A series of bis-[1,3,5]triazinyl diazenes and two metal complexes thereof were synthesized and tested as new flame retardants in polypropylene films. It was observed that electron withdrawing triazinyl ring substituent (i.e., Cl and phenyl moieties) improved the thermal stability of diazene compounds, whereas electron donating groups (i.e., methoxy, dimethylamino, and ethylthio moieties) had a destabilizing effect. TGA, DTA/DSC, NMR, and ATR-FTIR were used for the characterization. Quantum mechanical modeling (Gaussian 09) was also utilized to facilitate the interpretation of the NMR data. Bis-(4,6-dichloro-[1,3,5]triazin-2-yl)-diazene 1 and bis-(4,6-dimethoxy-[1,3,5]triazin-2-yl)-diazene 2 were found to be effective flame retardants in polypropylene films and DIN4102-1 B2 classification was reached already at a loading of 0.5 wt %. Interestingly, the copper complex of 2 showed even higher fire retardant activity than its precursor 2, whereas the corresponding zinc complex exhibited lower flame retardant efficacy than its precursor.