Trimeric silver(I) pyrazolates with isopropyl, bromo, and nitro substituents:synthesis and characterization of {[3,5-(i-Pr)2Pz]Ag}3, {[3,5-(i-Pr)2,4-(Br)Pz]Ag}3, and {[3,5-(i-Pr)2,4-(NO2)Pz]Ag}3