Design, Synthesis,\nMagnetic Properties, and Hydrogen\nEvolution Reaction of a Butterfly-like Heterometallic Trinuclear [Cu<sup>II</sup><sub>2</sub>Mn<sup>II</sup>] Cluster

A novel heterometallic trinuclear cluster [Cu<sup>II</sup><sub>2</sub>Mn<sup>II</sup>(cpdp)(NO<sub>3</sub>)<sub>2</sub>(Cl)]\n(<b>1</b>) has been designed and synthesized by employing a\nmolecular\nlibrary approach that uses CuCl<sub>2</sub>·2H<sub>2</sub>O and\nMn(NO<sub>3</sub>)<sub>2</sub>·4H<sub>2</sub>O as inorganic metal\nsalts and H<sub>3</sub>cpdp as a multifunctional organic scaffold\n(H<sub>3</sub>cpdp = <i>N</i>,<i>N</i>′-bis[2-carboxybenzomethyl]-<i>N</i>,<i>N</i>′-bis[2-pyridylmethyl]-1,3-diaminopropan-2-ol).\nThis heterometallic cluster has emerged as an unusual ferromagnetic\nmaterial and promising electrocatalyst for hydrogen evolution reaction\n(HER) in the domain of inorganic and materials chemistry. Crystal\nstructure analysis establishes the structural arrangement of <b>1</b>, revealing a butterfly-like topology with an unusual seven-coordinated\nMn(II) center. Formation of this cluster is accomplished by a self-assembly\nprocess through functionalization of <b>1</b> with one μ<sub>2</sub>:η<sup>1</sup>:η<sup>1</sup>-nitrate and two μ<sub>2</sub>:η<sup>2</sup>:η<sup>1</sup>-benzoate groups via\nthe Cu<sup>II</sup>(μ<sub>2</sub>-NO<sub>3</sub>)Cu<sup>II</sup>} and {Cu<sup>II</sup>(μ<sub>2</sub>-O<sub>2</sub>CC<sub>6</sub>H<sub>5</sub>)Mn<sup>II</sup>} linkages, respectively. Variable-temperature\nSQUID magnetometry revealed the coexistence of ferromagnetic and antiferromagnetic\ninteractions in <b>1</b>. The observed magnetic behavior in <b>1</b> is unexpected because of a large Cu–O–Mn angle\nwith a value of 132.05°, indicating that the correlation between\ncoupling constants and the structural parameters is a multifactor\nproblem. This cluster shows excellent electrocatalytic performance\nfor the HER attaining a current density of 10 mA/cm<sup>2</sup> with\na Tafel slope of 183 mV dec<sup>–1</sup> at a 310 mV overpotential\nvalue. Essentially, cluster <b>1</b> shows exceptional electrochemical\nstability at ambient temperature, accompanied by minimal degradation\nof the current density as examined by chronoamperometric studies.\nDensity functional theory calculations establish the mechanistic insight\ninto the HER process, indicating that the Cu<sup>II</sup>–OCO–Mn<sup>II</sup> site is the active site for formation of molecular hydrogen.