Intra­molecular π–π stacking in diaqua­bis(2-hydroxy­benzoato-κO)bis­(1,10-phenanthroline-κ²N,N′)strontium(II)

In the title compound, [Sr(C₇H₅O₃)₂(C₁₂H₈N₂)₂(H₂O)₂], the Sr^II ion is located on a twofold rotation axis and assumes a distorted square-anti­prism SrN₄O₄ coordination geometry, formed by two phenanthroline (phen) ligands, two 2-hydroxy­benzoate anions and two water mol­ecules. Within the mononuclear complex mol­ecule, intra­molecular π–π stacking is observed between nearly parallel coordinated phen ligands, while normal inter­molecular π–π stacking occurs between parallel phen ligands of adjacent complex mol­ecules. Classic O—H...O and weak C—H...O hydrogen bonding helps to stabilize the crystal structure.

S1. Comment

As π–π stacking between aromatic rings plays an important role in electron-transfer processes in some biological systems (Deisenhofer & Michel, 1989), it has attracted our attention in recent years. Because π–π stacking is usually observed between adjacent molecules, it is considered as a type of intermolecular interaction. In order to understand the nature of π–π stacking, a series of metal complexes incorporating aromatic ligands such as phenanthroline (phen) (Nie et al., 2001), benzimidazole (Chen et al., 2003), bipyridine (Xu et al., 1996), diaminobithiazole (Luo et al., 2004) and substituted benzoate (Cheng et al., 2000) has been prepared and their crystal structures have been determined in our laboratory. As part of our ongoing investigation into the nature of π–π stacking, the title Sr^II complex, (I), has recently been prepared, and its X-ray crystal structure, which shows unusual intramolecular π–π stacking, is presented here.

The molecular structure of (I) is shown in Fig. 1. The Sr^II ion is located on a twofold rotation axis and coordinated by two phen ligands, two 2-hydroxybenzoate anions and two water molecules in a distorted square-antiprism coordination geometry. Coordinated bond distances and angles are listed in Table 1. In order to compare the coordination geometry, a search of the Cambridge Structure Database (Version ?, updated in November 2006; Allen, 2002) was performed. Table 2 summarizes the Sr—N bond distances in (I) and the maximum and minimum Sr—N bond distances in the Sr^II complexes with a phen ligand reported previously. The atomic deviations of the Sr^II ion from the chelating phen ligands are also listed in Table 2 in order to compare coplanarities. These values show that in all these Sr^II complexes the Sr^II ion is nearly coplanar with the phen mean plane. In (I), however, the Sr^II ion deviates from the phen mean planes by 1.374 (3) Å, the chelating phen ligand tilting with respect to the Sr1/N1/N2 plane by a larger dihedral angle of 33.49 (7)°. This clearly indicates the poor overlap between the atomic orbitals of the Sr^II ion and phen N atoms in the structure. However, the average Sr—N bond of 2.795 (3) Å is similar to those found in related Sr^II complexes (Table 2). These facts suggest a more electrostatic nature of Sr—N bonds in (I).

The two phen ligands, which are related by the twofold rotation axis, are nearly parallel to each other, the dihedral angle being 10.47 (2)°. The partially overlapped arrangement between these two nearly parallel phen ligands is shown in Fig. 2. Atoms C11ⁱ, C14ⁱ, C16ⁱ, C17ⁱ and C19ⁱ of the N1ⁱ-containing phen ligand [symmetry code: (i) 1 - x, y, 3/2 - z] are displaced from the N1-phen mean plane by 3.470 (4), 3.537 (4), 3.413 (5), 3.139 (5) and 3.250 (4) Å, respectively, with an average value of 3.362 (5) Å, appreciably shorter than the van der Waals thickness of an aromatic ring (3.70 Å; Cotton & Wilkinson, 1972). A PLATON calculation (Spek, 2003) shows a short Cg···Cg separation of 3.511 (2) Å between the two phen ligands, involving the N1-pyridine and the C12ⁱ-benzene rings. These facts suggest the existence of intramolecular π–π stacking between the coordinated phen ligands within the mononuclear complex. As expected, intermolecular π–π stacking is also observed between parallel phen ligands of adjacent molecules of the complex (Fig. 3). The face-to-face separation between parallel N1-phen and N1ⁱⁱ-phen is 3.422 (8) Å [symmetry code: (ii) 1 - x, -y, 1 - z].

