Fluxionality in a Paramagnetic Seven-Coordinate Iron(II) Complex: A Variable-Temperature, Two-Dimensional NMR and DFT Study

The preparation and detailed characterizations of the high-spin seven-coordinate complexes [M(κ⁷N-L)](ClO₄)₂ (M = Mn(II), Fe(II); L = N,N,N',N'-tetrakis(2-pyridylmethyl)-2,6-bis(aminomethyl)pyridine) are described. The X-ray crystal structures reveal seven-coordinate metal complex ions. Consideration of continuous shape measures reveals that the coordination environments about the metal ions are better described as having (C_s) face-capped trigonal prismatic symmetry than (C₂) pentagonal bipyramidal symmetry. The (S = ⁵/₂) Mn(II) species shows complicated X-band electron paramagnetic resonance (EPR) spectra and broad, unrevealing ¹H NMR spectra. In contrast, the (S = 2) Fe(II) complex is EPR-silent and shows completely interpretable ¹H NMR spectra containing the requisite number of paramagnetically shifted peaks in the range δ +150 to −60. The ¹³C NMR spectra are likewise informative. Variable-temperature ¹H NMR spectra show coalescences and decoalescences indicative of an intramolecular process that pairwise-exchanges the nonequivalent pyridylmethyl “arms” of the two bis(pyridylmethyl)amine (bpa) domains. A suite of NMR techniques, including T₁ relaxation measurements and variable-temperature ¹H−¹H correlation spectroscopy, ¹H−¹H total correlation spectroscopy, ¹H−¹H nuclear Overhauser effect spectroscopy/exchange spectroscopy, and ¹H−¹³C heteronuclear multiple-quantum coherence experiments, has been used to assign the NMR spectra and characterize the exchange process. Analysis of the data from these experiments yields the following thermodynamic parameters for the exchange: ΔH^‡ = 53.6 ± 2.8 kJ mol⁻¹, ΔS^‡ = −10.0 ± 9.8 J K⁻¹ mol⁻¹, and ΔG^‡ (298 K) = 50.6 kJ mol⁻¹. Density functional theory (B3LYP) calculations have been used to explore the energetics of possible mechanistic pathways for the underlying fluxional process. The most plausible mechanism found involves dissociation of a pyridylmethyl arm to afford a strained six-coordinate species followed by rebinding of the arm in a different position to afford a new seven-coordinate transition state in which the pyridylmethyl arms within each bpa domain are essentially equivalent; the calculated energy barrier for this process is 53.5 kJ mol⁻¹, in good agreement with the observations.