Crystal structures of five gold(I) complexes with methylpiperidine ligands

Five structures with methylpiperidine ligands and gold(I) centres all exhibit linear geometry at the Au atom, and equatorial and axial positions, respectively, for the methyl groups and Au atoms at the piperidine rings. The packing involves hydrogen bonding and aurophilic interactions.


Structural commentary
At the outset we comment, as usual: for structures that contain more than one residue in the asymmetric unit, the distinction between the categories Structural commentary (which generally refers to the asymmetric unit) and Supramolecular features becomes blurred, especially when atoms occupy special positions (as for compound 5 here).
Selected molecular dimensions are presented in Tables 1-5, with hydrogen bonds in Tables 6-10.
Fig. 1 shows the asymmetric unit of compound 1, which consists of one formula unit [Au(4-Me-pip) 2 ]Cl and contains one classical hydrogen bond N21-H02� � �Cl1 (Table 6).All atoms lie on general positions.Selected geometric parameters are presented in Table 1.The geometry at the Au atom is, as expected, linear (as is the case for all structures presented in this paper).The substituents of the piperidine ring occupy different position types; whereas the methyl groups are equatorial, which would be expected, the Au atoms are axial (cf.absolute C methyl -C-C-C and Au-N-C-C torsion angles of approximately 180 or 60 � respectively in Table 1).

Figure 1
The asymmetric unit of compound 1 in the crystal, with ellipsoids at the 50% probability level.The dashed line represents a hydrogen bond.

Figure 2
The asymmetric unit of compound 2 in the crystal, with ellipsoids at the 50% probability level.The dashed lines represent hydrogen bonds (thin) or the aurophilic interaction (thick).
The axial configuration of Au atoms with respect to piperidine ligands has been noted in our previous papers (Do ¨ring & Jones, 2023a,b), although it is not always observed; in the AuCl(piperidine) tetramer (Guy et al., 1977), for instance, the Au atoms lie equatorially with respect to the ring.The piperidine rings eclipse each other when viewed along the direction N11� � �N21, with pseudo torsion angles C26-N21� � �N11-C12 = 2.9 (3) � and C22-N21� � �N11-C16 = 2.2 (3) � .Because of the contrast with compound 5 (see below), we comment here that the 4-Me-pip complexes 1-4 are achiral, because of the local mirror planes through the atoms N, C-4, C methyl and Au.
The structure of compound 2 is shown in Fig. 2; the asymmetric unit contains one    2.The configurations of the methyl group and the gold substituent at the piperidine ring are the same as for 1, namely equatorial and axial, respectively.The anion and cation are connected by two N-H� � �Cl hydrogen bonds (Table 7) and the aurophilic contact Au1� � �Au2, and the coordination axes are thus almost parallel, with torsion angles of ca 0 and 180 As in 1, the piperidine rings eclipse each other when viewed along the direction N11� � �N21, with pseudo torsion angles C12-N11� � � N21-C22 = 2.1 (3) � and C11-N11� � �N21-C26 = 1.0 (3) � .Compound 3 is isotypic to 2; it is shown in Fig. 3, with molecular dimensions and hydrogen bond details in Tables 3 and  8, but is not discussed further.
Compound 4 is a 1:1:1 mixture of the molecular complex [AuCl(4-Me-pip)], the ionic [Au(4-Me-pip  The asymmetric unit of compound 3 in the crystal, with ellipsoids at the 50% probability level.The dashed lines represent hydrogen bonds (thin) or the aurophilic interaction (thick).

