Substituent effects on Ni-S bond dissociation energies and kinetic stability of nickel arylthiolate complexes supported by a bis(phosphinite)-based pincer ligand

Article abstract of DOI:10.1039/c2dt30407d

Pincer complexes of the type [2,6-(R₂PO)₂C ₆H₃]NiSC₆H₄Z (R = Ph and i-Pr; Z = p-OCH₃, p-CH₃, H, p-Cl, and p-CF₃) have been synthesized from [2,6-(R₂PO)₂C₆H ₃]NiCl and sodium arylthiolate. X-ray structure determinations of these thiolate complexes have shown a somewhat constant Ni-S bond length (approx. 2.20 A) but an almost unpredictable orientation of the thiolate ligand. Equilibrium constants for various thiolate exchange (between a nickel thiolate complex and a free thiol, or between two different nickel thiolate complexes) reactions have been measured. Evidently, the thiolate ligand with an electron-withdrawing substituent prefers to bond with [2,6-(Ph ₂PO)₂C₆H₃]Ni rather than [2,6-(i-Pr₂PO)₂C₆H₃]Ni , and bonds least favourably with hydrogen. The reactions of the thiolate complexes with halogenated compounds such as PhCH₂Br, CH ₃I, CCl₄, and Ph₃CCl have been examined and several mechanistic pathways have been explored.

Full text of DOI:10.1039/c2dt30407d

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PAPER
Substituent effects on Ni–S bond dissociation energies and kinetic stability
of nickel arylthiolate complexes supported by a bis(phosphinite)-based
pincer ligand†
Jie Zhang, Anubendu Adhikary, Krista M. King, Jeanette A. Krause and Hairong Guan*
Received 21st February 2012, Accepted 29th March 2012
DOI: 10.1039/c2dt30407d
Pincer complexes of the type [2,6-(R2PO)2C6H3]NiSC6H4Z (R = Ph and i-Pr; Z = p-OCH3, p-CH3, H,
p-Cl, and p-CF3) have been synthesized from [2,6-(R2PO)2C6H3]NiCl and sodium arylthiolate. X-ray
structure determinations of these thiolate complexes have shown a somewhat constant Ni–S bond length
(approx. 2.20 Å) but an almost unpredictable orientation of the thiolate ligand. Equilibrium constants for
various thiolate exchange (between a nickel thiolate complex and a free thiol, or between two different
nickel thiolate complexes) reactions have been measured. Evidently, the thiolate ligand with an electron-
withdrawing substituent prefers to bond with “[2,6-(Ph2PO)2C6H3]Ni” rather than “[2,6-(i-Pr2PO)2C6H3]-
Ni”, and bonds least favourably with hydrogen. The reactions of the thiolate complexes with halogenated
compounds such as PhCH2Br, CH3I, CCl4, and Ph3CCl have been examined and several mechanistic
pathways have been explored.
Introduction
Late transition metal complexes bearing anionic heteroatomic
ligands such as RO−, R2N−, and RS− are fundamentally impor-
tant for their crucial roles in catalytic carbon–heteroatom bond
forming reactions.1 The reactivity of the MX moiety (X = OR,
NR2, and SR) toward carbon-based electrophiles is often higher
than that of the corresponding HX species. Molecular orbital
Chart 1
analysis has suggested that π-symmetry interaction between an
occupied metal d orbital and a filled heteroatom p orbital results
in a higher energy π-orbital and thereby enhances the nucleo-
philicity of the metal-bound heteroatomic ligands.2 An alterna-
diamagnetic nickel arylthiolate complexes that are shown in
Chart 1. In a related report, Morales-Morales and co-workers
have postulated that complex 1c can react with PhI readily to
tive explanation has echoed Drago’s electrostatic–covalent (E–C)
model3 and emphasized the electrostatic (or ionic) component of
form PhSPh (eqn (1)).6 The same type of reactions could be
involved in our catalytic system. However, our mechanistic
studies have suggested that it is too slow to be a viable
step under the catalytic conditions.5 Nevertheless, the mixture of
1c and PhI in DMF-d8 at 80 °C does produce PhSPh in 5% GC
yield after 24 h. We are thus curious to know how the Ni–S bond
of 1c is cleaved during this process. Given the fact that nickel
radicals supported by a pincer-type ligand are known in the
literature,7 homolytic Ni–S bond dissociation is an attractive
possibility. It is also likely that nucleophilic aromatic substitution
of PhI takes place with the nickel-bound thiolate. A similar reac-
tion pathway has been suggested in copper-catalysed formation
of carbon–heteroatom bonds.8,9 Prompted by these mechanistic
hypotheses, we decided to study Ni–S bond dissociation energies
(BDEs) and nucleophilicity of the nickel arylthiolate complexes
depicted in Chart 1, with an objective to delineate how the
substituents on the thiolate ligand and the phosphorus donor
atoms would impact the thermodynamic and kinetic stability of
bonding in M–X bonds.4 For complexes involving late transition
metals, especially those in the first row, M–X bonds are signifi-
cantly polarized as compared to H–X bonds. Accordingly,
anionic heteroatomic ligands attached to late transition metals
are more nucleophilic than the neutral, metal-free HX.
