Zirconium and hafnium polyhydrides. Preparation and characterization of M2H3(BH4)5(PMe3) 2, MH(BH4)3(dmpe), and MH2(BH4)2(dmpe)2

Article abstract of DOI:10.1021/ja00010a027

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 (J_PH ≈ 13 Hz) for the hydride ligands in their ¹H NMR spectra at δ 3.96 (Zr) and 8.53 (Hf). The ¹H, ¹¹B{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) A?, Zr-H_b = 1.92 (4) A?, Zr-P = 2.750 (1) A?, Zr-B = 2.346 (8) A? (η³-BH₄), Zr-B = 2.604 (8) A? (η²-BH₄), Zr-H-Zr = 109 (3)°; Hf?Hf = 3.076 (1) A?, Hf-P = 2.725 (6) A?, Hf-B = 2.34 (3) A? (η³-BH₄), and Hf-B = 2.53 (3) A? (η²-BH₄). Treatment of the M(BH₄)₄ complexes with the bidentate phosphine 1,2-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 (J_PH = 56 Hz) for M = Zr and 10.99 (J_PH = 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) A?, b = 12.255 (3) A?, c = 16.524 (6) A?, V = 2212 (1) A?³, Z = 4, R_f = 0.031, R_wf = 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) A?, b = 12.248 (7) A?, c = 16.490 (4) A?, V = 2193 (3) A?³, Z = 4, R_f = 0.042, R_wf = 0.051 for 112 variables and 1634 data for which I > 2.58σ(I).