High-pressure studies of palladium and platinum thioether macrocyclic dihalide complexes

# 2014 International Union of Crystallography The mononuclear macrocyclic Pd complex cis[PdCl2([9]aneS3)] ([9]aneS3 = 1,4,7-trithiacyclo-nonane) converts at 44 kbar into an intensely coloured chain polymer exhibiting distorted octahedral coordination at the metal centre and an unprecedented [1233] conformation for the thioether ligand. The evolution of an intramolecular axial sulfur–metal interaction and an intermolecular equatorial sulfur–metal interaction is central to these changes. Highpressure crystallographic experiments have also been undertaken on the related complexes [PtCl2([9]aneS3)], [PdBr2([9]aneS3)], [PtBr2([9]aneS3)], [PdI2([9]aneS3)] and [PtI2([9]aneS3)] in order to establish the effects of changing the halide ligands and the metal centre on the behaviour of these complexes under pressure. While all complexes undergo contraction of the various interaction distances with increasing pressure, only [PdCl2([9]aneS3)] undergoes a phase change. Pressure-induced I I interactions were observed for [PdI2([9]aneS3)] and [PtI2([9]aneS3)] at 19 kbar, but the corresponding Br Br interactions in [PdBr2([9]aneS3)] and [PtBr2([9]aneS3)] only become significant at much higher pressure (58 kbar). Accompanying density functional theory (DFT) calculations have yielded interaction energies and bond orders for the sulfur–metal interactions. Received 10 February 2014 Accepted 17 April 2014


Introduction
The number of coordination complexes being studied under high pressure is increasing rapidly, and a search of the Cambridge Structural Database (Allen, 2002;CSD Version 5.35 November 2013 plus two updates) reveals 259 entries for transition metal crystal structures determined above ambient pressure: 74 of these were published prior to 2009 commencing with the first report in 1987, with 70 in 2009-2010 and a further 115 published between 2011 and 2014. As each pressure point in a high-pressure experiment is recorded in the CSD as a separate entry, the number of unique complexes studied is substantially lower, with currently ca 60 systems in the literature.
High-pressure studies of organic compounds typically produce new phases by the rearrangement of their intermolecular interactions, with bonded distances and valence angles largely unaffected; in contrast, coordination compounds, where the metal geometry and bond distances are more flexible, can exhibit changes in bond distances which are an order of magnitude greater than in organic compounds.
A study of [GuH][Cu 2 (OH)(citrate)(guanidine) 2 ] (GuH = guanidinium cation) by Moggach et al. (2009) illustrates the wide range of geometric changes possible for a metal coordination complex. The coordination at the Cu II centres changes from [4 + 1] distorted square pyramidal to [4 + 2] distorted octahedral as part of a phase change at 29.5 kbar, with one of the Cu II centres reverting to a [4 + 1] coordination mode on passing through a second phase change at 42.3 kbar. These changes involve conversions between long intermolecular interactions and coordinate bonds. The conversion of intra-and intermolecular interactions to full covalent or coordinate bonds remains rare at pressures below 100 kbar.
We previously reported (Allan et al., 2006) that [PdCl 2 ([9]aneS 3 )], a mononuclear square-planar Pd II complex, undergoes a second-order phase transition at 44 kbar: the geometry becomes distorted octahedral via short SÁ Á ÁPd interactions, a chain polymer is formed with a new macrocyclic conformation, and the colour of the complex changes dramatically from orange to black. The observation of this array of phenomena led us to investigate further [PdCl 2 ([9]aneS 3 )] and five closely related complexes, described herein.  ([9]aneS 3 )]). The complex was prepared according to a published procedure (Blake et al., 1988(Blake et al., , 1996. Crystals were obtained from a solution of the complex in CH 3 NO 2 . 2.1.2. cis-Dichloro(1,4,7-trithiacyclononane)platinum(II) ([PtCl 2 ([9]aneS 3 )]). The complex was prepared according to the procedure described by Grant et al. (2001). Crystals were obtained from a solution of the complex in CH 3 NO 2 .

