Heterolysis of Dihydrogen by Nucleophilic Calcium Alkyls

Article abstract of DOI:10.1002/anie.201809833

β-Diketiminato (BDI) calcium alkyl derivatives undergo hydrogenolysis with H₂ to regenerate [(BDI)CaH]₂, allowing the catalytic hydrogenation of a wide range of 1-alkenes and norbornene under very mild conditions (2 bar H₂, 298 K). The reactions are deduced to take place with the retention of the dimeric structures of the calcium hydrido-alkyl and alkyl intermediates via a well-defined sequence of Ca?H/C=C insertion and Ca?C hydrogenation events. This latter deduction is strongly supported by DFT calculations (B3PW91) performed on the 1-hexene/H₂ system, which also indicates that the hydrogenation transition states display features which discriminate them from a classical σ-bond metathesis mechanism. In particular, NBO analysis identifies a strong second order interaction between the filled α-methylene sp³ orbital of the n-hexyl chain and the σ* orbital of the H₂ molecule, signifying that the H?H bond is broken by what is effectively the nucleophilic displacement of hydride by the organic substituent.

Full text of DOI:10.1002/anie.201809833

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Accepted Article
Title: Heterolysis of Dihydrogen by Nucleophilic Calcium Alkyls
Authors: Michael Stephen Hill, Laurent Maron, Mary Mahon, Andrew
Wilson, and Chiara Dinoi
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Heterolysis of Dihydrogen by Nucleophilic Calcium Alkyls
[a]
[b]
[a]
[a]
[b]
Andrew S. S. Wilson, Chiara Dinoi, Michael S. Hill, * Mary F. Mahon and Laurent Maron *
Abstract: -Diketiminato (BDI) calcium alkyl derivatives undergo
hydrogenolysis with H
catalytic hydrogenation of a wide range of 1-alkenes and norbornene
under very mild conditions (2 bar H , 298 K). The reactions are
2 2
to regenerate [(BDI)CaH] , allowing the
2
deduced to take place with the retention of the dimeric structures of
the calcium hydrido-alkyl and alkyl intermediates via a well-defined
sequence of Ca-H/C=C insertion and Ca-C hydrogenation events.
This latter deduction is strongly supported by DFT calculations
(
B3PW91) performed on the 1-hexene/H
indicates that the hydrogenation transition states display features
which discriminate them from classical -bond metathesis
2
system, which also
a
mechanism. In particular, NBO analysis identifies a strong second
3
order interaction between the filled -methylene sp orbital of the n-
Scheme 1: Compounds 1 and 2 and Okuda’s proposed catalytic
cycle for the hydrogenation of terminal alkenes by 2.
2
hexyl chain and the σ* orbital of the H molecule, signifying that the
H-H bond is broken by what is effectively the nucleophilic
displacement of hydride by the organic substituent.
Like the aforementioned organolanthanides, the calcium center
of 1 possesses a formal d configuration and was found to engage
0
in polarized insertion reactivity with a variety of unsaturated
The catalytic hydrogenation of alkenes and alkynes by mid/late
transition metal complexes has been developed to a high degree
of sophistication and is typically predicated on the cooperative
substrates.[18,19] Although compound
1
helped stimulate
widespread interest in the isolation of heavy s-block hydrides, its
reactivity with C=C bonds was limited to the conjugated and more
activated alkenes, 1,3-cyclohexadiene,[19] myrcene, and 1,1-
diphenylethene.[20] Addition of hydrogen (20 bar, 60C) to THF
solutions of the resultant organometallic compounds also
completely regenerated the parent hydride (1), albeit
incorporation of this hydrogenation reactivity into a catalytic
regime was similarly limited to these activated alkenes. While
binding and activation of both unsaturated hydrocarbons and H
2
at a redox-active metal center.[1-9] Albeit less well developed,
similar catalysis was achieved by group 3 and organolanthanide
complexes more than a quarter of a century ago.[10-14] In these
0
latter cases, a d electron configuration mitigates against redox-
based reactivity and turnover is derived from alkene insertion into
a polarised metal-hydrogen bond to form an alkyl intermediate,
related hydrogenation catalysis has also been extended to the
which is itself reactive toward H
2
through -bond metathesis to
[21]
reduction of more polarized imines,
the calcium-catalyzed
release the hydrocarbon and regenerate the active hydride
catalyst.
