3 research outputs found
A Simple Approach for Predicting the Spin State of Homoleptic Fe(II) Tris-diimine Complexes
We propose a simple
method for predicting the spin state of homoleptic
complexes of the FeĀ(II) d<sup>6</sup> ion with chelating diimine ligands.
The approach is based on the analysis of a single metric parameter
within a free (noncoordinated) ligand: the interatomic separation
between the N-donor metal-binding sites. An extensive analysis of
existing complexes allows the determination of critical NĀ·Ā·Ā·N
distances that dictate the regions of stability for the high-spin
and low-spin complexes, as well as the intermediate range in which
the magnetic bistability (spin crossover) can be observed. The prediction
has been tested on several complexes that demonstrate the validity
of our method
Probing the Impact of Solvation on Photoexcited Spin Crossover Complexes with High-Precision Xāray Transient Absorption Spectroscopy
Investigating the photoinduced electronic
and structural response
of bistable molecular building blocks incorporating transition metals
in solution phase constitutes a necessary stepping stone for steering
their properties toward applications and performance optimizations.
This work presents a detailed X-ray transient absorption (XTA) spectroscopy
study of a prototypical spin crossover (SCO) complex [Fe<sup>II</sup>(mbpy)<sub>3</sub>]<sup>2+</sup> (where mbpy = 4,4ā²-dimethyl-2,2ā²-bipyridine)
with an [Fe<sup>II</sup>N<sub>6</sub>] first coordination shell in
water (H<sub>2</sub>O) and acetonitrile (CH<sub>3</sub>CN). The unprecedented
data quality of the XTA spectra together with the direct fitting of
the difference spectra in <i>k</i> space using a large number
of scattering paths enables resolving the subtle difference in the
photoexcited structures of an Fe<sup>II</sup> complex in two solvents
for the first time. Compared to the low spin (LS) <sup>1</sup>A<sub>1</sub> state, the average FeāN bond elongations for the photoinduced
high spin (HS) <sup>5</sup>T<sub>2</sub> state are found to be 0.181
Ā± 0.003 Ć
in H<sub>2</sub>O and 0.199 Ā± 0.003 Ć
in CH<sub>3</sub>CN. This difference in structural response is attributed
to ligandāsolvent interactions that are stronger in H<sub>2</sub>O than in CH<sub>3</sub>CN for the HS excited state. Our studies
demonstrate that, although the metal center of [Fe<sup>II</sup>(mbpy)<sub>3</sub>]<sup>2+</sup> could have been expected to be rather shielded
by the three bidentate ligands with quasi-octahedral coordination,
the ligand field strength in the HS excited state is nevertheless
indirectly affected by solvation effects that modifies the charge
distribution within the FeāN covalent bonds. More generally,
this work highlights the importance of including solvation dynamics
in order to develop a generalized understanding of the spin-state
switching at the atomic level
Probing the Anisotropic Distortion of Photoexcited Spin Crossover Complexes with Picosecond Xāray Absorption Spectroscopy
For numerous spin crossover complexes,
the anisotropic distortion
of the first coordination shell around the transition metal center
governs the dynamics of the high-spin/low-spin interconversion. However,
this structural parameter remains elusive for samples that cannot
be investigated with crystallography. The present work demonstrates
how picosecond X-ray absorption spectroscopy is able to capture this
specific deformation in the photoinduced high-spin state of solvated
[FeĀ(terpy)<sub>2</sub>]<sup>2+</sup>, a complex which belongs to the
prominent family of spin crossover building blocks with nonequivalent
metalāligand bonds. The correlated changes in FeāN<sub>Axial</sub>, FeāN<sub>Distal</sub>, and bite angle N<sub>Distal</sub>āFeāN<sub>Axial</sub> extracted from the measurements
are in very good agreement with those predicted by DFT calculations
in <i>D</i><sub>2<i>d</i></sub> symmetry. The
outlined methodology is generally applicable to the characterization
of ultrafast nuclear rearrangements around metal centers in photoactive
molecular complexes and nanomaterials, including those that do not
display long-range order