Electrophilic alkylation of pseudotetrahedral nickel(II) arylthiolate complexes

Article abstract of DOI:10.1021/ic5018328

A kinetic study is reported for reactions of pseudotetrahedral nickel(II) arylthiolate complexes [(Tp^R,Me)Ni-SAr] (Tp^R,Me = hydrotris{3-R-5-methyl-1-pyrazolyl}borate, R = Me, Ph, and Ar = C₆H₅, C₆H₄-4-Cl, C₆H₄-4-Me, C₆H₄-4-OMe, 2,4,6-Me₃C₆H₂, 2,4,6-ⁱPr₃C₆H₂) with organic electrophiles R'X (i.e., MeI, EtI, BzBr) in low-polarity organic solvents (toluene, THF, chloroform, dichloromethane, or 1,2-dichloroethane), yielding a pseudotetrahedral halide complex [(Tp^R,Me)Ni-X] (X = Cl, Br, I) and the corresponding organosulfide R'SAr. Competitive reactions with halogenated solvents and adventitious air were also examined. Akin to reactions of analogous and biomimetic zinc complexes, a pertinent mechanistic question is the nature of the reactive nucleophile, either an intact thiolate complex or a free arylthiolate resulting from a dissociative pre-equilibrium. The observed kinetics conformed to a second-order rate law, first order with respect to the complex and electrophile, and no intermediate complexes were observed. In the absence of a mechanistically diagnostic rate law, a variety of mechanistic probes were examined, including kinetic effects of varying the metal, solvent, electrophile, and temperature, as well as the 3-pyrazolyl and arylthiolate substituents. Compared to zinc analogues, the effect of Ni-SAr covalency is also of interest herein. The results are broadly interpreted with respect to the disparate mechanistic pathways.

Full text of DOI:10.1021/ic5018328

Article
pubs.acs.org/IC
Electrophilic Alkylation of Pseudotetrahedral Nickel(II) Arylthiolate
Complexes
Tapash Deb and Michael P. Jensen*
Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701, United States
S
* Supporting Information
ABSTRACT: A kinetic study is reported for reactions of pseudotetrahedral
nickel(II) arylthiolate complexes [(TpR,Me)Ni−SAr] (TpR,Me = hydrotris{3-R-5-
methyl-1-pyrazolyl}borate, R = Me, Ph, and Ar = C6H5, C6H4-4-Cl, C6H4-4-
Me, C6H4-4-OMe, 2,4,6-Me3C6H2, 2,4,6-iPr3C6H2) with organic electrophiles
R′X (i.e., MeI, EtI, BzBr) in low-polarity organic solvents (toluene, THF,
chloroform, dichloromethane, or 1,2-dichloroethane), yielding a pseudote-
trahedral halide complex [(TpR,Me)Ni−X] (X = Cl, Br, I) and the
corresponding organosulfide R′SAr. Competitive reactions with halogenated
solvents and adventitious air were also examined. Akin to reactions of
analogous and biomimetic zinc complexes, a pertinent mechanistic question is
the nature of the reactive nucleophile, either an intact thiolate complex or a free
arylthiolate resulting from a dissociative pre-equilibrium. The observed kinetics
conformed to a second-order rate law, first order with respect to the complex
and electrophile, and no intermediate complexes were observed. In the absence of a mechanistically diagnostic rate law, a variety
of mechanistic probes were examined, including kinetic effects of varying the metal, solvent, electrophile, and temperature, as well
as the 3-pyrazolyl and arylthiolate substituents. Compared to zinc analogues, the effect of Ni−SAr covalency is also of interest
herein. The results are broadly interpreted with respect to the disparate mechanistic pathways.
either by a four-centered transition state (i.e., σ-bond
metathesis, pathway B) or by a classical SN2 mechanism
(pathway C).37 Relative to the dissociative and associative SN2
pathways, the reaction barrier for σ-bond metathesis was
calculated to be intrinsically unfavorable for several computa-
tional NS2 heteroscorpionate models,37 although an additional
possibility germane to scorpionate complexes is that of
anchimeric assistance involving donor arm dechelation.28
An associative reaction will exhibit overall second-order
kinetics, rigorously first order with respect to both the complex
and nucleophile.39 The kinetics of a reversible dissociative
mechanism are more complicated; first-order behavior with
respect to the complex is obtained under pre-equilibrium or
steady-state conditions,39 whereas the order with respect to the
electrophile would range from first-order at low concentrations
where alkylation is rate-limiting toward a zero-order limit where
thiolate dissociation is rate-limiting. Although observation of
saturating kinetics with respect to electrophile concentration
would be consistent with reversible thiolate dissociation,
second-order kinetics are not sufficient to distinguish the
mechanism.30 The associative SN2 pathway would be
distinguished by a diagnostic organosulfide complex inter-
mediate (Scheme 1, top right), but the organosulfide ligand is
rapidly displaced from substitution-labile metal ions by the
leaving group (i.e., X−).26,32−35
1. INTRODUCTION
Metal−thiolate (M−SR) bonds are a ubiquitous and essential
feature of many diverse metalloenzyme active sites.1 Several
prominent examples include heme iron-dependent cytochrome
P450,2 non-heme iron-dependent superoxide reductase,3−5
iron- and cobalt-dependent nitrile hydratases,6 thiolatocobala-
mins,7 Fe/Ni-hydrogenase,8 nickel-dependent superoxide dis-
mutase,3 acetyl-coenzyme A synthase9 and methyl-coenzyme M
reductase,10 Type I copper and CuA redox centers,11 and zinc-
dependent methyltransferase enzymes12 such as methionine
synthase,13 as well as the Ada DNA repair protein.14 M−SR
bonds support a rich array of biological reactivity, including
alkylation,15 protonation,16 oxidation,17 and oxygenation.18
Zinc-mediated biological alkyl group transfer has been
