The Ability of the α,α′-Diiminopyridine Ligand System to Accept Negative Charge: Isolation of Paramagnetic and Diamagnetic Trianions

Abstract

Recent work, both in this research group and elsewhere, has shown that the a,a'-diiminopyridine ligand [a,a'-{2,6-(iPr) 2 PhN¼C(Me)} 2 (C 5 H 3 N)], well known for its ability to form highly active polymerization catalysts, [1] can be involved in the organometallic transformations of metal centers. Alkylating agents may attack not only the imino function [2] but also all of the pyridine ring carbon atoms [3,4] and even the nitrogen atom. [5] In addition, dimerization may be achieved by imine reductive coupling [4] or by the cycloaddition of two pyridine rings to form a tricyclic system. [4] In parallel with all of these transformations, either one or both of the CH 3 groups attached to the imine functions may be partly deprotonated. [4,5] In this unusual variety of transformation, the metal center coordinated to the ligand may engage in redox reactions and may either lower or even increase its oxidation state. [3,6, 7] This behavior illustrates the unique ability of this ligand system to a) accept negative charge with preferential spin-density localization on the imine groups and on the pyridine N and C para atoms, and b) to engage in internal redox processes with the coordinated metal. The ability of the ligand to accept or to donate negative charge to the metal is paramount to the fine-tuning of the redox potential of the metal, and of its Lewis acidity which, in turn, determines the catalytic behavior of the metal complex.Given the above scenario, we became interested in clarifying the ability of this remarkable ligand system to accept negative charge. For this purpose, we have carried out the reduction of [a,a'-{2,6-(iPr) 2 PhN ¼ C(Me)} 2 (C 5 H 3 N)] with strong reductants such as Li and [Li(naphthalenide)], in the absence of transition metals. Herein we describe our findings.The reactions were carried out by treating a solution of [a,a'-{2,6-(iPr) 2 PhN¼C(Me)} 2 (C 5 H 3 N)] with either metallic Li under argon or [Li(naphthalenide)] under nitrogen in THF. Regardless of the stoichiometric ratio, the reduction with Li afforded a mixture of [a,a'-{2,6-(iPr) 2 PhN ¼ C(Me)} 2 -(C 5 H 3 N)][Li(thf)] 3 (1) and [{a-[2,6-(iPr) 2 PhN ¼ C(Me)]}-{a'-[2,6-(iPr) 2 PhN-C(¼CH 2 )]}C 5 H 3 N][Li(thf)] 2 [Li(thf) 2 ](2) through a readily reproducible process. Both species contain a trianionic ligand (Scheme 1). Separation of the two extremely air-sensitive species was possible because of their relative solubilities in hexane. However, we observed that reactions carried out at low temperature with 3 equiv of [Li(naphthalenide)] afforded 2 as the only isolated compound (67 %) and no evidence for the presence of 1.The connectivity of 1 was elucidated by an X-ray crystal structure (Figure 1). The complex contains the intact ligand, which adopts the usual chelating tridentate conformation ZUSCHRIFTEN

From Cyclic Iminophosphoranes to π‐Conjugated Materials

“…As expected, the computed HOMO and LUMO for 2 extend across the whole of the dication and are consistent with a quinoidal‐type structure, although only a small orbital component is located at the two formally cationic phosphorus centers as would be expected (Figure 2). For the HOMO–LUMO transition it is calculated that the electron density shifts from the central quinoidal core (P,N1,C6,C5,C4) to the imine moiety (C1,N2), which is consistent with previous reports of the reduction of iminopyridines 21. The HOMO–LUMO gap of 1.7 eV for 2 is considerably smaller than those reported for other N , N ′‐R 2 ‐dihydropyridines (e.g.…”

From Cyclic Iminophosphoranes to π‐Conjugated Materials

“…[6,20] The computed absorption maxima (l max = 761 nm, e = 20 000 dm 3 mol À1 cm À1 ) correspond well with those determined experimentally (see below), with the band at 761 nm being comprised largely of a transition from the HOMO to the LUMO (73 %). [21] The HOMO-LUMO gap of 1.7 eV for 2 is considerably smaller than those reported for other N,N'-R 2dihydropyridines (e.g. For the HOMO-LUMO transition it is calculated that the electron density shifts from the central quinoidal core (P,N1,C6,C5,C4) to the imine moiety (C1,N2), which is consistent with previous reports of the reduction of iminopyridines.…”

“…For the HOMO-LUMO transition it is calculated that the electron density shifts from the central quinoidal core (P,N1,C6,C5,C4) to the imine moiety (C1,N2), which is consistent with previous reports of the reduction of iminopyridines. [21] The HOMO-LUMO gap of 1.7 eV for 2 is considerably smaller than those reported for other N,N'-R 2dihydropyridines (e.g. R = Me, 3.0 eV; [22] R = Ph, [23] 2.7 eV; R = B(mesityl) 2 , 2.6 eV [24] ), consistent with the p-conjugation extending across the two imino groups as well as the bipyridylidene moiety.…”

The Role of 2,6‐Diaminopyridine Ligands in the Isolation of an Unprecedented, Low‐Valent Tin Complex

“…A prominent donor molecule in this context is the DIMPY ligand (2,6‐[ArNC(H)] 2 (NC 5 H 3 ) (Ar=C 6 H 3 ‐2,6‐ i Pr 2 )), which was used to obtain, for example, a series of groundbreaking, neutral DIMPYFe complexes 43–47. Furthermore, highly reactive DIMPY main group element complexes, including Group 1,48 2,49 1350–52 14,53, 54 15,55–58 and 1659 compounds, were reported. Of these compounds, the DIMPYAs + cation58 and the [DIMPYCh 2+ ] dications59 (Ch=S 2+ , Se 2+ , and Te 2+ ) were isolated (Scheme ).…”