Ultrasonic Clusterization Process to Prepare [(NNCO)6Co4Cl2] as a Novel Double-Open-Co4O6 Cubane Cluster: SXRD Interactions, DFT, Physicochemical, Thermal Behaviors, and Biomimicking of Catecholase Activity.

A novel double-open-cubane (NNCO)6Co4Cl2 cluster with a Co4O6 core was made available under aqua-ultrasonic open atmosphere conditions for the first time. The ultrasonic clusterization of the (3,5-dimethyl-1H-pyrazol-1-yl)methanol (NNCOH) ligand with CoCl2·6H2O salts in ethanol yielded a high-purity and high-yield cluster product. Energy-dispersive X-ray (EDX), Fourier transform infrared (FT-IR), and ultraviolet (UV)-visible techniques were used to elucidate the clusterization process. The double-open-Co4O6 cubane structure of the (NNCO)6Co4Cl2 cluster was solved by synchrotron single-crystal X-ray diffraction (SXRD) and supported by density functional theory (DFT) optimization and thermogravimetric/differential TG (TG/DTG) measurements; moreover, the DFT structural parameters correlated with the ones determined by SXRD. Molecular electrostatic potential (MEP), Mulliken atomic charge/natural population analysis (MAC/NPA), highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO), density of states (DOS), and GRD quantum analyses were computed at the DFT/B3LYP/6-311G(d,p) theory level. The thermal behavior of the cluster was characterized to support the formation of the Co4O6 core as a stable final product. The catalytic property of the (NNCO)6Co4Cl2 cluster was predestined for the oxidation process of 3,5-DTBC diol (3,5-di-tert-butylbenzene-1,2-diol) to 3,5-DTBQ dione (3,5-di-tert-butylcyclohexa-3,5-diene-1,2-dione).

Introduction

Lately, exceptional care for the coordination chemistry domain has been focused on nitrogen heteroaromatic alcohol molecules like a family of commercially accessible compounds acquiring adaptability in complexation potentiality.[1−8] A large category of fascinating engineering of complexes is built from N,O– donor alcohol building units.[9−14] Due to their optimal and fine characteristics, these molecules are extensively utilized like a crux for the construction of important materials based on polynuclear clusters.[15−18] The design and synthesis of polynuclear metal cluster-based coordination cages have attracted much interest, due to their esthetic structure and fascinating quantum mechanical properties, in applications including information storage, quantum computing, gas storage, drug delivery, conversion of CO2, and crucial contribution in the catalysis of the splitting of water via enzymatic photosystem II {PS-II} of photosynthetic organisms.[19−21] This economic and ecofriendly model is the foundation for all envisioned solar fuel-based green energy strategies; accordingly, the biomodel is considered to be the efficient, simple, and clear starting phase for any bioinspired strategy.[22−25] As a result, the draft and assembly of the polynuclear metal cluster-based coordination cages are already charming and yet remain a synthetic challenge.[26−28] While most advances concerned the challenging preparation and characterization of the Mn-cluster oxygen evaluation conversion {OEC} mimic,[29−35] {Co4O6} cubane water oxidation catalysts {WOCs} appear to be preferred and ideal typical plan model systems for the essential oxidation reaction[36−40] and the creation of excellent photoanodes.[41−48]

In our recent work, we have synthesized several types of clusters based on cobalt, copper, and nickel ions with a general form {M4O6}, in addition to mixed samples with copper and cadmium like a bimetallic {Cd–O–Cu} double-open-cubane cluster using pyrazole alcohol compounds. The structures of most clusters were identified by single-crystal X-ray diffraction, and their oxidation properties were tested by conversion of catechol to O-quinone as an oxidation model.[49−55] In this context, the multifaceted double-open-cubane [(NNCO)6Co4Cl2] cluster was achieved by ultrasonic clusterization of the 1-hydroxymethyl-3,5-dimethylpyrazole (NNCOH) ligand with Co(II). The crystal structure of the desired [(NNCO)6Co4Cl2] cubane cluster was determined by synchrotron single-crystal X-ray diffraction (SXRD); moreover, several physicochemical and density functional theory (DFT) analyses in addition to the catalytic capacity of the cluster have been evaluated.

