Crystal structures of the isotypic complexes bis(morpholine)gold(I) chloride and bis(morpholine)gold(I) bromide

In the title compounds, the gold atoms lie on inversion centres and are linearly coordinated by two morpholine ligands. The halide anions lie on twofold axes. Hydrogen bonds and Au⋯halide contacts lead to a layer structure.


Chemical context
We are interested in the synthesis and, particularly, the structures of amine complexes of gold halides and pseudohalides.These structures often display packing features such as aurophilic interactions (reviewed by Schmidbaur & Schier, 2008, 2012), hydrogen bonding (sometimes involving metalbonded halogens; Brammer, 2003), gold� � �halogen contacts or halogen� � �halogen contacts (see e.g.Metrangelo, 2008).Background material, including an extensive summary of our previous investigations, can be found in the previous article of this series (Do ¨ring & Jones, 2023), which presented complexes involving piperidine and pyrrolidine complexes.The ligand morpholine, C 4 H 9 NO, (sometimes referred to as 1,4-oxazinane or tetrahydro-1,4-oxazine, although morpholine is the preferred IUPAC name; here abbreviated in formulae as 'morph') is closely similar to piperidine (both are sixmembered rings involving secondary amine functions), but the presence of the oxygen atom in the ring might lead to additional possibilities for hydrogen bonding.Here we present the structures of the isotypic complexes bis(morpholine)gold(I) chloride, [Au(morph) 2 ]Cl, 1 and bis(morpholine)gold(I) bromide, [Au(morph) 2 ]Br, 2. We have already reported the synthesis of 1 (Ahrens et al., 1999), but the structure was not determined at that time.

Structural commentary
At the outset we comment that, 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.
Compounds 1 and 2 crystallize isotypically in space group C2/c with Z = 4.The gold atoms lie on inversion centres at (0.5, 0.5, 0.5) and the halide ions on twofold axes at (0.5, y, 0.75).Figs. 1 and 2 show the formula units, extended appropriately over the inversion centres.Selected molecular dimensions are presented in Tables 1 and 2. The Au-N bond lengths of 2.0631 ( 19) in 1 and 2.0598 (18) A ˚in 2 may be considered normal.The coordination geometry at gold is exactly linear by symmetry.Within the asymmetric units, a classical hydrogen bond connects the NH group and the halide ion.The morpholine rings are mutually rotated as viewed along the N11� � �N11 i vector, with C12-N11� � �N11 i -C12 i = 180 � by symmetry and C16-N11� � �N11 i -C12 i = 56.6 (2) � for 1 and 55.8 (2) � for 2.
One notable feature is the axial disposition of the gold centres at the morpholine ring, associated with C-C-N-Au torsion angles of around 68 � .This conformation is usually regarded as unfavourable for a single substituent of a sixmembered ring in the chair form; one would expect the conformation to be equatorial, with an antiperiplanar sequence C-C-N-Au, as was indeed observed for the piperidine complexes in our previous paper (Do ¨ring & Jones, 2023).See also Section 4.

Supramolecular features
Hydrogen bonds for 1 and 2 are presented in Tables 3 and 4 respectively.
For compound 1, the chloride ion accepts hydrogen bonds from two symmetry-equivalent NH donors (one in the asymmetric unit and the other with operator 1 À x, y, 3 2 À z); the H� � �Cl� � �H angle is 93.9 (12) � .The gold atom is involved in two symmetry-equivalent Au1� � �Cl1 contacts of 3.7187 (5) A (with operators x, 1 + y, z and 1 À x, À y, 1 À z for the chlorine atoms), with a Cl� � �Au� � �Cl angle of 180 � by symmetry; the corresponding Au� � �Cl� � �Au angle is 98.93 (2) � (with operators x, À 1 + y, z and x, À y, 1 2 + z for the gold atoms).The contacts combine to form a layer structure parallel to the bc plane (Fig. 3) in the region x ' 0.5.N-H� � �Cl hydrogen-bonded zigzag chains [� � �Cl� � �(morph)-Au-(morph)� � �] n , with overall direction parallel to the c axis, are crosslinked by the Au� � �Cl contacts.Within the layer, the chloride anion is involved in two C-H� � �Cl contacts that might be regarded as borderline 'weak' hydrogen bonds.The morpholine ligands project out of the layer to occupy the spaces at x ' 0.25 and 0.75.The morpholine oxygen atom is not involved in classical hydrogen bonding, but two C-H� � �O contacts connect the morpholine ligands of the layer at x ' 0.5 to those of adjacent layers at x ' 0 and 1.The significant role

Figure 1
The structure of compound 1 in the crystal, with ellipsoids at the 50% probability level.The asymmetric unit (labelled) is extended over the inversion centre at the gold atom.The dashed line represents the hydrogen bond.

