Controlling P–C/C–H Bond Cleavage in Nickel Bis(diphosphine) Complexes: Reactivity Scope, Mechanism, and Computations

Abstract

The synthesis of heteroleptic [Ni­(P2N2)­(diphosphine)]­[BF4]2 complexes and the cleavage of P–C and C–H bonds of the P2N2 ligand in those complexes are reported. The products are five-coordinate complexes in which Ni–C and P–H bonds have formed to give a cyclic moiety containing Ni–CHNR2. The reactivity of [Ni­(P2N2)­(diphosphine)]­[BF4]2 complexes is influenced by the rigidity of the diphosphine, the steric effect of the substituents, and length of the carbon linker of the diphosphine ligands. Diphosphine ligands bearing a rigid backbone (e.g., dmpbz, 1,2-bis­(dimethylphosphino)­benzene) or aromatic substituents (e.g., dppe, 1,2-bis­(diphenylphosphino)­ethane) react with [Ni­(P tBu 2NBn 2)­(CH3CN)2]­[BF4]2 to give P–C/C–H bond cleavage products. Both [Ni­(P tBu 2NBn 2)­(dmpe)­(MeCN)]­[BF4]2 and [Ni­(P tBu 2NBn 2)­(dmpm)­(MeCN)]­[BF4]2 (dmpm = 1,2-bis­(dimethylylphosphino)­methane) were prepared by the reaction of [Ni­(P tBu 2NBn 2)­(CH3CN)2]­[BF4]2 with the corresponding diphosphine ligands. [Ni­(P tBu 2NBn 2)­(dmpe)­(MeCN)]­[BF4]2 readily undergoes P–C/C–H bond cleavage in nitromethane. In sharp contrast, [Ni­(P tBu 2NBn 2)­(dmpm)]­[BF4]2 is stabilized by dmpm, a diphosphine with small bite angle, and does not show P–C/C–H bond cleavage reactivity. Computational results show that for complexes bearing less bulky diphosphine ligands, such as dmpm, the barriers for the rate-determining transition states are in some examples higher than 30 kcal/mol with the M06 functional, higher than those for complexes bearing more rigid or more bulky ligands, consistent with experimental studies. The calculated barriers for the first transition state correlated with increased values of the dihedral angle formed by the two NiP2 planes.

Hard to Handle: Extremely Labile Hydrido Nickel(II) Complexes of Cyclometalated C∧N∧N and N∧C∧N Ligands

“…Organometallic nickel­(II) hydrido complexes have gained enormous interest in recent years due to their role as catalysts or as important intermediates in organometallic processes such as C–H functionalizations, − hydroarylation C–C coupling reactions and related, ,,− the oligomerization of ethene to α-olefins, ,,− (de)­hydrogenation reactions, ,,,− photo- or electrolytical dihydrogen evolution or oxidation, ,,− including the [FeNi] depending hydrogenases. ,,, Ni­(II) hydrido complexes are frequently formed in C–H activation through oxidative addition steps using a Ni(0) species, − ,, or through X – → H – ligand exchange (X = halides or pseudohalides), ,,− and the reactivity of such species has been studied in model complexes. − ,− Despite their vital role in dihydrogen turnover and C–H activation, however, synthetic access to and bulk isolation of Ni­(II) hydrido species are still hampered by their intrinsic lability. Only scant examples of Ni­(II) hydrido complexes have been structurally characterized. ,,,,,…”

Pd(II)- and Pt(II)-Assisted P–C Activation/Cyclization Reactions with a Luminescent α-Aminophosphine

“…Metal ions in these cases are Ru(II), Rh(III), Ir(III), or Ni(II); α-aminophosphines bear morpholine, piperazine, or carboxyphenyl substituents at the N atom. 21,32,33 Meantime, metal-assisted synthesis can facilitate transformations in ligands, yielding novel architectures, which are otherwise difficult to access. 34,35 Therefore, our motivation in this work is to find novel metal-assisted P−C bond activation paths for aminophosphines, especially for lumines- cent ones, which comprise a photophysically active sulfur− nitrogen heterocycle.…”

“…Scarce metal-assisted reactions involve the P–C to P–H bond transformation. Metal ions in these cases are Ru­(II), Rh­(III), Ir­(III), or Ni­(II); α-aminophosphines bear morpholine, piperazine, or carboxyphenyl substituents at the N atom. ,, Meantime, metal-assisted synthesis can facilitate transformations in ligands, yielding novel architectures, which are otherwise difficult to access. , Therefore, our motivation in this work is to find novel metal-assisted P–C bond activation paths for aminophosphines, especially for luminescent ones, which comprise a photophysically active sulfur–nitrogen heterocycle. In this context, we chose a 2-phenylbenzothiazolyl ( Pbt ) chromophore, whose derivatives are widely used in the design of photoluminescent materials. − They reveal interesting photophysical features, such as excited-state intramolecular proton transfer (ESIPT) , and aggregate-induced emission (AIE). , A new Pbt -based α-methylaminophosphine ( PCN , 1 ) designed in our work has not been synthesized to date, although an oxidized congener, α-methylaminophosphine oxide, and its luminescent Zn complexes were reported recently .…”