Nickel(II) and zinc(II) dibenzoylmethanates: molecular and crystal structure, polymorphism, and guest- or temperature-induced oligomerization.

Four forms of nickel(II) and two of zinc(II) dibenzoylmethanates have been isolated and characterized with powder and single-crystal X-ray diffraction analyses, differential scanning calorimetry, magnetic susceptibility measurements, and solid-state 13C cross-polarization/magic angle spinning NMR. Nickel dibenzoylmethanate, Ni(DBM)2 (DBM = PhCOCHCOPh-), forms three polymorphic forms (light-green, brown, and green) and a fourth clathrate form with guest benzene included. The light-green polymorph is metastable. Substituted benzenes induce recrystallization of the polymorph into a stable brown form (C30H22NiO4; a = 26.502(3) A, b = 5.774(1) A, c = 16.456(2) A, beta = 116.03(1) degrees; monoclinic, C2/c; Z = 4). Unlike the other forms, the brown form is diamagnetic and is comprised of monomers of the low-spin [Ni(DBM)2] complex. The Ni(II) is chelated by two DBM ligands in a square planar environment by four donor oxygen atoms. When heated, the brown form transforms to a green form which is stable above 202 degrees C (C90H66Ni3O12; a = 13.819(2) A, b = 16.252(2) A, c = 17.358(2) A, beta = 108.28(1) degrees; monoclinic, P2(1)/n; Z = 2). This polymorph is formed by van der Waals packing of trimers [Ni3(DBM)6] containing linear Ni3 clusters with an Ni-Ni distance of 2.81 A. The cluster is surrounded by six DBM ligands, providing a distorted octahedral environment about each Ni by six oxygen atoms. Benzene stabilizes the trimeric structure at room temperature, forming a [Ni3(DBM)6].2(benzene) inclusion compound (Ni-Ni distance of 2.83 A) with guest benzene molecules located in channels (C90H66Ni3O12 + 2(C6H6); a = 17.670(2) A, b = 20.945(3) A, c=11.209(2) A, beta = 102.57(1) degrees; monoclinic, P2(1)/c; Z = 2). Zinc dibenzoylmethanate has been prepared in two polymorphic forms. The monomeric form contains [Zn(DBM)2] molecules with the zinc center in a distorted tetrahedral environment of four oxygens from the two chelated DBMs (C30H22O4Zn; a = 10.288(2) A, b = 10.716(2) A, c = 12.243(2) A, alpha = 89.19(1) degrees, beta = 75.39(1) degrees, gamma = 64.18(1) degrees; triclinic, P1; Z = 2). Another, dimeric form contains [Zn2(DBM)4] species, with two zinc atoms separated by a distance of 3.14 A and each zinc coordinated by five oxygen atoms (C60H44O8Zn2; a = 25.792(3) A, b = 7.274(1) A, c = 24.307(2) A, beta = 90.58(1) degrees; monoclinic, C2/c; Z = 4). The polymorphic variety of the title complexes and the peculiarities of the Ni(II) and Zn(II) coordination environments are discussed in the context of using the complexes as precursors for new metal complex hosts.