High-pressure studies
A Merrill-Bassett diamond anvil-cell (DAC) was constructed around Boehler-Almax diamonds with 600 mm culets. Laser-cut tungsten (Goodfellow Metals, thickness non-ambient crystallography 200 mm) was used as the gasket material, and gasket holes (200 mm diameter) were drilled using a BETSA electric discharge machine. A crystal was placed in the DAC along with a ruby sphere for pressure measurement and 4:1 methanol/ethanol as the pressure-transmitting medium (PTM). The pressure was allowed to equilibrate for at least 1 h after each ramp before being measured using the ruby fluorescence method (Barnett et al., 1973). The pressure was remeasured immediately after the frameset was collected and the average pressure calculated. Although individual pressure measurements were reproducible to within 0.5 kbar (Holzapfel, 2003), the pressure drifted slightly during the collection of the frames, with the result that average pressures are subject to an uncertainty of 2 kbar. A pressure of 0.001 kbar corresponds to atmospheric pressure: the crystal and the ruby are loaded into the DAC but no PTM has yet been added.  12.6, 23.5, 36, 48.7, 73.1, 76.8 and 53.5 kbar for crystal (1); 46.0 and 42.5 kbar for crystal (2). Cell refinement and data reduction was completed using Bruker SAINT (Bruker, 2002). Structure solution used SHELXS97 (Sheldrick, 2008), absorption corrections were applied by means of multi-scan methods (SADABS; Sheldrick, 2004), structure refinement used SHELXL97 (Sheldrick, 2008) and molecular graphics used PLATON (Spek, 2009). Because the completeness of the data was severely restricted by the geometry of the diamond-anvil cell, only Pd, Cl and S atoms could be refined anisotropically, even with the use of suitable restraints. This occurs even at lower pressures; with increasing pressure more atoms were refined isotropically; at the highest pressure all atoms were refined isotropically (see CIF and Table ED1 in the supporting information 1 ).
Framesets for crystal (3) were acquired on Diamond Light Source Beamline I19 at 3.0, 21.8 and 47.1 kbar using a Rigaku Saturn 724 CCD area detector and a CrystalLogic four-circle kappa goniometer, using a synchrotron radiation wavelength of 0.6889 Å . Cell refinement and data reduction were completed using CrysAlis PRO (Agilent, 2011) and absorption corrections, structure solution and refinement used the same programs as above for [PdCl 2 ([9]aneS 3 )]. Despite the limited completeness of the data all non-H atoms could be refined anisotropically with the application of suitable extensive restraints to molecular geometry and displacement parameters (see CIFs and Table ED2 ([9]aneS 3 )] measuring 0.14 Â 0.09 Â 0.07 mm. Framesets were collected at the following pressures: 0. 001, 4.6, 25.0, 102.6, 64.7, 50.1, 40.1, 30.4, 96.5, 71.3, 81.9, 116.9 kbar. Cell refinement, data reduction, absorption corrections, structure solution and structure refinement were carried out as for [PtCl 2 ([9]aneS 3 )]. At lower pressures, non-H atoms could be refined anisotropically with the application of suitable extensive restraints to molecular geometry and displacement parameters (see CIFs and Table ED3 in the supporting information); at higher pressures only isotropic refinement (with geometric restraints) was possible.
Diffraction data for [PtBr 2 ([9]aneS 3 )] were taken at Diamond Light Source Beamline I19 as for [PtCl 2 ([9]aneS 3 )] on a red/brown block measuring 0.10 Â 0.10 Â 0.15 mm. Framesets were collected at the following pressures: 0. 001, 2.7, 17.0, 42.5, 62.5, 74.9, 82.6, 88.8, 97.1, 48.2 and 32.7 kbar. Cell refinement, data reduction, absorption corrections, structure solution and structure refinement were carried out as for [PtCl 2 ([9]aneS 3 )]. At all pressures, non-H atoms could be refined anisotropically with the application of suitable extensive restraints to molecular geometry and displacement parameters (see CIFs and Table  ED4 ([9]aneS 3 )] on a dark red block measuring 0.10 Â 0.10 Â 0.15 mm. Framesets were collected at the following pressures: 0. 001, 7.6, 19.0, 30.0, 57.7, 36.6, 60.4, 79.4, 94.0, 66.2 and 41.9 kbar. Cell refinement, data reduction, absorption corrections, structure solution and structure refinement were carried out as for [PtCl 2 ([9]aneS 3 )]. No restraints were required to allow anisotropic refinement against data acquired at ambient pressure, but at all other pressures extensive restraints to molecular geometry and displacement parameters were required (see CIFs and Table ED5 Table ED6 in the supporting information). The Flack (1983) parameter was allowed to refine to values > 0.5 without the structure being inverted in order to allow direct comparison of the ligand conformations at different pressures.

DFT calculations
Single-point DFT calculations were performed on models derived from the high-pressure crystal structure determinations on [MX 2 ([9]aneS 3 )] (M = Pd, Pt; X = Cl, Br, I). For each complex and pressure, two symmetry-equivalent molecules were generated to give a chain of three molecules along the direction of the intermolecular MÁ Á ÁS4 interaction. The calculations were performed using the Amsterdam Density Functional (ADF) suite version 2007.01 (Fonseca-Guerra et al., 1998;Te Velde et al., 2001). The DFT calculations employed Slater-type orbital (STO) triple--plus polarization all-electron basis sets (from the ZORA/TZP database of the ADF suite). Scalar relativistic approaches were used within the ZORA Hamiltonian for the inclusion of relativistic effects and the local density approximation (LDA) with the correlation potential due to Vosko et al. (1980) was used in all of the calculations. Gradient corrections were performed using the functionals of Becke (1988) and Perdew (1986).

Effects of pressure on unit-cell parameters
Additional figures showing the effects of pressure on unitcell parameters appear in Appendix A of the supporting information: only the most significant features are described here. The molecules which eventually form extended chains are related by the 2 1 screw axis parallel to the crystallographic a axis.
3.1.1. [PdCl 2 ([9]aneS 3 )]. During the abrupt phase change at 44 kbar (see below) the decreasing length of the c axis of [PdCl 2 ([9]aneS 3 )] is reversed: it increases by 0.1594 (13) Å and this, along with the continuing decrease in the b axis, causes the lengths of these axes to cross over (Table 1, Fig. 1). Between ambient pressure and 76.8 kbar the unit-cell volume decreases by 23.5%, from 1128.2 (4) to 869.6 (2) Å 3 , with similar contractions of 8.5 and 9.0% for the a and b axes, respectively, and a slightly lower value of 7.4% for the c axis. Between 42.5 and 46.0 kbar, i.e. at pressures bracketing the phase change at 44 kbar, the unit-cell volume contracts by 14.5 (7) Å 3 .
3.1.2. Variations in unit-cell parameters for the six complexes. In contrast to [PdCl 2 ([9]aneS 3 )], the unit-cell volumes of the other five complexes tend to decrease smoothly with increasing pressure, and provide no indications of a phase change in a wide pressure range around 44 kbar ( Table 2). The relative contractions in the lengths of the a and b axes tend to be somewhat similar, but they are consistently larger than the reductions in the c axis lengths. There are no simple correlations between the relative variations in unit-cell dimensions and the pressure-induced structural changes in the complexes: we tentatively attribute this absence to the fact that multiple intramolecular and intermolecular changes take place concomitantly. The calculated bulk moduli in Table 2 are typical of molecular complexes (Tidey et al., 2014 and references therein), but the standard uncertainties preclude the identification of any meaningful trends.
At ambient pressure [PdCl 2 ([9]aneS 3 )] is an orange, square-planar, mononuclear complex which at 44 kbar transforms into an intensely black chain polymer with a distorted octahedral coordination at the metal centre. This phase transformation also involves a number of other structural changes to the complex: the distance between the apical S atom and the Pd II metal centre contracts sharply; the aforementioned chain polymer is formed by the shortening of an interaction between each Pd II centre and an S atom on an adjacent molecule. Finally, the macrocyclic ring undergoes significant conformational changes. These changes are analysed more fully in the following sections, but it is relevant to note here that the phase transformation appears to be fully reversible, with little or no hysteresis: reducing the pressure below 44 kbar restores the original mononuclear orange complex. Packing diagrams for the [PdCl 2 ([9]aneS 3 )] structure at 0.001, 42.5, 46.0 and 76.8 kbar appear as Figs. P1-P4 in the supporting information.
3.2.3. The intramolecular S1Á Á ÁPd1 interaction. The length of the interaction between the axial S1 and Pd1 shortens as the pressure increases, from 3.159 (10) Å at ambient pressure to 2.752 (17) Å at 76.8 kbar, an overall decrease of 0.41 (2) Å (Table 3, Fig. 3): between ambient pressure and 42.5 kbar it decreases from 3.159 (10) to 3.009 (5) Å , a change of 0.150 (11) Å ; during the phase change between 42.5 and 46 kbar it decreases by 0.160 (9) Å from 3.009 (5) to 2.849 (7) Å ; finally, between 46 and 76.8 kbar it decreases from 2.849 (7) to 2.752 (17) Å , a change of 0.097 (18) Å . It is noteworthy that the change observed over the relatively narrow range of 42.5-46 kbar comprises 39% of the total shortening and is in contrast to the behaviour of the other complexes studied (see Appendix B,  Table 2 Variations in unit-cell parameters and bulk moduli for the six complexes as a function of pressure.

