Synthesis, characterization, and reactivity of low valent technetium phosphine complexes

Abstract

Chapter 1. The chemistry of the metal-metal multiply bonded Tc2(II,II) core has been investigated with bis(diphenylphosphino)methane (dppm). The parent complex, Tc2Cl4(dppm)2, has been prepared from the reaction of Tc2C14(PEt3)4 with dppm. The reactivity of Tc2Cl4(dppm)2 with tert-butyl isocyanide has been studied and a neutral 1:1 adduct, Tc2Cl4(dppm)2CNBut, a cationic 1:2 adduct, [Tc2Cl3(dppm)4(CNBut)2](PF6), and a [mu]-iminyl complex, [Tc2Cl3(dppm)2(CNBut)2CNHBul(PF6), have been prepared. The parent compound and its reaction products have been characterized via a combination of spectroscopic techniques and single crystal X-ray crystallography. The metal-metal bonded ditechnetium bis(dppm) motif is retained in the reaction products. Chapter 2. The reduction of ammonium pertechnetate with bis(diphenylphosphino)methane (dppm), and with diphenyl-2-pyridyl phosphine (Ph2Ppy), has been investigated. The neutral Tc(II) complex, TcCl2(dppm)2, has been isolated from the reaction of (NH4)[TcO4] with excess dppm in refluxing EtOH/HCl. Chemical oxidation with ferricinium hexafluorophosphate results in formation of the cationic Tc(III) analogue, [TcCl2(dppm)2](PF6). The dppm ligands adopt the chelating bonding mode in both complexes, resulting in strained four member metallocycles. With excess PhPpy, the reduction of (NH4)[TcO4] in refluxing EtOH/HCl yields a complex with one chelating Ph2Ppy ligand and one unidentate Ph2Ppy ligand, TcCl3(Ph2Ppy-P,N)(Ph2Ppy-P).

(cont.) The cationic Tc(III) complexes, [TcCl2(dppmO-P, O)2](PF6) and [TcCl2(Ph2P(O)py-N, O)2](PF6) (dppmO = bis(diphenylphosphino)methane monoxide, Ph2P(O)py = diphenyl-2-pyridyl phosphine monoxide), have been isolated as by-products from the reactions of (NH4)[TcO4] with the corresponding phosphine. The products have been characterized in the solid state and in solution via a combination of single crystal X-ray crystallography and spectroscopic techniques. The solution state spectroscopic results are consistent with the retention of the bonding modes revealed in the crystal structures. Chapter 3. A new route to low valent technetium complexes containing multiple acetonitrile ligands has been developed. The reduction of TcCl4(PPh3)2 with zinc metal dust in acetonitrile results in the formation of [Tc(CH3CN)4(PPh3)2][Zn2Cl6]1/2. The hexafluorophosphate salt of the analogous Tc(II) cation can be prepared via chemical oxidation of the Tc(I) species, and the Tc(I) cation can be regenerated via chemical reduction. The compounds have been characterized in the solid state via single crystal X-ray crystallography, and in solution via a combination of spectroscopic techniques and cyclic voltammetry. The structural parameters found in the two complexes are similar to each other; however, the difference in oxidation state is reflected, as expected, in the spectroscopic results. The electrochemical data, obtained from cyclic voltammograms of [Tc(CH3CN)4(PPh3)2](PF6)n (n = 1,2), mirror the synthetic results in that both compounds possess a reversible redox couple at -0.55 V versus ferrocene, which has been assigned to the Tc(II)/Tc(I) couple.