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. 2023 Apr 25;62(18):7032–7044. doi: 10.1021/acs.inorgchem.3c00325

Site Selectivity for the Spin States and Spin Crossover in Undecanuclear Heterometallic Cyanido-Bridged Clusters

Le Shi †,‡,*, Jedrzej Kobylarczyk †,§, Katarzyna Dziedzic-Kocurek ∥, Jan J Stanek ∥, Barbara Sieklucka †, Robert Podgajny †,*
PMCID: PMC10170501  PMID: 37120844

Abstract

graphic file with name ic3c00325_0007.jpg

Polynuclear molecular clusters offer an opportunity to design new hierarchical switchable materials with collective properties, based on variation of the chemical composition, size, shapes, and overall building blocks organization. In this study, we rationally designed and constructed an unprecedented series of cyanido-bridged nanoclusters realizing new undecanuclear topology: FeII[FeII(bzbpen)]6[WV(CN)8]2[WIV(CN)8]2·18MeOH (1), NaI[CoII(bzbpen)]6[WV(CN)8]3[WIV(CN)8]·28MeOH (2), NaI[NiII(bzbpen)]6[WV(CN)8]3[WIV(CN)8]·27MeOH (3), and CoII[CoII(R/S-pabh)2]6[WV(CN)8]2[WIV(CN)8]2·26MeOH [4R and 4S; bzbpen = N1,N2-dibenzyl-N1,N2-bis(pyridin-2-ylmethyl)ethane-1,2-diamine; R/S-pabh = (R/S)-N-(1-naphthyl)-1-(pyridin-2-yl)methanimine], of size up to 11 nm3, ca. 2.0 × 2.2 × 2.5 nm (1–3) and ca. 1.4 × 2.5 × 2.5 nm (4). 1, 2, and 4 exhibit site selectivity for the spin states and spin transition related to the structural speciation based on subtle exogenous and endogenous effects imposed on similar but distinguishable 3d metal-ion-coordination moieties. 1 exhibits a mid-temperature-range spin-crossover (SCO) behavior that is more advanced than the previously reported SCO clusters based on octacyanidometallates and an onset of SCO behavior close to room temperature. The latter feature is also present in 2 and 4, which suggests the emergence of CoII-centered SCO not observed in previous bimetallic cyanido-bridged CoII–WV/IV systems. In addition, reversible switching of the SCO behavior in 1 via a single-crystal-to-single-crystal transformation during desolvation was also documented.

Short abstract

Self-assembled undecanuclear heterometallic cyanido-bridged clusters exhibit site selectivity for spin states and SCO.

Introduction

The rational design of new molecular clusters exhibiting multifunctionality has unceasingly attracted considerable attention owing to the demands for the miniaturization of electronic devices. In particular, clusters exhibiting spatial control over geometric electronic structures and multiple or heterofunctional groups might promote emergent properties.1,2 In this regard, the topology of clusters is appropriately controlled by the polyhedral features of metal-ion complexes and by the denticity (or hapticity) and connectivity of the bridging ligands, which enables cluster geometry ranging from molecular triangles3−5 to cages or capsules,6 and various superpolyhedral structures.7 Advanced physicochemical and functional properties can be achieved by modular modification of the overall shape, periphery zone, and metallic composition of cluster cores, which has led to many breakthroughs in the fields of catalysis,8 reactivity,9 fluorescence,10 host–guest recognition,11 and magnetism.12,13

One of the challenges in the field of molecular magnetism is the design of molecular spin-crossover (SCO) clusters with bistability controlled by external stimuli such as temperature, pressure, light irradiation, electric field, or guest inclusion and exchange.14−16 Moreover, specific arrangement of high-spin (HS) and low-spin (LS) sites in one cluster may provide more than two stable phases, being beneficial for site-selective switching and multistability,16 the property that provides a potential toward applications in nanotechnological devices such as memory storage units, quantum cellular automata, and molecular binary logic devices.17,18 Aiming at this goal, a number of SCO clusters of various nuclearities were prepared by combining the SCO-active ions with diverse bridging ligands that might cooperate in the fine-tuning of steric and electronic strain, e.g., multidentate organic ligands,19−29 metalloligands,30−32 cyanides,33,34 and cyanidometallates.35−38 Multistability and site-selective switching under thermal stimulation or photostimulation have been revealed in some dinuclear,20 square-like,33−35 grid-like,25−27 or metallocubane complexes28 as well as in the largest icosanuclear [Fe20] SCO clusters.29

