(2,2′-Biquinoline-κ²N,N′)bis­(nitrato-κ²O,O′)copper(II)

In the title monomer, [Cu(NO₃)₂(C₁₈H₁₂N₂)], the six-coordinated Cu^II atom lies on a twofold axis which bisects one of the ligands (a chelating biquinoline) and duplicates the remaining ligand, a chelating nitrate. The latter binds in a very asymmetric way, consistent with a Jahn–Teller distortion in the coordination polyhedron which, due to the triple chelation, is extremely distorted and difficult to describe in terms of any regular model.

S1. Comment

Hydrothermal synthesis refers to a chemical reaction taking part in a sealed heated solution above ambient pressure. The mechanisms involved are not yet fully understood, but the process is often able to generate unique products; see, for instance, Feng & Xu (2001), Moghimi et al. (2003) and Walton (2002). Efficient methods for optimizing the yield of crystalline material have also been developed; for a successful application see, for instance, Contreras et al. (2007). However, the exact outcome of such syntheses still seems to be a matter of `green fingers' and is often prone to serendipity. The Experimental section gives details of the fortuitous obtainment of the title complex, [Cu(NO₃)₂(C₁₈H₁₂N₂)], (I).

Compound (I) is a monomer built up around a twofold axis passing through the metal centre. The symmetry axis also halves a chelating biquinoline (biq) group which binds through its two N atoms, thus rendering only half of the ligand (and of the whole polyhedron) independent. The remaining four coordinating atoms are provided by one independent nitrate (nit) group and its symmetry-related image, acting also as a chelate (Fig. 1).

A very similar, though nonsymmetric, disposition has been found in a closely related analogue published recently, [Cu(NO₃)₂(C₁₈H₁₀N₄)]·CH₃CN, (II) (Fitchett & Steel, 2007), where the role of the biq group is fulfilled by 2,2'-bi-1,5-naphthyridine, an ambivergent ligand differing from the present biq in that the C5—H5 and C5ⁱ—H5ⁱ groups (Fig. 1) are replaced by N atoms, potentially active either in coordination or as hydrogen-bonding acceptors. Fig. 2 shows a superposition diagram where the differences and similarities between both molecules can be clearly appreciated.

Both structures show four shorter and two much longer bonds (Table 1), suggesting a Jahn–Teller distortion in a classical octahedral Cu environment. Although in both cases the bond distances are normal for six-coordinate copper complexes [Cambridge Structural Database, Version?; Allen (2002)], the chelating character of the three ligands involved induces highly distorted coordination polyhedra. Table 1 shows the departure of `trans' angles in (I) from their expected regular values of 180°.

These facts make the polyhedra difficult to describe by any regular model, and in this regard both compounds are quite suitable for testing the vectorial bond-valence model (VBVM) proposed by Harvey et al. (2006), a novel approach tending to a simpler description of multidentate binding in which the action of each ligand is replaced by a single interaction vector, or VBV, derived from the individual bond valences of the coordinating atoms.

For the present kind of three-ligand coordination geometry, the VBVM would predict a planar array of the three VBV vectors and a nil resultant of their vectorial sum. These requirements are quite satisfactorily fulfilled in both structures. The first condition (a planar array of vectors) applies in a strict sense in (I) due to the intrinsic twofold symmetry around Cu, and is well within experimental error in (II), where the calculated angles between bond-valence vectors differ from 360° by less than 0.005°. The second condition is also very satisfactorily complied with in both cases, with a resultant bond-valence vector of 0.02 valence units (v.u.) for (I) and 0.08 v.u. for (II). In both cases, a twofold interaction symmetry is evidenced [real in (I), pseudo in (II)].

The ligands of (I) look normal, without unprecedented features. Both are planar to within experimental error [maximum deviations from the least-squares planes are 0.07 (1) Å for C3/C3ⁱ in biq and 0.02 (1) Å for N2 in nit; symmetry code: (i) 1 − x, y, −z + 3/2]. The biq unit is in fact made up of two lateral hemi-planes (maximum deviation 0.02 Å for C3/C3A), rotated with respect to each other by 3.1 (1)° around the central C9—C9ⁱ axis; the coordination planes make dihedral angles with each other of 60.5 (1)° (biq–nit) and 83.4 (1)° (nit–nit). N—O distances in the nit group are compatible with double bonds at O2 and O3, and a single bond at O1 (Table 1).

