"These elements (rare earths) perplex us in our researches, baffle us in our speculations and haunt us in our dreams. They stretch like an unknown sea before us - mocking, mystifying and murmuring strange revelations and possibilities."
-Sir William Crookes, Address to the British Association, 1887
Early transition metals, lanthanides, and uranium
Early transition metals, lanthanides, and uranium (d0fn elements) have shown impressive reactivity in transformations ranging from the activation of small, inert molecules such as methane, dinitrogen, and carbon dioxide to reactions with applications in natural product synthesis (e.g. hydroamination) and formation of biodegradable polymers. The high reactivity of d0fn metal centers makes it difficult to tune their behavior without rendering them unreactive. Our solution to this problem involves the design and synthesis of complexes with specific geometric and electronic properties. For example, weak interactions, such as electronic assistance from an electron rich metal center, are used in small molecule activation to generate highly reactive metal species. In a uranium bisferrocene compound, uranium was found to mediate the electronic communication between the two iron centers better than zirconium did in an analogous complex.
Redox Processes in a Uranium Bis(1,1'-diamidoferrocene) Complex. Monreal, M. J., Carver, C. T., Diaconescu, P. L., Inorg. Chem. 2007, 46(18), 7226-7228.
Electrophilic Metal Centers For Small Molecule Activation
Activation and functionalization of inert molecules is essential to solve some of the world’s energy problems. Important examples of these molecules include aromatic compounds, as well as small molecules such as CO, CO2, CH4, and N2. Synthetic organometallic chemists have contributed majorly to advancing this field of research by designing compounds that display unusual properties and, consequently, reactivity toward these substrates. The basis of molecular design in organometallic chemistry is the study of ancillary ligands that enable specific characteristics to various metal centers.
An important new direction in ligand design is engineering direct ligand involvement in the reactions of a specific metal. This effect may be achieved through reversible processes: weak interactions (such as hydrogen bonds), protonation/deprotonation, and redox switches. Most systems that take advantage of these processes incorporate only one type of such an interaction. Our approach, however, is unique because the ligand platform can interact with the metal center through more than one process. Chelating ferrocene ligands can invoke two desirable characteristics: (i) the ligand backbone is redox active and (ii) a weak interaction of donor-acceptor type may occur between iron and an electrophilic metal center. This approach is distinct; we take advantage of weaker interactions between the metal center and iron than those delineated by others because they become important in influencing the behavior of a metal center during the course of a reaction.This stategy has led to unprecedented reactivity, as described below.
A. Isolation of a six-carbon, 10π-electron aromatic system. Aromaticity is a fundamental concept with implications spanning all the chemical sciences. Hückel's (4n+2)π-electron rule is the standard criterion to determine aromaticity and it applies well to neutral arenes as well as to charged species such as the cyclopentadienyl anion (Cp), the cyclooctatetraene dianion, and the cycloheptatrienyl cation (tropylium). In the series of all carbon aromatic compounds, no example of a benzene tetraanion, which theoretically is a 6C, 10π-electron aromatic system, had been reported prior to our work, although heteroatom analogues of such a system, known as "electron-rich aromatics", have been studied in detail for a long time. The hypothetical C6H64- was shown computationally to have the largest orbital overlap and the strongest bonds compared to its 10π-electron heteroaromatic analogues. Despite the successful synthesis of S3N3-, P64-, and Te62+, however, there had been no reports of C6H64- or its derivatives prior to our article in Nature Communications (2013), likely due to its high negative charge.
We relied on a successful strategy to stabilize benzene polyanions by using lanthanides and actinides to support the highly reduced benzene ring by coordination. The isolation of the first tetraanionic substituted benzene as a ligand coordinated to group 3 metals was achieved by taking advantage of the unique ancillary ligand employed commonly by our group. The nature of the benzene tetraanion and the aromaticity of the 6C, 10π-electron system were established by X-ray crystallographic studies, multi-nuclei nuclear magnetic resonance, X-ray absorption spectroscopy, and density functional theory calculations. The benzene tetraanionic fragment reported by us completes the series of all carbon aromatic systems. Although C6H64- in its free form may prove elusive to synthetic chemists, these examples further our understanding of aromaticity and of the ability of metal complexes to stabilize reactive fragments.
A six-carbon 10π-electron aromatic system supported by group 3 metals. Huang, Wenliang; Dulong, Florian; Wu, Tianpin; Khan, Saeed I.; Miller, Jeffrey T.; Cantat, Thibault; Diaconescu, Paula L. Nat. Commun. 2013, 4, 1448.
