Synthesis and Fluorescence Properties of a New Heterotrinuclear Co(II)-Ce(III)Complex Constructed from a bis(salamo)-Type Tetraoxime Ligand.

[Co₂(L)Ce(OAc)₃(CH₃CH₂OH)]·1.5CH₃OH∙0.5CH₂Cl₂, a heterotrinuclear Co(II)-Ce(III) bis(salamo)-type complex with a symmetric bi(salamo)-type ligand H₄L and an acyclic naphthalenediol moiety, was designed, synthesized and characterized by elemental analyses, FT-IR, UV-Vis and fluorescence spectroscopy and X-ray crystallography. The X-ray crystallographic investigation revealed the heterotrinuclear complex consisted of two Co(II) atoms, one Ce(III) atom, one (L)4‒ unit, three μ₂-acetate ions, one coordinated ethanol molecule, one and half crystallization methanol molecule and half crystallization dichloromethane molecule. Two Co(II) atoms located in the N₂O₂ coordination spheres, are both hexacoordinated, with slightly distorted octahedral geometries. The Ce(III) atom is nine-coordinated and located in the O₆ cavity possesses a single square antiprismatic geometry. In addition, supramolecular interactions exist in the Co(II)-Ce(III) complex. Two infinite 2D supramolecular structures are built via intermolecular O-H···O, C-H···O and C-H···π interactions, respectively.

1. Introduction

Salen (N,N-disalicylideneethylenediamine) is a versatile important compound that has been widely used in coordination chemistry and organometallic chemistry [1,2,3,4,5,6,7,8]. On the one hand, salen-type compounds are tetradentate ligands with a N2O2 coordination environment that can usually coordinate with transition metal ions to afford diverse metal complexes. On the other hand, a number of salen-type metal complexes have already been synthesized to study their structures and applied in various fields for their magnetic properties [9,10,11,12,13,14], catalytic action [15,16], electrochemistry [17,18], in biological systems [19,20,21,22,23,24,25,26,27,28], supramolecular architectures [29,30,31,32,33,34,35,36,37], their luminescent properties [38,39,40,41,42,43,44], and in optical sensors [45] and nonlinear optical materials [46].

More recently, salamo-type ligands [47,48,49,50,51,52,53,54,55,56] using an O-alkyloxime (–CH=N–O–(CH2)–O–N=CH–) have been reported. In our previous studies on salamo-type metal complexes, we exchanged salicylaldehyde for its derivatives to obtain some new salamo-type complexes with different structures. Our research group is focused on the synthesis and study of 3d–4f heterometallic salamo-type complexes [14,16,49].

Based on these points of view, we have now designed and synthesized a symmetric bi(salamo)-type ligand H4L and its corresponding heterotrinuclear Co(II)-Ce(III) 3d–4f complex [Co2(L)Ce(OAc)3(CH3CH2OH)]∙1.5CH3OH∙0.5CH2Cl2. The structure of H4L is depicted in Figure 1. Furthermore, the structure and fluorescence properties of the Co(II)-Ce(III) complex were studied.

Figure 1

Structural representation of the ligand H4L.

2. Results and Discussion

2.1. IR Spectra

The IR spectra of H4L and its corresponding Co(II)-Ce(III) complex exhibited various bands in the 4000–400 cm−1 region. The FT-IR spectrum and data of the ligand H4L and its corresponding Co(II)-Ce(III) complex are given in Figure 2 and Table 1.

Figure 2

IR spectra of the ligand H4L and its complex.

Table 1

Selected FT-IR bands for H4L and its Co(II)-Ce(III) complex (cm−1).

Compound	ν(O–H)	ν(C=N)	ν(Ar–O)
H4L	3169	1612	1255
Complex	–	1623	1239

A C=N stretching vibration band was observed at 1612 cm−1 in the IR spectrum of H4L. Upon complex formation, the strong characteristic C=N vibration band of the Co(II)-Ce(III) complex appeared at 1623 cm−1, which is slightly red-shifted in comparison to the free ligand H4L and is attributed to coordination of the nitrogen atoms of the C=N group and the metal(II) atoms [57]. The Ar–O stretching vibration band is detected in a range of 1220–1260 cm−1. In the free ligand H4L the characteristic Ar–O group absorption appeared at 1255 cm−1, a ca. 16 cm−1 shift to a lower frequency showing that the M–O bonds are formed between the metal atoms and the oxygen atoms from methoxy and phenolic groups of the free ligand H4L [58]. The above facts are consistent with the results determined by X-ray diffraction.

