Mechanism of proton transfer to coordinated thiolates: Encapsulation of acid stabilizes precursor intermediate

Article abstract of DOI:10.1039/c5dt01716e

Earlier kinetic studies on the protonation of the coordinated thiolate in the square-planar [Ni(SC₆H₄R′-4)(triphos)]⁺ (R′ = NO₂, Cl, H, Me or MeO) by lutH⁺ (lut = 2,6-dimethylpyridine) indicate a two-step mechanism involving initial formation of a (kinetically detectable) precursor intermediate, {[Ni(SC₆H₄R′-4)(triphos)]...Hlut}²⁺ (KR1), followed by an intramolecular proton transfer step (kR2). The analogous [Ni(SR)(triphos)]BPh₄ {R = Et, Bu^t or Cy; triphos = PhP(CH₂CH₂PPh₂)₂} have been prepared and characterized by spectroscopy and X-ray crystallography. Similar to the aryl thiolate complexes, [Ni(SR)(triphos)]⁺ are protonated by lutH⁺ in an equilibrium reaction but the observed rate law is simpler. Analysis of the kinetic data for both [Ni(SR)(triphos)]⁺ and [Ni(SC₆H₄R′-4)(triphos)]⁺ shows that both react by the same mechanism, but that KR1 is largest when the thiolate is poorly basic, or the 4-R′ substituent in the aryl thiolates is electron-withdrawing. These results indicate that it is both NH...S hydrogen bonding and encapsulation of the bound lutH⁺ (by the phenyl groups on triphos) which stabilize the precursor intermediate.

Full text of DOI:10.1039/c5dt01716e

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Mechanism of proton transfer to coordinated
thiolates: encapsulation of acid stabilizes
Cite this: DOI: 10.1039/c5dt01716e
precursor intermediate†
Ahmed Alwaaly,a,b William Clegg,a Ross W. Harrington,a Athinoula L. Petrouc and
Richard A. Henderson*a
Earlier kinetic studies on the protonation of the coordinated thiolate in the square-planar [Ni(SC6H4R’-4)-
(triphos)]+ (R’ = NO2, Cl, H, Me or MeO) by lutH+ (lut = 2,6-dimethylpyridine) indicate a two-step mecha-
nism involving initial formation of a (kinetically detectable) precursor intermediate, {[Ni(SC6H4R’-4)-
(triphos)]⋯Hlut}2+ (K1R), followed by an intramolecular proton transfer step (kR2). The analogous [Ni(SR)-
(triphos)]BPh4 {R = Et, But or Cy; triphos = PhP(CH2CH2PPh2)2} have been prepared and characterized by
spectroscopy and X-ray crystallography. Similar to the aryl thiolate complexes, [Ni(SR)(triphos)]+ are proto-
nated by lutH+ in an equilibrium reaction but the observed rate law is simpler. Analysis of the kinetic
data for both [Ni(SR)(triphos)]+ and [Ni(SC6H4R’-4)(triphos)]+ shows that both react by the same mecha-
nism, but that KR1 is largest when the thiolate is poorly basic, or the 4-R’ substituent in the aryl thiolates is
electron-withdrawing. These results indicate that it is both NH⋯S hydrogen bonding and encapsulation
of the bound lutH+ (by the phenyl groups on triphos) which stabilize the precursor intermediate.
Received 6th May 2015,
Accepted 8th June 2015
DOI: 10.1039/c5dt01716e
www.rsc.org/dalton
and the kinetics of these simple protonation reactions are
unusually complicated. In particular, a non-linear dependence
Introduction
The study of simple proton transfer reactions involving on the concentration of lutH+ indicates that a precursor inter-
metal complexes continues to be fundamental in understand- mediate, {[Ni(SC6H4R′-4)(triphos)]⋯Hlut}2+, which presumably
ing such reactions when they are part of complex multistage contains a NH⋯S hydrogen bond, accumulates at high concen-
processes, as in the reactions of natural and synthetic cata- trations of lutH+.2e Analysis of the kinetics allows determi-
lysts. Of particular current interest is the protonation at co- nation of the equilibrium constant for the formation of this
ordinated sulfur sites, since such ligands are common in intermediate (KR1) and the rate constant for the subsequent
metalloenzymes.1,2
intramolecular proton transfer step (kR2). The reason why the
Usually, the kinetics of proton transfer reactions are very precursor intermediate accumulates is because its formation
simple and give little mechanistic insight.1 However, studies from [Ni(SC6H4R′-4)(triphos)]+ and lutH+ is rapid but the sub-
on [Ni(SC6H4R′-4)(triphos)]+ {R′ = NO2, Cl, H, Me or MeO; sequent intramolecular proton transfer step is relatively slow.
triphos = PhP(CH2CH2PPh2)2} have shown that the reactions However, this is not the whole story and in this paper we
with mixtures of lutH+ and lut (lut = 2,6-dimethylpyridine) in describe the kinetics of the reactions of the analogous [Ni(SR)
MeCN involve protonation of the coordinated thiolate {eqn (1)}, (triphos)]+ {R = Et, But or Cy} with mixtures of lutH+ and lut in
MeCN. Analysis of the kinetics shows that the precursor inter-
mediate does not accumulate for these complexes, under con-
aSchool of Chemistry, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK.
ditions identical to those used with [Ni(SC6H4R′)(triphos)]+.
