Synthesis, structure, and properties of a pentanuclear cobalt(III) coordination cluster

The synthesis, X-ray structure and properties of a pentanuclear cobalt(III) coordination cluster [{L(O2CCH3)Co2O(OCH3)2}2Co](ClO4)3 (1) (L− = 2,6-bis((3-aminopropylimino)methyl)-4-methylphenolate) are described. The dinucleating L− is coordinated with two cobalt(III) centers to form the {L(O2CCH3)Co2O(OCH3)2} unit, where each metal center is in a distorted octahedral N2O4 environment. The oxo and the methoxo ligands of these two dinuclear units assemble a distorted octahedral O6 coordination sphere around the central cobalt(III). Elemental analysis and spectroscopic (IR, NMR, UV–vis, and HRMS) features are consistent with the pentanuclear structure of the complex. The diamagnetic complex is a 1 : 3 electrolyte in solution. It is redox-active and displays a metal-centered reduction at E1/2 = −0.04 V (vs. Ag/AgCl).


Introduction
Polynuclear transition metal coordination clusters are of interest due to their potential applications in catalysis, photonics, electrochemistry, molecular magnetism, and their biological activity [1][2][3][4][5][6][7][8][9][10][11][12]. Their diverse applications arise from the physical properties that can be attributed to their molecular architectures. Consequently, there is interest in synthesizing new structurally intricate transition metal coordination clusters. The nuclearity and topology of such a cluster, and hence its overall molecular architecture is generally determined by the type of ligands used. Acyclic compartmental Schiff bases derived from 2,6-diformyl-4methylphenol or analogous dialdehydes provide complexes with a bimetallic core that is supported by various additional ancillary bridging ligands such as oxide, hydroxide, alkoxide, halide, and carboxylate. Self-assembly of this type of dinuclear complex via formation of additional bridges by the ancillary ligands of the primary dinuclear unit or by small bridging ligands can lead to extended polynuclear coordination clusters involving additional metal cations [13][14][15][16][17]. The magnetic properties of cobalt(II) have led to considerable interest in cobalt(II) coordination clusters as single molecule magnets [10,11,[17][18][19][20][21][22][23][24][25]. In an attempt to prepare a coordination cluster using cobalt(II) acetate and the acyclic compartmental Schiff base 2,6-bis((3-aminopropylimino)methyl)-4-methylphenol, we have isolated the diamagnetic pentanuclear cobalt(III) species Herein, we describe the synthesis, characterization, crystal structure, and spectroscopic properties of the perchlorate salt of this complex cation.

Materials
All chemicals and solvents used in this work were of reagent grade and were used as received.

Physical measurements
A Thermo Finnigan Flash EA1112 series elemental analyzer was used for the elemental (C, H, and N) analysis measurements. A Sherwood Scientific balance was used for magnetic susceptibility measurement. Solution electrical conductivity was measured using a Digisun DI-909 conductivity meter. The mass spectrum was obtained using a Bruker Maxis HRMS (ESI-TOF analyzer) spectrometer and the infrared spectrum was recorded using a Thermo Scientific Nicolet 380 FT-IR spectrometer. A Shimadzu UV-3600 UV-Vis-NIR spectrophotometer was used to collect the electronic spectrum. The 1 H (400 MHz) NMR spectrum was recorded using a Bruker NMR spectrometer. A CH Instruments model 620A electrochemical analyzer was used for the cyclic voltammetric measurements.

Synthesis of [{L(O 2 CCH 3 )Co 2 O(OCH 3 ) 2 } 2 Co](ClO 4 ) 3 (1)
A solution of 2,6-diformyl-4-methyl-phenol (50 mg, 0.3 mM) in CH 3 OH (10 mL) was added dropwise over a period of 10 min to a solution of 1,3-diaminopropane (0.05 mL, 44 mg, 0.6 mM) in CH 3 OH (10 mL), and the mixture was stirred at room temperature for 1 h. To the resulting yellow solution, solid Co(O 2 CCH 3 ) 2 ·4H 2 O (187 mg, 0.75 mM) was added in one portion and the solution was stirred for 10 min. The yellow color of the reaction mixture turned dark brown. To this brown solution, a CH 3 OH solution (5 mL) of NaClO 4 ·H 2 O (85 mg, 0.6 mM) was added and the mixture was refluxed for 6 h. The solution was then cooled to room temperature and allowed to evaporate slowly to approximately half of the initial volume in 2 days. The resultant dark brown needle-shaped crystals were collected by filtration. This material was recrystallized three times from a CH 3 CN-C 6 H 5 CH 3 mixture (20 mL, 1 : 3 ratio) to remove the cobalt(II) impurity. The recrystallized product was dried in air, crushed to give a powder, and stored under vacuum. The yield of the dark green powder varied from 50 to 64 mg (24-30%) and was used for elemental analysis, spectroscopic, and electrochemical measurements. Anal. Calcd for

X-ray crystallography
Complex 1 crystallizes as 1·C 6 H 5 CH 3 as well as 1·C 6 H 5 CH 3 ·CH 3 CN from the CH 3 CN-C 6 H 5 CH 3 (1 : 3) solution. X-ray data for 1·C 6 H 5 CH 3 and 1·C 6 H 5 CH 3 ·CH 3 CN were collected on a Bruker-Nonius SMART APEX CCD and Oxford Diffraction Xcalibur Gemini single crystal X-ray diffractometers, respectively, using graphite monochromated-Mo Kα radiation (λ = 0.71073 Å). Data were collected at 100 K for both crystals. The SMART and the SAINT-Plus packages [26] were used for data acquisition and data extraction, respectively, and SADABS [27] was used for the absorption correction for 1·C 6 H 5 CH 3 . The CrysAlisPro software [28] was used for data collection, reduction, and absorption correction for 1·C 6 H 5 CH 3 ·CH 3 CN. The structures were solved by direct methods and refined on F 2 by a full-matrix least-squares procedure. In each structure, all non-hydrogen atoms were refined using anisotropic thermal parameters. The hydrogens were included in the structure factor calculation at idealized positions using a riding model. Structure solution and refinement were performed using the SHELX-97 programs [29] available in the WinGX package [30]. The Platon [31] and Mercury [32] packages were used for molecular graphics. Selected crystallographic data are summarized in table 1.