2.2. UV-Vis Absorption Spectra

In many studies, UV-Vis absorption spectra have been utilized to study lanthanide complexes. In this study, the UV-Vis spectra of the free ligand H4L in CHCl3:CH3OH (1:1) (c = 2.5 × 10−5 mol L−1) with its corresponding Co(II)-Ce(III) complex in CH3OH:H2O (10:1) (c = 1 × 10−3 mol L−1) were collected in the range of 250–550 nm. In the absorption spectrum of the free ligand H4L, there are four consecutive absorption peaks at ca. 269, 342, 360 and 375 nm. The absorption peak at 269 nm can be assigned to the π–π* transition of the benzene rings. The other three absorption peaks can be attributed to the π–π* transition of the oxime groups [57].

In the UV-Vis titration experiment, it can be clearly seen that the gradual addition of Co(OAc)2 solution caused absorption peak changes. Compared to the free ligand H4L, the absorption peaks are bathochromically shifted [59]. This phenomenon is due to the coordination of H4L with the Co(II) ions. Upon addition of Co(II) ions, the absorbance of the solution first increases. When Co(II) ions were added in excess of three equiv, the absorbance of the solution no longer changed. The spectroscopic titration clearly showed the formation of a 1:3 Co(II) complex (Figure 3a).

Figure 3

(a) UV-Vis spectral changes of the H4L (2.5 × 10−5 M) on addition of Co(II) (1.0 × 10−3 M) ions; (b) UV-Vis spectral changes of the [LCo3]2+ on addition of Ce(III) (1.0 × 10−3 M) ions.

The color of the solution changed unconspicuously when Ce(III) ions were added. Then, upon addition of 1 equiv of Ce(II) ions, the absorbance changed and showed three isoabsorptive points at about 313, 365 and 380 nm. The spectroscopic titration clearly exhibited that the ratio of the replacement reaction stoichiometry is 1:1 and is shown in Figure 3b.

The importance of the coordination of OAc− was confirmed by the following experiment. A 1:3:1 mixture of H4L, Co(NO3)2 and Ce(NO3)3 displayed an absorption spectrum identical to that of H4L, indicating no complexation. Spectrophotometric titration of the mixture with KOAc showed that nine equiv of KOAc were required to convert the mixture to [LCo2Eu(OAc)3]. The nine equiv of OAc-consists of six for deprotonation and three for coordination to the trinuclear core.

2.3. Crystal Structure Description

The crystal structure of the Co(II)-Ce(III) complex was determined by X-ray crystallography and is shown in Figure 4.

Figure 4

(a) View of the molecular structure of the Co(II)-Ce(III) complex; (b) Coordination polyhedrons for Co(II) and Ce(III) atoms.

X-ray crystallographic analysis revealed that the Co(II)-Ce(III) complex crystallizes in a triclinic system, space group of P-1, possessing a symmetric trinuclear structure. The Co(II)-Ce(III) complex consists of two Co(II) atoms, one Ce(III) atom, one (L)4‒ unit, three μ2-acetate ions, one coordinated ethanol molecule, one and half crystallization methanol molecules and half crystallization dichloromethane molecule. As shown in Figure 4, we can see that the coordination ratio of the ligand (L)4− unit to metal atoms (Co(II) and Ce(III)) in the Co(II)-Ce(III) complex is 1:2:1 [60,61]. Meanwhile, the terminal Co(II) atoms (Co1 and Co2) are located in the N2O2 compartments and they are both hexa-coordinated with slightly distorted octahedral geometries [62,63,64]. Differently, the Co1 atom is bonded to two μ2-acetate ions (O14 and O16), the nitrogen atoms (N1 and N2) and oxygen atoms (O1 and O2) of the oxime and phenolic groups. The Co2 atom is coordinated the nitrogen atoms (N3 and N4) and oxygen atoms (O6 and O7) of the oxime and phenolic groups, one μ2-acetate oxygen atom (O11) and one oxygen atom (O13) from the coordinated ethanol molecule. The central O6 site (O1, O2, O3, O6, O7 and O8) was occupied by one Ce(III) atom coordinating to three oxygen atoms (O12, O15 and O17) of three μ2-acetate ions. Hence, the Ce(III) atom is nine-coordinated with a single square antiprismatic geometry. In the crystal structure of the Co(II)-Ce(III) complex, three μ2-acetate ions bridge Co(II) and Ce(III) atoms in a familiar μ2-fashion mode. The high coordination number of Ce(III) atom is determined by its longer ionic radius and smaller winding angles. The Ce–O bond distances of the four phenolic oxygen (O1, O2, O6 and O7) atoms from the completely deprotonated (L)4− unit are in the range of 2.421(4)–2.523(3) Å and the Ce–O bond distances of the methoxy groups are about Ce1-O3, 2.688(4) and Ce1-O8 2.655(4) Å, which are clearly shorter than later. The crystallographic data and structural refinement paramenters are summarized in Table 2. Selected bond lengths and angles of the Co(II)-Ce(III) complex are listed in Table 3.

