cobalt(III) complex: Synthesis, crystal structure, photoluminescence and magnetic properties

Accepted Manuscript New mixed valence defect dicubane cobalt(II)/cobalt(III) complex: Synthesis, crystal structure, photoluminescence and magnetic properties

Mustafa Burak Coban, Elif Gungor, Hulya Kara, Ulrich Baisch, Yasemin Acar PII:

S0022-2860(17)31393-5

DOI:

10.1016/j.molstruc.2017.10.049

Reference:

MOLSTR 24420

To appear in:

Journal of Molecular Structure

Received Date:

19 June 2017

Revised Date:

12 October 2017

Accepted Date:

15 October 2017

Please cite this article as: Mustafa Burak Coban, Elif Gungor, Hulya Kara, Ulrich Baisch, Yasemin Acar, New mixed valence defect dicubane cobalt(II)/cobalt(III) complex: Synthesis, crystal structure, photoluminescence and magnetic properties, Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.10.049

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ACCEPTED MANUSCRIPT

Highlights 

Solvotermal synthesis of a new defect dicubane cobalt(II)/cobalt(III),



Characterization of the complex by elemental analysis, UV, IR, single crystal X-ray diffraction,

visible

photoluminescence

susceptibility measurements.

and

variable–temperature

magnetic

ACCEPTED MANUSCRIPT Graphical Abstract A new defect dicubane cobalt(II)/cobalt(III), [(CoII2CoIII2L4 2(H2O)(CH3COO)(CH3COOH]. 4H2O (1) where H2L=[1–(3–hydroxypropyliminomethyl)naphthalene–2–ol], has been synthesized and structurally characterized. The solid state photoluminescence properties of complex (1) display red emission while H2L exhibits blue emission at room temperature. Variable–temperature magnetic susceptibility measurements on the complex (1) in the range 2–300 K indicate a antiferromagnetic interaction.

ACCEPTED MANUSCRIPT New mixed valence defect dicubane cobalt(II)/cobalt(III) complex: Synthesis, crystal structure, photoluminescence and magnetic properties Mustafa Burak Coban2, Elif Gungor1, Hulya Kara1,3, Ulrich Baisch4,5, Yasemin Acar1 1Center

of Sci. and Tech. App. and Research, Balikesir University, 10145, Balikesir, Turkey

2Department

of Physics, Faculty of Science and Art, Balikesir University, 10145, Balikesir, Turkey

3Department

of Physics, Faculty of Science, Mugla Sıtkı Kocman University, 48170, Mugla, Turkey

4Department 5School

of Chemistry, University of Malta, Msida, MSD2080, Malta

of Chemistry, Newcastle University, Bedson Building, Newcastle upon Tyne, NE1 7RU, UK

* Corresponding author: Dr. Elif GUNGOR Balikesir University, Faculty of Science and Art, Department of Physics 10145, Balikesir, Turkey Phone: +902666121200; Fax: +902666121215 Email: [email protected]

ACCEPTED MANUSCRIPT Abstract A new defect dicubane cobalt(II)/cobalt(III), [(CoII2CoIII2L42(H2O)(CH3COO)(CH3COOH]. 4H2O complex (1) where H2L=[1–(3–hydroxypropyliminomethyl)naphthalene–2–ol], has been synthesized and characterized by element analysis, FT–IR, solid UV–Vis spectroscopy and single crystal X–ray diffraction. The crystal structure determination shows a cationic tetrameric arrangement consisting of a defect dicubane core with two missing vertexes. Each cobalt ion has a distorted octahedral geometry with six coordinate ordered CoII and CoIII ions. The solid state photoluminescence properties of complex (1) and its ligand H2L have been investigated under UV light at 349 nm in the visible region. H2L exhibits blue emission while complex (1) shows red emission at room temperature. Variable–temperature magnetic susceptibility measurements on the complex (1) in the range 2–300 K indicate an antiferromagnetic interaction.

