First example of a trimeric metalloporphyrin. Synthesis, electrochemical, and spectroelectrochemical studies of [(P)Th(OH)2]3 where P is the dianion of octaethyl- or tetraphenylporphyrin. Crystal structure of a dihydrated trinuclear complex of dihydroxy(5,10,15,20-tetraphenylporphinato)thorium(IV) h