Preparation and reactions of 1,3-diphosphacyclobutane-2,4-diyls that feature an amino substituent and/or a carbonyl group

The preparation and properties of a 1-amino-1,3-diphosphacyclobutane-2,4-diyl and a 1-benzoyl-1,3-diphosphacyclobutane-2,4-diyl, which can be regarded as functionalized cyclic biradical derivatives, were investigated. Hydrolysis of 1-diisopropylamino-3-methyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl (7), which is formed by reaction of Mes*C[triple chemical bond]P (4; Mes*=2,4,6-tBu(3)C(6)H(2)) with lithium diisopropylamide and iodomethane, resulted in ring-opening of the 1,3-diphosphacyclobutane-2,4-diyl skeleton, as well as de-aromatization of one of the Mes* rings. 3-Oxo-1,3-diphosphapropene 8 and 7-phosphabicyclo[4.2.0]octa-1(8),2,4-triene 9 were the resultant products, and these were subsequently characterized. Isomerization and oxidation of 7 occurred in the presence of TEMPO (2,2,6,6-tetramethyl-1-piperidinoxy) to give the first example of a cyclic dimethylenephosphorane derivative, namely 3-oxo-1,3-diphospha-1,4-diene 10. 1-Benzoyl-3-tert-butyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl (12) was isolated and characterized from the reaction of 4 with tert-butyllithium and benzoyl chloride. Compound 12 was subsequently heated and underwent rearrangement of the benzoyl group and ring-expansion to afford 1-oxo-1H-[1,3]diphosphole 13. Reaction of 4 with lithium diisopropylamide and benzoyl chloride afforded the 2H-[1,2,4]oxadiphosphinine 15, which was probably formed through the 1,3-diphosphacyclobutane-2,4-diyl intermediate 14. Thermolysis of 15 afforded 1-oxo-1H-[1,3]diphosphole 16 in an Arbuzov-type rearrangement.     

Using the diphosphanyl radical as a potential spin label: effect of motion on the EPR spectrum of an R1(R2)P--PR1 radical

The EPR spectrum of the novel radical Mes*(CH3)P--PMes* (Mes*=2,4,6-(tBu)3C6H2) was measured in the temperature range 100-300 K, and was found to be drastically temperature dependent as a result of the large anisotropy of the 31P hyperfine tensors. Below 180 K, a spectrum of the liquid solution is accurately simulated by calculating the spectral modifications due to slow tumbling of the radical. To achieve this simulation, an algorithm was developed by extending the well-known nitroxide slow-motion simulation technique for the coupling of one electron spin to two nuclear spins. An additional dynamic process responsible for the observed line broadening was found to occur between 180 K and room temperature; this broadening is consistent with an exchange between two conformations. The differences between the isotropic 31P couplings associated with the two conformers are shown to be probably due to an internal rotation about the P--P bond.     

Access to Air‐Stable 1,3‐Diphosphacyclobutane‐2,4‐diyls by an Arylation Reaction with Arynes

Tuning of the physicochemical properties of the 1,3‐diphosphacyclobutane‐2,4‐diyl unit is attractive in view of materials applications. The use of arynes is shown to be effective for installing relatively electron rich aryl substituents into the open‐shell singlet P‐heterocyclic system. Treatment of the sterically encumbered 1,3‐diphosphacyclobuten‐4‐yl anion with ortho‐silylated aryl triflates in the presence of fluoride under appropriate conditions afforded the corresponding 1‐aryl 1,3‐diphosphacyclobutane‐2,4‐diyls. The air‐stable open‐shell singlet P‐heterocycles exhibit considerable electron‐donating character, and the aromatic substituent influences the open‐shell character, which is thought to be related to the property of p‐type semiconductivity. The P‐arylated 1,3‐diphosphacyclobutane‐2,4‐diyl systems can be further utilized as detectors of hydrogen fluoride (HF), which causes a remarkable change in their photoabsorption properties.     

Structural Tailoring Effects on the Magnetic Behavior of Symmetric and Asymmetric Cubane‐Type Ni complexes

Using two kinds of carboxylate ligands with small but significant differences in steric size, symmetric and asymmetric Fe^II and Ni^II cubanes have been synthesized in a controlled fashion. Fast sweeping pulsed field measurements showed magnetization hysteresis loops for two cubane‐type molecular complexes, [Ni₄(μ‐OMe)₄(O₂CAr^4F‐Ph)₄(HOMe)₈] and [Ni₄(μ‐OMe)₄(O₂CAr^Tol)₄(HOMe)₆], thus suggesting single‐molecule magnet behavior. To differentiate the magnetic properties between the symmetric and asymmetric cubanes, detailed electron paramagnetic resonance (EPR) measurements were performed. From the EPR data, taken at various frequencies and temperatures, zero‐field splitting parameters D, E, and other higher‐order parameters for both cubane samples were extracted. Compared to the symmetric Ni‐cubane, the asymmetric one shows an increase in the D and E values by about 20 %, thereby suggesting structural engineering effects on the magnetic properties. By using the magnetic parameters determined by EPR, a static magnetization curve at 2 K and a temperature dependence of the magnetic susceptibility were simulated. A good agreement between theoretical and experimental data confirms the validity of the values obtained from EPR measurements.     