E-mail: richard.henderson@ncl.ac.uk
bDepartment of Chemistry, College of Science, University of Basrah, Basrah, Iraq
This observation indicates that the principal factor stabilizing
the precursor intermediate is not exclusively NH⋯S hydrogen
bonding but also the encapsulation of the bound lutH+ by the
phenyl groups on triphos.
cDepartment of Chemistry, Inorganic Chemistry Laboratory, National and
Kapodistrian University of Athens, Panepistimiopolis, 157 71 Athens, Greece
†Electronic supplementary information (ESI) available: Synthesis and character-
ization of reagents; summary of kinetic data for the reactions of [Ni(SR)
(triphos)]+ (R = Et, But or Cy) with mixtures of lutH+ and lut in MeCN, crystallo-
graphy results tables and pictures. CCDC 1042944–1042946. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5dt01716e
ð1Þ
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complex was accomplished by dissolving the solid in the
minimum amount of THF and then adding a large excess of
Experimental
All preparations and manipulations were routinely performed MeOH (dichloromethane and ether were used instead for the
under an atmosphere of dinitrogen using Schlenk or syringe But compound). Leaving the solution undisturbed at room
techniques as appropriate. All solvents were dried and distilled temperature for 48 h produced well-formed red crystals. The
under dinitrogen immediately prior to use. The following crystals were removed by filtration, washed with MeOH, and
chemicals were purchased from Sigma-Aldrich and used as dried in vacuo. Crystals grown in such a manner were suitable
received: NiCl2·6H2O, EtSH, CySH, ButSH, NaBPh4, lut, triphos for X-ray crystallographic analysis. The identity of each product
and D2O. CD3CN was purchased from Goss Scientific and used was confirmed by NMR spectroscopy and X-ray crystallography.
as received. Toluene was used as received. Acetonitrile was dis- [Ni(SEt)(triphos)]BPh4, 1H NMR: δ 8.24–7.39 (m, PPh, 25H),
tilled from calcium hydride and methanol from Mg(OMe)2. 7.33–6.75 (m, BPh, 20H), 2.1–1.9 (m, CH2, 10H), 0.69 (t, JHH
=
Tetrahydrofuran (THF) and diethyl ether were distilled from 7.3 Hz, CH3, 3H); 31P{1H} NMR: δ 105.5 (t, JPP = 26.2 Hz, PPh),
sodium. 1H NMR spectra chemical shifts are quoted relative to 54.0 (d, JPP = 24.9 Hz, PPh2); UV-visible: λ = 350 nm (ε = 3600
TMS. 31P{1H} NMR spectra chemical shifts are relative to H3PO4. dm3 mol−1 cm−1); λ = 500 nm (ε = 600 dm3 mol−1 cm−1).
[Ni(SBut)(triphos)]BPh4, 1H NMR: δ 8.30–7.37 (m, PPh, 25H),
7.35–6.79 (m, BPh, 20H), 2.1–1.95 (m, CH2, 8H), 0.8 (s, CH3,
Preparation of compounds
9H); 31P{1H} NMR: δ 104 (t, JPP = 26.3 Hz, PPh), 53.6 (d, JPP
=
[lutH]BPh4,3 NaSR (R = Et, Cy or But)4 and [NiCl(triphos)]-
26.2 Hz, PPh2); UV-visible: λ = 350 nm (ε = 3200 dm3 mol−1
cm−1); λ = 575 nm (ε = 360 dm3 mol−1 cm−1). [Ni(SCy)(triphos)]-
5
BPh4 were prepared by the methods described in the litera-
ture (see ESI†).
1
BPh4, H NMR: δ 8.26–7.45 (m, PPh, 25H), 7.35–6.79 (m, BPh,
20H), 1.87–1.76 (m, CH, 1H), 1.24–0.89 (m, CH2, 4H), 0.99–0.79
(m, CH2, 4H), 0.55–0.32 (m, CH2, 2H); 31P{1H} NMR: δ 104
(t, JPP = 21.1 Hz, PPh), 53.6 (d, JPP = 21.0 Hz, PPh2); UV-visible:
λ = 350 nm (ε = 3800 dm3 mol−1 cm−1); λ = 520 nm (ε = 730 dm3
mol−1 cm−1).
