In or out? Adaptive metal binding by a diphosphine-based Zr metal–organic cage

Abstract

A lantern-type Zr metal–organic cage (MOC), Zr–P₂N₂, has been synthesized using ditopic carboxylate linkers based on the 1,5-diaza-3,7-diphosphacyclooctane (P₂N₂) ligand platform. Single-crystal X-ray diffraction shows that the diphosphine groups of the P₂N₂ linkers point into the confined MOC cavity. Postsynthetic metalation of the diphosphine groups with Pd and Pt precursors generates the heterobimetallic cages Zr–P₂N₂–PdCl₂ and Zr–P₂N₂–PtCl₂. Structural characterization reveals that the linkers flip to accommodate metal chelation, resulting in outward facing metal diphosphine groups. Conversely, reaction of Zr–P₂N₂ with AgPF₆ results in formation of Zr–P₂N₂–Ag, containing a hexasilver core supported by phosphine complexation within the cage interior. This work highlights the adaptive structural behavior of Zr–P₂N₂ which provides new opportunities for designing responsive MOCs.

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Introduction

Metal–organic cages (MOCs) have attracted increasing interest for applications in catalysis and molecular separation owing to their chemical tunability and inherent porosity.^1–3 MOCs are considered molecular analogues of metal–organic frameworks (MOFs).⁴ Both are assembled from organic linkers and metal ions or clusters and exhibit well-defined cavities or pores. However, unlike MOFs, the molecular nature of MOCs confers solubility and solution-state processability, providing unique opportunities for heterogeneous and homogeneous catalyst design.⁵

A common strategy for designing MOC-based catalysts involves using linkers with secondary ligand groups for catalytic metal binding.^6–8 Hard–soft acid–base principles are often exploited to impart orthogonal reactivity between the primary donor groups necessary for cage self-assembly and the secondary ligand groups. For example, the combination of hard Lewis acidic metal ions such as Zr⁴⁺ with hard carboxylate ligands can facilitate cage assembly while softer Lewis basic ligands remain available for catalyst binding. Catalytically active metal fragments can be selectively complexed at the secondary donor sites, either via postsynthetic reactions or cage assembly with metallolinkers.^9,10 This general strategy has been used to design a variety of heterometallic MOCs with nitrogen-based secondary ligand groups such as bipyridines.^11–14

Phosphine ligands are widely used in homogeneous catalysis owing to their steric and electronic versatility and ability to support catalytically active late transition metal species.^15–18 However, there have been relatively few reports of MOCs containing phosphines as secondary ligand groups. Nitschke and co-workers reported a Zn-based MOC (Scheme 1A) assembled from triarylphosphine-based linkers.¹⁹ The phosphine groups were found to react with AuCl(SMe₂), resulting in binding of four AuCl units at the exterior of the tetrahedral cage. Surprisingly, the phosphine MOC exhibited no reaction with other late transition metal precursors such as [Pd(MeCN)₄](BF₄)₂ and [Cu(MeCN)₄]OTf. More recently, Cui, Liu, and co-workers reported the synthesis of a tetrahedral Zr MOC (Scheme 1B) using linkers based on chiral 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl (BINAP) ligands.²⁰ The BINAP groups were metalated with CuOTf and NiCl₂ to generate heterometallic MOCs that catalyze asymmetric alkylation and alkynyl amination reactions as homogeneous catalysts.

Scheme 1 Reported MOCs with phosphine-containing linkers.

Herein, we describe the synthesis and characterization of a novel, lantern-type Zr MOC, Zr–P₂N₂, assembled from diphosphine-based linkers (Scheme 2). The ditopic linkers are based on the 1,5-diaza-3,7-diphosphacyclooctane (P₂N₂) ligand platform, which has been widely studied in homogeneous catalyst design.^21–23 The new MOC undergoes postsynthetic metalation with PdCl₂(cod), PtCl₂(cod), and AgPF₆, revealing that metal chelation at the diphosphine sites requires marked structural changes including an outward flip of the linker groups.

Scheme 2 Synthesis of Zr–P₂N₂, Zr–P₂N₂–MCl₂, and Zr–P₂N₂–Ag.