As an essential contribution to the above studies, we and others applied [M(CN)8]3–/4– (M = W, Mo, Nb, Re) building blocks of diverse structural and redox nonrigidity39,40 to promote thermal-induced and photoinduced charge transfer (CT) within WV/IV–CoII/III, WV/IV–FeII/III, or MoIV–CuII pairs40 and the SCO effect in MV/IV–FeII (M = W, Mo, Nb, Re) coordination networks.41−46 Moreover, the SCO behavior and light-induced excited-spin-state trapping effects (LIESST) were also noted at the FeII sites in discrete structures, such as a [Fe2M2] molecular rhombus47,48 and a [Fe4M2] octahedron,49−51 with the latter one featuring also site-selective double photoswitching of the FeII and MoIV sites.50 Particular attention was paid to the family of 15-nuclear {M9M′6} (M = MnII, FeII, CoII, NiII) clusters easily affordable by crystallization from the methanol (MeOH) solution of the 3d metal salts and [M(CN)8].3−60 The composition of the core of these clusters might be appropriately adjusted to achieve HS in the ground state,52 slow magnetic relaxation,53,54 or phase transitions accompanied by CT55−57 and SCO.58,59 The SCO phenomena were systematically observed for the central [Fe(μ-NC)6] site owing to the favorable ligand-field stabilization energy, whereas the external [Fe(μ-NC)3(MeOH)3] sites remained in the HS state during the transition. This feature definitely precluded the observation of multistability, although multichannel bistability was achieved through a solid/solution approach.57,59 Furthermore, the weakly bonded MeOH molecules on the surface of a cluster allow facile ligand substitution chemistry at these specific positions to create extended clusters and polymers.61−66 However, until now, only the use of dedicated capping 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3tacn) ligands allowed one to extend the SCO behavior onto the peripheral cluster areas.64 Thus, in a further pursuit to activate as much as possible potential SCO sites and appointing the new topologies of heterometallic clusters of modular potential, we exploited the self-assembly between [W(CN)8]3– and FeII, CoII, or NiII complexes with bulky N donors of various degrees of flexibility, commonly used to construct SCO complexes.67,68 As a result, here we present an unprecedented series of cyanido-bridged clusters realizing new undecanuclear topology: FeII[FeII(bzbpen)]6[WV(CN)8]2[WIV(CN)8]2·18MeOH (1), NaI[CoII(bzbpen)]6[WV(CN)8]3[WIV(CN)8]·28MeOH (2), NaI[NiII(bzbpen)]6[WV(CN)8]3[WIV(CN)8]·27MeOH (3) (group I), and CoII[CoII(R/S-pabh)2]6[WV(CN)8]2[WIV(CN)8]2·26MeOH [4R and 4S) (group II); bzbpen = N1,N2-dibenzyl-N1,N2-bis(pyridin-2-ylmethyl)ethane-1,2-diamine; R/S-pabh = (R/S)-N-(1-naphthyl)-1-(pyridin-2-yl)methanimine]. The series was characterized by scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS), flame atomic absorption spectroscopy (FAAS), single-crystal X-ray diffraction (XRD), powder XRD (PXRD), superconducting quantum interference device (SQUID) magnetometry, UV–vis–NIR (in the solid state and in solution), IR and 57Fe Mössbauer spectroscopic techniques, and electrospray ionization mass spectrometry (ESI MS). We demonstrate modular features of all clusters: remarkably reproducible shape and topology, site-selective SCO activity, and corresponding site-selective spin states and bond lengths (the latter includes also SCO-inactive NiII ions), together with the overall stability and accessibility of clusters in organic media and in the gas phase. In addition, we describe the structure and SCO behavior modification associated with the single-crystal-to-single-crystal (SC–SC) transformation in 1.

Results and Discussion

A series of undecanuclear M7W4 clusters, Fe7W4 (1 and 1de), NaCo6W4 (2), NaNi6W4 (3), and (R/S)-Co7W4 (4R and 4S or just 4), were obtained by the self-assembly of 3d divalent metal-ion salts, [W(CN)8]3– precursors and bzbpen (group I), or R/S-pabh ligands (group II) in MeOH (see the Experimental details). The general formula {M′[MIILx]6[WV(CN)8]y[WIV(CN)8]4–y}·nMeOH, phase purity, and composition of all compounds were confirmed by IR spectra (Figure S1), SEM/EDS, FAAS (Figures S2–S6), PXRD (Figures S7–S13), thermogravimetric analysis (TGA; Figures S14–S17), and bond-valence-sum calculations (Tables S15–S17). For the sake of simplicity, we will consider the M7W4 forms as the fundamental structural components (see further description).

Molecular Structure

1–4 crystallize in various space groups, C2/c (isomorphous 1 and 2), P21/c (3), and P21212 (4) (Tables S1–S3), and show the relevant different symmetry-independent units (Figures S18–S22); however, in all of the cases, they are composed of topologically identical cluster motifs (Figure 1). The central pseudotetrahedral [M′(μ-NC)4] (group I: M′ = M4 = Fe, 1; Na, 2 and 3; group II: M′ = Co3, 4) moiety forms four cyanido bridges toward four neighboring [W(CN)8]n− units located in the periphery of the cluster. Their supertetrahedral arrangement reproduces the connectivity of the central unit. Each [W(μ-CN)4(CN)4] unit connects with three neighboring cis-[M(μ-NC)2(bzbpen)] (group I) or cis-[Co(μ-NC)2(R/S-pabh)2] (group II) pseudooctahedral moieties and one tetrahedral metal center, exploiting further the formation of W–CN–M/Co linkages. In total, six such general units, topologically identical within each group, are present in the related peripheral regions of the cluster in each case. As a result, within the undecanuclear skeleton, we distinguished the rhombus M2W2 fragments involving the single peripheral vertex sites (marked in Figure 1 as M1 or Co1) and trigonal-bipyramidal M3W2 fragments involving the pairs of lateral sites (marked in Figure 1 as M2 and M3 or Co2). This assignment is important and convenient for the systematic description of the structure–property scheme of SCO and will be further used.

Figure 1.

Figure 1

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Molecular structure of group I (a) and group II (b) complexes highlighting the undecanuclear metal core, together with the positioning of cis-[MII(μ-NC)2(bzbpen)] moieties (topologically identical for the M1, M2, and M3 sites in 1–3) (a) or of cis-[CoII(μ-NC)2(R/S-pabh)2] (topologically identical for Co1 and Co2 in 4S and 4R) (b). Note that, in the actual crystal structure, the central M4 (M′ in general) position in 2 and 3 is Na1. (c and e) Locations of the distinguished vertex 3d metal-ion site in the rhombus fragments and the lateral 3d metal-ion sites in the trigonal-bipyramidal fragments. (d and f) N6 coordination spheres of 3d metal-ion moieties. Atom color code: blue, W; orange, Fe, Co, or Ni in 1–3; brown, Co in 4; gray, C; pale violet, N. MeOH molecules and H atoms were omitted for clarity.

