Enhanced Catalytic Activity of Nickel Complexes of an Adaptive Diphosphine-Benzophenone Ligand in Alkyne Cyclotrimerization

Article abstract of DOI:10.1021/acscatal.8b05025

Adaptive ligands, which can adapt their coordination mode to the electronic structure of various catalytic intermediates, offer the potential to develop improved homogeneous catalysts in terms of activity and selectivity. 2,2′-Diphosphinobenzophenones have previously been shown to act as adaptive ligands, the central ketone moiety preferentially coordinating reduced metal centers. Herein, the utility of this scaffold in nickel-catalyzed alkyne cyclotrimerization is investigated. The complex [(^p-tolL1)Ni(BPI)] (^p-tolL1 = 2,2′-bis(di(para-tolyl)phosphino)-benzophenone; BPI = benzophenone imine) is an active catalyst in the [2 + 2 + 2] cyclotrimerization of terminal alkynes, selectively affording 1,2,4-substituted benzenes from terminal alkynes. In particular, this catalyst outperforms closely related bi- and tridentate phosphine-based Ni catalysts. This suggests a reaction pathway involving a hemilabile interaction of the C-O unit with the nickel center. This is further borne out by a comparative study of the observed resting states and DFT calculations.

Full text of DOI:10.1021/acscatal.8b05025

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Article
Enhanced Catalytic Activity of Nickel Complexes of an Adaptive
Diphosphine-Benzophenone Ligand in Alkyne Cyclotrimerization
Alessio F. Orsino, Manuel Gutiérrez del Campo, Martin Lutz, and Marc-Etienne Moret
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b05025 • Publication Date (Web): 31 Jan 2019
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Enhanced Catalytic Activity of Nickel Complexes
of an Adaptive Diphosphine-Benzophenone
Ligand in Alkyne Cyclotrimerization
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†
†
‡
Alessio F. Orsino, Manuel Gutiérrez del Campo, Martin Lutz, and
Marc-Etienne Moret*,
†
†Utrecht University, Organic Chemistry and Catalysis, Debye
Institute for Nanomaterials Science, Faculty of Science,
Universiteitsweg 99, 3584 CG, Utrecht, The Netherlands, E-mail:
*M.Moret@uu.nl
‡Utrecht
University,
Crystal
and
Structural
Chemistry, Bijvoet Center for Biomolecular Research, Faculty of
Science, Padualaan 8, 3584 CH, Utrecht, The Netherlands
ABSTRACT. Adaptive ligands, which can adapt their coordination
mode
intermediates, offer the potential to develop improved homogeneous
catalysts in terms of activity and selectivity. 2,2'-
to
the
electronic
structure
of
various
catalytic
diphosphinobenzophenones have previously been shown to act as
adaptive ligands, the central ketone moiety preferentially
coordinating reduced metal centers. Herein, the utility of this
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scaffold
in
nickel-catalyzed
alkyne
cyclotrimerization
is
investigated. The complex [(p-tolL1)Ni(BPI)] (p-tolL1 = 2,2’-bis(di-
(para-tolyl)phosphino)-benzophenone; BPI = benzophenone imine) is
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an active catalyst in the [2 + 2 + 2] cyclotrimerization of
terminal alkynes, selectively affording 1,2,4-substituted benzenes
from terminal alkynes. In particular, this catalyst outperforms
closely related bi- and tri-dentate phosphine-based Ni-catalysts.
This suggests
a
reaction pathway involving
a
hemilabile
interaction of the C=O unit with the nickel center. This is further
borne out by a comparative study of the observed resting states
and DFT calculations.
KEYWORDS. Alkyne Cyclotrimerization – Nickel Complexes – Adaptive
Ligand – -Acceptor Ligand – Hemilabile Ligand
INTRODUCTION
The search for improved activity and selectivity in transition-
metal based homogeneous catalysts strongly relies on the
development of steering ligands to tune the steric and electronic
properties of the metal center to the requirements of particular
transformations. Recent years have seen the development of more
sophisticated adaptive and/or cooperative ligands that can adjust
their binding modes or engage in chemical transformations along a
catalytic cycle.[1–4] The use of such ligands also plays an important
role in the development of catalysts based on inexpensive and non-
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toxic first-row transition metals that can compete with – or even
surpass
–
their
counterparts
based
on
precious
metals,
contributing to a more sustainable chemistry.[5] Perhaps one of the
simplest implementation of the concept of adaptive ligands are
hemilabile ligands, that are polydentate ligands featuring a
weakly binding moiety that can reversibly (de)coordinate the metal
center.[6] Since their introduction in synthetic chemistry by
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Jeffrey and Rauchfuss,[7] hemilabile ligands have played
a
significant role in the current transition from noble to base
metals in homogeneous catalysis. The hemilabile interaction is not
only important for opening up a masked coordination site, but also
to stabilize catalytic intermediates, by allowing the supporting
ligand to adapt its binding mode throughout the reaction
coordinate. Most commonly, weak donor groups such as ether
[7,8]
[9]
[8b,10]
(OR ),
amine (NR ),
pyridine derivatives (R C=NR),
are
2
3
2
employed as labile unit, often resulting in enhanced catalytic
activity for different kinds of reactions. Examples include olefin
oligomerizations
and
polymerizations,[8c,11]
(cyclo)trimerization,[9c,11] carbonylation of methanol and methyl
acetate,[12] (transfer) hydrogenation of ketones or dehydrogenation
of alcohols,[9b,13,14] hydroacylation of alkenes and alkynes,[8d,15] and
coupling reactions.[9c,11–19]
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In addition to weak -donor moieties, -acceptor groups[20] (e.g.
