Selective Metal-Ligand Bond-Breaking Driven by Weak Intermolecular Interactions: From Metamagnetic Mn(III)-Monomer to Hexacyanoferrate(II)-Bridged Metamagnetic Mn 2 Fe 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 Mn ₂ Fe complex {[Mn(L)(H ₂ O)] ₂ Fe(CN) ₆ }, ^2- starting from a mononuclear Mn(III)-Schiff base complex: [Mn(L)(H ₂ O)Cl] ( 1 ) and [Fe(CN) ₆ ] ^4- 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)(H ₂ O)Cl] ( 1 ) and of compound (H ₃ O)[Mn(L)(H ₂ O) ₂ ]{[Mn(L)(H ₂ O)] ₂ Fe(CN) ₆ } · 4H ₂ O ( 2 ) (H ₂ L = 2,2'-((1 E ,1' E )-(ethane-1,2-diylbis(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 H ₂ O 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) 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)(H ₂ O)Cl] complexes, giving rise to the linear anionic {[Mn(L)(H ₂ O)] ₂ Fe(CN) ₆ } ^2- trimer. This Mn ₂ Fe trimer crystallizes with an oxonium cation and a mononuclear [Mn(L)(H ₂ O) ₂ ] ⁺ cation, closely related to the precursor neutral complex [Mn(L)(H ₂ O)Cl]. In compound 2 , the Mn ₂ Fe trimers are assembled by several hydrogen-bonding and π-π 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 (| D | = 1.23 cm ^-1 ), and a very weak intermolecular interaction ( zJ = -0.03 cm ^-1 ). 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, | D | = 1.21 cm ^-1 , and J = -0.42 cm ^-1 . The metamagnetic behavior of both compounds is attributed to the weak intermolecular π-π and hydrogen-bonding interactions.