Neutral, cationic and anionic organonickel and -palladium complexes supported by iminophosphine/phosphinoenaminato ligands

Article abstract of DOI:10.1039/c9dt04062e

We report a series of organometallic nickel and palladium complexes containing iminophosphine ligands R₂PCH₂C(Ph) = N-Dipp (Dipp = 2,6-diisopropylphenyl; R = iPr, La; R = Ph, Lb; and R = o-C₆H₄OMe, Lc), synthesized by ligand exchange or oxidative addition reactions, and we investigate the capacity of such ligands to undergo reversible deprotonation to the corresponding phosphinoenaminato species. In the attempted ligand exchange reaction of the nickel bis(trimethylsilyl)methyl precursor [Ni(CH₂SiMe₃)₂Py₂] with Lb, the iminophosphine acts as a weak acid rather than a neutral ligand, cleaving one of the Ni-C bonds, to afford the phosphinoenaminato complex [Ni(CH₂SiMe₃)(L′b)(Py)] (L′b = conjugate base of Lb). We disclose a general method for the syntheses of complexes [Ni(CH₂SiMe₃)(L)(Py)]⁺ (L = La, Lb or Lc), and demonstrate that iminophosphine deprotonation is a general feature and occurs reversibly in the coordination sphere of the metal. By studying proton exchange reactions of the cation [Ni(CH₂SiMe₃)(Lb)(Py)]⁺ with bases of different strength we show that the conjugate phosphinoenaminato ligand in [Ni(CH₂SiMe₃)(L′b)(Py)] is a base with strength comparable to DBU in THF. The acyl group in the functionalized aryl complex [Ni(p-C₆H₄COCH₃)(Br)(La)] does not interfere in the iminophosphine deprotonation with NaH. The latter reaction affords the unusual anionic hydroxide species [Ni(p-C₆H₄COCH₃)(OH)(L′a)]^-Na⁺, which was isolated and fully characterized.

Full text of DOI:10.1039/c9dt04062e

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Neutral, cationic and anionic organonickel
and -palladium complexes supported by
iminophosphine/phosphinoenaminato ligands†‡
Cite this: Dalton Trans., 2020, 49,
322
Tomás G. Santiago,
Pilar Palma
Carmen Urbaneja, Eleuterio Álvarez,
*
Elena Ávila,
and Juan Cámpora
We report a series of organometallic nickel and palladium complexes containing iminophosphine ligands
R2PCH2C(Ph) = N-Dipp (Dipp = 2,6-diisopropylphenyl; R = iPr, La; R = Ph, Lb; and R = o-C6H4OMe, Lc),
synthesized by ligand exchange or oxidative addition reactions, and we investigate the capacity of such
ligands to undergo reversible deprotonation to the corresponding phosphinoenaminato species. In the
attempted ligand exchange reaction of the nickel bis(trimethylsilyl)methyl precursor [Ni(CH2SiMe3)2Py2]
with Lb, the iminophosphine acts as a weak acid rather than a neutral ligand, cleaving one of the Ni–C
bonds, to afford the phosphinoenaminato complex [Ni(CH2SiMe3)(L’b)(Py)] (L’b = conjugate base of Lb).
We disclose a general method for the syntheses of complexes [Ni(CH2SiMe3)(L)(Py)]+ (L = La, Lb or Lc),
and demonstrate that iminophosphine deprotonation is a general feature and occurs reversibly in the
coordination sphere of the metal. By studying proton exchange reactions of the cation [Ni(CH2SiMe3)(Lb)
(Py)]+ with bases of different strength we show that the conjugate phosphinoenaminato ligand in
[Ni(CH2SiMe3)(L’b)(Py)] is a base with strength comparable to DBU in THF. The acyl group in the function-
alized aryl complex [Ni(p-C6H4COCH3)(Br)(La)] does not interfere in the iminophosphine deprotonation with
NaH. The latter reaction affords the unusual anionic hydroxide species [Ni(p-C6H4COCH3)(OH)(L’a)]−Na+,
which was isolated and fully characterized.
Received 17th October 2019,
Accepted 19th November 2019
DOI: 10.1039/c9dt04062e
rsc.li/dalton
these catalysts, the o,o′ aryl groups control the molecular
Introduction
weights of the products.6 Not surprisingly, nickel and palla-
Iminophosphines are very attractive ligands used in the devel- dium iminophosphine complexes have attracted attention as
opment of new homogeneous catalysts for various processes,1 ethylene polymerization or oligomerization catalysts but unfor-
because their modular design2 allows systematic and indepen- tunately, their activities are significantly lower than those of
dent variation of the steric and electronic properties of the P the well-known α-diimine catalysts, and their products tend to
and N donor centers, and the structural properties of the exhibit broad or bimodal molecular weight distributions.7,8
ligand backbone. Among the most typical examples of this One of the potential problems identified in hybrid iminophos-
class of hybrid ligands are iminophosphines with a single phine catalyst systems is the weakly acidic character of the
carbon spacer connecting the phosphine and imino hydrogen atoms in the iminophosphine backbone. As shown
functionalities.3,4 Such iminophosphine ligands often bear in Scheme 1, iminophosphine complexes are deprotonated by
bulky o,o′-disubstituted N-aryl substituent groups, like the strong bases to afford phosphinoenaminates.9 Therefore, it
species A shown in Scheme 1. This substitution pattern is a was suggested that polymerization catalysts using this type of
common feature in many types of catalysts, particularly in ligand could be also deprotonated by aluminum alkyls used as
those used for late transition metal olefin poylmerization.5 In
Instituto de Investigaciones Químicas, CSIC-Universidad de Sevilla. CIC-Cartuja,
c/Américo Vespucio, 49, 41092 Sevilla, Spain. E-mail: campora@iiq.csic.es
†Dedicated to the memory of Prof. Richard A. Andersen.
‡Electronic supplementary information (ESI) available: Complete experimental
details, spectroscopic and crystallographic data; CIF files for compounds CCDC
1944621 (3b), 1944622 (4c), 1944623 (6b), 1944624 (7b) and 1944625 (16). For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
C9DT04062E
Scheme 1 The relationship between iminophosphine and phosphinoe-
naminato complexes.
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co-catalysts, turning the active species into more electron-rich
phosphinoenaminate species (shown as B in Scheme 1).8
However, the weak Brønsted acidity of iminophosphine
ligands could be turned into an advantage for other appli-
cations. In recent years, there has been growing interest in
ligand–metal cooperative behaviour as an alternative strategy
to extend or improve the capacities of metals in catalysis.10
Among the various sorts of metal–ligand cooperation, revers-
ible ligand protonation/deprotonation is a central step in some
important processes involving hydrogen transfer reactions,
such as transfer hydrogenation,11 or aerobic oxidations.10a
This context has brought some enolizable ligands such as
bis(phosphinomethyl)pyridine (PNP) pincers, closely related
to iminophosphines, under the spotlight.10b,e Although enoliz-
able iminophosphine chelates have been less extensively inves-
tigated in this regard, Fryzuk has recently demonstrated their
ability to promote hydrogen activation through metal–ligand
cooperative behaviour in Ir and Ru complexes.12 In addition,
first-row transition metal complexes with deprotonated phos-
phinoamidinates, iminophosphine-type ligands, have found
many useful applications in catalysis.13 These reports suggest
that iminophosphines could also induce similar cooperative
Scheme 2 Syntheses of iminophosphine ligands.
trophilic phosphination of the enaminate anion with the
corresponding chlorophosphine (Scheme 2). Ligand Lb has
been reported in the literature before.9b Deprotonation of aceto-
phenone imines is usually accomplished with lithium diiso-
propylamide (LDA), but we found that the same can be con-
veniently achieved using one equivalent of t-BuLi at −80 °C,
avoiding the need to generate the lithium amide. The ligands
were separated from lithium chloride by extraction in hexane.
La and Lb are obtained as pale-yellow oils, but Lc is obtained
as a yellow solid that is easier to handle and store. The NMR
spectra of all three ligands show that these exist as mixtures of
cis/trans isomers, with no detectable amounts of enamine
tautomers.
