Crystal structures of bis- and hexakis[(6,6′-dihydroxybipyridine)copper(II)] nitrate coordination complexes

Two copper(II) complexes, a dinuclear and a hexanuclear complex with bridging hydroxyl and nitrate ligands, were obtained from reaction of copper nitrate with dihydroxybipyridine at neutral and slightly acidic pH. Formation of multi-nuclear complexes contrasts with the equivalent sulfate compounds which formed discrete mononuclear complexes. The complexes feature intramolecular and intermolecular hydrogen bonding.


Chemical context
Catalytic processes in nature are often facilitated by enzymes that feature transition metals in their active sites. Many of these reactions would be of tremendous interest could they be copied using simpler and technologically feasible conditions. One such process is water oxidation as observed in photosynthesis, and the use of transition metal complexes to mimic the reactivity of photosystem II have captured the attention of an increasing number of research groups over the last few years (Kikuchi & Tanaka, 2014;Singh & Spiccia, 2013). One complex that especially caught our interest was [Cp*Ir(bpy)Cl] + (Blakemore et al., 2010) which features a bipyridine (bpy) type ligand. In our research into catalytic water oxidation, we are trying to enhance proton-coupled electron transfer (PCET) in metal-complex catalysts by incorporating hydrogen-bond donors and acceptors in near proximity to the potentially catalytic metal atoms to mimic the active center of a protein-metal complex. When applying this principal to the Blakemore-type [Cp*Ir(bpy)Cl] + complex by swapping normal bipyridine for dihydroxybipyridine (6,6 0dhbp), we were indeed able to increase the catalytic turnover rate and control water oxidation rates by adjusting pH levels (DePasquale et al., 2013). The ligand 6,6 0 -dhbp has also been used in combination with ruthenium terpyridine (tpy) fragments to yield the complex [(tpy)Ru(6,6 0 -dhbp)(H 2 O)] (Marelius et al., 2014).
Our focus has most recently shifted to the investigation of copper(II) bipyridine complexes analogous to [(bpy)Cu(OH) 2 ] 2 2+ (Barnett et al., 2012). We isolated discrete mono-copper complexes from copper(II) sulfate with a selection of modified bipyridine ligands and investigated the compounds spectroscopically, crystallographically and for their catalytic water oxidation capacity (Gerlach et al., 2014). When swapping sulfate for nitrate as the counter-ion we found that the resulting complexes are no longer mononuclear. Instead, larger aggregates with two or six copper(II) atoms formed that feature coordinating nitrate as well as hydroxyl ligands. As a result of their aggregation and the varied coordination environment of their copper atoms, these complexes are not ideally suited for homogenous water oxidation catalysis. Instead they feature quite intriguing and fascinating solid structures which we would like to describe and present.

Figure 1
The numbering scheme of complex (I), with donor-acceptor distances of intramolecular hydrogen bonds colored green and of intermolecular hydrogen bonds colored blue, represented with displacement ellipsoids at the 50% probability level. Additional symmetry-related atoms O5 and O11 were generated by translation along the a axis. Table 1 Hydrogen-bond geometry (Å , ) for (I). The hexanuclear copper(II) dhbp complex (II) is comprised of a dimer of the asymmetric portion of the molecule which contains three symmetry-unique copper atoms, two fully protonated and one fully deprotonated dhbp, two bridging hydroxide and two nitrate ligands (see Fig. 2). This asymmetric trinuclear unit is related through an inversion center to the full hexanuclear complex (see Fig. 3). Two copper atoms, Cu1 and Cu3, are hexa-coordinate with a distorted octahedral geometry whereas Cu2 is penta-coordinate with a distorted trigonal-pyramidal geometry with = 0.746 (Addison et al., 1984). Similar to the dinuclear complex, each copper atom is coordinated by one hydroxybipyridine ligand with one bridging hydroxyl ligand between Cu1 to Cu2 and Cu2 to Cu3. The dihydroxybipyridine ligand bound to Cu2 (dhbp2) is doubly deprotonated with each deprotonated oxygen bound to the flanking Cu1 and Cu3 metal sites, O3 and O4, respectively. The remaining coordination sphere of Cu1 entails one dhbp (N,N bound), one bridging hydroxide to Cu2 (O14), one bridging nitrate to Cu2 (O11), and one bridging nitrate (O7) which tethers the two asymmetric units. The coordination of Cu2 entails one deprotonated dhbp (N,N bound), one bridging nitrate to Cu1 (O12), and two bridging hydroxides to Cu1 and Cu3 (O14 and O13, respectively). The remaining coordination sphere of Cu3 entails one dhbp (N,N bound), one methanol (O15), one bridging hydroxide (O13), and one bridging nitrate  (a) The asymmetric unit of complex (II) represented with displacement ellipsoids at the 50% probability level. H atoms not involved in hydrogen bonding are omitted for clarity. The donor-acceptor distances are shown as green for intramolecular hydrogen bonds and shown as blue for intermolecular hydrogen bonds (O10-O15) and inter-asymmetric unit hydrogen bonds (O9 -O13). Additional symmetry-related atoms O10 and O15 were generated by the symmetry operator 1 À x, Ày, 1 À z, and O9 and O13 were generated via the inversion center 1 À x, 1 À y, 1 À z at the center of the hexanuclear complex. (b) The asymmetric unit of complex (II) oriented to show donor-acceptor distances of intramolecular hydrogen bonds, in green, of the protonated dhbp ligands.