[Ni(SR)(triphos)]BPh4 (R = Et, But or Cy)
The complexes were prepared by a method analogous to that
previously described for R = Et.5 To a suspension of [NiCl-
(triphos)]BPh4 (0.5 g; 0.53 mmol) in THF (ca. 20 mL) was
added a slight excess of NaSR (0.75 mmol). The colour
changed rapidly from bright yellow to dark red, and the
mixture became homogeneous. After the solution was stirred
X-ray crystallography6
for ca. 0.5 h, it was concentrated in vacuo to ca. 10 ml. Addition Diffraction data for the three complexes were measured at
of an excess of MeOH (ca. 50 ml) produced a dark red micro- 150 K with Mo-Kα radiation (λ = 0.71073 Å) on an Agilent
crystalline solid. The solid was removed by filtration, washed Technologies Gemini
A Ultra diffractometer. Absorption
with MeOH, and then dried in vacuo. Recrystallization of the corrections were based on repeated and symmetry-equivalent
Table 1 Crystal data
Compound
R = Et
R = But
R = Cy
Chem. formula
Formula mass
C36H38NiP3S+·C24H20B−
973.6
C38H42NiP3S+·C24H20B−·0.5CH2Cl2
1044.1
C40H34NiP3S+·C24H20B−·C4H8O
1089.7
Crystal system
a/Å
b/Å
c/Å
α/°
β/°
Triclinic
Triclinic
12.5663(6)
13.8894(6)
17.6784(8)
72.371(4)
75.534(4)
67.285(4)
2681.4(2)
150(2)
Triclinic
11.4239(7)
14.8484(7)
16.8232(8)
96.983(4)
106.192(5)
110.902(5)
2481.2(2)
11.3585(4)
14.9357(5)
19.0458(8)
79.270(3)
73.939(3)
68.720(3)
γ/°
V/Å3
2879.9(2)
T/K
Space group
150(2)
150(2)
ˉ
ˉ
ˉ
P1
P1
P1
Z
2
2
2
Reflections measured
Independent reflections
Refined parameters
Rint
25 830
10 547
779
0.0369
0.0381
0.0747
0.0578
0.0834
1.033
27 976
10 556
653
0.0430
0.0506
0.1120
0.0785
0.1278
1.021
44 424
11 699
631
0.0335
0.0377
0.0873
0.0505
0.0939
1.021
R1 (I > 2σ)
wR(F2) (I > 2σ)
R1 (all data)
wR(F2) (all data)
GoF on F2
Max, min diff el dens/e Å−1
CCDC ref.
0.37, −0.39
1042944
0.77, −0.70
1042945
0.92, −0.57
1042946
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reflections. The structures were solved by direct methods and
refined using all unique F2 values. Two-fold disorder was
resolved for the ethyl and some phenyl groups in the ethyl
complex, and for a dichloromethane molecule with alternative
Results and discussion
X-ray crystal structures of [Ni(SR)(triphos)]BPh4
(R = Et, But or Cy)
positions close to an inversion centre in the t-butyl complex; a The structures of [Ni(SR)(triphos)]BPh4 (R = Et, But or Cy, the
satisfactory model for disordered THF in the cyclohexyl last two incorporating also disordered solvent molecules) have
complex could not be achieved, so this was treated with the all been determined by X-ray crystallography (see ESI for
SQUEEZE procedure of PLATON (as a mono-solvate). Selected details†). The structure of the R = But cation is shown in Fig. 2
crystal data are shown in Table 1.
and the structures of the other cations are shown in the ESI.†
Selected bond lengths and angles (around Ni) for all three
cations are presented in Table 2, together with the dimensions
for the previously reported, structurally analogous R = Bn and
R = Ph complexes.2e The structure of the complexes are best
described as distorted square-planar. Inspection of the data in
Table 2 shows that there are no significant systematic differ-
ences in the dimensions or geometry at Ni. It is, however,
worth noting that, whilst the bond lengths and angles for
[Ni(SEt)(triphos)]+ and [Ni(SCy)(triphos)]+ are very similar,
there appear to be some minor differences in the [Ni(SBut)-
(triphos)]+ structure. Thus, the Ni–S bond in [Ni(SBut)-
(triphos)]+ (2.2384(8) Å) is significantly longer than those in
[Ni(SEt)(triphos)]+ (2.1600(6) Å) and [Ni(SCy)(triphos)]+ (2.1663
(6) Å). Furthermore, the Ni–S–C angle for [Ni(SBut)(triphos)]+
(110.57(11)°) is somewhat smaller than those in [Ni(SEt)-
(triphos)]+ (119.24(18)°) and [Ni(SCy)(triphos)]+ (117.08(7)°). It
seems unlikely that such small differences will have a signifi-
cant effect on the rates or kinetics of the protonation reactions
discussed below.
Kinetic studies
All kinetic studies were performed using an Applied Photo-
physics SX.18 MV stopped-flow spectrophotometer, modified to
handle air-sensitive solutions, connected to a RISC computer.
The temperature was maintained using a Grant LTD 6G thermo-
stat tank with combined recirculating pump. The experiments
were conducted at 25.0 °C. The wavelength used was λ =
350 nm. The solutions of complex and reactants were prepared
under an atmosphere of dinitrogen and transferred to the
stopped-flow apparatus using gas-tight, all-glass syringes. The
kinetics were studied in dry MeCN under pseudo-first-order
conditions, with [lutH]BPh4 and lut present in at least a
10-fold excess over the concentration of the complex. Mixtures
of [lutH]BPh4 and lut were prepared from stock solutions of
the two reagents. All solutions were used within 1 hour of
preparation.
Under all conditions, the stopped-flow absorbance-time
traces are a good fit to a single exponential curve (see ESI†).
