Kappa what? Insights into the coordination modes of N2P2 ligands

While first synthesized more than three decades ago, complexes supported by N2P2 ligands have seen renewed interest due to the synthesis of new ligands, expansion of their reactivity, and catalytic applications. Possessing both soft phosphines and hard nitrogen donors, N2P2 ligands can accommodate various metal geometries and coordination modes thanks to their capability to act as bidentate, tridentate or tetradentate ligands. This short review will explore how metals bind to these ligands and also highlight the complexes' reactivity and catalytic abilities.

Nickel-Templated Replacement of Phosphine Substituents in a Tetradentate Bis(amido)bis(phosphine) Ligand

The replacement of phosphine substituents in nickel-bound PNNP ligands is reported as an alternative method for preparing multidentate phosphine ligands with alkyl substituents. Treatment of the previously reported bis(phosphide) complex {K(THF)_x}₂^2Ph[PNNP]Ni (2) with 2 equiv of MeI, ⁱPrI, and 1,3-dibromoethane formed alkyl-substituted complexes ^2Ph,2Me[PNNP]Ni (3), ^2Ph,2iPr[PNNP]Ni (4), and ^{2Ph,propylene}[PNNP]Ni (5), respectively. The stereoselectivity (racemic vs meso) of these reactions can be controlled by varying the reaction temperature. The racemic mixtures of products with the new alkyl substituents in an anti configuration were favored at lower temperatures, whereas a larger proportion of meso compounds was acquired at higher temperatures. Further treatment of 3 with KH resulted in selective elimination of the remaining phenyl groups rather than the methyl substituents, affording bis(methylphosphide) complex {K(THF)_x}₂^2Me[PNNP]Ni (6). Subsequent treatment of 6 with additional MeI formed ^4Me[PNNP]Ni (7), in which all four phenyl groups were replaced with methyl substituents. As a proof of concept, demetalation of the ligand from 7 was achieved using aqueous KCN to form a free dimethylphosphine-substituted ligand H₂^4Me[PNNP] (8), and 8 was subsequently coordinated to a different metal, using PdCl₂ to form ^4Me[PNNP]Pd (9). Unlike the clean elimination of phenyl substituents from 3, the reactions of KH with 4 and 5 exhibited competitive elimination of both alkyl and phenyl substituents and/or attenuated reactivity.

Metal-ligand cooperation behaviour of Fe and Co complexes bearing a tetradentate phenanthroline-based PNNP ligand

This perspective article describes the synthesis of a series of Fe and Co complexes coordinated with a phenanthroline-based meridional PNNP ligand (2,9-bis((diphenylphosphino)methyl)-1,10-phenanthroline). PNNP-iron(ii) dichloride and -cobalt(i) chloride, [FeCl2(PNNP)] and [CoCl(PNNP)], underwent abstraction of the benzylic H-atom upon treatment with NaOtBu, forming the corresponding deprotonated products [FeCl(PNNP')] (1) and [Co(PNNP')] (2), respectively, each of which bears an asymmetrical PNNP' ligand with a dearomatized phenanthroline backbone as a good metal-ligand cooperation (MLC) scaffold. Complex 2 achieved facile H-H bond cleavage mediated by unique long-range MLC, where the PNNP backbone acts as a H-atom reservoir.

Co(I) complexes with a tetradentate phenanthroline-based PNNP ligand as a potent new metal-ligand cooperation platform

A series of low spin cobalt(i) complexes bearing a tetradentate phenanthroline-based PNNP ligand (2,9-bis((diphenylphosphanyl)methyl)-1,10-phenanthroline), [CoCl(PNNP)] (1), [CoMe(PNNP)] (2) and [Co(CH2SiMe3)(PNNP)] (3), were synthesized and structurally identified. Complex 3 underwent a structural rearrangement of the PNNP skeleton upon heating to form [Co(PNNP')] (4), which is supported by an asymmetrical PNNP' ligand with a dearomatized phenanthroline backbone. Mechanistic studies supported that the transformation from 3 to 4 was initiated by the homolysis of either a Co-CH2SiMe3 bond or a benzylic C-H bond. Complex 4 achieved H-H bond cleavage of H2 (1 atm) at ambient temperature, to form [Co(PNNP'')] (6), in which two H atoms were incorporated into the endocyclic double bond of the PNNP'' ligand backbone.