Supramolecular Interactions and Contacts

The crystal packing, supramolecular interactions, and contacts between the neighboring clusters in both groups are shown in Figures 2 and S23–S29. They are dominated by numerous weak hydrogen bonds involving Cphenyl–H···NCN and some perpendicular Cphenyl–H···ringcentroid synthons connecting the appropriate fragments of the clusters’ periphery (Figures S23a,c and S27). Besides, crystallization MeOH molecules create a proper medium for the propagation of hydrogen bonds involving the terminal cyanido ligands and protons of phenyl rings (Figure S23b,d). In compounds 1–3 (group I), clusters are arranged in two-dimensional layers, vertex moieties rather loosely oriented within the layer, and lateral moieties protruding outside the layer to form tighter interlayer lateral–lateral contacts oriented perpendicular to the cluster’s layers, along the a* direction (Figure 2b). In close connection with the above, the MeOH molecules in 1 are unequally distributed within two distinguished solvent-accessible spaces. About two-thirds of them are located in rather narrow channels weaving along the crystallographic direction c and definitely form tight supramolecular contacts with the interlayer lateral parts of the molecular surface of the clusters arranged in the adjacent layers (Figure S25b). The intercluster separation is represented by the Fe···Fe distances of 10.605 Å (Fe2···Fe3), 10.573 Å (Fe3···Fe3), and 14.163 Å (Fe2···Fe2), respectively (Figure S26c). The remaining one-third are trapped in the intralayer cages adjacent to the vertex part of the cluster and show more loose interactions (Figure S25a). The shortest Fe1···Fe1 distance is 11.170 Å (Figure S26a). These “tight” and “close” contacts are also present in complexes 2 and 3 (Figures S28 and S29). Contrary to group I, the crystal packing of the group II networks exhibits rather “isotropic” three-dimensional cluster arrangement and significantly larger solvent-accessible space compared to group I (Figure S30). The channels of the ca. 1 nm × 1 nm square-like cross section run along the b direction (Figure S31). Again, we observe a similar difference in the exposition of the vertex and lateralcis-[Co(μ-NC)2(R/S-pabh)2]2+ and [W(CN)8]3– moieties: the vertex moieties are involved in relatively less tight contacts compared to those of the lateral ones. The shortest intercluster separations in the vertex moieties are 16.519 and 16.591 Å for 4S and 4R, respectively, and are slightly longer than the lateral intercluster separations, 16.306 and 16.170 Å, respectively (Figure S32).

Figure 2.

Figure 2

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(a) Illustration of hydrogen-bond contacts between the CN– groups and MeOH molecules during the SCSC transformation in 1 and 1de. (b) Supramolecular arrangement of undecanuclear clusters of 1 and 1de within the ac plane. Color code: gray, cluster core; green, organic ligands attached to the vertex metal ions; pink, organic ligands attached to the lateral metal ions; red, MeOH molecules removed during the 1 → 1de process; yellow, MeOH molecules that were retained during the 1 → 1de process.

SC–SC Transformation 1 → 1de

A single crystal of 1 was dried for 20 min in a stream of dry N2 at 300 K, which led to a SCSC transformation assigned as 1 → 1de. During this process, the C2/c space group and the topology of the cluster remain intact; however, the crystal shows a significant 10% anisotropic contraction along the a* direction, perpendicular to the cluster layer, and 10% cell volume reduction (see the PXRD patterns in Figure S9). Elemental analysis and TGA indicated that 13 out of 18 MeOH molecules per one cluster in 1 (close to 2/3) were removed to leave the five MeOH molecules in 1de (close to 1/3) (Figure S14). This leads to modifications of the packing structure (Figures S23–S26 and Tables S5 and S6), hydrogen-bonding networks (Figure 2b), and specific bond lengths and angles (see below). The great majority of removed molecules were those located in the tight channels weaving along the c direction, while those located in the cages within the cluster layer underwent only minor rearrangement (Figure S25). Thus, the 1 → 1de process imposes more drastic changes at the lateral interlayer regions of the clusters compared to the vertex intralayer regions, leading to much closer interlayer contacts (Figure S24) compared to the intralayer contacts.

Site-Selective Spin States and SCO

Structural data collected for HT phases (at 250 or 300 K) and for LT phases (at 100 K) revealed significant structural differences, which allowed us to differentiate the spin states along the 6-coordinated FeII and CoII moieties and observe the selective thermal SCO effect at some of these sites (Tables 1 and S9–S12). On cooling of 1 from 250 to 100 K, the decrease of the average Fe1–N bond length in the vertex site from 2.169 to 2.025 Å is detected (Table 1; see details in Table S9), which suggests that thermal SCO behavior occurs at this site. In contrast, the average Fe–N bond lengths for the lateral Fe2 and Fe3 sites are slightly below 2.0 Å at both 250 K and 100 K, suggesting the LS state of these FeII centers. A similar image of the Fe–N bond lengths was noted for the desolvated form 1de (Table S12), for which the average Fe1–N bond length decrease from 2.144 to 2.001 Å was detected only at the Fe1 sites, on cooling from 300 to 100 K. The distinction of spin states was also confirmed by octahedral distortion bond-angle parameters Σ for both phases, ca. 50° for LS complexes and ca. 75° for HS complexes. Thus, despite the identical coordination spheres composed of two pyridines, two amines, and two isocyanides, the vertex FeII moieties and lateral FeII moieties exhibit different spin-state behavior. This indicates site selectivity at our new undecanuclear topology with respect to the spin states and SCO behavior. Finally, the Fe4–N bond lengths in the range of 2.01–2.04 Å and the angles N–Fe4–N in the general range of 98.0–129.0° (av. 109.8° in all cases) indicate unequivocally the HS tetrahedral [FeII(μ-NC)4] moiety in the central site.69 The representative overlays of the high-temperature (HT) and low-temperature (LT) forms of 1 and 1de are shown in Figure S33.

