Kinetic and theoretical studies on the protonation of [Ni(2-SC 6H4N){PhP(CH2CH2PPh 2)2}]+: Nitrogen versus sulfur as the protonation site

Article abstract of DOI:10.1021/ic101444d

The complexes [Ni(4-Spy)(triphos)]BPh₄ and [Ni(2-Spy)(triphos)] BPh₄ {triphos = PhP(CH₂CH₂PPh₂) ₂, 4-Spy = 4-pyridinethiolate, 2-Spy = 2-pyridinethiolate} have been prepared and characterized both spectroscopically and using X-ray crystallography. In both complexes the triphos is a tridentate ligand. However, [Ni(4-Spy)(triphos)]+ comprises a 4-coordinate, square-planar nickel with the 4-Spy ligand bound to the nickel through the sulfur while [Ni(2-Spy)(triphos)]+ contains a 5-coordinate, trigonal-bipyramidal nickel with a bidentate 2-Spy ligand bound to the nickel through both sulfur and nitrogen. The kinetics of the reactions of [Ni(4-Spy)(triphos)]+ and [Ni(2-Spy)(triphos)]+ with lutH+ (lut = 2,6-dimethylpyridine) in MeCN have been studied using stopped-flow spectrophotometry, and the two complexes show very different reactivities. The reaction of [Ni(4-Spy)(triphos)]+ with lutH+ is complete within the deadtime of the stopped-flow apparatus (2 ms) and corresponds to protonation of the nitrogen. However, upon mixing [Ni(2-Spy)(triphos)]+ and lutH+ a reaction is observed (on the seconds time scale) to produce an equilibrium mixture. The mechanistic interpretation of the rate law has been aided by the application of MSINDO semiempirical and ADF calculations. The kinetics and calculations are consistent with the reaction between [Ni(2-Spy)(triphos)]+ and lutH+ involving initial protonation of the sulfur followed by dissociation of the nitrogen and subsequent transfer of the proton from sulfur to nitrogen. The factors affecting the position of protonation and the coupling of the coordination state of the 2-pyridinethiolate ligand to the site of protonation are discussed.

Full text of DOI:10.1021/ic101444d

Inorg. Chem. 2011, 50, 847–857 847
DOI: 10.1021/ic101444d
Kinetic and Theoretical Studies on the Protonation of
[Ni(2-SC6H4N){PhP(CH2CH2PPh2)2}]þ: Nitrogen versus Sulfur as the Protonation Site
Athinoula L. Petrou,† Andreas D. Koutselos,‡ Hilal S. Wahab,‡,|| William Clegg,§ Ross W. Harrington,§ and
Richard A. Henderson*,§
†Laboratory of Inorganic Chemistry, and ‡Laboratory of Physical Chemistry, Department of Chemistry,
National and Kapodistrian University of Athens, Athens 15771, Greece, and §School of Chemistry,
Newcastle University, Newcastle upon Tyne, NE1 7RU, U.K. || Permanent address: Department of Chemistry,
College of Science, Al-Nahrain University, Baghdad, Iraq
Received July 20, 2010
The complexes [Ni(4-Spy)(triphos)]BPh4 and [Ni(2-Spy)(triphos)]BPh4 {triphos = PhP(CH2CH2PPh2)2, 4-Spy =
4-pyridinethiolate, 2-Spy = 2-pyridinethiolate} have been prepared and characterized both spectroscopically and using
X-ray crystallography. In both complexes the triphos is a tridentate ligand. However, [Ni(4-Spy)(triphos)]þ comprises a
4-coordinate, square-planar nickel with the 4-Spy ligand bound to the nickel through the sulfur while [Ni(2-
Spy)(triphos)]þ contains a 5-coordinate, trigonal-bipyramidal nickel with a bidentate 2-Spy ligand bound to the nickel
through both sulfur and nitrogen. The kinetics of the reactions of [Ni(4-Spy)(triphos)]þ and [Ni(2-Spy)(triphos)]þ with
lutHþ (lut = 2,6-dimethylpyridine) in MeCN have been studied using stopped-flow spectrophotometry, and the two
complexes show very different reactivities. The reaction of [Ni(4-Spy)(triphos)]þ with lutHþ is complete within the
deadtime of the stopped-flow apparatus (2 ms) and corresponds to protonation of the nitrogen. However, upon mixing
[Ni(2-Spy)(triphos)]þ and lutHþ a reaction is observed (on the seconds time scale) to produce an equilibrium mixture.
The mechanistic interpretation of the rate law has been aided by the application of MSINDO semiempirical and ADF
calculations. The kinetics and calculations are consistent with the reaction between [Ni(2-Spy)(triphos)]þ and lutHþ
involving initial protonation of the sulfur followed by dissociation of the nitrogen and subsequent transfer of the proton
from sulfur to nitrogen. The factors affecting the position of protonation and the coupling of the coordination state of the
2-pyridinethiolate ligand to the site of protonation are discussed.
Introduction
pair of electrons invariably occurs at the diffusion-controlled
limit while thermodynamically unfavorable reactions of the
same type are slower and occur at a rate defined by the
Proton transfer is a common feature in many complex,
multistep pathways, most notably those associated with
catalysis and biology.1 The complexity of these multistep
reactions often precludes direct study of the individual
component proton transfer steps. Consequently, understand-
ing proton transfer in such systems relies on studies of simple
model systems.2 The factors which control the rates of proton
transfer are, in general, relatively simple, resulting in easy
prediction of the rates of proton transfer. Thus, thermody-
namically favorable proton transfer to an atom (within a
molecule or ion) containing a stereochemically active lone
€
approximate Bronsted relationship which is shown in eq 1,
where kA is the rate constant for proton transfer, Ka is the
equilibrium constant for the proton dissociation reaction,
and GA and R are constants.1,3
R
kA ¼ GAKa
ð1Þ
Exceptions to this general behavior are rare. However, it
has been observed that thermodynamically favorable proton
transfer reactions involving metal and carbon sites are slow.2
It has been proposed that the reason for slow proton transfer
at these sites is attributable to reorganization of the coligands
(for metal sites) and rehybridization (for carbon sites).
The coordination of a molecule or ion (substrate) to a
metal site can significantly affect the protonation chemistry
of the substrate and, depending on the characteristics of the
*To whom correspondence should be addressed. Fax: 0191 222 6929.
Phone: 0191 222 6636. E-mail: r.a.henderson@ncl.ac.uk.
(1) Eigen, M. Angew. Chem., Int. Ed. Engl. 1964, 3, 1, and refs therein.
(2) (a) Kramarz, K. W.; Norton, J. R. Prog. Inorg. Chem. 1994, 42, 1, and
refs therein. (b) Kristjansdottir, S. S.; Norton, J. R. Transition Metal Hydrides:
Recent Advances in Theory and Experiment; Dedieu, A., Ed.; VCH: New York,
1992; Chapter 9, and refs therein; (c) Henderson, R. A. Angew. Chem., Int. Ed.
Engl. 1996, 35, 946, and refs therein. (d) Henderson, R. A. Recent Advances in
Hydride Chemistry; Peruzzini, M., Poli, R., Eds.; Elsevier: Amsterdam, 2001
p 3470, and refs therein.
(3) Bell, R. P. The Proton in Chemistry, 2nd ed.; Chapman and Hall: London,
1973; and refs therein.
r
2011 American Chemical Society
Published on Web 01/10/2011
pubs.acs.org/IC

848 Inorganic Chemistry, Vol. 50, No. 3, 2011
Petrou et al.
