Cyclische Gruppe-15-Radikalkationen

Radical Reactivity of the Biradical [⋅P(μ-NTer)2 P⋅] and Isolation of a Persistent Phosphorus-Cantered Monoradical [⋅P(μ-NTer)2 P-Et]

The activation of C-Br bonds in various bromoalkanes by the biradical [⋅P(μ-NTer)2 P⋅] (1) (Ter=2,6-bis-(2,4,6-trimethylphenyl)-phenyl) is reported, yielding trans-addition products of the type [Br-P(μ-NTer)2 P-R] (2), so-called 1,3-substituted cyclo-1,3-diphospha-2,4-diazanes. This addition reaction, which represents a new easy approach to asymmetrically substituted cyclo-1,3-diphospha-2,4-diazanes, was investigated mechanistically by different spectroscopic methods (NMR, EPR, IR, Raman); the results suggested a stepwise radical reaction mechanism, as evidenced by the in-situ detection of the phosphorus-centered monoradical [⋅P(μ-NTer)2 P-R].< To provide further evidence for the radical mechanism, [⋅P(μ-NTer)2 P-Et] (3Et⋅) was synthesized directly by reduction of the bromoethane addition product [Br-P(μ-NTer)2 P-Et] (2 a) with magnesium, resulting in the formation of the persistent phosphorus-centered monoradical [⋅P(μ-NTer)2 P-Et], which could be isolated and fully characterized, including single-crystal X-ray diffraction. Comparison of the EPR spectrum of the radical intermediate in the addition reaction with that of the synthesized new [⋅P(μ-NTer)2 P-Et] radical clearly proves the existence of radicals over the course of the reaction of biradical [⋅P(μ-NTer)2 P⋅] (1) with bromoethane. Extensive DFT and coupled cluster calculations corroborate the experimental data for a radical mechanism in the reaction of biradical [⋅P(μ-NTer)2 P⋅] with EtBr. In the field of hetero-cyclobutane-1,3-diyls, the demonstration of a stepwise radical reaction represents a new aspect and closes the gap between P-centered biradicals and P-centered monoradicals in terms of radical reactivity.

Isolation and Properties of the Long Elusive Deep Blue Soluble [K3 {(Nt Bu)3 S}2 ]. Cage Radical

The reaction of potassium metal with sulfurtriimide S(Nt Bu)3 (II) gives the long elusive deep blue cage radical [K3 {(Nt Bu)3 S}2 ]. (1_K) that crystallizes at -35 °C from toluene solution. The subsequent physical characterization via X-ray structure analysis, UV/Vis-, and EPR spectroscopy from solution reveals the existence of one unpaired electron delocalized within the whole cage, i.e. coupling with the six nitrogen atoms, as well as the three potassium atoms caped by the two SN3 ligands. The present X-ray structure analysis further supports previous assumptions made on the parent compound 1_Li obtained from [Li4 {(Nt Bu)3 S}2 ] (I) and finally elucidates the structural arrangement of the SN3 caps and alkali metals in such radical cage species.

Azadiphosphaindane-1,3-diyls: A Class of Resonance-Stabilized Biradicals

Conversion of 1,2-bis(dichlorophosphino)benzene with sterically demanding primary amines led to the formation of 1,3-dichloro-2-aza-1,3-diphosphaindanes of the type C6 H4 (μ-PCl)2 N-R. Reduction yielded the corresponding 2-aza-1,3-diphosphaindane-1,3-diyls (1), which can be described as phosphorus-centered singlet biradical(oid)s. Their stability depends on the size of the substituent R: While derivatives with R=Dmp (2,6-dimethylphenyl) or Ter (2,6-dimesitylphenyl) underwent oligomerization, the derivative with very bulky R=tBu Bhp (2,6-bis(benzhydryl)-4-tert-butylphenyl) was stable with respect to oligomerization in its monomeric form. Oligomerization involved activation of the fused benzene ring by a second equivalent of the monomeric biradical and can be regarded as formal [2+2] (poly)addition reaction. Calculations indicate that the biradical character in 1 is comparable with literature-known P-centered biradicals. Ring-current calculations show aromaticity within the entire ring system of 1.

