Turbasu
Sengupta
and
Shiv N.
Khanna
*
Department of Physics, Virginia Commonwealth University, Richmond, VA, 23284-2000, USA. E-mail: Snkhanna@vcu.edu
First published on 5th April 2022
We propose a new class of superatomic molecules where the bonding characteristics between two identical superatoms can be modified by using suitable ligands around metal-chalcogen clusters. We demonstrate these intriguing findings by considering the Rh6S8Ln class of superatoms. Our choice of ligands (L) is motivated by the condition that the ligand binds to the metal-chalcogen core via the same atom while the electronegativity of the ligand is altered by attaching various units to this connecting atom. For this purpose, we have chosen the phosphine ligands (PR3, R = functional groups) where the electronegativity of the ligands is changed via the functional (R) group ranging from NMe2 to F. It is also observed that this technique can be further used to induce controllable electronic features in the (PR3)5Rh6S8–Rh6S8(CO)5 series of dimers/salts. The differences in the location of electronic levels on the two sides result in internal charge transfer that not only adds an ionic component to bonding over the covalent component but also leads to dipole moments/internal electric field in an otherwise homo-cluster dimer. Thus, the strength of the dipole moment/internal field of the dimer can be adjusted by changing the electronegativity of the PR3 group offering a unique opportunity of making cluster assembled solids using superatomic molecules with adjustable dipolar strength, internal fields, and location of HOMO/LUMO, useful for various applications including light-harvesting.
In this work, we demonstrate a novel phenomenon whereby a molecule composed of two identical building blocks can simultaneously exhibit covalent and ionic bonding. We demonstrate this intriguing possibility by forming a supermolecule by linking two identical ligated metal-chalcogenide superatoms. This class of clusters is composed of transition metal and chalcogen atoms forming the inner core that is further protected by outer ligands. The units are highly stable, can be formed in solutions, and as first demonstrated by Roy, Nuckolls, and co-workers,2–9 they can form ionic solids when combined with counter-ions maintaining their identity. Unlike metallic clusters where a confined nearly free electron gas10–14 model can identify particularly stable species, the bonding in individual metal-chalcogenide cluster has a covalent component between metal and chalcogens and form a charge–transfer complex between metal and ligands. Consequently, a hybrid model including valence electrons of metals, chalcogens, and the electrons shared with ligands is needed to account for variations in chemical stability as seen through the gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).15 What is interesting is that for a given metal-chalcogen core, the redox properties can be controlled by the choice of ligands without altering the effective valence electron count.16–23 Thus, attaching strong σ-donor ligands, e.g., phosphines, to the cluster induces an upward shift of the electronic spectrum, which results in a superatom with enhanced charge donor properties. In contrast, a shift in the opposite direction is induced when strong π-acceptor ligands (e.g., CO) are introduced within the cluster framework. In such a situation, the ligated cluster act as a charge acceptor.
One of the important questions is how the assemblies of superatoms can lead to novel features. It is important to highlight that, unlike atoms, assemblies of superatoms are marked by intra-atomic and inter–atomic interactions. The purpose of the paper is to offer a step in this direction by considering a supermolecule built from the core of identical metal-chalcogen atoms. In this work, we address two questions: (1) How the redox properties of superatomic molecules can be controlled by changing ligands? (2) Is it possible to modify the covalent bonding between superatoms to have an ionic component? We are particularly interested in forming a superatomic salt that does not require distinct metal-chalcogenide units but where the change in ligands induces charge transfer and an internal electric field between otherwise identical cores. The current work, in our view, is a unique example of how intra–cluster and inter–cluster interactions can conspire to lead to novel units that has potential as building blocks for ferroelectric materials or where the individual units could be used for charge separation in photovoltaic applications.24,25