In the absence of strong hydrogen bonds, there are a number of second-order interactions in the structure of (I) which play definite roles (in spite of their intrinsic weakness), both in the molecular geometry and in the packing disposition. Full details are presented in Tables 2 and 3, and Fig. 1 [Should this be Fig. 4? No dashed lines in Fig. 1] shows their contribution to the molecular shape. Single broken lines indicate the intramolecular C—H···O bond connecting a biq aromatic H atom with nit atom O2 (first entry in Table 2), pulling the ligand towards the cation and partly contributing to the large asymmetry in the coordination of the anion.

As far as the packing is concerned, the (weak) intermolecular interactions are of two different types. Firstly, π–π bonds link the lateral wings of the biq groups (Fig. 3 and Table 3), thus giving rise to simple π-connected chains of monomers evolving along [101]. Secondly, a couple of non-classical C—H···O hydrogen bonds (second and third entries in Table 2) link these chains together into a weakly connected three-dimensional network (Fig. 4).

As expected from their molecular similarities, almost all of these types of interactions in (I) are also present in (II), though with different strengths and structural consequences. For example, the two intramolecular hydrogen bonds are equivalent by symmetry in (I), but are independent and midway in strength in (II), i.e. one is shorter and one longer than that in (I), leading to a slightly different deformation of the polyhedron (Fig. 2). On the other hand, π–π contacts between aromatic rings, though also present, are definitely second-order interactions compared with the C—H···N bonds, which are unique to (II) due to the outermost N atoms not being present in (I). These interactions define molecular ribbons which are linked via the π–π stacking, and this fact, and the presence of a solvent acetonitrile molecule, not present in the unsolvated (I), define a quite different packing assembly.

S2. Experimental

The title compound was unexpectedly generated in a hydrothermal bath, set up for the synthesis of a copper–vanadium hybrid complex. The synthesis also included phosphate, but the principal crystalline component in the multiphase outcome proved to be the title compound, (I), in a good yield of irregular light-brown crystals, suitable for single-crystal X-ray diffraction. For the synthesis, a mixture of Cu(NO₃)₂·3H₂O (1.375 mmol), V₂O₅ (0.5 mmol), 2,2-biquinoline (1.0 mmol) and H₃PO₄ (5 ml, 7.4 mmol) was sealed in a Teflon-lined acid digestion bomb and heated at 390 K for 3 d under autogenous pressure (pH = 2.5), followed by slow cooling at 20 K h⁻¹ to room temperature.

S3. Refinement

H atoms were placed in calculated positions, with C—H = 0.93 Å, and allowed to ride. They were assigned an isotropic displacement parameter U_iso(H) = 1.2U_eq(C)

Computing details

Data collection: SMART-NT (Bruker, 2001); cell refinement: SAINT-NT (Bruker, 2000); data reduction: SAINT-NT (Bruker, 2000); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL-NT (Sheldrick, 2000), ORTEPIII (Burnett & Johnson, 1996) and ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXTL-NT (Sheldrick, 2000) and PLATON (Spek, 2003).

Figures

	Fig. 1. A molecular view of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% level and H atoms are represented as small spheres of arbitrary radii. [Symmetry code: (i) −x + 1, y, −z + 3/2]
	Fig. 2. Schematic comparison between (I) (solid lines) and (II) (dashed lines).
	Fig. 3. Overlapping view of two neighbouring molecules. Double-dashed lines represent π–π bonds relating centrosymmetrically related units. Cg are the ring centroids.
	Fig. 4. A packing view of (I) down [101], showing the network of weak hydrogen bonds (dashed lines) around the reference molecule (bold solid lines) in the centre of the figure. [Symmetry codes: (i) 1 − x, y, 3/2 − z; (ii) x − 1/2, y − 1/2, z; (iii) 1 − x, y − 1, 3/2 − z.]

