If produced, these materials would represent a class of diamond-like compounds wherein the electronic structure is tunable by adjusting the occupancy of electron-donating (or withdrawing) atoms within the cages ( 25). Some proposed carbon clathrates are expected to exhibit exceptional mechanical properties with tensile and shear strengths exceeding those of diamond ( 22), while large electron-phonon coupling is predicted to give rise to conventional superconductivity with high transition temperatures ( 23, 24). Attempts to synthesize carbon clathrates go back at least 50 years since they were postulated following the formation of inorganic silicon clathrates ( 18, 19), and their possible structures and properties are of longstanding interest ( 20, 21). Despite their prominence in other systems with tetrahedral coordination ( 15– 18), carbon-based clathrates are tremendously challenging to synthesize. As sp 3-bonded frameworks, carbon-based clathrates potentially represent strong and lightweight materials that also offer tunable properties through manipulation of the occupancy and type of guest atoms within the cages. Carbon-based clathrates are open-framework structures composed of host cages that trap guest atoms, in which all host atoms are linked by four-coordinate bonds. Compared with the exquisite synthetic control over porous COF materials, the experimental progress in denser sp 3 carbon-based structures lags far behind.Īnother impressive 3D sp 3 material is the carbon clathrate. Of broader interest are 3D covalent organic frameworks (COFs), which are formed by linking sp 2-hybridized molecular building blocks, that have attracted attention for gas storage and separations ( 11). Boron carbide also contains sp 3-hybridized carbon, but these atoms serve as dopants within or linkages between B icosahedra ( 9, 10), rather than establishing the overall structural framework. These materials all have several attractive properties for applications that include hardness, strength, thermal conductivity, and electron mobility. The limited number of 3D sp 3 carbon-based structures includes diamond, lonsdaleite (a hexagonal diamond allotrope) ( 5), B-doped diamond ( 6), SiC ( 7), and BC 2N ( 8). 2D materials (e.g., graphene and metal borocarbides) attracted attention due to remarkable properties potentially useful for advanced technology ( 1– 4). One-dimensional (1D) carbon-based materials (e.g., polymers) have thoroughly reshaped society over the past century, in addition to providing the building blocks for life. As a fundamental building block of nature, carbon is unrivaled in its diversity to form stable structures with other elements and itself.