In this work, we focus on Rh6S8Ln (L: Ligand) class of clusters to demonstrate these intriguing findings. Our choice of ligands is motivated by the fact that the ligand atom coordinating to the metal-chalcogen cluster remains the same and that the charge transfer properties are altered by attaching different units to the linking atom. To accomplish this, we focus on tertiary phosphines (PR3, R = functional groups) that represent a unique class of ligands whose electronic and steric properties are directly related to the electronegativity of the attached functional groups (R).26,27 Thus, by using groups of varying electronegativity, one can effectively create a series of PR3 ligands with different donor/acceptor characteristics. For example, although ligands like PEt3 or PMe3 are strong σ donors, PCl3 or PF3 ligands are very strong π- acceptors. The electron-withdrawing capability of PF3 is often comparable to CO. In this present paper, we have used this strategy on the 5 and 6 ligated Rh6S8 metal chalcogenide clusters. It is observed that by changing the attached R groups, it is possible to transform a donor metal chalcogenide cluster to an acceptor without altering the effective valence count or the frontier orbitals. The HOMO−LUMO gaps of the clusters, which are a measure of kinetic stability, have also remained within a similar range for all cases. For a single cluster, in terms of the adiabatic ionization energy (AIE) and electron affinity (AEA), the performance of this technique is observed to be near equivalent to the ligand substitution method,16 which we have proposed earlier for the cobalt telluride clusters (Co6Te8(PEt3)m(CO)n, m + n = 6). Thus, instead of sequentially substituting the donor ligands with acceptors, one can simply use different R groups to attain a similar result. Consequently, this technique can also act as an alternative when the ligand substitution method is not viable or experimentally difficult to achieve. Furthermore, it is noticed that by utilizing a series of PR3 ligands to decorate the donor side of the metal-chalcogenide dimers, one can introduce a similar shift within the electronic spectrum of the dimers as well. Hence, by altering the ligands only on the donor side, a superatomic dimer can also be converted from donor to acceptor or vice versa without significantly changing the characteristics of the frontier orbitals or the HOMO−LUMO gap. Additionally, since the dipole moment of the dimer depends on the donor/acceptor properties of the attached ligands, the same approach also results in simultaneous alteration of the dipole moment of the dimer without changing the direction. The calculated results show that by using a range of PR3 ligands to decorate the donor side, one can create a series of superatomic salts with variable redox properties and dipole moments similar to the series of common ionic molecules (e.g., NaCl, KBr, etc.). The range of the dipole moment is observed to be altered from nearly 1 Debye to as high as 20 Debye and possibly higher for the Rh6S8 based dimer. We have also argued that a similar series can also be constructed simply by using only one type of ligand (i.e., PR3) with differential donor/acceptor characteristics on both sides, which completely eliminates the requirement of a specialized π-acceptor ligand like CO. As shown here, all of the modifications are also achievable by using identical metal chalcogenide cluster on both sides which in this case is Rh6S8. Lastly, since the group electronegativity can be altered simply by chemical design, unlike the ligand substitution method, this approach is not limited to the number of available metal sites on the cluster. Thus, the reported series can further be extended simply by introducing different functional groups within the PR3 series.
To obtain further insight, we have first investigated the effect of sequential ligand attachment on the magnetic and redox properties of the Rh6S8 cluster. For this purpose, we have chosen one donor (i.e., PMe3) and one acceptor (i.e., CO) ligand. The optimized structures of both series are presented in Fig. 1a. It is seen that for both PMe3 and CO ligands, the magnetic moment of the ligated clusters is reduced to 8 μB just after attaching one ligand. Except for Rh6S8(PMe3)3, the inclusion of more than one ligand results in a non-magnetic ligated cluster. Thus, for both PMe3 and CO, all ligated clusters with more than three ligands are found to be non-magnetic. It is noteworthy that even for Rh6S8(PMe3)3, the 2 μB ground-state structure is only marginally (0.03 eV) lower in energy compared to the 0 μB structure. For all the non-magnetic ligated clusters, we have not found any residual moment on any atoms, which proves that the absence of net moment is not due to any antiferromagnetic alignments. Although the effect of both PMe3 and CO ligands on the net moment of ligated clusters seems mostly similar, the electronic features of both series show striking dissimilarities. For example, in Fig. 1b, we have plotted the net Hirshfeld30 charge, which is accumulated on the Rh6S8 cluster as the number of ligands (PMe3 and CO) is sequentially increased. It is evident that being a very strong σ-donor, the negative charge on the cluster has increased consistently as the number of PMe3 ligands changes from 1 to 6. On the other hand, in the case of CO, the net charges