The dependences on the concentrations of lutH+ and lut were
determined from graphs of kobs/[lut] versus [lutH+]/[lut] (where
kobs = pseudo-first-order rate constant). The type of plot
obtained is shown in Fig. 1 and ESI.† The analysis of the plots
to obtain the rate laws is explained in the Results and discus-
sion section.
Kinetics of the reactions
The kinetics of the reactions between [Ni(SR)(triphos)]+ (R =
Et, Cy or But) and lutH+ in the presence of lut have been
studied in MeCN under conditions where the concentrations
of both lutH+ and lut are in an excess over the concentration of
complex. The stopped-flow absorbance–time curves associated
Fig. 1 Dependence of kobs/[lut] on [lutH+]/[lut] for the reaction of
[Ni(SEt)(triphos)]+ with mixtures of lutH+ and lut in MeCN at 25.0 °C.
Data points correspond to: [lutH+]
= , [lut] =
1.25 mmol dm−3
+
2.5–40 mmol dm−3
(
); [lutH ] = 2.5 mmol dm−3, [lut] = 2.5–40 mmol
▲
+
dm−3
); [lutH ] = 5.0 mmol dm−3, [lut] = 2.5–40 mmol dm−3
). The
●
(
■
(
line is that defined by eqn (2) and the parameters shown in Table 3. Error
bars correspond to 10%.
Fig. 2 Structure of the [Ni(SBut)(triphos)]+ cation, with
omitted. Selected bond lengths and angles are presented in Table 2.
H atoms
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Table 2 Comparison of selected bond lengths and angles for [Ni(SR)(triphos)]BPh4 (R = Et, But, Cy, Bn or Ph)
Complex
R = Et
R = But
R = Cy
R = Bna
R = Phb
Bond angles
P1–Ni–S
P2–Ni–S
90.22(2)
100.57(3)
173.78(4)
88.78(3)
85.59(3)
160.01(3)
85.10(3)
90.27(2)
165.92(9)
102.72(2)
84.71(2)
162.86(2)
85.04(2)
117.08(7)
103.95(4)
164.01(4)
89.66(4)
84.80(4)
161.90(4)
84.96(4)
99.03(3)
173.89(3)
89.76(3)
86.41(2)
161.50(3)
85.82(2)
99.20(8)
165.83(2)
102.43(2)
85.24(2)
162.10(2)
85.23(2)
P3–Ni–S
P1–Ni–P2
P1–Ni–P3
P2–Ni–P3
Ni–S–C
119.24(18)
110.57(11)
115.18(13)
Bond lengths
Ni–S
P1–Ni
P2–Ni
P3–Ni
2.1600(6)
2.1962(6)
2.1339(6)
2.1888(6)
2.2384(8)
2.2056(9)
2.1372(8)
2.1816(9)
2.1663(6)
2.1934(5)
2.1284(5)
2.1990(5)
2.1689(10)
2.2042(10)
2.1468(10)
2.1960(10)
2.2456(7)
2.2101(7)
2.1506(6)
2.1858(7)
a Dimensions correspond to the numbering scheme presented here, report of original structure had slightly different numbering scheme (P2 and
P3 interchanged);2e b Structure reported in ref. 2e.
Rate ¼ ðkf ½BHþꢀ þ kb½BꢀÞ½Aꢀ
ð4Þ
Table 3 Rate constants for reactions of [Ni(SR)(triphos)]+ (R = Et, Cy,
But or Bn) with mixtures of lutH+ and lut in MeCN at 25.0 °C
Earlier studies on the equilibrium reactions between
[Ni(SC6H4R′-4)(triphos)]+ (R′ = MeO, Me, H, Cl or NO2) and
mixtures of lutH+ and lut showed that the protonation of the
complex was associated with a complicated rate law, exhibiting
a non-linear dependence on [lutH+] {eqn (5)}.2e The mechan-
ism consistent with this rate law is shown in Fig. 3 and
involves two coupled equilibria. The first step is the rapid
association of lutH+ with the complex C, forming a hydrogen-
bonded precursor intermediate, D. The second step is the
intramolecular transfer of the proton to form E and diffusion
away of lut.
R
a/dm3 mol−1 s−1
b/dm3 mol−1 s−1
Et
12.0
5
13.0
3.2
0.6
1.2
Cy
But
Bn
0.7
2.02e
with these reactions can be fitted to a single exponential curve,
indicating a first-order dependence on the concentration of
[Ni(SR)(triphos)]+ (see ESI†). In experiments where the concen-
trations of lutH+ and lut are varied, graphs of kobs/[lut] versus
[lutH+]/[lut] are linear with a small positive intercept (Fig. 1
and ESI†). Consequently, the rate law for these reactions of
[Ni(SR)(triphos)]+ is that shown in eqn (2). The values of a and b
are collected in Table 3. Also included in this table are the rate
constants for the analogous reaction of [Ni(SCH2Ph)(triphos)]+
which were reported earlier.2e
ꢀ
ꢁ
K1Rk2R½lutHþꢀ
Rate ¼
þ kꢂR2½lutꢀ ½NiðthiolateÞðtriphosÞþꢀ ð5Þ
1 þ K1R½lutHþꢀ
In deriving eqn (5) it is assumed that the formation of D is
a rapidly established equilibrium (KR1), and the subsequent
intramolecular proton transfer step (kR2) is rate-limiting (i.e. k−R1
> kR2). Thus, when KR1[lutH+] < 1 (because the association of
lutH+ with [Ni(SR)(triphos)]+ is weak and/or the concentration
of lutH+ is low), eqn (5) simplifies to eqn (6). Under these con-
ditions, the rate exhibits a linear dependence on [lutH+].
