An characteristic of a metal atom in a compound is its coordination number, the number of atoms to which it is attached. Transition metals typically have coordination numbers of five or six. Metals with lower coordination numbers are reactive, and are important in catalytic transformations of organic and inorganic molecules. Some metal-containing enzymes also feature unusually low-coordinate metal sites.
The Holland research group is inspired by these catalysts - especially the natural enzymes - to study the reactions of iron, cobalt, and nickel complexes that have a coordination number of 3 or 4. We especially focus on reactions that have a direct analogy to those in metalloenzymes, and also on reactions that are useful in industrial or environmental catalysis. Because of the analogy, the compounds can be viewed as simple "models" of enzymes or other catalysts.
Because of the importance of iron in catalytic oxidation (metabolism of organic compounds) and reduction (converting nitrogen to ammonia) reaction, we have synthesized a large number of three-cooordinate iron complexes. Especially notable have been the first structures of iron(II) fluoride and diiron(II) oxo complexes, and an iron(III) imide complex that oxidizes hydrocarbons (Figure 1). We have also studied rare examples of isolable 12-electron organometallic compounds. Some of the complexes bring about catalytic reactions, such as the reduction of C-F bonds in fluorinated aromatic compounds. Our goals in this area are (a) to push back the frontiers of coordination chemistry, (b) to discover unusual reactions, (c) to study the electronic structure of the compounds and mechanism of the reactions in detail, and (d) to develop new catalytic reactions based on what we have learned.
A topic of special interest to us is reduction of N2 to NH3, which provides “fixed” nitrogen for biosynthesis of nucleic acids and proteins. The N-N triple bond is cleaved in nature by the “iron-molybdenum cofactor” of the enzyme nitrogenase. Intrigued by the low-coordinate iron sites in the cofactor, we are studying 3-coordinate and 4-coordinate iron complexes that bind and weaken N2. The illustration below shows the stepwise stretching of N2 from 1.10 Å to 1.23 Å by our low-coordinate iron complexes. We have done detailed research on the synthesis, spectroscopic properties, bonding, and reactivity of these complexes, which showed that low-coordinate iron is unusually well- suited for weakening the N-N bond of bound N2. In other reactions, we have broken N-N single and double bonds.
A group member's adventures usually involve organic synthesis, inorganic synthesis (in an inert-atmosphere glove box), and mechanistic studies. We use a variety of techniques including NMR, EPR, UV/visible, infrared, resonance Raman, Mössbauer, magnetic measurements, X-ray crystallography, and kinetics. Our research develops understanding of organic chemistry, biochemistry, organometallic chemistry, coordination chemistry, spectroscopy, synthesis, and mechanism. Therefore, members and graduates of this group acquire broad-based experience that is necessary for addressing the wide range of chemical problems they are likely to come across in their careers.