During our study of cross-coupling of aryl iodides and aryl
thiols catalysed by nickel bis(phosphinite) pincer complexes,5
we became interested in the chemistry of square-planar
Department of Chemistry, University of Cincinnati, P. O. Box 210172,
Cincinnati, OH 45221-0172, USA. E-mail: hairong.guan@uc.edu;
Fax: +1-513-556-9239; Tel: +1-513-556-6377
†Electronic supplementary information (ESI) available: X-ray crystallo-
graphic data in CIF and PDF formats, kinetic data, and plots of these
data. CCDC 872168–872175 (complexes 1b, 1d, 1e, and 2a–e). For ESI
and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2dt30407d
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the Ni–S bonds. Such information should not only help us to
modulate the reactivity of metal thiolate complexes, but
also have important mechanistic implications in various C–S
bond formation reactions catalysed by transition metal
complexes.10
ð1Þ
ð3Þ
The strength of an M–S bond may be inferred from its bond
length, which is sensitive to the coordination number for the
metal, molecular geometry, as well as the steric and electronic
properties of the ancillary ligands. When these structural features
are kept the same, the bond length can be influenced by the basi-
city of the thiolate ligand. For example, over a range of pseudo-
tetrahedral zinc thiolate complexes with an identical scorpionate-
type ligand, the shortest Zn–S bond has been found in the com-
pound bearing the most basic thiolate ligand.18 Similar results
have been described by Jensen and co-workers in their study of
nickel arylthiolate complexes containing a tris(pyrazolyl)borate
(Tp) ligand, although the comparison was made between com-
plexes with different Tp ligands.13c
The nickel thiolate complexes reported in this paper crystallize
readily, providing an excellent opportunity for a more systematic
comparison of the Ni–S bond lengths. The structures of 1a and
1c have been reported in our previous study.5 As representative
examples, the structures of 1b and 2b are shown in Fig. 1 and 2,
respectively. Structures and crystallographic data of other thiolate
complexes are provided in ESI.† For ease of comparison, all the
Ni–S bond lengths including those of 1a and 1c are compiled in
Table 1. When the basicity of the thiolate ligand19 decreases in
going from 1a to 1e, Ni–S bond length is almost invariable
(approx. 2.20 Å), except in 1c where the bond [2.2338(6) Å] is
slightly longer. To compensate for the longer Ni–S bond, the
S1–C31 bond in 1c [1.763(2) Å] is about 0.02 Å shorter than
those of other nickel thiolate complexes. Another anomaly of 1c
Thermodynamic data of M–S bonds of transition metal
thiolate complexes are surprisingly limited in the literature,
particularly for compounds containing late transition metals.11
The nucleophilicity of metal thiolate complexes, on the other
hand, are better understood due to the efforts of bioinorganic
chemists to model cysteine-ligated metalloenzymes.12 However,
studies on nickel systems have been primarily focused on
the electrophilic alkylation of paramagnetic thiolate complexes.13
Of the known diamagnetic Ni(II) complexes with a terminal
thiolate ligand,14 neither BDEs nor nucleophilicity of Ni–S
bonds have been examined. In this work, we will report the syn-
thesis and structures of pincer-ligated nickel arylthiolate com-
plexes that are shown in Chart 1. Using well-established
equilibrium constant measurement4a,15 in combination with
reported S–H BDEs of substituted and unsubstituted thiophe-
nols,16 we will describe the electronic effects of the ligand sub-
stituents on relative Ni–S BDEs of the nickel thiolate
complexes and analyse the nature of the Ni–S bonds with the
E–C model. We will also compare the nucleophilicity of these
complexes by studying the kinetics of their reaction with benzyl
bromide.
Results and discussion
Synthesis and structures of nickel thiolate complexes
Following our previously reported procedures for the synthesis
of 1a–c,5 room temperature reaction of [2,6-(Ph2PO)2C6H3]NiCl
with p-ZC6H4SNa (Z = Cl and CF3) in THF afforded the analo-
gous complexes 1d and 1e in good yield (eqn (2)). A different
series of nickel arylthiolate complexes containing isopropyl
groups on the phosphorus donor atoms were prepared using a
similar approach, but under refluxing conditions (eqn (3)). In
contrast to many other reported nickel thiolate complexes that
are prone to S-oxygenation by O2,17 all the thiolate complexes
reported here are remarkably air stable both in solution and in
the solid state.
Fig. 1 ORTEP drawing of [2,6-(Ph2PO)2C6H3]NiSC6H4CH3 (1b) at
the 50% probability level. Hydrogen atoms are omitted for clarity.
Selected bond length (Å) and angles (°): Ni–S1 = 2.1947(7), Ni–C1 =
1.907(2), Ni–P1 = 2.1555(7), Ni–P2 = 2.1452(7), S1–C31 = 1.784(3);
P1–Ni–P2 = 163.10(3), P1–Ni–S1 = 103.11(3), P2–Ni–S1 = 93.79(3),
C1–Ni–S1 = 174.45(7), C31–S1–Ni = 110.85(8).
ð2Þ
7960 | Dalton Trans., 2012, 41, 7959–7968
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is the orientation of the thiolate ligand relative to the coordi-
nation plane, which results in a dihedral angle of 72.76(5)°
between planes P1–Ni–P2 and Ni–S1–C31. In contrast to this
“perpendicular” geometry (Fig. 3), the thiolate ligand in other
nickel complexes of the same series adopts an “in-plane” geome-
try with a significantly smaller dihedral angle (24.43–31.30°, see
Table 2). Interestingly, in the isopropyl series 2c has the shortest
Ni–S bond [2.1734(6) Å], but other Ni–S bond lengths fall in
the narrow range of 2.1908(7)–2.2191(6) Å (Table 1). The orien-
tation of the thiolate ligand in the isopropyl series is not easily
predicted either (Table 2); relatively smaller dihedral angles are
found in complexes 2b and 2c (Fig. 4). The ipso-carbon–nickel
bond length may also be impacted by the thiolate ligand;
perhaps the ligand inducing a shorter Ni–S bond exerts greater
trans influence on the Ni–C distance. However, as suggested by
Table 3, the Ni–C bond length is rather constant, regardless of
the substituents on the phosphorus atoms or the thiolate aromatic
ring. Taken together, these results suggest that the electronic
effects of the supporting ligand on the bond lengths are very
small and can be smaller than the uncertainty of crystal struc-
ture determination. Additionally, crystal packing effects, offset
face-to-face π-stacking interactions,20 and C–H/π interactions21
may play bigger roles in maintaining a relatively constant Ni–S
bond length and affecting the orientation of the thiolate ligand.
More importantly, the reactivity of these nickel arylthiolate com-
plexes in solution (vide infra) has no correlation with the Ni–S
bond lengths revealed by the X-ray studies, and the rotation of
the Ni–S bonds in solution is not restricted as all the thiolate
complexes display a singlet in their 31P{1H} NMR spectra.
