Kinetic evidence for intramolecular proton transfer between nickel and coordinated thiolate

Article abstract of DOI:10.1021/ic0104306

The complexes [Ni(YR)(triphos)]BPh₄ {Y = S, R = Ph or Et or Y = Se, R = Ph; triphos = (Ph₂PCH₂CH₂)₂PPh} have been prepared and characterized, and the X-ray crystal structure of [Ni(SPh)(triphos)]BPh₄ has been solved. In MeCN, [Ni(YR)(triphos)]⁺ are protonated by [lutH]⁺ (lut = 2,6-dimethylpyridine) to give [Ni(YHR)(triphos)]²⁺. Studies on the kinetics of these equilibrium reactions reveal an unexpected difference in the reactivities of [Ni(SPh)(triphos)]⁺ and [Ni(SEt)(triphos)]⁺. In both cases, the reactions exhibit a first-order dependence on the concentration of complex. When R = Ph, the dependence on the concentrations of [lutH⁺] and lut is given by k_obs = k₁^Ph[lutH⁺] + k_-1^Ph[lut], which is typical of an equilibrium reaction where k₁^Ph and k_-1^Ph correspond to the forward and back reactions, respectively. Analogous behavior is observed for [Ni(SePh)(triphos)]⁺. However, for [Ni(SEt)(triphos)]⁺, the kinetics are more complicated, and k_obs = {k₁k₂[lutH⁺] + (k_-2 + k₂)}/(k₁[lutH⁺] + k-_-1[lut]), which is indicative of a mechanism involving two coupled equilibria in which the initial protonation of the thiolate is followed by a unimolecular equilibrium reaction that is assumed to involve the formation of an η²-EtS-H ligand. The difference in reactivity between the complexes with alkyl and aryl thiolate ligands is a consequence of the {Ni(triphos)}²⁺ site "leveling" the basicities of these ligands. The pK_a's of the PhSH and EtSH constituents coordinated to the {Ni(triphos)}²⁺ are 16.0 and 14.6, respectively, whereas the difference in pK_a's of free PhSH and EtSH differ by ca. 4 units. The pK_a of [Ni-(SeHPh)(triphos)]⁺ is 14.4. The more strongly σ-donating EtS ligand makes the {Ni(triphos)}²⁺ core sufficiently electron-rich that the basicities of the sulfur and nickel in [Ni(SEt)(triphos)]⁺ are very similar; therefore, the proton serves as a bridge between the two sites. The relevance of these observations to the proposed mechanisms of nickel-based hydrogenases is discussed.

Full text of DOI:10.1021/ic0104306

Inorg. Chem. 2002, 41, 1128−1135
Kinetic Evidence for Intramolecular Proton Transfer Between Nickel and
Coordinated Thiolate
William Clegg and Richard A Henderson*
Department of Chemistry, Bedson Building, UniVersity of Newcastle,
Newcastle-upon-Tyne, NE1 7RU, U.K.
Received April 24, 2001
The complexes [Ni(YR)(triphos)]BPh4 {Y ) S, R ) Ph or Et or Y ) Se, R ) Ph; triphos ) (Ph2PCH CH ) PPh}
2
2 2
have been prepared and characterized, and the X-ray crystal structure of [Ni(SPh)(triphos)]BPh4 has been solved.
In MeCN, [Ni(YR)(triphos)]+ are protonated by [lutH]+ (lut ) 2,6-dimethylpyridine) to give [Ni(YHR)(triphos)]2+. Studies
on the kinetics of these equilibrium reactions reveal an unexpected difference in the reactivities of [Ni(SPh)(triphos)]+
and [Ni(SEt)(triphos)]+. In both cases, the reactions exhibit a first-order dependence on the concentration of complex.