triarylborane),[21] or -ligands[2] (e.g. C=C double bond),[22] can
also act as hemilabile fragments. Our group recently reported the
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hemilabile
character
of
the
ligand
2,2’-
bis(diphenylphosphino)benzophenone[23] (PhL1) bound to nickel[24] and
other first-row transition metals.[25] PhL1 was shown to function as
a
hemilabile -acceptor ligand, with the central C=O bond
coordinating to the metal center in its more reduced forms (Figure
). Ru[23,26], Rh[27–29] Os,[30] and Ir[31] complexes of PhL1 were also
1
studied, more specifically in the catalytic hydrogenation of
ketones,[23,26,28,30] in which the formation of an hydroxyalkyl specie
was accessible via the hydrogenation of the ketone moiety of the
ligand.[28]
Figure 1. Previous work.[24] Coordination of
a
diphosphine
benzophenone ligand, PhL1, to nickel(0), nickel(I), and nickel(II).
In this context, we set out to assess the potential utility of
2
an  -bound hemilabile -acceptor moiety incorporated in a pincer
type architecture for catalysis, using the cyclotrimerization of
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alkynes as a benchmark reaction. Since its discovery by Reppe et
al.[32] using a [Ni(CO) (PCl ) ] catalyst, intermolecular [2 + 2 +
2
3 2
2
] cyclotrimerization of alkynes have been widely studied.[33] Metal
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catalyzed cyclotrimerization is an elegant method for the
formation of cyclic frameworks, particularly convenient when the
desired product is not accessible via traditional aromatic
substitution reactions. Many transition metal systems have been
developed (e.g. Ti,[34] Fe,[35] Co,[36] Ni,[37] Mo,[38] Ru,[39] Rh,[40]
Pd,[41] Ir[42]). Literature precedents for the nickel-catalyzed
conversion of alkynes to substituted benzenes regio-isomers are
known,
both
from
an
intramolecular[36c,37g–37l,37n,37p,37r,37s]
and
intermolecular approach.[9c,32,36a,37a–37f,37m,37o,37q,37t–37z] For the [2 + 2
+
2] intermolecular alkyne cyclotrimerization, it starts from the
early example by Reppe et al.,[32] to the combination of divalent
nickel halide salts and reductants,[36a,37f] or even activated Ni-
particles.[37b]
Phosphine-,[9c,36a,37j,37m,37t,37v]
NHC-,[37v,37x]
mixed
Phosphine, Imino (P,N)-,[9c] or 1,4-diazadiene-[37c,37e,37u,37w,37y,37z]
ligands, have been introduced to Ni-systems in order to improve
selectivity and catalytic activity. Phosphine complexes generally
favor the formation of benzene derivatives while 1,4-diazadiene
complexes yield to cyclooctatetraenes.
Besides their synthetic utility, alkyne condensations are also
useful probes for ligand effects in catalysis:[43] The possible
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formation of different products and isomers, such as cyclotrimers,
cyclotetramers, linear oligomers, polymers, and other compounds
provides useful information about the reactivity of a particular
catalyst. For example, Uyeda[37w,44] and Ess[44] recently applied this
approach to the evaluation of the effect of catalyst nuclearity,
and Jones[9c] to assessing differences in catalytic performance
between bidentate PP and PN ligands (Figure 2). In this study, we
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assess the effect of using hemilabile -acceptor tridentate ligand
(p-tolL1; Figure 3) in alkyne cyclotrimerization through comparison
with diverse nickel systems. Ni(0) adopting a strong tridentate
architecture (ligand = p-tolL2; Figure 3) and Ni-complexes supported
Ph
by a bidentate ligand ( L3; Figure 3) were selected for comparison
with p-tolL1.
Figure 2. Examples of Ni(0)-complexes used to evaluate the
catalytic properties of a ligand-metal system by their reactivity
with alkynes.[9c,37w] Blue bold font = property studied. R = Ph, CF3,
tert-butyl.
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Here we report the synthesis of nickel(0) complexes of p-tolL1, p-
tolL2, and PhL3 featuring benzophenone imine (BPI) as a labile
protecting ligand. Their stoichiometric reactivity with terminal
alkynes is investigated, and the resulting Ni-alkyne complexes are
thoroughly characterized. Then, a comparative study of these bi-
and tridentate Ni(0)-catalysts in the cyclotrimerization of
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terminal alkynes is described, aiming to study the effect a
hemilabile -acceptor system. For all tested alkyne substrates, p-
tolL1 outperforms p-tolL2 and PhL3 as well as the structurally more
Ph
different diphosphine rac-BINAP ( L4) in terms of selectivity and
activity. Reactivity studies combined with DFT calculations
provide an insight into the increased catalytic activity,
suggesting that p-tolL1 can adapt its binding mode throughout the
reaction pathway, by the labile (de)coordination of its ketone
unit.
Figure 3. Tridentate and bidentate ligands of comparison in this
study.
