Spin Delocalization in Electron-Rich Iron(III) Piano-Stool σ-Acetylides. An Experimental (NMR) and Theoretical (DFT) Investigation

Abstract

Several paramagnetic electron-rich Fe(III) mononuclear arylacetylide complexes of formula [(η2-dppe)(η5-C5Me5)Fe(C⋮C−Ar)]+ in which Ar represents a functional aryl group were studied by means of multinuclear NMR. All signals detected for the various nuclei were assigned. Hyperfine coupling constants for selected nuclei of the arylacetylide ligand were derived from 1H or 19F NMR contact shifts. These NMR data are diagnostic of a metal-centered unpaired electron partly residing in a π molecular orbital on the arylacetylide ligand, in line with DFT computations. We show here that the 1H NMR paramagnetic shifts of the ortho (H1) and meta (H2) arylacetylide protons convey decisive information on the charge distribution in the aryl ring. Estimates of the relaxation rates of the unpaired electron were also derived from half-widths of the 1H NMR signals. Finally, line-broadening studies of Fe(II)/Fe(III) mixtures allowed extracting the self-exchange rates for several redox couples among these complexes. The self-exchange rates appear slightly substituent dependent and are apparently larger for compounds with electron-withdrawing substituents on the aryl ring. Reorganization energies of ca. 4000 cm-1 could be derived for these outer-sphere electron-transfer processes.

Electron‐Rich Iron/Ruthenium Arylalkynyl Complexes for Third‐Order Nonlinear Optics: Redox‐Switching between Three States

“…[8] Except for two protons on dppe aryl groups on iron, which overlap with the dppe methylene protons on ruthenium, the temperature-dependent shifts of the signals of the protons around the iron center (H1-H11) correspond to those of classic Fe III -arylA C H T U N G T R E N N U N G alkynyl radicals (see the Supporting Information). [19] The dppe protons on ruthenium are only slightly affected by temperature, in line with a small paramagnetic contribution to the shift on that metal center in the dinuclear complex J HH = 7.3 Hz, 2 H). As evidenced by their characteristic multiplicity and intensity pattern, they correspond to the aromatic protons on the phenylalkynyl ligand on ruthenium.…”

“…It is mainly localized on the iron atom (0.52e) and, to a lesser extent, on the conjugated carbon chain and the ruthenium center (0.13e) and is in line with earlier calculations on related complexes. [19,29] The optimized (gas-phase) structure shows slight modifications of the torsion angles along the conjugated chain when compared with those of the experimental (solid-state) arrangement. To evaluate the role of these changes of orientation of the Fe-A C H T U N G T R E N N U N G (dppe)Cp* and RuA C H T U N G T R E N N U N G (dppe) 2 fragments in localizing the unpaired electron, a partial conformational study was performed.…”

“…The modest 31 P NMR shift to lower field suggests the presence of a very weak spin density (if any) on these nuclei, as confirmed by our calculations. [19] The dioxidized species were also investigated theoretically. Three different spin states were optimized, that is, the closed-shell singlet state (low-spin (S) diamagnetic state), the triplet state (high-spin (T) magnetic state), and the broken-symmetry state ("antiferromagnetic" state (BS)).…”

Ligand Redox Non‐Innocence in Transition‐Metal σ‐Alkynyl and Related Complexes

“…[102,103] The reaction of the vinylidene complex 13 with [FeCp 2 ]PF 6 at room temperature affords varying mixtures of the bis(carbyne) complexes 14 and 17. The reaction proceeds by the initial oxidation of the vinylidene to give the radical cation [13] ·+ , which may then undergo immediate C β -C β coupling to afford complex 14. This process results in almost quantitative conversion of the mononuclear vinylidene 13 into the bis-(carbyne) 14 for complexes with R = H or Me.…”

Bis‐alkynyl Diruthenium Compounds with Built‐in Electronic Asymmetry: Toward an Organometallic Aviram–Ratner Diode

“…Prior studies from groups of Lapinte [43,44] and Ren [25] revealed that both the electrode potentials and optical transition energy of {M-CC-ArX} species exhibit a linear dependence on the Hammett constant of aryl-substituent X. Electrode potentials for all compounds except 3 were analyzed by the use of the Equation (1) for which the composite Hammett constants AEs X are also listed in Table 4, and 1 is the reactivity constant.…”