands of two complexes in 6AE chains, which are different from that on the right providing the (phen)₂ groove. One C–H⋯I interaction from an adjacent complex in the right 6AE chain is marked.

The combination of the features described is very effective crystal packing in [Fe(phen)₃]I₈. Three types of supramolecular motif are dominant and recurrent: (1) the 6AE chain of [Fe(phen)₃]²⁺ ions; (2) the occupancy of all (phen)₂ grooves by linear I₃ or I₄ segments of the I₈^2– polyiodide; and (3) enclosure of the other side of polyiodide segments by C–H⋯I interactions from the peripheries of phen ligands.

The crystal packing of this crystal approaches threefold symmetry, in that the axial array of 6AE chains and polyiodide helices is approximately hexagonal (Fig. 2), that the 6AE chain is very close to the threefold symmetry of its component complexes and propagating embraces, and that the I₈^2– ion has three linear segments. The overall threefold symmetry and deviations from it are evident in the representations (Figs. 7–9) of the surroundings of the polyiodide segments. The question arises as to whether there could be a similar compound with exact trigonal symmetry. This would require a polyiodide component of similar size but with three equal components, and charge 2–, such as I₉^2–, but this species is neither known nor expected in polyiodide chemistry. The challenge remains.

Fig. 9 Surroundings of the I6–I7–I8 segment of the polyiodide chain in the groove of the complex on the right, showing the C–H⋯I interactions (black and white candystripes, drawn for H⋯I < 3.4 Å) from the two complexes (left) in the other two two chains, and also C–H⋯I from the upper complex on the right.

Crystal structure of [Mn(phen)₃]I₈

This crystal contains two formula units in the asymmetric unit of a centrosymmetric triclinic lattice (P1̄). The polyiodide structure is straightforward. The 16 crystallographically unique I atoms combine as two I₃^– and two bent I₅^– ions. The bond distances in the I₃^– ions are 2.87, 2.95 Å and 2.92, 2.95 Å, with small deviations from linearity (angles 178.2 and 171.9°, respectively). The two I₅^– ions are pictured in Fig. 10. All other I⋯I distances are ≥3.87 Å, so the assignment of I₃^– and I₅^– ions is unambiguous. The variability of distance and angle (Fig. 11) manifests the softness of polyiodide energy potentials.¹⁴

Fig. 10 Atom labels, bond distances (Å), and the central bond angle (°) for the obtuse and acute I₅^– ions in [Mn(phen)₃]I₈.

Fig. 11 Orthogonal views of the homo-chiral P4AE that occurs in crystalline [Mn(phen)₃]I₈. The Mn⋯Mn separation is 8.67 Å.

Before presenting the arrangement of the species [Mn(phen)₃]²⁺, I₃^– and I₅^– in the lattice, we identify and describe three significant interactions between [Mn(phen)₃]²⁺ complexes, which are unusual. None of the standard embraces involving [M(phen)₃] complexes is present. There is a variant of the P4AE, shown in Fig. 11. As in the P4AE, the variant comprises one OFF and two EF local motifs, using two phen ligands per complex, but the difference is that the two [Mn(phen)₃]²⁺ complexes have the same chirality. The conventional P4AE is exactly or approximately centrosymmetric, involving enantiomeric [M(phen)₃] complexes. We name the embrace in crystalline [Mn(phen)₃]I₈ as a homo-chiral P4AE, as differentiated from the normal hetero-chiral P4AE. Fig. 12 illustrates the differences between the homo- and hetero-chiral P4AEs. The Mn⋯Mn separation is 8.7 Å, slightly shorter than M⋯M in the hetero-P4AE.

Fig. 12 Comparative representations of (left) the normal, hetero-chiral P4AE, and (right) the homo-chiral P4AE which occurs in crystalline [Mn(phen)₃]I₈.

The second unusual embrace between two [Mn(phen)₃]²⁺ complexes, shown in Fig. 13, is built from three EF interactions involving four phen ligands. One phen ligand functions as C–H donor to two orthogonal phen ligands in the other complex. This motif is non-centrosymmetric, with homo-chiral complexes. Two I₅^– ions are closely associated with this embrace, as shown in Fig. 14, lying in the grooves of both complexes, and also separating what would otherwise be an EF interaction. The effectiveness of this packing of two [Mn(phen)₃]²⁺ complexes and two I₅^– ions leads us to regard the combination of all four ions as the crystal supramolecular motif. The Mn⋯Mn separation is 10.1 Å, and this (phen)₄(I₅)₂ motif will be labelled as ‘10.1’ in subsequent descriptions and figures.

Fig. 13 The (EF)₃ embrace between two crystallographically independent homo-chiral [Mn(phen)₃]²⁺ complexes in [Mn(phen)₃]I₈. The EF interactions are arrowed. The Mn⋯Mn separation is 10.1 Å.

