# Understanding the impact of M-OH activation on oxygen evolution in Hofmann-type 2D coordination polymers: Insights from experiments and theory

Gloria Flores, Kerry Wrighton-Araneda, Jose Miguel Ayar, Nathalie Audebrand, Veronica Paredes-Garcia, Ricardo Venegas, Karina Munoz-Becerra, Carlos Cruz

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Gloria Flores, Kerry Wrighton-Araneda, Jose Miguel Ayar, Nathalie Audebrand, Veronica Paredes-Garcia, et al.. Understanding the impact of M-OH activation on oxygen evolution in Hofmann-type 2D coordination polymers: Insights from experiments and theory. International Journal of Hydrogen Energy, 2025, 157, pp.150385. 10.1016/j.ijhydene.2025.150385. hal-05246448

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Submitted on 28 Jan 2026

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<figure><img src="image_1.png" /></figure>
Hofmann-type 2D Coordination Polymers as new catalysts for the oxygen
evolution in alkaline media.

Gloria Flores<sup>a</sup>, Kerry Wrighton-Araneda<sup>a,b</sup>, Felipe Barría<sup>a</sup>, Nathalie Audebrand<sup>c</sup>, Verónica Paredes-García<sup>d,e</sup>, Ricardo Venegas<sup>a,f*</sup>, Karina Muñoz-Becerra<sup>a,b*</sup>, Carlos Cruz<sup>d,e*</sup>

<sup>a</sup>Centro Integrativo de Biología y Química Aplicada (CIBQA), Facultad de Ciencias de la Salud, Universidad Bernardo O'Higgins, General Gana 1702, Región Metropolitana, Santiago 8370854, Chile.

<sup>b</sup>Escuela de Kinesiología, Facultad de Ciencias de la Salud, Universidad Bernardo O'Higgins, General Gana 1702, Región Metropolitana, Santiago 8370854, Chile.

<sup>c</sup>Univ. Rennes, CNRS, INSA Rennes, ISCR (Institut des Sciences Chimiques de Rennes) - UMR 6226, Rennes F-35000, France.

<sup>d</sup>Departamento de Ciencias Químicas, Facultad de Ciencias Exactas, Universidad Andrés Bello, Químicas, Santiago 8370146, Chile.

<sup>e</sup>Centro para el Desarrollo de la Nanociencia y Nanotecnología, CEDENNA, Santiago 9170022, Chile.

<sup>f</sup>Escuela de Tecnología Médica, Facultad de Ciencias de la Salud, Universidad Bernardo O'Higgins, General Gana 1702, Región Metropolitana, Santiago 8370854, Chile.

**Corresponding authors:** karina.munoz@ubo.cl, c.cruzherrera@uandresbello.edu, ricardo.venegas@ubo.cl
**Abstract**

The oxygen evolution reaction (OER) is a key process in water electrolyzers and metal-air batteries, both of which are promising alternatives to fossil fuels. However, the slow kinetics of this reaction demand efficient electrocatalysts. 2D coordination polymers are strong candidates due to their structural and electronic advantages: high crystallinity, porosity, large surface area, abundant active centers, and tunable electronic properties. In this study, we evaluated four Hofmann-type 2D-coordination polymers (HCPs) {M(4acpy)₂ [Ag(CN)₂]₂}n (M = Mn, Fe, Co, and Ni; 4acpy = 4-acetylpyridine), as OER electrocatalysts. The HCPs, referred to as **M-Ag**, were synthesized by incorporating different transition metals strategically selected based on their periodic electronegativity and external 3d electronic configuration. Our results reveal a periodic trend in activity: **Ni-Ag > Co-Ag > Fe-Ag > Mn-Ag**, with **Ni-Ag** outperforming the series and surpassing the standard RuO₂ by 89 mV at 100 mA cm⁻². The superior activity of the **Ni-Ag** HCP is attributed to partial surface structural reconstruction induced by the alkaline electrolyte and applied anodic potential. Experimental results, combined with DFT calculations, support an adsorption-based OER mechanism initiated by the formation of surface Ni-hydroxide species with balanced adsorption energy compared to the rest of the series, which boosts the catalytic process. **Under the applied potential, these species transform into Ni(III)-OH, whose high acidity facilitates the proton-coupled electron transfer process.** As a pioneering study, this research demonstrates, for the first time, the tunability of **M-Ag** HCPs, enabling fine control over their OER catalytic performance and paving the way for the development of promising alternatives for application under operando conditions.
**Keywords:** 2D Hofmann-type coordination polymers; oxygen evolution reaction; electrocatalysis; DFT calculations.

# 1. Introduction.

Energy conversion devices such as water electrolyzers and metal-air batteries are promising solutions to reduce reliance on carbon-based energy. These devices convert chemical energy into electrical energy through electrochemical processes. In electrolyzers, this involves hydrogen reduction and oxygen evolution reactions (HER and OER, respectively). In metal-air batteries involve metal oxidation and oxygen reduction reactions (ORR/OER). Nevertheless, the oxygen-based reactions are kinetically limited due to critical factors, including a four-electron transfer, multi-intermediate adsorption steps, O=O bond forming/activation, and multiplicity singlet-triplet interconversion [1]. Efficient catalysts address these limitations, with noble metals like Pt (for ORR) and Ru/Ir oxides (for OER) as benchmarks. Specifically, RuO₂ and IrO₂ stand out nowadays as top-performing electrocatalysts for the anodic OER. However, their scarcity, high price, and limited stability at high pH values have restricted their large-scale implementation [2–4]. These problems have inspired the development of stable, efficient, and inexpensive alternative catalysts for oxygen-based reactions. Among the alternatives, semiconducting 2D materials, such as graphene and layered double hydroxides (LDH), have emerged as transformative options for developing noble-metal-free catalysts [5,6]. Their unique electronic distribution and high specific surface area increase the availability of active sites and enhance mass transport during catalysis [7]. These advancements have inspired the rational design of next-generation 2D materials, such as 2D coordination polymers (CPs), including metal-organic frameworks (MOFs). These materials have recently gained attention as
electrocatalysts, especially in OER in alkaline media. The superior performance of CPs and MOFs has been attributed to their high crystallinity, porosity and surface area, active sites availability, and the possibility of tuning the electronic properties by a structural design [8–15].

CPs correspond to a class of metal-organic crystalline solids in which metallic ions (Mⁿ⁺) are coordinated with organic ligands (L) [9,16,17]. 120 years ago, Hofmann reported the cyanido CPs [Ni(CN)₂(NH₃)]·(bz) (bz = benzene) [18]. Since then, a large number of analogous compounds [MLₙ(M'(CN)₄)] (M= divalent cation, often a first transition metal; M' = Ni(II), Pt(II), Pd(II)) have been designed and obtained, giving rise to a family of CPs known as Hofmann-type coordination polymers (HCPs). The structure of the [MLₙ(M'(CN)₄] HCPs is often described as pillared 2D structures comprising layers of [M(M'(CN)₄)] with the axial position of the M cation occupied by an N-donor ligand. In this frame, the M cation presents an N₆ octahedral geometry, being the M' cation in a square-planar geometry. Moreover, the structural features of the HCPs are defined by the organic L ligands. Using ditopic N-donor ligands, 3D frameworks can be obtained with the possibility of porous structures depending on the length of the ligand, while monotopic N-donor ligands afford 2D structures [19–21]. The structural and electronic diversity of the HCPs have rendered important features such as gas adsorption and magnetic properties [19,20,22–24]. Despite the outstanding properties of these HCPs in terms of low-cost building blocks (for noble metal-free CPs), periodic ordering, active sites availability, and tunable electronic conjugation, their application as electrocatalysts for energy conversion reactions in their pristine phase is scarce [25–29]. Gao *et al.* [25] have been pioneers in the use of HCPs in electrocatalysis, reporting for the first time in 2018 the use of a series of as-
synthesized HCPs with M' = Fe, Co, Ni, as electrocatalysts for OER in alkaline media. The superior performance was achieved with CoFe-PYZ and attributed to the synergy of the constituted metals, where Co²⁺ influences the local electronic structure of the catalytic Fe²⁺ active centers [25]. Afterward, Wang *et al.* [26] studied the OER activity of HCPs denoted as M-NiHF, being M= Fe, Co, Ni, and L= 2-methylpyrazine. The outstanding OER performance of Fe-NiHF was attributed to the bimetallic Fe-Ni synergy that favors the electron transfer and the thin morphology of the compound, which allows a high active site exposure. The proposed active sites were Ni²⁺ centers of the [Ni(CN₄)]²⁻ units, whose electron density is decreased by the presence of adjacent Fe²⁺ sites favoring the Ni²⁺-OH adsorption [26].

