Chemistry and Electrochemistry of Phosphonium-functionalized Isocyanide and Derived Carbene and Indole Complexes of Group-6 Transition-metal Carbonyls

A series of [M(CO)5L] [M = Cr, Mo or W) complexes, where L is a phosphonium-substituted phenyl isocyanide ligand of the type o-R3PCHC6H4NC [PR3 = PMe3(L1), PPh2(CH2Ph)(L2) or PPh3(L3)], have been prepared by reaction of [NEt4][M(CO)5I] with the isocyanides in the presence of AgBF4 in acetone and they have been characterized by IR, H-1 and P-31 NMR spectroscopies. The co-ordinated ligands L react in tetrahydrofuran at 0-degrees-C with Na[N(SiMe3)2] to give in most cases a mixture of different cyclization products whose structures have been assigned, on the basis of spectroscopic and electrochemical data, as the carbene-type [M(CO)5{o-CNHC6H4C(PR3)}] A, indole-type [M(CO)5{o-NCHC(PR3)(C6H4)}] B or protonated indole-type species [M(CO)5{o-NHCHC(PR3)(C6H4)}] C. The relative abundance of the final products is strictly related to the nature of the metal and the type of R group in the phosphonium moiety: as a general trend the [W(CO)5L] complexes yield mostly type A derivatives, while those of Cr and Mo give primarily types B and C. A possible mechanism for the formation of these different species is discussed. These complexes, in aprotic media, exhibit upon cyclic voltammetry, a single-electron (reversible or irreversible) anodic wave, the oxidation potential of which has been applied to estimate the ligand parameters P(L) and E(L) for the isocyanide and derived ligands. According to their net electron-donor ability, the ligands can be ordered as follows: indoles > carbenes > protonated carbenes or protonated indoles > isocyanides. For the ligating metal centres M(CO)5 (M = Mo or W) the electrochemical parameter beta (polarizability) has also been estimated.