Table 1. Comparison of M–NCN, M–Npy, and M–Nami or M–Nimi Bond Lengths, Average M–N Bond Lengths, and Octahedral Distortion Bond Angle Parameters Σ in the HT and LT Phases of Complexes 1, 2, 4S, and 4R, together with the Fe or Co Position in the Cluster and Spin-State Assignmenta.

  1
   2
  250 K 100 K   250 K 100 K
Fe1–NCN 2.082 ± 0.005 1.962 ± 0.013 Co1–NCN 2.109 ± 0.007 2.105 ± 0.012
Fe1–NPy 2.159 ± 0.004 2.007 ± 0.009 Co1–NPy 2.159 ± 0.013 2.141 ± 0.008
Fe1–Nami 2.262 ± 0.012 2.106 ± 0.002 Co1–Nami 2.255 ± 0.003 2.247 ± 0.009
Fe1–Nav 2.169 2.025 Co1–Nav 2.175 2.164
Σ (deg) 73.08 53.49 Σ (deg) 77.38 76.43
vertex HS LS vertex HS HS
Fe2–NCN 1.912 ± 0.003 1.909 ± 0.007 Co2–NCN 1.973 ± 0.004 1.964 ± 0.003
Fe2–NPy 1.981 ± 0.006 1.978 ± 0.009 Co2–NPy 2.013 ± 0.007 2.012 ± 0.001
Fe2–Nami 2.051 ± 0.003 2.046 ± 0.004 Co2–Nami 2.096 ± 0.007 2.076 ± 0.006
Fe2–Nav 1.982 1.978 Co2–Nav 2.027 2.017
Σ (deg) 48.44 47.44 Σ (deg) 50.49 49.23
lateral LS LS lateral LS LS
Fe3–NCN 1.915 ± 0.010 1.914 ± 0.003 Co3–NCN 1.929 ± 0.003 1.917 ± 0.005
Fe3–NPy 1.982 ± 0.009 1.978 ± 0.008 Co3–NPy 1.968 ± 0.006 1.980 ± 0.002
Fe3–Nami 2.053 ± 0.004 2.055 ± 0.007 Co3–Nami 2.036 ± 0.002 2.024 ± 0.006
Fe3–Nav 1.984 1.983 Co3–Nav 1.977 1.974
Σ (deg) 52.14 49.94 Σ (deg) 47.22 45.34
lateral LS LS lateral LS LS
  4S
   4R
  250 K 100 K   250 K 100 K
Co1–NCN 2.127 2.131 Co1–NCN 2.121 2.129
Co1–NPy 2.111 2.092 Co1–NPy 2.090 2.116
Co1–Nimi 2.225 2.213 Co1–Nimi 2.244 2.207
Co1–Nav 2.154 2.145 Co1–Nav 2.152 2.151
Σ (deg) 85.32 79.64 Σ (deg) 79.93 80.45
vertex HS HS vertex HS HS
Co2–NCN 1.974 ± 0.008 1.977 ± 0.002 Co2–NCN 1.984 ± 0.002 1.972 ± 0.005
Co2–NPy 2.032 ± 0.01 2.044 ± 0.004 Co2–NPy 2.046 ± 0.001 2.021 ± 0.001
Co2–Nimi 2.101 ± 0.001 2.072 ± 0.013 Co2–Nimi 2.072 ± 0.001 2.046 ± 0.004
Co2–Nav 2.036 2.031 Co2–Nav 2.034 2.013
Σb(deg) 57.94 56.33 Σ (deg) 58.76 62.55
lateral LS LS lateral LS LS
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a

The assignment for 1 is also representative for 1de (Table S12).

b

Σ is the sum of the deviations of the 12 cis angles of the MN6 octahedron from 90°.

The above coordination site speciation is also reflected by the relevant structural parameters of the Co analogues, 2 in group I, and 4S and 4R in group II (Tables 1 and S10 and S11). However, the average Co1–N bond lengths in 2 indicate the presence of HS CoII complexes in the vertex sites, both at the HT phase (2.175 Å) and at the LT phase (2.164 Å), whereas the average Co–N bond lengths at the lateral sites approach 2.02 Å (Co2) or 1.98 Å (Co3), which suggest the occurrence of LS CoII complexes. A very similar scenario was observed for the pair 4R and 4S, despite the alternative coordination sphere of two imines, two pyridines, and two isocyanides: the average Co–N bond lengths at the vertex Co1 site are close to 2.15 Å, while at the lateral Co2 site, they are close to 2.02 or 2.03 Å at both examined temperatures. The values of Σ confirm the above spin-state distribution. At the central M′ sites, 4R and 4S reveal the average Co–Nisocyanide bond lengths of 1.93–1.95 Å, in agreement with the values observed previously for tetrahedral HS [Co(μ-NC)4] moieties,70 whereas 2 accommodates Na+ cations with reasonable Na–Ncyanide distances of ca. 2.27 and 2.30 Å,71 which leads to a slight expansion of the cluster core along the W–CN–Na direction compared to complexes 1 and 4R/4S (Figure 3). The absence of SCO in the temperature range 100–250 K in 2 and 4 might be understood in terms of the hard-to-overcome steric demands associated with the significant Jahn–Teller distortion expected for the LS state. This is in line with the fact that a pure CoII-centered SCO was not realized in the bimetallic cyanido-bridged CoII–WV/IV systems to date, although some systems revealed switchable CT-induced spin transition phenomena involving the HSCoIIWV ⇄ LSCoIIIWIV equilibrium in the solid state.72−76

