Urea Azines (Bisguanidines): Electronic Structure, Redox Properties, and Coordination Chemistry

Abstract

Urea azines are a largely neglected class of compounds. We show herein that they can easily be synthesized and sublimed at higher temperatures (90–100 °C) without decomposition. Our discussion includes the derivatives N,N′‐diisopropylurea azine (2), tetramethylurea azine (3), and N,N′‐dimethylethyleneurea azine (4) as examples. Vibrational spectroscopy and quantum chemical calculations were used to study their electronic structure in detail. A clear trend in the calculated decomposition to N2 and carbene is found, but the analysis reveals that this trend reflects the stability of the carbene decomposition product rather than changes in the electronic structure of the urea azines. Using cyclic voltammetry (CV) measurements we show that the urea azines are strong organic electron donors, which can be oxidized reversibly in two well‐separated one‐electron steps. The radical salt 4(TCNQ), featuring radical monocations and radical monoanions, which form mixed stacks in the solid state, was prepared by reaction of neutral 4 with tetracyanoquinodimethane (TCNQ). Oxidation of 4 with silver salts Ag+X– (X = BF4 or PF6) was accompanied by dehydrogenation, leading to intensively red‐brown colored radical azoimidazolium dyes. Furthermore, urea azines were used as chelating ligands. In the case of 2, coordination to late transition metals initiates tautomerization. Reaction of 2 with boron hydrides leads to hydrogen elimination and formation of new B,N bi‐heterocyclic ring structures.

Hydrogen‐Atom Transfer (HAT) Initiated by Intramolecular Ligand–Metal Electron Transfer

“…[13][14][15][16] The discussioni nt his work concentrates on the two derivatives 4 and 5 (see the Supporting Informationf or somer esults with 3)t hat can be reversiblyo xidized electrochemically (cyclic voltammetry measurements)i nt wo one-electron waves (E 1/2 (CH 2 Cl 2 ) = À0.29 and 0.16 Vv s. Fc/Fc + for 4 and À0.40 and 0.22 Vv s. Fc/Fc + for 5;F c = ferrocene). [15] The focus of this work is on the evaluation of the electron transfer processes that initiate HATi nu rea azines (Scheme 2) and the evaluation of HAT-induced radical recombination. Applications of this novelH AT variant for synthetic chemistry are not explored here, but the first preliminary experiments indicatet hat the carbon-centered radicalf ormed by HATc an be trapped with 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO).…”

Tuneable Redox Chemistry and Electrochromism of Persistent Symmetric and Asymmetric Azine Radical Cations

“…The redox potentials for compounds 1-3 were taken from the literature. [8] All compounds are oxidised in two reversible, clearly separated one-electron redox steps, indicatingt hat the radical monocations are thermodynamically stable towards disproportionation. Compound 4 exhibits the lowest redox potential (E 1/2 = À0.57 Vv s. Fc + /Fc for the redox couple 4C + /4), and (2)1 .306 (2)1 .415 (2) 1.371 (2) 1.403 (2) 1.371 (2) 1.403 (2) (2) 1.295 (2) 1.407 (2) 1.400 (2) 1.392 (2) 1.400 (2) 1.392(2) 5 [a] 1.301 (2) 1.300 (2)1 .407 (2)1.394 (2) 1.386 (2) 1.391 (2) 1.393 (1) RAA C=NC =NNN CÀNC ÀSC ÀNC ÀS 6…”

“…Quantum-chemical calculations (B3LYP/TZVP) show that the HOMO of the neutral RAAs is predominantly located on the bisguandine unit;t hus modifications in this part should be most effective. The introduction of sulfur (6)(7)(8)w ith its mismatching orbitale nergies and shapes leads to less electronrich azines.T his trend is also evident from the calculated first adiabatic ionisation energies, showing higher values for the thiourea azines (Table 3). Al inear relationship was found between the experimentally determined E 1/2 values of the redox couples RAAC + /RAAa nd the calculated first (adiabatic)i onisation energies I 1 (see Supporting Information).…”

Electropolymerization of N,N'-Diphenylguanidine in Non-Aqueous Aprotic Solvents and Alcohols

“…Guanidine benzene derivatives can build up Ag + bridged multicore metal complexes which have semiconducting properties [12] and metal ion bridged polymers also form with silver dicyanamide [13]. Their color changes remarkably by the degree of protonation and due to electrochemical reactions [14][15][16]. Guanidines having ferrocene moiety are redox active and they have attracted wide interest in anion sensing, mainly acetate [17], and showed anticancer activity [18].…”