The diiminepyridine ligand, made famous by Brookhart and Gibson a decade ago through its use in the first iron-containing olefin polymerization catalyst, also displays an amazing variety of ligand-centred reactions, including electron transfer, alkylation, dimerization and deprotonation. The present Perspective summarizes these reactions and also speculates on the links between ligand- and metal-centred reactivity.

The synthesis of homoleptic aryl derivatives of group 11 metals has been performed by reacting the monochlorides [CuCl], [AgCl], and [AuCOCl] with MesMgBr (Mes = 2,4,6-Me3C6H2). The resulting products [Cu6Mes5] (1), [Ag4Mes4] (2), and [Au6Mes5]-2THF (3) have been isolated as crystalline thermally stable compounds. They have, in the solid state, cyclic structures whose molecular geometries are discussed within. MW determination by cryoscopy in benzene and NMR spectra in both C6D6 and C6D12 showed that the three oligomeric structures were in equilibrium with the respective dimeric form of the molecule. In each case the dimeric form was the prevailing species in solution. The solution equilibrium was very rapidly established for the silver and gold complexes and was relatively slower for the copper complex. In the case of the copper complex the rate of equilibrium was solvent dependent, being much faster in toluene (tX/2 for CugMesg being ca. 0.66 h) than in cyclohexane (t1/2 being ca. 56 h). The addition of electron-rich arenes to cyclohexane solutions of 1 greatly enhanced the rate of conversion of the pentamer to dimer.

Reaction of {2,6-bis[2,6-(i-Pr)2PhNC(Me)]2(C5H3N)}VCl3·1.3(CH2Cl2) (1) with stoichiometric amount of methyl alumoxane (PMAO) in toluene resulted in the methylation of the pyridine ring ortho position affording {2,6-bis[2,6-(i-Pr)2PhNC(Me)]2(2-MeC5H3N)}VCl2·0.5 (toluene) (2). In the process the ligand became an anionic amide, one chlorine atom was eliminated by the metal center, and the vanadium coordination number decreased by one unit. This new trivalent compound is a potent ethylene polymerization precatalyst, and polymers produced by the complex activated with PMAO showed a bimodal character in the GPC. Its bimodality is tentatively explained with the existence of two catalytically active species (mono and dialkylation of the vanadium center). Further attack of a strong alkylating agent such as MeLi occurs at the pyridine ring to either remove the methyl group, or to place an additional methyl group on the ring meta position. Both processes imply two-electron reduction of the metal center and formation of the corresponding V(I) derivatives {2,6-bis[2,6-(i-Pr)2PhNC (Me)]2(C5H3N)}V(CH3)(μ-CH3)Li(Et2O)3 (3) and the ionic [{2,6-bis[2,6-(i-Pr)2PhNC(Me)]2 (2,3-Me2C5H3N)}V(CH3)2][Li(THF)2(TMEDA)2]·0.5(Et2O) (4) which were isolated in crystalline form. Crystal data are as follows. 1:  monoclinic, space group P2 1 /n, a = 18.267(5) Å, b = 16.643(4) Å, c = 38.195(9) Å, β = 96.061(5)°, Z = 12; 2:  monoclinic, space group P2 1 /c, a = 15.741(2) Å, b = 16.565(2) Å, c = 15.849(2) Å, β = 100.316(3)°, Z = 4; 3:  orthorhombic space group Pnma, a = 19.724(6) Å, b = 22.001(7) Å, c = 11.035(3) Å, Z = 4; 4:  monoclinic space group P2 1 /n, a = 11.932(3) Å, b = 25.363(6) Å, c = 18.957(4) Å, β = 96.587(4)°, Z = 4.

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