Syntheses, Crystal Structures, and Thermal Stabilities of Polymorphs of Cr(thd)3

The syntheses, crystal structures, and thermal stabilities of six polymorphs of Cr(thd)3 [(thd)– = anion of H(thd) = C11H20O2 = 2,2,6,6-tetramethylheptane-3,5-dione] were studied using X-ray diffraction and differential scanning calorimetry. Compound 1 is thermodynamically stable below ca. 110 °C and forms plate-shaped crystals [a = 9.927(5), b = 18.010(5), c = 21.427(5) Å, and β = 95.461(5)° at 295 K; space group C2/c]. Its crystal structure is isotypic with Mn(thd)3. Polymorph 4 is metastable to 1, and the only way to prepare 4 appears to be by precipitation from solution. Crystals of 4 are needle shaped [a = 28.54(3), b = 19.14(2), c = 21.92(2) Å, and β = 97.31(2)° at 295 K; space group C2/c] and this structure is isotypic with monoclinic Co(thd)3. Modifications 2, 2*, and 5 also appear as needle-shaped crystals [orthorhombic: a = 18.97(7), b = 18.69(7), and c = 10.62(5) Å for 2 and tetragonal: a = b = 18.93(5) and c = 10.57(4) Å for 5, both at 110 K]. Crystals of 2 and 5 have limited lifetime, which depends on storage temperature and exposure to X-rays. The conversion sequence is: 5 → 2 → 1, where the first step takes 3 to 4 h and the second some 20 h. Attempts to establish detailed accounts of the orthorhombic Co(thd)3-type structure of 2 and 2* by SXD were unsuccessful due to disorder. The structural distinction between 2 and 2* appears to be associated with rotational disorder among the Cr(thd)3 molecules. 2* and 3 are high-temperature modifications with stability range ca. 155–200 and ca. 200–235 °C, respectively [Cr(thd)3 melts at ca. 235 °C]. Quenched 2* can be retained as metastable at room temp. for some weeks, whereas 3 is not quenchable. High-temperature PXD data show that 3 is cubic [a = 13.357(4) Å at 225 °C; with strong rotational disorder of the molecules]. There are only minor variations in bond lengths and angles between 1 and 4 or from 100 to 295 K, and disorder is limited.