Tetra-, Penta- and Hexa-Coordinated Transition Metal Complexes Constructed from Coumarin-Containing N₂O₂ Ligand

Abstract

Three newly designed complexes, [Cu(L)]·CHCl₃ (1), [Co(L)(MeOH)]·CHCl₃ (2) and [{Ni(L)(MeOH)(PhCOO)}₂Ni] (3) a coumarin-containing Salamo-type chelating ligand (H₂L) have been synthesized and characterized by elemental analyses, IR and UV-VIS spectra, and X-ray crystallography. Complex 1 includes one Cu(II) atom, one completely deprotonated (L)²⁻ unit and one crystalling chloroform molecule, the Cu(II) atom shows a square-planar geometry. Complex 2 includes one Co(II) atom, one completely deprotonated (L)²⁻ unit, one coordinated methanol molecule and one crystalling chloroform molecule. The Co(II) atom is a distorted trigonal-bipyramidal geometry. While complex 3 includes three Ni(II) atoms, two completely deprotonated (L)²⁻ units, two benzoates and two coordinated methanol molecules. The complexes 1 and 2 are both possess three-dimensional supra-molecular structures by abundant noncovalent interactions. But, complex 3 formed a two-dimensional supra-molecular structure by intra-molecular hydrogen bonds. In addition, the antimicrobial and fluorescence properties of H₂L and its complexes 1, 2 and 3 were also investigated.

1. Introduction

Salen compound is a kind of versatile tetradentate N₂O₂ chelating ligand in modern coordination chemistry [1,2,3,4,5,6,7], and its metal complexes have been diffusely investigated in biological fields [8,9,10], ion recognitions [11], luminescent [12,13,14,15,16,17] and magnetic [18,19,20,21] materials, supra-molecular buildings [22,23,24,25,26] and so on. Especially, some substitution of atoms of the ligand with other elements often evidently improves its properties. When an O-alkyl oxime unit substitutes the imine moiety, the larger electronegativity of the O atoms is predicted to improve significantly the electronic behaviors of Salamo-type compounds, which may give rise to novel structures and better properties of the metal(II) complexes [27,28,29,30,31,32,33].

The introduction of different substituted groups into Salamo-type compounds may give rise to novel structures and the uncoordinated crystalling molecule can also affect the spatial structures [34,35,36]. Although these Salamo-type complexes have been in the process of development, the fluorescence and antimicrobial properties with coumarin-containing transition metal complexes are still unreported. In this study, three newly designed complexes [Cu(L)]·CHCl₃ (1), [Co(L)(MeOH)]·CHCl₃ (2) and [{NiL(MeOH)(PhCOO)}₂Ni] (3) have been prepared with a coumarin-containing Salamo-type N₂O₂ ligand, in particular the research on the Salamo-type complex contained benzoate ligands is reported firstly.

2. Experimental

2.1. Materials and Methods

7-Hydroxyl-4-methyl-coumarin (98%) was obtained from Alfa Aesar (New York, NY, USA). C, H and N analyses were gained by a GmbH VarioEL V3.00 automatic elemental analysis instrument (Elementar, Berlin, Germany). Elemental analyses for metals were obtained using an IRIS ER/S-WP-ICP atomic emission spectrometer (Elementar, Berlin, Germany). Melting points were measured by the use of a microscopic melting point apparatus made by Beijing Taike Instrument Limited Company (Beijing, China) and were uncorrected. IR spectra were recorded on a Vertex70 FT-IR spectrophotometer, with samples prepared as KBr (400–4000 cm⁻¹) pellets (Bruker AVANCE, Billerica, MA, USA). UV-VIS absorption spectra were measured on a Shimadzu UV-3900 spectrometer (Shimadzu, Tokyo, Japan). Luminescence spectra in solution were recorded on a Hitachi F-7000 spectrometer (Shimadzu, Tokyo, Japan). ¹H-NMR spectra were measured by a German Bruker AVANCE DRX-400 spectrometer (Bruker AVANCE, Billerica, MA, USA). X-ray single-crystal structures were determined by a SuperNova Dual (Cu at zero) and Bruker APEX-II CCD diffractometers (Bruker AVANCE, Billerica, MA, USA), respectively.

