Bis{2,4-dibromo-6-[(2-phenyl­eth­yl)imino­meth­yl]phenolato-κ2N,O}cobalt(II)

Yin, Y.; Wang, J.; Zhao, Y.; Huang, L.

doi:10.1107/S160053681104459X

metal-organic compounds

CRYSTALLOGRAPHIC
COMMUNICATIONS

ISSN: 2056-9890

Volume 67| Part 11| November 2011| Page m1642

doi:10.1107/S160053681104459X

Open

access

Bis{2,4-dibromo-6-[(2-phenylethyl)iminomethyl]phenolato-κ²N,O}cobalt(II)

Yanli Yin,^a ^* Jinrong Wang,^a Yongliang Zhao ^a and Liang Huang ^a

^aCollege of Biological Engineering, Henan University of Technology, Zhengzhou, Henan 450001, People's Republic of China
^*Correspondence e-mail: yinzihust@163.com

(Received 18 September 2011; accepted 25 October 2011; online 29 October 2011)

In the title complex, [Co(C₁₅H₁₂Br₂NO)₂], the Co^II atom is four-coordinated by two N,O-bidentate chelate Schiff base ligands, displaying a flattened tetrahedral coordination environment. The Co^II atom occupies a special position on a twofold rotation axis. In the crystal, molecules are linked via weak C—H⋯Br interactions.

Related literature

For background to vitamin B12, see: Randaccio et al. (2010 ). For the antitumour activity of Schiff base–metal complexes, see: Ren et al. (2002 ) and for their anti-microbial activity, see: Panneerselvam et al. (2005 ). For related structures, see: Chen et al. (2010 ); Li et al. (2010 ); Jiang et al. (2008 ); For standard bond lengths, see: Allen et al. (1987 ).

Experimental

Crystal data

[Co(C₁₅H₁₂Br₂NO)₂]
M_r = 823.08
Monoclinic, C 2/c
a = 22.5087 (16) Å
b = 4.8717 (4) Å
c = 28.864 (2) Å
β = 111.505 (1)°
V = 2944.8 (4) Å³
Z = 4
Mo Kα radiation
μ = 6.04 mm⁻¹
T = 291 K
0.24 × 0.23 × 0.22 mm

Data collection

Bruker SMART APEX CCD diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2000 ) T_min = 0.325, T_max = 0.350
14453 measured reflections
2876 independent reflections
2478 reflections with I > 2σ(I)
R_int = 0.031

Refinement

R[F² > 2σ(F²)] = 0.032
wR(F²) = 0.108
S = 1.01
2876 reflections
177 parameters
H-atom parameters constrained
Δρ_max = 0.79 e Å⁻³
Δρ_min = −0.78 e Å⁻³

Table 1
Selected bond lengths (Å)

Co1—O1	1.916 (2)
Co1—N1	1.986 (3)

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C7—H7⋯Br2ⁱ	0.93	3.01	3.940 (3)	173
C8—H8B⋯Br1ⁱⁱ	0.97	2.93	3.814 (3)	151
C9—H9B⋯Br2ⁱⁱⁱ	0.97	2.94	3.854 (3)	157
Symmetry codes: (i) $[-x+{\script{1\over 2}}, y-{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (ii) $[-x, y-1, -z+{\script{1\over 2}}]$ ; (iii) $[-x+{\script{1\over 2}}, y+{\script{1\over 2}}, -z+{\script{1\over 2}}]$ .

Data collection: SMART (Bruker, 2000); cell refinement: SAINT-Plus (Bruker, 2000); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S160053681104459X/br2176sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S160053681104459X/br2176Isup2.hkl

checkCIF report

Comment top

Cobalt is an important life-required element. For example, vitamin B12, also called cobalamin, is a water soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood (Randaccio et al., 2010). In addition, the Schiff base metal complexes generally possess antitumour activities (Ren et al., 2002) and antimicrobial activities (Panneerselvam et al., 2005). By taking the biological importance of element cobalt into account, we synthesized the title complex with the bidentate N,O-donor Schiff base ligands (Scheme I).