Experimental Section

Computational and SXRD

In the gaseous phase, all the DFT calculations were carried out via Gaussian09 software at the B3LYP/6-311G (d,p) level of theory.[56] Single-crystal X-ray diffraction experiments were performed at ID11, the Materials Science Beamline of the ESRF, Grenoble, France,[57] at room temperature, using a monochromatic beam with a wavelength (λ) of 29339 Å, (≈42.26 keV, relative bandwidth Δλ/λ ∼10–3) and a sample–detector distance of 118.81 mm. The beam was focused with a Si refractive compound lens system[58] to 500 nm. The detector beamline has been recently upgraded to a Dectris photon counting Eiger2 4M CdTe. The images were then converted into the “Esperanto” format using the script Eiger2esperanto, a portable image converter based on the FabIO library,[59] to export Eiger frames to a set of Esperanto frames, which can be imported into CrysalisPro software.[60] The converted detector images were successively indexed, and the intensities were estimated and corrected for Lorentz polarization effects using the CrysalisPro package. Scaling and correction for absorption were carried out by the semiempirical ABSPACK routine implemented in CrysalisPro. The crystal structure was solved by direct methods via the package SIR2019[61] and refined using full-matrix least-squares techniques using SHELXL2014/7.[62] All non-hydrogen atoms were refined anisotropically; the H atoms were placed at calculated positions, and their atomic coordinates were refined according to a riding model; the constraints on the isotropic U value of H atoms in the case of C–H and C–H2 groups were Uiso(H) = 1.2 Ueq(C), and in the case of the methyl group, it was Uiso(H) = 1.5 Ueq(C). Additionally used computer programs were Mercury[63] for molecular graphics and WinGX[64] and publCIF[65] for preparing the published material.

Materials and Synthesis

Commercially available solvents and materials used in this study were purchased from Sigma-Aldrich.

Cluster synthesis: In ultrasonic medium, an ethanolic solution of CoCl2·6H2O (94.25 mg, 1.0 mmol in 20 mL) was mixed to a suspension solution of NNCOH (50 mg, 1.0 mmol in 12 mL). The change in the color of the mixture to dark brown by ∼2 h supported the possibility of ligand–metal clusterization; then, the reaction mixture was stirred for ∼5 min at RT before it was filtered. The filtrate was allowed to stand for 5 days, after the solvent was evaporated; suitable X-ray diffraction (XRD) crystals were collected with ∼81% yield.

Results and Discussion

Synthesis, CHN-EA, and EDX of the Cluster

The multifaceted tetranuclear cubane (NNCO)6Co4Cl2 cluster was synthesized by one to one equivalent amounts of CoCl2·6H2O with 1-hydroxymethyl-3,5-dimethylpyrazole {NNCOH} under ambient conditions for 2 h (Scheme ). The clusterization reaction to prepare the [(NNCO)6Co4Cl2] cluster was performed in ethanol and in free oxygen atmosphere, resulting in 80% yield with no side products. The clusterization reaction of NNCOH with CoCl2·6H2O was successfully monitored via FT-IR, UV–vis, and energy-dispersive X-ray (EDX) spectroscopy; the nonclassical double-open-Co4O6 cubane structure was confirmed by SXRD for the first time.

Scheme 1

Schematic Representation of the Synthesis of the [(NNCO)6Co4Cl2] Cluster

The atomic content of the [(NNCO)6Co4Cl2] cluster was verified by CHN-EA and EDX. The calculated CHN-EA data from C36H54Cl2Co4N12O6 molecular formula are C, 40.89; N, 15.89; and H, 5.15% and found to be C, 40.76; N, 15.69; and H, 5.24% (Table ). EDX reflected only the signals of Co, C, O, N, and Cl that corresponded to the elemental composition of the (NNCO)6Co4Cl2 desired cluster; moreover, a high degree of purity was achieved since no unknown signals were observed, as can be seen from Figure .