Figure 1. Mechanism for the reaction of [Ni(SC6H4R-4)(triphos)]þ with lutHþ.
metal site (the metal and its coligands), can increase or
decrease the acidity of the bound substrate compared to its
free state. Thus, when bound to an electron-poor site (Hard
Lewis acid) protonated substrates are more acidic than when
free but when bound to an electron-rich site (Soft Lewis acid)
protonation of the substrate can become more favorable. For
the most part, this behavior is well understood only in
thermodynamic terms (i.e., changes to pKa of substrate when
bound and when free). Little is known of how coordination
affects the rates of proton transfer although there have been a
variety of studies involving protonation of organometallic
compounds where the rates of proton transfer to either the
metal or the carbon-based ligand have been measured as well
as any subsequent rearrangements (either inter- or in-
tramolecular) resulting in the transfer of the proton to the
thermodynamically favored product.2
In this paper we report studies on the protonation of 2- and
4-pyridinethiolates coordinated to the robust {Ni(triphos)}2þ
{triphos=PhP(CH2CH2PPh2)2} site. In particular, for [Ni(2-
Spy)(triphos)]þ the site of protonation is coupled to the
coordination state of the 2-pyridinethiolate ligand. The
X-ray crystal structure of a salt of [Ni(2-Spy)(triphos)]þ
shows that both the nitrogen and sulfur are bound to nickel.
Consequently, protonation by lutHþ (lut = 2,6-dimethyl-
pyridine) has to occur at the sulfur. The pKas of free
pyridinethiols indicate that protonation of sulfur by lutHþ is
unfavorable. Clearly, coordination facilitates protonation of
the sulfur site. However, upon protonation of the sulfur, the
2-pyridinethiol chelate undergoes ring-opening, making
available the lone pair of electrons on the nitrogen for
protonation. Both calculations and the pKas of the free
pyridinethiols indicate that the most favored protonation
site is the nitrogen. Consequently, the proton transfers from
sulfur to nitrogen.
[Ni(4-thiopyridine)(triphos)]BPh4, their structural character-
ization by X-ray crystallography, together with kinetic
and computational studies on the protonation of these
complexes.
Protonation of thiolate ligands bound to {Ni(tertiary
phosphine)}2þ sites has been studied earlier.5,6 In particular,
previous studies have shown that [Ni(SC6H4R-4)(triphos)]þ
(triphos = PhP(CH2CH2PPh2)2; R = MeO, Me, H, Cl, or
NO2) can be protonated with lutHþ to form [Ni(HSC6H4R-
4)(triphos)]2þ in MeCN.5 These are equilibrium reactions,
and the kinetics exhibit a complex dependence on the con-
centrations of lutHþ and lut, consistent with the mechanism
involving two coupled equilibria as shown in Figure 1. Using
stopped-flow spectrophotometry, the initial hydrogen-bond-
ing of lutHþ to the sulfur of the thiolate group can be
detected, and the intramolecular proton transfer within the
hydrogen-bonded adduct to transfer the proton completely
to the sulfur (k2R) can be followed.
It is important to appreciate that there is a large difference
in the basicities of the sulfur sites in thiophenols and pyr-
idinethiolates. Protonation of the sulfur in the complexes
[Ni(SC6H4R-4)2(Ph2PCH2CH2PPh2)] and [Ni(SC6H4R-4)-
(triphos)]þ can be accomplished using lutHþ, and kinetic
studies indicate the thiols coordinated to these NiII sites are
associated with pKa =about 14.3.5,6 This is consistent with
the calculated pKa=13.9 of free C6H5SH in MeCN, which is
estimated using the literature value for the pKa in water (pKa
=6.6)7 together with the relationship shown in eq 2.8
MeCN
H2O
pKa
¼ 1:313pKa
þ 5:20
ð2Þ
In contrast, consideration of the pKas of free pyridinethio-
lates indicates that protonation of the sulfur in these mole-
cules by lutHþ is significantly less favorable than protonation
of thiophenolate. The arguments presented here are not
precise; however, they do highlight the significant thermo-
dynamic difference in protonating a thiolate and a pyridi-
nethiolate. The imprecision is because the pKas of pyridi-
nethiolates in water are calculated (not experimental), and to
calculate the pKa in MeCN, another approximate calculation
has to be performed.
Results and Discussion
Although there is work in the literature reporting the synth-
esis and structural characteristics of a wide range of complexes
containing pyridinethiolate ligands, particularly describing the
different modes of coordination (S-bonded, N-bonded, and
bidentate),4 there are no reports on the kinetics and mechanism
of protonation of this type of complex. In this paper we
report the synthesis of [Ni(2-thiopyridine)(triphos)]BPh4 and
Pyridinethiols have two associated pKas: one correspond-
ing to protonation of the sulfur and the other corresponding
to protonation of the nitrogen. The two pKa’s of the cation
HNC5H4SHþ have been reported in water9 and, using eq 2, it
(4) Selected examples include: (a) Onaka, S.; Yaguchi, M.; Yamauchi, R.;
Ozeki, T.; Ho, M.; Sunahara, T.; Sugiura, Y.; Shiotsuka, M.; Nunokawa, K.;
Horibe, M.; Okazaki, K.; Iida, A.; Chiba, H.; Inoue, K.; Imai, H.; Sako, K.
J. Organomet. Chem. 2005, 690, 57. (b) Zhang, L.; Zhang, H.-X.; Chen, C.-L.;
Deng, L.-R.; Kang, B.-S. Inorg. Chim. Acta 2003, 355, 49. (c) Liaw, W.-F.; Lee,
J.-H.; Gau, H.-B.; Chen, C.-H.; Jung, S.-J.; Hung, C.-H.; Chen, W.-Y.; Hu, C.-H.;
Lee, G.-H. J. Am. Chem. Soc. 2002, 124, 1680. (d) Lopes, I.; Hillier, A. C.; Liu,
S. Y.; Domingos, A.; Ascenso, J.; Galvao, A.; Sella, A.; Marques, N. Inorg.
Chem. 2001, 40, 1116. (e) Deeming, A. J.; Meah, M. N.; Bates, P. A.; Hursthouse,
M. B. Inorg. Chim. Acta 1988, 142, 33–37.
(5) Autissier, V.; Zarza, P. M.; Petrou, A.; Henderson, R. A.; Harrington,
R. W.; Clegg, W. Inorg. Chem. 2004, 43, 3106.
(6) Autissier, V.; Clegg, W.; Harrington, R. W.; Henderson, R. A. Inorg.
Chem. 2004, 43, 3098.
(7) www.cem.msu.edu/∼reusch/OrgPage/acidity2.htm
€
€
€
(8) Kaljurand, I.; Kutt, A.; Soovali, L.; Rodima, T.; Maemets, V.; Leito,
I.; Koppel, I. A. J. Org. Chem. 2005, 70, 1019.
(9) Albert, A.; Barlin, G. B. J. Chem. Soc. 1959, 2384.

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 849
Table 1. Analytical and Spectroscopic Data for [Ni(2-Spy)(triphos)]BPh4 and [Ni(4-Spy)(triphos)]BPh4
elemental analysisa
complex
C
H
N
1H NMR spectrum
31P{1H} NMR spectrum
b
[Ni(4-Spy)(triphos)]BPh4
71.8
5.68
1.37
2.6 (br, s, 8H) CH2
53.8 (d) terminal PPh2
106.0 (t) central PPh
51.4 (d) terminal PPh2
115.8 (t) central PPh
(71.6)
73.9
(74.0)
(6.10)
5.62
(5.57)
(1.25)
1.37
(1.36)
6.7-8.2 (m, 49H) aromatic CH
2.6, 2.7, 2.1 (br, s, 8H) CH2
6.2-7.3 (m, 49H) aromatic CH
[Ni(2-Spy)(triphos)]BPh4
a Calculated values in parentheses. b Isolated with 2MeOH and 0.5 thf in crystals.
Figure 2. Structures of [Ni(4-Spy)(triphos)]þ (left) and [Ni(2-Spy)(triphos)]þ (right) with 50% probability displacement ellipsoids and selected atom
labels. H atoms are omitted for clarity.
