Electronic Transition and Magnetic Coupling Regulation in Trimetallic Complexes Featuring a New Bridging Ligand Obtained by Oxidative Addition

Wesynthesized trimetallic homo- anwd heterotrimetalliccomplexes [Fe-III(& mu;-L)(py)](2)M-II(py)( n ) with a new bridgingligand by two-step metalation strategy. They present two distinctcharacteristics of ferromagnetic interaction (M-II = Mn-II, Fe-II, Co-II) and paramagnetism (M-II = Zn-II, Cd-II), and the order of ferromagneticcoupling constant J is J ( FeFe ) > J ( FeCo ) > J ( FeMn ) . A seriesof trimetallic complexes [Fe-III(& mu;-L)(py)](2)M-II(py)( n ) (n = 2, M-II = Mn-II, 1;Fe-II, 2; Co-II, 3;Zn-II, 4; n = 3,M-II = Cd-II, 5) with a new bridgingligand L4- (deprotonated 1,2-N-1,N-2 -bis(2-mercaptoanil) oxalimidic acid) weresynthesized and fully characterized by elemental analysis, single-crystalX-ray crystallography, IR, and Mo''ssbauer spectra. Interestingly,the bridging ligand was obtained by oxidative addition of the (gma(& BULL;))(3-) ligand from the mononuclear precursorFe(gma)py (gma = glyoxal-bis(2-mercaptoanil)). Inthe obtained complexes, the bridging ligand L4- coordinatesto the terminal Fe-III ions (intermediate-spin with S-Fe = 3/2) by the N, S atoms, and coordinate to the centralmetal M-II ion by the four O atoms. The resonance structureof the bridging ligand can be described as the two 4 & pi;-electrondelocalized systems connected by one single-bond (C-1-C-2), which is different from the electronic structure of theprecursor Fe(gma)py. Remarkably, the magnetic couplinginteraction can be regulated through the central metal. The ferromagneticcoupling constant J gradually decreases as M-II changes from Fe-II to Co-II and Mn-II, while the paramagnetic behaviors are presented when M-II = Zn-II and Cd-II, confirmed by themagnetic susceptibility measurements and further supported by usingthe PHI program. Furthermore, the bridging ligand to the terminalFe(III) charge transfer (LMCT) transitions emerged in allcomplexes but the central Fe-II to terminal Fe-III charge transfer (MMCT) only presented in complex 2,strongly supported by the UV/vis-NIR electronic spectra andTDDFT calculations.