However, in reactions associated with eqn (6), D does not
attain concentrations sufficient to perturb the kinetics. The
Rate ¼ fa½lutHþꢀ þ b½lutꢀg½NiðSRÞðtriphosÞþꢀ
ð2Þ
Kinetics and mechanism: general considerations
The kinetics of most proton transfer reactions are simple. The
generic mechanism of a proton transfer reaction is shown in
eqn (3).7 The diffusion together of the reactants and the
diffusion apart of the products are fast, and normally the
hydrogen-bonded species {(a) and (b)} do not accumulate to
concentrations which result in a perturbation to the rate law.
Consequently, the rate law for proton transfer reactions is nor-
mally that shown in eqn (4), where kf and kb are the second-
order rate constants for the forward (protonation) and reverse
(deprotonation) reactions, respectively.
kf
Fig. 3 Mechanism for the reaction between [Ni(SR)(triphos)]+ and mix-
tures of lutH+ and lut; triphos ligand omitted for clarity.
BHþ þ A Ð BHþ ꢁ ꢁ ꢁ A Ð B ꢁ ꢁ ꢁ HAþ Ð B þ HAþ
ð3Þ
kb
ðaÞ
ðbÞ
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kinetics of the reactions of [Ni(SR)(triphos)]+ {eqn (2)} are con-
Table 4 Summary of the kinetic data for the reactions between
[Ni(thiolate)(triphos)]+ and mixtures of lutH+ and lut in MeCN at 25.0 °C
sistent with eqn (6), where a = KR1k2R and b = k−R2
.
KR1/dm3
KR1kR2/dm3
kR−2/dm3
Rate ¼ fKR1 k2R½lutHþꢀ þ kRꢂ2½lutꢀg½NiðthiolateÞðtriphosÞþꢀ ð6Þ
R or R′
mol−1
kR2/s−1
mol−1 s−1
mol−1 s−1
KR1KR2
pKRa
The derivation of eqn (5), and consequently eqn (6),
assumes that KR1 is a rapid equilibrium established prior to the
rate-limiting (k2R) step (i.e. k−R1 > kR2). However, if k2R > k−R1 then D
must be considered a steady state intermediate and the
[Ni(SR)(triphos)]+
Et
≤20
≤20
≤20
≤20
≥0.6
12.0
5.0
13.0
3.2
0.6
1.2
0.7
2.0
20.0
4.2
18.6
1.6
16.7
16.0
16.7
15.62e
Cy
But
Bn
≥0.25
≥0.65
≥0.16
derived rate law would be that shown in eqn (7). When k2R
>
[Ni(SC6H4R′-4)(triphos)]+2e
kR−1, the kinetics would, under all conditions, exhibit a first-
NO2
Cl
H
>160
>160
200
7.0
1 × 104
6
4
1.5
2.5
order dependence on the concentration of lutH+. Eqn (6) and
0.19
0.10
0.05
0.07
(7) are experimentally indistinguishable but it is only if k−R1
>
20.0
3.9
5.0
1.6
14.8
14.3
Me
MeO
>160
55.5
kR2 that D would, under certain conditions, accumulate, and so
it seems most appropriate to interpret all kinetics using eqn
(5) and (6). Nonetheless, it is possible that, at least in some
cases, the experimentally observed simple rate law is because
kR2 > kR−1
.
Thus, the complexes containing the less basic thiolates (or the
more electron-withdrawing 4-R′-substituents) are associated
with the largest KR1.
ꢀ
ꢁ
k1Rk2R½lutHþꢀ
Rate ¼
þ kꢂR2½lutꢀ ½NiðthiolateÞðtriphosÞþꢀ ð7Þ
kꢂR1 þ k2R
That the less basic thiolates most favour the formation of D
(i.e. highest value of KR1) is counter-intuitive. It seems reason-
able that the precursor intermediates {[Ni(thiolate)(triphos)]-
⋯Hlut}2+ (D) involve a NH⋯S hydrogen bond and this propo-
sal is consistent with earlier DFT calculations on {[Ni(SPh)-
(triphos)]⋯Hlut}2+.8 If the strength of the NH⋯S hydrogen
bond was the principal factor in stabilizing D, It would be
anticipated that KR1 would increase with the basicity of the thio-
late or the electron-donating capability of the 4-R′-substituent
in aryl thiolates. Clearly, the (electronic) factors which stabilize
D are not those which result in stronger NH⋯S hydrogen
bonds. The hydrogen bonded lutH+ cannot be bound to any
other part of the complex in D and so non-bonding inter-
actions within D need to be considered to understand why
lutH+ binds most strongly to complexes containing weakly
basic thiolates. In particular, we need to consider the local
environment, created by the phenyl substituents on triphos
that encapsulates the bound lutH+.