In the title compound, the Co^II atom occupies a special position on a twofold rotation axis to form a distorted tetrahedral coordination sphere. Cobalt(II) atom is four-coordinated by two imino N atoms and two phenolic O atoms from two bidentate Schiff-base ligands derived from the condensation of 3,5-dibromosalicylaldehyde and 2-phenylethylamine (Fig. 1). All bond lengths are within normal ranges (Allen et al., 1987). The C7═N1 bond length of 1.284 (4) Å is within the range of 1.256 (14)–1.310 (15) Å observed in the analogous tetrahedral Co(II) species (Chen et al., 2010; Li et al., 2010). The Co–O and Co–N bond distances of 1.916 (2) and 1.986 (3) Å are also similar to those of 1.935 (2) and 2.006 (3) Å previously reported in the related cobalt(II) complex of a Schiff base ligand derived from the condensation of 3,5-dibromosalicylaldehyde and benzylamine (Jiang et al., 2008).

In the crystal structure, the molecules are linked via weak C–H···Br interactions (Fig.2).

Related literature top

For background to vitamin B12, see: Randaccio et al. (2010). For the antitumour activity of Schiff base–metal complexes, see: Ren et al. (2002) and for their anti-microbial activity, see: Panneerselvam et al. (2005). For related structures, see: Chen et al. (2010); Li et al. (2010); Jiang et al. (2008); For standard bond lengths, see: Allen et al. (1987).

Experimental top

3,5-Dibromosalicylaldehyde (560 mg, 2 mmol) and 2-phenylethylamine (242 mg, 2 mmol) were dissolved in a methanol solution (25 mL).The mixture was stirred at room temperature for 1 h to give an orange solution, which was added to a methanol solution (15 mL) of Co(NO₃)₂.6H₂O (280 mg, 1 mmol). The mixture was stirred for another 25 min at room temperature to give a red solution and then filtered. The filtrate was kept in air for 7 days, forming red blocky crystals. The crystals were isolated and dried in a vacuum desiccator containing anhydrous CaCl₂, in about 64% yield. Anal. Calcd for C₃₀H₂₄Br₄CoN₂O₂: C, 43.78; H, 2.94; N, 3.40. Found: C, 43.66; H, 2.99; N, 3.31%. IR (KBr, cm^-1): 3423, 2909, 2361, 1614, 1502, 1433, 1410, 1310, 1210, 1152, 865, 749, 703, 486, 437.

Computing details top

Data collection: SMART (Bruker, 2000); cell refinement: SAINT-Plus (Bruker, 2000); data reduction: SAINT-Plus (Bruker, 2000); program(s) used to solve structure: SHELXL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures top

	Fig. 1. The structure of the title compound, with the atom numbering scheme of the unique atoms (30% probability ellipsoids).
	Fig. 2. Partial packing view showing the chain formed through weak C–H···Br interactions.

Bis{2,4-dibromo-6-[(2-phenylethyl)iminomethyl]phenolato- κ²N,O}cobalt(II) top

Crystal data top

[Co(C₁₅H₁₂Br₂NO)₂]	F(000) = 1604
M_r = 823.08	D_x = 1.857 Mg m⁻³
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 5864 reflections
a = 22.5087 (16) Å	θ = 2.9–28.1°
b = 4.8717 (4) Å	µ = 6.04 mm⁻¹
c = 28.864 (2) Å	T = 291 K
β = 111.505 (1)°	Block, red
V = 2944.8 (4) Å³	0.24 × 0.23 × 0.22 mm
Z = 4

Data collection top

Bruker SMART APEX CCD diffractometer	2876 independent reflections
Radiation source: fine-focus sealed tube	2478 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.031
ϕ and ω scans	θ_max = 26.0°, θ_min = 1.9°
Absorption correction: multi-scan (SADABS; Bruker, 2000)	h = −27→27
T_min = 0.325, T_max = 0.350	k = −6→6
14453 measured reflections	l = −35→35

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.032	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.108	H-atom parameters constrained
S = 1.01	w = 1/[σ²(F_o²) + (0.082P)² + 0.812P] where P = (F_o² + 2F_c²)/3
2876 reflections	(Δ/σ)_max < 0.001
177 parameters	Δρ_max = 0.79 e Å⁻³
0 restraints	Δρ_min = −0.78 e Å⁻³

Crystal data top

[Co(C₁₅H₁₂Br₂NO)₂]	V = 2944.8 (4) Å³
M_r = 823.08	Z = 4
Monoclinic, C2/c	Mo Kα radiation
a = 22.5087 (16) Å	µ = 6.04 mm⁻¹
b = 4.8717 (4) Å	T = 291 K
c = 28.864 (2) Å	0.24 × 0.23 × 0.22 mm
β = 111.505 (1)°