RESULTS & DISCUSSION
Ni(0)-benzophenone imine complexes
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The diphosphine-benzophenone pincer-type ligand[23] 2,2’-bis(di-
(
para-tolyl)phosphino)-benzophenone[25] (p-tolL1; Figure 3) and the
triphosphine
pincer-type
ligand,[45]
bis(2-(di-(para-
tolyl)phosphino)phenyl)phenylphosphine (p-tolL2; Figure 3) were
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synthesized according to (adapted) literature procedures,[24,25,46]
by lithiation of o-bromo(diarylphosphino)benzene[47] with n-BuLi
followed by reaction with the appropriate electrophile (Scheme 1).
Para-tolyl substituents on the phosphine were introduced both to
improve the solubility of the complexes and to provide a convenient
1H NMR handle for characterization. The 31P NMR spectrum of p-tolL2
indicates the presence of an AB2 system (  JA,B)[48] with a
coupling constant of 3JA,B = 155 Hz, as previously observed for its
phenyl-substituted analogue.[46] The diphosphine ether ligand L3
Ph
[49]
was obtained from commercial sources and used as received.
Scheme 1. Synthesis of p-tolL1 and p-tolL2 from o-bromo(di-(para-
tolyl)phosphino)benzene.
Coordination of the chelating phosphine ligands to a nickel(0)
center
was
achieved
by
reaction
with
Ni(cod)2 (cod
=
cyclooctatetraene) in the presence of benzophenone imine (BPI), in
a stoichiometric ratio, affording respectively complex (i) [(p-
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tolL1)Ni(BPI)] (p-tol1; Scheme 2), (ii) [(p-tolL2)Ni(BPI)]
(p-tol2;
Ph
Ph
Scheme 2) and (iii) [( L3)Ni(BPI)] ( 3; Scheme 2). BPI was chosen
as co-ligand since it binds strongly enough to Ni(0) to prevent
the formation of dimeric species[24] but weakly enough to be
exchanged with different types of potential substrates such as
alkenes, nitriles, and alkynes (see below; Scheme 4).
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Scheme 2. Ni -benzophenone imine complexes. A. General synthetic
route. B. Isolated complexes. BPI = benzophenone imine.
NMR characterization of p-tol1 indicates that the BPI ligand is
bound in an 1 fashion. The 13C NMR signal corresponding to the
imine appears as a triplet (169.8 ppm,3JC,P = 6.1 Hz) close to that
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of the free imine at 177.3 ppm, consistent with retention of C=N
1
double bond character. In the H NMR spectra, slight deshielding
of the C=N–H proton by 0.24 ppm with respect to free BPI is also
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in accordance with  (N) binding. Moreover, the ATR-IR spectrum
displays a signal at 3152 cm-1 in the region corresponding the N–
H stretch. In contrast, the backbone C=O moiety is bound side-on:
the corresponding 3C NMR signal appears at 119.0 ppm (t, 3JC,P
1
=
5.1 Hz), considerably shifted from the value of 197.4 ppm found in
the free ligand.[25] This large shift is useful as a diagnostic tool
to determine whether the CO moiety is coordinated to the Ni center.
The 31P NMR spectra of p-tol1 consist of a single singlet signal at
15.5 ppm, indicating that the two phosphorous atoms are equivalent
on the NMR time scale. However, from 1H and 13C NMR analysis it
appears that complex p-tol1 contains two chemically different methyl
groups from the para-tolyl substituents (respectively 2.01 and
1
1
.91 ppm in H NMR, 21.3 and 21.1 ppm in 13C NMR). Therefore, the
para-tolyl substituents belonging to a single P-donor site are not
equivalent on the NMR time scale, which can be used as a secondary
indication for ketone coordination.
In a similar fashion, NMR data support 1 binding of the BPI
ligand in the triphosphine complex p-tol2. The 13C NMR signal
corresponding to the N=C bond is a doublet of triplet at 168.9 ppm
(3JC,P = 8.1 Hz, 3JC,P = 7.4 Hz). The imine proton appears at 10.25
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ppm in the 1H NMR spectrum (deshielding of 0.48 ppm compared to
the free BPI ligand), and couples with the phosphorous nuclei (dt,
3JH,P = 3.2 Hz, 3JH,P = 2.7 Hz). The 31P NMR spectrum is consistent
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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with an AK system (  5 JA,K)[48] with a coupling constant 2JA,K
=
2
1
8
5 Hz. From H and 13C NMR analysis, it appears that the complex
contains
two
chemically
different
para-tolyl
substituents
1
(
respectively 2.14 and 1.95 ppm in H NMR, 21.4 and 21.1 ppm in 13C
NMR), as also observed with complex p-tol1.
In the H NMR of Ph3, the N–H signal from the coordinated imine
1
appears as a broad singlet at  = 9.71 in the expected 1:38 ratio
H
compared to the aromatic region of the spectrum. The 31P NMR
spectrum exhibits a single resonance at 32.4 ppm. No satisfactory
13C NMR data could be obtained due to the low solubility of the
complex and its progressive decomposition, making the assignment
of the binding mode of BPI in solution uncertain. The side-on
binding of BPI observed in the solid state (see below) may be
preserved in solution, in which case fast rotation of BPI ligand
on the 1H NMR timescale would render the two phosphorus atoms
equivalent. The N–H stretch from the bound imine appears as a broad
-1
weak band in the ATR-IR spectrum, located at 3200 cm .
More insights into the structural and electronic properties of
nickel complexes p-tol1 and Ph3 were obtained by X-ray crystal
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structure determination (Figure 4). Crystals suitable for X-ray
diffraction were not accessible with p-tol1 but with the
structurally related complex Ph1, in which the para-tolyl ligand p-
tolL1
is
replaced
by
the
phenyl
analogue
2,2’-bis(di-
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Ph
(
phenyl)phosphino)-benzophenone ( L1).