Fig. 14 Two views from opposite sides of the (EF)₃ interaction, showing the two crystallographically independent I₅^– ions involved. There are facial interactions between the polyiodide arms and the ligand planes, C–H⋯I peripheral interactions (partly obscured). The acute I₅^– shown on the right penetrates (and separates) what would otherwise be an EF interaction. Click image or fig14.htm to access a 3D representation.

The third significant motif in this crystal also involves a combination of complex cations and polyiodide, in this case I₃^–. As shown in Fig. 15, this motif is a zig-zag chain, in which each [Mn(phen)₃]²⁺ complex uses two phen ligands, with an I₃^– in their groove. The association is such that contiguous phen ligands are parallel, and in fact all of the phen ligands in this one-dimensional motif are effectively parallel to the direction of all I₃^– ions. Each I₃^–, in addition to being facial to two phen ligands from one complex, is acceptor for the well formed C–H⋯I interaction with the peripheries of phen ligands from the contiguous complexes in the chain. We refer to this motif as the parallel (phen)₄(I₃)₂ embrace, since these are the components between each pair of complexes.

Fig. 15 The zig-zag chain of parallel (phen)₄(I₃)₂ embraces in [Mn(phen)₃]I₈. The peripheral C–H⋯I interactions are marked as black and white candystripes, and the two crystallographically independent motifs are marked with red and white candystripes and identified by their Mn⋯Mn distances marked.

There are two crystallographically independent parallel (phen)₄(I₃)₂ embraces in the zig-zag chain (identified by their Mn⋯Mn separations, see Fig. 15), and each is centrosymmetric. Both embraces involve only one of the crystallographically independent cations and one of the crystallographically independent I₃^– ions.

We next describe the placement of these motifs in the lattice. Fig. 16 shows that there are chains of complexes in which the homo-chiral P4AE (labelled as 8.7) alternates with the (EF)₃ embrace (labelled as 10.1). The association of the two types of I₅^– ion with the (EF)₃ embrace, as (phen)₄(I₅)₂, is evident in Fig. 16(a), in which the I₃^– ions have been omitted. There are two different interchain domains, at z = 0.5 and 1, with the parallel (phen)₄(I₃)₂ embraces straddling the centres of inversion at z = 0.5, as marked in Fig. 16(b). The interchain domain at z = 0 and 1 is occupied by the second type of I₃^– ion, in pairs across a centre of inversion, and associated with six [Mn(phen)₃]²⁺ complexes. In this assembly the shortest Mn⋯Mn separation is 10.0 Å, and so the assembly is labelled ‘10.0’ in Fig. 16 and the following descriptions.

Fig. 16 Two representations (with the same view direction) of aspects of the crystal packing in [Mn(phen)₃]I₈. I₅^– are drawn purple, and I₃^– ions blue. Chains of [Mn(phen)₃]²⁺ complexes in the y-direction are comprised of alternating homo-chiral P4AE (red Mn···Mn connection labelled ‘8.7’) and the (phen)₄(I₅)₂ embrace (red Mn···Mn connection labelled ‘10.1’). The association of the two types of I₅^– ion with four phen ligands from two complexes is evident in (a). Both parts show that there are two different regions between these chains, at z = 0.5 and z = 0, 1. The two types of I₃^– are included in (b), together with the parallel (phen)₄(I₃)₂ embraces which are the red Mn··Mn connections labelled ‘12.2’ and ‘12.6’. These latter embraces straddle the centres of inversion at z = 0.5. The red Mn··Mn connections across the centres of inversion at cell vertices are the ‘10.0 motifs’ involving two I₃^– and six surrounding [Mn(phen)₃]²⁺ complexes.

The crystal packing in the ‘10.0’ domain is shown in Fig. 17. There is no significant additional [Mn(phen)₃]²⁺⋯[Mn(phen)₃]²⁺ embrace, but each I₃^– assembles around it the phen ligands of five different [Mn(phen)₃]²⁺ complexes, using a large number of local motifs of three types: (a) I₃^– side ··· phen face; (b) I₃^– end ··· phen face; (c) phen peripheral C–H⋯I, close to linear. There is efficient crystal packing in this region.

Fig. 17 The centrosymmetric assembly of two I₃^– ions and six [Mn(phen)₃]²⁺ complexes in the ‘10.0’ domain of [Mn(phen)₃]I₈. The 10.0 Å connection between two Mn atoms (see Fig. 16) is marked across the centre, and the orientation of two ‘8.7’ motifs is also marked. Note many local contacts in which the side of I₃^– or the end of I₃^– is facial to a phen ligand, and also a number of peripheral C–H⋯I interactions which are close to linear. Each I₃^– forms interactions with five surrounding cations.