on the cluster have remained close to 0 throughout, and accumulation of marginal negative charge (−0.04 e to −0.06 e) is only observed near the end of the series. This is because the CO ligand is not only a strong σ-donor but also a good π-acceptor.27 Both of these effects neutralize each other resulting in the near-zero charge on the cluster core. The striking difference is also reflected in the redox properties of the ligated clusters. Due to the transfer of electrons from ligand to cluster, attaching PMe3 ligands has raised the electronic spectrum of the overall system. Hence, as the number of PMe3 ligands is sequentially increased, the HOMO and LUMO levels are shifted upward (see Table S2 of the ESI†). Consequently, the AIEs of the 5 and 6 PMe3 ligated clusters are calculated as 5.12 and 5.00 eV, respectively, which are much lower compared to the pristine Rh6S8 cluster. In contrast, due to the transfer of electrons to the ligand via back donation, the electronic spectrum of the CO ligated system has lowered. Hence, the AIEs of 5 and 6 CO ligated Rh6S8 cluster has increased to 7.65 and 7.85 eV, respectively. As we sequentially increase the number of the ligands, the HOMO−LUMO gap shows a non-monotonic trend (Fig. 1c) for both ligands; however, a large leap is observed when the last (6th) ligand (PMe3 or CO) is attached, showing the high kinetic stability of the fully ligated Rh6S8 cluster. The HOMO−LUMO gap of Rh6S8(PMe3)6 and Rh6S8(CO)6 clusters are calculated as 1.65 and 1.83 eV, respectively. Our previous investigations15 have shown that the closed-shell metal chalcogenide cluster, i.e., Co6Te8(PEt3)6 with 114 valence electrons, is a closed-shell magic cluster with a large HOMO−LUMO gap. The total valence electron count for Co6Te8(PEt3)6 is obtained by counting 6 electrons per chalcogen, 2 electrons per ligand, and including 9 electrons for each cobalt. Since rhodium shares the same group (Group 9) as cobalt in the periodic table,31 it is expected that the same electron counting rule is equally valid for here as well.
To test this, we have chosen seven such functional groups viz. 1. dimethylamine (–NMe2) 2. isopropyl (–CH(CH3)2 or –iPr) 3. ethenyl or vinyl (–CH = CH2) 4. ethynyl (–CCH) 5. thiol (–SH) 6. chlorine (–Cl) and 7. fluorine (–F). The groups are chosen in such a way that the group electronegativity increases as we move from 1 to 7. As a result, P(NMe2)3 is expected to act as the strongest σ-donor and PF3 should behave as the best π-acceptor, similar to CO. As a next step, we have decorated the Rh6S8 cluster by 5 of each ligand from the above series, and their optimized ground state structures are shown in Fig. 2a. The electronic structure and associated properties, e.g., HOMO−LUMO gaps, AIEs, and AEAs, are also evaluated. Similar calculations are also performed for the six ligated Rh6S8 clusters. However, the relative trend of the calculated properties for the six ligated series is found to be very similar to the five ligated clusters and hence included in Table S3 of the ESI.†Fig. 2b and Table 1 show the calculated electronic properties of the five ligated Rh6S8 series. It is important to note that upon changing the ligands from P(NMe2)3 to PF3, the energies of both HOMO and LUMO show a near parallel decrease. As a result, the HOMO−LUMO gaps vary within a narrow range (0.39–0.53 eV) throughout the series. This observation is similar to our ligand substitution method,16 which we have proposed before. The energies of HOMO (−3.69 eV) and LUMO (−3.20 eV) are observed to be highest for Rh6S8[P(NMe2)3]5. As we move along the series, both values show a steady decrease and finally reach the lowest for Rh6S8(PF3)5, for which the HOMO and LUMO values are calculated to be −5.89 and −5.46 eV, respectively. As expected, the AIE and AEA show a reverse trend (Fig. 2b), i.e., both the properties show a consistent increase upon moving from left to right. Hence, Rh6S8[P(NMe2)3]5 shows the lowest AIE (4.78 eV) and AEA (1.98 eV), whereas, Rh6S8(PF3)5 shows the highest AIE (7.36 eV) and AEA (3.98 eV) among the series. It is important to note the overall range of AIE and AEA as obtained for both 5 and 6 ligated clusters are similar to our previously reported results16 for the Co6Te8(PEt3)m(CO)n, (m + n = 6) cluster series. In that work, the electron donor PEt3 groups were sequentially substituted by CO. This shows that the current approach can act as an effective alternative to the ligand substitution proposed earlier. If compared with the experimental database,31 then it can be seen that the computed AIE of Rh6S8[P(NMe2)3]5 is between sodium (5.14 eV) and potassium (4.34 eV), and the AEA of Rh6S8(PF3)5 cluster is even higher than the Cl atom (i.e., 3.61 eV), which has the highest electron affinity among all elements. Thus, it is evident that the Rh6S8 cluster can be converted from a strong donor to a strong acceptor simply by altering the nature and the type of the R groups which are attached to the phosphorus atom.
