Difference in the Reactivities of H- and Me-Substituted Dinucleating Bis(iminopyridine) Ligands with Nickel(0)

The react iv i ty of dinucleating bis(iminopyridine) ligands bearing H (L, (N,N′)-1,1′-(1,4phenylene)bis(N-(pyridin-2-ylmethylene)methanamine)) or Me substituents (L, (N,N′)-1,1′-(1,4-phenylene)bis(N-(1(pyridin-2-yl)ethylidene)methanamine)) on the imine carbon atom with Ni(COD)2 (COD = 1,5-cyclooctadiene) has been investigated. Treatment of L with 2 equiv of Ni(COD)2 forms dinuclear Ni2(L )(COD)2, whereas the reaction of L 2 with 2 equiv of Ni(COD)2 leads to Ni2(L )2, along with 1 equiv of Ni(COD)2. The compounds were characterized by H and C NMR spectroscopy, mass spectrometry, and elemental analysis; the structure of Ni2(L )2 was determined by XRD. Ni2(L )2 exists as syn and anti stereoisomers in the solid state and in solution. DFT calculations suggest Ni(I) for both Ni2(L )(COD)2 and Ni2(L )2, with the radical anion localized on one iminopyridine fragment in Ni2(L )(COD)2 and delocalized over two iminopyridine fragments in Ni2(L )2. Both Ni2(L )(COD)2 and Ni2(L )2 undergo a reaction with excess diphenylacetylene, forming diphenylacetylene complexes. However, whereas Ni2(L )(diphenylacetylene)2 decomposes upon removal of the excess diphenylacetylene, Ni2(L )2 demonstrates a reversible disassembly/reassembly sequence upon the addition/removal of diphenylacetylene. R1 ligands play an active role in redox transformations mediated by a metal−ligand system. Delineation of the factors that control the reactivity of the redox-active ligand systems is important for their application in catalysis. Iminopyridines have been recently demonstrated to possess noninnocent ligand character, closely related to that of the much studied α-diimines, bis(imino)pyridines, and bipyridines. The majority of the iminopyridine systems investigated so far had an H substituent on the imine carbon atom. Herein, we demonstrate that the nature of the substituent can have a profound impact on the reactivity of these ligands with reduced metal centers. We are targeting dinucleating redox-active ligand platforms for cooperative dinuclear and multinuclear catalysis. As our first goal, we aim to develop the chemistry of the dinucleating bis(iminopyridine) ligands, depicted in Figure 1, that are capable of binding reduced metal centers. As part of the ligand design, we decided to investigate two different substituents attached to the imine carbon atom: H (L) and Me (L). L and L are flexible ligands, enabling syn and anti relative orientations of the iminopyridine chelating units. L has been previously shown to form both dinuclear complexes and metallosupramolecular systems, depending on the reaction conditions; L has not been synthesized before. L and L were obtained by condensation of 1,4-xylylenediamine with carboxypyridine (L) or acetylpyridine (L) and were isolated as crystalline solids from MeOH, in 81% (L) and 76% (L) yields. Next, we targeted dinuclear Ni species. Treatment of 2 equiv of Ni(COD)2 with L 1 forms blue-violet 1a, isolated in 57% yield (Figure 2). The H NMR spectrum of 1a is consistent with the expected Ni2(L )(COD)2 formulation. Most characteristically, COD signals are observed at 3.8, 2.7, and 1.7 ppm, supporting its rigid binding to the metal (Figure S5, Supporting Information). Furthermore, the asymmetric nature of the iminopyridine ligand leads to two sets of signals for the alkene CH (around 3.8 ppm) and for one of the methylene (2.7 ppm) protons. The C NMR is consistent with these observations (Figure S6), displaying two sets of CH (82.8 and 81.9 ppm) and CH2 (31.4, 31.3 ppm) signals. The methylene (NCH2Ph) protons of the ligand backbone appear as a sharp singlet (5.26 ppm), consistent with a flexible behavior of 1a. Further support for the formation of 1a was obtained by mass spectrometry. The compound displays limited stability under vacuum or in solution for prolonged Received: January 26, 2012 Published: March 8, 2012 Figure 1. Syn and anti conformations of the bis(iminopyridine) ligands L and L. Communication pubs.acs.org/Organometallics © 2012 American Chemical Society 2120 dx.doi.org/10.1021/om300067z | Organometallics 2012, 31, 2120−2123 periods of time, forming an insoluble brown material. As a result, our multiple attempts to obtain its crystal structure proved unsuccessful. The reaction of L with 2 equiv of Ni(COD)2 took a different path. Under identical reaction conditions, purple product 2 was formed, along with an equimolar amount of Ni(COD)2. The H NMR spectrum of 2 in C6D6 at room temperature exhibits two sets of resonances attributable to two different species (2b,c) in a ca. 2:1 ratio and no signals attributable to COD (Figure S7). The imino Me groups gave rise to two peaks at −0.3 and −0.5 ppm. This unusual chemical shift for a Me group is consistent with a Me group in the vicinity of a radical anion, indicating noninnocent behavior of L upon coordination to the reduced Ni centers. The presence of two different structural isomers (2b,c) was confirmed by an X-ray structure determination (Figure 3). Two isomers are present in the asymmetric unit in a 2:1 ratio (nearly identical with the NMR ratio). The prevalent isomer, 2b, features an anti conformation of the chelating iminopyridine units in L, whereas 2c has a syn conformation of the iminopyridine units. L has been reported to feature either syn or a mixture of syn and anti isomers in supramolecular assemblies. Different isomers have been proposed to be stabilized by noncovalent packing interactions. Herein, we show that discrete, molecular species can display similar isomerism both in solution (H NMR) and in the solid state. The average imine C−C (1.43 Å) and C−N (1.33 Å) bonds are similar between the structures and are intermediate between those of singly reduced (1.41 Å and 1.34 Å) and neutral (1.47 Å and 1.28 Å) iminopyridine. Dihedral angles between Ni−N−N planes are similar for the anti (2b) and syn (2c) structures (51°). We note that a dinucleating bis(iminopyridine) ligand with a shorter linker, N,N′-bis(6-methyl-2-pyridylmethylene)ethane-1,2-diamine, has been shown to form di-Ni complexes featuring the anti geometry exclusively. We monitored both transformations by H NMR spectroscopy in toluene-d8 at 23 °C (Supporting Information). For L , only reactants (L and Ni(COD)2) and products (1a and COD) are observed. The formation of 1a is fast: it is almost complete within several minutes (Figure S22). No traces of the 1b or 1c dimer (see below) were detected during the time the reaction was monitored by H NMR (ca. 6 h). In contrast, the reaction of L with Ni(COD)2 is slow: signals of free L 2 were present in the spectrum after ca. 2 h (δ 4.59 ppm, Figure S19). Furthermore, signals attributable to Ni2(L )(COD)2 were observed in the reaction of L with 2 equiv of Ni(COD)2. Initially, Ni2(L )(COD)2 was observed as a predominant product, but as the reaction progressed the concentration of Ni2(L )(COD)2 decreased and the concentration of Ni2(L )2 increased. After 5 h, Ni2(L )(COD)2 constituted <10% of Ni2(L )2 in the reaction mixture. These concentration profiles suggest that Ni2(L )(COD)2 is an intermediate in the formation of Ni2(L )2. (see eq 1).