Ligand exchange reactions
phenomena when coordinated to 16e, square-planar com- Methods for the synthesis of organometallic derivatives of
plexes of Ni, enhancing H-transfer reactions that are usually ligands containing active (acidic) CHn groups usually avoid
facile in the chemistry of the heavier group 10 metals, particu- transmetallation reactions with organolithium or organo-
larly Pd.
magnesium reagents, as these behave as strong bases susceptible
One of our current interests focuses on the ability of Ni to deprotonation of the coordinated ligand. Instead, ligand
complexes to emulate Pd in fundamental processes that are exchange reactions are usually preferred for this purpose.15 In
relevant to catalysis. Whereas a number of organometallic our previous study, we found that pyridine-stabilized nickel
complexes of the heavier group 10 elements (Pd and Pt) sup- dialkyls, as well as equivalent pyridine or COD-bearing dialkyl
ported by deprotonable iminophosphine ligands have been and metallacyclic palladium complexes, are very convenient
reported, the chemistry of their analogues was examined in precursors that provide access to a plethora of organometallic
less detail.9 Some years ago, we reported a series of Ni and Pd derivatives via exchange reactions.16 Since (trialkylsilyl)methyl
complexes containing non-enolizable iminophosphinite derivatives often exhibit enhanced stability compared with
hybrid ligands,14 similar to iminophosphines, except that the “normal” alkyl derivatives, we set to synthesize a series of bis
linker between the PR2 unit and the imino group is an oxygen (trimethylsilyl)methyl derivatives of Ni and Pd, starting out
atom. These are excellent ligands for both Ni and Pd organo- from the precursor complexes [Ni(CH2SiMe3)2(Py)2]17a (1) and
metallic complexes. This prompted us to develop pathways to [Pd(CH2SiMe3)2(COD)]17b (2), respectively. Interestingly, the
well-defined organometallic derivatives of Ni ligated by weakly reactions of these nickel and palladium precursors with
acidic iminophosphines, and to compare these Ni derivatives ligands La, Lb and Lc had divergent results (Scheme 3).
to their Pd counterparts. Herein we report our initial results
As discussed below in more detail, the reactions of precur-
on this topic, including the synthesis of organometallic sor 1 with La–Lc fail to produce the expected dialkylnickel
species supported by three bulky iminophosphine ligands, derivatives. Only in the case of Lb, a different type of product
and for the first time we illustrate reversible iminophosphine (3b, Scheme 3 and Fig. 1) could be isolated after the reaction
deprotonation chemistry on the coordination sphere of Ni(II).
mixture was allowed to evolve for 20 h at room temperature. In
contrast, the exchange reactions of the iminophosphine
ligands with the Pd precursor 2 proceed as expected affording
the corresponding dialkyls 4a–c, which were successfully iso-
lated as dark brown solids. Although essentially quantitative
conversions were observed in the NMR spectra of the crude
Results and discussion
Iminophosphine ligands
In this study, we chose to focus on the iminophosphine reaction mixtures, the high solubility of the products in hydro-
ligands La, Lb and Lc, with an enolizable skeleton based on carbon solvents (particularly 4a), combined with some
acetophenone-imine, and differing only in the PR2 fragment thermal instability in solution, hampered their crystallization,
(R = i-propyl, phenyl or 2-anisyl, respectively). Such ligands are thus reducing their final isolated yields. The dialkyl derivative
readily obtained using a one-pot procedure that involves 4c is thermally more stable and quality crystals suitable for
sequential deprotonation of the acetophenone imine, and elec- X-ray diffraction could be grown; its crystal structure is shown
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Fig. 2 The crystal structure of 4c with 50% probability thermal ellip-
soids. The selected bond distances (Å) and angles (°): Pd(1)–C(35), 2.046(6);
Pd(1)–C(39), 2.103(6); Pd(1)–N(1), 2.235(5); Pd(1)–P(1), 2.3017(17); C(1)–N(1),
1.301(8); C(1)–C(2), 1.505(8); C(35)–Pd(1)–C(39), 88.0(3); and P(1)–
Pd(1)–N(1), 78.50(14); torsion N(1) = C(1)–C(2)–P(1), 41.3(7); dihedral
between DiPP ring and plane N(1)C(1)Pd(1), 71.0; dihedral between
Ph ring and plane C(1)N(1)C(2), 44.5; and dihedral between planes
Pd(1)C(35)C(39) and Pd(1)N(1)P(1), 4.9.
Scheme 3 Divergent reactivity of precursors 1 and 2 with iminophos-
phine ligands.
course. 31P{1H} NMR monitoring showed that the reactions
initially afford mixtures of several P-containing species that
slowly evolve over a time lapse of ca. one day. Only for Lb the
31P{1H} spectra of the reaction mixture gradually simplified
into a single P-containing species that was eventually isolated
and identified as the monoalkylnickel phosphinoenaminate
complex 3b. The identity of 3b was confirmed by its crystal
structure shown in Fig. 1. The molecule exhibits a single alkyl
ligand that, as deduced from the solution NMR data, is placed
in cis to the P donor, while the pyridine unit occupies the trans
position. The anionic phosphinoenaminate ligand is almost
planar (N–CvC–P, dihedral = 5.6°) and the chelate lacks the
characteristic puckering observed in the structure of 4c. The
flat geometry of the chelate ring and the C1–C2 and C1–N1
bond lengths within (ca. 1.38 and 1.35 Å, respectively) are
typical of a delocalized enamine-type system.
Fig. 1 The crystal structure of 3b with 50% probability thermal ellip-
soids. The selected bond distances (Å) and angles (°): Ni(1)–C(33),
1.9593(13); Ni(1)–P(1), 2.1153(4); Ni(1)–N(1), 2.0015(11); Ni(1)–N(2),
1.9552(11); C(1)–N(1), 1.3530(16); C(1)–C(2), 1.3842(17); C(33)–Ni(1)–
N(2), 89.91(5); and P(1)–Ni(1)–N(1), 85.33(3); torsion N(1)–C(1)–C(2)–
P(1), 5.76(17); dihedral between DiPP ring and plane N(1)C(1)Ni(1), 68.4;
dihedral between Ph ring and plane C(1)N(1)C(2), 46.2; and dihedral
between planes Ni(1)C(33)N(2) and Ni(1)N(1)P(1), 6.4.
The complex outcome of the reactions of the dialkyl 1 with
iminophosphine ligands was not entirely unexpected. In pre-
vious research we found that, whilst exchange reactions of pyri-
dine-stabilized dialkyls [MR2Py2] (M = Ni and Pd; and R = Me,
CH2Ph, CH2SiMe3, etc.) with various N,N bidentate donors
were in general clean and quantitative, the reaction of 1 with
in Fig. 2. The crystal structures of simple Pd(II) dialkyls are not the bulky α-diimine ligand MeC(vN–DiPP)–C(vN–DiPP)CMe
particularly frequent, and the CSD database only contains a (DiPP = 2,6-diisopropylphenyl) was incomplete, which we
couple of examples stabilized with hybrid P,N donors.9a,18 As attributed to the steric crowding when two CH2SiMe3 groups
expected, the stronger trans influence of the P atom is reflected are simultaneously attached to the relatively small Ni atom.16
in a significant lengthening of the Pd–C39 bond with regard to Very likely, the reaction of 1 with bulky iminophosphines is
the bond in trans to the nitrogen (2.103(6) vs. 2.046(6) Å). The also sterically disfavored. This allows that a slow but irrevers-
concurrent trans effect could be translated into a significant ible process like proteolytic cleavage of a Ni–C bond by the
kinetic labilization of the alkyl sitting in trans to the P donor.