Table 1

Cluster Refinement Data

chemical formula	C36H56Cl2Co4N12O6
Mr	1059.53
crystal system, space group	monoclinic, P21/n
temperature (K)	293
a, b, c (Å)	10.4826 (2), 18.5156 (2), 11.6440 (3)
β(°)	95.603 (2)
V(Å3)	2249.20 (S)
Z	2
radiation type	synchrotron, λ = 0.29339 Å
μ (mm–1)	0.15
crystal size (mm)	0.09 × 0.06 × 0.02
absorption correction	empirical (using intensity measurements)
no. of measured, independent, and observed [I > 2σ(I)] reflections	38068, 5410, 5008
Rint	0.059
(sin θ/λ)max (Å–1)	0.667
R[F2 > 2σ(F2)], wR (F2), S	0.051, 0.145, 1.14
no. of reflections	5410
no. of parameters	278
H-atom treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å–3)	1.01, −0.60
CCDC	2099518

Figure 1

EDX of the [(NNCO)6Co4Cl2] cluster.

SXRD and DFT Optimization

The formation of the studied [(NNCO)6Co4Cl2] cluster was proved via SXRD and DFT optimization analysis (Figure ); moreover, the comparison of the geometric parameters of the crystal structure determined by SXRD with those of the DFT-B3LYP/6-311G(d,p)-optimized structure is shown in Figures and Tables and 3. The tetranuclear Co(II) double-open-cubane [(NNCO)6Co4Cl2] cluster C36H56Cl2Co4N12O6 crystallizes as a dichloride neutral cluster, in the monoclinic crystal system, with the P21/n space group and unit cell parameters a = 10.4826 (2), b = 18.5156 (2), c = 11.6440 (3) Å, and β = 95.603 (2)°.

Figure 2

[(NNCO)6Co4Cl2] cluster: (a) View of the asymmetric unit with the atomic labeling scheme, (b) view of the local environment of the asymmetric unit showing the coordination of the Co(II) centers; and (c) ORTEP view showing all the H-bond types and lengths.

Figure 3

(a) Histogram of DFT/XRD bond distances and its (b) correlation coefficient and (c) histogram of angles and its (d) correlation coefficient.

Table 2

DFT/SXRD- Bonds Lengths (Å) and Angles (°)

no.	bond		XRD	DFT	no.	angles			XRD	DFT
1	Co1	O2	2.120(2)	2.1165	1	O2	Co1	O3	80.60(8)	95.12
2	Co1	O3	2.020(2)	1.8926	2	O2	Co1	O5	100.45(8)	104.66
3	Co1	O5	2.027(2)	1.9499	3	O2	Co1	N6	94.95(8)	93.54
4	Co1	O2	2.109(2)	1.9168	4	O2	Co1	N3	156.40(8)	169.42
5	Co1	N6	2.105(2)	2.0404	5	O2	Co1	O2	76.61(7)	79.61
6	Co1	N3	2.139(2)	1.919	6	O3	Co1	O5	178.90(8)	169.22
7	Co2	O2	2.326(2)	1.9165	7	O3	Co1	N6	100.99(9)	102.32
8	Co2	O3	1.970(2)	1.8592	8	O3	Co1	N3	79.35(9)	79.61
9	Co2	N2	2.059(2)	1.9191	9	O3	Co1	O2	99.04(8)	101.05
10	Co2	Cl1	2.333(1)	2.399	10	O5	Co1	N6	78.65(8)	77.24
11	Co2	O5	1.967(2)	1.9498	11	O5	Co1	N3	99.69(8)	101.05
12	O2	C7	1.376(3)	1.386	12	O5	Co1	O2	81.53(7)	84.35

Table 3

[(NNCO)6Co4Cl2] Cluster: H-Bond Interactions (Å, °)a

D—H···A	D—H	H···A	D···A	D—H···A
C7—H7A···N3	0.97	2.55	3.204 (4)	125
C7—H7B···N6i	0.97	2.53	3.196 (4)	126
C22—H22A···Cl1	0.96	2.66	3.585 (4)	163
C24—H24A···O5	0.96	2.64	3.419 (4)	139
C28—H28B···Cl1	0.97	2.91	3.512 (4)	121

Symmetry code: (i) −x + 2, −y, −60z + 2.

