Sodium and Potassium Salts of Mono‐ and Dianionic α‐Iminopyridines

Abstract

a-Diimine (1,4-diaza-1,3-butadiene) ligands are well established in main-group, transition-metal, and lanthanide chemistry. Beside their application as ligands for latetransition-metal complexes, which can function as active and selective catalysts for a-olefin polymerization, a-diimine and related ligands are of special interest due to their ability to undergo redox chemistry. They have frequently been described as “non-innocent” ligands. For example, in lanthanide chemistry, it has been found that the formal oxidation state of Eu and Yb depends on the nature of the substituents on the a-diimine ligand and on the ligand on the metal center. Thus, in both cases, the a-diimine acts either as neutral ligand or as radical anion. Reduction of adiimines, using either alkali metals or electrochemical methods, leads in each case to the corresponding radical anions and eventually to diamagnetic dianions, albeit at much more negative potentials. In contrast to the well-established chemistry of a-diimines, the related a-iminopyridines are much less investigated. The three different redox states of a-iminopyridines are shown in Scheme 1. The neutral system L has been extensively used as ligand for late transition metals. Only recently, the monoanion LC and the doubly reduced dianion L were investigated as ligands in more detail. Recent reports showed a series of bis(a-iminopyridine)–metal complexes featuring aluminum and the first-row transition ions (Cr, Mn, Fe, Co, Ni, and Zn) with the monoanionic, and thus paramagnetic p-radical form of the ligand in coordination complexes. The a-iminopyridine N-2,6-diisopropylphenylimino-2-pyridine (IPy) has also been introduced in lanthanide chemistry. In this context, a potassium derivative of the monoanionic form of a-iminopyridine was prepared in situ, but not further characterized. The doubly reduced dianion has only been observed in two magnesium complexes so far. In contrast, the reaction of “GaI” with a-iminopyridine leads to reductive coupling of the coordinated ligand to give neutral diamido-digallium complexes. Based on this information, we were interested in a comprehensive study of the properties of the alkali metal salts of the monoanionic p-radical and the doubly reduced dianion of a-iminopyridine in solution and in the solid state. We report here the synthesis, characterization and magnetic properties of sodium and potassium salts of the monoand dianionic a-iminopyridines. N-2,6-Diisopropylphenylimino-2-pyridine (IPy) was prepared according to literature procedures. As a first entry point, we examined the electrochemical properties of IPy using cyclic voltammetry at room temperature in THF. As can be seen from Figure 1, we detected a quasi-reversible one-electron reduction process centered at E1=2= 2.57 V (vs. the ferrocene/ferrocenium couple, Fc/Fc), with a peakto-peak separation of 158 mV and ipa/ipc 1. It is reasonable to assume that this reduction process furnishes the radical anion LC . In addition, we observed two irreversible reduction processes at more negative values of Epc= 3.12 and 3.44 V, respectively. Based on these data we speculated that reactions of IPy with alkali metals should yield the radical anion LC and eventually the dianion L . Indeed, the reaction of IPy with one equivalent of metallic potassium in THF or diethyl ether resulted in the formation of dimeric, p-radical monoanionic potassium salts [{K ACHTUNGTRENNUNG(thf)2IPy}2] (1) and [{KACHTUNGTRENNUNG(Et2O)IPy}2] (2), respectively (Scheme 2). As evidenced by NMR spectroscopy, an excess of potassium surprisingly does [a] Dr. H. P. Nayek, Dipl.-Chem. N. Arleth, Dipl.-Chem. I. Trapp, Dipl.-Chem. M. Lcble, Dr. P. OCa-Burgos, M. Sc. M. Kuzdrowska, Dr. Y. Lan, Prof. Dr. A. K. Powell, Prof. Dr. F. Breher, Prof. Dr. P. W. Roesky Institut f r Anorganische Chemie Karlsruhe Institute of Technology (KIT) Engesserstrasse 15, 76131 Karlsruhe (Germany) Fax: (+49)721-6084-4854 E-mail : breher@kit.edu roesky@kit.edu [b] Dr. H. P. Nayek Present address: Department of Applied Chemistry Indian School of Mines, Dhanbad-826004 (India) [c] M. Sc. M. Kuzdrowska Helmholtz Research School Energy-Related Catalysis Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201101646. Scheme 1. Different redox state of a-iminopyridines.