(2,2'-Biquinoline-κ²N,N')bis(nitrato-κ²O,O')copper(II)

Crystal data

[Cu(NO3)2(C18H12N2)]	F(000) = 900
Mr = 443.86	Dx = 1.700 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 4568 reflections
a = 19.310 (8) Å	θ = 3.4–26.1°
b = 8.342 (4) Å	µ = 1.31 mm^−1
c = 13.160 (6) Å	T = 291 K
β = 125.084 (5)°	Block, light brown
V = 1734.7 (14) Å^3	0.45 × 0.25 × 0.20 mm
Z = 4

Data collection

Bruker SMART CCD area-detector diffractometer	1914 independent reflections
Radiation source: fine-focus sealed tube	1125 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.061
ϕ and ω scans	θmax = 27.9°, θmin = 2.6°
Absorption correction: multi-scan (SADABS; Sheldrick, 2001)	h = −24→25
Tmin = 0.66, Tmax = 0.77	k = −10→10
6551 measured reflections	l = −17→17

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.056	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.153	H-atom parameters constrained
S = 1.11	w = 1/[σ^2(Fo^2) + (0.0651P)^2] where P = (Fo^2 + 2Fc^2)/3
1914 reflections	(Δ/σ)max < 0.001
132 parameters	Δρmax = 0.35 e Å^−3
0 restraints	Δρmin = −0.30 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cu1	0.5000	0.50876 (7)	0.7500	0.0662 (3)
N1	0.4215 (2)	0.3341 (3)	0.7158 (3)	0.0529 (8)
C1	0.3386 (3)	0.3477 (5)	0.6759 (4)	0.0593 (11)
C2	0.3017 (3)	0.4988 (5)	0.6536 (4)	0.0650 (11)
H2	0.3335	0.5903	0.6672	0.078*
C3	0.2188 (3)	0.5119 (6)	0.6118 (4)	0.0756 (14)
H3	0.1942	0.6130	0.5959	0.091*
C4	0.1705 (3)	0.3766 (8)	0.5925 (4)	0.0910 (18)
H4	0.1145	0.3880	0.5660	0.109*
C5	0.2044 (4)	0.2292 (8)	0.6123 (4)	0.0897 (18)
H5	0.1714	0.1395	0.5984	0.108*
C6	0.2901 (3)	0.2093 (6)	0.6541 (4)	0.0716 (13)
C7	0.3288 (4)	0.0615 (6)	0.6749 (4)	0.0841 (16)
H7	0.2983	−0.0314	0.6628	0.101*
C8	0.4103 (4)	0.0497 (5)	0.7127 (4)	0.0824 (16)
H8	0.4358	−0.0500	0.7263	0.099*
C9	0.4555 (3)	0.1906 (4)	0.7309 (4)	0.0607 (11)
N2	0.4674 (2)	0.6899 (4)	0.5622 (3)	0.0684 (10)
O3	0.4669 (2)	0.7863 (5)	0.4936 (3)	0.1085 (13)
O1	0.4056 (2)	0.6190 (4)	0.5398 (3)	0.1082 (13)
O2	0.5320 (2)	0.6714 (4)	0.6719 (3)	0.1061 (13)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cu1	0.0522 (5)	0.0400 (4)	0.0939 (6)	0.000	0.0347 (4)	0.000
N1	0.066 (2)	0.0433 (16)	0.0486 (19)	−0.0085 (15)	0.0321 (18)	−0.0048 (14)
C1	0.061 (3)	0.069 (3)	0.049 (2)	−0.022 (2)	0.032 (2)	−0.0093 (19)
C2	0.055 (3)	0.073 (3)	0.059 (3)	−0.010 (2)	0.029 (2)	−0.006 (2)
C3	0.051 (3)	0.112 (4)	0.061 (3)	−0.007 (3)	0.030 (2)	−0.015 (3)
C4	0.071 (3)	0.151 (5)	0.058 (3)	−0.039 (4)	0.041 (3)	−0.019 (4)
C5	0.082 (4)	0.131 (5)	0.059 (3)	−0.060 (4)	0.042 (3)	−0.024 (3)
C6	0.092 (4)	0.078 (3)	0.053 (3)	−0.038 (3)	0.047 (3)	−0.017 (2)
C7	0.123 (5)	0.070 (3)	0.058 (3)	−0.052 (3)	0.052 (3)	−0.018 (2)
C8	0.129 (5)	0.047 (2)	0.068 (3)	−0.025 (3)	0.054 (4)	−0.010 (2)
C9	0.090 (3)	0.0429 (19)	0.044 (2)	−0.0085 (19)	0.035 (2)	−0.0038 (18)
N2	0.057 (2)	0.076 (2)	0.061 (2)	−0.002 (2)	0.027 (2)	0.015 (2)
O3	0.095 (3)	0.122 (3)	0.101 (3)	0.000 (2)	0.053 (2)	0.051 (2)
O1	0.074 (2)	0.119 (3)	0.097 (3)	−0.034 (2)	0.029 (2)	0.023 (2)
O2	0.074 (2)	0.077 (2)	0.096 (3)	−0.0177 (18)	0.008 (2)	0.033 (2)

