Direct observation of low spin – high spin electronic ground states and cross-over exchange in manganocene derivatives, (η⁵-C₅H₄R)₂Mn, R = H, CH₃, C₂H₅ by paramagnetic nuclear magnetic resonance

Abstract

DANIEL COZAK, FRANFOIS GAUVIN, and JACQUES DEMERS. Can. J. Chem. 64, 71 (1986). The paramagnetic 'H nmr spectra for manganocene (I) and 1 ,I '-dimethylmanganocene (2), and the I3C nrnr spectra for 1,l '-diethylmanganocene (3) have been recorded in toluene solvent over a -90 to 90°C temperature range. 1 shows a low field and a high field ring proton resonance in its spectrum near -59°C. At higher temperatures the low field resonance is prevalent and becomes gradually averaged due to a fast spin exchange process that dominates the spectrum at 90°C. For the ring substituted derivatives 2 and 3, resonances due to only one paramagnetic species were detected in the low temperature range. Above ambient temperature a new spectrum due to rapid spin exchange averaging is observed for these complexes. Results are readily interpreted in terms of ground state molecular cross-over exchanges between the 2~2 , , Chem. 64, 71 (1986). Nous avons enregistrC les spectres de larmn 'H du manganockne (I), du dimCthyl-1,l' manganockne (2) ainsi que les spectres de rksonance du 13C pour le diCthyl-1 , 1 ' manganocene 1 differentes temperatures entre -90 et 90°C. Le spectre du composC 1, dCterminC i -59"C, prCsente deux resonances paramagnCtiques dues,aux protons annulaires. La rCsonance se trouvant a champs faible domine les spectres dCterminCs i plus hautes temperatures. A 90°C, ce signal est dO a un spectre moyen cause par un Cchange rapide entre plusieurs Ctats paramagnktiques de la molCcule. Les spectres des composCs 2 et 3, dCterminCs B basse tempCrature, prCsentent un seul Ctat paramagnktique. Aux tempkratures superieures i celles de la pikce, un nouveau spectre, dO aussi i un Cchange rapide entre plusieurs Ctats paramagnCtiques, remplace graduellement le premier. Les rCsultats expCrimentaux sont interprCtCs en fonction de la dynamique des Ctats Clectroniques fondamentaux *E~,, 6~I g et 2~I g connus pour ces molCcules.

Phase-Dependent Diagonal and Lateral Isomerism of the Rhenium Complexes (η5-C5H4Me)Re(CO)2Br2

“…The ligands ethylcyclopentadiene and tert -butylcyclopentadiene and the complex CpRe(CO) 3 were prepared by the literature methods. − All reactions were carried out under dry N 2 in either a Schlenk apparatus or a flask connected to a double manifold providing low vacuum or nitrogen. Solvents were dried by conventional methods, distilled under nitrogen, and used immediately.…”

Electronic and Steric Control of the Spin-Crossover Behavior in [(CpR)2Mn] Manganocenes

“…While some of these molecules are high-spin systems, such as the [(Cp 1,3,4‑tBu ) 2 Mn] molecule, with a magnetic moment of 5.9 μ B , others have been reported to be low-spin systems, such as the [(Cp*) 2 Mn] molecule, with a temperature-independent magnetic moment of 2.18 μ B . The possibility of accessing two alternative spin states points, therefore, to the potential design of manganocene molecules exhibiting spin crossover, and in fact this has been reported in the literature through the years. ,, However, as also occurs for many other families of spin-crossover systems, the link between the chemical functionalization of the ligand and the physical properties of the system is buried under several competing effects. In fact, as was already introduced by Andersen and co-workers, two competing effects are in play for the [(Cp R ) 2 Mn] (R = Me, i Pr, t Bu) family.…”

Electronic ground states and isotropic proton NMR shifts of manganocene and its derivatives

“…Preparation and characterization of the studied compounds is given in detail elsewhere. 11 The tetraethyl derivative was isolated as an impurity during the synthesis of diethylmanganocene. The doubly substituted cyclopentadienyl ligand was identified as the 1,2-ring isomer product from the more easy to handle ferrocene analogue (j76-CsH3(C2H5)2)2Fe: NMR (C6D6) S 3.85 (m, ring), 3.80 (m, ring), 2.28 (q, 3J = 7.5 Hz, CH2), 1.10 (t, 3J = 7.5 Hz, CH3); 13C NMR (C6D6) 5, 88.5 (substituted C), 67.9 and 66.6 (CH), 20.7 (CH2), 15.1 (CH3); MS, [M]+ m/z 298 (relative intensity 100).…”