A new coordination mode of 6-methyl­nicotinic acid in trans-tetra­aquabis­(6-methyl­pyridine-3-carboxyl­ato-κO)cobalt(II) tetra­hydrate

The title compound, [Co(C₇H₆NO₂)₂(H₂O)₄]·4H₂O, contains a Co^II ion lying on a crystallographic inversion centre. The Co^II ion is octa­hedrally coordinated by two 6-methyl­pyridine-3-carboxyl­ate ligands in axial positions [Co—O = 2.0621 (9) Å] and by four water mol­ecules in the equatorial plane [Co—O = 2.1169 (9) and 2.1223 (11) Å]. There are also four uncoordinated water mol­ecules. The 6-methyl­pyridine-3-carboxyl­ate ligands are bound to the Co^II ion in a monodentate manner through a carboxyl­ate O atom. There is one strong intra­molecular O—H...O hydrogen bond, and six strong inter­molecular hydrogen bonds of type O—H...O and one of type O—H...N in the packing, resulting in a complex three-dimensional supra­molecular structure.

S1. Comment

As a part of our wider research programme, we have recently studied (Popović et al., 2006; Kukovec et al., 2006; Kukovec, Popović, Pavlović & Rajić Linarić, 2007) the coordination modes of N,O-chelating ligands such as 3- and 6-hydroxypicolinic acids with late 3d-block metals, namely cobalt(II) and nickel(II). Complexes of these ligands with many transition metals have been extensively investigated owing to their biological significance and their great coordination potential.

In order to develop this structural diversity, we have extended our investigations to 3- and 6-methyl-substituted picolinic acid (Kukovec, Popović, Pavlović, Calhorda & Vaz, 2007). There are only a few such complexes reported in the literature, for example, complexes of zinc (Yoshikawa et al., 2002; Pons et al., 2004), cadmium (Kukovec, Popović & Pavlović, 2007), mercury(II) (González-Duarte et al., 1998), cobalt(II) (March et al., 2003) and silver (Leiva et al., 1999), and a heteroleptic binuclear complex containing Cr^III and Sn^IV (Xu et al., 2006). Consequently, we began to study more systematically the structural features of transition metal complexes derived from 6-methylnicotinic acid as there are few structural data on complexes of nicotinic acid derivatives containing electron-donating substituents. To the best of our knowledge, only two such complexes, with copper(I), have been reported (Aakeröy et al., 2000).

Compound (I) contains a Co^II ion lying on a crystallographic inversion centre. The Co^II ion is octahedrally coordinated by two 6-methylpyridine-3- carboxylate ligands in the apical positions and by four water molecules in the equatorial plane (Table 1 and Fig. 1). The bond angles around the Co centre lie within the range of 87–93° for the formally cis pairs of ligating atoms (Table 1). There are also four non-coordinated water molecules in (I). The 6-methylpyridine-3-carboxylate ligands are bound to the Co^II ion in a monodentate mode through a carboxylate O atom. This coordination mode has not previously been reported. There are only two polymeric copper(I) complexes reported hitherto containing 6-methylnicotinic acid bound to Cu^I through the pyridine N atom (Aakeröy et al., 2000). By contrast, several papers (Waizumi et al., 1998; Jia et al., 2002; Yu et al., 2006) report a cobalt(II) complex of nicotinic acid, [Co(nic)₂(H₂O)₄], where the nicotinate ligand is bound to cobalt(II) through the pyridine N atom.

The Co—O(water) bond distance found here corresponds to the values found in the literature (Waizumi et al., 1998; Jia et al., 2002; Yu et al., 2006). However, nicotinic acid N-oxide is bound monodentately through a carboxylate O atom to the Co^II ion, as with 6-methylnicotinic acid in (I), in the similar complex [Co(N-nicO)₂(H₂O)₄] (Knuuttila, 1983); the Co—O(carboxylate) bond distance in (I) is slightly shorter than the analogous bond in the latter complex [2.102 (1) Å; Knuuttila, 1983], while the Co—O(water) bond distances are slightly longer [2.072 (1) and 2.113 (2) Å, Knuuttila, 1983].

