Crystal structures of seven gold(III) complexes of the form LAuX3 (L = substituted pyridine, X = Cl or Br)

The structures of seven complexes of general formula LAuX3 (L = methylpyridines or dimethylpyridines, X = Cl or Br) are presented. In the crystal packing, a frequent feature is the offset-stacked and approximately rectangular dimeric moiety (Au—X)2, linked by Au⋯X contacts.


Chemical context
In the previous part (Do ¨ring & Jones, 2024a) of our series of publications 'Gold complexes with amine ligands', we reported the structures of four gold(I) halide complexes involving methylpyridine (picoline) and dimethylpyridine (lutidine) ligands.That publication presents much introductory material that we do not repeat here.For convenience, we have interpreted the term 'amine' liberally to include azaaromatics.

Note added during revision:
A referee commented that 8 might be referred to as a co-crystal rather than an adduct.This is certainly a reasonable suggestion in view of the IUCr definition of a co-crystal (https://dictionary.iucr.org/Co-crystal):'Solid consisting of a crystalline single-phase material composed of two or more different molecular and/or ionic compounds, generally in a stoichiometric ratio, which are neither solvates nor simple salts.'The problem in our view is that a solid is not necessarily the same as a crystal.We would therefore prefer to say that we studied a co-crystal of the adduct 8.The IUCr dictionary is an extremely useful document, but it is often difficult to provide watertight definitions of any given concept.For example, Bombicz (2024) recently offered reasoned criticism of the IUCr definition of 'isostructural/isotypic', and we supported her views in our previous paper (Do ¨ring & Jones, 2024a).

Structural commentary
All the structures crystallize solvent-free; Z 0 values are 0.5 for 5 and 7, which display crystallographic twofold symmetry (with atoms N11, C14, Au1 and Cl1 on the twofold rotation axes 0.5, y, 0.75 and 0.5, y, 0.25, respectively), 2 for 4 and 1 for all other structures.Structures 1a and 2 are isotypic, but 5 and 6, which also differ only in the halogen, are not.Figs.1-9 show the molecules of these compounds in the crystal, with ellipsoids drawn at the 50% probability level.Selected bond lengths and angles are given in Tables 1-9.The molecules are numbered such that atoms X1 (and X4, where two independent molecules are present) are trans to the pyridinic nitrogen atoms.The numbering of X2/X3, cis to the pyridinic nitrogen, is chosen to make X2-Au1-N11-C12 the smallest absolute torsion angle (with appropriately altered numbering for structures with two residues).This does not apply to 5 and 7, for which the cis sites are symmetry-related.The ring numbering of 6 (C12 to C16), otherwise ambiguous, is assigned by the same criterion.
The pyridine rings are as expected planar, with r.m.s.deviations of the six ring atoms between 0.002 and 0.01 A ˚.The coordination geometry at the central gold(III) atoms is, also as expected, square-planar; the r.m.s.deviations from the plane of Au, N and the three X atoms range from zero for 5 and 7 (by symmetry) to 0.058 A ˚for 3, whereby the donor atoms alternate above and below the plane by ca 0.06 A ˚; a similar alternation is observed for the dimethylpyridine component of the adduct 8, whereas the same molecule alone (structure 6) has a much lower r.m.s.deviation of 0.012 A ˚.The angles between these two planes are 78.4 (1) � for 1a, 84.7 (2) � for 1b, 78.7 (2) � for 2, 57.2 (1) � for 3, 84.5 (1) � and 74.8 (1) � for the two molecules of 4, 51.0 (1) � for 5, 56.0 (1) � for 6, 83.4 (1) � for 7 and 58.2 (2) and 84. 3 (2) � for the two components of the adduct 8, corresponding to compounds 6 and 2. The largest angles are thus observed for those structures with a 2-methyl substituent of the pyridine ring, and presumably serve to reduce steric stress between these substituents and the X atoms cis to the nitrogen donor atom at Au.The gold atoms lie up to 0.15 (1) A ˚(for 1b) out of the pyridine plane, but lie exactly in this plane (by symmetry) for 5 and 7.
Bond lengths and angles may be regarded as normal.The Au-N bonds are consistently longer trans to Br [average (av.) of seven bonds: 2.059 A ˚] than trans to Cl (av. of four bonds: 2.036 A ˚), reflecting a greater trans influence of the bromido ligand compared to chlorido.There is no clear difference between Au-Cl bond lengths trans to N compared with those cis to N, whereas Au-Br bonds trans to N (av. of seven bonds: 2.395 A ˚) are significantly shorter than those cis to N (av. of fourteen bonds: 2.421 A ˚).The bond angles at Au are close to the ideal 90 � /180 � ; the angles with the largest deviations for the former are 88.25 (10) � for N1-Au1-Br2 of 3 and 91.17 (5) � for Cl3-Au1-Cl1 of 1b, and for the latter 176.590 (17) � for Br6-Au2-Br5 of 4. The C-N-C angles of the py ligands are all close to 120 � (av. of eleven angles: 120.8 � ).
A least-squares fit of the polymorphs 1a and 1b gave an r.m.s.deviation of 0.08 A ˚; a similar fit of the two independent molecules of 4 (one inverted) gave a deviation of 0.16 A ˚. Fits The molecular structure of compound 3 in the crystal.

