Atomically accurate site-specific ligand tailoring of highly acid- and alkali-resistant Ti(iv)-based metallamacrocycle for enhanced CO2 photoreduction

Skillfully engineering surface ligands at specific sites within robust clusters presents both a formidable challenge and a captivating opportunity. Herein we unveil an unprecedented titanium-oxo cluster: a calix[8]arene-stabilized metallamacrocycle (Ti16L4), uniquely crafted through the fusion of four “core–shell” {Ti4@(TBC[8])(L)} subunits with four oxalate moieties. Notably, this cluster showcases an exceptional level of chemical stability, retaining its crystalline integrity even when immersed in highly concentrated acid (1 M HNO3) and alkali (20 M NaOH). The macrocycle's surface unveils four specific, customizable μ2-bridging sites, primed to accommodate diverse carboxylate ligands. This adaptability is highlighted through deliberate modifications achieved by alternating crystal soaking in alkali and carboxylic acid solutions. Furthermore, Ti16L4 macrocycles autonomously self-assemble into one-dimensional nanotubes, which subsequently organize into three distinct solid phases, contingent upon the specific nature of the four μ2-bridging ligands. Notably, the Ti16L4 exhibit a remarkable capacity for photocatalytic activity in selectively reducing CO2 to CO. Exploiting the macrocycle's modifiable shell yields a significant boost in performance, achieving an exceptional maximum CO release rate of 4.047 ± 0.243 mmol g−1 h−1. This study serves as a striking testament to the latent potential of precision-guided surface ligand manipulation within robust clusters, while also underpinning a platform for producing microporous materials endowed with a myriad of surface functionalities.


Introduction
2][3] The allure of structurally precise titanium-oxo clusters (TOCs) stems from their tunable geometry and intriguing photoelectrical properties.[6][7][8][9][10] Unlike conventional TiO 2 , TOCs exhibit a rich diversity of structural types, carrying with them a distinctive palette of traits such as stability, semiconductor-like attributes, light absorption capacity, and band structure.2][13][14][15][16][17][18][19][20][21] Yet, the pursuit of precision in ligand modications on specic TOCs comes with its challenges, requiring the careful negotiation of unstable coordination sites while upholding the core's inorganic integrity.Regrettably, altering protective ligands oen triggers partial or complete dissection, resection, or structural reconstruction of the clusters. 22,23To surmount this hurdle, the creation of exceptionally stable TOCs emerges as a pivotal prerequisite.Enter the realm of macrometallocycles (MMCs), renowned for their formidable stability rooted in robust ring structures.This inherent resilience safeguards the chemical essence of TOCs, even under rigorous conditions, paving the way for subsequent adaptations.][26] These intricate macrocycles, composed of multiple phenol a Hunan Provincial Key Laboratory of Chemical Power Sources, College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, P. R. China.E-mail: chaoliu@csu.edu.cnb China College of Science, Hunan University of Technology and Business, Changsha 410000, P. R.8][29][30][31][32][33] By fusing TOCs with a macrocyclic scaffold shielded by calixarene ligands, an uncharted territory opens to explore the impact of ligand tailoring at precise sites on their physicochemical attributes.While cyclic clusters abound, [34][35][36][37][38][39][40] the realm of Ti IV -based rings remains relatively untapped due to the hydrolysis susceptibility of Ti 4+ ions.Notably, calixarene-stabilized Ti IV -MMCs have yet to grace the literature.The envisioned assembly of atomically precise MMCs of Ti IV , secured by calixarene guardians, holds immense promise for ushering rational design and optimizing performance across future applications.