extensively modeled by electrophilic alkylation of synthetic
complexes.19 Several classes of zinc complexes have been
studied, including tetrahedral homoleptic arylthiolate ad-
ducts,20,21 square-pyramidal complexes with macrocyclic
supporting ligands,22,23 pseudotetrahedral complexes supported
by hydrotris(pyrazolyl)borate24 ligands [(Tp)Zn−SR] and
related facially tridentate “scorpionate” ligands,25−34 as well as
complexes of polydentate ligands with incorporated thiolate
donors.33−36
Two limiting alkylation mechanisms have been proposed that
involve distinct nucleophiles:37 predissociation of strongly
nucleophilic free thiolate (Scheme 1, pathway A)21,22,32,38 and
an associative reaction of intact thiolate complexes. The
associative mechanism may proceed by distinct pathways:
Received: July 29, 2014
© XXXX American Chemical Society
A
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Scheme 1
A prototypical kinetic study of (MeO)3PO demethylation
in DMSO solution by a series of arylthiolate complexes
[NMe4]2−n[Zn(SPh)4−n(1-methylimidazole)n] (n = 0, 1, 2)
found that reactivity increased with the anionic charge of the
reported for other scorpionates;32,44 in one such case, that of
a sulfur-rich hydrotris(2-mercapto-imidazolyl)borato scorpio-
nate incorporating an intramolecular hydrogen bond to the
thiolate sulfur, a dissociative mechanism was assigned for
reaction with MeI in chloroform on the basis of rapid thiolate
exchange and observed kinetic isotope effects.30,44
The reactivity of transition metal analogues is of interest with
respect to the enhanced covalency of M−SR bonding, as
influenced by disparate ligand field geometries, variable d
electron counts, and accessible spin states.45 Kinetic studies
have been reported for reactions of neutral square-planar and
cationic square-pyramidal nickel(II) complexes with various
organic electrophiles,22,36,46,47 but pseudotetrahedral scorpio-
nate complexes have not been examined in any detail.45,48−54
Because a common pseudotetrahedral geometry and a
significant ionic component are retained in M−SAr bonding,45
the mechanistic discussion of the zinc(II) complexes (Scheme
1) is also relevant for nickel(II) analogues, although alternative
redox mechanisms are possible.46
We recently prepared and characterized a number of such
arylthiolate complexes [(TpR,Me)Ni−SAr] (R = Me, Ph).51−53
A key feature of these complexes is the steric interaction
between the arylthiolate substituent and the proximal 3-
pyrazole substituents,24,28 which leads to significant structural
effects.49−54 Moreover, electronic properties of the arylthiolate
are readily manipulated. A preliminary demonstration of
electrophilic alkylation by MeI (Ar = Ph and Mes, 2,4,6-
Me3C6H2) revealed a second-order rate law.51 In the present
work, additional insights into the relevant alkylation mechanism
were sought using a suite of kinetic and extrakinetic probes of
mechanism,39 including Arrhenius and Hammett analyses, as
well as the elucidation of solvent, substituent, and conforma-
tional effects.
complexes: [Zn(SPh)2(MeIm)2] < [Zn(SPh)3(MeIm)]−
<
[Zn(SPh)4]2−.20,21 The observed second-order rate constant
of the homoleptic dianion approaches that of free phenyl-
1
thiolate, and H NMR line broadening provided evidence for
facile thiolate dissociation and exchange. A dissociative
equilibrium was proposed for [Zn(SPh)4]2−, with subsequent
alkylation of the free arylthiolate anion; the kinetic data were fit
according to this model.21
In contrast, alkylation of the substitution-inert organometallic
complexes [CpFe(CO)2SR] (Cp = η5-C5H5 and R = Me, Et,
Ph) with the potent electrophile MeI proceeds by an associative
mechanism in DMSO or acetone solutions. Second-order
kinetics were observed, as well as metastable thioether complex
intermediate salts [CpFe(CO)2(MeSR)]+(I−);40−42 under
more vigorous conditions, subsequent ligand substitution
occurs to give [CpFe(CO)2I] and free organosulfide MeSR
as the final products.40 Redox chemistry is also possible for
iron(II), but no evidence was found for complex oxidation or
free radical formation.40
Mechanistic assignment for reactivity of neutral scorpionate
complexes (e.g., [(Tp)Zn−SR]) with strong organic electro-
philes (e.g., MeI, BzBr) in low-polarity solvents is more
challenging.25−34 Overall second-order behavior is typically
observed,25 usually under pseudo-first-order conditions with
excess electrophile.26−28,30−32 Observed rates are much slower
than that of free thiolate anion,32 organosulfide complexes are
not observed,25−28,31,32 and exchange reactions with free
thiolate salts are relatively slow for [(TpPh,Me)Zn−SR] in
chloroform and DMSO.26,43 Therefore, an associative mecha-
nism is frequently invoked.25−28,31,32 On the other hand, the
assignment of an associative mechanism has been questioned in
some instances.28,30,35 Facile thiolate exchange has been
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Figure 1. UV−vis−NIR spectra of authentic [(TpPh,Me)Ni−X] (X = Cl (green), Br (orange), I (violet)) in 1,2-dichloroethane at 298 K, with 10-fold
vertical expansion from 700 to 1000 nm.56
Figure 2. Time-dependent UV−vis−NIR spectra (gray) for the reaction of [(TpPh,Me)Ni−SMes] with MeI (78 mM) in 1,2-dichloroethane at 326.2
K with limiting spectra (blue, violet) obtained from a global fit (kobs = 6.64(1) × 10−4 s−1). Inset shows single-wavelength exponential fits for decay
of [(TpPh,Me)Ni−SMes] observed at 557 nm (kobs = 6.59(1) × 10−4 s−1; r2 = 1.00) and accumulation of [(TpPh,Me)Ni−I] observed at 427 nm (kobs
6.63(2) × 10−4 s−1; r2 = 1.00).