Figure 3
The molecular structure of compound 2 in the crystal.

Figure 1
The molecular structure of compound 1 (polymorph 1a) in the crystal.

Figure 2
The molecular structure of compound 1 (polymorph 1b) in the crystal.

Figure 6
The molecular structure of compound 5 in the crystal.Only the asymmetric unit is numbered.

Figure 5
The molecular structure of compound 4 (with two independent molecules) in the crystal.

Figure 7
The molecular structure of compound 6 in the crystal.

Figure 8
The molecular structure of compound 7 in the crystal.Only the asymmetric unit is numbered.The molecular structures of compound 8 (an adduct of 2 and 6) in the crystal.
angle, leads to significant differences in the positions of the bromine atoms (0.39, 0.50, 0.51 A ˚, respectively for Br1-3; Fig. 11).A similar effect, although the interplanar angles are almost equal, is seen for the fit of 1b with its counterpart in the adduct 8 (deviations 0.50, 0.43, 0.40 A ˚; Fig. 12), whereas the largest difference for the fit of 6 with its counterpart in 8 is for Br1 (0.22 A ˚; Fig. 13).

Supramolecular features
Hydrogen bonds of the type C-H� � �X for all structures are given in Tables 10-18.These include several borderline cases that are not discussed explicitly.For all packing diagrams, the labelling indicates the asymmetric unit, and hydrogen atoms not involved in secondary contacts are omitted for clarity.The choice of 'important' interactions and their hierarchy is necessarily subjective, at least to some extent; diagrams with a  Cl2-Au1-N11-C12 i À 92.76 (10) Cl2-Au1-N11-C12 87.24 (10)

Figure 10
A least-squares fit of the pyridinic ligands of 1a and 1b (excluding H atoms). 1a is the dotted molecule.

Figure 12
A least-squares fit of the pyridinic ligands of 1b and its counterpart in the adduct 8 (excluding H atoms). 1b is the dotted molecule.

Figure 11
A least-squares fit of the pyridinic ligands of both molecules of 4 (excluding H atoms). Molecule 1 (centred on Au1) is dotted.