Results and discussion

Synthesis and characterization of Zr–P₂N₂

The H₂(P₂N₂) linker was synthesized following a reported procedure.²⁴ Zr–P₂N₂ was obtained as colourless crystals by solvothermal reaction of H₂(P₂N₂) with Cp₂ZrCl₂ in a N,N-diethylformamide (DEF) and H₂O solvent mixture (Scheme 2). Single crystal X-ray diffraction (SCXRD) revealed formation of a lantern-type cage comprised of a pair of [(CpZr)₃(OH)₃O]⁴⁺ clusters bridged by three (P₂N₂)²⁻ linkers (Fig. 1a, Table S1). The 8-membered P₂N₂ heterocycle adopts a crown conformation with the lone pairs of the phosphine groups facing inward to the cage. Previous studies have shown that this is the most energetically favourable ligand conformation, and it is commonly observed in solid state structures of P₂N₂ ligands.²⁵ The sum of the bond angles around the N atoms (ave. 119.95°) reflects a nearly trigonal planar geometry arising from delocalization of the N lone pair into the aromatic ring of the benzoate group (Table S2). These features result in formation of a trigonal bipyramidal cage with a solvent-accessible cavity that is ∼10 Å in diameter. Disordered DEF solvent molecules residing in the cage and interstitial space could not be adequately modelled in the structure refinement. The associated electron density was removed from the calculation using SQUEEZE (3715 electrons, 64 DEF molecules per unit cell).²⁶ The electron density map and ¹H NMR data indicate the presence of approximately one DEF molecule inside the cavity of each cage (Fig. S1).

Fig. 1 (a) Molecular structure of Zr–P₂N₂. (b) Structure of adjacent Zr nodes showing hydrogen-bonding interactions between hydroxyl groups and chloride anions. (c) Packing of Zr–P₂N₂ molecules (viewed along the b-axis). (Hydrogen atoms are omitted for clarity. Cyan: Zr, red: O, gray: C, green: Cl, orange: P, blue: N.)

The packing diagram of Zr–P₂N₂ viewed along the b-axis shows a slipped-stacking arrangement with adjacent molecules displaced by approximately half a molecular length (Fig. 1c). Disordered chloride anions were located in pockets created by the µ-OH groups of the Zr clusters, supporting an empirical formula of {[Cp₃Zr₃O(OH)₃]₂(P₂N₂)₃}Cl₂ (Fig. 1b). Although the structure contains DEF solvent molecules in the interstitial space between the cages, Zr–P₂N₂ does not retain permanent, gas accessible porosity after desolvation, and N₂ adsorption isotherm measurements showed no measurable gas uptake. The absence of gas-accessible porosity in Zr–P₂N₂ is consistent with other lantern-type Zr cages.²

Zr–P₂N₂ is slightly soluble in DMSO, EtOH and MeOH, but is insoluble in H₂O and most other polar organic solvents including DMF, acetone, and acetonitrile. The poor solubility of Zr–P₂N₂ is at least partly attributed to strong hydrogen bonding interactions between the hydroxyl groups of the Zr clusters and the chloride counteranions. The ³¹P NMR spectrum measured in DMSO-d₆ shows a single resonance at −40.9 ppm, which is shifted slightly downfield from the free linker (−47.4 ppm) (Fig. S2 and S3). The ¹H NMR spectrum exhibits all expected resonances for the η⁵-Cp groups and (P₂N₂)²⁻ linkers. The linker resonances experience very subtle changes in chemical shift compared to H₂(P₂N₂). The µ-OH groups also appear as a sharp singlet at 9.76 ppm, supporting retention of the lantern cage structure in solution.³ The ¹H DOSY NMR spectrum of Zr–P₂N₂ in DMSO-d₆ shows an average diffusion coefficient of 7.98 × 10⁻¹¹ m² s⁻¹, which is consistent with other reported lantern-type Zr MOCs (Fig. S4).^3,27

ESI-MS data obtained for Zr–P₂N₂ in MeOH solution show that the lantern cage structure is retained, but the phosphine groups undergo partial oxidation (Fig. S5). A mass signal corresponding to the parent cage (z = +2) is observed at m/z = 1346.04 and accompanied by a progression of minor species corresponding to sequential oxidation of the three diphosphine groups. The ³¹P NMR spectrum of Zr–P₂N₂ in MeOD-d₄ exhibits a single resonance at −43 ppm, even after exposure to ambient conditions for several days (Fig. S6). This result suggests that phosphine oxidation occurs in situ during the ESI-MS experiment rather than during sample preparation. Signals corresponding to the +1 (–H⁺), +3 (+H⁺), and +4 (+2H⁺) forms of Zr–P₂N₂ are also observed. Importantly, the isotopic distributions of these signals are consistent with the lantern-type structure, and no mass signals are observed in the m/z = 1789–1806 range, which would indicate the presence of a tetrahedral cage impurity with z = +3.

