The synthesis and coordination chemistry of a new asymmetric multidentate ligand designed for modeling coordination number asymmetry at metal sites in binuclear metalloproteins are described., A binuclear copper complex of this ligand demonstrates proof-of-concept for inducing coordinative unsaturation at one metal of the binuclear pair, and subsequent reaction with azide illustrates site-directed reactivity. The ligand N,N,N’-tris((N-methyl-2-benzimidazolyl)methyl)-N’-methyl- 1,3-diamino-2-propanol (HBMDP) was prepared in good yield by a five-step procedure. HBMDP reacts with the copper(II) ion in methanol to produce a mononuclear copper complex, [CuHBMDP](ClO4)(2) . CH2CN, that crystallizes in the triclinic space group with unit cell dimensions a = 10.833(3) Angstrom, b = 12.457(3) Angstrom, c = 15.214(3) Angstrom, alpha = 78.28(2)degrees, beta = 79.26(2)degrees, and gamma = 68.10(2)degrees. A single-crystal X-ray structure shows that the copper ion is coordinated to the five nitrogen donor atoms in a distorted trigonal bipyramid. The hydroxo oxygen of the ligand does not participate in metal coordination.On the other hand, the reaction of HBMDP with 2 equiv each of copper(II) ion and acetate in methanol produces the binuclear copper complex [Cu(2)BMDP(OAc)](ClO4)(2) . 2H(2)O, which crystallizes in the monoclinic space group C2/m with unit cell dimensions a = 19.934(4) Angstrom, b = 23.460(9) Angstrom, c = 20.039(4) Angstrom, and beta = 110.65(2)degrees. A single-crystal X-ray structure reveals that the two copper ions are coordinated to the aromatic and aliphatic nitrogens as well as being bridged by the acetate anion and the alkoxide oxygen of the ligand. As a result of the inherent asymmetry of the ligand, one copper ion is five-coordinate (distorted trigonal bipyramidal) while the other copper ion is four-coordinate (distorted square planar). The crystal structure also reveals that in the solid state an oxygen from one of the perchlorate counterions is weakly bound to the four-coordinate copper (similar to 2.6 Angstrom), suggesting that a vacant coordination site exists at this formally four-coordinate copper center. The potential for site-directed reactivity at this lower coordinated copper center was demonstrated by adding 0.5 equiv of azide ion to the binuclear copper complex in acetonitrile. The resulting azido-bridged complex, {[Cu(2)BMDP(OAc)]N-2(3)}(ClO4)(3) . 8.5H(2)O, crystallizes in the orthorhombic space group Cmca with unit cell dimensions a = 22.603(3) Angstrom, b = 19.565(3) Angstrom, and c = 36.965(3) Angstrom. A single-crystal X-ray structure shows that the two copper ions within the binuclear unit; are still coordinated by the ligand nitrogens, the acetate, and the ligand alkoxy oxygen. In addition, the copper ion that was four-coordinate in the binuclear complex is now bridged by azide to its companion four-coordinate copper in a separate binuclear unit. As a result, both coppers within the binuclear unit are now five-coordinate, one remaining distorted trigonal bipyramidal while the other adopts square pyramidal geometry. In this reaction, azide specifically fills the vacant coordination site of the four-coordinate copper ion in the binuclear complex. Although these two binuclear paramagnetic copper(II) complexes are magnetically weakly coupled, their H-1 NMR spectra show relatively sharp lines that provide solution assignments consistent with the solid state structures. To our knowledge this is the first example of a binuclear copper complex to exhibit both coordination number asymmetry and directed reactivity at one metal center by virtue of ligand design.X,?