New derivatives of trovacene (heta7-C7H7)V(heta5-C5H5): studies of redox splitting, exchange coupling and hydrogen bonding

The strategy followed in this work is based on the utilization of hydrogen bonds, master key interaction in crystal engineering, between adequately functionalised organometallic molecules. Exchange interactions mediated by hydrogen bridges, i.e. O-H---O, in paramagnetic complexes can be studied employing EPR spectroscopy. Hence, a study of selective metalation of trovacene, in the five-membered ring, in both rings, and in the seven-membered ring obtained employing (heta7-cyclo-heptatrienyl)(heta5-pentamethylcyclopentadienyl)vanadium, was undertaken and subsequent reactions of carboxylation and formylation were performed. Distortion of the molecular geometry, such as ring tilting or deviations of the individual rings from the planarity can be studied spectroscopically to furnish understanding of the bonding in tilted sandwich complexes. Strained, ring-tilted sila-metallocenophanes are of interest because of the fact that these species readily undergo thermally induced ring-opening polymerisation (ROP). For this purpose, via dilithiation of trovacene, monosila- and disila-trovacenophanes were synthesized, and, thermal or catalytic ring-opening polymerisation was attempted. In organometallic carboxylic acids the hydrogen bonding capacity of the –COOH groups can be combined with the topology of the ligand coordination about the metal centers. In this study the chromium sandwich complex of benzoic acid, [Cr(h6-C6H5COOH)2], was used as such as well as in the oxidized form, containing CrI as the central metal and in the deprotonated, zwitterionic form. The different packing modes, governed by hydrogen bonding, have been elucidated by means of X-ray diffraction

1,1′-Diarsaferrocene This work was supported by the Petroleum Research Fund (administered by the American Chemical Society) and the Horace H. Rackham Fund.

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1,1′-Diarsaferrocen Diese Arbeit wurde vom Petroleum Research Fund (verwaltet von der American Chemical Society) sowie dem Horace H. Rackham Fund gefÖrdert.

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Magnetic face-to-face interaction and electrocommunication in chromium sandwich compounds

Elschenbroich C, Kanellakopulos B, Koehler FH, et al. Magnetic face-to-face interaction and electrocommunication in chromium sandwich compounds. CHEMISTRY-A EUROPEAN JOURNAL. 2007;13(4):1191-1200.The reaction of [{(C5Me5)-CrCl2}(2)] with [2.2](1,4)cyclophane gave [(C5Me5)Cr{[2.2](1,4)cyclophane}] (1) and [(C5Me5)Cr{[2.2](1,4)cyclophane}-Cr(C5Me5)] (2), depending on the reaction conditions. X-ray structure analysis showed 2 to be a ministack which in turn is stacked in the lattice. The chromium atoms are 6.035 angstrom apart, and the distortion of the benzene rings to boat-shaped moieties is less pronounced than in parent [2.2](1,4)cyclophane. The NMR and EPR spectra were consistent with a S = 1/2 ground state for 1 and with two interacting S = 1/2 centers in 2. Spin density was found in the ligand pi systems, where its sign was negative when the pi system was adjacent to chromium, while on the non-bonded benzene moiety of 1 it was positive. Cyclic voltammograms showed reductions to 1(-) and 2(2-), as well as oxidations to 1(+), 2(+), and 2(2+) which were quasireversible, whereas oxidations to 1(+), 2(+) and 2(2+) were irreversible. Interaction between the metal ions was revealed by a 260 mV separation of the redox waves belonging to 2(+), and 2(2+). Both cations were isolated as [B-(C6H5)(4)](-) salts, which in solution decomposed to [2.2](1,4)cyclophane and [(C5Me5)Cr{(eta(6)-C6H5)B(C6H5)(3)}] (3). The H-1 and C-13 NMR spectra of 3 were in accordance with an S=1 ground state. Solid-state magnetic measurements of the dimetallic compounds showed antiferromagnetic interaction with J = -122 cm(-1) for 2, J = -31 cm(-1) for 2(+) (ground state S = 1/2), and J = -23.5 cm(-1) for 2(2+) (with H = -JS(1)S(2)). The decrease of J in the series 2, 2(+), and 2(2+) was traced to the number of impaired electrons and, for the mixed-valent cation 2(+), to additional double exchange