Synthesis and characterization of Zr–P₂N₂–MCl₂ (M = Pd, Pt)

The inward facing diphosphine groups of Zr–P₂N₂ are not well poised to accommodate metal chelation. In addition to the small pore size (10 Å diameter, 589 ų volume), the phosphine lone pairs diverge, precluding metal chelation without a significant distortion of the cage. Thus, we hypothesized that postsynthetic metalation would require either a linker flip such that the diphosphine groups point outward from the cage or restructuring to give a tetrahedral cage with more sterically accessible phosphine sites. Accordingly, suspensions of Zr–P₂N₂ in MeOH were treated with 3 equiv. of PdCl₂(cod) (cod = η⁴-1,5-cyclooctadiene) or PtCl₂(cod) and gently stirred for 16 h at room temperature. After isolation and washing, the solid products, Zr–P₂N₂–PdCl₂ and Zr–P₂N₂–PtCl₂, were analysed by ³¹P and ¹H NMR spectroscopy in DMSO-d₆. The ³¹P NMR spectra showed a single resonance at +9.5 and −7.4 ppm for Zr–P₂N₂–PdCl₂ and Zr–P₂N₂–PtCl₂, respectively (Fig. S7 and S8). The presence of a single, downfield shifted signal indicates formation of symmetric cages containing equivalent P₂N₂–MCl₂ metallolinkers (Fig. S9 and S10). The ¹H NMR spectra also exhibit signals consistent with symmetric, metalated linkers.

The postsynthetic metalation was further investigated by titrating Zr–P₂N₂ solutions in DMSO-d₆ with up to 4 equiv. of MCl₂(cod) (M = Pd, Pt) and monitoring the reactions by ¹H and ³¹P NMR spectroscopy (Fig. S11 and S12). Upon addition of 1–2 equiv. of the metal precursors, the ³¹P NMR spectra show the appearance of product resonances at +10 ppm (Zr–P₂N₂–PdCl₂) and −6.8 ppm (Zr–P₂N₂–PtCl₂) with a concomitant decrease in the parent phosphine resonance at −41 ppm. The product signals appear abruptly and without the appearance of any discernible intermediate species.

We have been unable to obtain single crystals of Zr–P₂N₂–PdCl₂ and Zr–P₂N₂–PtCl₂ suitable for X-ray diffraction using the postsynthetic metalation approach. Consequently, H₂(P₂N₂–PdCl₂) and H₂(P₂N₂–PtCl₂) metallolinkers were prepared (Fig. S9 and S10) and used for direct assembly of Zr–P₂N₂–PdCl₂ and Zr–P₂N₂–PtCl₂. The ³¹P and ¹H NMR spectra of the products match those obtained from postsynthetic metalation reactions, supporting the formation of the same products in both cases (Fig. S13 and S14). The direct assembly gave single crystals suitable for X-ray diffraction, revealing formation of isostructural, lantern-shaped MOCs with P₂N₂–MCl₂ metallolinkers. As predicted, the linkers undergo a flip, resulting in square planar PdCl₂ and PtCl₂ diphosphine groups that point outward from the cage (Fig. 2, Tables S3 and S4, Fig. S15 and S16). The heterocyclic P₂N₂ rings adopt chair–boat conformations with two of the MCl₂ units pointed up and the third pointed downward with respect to the primary axis of the lantern cage.²⁵ The presence of a single resonance in the ³¹P NMR spectra indicates that the metallolinkers are dynamic in solution, undergoing rapid exchange of the up/down orientations of the MCl₂ units. The change in conformation of the P₂N₂ linkers from crown-type in Zr–P₂N₂ to chair–boat in the Zr–P₂N₂–MCl₂ structures also has a pronounced effect on the shape of the cages. The Zr⋯Zr distances between the metal nodes in Zr–P₂N₂–MCl₂ (17.6 Å) are significantly longer than for the parent cage (15.1 Å) (Fig. S17). A concomitant decrease in the equatorial diameter diminishes the internal volume of the Zr–P₂N₂–MCl₂ cages. Residual electron density in the interstitial spaces between the Zr–P₂N₂–MCl₂ cages, attributed to disordered solvent molecules, was removed from the structure refinement using SQUEEZE. The calculation indicated 1839 electrons for Zr–P₂N₂–PdCl₂ and 1596 electrons for Zr–P₂N₂–PtCl₂ within the masks, corresponding to ∼32 and ∼28 DEF molecules per unit cell, respectively. The electron density maps suggest that no DEF molecules are present within the cavities of the cages, consistent with their decreased pore size compared to Zr–P₂N₂. Overall, the SCXRD data reveal significant structural changes, including a linker flip to generate outward facing Pd/Pt-diphosphine groups and a notable increase in the Zr⋯Zr intercluster distances.

Fig. 2 Structures of (a) Zr–P₂N₂–PdCl₂ and (b) Zr–P₂N₂–PtCl₂. (Hydrogen atoms are omitted for clarity. Cyan: Zr, red: O, gray: C, green: Cl, magenta: Pd, orange: P, blue: N, yellow: Pt.)