Ph
Ph
Figure 4. Molecular structures of [( L1)Ni(BPI)] ( 1) and
Ph
Ph
[( L3)Ni(BPI)] ( 3) in the crystal (50% probability level). Hexane
Ph
solvent molecules ( 1) and hydrogen atoms are omitted for clarity.
In accord with NMR data, the structure of Ph1 (Figure 4; left)
exhibits a side-on bound ketone moiety with Ni1–O11 and Ni1–C71
bond lengths of 2.0217(14) and 1.9760(19) Å, respectively. The
C71–O11 bond length (1.320(2) Å) is found in between those of
unbound PhL1 (1.213(3) Å)[23] and a C–O single bond (1.43 Å in
ethanol).[50] The ketone is slightly less activated than in
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Ph
[24]
[
( L1)Ni(PPh )],
which displays a C–O bond length of 1.310(2)
3
Å, consistent with less pronounced -backdonation arising from the
weaker donor character of BPI with respect to PPh . The sum of bond
3
1
1
1
1
1
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1
1
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0
angles (351.3(3)˚) around the carbon atom lies between those
expected for sp2 (328.5˚) and sp3 (360˚) hybridization at the
carbon, in accordance with the Dewar-Chatt-Duncanson model. The
N12–C12 bond length of 1.289(2) Å is close to the value of a C–N
double bond (ca. 1.28 Å)[50] and comparable to the value found by
Zhao et al. (1.294(3) Å)[51a] for  -coordination mode in their
1
2
1
synthesized NHC-based nickel(0) complex, [(IPr)Ni( -BPI)( -BPI)]
IPr = 1,3-bis(diisopropylphenyl)-imidazolium), showing that the
-backdonation to the BPI ligand is small. Accordingly, the valence
(

angles around the imine carbon add up to 360.0(3)˚. Finally, the
torsion angle ( Ni1–N12–C12–CPh) of 20.0(3)˚ shows that the
nickel atom is slightly out of the plane of BPI, which is probably
sterically driven.
In contrast to that of Ph1, the crystal structure of Ph3 (Figure
4
; right) reveals a trigonal planar complex in which the imine co-
ligand is coordinated to the nickel in an η2 fashion (Ni1–C37 =
2.018(2) Å, Ni1–N1 = 1.8985(19) Å). The C–N bond axis is parallel
to the metal coordination plane, which is thought to maximize -
backdonation from the high-lying in-plane
d
orbital with
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significant *(P–Ni) character. Accordingly, strong π-backdonation
into the *(N–C) orbital is evidenced by an elongated C37–N1 bond
(
1.373(3) Å vs 1.28 Å[50] in the free imine) and the pyramidalization
0
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of the C37 atom, which displays a sum of valence angles of
2
3
55.3(3)˚, in line with known η (C,N)-Ni(0) complexes.[51] As
expected, the central oxygen atom from the ligand is not binding
to the metal center (Ni1–O1 = 3.3668(16) Å), as also confirmed by
a Wiberg bond index lower than 0.01 calculated by NBO analysis at
a B3LYP/def2TZVP level of theory. The distance between the nickel
and oxygen atoms is similar to the values reported from Ni(0)
complexes bearing this ligand.[52] Finally, the measured P1–Ni1–P2
bite angle of 108.59(2)˚ and the Ni1–O1 distance of 3.3668(16) Å
Ph
[24]
are in the same range as those found for [( L1)Ni(Cl )],
in
which the C=O unit is not bound to the Ni ion (P1–Ni–P2
12.996(13)˚; Ni–C = 3.4031(12) Å; Ni–O = 3.1012(10) Å), supporting
2
1
the use of Ph3 as a model complex for the unbound state of p-tolL1.
In summary, complexes p-tol1 and p-tol2 are geometrically similar
and mainly differ by the central coordinating unit the ligand (C=O
vs P). Complex Ph3 differs in that the central oxygen atom is not
coordinated, and can be seen as a structural analogue of the
unbound state of ligand RL1 (R = p-tolyl or Ph). Interestingly,
moving from tridentate to a bidentate mode is coupled to a change
in binding mode of the BPI ligand from end-on to side-on, which
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can be understood from enhanced -backdonation from a d10 L2Ni
fragment (see Supporting Information for more insight by NBO
analysis into the different coordination modes of the imine co-
ligand; Table S8).
1
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1
1
1
1
1
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1
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Ni(0)-alkyne complexes
With complexes p-tol1 – Ph3 in hand, their reactivity towards
terminal alkynes was investigated, leading to the generation of
Ni-alkynes
complexes
[(p-tolL1)Ni(HCCPh)]
(p-tol4-Ph),
[(p-
tol
L1)Ni(HCCCH OMe)] (p-tol4-CH OMe), [(p-tolL2)Ni(HCCPh)] (p-tol5-
2
2
Ph), and [( L3)Ni(HCCPh)] ( 6-Ph) (Scheme 3). Ni-complexes p-tol5-
Ph and Ph6-Ph, bearing phenylacetylene as co-ligand, are
sufficiently stable to be isolated and are discussed first.
