Oxidative addition reactions of bis-aminophosphine and bis-phosphinite nickel(0) pincer complexes

Article abstract of DOI:10.1021/om201229x

The tridentate ligands S(CH₂CH₂NHPiPr ₂)₂ and S(CH₂CH₂OPiPr ₂)₂ undergo C-S oxidative addition to give (SCH ₂CH₂EPiPr₂)Ni(CH₂CH ₂EPiPr₂) (E = NH (2), O (4)). The corresponding reaction of the aminophosphine ligand HN(CH₂CH₂NHPiPr ₂)₂ is reversible, affording access to a transient Ni(0) synthon that undergoes oxidative addition with HCl, MeI, or o-ClC ₆H₄F to give the Ni(II) complexes [HN(CH ₂CH₂NHPiPr₂)₂NiR]X (R = H, X = Cl (7), PF₆ (8); R = Me, X = I (9); R = o-C₆H₄F, X = Cl (10)). These observations are supported via computations.

Full text of DOI:10.1021/om201229x

Communication
pubs.acs.org/Organometallics
Oxidative Addition Reactions of Bis-Aminophosphine and Bis-
Phosphinite Nickel(0) Pincer Complexes
Michael J. Sgro and Douglas W. Stephan*
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
S
* Supporting Information
ABSTRACT: The tridentate ligands S(CH2CH2NHPiPr2)2 and S-
(CH2CH2OPiPr2)2 undergo C−S oxidative addition to give (SCH2CH2EPiPr2)-
Ni(CH2CH2EPiPr2) (E = NH (2), O (4)). The corresponding reaction of the
aminophosphine ligand HN(CH2CH2NHPiPr2)2 is reversible, affording access to
a transient Ni(0) synthon that undergoes oxidative addition with HCl, MeI, or o-
ClC6H4F to give the Ni(II) complexes [HN(CH2CH2NHPiPr2)2NiR]X (R = H, X
= Cl (7), PF6 (8); R = Me, X = I (9); R = o-C6H4F, X = Cl (10)). These
observations are supported via computations.
ridentate ligand complexes have attracted great interest, as
a vast array of possible variation affords uses in a broad
coupling constant of 270.0 Hz. These data suggest a trans
disposition of inequivalent P atoms. H NMR data included
1
T
range of applications, including sensors, switches, and
catalysts.1,2 PCP2 and PNP3 type ligands have garnered the
most attention, demonstrating electronic and steric tunability,
thus rendering highly reactive species stable.4−8 In addition,
such complexes have displayed interesting reactivity, including
oxidative addition of C−halogen bonds,7,9 C−H bond
activation,6 N2 activation, and homolytic cleavage of H2.8
Much of this work has been described in a text.10
resonances from the ligand constituents; however, of particular
note are the resonances at 1.21 and 0.97 ppm attributable to
NH and NiCH2 fragments. The corresponding 13C{1H} signal
for this latter group was observed as a doublet of doublets at
19.63 ppm with C−P couplings of 21.3 and 16.9 Hz.
Collectively these data suggest the formulation of 2 as the
Ni(II) species (SCH2CH2NHPiPr2)Ni(CH2CH2NHPiPr2)
(Scheme 1). This was unambiguously confirmed by X-ray
In seeking new systems for potential applications in catalysis,
we are exploring a variety of new tridentate ligand systems. For
example, we have recently developed strategies based on
tridentate bis-phosphinimine ligands, demonstrating that such
ligands afford both unique binding modes and the ability to
access bimetallic systems.11−14 An alternative approach is based
on the use of phosphine donor fragments derived from
aminophosphines or -phosphinites. Such ligands are expected
to be strongly donating, rendering the metal centers electron
rich and thus potentially reactive. In this communication, we
report reactions in which the ligands D(CH2CH2NHPiPr2)2 (D
= S, HN) are shown to undergo oxidative addition at Ni(0) to
effect ligand cleavage. In the case of C−S bond activation (D =
S) the formation of the resulting Ni alkyl-thiolate complex is
irreversible. In contrast, for the ligand with D = NH the
corresponding N−C activation is reversible, permitting access
to Ni(II) products by addition of acid or an alkyl or aryl halide.
The tridentate ligand S(CH2CH2NHPiPr2)2 (1) is readily
prepared in high yields from the corresponding diamine and
ClPiPr2. Subsequent reaction with Ni(COD)2 in THF resulted
in a clear orange solution from which a pale orange product (2)
could be isolated in 83% yield. The 31P{1H} NMR spectrum of
2 exhibited two doublets at 121.8 and 80.3 ppm with a P−P
Scheme 1. Synthesis of 2 and 4
methods (Figure 1). The pseudo-square-planar Ni center is
bound to two trans phosphorus atoms and a C and an S atom.
This gives rise to Ni−S, Ni−C, and Ni−P bond lengths that
average 2.2019(8), 1.990(3), and 2.1750(9) Å, respectively.