The ¹H DOSY NMR spectra of Zr–P₂N₂–MCl₂ in DMSO-d₆ show average diffusion coefficients of 8.89 × 10⁻¹¹ m² s⁻¹ for Pd and 8.61 × 10⁻¹¹ m² s⁻¹ for Pt (Fig. S18 and S19). These values are slightly larger than those of the parent Zr–P₂N₂ (7.98 × 10⁻¹¹ m² s⁻¹), indicating that the elongated structures of the Zr–P₂N₂–MCl₂ cages reduces their effective hydrodynamic radius (Table S5).^27,28

Zr–P₂N₂–PdCl₂ and Zr–P₂N₂–PtCl₂ exhibit limited solubility in MeOH and DMSO and are insoluble in water and all other polar organic solvents tested. Moreover, the ¹H and ³¹P NMR spectra measured in methanol-d₄ showed obvious decomposition, as indicated by the appearance of PhPH₂ (³¹P: −124 ppm) and multiple new ¹H NMR resonances in the 6–9 ppm region (Fig. S20 and S21). This solvent sensitivity and limited solubility precluded characterization of the Zr–P₂N₂–MCl₂ cages using ESI-MS. Attempts to improve their solubility via PF₆⁻/Cl⁻ salt metathesis were largely unsuccessful due to increased solvent sensitivity (see SI for additional details). Consequently, Zr–P₂N₂–PdCl₂ and Zr–P₂N₂–PtCl₂ were analysed by MALDI-TOF mass spectrometry to verify that the lantern-type cage is maintained in bulk samples (Fig. S22 and S23). The MALDI spectrum of Zr–P₂N₂–PdCl₂ displayed two mass signals (z = +2, 1612.29 amu and 1593.77 amu) that correspond to the parent lantern-type cage and the same ion with loss of an HCl equivalent from the Zr–OH and PdCl₂ groups. The MALDI spectrum of Zr–P₂N₂–PtCl₂ showed only a single mass signal (z = +2, 1745.39 amu) associated with the parent ion. No trace signals corresponding to tetrahedral cages were observed for either sample. These results indicate that Zr–P₂N₂–PdCl₂ and Zr–P₂N₂–PtCl₂ exclusively form lantern-type cages, consistent with the structures determined by SCXRD.

Synthesis and characterization of Zr–P₂N₂–Ag

The solid-state structures of Zr–P₂N₂–PdCl₂ and Zr–P₂N₂–PtCl₂ raised the question of whether Zr–P₂N₂ could accommodate metal coordination at the diphosphine groups in the cage interior without undergoing a linker flip. To test this possibility, we considered silver phosphine species which present a minimal steric profile. Accordingly, a suspension of Zr–P₂N₂ in DMSO was treated with 6 equiv. of AgPF₆. Colourless crystals of Zr–P₂N₂–Ag suitable for X-ray diffraction were obtained by ethyl acetate vapor diffusion into the reaction solution. However, the structure solution and refinement were limited by weak diffraction and significant disorder of the guest solvent molecules and counteranions, resulting in high R-factors (R₁ = 0.2131, wR₂ = 0.5733, see SI for additional details). The resulting data preclude detailed analysis of bond metrics but establish connectivity of the core structure. Zr–P₂N₂–Ag adopts a lantern-type structure resembling the parent Zr–P₂N₂ but with dinuclear Ag species complexed at each of the diphosphine ligands in the interior of the cage (Fig. 3, Fig. S30, and Tables S6 and S7). The cage is bisected by a mirror plane in the Pbcm space group, resulting in crystallographically inequivalent linkers. Two of the diphosphine ligands are equivalent and support unsymmetric, dinuclear species (Ag2 and Ag3) capped by a µ-Cl ligand originating from the Zr–P₂N₂ counteranion. The remaining diphosphine ligand is bisected by the mirror plane and supports a symmetric dinuclear Ag species with two bridging κ-O-DMSO ligands. As a result, each Ag1 atom is three-coordinate and adopts a distorted Y-shaped geometry. The presence of two chloride ligands in the hexanuclear Ag core suggests that each Zr–P₂N₂–Ag molecule should bear an overall charge of +6, which is balanced by disordered PF₆⁻ or Cl⁻ ions residing outside of the cage.

Fig. 3 (a) Molecular structure of the Zr–P₂N₂–Ag cage viewed along the equatorial plane and (b) truncated structure of the cage viewed down the axis containing the metal nodes. (Hydrogen atoms are omitted for clarity. Cyan: Zr, red: O, gray: C, green: Cl, purple: Ag, orange: P, blue: N, yellow: S.)