Table 2. Crystal Data and Structure Refinement Parameters for [Ni(2-Spy)-
(triphos)]BPh4 and Solvated [Ni(4-Spy)(triphos)]BPh4
can be calculated that, for the 2-pyridinethiol cation in
S
N
MeCN, pKa = 3.9 and pKa = 17.9 and for 4-pyridi-
nethiol:cation pKaS = 8.1 and pKaN = 20.8. However, we
cannot calculate the equilibrium constants for protonation
(by lutHþ) of the sulfur and nitrogen sites in the pyridi-
nethiols using these pKas, since we require the pKas of the
neutral species HNC5H4S and NC5H4SH which are unavail-
able. However, (simply based on the charge) it would be
expected that the pKas of pyridinethiol would be higher than
the corresponding pKas of HNC5H4SHþ (i.e., both the
nitrogen and the sulfur in the neutral species would be more
basic than in the cation). These arguments indicate that in
free 2-pyridinethiol (i) the sulfur in pyridinethiols is signifi-
cantly less basic than in thiophenols and is not capable of
being protonated by lutHþ (pKa = 14.13 in MeCN)8,10 but
that (ii) the nitrogen is much more basic than the sulfur.
X-ray Crystal Structures of [Ni(2-Spy)(triphos)]BPh4
and [Ni(4-Spy)(triphos)]BPh4 2MeOH 0.5thf. The lithium
[Ni(2-Spy) [Ni(4-Spy)(triphos)]
(triphos)]BPh4 BPh4 2CH3OH 0.5C4H8O
3
chemical formula
C
63H57
BNNiP3S
C67H69BNNiO2.5P3S
molecular weight
crystal system
space group
T, K
1022.59
triclinic
P1
1122.7
triclinic
P1
160
120
˚
a, A
12.6807(15)
13.1347(15)
17.164(2)
105.0751(9)
97.1106(9)
97.4014(10)
2700.2(5)
2
11.6661(6)
14.3526(7)
20.1381(10)
73.159(2)
73.332(2)
67.844(2)
2928.4(3)
2
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
3
˚
V, A
Z
˚
λ, A
0.6727
0.527
22208
0.71073
0.495
24838
μ, mm-1
3
3
reflections measured
unique data, Rint
refined params
R (F, F2 > 2σ)
RW (F2, all data)
GoF (F2)
salts of 2-pyridinethiolate and 4-pyridinethiolate both react
with [NiCl(triphos)]BPh4 to produce (after workup and
recrystallization from CH2Cl2/Et2O) red crystalline solids.
Analysis and spectroscopic data of both compounds (Table 1)
show that they have the formulation [Ni(pyridinethiolate)-
(triphos)]BPh4 (pyridinethiolate: 2-pyridinethiolate=2-Spy;
4-pyridinethiolate=4-Spy). However, X-ray crystallography
shows that their structures are different. The structures of
the cations [Ni(4-Spy)(triphos)]þ and [Ni(2-Spy)(triphos)]þ
are shown in Figure 2. The crystal data and structure
refinement parameters for both complexes are presented
in Table 2.
10862, 0.0419 13054, 0.0331
631
712
0.0451
0.1069
1.012
0.0460
0.1334
1.036
-3
˚
max, min el dens (e A
)
0.59, -0.34
0.95, -0.56
4-coordinate with the geometry best described as square
planar {angles: P(2)-Ni-P(3)=85.04(2)°, P(2)-Ni-P-
(1) = 84.99(2)°, P(3)-Ni-P(1) = 159.79(3)°, P(3)-Ni-
S = 91.22(3)°, P(1)-Ni-S = 103.44(3)°, P(2)-Ni-S =
162.02(3)°}. The pyridinethiolate is coordinated through
˚
The structure of [Ni(4-Spy)(triphos)]þ is similar to that
of [Ni(SC6H5)(triphos)]þ reported earlier.15 The Ni is
the sulfur atom to Ni {Ni-S = 2.1830(7) A}, with the
angle Ni-S-C(35)=115.54(8)°. The sulfur is trans to the
central phosphorus of the triphos ligand. The Ni-P bond
˚
lengths are unremarkable {Ni-P(1)=2.2096(7) A, Ni-
(10) Izutsu, K. Acid-Base Dissociation Constants in Dipolar Aprotic
Solvents; Blackwells Scientific: Oxford, U.K., 1990.
˚
˚
P(2)=2.1405(7) A, Ni-P(3)=2.2029(7) A}.

850 Inorganic Chemistry, Vol. 50, No. 3, 2011
Petrou et al.
Figure 3. Typical stopped-flow absorbance-time curves for the reaction of [Ni(2-Spy)(triphos)]þ (0.1 mmol dm-3) with lutHþ in the presence of lut in
MeCN at 25.0 °C (λ = 385 nm). The upper curve was recorded with [lutHþ] = 2.5 mmol dm-3 and [lut] = 2.0 mmol dm-3 and the lower curve with [lutHþ]
=2.5mmoldm-3 and[lut] =10.0mmoldm-3. Thegray curvesare the exponentialfitstoeach curve. Fortheupper curvethefitisdefinedbythe equationAt
= 0.0825 - 0.0715e-25.7t, and for the lower curve the fit is defined by the equation At = 0.0226 - 0.0133e-15.2t
.
In contrast, the structure of [Ni(2-Spy)(triphos)]þ con-
tains a 5-coordinate Ni. The structure is best described as
a distorted trigonal bipyramid with the trigonal plane
comprising the two terminal phosphorus atoms of the
steric factors. Rather, the inability of lutHþ to protonate
the sulfur of [Ni(4-Spy)(triphos)]þ is consistent with the
sulfur in coordinated 4-pyridinethiolate being insuffi-
ciently basic to be protonated by lutHþ. The rapid reaction
associated with the observed small absorbance change is
most probably due to protonation of the nitrogen (remote
from the metal) by lutHþ. It would be anticipated that
protonation of the nitrogen to form [Ni(4-SpyH)(tri-
phos)]2þ would decrease the basicity of the coordinated
sulfur even further, making this site even less favorable to
protonation by lutHþ.
˚
triphos ligand {Ni-P(2) = 2.1637(9) A, Ni-P(3) =
˚
2.1996(9) A}and the nitrogen of the 2-Spy ligand {Ni-
N=2.164(2) A}. The two axial positions are occupied by
˚
the central phosphorus of the triphos ligand {Ni-P(1)=
˚
˚
2.1310(8) A} and the sulfur {Ni-S = 2.2644(8) A}. The
principal origin of the distortion is the small bite angle
associated with the 2-Spy ligand {S-Ni-N=70.81(6)°}.
Other bond angles that define the coordination geometry
of the Ni atom in [Ni(2-Spy)(triphos)]þ are as follows:
P(1)-Ni-P(2) = 87.08(3)°, P(1)-Ni-P(3) = 87.08(3)°,
P(2)-Ni-P(3) = 146.34(3)°; P(1)-Ni-S = 178.98(4)°,
P(2)-Ni-S = 92.08(3)°, P(3)-Ni-S = 93.93(3)°; P(1)-
Ni-N=109.12(7)°, P(2)-Ni-N=119.08(7)°, P(3)-Ni-
S=94.11(7)°.
Reaction between [Ni(2-Spy)(triphos)]þ and lutHþ.