There are two polymeric cobalt(II) complexes with nicotinic acid, both with the carboxylate group bound in a bidentate manner to one Co^II ion (Yeh et al., 2004; Feng et al., 2006). One Co—O(carboxylate) bond distance in these complexes [2.054 (6) Å (Feng et al., 2006) and 2.059 (6) Å (Yeh et al., 2004)] corresponds to that in (I), while the other is significantly longer [2.441 (6) Å (Feng et al., 2006) and 2.291 (7) Å (Yeh et al., 2004)].

Two similar dinuclear cobalt(II) complexes (Ayyappan et al., 2001; Xu et al., 2007), containing a bridging water ligand, contain nicotinic acid bound in three different coordination modes, viz. monodentate through a carboxylate O atom or pyridine N atom, and bidentate through carboxylate O atoms bridging two Co^II ions. The Co—O(carboxylate) bond distance in (I) is similar to various Co—O bond distances [in the ranges 2.052 (4)–2.112 (4) Å (Ayyappan et al., 2001) and 2.035 (2)–2.136 (2) Å (Xu et al., 2007)] in these cobalt(II) complexes. The carboxylate group geometry is not affected by coordination of one carboxylate O atom to the Co^II ion since the O1—C6 and O2—C6 bond distances of the carboxylate group [1.2573 (14) and 1.2595 (15) Å] are very similar.

There is one strong intramolecular hydrogen bond of O—H···O type, formed by coordinated water molecule O3 atom and uncoordinated carboxylate atom O2, as well as six strong intermolecular hydrogen bonds of O—H···O type and one of O—H···N type in the packing of (I) (Table 2 and Fig. 2). Three of these are formed by coordinated water atoms O3 or O4 as donors and the uncoordinated water atoms O5 or O6 as acceptors; two of them are formed by water atoms O5 or O6 as donors and water atoms O3 or O2 as acceptors; and one of them is formed between water atom O6 and the pyridine N atom. There are also an intermolecular O—H···O hydrogen bond between the water atoms O5 and O6. Thus, each of atoms O3, O4, O5 and O6 acts as a double donor, while only atoms O5 and O6 act as a double proton acceptor. The uncoordinated carboxylate atom O2, acting as a double acceptor, is the only O atom in the complex that participates in the hydrogen-bond formation (Table 2). This complex hydrogen-bonding scheme links the molecular components into a three-dimensional supramolecular structure.

Related literature

For related literature, see: Aakeröy et al. (2000); Ayyappan et al. (2001); Feng et al. (2006); Jia et al. (2002); Knuuttila (1983); Kukovec et al. (2006); Kukovec, Popović & Pavlović (2007); Kukovec, Popović, Pavlović & Rajić Linarić (2007); Kukovec, Popović, Pavlović, Calhorda & Vaz (2007); Leiva et al. (1999); March et al. (2003); Pons et al. (2004); Popović et al. (2006); Waizumi et al. (1998); Xu et al. (2006, 2007); Yeh et al. (2004); Yoshikawa et al. (2002); Yu et al. (2006).

S2. Experimental

A solution of 6-methylnicotinic acid (0.10 g, 0.70 mmol) in 10 ml of water was added to a solution of cobalt(II) acetate tetrahydrate (0.09 g, 0.36 mmol) in 5 ml of water. The pH value of the resulting pink solution was 6, and concentrated ammonia solution was added dropwise until the pH of the solution was 7. The solution was left to stand for a month until a pink crystalline product, (I), was formed. It was filtered off, washed with a small portion of water and dried in air (yield 0.11 g, 64.7%). IR (KBr pellet, cm^-1): 3390 (s), 1604 (s), 1549 (s), 1404 (s), 1370 (m), 1286 (m), 1037 (m), 852 (m), 787 (m), 720 (m), 668 (m), 653 (m), 634 (m).

S3. Refinement

H atoms bonded to C atoms were introduced in calculated positions and refined using a riding model [C—H = 0.93 Å and U_iso(H) = 1.2 U_eq(C) for aromatic H atoms, and C—H = 0.96 Å and U_iso(H) = 1.5 U_eq(C) for methyl H atoms]. H atoms belonging to water molecules were found in difference Fourier maps, and the O—H distances were restrained to an average value of 0.90 (5) Å during refinement, using the SHELXL97 (Sheldrick, 1997) DFIX instruction.