Keywords: Crystal structure; Defect dicubane; Tetranuclear cobalt(II)/cobalt(III) complex; Photoluminescence; Antiferromagnetic interaction

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1. Introduction Metal–chelate Schiff–base complexes are a well studied area of research in various fields such as coordination chemistry [1,2], fluorescence [3–6], catalysis [7,8], sensors [9,10], magnetic properties [11,12] and antitumor activities [13,14] etc. These metal complexes have shown high thermal stability, and good photoluminescent and electroluminescent properties [15,16]. They have absorption bands in the UV or near–UV region where most aromatic molecules show absorptions [16]. Therefore, 3d metal complexes can be used as fluorescent sensors for the detection of aromatic molecules, since the luminescence intensity is sensitive to the environment. Besides, different magnetic behaviour ranging from ferro– to antiferromagnetic interactions of metal complexes has been extensively investigated [17–19]. They are of particular interest since molecule–based magnets of these complexes have a variety of applications including their use in low–dimensional magnetic systems such as single–molecule magnets (SMMs) [19–21], quantum computing and magnetic refrigeration [21,22], data storage and memory devices [23]. Various coordination motifs of cobalt complexes have been described in the literature, in which the most important Co4O4 cores are butterfly, incomplete cubane, cubane, or defect dicubane topologies (Scheme 1)[8,12]. A number of butterfly, incomplete cubane, cubane, and defect dicubane cobalt complexes have also been studied in great detail for their various structural and interesting magnetic properties [24–27]. To date, the structural and magnetic characterization of few mixed-valence cobalt (II/III) complexes has been published by several research groups [12,20,28–30]. However, to the best of our knowledge, photoluminescence properties of Co(II) and Co(III) complexes are reported [31–35] while there are no reports on the solid state photoluminescence of mixed valence CoII/CoIII compounds in defect dicubane motifs. O Co Co

O

Co O

Co

Co

Co

O

Co Co O

O Co

Co O

Co

O

O

Co O

O

Co

O O Co

O

Co

O

Co

Butterfly

Incomplete cubane

Cubane

Defect dicubane

Scheme 1 Selected cores found in tetranuclear clusters

ACCEPTED MANUSCRIPT Our research group has successfully synthesized Schiff base metal complexes with different nuclearities and investigated their magneto–structural properties [36–41]. Here, we describe the synthesis, crystal structure, spectroscopic, photoluminescence and magnetic properties of a new mixed valence CoII/CoIII complex in a defect dicubane motif. 2. Experimental Section 2.1. Materials and Instrumentation All chemical reagents and solvents were purchased from Merck or Aldrich and used without further purification. Elemental (C, H, N) analyses were carried out by standard methods with a LECO, CHNS–932 analyzer. Solid state UV–vis spectra were measured with an Ocean Optics Maya 2000–PRO spectrometer. FT–IR spectra were measured with a Perkin–Elmer Spectrum 65 instrument in the range of 4000–600 cm–1. Powder X–ray diffraction measurements were performed using Cu–K radiation (λ = 1.5418 Å) on a Bruker– AXS D8–Advance diffractometer equipped with a secondary monochromator. The data were collected in the range 5< 2 <50 in − mode with a step time of ns (5 s < n < 10 s) and step width of 0.02. Solid state photoluminescence spectra in the visible region were measured at room temperature with an ANDOR SR500i–BL Photoluminescence Spectrometer, equipped with a triple grating and an air–cooled CCD camera as detector. The measurements were done using the excitation source (349 nm) of a Spectra–physics Nd:YLF laser with a 5 ns pulse width and 1.3 mJ of energy per pulse as the source. The temperature dependence of the magnetic susceptibility of polycrystalline samples was measured between 3 and 300 K at a field of 1.0 T using a Quantum Design model MPMS computer–controlled SQUID magnetometer. The effective magnetic moments were calculated by the equation µeff = 2.828 (χmT)1/2[42] where χm, the molar magnetic susceptibility, was set equal to Mm/H [42]. The synthetic route of the ligand is outlined in Scheme 2. X-ray single crystal diffraction data for complex (1) was collected on an Oxford Diffraction Gemini a Ultra diffractometer using graphite monochromated Mo–K radiation (λ = 0.71073 Å) at 150 K. Diffraction data of single-crystals was intrinsically weak lacking high resolution data. The raw data were reduced for Lorentz, polarization and absorption effects using the analytical numeric absorption correction technique [43].