Ph
Ph
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2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
4
4
4
4
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5
5
5
5
5
5
5
5
6
0
Scheme 3. Ni -terminal alkyne complexes. A. General synthetic
route. B. Generated complexes. p-tol4-Ph and
p-tol
4-CH OMe were
2
characterized in-situ in solution while p-tol5-Ph and Ph6-Ph were
0
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4
5
6
7
8
9
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2
3
4
5
6
7
8
9
0
isolated. BPI = benzophenone imine.
The 31P NMR spectrum of p-tol5-Ph displays two signals at 78.7 ppm
(
t, 2JP,P = 35.4 Hz) and at 28.1 ppm (d, 2JP,P = 35.4 Hz) in a ratio
of 2:1, indicating that the ligand binds in a tridentate fashion.
Two 1H NMR singlets of same intensity at 2.12 and 1.91 ppm,
correspond to the methyl group from diastereotopic para-tolyl
substituents bound to the same phosphorus atom. A single alkyne is
bound to Ni as indicated by the terminal proton signal at 6.30
ppm, which appears as a doublet of triplet (3JH,P = 25.6 Hz, 3JH,P
=
1
7
.2 Hz) and couples with a carbon nucleus at 92.9 ppm in HMQC ( H-
13C). Further confirmation of this assignment is provided by
treatment of p-tol2 with d-phenylacetylene, resulting in the
disappearance of the signal at 6.30 ppm in an otherwise identical
1H NMR spectrum (Supporting Information; Figure S68 and S76). In
agreement with the spectroscopic characterization, the solid-state
structure of p-tol5-Ph (Figure 5; left) reveals a pseudo tetrahedral
2
geometry in which the alkyne moiety is coordinated a  fashion.
Compound Ph6-Ph, supported by the bidentate diphosphine ether
ligand PhL3, displays two signals at 29.2 ppm and at 27.3 ppm
coupling to each other (2JP,P = 14.0 Hz) and in a ratio of 1:1 in
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the 31P NMR spectrum. This is consistent with a trigonal planar
geometry in which rotation of the alkyne ligand is slow on the 1H
NMR timescale; warming up the sample up to 100 °C does not result
in coalescence of the two 31P NMR signals. This interpretation is
supported by reaction of Ph3 with one equivalent of the symmetric
alkyne diphenylacetylene, resulting in a symmetrical analogue of
Ph6-Ph displaying a single 31P NMR singlet at 28.1 ppm (Supporting
Information; Figure S98). The proton from the alkyne gives a 1H
1
1
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2
3
4
5
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0
1
13
NMR signal at 6.9 ppm, as revealed by HMQC ( H- C) and comparison
with deuterated analogue. The solid state structure (Figure 5;
right) of Ph6-Ph is in accord with the NMR data and reveals a
trigonal planar geometry with a P1–Ni–P2 bite angle of 104.94(2)˚,
and where the oxygen atom is not bound to the nickel center (Ni1–O1
2
= 3.3317(15) Å). The alkyne is bound in an  -coordination mode,
the C37–C38 axis being in the Ni coordination plane.
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5
5
5
5
5
6
Figure 5. Molecular structure of [(p-tolL2)Ni(HCCPh)] (p-tol5-Ph)
Ph
Ph
and [( L3)Ni(HCCPH)] ( 6-Ph) in the crystal (50% probability
Ph
level). Toluene solvent molecules ( 6-Ph) and hydrogen atoms have
0
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8
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2
3
4
5
6
7
8
9
0
been omitted for clarity.
While stable tricoordinate Ni(0) alkyne complexes with formally
16 valence electrons analogous to Ph6-Ph are well documented,[53,54]
compound p-tol5-Ph represents an unusual example of tetracoordinate,
18 VE Ni(0) alkyne complex, to the best of our knowledge the first
to be structurally characterized. Spectroscopic and structural
data point towards a considerably weaker activation of the C–C
triple bond in this geometry (Table 1). First, the 13C NMR chemical
shifts of the two alkyne carbons (δ = 92.9 and 101.0) in p-tol5-Ph
C
are moderately deshielded in comparison with the free alkyne (δ =
C
77.5 and 83.3), indicating relatively weak rehybridization from sp
2
to sp . This contrasts with a stronger activation of the C–C triple
bond of phenylacetylene in Ph6-Ph evidenced by the 13C NMR signals
from the coordinated phenylacetylene at 125.7 ppm (CC–H, dd, 2JC,P
=
40.1 Hz, 2JC,P = 7.1 Hz ) and 136.4 ppm (CC–Ph, dd, 2JC,P = 38.9
Hz, 2JC,P = 8.4 Hz ), which were assigned with the assistance of
APT 13C NMR and by comparison with the d-phenylacetylene complex.
Second, the acetylic hydrogen signal of p-tol5-Ph (H = 6.30) is
less shifted than that of Ph6-Ph (H = 6.9), consistent with a
2
stronger rehybridization towards sp . Third, the CC stretch
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vibration from the alkyne in p-tol5-Ph is found at 1823 cm-1 in the
FT-IR spectrum vs 2126 cm-1 for free phenylacetylene and vs 1749
cm-1 for Ph6-Ph. Finally, the slightly elongated C47C48 distance
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
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4
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4
4
4
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5
5
5
5
5
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5
6
0
1
2
3
4
5
6
7
8
9
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1
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3
4
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8
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2
3
4
5
6
7
8
9
0
at 1.231(8) Å (vs 1.182 – 1.190 Å in free phenylacetylene)[55] and
the rather large C47–C48–C49 angle
at 152.0(6)˚ are also
consistent with a relatively weak activation of the C–C triple
bond, contrasting with the longer C37–C38 bond length (1.269(3)
Å) and the more acute the C37–C38–C39 angle (144.07(19)˚) found
for Ph6-Ph. The C47–C48 bond length in p-tol5-Ph is the shortest we
are aware of for nickel(0)-alkyne complexes. Together, these data
suggest that the π-back donation from the nickel to phenylacetylene
is
more
pronounced
in
Ph6-Ph,
resulting
in
a
stronger
rehybridization towards sp2.