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction, 2007); cell refinement: CrysAlis CCD or RED? (Oxford Diffraction, 2007); data reduction: CrysAlis RED (Oxford Diffraction, 2007); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 (Farrugia, 1997) and Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXL97 (Sheldrick, 1997).

Figures

	Fig. 1. The molecular structure of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. The crystal packing of (I), showing the three-dimensional hydrogen-bond network (represented by thin lines) formed by hydrogen bonds of the O—H···O type between water molecules themselves, and between water molecules and carboxylate atom O2. The O—H···N hydrogen bond between water atom O6 and pyridine atom N1 aslo takes part in the network.

tetraaqua-trans-bis(6-methylpyridine-3-carboxylato-κO)cobalt(II) tetrahydrate

Crystal data

[Co(C7H6NO2)2(H2O)4]·4H2O	Z = 1
Mr = 475.31	F(000) = 249
Triclinic, P1	Dx = 1.489 Mg m^−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 7.0284 (2) Å	Cell parameters from 12287 reflections
b = 7.3655 (2) Å	θ = 4.0–34.9°
c = 11.4912 (4) Å	µ = 0.87 mm^−1
α = 86.829 (2)°	T = 296 K
β = 78.746 (3)°	Block, red
γ = 65.357 (3)°	0.53 × 0.33 × 0.07 mm
V = 530.07 (3) Å^3

Data collection

Oxford Diffraction Xcalibur2 diffractometer with Sapphire 3 CCD detector	3062 independent reflections
Radiation source: fine-focus sealed tube	2783 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.016
ω–scan	θmax = 30.0°, θmin = 4.0°
Absorption correction: multi-scan (CrysAlis RED; Oxford Diffraction, 2007)	h = −9→9
Tmin = 0.720, Tmax = 0.940	k = −10→10
14868 measured reflections	l = −16→16

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)] = 0.034	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.089	H atoms treated by a mixture of independent and constrained refinement
S = 1.12	w = 1/[σ^2(Fo^2) + (0.0592P)^2 + 0.0689P] where P = (Fo^2 + 2Fc^2)/3
3062 reflections	(Δ/σ)max < 0.001
166 parameters	Δρmax = 0.30 e Å^−3
8 restraints	Δρmin = −0.82 e Å^−3