Following this line, Cheng *et al.* investigated the morphology-dependent OER activity of bimetallic nanostructured HCPs, including nanoboxes, nanocubes, nanoplates, and nanosheets [28]. Their findings revealed that the hollow nanobox morphology demonstrated superior performance with an overpotential of 285 mV @10 mA cm⁻² in 1.0 M KOH [28]. In order to broaden the understanding of HCPs as OER electrocatalysts, this group deep into the importance of the reconstruction phenomena. These processes, often inevitable during electrocatalytic reactions, occur due to material oxidation caused by applied overpotentials and/or interactions leading to catalyst dissolution [30,31]. XPS analysis after 24 h of exposure to OER, reveals the formation of surface hydroxide species, such as Fe(OH)₃ and Ni(OH)₃, confirmed by FTIR analysis. Additionally, the collapse of the nanostructures was observed by FESEM analysis.

Leveraging the 2D architectures and transformation abilities of HCPs in alkaline media, Huo *et al.* [27] prepared FeNi layered double hydroxides (FeNi-LDHs) using the
bimetallic [Fe(L)₂Ni(CN)₄] CP, in which L=pyridine (py) or isoquinoline (Isoq). The transformation process involved the attack of OH⁻ ions to coordination bonds of the HCP, resulting in the displacement of ligands from the structure. In this context, the π-π interactions of the ligands, weaker in the pyridine ligand compared to the Isoq ligand, significantly influence the catalyst's final morphology and, consequently, its OER performance [27].

An interested family of HCPs comprehends those containing dicyanometalate [M'(CN)₂]⁻ fragments instead of [M'(CN)₄]²⁻ ones, forming structures with the general formula [MLₙ(M'(CN)₂] (M' = Ag, Au, Cu). The group of J. A. Real published a series of structures of this family for the first time in 2002, with M = Fe, M' = Ag, L = pyrazine, 4,4'-bipyridine, or bis-pyridyl-ethylene [32]. The low coordination of the dicyanidometallate fragments leads to compounds with novel extended structures and unique electronic properties. Although these materials typically display interpenetrated structures, they usually can host guest molecules exhibiting an interplay between host-guest chemistry and magnetic behavior [31,33]. Hill et al. have defined dicyanometalate-based HCPs as the natural extension of oxide frameworks as perovskite and cyanido frameworks as Prussian-Blue analogs (Scheme 1) [30,34], which have exhibited competitive electrocatalytic activity in energy conversion, such as in the OER [35–37]. However, to the best of our knowledge, their electrocatalytic properties in their pristine form have not been studied to date.
<figure><img src="image_2.png" /></figure>

**Scheme 1.** The Hofmann CP can be defined as extensions of cyanido-based Prussian Blue networks ([M(M'(CN)₆)]). [MLₙ(M'(CN)₄)] HCPs (left side of the scheme) are obtained by inserting an L ligand and changing the M' for divalent Ni, Pt, and Pd. On the other hand, [MLₙ(M'(CN)₂)] HCPs (right side of the scheme) are obtained by changing the M' for monovalent Cu, Ag, or Au and inserting pyridine ligands.

Inspired by the state-of-the-art of HCPs as electrocatalysts, four crystalline Hofmann-type coordination polymers, {M(4acpy)₂[Ag(CN)₂]₂}n [M= Mn(II), Fe(II), Co(II), and Ni(II), 4acpy = 4-acetylpyridine], have been synthesized, characterized, and evaluated for their catalytic activity in the OER for the first time. The four compounds, named **M-Ag** (M=Mn, Fe, Co, Ni), are isostructural presenting 2D structures. The electrocatalytic study showed that the OER activity follows the trend **Ni-Ag** > **Co-Ag** > **Fe-Ag** > **Mn-Ag**, with overpotentials of 381, 441, 481, and 506 mV, and Tafel slopes values of 42, 59, 70, and 78
mV dec⁻¹, respectively. The superior activity of the **Ni-Ag** CP is attributed to the fact that Ni-centers are able to partially oxidize to Ni²⁺ → Ni³⁺ to form active Ni-OH species (in the catalyst interface) that coexists with the bulk 2D structure of the CP, enhancing the overall OER activity. Theoretical DFT calculations supported the experimental results.

## 2. Experimental.

### 2.1 Synthesis of {M(4acpy)₂[Ag(CN)₂]₂}n (M= Mn(II) (Mn-Ag), Fe(II) (Fe-Ag), Co(II) (Co-Ag) and Ni(II) (Ni-Ag)).
The solvents and reagents used for the synthesis were purchased from Sigma-Aldrich® and Merck® and used without any previous purification.

**Synthesis of Mn-Ag.** Single crystals of **Mn-Ag** were grown using the following procedure: A mixture of a water solution of 0.5 mmol of Mn(ClO₄)₂·nH₂O (5 mL) and an ethanol solution of 1 mmol of 4-acetylpyridine (5 mL) was added to 1 mmol of K[Ag(CN)₂] dissolved in 10 mL of water. The mixture was refluxed for 3 h, and the resulting clear solution was left undisturbed, giving colorless plate crystals after one week. MW: 617.03 g/mol. Yield of 51% based on the metallic salt. % Calculated from the empirical formula C₁₈H₁₄N₆O₂MnAg₂ for **Mn-Ag** (in %): C, 35.04; H, 2.29; N, 13.62. % Obtained from Elemental Analysis (in %): C, 36.59; H, 2.16; N, 13.55.

**Synthesis of Fe-Ag.** Single crystals of **Fe-Ag** were grown using a straight tube with an initial layer of a water solution of K[Ag(CN)₂] (1 mmol in 10 mL of water), 2:1 water-acetonitrile as buffer solution, and a superior layer of a mixture of Fe(ClO₄)₂ and 4-acetylpyridine in pure acetonitrile (0.5 and 1 mmol in 10 mL of acetonitrile, respectively). The filled tube was closed with paraffin film. MW: 617.94 g/mol. Yield of 45% based on the metallic salt. % Calculated from the empirical formula C₁₈H₁₄N₆O₂FeAg₂ for **Fe-Ag** (in %):
): C, 34.98; H, 2.29; N, 13.60. % Obtained from Elemental Analysis (in %): C, 35.80; H, 2.50; N, 13.71.

**Synthesis of Co-Ag and Ni-Ag.** Compounds **Co-Ag** and **Ni-Ag** were synthesized as polycrystalline powder by direct mixing of 0.5 mmol of M(ClO₄)₂·nH₂O (M=Co, Ni) and 1 mmol of 4-acetylpyridine in 10 mL of water and 1 mmol of K[Ag(CN)₂] in 10 mL of water. The powder was washed with water and ethanol. For **Co-Ag**, MW: 621.01 g/mol. Yield of 88% based on the metallic salt. % Calculated from the empirical formula C₁₈H₁₄N₆O₂CoAg₂ (in %): C, 35.81; H, 2.27; N, 13.53. % Obtained from Elemental Analysis (in %): C, 37.87; H, 2.22; N, 13.87. For **Ni-Ag**, MW: 620.77 g/mol. Yield of 84% based on the metallic salt. % Calculated from the empirical formula C₁₈H₁₄N₆O₂NiAg₂ (in %): C, 35.83; H, 2.27; N, 13.54. % Obtained from Elemental Analysis (in %): C, 38.42; H, 2.16; N, 13.98. **Ni-Ag** CP was treated in a 1.0 M KOH solution for 24 hours to test the chemical stability of the pristine phase. This sample was labeled as **Ni-Ag***.