Systems | Mag. moment (μB) | Electronic properties (eV) | ||||
---|---|---|---|---|---|---|
HOMO | LUMO | HL gap | AIE | AEA | ||
Rh6S8[P(NMe2)3]5 | 0 | −3.69 | −3.20 | 0.49 | 4.78 | 1.98 |
Rh6S8[P(iPr)3]5 | 0 | −3.84 | −3.45 | 0.39 | 5.00 | 2.16 |
Rh6S8[(P(CHCH2)3]5 | 0 | −4.17 | −3.71 | 0.46 | 5.24 | 2.41 |
Rh6S8[(P(CCH)3]5 | 0 | −4.45 | −3.95 | 0.50 | 5.64 | 2.72 |
Rh6S8[(P(SH)3]5 | 0 | −5.06 | −4.54 | 0.52 | 6.34 | 3.36 |
Rh6S8(PCl3)5 | 0 | −5.67 | −5.14 | 0.53 | 6.93 | 3.89 |
Rh6S8(PF3)5 | 0 | −5.89 | −5.46 | 0.43 | 7.36 | 3.98 |
Next, we have plotted (Fig. 3) the molecular orbital (MO) diagrams for all seven clusters, each of which are decorated by five different PR3 ligands. It can be clearly seen from Fig. 3 that as the electronegativity of the R group increases from left to right, the whole electronic spectrum is shifting downward to lower energy, preserving the HOMO−LUMO gaps within a small range of 0.39–0.53 eV. The HOMO and LUMO isosurfaces are also included at the right-hand side of Fig. 3. Despite the change in the attached R groups throughout the series, the isosurfaces of both HOMO and LUMO have remained identical for all cases. This can be confirmed by looking into the salient features (i.e., the location and shape of the positive and negative lobes) of the plotted isosurfaces in Fig. 3. This is direct proof that altering the R groups results in a Coulomb-like shift of the whole electronic spectrum without altering the relative ordering and the occupation numbers of the orbitals. The same conclusion can be reconfirmed from the projected density of states (PDOS) diagram (Fig. 4) as well. Firstly, for all the seven systems, both the HOMO and LUMO show a very similar composition of Rh and S. The attached ligands (PR3) show nearly zero contribution to both HOMO and LUMO in all cases. Secondly, the shift of the whole electronic spectrum can be easily visualized simply by tracking the locations of the PDOS peaks in Fig. 4. As we alter the R groups from NMe2 to F, all the PDOS peaks have simultaneously shifted toward lower energy, clearly showing that it is the overall electronic spectrum that is shifting and not some specific molecular orbitals.
Fig. 5 The optimized ground state structures of (PR3)5Rh6S8–Rh6S8(CO)5 (R = NMe2 to F) dimers. The AIE (red) and AEA (blue) values (in eV) of each dimer are included within parenthesis. |
Dimers | Mag. moment (μB) | Dipole moment (Debye) | Hirshfeld charge |e| | Electronic properties (eV) | ||||
---|---|---|---|---|---|---|---|---|
HOMO | LUMO | HL gap | AIE | AEA | ||||
[(P(NMe2)3]5Rh6S8–Rh6S8(CO)5 | 0 | 19.59 | 0.42 | −4.22 | −3.61 | 0.61 | 5.07 | 2.62 |
[(P(iPr)3]5Rh6S8–Rh6S8(CO)5 | 0 | 18.02 | 0.37 | −4.36 | −3.77 | 0.59 | 5.28 | 2.64 |
[(P(CH = CH2)3]5Rh6S8–Rh6S8(CO)5 | 0 | 15.46 | 0.32 | −4.59 | −3.99 | 0.60 | 5.69 | 2.74 |
[(P(C ≡ CH)3]5Rh6S8–Rh6S8(CO)5 | 0 | 13.45 | 0.29 | −4.78 | −4.09 | 0.69 | 5.76 | 2.99 |
(PH3)5Rh6S8–Rh6S8(CO)5 | 0 | 11.42 | 0.27 | −4.96 | −4.25 | 0.71 | 6.08 | 3.05 |
[(P(SH)3]5Rh6S8–Rh6S8(CO)5 | 0 | 6.15 | 0.21 | −5.28 | −4.57 | 0.71 | 6.27 | 3.50 |
(PCl3)5Rh6S8–Rh6S8(CO)5 | 0 | 3.45 | 0.09 | −5.55 | −4.89 | 0.66 | 6.57 | 3.79 |
(PF3)5Rh6S8–Rh6S8(CO)5 | 0 | 0.98 | 0.02 | −5.73 | −5.07 | 0.66 | 6.84 | 3.85 |
The influence of the charge accumulation on the monomeric fragments of the dimer also affects the binding energies between the donor and acceptor units. To further illustrate the problem, we show in Fig. 6d the binding energy of the cluster dimer as a function of ligands. The binding energies between the Rh6S8(PR3)5 and Rh6S8(CO)5 monomeric units are higher for the first few members of the series compared to the dimers near the end. Since the charge accumulation on the monomers is significantly higher at the beginning of the series, that results in a strong electrostatic/ionic attraction between the donor and acceptor sides. As we move down the series, the electrostatic attraction between the monomeric units has gotten weaker, and thereby the binding energy as expected for a covalent-dominant interaction. The energy decomposition analysis further proves this point (see Table S4 of ESI†). The electrostatic component (−7.71 eV) between the monomers for [(P(NMe2)3]5Rh6S8–Rh6S8(CO)5 is significantly more negative compared to the values obtained (−6.65 to −7.03 eV) for the dimers near the end. Thus, it can be said with certainty that in addition to the redox properties, the current technique is equally useful to predictably alter the charge accumulation, dipole moments, and binding energies of the superatomic dimers.