The observations made for complex Ph6-Ph parallel those reported
for mononuclear, 16 valence-electrons, tricoordinate Ni-alkyne, in
which the C–C bond is significantly more elongated than in p-tol5-
Ph, regardless of substituents group on the alkyne. For example,
strong activation of the alkyne is observed for bidentate diimine
and
mixed
P,N
supported
species
(up
to
1.296(6)
Å)[9c,53a,53b,53i,53l,53n,53r] while bidentate phosphorous ligands afford
C–C bond lengths from 1.260(4) to 1.283(3) Å.[52b,53d–53f,53h,53j,53m,53o–
53q]
The substantially weaker rehybridization observed in p-tol5-Ph
can be assigned to its unique coordination geometry (i.e.
tetracoordinate complex; 3 donor ligands; pseudo Td geometry)
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compared to Ph6-Ph and the reported Ni-alkynes complexes (i.e.
tricoordinate complexes;
2
donor ligands; trigonal planar
geometry). The weak activation of phenylacetylene in p-tol5-Ph is
presumably a consequence of the smaller splitting of the d-orbitals
in its pseudotetrahedral geometry (3 donor ligands), resulting in
0
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2
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a less pronounced -back donation from Ni to the antibonding
orbitals of the alkyne. Similar considerations were used by Lee
and co-workers to explain the rather weak activation of CO in the
2
Me
2
tetracoordinate pseudo tetrahedral nickel complex, [(PP P)Ni( -
Me
i
[56]
CO )] (PP P = PMe[2-P Pr -C H ] )
with respect to tricoordinate
2
2
6 4 2
2
Ni- -CO
adducts supported by bidentate phosphines such as
2
2
[
(dtbpe)Ni( -CO )]
(dtbpe
=
1,2-bis(di-tert-butylphophino)-
2
ethane).[57]
Table 1. Selected spectroscopic and X-ray crystal structure values
of phenylacetylene, p-tol5-Ph, and Ph6-Ph. Vibrational frequencies
are measured with ATR-IR (neat). NMR chemical shifts are given in
C D .
6
6
p-tol5-Ph
92.9
Ph6-Ph
125.7
136.4
6.9
phenylacetylene
77.5
C(CC–H) [ppm]
C(CC–Ph)
83.3
101.1
6.30
[
ppm]
H(CC–H) [ppm]
2.73
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-
1

(CC) [cm ]
CC [Å]
CC-CPh [˚]
2
126
1823
1749
1.182 –
1.190
1
.231(8)
1.269(3)
[55]
177.39 –
79.49

1
52.0(6)
144.07(19)
[
55]
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Finally, reaction of the diphosphine-ketone complex p-tol1 with
terminal alkynes generally led to a mixture containing the alkyne
complex, [(p-tolL1)Ni(HCCR )] (
1
p-tol
4-R ; R = substituent on the
1
1
alkyne), the starting material p-tol1, BPI, the substrate, and
cyclotrimerization
products.
This
indicates
that
cyclotrimerization occurs concomitantly with ligand exchange,
precluding the isolation of pure alkyne complexes. Nonetheless,
in-situ characterization of alkyne complexes was possible.
Interestingly, 13C NMR of p-tol4-Ph indicates that the ketone moiety
is not bound to the metal center: addition of phenylacetylene to
p-tol1 causes the disappearance of the triplet at 119.0 ppm,
2
characteristic of the  (C,O) coordination to nickel and the
appearance of a triplet at 202.7 ppm (3JC,P = 4.9 Hz). In addition,
all the para-tolyl groups in p-tol4-Ph are equivalent at room
temperature, with singlets appearing at 2.00 and at 21.2 ppm in
respectively 1H and 13C NMR. This is consistent with an unbound
ketone allowing fast ring inversion of the chelate macrocycle. A
single 31P NMR singlet at 33.0 ppm down to –80 ˚C indicates fast
rotation of the alkyne, exchanging the two phosphorous atoms on
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the NMR timescale even at low temperature (Supporting Information;
Figure S59).