=
2. EXPERIMENTAL SECTION
3. RESULTS AND DISCUSSION
The synthesis and characterization of all thiolate complexes [(TpR,Me)-
Ni−SAr] (R = Me, Ph)51−54 utilized herein, as well as the halide
product complexes [(TpR,Me)Ni−X] (R = Me,55 Ph;56 X = Cl, Br, I),
were reported previously. All reaction solutions were prepared under
argon in a glovebox (MBraun Unilab), loaded into cuvettes, and sealed
under a rubber septum with a threaded screw cap (Starna, Atascadero,
CA). Control solutions were oxygenated as previously described57 by
brief bubbling with O2 gas predried by passage through a bed of
MgSO4 and CaH2. Time-dependent UV−vis−NIR spectra were
recorded on an Agilent HP-8453 diode-array spectrophotometer
using a jacketed cuvette holder interfaced to a circulating bath
equipped with heating and cooling elements under control of a
thermostat (VWR). Multivariate nonlinear least-squares fits were
calculated for time-dependent UV−vis−NIR spectra using SPECFIT/
32.58,59 Observed rates of electrophilic alkylations are listed in Table
S1 in the Supporting Information (SI). Alternative single-wavelength
exponential fits were calculated by nonlinear least-squares regression
using SigmaPlot 8.02 (SPSS, Inc.).60 Gas chromatography−mass
spectrometry (GC−MS) data were obtained on a Shimadzu QP2010
SE instrument after passage of reaction solutions through a silica plug
to remove inorganic components; the initial column temperature of 50
°C was increased by 12 °C/min to 210 °C and then 15 °C/min to 315
°C.
3.1. General Remarks Regarding the Reactivity of
[(TpR,Me)Ni−SAr] with Electrophiles. The arylthiolate
complexes [(TpR,Me)Ni−SAr] (R = Me, Ph) react with organic
electrophiles (R′X = MeI, EtI, BzBr, ClCH2CH2Cl) to form the
corresponding halide complex [(TpR,Me)Ni−X] (Figure 1) and
free organosulfide R′SAr (eq 1). Decompositions of [(TpR,Me)-
Ni−SAr] were monitored by time-dependent UV−vis−NIR
spectroscopy (Figure 2), and observed kinetics conformed to
an overall second-order rate law (eq 2). Values of k0 and k1
were extracted from the observed rate (kobs) by variation of
electrophile concentration (i.e., [R′X]0, eq 3) under pseudo-
first-order conditions. A preliminary demonstration of
[(TpR,Me)Ni−SAr] alkylation with MeI in CH2Cl2 solutions
revealed a small positive intercept at 297 K (i.e., k0 > 0), which
was speculatively assigned to rate-limiting thiolate dissociation
under saturating [MeI]0.51 The more extensive investigation
herein shows that k0 actually reflects a composite of minor side
reactions with halogenated solvent (eq 4) and adventitious air
(eq 5 and/or eq 6), as well as any systematic error in
determining [MeI]0 (section 3.8, SI).
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Figure 3. GC−MS data showing organic products from thermolysis of [(TpPh,Me)Ni−SMes] at 336 K in 1,2-dichloroethane with added MeI (0.5 M,
top) and without (bottom). Peak labels: (A) naphthalene standard (m/z = 128), (B) MesSH (m/z = 152), (C) MeSMes (m/z = 166), (D) HpzPh,Me
(m/z = 158), (E) MesSCH2CH2Cl (m/z = 214), and (F) MesSSMes (m/z = 302).
product arene resonances with the [(TpPh,Me)Ni−SMes]
analogue, formation of the organosulfide coproduct MeSMes
was alternatively confirmed by GC−MS measurements on the
equilibrium solution (Figure 3).
[( TpR,Me)Ni−SAr] + R′X →[(TpR,Me)Ni−X] + R′SAr
(1)
−d[(TpR,Me)Ni−SAr]/dt
= (k0 + k1[R′X])[(TpR,Me)Ni−SAr]
The time-dependent absorption data were fit by global least-
squares regression to the first-order integrated rate law to
obtain the kobs value (observed alkylation kinetics are compiled
in Table S1 in the SI, and details of the global fitting analysis for
the specific data shown in Figure 2 are given in Figures S1−S10
in the SI). Coincident results were obtained by least-squares
fitting of exponentials to single-wavelength data at the maximal
absorption change for both [(TpPh,Me)Ni−SMes] decay and
[(TpPh,Me)Ni−I] accumulation (557 and 427 nm, respectively;
Figure 2, inset). Kinetic behavior conforming to the overall
second-order rate law (eq 2) was demonstrated by
independently varying the initial concentrations of [(TpPh,Me)-
Ni−SMes] and MeI at 308(1) K (Figure S11, SI). Linear least-
squares regression of kobs versus [MeI]0 gave values of k0 and k1
from the intercept and slope, respectively (Figure 4 and Table
S1 in the SI). Analyses of all other reactions involving a variety
of arylthiolate complexes and organic electrophiles were
conducted analogously (representative time-dependent spec-
(2)
(3)
−d[(TpR,Me)Ni−SAr]/dt = kobs[(TpR,Me)Ni−SAr]
where kobs = k0 + k1[R′X]0 and [R′X]0 ≫ [(TpR,Me)Ni−SAr]0
[(TpR,Me)Ni−SAr] + ClCH2CH2Cl
→ [(TpR,Me)Ni−Cl] + ArSCH2CH2Cl
(4)
[(TpR,Me)Ni−SAr] + H2O
→ 1/n[(TpR,Me)Ni−OH]n + ArSH
(5)
[(TpR,Me)Ni−SAr] + O2
1
2
→ [(TpR,Me)Ni]+ + O2
+
ArSSAr
−
(6)
3.2. Kinetics for Electrophilic Alkylation of [(TpPh,Me)-
Ni−SMes]. Alkylation of [(TpPh,Me)Ni−SMes] with MeI was
selected as the prototypical reaction because this reaction
occurs on a convenient time scale near room temperature
under pseudo-first-order conditions in nonpolar solvents (e.g.,
100-fold excess MeI under argon in dry 1,2-dichloroethane)
and is easily monitored by electronic spectroscopy (Figure 2).
