Polymorphism in Coordination Chemistry. Selective Syntheses and ab Initio X-ray Powder Diffraction Characterization of Two New Crystalline Phases of Solid [Pd(dmpz)2(Hdmpz)2]2 (Hdmpz = 3,5-Dimethylpyrazole)

[Pd(dmpz)(2)(Hdmpz)(2)](2), which is known to crystallize in the monoclinic system (alpha phase), possesses two polymorphs which could only be produced as microcrystalline powders. Selective syntheses of these new (beta and gamma) polymorphs and their structural characterization from X-ray powder diffraction data have been successfully accomplished by ab initio methods and Rietveld refinement. beta-[Pd(dmpz)(2)(Hdmpz)(2)](2) is tetragonal, space group I422, a = 13.1342(7) Angstrom, c = 13.7920(9) Angstrom, Z = 2; R(p), R(wp) and R(F) being 0.106, 0.143, and 0.075, respectively, gamma-[Pd-(dmpz)(2)(Hdmpz)(2)](2) is triclinic, space group <P(1)over bar>, a = 11.470(1) Angstrom, b = 11.619(2) Angstrom, c = 19.221(2) Angstrom, alpha 80.928(6)degrees, beta = 81.434(7)degrees, and gamma = 69.033(5)degrees, Z = 2; R(p), R(wp), and R(F) being 0.104, 0.136, and 0.085, respectively. In all three phases the identity of the [Pd(dmpz)(2)(Hdmpz)(2)](2) dimer, containing four strong intramolecular N-H ... N bonds, is retained. The alpha and gamma phases are a racemate of chiral molecules, while during the formation of the beta phase, spontaneous resolution of chiral molecules occurs, thus generating a conglomerate of enantiomeric crystals.