Spectroscopic characteristics
The infrared spectrum of 1 recorded in a KBr pellet (figure S1, Supplementary data) shows two closely spaced bands at 3600 and 3540 cm −1 and a group of bands from 3260 to 2820 cm −1 due to the NH 2 and the C−H vibrations, respectively. The C=N stretch is located at 1640 cm −1 [41,42]. The strong band at 1580 cm −1 is attributed to the bridging acetate asymmetric stretch. The band at 1470 cm −1 that overlaps with the broad and strong band centered at 1460 cm −1 is tentatively assigned to the symmetric stretch of the bridging acetate. The ClO À 4 anions in 1 show a characteristic strong broad band centered at 1080 cm −1 and the sharp strong band at 623 cm −1 .
The electronic spectrum of 1 in CH 3 CN displays multiple bands from 400 to 200 nm ( figure S2, Supplementary data). All the absorptions are intense, with the extinction coefficients (ε) in the order of 10 4 -10 5 M −1 cm −1 . These bands are therefore likely to arise primarily from ligand-to-metal charge transfer and ligand-centered transitions [34,43]. The 1 H NMR (400 MHz) spectrum of 1 was recorded in CD 3 CN (figure 4). The azomethine protons and the phenolate meta-protons of the two L − appear as two fourproton singlets at 8.44 and 7.52 ppm, respectively. The protons of the methyl groups at the para positions of the phenolate rings and the methyl protons of the two acetate bridges resonate as two singlets at 1.68 and 1.82 ppm, respectively. In contrast, the protons of the four bridging methoxo groups do not appear as a single signal, but instead as two singlets at 2.51 and 2.33 ppm. The multiplet (8H) centered at 4.01 ppm is attributed to the protons of the four methylene groups adjacent to the four azomethine groups. The broad singlet (8H) at 2.59 ppm is tentatively assigned to the protons of the central methylene groups of the four propylene fragments of the two dinucleating L − . The protons of the methylene groups adjacent to the amine functionalities are observed at 3.85, 2.86, 2.42, and 2.25 ppm as a two-proton multiplet or a broad singlet. Four somewhat broad signals are observed at 6.61, 5.45, 4.87, and 3.75 ppm. The absence of these signals in the presence of D 2 O indicates that these are the four NH 2 protons. The non-equivalent disposition is only observed for -CH 2 NH 2 and the bridging methoxo group protons, which are nearest to the Co 5 core at the center of the cationic complex.

Redox features
The electron transfer characteristics of 1 have been studied by cyclic voltammetry. The measurements were carried out with a platinum disk working electrode, a platinum wire auxiliary electrode, and an Ag/AgCl reference electrode at 298 K under nitrogen for ã 10 −3 M solution of 1 in CH 3 CN with tetrabutylammonium perchlorate as the supporting electrolyte. The ferrocenium/ferrocene (Fc + /Fc) couple was observed at E 1/2 = 0.50 V under identical conditions. The cyclic voltammogram of 1 displays a reduction response on the cathode side of the reference electrode ( figure 5). The instability of the reduced species is indicated by the large cathodic peak current (i pc ) at E pc = −0.28 V compared with the anodic peak current (i pa ) at E pa = 0.20 V and the large ΔE p (= E pa − E pc ) value. Generally, ligand-based reductions of free as well as metal-coordinated Schiff bases are observed at much more negative potentials than the reduction response observed for 1 [43][44][45][46][47]. Indeed, a second irreversible broad reduction peak with E pc centered at −1.25 V was observed on scanning the solution of 1 further, up to −2.0 V (figure S3, Supplementary data); thus this second reduction is most likely a ligand-centered process. The bond lengths in the X-ray structure of 1 indicate that the central cobalt(III) with a O 6 environment is somewhat weakly bonded, and hence less stable and more prone to reduction than the four-terminal cobalt(III) centers with N 2 O 4 coordination geometry (vide supra). In addition, the i pc of the first reduction is also comparable with the peak currents of Fc + /Fc and other known one-electron redox processes [34,48,49]. Based on the above, the first response of 1 with E 1/2 = −0.04 V is assigned to the reduction of the central Co III O 6 to Co II O 6 .

Conclusion
A diamagnetic redox-active pentanuclear cobalt(III) coordination cluster [{L(O 2 CCH 3 ) Co 2 O(OCH 3 ) 2 } 2 Co] 3+ has been synthesized by a one-pot reaction. The cobalt(III) cluster comprises two dinucleating N 2 ON 2 -donor ligands L − , two μ-acetato, two μ 3 -oxo, and four μ-methoxo ligands. The X-ray crystal structure shows that the two neutral dinuclear {L(O 2 CCH 3 )Co 2 O(OCH 3 ) 2 } units create a distorted octahedral O 6 environment through their facially disposed oxo and methoxo groups, which coordinate the fifth cobalt(III) center. The bond parameters in addition to the redox behavior indicate that the central metal center is relatively weakly bonded and its +3 oxidation state is less stabilized compared with the metal centers in the dinuclear units. Our current efforts are focused on the synthesis of analogous hetero-and homometallic coordination clusters of other transition metal cations using HL and related Schiff bases.