acidic ligand may become competitive, this being selective
As mentioned above, the reaction of the nickel dialkyl pre- only in the case of 3b. Nevertheless, the successful isolation of
cursor with iminophosphines La–Lc follows an unexpected the latter as a stable complex inspired us to achieve the goal of
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synthesizing its analogues with ligands L′a (3a) and L′c (3c). To
this end, we devised a general strategy that would begin with
the syntheses of cationic iminophosphine monoalkyl-pyridine
derivatives [Ni(CH2SiMe3)(L)(Py)]+ (L = La, Lb or Lc), followed
by the removal of one of the weakly acidic H protons from
L with a strong base, as reported by Green and Pascu
for several Pd and Pt complexes.9 To prepare the desired
cationic precursors, we initially considered the reaction of 1
with the conjugate acid of the corresponding ligand, i.e.,
HL+. Disappointingly, attempts to generate the triflate salt
[HLb]+OTf− from Lb and one equivalent of trifluoromethane-
sulfonic (triflic) acid led to decomposition and a phosphorus-
free product was obtained instead, presumably the iminium
salt [PhC(vNHDiPP)Me]+OTf−. Consequently, we adopted the
alternative plan depicted in Scheme 4. Its key feature is a less
hindered alkylnickel precursor complex: the tris-pyridine
monoalkyl cation 5. It is worth mentioning that no Ni(II)
monoalkyl species ligated uniquely by pyridine co-ligands has
been reported to date. As shown in Scheme 4, this compound
is accessible by reacting 1 with one equivalent of pyridinium
triflate. Complex 5 can be isolated in high yield as an analyti-
cally pure greenish-yellow solid and, in contrast to the parent
Fig. 3 The crystal structure of 6b with 50% probability thermal ellip-
soids. The selected bond distances (Å) and angles (°): Ni(1)–C(33),
1.9512(17); Ni(1)–P(1), 2.0943(4); Ni(1)–N(1), 2.0329(13); Ni(1)–N(2),
1.9572(13); C(13)–N(1), 1.298(2); C(13)–C(14), 1.510(2); C(33)–Ni(1)–N(2),
90.48(6); and P(1)–Ni(1)–N(1), 84.26(4); torsion N(1)–C(13)–C(14)–P(1),
16.53(17); dihedral between DiPP ring and plane N(1)C(13)Ni(1), 72.2; di-
hedral between Ph and plane C(13)N(1)C(14), 41.6; and dihedral between
planes Ni(1)C(33)N(2) and Ni(1)N(1)P(1), 12.8.
(major) and trans (minor) isomers. The crystal structure of 6b
was determined (Fig. 3) for comparison with its neutral ana-
logue 3b. Except for the logical differences within the phosphino-
enaminate/iminophosphine systems, the structures of 6b
and 3b show a remarkable similarity, both providing nearly
identical coordination environments of the Ni(II) center. Thus,
albeit the overall positive charge would be expected to
strengthen metal–ligand interactions in the cationic derivative,
both Ni–N bonds are marginally longer in the cation 6b than
in neutral 3b, whilst both Ni–C distances are nearly the same.
Only the dative Ni–P bond exhibits a slight shortening in 6b,
and both complexes exhibit a similar degree of tetrahedral dis-
tortion at the Ni center. The similarity of the coordination
sphere of Ni in the cationic and neutral complexes suggests
that the influence of the protonated/deprotonated state of the
P–N unit is mostly limited to the PN ligand, and has only a
minor influence on the electronic state of the metal atom.
A suspension of solid NaH in THF deprotonates complexes
6a–c, cleanly affording the corresponding phosphinoenami-
nates 3a–c. The heterogeneous reaction is slow and takes
several hours to complete. However, it is also very selective,
cleanly affording a single product in each case, even if the cat-
ionic precursor (6c) existed as a cis/trans mixture. In addition,
we also investigated the deprotonation of 6b with a set of
bases of different strength (see the last point in this section,
“reversibility of acid–base reactions”). The spectra of 3b
obtained by this method are undistinguishable from those of
the sample isolated from the reaction of 1 with Lb.
1
dialkyl 1, it is fairly stable in dichloromethane solution. Its H
NMR and 19F spectra show broad signals at room temperature,
which is consistent with rapid exchange with the labile pyri-
dine in trans to the alkyl, while the pair of ligands in the cis
position are static, or exchange at a much slower rate. The labi-
lity of the pyridine ligand was also noticed in the ESI-MS spec-
trum of this compound, which only shows a faint molecular
ion signal ([Ni(CH2SiMe3)Py3]+, m/z = 382.1), and two much
stronger signals corresponding to the loss of one and two pyri-
dine ligands ([Ni(CH2SiMe3)Py2]+ and [Ni(CH2SiMe3)Py]+, m/z =
303.1 and 224.1, respectively), with correct isotope peak
distributions.
As shown in Scheme 4, complex 5 reacts with one equi-
valent of either La, Lb or Lc, cleanly affording the corres-
ponding cationic complexes 6a–6c, which were isolated as ana-
lytically pure products and fully characterized. Their NMR
spectra showed that they exist in a single geometric configur-
ation, except 6c that was obtained as a mixture of P,C cis
Oxidative addition of aryl halides to zero-valent Ni and Pd
iminophosphine species
Oxidative addition of aryl halides and sulfonates to Ni(0) and
Pd(0) complexes is a key step in many catalytic reactions
leading to C–C or C-heteroatom bond formation. Although a
large number of Ni(II) and Pd(II) aryl complexes containing
various ligands (phosphine, N-heterocyclic carbenes, etc.) have
Scheme 4 General approach to the syntheses of nickel phosphinoena-
minate complexes.
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been synthesized by oxidative addition reactions, this type of
reaction has been rarely used with iminophosphines.19–21 In
the case of Ni complexes, the availability of convenient sources
of Ni(0) such as Ni(COD)2 considerably facilitates the gene-
ration of zero-valent precursors. The oxidative addition of
stabilized phosphonium ylides has been used to prepare
phenyl-nickel enaminate complexes.19 However, to the best of
our knowledge, the oxidative addition of aryl halides or sulfo-
nates to Ni(0)-iminophosphine complexes has not been
hitherto demonstrated. Several examples of this type of reac-
tion involving Pd(0) have been reported, using either Pd(DBA)2
(DBA = dibenzylideneacetone) in the presence of the imino-
phosphine ligand20 or pre-formed Pd(0) species.21 However,
these methodologies are usually complicated by the difficulty
of removing DBA. In recent years, complex 2 has been used as
a versatile source of Pd(0), much as a synthetic equivalent of
Ni(COD)2, without the practical drawbacks of DBA com-
plexes.22 Considering these antecedents, we deemed it useful
to check whether iminophosphines La–Lc are competent
ligands to support oxidative addition reactions leading to
stable organometallic complexes of both Ni and Pd.
Fig. 4 The crystal structure of 7b with 50% probability thermal
ellipsoids. The selected bond distances (Å) and angles (°): Ni(1)–C(33),
1.8855(19); Ni(1)–P(1), 2.1058(6); Ni(1)–N(1), 2.0255(16); Ni(1)–Br(1),
2.3239(3); C(13)–N(1), 1.289(3); C(13)–C(14), 1.516(3); C(33)–Ni(1)–Br(1),
89.32(6); and P(1)–Ni(1)–N(1), 85.34(5); torsion: N(1)–C(13)–C(14)–P(1),
24.5(2); dihedral between DiPP ring and plane N(1)C(12)Ni(1), 75.7; di-
hedral between Ph ring and plane C(13)N(1)C(14), 38.4; and dihedral
between planes Ni(1)C(33)Br(1) and Ni(1)N(1)P(1), 13.7.
We first investigated the oxidative addition of 4-acetylphe- These conclusions were confirmed by the crystal structure of
nyl halides or triflate, p-AcOC6H4X (X = Br, Cl and OTf), to 7b (Fig. 4).
Ni(0). Preliminary studies showed that iminophosphines La–Lc
The oxidative addition reactions of the corresponding aryl
interact only weakly with Ni(COD)2 in THF at room tempera- triflate to Ni(COD)2/L proceed with similar colour changes.
ture, forming blue coloured solutions whose 31P{1H} NMR However, the triflate complexes 9 proved to be sensitive
spectra indicate the presence of substantial amounts of materials and their isolation is problematic. For each ligand, a
free ligands. Therefore, we combined equimolar amounts of broad resonance is observed for the main product in the 31P
Ni(COD)2, ligand (La, Lb or Lc) and the corresponding aryl {1H} spectrum of the mixture but as the reaction advances one
halide/triflate in THF at low temperatures (−80 °C), and then or two additional signals gradually gain intensity. We attribute
allowed the mixtures to react at room temperature (Scheme 5). the extra signals to the cationic species arising from the re-
Upon warming, the solutions become dark brown, evolving to placement of the triflate anion with solvent (THF), adventi-
somewhat clearer brownish-orange tones within 60–90 min. tious water, or any other potential donor present in the system.
