Redox-Switchable Allosteric Effects in Molecular Clusters.

We demonstrate that allosteric effects and redox state changes can be harnessed to create a switch that selectively and reversibly regulates the coordination chemistry of a single site on the surface of a molecular cluster. This redox-switchable allostery is employed as a guiding force to assemble the molecular clusters Zn3Co6Se8L'6 (L' = Ph2PN(H)Tol, Ph = phenyl, Tol = 4-tolyl) into materials of predetermined dimensionality (1- or 2-D) and to encode them with emissive properties. This work paves the path to program the assembly and function of inorganic clusters into stimuli-responsive, atomically precise materials.

Reversibly activating a single coordination site on the surface of an inorganic cluster could enable unprecedented control over reactivity, opening uncharted paths for catalyst development and functional materials synthesis.[1,2] While redox cooperativity and charge redistribution among neighboring metals have been shown to lead to site-selective reactivity in some molecular clusters,[3−5] reversibly regulating the coordination chemistry of a single site on the surface of a molecular cluster remains elusive. More often, chemical differentiation, achieved either using the supporting ligand or by changing the identity of the metal itself, is required to direct the chemistry to a single site.[6−15] On an extended surface, the reactivity of chemically degenerate surface sites can become differentiated as a result of ligand binding via “inter-adsorbate interactions”, giving rise to important phenomena such as altered Sabatier volcano curves[16,17] or nanopatterning.[18] When a substrate binds at one site on a surface, it induces subtle structural and electronic changes at neighboring sites that alter their reactivity primarily by modifying adsorption energies. For example, inter-adsorbate interactions inhibit CO adsorption at nearest-neighbor sites on Cu(100) due to decreased electron density around the initial CO adsorption site.[19] Similar effects are invoked as the major contributors for the coverage-dependent adsorption energy of H2S on FeS2.[20] Inspired by inter-adsorbate effects in extended materials, we set out to probe if site-differentiation of a molecular cluster could also be achieved based solely on ligand binding and to investigate if redox state changes can be used as a switch for this differentiation.

Our group previously introduced a family of molecular clusters M3Co6Se8L′6 (M, L′ = Ph2PN(H)R, Ph = phenyl, R = 4-tolyl; M*, R = isopropyl, M = Fe, Co, Zn, Sn) with three degenerate surface sites (M) that can engage with substrates and linkers,[21−24] enabling them to function as catalysts[21] and building blocks alike.[23] In this study, a redox innocent metal, Zn, was selected to decorate the cluster; therefore, a redox change in the Co6Se8 support would be unambiguously responsible for modifying the affinity of the cluster for linkers. Two main discoveries were made. First, the equivalent edge sites of Zn become site-differentiated when exposed to ligands (Scheme ). This is an illustration of inter-adsorbate effects wherein through-cluster electronic and structural rearrangements are propagated between the edge Zn sites upon ligand binding. Due to its similarity to allosteric effects in biological systems,[25,26] we refer to it as allosteric site-differentiation. Second, the site-differentiation can be reversibly switched “on” or “off” by simply changing the oxidation state of the cobalt selenide support. The utility of this redox-switchable allosteric effect is illustrated in the assembly of the atomically precise materials of predetermined dimensionality, namely one-dimensional nanowires and two-dimensional nanosheets. While electrons are a convenient way to operate a molecular switch, we also demonstrate that an inner-sphere electron transfer pathway relying on quinone reduction can be employed to tag the Zn nanocluster. We use this strategy to equip the ditopic nanocluster building block ZnL2 with a fluorophore, opening a new path to access stimuli-responsive, atomically precise nanomaterials.[27−29]

Scheme 1

Redox-Switchable Allosteric Effects: A Powerful Strategy to Reversibly Site-Differentiate Degenerate Surface Sites in Inorganic Clusters