When R ) Ph, the dependence on the concentrations of [lutH ] and lut is given by kobs ) k1Ph[lutH ] + k-1Ph[lut],
+
+
Ph
which is typical of an equilibrium reaction where k1Ph and k-1 correspond to the forward and back reactions,
respectively. Analogous behavior is observed for [Ni(SePh)(triphos)]+. However, for [Ni(SEt)(triphos)]+, the kinetics
+
+
are more complicated , and kobs ) {k1k2[lutH ] + (k-2 + k2)}/(k1[lutH ] + k-1[lut]), which is indicative of a mechanism
involving two coupled equilibria in which the initial protonation of the thiolate is followed by a unimolecular equilibrium
reaction that is assumed to involve the formation of an η2-EtS−H ligand. The difference in reactivity between the
complexes with alkyl and aryl thiolate ligands is a consequence of the {Ni(triphos)}2+ site “leveling” the basicities
of these ligands. The pK ’s of the PhSH and EtSH constituents coordinated to the {Ni(triphos)}2+ are 16.0 and
a
14.6, respectively, whereas the difference in pK ’s of free PhSH and EtSH differ by ca. 4 units. The pK of [Ni-
a
a
(SeHPh)(triphos)]+ is 14.4. The more strongly σ-donating EtS ligand makes the {Ni(triphos)}2+ core sufficiently
electron-rich that the basicities of the sulfur and nickel in [Ni(SEt)(triphos)]+ are very similar; therefore, the proton
serves as a bridge between the two sites. The relevance of these observations to the proposed mechanisms of
nickel-based hydrogenases is discussed.
Introduction
The discovery of several diverse biological roles for nickel1
has led to a resurgence in interest in nickel thiolate
complexes.2,3 While much attention has been devoted to
synthesizing complexes that model the structure of nickel
centers in metalloenzymes, there are few studies mimicking
the proposed reactivity of these biological sites. One
elementary reaction of particular interest is the migration of
protons between sulfur and metal atoms that have been
proposed to play key roles in the action of the nickel-based
hydrogenases (Figure 1).
Figure 1. Intramolecular transfer of hydrogen from sulfur to metal as
proposed by the action of hydrogenase from theoretical studies.4,5
The movement of protons between metal and sulfur ligands
has been proposed in the actions of other metalloenzymes.4-6
Thus, both the nitrogenases7 and hydrogenases8 catalyze the
* Corresponding author. E-mail: r.a.henderson@ncl.ac.uk.
(1) Cammack, R. Bioinorganic Chemistry of Nickel; Lancaster, J. R., Ed.;
VCH: New York, 1989; Chapter 8 and references therein.
(2) Davies, S. C.; Evans, D. J.; Hughes, D. L.; Longhurst, S.; Sanders, J.
R. Chem. Commun. 1999, 1935.
(3) Sa´nchez, G.; Ruiz, F.; Serrano, J. L.; Ramirez de Arellano, M. C.;
Lo´pez, G. Eur. J. Inorg. Chem. 2000, 2185 and references therein.
(4) DeGioia, L.; Fantucci, P.; Guigliarelli, B.; Bertrand, P. Inorg. Chem.
1999, 38, 2658.
(5) Pavlov, M.; Siegbahn, P. E. M.; Blomberg, M. R. A.; Crabtree, R. H.
J. Am. Chem. Soc. 1998, 120, 548.
(6) Dance, I. G. Chem. Commun. 1998, 523 and references therein.
1128 Inorganic Chemistry, Vol. 41, No. 5, 2002
10.1021/ic0104306 CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/02/2002

Proton Transfer Between Nickel and Thiolate
Figure 2. Possible intramolecular and acid-base-catalyzed pathways for
the transfer of a proton between sulfur and metal.
reduction of protons to dihydrogen. The structures of the
active sites of these enzymes have been determined, and for
the Mo-based nitrogenase, this active site is a cluster
comprising seven Fe, nine S, and a single Mo (FeMoco).
The active site of the Ni-based hydrogenase is a binuclear
species in which the Ni and Fe are bridged by cysteinate
ligands. These structures give little indication of how protons
are reduced to dihydrogen, but the predominant sulfur
ligation at both sites has led to proposals that sulfur plays a
key role in the process (Figure 2).