2.2. Synthesis of H₂L

Major reaction step, O,O′-ethane-1,2-diyl-bis-hydroxylamine was prepared following the literature [37,38,39]. 7-Hydroxy-4-methyl-2-oxo-2H-chromene-8-carbonitrile was synthesized in accordance with the reported procedures [40]. The major reaction steps participated in the preparation of H₂L are shown in Scheme 1.

An ethanol solution (15 mL) of 7-Hydroxy-4-methyl-2-oxo-2H-chromene-8-carbonitrile (384.38 mg, 1.45 mmol) was added dropwise to an ethanol solution (15 mL) of O,O′-ethane-1, 2-diyl-bis-hydroxylamine (46.00 mg, 0.5 mmol) in ethanol (15 mL) and the mixture was subjected to heating at 65~70 °C for 3 h. After cooling to the room temperature, the resulting white solid was collected. Yield, 186.47 mg, 0.40 mmol (80.3%). Anal. Calcd for C₂₄H₂₀N₂O₈ (%): C, 62.07; H, 4.34; N, 6.03; Found: C, 62.25; H, 4.47; N, 5.89. ¹H-NMR (400 MHz, CDCl₃), δ 10.72 (s, 2H, OH), 8.95 (s, 2H, CH=N), 7.50 (d, J = 8.9 Hz, 2H, ArH), 6.93 (d, J = 8.9 Hz, 2H, ArH), 6.14 (s, 2H, ArH), 4.54 (s, 4H, CH₂), 2.40 (s, 6H, CH₃).

2.3. Synthesis of Complex 1

A chloroform solution (4 mL) of H₂L (4.64 mg, 0.01 mmol) was added dropwise to a methanol solution (4 mL) of Cu(OAc)₂·H₂O (1.99 mg, 0.01 mmol), the color of the mixed solution changes immediately to dark green. The mixed solution was filtered, and the filtrate was kept undisturbed in the dark to avoid decomposition of the coumarin-containing building blocks. Single-crystals suitable for X-ray crystallography were grown up by partial solvent evaporation after about two weeks, and collected carefully by filtration, washed with n-hexane, and dried at room temperature. Yield, 3.01 mg, 0.0047 mmol (46.7%). Anal. Calc. for C₂₅H₁₉Cl₃CuN₂O₈ (%): C, 46.53; H, 2.97; N, 4.34; Cu, 9.85; Found: C, 46.69; H, 3.02; N, 4.17; Cu, 9.71.

2.4. Synthesis of Complex 2

A solution of H₂L (4.64 mg, 0.01 mmol) in 4 mL of chloroform was added dropwise to a methanol solution (6 mL) of Co(NO₃)₂·6H₂O (2.91 mg, 0.01 mmol). The color of the mixed solution changes immediately to brown. The mixed solution was filtered, and the filtrate kept undisturbed in the dark to avoid decomposition of the coumarin-containing building blocks. Single-crystals suitable for X-ray crystallography were grown up by partial solvent evaporation after about one week, and collected carefully by filtration, washed with n-hexane, and dried at room temperature. Yield, 3.32 mg, 0.0049 mmol (49.3 %). Anal. Calc. for C₂₆H₂₃Cl₃CoN₂O₉ (%): C, 46.42; H, 3.45; N, 4.16; Co, 8.76; Found: C, 46.61; H, 3.57; N, 4.02; Co, 8.61.

2.5. Synthesis of Complex 3

A chloroform solution (4 mL) of H₂L (4.64 mg, 0.01 mmol) was added dropwise to a methanol solution (2 mL) of Ni(NO₃)₂·6H₂O (2.90 mg, 0.01 mmol). Then, a methanol solution (5 mL) of sodium benzoic (1.22 mg, 0.01 mmol) was added, and the mixed sulution was kept stirring for 5 min at room temperature. The mixed solution was filtered, and the filtrate kept undisturbed in the dark to avoid decomposition of the coumarin-containing building blocks. Single-crystals suitable for X-ray crystallography were grown up by partial solvent evaporation after about three weeks, and collected carefully by filtration, washed with n-hexane, and dried at room temperature. Yield, 2.05 mg, 0.0015 mmol (43.7%). Anal. Calc. for C₆₄H₅₄N₄Ni₃O₂₂ (%): C, 54.62; H, 3.87; N, 3.98; Ni, 12.51; Found: C, 54.81; H, 3.93; N, 3.83; Ni, 12.33.

