**1. Introduction**

Allosterism is commonly observed in proteins that suffer conformational changes induced by ligand binding to an orthosteric site, producing an activation, inhibition, or regulation of the enzymatic

activity [1]. Allosteric enzymes optimize the interactions between the ligand and host to tune the populations of active and inactive states for a specific metabolic function [2]. Therefore, allosterism control leads to a high specificity of these enzymes [3,4], which can inspire the design and development of supramolecular devices with high selectivity toward a substrate or analyte. Therein, abiotic allosteric catalysts have shown that metal ions can induce the control of conformation and reactivity of dinuclear catalytic sites [5–7]. The use of redox switching [8] and anion binding a ffinity [9–11] has also been explored in allosteric coordination chemistry, in which the weak-link approach (WLA) is a fforded by the employment of hemilabile ligands to obtain systems that are stimuli-responsive [10].

The WLA approach allows for reversibility in enzyme mimics, leading to allosteric responses [12] as well as to the development of small-molecule sensors such as enzyme-linked immunosorbent assays (ELISA) [13,14] and polymerase chain reaction (PCR) [15]. In the WLA approach, systems with ditopic ligands are commonly used since they enable the interaction of the complex with other molecular species such as anions and cations, causing the change in the overall conformation of the complex [16]. This change in conformation induces an alteration in the properties of one of the metal centers. Classical examples of systems employing WLA in an allosteric conformational manner can be found in the literature, in which the molecular structure [17], binding specificity [18], and catalytic activity [19,20] are modulated upon the interaction with a regulator.

Ligands based on Schi ff bases are common moieties in complexes bearing allosteric behaviors [12,21,22] and homo-bimetallic complexes have been shown to be e ffective in several catalytic reactions [23–26], with special attention to the supramolecular assemblies forming dimeric structures that exhibit significant rate acceleration when compared to the corresponding monomeric catalyst [27,28]. However, these systems are based on coordination compounds with rather sophisticated structures and it would be valuable to find simpler structural complexes bearing abiotic allosterism or catalytic regulation. In this aspect, dimeric or polymeric structures can be easily achieved with chlorido bridges [29–33], which can su ffer ligand substitution reactions to form monomeric species in solution, enabling an easy approach to obtain di fferent reactivities of the complex in an allosteric manner.

In this work, a simple coordination system was designed to achieve allosteric behavior by the regulation of the equilibrium between monomeric and dimeric species. For this purpose, Schi ff base ligands based on *<sup>L</sup>*-proline were designed and coordinated to CuII, as shown in Figure 1. The equilibrium between monomeric and dimeric species was dependent on the solvent mixture used in the reaction, enabling their use in a model reaction. As a proof-of-concept, urea hydrolysis was performed by these complexes, and water was shown to act as an allosteric regulator, since it induced the formation of active monomeric structures. Hence, we demonstrate that coordination systems with less complexity can also be used in regulatory reactions, serving as an inspiration to the development of cheaper sensors.