Figure 4
The asymmetric unit of compound 4 in the crystal, with ellipsoids at the 50% probability level.The dashed lines represent hydrogen bonds (thin) or aurophilic contacts (thick).
All atoms lie on general positions.The asymmetric unit consists of two closely similar formula units and is shown in Fig. 4. Selected geometric parameters are presented in Table 4.As usual in this series of compounds, all Au atoms occupy an axial position at the piperidine rings, and all methyl groups are equatorial.In each formula unit, the chloride anion accepts one hydrogen bond (Table 9) each from the cation and the neutral molecule, and the two Au atoms are linked via an aurophilic interaction.The H� � �Cl À � � �H angles are 83 (3) � at Cl3 and 80 (3) � at Cl4.The coordination axes at the linked Au atoms are approximately perpendicular to each other, with torsion angles e.g.N11-Au1� � �Au2-N31 = 85.3 (3) � , N11-Au1� � �Au2-Cl1 = À 93.2 (2) � for the first formula unit and N41-Au3� � �Au4-N61 = À 84.6 (3) � , N41-Au3� � �Au4-Cl2 = 95.0 (2) � for the second.The dichloromethane molecules form short C-H� � �Cl hydrogen bonds to the chlorido ligands of the same formula unit and to the chloride anion of the other formula unit (for C1-H1A� � �Cl4 within the asymmetric unit but for C2-H2B� � �Cl3 via a glide plane, see section 3).The short contacts H16A� � �Au2 and H46B� � �Au4 (Table 9) might be regarded as forced by the formation of the hydrogenbonded dimer (see Section 3).
Compound 5 is the only complex of 2-Me-pip for which a structure was obtained.Selected geometric parameters are presented in Table 5.The asymmetric unit contains two [Au(2-Me-pip) 2 ] + cations, for both of which the Au atoms lie on the twofold axis 0.5, y, 0.25, and one chloride ion on a general position.Fig. 5 shows the twofold-symmetric dimer, which involves two N-H� � �Cl À � � �H-N units [H-Cl À � � �H = 75.4( 16) � ] and an aurophilic contact between the two Au atoms.Again, both Au atoms occupy an axial position at the piperidine rings, and both methyl groups are equatorial.The presence of two stereocentres in each piperidine ring, at the nitrogen and the methyl-substituted carbon atom, means that various diastereomers of the cation of 5 are formally possible, but their number is limited (i) by the preferences of gold for an axial and of the methyl group for an equatorial position, leading to configurations of R,S at N11 and C12 respectively, and (ii) by the twofold axis through the Au atom, so that the second piperidine of each cation is also R,S (it is conceivable that a different form of 5 might be obtained in which the two ligands of the cation have opposite configurations, for instance if the Au atom lay on an inversion centre).The same relative configurations would apply to any 2-methylpiperidine complex with an axially positioned metal, whereas an equatorially placed metal would lead to the same configuration for both centres (see Database Survey below).Of course, in the centrosymmetric space group C2/c the overall composition of 5 is a racemate.The coordination axes are inclined to each other at an angle of ca 64 � [cf.torsion angle N11-Au1� � � Au2-N21 = À 64.21 ( 14) � ].The central hydrogen-bonded ring has graph set R 2 4 (12) (Bernstein et al., 1995).The piperidine rings at each Au atom are mutually rotated, as viewed along the direction N� � �N 0 , but to a different extent [cf.pseudo torsion angles C12-N11� � �N11 0 -C12 0 = À 51.28 ( 5

Figure 5
The hydrogen-bonded dimer of compound 5 in the crystal, with ellipsoids at the 30% probability level.The dashed lines represent hydrogen bonds (thin) or an aurophilic contact (thick).

Figure 6
The hydrogen-bonded dimer of compound 1.Dashed lines indicate hydrogen bonds.

Supramolecular features
Supramolecular features within the asymmetric units have already been discussed in the Structural commentary section.
Compound 1 forms inversion-symmetric dimers with hydrogen bonding of the form N-H� � �Cl À � � �H-N (Fig. 6, Table 6); the central hydrogen-bonded ring has graph set R 2 4 (12) (Bernstein et al., 1995).The same applies to the [Au(pip) 2 ]Cl dimer (Ahrens et al., 1999), and yet, despite the topological similarity, there are major differences between these dimeric substructures.For 1, the H� � �Cl À � � �H angle is wider at 135 (1) � , the Au� � �Au distance is much longer at 5.9269 (3) A ˚, and the piperidine rings are approximately eclipsed, whereas in [Au(pip) 2 ]Cl (Fig. 7) the H� � �Cl À � � �H angle of 82 � is much narrower, the Au� � �Au distance of 4.085 A ˚is shorter and the piperidine rings are significantly rotated around the N� � �N vector, with three absolute C-N� � �N-C pseudo torsion angles of approximately 60 � and one approximately antiperiplanar (values calculated from deposited coordinates).It is tempting to suggest that the axial configuration of the Au atoms may facilitate the formation of the dimers, but detailed theoretical calculations would be necessary to provide corroborative evidence for this.Apart from the classical hydrogen bonds, three short contacts (Table 6) might be regarded as 'weak' hydrogen bonds.Two of these involve the gold atom as hydrogen-bond acceptor, a topic that has been reviewed by Schmidbaur et al. (2014) and Schmidbaur (2019), and lead to ribbons of cations parallel to the a axis (Fig. 8).
Compound 2 also forms inversion-symmetric dimers (Fig. 9) with a quadrilateral of Au atoms, exactly planar by symmetry, connected via two independent aurophilic interactions, and two H 2 Cl 2 quadrilaterals, above and below this plane, involving three-centre NH(� � �Cl) 2 interactions (Table 7).This motif is topologically analogous to that of the AuCl(piperidine) tetramer (Guy et al., 1977; for an improved

Figure 8
Compound 1: The short H� � �Au contacts (dashed lines) combine to form a ribbon of cations parallel to the a axis.Hydrogen atoms not involved in these interactions are omitted.

