Zirconium and Hafnium Polyhydrides. Preparation and Characterization of M2H3(BH4)5(PMe3)2, MH(BH4)3(dmpe), and MH2(BH4)2(dmpe)2

John E. Gozum, Gregory S. Girolami

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Treatment of the zirconium and hafnium tetrahydroborate complexes M(BH4)4 with trimethylphosphine yields amber (M = Zr) or colorless (M = Hf) crystals of the new polyhydride complexes Zr2H3(BH4)5(PMe3)2 and Hf2H3(BH4)5(PMe3)2. These d0 complexes exhibit triplets (JPH ≈ 13 Hz) for the hydride ligands in their 1H NMR spectra at δ 3.96 (Zr) and 8.53 (Hf). the 1H, 11BI{1H}, and 31P{1H} NMR spectra show that there is only one phosphine environment but two BH4 environments in a 2:3 ratio. Single-crystal X-ray diffraction studies of the two complexes in each case reveal a distinctly asymmetric dinuclear structure bridged by three hydrogen atoms. One metal center is ligated by three terminal tridentate BH4 groups, while the other is ligated by the two phosphines and by one tridentate and one bidentate BH4 group. All the metal-ligand distances in the zirconium complex are slightly longer than those in the hafnium complex, as expected from the relative ionic radii: Zr…Zr = 3.124 (1) Å, Zr-−Hb = 1.92 (4) Å, Zr−P = 2.750 (1) Å, Zr−B = 2.346 (8) Å (η3-BH4), Zr−B = 2.604 (8) Å (η2-BH4), Zr-H−Zr = 109 (3)°; Hf…Hf = 3.076 (1) Å, Hf−P = 2.725 (6) Å, Hf−B = 2.34 (3) Å (η3−BH4), and Hf−B = 2.53 (3) Å (η2−BH4). Treatment of the M(BH4)4 complexes with the bidentate phosphine l,2-bis(dimethylphosphino)ethane gives mononuclear hydrides of stoichiometry MH(BH4)3(dmpe) or MH2(BH4)2(dmpe)2, depending on the conditions. the monohydride complexes MH(BH4)3(dmpe) contain both bidentate and tridentate BH4 groups as judged by IR spectroscopy. the 1H NMR spectra show a triplet for the terminal hydride groups at δ 6.08 (JPH = 56 Hz) for M = Zr and 10.99 (JPH = 45 Hz) for M = Hf. the 1H NMR spectra also show that there are two PMe2 environments, while the 11B{1H} and 31P{1H} NMR spectra show that there is a single BH4 and a single phosphorus environment. These data are consistent with a pseudooctahedral structure in which the three mutually fac BH4 groups are exchanging with each other. the BH4 groups in the dihydride complexes MH2(BH4)2(dmpe)2 are bidentate by IR spectroscopy. the 1H NMR spectra show that there is one hydride environment and one BH4 environment but that the two ends of each dmpe ligand are inequivalent. the terminal hydride resonances at δ 3.38 (Zr) and 6.65 (Hf) are complex multiplets arising from the X part of XX′AA′BB′ spin systems. Simulation of the resonances suggests that the coupling constant between the two hydride ligands may be unusually large at ca. 30 Hz. the possibility that nuclear exchange processes are responsible for this large coupling constant was ruled out on the basis of deuteration studies. Interestingly, these MH2(BH4)2(dmpe)2 molecules are nonfluxional at room temperature, despite the high coordination numbers (8 or 10, depending on how the BH4 groups are counted). Variable-temperature NMR spectroscopy shows that the molecules undergo an intramolecular fluxional process with an activation energy of 15.6 kcal mol−1; this value is unusually high for a molecule with such a high coordination number. Crystal data for Zr2C6H41B5P2 at −75 °C are as follows: orthorhombiC.; P212121; a = 10.921 (2) Å,b= 12.255 (3) Å, c = 16.524 (6) Å, V= 2212 (1) Å3, Z = 4, Rf = 0.031, Rwf= 0.029 for 301 variables and 2821 data for which I > 2.58σ(I). Crystal data for Hf2C6H41B5P2 at −75 °C are as follows orthorhombiC.; P212121; a = 10.860 (7) Å,b = 12.248 (7) Å, c = 16.490 (4) Å, V = 2193 (3) Å3, Z = 4, Rf = 0.042, Rwf= 0.051 for 112 variables and 1634 data for which I > 2.58σ(I).

Original languageEnglish (US)
Pages (from-to)3829-3837
Number of pages9
JournalJournal of the American Chemical Society
Issue number10
StatePublished - May 1 1991

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry


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