**Figure 1.** Synthesis of CuIIL1-L5(X) complexes. \* Note: The perchlorate anion was used only for ligand L2.

### **2. Materials and Instruments**

The reagents were of analytical grade and were used without prior purification. Thionylchloride (SOCl2(l)) used in the esterification of L-proline, as described in the literature, was previously distilled under argon. The solvents used in the synthesis and experiments were previously distilled. High Resolution Mass Spectra (HRMS) were obtained with a MICROTOF–Bruker Daltonics (Billerica, MA, USA) in the positive mode. Nebulizer: 0.3 bar, Dry gas: 4 mL/min, temperature: 180 ◦C, High Voltage: 4500 V. Nuclear Magnetic Resonance (NMR) spectra were recorded on a BRUKER DRX 400 MHz using CDCl3 as the solvent. X-ray di ffraction data were obtained on a Rigaku XtaLAB Mini di ffractometer (Rigaku, Tokio, Japan) with an X-ray generator operating at 50 kV and 12 mA with a graphite monochromatic Mo-K (λ = 0.71073 Å) using the Olex2 program (version Olex2 v1.3 © OlexSys Ltd. 2004–2020, Chemistry Department, Durkham University, Durkham, UK). EPR spectra were obtained on a Varian E109 EPR X-Band (Varian, Palo Alto, CA, USA) using the rectangular cavity field modulation at 100 kMz. Parameters: microwave power of 20 mW, modulation amplitude of 0.4 mT peak to peak, gain adjustable for each sample, field scan of 160 mT, time constant of 0.064 s, scan time of 3 min. For the measurements of liquid, N2 was used in the Dewar immersion method. An EPR standard was used to calibrate the magnetic field (MgO crystal: CrIII g = 1.9797) and the resonance frequency was measured with a microwave frequency meter. FTIR spectra were recorded on a Bomen-Michelson FT model MB-102 spectrometer (ABB BOMEM, Quebec, QC, Canada) in the 4000–200 cm<sup>−</sup><sup>1</sup> region. In situ FTIR were recorded on a Nicolet 6700 FTIR spectrometer equipped with a Mercury-Cadmium-Telluride (MCT) detector using p-polarized light and employed a 60 CaF2 prism located in the bottom of a gas cell in the configuration described. Each spectrum consisted of 32 interferograms, recorded with a spectral resolution of 4 cm<sup>−</sup>1. UV–Vis spectra were recorded on a HP-Hewlett Packard 8452 A spectrophotometer (Hewlett Packard, Palo Alto, CA, USA) in the 190–800 nm range.

### *2.1. Synthesis of Ligands (L1–L5)*

The ligands were synthesized starting from L-proline. Five synthetic steps are necessary, according to the method reported in the literature [34–36] for the synthesis of L1. Some minor modifications were performed. Essentially, 205 mg (0.60 mmol) of ((*S*)-1-benzylpirrolidin-2-yl)diphenylmethanamine (6) was added in a reaction flask containing 2.0 mL anhydrous methanol. Then, 1.05 eq of the salicylaldehyde derivative was added (63.94 μL (0.63 mmol) of 2-hydroxybenzaldehyde (L1), 96.00 mg (0.63 mmol) of 2-hydroxy-3-methoxybenzaldehyde (L2), 76.58 μL (0.63 mmol) of 2-hydroxy-3-methylbenzaldehyde (L3), 107.60 μL (0.63 mmol) of 3-tert-butyl-2-hydroxybenzaldehyde (L4), and 105.00 mg (0.63 mmol) of 3-ethoxy-2-hydroxybenzaldehyde (L5)), in addition to 20.00 mg (0.23 eq) of anhydrous Na2SO4(s). The reactions were left under stirring at 40 ◦C and followed by thin layer chromatography until no change in reagen<sup>t</sup> consumption was observed. L1, L2, and L3 were filtered after 20 h of reaction and washed extensively with methanol to obtain yellowish solids. The solids were dissolved in dichloromethane and methanol was added to obtain yellowish crystals by slow evaporation for 24 h. L4 and L5 were left in reaction for 48 and 10 h, respectively, with the formation of a viscous yellowish solid. The solid was purified by column chromatography on silica gel with ethyl acetate/hexane (5:95) and (20:80) mixture as the eluent, respectively, for ligands L4 and L5.

(*E*)-2-((((1-benzylpyrrolidin-2-yl)diphenylmethyl)imino)methyl)phenol (L1). Yield: 75%. 1HNMR (400 MHz, CDCl3, 298 K): δ 14.71 (s, 1H), 7.97 (d, 2H), 7.33–7.20 (m, 7H), 7.18–7.01 (m, 8H), 6.96 (d, 1H), 6.75 (t, 1H), 3.96 (dd, 1H), 3.34 (d, 1H), 3.10 (d, 1H), 2.75–2.63 (m, 1H), 2.18–2.02 (m, 2H), 1.72–1.62 (m, 1H), 1.43–1.32 (m, 1H), 0.84–0.72 (m, 1H) ppm. 13C NMR (400 MHz, CDCl3, 298 K): δ 164.55, 162.06, 144.55, 142.85, 140.44, 132.60, 132.09, 130.19, 129.10, 128.48, 128.12, 127.98, 127.83, 127.18, 126.86, 126.44, 118.87, 118.22, 117.51, 77.76, 71.96, 62.03, 55.10, 30.72, 24.00 ppm. HRMS (ESI<sup>+</sup>, CH3OH) *m*/*z* calculated for C31 H31 N2O 447.2436 [M+H]<sup>+</sup>; found 447.2413. IR (KBr): 3347 (ν OH), 3056 (ν Csp2H), 2969 (v Csp3H), 2818 (v Csp3H), 1620 (ν C=N), 1490 (ν C=C), 1280 (ν C–O) cm<sup>−</sup>1. UV–Vis ε(L mol−<sup>1</sup> cm<sup>−</sup>1) in CH2Cl2: 240 (8104), 260 (9513), 320 (3729), and 414 (540) nm.