When studied using stopped-flow spectrophotometry,
the reaction between [Ni(2-Spy)(triphos)]þ and lutHþ in
the presence of lut shows absorbance-time curves with a
significant amplitude over the seconds time scale. Typical
absorbance-time curves (measured at λ = 385 nm) are
shown in Figure 3. Furthermore, the magnitude of the
absorbance-time curves are characteristic of an equilibri-
um protonation reaction as indicated in eq 3. Thus, under
all conditions, the initial absorbance of the absorbance-
time curves corresponds to that of [Ni(2-Spy)(triphos)]þ
at the concentration used ([Ni]=0.5 mmol dm-3), but the
final absorbance varies. Consistent with the equilibrium
reaction shown in eq 3, the spectrophotometric changes
increase in magnitude as the concentration of lutHþ is
increased (at a constant concentration of lut), and de-
crease as the concentration of lut increases (at a constant
concentration of lutHþ). This behavior is markedly dif-
ferent to that observed in the irreversible reaction of
[Ni(2-Spy)(triphos)]þ with anhydrous HCl ([HCl] =
2-10 mmol dm-3) in MeCN, to form [NiCl(triphos)]þ
and 2-pyridinethiol. In limited studies using [HCl]=2-10
mmol dm-3, the reaction with the stronger acid HCl (pKa
in MeCN=8.9)10 is associated with a single exponential
absorbance decrease, with both the initial and the final
Reaction between [Ni(4-Spy)(triphos)]þ and lutHþ.
When monitored using a stopped-flow spectrophotom-
eter (at several wavelengths in the range λ = 350-
450 nm), mixing [Ni(4-Spy)(triphos)]þ (0.1 mmol dm-3
)
and lutHþ is associated with a very small absorbance
decrease complete within the dead-time of the stopped-
flow apparatus (2 ms). We have studied this reaction with
[lutHþ] = 0.1-5.0 mmol dm-3, [lut] = 0.2 mmol dm-3
,
and monitored over short and protracted times (5 ms-5
min). Under all conditions, even using equimolar
amounts of [Ni(4-Spy)(triphos)]þ and lutHþ and in the
presence of a large excess of lut, no reaction other than the
small absorbance decrease complete within the dead-time
is observed. The structure of [Ni(4-Spy)(triphos)]þ is very
similar to that of [Ni(SC6H5)(triphos)]þ which is readily
protonated at the sulfur by lutHþ. Consequently, it seems
unlikely that the reason the sulfur atom in [Ni(4-Spy)-
(triphos)]þ is not protonated by lutHþ is attributable to

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 851
Figure 4. Kinetic data for the reaction between [Ni(2-Spy)(triphos)]þ (0.1 mmol dm-3) and lutHþ in the presence of lut (solvent = MeCN, λ = 385 nm,
T = 25.0 °C). Data points correspond to: [lutHþ]/[lut] = 0.25, [lut] = 5-20 mmol dm-3 (b); [lutHþ]/[lut] = 0.50, [lut] = 2-20 mmol dm-3 (2); [lutHþ]/
[lut] =1.0, [lut] = 2-20 mmoldm-3 (9); [lutHþ]/[lut] = 2.0, [lut] =1-20mmoldm-3 ((); [lutHþ]/[lut] = 5.0, [lut] = 1-5mmoldm-3 (O). The straightline
fitsarethose defined byeq5. Additional dataispresented in SupportingInformation whichalso is agoodfit toeq 5. See textfordetails ofthe analysisofthese
data.
absorbances unaffected by the concentration of acid.
analysis difficult.
- d½Nið2-SpyÞðtriphosÞþꢀ
dt
½Nið2-SpyÞðtriphosÞꢀþ þ lutHþ
¼
K0
h ½Nið2-SpyHÞðtriphosÞꢀ2þ þ lut
ð3Þ
fð1:38 ( 0:20 ꢁ 104Þ½lutHþꢀ þ ð2:23 ( 0:20 ꢁ 103Þ½lutꢀg½Nið2-SpyÞðtriphosÞþꢀ
f1 þ ð2:69 ( 0:20 ꢁ 102Þ½lutꢀg
Kinetics of the Protonation Reaction. The spectropho-
tometric changes observed in the reactions between [Ni(2-
Spy)(triphos)]þ and lutHþ in the presence of lut indicates
this is an equilibrium reaction. If the reaction were a
simple single-step equilibrium then the rate law for the
reaction would be that described by eq 4, where the rate of
the reaction would increase as the concentration of either
lutHþ or lut is increased.11 However, analysis of the
kinetic results for the reaction between [Ni(2-Spy)-
(triphos)]þ and an excess of lutHþ and lut shows that
the rate of the reaction increases as the concentration of
lutHþ increases but decreases as the concentration of lut
increases.
ð5Þ
While these kinetics and the spectrophotometric
changes indicate that [Ni(2-Spy)(triphos)]þ is protonated
by lutHþ, we will defer until later identification of the
protonation site of [Ni(2-Spy)(triphos)]þ.
Mechanism of Reaction between [Ni(2-Spy)(triphos)]þ
and lutHþ in the Presence of lut. We will now discuss the
details of the protonation chemistry of [Ni(2-Spy)-
(triphos)]þ as revealed by the kinetic analysis, the pKas,
and calculations.
Sites of Protonation. The key issue concerns where
[Ni(2-Spy)(triphos)]þ binds protons. We have been un-
able to isolate the protonated complex because of the
equilibrium nature of this protonation reaction. Attempts
to isolate the product by using a stronger acid (HCl), and
hence drive the equilibrium shown in eq 3 to the right-
hand side, resulted in the loss of 2-HSpy and formation of
[NiCl(triphos)]þ (vide supra). Similar behavior has been
observed in the reaction of [Ni(SC6H5)(triphos)]þ with
HCl.15 In principle, there are four possibilities as shown in
Figure 5: at sulfur or nitrogen in the closed or open
structures. Alternative structures with the nitrogen co-
ordinated and the sulfur free seem unreasonable because
in [Ni(4-Spy)(triphos)]þ, where either nitrogen or sulfur
could bind, coordination occurs at the sulfur. Further-
more, our calculation studies show no indication that an
N-bonded (S-open) structure of [Ni(2-Spy)(triphos)]þ
represents an energy minimum.
- d½Nið2-SpyÞðtriphosÞþꢀ
dt
¼ fkforward½lutHþꢀ þ kback½lutꢀg½Nið2-SpyÞðtriphosÞþꢀ
ð4Þ
When [lutHþ]/[lut] is kept constant, plots of [lut]/kobs
versus [lut] are a straight line. Varying [lutHþ]/[lut]
produces a series of straight lines and as [lutHþ]/[lut]
increases the straight lines have decreasing gradient and
intercept, as shown in Figure 4. A least-squares analysis
of these data yields the rate law shown in eq 5. It is
important to note that the data in the graphical analysis
of Figure 4 show significant scatter making precise
(11) Espenson, J. H. Chemical Kinetics and Reaction Mechanism;
McGraw-Hill: New York, 1981.

852 Inorganic Chemistry, Vol. 50, No. 3, 2011
Petrou et al.
Figure 5. Possible structures of protonated [Ni(2-Spy)(triphos)]þ (triphos ligand omitted for clarity).
To explore the protonation chemistry of [Ni(2-Spy)-
(triphos)]þ we have applied a variety of theoretical meth-
ods (MSINDO, ADF, and CPMD). Initially, we studied
the structure of [Ni(2-Spy)(triphos)]þ in the gas phase and
explored the sites of protonation, especially sulfur and
nitrogen, through MSINDO and Amsterdam Density
Functional (ADF) calculations. As detailed below, both
methods indicate that the initial site of protonation of
[Ni(2-Spy)(triphos)]þ is the sulfur but the energy-favored
product is the “open” structure with the nitrogen proto-
nated. However, MSINDO and ADF calculations do not
give a consistent answer about the relative energies of
these two protonated species. Parenthetically, we should
mention that the Car-Parrinello Molecular Dynamics
(CPMD) method (which was used for molecular dy-
namics (MD) simulation of H transfer in a simplified
molecular structure, with phenyl groups replaced by H),
produces a different result, with the “closed” structure
where sulfur is protonated appearing to be more stable
than the “open” structure where nitrogen is protonated.
However, this result should be considered to be less
reliable than those with MSINDO or ADF because of
the drastic modifications applied to the molecular
structure.