The isolation of the benzene tetraanion described above is based on work conducted with other aromatic hydrocarbons such as naphthalene and anthracene. In 2011, we reported the synthesis and characterization of the first inverted sandwiches of these arene scandium complexes (J. Am. Chem. Soc. 2011, p. 10410). Prior to our work, the series of naphthalene and anthracene rare earth metal (group 3 metals and lanthanides) complexes was missing examples of scandium compounds. Based on structural and computational studies, we proposed that the presence of the ferrocene backbone was instrumental in stabilizing these motifs. Besides the fundamental importance of such complexes, we also showed that they exhibit rich redox chemistry, thus enabling redox-inactive metal centers (group 3 metals) to carry out electron transfer reactions (Chem. Commun. 2012, p. 2216).
Scandium Arene Inverted-Sandwich Complexes Supported by a Ferrocene Diamide Ligand. Huang, Wenliang; Khan, Saeed I.; Diaconescu, Paula L. J. Am. Chem. Soc. 2011, 133(24), 10410-10413.
P4 activation by group 3 metal arene complexes. Huang, Wenliang; Diaconescu, Paula L. Chem. Commun. 2012, 48(16), 2216-2218.
P4 Activation by Lanthanum and Lutetium Naphthalene Complexes Supported by a Ferrocene Diamide Ligand. Huang, Wenliang; Diaconescu, Paula L. Eur. J. Inorg. Chem. 2013, 2013(22-23, SI: Small-Molecule Activation by Reactive Metal Complexes), 4090-4096.
Group 3 metal stilbene complexes: synthesis, reactivity, and electronic structure studies. Huang, Wenliang; Abukhalil, Paul, M.; Khan, Saeed I.; Diaconescu, Paula L. Chem. Commun. 2014, 50(SI: 2014 Emerging Investigators), 5221-5223.
B. Characterization of weak iron-metal interactions. As described in the preceding section, the new bonding motifs isolated by us were possible because of the presence of the ferrocene backbone and the interaction between iron and the group 3 metal. Metal-metal bonding has been the subject of intense research and discussion, and interactions between metal centers in bimetallic complexes can give rise to interesting electronic and magnetic properties and novel reactivity. Additionally, weak metal-metal bonding is important in developing and understanding the concept of transition metal Lewis basicity, which, in turn, is of fundamental interest in understanding the reactivity of corresponding complexes. Prior to our work, only X-ray crystallography, Mössbauer, X-ray absorption spectroscopy, and computational methods were employed to determine the nature of weak metal-metal interactions. In collaboration with Professor Zink's group, we added the use of resonance Raman spectroscopy to this list (Inorg. Chem. 2013). Resonance Raman spectroscopy utilizes photons that are in resonance with an electronic transition of interest. When an electron is promoted from a bonding orbital to either a non-bonding or anti-bonding orbital, the change in bond order incites motion along the bond axis. This increases the Raman signal of vibrations with movement along the coordinate of the bond. Additionally, symmetric transitions are most strongly resonantly enhanced since the intensity of scattering scales with the square of the distortion. The ability to amplify the intensity of vibrations when in resonance with an electronic transition enables resonance Raman spectroscopy to assist in the assignment of these specific transitions.
Characterization of an Iron-Ruthenium Interaction in a Ferrocene Diamide Complex. Green, Aaron G.; Kiesz, Matthew D.; Oria, Jeremy V.; Elliott, Andrew G.; Buechler, Andrew K.; Hohenberger, Johannes; Meyer, Karsten; Zink, Jeffrey I.; Diaconescu, Paula L. Inorg. Chem. 2013, 52(9), 5603-5610.
Redox Switches For Polymerization Reactions
Redox-switchable catalysis is an atom-economical method that generates multiple catalytically active species with different reactivity; because these species originate from a single precursor, the cost of chemical synthesis is reduced. Metal complexes containing redox-active groups are being increasingly studied because the electronic properties of a metal center can be altered without the need for further, extensive synthetic steps to achieve ligand modification. The goal of this research is to design a compound that exhibits orthogonal reactivity for different substrates by switching between the oxidized and reduced forms of a catalyst. Ultimately, this research project is pushing the boundaries of understanding the factors that influence catalytic activity.
The synthesis of new polymeric materials is motivated by the limitations of current materials. Interest in copolymers containing blocks that display different or complementary properties has been increasing since these materials have potential for further performance enhancements. They could also prove cost effective and competitive even against existing, inexpensive polymers.
We showed (J. Am. Chem. Soc. 2011, p. 9278) that by changing the oxidation state of iron in the ferrocene-based ligand, the reactivity of the corresponding metal complex toward various monomers was modified. Although a change in reactivity had been previously observed for systems containing ferrocene-derivatized ligands, we reported for the first time that the redox switch is dependent on the nature of the metal carrying out the polymerization reaction: yttrium experiences a decrease in reactivity toward lactide when ferrocene is oxidized, while indium shows the opposite behavior.