**2.2 X-ray Data Collection and Structure Determination.** Single crystals (SC) of **Mn-Ag** and **Fe-Ag** were directly picked up from reaction media, immediately immersed in a drop of Paratone® oil, and mounted in a tip of a capillary pin in D8 Venture Bruker diffractometer using Mo-Kα radiation (λ = 0.71073 Å). The intensities were measured at 150 for **Mn-Ag** and **Fe-Ag**. Frame integration and data reduction were carried out with SAINT [38], and the SADABS program was employed for multiscan-type absorption corrections [39]. Using the OLEXs package [40], the crystal structures were solved with the ShelXT [41] structure solution program using Dual Methods and refined with the ShelXL [42] package using least-squares minimization based on F². Crystallographic data details on data collection and refinement parameters of the crystal structure are summarized in **Table**
1. Structure drawings have been made with TOPOS software [43]. Additional data concerning the crystals and the refinement parameters are detailed in the supporting information. The powder X-ray diffraction data were collected at room temperature on a Bruker D8 ADVANCE with Cu Kα₁,₂ radiation in the range of 5°< 2θ< 80°. Indexing of the powder patterns of **Co-Ag**, **Ni-Ag** and **Ni-Ag*** has been made with the program DICVOL06 [44]. The whole powder pattern fittings (with the Le Bail algorithm) were done employing the FULLPROF [45] program available in the software package WinPLOTR [46]. The experimental powder X-ray diffraction (P-XRD) patterns of **Co-Ag**, **Ni-Ag** and **Ni-Ag*** agree well with the calculated patterns generated by the SC-XRD data from the crystal structures of **Mn-Ag** and **Fe-Ag** (Figure S1 and S2). The unit-cell parameters calculated from the indexing of the P-XRD patterns and refined by the Le Bail fittings (Table S1a) are very similar to those of **Mn-Ag** and **Fe-Ag** (Table 1). These results prove that the synthesized polycrystalline samples and the obtained single crystals belong all to the same family of crystalline materials. Furthermore, the whole powder pattern fitting demonstrates the purity of the powder samples (a small impurity is observed in **Co-Ag**). It can be noted that the Le Bail refinements for **Co-Ag** and **Ni-Ag** have been made with the space group *P*2₁/*n* (N° 14), which is the same as for **Fe-Ag**, whereas for **Ni-Ag*** and **Mn-Ag** the space group is *P*2₁/*c* (N° 14) in order to keep close unit-cell parameters for all compounds of this series.

<caption>**Table 1.** Crystal data and structure refinement for **Mn-Ag** and **Fe-Ag**.</caption>

**Crystal data and structure refinement for Mn-Ag and Fe-Ag**

<table><thead><tr><th></th><th>Mn-Ag</th><th>Fe-Ag</th></tr></thead><tbody><tr><td>Identification code</td><td></td><td></td></tr><tr><td>Empirical formula</td><td>C<sub>18</sub>H<sub>14</sub>Ag<sub>2</sub>MnN<sub>6</sub>O<sub>2</sub></td><td>C<sub>18</sub>H<sub>14</sub>Ag<sub>2</sub>FeN<sub>6</sub>O<sub>2</sub></td></tr><tr><td>Formula weight</td><td>617.03</td><td>617.94</td></tr><tr><td>Temperature/K</td><td></td><td></td></tr></tbody></table>

150
<table><tr><td>Crystal system</td><td>monoclinic</td><td></td></tr><tr><td>Space group</td><td>P2<sub>1</sub>/c</td><td>P2<sub>1</sub>/n</td></tr><tr><td>a/Å</td><td>7.4175(14)</td><td>7.5830(6)</td></tr><tr><td>b/Å</td><td>13.698(2)</td><td>13.6209(11)</td></tr><tr><td>c/Å</td><td>11.184(2)</td><td>10.4943(9)</td></tr><tr><td>β/°</td><td>106.223(7)</td><td>110.131(3)</td></tr><tr><td>Volume/Å<sup>3</sup></td><td>1091.1(3)</td><td>1017.71(15)</td></tr><tr><td>Z</td><td>2</td><td>2</td></tr><tr><td>ρ<sub>calc</sub>g/cm<sup>3</sup></td><td>1.878</td><td>2.017</td></tr><tr><td>μ/mm<sup>-1</sup></td><td>2.372</td><td>2.635</td></tr><tr><td>Crystal size/mm<sup>3</sup></td><td>0.48 × 0.22 × 0.14</td><td>0.12 × 0.07 × 0.05</td></tr><tr><td>Radiation</td><td>MoKα (λ = 0.71073)</td><td>MoKα (λ = 0.71073)</td></tr><tr><td>2Θ range for data collection/°</td><td>4.82 to 55.056</td><td>5.102 to 54.948</td></tr><tr><td>Reflections collected</td><td>7450</td><td>11405</td></tr><tr><td>Independent reflections</td><td>2447 [R<sub>int</sub> = 0.0321, R<sub>sigma</sub> = 0.0305]</td><td>2332 [R<sub>int</sub> = 0.0308, R<sub>sigma</sub> = 0.0259]</td></tr><tr><td>Data/restraints/parameters</td><td>2447/0/134</td><td>2332/0/134</td></tr><tr><td>Goodness-of-fit on F<sup>2</sup></td><td>1.132</td><td>1.086</td></tr><tr><td>Final R indexes [I>=2σ (I)]</td><td>R<sub>1</sub> = 0.0221, wR<sub>2</sub> = 0.0516</td><td>R<sub>1</sub> = 0.0198, wR<sub>2</sub> = 0.0466</td></tr><tr><td>Final R indexes [all data]</td><td>R<sub>1</sub> = 0.0235, wR<sub>2</sub> = 0.0522</td><td>R<sub>1</sub> = 0.0212, wR<sub>2</sub> = 0.0474</td></tr><tr><td>Largest diff. peak/hole / e Å<sup>-3</sup></td><td>0.49/-0.81</td><td>1.18/-0.53</td></tr></table>

**2.3. FESEM-EDS.** The morphological analyses of **M-Ag** were performed by field emission scanning electron microscope using FEI Quanta FEG250 equipment coupled with an energy-dispersive detector (EDXS) (Figure S3).

**2.4. Thermogravimetric Analysis.** Thermogravimetric analyses of **M-Ag** were performed on a Mettler Toledo TGA/DSC-II system. The samples were introduced in an alumina holder and heated under an air atmosphere from room temperature to 900 °C with a heating rate of 5 °C/min (Figure S4).

**2.5. Infrared Spectroscopy.** Fourier transform infrared spectra of **M-Ag**, the organic ligand 4-acetylpyridine, and the potassium cyanoargentate were recorded on an ATR-FTIR-4000 Jasco in the 4000–400 cm<sup>-1</sup> range without using any support (Figure S5).
**2.6. Magnetic Measurement.** Magnetic measurements were carried out using a Quantum Design Dynacool Physical Properties Measurement System (PPMS) equipped with a Vibrating Sample Magnetometer (VSM). The dc data were collected under an externally applied field of 10 kOe in the 2-300 K temperature range. Isothermal magnetization measurements were performed between 0 and 90 kOe at 2 and 5 K. Diamagnetic corrections (estimated from Pascal constants) were considered [47].

**2.7 X-ray Photoelectron Spectroscopy (XPS).** The analysis was carried out with a PHOIBOS 150 electron analyzer (SPECS) under a pressure lower than $2 \times 10^{-9}$ mbar using Ag $K_\alpha$ 2984.3 eV radiation and a constant pass energy of 30 eV. C-C C1s binding energy at 284.8 eV was employed as a reference.

