Iodomethane is a far more reactive electrophile than diiodomethane for bothsteric and electronic reasons. This reaction should be harder. But more importantly, I think the reaction you are trying should give a polymer. If you want monoalkylation, try with iodo-chloro alkyl. Even then, it will be hard to get only monoalkylated product as hydrazine is an amazing nucleophile...You can try out the reaction with mono-boc hydrazine to help out with polymerization issues too.

Thank you so much

hello I think the problem has been mentioned by my colleague, but sometimes it's better to say you want to produce so it's easier to help

  and if you want to use a polymer dibromo ethyl or propyl, anyway the diiodomethano is very reactive and should react to form a product not even want!

The goal is pretty clearly H2NNH-C(=S)S-(CH2)n-SC(=S)NHNH2.

The intermediate is the hydrazinium salt H2NN-C(=S)S- NH2NH3+

Secondary iodides (and CH2I2 is effectively a secondary iodide) evidently aren't reactive with this particular nucleophile. I would just try a better leaving group, like triflate, at least for the cases where n>1.

thanks James. would you suggest me any better electrophile instead of diiodo methane?

There's really not much to try, other than mesylates and triflates, so your case where n=1 is quite the challenge. You might try formaldehyde, with TMSCl (or TMS triflate) as an activator. Sort of far-fetched, but that's what grad students are for. :-)

CS2 is very non-polar, and doesn't help stabilize the transition state at all (nor does it help separate the ion pair that's tying up your nucleophile.) Perhaps adding a polar solvent will get things moving with CH2I2. THF, MeCN, even MeOH are worth a try.

If I use Diiodobenze instead of diodomethane , what can happen?? Will that react?

Unlikely - you're trying to do an SN2 displacement. On the other hand, o-, m- and p-xylylene dibromides, with two benzyl leaving groups, should react.