The 31P{1H} NMR spectra of the reaction mixtures indicate The treatment of such mixtures with lithium chloride causes
that oxidative additions proceed cleanly for X = Br or Cl, the quantitative exchange of the labile triflate anion for the
affording in each case a single P-containing species. A workup halide. The 31P spectrum gets simplified, with all signals
of these solutions affords the corresponding oxidative addition being replaced with that of the corresponding chloro-complex
products as yellow solids in good yields. The spectroscopic pro- (i.e. 9 → 8). This confirms that the aryl triflate undergoes oxi-
perties of these complexes are fully consistent with their dative addition similar to its bromo and chloro analogues.
expected geometries, with their NMR spectra showing that the
Next, we investigated a similar oxidative addition of 4-acetyl-
functionalized 4-acetylphenyl fragment is incorporated into phenyl derivatives to prepare Pd aryl complexes. Initial experi-
the Ni(II) complex and is placed in cis to the phosphine donor. ments using Pd(DBA)2 and ligands La–Lc showed that the
desired products were formed, but purifying them from the
DBA released in the reaction proved a tedious and unpractical
task. We then turned to complexes 4a–c as seemingly ideal
starting materials. NMR tests using pure samples of complexes
4 with 4-bromoacetophenone showed that these reactions pro-
ceeded smoothly in THF at 60 °C, but they led to mixtures of
two P-containing products (Scheme 6). No other P resonances
were generated in the process. The integration of the 31P reso-
nances indicates that the combined yields were close to quan-
titative. Virtually the same results were obtained when the
starting materials 4a–c were generated in situ from 2 and La,
Lb or Lc, and, therefore this method was systematically
applied in further experiments, as described in the experi-
mental section. Evaporation and washing with hexane
Scheme 5 Oxidative addition of aryl halides and triflate to Ni(0)/L
combinations.
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products than observed in the reactions with aryl bromide
(14/11 ratios shown in Scheme 6 have been determined after anion
exchange with LiBr), but the alkyl/aryl product ratio exhibits
the same general trend for X = Br and OTf, decreasing in the
order La ≈ Lb < Lc. The oxidative addition of aryl triflate
becomes essentially selective for Lc, as after treatment with
LiBr, the 31P{1H} spectrum showed a strongly prevalent reso-
nance for 10c that was subsequently isolated as an analytically
pure product and fully characterized. Significantly, GC-MS ana-
lysis of the corresponding reaction mixture showed the pres-
ence of 1,2-bistrimethylsilylethane, but the cross-coupled
organic product was not detected. In contrast, the reaction of
4c with p-iodoacetophenone afforded even higher content of
the alkyl product than 4-bromoacetophenone. This obser-
vation implies that the alkyl ratio increases with the halide or
pseudohalide group in the order I > Br > OTf.
Scheme 6 Oxidative addition reactions using complexes 4 as Pd(0)
sources.
afforded solids containing mixtures of two organometallic pro-
We proposed the mechanism shown in Scheme 7 to explain
ducts. A careful analysis of the NMR spectra of these mixtures our experimental observations. The process begins with the
showed that the products are as shown in Scheme 6. The iden- irreversible reductive elimination of 1,2-bistrimethylsilylethane
tity of the Pd-aryl products 10a–c was confirmed by the pres- from complexes 4 to generate a Pd(0) species. Note that the
ence of the characteristic acetyl signal in the spectra, and by a [(P ∼ N)Pd] notation does not mean any explicit guess on the
comparison of the chemical shifts of their 31P resonances with actual structure of such a species. The essentially quantitative
those of the same compounds generated from Pd(DBA)2. The spectroscopic yields show that the Pd(0) intermediate is
identity of the mono-trimethylsilyl species 13a–c was ascer- efficiently trapped by the aryl halide. The fact that these inter-
tained from the characteristic high-field signals of the mediates were not detected in the 31P{1H} monitoring indi-
CH2SiMe3 group, and the splitting of the CH2 resonance by cates that reductive elimination is slower than any subsequent
coupling with the 31P nucleus (2JHP ≈ 6 Hz). The crude reaction step or, in other words, the rate determining step. The for-
mixtures were analysed by GC-MS, which led to the detection mation of monalkyl complexes (13, 14 and 15) could arise by
of the homo-coupling product 1,2-bis(trimethylsily)ethane, two different pathways. One of them, represented in the
along with the cross-coupled 4-(trimethylsilyl)methyl-acetophe- middle line of Scheme 7, is Pd–Pd alkyl-halide exchange (or
none. However, neither 4,4′-diacetylbiphenyl nor acetophenone transmetallation) between the oxidative addition products
(hydrodehalogenation product) was detected in the mixtures.
As mentioned above, the use of complex 2 as a practical
Pd(0) precursor is becoming a rather common option in homo-
geneous catalysis, with preference over conventional sources of
Pd(0). The actual outcome of such “catalyst activation” reac-
tions is seldom investigated, but it has been recently shown
that in some cases it can be more complex than foreseen, and
affords a number of unexpected side products.23 Therefore, we
deemed it worthwhile to perform some additional experiments
in our system in order to gain some insight into the mecha-
nism of this reaction. First, we examined the reactions of com-
plexes 4 with p-acetylphenyltriflate and 4-iodoaceto-phenone
(the latter only with 4c). 4-Acetylphenyl triflate reacts with 4a–c
under the same conditions tested with the bromide and, as
observed for Ni, the 31P{1H} spectrum of the final reaction
mixture contains several broad signals (up to four), presum-
ably due to the partial replacement of the labile triflate anion
of the initial products (11 and 14) with potential ligands
present in the mixture (solvent, H2O traces, COD, etc.). As
observed with the Ni triflates, the treatment of the final reac-
tion mixtures with solid LiBr in excess simplified the 31P{1H}
spectra, with the original broad signals being replaced by the
signals of products 10 and 13 previously observed in the reac-
tions with p-bromoacetophenone. Interestingly, the oxidative
(10–12) and starting dialkyl 4 to yield an unstable alkyl–aryl
Scheme 7 The possible mechanisms leading to the competitive for-
mation of aryl and alkyl products. Top, generation of a Pd(0) precursor,
followed by oxidative addition. Center, bimolecular alkyl/halide
exchange. Bottom, direct reaction of the dialkyl 4 with the electrophile
addition of the aryl triflate yields a larger fraction of the aryl (not operative).
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intermediate. This rapidly undergoes reductive elimination, further control experiments. In these experiments complex 4b
affording the cross-coupling organic product and regenerating was allowed to decompose at 40 °C in the presence of 2 and 4
the Pd(0) species that is recycled through oxidative addition. A equivalents of methyl acrylate, an electron-poor olefin that
similar Pd–Pd aryl-halide transmetallation step was identified could also act as a reductive elimination promotor.26 The half-
by A. Yamamoto in the reaction of palladium diaryls with MeI lives of 4b in these experiments were 60 and 49 min, fairly
or aryl halides,24 and Pd(II)–Pd(II) aryl exchanges have been similar to those measured in the blank experiment, which con-
subsequently proposed by other authors to occur in Pd-cata- firmed that the role of reductive elimination enhancement by
lyzed reactions.25 The bimolecular alkyl-halide exchange is the electrophile is marginal, at best.
probably driven by the special activation of the Pd–C bond
In closing this section, it is worth highlighting the marked
placed in trans to the P donor atom in 4. This path is consist- effect of temperature on the 13b/10b ratio: the selectivity of the
ent with the observed influence of X (halide or triflate) on the oxidative addition product actually decreased by a factor of 2
alkyl/aryl ratio, as the better the ability of X to bridge metal when the reaction temperature was lowered from 60 to 40 °C,
centres, the more efficient the exchange (hence the I > Br > giving a cautionary warning on the practical applications of
OTf trend). The lower alkyl/aryl ratios observed with Lc can be bis(alkyl)Pd(II) complexes as a source of Pd(0). According to our
attributed to the lower rate of bimolecular exchange, hampered mechanistic proposal, the product ratio is dictated by the rela-
by the steric hindrance of this bulky ligand. In agreement with tive rates of the alkyl/halide exchange step (a bimolecular reac-
the Pd–Pd transmetallation proposal, we confirmed that react- tion) and reductive elimination (a unimolecular process).
ing 1 equiv. of complex 4b with a 3 : 1 10b/13b mixture at Bimolecular processes have strongly negative activation entro-
60 °C, in the absence of additional ArBr, caused a 30% pies, and therefore their rates scale more slowly with tempera-
enhancement of the signal 13b at the expense of 10b, as ture compared with those of the unimolecular reactions. Thus,
expected if alkyl-halide exchange occurs between 10b and 4b the formation of the alkyl side product is expected to decrease
concurrently with reductive elimination processes.
