In 2011, a new type of hyper-cross-linked polymers (HCPs) arose, known as knitting aromatic polymers (KAPs), characterized by their extraordinary chemical and thermal stability, by their porosity properties, and above all, for the simplicity of their synthesis based on the union of aromatic monomers, without any previous functionalization. The next logical step was the incorporation of metals within these networks, as support for different soluble molecular catalysts or metal nanoparticles (NPs). Thus, the number of metal-containing KAPs has been gradually growing in the past decade, and we have considered that now, in the 10th anniversary of the first KAPs reported, a review of all the metal-containing KAPs and their application as heterogeneous metal catalysts is mandatory. In this review, the most relevant characteristics of all KAPs that contain metals are summarized, divided into two large groups, either as metal complexes or as metal NPs, and classified according to the type of metal incorporated. Finally, the catalytic activities were compared based on the metal employed in each studied reaction, and the future goals of these types of materials are commented on.

Commercial iron (II) phthalocyanine (FePc) was knitted with biphenyl using a Friedel–Crafts reaction to yield a micro-meso porous organic polymer (FePc-POP) with a specific surface area of 427 m²/g and 5.42% of iron loading. This strategy allowed for the direct synthesis of a heterogeneous catalyst from an iron containing monomer. The catalytic system, formed by the knitted polymer containing FePc and DMAP (4-dimethylamino pyridine) as base, results in an efficient heterogeneous catalyst in the cycloaddition of CO₂ to epichlorohydrin to selectively obtain the corresponding cyclic carbonate. Thus, a TON (mmol substrate converted/mmol catalysts used) value of 2700 was reached in 3 h under mild reaction conditions (solvent free, 90 °C, 3 bar of CO₂). The catalyst does not exhibit leaching during the reactions, which was attributed to the excellent stability of the metal in the macrocycle.

The development of photocatalysts that are effective in different organic processes is a topic of great interest to researchers in materials chemistry. Herein, we report the design and synthesis of a new covalent triazine framework (CTF) built by acid-catalyzed trimerization of 4,4′-(benzo[b]benzo[4,5]thieno[2,3-d]thiophene-2,7-diyl)dibenzonitrile activated by microwaves that generates a new organic polymer, DPhBTBT-CTF. This CTF exhibits visible-light absorption due to the electron-donating BTBT units and the extended π-conjugated framework with the electron-acceptor triazine node and proves to be an efficient and versatile metal-free heterogeneous photocatalyst for different organic processes such as selective oxidations of both sulfides and benzyl alcohols to sulfoxides and benzaldehydes, respectively or the oxidative bromination of electron rich aromatic compounds. Moreover, two different proof of concept reactions were tested: the Diels–Alder cycloaddition and the hydroxylation of boronic acids. Heterogeneity studies confirmed that this photocatalyst can be recycled without significant loss of selectivity, and also some mechanistic tests suggested the preferred oxidation pathway.

Porous polymers based on ruthenium and gold triphenylphosphine complexes (KPhos(Ru), KPhos(Ru)Bi, KPhos(AuCl) and KPhos(AuNTf₂)) were prepared via a cost-effective solvent knitting method with [RuHClCO(PPh₃)₃] or AuXPPh₃ (X = Cl, NTf₂) as single monomers or combined with biphenyl, which represents a further approach to obtain heterogenized catalysts. The resulting materials mainly preserve the metal coordination environment of their parent complexes, are stable up to 350 °C and have reasonable surface areas (250–300 m² g⁻¹ for KPhos(Ru)-polymers). KPhos(Ru)s selectively catalyze the imination of alcohols in the presence of base and the results for KPhos(Au)s show they are effective for the intermolecular hydration and hydroamination of alkynes. These materials can be reused several times without significant loss of activity. This novel and simple method affords heterogenized catalysts that combine the reactivity and selectivity of their homogeneous counterparts with the stability and reusability of a heterogeneous framework.