Herein, an exceptionally stable p-tert-butylcalix [8]areneprotected Ti IV -based metallamacrocycle (MMC), denoted as [H 4 Ti 16 O 8 (TBC [8]) 4 (Oa) 4 (Ac) 4 ( i PrO) 8 ], was synthesized.This MMC showcases remarkable resilience, displaying resistance against an array of organic solvents, concentrated acid (1 M HNO 3 ), and alkali (20 M NaOH), thus establishing itself as a paramount example of cluster stability.The gigantic cluster has a "donut" shape with an inner diameter of 12 Å, an outer diameter of 30 Å, and a height of 18 Å, making it the largest known TOC in the metal-calixarene system.A pivotal discovery lies in the identication of four modiable coordination sites on the Ti 16 L 4 surface, which has led to further exploration of its exchange activity and applications (Scheme 1).These sites, intriguingly, offer two distinct avenues for functionalization: (1) utilizing a one-pot synthesis method, the m 2 -bridging sites can be occupied by acetate and Cl − .The ratio of these ligands can be precisely tailored from 4 : 0 to 2 : 2, and further to 0 : 4. Noteworthy is the emergence of a 3D network in Ti 16 L 4-3, modied with four Cl − , yielding innite cylindrical channels with diameters of approximately 2 nm.
(2) An alternative post-modication strategy involves a sequential immersion of the cluster in concentrated alkali and carboxylic acid solutions, facilitating reversible exchange of the m 2 -bridging sites by OH − and diverse carboxylates.The molecular-level understanding of this exchange process is unveiled through crystal-to-crystal diffraction studies, shedding light on structural transformations and ligand dynamics.Signicantly, modifying the Ti 16 L 4 shell distinctly inuences physicochemical attributes, encompassing photocurrent response, hydrophilicity and energy levels, with ultimate implications for the photocatalytic CO 2 reduction potential of the clusters.The ability to organically modify the shell while preserving macrocycle integrity introduces a pioneering avenue for ne-tuning chemical properties, setting the stage for these promising Ti IV -based MMCs to be harnessed across diverse applications.

Ligand modication
One-spot method.As demonstrated in prior studies, the unique structural motifs of the four {Ti 4 @TBC [8]} subunits encompass embellished Ti2 and Ti3 sites, each adorned with a single acetate and two weakly coordinated i PrO − groups.Intriguingly, through a deliberate alteration involving the ].Remarkably, these two clusters mirror the macrocyclic framework of Ti 16 L 4-1, while diverging in their surface ligands.A notable feature is the dynamic occupancy of the four m 2 -bridging sites within the macrocycles, where acetate and Cl − nd a variable equilibrium.The precise modulation of this ligand interplay is evident, transitioning from a 4 : 0 ratio in Ti 16 L 4-1, to a balanced 2 : 2 ratio in Ti 16 L 4-2 (Fig. 2A), culminating in an exclusive 0 : 4 ratio in Ti 16 L 4-3 (Fig. 2B).
Beyond the intricate geometries that clusters adopt, the interplay between surface ligands emerges as a pivotal determinant of their crystalline packing and consequential material attributes.Mastery over these multifaceted factors remains a formidable task.Our study, however, illuminates a path to precision control in this arena.Strikingly, while the shared macrocyclic scaffold remains constant, variations in surface ligands orchestrate disparate packing congurations within the crystal lattice of Ti 16 L 4-1/2/3.Within all phases, the metallacycles ingeniously arrange face-to-face, thus sculpting 1D nanotubes that harmonize through van der Waals forces, culminating in their assemblage (Fig. 2C and D).The meticulous orchestration of this assembly begins by staggering neighboring nanotubes to alleviate steric hindrance stemming  from tert-butyl moieties of adjacent macrocycles (Fig. 2E).This choreographed dance then evolves into a parallel stacking, fashioning an assorted array of 3D architectures.Remarkably, Ti 16 L 4-1's nanotubes organize into a square matrix (Fig. 2F), whereas Ti 16 L 4-2's nanotubes adopt a hexagonal pattern (Fig. 2G).This intriguing divergence traces its origins back to the precise occupancy of the four m 2 -bridging sites.The substitution of acetate with Cl − profoundly impacts C-H/C-H interactions among the clusters.Additionally, the presence of polar Ti-Cl bonds endows the surface of Ti 16 L 4-2, now embellished with two Cl − , with a discernible negative charge.This newfound electrostatic repulsion between the nanotubes impels their separation within the lattice.Evidently, the centroid distance between adjacent nanotubes escalates from 2.32 nm (as observed in Ti 16 L 4-1) to 2.56 and 2.66 nm in succession.This captivating phenomenon nds further exem-plication in the assembly motif of Ti 16 L 4-3, wherein all four m 2bridging sites embrace Cl − , ensuing in yet another square arrangement of macrocycles.Notably, a subtle 45°rotation between neighboring nanotubes in Ti 16 L 4-1 mitigates the dominant electrostatic repulsion, paving the way for the emergence of 1D channels along the c-axis.These channels, a noteworthy 2 × 2 nm in dimension, dene Ti 16 L 4-3's unique character (Fig. 2H).The resplendent novelty of these ndings lies in the precise manipulation of ligand dynamics to orchestrate such diverse and intricate packing phenomena.
