Ambient Photocatalytic Conversion of Atmospheric CO₂ and H₂O to Polyoxymethylene Via a Redox-Active Titanium–Phenanthroline Matrix

We report a light-driven, ambient-temperature method for converting atmospheric CO₂ and water directly into polyoxymethylene-type (POM) oligomers containing up to 30 –CH₂O– units, mediated by a titanium (IV) chloride–1,10-phenanthroline (phen) matrix. Spontaneous hydrolysis of the matrix with trace water generates a disordered yet catalytically competent mixture of Ti–oxo, Ti–hydroxy, Ti–Cl, and phen-supported Ti species. Water thus plays a dual role, driving hydrolytic assembly of redox-active titanium clusters while simultaneously providing the protons and oxygen atoms required for CO₂ activation and subsequent chain growth. The catalytic network evolves into two distinct but redox-linked families: (i) monomeric, carbonate-bound Ti complexes that capture CO₂ and undergo photoreduction to formaldehyde, and (ii) dimeric, mixed-valent Ti(II/III/IV) species bridged by Cl⁻ and oxo/hydroxo ligands, which catalyze the oligomerization of formaldehyde into acetal chains. Visible-light illumination while enabling CO₂ activation through Ti–OH intermediates promotes hydroxyl-radical (•OH) release from Ti–OH groups, driving Ti(IV) reduction and sustaining the catalytic cycle. In contrast to earlier ligand systems based on π-bonded indolenines or benzoxazoles, which generate complex C₁–C₁₇ oxygenate mixtures, the phen·TiCl₄ matrix displays remarkable formaldehyde selectivity. Only trace formic acid and methanol are detected, bound to phen·Ti₂Cl₅ complexes, where they enhance hydroxyl acidity and assist in protonating formaldehyde. This electrophilic activation facilitates nucleophilic attack by Ti-bound hydroxyls or chain ends, enabling stepwise CH₂O insertion and controlled POM growth. MALDI-TOF mass spectrometry reveals repeating m/z intervals consistent with CH₂O-based chain extension, corroborated by ¹H/¹³C NMR and FTIR spectra of hydroxyl-terminated acetal chains with the general formula HCOO–(CH₂–O)ₙ–H. This work establishes a new paradigm in artificial photosynthesis: a radical-driven, acetal-based pathway that couples CO₂ and H₂O under visible light to construct polymeric carbon frameworks directly from atmospheric inputs.