Synthesis of (eta^5-cyclopentadienyl)-1-(4-benzyloxycarbonyl-3,4-dihydroquinoxalin-2-yl)ethene-1,2-dithiolatocobalt(III) and (eta^5-cyclopentadienyl)-1-[2-(N,N-dimethylaminomethyleneamino)-3-methyl-4-oxo-pteridin-6-yl]ethene-1,2-dithiolatocobalt(III)