This structure determination reveals that π–π stacking interactions can be not only intermolecular but also intramolecular in nature, somehow correlating to their hydrogen-bonding counterparts. Classic O—H···O and weak C—H···O hydrogen bonds also occur in the crystal structure of (I) (Table 3). Intramolecular O—H···O hydrogen bonds form internal loops (Fig. 1). Intermolecular O—H···O and weak C—H···O hydrogen bonding, in turn, helps to stabilize the crystal structure.

Related literature

For related literature, see: Allen (2002); Chen et al. (2003); Cheng et al. (2000); Cotton & Wilkinson (1972); Deisenhofer & Michel (1989); Luo et al. (2004); Nie et al. (2001); Spek (2003); Xu et al. (1996).

S2. Experimental

A water–ethanol solution (20 ml, 1:1 v/v) containing SrCO₃ (0.15 g, 1 mmol), 2-hydroxybenzoic acid (0.27 g, 2 mmol) and Na₂CO₃ (0.05 g, 0.5 mmol) was refluxed for 0.5 h. Then phen (0.20 g, 1 mmol) was added to the solution and the mixture was refluxed for a further 4 h. After cooling to room temperature, the solution was filtered and the filtrate was kept at room temperature. Single crystals of (I) were obtained from the filtrate after 3 d.

S3. Refinement

Water H atoms and hydroxy H atoms were located in a difference Fourier map and refined as riding in their as-found relative positions, with U_iso(H) = 1.5U_eq(O). Water atom H4B is disordered over two sites (H4B1 and H4B2) and their occupancies were set at 0.5. Aromatic H atoms were placed in calculated positions, with C—H = 0.93 Å, and refined in riding mode, with U_iso(H) = 1.2U_eq(C).

Computing details

Data collection: PROCESS-AUTO (Rigaku, 1998); cell refinement: PROCESS-AUTO (Rigaku, 1998); data reduction: CrystalStructure (Rigaku/MSC, 2002); program(s) used to solve structure: SIR92 (Altomare et al., 1993); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

	Fig. 1. The molecular structure of (I), with the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. One disordered water H atom has been omitted for clarity; the remaining H atoms are shown as small spheres of arbitrary radii. Dashed lines indicate hydrogen bonding. [symmetry code: (i) 1 - x,y,3/2 - z].
	Fig. 2. A diagram showing intra-molecular π···π stacking between N1-phen and N1^i-phen [symmetry code: (i) 1 - x,y,3/2 - z].
	Fig. 3. The unit cell packing diagram showing both intra-molecular and inter-molecular π-π stacking [symmetry code: (i) 1 - x,y,3/2 - z; (ii) 1 - x,-y,1 - z].

diaquabis(2-hydroxybenzoato-κO)bis(1,10-phenanthroline-κ²N,N')strontium(II)

Crystal data

[Sr(C7H5O3)2(C12H8N2)2(H2O)2]	F(000) = 1552
Mr = 758.28	Dx = 1.506 Mg m^−3
Orthorhombic, Pbcn	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -P 2n 2ab	Cell parameters from 16753 reflections
a = 23.411 (4) Å	θ = 2.1–24.2°
b = 10.409 (2) Å	µ = 1.67 mm^−1
c = 13.722 (3) Å	T = 295 K
V = 3343.8 (11) Å^3	Block, colourless
Z = 4	0.35 × 0.29 × 0.27 mm

Data collection

Rigaku R-AXIS RAPID diffractometer	3111 independent reflections
Radiation source: fine-focus sealed tube	2383 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.083
Detector resolution: 10.00 pixels mm^-1	θmax = 25.5°, θmin = 1.7°
ω scans	h = −28→28
Absorption correction: multi-scan (ABSCOR; Higashi, 1995)	k = −12→12
Tmin = 0.548, Tmax = 0.640	l = −15→16
25451 measured reflections

