Nickel(II) and Palladium(II) Bis-Aminophosphine Pincer Complexes

Abstract

Chelating ligands have proven instrumental in the development of new catalysts; furthermore trans-chelating "pincer" ligands have garnered considerable attention over the years. 1À13 The most extensively investigated pincer complexes feature ligands based on a meta-substituted arene skeleton (PCP). Variations of the PCP-pincer framework have been shown to alter the electronic and steric properties of the corresponding metal complexes, resulting in unique reactivity. 14,15 Aminophosphine chelating ligands of the form (linker)-(NR 0 PR 2 ) 2 have also garnered attention, as such ligands are readily prepared from diamines and chlorophosphines. 5,16 While linked diaminophosphine ligands such as (CH 2 N(R 0 )PR 2 ) 2 (R 0 = H, alkyl; R = alkyl, aryl) have been reported by Wollins, 17 Gusev, 9 Vogt, 16 and others, 18À21 investigations of PCP aminophosphine-based pincer ligand complexes have received less attention. 10 We were drawn to these systems, as such ligands offer adjacent P and N donors, suggesting the possibility of unique binding modes and thus subsequent reactivity. In this regard, the only report in which the N atom of an aminophosphine has been shown to participate in binding to a metal was recently reported by Palacios et al. In this case a bis-aminophosphine ligand was shown to bind to Ru via both P atoms as well as one of the amino NH groups, affording the Ru cation [Cp*Ru(k 3 P,N,P-(iPr 2 PNH) 2 C 6 H 10 ] + . 21 In this paper, we describe the synthesis and characterization of Ni and Pd complexes of bis-aminophosphine chelating ligands. Interestingly, these ligands are shown to readily adopt two tridentate binding modes. Participation of the secondary N adjacent to P affords k 3 P,N,P ligands that incorporate three-membered NPM metallacycles. Alternatively, metalation of the alkyl chain in the ligand backbone affords k 3 P,C,P pincer complexes.

Synthesis of unsymmetrical 5,6-POCOP′-type pincer complexes of nickel(ii): impact of nickelacycle size on structures and spectroscopic properties

“…On the other hand, recent reports have shown a growing interest in the structures and reactivities of nonsymmetrical EXE´-type ligands based on both different donor moieties and differently-sized metallacycles, including : 5,6-NCS; 8 5,6-SCN; 9 5,6-POCN; 10 3,6-, 3,7-, and 4,5-PNP´; 11 5,6-PCN; 12 5,6-POCOP´. On the other hand, recent reports have shown a growing interest in the structures and reactivities of nonsymmetrical EXE´-type ligands based on both different donor moieties and differently-sized metallacycles, including : 5,6-NCS; 8 5,6-SCN; 9 5,6-POCN; 10 3,6-, 3,7-, and 4,5-PNP´; 11 5,6-PCN; 12 5,6-POCOP´.…”

Low symmetry Pincer Ligands designed for applications in coordination chemistry and catalysis

“…Over the years chelating ligands have been of much interest and use for catalytic purposes. 4,5 An important class of chelate chemistry is comprised of pincer ligands and 2 science. [7][8][9][10][11][12] But it was not until late 1980s that this class of ligands received the name of the 'pincer', a term coined by the pioneer in the field -Gerard van Koten.…”

Ammonia Activation by a Nickel NCN‐Pincer Complex featuring a Non‐Innocent N‐Heterocyclic Carbene: Ammine and Amido Complexes in Equilibrium

“…Thec rystal structure confirmed the distorted tetrahedral geometry of the N 2 CH carbon atom on the NCN pincer complex, with the sum of the bond angles formed by the heavier elements at C1 measuring 332.58 8 (Figure 1). With 1.848(5) , the C1 À Ni1 bond is short when compared to similar bonds in Ni II pincer complexes featuring dialkylmethyl [16] or diarylmethyl [11] central platforms (av.N i À C 1.98(2) and 1.986(3) , respectively). Although the preference for alkyl versus carbene coordination is common for complexes of dialkyl and diaryl carbenes,i ti sv ery rare for complexes of the more stable NHCs.O nly af ew such examples have been structurally characterized to date, [17] and we propose that the competitive delocalization of the nitrogen lone pairs of electrons onto the exocyclic amidine moieties is responsible for ar educed stabilization of the carbene structure.…”