**2.8. Electrochemical Measurements.** The electrochemical analyses were carried out in a conventional glass cell using an Ag/AgCl($3.0 \text{ M KCl}$) electrode as a reference and a graphite bar as the counter electrode. The working electrode was a glassy carbon (GC)-rotating disk electrode (PINE research E6R1, area = 0.196 $\text{cm}^2$) modified with HCPs catalytic inks of each. The catalytic inks were prepared according to a methodology reported in the literature as follows [25,26,48]: 1 mg of the synthesized **HCP** (**Figure S3**) was carefully grounded, mixed with 4 mg of carbon **Vulcan XC72®** and dispersed by sonication in a mixture of 1 mL of Milli-Q water and 4 mL of isopropanol for 20 minutes. Then, 20 µL of Nafion was added, and the mixture was sonicated for 10 minutes. Before the electrode modification, the working electrode surface was polished with a 0.5 µm alumina slurry and sonicated in Milli-Q water for 1 minute to eliminate impurities. The GC disk was modified by dropping 10 µL of the catalytic ink on its surface and drying under a nitrogen flow. The modified electrodes were characterized by cyclic voltammetry (CV) in $N_2$-saturated 1.0 M KOH
solution at a scan rate of 50 mV s⁻¹, while the catalytic studies for the OER were conducted by linear sweep voltammetry (LSV) in O₂-saturated 1.0 M KOH (pH 14) at a rotation of 1600 rpm and a scan rate of 5 mV s⁻¹. Before the electrocatalytic analyses, the modified GCE was activated through several cycles (500 cycles) in N₂-purged 1.0 M KOH, at a scan rate of 500 mV s⁻¹ until a stable CV was obtained. Finally, the stability of the catalysts with the best OER performance was evaluated by chronoamperometry in an O₂-saturated 1.0 M KOH for 5 hours at a current density of 10 mA cm⁻². The potentials were referred to RHE according to the following Equation 1:

$$E(RHE) = E_{applied} + E_{Ag/AgCl(3.0M KCl)} + 0.059 * pH \quad (Equation 1)$$

**2.9. Computational Modeling.** Quantum chemical modeling was applied to explore the influence of electronic structures on catalytic properties through the DFT framework. Periodic condition models were developed using CP2K to evaluate solid-state properties using a PBE functional while all atoms were treated using Gaussian plane waves for basis set and norm-conserving Goedecker-Tetter Hutter for pseudopotentials, DZVP-MOLOPT-PBE-GTH and GTH, respectively [49,50]. Relaxed structures of **Mn-Ag**, **Fe-Ag**, **Co-Ag**, and **Ni-Ag** were employed to build up the molecular models to evaluate molecular properties at higher theory level. Molecular models were treated using the Gaussian16 electronic structure program with the hybrid functional B3LYP. Dispersion force corrections were evaluated using the Grimme D3 model with the Becke-Johnson damping function (D3BJ), and the Def2-TZVP basis set was applied to all atoms [51–54]. The total electronic energies were obtained under tight convergence criteria using the quadratically convergent SCF procedure for the relaxed structures. These molecular designs, considered as possible active species, included the coordinative vacant structure (Ag-M-V), the
hydroxide structure (Ag-M-OH), and the (oxy)hydroxide structure (Ag-M-OOH) **(Scheme 2)**.

<figure><img src="image_3.png" /></figure>

**Scheme 2.** Periodic and molecular models used for electronic structure analysis: M= Mn, Fe, Co, and Ni; L= 4acpy.

Stabilization studies were conducted to evaluate the competitive interaction between -OH and -OOH. The hydroxide and (oxy)hydroxide formations were analyzed using adsorption energy ($E_{ads}$), calculated as described in **Equation 2**:
$$E_{ads} = E_{OH/OOH} + E_{M-Ag} - E_{Ag-M-OH/OOH} + BSSE \quad (Equation 2)$$

Here, $E_{OH/OOH}$, $E_{M-Ag}$, and $E_{Ag-M-OH/OOH}$ represent the total electronic energy of hydroxide/(oxy)hydroxide anions, the M-Ag molecular model, and the Ag-M-OH/OOH complex, respectively. Basis Set Superposition Error (BSSE) was corrected using the counterpoise method described by Boys and Bernardi [55]. The quadratically convergent method was employed to ensure consistency in energy calculations during the BSSE procedure. Notably, higher $E_{ads}$ values (Equation 2) indicate more stable species, reflecting higher stabilization during the adsorption processes. To characterize the nature of interactions between fragments, the Atoms-in-Molecules (AIM) method was employed. This approach enables the determination of electron density at specific bond critical points ($\rho_{BCP}$) between molecules [56]. The magnitude of $\rho_{BCP}$ provides insights into the strength of the interaction: strong covalent bonds exhibit higher electron densities, while weaker interactions such as hydrogen bonds or electrostatic attractions display lower values. Specifically, $\rho_{BCP}$ ranges were categorized as follows: ~0.1–0.5 $|e|$/Bohr$^3$ for covalent bonds, ~0.04–0.10 $|e|$/Bohr$^3$ for coordinative bonds, and $\rho_{BCP} \le 0.01|e|$/Bohr$^3$ for weak electrostatic interactions. For these calculations, the following critical point search parameters were applied: a maximum of 120 iterations, a gradient normalized convergence threshold of $1.0 \times 10^{-6}$, and a Hessian matrix singularity criterion of $5.0 \times 10^{-22}$ a.u., with all length units expressed in Bohr. To validate the AIM topological analysis on isolated molecules, the Poincaré-Hopf relationship was verified in all cases [56]. Additionally, a correlation analysis of molecular properties was performed and contrasted with experimental data to identify the key properties influencing electrocatalytic behavior through the analyses of correlation matrices. The primary objective is to uncover structural-
electronic relationships that can explain the fundamental steps of the electrocatalytic
process.

**3. Results and discussion.**

**3.1 Structural Characterization of M-Ag HCPs.**

To investigate the electrocatalytic properties of HCP-based electrocatalysts, a series
of 2D Hofmann coordination polymers with the general formula {M(4acpy)₂[Ag(CN)₂]₂}n
(M-Ag) have been synthesized, incorporating four different 3d transition metals along with
Ag(I) cation. These metals were strategically selected based on their increasing
electronegativity across the period and their distinct d-electron configurations, which are
expected to influence their catalytic behavior rationally. The studied series comprehends
M-Ag structures with M = Mn, Fe, Co, and Ni, where the M(II) centers present an [MN₆]-
octahedral environment, as determined by SC-XRD for Mn-Ag and Fe-Ag (Figure 1a),
solved at 150 K (Table 1). This geometry is defined by four CN⁻ anions belonging to four
linear [Ag(CN)₂]⁻ fragments and two 4acpy ligands in the axial positions. Both compounds
crystallize in the space group N° 14 (P2₁/c or P2₁/n).

In the extended structure of the as-synthesized HCP, the Mn(II) centers [Mn(II)
taken as reference] are interconnected through [Ag(CN)₂]⁻ units forming an extended
{Mn[Ag(CN)₂]₂} square grid (Figure 1a). The square windows in the grid are penetrated
by two 4acpy ligands belonging to two adjacent layers, in which 4acpy interacts with the
Ag(I) through the ketone with an Ag1···O1=2.9484(16) Å, forming a supramolecular 3D
stacked structure (Figure 1b). The Co-Ag and Ni-Ag polycrystalline powders were
unequivocally confirmed to be isostructural by comparing their experimental P-XRD
patterns with the simulated ones for Mn-Ag and Fe-Ag (Figures S1, S2a-b and Table
**S1a).** In addition, the microstructure of the HCPs studied by FESEM show that all four **M-Ag** HCPs exhibit similar aggregate morphologies, characterized by particles with rough surface textures and sizes ranging on the order of a few micrometers (**Figure S3**).

The thermal stability of the **M-Ag** HCPs was evaluated using thermogravimetric analysis in an air atmosphere (**Figure S4**). From room temperature to ~180°C, no weight loss was observed, confirming the absence of interstitial guest molecules and indicating structural stability within this range. A gradual weight loss of ~50% occurs beyond 180°C, reaching a plateau at ~400°C and marking complete decomposition (**Figure S4**). A closer inspection reveals that **Mn-Ag**, **Co-Ag**, and **Fe-Ag** decompose in two-steps, while **Ni-Ag** decomposes in three steps, associated with the sequential release of the 4acpy molecules followed by the loss of the cyanido groups. The two-step release of the axial 4acpy ligands suggests that **Ni-Ag** stabilized more effectively than the other series members (more details are provided in Supporting Information).