It is important to note that the reactions between [Ni(SR)
(triphos)]+ or [Ni(SC6H4R′-4)(triphos)]+ with mixtures of lutH+
and lut were all studied using similar concentration ranges
(for [Ni(SC6H4R′-4)(triphos)]+, [lutH+]
1.0–50.0 mmol dm−3
=
2.5–50.0, [lut]
=
=
;
for [Ni(SR)(triphos)]+, [lutH+]
1.25–25.0, [lut] = 2.5–40.0 mmol dm−3). Thus the simpler kine-
tics observed for [Ni(SR)(triphos)]+ are not due to use of a
lower concentration of lutH+, but are a consequence of a
smaller value of KR1 (i.e. weaker binding of lutH+ to [Ni(SR)
(triphos)]+ than to [Ni(SC6H4R′-4)(triphos)]+). That the for-
mation of D is favoured by the less basic arylthiolate ligands
indicates that NH⋯S hydrogen bonding is not the principal
factor stabilizing D. Rather, it suggests (as discussed below)
that when lutH+ is hydrogen bonded to the complex the local
environment generated by the phenyl groups on triphos which
surround and encapsulate the hydrogen bonded lutH+ is
important in stabilizing D. The electronic characteristics of
the phenyl groups on triphos (and hence the environment
surrounding the bound lutH+) will be modulated by the nature
of the thiolate ligand.
The X-ray crystal structures of [Ni(SR)(triphos)]+ (R = Ph,5
C6H4NO2-4,2e Bn,2e Et, Cy and But) have been reported and in
all structures the conformations of the two chelate rings of
triphos are identical; there is a pseudo mirror plane contain-
ing P2–Ni–S, as shown in Fig. 4. These conformations result in
the two faces of the square plane being different. Thus, the
side of the square plane containing the phenyl substituent on
the central phosphorus (P2) is more open (i.e. less congested),
with the distance between phenyl substituents on the two
terminal phosphorus atoms being ca. 5.2 Å. In contrast, on the
other side of the molecule, the distance between phenyl substi-
tuents on the two terminal phosphorus atoms is only ca. 2.8 Å.
In all the X-ray crystal structures the R group of the thiolate
sits on the side of the complex with the more open face.
Formation of the precursor intermediate {[Ni(thiolate)-
(triphos)]⋯Hlut}2+ (D) necessitates that the acid binds to a
lone pair of electrons on sulfur and hence the lutH+ sits in the
closed face. Alternatively, after Ni–S bond rotation, lutH+ could
Factors stabilizing the precursor intermediate (D)
The values of KR1, k2R and k−R2 for the reactions of [Ni(thiolate)
(triphos)]+ with mixtures of lutH+ and lut are summarised
in Table 4. Inspection of these data shows the following fea-
tures. (i) Equilibrium constants for binding of lutH+ to all
[Ni(SC6H4R′-4)(triphos)]+ (K1R) are significantly larger than the
binding constants to [Ni(SR)(triphos)]+ (R = Et, Cy or But).
(ii) The experimental kinetic data for the reactions of
[Ni(SC6H4R′-4)(triphos)]+ with lutH+ do not allow KR1 to be cal-
culated for all complexes (only limits to KR1 can be estimated in
some cases). However, a notable feature of the data in Table 4
is that KR1 is larger when the 4-R′-substituent is electron-with-
drawing (e.g. [Ni(SC6H4NO2-4)(triphos)]+, K1NO2 ≥ 160 dm3
mol−1; [Ni(SC6H4OMe-4)(triphos)]+, K1MeO = 55.5 dm3 mol−1).
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which is important in stabilizing D also rationalizes the reac-
tivity of other similar complexes. Thus, for the reactions of
mixtures of lutH+ and lut with [Ni(XPh)(triphos)]+ (X = Se or
O)9 the kinetics with X = Se (poorly basic) exhibit a non-linear
dependence on [lutH+] {eqn (5)} but the kinetics for X = O
(strongly basic) exhibit a linear dependence on [lutH+] {eqn
(6)}. Furthermore, we have previously shown that the reactions
of [Ni(SC6H4R′-4)2(dppe)] (dppe = Ph2PCH2CH2PPh2) with mix-
tures of lutH+ and lut follow a similar reactivity pattern to
[Ni(SC6H4R′-4)(triphos)]+.2d Thus, for [Ni(SC6H4NO2-4)2(dppe)],
the kinetics of the protonation reactions exhibit a non-linear
dependence on [lutH+] {eqn (5)}. However, with [Ni(SC6H4R′-
4)2(dppe)] (R′ = MeO, Me, H or Cl) containing thiolates with
more electron-releasing 4-R′-substituents, the reactions show a
linear dependence on [lutH+] {eqn (6)}.