Using the unaltered “closed” structure of [Ni(2-Spy)-
(triphos)]þ established by X-ray crystallography, MSIN-
DO semiempirical calculations indicate that the bound
sulfur is more basic than the bound nitrogen, and conse-
quently the sulfur site would protonate preferentially (i.e.,
there is a lone pair of electrons available on sulfur but
none on the bound nitrogen). However, in the “open”
structure where the 2-pyridinethiolate is bound to nickel
only by the sulfur, nitrogen was found to be more basic
than the coordinated sulfur. Energy calculations have
shown that the thermodynamically favored protonated
species contains the “open” 2-pyridinethiolate ligand
with a protonated nitrogen.
protonated. The MSINDO method predicts the same
relative stability property of the two open structures
but with a quite smaller energy difference, of about
0.19 kcal mol-1
.
The numerical difference in these energy calculations
originates from the approximation of the two ap-
proaches. ADF is a density functional method that is
based on approximate functional integrals about the
correlation and exchange energy, while MSINDO is
based on the neglect of complex interaction matrix
elements of Molecular Orbital theory. The different
nature of the two approaches makes comparison of the
results difficult. Further, since the calculations refer to
gas-phase structures it is not justifiable to relate the
energies to the kinetic studies that were performed in
MeCN as the solvent. However, the experimentally
observed rapidity and reversibility of the reaction be-
tween [Ni(2-Spy)(triphos)]þ and [lutH]þ favors the
MSINDO result, since the ADF result indicates K4 ∼
10-10 at 25 °C.
Mechanism of Protonation. For the reaction of [Ni(2-
Spy)(triphos)]þ with lutHþ in the presence of lut, we
suggest that the experimentally observed rate law {eq 5}
together with the calculations presented above, are con-
sistent with the mechanism shown in Figure 6. This
mechanism consists of two coupled equilibrium reactions
in which the first step is protonation of [Ni(2-Spy)-
(triphos)]þ at the sulfur to form an intermediate, which
then undergoes a unimolecular equilibrium reaction to
form the “open” structure with nitrogen protonated.
Consideration of this mechanism, assuming that the
sulfur-protonated species is a steady state intermediate,
shows that the associated rate law is that shown in eq 6.
Comparison of eqs 5 and 6 gives the values: k3=(1.38 (
0.20) ꢁ 104 dm3 mol-1 s-1; k-3/k4 =(2.69 ( 0.20) ꢁ 102
dm3 mol-1, and k-4 =8.29 ( 2 s-1. However, as we will
see when we consider the second step in this equilibrium,
it seems unlikely that k4 and k-4 are true elementary rate
constants. Rather, the transfer of the proton from sulfur
to nitrogen most likely involves several steps.
To further substantiate the results of the MSINDO
semiempirical calculations, we have applied a density
functional computational method, ADF, at the level of
TZP basis set, to the calculation of the protonated
structures of [Ni(2-Spy)(triphos)]þ. The ADF method
converged only when the starting configuration was
approximated by an “open” complex (i.e., nitrogen not
coordinated). All attempts to obtain a converged struc-
ture with the nitrogen both protonated and coordinated
to nickel failed, indicating that such a structure may be
impossible to obtain numerically and therefore to exist.
Using the ADF method, the “open” structure with the
nitrogen protonated is found to be about 13.2 kcal mol-1
more stable than the “closed” structure with sulfur
- d½Nið2-SpyÞðtriphosÞþꢀ
dt
fðk3½lutHþꢀ þ ðk- 3k- 4=k4Þ½lutꢀg½Nið2-SpyÞðtriphosÞþꢀ
¼
1 þ ðk- 3=k4Þ½lutꢀ
ð6Þ
Previous work5 on the protonation of other, related,
Ni-thiolate complexes [Ni(SC6H4R-4)(triphos)]þ indi-
cated a mechanism involving the rapid equilibrium

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 853
Figure 6. Top line shows the mechanism for the protonation of [Ni(2-Spy)(triphos)]þ by lutHþ in MeCN. The pathway presented in the box shows the
proposed detail for transfer of the proton involving initial hydrogen bonding of lutHþ to the complex (K1) prior to intramolecular proton transfer (k2). Such
hydrogen bonding has been detected previously in the reactions of lutHþ with [Ni(SC6H4R-4)(triphos)]þ (see Figure 1). However, in the reaction with [Ni(2-
Spy)(triphos)]þ the value of K1 is small.
formation of a hydrogen-bonded adduct between the
complex and lutHþ (Figure 1). It seems likely that a
similar hydrogen-bonded adduct is formed between
[Ni(2-Spy)(triphos)]þ and lutHþ and that in reality the
k3 step involves the initial formation of a hydrogen-
bonded adduct (K1) prior to the complete transfer of the
proton to the sulfur (k2) as shown in the inset of Figure 6.
For such a sequence of reactions, k3[lutHþ] = K1k2-
[lutHþ]/(1 þ K1[lutHþ]). However, in the reaction with
[Ni(2-Spy)(triphos)]þ the concentration of the hydrogen
bonded adduct does not accumulate and so K1[lutHþ] <
1 and k3[lutHþ] = K1k2[lutHþ]. Thus, the kinetics and
mechanism presented here for the reaction between [Ni(2-
Spy)(triphos)]þ and lutHþ are consistent with the kinetics
and mechanism proposed earlier for the reactions of
[Ni(SC6H4R-4)(triphos)]þ with lutHþ.
transferred to [Ni(2-Spy)(triphos)]þ.
1
Af - εNi½Nið2-SpyÞðtriphosÞþꢀ
i
1
¼
K0ðεNiH - εNiÞ½Nið2-SpyÞðtriphosÞþꢀi½lutHþꢀ=½lutꢀ
1
þ
ð7Þ
ðεNiH - εNiÞ½Nið2-SpyÞðtriphosÞþꢀ
i
The plot of 1/(Af - εNi[Ni(2-Spy)(triphos)þ]i) versus
[lut]/[lutHþ] is shown in Figure 7. To measure absorbance
differences more accurately, we have used a larger con-
centration of [Ni(2-Spy)(triphos)]þ = 0.5 mmol dm-3 in
the studies to determine the equilibrium constant than
in the kinetic studies ([Ni(2-Spy)(triphos)]þ=0.1 mmol
dm-3). Although the plot is a straight line, it is clear that
there is a high degree of scatter on the data and, in
particular, that there appears to be a systematic deviation
from the line at low [lut]/[lutHþ]. These features result in
large uncertainties to the measured gradient and intercept
of the line. Since the gradient=1/{K0(εNiH - εNi)[Ni(2-
Spy)(triphos)þ]i} and the intercept=1/{(εNiH - εNi)[Ni-
(2-Spy)(triphos)þ]i}, K0=intercept/gradient=3.0 ( 1.0.
Clearly, the agreement between the value of K0 deter-
mined from the kinetics and that determined from the
spectrophotometric analysis is not very good. We believe
the reason for this poor agreement is that at the wave-
length used (λ=385 nm) there is a small contribution to
the absorbance from lutHþ and/or lut (both of which are
present in a large excess over the concentration of com-
plex). This would affect the spectrophotometric analysis,
in particular, increasing the gradient of the plot at low
[lut]/[lutHþ] and leading to a lower calculated value of K0
when determined spectrophotometrically.
The overall equilibrium constant for the mechanism
shown in Figure 6 (K0) is related to the elementary rate
constants by the expression K0=k3k4/k-3k-4. Using the
values of the rate constants determined from the kinetic
analysis, we can calculate K0=6.19 ( 0.9.
We can independently estimate the overall equilibrium
constant (K0) for the reaction from spectrophotometric
analysis of the effects that the concentrations of lutHþ
and lut have on the absorbance changes in the reaction
with [Ni(2-Spy)(triphos)]þ (Figure 3). By considering the
equilibrium reaction between [Ni(2-Spy)(triphos)]þ and
lutHþ to form [Ni(2-SpyH)(triphos)]2þ and lut, and using
Beer’s Law, it can be shown that the variation of the final
absorbance of the stopped-flow trace, Aeq (i.e., absor-
bance of the equilibrium mixture of [Ni(2-Spy)(triphos)]þ
and [Ni(2-SpyH)(triphos)]2þ), is related to the ratio
[lutHþ]/[lut] as shown in eq 7, provided that neither lutHþ
nor lut absorb at the wavelength being used (λ=385 nm).