Figure 3
The full hexanuclear complex (II) represented with displacement ellipsoids at the 50% probability level and H atoms not involved in hydrogen bonding are omitted for clarity. The two symmetry-related units of the hexamer are shown in blue and green to better visualize the relation of the asymmetric unit through the inversion center.

Figure 4
The three unique copper atoms of complex (II) are displayed with the coordination relevant atoms of the bound dhbp ligands and bridging hydroxides with displacement ellipsoids at the 50% probability level and the donor-acceptor distances of intramolecular hydrogen bonds of the protonated dhbps represented in green.
(O8) which tethers the two asymmetric units. Each deprotonated oxygen of dhbp acts as an acceptor for intramolecular hydrogen bonds from the two protonated dhbp ligands, O2 to O3 at 2.499 (3) (4) ], respectively. C-O bond lengths of the protonated and deprotonated dhbp ligands of the hexa and dinuclear complexes are similar; CÁ Á ÁO distances (Å ) are 1.302 (4) and 1.312 (4) for the copper coordinating oxygen atoms, and 1.324 (4), 1.304 (4), 1.330 (4), and 1.321 (4) for the hydroxyl O atoms, with the longer of the four values belonging to the hydroxyl groups hydrogen-bound to the neighboring deprotonated dhbp ligand, and the shorter two being associated with those hydrogen-bound to the bridging hydroxyl groups. These reduced lengths reflect increased C-O double-bond character upon deprotonation. One intermolecular hydrogen bond of intermediate strength connects the bound methanol to the non-coordinating oxygen of the Cu1-Cu2 bridging nitrate of another molecule, O15 to O10 at 2.790 (4) Å [O-HÁ Á ÁO bond angle of 107 (4) ].
Comparison of complex (I) to the asymmetric component of complex (II) indicates some structural similarities, Fig. 5. The overall structure of complex (I) can be reasonably well overlaid with the dinuclear component of (II) including Cu2 and Cu3, with the main differences resulting from one nitrate ligand that is bridging between the two copper ions in complex (I) being rotated so that in complex (II) it instead bridges one of these copper ions to the third that has no counterpart in complex (I). The two copper ions that are bridged by a nitrate ion in complex (I) are thus not bridged in complex (II) (featuring a methanol molecule and a nitrate bridging to the third copper instead), leading to a larger distance between the copper ions in complex (II) and a different tilt angle of the fully protonated dhbp ligand (left dhbp in Fig. 5).
The bridging oxygen species for both complexes (I) and (II) are correctly assigned as hydroxides to balance the overall neutral charge of the complex. Complex (I) with two Cu II ions is charge balanced with one terminal and one bridging nitrate each with a single negative charge, one deprotonated hydroxyl group of dhbp, and one bridging hydroxide. The bond lengths to the bridging hydroxide from Cu1 to O5 is 1.964 (3) Å and Cu2 to O5 is 1.939 (3) Å where the proton of O5 hydrogen bonds to one acceptor. A comparable bond length is 1.946 (3) Å from Cu2 to O2 of the deprotonated dhbp ligand. The remaining oxygen atoms of the dhbp ligands are each protonated and engaged in hydrogen bonding as described above. The asymmetric unit of complex (II) balances similarly with three Cu (II) ions against one bridging nitrate, one nitrate bridging the two asymmetric units, two deprotonated dhbp hydroxyl groups, and two bridging hydroxides. The bond lengths to the bridging hydroxide, Cu2 and Cu3 to O13: 1.933 (2) and 1.951 (2) Å , respectively, are comparable to those observed in complex (I) where each of these hydroxides have one hydrogen bond. Alternatively, the bond lengths to the bridging hydroxide are longer where Cu1 and Cu2 to O14 are 1.970 (2) and 2.062 (3) Å , respectively, likely due to the two hydrogen-bonding interactions described above weakening the orbital overlap with the copper and lengthening these bonds. The two copper-hydroxyl dhbp bonds in complex (II) are comparable to this bond in complex (I) at 1.966 (3) Å from Cu1 to O3 of dhbp and 1.960 (2) Å from Cu3 to O4 of dhbp.