or even to be suppressed as the temperature is increased. A
An alternative route to the alkyl complexes 13–15, shown in similar argument leads to the conclusion that the alkyl/aryl
the lower part of Scheme 7, would entail a competitive reaction product ratio will increase with the overall Pd concentration
of the starting dialkyl 4 with the aryl halide to afford the (i.e., the initial concentration of the starting dialkyl 4), as the
corresponding monoalkyl (13–15) and the cross-coupled rate of the comproportionation step has a quadratic depen-
organic product in a single step. To gather some additional dency on concentration, vs. simple dependency for the reduc-
mechanistic information on this regard, we investigated the tive elimination rate. Low temperatures and high concen-
overall rate of disappearance of 4b in the presence of 0, 2 and tration of the starting dialkyl are conditions favorable for
4 equiv. of 4-bromoacetophenone. At 40 °C (the lower tempera- organometallic synthesis, whilst in catalysis very diluted metal
ture was selected in order to facilitate data collection) 4b complexes and relatively high temperatures are commonly
decays with estimated half-lives of 61, 19 and 17 min, respect- used. The conclusion is that using complex 2 as a Pd(0) equi-
ively.§ The blank reaction (no electrophile added) leads to a valent may be appropriate for catalytic applications, but can be
very dark solution probably containing colloidal Pd(0), and a inconvenient for synthetic purposes, due to the formation of
number of low intensity resonances in the 31P{1H} spectrum. the mono(trimethylsilyl)methyl product as an awkward
Thus, albeit the decomposition of 4b alone is somewhat byproduct.
slower, the half-life of 4b does not change appreciably when
the concentration of the electrophile is increased twofold (2 to
4 equiv.), nor does the product ratio (13b/10b = 2.1 : 1 in the
Reversible acid–base reactions of nickel iminophosphine/
enaminate complexes
latter two experiments). This is incompatible with the direct In addition to NaH, other strong bases such as tBuOK, Cs2CO3
reaction of 4b with bromoacetophenone, as in this case the and DBU deprotonate 6b in THF (Scheme 8). When these reac-
reaction rate should exhibit a direct dependency on the con- tions were monitored using 31P{1H} spectra, the resonance of
centration of the aryl bromide. The bimolecular exchange the starting material at 42.5 ppm was replaced in all cases with
mechanism also accounts for the slower decomposition rate in that of 3b at 33.2 ppm (both in THF). In contrast, triethyl-
the blank reaction, since the overall rate of decay of 4b is the amine, diisopropyl(ethyl)amine (Hunig’s base) or proton
sum of the reductive elimination and the bimolecular sponge fail to induce any change in the spectrum, either at
exchange reactions, and the latter does not occur in the room temperature or after heating to 60 °C. The addition of
absence of the electrophile. To confirm that the electron-poor one equivalent of HOTf to solution of 3b generated with either
aryl halide is not accelerating reductive elimination on 4b, via NaH or tBuOK caused the disappearance of its 31P resonance
transient coordination of the electrophile, we carried out two and restored that of 6b, demonstrating the full reversibility of
these acid–base processes.
As an insoluble, hence heterogeneous base, NaH is efficient
but slow. In contrast, soluble bases (tBuOK and DBU) or par-
tially soluble ones (Cs2CO3) deprotonate 6b rapidly. Upon treat-
§The half-lives were estimated from approximate first-order kinetic plots, but
this reaction is not expected to obey a simple kinetic law. At this stage, a com-
parison of overall reaction rates suffices to draw some valuable conclusions
ment with such bases, 6b is consumed within the time
required to carry the sample into the NMR probe. Whereas
regarding the influence of the aryl bromide concentration on the reaction rate
and product distribution.
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Scheme 9 Deprotonation of complex 7a.
of a nickel aryl complex 7a with NaH. Note that 7a has at least
two potentially reactive points for a strong base, the iminophos-
phine ligand, and the acetyl group. In addition, the deproto-
nation of a neutral complex should lead to an anionic species.
The latter could eliminate insoluble sodium bromide leaving
an unstable, coordinatively unsaturated complex. Thus, we
were pleased to observe that 7a reacts cleanly in THF solution
with a large excess of solid NaH (>10 eq.), affording a single
31P-containing product 16. This was isolated as a highly sensi-
tive brick-red solid, nearly insoluble in C6D6, that decomposed
in CD2Cl2. The 1H NMR spectrum, recorded in THF-d8, showed
a signal for the enaminate CH at 3.04 ppm with a small coup-
ling to 31P (1.5 Hz), and the usual 3H-intensity of the p-acetyl
resonance at 2.33 ppm that confirmed that only the iminophos-
phine ligand had been deprotonated. These spectroscopic
properties, in addition to its insolubility in hydrocarbon sol-
vents, suggested that 16 might be an anionic complex of the
type [Ni(L′a)(X)(p-C6H4COCH3)]−Na+, with L′a representing the
conjugate base of ligand La, as shown in Scheme 9.
Surprisingly, the 1H NMR spectrum also shows a high-field
resonance (−2.78 ppm) integrating for 1H, which suggests that
X could be a hydroxide ligand. Accordingly, the IR spectrum of
16 shows a weak, but sharp band at 3584 cm−1, overlapping an
intense but broader one centered at 3422 cm−1, which is con-
sistent with a partially associated hydroxide ligand. The
hydroxide could come from Br exchange with a small amount
of NaOH in sodium hydride, used in a large excess (10-fold), or
may be generated by the interaction of NaH with moisture
traces. As a matter of fact, we confirmed that 7a is also cleanly
deprotonated by a suspension of NaOH in anhydrous THF,
directly yielding the same product 16 in essentially quantitat-
ive yield. Complex 16 was crystallized by layering THF solution
with diethyl ether, and its crystal structure was determined
(Fig. 5).
Scheme 8 Reversible deprotonation of cationic nickel iminophosphine
complexes to neutral phosphinoenaminate with different base strengths.
reactions with NaH, tBuOK, and Cs2CO3 are essentially quanti-
tative, the intensity of the signal of 3b in the experiment with
1 equiv. of DBU was ca. 25% of the expected value, though it
was the only detected resonance in the final spectrum. An
excess of this base (5 equiv.) causes irreversible decompo-
sition, the final spectrum showing a number of low intensity
31P resonances different from those of 3b or 6b, those of the
free ligand Lb amongst them.
A basicity scale is displayed at the bottom of Scheme 8
(pKaH is the pKa of the acid conjugates in organic, non-protic
solvents¶ 27). Consistent with our qualitative observations,
bases placed below DBU in the scale fail to perform the depro-
tonation of 6b, at least to a noticeable extent. Thus, it can be
assumed that the basicity of 3b is comparable with that of
DBU. Very likely, the decomposition problems observed with
this base relate to incomplete deprotonation of the iminophos-
phine ligand, allowing some other undisclosed processes to
compete and, eventually, causing decomposition of the
material (note that DBU is a non-innocent base, as it is also a
potent coordinating ligand towards Ni28). Interestingly, whilst
the transformations associated with the stronger bases NaH or
KtBuO generate a narrow signal of 3b, the same signal appears
broad in the spectra generated with Cs2CO3 or DBU,
suggesting that in these cases reversible proton exchange equi-
libria are established.
In a further test, we explored the capacity of imino-
phosphine ligands to support reversible deprotonation in a
different coordination environment, carrying out the reaction
The structure of 16 confirmed the presence of the anionic
enaminato(aryl)nickel moiety, with a hydroxo ligand occupying
the fourth coordination position. In the solid state, the
sodium cation coordinates to the hydroxo ligand, through a
strong (short) Na–O bond (2.160(2) Å), and completes its
coordination sphere with two ether molecules (one THF and
¶The approximate basicity scales shown in Scheme 8 are based on the pKa
values measured for the corresponding conjugate acids in different nonaqueous
acids. The largest series of pKa values are available in MeCN (DBU-H+, 24.3;
H-proton sponge+, 18.6; and H-NEt3+, 18.6) (see ref. 28a). Other values can be one Et2O), and a dative interaction with the acetyl oxygen atom
estimated from other solvents using empirical correlations for X-OH acids (ref.
belonging to a second coordination unit. This gives rise to a
dimeric molecular entity in the solid state. This arrangement
presents a contact (3.080(3) Å) between the sodium atom and
C28, one of the carbon atoms of the aromatic ring bound to
the Ni center. The coordination environment of Na in this
28b): pKa in MeCN = pKa in DMSO +10.5 (used for tBuOH, pKa = 29.4 in DMSO,
ref. 28c) and pKa in MeCN = 1.6·pKa in H2O + 14.9 for HCO3− (pKa = 10.3 in H2O)
as an estimation for Cs2CO3. Similar correlations are not available for THF but
+
the pKa values in this solvent are also given in ref. 28a for HNEt3 (12.5),
H-proton sponge+ (11.1) and DBU (16.6).