ACCEPTED MANUSCRIPT O

H N

OH + H2N

OH

Ethanol OH OH

Scheme 2. The ligand H2L evaluated in this study. 2.2. Synthesis of Schiff base H2L and complex (1) The tridentate Schiff base ligand, H2L= [1– (3–hydroxypropyliminomethyl)naphthalene–2–ol] was prepared by reaction of 3 amino–1–propanol (0.0751ml, 1 mmol) with 2–hydroxy–1– naphthaldehyde (0.1721g, 1 mmol) in hot ethanol (60 cm3). The solution obtained was stirred at 65 °C for 10 min. The yellow complex was precipitated from solution on cooling. Analysis calculated for C14H14NO2: Calcd. C, 73.66; H, 6.18; N, 6.14%; Found: C, 73.59; H, 6.23; N, 6.11%. Yield: 80%. Complex (1) was prepared by addition of cobalt(II) acetate tetrahydrate (1 mmol, 0.249 g) in 20 cm3 of hot methanol to the ligand (1 mmol, 0.229 g) in 30 cm3 of hot ethanol. This solution has been warmed to 78 °C and stirred for 15 min. The resulting solution was filtered rapidly and then allowed to stand at room temperature. Several weeks of standing have led to the growth of clear dark violet crystals of the complex (1) suitable for X–ray analysis. Analysis calculated for C60H68.5Co4N4O17.5: Calcd. C, 52.93; H, 5.07; N, 4.12%; Found: C, 52.90; H, 5.05; N, 4.13%.Yield 75%. 2.3. X–ray Structure Determinations In the Olex2 program [44], the structure was solved by direct methods using SHELXS [45] and refined by full-matrix least-squares based on |Fobs|2 using SHELXL [46]. The nonhydrogen atoms were refined anisotropically, while the hydrogen atoms, generated using idealized geometry, were made to “ride” on their parent atoms and used in the structure factor calculations. Water molecules were entirely treated with rigid body restraints using RIGU in SHELXL [46]. The absolute structure could not be determined on the basis of the Flack [47] parameter as x = 0.46 (4) and thus, pseudo-inversion and/or inversion twinning is present and was considered during the refinement.

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3. Results and discussion 3.1. Crystal structure description of (1) The asymmetric unit of the tetranuclear complex (1) includes the [(CoII2CoIII2L42(H2O)(CH3COO)(CH3COOH] and four water molecules. The crystal structure determination shows that complex (1) is a defect dicubane structure with two missing vertices as shown in Fig. 1. All the cobalt ions show a distorted octahedral geometry, while the coordination environments for the cobalt ions are different. The two cobalt ions Co1 and Co3 exhibit identical N2O4 environments, in which two imine nitrogen atoms, and two alkoxy and two phenoxy oxygen atoms are from the Schiff base ligands. The remaining Co2 and Co4 display O6 environments, of which one oxygen atom derives from the water molecule, one from the acetate group, and three alkoxy and one phenoxy oxygen atoms from the Schiff base ligands, respectively. The charge balance requires a formal Co2IICo2III mixed valence description for the four Co ions in the cluster, a feature supported by the examination of the coordination Co–O bond distances. An analysis of the Co–O bond lengths, which are shorter for Co1 and Co3 (1.900 (9) – 1.963 (8) Å) than for Co2 and Co4 (1.985 (7) – 2.149 (9) Å) indicates that Co1 and Co3 are in a lower oxidation state than Co2 and Co4. Thus, the 3+ valence state is assigned to Co1 and Co3 and the 2+ valence state to Co2 and Co4 [10, 26]. Co···Co distances on different cubic faces of (1) are also different, varying from 3.035 to 3.206 Å, in a manner comparable to those of the similar tetranuclear cobalt complexes reported in the literature [20,30]. Moreover, the oxidation state of cobalt is analysed by bond valence sum using the published parameters [48] for Co-O and Co-N bonds. This analysis results in the assignment of the following values: Co1 = 3.408, Co2 = 1.984, Co3 = 3.533 and Co4 = 2.025, in clear agreement with the assumed +3 (for Co1 and Co3) and +2 charge for the latter [30,48]. In the crystal structure of (1), adjacent molecules are linked through intermolecular O– H···O hydrogen bonds (Table 3, Fig. 2). Molecules interacting through intramolecular and intermolecular O–H···O hydrogen bonds in (1) give rise to a 2D coordination network which lies in the ab–plane.