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The CC–H H NMR signal of the coordinating phenylacetylene could
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not be located, presumably hidden in the crowded aromatic region
of the spectrum. In contrast, the assignment of the acetylic proton
of the bound alkyne was possible in the H NMR spectrum of p-tol4-
1
3
CH OMe where it appears as a triplet of triplets at 6.34 ppm ( J
2
H,P
=
15.6, 4JH,H = 1.6 Hz) and integrates in a ratio of 1:12 compared
to the four CH3 groups of the ligand, confirming that a single
alkyne molecule is bound. The integrals, coupling constants, and
4
multiplicity of C–H (4.63 ppm, d, J
= 1.6 Hz) and OC–H (3.32
3
2
H,H
ppm, s) of the bound alkyne are in agreement with the proposed
p-tol
4-Ph, both H and 13C NMR of
1
p-tol
structure. Similar to
4-CH OMe
2
reveal that the ketone moiety is not bound to the metal center:
the ketone resonance appears as triplet at 203.1 ppm (3JC,P = 4.7
Hz) and the methyl groups from the para-tolyl are all equivalent,
with a singlet at 2.03 and a doublet 22.2 ppm (4JC,P = 2.3 Hz) in
respectively H and 13C NMR. A singlet 31P NMR signal at 32.7 ppm
at room temperature is consistent with fast rotation of the methyl
propargyl ether similar to p-tol4-Ph. However, gradually cooling the
1
sample down to –85˚C first shows the apparition of
a
new
unsymmetrical specie at c.a. –10 ˚C, as characterized by two
doublets (2JP,P = 23.3 Hz) at 32.1 and 34.1 ppm (Supporting
p-
Information; Figure S67). The central signal corresponding to
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tol4-CH OMe does split into two new doublets ( J
2
= 23.3 Hz) with
P,P
2
a coalescence temperature between –50 ˚C and –60 ˚C. This indicates
that the rotation of methyl propargyl ether can be frozen on the
NMR timescale, but also that two distinct species are formed at
low temperature. This is tentatively attributed to a secondary
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interaction of the oxygen atom from H–CC–CH OMe with Ni. In
2
addition, the second species observed at low temperature may be
either a diastereomeric conformer or an isomeric structure in which
the central ketone fragment binds to the nickel center.
The fast rotation of the alkyne fragment in complex p-tol4-Ph
contrasts with the observed behavior of the tricoordinate Ph6-Ph.
DFT calculations on the acetylene complex p-tol4-H reveal that the
rotation of acetylene is assisted by transient coordination of the
ketone moiety to the Ni center, resulting in a low overall rotation
energy barrier of 5.5 kcal/mol (Figure 6). The first transition
state, p-tol4-H (TS1), from the unbound ketone state p-tol4-H to the
bound state p-tol4-H (I1) is characterized by the coordination of
-1
TS1
the ketone moiety (ν = –113 cm ; G
= 4.3 kcal/mol) to generate
a four-coordinate intermediate p-tol4-H (I1). The second transition
state, p-tol4-H (TS2), corresponds to the rotation of acetylene (ν
-
1
=
–95 cm ; GTS2
=
4.1 kcal/mol) accompanied by partial
decoordination of one of the phosphine arms. The low overall energy
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barrier of GTS,overall = 5.5 kcal/mol is in qualitative agreement
with the observed fast rotation of alkynes at room temperature.
For comparison, a rotation energy barrier of 25.0 kcal/mol was
calculated for the rotation of acetylene in the diphosphine ether
complex Ph6-H (Supporting Information; Figure S112), in qualitative
agreement with the observation of two 31P NMR signals for Ph6-Ph up
to 100 °C.
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Figure 6. Top. Energy diagram for the rotation of acetylene around
the Ni-coordination plane ( C1–C2–Ni–P1) of p-tol4-H through two
different transition states, p-tol4-H (TS1) = p-tol4-H (TS1’) and
p-
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tol4-H (TS2), and one intermediate, p-tol4-H (I1) = p-tol4-H (1’).
Numbers into bracket are G˚ values given in kcal/mol. GTS1 = 4.3
kcal/mol. GTS1’ = 2.9 kcal/mol. GTS2 = 4.1 kcal/mol. GTS,overall = 5.5
kcal/mol. Bottom. Optimized geometry of transition states p-tol4-H
(TS1) and p-tol4-H (TS2) as well as intermediate p-tol4-H (I1) at a
B3LYP/6-31g(d,p) level of theory under vacuum. The imaginary
frequency (–113 cm ) of p-tol4-H (TS1) shows the coordination of
-1
-1
the ketone (C3–O) to Ni while the imaginary frequency (–95 cm )
of p-tol4-H (TS2) is the rotation of acetylene around the C1–C2–
Ni–P1 torsion angle. Selected bond lengths [Å] for p-tol4-H (TS1).
C3–O = 1.25; Ni–C3 = 2.35; Ni–O = 2.55; Ni–P1 = 2.17; Ni–P2 = 2.19;
C1–C2 = 1.27. Selected bond lengths [Å] for p-tol4-H (I1). C3–O =
1.30; Ni–C3 = 2.01; Ni–O = 2.06; Ni–P1 = 2.18; Ni–P2 = 2.24; C1–
C2 = 1.25. Selected bond lengths [Å] for p-tol4-H (TS2). C3–O =
1.32; Ni–C3 = 1.96; Ni–O = 1.92; Ni–P1 = 2.14; Ni–P2 = 2.45; C1–
C2 = 1.24. Hydrogen atoms have been omitted for clarity.
The differences in electronic and structural properties of Ni-
alkynes complexes p-tol4-Ph – Ph6-Ph were further investigated by
DFT calculations (geometry optimization and NBO analysis), as
summarized in Table 2. The optimized geometries of p-tol4-Ph, p-tol5-
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Ph and Ph6-Ph (Tables 2; Entries 1 to 4) are consistent with the
obtained crystal structures in which the C1–C2 bond length, C1–
C2–C3 angle and Ni–C1, Ni–C2 distances calculated for p-tol5-Ph
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indicate weaker activation of the HCCPh substrate. Accordingly,
Wiberg bond indexes (WBIs) calculated in the NBO basis at a
B3LYP/def2TZVP level of theory show that the binding of
phenylacetylene induces a decrease of the C1C2 WBI (Table 2; Entry
5
) from 2.82 to 2.30 in p-tol5-Ph versus 2.20 in p-tol4-Ph and 2.17
in Ph6-Ph. The smaller Ni–C1 and Ni–C2 WBIs (Table 2; Entry 6 and
) of 0.31 in p-tol5-Ph corroborates a weaker orbital interaction
7
between the HCCPh ligand and the Ni-center in the triphosphine
complex p-tol5-Ph compared p-tol4-Ph (WBIs of 0.43 and 0.39) and Ph6-Ph
(WBIs of 0.42 and 0.39).