Relative bond dissociation energies (BDEs)
The absolute BDE value for an M–S bond may be determined
from solution calorimetry by combining the metal thiolate
complex with HCl.11a–c However, to address specifically the
effects of ligand substituents on M–S BDEs of a series of related
complexes, only the relative BDE values are needed. Such infor-
mation can be conveniently obtained through equilibrium
Fig. 2 ORTEP drawing of {2,6-[(i-Pr)2PO]2C6H3}NiSC6H4CH3 (2b)
at the 50% probability level. Hydrogen atoms are omitted for clarity.
Selected bond length (Å) and angles (°): Ni–S1 = 2.1908(7), Ni–C1 =
1.907(2), Ni–P1 = 2.1566(7), Ni–P2 = 2.1538(7), S1–C31 = 1.789(3);
P1–Ni–P2 = 163.27(3), P1–Ni–S1 = 104.37(3), P2–Ni–S1 = 92.24(3),
C1–Ni–S1 = 173.99(8), C31–S1–Ni = 114.07(8).
Fig. 3 Side view of 1c illustrating the “perpendicular” orientation of
the thiolate ligand.
Table 1 Ni–S bond lengths (Å) of nickel arylthiolate complexes
Z = OCH3 (a)
Z = CH3 (b)
2.1947(7)
2.1908(7)
Z = H (c)
2.2338(6)a
2.1734(6)
Z = Cl (d)
2.1961(5)
2.2083(6)
Z = CF3 (e)
2.2030(10)
2.2075(8)
Ph series (1a–e)
i-Pr series (2a–e)
2.1965(6)a,b
2.1979(10)a,b
2.2191(6)
a From ref. 5. b Complex 1a crystallizes in two different crystal forms.
Table 2 Dihedral angles (°) between P1–Ni–P2 and Ni–S1–C31 planes in nickel arylthiolate complexes
Z = OCH3 (a)
Z = CH3 (b)
26.97(13)
8.49(23)
Z = H (c)
72.76(5)a
19.64(10)
Z = Cl (d)
24.43(11)
42.95(7)
Z = CF3 (e)
27.48(19)
42.25(10)
Ph series (1a–e)
i-Pr series (2a–e)
26.00(12)a,b
31.30(20)a,b
54.82(9)
a From ref. 5. b Complex 1a crystallizes in two different crystal forms.
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Table 3 Ni–C bond lengths (Å) of nickel arylthiolate complexes
Z = OCH3 (a)
Z = CH3 (b)
Z = H (c)
1.898(2)a
1.899(2)
Z = Cl (d)
1.909(2)
1.900(2)
Z = CF3 (e)
1.910(3)
Ph series (1a–e)
i-Pr series (2a–e)
1.909(2)a,b
1.907(3)a,b
1.899(2)
1.907(2)
1.907(2)
1.899(3)
a From ref. 5. b Complex 1a crystallizes in two different crystal forms.
Table 4 Equilibrium constants for the exchange reactions between 1c
and para-substituted thiophenols at 23 °C
Z
Keq (in toluene-d8)a
Keq (in THF-d8)a
OCH3
CH3
Cl
0.92 0.03
0.91 0.05
5.6 0.6
0.91 0.02
0.87 0.03
5.7 0.9
CF3
18.9 0.4
13.8 0.4
a Average of three individual experiments.
Fig. 4 Side view of 2c illustrating the “in plane” orientation of the
thiolate ligand.
Table 5 Relative S–H BDEs of thiols and relative Ni–S BDEs of
nickel thiolate complexes 1a–e
constant measurement for the exchange reactions between a
metal thiolate complex (e.g., 1c) and other free thiols (eqn (4)),
considering that S–H BDEs are available in the literature.16,22
Z
ΔDH°(S–H) (kJ mol−1 a
)
ΔDH°(PhLNi–S) (kJ mol−1
)
OCH3
CH3
Cl
−8.3 2.9
−1.9 2.9
3.6 2.7
3.1 2.8
−8.5 3.0
−2.1 3.0
7.8 3.0
CF3
10.3 2.9
ð4Þ
a Data from ref. 16a.
At room temperature, the equilibria in eqn (4) were
approached from either direction within several days. Analogous
to a recent study on a related ligand exchange reaction between a
PNP-pincer nickel anilide complex and PhSH,14h the thiolate
exchange reactions shown here may proceed via a concerted
mechanism involving thiol coordination to nickel and concurrent
deprotonation by the nickel-bound thiolate. The equilibrium con-
calculations along with the Keq values measured in toluene-d8.
As shown in Table 5, electron-releasing groups (OCH3 and CH3)
weaken the Ni–S bonds whereas electron-withdrawing groups
(Cl and CF3) strengthen the Ni–S bonds. Compared to relative
S–H BDEs, relative Ni–S BDEs span a wider range, a conse-
quence of less basic thiolate ligands favoring the binding of the
metal over hydrogen. From an E–C model point of view, the sub-
stituent effect on the relative Ni–S BDEs highlights the impor-
tance of the electrostatic contribution in the bonding, which is
present to a lesser extent in S–H bonds.
1
stants were calculated based on the integrations of H and 31P
{1H} NMR spectra. A potential hydrogen-bonding interaction
between a thiol and a nickel thiolate could complicate the equili-
brium constant measurement; however, it is not involved in our
system as the SH resonances are not significantly broadened and
not shifted from those of the free thiols. In addition, the equili-
brium constants are temperature independent from 23 °C to
60 °C. Solvation does not appear to play a significant role either
in altering the equilibria, as similar Keq values have been
obtained in toluene-d8 and THF-d8 (Table 4).
