On the stability of a POCsp3OP-type pincer ligand in nickel(II) complexes

Article abstract of DOI:10.1002/anie.201310386

We describe the results of a study on the stabilities of pincer-type nickel complexes relevant to catalytic hydroalkoxylation and hydroamination of olefins, Ci-C and Ci-X couplings, and fluorination of alkyl halides. Complexes [(POC_sp3OP)NiX] are stable for X=OSiMe₃, OMes (Mes=1,3,5-Me₃C₆H₂), NPh₂, and CCi-H, whereas the O(tBu) and N(SiMe₃)₂ derivatives decompose readily. The phenylacetylide derivative transforms gradually into the zero-valent species cis-[{κ^P,κ^C, κ^C′-(iPr₂POCH₂CHCH ₂)}Ni{η²,κ^C,κ ^C′-(iPr₂P(O)CCPh)}]. Likewise, attempts to prepare [(POC_sp3OP)NiF] gave instead the zwitterionic trinuclear species [{(η³-allyl)Ni}₂-{μ,κ^P, κ^O-(iPr₂PO)₄Ni}]. Characterization of these two complexes provides concrete examples of decomposition processes that can dismantle POC_sp3OP-type pincer ligands by facile Ci-O bond rupture. These results serve as a cautionary tale for the inherent structural fragility of pincer systems bearing phosphinite donor moieties, and provide guidelines on how to design more robust analogues. Survival of the fittest: One or both of the Ci-O linkages in the complex [(POC_sp3OP)Ni(OSiMe ₃)] break under fairly mild conditions to generate unprecedented mono- and trinuclear species. These decomposition processes can serve as models for predicting the stabilities of these complexes in catalytic settings.

Full text of DOI:10.1002/anie.201310386

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Angewandte
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DOI: 10.1002/anie.201310386
Quasi-Stable Pincer Ligands
On the Stability of a POCsp3OP-Type Pincer Ligand in
Nickel(II) Complexes**
Jingjun Hao, Berline Mougang-Soumꢀ, Boris Vabre, and Davit Zargarian*
Abstract: We describe the results of a study on the stabilities of
In this context, there have been sporadic reports describ-
pincer-type nickel complexes relevant to catalytic hydroalkox-
ing pincer complexes in which one or more binding moiety
undergoes unexpected reactions that introduce major modi-
fications in the pincer framework. A few recent examples
include reports on complexes featuring PCP-,[4] PNP-,[5] and
PSiP-type[6] pincer ligands, whereas Ni complexes featuring
resorcinol-derived POCOP ligands have been shown to be
prone to oxidative degradation.[7] Indeed, the aliphatic
analogues of POCOP ligands, POCsp3OP (Scheme 1), are
ꢀ
ꢀ
ylation and hydroamination of olefins, C C and C X
couplings, and fluorination of alkyl halides. Complexes
[(POCsp3OP)NiX] are stable for X = OSiMe3, OMes (Mes =
ꢀ
1,3,5-Me3C6H2), NPh2, and CC H, whereas the O(tBu) and
N(SiMe3)2 derivatives decompose readily. The phenylacetylide
derivative transforms gradually into the zero-valent species
cis-[{kP,kC,kC’-(iPr2POCH2CHCH2)}Ni{h2,kC,kC’-(iPr2P(O)C-
CPh)}]. Likewise, attempts to prepare [(POCsp3OP)NiF] gave
instead the zwitterionic trinuclear species [{(h3-allyl)Ni}2-
{m,kP,kO-(iPr2PO)4Ni}]. Characterization of these two com-
plexes provides concrete examples of decomposition processes
ꢀ
that can dismantle POCsp3OP-type pincer ligands by facile C
O bond rupture. These results serve as a cautionary tale for the
inherent structural fragility of pincer systems bearing phos-
phinite donor moieties, and provide guidelines on how to
design more robust analogues.
Scheme 1. POCOP- and POCsp3OP-type complexes of nickel.