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Fig. 1. Molecular structure of (1). Dashed lines indicate hydrogen bonds.

Fig. 2. A two–dimensional network structure formed by hydrogen bonds of complex (1). The hydrogen atoms were omitted for clarity

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Table 1: Crystal data and structure refinement information for complex (1). Empirical formula Formula weight Crystal system Space group a/Å b/Å c/Å α/° β/° γ/° Volume/Å3 Z ρcalc g/cm3 μ/mm–1 Radiation 2Θ range for data collection/° Index ranges Reflections collected Independent reflections Data/Restraints/Parameters Goodness–of–fit on F2 Final R indexes [I>=2σ (I)] Flack parameter

C60H67Co4N4O17.5 1359.89 Monoclinic P21 11.7233(6) 22.3875(11) 12.2351(7) 90 106.152(6) 90 3084.4(3) 2 1.464 1.129 MoKα (λ = 0.71073) 5.95 to 58.938 –16 ≤ h ≤ 16, –30 ≤ k ≤ 26, –15 ≤ l ≤ 16 27528 13870 [Rint = 0.0508, Rsigma = 0.0754] 13870/53/739 1.059 R1 = 0.0897, wR2 = 0.1951 0.46(4)

Table 2: Selected geometric parameters (Å, º) and oxidation states for complex (1).

ACCEPTED MANUSCRIPT Co1–N1 Co1–N2 Co1–O1 Co1–O2 Co1–O3 Co1–O4 Co2–O3 Co2–O4 Co2–O5 Co2–O6 Co2–O12 Co2–O14

1.900 (10) 1.935 (11) 1.901 (10) 1.944 (9) 1.905 (9) 1.963 (8) 1.993 (7) 2.142 (9) 2.093 (8) 2.149 (9) 2.065 (11) 2.118 (11)

Co3–N3 Co3–N4 Co3–O5 Co3–O6 Co3–O7 Co3–O8 Co4–O2 Co4–O4 Co4–O6 Co4–O7 Co4–O9 Co4–O10

1.923 (11) 1.891 (11) 1.924 (9) 1.957 (8) 1.913 (9) 1.900 (9) 2.089 (8) 2.114 (9) 2.132 (9) 1.985 (7) 2.134 (10) 2.060 (10)

Co1–O2–Co4 Co1–O3–Co2 C14–O3–Co1 C14–O3–Co2 Co1–O4–Co2 Co1–O4–Co4

100.6 (4) 102.2 (4) 117.4 (8) 131.7 (9) 95.3 (4) 99.1 (3) oxidation state of Co 3+ 3.408 3.533

Co4–O4–Co2 Co3–O5–Co2 Co3–O6–Co2 Co3–O6–Co4 Co4–O6–Co2 Co3–O7–Co4

97.8 (4) 100.8 (4) 97.8 (3) 95.8 (4) 97.0 (3) 102.2 (4)

Co1 Co3 Co2 Co4

2+ 1.984 2.025

Table 3: Hydrogen bond geometry (Å, ) of complex (1). D—H···A D—H H···A D···A O9—H9A···O18 0.86 2.14 2.858 (17) O9—H9B···O8 0.86 2.16 2.834 (12) O14—H14A···O13 0.89 1.77 2.571 (17) O14—H14B···O1 0.90 2.10 2.849 (13) O16—H16A···O18i 0.87 2.00 2.83 (2) O16—H16B···O13 0.87 2.03 2.84 (2) O17—H17A···O15 0.85 1.85 2.68 (4) ii O17—H17B···O11 0.85 1.93 2.78 (2)

D—H···A 140 135 148 140 162 156 164 171

Symmetry code

−x, y−1/2, −z −x−1, y−1/2, −z

ACCEPTED MANUSCRIPT 3.2. X–ray powder diffraction pattern Before

proceeding

to

the

spectroscopic,

photoluminescence

and

magnetic

characterization, we note that experimental powder X–ray patterns for (1) are well in position with those of simulated patterns on the basis of single crystal structure of (1) (Fig. 3).