Ph
Ph
Ph
ΔDH°ð LNi–SÞ ¼ DH°ð LNi–SArÞ ꢀ DH°ð LNi–SPhÞ
¼ DH°ðH–SArÞ ꢀ DH°ðH–SPhÞ ꢀ ΔH
ð5Þ
Since ΔS ≈ 0, ΔH ≈ ΔG
Ph
Relative Ni–S BDE, defined as the change in BDE for a
nickel arylthiolate complex relative to the corresponding nickel
thiophenolate complex, can be calculated from eqn (5). For reac-
tions in which the number of particles is conserved,23 the ΔS
values are negligible, and therefore the relationship between rela-
tive Ni–S BDEs, relative S–H BDEs, and equilibrium constants
is established by eqn (6). The computed gas-phase relative BDEs
for different thiols16a,22 could be used, as the equilibrium con-
stants are insensitive to the solvent. However, the solution data
(in benzene) are known,16a and therefore preferred in our
ΔDH°ð LNi–SÞ ¼ ΔDH°ðS–HÞ þ RT ln Keq
ð6Þ
Interestingly, nickel thiolate complexes undergo thiolate
exchange with complexes bearing a different pincer ligand, as
demonstrated in eqn (7). The reactions are markedly faster than
those in eqn (4) and the equilibria are typically reached from
either direction within just several hours. Mechanistic details of
the exchange process, however, remain unclear to us at the
moment. Fortunately, the four pincer complexes involved in each
equilibrium show distinctively different 31P resonances; thus
7962 | Dalton Trans., 2012, 41, 7959–7968
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Table 6 Equilibrium constants Keq′ for thiolate exchange and relative
Ni–S BDEs of nickel thiolate complexes 2a–e
Table 7 Kinetic data for the reaction of nickel thiolate complexes with
PhCH2Br at 60 °C in toluene-d8
Z
Keq′ (in toluene-d8)a
ΔDH°(iPrLNi–S) (kJ mol−1
)
Complex
Substituent Z
Rate constant k (M−1 s−1
)
krel
OCH3
CH3
Cl
1.0 0.1
−8.5 3.2
−2.5 3.1
5.5 3.2
7.0 3.1
1a
1b
1c
1d
1e
2a
2c
OCH3
CH3
H
2.5(2) × 10−4
1.6(2) × 10−4
1.4(1) × 10−4
4.4(2) × 10−5
1.7(1) × 10−5
4.5(5) × 10−4
1.7(1) × 10−4
15
9.4
8.2
2.6
1
26
10
0.86 0.04
0.39 0.03
0.26 0.02
CF3
Cl
CF3
OCH3
H
a Average of three individual experiments at 23 °C.
for their reactions with alkyl halides.13,25 Iodomethane, a typical
alkylating reagent for these studies, was initially chosen to react
with 1c. The reaction yields the expected nickel iodide complex
3 and PhSCH3 (Scheme 1); however, the required temperature
for this process (50–80 °C) is above the boiling point of CH3I
(42 °C), preventing reliable measurement of the rate constant.
We then resorted to using higher boiling PhCH2Br as the alkylat-
ing reagent. In the presence of a large excess of PhCH2Br
(10–20 equiv.), monitoring the disappearance of 1c in toluene-d8
by 1H NMR gives the pseudo-first-order rate constant kobs. Varia-
tion of [PhCH2Br] establishes a linear relationship between
kobs and [PhCH2Br], implying an overall second-order reaction
(see ESI†). The second-order rate constant k for 1c at 60 °C was
determined to be 1.4(1) × 10−4 M−1 s−1. Rate constant measure-
ments between 50 °C and 80 °C give ΔH‡ = 61.5 0.8 kJ mol−1
and ΔS‡ = −135.1 2.9 J K−1 mol−1. The large negative entropy
of activation is consistent with a bimolecular process. The
kinetic behaviors of other nickel thiolate complexes toward
PhCH2Br were found to be similar to that of 1c. The second-
order rate constants at 60 °C are summarized in Table 7. As seen
from the phenyl series 1a–e, the rate constant decreases when
the thiolate ligand becomes less donating; under the same con-
ditions, the reaction of 1a is about 15 times as fast as that of 1e.
Scheme 1
the equilibrium constants were conveniently measured by
31P{1H} NMR spectroscopy. In the cases of thiolate complexes
possessing electron-releasing substituents (Table 6), the near-
unity Keq′ values imply that relative Ni–S BDEs are constant
irrespective of the pincer ligand used. On the other hand, thiolate
ligands with electron-withdrawing groups clearly prefer the
nickel center supported by the phenyl-substituted pincer ligand,
resulting in Keq′ values much less than one. Relative Ni–S BDEs
for complexes in the isopropyl series were calculated according
to eqn (8) and are listed in Table 6. Compared to the phenyl
series, these data span a more narrow range, suggesting that
electrostatic interaction contributes less in the overall bonding.
Salah and Zargarian have recently compared the redox potentials
of nickel bis(phosphinite)-based pincer complexes with different
P-substituents and have concluded that the pincer ligand with
phenyl substituents is less electron donating than the one with
isopropyl substituents.24 One can thus view the PhLNi moiety as
a harder acid than iPrLNi, and therefore more likely to bind to a
harder base such as p-CF3C6H4S−.
The obtained Hammett reaction constant (ρ) of −1.5
0.3
implies a positive charge being built on sulfur during the reac-
tion. Comparisons between complexes with an identical thiolate
ligand show that complexes in the isopropyl series are more reac-
tive than those in the phenyl series. For instance, the rate con-
stant for 2a is 1.8 times the rate constant for 1a. This
observation is consistent with the notion that the i-Pr groups are
more electron donating than the Ph groups24 and presumably
help to stabilize the transition state.
Although from the net chemical transformation point of view
we have described the reactions in Scheme 1 as nucleophilic
reactivity of the nickel-bound thiolate, it does not necessarily
define the reaction mechanism. To better understand the alky-
lation reactions, we have considered several mechanistic scena-
rios as illustrated in Scheme 2. A rate-determining homolytic or
heterolytic cleavage of the Ni–S bond (mechanism I) would be
inconsistent with the observed second-order reaction. Mecha-
nism II involves an SN2-like transition state leading to the for-
mation of a cationic nickel complex with a thioether, which may
be rapidly displaced by Br−. Such a mechanism is supported by
the negative ρ value of −1.5 determined from the kinetic studies.
