Crystal structures of four gold(I) complexes [AuL2]+[AuX2]− and a by-product (L·LH+)[AuBr2]− (L = substituted pyridine, X = Cl or Br)

In the four main structures, the anions and cations are connected by aurophilic contacts, hydrogen bonds C—H⋯halogen and (in two cases) C—H⋯Au contacts. In the by-product without pyridinic N coordination a N—H⋯N bond dominates the packing pattern supported by Br⋯Br, H⋯Br, and Au⋯Br contacts.

Bis(2-methylpyridine)gold(I) dibromidoaurate(I), [Au(C 6 H 7 N) 2 ][AuBr 2 ], (1), crystallizes in space group C2/c with Z = 4.Both gold atoms lie on twofold axes and are connected by an aurophilic contact.A second aurophilic contact leads to infinite chains of alternating cations and anions parallel to the b axis, and the residues are further connected by a short H� � �Au contact and a borderline Br� � �Br contact.

Chemical context
The first X-ray structural results on pyridine complexes of gold(I) were reported by the group of Stra ¨hle (Adams et al., 1982), one of the pioneers of structural gold chemistry, who established that the compounds with stoichiometry (py)AuX (py = pyridine, X = Cl and I) were in fact ionic, [Au(py) 2 ] + [AuX 2 ] À .In both compounds, the ions were linked by short Au� � �Au contacts to form tetranuclear chains anion� � �cation� � �cation� � �anion, with a linear sequence Au� � �Au� � �Au� � �Au for X = I but a zigzag for X = Cl.For X = I, the Au� � �Au distances were shorter (peripheral 2.990, central 3.291 A ˚) than for X = Cl (3.249, 3.416 A ˚).Such contacts have now proved to be quite common for Au I centres and have been intensively studied, in particular by Schmidbaur, who termed them 'aurophilic contacts' (see e.g.Schmidbaur & Schier, 2008, 2012).We recently redetermined the structure of the iodine derivative, using the improved methods now available, as a student project and obtained Au� � �Au distances of 2.9784 (3) and 3.2575 (5) A ˚ (Do ¨ring et al., 2018).
We have now returned to complexes involving pyridine ligands.In this publication we describe the structures of four gold(I) halide derivatives of empirical formula LAuX, all of which proved to be ionic compounds of the form [AuL 2 ] + [AuX 2 ] À (L = substituted pyridine, X = Cl or Br), together with one by-product.The next publication (in preparation) will describe complexes of the form LAuX 3 for the same ligand type.
The reader should note that the trivial names picoline (= methylpyridine) and lutidine (= dimethylpyridine) have often been used (also by us) in the literature.

Structural commentary
We note at the outset that, for compounds consisting of more than one residue, it is to some extent arbitrary which aspects belong to the Structural commentary and which to the Supramolecular features (next section).In this section we describe only structural aspects within the asymmetric unit, extended where necessary to generate complete ions.
The structure of bis(2-methylpyridine)gold(I) dibromidoaurate(I) (1), which crystallizes in space group C2/c with Z = 4, is shown in Fig. 1, with selected dimensions in Table 1.The corresponding chlorido derivative (Jones & Ahrens, 1998) is molecular rather than ionic; it is not clear which factors determine the ionic or molecular nature of compounds with stoichiometry LAuX, and we did not attempt to find alternative forms of the compounds described here (e.g. by carrying out extensive recrystallization experiments).Both gold atoms lie on the twofold axis (0, y, 0.75) and are connected by an aurophilic contact of 3.1904 (4) A ˚.The coordination axes N11-Au1-N11 0 and Br1-Au2-Br1 0 are

