Reversible H2 addition across a nickel-borane unit as a promising strategy for catalysis

Article abstract of DOI:10.1021/ja211419t

We report the synthesis and characterization of a series of nickel complexes of the chelating diphosphine-borane ligands ArB(o-Ph ₂PC₆H₄)₂ ([^ArDPB ^Ph]; Ar = Ph, Mes). The [^ArDPB<

Full text of DOI:10.1021/ja211419t

Communication
pubs.acs.org/JACS
Reversible H2 Addition across a Nickel−Borane Unit as a Promising
Strategy for Catalysis
W. Hill Harman and Jonas C. Peters*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
S
* Supporting Information
ABSTRACT: We report the synthesis and character-
ization of a series of nickel complexes of the chelating
diphosphine-borane ligands ArB(o-Ph2PC6H4)2
([ArDPBPh]; Ar = Ph, Mes). The [ArDPBPh] framework
supports pseudo-tetrahedral nickel complexes featuring η2-
Figure 1. Schematic representation of (a) H2 activation across a
B,C coordination from the ligand backbone. For the B-
nickel−borane pair and (b) conventional (left) versus polarity-inverted
phenyl derivative, the THF adduct [PhDPBPh]Ni(THF)
(right) H2 heterolysis at a TM−ligand pair.
has been characterized by X-ray diffraction and features a
very short interaction between nickel and the η2-B,C
oxidative addition of H2 and catalyzes the hydrogenation of
ligand. For the B-mesityl derivative, the reduced nickel
complex [MesDPBPh]Ni is isolated as a pseudo-three-
coordinate “naked” species that undergoes reversible,
nearly thermoneutral oxidative addition of dihydrogen to
give a borohydrido-hydride complex of nickel(II) which
has been characterized in solution by multinuclear NMR.
Furthermore, [MesDPBPh]Ni is an efficient catalyst for the
hydrogenation of olefin substrates under mild conditions.
olefins under mild conditions.
We initially investigated nickel complexes of Bourissou’s
phenyl-substituted ligand PhB(o-PPh2C6H4)2 ([PhDPBPh], 1).11
Deep red-orange [PhDPBPh]NiBr (2) was synthesized by the
comproportionation of NiBr2 and Ni(cod)2 in the presence of 1
in THF (Scheme 1). Reduction of 2 in THF by Na/Hg
Scheme 1
he noble metals are prominent in organometallic catalysis
T
owing to their predisposition for controlled multielectron
reactivity (e.g., oxidative addition).1 Of late, increased focus has
been given to developing catalyst systems using more abundant
first-row transition metals (TMs).2 This task entails special
challenges given the propensity of the late 3d metals to undergo
single-electron processes. Redox active ligands have emerged as
one strategy to overcome this issue.3
afforded the diamagnetic pseudo-tetrahedral nickel−THF
complex [PhDPBPh]Ni(THF) (3). Complexes 2 and 3 feature
short Ni−(η2-B,C) interactions, as demonstrated by their solid-
state structures (Figure 2).12
We have recently explored the reactivity of first-row TM
complexes featuring a silyl4 or borane5 moiety in the supporting
ligand scaffold. In principle, the Lewis acidic functionality can
serve to accommodate lower oxidation states via direct
interaction with the metal center6 and/or to operate in tandem
with the TM to activate a small-molecule substrate.7 The latter
arrangement can be thought of as a minimal heterobimetallic
system, wherein the main-group atom mimics a second metal
and is preinstalled in the ligand framework.7c
As a prototypical two-electron organometallic reaction, H2
activation provides a suitable test case for this strategy.
Heterolysis of the H2 bond across a low-valent TM−borane
interaction would provide a metal hydride/borohydride pair
that could ideally be intercepted by substrate to achieve
catalysis (Figure 1). Although H2 addition across an M−B bond
for any TM is rare,7a,8 it is conceptually related to H2 activation
by frustrated Lewis pairs (FLPs)9,10 and constitutes an
attractive approach to extend to the base metals. We herein
disclose an organometallic nickel complex supported by a
diphosphine-borane ligand that undergoes facile, reversible
Figure 2. Thermal ellipsoid representations (50%) of (a) [PhDPBPh]-
NiBr (2) and (b) [PhDPBPh]Ni(THF) (3). Hydrogen atoms are
omitted for clarity.