In addition, the reaction of 1a with PhCH2Br in THF is approxi-
mately twice as fast as the one in toluene, presumably due to
ð7Þ
0
Ph
ΔDH°ðiPrLNi–SÞ ¼ ΔDH°ð LNi–SÞ þ RT ln Keq
ð8Þ
Reactivity of nickel thiolate complexes with halogenated
compounds
The kinetic stability and nucleophilicity of metal thiolate com-
plexes have often been probed by measuring the rate constants
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Scheme 3
trityl radical, which is known to dimerize to make the Gomberg
dimer.34 In an attempt to generate a radical clock,35 we also
examined the reaction between 1a and cyclopropylmethyl
bromide (eqn (9)). The alkylation reaction is significantly slower
than the one with PhCH2Br. More importantly, no ring-opening
product was observed, which is inconsistent with the halogen
abstraction and SET mechanisms. In addition to the SN2-type
mechanism described earlier, we cannot rule out a σ-bond-
metathesis-like mechanism36 in which Ni–S bond breaking and
C–S bond making take place simultaneously (mechanism VI).
Both mechanisms would suggest that the alkylation reaction is
very sensitive to the groups around the carbon center. Indeed, we
have observed that the reaction of 1a with PhCH2Br at 60 °C is
twice as fast as the one with PhCHBrCH3 under the same
conditions.
Scheme 2
better stabilization of a charged species in a more polar solvent.
An alternative mechanism features (concerted or SN2-type) oxi-
dative addition of PhCH2–Br to a nickel thiolate complex
(mechanism III) and invokes a formally six-coordinate Ni(IV)
intermediate. Although such species are not unprecedented,26,27
they are rarely formed via oxidative addition reactions.26b,c At a
nickel center supported by a relatively electron-deficient bis
(phosphinite) ligand, this is even more of a remote possibility.
An outer-sphere single electron transfer (SET) mechanism28 has
been previously proposed for the reactions of alkyl halides
with lithium thiolates,29 and it may be applied to transition-
metal-bound thiolates as well (mechanism IV). The large nega-
tive activation entropy (−135.1 2.9 J K−1 mol−1) obtained,
however, argues against such a mechanism as the value for an
outer-sphere SET process is typically small or even positive.30
Another possible mechanism proceeds via the abstraction of
bromine atom from PhCH2Br by the nickel thiolate complex,
leading to the formation of a formally Ni(III) species (mechanism
V). Analogous halogen abstraction has been proposed as the key
step in nickel-catalyzed Kharasch addition reactions.31 Further-
more, Ni(III) species bearing various pincer ligands have been
spectroscopically observed or crystallographically characterized
by van Koten32 and Zargarian.33 Such a mechanism is certainly
valid in the case of reactions involving weak carbon–halogen
bonds. When a solution of thiolate complex 1b in toluene-d8
was treated with CCl4 at 100 °C, in addition to the expected
nickel chloride complex 5,6 both Cl3CCCl3 and ArSSAr were
cleanly produced (Scheme 3), as suggested by NMR and
GC-MS. This result supports the formation of radical intermedi-
ates ˙CCl3 and ArS˙ most likely as a result of halogen abstrac-
tion. Similarly, 1b can abstract chlorine from Ph3CCl to generate
ð9Þ
Conclusions
We have synthesized several new nickel thiolate complexes by
varying substituents on the thiolate ligand and phosphorus
donors of a bis(phosphinite)-based pincer ligand, and we have
systematically compared their Ni–S bond lengths, Ni–S BDEs,
as well as their kinetic stability toward halogenated compounds.
In contrast to previous studies on other metal thiolate complex-
es,13c,18 Ni–S bond lengths in our nickel pincer complexes are
neither affected by the basicity of the thiolate ligand nor by the
pincer P-substituents. However, Ni–S bond strengths are clearly
influenced by the electronic effect of the thiolate para-substitu-
ent; in general, having an electron withdrawing group leads to a
stronger Ni–S bond. The sensitivity of Ni–S bond strength
toward changes in the thiolate substituent is dependent on the
electronic property conferred by the pincer unit. A less electron-
donating pincer ligand makes the Ni–S bond more sensitive to
changes in the thiolate substituent, which can be rationalized by
increased electrostatic contribution in the bonding. The reaction
of nickel thiolate complexes with PhCH2Br is more favourable
when the thiolate ligand or the pincer ligand is more electron
donating. Mechanistic studies have suggested that either an
7964 | Dalton Trans., 2012, 41, 7959–7968
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SN2-type mechanism or a σ-bond-metathesis-like mechanism is
involved in these alkylation reactions.
temperature, the volatiles were removed under vacuum and the
residue was extracted with toluene and filtered through a pad
of Celite. Removal of toluene under vacuum followed by recrys-
tallization in hexanes produced an orange solid of 2a (421 mg,
78% yield). 1H NMR (400 MHz, CDCl3, δ): 7.35 (d, JH–H = 8.6
Hz, Ar, 2H), 6.94 (t, JH–H = 8.0 Hz, Ar, 1H), 6.63 (d, JH–H = 8.6
Hz, Ar, 2H), 6.43 (d, JH–H = 8.0 Hz, Ar, 2H), 3.75 (s, OCH3,
3H), 2.10–2.03 (m, CH(CH3)2, 4H), 1.37–1.23 (m, CH(CH3)2,
24H). 13C{1H} NMR (101 MHz, CDCl3, δ): 168.0 (t, JC–P = 9.1
Hz), 156.8, 136.2 (t, JC–P = 9.1 Hz), 135.4, 130.9 (t, JC–P = 20.2
Hz), 128.4, 113.3, 104.6 (t, JC–P = 6.1 Hz), 55.4 (OCH3), 28.4
(t, JC–P = 11.1 Hz, CH(CH3)2), 17.8 (t, JC–P = 3.0 Hz, CH
(CH3)2), 16.8 (s, CH(CH3)2). 31P{1H} NMR (162 MHz, CDCl3,
δ): 189.4. Anal. Calcd for C25H38NiO3P2S: C, 55.68; H, 7.10.
