Dennis Rottschäfer

Crystalline Radicals Derived from Classical N-Heterocyclic Carbenes

One-electron reduction of C2-arylated 1,3-imidazoli(ni)um salts (IPrAr )Br (Ar=Ph, 3 a; 4-DMP, 3 b; 4-DMP=4-Me2 NC6 H4 ) and (SIPrAr )I (Ar=Ph, 4 a; 4-Tol, 4 b) derived from classical NHCs (IPr=:C{N(2,6-iPr2 C6 H3 )}2 CHCH, 1; SIPr=:C{N(2,6-iPr2 C6 H3 )}2 CH2 CH2 , 2) gave radicals [(IPrAr )]. (Ar=Ph, 5 a; 4-DMP, 5 b) and [(SIPrAr )]. (Ar=Ph, 6 a; 4-Tol, 6 b). Each of 5 a,b and 6 a,b exhibited a doublet EPR signal, a characteristic of monoradical species. The first solid-state characterization of NHC-derived carbon-centered radicals 6 a,b by single-crystal X-ray diffraction is reported. DFT calculations indicate that the unpaired electron is mainly located at the original carbene carbon atom and stabilized by partial delocalization over the adjacent aryl group.

N-Heterocyclic Vinylidene-Stabilized Phosphorus Biradicaloid

Four-membered biradicaloid compounds containing a N2 E2 (E=main group element) framework have been thoroughly investigated; however, the synthesis of stable analogues with a C2 P2 skeleton remains a challenge. Base-mediated double C-H functionalization of IPr=CH2 (1) (IPr=C[N(2,6-iPr2 -C6 H3 )CH]2 ) with PCl3 affords [{(IPr)CP}2 Cl]Cl (2) as a royal blue solid. Treatment of 2 with KC8 yields the stable phosphorus biradicaloid [(IPr)CP]2 (3) featuring a four-membered C2 P2 ring. Compound 3 is diamagnetic and shows sharp and temperature-independent NMR resonances, revealing its singlet biradicaloid nature. The stability of 3 is attributed to the σ- and π-electron-donating property of the N-heterocyclic vinylidene [(IPr)C] group.

The Viability of C5-Protonated- and C4,C5-Ditopic Carbanionic Abnormal NHCs: A New Dimension in NHC Chemistry

The first C5-protonated abnormal N-heterocyclic carbene (aNHC), PhC{N(2,6-iPr2 C6 H3 )}2 CHC: (4) is readily accessible by C4-deprotonation of [ArC{N(2,6-iPr2 C6 H3 )}2 CHCH]X (3 a-X) (Ar=Ph, X=Br or I) with a base. The aNHC 4 is stable at 298 K in [D8 ]THF solution and has been spectroscopically characterized. The facile availability of 4 enables the synthesis of a series of main-group compounds as well as transition-metal complexes featuring a new phosphorus-aNHC hybrid ligand. Double deprotonation of [ArC{N(2,6-iPr2 C6 H3 )}2 CHCH]X (Ar=Ph, 3 a-X (X=Br or I); 4-Tol, 3 b-Br; 4-DMP, 3 c-Br; Tol=MeC6 H4 , DMP=Me2 NC6 H4 ) with nBuLi yields the C4,C5-ditopic carbanionic aNHCs, [ArC{N(2,6-iPr2 C6 H3 )}2 (C:)2 ]Li(THF)n (Ar=Ph, 13 a; 4-Tol, 13 b; 4-DMP, 13 c), which on treatment with Ph2 PCl affords cationic vicinal bisphosphine derivatives [ArC{N(2,6-iPr2 C6 H3 )}2 {C(PPh2 }2 ]X (Ar=Ph, 14 a-X, X=Br or I; 4-Tol, 14 b-Br; 4-DMP, 14 c-Br).

Kekulé diradicaloids derived from a classical N-heterocyclic carbene

Two-electron reduction of bis(1,3-imidazolinium) salts 2 and 3 with KC8 gives rise to stable diradicaloids 4 and 5, respectively. Calculations reveal a very low singlet–triplet energy gap ΔES–T for 5 (10.7 kcal mol–1), while ΔES–T for 4 (29.1 kcal mol–1) is rather large.