The diiminopyridine (DIP) class of ligands are pyridine derivative with imine sidearms appended to the 2,6–positions. The three nitrogen centres can bind metals in a tridentate fashion, forming pincer complexes. Diiminopyridines are notable as non-innocent ligand that can assume more than one oxidation state. Complexes of DIPs participate in a range of chemical reactions, including ethylene polymerization, hydrosilylation, and hydrogenation.
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Synthesis and properties of DIP ligands
Many 2,6-diiminopyridines have been prepared. The dianilido ligands can be synthesized by Schiff base condensation of commercially available 2,6-diacetylpyridine with two equivalents of substituted anilines. Using substituted anilines, complexes one can obtain DIPs with diverse steric environments. Commonly used anilines are 2,4,6-trimethylaniline and 2,6-diisopropylaniline. Unsymmetric variations have been established by successive condensation of different anilines. The dicarbonyl portion of the backbone can be further modified, as with 2,6-dipyridecarboxaldehyde and 2,6-dibenzoylpyridine. Most commonly, variations in the DIP arise from changes in the anilines.
Effect of steric bulk
Depending on its steric bulk, DIP ligands form complexes of 2:1 and 1:1 ratios, M(DIP)Lx and M(DIP)2, respectively. The 2:1 complexes occur for unhindered DIP ligands. Although such complexes are coordinatively saturated, they have been studied for their electronic and structural properties. Formation of 2:1 complexation is suppressed with bulky DIP ligands. Complexes of the type M(DIP)Ln exhibit diverse reactivity.
Fe and Co complexes
The reduction of the Fe(II)bis(imino)pyridineX2 with sodium amalgam under nitrogen yields a square-pyramidal) bis(nitrogen) complex. This complex is a useful precursor to other derivatives by exchange of the dinitrogen ligands with H2 and CO, to give the monohydrogen or dicarbonyl complexes. Arylazides give imido complexs. Fe(DIP)(N2)2 is a precursor to highly active catalysts for hydrosilylation and hydrogenation reactions.
Dissociation of N2 from Fe(DIP)(N2)2 results in binding of the anilino arene in an η6-fashion. This binding mode may play a role in the catalytic hydrogenation cycle.
The reactivity of cobalt- and iron-DIP complexes are similar. Cobalt DIP complexes with azide ligands have been shown to lose N2 to give reactive nitrido complexes that undergo C-H activation of benzylic sites of the aryl substituents . The resulting cyclometalated amide adopt a roughly planar geometry.
Non-innocence of DIP complexes
The highly conjugated ligand framework of bis(imino)pyridine stabilizes metals in unusual oxidation states. The ability of the neutral complex to accept up to three electrons leads to ambiguity about the oxidation states of the metal center. The complex Fe(DIP)(N2)2 complex is ostensibly a 18e complex, consisting of Fe(0) with five 2-electron ligands. Mössbauer spectroscopy indicates, however, that this complex is better described as a ferrous derivative of DIP2−. This assignment is corroborated by the high frequency of the νNN vibration in the infrared spectrum, which is more consistent with Fe(II). Thus, reduction of Fe(DIP)Br2 is ligand-centered, not Fe-centered.
This non-innocent behavior allows iron-DIP complexes to participate in 2e redox reactions, which is a pattern more usually seen for complexes of platinum group metals.
Catalytic reactions of M-DIP complexes
The catalytic properties of DIP complexes of Fe, Co, and Ni have attracted much attention. In principle, catalyst derived from "base metals" are preferred to noble transition metal catalysis due to low environmental impact and cost effectiveness. Furthermore, owing to its modular synthesis, the DIP ligand is easily modifiable allowing diversity in ligand screening. and nickel(II) Complexes of the type M(DIP)Xn serve as precatalysts for ethylene polymerization. The precatalysts are activated by treatment with methylaluminoxane (MAO), which serves as a co-catalyst. Activities for 2,6-bis(imino)pyridine iron complexes are often comparable to or greater than group 4 metallocenes. The aryl substituents greatly affect the products. Small aryl substitutents allow for highly selective production of oligomeric α-olefins, whereas bulky groups provide strictly linear, high molecular weight polyethylene. Silica-supported and homogeneous catalysts have been reported.
Traditionally catalyzed by Pt and other precious metals, hydrosilylation is also catalyzed by Fe-DIP complexes. Reactions proceed under mild conditions and give exclusively anti-Markovnikov product. The catalysts exhibit some tolerance for functional group. Depending on the steric properties of the ligand, Fe-DIP complexes catalyze hydrogenation of terminal olefins.
Variations of DIP ligands
In N-heterocyclic carbene variations of the diiminopyridine complex, the pyridine or imine substituents is replaced with an NHC group. The aryl substituted bis(imino)NHC complexes produce tridentate ligands, while the pyridine exchanged NHC forms exclusively bidentate compexes. This is presumably due to the additional strain from the 5 member ring of the central carbene.