In eq 7, [Ni(2-Spy)(triphos)]i = 0.5 mmol dm-3, εNi
=
38.0 dm3 mol-1 cm-1 (extinction coefficient [Ni(2-Spy)-
(triphos)]þ, λ = 385 nm), and εNiH = 228.5 dm3 mol-1
cm-1 (extinction coefficient [Ni(2-SpyH)(triphos)]2þ, λ=
385 nm). It is important to emphasize that the derivation
of eq 7 is consistent with only a single proton being
We will now consider each of the steps in the mechanism
shown in Figure 6 (the protonation and the rearrangement)
in turn. This mechanism indicates that, upon coordination
as a bidentate ligand, the sulfur in 2-pyridinethiolate of the
“closed” structure of [Ni(2-Spy)(triphos)]þ is sufficiently

854 Inorganic Chemistry, Vol. 50, No. 3, 2011
Petrou et al.
Figure 7. Analysis of the spectrophotometric data to determine the overall equilibrium constant (K0) for the reaction between [Ni(2-Spy)(triphos)]þ and
lutHþ in the presence of lut (solvent = MeCN, λ = 385 nm, T = 25.0 °C). Plot of 1/(Af - εNi[Ni(2-Spy)(triphos)þ]i) versus [lut]/[lutHþ] {Af = final
absorbance of absorbance-time curve (see Figure 3) measured in the presence of known concentrations of lutHþ and lut; εNi = molar extinction coefficient
of [Ni(2-Spy)(triphos)]þ; [Ni(2-Spy)(triphos)þ] = 0.5 mmol dm-3}. Data points correspond to: [lutHþ] = 0.5-10 mmol dm-3, [lut] = 1.0 mmol dm-3 (b);
[lutHþ] = 10 mmol dm-3, [lut] = 1.0-30 mmol dm-3 (9). The straight line fit is that defined by eq 7 and by the parameters presented in the text.
basic to be protonated by lutHþ (pKa
= 14.13 in
with no bond cleavage, (ii) cleavage of a Ni-P bond, or
(iii) cleavage of the Ni-N bond. We conclude that the
second step involves Ni-N bond cleavage for the follow-
ing reasons. First, we have used calculations to explore
whether geometrical changes occur to [Ni(2-SpyH)-
(triphos)]2þ, and have found no evidence of such struc-
tural reorganization. Second, there is no evidence that the
phosphine dissociates from the complex. Thus, 31P{1H}
NMR spectra of mixtures comprising [Ni(2-Spy)-
(triphos)]þ and lutHþ show patterns characteristic of
the triphos ligand where two terminal phosphorus and
one central phosphorus are all coordinated to the nickel.
The MSINDO and ADF calculations described above
indicate that the thermodynamically favored product of
the reaction between [Ni(2-Spy)(triphos)]þ and lutHþ
contains the “open” 2-pyridinethiolate ligand with the
nitrogen protonated. We therefore propose that, after
initial protonation of the sulfur to form the “closed”
[Ni(2-SpyH)(triphos)]2þ, that the nitrogen dissociates
from the nickel, thus freeing the lone-pair of electrons
on nitrogen to be protonated. In line with the results of
the MSINDO and ADF calculations, we suggest that the
pendant nitrogen in the “open” 2-pyridinethiol ligand is
more basic than the coordinated sulfur, and so the proton
transfers from sulfur to nitrogen. The possible pathways
for the proton transfer between sulfur and nitrogen are
summarized in Figure 8. Because the kinetics of the
second step in the mechanism (Figure 6) occur at a rate
independent of the concentrations of lutHþ or lut, proton
transfer must either be intramolecular (i.e., direct transfer
from sulfur to nitrogen), in which, after dissociation of
lutH
MeCN).8 The observation that [Ni(4-Spy)(triphos)]þ is
not protonated at sulfur by lutHþ indicates that coordi-
nation of only the sulfur of a pyridinethiolate to the
{Ni(triphos)}2þ site is insufficient to make the sulfur basic
enough to be protonated by lutHþ. The increased basicity
of the sulfur in the 2-pyridinethiolate ligand must be a
consequence of the bidentate ligation of 2-pyridinethio-
late resulting in redistribution of the electron density
toward sulfur.
The kinetic analysis presented above allows us to
calculate the rate constant for protonation of the sulfur
in [Ni(2-Spy)(triphos)]þ by lutHþ (k3=1.38 ( 0.20 ꢁ 104
dm3 mol-1 s-1). Previous studies have shown that the
rates of proton transfer from lutHþ to thiolate sulfurs
bound to NiII sites in [Ni(SC6H5)(triphos)]þ and [Ni-
(SC6H5)2(Ph2PCH2CH2PPh2)] fall in the range k ∼
20-200 dm3 mol-1 s-1. The rate of proton transfer to
[Ni(2-Spy)(triphos)]þ is appreciably faster than that mea-
sured for [Ni(SC6H4R)(triphos)]þ and [Ni(SC6H4R)2-
(dppe)] with lutHþ. It is not entirely clear why this should
be so, but it could be that the juxtaposition of the nitrogen
and sulfur atoms in [Ni(2Spy)(triphos)]þ results in an
enhanced rate of proton transfer to the sulfur through
anchimeric assistance,12 in which the approach of the acid
toward the sulfur is aided by the proximal electronegative
nitrogen.
The experimental (kinetics) results give little informa-
tion about the nature of the second step shown in Figure 6,
except that it occurs at a rate independent of the concen-
trations of lutHþ or lut. Processes consistent with such
kinetics could be (i) a geometrical change at the nickel
the nitrogen from the “closed” [Ni(2-SpyH)(triphos)]2þ
,
rotation about the C-S bond occurs and the proton hops
from sulfur to the more basic nitrogen, or intermolecular
involving rapid movement of the proton mediated by
(12) Calveresi, M.; Rinaldi, S.; Arcelli, A.; Garavelli, M. J. Org. Chem.
2008, 73, 2066, and refs therein.

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 855
Figure 8. Possible intramolecular and intermolecular pathways for the transfer of proton from sulfur to nitrogen in the reaction of [Ni(2-Spy)(triphos)]þ
with lutHþ in MeCN. Another intermolecular pathway (not shown) is possible in which deprotonation of the sulfur precedes protonation of the pendant
nitrogen. Forthis pathwaytobeviable, the protonation ofthe pendantnitrogenwouldhavetobefasterthanthe rateatwhichthe nitrogencoordinatestothe
nickel.
lutHþ and lut, after the slow dissociation of the Ni-N
bond.
because of the availability of the lone pair of electrons. On
the basis of purely charge considerations, this proton-
ation would be expected to decrease the basicity of the
sulfur, resulting in proton dissociation from this site.
Isotope Experiments. We have studied the effect on the
kinetics of using lutDþ in the reaction with [Ni(2-Spy)-
(triphos)]þ in the presence of lut. The results are presented
in Figure 9. This figure shows plots of [lut]/kobs versus [lut]
at constant [lutHþ]/[lut] = 2. Clearly, because of the
multistep nature of the reaction, the interpretation of
the effect is not simple.14 Considering the mechanism
shown in Figure 6, it seems most reasonable to propose
that both k3 and k-3 would be associated with primary
isotope effects. Although the second stage must also
involve proton movement we suggest that the rate of this
stage is limited by dissociation of the Ni-N bond and so
has no, or very minor, deuterium isotope effects. Several
lines are presented in Figure 9. In addition to the line
defined by eq 6, there are also lines defined by the same
rate law but which show the influence of different isotope
Despite the apparent reasonableness of the intramole-
cular processes, it is not supported by calculations. Pre-
vious theoretical studies on free pyridones indicated that
the intramolecular pathway was of very high energy.13 In
addition, direct transfer of a proton between nitrogen and
sulfur has been studied using the CPMD method for a
simplified complex where phenyl groups are replaced by
hydrogen for computational convergence and efficiency.