Complexes 2 and 3 were found to be highly stable with
respect to cleavage of the Ni−(η2-B,C) interaction. In
Received: December 6, 2011
Published: March 1, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
particular, 3 fails to react with H2 after days at 60 °C. To
facilitate the desired bifunctional reactivity, we therefore
installed a bulky mesityl substituent in the place of phenyl at
boron to give MesB(o-PPh2C6H4)2 ([MesDPBPh], 4, see SI for
details).
Deep maroon [MesDPBPh]NiBr (5) was accessed by
comproportionation analogously to 2 (Scheme 2). The solid-
Scheme 2
Figure 4. (a) Hydridic region of the 1H NMR spectrum of 7 in C6D6
under 4 atm of H2 at 298 K. (b) DFT-minimized structure of 7
(B3LYP/6-31G(d)) and selected bond lengths (Å) and angles of the
core structure. Hydrogen atoms bound to carbon have been omitted
for clarity.
features two signature hydridic resonances that each integrate
to one proton: a broad resonance centered at −6.16 ppm and a
sharp triplet of doublets at −15.5 ppm (2JPH = 57.8 Hz, 2JH(B−H)
14.6 Hz). The proton-coupled 31P NMR spectrum confirms
that this most upfield hydride peak is coupled to two equivalent
31P nuclei, and the smaller coupling interaction was assigned via
two-dimensional 1H NMR to the broad 1H resonance at −6.16
ppm. The 11B resonance of 7 (−5.0 ppm) is shifted upfield
versus that of 6 (21.6 ppm), consistent with increased electron
density at boron. On the basis of these spectral features, we
propose that 7 is the hydrido-borohydrido complex
Figure 3. Thermal ellipsoid representations (50%) of (a) [MesDPBPh]-
NiBr (5) and (b) [MesDPBPh]Ni (6). Hydrogen atoms are omitted for
clarity.
state molecular structure of 5 (Figure 3a) is crudely similar to
that of the phenyl analogue 2. The Ni−Cipso distance
(2.4329(10) Å) is lengthened by ∼0.2 Å relative to that in 2,
suggesting some steric disruption of the η2-B,C interaction by
the flanking methyl groups of the mesityl substituent.
Reduction of 5 with Na/Hg affords the [MesDPBPh]Ni complex
(6), as confirmed by single-crystal X-ray diffraction (Figure 3b).
Noteworthy is the absence of a solvent ligand. The
coordination sphere features a shortened Ni−B distance as
well as close contacts with both the ipso and one of the ortho
carbons of the mesityl unit (Ni−Cipso = 2.0751(8) Å, Ni−Cortho
= 2.1616(8) Å). The coordinated aryl ring exhibits bond length
alternation consistent with partial dearomatization from the
strongly back-donating nickel center.12
[
MesDPBPh](μ-H)NiH. Due to the symmetry indicated by the
solution NMR data as well as DFT optimization of several
potential isomers (Figure 4), we favor a square planar geometry
for this species with two trans-phosphine donors, a terminal
Ni−H, and a hydride ligand that bridges between B and Ni.
This assignment is supported by the existence of structural
analogues (R3P)2NiH(BH4 ) (R = Cy, i-Pr),13 which have been
characterized by IR and NMR13a and X-ray crystallography.13b
Under 1 atm of H2 in C6D6 at 298 K, 6 and 7 exist in an
equilibrium ratio of ∼1:5 (6:7). Taking into account the
concentration of dissolved H2, the equilibrium constant for the
addition of H2 to 6 to give 7 is Kobs ≈ 5. This process is
reversible, and removal of the H2 by successive freeze−pump−
thaw cycles regenerates 6 quantitatively. Under 4 atm of H2 in
C6D6, a solution containing 95% 7 can be obtained. A van’t
Hoff analysis over a 60 K range (see SI) afforded thermal
parameters of ΔH = −9.0 1 kcal/mol and ΔS = −28 3 eu.