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

	x	y	z	Uiso*/Ueq
Co1	0.0000	0.5000	0.5000	0.02579 (9)
N1	−0.19146 (18)	0.77732 (17)	−0.04924 (9)	0.0299 (2)
O1	−0.10712 (16)	0.56022 (16)	0.34141 (8)	0.0342 (2)
O2	0.10956 (16)	0.68928 (16)	0.23789 (9)	0.0342 (2)
O3	0.27610 (15)	0.54722 (16)	0.42744 (9)	0.0336 (2)
H31	0.303 (4)	0.628 (4)	0.470 (2)	0.065 (7)*
H32	0.241 (4)	0.610 (4)	0.356 (2)	0.071 (7)*
O4	0.1713 (2)	0.19649 (16)	0.44155 (10)	0.0421 (2)
H41	0.136 (3)	0.153 (3)	0.390 (2)	0.049 (6)*
H42	0.223 (3)	0.103 (3)	0.4783 (19)	0.048 (6)*
O5	0.3325 (2)	0.8246 (2)	0.56128 (11)	0.0454 (3)
H51	0.447 (4)	0.759 (4)	0.576 (2)	0.075 (8)*
H52	0.244 (5)	0.831 (5)	0.622 (3)	0.089 (9)*
O6	0.0242 (2)	0.08979 (18)	0.26089 (10)	0.0432 (3)
H61	0.082 (3)	0.136 (3)	0.1935 (19)	0.050 (5)*
H62	0.058 (4)	−0.030 (4)	0.253 (2)	0.069 (7)*
C1	−0.09323 (19)	0.74354 (19)	0.04328 (11)	0.0279 (2)
H1	0.0301	0.7649	0.0346	0.034*
C2	−0.16503 (18)	0.67819 (17)	0.15192 (10)	0.0248 (2)
C3	−0.3510 (2)	0.6491 (2)	0.16393 (11)	0.0318 (2)
H3	−0.4043	0.6047	0.2349	0.038*
C4	−0.4557 (2)	0.6867 (2)	0.06949 (12)	0.0336 (3)
H4	−0.5817	0.6704	0.0768	0.040*
C5	−0.3711 (2)	0.74921 (18)	−0.03710 (11)	0.0284 (2)
C6	−0.04491 (18)	0.64040 (17)	0.25142 (10)	0.0249 (2)
C7	−0.4755 (2)	0.7869 (2)	−0.14280 (13)	0.0378 (3)
H71	−0.3905	0.6836	−0.2022	0.057*
H72	−0.4888	0.9141	−0.1744	0.057*
H73	−0.6145	0.7872	−0.1196	0.057*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Co1	0.02864 (13)	0.02988 (13)	0.02113 (12)	−0.01411 (9)	−0.00647 (8)	0.00500 (8)
N1	0.0331 (5)	0.0315 (5)	0.0235 (5)	−0.0122 (4)	−0.0061 (4)	0.0056 (4)
O1	0.0423 (5)	0.0454 (5)	0.0255 (4)	−0.0270 (4)	−0.0128 (4)	0.0118 (4)
O2	0.0374 (5)	0.0416 (5)	0.0321 (5)	−0.0235 (4)	−0.0119 (4)	0.0098 (4)
O3	0.0340 (5)	0.0419 (5)	0.0306 (4)	−0.0208 (4)	−0.0086 (4)	0.0057 (4)
O4	0.0577 (7)	0.0297 (5)	0.0366 (5)	−0.0128 (5)	−0.0163 (5)	0.0017 (4)
O5	0.0391 (6)	0.0522 (7)	0.0429 (6)	−0.0155 (5)	−0.0127 (5)	0.0075 (5)
O6	0.0615 (7)	0.0391 (6)	0.0315 (5)	−0.0266 (5)	−0.0010 (5)	0.0009 (4)
C1	0.0283 (5)	0.0310 (6)	0.0247 (5)	−0.0128 (4)	−0.0055 (4)	0.0053 (4)
C2	0.0271 (5)	0.0234 (5)	0.0221 (5)	−0.0091 (4)	−0.0043 (4)	0.0022 (4)
C3	0.0340 (6)	0.0390 (6)	0.0259 (5)	−0.0195 (5)	−0.0048 (4)	0.0055 (5)
C4	0.0311 (6)	0.0419 (7)	0.0317 (6)	−0.0186 (5)	−0.0073 (5)	0.0032 (5)
C5	0.0305 (5)	0.0253 (5)	0.0263 (5)	−0.0074 (4)	−0.0080 (4)	0.0011 (4)
C6	0.0287 (5)	0.0223 (5)	0.0232 (5)	−0.0098 (4)	−0.0055 (4)	0.0016 (4)
C7	0.0417 (7)	0.0361 (7)	0.0328 (6)	−0.0098 (6)	−0.0159 (5)	0.0017 (5)

Geometric parameters (Å, º)