Zn is prepared by salt metathesis from Li6(py)6Co6Se8L′6 (1.0 equiv) and ZnCl2 (3.3 equiv).[21,22] In the presence of coordinating ligands (L), the cluster is isolated exclusively as the site-differentiated bis-adduct Zn3L2 (L = py; 4.5 g, 80% yield, Figure a and b). Single crystal X-ray diffraction shines light on the site-differentiation, revealing an allosteric effect in which THF binding at two of the Zn edge sites effectively strengthens the Zn–Co6Se8 interactions at the third. Two Zn(THF) sites are chelated κ3 by the cluster via two amides and one Se, while the third is bound κ4, featuring a second Zn–Se bond (Figure c). The site-differentiation is retained in the presence of excess 1,10-phenanthroline (phen), a chelating ligand with strong σ-donating properties. In the solid state, the bis-adduct Zn(phen)2 (78% yield; Figure a) features one naked κ4-Zn edge and two κ2-Zn sites in which both Zn–Se interactions have been replaced by coordination to phenanthroline (Figure d).[30−32] The allosteric effect set off by ligand binding in Zn is enabled by the hemilability of the metal–support interactions.[21,22] We propose that breaking a Zn–Se bond upon ligand coordination begins a cascading effect through the Co6Se8 support which regains some electron density and presents at another Zn edge site by strengthening the Zn–Se interaction and diminishing its affinity for exogenous ligands.

Figure 1

(a) Synthesis of a ditopic nanoblock and (b) structure of ZnL2 (L = THF or py). Single crystal X-ray diffraction structures of (c) Zn3(THF)2 and (d) Zn(phen)2. Hydrogen atoms, cocrystallized solvent, and any disorder are omitted for clarity.

Here, the utility of the site-differentiation is harnessed to control the dimensionality of cluster-assembled nanostructures. Typically, the chemically degenerate sites on the surface of inorganic clusters bind linkers indiscriminately, precluding dimensional control over the assembly. Synthetic pathways that enable deterministic assembly routes of low-dimensional structures are an active area of research.[23,33−37] With only two edge sites available to engage linkers, the Zn(py)2 cluster is poised to encode the formation of a one-dimensional wire when mixed with a linear ditopic ligand. To mimic the coordination of pyridine and phenanthroline at the Zn edge sites, two types of nitrogen based linear linkers are employed: the monodentate 4,4′-bipyridine (bipy) and the chelating tetrapyrido[3,2-a:2′,3′-c:3″,2″-h:2‴,3‴-j] phenazine (tpphz), respectively. Black, prismatic crystals of the nanowires [Zn(bipy)] (1-bipy, 48% yield) and [Zn(tpphz)] (1-tpphz, 65% yield) grew over the course of 2–3 days of mixing Zn(py)2 with excess bipy or tpphz (Figure a). Single crystal X-ray diffraction reveals the similarity of the cluster nodes with their molecular counterparts, which retain one edge site in the “off”, κ4-Zn state, and two linker-bound edge sites that enable catenation of the wires (Figure b and c). While both nanowires exhibit a zigzag motif, the κ2-Zn versus κ3-Zn linkages produce obvious structural differences. Most notably, the kinks of the wire measure 62° in 1-bipy and double, 124°, in 1-tpphz, where the chelating effect of tpphz disengages the Zn from the cluster enabling a more relaxed chain. Interestingly, in both wires, the racemic mixture of helical nanoclusters (Λ, Δ) of Zn assembles with alternating enantiomers, giving rise to the first examples of syndiotactic nanocluster wires. Typically, the tacticity of hybrid inorganic/organic polymers is encoded based on the chirality of the organic linker rather than the intrinsic chirality of an inorganic node.[38−40] Tacticity in organic polymers is associated with drastic differences in physical properties; exploring the consequences of tacticity in inorganic hybrid materials could likewise be tied to interesting properties, but in part due to lack of synthetic access, this remains a little explored frontier.

Figure 2

(a) Synthesis of nanowires 1-bipy and 1-tpphz. Single crystal X-ray data of (b) 1-bipy and (c) 1-tpphz, with insets depicting a single cluster node of Δ-helicity. Organic ligands, hydrogen atoms, and any cocrystallized solvent are not depicted for clarity.

