Paramagnetic Titanium(III) and Zirconium(III) Metallocene complexes as precatalysts for the dehydrocoupling/dehydrogenation of Amine-Boranes

Article abstract of DOI:10.1002/anie.201207903

Complexes of Group 4 metallocenes in the +3 oxidation state and amidoborane or phosphidoborane function as efficient precatalysts for the dehydrocoupling/dehydrogenation of amine-boranes, such as Me ₂NH·BH₃ (see scheme). Such Ti

Full text of DOI:10.1002/anie.201207903

Angewandte
Chemie
DOI: 10.1002/anie.201207903
Dehydrocoupling
Paramagnetic Titanium(III) and Zirconium(III) Metallocene
Complexes as Precatalysts for the Dehydrocoupling/Dehydrogenation
of Amine–Boranes**
Holger Helten, Barnali Dutta, James R. Vance, Matthew E. Sloan, Mairi F. Haddow,
Stephen Sproules, David Collison, George R. Whittell, Guy C. Lloyd-Jones, and Ian Manners*
Catalytic dehydrocoupling/dehydrogenation of amine–
borane adducts has become a field of rapid growth over the
past decade.[1] This development has been driven by potential
applications with respect to hydrogen storage,[1,2] transfer
hydrogenations and reductions of organic substrates,[3] and
Scheme 1. Titanocene-catalyzed dehydrogenation of 1.
the preparation of new inorganic polymeric and solid-state
materials.[4,5] A wide variety of catalyst systems have been
developed that promote this reaction,[6–10] most of which are
based on complexes of second- and third-row late transition
metals. Mechanistic interest in these transformations has also
led to the emergence of an interesting new area of coordi-
nation chemistry associated with amine–borane and amino-
borane ligands.[11–13] Although much less developed, the
catalytic dehydrogenation of phosphine–boranes appears to
offer similar potential.[14]
based precatalysts studied to date were found to be far more
active than their Zr-based congeners.
Mechanistic proposals for the titanocene-catalyzed dehy-
drogenation of 1 have differed in many key details, but in all
cases, diamagnetic TiII or TiIV precatalysts and intermediates
have been invoked. For example, based on DFT calculations,
Luo and Ohno suggested that interaction of 1 with [Cp2Ti]
IV
À
leads to N H bond activation and formation of the Ti
intermediate 3 (Scheme 2). Subsequent hydride transfer
We have previously reported that the first-row Group 4
metallocene [Cp2Ti], generated in situ from [Cp2TiCl2] and
nBuLi (2 equiv) or, alternatively, from the isolable TiII
precatalyst [Cp2Ti(PMe3)2], functions as an efficient homoge-
neous catalyst for the dehydrogenation of secondary amine–
borane adduct 1 to give the cyclodiborazane 2 (Scheme 1).[7]
Chirik[8] and, more recently, Rosenthal[9] and co-workers have
described analogous studies of a series of active TiII and ZrII
precatalysts for the dehydrocoupling of 1. In general, the Ti-
Scheme 2. Proposed TiIV (3), and isolated ZrIV (6, 7) and TiIII (8)
amidoborane species.
from boron to the metal was proposed to give the monomeric
IV
=
aminoborane Me2N BH2 (4) and the Ti species [Cp2TiH2].
The former was proposed to dimerize to 2 in an off-metal
process, while the latter was expected to release hydrogen to
re-form [Cp2Ti].[15] Chirik and co-workers suggested a similar
[*] Dr. H. Helten,[$] [+] Dr. B. Dutta,[+] J. R. Vance, Dr. M. E. Sloan,
Dr. M. F. Haddow, Dr. G. R. Whittell, Prof. G. C. Lloyd-Jones,
Prof. I. Manners
À
mechanism, except that B H bond activation was involved in
School of Chemistry, University of Bristol
Cantock’s Close, Bristol BS8 1TS (UK)
E-mail: ian.manners@bristol.ac.uk
the first step.[8] In contrast, on the basis of detailed kinetic
studies, Manners, Lloyd-Jones, and co-workers proposed
a two-stage catalytic cycle, with initial formation of the
detected linear diborazane Me2NH-BH2-NMe2-BH3 (5) after
Dr. S. Sproules, Prof. D. Collison
School of Chemistry and Photon Science Institute
The University of Manchester
À
an N H bond activation step, and subsequent on-metal ring-
Oxford Road, Manchester M13 9PL (UK)
closing dehydrogenation to give 2. Once again, the proposed
catalytic cycle was postulated to involve the interplay of Ti
intermediates in the + 2 and + 4 oxidation states.[7b]
[$] Current address: Institute of Inorganic Chemistry
RWTH Aachen
Landoltweg 1, 52056 Aachen (Germany)
[+] These authors contributed equally to this work.