hydrogenation of unactivated terminal alkenes has only been
described very recently. Okuda and co-workers have reported
that the catalytic hydrogenation of linear alkenes, including 1-
hexene and 1-octene, can be mediated by the dicationic calcium
Reports of analogous catalysis by main group (s- or p-block)
compounds are more recent in origin and largely restricted to
conjugated C=C and CC bonded substrates.[15,16] Reactivity is
again, however, dependent on the intermediacy of highly reactive
hydride derivatives. While these species may be formed by
frustrated Lewis pair (FLP)-derived heterolysis of the dihydrogen
H-H  bond,[17] a seminal report in s-block chemistry was provided
by Harder’s molecular -diketiminato calcium hydride complex,
hydride [Ca
2
H
2
(Me
4
TACD)
2
][BAr
4
]
2
(Me
4
TACD
=
1,4,7,10-
t
tetramethyl-1,4,7,10-tetraazacyclododecane; Ar = C
6
H
4
-4- Bu)
(
2) in THF. The catalysis was proposed to proceed via linear
calcium -n-alkyls that either rapidly cleave dihydrogen or
themselves undergo β-hydride elimination to reform the active
calcium hydride species (Scheme 1).[22] The necessary calcium
n-alkyl intermediates could only be inferred, however, and the
nature of the hydrogenolysis step was consequently unamenable
to direct study.
[
2 2 6 3 2
(BDI)CaH(THF)] (BDI = HC{(Me)CN-2,6-i-Pr C H } ; 1).
[
[
a]
b]
Andrew S. S. Wilson, Dr Mary F. Mahon and Prof. Michael S. Hill
Department of Chemistry
University of Bath
Bath, BA2 7AY, UK.
E-mail: msh27@bath.ac.uk
Prof. Laurent Maron, Dr Chiara Dinoi
Université de Toulouse et CNRS, INSA, UPS, UMR 5215, LPCNO,
135 Avenue de Rangueil,
F-31077 Toulouse, France
E-mail: laurent.maron@irsamc.ups-tlse.fr
Scheme 2: Synthesis of compounds 4 – 9.
Supporting information for this article is given via a link at the end of
the document and in CCDC 1857821.
In contrast, we have recently reported that a non-THF solvated
analogue of compound 1, compound 3, reacts readily with the
terminal alkenes, ethene, 1-butene and 1-hexene to provide the
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COMMUNICATION
corresponding calcium ethyl, n-butyl and n-hexyl derivatives (7 –
9
was observed to ensue in a stepwise fashion through the
intermediacy of mixed hydride-alkyl species (4 – 6). Although 7 –
process, in which 6 is formed but more rapidly converted to the
hydride (3). After 48 hours at room temperature, the complete
conversion of 9 to 3 and the quantitative formation of n-hexane
were confirmed by the emergence of the BDI methine and hydride
resonances of 3 (δ 4.83 and 4.27 ppm) and the triplet of n-hexane
(δ 0.89 ppm), which displayed a 1:1:6 ratio of intensities in the
resultant 1H NMR spectrum. Although slower due to a likely
, Scheme 2).[23] The formation of the dimeric compounds 7 - 9
9
were stable in aliphatic solvents, solutions in benzene at even
mildly elevated temperatures (ca. 50C) were found effect the
unprecedented nucleophilic alkylation of a C-H bond of the arene
solvent. In this contribution, we further examine the reactivity of
these heteroleptic calcium alkyls with dihydrogen and report our
observations with regard to the activity of compound 3 for the
catalytic hydrogenation of alkenes.
primary kinetic isotope effect, analogous addition of D
solution of 9 similarly afforded 3-d and CH D(CH CH
3
2
to a C
, which
6 6
D
2
2
2
)
3
could be clearly detected by the emergence of its BDI methine
1
signal at δ 4.83 ppm in the resultant H NMR spectrum and by the
relevant deuterated Ca-D and CH
and 0.85 ppm) in the corresponding H NMR spectrum.
