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Ind. Eng. Chem. Res. 2000, 39, 3536-3540
Design of Ligands Which Improve Cu(I) Catalysis Nadav Navon,† Haim Cohen,†,‡ Piero Paoletti,§ Barbara Valtancoli,§ Andrea Bencini,§ and Dan Meyerstein*,†,| Chemistry Department, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel, Nuclear Research Centre Negev, Beer-Sheva, Israel, The College of Judea and Samaria, Ariel, Israel, and Chemistry Department, University of Florence, Florence, Italy
It seemed plausible that tri- and tetraamine ligands with a substituent which binds to Cu(I) but not to Cu(II), e.g., an allyl, stabilize Cu(I) in aqueous solutions and shift its redox potential cathodically relative to Cu+(aq). Therefore, the ligands N(CH2CH2NR2)2(CH2CH2NRCH2CHd CH2) (R ) H or CH3) were synthesized. These ligands indeed stabilize Cu(I) in aqueous solutions and shift the redox potential of the Cu(II)/Cu(I) and Cu(I)/Cu(0) couples cathodically relative to Cu2+/+(aq) and the complexes CuIIL/CuIL where L ) R2NCH2CH2NRCH2CH2NR(CH2CHdCH2). Therefore, the copper complexes with the new ligands are expected to be better catalysts to processes in which the redox step is the rate-determining step. Introduction Copper(I) complexes catalyze a large variety of processes, some of which are of industrial importance.1 Among these processes are the Ullmann reaction,2 the Sandmeyer reaction,3 the Meerwein reaction,4 and the addition of polyhalides to alkenes5 which are catalyzed by Cu0/Cu(II) and/or by Cu(I) salts. In many of these processes, the rate-determining step is a redox reaction, e.g., the abstraction of a halogen atom from the substrate by the Cu(I) complex. Therefore, the redox properties of the copper complexes play a crucial role in these processes. Some of these processes are carried out in aqueous, or in protic, media. However, because of the low solubility of Cu(I) halides and of CuOH and because of the disproportionation of Cu+(aq) (reaction 1), the concentration of Cu(I) in these solutions is very small and usually unknown.
2Cu+(aq) S Cu2+(aq) + Cu0(s)
K ) 1 × 106 M-1 6 (1)
It is possible to increase the concentration of Cu(I) species in the solutions by adding ligands which stabilize it, e.g. alkenes,7 CH3CN,8 and NH3.9 Adding such ligands to a catalytic system containing Cu0/Cu(II) is expected to increase the rate of the catalytic process because of the increase in the concentration of the Cu(I) complexes in the system. However, these ligands clearly also affect the reactivity of the Cu(I); i.e., the higher the concentration of the stabilizing ligand, the poorer Cu(I) is as a reducing agent. In principle, ligands that stabilize Cu(I) in aqueous solutions while shifting its redox potential cathodically and, therefore, improve the catalytic properties of Cu0/Cu(II) and/or Cu(I) can be designed. These ligands should have the following properties:10 1. KII > KI (KII ) stability constant of CuIIL, KI ) stability constant of CuIL), i.e., that CuIL will be a stronger reducing agent than Cu+(aq) and therefore a †
Ben-Gurion University of the Negev. Nuclear Research Centre Negev. § University of Florence. | The College of Judea and Samaria. ‡
more potent catalyst in systems in which the catalytic role of Cu(I) involves a redox process. 2. (KI)2 > KII; i.e., the ligand will shift the equilibrium of the disproportionation reaction to the left, thus increasing the concentration of the catalytic active species in the reaction mixture. 3. So that stable CuL+(aq) will be a good catalyst, it should also not be too crowded sterically and, if possible, should have an available coordination site for inner sphere processes. 4. The substituents on the ligand have to be stable during the catalytic processes. Clearly, this requirement points out that different ligands are required for different catalytic processes. Recently, such ligands (L1 and L2) were synthesized, and the properties of their Cu(I) and Cu(II) complexes were studied.10 The principle applied in the design of these ligands is a combination of two kinds of binding sites in the same ligand, nitrogen atoms, which are strong σ donors, and an allyl group, which is a π acceptor. L1 and L2 form Cu(II) complexes using only the nitrogen atoms as the binding sites, while Cu(I) binds these ligands through the allyl group and the nitrogen atoms. The allyl group does not decrease significantly the stability constant of the Cu(II) complexes, whereas it stabilizes considerably the Cu(I) complexes. In this way, properties 1 and 2 are achieved. The redox potentials of the copper complexes [CuIIL1/ CuIL1, -0.035 V; CuIL1/(Cu0 + L1), -0.12 V; CuIIL2/ CuIL2, +0.095 V; CuIL2/(Cu0 + L2), -0.040 V (vs NHE)] are shifted cathodically in comparison to the redox potentials of the couples Cu2+(aq)/Cu+(aq) and Cu+(aq)/ Cu0, which are 0.16 and 0.51 V, respectively.11 To shift the redox potential of the CuIIL/CuIL even more cathodically and make the CuIL complex a better reducing agent, it has been decided to synthesize L3 and L4 with the expectation that they will form Cu(II) complexes through the four nitrogen atoms and Cu(I) complexes through the four nitrogen atoms and the allyl substituent. An increase of the number of nitrogen atoms, from three in L1 and L2 to four in L3 and L,4 is expected to cause a considerable increase in the stability constants of the Cu(II) complexes with these ligands,
10.1021/ie990840q CCC: $19.00 © 2000 American Chemical Society Published on Web 09/14/2000
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while the stability constants of the Cu(I) complexes are expected to be less affected.
Table 1. Basicity of the Ligands and Stability Constants of Their Copper Complexes in Aqueous Solutions. log Kc L6 a
L5 a
L4
L3
L2 a
L1 a
reaction
9.50 8.68 7.44