Table 2

Crystallographic data and structural refinement paramenters for the Co(II)-Ce(III) complex.

Formula	C42H50ClCo2CeN4O18.50
Formula weight, g·mol−1	1200.29
Temperature, K	293.66(10)
Wavelength, Å	0.71073
Crystal system	Triclinic
Space group	P-1
a, Å	12.0521(5)
b, Å	13.7804(5)
c, Å	15.2909(4)
α, °	95.678(3)
β, °	99.471(3)
γ, °	90.210(3)
Volume, Å3	2492.13(15)
Z	2
Calculated density, mg·m−3	1.600
Absorption coefficient, mm−1	1.685
F (000)	1214
θ range for data collection, °	3.390–25.677°
h/k/l(min, max)	−12, 14/−16, 16/−18, 18
Reflections collected	17180
Independent reflections	9641
Rint	0.034
Completeness to θ = 26.32	99.78%
Data/restraints/parameters	9641/60/649
Final R indices [I > 2σ (I)] a[I > 2σ(I)]	R1 = 0.0519, wR2 = 0.1377
R indices (all data) b	R1 = 0.0693, wR2 = 0.1537
Goodness-of-fit for F2 c	1.020
Largest differences peak and hole (e Å−3)	1.130 and −1.300

a R1 = Σ‖Fo| − |Fc‖/Σ|Fo|. b wR2 = [Σw(Fo2 − Fc2)2/w(Fo2)2]1/2, w = [σ2(Fo2) + (0.0784P)2 + 1.3233P]−1, where P = Fo2 + 2Fc2)/3. c GOF = [Σw(Fo2 − Fc2)2/nobs − nparam)]1/2.

Table 3

Selected bond lengths (Å) and angles (°) for the Co(II)-Ce(III) complex.

