Coordination compounds in the entatic state

Much of the selectivity and efficiency of chemical transformations, which involve metalloproteins is due to a modulation of the properties of the metal ions by the protein to which they are bound. Geometric and electronic distortions, enforced by the protein backbone, and specific electrostatic fields and solvation patterns (e.g. hydrophobic pockets) may lead to a destabilization of the catalytically active site (metal center(s) and/or enzyme-substrate complex(es)) and, therefore, to a (selective) activation of certain reaction channels. This is known as the 'energization theory' or, for specific cases, the 'entatic state' principle. Examples, where specifically enforced coordination geometries lead to stresses and enhanced reactivities range from biological systems (metalloproteins and enzymes) to classical coordination compounds and processes of industrial importance (catalytic systems which involve organometallic and transition metal coordination compounds). Hence, entasis is not refined to metalloproteins; reactions induced by metal-free enzymes or by small coordination compounds may also involve strained, that is entatic states. Based on few selected examples, which include the classical cases of the blue copper proteins and of electron transfer in general, as well as simple coordination compounds and organometallic catalysis, it is shown that a thorough analysis and interpretation of enhanced reactivities may in general not be assigned exclusively to steric strain. However, specific coordination geometries, enforced by the ligand sphere (protein or simple organic compounds) which may or may not be strained are often of importance. One of the main conclusions is that the understanding, design and synthesis of new compounds with specific and enhanced reactivities may involve similar thoughts, tools and difficulties as the design and study of highly preorganized ligands in areas such as metal ion recognition and host-guest interactions in general. (C) 2000 Elsevier Science S.A. All rights reserved.