Metalation of Aryl-bis(3-alkyl-5-methylpyrazol-1-yl)- methane (Alkyl=Me, Ad; Aryl=Ph, C6H4-2 OH) with NaN(SiMe3)(2), KN(SiMe3)(2), and Ca{N(SiMe3)(2)}(2)

Deprotonation of phenyl-bis(3,5-dimethylpyrazolyl)methane (HC(Ph)(Pz(Me2))(2), 1a) with [(thf)2Ca{N(SiMe3)(2)}(2)] in THF at -40 degrees C yields heteroleptic [(thf)(2)Ca{N(SiMe3)(2)}{C(Ph)(Pz(Me2))(2)}] (2a) and finally homoleptic [(thf)Ca{C(Ph)(Pz(Me2))(2)}(2)] (2b). However, these complexes degrade at room temperature to calcium bis(pyrazolate) and 2-phenyl-4,6-dimethylpyrimidine (3a). This degradation is accelerated when bulky phenyl-bis(3-adamantyl-5-methylpyrazolyl)methane (HC(Ph)(Pz(Ad,Me))(2), 1b) is treated similarly and 2-phenyl-4-adamantyl-6-methylpyrimidine (3b) forms quickly. Sodiation of 1a and 1b with NaN(SiMe3)(2) yields [{HC(Ph)(Pz(Me2))(2)}Na{C(Ph)(Pz(Me2))(2)}] (4a) and [{HC(Ph)(Pz(Ad,Me))(2)}Na {C(Ph)(Pz(Ad,Me))(2)}] (4b). The stronger metalation reagent benzyl-potassium is able to quantitatively deprotonate 1b leading to dinuclear [K{C(Ph)(Pz(Ad,Me))(2)}](2) (4c). The metathetical approach and the reaction of 4c with metal(II) halides of Mg, Ca and Zn again leads to degradation and formation of pyrimidine 3b. This decomposition of the ligand can be circumvented when the sidearm contains the anionic charge. Thus, 2-hydroxyphenyl-bis(3-adamantyl-5-methylpyrazol-1-yl)methane (1c) reacts with [(thf)(2)Ca{N(SiMe3)(2)}(2)] in THF to the homoleptic complex [(thf)(4)Ca{O C6H4 CH(Pz(Ad,Me))(2)}] (2c). In this complex, THF and pyrazolyl bases compete in the coordination sphere and preparation of a heteroleptic complex fails. To avoid the binding competition between THF and pyrazolyl bases and to stabilize heteroleptic calcium complexes, the reaction has been repeated in toluene yielding [Ca{N(SiMe3)(2)}{O-C6H4-CH-(Pz(Ad,Me))(2)}] 2 (2d).

Automated summary

This study investigates the stability of calcium complexes with different ligands, showing that ligands with negative charge can prevent degradation reactions from occurring.