**Figure S5a** shows the ATR-IR spectra of the **M-Ag** series, revealing similar fingerprints across the compounds. The spectra show the characteristic $\bar{\nu}$(C=O) vibration of the free acetyl group from the 4acpy ligands at approximately 1688 cm⁻¹ together with the $\bar{\nu}$(C≡N) vibration, which is observed in the range of 2169-2159 cm⁻¹ (**Figure S5b**). The $\bar{\nu}$(C≡N) vibrations for **M-Ag** compounds are blue-shifted in comparison to K[Ag(CN)₂] (2136 cm⁻¹), where **Mn-Ag** and **Fe-Ag** present almost the same $\bar{\nu}$(C≡N), while **Co-Ag** and **Ni-Ag** present a higher blue shift to 2164 and 2169 cm⁻¹ respectively (**Figure S5c**). Since CN⁻ acts as both a σ-donor and π-acceptor, its $\bar{\nu}$(C≡N) vibration is sensitive to coordination with a metal cation and influenced by (i) the electronegativity and (ii) the charge of the cation. In the **M-Ag** compounds, where the 3d metal cations possess
the same charge and coordination number, electronegativity is the only factor influencing
the $\bar{\nu}(C\equiv N)$ [57]. In this sense, donation from the $\sigma$-antibonding orbitals increases the
$\bar{\nu}(C\equiv N)$, while backbonding from the $\pi$-antibonding orbitals reduces the $\bar{\nu}(C\equiv N)$ vibration
[58]. Thus, the **M-Ag** $\bar{\nu}(C\equiv N)$ vibrations exhibit the following trend: $\bar{\nu}_{Mn} \sim \bar{\nu}_{Fe} > \bar{\nu}_{Co} > \bar{\nu}_{Ni}$.
Therefore, it can be inferred that $\pi$-backbonding from the M cations becomes progressively
less significant in the Mn ~ Fe > Co > Ni series compared to $\sigma$-donation, which correlates
with the effect on the covalency of the M-N bonds. This trend is supported by the shorter
Fe-N bond distances relative to Mn-N (**Table S1b**), consistent with the higher
electronegativity of Fe(II) compared to Mn(II) [59].

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>
**Figure 1.** Views of the crystal structure of **Mn-Ag** at 150 K (P2₁/c): (a) showing an {Mn[Ag(CN)₂]₂} layer exhibiting a quasi-square 2D structure decorated by the 4acpy ligand in the axial coordination of each Mn(II) center. The inset shows the [MnN₆] coordination around the metal cation formed by the four cyanido groups and the two 4acpy ligands. (b) c-axis crystal view and schematic figure showing the packing of three successive layers interacting via Ag···O interactions to form a triple interlocked supramolecular structure.

The 2D structure of the as-synthesized **M-Ag** HCPs provides a high surface-to-volume ratio, enhancing the exposure of the active sites [60]. This structural feature facilitates efficient interaction between the surface atoms of these layered CPs and the reaction media, which can enhance catalytic activity [7,15]. Moreover, the heterometallic nature of the **M-Ag** materials allows precise control over their catalytic behavior [61]. Furthermore, their robust structure makes the **M-Ag** HCPs ideal candidates for electrocatalyst applications.

**3.2 Electrochemical and Electrocatalytic Studies of M-Ag HCPs.**

To explore the influence of different metals on the catalytic properties of these **M-Ag** systems, electrochemical and electrocatalytic experiments were performed in alkaline media (1.0 M KOH; pH 14) using the HCPs crystalline powder samples immobilized on glassy carbon electrode (GCE) surfaces. The GCEs were modified using a drop-casting method with a catalyst loading of c.a. 51 µg/cm² (see Experimental details). The electrochemical characterization of the modified electrodes was conducted through cyclic voltammetry. For comparison, we included GCEs modified with the organic ligand 4acpy
and the K[Ag(CN)₂] precursor. The voltammetric profiles of each modified electrode are presented in **Figure 2a**.

<figure><img src="image_6.png" /><figcaption>**Figure 2.** a) Cyclic voltammetry of GCE modified with catalytic inks containing the 4acpy ligand, the K[Ag(CN)₂] precursor, and the as-synthesized HCPs. CVs were conducted in 1 M KOH purged with N₂, scan rate: 50 mV s⁻¹. b) Minimum unit of the HCP crystalline</figcaption></figure>
structure. c) Polarization curves for the OER on the HCPs modified electrodes and (at bottom-right) zoom of the kinetic segment of the LSVs showed in c) (LSVs were recorded in 1 M KOH saturated with O₂ at 1600 r.p.m., scan rater: 5 mV s⁻¹).

The 4acpy ligand exhibits a faradaic redox process in the cathodic scan direction with a reduction peak potential (E_c) of 0.730 V<sub>RHE</sub>, corresponding to a one-electron reduction process centered on the acetyl group (C=O, blue area in **Figure 2a**) [62,63]. The K[Ag(CN)₂] precursor shows a cathodic peak (E_c = 1.09 V<sub>RHE</sub>; red area in **Figure 2a**) associated with its one-electron reduction process, which is in agreement with previous studies [64,65]. Comparing the CVs of the precursors with those of the as-synthesized **M-Ag** HCPs, clear redox differences can be seen due to the integration of 4acpy ligands and [Ag(CN)₂]⁻ fragments into the structures (**Figure 2b**). Oxidation processes for the M(III)/M(II) couples were observed in all the **M-Ag** HCPs [66] (green area in **Figure 2a**) being the **Ni-Ag** with the most pronounced anodic Ni(III)/Ni(II) peak, related to a high amount of surface Ni sites that might be transformed to Ni-hydroxide species, as discussed below (calculated charges of 3.7 µC cm⁻² for **Ni-Ag**, 0.3 µC cm⁻² for **Co-Ag**, 0.1 µC cm⁻² for **Fe-Ag**, and 0.1 µC cm⁻² for **Mn-Ag**) [66–69]. The cathodic peaks observed close to ~1.1 V<sub>RHE</sub> match with the one-electron reduction process of the [Ag(CN)₂]⁻ unit (red area in **Figure 2a**) [64,65]. In the case of the **Co-Ag** HCP, two anodic processes are observed with oxidation peak potentials of 1.22 V and 1.34 V<sub>RHE</sub>, which are attributed to Co(III)/Co(II) and Co(IV)/Co(III) redox couples, respectively. Only the first process, at a lower anodic potential, shows a corresponding cathodic peak at 1.084 V<sub>RHE</sub>, adjacent to the cathodic process associated with the [Ag(CN)₂]⁻ units. This behavior is attributed to the instability of Co(IV) species, which are rapidly reduced back to Co(III) [29,70].
Subsequently, the electrocatalytic activity of the **M-Ag** series for the OER was
tested in alkaline media saturated with dioxygen. The catalysts were first activated through
various CV cycles (500 cycles) in an N₂-purged electrolyte within the potential window
0.3-1.5 V<sub>RHE</sub> until stable CV curves were observed. The LSV curves in **Figure 2c**
corroborate a higher catalytic activity for the **M-Ag** systems integrating the 4acp ligands
and [Ag(CN)₂]⁻ fragments to the MN₆ units (**Figure 2b**), compared to the free precursors.
The variations in onset potentials follow the **Ni-Ag** > **Co-Ag** > **Fe-Ag** > **Mn-Ag** trend,
which can be correlated with the periodic electronegativity increasing of the M cations in
the MN₆ units, which modifies the covalency of the M-N bonds due to the prevalence of σ-
donation (*vide supra*). Thus, the **Ni-Ag** system exhibits the highest catalytic activity among
the series, with an overpotential of 381 mV @10 mA cm⁻² and the lowest Tafel slope,
indicating enhanced charge transfer (**Figure S6** and **Table S2**). The superior OER
performance of the **Ni-Ag** catalyst is further supported by the calculated turnover frequency
(TOF) at η = 381 mV, assuming that all Ni centers are catalytically active (**Table S2**).
Although the overpotential of **Ni-Ag** is 46 mV higher than that of the reference RuO₂ (335
mV @10 mA cm⁻²), at a higher current density of 100 mA cm⁻², **Ni-Ag** outperforms RuO₂
by 89 mV (**Figure 2c**). **Table S2** also includes the electrocatalytic parameters for some
HCPs and Ni-based MOFs previously reported in the literature. From these data, it can be
extracted that the monometallic Ni-carboxylate-derived MOFs [66,70] show higher OER
overpotential values than the **Ni-Ag** HCP. The superior activity of the **Ni-Ag** system
compared with the Ni-carboxylate-derived MOFs can be attributed to factors such as the N₆
environment of the Ni(II)-cations, conformed by 4acpy and CN⁻ ligands, which influences a
lower electron density into Ni(II) sites favoring the OH-coordination and inducing a high
polarity of the interstices [31], as it was corroborated by the blue shift of ν̄ (C≡N)
vibrations due to the favored M-N σ-donation (Figure S5b-c). This also explains the catalytic performance of the reported family of **M-Ag** HCPs compared to mono- and bimetallic coordination polymers obtained with the ligand pyrazine-2,3,5,6-tetracarboxilate ($H_4pztc$) [71].