Data collection top

Bruker SMART APEX CCD diffractometer	2876 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2000)	2478 reflections with I > 2σ(I)
T_min = 0.325, T_max = 0.350	R_int = 0.031
14453 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.032	0 restraints
wR(F²) = 0.108	H-atom parameters constrained
S = 1.01	Δρ_max = 0.79 e Å⁻³
2876 reflections	Δρ_min = −0.78 e Å⁻³
177 parameters

Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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 (Å²) top

	x	y	z	U_iso*/U_eq
C1	0.12749 (15)	0.3862 (7)	0.22685 (12)	0.0389 (7)
C2	0.08497 (15)	0.5880 (6)	0.19768 (11)	0.0367 (7)
C3	0.09703 (15)	0.6772 (6)	0.15481 (11)	0.0365 (6)
C4	0.14508 (15)	0.5753 (6)	0.14172 (12)	0.0409 (7)
H4	0.1509	0.6379	0.1132	0.049*
C5	0.18509 (15)	0.3773 (7)	0.17151 (12)	0.0430 (7)
C6	0.17739 (16)	0.2844 (7)	0.21350 (12)	0.0435 (8)
H6	0.2052	0.1534	0.2333	0.052*
C7	0.12310 (17)	0.2717 (7)	0.27173 (12)	0.0431 (7)
H7	0.1561	0.1560	0.2903	0.052*
C8	0.08535 (19)	0.1846 (8)	0.33657 (12)	0.0492 (8)
H8A	0.1125	0.0239	0.3418	0.059*
H8B	0.0438	0.1257	0.3358	0.059*
C9	0.1142 (2)	0.3836 (9)	0.37889 (14)	0.0656 (11)
H9A	0.0876	0.5464	0.3729	0.079*
H9B	0.1560	0.4390	0.3799	0.079*
C10	0.12060 (19)	0.2619 (7)	0.42885 (13)	0.0503 (9)
C11	0.0790 (2)	0.3341 (10)	0.45192 (14)	0.0613 (10)
H11	0.0479	0.4661	0.4374	0.074*
C12	0.0828 (3)	0.2136 (10)	0.49615 (16)	0.0701 (12)
H12	0.0536	0.2616	0.5107	0.084*
C13	0.1281 (3)	0.0287 (10)	0.51828 (15)	0.0702 (12)
H13	0.1309	−0.0479	0.5485	0.084*
C14	0.1703 (3)	−0.0485 (13)	0.49693 (19)	0.0972 (19)
H14	0.2012	−0.1803	0.5121	0.117*
C15	0.1668 (2)	0.0710 (11)	0.45224 (17)	0.0773 (14)
H15	0.1962	0.0209	0.4380	0.093*
Br1	0.040636 (17)	0.94006 (7)	0.113456 (12)	0.04757 (15)
Br2	0.250012 (17)	0.22456 (9)	0.151646 (14)	0.05805 (16)
Co1	0.0000	0.52246 (15)	0.2500	0.04521 (19)
N1	0.07834 (13)	0.3134 (6)	0.28853 (10)	0.0425 (6)
O1	0.03706 (12)	0.6888 (5)	0.20677 (9)	0.0468 (6)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
C1	0.0389 (16)	0.0510 (18)	0.0320 (16)	−0.0033 (13)	0.0192 (13)	−0.0013 (13)
C2	0.0394 (16)	0.0439 (17)	0.0338 (16)	−0.0048 (12)	0.0219 (13)	−0.0012 (12)
C3	0.0406 (16)	0.0432 (16)	0.0303 (15)	−0.0029 (13)	0.0182 (13)	0.0005 (12)
C4	0.0460 (18)	0.0523 (19)	0.0324 (16)	−0.0078 (15)	0.0239 (14)	−0.0023 (13)
C5	0.0360 (16)	0.0589 (19)	0.0414 (18)	−0.0039 (14)	0.0226 (14)	−0.0086 (15)
C6	0.0364 (16)	0.059 (2)	0.0370 (17)	0.0031 (14)	0.0155 (14)	0.0028 (14)
C7	0.0444 (18)	0.0548 (19)	0.0336 (16)	0.0020 (14)	0.0184 (14)	0.0063 (14)
C8	0.054 (2)	0.064 (2)	0.0345 (17)	−0.0061 (17)	0.0213 (15)	0.0100 (15)
C9	0.091 (3)	0.069 (2)	0.038 (2)	−0.021 (2)	0.026 (2)	0.0042 (17)
C10	0.062 (2)	0.057 (2)	0.0324 (17)	−0.0141 (17)	0.0174 (16)	−0.0011 (14)
C11	0.067 (2)	0.074 (3)	0.040 (2)	0.005 (2)	0.0150 (18)	0.0093 (18)
C12	0.089 (3)	0.087 (3)	0.044 (2)	0.001 (3)	0.035 (2)	−0.004 (2)
C13	0.095 (3)	0.083 (3)	0.036 (2)	0.004 (3)	0.027 (2)	0.0095 (19)
C14	0.114 (4)	0.133 (5)	0.050 (3)	0.052 (4)	0.036 (3)	0.032 (3)
C15	0.076 (3)	0.116 (4)	0.047 (2)	0.022 (3)	0.032 (2)	0.013 (2)
Br1	0.0584 (2)	0.0515 (2)	0.0413 (2)	0.00711 (15)	0.02841 (17)	0.00863 (13)
Br2	0.0447 (2)	0.0830 (3)	0.0575 (3)	0.00539 (17)	0.03169 (19)	−0.00833 (18)
Co1	0.0441 (4)	0.0638 (4)	0.0359 (4)	0.000	0.0243 (3)	0.000
N1	0.0470 (16)	0.0554 (16)	0.0312 (14)	−0.0017 (13)	0.0216 (12)	0.0052 (12)
O1	0.0522 (14)	0.0575 (14)	0.0442 (13)	0.0103 (11)	0.0337 (11)	0.0089 (11)