The equilibrium defined in Scheme 3, whereby reversible
oxidative addition occurs at a mononuclear nickel center, is to
our knowledge without precedent.14 Mononuclear dihydrides
of nickel are in general not stable and would instead be
expected to favor H2 release by reductive elimination. The
presence of the Lewis acidic borane center in the ancillary
ligand of 6, allowing for a B−H−Ni bridge to form in 7 via net
H2 heterolysis (possibly following an OA step at Ni),
presumably facilitates the observed equilibrium. H2 heterolysis
reactions are common for Ni−H2 chemistry; indeed, only a
single thermally stable example of an H2 adduct of Ni is
In sharp contrast to [PhDPBPh] complex 3, 6 reacts readily
with H2 in C6D6 at room temperature (Scheme 3). The
product of this reaction has been characterized by a suite of
1
solution NMR techniques. The H, 31P, and 11B NMR spectra
of the solution resulting from the addition of H2 to 6 indicate
the formation a new diamagnetic product (7) in addition to
residual 6 and free H2 (Figure 4). The 1H NMR spectrum of 7
Scheme 3
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Journal of the American Chemical Society
Communication
presently known.15,16 Heterolysis of H2 at Ni typically involves
Ni(II) as a Lewis acid that accepts H− while an exogenous or
internal base accepts H+.17 FLPs that incorporate TMs likewise
typically exploit the TM as the Lewis acid.10 The present
chemistry invokes an alternative scenario in which a Lewis basic
Ni center accepts H+ and a Lewis acidic borane accepts H−
(Figure 1).
AUTHOR INFORMATION
Corresponding Author
jpeters@caltech.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Given the ease with which 6 reversibly activates H2, we were
hopeful that a substrate might intercept 7 to regenerate 6,
providing a turnover step for catalysis (Scheme 3). The
addition of 20 equiv of styrene to 6 in C6D6 results in a deep
This work was supported by the NSF Center for Chemical
Innovation: Powering the Planet grant CHE-0802907, and by
the Gordon and Betty Moore Foundation. We thank Professor
Greg Fu for a helpful suggestion.
1
maroon solution, the H NMR spectrum of which contains
broadened peaks corresponding to free styrene as well as a new
diamagnetic nickel complex that we presume to be [MesDPBPh]-
Ni(η2-H2CCHPh) (8) (see SI). After exposure to an
atmosphere of H2 at room temperature, the formation of
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ethyl benzene was apparent as soon as a H NMR spectrum
could be acquired. During the course of catalysis, the solution
1
remained dark maroon, and the H NMR spectrum exhibited
peaks corresponding only to free styrene, ethyl benzene, and 8.
1
Notably, neither 6, 7, nor free H2 was observed by H NMR
spectroscopy so long as styrene was present. Provided the
reaction was agitated sufficiently so as to replenish dissolved
H2, catalysis proceeded rapidly, with complete conversion after
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1
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the acceptor substrate confirmed syn addition to the exo
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mixture of H2 and D2 (1:1, 1 atm) in C6D6 at room
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1
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In conclusion, nickel hydrogenation catalyst 6 mediates the
facile activation of H2 via net oxidative addition across a Ni−B
unit. The strategy outlined herein facilitates two-electron
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thermodynamic traps in other Ni-based systems.14b Given the
facile H2 activation carried out by 6, the possibility that
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studies will (i) map the substrate scope of hydrogenation
catalyst 6 and (ii) explore the outlined approach in the context
of related two-electron organometallic reactions catalyzed by
noble metals. The efficient olefin hydrogenation catalysis
described herein offers a promising lead toward these goals.
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ASSOCIATED CONTENT
* Supporting Information
(21) Vincent, K. A.; Parkin, A.; Armstrong, J. F. Chem. Rev. 2007,
107, 4366.
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Experimental procedures, characterization, and crystallographic
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
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