Possible pathways for dihydrogen oxidation by hydroge-
nase have been investigated in quantum mechanical calcula-
tions, and a common feature in all of the studies is the
heterolytic cleavage of coordinated dihydrogen, followed by
proton transfer to a cysteinate sulfur. Specifically, in the Ni-
based hydrogenases, the proposed mechanism involves
binding of dihydrogen to iron with H- transfer to iron and
H+ transfer to the cysteinate ligand and subsequent proton
transfer to nickel, as shown in Figure 1. Although thiol and
hydride/thiolate complexes are known, there are few studies
that show that the hydrogen can move between metal and
sulfur.11-14 In those studies where there is evidence for such
transfer, no kinetic studies have been performed; conse-
quently, we have no basic understanding of the electronic
factors that facilitate this transfer nor any direct evidence
that the reaction is truly intramolecular. While the intramo-
lecular migration of protons between metal and ligand is a
reaction that is widespread among carbon-based ligands,9 this
pathway is less evident with more electronegative donor
atoms, where acid-base-catalyzed mechanisms may be
energetically more favorable.10
Figure 3. X-ray crystal structure of [Ni(SPh)(triphos)]+ with 50%
probability ellipsoids. Important dimensions associated with this cation are
listed in the text.
In this contribution, we report studies on the protonation
of [Ni(YR)(triphos)]+ (Figure 3 {Y ) S, R ) Ph or Et or Y
) Se, R ) Ph; triphos ) (Ph2PCH2CH2)2PPh} by [lutH]+
(lut ) 2,6-dimethylpyridine) as shown in eq 1, which shows
that the {Ni(triphos)}2+ site effectively levels the basicities
of alkyl and aryl thiolates. With the more electron-releasing
EtS ligand, this process has the effect that in [Ni(SEt)-
(triphos)]+, initial protonation at the sulfur is followed by
intramolecular equilibration. Evidence is presented that this
process involves the formation of an η2-EtS-H complex.
[Ni(YR)(triphos)]+ + [lutH+] h
[Ni(YHR)(triphos)]2+ + [lut] (1)
Experimental Section
All manipulations were performed under an atmosphere of
dinitrogen using Schlenk and vacuum lines or syringe techniques,
as appropriate. Lithium, sodium, triphos, lutidine, Me3SiCl, NaBPh4,
PhSH, EtSH, and NiCl2‚6H2O were purchased from Aldrich and
used as received.
All solvents were dried and distilled under dinitrogen im-
mediately prior to use. MeCN was distilled from CaH2, THF, from
Na/benzophenone, and diethyl ether, from Na.
The compounds [lutH]BPh4 and [lutD]BPh4 were prepared by
the methods reported in the literature.15 1H NMR studies on [lutD]-
BPh4 demonstrated that the material was only 70% D-labeled.
Preparation of Complexes. [NiCl(triphos)]BPh4. To a solution
of NiCl2‚6H2O (1.6 g; 6.7 mmol) in MeOH (ca. 20 mL) was added
a solution of triphos (3.5 g; 6.7 mmol) in a 50:50 toluene/MeOH
mixture, and the resulting red solution was stirred for 0.5 h. After
allowing all the solid to dissolve (ca. 1 h), the solvent was removed
in vacuo until a yellow solid started to precipitate from solution.
The solid was removed by filtration, washed with diethyl ether,
and then dried in vacuo.
(7) Evans, D. J.; Henderson, R. A.; Smith, B. E. In Bioinorganic Catalysis,
2nd ed.; Reedijk, J., Bouwman, E., Eds.; Marcel Dekker: New York,
1999; p 153 and references therein.
(8) Frey, M. Struct. Bonding (Berlin) 1998, 90, 97 and references therein.
(9) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 2nd ed.; Wiley & Sons: New York, 1994; p 161.
(10) Henderson, R. A. J. Chem. Soc., Dalton Trans. 1995, 503 and
references therein.
(11) Burrows, T. E.; Hills, A.; Hughes, D. L.; Lane, J. D.; Morris, R. H.;
Richards, R. L. J. Chem. Soc., Dalton Trans. 1991, 1813 and references
therein.
(12) Fandos, R.; Lanfranchi, M.; Otero, A.; Pellinghelli, M. A.; Ruiz, M.
J.; Terreros, P. Organometallics 1996, 15, 4725.