Figure 5
Complex (I) in yellow is overlaid on the Cu2-Cu3 dimer portion of the asymmetric unit of complex (II) in green.
molecule across the symmetry operation 1 À x, Ày, Àz at a distance of 3.894 (3) Å between the centroids of the rings. The pyridine ring containing N1 interacts with its symmetryequivalent ring across the symmetry operation 1 À x, 1 À y, Àz with a centroid-to-centroid distance of 3.969 (3) Å . The dhbp ligand coordinating to Cu2 also shows -stacking via two alternating inversion-symmetry operations, forming chains along [100]. The pyridine rings containing N3 and N4 intercross by the symmetry operation 1 À x, Ày, 1 À z where the centroid of the ring defined by N3 is at a distance of 3.604 (2) Å from the centroid of the ring defined as N4 on side of the dhbp plane with the bridging hydroxide. These rings also -stack on the opposite face of the plane at a distance of 3.768 (2) Å from the centroid of the ring defined by N3 to N4 through the symmetry operation Àx, Ày, 1 À z. Intermolecular hydrogen bonding from the bridging hydroxide ligand to the terminal oxygen of the bridging nitrate ligand interlinks neighboring molecules primarily along [100]. See Fig. 6 for extended intermolecular interactions of complex (I).
The hexanuclear complex (II) progresses along [010] through two symmetry-related hydrogen bonds between O15 of the bound methanol molecule of Cu3 to O10 of the Cu1-Cu2 bridging nitrate (Fig. 7). The dhbp ligands are primarily within the ac plane and exhibit -stacking but in a less regular fashion than for complex (I), primarily in the [010] direction without forming chains. Off-set -stacking of the dhbp ligand bound to Cu1 are related through the symmetry operation 1 À x, Ày, Àz with a centroid-to-centroid distance of 3.784 (2) Å of the pyridine rings containing N1 to the ring containing N2 and vice versa. A single ring of each dhbp ligand bound to Cu2 and Cu3 -stack via translation at a distance of 3.551 (2) Å between the centroids of the pyridine rings defined by N3 and N5, respectively. Additionally, the Cu3 dhbp ligand -stacks via the symmetry-equivalent ring defined by N5 of a neighboring molecule across the symmetry operation Àx, Ày, 1 À z at a centroid-to-centroid distance of 3.887 (2) Å . Close proximity occurs in plane between the pyridine ring containing N4 of the dhbp ligand bound to Cu2 at a distance of 3.818 (2) Å between C18 to C19 and vice versa across the symmetry operation 1 À x, 1 À y, Àz.
The most relevant structure reported in the database contains a dinuclear copper 6-hydroxybipyridine complex with The packing arrangement of complex (I) propagates along [100] via intermolecular hydrogen bonding (blue) and in the bc plane bystacking of the dhbp pyridyl rings.

Figure 7
The packing arrangement of complex (II) propagates along [010] via intermolecular hydrogen bonding (blue) and in the ac plane by -stacking of the dhbp pyridyl rings. a nitrate ligand bridging the copper ions, IBOXAZ (Zhang et al., 2004). No examples of hydroxybipyridine-ligated copper compounds with oxide or hydroxide bridges have been reported.