The intricate interlocking of Ti 16 L 4 macrocycles intricately assembles into meticulously ordered nanotubular frameworks across all three distinct phases.To validate the porosity of these architectures, we conducted N 2 adsorption/desorption analyses that unveiled characteristic type I isotherms for each phase, unmistakably indicating their inherent porous nature.Concretely, our evaluation yielded Brunauer-Emmett-Teller (BET) surface areas of 209.24 m 2 g −1 for Ti 16 L 4-1, 389.92 m 2 g −1 for Ti 16 L 4-2, and a remarkable 813.46 m 2 g −1 for Ti 16 L 4-3 (Fig. 3A).The underlying absorbance potential was corroborated by diffuse reectance spectra, revealing robust light absorption in the visible spectrum (Fig. S39 †).Capitalizing on this light absorption attribute, we ventured into assessing the visible-light-driven photocurrent responses of Ti 16 L 4 .Through repeated irradiation cycles, steady photocurrent responses were uniformly observed across all clusters.Notably, the substitution of acetate with Cl − yielded an enhancement in photocurrent density.Among the clusters, Ti 16 L 4-3 emerged as the standout, demonstrating the most optimal photocurrent response.Its recorded current density surged to an impressive 3.2 mA cm −2 , which was approximately threefold higher than that of Ti 16 L 4-1 (1.0 mA cm −2 ) (Fig. 3B).Interestingly, the hydrophobicity of Ti 16 L 4 crystals emerged as an outcome intricately linked to their specic stacking arrangements.Evidently, water droplets exhibited distinct behaviors on the surfaces of Ti 16 L 4-1/2 powders, maintaining rounded shapes with contact angles measuring 138.5°and 117.5°, respectively (Fig. 3C).In a striking departure, the contact angle observed for Ti 16 L 4-3 was notably reduced to a mere 49.5°, indicative of its pronounced hydrophilic character.This intriguing contrast owes its origins to the presence of polar Ti-Cl bonds and the substantial presence of expansive cylindrical channels within the structural matrix, collectively orchestrating this distinctive behavior.