Conversion of the arylthiolate complex to [(TpPh,Me)Ni−I] is
marked by an obvious bleaching of the reaction solution from
dark purple to pale pink. Time-dependent UV−vis−NIR
spectra at 326 K (Figure 2) showed a monotonic conversion
to [(TpPh,Me)Ni−I] with bleaching of SAr → Ni ligand-to-metal
charge-transfer (LMCT) bands and growth of the characteristic
I → Ni LMCT bands (cf., Figure 1). A clear isosbestic point
was maintained directly between these features at 461 nm;
therefore, transient accumulation of any intermediate complex
was not evident. Ligand field bands between 800 and 1100 nm
sharpened with the increase in ideal point symmetry from Cs-
[(TpPh,Me)Ni−SMes] to C3v-[(TpPh,Me)Ni−I] but did not shift
considerably, which is consistent with the comparable position
of π-donating arylthiolates with respect to that of halides in the
spectrochemical series.61,62 Clean formation of the expected
organosulfide coproduct MeSMes was previously demonstrated
Figure 4. Plot of kobs (s−1) vs [MeI]0 for alkylation of [(TpPh,Me)Ni−
SMes] in 1,2-dichloroethane solution with linear least-squares
regressions at the following temperatures: 299 K (black), k0 = 2(1)
× 10−5 s−1 and k1 = 1.31(6) × 10−3 M−1 s−1; 308 K (pink), k0 = 1(2) ×
10−5 s−1 and k1 = 3.1(2) × 10−3 M−1 s−1; 318 K (blue), k0 = 6(2) ×
10−5 s−1 and k1 = 4.7(3) × 10−3 M−1 s−1; 326 K (green), k0 = 1.3(4) ×
10−4 s−1 and k1 = 7.3(5) × 10−3 M−1 s−1; and 334 K (red), k0 = 2.5(2)
× 10−4 s−1 and k1 = 1.01(3) × 10−2 M−1 s−1.
1
by H NMR spectroscopy for alkylation of [(TpMe,Me)Ni−
SMes].51 Given the expected overlap of the reactant and
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trophotometric data and pseudo-first-order fits are shown in
Figures S12−S19 in the SI).
were ΔH⧧
= 11(1) kcal/mol and ΔS⧧
= −37(4) cal
mol−1 K−11.,obSs imilar values were reported for electrophilic
alkylations of several neutral Zn(II) complexes,30−32,36 as well
as square-planar and square-pyramidal Ni(II) (Table 1).22,36,46
The small enthalpy and negative entropy are seemingly
consistent with the bimolecular transition state of an SN2
reaction mechanism; however, a dissociative mechanism was
assigned to one of the zinc complexes, with solvent electro-
striction proposed to account for the unfavorable entropy.30
3.4. Reaction of [(TpPh,Me)Ni−SMes] with Alternate
Electrophiles. Reaction rates of [(TpPh,Me)Ni−SMes] with the
alternative electrophiles benzyl bromide (Figure S12, SI) and
ethyl iodide (data not shown) were also determined, but the
elucidation of Arrhenius constants was constrained by the
higher concentrations and temperatures required to obtain
convenient reaction rates (Table S1, SI). Nevertheless, relative
reactivities of the various electrophiles, including the con-
tribution of 1,2-dichloroethane solvent to k0 already described,
can be compared using observed or interpolated k1 values at
326 K: MeI (7.3(5) × 10−3 M−1 s−1) > BzBr (3 × 10−4 M−1
s−1) ≈ EtI (1.9(1) × 10−4 M−1 s−1) > ClCH2CH2Cl (≤1 ×
10−5 M−1 s−1). This trend is consistent with the participation of
the R′X electrophiles in a common SN2 reaction mechanism
with respect to the order-of-magnitude kinetic effects induced
by steric (R′: Me > Et)40,47,63 and leaving group (X: I ≥ Br >
Cl)63 properties; however, the identity of the reactive
nucleophile is not revealed by this comparison.
1,obs
3.3. Arrhenius Behavior for Electrophilic Alkylation of
[(TpPh,Me)Ni−SMes]. Observed reaction rates of [(TpPh,Me)-
Ni−SMes] were determined as a function of MeI concentration
at five different temperatures between 299 and 334 K (Figure
4). A linear dependence of kobs on [MeI]0 was maintained at all
temperatures, and Arrhenius plots were constructed for both
the intercept and slope values (i.e., k0 and k1, respectively;
Figure 5). Notwithstanding the volatility of MeI, both plots
Figure 5. Arrhenius plot for k0 (green, s−1) and k1 (red, M−1 s−1)
values for the reaction of MeI and [(TpPh,Me)Ni−SMes] in 1,2-
dichloroethane using the respective intercepts and slopes fitted to the
data in Figure 4. Open circles were not included in the linear
regression to k0 on the basis of a lack of statistical significance in the
intercept values. The vertical dashed line represents the normal boiling
point of MeI (1 atm).
