Neutron Diffraction Studies of Metal Hydride Complexes

Our main research involves the neutron diffraction studies of metal hydride complexes, especially H atoms in metal clusters. The usual technique for structural determination, X-ray diffraction, is normally incapable of determining the positions of the hydride ligands reliably in such molecules, and over the years we have used the technique of neutron diffraction to accurately characterize such compounds. Examples include the discovery of five-coordinate hydrogen atoms in the cluster complex [H₂Rh₁₃(CO)₂₄]^3- (Fig. 1),^[1] and the more recent characterization of a H4Co4 cluster in the core of H₄Co₄(Cp*)₄ (Fig. 2),^[2] Other recent results in the neutron structures of metal hydrides are given in the references below,^[3-9] It is hoped that our investigations will lead to a better understanding of metal-hydrogen bonding, and this work has been summarized in a review article.^[10]

Neutron Diffraction Studies on Small Proteins

In an attempt to understand the unusual thermostability of proteins from hyperthermophiles such as Pyrococcus furiosus, we have recently been engaged in the neutron diffraction study of the small iron/sulfur protein rubredoxin isolated from this organism. Pyrococcus furiosus is a microorganism that lives in deep undersea vents (Fig 3), under conditions of extremely high temperature and pressure. We have recently published the 1.5-Å resolution neutron diffraction study of the wild-type protein^[11] as well as one of its mutants,^[12] which revealed details of its molecular structure at the atomic level (e.g., see Fig 4). In addition to finding a tight series of hydrogen bonds at the N-terminus of this protein that may explain part of its remarkable thermostability, we were also able to map out the H/D exchange behavior of the N-H bonds of the protein backbone.

Construction of a Single Crystal Diffractometer at the new Spallation Neutron Source

Finally, we are involved in the design and building of a Single Crystal Diffractometer at the new high-intensity Spallation Neutron Source (SNS), currently under construction at Oak Ridge National Laboratory in Tennessee. The significance of this project is that the SNS is capable of delivering neutron beams with intensities 50-100 times higher than existing neutron sources. The concomitant reduction in crystal size necessary for a structural analysis (currently one of the major disadvantages of neutron diffraction) will hopefully make the technique much more accessible to the practicing chemist or structural biologist. The ability to routinely locate H atoms in biological macromolecules will make neutron diffraction an extremely powerful technique in structural biology. The SNS itself is scheduled for completion in late 2006, and our instrument by late 2008.
(For more information about the SNS, see the following links:
http://www.sns.gov/aboutsns/aboutsns.htm
http://www.sns.gov/users/instrument_systems/instruments/elastic/crystal.shtml
http://www.sns.gov/users/instrument_systems/iatidt/idt/idtmembers/crystal.shtml)

References

 

1.	R. Bau, M. H. Drabnis, Z. Xie, L. Garlaschelli, W. T. Klooster, T. F. Koetzle and S. Martinengo, “Five-Coordinate Hydrogen: Neutron Diffraction Analysis of the Hydrido Cluster Complex [H2Rh13(CO)24]3-”, Science, 275, 1099 (1997).
2.	R. Bau, N. N. Ho, J. J. Schneider, S.A. Mason, and G. J. McIntyre, Garlaschelli, W. T. Klooster, T. F. Koetzle and S. Martinengo, “Neutron Diffraction Study of [H4Co4(C5Me4Et)4], a Tetrahedral Metal Cluster Complex with Four Face-Bridging Hydride Ligands”, Inorganic Chemistry, 43, 555 (2004).
3.	K.Y. Dorogov, E. Dumont, N.N. Ho, A.V. Churakov, L.G. Kuzmina, J.M. Poblet, A.J. Schultz, J.A.K. Howard, R. Bau, A. Lledos and G.I. Nikonov, “Neutron and X-ray Diffraction Studies and DFT Calculations of Asymmetric Bis(silyl) Niobocene Hydrides”, Organometallics, 23, 2845-2847 (2004).
4.	N.N. Ho, R. Bau, and S.A. Mason, “Neutron diffraction study of the highly distorted octahedral complex FeH2(CO)2[P(OPh)3]2”, J. Organometal. Chem., 676, 85-88 (2003).
5.	N.N. Ho, R. Bau, C. Plecnik, S.G. Shore, X. Wang, and A.J. Schultz, “A neutron diffraction study of Cp2Ti{(m–H)2BC8H14}”, J. Organometal. Chem., 654, 216-220 (2002).
6.	R. Bau, S.A. Mason, B.O. Patrick, C.S. Adams, W.B. Sharp and P. Legzdins, ”a-Agostic Interactions in Cp*W(NO)(CH2CMe3)2 and Related Nitrosyl Complexes”, Organometallics, 20, 4492-4501 (2001).
7.	M. Bortz, R. Bau, J.J. Schneider and S.A. Mason, Neutron diffraction analysis of H3Co2[C5H2(t-Bu)3]2, a molecule with a triply hydrogen-bridged metal-metal bond: some comments on structural patterns in M(m–H)2M systems (n = 1, 2, 3, 4)”, Journal of Cluster Science, 12, 285-291 (2001).
8.	J.J. Schneider, N. Czap, J. Hagen, C. Krüger, S.A. Mason, R. Bau, J. Ensling, P. Gütlich, and B. Wrackmeyer, "Hydroxo Hydrido Complexes of Iron and Cobalt: Probing Agostic Sn...H–M Interactions in Solution and in the Solid State", Chem. Eur. J., 6, 625 (2000)
9.	I.Tanaka, T. Ohhara, N. Niimura, Y. Ohashi, Q. Jiang, D.H. Berry and R. Bau, "The Classical Structure of TaCp2(H)(SiMe2H)2: A Neutron Structure Determination", J. Chem. Res (S), 180-192 (1999)
10.	R. Bau and M. Drabnis, "Structures of Transition Metal Hydrides by Neutron Diffraction", Inorg. Chim. Acta, 259, 27 (1997)
11.	K. Kurihara, I. Tanaka, T. Chatake, M.W.W. Adams, F.E. Jenney, Jr., N. Moiseeva, R. Bau, Robert and N. Niimura, “Neutron crystallographic study on rubredoxin from Pyrococcus furiosus by BIX-3, a single-crystal diffractometer for biomacromolecules”, Proc. Nat. Acad. Sci., USA, 101, 11215-1120 (2004)
12.	T. Chatake, K. Kurihara, I. Tanaka, I. Tsyba, R. Bau, F.E. Jenney, Jr., M.W.W. Adams and N. Niimura, “A neutron crystallographic analysis of a rubredoxin mutant at 1.6 Å resolution”, Acta Cryst., D60, 1364-1373 (2004)