Coordination “Oligomers” in Self-Assembly Reactions of Some “Tritopic” Picolinic Dihydrazone LigandsMononuclear, Dinuclear, Hexanuclear, Heptanuclear, and Nonanuclear Examples

Abstract

"Tritopic" picolinic dihydrazone ligands with tridentate coordination pockets are designed to produce homoleptic [3 x 3] nonanuclear square grid complexes on reaction with transition-metal salts, and many structurally documented examples have been obtained with Mn(II), Cu(II), and Zn(II) ions. However, other oligomeric complexes with smaller nuclearities have also been discovered and identified structurally in some reactions involving Fe(II), Co(II), Ni(II), and Cu(II), with certain tritopic ligands. This illustrates the dynamic nature of the metal-ligand interaction and the conformationally flexible nature of the ligands and points to the possible involvement of some of these species as intermediates in the [3 x 3] grid formation process. Examples of mononuclear, dinuclear, hexanuclear, heptanuclear, and nonanuclear species involving Fe(II), Co(II), Ni(II), and Cu(II) salts with a series of potentially heptadentate picolinic dihydrazone ligands with pyrazine, pyrimidine, and pyridine end groups are described in the present study. Iron and cobalt complexation reactions are complicated by redox processes, which lead to mixed-oxidation-state Co(II)/Co(III) systems when starting with Co(II) salts, and reduction of Fe(III) to Fe(II) when starting with Fe(III). Magnetic exchange within the polynuclear structural frameworks is discussed and related to the structural features.

A Spin‐Crossover Cluster of Iron(II) Exhibiting a Mixed‐Spin Structure and Synergy between Spin Transition and Magnetic Interaction

“…[30] With decreasing temperature, the chiral space group P2 1 2 1 2 1 is unchanged and the cell dimensions decrease slightly. Investigation of the other crystals in the same batch revealed that the compound is composed of a racemic mixture of opposite handedness.…”

“…Neighboring iron ions are bridged by the single oxygen atoms of the ligand "edges" with M···M separations of approximately 3.8 and M-O-M angles of about 1308, facilitating the antiferromagnetic superexchange pathway. [31] On the other hand, since the coordination bond lengths around the Fe II centers differ for low-spin (LS) and high-spin (HS) states (Dd = 0.14-0.24 ), [32] LS and HS states can be identified by the metal-ligand bond length. In fact, at 123 K, the average FeÀN coordination bond lengths involving 2 , two possible spin geometries may exist, namely, HS species exist either on the same side (a cis structure) or on the diagonal (trans geometry) of the square.…”

Liganddirected self-assembly of polymetallic [n × n] grids: rational routes to large functional molecular subunits?

“…It effectively competes with nickel(II) in the sense that it occupies the central cavity with tight hydrogen bonding interactions to six ligand NH protons. 32 The reaction could in fact be regarded as an anion templated dinuclear assembly. Interestingly, in a similar reaction of 2popp with Fe(III) in the presence of PF 6 − , the same type of fluoride centered dinuclear structure results, but is also accompanied by reduction of Fe(III) to Fe(II).…”

“…Interestingly, in a similar reaction of 2popp with Fe(III) in the presence of PF 6 − , the same type of fluoride centered dinuclear structure results, but is also accompanied by reduction of Fe(III) to Fe(II). 32 Higher order grid based oligomers have also been observed, and may represent logical intermediates en route to the assembly of a full [3 × 3] grid. [Ni 6 (Cl2poap) 5 ][Ni(H 2 O) 6 ](BF 4 ) 4 is observed in the same reaction as [Ni 2 (Cl2poap) 3 F](BF 4 ) 3 ·7.5H 2 O.…”

A 1D zinc(II) polymer with W-like pentanuclear secondary building blocks constructed by 2,6-pyridine-diacylhydrazone ligand

“…Indeed, many polynuclear complexes of 2,6-pyridinediacylhydrazone have been reported [21][22][23][24][25][26][27][28][29]. It is interest to see that most of which are trinuclear complexes [2,30,31] and nonanuclear clusters with [3 9 3] grid-type architectures [23,[32][33][34][35][36][37][38][39][40].…”