Figure 1
The structure of compound 1 in the crystal, showing the asymmetric unit (labelled) extended by symmetry.The dashed line represents an aurophilic attraction.Ellipsoids correspond to 50% probability levels.
approximately perpendicular to each other across the Au� � �Au contact (see torsion angles in Table 1).The interplanar angle of the two rings is 4.31 (2) � , with the methyl groups on opposite sides of the rings.
The structure of bis(3-methylpyridine)gold(I) dibromidoaurate(I) (2), which crystallizes in space group C2/m with Z = 2, is shown in Fig. 2, with selected dimensions in Table 2.It is not isotypic to the chlorido derivative (Jones & Ahrens, 1998; see next section).Both gold atoms of 2 lie on special positions with symmetry 2/m, and all other atoms except for one methyl hydrogen (see Refinement) in mirror planes at y = 0 or 0.5.The gold atoms are connected by an aurophilic contact of 3.22048 (6) A ˚. Again, the coordination axes at the gold atoms are approximately perpendicular to each other (see torsion angles in Table 2).The crystallographic symmetry means that the coordination at both gold atoms is exactly linear, the rings are exactly coplanar, and the coordination axes are exactly perpendicular to the Au� � �Au contacts while roughly perpendicular to each other.
The structure of bis(3,5-dimethylpyridine)gold(I) dichloridoaurate(I) (3), which crystallizes in space group P1 with Z = 2, is shown in Fig. 3, with selected dimensions in Table 3.The cation lies on a general position, and there are two independent anions in which the gold atoms lie on inversion centres.There are no aurophilic contacts within the asymmetric unit.The interplanar angle between the rings is 8.61 (9) � , which corresponds to an out-of-plane bend about the gold atom rather than a mutual rotation around the N-Au-N coordination axis.Bis(3,5-dimethylpyridine)gold(I) dibromidoaurate(I) (4; Fig. 4, Table 4) is isotypic to 3; its interplanar angle is 7.8 (1) � .
We did not succeed in making any further compounds (cf.Freytag & Jones, 2000) in which a bromopyridine was coor-

Figure 2
The structure of compound 2 in the crystal, showing the asymmetric unit (labelled) extended by symmetry.The dashed line represents an aurophilic attraction.Ellipsoids correspond to 50% probability levels.

Figure 3
The structure of compound 3 in the crystal, showing the asymmetric unit (labelled) extended by symmetry.Ellipsoids correspond to 50% probability levels.Dashed lines indicate short H� � �Cl contacts.

Figure 4
The structure of compound 4 in the crystal, showing the asymmetric unit (labelled) extended by symmetry.Ellipsoids correspond to 50% probability levels.Dashed lines indicate short H� � �Br contacts.
dinated to gold.Attempts to make bis(2-bromopyridine)gold(I) dibromidoaurate(I) (or the corresponding neutral molecule) led instead to 2-bromopyridine 2-bromopyridinium dibromidoaurate(I) (5; Fig. 5), possibly because of small amounts of adventitious water.Compound 5 crystallizes in space group P1 with Z = 2; all atoms lie on general positions.The 2-bromopyridinium cation is linked to the 2-bromopyridine molecule by an N-H� � �N hydrogen bond.The NH hydrogen atom was refined freely, and there are no signs of disorder of this atom.The ring angle at the nitrogen atom is 5 � larger for the cation than for the neutral molecule (Table 5), and the interplanar angle between the rings is 1.9 (2) � .The bond lengths and angles in compounds 1-5 may be considered normal.The [L 2 Au] + cations and the [AuX 2 ] À anions are linear at the gold atom, with maximum deviations of ca 3.5 � for the cations of 3 and 4. The six independent Au-Br bond lengths range from 2.3775 (5) to 2.3951 (4) A ˚.The Au-N bond lengths in 1-4 are almost constant at 2.012 (3)-2.027(3) A ˚, as are the C-N-C angles at 118.9 (3)-119.7 (3) � , appreciably wider than in free pyridine (116.4-116.8� in four independent molecules; Mootz & Wussow, 1981).
The related structure of 3-bromopyridine 3-bromopyridinium dibromidoaurate(I) (6) was determined; it crystallizes in space group C2/c with Z = 4, with the gold atom on an inversion centre at (0.25, 0.25, 0.5).However, the NH hydrogen atom is disordered over a twofold axis connecting both bromopyridine residues (and was refined freely as a 'half' hydrogen atom).The U values of the bromopyridine site were somewhat high, which probably indicates that this residue is also disordered, over two closely adjacent positions corresponding to a superposition of the cation and the neutral molecule.For this reason, we do not discuss this structure here, but have deposited it (with all faults) for the interested reader (Do ¨ring & Jones, 2024c).