2.6. X-ray Crystal Structure Determinations for Complexes 1, 2 and 3

X-ray single crystal diffraction data of the complexes 1, 2 and 3 were recorded using a SuperNova Dual (Cu at zero) and Bruker APEX-II CCD diffractometers with a monochromated Mo-Kα radiation (λ = 0.71073 Å) source at 296(2), 173.00(10) and 153(2) K, respectively. The LP corrections were applied to the SAINT program [41] and Semi-empirical correction were applied to the SADABS program [42]. The single crystal structures were solved by the direct methods (SHELXS-2014) [43]. All hydrogen atoms were added theoretically and difference-Fourier map exhibited the positions of the remaining atoms. The H atoms were included at the calculated positions and constrained to ride on their parent atoms. All non-hydrogen atoms were refined anisotropically by a full-matrix least-squares procedure on F² with SHELXL-2014 [43]. Crystal data and refinement parameters involved the structure determinations are presented in

Table 1. Crystal data and refinement parameters for complexes 1, 2 and 3.

Complex	1	2	3
Formula	C25H19Cl3CuN2O8	C26H23Cl3CoN2O9	C64H54N4Ni3O22
Formula weight	645.31	672.74	1407.24
Temperature (K)	296(2)	173.00(10)	153(2)
Wavelength (Å)	0.71073	0.71073	0.71073
Crystal system	Monoclinic	Monoclinic	Triclinic
Space group	P21/c	P21/c	P–1
Unit cell dimensions
a (Å)	13.5401(9)	12.1129(5)	12.6250(18)
b (Å)	6.8318(5)	13.5779(3)	12.6344(17)
c (Å)	27.1822(16)	16.7455(6)	14.7046(18)
α (°)	90	90	113.420(4)
β (°)	94.8504(19)	100.038(3)	95.978(5)
γ (°)	90	90	107.557(2)
V (Å3)	2505.4(3)	2711.93(15)	1983.6(5)
Z	4	4	1
Dc (g cm−3)	1.711	1.648	1.178
μ (mm−1)	1.247	0.987	0.770
F (000)	1308	1372	726
Crystal size (mm)	0.26 × 0.22 × 0.19	0.16 × 0.07 × 0.04	0.22 × 0.19 × 0.18
θ Range (°)	2.219–25.005	3.416–26.020	1.750–25.010
	−16 ≤ h ≤ 16,	−9 ≤ h ≤ 14,	−13 ≤ h ≤ 15
Index ranges	−8 ≤ k ≤ 6	−16 ≤ k ≤ 15	−15 ≤ k ≤ 15
	−32 ≤ l ≤ 31	−20 ≤ l ≤ 20	−17 ≤ l ≤ 16
Reflections collected	16385	10655	14781
Independent reflections	4402	5343	6919
Rint	0.0746	0.0410	0.0661
Completeness	99.9%	99.7%	99.0%
Data/restraints/parameters	4402/0/354	5343/4/376	6919/69/319
GOF	1.097	1.053	1.050
Final R1, wR2 indices	0.0659, 0.1066	0.0585, 0.1315	0.0579, 0.1580
R1, wR2 indices (all data)	0.1058, 0.1207	0.0861, 0.1530	0.0890, 0.1779
Largest differences	0.364/−0.439	1.211/−0.653	0.935/−0.828
peak and hole (e Å−3)

.

Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication, No. CCDC 1,816,001, 1,816,000 and 1,816,002 for complexes 1, 2 and 3, respectively. Copies of the data can be gained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (Telephone: (44) 01223 762910; Fax: +44-1223-336033; E-mail: deposit@ccdc.cam.ac.uk). These data can be also obtained free of charge at www.ccdc.cam. Ac.uk/conts/retrieving.html.

3. Results and Discussion

Complexes 1, 2 and 3 a coumarin-containing Salamo-type ligand have been prepared, and characterized by IR, UV-VIS and X-ray crystallography methods. In addition, the fluorescence properties of complexes 1, 2 and 3 and antimicrobial activities of H₂L and its complexes 1, 2 and 3 were also investigated.

