Two Linear Undecanickel Mixed‐Valence Complexes: Increasing the Size and the Scope of the Electronic Properties of Nickel Metal Strings

Abstract

The importance of one-dimensional (1D) transition-metal complexes stems from their ability to provide a fundamental understanding of metal–metal interactions and electron transport along an extended metal-atom chain (EMAC), and from the perspective of taking advantage of their specific properties for potential applications, such as molecular metal wires and switches. A series of string complexes of oligo-apyridylamino ligands ranging from 3 to 9 core metal atoms has been synthesized and characterized by Cotton s group and our group. Attempts to characterize such very long EMACs with high electron conductivity were hindered by the synthetic difficulties rapidly increasing with the size of the metal chain. We synthesized [Ni9(m9-peptea)4Cl2] ten years ago, but all attempts to characterize a longer chain of Ni atoms have, to date, been unsuccessful, owing to very low yields and to the instability of the target compound, probably because of the high flexibility of large pyridylamino ligands. Recently we developed a new family of ligands by substituting rigid and potentially redox active naphthyridine (na) groups for the pyridine (py) rings. Naphthyridinemodulated ligands stabilize nickel ions in a low oxidation state, giving rise to mixed-valent [Ni2(napy)4] 3+ units (napy= naphthyridine). Using this strategy, we obtained a series of stable, low-oxidation-state-nickel string complexes combining mixed-valency, a property important in the development of novel electronic materials, with an enhanced electron mobility, which is able to increase the conductance of molecular metal wires. We report a new tetranaphthyridyltriamine ligand, N-(2(1,8-naphthyridin-7-ylamino)-1,8-naphthyridin-7-yl)-N-(1,8naphthy-ridin-2-yl)-1,8-naphthyridine-2,7-diamine (H3tentra) and two undecanickel complexes of the deprotonated tentra trianion, [Ni11(tentra)4Cl2](PF6)4 (1) and [Ni11(tentra)4(NCS)2](PF6)4 (2). The ligand H3tentra was synthesized on the basis of Buchwald s palladium-catalyzed procedures by the crosscoupling of bis(2-chloro-1,8-naphthyridin-7-yl)amine and 2amino-1,8-naphthyridine. Undecametallic complex [Ni11(tentra)4Cl2](PF6)4 (1) was obtained by the reaction of anhydrous NiCl2 with the H3tentra ligand in an argon atmosphere employing naphthalene as solvent and tBuOK as a base to deprotonate the amine groups. The thiocyanate species (2) was obtained from 1 by an axial ligand exchange reaction. The crystal structures of 1 and 2 are shown in Figure 1 and the Supporting Information Figure 1S, respectively. Both 1 and 2 are tetracationic molecules associated each with four PF6 counterions. They crystallize in unusually large cells, with one dimension exceeding 50 . The Ni11 chain of 1 and 2 is linear and wrapped in a helical manner by four tentra trianions. In both complexes, the atoms of the axial ligands are collinear with the Ni11 axis; the molecular lengths are 27.7 and 32.4 for 1 and 2, respectively. These are the longest EMAC complexes reported to date. The nature of the axial ligand does not significantly affect the metal–metal bond length, and no obvious structural change is observed for compound 2 with respect to 1. Therefore, we will only analyze the structure of 1 in detail. Selected bond lengths for 1 are displayed in Figure 1c together with the corresponding values obtained from geometry optimization at the DFT/B3LYP level. Molecule 1 consists of eleven nickel atoms in a linear chain with the Ni-Ni-Ni bond angles in the range of 179–1808. The N-Ni-Ni-N torsion angles for adjacent nickel are between 13.0 and 18.78, much smaller than those in oligo-a-pyridylamino ligand EMAC complexes (ca. 22.58). Metal–metal distances usually decrease from the end to the center of the chain in both nickel and cobalt EMACs of oligo-a-pyridyl[*] Dr. R. H. Ismayilov, Dr. W.-Z. Wang, Dr. G.-H. Lee, S.-A. Hua, Prof. Dr. S.-M. Peng Department of Chemistry, National Taiwan University 1, Sec. 4, Roosevelt Road, Taipei, 106 (Taiwan, ROC) Fax: (+886)2-8369-3765 E-mail: smpeng@ntu.edu.tw