Figure 13
A least-squares fit of the pyridinic ligands of 6 and its counterpart in the adduct 8 (excluding H atoms). 6 is the dotted molecule.
small number of heavy-atom contacts are easier to interpret than those involving a larger number of hydrogen bonds, and this is especially true for H� � �Br contacts, which are probably weaker than H� � �Cl.Primes ( 0 , 00 ) indicate symmetry-equivalent atoms; operators are not given in full each time.A summary of the packing features is given in Table 19.Before discussing the packing of 1-8 in detail, it is useful to look back on the packing of (py)AuCl 3 (Adams & Stra ¨hle, 1982; space group C2/c, Z = 8), to see what types of secondary interaction can arise.Short non-bonded contacts were observed between the gold atom and two chlorine atoms, positioned axially to the main coordination plane in such a way as to complete a highly stretched octahedron at the gold atom (Au� � �Cl 3.636 and 3.648 A ˚, Cl� � �Au� � �Cl 173.0 � ; operators 1 2 À x, À 1 2 + y, 1 À z and 1 2 À x, 1 2 + y, 1 À z ).This leads to ladder-like double chains of residues (Fig. 14), parallel to the b axis, in which the molecules display offset stacking of the AuX 3 groups; one Au-Cl bond of each molecule (the rungs of the ladder) shares two Au� � �Cl contacts with antiparallel Cl-Au bonds of each neighbouring molecule (the side rails of the ladder).The (Au-X) 2 quadrilaterals, approximately rectangular and with side lengths corresponding to the Au-Cl bond length and the Au� � �Cl contact distance, are a recurring feature in the structures discussed here.Offset stacking of this type is a common feature in AuX 3 complexes, and we have observed it e.g. in four modifications of (tetrahydrothiophene)AuCl 3 (Upmann et al., 2017).In general, any suitable donor atoms can occupy these two contact sites.It might be argued that such contacts are merely connected with the steric ease of approach to the two sides of research communications Acta Cryst. (2024).E80, 894-909
the coordination plane; this has also been argued for short contacts to the linearly coordinated gold atom of gold(I) complexes, although H� � �Au hydrogen bonding in such systems is reasonably well established (Schmidbaur, 2019;Schmidbaur et al., 2014).However, recent studies and calculations (Daolio et al., 2021;Pizzi et al., 2022) have indicated that there is a �-hole at the gold atom, and that there is thus a definite attractive interaction, a 'coinage bond', between the gold atom and the additional donor(s) (see below).
At the time of publication of the (py)AuCl 3 structure, more than 40 years ago (the data were probably recorded in the late 1970s), the main interest in crystal structure determinations generally centred on the molecule being studied, whereas intermolecular contacts were often neglected.The analysis of the Au� � �Cl contacts in (py)AuCl 3 constituted a welcome exception.However, the structure contains other secondary contacts that were not mentioned, probably because at the time such contacts were not regarded as significant.First, there is a short Cl� � �Cl contact of 3.462 A ˚connecting the 'ladders'.Such formally non-bonding contacts between halogen atoms have been the subject of considerable interest for some time and are usually termed 'halogen bonds'.For C-X� � �X-C systems, they are considered to involve a small region of positive charge in the extension of the C-X bond vectors beyond the atom X, often leading to one C-X� � �X angle of ca 90 � and one of ca 180 � (see e.g.Metrangolo et al., 2008, or Cavallo et al., 2016, for   The packing of (py)AuCl 3 (Adams & Stra ¨hle, 1982), showing two adjacent ladder-like double chains parallel to the b axis at (x, z) = (0.25, 0) and (0.75, 0.5).The view direction is approximately parallel to the c axis (but rotated slightly to reduce overlap).Thick dashed lines indicate Au� � �Cl contacts; thin dashed lines indicate 'weak' H� � �Cl hydrogen bonds or short Cl� � �Cl contacts.Atomic coordinates were taken from the database (refcode PYAUCL10); hydrogen-atom positions were calculated using the HADD option of XP (Bruker, 1998).Colour codes for this  the type C-H� � �Cl that are now regarded as hydrogen bonds and are, somewhat misleadingly, often termed 'weak' hydrogen bonds.These were not mentioned in the 1982 publication, and indeed no hydrogen atoms were included in the refinement, which was not unusual at the time for heavyatom structures.We used the program XP (Bruker, 1998) to calculate the hydrogen-atom positions, and established that there are three short H� � �Cl contacts, one as short as 2.79 A ˚; this connects neighbouring molecules in the ladders.Two further such contacts (2.85 and 2.86 A ˚) connect the ladders; these are omitted from Fig. 14 for clarity.