The calculations have identified an apparent saddle point
with the proton lying between the nitrogen and sulfur
atoms (at a line positioned in between sulfur and nitrogen
and inclined at 30° toward nickel). However, the energy
barrier for this pathway is calculated to be greater than 45
kcal mol-1 and so appears not to be feasible.
In the absence of support for an intramolecular path-
way, we suggest that the prototropic shift is accomplished
by an acid-base-catalyzed pathway similar to that sug-
gested earlier for pyridones. In solutions containing an
excess of lutHþ and lut, the “transfer” of the proton is
accomplished rapidly, after the dissociation of the nitro-
gen from nickel. Such an acid-base-catalyzed transfer is
shown in the bottom line of Figure 8. After dissociation of
the Ni-N bond, rapid protonation of the nitrogen ensues
effects for k3 and k-3, ranging from the extremes where
=
D
k3H/k3D=2, k-3H/k-3D=1 to k3H/k3D=1, k-3H/k-3
H
2 and where k3 and k-3 have the same isotope effect, k3
/
k3 = 2, k-3H/k-3 = 2. Two features of these plots
are worth comment. First, it is evident that, depending on
the relative isotope effects associated with k3 and k-3, the
overall effect can appear to be a normal or an inverse
D
D
(13) (a) Lowry, T. H.; Richardson, K. S. Mechanisms and Theory in
Organic Chemistry; Harper and Row: New York, 1976; p 426; (b) Swain, C. G.;
Brown, J. F., Jr. J. Am. Chem. Soc. 1952, 74, 2534. (c) Swain, C. G.; Brown, J. F.,
Jr. J. Am. Chem. Soc. 1952, 74, 2538. (d) Rony, P. R. J. Am. Chem. Soc. 1968,
90, 2824. (e) Rony, P. R. J. Am. Chem. Soc. 1969, 91, 6090. (f) Rony, P. R.; Neff,
R. O. J. Am. Chem. Soc. 1973, 95, 2896. (g) Li, Y.; Guo, Y.; Li, B. Huaxue
Yanjiu. 2006, 17, 71 (chinese, abstract only).
(14) For discussions of normal and inverse primary isotope effects see, for
example: (a) Parkin, G. Acc. Chem. Res. 2009, 42, 315, and refs therein.
(b) Truhlar, D. C.; Garrett, B. C. Isotope Effects in Chemistry and Biology;
Kohen, A., Limbach, H.-H., Eds.; Taylor and Francis: New York, 2006; reference ,
Chapters 11 and 12.

856 Inorganic Chemistry, Vol. 50, No. 3, 2011
Petrou et al.
Figure 9. Plot of [lut]/kobs versus [lut] for the reaction between [Ni(2-Spy)(triphos)]þ and lutDþ in the presence of lut in MeCN at 25.0 °C, illustrating how
various deuterium isotope effects associated with k3 and k-3 affect the kinetics. In all plots [lutHþ]/[lut] = 2.0. The dark, solid line and the data points shown
as solid diamonds (() correspond to that for the reaction with lutHþ as defined by eq 6. The gray lines correspond to simulations of observed dependences,
using eq 6 when k3H/k3D = 2.0, k-3H/k-3D = 1.1 (gray long-dashed line); k3H/k3D = 2.0, k-3H/k-3D = 2.0 (gray short-dashed line); and k3H/k3D = 1.1,
k-3H/k-3D = 2.0 (gray dashed-dotted line). For the reaction between [Ni(2-Spy)(triphos)]þ and lutDþ in the presence of lut the data points are shown as
open diamonds (]).
isotope effect, and, second, the isotope effect due to k3 is
primarily reflected in the intercept and the isotope effect
due to k-3 is mostly reflected in the slope.
consequence of the coordination of the nitrogen. Protonation
of the sulfur results in breaking of the Ni-N bond, and when
free, the proton effectively transfers to the nitrogen (now the
most basic site). However, in the presence of lut the whole
reaction is reversible, with deprotonation of the pendant
N-H group allowing recoordination of the nitrogen to the
Ni site. Thus, there is a mutual control of ligation mode and
protonation site of the coordinated pyridinethiolate.
The quality of the experimental data and the complex-
ity of the kinetic analysis, together with the way that
subtle changes to the values of k3 and k-3 (because of
isotopic substitution) affect the plots (as shown in
Figure 9) mean that it is difficult to accurately measure
the isotope effects. However, the data indicate that the
isotope effect for k-3 is larger than k3, and we can estimate
k3H/k3D = 1.1, k-3H/k-3D = 2.0. Thus, the apparent
inverse isotope effect is actually attributable to the k-3
step having a larger primary isotope effect than k3.
Experimental Section
All preparations and manipulations were performed under
an atmosphere of dinitrogen using Schlenk and syringe
techniques as appropriate. All solvents were dried and dis-
tilled from the appropriate drying agent and were used
immediately after their preparation. 2-Pyridinethiol, 4-pyr-
idinethiol, and lut were purchased from Aldrich and were
used as received. [NiCl(triphos)]BPh4,15 [lutH]BPh4, and
Conclusions
In considering the protonation chemistry of complexes
where the ligands can undergo intramolecular changes, the
question of how protons move between various basic sites on
the ligand needs to be addressed. This paper describes a
combined study using calculation methods and experimental
kinetic studies to understand the protonation chemistry of
coordinated pyridinethiolates. The pyridinethiolate is just
one example of a type of ligand in which there is more than
one potential protonation site and more than one type of
coordination mode.
Our studies show, not unexpectedly, that, even in com-
plexes, protonation of the nitrogen is preferred thermodynam-
ically over protonation of sulfur. However, when the
nitrogen is coordinated to the metal (as in [Ni(2-Spy)-
(triphos)]þ), the lone pair of electrons is unavailable. Conse-
quently, protonation at the sulfur is enforced. Coordination
to the {Ni(triphos)}2þ site must make the sulfur sufficiently
basic to be protonated by lutHþ, and it seems likely that, at
least in part, the basicity of the coordinated sulfur is a
16
[lutD]BPh4 were prepared by methods reported in the
literature. H NMR spectroscopy showed that [lutD]BPh4
was at least 90% deuterium-labeled.
1
Preparation of [Ni(2-Spy)(triphos)]BPh4 and [Ni(4-Spy)-
(triphos)]BPh4. Both complexes were prepared by the same
method. Only the preparation of [Ni(2-Spy)(triphos)]BPh4 will
be described here.
A slurry of [NiCl(triphos)]BPh4 (2.0 g, 1.96 mmol) in tetra-
hydrofuran (thf, ca. 20 mL) was stirred rapidly, and Na[2-Spy]
(0.30 g, 2.2 mmol) was added. The solution rapidly turned from
yellow to dark red. After stirring at room temperature for 2 h,
the solution was concentrated in vacuo. An excess of methanol
(3-4 volumes) was added and, upon standing, red crystals of the
product were formed slowly. The crystals were removed by
filtration, washed with methanol and diethyl ether, then dried
(15) Clegg, W.; Henderson, R. A. Inorg. Chem. 2002, 41, 1128.
€
(16) Gronberg, K. L. C.; Henderson, R. A.; Oglieve, K. E. J. Chem. Soc.,
Dalton Trans. 1998, 3093.

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 857
in vacuo. The crude solid was recrystallized by dissolving the
solid in dichloromethane, filtering to remove any insoluble
material, then layering methanol (2-3 volumes) on the solution.