A view of the asymmetric unit (consisting of 2Co,1Cl, 3O, 6N, 18C, and 27H atoms) and its local environment showing the molecular structure of the [(NNCO)6Co4Cl2] cluster together with the double-open-cubane [Co4O6] core is given in Figure a,b, respectively.

The asymmetric unit included two crystallographically independent cobalt(II) cations and three NNCO– anion ligands. One of these two Co(II) centers (i.e., Co1) was octahedrally coordinated (five of the six atoms at the vertices of the Co1-centered octahedron were symmetry-independent and the sixth one was symmetry-dependent), and the second Co(II) center had a trigonal bipyramid coordination. No Co–Co direct bonds were detected; four oxide atoms acted as trigonal bridges and two acted as tetrahedral bridges to connect Co(II) centers in the double-open-cubane core. The bond lengths concerning the Co1 atom belonged to the range 2.020(2)–2.139(2) Å, and the octahedron centered at Co1 was distorted, with two bond angles involving opposite vertices, N6—Co1—O2 and O2—Co1—N3, equal to 156.67 (8) and 156.41 (9), respectively, and far from the ideal value of 180° (i.e., the typical value of undistorted octahedra). The presence of some H-bond interactions such as H...O, H...N, and H...Cl stabilizing the crystal packing was detected (see Table and Figure c).

To study the compatibility of the crystal structural parameter data with their DFT- computational counterparts, a group of the selected bonds and angles (Table ) was compared, and the results are illustrated in Figure . The XRD-structural parameters are in high agreement with the DFT-results, as can be deduced from Figure . The XRD and DFT bond distances were very similar, showing a linear relation with very good agreement (Figure a), characterized by a correlation coefficient of 0.976 (Figure b). Moreover, XRD and DFT angles well agreed as shown in Figure c, with 0.961 correlation coefficient (Figure d).

FT-IR

The synthesis of the double-open-cubane [(NNCO)6Co4Cl2] cluster was tracked via FT-IR as seen in Figure . The IR spectra of the (NNCOH) free ligand before and after coordinating to the CoCl2·6H2O center to prepare the [(NNCO)6Co4Cl2] cluster have been recorded as seen in Figure S2 (see the Supporting Information). The biggest change that supported the clusterization interaction is the disappearance of the O–H band of the NNOH free ligand at 3161 cm–1 due to ionic bonding with the Co(II) center, resulting in a new Co–O band at 880 cm–1 in addition to the Co–N peak at 480 cm–1. Moreover, taking into account the slight difference in the chemical shifts due to the bonding, various elongate frequency vibrations were apparent in the free ligand and the [(NNCO)6Co4Cl2] cluster, for example, C–H aliphatic and aromatic, C=C, N=N, C=C, C–O, Co–O, Co-N, and Co–Cl

Figure 4

(a) Highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) and (b) DOS of the desired cluster.

UV–visible Behavior

To get more information about the absorbance behavior of the cluster, UV–vis of both the free ligand and the cluster was measured at 200–800 nm using methanol as a solvent. The free ligand in methanol reflected a π-to-π electron transfer at 280 nm; this peak is present also in the cluster at the same wavelength (Figure S3). Two new signs in the 600–750 nm range appeared when the free ligand clustered the CoCl2·6H2O to form the desired double-open-cubane (NNCO)6Co4Cl2 cluster. The new two visible bands at 625 and 730 nm of the cluster can be attributed to metal d-to-d transitions as seen in Figure S3 (see the Supporting Information).

