Pt(II)-based molecular catalysts stand as a prototypical system in hydrogen evolution reactions (HER) owing to their consistently elevated activity levels. Their integration into heterogeneous systems thus provides an ideal platform to develop catalytic materials with optimal atom economy. In this work, by rational molecular design, we have synthesized a novel two-dimensional photoactive Covalent Organic Framework (COF), wherein the pore walls host a quinoline-based alpha-diimine ligand serving as a coordination site for anchoring a Pt(II) molecular catalyst. Thorough structural analyses, employing X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (FT-IR), diffuse reflectance spectroscopy (DR), coupled with DFT calculations, distinctly confirm the Pt(II) complexation through the coordination site of the alpha-diimine ligand. The Pt(II)-metalated COF exhibits photocatalytic activity with a hydrogen evolution rate reaching up to 1300 mu mol g-1 h-1. Nevertheless, the occurrence of platinum nanoparticles in post-catalysis samples, along with reduced photocatalytic activity in the presence of chloride ions, suggests that Pt(II) anchored into the COF backbone might not be the primary catalytic site.|An innovative photoactive covalent organic framework (COF) with pore walls featuring a quinoline-based alpha-diimine ligand as a coordination site was synthesized for anchoring a Pt(II) molecular catalyst for hydrogen evolution reaction (HER). Extensive characterization, supported by DFT calculations, shows that H2 evolution is likely driven by the platinum nanoparticles generated through the photocatalytic processes, rather than by Pt(II) species. image