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)] = 0.054	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.113	H-atom parameters constrained
S = 1.18	w = 1/[σ^2(Fo^2) + (0.0331P)^2 + 3.9974P] where P = (Fo^2 + 2Fc^2)/3
3111 reflections	(Δ/σ)max < 0.001
231 parameters	Δρmax = 0.30 e Å^−3
0 restraints	Δρmin = −0.60 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.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Sr1	0.5000	0.32513 (4)	0.7500	0.04858 (18)
N1	0.56638 (12)	0.1605 (3)	0.6259 (2)	0.0408 (7)
N2	0.45108 (12)	0.1306 (3)	0.6475 (2)	0.0404 (7)
O1	0.59582 (12)	0.4250 (3)	0.7793 (2)	0.0595 (8)
O2	0.63015 (12)	0.5102 (3)	0.6429 (2)	0.0619 (8)
O3	0.70757 (12)	0.6744 (3)	0.6475 (2)	0.0635 (8)
H3	0.6739	0.6294	0.6234	0.095*
O4	0.47715 (12)	0.4445 (3)	0.9133 (2)	0.0695 (9)
H4A	0.4388	0.4614	0.9116	0.104*
H4B1	0.4980	0.4617	0.9582	0.104*	0.50
H4B2	0.4804	0.3805	0.9570	0.104*	0.50
C1	0.62949 (15)	0.4969 (3)	0.7347 (3)	0.0455 (9)
C2	0.67216 (14)	0.5724 (3)	0.7920 (3)	0.0392 (8)
C3	0.70825 (14)	0.6612 (3)	0.7462 (3)	0.0471 (9)
C4	0.74526 (16)	0.7365 (4)	0.8010 (4)	0.0600 (11)
H4	0.7686	0.7965	0.7703	0.072*
C5	0.74730 (18)	0.7222 (4)	0.8997 (4)	0.0653 (12)
H5	0.7722	0.7730	0.9358	0.078*
C6	0.71325 (17)	0.6340 (4)	0.9470 (3)	0.0598 (11)
H6	0.7155	0.6241	1.0142	0.072*
C7	0.67561 (15)	0.5606 (4)	0.8927 (3)	0.0478 (9)
H7	0.6521	0.5019	0.9243	0.057*
C8	0.62195 (15)	0.1738 (4)	0.6138 (3)	0.0478 (9)
H8	0.6372	0.2563	0.6158	0.057*
C9	0.65907 (16)	0.0719 (4)	0.5981 (3)	0.0525 (10)
H9	0.6980	0.0865	0.5901	0.063*
C10	0.63777 (17)	−0.0490 (4)	0.5945 (3)	0.0530 (10)
H10	0.6621	−0.1185	0.5847	0.064*
C11	0.57861 (16)	−0.0692 (3)	0.6057 (3)	0.0419 (8)
C12	0.55293 (19)	−0.1938 (4)	0.6034 (3)	0.0568 (11)
H12	0.5758	−0.2659	0.5947	0.068*
C13	0.4960 (2)	−0.2084 (3)	0.6138 (3)	0.0571 (11)
H13	0.4803	−0.2904	0.6122	0.069*
C14	0.45953 (16)	−0.1000 (3)	0.6273 (3)	0.0430 (9)
C15	0.39992 (18)	−0.1094 (4)	0.6365 (3)	0.0536 (11)
H15	0.3825	−0.1897	0.6348	0.064*
C16	0.36765 (16)	−0.0022 (4)	0.6479 (3)	0.0532 (10)
H16	0.3281	−0.0076	0.6515	0.064*