**3.3 Insights into the M-Ag Electrochemical and Electrocatalytic Activity.**

Based on the CV tests (Figure 2a) and the zoomed LSV curves (inset of Figure 2c), it was confirmed that the studied **M-Ag** HCPs undergo oxidation prior to the OER onset. This suggests that the HCPs function as pre-catalysts; during the oxidation process of $3d$ M(II) → M(III)/(IV), oxidized electroactive species such as M-OH and M-OOH are formed. These species act as the active sites for driving the OER, consistent with similar findings reported for related structures in the literature [28,31,35,67,68,70,72–74]. These species are thermodynamically favored under high anodic overpotentials in alkaline environments exhibiting high stability [66,75,76]. For instance, the systematic study of Ni-carboxylate-derivatives MOF catalysts demonstrated that the active OER species correspond to the $Ni(OH)_2$/NiOOH phases produced by the partial structural-reconstruction process that leads to a surface-structural conversion [67]. In the same way, the superior OER catalytic activity for a Ni-BTC MOF (BTC = 1,3,5-benzenetricarboxylic acid) was attributed to the formation of $Ni(OH)_2$/NiOOH surface phases as a result of the electrochemical aging during the OER, evidencing the structural reconstruction phenomena [77]. This phenomenon has also been documented for Prussian-Blue analogs [30,37,78–80] with structural similarities to the cyanido-based HCPs studied here [34] (Scheme 1) and for other similar HCPs such as $[Fe(pyridine)_2Ni(CN)_4]$. For this, surface hydroxide species $Fe(OH)_3$ and $Ni(OH)_3$ were characterized by FT-IR after OER [28]. Another example
corresponds to crosslinked Co-based complexes, which undergo a reconstruction phenomenon during the OER, involving two steps to form hydroxy and (oxy)hydroxy species on the electrode surface [81]. Building on the previous findings and delving further into the above-proposed hypothesis, additional experimental and theoretical studies were conducted.

<figure><img src="image_7.png" /><figcaption>**Figure 3.** a) P-XRD pattern of Ni-Ag\* (red) and the pristine phase Ni-Ag (black). Inset:</figcaption></figure>
Photograph of the aspect of each phase. b)-d) Superficial ATR-FTIR spectrum of **Ni-Ag*** (red) and the pristine phase **Ni-Ag** (black) showing the OH-stretching and (c) Ni-OH stretching bands. (d) Full wavelength ATR-FTIR spectra for **Ni-Ag** and **Ni-Ag***. (e) XPS Ni(2p) spectra recorded from **Ni-Ag** and **Ni-Ag***; (f) XPS O(1s) spectra recorded from **Ni-Ag** and **Ni-Ag***.

A schematic representation is provided between (e) and (f) figures illustrating the release of 4acyl ligands and the incorporation of hydroxyl species into the coordination environment of Ni sites.

First, **Ni-Ag** CP was treated in a 1.0 M KOH solution for 24 hours [31,82]; this sample was labeled as **Ni-Ag***.

As shown in **Figure 3a**, a visible color change occurred from the pale purple of pristine **Ni-Ag** to light-brown of **Ni-Ag*** due to KOH aging. This behavior has been previously reported for similar MOFs [77]. Despite this, P-XRD diffractograms show no significant bulk structural changes between **Ni-Ag** and KOH-treated **Ni-Ag***, though a slight loss of crystallinity (**Figure 3a**). Le Bail fit of the whole polycrystalline pattern shows that cell parameters of **Ni-Ag*** are comparable to the pristine phase, confirming that the structure of the compound is stable and remains unchanged (**Figure S2c** and **Table S1a**). On the other hand, the surface ATR-FTIR of **Ni-Ag*** (**Figure 3b-d**) revealed the appearance of two bands at 3646 cm⁻¹ and 508 cm⁻¹, absent in the untreated **Ni-Ag**. These bands are attributed to the formation of surface Ni-hydroxide species, providing evidence of a surface reconstruction phenomenon [83]. The other characteristic bands of the pristine **Ni-Ag** sample remained mostly unchanged [70]. XPS analyses further confirmed the surface reconstruction of **Ni-Ag** (**Figure 3**, **Figure S7**). Ni(2p) and O(1s) XPS spectra were included in **Figure 3e-f** for both the pristine **Ni-Ag** and KOH-treated **Ni-Ag*** phases. The Ni(2p) spectrum of **Ni-Ag** (**Figure 3e**) consists of an
asymmetric doublet with binding energies of Ni(2p<sub>3/2</sub>) and Ni(2p<sub>1/2</sub>) core levels at 855.34 and 872.74 eV, respectively (Table S3), which are indicative of Ni(II) [26,27]. Considering that Ni(II) cations in **Ni-Ag** are coordinated solely by nitrogen atoms from pyridine and cyanido ligands, these signals can be assigned to Ni-N bonds. However, the spectra also reveal two low-intensity peaks at 858.14 eV and 875.54 eV (Table S3), which are associated with Ni(III) [84]. These peaks indicate the presence of a small amount of Ni(III) oxide, attributed to the partial aerobic oxidation of the **Ni-Ag** surface. In the KOH-treated **Ni-Ag*** phase, the Ni(2p<sub>3/2</sub>) and Ni(2p<sub>1/2</sub>) peaks exhibit a slight shift towards higher binding energies, concurrent with an increase in the relative atomic content of Ni(III) (Table S3). This shift suggests partial Ni(II)/Ni(III) oxidation at the electrolyte-HCP interface. Further analysis of the O(1s) spectra provides insights into the surface modification induced by KOH aging. The O-M signal in the treated **Ni-Ag*** phase increases (Figure 3f), indicating that Ni(II)/Ni(III) oxidation is accompanied by the formation of a hydroxy-surface layer. This observation aligns with the decrease in the atomic content of the M-N signal (Table S3, Figure S7, N(1s) XPS spectra), which likely results from partial ligand release.

As the reconstruction phenomenon is confined to the surface, **Ni-Ag** retains its bulk structural integrity, as demonstrated by P-XRD (Figure 3a) [35,74]. This structural robustness is unusual in 2D HCPs, such as [Fe(pyridine)₂Ni(CN)₄], where alkaline aging leads to the loss of pyridine and cyanido IR-vibrations and the appearance of strong Ni-O and Fe-O bond vibrations, indicating a near-complete transformation of the pristine phase into (oxy)hydroxides phases [30,31,35]. These findings suggest that the silver-ketone supramolecular interactions in the 2D **M-Ag** systems provide greater chemical stability
compared to the π-π interlayer interactions in [Fe(pyridine)₂Ni(CN)₄], offering a promising foundation for the rational design of highly stable OER catalysis.

