2 research outputs found
Bis‑<i>N</i>‑Heterocyclic Carbene (NHC) Stabilized η<sup>6</sup>‑Arene Iron(0) Complexes: Synthesis, Structure, Reactivity, and Catalytic Activity
Reaction of FeCl<sub>2</sub> with
the chelating bis-<i>N</i>-heterocyclic carbene (NHC) bis-(<i>N</i>-Dipp-imidazole-2-ylidene)Âmethylene
(abbreviated {(<sup>Dipp</sup>C:)<sub>2</sub>CH<sub>2</sub>}) (Dipp
= 2,6-di-isopropylphenyl) affords the complex [FeCl<sub>2</sub>{(<sup>Dipp</sup>C:)<sub>2</sub>CH<sub>2</sub>}] (<b>1</b>) in high
yield. Reduction of complex <b>1</b> with excess KC<sub>8</sub> with a 10-fold molar excess of PMe<sub>3</sub> affords the FeÂ(II)
complex [FeHÂ{(<sup>Dipp</sup>C:)<sub>2</sub>CH<sub>2</sub>}Â(PMe<sub>3</sub>)Â(η<sup>2</sup>-PMe<sub>2</sub>CH<sub>2</sub>)] (<b>2</b>) as a mixture of three stereoisomers. Complex <b>2</b>, the first example of any ironÂ(II) complex bearing mutually an NHC
and PMe<sub>3</sub> ligand, is likely obtained from the <i>in
situ</i>, reductively generated 16 VE Fe(0) complex, [FeÂ{(<sup>Dipp</sup>C:)<sub>2</sub>CH<sub>2</sub>}Â(PMe<sub>3</sub>)<sub>2</sub>] (<b>2</b>′), following intramolecular C–H activation
of one of the phosphorus-bound CH<sub>3</sub> groups. Complex <b>2</b> is unstable in aromatic solvents and forms, <i>via</i> a novel synthetic transformation involving intramolecular reductive
elimination and concomitant PMe<sub>3</sub> elimination, the Fe (0)
arene complex [FeÂ{(<sup>Dipp</sup>C:)<sub>2</sub>CH<sub>2</sub>}Â(η<sup>6</sup>-C<sub>6</sub>D<sub>6</sub>)] (<b>4-<i>d</i></b><sub><b>6</b></sub>) in C<sub>6</sub>D<sub>6.</sub> Complex <b>4-<i>d</i></b><sub><b>6</b></sub> represents the
first example of an NHC stabilized iron (0) arene complex. The transformation
from <b>2</b> to <b>4-<i>d</i></b><sub><b>6</b></sub> can be accelerated at higher temperature and at 60
°C forms immediately. Alternatively, the reduction of <b>1</b> in the presence of toluene or benzene affords the complexes [FeÂ{(<sup>Dipp</sup>C:)<sub>2</sub>CH<sub>2</sub>}Â(η<sup>6</sup>-C<sub>7</sub>H<sub>8</sub>)] (<b>3</b>) and [FeÂ{(<sup>Dipp</sup>C:)<sub>2</sub>CH<sub>2</sub>}Â(η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)] (<b>4</b>), selectively and in good yields. DFT calculations
characterizing the bonding situation in <b>3</b> and <b>4</b> reveal similar energies of the HOMO and LUMO orbitals, with the
LUMO orbital of both complexes located on the Dipp rings of the bis-NHC.
The HOMO orbital reflects a π-back-bonding interaction between
the Fe(0) center and the chelating NHC ligand, while the HOMO-1 is
associated with the arene interaction with the Fe(0) site. The calculations
do not suggest any noninnocence of the coordinated arene in either
complex. Moreover, the <sup>57</sup>Fe Mössbauer spectrum of <b>4</b> at 80K exhibits parameters (δ = 0.43 mm·s<sup>–1</sup>; Δ<i>E</i><sub>Q</sub> = 1.37 mm·s<sup>–1</sup>) which are consistent with a five-coordinate Fe(0)
system, rendering <b>3</b> and <b>4</b> the first examples
of well-defined authentic Fe(0)-η<sup>6</sup>-arene complexes
of the type [FeÂ(η<sup>6</sup>-arene)ÂL<sub>2</sub>] (L = η<sup>1 or 2</sup> neutral ligand, mono or bidentate). Some reactivitiy
studies of <b>3</b> are also reported: The reaction of <b>3</b> with excess CO selectively yields the five-coordinate piano-stool
complex [FeÂ{(<sup>Dipp</sup>C:)<sub>2</sub>CH<sub>2</sub>}Â(CO)<sub>3</sub>] (<b>6</b>) in near quantitative yields, while the
reaction of complex <b>3</b> with C<sub>6</sub>D<sub>6</sub> under heating affords by toluene elimination <b>4-d</b><sub><b>6.</b></sub> The catalytic ability of <b>4</b> was
also investigated with respect to amide reduction to amines, for a
variety of substrates using Ph<sub>2</sub>SiH<sub>2</sub> as a hydride
source. In all cases good to excellent yields to the corresponding
amines were obtained. The use of <b>4</b> as a precatalyst represents
the first example of a well-defined Fe(0) complex to effect this catalytic
process
Spin-state dynamics of a photochromic iron(II) complex and its immobilization on oxide surfaces via phenol anchors
<div><p>This work presents a detailed study of the photo-induced spin-state dynamics of the photochromic iron(II) complex <b>1</b>, where the metal ion is in the field of a tripodal hexa-imine ligand with protolysable phenol groups. The nature of the complex’s ground state has been identified as a spin singlet by <sup>1</sup>H NMR and steady-state UV/vis spectroscopies, and its distorted octahedral structure was analyzed via crystal structure determination. Sub-picosecond and nanosecond time-resolved laser flash photolysis experiments identify the long-lived quintet state of <b>1</b> as the selective product of photoexcitation in the UV/vis spectral region. Thermal barriers of spin-state interconversion as a function of solvent and added base are derived from temperature-dependent rates of transient decay. Ground-state recovery is found to be significantly affected by the solvent and is strongly enhanced, in particular, by base-driven solvolysis of the ligand’s phenol groups. Partial spontaneous deprotonation of the phenolic hydroxyl groups of <b>1</b> seems to prevail on metal oxide surfaces, <i>i.e.</i> on alumina. Composite materials, like <b>1</b> at Al<sub>2</sub>O<sub>3</sub>, that retain the characteristic spectral features of the parent iron(II) complex can be readily obtained by wet impregnation of hydrous alumina with solutions of <b>1</b>.</p></div