Fig. 3. X–ray powder diffraction pattern of complex (1). (Red– simulated from CIFs, Black Experimental). 3.3. IR spectroscopy Fig. 4 shows the IR spectrum of (1) in comparison with that of its free ligand H2L. The H2L ligand is verified mainly by the intense bands at around 3276–3170 for their stretching vibrations of aromatic ν(O–H) [43]. The presence of several weak peaks observed in the range 3029–2869 cm–1 is likely to originate from aromatic and aliphatic C–H stretches[31]. The strong absorption band occurring at 1614 cm–1 for (1) can be assigned to the C=N stretching frequency of the coordinated ligand, whereas the same band is observed at ca. 1616 cm–1 for H2L. The shift of this band towards lower frequency on complexation with the metals suggests coordination via the imino nitrogen atom in all the complexes[49]. The bands around 1493–1402 cm−1 in the free ligands can be attributed to phenolic C–O group vibrations. These bands move to higher or lower frequencies in complexes which indicate that oxygen atoms coordinate to metal ions [50]. The IR spectra of υ(H2O) of coordinated water appears at 866– 836 cm–1, indicating the binding of water molecules to the metal ions [51]. The peak around 1542 cm−1 can be assigned to the acetate group in complex (1) [11].

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Fig. 4. The IR spectrum of H2L ligand and complex (1) 3.4. Solid state UV–Vis Spectra The solid state UV–Vis spectrum of complex (1) have been analyzed in comparison with that of its free ligand H2L (Fig. 5). The UV–Vis spectrum of the H2L ligand displays a band at 345 and 464 nm whereas complex (1) shows two bands at 303 and 444 nm. The first band can be attributed to n–π∗ transition within the aromatic ring, while the second band would be due to π–π∗ transition within –C=N group. These bands at the high energy region are probably obscured by the intense charge transfer transitions [52]. It was not possible to identify d–d transitions due to a strong charge transfer band tailing from UV to the visible region.

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Fig. 5. The absorption spectrum of H2L and complex (1).

3.5. Photoluminescence Properties Coordination compounds have been reported to have the ability to adjust the emission wavelength of organic materials through incorporation of metal centres [40,53–55]. Therefore, it is important to investigate the luminescence properties of coordination compounds in view of potential applications as light–emitting diodes (LEDs). Owing to their ability to affect the emission wavelength and strength of organic materials, transition–metal centres can be an efficient method for obtaining new types of photoluminescent materials, especially for d10 or d10–d10 systems [54,56–58]. The solid–state photoluminescence properties of H2L and complex (1) have been investigated at room temperature in the visible region upon excitation at ex = 349 nm (Fig. 6). The free ligand H2L shows a strong blue emission band at max =482 nm which may be assigned to the nπ or ππ* electronic transition (ILCT) [59,60], while complex (1) shows two emission bands at max =469 and 682 nm, respectively. Complex (1) exhibits strong emission bands in comparison to the corresponding free ligand. The enhancement of luminescence in complex (1) may be attributed to the chelation of the ligand to the metal centre [60,61]. This enhances the “rigidity” of the ligand and thus reduces energy lost via a

ACCEPTED MANUSCRIPT radiationless pathway. The observed emissions of (1) are probably contributed by the nπ or ππ* intraligand fluorescence since similar emission was also observed for its ligand [59– 62].