2
D(CH
2
)
3
CH
3
signals (δ 4.30
2
Table 1: Catalytic hydrogenation of alkenes with 3 (10 mol%, 25C,
6 6
C D ).
An initial catalytic reaction mediated by 10 mol% of 3 with 1-
6 6
hexene under a dihydrogen atmosphere (2 bar) in C D provided
t
%
Conv.
TOF
h
Entry
1
Substrate
Product
a
-1
days
slow turnover at room temperature to hexane over the duration of
three weeks as confirmed by the emergence of the diagnostic
21
21
99
0.02
0.02
1
triplet signal of hexane at δ 0.89 in the resultant H NMR spectrum.
Encouraged by this successful reactivity, the catalytic
hydrogenation of a range of alkenes by 3 was investigated. The
results of this study are summarized in Table 1.
2
99
All substrates proceeded with good to excellent conversions (ca.
7
7
65
65
99
99
0.04
0.04
0.03
0.03
3
4
5
≥
65%) at room temperature, albeit with similar turnover
-1
frequencies (0.02 – 0.04 h ) and prolonged reactions times. This
latter observation is interpreted to indicate that the slow rate of
catalysis is a result of the limited solubility and availability of
1
4
4
1
6
7
hydrogen in C
rather than any variation in the nature of the substrates. At
elevated temperatures, hydrogenation catalysis by was
6 6
D under the conditions of the experiments (2 bar)
21
21
99
99
0.02
0.02
3
8
suppressed by facile redistribution of the ligand system to the
catalytically inactive homoleptic β-diketiminate species,
[
(BDI)
2
Ca].[24,25] Consistent with the lower conversions, alongside
the desired alkanes, unidentified products were observed in the
1H NMR spectra throughout the hydrogenation of
vinyltrimethylsilane, triphenyl(vinyl)silane and norbornene
95 b
9
14
21
0.03
(
Entries 3, 4 and 11). Notably, a significant quantity of the cyclized
alkane, methylcyclopentane (56%) was observed to form during
the hydrogenation of 1,5-hexadiene (Entry 9), which suggested
the intramolecular cyclization is competitive with hydrogenation.
In contrast, markedly less methylcyclopentane (4 %) was formed
90 c
1
0
1
0.02
0.02
14
69
1
during Okuda’s hydrogenation of 1,5-hexadiene catalyzed by
2 [
.
22]
To provide further insight into this observation, three
a
1
Determined by H NMR spectroscopy by integration against C
6
Me
6
equivalents of 1,5-hexadiene were added to a C D solution of
6 6
b
c
as an internal standard; ca. 5% 2-hexene also observed; ca. 10%
-octene also observed.
compound 3. Monitoring of the reaction at room temperature for
24 hours revealed the production of a mixture of unidentified
products in addition to a dicalcium cyclopentylmethyl-hydride
2
An atmosphere of dihydrogen (2 bar) was added to a C
6
D
6
species
(10)
and
the
corresponding
dicalcium-
solution of the calcium n-hexyl derivative (9). Immediate analysis
of the resultant 1H NMR spectrum confirmed that the solution
comprised primarily a mixture of unreacted 9 (δ 4.69 ppm) and
hydrogen (δ 4.46 ppm). Small quantities of the hydride (3) and
the dicalcium n-hexyl-hydride (6), however, were also detected
by their respective BDI methine resonances at δ 4.83 and 4.77
ppm. After storage at room temperature for 24 hours, the reaction
was observed to contain a mixture of 3 and 9, whilst no significant
increase in the intensity of the signals associated with the
dicalcium hexyl-hydride intermediate (6) was observed. The
apparent steady-state concentration of the dicalcium hexyl-
hydride suggested hydrogenation was occurring via a two-step
dicyclopentylmethyl derivative (11), identified by the emergence
of BDI methine and upfield methylene doublet signals with
relative intensities of 1:2 at δ 4.72 and 0.55 ppm, respectively.
Despite the mixture of products formed, single crystals deposited
from the reaction mixture and the resultant X-ray diffraction
analysis allowed the structure of 11 to be confirmed as the result
of facile carbocalciation and 5-exo-trig cyclization (Figure 1).
