Dinuclear cationic zirconium hydrides stabilized by the N,N-dibenzylcyclam ancillary ligand

Article abstract of DOI:10.1021/om300504q

The reaction of [Bn₂Cyclam]ZrCl₂ (Bn = benzyl) with 2 equiv of KHBEt₃ generates the ionic dinuclear zirconium derivative [([Bn₂Cyclam]Zr)₂(μ-H)₃][(Et ₃B)₂H]. An analogous trihydride is obtained when [Bn ₂Cyclam]Zr(CH₂Ph)₂ reacts with BEt₃ in the presence of dihydrogen, whereas in the absence of BEt₃, the bis(ortho-metalated) derivative [(C₆H₄CH₂) ₂Cyclam]Zr is obtained. DFT calculations support the stability of a cationic dinuclear trihydride complex in comparison to the neutral dinuclear tetrahydride.

Full text of DOI:10.1021/om300504q

Communication
pubs.acs.org/Organometallics
Dinuclear Cationic Zirconium Hydrides Stabilized by the N,N-
Dibenzylcyclam Ancillary Ligand
Rui F. Munha,†,‡ Joachim Ballmann,‡ Luis F. Veiros,† Brian O. Patrick,‡ Michael D. Fryzuk,*,‡
́
and Ana M. Martins*,†
†Centro de Química Estrutural, Instituto Superior Tec
Portugal
́ ́
nico, Universidade Tecnica de Lisboa, Av. Rovisco Pais No. 1, 1049-001 Lisboa,
‡Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1
S
* Supporting Information
ABSTRACT: The reaction of [Bn2Cyclam]ZrCl2 (Bn =
benzyl) with 2 equiv of KHBEt3 generates the ionic dinuclear
zirconium derivative [([Bn2Cyclam]Zr)2(μ-H)3][(Et3B)2H].
An analogous trihydride is obtained when [Bn2Cyclam]Zr-
(CH2Ph)2 reacts with BEt3 in the presence of dihydrogen,
whereas in the absence of BEt3, the bis(ortho-metalated)
derivative [(C6H4CH2)2Cyclam]Zr is obtained. DFT calcu-
lations support the stability of a cationic dinuclear trihydride complex in comparison to the neutral dinuclear tetrahydride.
ot only are zirconium hydride complexes important
reagents in organic synthesis1−10 but they have also been
Scheme 1
N
used in the reduction of small molecules such as CO2 and
CO;11−20 in addition, certain zirconium hydride species have
been found useful in the preparation of dinitrogen complexes
via dihydrogen elimination.21,22 For the vast majority of
structurally characterized zirconium hydride complexes, cyclo-
pentadienyl-type ligands are typically found to stabilize these
derivatives; in fact, the bis(cyclopentadienyl) motif dominates
this chemistry.14,23−36 For some time we have been interested
in alternative ligand environments37−42 in an attempt to expand
early-transition-metal chemistry and, in particular, to explore
the use of group 4 and 5 hydride complexes that can serve as
entry points into N2 activation.43 For this reason, we
investigated the formation of zirconium hydride derivatives
stabilized by the N,N-dibenzylcyclam (Bn2Cyclam) ancillary
ligand,44 in an effort to build on the alkyl and amido chemistry
already reported for complexes of this type.45−47 Herein we
report the synthesis and characterization of a unique dinuclear
zirconium hydride species that is unexpectedly cationic.
We have previously reported the preparation of the
zirconium(IV) N,N-dibenzylcyclam dichloride complex 1
(Scheme 1) and its further reactivity with a variety of alkyl
Grignard or lithium reagents that allows access to the
corresponding dialkyl derivatives [Bn2Cyclam]ZrR2 (R = Me,
Bu, CH2Ph).46 Unfortunately, attempts to hydrogenate these
dialkyl species directly to putative dihydride species of the
general formula ([Bn2Cyclam]ZrH2)x failed even under 4 atm
pressure of H2. This is in contrast with the many known
zirconocene dialkyl complexes and related derivatives
(Cp′2ZrR2) that have been reported to react with H2 under a
variety of conditions via a σ-bond metathesis type process to
generate hydrides.26,31,48−51 In the case of the aforementioned
cyclam-ligated zirconium dialkyl derivative species, there is no
apparent reaction with H2, only the observation of the
bis(ortho-metalated) derivative [(C6H4CH2)2Cyclam]Zr (3)46
(Scheme 1) via a competing slow alkane elimination; the rate of
Received: June 7, 2012
Published: July 6, 2012
© 2012 American Chemical Society
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formation of 3 is independent of the H2 pressure, which
strongly precludes the formation of hydride intermediates.
Interestingly, the rate of formation of 3 via thermolysis depends
on the nature of the alkyl group and is slowest for R = CH2Ph.
