Synthesis of Magnetic Fe
3
O
4
@polyethyleneimine.Mn(II) from Fe
3
O
4
, [3-(2,3-Epoxypropoxy)propyl]trimethoxysilane, Polyethyleneimine and Mn(II) Acetate as a Novel Silicon-Containing Polymeric Organic-Inorganic Hybrid Nanomaterial and Its Catalytic Investigation Towards the Oxidation of Cyclohexene, Ethyl Benzene and Toluene in the Presence of H
2
O
2
as an Oxidant