Synthesis and crystallization
The neutral copper dinuclear complex (I) Copper(II) nitrate trihydrate (128 mg, 0.530 mmol) and 6,6 0 -dhbp (100 mg, 0.531 mmol) were combined in 50/50 ethanol and water solvent (10 mL) and stirred two days. Green plate crystals were grown from an ethanol solution in a freezer. This complex was analyzed exclusively by X-ray diffraction.
The neutral copper hexanuclear complex (II) Copper(II) nitrate hemipentahydrate (124 mg, 0.533 mmol) and 6,6 0 -dhbp (100 mg, 0.531 mmol) were combined in 10 mL of 0.1 M NaOAc adjusted to pH 3 by acetic acid. The mixture was stirred for three days at room temperature. The resulting solution was dried under high vacuum and recrystallized twice from methanol to afford green prismatic crystals. This complex was analyzed exclusively by x-ray diffraction.

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 3.
The crystal under investigation for complex (II) was found to be split with two major domains not related by any obvious twin operation. The orientation matrices for the two components were identified using the program Cell Now (Sheldrick, 2008), with the two components being related by a 2.9 rotation about either the reciprocal axis 1.000 À 0.363 À 0.339 or the real axis 1.000 À 0.178 À 0.265. The two components were integrated using SAINT (Bruker, 2012), resulting in the following statistics: 17535 data (5769 unique) involve domain 1 only, mean I/ 8.6, 17271 data (5689 unique) involve domain 2 only, mean I/ 8.2, 34813 data (9811 unique) involve 2 domains, mean I/sigma 9.5, 11 data (11 unique) involve 3 domains, mean I/ 8.7 and 4 data (2 unique) involve 4 domains, mean I/ 57.6 The exact correlation matrix as identified by the integration program was found to be 1.00336 0.02923 À0.02720, À0.01894 1.02272 À0.04903, 0.02520 0.05747 0.97055. The data were corrected for absorption using TWINABS (Sheldrick, 2009), and the structure was solved using direct methods with only the non-overlapping reflections of component 1. The structure was refined using the HKLF5 routine with all reflections of component 1 (including the overlapping ones), resulting in a BASF value of 0.486 (1). The R int value given is for all reflections and is based on agreement between observed single and composite intensities and those calculated from refined unique intensities and twin fractions (TWINABS; Sheldrick, 2009  C-and O-bound H atoms were placed in calculated positions and allowed to ride on their carrier atoms: aromatic C-H arom = 0.95 Å with U iso (H) = 1.2U eq (C), C-H methyl = 0.98 Å with U iso (H) = 1.5U eq (C). O-H were refined for complex (I) and for hydroxide and methanol H atoms of complex (II), with O-H distances restrained to 0.84 (2) Å for O1, O3 and O4 of complex (I), and O13 and O14 of complex (II) yielding O-H distances of 0.748-0.828 Å . The remainder of the hydroxyl atoms were placed in calculated positions with O-H = 0.84 Å , and all U iso (H OH ) were set to 1.5U eq (O).

Special details
Experimental. The crystal under investigation was found to be split with two major domains not related by any obvious twin operation. The orientation matrices for the two components were identified using the program Cell_Now, with the two components being related by a 2.9 degrees rotation about either the reciprocal axis 1.000 -0.363 -0.339 or the real axis 1.000 -0.178 -0.265 degree. The two components were integrated using Saint, resulting in in the following statistics: 17535 data (5769 unique) involve domain 1 only, mean I/sigma 8.6 17271 data (5689 unique) involve domain 2 only, mean I/sigma 8.2 34813 data (9811 unique) involve 2 domains, mean I/sigma 9.5 11 data (11 unique) involve 3 domains, mean I/sigma 8.7 4 data (2 unique) involve 4 domains, mean I/sigma 57.6 The exact correlation matrix was identified by the integration program was found to be Transforms h1.1(1)->h1.2(2) 1.00336 0.02923 -0.02720 -0.01894 1.02272 -0.04903 0.02520 0.05747 0.97055. The data were corrected for absorption using twinabs, and the structure was solved using direct methods with only the non-overlapping reflections of component 1. The structure was refined using the hklf 5 routine with all reflections of component 1 (including the overlapping ones), resulting in a BASF value of 0.486 (1). The R int value given is for all reflections and is based on agreement between observed single and composite intensities and those calculated from refined unique intensities and twin fractions (TWINABS (Sheldrick, 2009) (2) Geometric parameters (Å, º)