Research on Synthesis, Structure, and Catalytic Performance of Tetranuclear Copper(I) Clusters Supported by 2-Mercaptobenz-zole-Type Ligands

Tetrahedral copper(I) clusters [Cu4(MBIZ)4(PPh3)2] (2), [Cu4(MBOZ)4(PPh3)4] (6) (MBIZ = 2-mercaptobenzimidazole, MBOZ = 2-mercaptobenzoxazole) were prepared by regulation of the copper-thiolate clusters [Cu6(MBIZ)6] (1) and [Cu8(MBOZ)8I]− (5) with PPh3. With the presence of iodide anion, the regulation provided the iodide-containing clusters [CuI4(MBIZ)3(PPh3)3I] (3) and [CuI4(MBOZ)3(PPh3)3I] (7). The cyclic voltammogram of 3 in MeCN (0.1 M nBu4NPF6, 298 K) at a scan rate of 100 mV s−1 shows two oxidation processes at Epa = +0.11 and +0.45 V with return waves observed at Epc = +0.25 V (vs. Fc+/Fc). Complex 3 has a higher capability to lose and gain electrons in the redox processes than complexes 2, 4, 4′, 6, and 7. Its thermal stability was confirmed by thermogravimetric analysis. The catalytic performance of 3 was demonstrated by the catalytic transformation of iodobenzenes to benzonitriles using AIBN as the cyanide source. The nitrile products show potential applications in the preparation of 1,3,5-triazine compounds for organic fluorescence materials.

Encouraged by these reasons, we start to study the synthesis of tetranucl per(I)-thiolate clusters with nitrogen-containing mercaptan ligands (2-mercapto idazole (MBIZ), 2-mercaptobenzoxazole (MBOZ), and 2-mercaptobenzothiazole The clusters are studied to catalyze the transformation of aryl iodides to aryl nitr latters are used to produce octupolar π-conjugated molecules for the study of flu materials.Aryl nitriles have wide applications not only in organic synthesis but also in materials sciences.The nitrile group can be easily transformed into a variety of functional groups, such as aldehydes, amines, amides, tetrazoles, triazoles, pyrimidines, triazine, etc. 1,3,5-triazine core was recognized as an electron acceptor that can make π-conjugated D-A molecules for application in organic electronic devices.The great significance has consistently stimulated the development of research methods for their preparation.
Encouraged by these reasons, we start to study the synthesis of tetranuclear copper(I)thiolate clusters with nitrogen-containing mercaptan ligands (2-mercaptobenzimidazole (MBIZ), 2-mercaptobenzoxazole (MBOZ), and 2-mercaptobenzothiazole (MBTZ).The clusters are studied to catalyze the transformation of aryl iodides to aryl nitriles.The latters are used to produce octupolar π-conjugated molecules for the study of fluorescent materials.