In a similar fashion the ligand S(CH2CH2OPiPr2)2 (3) was
prepared from 2,2′-thiodiethanol and was reacted with
Ni(COD)2 to give a light orange product (4) in 84% yield.
Similar to the case for 2, the 31P{1H} NMR spectrum of 4
showed two strongly coupled doublets as 187.00 and 152.91
1
ppm with a P−P coupling constant of 286 Hz. The H and
13C{1H} NMR spectra were also similar to those of 2, with
Received: December 10, 2011
Published: February 9, 2012
© 2012 American Chemical Society
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Communication
in which S−C has undergone oxidation addition, afforded the
model cis-2, which was computed to be 19.3 kcal/mol more
stable than 2a. In considering the isomerization of cis-2 to trans-
2, the tetrahedral transition state 2-TS was envisioned. Indeed,
this geometry resulted in a single imaginary frequency and was
found to be 7.6 kcal/mol above cis-2. trans-2 was also
minimized and found to be 19.9 kcal/mol more stable than
2-TS. Thus, these calculations suggest that the observed
geometry of 2 is some 31.6 kcal/mol more stable than that of a
transient Ni(0) species generated from the combination of the
ligand 1 and Ni(COD)2 (Scheme 2a).
Scheme 2. Computed Energy Level Diagrams for Reactions
a
of (a) 1 and (b) 5 with Ni(COD)2
Figure 1. POV-ray depictions of (a) 2 and (b) 4: C, black; S, yellow;
N, aquamarine; O, red; P, orange; Ni, maroon. Hydrogen atoms are
omitted for clarity.
multiplets at 1.13 and 23.12 ppm, respectively, attributed to a
Ni-bound methylene fragment. Collectively these data were
consistent with the formulation of 4 as the product
(SCH2CH2OPiPr2)Ni(CH2CH2OPiPr2) (Scheme 1). X-ray
methods confirmed the pseudo-square-planar geometry about
Ni (Figure 1). The Ni−S, Ni−C, and Ni−P bond lengths in 4
are 2.1881(4), 1.9873(15), 2.1537(4), and 2.1572(5) Å,
respectively. These metric parameters are consistent with
stronger Ni−P bonds and consequently slightly longer Ni−S
and Ni−C bonds in 4 compared to 2.
The formation of 2 and 4 clearly result from the binding of
the corresponding ligand to Ni(0) followed by oxidative
addition of the S−C bonds. It is noteworthy that Yamamoto et
al.15 have previously reported that the reaction of Ni(COD)2,
PEt3, and Ph2S proceeds to afford (Et3P)2Ni(SPh)Ph, while
Darensbourg and co-workers have described light-induced
dealkylation of sulfur in Ni(0) phosphine-thiolate complexes.16
The noninnocence of pincer ligands in nickel complexes
resulting in the cleavage of C(sp3)−heteratom bonds has
recently received attention from Caulton and co-workers, who
showed the cleavage of a Si−C(sp3) bond in their PNPNi
system.17,18 However, to our knowledge this is the first
intramolecular S−C cleavage affording a Ni−alkyl-thiolate
derivative. It is reasonable to suggest that, in the present case,
the initial interaction of the ligand with Ni(0) affords an
approximately trans binding of phosphines, not unlike the the
case for the known (R3P)2Ni complexes.19 However, this
geometry requires the proximity of the thioether fragment to
Ni(0). This, in addition to the strong donor ability of the
aminophosphine and phosphinite ligands, generates an
electron-rich Ni(0) center, prompting oxidative addition of
the S−C bond. Subsequent isomerization is required to
generate the trans-phosphine disposition observed.
a
Energies are given in kcal/mol.
To further probe related systems, the analogous ligand
HN(CH2CH2NHPiPr2)2 (5) was prepared in a conventional
manner. The stoichiometric reaction of 5 with Ni(COD)2
generated a red solution in THF. Monitoring this mixture by
31P NMR spectroscopy revealed a mixture of products that
could not be separated. The major products consisted of a set
of two resonances at 84.29 and 79.65 ppm which exhibited a
P−P coupling constant of 10.3 Hz and a broad singlet at 65
ppm consistent with 5. In the corresponding 1H NMR
spectrum, a doublet of multiplets was observed at −1.42
ppm, similar to that seen in 2 and 4, as well as resonances
attributable to free COD. These data suggest similar reactivity
involving C−N oxidative addition which generates inequivalent
P environments. For comparative purposes computations
analogous to those described above were performed for the
interaction of 5 with Ni(COD)2. In this case, the product of
oxidation addition of a C−N bond to Ni to give cis-6 was also
found to be more stable than that the corresponding Ni(0)
species 6a; the difference in energy was only 12.8 kcal/mol.