Tuning the Conformation and Color of Conjugated Polyheterocyclic Skeletons by Installing ortho‐Methyl Groups

ortho‐Methyl effects are exploited to tune steric hindrance between side‐chain N,N′‐diaryls and polycyclic dihydrodibenzo[a,c]phenazine, and in turn control the conformations of N,N′‐diphenyl‐dihydrodibenzo[a,c]phenazine (DPAC) and its ortho‐methyl derivatives M x ‐M y (x=0, 1 or 2, y=1 or 2, x and y correlate with the number of methyl groups in the ortho‐positiond of N,N′‐diphenyl). The magnitude of steric hindrance increases as x and y increase, and the V‐shaped dihydrodibenzo[a,c]phenazine skeleton is gradually tuned from a bent (DPAC) to planar ( M2‐M2 ) structure in the ground state. As a result, the relaxation of the excited‐state structure of DPAC and its numerous analogues could be mimicked by model structures M x ‐M y, demonstrating for the first time the the conformation change from bent‐to‐planar and hence a large range of energy‐gap tuning of polycyclic conjugated structures controlled by the steric hindrance.     

Synthesis and EPR/UV/Vis‐NIR Spectroelectrochemical Investigation of a Persistent Phosphanyl Radical Dication

The reaction of the bis(imidazoliumyl)‐substituted P^I cation [(2‐Im^Dipp)P(4‐Im^Dipp)]⁺ ( 10 ⁺) (2‐Im=imidazolium‐2‐yl; 4‐Im=imidazolium‐4‐yl; Dipp=2,6‐di‐isopropylphenyl) with trifluoromethanesulfonic acid (HOTf) or methyl trifluoromethylsulfonate (MeOTf) yields the corresponding protonated [(2‐Im^Dipp)PH(4‐Im^Dipp)]²⁺ ( 11 ²⁺) and methylated [(2‐Im^Dipp)PMe(4‐Im^Dipp)]²⁺ ( 12 ²⁺) dications, respectively. EPR/UV/Vis‐NIR spectroelectrochemical investigation of the low‐coordinated P^I cation 10 ⁺ predicted a stable and “bottleable” P‐centered radical dication [(2‐Im^Dipp)P(4‐Im^Dipp)]^2+. ( 13 ^2+.). The reaction of 10 ⁺ with the nitrosyl salt NO[OTf] yields the persistent phosphanyl radical dication 13 ^2+. as triflate salt in crystalline form. Quantum chemical investigation revealed an exceptional high spin density at the P atom.     

Investigations of the Vicarious C‐Aminations of 5,7‐Dinitrobenzotriazole and 4,6‐Dinitrobenzotriazol‐3‐ium‐1‐oxide and Their Energetic Properties

This combined experimental and theoretical study details the vicarious nucleophilic substitution by amination of 5,7‐dinitrobenzotriazol ( 1 ) and 4,6‐dinitrobenzotriazole‐3‐ium‐1‐oxide ( 4 ) with trimethylhydrazinium iodide to afford the new corresponding one‐ and two‐time aminated compounds and investigations of its mechanism by EPR spectroscopy. The preferred position for the first amination is computed by spin density population and verified by X‐ray crystallography. The zwitterionic structure of 4 is investigated in solution by ¹H NMR spectroscopy and in solid state by X‐ray diffraction. Furthermore, the crystal structure of 1 is presented. The energetic behavior of the aminated products as well as the starting materials 1 and 4 was investigated, regarding sensitivities and performance.     

New Insights into the Hexaphenylethane Riddle: Formation of an α,o‐Dimer

Upon reduction of a 1H‐cyclobuta[de]naphthalene‐4,5‐diylbis(diarylmethylium) species, a new C? C bond is formed between the C_α and C_ortho atoms of the two chromophores, which presents an unprecedented coupling pattern for the dimerization of two trityl units. By attaching an annulated cyclobutane ring at the opposite peri position of the naphthalene core, the distance between the C_α carbon atoms was elongated beyond the limit of σ‐bond formation through “scissor effects”. The suppression of C_α? C_α bond formation, which would lead to hexaphenylethane‐type compounds, is key to the first successful isolation of the α,o‐adducts. The 5‐diarylmethylene‐6‐triarylmethyl‐1,3‐cyclohexadiene unit in the α,o‐adducts is stable, and isomerization of the cyclohexadiene unit into an aromatic system was not observed. The newly formed C_α? C_ortho bond was cleaved upon two‐electron oxidation to regenerate the dicationic dye.