Ligands have a major impact on applications of the resulting complexes, so their design is crucial in coordination chemistry [1,2]. In this review, the specific properties of bidentate ligands containing both the pyridine group and a hydroxyl group are described. Research on this group of ligands began at the turn of the 20th and 21st centuries. However, interest in these ligands has been revived, and reports on both the structures and properties of their metal complexes have been published [[3], [4], [5], [6], [7], [8], [9], [10], [11]]. For example, 2-pyridinemethanol (A) and 2-pyridineethanol (B) ligands (Fig. 1) that coordinate to metal centers through the nitrogen atom of the pyridine ring form metal complexes that have particularly desirable physical properties and reactivity [[12], [13], [14], [15], [16]].

The electronic structure of the pyridyl group combined with the hydroxyl group stabilizes various metal oxidation states and forms complexes with diverse geometric configurations. Furthermore, the presence of an ethanol or methanol group allows for greater flexibility and potentially different spatial arrangements in coordination complexes, making them particularly valuable in coordination chemistry [[3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. As early as the 1970s, pyridine-based alcohol derivatives, such as alpha-dibutylaminomethyl-2,6-bis(p-trifluoromethylphenyl)-4-pyridinemethanol, mefloquine (quinolinemethanol), and other pyridinemethanol derivatives, were highlighted for their potential in various applications, including antimalarial properties [31,32]. Pyridyl alcohol ligands are particularly useful for complexing transition metals, where they can form stable complexes with metals such as copper, cobalt, nickel, and palladium, where the metal is coordinated by both the nitrogen and oxygen atoms [4,17,28,[33], [34], [35]]. Fig. 2. presents the popular metal complexes coordinated to the two most popular ligands, A and B, both as neutral and deprotonated ligands from the CCDC database [20]. There have been formed 1000 metal complexes so far (December 2024) with A but almost 5 times fewer (189) complexes have been formed with B, presumably because of the lower stability of the six-membered ring formed between B and the metal ion (Fig. 1). Many of these complexes have been found to possess catalytic and antimicrobial properties, and can be used for developing new materials with specific electronic properties. For example, copper(II) complexes have been used as catalysts in oxidation reactions, such as the selective oxidation of alcohols to aldehydes, and silver(I) complexes with Schiff base ligands have shown strong antibacterial activity against E. coli and S. aureus. Also, manganese(III) complexes have been utilized in the fabrication of molecular magnets, and nickel(II) complexes are incorporated into organic semiconductors for use in light-emitting diodes (LEDs) or chromotropic indicators [24].

Pyridyl alcohol ligands are frequently used as scaffolds in catalysis in a range of organic reactions, including hydrogenation, cross-coupling, and oxidation reactions [21,29,[36], [37], [38], [39]]. The ability of these ligands to stabilize metal centers and promote catalytic cycles makes them valuable in both industrial and laboratory settings. The magnetic properties of metal complexes with pyridyl alcohol ligands have also been described, and such compounds can be utilized as single-molecule magnets (SMMs) [7,8,21,40]. In materials science, pyridyl alcohol ligands are investigated for their role in developing novel materials, such as metal-organic frameworks (MOFs) and coordination polymers [22,41,42]. These materials have potential applications in gas storage, separation technologies, and as components in electronic devices due to their tunable porosity and electronic properties [43,44]. The pyridyl alcohol ligands and their metal complexes exhibit antibacterial, antifungal, and antiviral properties, which are leveraged in drug development [9,23,27,[45], [46], [47], [48]]. The hydroxyl group's presence often enhances solubility and bioavailability, crucial for pharmaceutical applications, which we highlight. There is also significant application of larger ligands with alcohol-based pyridine fragments [32,49]. Recent studies also indicate the application of pyridine-based alcohol derivatives, such as B, in efficient SO2 capture. These compounds exhibit high SO2 absorption capacity, excellent reversibility, and low-energy regeneration, making them promising candidates for the development of advanced SO2 absorbents [50]. Besides, A acts as a ligand in a reaction with dicarboxylic acids, forming new compounds, including mononuclear chains and a metal cluster. These materials, due to their low dimensionality and the presence of functional groups, exhibit the ability to remove metal ions, such as Co2+, from the environment [51].

In this review, we present recently reported pyridine-based alcohols and their metal complexes. We demonstrate how these ligands make it possible to plan synthesis of metal complexes with various properties, by highlighting their structural aspects. We also identify the knowledge gaps needed for comprehensive and complete understanding of this class of ligands, which can then be used to design more advanced structural motifs with potentially favorable applications.