(*E*)-2-((((1-benzylpyrrolidin-2-yl)diphenylmethyl)imino)methyl)-6-methoxyphenol (L2) Yield: 69%. 1HNMR (400 MHz, CDCl3, 298 K): δ 15.23 (s, 1H), 7.91 (s, 1H), 7.45–7.40 (m, 2H), 7.32–7.20 (m, 6H), 7.17–7.00 (m, 7H), 6.82 (dd, 1H), 6.65–6.58 (m, 2H), 3.96 (dd, 1H), 3.31 (d, 1H), 3.08 (d, 1H), 2.76–2.69 (m, 1H), 2.18–2.06 (m, 2H), 1.72–1.63 (m, 1H), 1.39 (dd, 1H), 1.09 (dd, 1H) ppm. 13C NMR (400 MHz, CDCl3, 298 K): δ 164.31, 155.95, 149.95, 144.01, 142.30, 139.91, 130.05, 128.87, 128.58, 128.30, 128.00, 127.95, 127.43, 127.00, 126.53, 123.74, 117.63, 116.63, 113.61, 77.22, 71.75, 62.00, 55.05, 30.70, 24.03 ppm. HRMS (ESI<sup>+</sup>, CH3OH) *m*/*z* calculated 477.2542 [M+H]<sup>+</sup>; found 477.2518. IR (KBr): 3417 (ν OH), 3020 (ν Csp2H), 2966 (ν Csp3H), 2805 (ν Csp3H), 1623 (ν C=N), 1491 (ν C=C), 1253 (ν C–O) cm<sup>−</sup>1. UV–Vis (L mol−<sup>1</sup> cm<sup>−</sup>1) in CH2Cl2: 232 (27,271), 266 (15,775), 324 (3046), and 432 (872) nm.

(*E*)-2-((((1-benzylpyrrolidin-2-yl)diphenylmethyl)imino)methyl)-6-methylphenol (L3). Yield: 72%. 1H NMR (400 MHz, CDCl3, 298 K): δ 14.63 (s, 1H), 7.95 (s, 1H), 7.44 (dt, 2H), 7.32–7.02 (m, 14H), 6.87 (dd, 1H), 6.65 (t, 1H), 3.98 (dd, 1H), 3.49 (d, 1H), 3.15 (d, 1H), 2.64 (m, 1H), 2.26 (s, 3H), 2.15 (m, 1H), 2.07 (m, 1H), 1.72 (m, 1H), 1.35 (m, 1H), 0.97 (m, 1H) ppm. 13C NMR (400 MHz, CDCl3, 298 K): δ 165.42, 160.18, 144.30, 143.29, 140.51, 133.45, 130.11, 129.78, 129.38, 128.57, 128.01, 127.73, 127.05, 126.79, 126.41, 126.28, 118.16, 117.74, 77.64, 72.13, 62.08, 54.98, 30.61, 23.89, 15.66 ppm. HRMS (ESI<sup>+</sup>, CH3OH) *m*/*z* calculated for C32H33N2O461.2592 [M+H]<sup>+</sup>; found 461.2569. IR (KBr): 3412 (ν OH), 3052 (ν Csp2H), 2976 (ν Csp3H), 2813 (ν Csp3H), 1619 (ν C=N), 1491 (ν C=C), 1264 (ν C–O) cm<sup>−</sup>1. UV–Vis ε(L mol−<sup>1</sup> cm<sup>−</sup>1) in CH2Cl2: 232 (24,009), 262 (18,426), 326 (5085), and 420 (358) nm.