MEP and MAC/NPA Charges

The B3LYP/6311G(d,P) MEP map, natural population analysis (NPA), and Mulliken atomic charge (MAC) calculations of each atom in the (NNCO)6Co4Cl2 cluster are illustrated in Figure S4 (see the Supporting Information) and Table . The result of MEP showed the presence of several positions that were characterized by the presence of electronic abundance nucleophiles (in red color) and the lack of electronic electrophiles (in blue color), but most of the functional groups were neutral green in color as shown in Figure S4a. The chlorine, oxygen, and nitrogen atoms have high nucleophilic properties; meanwhile, the cobalt and many hydrogen atoms have high electrophilic properties, as seen in Figure S4b. The MAC and NPA charges of each atom showed N, O, Cl, and some C atoms with negative charge also (Figure S4c and Table ). Moreover, the Co and all H and most of the C atoms have positive charge. Moreover, a linear relation between MAC and NPA charges with a very good correlation coefficient (0.937) has been recorded, as shown in Figure S4d. The NPA/MAC charge MPE map results show strong correlation with the XRD interaction results.

Table 4

NPA and MAC Charge Population

no.	atom	MAC	NPA	no.	atom	MAC	NPA
1	O	–0.61192	–0.71664	58	N	–0.41002	–0.23065
2	O	–0.74039	–0.73435	59	C	–0.63462	–0.72028
3	O	–0.59988	–0.74906	60	C	–0.69888	–0.70459
4	Co	0.749812	0.90812	61	H	0.171175	0.2415
5	Co	1.313264	1.16056	62	H	0.15928	0.2415
6	Co	0.69633	0.9081	63	H	0.296498	0.25902
7	Co	1.198007	1.1355	64	H	0.191114	0.22977
8	O	–0.63176	–0.71662	65	H	0.210016	0.25055
9	O	–0.62157	–0.7491	66	H	0.277143	0.23287
10	O	–0.72395	–0.73435	67	H	0.206697	0.25355
11	N	–0.4359	–0.27402	68	H	0.201031	0.23852
12	N	–0.3588	–0.27405	69	H	0.191371	0.22977
13	C	0.228433	0.13631	70	H	0.303718	0.25054
14	C	–0.30213	–0.31998	71	H	0.223622	0.25904
15	C	0.347492	0.12595	72	H	0.217928	0.25356
16	N	–0.58023	–0.44436	73	H	0.219633	0.23853
17	C	0.212681	0.13629	74	H	0.212682	0.23287
18	C	–0.28762	–0.31998	75	H	0.323296	0.22879
19	C	0.348974	0.12592	76	H	0.264224	0.23983
20	N	–0.57808	–0.44439	77	H	0.234604	0.23982
21	C	–0.64475	–0.69986	78	H	0.321058	0.22878
22	C	–0.64813	–0.70736	79	H	0.19268	0.25315
23	C	–0.65064	–0.69987	80	H	0.371661	0.25979
24	C	–0.63962	–0.70736	81	H	0.22736	0.25138
25	Cl	–0.48207	–0.65481	82	H	0.20611	0.24657
26	C	–0.07934	0.06848	83	H	0.226174	0.2585
27	Cl	–0.48869	–0.65482	84	H	0.233265	0.25236
28	C	–0.08533	0.06846	85	H	0.249471	0.27029
29	N	–0.39312	–0.21297	86	H	0.192307	0.25315
30	C	0.268381	0.21808	87	H	0.225657	0.2585
31	C	–0.28125	–0.32847	88	H	0.227319	0.25236
32	C	0.359936	0.19949	89	H	0.238174	0.2703
33	N	–0.5649	–0.36161	90	H	0.202812	0.24658
34	C	–0.74427	–0.71543	91	H	0.314703	0.25978
35	C	–0.63493	–0.71983	92	H	0.261264	0.25137
36	N	–0.57786	–0.36164	93	H	0.361865	0.2573
37	C	0.283897	0.1995	94	H	0.252158	0.20743
38	C	–0.29263	–0.32846	95	H	0.235215	0.20743
39	C	0.275884	0.21812	96	H	0.26782	0.2573
40	N	–0.38704	–0.21297	97	H	0.278851	0.24014
41	C	–0.64342	–0.71983	98	H	0.259978	0.21831
42	C	–0.66239	–0.71542	99	H	0.187784	0.25352
43	C	–0.05727	0.04911	100	H	0.244682	0.24706
44	C	–0.02998	0.04909	101	H	0.220848	0.24252
45	C	0.025032	0.08098	102	H	0.327339	0.27302
46	N	–0.40379	–0.23065	103	H	0.234011	0.25553
47	C	0.237129	0.19036	104	H	0.225788	0.25943
48	C	–0.27384	–0.33345	105	H	0.231886	0.25634
49	C	0.295062	0.19908	106	H	0.255377	0.21832
50	N	–0.57309	–0.37329	107	H	0.278613	0.24015
51	C	–0.69018	–0.70457	108	H	0.185523	0.25352
52	C	–0.63256	–0.72028	109	H	0.246257	0.25634
53	C	–0.00798	0.08096	110	H	0.224411	0.25553
54	N	–0.56859	–0.37326	111	H	0.222022	0.25944
55	C	0.406898	0.19911	112	H	0.203215	0.24253
56	C	–0.29802	–0.33345	113	H	0.314655	0.27304
57	C	0.277697	0.19037	114	H	0.27	0.24705

