Catecholborane bound to titanocene. Unusual coordination of ligand σ-bonds

Full text of DOI:10.1021/ja962086v

10936
J. Am. Chem. Soc. 1996, 118, 10936-10937
Catecholborane Bound to Titanocene. Unusual
Coordination of Ligand σ-Bonds
John F. Hartwig,* Clare N. Muhoro, and Xiaoming He
Department of Chemistry, Yale UniVersity
P.O. Box 208107, New HaVen, Connecticut 06520-8107
Odile Eisenstein,* Ramon Bosque, and Feliu Maseras
Laboratoire de Structure et Dynamique
des Systemes` Mole´culaires et Solides, UMR 5636
UniVersite´ de Montpellier II, Montpellier, France
ReceiVed June 21, 1996
Figure 1. ORTEP drawing of 1a. Hydrogen atoms (except B-H) were
omitted for clarity. Selected distances (Å) and angles (deg): Ti-B
2.335(5), Ti-H 1.74(4), B-H 1.25(3), B-Ti-B 53.8(2), H-Ti-H
117(2), B-Ti-H1 32(1), B-Ti-H1A 86(1), Ti-B-H1 47(2), Ti-
H-B 101(2), Ti-B-O 125.4(3), 126.0(3), O-B-O 108.5(4).
Isolated examples of complexes formed by coordination of
small molecules only through an X-H bond or “σ complexes”
are rare and are limited to those involving dihydrogen or
silanes.1,2 Closely related “agostic” complexes, involving coor-
dination of an X-H bond of a ligand with another point of
attachment to the metal, are more generally known and include
those for X-H bonds of first row elements. Isotope effect
measurements and labeling studies along with gas phase
transition metal chemistry and organometallic reactions in alkane
matrices suggest that σ- and agostic complexes are important
intermediates in C-H activations.3,4
We report the synthesis, structure, and preliminary reaction
chemistry of set of complexes with unusual coordination of
neutral boranes through σ-bonds. The compounds are distinct
from those containing anionic borates stabilized by electrostatic
effects5 or metallocarboranes and polyboranes with agostic B-H
bonds.6-9 The borane complexes we report are likely to be
important in oxidative additions of B-H bonds, and their
geometry depicts partial X-H bond cleavage of sp2-hybridized
X-H bonds.
Reaction of 3-5 equiv of catecholborane (HBcat) or 4-(me-
thylcatechol)borane (HBcat′) and titanocene dimethyl (2) at -35
°C for 1-2 d evolved methane, formed B-(methylcatechol)-
borane, and precipitated the compounds Cp2Ti(HBcat)2 (1a, cat
) O2C6H4; 1b, 4-MeC6H4) in 60-95% yields. 1a is a yellow
solid that is insoluble in aromatic solvents, extremely temper-
ature sensitive in solution, and reactive toward both THF and
methylene chloride at low temperatures. 1b is equally sensitive
thermally, but is slightly soluble in toluene at low temperatures.
These properties precluded NMR spectroscopic identification
of isolated 1a, but the greater solubility of 1b made it amenable
to spectroscopic characterization at -45 °C. NMR spectral data
for 1a were obtained during reaction of 2 with catecholborane
by simple one- and two-dimensional NMR techniques. Both
1a and 1b showed 11B NMR signals at 45 ppm, which are
located downfield of free catecholborane but upfield of isolated
metallocene boryl complexes and suggest partial metal-boron
bond character.10 Compound 1a showed a single Cp resonance.
Compound 1b consisted of two isomers with different orienta-
tions of the aromatic methyl groups. Three cyclopentadienyl
resonances were observed in an approximate ratio of 1:1:2
corresponding to equal ratios of the C2 and Cs isomers of 1b.
Thus, interconversion between coordination to the two faces of
the borane is slow on the NMR time scale at -45 °C. A single
set of catecholate resonances were observed for 1a, but again
two sets of aromatic resonances and two methyl groups of
almost equal intensity were observed for 1b. The hydride signal
for both complexes was broad due to remaining B-H interac-
tion, and separate signals for 1b-C2 and 1b-Cs were not resolved.
Infrared vibrations involving the hydrides of 1a and 1b were
observed between 1611 and 1680 cm-1. This assignment was
confirmed by synthesis of 1b-d2 from DB(O2C6H4-4-Me).
Compound 1b-d2 displayed no bands between 1611 and 1680
and showed bands in the region between 1160 and 1250 cm-1
.
The low νB-H of 1a and 1b suggests a low B-H bond order
relative to free catecholborane (νB-H of 2660 cm-1).