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Ni(COD)2 + L. The use of complexes 4a–c as Pd(0) sources for
the analogous oxidative addition reactions was found unpracti-
cal for synthetic purposes, due to the formation of undesired
side products arising from competitive Pd(II)–Pd(II) alkyl/halide
exchange. The weak acidic character of the phosphino-imine
ligand is a source of interesting reactivity with potential
applications in catalysis. We have demonstrated the reversibi-
lity of the deprotonation of this ligand for Ni, a potential route
for cooperative metal–ligand behavior. Tests with bases of
different strengths indicate that the iminophosphine/phosphi-
noenaminate pair in the coordination sphere of Ni is an acid/
base pair comparable to H-DBU+/DBU pair. Interestingly,
neutral halo(aryl) complexes arising from the oxidative
addition of 4-bromoacetophenone are selectively deprotonated
at the iminophosphine ligand, even with strong bases,
Fig. 5 The crystal structure of complex 16·(THF, Et2O) with 50% prob-
ability thermal ellipsoids. The selected bond distances (Å): Ni1–O1,
1.8916(18); Ni1–C27, 1.896(3); Ni1–N1, 1.981(2); Ni1–P1, 2.1438(12); N1–
C2, 1.363(3); C(33)–O(2), 1.214(4); C(33)–C(30), 1.483(4); C(33)–C(34),
1.510(5); O1–Na1, 2.160(2); O2–Na1’, 2.271(3); O3A–Na1, 2.335(4); O4A–
Na1, 2.384(4); and Na1–C(28), 3.080(3). The selected angles (°): O1–N1– affording an unusual type of anionic arylnickel complex. These
C27, 87.41(19) and N1–Ni1–P1, 85.49(6); torsion: N1vC2–C1–P1, 4.0(3);
and angle between planes O1–Ni1–C27 and N1–Ni1–P1, 9.2.
results open interesting prospects in the application of nickel
complexes as catalysts for coupling reactions such as the Heck
or Suzuki reactions, with iminophosphine ligands acting as
auxiliaries in proton transfer steps, or base-promoted acti-
vation of the electrophile.
molecule is most likely contingent upon the choice of the
solvent and crystallization conditions. In solution, it would be
reasonable to assume that 16 has the ion pair structure shown
in Scheme 9, with an external Na+ cation fully solvated with
THF. The Ni–O distance (1.889(2) Å) is similar to that found in
other square-planar, monomeric Ni(II) hydroxides reported in
the literature, either neutral or anionic.29
It is remarkable that the deprotonation of 7a should afford
selectively complex 16, as NaH is one of the bases commonly
used to generate enolates from ketones. The low acidity of the
acetyl group of this organometallic complex is probably due to
the influence of the electron-rich Ni fragment connected to the
para position in the aryl ring. Since 7a arises from the oxi-
dative addition of a functional aryl halide to a Ni(0) species,
the outcome of this experiment is relevant for potential appli-
cations of the nickel-iminophosphine system in cross-coupling
catalysis.
Experimental section
General considerations
All manipulations were carried out under an inert atmosphere,
either using the standard Schlenk technique in a vacuum line
operating with purified argon, or a nitrogen-filled glove box
equipped with a pump for solvent evaporation, an externally
cooled (with liquid N2) cavity, a freezer and an analytical
balance. The following paragraphs provide a summary of the
general procedures, and a full description of the syntheses and
complete spectroscopic and analytical data are given in the
ESI‡ for all ligands and complexes.
General procedure for the synthesis of iminophosphine
ligands (La, Lb, and Lc). To a solution of imine Ph(Me)CvN
(DiPP) (0.560 g, 2 mmol) in THF (10 mL), stirred at −80 °C,
t
was added 1.6 mL of a 1.7 M solution of BuLi in pentane
(2.72 mmol). The stirring was continued for 30 minutes, and
then the cooling bath was removed allowing the mixture to
warm to room temperature. Then, the volume corresponding
Conclusions
In this article we explored the ability of iminophosphine
ligands to stabilize a variety of organonickel and organopalla-
dium complexes, supporting reversible deprotonation and oxi-
dative addition reactions. The syntheses of such complexes
have been adapted to each of these metals. Thus, ligand
exchange reactions from suitable bis(trimethylsilyl)methyl pre-
cursor complexes with La–Lc have divergent results for M = Ni
(1) or Pd (2) probably due to the different sizes of the metal
atoms. The exchange is straightforward for the latter affording
dialkylpalladium complexes 4a–c. In contrast, dialkyl precur-
sor 1 reacts with iminophosphine Lb affording the phosphi-
noenaminate complex 3b. We devised a general synthesis of
to
1 equivalent of the corresponding chlorophosphine
(2 mmol) was added, and the stirring was continued, monitor-
ing the reaction by 31P NMR spectroscopy. Usually, the reac-
tions were complete within ca. 30 min. The solvent was
removed under vacuum and the residue was then extracted
with n-hexane (40 mL) and the extract was filtered through
Celite. Evaporation under reduced pressure leaves a pale-
yellow oil that can be stored at 5 °C under an Ar atmosphere.
Ligand exchange reactions
Reaction of iminophosphine ligands La–Lc with nickel pre-
nickel phosphinoenaminates 3, starting from the new cationic cursor 1
monoalkylnickel precusor 5. Neutral arylnickel complexes
Isolation of 3b. To a solution of complex 1 (0.115 g,
were readily obtained via oxidative additions to mixtures 0.3 mmol) in 10 mL of THF, stirred at −80 °C, was added an
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equimolar amount of the corresponding ligand (La, Lb or Lc), precursor (6a, 6b or 6c) in 2 mL of THF, generated as described
dissolved in 1.5 mL of THF. The mixture was stirred for 5 min, above, starting from equimolar amounts (ca. 0.250 mmol) of 1,
and then allowed to stir at room temperature. The 31P{1H} [HPy2]+OTf− and the corresponding ligand, was transferred to
spectra of the reaction mixtures were recorded periodically. a scintillation vial charged with a 10-fold excess of NaH and a
The initial spectra, recorded after 1 h, showed several signals, small magnetic bar. The mixture was stirred for 1 h at room
including some unreacted free ligands. After 20 h, the spec- temperature, after which time the solution was separated from
trum of the mixture generated from Lb showed a single signal the solids using a syringe capped with a PTFE 0.45 μM HPLC
at 32.3 ppm (together with a small amount of unreacted Lb), filter. Next, the clear solution was dried under vacuum and the
but those generated from La and Lc still showed a number of residue was extracted with a suitable low-polarity solvent (see
peaks that revealed the existence of a number of different pro- specific descriptions) to remove the NaOTf that might remain
ducts. After this time, the mixtures were evaporated, the resi- in the sample. The solution was filtered through the HPLC
dues taken up in Et2O (15 mL) and the solutions filtered filter and evaporated. If necessary, the residue can be recrystal-
through Celite. These were concentrated and some hexane was lized by taking it up in a minimum amount of dichloro-
added until becoming slightly turbid. Cooling in the freezer methane, layering the solution with hexane and allowing the
(−25 °C) led to the crystallization of 3b (from Lb), but no solid liquids to slowly diffuse one into another at the box freezer
products crystallized from the mixtures generated from La or temperature (−25 °C). The solids were separated, washed with
Lc. The red-orange crystals of 3b were collected by filtration, a small amount of hexane and dried under vacuum.
and a small sample of crystals was reserved for X-ray analysis.