Figure 9
The hydrogen-bonded dimer of compound 2. Dashed lines indicate hydrogen bonds (thin) or aurophilic interactions (thick).Atom labels indicate the asymmetric unit.
rilateral displays widely differing angles (Table 2), whereby the transannular Au2� � �Au2 0 distance is much smaller than Au1� � �Au1 0 [3.5051 (8) and 5.3963 (6) A ˚, respectively].The angles in the H 2 Cl 2 quadrilateral are approximately equal [89 (3) � at the chlorine atoms and 91 (2) � at the hydrogen atoms].There are no C-H� � �Cl or C-H� � �Au contacts shorter than 2.9 or 3.1 A ˚, respectively, so that one may loosely speak of a packing of dimers that involves only van der Waals interactions; the dimers lie in layers parallel to (011), whereby neighbouring dimers are related by translational symmetry parallel to [100] and [111] (approximately vertical and horizontal, respectively, in Fig. 10).Both formula units of compound 4 form closely similar inversion-symmetric dimers via additional hydrogen bonds from H21 or H51 to the chloride ion (Table 9, Fig. 11).The centre of the dimer is a hydrogen-bonded ring of graph set R 2 4 (12).There are many short contacts of the type H� � �Cl or H� � �Au that might be regarded as 'weak' hydrogen bonds (Table 9).The contacts H34� � �Cl1 and H36B� � �Au1 connect the dimers of the first formula unit to form a layer parallel to the bc plane (Fig. 12), whereas H64� � �Cl2 and H66A� � �Au2 do the same for the second unit, although it is often a moot point whether short contacts to Au(I) centres are of structural significance, or whether they are simply a consequence of the sterically exposed nature of a linearly coordinated atom.A projection of the structure parallel to the b axis (Fig. 13) shows that the gold complexes of the first formula unit occupy the regions x ' 0 and 1, whereas those of formula unit 2 lie in the region x ' 0.5, with the solvent molecules between these broad layers.
General packing of compound 2, showing a layer parallel to (011), with view direction perpendicular to the layer.This layer passes through the region at y ' 0.25, z ' 0.25; a further such layer passes through the region at y ' 0.75, z ' 0.75.

Figure 11
The inversion-symmetric dimer of the first formula unit of compound 4.
Dashed lines indicate hydrogen bonds (thin) or aurophilic interactions (thick).Atom labels indicate the asymmetric unit.

Figure 12
Compound 4: Connection of the dimers of the first formula unit by the 'weak' hydrogen bonds of the form H� � �Au and H� � �Cl (thick dashed lines).Classical hydrogen bonds are represented by thin dashed lines.The view direction is perpendicular to the bc plane, and the region is x ' 0.
The packing of the dimers of compound 5 is essentially featureless.Three of the four shortest C-H� � �Cl contacts involve methyl hydrogens (whose position is always somewhat unreliable for heavy-atom structures) and all short C-H� � �Au contacts are intramolecular.However, the two shortest C-H� � �Cl contacts (Table 10) serve to link the dimers, forming a layer structure parallel to (101) (Fig. 14).The hexagonal packing of the dimers may nevertheless be determined more by steric or van der Waals effects.

Database survey
The searches employed the routine ConQuest (Bruno et al., 2002), part of Version 2022.3.0 of the CSD (Groom et al., 2016).
Few structures of transition-metal complexes involving alkylpiperidine ligands have been reported, and most of these involved methyl substituents.For 3-Me-pip (which we did not study) there is only [Pt(malonate)(3-Me-pip) 2 ], with inversion symmetry, in which both the metal and the methyl group are equatorial (QUBFOI; Khan et al., 2000).For 2-Me-pip there are two structures: in the structure of enantiomerically pure [W(CO) 5 (2-Me-pip)], with S,S configuration at C2 and the nitrogen atom (CAPSOB; Korp et al., 1983), the metal and the methyl group are also equatorial, and this is also the case for the cubane-type tetramer [CuI(2-Me-pip)] 4 (ZAYYAD; Wang et al., 2022).
Nine complexes of 4-Me-pip appear in the CSD, seven of which display the usual equatorial positions of the metal atoms and methyl groups.The exceptions, with equatorial methyl groups but axially positioned metal atoms, are provided by two silver complexes studied by us (Jones & Wo ¨lper, 1975;Wo ¨lper et al., 2010), namely [(AgCl) 5 (4-Me-pip) 4 ] (GAQLEQ) and the polymeric [(AgBr) 3 (4-Me-pip) 2 ] (YUXWOE), which contain one and six independent ligands, respectively.This reinforces our observation that coinage metals have a higher tendency to be axial at piperidine ligands.In this context, the 4-benzylpiperidine complex [(AgCN) 2 (4-Bz-pip)] (CITWOU; Bz = benzyl; Strey & Do ¨ring, 2018) is interesting; the silver atom is axial at one of the two independent ligands but equatorial at the other.

