Synthesis of molecular adducts of beryllium, boron, and gallium cyanides: Theoretical and experimental correlations between solid-state and molecular analogues

Metal cyanide framework materials with stoichiometries M(CN) ₂ and M′(CN) ₃ represent an intriguing family of inclusion compounds with technological potential in materials science and energy storage applications. In this paper we develop fundamental new insights by comparing, experimentally and theoretically, the structure and bonding trends in several molecular and solid-state main group compounds containing the same basic M-C≡N "building blocks". In particular we describe for the first time the synthesis and structural characterization of molecular analogues of the Be(CN) ₂ and Ga(CN) ₃ frameworks such as Be(CN) ₂(NC ₅H ₅) ₂ and Ga(CN) ₃(NC ₅H ₅) ₂, which represent prototypical examples of simple binary cyanides of the main group element class. We also describe the formation of closely related analogues of boron such as B(CN) ₃·NC ₅H ₅ and B(CN) ₄·HNC ₅H ₅ and report their molecular crystal structures. Complementary density functional theory simulations are then used to elucidate: (i) the origin of the structural differences between the Ga(CN) ₃(NC ₅H ₅) ₂ and the corresponding Ga(CN) ₃ framework solid, (ii) bonding and energetic trends in the M(CN) ₃(NC ₅H ₅) ₂ series of molecules (M = Al, Ga, In), (iii) deviations from idealized structure in the M - CN - M units within the framework solids, and (iv) bond distributions in orientationally disordered framework solids.