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Electronic and Magnetic Couplings in Free and π-Coordinated 1,4-Diethynylnaphthalene-Bridged [Cp*(dppe)Fe]n+ (n = 0, 1) Units

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journal contribution
posted on 24.08.2009, 00:00 by Yuya Tanaka, Jennifer A. Shaw-Taberlet, Frédéric Justaud, Olivier Cador, Thierry Roisnel, Munetaka Akita, Jean-René Hamon, Claude Lapinte
Treatment of freshly made 1,4-diethynylnaphthalene with Cp*(dppe)FeCl in the presence of either NaPF6 or NaBPh4 salts provides the new binuclear organoiron bis(vinylidenes) [Cp*(dppe)FeCCH-1,4-nph-CHCFe(dppe)Cp*][X]2 (X = BPh4, PF6, Nph = naphthalenediyl, Cp* = η5-C5Me5, dppe = 1,2-bis(diphenylphosphino)ethane), 4H2[PF6]2 and 4H2[BPh4]2. The daughter bis(acetylide) complex 4 was obtained upon deprotonation of the vinylidene precursor 4H2[PF6]2 by an excess of potassium tert-butoxide (80%). The heterotrimetallic complex [{Cp*(dppe)FeCCH}2-{μ-(η6-1,4-nph)RuCp*}][PF6]3 (5H2[PF6]3) was obtained upon reaction of the bis(vinylidene) 4H2[PF6]2 with [Cp*Ru(CH3CN)3][PF6] (82%). The regiopure bisacetylide isomer [Cp*(dppe)FeCC-1,4-nph(RuCp*)-CCFe(dppe)Cp*][PF6] (5B[PF6]) was isolated as a dark brown powder (61%). The IR, 1H, 13C, and 31P NMR, cyclic voltammetry, Mössbauer, and UV−vis data were obtained for the bisiron(II) complexes 4 and 5B[PF6]. All these data and the X-ray crystal structure determined for 5B[PF6] reveal a strong metal−metal interaction across the bridge reinforced by the Cp*Ru+ arenophile. When reacted with 2 equiv of [(C5H5)2Fe][PF6], 4 and 5B[PF6] provided the stable bisiron(III) complexes 4[PF6]2 and 5B[PF6]3 (60−80%), while treatment of 4 and 5B[PF6] with 1 equiv of 4[PF6]2 and 5B[PF6]3, respectively, gave the mixed valence (MV) compounds 4[PF6] and 5B[PF6]2 (88−98%). All the iron(III) complexes can be isolated and stored at 20 °C. ESR spectroscopy and magnetization measurements established that strong superexchange interaction propagates through the 1,4-diethynylnaphthalene bridge (J = −517 cm−1 for 4[PF6]2 and J = −889 cm−1 for 5B[PF6]2). UV−vis−NIR spectra were obtained for the MV complexes, showing that a photodriven metal−metal electron-transfer process takes place over ca. 12 Å with a reorganization energy of ca. 4250 and 4410 cm−1 for 4[PF6] and 5B[PF6]2, respectively. According to the Robin and Day classification, both MV compounds constitute nice examples of strongly coupled class III organometallic MV compounds (HAD = 2140 cm−1 for 4[PF6] and 2230 cm−1 for 5B[PF6]2).