Table 2. Selected DFT bond distances, angles, and Wiberg bond
indexes (WBI). Geometry optimizations of [(p-tolL1)Ni(HCCCPh)] (p-
tol4-Ph), [(p-tolL2)Ni(HCCPh)] (p-tol5-Ph), and [( L3)Ni(HCCPh)],
(Ph6-Ph) were performed at a B3LYP/6-31g(d,p) level of theory. WBIs
were calculated by NBO analysis at a B3LYP/def2TZVP level of theory
from the optimized geometries. Hydrogen atoms have been omitted
for clarity.
Ph
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DFT Bond Length and Angle [in Å and ˚]
Entry
Property
HCCPh
p-tol4-Ph
p-tol5-Ph
Ph6-Ph
1
C1–C2
1.21
1.29
1.26
1.30

C1–C2–
C3
2
180
141
150
141
3
4
Ni–C1
Ni–C2
–
–
1.84
1.89
1.94
1.97
1.84
1.89
Wiberg Bond Index (WBI)
5
6
7
C1–C2
Ni–C1
Ni–C2
2.82
2.20
0.43
0.39
2.30
0.31
0.31
2.17
0.42
0.39
–
–
In summary, we showed in this section that ligands p-tolL1 – PhL3
all form Ni(0) alkyne complexes with phenylacetylene. The
triphosphine p-tolL2 binds in
tridentate mode in p-tol5-R1,
a
resulting in unusually weak activation of the alkyne, while PhL3
acts as a bidentate ligand in the tricoordinate Ph6-R , in which a
1
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strong Ni–alkyne interaction causes a high rotation barrier around
the Ni–alkyne axis. The hemilabile ligand p-tolL1 is able to sample
both binding modes: it binds as a bidentate ligand in p-tol4-R1,
resulting in a similar geometry as that of Ph6-R , but the observed
low rotation barrier around the Ni–alkyne axis is indicative of
the ability of p-tolL1 to transiently adopt a tridentate mode.
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Ligand exchange reactions on [(p-tolL1)Ni(BPI)]
Having observed that the ketone moiety decoordinates upon binding
of an alkyne to the (p-tolL1)Ni fragment, we sought to probe the
generality of this hemilabile behavior. NMR-tube experiments
showed that the BPI co-ligand can be reversibly exchanged with
-3
several types of ligands, such as benzonitrile (Keq  2.7  10 ;
-
2

G298,15  3.5 Kcal/mol), styrene (Keq  1.5  10 ; G
 2.4
 3.1
298,15
-3
Kcal/mol), and diphenylacetylene (Keq  6.1  10 ; G
298,15
Kcal/mol), as depicted in Scheme 4 (see Supporting Information for
more details). The proposed mode of coordination of the
benzonitrile ligand in p-tol7-PhCN relies on DFT calculations, which
2
predict the  -(C,N) binding to be 9.4 kcal/mol energetically less
1
stable than the  -(N) coordination mode (Supporting Information;
Table S11). With triphenylphosphine, the exchange is irreversible.
In every case, with exception of diphenylacetylene, 13C NMR
demonstrates that the ketone moiety is still bound to nickel (no
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peaks between 190 and 210 ppm). The coordinated ketone appears at
1
20.4 ppm (dt, 2JC,P = 13.8 Hz, 2JC,P = 4.4 Hz) for the PPh complex
3
p-tol7-PPh , and shows two chemically different methyl groups from
3
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the para-tolyl substituents at 1.97 ppm (s, 6H) and 1.96 ppm (s,
H) in H NMR and respectively at 21.2 and 21.1 ppm in 13C NMR.
1
6
Unfortunately for the PhCN complex p-tol7-PhCN and for the styrene
complex p-tol7-C H Ph, the carbon resonance of the bound ketone could
2
3
not be assigned, as the weak signal is covered by those of the
substrate and the starting complex p-tol1. However, for all of these
1
complexes, H NMR shows that the two methyl groups from the para-
tolyl substituents on the phosphine are not equivalent ( = 2.04
H
and 2.03 for p-tol7-PhCN, 2.05 and 2.01 for
p-tol
7-C H Ph) which is
2 3
also indicative of the pincer-type mode of p-tolL1 to nickel, with
the central ketone moiety bound, as observed with p-tol1 and p-tol7-
p-tol
1
PPh3 and in opposition to
4-R . With the diphenylacetylene
p-tol
complex
7-C Ph , the absence of interaction of the ketone is
2 2
demonstrated by the presence of a triplet at 198.3 of 3JC,P = 4.7
Hz and with all the four C–H3 being chemically equivalent ( =
H
1
.95;  = 21.2), similar to the closely related terminal alkyne
C
p-tol
complex
7-CH OMe. The difference in coordination between
2
olefin- and alkyne-complexes is consistent with alkynes being
stronger π-acceptors than olefins, and illustrates the ability of
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