Fig. 6. The emission spectrum of H2L ligand and complex (1) in solid samples at room temperature (λexc.= 349 nm). 3.6. Magnetic Properties Magnetic susceptibility measurements were performed on polycrystalline samples of complex (1) in the 2.34–300 K temperature range (Fig. 7). The main core of the cluster is a deformed cubane-like Co4O4 consisting of two CoII and two CoIII centres, which are bridged by four alkoxy and two phenoxy oxygen atoms. From a magnetic viewpoint, complex (1) is effectively a binuclear CoII complex since the CoIII sites are low spin and will be diamagnetic. The χmT value per CoII2CoIII unit at room temperature is 6.01 cm3 K mol-1, which is higher than the spin-only value of 3.75 cm3 K mol-1 expected for four independent spin systems (SCo(III), SCo(II), SCo(III), SCo(II)) = (0, 3/2, 0, 3/2) assuming g = 2.0. However, this is not surprising, and quite common for CoII clusters, indicating the persistence of a residual unquenched orbital contribution due to the 4T1 state of the high-spin Co(II), the effective g value of which is often found to be different from 2.00 [29]. We found g = 2.45 per ion from

ACCEPTED MANUSCRIPT the room temperature χmT value. Similar g values have been reported for Co(II) complexes [29]. The mT value gradually decreases with decreasing temperature until 21.55 K, reaching a maximum at 5.14 cm3 K mol−1 at 4.42 K and decreasing sharply to 4.27 cm3 K mol−1 at 2.34 K. (Fig. 9). Note that the decrease observed at low temperatures in the χmT value can be attributed to the combined action of spin–orbit coupling common for the HS-CoII ion and antiferromagnetic exchange interactions between the two CoII ions mediated by two alkoxidobridges [62]. The magnetic susceptibilities conform well with the Curie-Weiss law (m = C / (T )) in the range of 2.34–300 K for complex (1) with a negative Weiss constant ( = – 5.11 K and C = 6.082 cm3 K mol-1). The negative θ confirms the antiferromagnetic interactions between the CoII ions. As a complementary characterization, the field dependence of magnetization for complex (1) has been measured at 2.5 K (Fig. 8). Magnetization increases rapidly at low field and eventually reaches 4.36 NµB at 6 T. The magnetization curve shows a growing trend, which agrees with the assumption of the antiferromagnetic exchange interactions and the presence of low-lying excited states [62]. All these results indicate the presence of dominant antiferromagnetic interactions between the Co(II) ions together with magnetic anisotropy. Due to the significant orbital contribution to the magnetic moment, the magnetic exchange interaction is not possible to be accurately estimated and a satisfactory fit could not be achieved to describe the magnetic coupling between the CoII ions.

ACCEPTED MANUSCRIPT Fig. 7. Temperature dependence of χMT vs T and χM-1 vs T for complex (1). The solid red line represents the best fit using the Curie-Weiss law. Inset figure shows the magnetic exchange pathways of complex (1).

5

M / NμB

4 3 2 1 0

0

1

2

3 4 H / Tesla

5

6

Fig. 8. Magnetization as a function of the applied magnetic field for complex (1), measured at 2.5 K.

Conclusions A new defect dicubane cobalt(II)/cobalt(III) complex has been synthesised. Complex (1) was characterized by single crystal X–ray diffraction analysis, spectroscopic, photoluminescence and magnetic measurements. The photoluminescent studies of complex (1) indicate the red shift compared with the ligand and the emission intensity of complex (1) is stronger than that of its free ligand. They are novel potential candidates for applications in optoelectronic devices. Magnetic studies indicate that complex (1) exhibits an antiferromagnetic interaction between CoII ions together with magnetic anisotropy. Acknowledgements The authors are grateful to the Research Funds of Balikesir University (BAP-2017/161) for the financial support and Balikesir University, Science and Technology Application and Research Center (BUBTAM) for the use of the Photoluminescence Spectrometer. The authors are also very grateful to Prof. Dr. Andrea Caneschi (Laboratory of Molecular Magnetism,

ACCEPTED MANUSCRIPT Department of Chemistry, University of Florence) for the use of SQUID magnetometer and helpful suggestions.

Supplementary data Crystallographic data for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre (The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK; e–mail: [email protected]; www: http://www.ccdc.cam.ac.uk; fax: +44 1223 336033) and are available free of charge on request, quoting the Deposition No. CCDC 1579520. References [1] [2] [3] [4] [5] [6]

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