Found: C, 55.69; H, 6.96.
Experimental section
General procedures
Nickel thiolate complexes were prepared under an argon atmos-
phere using standard Schlenk techniques. Once formed, they can
be handled in air without noticeable decomposition. Dry and
oxygen-free solvents (THF, toluene, and CH2Cl2) were collected
from an Innovative Technology solvent purification system
and used throughout the experiments. Hexanes were purchased
from commercial sources and used without purification or degas-
sing. Toluene-d8 was distilled from Na and benzophenone
under an argon atmosphere. [2,6-(Ph2PO)2C6H3]NiCl,6 {2,6-[(i-
Pr)2PO]2C6H3}NiCl,33b and 1a–c,5 were prepared as described
in the literature.
{2,6-[(i-Pr)2PO]2C6H3}NiSC6H4CH3 (2b) was prepared in
1
78% yield by a procedure similar to that used for 2a. H NMR
(400 MHz, CDCl3, δ): 7.33 (d, JH–H = 8.0 Hz, Ar, 2H), 6.95 (t,
J
H–H = 8.0 Hz, Ar, 1H), 6.84 (d, JH–H = 8.0 Hz, Ar, 2H), 6.44 (d,
Synthesis of [2,6-(Ph2PO)2C6H3]NiSC6H4Cl (1d). To a sus-
pension of NaH (48 mg, 2.0 mmol) in 30 mL of THF was added
4-chlorobenzenethiol (289 mg, 2.0 mmol) at 0 °C under an
argon atmosphere. The resulting mixture was warmed to
room temperature and stirred for 1 h followed by the addition of
[2,6-(Ph2PO)2C6H3]NiCl (572 mg, 1.0 mmol). After stirring at
room temperature for another 2 h, the volatiles were removed
under vacuum and the residue was extracted with toluene and
filtered through a pad of Celite. Removal of toluene under
vacuum produced a red solid, which was recrystallized from
CH2Cl2–hexanes (1 : 2) to give 1d as deep red crystals (476 mg,
JH–H = 8.0 Hz, Ar, 2H), 2.24 (s, CH3, 3H), 2.12–2.05 (m, CH
(CH3)2, 4H), 1.36–1.23 (m, CH(CH3)2, 24H). 13C{1H} NMR
(101 MHz, CDCl3, δ): 168.1 (t, JC–P = 9.8 Hz), 142.0 (t,
JC–P = 7.9 Hz), 134.3, 132.8, 131.0 (t, JC–P = 20.4 Hz), 128.5,
128.2, 104.6 (t, JC–P = 6.1 Hz), 28.4 (t, JC–P = 11.5 Hz,
CH(CH3)2), 20.9 (ArCH3), 17.8 (t, JC–P = 2.5 Hz, CH(CH3)2),
16.8 (CH(CH3)2). 31P{1H} NMR (162 MHz, CDCl3, δ): 189.2.
Anal. Calcd for C25H38NiO2P2S: C, 57.38; H, 7.32. Found: C,
57.50; H, 7.15.
1
70% yield). H NMR (400 MHz, CDCl3, δ): 7.83–7.80 (m, Ar,
Synthesis of {2,6-[(i-Pr)2PO]2C6H3}NiSPh (2c). A mixture of
{2,6-[(i-Pr)2PO]2C6H3}NiCl (152 mg, 0.35 mmol) and NaSPh
(90%, technical grade, purchased from Sigma-Aldrich, 231 mg,
1.57 mmol) in 30 mL of THF was refluxed under an argon
atmosphere for 24 h. After cooling, the volatiles were removed
under vacuum and the residue was treated with 40 mL of toluene
and then filtered through a pad of Celite. Removal of toluene
under vacuum afforded 2c as an orange solid, which was further
8H), 7.52–7.39 (m, Ar, 12H), 7.10 (t, JH–H = 8.0 Hz, Ar, 1H),
6.94 (d, JH–H = 8.3 Hz, Ar, 2H), 6.67 (d, JH–H = 8.0 Hz, Ar, 2H),
6.37 (d, JH–H = 8.3 Hz, Ar, 2H). 13C{1H} NMR (101 MHz,
CDCl3, δ): 166.3 (t, JC–P = 11.1 Hz), 142.7 (t, JC–P = 7.7 Hz),
135.0, 132.6, 132.4, 132.2 (t, JC–P = 6.9 Hz), 131.6, 129.6,
128.7, 128.6 (t, JC–P = 5.1 Hz), 127.0, 106.3 (t, JC–P = 6.8 Hz).
31P{1H} NMR (162 MHz, CDCl3, δ): 148.2. Anal. Calcd
for C36H27ClNiO2P2S: C, 63.61; H, 4.00; Cl, 5.22. Found:
C, 63.35; H, 3.92; Cl, 5.47.
[2,6-(Ph2PO)2C6H3]NiSC6H4CF3 (1e) was prepared in 81%
yield by a procedure similar to that used for 1d. 1H NMR
(400 MHz, CDCl3, δ): 7.86–7.81 (m, Ar, 8H), 7.50–7.37 (m, Ar,
12H), 7.16–7.14 (m, Ar, 3H), 6.71 (d, JH–H = 8.0 Hz, Ar, 2H),
6.66 (d, JH–H = 8.1 Hz, Ar, 2H). 13C{1H} NMR (101 MHz,
CDCl3, δ): 166.5 (t, JC–P = 11.2 Hz), 150.4 (t, JC–P = 5.6 Hz),
133.6, 132.6, 132.4, 132.1 (t, JC–P = 7.1 Hz), 131.8, 130.0,
128.7 (t, JC–P = 5.1 Hz), 124.8 (q, JC–F = 272.0 Hz, CF3),
124.3 (q, JC–F = 32.0 Hz, CCF3), 123.7 (q, JC–F = 3.8 Hz),
106.5 (t, JC–P = 6.8 Hz). 31P{1H} NMR (162 MHz, CDCl3, δ):
147.0. Anal. Calcd for C37H27F3NiO2P2S: C, 62.30; H, 3.82;
F, 7.99. Found: C, 62.12; H, 3.70; F, 8.10.