HOMO → LUMO, DOS, and GRD

The highest occupied molecular orbital(HOMO), lowest unoccupied molecular orbital (LUMO), and frontier molecular orbitals played a considerable role in the evaluation of the optical and chemical relativities of the prepared molecules.[51−55] The ΔEHOMO/LUMO energy bandgap is a helpful parameter for determining activity and stability molecular properties.[56] HOMO/LUMO shapes and energies have been characterized and then compared to DOS values; the elaborated HOMO and LUMO energy levels were found to be −2.0003 and −1.2131 eV, respectively. The calculations reflected a little amount of energy needed for the electron to be transferred from the HOMO to the LUMO with ΔEHOMO/LUMO = 0.7878 eV (Figure a) that is consistent with a recently similar reported system.[53] Moreover, the energy gap was supported also via DOS calculation; the ΔEDOS was 0.9011 eV that is very close to the ΔEHOMO/LUMO result as represented in Figure b.

The main GRD parameters such as the electrophilicity (ω), softness (σ), chemical potential (μ), hardness (η), and electronegativity (χ) of the desired cluster were calculated viaeqs –8 listed below, and the results are summarized in Table .

Table 5

GRD Quantum Parameters

GRD		value
global total energy	ET	–8919.9848 a.u.
low unoccupied molecular orbital	LUMO	–0.0446 a.u.
high occupied molecular orbital	HOMO	–0.0735 a.u.
energy gap	ΔEgap	0.0289 a.u.(0.7873 eV)
electron affinity	A	1.2131 eV
ionization potential	I	2.0003 eV
global hardness	η	0.3936 eV
global softness	σ	2.5406 eV
chemical potential	μ	–1.6123 eV
absolute electronegativity	X	1.6123 eV
electrophilicity	ω	3.2895 eV
dipole moment	u	1.4242 D

Thermal Behavior

The TG and DTG signals belonging to the [(NNCO)6Co4Cl2] cluster are illustrated in Figure . The cluster was decomposed via three steps; no small groups like methyl that decomposed early before 180 °C have been recorded. The first thermal decay of the cluster was recorded above 180–250 °C with TDTG = 230 °C and 31% mass lost (theoretical 30.8%). Depending on the lost mass calculation, such a step can be attributed to 3NNC fragmented from the NNCO ligand resulting in the [(NNCO)6Co4Cl2] → [(NNCO)3Co4O3Cl2] decomposition step. The second step was also due to the next 3NNC fragmented from the NNCO ligand resulting in [(NNCO)3Co4O3Cl2] → [Co4O6Cl2] decomposed at 300–380 °C with TDTG = 348 °C and 31% mass lost (theoretical 30.8%). The third step was attributed to loss of 2Cl resulting in the [Co4O6Cl2] → [Co4O6] decomposition step from 420 to 460 °C with TDTG = 424 °C and 7.1% mass lost (theoretical 6.8%). The stability (460–800 °C) and the quantity of the final mass residue (32.8%) greatly supported the possibility of cobalt oxide Co4O6 matrix formation (Figure ).