Highly dilute reaction solutions deposited single crystals of
1a over the course of several days at 35 °C. An ORTEP
drawing of 1a is provided in Figure 1; the two halves of the
molecule are related by crystallographic symmetry. In addition
to the spectroscopic data and the the reaction chemistry
described below, several structural features confirmed the
presence of hydrides in 1a, which were located in the difference
map and were refined isotropically. The B-Ti-B angle of 55°
was too small for the compound to be a Ti(IV) bis-boryl
complex, considering the typical angles in metallocene sys-
tems.11 Further, the Ti-B distance of 2.335(5) Å was signifi-
cantly longer than that of metallocene boryl complexes.12-14
Indeed, the hydrides were located and observed in the difference
map and are displayed in the difference map position.
The B-B distance was 2.11 Å, 0.23-0.25 Å longer than
those in polynuclear B2H62- complexes15-17 and more than 0.4
Å longer than those in catBBcat molecules or their Lewis base
adducts.18,19 The H-Ti-H angle was 117°, much greater than
the typical 94-97° angles in d0 Cp2ML2 compounds.11 In
contrast, the angle between the midpoint of the B-H bond, the
titanium, and the midpoint of the other B-H bond is 81°, exactly
in the range of L-M-L angles typically observed for d2 Cp2-
ML2 compounds.11 These data are consistent with a d2
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(5) A side-bound BH4 complex identified by X-ray diffraction was
reported. The IR frequencies for the B-H bonds in this compound are much
higher than those reported for complexes 1a and 1b. Jensen, J. A.; Wilson,
S. R.; Girolami, G. S. J. Am. Chem. Soc. 1988, 110, 4977.
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(10) In contrast to the resonances of the metal-bound carbon in many
transition metal alkyl groups being located upfield of alkanes, the boron
resonances of metal boryl groups are significantly downfield of those for
the corresponding boranes.
(17) Ting, C.; Messerle, L. J. Am. Chem. Soc. 1989, 111, 3449.
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S0002-7863(96)02086-0 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 44, 1996 10937
Scheme 1
Figure 2. Two dominant orbital interactions in 1a and 1b and a
localized bond rationalization for the planar geometry at boron.
contrast, 1a and 1b can be described as as Ti(II) complexes
containing two neutral coordinated boranes.20 This description
would predict some M-B bond character, a decrease in the
B-H bond order, and facile dissociation of borane. Although
1a and 1b surely contain characteristics of both structural
extremes, the spectroscopic and reactivity data for 1a and 1b
suggest that Ti(II) is a more appropriate description.
titanocene complex with coordination of the B-H bond in
neutral catecholborane. The B-H distance was 1.268 Å, while
the Ti-H distance was 1.779 Å, typical for bridging hydrides.20
The geometry at boron is distinct from a Lewis acid/base
complex. Interestingly, the boron lay in a plane containing the
Ti and the two oxygens, with the hydride lying above this plane.
The reaction chemistry (Scheme 1) of 1a and 1b was dom-
inated by borane displacement and additions to C-C and C-N
multiple bonds. For example, 2 atm of CO displaced HBcat
Core potential ab initio MP2 calculations26 on the model
system Cp2Ti(HB(OH)2)2 produced an optimized geometry that
was very close to the experimental results. Important for stru-
ctural confirmation, the computed positions of the bridging
hydrogens were close to those determined by X-ray diffraction.
Further, the calculations confirmed that the boron atom is not
tetrahedral. It lies in the plane defined by the titanium and two
oxygens. This planarity was also observed after optimization
of Cp2Ti(HBH2)2 without the catecholate oxygens.27 An extend-
ed Hu¨ckel calculation of Cp2Ti(HB(OH)2)2 yields the MO dia-
gram in Figure 2.11,28,29 There is good overlap between the
HOMO and LUMO of both fragments, with one interaction
involving donation of a B-H σ-bond and the other back-
donation into the boron p-orbital. The planarity at boron is dif-
ficult to explain qualitatively. However, the MO analysis shows
that such a geometry does maximize back-donation from Ti to
B. This structure can also be generated formally by protonation
21
after 1 d at -10 °C. Cp2Ti(CO)2 was formed in 88% yield
and HBcat in 93% yield, as determined by 1H NMR integration
of the products against an internal standard. Reaction of tri-
ethylamine with a slurry of 1a in C6D6 gave amine-ligated cat-
echolborane (Et3NB(H)cat) in 70% yield by 1H NMR spectros-
copy. The doublet in the 11B NMR spectrum located upfield
of catecholborane identified Et3NB(H)cat, along with its ind-
pendent synthesis by addition of Et3N to catecholborane. The
titanium products of this reaction were paramagnetic and have
not yet been firmly identified.