Database survey
The searches employed the routine ConQuest (Bruno et al., 2002), part of Version 2024.1.0 of the CSD (Groom et al., 2016).A search for 'simple' compounds of the form LAuCl 3 (L = pyridine ligand with no substituents involved in further rings, X = halogen) gave 21 hits.The Au-N bond lengths were 2.015-2.073,av.2.043 ( 13 (WEFRAE, Pizzi et al., 2022).All showed Au-Br trans bonds significantly shorter than Au-Br cis , by ca 0.02-0.03A ˚, but the Au-N bond lengths were variable at 2.040-2.098A ˚.The sample is probably too small to draw reliable conclusions.

Figure 28
The links between the layers (seen edge-on) of adduct 8 are provided by the contacts Br1� � �Br1 0 and Br4� � �Br4 0 , drawn with thick dashed lines.The former are almost exactly vertical in this diagram; the latter are slightly angled to the vertical direction.The view direction is parallel to the c axis.Pizzi et al., 2022).In all cases, the authors drew attention to the short Au� � �X contacts.These compounds are included in Table 19.C-H� � �X hydrogen bonding is neglected.
ESITIM crystallizes in Pcab with Z = 8.In the original publication, the Au� � �Cl contacts (3.244, 3.409 A ˚) were described as linking the molecules to form infinite chains.In fact, they combine to form a layer structure, involving Au 4 Cl 4 rings, parallel to the ab plane at z ' 0.25, 0.75 (Fig. 29).In the series of 3-halopyridine complexes, the halogen substituents of the pyridine rings are 'non-innocent' atoms as regards to intermolecular interactions.In WEFQAD (P1, Z = 2), the Au� � �Cl contacts (3.492, 3.

Trichlorido(2-methylpyridine)gold(III)
(1): 114.2 mg (0.351 mmol) of the gold(I) precursor chlorido(2-methylpyridine)gold(I) was prepared by the method of Ahrens (1999).This was dissolved in 5 ml of dichloromethane, and the solution was added to a solution of 100 mg (0.363 mmol) of PhICl 2 in 5 ml of dichloromethane.Equal (0.4 ml) portions of the solution were transferred to five ignition tubes and overlayered with the five precipitants n-pentane, n-heptane, diethyl ether, diisopropyl ether and petroleum ether (b.p. 313-333 K).The tubes were stoppered and transferred to the refrigerator overnight.Crystals of compound 1, polymorph a, were obtained as yellow prisms and tablets from the tube with diisopropyl ether.In general for these syntheses, crystals also formed in at least some of the other tubes, but the best, judged by inspection under a microscope, were selected for X-ray measurements.Elemental analysis [%]: calc.: C 18.18, H 1.78, N 3.53; found C 17.78, H 1.79, N 3.59.Because of the problem of incomplete oxidation that we have sometimes encountered using PhICl 2 , the procedure was repeated in parallel using two equivalents of PhICl 2 , although this precaution later proved to have been unnecessary for the reactions presented here.The same crystallization experiments were carried out.Crystals of compound 1, polymorph b, were obtained as yellow plates from the tube with n-pentane.Elemental analysis [%]: calc.: C 18.18, H 1.78, N 3.53; found: C 17.63, H 1.78, N 3.58.
Tribromido(2,4-dimethylpyridine)gold(III) (4): 45,2 mg (0.124 mmol) of (tht)AuBr were dissolved in 2 ml of 2,4-dimethylpyridine.The solution was transferred to a 5 ml glass vial and overlayered with diisopropyl ether.The vial was closed and stored in the refrigerator.The supernatant was then pipetted off and the remaining colourless crystals, assumed to be bis(2,4-dimethylpyridine)gold(I) dibromidoaurate(I), dried in vacuo, yielded 32.5 mg (48%).The crystals proved to be unsuitable for structure determination because of streaking of the diffraction peaks.They were dissolved in 2 ml of dichloromethane and 3 drops of elemental bromine  Trichlorido (3,5-dimethylpyridine)gold(III) (5): 166 mg (0.518 mmol) of (tht)AuCl were converted to bis(3,5-dimethylpyridine)gold(I) dichloridoaurate(I) (Do ¨ring & Jones, 2024a).The sample was divided in half; each half was dissolved in 5 ml of dichloromethane, and then treated with one or two equivalents of PhICl 2 .The solutions were subjected to the usual crystallization experiments.Crystals of 5 were obtained in the form of yellow blocks from all tubes; those chosen were from the 1:2 experiment using diethyl ether.Elemental analysis [%]: calc.: C 20.48, H 2.21, N 3.41; found: C 20.23, H 2.121, N 3.58.
Tribromido(2-methylpyridine)gold(III)/tribromido (3,5-dimethylpyridine)gold(III) (1/1) (8): Crystals of compounds 8 and 6 arose serendipitously, partly as a result of human error, as follows.137.3 mg (0.376 mmol) of (tht)AuBr were converted to 84.0 mg (0.114 mmol) of bis(2-methylpyridine)gold(I) dibromoaurate(I) as above, of which 75.1 mg (0.102 mmol) were dissolved in 5 ml of dichloromethane.Five drops of elemental bromine were added.Half of the resulting red solution was overlayered with n-pentane.At some stage, which can no longer be identified (but the 2-picoline was checked by NMR and was pure), the system became contaminated with 3,5-dimethylpyridine.One of the red crystals that formed was investigated and proved to be the 1/1 adduct 8.The 1 H NMR spectrum of the sample showed the expected two methyl singlets, but in the ratio 4:1 rather than the expected 2:1 for a 1/1 mixture of 2 and 6; this would suggest that the sample of red crystals from which 8 was taken consisted of both 6 and 8. Consistent with this, the solution of the red crystals in CDCl 3 , left to stand for some time, deposited a few red crystals that proved on X-ray examination to be compound 6.