Supramolecular features
Hydrogen bonds, mostly of the type C-H� � �X, for all structures are given in Tables 6-10.These include several borderline cases that are not discussed explicitly.
Compound 1: A second aurophilic contact, Au1� � �Au2(x, À 1 + y, z) = 3.1937 (4) A ˚, connects the gold atoms to form      Compound 2: The packing is closely related to that of 1.The aurophilic contacts are now equivalent and again connect the gold atoms to form infinite chains parallel to the b axis (Fig. 8).Adjacent chains are again linked by the short contact H14� � �Au2 (2.66A ˚) to complete the layer structure parallel to the ab plane.The packing in layers parallel to the ac plane is also repeated, but the c axis is halved, so that adjacent cations (vertically displaced in Fig. 9) are translationally equivalent.The Br1� � �Br1 0 contact is 3.8489 (7) A ˚via the operator À x, 1 À y, 1 À z.

Table 9
Hydrogen-bond geometry (A ˚, � ) for 4.   The close similarity between Figs. 7 and 9 is evident.The structures of compounds 1 and 2 are effectively the same (except for the position of the methyl substituent and the small shifts associated with this), except that 1 has the higher formal symmetry.The usage of the term 'isostructural' in the crystallographic literature has often been inconsistent, but previously one might have defined the two structures as (nearly) isostructural (closely similar connectivity including the secondary contacts) but not isotypic (because of the different cells and space group).The IUCr (2019) has however defined the terms 'isostructural' and 'isotypic' as synonymous: 'Two crystals are said to be isostructural if they have the same structure, but not necessarily the same cell dimensions nor the same chemical composition, and with a 'comparable' variability in the atomic coordinates to that of the cell dimensions and chemical composition . . .One also speaks of isostructural series, or of isostructural polymorphs or isostructural phase transitions.The term isotypic is synonymous with isostructural' (their italics).Bombicz (2024) has recently commented: ' . . . the definition of isostructurality is not explicit about several issues.Are the corresponding structures required to have the same stoichiometry, Z 0 , symmetry elements and the same space group?', and we have pointed out the presence of some significant differences in formally isotypic structures (Upmann et al., 2024).We too would suggest that the definitions need further amendment and/or clarification.
The packing of bis(3-methylpyridine)gold(I) dichloridoaurate(I) (Jones & Ahrens, 1998) is not closely related to those of compounds 1 and 2, although it too crystallizes in a Ccentred monoclinic space group (C2/c).The chains of alternating cations and anions parallel to the c axis were described in the original publication.However, at the time 'weak' hydrogen bonds were not generally discussed, so we rectify that omission here.Fig. 10 shows the formation of a layer structure parallel to (101), whereby two 'weak' H� � �Cl hydrogen bonds (2.71, 2.81 A ˚) connect the ions.In contrast to 1 and 2, there are no very short and linear C-H� � �Au contacts.

Database survey
The searches employed the routine ConQuest (Bruno et al., 2002), part of Version 2024.1.0 of the Cambridge Structural Database (Groom et al., 2016).
A search for all gold(I) complexes involving pyridines (any substitution, but no fused rings) led to 116 hits.The average angle at nitrogen was 118.4 (17) � for 187 values and the average Au-N bond length was 2.058 (30) A ˚, but the latter values showed a considerable spread (2.003-2.137A ˚); as would be expected from the known trans influences, the shortest Au-N bonds were observed trans to halogen or nitrogen donors and the longest trans to phosphorus donors.Two further 'simple' derivatives involving only alkylpyridine and halogenido ligands were found: bis(2,6-dimethylpyridine)gold(I) dichloridoaurate(I), which displays the known structure type with alternating cations and anions connected by Au� � �Au contacts (3.334 and 3.328 A ˚; refcode BUVTUI, Hashmi et al., 2010) and chlorido(4-ethylpyridine)gold(I), a molecular structure without aurophilic contacts (ESITAE; Hobbollahi et al., 2019).

