Prolonged treatment of the tetrakis(tetrahydroborate) complexes Zr(BH4)4 or Hf(BH4)4 with trimethylphosphine has given the first trinuclear group 4 polyhydrides, M3H6(BH4)6(PMe3)4, where M is Zr or Hf. The 1H, 31P, and11B NMR data suggest that these trinuclear compounds contain noncyclic M(μ-H)3M(μ- H)3M backbones with the phosphine and tetrahydroborate ligands distributed in 2:2:0 and 2:1:3 ratios among the three metal centers. This suggestion has been confirmed by the X-ray crystal structure of Zr3H6(BH4)6(PMe3)4. The metal-metal vectors are each bridged by three hydride ligands: the average Zr-H-Zr angle is 108 (2)°, and the Zr…Zr…Zr angle is 124.14 (1)°. The Zr-B distances average 2.633 (4) Å for the η2-BH4 groups and 2.368 (6) Å for the η3-BH4 groups, while the Zr-P distances average 2.761 (1) Å. The average Zr…Zr distance is 3.164 (1) Å. Interestingly, several of the η3-BH4 groups are bonded asymmetrically, so that of the three Zr-H bonds to each BH4 ligand, one Zr-H bond is longer than the other two. Addition of 1,2-bis(dimethylphosphino)ethane(dmpe) to the previously reported polyhydrides of stoichiometry M2H3(BH4)5(PMe3)2 results in phosphine exchange and loss of one BH3 unit to yield the new dinuclear hydrides M2H4(BH4)4(dmpe)2. The NMR and X-ray crystallographic data show that three of the hydrides bridge the Zr…Zr axis; the fourth hydride, one η2-BH4 group, and the two dmpe ligands are coordinated to one of the zirconium centers, while three asymmetrically-bonded η3-BH4 groups are coordinated to the other. The variable-temperature NMR data show that the terminal and bridging hydrides on zirconium exchange with each other via a “windshield wiper” type of motion with an activation energy of 12.6 • 0.1 kcal mol-1. The X-ray crystal structure of this molecule gives the following distances and angles: Zr…Zr = 3.150 (1) Å, Zr-Hb = 2.03 (8) Å, Zr-Ht = 1.74 (9) Å, Zr-P = 2.715 (3), 2.836 (3) Å, Zr-B = 2.70 (1) Å (η2-BH4), Zr-B = 2.39 (2) Å (η3-BH4), Zr-H-Zr = 107 (4)°. X-ray data for C12H66B6P4Zr3 at 198 K: space group P21/n, a = 10.142 (5) Å, b= 18.499 (9) Å, c = 19.088 (8) Å, β = 90.49 (4)°, V = 3581 (5) Å3, Z = 4, RF = 0.022, and RwF = 0.024 for 426 variables and 4000 unique data for which I > 2.58σ (1). X-ray data for C12H52B4P4Zr2 at 198 K: space group Pna21, a = 20.736 (4) Å, b = 9.894 (2) Å, c = 13.788 (4) Å, V= 2829 (2) Å3, Z = 4, RF = 0.046, and RwF = 0.036 for 241 variables and 1965 unique data for which I > 2.58 σ(I).
ASJC Scopus subject areas
- Colloid and Surface Chemistry