Figure 3.

Figure 3

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Superimposed structures of 1/2, 1/4, and 2/4 illustrating spatial convergence between the related cyanido-bridged skeletons along the same undecanuclear topology. The chelating organic ligands, terminal cyanido ligands, solvent molecules, and H atoms were omitted for clarity.

The spin-state site selectivity disclosed above is independent of the metal ions (FeII or CoII) and, to a reasonable extent, of the ligand field. Therefore, SCO transitions in the case of FeII compounds most likely stem from geometric constraints inherent in the undecanuclear topology and from the bulk ligand steric effect. The vertex 3d metal-ion moieties in rhombus W2M2 fragments possess a relatively large degree of freedom, which is conducive to stabilization of the HS state. Moreover, the organic ligands coordinated at those sites as well as the terminal cyanido ligands located nearby are definitely exposed toward the less crowded and less rigid solvent-accessible space, which provides the appropriate molecular arrangement supportive for the expanded HS complexes and for the rearrangements accompanying the transition. On the contrary, metal ions located in the lateral sites are more confined and thus conditioned to remain at the LS state, not only by the rigidity of the trigonal-bipyramidal geometry but also by bulk ligand hindrance due to more intense intercluster interactions. The impact of the geometric constraints on the HS and LS states in 1, 2, and 4 can be further supported by the structural data for Ni analogue 3 within group I. All peripheral octahedral NiII ions possess the t2g6eg2 configuration, to show slightly longer Ni–N bond lengths at the vertex sites, 2.12–2.13 Å, compared to those located at the lateral sites, 2.09–2.10 Å, both at 250 K (Table S13).

Magnetic Properties

The χMT(T) products for compounds 1–4 in the range 330–2 K suggest that 1, 1de, 2, and 4 are SCO complexes, whereas the Ni compound 3 is an SCO-inactive HS molecule with Sgr = 15/2 (Figures 4 and S34 and Table S14). For 1, the χMT value at 330 K is 12.44 cm3 K mol–1 and gradually decreases to 12.09 cm3 K mol–1 at 300 K, in good agreement with the value of 12.14 cm3 K mol–1 predicted for magnetically uncorrelated three HS FeII ions of S = 2 and gFe = 2.25 and two paramagnetic [W(CN)8]3– units of S = 1/2 and gW = 2.0, accompanied by diamagnetic moieties provided by four LS FeII complexes and two [W(CN)8]4– complexes, in agreement with the structural data. The decrease of χMT in the 330–300 K temperature range is attributed to the onset of spin transition of the lateralcis-[Fe(μ-NC)2(bzbpen)] moieties occurring most probably in the higher temperatures (Figures 4a and S34a). Upon further cooling, the χMT value remains constant until 250 K and then notably decreases, tending toward a shallow plateau represented by values 7.34 cm3 K mol–1 at 100 K and 6.58 cm3 K mol–1 in 50 K. The decrease in χMT of ca. 4.75–5.51 cm3 K mol–1 is evidently smaller than 7.25–7.93 cm3 K mol–1 expected for a complete transition HS → LS of two FeII complexes, assuming gFe in the range 2.2–2.3. The above decline indicates a partial transition occurring on ca. 60–75% SCO-active FeII centers and thus should be assigned to the incomplete SCO process occurring at the vertexcis-[Fe(μ-NC)2(bzbpen)]2+ moieties. While the 100 K temperature point was given as a reference, the onset of the mid-temperature LS → HS transition might be located slightly above this point. The exact indication of this onset is blurred by the contribution from the zero-field-splitting (ZFS) properties expected for the HS FeII ions.46−51,581de shows a similar magnetic behavior, but it is a bit less advanced in the range of 50–300 K. The χMT value of 11.46 cm3 K mol–1 at 300 K is very close to the values predicted for three HS FeII ions and four LS FeII ions. As the temperature is lowered, the χMT curve shows a gradual decrease to 6.03 cm3 K mol–1 at 100 K and 5.53 cm3 K mol–1 at 50 K. The decrease in the amplitude of χMT of ca. 5.03–5.93 cm3 K mol–1 indicates at least 64–82% SCO completion. The overall course of the χMT(T) curves below 100 K indicates the expected essential contribution of the ZFS from the remaining FeII HS centers and weak antiferromagnetic (AF) interactions along the WV–CN–FeII linkages.46,48,55,59,64 In addition, the M(H) curves at 2 K for both complexes show relatively large magnetization values of 5.39 and 4.79 μB for 1 and 1de, respectively, further supporting the presence of the remaining HS FeII complexes at low temperatures (see details in Figure S34). Because the preparation means for sample 1de (foil bag) preclude reliable examination of its magnetic properties above 300 K, we are not able to judge the development of the SCO properties at these conditions.

