Multiple Dimerization Modes in Thiocarboxylate Paddlewheel Complexes: a Comprehensive View of Energy Landscapes from DFT Calculations and Statistics

Thiocarboxylate paddlewheels (PWs) [MTr(SOCR)4L] (M = Pt, Pd; Tr = first-row transition metal; L = Tr-coordinated axial ligand) form a variety of dimeric structures via M⋅⋅⋅M’ and M⋅⋅⋅S’ contacts. We found that [PtVO(SOCPh)4] (1), a molecular spin qubit, yields three crystalline toluene (tol) solvates, namely 1⋅0.875tol and two polymorphic 1⋅0.5tol phases. The crystals contain either staggered quasi-coaxial dimers with short Pt⋅⋅⋅Pt’ distances (3.17-3.23 Å) or heavily bent noncoaxial molecular pairs supported by Pt⋅⋅⋅S’ contacts (3.34-3.38 Å). By contrast, in the known solvatomorphs 1⋅CH2Cl2 and 1⋅0.5hex (hex = n-hexane), two collinear molecules compose a “square” dimer via a pair of reciprocating Pt⋅⋅⋅S’ contacts (3.13-3.16 Å). According to gas-phase DFT calculations (PBE0/def2-TZVPP/D3BJ), dimerization is energetically favored by 15–20 kcal mol−1 and is guided by a shallow potential energy surface, with staggered dimers as ground configurations but eclipsed and square dimers well within energetic reach. Inspection of the local energy minima also disclosed a previously unrecognized eclipsed arrangement with ∼45◦ twisting of both PWs relative to the metal plane, whose existence was confirmed by statistical analysis of PW structures in the Cambridge Structural Database. Our results led to a new classification scheme for these PW dimers relevant to molecular magnetism and quantum technologies.