A FAMILY OF STABLE IRON(I) SIGMA-ALKYNYL COMPLEXES - SYNTHESIS, CHARACTERIZATION, STRUCTURE, AND ELECTRON-TRANSFER CHEMISTRY

Fe(II) and Fe(I) sigma-alkynyl complexes of the general formula [(PP3)Fe(C=CR)]n+ (n = 1, 0) have been synthesized as BPh4- salts or neutral molecules and characterized by chemical, spectroscopic, X-ray, and electrochemical techniques [R = Ph, SiMe3, n-C3H7, n-C5H11, CMe3; PP3 = P(CH2CH2PPh2)3]. All of the compounds undergo electron-transfer reactions that encompass Fe(0), Fe(I), Fe(II), and Fe(III) oxidation states of the metal. X-ray crystal structures of the 16- and 17-electron complexes [(PP3)Fe(C=CPh)]BPh4.C4H8O and [(PP3)Fe(C=CPh)] have been determined. The Fe(II) compound crystallizes in the space group P2(1)/c, and the cation assumes an almost regular trigonal-bipyramidal structure with the alkynyl ligand trans to the bridgehead phosphorus atom of PP3 (P4-Fe-C7 bond angle = 177.2(6)degrees). The Fe(I) compound crystallizes in the space group P2(1)/n and assumes a strongly distorted trigonal-bipyramidal structure with the P4-Fe-C7 bond angle of 170.3(3)degrees and equatorial bond angles of 143.9(1)degrees, 102.4(1)degrees, and 111.1(1)degrees. A decrease in the Fe-P bond distances on going from Fe(II) to Fe(I) is interpreted in terms of significant metal --> phosphorus pi-back-bonding. In contrast, from a perusal of IR, structural, and electrochemical data, no significant dpi (metal) --> pi* (alkynyl) interaction occurs. All compounds are paramagnetic and have been characterized by X-band ESR spectroscopy (powder, frozen solution, fluid solution). The powder and frozen solution spectra of the Fe(I) alkynyls are interpreted in terms of S = 1/2 and a rhombic g tensor. The fluid solution spectra show that the compounds exist in tetrahydrofuran solution as two isomeric forms exhibiting distorted trigonal-bipyramidal structures in a ratio that depends on the temperature. The ESR spectra of the Fe(II) derivatives (powder and frozen solution) display unresolved line shape consistent with a S = 1 Hamiltonian with noticeable zero-field splitting effects at room temperature.