Results and Discussions
We characterized all tetranuclear copper clusters by SC-XRD, XRD, and NMR (showed in Supplementary Materials).In air, the stirring of [Cu(CH  The cyclic voltammogram of complex 3 in MeCN (0.1 M n Bu4NPF6, 298 K) at a scan rate of 100 mV s −1 shows two oxidation processes at Ep a = +0.11and +0.45 V with return waves observed at Ep c = +0.25 V (vs.Fc + /Fc) (Figure 3).It has inferior comparability with the fully irreversible process, with no return wave observed at Ep a = +0.15 and +0.21 V for complex 7 under the same experimental conditions.This suggests a degree of decreased kinetic activity for the [Cu I 4] cluster with the ligand MBOZ.Without the support of iodide, the [Cu I 4] cluster 2 shows two electrochemical oxidation processes at Ep a = −0.03and +0.36 V without a reduction process.This contrasts with the oxidation process observed at Ep a = −0.03and +0.22 V for complex 6.As for complex 4 and 4′, their oxidation potentials are similar; they show two electrochemical oxidation processes at Ep a = +0.36 and +0.49V (Ep a of 4′ = +0.35 and +0.48 V) without a reduction process.(The oxidation-reduction potentials of complex 2, 3, 4, 4′, 6, and 7 are shown in Table 1) It is not difficult to observe through a cyclic voltammogram that the oxidation and reduction peak area of complex 3 is larger than others, which means that when complex 3 reacts on the electrode surface, its electron transfer amount is maximized (Figure S11).And complex 3 is the only one that undergoes oxidation processes at both the ligand center and cluster core (Figures S7 and S10).The above result indicates that complex 3 has a higher capability to lose and gain electrons upon oxidation/reduction than complexes 2, 4, 4′, 6, and 7.The chemical redox stability of complex 3 was checked by electrochemical cycling tests (Figure S8).The cyclic voltammogram of complex 3 in MeCN (0.1 M n Bu 4 NPF 6 , 298 K) at a scan rate of 100 mV s −1 shows two oxidation processes at E p a = +0.11and +0.45 V with return waves observed at E p c = +0.25 V (vs.Fc + /Fc) (Figure 3).It has inferior comparability with the fully irreversible process, with no return wave observed at E p a = +0.15 and +0.21 V for complex 7 under the same experimental conditions.This suggests a degree of decreased kinetic activity for the [Cu I  4 ] cluster with the ligand MBOZ.Without the support of iodide, the [Cu I  4 ] cluster 2 shows two electrochemical oxidation processes at E p a = −0.03and +0.36 V without a reduction process.This contrasts with the oxidation process observed at E p a = −0.03and +0.22 V for complex 6.As for complex 4 and 4 ′ , their oxidation potentials are similar; they show two electrochemical oxidation processes at E p a = +0.36 and +0.49V (E p a of 4 ′ = +0.35 and +0.48 V) without a reduction process.(The oxidation-reduction potentials of complex 2, 3, 4, 4 ′ , 6, and 7 are shown in Table 1) It is not difficult to observe through a cyclic voltammogram that the oxidation and reduction peak area of complex 3 is larger than others, which means that when complex 3 reacts on the electrode surface, its electron transfer amount is maximized (Figure S11).And complex 3 is the only one that undergoes oxidation processes at both the ligand center and cluster core (Figures S7 and S10).The above result indicates that complex 3 has a higher capability to lose and gain electrons upon oxidation/reduction than complexes 2, 4, 4 ′ , 6, and 7.The chemical redox stability of complex 3 was checked by electrochemical cycling tests (Figure S8).
The thermal stability of complex 3 was examined by the technique of thermogravimetric analysis (Figure 4).The first weight loss process occurs at 25-190 • C with a weight loss of about 6.38%.It is attributed to the escape of solvent molecules in the crystal sample.The second weight loss process occurs at 220-450 • C with a weight loss of about 48.62%.The significant weight loss is believed to result from the damage to the complex.According to the test results of differential scanning calorimetry, we can conclude that complex 3 did not undergo a significant phase transition below 180 • C (Figure S12).The result shows that complex 3 is stable for thermal catalytic reactions.The thermal stability of complex 3 was examined by the technique of thermogravimetric analysis (Figure 4).The first weight loss process occurs at 25-190 °C with a weight loss of about 6.38%.It is attributed to the escape of solvent molecules in the crystal sample.The second weight loss process occurs at 220-450 °C with a weight loss of about 48.62%.The significant weight loss is believed to result from the damage to the complex.According to the test results of differential scanning calorimetry, we can conclude that complex 3 did not undergo a significant phase transition below 180 °C (Figure S12).The result shows that complex 3 is stable for thermal catalytic reactions.Research on organic catalysis by transition-metal clusters has attracted more and more interest, for example, the catalytic cyanation reaction.Excessive cyanide anion might  The thermal stability of complex 3 was examined by the technique of metric analysis (Figure 4).The first weight loss process occurs at 25-190 °C loss of about 6.38%.It is attributed to the escape of solvent molecules in the c The second weight loss process occurs at 220-450 °C with a weight loss of a The significant weight loss is believed to result from the damage to the com ing to the test results of differential scanning calorimetry, we can conclude 3 did not undergo a significant phase transition below 180 °C (Figure S1 shows that complex 3 is stable for thermal catalytic reactions.Research on organic catalysis by transition-metal clusters has attract more interest, for example, the catalytic cyanation reaction.Excessive cyanid be unfavorable for catalytic cyanation [77].The cyanide source has always b tor.2,2′-Azobis(2-methylpropionitrile) (AIBN) was considered as one of th cyanide sources for coupling reactions [78][79][80].Thus, the stirring of iodo mmol), AIBN (0.3 mmol), complex 3 (10 mol%), KI (0.6 mmol), and 1,8 Research on organic catalysis by transition-metal clusters has attracted more and more interest, for example, the catalytic cyanation reaction.Excessive cyanide anion might be unfavorable for catalytic cyanation [77].The cyanide source has always been a key factor.2,2 ′ -Azobis(2-methylpropionitrile) (AIBN) was considered as one of the lowtoxicity cyanide sources for coupling reactions [78][79][80].Thus, the stirring of iodobenzenes (0.2 mmol), AIBN (0.3 mmol), complex 3 (10 mol%), KI (0.6 mmol), and 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU, 0.6 mmol) in CH 3 CN for 12 h at 120 • C provided the corresponding products 10 in normal yields of 40-56% (Figure 5).The low yield of 10l could be due to the variability of the bromide group on the substrate.
[5.4.0]undec-7-ene (DBU, 0.6 mmol) in CH3CN for 12 h at 120 °C provided the correspond ing products 10 in normal yields of 40-56% (Figure 5).The low yield of 10l could be du to the variability of the bromide group on the substrate.6), demonstrating its application potential in photo-electrochemistry.
[5.4.0]undec-7-ene (DBU, 0.6 mmol) in CH3CN for 12 h at 120 °C provided the corresponding products 10 in normal yields of 40-56% (Figure 5).The low yield of 10l could be due to the variability of the bromide group on the substrate.

Synthesis Procedures
The synthesis procedure of complex 1, 5, and 8 can be found in the Supporting Information.

Conclusions
In summary, we described the synthesis of a series of tetrahedral copper-thiolate tetramers by the regulation of multi-nuclear copper clusters with PPh 3 .The insertion of iodide anion to the cluster improved the redox activity and stability, which was supported by the study of electrochemistry and thermogravimetric analysis.The reaction performance was demonstrated by the catalytic cyanation of iodobenzenes using AIBN as the cyanide source.The products show potential applications as luminescent layer materials.