Although reversible covalent activation of molecular hydrogen (H2) is a common reaction at transition metal centers, it has proven elusive in compounds of the lighter elements. We report that the compound (C6H2Me3)2PH(C6F4)BH(C6F5)2 (Me, methyl), which we derived through an unusual reaction involving dimesitylphosphine substitution at a para carbon of tris(pentafluorophenyl) borane, cleanly loses H2 at temperatures above 100°C. Preliminary kinetic studies reveal this process to be first order. Remarkably, the dehydrogenated product (C6H2Me3)2P(C6F4)B(C6F5)2 is stable and reacts with 1 atmosphere of H2 at 25°C to reform the starting complex. Deuteration studies were also carried out to probe the mechanism.

Reversible, Metal-Free Hydrogen Activation

There is increased recognition by the world’s scientific, industrial, and political communities that the concentrations of greenhouse gases in the earth’s
atmosphere, particularly CO_2, are increasing. For
example, recent studies of Antarctic ice cores to
depths of over 3600 m, spanning over 420 000 years,
indicate an 80 ppm increase in atmospheric CO_2 in
the past 200 years (with most of this increase
occurring in the past 50 years) compared to the
previous 80 ppm increase that required 10 000 years.2
The 160 nation Framework Convention for Climate
Change (FCCC) in Kyoto focused world attention on
possible links between CO2 and future climate change
and active discussion of these issues continues.3 In
the United States, the PCAST report4 “Federal
Energy Research and Development for the Challenges
of the Twenty First Century” focused attention
on the growing worldwide demand for energy and the
need to move away from current fossil fuel utilization.
According to the U.S. DOE Energy Information
Administration,5 carbon emission from the transportation
(air, ground, sea), industrial (heavy manufacturing,
agriculture, construction, mining, chemicals,
petroleum), buildings (internal heating, cooling, lighting),
and electrical (power generation) sectors of the
World economy amounted to ca. 1823 million metric
tons (MMT) in 1990, with an estimated increase to
2466 MMT in 2008-2012 (Table 1).

Catalysis Research of Relevance to Carbon Management: Progress, Challenges, and Opportunities

Review and analysis of molecular simulations of methane, hydrogen, and acetylene storage in metal-organic frameworks.

A new type of hydrogen bond, called a dihydrogen bond, has recently been introduced. In this bond a hydrogen is donated to another (hydridic) hydrogen. We apply a set of criteria developed in the context of the theory of “atoms in molecules” that were previously successfully used to study conventional hydrogen bonds. This method enables one to characterize the dihydrogen bond on the basis of the electron density only. We investigated a dimer structure of BH3NH3 at the ab initio level which contains two dihydrogen bonds that differ in strength. The combination of a theoretical density with our hydrogen-bonding criteria turns out to be a valuable new and independent source of information complementary to techniques such as NMR, IR, and structural crystallography.

Characterization of a Dihydrogen Bond on the Basis of the Electron Density

The reaction of [Cp*RuCl]4 (1; Cp* = η5-C5Me5) with the carbene ligand 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) affords a coordinatively unsaturated Cp*Ru(IMes)Cl (3) complex in 86% isolated yield. Solution calorimetric results in this system provide information concerning the electron donor properties of the carbene ligand, which are very similar to those of PCy3. Structural information from single-crystal X-ray studies for complex 3 allows the determination of steric parameters associated with this ligand. The thermochemical information is used to predict the magnitude of the enthalpic driving force behind substitution reactions involving RuCl2(C(H)Ph)(PCy3)2 (1) and the carbene ligand, IMes, affording the RuCl2(C(H)Ph)(PCy3)(IMes) (6) complex in high yield. A similar mixed carbene/phosphine ruthenium complex, RuCl2(C(H)Ph)(PPh3)(IMes), can also be isolated from RuCl2(C(H)Ph)(PPh3)2 and the IMes ligand. A single-crystal X-ray diffraction study has been performed on 6. The thermal stabilit...

Olefin Metathesis-Active Ruthenium Complexes Bearing a Nucleophilic Carbene Ligand

Molecular hydrogen complexes: coordination of a .sigma. bond to transition metals

Aside from its renown as a source of acid precipitation, sulfur dioxide is remarkable in possessing physicochemical and coordination properties that are more diverse than those of any other small molecule. SO{sub 2} is amphoteric, behaving as a Lewis acid or base, mild oxidant or reductant, or oxygen donor or acceptor. It is an excellent nonaqueous solvent when liquefied at -10{degrees}C and coordinates to many types of compounds, including metal complexes at both metal and ligand sites, strong Lewis acids, and virtually all nucleophiles, even halide ion. SO{sub 2} can bind strongly to low-valent metals like CO or NO or completely reversibly like O{sub 2} or H{sub 2}. The diversity of metal-SO{sub 2} bonding geometries is unmatched and has been reviewed. This Account will focus on the reactivity of SO{sub 2} e.g. SO double bond cleavage. 72 refs., 7 figs., 1 tab.

Gregory J. Kubas

Papers

Molecular hydrogen complexes: coordination of a .sigma. bond to transition metals

Chemical Transformations and Facile Disproportionation of Sulfur Dioxide on Transition Metal Complexes