Cyano-substituted polypyrazolylborate metal complexes
Polypyrazolylborates have become one of the most popular ligands used in inorganic and organometallic chemistry. Because of the electron-withdrawing and potential coordination property of the cyano group, as well as the steric effect of bulky substituents on metal complexes, we were trying to synthesize new trispyrazolylborates carrying both these substituents, which will potentially produce molecular materials with desired electronic and magnetic properties. Successful syntheses of TpPh,4CN, Tpt-Bu,4CN and TpMe2,4CN are reported here. Thallium salts of TpPh,4CN and Tpt-Bu,4CN were made for structure characterization. Steric effects of these bulky groups and short contacts between CN substituents and metal ion were observed. The instability of the TpMe2,4CN ligand was detected. Attempts were also made to produce some heteroscorpionates. Although the desired product was detected, final yields were too random to control. Manganese, iron, cobalt and copper complexes of these new ligands were synthesized and studied by X-ray crystallography. Ligand isomerization was revealed in (TpPh,4CN)*2Co, (TpPh,4CN)*2Mn and (TpPh,4CN)*2Fe complexes, possibly due to reaction involving solvent or oxygen. The rearranged pyrazole in the ligand has a much shorter metal-N bond thus reducing the symmetry around the metal ion. Both copper(I) and copper(II) complexes of TpPh,4CN and Tpt-Bu,4CN ligands were synthesized. The cyano substituents in some of these complexes show strong electron-withdrawing property by reducing the electron density on the pyrazole rings and the metal center. X-ray crystallography shows that [TpPh,4CNCu]n and [Tpt-Bu,4CNCu]n have 1-dimension polymer structures with one CN substituent coordinate to the neighboring metal. They represent the first crystallographically characterized examples of this kind of cyano bridged polymers. Some metal pyrazole complexes were also isolated due to the decomposition of the ligand. CN-metal contacts were revealed in {[(HpzPh,4CN)2Cu(CH3CN)2][ClO4]2}n and (Hpzt-Bu,CN)2CuCl2, reaffirming the coordination potential of the cyano substituent. Attempts were made to construct heterogeneous cyano bridging coordination polymers. Mass spectroscopy demonstrates the formation of a dinuclear copper species although the only isolated crystalline product was TpPhCu(HpzPh,4CN)(NO3). Rh2(CF3COO)4 or Ni(cyclam)(ClO4)2 were used as one building block of the coordination polymer, reacting with TlTpR,4CN. Crystals isolated from these products, instead of the coordination polymers we desired, are metal pyrazole complexes with the ring N atom coordination, suggesting decomposition of the ligand.
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