Figure 2
The structure of compound 2 in the crystal, with ellipsoids at the 50% probability level.The asymmetric unit (labelled) is extended over the inversion centre at the gold atom.The dashed line represents the hydrogen bond.

Database survey
The searches employed the routine ConQuest (Bruno et al., 2002), part of Version 2022.3.0 of the Cambridge Database (Groom et al., 2016).
Only four other complexes of gold with morpholine are present in the CSD.Two of these involve our own work: A search for morpholine complexes of any transition metal gave 120 hits that included atom coordinates.A total of 117 structures displayed absolute C-C-N-TM torsion angles of 160-180 � (i.e. with the metal atom equatorial to the morpholine ring), whereas just six lay in the range 68-78 � , representing an axial position for the metal residue (with seven further cases in the range 78-90 � , but none with angles < 68 � ; because of structures containing more than one morpholine and/or differing torsion angles, the sum of these exceeds the number of hits).All six axial systems (DUHKAY, Ahrens et al., 2000;FIMSUR, Do ¨ring & Jones, 2013;ICADIB, Miller et al., 2011;REZKUE, Wang & Lian, 2013;YUXWUK and YUXXAR, Wo ¨lper et al., 2010) involved the coinage metals.We made similar observations for piperidine complexes in the CSD (Do ¨ring & Jones, 2023).It is unclear whether the generally lower coordination numbers of these metals, especially silver and gold, might promote the axial geometry (by reducing steric repulsions), whether electronic effects may play a role, or whether packing effects are involved.
A search for any structure containing morpholine (including those with four-coordinated nitrogen, but only where the NH function is retained) gave 766 hits.All necessarily contained an NH group, and 378 an additional OH group.Only 144 structures displayed an N-H� � �O morpholine or O-H� � �O morpholine contact shorter then the sum of the van der Waals radii (2.68A ˚in the CCDC system), and only 83 of these had a short H� � �O contact < 2.2 A ˚.This of course merely confirms the general principle that the oxygen atoms of ether groups have a limited tendency to form hydrogen bonds.In an investigation of the frequency of various hydrogen-bonded motifs, Allen et al. (1999)      oxygen atom is two-coordinate'.A typical example, drawn from the hit-list and showing both possible roles of the morpholine oxygen atom, is the complex dichloridobis-(morpholine)zinc (WIQRIA; Kinens et al., 2018), with two crystallographically independent morpholine ligands in the molecule, where a short N-H� � �O hydrogen bond (H� � �O 2.08 A ˚) connects the morpholine NH group of one ligand to the oxygen atom of the other ligand in a neighbouring molecule related by translational symmetry, forming chains of molecules (Fig. 4).The second independent NH group, however, forms three-centre hydrogen bonds to two chloride ligands of an adjacent chain, whereby the second oxygen atom 'misses out' on classical hydrogen-bond formation.We note in passing, after a random check of the hit-list, that the hydrogen bonding is often not discussed in the original references (nor in the corresponding Supplementary Material).
The above searches were limited to structures without disorder.One further relevant structure, which has disordered bridging cyano groups (with alternative orientations C N or N C), is the polymeric [Ag(CN)(morph)] (CITXAH; Strey & Do ¨ring, 2018).This too has axial positions for the silver atoms at all three independent morpholine ligands, and the packing involves classical N-H� � �N hydrogen bonds and short C-H� � �O contacts, but no N-H� � �O hydrogen bonds.

Synthesis and crystallization
Single crystals of compound 1 (Ahrens et al., 1999) were obtained by adding 40 mg (0.125 mmol) of chlorido(tetra-hydrothiophene)gold(I) to 2 mL of morpholine and overlayering portions of the solution thus obtained with various precipitants.The crystal chosen for structure determination was obtained using petroleum ether.Analysis: calculated C 23.63, H 4.46, N 6.89; found C 23.29, H 4.45, N 6.94%.Crystals of 2 were obtained analogously from 45.6 mg (0.125 mmol) of bromido(tetrahydrothiophene)gold(I); again, the measured crystal was obtained using petroleum ether as precipitant.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. Structures were refined anisotropically on F 2 .Hydrogen atoms of the NH groups were refined freely [but for 2 with U iso (H) set to 1.2 � U eq (N), because the value otherwise refined to below zero].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� , and with U iso (H) set to 1.2 � U eq (C).
concluded that particular motifs involving oxygen atoms were 'much less likely to occur if the research communications Acta Cryst.(2023).E79, 1161-1165 Do ¨ring and Jones � [Au(C 4 H 9 NO) 2 ]Cl and [Au(C 4 H 9 NO) 2 ]Br 1163

Figure 3
Figure 3 Packing diagram of compound 1, viewed perpendicular to the bc plane in the region x ' 0.5.Hydrogen atoms bonded to carbon are omitted for clarity.Thick dashed lines indicate hydrogen bonds; thin dashed lines indicate Au� � �Cl contacts.

Table 5
Experimental details.
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