Hydride complexes could be accessed, however, by a
metathesis reaction of the dichloro species 1 with potassium
triethylborohydride (KBEt3H). Thus, the reaction of 2 equiv of
KBEt3H with 1 results in the clean formation of the new
dinuclear hydrido species 4-[(Et3B)2H] in 95% yield (Scheme
1). The 1H NMR spectrum of 4-[(Et3B)2H] is quite
complicated and is suggestive of low symmetry.45,46 This in
itself was strong evidence that the species formed was not a
dihydride derivative, e.g. ([Bn2Cyclam]ZrH2)x, as such species
would be expected to have C2 symmetry if mononuclear, on the
basis of previous studies.45,46 Of particular note are the hydride
signals that appear as two nearly coincident singlets at
approximately 6 ppm in d8-THF;15,52 interestingly, these two
singlets are in a 2:1 ratio and integrate to three H's per two
[Bn2Cyclam]Zr units, which suggests a dinuclear structure with
a core corresponding to Zr2(μ-H)3. The ionic nature of the
complex was also suggested by the presence of a set of signals
assigned to [(Et3B)2H]−; this included a characteristic
resonance assigned to the bridging B−H of [Et3B−H−BEt3]−
at −2.31 ppm and a 11B NMR spectrum displaying a broad
peak at approximately 12 ppm.53 As observed in other
systems,53 a compound of the formula 4-[(Et3B)2H] is in
equilibrium with 4-[Et3BH] and free BEt3. In fact, yellow X-
ray-quality crystals of 4-[Et3BH] could be grown from THF
that provided information on the structure of this latter
derivative.
cation and a triethylborohydride anion. The coordination
geometry of each seven-coordinate zirconium center is a
distorted capped trigonal prism, with three symmetrically
bridging hydrides (1.5 independent atoms) with Zr−H bond
lengths of 1.95(4) and 1.99(3) Å. The latter are comparable to
those in other zirconium hydride complexes30,54 as are the Zr−
Namine bond lengths at 2.384(3) and 2.390(3) Å and the Zr−
Namido distances at 2.059(3) and 2.062(2) Å, all within the
previously observed values for zirconium complexes incorpo-
rating this ligand set.44−46 The two macrocyclic ligands are
twisted with respect to each other, with the amine units of one
zirconium center facing the amido donors of the other metal
center (Figure 1). While cationic zirconium hydride complexes
are known, structurally characterized examples are relatively
rare.55−59
The formation of this cationic dizirconium trihydride was
surprising, as metathesis of the dichloride 1 with 2 equiv of
KBEt3H would normally be expected to generate the neutral
dihydride [Bn2Cyclam]ZrH2, on the basis of simple stoichiom-
etry.60 In an effort to understand how a cationic dinuclear
trihydride is formed, a computational analysis61 was performed
on the observed dinuclear trihydride cation and likely neutral
intermediates in the presence of BEt3. Calculations show that
the reaction of BEt3 with the putative neutral dihydride
[Bn2Cyclam]ZrH2 to generate the monohydride
([Bn2Cyclam]ZrH)(HBEt3) is uphill energy-wise by +26.9
kcal/mol. The dimerization of [Bn2Cyclam]ZrH2 to give the
dizirconium tetrahydride ([Bn2Cyclam]Zr)2(μ-H)4 is favorable
by −21.4 kcal/mol, indicating that the dinuclear species is more
stable than the mononuclear dihydride. Interestingly, the
calculations also show that BEt3 is a strong enough Lewis
acid to abstract a hydride ligand from dinuclear ([Bn2Cyclam]-
Zr)2(μ-H)4 to give [([Bn2Cyclam]Zr)2(μ-H)3][BEt3H], albeit
by a stabilization of only ΔE = −0.9 kcal/mol. Overall these
calculations show that the formation of the dinuclear trihydride
derivative is a favorable process.
The ORTEP diagram of 4-[Et3B-H] is shown in Figure 1
and shows that this species consists of a dinuclear trihydride
As already mentioned (Scheme 1), the direct reaction of the
dibenzyl complex 2 with H2 did not lead to hydride derivatives;
rather, intramolecular cyclometalation to generate 3 was
observed. On the assumption that cationic alkyl complexes
should be more reactive to H2 addition,62−64 we reexamined
the reaction of 2 with dihydrogen in the presence of Lewis
acids. Treatment of [Bn2Cyclam]Zr(CH2Ph)2 with 1 equiv of
BEt3 under H2 gave [([Bn2Cyclam]Zr)2(μ-H)3]-
[BEt4−n(CH2Ph)n] (4-[BEt(R)3]: R = Et, CH2Ph) as the
major product (Scheme 1). The NMR spectrum of 4
unequivocally shows that a cationic trihydride identical with
4-[(Et3B)2H] was isolated, although the stoichiometry of the
1
anion is ambiguous. The H NMR spectra of 4-[(Et3B)2H]
(bottom, black) and 4-[BEt(R)3] (R = Et, CH2Ph) (blue) are
depicted in Figure 2. The spectral overlay makes it easy to
identify the resonances corresponding to the BCH2Ph and BEt
moieties. However, integration of the signals in comparison to
the macrocyclic NCH2 resonances does not allow a clear
identification of the stoichiometry of the anion; it is suggested
that a mixture of anions of the general formula
[BEt4−n(CH2Ph)n]− (n = 1−3) is present on the basis of the
analysis of the aromatic region of the spectrum (insert area,
blue), where different sets of signals assigned to the ortho, meta,
and para protons of BCH2Ph are observed.