C17	0.39512 (15)	0.1152 (4)	0.6541 (3)	0.0480 (10)
H17	0.3728	0.1881	0.6634	0.058*
C18	0.48331 (14)	0.0243 (3)	0.6326 (2)	0.0348 (8)
C19	0.54446 (15)	0.0399 (3)	0.6210 (2)	0.0346 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sr1	0.0414 (3)	0.0261 (2)	0.0783 (4)	0.000	−0.0193 (3)	0.000
N1	0.0419 (16)	0.0403 (17)	0.0403 (17)	−0.0054 (14)	0.0011 (13)	−0.0064 (14)
N2	0.0383 (16)	0.0435 (18)	0.0394 (17)	−0.0017 (14)	−0.0046 (14)	−0.0064 (14)
O1	0.0577 (16)	0.0526 (17)	0.068 (2)	−0.0187 (14)	−0.0128 (14)	0.0050 (14)
O2	0.0714 (19)	0.0590 (18)	0.0554 (18)	−0.0084 (15)	−0.0113 (15)	−0.0065 (15)
O3	0.0546 (16)	0.0676 (19)	0.068 (2)	0.0007 (15)	0.0070 (15)	0.0124 (17)
O4	0.0631 (17)	0.095 (2)	0.0500 (17)	0.0018 (17)	−0.0160 (14)	−0.0190 (17)
C1	0.0460 (19)	0.0344 (18)	0.056 (3)	0.0026 (16)	−0.0040 (19)	−0.0015 (19)
C2	0.0324 (17)	0.0325 (18)	0.053 (2)	0.0066 (15)	−0.0033 (16)	−0.0055 (17)
C3	0.0364 (17)	0.042 (2)	0.063 (2)	0.0061 (15)	0.005 (2)	−0.001 (2)
C4	0.034 (2)	0.053 (3)	0.093 (3)	−0.0101 (18)	0.004 (2)	−0.007 (2)
C5	0.046 (2)	0.065 (3)	0.085 (3)	−0.003 (2)	−0.012 (3)	−0.028 (3)
C6	0.048 (2)	0.073 (3)	0.059 (3)	0.010 (2)	−0.007 (2)	−0.015 (2)
C7	0.0381 (19)	0.048 (2)	0.058 (2)	0.0048 (17)	−0.0002 (18)	−0.004 (2)
C8	0.043 (2)	0.050 (2)	0.050 (2)	−0.0053 (18)	0.0010 (17)	−0.004 (2)
C9	0.040 (2)	0.066 (3)	0.051 (2)	0.001 (2)	0.0006 (19)	−0.003 (2)
C10	0.055 (2)	0.056 (3)	0.048 (2)	0.017 (2)	0.0023 (19)	−0.002 (2)
C11	0.052 (2)	0.041 (2)	0.0328 (18)	0.0069 (17)	−0.0004 (17)	0.0006 (16)
C12	0.071 (3)	0.034 (2)	0.065 (3)	0.010 (2)	0.000 (2)	0.001 (2)
C13	0.083 (3)	0.0336 (19)	0.055 (2)	−0.011 (2)	−0.006 (2)	0.0012 (17)
C14	0.057 (2)	0.040 (2)	0.0320 (19)	−0.0074 (18)	−0.0038 (17)	−0.0025 (16)
C15	0.060 (2)	0.054 (3)	0.046 (2)	−0.026 (2)	−0.006 (2)	0.001 (2)
C16	0.040 (2)	0.069 (3)	0.050 (2)	−0.013 (2)	−0.0054 (18)	−0.005 (2)
C17	0.040 (2)	0.060 (3)	0.044 (2)	−0.0028 (19)	−0.0078 (17)	−0.009 (2)
C18	0.0420 (19)	0.0364 (19)	0.0261 (17)	−0.0018 (15)	−0.0062 (14)	−0.0033 (15)
C19	0.0433 (18)	0.0327 (18)	0.0277 (17)	−0.0033 (16)	0.0004 (15)	−0.0016 (15)