The Zr⋯Zr distance between metal nodes in Zr–P₂N₂–Ag (ave. 15.7 Å) is comparable to that of the parent Zr–P₂N₂ (15.1 Å), indicating that the cage does not require significant distortion to accommodate complexation of the Ag species. The preferential accumulation of the three Ag₂ units inside the cage appears to arise from a combination of steric and geometric factors inherent to the P₂N₂ ligands and the cage structure. The intraligand P⋯P distances are relatively short for the chelate species in Zr–P₂N₂–MCl₂ (ave. 3.039 Å for Pd and 3.029 Å for Pt) and much longer for the dinucleating ligands in Zr–P₂N₂–Ag (ave. 3.912 Å) owing to the diverging metal–phosphine bond vectors. The different coordination modes affect the relative orientation of the phenyl substituents, which splay toward the P₂MCl₂ chelate groups in Zr–P₂N₂–MCl₂ but bend away from the P₂Ag₂ units in Zr–P₂N₂–Ag. As a result, the inward-facing P₂Ag₂ groups alleviate unfavorable steric interactions that would occur if they were outward-facing.

Crystalline samples of Zr–P₂N₂–Ag were dissolved in DMSO-d₆ and analysed by ¹H and ³¹P NMR spectroscopy. The ³¹P NMR spectrum is consistent with the presence of the three crystallographically unique silver-phosphine sites observed in the solid-state structure (Fig. 3b). The three phosphine resonances centered around −19, −23, and −28 ppm each appear as two doublets owing to spin–spin coupling with ¹⁰⁷Ag (I = 1/2, ∼52% abundance) and ¹⁰⁹Ag (I = 1/2, ∼48% abundance) isotopes (Fig. 4). The ³¹P NMR spectrum was also measured at a higher magnetic field strength to clearly resolve the individual signals and coupling constants of the isotopologues (Fig. S34). The ¹J(¹⁰⁷Ag–³¹P) and ¹J(¹⁰⁹Ag–³¹P) coupling constants were assigned based on the relative gyromagnetic ratios of the isotopes (γ(¹⁰⁹Ag)/γ(¹⁰⁷Ag) ≈ 1.15).²⁹ The ¹J(¹⁰⁷Ag–³¹P) (582–617 Hz) and ¹J(¹⁰⁹Ag–³¹P) (676–710 Hz) values are comparable to those reported for other diphosphine bridged, dinuclear Ag complexes.^30,31 However, given the similarity in the Ag coordination environments and NMR chemical shifts, the resonances cannot be unambiguously assigned to the corresponding phosphine groups in the structure of Zr–P₂N₂–Ag. The ¹H NMR spectrum displays three Zr–OH group resonances and overlapping multiplets in both the aromatic and aliphatic regions, which is consistent with the lower symmetry of Zr–P₂N₂–Ag relative to the parent cage (Fig. S31). Zr–P₂N₂–Ag was also analyzed by mass spectrometry using MALDI-TOF and ESI-MS in acetonitrile solution (Fig. S35–S37). Both spectra show signals for z = +2 ions corresponding to the hexasilver parent cage with solvent exchange or adduct formation with the MALDI matrix. In addition, inductively-coupled plasma optical emission spectroscopy (ICP-OES) provided a Zr : Ag ratio of 1 : 0.98 which is consistent with the expected stoichiometry (6 Ag and 6 Zr per cage) for Zr–P₂N₂–Ag (Fig. S38).

Fig. 4 ³¹P{¹H} NMR spectrum of Zr–P₂N₂–Ag (DMSO-d₆, 162 MHz) showing the Ag–P coupling and chemical shifts of the doublets.

Conclusions

In summary, we report the synthesis and structural characterization of a new lantern-type MOC, Zr–P₂N₂, comprised of trinuclear Zr metal nodes and linkers based on the known P₂N₂ ligand motif. There are only two previously reported MOCs with phosphine-functionalized linkers, and both contain sterically accessible phosphine groups poised for metal coordination.^19,20 However, in Zr–P₂N₂, the diphosphine donor groups point into the small cavity of the lantern-type cage. The limited pore space and divergent phosphine lone pairs preclude diphosphine chelation of metal fragments inside the cage. Consequently, reaction of Zr–P₂N₂ with MCl₂(cod) (M = Pd, Pt) induces a linker flip, resulting in outward-facing (P₂N₂)PdCl₂ and (P₂N₂)PtCl₂ groups. This adaptive behaviour points to dynamic linker exchange processes reminiscent of those observed for related Zr MOFs.^32,33 In contrast, metalation of Zr–P₂N₂ with AgPF₆ affords a hexasilver cage, Zr–P₂N₂–Ag, with dinuclear Ag species complexed in the interior of the cage. Retention of the inward-facing diphosphine groups is enabled by the smaller steric profile of the dinuclear silver fragments along with the bridging, rather than chelating, mode of the diphosphine ligands. The adaptive structure of Zr–P₂N₂ offers possibilities for designing switchable catalysts, sensors, or selective metal capture systems. Our ongoing efforts are focused on improving the solubility and stability of Zr–P₂N₂ and related MOCs to further explore these applications.