The stability of the nanocluster emerges as a pivotal facet, particularly in the context of post-synthetic modications, warranting meticulous consideration.Signicantly, the calixarene modication of the Ti 16 L 4 confers a profound enhancement in chemical stability.Evidently, our PXRD analysis substantiated the robust stability of the Ti 16 L 4-1 crystal across an array of solvents such as toluene, methanol, CH 3 CN, and DMF (Fig. S26 †).To further probe its solution behavior, we harnessed matrix-assisted laser desorption/ionization time-of-ight mass spectrometry (MALDI-TOF-MS) on Ti 16 L 4-1 dissolved within a mixed solvent of CHCl 3 and MeOH (Fig. 4A).Encouragingly, the dominant peaks in the spectrum unequivocally corresponded to species like [H 4 Ti 16 O 8 (TBC [8]) 4 (Oa) 4 (-Ac) 4 ( i PrO) x (MeO) y ] + (x + y = 7).This nding underscores that, while retaining the overall architecture of Ti 16 L 4 , the i PrO − on the cluster exhibit high exchangeability with MeO − ions, while the chelating acetate sites remain resolutely stable.Dynamic  light scattering measurements further conrmed a singular size distribution with an average diameter of 4.5 nm, a tting match to the cluster size, concretely affirming the presence of well-dened Ti 16 L 4 entities in solution (Fig. S37 †).Another distinctive feature of Ti 16 L 4-1 crystals comes to the fore in the realm of water stability-an attribute oen fraught with challenges in many cluster crystals.In stark contrast, the exceptional pH stability of Ti 16 L 4-1 crystals prevails over a broad pH values spanning from 1 to 14 (Fig. S27 †).Subsequent testing reinforces this extraordinary stability, with the crystals remaining unaltered even in the face of concentrated acids (1 M HCl, 1 M H 2 SO 4 , and 1 M HNO 3 ) and alkali (20 M NaOH) over a 24 hour period (Fig. 4B).This robust endurance nds its basis in the hydrophobic calixarene outer chamber enveloping Ti 16 L 4 , serving as an effective barrier against acid/alkali corrosion of the hydrophilic Ti 4+ .This ingeniously designed spatial protection imparts a remarkably high level of water stability to the cluster.
Post-synthetic modication.Through an in-depth analysis of the Ti 16 L 4-1 structure, we nd that it does present 4 × 3 = 12 labile coordination sites, comprising four m 2 -acetate sites and eight weakly coordinated i PrO − sites.Complementary mass spectrometry experiments have intriguingly demonstrated the dynamic nature of i PrO − in solution, allowing rapid replacement by MeO − .Eager to bolster the surface functionality of Ti 16 L 4 , our focus shied towards investigating the substitution of the four m 2 -acetate sites through post-synthetic modication.The exceptional stability of Ti 16 L 4 crystals offered us an opportunity to pinpoint the exchange sites at the molecular level, employing the powerful SCXRD techniques that provided nuanced insights into the intricacies of the ligand exchange process (Fig. 5A).Embarking on a journey of formic acid (HFa) immersion, a carefully orchestrated sequence unveiled fascinating revelations.SCXRD scrutiny aer immersing the Ti 16 L 4-1 crystal in 1 M formic acid for 12 h highlighted the replacement of only the four innermost labile i PrO − sites within the ring, now replaced by H 2 O molecules (yielding Ti 16 L 4-1(a), 44 Fig. 5B).Elevated HFa concentration (5 M) and prolonged soaking (24 h) triggered further substitution, this time of the four acetate sites by formate ligands (resulting in Ti 16 L 4-1/HFa(a), Fig. 5C).Notably, the stability of the m 2 -(O,O ′ ) chelating form conferred a thermodynamically challenging nature to the exchange of these four acetate sites, mandating a high-concentration, extended duration immersion for successful replacement.In parallel, the terminal i PrO − sites, inherently labile, seamlessly surrendered to H 2 O molecules, while the external i PrO − sites, shrouded within the hydrophobic outer calixarene cavity, remained impervious to H 2 O access.An interesting color transformation emerged from the immersion of Ti 16 L 4-1 crystals in 5 M NaOH, with a rapid shi from red to yellow within 30 minutes.SCXRD analysis of the resulting yellow crystal (now Ti 16 L 4-1/NaOH, 45 Fig. 5D) illuminated a thorough overhaulcomplete replacement of all four acetates and eight i PrO − ligands by OH − , bolstered by numerous Na + that connected through OH − bridging.Subsequent immersion in 1 M HFa for 30 minutes not only restored the red hue but also the crystalline phase (now Ti 16 L 4-1/HFa(b)). 46SCXRD verication elucidated the reintroduction of the four m 2 -sites via formate ions, reverting to the m 2 -(O,O ′ ) conguration (Fig. 5E).Encouragingly, this intricate exchange process proved fully reversible, evidenced by the cycle of color transformation-the red crystals of Ti 16 L 4-1/ HFa(b) reverting to yellow during re-immersion in 5 M NaOH, indicative of formate ions being replaced by OH − ions.Impressively, even aer undergoing ve alternating cycles of immersion in 1 M HFa and 5 M NaOH, the crystal retained its structural stability (Fig. S28 †).Remarkably, these color shis in response to acid and alkali serve as tangible indicators of the comprehensiveness of the ligand exchange process-a visually compelling testament to the groundbreaking strides of our work.