Figure 2
Ball-and-stick representation of the structure of [PdCl 2 ([9]aneS 3 )] at ambient pressure showing the atom-numbering scheme used.
3.2.4. Effect on the S 2 Cl 2 Pd equatorial plane. As with the Pd-S distances described above, the Pd-Cl distances are not significantly affected by increasing pressure: over the pressure range 0-76.8 kbar, Pd1-Cl1 decreases by only 0.022 (11) Å and Pd1-Cl2 by only 0.043 (9) Å . During the phase change Cl1 appears to be displaced from the least-squares mean plane defined by S4, S7 and Cl2 by 0.59 (2) Å . However, this displacement occurs in a direction approximately parallel to the c-axis direction, coinciding with the region in the diffrac-tion pattern which is most shaded by the DAC; consequently we do not regard this displacement as a reliable feature of the evolving molecular geometry. This apparent displacement of Cl1 led us to define S4/S7/Cl2 as the reference coordination plane in preference to S4/S7/Cl1/Cl2. The resulting displacement of Pd1 [0.080 (5) Å at ambient pressure] shows no significant change up to 42.5 kbar, but decreases sharply by 0.091 (7) Å [from 0.096 (3) to 0.005 (7) Å ] between that pressure and 46 kbar, remaining in the S4/S7/Cl2 plane up to 76.8 kbar (Fig. 5).
3.2.5. Effect of pressure on the conformation of the macrocycle. Although restraints were applied to the S-C and C-C distances for all six complexes, none were applied to the conformation of the macrocycle; the conformational features of the macrocycle can therefore be discussed. The phase change at 44 kbar is accompanied by a series of changes from the overall [234] conformation of the macrocycle observed at ambient pressure. The conformation of the C9-S1-C2-C3-S4-C5 section is essentially unchanged, but the S4-  Variation in the angles S1Á Á ÁPd1-S4 (red trendlines) and S1Á Á ÁPd1-S7 (blue trendlines) for [PdCl 2 ([9]aneS 3 )], indicating the change in position of the axial sulfur donor atom S1 relative to S4, S7 and Pd1. Note that these changes occur largely over the narrow pressure range 42.5-46 kbar; the data points to either side of 44 kbar are indicated by different symbols.

Figure 6
Pressure dependence of the torsion angles of the macrocyclic ring in [PdCl 2 ([9]aneS 3 )], showing that major changes of 40-60 occur in six of the nine torsion angles at 44 kbar. Standard uncertainties are in the range 1-5 and error bars have been suppressed for clarity.  (17) 3.006 (10) C5-C6-S7 section adopts a more eclipsed, sterically less favourable arrangement, as seen by the change in the S4-C5-C6-S7 torsion angle from À37.9 (14) to 18 (3) (Fig. 6): this torsion angle approaches zero as the pressure is increased from 46 kbar to 76.8 kbar. The apical S atom moves to a more symmetrical position above the metal, and the S7-C8-C9-S1 arm adopts a visibly different conformation. The conformational changes in the [9]aneS 3 ligand are clearly visible in Fig. 7. At 46 kbar the Dale notation (Dale, 1973) for the conformation of the macrocycle is close to [333] based on corners at C3, C6 and C9, but a less strict definition allowing pseudo-Dale corners gives the descriptor [1233], which better reflects the asymmetry in the macrocyclic conformation; further increases in pressure lead to even less symmetry within the macrocyclic ring.
3.2.6. Effect on intermolecular Pd . . . S interaction. The potential of the title compounds for polymerization via chain formation has been recognized by us and others (e.g. Grant et al., 2001). An intermolecular interaction between Pd1 in the reference molecule and an equatorially coordinated S atom S4 i [symmetry code (i): x þ 1 2 ; Ày þ 1 2 ; 1 À z, indicating the operation of the 2 1 screw axis parallel to the a axis] completes the distorted [4 + 2] octahedral coordination around Pd1 and links the molecules into polymeric chains. The length of this interaction decreases markedly with pressure, from 3.525 (8) Å at ambient pressure to 3.006 (10) Å at 76.8 kbar, a change of 0.519 (13) Å . Most of the compression [0.321 (9) Å ] occurs between ambient pressure and 42.5 kbar; the decreases across the phase change and between 46 and 76.8 kbar are only 0.087 (9) and 0.111 (13) Å , respectively (Table 3, Fig. 8). This behaviour is unique in the family of complexes studied (see Appendix B, Fig. B2 of the supporting information). We employed the van der Waals radii for Pd and S (1.63 and 1.80 Å , respectively, giving AE vdW = 3.43 Å ; Bondi, 1964) as a criterion of whether there was a real interaction between Pd1 and S4 i and if so the pressure at which this occurs: an interaction is deemed to be present if the Pd1Á Á ÁS4 i distance is within AE vdW . At ambient pressure the distance is 3.525 (8) Å , so we conclude that no interaction is present, but by 12.6 kbar this distance has decreased to 3.367 (8) Å and at this point we judge the chain-forming Pd1Á Á ÁS4 i interaction to be present.
The shortening of the Pd1Á Á ÁS4 i interaction with increasing pressure is accompanied by a slight rotation of the reference and adjacent molecules relative to each other, which has the effect of aligning S4 i and Pd1 and reducing steric clashes as the molecules approach each other. The rotation is linked with an increase in the Pd1-S4Á Á ÁPd1 i -S4 i torsion angle (Figs. 9 and 10) from À82.2 (4) at ambient pressure to À86.1 (4) at 42.5 kbar. During the phase change at 44 kbar the compression of the Pd1Á Á ÁS4 i distance is accompanied by a more rapid change in this torsion angle to À92.9 (4) at 46 kbar, but subsequent increases in pressure have little effect on the angle.
The contraction in the length of the Pd1Á Á ÁS4 i interaction at a pressure of 12.6 kbar is also judged to indicate the onset of the change from the square-planar coordination for Pd1 seen at ambient pressure to a distorted octahedral coordination. Many of the valence angles around Pd1 show little variation with pressure (see the supporting information), but several exhibit marked changes across the phase transition at 44 bar (  The change in conformation for [PdCl 2 ([9]aneS 3 )] between (a) ambient pressure and (b) 76.8 kbar, showing the more symmetrical position of S1, the flattening of the S4-C5-C6-S7 region and the movement of C8. trend towards the adoption of a more symmetric coordination sphere with increasing compression.

