Hydroxylation of N-Heterocycle Ligands Observed in Two Unusual Mixed-Valence CuI/CuII Complexes

Abstract

Dedicated to Professor Thomas C. W. Mak on the occasion of his 65th birthdayThe chelate ligands 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), and their substituted derivatives have played an important role in the development of coordination chemistry, [1,2] and over the decades a number of so-called anomalies have been reported in reactions of their metal complexes. To rationalize the anomalous properties of complexes such as [M(bpy) 3 ] n and [M(phen) 3 ] n , Gillard borrowed the idea of ™covalent hydrates∫ from the organic literature and proposed an interesting mechanism, [3] the core of which is nucleophilic attack of a hydroxide ion on a coordinated heterocyclic ligand to form a covalent hydrate (CH); this may react further by intramolecular shift of OH À or H 2 O onto the metal before the formation of the final products (Scheme 1). The mechanism Scheme 1. Gillard mechanism for the hydroxylation of [L 2 M(phen)] 2 . PB pseudo-base.was questioned [4±8] because of to the lack of structural evidence [6,7] for the CH intermediate. [8] However, if the proposed covalent hydrates of phen and bpy could be isolated as hydroxylated products, they would be novel heterocyclic ligands capable of chelating and bridging ligation for the synthesis of polynuclear metal complexes. Therefore, the hydroxylation of phen and bpy is important both in organic heterocyclic chemistry and as a possible bridge to coordination chemistry.We report here on our observation of hydroxylated bpy and phen ligands in the two delocalized mixed-valence Cu I /Cu II complexes 1 and 2 (Hophen 2-hydroxy-1,10-phenanthroline, Hobpy 6-hydroxy-2,2'-bipyridine, tp terephthalate).[Cu 4 (obpy) 4 (tp)] 1[Cu 4 (ophen) 4 (tp)] 2Dark brown platelike crystals of 1 and 2 (ca. 50 % yield) were obtained by heating Cu(NO 3 ) 2 , phen or bpy, respectively, H 2 tp, NaOH, and water in the molar ratio 1:1.3:0.25:0.5:1100 at 160 8C for 144 h. Both 1 and 2 are air stable and insoluble in water and most organic solvents. Maintaining the initial pH at > 8 is critical to the formation of the products. The X-ray crystal structures obtained for 1 and 2 (Figure 1) reveal unequivocally that a hydroxy group is attached at the carbon atom adjacent to a nitrogen atom (C À O 1.286(7) ± 1.294(7) ä) of the heterocycle ligands. There are two crystallographically independent but chemically very similar copper Figure 1. X-ray crystal structures of 1 (a) and 2 (b) with thermal ellipsoids set at 35 % probability. Distances of interest are discussed in the text.atoms in 1 and 2, as both the bond lengths and bond angles for the two metal atoms are very similar. The two metal atoms approach closely with a CuÀCu separation of 2.415(2) ä in 1 and 2.443(2) ä in 2. The coordination geometry of each metal atom is roughly a square pyramid composed of a pair of nitrogen atoms (CuÀN 1.950(5) ± 2.096(5) ä) from an obpy or ophen ligand, a deprotonated hydroxy group (Cu À O 1.921(5) ± 1.943(4) ä) from another obpy or ophen, and the adjacent copper atom at the equatorial positions, and by a tp carbo...

Cu2+‐Mediated Dehydrogenative Coupling and Hydroxylation of an N‐Heterocyclic Ligand: From Generation of a New Tetratopic Ligand to the Designed Assembly of Three‐Dimensional Copper(I) Coordination Polymers

“…Recently, we have been investigating a few in situ metal/organic reactions such as ligand oxidative coupling, hydrolysis, and substitution by hydrothermal methods,3 which provide an intriguing pathway for generating new, functional building blocks and coordination polymers. In particular, we have isolated and structurally characterized covalent hydrates of N‐heterocyclic ligands in delocalized mixed‐valence Cu I –Cu II complexes, which are not only an important bridge between coordination chemistry and organic heterocyclic chemistry, but can also be successfully applied to the assembly of functional molecular solids 3a,b. Following our recent report on the in situ metal/ligand redox reaction, in this communication we report a new Cu 2+ ‐mediated dehydrogenative coupling and hydroxylation of an N‐heterocyclic ligand (chtpy ) in four Cu I coordination polymers, namely [Cu 2 (chtpy) 3 ](NO 3 ) 2 ( 1 ), [Cu 2 (chtpy) 2 ]Cl(NO 3 )⋅4/3 C 6 H 6 ( 2 ), [Cu 2 Cl 2 (chtpy)] ( 3 ), and [Cu 2 Cl(chtpy)](NO 3 ) ( 4 ; chtp y = a , a ‐1,4‐dihydroxy‐ e , e , e , e ‐1,2,4,5‐tetra(4‐pyridyl)cyclohexane).…”

Crystal structure of bis(μ2-4-carboxy-2-sulfonatobenzoato)- bis[aqua(2,2′-bipyridyl)copper(II)] hexahydrate, [Cu(H2O)(C10H8N2)(C8H4O7S)]2 · 6H2O

“…Terephthalate is an important rigid multicaboxylate ligand for construction of extended coordination solids owing to its high symmetry and coordination ability for abroad range of metal ions [1][2][3][4][5]. Introduction of asulfonate group into rigid multicarboxlate ligands may result in the formation of unexpected frameworks as the sulfonic group has adifferent shape and properties in terms of its coordination ability compared to the carboxylate group.…”

Syntheses, Crystal Structure and Fluorescence Property of Copper (II) Complex Constructed from 5-Sulfosalicylic Acid

“…Among the achievements which have been reported, organic ligands with both carboxylate and sulfonate groups are especially fascinating, because they can adopt a variety of coordination modes and result in diverse multidimensional architectures. As a result of the transition metal coordination polymers not only show a rich and interesting structure of the topology, but also in the hole material, light and magnetic aspects of the unique nature still attracted great interest [3][4][5].…”