Drawing from the above insightful ndings, the substitution of the four m 2 -acetate sites within Ti 16 L 4 emerges as a dynamic process, amenable to two distinct pathways.The rst, path I, involves the direct immersion of crystals in a concentrated formic acid solution for a minimum of 24 hours.The second, path II, unfolds as a sequential immersion, commencing with a solution of NaOH, followed by a carboxylic acid.This latter path capitalizes on acid-base neutralization reactions, rendering the exchange process swi, completed within a mere hour.Evaluating the efficacy of these approaches, we embarked on a mission to extend the carboxylate modications on the Ti 16 L 4 surface.Encouragingly, the gamut encompassed successful modications: chloroacetate (Cla − ) and bromoacetate (Bra − ) through path I, and acetate (Ac − ), 47 aminoacetate (Ama − ), glycinate (Ga − ), 48 and oxalate (Oa 2− ) 49 via path II.A comprehensive SCXRD analysis discerned the occupation of all four sites by these carboxylates.Delving into specics, the bridging modes of Ac − , Ama − , Cla − , and Bra − ligands aligned uniformly, establishing m 2 -(O,O ′ ) bridges connecting two Ti sites (as depicted in Fig. 6A-D).In marked contrast, Oa 2− and Ga − ligands exhibited an intriguingly different binding pattern (as showcased in Fig. 6E andF), with the ligands forming a chelating arrangement around the Ti3 site in a bidentate m 1 -(O,O) mode, concomitantly coordinating the Ti2 site with two OH − ions.Notably, the structural analysis of Ti 16 L 4-1 modied with triuoroacetate (Tfa − ) encountered challenges due to poor crystal quality.This phenomenon can be attributed to the introduction of a signicant number of F atoms to the cluster's surface, engendering repulsion among clusters within the tetragonal phase and leading to lattice displacement.However, this issue was effectively surmounted in the case of the Tfa − modied Ti 16 L 4-2 cluster, revealing a denitive structural determination within the hexagonal lattice of Ti 16 L 4-2 (Fig. 6G), underscoring the adaptability of the Tfa − to occupy these sites.
Interestingly, our explorations extended to the realm of inorganic oxyanions.When Ti 16 L 4-1/NaOH crystals were immersed in 1 M H 3 PO 4 for 30 minutes, red crystals of Ti 16 L 4-1/ H 3 PO 4 were obtained, retaining crystallinity.SCXRD analysis brought forth an intriguing revelation-PO 4 3− did not interact with the clusters.Instead, OH − persisted as occupants within the 12 labile sites, with Na + ions being entirely dislodged from the structure.Notably, the cumulative occupancy rate of the four m 2 -OH − sites reached 2.80 (Fig. S22 †).This intriguing outcome underscores our capacity to effect partial ligand removal from these four m 2 -sites through a meticulous interplay of two acid-base neutralization reactions.This multifaceted exploration, together with the systematic elucidation of intricate ligand exchange pathways, reects the groundbreaking dimension of our research endeavor.

