Synthesis, Spectroscopic Characterization, and Redox Reactivity of a Cyclic (Alkyl) Amino Carbene‐Derived Arsamethine Cyanine Dye

Abstract

In our efforts to prepare a diarsenic allotrope supported by two cyclic alkyl amino carbene (CAAC) ligands we stumbled upon the synthesis of the first carbene-supported chloroarsinidene 3, which has been fully characterized by a combination of NMR spectroscopic and XRD methods. Although further reduction of 3 was not possible, we found that addition of a second equivalent of CAAC in refluxing toluene afforded the first example of a crystallographically characterized arsamethine cyanine dye (4). The arsenic(I) dye is structurally similar to phosphorus analogues, and contains an arsenide anion with two stereochemically active lone pairs supported by two iminium ligands. The UV/Visible spectrum and redox chemistry of 4 were also explored. Upon reduction with one equivalent of KC , 3 is reduced to the originally targeted CAAC As allotrope 6, whereas oxidation provides access to the first example of an arsenic(II) radical dication (5).

Long‐Lived Radicals of the Heavier Group 15 Elements Arsenic, Antimony, and Bismuth

“…A cAAC‐stabilized arsenic radical dication [( Et cAAC) 2 As] · 2+ ( 23 2+ ) was synthesized by Hudnall and co‐workers as a [PF 6 ] – based salt by further oxidation of the monocationic counterpart 23 + (Scheme ). 23 2+ was found to be unstable in solution, hence no suitable single crystals for X‐ray diffraction could be obtained …”

“…The radical dication 23 2+ was characterized by EPR spectroscopy in conjunction with DFT calculations. The results suggest an As‐centered radical with the spin density primarily located on As (0.87) and a SOMO that is essentially the As p orbital with small contributions from the π ‐systems of the Dip groups …”

Crystalline Divinyldiarsenes and Cleavage of the As=As Bond

“…The As=As bond length of 3 a and 3 b compares well with those of the known diarsenes (2.276(3)–2.358(4) Å) . Nonetheless, these are considerably shorter compared with that of Robinson's (IPr) 2 As 2 (2.442(1) Å) as well as Hudnall's (CAAC) 2 As 2 (2.433(1) Å) carbene‐stabilized diatomic arsenic compounds. The As=As bond length of 3 a and 3 b is similar to that of Tamm's diarsene III (2.289(8) Å) derived from an anionic NHC (Figure ).…”

London Dispersion Interactions in Pnictogen Cations [ECl2]+ and [E=E]2+ (E=P, As, Sb) Supported by Anionic N‐Heterocyclic Carbenes

“…Although NHC‐stabilized diantimony and dibismuth were also calculated to be stable entities, the isolation of (IDipp) 2 E 2 (E=Sb, Bi) by reduction of the carbene adducts (IDipp)ECl 3 failed, suggesting “that the ′carbene‐stabilization′ strategy may be less effective with the heavier pnictogens.” Likewise, the attempted reduction of a series of six‐membered tetrahydropyrimidin‐2‐ylidene adducts of group 15 element trichlorides only returned a mixture of products, including significant amounts of the free carbene . Cyclic (alkyl)(amino)carbenes (CAACs), however, which were also employed for the preparation of the (CAAC) 2 E 2 species X (E=P) and XI (E=As), allowed the preparation of (CAAC) 2 Sb 2 ( XI ) by three‐electron reduction of (CAAC)SbCl 3 with KC 8 (Figure ) . It is noteworthy that XI represents the only compound reported to date in which a diantimony unit is flanked by two carbene ligands.…”

“…[29] Likewise, the attempted reduction of as eries of sixmembered tetrahydropyrimidin-2-ylidenea dducts of group 15 elementt richlorides only returned am ixture of products, including significant amountso ft he free carbene. [31] Cyclic (alkyl)(amino)carbenes (CAACs), [32] however,w hichw ere also employed for the preparation of the (CAAC) 2 E 2 species X (E = P) and XI (E = As), [33,34] allowed the preparation of (CAAC) 2 Sb 2 (XI) by three-electron reduction of (CAAC)SbCl 3 with KC 8 (Figure 1). [35] It is noteworthy that XI represents the only compound reported to date in whichadiantimony unit is flanked by two carbene ligands.…”