Dihydrogen complex

Formation and equilibrium structures of metal dihydrogen and dihydride complexes (L = ligand)

Dihydrogen complexes are coordination complexes containing intact H₂ as a ligand. They are a subset of sigma complexes.^[1] The prototypical complex is W(CO)₃(PCy₃)₂(H₂). This class of compounds represent intermediates in metal-catalyzed reactions involving hydrogen. Hundreds of dihydrogen complexes have been reported. Most examples are cationic transition metals complexes with octahedral geometry.

Upon complexation, the H−H bond is extended to 0.81–0.82 Å as indicated by neutron diffraction, about a 10% extension relative to the H−H bond in free H₂. Some complexes containing multiple hydrogen ligands, i.e. polyhydrides, also exhibit short H−H contacts. It has been suggested that distances < 1.00 Å indicates significant dihydrogen character, where separations > 1 Å are better described as dihydride complexes (see figure).

Characterization

The usual method for characterization is ¹H NMR spectroscopy. The magnitude of spin-spin coupling, J_HD, is a useful indicator of the strength of the bond between the hydrogen and deuterium in HD complexes. For example, J_HD is 43.2 Hz in HD but 33.5 Hz in W(HD)(CO)₃(PⁱPr₃)₂. Dihydrogen complexes typically have shorter ¹H-spin-lattice relaxation times than the corresponding dihydrides.^[2]

An ideal if nontrivial method of characterization of dihydrogen complexes is neutron diffraction. Neutrons interact strongly with hydrogen atoms, which allows one to infer their location in a crystal. In some cases, hydrogen ligands are usefully characterized by X-ray crystallography, but often the presence of metals, which strongly scatter X-rays, complicates the analysis.

The triangular MH₂ subunit has six normal modes of vibration, one of which is mainly of ν_H−H character. In free H₂, this very strong bond absorbs at 4300 cm⁻¹, whereas in dihydrogen complexes the frequency drops to around 2800 cm⁻¹.

Synthesis

Two preparation methods involve the direct reactions with H₂ gas. The first entails the addition of H₂ to an unsaturated metal center, as originally reported for W(CO)₃(P-i-Pr₃)₂(H₂). In such cases, the unsaturated complex in fact features an agostic interaction that is displaced by the H₂.

In other cases, H₂ will displace anionic ligands, sometimes even halides. Treatment of chlorobis(dppe)iron hydride with sodium tetrafluorborate under an atmosphere of hydrogen is one example:^[3]

HFeCl(dppe)₂ + NaBF₄ + H₂ → [HFe(H₂)(dppe)₂][BF₄] + NaCl

Many metal hydrides can be protonated to give dihydrogen complexes:^[4]

H₂Fe(dppe)₂ + H⁺ → [HFe(H₂)(dppe)₂]⁺

In such cases, the acid usually is derived from a weakly coordinating anion, e.g., Brookhart's acid.

History

In 1984, Kubas et al. discovered that the addition of H₂ to the purple-colored species M(CO)₃(PR₃)₂ gave a yellow precipitate of mer-trans-M(CO)₃(PR₃)₂(H₂) (M = Mo or W; R = cyclohexyl, iso-propyl).^[5] This result rapidly led to the discovery of a variety of related complexes such as Cr(H₂)(CO)₅^[6] and [Fe(H₂)(H)(dppe)₂]⁺.^[7] Kubas et al.'s findings also led to a reevaluation of previously described compounds. For example, the complex "RuH₄(PPh₃)₃" described in 1968 was reformulated as a dihydrogen complex.

References

^ Kubas, Gregory J. (2001-08-31). Metal Dihydrogen and σ-Bond Complexes: Structure, Theory, and Reactivity (1 ed.). Springer. ISBN 0-306-46465-9.
^ Crabtree, R. H. (1990). "Dihydrogen complexes: some structural and chemical studies". Accounts of Chemical Research. 23 (4): 95–101. doi:10.1021/ar00172a001.
^ Bautista, M. T.; Bynum, L. D.; Schauer, C. K. (1996). "Synthesis of η²-Dihydrogen Complex, trans-{Fe(η²-H₂)(H)[1,2-bis(diphenylphosphino)ethane]₂}[BF₄]: An Experiment for an Advanced Inorganic Chemistry Laboratory Involving Synthesis and NMR Properties of an η²-H₂ Complex". Journal of Chemical Education. 73 (10): 988–991. Bibcode:1996JChEd..73..988B. doi:10.1021/ed073p988.
^ Morris, R. H. (2008). "Dihydrogen, Dihydride and in Between: Nmr and Structural Properties of Iron Group Complexes". Coord. Chem. Rev. 2252 (21–22): 2381–2394. doi:10.1016/j.ccr.2008.01.010.
^ Kubas, G. J.; R. R. Ryan; B. I. Swanson; P. J. Vergamini; H. J. Wasserman (1984-01-01). "Characterization of the first examples of isolable molecular hydrogen complexes, M( CO)₃(PR₃)₂(H₂) (M = molybdenum or tungsten; R = Cy or isopropyl). Evidence for a side-on bonded dihydrogen ligand". Journal of the American Chemical Society. 106 (2): 451–452. doi:10.1021/ja00314a049.
^ Sweany, Ray L. (1985-04-01). "Photolysis of hexacarbonylchromium in hydrogen-containing matrixes: evidence of simple adducts of molecular hydrogen". Journal of the American Chemical Society. 107 (8): 2374–2379. doi:10.1021/ja00294a030.
^ Morris, Robert H.; Jeffery F. Sawyer; Mahmoud Shiralian; Jeffrey Zubkowski (1985). "Two molecular hydrogen complexes: trans-[M(η²-H₂)(H)(PPh₂CH₂CH₂PPh₂)2]BF₄ (M = Fe, Ru). The crystal structure determination of the iron complex". Journal of the American Chemical Society. 107 (19): 5581–5582. doi:10.1021/ja00305a071.