In chemistry, a complex, also called a “coordination compound” or “metal complex”, is a structure consisting of a central atom or molecule connected to surrounding atoms or molecules. Originally, a complex implied a reversible association of molecules, atoms, or ions through weak chemical bonds. As applied to coordination chemistry, this meaning has evolved. Some metal complexes are formed virtually irreversibly and many are bound together by bonds that are quite strong.
Copper is a transition metal, which in the zero oxidation state has an electron configuration of [Ar]4s24p63d9. Copper is found in three different oxidation states: Cu(I), Cu(II), and Cu(III). Copper (I) atoms have 10 d electrons. Cu(I) complexes being d10 have no Jahn-Teller distortion. Cu (I) complexes are diamagnetic and typically colorless. If a Cu(I) complex is colored, the color is a result of a charge transfer band or an internal transition in a ligand. In the copper(II) oxidation state, the metal has 9 d electrons. Jahn- Teller distortion causes a splitting of eg and t2g orbitals. Most Cu(II) complexes are square planar for this reason. Usually observed in the electronic spectra of Cu(II) complexes is a single broad, poorly resolved band envelope. This envelope is typical of Cu(II) complexes in tetragonal complexes. These complexes are generally blue or green because of an absorption band in the 600-900 nm region of the spectrum.
Reproducing complex biological reactivity within a simple synthetic molecule is a challenging endeavor with both intellectual and aesthetic goals. The sequence of examining biological reactivity, creating similar chemical architectures, and determining functional reaction conditions for model systems is a process that allows the biological code of reactivity to be deciphered.
In the past years the report on the crystal structures of type 3 copper enzymes (e.g. catechol oxidase, hemocyanins, and tyrosinase), as too type 2 – type 3 copper enzymes (e.g. ascorbate oxidase, laccase, ceruloplasmin) has taken a new turn. The greater availability of such structural information now allows a shift in the role of synthetic modeling from structural and spectroscopic endeavors to development of functional and catalytic models. Functional models can provide an opportunity to examine a biological reactivity at a small-molecule level of detail through systematic and comparative studies. Although one goal of modeling is reproduction of reactivity, extension of this reactivity beyond the scope of the inspiring system is perhaps an even more important objective. Adequate synthetic models that have similar structural, spectroscopic and functional properties of active sites of copper proteins are done. These models provide many elegant examples of selective and environmentally benign oxidants capable of performing interesting organic transformations and many of these are copper complexes that use dioxygen as the ultimate oxidant above all in the catecholase activity.
The coordination chemistry of copper(II) attracts much attention because of its biological relevance and its own interesting coordination chemistry such as geometry, flexible redox property, and oxidation state.1-3 Nowadays, coordination compounds have been known to be useful in constructing molecular information processing systems, particularly by biological self-organizing processes. 4,5 Especially for this purpose, synthesis, and structural and chemical characterization of copper complexes has been attempted to mimic metalloenzyme.6-8 Recently, we tried to prepared new materials for electric devices.
In a series of those compounds, we have prepared copper (II) complexes containing two nitrato ligands and a 2,2′-dipyridylamine(dpa) derivative ligand. The 2,2′- dipyridylamine and its derivatives have been widely used for metal complexes because of their good chelating property, structural flexibility, and feasible reduction property compared with other N,N’-chelating ligand including bipyridine and phenanthroline. Copper(II) complexes ligated by two nitrato ligands and a N.N’-chelating ligand (L), CuL(NO3)2, are common and some XRD structures have been reported. In those complexes, the coordination numbers varied from 4 to 6 according to nitrato ligating properties.
The aqueous solution coordination chemistry of the transition metal copper is limited to its three accessible oxidation states (I-III) [8-12]. The lowest oxidation state, Cu(I) has a diamagnetic d10 configuration and forms complexes without any crystal-field stabilization energy. Complexes of this type are readily prepared using relatively soft polarizable ligands like thioethers, phosphines, nitriles, isonitriles, iodide, cyanide and thiolates. A broad range of coordination geometries
is observed. Cu(I) complexes are biologically relevant because they are able to reductively activate molecular oxygen (O2). However, due to the lability of most Cu(I) complexes,
they typically lack sufficient kinetic stability for radiopharmaceutical applications.
Copper (II) exists as a d9 metal center of borderline softness which favors amines, imines, and bidentate ligands like bipyridine to form square planar, distorted square planar, trigonal pyramidal, square pyramidal, as well as distorted octahedral geometries. Jahn-Teller distortions in six-coordinate Cu(II) complexes are often observed as an axial elongation or a tetragonal compression. Due to the presence of some crystal-field stabilization energy, Cu(II) is generally
less labile toward ligand exchange and is the best candidate for incorporation into radiopharmaceuticals. A third oxidation state Cu(III) is relatively rare and difficult
to attain without the use of strong π-donating ligands. These complexes usually adopt a square planar geometry due to the d8 Cu(III) electron configuration.
Reactions of cupric chloride, nitrate and perchlorate group with dibenzoylmethane and neutral uni and bidentate nitrogen donor ligands in ethanolic medium give series of complexes. The complexes have been characterized on the basis of elemental analysis, molar conductance, magnetic moment, infrared and electronic spectral data.
Copper (II) forms bis-β- diketonate when a cupric salt is treated with a β- diketone in 1:2 molar ratio followed by addition of ammonia. However, if the metal ion and β- diketone are taken in 1:1 ratio instead of 1:2 and then treated with nitrogen donor ligands, mixed ligand complexes are formed. It was therefore thought worthwhile to study the reaction of cupric salts with dibenzoyl methane and some nitrogen donor ligands with a view to preparing mixed ligand complexes.