E
XE-type pincer ligands, where E and X are the elements
particularly vulnerable to complex side reactions leading to
decomposition. For instance, whereas [(POCOP)NiR] com-
plexes (R = Me, Et) are isolable and stable, their POCsp3OP
analogues undergo decomposition processes that remain
obscure owing to difficulties in identifying the decomposition
products.[8]
During the course of our studies aimed at developing
catalytic transformations promoted by pincer nickel com-
plexes,[9] we were prompted by the above findings to examine
the stabilities of various ligand frameworks and search for
clues on the nature of decomposition processes undermining
their integrity. Reported herein are results that establish the
relative thermal stabilities of [(POCsp3OP)NiX] complexes
(X = OR, NR2, F, CCR) and identify two unprecedented
decomposition products arising from the attempted prepara-
tion of the fluoro and phenylacetylide derivatives.
representing, respectively, the neutral and anionic donor
moieties in the ligand framework, are considered to be
“privileged” ligand platforms since they provide highly
tunable and stable frameworks for supporting and sustaining
the action of a metal center in diverse settings.[1] Thus, the
structurally robust and thermally stable architecture of most
EXE-type pincer ligands is thought to be a major contributor
to the excellent catalytic reactivities displayed by their
complexes at unusually high temperatures.[2] On the other
hand, there have also been instances where the survival of the
pincer framework under forcing conditions has been called
into question, and the observed catalytic reactivities have
been attributed to decomposed fragments of the pincer-type
pre-catalysts used.[3] Given the continued use and increasing
prominence of pincer complexes in various catalytic reactions,
it is of importance from both practical and fundamental
perspectives to better understand the decomposition path-
ways available to pincer complexes.
Treatment of [(POCsp3OP)NiBr][9a] with excess KOSiMe3
or NaOMes (Mes = 2,4,6,-Me3C6H2) in dioxane or THF gave
the corresponding derivatives 1-OSiMe3 and 1-OMes in
nearly quantitative yields (Scheme 2). In contrast to the
clean formation of these derivatives, the analogous reaction
with tBuOK, under the same conditions, led instead to
complete decomposition. Based on the assumption that this
decomposition is due to the tendency of the “harder” tBuO
anion to attack the phosphinite moiety instead of the “softer”
Ni center, we attempted to prepare the target {Ni-O(tBu)}
derivative indirectly, by treating 1-OSiMe3 with tBuOH.
Unfortunately, this approach also led to decomposition of
the starting material, whereas 1-OMes was accessible by
protonolysis with MesOH (Scheme 2). We conclude that the
{Ni-O(tBu)} derivative forms initially but decomposes sub-
[*] Dr. J. Hao, B. Mougang-Soumꢀ, B. Vabre, Prof. Dr. D. Zargarian
Dꢀpartment de chimie, Univerrsitꢀ de Montrꢀal
2900 Boul. Edouard-Montpetit
Montrꢀal, Quꢀbec, H3C 3J7 (Canada)
E-mail: zargarian.davit@umontreal.ca
[**] We gratefully acknowledge the financial support provided by NSERC
(Canada) and FRQNT (Quꢀbec).
Supporting information for this article (details on experimental
procedures as well as spectral and analytical data used to establish
the identities and purities of the products) is available on the WWW
under http://dx.doi.org/10.1002/anie.201310386.
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a toluene mixture of 1-OSiMe3 and AgF (1.0:1.3) at room
temperature and under ambient light caused a slow color
change from yellow to green over 4 days; significantly, little or
no reactivity was observed in the absence of light. Analysis of
the final mixture by 31P NMR spectroscopy revealed the
presence of multiple minor peaks and a major one at d =
125 ppm, which is significantly upfield of the region charac-
teristic of [(POCsp3OP)NiX] compounds (d = 200–170 ppm).
Repeated recrystallization of the mixture from hexane
solutions allowed us to isolate single crystals of 2 in
approximately 5% yield.
Scheme 2. Synthesis of [(POCsp3OP)Ni(OR)].
