Strong Evidence of a Phosphanoxyl Complex: Formation,Bonding, and Reactivity of Ligated Phosphorus Analogues of Nitroxides

Facile access to [W(CO)₅(Ph₂P‐OTEMP)] is used to initiate a study on the generation, properties, and reactions of transient phosphanoxyl complexes [ML_n(R₂PO)], the first example of which could be trapped via heterocoupling with the trityl radical. It is also demonstrated that the phosphorus nitroxyl complex acts as radical initiator in the polymerization of styrene. The quest for P?O versus O?N bond homolysis, as well as the initial steps of the polymerization were studied by DFT methods.     

H(OEt2)2[P(1,2‐O2C6Cl4)3]: Synthesis,Characterization, and Application as a Single‐Component Initiator for the Carbocationic Polymerization of Olefins

The development of novel Brønsted acids featuring the hexacoordinate phosphorus(V) anion [TRISPHAT]^? {[ 1 ]^?=[P(1,2‐O₂C₆Cl₄)₃]^?} are reported. The title compound, H(OEt₂)₂[ 1 ], was synthesized from 1,2‐(HO)₂C₆Cl₄ (3 equiv) and PCl₅ in the presence of diethyl ether. This compound was fully characterized by ¹H, ³¹P and ¹³C NMR spectroscopy, X‐ray crystallography and elemental microanalysis. Dissolution of H(OEt₂)₂[ 1 ] in acetonitrile results in the slow precipitation of crystalline H(OEt₂)(NCMe)[ 1 ], which was characterized by X‐ray diffraction; however, in CD₂Cl₂ solution the [TRISPHAT]^? anion protonated and ring‐opened. The weighable, solid H(OEt₂)₂ [ 1 ] was found to be a competent initiator for the polymerization of n‐butyl vinyl ether, α‐methylstyrene, styrene and isoprene at a variety of temperatures and monomer‐to‐initiator ratios. At low temperatures, polymers with M_n>10⁵ were obtained for n‐butyl vinyl ether and α‐methylstyrene whereas slightly lower molecular weights were obtained with styrene and isoprene (10⁴<M_n<10⁵). The poly(α‐methylstyrene) synthesized at ?78 °C is syndiotactic‐rich (ca. 87 % rr) whereas the polystyrene obtained at ?50 °C is atactic. The polyisoprene obtained possessed all possible modes of enchainment as well as branched and/or cyclic structures that are often observed in polyisoprene.     

Class III Delocalization and Exciton Coupling in a Bimetallic Bis‐ligand Radical Complex

The geometric and electronic structure of an oxidized bimetallic Ni complex incorporating two redox‐active Schiff‐base ligands connected via a 1,2‐phenylene linker has been investigated and compared to a monomeric analogue. Information from UV/Vis/NIR spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, and density functional theory (DFT) calculations provides important information on the locus of oxidation for the bimetallic complex. The neutral bimetallic complex is conformationally dynamic at room temperature, which complicates characterization of the oxidized forms. Comparison to an oxidized monomer analogue 1 provides critical insight into the electronic structure of the oxidized bimetallic complex 2 . Oxidation of 1 provides [ 1 ^.]⁺, which is characterized as a fully delocalized ligand radical complex; the spectroscopic signature of this derivative includes an intense NIR band at 4500 cm^?1. Oxidation of 2 to the bis‐oxidized form affords a bis‐ligand radical species [ 2 ^..]²⁺. Variable temperature EPR spectroscopy of [ 2 ^..]²⁺ shows no evidence of coupling, and the triplet and broken symmetry solutions afforded by theoretical calculations are essentially isoenergetic. [ 2 ^..]²⁺ is thus best described as incorporating two non‐interacting ligand radicals. Interestingly, the intense NIR intervalence charge transfer band observed for the delocalized ligand‐radical [ 1 ^.]⁺ exhibits exciton splitting in [ 2 ^..]²⁺, due to coupling of the monomer transition dipoles in the enforced oblique dimer geometry. Evaluating the splitting of the intense intervalence charge transfer band can thus provide significant geometric and electronic information in less rigid bis‐ligand radical systems. Addition of excess pyridine to [ 2 ^..]²⁺ results in a shift in the oxidation locus from a bis‐ligand radical species to the Ni^III/Ni^III derivative [ 2 (py)₄]^{2 +} , demonstrating that the ligand system can incorporate significant bulk in the axial positions.