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.

Insertion of CS2 into a Phosphorus-Arsenic Single Bond and Investigations on Phosphane Arsanyldithiocarboxylates.

The synthesis and reactivity of sterically demanding phosphaarsanes TerR1P-AsR2 (3) is described. These species were selectively synthesized via metathesis reactions of Ter-stabilized [Ter = 2,6-bis(2,4,6-trimethylphenyl)phenyl] potassium phosphides TerR1PK (1) with the N-heterocyclic chloroarsane ClAs{N(tBu)CH2}2 (2). Conversion of the n-butyl-substituted phosphaarsane 3c with the reactive heterocumulene CS2 leads to an insertion into the P-As bond, yielding the phosphane arsanyldithiocarboxylate TerR1P-C(S)S-AsR2 (4c) as a new structural motif. Because full conversion of 3c with CS2 requires long reaction times, an alternative synthetic route is reported herein, involving the conversion of 2 with phosphane dithiocarboxylates TerR1-C(S)SK (5), enabling synthetic access to a wider range of phosphane arsanyldithiocarboxylates. These interpnictogen dithiocarboxylates show an interesting bonding situation with a significantly elongated As-S bond due to negative hyperconjugation within the molecule. All products along the reaction path were fully characterized.

Synthesis of Sterically Demanding Secondary Phosphides and Diphosphanes and Their Utilization in Small-Molecule Activation.

Sterically demanding secondary potassium phosphides (4) were synthesized and investigated. Reaction with halophosphanes (5) yields diphosphanes (6), whereas reaction with CS2 yields phosphanyl dithioformates (10). These can be further converted to the corresponding phosphanyl esters of dithioformic acid R2P-C(S)S-PR2 (8). One of these thioesters (8) was found to undergo a migration reaction, resulting in the formation of a phosphanylthioketone with an additional phosphanylthiolate group (9), which was used as a chiral ligand in gold coordination chemistry. The phosphanyl migration reaction was investigated by spectroscopic and theoretical methods, revealing a first-order reaction via a cyclic transition state. All species mentioned were fully characterized.

Heterocyclopentanediyls vs Heterocyclopentadienes: A Question of Silyl Group Migration.

The reaction of the singlet biradical [P(µ-NHyp)]2 (Hyp = hypersilyl, (Me3Si)3Si) with different isonitriles afforded a series of five-membered N2P2C heterocycles. Depending on the steric bulk of the substituent at the isonitrile, migration of a Hyp group was observed, resulting in two structurally similar but electronically very different isomers. As evidenced by comprehensive spectroscopic and theoretical studies, the heterocyclopentadiene isomer may be regarded as a rather unreactive closed-shell singlet species with one localized N═P and one C═P double bond, whereas the heterocyclopentanediyl isomer represents an open-shell singlet biradical with interesting photochemical properties, such as photoisomerization under irradiation with red light to a [2.1.0]-housane-type species.

[E(µ-NBbp)]2 (E = P, As) - group 15 biradicals synthesized from acyclic precursors.

The synthesis of Bbp stabilized biradicals of the type [E(µ-NBbp)]2 (E = P, As) (Bbp = 2,6-bis[bis(trimethylsilyl)methyl]phenyl) was investigated. Contrary to the established synthetic protocol for terphenyl substituted biradicals [E(µ-NTer)]2 by reduction of [ClE(µ-NTer)]2, the analogous Bbp substituted precursor [ClE(µ-NBbp)]2 could only be obtained in low yields. For this reason, a new synthesis had to be found to generate [E(µ-NBbp)]2. Instead of using cyclic [ClE(µ-NBbp)]2 we prepared the acyclic Bbp-N(ECl2)2 species starting from Bbp-NH2 and ECl3 and studied the reduction with magnesium chips, which led to the formation of the desired biradicals [E(µ-NBbp)]2. All species along the reaction path were fully characterized.

Lewis acid assisted methyl/chlorine exchange in silylated hydrazinochlorophosphanes.

Differently substituted hydrazinophosphanes of the type (Me(3)Si)(2)N-N(SiMe(3))-PR(1)R(2) (R(1) = Cl with R(2) = Me, C(6)F(5) and R(1) = Me, R(2) = C(6)H(5)) have been studied in the reaction with Lewis acids such as ECl(3) (E = Al, Ga). For (Me(3)Si)(2)N-N(SiMe(3))-P(Cl)(Me) and (Me(3)Si)(2)N-N(SiMe(3))-P(Me)(C(6)H(5)), only adduct formation was found while a chlorine/methyl exchange reaction was observed for (Me(3)Si)(2)N-N(SiMe(3))-P(Cl)R (R = C(6)H(5) and C(6)F(5)) leading to the formation of (Me(2)ClSi)(Me(3)Si)N-N(SiMe(3))-P(Me)R, which crystallize as ECl(3) adducts. The free hydrazinophosphanes can be obtained by removal of the Lewis acid with the help of a strong base such as 4-(dimethylamino)pyridine (DMAP).

Hypersilylated cyclodiphosphadiazanes and cyclodiphosphadiazenium salts.

Starting from tris(trimethylsilyl)silane the novel 2-chloro-3,4-bis-tris(trimethylsilyl)silyl-cyclo-diphospha-diazenium-tetrachloridogallate salt (7) was prepared in a six step synthetic procedure including the synthesis of N-tris(trimethylsilyl)silyl-aminodichlorophosphine (4) and 1,3-dichloro-2,4-bis-tris(trimethylsilyl)silyl-cyclo-diphospha-diazane (5). All intermediate products along the reaction path have been isolated and fully characterized. The thermally stable cyclo-diphospha-diazenium-tetrachloridogallate (7) represents an interesting example of a binary cyclic P(III)/N four-membered heterocyclic cation, with a di- and tricoordinated P atom and a delocalized bond along the NP(+)N unit. The structures of all compounds are discussed on the basis of experimental X-ray data. Moreover, for comparison the hitherto unknown 1-chloro-3-trifluoromethanesulfonato-2,4-bis-tris(trimethylsilyl)silyl-cyclo-diphosphadiazane (6) has been synthesized and fully characterized.

GaCl3-assisted cyclization reactions in hypersilyl(trimethylsilyl)aminodichlorophosphine.

Hypersilyl(trimethylsilyl)aminodichlorophosphine, (hyp)N(SiMe3)PCl2, was treated with GaCl3, which resulted in the formation of an interesting novel bicycle, composed of a four-membered SiNP2 ring and a five-membered P2Ga2Cl ring. In the presence of Me3SiN3, the same reaction provided access to a cyclo-2-phospha-4-sila-1,3-diazenium tetrachlorogallate. The free chloro-cyclo-phosphasiladiazane was obtained by the addition of nucleophilic bases.