Isomerization of cis-6 to trans-6 requires an activation energy of
9.8 kcal/mol to reach a pseudotetrahedral transition state, 6-TS,
but results in only a small gain in stability of 6.1 kcal/mol over
6a. These data support the view that the barriers to C−N
activation and isomerization are low and suggest the possibility
In order to probe this view of the reactions, a computational
study was undertaken. Using Gaussian0320,21 and the B3LYP-6-
31g(d,p) basis set, the geometry of the three-coordinate Ni(0)
species 2a derived from the ligand 1 was optimized. The
optimized geometry of this species was three-coordinate.
Computation of the minimized pseudo-square-planar Ni(II),
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that PNP ligands may undergo reversible C−N activation, thus
accounting for the mixture of products.
To probe this possibility further, mixtures of 5 and
Ni(COD)2 were subsequently reacted with NEt3HCl to give
an orange solid (7) in 87% yield (Figure 2). The 31P{1H} NMR
planar geometries, with the ligand 3 acting as a tridentate
chelate. Thus, it appears that the low energy barrier to the
reversibility of the ligand N−C oxidative addition in 6a
generating cis-6/trans-6 provides access to the Ni(0) species 6a.
This species is irreversibly oxidized to give the Ni(II) species
7−10. Consistent with this view are computations which
suggest that the reaction of the Ni(0) species 6a with o-
ClC6H4F to give the Ni aryl Cl complex 10 is thermodynami-
cally downhill by 40.9 kcal/mol.
While the oxidation of Ni(0) to Ni(II) hydrides, alkyls, or
aryls is well documented in the literature,16,22−25 the present
reactions of 5 with Ni(COD)2 illustrate examples where
reversible oxidative addition of a ligand, combined with
oxidative addition of HX, RX, or ArX, permit access to the
more thermodynamically stable Ni(II) products, in which the
tridentate nature of the ligand is retained.
In summary, tridentate bis-aminophosphine and bis-amino-
phosphinite ligands 1 and 3 undergo oxidative addition with
Ni(0). In the case of these thioether ligands, the C−S cleavage
is irreversible, providing unique Ni(II) alkyl-thiolate complexes.
In contrast, the analogous pathway for the corresponding C−N
cleavage of the amine-based ligand 5 is inferred to be reversible,
allowing the Ni(0) synthons to provide a convenient route to
Ni(II) cations via oxidative additions. The chemistry of
transition-metal complexes of these electron-rich tridentate
ligands continues to be the subject of study in our laboratories.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, tables, and CIF files giving synthetic, experimental, and
crystallographic details. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 2. Synthesis of 7−10 and POV-ray depictions of the cations of
7/8 and 10.
AUTHOR INFORMATION
Corresponding Author
*E-mail: dstephan@chem.utoronto.ca.
Notes
■
spectrum of 7 revealed a single resonance at 80.00 ppm, while
the 1H NMR exhibited a triplet resonance at −20.90 ppm with
a P−H coupling of 81.2 Hz attributable to a Ni hydride.
Collectively the NMR data were consistent with the
formulation of 7 as the diamagnetic Ni(II) complex [HN-
(CH2CH2NHPiPr2)2NiH]Cl. While simple anion exchange
affords the species [HN(CH2CH2NHPiPr2)2NiH]PF6 (8), 8
can be prepared directly from the reaction of [NH4][PF6] with
the Ni(0) mixture. In a similar fashion, addition of MeI to a
mixture of 5 and Ni(COD)2 afforded the analogous Ni(II)
species [HN(CH2CH2NHPiPr2)2NiMe]I (9) in 79% yield.
This species exhibited a single 31P signal at 72.4 ppm, while the
1H NMR triplet at −0.79 ppm, which showed a P−H coupling
of 10.8 Hz, was consistent with the presence of a Ni−Me
fragment. Previous work in our group has shown that the
addition of o-ClC6H4F to a mixture of Ni(0) and tridentate
phosphinimine ligands yields the nickel aryl complex.11,12 In an
experiment akin to those, the addition of o-ClC6H4F to the
Ni(0) mixture led to the isolation of [HN-
(CH2CH2NHPiPr2)2Ni(o-C6H4F)]Cl (10) in 86% yield. The
19F NMR spectrum of 10 shows resonances at −84.11 and
−84.23 ppm, while the 31P{1H} NMR spectra gave rise to
signals at 70.58 and 69.53 ppm. These data, together with
analogous 1H NMR data were consistent with the presence of a
50:50 mixture of the two conformational isomers, attributable
to the relative orientation of the o-F and the NH fragments.
The formulations of 8 and 10 were confirmed via a
crystallographic study (see the Supporting Information). Each
of these compounds exhibited the expected pseudo-square-
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The financial support of LANXESS Inc., the NSERC of
Canada, and the Ontario Centres of Excellence is gratefully
acknowledged. M.J.S. is grateful for the award of an NSERC
postgraduate scholarship. D.W.S. is grateful for the award of a
Killam Research Fellowship 2009−2011 and a Canada
Research Chair.
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