Solid-state samples and solutions of 1-OSiMe3, 1-OMes,
and 1-NPh2 decompose in air, but under inert atmosphere
they are stable indefinitely as solids and their solutions are
thermally stable up to 1008C for more than 5 h. These
diamagnetic complexes have been fully characterized by
NMR spectroscopy, elemental analysis, and X-ray diffraction
analysis. Many of their NMR spectroscopic features (e.g.,
observation of virtual triplets for OCH2 groups and a singlet
31P resonance for the equivalent phosphinite moieties) are
similar to those of previously studied analogues and consis-
tent with the proposed structures. The OSiCH3 resonances
signals in the 1H and 13C NMR spectrum at d = 0.24 ppm (1H)
and d = 5.46 ppm (13C) fall into the expected range for
OSiMe3 complexes.[11]
sequently because of the destabilizing dp–pp interactions that
are more prevalent in this case relative to the siloxide or
aryloxide derivatives.[10]
Consistent with the above observations, potentially strong
p-donor ligands, such as amides bearing aliphatic N-substitu-
ents gave unstable derivatives, whereas the diphenylamido
derivative 1-NPh2 could be prepared and isolated in around
70% yield (Scheme 3). To our surprise, the analogous
The Ni centers in 1-OSiMe3, 1-OMes, and 1-NPh2 adopt
a square-planar geometry in the solid state (Figure 1),[12] and
most of the structural parameters are comparable to those
found in 1-Br.[9a] The Ni N distance of 1.961(3) ꢀ and the Ni-
ꢀ
N-C angles of 120–1228 in 1-NPh2 are comparable to the
Scheme 3. Synthesis of 1-NPh2 and attempted syntheses of 1-F and 1-
N(SiMe3)2.
corresponding parameters in reported nickel amido com-
plexes,[13] whereas the Ni O bond is longer in 1-OSiMe3
ꢀ
(1.873(3) ꢀ) and 1-OMes (1.895(1) ꢀ) than in two other
[14a]
ꢀ
hexamethyldisilazide derivative 1-N(SiMe3)2 also proved to
be thermally unstable, leading to intractable yellow solids.
Similarly, the fluoride derivative proved inaccessible as the
reaction of 1-Br with AgF led to a colorless solution and an
intractable black precipitate. A slow decomposition also
resulted from the reaction of 1-OSiMe3 with AgF, but in this
case an unusual zwitterionic trinuclear complex 2 could be
isolated from the reaction mixture (Scheme 3). Thus, stirring
{Ni OSiR3} complexes (1.820(2)
and 1.831(3)[14b] ꢀ) and
the closely related {(PNP)Ni-OPh} (1.863(2) ꢀ).[15] The
ꢀ
longer Ni O bonds in 1-OSiMe3 and 1-OMes are presumably
due to the very strong trans influence of nickel-bound central
carbon atom of the POCsp3OP ligand. The relative sizes of the
Ni-O-Si/C angles in 1-OSiMe3 and 1-OMes (1518 vs. 1368) are
consistent with a greater degree of O!Si electron delocal-
ization that serves presumably to minimize the dp–pp desta-
Figure 1. ORTEP diagrams for 1-OSiMe3 (left), 1-OMes (middle), and 1-NPh2 (right). Thermal ellipsoids set at 50 % probability.
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bilizing interactions anticipated in such square-planar d8
out the Achillesꢁ heel of the 1,3-propanediol-based POCsp3OP
systems featuring metal--heteroatom linkages.[10] The O Si
ligands. The C O bonds of the resorcinol-based aromatic
ꢀ
ꢀ
bond in 1-OSiMe3 (1.592(3) ꢀ) is comparable to the corre-
POCOP ligand are also susceptible to rupture,[7] but much less
so. For instance, reaction of [(POCOP)NiBr] with AgF gives
[(POCOP)NiF], which is stable to light and heating.[18]
A different but related decomposition of the POCsp3OP
ligand was also noted during the synthesis of the alkynyl
derivatives, as follows. Reaction of 1-Br with sodium acety-
lides at room temperature led to full conversion of the starting
material into the anticipated alkynyl derivatives 1-CCH
and 1-CCPh displaying singlet 31P resonance signals at
approximately d = 192.7 ppm and 192.9 ppm, respectively
(Scheme 4). Complex 1-CCPh was also accessible by reaction
ꢀ
sponding distance found in complexes featuring a M OSiMe3
moiety.[11c,16] The coordination plane in 1-OMes (P/Ni/P/C/O)
makes an 858 angle with the mesityl ring, and that in 1-NPh2
(P/Ni/P/C/N) makes angles of approximately 77–798 with the
Ph rings.
