The Effect of Environment on Molecular Structure: The Crystalline-State Stereochemistry of N-Methylated Adamantane 1,3-Diammonium Salts

The expected achiral C_2v-symmetry was observed by solution-state ¹H and ¹³C NMR spectroscopic measurements performed for each of the N-methylated 1,3-diaminoadamantane diammonium salts studied. 1,3-Diaminoadamantane dihydrochloride crystallized in the orthorhombic achiral system Pna2₁ and at 294(2) K: a = 10.910(1) Å, b = 15.515(2) Å, c = 7.246(1) Å, V = 1226.5(3) ų, and Z = 4. N,N,N′,N′-Tetramethyl-1,3-diaminoadamantane dihydrochloride gave monohydrate crystals in the monoclinic achiral system C2/c and at 173(2) K: a = 17.482(2) Å, b = 22.194(2) Å, c = 9.660(1) Å, β = 120.173(2)°, V = 3240.2(6) ų, and Z = 8. N,N,N,N′,N′,N′-Hexamethyl-1,3-diaminoadamantane diiodide afforded monohydrate crystals in the orthorhombic chiral system P2₁2₁2₁ and at 298(2) K: a = 9.6400(10) Å, b = 13.982(2) Å, c = 15.601(2) Å, V = 2102.8(5) ų, and Z = 4. 5,7,N,N,N,N′,N′,N′-Octamethyl-1,3-diaminoadamantane dibromide gave dihydrate crystals belonging to the triclinic achiral system P1̅ and at 294(2) K: a = 9.154(1) Å, b = 14.825(2) Å, c = 16.722(2) Å, α = 108.774(2)°, β = 90.969(2)°, γ = 90.791(2)°, V = 2147.8(5) ų, and Z = 4. All the diammonium dications in these crystals occupied “general positions of symmetry” within the crystal lattice, which resulted in their complete desymmetrization to C₁-symmetry. Their molecular geometries still exhibited some degree of “pseudosymmetry”, since the distortions were fairly small in magnitude. Continuous symmetry measures (CSM) were made to study the effect of the crystal lattice environment upon the distortion of the molecular structure. The CSM showed that the crystalline diammonium salts were either predominantly C₂ pseudosymmetric if the N-methylated ammonium units were skewed in the same directions, or predominantly C_s pseudosymmetric (σ_⊥) if these units were twisted in opposite directions. In the crystalline compounds studied in this series, there were no examples with predominantly pseudomirror symmetry via a σ_//-plane encompassing both nitrogen atoms, since this would have required a chemical (structural) constraint in favor a staggered arrangement for the C–N⁺–C–C–C–N⁺–C fragment that was not obligated by the local site asymmetric environments in the various crystals. The values of S(total), the total CSM distortion from solution-state ideal C_2v-symmetry exhibited by the general positioned dication moieties, were augmented with increasing N-methylation, while the relationships between the two external periphery subunits [R₁R₂R₃N(1) and N(2)R₁R₂R₃] became more distorted than those of the adamantane C₁₀ core fragments.