Effects of pressure on the molecular geometry of [PtCl 2 ([9]aneS 3 )]
3.3.1. The structure of [PtCl 2 ([9]aneS 3 )] at ambient pressure. The complex cis-[PtCl 2 ([9]aneS 3 )] crystallizes in the chiral orthorhombic space group P2 1 2 1 2 1 with a = 7.564 (2), b = 12.223 (4), c = 12.248 (2) Å and V = 1132.1 (5) Å 3 (Grant et al., 2001;Fig. 12). The structure is isomorphous and isostructural with the Pd II analogue [PdCl 2 ([9]aneS 3 )] described above. The Pt II centre (Pt1) in [PtCl 2 ([9]aneS 3 )] is surrounded by a distorted square-planar array of two cis-coordinating S atoms (S4 and S7) at 2.221 (3) and 2.237 (3) Å , respectively, and two chloride ions (Cl1 and Cl2) at 2.336 (4) and 2.321 (3) Å , respectively. The apical S atom (S1) is endo with respect to the metal centre and participates in a long Pt1Á Á ÁS1 interaction of 3.264 (2) Å , which is within AE vdW for Pt and S (3.52 Å ; Bondi, 1964): the resulting coordination around Pt1 is described as [4 + 1], with equatorially bound donors S4, S7, Cl1 and Cl2 defining the base and S1 at the elongated apex. Pt1 is displaced from the plane of the two sulfur and two chloride donors by 0.0762 (8) Å in the direction of S1. The [9]aneS 3 macrocycle is endodentate with respect to Pt1 and adopts a [234] conformation, with the Dale corners (Dale, 1973) occurring at C3, C5 and C8. The apical S atom S1 does not occupy a position which is symmetrically above the metal centre and relative to the two equatorial S atoms, as shown by the different respective values for the angles S1Á Á ÁPt1-S4 and S1Á Á ÁPt1-S7 of 79.9 (1) and 77.5 (1) .      ([9]aneS 3 )], nor are any discontinuous changes observed in the conformation of the macrocycle or the colour of the crystal. The significant changes which occur upon compression are therefore those associated with the distance between the apical S atom and the Pt II metal centre and the length of the chain-forming interaction between each Pt II centre and an S atom in an adjacent molecule. 3.3.3. Effect on the intramolecular SÁ Á ÁPt interaction. The length of the interaction between the axial sulfur S1 and the Pt1 metal centre shortens smoothly and linearly as the pressure increases, from 3.264 (2) Å at ambient pressure to 3.073 (3) Å at 54 kbar (Table 4, Fig. S1). This decrease of 0.191 (4) Å is much larger than those observed in the equatorial distances (see x3.3.4).
The change in the S1Á Á ÁPt1-S4 angle, from 79.93 (7) at ambient pressure to 83.2 (1) at 54 kbar and the S1Á Á ÁPt1-S7 angle which changes from 77.36 (7) at ambient pressure to 80.1 (14) , shows that as the axial S1Á Á ÁPt1 distance decreases with pressure S1 moves to a position which is more symmetrically located over the Pt II centre. However, the difference between the S1Á Á ÁS4 and S1Á Á ÁS7 distances remains constant at around 0.100 Å throughout, indicating that there is no lateral movement of S1.
3.3.5. Effect on the conformation of the macrocycle. The conformation of the macrocycle in [PtCl 2 ([9]aneS 3 )] does not change under compression. Although the values of individual torsion angles fluctuate slightly (Fig. S2), the standard uncertainties are such that these changes cannot be regarded as significant. This retention of the [234] conformation is in clear contrast to the flattening of the S4-C5-C6-S7 section of the macrocycle observed in [PdCl 2 ([9]aneS 3 )] (see x3.2.5 above).
3.3.6. Effect on the intermolecular SÁ Á ÁPt interaction. The distance between Pt1 and the equatorial S atom S4 i in a neighbouring molecule related by the 2 1 screw axis parallel to the a axis [symmetry code (i) x þ 1 2 ; Ày þ 1 2 ; 1 À z] decreases with pressure, from 3.593 (2) Å at ambient pressure to 3.244 (5) Å at 54 kbar, a change of 0.349 (5) Å (Table 4, Fig.  S3). Mirroring the trends in the unit-cell dimensions, over half of this compression [0.196 (7) Å ] occurs before 16.3 kbar, compared with 0.153 (9) Å between 16.3 and 54 kbar. The Pt1Á Á ÁS4 i distance at ambient pressure slightly exceeds AE vdW for Pt and S (3.52 Å ; Bondi, 1964), but by ca 7 kbar (Table 4, Fig. S3, by interpolation) it has decreased to below this limit, at which point we determine that the chain-forming Pt1Á Á ÁS4 i interaction is present. This decrease in distance confers a distorted octahedral [4 + 2] coordination on the metal centre, but in contrast to [PdCl 2 ([9]aneS 3 )] the valence angles around the metal are essentially invariant with pressure.  Wieghardt et al., 1986). It is isostructural and isomorphous with the dichloro analogues [PdCl 2 ([9]aneS 3 )] and [PtCl 2 ([9]aneS 3 )] described above. The Pd II centre Pd1 is coordinated by two cis-coordinating S atoms (S4 and S7) at 2.257 (2) and 2.275 (2) Å , respectively, and two bromide ions (Br1 and Br2) at 2.456 (1) and 2.468 (1) Å , respectively, forming a distorted square-planar arrangement. This coordination is supplemented by a long interaction from the apical, endodentate sulfur donor S1 located 3.125 (1) Å from the metal. The length of this apical PdÁ Á ÁS interaction is shorter than the corresponding distance of 3.159 (10) Å in [PdCl 2 ([9]aneS 3 )], consistent with bromide being a poorerdonor than chloride. The coordination of Pd1 at ambient pressure can therefore be described as [4 + 1], with the base defined by the equatorial donors S4, S7, Br1 and Br2, and S1 at the apex. The metal centre sits above the plane defined by four donor atoms, by 0.0957 (10) Å in the direction of S1. The Ball-and-stick representation of the structure of [PtCl 2 ([9]aneS 3 )] at ambient pressure showing the atom-numbering scheme used. (Dale, 1973) which define the conformation again occur at C3, C5 and C8.
3.4.4. Effect on the S 2 Br 2 Pd equatorial plane. Although the lengths of the Pd-Br bonds are unaffected by compression, they do move further apart, as shown by the increase of 3.3 (4) in the Br1-Pd1-Br2 angle, from 93.5 (2) at ambient pressure to 96.8 (4) at 116.4 kbar: corresponding decreases are observed in the S-Pd-Br angles (Fig. S6). The displacement of Pd1 from the coordination plane formed by Br1, Br2, S4 and S7 increases smoothly by 0.046 (3) Å over the pressure range studied, from 0.0957 (10) Å at ambient pressure to 0.142 (3) Å at 116.9 kbar.
3.4.5. Effect on the conformation of the macrocycle. The conformation of the macrocycle remains unchanged across the pressure range studied, as indicated by the values of the torsion angles and their associated standard uncertainties (see Fig. S7). The [234] conformation of the macrocycle is, therefore, retained throughout.

