Coordination Chemistry of a Novel Tetramacrocyclic Ligand Derived from 1,4,7-Triazacyclononane: Synthesis, Structure, and Properties of Nickel(II) and Copper(II) Complexes

A new polynucleating ligand, 1,2,4,5-tetrakis(1,4,7-triazacyclonon-1-ylmethyl)benzene (Ldur), has been prepared
and characterized as its dodecahydrobromide salt. Addition of base to an aqueous solution of this salt and 4
molar equivalents (m.e.) of a Ni(II) salt produces a mixture of bi- and trinuclear complexes, which can be separated
by cation-exchange chromatography (CEC) and crystallized as [Ni2Ldur](ClO4)4·2H2O (1) and [Ni3Ldur(H2O)6](ClO4)6·9H2O (2). The “full capacity” tetranuclear complex, [Ni4Ldur(H2O)12](ClO4)8·8H2O (3), is obtained by
slow addition of Ldur to a refluxing aqueous solution of excess Ni2+ ions, followed by CEC purification. Treatment
of Ldur with 4 m.e. of a copper(II) salt produces exclusively the tetranuclear complex, [Cu4Ldur(H2O)8](ClO4)8·9H2O (4), while reaction with only 2 m.e. of Cu2+ ions yields the binuclear complex, [Cu2Ldur](ClO4)4·4H2O (5).
The X-ray structures of complexes 1, 2, 4, and [Cu2Ldur](ClO4)4·3H2O (5‘) have been determined; all are monoclinic,
P21/c:  for 1, a = 9.497(3) Å, b = 13.665(5) Å, c = 19.355(6) Å, β = 100.57(2)°, V = 2469(1) Å3, and Z = 2;
for 2, a = 22.883(7) Å, b = 15.131(6) Å, c = 20.298(8) Å, β = 97.20(3)°, V = 6973(4) Å3, and Z = 4; for 4,
a = 16.713(7) Å, b = 16.714(6) Å, c = 14.775(11) Å, β = 108.24(5)°, V = 3920(4) Å3, and Z = 2; and for 5‘,
a = 9.5705(1) Å, b = 13.0646(1) Å, c = 20.1298(2) Å, β = 103.1618(8)°, V = 2450.81(4) Å3, and Z = 2. The
metal centers in 1 and 5‘ lie in distorted octahedral environments, each facially coordinated by two of the triamine
rings of Ldur, the cation in each case being centrosymmetric. In 2, one of the nickel(II) centers is similarly
sandwiched by two triamine rings, while the other two nickel(II) centers are each coordinated by a single triamine
ring from the ligand, with their distorted octahedral coordination spheres each being completed by three water
molecules. In 4, the four triamine rings of Ldur bind to separate copper(II) centers, with two water molecules
occupying the remaining two sites of the distorted square pyramidal (SP) coordination spheres, the cation again
being centrosymmetric.