1
purified by recrystallization in hexanes (146 mg, 82% yield). H
NMR (400 MHz, CDCl3, δ): 7.46 (d, JH–H = 7.0 Hz, Ar, 2H),
7.05–6.93 (m, Ar, 4H), 6.44 (d, JH–H = 7.9 Hz, Ar, 2H),
2.12–2.05 (m, CH(CH3)2, 4H), 1.37–1.24 (m, CH(CH3)2, 24H).
13C{1H} NMR (101 MHz, CDCl3, δ): 168.1 (t, JC–P = 9.8 Hz),
146.3 (t, JC–P = 7.7 Hz), 134.5, 130.8 (t, JC–P = 20.5 Hz), 128.6,
127.4, 123.4, 104.7 (t, JC–P = 6.1 Hz), 28.4 (t, JC–P = 11.7 Hz,
CH(CH3)2), 17.8 (t, JC–P
= 2.3 Hz, CH(CH3)2), 16.8
(CH(CH3)2). 31P{1H} NMR (162 MHz, CDCl3, δ): 189.0. Anal.
Calcd for C24H36NiO2P2S: C, 56.61; H, 7.13. Found: C, 56.81;
H, 7.26.
{2,6-[(i-Pr)2PO]2C6H3}NiSC6H4Cl (2d) was prepared in
1
76% yield by a procedure similar to that used for 2a. H NMR
(400 MHz, CDCl3, δ): 7.37 (d, JH–H = 8.4 Hz, Ar, 2H),
7.00–6.94 (m, Ar, 3H), 6.45 (d, JH–H = 7.9 Hz, Ar, 2H),
2.14–2.07 (m, CH(CH3)2, 4H), 1.35–1.25 (m, CH(CH3)2, 24H).
13C{1H} NMR (101 MHz, CDCl3, δ): 168.1 (t, JC–P = 9.6 Hz),
145.2 (t, JC–P = 7.7 Hz), 135.4, 130.4 (t, JC–P = 20.2 Hz), 129.0,
128.7, 127.4, 104.8 (t, JC–P = 6.1 Hz), 28.5 (t, JC–P = 12.1 Hz,
Synthesis of {2,6-[(i-Pr)2PO]2C6H3}NiSC6H4OCH3 (2a). To a
suspension of NaH (120 mg, 5.0 mmol) in 30 mL of THF was
added 4-methoxybenzenethiol (700 mg, 5.0 mmol) at 0 °C
under an argon atmosphere. The resulting mixture was warmed
to room temperature and stirred for 1 h. {2,6-[(i-Pr)2PO]2C6H3}-
NiCl (436 mg, 1.0 mmol) was then added, and the reaction
mixture was refluxed for 24 h. After cooling down to room
CH(CH3)2), 17.7 (t, JC–P
= 2.3 Hz, CH(CH3)2), 16.8
(CH(CH3)2). 31P{1H} NMR (162 MHz, CDCl3, δ): 188.8. Anal.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 7959–7968 | 7965

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Calcd for C24H35ClNiO2P2S: C, 53.02; H, 6.49; Cl, 6.52.
Found: C, 53.29; H, 6.41; Cl, 6.51.
(0.25 g, 0.39 mmol) and benzyl bromide (1.0 mL, 8.4 mmol) in
20 mL of toluene was stirred at 80 °C for 16 h. After cooling to
room temperature, the reaction mixture was filtered through a
pad of Celite, followed by evaporation of the solvent under
vacuum. The resulting solid was recrystallized from CH2Cl2–
hexanes (1 : 2) to produce [2,6-(Ph2PO)2C6H3]NiBr24 as green-
{2,6-[(i-Pr)2PO]2C6H3}NiSC6H4CF3 (2e) was prepared in
1
72% yield by a procedure similar to that used for 2a. H NMR
(400 MHz, CDCl3, δ): 7.53 (d, JH–H = 8.0 Hz, Ar, 2H), 7.25 (d,
JH–H = 8.0, Ar, 2H), 6.98 (t, JH–H = 8.0 Hz, Ar, 1H), 6.46 (d,
JH–H = 8.0 Hz, Ar, 2H), 2.17–2.10 (m, CH(CH3)2, 4H),
1.34–1.25 (m, CH(CH3)2, 24H). 13C{1H} NMR (101 MHz,
CDCl3, δ): 168.1 (t, JC–P = 9.6 Hz), 153.2 (t, JC–P = 6.9 Hz),
1
ish-yellow crystals (220 mg, 92% yield). H NMR (400 MHz,
CDCl3, δ): 8.00–7.95 (m, Ar, 8H), 7.51–7.42 (m, Ar, 12H), 7.08
(t, JH–H = 8.0 Hz, Ar, 1H), 6.63 (d, JH–H = 8.0 Hz, Ar, 2H). 13C-
{1H} NMR (101 MHz, CDCl3, δ): 167.0 (t, JC–P = 11.5 Hz, Ar),
132.7, 132.4, 132.2 (t, JC–P = 7.1 Hz, Ar), 131.8, 129.8, 128.7
(t, JC–P = 5.5 Hz, Ar), 106.6 (t, JC–P = 6.8 Hz, Ar). 31P{1H}
NMR (162 MHz, CDCl3, δ): 144.8. Anal. Calcd for
C30H23P2O2NiBr: C, 58.49; H, 3.76. Found: C, 58.37; H, 3.78.