3.1. IR Spectra

The FT-IR spectra of H₂L with its corresponding complexes 1, 2 and 3 possess different bands in the 4000–400 cm⁻¹ region (Figure 1 and

Table 2. The major FT-IR spectra of H₂L and its complexes 1, 2 and 3 (cm⁻¹).

Compound	ν(C=N)	ν(Ar-O)	ν(C=O)	ν(C=C)
H2L	1616	1282	1734	1386
Complex 1	1601	1242	1722	1320
Complex 2	1597	1238	1726	1312
Complex 3	1605	1222	1719	1325

).

The characteristic C=O stretching band at 1734 cm⁻¹ of the ligand H₂L, and those of the complexes 1, 2 and 3 emerge at 1722, 1726 and 1719 cm⁻¹, respectively [44]. Meanwhile, a characteristic C=N stretching band of H₂L emerges at 1616 cm⁻¹, and those of complexes 1, 2 and 3 show at 1601, 1597 and 1605 cm⁻¹, respectively [45]. The characteristic C=N stretching frequencies are moved to low frequencies, which indicates that the metal(II) atoms are bonded by azomethine N atoms of the ligand (L)²⁻ moieties [46]. H₂L presents a characteristic Ar–O stretching frequency at 1282 cm^-1, while those of complexes 1, 2 and 3 emerge at 1242, 1238 and 1222 cm⁻¹, respectively. The characteristic Ar–O stretching frequencies are moved to low frequencies, which can be evidence for formation of Cu–O, Co–O and Ni–O bonds between Cu(II), Co(II) and Ni(II) atoms with O atoms of phenolic groups [47].

3.2. UV-VIS Spectra

The UV–VIS absorption spectra of H₂L with its complexes 1, 2 and 3 in the dichloromethane solutions (2.0 × 10⁻⁵ M) at 298 K are shown in Figure 2 and

Table 3. Absorption maxima and molar extinction coefficients for H₂L and its complexes 1, 2 and 3.

Compound	λmax1, nm	εmax1, M−1·cm−1	λmax2, nm	εmax2, M−1·cm−1	λmax3, nm	εmax3, M−1·cm−1
H2L	291	4.9 × 10−4	330	3.3 × 10−4	345	2.9 × 10−4
Complex 1	299	2.5 × 10−4	343	2.7 × 10–4	396	0.7 × 10−4
Complex 2	297	3.2 × 10−4	347	3.9 × 10−4	399	0.9 × 10−4
Complex 3	338	4.3 × 10−4	391	0.7 × 10−4

. The absorption peaks of H₂L are different from its complexes 1, 2 and 3. The absorption spectrum of H₂L comprises three relatively intense absorption peaks centered at 291, 330 and 345 nm, the first peak at 291 nm can be assigned to the π–π* transitions of the phenyl rings [48], the second peak at 330 nm can be assigned to the π–π* transitions of the oxime group [49], and the third peak at 345 nm can be assigned to the n–π* transitions of lactone carbonyl group [50]. Upon coordination of H₂L, the π–π* transitions of the phenyl rings in complexes 1 and 2 are bathochromically shifted to 299 and 297 nm, respectively, and the absorption peak at 291 nm vanishes from the UV–VIS spectrum of complex 3, indicating the coordination of Co(II), Cu(II) and Ni(II) atoms with the (L)²⁻ units [48]. Compared with H₂L, the absorption peak at 330 nm vanishes from the UV–VIS spectra of complexes 1, 2 and 3, indicating that the oxime N atoms are participated in coordination to the Cu(II), Co(II) and Ni(II) atoms [49]. Meanwhile, the n–π* transitions of the lactone carbonyl group in complexes 1, 2 and 3 assumes a hypsochromic shift to 343, 347 and 338 nm exhibiting that the coordination of the (L)²⁻ units with Cu(II), Co(II) and Ni(II) atoms, respectively. Meanwhile, three weak broad absorption peaks are gained at 396, 399 and 391 nm for complexes 1, 2 and 3, respectively, these new absorption peaks can be attributed to L→M charge-transfer transitions (LMCT). This is characteristic of N₂O₂-donors sphere with the transition metal complexes [49].