The solid ([NiCl(triphos)]Cl) was weighed and then dissolved
in the minimum amount of MeOH. A solution containing 1 mol
equiv of Na[BPh4] in MeOH was added to the MeOH solution
dropwise to produce a bright yellow solid of [NiCl(triphos)]BPh4.
(13) Hitchcock, P. B.; Hughes, D. L.; Maguire, M. J.; Marjani, K.; Richards,
R. L. J. Chem. Soc., Dalton Trans. 1997, 4747.
(14) Darensbourg, M. Y.; Liaw, W. F.; Riordan, C. G. J. Am. Chem. Soc.
1989, 111, 8051.
(15) Gro¨nberg, K. L. C.; Henderson, R. A.; Oglieve, K. E. J. Chem. Soc.,
Dalton Trans. 1998, 3093.
Inorganic Chemistry, Vol. 41, No. 5, 2002 1129

Clegg and Henderson
Table 1. Elemental Analysis and Spectroscopic Characterization of Nickel Complexes
formula
elemental analysisa
1H NMRb
31P NMRc
C
H
[Ni(SPh)(triphos)]BPh4
76.4
(76.0)
5.7
(5.7)
7.3-8.4 (m, triphos and PhS Ph)
6.5-7.2 (m, BPh4)
107.5 (t, JPP ) 36.5 Hz; mid P)
54.2 (d, JPP ) 36.5 Hz; end P)
3.0, 3.25 (br, -CH2-)
7.3-8.4 (m, triphos and PhSe Ph)
6.5-7.2 (m, BPh4)
3.0, 3.25 (br, -CH2-)
7.3-7.9 (m, triphos Ph)
6.6-7.0 (m, BPh4)
2.6, 2.8 (br, -CH2-)
0.7 (t, JHH ) 7.3 Hz, CH3)
1.9 (q, JHH ) 7.3 Hz, CH2CH3)
[Ni(SePh)(triphos)]BPh4
[Ni(SEt)(triphos)]BPh4
73.0
(76.0)
5.5
(5.7)
117.5 (t, JPP ) 36.5 Hz; mid P)
50.0 (d, JPP ) 36.5 Hz; end P)
74.8
(76.0)
5.6
(5.7)
113.1 (t, JPP ) 36.5 Hz; mid P)
64.7 (d, JPP ) 36.5 Hz; end P
a Calculated values shown in parentheses. b Shifts relative to TMS. c Shifts relative to TMP.
[Ni(YR)(triphos)]BPh4 (Y ) S, R ) Et or Ph; Y ) Se, R )
Ph). These complexes were all prepared by essentially the same
route. LiYR was prepared by the reaction between Li and RYH in
THF and isolated as a white solid. To a suspension of [NiCl-
(triphos)]BPh4 (0.5 g; 0.53 mmol) in THF (ca. 20 mL) was added
LiYR (0.35 g; 3.0 mmol). The color changed rapidly from bright
yellow to dark red, and the mixture became homogeneous. After
the solution was stirred for ca. 0.5 h, it was concentrated in vacuo
to ca. 10 mL. Addition of an excess of MeOH produced a dark red
microcrystalline solid. The solid was removed by filtration, washed
with MeOH, and then dried in vacuo.
Recrystallization of the complex was accomplished by dissolving
the solid in the minimum amount of THF and then adding a large
excess of MeOH. Leaving the solution undisturbed at room
temperature for 48 h produced well-formed crystalline needles.
These crystals were removed by filtration, washed with MeOH,
and dried in vacuo. Crystals grown in such a manner were suitable
for X-ray crystallographic analysis (vide infra).
handle air-sensitive solutions. All studies were conducted at 25.0
°C, a temperature that was maintained using a Grant LTD6G
recirculating thermostat tank.
The kinetics were studied in dry MeCN under pseudo-first-order
conditions, with [lutH+] and lut present in at least a 10-fold excess
over the concentration of complex. Mixtures of [lutH]BPh4 and lut
were prepared from stock solutions of the two reagents. All solutions
were used within 1 h of preparation.