We previously observed that oxidation state changes affect the affinity for ligands of Fe clusters.[21] Here, we set out to explore if an increased affinity for ligands upon oxidation could turn the κ4-Zn site into a redox switch. Electrochemically, Zn exhibits three single electron oxidation events (−0.37, 0.12, and 0.49 V) and one reduction (−1.59 V) all of which are chemically reversible (Figures S16–S18; referenced to Fc/Fc+). The chemical oxidation of the neutral bis-adduct Zn(py)2 in the presence of phenanthroline (3.3 equiv) is revealing. The tris-adduct [Zn(phen)3][PF6] (80%; Figure a) is formed as the sole product, and its solid state structure confirms the chemical equivalence of its three κ2-Zn edge sites (Figure b). The formation of [Zn(phen)3]+ illustrates how removing a single electron from the cobalt core can be used to switch “on” the affinity for ligands at the κ4-Zn site in the neutral cluster Zn(phen)2. Due to the redox resilience of the Co6Se8 support, this process is electrochemically reversible (Figure S17). The ability to switch between the neutral and mono-oxidized nanoclusters using electrons in the transformation Zn(L)2 + L ⇌ [Zn(L)3]+ + e– makes the zinc nanocluster an exceptional candidate for the assembly of stimuli-responsive materials.

Figure 3

(a) Synthesis of a tritopic nanoblock. (b) Single crystal X-ray diffraction structure of [Zn(phen)3][PF6]. Hydrogen atoms, cocrystallized solvent, and any disorder are omitted for clarity.

Redox-switching “on” the affinity for ligands at the κ4-Zn site turns the ditopic Zn3(py)2 cluster into a tritopic nanoblock, which in combination with linear linkers would give rise to two-dimensional nanosheets. Mixing the Zn(py)2 nanocluster, the FcPF6 oxidant, and the bipy linker gives rise over the course of 2 days to black, prismatic crystals identified as [Zn(bipy)1.5][PF6] (2-bipy, 44% yield; Figure a). This material is identified as a layered two-dimensional van der Waals material, with each nanoblock in the +1 oxidation state, using single crystal X-ray diffraction (Figure b). The structure of the individual nanosheets is remarkably similar to that of the previously reported Co3(bipy)1.5Co6Se8L′6 nanosheet,[23] with the only major differences being the presence of the PF6– anion and the orientation of the sheets within the crystal lattice.

Figure 4

(a) Synthesis of 2-bipy nanosheets. (b) Single crystal X-ray diffraction structure of 2-bipy, with an inset depicting a tritopic node of Δ-helicity. Counterion PF6–, organic ligands, hydrogen atoms, and any cocrystallized solvent are not depicted for clarity.

Beyond controlling dimensionality, electron transfer can be used to endow Zn(py)2 with orthogonal emissive properties. The ruthenium complex [Ru(bipy′)2(phendione)][PF6]2 (bipy′ = 2,2′-bipyridine, phendione = 1,10-phenanthroline-5,6-dione) is uniquely equipped to serve as both a fluorophore and an inner-sphere oxidant due to the dione moiety.[41−43] Treatment of Zn(py)2 (1 equiv) with [Ru(bipy′)2(phendione)][PF6]2 (1 equiv) produces a new species 3 (Figure ), which is most consistent with the structure proposed in Figure b. Signaling the oxidation of the Co6Se8 core is the emergence of a single paramagnetic 31P NMR resonance in 3 at −415 ppm. The monoreduction of the quinone to a semiquinone is reflected in the disappearance of the C=O stretch (1770 cm–1) of the starting material and the appearance of a lower energy feature at 1464 cm–1, attributed to the C–O stretch.[42,44] Upon excitation at λmax (438 nm), 3 exhibits a broad emission band centered at 677 nm. Not only is this feature absent in Zn(py)2, but it is 50 nm red-shifted from the parent Ru complex (Figure c). Preliminary studies reveal that 3 has potential as a nanoblock; treating it with bipy results in the formation of a material insoluble in common organic solvents—the hallmark of an extended framework.

Figure 5

(a) Synthesis of 3 and (b) structural model highlighting the inner-sphere electron transfer from the cobalt core to the phendione unit. (c) Absorption and emission (438 nm excitation) profiles of 3 and Zn(py)2.

Traditionally, allosteric effects describe how the binding of a ligand at one site of a protein alters the properties of a distant site on the same protein.[25,26] In Zn, ligand binding at one edge site sets off a domino effect that changes the affinity for ligands at another Zn site. The hemilability of the interactions between the edge (Zn) and the redox-active support (Co6Se8) enables the creation of a switch, wherein electron transfer controls the affinity for ligands at a single Zn edge site. The redox-switchable allosteric site-differentiation of the Zn nanocluster turns it into a versatile building block which can be pre-encoded with desirable physicochemical information. This work illustrates a new approach to program dimensionality and function into stimuli-responsive atomically precise materials.