Studies on model compounds are expected to provide
further insight into the mechanism of these dehydrogenation
reactions catalyzed by Group 4 metallocenes. Roesler and co-
workers reported a zirconocene(IV) amidoborane complex 6
(Scheme 2), synthesized by reaction of [Cp2ZrCl2] with
H3N·BH3 (2 equiv) in the presence of nBuLi.[12] Complex 6,
an analogue of 3, is formally the product of oxidative addition
[**] This work was supported by the EPSRC, including the UK National
EPR Service at The University of Manchester. H. Helten and B. Dutta
thank the German Research Foundation (DFG) and the Swiss
National Science Foundation (SNSF), respectively, for postdoctoral
fellowships. GCLJ is the recipient of a Royal Society Wolfson
Research Merit Award.
À
of the N H bond of ammonia–borane to zirconocene. Very
recently, a B-disubstituted analogue of 6, complex [Cp2Zr(H)-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207903.
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{NH2B(C6F5)2H}] (7), has been reported by Lancaster and co-
workers.[11g] Interestingly, Wolstenholme, McGrady, and co-
workers have isolated a paramagnetic TiIII amidoborane
complex (8) from the reaction of [Cp2TiCl2] with Li[NH2BH3]
(2 equiv).[13] The latter result suggested that the presence of
an MIII species under the catalytic dehydrogenation condi-
tions should not be dismissed.
Herein, we report our preliminary studies of the catalytic
behavior of a series of Group 4 metallocene complexes with
ligands derived from either amine–boranes or the related
phosphine–boranes with Ti and Zr in the + 3 and + 4
oxidation state. Our results suggest that paramagnetic MIII
species may play a key catalytic role in the dehydrogenation
chemistry.
We initially focused on the isolation and characterization
of products formed in the stoichiometric version of the
catalytic dehydrogenation reaction of 1 (Scheme 1) involving
the generation of [Cp2Ti] in situ. We postulated that the use of
a phosphine–borane as substrate might facilitate the isolation
of products relevant to prospective reaction intermediates, as
phosphine–boranes do not undergo dehydrogenation under
these conditions. Indeed, in situ generation of titanocene by
the reaction of [Cp2TiCl2] with nBuLi (2 equiv) in toluene at
À158C, followed by addition of Ph2PH·BH3 (9) at 228C led to
the isolation of a green solid, which was identified as the
titanocene(III) phosphidoborane complex 10 (63% yield;
Scheme 3). The same product was obtained by the reaction of
[Cp2TiCl] with Li(PPh2BH3) in THF at À788C.
Figure 1. Molecular structure of TiIII complex 10 in the solid state (only
one of two independent molecules shown; C-bonded hydrogen atoms
omitted for clarity).
Figure 2. Molecular structure of TiIII complex 11 in the solid state (C-
In contrast, attempts to isolate a clean product from the
stoichiometric reaction of in situ generated [Cp2Ti] with
Me2NH·BH3 were not successful, a result that is not surprising
with regard to the efficiency of the analogous catalytic
reaction to form 2 (Scheme 1). However, the TiIII complex 11,
an amidoborane analogue of 10, was obtained from the
reaction of [Cp2TiCl2] with Li[NMe2BH3] (2 equiv) in THF at
À788C (Scheme 3). Complex 11 was isolated as a blue solid in
65% yield, and the aminoborane by-products, primarily 2,
were removed by sublimation.