1a and 1b underwent interconversion upon addition of free
borane. For example, reaction between 4-methylcatecholborane
and a slurry of 1a in toluene-d8 at -35 °C generated 1b, while
addition of 20 equiv of catecholborane to a toluene solution of
1b at -35 °C deposited 1a and formed free 4-methylcatechobo-
rane in 98% yield by 1H NMR spectroscopy. Reaction between
10 equiv of DB(O2C6H3-4-Me) and 1b at -10 °C led to
exchange of deuterium into the hydride position, as determined
2-
of Cp2Ti(Bcat)2 (with planar boron and π-bonding), while
keeping in mind that the proton and π-bond draws analogies to
a nonclassical ethyl cation that has relatively planar carbons.30
Acknowledgment. We are grateful for support from the National
Science Foundation (CHE 1921109 and NYI Award program), the
Dreyfus Foundation for a New Faculty Award, DuPont for a Young
Faculty Award, Union Carbide for an Innovative Recognition Award,
and Yale University for a Junior Faculty Fellowship. J.F.H. is a fellow
of the Alfred P. Sloan Foundation. F.M. thanks the CNRS for an
associate researcher position. R.B. thanks the Spanish Ministerio de
Educacion y Cultura for a postdoctoral fellowship.
2
by H NMR spectroscopy of the reaction mixture at -10 °C.
Compound 1a also transferred boranes to alkynes and imines.
Addition of a benzene-d6 solution of p-tolylacetylene to solid
1a led to the formation of trans-(p-tolyl)CHdCHBcat22 in 71%
yield. Addition of PhCCPh at -35 °C gave the hydroboration
Supporting Information Available: Spectroscopic data for 1a,b
and X-ray data for 1a (19 pages). See any current masthead page for
ordering and Internet access instructions.
product in 78% yield and Cp2Ti(C(Ph)dC(Ph)C(Ph)dC(Ph))
in 45% yield. Reaction of N-methylbenzylidene imine with a
slurry of 1a in C6D6 at room temperature gave the hydroborated
imine in 74% yield, which was identified by comparison of 1H
and 11B NMR spectra of reaction solutions to 1H and 11B NMR
spectra of the product amidoborane generated by direct hy-
droboration of N-methylbenzylidene imine and by addition of
HNMeBz to catecholborane at 80 °C. These alkyne and imine
hydroborations could occur by either direct reaction with 1a or
by borane dissociation and titanocene-catalyzed addition.
Compounds 1a and 1b can be described as Ti(IV) complexes
of B2H2cat22-, which is analogous to B2H62-, and 1a and 1b
would be the first monomeric examples of a B2H62--type
compound.23,24 However, the 11B NMR signal of such a
complex should be upfield of catecholborane, the hydride would
be in a region characteristic of Ti(IV) complexes, and the
infrared vibration would be near those of d0 borohydrides.25 In
JA962086V
(23) A stoichiometrically related mononuclear complex is Cp2Mo(H)-
(B2H5): Grebenik, P. D.; Green, M. L. H.; Kelland, M. A.; Leach, J. B.;
Mountford, P.; Stringer, G.; Walker, N. M.; Wong, L.-L. J. Chem. Soc.,
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Chem. Soc. 1996, 118, 4830.
(25) Marks, T. J.; Kolb, J. R. Chem. ReV. 1977, 77, 263.
(26) The calculations were performed with the Gaussian 92 package.The
pseudopotential and basis sets for Ti and bridging H, B, and O are those of
LANL2DZ. Polarization functions were added to B, O, and the bridging
hydride. The carbon and H of the cyclopentadienyl ring have been calculated
with a minimal STO-3G basis set. Optimization was carried out at the MP2
level. Gaussian 92, Revision A; Frisch, M. J.; Trucks, G. W.; Head-Gordon,
M.; Gill, P. M. W.; Wong, M.W.; Foresman, J. B.; Johnson, B. G.; Schlegel,
H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.;
Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.;
Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian Inc.
Pittsburgh, PA, 1992.
(20) Spaltenstein, E.; Palma, P.; Kreutzer, K. A.; Willoughby, C. A.;
Davis, W. M.; Buchwald, S. L. J. Am. Chem. Soc. 1994, 116, 10308.
(21) Sikora, D. J.; Moriarty, K. J.; Rausch, M. D. In Inorganic Syntheses;
Angelici, R. J., Ed.; John Wiley and Sons, Inc.: New York, 1990; Vol. 28,
pp 248.
(27) See also: Demachy, I.; Esteruelas, M. A.; Jean, Y.; Lledos, A.;
Maseras, F.; Oro, L. A.; Valero, C.; Volatron, F. J. Am. Chem. Soc. 1996,
118, 8388.
(28) Anderson, A. B.; Hoffmann, R. J. Chem. Phys. 1974, 60, 4271.
(29) Hoffmann, R. J. Chem. Phys. 1963, 39, 1397.
(22) He, X.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 1696.
(30) Lammertsma, K.; Ohwada, T. J. Am. Chem. Soc. 1996, 118, 7247.