Synthesis and crystallization
Bis(4-methylpiperidine)gold(I) chloride 1. 40 mg (0.125 mmol) of AuCl(tht) (tht = tetrahydrothiophene) were dissolved in 2 mL of 4-methylpiperidine.The solution was divided into five portions in small test-tubes and overlaid with various precipitants [see Do ¨ring and Jones (2023a) for details], before being stoppered and stored in a refrigerator overnight.The portion with petroleum ether as precipitant yielded crystals in the form of colourless blocks, one of which was used for the structure analysis, in approximately quantitative yield.Analysis: calculated: C 33.46, H 6.08, N 6.50; found: C 33.09, H 5.94, N 6.33%.
Bis(4-methylpiperidine)gold(I) dichloridoaurate(I) 2. 40 mg (0.093 mmol) of 1 were dissolved in 2 mL of dichloromethane.The solution was then treated as above for 1.The portion with n-pentane as precipitant yielded crystals in the form of colourless blocks, one of which was used for the structure analysis, in approximately 90% yield.Analysis: calculated: C 21.73, H 3.95, N 4.22; found: C 22.07, H 4.01, N 4 is at (1/2, 1/2, 1/4).Bis(4-methylpiperidine)gold(I) dibromidoaurate(I) 3. 90 mg (0.247 mmol) of AuBr(tht) were dissolved in 2 mL of 4-methylpiperidine.The solution was then treated as above for 1.The portion with n-pentane as precipitant yielded crystals in the form of colourless blocks, one of which was used for the structure analysis, in approximately quantitative yield.Analysis: calculated: C 19.16, H 3.48, N 3.73; found: C 19.38, H 3.55, N 3.57%.
Bis(2-methylpiperidine)gold(I) chloride 5. 40 mg (0.125 mmol) of AuCl(tht) were dissolved in 2 mL of 2-methylpiperidine.The solution was then treated as above for 1.The portion with n-pentane as precipitant yielded crystals in the form of colourless blocks, one of which was used for the structure analysis, in approximately quantitative yield.Analysis: calculated: C 33.46, H 6.08, N 6.50; found: C 32.99, H 6.26, N 6.22%.

Refinement
Details of the measurements and refinements are given in Table 11.Structures were refined anisotropically on F 2 .Methylene hydrogens were included at calculated positions and refined using a riding model with C-H = 0.99 A ˚and H-C-H = 109.5� .Methine hydrogens were included similarly, but with C-H = 0.99 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.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 all compounds, the NH hydrogen atoms were refined freely but with N-H distances restrained to be approximately equal.For compound 4, U values of the NH hydrogen atoms were fixed at 1.2 � U eq of the parent nitrogen atoms, because the values were otherwise too small (close to or slightly below zero).
The crystal of compound 2 was pseudo-merohedrally twinned by interchange of the a and b axes, with the twin matrix [010 / 100 / 001].The relative volume of the smaller component refined to 0.3023 (7).Five badly-fitting reflections were omitted from the refinement.
For compound 3, the cell is presented in a non-standard form (b > a) in order to allow a direct comparison with the isotypic chlorine analogue 2.
The crystal of compound 4 was pseudo-merohedrally twinned (via an apparently orthorhombic cell) with the twin matrix [101 / 010 / 001].The relative volume of the smaller component refined to 0.4614 (5).Six badly-fitting reflections were omitted from the refinement.
For compound 5, the U values are rather high for a structure measured at 100 K. Accordingly, Fig. 5 shows ellipsoids at the 30% level.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å
2 ] + cation and one [AuCl 2 ] À anion, a composition corresponding to type III in our arbitrary classification of products (Do ¨ring & Jones, 2023a), and already observed during our studies of secondary amine complexes (Do ¨ring & Jones, 2018) for [Au(Et 2 NH) 2 ] [AuBr 2 ].All atoms lie on general positions.Selected geometric parameters are presented in Table ) 2 ]Cl (thus corresponding to a mixture of types I and II, as established for the corresponding pyrrolidine derivative; Do ¨ring & Jones, 2023a) and dichloromethane.The dichloromethane is well-ordered.research communications Acta Cryst.(2024).E80, 157-165 Do ¨ring and Jones � Five gold(I) complexes with amine ligands 159 and Jones � Five gold(I) complexes with amine ligands Acta Cryst.(2024).E80, 157-165 Figure of this structure, see Do ¨ring & Jones, 2023a).The Au 4 quad-

Figure 13 Compound 4 :
Figure 13Compound 4: Projection of the structure parallel to the b axis.Hydrogen atoms (except for those of the solvent) are omitted.Dashed lines indicate hydrogen bonds.The layers of gold-containing residues at x ' 0, 0.5 and 1 can be clearly recognized.

Table 11
Experimental details.