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.