Geometric parameters (Å, º)

Sr1—O1	2.505 (3)	C5—C6	1.378 (6)
Sr1—O1^i	2.505 (3)	C5—H5	0.9300
Sr1—O4	2.618 (3)	C6—C7	1.384 (5)
Sr1—O4^i	2.618 (3)	C6—H6	0.9300
Sr1—N1^i	2.872 (3)	C7—H7	0.9300
Sr1—N1	2.872 (3)	C8—C9	1.388 (5)
Sr1—N2	2.718 (3)	C8—H8	0.9300
Sr1—N2^i	2.718 (3)	C9—C10	1.355 (5)
Sr1—H4B2	2.9341	C9—H9	0.9300
N1—C8	1.319 (4)	C10—C11	1.409 (5)
N1—C19	1.358 (4)	C10—H10	0.9300
N2—C17	1.323 (4)	C11—C19	1.404 (5)
N2—C18	1.354 (4)	C11—C12	1.430 (5)
O1—C1	1.247 (4)	C12—C13	1.348 (6)
O2—C1	1.267 (5)	C12—H12	0.9300
O3—C3	1.362 (5)	C13—C14	1.428 (5)
O3—H3	0.9735	C13—H13	0.9300
O4—H4A	0.9149	C14—C15	1.405 (5)
O4—H4B1	0.8069	C14—C18	1.410 (5)
O4—H4B2	0.8998	C15—C16	1.357 (6)
C1—C2	1.495 (5)	C15—H15	0.9300
C2—C7	1.389 (5)	C16—C17	1.384 (5)
C2—C3	1.401 (5)	C16—H16	0.9300
C3—C4	1.388 (6)	C17—H17	0.9300
C4—C5	1.364 (6)	C18—C19	1.449 (5)
C4—H4	0.9300
O1—Sr1—O4	81.30 (9)	C7—C2—C1	121.0 (3)
O1—Sr1—N1	81.84 (9)	C3—C2—C1	121.0 (4)
O1—Sr1—N2	140.30 (9)	O3—C3—C4	119.2 (4)
O1—Sr1—O1^i	130.96 (13)	O3—C3—C2	120.4 (3)
O1—Sr1—O4^i	75.94 (9)	C4—C3—C2	120.4 (4)
O1—Sr1—N1^i	129.58 (9)	C5—C4—C3	119.8 (4)
O1—Sr1—N2^i	81.31 (9)	C5—C4—H4	120.1
O1^i—Sr1—O4	75.94 (9)	C3—C4—H4	120.1
O1^i—Sr1—O4^i	81.30 (9)	C4—C5—C6	121.4 (4)
O1^i—Sr1—N2	81.31 (9)	C4—C5—H5	119.3
O1^i—Sr1—N2^i	140.30 (9)	C6—C5—H5	119.3
O1^i—Sr1—N1^i	81.84 (9)	C5—C6—C7	118.8 (4)
O1^i—Sr1—N1	129.58 (9)	C5—C6—H6	120.6
O4—Sr1—N1	154.32 (9)	C7—C6—H6	120.6
O4—Sr1—N2	135.26 (9)	C6—C7—C2	121.6 (4)
O4—Sr1—O4^i	123.31 (15)	C6—C7—H7	119.2
O4—Sr1—N1^i	70.45 (9)	C2—C7—H7	119.2
O4—Sr1—N2^i	89.83 (9)	N1—C8—C9	123.8 (4)
O4^i—Sr1—N1^i	154.32 (9)	N1—C8—H8	118.1
O4^i—Sr1—N2	89.83 (9)	C9—C8—H8	118.1
O4^i—Sr1—N2^i	135.26 (9)	C10—C9—C8	119.0 (4)
O4^i—Sr1—N1	70.45 (9)	C10—C9—H9	120.5
N1—Sr1—N2	58.46 (9)	C8—C9—H9	120.5
N1—Sr1—N1^i	106.74 (12)	C9—C10—C11	119.7 (4)
N2—Sr1—N1^i	68.54 (9)	C9—C10—H10	120.1
N2—Sr1—N2^i	83.68 (13)	C11—C10—H10	120.1
N2^i—Sr1—N1^i	58.46 (9)	C19—C11—C10	117.1 (3)
N2^i—Sr1—N1	68.54 (9)	C19—C11—C12	119.8 (3)
O1—Sr1—H4B2	84.4	C10—C11—C12	123.1 (4)
O1^i—Sr1—H4B2	86.2	C13—C12—C11	121.0 (4)
O4—Sr1—H4B2	17.5	C13—C12—H12	119.5
O4^i—Sr1—H4B2	140.1	C11—C12—H12	119.5
N2—Sr1—H4B2	125.5	C12—C13—C14	121.1 (3)
N2^i—Sr1—H4B2	73.2	C12—C13—H13	119.5
N1^i—Sr1—H4B2	57.2	C14—C13—H13	119.5
N1—Sr1—H4B2	140.8	C15—C14—C18	116.9 (4)
C8—N1—C19	117.6 (3)	C15—C14—C13	123.4 (4)
C8—N1—Sr1	123.1 (2)	C18—C14—C13	119.7 (3)
C19—N1—Sr1	112.1 (2)	C16—C15—C14	120.4 (4)
C17—N2—C18	117.6 (3)	C16—C15—H15	119.8
C17—N2—Sr1	118.2 (2)	C14—C15—H15	119.8
C18—N2—Sr1	116.9 (2)	C15—C16—C17	118.4 (3)
C1—O1—Sr1	137.5 (3)	C15—C16—H16	120.8
C3—O3—H3	107.4	C17—C16—H16	120.8
Sr1—O4—H4A	105.6	N2—C17—C16	124.2 (4)
Sr1—O4—H4B1	129.5	N2—C17—H17	117.9
H4A—O4—H4B1	124.9	C16—C17—H17	117.9
Sr1—O4—H4B2	101.6	N2—C18—C14	122.5 (3)
H4A—O4—H4B2	104.1	N2—C18—C19	118.4 (3)
H4B1—O4—H4B2	66.6	C14—C18—C19	119.1 (3)
O1—C1—O2	124.1 (4)	N1—C19—C11	122.7 (3)
O1—C1—C2	118.7 (4)	N1—C19—C18	118.1 (3)
O2—C1—C2	117.2 (3)	C11—C19—C18	119.2 (3)
C7—C2—C3	118.0 (4)

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
O3—H3···O2	0.97	1.63	2.492 (4)	145
O4—H4A···O2^i	0.92	1.85	2.715 (4)	157
O4—H4B1···O4^ii	0.81	2.10	2.853 (4)	156
C9—H9···O3^iii	0.93	2.52	3.368 (5)	152
C10—H10···O3^iv	0.93	2.55	3.389 (5)	150

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