An Fe(III) derivative of (<i>R</i>)-sirtinol: bis(nitrato)-(2-(((2-(oxy)-1-naphthyl)methylene)amino)-N-(1-phenylethyl)-benzamidato)-iron(III) tetrahydrofuran solvate. Its crystallization mode, chirality nomenclature, and biochemical implications ther

Abstract The chiral ligand (R)-sirtinol and its ferric derivative, compound (I), have been used in a wide range of chemical and biological studies which should be sensitive to their chirality inasmuch as enantioselectivity is of primordial importance in such processes. It is, therefore, imperative that the chiral nature of the iron-containing molecule should be recognized; and, if the metal is chiral, the expectation is that its function(s) may be stereospecific. Surprisingly enough, until now little or nothing has been elucidated concerning its stereochemical character, probably because the chiral nature of a seven-fold coordinated environment is often overlooked and not commonly encountered in 3d compounds. Below, we address these issues because (I) crystallizes with Z ′ = 2.0; thus, the chiral nature of its two diastereoisomers must be clearly understood in order to properly interpret the results of future chemical and biochemical investigations. We found that there is a novel form of crystallization in (I) characterized by: (1) there are two distinct diastereoisomeric species in the asymmetric unit differing in the magnitude and/or sign of relevant torsional angles; (2) the sirtinol ligand itself bears an (R) carbon in both, as expected from Z ′ = 2.0. Thus, if the chirality of the ligand carbon is momentarily ignored , the crystals are simple examples of a kryptoracemate. If not, we have discovered a new crystalline form for which we propose a suitable nomenclature as this is now needed. In analogy to the classical example of “racemic mimics” recognized a long time ago, we name it kryptoracemic mimic. In essence, there are two different conditions to consider: the kryptoracemic character of the iron (in this case), and the pure enantiomeric character of the sirtinol ligand, and they must be properly described in the name of such species.