Isolation of an Arsenic Diradicaloid with a Cyclic C2 As2 -Core

Heteroatom-Based Diradical(oid)s

Heteroatom-centered diradical(oid)s have been in the focus of molecular main group chemistry for nearly 30 years. During this time, the diradical concept has evolved and the focus has shifted to the rational design of diradical(oid)s for specific applications. This review article begins with some important theoretical considerations of the diradical and tetraradical concept. Based on these theoretical considerations, the design of diradical(oid)s in terms of ligand choice, steric, symmetry, electronic situation, element choice, and reactivity is highlighted with examples. In particular, heteroatom-centered diradical reactions are discussed and compared with closed-shell reactions such as pericyclic additions. The comparison between closed-shell reactivity, which proceeds in a concerted manner, and open-shell reactivity, which proceeds in a stepwise fashion, along with considerations of diradical(oid) design, provides a rational understanding of this interesting and unusual class of compounds. The application of diradical(oid)s, for example in small molecule activation or as molecular switches, is also highlighted. The final part of this review begins with application-related details of the spectroscopy of diradical(oid)s, followed by an update of the heteroatom-centered diradical(oid)s and tetraradical(oid)s published in the last 10 years since 2013.

FP(μ-N)2 S: A Sulfur-Pnictogen Four-Membered Ring with 6π Electrons

The new 6π-electron four-membered ring compound 3-fluoro-1λ2 ,2,4,3λ3 -thiadiazaphosphetidine, FP(μ-N)2 S, has been generated in the gas phase through high-vacuum flash pyrolysis (HVFP) of thiophosphoryl diazide, FP(S)(N3 )2 , at 1000 K. Subsequent isolation of FP(μ-N)2 S in cryogenic matrices (Ar, Ne, and N2 ) allows its characterization with matrix-isolation IR and UV-vis spectroscopy by combination with 15 N-isotope labeling and computations at the CCSD(T)-F12a/VTZ-F12 level of theory. Upon visible-light irradiation at 550 nm, this cyclic compound undergoes ring-opening to the thiazyl isomer FPNSN, followed by dissociation to FP and SN2 under subsequent UV-irradiation at 365 nm. In sharp contrast to the square planar structure for the isolobal four-membered ring S2 N2 , a puckered structure with significant biradical character has been found for FP(μ-N)2 S.

An indium(I) tetramer bound by anionic N-heterocyclic olefins: ambiphilic reactivity, transmetallation and a rare indium-imide

An organometallic tetrahedron-shaped indium(I) tetramer [(^MeIPrCH)In]₄ (^MeIPrCH = [(MeCNDipp)₂CCH]^-; Dipp = 2,6-ⁱPr₂C₆H₃) supported by anionic N-heterocyclic olefin (aNHO) ligands is reported. The monomeric unit of this species exhibits both Lewis acidic and basic character at indium, while the steric profile of the aNHO ligand enables isolation of a rare monomeric imide, RInNR'.

Oxidations of N-coordinated Arsinidene and Stibinidene by Substituted Quinones: A Remarkable Follow-Up Reactivity

The reactivity of pnictinidenes [2-(DippN=CH)-6-(DippNHCH₂ )C₆ H₃ ]E (where E=As (1) or Sb (2)) toward substituted ortho- and para-quinones is reported. The central pnictogen atom is easily oxidized by ortho-quinones closing five-membered EO₂ C₂ ring. The oxidized antimony derivatives are stable species, while in the case of arsenic compounds the hydrogen of the pendant amino NHCH₂ group cleaves one newly formed As-O bonds leading to the closure of a new azaarsole ring. Furthermore, a heating of these arsenic heterocycles resulted in a C-H bond activation at the NCH₂ group involved in this heterocycle followed by a reductive elimination of corresponding catechols and arsinidene [2,6-(DippN=CH)C₆ H₃ ]As. Using of para-quinones, resulted in the oxidation of the central atom with a concomitant hydrogen migration from NHCH₂ group even in the case of the antimony derivatives. The reductive elimination of hydroquinones is in this case feasible for all compounds. Studied compounds were characterized by multi-nuclear NMR, IR and Raman spectroscopy and single-crystal X-ray diffraction analysis. The theoretical study focusing the key compounds and reactions is also included.

Isolation of an Annulated 1,4-Distibabenzene Diradicaloid

The first 1,4-distibabenzene-1,4-diide compound [(ADC)Sb]₂ (5) based on an anionic dicarbene (ADC) (ADC=PhC{N(Dipp)C}₂ , Dipp=2,6-iPr₂ C₆ H₃ ) is reported as a bordeaux-red solid. Compound 5, featuring a central six-membered C₄ Sb₂ ring with formally Sb^I atoms may be regarded as a base-stabilized cyclic bis-stibinidene in which each of the Sb atoms bears two lone-pairs of electrons. 5 undergoes 2 e-oxidation with Ph₃ C[B(C₆ F₅ )₄ ] to afford [(ADC)Sb]₂ [B(C₆ F₅ )₄ ]₂ (6) as a brick-red solid. Each of the Sb atoms of 6 has an unpaired electron and a lone-pair. The broken-symmetry open-shell singlet diradical solution for (6)²⁺ is calculated to be 2.13 kcal mol^-1 more stable than the closed-shell singlet. The diradical character of (6)²⁺ according to SS-CASSCF (state-specific complete active space self-consistent field) and UHF (unrestricted Hartree-Fock) methods amounts to 36 % and 39 %, respectively. Treatments of 6 with (PhE)₂ yield [(ADC)Sb(EPh)]₂ [B(C₆ F₅ )₄ ]₂ (7-E) (E=S or Se). Reaction of 5 with (cod)Mo(CO)₄ affords [(ADC)Sb]₂ Mo(CO)₄ (8).

Concerted addition of aldehydes to the singlet biradical [P(μ-NTer)]2

The reaction of the singlet biradical [P(μ-NTer)]₂ with various aldehydes selectively yielded the corresponding [2.1.1]-bicyclic addition products in a very fast reaction. All products were fully characterized, including by NMR and vibrational spectroscopy as well as single-crystal X-ray diffraction. The mechanism of the addition was investigated theoretically using high-level ab initio methods (CCSD(T) with triple- and quadruple-zeta basis sets) and corresponds to a concerted cycloaddition reaction with a very low activation barrier. For comparison, the mechanisms of the literature-known cycloadditions of H₂, alkenes, and alkynes were also studied, indicating a similar reaction profile for all unsaturated reactants.