It is unusual that the principal factor controlling the stabi-
lity of {[Ni(thiolate)(triphos)]⋯Hlut}2+ (D) is the local environ-
ment surrounding the hydrogen bonded acid rather than the
NH⋯S hydrogen bond strength. The reason why this behav-
iour is observed for D could, at least in part, be because the
basicities (and hence presumably the NH⋯S hydrogen
bonding strengths) of the coordinated thiolates in [Ni(thiolate)
(triphos)]+ are very similar, as described in the next section.
Fig. 4 Representations of the binding of lutH+ to [Ni(SR)(triphos)]+ (C)
to form D, showing how the conformation of the triphos chelate rings
differentiates the two faces of C. The view is along the P2–Ni–S axis (P2
= central P of triphos; in the orientation shown, Ni and S are behind P2).
hydrogen bond to the sulfur whilst siting in the more open
face of the complex (Fig. 4). In either case the structure of D is
congested and in particular the phenyl substituents of the
triphos ligand surround the bound lutH+.8 This observation
Rates of proton transfer and pKas of [Ni(thiol)(triphos)]2+
intimates that the local environment that encapsulates the The rate constants for protonation of [Ni(thiolate)(triphos)]+ by
bound lutH+ may affect the stability of D.
lutH+ (K1Rk2R) and deprotonation of [Ni(thiol)(triphos)]2+ by lut
Because of the congestion in D the lutH+ binds in a cavity (kR−2) are presented in Table 4. It is notable that both K1RkR2 and
created by the phenyl groups of triphos and consequently the kR−2 show little variation with the thiolate. Furthermore, this
binding affinity of lutH+ is affected by the local environment pattern extends to [Ni(XPh)(triphos)]+ (X = O, KO1k2O = 1.65, kO−2
=
created by these phenyl groups. In [Ni(thiolate)(triphos)]+ it 1.5 dm3 mol−1
s
−1; X = Se, K1Sek2Se = 14.9, k = 0.4 dm3 mol−1
Se
−2
seems likely that the electronic characteristics of the thiolate s−1). We have noted this feature before and suggested it is
ligand are transmitted, through the Ni, to the phenyl groups of because a significant barrier to proton transfer in these
the triphos ligand and this modulates the local environment systems is the sterically demanding lutH+ and lut accessing
encapsulating the bound lutH+.9
Studies on simple organic molecules have shown that aro- ligand.8
the basic site through the phenyl substituents on the triphos
matics can associate (π-stack) with one another and that this
The equilibrium constant (KR0) for eqn (1) can be calculated
association is stabilized by electron-withdrawing substituents from the kinetic data using eqn (8). Since, for lutH+, pKalut
=
on the aromatics.10 The same reactivity pattern is reflected in 14.1 in MeCN,11 the pKRa of the coordinated thiol can be calcu-
the variation of KR1 with changes in the thiolate, suggesting lated using eqn (9).
that the binding of lutH+ to [Ni(thiolate)(triphos)]+ is affected
½NiðSHRÞðtriphosÞ2þꢀ½lutꢀ
K1Rk2R
¼ K0R ¼ K1RK2R
¼
ð8Þ
by the local encapsulating environment created by the phenyl
groups of triphos. The electronic characteristics of the thiolate
ligand will modulate both the NH⋯S hydrogen bonding and
the encapsulation, but in opposite ways: hydrogen bonding is
favoured by basic thiolates, or aryl thiolates with an electron-
donating substituent, whilst the non-bonding interactions
associated with the encapsulation is facilitated by thiolates
with the opposite characteristics. The results presented in this
paper indicate that modulating the local environment encap-
sulating the bound lutH+ is more important in stabilizing D
than changing the basicity of the coordinated thiolate (which
would affect the NH⋯S bond strength).
½NiðSRÞðtriphosÞþꢀ½lutHþꢀ
kR
ꢂ2
KR0 ¼ Klaut=KRa
ð9Þ
Calculations show that pKRa ′ = 14.5 0.3 for [Ni(SC6H4R′-4)
(triphos)]+ (R′ = H or MeO, Table 4). This value is in good
agreement with pKRa′ = 14.3
0.4, reported earlier for the
similar [Ni(SC6H4R′-4)2(dppe)].2d For [Ni(SR)(triphos)]+ (R =
alkyl) pKRa = 16.3
0.5. Thus, as expected, the coordinated
alkyl thiol is less acidic than coordinated aryl thiols. However,
the difference is smaller than that of the free thiols (pKaPhSH
=
21.5; pKEatSH = 28.6, in MeCN).12,13 This ‘levelling’ of the pKas
of coordinated thiols has been observed before2d and also for
other types of ligands.14 As we have discussed before,8 these
The proposal that it is the surroundings encapsulating
lutH+, rather than the strength of the NH⋯S hydrogen bond,
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Paper
pKas (calculated from the kinetic data) need to be treated with
some caution. However, the rather small difference in the pKas
of coordinated alkyl and aryl thiols (ΔpKa ∼ 2) indicates that
NH⋯S hydrogen bonding in D would be relatively insensitive
to the nature of the coordinated thiolate and hence rather
minor non-bonding interactions associated with the encapsu-
lation of the bound lutH+ can make a relatively significant con-
tribution to the stability of D.