3.3. Fluorescence Properties

The fluorescent properties of H₂L and its complexes 1, 2 and 3 were measured at room temperature (Figure 3). The ligand H₂L exhibits a strong and broad emission at 432 nm upon excitation at 351 nm, which should be assigned to intraligand π–π* transition [12]. The complexes 1, 2 and 3 display weakened photoluminescence with maximum emission peaks at ca. 417, 462 and 469 nm upon excitation at 351 nm, respectively. The absorption peaks are bathochromically-shifted of complexes 1 and 2, and hypsochromically-shifted of complex 3, which could be attributed to LMCT [12].

3.4. Crystal Structure Description of Complex 1

As presented in Figure 4 and

Table 4. Selected bond lengths (Å) and angles (°) for complex 1.

Bond
Cu1-O1	1.891(3)	Cu1-N1	1.950(4)
Cu1-O2	1.909(3)	Cu1-N2	1.976(4)
Angles
O1-Cu1-O2	83.86(14)	O1-Cu1-N2	90.30(15)
O1-Cu1-N1	162.19(18)	O2-Cu1-N2	161.44(17)
O2-Cu1-N1	89.28(15)	N1-Cu1-N2	100.99(16)

, complex 1 crystallizes in the monoclinic system, space group P 2₁/c, which comprises one Cu(II) atom, one deprotonated (L)²⁻ unit and one crystalling chloroform molecule.

The Cu(II) atom is tetra-coordinated by two oxime N (N1 and N2) atoms and two deprotonated phenoxo O (O1 and O2) atoms, the four atoms are all from one deprotonated (L)²⁻ unit (Figure 4a). Geometry of Cu(II) atom can be best described as a slightly distorted square-planar with CuN₂O₂ coordination and deduced by using τ₄ index, τ₄ = 0.258 (Figure 4b) [51]. The two phenolic O (O1 and O2) atoms and the two oxime N (N1 and N2) atoms of the (L)²⁻ unit compose together the basal (Cu1-O1, 1.891(3); Cu1-O2, 1.909(3); Cu1-N1, 1.950(4) and Cu1-N2, 1.976(4) Å) with N2 and O2 above average by 0.239(2) and 0.292(2) Å, and N1 and O1 below average by 0.241(2) and 0.289(2) Å, respectively. Additionally, dihedral angles between the basal planes (N₂O₂ plane) and the benzene rings are 13.17(2)° and 14.74(2)°, respectively, which defined as shown in Figure 5a. Dihedral angle of planes N1-Cu1-N2 and O1-Cu1-O2 is 22.37(2)° (Figure 5b) [52].

As illustrated in Figure 6 and

Table 5. Hydrogen bondings (Å, °) for complex 1.

D-H · A	D · A	D-H · A
C10-H10 · O4	2.724(6)	103
C13-H13 · O7	2.715(5)	103
C12-H12B · O1	3.495(6)	156
C20-H20 · O3	3.284(6)	133
C25-H25 · O8	3.071(7)	141
C12-H12A · Cl1	3.561(6)	136

, complex 1 molecules form an infinite three-dimensional supra-molecular structure by inter-molecular hydrogen bonds, four pairs of inter-molecular hydrogen bonds C12-H12B···O1, C20-H20···O3, C25-H25···O8 and C12-H12A···Cl1 are formed [53].

3.5. Crystal Structure Description of Complex 2

Complex 2 presents a symmetric mononuclear structure, crystallizes in the monoclinic system, space group P 2₁/c, composes one Co(II) atom, one deprotonated (L)²⁻ unit, one coordinated methanol molecule and one crystalling chloroform molecule. Selected bond lengths and angles are listed in

Table 6. Selected bond lengths (Å) and angles (°) for complex 2.

Bond
Co1-O1	2.008(3)	Co1-N1	2.014(3)
Co1-O6	1.926(3)	Co1-N2	2.150(3)
Co1-O9	2.039(3)
Angles
O1-Co1-O9	90.67(12)	O6-Co1-N1	124.97(14)
O1-Co1-N1	88.52(12)	O6-Co1-N2	86.65(12)
O1-Co1-N2	178.25(13)	O9-Co1-N2	91.00(13)
O6-Co1-O1	93.23(11)	N1-Co1-O9	124.07(14)
O6-Co1-O9	110.91(13)	N1-Co1-N2	90.12(12)

.

