A chelating diphosphine ligand with a central N-heterocyclic phosphenium cation (NHP(+)) has been used to explore the coordination chemistry of NHPs with nickel. Treatment of the chlorophosphine precursor [PPP]Cl (1) with stoichiometric Ni(COD)2 affords (PPP)NiCl (8), which is best described as a Ni(II)/NHP(-) phosphido complex formed via oxidative addition of the P-Cl bond. In contrast, treating [PPP]Cl (1) with excess Ni(COD)2 results in a mixture of the trimetallic complex (PPP)2Ni3Cl2 (9) and the reduced NHP-bridged dimer [(PPP)Ni]2 (10). Compound 9 is found to be a Ni(II)Ni(II)Ni(0) complex in which the two NHP ligands act as bridging NHP(-) phosphidos, while complex 10 is a Ni(I)Ni(I) complex that is highly delocalized throughout the symmetric Ni2P2 core. In contrast, the reaction of [PPP][PF6] (11) with Ni(COD)2 affords an asymmetrically-bridged dication [(PPP)Ni]2[PF6]2 (12), which is found to contain two bridging NHP(+) cations bridging two Ni(0) centers. Comproportionation of 10 and 12 affords monocationic [(PPP)Ni]2[PF6] (13), completing the redox series. Nickel complexes 8-10 and 12 are largely similar to their Pd and Pt analogues, but a paramagnetic monocation such as 13 was not observed in the Pd and Pt case. Computational studies lend further insight into the electronic structure and bonding in complexes 8-10 and 12-13, and further support the potential redox non-innocent properties of NHP ligands.

By incorporating an N-heterocyclic phosphenium/phosphide (NHP) ligand into a chelating pincer ligand framework (PPP(+)/PPP(-)), we have elucidated several different and unprecedented binding modes of NHP ligands in homobimetallic, heterobimetallic, and trimetallic metal complexes. One-electron reduction of the previously reported (PPP)(-)/M(II) complexes (PPP)M-Cl (M = Pd (1), Pt (2)) results in clean formation of the symmetric homobimetallic M(I)/M(I) complexes [(μ-PPP)Pd]2 (5) and [(μ-PPP)Pt]2 (6). The tridentate NHP ligand has also been utilized as a bridging linker in the M/Co heterobimetallic compounds (OC)3Co(u-PPP)M(CO) (M = Pd (7), Pt (8)), synthesized via salt elimination from mixtures of 1 and 2 and Na[Co(CO)4]. Furthermore, an NHP-bridged trimetallic complex (PPP)2Pd3Cl2 (9) can be synthesized in a manner similar to precursor 1 (Pd(PPh3)4 + (PPP)Cl) via careful adjustment of reaction stoichiometry. Examination of the interatomic distances and angles in complexes 5-9, in tandem with density functional theory calculations have been used to evaluate and characterize the bonding interactions in these complexes.

Treatment of the N-heterocyclic chlorophosphine precursor (PPP)Cl (1) with two equivalents of nickelocene (NiCp 2 ) affords the phosphorus-bridged dimer [(µ-PPP)Ni 2 Cp 2 ]Cl (2). In contrast, an equimolar mixture of 1 and NiCp 2 in the presence of PPh 3 generates a different product, (PP(C 5 H 5 )P)NiCl 2 (3), in which a cyclopentadienyl anion has migrated to the N-heterocyclic phosphenium center. The phosphorus-bound Cp ring in complex 3 has undergone a [1,5]-hydride shift to afford a vinylic C 5 H 5 ring, and can be subsequently deprotonated to produce [(PP(C 5 H 4 )P)NiCl] (4).