A Room-Temperature Stable Distonic Radical Cation

Distonic radical cations (DRCs) with spatially separated charge and radical sites have, so far, largely been observed by gas-phase mass spectrometry and/or matrix isolation spectroscopy work. Herein, we disclose the isolation of a crystalline dicarbondiphosphide-based β-distonic radical cation salt 3^.+ (BARF) (BARF=[B(3,5-(CF₃ )₂ C₆ H₃ )₄ )]^- ) stable at room temperature and formed by a one-electron-oxidation-induced intramolecular skeletal rearrangement reaction. Such a species has been validated by electron paramagnetic resonance (EPR) spectroscopy, single-crystal X-ray diffraction, UV/Vis spectroscopy and density functional theory (DFT) calculations. Compound 3^.+ (BARF) exhibits a large majority of spin density at a two-coordinate phosphorus atom (0.74 a.u.) and a cationic charge located predominantly at the four-coordinate phosphorus atom (1.53 a.u.), which are separated by one carbon atom. This species represents an isolable entity of a phosphorus radical cation that is the closest to a genuine phosphorus DRC to date.

Ligand Effects on the Electronic Structure of Heteroleptic Antimony-Centered Radicals

We report on the structures of three unprecedented heteroleptic Sb-centered radicals [L(Cl)Ga](R)Sb. (2-R, R=B[N(Dip)CH]2 2-B, 2,6-Mes2 C6 H3 2-C, N(SiMe3 )Dip 2-N) stabilized by one electropositive metal fragment [L(Cl)Ga] (L=HC[C(Me)N(Dip)]2 , Dip=2,6-i-Pr2 C6 H3 ) and one bulky B- (2-B), C- (2-C), or N-based (2-N) substituent. Compounds 2-R are predominantly metal-centered radicals. Their electronic properties are largely influenced by the electronic nature of the ligands R, and significant delocalization of unpaired-spin density onto the ligands was observed in 2-B and 2-N. Cyclic voltammetry (CV) studies showed that 2-B undergoes a quasi-reversible one-electron reduction, which was confirmed by the synthesis of [K([2.2.2]crypt)][L(Cl)GaSbB[N(Dip)CH]2 ] ([K([2.2.2]crypt)][2-B]) containing the stibanyl anion [2-B]- , which was shown to possess significant Sb-B multiple-bonding character.

Crystalline Divinyldiarsene Radical Cations and Dications

The divinyldiarsene radical cations [{(NHC)C(Ph)}As]2 (GaCl4 ) (NHC=IPr: C{(NDipp)CH}2 3; SIPr: C{(NDipp)CH2 }2 4; Dipp=2,6-iPr2 C6 H3 ) and dications [{(NHC)C(Ph)}As]2 (GaCl4 )2 (NHC=IPr 5; SIPr 6) are readily accessible as crystalline solids on sequential one-electron oxidation of the corresponding divinyldiarsenes [{(NHC)C(Ph)}As]2 (NHC=IPr 1; SIPr 2) with GaCl3 . Compounds 3-6 have been characterized by X-ray diffraction, cyclic voltammetry, EPR/NMR spectroscopy, and UV/vis absorption spectroscopy as well as DFT calculations. The sequential removal of one electron from the HOMO, that is mainly the As-As π-bond, of 1 and 2 leads to successive elongation of the As=As bond and contraction of the C-As bonds from 1/2→3/4→5/6. The UV/vis spectrum of 3 and 4 each exhibits a strong absorption in the visible region associated with SOMO-related transitions. The EPR spectrum of 3 and 4 each shows a broadened septet owing to coupling of the unpaired electron with two 75 As (I=3/2) nuclei.

The Charge Transfer Approach to Heavier Main-Group Element Radicals in Transition-Metal Complexes

The Co^II and Fe^II complexes 1Co and 1Fe with a coordinated phosphorus radical were easily obtained through a charge-transfer approach from the M^I precursors LM^I (tol) (M=Co, Fe; L=CH(MeC=NDipp)₂ , Dipp=2,6-iPr₂ C₆ H₃ ) to the diazafluorenylidene-substituted phosphaalkene 1. Structural, magnetic, and computational studies on 1Co and 1Fe indicate a weak antiferromagnetic interaction between the high-spin M^II ion and the phosphorus radical, resulting in a triplet and quartet ground state, respectively. Complexes 1Co and 1Fe are the first examples of phosphorus-radical-coordinated transition-metal complexes synthesized by charge transfer, providing a new approach to access radicals of heavier main-group elements.

On the behaviour of biradicaloid [P(μ-NTer)]2 towards Lewis acids and bases

The well-known diphosphadiazane-1,3-diyl [P(μ-NTer)]₂ (Ter = 2,6-bis(2,4,6-trimethyl-phenyl)-phenyl) was treated with Lewis bases such as N-heterocyclic carbenes and Lewis acids e.g. gold(i) chloride complexes.

Accessing heavy allyl-analogous [(TerN)2E]− (E = Sb, Bi) ions and their reactivity towards ECl3

Highly reactive heavy heteroatom allyl analogues were obtained by reduction of 1,3-dichloro-cyclo-1,3-dipnicta-2,4-diazanes.