The molecular structures of complexes 10 and 11 in the
solid state were determined by single-crystal X-ray diffraction
(Figure 1 and 2). Similar to 6–8, complexes 10 and 11 feature
a planar four-membered Ti-E-B-H ring system involving the
metal, the bridging hydrogen atom, the boron atom, and the
Group 15 atom (E = P or N).
bonded hydrogen atoms omitted for clarity).
signals in C6D6 were detected at d = À36 (full width at half
maximum (FWHM) = 400 Hz) for 10 and d = À48 (FWHM =
2200 Hz) for 11, respectively (cf. 8: dB = À49, broad signal[13]).
EPR measurements of 10 and 11 in toluene provided giso
values of 1.9923 and 1.9899, respectively. The hyperfine
splitting pattern showed coupling to a single H nucleus
(Aiso{1H} = 4.2 ꢀ 10À4 cmÀ1 in 10; 4.6 ꢀ 10À4 cmÀ1 in 11), thus
confirming that the solid-state structure persists in toluene
solution. The spectrum of 10 is dominated by the P hyperfine
splitting (Aiso{31P} = 10.6 ꢀ 10À4 cmÀ1), whereas 14N coupling
remains unobserved even at S-band frequency. For compar-
ison, we also recorded the EPR spectrum of complex 8. This
spectrum showed a giso value of 1.9917 and hyperfine coupling
to the bridging H (4.6 ꢀ 10À4 cmÀ1), consistent with structural
similarity to complexes 10 and 11 (see Table S1). IR spectra of
8, 10, and 11 (in THF) displayed bands characteristic of
vibrations of terminal and bridging BH bonds at 2310–2430
and 1810–1820 cmÀ1, respectively. The UV/Vis spectra (in
THF) showed a single broad low-energy absorption band
(lmax = 585 (8), 596 (10), and 577 nm (11); Figure S8, S2, S4),
assigned to excitation of the unpaired d electron. When the
samples were exposed to air, a color change from blue (8 and
11) or green (10) to yellow was observed, and the long-
wavelength absorption disappeared (see Figure S9).
Because of their paramagnetic nature, complexes 10 and
11 showed no discernible 1H, 13C, and 31P NMR resonances in
solution, even at À808C. However, broad 11B{1H} NMR
In an analogous reaction of [Cp2ZrCl2] with nBuLi
(2 equiv) and 9 at À788C in THF, a brown solid was obtained
after work-up and identified as the ZrIII phosphidoborane
complex 12 (69% yield; Scheme 4). As with 10 and 11, there
Scheme 3. Synthesis of TiIII phosphidoborane complex 10 and TiIII
amidoborane complex. By-products have been proposed based on
reaction stoichiometry.[16]
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Table 1: Turnover frequencies (TOF) [hÀ1] and conversion after 2 h for
catalytic dehydrocoupling of 1 to give 2.[a]
10
11
12
13
[Cp2Ti][b]
TiIII
0.13
83%
TiIII
10.7
97%
ZrIII
0.07
1.5%
ZrIV
0.06
0
TOF
11.1
88%
Conversion[c]
[a] 1 (1.3m) in toluene, 208C, precatalyst (2 mol%, 0.027m). [b] Gen-
erated in situ from [Cp2TiCl2]/2nBuLi. [c] Conversion of 1 after 2 h,
measured by 11B{1H} NMR spectroscopy.
Scheme 4. Synthesis of ZrIII complex 12 and ZrIV complex 13.
were no discernible 1H, 13C, and 31P NMR signals, but a broad
11B{1H} NMR resonance was detected for 12 (d = À34;
FWHM = 471 Hz). From EPR measurements, a giso value of
1.9884 was obtained and coupling with P and H was observed
(Aiso{31P} = 25.9 ꢀ 10À4 cmÀ1, Aiso{1H} = 15.5 ꢀ 10À4 cmÀ1). Fur-
ther characterization was achieved by IR and UV/Vis
spectroscopy, which gave analogous results to 8, 10, and 11.
