On the energetics of P–P bond dissociation of sterically strained tetraamino-diphosphanes

Abstract

The homolytic P-P bond fission in a series of sterically congested tetraaminodiphosphanes (R2N)2P-P(NR2)2 ({4}2-{9}2, two of which were newly synthesized and fully characterized) into diaminophosphanyl radicals (R2N)2P˙ (4-9) was monitored by VT EPR spectroscopy. Determination of the radical concentration from the EPR spectra permitted to calculate free dissociation energies ΔGDiss(295) as well as dissociation enthalpies ΔHDiss and entropies ΔSDiss, respectively. Large positive values of ΔGDiss(295) indicate that the degree of dissociation is in most cases low, and the concentration of persistent radicals--even if they are spectroscopically observable at ambient temperature--remains small. Appreciable dissociation was established only for the sterically highly congested acyclic derivative {9}2. Analysis of the trends in experimental data in connection with DFT studies indicate that radical formation is favoured by large entropy contributions and the energetic effect of structural relaxation (geometrical distortions and conformational changes in acyclic derivatives) in the radicals, and disfavoured by attractive dispersion forces. Comparison of the energetics of formation for CC-saturated N-heterocyclic diphosphanes and the 7π-radical 3c indicates that the effect of energetic stabilization by π-electron delocalization in the latter is visible, but stands back behind those of steric and entropic contributions. Evaluation of spectroscopic and computational data indicates that diaminophosphanyl radicals exhibit, in contrast to aminophosphenium cations, no strong energetic preference for a planar arrangement of the (R2N)2P unit.

Bis‐[3]Ferrocenophanes with Central >E−E’< Bonds (E, E’=P, SiH): Preparation, Properties, and Thermal Activation

“…The observed temperature dependence of the EPR spectra suggests that the radicals 11 recombine at lower temperature and exist in a dynamic equilibrium with the diphosphane. Similar reversible equilibria have precedence for amino‐diphosphanes –. Prolonged heating of toluene solutions of 4 under inert conditions affords detectable amounts of secondary phosphane 2 , suggesting that radicals 11 may be irreversibly quenched by hydrogen abstraction, e. g. from the solvent.…”

“…Single crystal XRD studies of 8 and 9 show a twisted arrangement with respect to the P−P bond for 8 , while 9 adopts a nearly antiperiplanar orientation (Figures and ). The P–P distance of the central bond is 2.3019(11) Å for 8 and 2.281(3) Å for 9 , both exceeding the respective bond length in Hittorf's phosphorus but well within the range observed for bulkily substituted tetraamino‐diphosphanes (2.24–2.34 Å). The pyramidal coordination geometry around the phosphorus atoms of 8 (sum of bond angles 313(1)°) is slightly flattened compared to 4 (sum of bond angles 304–308°) and becomes even more planarized in 9 (sum of angles 322(1)°/323(1)°).…”

“…This methodology works particularly well for the generation of amino‐phosphanyl radicals (X 2 P . ; X=R 2 N) which entails a propensity for bond fragmentation in sterically strained tetraaminodiphosphanes . In this context, we explored the potential of the above mentioned bis‐[3]ferrocenophanes to undergo thermally induced homolytic cleavage of the central P−P, P−Si or Si−Si bond, respectively.…”

Functionalization of Bis‐Diazaphospholene P–P Bonds with Diverse Electrophiles

“…Wright and co‐workers showed that readily available diazaphospholene chloride 3‐Cl is reductively coupled to form bis‐diazaphospholene 2 in the presence of magnesium dust (Equation 1, Scheme ) . Bis‐diazaphospholene 2 was shown to be a source of persistent 7 pi radical 5 , however the solution concentration of these radicals is typically low relative to that of dimer 2 at ambient temperature . We found that 2 could be accessed in high yield and purity via reductive coupling of readily accessible 3‐Br with magnesium turnings in THF .…”

Bond fission in monocationic frameworks: diverse fragmentation pathways for phosphinophosphonium cations

“…S22†). In computational studies of diphosphines the inclusion of dispersion correction is reportedly critical to the determination of P–P homolytic dissociation energies 35. We have considered dispersion corrections through use of Grimme's DFT-D3 correction36 in PBE1PBE/6-311++G(d,p) optimization and frequency analysis of the phosphinophosphoniums [Et 2 PPMe 3 ] + and [ t Bu 2 PPMe 3 ] + .…”