As shown in Figure 7, the Co(II) atom is penta-coordinated by two oxime N (N1 and N2) and phenoxo O (O1 and O6) atoms from one deprotonated (L)²⁻ unit, and one O (O9) atom from the coordinated methanol molecule (Figure 7a). The coordination around the Co(II) atom is depicted as a trigonal bipyramid [54], and the τ value was estimated to be τ = 0.888 (Figure 7b) [55]. The phenolic O (O6) atom and the oxime N (N1) atom of the (L)²⁻ unit and one O (O9) atom of the coordinated methanol molecule compose together the basal plane (Co1-O6, 1.926(3); Co1-N1, 2.014(3) and Co1-O9, 2.039(3) Å), and other phenolic O (O6) atom and oxime N (N1) atom of the (L)²⁻ unit hold the axial positions (Co1-O1, 2.008(3) and Co1-N2, 2.150(3) Å). The dihedral angles of the N₂O₂ basal plane and the benzene rings are 30.35(2)° and 33.16(2)°, respectively, which defined as shown in Figure 8a. The dihedral angle of the planes N1-Co1-N2 and O1-Co1-O2 is 55.11(2)° (Figure 8b) [56].

As shown in Figure 9 and

Table 7. Hydrogen bondings (Å, °) for complex 2.

D-H···A	D···A		D-H···A
C10-H10···O3	2.682(4)		104
C13-H13A···O9	3.433(6)		145
C14-H14···O7	2.712(5)		103
C25-H25A···O1	3.041(7)		125
O9-H9···O2	2.651(4)		166
C6-H6···O6	3.315(4)		139
C13-H13B···O2	3.309(5)		134
C26-H26···O8	3.146(6)		156
C25-H25C···Cl3	3.642(6)		159
D-X···A	D-X	X···A	D···A	D-X···A
C26-Cl1···Cg6	1.757(5)	3.824	4.415(5)	97.59
C26-Cl2···Cg6	1.757(5)	3.849	4.415(5)	96.78
C26-Cl2···Cg4	1.757(5)	3.655	4.557(5)	109.67

Note: Cg6 = C₁₅–C₁₆–C₁₇–C₁₈–C₂₀–C₂₃; Cg4 = O₇–C₁₉–C₂₃.

, the three-dimensional supra-molecular network of complex 2 is made up of two parts. The first part is linked by inter-molecular hydrogen bond interactions, and five pairs of inter-molecular hydrogen bonds, C9-H9···O2, C6-H6···O6, C13-H13B···O2, C26-H26···O8 and C25-H25C···Cl3 are formed. The another part is made up of the C-Cl···π interactions. The Cg6 (C₁₅–C₁₆–C₁₇–C₁₈–C₂₀–C₂₃) of phenyl as acceptors forms two stacking interactions with the protons (-C26Cl1 and -C26Cl2). The Cg4 (O₇–C₁₉–C₂₃) of pyrone ring as acceptor forms one stacking interaction with -C26Cl2. In addition, complex 2 molecules form a three-dimensional infinite structure by O-H···O, C-H···O, C-H···Cl hydrogen bonds and C-Cl···π stacking interactions [57,58].

3.6. Crystal Structure Description of Complex 3

As illustrated in Figure 10 and

Table 8. Selected bond lengths (Å) and angles (°) for complex 3.