Experimental

Materials and methods

Bis(hydroxymethyl)phenylphosphine, PdCl₂(cod), and PtCl₂(cod) were prepared following reported procedures.^34–36 4-aminobenzoic acid (Alfa Aesar, 99%), trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB, Fisher, 98+%), N,N-dimethylformamide (DMF, Fisher Chemical), N,N-diethylformamide (DEF, TCI America), acetone (Fisher Chemical), ethyl acetate (Fisher Chemical), and dimethyl sulfoxide (VWR Chemicals) were used as received. Cp₂ZrCl₂ (Thermo Scientific, 98%) was used as received and stored in a N₂-filled glovebox. Phenylphosphine (Beantown Chemical) and AgPF₆ (TCI America) were used as received and stored at −20 °C in a N₂-filled glovebox. All other solvents and reagents were of reagent grade and used as received.

Solution-state NMR spectra were collected using a Bruker DPX 400 or AVIII 600 MHz spectrometer. For ¹H NMR spectra, the solvent resonance was referenced as an internal standard. For ³¹P{¹H} NMR spectra, 85% H₃PO₄ was used as an external standard (0 ppm). For ¹H DOSY NMR spectroscopy, saturated solutions of the analyte in DMSO-d₆ or MeOD-d₄ were analysed using a Bruker AVIII 600 MHz spectrometer. The μ-OH resonances were excluded for average diffusion coefficients (D) calculations due to their potential exchange with H₂O.

Single crystal X-ray diffraction analyses were performed using a Bruker Kappa Photon III Diffractometer equipped with Mo Kα radiation (λ = 0.71073 Å). Each crystal was mounted on a MiTeGen micromount with paratone oil. The analyses were conducted under N₂ flow at temperatures of 100 K or 240 K. The structures were solved using ShelXT and refined with SHELXL implemented in the Olex2 1.5 software package.^37–39

ICP-OES analyses were performed using an Agilent 5100 ICP-OES spectrometer. Zr–P₂N₂–Ag samples were digested in a 3 : 1 (v/v) mixture of H₂SO₄ (ThermoSci, 99.999% metal-basis) and H₂O₂ (Ricca Chemical). Calibration curves were prepared using commercial ICP standards (Ag, VWR Chemicals; Zr, Inorganic Ventures). Powder X-ray diffraction patterns were collected using a Rigaku Miniflex 600 Diffractometer equipped with nickel-filtered Cu Kα radiation (λ = 1.5418 Å).

Mass spectrometry

Electrospray ionization (ESI) mass spectrometry data were collected with a ThermoFisher Orbitrap Exploris MX and Vanquish HPLC mass spectrometer. MALDI-TOF mass spectrometry data were collected using a Bruker Microflex LRF in reflection mode. Saturated solutions of analyte and matrix (DCTB) were prepared separately in 100 µL of DMSO. The solutions were then combined in an Eppendorf tube, and the resulting mixture was spotted onto a Bruker MSP 96 polished steel BC plate. After drying completely, the spots formed light yellow solids. ESI-MS and MALDI-TOF MS spectra were simulated using the molecular mass calculator and “Soup of Peaks” tools available via the open-access EPFL Mass Spectrometry Toolbox.⁴⁰

Synthetic procedures

Synthesis of H₂(P₂N₂). The linker was synthesized following a modified literature procedure.²⁴ In a N₂-filled glovebox, bis(hydroxymethyl)phenylphosphine (481 mg, 2.83 mmol), 4-aminobenzoic acid (388 mg, 2.83 mmol), and degassed ethanol (20 mL) were charged in a 35 mL CEM microwave vessel with a Teflon stir bar. The tube was sealed and heated at 100 °C for 18 hours in a CEM Discover SP microwave reactor. Following the reaction, the vessel was cooled in a −4 °C freezer for 15 min. The resulting white precipitate was collected via vacuum-filtration and washed with cold ethanol (5 × 10 mL). The solid was dried in vacuo and characterized via ¹H and ³¹P{¹H} NMR spectroscopy. Yield: 710 mg (93%). ¹H NMR (600 MHz, DMSO-d₆): δ 12.08 (s, 2H, –COOH), 7.75–7.72 (m, 8H, Ar), 7.55–7.49 (m, 6H, Ar), 6.75 (d, 4H, Ar), 4.65 (dd, ²J_H–H = 15.2, ²J_H–P = 5.7 Hz, 4H, alkyl), 4.22 (d, ²J_H–H = 14.1 Hz, 4H, alkyl). ³¹P{¹H} NMR (162 MHz, DMSO-d₆): δ −47.44 (s).