In contrast to the aforementioned reactions that afforded
MIII complexes 10–12, the reaction of [Cp2ZrCl2] with Li-
[NMe2BH3] (2 equiv) in THF at À788C afforded the off-
white, diamagnetic zirconocene(IV) amidoborane complex 13
in 69% yield of isolated product (Scheme 4). Complex 13 was
characterized by multinuclear NMR, IR, and UV/Vis spec-
in situ from [Cp2TiCl2] and nBuLi (2 equiv). The UV/Vis
spectra of both reaction mixtures showed broad peaks at circa
560 nm, which is similar to the spectra of solutions of the
isolated TiIII complex 11, within experimental error (see
above). Moreover, EPR spectra that were obtained from both
reaction solutions (Figure 4) were virtually identical to that of
the pure complex 11 (compare with Figure S26). These results
are consistent with the presence of the TiIII species 11 in both
reaction mixtures.
troscopy, and
a single-crystal X-ray diffraction study
1
(Figure 3). The NMR spectra of 13 showed one set of H,
11B, and 13C NMR signals both at ambient temperature and at
À808C, irrespective of the solvent employed, consistent with
the formation of a single isomer.
Figure 4. X-band EPR spectrum (toluene, 200 K) recorded in situ from
the reaction solution of the dehydrocoupling of 1 catalyzed by a) 11
(2 mol%) and b) [Cp2Ti] (2 mol%). For experimental conditions, see
Figure S32 in the Supporting Information.
Figure 3. Molecular structure of ZrIV complex 13 in the solid state (C-
bonded hydrogen atoms omitted for clarity).
The catalytic activity of the new complexes 10–13 was
investigated with respect to the dehydrocoupling/dehydro-
genation of 1 in toluene at 208C at a 2 mol% precatalyst
loading, conditions analogous to those previously used for
[Cp2Ti]-catalyzed reactions[7] (compare with Scheme 1). In
each case, reaction monitoring by 11B NMR spectroscopy
indicated the formation of linear diborazane 5 as an
intermediate, followed by formation of 2 as the final product.
The TiIII complexes 10 and especially 11 were found to be
highly active precatalysts. Significantly, the activity of 11
toward 1 was comparable to that of in situ generated [Cp2Ti]
(see Table 1). Furthermore, UV/Vis and EPR spectra were
recorded on the reaction solutions derived from 1 and
2 mol% of either complex 11 or [Cp2Ti], the latter generated
The ZrIII complex 12 also functioned as a precatalyst for
the dehydrocoupling of 1, but was much less active than 10 or
11 (Table 1), and the catalytic efficiency decreased at longer
reaction times (Figure S16). By comparison, the ZrIV complex
13 showed only very poor activity for the dehydrocoupling of
1. Thus, while conversions of 83% and 97% for 1 were
observed after 2 h with 10 and 11, respectively, negligible
conversion was detected with 13 within the same period
(Table 1).
In summary, paramagnetic TiIII species, such as 11, can be
isolated from stoichiometric reactions that are analogous to
the previously reported catalytic conditions for amine–borane
dehydrocoupling using in situ generated [Cp2Ti], for which
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only TiII and TiIV precatalysts and intermediates had been
previously invoked. Moreover, the isolated TiIII complexes
show activities as precatalysts toward 1 that are similar to
those for the latter system.[17] Further studies also showed that
UV/Vis absorption bands and EPR spectra characteristic of
the d1 TiIII species 11 could be detected in the in situ based
[Cp2Ti] reaction mixtures involving 1. These results indicate
that paramagnetic TiIII species may play a key catalytic role in
the dehydrogenation chemistry. Our results for isolated
model complexes are also consistent with the previous
general observations that zirconocene-based precatalysts are
less active than those based on Ti. Reasons for this difference
will be explored in detail in our future work.
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Received: October 1, 2012
Published online: November 29, 2012
Keywords: amine–boranes · dehydrogenation · metallocenes ·
.
phosphine–boranes · transition metals
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