Heteroatom-Based Diradical(oid)s

Abstract

Published as part of the Chemical Reviews virtual special issue "Persistent and Stable Organic Radicals".

Variation from closed-shell to open shell electronic structures in oligothiophene bis(dioxolene) complexes

“…This model yields 4-states, 69 and has been used for understanding biradicaloid character in similar systems. 1,6,69,70 The two-orbital model was originally developed using hydrogenic orbitals that allow for the configurational mixing of open- and closed-shell singlet configurations, since there is only one triplet configuration. Fig.…”

Substituent Effects on the Electronic Ground State (Singlet versus Triplet) of Indenyl Cations: DFT and CASPT2 Studies

“…It is well known that such species with diradical character obtained from mixing of two closed shell configurations are not properly described by the restricted DFT methods and therefore the energies of singlets profit a lot using broken symmetry unrestricted approach. [11,13,[26][27][28][29][30] Indeed singlets optimized by BS-UB3LYP for cations 3, 5 and 8 are more stable by 4.3, 6.2 and 6.3 kcal mol À 1 , respectively. On the other side, restricted and unrestricted calculations resulted in the same geometries and energies for singlet states of 2 (R 1 ,R 2 = Me,H; entry 2), 6 (R 1 ,R 2 = CN,H; entry 6), 7 (R 1 ,R 2 = NO 2 ,H; entry 7) and 10 (R 1 ,R 2 = 4-MeOC 6 H 4 ,NH 2 ; entry 10).…”

On the reactivity of a masked N‐heterocyclic phosphinidene towards carbonyls and isonitriles

“…[4] Based on Niecke's pioneering work in biradical chemistry, [5,6] many biradical four-membered rings have been isolated in recent years. [7][8][9][10] While Niecke's biradical is carbon-centered and has two trivalent phosphorus centers, 1,3-diphospha-2,4-diaza-cyclobutane-1,3-diyls [P(μ-NR) 2 P] (species B) [11] are found to have the radical character dominant on both phosphorus atoms. Such open-shell singlet biradicals are also called diradical(oid)s or biradicaloids [12] displaying a biradical character commonly between 20-30 %.…”

“…Such open-shell singlet biradicals are also called diradical(oid)s or biradicaloids [12] displaying a biradical character commonly between 20-30 %. [9,10] The phosphorus-centered biradical C with a carbon-phosphorus four-membered ring was isolated independently by Grützmacher [13,14] and Ghadwal [15] and can be regarded as a formal carbon analog to compound B and a phosphoruscentered biradical analog to species A, respectively. What was still missing was a "mixture" of biradicals B and C that has a radical carbon and phosphorus atom (species D).…”

“…While biradicals of the type [P(μ-NR) 2 P] (B; e. g. R = H, Ph, Ter, etc.) have a biradical character of 20-30 %, [10] a biradical character of only 9 % was calculated for model compound [P(μ-NH) 2 CH] (D; R, R' = H). [16] Thus, [P(μ-NH) 2 CH] should be considered a "prevented" biradical, which is also visible in the Lewis resonance scheme (Scheme 2), since the zwitterionic formula I has a significantly higher weight than the biradical (II).…”