Synthesis of Zr–P₂N₂. A 20 mL scintillation vial was charged with H₂(P₂N₂) (50 mg, 0.092 mmol), Cp₂ZrCl₂ (54.0 mg, 0.185 mmol), H₂O (0.3 mL), and DEF (3 mL) and sealed with a Teflon-lined screw cap. The mixture was sonicated for 10 min, resulting in a pale yellow suspension. The vial was placed in a room temperature oven and heated to 60 °C at a ramp rate of 10 °C h⁻¹. After 16 hours, the vial was slowly cooled to room temperature at a ramp rate of 15 °C/hr. The resulting white crystalline solid was collected via vacuum filtration and washed with DMF (5 × 5 mL) and acetone (5 × 5 mL). The solid was dried in vacuo and characterized via ¹H and ³¹P{¹H} NMR spectroscopy. Yield: 44 mg (51.9%). For SCXRD data collection, the colourless block-like crystals were isolated without washing. ¹H NMR (400 MHz, DMSO-d₆): δ 9.76 (s, 2H, –OH), 7.83–7.76 (m, 8H, Ar), 7.59–7.58 (m, 6H, Ar), 6.68 (d, ²J_H–H = 9.0 Hz, 4H, Ar), 6.41 (s, 10H, Cp), 4.69 (d, ²J_H–H = 15.7 Hz, 4H, alkyl), 4.33–4.27 (m, 4H, alkyl). ³¹P{¹H} NMR (162 MHz, DMSO-d₆): δ −40.89 (s). See SI for mass spectrometry data.

Direct synthesis of Zr–P₂N₂–PdCl₂. The H₂(P₂N₂–PdCl₂) metallolinker was prepared in situ by dissolving H₂(P₂N₂) (25 mg, 0.046 mmol) and PdCl₂(cod) (13 mg, 0.046 mmol) in DEF (9 mL) and H₂O (0.9 mL) in a 20 mL scintillation vial. Solid Cp₂ZrCl₂ (27 mg, 0.092 mmol) was added to the light yellow H₂(P₂N₂–PdCl₂) solution. The vial was sealed with a Teflon-lined screw cap and sonicated for 5 min resulting in a yellow solution. The vial was placed in a room temperature oven and heated to 60 °C at a ramp rate of 10 °C h⁻¹. After 18 hours, the vial was slowly cooled to room temperature at a ramp rate of 15 °C h⁻¹. The resulting yellow plate-like crystals were washed with acetone (5 mL) and characterized by ¹H and ³¹P{¹H} NMR spectroscopy and SCXRD. Yield: 20 mg (41%). ¹H NMR (600 MHz, DMSO-d₆) δ 10.08 (s, 2H), 7.84–7.83 (m, 4H), 7.78 (d, ²J_H–H = 8.9 Hz, 4H), 7.72–7.70 (m, 2H), 7.67–7.65 (m, 4H), 6.99 (d, ²J_H–H = 8.3 Hz, 4H), 6.55 (s, 10H), 4.75 (d, J = 14.6 Hz, 4H), 4.15 (s, 4H). ³¹P{¹H} NMR (243 MHz, DMSO-d₆) δ 9.94. See SI for mass spectrometry data.

Direct synthesis of Zr–P₂N₂–PtCl₂. The H₂(P₂N₂–PtCl₂) metallolinker was prepared in situ by dissolving H₂(P₂N₂) (25 mg, 0.046 mmol) and PtCl₂(cod) (17 mg, 0.046 mmol) in DEF (9 mL) and H₂O (0.9 mL) in a 20 mL scintillation vial. The mixture was stirred and heated at 50 °C for 16 h. Solid Cp₂ZrCl₂ (27 mg, 0.092 mmol) was added to the light yellow H₂(P₂N₂–PtCl₂) solution. The vial sealed with a Teflon-lined screw cap and sonicated for 5 min resulting in a yellow solution. The vial was placed in a room temperature oven and heated to 60 °C at a ramp rate of 10 °C h⁻¹. After 18 hours, the vial was slowly cooled to room temperature at a ramp rate of 15 °C h⁻¹. The resulting colourless plate-like crystals were washed with acetone (5 mL) and characterized by ¹H and ³¹P{¹H} NMR spectroscopy and SCXRD. Yield: 34 mg (63%) ¹H NMR (600 MHz, DMSO-d₆) δ 10.08 (s, 2H), 7.84–7.82 (m, 4H), 7.78 (d, ²J_H–H = 8.6 Hz, 4H), 7.69–7.67 (m, 6H), 6.99 (d, ²J_H–H = 8.8 Hz, 4H), 6.55 (s, 10H), 4.67 (d, J = 15.4 Hz, 4H), 4.31 (s, 4H). ³¹P{¹H} NMR (243 MHz, DMSO-d₆) δ −6.84. See SI for mass spectrometry data.

