Force-Modulated C–C Reductive Elimination from Nickel Bis(polyfluorophenyl) Complexes

Abstract

We have analyzed the rate of C(sp2)–C(sp2) reductive elimination from nickel(II) bis(2,4,6-trifluorophenyl) complexes (P–P)Ni(2,4,6-C6H2F3)2 containing either MeOBiPhep (3a) or a macrocyclic bisphosphine ligand (3b–3e) as a function of force applied to the biaryl backbone of these ligands through intramolecular tension generated by a molecular force probe. Nickel complexes 3 were isolated in 22–60% yield from the reaction of bisphosphine with the bis(tetrahydrofuranyl) complex (THF)2Ni(2,4,6-C6H2F3)2 followed by chromatography. Thermolysis of complexes 3 in C6D6 at 68 °C leads to first-order decay through >3 half-lives to the form 2,2′,4,4′,6,6′-hexafluoro­biphenyl as the exclusive fluorine-containing product in ≥93% yield. Whereas compressive forces up to −65 pN have no significant effect on the rate of reductive elimination, extension forces increase the rate of reductive elimination by a factor of 3 over an ∼230 pN range of restoring forces relative to the strain-free MeOBiphep complex. The rate response of reductive elimination from nickel(II) bis(trifluorophenyl) complexes as a function of extension force is similar to the previously reported 2.8-fold increase in the rate of reductive elimination from platinum diaryl complexes (P–P)Pt(4-C6H4NMe2)2 over the same range of forces.

Modulating Transition Metal Reactivity with Force

“…Here, we have investigated the rate of C(sp 2 )À C(sp 2 ) reductive elimination from platinum(II) bis(dimethylamino)phenyl complexes (PÀ P)Pt(4-C 6 H 4 NMe 2 ) 2 and nickel bis(trifluorophenyl) complexes (PÀ P)Ni-(2,4,6-C 6 H 2 F 3 ) 2 (Scheme 4b, c). [15,19] Compressive forces increase the rate of oxidative addition, whereas tensile forces decrease the rate, relative to that of the MeOBiphep complex. Rates vary by a factor of ~6 across ~340 pN of force applied to the bound ligands.…”

“…Toward this objective, we focused our attention on oxidative addition and reductive elimination -two of the most important elementary steps associated with transition metal catalysis and characterized the change in rates of these processes as a function of restoring force. [13,15,19] Specifically, we studied the oxidative addition of bromobenzene to the low ligated Pd(0) bisphosphine complexes generated via sodium borohydride reduction of (PÀ P)PdCl 2 in THF at À 10 °C (Scheme 4a). [13] This transformation mimics the key MÀ C bond forming step in the synthetically relevant Kumada coupling and is free of any potentially complicating ligand dissociation processes.…”

“…Having demonstrated force modulated transition metal‐catalysis, we sought to gain fundamental information regarding the effects of force on the energetics of elementary transformations occurring within the first coordination sphere. Toward this objective, we focused our attention on oxidative addition and reductive elimination – two of the most important elementary steps associated with transition metal catalysis and characterized the change in rates of these processes as a function of restoring force [13,15,19] . Specifically, we studied the oxidative addition of bromobenzene to the low ligated Pd(0) bisphosphine complexes generated via sodium borohydride reduction of (P−P)PdCl 2 in THF at −10 °C (Scheme 4a) [13] .…”

“…Here, we have investigated the rate of C(sp 2 )−C(sp 2 ) reductive elimination from platinum(II) bis(dimethylamino)phenyl complexes (P−P)Pt(4‐C 6 H 4 NMe 2 ) 2 and nickel bis(trifluorophenyl) complexes (P−P)Ni(2,4,6‐C 6 H 2 F 3 ) 2 (Scheme 4b, c). [15,19] …”