Chronoamperometry test at $j = 10 \text{ mA cm}^{-2}$ confirms the stability of the **Ni-Ag**, as shown in **Figure 4a**, with the overpotential remaining stable for 5 hours. In a previous study by Ma *et al.* [67], the transformation of MOF structures under KOH-aging and the applied OER overpotential was described as a gradual displacement of ligands from the surface, exposing Ni-sites and incorporating $OH^-$ ions of the electrolyte into the Ni coordination environment.

Based on this description and our findings, the superior activity of the **Ni-Ag HCP** can be attributed to its partial structural reconstruction, driven by the electrolyte-surface interaction and the applied anodic potential during OER. The coexistence of 2D-pristine and surface-oxidized phases synergistically enhances oxygen evolution, with the silver-ketone 2D-supramolecular interactions playing a pivotal role in maintaining this synergy. This synergy may also contribute to the observed stability of the catalyst (**Figure 4a**).

**Figure 4b** illustrates a schematic representation of the structural transformation occurring in the **Ni-Ag** catalyst alongside the proposed adsorption-based OER mechanism, which is inferred from the calculated Tafel slopes (**Figure S6** and **Table S2**) [85]. To explore these assumptions and to confirm the proposed mechanism, DFT calculations were carried out considering simplified models for the **M-Ag** systems (see Computational Details).
<figure><img src="image_8.png" /><figcaption>**Figure 4.** a) Chronopotentiometry measurements at a *j*=10 mA cm⁻², recorded for Ni-Ag/GC electrode in 1M KOH. The η@10 for RuO₂ was plotted for comparison purposes.
b) OER mechanism on the active sites (left) along with a scheme of the partial surface reconstruction of HPC under OER conditions in alkaline media (right).</figcaption></figure>

The **M-Ag** simplified models were suitable for the analysis since the tested periodic models demonstrated no significant structural differences in the crystal structure after relaxation, reaching root mean square deviation values from 0.08 to 0.12 Å. Furthermore, the distribution of crystalline orbitals demonstrated that Ag centers are not active while ligands and transition metals played a central role (Figure S8). The above is consistent with the experimental magnetic properties measured for the **M-Ag** series of HCPs. Magnetic
results showed that the compounds exhibit a very weak magnetic coupling between M
centers, suggesting that magnetic metal centers (M= Mn, Co, Fe, and Ni) behave as isolated
paramagnetic centers with zJ terms that did not overcome the 0.10 cm⁻¹ (Table S3). This
experimental evidence confirms that the electronic properties of the synthesized M-Ag
compounds may be represented using isolated units that contain the main fragment
involved in electronic properties. From the χT(T) curves (Figure S9), it is observed that all
compounds are magnetically uncoupled at room temperature, exhibiting a simple
paramagnetic behavior, with χT values at room temperature of 4.40, 3.43, 3.00, and 1.03
cm³ K mol⁻¹, respectively. These values are close to the expected theoretical values
considering a Curie law for uncoupled Mn(II), Fe(II), Co(II), and Ni(II) cations, with S =
5/2 (4.38 cm³ K mol⁻¹), S = 2 (3 cm³ K mol⁻¹), S = 3/2 (1.88 cm³ K mol⁻¹) and S = 1 (1 cm³
K mol⁻¹), respectively. The decrease of the χT by lowering the temperature for Mn-Ag, Co-
Ag, and Ni-Ag can be associated with zero-field splitting (zfs) due to the axially distorted
octahedral surroundings of the divalent 3dⁿ cations but also due to M(II)···M(II) super-
exchange by the cyanido-metal -C≡N-Ag-C≡N- bridges [86], or by interlayer interactions.
Keeping this in mind, the spin-only Hamiltonian, including the Zeeman effect, zero-field
splitting (zfs), and the molecular field perturbation, can be used to describe their magnetic
data (see Supporting Information for more details). For Fe-Ag, as the temperature is down,
a smooth decrease of χT is observed in the 300-270 K range, followed by an abrupt
decrease reaching a minimum near zero, i.e. the FeN₆ environment under the Fe(II) makes
possible a one-step HS→LS thermally activated spin crossover being complete at 120 K
approximately (T₁/₂ = 200 K). Thus, below 120 K the Fe-Ag compound is essentially
diamagnetic. Very weak antiferromagnetic interactions are found in the four compounds,
proving that the spin carriers are essentially magnetically isolated.
Furthermore, to corroborate that the computed model is suitable for explaining the electronic behavior of the synthesized compound, a correlation of the experimental vibrational trend $\bar{v}_{Mn} \sim \bar{v}_{Fe} > \bar{v}_{Co} > \bar{v}_{Ni}$ observed by ATR-IR analysis (Figure S5c) was made with the calculated Wiberg bond order (BOw) and cations spin density [$\rho_s(r)$] obtaining $R^2$ values of 0.959 and 0.936, respectively (Figure S10a). The BOw values from 2.37 (Mn) to 1.00 (Ni) relate with the partial population of anti-bonding orbital as the number of $3d^n$ electron population increases, and the $\pi$-back bonding from the M cations series weakens the strength of the bond (Figure S10b).

At this point, it is interesting to note that the observed catalytic activity for the **Ni-Ag > Co-Ag > Fe-Ag > Mn-Ag** resembles the tendencies described for the highly OER active 3d-metal-based layer double hydroxide-based catalysts [87]. In the seminal study of Subbaraman *et al.* [87], the superior OER activity of Ni in the series $NiO_xH_y > CoO_xH_y > FeO_xH_y > MnO_xH_y$ was attributed to the strength of $M^{2+}_\delta$-OH adsorption, where Ni shows moderate adsorption energy in comparison with the rest of the 3d-cations (Mn, Fe, and Co). This aligns with the previous statement that the **M-Ag** HCPs act as pre-catalysts, as the M sites are partially transformed into M-oxidized species. These oxidized species, in synergy with the concomitant pristine structures, constitute the real active species responsible for the enhanced catalytic activity. The superior activity of **Ni-Ag** CP is attributed to the $Ni-3d$ occupation and the Ni-OH adsorption energy. To test this hypothesis, three optimized models are proposed: one assuming the displacement of the apical 4acyl ligand, creating a vacancy (**Ag-M-V**, *Scheme 2c*), and two others involving the coordination of -OH and -OOH anions to form both **Ag-M-OH** and **Ag-M-OOH** adducts, respectively (*Scheme 2d-e*). The competition between these adducts was evaluated using $E_{ads}$ as a stability criterion
(Figure 5a). The results indicate that **Ag-M-OH** configurations are more stable and thermodynamically favored than **Ag-M-OOH** across the series. This finding corroborates the proposed mechanism in **Figure 4b** for the studied HCPs, where the initial catalytic species corresponds to M-OH. Furthermore, a volcano trend was found when comparing the stability of the adducts [ΔE<sub>ads</sub>=E<sub>ads(M-Ag-OH)</sub> – E<sub>ads(M-Ag-OOH)</sub>], with the **Co-Ag-OH** adduct showing the highest stabilization energy among the series (Figure 5). The above helps explain the lower experimental TOF for **Co-Ag** compared to **Ni-Ag**. While **Co-Ag** exhibits stronger interactions with -OH and -OOH species, the over-stabilization of the final intermediate, i.e. the Ag-M-OOH species in the mechanism of **Figure 4b**, slows down catalyst recycling, as demonstrated in the electrocatalytic experiments.
<figure><img src="image_9.png" /><figcaption>**Figure 5.** (a) M-Ag-OH and M-Ag-OOH adsorption energy for the series with M=Mn, Fe, Co, Ni. The difference in the Adsorption energy ($\Delta E_{ads}$) expressed in meV. (b) $\rho_{BCP}(r)$ M-OH, and (c) $\rho_{BCP}(r)$ MO-H bonds for the M-Ag-OH complexes.</figcaption></figure>