Figure 4.

Figure 4

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Magnetic χMT(T) curves for 1 and 1de (a), 2 (b), 3 (c), 4R and 4S (d). The gray dashed lines show estimated χMT values or their ranges for different spin-state populations of the undecanuclear clusters. Insets: (a, b, and d) χMT(T) data in the extended range up to 330 K collected in the second cycle; (c) M(H) plot at 2.0 K (dark-yellow circles) compared with the Brillouin functions for S = 15/2 and gav = 1.99 (red line).

For structure–property relationship for 1 and 1de, careful inspection of the structures provides some clues. (i) The spin-transition temperature (T1/2 values) for 1 (T1/2 = 157 K) is significantly lower than that for 1de (T1/2 = 224 K), which indicates stronger stabilization of the LS states in the latter phase. This might be correlated with the increased intercluster steric crowding because of the loss of crystallization solvents in the channel space favoring LS states, thus raising T1/2. (ii) No hysteretic effect was observed in any of the crystals; however. 1 exhibited slightly better cooperativity than 1de. It is reasonable to attribute weak cooperativity to the domination of Cphenyl–H···NCN and perpendicular Cphenyl–H···ringcentroid contacts, while the presence of crystallization MeOH molecules might just tune that property to some extent; actually, in our case, the increased contribution of intercluster noncovalent interactions involving MeOH as a reasonable mediator of hydrogen bonds visibly enhances the cooperativity.

The χMT(T) curve for 2 at the HT region presents a behavior similar to that of 1. The χMT values gradually decreased from 9.56 cm3 K mol–1 at 330 K to 9.31 cm3 K mol–1 at 300 K, which approaches reasonably the range of 8.03–9.46 cm3 K mol–1, predicted for the presence of two HS cis-[Co(μ-NC)2(bzbpen)2] with SHS-Co = 3/2 and gHS-Co in the range 2.4–2.7, four LS cis-[Co(μ-NC)2(bzbpen)2] with SLS-Co = 1/2 and gLS-Co = 2.0, and three [W(CN)8]3– moieties with SW = 1/2 and gW = 2.0, together with one diamagnetic [W(CN)8]4– moiety and one Na+ cation. Below 300 K, χMT(T) does not significantly decrease until 50 K and then diminishes more rapidly to reach a value of 6.25 at 2.0 K. For 4S and 4R, the χMT values are 12.93 cm3 K mol–1 at 330 K and 12.22 cm3 K mol–1 at 300 K, respectively. Then, the curves gradually decrease to 11.78 and 11.34 cm3 K mol–1 at 250 K, respectively, approaching reasonably the range of 9.92–11.35 cm3 K mol–1, predicted for the presence of one tetrahedral [Co(μ-NC)4] moiety with STd-Co = 3/2 and gTd-Co = 2.2, two HS cis-[Co(μ-NC)2(R/S-pabh)2] with SHS-Co = 3/2 and gHS-Co in the range 2.4–2.7, four LS cis-[Co(μ-NC)2(R/S-pabh)2] with SLS-Co = 1/2 and gLS-Co = 2.0, two [W(CN)8]3– ions with SW = 1/2 and gW = 2.0, and two diamagnetic [W(CN)8]4– moieties. As the temperature is lowered, the χMT values for both complexes decrease smoothly in the HT regime and then decrease rapidly below 50 K to reach the values of 7.01 and 7.36 cm3 K mol–1 for 4S and 4R, respectively. In both 2 and 4, the HT behavior may indicate the onset of a SCO transition (Figures 4b,d and S34b,c) most probably centered on the lateral LS cis-[Co(μ-NC)2(bzbpen)2] and cis-[Co(μ-NC)2(R/S-pabh)2] moieties, respectively. The smooth decrease of the χMT curves at the HT and mid-temperature regions should be interpreted in terms of the SOC/ZFS properties of octahedral CoII complexes,77 whereas at the LT region, the component of magnetic exchange coupling within WV–CN–CoII linkages might be important.72,78−82 However, the single-ion properties dominate the magnetic behavior of the clusters. All presented magnetic data for 2 and 4 are in good agreement with the relevant structural data.

The χMT value of 3 at 300 K is 8.70 cm3 K mol–1, which is close to the predicted value of 8.39 cm3 K mol–1 calculated for the uncoupled six HS NiII ions of S = 1 and gNi = 2.2 and three paramagnetic [W(CN)8]3– units of S = 1/2 and gW = 2.0 (Figure 4c). Upon cooling, χMT gradually increases up to a maximum of 33.21 cm3 K mol–1 at 4.0 K. This can be attributed to the ferromagnetic coupling along the WV–CN–NiII linkages, in line with the cyanido-bridged WVNiII complexes reported previously.53,62,83−85 The drop of the signal below 4.0 K is due to the combined effects of the single-ion anisotropy on NiII ions and possible intercluster AF interactions. The maximal value of χMT may be compared with the 31.56 cm3 K mol–1 expected for an exchange-coupled cluster with a S = 15/2 ground state spin value and gav = 1.99. The ferromagnetic character is further supported by the field dependence of magnetization gathered at T = 2.0 K. It shows a saturation value of 16.38 μB at 70 kOe, which is slightly higher than the expected values of 16.2 μB for the parallel alignment of all magnetic moments. No alternating-current signal was detected. Based on the Ni–N≡C angle range of 161–173° (av. value 167.4°; see details in Table S18) within the coordination skeleton of 3, a rough estimation of J ∼ +10 cm–1 (2J formalism) might be inferred.53,62,83,84

We definitely exclude the loss of solvent and sample degradation in 1–4 expected and frequently observed while approaching the boiling point of the solvent used, in case the sample is not protected properly. This is supported by perfect reversibility of the curves during applied thermal cycles, thanks to the effective protection in sealed glass tubes.