Bonds Lengths (Å)			Bonds Lengths (Å)
Co1–O1	2.018(3)	Co1–O2	2.081(4)	Co1–O14	2.088(4)
Co1–O16	2.123(4)	Co1–N1	2.095(5)	Co1–N2	2.116(4)
Co2–O6	2.021(4)	Co2–O7	2.052(4)	Co2–O11	2.058(5)
Co2–O13	2.168(6)	Co2–N3	2.123(5)	Co2–N4	2.139(7)
Ce1–O1	2.421(4)	Ce1–O2	2.461(3)	Ce1–O3	2.688(4)
Ce1–O6	2.467(4)	Ce1–O7	2.523(3)	Ce1–O8	2.655(4)
Ce1–O12	2.437(5)	Ce1–O15	2.528(5)	Ce1–O17	2.526(4)
Angle (°)			Angle (°)
O1–Co1–O2	90.51(15)	O1–Co1–O14	87.26(16)	O1–Co1–O16	88.60(16)
O1–Co1–N1	176.65(19)	O1–Co1–N2	83.73(17)	O2–Co1–O14	88.61(15)
O2–Co1–O14	88.61(15)	O2–Co1–O16	85.93(15)	O2–Co1–N1	86.51(17)
O2–Co1–N2	171.58(16)	O14–Co1–O16	173.12(17)	O14–Co1–N1	94.18(18)
O14–Co1–N2	97.21(18)	O16–Co1–N1	89.67(18)	O16–Co1–N2	87.79(18)
N1–Co1–N2	99.1(2)	O1–Ce1–O2	73.22(12)	O1–Ce1–O3	128.52(12)
O1–Ce1–O6	62.41(11)	O1–Ce1–O7	115.41(12)	O1–Ce1–O8	136.97(13)
O1–Ce1–O12	129.53(13)	O1–Ce1–O15	68.24(13)	O1–Ce1–O17	69.66(13)
O2–Ce1–O3	60.44(13)	O2–Ce1–O6	135.35(13)	O2–Ce1–O7	147.78(12)
O2–Ce1–O8	91.55(11)	O2–Ce1–O12	130.03(14)	O2–Ce1–O15	71.51(13)
O2–Ce1–O17	72.66(13)	O3–Ce1–O6	151.91(15)	O3–Ce1–O7	115.69(13)
O3–Ce1–O8	68.54(13)	O3–Ce1–O12	74.57(14)	O3–Ce1–O15	112.71(14)
O3–Ce1–O17	75.97(14)	O6–Ce1–O7	64.67(13)	O6–Ce1–O8	124.04(12)
O6–Ce1–O12	79.85(14)	O6–Ce1–O15	95.34(15)	O6–Ce1–O17	87.10(15)
O7–Ce1–O12	70.43(14)	O7–Ce1–O8	60.38(11)	O7–Ce1–O15	82.89(13)
O7–Ce1–O17	139.42(13)	O8–Ce1–O12	91.27(14)	O8–Ce1–O15	68.78(14)
O8–Ce1–O17	144.42(13)	O12–Ce1–O15	152.40(15)	O12–Ce1–O17	76.59(15)
O15–Ce1–O17	130.63(14)

2.4. Supramolecular Interactions

Notably, supramolecular interactions exist in the Co(II)-Ce(III) complex. The hydrogen bonds and C–H···π stacking interactions are listed in Table 4.

Table 4

Hydrogen bonds (Å, deg) and C–H···π stacking interactions for the Co(II)-Ce(III) complex.

D–H···A	d(D–H)	d(H···A)	d(D···A)	∠DHA	Symmetry Code A
C9–H9B···O14	0.97	2.32	3.249(8)	161
C36–H36C···O15	0.96	2.20	2.879(16)	126
C23–H23B···N4	0.97	2.46	2.865(11)	105
C24–H24B···N3	0.97	2.56	2.957(11)	104
O13–H13···O18	0.87	1.88	2.733(18)	170	x, y, 1 + z
O19–H19A···O16	0.82	2.05	2.834(9)	160	−x, −y, 1 − z
C34–H34A···Cl1	0.96	2.76	3.446(16)	129	−1 + x, y, z
C36–H36A···Cl1	0.96	2.34	2.955(17)	121	x, y, 1 + z
C41–H41B···O17	0.97	2.38	3.345(16)	173	1 + x, y, z
C24–H24A···Cg1		2.97	3.599(10)	124	−x, −y, −z

Symmetry codes: Cg1 for the Co(II)-Ce(III) complex is the centroid of C27–C32 atoms.

In the crystal structure of the Co(II)-Ce(III) complex, there are four significant intramolecular hydrogen bonds (C9–H9B···O14, C23–H23B···N4, C24–H24B···N3 and C36–H36C···O15) (Figure 5) [65,66,67,68,69]. As illustrated in Figure 6 and Figure 7, three pairs of intermolecular hydrogen bonds (O13–H13···O18, O19–H19A···O16 and C41–H41B···O17) and two intermolecular C-H···Cl interactions (C34–H34A···Cl1 and C36–H36A···Cl1) are formed, respectively [70,71,72,73,74]. Especially, an infinite 2D supramolecular structure are interlinked by one significant C–H···π interactions (C24–H24A···Cg1 (C27–C32)) and is shown in Figure 8.

Figure 5

View of the intramolecular hydrogen bonds for the Co(II)-Ce(III) complex.