Co1—O1	2.0621 (9)	O5—H52	0.83 (3)
Co1—O1^i	2.0621 (9)	O6—H61	0.92 (2)
Co1—O3	2.1169 (9)	O6—H62	0.82 (3)
Co1—O3^i	2.1169 (9)	C1—C2	1.3910 (16)
Co1—O4	2.1223 (11)	C1—H1	0.9300
Co1—O4^i	2.1223 (11)	C2—C3	1.3907 (17)
N1—C1	1.3352 (16)	C2—C6	1.4977 (16)
N1—C5	1.3424 (17)	C3—C4	1.3797 (18)
O1—C6	1.2572 (14)	C3—H3	0.9300
O2—C6	1.2595 (15)	C4—C5	1.3955 (18)
O3—H31	0.89 (2)	C4—H4	0.9300
O3—H32	0.94 (2)	C5—C7	1.4941 (18)
O4—H41	0.82 (2)	C7—H71	0.9600
O4—H42	0.78 (2)	C7—H72	0.9600
O5—H51	0.79 (3)	C7—H73	0.9600
O1—Co1—O1^i	180.0	N1—C1—C2	123.70 (11)
O1—Co1—O3	92.11 (4)	N1—C1—H1	118.2
O1^i—Co1—O3	87.89 (4)	C2—C1—H1	118.2
O1—Co1—O3^i	87.89 (4)	C3—C2—C1	117.46 (11)
O1^i—Co1—O3^i	92.11 (4)	C3—C2—C6	121.70 (10)
O3—Co1—O3^i	180.0	C1—C2—C6	120.83 (11)
O1—Co1—O4	88.30 (4)	C4—C3—C2	119.30 (11)
O1^i—Co1—O4	91.70 (4)	C4—C3—H3	120.4
O3—Co1—O4	88.41 (5)	C2—C3—H3	120.4
O3^i—Co1—O4	91.59 (5)	C3—C4—C5	119.58 (12)
O1—Co1—O4^i	91.70 (4)	C3—C4—H4	120.2
O1^i—Co1—O4^i	88.30 (4)	C5—C4—H4	120.2
O3—Co1—O4^i	91.59 (5)	N1—C5—C4	121.36 (11)
O3^i—Co1—O4^i	88.41 (5)	N1—C5—C7	117.06 (12)
O4—Co1—O4^i	180.0	C4—C5—C7	121.58 (12)
C1—N1—C5	118.58 (11)	O1—C6—O2	125.66 (11)
C6—O1—Co1	128.79 (8)	O1—C6—C2	116.49 (10)
Co1—O3—H31	115.6 (16)	O2—C6—C2	117.85 (10)
Co1—O3—H32	101.6 (17)	C5—C7—H71	109.5
H31—O3—H32	108 (2)	C5—C7—H72	109.5
Co1—O4—H41	120.2 (15)	H71—C7—H72	109.5
Co1—O4—H42	128.7 (16)	C5—C7—H73	109.5
H41—O4—H42	103 (2)	H71—C7—H73	109.5
H51—O5—H52	107 (3)	H72—C7—H73	109.5
H61—O6—H62	110 (2)
O3—Co1—O1—C6	−14.28 (11)	C1—N1—C5—C4	0.20 (19)
O3^i—Co1—O1—C6	165.72 (11)	C1—N1—C5—C7	−179.52 (11)
O4—Co1—O1—C6	−102.63 (12)	C3—C4—C5—N1	−1.2 (2)
O4^i—Co1—O1—C6	77.37 (12)	C3—C4—C5—C7	178.47 (12)
C5—N1—C1—C2	0.79 (19)	Co1—O1—C6—O2	5.01 (19)
N1—C1—C2—C3	−0.70 (19)	Co1—O1—C6—C2	−175.66 (8)
N1—C1—C2—C6	179.09 (11)	C3—C2—C6—O1	7.65 (17)
C1—C2—C3—C4	−0.37 (19)	C1—C2—C6—O1	−172.13 (11)
C6—C2—C3—C4	179.84 (11)	C3—C2—C6—O2	−172.97 (12)
C2—C3—C4—C5	1.3 (2)	C1—C2—C6—O2	7.24 (17)

Symmetry code: (i) −x, −y+1, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H31···O5	0.90 (3)	1.93 (3)	2.821 (2)	174 (3)
O3—H32···O2	0.94 (2)	1.73 (3)	2.633 (2)	159 (3)
O4—H41···O6	0.81 (2)	1.97 (2)	2.774 (2)	171 (2)
O4—H42···O5^ii	0.78 (2)	2.11 (2)	2.878 (2)	169 (2)
O5—H51···O3^iii	0.79 (3)	2.28 (3)	2.984 (2)	148 (3)
O5—H52···O6^i	0.83 (4)	1.97 (4)	2.772 (2)	163 (3)
O6—H61···N1^iv	0.92 (2)	1.88 (2)	2.799 (2)	176 (2)
O6—H62···O2^ii	0.82 (3)	1.96 (3)	2.771 (2)	175 (3)

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