Finally, we describe the surrounds of the I₅^– ions. Fig. 14 shows the association of two I₅^– with the ‘10.1’ embrace, (EF)₃, of two [Mn(phen)₃]²⁺ complexes, but not the surrounds of each type of I₅^–. Fig. 18 shows four [Mn(phen)₃]²⁺ complexes (in two ‘10.1’ motifs) associated with a centrosymmetric pair of the obtuse I₅^– ions. This assembly contains a good example of I₅^– wrapping around the periphery of one phen ligand, generating a series of C–H⋯I motifs, including bifurcated C–H(⋯I)₂ motifs. It is possible that the larger angle (102°) at the bend of this I₅^– is caused by its wrapping around the periphery of one phen ligand.

Fig. 18 The surrounds of a centrosymmetric pair of obtuse I₅^– ions in [Mn(phen)₃]I₈. The locations of two surrounding ‘10.1’ motifs are marked. A large number of C–H⋯I local motifs occur in this assembly: the ellipses draw attention to these where the upper I₅^– lies around the periphery of one phen ligand, and other C–H⋯I motifs are vertical in this orientation.

Fig. 19 shows the eight complexes surrounding a centrosymmetric pair of the acute I₅^– ions. Again, the complementary orthogonality of the phen ligands in all of the complexes, and of the arms of the I₅^– ions, is obvious. The I₅^– side⋯phen face, and C–H⋯I local motifs are prevalent. Close examination shows that some pairs of phen rings in the complex are not exactly orthogonal, and the direction of deviation is the same as that in the slight deviation from 90° of the central angle of I₅^–. The complementarity of both I₅^– and the [Mn(phen)₃]²⁺ complex extends to slight deviations. This observation raises the question as to whether the intramolecular geometry of [Mn(phen)₃]²⁺ causes this deviation, or the angle at I₅^–.

Fig. 19 The surrounds of a centrosymmetric pair of acute I₅^– ions in [Mn(phen)₃]I₈. The locations of surrounding ‘8.7’ and ‘10.1’ motifs are marked.

The different molecular structures of [Fe(phen)₃]²⁺ and [Mn(phen)₃]²⁺

There are pronounced differences in the crystal packing of [Fe(phen)₃]I₈ and [Mn(phen)₃]I₈, and the question that arises is whether these could be due to, or induce, differences in the molecular structures of the complex cations. The low-spin [Fe(phen)₃]²⁺ complex is regular, with Fe–N distances ranging between 1.972(6) and 1.986(6) Å, and inter-ligand N–Fe–N angles ranging between 90.7(2) and 94.4(2)°. The angles between the normals to the ligand planes are 84.6, 89.6 and 85.2°. In contrast, the high-spin [Mn(phen)₃]²⁺ complexes have some asymmetrically coordinated phen ligands, with the dimensions listed in Table 3.

Table 3 Intramolecular dimensions for [Mn(phen)₃]²⁺ complexes in [Mn(phen)₃]I₈

		Complex A	Complex B
Mn–N distances	Mn–phenA	2.27(2), 2.17(2)	2.23(2), 2.25(2)
Mn–N distances	Mn–phenB	2.17(2), 2.30(2)	2.20(2), 2.27(2)
Mn–N distances	Mn–phenC	2.23(2), 2.20(2)	2.22(2), 2.19(2)
Inter-ligand N–Mn–N angles		89.4(7)–100.6(6)	88.7(6)–103.1(6)
Phen–phen interplanar angles		75.4, 87.9, 80.4	83.0, 89.4, 74.4

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The only other [Mn(phen)₃]²⁺ complex in the Cambridge Structural Database (CSD) is that with the anion diethyldithiophosphate [CSD refcode SOMZOJ].¹⁵ The Mn–N distances are similar, falling in the range 2.21–2.26 Å. However, the inter-ligand N–Mn–N angles show an even wider range than those shown above, of 86.6–107.6°. More significantly, the angles between the normals to the ligand planes are 60.0, 61.6 and 66.4°, showing large deviation from octahedral coordination. The crystal structure of [Mn(phen)₃](I₃)₂ has been reported (although it is not in the CSD),¹⁶ and the [Mn(phen)₃]²⁺ complex has exact C₃ and almost D₃ symmetry: the Mn–N distances are 2.25 and 2.26 Å; the inter-ligand N–Mn–N angles range from 92.3 to 103.3°; and the angle between the normals to the phen planes is 74.6°, indicating trigonal distortion. Therefore, on the basis of four different [Mn(phen)₃]²⁺ molecular structures in three crystals, we conclude that the metric properties of high-spin [Mn(phen)₃]²⁺ are quite variable.

