Research

Zintl-ion clusters

In collaboration with Prof. Zhong-Ming Sun (Nankai University, China), we are exploring the relationship between structure, bonding and cluster growth in anionic Zintl clusters. These clusters typically contain a mixture of transition metals and tetrel elements (Ge, Sn, Pb), and the degree to which the elements segregate offers important insights into alloying processes. Our latest work in this area has looked at the nature of Fe-Fe interactions in Sn/Pb clusters containg three or four Fe centers. The particular structural motifs, [Fe₃Sn₁₈]^4- and [Fe₄(Sn/Pb)₁₈]^4- have precedent in either Pd or Cu chemistry, where the metal ions have d¹⁰ configurations, so the comparison of structure and magnetism offers an opportunity to explore the impact of a partially filled d shell. Earlier papers in Angewandte Chemie explored the nature of mixed valency in the formally In^III₇In^I cluster [Sb@In₈Sb₁₂]^5-, and showed that the crystallographic disorder could be understood in terms of pairwise delocalisation over In^II₂ units. A 2020 Nature Comm. paper reports a family of new mixed gold/lead clusters. Gold and lead are notoriously immiscible, and this appears to be reflected in the discrete clusters, which are based on a Au_x core surrounded by lead icosahedra. 
Our work in this area was summarised in a recent review article in Chem. Soc. Rev. co-authored with Professor Stefanie Dehnen and Professor Florian Weigend.

Electronic structure and bonding in endohedral Zintl clusters,
J. E. McGrady, F. Weigend and S. Dehnen, Chem. Soc. Rev., 2022, 51,628.

Fe-Fe bonding in the rhombic Fe₄ cores of the Zintl clusters [Fe₄E₁₈]^4- (E = Sn and Pb),
W.-X. Chen, Z.-S. Li, H.W.T. Morgan, C.-C. Shu, Z.-M. Sun and J. E. McGrady, Chem. Sci., 2024, 15, 4981

Snap-shots of cluster growth: structure and properties of a Zintl ion with an Fe₃ core, [Fe₃Sn₁₈]^4-,
Z. Li, W.-X. Chen, H. W. T. Morgan, C.-C. Shu, J. E. McGrady and Z.-M. Sun, , Chem. Sci., 2024, 15, 1018

[Cu₄@E₁₈]^4- (E = Sn, Pb): Fused Derivatives of Endohedral Stannaspherene and Plumbaspherene,
L. Qiao, C. Zhang, C.-C. Shu, H. W. T. Morgan, J. E. McGrady and Z.-M. Sun, J. Am. Chem. Soc., 2020, 142, 31, 13288–13293.

A family of lead clusters with precious metal cores, C.-C. Shu, H.W.T. Morgan. L. Qiao, J.E. McGrady and Z.-M. Sun, Nature Comm., 2020, 11, 3477.
Featured as an editor's highlight:

Structure and Bonding in [Sb@In₈Sb₁₂]^3- and [Sb@In₈Sb₁₂]^5-, C. Liu, N.V. Tkachenko, I.A. Popov, N. Fedik, X. Min, C.Q. Xu, J. Li, J.E. McGrady, A.I. Boldyrev and Z.-M. Sun, Angew. Chem. Int. Ed., 2019, 58, 8367.

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Endohedral Clusters

As a complement to the Zintl-ion work in the previous box, we have a long-standing interest in the electronic structure of small clusters of Si and Ge which contain endohedral transition metal ions. This family is remarkably diverse, and examples are known for almost all of the transition elements. These are typically characterised only in the gas phase through various spectroscopies, and theory has an important role to play in establishing structure. More importantly from our perspective, the nature of the interaction between the cage and the metal can vary enormously, and in fact samples the entire spectrum of chemical bonding. We have used a range of theoretical tools to probe these systems, including DFT and multi-configurational ab initio methods (CASSCF). Establishing the link between structure and electronic properties offers a unique insight into periodic trends. We have also considered how these interactions change when clusters are absorbed onto a silicon surface - this has some relevance to the behaviour of impurities in bulk silicon, a matter of great industrial significance.

Quantum chemical models for the absorption of endohedral clusters, X. Jin and J.E. McGrady, Adv. Inorg. Chem., 2019, 73, 265
Structure and bonding in endohedral metal clusters, X. Jin, V. Arcisauskaite and J.E. McGrady, Phys. Chem. Chem. Phys, 2019, 21, 13686

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Vibrational Spectroscopy of Clusters

Our focus in the structure and bonding in silicon clusters has led to a collaboration with Peter Lievens and Ewald Janssens at KU Leuven where we have looked at the vibrational spectroscopy of Re/Si clusters, [ReSi_n]⁺. The experiments (performed in Leuven) involve infra-red multi-photon dissociation (IR-MPD) spectroscopy, wherein an inert gas tag Xe or Ar) is displaced from the cluster upon absorption of radiation. The results offer a rich source of information on structure and bonding, as well as the growth patterns of the family.

