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) = C 11 H 20 O 2 = 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 C 2/ 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 C 2/ 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.