Geometric parameters (Å, º) top

C1—C6	1.405 (5)	C9—C10	1.516 (5)
C1—C2	1.415 (5)	C9—H9A	0.9700
C1—C7	1.447 (4)	C9—H9B	0.9700
C2—O1	1.296 (4)	C10—C15	1.374 (6)
C2—C3	1.428 (4)	C10—C11	1.378 (6)
C3—C4	1.363 (4)	C11—C12	1.379 (6)
C3—Br1	1.888 (3)	C11—H11	0.9300
C4—C5	1.383 (5)	C12—C13	1.335 (7)
C4—H4	0.9300	C12—H12	0.9300
C5—C6	1.363 (5)	C13—C14	1.360 (7)
C5—Br2	1.906 (3)	C13—H13	0.9300
C6—H6	0.9300	C14—C15	1.391 (7)
C7—N1	1.284 (4)	C14—H14	0.9300
C7—H7	0.9300	C15—H15	0.9300
C8—N1	1.477 (4)	Co1—O1	1.916 (2)
C8—C9	1.507 (5)	Co1—O1ⁱ	1.916 (2)
C8—H8A	0.9700	Co1—N1	1.986 (3)
C8—H8B	0.9700	Co1—N1ⁱ	1.986 (3)

C6—C1—C2	121.1 (3)	C10—C9—H9B	109.1
C6—C1—C7	115.7 (3)	H9A—C9—H9B	107.8
C2—C1—C7	123.2 (3)	C15—C10—C11	117.4 (3)
O1—C2—C1	125.0 (3)	C15—C10—C9	121.6 (4)
O1—C2—C3	119.8 (3)	C11—C10—C9	121.0 (4)
C1—C2—C3	115.2 (3)	C10—C11—C12	121.1 (4)
C4—C3—C2	123.5 (3)	C10—C11—H11	119.5
C4—C3—Br1	119.3 (2)	C12—C11—H11	119.5
C2—C3—Br1	117.2 (2)	C13—C12—C11	120.5 (4)
C3—C4—C5	118.9 (3)	C13—C12—H12	119.8
C3—C4—H4	120.5	C11—C12—H12	119.8
C5—C4—H4	120.5	C12—C13—C14	120.5 (4)
C6—C5—C4	121.2 (3)	C12—C13—H13	119.8
C6—C5—Br2	120.0 (3)	C14—C13—H13	119.8
C4—C5—Br2	118.8 (2)	C13—C14—C15	119.5 (5)
C5—C6—C1	120.1 (3)	C13—C14—H14	120.2
C5—C6—H6	119.9	C15—C14—H14	120.2
C1—C6—H6	119.9	C10—C15—C14	121.0 (5)
N1—C7—C1	126.7 (3)	C10—C15—H15	119.5
N1—C7—H7	116.7	C14—C15—H15	119.5
C1—C7—H7	116.7	O1—Co1—O1ⁱ	129.98 (15)
N1—C8—C9	110.7 (3)	O1—Co1—N1	94.15 (10)
N1—C8—H8A	109.5	O1ⁱ—Co1—N1	111.18 (11)
C9—C8—H8A	109.5	O1—Co1—N1ⁱ	111.18 (11)
N1—C8—H8B	109.5	O1ⁱ—Co1—N1ⁱ	94.14 (10)
C9—C8—H8B	109.5	N1—Co1—N1ⁱ	118.31 (17)
H8A—C8—H8B	108.1	C7—N1—C8	117.3 (3)
C8—C9—C10	112.5 (3)	C7—N1—Co1	121.9 (2)
C8—C9—H9A	109.1	C8—N1—Co1	120.7 (2)
C10—C9—H9A	109.1	C2—O1—Co1	124.4 (2)
C8—C9—H9B	109.1

