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Air-Stable Chiral Four-Coordinate Cobalt(II) Guanidinium BINOLate Single-Molecule Magnets
Tingting Feng1,2, Xu Ying1,3, Chun-Yang Gu4
1State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China.
Abstract:
Two pairs of enantiopure mononuclear Co(II) complexes with a CoO4 coordination motif, (S/R-F2BINOL)2Co[TMG-H+]2 (S/R-Co-F, where F2BINOL refers to 3,3'-difluoro-BINOL, TMG = 1,1,3,3-tetramethylguanidine) and (S/R-N2BINOL)2Co[TMG-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.
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