Oxidative addition of 4-X-C6H4COMe to Ni(0)/L (L = La, Lb or
Lc; and X = Br or Cl)
The remaining product was washed with some hexane and
dried in vacuo. Yield, 0.084 g, 0.12 mmol, 40%. Complete spec-
troscopic data for 3b can be found in the ESI.‡
Synthesis of the aryl derivatives 7a–c (X = Br) and 8a–c
General procedure for the preparation of dialkyl palladium(II) (X = Cl)
complexes 4a–c. A solution of ca. 0.1 mmol complex bis[(tri-
General procedure. A 100 mL round-bottom Schlenk flask
methylsilyl)methyl] (1,5-cyclooctadiene) palladium(II) (2) in loaded with Ni(COD)2 (ca. 1 mmol), THF (ca. 10 mL) and a stir-
3 mL of THF, cooled at −80 °C, was treated with a solution ring bar was cooled to −80 °C. To the stirred suspension was
containing an equimolar amount of the corresponding ligand added a solution containing equimolar amounts of the corres-
(La, Lb or Lc) in 1 mL of THF. The mixtures were stirred at this ponding ligand (La, Lb or Lc) and the aryl halide. After 5 min,
temperature for 5 minutes, then the cold bath was removed the cooling bath was removed. As the reaction mixture
and the mixtures stirred at room temperature for 1 h. Upon approached the room temperature, the colour became dark
completion of the reaction, the products were separated by fil- brown. As the stirring was continued at room temperature, the
tration, washed with cold hexane, and dried under vacuum. color turned gradually to dark yellow. After 2 h, the mixture
These were crystallized either from n-pentane or from a was evaporated under reduced pressure. The oily residue was
CH2Cl2/n-hexane mixture, and stored in glass vials in the extracted with CH2Cl2, and the solution was filtered through a
glove-box freezer at −25 °C.
Celite plug. The clear solution was evaporated and the oily
General procedure for the preparation of cationic monoalkyl residue cooled to −80 °C and vigorously washed with 2 × 20
nickel(II) complexes 6a–c. First, complex 5 was generated mL of diethyl ether. The ether washings were discarded,
in situ: in the glove box, a vial containing a solution of and the resulting powdery solid was dried under vacuum. The
Ni(CH2SiMe3)2Py2 in THF (e.g. 0.25 mmol in 2 mL) was placed powders were spectroscopically pure, but they were recrystal-
in a well cooled to N2(l) temperature, and treated with a solution lized by slow diffusion of Et2O or hexane into a cold, concen-
containing the exact equimolar amount of [Py2H]+[TfO]− in the trated CH2Cl2 solution, in the glove box freezer.
same amount of THF. The mixture was shaken and allowed to
Oxidative addition of 4-TfO-C6H4COMe to Ni(0)/L (L = La, Lb
or Lc)
stir for 1 h at 25 °C. Once the reaction was completed, to the
resulting solution of 5 was added a solution containing the
equivalent amount of the corresponding ligand La, Lb or Lc,
General procedure. In the glove box, a scintillation vial was
and the stirring was continued for one more hour. The solvent charged with an amount close to 0.05 mmol Ni(COD)2 and a
was evaporated under vacuum and the residue washed/solidi- small stirring bar. The vial was placed in a well cooled to
fied with 5 mL of cold hexane (precooled at N2(l) temperature). liquid nitrogen temperature, a THF solution containing equi-
The products were recrystallized by slow diffusion of hexane molar amounts of the corresponding ligand (La, Lb or Lc) and
into a solution of the complex in CH2Cl2 at −25 °C.
4-TfOC6H4COCH3 was added, and the total volume was
adjusted to 1 mL with THF. The vial was placed on a magnetic
stirrer and allowed to stir at room temperature. As the mixture
warmed, its color changed similarly to the reactions with
Syntheses of neutral alkylnickel complexes [Ni(CH2SiMe3)(L′)
(Py)] (L′ = deprotonated L), 3a–c
General procedure. The syntheses of these complexes were 4-bromo- or 4-chloroacetophenone, becoming first dark
carried out using a one-pot procedure starting from precursor brown, and then changing gradually to a yellow or greenish-
1, [HPy2]+OTf−, and the corresponding ligand La, Lb or Lc. In yellow hue. After 1 h, the mixture was transferred to a gas-tight
the glove box, a solution containing the appropriate cationic (PTFE-valve) NMR tube containing a sealed glass capillary with
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the external standard (PPh3 and C6F6 in C6D6), in order to troscopy. The mixtures produced by reaction with 4-acetyl-
record its 31P{1H} and 19F NMR spectra. These spectra phenyl triflate were also analyzed by GC-MS, but the main frac-
showed a number of broad signals different from those of the tion was treated with a two-fold excess of LiBr and stirred for
starting materials (ligand (31P) and the organic triflate (19F, over 1 h. The 31P{1H} NMR spectra of these mixtures showed
δ −76.1 ppm in THF). In order to confirm that these samples the same signals observed in the reactions with 4-bromoaceto-
contain a mixture of the oxidative addition complex (9a, 9b or phenone. Following this procedure, 10c was isolated as a pure
9c), and the related ionic species arising from the displace- product as specified below, and complete NMR data were col-
ment of the triflate anion, these mixtures were converted into lected for this compound. In order to assign the signals corres-
the previously known chloro complexes (8a, 8b or 8c), as ponding to the aryl oxidative addition products, 10a and 10b,
follows: the NMR tube was brought back to the glove box, separate NMR tests were carried out in which equimolar
where it was open and a 10-fold excess of solid LiCl was amounts of ca. 0.05 mmol Pd(DBA)2 and the corresponding
added. The mixture was gently shaken and placed in the ultra- ligand (La or Lb) in THF (external PPh3/C6D6 glass capillary
sound bath to activate the solid LiCl. After allowing the solids reference) were heated to 60 °C until the reaction proved com-
to settle, the 31P{1H} spectra were recorded, which showed a pleted (ca. 3 h), and the 31P{1H} spectrum of the resulting oxi-
single, sharp signal for the corresponding chloro derivative dative addition product was compared with that of the sample.
(31P {1H} in THF, reference vs. external PPh3: δ 46.4 (8a), 34.1 The selected NMR data for the monoaryl and monoalkyl com-
(8b) and 25.0 (8c)). The integration of these signals vs. the plexes are listed below.
external reference showed that the total 31P intensity was held
constant after the triflate/chloride exchange.
In a separate experiment, a solution of complex 4b in THF
was generated as indicated above, and then split in two equal
In order to confirm that the final products were chloro parts. To one of them was added the stoichiometric amount of
derivatives, the oxidative addition reaction of Ni(COD)2 with p-bromoacetophenone, and heated at 60 °C until completion.
Lb and 4-TfOC6H4COCH3 was carried out on a preparative
A
31P{1H} NMR spectrum showed that the corresponding
scale (1 mmol), following the same procedure described above mixture of complexes 10b and 13b had been formed. To this
for the oxidative addition of 4-haloacetophenones, except that mixture was added the second half of the 4b solution, and the
a ten-fold excess of LiCl was added once the oxidative addition combination was heated again to 60 °C, until the 31P reso-
reaction was complete. The usual workup yielded the expected nance of 4b disappeared. The final 31P spectrum showed an
complex 8b, which was identical to the sample obtained from enhancement of the signal of 13b (ca. 30%) and a decrease of
4-chloroacetophenone. The 31P{1H} spectrum of the intermedi- that of 10b by a similar amount.
ate triflate complex 9b (THF solution, external PPh3/C6D6 refer-
The available NMR data for the mixtures 10/13 are provided
ence) showed two broad signals at δ 37.3 (major) and 40.4 in the ESI.‡
(minor), and the 19F spectrum showed a prominent signal at
NMR monitoring of the thermal behaviour of 4b in the pres-
δ 80.3, probably due to the coordinated or ion-paired triflate ence of different additives. In the glove box, a solution contain-
anion. All attempts to isolate the intermediate triflate complex ing 0.234 g (1 mmol) of 4b in 1 mL of THF was prepared. This
9b were unsuccessful.
was divided into five portions of 0.2 ml, and each one was
transferred to a different gas-tight NMR tube (PTFE valve), con-
taining a sealed glass capillary with a PPh3/C6D6 external refer-
ence. One of the tubes was used to prepare a blank (control)
Studies on the oxidative addition of p-XC6H4COCH3 to Pd(0)
using complex 2 as a precursor
Qualitative experiments. A solution containing a precisely sample with no other reagent added. Two samples were
weighed amount of the precursor 2 (ca. 0.2 mmol) was dis- treated with, respectively, 2 equiv. (0.018 g, 0.093 mmol) and 4
solved in ca. 15 mL of THF, and transferred to a glass ampoule equiv. (0.018 g, 0.093 mmol) of 4-bromoacetophenone, each
with PTFE valve, and the content was cooled to −80 °C. To this dissolved in 0.2 mL of THF. The remaining two samples were
solution was added 1 equiv. of the corresponding iminopho- treated with, respectively, 2 equiv. (8.50 μL) and 4 equiv.
