Metal‐Anion Coordination and Linker‐Anion Hydrogen Bonding in the Construction of Metal‐Organic Frameworks from Bipyrazole

Abstract

Two new two-dimensional frameworks based on packing of square grids, Cu(Me 4 bpz) 2 (NO 3 ) 2 (1) and Zn(Me 4 bpz)(SO 4 ) (2) (Me 4 bpz＝3,3',5,5'-tetramethyl-4,4'-bipyrazole), have been synthesized by mild solvothermal method. In addition to the major metal-organic linker coordination, they allow us to study how the metal-inorganic anion coordination and organic linker-inorganic anion hydrogen bonding affect the resulting structures during the framework assembly.

Synthetic Routes to Bioactive Bipyrazole Derivatives

“…The latter Me 4 bpz 229 was employed in the synthesis of two twodimensional metal-organic frameworks (MOF) structures [Cu (Me 4 bpz) 2 (NO 3 ) 2 ] and [Zn(Me 4 bpz) 2 SO 4 ] and the 1-D helical tubular cadmium coordination polymer [Cd(H 2 Me 4 bpz)(H 2 bidc) 2] n . [110][111][112] Regioselective cycloaddition reaction between 2,5-dinitro-2,4-hexadiene 230 and diazomethane proceeded smoothly under mild condition and gave 3,3'-dimethyl-3,3'-dinitro-4,4'bipyrazole 231 in 91 % yield. Aromatization of the bipyrazolines 231 readily underwent upon heating with sodium hydroxide in aqueous methanol to give 3,3'-dimethyl-4,4'-bipyrazole 232 in 79 % yield (Scheme 60).…”

“…3,3′,5,5′‐Tetramethyl‐4,4′‐bipyrazole 229 (Me 4 bpz) was prepared by treatment of the tetraketone 228 with N 2 H 4 .H 2 O in methanol/water at room temperature (Scheme 59). The latter Me 4 bpz 229 was employed in the synthesis of two two‐dimensional metal‐organic frameworks (MOF) structures [Cu(Me 4 bpz) 2 (NO 3 ) 2 ] and [Zn(Me 4 bpz) 2 SO 4 ] and the 1‐D helical tubular cadmium coordination polymer [Cd(H 2 Me 4 bpz)(H 2 bidc)2] n [110–112] …”

Metal‐4,4′‐azobis(3,5‐dimethyl‐1H‐pyrazole) Complexes with Seven‐ and Nine‐membered Hydrogen‐bonded Rings Originating from the Pyrrolic NH Function

“…In such metal‐coordinated pyrazoles the acidity of the pyrrolic NH function often increases, making it a better hydrogen‐bond donor . In this regard, especially seven‐membered hydrogen‐bonded rings around the central metal atom in pyrazole coordination compounds with coordinating counter‐balancing oxo‐anions like carboxylates, nitrate or sulfate as structural motifs in the so‐called second‐sphere coordination are frequently observed (Scheme ) , …”

“…The two pyrazolyl rings in bipyrazoles can rotate freely around the central C–C bond such that a metal‐bridging neutral bipyrazole can adopt a cis or trans coordination configuration, in the co‐planar ring limit, both with respect to the two metal or the two N–H hydrogen atoms (Scheme ). Even with an azo spacer between the two pyrazolyl rings and with only one coordinating metal atom in the monodentate ligand 4,4′‐azobis(3,5‐dimethyl‐1H‐pyrazole) (H 2 azbpz) we can still discern the cis and trans form for the planar ligand based on the relative position of the N–H atoms , , . In the H 2 azbpz ligand there are then two cis and trans forms, each, due to an additional different orientation of the bent azo spacer (Scheme ).…”

“…When such cis and trans forms are seen in bipyrazole coordination compounds hydrogen bonds have been identified to play an important role via the so‐called second sphere coordination to lock the conformers into place and to localize the NH group through these supramolecular assemblies . Referring to this concept, the anionic ring around the central metal atom seen in metal‐pyrazole‐coordination compounds was regarded as a possible strategy to observe the neutral, NH group versions of H 2 azbpz in a supramolecular matrix , , , . In addition the synergy of the hydrogen‐bond interactions affects the cis‐trans orientation of bipyrazoles in such complexes or coordination polymers.…”