Effect on the intermolecular SÁ Á ÁPd interaction.
There is potential for an additional, long-range PdÁ Á ÁS interaction between the metal centre and an equatorially coordinated S4 donor atom in a neighbouring molecule related by the operation of a 2 1 screw axis parallel to the a axis of the unit cell. At ambient pressure the distance between the relevant atoms is 3.549 (3) Å : as this lies beyond AE vdW for Pd and S (3.43 Å ; Bondi, 1964) by some 0.119 (3) Å , no interaction is deemed to exist. However, the distance decreases smoothly with increasing pressure to reach a value of 3.058 (9) Å by 116.9 kbar, representing an overall change of 0.491 (9) Å (Table 5, Fig. S8); by ca 15 kbar it had fallen below AE vdW , at which point we consider the chain-forming Pd1Á Á ÁS4 i interaction to be present and a distorted octahedral [4 + 2] coordination conferred on the metal centre. As with [PtCl 2 ([9]aneS 3 )], the valence angles around the metal in [PdBr 2 ([9]aneS 3 )] appear essentially invariant with pressure. The Pd-S and Pd-Br distances are marginally but systematically shorter at 116.9 kbar than at ambient pressure.   (5) 3.058 (9) Symmetry code: (i) x þ 1 2 ; Ày þ 1 2 ; 1 À z.

Figure 13
Ball-and-stick representation of the structure of [PdBr 2 ([9]aneS 3 )] at ambient pressure showing the atom-numbering scheme used.
comprising two cis-coordinating S atoms S4 and S7 at 2.263 (6) and 2.241 (6) Å , respectively, and two bromide ions Br1 and Br2 at 2.480 (4) and 2.467 (4) Å , respectively (Fig. 14). A long SÁ Á ÁPt interaction from the endodentate apical sulfur S1 at 3.2054 (17) Å lies within AE vdW for Pt and S (3.52 Å ; Bondi, 1964): the length of the apical PtÁ Á ÁS interaction in [PtBr 2 ([9]aneS 3 )] is noticeably shorter than the corresponding distance of 3.264 (2) Å in [PtCl 2 ([9]aneS 3 )]. The metal centre sits above the least-squares mean plane defined by these four donor atoms, by 0.0739 (10) Å in the direction of S1. The [9]aneS 3 macrocycle is endodentate to the Pt II centre and adopts a [234] conformation, with Dale corners (Dale, 1973) located at the atoms C3, C5 and C8. A comparison of the angles S1Á Á ÁPt1-S4 and S1Á Á ÁPt1-S7 [77.88 (11) and 80.53 (11) , respectively] shows them to differ by 2.65 (15) , indicating that the position of S1 is not symmetrical with respect to the two equatorial S atoms. Correspondingly, the S1Á Á ÁS4 and S1Á Á ÁS7 distances [3.514 (5) and 3.596 (5) Å , respectively] differ by 0.084 (7) Å , placing S1 slightly closer to S4 than to S7. The coordination at the metal centre can be best described as [4 + 1] with S4, S7, Br1 and Br2 defining a basal plane and S1 the more remote apex. As for other members of this family, molecules of [PtBr 2 ([9]aneS 3 )] are pre-organized for a long-range S4Á Á ÁPt interaction between Pt1 in the reference molecule and the equatorial S-donor S4 in a neighbouring molecule related by the operation of a screw axis parallel to the a axis of the unit cell (Fig. S9). At ambient pressure the distance between these two atoms is 3.575 (3) Å , slightly [0.055 (3) Å ] outside AE vdW for Pt and S (3.52 Å ; Bondi, 1964), so no interaction is yet deemed to exist. The crystal undergoes no discontinuous changes, for example in the conformation of the macrocycle or the colour of the crystal. The significant changes which occur involve the lengths of the apical SÁ Á ÁPt interaction and the chain-forming SÁ Á ÁPt interaction between the metal centre in the reference molecule and an S atom in an adjacent molecule.
3.5.4. Effects of pressure in the S 2 Br 2 Pt equatorial plane.
As the pressure increases the Pt II centre moves further away from the equatorial plane defined by Br1, Br2, S4 and S7: the displacement increases from 0.0739 (10) Å at ambient pressure to 0.0995 (10) Å at 97.1 kbar. The positions of C5 and C6 also change differently in relation to this plane: C5 lies 0.46 (9) Å below the plane at ambient pressure and moves to lie only 0.32 (1) Å above the plane at 97.1 kbar, while C6 lies 0.01 (1) Å above the plane at ambient pressure and moves to 0.20 (2) Å above the plane at 97.1 kbar.   (5) Symmetry code: (i) x þ 1 2 ; Ày þ 1 2 ; 1 À z.