CO 2 photoreduction activities
The promising avenue of photocatalytically reducing CO 2 into reusable chemicals looms large as a critical step toward carbon neutrality. 502][53][54][55] Harnessing the unparalleled stability, controllable ligands, and adaptable packing modes inherent to Ti 16 L 4 , we ventured into their application as heterogeneous photocatalysts for CO 2 reduction.Conceiving the intricate setup, CO 2 photoreduction over Ti 16 L 4 unfolded under visible light irradiation (l $ 420 nm) in the presence of [Ru(bpy) 3 ]Cl 2 $6H 2 O as a photosensitizer, triethanolamine (TEOA) as a sacricial agent, and a CH 3 CN/H 2 O (4 : 1, v/v) solvent composition.A comprehensive analysis of the product, employing gas chromatography (GC, Fig. S47 †), ion chromatography (IC, Fig. S51 †) and 1 H NMR (Fig. S52 †), lucidly affirmed the exclusive generation of CO via CO 2 RR, with a marginal presence of H 2 .It's noteworthy that no products were observed in the absence of irradiation, CO 2 or clusters, or the yields were exceptionally low (Table S4 †).This unequivocally underscores the orchestrated interplay of each component within the photocatalytic system.Comparative assessment of the catalytic activity of Ti 16 L 4-1/2/3 in CO 2 photoreduction revealed a remarkable hierarchy in CO yields.Ti 16 L 4-1 produced a modest CO yield of 11.97 ± 0.67 mmol within 5 hours, with Ti 16 L 4-2 and Ti 16 L 4-3 exhibiting substantially enhanced CO yields of 29.97 ± 2.17 mmol and 60.70 ± 3.65 mmol, respectively (Fig. 7A).Ti 16 L 4-3 emerged as the frontrunner in catalytic prowess, displaying an impressive CO production rate of 4046.67 ± 243.33 mmol g −1 h −1 , coupled with an exceptional selectivity of 96.28%.7][58][59][60][61] The catalytic prociency of this macrocyclic series aligns with the potentially catalytically active Ti IV sites boasting exible coordination spaces (specically the Ti2 and Ti3 sites), which stand poised for CO 2 adsorption.In Ti 16 L 4-3, Cl − exhibit a greater tendency to vacate compared to acetates in Ti 16 L 4-1.Consequently, the Ti sites in Ti 16 L 4-3 can showcase higher catalytic activity.This assessment of catalytic activity, supported by Table S5, † revealed that each potential Ti 4+ catalytic site (TON Ti ) in Ti 16 L 4-3 exhibited higher activity than those in Ti 16 L 4-1/2.Additionally, the strategic advantage of the 2 nm macrochannel within Ti 16 L 4-3 unveiled a larger surface area, fostering heightened CO 2 adsorption and exposure to additional catalytic enclaves.Experimental ndings in CO 2 adsorption bolstered these revelations (Fig. S36 †), where Ti 16 L 4-3 showcased superior performance over Ti 16 L 4-1/2, with CO 2 uptake values of 57.38 and 48.79 cm 3 g −1 , respectively, at 273 K and 298 K under 1 bar pressure.Intriguingly, functionalization of ligands on Ti 16 L 4 also had a notable inuence on catalytic activity (Fig. 7B).In the case of Ti 16 L 4-1/NaOH, all potential catalytic sites on the cluster's surface are occupied by OH − ions.Due to the presence of strong Ti-OH bonds, CO 2 is unable to access the catalytic Ti sites, resulting in a decrease in catalytic activity (2.35 ± 0.89 mmol for 5 hours).Noteworthy examples include Ti 16 L 4-1/HBra and Ti 16 L 4-1/HCla, unveiling CO yields of 36.25 ± 2.59 mmol and 39.48 ± 3.16 mmol, respectively, aer 5 hours, approximately threefold higher than pristine Ti 16 L 4-1 (Fig. 7C).Similarly, Ti 16 L 4-2/HTfa exhibited double the CO production of its untouched counterpart (58.18 ± 3.10 mmol for 5 hours), Ti 16 L 4-2.The higher photocatalytic performance of those derived clusters is mainly due to their modication with halogenated carboxylates that have the ability to quickly transfer charges.These enhancements nd resonance in the modication-induced shis within the cluster's bandgap structures, catalyzing accelerated electron transfers, and amplifying catalytic efficiency.
