Impact of Oxidation State on Reactivity and Selectivity Differences between Nickel(III) and Nickel(IV) Alkyl Complexes

Abstract

Described is as ystematic comparison of factors impacting the relative rates and selectivities of C(sp 3 ) À Ca nd C(sp 3 )ÀObond-forming reactions at high-valent Ni as afunction of oxidation state.T wo Ni complexes are compared: ac ationic octahedral Ni IV complex ligated by tris(pyrazolyl)borate and ac ationic octahedral Ni III complex ligated by tris(pyrazolyl)methane.K ey features of reactivity/selectivity are revealed:1 )C(sp 3 ) À C(sp 2 )b ond-forming reductive elimination occurs from both centers,b ut the Ni III complex reacts up to 300-fold faster than the Ni IV ,d epending on the reaction conditions.T he relative reactivity is proposed to derive from ligand dissociation kinetics,w hich vary as af unction of oxidation state and the presence/absence of visible light. 2) Upon the addition of acetate (AcO À ), the Ni IV complex exclusively undergoes C(sp 3 ) À OAcb ond formation, while the Ni III analogue forms the C(sp 3 )ÀC(sp 2 )c oupled product selectively.This difference is rationalized based on the electrophilicity of the respective MÀC(sp 3 )b onds,a nd thus their relative reactivity towards outer-sphere S N 2-type bond-forming reactions.

σ‐Noninnocence: Masked Phenyl‐Cation Transfer at Formal NiIV

“…We became interested in these findings as these high-valent formal Ni IV species have become suspect of featuring an inverted ligand field, [3] rendering the assignment of oxidation states in these complexes possibly challenging. [4] Thec oncept of inverted ligand fields was recently reviewed by Hoffmann et al [5] and is most prominently discussed for the prototypical example of the [Cu(CF 3 ) 4 ] À anion. [3a,6] Already in 1995, Snyder proposed that this formal Cu III (d 8 )c omplex would be better described as aC u I (d 10 ) complex.…”

“…A Hammett analysis of the aryl–CF 3 bond‐forming reductive elimination from I R=H suggests that the reaction proceeds via a nucleophilic attack of the aryl ligand on an electrophilic CF 3 ligand. We became interested in these findings as these high‐valent formal Ni IV species have become suspect of featuring an inverted ligand field, rendering the assignment of oxidation states in these complexes possibly challenging …”

Focusing on the Catalysts of the Pd- and Ni-Catalyzed Hirao Reactions

“…It was another surprise that in the “reductive” variation, again a Ni(II) → Ni(IV) oxidation was substantiated by the preparative experiments and calculations [ 72 ]. However, surveying the literature, the Ni(II) → Ni(IV) transition is well-known during the interconversion of different Ni complexes [ 73 , 74 , 75 ]. Moreover, a Ni(II) → Ni(IV) conversion was observed in the Ni-catalyzed alkylation of benzamides with alkyl halides [ 76 ].…”

High-Valent NiIII and NiIV Species Relevant to C–C and C–Heteroatom Cross-Coupling Reactions: State of the Art

“…As shown earlier, Ni III and Ni IV complexes are engaged in C-C bond forming reactions, although limited knowledge is available concerning their comparative efficiency upon similar environments such as identical geometry, type of ligands or ligand set, and global charge. In this sense, Sanford's group has evaluated the feasibility of C-C and C-heteroatom bond formation depending on: i) the nature of the transition metal (Ni vs. Pd) [80]; ii) the nature of the surrounding ligands (MeCN vs. CF3) [80]; and iii) the oxidation state at Ni (+3 vs. +4; see Scheme 12) [81].…”

“…Kinetic studies performed for the elimination of 21 from the M IV species 50, 51, and 52 proved the higher stability of 50 vs. 52, most likely due to the strong σdonation of the CF3-group. The nature of the cationic species (Tp)-Ni IV 52 and (Py3CH)-Ni III 53 was authenticated using XRD, EPR (53) or NMR (52), and cyclic voltammetry [80,81]. Their ability to release 21 was then compared at r.t. in the dark or when exposed to daylight (Scheme 12), thus reflecting the higher activity of the (Py3CH)-Ni III complex 53.…”