Complex 2 is a zwitterionic trinuclear species featuring
two [(h3-C3H5)NiII]+ fragments (Figure 2),[12] each coordi-
nated by two phosphorus atoms of the central unit
[(iPr2PO)4NiII]2ꢀ. The central Ni atom adopts a tetrahedral
coordination geometry defined by four oxygen atoms.
Figure 2. ORTEP diagram for complex 2. Thermal ellipsoids set at
50 % probability.
Scheme 4. Syntheses of 1-CCH, 1-CCPh, and 3.
of 1-OSiMe3 with PhCCH. The new alkynyl complexes were
isolated in nearly quantitative yields as yellow microcrystals
(1-CCH) or an oily yellow solid (1-CCPh), and characterized
by NMR spectroscopy. Both compounds showed the expected
aliphatic 1H NMR resonance signals for the POCsp3OP ligand,
Although the average of O-Ni-O angles is close to the ideal
value of 109.58, there are significant variations in these angles:
the ones inside the two 6-membered metallacycles are
approximately 988 while the remaining four angles defined
by the two spirocyclic rings are approximately 112–1198. The
two Ni centers in the terminal {(allyl)NiP2} fragments adopt
a square-planar geometry defined by h3-coordination of the
allyl ligand and two monodentate phosphinite moieties;
accordingly, the sum of angles defining the square plane
ꢀ
in addition to the signals for Csp H (d = 2.78 ppm) and three
aromatic peaks due to Ni-CC-C6H5. Triplet resonances
observed at d = 104 ppm and 112 ppm (2JP-C = 28–29 Hz) in
the 13C NMR spectra of 1-CCH and 1-CCPh, respectively,
ꢀ
were assigned to the Ni Csp nuclei by comparison to the
corresponding spectrum of the reported alkynyl complex
ꢀ
ꢀ
adds up to approximately 3608. The Ni P and Ni O distances
are somewhat longer in 2 relative to the pincer complexes 1-
[(PCP)Ni(CCPh)].[9d]
Single crystals suitable for X-ray analysis were obtained
for 1-CCH, allowing us to examine its solid-state structure
(Figure 3).[12] The main structural parameters of 1-CCH are
ꢀ
OSiMe3 and 1-OMes, whereas the Ni C distances and C-Ni-C
angles are comparable to the corresponding parameters in
allyl nickel complexes.[17]
The mechanism for
the unusual transforma-
tion of 1-OSiMe3 to 2
remains elusive, but the
finding that ambient
light plays
a crucial
role in initiating the
decomposition process
implies a radical-initi-
ated reaction sequence.
That this decomposi-
tion process leads to
dismantling of the
POCsp3OP
ligand
ꢀ
framework by C O
bond rupture points
Figure 3. ORTEP diagram for complex 1-CCH (left) and cis-3 (right). Thermal ellipsoids set at 50 % probability.
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similar to those observed for the derivatives reported
observed a similar rearrangement when the Lewis acidic
previously and discussed above, and the Csp Csp bond is
fragment [(PNP)Ni]+ was treated with terminal alkynes.[5c] In
ꢀ
[19]
ꢀ
ꢀ
ꢀ
comparable to a reported Ni CCH complex. The Ni Csp
distance (1.913(2) ꢀ) is intermediate between the corre-
sponding distances found in [(POCOPPh)Ni(CCPh)][9l]
(1.878(2) ꢀ) and [(PCP)Ni(CCPh)][9d] (1.944(2) ꢀ). Observa-
this case, the P C bond making step generates a phosphonium
group linking the pincer backbone to the newly formed
alkyne moiety, whereas in our system the alkyne ligand
ꢀ
detaches from the pincer backbone following the C O bond
ꢀ
tion of comparable Ni Cbridgehead distances in 1-CCH and the
rupture. The relatively facile “flipping” of the non-chelating
phosphinoalkyne ligand in 3 helps explain the fluxional
process alluded to above.
1-X derivatives discussed above (1.97–1.98 ꢀ) implies com-
parable trans influences for the acetylide, siloxide and
aryloxide, and NPh2 ligands.