Refinement
Details of the measurements and refinements are given in Table 20.For all structures, multi-scan absorption corrections were applied using spherical harmonics, as implemented in the SCALE3 ABSPACK scaling algorithm (Rigaku OD, 2020).For compound 6, analytical numeric absorption corrections using a face-indexed crystal model, based on expressions derived by Clark & Reid (1995), were applied first.
Aromatic hydrogen atoms were included at calculated positions and refined using a riding model with C-H = 0.95 A ˚. Methyl groups were included as idealized rigid groups with C-H = 0.98 A ˚and H-C-H = 109.5� , and were allowed to rotate but not tip (command 'AFIX 137').U values of the hydrogen atoms were fixed at 1.5 � U eq of the parent carbon atoms for methyl groups and 1.2 � U eq of the parent carbon atoms for other hydrogens.A small number of badly fitting reflections were omitted (1a, two reflections with deviations > 8�; 1b, seven reflections > 7�; 2, one reflection > 15�; 5, one reflection > 6�; 8, one reflection > 29�).
Four of the crystals (1a, 1b, 2 and 8) were non-merohedral twins, with twinning by 180 � rotation about the a axis for 1a, 1b and 2 and about [111] for 8.These structures were refined using the 'HKLF 5' method (Sheldrick, 2015).The relative volumes of the smaller twinning components refined to 0.4710 (6), 0.4583 (6), 0.4641 (5) and 0.4440 (5), respectively.The twin data reduction merges equivalent reflections before writing the intensity file, so that R int is meaningless (and is not given in Table 20).The intensity datasets comprise all nonoverlapped reflections from both components and all overlapped reflections, so that the number of reflections should be interpreted with caution.More stringent checks during the data reduction of twins (e.g. the command 'remove outliers') mean that the completeness of some datasets is less than ideal, typically around 95%.
Special features and exceptions: For 1b, the large difference peak of 4.5 e A ˚À 3 has coordinates that are arithmetically related to those of the gold atom and thus may represent residual twinning errors.For 3, the x and y coordinates of the gold atom are ca 0.25, which leads to systematically weak Computer programs: CrysAlis PRO (Rigaku OD, 2020), SHELXS97 (Sheldrick, 2008), SHELXL2019/3 (Sheldrick, 2015) and XP (Bruker, 1998).
reflection classes; checkCIF comments on (pseudo-) Bcentring.The second weighting parameter b (Sheldrick, 2015) does not converge, but oscillates over a small range.For 6, the methyl hydrogen atoms at C18 were unclear, and were therefore refined as an ideal hexagon of half-occupied sites (command 'AFIX 127').However, the disorder may be more extensive than this simple model.

