Two-Dimensional Magnetic Phosphite Oxalate K₂Co₂(HPO₃)₂(C₂O₄)(H₂O)₂ with Triangular Arrangement of Spin Dimers

Two-dimensional magnetic systems have attracted increasing attention recently due to their unconventional quantum ground states. In this work, directed by utilizing the triangular PO₃ unit and oxalate group, a novel two-dimensional magnetic phosphite K₂Co₂(HPO₃)₂(C₂O₄)(H₂O)₂1 with bitriangular layers has been hydrothermally synthesized and characterized by single-crystal X-ray diffraction, optical spectroscopy, magnetic susceptibility, heat capacity, and electron spin resonance measurements, as well as DFT calculations. Magnetic susceptibility results provide a larger effective magnetic moment μ_eff of 5.15(1) μ_B than the theoretical value due to significant orbital contribution and negative Curie–Weiss temperature, indicating predominant antiferromagnetic interactions in 1. However, no evidence of long-range order is observed down to 2 K, due to strong antiferromagnetic dimerization and weak interdimer interactions. DFT calculation results indicate that the intralayer spin exchange interaction through the oxalate group with −20.5 K is stronger than that through the HPO₃^2– group, suggesting that each layer of 1 can be considered as a magnetic layer of dimers stacked in a triangular manner with their bonds perpendicular to the layer. Our work shows promising avenues to rational chemical design and experimentally realize novel specific magnetic lattices including triangular lattices based on a triangular unit such as the PO₃ unit.