(*E*)-2-((((1-benzylpyrrolidin-2-yl)diphenylmethyl)imino)methyl)-6-(tert-butyl)phenol (L4). Yield: 45%. 1H NMR (400 MHz, CDCl3, 298 K): δ 15.13 (s, 1H), 7.96 (s, 1H), 7.43 (dd, 2H), 7.35–7.06 (m, 14H), 6.88 (dd, 1H), 6.67 (dd, 1H), 3.96 (dd, 1H), 3.49 (d, 1H), 3.16 (d, 1H), 2.65 (ddd, 1H), 2.16–2.03 (m, 2H), 1.75–1.66 (m, 1H), 1.39 (s, 9H), 0.92–0.85 (m, 2H) ppm. 13C NMR (400 MHz, CDCl3, 298 K): δ 165.45, 161.15, 144.44, 143.08, 140.63, 137.75, 130.28, 130.23, 129.40, 128.47, 127.89, 127.77, 126.99, 126.81, 126.33, 118.80, 117.30, 77.66, 72.23, 62.04, 54.97, 34.93, 30.64, 29.33, 23.81 ppm. HRMS (ESI<sup>+</sup>, CH3OH) *m*/*z* calculated for C35H39N2O 503.3062 [M+H]<sup>+</sup>; found 503.3035. IR (KBr): 3413 (ν OH), 3058 (ν Csp2H), 2954 (ν Csp3H), 1619 (νC=N), 1264 (ν C–O) cm<sup>−</sup>1. UV–Vis ε(L mol−<sup>1</sup> cm<sup>−</sup>1) in CH2Cl2: 232 (22,025), 264 (16,192), 330 (5320), and 400 (269) nm.

(*E*)-2-((((1-benzylpyrrolidin-2-yl)diphenylmethyl)imino)methyl)-6 ethoxy phenol (L5). Yield: 38%. 1H NMR (400 MHz, CDCl3, 298 K): δ 14.92 (s, 1H), 7.93 (s, 1H), 7.44 (dd, 2H), 7.32–7.05 (m, 13H), 7.01 (dd, 2H), 6.84 (t, 1H), 6.63 (d, 2H), 4.08 (q, 2H), 3.99 (dd, 1H), 3.42 (d, 1H), 3.12 (d, 1H), 2.66 (m, 1H), 2.10 (m, 2H), 1.70 (m, 1H), 1.45 (t, 3H), 1.35 (m, 1H), 1.03 (m, 1H) ppm. 13C NMR (400 MHz, CDCl3, 298 K): δ 165.11, 154.02, 148.19, 144.16, 143.05, 140.33, 132.44, 130.08, 130.00, 129.19, 128.59, 128.15, 127.96, 127.79, 127.18, 126.85, 126.45, 123.76, 118.48, 117.15, 115.08, 77.50, 71.86, 64.38, 62.06, 54.99, 30.63, 23.95, 14.88 ppm. HRMS (ESI<sup>+</sup>, CH3OH) *m*/*z* calculated for C32H33N2O2 477.2537 [M+H]<sup>+</sup>; found 477.2503. IR (KBr): 3421 (ν OH), 3055 (νCsp2H), 2965 (ν Csp3H), 1621 (ν C=N), 1492 (ν C=C), 1272 (ν C–O) cm<sup>−</sup>1. UV–Vis ε (L mol−<sup>1</sup> cm<sup>−</sup>1) in CH2Cl2: 232 (15,984), 264 (9288), 332 (2716), and 430 (622) nm.

### *2.2. General Procedure of Synthesis of CuII Chlorido Complexes*

In a reaction flask, 50.00 mg of CuCl2 (0.37 mmol, 1.1 eq) wasadded to 3.0 mL of anhydrous methanol. The methanolic solution was heated at reflux temperature for 10 min, followed by the addition of 1.0 eq of the ligands (150.00 mg (0.33 mmol) of HL1, 157.00 mg (0.33 mmol) of HL2, 152.00 mg (0.33 mmol) of HL3, 166.00 mg (0.33 mmol) of HL4 and 162.00 mg (0.33 mmol) of HL5). After 4 h, the reaction mixture was cooled to room temperature and filtered. The filtrate was evaporated to dryness and suspended in dichloromethane. The mixture was centrifuged and the supernatant was removed. The complexes were obtained after removal of the solvents by rotatory evaporation under vacuum.