Under all the conditions described herein, the absorbance-time
curves were of the form of a single exponential, with an initial
absorbance corresponding to that of [Ni(SR)(triphos)]2+ and a final
absorbance corresponding to that of the equilibrium mixture of [Ni-
(SR)(triphos)]+ and [Ni(SHR)(triphos)]2+. Typical absorbance-time
curves are shown in Figure 4. The associated rate constants (kobs
)
were determined by a computer fit to the exponential absorbance-
time curve. In all cases, the curve was an exponential for more
than four half-lives. The dependence of kobs on [lutH]+ and [lut]
were determined graphically, as illustrated in Figures 5 and 6.
X-ray Crystal Structure Determination of [Ni(SPh)(triphos)]-
BPh4. Crystal data for C64H58BNiP3S (formula weight 1021.59,
burgundy crystals, triclinic, space group P1): a ) 14.6807(11) Å,
b ) 18.6897(14) Å, c ) 20.9971(16) Å; R ) 111.050(2)°, â )
101.407(2)°, γ ) 96.042(2)°. V ) 5171.7(7) Å3, T ) 160 K, Z )
4, R(F, F2 > 2σ) ) 0.0431, Rw(F2, all data) ) 0.1044, GOF )
1.007. The data collection and structure determination followed
standard procedures using a Bruker AXS SMART CCD diffrac-
tometer and Mo KR radiation (θmax ) 28.6°, 44882 reflections
measured, 23463 unique, semiempirical absorption corrections were
based on symmetry-equivalents), direct methods, and full-matrix
least-squares refinement on all unique F2 values. The programs used
were Bruker SMART (data collection), SAINT (integration), and
SHELXTL (structure solution).
Results from microanalysis and spectroscopic characterization
of the complexes are presented in Table 1.
Characterization of the Products of the Protonation Reac-
tions. We have been unable to isolate the products [Ni(SHPh)-
(triphos)]{BPh4}2 and [Ni(SHEt)(triphos)]{BPh4}2 from the reac-
tions with [lutH]BPh4. This problem is, in part, due to the large
excess of [lutH]+ necessary to drive the equilibrium protonation to
completion. Using stronger acids such as anhydrous HCl and HBF4‚
OEt2 resulted in the dissociation of thiol because of either multiple
protonation of the complex or attack by the nucleophilic conjugate
base. Thus, in the reaction of [Ni(SPh)(triphos)]+ with HCl, [NiCl-
(triphos)]BPh4 was isolated and identified by elemental analysis
and 31P{1H} NMR spectroscopy: δ 113.1 (t, JPP ) 35.0 Hz; mid
P), 64.7 (d, JPP ) 35.0 Hz; terminal P). This product was shown to
be identical to an authentic sample of [NiCl(triphos)]BPh4.
1
Results and Discussion
We have characterized the product in solution using H NMR
spectroscopy. Solutions containing mixtures of [Ni(SEt)(triphos)]+
and a 20-fold excess of [lutH]+ in CD3CN exhibited a broad
peak at δ 3.8, which we tentatively attribute to the S-H group.16
The transfer of a proton between nickel and sulfur could,
in principle, occur by any of the three mechanisms shown
in Figure 2. In addition to the intramolecular pathway, there
is the direct route involving competitive protonation of nickel
and sulfur and an acid-base-catalyzed pathway involving
an intermediate in which both nickel and sulfur are proto-
nated. Analogous pathways have been discussed for nitrogen-
based ligands.7 Currently, there are no studies that allow us
to establish the relative merits of these pathways.
A major problem in investigating the mechanism of
protonation of metal thiolates and establishing the way in
which the proton moves about the complex is that S-H
bonds are sufficiently acidic that they undergo rapid exchange
In addition, the resonances of the ethyl group (δ 0.74 (t, JHH
)
7.4 Hz, CH3), 1.34 (q, JHH ) 7.4 Hz, CH2)) are significantly shifted
from those of [Ni(SEt)(triphos)]+ (Table 1). The 31P NMR spectrum
of the protonated species (δ 115 (t, JPP ) 36.5 Hz, mid P), 66
(d, JPP ) 36.5 Hz, end P) is little different from that of
[Ni(SEt)(triphos)]+.