The results described in this report establish the stabilities
of [(POCsp3OP)NiX] as a function of the ligand X. Potentially
strong p-donor ligands OR can form stable and isolable
derivatives if the R substituent is “tuned” to attenuate the
destabilizing pp–dp interactions with the filled nickel-based
orbitals. The analogous NR2 derivatives are also sensitive to
the nature of N-substituents, but the observed instability of
the N(SiMe3)2 derivative indicates that factors other than pp–
dp interactions must contribute to the stability of these
complexes.[22] Similarly, other factors, such as ambient light
All attempts at growing single crystals of 1-CCPh were
circumvented by a gradual decomposition process as revealed
by 31P NMR spectroscopic monitoring of solutions set aside
for crystallization. Thus, the singlet resonance signal attrib-
uted to this compound was gradually replaced by two sets of
doublets of doublets, one centered at approximately d = 43
and 201 ppm (J = 34 Hz) and the other at approximately d =
45 and 193 ppm (J = 9 Hz). That these sets of signals represent
interconverting isomers as opposed to unrelated, different
species was inferred from the observation that the ratios of
the two sets of signals varied with the nature of the solvent, as
follows: CD2Cl2 and C6D6/C7D8 samples showed a 1:2 and
2.5:1 ratio, respectively, whereas a sample made from equal
volumes of CD2Cl2 and C6D6 gave a 1:1 ratio of the two
doublets of doublets. The thermal stability of the isomeric
products was also solvent-dependent: solutions in hexane,
benzene, or toluene proved to be fairly stable over extended
periods at ambient temperature or over hours at higher
temperatures, whereas solutions of chlorinated solvents
decomposed over several hours at ambient temperature.
Curiously, however, variable temperature (VT) NMR experi-
ments performed on a C7D8 sample over the range of ꢀ70 to
908C showed no intensity variation for the observed reso-
nances. These two isomers were also analyzed by 1H and
13C NMR spectroscopy (Supporting Information).
ꢀ
appear to destabilize the Ni F derivative. Finally, an even
more subtle interplay of factors appears to govern the stability
of alkynyl derivatives, the normally more-stable phenylace-
tylide derivative being less stable than the unsubstituted
acetylide.
The ligand decomposition reactions noted above proceed
by complex and as yet obscure pathways, but the isolation and
characterization of complexes 2 and cis-3 from the reaction
mixtures has helped identify the potential structural weak-
nesses of this family of pincer-type ligands. This information
will serve as a cautionary tale alerting us to the possible
pitfalls in the use of POCsp3OP complexes in various catalytic
ꢀ
applications. On the other hand, the seemingly facile C O
bond cleavage reactions generating allylic and phosphate
fragments from this ligand framework constitute new reac-
tivities that might be exploited in the context of transforming
highly oxygenated feedstocks into value-added petrochemi-
cals. Our future investigations will be directed toward taking
advantage of the opportunities afforded to us by the decom-
position reactions described herein.
Isolation of X-ray quality single crystals from the above
reaction mixture allowed us to unambiguously identify one of
the above-mentioned isomers, which in turn let us shed some
light on the decomposition of 1-CCPh. Figure 3, right[12] shows
that the new product, cis-3, consists of a Ni0 center ligated by
a bidentate phosphinite-alkene and a p-coordinated phosphi-
noxy alkyne, both originating from the POCsp3OP and
phenylacetylide ligands. The C22-Ni-C23 and C13-Ni-C14
Received: November 29, 2013
Published online: February 14, 2014
ꢀ
Keywords: C O bond cleavage · nickel · pincer complexes
ꢀ
angles are equivalent (39–408), the Ni CCcentroid distances are
.
=
ꢁ
very similar (1.902 ꢀ for C C and 1.801 ꢀ for C C), and sum
of two P-Ni-C and three C-Ni-C angles is 359.828, all
indicating that the geometry around the nickel atom in
complex cis-3 is trigonal planar. The alkyne and alkene
fragments can be treated as two-p-electron-donor moieties
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[22] A reviewer has suggested that a strong Brønsted base, such as
ꢀ
ꢀ
(Me3Si)2N , can deprotonate the Ni-bound C H moiety to
[12] All H atoms and methyl groups of P(iPr)2 are omitted in all
ORTEP diagrams for clarity. CCDC 936524 (1-OSiMe3), 972243
generate an unstable alkylidene that would decompose subse-
quently.
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