Homoleptic rare earth dipyridylamides [Ln2(N(NC5H4)2)6], Ln = Ce, Nd, Sm, Ho, Er, Tm, Yb, and Sc:: Metal oxidation by the amine melt and in 1,2,3,4-tetrahydroquinoline with the focus of different metal activation by amalgams, liquid ammonia, and microwaves