Figure 5

TG/DTG of the (NNCO)6Co4Cl2 cluster with heating for 5 min.

Catalytic Activity of 3,5-DTB to 3,5-DTB Using the (NNCO)6Co4Cl2 Cluster

One of the main objectives of this work is to test the ambient aerobic oxidation catalytic properties of 1,2-diol to 1,2-dione using the (NNCO)6Co4Cl2 cluster, as seen in Scheme .

Scheme 2

Aerobic Oxidation of 3,5-DTBC to 3,5-BTBQ Catalyzed by the (NNCO)6Co4Cl2 Cluster

To achieve our objective, we mixed 0.1 M 3,5-di-tert-butylbenzene-1,2-diol (3,5-DTBC) with 1 × 10–5 M prepared cluster in the DMSO solvent under aerobic conditions for about 1h (Scheme ). The production of the 3,5-di-tert-butylcyclohexa-3,5-diene-1,2-dione (3,5-DTBQ) product was tracked via UV–visible analysis at 400 nm in 350–500 nm (Abs. vs λ), as seen in Figure a. The catalytic capacity toward the oxidation of 3,5-DTBC was compared to a reference matrix with no cluster, as observed in Figure b. The sample containing the catalyst showed a very high activity; completeness was achieved within half an hour (Figure b), and meanwhile, no oxidation process was recorded under the same reaction conditions in the absence of the cluster.[48−53,66−68]

Figure 6

Aerobic oxidation of 0.1 M 3,5-DTBC using 1 × 10–5 M cluster as a catalyst in DMSO at RT, (a) Abs. vs wavelength (nm), the spectra were noted in the 5 min range, (b) Abs. (at λ = 400 nm) vs time for oxidation processes with and without the cluster, and (c) Michaelis–Menten rate vs [Sub.] and (d)1/rate vs 1/[Sub].

An important aspect of obtaining the catalyst kinetic data is the elucidation of pertinent kinetic parameters, Km and Vmax, across the aerobic oxidation process via Michaelis–Menten graphs, as seen in Figure c,d. In DMSO, promising Vmax = 17.53 μmol L–1 min–1 and Km = 0.04 mol L–1 values have been calculated for the aerobic RT oxidation of 0.1 M 3,5-DTBC using 1 × 10–5M cluster concentration as a catalyst.

Conclusions

For the first time, a novel double-open-cubane (NNCO)6Co4Cl2 cluster has been made available under ultrasonic media. The double-open Co4O6 cubane core in the (NNCO)6Co4Cl2 cluster was clearly confirmed by SXRD and thermally proved by TG/DTG. The clusterization process of NNCOH with CoCl2 salts was monitored by EDX, FT-IR, and UV–visible methods. In the crystal structure, the double-open-cubane cluster, both octahedral and trigonal bipyramids around the Co(II) center geometries, has been observed; moreover, the oxygen atoms acted as trigonal and tetrahedral bridges to connect the Co(II) centers. The SXRD characterization showed the presence of polar H...O, H...N, and H...Cl interactions that stabilized the Co4O6 core cluster lattice. The DFT optimization structural parameters match very well the corresponding SXRD-refined parameters; MAC/NPA and MEP electrostatics computations agree with the outcomes of the SXRD study, concerning the charge of each atom and their binding ability with their surroundings. GRD, HOMO/LUMO, and DOS quantum analyses confirmed the stability of the cluster and proved the convergence of the energy gap. The double-open-cubane (NNCO)6Co4Cl2 cluster reflected a high thermal stability; three-step thermal decomposition was needed to reach the Co4O6 final stable core. The cluster showed a very high ability to oxidize 1,2-diol to 1,2-dione with a high TOF.