Synthesis, characterization, and crystal structure of aquabis(4,4′-dimethoxy-2,2′-bipyridine)[μ-(2R,3R)-tartrato(4−)]dicopper(II) octahydrate

The title compound crystallized from the mock-up of a typical electroless copper bath (ECB) as used to deposit copper on printed circuit boards, consisting of a copper(II) salt, soda lye, l-(+)-tartrate as a complexing agent, and 2,2′-bypyridine derivative as a stabilizer. Its layer-like crystal structure is dominated by extensive π stacking and classical hydrogen bonding.

Typical electroless copper baths (ECBs), which are used to chemically deposit copper on printed circuit boards, consist of an aqueous alkali hydroxide solution, a copper(II) salt, formaldehyde as reducing agent, an l-(+)-tartrate as complexing agent, and a 2,2 0 -bipyridine derivative as stabilizer. Actual speciation and reactivity are, however, largely unknown. Herein, we report on the synthesis and crystal structure of aqua-1O-bis(4,4 0 -dimethoxy-2,2 0 -bipyridine)-1 2 N,N 0 ;2 2 N,N 0 -[-(2R,3R)-2,3-dioxidosuccinato-1 2 O 1 ,O 2 :2 2 O 3 ,O 4 ]dicopper(II) octahydrate, [Cu 2 (C 12 H 12 N 2 O 2 ) 2 (C 4 H 2 O 6 )(H 2 O)]Á8H 2 O, from an ECB mock-up. The title compound crystallizes in the Sohncke group P2 1 with one chiral dinuclear complex and eight molecules of hydrate water in the asymmetric unit. The expected retention of the tartrato ligand's absolute configuration was confirmed via determination of the absolute structure. The complex molecules exhibit an ansa-like structure with two planar, nearly parallel bipyridine ligands, each bound to a copper atom that is connected to the other by a bridging tartrato 'handle'. The complex and water molecules give rise to a layered supramolecular structure dominated by alternating stacks and hydrogen bonds. The understanding of structures ex situ is a first step on the way to prolonged stability and improved coating behavior of ECBs.

Chemical context
The production of printed circuit boards (PCB) starts with electroless copper deposition (ECD) on electrically nonconductive plastics. Copper is deposited from an alkaline solution of a copper(II) salt and a reducing agent (in general, formaldehyde). The reduction of copper(II) ions proceeds only at pH > 10, thus making methanediolate (deprotonated formaldehyde hydrate) the actual reactant (Van Den Meerakker, 1981;Jusys & Vaskelis, 1992). A complexing agent prevents the precipitation of copper(II) hydroxide (K L = 0.16 mmol 3 L À3 ), which would otherwise occur at pH > 5.7. Since the early development of ECD in 1946, l-(+)-tartrate has commonly been used as complexing agent (Narcus, 1947). Between pH 11 and 13, it forms bis(tartrato)copper(II), [Cu(C 4 H 2 O 6 ) 2 ] 6-, where each tartaric-acid-derived ligand is quadruply deprotonated. This complex is also known from Fehling's solution (Fehling, 1848;Hö rner & Klü fers, 2016). Reactant solutions facilitating ECD, so-called electroless copper baths (ECB), are metastable with respect to the precipitation of metallic copper, making additional stabilizers necessary. Over the past 60 years, a plethora of compounds has been used for this purpose (Agens, 1960;Saubestre, 1972), affecting not only the lifetime of ECBs but also the rate of ISSN 2056-9890 ECD and the physical properties of the deposited copper. Amongst the stabilizers, 2,2 0 -bipyridine and its derivatives are especially popular (Oita et al., 1997).
Herein, we report on the crystal structure of a compound that formed from an alkaline solution of a copper(II) salt, a tartrate, and 4,4 0 -dimethoxy-2,2 0 -bipyridine (dmobpy) during the investigation of stabilities of various copper(II) complexes with ligands derived from 2,2 0 -bipyridine (bpy).

