Synthesis, Crystal Structure, and Magnetic Properties of the Two-Dimensional Nickel(II) Complex via Covalent and Hydrogen Bonds: [Ni(L)(H2O)2][Ni2(L)(NTA)2]ċ6H2O (L = 3,14-di-methyl-2,6,13,17-tetra- azatricyclo[14,4,01.18,07.12]docosane, NTA = Nitrillotriacetate)