ELEMENTAL CARBON-CHAIN BRIDGING 2 IRON CENTERS - SYNTHESES AND SPECTROSCOPIC PROPERTIES OF [CP-ASTERISK(DPPE)FE-C-4-FECP-ASTERISK(DPPE)](N+)CENTER-DOT-N[PF6](-) - X-RAY CRYSTAL-STRUCTURE OF THE MIXED-VALENCE COMPLEX (N=1)

Reaction of [Fe(Cp*)(dppe)Cl] [1, Cp* = eta(5)-C(5)Me(5), dppe = eta(2)-ethylenebis(diphenylphosphine)] and NH4-PF6 in methanol with equiv of Me(3)SiC=CH produced the vinylidene complex [Fe(Cp*)(dppe)(=C=CH2)][PF6] (2) isolated in 89% yield. The complex 2 was readily deprotonated by KOBu(t) in tetrahydrofuran (THF), giving the ethynyl iron complex Fe(Cp*)(dppe)(-C=CH) (3) isolated after workup as a thermally stable orange solid in 98% yield. The addition of 0.95 equiv of [Cp(2)Fe][PF6] to a -80 degrees C solution of 3 gave the low-spin Fe(III) intermediate 3[PF6] characterized by ESR spectroscopy. Upon stirring for 5 h, 3[PF6] produced a brown solution from which the bis(vinylidene) complex [Fe(Cp*)(dppe)](=C=CH-HC=C=)[(dppe)(Cp*)Fe][PF6](2) was isolated (4, 87 %) as a stable compound. The compound 4 was reacted with 2 equiv of potassium tert-butoxide to give the mu-eta(1):eta(1)-butadiyndiyl C-4 complex 5, isolated as a brown powder in 92% yield. Complex 5 is characterized by H-1, C-13, and P-31 NMR and by IR, Mossbauer, and microanalysis. A variable-temperature P-31 NMR experiment evidenced a rotation barrier (Delta G double dagger = 41.4 kJ . mol(-1) (9.90 kcal . mol(-1))) in agreement with a slow rotation about the iron-C-4-iron axis. CV analysis of the butadiyne bridged complex 5 from -1.2 to 0.6 V displays two reversible one-electron oxidation waves. The two redox processes are separated by 0.720 V corresponding to a large comproportionation constant (K-c = 1.6 x 10(12)). Oxidation of 5 with 2 equiv of [Cp(2)Fe][PF6] gave the salt 5[PF16]2, isolated in 91% yield. It is a thermally and air stable compound which is characterized by IR, ESR, NMR, and Mossbauer spectroscopies. Its CV waves are identical to those of its parent complex 5. The addition of 1 equiv of [FeCp(2)][PF6] to 5 in CH2Cl2 produced the Fe(III)-Fe(II) complex 5[PF6] (92% yield). The air stable mixed-valence compound 5[PF6] is characterized by elemental analysis, IR, NIR, Mossbauer, and ESR spectroscopies, and magnetic susceptibility. The X-ray crystal structure of 5[PF6] shows that it crystallizes in the monoclinic space group P21/n with unit cell parameters a = 19.401(3) Angstrom, b = 20.205(7) Angstrom, c = 8.458(3) Angstrom, beta = 90.08(2)degrees, and Z = 2. The structure was solved and refined (5732 reflexions) to the final values R = 0.051 and R(w) = 0.047. It is established that 5[PF6] is a nontrapped mixed-valence complex on the infrared timescale and the intervalence transition band allowed the determination of a strong electronic coupling (V-ab = 0.47 eV).