Pathways of cluster growth: infra-red multi-photon dissociation spectroscopy of a series of Re-Si clusters, [ReSi_n]⁺, n=3-9 , R. Singh, P.-J. Claes, A Fielicke, E. Janssens, P. Lievens and J.E. McGrady,
Phys. Chem. Chem. Phys., 2024, 26, 22611.

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Low-valent metal oxides

Much of our work in the field of low-valent metal oxide chemistry is done in collaboration with Professor Michael Hayward, whose group synthesis novel materials through topotactic reduction of perovskite-type materials. The result is either replacement of oxide with hydride, or complete removal of layers of oxides, leaving 2-dimensional materials. The transition metals ions are then left in unusually low oxidation states (for an oxide lattice) and, often, also unusual square planar coordination environments. Our work involves the application of plane-wave density functional theory (VASP) along with post analysis tools such as LOBSTER to analyse magnetic and electronic properties. In one recent case study, also in collaboration with Professor Hiroshi Kageyama, we showed that the pressure dependence of conductivity in a a V(III) oxyhydride, SrVO₂H, arises through compression in the xy (VO₂) plane, despite the fact the crystals is intrinsically more compressible along the z (H-V-H) axis.

The role of π-blocking hydride ligands in a pressure-induced insulator-to-metal phase transition in SrVO₂H, T. Yamamoto, D. Zeng, T. Kawakami, V. Arcisauskaite, K. Yata, M. Amano Patino, N. Izumo, J.E. McGrady, H. Kageyama and M.A. Hayward, Nature Comm., 2017, 8, 1.

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Electron transport properties of molecules

Another area of interest to the group is to the link between electronic structure and electron transport properties of extended metal atom chain (EMAC) complexes, where hlical array of oligo-α-pyridyl ligands is used to support a chain of metal centres. These systems have been the subject of a protracted debate in the inorganic chemistry community due to their polymorphism. They exist in symmetric and unsymmetic forms. Our current objective is to relate the fundamental electronic structure of these EMAC complexes to their conductance as measured, for example, by STM. Ultimately, an understanding of these phenomena will be essential to the development of new computer architectures based on molecular-scale components.

Can heterometallic 1-dimensional chains support current rectification?, D. DeBrincat, O. Keers and J. E. McGrady, Chem. Comm., 2013, 49, 9116.

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Mechanisms

We have a number of collaborative projects that focus on the mechanisms of chemical reactions. These involve either transition metal based systems or main-group reactivity, and our goal is to identify the fundamental electronic properties that lead to low barriers. Most recently, in collaboration with the Mehta group in Oxford we have explored the reaction of CO₂ with boranes, catalysed by a boron-functionalised P₇ cluster. The goal here is to establish whether the making and breaking of P-P bonds, a well-established phenomenon in the parent P₇^3- cluster, plays a role in the reaction. Earlier work in collaboration with Chris Russell at Bristol looked at H-H and Si-H bond activation by the triphosphabenzene (^tBuC)₃P₃ highlights the degree to which main group compounds can mimic reactivity (oxidative additions) typically associated with transition elements. A very recent new area in this field is the exploration of metal clusters on surfaces, and their ability to catalyse the activation of small molecules.

Transforming carbon dioxide into a methanol surrogate using modular transition metal-free Zintl ions , B. van IJzendoorn, S.F. Albawardi, W.D. Jobbins, G.F.S. Whitehead, J.E. McGrady and M. Mehta,
Nat. Commun., 2024, 15, 10030.

A Zintl Cluster for Transition Metal-Free Catalysis: C=O Bond Reductions
B. van Ijzendoorn, S. F. Albawardi, I. J. Victorica-Yrezabal, G. F. S. Whitehead, J. E. McGrady, M. Mehta,
J. Am. Chem. Soc., 2022, 144, 21213.

Hydrogen Activation by an Aromatic Triphosphabenzene, L. E. Longobardi, C. A. Russell, M. Green, N. S. Townsend, K. Wang, A. J. Holmes, S. B. Duckett, J. E. McGrady and D. W. Stephan, J. Am. Chem. Soc., 2014, 136, 13453-13457.
Oxidative Addition, Transmetalation, and Reductive Elimination at a 2,2'-Bipyridyl-Ligated Gold Center, M. Harper, C.J. Arthur, J. Crosby, E.J. Emmett, R.L. Falconer, A.J. Fensham-Smith, P.J. Gates, T. Leman, J.E. McGrady, J.F. Bower and C.A. Russell, J. Am. Chem. Soc., 2018, 140, 4440.

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