Triggering Redox Activity in a Thiophene Compound: Radical Stabilization and Coordination Chemistry

Abstract

Abstract:The synthesis, metalation, and redox properties of an acyclic bis(iminothienyl)methene L − are presented. This π-conjugated anion displays pronounced redox activity, undergoing facile oneelectron oxidation to the acyclic, metal-free, neutral radical L • on reaction with FeBr2. In contrast, reaction of L − with CuI forms the unique, neutral Cu2I2(L • ) complex of a ligand-centered radical, whereas reaction with the stronger oxidant AgBF4 forms the metalfree radical dication L •2+ .Since the first reports on dithiolate metal complexes, [1] interest in redox-active ligands has burgeoned due to their relevance to enzymatic processes [2] and access to unusual chemical properties by coupling the redox activity of the ligand to the coordination chemistry of a metal. [3] In these cases, the ligand is no longer a classical "spectator", [4] and a large number of ligands have been shown to exhibit redox activity and stabilize the radical species through an inductive effect or by delocalization in a conjugated π-system. Accordingly, we have shown that an N-donor-expanded dipyrrin ligand [5] is redox active and able to mediate sequential electron transfer to a uranyl(VI) center (Figure 1). The initial reduction occurs at the ligand, forming a U(VI) ligand-centered radical prior to reduction of the uranium center, ultimately to U(IV). [6] Ligandcentered oxidation was also seen in Ni complexes of a similar bis(phenolate)dipyrrin ligand, with the one-electron oxidation product characterized as a ligand-centered radical. [7] In contrast to the nitrogen-containing heterocycles found in dipyrrins, porphyrinoids and sub-porphyrins, [8] studies on the redox activity of sulfur-containing heterocycles such as thiophene are more limited, despite their use in tuning the electronic properties of molecular compounds [9] and polymeric materials. [10] Expanded porphyrinoids featuring five thiophene units undergo single-electron oxidations, from the aromatic mono-anion to an isolable, air-stable neutral radical, and further to an anti-aromatic mono-cation. [11] In contrast, radical cations of simple thiophenes or their analogues are stable only at low temperatures or their identity inferred from quenching reactions. [12,13] We were keen to see if we could exploit redox activity and the 'softer' donor properties of the sulfur atoms in methylene-bridged thiophenes to access new transition-metal chemistry and reactivity. As such, we show here that the bis(iminothienyl)methene L − reacts with metal salts to generate the neutral radical L • , the dicationic radical L •2+ or the dinuclear copper(I) complex Cu2I2(L • ) of a ligand-based iminothienyl radical. Scheme 1. Synthetic pathway to the monoanionic iminodithiophene KL and its redox reactions with metal salts (isolated yields in parentheses).Studies on the meso-C lithiation of dithiophenemethane compounds have found thermodynamic versus kinetic selectivity issues along with the formation of meso-C coupled products. [14] However, we find that deprotonation of HL with KH in THF ...

Molybdenum(VI) bis-imido Complexes of Dipyrromethene Ligands

“…The ligand precursors HL 1 (R = Mes) and HL 2 (R = t Bu) were both obtained following literature known procedures , using an acid-catalyzed condensation of the substituted pyrroles with mesityl dimethyl acetal. Both precursors could be conveniently converted to their corresponding potassium salts by deprotonation with KHMDS in n -pentane, causing them to precipitate in isolated yields of ∼90%. , Deprotonation of the ligand salt was apparent by the absence of the characteristic N H resonance above δ 10 ppm in the 1 H NMR spectra of the corresponding salts KL 1 and KL 2 , respectively. The desired complexes 1 – 4 were then synthesized following a salt metathesis protocol using the dipyrromethene salts KL 1 and KL 2 and the corresponding bis-imido precursors (DME)­Mo­(NR′) 2 Cl 2 (R′ = t Bu , and Mes; DME = 1,2-dimethoxyethane, Scheme ).…”

Radical‐Type Reactivity and Catalysis by Single‐Electron Transfer to or from Redox‐Active Ligands

“…[1] Well-known examples of redox-activel igandsa re dithiolenes, [2] donora ppended diarylamines, [3] heterodienes such as 1,4-diimine and related 2iminopyridines as wella sb is(imino)pyridines, [4] olefin-appended diazadienes, [5] salensa nd derivatives thereof, [3j, 6] and dipyrrins as well as extended systems such as porphyrins. [7] Newer additions have appeared as well, includingv erdazyl, [8] and nindigo, [9] formazanate, [10] aminotropiminate, [11] arylazopyridine, [12] p-coordinated arene and derivatives thereof, [13] b-diketiminate [14] and variouso thers. [15] Particularly well-studied are ligands bearing the redox-active 1,2-catechol or o-phenylenediamine [16,17] moiety or the hybrid o-aminophenol 18,19] fragment, which can bind to am etal center in three different oxidation states:t he two-electron reduced (dianionic), one-electron reduced/oxidized (monoanionic ligand radical)a nd two-electron oxidizedf orm (neutral).…”

Radical Relatives: Facile Oxidation of Hetero-Diarylmethene Anions to Neutral Radicals

“…Upon oxidation of K L S into L S· a weak band at 455 nm is seen, similar in shape to that of K L S , and due to combined α/β singly occupied molecular orbital (SOMO)–LUMO transitions. As in the related thiophene compounds, , the oxidation of the anion to the radical is accompanied by a hypsochromic shift that is indicative of a larger orbital gap in the radical of 5.50 eV (α-spin) and 5.31 eV (β-spin) against 4.24 eV for the anion. The electronic absorption spectra of the furan-containing compounds are different, in which the HOMO–LUMO transition observed at 284 nm for H L NO (Figure S11) is modified to multiple bands on deprotonation to K L NO , with the low-energy band at 630 nm associated with the HOMO–LUMO transition (Figure ).…”

“…In the structure, the meso carbon C17 is sp 2 hybridized with essentially coplanar iminofuran linkages (torsion angle 8.7°). In contrast to the iminothiophene relative K L NS , which adopts a dimeric structure, the N 2 O 2 compartment in K L NO binds a single potassium cation due to the shorter N1···N2 separation between the imine nitrogen atoms of 4.785(4) Å (compared with 7.357(1) Å in K L NS ). Such a feature is also likely responsible for the positioning of the K cation above the N 2 O 2 plane (N 2 O 2 plane···K1 1.222 Å), although influence by the adjacent π-coordinated furan cannot be excluded (centroid···K1 3.029 Å).…”

“…We also reported the redox-active bis­(iminothienyl)­methene anion L NS – (Figure ) that undergoes facile single-electron oxidation to form the stable and isolable neutral iminothiophene radical L NS· . This radical formed the dinuclear Cu­(I) ligand-radical complex {CuI} 2 ( L NS· ) upon reaction with CuI, and under more oxidizing conditions, the radical dication L NS 2+ · was generated . The mild potentials at which these redox processes take place and the poor coordinating ability of the sulfur donors of the thiophene heterocycles prompted us to study additional variants of the diarylmethene scaffold.…”