Geometric parameters (Å, º)

Cu1—N1	1.959 (3)	C4—C5	1.346 (7)
Cu1—N1^i	1.959 (3)	C4—H4	0.9300
Cu1—O2^i	2.002 (3)	C5—C6	1.422 (7)
Cu1—O2	2.002 (4)	C5—H5	0.9300
Cu1—O1^i	2.455 (4)	C6—C7	1.385 (7)
Cu1—O1	2.455 (4)	C7—C8	1.354 (8)
N1—C9	1.324 (4)	C7—H7	0.9300
N1—C1	1.374 (5)	C8—C9	1.399 (5)
C1—C2	1.393 (5)	C8—H8	0.9300
C1—C6	1.408 (5)	C9—C9^i	1.489 (9)
C2—C3	1.366 (7)	N2—O3	1.205 (4)
C2—H2	0.9300	N2—O1	1.206 (4)
C3—C4	1.393 (7)	N2—O2	1.262 (4)
C3—H3	0.9300
N1—Cu1—N1^i	83.9 (2)	C5—C4—C3	120.3 (5)
N1—Cu1—O2^i	101.32 (16)	C5—C4—H4	119.9
N1^i—Cu1—O2^i	144.31 (14)	C3—C4—H4	119.9
N1—Cu1—O2	144.31 (14)	C4—C5—C6	120.7 (5)
N1^i—Cu1—O2	101.32 (16)	C4—C5—H5	119.7
O2^i—Cu1—O2	94.7 (3)	C6—C5—H5	119.7
N1—Cu1—O1^i	120.84 (13)	C7—C6—C1	118.0 (5)
N1^i—Cu1—O1^i	92.56 (13)	C7—C6—C5	123.8 (5)
O2^i—Cu1—O1^i	54.23 (13)	C1—C6—C5	118.2 (5)
O2—Cu1—O1^i	94.40 (16)	C8—C7—C6	121.2 (4)
N1—Cu1—O1	92.56 (13)	C8—C7—H7	119.4
N1^i—Cu1—O1	120.84 (13)	C6—C7—H7	119.4
O2^i—Cu1—O1	94.40 (16)	C7—C8—C9	118.7 (5)
O2—Cu1—O1	54.23 (13)	C7—C8—H8	120.7
O1^i—Cu1—O1	136.00 (18)	C9—C8—H8	120.7
C9—N1—C1	120.0 (3)	N1—C9—C8	121.8 (4)
C9—N1—Cu1	112.8 (3)	N1—C9—C9^i	115.3 (2)
C1—N1—Cu1	127.2 (3)	C8—C9—C9^i	122.9 (3)
N1—C1—C2	119.9 (3)	O3—N2—O1	124.3 (4)
N1—C1—C6	120.2 (4)	O3—N2—O2	121.0 (4)
C2—C1—C6	119.9 (4)	O1—N2—O2	114.0 (4)
C3—C2—C1	119.8 (4)	O3—N2—Cu1	167.5 (3)
C3—C2—H2	120.1	O1—N2—Cu1	67.6 (2)
C1—C2—H2	120.1	O2—N2—Cu1	46.6 (2)
C2—C3—C4	121.1 (5)	N2—O1—Cu1	85.4 (3)
C2—C3—H3	119.5	N2—O2—Cu1	106.1 (3)
C4—C3—H3	119.5
N1^i—Cu1—N1—C9	0.2 (2)	N1^i—Cu1—C9—C8	177.8 (11)
O2^i—Cu1—N1—C9	−144.0 (3)	O2^i—Cu1—C9—C8	40.2 (11)
O2—Cu1—N1—C9	101.1 (3)	O2—Cu1—C9—C8	−116.8 (10)
O1^i—Cu1—N1—C9	−89.1 (3)	O1^i—Cu1—C9—C8	103.1 (10)
O1—Cu1—N1—C9	120.9 (3)	O1—Cu1—C9—C8	−65.2 (10)
N1^i—Cu1—N1—C1	−177.5 (4)	N1—Cu1—C9—C9^i	179.5 (5)
O2^i—Cu1—N1—C1	38.2 (3)	N1^i—Cu1—C9—C9^i	−0.3 (3)
O2—Cu1—N1—C1	−76.7 (4)	O2^i—Cu1—C9—C9^i	−137.8 (3)