Effects of pressure on the intermolecular SÁ Á ÁPt
interaction. The length of the intermolecular interaction between Pt1 and an equatorial S atom (S4 i ) in a neighbouring molecule related by the 2 1 screw axis parallel to the a axis [symmetry code: (i) x þ 1 2 ; Ày þ 1 2 ; 1 À z] decreases with pressure, from 3.575 (3) Å at ambient pressure to 3.136 (5) Å at 97.1 kbar (Table 6, Fig. S14). Taking AE vdW for Pt and S as 3.52 Å (Bondi, 1964), we observe that by ca 5 kbar the separation of the two atoms lies within this sum and a PtÁ Á ÁS interaction exists. Between ambient pressure and 97.1 kbar, the S4-Pt1Á Á ÁS4 i angle decreases slightly from 88.92 (9) to 85.23 (11) . There is no systematic shortening of the Pt-S or Pt-Br distances across the pressure range studied.
Pd II tends to adopt a square-planar geometry, but in [PdI 2 ([9]aneS 3 )] its coordination is best described as [4 + 1] with S4, S7, I1 and I2 as the base and S1 defining the apex. There is potential for a long-range intermolecular interaction between the Pd1 and an equatorial sulfur S4 in a neighbouring molecule related by the 2 1 screw axis parallel to the a axis of the unit cell. At ambient pressure their separation of 3.596 (3) Å lies 0.166 (3) outside AE vdW for Pd and S (3.43 Å ; Bondi, 1964), so no interaction is deemed to exist.  ([9]aneS 3 )] are absent. The significant changes which occur involve the apical SÁ Á ÁPd interaction, the chain-forming SÁ Á ÁPd interaction and IÁ Á ÁI contacts which cross-link the chains into a two-dimensional extended structure.
3.6.4. Effects of pressure on the conformation of the macrocycle. The conformation of the macrocycle is essentially unchanged with increased pressure (see Fig. S17) and the conformational descriptor [234] is therefore maintained.
3.6.5. Effects of pressure in the S 2 I 2 Pd equatorial plane.
3.6.6. Effect of pressure on the intermolecular SÁ Á ÁPd interaction. The length of the intermolecular interaction between the Pd1 and an equatorial S atom S4 i in a neighbouring molecule related by the 2 1 screw axis parallel to a [symmetry code (i) x þ 1 2 ; Ày þ 1 2 ; 1 À z] decreases from 3.596 (3) Å at ambient pressure to 3.034 (10) Å at 94 kbar, a change of 0.562 (10) Å (Table 7, Fig. S18). The value of AE vdW for Pd and S is 3.43 Å (Bondi, 1964), and the shortening of the Pd1Á Á ÁS4 i distance falls below this limit at a pressure of ca 7.9 kbar, at which point the interaction is deemed to exist and that the chain formation has occurred, resulting in distorted octahedral coordination at Pd1.
Each iodide ligand can potentially interact with iodides in two neighbouring molecules to give a total of four adjacent, symmetry-related molecules (Fig. 16). The value of AE vdW for an IÁ Á ÁI interaction is 3.96 Å (Bondi, 1964), and at ambient pressure neither the shorter [4.2447 (12) Å ] nor the longer [5.013 (2) Å ] interaction lies within this limit. At ambient pressure the difference between the shorter and longer distances is 0.768 (2) Å ; as the pressure increases to 94.0 kbar very similar decreases in the two distances are observed, the shorter contracting by 0.613 (4) Å and the longer by 0.597 (3) Å , reductions of 14.4 and 11.8%, respectively (Fig.  S19). By ca 20 kbar the shorter distance has reached AE vdW , and an IÁ Á ÁI interaction between molecules related by the 2 1 screw axis parallel to b can be said to exist; this distance continues to contract, and at 94.0 kbar is 0.328 (4) Å shorter than AE vdW . In contrast, its initial greater length and somewhat smaller degree of contraction of the longer distance with increasing pressure means that it never approaches AE vdW , even at 94.0 kbar: at this point its length of 4.416 (3) Å lies 0.456 (3) Å outside the van der Waals limit. The combination of the chain-forming Pd1Á Á ÁS4(x þ 1 2 ; Ày þ 1 2 ; z À 1) and I1Á Á ÁI2(2 À x; y þ 1 2 ; Àz þ 3 2 ) interactions (Fig. 17) generates a two-dimensional sheet of molecules in the (110) plane. The dimensionality of [PdI 2 ([9]aneS 3 )] is, therefore, different from that of the four previous complexes.

