Trinuclear nickel coordination complexes of phenanthrene-9,10-dione dioxime

Trinuclear nickel complexes have been isolated and characterized. These complexes resulted from reaction with the constrained α–β dioxime congener of phenanthrene-9,10-dione.


Chemical context
Oxime functional groups can coordinate to transition metal ions in a variety of ways, due to the presence of both nitrogen and oxygen donors. On account of the multitude of possible coordinations, these ligands, and particularlydioximes, have the capability of forming bridging multinuclear complexes with many transition metals, including nickel (Chaudhuri, 2003). From the standpoint of single-molecule magnets, these multi-nuclear complexes play an important role due to their ability to facilitate spin-frustration in magnetic transition-metal clusters (Aromí & Brechin, 2006). Other nickel polynuclear compounds supported by oxime ligands have been reported (Jiang et al., 2005;Biswas et al., 2009). ISSN 2056-9890 Phenanthrenequinone dioxime (pqdH 2 ) is andioxime ligand that incorporates a constrained ring system. Similar to other dioximes, however, it exists as three separate stereoisomers (E-E, E-Z, and Z-Z), as confirmed by liquid chromatography -mass spectrometry. Interestingly, although this compound was synthesized over 100 years ago (Schmidt & Sö ll, 1907), no coordination complexes of this ligand have been structurally characterized to date. Fig. 1 shows the structure of [Ni 3 (H 2 pqd)(Hpqd) 2 (pqd) 2 ], (1). This complex consists of three Ni II atoms in a triangular arrangement, two of which are in a square-planar coordination environment, while the third is in a pseudo-octahedral coordination environment. The square-planar Ni II atoms (Ni1 and Ni2) consist of one N,N-coordinating and one N,O-coordinating ligand. These ligands form bridges with the pseudooctahedral Ni II atom (Ni3) by means of their oxime O atoms.

Structural commentary
This arrangement permits the formation of Ni-N-O-Ni and Ni-O-Ni bridges between each square-planar Ni II atom and the pseudo-octahedral Ni II atom.

Supramolecular features
The proton-bridged complex completes the macrocyclic coordination around the square-planar Ni II atoms by means of hydrogen bonds. Furthermore, the ligand that coordinates the pseudo-octahedral Ni II atom features hydrogen-bonding interactions (Table 1) between the oxime hydroxy groups and the ligands of the square-planar Ni II atoms. The nickel units show no direct interaction with their nearest neighbors in the extended lattice. Some -stacking between adjacent molecules is, however, evident (Fig. 3). Two interactions were found, one with a centroid-centroid distance of 3.886 (2) Å (symmetry code: 1 À x, À 1 2 + y, 3 2 À z) and the other with a centroidcentroid distance of 4.256 (3) Å (symmetry code: Àx, Ày, 2 À z). In the latter case, although not aromatic, the distance to the centroid of the central ring of phenanthrene is shorter, with a distance of 3.528 (3) Å . Toluene molecules occupy the solvent channels that are oriented along the c axis. Displacement ellipsoid plot at the 50% probability level of [Ni 3 (H 2 pqd)(BF 2 pqd) 2 (pqd) 2 ]. H atoms (with the exception of hydrogen-bonded atoms) and solvent molecules have been omitted for clarity.

Figure 3
Packing diagram of [Ni 3 (H 2 pqd)(Hpqd) 2 (pqd) 2 ], viewed approximately down the c-axis direction. Table 1 Hydrogen-bond geometry (Å , ) for (1).  Hydrogen-bond geometry (Å , ) for (2). The BF 2 -bridged complex completes the macrocyclic coordination around the square-planar Ni II atoms by means of covalent O-B-O bonds. However, the hydrogen-bonding interactions ( Table 2) that lock the pseudo-octahedral Ni II atom remain in place. The nickel units show no direct inter-action with their nearest neighbors in the extended lattice. A solvent channel oriented along the c axis is also evident (Fig. 4). However, the extreme disorder of the solvent does not permit the determination of a suitable model. Packing diagram of Ni 3 (H 2 pqd)(BF 2 pqd) 2 (pqd) 2 , viewed approximately down the c-axis direction. Voids presented in brown were calculated in Mercury (Macrae et al., 2006) using a probe radius of 1.2 Å .

Synthesis and crystallization
The parent ligand, pqdH 2 (0.75 g; 3.1 mmol), was dissolved in 100 ml of ethanol, to which nickel(II) acetate (0.33 g, 1.3 mmol) was added. A red precipitate began to form after approximately 30 min. The solution was then allowed to stir for 1 h, followed by cooling in a freezer and filtration of the crude product (yield: 272 mg, 0.2 mmol, 32%). The resulting product was dissolved in DMF solution and layered with toluene, resulting in the formation of crystals of [Ni 3 (H 2 pqd)(Hpqd) 2 (pqd) 2 ] after a period of 3-4 d. The crystals grew as red blocks with an asymmetric unit consisting of a complete [Ni 3 (H 2 pqd)(Hpqd) 2 (pqd) 2 ] molecule and two toluene solvent molecules. The foregoing complex is stable enough to undergo a fluoridoboration reaction with boron trifluoride, thereby affording the compound [Ni 3 (H 2 pqd)(BF 2 pqd) 2 (pqd) 2 ].
[Ni 3 (H 2 pqd)(Hpqd) 2 (pqd) 2 ] was diluted in diethyl ether, thereby creating a slurry. One ml of 1.0 molar BF 3 -OEt 2 (in ether) was then added and the mixture was allowed to react overnight. The resulting precipitate was then filtered off and washed thoroughly with EtOH and Et 2 O. The resulting precipitate was then dissolved in dichloromethane (DCM) and filtered through Celite (yield: 43 mg, 30 mmol, 79%). Subsequently, a crop of red block-shaped crystals was grown by solvent evaporation over a period of one day.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. In proton-bridged structure (1), atoms H1A, H2A and H4A were found by assignment of difference map peaks and refined isotropically without geometrical constraints. The proton H3A was initially placed with the SHELXL HFIX 147 command (refinement on rotation) on O9, but was refined freely. Four distinct hydrogenbonding interactions were evident in the trinuclear cluster. Finally, there were two O-H-O interactions between an oxime and oximato of each [Ni(Hpqd)(pqd)] À unit that could not be resolved due to rapid conversion to [Ni(pqd)(Hpqd)] À .
All the restraints that are reported were included for the modelling of the disordered toluene solvent molecules.
In the case of BF 2 -bridged structure (2), atoms H1A and H2A were affixed to O9 and O10, respectively. They were then refined isotropically without rotational constraints. The SQUEEZE routine (Spek, 2015) as implemented in PLATON (Spek, 2009) was used to remove the electron density of three solvent DCM molecules per unit cell (calculated: 134 e À ; 593 Å 3 ).
Three molecules of what appeared to be dichloromethane were found to be badly disordered. Attempts to model the disorder were unsatisfactory. The contributions to the scattering factors due to these solvent molecules were removed by use of the utility SQUEEZE (Sluis and Spek, 1990) in PLATON98 (Spek, 1998). PLATON98 was used as incorporated in WinGX (Farrugia, 1999).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.3411 (