To further investigate the proposed mechanism (Figure 4b) and the catalytic trend observed experimentally where **Ni-Ag** overpass the studied series, an analysis of the structural parameters and chemical nature of the M-OH bonds using the bond critical point ($\rho_{BCP}$) derived from the Atoms in Molecules (AIM) theory was made. This study reveals that OH adsorption at the metal site significantly stabilizes the overall structures by forming
M-OH bonds and subsequently oxidizing the metal center [M(II) → M(III) + ē]. At this stage, the influence of the metal oxidation state on the M-OH bond strength highlights two opposing effects: the clash of covalent- versus electrostatic-interactions. On the one hand, the filling of antibonding orbitals destabilizes the covalent nature of the M-OH bond across the series, leading to a weakening of M-OH bonds, particularly in the case of **Ag-Ni(II)-OH** and **Ag-Ni(III)-OH**. This trend is confirmed by the ρ<sub>BCP</sub>(r) descriptor (**Figure 5b**), which displayed a transition from highly covalent to more coordinative bonding interactions for **Ni-Ag** catalysts. On the other hand, the electrostatic interactions should be increased for M(III) compared to M(II) due to the higher charge, but this effect reaches a turning point for Co(II) and Co(III) (**Figure 5b**) where both present almost the same strength for Co-OH bond. For Ni(II) and Ni(III), the trend is reversed, with the Ni(II)-OH bond being stronger than the Ni(III)-OH bond, indicating a weakening of the bond strength as it transitions from covalent to coordinative interaction. As can be seen, these results align well with the OER activity trend observed experimentally (**Ni-Ag** > **Co-Ag** > **Fe-Ag** > **Mn-Ag**), supporting the higher performance for **Ni-Ag**.

Afterward, a deeper analysis of O-H bond activation was made examining the ρ<sub>BCP</sub>(r)
for [M-OH], focusing on the influence of the identity of the metal and oxidation state
*(Figure 5c)*. The analysis revealed that, although the O-H bonds generally retain their
covalent character, the Ni(III)O-H bond exhibits a significant weakening, which is not
observed for the other metals in the series. This finding also supports the higher OER
activity for **Ni-Ag** and exhibits two key implications: (i) The weakening of the Ni(III)-O-H
bond suggests an enhanced propensity for deprotonation that facilitates the proton-coupled
electron transfer process and indicates increased Lewis acidity for **Ni-Ag**, favoring the
second step of the proposed mechanism (Figure 4b); (ii) The trivalent M(III) oxidation state systematically reduces the O-H bond strength across the series, exemplifying a coordination-induced bond weakening (CIBW) effect. The CIBW phenomenon, which depends on the oxidation state of metal centers, is particularly pronounced in lower-valent metals and enables the activation of otherwise unreactive small molecules [88]. In biological systems, notable examples include photosystem II, where the Mn-based oxygen-evolving complex weakens the O-H bond of coordinated water molecules, facilitating dehydrogenation and deprotonation driven by chlorophyll photoactivation [89]. CIBW is highly relevant in energy production and storage technologies, particularly in catalysis, photocatalysis, and electrocatalysis. For instance, low-valent titanium systems have shown success in activating unreactive alkenes via aqua-coordinated complexes [90–92]. Similar CIBW effects have been observed in coordination and organometallic compounds of Co and Fe, further extending the applicability of this phenomenon to various catalytic systems [83]. Our analysis demonstrates that the CIBW effect occurs across the M-Ag series. However, a crossing point is found for the Co(II)/Co(III) pair, reaching almost the same bond strength for the M-O bond (Figure 5b). Beyond this point, the effect reaches an inflection with the Ni compound, where the bond strength reverses, leading to a quantitative decrease in Ni(III)-OH bond strength. Under our scope, the CIBW effect integrates electronic configuration, bond valence, and electrostatic effects. Therefore, CIBW phenomena can be introduced as an OER reactivity descriptor for the oxygen evolution reaction where the formation of M-OH species is crucial.

Finally, the combined experimental-theoretical approach confirms that the factors governing Lewis acidity, orbital filling, bond strength, bond character, interaction
mechanisms, surface activation, and electrocatalytic activity are all linked to the stabilization and competition between the **Ag-M-OH** and **Ag-M-OOH** adducts. The superior performance of **Ni-Ag** in terms of TOF can be attributed to the balance between stabilization and CIBW during the adsorption, metal oxidation, and deprotonation steps of the electrocatalytic cycle. These findings are consistent with experimental results and align with key theoretical principles. Specifically, the destabilizing effect of antibonding orbital filling, as described by Ligand Field Theory and Sabatier's principle, which predicts that over-stabilization leads to catalytic site poisoning, explains the suboptimal performance of **Mn-Ag**, **Fe-Ag**, and **Co-Ag** HCPs. In contrast, **Ni-Ag** achieves optimal conditions due to the facilitation of the first and second stages of the electrocatalytic cycle (proposed in **Figure 4b**) by the CIBW effect.

## 4. Conclusion.

This study highlights the potential of Hofmann-type 2D-coordination polymers as highly efficient electrocatalysts for the OER in alkaline media. The catalytic performance of the **M-Ag** series follows a periodic trend driven by factors such as 3d-electron occupancy, Lewis acidity, and surface activation linked to the M-OH and M-OOH energy adsorption and bond strengths. Among the series, **Ni-Ag** demonstrated superior activity over **Co-Ag** > **Fe-Ag** > **Mn-Ag** systems. Based on the experimental and theoretical results, we were able to propose an OER mechanism where the surface structural reconstruction of the studied HCPs leads to the forming of M-OH species, which triggers the catalytic cycle. Structural characterizations of the outperforming **Ni-Ag** system, including Le Bail fit P-XRD, FTIR, and XPS analyses conducted before and after its exposure to the KOH electrolyte, confirmed its partial structural transformation and formation of surface Ni-OH
species. Furthermore, the computational studies allow to explain the experimental **Ni-Ag > Co-Ag > Fe-Ag > Mn-Ag** catalytic trend by the calculated $\Delta E_{ads}$ for the M-OH and MOOH species, further supporting the proposed catalytic mechanism, where the formation of M-OH species as initial catalytic species is crucial. In addition, the CIBW effect, as an indicator of O-H bond strength, evidences that the deprotonation of the **Ag-Ni-OH** adduct is facilitated, promoting the second step of the catalytic mechanism. As a result, the CIBW effect can be considered an OER descriptor when the M-OH species are the initial mechanistic species. Finally, through this novel and systematic study, it is proved that 2D Ni-based HCPs act as pre-catalysts, where the formation of surface Ni-OH species works synergistically with the remaining 2D-pristine structures stabilized by the silver-ketone supramolecular interactions, to initiate and sustain the OER, outperforming $RuO_2$. In conclusion, the surface chemical modification observed in these coordination compounds is crucial for developing strategies focused on interfacial engineering to improve the performance of competitive electrocatalysts.

**Supporting Information.**

A PDF with Supporting Information is available free of charge.

**Accession Codes**

CCDC 2368203 (**Mn-Ag**) and 2368204 (**Fe-Ag**), contains the supplementary crystallographic data for this publication. This data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

**Acknowledgments.**
C.C. acknowledges the financial support of ANID FONDECYT Iniciación N° 11241257.
V.P-G. acknowledges ANID FONDECYT Regular N° 1211394, CONICYT-
FONDEQUIP/PPMS/EQM130086-UNAB, French-Chilean International Research project
"Cooperation in Inorganic Chemistry" (CoopIC), CEDENNA. K.M-B and R.V.
acknowledge ANID FONDECYT Iniciación N° 11221073 and N°11241509. This research
was partially supported by the supercomputing infrastructure of the NLHPC (ECM-02).
The authors are thankful for the computational resources through the CONICYT-
FONDEQUIP-EQM180180 N. A. acknowledges Dr. T. Roisnel from CDIFX (Centre de
Diffractométrie X), UMR CNRS 6226 Rennes, for single-crystal X-ray data collection.

**Corresponding Authors***

*C. Cruz c.cruzherrera@uandresbello.edu.; *K. Muñoz-Becerra karina.munoz@ubo.cl; *R. Venegas ricardo.venegas@ubo.cl
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