# Magnetic Energy Landscape of Dimolybdenum Tetraacetate on a Bulk Insulator Surface
###### Abstract
Since the early 2000s, SMMs synthesized in the gas-phase and as bulk materials have been deposited on substrates [9; 11; 28], such as Au(111), where SMMs self-assemble, forming molecular chains on terraces [29]. Examples include on-surface lanthanide and transition-metal phthalocyanines and porphyrins, which showed a high magnetic tunability by changing ligands, metal-centers, end-groups and underlying substrates [11; 28]. These complexes exhibit a high thermal stability and versatility [11], and besides being synthesised in gas-phase as catalysts [30], they were sandwiched on thin films [31], deposited on metallic substrates [32] or used as thin films on silicon surfaces [33; 34]. Metallophthancyamines magnetic properties on substrates exhibit both a ferromagnetic-like, collective behaviour (due to exchange interactions) and an individual, SMM behavior with no collective order [28; 35]. Investigations on SMMs include also computational studies, based on CASSCF methods [36] and DFT [37; 38] in the gas-phase, on thin films [39] and on metallic [29; 32] and semiconducting substrates [34; 40]. The DFT + \(U\) method was used to investigate Mn-porphyrins on ferromagnetic substrates [41]. First-principles methods, based on wavefunctions and density functionals were also reviewed [38; 42]. A relevant issue is that SMMs, when deposited on conductive nonmagnetic substrates, might loose their magnetic properties, making their magnetic remanence vanishingly small and exhibiting narrow hysteresis loops [43]. In general, the magnetization and the hysteresis of a molecule depend on the chemical environment surrounding the magnetic ion(s). This includes the crystal field of the molecular complex but also the presence of the substrate, via electronic screening, molecule-surface electronic hybridization and charge transfer [44; 45]. Substrates-molecules combinations show cases where the molecular magnetic moment is unaffected or partially/completely quenched. Interestingly, sometimes this is not only related to the ion magnetic moment, but to an overall change in magnetization, where also other atoms (e.g., N or C) contribute to the total moment [46]. Strategies to preserve the SMMs properties include the decoupling between the SMM and a metallic surface, through the insertion of nonmagnetic insulator films [43]. Clearlythen, the successful attempts in keeping the properties of SMMs when adsorbed on insulating films such as MgO [43; 47], pose the question as whether the adsorption on bulk insulating surfaces represents a valid alternative to preserve the molecular magnetic properties of SMMs. Recently, a coordination complex, dimolybdenum tetraacetate Mo\({}_{2}\)(O\({}_{2}\)CCH\({}_{3}\))\({}_{4}\), denoted in the following as \"MoMo\"), was deposited on the bulk insulating calcite (10.4) surface (CaCO\({}_{3}\)) [48] and other metallic and insulating surfaces [49]. Despite the well-known challenges in having stable molecular self-assemblies on insulators, MoMo was observed to form stable and locally regular arrays at high coverage [48], adopting an adsorption geometry on calcite that allows the molecule to be immobile at room temperature, as also confirmed by ab initio DFT calculations [48]. Realizing stable and ordered molecular structures that spontaneously assemble on bulk insulators is an important step forward in the field of molecular electronics [50; 51; 52; 53]. The ordered MoMo arrays open up this possibility, with the extra feature of containing transition metals, regularly spaced by the acetate groups belonging to the molecule [48]. Due to the TM presence, a relevant question is whether these complexes can carry magnetic moments, resulting from stable or meta-stable electronic configurations. A related issue is whether MoMo can be assigned SMM features. Di-molybdenum complexes are known to feature triple or quadruple bonds between the Mo centers (depending on the strength of the \(\pi\)-donation from their ligands) [54; 55] which imply the pairing of the involved electrons and inhibit the formation of magnetic moments.