Synthesis and crystallization
Bis(2-methylpyridine)gold(I) dibromidodaurate(I) (1): 55 mg (0.104 mmol) of (tht)AuBr 3 (tht = tetrahydrothiophene) were dissolved in 2 mL of 2-methylpyridine.The clear, deep red solution was divided amongst five ignition tubes, overlayered with the five precipitants n-pentane, n-heptane, diethyl ether, diisopropyl ether and petroleum ether (b.p. 313-333 K) and transferred to a refrigerator (276 K).A red oil formed, in which some colourless blocks of compound 1 were observed and removed for investigation.The measured crystal was taken from the tube with n-pentane as precipitant.Elemental analysis [%]: calculated C 19.48, H 1.91, N 3.79; found C 18.89, H 1.89, N 3.91.This synthesis was intended to lead to tribromido(2-methylpyridine)gold(III), which we later obtained in crystalline form using a different method (to be published), and which probably corresponds to the red oil.We can see no obvious reason for the observed reduction to gold(I); the 2-methylpyridine had been recently redistilled.

Refinement
Details of the measurements and refinements are given in Table 11.Structures were refined anisotropically on F 2 .For compound 5, the NH hydrogen atom was refined freely.Aromatic hydrogens were included at calculated positions and refined using a riding model with C-H = 0.95 A ˚. Methyl groups were included as idealised rigid groups with C-H = Several dimeric units of compound 5, connected into chains parallel to the a axis by the Br2� � �Br4 contact.This view is a projection parallel to the c axis.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.For compounds 1, 2 and 3, three, one and one badly fitting reflection(s), respectively, were omitted.
Special aspects for compound 2: The structure was refined in a non-reduced setting of C2/m to facilitate comparison with structure 1 (see Supramolecular features), The reorientation matrix À 1 0 À 2 / 0 À 1 0 / 0 0 1 converts the cell to a C-centred cell with a = 16.460,b = 6.441, c = 8.192A ˚and a lower � angle of 118.72 � , whereas the matrix 0 0 À 1 / 0 À 1 0 / À 1 0 À 1 leads to an I-centred cell with a = 8.192, b = 6.441, c = 14.437A ˚and � = 91.12� .The carbon atom of the methyl group (C17) lies in a mirror plane; its hydrogen atoms (one in the mirror plane and one on a general position) were refined freely, but with C-H distances restrained to be approximately equal (command SADI).

Figure 5
Figure 5The structure of compound 5 in the crystal.Ellipsoids correspond to 50% probability levels.The dashed lines indicate a hydrogen bond (thick) and short Au� � �Br and Br� � �Br contacts (thin).

Figure 6
Figure 6 Packing diagram of compound 1 viewed perpendicular to the ab plane in the region z ' 0.75.Dashed lines indicate Au� � �Au contacts (thick) or H� � �Au contacts (thin).Atom labels indicate the asymmetric unit.In all packing diagrams, the hydrogen atoms not involved in significant contacts are omitted.

Figure 7
Figure 7 Packing diagram of compound 1 viewed perpendicular to the bc plane in the region y ' 0.5.Dashed lines indicate Br� � �Br or H� � �Au contacts.

Figure 8
Figure 8 Packing diagram of compound 2 viewed perpendicular to the ab plane in the region z ' 0. Dashed lines indicate Au� � �Au contacts (thick) or H� � �Au contacts (thin).Atom labels indicate the asymmetric unit.Hydrogen atoms not involved in H� � �Au contacts are omitted.

Figure 9
Figure 9 Packing diagram of compound 2 viewed perpendicular to the bc plane in the region y ' 0. Dashed lines indicate Br� � �Br or H� � �Au contacts.

Figure 12
Figure 12 Packing diagram of compound 3 viewed parallel to the b axis.Hydrogen atoms are omitted.Dashed lines indicate Au� � �Au contacts; the Au3 anions left and right have been omitted to show these contacts more clearly.

Table 11
Experimental details.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters(Å 2 )