Effect of pressure on the intramolecular SÁ Á ÁPt
interaction. The interaction between the axial sulfur S1 and the Pt1 shortens as the pressure increases, from 3.203 (13) Å at ambient pressure to 2.969 (13) Å at 69.7 kbar, a decrease of 0.234 (18) Å (see Fig. S20). In comparison, the equatorial Pt1-S4 and Pt1-S7 bonds show no statistically significant changes. The precision of this study is insufficient to determine whether there are any real changes in the values of the S1Á Á ÁPt1-S4 or S1Á Á ÁPt1-S7 angles.
3.7.4. Effects of pressure on the conformation of the macrocycle. Within the high standard uncertainties on the torsion angles, the [234] conformation of the macrocycle is retained across the range of pressures studied (see Fig. S21).
3.7.5. Effects of pressure in the S 2 I 2 Pt equatorial plane.
There are no significant changes to the Pt1-I distances or to the position of Pt1 relative to the coordination plane in the pressure range studied.
3.7.6. Effect of pressure on the intermolecular SÁ Á ÁPt interaction. The length of the intermolecular interaction between the Pt1 and an equatorial S atom S4 i [symmetry code: (i) x þ 1 2 ; Ày þ 1 2 ; 1 À z], which forms part of a neighbouring molecule related by the 2 1 screw axis parallel to a, decreases from 3.640 (17) Å at ambient pressure to 3.154 (16) Å at 69.7 kbar, a change of 0.49 (2) Å (Fig. S22). By ca 5 kbar the distance has fallen below AE vdW for Pt and S (3.52 Å ; Bondi, 1964), at which point Pt1Á Á ÁS4 i interactions form chains of molecules within the structure and complete distorted octahedral coordination at Pt1.
3.7.7. Effects of pressure on iodideÁ Á Áiodide catenation. At ambient pressure the difference between the shorter and longer IÁ Á ÁI distances as defined in Fig. 16 is 0.715 (8) Å . As the pressure is increased to 69.7 kbar, the distances decrease to similar degrees, the shorter by 0.585 (9) Å and the longer by 0.531 (8) Å , corresponding to reductions of 13.8 and 10.7%, respectively (Fig. S23). By ca 16 kbar the shorter distance has contracted sufficiently to bring it below AE vdW for two I atoms, and it continues to contract as pressure increases, at 69.7 kbar reaching 3.655 (7) and 0.305 (7) Å within AE vdW (3.96 Å ). The longer distance also contracts with increasing pressure, but even at 69.7 kbar its length lies well beyond AE vdW .    Ball-and-stick representation of the structure of [PtI 2 ([9]aneS 3 )] at ambient pressure showing the atom-numbering scheme used.

Overview of catenation in [MX 2 ([9]aneS 3 )] complexes
At ambient pressure all six complexes [MX 2 ([9]aneS 3 )], M = Pd, Pt; X = Cl, Br, I, are isomorphous and isostructural. However, with the application of pressure the complexes do not all remain isostructural with each other. Within the extended structure of the crystal the dihalides have the potential to exhibit halogenÁ Á Áhalogen interactions which generate sheets normal to the c axis. Iodine is well known to catenate, i.e. to form bonds or significant contacts with itself. In this section we explore the question of whether pressure can be used to instigate such catenation in the diiodide complexes and whether increased pressure can induce catenation in the dibromide and dichloride complexes, such that they form chains through BrÁ Á ÁBr or ClÁ Á ÁCl interactions, respectively.
The values of AE vdW for ClÁ Á ÁCl, BrÁ Á ÁBr and IÁ Á ÁI are 3.50, 3.70 and 3.96 Å , respectively (Bondi, 1964). At ambient pressure none of the halogenÁ Á Áhalogen distances lie within the corresponding AE vdW , but by ca 20 kbar catenation is established for the diiodide complexes (see above and Table  7). Catenation appears in the dibromide complexes only at much higher pressures of ca 58 kbar, an observation which suggests that the application of higher pressure can induce bromide to imitate the behaviour of iodide. As discussed below, no catenation is observed for the dichloride complexes in the pressure range studied.
The halogenÁ Á Áhalogen interactions are of Type I, which are characterized by approximately similar M-XÁ Á ÁX angles ( 1 and 2 ; see Table 8) rather than Type II where one of these angles is close to 180 and the other is near 90 (Desiraju & Parthasarathy, 1989). For the four dibromo and diiodo complexes the separation between 1 and 2 gradually reduces, from around 10 at low pressures to half that value or less at the highest pressures studied. The shortening XÁ Á ÁX contact distances and the converging of 1 and 2 are indicators of XÁ Á ÁX interactions strengthening with increasing pressure.
The behaviour of the XÁ Á ÁX interactions in [PdCl 2 ([9]aneS 3 )] is unique and anomalous within the group of six complexes. Although both distances contract as expected between ambient pressure and 42.5 kbar, across the phase change at 44 kbar the shorter distance contracts by 0.294 (9) Å [from 3.761 (4) to 3.467 (8) Å ], but the longer distance increases by 0.59 (2) Å [from 4.688 (9) to 5.28 (2) Å ] (Fig. 19). Although the contraction brings the shorter ClÁ Á ÁCl distance marginally below AE vdW (3.5 Å ), increasing the pressure to 76.8 kbar results in only minimal further contraction. This behaviour is related to trends in 1 and 2 which differ sharply from those seen for the dibromo and diiodo complexes: for [PdCl 2 ([9]aneS 3 )], with increasing pressure the values of these parameters diverge rather than converge. We conclude that the shortened contact is a consequence of the formation of the chains at 44 kbar and does not represent catenation of chlorides. Coupled with the fact that we were unable to study [PtCl 2 ([9]aneS 3 )] beyond a pressure of 54.0 kbar, we cannot confidently identify catenation in either [PdCl 2 ([9]aneS 3 )] or [PtCl 2 ([9]aneS 3 )] across the pressure ranges studied.
3.9. Density functional theory calculations 3.9.1. Introduction. In parallel with the crystallographic investigations, density functional theory (DFT) calculations were performed on each high-pressure crystal structure to estimate the effect of the structural changes on the total bonding, steric and orbital interaction energies, and metalligand bond orders for each [MX 2 ([9] Figure 19 Variation of the shorter and longer ClÁ Á ÁCl distances for [PdCl 2 ([9]aneS 3 ). The horizontal black line indicates AE vdW at 3.5 Å . Note the anomalous behaviour of both ClÁ Á ÁCl distances.
are reported for the central [MX 2 ([9]aneS 3 )] complex in each chain of three molecules.
We performed a bond-energy analysis using the energy decomposition scheme as implemented in ADF in which the internal energy (ÁE int ) is calculated relative to the energies of the free atoms from which each model is constructed. The internal energy (ÁE int ) has contributions from the Pauli repulsions between the occupied orbitals (ÁE Pauli ), attractive electrostatic interactions (ÁE El ) and orbital interactions (ÁE oi ) containing contributions from the donation of charge from the occupied orbitals of one fragment and the virtual orbitals of the other fragment and the mixing of occupied and virtual orbitals of the same fragment Together ÁE Pauli + ÁE El comprise the steric interaction energy (ÁE 0 ) of the fragments.  ([9]aneS 3 )] 3 shows a feature across the phase transition involving a decrease in ÁE int of 352 kJ mol À1 between 36 and 42.5 kbar and an increase in ÁE int by 433 kJ mol À1 between 42.5 and 46 kbar coincident with the phase transition. There is a continued increase in energy (by a further 130 kJ mol À1 ) up to 76.8 kbar.
3.9.2. Steric interaction energies (DE 0  An increase in ÁE 0 is consistent with the observed compressions of the unit cells for each complex, resulting in an increase in steric repulsion between each atomic fragment. For ([PdCl 2 ([9]aneS 3 )]) 3 ÁE 0 increases in energy between 19.0 and 23.5 kbar before decreasing in energy across the phase change. This suggests that the effects of the phase change, including the compression of the S1Á Á ÁPd1 and S4 i Á Á ÁPd1 distances, displacement of the Cl1 out of the coordination plane and the rearrangement of the macrocycle, could relieve the steric repulsion in the system.