Bond
Ni1-O3	2.082(3)	Ni2-O3	2.014(3)
Ni1-O3#	2.082(3)	Ni2-O6	2.150(3)
Ni1-O8	2.031(3)	Ni2-O7	2.018(3)
Ni1-O8#	2.031(3)	Ni2-O9	2.040(3)
Ni1-O9#	2.089(3)	Ni2-N1	2.068(4)
Ni1-O9	2.089(3)	Ni2-N2	2.069(4)
Angles
O3-Ni1-O3#	180.0	O3-Ni2-O6	93.13(12)
O3#-Ni1-O9	101.51(11)	O3-Ni2-O9	80.97(11)
O3-Ni1-O9#	101.51(11)	O3-Ni2-N1	87.44(13)
O3-Ni1-O9	78.49(11)	O7-Ni2-O3	90.35(13)
O3#-Ni1-O9#	78.49(11)	O7-Ni2-O6	175.38(11)
O8#-Ni1-O3#	90.07(12)	O7-Ni2-O9	94.69(12)
O8#-Ni1-O3	89.93(12)	O7-Ni2-N1	91.74(15)
O8-Ni1-O3#	89.93(12)	O7-Ni2-N2	90.06(15)
O8-Ni1-O3	90.07(12)	O9-Ni2-O6	88.86(12)
O8#-Ni1-O8	180.0	O9-Ni2-N1	166.76(13)
O8#-Ni1-O9#	88.56(12)	O9-Ni2-N2	85.38(13)
O8-Ni1-O9	88.56(12)	N1-Ni2-O6	85.38(14)
O8#-Ni1-O9	91.44(12)	N1-Ni2-N2	106.20(14)
O8-Ni1-O9#	91.44(12)	N2-Ni2-O6	87.29(15)
O9#-Ni1-O9	180.0

, complex 3 exhibits a symmetric trinuclear structure, crystallizes in the triclinic system, space group P–1, includes two completely deprotonated (L)²⁻ units, three Ni(II) atoms, two benzoates and two coordinated methanol molecules.

The terminal Ni2 atom is hexa-coordinated by two phenolic O (O3 and O9) and two oxime N (N1 and N2) atoms of the completely deprotonated (L)²⁻ moity, one O (O7) atom from the coordinated benzoate group and one O (O6) atom from the coordinated methanol molecule. The terminal Ni2 atom possesses a distorted octahedron geometry [59]. The two oxime N (N1 and N2) and phenolic O (O3 and O9) atoms are in mutually cis-positions. Then, the central Ni1 atom is completed by double μ₂-phenoxo O (O3 and O9) atoms from two (L)²⁻ moities and two O (O8 and O8#) atoms from two benzoate groups. Each of the benzoate groups bridges the central Ni1 and terminal Ni2 atoms in the syn-syn bridging mode, as a result the central Ni1 atom finally possesses an O₂O₂O₂ coordination environment (Figure 10b) [59]. The dihedral angles between the benzene rings of (L)²⁻ moities and the N₂O₂ basal plane are 32.87(2)° and 23.66(2)°, respectively, which defined as shown in Figure 11a. The dihedral angle between the planes of N1–Ni2–N2 and O3–Ni2–O9 is 6.50(2)° (Figure 11b) [59].

As depicted in Figure 12 and

Table 9. Hydrogen bondings (Å, °) for complex 3.

D-H···A	D···A	D-H···A
C8-H8···O8	3.176(6)	122
C8-H8···O9	3.289(5)	133
C11-H11···O2	2.661(6)	103
C12-H12B···N2	2.878(9)	102
C13-H13B···O7	3.110(8)	142
C14-H14···O10	2.673(6)	101
C32-H32C···O8	3.362(6)	155
O6-H6···O1	2.741(5)	164
C12-H12A···O11	3.180(8)	154

, in complex 3, seven pairs of intra-molecular hydrogen bonds C8-H8···O8, C8-H8···O9, C11-H11···O2, C12-H12B···N2, C13-H13B···O7, C14-H14···O10 and C32-H32C···O8 are formed [60,61]. The protons (-C11H11) and (-C14H14) of (L)²⁻ moities form hydrogen bondings with two ester O (O2 and O10) atoms of (L)²⁻ moities, respectively. The proton (-C12H12B) from ethylenedioxime carbon atom of (L)²⁻ moities forms hydrogen bonding with oxime N (N2) atom. The proton (-C13H13B) from ethylenedioxime carbon atom of the (L)²⁻ moitie forms hydrogen bonding with carboxylate O (O7) atom of coordinated benzoate group. The proton (-C32H32C) from coordinated methanol molecule form hydrogen bonding with O (O8) atom of coordinated benzoate group. The proton (-C8H8) of the (L)²⁻ moity forms hydrogen bonds with phenoxo O (O9) atom of (L)²⁻ moity and carboxylate O (O8) atom of coordinated benzoate group, respectively [62,63].