Spectroscopic data of CuIIL1. Dark green powder, yield 75%. HRMS (ESI<sup>+</sup>, CH2Cl2/CH3CN) *m*/*z* 566.1162 calculated for [M+Na]<sup>+</sup>, found 566.1122; IR (KBr): 3546, 3472, 3412 (ν OH), 3080, 3057 (ν Csp2H), 3026 (ν N=Csp2H), 2959, 2922, 2851 (ν Csp3H), 1654, 1637, 1617(ν C=N), 1598, 1580 (ν C=C), 1455, 1445 (δ CH2), 1317, 1276, 1261 (ν C–O), 1089, 1074, 1028 (ν C–N), 760, 704 (γ Csp2H), 638 (ν Cu–O), 474 (ν Cu–N) cm<sup>−</sup>1. UV–Vis Vis ε L mol−<sup>1</sup> cm<sup>−</sup><sup>1</sup> (CH2Cl2): 248 (18,246), 276 (16,002), 380 (4383), 636 (265) nm. C31.5H30.5Cl1.5CuN2O1.25[Cu(L1)Cl]·(0.25CH3OH)·(0.25CH2Cl2) calculated C, 65.93; H, 5.36; N, 4.88. Found: C, 65.95; H, 5.23; N, 4.95.

Spectroscopic data of CuIIL2. Dark brown powder, yield 91%. HRMS (ESI<sup>+</sup>, CH2Cl2/CH3CN) *m*/*z* calculated for [M–Cl]+ 538.1676, found 538.1669; HRMS (ESI<sup>+</sup>, CH2Cl2/CH3OH) *m*/*z* calculated 1169.2638 for [2M + Na<sup>+</sup>]<sup>+</sup>, found *m*/*z* 1169.2410; IR (KBr): 3458, 3410 (ν OH), 3055 (ν Csp2H), 3026 (ν N=Csp2H), 2959, 2926 (ν Csp3H), 1619 (ν C=N), 1577, 1544 (ν C=C), 1469, 1444 (δ CH2), 1316, 1276 (ν C–O), 1245, 1218 (ν C–O–C), 1081, 1004 (ν C–N), 748, 704 (γ Csp2H), 638 (ν Cu–O), 557 (ν C–N) cm −1. UV–Vis Vis ε(L mol−<sup>1</sup> cm<sup>−</sup>1) in CH2Cl2: 234 (15,660), 284 (13,953), 362 (2479), ~600 (–) nm. C32.6H32.2Cl2.2CuN2O2[Cu(L2)Cl]·(0.66CH2Cl2) calculated C, 62.16; H, 5.16; N, 4.44. Found: C, 62.14; H, 4.82; N, 4.71

Spectroscopic data of CuIIL3. Green brownish powder, yield 91%. HRMS (ESI<sup>+</sup>, CH2Cl2/CH3OH) *m*/*z* calculated for [M–HCl]+ 522.1727, found 522.1691. IR (KBr): 3549, 3450, 3410 (ν OH), 3082, 3057 (ν Csp2H), 3026 (ν N=Csp2H), 2949, 2920 (ν Csp3H), 1654, 1615 (ν C=N), 1600, 1577, 1544 (ν C=C), 1467, 1446, 1421 (δ CH2), 1317, 1276 (ν C–O), 1087, 1028 (ν C–N), 748, 704 (γ Csp2H), 638 (ν Cu–O), 567 (νCu–N) cm<sup>−</sup>1. UV–Vis Vis ε(L mol−<sup>1</sup> cm<sup>−</sup>1) in CH2Cl2:252 (19,765), 280 (12,308), 378 (3053), 600–700 (–) nm. C33H34Cl2CuN2O1.5 [Cu(L3)Cl]·(0.5CH3OH)·(0.5CH2Cl2) calculated C, 64.23; H, 5.55; N, 4.54. Found: C, 64.00; H, 5.78; N, 4.48.

