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

Abstract

[a] a-Diimine (1,4-diaza-1,3-butadiene) ligands are well established in main-group, transition-metal, and lanthanide chemistry. [1][2][3][4][5][6][7] Beside their application as ligands for latetransition-metal complexes, which can function as active and selective catalysts for a-olefin polymerization, [8] a-diimine and related ligands are of special interest due to their ability to undergo redox chemistry.[9] They have frequently been described as "non-innocent" ligands. [10][11][12] For example, in lanthanide chemistry, it has been found that the formal oxidation state of Eu [13] and Yb [14] 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 [15][16][17][18] and eventually to diamagnetic dianions, [17][18][19][20][21][22][23][24] 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 0 has been extensively used as ligand for late transition metals. [4,[25][26][27][28][29] Only recently, the monoanion LC À and the doubly reduced dianion L 2À were investigated as ligands in more detail. Recent reports showed a series of bis(a-iminopyridine)-metal complexes featuring aluminum [30] and the first-row transition ions (Cr, Mn, Fe, Co, Ni, and Zn) [31,32] 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. [33,34] In this context, a potassium derivative of the monoanionic form of a-iminopyridine was prepared in situ, but not further characterized.[34] The doubly reduced dianion has only been observed in two magnesium complexes so far. [35] In contrast, the reaction of "GaI" with a-iminopyridine leads to reductive coupling of the coordinated ligand to give neutral diamido-digallium complexes. [36] 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 mono-and dianionic a-iminopyridines.N-2,6-Diisopropylphenylimino-2-pyridine (IPy) was prepared according to literature procedures.[37] 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 E 0 1=2 = À2.57 V (vs. the ferrocene/ferrocenium couple, Fc/Fc + ), with a peakto-peak separation of 15...

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

“…The resonances of the in‐plane protons at the pyridine moiety and the HCN groups are similar to the signals observed in diamagnetic Sn II compound 3 and thus rule out both temperature‐dependent and ‐independent paramagnetic contributions to NMR spectroscopic shifts that would arise from orbital mixing with excited triplet states. The in‐plane 1 H NMR spectroscopic shifts observed in the diamagnetic compound [SIMPY]Na 2 are significantly shifted to high field as a consequence of charge delocalization to the pyridine ring, and are diagnostic of charge delocalization onto the ligand backbone 61. Compound 1 does not exhibit similarly shifted 1 H resonances, which indicates the absence of delocalization of negative charges on the DIMPY ligand in this case.…”

“…Compound 1 slowly decomposes in solution to give elemental Sn and DIMPY ligand and was additionally studied by solid‐state NMR methods. Unlike paramagnetic ligand‐centered radicals DIMPYAlMe 2 60 and [SIMPY] .− K + ,61 no resonances were observed in the ESR spectra for compound 1 in solution and the solid state at temperatures between 77 K and room temperature. The 1 H NMR spectrum of 1 displays a characteristic H CN resonance at δ =8.45 ppm that does not change with temperature.…”

“…The 1 H NMR spectrum of 1 displays a characteristic H CN resonance at δ =8.45 ppm that does not change with temperature. The 1 H and 13 C NMR spectroscopic resonances of 1 are similar to compound 3 and the free ligand, which is in sharp contrast to H CN resonances significantly shifted upfield in the ligand‐centered diamagnetic dianion [SIMPY]Na 2 61 . 1 H NMR spectroscopic data and results from 1 H‐coupled 119 Sn NMR spectra of 1 in solution also clearly rule out the existence of SnH bonds in 1 .…”

“…In addition to diamagnetic compounds, several paramagnetic iminopyridine‐based species, including DIMPYLi 3 ,48 DIMPYAlMe 2 ,60 and, more recently, [SIMPY] − K + (SIMPY=2‐[ArNC(H)](NC 5 H 3 ) (Ar=C 6 H 3 ‐2,6‐ i Pr 2 )),61 have been reported. Examples for Group 14 elements are the Sn II and Pb II complexes coordinated by a DIMPY pentadentate bishydrazine ligand62 and the [MeDIMPYSn II Cl] + cation stabilized by 2,6‐[ArNC(Me)] 2 (NC 5 H 3 ) (Ar=C 6 H 3 ‐2,6‐ i Pr 2 ) 54.…”

Anorganische Chemie 2011

“…Die Alkalimetallsalze (Na, K) des monoanionischen Iminopyridinradikals (3) sowie des dianionischen Iminopyridin‐Derivats von N‐2,6‐Di(iso‐propyl)phenylimino‐2‐pyridin lassen sich gezielt darstellen und zeigen ein interessantes Reaktionsverhalten. Das Radikalanion wurde vollständig charakterisiert und mit magnetischen Messungen, EPR‐Studien sowie quantenmechanischen Berechnungen untersucht 1_6.…”