J. Phys. Chem., 1965, 69, 2287; (c) C. F. Bernasconi
and R. L. Montanez, J. Org. Chem., 1997, 62, 8162;
(d) C. F. Bernasconi, D. M. Weirsema and W. Stronach,
J. Org. Chem., 1993, 58, 217, and refs therein.
2 (a) R. A. Henderson, D. L. Hughes, R. L. Richards and
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M. Y. Darensbourg, Inorg. Chem., 1994, 33, 1421;
(c) C. B. Allan, G. Davidson, S. B. Choudhury, Z. Gu,
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R. A. Henderson, Inorg. Chem., 2004, 43, 3098;
(e) V. Autissier, P. M. Zarza, A. Petrou, R. A. Henderson,
R. W. Harrington and W. Clegg, Inorg. Chem., 2004, 43,
3106; (f) A. Petrou, A. D. Koutselos, H. S. Wahab, W. Clegg,
R. W. Harrington and R. A. Henderson, Inorg. Chem., 2011,
50, 847; (g) R. A. Henderson, Bioinorg. React. Mech., 2012, 8, 1.
3 K. L. C. Grönberg, R. A. Henderson and K. E. Oglieve,
J. Chem. Soc., Dalton Trans., 1998, 3093.
4 K. S. Hagen and R. H. Holm, Inorg. Chem., 1984, 23, 418.
5 W. Clegg and R. A. Henderson, Inorg. Chem., 2002, 41,
1128.
6 (a) CrysAlisPro software, Agilent Technologies Ltd, Oxford,
UK, 2013; (b) SHELXTL, Bruker AXS Inc, Madison, Wiscon-
sin, USA, 2014; (c) G. M. Sheldrick, Acta Crystallogr., Sect. A:
Fundam. Crystallogr., 2008, 64, 112; (d) G. M. Sheldrick,
Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 2015, 71, 3;
(e) A. L. Spek, Acta Crystallogr., Sect. C: Cryst. Struct.
Commun., 2015, 71, 9.
Conclusions
The studies described in this paper show that the different
kinetics observed for the equilibrium protonation reactions of
[Ni(SC6H4R′-4)(triphos)]+ and [Ni(SR)(triphos)]+ with mixtures
of lutH+ and lut are consistent with a single mechanism invol-
ving initial formation of a precursor hydrogen-bonded inter-
mediate {[Ni(thiolate)(triphos)]⋯Hlut}2+ (D) followed by
intramolecular proton transfer (Fig. 3). It might be expected
that D is most stable (highest KR1) with complexes containing
the most basic thiolates because this would increase the
strength of the NH⋯S hydrogen bonding in D. However, the
results presented herein show that quite the opposite is true,
and that changing the electronic character of the coordinated
thiolate results in a reactivity pattern which is not consistent
with hydrogen bonding being the only (or indeed the princi-
pal) factor stabilising D. Rather the electronic influences that
the thiolate has on KR1 indicate that an important factor stabi-
lizing D is the local environment created by the phenyl groups
of triphos which encapsulate the bound lutH+. This local
environment is modulated by the electronic characteristics of
the coordinated thiolate. The results and conclusions pre-
sented in this paper suggest that preparing elaborate ligands
(which generate extended cavities around reaction sites) could
be used to control the rates of proton transfer to coordinated
molecules and ions in transition metal complexes.
7 R. P. Bell, The Proton in Chemistry, Chapman and Hall,
London, 2nd edn, 1973.
8 A. Alwaaly and R. A. Henderson, Chem. Commun., 2014, 50,
9669.
9 Note that the 4-R′-substituted phenyl group in [Ni(SC6H4R′-
4)(triphos)]+ is not involved in creating the cavity where
lutH+ binds.
10 J. Hwang, P. Li, W. R. Carroll, M. D. Smith, P. J. Pellechia
and K. D. Shimizu, J. Am. Chem. Soc., 2014, 136, 14060, and
refs therein.
Acknowledgements
11 C. Cauquis, A. Deronzier, D. Serve and E. Vieil, J. Electro-
anal. Chem. Interfacial Electrochem., 1975, 60, 205.
12 Advanced Chemistry Development (ACD/Labs) Software
V11.02 (© 1994–2014 ACD/Labs).
13 The calculated pKa in water was obtained from ref. 12. The
pKa in MeCN was then calculated using the equations in,
B. G. Cox, Acids and Bases Solvent Effects on Acid-Base
Strength, Oxford University Press, UK, 2013.
A.A. thanks the Iraqi Ministry of Higher Education and Scienti-
fic Research for a studentship and A.L.P. thanks the University
of Athens for study leave. We thank EPSRC for a crystallo-
graphy equipment grant (EP/F03637X/1).
Notes and references
1 (a) M. Eigen, Angew. Chem., Int. Ed. Engl., 1964, 3, 1; 14 B. L. Westcott, N. E. Gruhn and J. H. Enemark, J. Am.
(b) M. M. Kreevoy, D. S. Sappenfield and W. Schwabacher,
Chem. Soc., 1998, 120, 3382.
This journal is © The Royal Society of Chemistry 2015
Dalton Trans.