Kinetic Studies. The kinetic studies on the compounds described
in this contribution were performed on an Applied Photophysics
stopped-flow SX.18V spectrophotometer, which was modified to
(16) Schlaf, M.; Lough, A. J.; Morris, R. H. Organometallics 1996, 15,
4423.
1130 Inorganic Chemistry, Vol. 41, No. 5, 2002

Proton Transfer Between Nickel and Thiolate
Figure 4. Typical absorbance-time curves for the reaction of [Ni(SPh)(triphos)]+ with [lutH]+ in MeCN at 25.0 °C, showing the effect of increasing
[lutH+]/[lut]. Curves were recorded at λ ) 350 nm and [lutH+] ) 5.0 mmol dm-3. Analysis of the magnitude of the absorbance changes from such curves
allows determination of K1SPh using the molar extinction coefficients of [Ni(SPh)(triphos)]+ (ꢀ ) 5.2 × 103 dm3 mol-1 cm-1) and [Ni(SHPh)(triphos)]2+
(ꢀ ) 1.8 × 103 dm3 mol-1 cm-1) at λ ) 350 nm.
Figure 5. Graph of kobs/[lut] versus [lutH+]/[lut] for the reaction of [Ni(SPh)(triphos)]+ with [lutH]+ in MeCN at 25.0 °C. The data points correspond to
[lutH+] ) 5.0 mmol dm-3, [lut] ) 1.0-30.0 mmol dm-3 (2); [lutH+] ) 10.0 mmol dm-3, [lut] ) 1.0-30.0 mmol dm-3 (9); and [lutH+] ) 1.0-40.0 mmol
dm-3, [lut] ) 5.0 mmol dm-3 (b). The line drawn is that defined by eq 2.
with protons in solution. Consequently, isotopic labeling
cannot be used to follow the course of the reaction, which
necessitates relying on kinetics to distinguish between the
pathways shown in Figure 2.
In this work, we have prepared structurally simple
complexes of the type [Ni(YR)(triphos)]+ and have shown
they are mononuclear species. We have also studied the
kinetics of reversible protonation of these complexes in
MeCN. Only the sulfur atoms in [Ni(SPh)(triphos)]+ and [Ni-
(SEt)(triphos)]+ contain lone pairs of electrons that can be
protonated, yet these complexes show surprisingly different
kinetic behavior. We attribute this difference to [Ni(SHEt)-
(triphos)]2+ undergoing intramolecular proton transfer be-
tween sulfur and nickel.
The Structure of [Ni(SPh)(triphos)]BPh4. One of the
major requirements of these studies is to keep the system as
uncomplicated as possible to simplify the interpretation of
the kinetics. To this end, it is clearly advantageous if the
complexes are mononuclear. The triphos ligand, with the
sterically demanding phenyl groups, has been employed to
discourage the thiolates from acting as bridging ligands and
thus aiding the formation of dimeric species. Confirmation
Inorganic Chemistry, Vol. 41, No. 5, 2002 1131

Clegg and Henderson
Figure 6. (Top) Graph of 1/kobs[lut] versus [lutH+]/[lut], at constant [lutH+], for the reaction of [Ni(SEt)(triphos)]+ with [lutH]+ in MeCN at 25.0 °C. The
data points correspond to [lutH+] ) 10.0 mmol dm-3, [lut] ) 1.0-20.0 mmol dm-3 (4); [lutH+] ) 20.0 mmol dm-3, [lut] ) 1.0-20.0 mmol dm-3 (0);
[lutH+] ) 2.5 mmol dm-3, [lut] ) 1.25-5.0 mmol dm-3 ([); [lutH+] ) 5.0 mmol dm-3, [lut] ) 1.25-5.0 mmol dm-3 (b); [lutH+] ) 10.0 mmol dm-3
,
[lut] ) 1.25-5.0 mmol dm-3 (2); and [lutH+] ) 20.0 mmol dm-3, [lut] ) 1.25-5.0 mmol dm-3 (9). (Bottom) Graph of 1/kobs[lut] versus [lutH+] at
constant [lutH+]/[lut] for the same reaction. Data points correspond to [lutH+]/[lut] ) 2.0 (9) and [lutH+]/[lut] ) 4.0 (b). All lines drawn are those defined
by eq 4.