Collectively, these revelations converge to spotlight the photocatalytic acumen inherent within the Ti 16 L 4 macrocycles, unfurling their supremacy in CO 2 reduction and accentuating their proclivity for selectively generating CO.What's more, the strategic orchestration of the Ti 16 L 4 's spatial conguration, coupled with tailored enhancements infused into its outer ligands, emerges as a potent avenue for catapulting its catalytic performance to new heights.A tangible testament to the robustness of these ndings is the recyclability assessment, which resoundingly affirmed the enduring vigor of Ti 16 L 4-1's photocatalytic activity even across ve successive cycles (Fig. 7D).It's worth highlighting that Ti 16 L 4-1 not only retained its remarkable photocatalytic stability but also withstood scrutiny through XPS, MALDI-TOF-MS and TEM morphology studies (Fig. S58-S60 †).Subsequent SCXRD analysis of Ti 16 L 4-1 post photocatalysis revealed subtle changes in the coordination environment of potential catalytic sites (Ti2 and Ti3) within the cluster, which further substantiates the catalytic activity of these exible coordination sites (Fig. S57 †).Following the culmination of ve cycles, the catalyst could be efficiently reclaimed from the reaction milieu.A analysis of the resultant supernatant via Inductively-Coupled Plasma (ICP) revealed an astonishingly low Ti leakage rate from Ti 16 L 4-1, accounting for a mere 0.01% of the total Ti content.This revelation underscores not only the catalyst's enduring stability but also its marked potential for pragmatic, real-world applications.To unravel the origin of the carbon in the generated CO, an isotopic experiment leveraging 13 CO 2 as the carbon source was conducted.The gas chromatography-mass spectrometry (GC-MS) identied the production of 13 CO, conrming the unequivocal derivation of CO from CO 2 (Fig. 7E).
The investigation into electron transfer in the catalytic process unveiled the at band potentials of Ti 16 L 4-1/2/3 via Mott-Schottky plots, pinpointing their calculated conduction band minimum (CBM) values.These CBM values resided at −0.86, −0.91, and −1.08 V vs. the normal hydrogen electrode (NHE), respectively.Strikingly, all LUMO potentials exhibited a notably more negative prole in contrast to CO 2 /CO (−0.51 V vs. NHE), signifying their inherent suitability for CO 2 RR (Fig. 7F).Previous reports have indicated that the LUMO of [Ru(bpy) 3 ]Cl 2 $6H 2 O is −1.27V vs. NHE. 62Under irradiation, [Ru(bpy) 3 ]Cl 2 $6H 2 O transits to an excited state, subsequently reductively quenched by TEOA, yielding a reducing photosensitizer.Given that the CBM values of Ti 16 L 4-1/2/3 lie below the LUMO of [Ru(bpy) 3 ]Cl 2 $6H 2 O, the excited electrons emanating from the reduced photosensitizer migrate to Ti 16 L 4 , setting off the activation of adsorbed CO 2 on its surface.This orchestrated sequence culminates in the reduction of CO 2 to CO, subsequently liberating the product.We further substantiated the photocatalytic mechanism through electron paramagnetic resonance (EPR) experiments (Fig. S53 †).The experimental ndings reveal that in the absence of light irradiation within an N 2 atmosphere, the reaction system involving Ti 16 L 4-1 and the sacricial agent did not exhibit any ESR signals.Nevertheless, when subjected to visible light irradiation, distinct Ti 3+ signals were observed, corresponding to g value of 1.945.This observation implies that the photoexcited electrons transfers to Ti 4+ , leading to their reduction to Ti 3+ .Concurrently, TEOA serves as the sacricial agent to neutralize the photogenerated holes.The intensity of the Ti 3+ signal gradually increased with extended irradiation time.Upon exposure of the reaction system to a CO 2 atmosphere, the ESR signal of Ti 3+ diminished, signifying the involvement of photogenerated Ti 3+ in CO 2 RR.ESR results affirm that Ti 4+ within Ti 16 L 4-1 function as the active centers for photocatalytic CO 2 RR, providing further support for this mechanism.