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chain walking mechanism involving the β-hydride elimination
reaction of 9, 2,1-insertion of a molecule of 1-butene into the
reformed Ca-H bond and a subsequent β-hydride elimination
reaction.
Figure 1: ORTEP representation of compound 11 with thermal
ellipsoids at 25% probability. Iso-propyl groups, and hydrogen atoms
except those attached to C30 – C32 have been removed for clarity.
Selected bond lengths (Å) angles (): Ca1-N1 2.3541(12), Ca1-N2
Scheme 3: Proposed mechanism for catalytic hydrogenation of
alkenes with 3.
On the basis of these observations, the mechanism shown in
Scheme 3 may be invoked for the catalytic hydrogenation of
alkenes with compound 3. The mechanism is again predicated
upon a cascade of polarized insertion reactions in conjunction
with irreversible hydrogenolysis and occurs with the maintenance
2.3391(12), Ca1-C30 2.5164(18), Ca1-C30' 2.5759(18), Ca1∙∙∙C31'
2.915(2), N2-Ca1-N1 83.16(4), Ca1-C30-Ca1' 82.03(5); Primed
labelled atoms related to those in the asymmetric unit by the 3/2 – x,
/2 – y, 1 – z symmetry operator.
3
The hydrogenation of the activated alkenes, 1,1-diphenylethylene, of dimeric calcium species throughout the catalysis. The
styrene and α-methylstyrene catalyzed by compound 1 was
reported to occur over a shorter timeframe (17-25 hours) than the
corresponding results shown in Table 1. This previous study,
however, was performed with a greater pressure of dihydrogen
stoichiometric reactivity studies described above indicate that
insertion of alkenes occurs in a stepwise fashion, whereupon the
resultant dicalcium alkyl-hydride intermediates are produced in
an effective steady state throughout the course of catalysis.
Although these latter species have the potential to undergo a
second alkene insertion, we suggest that turnover is largely
based on their interception and hydrogenolysis by H2.
(
20 bar) and at an elevated temperature (60 °C) and provided
significantly lower selectivity for conversion to the corresponding
alkanes, 1,1-diphenylethane (49%), ethylbenzene (99% but 19%
oligomers) and isopropylbenzene (60%).[20] Okuda’s recent study
Further insight into this proposed mechanism was provided by
density functional theory (DFT) calculations carried out with the
same computational approach (B3PW91, see SI) previously used
to describe the stepwise formation of the dicalcium di-n-hexyl
derivative (9), through the reaction of compound 3 with 1-
hexene.23 The first 1-hexene insertion with compound 3 was
described in our previous report and the associated barrier
yielding the dicalcium n-hexyl-hydride complex C (6) is 20.1 kcal
of catalytic alkene hydrogenation with the cationic calcium
t
hydride, [Ca
2
H
2
(Me
4
TACD)
2
][B(C
6
H
4
-4- Bu)
4
]
2
(2) significantly
expanded this substrate scope to include less activated terminal
alkenes.[ Hydrogenation was also observed to ensue at a faster
rate (16 - 36 hours) and to provide conversions of 92 – 98% under
a 1 bar atmosphere of dihydrogen, albeit these experiments were
22]
8
also performed at 60 °C in d -THF. Catalysis performed by 2,
-1
however, also ensued without any observable formation of the
necessary alkyl intermediates, an observation which was
ascribed to their instantaneous consumption either by
hydrogenolysis or competitive β-hydride elimination (Scheme 1).
We have observed only limited potential for -hydride elimination
from monitoring of solutions of compound 9 alone or a 1:1 mixture
of compounds 3 and 9. This latter solution equilibrated to an
approximate 1:1:0.7 mixture of 3:9:6 over a 24 hour period but
evidenced no elimination of 1-hexene. Similarly, monitoring of a
mol . From C (6), either hydrogenation by H (left part of Figure
2
2) or further 1-hexene insertion (right part of Figure 2) was
considered computationally. It should first be noted that both
reactions occur on the bimetallic species as the hydride dimer
dissociation was computed to be endothermic by 40.4 kcal mol-1
(Figure S20).