O1^i—Cu1—N1—C1	93.1 (3)	O2—Cu1—C9—C9^i	65.2 (4)
O1—Cu1—N1—C1	−56.8 (3)	O1^i—Cu1—C9—C9^i	−75.0 (3)
C9—N1—C1—C2	−176.5 (4)	O1—Cu1—C9—C9^i	116.7 (3)
Cu1—N1—C1—C2	1.1 (5)	N1—Cu1—N2—O3	−176.9 (15)
C9—N1—C1—C6	1.7 (6)	N1^i—Cu1—N2—O3	−81.9 (15)
Cu1—N1—C1—C6	179.3 (3)	O2^i—Cu1—N2—O3	76.4 (15)
N1—C1—C2—C3	178.8 (4)	O2—Cu1—N2—O3	−11.0 (14)
C6—C1—C2—C3	0.6 (6)	O1^i—Cu1—N2—O3	23.6 (16)
C1—C2—C3—C4	0.9 (7)	O1—Cu1—N2—O3	163.2 (17)
C2—C3—C4—C5	−1.6 (7)	N1—Cu1—N2—O1	19.8 (3)
C3—C4—C5—C6	0.6 (7)	N1^i—Cu1—N2—O1	114.8 (3)
N1—C1—C6—C7	0.4 (6)	O2^i—Cu1—N2—O1	−86.8 (3)
C2—C1—C6—C7	178.6 (4)	O2—Cu1—N2—O1	−174.2 (5)
N1—C1—C6—C5	−179.7 (4)	O1^i—Cu1—N2—O1	−139.7 (2)
C2—C1—C6—C5	−1.5 (6)	N1—Cu1—N2—O2	−166.0 (3)
C4—C5—C6—C7	−179.3 (4)	N1^i—Cu1—N2—O2	−70.9 (4)
C4—C5—C6—C1	0.9 (7)	O2^i—Cu1—N2—O2	87.4 (4)
C1—C6—C7—C8	−1.3 (7)	O1^i—Cu1—N2—O2	34.6 (3)
C5—C6—C7—C8	178.8 (4)	O1—Cu1—N2—O2	174.2 (5)
C6—C7—C8—C9	0.2 (7)	O3—N2—O1—Cu1	−175.7 (4)
C1—N1—C9—C8	−2.9 (6)	O2—N2—O1—Cu1	−4.6 (4)
Cu1—N1—C9—C8	179.1 (3)	N1—Cu1—O1—N2	−162.6 (3)
C1—N1—C9—C9^i	177.4 (4)	N1^i—Cu1—O1—N2	−78.2 (3)
Cu1—N1—C9—C9^i	−0.5 (5)	O2^i—Cu1—O1—N2	95.8 (3)
C1—N1—C9—Cu1	177.9 (5)	O2—Cu1—O1—N2	3.3 (3)
C7—C8—C9—N1	2.0 (7)	O1^i—Cu1—O1—N2	55.7 (2)
C7—C8—C9—C9^i	−178.3 (5)	O3—N2—O2—Cu1	177.2 (4)
C7—C8—C9—Cu1	3.9 (13)	O1—N2—O2—Cu1	5.9 (5)
N1^i—Cu1—C9—N1	−179.8 (2)	N1—Cu1—O2—N2	21.5 (5)
O2^i—Cu1—C9—N1	42.7 (3)	N1^i—Cu1—O2—N2	116.8 (3)
O2—Cu1—C9—N1	−114.3 (3)	O2^i—Cu1—O2—N2	−95.3 (3)
O1^i—Cu1—C9—N1	105.5 (3)	O1^i—Cu1—O2—N2	−149.7 (3)
O1—Cu1—C9—N1	−62.8 (3)	O1—Cu1—O2—N2	−3.2 (3)
N1—Cu1—C9—C8	−2.5 (9)

Symmetry code: (i) −x+1, y, −z+3/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C2—H2···O2^i	0.93	2.32	3.030 (7)	133
C4—H4···O3^ii	0.93	2.57	3.440 (8)	155
C8—H8···O2^iii	0.93	2.57	3.400 (6)	148

Symmetry codes: (i) −x+1, y, −z+3/2; (ii) x−1/2, y−1/2, z; (iii) −x+1, y−1, −z+3/2.