133.9, 130.1 (t, JC–P = 20.4 Hz), 129.0, 125.2 (q, JC–F
=
32.4 Hz), 124.8 (q, JC–F = 272.1 Hz, CF3), 124.0 (q, JC–F = 3.8
Hz), 104.8 (t, JC–P = 6.1 Hz), 28.5 (t, JC–P = 12.1 Hz,
CH(CH3)2), 17.6 (t, JC–P = 2.3 Hz, CH(CH3)2), 16.8 (CH-
(CH3)2). 31P{1H} NMR (162 MHz, CDCl3, δ): 188.2. Anal.
Calcd for C25H35F3NiO2P2S: C, 52.02; H, 6.11; F, 9.87. Found:
C, 52.21; H, 6.07; F, 9.98.
Synthesis of {2,6-[(i-Pr)2PO]2C6H3}NiBr from the reaction of
2c with PhCH2Br. Following the same procedure as above,
except recrystallization was from hexanes, {2,6-[(i-Pr)2PO]2-
C6H3}NiBr was isolated in 87% yield as light brown crystals.
The NMR spectra of this compound are consistent with those
reported in the literature.33a,b
Equilibrium constant measurement for the exchange between
a nickel thiolate complex and a free thiol. Approximately equi-
molar amounts of 1c and p-Z-C6H4SH (Z = OCH3, CH3, Cl, and
CF3) were mixed in toluene-d8 (∼0.5 mL) at room temperature
(23 °C). Typically, the thiolate exchange reaction reached its
equilibrium in several days. The same equilibrium was also
reached from the reverse direction by mixing approximately
equimolar amounts of [2,6-(Ph2PO)2C6H3]NiSC6H4Z (Z =
p-OCH3, p-CH3, p-Cl, and p-CF3) and PhSH in toluene-d8 at
room temperature. Equilibrium constants were calculated based
on the integrations of 1H and 31P{1H} NMR spectra. The
average value from three independent experiments is reported.
X-ray structure determinations
Single crystals of nickel thiolate complexes in the phenyl series
(1b, 1d, and 1e), and those in the isopropyl series (2a–e) were
obtained from recrystallization in CH2Cl2–hexanes, and hexanes,
respectively. Crystal data collection and refinement parameters of
1b and 2b are summarized in Table 8. The data for other nickel
thiolate complexes can be found in ESI. Intensity data were col-
lected at 150 K on a Bruker SMART6000 CCD diffractometer
using graphite-monochromated Cu Kα radiation, λ = 1.54178 Å.
The data frames were processed using the program SAINT. The
data were corrected for decay, Lorentz, and polarization effects
as well as absorption and beam corrections based on the multi-
Equilibrium constant measurement for the exchange between
two nickel thiolate complexes. Approximately equimolar
amounts of 2c and [2,6-(Ph2PO)2C6H3]NiSC6H4Z (Z = p-OCH3,
p-CH3, p-Cl, and p-CF3) were mixed in toluene-d8 at room temp-
erature. Typically, the thiolate exchange reaction reached its equi-
librium in several hours. Equilibrium constants were calculated
based on the integrations of 31P{1H} NMR spectra. The average
value from three independent experiments is reported.
Table 8 Summary of crystallographic data for representative nickel
thiolate complexes
Rate constant measurement for the alkylation of nickel thio-
late complexes with PhCH2Br. In a typical experiment, a
toluene-d8 (0.50 mL) solution of nickel thiolate complex
(15–20 mM) was transferred to a resealable NMR tube, followed
by the addition of 1,4-dioxane (2.0 μL, as an internal standard)
and 10–20 equiv. of benzyl bromide. The sealed NMR tube was
placed in a constant temperature oil bath (50–80 °C). Every
20 min to 5 h, the NMR tube was quickly cooled (with cold
water) to room temperature and the 1H NMR spectrum was
recorded. Control experiments showed that at room temperature
the alkylation of nickel thiolate complexes with benzyl bromide
was negligible. All the reactions were carried out until they
reached 3–5 half-lives. The integration of one of the thiolate aro-
matic resonances was compared to that of the internal standard.
Diaryl sulfides produced in these processes were verified by
GC-MS. The nickel products ([2,6-(Ph2PO)2C6H3]NiBr and
{2,6-[(i-Pr)2PO]2C6H3}NiBr were isolated from preparative scale
reactions and fully characterized.
1b
2b
Empirical formula
Formula weight
Temp, K
Crystal system
Space group
a, Å
b, Å
c, Å
α (°)
β (°)
C37H30O2P2SNi
659.32
150(2)
C25H38O2P2SNi
523.26
150(2)
Monoclinic
P21/n
14.8305(4)
9.1267(2)
24.2462(6)
90
105.642(1)
90
3160.27(13)
4
Triclinic
P1
ˉ
10.2691(2)
11.3934(2)
13.1019(2)
88.437(1)
67.681(1)
66.738(1)
1289.15(4)
2
γ (°)
Volume, Å3
Z
dcalc, g cm−3
λ, Å
1.386
1.54178
2.714
1.348
1.54178
3.160
μ, mm−1
No. of data collected
No. of unique data
Rint
26 302
5650
11 035
4444
0.0622
1.024
0.0288
1.032
Goodness-of-fit on F2
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
0.0390, 0.0987
0.0519, 0.1069
0.0377, 0.0957
0.0477, 0.1019
Synthesis of [2,6-(Ph2PO)2C6H3]NiBr from the reaction of 1c
with PhCH2Br. Under an argon atmosphere, the mixture of 1c
7966 | Dalton Trans., 2012, 41, 7959–7968
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(70 : 30 occupancy, see ESI†). The crystal structures for 1b, 1d,
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Data Centre (CCDC) and allocated the deposition numbers
CCDC 872168–872175.
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Acknowledgements
We thank the US National Science Foundation (CHE-0952083)
and the donors of the American Chemical Society Petroleum
Research Fund (49646-DNI3) for support of this research. J. Z.
thanks the University of Cincinnati University Research Council
for a postdoctoral research fellowship. X-ray data were collected
on a Bruker SMART6000 diffractometer which was funded
through an NSF-MRI grant (CHE-0215950).
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