Figure 6

View of 2D supramolecular structure by the intermolecular O–H···O and C–H···O interactions of the Co(II)-Ce(III) complex.

Figure 7

View of 1D supramolecular structure by the intermolecular C–H···Cl interactions of the Co(II)-Ce(III) complex.

Figure 8

View of 2D supramolecular structure by C–H···π interaction of the Co(II)-Ce(III) complex.

2.5. Fluorescence Tests

Recently, synthesis of several examples of lanthanide complexes of salamo-type ligands to study their luminescence properties have been reported [75,76]. The fluorescence emission spectra of H4L in CHCl3:CH3OH solution and its corresponding Co(II)-Ce(III) complex in methanol solution were investigated at room temperature. In the fluorescence titration experiment, the emission spectrum of the free ligand H4L exhibited broad visible photoluminescence with maximum emission at ca. 416 nm upon excitation at 340 nm (Figure 9a). When Co(II) ions was added, the fluorescence emission intensity quenches. The fluorescence emission intensity of the solution no longer changed after the Co(II) ions was added in excess of three equiv. The spectroscopic titration indicated that the stoichiometric ratio between Co(II) and the ligand H4L is 3:1 (Figure 9b). Then, the fluorescence intensity enhances upon addition of Ce(III) ions and showed a maximum at ca. 433 nm upon excitation at 341 nm (Figure 10a). When the amount of Ce(III) ions reaches one equiv, the fluorescence emission intensity of the solution becomes stable. Obviously, the spectroscopic titration indicated that the ratio of the replacement reaction was 1:1 (Figure 10b), which obtained the same conclusion with UV-Vis titration experiments.

Figure 9

(a) The excitation and emission spectra of the Co(II) complex. (b) fluorescence emission spectrum changes of H4L in methanol solution by the addition of Co(II) ions.

Figure 10

(a) The excitation and emission spectra of the Co(II)-Ce(III) complex; (b) Fluorescence spectrum changes of Co(II) complex in methanol solution by the addition of Ce(III) ions.

3. Experimental Section

3.1. Materials and Methods

2-Hydroxy-3-methoxybenzaldehyde (99%), methyltrioctylammonium chloride (90%), pyridinium chlorochromate (98%) and boron tribromide (99.9%) were bought from Alfa Aesar (New York, NY, USA) 33 wt % hydrobromic acid solution in acetic acid was purchased from J&K Scientific Ltd. (Beijing, China). The other general reagents and solvents in this work were used directly without further purification in the preparation of the free ligand and its complex. Elemental analyses (C, H and N) were carried out using a VarioEL V3.00 automatic elemental analysis instrument (Elementar, Berlin, Germany).Elemental analyses for metals were performed on an ER/S·WP-1 ICP atomic emission spectrometer (IRIS, Elementar, Berlin, Germany).Melting points were measured using a microscopic melting point apparatus made by Beijing Taike Instrument Limited Company(Beijing, China) and were uncorrected. Infrared spectra were recorded between 500 and 4000 cm−1 on a VERTEX 70FT-IR spectrophotometer (Bruker, Billerica, MA, USA)for samples prepared as KBr pellets. UV-Vis spectra in the 250–550 nm range were recorded by a U–3900H spectrometer (Hitachi, Shimadzu, Tokyo, Japan). Fluorescence spectra were taken on a Hitachi F–7000 fluorescence photometer.(Hitachi, Tokyo, Japan). 1H-NMR spectra were determined with a German Bruker AVANCE DRX-400 spectrometer (Bruker AVANCE, Billerica, MA, USA). X-ray single crystal structure determination was carried out on a SuperNova Dual (Cu at zero) four-circle diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 293.66(10) K (Bruker, Billerica, MA, USA). Reflection data were corrected for Lorentzian and polarization effects and for absorption using the multi-scan method.

