화학공학소재연구정보센터
Inorganic Chemistry, Vol.59, No.12, 8487-8497, 2020
Selective Metal-Ligand Bond-Breaking Driven by Weak Intermolecular Interactions: From Metamagnetic Mn(III)-Monomer to Hexacyanoferrate(II)-Bridged Metamagnetic Mn2Fe Trimer
Metal-ligand coordination interactions are usually much stronger than weak intermolecular interactions. Nevertheless, here, we show experimental evidence and theoretical confirmation of a very rare example where metal-ligand bonds dissociate in an irreversible way, helped by a large number of weak intermolecular interactions that surpass the energy of the metal-ligand bond. Thus, we describe the design and synthesis of trinuclear Mn2Fe complex {[Mn(L)-(H2O)](2)Fe(CN)(6)},(2-) starting from a mononuclear Mn(III)-Schiff base complex: [Mn(L)(H2O)Cl] (1) and [Fe(CN)(6)](4-) anions. This reaction implies the dissociation of Mn(III)-CI coordination bonds and the formation of Mn(III)-NC bonds with the help of several intermolecular interactions. Here, we present the synthesis, crystal structure, and magnetic characterization of the monomeric Mn(III) complex [Mn(L)(H2O)Cl] (1) and of compound (H3O) [Mn(L)(H2O)(2)]-{[Mn(L) (H2O)](2) Fe(CN)(6)}center dot 4H(2)O (2) (H2L = 2,2'-((1E, 1'E)- (ethane-1,2-diyIbis(azaneylylidene))bis(methaneylylidene))bis(4-methoxyphenol)). Complex 1 is a monomer where the Schiff base ligand (L) is coordinated to the four equatorial positions of the Mn(III) center with a H2O molecule and a Cl- ion at the axial sites and the monomeric units are assembled by pi-pi and hydrogen-bonding interactions to build supramolecular dimers. The combination of [Fe(CN)(6)](4-) with complex 1 leads to the formation of linear Mn-NC-Fe-CN-Mn trimers where two trans cyano groups of the [Fe(CN)(6)](4-) anion replace the labile chloride from the coordination sphere of two [Mn(L)(H2O)Cl] complexes, giving rise to the linear anionic {[Mn(L)(H2O)](2) Fe(CN)(6)}(2-) trimer. This Mn2Fe trimer crystallizes w ith an oxonium cation and a mononudear [Mn(L)(H2O)(2)] . cation, closely related to the precursor neutral complex [Mn(L)(H2O)Cl]. In compound 2, the Mn2Fe trimers are assembled by several hydrogen-bonding and pi-pi interactions to frame an extended structure similar to that of complex 1. Density functional theoretical (DFT) calculations at the PBE1PBE-D3/def2-TZVP level show that the bond dissociation energy (-29.3 kcal/mol) for the Mn(III)-Cl bond is smaller than the summation of all the weak intermolecular interactions (-30.1 kcal/mol). Variable-temperature magnetic studies imply the existence of weak intermolecular antiferromagnetic couplings in both compounds, which can be can cancelled with a critical field of ca. 2.0 and 2.5 T at 2 K for compounds 1 and 2, respectively. The magnetic properties of compound 1 have been fit with a simple S = 2 monomer with g = 1.959, a weak zero-field splitting (vertical bar D vertical bar = 1.23 cm(-1)), and a very weak intermolecular interaction (zJ = -0.03 cm For compound 2, we have used a model with an S = 2 monomer with ZFS plus an S = 2 antiferromagnetically coupled dimer with g = 2.009, vertical bar D vertical bar = 1.21 cm(-1), and J = -0.42 cm(-1). The metamagnetic behavior of both compounds is attributed to the weak intermolecular pi-pi and hydrogen-bonding interactions.