A cerium(III)/(IV) redox switch presented analogous behavior to the yttrium system and allowed us to study it using DFT calculations (Chem. Commun. 2011, p. 9897). Based on those results, we interpreted the difference between the two oxidation states to be the result in large changes of the binding profile to the two oxidation states, i.e., for early transition metals, cationic complexes make stronger bonds with the polar substrates of interest than their neutral counterparts. Guided by these results and because of their less Lewis acidic character than rare earths, we turned to group 4 metal complexes. This choice allowed us to test whether a better balance between the oxidized and reduced complexes exists and whether the cationic/oxidized states would still show activity toward polar substrates. Indeed, the activity of several group 4 metal alkoxide complexes supported by ferrocene-based ligands was controlled using redox reagents during the ring-opening polymerization of L-lactide and ε-caprolactone. Switching in situ between the oxidized and reduced forms of a metal complex resulted in a change in the rate of polymerization of each monomer. Opposite behavior was observed for each monomer used; this result represented the first example when both the oxidized and reduced forms of a catalyst showed activity and selectivity toward different monomers. Even more importantly, we also demonstrated that one-pot copolymerization of the two monomers to give a block copolymer could be achieved.
Redox Control of a Ring-Opening Polymerization Catalyst. Broderick, Erin M.; Guo, Neng; Vogel, Carola; Xu, Cuiling; Sutter, Jorg; Miller, Jeffrey T.; Meyer, Karsten; Mehrkhodavandi, Parisa; Diaconescu, Paula L. J. Am. Chem. Soc. 2011, 133(24), 9278-9281.
Redox control of a polymerization catalyst by changing the oxidation state of the metal center. Broderick, Erin M.; Guo, Neng; Wu, Tianpin; Vogel, Carola; Xu, Cuiling; Sutter, Jorg; Miller, Jeffrey T.; Meyer, Karsten; Cantat, Thibault; Diaconescu, Paula L. Chem. Commun. 2011, 47, 9897-9899.
Redox Control of Group 4 Metal Ring-Opening Polymerization Activity toward l-Lactide and ε-Caprolactone. Wang, Xinke; Thevenon, Arnaud; Brosmer, Jonathan L.; Yu, Insun; Khan, Saeed I.; Mehrkhodavandi, Parisa; Diaconescu, Paula L. J. Am. Chem. Soc. 2014, 136(32), 11264-11267.
Reactions with Aromatic N-Heterocycles
The activation of C-N bonds in heterocycles is important in the context of hydrodenitrogenation processes. Our group studies the reactions of electrophilic metal-carbon bonds with substrates featuring strong C-N bonds in order to understand the mechanism of intimate reaction steps. Ring-opening of aromatic N-heterocycles can be initiated by d0fn metal-carbon sigma-bonded fragments. These results are different from existing examples, which all involve metal-element multiple bonds. DFT calculations indicate that the ligand system employed by us, 1,1'-ferrocene diamides, may be unique in supporting electrophilic metal centers. Our goal is to understand specific examples of strong bond cleavage in order to develop general transformations.
Reactions of Aromatic N-Heterocycles with d0fn-Metal Alkyl Complexes Supported by Chelating Diamide Ligands. Diaconescu, P. L., Acc. Chem. Res. 2010, 43(10), 1352-1363.
Ring-Opening Reactions of Aromatic N-Heterocycles by Scandium and Yttrium Alkyl Complexes. Carver, C. T., Diaconescu, P. L., J. Am. Chem. Soc. 2008, 130(24), 7558-7559.
Metal Nanoparticles Stabilized by Polyaniline Nanofibers as Catalysts for Organic Synthesis
This project is shaped by efforts to develop "green chemistry" by recovery and reuse of catalysts and the ability to use water as a reaction medium. Metal nanoparticles supported by polyaniline nanofibers meet both criteria since the polyaniline nanofibers act as a support and allow their generation in water. Given the possibility to stabilize metal nanoparticles in water, organic synthesis in this medium becomes tangible. We have already reported that palladium nanoparticles supported by polyaniline nanofibers are very active catalysts in the C-C and C-O bond forming reactions conducted in water. These results open the question of how this catalyst system works, how general it is (i.e. how many different reactions can be catalyzed by the same catalyst) and if tandem catalysis is possible. The work targeted will allow the development of synthetic processes that are usually confined to the field of organometallic chemistry to be possible in environmentally friendly conditions.
Palladium Nanoparticles Supported on Polyaniline Nanofibers as a Semi-Heterogeneous Catalyst in Water. Gallon, Benjamin J.; Kojima, Robert W.; Kaner, Richard B.; Diaconescu, Paula L. Angew. Chem. Int. Ed. 2007, 46(38), 7251-7254.