Figure 4
Figure 4 of molecules 2 and 6 (the latter inverted) to the same molecules of the adduct 8 gave r.m.s.deviations of 0.091 and 0.061 A ˚, respectively.More informative figures are however obtained by fitting only the pyridine ligands, which are closely similar; the differences associated with the AuX 3 moieties are then shown more clearly.For 1a/1b, the atoms Cl2 and Cl3 differ in position by 0.26 and 0.20 A ˚respectively (Fig. 10).For 4, the gold atoms lie on opposite sides of the pyridine plane, and this, coupled with the 10 � difference in the interplanar research communications Acta Cryst.(2024).E80, 894-909 Do ¨ring and Jones � Gold complexes with amine ligands 897 -N11-C12 51.15 (11) Cl2-Au1-N11-C12 i À 128.85 (11) Symmetry code:

Figure 14
Figure 14 Figure and for Figs.29-34 are the same as for those of 1-8 (C and H black, N dark blue, Au yellow, Cl green, Br brick-red), but we do not number the atoms in these Figures because the database numbering is not consistent e.g. for cis and trans halogen atoms.

Figure 19 A
Figure 19 A double chain of molecules 1 for compound 4, with view direction parallel to the b axis.Dashed lines indicate Au� � �Br and Br� � �Br contacts.

Figure 20 A
Figure 20 A double chain of molecules 2 for compound 4, with view direction parallel to the b axis.Dashed lines indicate Au� � �Br and Br� � �Br contacts.

Figure 18
Figure 18 Packing diagram of compound 3, projected parallel to the b axis, showing the linking of the layers of Fig. 17 by the Br1� � �Br1 contacts (thick dashed lines, vertical).

Figure 21
Figure 21Projection of the structure of compound 4 parallel to the a axis.Molecules 2 are indicated by the thicker bonds of the rings.Thick dashed lines indicate Br� � �Br contacts between the double chains of molecule 1 and 2 (in the regions y ' 0.5, z ' 0 and y ' 0.5, z ' 0.5 respectively, and in regions related to these by symmetry).Thin dashed lines are contacts within the chains, as seen in the previous two figures.

Figure 22
Figure 22The layer structure of compound 5 viewed parallel to the b axis.Dashed lines indicate Au� � �Cl and Cl� � �Cl interactions.The Au coordination planes are seen edge-on, so that Au1 obscures Cl1 or vice versa.

Figure 23
Figure 23Projection of the structure of compound 5 viewed perpendicular to the ab plane.The dashed lines connecting the layers (see Fig.22) are the threecentre hydrogen bonds H14� � �Cl2 (with two symmetry-equivalent Cl2 atoms).

Figure 24 A
Figure 24 A double chain of compound 6, viewed parallel to the c axis.Dashed bonds indicate Au� � �Br or Br� � �Br interactions.

Figure 25 A
Figure 25 A double layer of compound 6 parallel to the plane (021).The double chains of Fig. 24 are linked by Br1� � �Br1 contacts [approximately vertical in this view, which is rotated by ca 20 � around the horizontal axis from the direction perpendicular to (021)].
Figure 31 The packing of WEFQEH, trichlorido(3-fluoropyridine)gold(III), viewed perpendicular to the ab plane in the region z ' 0.25.Dashed lines indicate Au� � �Cl contacts.Fluorine atoms are the smaller green circles.This is a redrawn version of Fig. 2 of Pizzi et al. (2022).

Table 19
Summary of packing features.

Table 20
Experimental details.