In chemistry, it is well known that combining a strong electron donor (e.g., alkali metals) element with a strong electron acceptor (e.g., halogen) results in a salt (e.g., NaCl).32 Arranging such molecules according to their dipole moment produces an ionic series in the molecular domain. Since the dimer series reported herewith are also constructed similarly, i.e., by combining an electron donor superatom with an acceptor, we have asked ourselves if an equivalent series can be constructed in the superatomic domain as well. Fig. 9 shows such a series in which the respective dimers are positioned on a dipole moment scale ranging from 0 to 20 Debye. It is noteworthy that in the superatomic series, although the dipole moment increases from right to left, the AIE and AEA increase in the opposite direction (i.e., from left to right). For qualitative comparison, an equivalent ionic series composed of a few well-known ionic molecules is included on the top-right corner of Fig. 9. The similarity between both series is certainly interesting, especially since we have also observed that such metal-chalcogenide dimers reported herewith show a similar dissociation curve as the ionic molecules showing further resemblance to the molecular scale. An example comparison plot of such is included in the Fig. S2 of ESI†). Moreover, in the series of ionic molecules, the charge on the ions also show a similar trend as the superatomic series (Fig. 9). It also needs to be mentioned that the superionic series presented herewith represents only a fraction of the overall scale. The dipole range can be further extended just by introducing better electron donors than P(NMe2)3 or just by using a different metal chalcogenide cluster. Similarly, attaching stronger electron acceptors (compared to PF3) to the donor side will produce dimers with dipole moments closer to zero. The AIE and AEA range will also extend accordingly. This is an additional advantage compared to our ligand substitution method. In that procedure, for a particular metal chalcogenide dimer and a particular pair of donor–acceptor ligands (e.g., PEt3 and CO), the maximum dipole moment is reached when one side is fully decorated by the donor ligands and the other side is decorated by acceptor ligand. Moreover, precise ligand substitution on specific sites on a small size cluster is always a difficult task. Additionally, not all ligands work in inducing the Coulombic well that is shifting the electronic spectrum. Strong ligands can often perturb the electronic structure of the cluster resulting in undesired results. In the present approach, the functional groups (R) are utilized to alter the donor–acceptor properties of the PR3 group as a whole and are not directly bonded to the cluster. Thus, the chances of perturbing the shell structure of the cluster are largely reduced. It is important to note that although we have used CO ligands on one side of the dimer, it is not necessary. As shown before, since Rh6S8(PF3)5 is also an effective electron acceptor, analogous series can be constructed solely with the PR3 groups without using CO or any different type of π-acceptor ligands. Moreover, if one requires a dimer with some intermediate dipole moment, that is also achievable by using a different combination of the PR3 groups. For example, the dipole moment of (PH3)5Rh6S8–Rh6S8(PCl3)5 dimer is calculated as 9.78 D, which is just between the dipole moment of (PH3)5Rh6S8–Rh6S8(CO)5 and [P(SH)3]5Rh6S8–Rh6S8(CO)5 dimers, respectively (Table 2).
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d1ma01154e |
This journal is © The Royal Society of Chemistry 2022 |