Selective Metal-Ligand Bond-Breaking Driven by Weak Intermolecular Interactions: From Metamagnetic Mn(III)-Monomer to Hexacyanoferrate(II)-Bridged Metamagnetic MnFe 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 MnFe complex {[Mn(L)(HO)]Fe(CN)}, starting from a mononuclear Mn(III)-Schiff base complex: [Mn(L)(HO)Cl] () and [Fe(CN)] anions. This reaction implies the dissociation of Mn(III)-Cl 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)(HO)Cl] () and of compound (HO)[Mn(L)(HO)]{[Mn(L)(HO)]Fe(CN)}4HO () (HL = 2,2'-((1,1')-(ethane-1,2-diylbis(azaneylylidene))bis(methaneylylidene))bis(4-methoxyphenol)). Complex is a monomer where the Schiff base ligand (L) is coordinated to the four equatorial positions of the Mn(III) center with a HO molecule and a Cl ion at the axial sites and the monomeric units are assembled by π-π and hydrogen-bonding interactions to build supramolecular dimers. The combination of [Fe(CN)] with complex leads to the formation of linear Mn-NC-Fe-CN-Mn trimers where two cyano groups of the [Fe(CN)] anion replace the labile chloride from the coordination sphere of two [Mn(L)(HO)Cl] complexes, giving rise to the linear anionic {[Mn(L)(HO)]Fe(CN)} trimer. This MnFe trimer crystallizes with an oxonium cation and a mononuclear [Mn(L)(HO)] cation, closely related to the precursor neutral complex [Mn(L)(HO)Cl]. In compound , the MnFe trimers are assembled by several hydrogen-bonding and π-π interactions to frame an extended structure similar to that of complex . 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 and , respectively. The magnetic properties of compound have been fit with a simple = 2 monomer with = 1.959, a weak zero-field splitting (|| = 1.23 cm), and a very weak intermolecular interaction ( = -0.03 cm). For compound , we have used a model with an = 2 monomer with ZFS plus an = 2 antiferromagnetically coupled dimer with = 2.009, || = 1.21 cm, and = -0.42 cm. The metamagnetic behavior of both compounds is attributed to the weak intermolecular π-π and hydrogen-bonding interactions.