Investigation of Transition Metal Compounds of Cyclam Derivatives for Applications in Catalysis, Electronics, and Magnetism
Described herein are the preparation and characterization of several alkynyl complexes utilizing earth abundant 3d transition metals coordinated by cyclam like macrocycles (cyclam = 1,4,8,11 – tetraazacyclotetradecane), namely TMC (TMC = 1,4,8,11 – tetramethyl – 1,4,8,11 – tetraazacyclotetradecane) and HMC (HMC = 5,5,7,12,12,14 – hexamethyl – 1,4,8,11 – tetraazacyclotetradecane). To begin, monomeric Ni(II) complexes having a single alkynyl moiety, specifically [Ni(TMC)C2nR]X type complexes with n = 1 and 2, were prepared from Ni(TMC)X2 (X = Cl – or OTf–) and the appropriate lithium acetylide. These complexes were characterized with single crystal X-ray diffraction, showing that the TMC macrocycle is in theRSRS conformation for all the complexes. The electronic structures of these complexes were investigated using both density functional theory (DFT) calculations as well as complete active space self-consistent field calculations with second order perturbation theory corrections (CASSCF/CASPT2). Moving forward, monomeric Cr(III) complexes were then explored. Cis-/trans-[Cr(rac/meso-HMC)(C2nR)2]Cl complexes were prepared from cis-/trans-[Cr(rac/meso - HMC)Cl2]Cl and the appropriate lithium acetylide. The cis or trans orientation is dependent on the starting material, i.e. trans-[Cr(meso-HMC)Cl 2]Cl yields trans-[Cr(meso-HMC)(C2nR) 2]Cl and cis-[Cr(rac-HMC)Cl2 ]Cl yields cis-[Cr(rac-HMC)(C 2nR)2]Cl. This finding is further corroborated by single crystal X-ray diffraction studies, which not only shows the cis/rac or trans/meso nature of the complexes, but also that each bis-alkynyl complex adopts a pseudo-octahedral geometry. Perhaps the most interesting feature of these complexes is the fine structured nature of the d-d bands observed in their UV-vis absorption spectra. These bis-alkynyl Cr(III) complexes also exhibit phosphorescence, which has been extensively studied. Mono-alkynyl Cr(III) complexes, namely cis-/trans-[Cr(rac/meso-HMC)(C2nR)Cl]Cl, can also be prepared from cis-/trans-[Cr(rac/meso - HMC)Cl2]Cl and 0.9 to 1.2 equiv of the appropriate lithium acetylide. These complexes can also be prepared by reacting cis-/trans -[Cr(rac/meso-HMC)(C2nR)2]Cl complexes with an under equiv of a dilute acid in methanol. Single crystal X-ray diffraction studies reveal that reactions using cis starting materials can yield both cis and trans products, where the trans products have an atypical HMC conformation. Similar to the bis-alkynyl species, the mono-alkynyl species also exhibit d-d bands with pronounced vibronic progressions in their electronic spectra. Lastly, the electrocatalytic ability of several Ni(II) complexes coordinated with cyclam derivatives have been investigated for their ability to reduce carbon dioxide at a glassy carbon working electrode. Several preliminary studies have been performed, as well as a thorough kinetic study on our laboratory’s leading catalyst, Ni(MPC)(OTf)2 (MPC = 5,12 – dimethyl – 7,14 – diphenyl – 1,4,8,11 – tetraazacyclotetradecane)^
Tong Ren, Purdue University.
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