Nickel(II) Complexes Containing a Pyrrole–Diphosphine Pincer Ligand

Abstract

A new pincer ligand, (P(2)(Ph)Pyr)(-), based on the anion of 2,5-bis[(diphenylphosphino)methyl]pyrrole has been prepared in four steps from pyrrole. The ligand undergoes oxidation to diphosphine oxide under ambient conditions and was therefore isolated as its borane adduct, H(P(2)(Ph)Pyr)·2BH(3) (2). Delivery of the ligand to nickel(II) was accomplished by the direct reaction of NiCl(2) with 2 in the presence of Et(2)NH to afford [NiCl(P(2)(Ph)Pyr)]. Salt metathesis reactions of the chloro complex afford new compounds including [Ni(CH(3))(P(2)(Ph)Pyr)] and [Ni(NCCH(3))(P(2)(Ph)Pyr)](OTf). In all cases, the ligand gives rise to diamagnetic square-planar complexes, which have been fully characterized in solution and the solid state. All complexes examined display an irreversible oxidation to nickel(III) according to cyclic voltammetry. Reduction of the chloro complex in dichloromethane results in an electrocatalytic process, whereas reduction in tetrahydrofuran leads to the irreversible formation of a nickel(I) species.

Synthesis, Characterization, and Catalytic Activity of Nickel(II) Alkyl Complexes Supported by Pyrrole–Diphosphine Ligands

“…21 Concurrent reports by the groups of Gade 19 and Mani 20 demonstrated an alternative method for the preparation of 1 using the secondary phosphines (or potassium salts thereof) directly in displacement reactions with bis (aminomethyl)pyrrole precursors. We have adopted this synthetic methodology for the preparation of 1 and have extended it to include the formation of the dicyclohexylphosphine derivative ( 2 ) as shown in Scheme 1.…”

“…The remaining bond metrics are unremarkable, and consistent with those of other Ni(II) complexes bearing the P 2 Ph Pyr − ligand. 21 …”

“…19–21 These complexes were prepared in a straightforward fashion and several examples containing different X-type ligands were accessed readily through salt metatheses. We were thus intrigued to learn whether such compounds display an interesting organometallic chemistry, especially with hydrocarbyl-derived ligands.…”

Phosphines and N‐Heterocycles Joining Forces: an Emerging Structural Motif in PNP‐Pincer Chemistry

“…In 2012, the pyrrole‐based ligand Pyr ( R PNP)H was reported independently by the groups of Gade, Mani and Tonzetich . Similar to the carbazole‐based pincer ligand mentioned above, Pyr ( R PNP)H was designed to form five‐membered chelates upon coordination of a metal ion, however, the methylene‐linkages between the phosphines and the central pyrrole unit render this tridentate ligand significantly more flexible and thus adaptable to different metal atom radii.…”

Asymmetric bis-PNP pincer complexes of zirconium and hafnium – a measure of hemilability

“…Since its first syn-thesis in 2012 the phenyl substituted pyrrolide-PNP pincer ligand has been relatively underused, especially given both the commercial availability of the precursors and its varied reactivity. [6][7][8][9][10][11][12][13][14] Our interest was also piqued by the recent resurgence of early transition metal pincer chemistry. Initially pioneered by Fryzuk and others from the 1970s onwards, 15 it remains remarkably underdeveloped when compared with the later transition metals.…”