- Electron Spin Adjustment in Manganocenes. Preparative, Paramagnetic NMR, and X-ray Study on Substituent and Solvent Effects
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The new manganocenes (EtCp)2Mn (3), (Me3SiCp)2Mn (6), (Me2Cp)2Mn (7), (Me4Cp)2Mn (8), and (EtMe4Cp)2Mn (10) have been prepared together with Cp2Mn (1), (MeCp)2Mn (2), (i-PrCp)2Mn (4), (t-BuCp)2Mn (5), and (Me5Cp)2Mn (9).Their 1H, 13C, and 29Si NMR signals experience shifts up to 1340 ppm at 390 K and provide a systematology of shifts which allows a fast and detailed characterization.Low-spin (2E2g) and high-spin (6A1g) manganocenes are recognized easily, even in a mixture, due to very different shift ranges.These ranges and further particularities are explained in terms of different electron spin delocalization.The spin state of manganocenes depends on the nature of substituents R, the number of R, the solvent, and the temperature. 2E2g is favored by donor substituents and 6A1g by acceptors. 2E2g is also favored by increasing the number of donors: for (RCp)2Mn 2E2g and 6A1g species are found to be in equilibrium with 13percent 2E2g for 2 (ΔH0 = 11.6 kJ mol-1, ΔS0 = 44.5 J mol-1 K-1), 30percent for 7, and no detectable 6A1g population for 8-10, all at 400 K.The mixtures of 2E2g and 6A1g manganocenes in toluene are transformed to pure high-spin species when donor solvents are used.At low temperatures the NMR spectra of both manganocene spin isomers are observed in toluene.Coalescence is found for 2-6 near room temperature with a lifetime of 9.8x10-6 s for the spin isomers of 2 and ΔG = 49 kJ mol-1 for 2E2g -> 6A1g. (Me3SiCp)2Mn turns out to be the first pure high-spin manganocene at ambient temperature.The X-ray analysis of 6 at -35 deg C shows discrete centrosymmetric monomers (P21/n, a = 6.362 (2) Angstroem, b = 11.725 (3) Angstroem, c = 12.628 (3) Angstroem, β = 94.48 (2) deg, V = 939.10 Angstroem3, dcalcd = 1.166 g/cm3 for Z = 2, Rw = 0.040 for 104 parameters and 1379 observables).The crystal packing is largely determined by the silyl groups, and unlike in Cp2Mn no bridging ligands are present.The distance Mn-Cp(centroid) is 2.050 (3) Angstroem.The rings are slightly slipped such that the silyl bearing carbons form the shortest bonds to Mn.This is explained by MO arguments.
- Hebendanz, Nikolaus,Koehler, Frank H.,Mueller, Gerhard,Riede, Juergen
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- 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
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The paramagnetic 1H nmr spectra for manganocene (1) and 1,1'-dimethylmanganocene (2), and the 13C nmr spectra for 1,1'-diethylmanganocene (3) have been recorded in toluene solvent over a -90 to 90 deg C temperature range. 1 shows a low field and a high field ring proton resonance in its spectrum near -59 deg C.At higher temperatures the low field resonance is prevalent and becomes gradually avaraged due to a fast spin exchange process that dominates the spectrum at 90 deg 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 2E2g, 6A1g, and 2A1g spin states of the complexes.
- Cozak, Daniel,Gauvin, Francois,Demers, Jacques
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