Giant Heterometallic [Mn36Ni4]0/2- and [Mn32Co8] "Loops-of-Loops-and-Supertetrahedra" Molecular Aggregates

Maria Charalambous; Eleni E Moushi; Tu N Nguyen; Constantina Papatriantafyllopoulou; Vassilios Nastopoulos; George Christou; Anastasios J Tasiopoulos

doi:10.3389/fchem.2019.00096

Giant Heterometallic [Mn₃₆Ni₄]^0/2- and [Mn₃₂Co₈] "Loops-of-Loops-and-Supertetrahedra" Molecular Aggregates

Front Chem. 2019 Mar 5:7:96. doi: 10.3389/fchem.2019.00096. eCollection 2019.

Authors

Maria Charalambous¹, Eleni E Moushi¹, Tu N Nguyen², Constantina Papatriantafyllopoulou^{1

2}, Vassilios Nastopoulos³, George Christou², Anastasios J Tasiopoulos¹

Affiliations

¹ Department of Chemistry, University of Cyprus, Nicosia, Cyprus.
² Department of Chemistry, University of Florida, Gainesville, FL, United States.
³ Department of Chemistry, University of Patras, Patras, Greece.

Abstract

We report the synthesis, crystal structures and magnetic properties of the giant heterometallic [Mn₃₆Ni₄]^2-/0 (compounds 1, 2)/[Mn₃₂Co₈] (compound 3) "loops-of-loops-and-supertetrahedra" molecular aggregates and of a [Mn₂Ni₆]²⁺ compound (cation of 4) that is structurally related with the cation co-crystallizing with the anion of 1. In particular, after the initial preparation and characterization of compound [Mn₂Ni₆(μ₄-O)₂(μ₃-OH)₃(μ₃-Cl)₃(O₂CCH₃)₆(py)₈]²⁺[Mn₃₆Ni₄(μ₄-O)₈(μ₃-O)₄(μ₃-Cl)₈Cl₄(O₂CCH₃)₂₆(pd)₂₄(py)₄]^2- (1) we targeted the isolation of (i) both the cationic and the anionic aggregates of 1 in a discrete form and (ii) the Mn/Co analog of [Mn₃₆Ni₄]^2- aggregate. Our synthetic efforts toward these directions afforded the discrete [Mn₃₆Ni₄] "loops-of-loops-and-supertetrahedra" aggregate [Mn₃₆Ni₄(μ₄-O)₈(μ₃-O)₄(μ₃-Cl)₈Cl₂(O₂CCH₃)₂₆(pd)₂₄(py)₄(H₂O)₂] (2), the heterometallic Mn/Co analog [Mn₃₂Co₈(μ₄-O)₈(μ₃-O)₄(μ₃-Cl)₈Cl₂(μ₂-OCH₂CH₃)₂(O₂CCH₃)₂₈(pd)₂₂(py)₆] (3) and the discrete [Mn₂Ni₆]²⁺ cation [Mn₂Ni₆(μ₄-O)₂(μ₃-OH)₄(μ₃-Cl)₂(O₂CCH₃)₆(py)₈](ClO₄)(OH) (4). The structure of 1 consists of a mixed valence [ ${Mn}_{28}^{III}$ ${Mn}_{8}^{II}$ ${Ni}_{4}^{II}$ ]^2- molecular aggregate that contains two ${Mn}_{8}^{III}$ ${Ni}_{2}^{II}$ loops separated by two ${Mn}_{6}^{III}$ ${Mn}_{4}^{II}$ supertetrahedral units and a [ ${Mn}_{2}^{III}$ ${Ni}_{6}^{II}$ ]²⁺ cation based on two [Mn^III ${Ni}_{3}^{II}$ (μ₄-O)(μ₃-OH)_1.5(μ₃-Cl)_1.5]⁴⁺ cubane sub-units connected through both mono- and tri-atomic bridges provided by the μ₄-O^2- and carboxylate anions. The structures of 2-4 are related to those of the compounds co-crystallized in 1 exhibiting however some differences that shall be discussed in detail in the manuscript. Magnetism studies revealed the presence of dominant ferromagnetic interactions in 1-3 that lead to large ground state spin (S_T) values for the "loops-of-loops-and-supertetrahedra" aggregates and antiferromagnetic exchange interactions in 4 that lead to a low (and possibly zero) S_T value. In particular, dc and ac magnetic susceptibility studies revealed that the discrete [Mn₃₆Ni₄] aggregate exhibits a large S_T value ~ 26 but is not a new SMM. The ac magnetic susceptibility studies of the [Mn₃₂Co₈] analog revealed an extremely weak beginning of an out-of-phase tail indicating the presence of a very small relaxation barrier assignable to the anisotropic Co²⁺ions and a resulting out-of-phase ac signal whose peak is at very low T.

Keywords: Mn; crystal structures; diols; heterometallic clusters; magnetic properties.