Reactivity of a Strictly T‐Shaped Phosphine Ligated by an Acridane Derived NNN Pincer Ligand

Abstract

The steric tuning of a tridentate acridane-derived NNN pincer ligand allows for the isolation of a strictly Tshaped phosphine that exhibits ambiphilic reactivity. Welldefined phosphorus-centered reactivity towards nucleophiles and electrophiles is reported, contrasting with prior reports on this class of compounds. Reactions towards oxidants are also described. The latter result in the two-electron oxidation of the phosphorus atom from + III to + V and are accom-panied by a strong geometric distortion of the NNN pincer ligand. By contrast, cooperative activation of EÀ H (HCl, HBcat, HOMe) bonds proceeds with retention of the phosphorus redox state. When using H 2 O as a substrate, the reaction results in the full disassembly of H 2 O to its constituent atoms, highlighting the potential of this platform for small molecule activation reactions.

Geometrically Deformed and Conformationally Rigid Phosphorus Trisamides Featuring an Unsymmetrical Backbone

“…Two-electron oxidation of the P-center in 1 by reaction with an equimolar amount of p-anisylazide (4-methoxyphenylazide) also proceeded smoothly upon heating at 90 °C for several hours, cleanly forming iminophosphorane 5. This species was characterized by a 31 P NMR chemical shift of 1.8 ppm, comparable to the previously reported iminophosphoranes of PN3 (at 14.5 ppm) [39] and PN3 acr (at 18.1 ppm), [28] although it must be noted that the specific reaction conditions (temperature required and aryl azide used) distinctly vary between these three systems. The molecular composition of species 5 was also confirmed by high resolution mass spectrometry and we obtained a connectivity plot from an incomplete X-ray diffraction analysis (Fig S88 ).…”

“…Equimolar reactivity of 1 with methyl triflate did lead to a single new species in the 31 P NMR spectrum at 70.2 ppm, with a 2 J P-H coupling of 16 Hz. Taken together with the new doublet at 3.06 ppm in the 1 H NMR spectrum and related data obtained by Abbenseth and Goicoechea with their version of the acridinebased PN3 acr system, [28] these data support the formation of the cationic λ 4 σ 4 -phosphorane 4 with a formal P(V) center, rather than methylation of the ligand backbone. [38] X-ray diffraction of single crystals grown from CDCl 3 -pentane corroborated this assignment (Figure 6, right) and showed that the anion is not interacting with the P(V) center in the solid state.…”

“…The geometry of the PN3 motif is only slightly altered relative to 1, with the ffN2-P1-N3 widening somewhat to 126.23(3)°. Furthermore, the central sulfonamido-nitrogen of the new N 3 motif also occupies an axial position in the distorted trigonal bipyramidal (tbp) structure (τ 5 = 0.79), [26] rather than an equatorial position, as previously observed with dihalidoanalogs of either a (semi)flexible (X = Cl [11c,12a,c,27] ) or a strictly Tshaped trisamido backbone (X = Br), [28] due to the conforma-tionally inflexible N3 Me2 backbone that does not allow for a planar, meridional coordination of the three nitrogen donors. Very few crystallographically characterized difluorophosphoranes with three nitrogen donors are known to date.…”

“…The magnitude of this coupling is commonly taken as indication for the Lewisbasicity of the P-center, with the J Se-P -value decreasing with increasingbasicity at phosphorus. [34] For comparison, the values of related geometrically deformed species reported by Radosevich (PN3 = Se) [12c] and Abbenseth and Goicoechea (PN2S2 = Se [18b] and PN3 acr = Se) [28] as well as a rare phosphinine-selenide P inine = Se [35] are all lower in comparison (although all J Se-P are > 900 Hz) (Figure 5) but still higher than for the non-geometrically deformed selenide of HMPT. [36] These data show that the lone pair of the P-center in 1 bears a high degree of s-character.…”

Metallomimetic C–F Activation Catalysis by Simple Phosphines

“…However, among the main group elements the pnictogens stand out in this regard, often allowing access to stable species in different oxidation states, and as such they are ideal candidates for the development of metallomimetic reactivity and catalysis . A key design concept that has led to observations of redox-cycling within these systems is the introduction of geometrical constraints around the pnictogen atoms, moving them away from their preferred pyramidal to more planar geometries, resulting in changes in orbital energies and reactivity. − Pincer ligands are often used for this purpose, promoting ambiphilic behavior and unusual reactivity of the pnictogen, − although metal–ligand cooperativity is also frequently observed in these systems. − …”