Solid State Dynamics of Tricarbonyl(η-1,5-cyclohexadienylium)iron Tetrafluoroborate and Tricarbonyl(η-1,5-cycloheptadienylium)iron Tetrafluoroborate

Abstract

The dynamic behavior of [(C(6)H(7))Fe(CO)(3)]BF(4) (I) and [(C(7)H(9))Fe(CO)(3)]BF(4) (II) in the solid state has been investigated principally by NMR spectroscopy. High-resolution variable-temperature (1)H and (13)C NMR spectra indicate that both complexes have a solid state phase transition above which there is rapid reorientation of the cyclodienylium rings and fast exchange of the carbonyl groups. The transition occurs between 253 and 263 K for I and between 329 and 341 K for II. The presence of the phase transition is confirmed by differential scanning calorimetry (DSC). (57)Fe Mössbauer spectroscopy supports the notion that complex I is highly mobile at room temperature, while II is relatively static. The activation energy for the cyclodienylium group rotation in the high-temperature phase of I is estimated from (1)H spin-lattice relaxation time measurements to be 17.5 kJ mol(-)(1). Static (13)C NMR measurements of the solid complexes in the high-temperature phase indicate that the (13)C chemical shift anisotropies are only 20-30 ppm. This is significantly less than that expected to result from motion of individual groups and thus suggests that rotation of the whole molecule is involved. A single-crystal X-ray structural determination of complex II, at 295 K, showed that the complex is tetragonal (space group P4(1), a = 10.610(1) Å, c = 21.761(3) Å, V = 2449.7(5) Å(3), rho(calc) = 1.734 g cm(-)(3)), with eight cycloheptadienyl cations and eight tetrafluoroborate anions per unit cell. In addition, powder X-ray diffraction studies of both I and II confirm that at low temperatures both complexes have a tetragonal unit cell, which transforms to a cubic unit cell above the phase transition. The powder patterns, recorded above the phase transition, support the proposal that the complexes are undergoing whole-molecule tumbling in their dynamic regimes.

Comparison of the Electrophilicities of the Free and the (Tricarbonyl)iron-Coordinated Tropylium Ion

“…The calculated minimum structure of 22 shows a normal C(sp 3 )−C(sp 3 ) bond length (1.534 Å) and a staggered conformation of the ethano bridge (Figure , bottom). In contrast, a nearly eclipsed conformation of the ethano bridge and very short C(sp 3 )−C(sp 3 ) distances (down to 1.400 Å) were reported for the solid state of 22-BF 4 …”

“…In contrast, a nearly eclipsed conformation of the ethano bridge and very short C(sp 3 )-C(sp 3 ) distances (down to 1.400 Å) were reported for the solid state of 22-BF 4 . 36 A possible explanation for this apparent discrepancy is torsional vibrational averaging in the solid state. The isodesmic reaction eq 6 indicates that the hydride affinity of 6 is greater than that of the free tropylium ion 1 by 13.1 kcal mol -1 , reflecting the aromatic stabilization of the latter.…”

Solid‐State NMR Investigation of Ligand Mobility and Reactivity in Transition Metal Complexes

“…[52] In both cases rapid reorientation of the rings and fast exchange of the carbonyl ligands have been demonstrated by a 1 H/ 13 C variable temperature study.…”

Molecular dynamics of (η6-arene)tricarbonylchromium complexes in the solid state: carbon-13 nuclear magnetic resonance and crystal structure of [Cr(η6-C6HMe5)(CO)3]

“…For instance, organometallic complexes containing the cyclohexa-and cyclohepta-dienyl groups have recently been found to show complicated dynamic behaviour: whole-molecule tumbling has been observed to occur for [Fe(C 6 H 7 )(CO) 3 ]BF 4 and [Fe(C 7 H 9 )(CO) 3 ]BF 4 above a solidstate phase transition, notwithstanding the steric protuberance of the diene ligands. 24 Braga and Grepioni 25,26 have calculated barriers to carbonyl rotation for [M(η 6 -arene)(CO) 3 ] complexes using atom-atom potential-energy calculations and concluded that such rotations were unlikely. However, our results show that exchange does indeed occur.…”