3.5. Solvent Effects for Electrophilic Alkylation of
[(TpPh,Me)Ni−SMes]. Reaction of [(TpPh,Me)Ni−SMes] with
MeI was determined near room temperature (299 K) in a range
of other aprotic solvents (i.e., toluene, THF, CHCl3, and
CH2Cl2). These solvents were selected with the intention of
varying the solvent polarity as much as possible while avoiding
ionization of either the initial arylthiolate complex or the
product halide complex. Time-dependent UV−vis−NIR
spectra were comparable to those recorded in 1,2-dichloro-
ethane, particularly with respect to the accumulation of
were linear and did not exhibit any obvious discontinuity. The
minor k0 pathway is a composite of reactions with halogenated
solvent and adventitious O2 (section 3.8 in the SI), and further
consideration of the associated activation parameters is not
warranted. Parameters calculated for the k1 alkylation pathway
Table 1. Observed Activation Parameters for Bimolecular Electrophilic Reactions
nucleophile
geometry
electrophile
solvent
ΔH⧧
(kcal/mol)
ΔS⧧
(cal mol−1 K−1
)
ref
1,obs
1,obs
a
[(L)Zn−SC6H4-2-NHC(O)tBu]
pseudo-Td
pseudo-Td
sq. planar
sq. pyramid
BzBr
BzBr
BzBr
BzBr
d8-toluene
d8-toluene
d8-toluene
CH3CN
19(1)
16(1)
−21(3)
−22(4)
−32.3(7)
−32(2)
31
31
46
22
a
[(L)Zn−SPh]
b
[(L)Ni−SPh]
14.7(2)
10.3(7)
c
[(L8)Ni−S−C6H4-4-Me]
a
[(L)Zn−SPh]
pseudo-Td
pseudo-Td
pseudo-Td
pseudo-Td
sq. planar
pseudo-Td
MeI
MeI
MeI
MeI
MeI
MeI
d8-toluene
CDCl3
17(1)
14(1)
8(2)
−22(3)
−23(4)
−35(6)
−27(3)
−29(3)
31
30
32
36
36
g
d
[(TmPh)Zn−SCH2C(O)NHPh]
[(L1O)Zn−SBz]
e
CDCl3
f
[(L)Zn]
CDCl3
12.9(7)
11(1)
11(1)
f
[(L)Ni]
CDCl3
[(TpPh,Me)Ni−SMes]
ClCH2CH2Cl
−37(4)
ΔH⧧
(kcal/mol)
ΔS⧧
(cal mol−1 K−1
−49(1)
)
7,obs
7,obs
NEt3
NEt3
NEt3
MeI
MeI
MeI
n-hexane
benzene
acetone
13.4(3)
8.5(3)
8.2(3)
67
67
67
−52(1)
−48(1)
NEt3
EtI
acetone
11.6(2)
−38.8(5)
68
a
b
c
L = phenyl[3-tert-butyl-pyrazol-1-yl]bis[(tert-butylthio)methyl]borate. L = κ3-2,6-(OPPh2)C6H3-1-ylate. L8 = 1,5-bis(2-pyridylmethyl)-1,5-
d
e
diazacyclooctane. TmPh = hydrotris(2-mercapto-1-phenylimidazolyl)borate. L1O = bis(3,5-dimethylpyrazol-1-yl)-6-(1-tert-butyl-4-
methylphenolato)methane. L = 6,6′-bis(2,2-diphenyl-2-ethylthiolato)-2,2′-bipyridine. This Article.
f
g
E
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[(TpPh,Me)Ni−I] in the presence of excess MeI (data not
shown). Linear correlations of kobs to [MeI]0 were observed in
each solvent, and values of k0 and k1 were extracted as before
(Figure 6).
provided by positive correlation with Gutmann’s solvent
acceptor number (r2 = 0.55, Figure 7).64,65 A comparison can
be made between the reaction of [(TpPh,Me)Ni−SMes] with
MeI and the Menschutkin reaction of NEt3 and EtI, which is
the prototypical ionogenic bimolecular reaction of neutral polar
molecules by an SN2 mechanism (eq 7).66 Comparison of k1 at
299 K versus k7 at 298 K in the same solvents reveals some
correlation (r2 = 0.65, Figure 8), suggesting a common
Figure 6. Plot of kobs (s−1) vs [MeI]0 for alkylation of [(TpPh,Me)Ni−
SMes] at 299 K with linear least-squares fits in the following solvents:
CHCl3 (red), k0 = 1.1(3) × 10−4 s−1 and k1 = 1.80(7) × 10−3 M−1 s−1;
1,2-dichloroethane (black), k0 = 2(1) × 10−5 s−1 and k1 = 1.31(6) ×
10−3 M−1 s−1; CH2Cl2 (green), k0 = 4.1(4) × 10−5 s−1 and k1 = 8.4(1)
× 10−4 M−1 s−1; THF (pink), k0 = 6(1) × 10−5 s−1 and k1 = 7.2(4) ×
10−4 M−1 s−1; and toluene (blue), k0 = 2.1(3) × 10−5 s−1 and k1 =
1.3(2) × 10−4 M−1 s−1.
Figure 8. Correlation of the bimolecular rate constants k1 for the
reaction of [(TpPh,Me)Ni−SMes] with MeI, observed at 299 K in the
various solvents shown in Figure 6, with k7 for the reaction of Et3N
with EtI at 298 K in identical solvents.66
mechanism for the two reactions. A particular analogy is
drawn between the product ion pair [R′NEt3]+(X−) and the
putative associative intermediate [(TpR,Me)Ni(R′SAr)]+(X−) of
interest herein (Scheme 1); covalent interaction between the
incipient I− and [(TpPh,Me)Ni]+ ions in the transition state (i.e.,
σ-bond metathesis) has no analogy in the Menschutkin
reaction, and a four-centered associative transition state is
disfavored as a mechanistic feature in the reaction of
[(TpPh,Me)Ni−SMes] on this basis.