3.9.3.
Orbital interaction energies (DE oi (Mayer, 1983(Mayer, , 1984 were calculated to compare the nature of the metal-ligand interactions about the Pd II or Pt II centre in the central molecule of each ([MX 2 ([9]aneS 3 )]) 3 (M = Pd, Pt; X = Cl, Br, I) unit.
3.9.6. Metal-halide bond orders. The trends in the metalhalide bond orders are shown in Figs. S26 and S27. General observations include variations in the M-X bond order of Br < Cl < I which do not mirror the variations in the M-X1 bond distances (M = Pd, Pt; X1 = Cl, Br, I; see supplementary molecular geometry information); bond distances in the Pd II complexes are shorter than in their Pt II counterparts and the halides follow the trend Cl < Br < I. There is little change in the Pd-X1 bond order of [PdCl 2 ([9]aneS 3 )] over its phase change, despite the movement of Cl1 out of the coordination plane; there is little change in the bond order for any M-X2 bond as pressure increases.  3.9.8. Bond order for the intermolecular sulfur-metal contact in ([MX 2 ([9]aneS 3 )]) 3 . At ambient pressure the bond orders for the intermolecular MÁ Á ÁS interaction (Fig. 22) are lower than those for the intramolecular MÁ Á ÁS axial interaction (Fig. 21) and considerably lower than the bond orders for the equatorial M-S bonds (ca 0.8). The bond order associated with the intermolecular MÁ Á ÁS interaction increases with pressure for all [MX 2 ([9]aneS 3 )] (M = Pd, Pt; X = Cl, Br, I), consistent with the shortening of the S4 i Á Á ÁM1 distance (see Fig. B2 of the supporting information). For [PdCl 2 ([9]aneS 3 )] there is a significant increase in the MÁ Á ÁS bond order across the phase change as the intermolecular MÁ Á ÁS interaction shortens from 3.204 (5) to 3.117 (8) ([9]aneS 3 )] attributed to the longer Pt1Á Á ÁS4 i distance found at ambient pressure (see Fig. B2 of the supporting information).
As the pressure is increased, the MÁ Á ÁS4 i bond order in [PdCl 2 ([9]aneS 3 )] increases, reaching a value of 0.093 at 42.5 kbar before increasing sharply to 0.101 through the phase change at 44 kbar, then continuing to increase after 46 kbar to reach a value of 0.116 at 76.8 kbar. Although similar increases in bond order with increasing pressure are observed for [MX 2 ([9]aneS 3 )] (M = Pd, X = Br, I and M = Pt, X = Cl, Br, I), none show any features related to a significant structural change.
[PtCl 2 ([9]aneS 3 )] appears to reach a limit in bond order for the interaction at ca 39 kbar.
3.9.9. Summary of DFT studies. Single-point gas-phase DFT calculations on models of [MX 2 ([9]aneS 3 )] 3 (M = Pd, Pt; X = Cl, Br, I) suggest that within these units ÁE int is dominated by steric considerations, in line with the decrease in unitcell parameters for each complex. Trends in axial MÁ Á ÁS, equatorial M-S and intermolecular MÁ Á ÁS bond orders with pressure for the central [MX 2 ([9]aneS 3 )] (M = Pd, Pt; X = Cl, Br, I) also mirror the variation of geometrical parameters with pressure for each complex. Calculations on ([MX 2 ([9]-aneS 3 )]) 3 corresponding to ambient pressure reveal a weak M1Á Á ÁS4 i interaction that develops as the pressure is increased. Only those calculations carried out for [PdCl 2 ([9]aneS 3 )] 3 predict significant changes in energy and bond orders: these correspond to the experimentally observed phase change in [PdCl 2 ([9]aneS 3 )] between 42.5 and 46 kbar. We are currently searching for other coordination complexes which show unusual behaviour under compression.

Conclusions
Although all exhibit contraction of both the intramolecular axial sulfur-metal interaction and the intermolecular equatorial sulfur-metal interaction to yield a chain polymer with distorted octahedral coordination at the metal centre, the six members of the family of M II complexes [MX 2 ([9]    the geometric changes within the S 2 X 2 M equatorial plane, if any, are relatively minor and follow no consistent trends. It is striking that only one of the six isomorphous compounds in this study displays an abrupt phase change upon the application of pressure. Each compound is set up for a change from four-coordination of the central metal to sixcoordination, but only [PdCl 2 ([9]aneS 3 )] undergoes a distinct structural change. The metal ions Pd II and Pt II are both well known for their strong preference for square-planar geometry, with six-coordinate complexes of the 4d ion Pd II being relatively rare: excluding complexes exhibiting metal-metal interactions, for Pt there are 11 580, 2698 and 1501 occurrences of four-, five-and six-coordinate complexes, respectively, while for Pd the corresponding numbers are 14 086, 1739 and only 135 (Allen, 2002; CSD version 5.34 November 2012 plus three updates). The halides become more polarizable as their size increases and in turn they become betterdonors. Thus, the ligand field stabilization of the six-coordinate species will be greater with chloride than with bromide or iodide. Complementary to this is the effect of the increasing covalency of the halides: as size increases there is more effective -donation to the metal which will reduce any partial positive charge at the metal centre (electroneutrality principle). These two effects favour the adoption of a six-coordinate geometry by [PdCl 2 ([9]aneS 3 )]. Furthermore, the formation of short IÁ Á ÁI and BrÁ Á ÁBr interactions in the diiodo and dibromo complexes will potentially have an impact on the subtle energetics of the system.