David Mills likes to make connections that – in theory – have no right to exist. His group synthesizes compounds with unusual oxidation states, unusual geometries, and unusual bonding motifs. Most of these compounds contain elements from the f-block of the periodic table.
“The connections we make are always sensitive to air and moisture. Sometimes they are also sensitive to light and temperature, ”explains Mills, a proud Welshman. F elements are strongly oxophilic and prefer high coordination numbers and ionic bonding regimes. Mills’ group targets compounds that are coordinatively unsaturated and contain atoms softer than oxygen, making them especially difficult to work with. “The big electronegativity difference means that you have to cross the i-points and the T-points with your Schlenk lines and glovebox techniques … I think sometimes our employees want our connections to behave better. But for us it’s part of the fun. ‘
As a subject, the chemistry of the f elements is less developed than that of the s, d, and p blocks. “Our understanding is not yet fully developed, but that means it’s an area where so many things are just waiting to be discovered,” says Mills. “There used to be a lot of assumptions. But for the past several decades chemists have challenged those assumptions and done things that people said were impossible. For example, a textbook from 25 years ago would state that all lanthanides in solution are in the +3 oxidation state, with the exception of cerium, which can be +4, and samarium, europium, and ytterbium, which can be +2. ‘And that was it. But for the past 20 years, every lanthanide – apart from promethium, which is massively radioactive so you can’t get your hands on enough to carry out a reaction – has been isolated in the +2 oxidation state. Chemists have really opened up the feasible, ”he says.
“Expanding an element’s oxidation state range helps you learn more about its underlying properties. And if you can open up a wide range of oxidation states, then move into the realm of transition metal chemistry and they do these very nice two-electron oxidative additions and reductive eliminations for catalytic processes. So expanding the redox chemistry of each element can lend itself to future applications. ‘
I think sometimes our employees wish our connections would behave better
Much of Mills’ research seeks to manipulate the optical and magnetic properties of f-elements. “One of the funniest things you can do with their non-aqueous chemistry is give them a shape. For example, if you can make a dysprosium compound with atoms on top, atoms below, and no atoms around the equator, these are the best single molecule magnets. But it is very difficult to make an f element component with a sufficiently low coordination number that has no equatorial atoms because the bond is so electrostatic, it is so ionic that it tries to bond to as many atoms as possible, so you have you have to work pretty hard on your ligand design and synthetic methods to prevent this from happening. ‘
Dysprosium and dates
As early as 2017, Mills and his colleagues synthesized a complex with a dysprosium atom sandwiched between two aromatic rings.1 The complex, known as dysprosozenium, remains magnetic down to –213 ° C, which was much warmer than previous efforts and makes single-molecule magnets closer to being used for data storage.
Four years later, dysprosocenium cations are still the family of single-molecule magnets with the highest hysteresis temperature. “Since that discovery, we and others have had different substituents, we have made a larger family of similar molecules, made structurally analogous compounds, and used other lanthanides.” Mills stresses, however, that his group’s work isn’t just about making certain connections. They also develop imaginative synthetic methods that other researchers can then apply.
For his work with single molecule magnets, Mills became a chemistry finalist in the UK’s 2021 Blavatnik Awards for Young Scientists, for which he received an unrestricted $ 30,000 (£ 22,000) award. “It’s an award that results from teamwork,” he says. “I will try to spend as much money as possible on people who have helped me on my way: my group, co-workers, mentors, family, friends. I would like to spend it on activities where we just spend some time together. “
Mills speaks very warmly about his research group. “We use a motto from the Bill and Ted films that is,“ Be excellent to one another ”. You don’t need a set of rules – it covers everything. We treat each other with respect and make people feel welcome. I expect my group members to do their best, but also to give the same importance to their own research activities as to those of their colleagues. It is important to me to maintain a positive, professional and selfless research environment, ”he explains.
“Like any chemist, I’m not a trained manager,” admits Mills. Nevertheless, in 2020 he received a student union award for the best student advisory service. “The students would have nominated me. And that is one of the reasons why I was very happy. I am not an expert [at advising] . But I enjoy spending time with our students to understand what they want to achieve and see if there is anything I can do to help. ‘