Solvent medium effects for an SN2 mechanism involving two
neutral molecules is attributed to differing dipole interactions
between solvent and solute molecules in the ground and
transition states; an inverse logarithmic relationship between
rate and the solvent dielectric (ε) is predicted, but the weak
interaction and the contributions of other forces typically result
in a poor correlation.39 Such a correlation is somewhat evident
in the present case (r2 = 0.67, Figure 7) in which the negative
slope implies charge accumulation in the transition state,
consistent with an SN2 mechanism. Further evidence for the
accumulation of negative charge on the incipient iodide anion is
Et3N + EtI → (NEt4)+I−
(7)
A compensation effect is suggested by solvent-dependent
activation parameters (Table 1),39,67,68 in which reactions in
more polar solvents tend to exhibit lower activation enthalpies
and more negative activation entropies. Although the
significance of such a correlation can be questioned,69
a
genuine compensation effect might arise through differential
solvation of accumulating charge in the transition state.69−72 In
the alternative dissociative mechanism, k1 would represent a
composite of the pre-equilibrium and thiolate alkylation steps,39
and the apparent solvent compensation would reflect some
modulation of this convolution. In this event, the comparable
reactivities of [(TpPh,Me)Ni−SMes] and NEt3 would be
coincidental.
3.6. Electronic Effects for Alkylation of [(TpPh,Me)Ni−
S−C6H4-4-Y] with MeI. The kinetics for reaction of a series of
complexes [(TpPh,Me)Ni−S−C6H4-4-Y] (Y = OMe, Me, H,
Cl)53 with MeI were determined at 298 K in 1,2-dichloroethane
(Figures S13−S16, SI). Linear dependences of kobs on [MeI]0
were again found for all four complexes, with increasing slopes
for the more electron-releasing substituents (Figure 9). A
Hammett plot of k1 against standard σp values yielded ρ =
−1.7(4) (Figure 10). In the context of an associative
mechanism, the negative ρ value may reflect significant
stabilization of positive charge accumulation on sulfur at the
Figure 7. Plot of log k1 (M−1 s−1) for alkylation of [(TpPh,Me)Ni−
SMes] with MeI at 299 K, obtained from the slopes shown in Figure 6,
vs the inverse solvent dielectric constant (blue, upper horizontal axis)
and Gutmann’s solvent acceptor number (red, lower horizontal axis)64
with linear least-squares fits. Because a solvent acceptor number has
not been assigned for toluene, the value reported for benzene is used
instead.
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measured the rates of MeI alkylation over a range of arylthiolate
complexes [(TpR,Me)Ni−S−2,4,6-R″3C6H2] (R = Ph and R″ =
i
i
H, Me, Pr, or R = Me and R″ = H, Pr). The data were
obtained at 298−299 K (R = Ph: R″ = H, Figure S15, SI; Me,
data not shown; iPr, Figure S17, SI) or at 319 K (R = Me: R″ =
i
H, Figure S18, SI; Pr, Figure S19, SI) in 1,2-dichloroethane.
The results afford some comparative insights into substituent
effects.
For the series [(TpPh,Me)Ni−S−2,4,6-R″3C6H2], the trend
observed at 298−299 K is a decrease in k1 with increasing size
of the arylthiolate substituent: R″ = H, 18.3(4) × 10−4 M−1 s−1;
i
Me, 13.1(6) × 10−4 M−1 s−1; and Pr, 7.6(9) × 10−4 M−1 s−1
(Table S1 in the SI and Figure 11). Notwithstanding the
Figure 9. Plot of kobs (s−1) vs [MeI]0 for alkylation of [(TpPh,Me)Ni−
S−C6H4-4-Y] in 1,2-dichloroethane at 298 K with linear least-squares
fits: Y = OMe (green), k0 = 9(2) × 10−5 s−1 and k1 = 4.3(2) × 10−3
M−1 s−1; Y = Me (red), k0 = 1.1(1) × 10−4 s−1 and k1 = 1.85(9) × 10−3
M−1 s−1; Y = H (black), k0 = 2.8(6) × 10−5 s−1 and k1 = 1.83(4) ×
10−3 M−1 s−1; and Y = Cl (blue), k0 = −2(3) × 10−5 s−1 and k1 =
4.9(8) × 10−4 M−1 s−1.
Figure 11. Plot of kobs (s−1) vs [MeI]0 in 1,2-dichloroethane solutions
for [(TpPh,Me)Ni−SPh] at 298 K (red), k0 = 2.8(6) × 10−5 s−1 and k1 =
1.83(4) × 10−3 M−1 s−1; [(TpPh,Me)Ni−SMes] at 299 K (green), k0 =
2(1) × 10−5 s−1 and k1 = 1.31(6) × 10−3 M−1 s−1; and [(TpPh,Me)Ni−
S−2,4,6-iPr3C6H2] at 298 K (blue), k0 = 4.1(7) × 10−5 s−1 and k1 =
7.6(9) × 10−4 M−1 s−1.
electron-releasing properties of the added alkyl substituents,
this trend suggests that ortho substituents may inhibit reactivity
through a modest steric effect. The complexes just enumerated
adopt a trigonal-pyramidal conformation with the arylthiolate
substituent disposed between opposing pyrazole substituents.53
Even with some rotation, the o-arylthiolate substituents are well
positioned in this conformation to occlude the reactive sulfur
lone pair from an approaching electrophile in an associative
mechanism. In the alternative mechanism, dissociation of the
more electron-rich thiolates would be less favorable.
Figure 10. Hammett plot for reaction of [(TpPh,Me)Ni−S−C6H4-4-Y]
(Y = OMe, Me, H, Cl) with MeI in 1,2-dichloroethane at 298 K.
Values of k1Y are obtained from the slopes shown in Figure 9.