As illustrated in Figure 13, two pairs of inter-molecular hydrogen bondings, O6-H6···O1 and C12-H12A···O11 are formed. The proton (-C12H12A) of the ((L)²⁻ moity forms hydrogen bondings with the O (O11) atom of four adjacent complex 3 molecules. Meanwhile, the proton (-O6H6) of the coordinated methanol molecule forms hydrogen bondings with the O (O1) atom from the (L)²⁻ moity of four adjacent complex 3 molecules. The space skeleton of complex 3 possesses a two-dimensional supra-molecular structure by the hydrogen bonding interactions [64].

Complexes 1, 2 and 3 with a coumarin-containing Salamo-type ligand H₂L have been synthesized, and have different structures depending on the anions and cations used. In complexes 1, 2 and 3, the metal atom located at the Salamo N₂O₂ unit is tetra-, penta- and hexa-coordinated, respectively. As widely known, the coordination number of Cu(II) atom is generally four or five in Salamo-type Cu(II) complexes and the coordination geometry around Cu(II) atom is planar quadrilateral, tetrahedron or tetragonal pyramid [26,37,49,51]. In complex 1, the coordination number of Cu(II) atom is four, the anions and the solvent molecules were not involved in the coordination. The coordination number of Co(II) atom is generally six, and the coordination geometry around Co(II) atom is octahedron [47,56,63,65]. While in complex 2, the methanol molecule was involved in the coordination forming a mono-nuclear Co(II) complex. The coordination geometry around Co(II) atom is trigonal bipyramid, which is rare in the previously reported Salamo-type Co(II) complexes. In complex 3, both of the anions and the solvent molecules were involved in the coordination, this coordination pattern is frequently appeared in the Salamo-type trinuclear Ni(II) complexes, in which the three Ni(II) atoms are all hexa-coordinated with slightly distorted octahedral geometries [12,31,44,59], but the research on the Salamo-type trinuclear Ni(II) complex contained benzoate ligands is reported firstly.

3.7. Antimicobial Activities

Bacteriostasis tests on common bacteria, such as E. coli and S. aureus, were carried out by the punch method. A small amount (0.1 mL) of a fresh overnight bacterial suspension was added into autoclaved LB (lysogeny broth) agar, then the agar was poured into sterile dishes. The concentration of the test compounds were 0.6, 1.2 and 2.4 mg/mL. 70 µL of samples were added into a burrowed hole measuring 5 mm in diameter with transfer liquid gun when the medium underwent solidification [65,66].

As shown in Figure 14 and Figure 15, the zones of DMF, metal salts, H₂L and complexes 1, 2 and 3 have apparent differences in antibacterial activity among two kinds of bacteria. The complexes 1, 2 and 3 displayed more enhanced antimicrobial activities than H₂L under the same conditions (2.5 mg/mL), and the DMF and the metal salts also have a weak biological activity. Moreover, E. coli shows stronger antibacterial activity, whereas S. aureus possesses weaker antibacterial activity. This increase in the antibacterial activities of complexes 1, 2 and 3 were accompanied with an increase in concentration and can be explained on the basis of the chelation theory [65,66]. Chelation reduces the polarity of the metal atom mainly due to partial share of positive charge of metal atom with donor groups and possible delocalization of π-electron within the whole chelate ring. Further, it enhances the lipophilic character of the central atom [66].

4. Conclusions

In summary, three newly designed complexes [Cu(L)]·CHCl₃ (1), [Co(L)(MeOH)]·CHCl₃ (2) and [{Ni(L)(MeOH)(PhCOO)}₂Ni] (3) derived from a Salamo-type coumarin-containing ligand (H₂L) have been successfully prepared and well characterized. For the central metals, the Cu(II) atom of complex 1 is tetra-coordinated and the Co(II) atom of complex 2 is penta-coordinated with trigonal-bipyramidal geometry, and the Ni(II) atoms of complex 3 are hexa-coordinated possessing slightly distorted octahedral geometries. The complexes 1 and 2 are both possess the three-dimensional supra-molecular structures by abundant noncovalent interactions. But complex 3 is formed a two-dimensional supra-molecular structure by intra-molecular hydrogen bonds. Furthermore, the antimicrobial and fluorescence properties of H₂L and complexes 1, 2 and 3 were also studied.