Synthesis of Zr–P₂N₂–Ag. Zr–P₂N₂ (0.018 mmol) was suspended in DMSO (2 mL). A solution of AgPF₆ (27 mg, 0.108 mmol) in DMSO (0.5 mL) was added to the Zr–P₂N₂ suspension, resulting in a pale-yellow solution and some colourless solids. The reaction was filtered using a PTFE syringe filter (0.45 µm), and the filtrate was subjected to vapor diffusion of ethyl acetate. After one month, colourless block-like crystals were obtained and analyzed via SCXRD. Rapid addition of EtOAc (20 mL) to the filtrate resulted in precipitation of the product as a colorless powder. The powder was collected via centrifugation and washed with ethyl acetate. Yield: 32 mg (44.4%). ¹H NMR (600 MHz, DMSO-d₆) δ 10.03–9.91 (m, 2H), 7.95–7.75 (m, 13H), 6.76–6.71 (m, 3H), 6.59–6.57 (m, 1H), 6.43–6.37 (m, 10H), 5.22 (s, 3H), 5.03 (m, 1H), 4.78–4.63 (m, 3H), 4.35 (s, 1H). ³¹P{¹H} NMR (162 MHz, DMSO-d₆) −19.44 (d, ¹J_Ag–P = 593 Hz), −19.45 (d, ¹J_Ag–P = 685 Hz), −22.72 (d, ¹J_Ag–P = 614 Hz), −22.71 (d, ¹J_Ag–P = 706 Hz), −27.93 (d, ¹J_Ag–P = 582 Hz), −27.93 (d, ¹J_Ag–P = 676 Hz). See SI for mass spectrometry data.

NMR titration procedures

NMR titration of Zr–P₂N₂ with MCl₂(cod) (M = Pd, Pt). Zr–P₂N₂ (0.0018 mmol) was suspended in DMSO-d₆ (0.4 mL) and transferred to an NMR tube. ¹H and ³¹P{¹H} NMR spectra were collected. A 0.05 M solution of MCl₂(cod) was prepared in DMSO-d₆ (3 mL). The NMR tube containing Zr–P₂N₂ was titrated with aliquots of the 0.05 M MCl₂(cod) solutions in increments of 0.036 mL (i.e. 1 equiv. of M per Zr–P₂N₂). After each addition, the sample was analysed via ¹H and ³¹P{¹H} NMR spectroscopy. The reaction was analyzed up to a total of 4 equiv. of MCl₂(cod). The Zr–P₂N₂ suspension became a yellow solution upon addition of the MCl₂(cod) solution.

NMR titration of Zr–P₂N₂ with AgPF₆. Zr–P₂N₂ (0.0018 mmol) was suspended in DMSO-d₆ (0.4 mL) and transferred to an NMR tube. ¹H and ³¹P{¹H} NMR spectra were collected. A 0.05 M AgPF₆ solution was prepared by dissolving AgPF₆ (38 mg) in DMSO-d₆ (3 mL). The NMR tube containing Zr–P₂N₂ was titrated with aliquots of the 0.05 M AgPF₆ solution in increments of 0.036 mL (i.e. 1 equiv. of Ag per Zr–P₂N₂). After each addition, the sample was analysed by ¹H and ³¹P{¹H} NMR spectroscopy. The reaction was analysed up to a total of 6 equiv. of AgPF₆. The Zr–P₂N₂ suspension became a yellow solution upon addition of the AgPF₆ solution.

Author contributions

J. S.: investigation, formal analysis, writing; C. R. W.: conceptualization, formal analysis, supervision, writing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: NMR spectral data, mass spectrometry data, crystallographic data, PXRD data, ICP-OES data, additional experimental details. See DOI: https://doi.org/10.1039/d6dt00536e.

CCDC 2535025–2535029 (Zr–P₂N₂, Zr–P₂N₂–PdCl₂, Zr–P₂N₂–PtCl₂, Zr–P₂N₂–Ag (masked counteranions) and Zr–P₂N₂–Ag) contain the supplementary crystallographic data for this paper.^41a–e

Acknowledgements

This research was supported by the U.S. National Science Foundation under Grant No. CHE-2044904. We acknowledge resources from the Campus Chemical Instrumentation Center Nuclear Magnetic Resonance Facility and Mass Spectrometry and Proteomics Facility at The Ohio State University. These facilities are supported in part by funding from OSU's Enterprise for Research, Innovation and Knowledge.

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