The difference between the barrier heights associated with the
-1
direct hydrogenation (15.9 kcal mol ) of C (6) and 1-hexene
-1
-1
insertion (19.9 kcal mol ) is 4.0 kcal mol , indicating a marginal
preference for the direct hydrogenation. Within the precision of
the method, however, the possibility of 1-hexene insertion cannot
be ruled out completely. The complete hydrogenation
subsequent to the second 1-hexene insertion, i.e. from the
dimeric n-hexyl complex F (9), is also reported on Figure S21.
The two barriers found for the hydrogenation from F (9) (17.3 and
C
6
D
6
solution of the calcium n-hexyl (9) under 1 atmosphere of 1-
1
butene by H NMR spectroscopy demonstrated that the 1-butene
was only very slowly consumed over the course of four weeks.
This process occurred with the simultaneous emergence of a new
alkene multiplet at δ 5.48 – 5.53 ppm and an aliphatic doublet
signal at 1.55 ppm, which were assigned to the generation of the
internal alkene, 2-butene. Although not investigated further, the
formation of 2-butene is proposed to have arisen from an effective
-1
19.1 kcal mol ) are of a similar magnitude as the barrier for the
hydrogenation from C (6). The hydrogenation, therefore, may be
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COMMUNICATION
deduced to be similarly kinetically accessible for both possible
pathways.
Figure 2: DFT (B3PW91) computed enthalpy reaction profile at room temperature for the direct hydrogenation vs the 1-hexene insertion of the
dicalcium n-hexyl-hydride complex C (6).
monometallic species.[28,29] Although largely a consequence of
The hydrogenation transition states are particularly notable and
display features which discriminate them from a classical -bond
metathesis. Examination of the relevant molecular orbitals and
the NBO analysis of the hydrogenation of the n-hexyl-hydride
species C (6) is clearly indicative of a heterolytic cleavage of the
the highly polarized nature of the Ca-C bonding, more generally
these observations underscore the view that group
organometallic reactivity is not simply ‘lanthanide mimetic’, but
displays features that are unique to each of the available
elements. We are continuing to examine these possibilities.
2
H
2
molecule. The HOMO-2 (Figure S22) involves the filled sp3
orbital of the n-hexyl chain, the H σ* and a 3d orbital of both Ca
2
Acknowledgements
centers. The involvement of 3d orbitals on Ca has recently been
invoked as a potentially significant factor in the structural and
reaction chemistry of organocalcium species.[26,27] This analysis
indicates, therefore, that the hydrogen atom interacting with the
n-hexyl unit is better viewed as a proton, whereas the hydrogen
atom interacting with the two Ca centers is much more hydridic in
nature. This interpretation is further corroborated by the
corresponding NBO analysis in which a strong second order
We thank the EPSRC (UK) for the support of a DTP studentship
(ASSW).
Conflict of interest
The authors declare no conflict of interest.
Keywords: calcium • hydrogenation • catalysis • main group
chemistry • density functional theory
3
interaction is observed between the filled -methylene sp orbital
2
of the n-hexyl chain and the σ* orbital of the H molecule,
[
1]
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nucleophilic displacement of hydride by the organic substituent.
This electron density is then redistributed via by a less strong
second order interaction between the σ*H-H orbital and the two Ca
atoms. The highly polarized nature of this process is underscored
by the NPA charge analysis, which indicates a charge of 1.0 on
the -methylene carbon of the n-hexyl chain, charges of +1.50
and +1.43 for the two Ca centers and charges of 0.23 and +0.03
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2
hydrogen atoms. An effectively identical
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5
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Entry for the Table of Contents (Please choose one layout)
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Andrew S. S. Wilson, Chiara Dinoi,
Michael S. Hill, Mary F. Mahon and
Laurent Maron
Page No. – Page No.
Heterolysis of Dihydrogen by
Nucleophilic Calcium Alkyls
Hydrogen gets nuked: Hydrogenation of dimeric -diketiminato calcium alkyls occurs by nucleophilic attack and heterolysis of the H-
H bond.
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[
a]
DA nr .d Lr eu wc i aS G. Sa .r cWi ai ,l s Do rn. , MD ar t Mh ea wr y DF .. AM n ak he or ,n D ar n. dM Pa rr yo fF. .M Mi ca hh ao en l