Discussion

Both of these crystals are unusual because they contain crystal supramolecular motifs which do not normally occur for [M(phen)₃] complexes, and, further, they do not contain the typical P4AE embrace.⁵ The 6AE and its one-dimensional extension occurs for a number of crystals containing [M(bipy)₃]^z complexes, but [Fe(phen)₃]I₈ is the first crystal in which (6AE)_∞ has been observed with [M(phen)₃]^z complexes, which more commonly engage in the P4AE. An isolated 6AE with exact trigonal symmetry occurs between [Mn(phen)₃]²⁺ complexes in [Mn(phen)₃](I₃)₂, although it was not recognised as such and was referred to as a ‘snow flake-like dimeric unit.’¹⁶ We are not aware of the occurrence of the 6AE between [M(phen)₃]^z complexes in any other crystals.⁵ The presence of the OFF in the P4AE but not in the 6AE is believed to favour the P4AE for phen complexes, due to the larger ligand surface area which favours the OFF local motif. However this may be too simplistic an interpretation, particularly for crystals containing polyiodide anions, because the 6AE allows all phen faces and the three inter-phen grooves of [M(phen)₃] to be occupied by linear segments of polyiodide chains, which is a very common occurrence,^6,7,9,10 and is demonstrated well by [Fe(phen)₃]I₈.

Peripheral C–H⋯I interactions are common in these crystals, as in others.^6,7,9,10 A higher level association involving these has been recognised in the crystals of both [Fe(phen)₃]I₈ and [Mn(phen)₃]I₈, in which an I₃ segment lying facial to (phen)₂ in the groove of one complex is bound by concerted C–H⋯I interactions extending from the peripheries of phen ligands of two other complexes. Notice the similarity between Fig. 8 for [Fe(phen)₃]I₈ and Fig. 15 showing the parallel (phen)₄(I₃)₂ embrace in [Mn(phen)₃]I₈. This motif can be propagated in one dimension (Fig. 15). The presence of this crystal supramolecular motif in other related crystals needs to be investigated.

The overall crystal packing in [Fe(phen)₃]I₈ can be described as complementary helicity, between the helical grooves of the complexes in the 6AE chain and the segments of the polyiodide helices. In [Mn(phen)₃]I₈ the overall packing is complementary orthogonality between the planes of the phen ligands and the directions of the I₃^– ions and the arms of the I₅^– ions.

A number of questions revolve around the relationships between these two structure types. Which is more favourable? To what extent do the relative inflexibility of the low-spin [Fe(phen)₃]²⁺ cation and the geometrical variability of the [Mn(phen)₃]²⁺ cation influence the crystal packing? Can the relationship between [Fe(phen)₃]I₈ and [Mn(phen)₃]I₈ be regarded as substitutional dimorphism, or is the difference between the cations too large? Can each compound be crystallised with the packing of the other?

Calculations of energies for the supramolecular motifs, and of lattice energies, will be needed to determine which is the better crystal packing. Neither of the lattices contains solvent, indicating that both are favourable. The presence of two formula units in the asymmetric unit of [Mn(phen)₃]I₈ could be regarded as an indication of less satisfactory crystal packing, just as the approach to relatively high symmetry in [Fe(phen)₃]I₈ could be regarded as an indication of favourable packing. Contrary to this, the flexibility of the [Mn(phen)₃]²⁺ cation seems to allow it to adjust to give effective local interactions with the polyiodides. But, [Mn(phen)₃]I₈ contains none of the motifs that are standard for other [M(phen)₃]I_x crystals. We conclude that both of the lattice packings are favourable, but reserve judgement of their relative energies until calculations are completed. The experimental approach to this issue, that is the crystallisation of one or both compounds in the other crystal structure type, has been investigated but has not yet yielded the dimorphic crystals for either compound. Given that [Mn(phen)₃](I₃)₂ crystallises with a single 6AE motif between pairs of threefold [Mn(phen)₃]²⁺ cations, and therefore that chains of [Mn(phen)₃]²⁺ to give (6AE)_∞ are possible, there is reason to expect that [Mn(phen)₃]I₈ could crystallise with the crystal supramolecularity of [Fe(phen)₃]I₈. However, until further experiments show otherwise, we view [Fe(phen)₃]I₈ and [Mn(phen)₃]I₈ as distinct crystal structures with different cations, and on the fringe of substitutional dimorphism.

There exist two other crystal structures which contain [M(phen)₃]I₈, with other species in the lattice. The structure of [Ni(phen)₃]I₈(CHCl₃)₂ [CSD refcode HILLUJ] has been reported,¹⁷ and we have determined the structure of [Mn(phen)₃]I₈(phen). These are not directly comparable with the two compounds reported in this paper, and we defer description and analysis of their crystal packing until consideration in the larger families of compounds [Fe(phen)₃]I_x and [Mn(phen)₃]I_x.¹⁸

Acknowledgements

This research is supported by the Australian Research Council and the University of New South Wales. We thank D. C. Craig for crystallography.

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