SN2 transition state by electron-releasing substituents,40,46 as
well as ground-state destabilization of the reactive lone pair.32
For example, a series of zinc complexes [(L1O)Zn−SR] (L1O
=
bis(3,5-dimethylpyrazol-1-yl)-6-(1-tert-butyl-4-
methylphenolato)methane) was reacted with MeI in chloro-
form at 298 K, and a linear logarithmic correlation was found
between the second-order observed rate constant and the
aqueous pKa of the free thiol, as well as an inverse correlation
with the calculated HOMO energies of the thiolate
complexes.32 Our results also compare to ρ = −1.8 for
alkylations of [CpFe(CO)2SAr] with MeI in acetone at 293 K40
and to ρ = −1.5 for square-planar arylthiolate nickel(II) pincer
complexes reacted with BzBr in toluene at 333 K.46
The steric trend is inverted for less-reactive [(TpMe,Me)Ni−
S−2,4,6-R″3C6H2] analogues at 319 K as ortho substituents
increase the rate of reaction: R″ = H, 4.4(4) × 10−4 M−1 s−1;
iPr, 7.3(6) × 10−4 M−1 s−1 (Table S1 in the SI and Figure 12).
Spectroscopic data indicate that [(TpMe,Me)Ni−S−
2,4,6-iPr3C6H2] adopts a unique sawhorse conformation, in
which the arylthiolate ring is disposed over a single 3-pyrazole
methyl substituent and rotated 90° perpendicular to the Ni−S
bond, displacing the ortho substituents away from the proximal
electrophile; the Ni−SAr bond may also lengthen slightly, as
observed between [(TpMe,Me)Ni−S−2,6-Ph2C6H3] (2.2589(6)
Å) and [(TpMe,Me)Ni−SPh] (2.2162(8) Å).52 The greater
reactivity of the substituted arylthiolate complex would reflect
electronic effects in an associative mechanism, or more
favorable arylthiolate dissociation in the sawhorse conforma-
tion, given a longer Ni−SAr bond.
Two opposing effects would operate in a dissociative
mechanism exhibiting second-order kinetics; a relatively
electron-rich thiolate should exhibit less favorable dissociation,
but would constitute a more potent nucleophile. The observed
substituent effect would be a composite of these effects. In view
of the larger ρ values observed for some nucleophilic
substitutions,39 this possibility cannot be excluded.
3.7. Substituent Effects for Reaction of [(TpR,Me)Ni−
SAr] with MeI. To further elucidate the steric effects of
proximal o-arylthiolate and 3-pyrazole substituents, we
Also noteworthy is the significant kinetic effect of the 3-
pyrazole substituents. [(TpPh,Me)Ni−SPh] exhibits four-fold
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If we extend this argument, the rate enhancement observed
for [(TpR,Me)Ni−SPh] (R = Ph > R = Me) might arise within
an associative pathway partly as a result of increased Ni−SPh
covalency at the expense of weaker tripodal scorpionate ligation
in the presence of the larger 3-pyrazole phenyl substituents.
Red-shifted ligand field bands observed in the corresponding
halide complexes [(TpR,Me)Ni−Cl] (R = Me, Ph) are consistent
with this proposal.55,56 However, such an effect is not fully
resolved in the X-ray crystal structures: Ni−N bond lengths
range from 1.978(2)−1.992(2) Å for [(TpMe,Me)Ni−SPh] to
1.997(2)−2.019(2) Å for [(TpPh,Me)Ni−SPh]; the Ni−SAr
bond lengths are coincident at 2.2162(8) and 2.2160(7)−
2.2224(7) Å for these complexes, respectively.51−53
Figure 12. Plot of kobs (s−1) vs [MeI]0 in 1,2-dichloroethane solutions
at 319 K for [(TpMe,Me)Ni−SPh] (red), k0 = 4(1) × 10−5 s−1 and k1 =
4.4(4) × 10−4 M−1 s−1; and [(TpMe,Me)Ni−S−2,4,6-iPr3C6H2] (blue),
k0 = 6(2) × 10−5 s−1 and k1 = 7.3(6) × 10−4 M−1 s−1.
4. CONCLUSIONS
This study determined the kinetics for reactions of organic
electrophiles R′X with pseudotetrahedral nickel(II) scorpionate
complexes [(TpR,Me)Ni−SAr] (R = Me, Ph). Our results are
intended to complement previous studies on zinc analogues,28
as well as nickel(II) complexes in alternative square-planar and
square-pyramidal geometries.22,36,46,47 Our findings are briefly
summarized as follows.
greater observed rates in comparison with those of [(TpMe,Me)-
Ni−SPh], notwithstanding a drop of 21 °C in temperature,
which plausibly extrapolates to an order-of-magnitude differ-
ence under identical conditions. A comparable rate difference
was observed in the zinc analogues [(TpR,Me)Zn−SAr] (R = Ph
> R = Me), and the greater reactivity of the phenyl-substituted
complex was tentatively ascribed to formation of a unique
hydrophobic cavity around the arylthiolate coligand.28 The
comparison otherwise seems surprising in the context of an
associative mechanism given that the sulfur atom should be
more sterically accessible to electrophiles with smaller methyl
substituents at the proximal 3-pyrazole positions. Overlooking
minor differences in solvent and reaction temperature, reaction
rates of MeI with [(TpPh,Me)M−SPh] (M = Zn, Ni) are nearly
identical (Zn, 2.0(1) × 10−3 M−1 s−1 at 300 K in chloroform;28
Ni, 1.83(4) × 10−3 M−1 s−1 at 298 K in 1,2-dichloroethane),
whereas the reaction of [(TpPh,Ph)Zn−SPh] is modestly faster
(7(1) × 10−3 M−1 s−1 in chloroform at 298 K).32 The
consistency of the observed rates for the Zn(II) analogues
argues for a common mechanism, precluding an alternative
redox pathway for the nickel complex.
