C O M M U N I C A T I O N S
2008, 27, 418. (g) Braunschweig, H.; Fuss, M.; Radacki, K.; Uttinger, K.
Z. Anorg. Allg. Chem. 2009, 635, 208.
HOMO and HOMO-1 orbitals in Figure 2. Both 5 and 6 have
HOMO character similar to a π orbital of the diazaborole ring,
like hydroborane 4.10,11 The shape of HOMO-1 seems to be similar
to the HOMO of boryllithium 1·(THF)2, which has lone-pair
character on the central boron atom.10,11 Natural bond order (NBO)
analyses of 5 and 6 suggest that both of the HOMO-1 orbitals
have a shared two-center-two-electron bonding character with
hybridizations of B-Ti ) 0.5668(sp1.49)B + 0.4332(sp0.92d1.06)Ti
for 5 and B-Hf ) 0.6265(sp1.06)B + 0.3745(sp0.54d2.01)Hf for 6.22
Atoms-in-molecule analyses also afforded the same conclusion.
Negative ∇2F(r) values (-0.03079 e/a05 in 5; -0.33061 e/a05 in 6)
at the bond critical point between the boron and metal atoms
indicated covalent character for these B-metal bonds.23
(3) Segawa, Y.; Yamashita, M.; Nozaki, K. Angew. Chem., Int. Ed. 2007, 46,
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(7) A gallylzirconium complex possessing a heavier group-13 element, gallium,
has also been synthesized via oxidative addition of digallane to low-valent
zirconium. See: Baker, R. J.; Jones, C.; Murphy, D. M. Chem. Commun.
2005, 1339.
(8) Group-3 gallyl complexes have been synthesized using a gallyl anion. See:
(a) Arnold, P. L.; Liddle, S. T.; McMaster, J.; Jones, C.; Mills, D. P. J. Am.
Chem. Soc. 2007, 129, 5360. (b) Jones, C.; Stasch, A.; Woodul, W. D.
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1077. (d) Liddle, S. T.; Mills, D. P.; Gardner, B. M.; McMaster, J.; Jones,
C.; Woodul, W. D. Inorg. Chem. 2009, 48, 3520.
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Figure 2. HOMO and HOMO-1 of (left) 5 and (right) 6.
Preliminary studies of the catalytic activity of 3 for polymeri-
zation of ethylene and hex-1-ene were performed (see the Sup-
porting Information for details). An admixture of 3 with
Ph3CB(C6F5)4 in toluene could polymerize ethylene to form a linear
polyethylene (PE) [turnover frequency (TOF) ) 110 kg of PE (mol
of Hf)-1 h-1, Mn ) 4800, polydispersity index (PDI) ) 2.1, 2
branches per 1000 C]. The present system was also active for
polymerization of hex-1-ene to afford an atactic poly(hex-1-ene)
(PHex) (TOF ) 21 kg of PHex (mol of Hf)-1 h-1, Mn ) 3100,
PDI ) 2.2). Activities of 3/Ph3CB(C6F5)4 toward polymerization
were comparable to those of previously reported hafnium half-
sandwich complex-derived catalyst systems.24
(11) Yamashita, M.; Suzuki, Y.; Segawa, Y.; Nozaki, K. Chem. Lett. 2008, 37, 802.
(12) (a) Marder, T. B. Science 2006, 314, 69. (b) Braunschweig, H. Angew.
Chem., Int. Ed. 2007, 46, 1946.
In conclusion, two group-4 boryl complexes, boryltitanium 2
and borylhafnium 3, were synthesized via nucleophilic borylation
using boryllithium 1. Complexes 2 and 3 are the first examples of
group-4 borylmetals. Theoretical calculations on model molecules
5 and 6 indicated that the boron-metal bond in both complexes
has covalent character. Complex 3 has an activity for polymerization
of ethylene and hex-1-ene. Further studies on polymerization and
modification of the boryl ligand are ongoing.
(13) The reaction of 1 with Cp*HfCl3 presumably afforded Cp*HfCl2(boryl) or
Cp*HfCl3(boryl)-. It is noteworthy that the following reaction of PhCH2K
with them occured at the metal center, not on the boron atom.
(14) The stability of 2 and 3 shows that the boryl ligand can coexist in the
coordination sphere with other nucleophilic ligands such as alkoxides and alkyls.
(15) There has been one report of a borylosmium complex having intramolecular
coordination of an alcoholic OH group to the metal center. See: Rickard,
C. E. F.; Roper, W. R.; Williamson, A.; Wright, L. J. J. Organomet. Chem.
2004, 689, 1609.
(16) (a) Alcaraz, G.; Sabo-Etienne, S. Coord. Chem. ReV. 2008, 252, 2395. (b)
Lin, Z. In Contemporary Metal Boron Chemistry I: Borylenes, Boryls,
Borane; Springer-Verlag: Berlin, 2008; Vol. 130, pp 123-148.
(17) The Cambridge Crystallographic Database listed all of the structurally
characterized Ti and Hf complexes having relatively short interatomic metal-
boron distances. For all of these references, see the Supporting Information.
(18) Emsley, J. The Elements; Oxford University Press: New York, 1998.
(19) A similar shortening of the Ti-O bond was observed in the structure of
the alkyltitanium triaryloxide species (Me3SiCH2)Ti(O-2,6-Ph2C6H3)3. See:
Chesnut, R. W.; Durfee, L. D.; Fanwick, P. E.; Rothwell, I. P.; Folting,
K.; Huffman, J. C. Polyhedron 1987, 6, 2019.
Acknowledgment. This work was supported by KAKENHI
(21245023, 21685006, and 19027015, “Synergy of Elements”) from
MEXT, Japan.
Supporting Information Available: Details about preparations and
characterizations of 2 and 3, polymerization procedures, X-ray crystal-
lography (CIF), and the computational study. This material is available
(20) (a) Yasumoto, T.; Yamagata, T.; Mashima, K. Organometallics 2005, 24,
3375. (b) Swenson, D. C.; Guo, Z.; Crowther, D. J.; Baenzigera, N. C.;
Jordan, R. F. Acta Crystallogr. 2000, C56, E313. (c) Kissounko, D. A.;
Zhang, Y.; Harney, M. B.; Sita, L. R. AdV. Synth. Catal. 2005, 347, 426.
(21) It was confirmed that this difference in the orientation angles of the
diazaborole rings in 3 and 6 did not affect the bonding character. See the
Supporting Information.
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