Copper(II) Complexes with Schiff Bases Containing a Disiloxane Unit: Synthesis, Structure, Bonding Features and Catalytic Activity for Aerobic Oxidation of Benzyl Alcohol

Abstract

Mononuclear copper(II) salen‐type Schiff base complexes, CuIIL1–5 [H2L1 to H2L5 = tetradentate N,N,O,O ligands derived from 2‐hydroxybenzaldehyde, 2,4‐dihydroxybenzaldehyde, 3,5‐dibromo‐2‐hydroxybenzaldehyde, 2‐hydroxy‐5‐nitrobenzaldehyde, 5‐chloro‐2‐hydroxybenzaldehyde and 1,3‐bis(3‐aminopropyl)tetramethyldisiloxane, respectively] were prepared in situ in the presence of a copper(II) salt or by direct complexation between a copper(II) salt and a presynthesised Schiff base. The compounds {CuL1, CuL1·0.5Py, CuL2·0.375CH2Cl2, (CuL3)[Cu(4‐Me‐Py)4Cl]Cl·2H2O, CuL4, CuL4·CHCl3 and CuL5, as well as the isolated ligand H2L3} were characterised by elemental analysis, spectroscopic methods (IR, UV/Vis, 1H NMR, EPR) and X‐ray crystallography. The formation of a 12‐membered central chelate ring in these complexes is effected by the tetramethyldisiloxane unit, which separates the aliphatic chains, thus significantly reducing the mechanical strain in such a chelate ring. We dub this a “shoulder yoke effect” by analogy with the load‐spreading ability of such an ancient device. The coordination geometry of copper(II) in CuIIL1–5 can be described as tetrahedrally distorted square‐planar. Maximum tetrahedral distortion of the coordination geometry expressed by the parameter τ4 was observed for CuL1 (0.460), while distortion was minimal for the two crystallographically independent molecules of CuL2 (0.219 and 0.284). The Si–O–Si bond angle varies markedly between 169.75(2)° for CuL1 and 154.2(3)° for CuL4·CHCl3. Charge‐density and DFT calculations on CuL1 indicate high ionic character of the Si–O bonds in the tetramethyldisiloxane fragment. The new copper(II) complexes bearing the disiloxane moiety have been shown to act as catalyst precursors for the aerobic oxidation of benzyl alcohol to benzaldehyde mediated by the TEMPO radical, reaching yields and TONs up to 99 % and 990, respectively, under mild and environmentally friendly conditions (50 °C; MeCN/H2O, 1:1).

Chiral tetranuclear and dinuclear copper(ii) complexes for TEMPO-mediated aerobic oxidation of alcohols: are four metal centres better than two?

“…18,46,47 It was recently reported that copper(II) complexes of a terpyridine derived ligand catalyzed the aerobic oxidation of benzylic alcohols in aqueous basic media with up to 94% yield. 18,48 Initially, we performed the oxidation of benzylic alcohol by using copper complex (R)-1 as catalyst with a low load (0.25%) at room temperature and a 0.2 M of K 2 CO 3 solution in water as reaction media in the presence of TEMPO (5 mol%), by taking advantage of water as a green solvent. Pleasingly, 31% yield of the only product, benzaldehyde was detected after 22 h (entry 1, Table 1).…”

“…However, aliphatic or secondary benzylic alcohols were confirmed to be challenging substrates for oxidation under the optimized conditions (entries 17 and 18), as also found for other Cu II /TEMPO catalysts. 18,48 The complex (S,R)-2 was also examined for the oxidation of benzylic alcohols. It was noted that the oxidation of benzyl alcohol resulted in a 78% yield of benzaldehyde, in the presence of 0.5 mol% of (S,R)-2 under the same conditions (5 mol% TEMPO, 5 cm 3 of 0.2 M K 2 CO 3 in water, 70 ºC and 22 h), and the TON was much lower than that found for (R)-1 (156 vs. 770) (entry 19, Table 1).…”

“…Reducing catalyst loading (substrate/catalyst ratio up to 1000/1) led to slightly lower yields, but the TON was improved to 770 (entries 4, 5, Table 1), which is well comparable to the highest TON value reported for this reaction. 48 The use of TEMPO radical was crucial for this reaction, as almost no reaction was revealed when it was run without the presence of TEMPO (entry 6, Table 1). The role of base additive was also tested.…”

Dielectric elastomers based on silicones filled with transitional metal complexes

“…Three metal complexes containing siloxane bond and manganese (M 1 L), iron (M 2 L), and chromium (M 3 L) in their structure, were used as fillers and have been prepared according to procedure reported in [19] and described in Supplementary Information Purolite CT-175, a styrene-divinylbenzene ion exchanger with -SO 3 H groups (4.1 mequiv./g) was dehydrated by azeotrope distillation with toluene and vacuumation at 110 °C/10 mm Hg before use.…”

Synthesis, Structural Characterization and Catalytic Oxidation Property of Schiff Base Copper(ii) Complexes

“…[1][2][3][4] Copper(II) complexes have been proved to possess excellent catalytic activity towards a number of organic reactions. [5][6][7][8] In particular, copper complexes derived from Schiff bases are amongst the most versatile catalysts known for oxygenation reactions. [9][10][11] The role played by copper ions in the active sites of a large number of metalloproteins has stimulated efforts to design and characterize copper complexes as models for a better understanding of biological systems.…”