A similar correspondence of observed rates was reported for
reactions of the cationic square-pyramidal complexes incorpo-
rating a planar tetradentate pyridinane macrocyclic ligand
[(L8)M−S−C6H4-4-Me](BPh4) (M = Ni, Zn; L8 = 1,5-bis(2-
pyridylmethyl)-1,5-diazacyclooctane) with a fixed concentra-
tion of benzyl bromide in CD3CN at 303 K.22 On the basis of
DFT calculations, it was proposed that destabilization of the
filled and ligand-centered Ni−SAr σ* orbital in Ni(II) offsets
the greater anionic charge on sulfur in the Zn(II) complex,
resulting in comparable nucleophilicity. Slower reactions were
observed for iron(II) and cobalt(II) analogues, which is
consistent with enhanced σ bonding arising from decreased
occupancy of destabilized d orbitals. However, this argument
does not clearly distinguish a reaction mechanism; enhanced
M−SAr bonding would also suppress the rate of thiolate
dissociation. An even greater kinetic effect was obtained for the
pair of distorted square-planar complexes [(L)M] (M = Ni, Zn;
L = κ4-6,6′-bis(2,2-diphenyl-2-ethylthiolato)-2,2′-bipyridine)
reacting with MeI in chloroform; in this geometry, the
HOMO exhibits Ni−SAr π* character, and the nickel complex
is more reactive than the zinc analogue by nearly an order of
magnitude.36
• Arylthiolate complexes [(TpR,Me)Ni−SAr] (R = Me, Ph)
react with organic electrophiles R′X to form the free
organosulfide R′SAr and the halide complex [(TpR,Me)-
Ni−X] without generation of observable complex
intermediate(s);
• Observed kinetics conform to an overall second-order
rate law, first-order each in complex and electrophile;
• The reaction rate accelerates in more polar solvents;
• The reaction rate increases for donating p-arylthiolate
substituents in the isosteric series [(TpPh,Me)Ni−S−
C6H4-4-Y] (Y: Cl < H < Me < OMe);
• The reaction rate depends on the 3-pyrazole substituent
and is generally greater for R = Ph than R = Me;
• Electron-donating o-arylthiolate alkyl substituents in
[(TpR,Me)Ni−S−2,4,6-R″3C6H2] (R″ = H, Me, Pr) also
i
enhance the rate for R = Me but inhibit the rate for R =
Ph, which is consistent with differing conformational
effects;52,53
• Observed rates for [(TpR,Me)Ni−SPh] (R = Me, Ph) are
comparable to those of zinc analogues.28
The key mechanistic question concerns the identity of the
reactive nucleophile, either the intact complex or a dissociated
thiolate (Scheme 1). Reactions of [(TpR,Me)Ni−SAr] gave
highly linear rate dependences on [MeI]0 without obvious
saturation. No direct evidence can be inferred for a dissociative
mechanism, but neither is this precluded.30 Therefore, we have
elucidated a variety of kinetic and extrakinetic probes of
mechanism to better constrain these possibilities. These
phenomena include Arrhenius and Hammett analyses, as well
as solvent and substituent effects. The large negative reaction
entropy and moderate ρ value are consistent with an associative
mechanism. Other correlations are more tenuous and limited in
scope but may provide a fruitful subject for further
investigation. Solvent effects, including a comparison to the
Menschutkin reaction as well as a possible compensation effect,
also provide evidence for an associative mechanism in the
present study. Differential conformational effects of o-
arylthiolate substituents observed for [(TpR,Me)Ni−S−2,4,6-
i
R″3C6H2] (R″ = H, Me, Pr) are also consistent with reactions
of intact complexes. An order-of-magnitude kinetic effect
H
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arising from 3-pyrazole ligand substituents was observed for
[(TpR,Me)Ni−SPh] (R = Me, Ph) complexes, as previously
reported for zinc analogues.28 Indeed, the enhanced covalency
of the Ni−SAr bond exerts surprisingly little kinetic effect.22,45
In summary, although the entirety of our data is consistent
with an associative mechanism involving intact thiolate
complexes (Scheme 1, pathway C), some uncertainty remains.
No single result definitively excludes a dissociative mechanism
for electrophilic alkylation of neutral thiolate complexes in
relatively nonpolar media. However, we have elucidated a rich
array of kinetic phenomena for further investigation, including
new conformational and solvent effects. Future work may
provide additional mechanistic insights, promoting a better
understanding of factors relevant for controlling biological
methyltransferase reactivity.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Tabulated kinetic data and miscellaneous spectroscopic and
GC−MS data. This material is available free of charge via the
Internet at http://pubs.acs.org.
(29) Bridgewater, B. M.; Fillebeen, T.; Friesner, R. A.; Parkin, G. J.
Chem. Soc., Dalton Trans. 2000, 4494−4496.
(30) Morlok, M. M.; Janak, K. E.; Zhu, G.; Quarless, D. A.; Parkin, G.
J. Am. Chem. Soc. 2005, 127, 14039−14050.
(31) Chiou, S.-J.; Riordan, C. G.; Rheingold, A. L. Proc. Natl. Acad.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: jensenm@ohio.edu.
■
(32) Warthen, C. R.; Hammes, B. S.; Carrano, C. J.; Crans, D. C.
JBIC, J. Biol. Inorg. Chem. 2001, 6, 82−90.
Notes
(33) Hammes, B. S.; Carrano, C. J. Chem. Commun. (Cambridge,
U.K.) 2000, 1635−1636.
The authors declare no competing financial interest.
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(35) Grapperhaus, C. A.; Tuntulani, T.; Reibenspies, J. H.;
Darensbourg, M. Y. Inorg. Chem. 1998, 37, 4052−4058.
ACKNOWLEDGMENTS
■
We acknowledge the donors of the American Chemical Society
Petroleum Research Fund (49296-DNI3) for support of this
research. We also thank the Ohio University 1804 Fund for
support in acquiring the DFT computational facility.
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