After 2-3 days the dark red crystals were removed by filtration,
washed with a small amount of cold methanol, then diethyl
ether, and dried in vacuo. Yield=0.52 g, 26%. To grow crystals
of [Ni(4-Spy)(triphos)]BPh4 suitable for X-ray crystallography
the recrystallization was performed by dissolving in the mini-
mum of thf, then layering with methanol.
energy surface. Here, the energy was calculated with the struc-
ture optimized alongside, with energy convergence criterion
below 10-9 hartree. In one case MD was performed at constant
ꢀ
temperature using Nose-Hoover-Chain (NHC) thermostat.
All complex structure calculations were based on the structure
of [Ni(2-Spy)(triphos)]þ as determined by X-ray crystallogra-
phy. Conformation and intramolecular H-transfer barrier cal-
culations were based on minimal pyridinethiolate deformations.
ADF (Version 2004.01) is the Amsterdam Density Functional
program21 based on the density functional theory, (DFT), and
implemented through the use of Slater type basis orbitals. We
have employed the TZP (core double-ζ, valence triple-ζ with
polarization) basis set, which is one of the most extended sets of
the method, because it can provide efficiently convergent results.
Technically, the method is based on Vosko-Wilk-Nusair
(VWN) functional and the Perdew-Zunger self-interaction
correction (SIC) which is implemented self-consistently through
the use of the Krieger-Li-Iafrate approximation to the opti-
mized effective potential.22 Difficulties in the convergence of the
whole optimized structures did not allow us to proceed beyond
the Local Density Approximation level of calculation.
X-ray Crystallography. Data were collected on a Bruker
APEX2 diffractometer with synchrotron radiation (at Daresbury
Laboratory SRS station 9.8) for the 2-Spy complex and with a
Bruker SMART 1K diffractometer with MoKR radiation for
the 4-Spy complex. Semiempirical absorption corrections were
applied, based on repeated and symmetry-equivalent data. The
structures were solved by direct methods and refined on all
unique F2 data, with anisotropic displacement parameters for
non-H atoms, and isotropic displacement parameters for riding
H atoms. In the case of the 4-Spy complex, solvent molecules
were identified and refined, without H atoms as these could not
be unambiguously positioned; the asymmetric unit contains two
methanol molecules and half of a thf molecule, which is dis-
ordered over an inversion center such that it is not possible to
distinguish C and O atoms (all were refined as C, with restraints
on geometry and displacement parameters). Programs were
standard Bruker control and integration software, SHELXTL,
and local programs.
CPMD (Version 3.11.1)23 is a program for Car-Parrinello
molecular dynamics method, at the level of DFT through the use
of BLYP24,25 functional that includes a gradient correction
contribution as proposed by Becke24 and the correlation energy
term of Lee et al.25 The core-valence interactions were de-
scribed by semilocal norm-conserving Martins-Troullier pseu-
dopotentials,26 and the plane-wave expansion of the electronic
states included waves up to 100 Ry.27 During the calculations,
first, the energy was minimized for an initial structure, then the
structure was optimized and MD simulation was followed with
attention that the electron temperature would not increase. Only
a simplified structure of the complex was amenable to calcula-
tions. The full structure did not converge.
Kinetic Studies. All kinetic studies were conducted using an
Applied Photophysics SX 18MV stopped-flow spectrophotom-
eter, constructed to handle oxygen-sensitive solutions. The
temperature was maintained at 25.0 ( 0.1 °C using a Grant
LT D6G recirculating thermostatted tank. All solutions were
prepared under an atmosphere of dinitrogen and were trans-
ferred by gastight, all-glass syringes into the stopped-flow
spectrophotometer. The solutions of the Ni complexes and those
comprising mixtures of lut and [lutH]BPh4 (or [lutD]BPh4)
solutions were prepared from freshly made stock solutions
and were used within 2 h.
Acknowledgment. One of the authors (A.D.K.) thanks the
Computer Center of the National and Kapodistrian University
of Athens for their support in the computations. We thank
EPSRC for funding of crystallographic equipment and of the
National Crystallography Service, and STFC for access to
synchrotron facilities at Daresbury Laboratory.
All kinetics were studied under pseudo first order conditions
in acetonitrile. The concentration of the prepared stock solu-
tions of the nickel complexes was always 0.2 mmol dm-3, so that
when studied by stopped-flow spectrophotometry the concen-
tration was 0.1 mmol dm-3. The concentrations of lut and lutHþ
Supporting Information Available: X-raycrystallographic files
for [Ni(2-Spy)(triphos)]BPh4 and [Ni(4-Spy)(triphos)]BPh4
ranged from 1 to 30 mmol dm-3 and from 2 to 12 mmol dm-3
,
3
respectively. The ratio [lutHþ]/[lut] ranged from 0.25 to 5. The
reaction of the [Ni(S-2py)(triphos)]þ with lutHþ in the presence
of lut was studied at λ=385 nm. At this wavelength there is an
increase in absorbance, as shown in Figure 3. Presumably, this
increase in absorbance at this wavelength is a consequence of the
reaction involving the chelate ring-opening of the 2-thiopyridine
ligand with a necessary decrease in coordination number of the
Ni. Interestingly, the reaction of [Ni(SC6H4R-4)(triphos)]þ with
[lutH]þ, at λ = 350 nm, is associated with an absorbance
decrease, but this reaction only involves protonation of the
sulfur and no gross structural changes at Ni.5
2MeOH 0.5thf. Listings of tables containing the observed rate
3
constants for the reactions between [Ni(2-Spy)(triphos)]þ and
lutHþ in the presence of lut. Details of the theoretical calcula-
tions and video films showing calculated movement of proton
between the saddle point and nitrogen or sulfur. This material is
available free of charge via the Internet at http://pubs.acs.org.
(17) Ahlswede, B.; Jug, K. J. Comput. Chem. 1999, 20, 563.
(18) Ahlswede, B.; Jug, K. J. Comput. Chem. 1999, 20, 572.
(19) Bredow, T.; Geudtner, G.; Jug, K. J. Comput. Chem. 2001, 22, 861.
(20) Steveson, M.; Bredow, T.; Gerson, A. Phys. Chem. Chem. Phys. 2002,
4, 358.
(21) ADF, Amsterdam Density Functional software; Scientific Computing and
Modelling N. V., Faculty of Sciences, Theoretical Chemistry, Vrije Universiteit: De
Boelelaan 1083, 1081 HVAmsterdam, The Netherlands (www.scm.com).
(22) (a) Patchkovskii, S.; Ziegler, T. J. Chem. Phys. 2001, 115, 26.
(b) Patchkovskii, S.; Ziegler, T. J. Chem. Phys. 2002, 116, 7806. (c) Patchkovs-
kii, S.; Ziegler, T. J. Phys. Chem. A 2002, 106, 1088.
(23) CPMD code, Version 3.11.1; Copyright IBM Corp. 1990-2006, Copy-
right MPI fur Festkorperforschung, Stuttgart, Germany, 1997-2001.
(24) Becke, A. D. Phys. Rev. A. 1988, 38, 3098.
(25) Lee, C. T.; Yang, W.; Parr, R. G. Phys. Rev. B. 1988, 37, 785.
(26) Troullier, N.; Martins, J. L. Phys. Rev. B. 1991, 43, 1993.
(27) Laasonen, K.; Pasquarello, A.; Car, R.; Lee, C.; Vanderbilt, D. Phys.
Rev. B. 1993, 47, 10142.
The dependences of kobs on the concentrations of lut and
lutHþ were determined graphically as described in the text. The
values of kobs presented are the average of at least three duplicate
experiments where the values of kobs all agree within 10%, and
this is reflected in the error bars presented in Figures 4, 7,
and 9.
Calculation Methods. MSINDO (Version 3.2.1)17-20 is a
method for quantum molecular dynamics at the level of the
Intermediate Neglect of Differential Overlap (INDO) semiem-
pirical self-consistent field (SCF) Molecular Orbital method
with MD calculations based on the lower Born-Oppenheimer