sphine ligand (La, Lb or Lc) dissolved in THF (ca. 1 mL). The (17.0 μL) of methyl acrylate. Subsequently, the total volume of
solution was stirred for 1 h at room temperature to ensure the the five samples was adjusted to 0.6 ml with THF. The samples
complete displacement of COD and formation of the corres- were analysed by 31P{1H} NMR and then they were heated at
ponding complex, 4a, 4b or 4c, respectively. To this solution 40 °C in an oil bath, removing all of them and bringing them
was added 1 equiv. of the corresponding aryl halide or triflate back simultaneously to record their spectra at predefined time
dissolved in ca. 1 equiv. of THF. The ampoule was heated in an lapses: 15, 45, 105, 225, and 345 min. The decay of the 31P
oil bath at the prescribed temperature, and the contents were NMR signal of 4b was fitted to a first-order plot, from which
periodically analyzed by recording their 31P{1H} NMR spectra the half-live of the complex in each sample was computed.
in THF with an external PPh3/C6D6 standard sealed in a glass
NMR studies on the reversible deprotonation of [Ni(CH2SiMe3)
capillary. When the reaction was completed, the contents were
worked-up as follows: for the reactions with 4-bromoacetophe-
(py)(Lb)]+[OTf]− (6b) with different bases
none, a small sample was removed and subjected to GC-MS
General procedure. In the glove box, a scintillation vial
analysis. The remaining sample was dried, washed with cold charged with 0.030 g (35 µmol) of complex 6b was dissolved
1
hexane, and the residue was analyzed by H and 31P{1H} spec- in 1 mL of THF. When the base to be tested was soluble in
332 | Dalton Trans., 2020, 49, 322–335
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THF, an exact equimolar amount was added to the mixture remained clear orange, no darkening was observed as with
(triethylamine (5 µL), diisopropylethylamine (6 µL), proton NaH. 31P{1H} NMR (161.9 MHz, 25 °C, THF-d8): δ 47.7 (s).
sponge (7.5 mg), DBU (5 µl), or KtBuO (3.9 mg)). In the case 1H NMR (400 MHz, 25 °C, THF-d8): δ −2.79 (s, 1 H, –OH), 0.96
3
3
of the partially or totally insoluble bases Cs2CO3 and NaH, a (d, 6 H, JHH = 7.0 Hz, ArCHMeMe), 1.11 (dd, 6 H, JHP
=
=
3
3
10-fold excess of the base was used and the mixture was 15.1 Hz, JHH = 7.3 Hz, P-CHMeMe), 1.29 (dd, 6 H, JHP
stirred for 18 h. The resulting solutions were transferred to a 13.3 Hz, 3JHH = 7.0 Hz, P-CHMeMe), 1.55 (d, 6 H, 3JHH = 6.8 Hz,
2
3
gas-tight NMR tube, provided with an external reference ArCHMeMe), 1.82 (d sept, 2 H, JHP = 9.2 Hz, JHH = 6.9 Hz,
(PPh3/C6D6) sealed in a glass capillary. Triethylamine, diiso- P-CHMe2), 2.33 (s, 3 H, –COCH3), 3.03 (d, 1 H, JHP = 1.7 Hz,
2
3
propylethylamine, and proton sponge failed to induce any P-CH), 3.86 (sept, 2 H, JHH = 7.2 Hz, CHMe2), 6.73 (m, 2 H,
change on the 31P{1H} spectrum of 6b, even after 18 h at m-N-Ar), 6.75 (m, 1 H, p-N-Ar), 6.87 (m, 2 H, m-Ph), 6.87 (m, 1
3
room temperature. The samples containing triethylamine H, p-Ph), 7.02 (m, 2 H, o-Ph), 7.31 (d, 2 H, JHH = 8.1 Hz,
and diisopropylethylamine were taken to the glove box and m-Ni-Ar), 7.88 (dd, 2 H, JHP = 1.1 Hz, JHH = 8.1 Hz, o-Ni-Ar).
an additional amount (4 equiv.) of the corresponding base 13C{1H} NMR (100.6 MHz, 25 °C, THF-d8): δ 17.7 (P-CHMeMe),
4
3
1
was added. Although some color darkening was observed, 18.5 (P-CHMeMe), 23.4 (ArCHMeMe), 23.8 (d, JCP = 30.3 Hz,
their spectra showed no noticeable changes. The sample con- PCHMe2), 25.8 (ArCHMeMe), 25.8 (–COCH3), 28.8 (ArCHMe2),
taining the proton sponge was heated at 60 °C for 1 h, but its 67.9 (hidden under the solvent residual signal, P-CH), 122.7
31P{1H} spectrum remained unaltered. In the case of DBU, an (m-CH, N-Ar), 123.2 (p-CH, N-Ar), 123.7 (m-CH, Ni-Ar), 126.5
immediate change to orange occurred on mixing, the 31P{1H} (p-CH, Ph), 126.8 (m-CH, Ph), 129.3 (o-CH, Ph), 131.8 (p-Cq, Ni-
3
signal of 6b was replaced with a broad resonance in the posi- Ar), 139.1 (o-CH, NiAr), 142.7 (d, JCP = 16.2 Hz, ipso-Cq, Ph),
tion expected for 3b, albeit with lower intensity than 147.2 (o-Cq, N-Ar), 148.3 (ipso-Cq N-Ar), 175.8 (d, 2JCP = 22.5 Hz,
expected. The sample remained stable at room temperature vC–N), 182.4 (d, 2JCP = 43.0 Hz, ipso-Cq, Ni-Ar), 197.2 (–COMe).
for 18 h, then it was taken to the box and an extra amount of IR (nujol mull, cm−1): 3584 (sh, w, (O–H)), 3422 (br, st, (O–H)),
DBU (4 equiv.) was added, prior to recording its 31P{1H} spec- 3053 (sh, w, ν(C–H arom.), 1669, 1657 (ν(CvO, νCvC). Anal.
trum. The reactions with Cs2CO3, KtBuO or NaH caused a calcd for C34H45NNaNiO2P: C, 66.68; H, 7.41; N, 2.29. Found:
rapid color change to orange, associated with the deprotona- C, 66.93; H, 7.68; N, 1.96.
tion of 6b, as confirmed by the 31P {1H} spectra of the mix-
tures, which showed a single resonance for 3b. The experi-
ment was repeated with NaH, and after confirming that full
conversion had taken place, the sample was brought back to
Conflicts of interest
There are no conflicts to declare.
the box, any excess of NaH or solid salts was filtered out
using a 0.45 µm PTFE filter and an equimolar amount of
triflic acid was added, resulting in the clean regeneration of
the 31P signal of 6b. The same operation was repeated with
the KtBuO sample, with an identical result.
Acknowledgements
The Spanish Ministries of Economy and Innovation (MINECO)
and Ciencia, Innovación y Universidades (MCIU/AEI) and the
EU FEDER funds of the European Union supported this work
through grants CTQ2015-68978-P and PGC2018-095768-B-I00.
Deprotonation of complex 7a with NaH or NaOH
Syntheses of [Ni(p-C6H4COMe)(OH)(L′a)]−Na+ 16. In the
glove box, NaH (0.037 g, 1.53 mmol) was placed in a 7 mL scin-
tillation vial with a small magnetic sitting bar. Then, a solu-
tion of complex 7a (0.100 g, 0.153 mmol) in 4.5 mL of THF
was added dropwise to the NaH reagent at room temperature.
The reaction mixture was stirred for 1 h, during which time its
colour changed from yellow to dark orange. Once the pre-
scribed time was over, the mixture was filtered through a PTFE
0.45 μM HPLC syringe filter and dried under reduced pressure
to obtain a dark orange oily residue. This was washed with
n-hexane (3 mL), and dried under vacuum giving a fine yellow
powder. The latter was purified by subsequent crystallization
by slow diffusion of a hexane layer into THF solution in the
box freezer (−25 °C). Compound (16) was obtained as clear
orange crystals in 33% yield (0.030 g, 0.048 mmol). X-ray
quality crystals were obtained by slow diffusion of hexane into
a THF/Et2O mixture of 16. The same product 16 was obtained
in 97% yield (0.091 g, 0.15 mmol) using finely ground NaOH
(0.061 g, 1.53 mmol) instead of NaH, and following exactly the
same protocol. In this case, the colour of the reaction mixture
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