Structural commentary
The compound crystallizes in the Sohncke group P2 1 with one chiral complex molecule and eight molecules of hydration water in the asymmetric unit. The copper(II) ions in the dinuclear complex (see Fig. 1) are each coordinated by two azine nitrogen donors, one alcoholate and one carboxylate oxygen donor. The lengths of the respective short bonds (ca 1.89-2.00 Å ) reflect the formal charge of the donor atoms, while a cis configuration is enforced by the structure of the ligand. An additional longer bond to an aqua ligand [d(Cu1-O60) = 2.322 (3) Å ] augments the coordination environment of Cu1 to a distorted square pyramid. Cu2, on the other hand, is coordinated in a square planar fashion with a short contact to a second alcoholate oxygen atom [d(Cu2Á Á ÁO55) = 2.549 (2) Å ].
The 4,4 0 -dimethoxy-2,2 0 -bipyridine ligands are nearly planar [positional root-mean-square (r.m.s.) deviation excluding hydrogen atoms: 0.032 Å for ligand containing N10 and N20, 0.041 Å for ligand containing N30 and N40], almost parallel [interplanar angle: 2.70 (4) ], and give rise to intramolecular stacks with an average centroid-plane distance of 3.36 (5) Å . Because of this, the overall molecular structure resembles that of ansa compounds, with the tartrato ligand representing the 'handle'. The tartrato ligand assumes an antiperiplanar (ap) conformation with respect to the central bond of the carbonatom chain. The C-O bonds at the carboxylate donors are synperiplanar (sp) to the C-O bonds at the neighboring alcoholate donors.
The absolute structure of the crystal was established via anomalous-dispersion effects [the inversion-distinguishing power of the experiment is strong according to Flack & Bernardinelli 2000)] and matches the absolute configuration of the employed l-(+)-(2R,3R)-tartrate. The Flack parameter is within the statistical range for an untwinned crystal, thus confirming the enantiopurity of the complex molecules (Flack & Bernardinelli, 2000).

Supramolecular features
Roughly parallel to {111}, complexes form infinite stacks, in which the intermolecular distance of 3.37 (6) Å (average centroid-plane distance) equals the intramolecular one (see Fig. 2a). A hydrogen bond from the aqua ligand to the carboxylato oxygen atom O50 of the neighboring molecule in the stack connects the tartrato(4À) ligands, forming an infinite hydrophilic backbone along the a direction.

Figure 1
Molecular structure of the title compound as an ORTEP plot (complex molecule only, solvent water molecules omitted for clarity). Hydrogen atoms are depicted as spheres with arbitrary radius, all other atoms as displacement ellipsoids of 50% probability. The dashed line indicates a non-bonding short contact.
(donors only) and tartrato ligands (all oxygen atoms as acceptors). The methoxy groups do not partake in hydrogen bonding but build a hydrophobic lining of the pocket. In this way, a front-to-back arrangement of alternating water and complex layers along b is formed (see Fig. 2b).
After two days of standing unsealed at ambient temperature, dark-blue crystals of [Cu 2 (dmobpy) 2 (- An infrared (IR) spectrum in attenuated total reflectance (ATR) was acquired from a ground crystal using a Thermo Nicolet iS5 equipped with a Thermo Nicolet iD5 ZnSe sample holder. Bands (vs: very strong, s: strong, m: medium, w: weak, br: broad) were assigned using literature data (Hesse et al., 1979;Socrates, 2001), as well as reference spectra of the dmobpy ligand and potassium sodium l-(+)-tartrate. The crystals were insoluble in common laboratory solvents (alkanes, ethers, alcohols, dimethylformamide, dimethyl sulfoxide, and water) at ambient and elevated temperature and decomposed in boiling coordinating solvents. Therefore, we cannot provide data of analyses relying on solutions. IR

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were located in difference-Fourier maps (for the complex and most water molecules) or their positions were inferred from neighboring sites (for the water molecule containing O68). Carbon-bound hydrogen atoms were refined with standard riding models. After close inspection of the reflection statistics, data with 2 > 60 (essentially noise) and the high-angle reflection 1 21 0 (mismeasurement) were excluded from the final refinement. The somewhat lower Friedel pair coverage is due to an inadequate choice of data-collection strategy. Unfortunately, we could not repeat the experiment because of sample loss.  Data collection: CrysAlis PRO (Rigaku OD, 2015); cell refinement: CrysAlis PRO (Rigaku OD, 2015); data reduction: CrysAlis PRO (Rigaku OD, 2015); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2008); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009) and publCIF (Westrip, 2010).  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Hydrogen atoms were located on difference Fourier maps for the complex and most water molecules or inferred from neighbouring sites for the water molecule containing O68. Hydrogen positions were refined semi-freely for oxygen-bound atoms with d(O-H) ≈ 0.84 (2) Å, d(H···H) ≈ 1.33 Å, and U iso (H) = 1.2U eq (O).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C11  (2)