Air-Stable Chiral Four-Coordinate Cobalt(II) Guanidinium BINOLate Single-Molecule Magnets

Two pairs of enantiopure mononuclear Co(II) complexes with a CoO4 coordination motif, (S/R-F2BINOL)2CoTMG-H+2 (S/R–Co–F, where F2BINOL refers to 3,3′-difluoro-BINOL, TMG = 1,1,3,3-tetramethylguanidine) and (S/R-N2BINOL)2CoTMG-H+2 (S/R–Co–N, where N2BINOL refers to 3,3′-dinaphthyl-BINOL), were constructed from BINOLate ligands. The introduction of TMG-H+ cations into the second coordination sphere induces intramolecular N–H···O hydrogen-bonding interactions, which enhance the air stability of the complexes. The different substituents at the 3,3′-positions of the BINOLate ligands modulate the molecular geometries of these two pairs of compounds. Compared with S/R–Co–F, which exhibits a distorted tetrahedral coordination geometry, the bulkier naphthyl substituents in S/R–Co–N give rise to a distorted seesaw geometry accompanied by smaller dihedral (δ) and bite angles (θ). Such structural differences result in their distinct magnetic anisotropies with S/R–Co–F and S/R–Co–N being easy-plane and easy-axis, respectively, as supported by the opposite signs of the zero-field splitting (ZFS) parameter D from CASSCF/RASSI-SO calculations and the experimental fitting. Additionally, both S/R–Co–F and S/R–Co–N behave as single-molecule magnets (SMMs) under a 1 kOe dc field, with effective barriers of 46(1)/41(1) K and 63(6)/48(2) K, respectively. These findings provide new insights into the design of air-stable four-coordinate Co(II) SMMs.