Syntheses, properties, and electrochemistry of transition-metal complexes of the macrocycle 1,4,7-tris(2-pyridylmethyl)-1,4,7-triazacyclononane (L). Crystal structures of [NiL](ClO4)2, [MnL](ClO4)2, and [PdL](PF6)2 containing a distorted-square-base-pyramidal PdIIN5 core