Crystal structure and theoretical investigation of the configurations of 1,3‐dichloro‐1,3‐diazetidine‐2,4‐dione and 1,3‐bis(trimethylsilyl)‐1,3‐diazetidine‐2,4‐dione

Abstract

AbstractCrystal structures of 1,3‐dichloro‐1,3‐diazetidine‐2,4‐dione (1) and the hitherto unknown compound 1,3‐bis(trimethylsilyl)‐1,3‐diazetidine‐2,4‐dione (2) have been determined by X‐‐ray crystallography: 1: (CINCO)2, Mr = 154.94, T = 295 K, orthorhombic, Pbca, a = 7.699(1), b = 6.706(1), c = 10.598(2) Å, V = 547.2(2) Å3, Z = 4, dx = 1.881 g/cm3, μ = 10.9 cm−1, R = 3.14%, Rw = 2.82% (660 observations, 38 parameters). 2: [(CH3)3SiNCO]2, Mr = 230.41, T = 100 K, monoclinic, I2/a, a = 20.257(2), b = 6.416(1), c = 21.260(3) Å, β = 101.29(1)°, V = 2709.7(6) Å3, Z = 8, dx = 1.130 g/cm3, μ = 2.4 cm−1, R = 4.86%, Rw = 4.39% (2375 observations, 151 parameters). In both compounds, the symmetry of the (XNCO)2 framework (X = Cl, Si) was determined to be nearly C2h with trans configuration of the exocyclic X atoms. Extreme values were observed for the angles between the ring plane and the exocyclic N–X bonds: 32.5(1)° in 1 and 2.5(2)° and 0.8(2)° in 2, respectively. Quantum chemical procedures at various levels of theory (ab initio SCF and semi‐empirical PM3) applied to 1 revealed the possible appearance of two isomers, a lower energy trans form and a higher energy cis form (approx. 2.4 kcal/mol above trans) differing mainly in the spatial arrangement of the chlorine atoms. The calculations excluded a planar heavy‐atom configuration by missing a local energy minimum.