Zinc-Containing Carboxylate-Bridged Heterodimetallic Complexes and Their Reactions with Phosphodiester Ligands

A series of zinc-containing heterodimetallic complexes were prepared by using the dinucleating ligand XDK [H₂XDK = m-xylylenediamine bis(Kemp's triacid imide)]. Mononuclear [Zn(XDK)(H₂O)] (1) reacts with 1 equiv of M(acac)₂·2H₂O (acac = 2,4-pentanedionate) to afford the heterodimetallic compounds [Zn^IIM^II(XDK)(acac)₂(CH₃OH)₂]·H₂O (2·H₂O, M = Co, 65% isolated yield; 3·H₂O, M = Mn, 54%; 4·H₂O, M = Fe, 30%; 5·H₂O, M = Ni, 32%). As determined by X-ray crystallography, 2−5 each contain a Zn^IIM^II dinuclear core bridged by XDK and acac ligands. The zinc and M atoms have trigonal bipyramidal and octahedral geometries, respectively. The Zn···M separations depend on the metal ion in the octahedral site (2, M = Co, 3.440(2) Å; 3, M = Mn, 3.517(1) Å; 4, M = Fe, 3.492(1) Å; 5, M = Ni, 3.397(1) Å), a variation which is correlated with the ionic radius of the high-spin octahedral metal ion. The reaction of 2 with diphenyl hydrogen phosphate afforded the bis(phosphate) complex [ZnCo(XDK){μ-η²-(PhO)₂PO₂}{η¹-(PhO)₂PO₂}(CH₃OH)₂(H₂O)] (7, 49% yield). The ZnCo center is bridged by the XDK through its two carboxylate groups and by a diphenyl phosphate ligand. The other diphenyl phosphate is terminally coordinated to the zinc atom in a monodentate fashion. The Zn···Co interatomic distance is 3.846(1) Å. An analogous bis(phosphate) homodinuclear complex, [Zn₂(XDK){μ-η²-(PhO)₂PO₂}{η¹-(PhO)₂PO₂}(CH₃OH)₂(H₂O)] (8), was prepared by reacting [Zn₂(XDK)(acac)₂(CH₃OH)₂] (6) with diphenyl hydrogen phosphate in 64% yield. Compound 8, which is isomorphous with 7, has an asymmetrical dizinc core bridged by XDK and a phosphate ligand (Zn···Zn = 3.869(2) Å). The monodentate diphenyl phosphate ligand dissociates from the dimetallic center of 8 in solution, as revealed by molar conductivity and ¹H and ³¹P{¹H} NMR spectroscopic studies. The resulting free phosphate ligand exchanges with the bridging one in methanol-d₄. The present results provide a useful synthetic route to carboxylate-bridged heterodimetallic compounds, which are potential models for the active centers in nonredox metalloproteins. Crystal data are as follows. 2:  monoclinic, P2₁/c, a = 17.725(7) Å, b = 12.354(4) Å, c = 21.815(6) Å, β = 90.45(3)°, V = 4777(2) ų, Z = 4; R = 0.049 and R_w = 0.051 for 3884 reflections with I > 3σ(I). 3:  monoclinic, P2₁/c, a = 17.292(2) Å, b = 12.450(1) Å, c = 21.717(2) Å, β = 91.464(7)°, V = 4673.9(7) ų, Z = 4; R = 0.046 and R_w = 0.051 for 3876 reflections. 4:  monoclinic, P2₁/c, a = 17.630(1) Å, b = 12.374(2) Å, c = 21.771(2) Å, β = 90.306(8)°, V = 4749.4(7) ų, Z = 4; R = 0.041 and R_w = 0.049 for 3248 reflections. 5:  monoclinic, P2₁/c, a = 17.817(2) Å, b = 12.241(3) Å, c = 21.786(2) Å, β = 91.043(9)°, V = 4751(1) ų, Z = 4; R = 0.050 and R_w = 0.058 for 4497 reflections. 7·CH₃OH:  monoclinic, P2₁/n, a = 18.812(2) Å, b = 16.156(2) Å, c = 21.760(2) Å, β = 112.836(9)°, V = 6095(1) ų, Z = 4; R = 0.055 and R_w = 0.064 for 5843 reflections. 8·CH₃OH:  monoclinic, P2₁/n, a = 18.845(6) Å, b = 16.105(3) Å, c = 21.776(3) Å, β = 112.78(2)°, V = 6091(2) ų, Z = 4; R = 0.055 and R_w = 0.061 for 5189 reflections.