Molecular sensing behavior of the first Mn(I)-compound of di-2-pyridylketone-p-nitrophenylhydrazone (dpknph), fac-[Mn(CO) 3 (dpknph)Br] Mohammed Bakir * , Orville Green, Colin Gyles Department of Chemistry, The University of the West Indies, Mona Campus, Kingston 7, WI, Jamaica Received 8 July 2004; accepted 30 December 2004 Available online 11 February 2005 Abstract The reaction between [Mn(CO) 5 Br] and di-2-pyridylketone-p-nitrophenylhydrazone (dpknph) in diethyl ether under ultrasonic conditions gave fac-[Mn(CO) 3 (dpknph)Br] in good yield. Optical and thermodynamic measurements on fac-[Mn(CO) 3 (dpknph)Br] in non-aqueous polar solvents revealed reversible interconversion between two intense charge transfer absorption bands due to p–p* (dpk), followed by dpk ! nitro intraligand charge transfer transition (ILCT), mixed with metal ligand charge transfer transition (MLCT) due to dp ðMnÞ ! p  ðdpkÞ . In non-polar solvents, a single absorption band appeared. Extinction coefficients of 46 200 ± 2000 and 28 400 ± 2000 M 1 cm 1 were calculated in DMSO for the low- and high-energy electronic states of fac- [Mn(CO) 3 (dpknph)Br] using excess NaBF 4 . Changes in enthalpy (DH ø ) of +14.0 and 12.1 kJ mol 1 , entropy (DS ø ) of +28.65 and 64.30 J mol 1 K 1 , and free energy (DG ø ) of +5.48 and +7.08 kJ mol 1 at 298 K were calculated for the interconversion between the high and low energy electronic states of fac-[Mn(CO) 3 (dpknph)Br]. These results allow for the use of these systems (fac-[Mn(CO) 3 (dpknph)Br] and surrounding solvent or solute molecules) as optical sensors for a variety of physical and chemical stimuli that include metal ions. Group 12 metal ions in concentrations as low as 1.00 · 10 9 M can be detected and determined using fac-[Mn(CO) 3 (dpknph)Br] in dmso in the presence and absence of NaBH 4 . Ó 2005 Elsevier B.V. All rights reserved. Keywords: Manganese; Carbonyl; Hydrazones; Sensors; Di-2-pyridyl ketone 1. Introduction Manganese carbonyl compounds are of interest in part because of their rich physico-chemical properties, reactivity patterns and applications in many important chemical processes [1–10]. We have been interested in the chemistry of di-2-pyridyl ketone and its oxime and hydrazone derivatives (see below) and their metal com- pounds and reported on the synthesis and characteriza- tion of a variety of polypyridyl-like ligands and their metal compounds [11–25]. dpk dpk.oxime dpkhydrazone N O N N N N O H N N N N R H H The electrochemical behavior of fac-[Re-(CO) 3 - (dpk)Cl] in the presence and absence of different sub- strates that include CO 2 and Groups I and II metal ions shows high sensitivity to solvent and substrate concen- tration and the use of fac-Re(CO) 3 (dpk)Cl as an electro- chemical sensor for Groups I and II metal ions [17,18]. In the case of CO 2 , electrochemical reactions at metal and ligand sites of fac-Re(CO) 3 (dpk)Cl were envisaged and 0020-1693/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2004.12.050 * Corresponding author. Tel.: +1 876 935 8164; fax: +1 876 977 1835. E-mail address: mohammed.bakir@uwimona.edu.jm (M. Bakir). www.elsevier.com/locate/ica Inorganica Chimica Acta 358 (2005) 1835–1840