Bimetallic Nickel Complexes Supported by a Planar Macrocyclic Diphosphoranide Ligand

Abstract

Metal-metal cooperativity is emerging as an important strategy in catalysis. This requires appropriate ligand scaffolds that can support two metals in close proximity. Here we report nickel-promoted formation of a dinucleating planar macrocyclic ligand that can support bimetallic dinickel(II) and dinickel(I) complexes. Reaction outcomes can be tuned by variation of the substituents and reaction conditions to favour dinucleating macrocyclic, mononucleating macrocyclic or conventional pincer architectures.

Oxidative Addition and β‐Hydride Elimination by a Macrocyclic Dinickel Complex: Observing Bimetallic Elementary Reactions

“…The presence of solid in the NMR tube suggests that the reason the integration of this peak is small compared to ethene (1 : 0.08 ethene : 8 ; should be 1 : 0.25 for equimolar) is due to the insolubility of 8 in C 6 D 6 (attempts to observe this product in THF were also unsuccessful). The appearance of Ni 2 (μ‐Br) 2 ( tBu PONNOPONNO) ( 7 ) was also observed from a signal in the 31 P{ 1 H} NMR appearing around δ P =19.9 (C 6 D 6 ), [28] suggesting that 7 is formed from decomposition of one (or more) of the species present. Once more, peaks corresponding to n ‐butane were not observed in the 1 H NMR spectra for these reactions with EtBr [49]…”

“…It is worth noting that this species is the first dinickel PONNOP complex that has been characterized. The strong tendency of the PONNOP ligand to rearrange in the presence of nickel has led to mononucleating naphthyridone pincer complexes of the type NiX 2 (PONNP), as well as mono‐ and dinucleating macrocyclic phosphoranide ligands [28,57] …”

“…This proximity is enforced by the PONNOPONNO ligand, which results in similar Ni⋯Ni distances in both 1 (2.3836(5) Å) and its Ni II 2 precursor, Ni 2 (μ‐Br) 2 ( tBu PONNOPONNO) (2.401(3) Å) [28] . We anticipate that this enforced proximity will facilitate cooperative bond activation between the two nickel sites that can aid in understanding bimetallic oxidative addition and β‐hydride elimination.…”

“…MeI, EtI, EtBr, I 2 , [Me 3 O]BF 4 and [Et 3 O]BF 4 were used as purchased from Sigma‐Aldrich without further purification. The compounds Ni 2 ( tBu PONNOPONNO), [28] [H(OEt 2 ) 2 ]BAr F 4 [63] and NaBAr F 4 [70] were synthesized according to literature procedures.…”

“…Our group recently reported the synthesis of a dinickel(I) complex supported by a planar, macrocyclic diphosphoranide ligand, Ni 2 ( tBu PONNOPONNO) ( 1 ), by reduction of the dinickel(II) species Ni 2 (μ‐Br) 2 ( tBu PONNOPONNO), which features two symmetrically bridging bromide ligands [28] . The unique structure of 1 prompted us to explore its reactivity, with the intention of determining the methods by which it activates substrates that are important in catalytic processes.…”