57Fe Mössbauer Spectroscopy

The above overall observations for 1 and 1de were further confirmed by 57Fe Mössbauer spectroscopy studies (Figure 5a–c; see details in the Supporting Information, SI). The spectra measured at 80 K (Figure 5a) were in both cases reasonably reproduced as a sum of a weakly split doublet assignable to the LS state (green fit component) and a strongly split doublet assigned to the HS state (red fit component). With increasing temperature, first no significant changes are noted; then above ca. 150 K, the LS contributions begin to decrease (Figure 5b), which is followed by the visible systematic emergence of the third moderately split component HS2 (pale-blue component; Figures 5b and S35 and S36 and Tables S19 and S20). Such behavior beyond a doubt indicates the occurrence of spin transition, in line with the structural and magnetic data. The percentage contributions of LS components in the LT phase are 80% for 1 and 82% for 1de, which might be recalculated into 5.6 (1) and 5.74 (1de), representing the number of LS complexes per seven FeII centers in one cluster in these conditions. The resulting HS/LS ratios in the LT phase, 1.4/5.6 (1) and 1.26/5.74 (1de), indicate some excess over the ideal ratio 1/6 expected for one tetrahedral HS [FeII(μ-NC)4] moiety and six LS [Fe(bzbpen)(μ-NC)2] moieties; however, it indeed conforms with some excess of HS complexes (over one per cluster) deduced from the LT magnetic data. As the result of spin transition, at room temperature, the percentage contributions of LS are decreased to 63% (1) and 56% (1de), which might be recalculated as 4.4 (1) and 3.92 (1de). These values result in the HS/LS ratios being very close to the ideal 3/4 (per one cluster) expected for one tetrahedral HS [FeII(μ-NC)4] moiety, two HS [Fe(bzbpen)(μ-NC)2] moieties assigned to the vertex sites of the cluster, and four LS [Fe(bzbpen)(μ-NC)2] moieties assigned to the lateral sites of the cluster, based on the structural data. It is important to note that the Mössbauer data decently reproduce the difference in the χMT(T) curves for 1 and 1de. For 1, the transition occurs in a more pronounced manner and visibly attains well-resolved saturation at room temperature. In contrast, for 1de, the decrease of the LS contribution occurs more slowly, and saturation is definitely not achieved in these conditions, which clearly confirms better stabilization of the vertex HS complexes in 1, illustrated by T1/2(1) definitely being lower than T1/2(1de) in the Mössbauer data. It must be, however, stressed that in our case the Mössbauer data cannot be used for an exact determination of the onset of the LS → HS transition temperature (like the SQUID data can) in the mid-temperature range. This is due to the relatively small “active space” for spin-state change, 1.5–2 per seven Fe centers per whole molecule, keeping in mind the limited accuracy of this technique in tandem with sample preparation: the limited sample mass and the requirement to use protecting means. Nevertheless, we definitely excluded sample degradation during both measurements.

Figure 5.

Figure 5

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Representative 57Fe Mössbauer spectra of 1 in T = 80 K (a) and T = 293 K (b), together with the best fits. (c) Course of the SCO transitions illustrated by the amount of LS complexes for 1 (black-filled squares) and 1de (green-filled squares) derived from the fits in various temperatures. The dashed lines indicate the limiting amount of LS complexes, blue is for the LT phase, and red is for the HT phase. UV–vis–NIR spectroscopic studies on undecanuclear clusters: diffuse-reflectance UV–vis–NIR spectra of 1 and 1de at room temperature (d); diffuse-reflectance UV–vis–NIR spectra of 2 (e) and 4S and 4R (f) in the 200–700 nm range (see details in the SI); (g) NCD spectra for 4R and 4S; UV–vis–NIR spectra of 1 (h) and 2 (i) (0.1 mmol/L) in different solvents at room temperature.

Optical Spectra and Stability of Clusters

The solid samples of complexes 1–4 were further examined by UV–vis–NIR spectroscopy, and the expected spectral features were identified (Figures 5d–f and S37–S40). The strong absorption in the 200–400 nm range is assigned to the sum of the CT bands of the [W(CN)8]n− moieties,86 CT absorption of the 3d metal-ion complex moiety, and intraligand transitions of the involved ligands.62,87 In the lower-energy range, the ligand-field bands62,78 of the complex moieties and metal-to-metal CT transitions MII–WV → MIII–WIV (1, 2, and 4)55,72,78,87 along the M–NC–W linkages were recognized. The natural circular dichroism (NCD) spectra of 4R and 4S are complementary to each other in the examined 200–700 nm range, which confirms the enantiopurity of both phases and indicates nonzero optical activity of the clusters within all related absorption bands (Figure 5g). The solution spectra in acetonitrile (MeCN), MeNO2 (1–3), or N,N-dimethylformamide (DMF; 4) present compositions of bands similar to those of the solid-state spectra (Figures 5h,i and S41 and S42), which suggests that the described polynuclear complexes retain their identity in the solution state, despite 4R and 4S showing a gradual decomposition in DMF (Figure S42). The stability of the clusters in solution is also supported by ESI MS (Figures S43 and S44). In particular, the MS spectrum of 1 in MeCN shows the peak-set patterns assigned to the singly reduced {Fe7(bzbpen)6[W(CN)8]4]− and doubly reduced {Fe7(bzbpen)6[W(CN)8]4]2– complete undecanuclear clusters, although the strongest peak-set pattern is definitely assignable to the {Fe2(bzbpen)2[W(CN)8]2]}2– motif.