3.2. Synthesis of the Bi(salamo)-Type Ligand H4L

The synthesis of the bi(salamo)-type ligand H4L is shown in Scheme 1. 2,3-Dihydroxynaphthalene-1,4-dicarbaldehyde was prepared according to a literature procedure [77]. 2,3-Dihydroxynaphthalene-1,4-dicarbaldehyde and 2-[O-(1-ethyloxyamide)]oxime-6-methoxy- phenol were synthesized according to an analogous method [78,79]. A mixed solution of 2,3-dihydroxynaphthalene-1,4-dicarbaldehyde (108.1 mg, 0.50 mmol) in ethanol (10 mL) and 2-[O-(1-ethyloxyamide)]oxime-6-methoxyphenol (226.1 mg, 1.00 mmol) in ethanol (10 mL) was heated at 55 °C for 4 h. After cooling to room temperature, the obtained yellow precipitate was filtered off and dried under vacuum to obtain light yellow crystalline solid. Yield: 59%. m.p. 170–171 °C. Anal. Calcd. (%) for C32H32N4O10 (632.42): C, 60.75; H, 5.10; N, 8.86. Found (%): C, 60.93; H, 5.23; N, 8.74. 1H-NMR (400 MHz, CDCl3): no instrument listed in 3.1 δ (ppm) = 11.03 (s, 2H), 9.82 (s, 2H), 9.14 (s, 2H), 8.29 (s, 2H), 7.97 (q, J = 3.2 Hz, 2H), 7.41 (q, J = 6.0, 2.9 Hz, 2H), 7.06–6.68 (m, 6H), 4.58 (t, 8H), 3.89 (s, 6H). UV-Vis [in methanol/chloroform (1:1)], λmax (nm) [2.5 × 10−5 M]: 342, 360, 375.

Scheme 1

Synthetic route to the bi(salamo)-type ligand H4L.

3.3. Synthesis of the Heterotrinuclear Co(II)-Ce(III) Complex

A mixed solution of Co(OAc)2·4H2O (4.98 mg, 0.02 mmol) in ethanol (2 mL) and Ce(OAc)3·H2O (3.35 mg, 0.01 mmol)) in water/methanol (1:1, 2 mL) was added to a solution of the ligand H4L (6.32 mg, 0.01 mmol) in dichloromethane (2 mL). Then the resulting mixed solution immediately turned yellow and was stirred at room temperature for 30 min. The mixture was filtered and the filtrate was allowed to stand at room temperature for approximately four weeks, giving colorless prismatic single crystals suitable for X-ray crystallographic analysis. Yield: 37%. Anal. Calcd. (%) for C42H50ClCo2CeN4O18.50 (1200.29): C, 42.03; H, 4.20; N, 4.67; Co, 9.82; Ce, 11.67. Found (%): C, 42.25; H, 4.47; N, 4.51; Co, 9.64; Ce, 11.49. UV-Vis [in methanol/H2O (10:1 v/v)], λmax (nm) [1.0 × 10−3 M]: 346, 364, 380.

3.4. Crystal Structure Determination and Refinement

The structure was solved by direct methods (SHELX-2014) and refined anisotropically using full-matrix least-squares methods on F2 with the SHELX-2014 program package. Lp and semi-empirical absorption corrections by SADABS were applied to the intensity data. The non-hydrogen atoms were refined anisotropically except for the solvent molecules of the crystal of the Co(II)-Ce(III) complex. All hydrogen atoms were added in calculated positions. Supplementary crystallographic data for this paper have been deposited at Cambridge Crystallographic Data Centre (1814394) and can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.

4. Conclusions

The synthesis, structural characterization, and fluorescence properties of the bis(salamo)-type ligand H4L and its corresponding heterotrinuclear Co(II)-Ce(III) complex were described. The X-ray crystal structure revealed that two Co(II) atoms are located in the N2O2 coordination environment and are both hexacoordinated with slightly distorted octahedral geometries. Simultaneously, the O6 cavity of the completely deprotonated (L)4− unit is occupied by the Ce(III) atom which is nine-coordinated with a single square antiprismatic geometry. The UV-Vis titration experiments revealed the ratio of the heterotrinuclear Co(II)-Ce(III) complex is 1:2:1 (ligand/Co(II)/Ce(III)). Ethanol as a coordinating solvent participates in the coordination in the Co(II)-Ce(III) complex. The different peak wavelength variation of the heterotrinuclear complex clearly showed the success of transformation from homotrinuclear to heteronuclear complex, which could be used in host-guest systems. Furthermore, the Co(II)-Ce(III) complex showed weak photoluminescence and exhibiting a hypsochromic-shift.