Reaction of H2 with IrHCl2P2 (P = PiPr3 or PtBu2Ph) : stereoelectronic control of the stability of molecular H2 transition metal complexes

IrHCl2P2 (P = PiPr3) reacts rapidly with H-2 at 25-degrees-C to set up an equilibrium where H-2 binds trans to the original hydride ligand (trans-2). A second slower reaction forms IrH(H2)Cl2P2 (cis-2), where the cis disposition of the chlorides, and also H cis to H-2, was established by neutron dif...

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Published in:Journal of the American Chemical Society 1993-08, Vol.115 (16), p.7300-7312
Main Authors: ALBINATI, A, BAKHMUTOV, V. I, KOETZLE, T. F, MCMULLAN, R. K, O'LOUGHLIN, T. J, PELISSIER, M, RICCI, J. S, SIGALAS, M. P, VYMENITS, A. B, CAULTON, K. G, CLOT, E, ECKERT, J, EISENSTEIN, O, GUSEV, D. G, GRUSHIN, V. V, HAUGER, B. E, KLOOSTER, W. T
Format: Article
Language:English
Subjects:
Chemical Physics
Chemistry
Condensed matter: structure, mechanical and thermal properties
Exact sciences and technology
Inorganic chemistry and origins of life
Inorganic compounds
Kinetics and mechanism of reactions
Metal complexes
Physics
Structure of solids and liquids
crystallography
Structure of specific crystalline solids
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Summary:IrHCl2P2 (P = PiPr3) reacts rapidly with H-2 at 25-degrees-C to set up an equilibrium where H-2 binds trans to the original hydride ligand (trans-2). A second slower reaction forms IrH(H2)Cl2P2 (cis-2), where the cis disposition of the chlorides, and also H cis to H-2, was established by neutron diffraction. This molecule (unlike trans-2), shows rapid site exchange between coordinated H and H-2. cis-2 can be induced to lose HCl to form Ir(H)2ClP2 (3). The structure of Ir(H)2Cl(PtBu2Ph)2, an analog of 3, was shown by neutron diffraction to have a planar H2IrCl in a Y shape, with Cl at the base of the Y and a H-Ir-H angle of only 73-degrees. ECP ab initio calculations of IrH2Cl(PH3)2 show that the Y shape with a H-Ir-H angle close to the experimental value has the minimum energy. They also show that the trans-2 isomer of IrH(H2)Cl2(PH3)2 is less stable than the cis-2 isomer by 10.3 kcal/mol. The Ir-H-2 interaction is stronger in cis-2. The rotational barrier has been calculated in the two isomers as 2.3 (trans) and 6.5 (cis) kcal/mol. In agreement with the experimental structure, the H-H bond is found to eclipse preferentially the Ir-H bond in cis-2. The calculations also show that the Ir-H-2 bond dissociation energy is greater in cis-2. It thus appears that the binding ability of a metal fragment not only depends on its ligands but is also linked in a subtle way to its stereochemistry. The J(HD) value for coordinated H-2 in cis-2 is 12 +/- 3 Hz. The implication of this small value and of a T1min(200 MHz) of 38 ms is an H/H distance of 1.07-1.35 angstrom, which compares to the neutron diffraction distance of 1.11(3) angstrom. The Ir-H distances of cis-2 are unprecedented in that the hydride-Ir distance (1.584(13) angstrom) is not shorter than the distances to the H-2 hydrogens (1.537(19) and 1.550(17) angstrom). One of the H-2 hydrogens interacts with chloride of an adjacent molecule to give an infinite hydrogen-bonded polymer. An inelastic neutron scattering spectroscopic study on solid IrHCl2(H2)(PiPr3)2 sets a lower limit on the rotational barrier of the Ir(H2) unit of 2.0 kcal/mol. Ab initio calculations on IrHCl2(H2)(PH3)2 yield a H-H distance in these two isomers of 0.81 and 1.4 angstrom, respectively, showing that the moiety IrHCl2(PH3)2 with chlorides mutually cis is a much stronger reducing agent than that with chlorides trans (and thus H trans to H-2). Crystallographic data: For cis-2 (at 15K), a = 13.008(4) angstrom, b = 11.296 (4) angstrom, c = 1
ISSN:0002-7863
1520-5126
DOI:10.1021/ja00069a032