Concluding Remarks

The presented series of undecanuclear M7W4 (M = Fe, Co) and NaM6W4 (M = Co, Ni) clusters provides novel, versatile modules for the generation of diverse magnetic properties. The NaNi6W4 complex 3 exhibits HS in the ground state, whereas the Fe and Co complexes 1, 1de, 2, 4R, and 4S reveal various scenarios of site-selective SCO and spin state at specific metal sites, which are summarized in Figure 6. Our results provide an important contribution toward knowledge on multistable molecular materials and the development of their acquisition. In particular, the speciation of sites important from the viewpoint of switchable behavior was achieved, exploiting the same type of coordination environment. As the crucial factors determining the spin-state distribution and SCO properties, we indicated the geometric constraints imposed on various regions of the cluster core (as the endogenous, inner factor) and intermolecular noncovalent interaction schemes in the related areas of the crystal architectures (as the exogenous, outer factor). As the novelty to the field, 2 and 4 represent the emergence of CoII-centered SCO compounds within the Co-[M(CN)8] family, not observed in previous bimetallic cyanido-bridged CoII–WV/IV systems, whereas 1 exhibits the first well-resolved two-step SCO system among the Fe-[W(CN)8] clusters. Moreover, these clusters unquestionably belong to the largest molecules known to exhibit site-selective spin state and SCO, with the dimensions of ca. 2.0 × 2.2 × 2.5 nm (1–3) and ca. 1.4 × 2.5 × 2.5 nm (4R/4S), considering the distance between the most remote atoms along three cluster axes. Various metallic core compositions embedded within the same topological scaffold and the related SCO scenarios open a promising route for molecular design toward advanced cocrystal salts involving switchable polynuclear clusters88,89 and molecular hierarchical systems such as heterotrimetallic solid solutions55,56,59,60 or core–shell composites;90 the solubility and stability of these forms in various organic solvents might be one of the prerequisites in further research. Moreover, the research on SCO transitions in multisite clusters might provide better characteristics (e.g., larger cooperativity, SCO site speciation, multistep transitions, etc.).

Figure 6.

Figure 6

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Summary of the properties in the 1–4 clusters family. (Top) FeII7WV2WIV2 (1) is a double-step SCO compound with distinct spin transition occurring at the vertex [Fe(bzbpen)(μ-NC)2] moieties in the temperature range 100–240 K and the onset of another spin transition at the lateral [Fe(bzbpen)(μ-NC)2] positions occurring just above room temperature. Its partially desolvated form 1de shows only one SCO transition step within the accessible T range occurring at the vertex [Fe(bzbpen)(μ-NC)2] moieties, starting from 150 K and expected to be finished just above room temperature. (Bottom, left panel) NaCoII6WV3WIV (2) and CoII7WV2WIV2 (4R and 4S) exhibit distinct HS forms at the vertex [Co(bzbpen)(μ-NC)2] or [Co(pabh)2(μ-NC)2] moieties and LS forms at the lateral moieties; the onset of spin transition at the lateral [Fe(bzbpen)(μ-NC)2] is ongoing close to room temperature. (Bottom, right panel) NaNiII6WV3WIV (3) is a paramagnet composed of HS clusters of the spin in the ground state SGS = 15/2. However, even this compound reveals slightly shorter Ni–N bond lengths at the lateral sites than at the vertex sites, in line with the trends observed in 1, 2, 4R, and 4S. The forms expected above room temperature are covered with a white semitranslucent mask.

Acknowledgments

We gratefully acknowledge financial support from the National Science Centre (Poland) research project UMO-2019/35/B/ST5/01481 (to R.P.). We also acknowledge Dr. Dawid Pinkowicz and Michał Magott (Faculty of Chemistry, Jagiellonian University) for supportive discussion and Dr. Anna M. Majcher-Fitas (Faculty of Physics, Astronomy and Applied Computer Science, Jagiellonian University) for her assistance in NCD measurement. Measurements were carried out with equipment funded by the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (Contract POIG.02.01.00-12-023/08). Magnetic measurements were performed using equipment funded by the Polish Ministry of Science and Higher Education in the framework of the Large Research Infrastructure Fund (Decision 6350/IA/158/2013.1). The maintenance and service costs of the SQUID magnetometer was supported by a grant of the Faculty of Chemistry under the Strategic Programme Excellence Initiative at Jagiellonian University.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.3c00325.

  • Detailed description of the used materials, synthetic procedures, physical techniques, crystal structure determination, and structural data presentation and additional figures and tables (PDF)

Author Contributions

L.S.: conceptualization; syntheses, measurements, and analyses; data curation, interpretation, and discussion; preparation of the draft manuscript. J.K.: measurements, analyses and data curation, manuscript editing. K.D.-K.: measurements and analyses (Mossbauer spectra); data curation and discussion; manuscript editing. J.J.S.: measurements and analyses (Mossbauer spectra); data curation and discussion; manuscript editing. B.S.: literature preview; data consultation; manuscript editing. R.P.: funding acquisition; project management; conceptualization; data curation, interpretation, and discussion; preparation of the draft manuscript.

The authors declare no competing financial interest.

Supplementary Material

ic3c00325_si_001.pdf (8.5MB, pdf)

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