To probe the CO 2 radical and other reaction intermediates in the photocatalytic reaction, the Ti 16 L 4-1 is investigated by in situ diffuse reectance infrared Fourier transform spectroscopy (Fig. 8).Under dark conditions post CO 2 pretreatment, Ti 16 L 4-1 displayed prominent peaks around 2348 cm −1 , which is associated with the asymmetric stretching of absorbed CO 2 . 59xposure to a CO 2 atmosphere under light for 10 minutes resulted in several new peaks: monodentate carbonate (m-CO 3 2− ) at 1351, 1451, and 1508 cm −1 ; bidentate carbonate (b-CO 3 2− ) at 1290 and 1543 cm −1 ; and bicarbonate (HCO 3 − ) at 1406 and 1656 cm −1 .These carbonates and bicarbonates likely formed from CO 2 reacting with water vapor.Notably, the CO 2 c − signal at 1713 cm −1 intensied with prolonged irradiation, indicating the formation of the CO 2 radical, a key intermediate in CO 2 photoreduction to *COOH. 52,53Furthermore, *COOH groups, crucial in CO 2 reduction to CO, were identied at 1338 and 1584 cm −1 , with increasing peak intensities under light exposure, suggesting light-induced formation. 63Additionally, absorption peaks for *CO and gaseous CO at 1708 and 2116 cm −1 respectively, provided further evidence of the reaction pathway. 64Therefore, according to the above analysis, a rational CO 2 photoreduction mechanism was proposed (Fig. S54 †): CO 2 was initially adsorbed on the Ti 3+ .Subsequently, the adsorbed *CO 2 received electrons from Ti 3+ and with protons to form the *COOH during irradiation.Thereaer, the deprotonation of the *COOH intermediate further generation of CO, and ultimately desorbs to obtain CO molecules.

Conclusions
In summary, this research marks a pioneering exploration into the realm of Ti IV -based metallamacrocycles, unveiling a host of novel insights and underlining its signicance in the broader landscape of scientic inquiry.Foremost, the unparalleled stability demonstrated by the Ti 16 L 4 stands out as a groundbreaking revelation.Its exceptional resilience against a spectrum of challenges, encompassing organic solvents, concentrated acids, and alkali, attests to a robustness that guarantees structural integrity and endurance across diverse environmental contexts.This stability not only solidies the cluster's foundation but also paves the way for its application across a multitude of conditions.Equally groundbreaking is the abundance of coordination anchors inherent on the Ti 16 L 4 's surface, effectively serving as a fertile ground for subsequent ligand adaptations.The dual pathways for facile ligand exchange form the bedrock of a versatile platform, facilitating the creation of microporous materials endowed with a plethora of surface functionalities.These microporous materials with coordinating sites on their surfaces are promising as carriers for loading noble metal nanoparticles, constructing heterogeneous catalysts with highly efficient catalytic activity.Another revelation lies in the crystallization potential of the macrocycle, giving rise to three distinct phases contingent upon the employed surface ligands.This revelation casts a revelatory spotlight on the predominant forces orchestrating the formation of diverse cluster-packing modes within the crystal lattice.The capability to manipulate surface functionalities and packing arrangements not only broadens our fundamental understanding but also unlocks a trove of opportunities for nely honing chemical and physical attributes to align with specic applications.

Fig. 5 (
Fig. 5 (A) Schematic representation of two different post-modification paths: (B-E) crystal structures of the derived clusters of Ti 16 L 4-1.