Symmetry code: (i) −x, y, −z+1/2.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C7—H7···Br2ⁱⁱ	0.93	3.01	3.940 (3)	173
C8—H8B···Br1ⁱⁱⁱ	0.97	2.93	3.814 (3)	151
C9—H9B···Br2^iv	0.97	2.94	3.854 (3)	157

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

Experimental details

Crystal data
Chemical formula	[Co(C₁₅H₁₂Br₂NO)₂]
M_r	823.08
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	291
a, b, c (Å)	22.5087 (16), 4.8717 (4), 28.864 (2)
β (°)	111.505 (1)
V (Å³)	2944.8 (4)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	6.04
Crystal size (mm)	0.24 × 0.23 × 0.22

Data collection
Diffractometer	Bruker SMART APEX CCD diffractometer
Absorption correction	Multi-scan (SADABS; Bruker, 2000)
T_min, T_max	0.325, 0.350
No. of measured, independent and observed [I > 2σ(I)] reflections	14453, 2876, 2478
R_int	0.031
(sin θ/λ)_max (Å⁻¹)	0.617

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.032, 0.108, 1.01
No. of reflections	2876
No. of parameters	177
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.79, −0.78

Computer programs: SMART (Bruker, 2000), SAINT-Plus (Bruker, 2000), SHELXL (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

Selected bond lengths (Å) top

Co1—O1

1.916 (2)

Co1—N1

1.986 (3)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C7—H7···Br2ⁱ	0.93	3.01	3.940 (3)	172.5
C8—H8B···Br1ⁱⁱ	0.97	2.93	3.814 (3)	151.0
C9—H9B···Br2ⁱⁱⁱ	0.97	2.94	3.854 (3)	157.4

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

Acknowledgements

This work was supported by the school scientific research fund of Henan University of Technology.

References

Allen, F. H., Kennard, O., Watson, D. G., Brammer, L., Orpen, A. G. & Taylor, R. (1987). J. Chem. Soc. Perkin Trans. 2, pp. S1–19. CrossRef Web of Science Google Scholar
Bruker (2000). SMART, SAINT-Plus and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Chen, W., Li, Y., Cui, Y., Zhang, X., Zhu, H.-L. & Zeng, Q. (2010). Eur. J. Med. Chem. 45, 4473–4478. Web of Science CSD CrossRef CAS PubMed Google Scholar
Jiang, W., Mo, G.-D. & Jin, L. (2008). Acta Cryst. E64, m1394. Web of Science CSD CrossRef IUCr Journals Google Scholar
Li, C., Li, R. & Zhang, S. (2010). Acta Cryst. E66, m1123. Web of Science CSD CrossRef IUCr Journals Google Scholar
Panneerselvam, P., Nair, R. R., Vijayalakshmi, G., Subramanian, E. H. & Sridhar, S. K. (2005). Eur. J. Med. Chem. 40, 225–229. Web of Science CrossRef PubMed CAS Google Scholar
Randaccio, L., Geremia, S., Demitri, N. & Wuerges, J. (2010). Molecules, 15, 3228–3259. Web of Science CrossRef CAS PubMed Google Scholar
Ren, S., Wang, R., Komatsu, K., Bonaz-Krause, P., Zyrianov, Y., McKenna, C. E., Csipke, C., Tokes, Z. A. & Lien, E. J. (2002). J. Med. Chem. 45, 410–419. Web of Science CrossRef PubMed CAS Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar