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

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 o f 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 o range 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) intermedia te 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 p otassium tert-butoxide to give the mu-eta(1):eta(1)-butadiyndiyl C-4 c omplex 5, isolated as a brown powder in 92% yield. Complex 5 is charac terized by H-1, C-13, and P-31 NMR and by IR, Mossbauer, and microanal ysis. 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 displa ys two reversible one-electron oxidation waves. The two redox processe s 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)F e][PF6] gave the salt 5[PF16]2, isolated in 91% yield. It is a thermal ly 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 CH 2Cl2 produced the Fe(III)-Fe(II) complex 5[PF6] (92% yield). The air s table mixed-valence compound 5[PF6] is characterized by elemental anal ysis, IR, NIR, Mossbauer, and ESR spectroscopies, and magnetic suscept ibility. The X-ray crystal structure of 5[PF6] shows that it crystalli zes 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, bet a = 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 de termination of a strong electronic coupling (V-ab = 0.47 eV).