Spectroscopic data of CuIIL4. Dark green, yield 72%. HRMS (ESI<sup>+</sup>, CH2Cl2/CH3OH) *m*/*z* calculated for [M–HCl]+ 564.2196, found 564.2168. IR (KBr): 3545, 3472, 3412 (νOH), 3084, 3054 (ν Csp2H), 3026 (ν <sup>N</sup>=Csp2H), 2949, 2920 (ν Csp3H), 1654, 1615 (ν C=N), 1596, 1534, 1492 (ν C=C), 1465, 1443, 1415 (δ CH2), 1336, 1326 (ν C–O), 1143, 1085 (ν C–N), 748, 702 (γ Csp2H), 567 (ν Cu–N) cm −1. UV–Vis Vis ε(L mol−<sup>1</sup> cm<sup>−</sup>1) in CH2Cl2:250 (18,922), 278 (12,694), 332 (3959), 388 (4499) e 650 (294) nm. C36H39.8Cl2.2CuN2O1.4[Cu(L4)Cl]·(0.4CH3OH)·(0.6CH2Cl2) calculated C, 65.07; H, 6.04; N, 4.22. Found: C, 64.84; H, 5.97; N, 4.71.

Spectroscopic data of CuIIL5. Dark brown, yield 96%. HRMS (ESI<sup>+</sup>, CH2Cl2/CH3OH) *m*/*z* calculated for [M–HCl]+ 552.1833, found 552.1784; [M–CH4–HCl]+ 536.1519, found 536.1691. IR (KBr): 3458, 3414(ν OH), 3080, 3057 (ν Csp2H), 3026 (ν <sup>N</sup>=Csp2H), 2974, 2924, 2853 (ν Csp3H), 1654, 1615 (ν C=N), 1602, 1577, 1560 (ν C=C), 1465, 1448 (δ CH2), 1317, 1278 (ν C–O), 1245, 1216 (ν C–O–C), 1073, 1028 (ν C–N), 763, 740, 704 (γ Csp2H), 638 (ν Cu–O) cm<sup>−</sup>1. UV–Vis Vis ε(L mol−<sup>1</sup> cm<sup>−</sup>1) in CH2Cl2: 236 (10,803), 252 (15,770) 356 (2348), ~600 (n.d.) nm. C34.1H35.7Cl2.7CuN2O2.25 [2Cu(L5)Cl]·(1.6CH2Cl2) calculated C, 61.57; H, 5.21; N, 4.31. Found: C, 61.64; H, 4.87; N, 4.44.

### *2.3. Synthesis of Cu<sup>I</sup> IPerchlorate Complex*

The perchlorate complex was synthesized similarly to the chlorido complex, but using the precursor Cu(ClO4)·6H2O. After 4 h of reaction, cold distilled water was added and the obtained solid was centrifuged, filtered, and washed with cold distilled water. The solid was left in a desiccator at high vacuum. Dark green. Yield: 89%. HRMS (ESI<sup>+</sup>, CH2Cl2/CH3OH) *m*/*z* calculated for [2M–ClO4−]<sup>+</sup> 1175.2848, found 1175.2834. IR (KBr): 3530, 3446, 3317 (ν OH), 3060 (ν Csp2H), 3033 (ν <sup>N</sup>=Csp2H), 2967, 2841 (ν Csp3H), 1655, 1623 (ν C=N), 1606, 1577, 1545 (ν C=C), 1493, 1470, 1440 (δ CH2), 1319, 1279 (ν C–O), 1246, 1220 (ν C–O–C), 1120, 1108, 1087 (<sup>ν</sup>3 ClO4−), 1005 (ν C–N), 943, 921 (<sup>ν</sup>4 ClO4−) 748, 706 (γ Csp2H), 638, 624 (ν Cu–O), 556, 522 (ν Cu–N)cm−1. UV–Vis (CH2Cl2): 244, 286, 392, 592 nm. C65.3H66.6Cl2.6Cu2N4O13 [2CuIIL2ClO4·(CH3OH)](0.3CH2Cl2) calculated C, 58.76; H, 5.03; N, 4.20. Found: C, 58.87; H, 5.15; N, 4.16. Conductivity: 13 μS cm<sup>−</sup><sup>1</sup> in dichloromethane.
