Exchange Interaction Mediated by O−H···O Hydrogen Bonds:  Synthesis, Structure, and EPR Study of the Paramagnetic Organometallic Carboxylic Acid (η⁷-C₇H₇)V(η⁵-C₅H₄COOH)

Abstract

Lithiation of trovacene [(η 7 -tropylium)vanadium(η 5 -cyclopentadienyl)] (1 • ) and appropriate subsequent reactions afforded trovacenyl carboxylic acid (η 7 The deuterated acid 2 • -d 1 was also prepared; 2 • , 2 • -d 1 , and 3 •• were characterized by X-ray structural analysis. The dimeric hydrogen bonded structure of 2 • which is found in the crystal also prevails in aprotic, nonpolar solvents and gives rise to a biradical EPR spectrum from which the electron exchange coupling constant J ) -0.011 cm -1 can be determined via computer simulation. Thus, exchange interaction across eight bonds is observable even though the spacer contains weak hydrogen bonds. Considerably stronger exchange coupling (J ) -0.345 cm -1 ) is displayed by the anhydride 3 •• . The EPR spectrum of 2 • in toluene is temperature dependent as expected for the equilibrium 2 (2 • ) h (2 • ) 2 which leads to the superposition of monomer and dimer spectra at ratios governed by temperature. The enthalpy of dimerization can thus be derived; it amounts to -11.8 kcal mol -1 for 2 • and -15.2 kcal mol -1 for 2 • -d 1 . The 1 H NMR spectrum of the radical 2 • reveals the positive contact shift ∆δ ) 3 ppm for the carboxylic proton, relative to the chemical shift observed for the diamagnetic organometallic acid Cr(η 6 -C 6 H 5 COOH) 2 . Conceivably, the finite spin density on the carboxyl group, present in the monomer 2 • , contributes to the exchange interaction displayed by the hydrogen bonded dimer (2 • ) 2 . The redox potential E 1/2 (2 +/0 ) and the hyperfine coupling constant a( 51 V, 2 • ) are pH dependent. In the former case, a Nernst-Clark plot furnishes the pK s values of 4.4 for neutral 2 • and 1.85 for the cation 2 + . A similar value, pK s (2 • ) ) 3.7, is read off the plot a( 51 V, 2 • ) versus pH.

Electro‐ and Magnetocommunication in [5,5]Ditrovacenyls, [(η7‐C7H7)V(η5‐C5H4‐X‐η5‐C5H4)V(η7‐C7H7)], Mediated by the Spacers X=(Z)CHCH, (E)CHCH, >CCH2, CH2CH2, and CH2

“…Paramagnetic trovacene lends itself admirably to this type of investigation, since it possesses a nondegenerate 2 A 1 ground state giving rise to well-resolved 51 V hyperfine structure in EPR spectra; in addition it features two adjacent reversible redox couples (1 + /0/À ) and can be lithiated regioselectively at the cyclopentadienyl ring, thereby rendering a variety of derivatives accessible. In the past we have described, amongst others, [5]trovacenyl carboxylic acid [(C 7 H 7 )V(C 5 H 4 -COOH)], which features exchange coupling in the hydrogen-bonded dimer (2C) 2 ; [5] the isomers bitrovacene 3CC [6] and bitrovacene 4CC, [7] which display a pronounced difference in the extent of exchange coupling; and the pair of [5,5]bitrovacenes 5CC and 6CC, which were conceived in order to assess the influence a side-on versus a head-on p-stacked disposition of two trovacene units may exert on intramolecular interactions. [8] A very recent study addressed the role of ethynyl-and butadiynyl units in interAbstract: Five new paramagnetic dinuclear complexes containing [5]trovacenyl groups, (h 7 -C 7 H 7 )V(h 5 -C 5 H 4 À), have been prepared and characterized, including by single-crystal X-ray diffraction.…”

“…In the past we have described, amongst others, [5]trovacenyl carboxylic acid [(C 7 H 7 )V(C 5 H 4 -COOH)], which features exchange coupling in the hydrogen-bonded dimer (2C) 2 ; [5] the isomers bitrovacene 3CC [6] and bitrovacene 4CC, [7] which display a pronounced difference in the extent of exchange coupling; and the pair of [5,5]bitrovacenes 5CC and 6CC, which were conceived in order to assess the influence a side-on versus a head-on p-stacked disposition of two trovacene units may exert on intramolecular interactions. [8] A very recent study addressed the role of ethynyl-and butadiynyl units in interAbstract: Five new paramagnetic dinuclear complexes containing [5]trovacenyl groups, (h 7 -C 7 H 7 )V(h 5 -C 5 H 4 À), have been prepared and characterized, including by single-crystal X-ray diffraction. As intervening spacers, ethenediyl units in the geminal and vicinal (Z)-and (E)-bridging modes as well as methanediyl and ethanediyl units have been included with the aim of studying their propensity to transmit electric and magnetic information.…”

“…It is concluded that orientation-dependent mechanisms of spinspin interactions (p-orbital overlap, hyperconjugation) contribute extensively although, as implied by sizeable J values for À CH 2 À and À C 2 H 4 À linked di [5]trovacenyl groups, coupling mediated by the s-orbital chain must also be considered.…”

“…It is almost a truism now that, after the inception of the Creutz-Taube ion [(H 3 N) 5 Ru(pyrazine)Ru(NH 3 ) 3 ] 5 + in 1969, [2] studies of mixed/intermediate valence complexes and their role in intramolecular electro-and magnetocommunication have become vital parts of contemporary research in coordination chemistry.…”

“…Keywords: magnetic properties · organic spacers · redox chemistry · sandwich complexes · vanadium [5]trovacenyl communication. [1] The present paper focusses on ethylene as a spacer: the question raised is how the three different ways (1,1-, (E)-1,2-, and (Z)-1,2-) of connecting two [5]trovacenyl groups to ethylene are reflected in the redox properties and the magnetic behavior of the products?…”