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41267-B3
Electronic Structure, Molecular Structure, and Site of Bond Breaking in Facial- and Meridional-(Dihapto-[60]Fullerene)(Dihapto-Bidentate Ligand)Transition Metal(0) Complexes

Jose E. Cortes-Figueroa, University of Puerto Rico

Electronically-unsaturated organometallic catalysts such as the Vaska's (Ir(CO)(PPh₃)₂(Cl) 1) and the Wilkinson's complexes are candidates for chemical modification by [60]fullerene. The preparation of the complex (Ir(CO)(PPh₃)₂(Cl)(C₆₀) 2) is reported in the chemical literature. The chemical characterization of 2 was based on crystallographic data. However, no characterization of the complex in solution has been reported. It seems that in solution the equilibrium described by equation (1) favors the dissociated species.

Ir(CO)(PPh₃)₂(Cl)(C₆₀) « Ir(CO)(PPh₃)₂(Cl) + C₆₀ (1)

If a solid sample of 2 is dissolved in C₆₀-saturated solutions of solvents where C₆₀ has a relatively high solubility such as benzene and toluene, then one may expect to observe mixtures of 1 and 2. The nCO spectrum of 2 dissolved in benzene corresponds to a non-equilibrium mixture of 1 and 2. The nCO spectrum of 1 in solid phase shows a band at 1967 cm^-1, while the corresponding spectrum of 2 shows a band at 2014 cm^-1. Each nCO and nC=C band in the liquid solution was unequivocally assigned by comparing the spectrum in the mixture to IR spectra of actual samples or literature values. Since the extinction coefficient of 2 at 550 nm is larger than the corresponding extinction coefficient of 1, the dissociation of C₆₀ from 2 was followed by monitoring the decrease of absorbance values at 550 nm of solutions prepared by dissolving solid samples of 2 in C₆₀-saturated benzene. Decreasing exponential plots of absorbance vs. time were obtained. The estimated rate constant values were [C₆₀]-independent, suggesting that k_forward >> k_reverse (eq 1).

Attempts were made to form adducts with C₆₀^n- fullerides (n =1, 2, 3 ,4, 5, 6). Experiments for electrochemical characterization of Ir(CO)(PPh₃)₂Cl(C₆₀ ^n-) were conducted under flooding conditions where [C₆₀] >> [Ir(CO)(PPh₃)₂Cl(C₆₀)]. Results from cyclic voltammetry and Osteryoung voltammetry confirmed that in solution C₆₀ is not coordinated to 1. However, C₆₀^4- and C₆₀^5- anions produced during cyclic voltammetry experiments seems to be coordinated to 1 forming the complexes Ir(CO)(PPh₃)₂Cl(C₆₀^4-) and Ir(CO)(PPh₃)₂Cl(C₆₀^5-), respectively.

The cyclic voltammogram of a saturated solution of C₆₀ in a 1:5 acetonitrile:toluene mixture at 25 șC showed five reduction waves at potential E_n^1/2 (n = 1-5) values relative to ferrocene (Fc/Fc⁺) (-986, -1397, -1913, -2429, and -2921 mV) are close to values reported for experiments under similar conditions but at lower temperatures. The cyclic voltammogram of the saturated solution of free C₆₀ in 1:5 acetonitrile:toluene mixture after addition of Ir(CO)(PPh₃)₂Cl showed new reduction waves at E_1/2 values close to -2617 and -3084 mV relative to ferrocene/ferrocinium. The preliminary interpretation is that these new reduction waves correspond to consecutive one-electron reductions of the complexes Ir(CO)(PPh₃)₂Cl(C₆₀^4-) and Ir(CO)(PPh₃)₂Cl(C₆₀^5-) forming Ir(CO)(PPh₃)₂Cl(C₆₀^5-) and Ir(CO)(PPh₃)₂Cl(C₆₀^6-), respectively. It seems likely that formation of Ir(CO)(PPh₃)₂Cl(C₆₀^4-) and Ir(CO)(PPh₃)₂Cl(C₆₀^5-) takes place after formation of C₆₀^4- and C₆₀^5-, respectively. Results from Osteryoung square wave voltammetry are consistent with this interpretation, suggesting that a fraction of C₆₀^n- (n = 4, 5) is depleted before they are reduced to C₆₀^(n+1)- by presumably forming Ir(CO)(PPh₃)₂Cl(C₆₀^n-) followed by one-electron reductions according to equations 2 and 3.

Ir(CO)(PPh₃)₂Cl + C₆₀^n- → Ir(CO)(PPh₃)₂Cl(C₆₀^n-) (2)

Ir(CO)(PPh₃)₂Cl(C₆₀^n-) + e- →Ir(CO)(PPh₃)₂Cl(C₆₀^(n+1)-) (3)

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