Silazanes

Silazanes are compounds that contain silicon–nitrogen (Si–N) bonds. They span a range from small molecules with Si–N–Si linkages to high-molecular-weight polymers known as polysilazanes. The silicon–nitrogen core can be substituted with various organic groups, giving a broad spectrum of reactivity and properties. Hexamethyldisilazane (HMDS), with the formula (CH3)3Si–NH–Si(CH3)3, is one of the most widely used silazanes and serves as a silylating agent and a ceramic-precursor stock.

Structural varieties include disilazanes (two silicon centers linked by Si–N bonds), cyclic silazanes, and linear or branched polysilazanes. These materials can form extended Si–N networks and are employed as precursors to silicon-based ceramics. The degree of condensation and the nature of substituents on silicon and nitrogen influence volatility, hydrolytic stability, and reactivity.

Synthesis generally involves the condensation of chlorosilanes with ammonia or amines, or redistribution of silazane units, to establish Si–N linkages. Many silazanes are moisture sensitive and hydrolyze to silanols and ammonia upon exposure to water. They can undergo dehydrogenative coupling to build higher molecular weight species and may be crosslinked or cured to form resin-like networks.

Applications include surface modification and protection, where HMDS is used to render surfaces hydrophobic and to improve adhesion in electronics and optics. Polyorganosilazanes are important ceramic precursors: upon pyrolysis in nitrogen, they yield silicon nitride and related ceramic materials. Safety considerations include ammonia evolution on hydrolysis and flammability of some silazane formulations.