Figure 1. ORTEP (50% thermal ellipsoids) of [([Bn2Cyclam]Zr)2(μ-
H)3][Et3BH] (4-[Et3BH]). Half of the molecule is generated by the
symmetry operation −x, y, −z + /2. Not shown are the hydrogen
1
atoms, except for the zirconium hydrido ligands, the disordered
triethylborohydride anion, and cocrystallized THF, for the sake of
clarity. Selected bond lengths (Å) and angles (deg): [N4Plane]- - -
Zr(1) = 0.968(1), Zr(1)- - -Zr(1_$1) = 3.1567(5), Zr(1)−H(11) =
1.99(3), Zr(1)−H(21) = 1.95(4), Zr(1)−N(1) = 2.384(3), Zr(1)−
N(2) = 2.059(3), Zr(1)−N(3) = 2.390(3), Zr(1)−N(4) = 2.062(2);
N(1)−Zr(1)−N(3) = 140.12(10), N(2)−Zr(1)−N(4) = 113.97(11),
H(1)−Zr(1)−H(21) = 67.3(14).
A possible origin of the scrambling of the benzyl and ethyl
groups in the anion is described in Scheme 2. Reaction of BEt3
with dibenzyl 2 would likely generate the benzyl cation
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Figure 2. 400 MHz 1H NMR spectra in d8-THF of [([Bn2Cyclam]Zr)2(μ-H)3][(Et3B)2H] (4-[(Et3B)2H]) (bottom, black), and
[([Bn2Cyclam]Zr)2(μ-H)3][BEt4−n(CH2Ph)n] (4-[BEt(R)3]) (top, blue) at 25 °C. The region from 0 to −2 ppm where the signal corresponding
to the bridging borohydride of 4-[(Et3B)2H] is observed is omitted. The overlay of the two spectra allows clear identification of the benzyl and ethyl
boron environments. The insert in the top spectrum (blue) shows two sets of signals for the benzyl aryl protons in a 1:2 ratio.
stoichiometry of the anion in 4. We also examined the reaction
of 2 with B(C6F5)3 in the presence of H2. While we have
already reported46 that [[Bn2Cyclam]Zr(CH2Ph)S]+[B-
(C6F5)3CH2Ph]− (S = THF) forms in the absence of H2,
when dihydrogen is present, there is no evidence of the
formation of the dinuclear trihydride. We suggest that the
cascade of equilibria in Scheme 2 necessary to produce the
cationic trihydride 4+ is somehow interrupted or prevented by
the strongly Lewis acidic B(C6F5)3.
Scheme 2
We have demonstrated that a dinuclear zirconium trihydride
complex incorporating a dianionic cyclam-based ligand can be
prepared via a salt metathesis route using a hydride reagent or
from hydrogenolysis of a zirconium dibenzyl derivative in the
presence of BEt3. That BEt3 is essential to the latter process is
counterintuitive, at least to us, given that it is not as Lewis
acidic as B(C6F5)3; nevertheless, we suggest that it can better
participate in alkyl group and hydride transfers from boron to
zirconium, which allows the final cationic trihydride complex to
form. DFT studies show that the cation [([Bn2Cyclam]Zr)2(μ-
H)3]+ (4+) is slightly more stable than the corresponding
dinuclear neutral tetrahydride species ([Bn2Cyclam]Zr)2(μ-
H)4. To the best of our knowledge, this represents a rare
example of a non-cyclopentadienyl zirconium system where a
zirconium hydride is isolated via dihydrogen activation, but
only in the presence of BEt3. While the reaction of this
dinuclear hydride with N2 does not generate a dinitrogen
complex, studies are continuing on the mechanisms that govern
the formation of these zirconium hydrides and the reactivity
displayed by nitrogen-rich ligated complexes of this type.
[[Bn2Cyclam]Zr(CH2Ph)][BEt3(CH2Ph)] (A). In the pres-
ence of dihydrogen in a THF solution, A likely undergoes σ-
bond metathesis to generate the hydride cation B. Hydride for
alkyl exchange from the cation to the anion will result in the
formation of C, which has a B−H moiety in the anion. Further
reaction of this new alkyl cation with H2 results in the
formation of hydride cation B′, which differs from B only in the
makeup of the anion. Hydride transfer from the anion back to
the cation generates the neutral dihydride D, which is a key
species in the formation of the dinuclear trihydride cation, via a
number of different pathways, some of which are indicated in
the scheme. The conversion of B to C can occur by either an
ethyl or a benzyl group transfer by a hydride ligand, giving BEt3
or BEt2(CH2Ph), which may explain the ambiguity in the
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, tables, and CIF files giving full experimental
procedures, representative NMR spectra, details of the
preparation of 4-[BEt3H], and Cartesian coordinates. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*M.D.F.: e-mail, fryzuk@chem.ubc.ca; fax, +1 604 822-8710;
tel, +1 604 822-2471.
■
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
M.D.F. thanks NSERC of Canada for a Discovery Grant.
A.A.M. and R.F.M. thank the Fundaca
Tecnologia for funding. We thank Nathan Halcovitch for his
■
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