that the complexes are indeed mononuclear was established
by the X-ray crystal structure of [Ni(SPh)(triphos)]BPh4. The
structure of the cation is shown in Figure 3. Although there
are few mononuclear Ni complexes with which to compare
this structure, there is nothing exceptional about the structural
parameters of this complex compared to those of other
thiophenolate complexes.
equal to or less than the ideal sp2 hybridized angle.17 Only
when steric hindrance becomes a problem is this angle greater
than 120°.
Reaction of [lutH]+ with [Ni(SPh)(triphos)]+. When
studied on a stopped-flow spectrophotometer, the reaction
between [Ni(SPh)(triphos)]+ and an excess of [lutH]+ and
[lut] in MeCN exhibits a single-exponential absorbance-
time curve. The magnitude of the absorbance change
increases with [lutH+]/[lut] (up to a maximum value), which
is typical of an equilibrium reaction (Figure 4). This behavior
is observed in all the reactions with [lutH]+ described in this
contribution.
The Ni center has a slightly distorted square-planar
geometry with P(1)-Ni-P(2) ) 86.41(2)°; P(2)-Ni-P(3)
) 85.82(2)°; P(3)-Ni-S(1) ) 89.76(3)°; P(1)-Ni-S(1) )
99.03(3)°; P(1)-Ni-P(3) ) 161.50(3)°, and P(2)-Ni-S(1)
) 173.89(3)°. The Ni-P(2) bond distance (2.1506(6) Å),
where the trans atom is sulfur, is shorter than the Ni-P(1)
(2.2101(7) Å) and Ni-P(3) (2.1858(7) Å) bond distances.
This effect has been observed before.17 Finally, the Ni-
S(1)-C(35) angle is 99.20(8)°, a value that also fits into
the generally observed pattern that the M-S-C angle is
The exponential shape of the absorbance-time curves
indicates that the reaction exhibits a first-order dependence
on the concentration of [Ni(SPh)(triphos)]+. Consistent with
this interpretation, experiments were performed in which the
concentration of the complex was varied ([Ni] ) 0.05-0.25
mmol dm-3) and the concentrations of [lutH]+ (10.0 mmol
dm-3) and lut (5.0 mol dm-3) were constant, but there was
no effect on kobs (0.22 ( 0.01s-1).
(17) Henderson, R. A.; Hughes, D. L.; Richards, R. L.; Shortman, C. J.
Chem. Soc., Dalton Trans. 1987, 1115.
1132 Inorganic Chemistry, Vol. 41, No. 5, 2002

Proton Transfer Between Nickel and Thiolate
The dependence of kobs on the concentrations of [lutH]+
and lut is shown in Figure 5, and analysis of the data yields
the rate law shown in eq 2.
of [Ni(SEt)(triphos)]+, and consistent with this behavior, the
value of kobs did not change in experiments in which the
concentration of [Ni(SEt)(triphos)]+ was varied from 0.05
to 0.25 mmol dm-3 at constant concentrations of [lutH]+
(10.0 mmol dm-3) and lut (5.0 mmol dm-3).
The dependence of the rate constant on the concentrations
of [lutH]+ and [lut] is more complicated than that observed
with [Ni(SPh)(triphos)]+. In particular, increasing [lutH+]/
[lut] results in a decrease in the rate. To analyze the data, it
is necessary to consider two conditions: (i) the variation of
the rate with [lutH+]/[lut] at constant [lutH+], for which the
top graph in Figure 6 shows a linear relationship between
1/kobs[lut] and [lutH+]/[lut] and (ii) the variation of the rate
with [lutH+] at constant [lutH+]/[lut], for which the bottom
graph in Figure 6 shows the variation of 1/kobs[lut] with
[lutH+].
-d[Ni(SPh)(triphos)+]
) {k1PhS[lutH+] +
dt
k-1PhS[lut]}[Ni(SPh)(triphos)+] (2)
This rate law is consistent with a single-step equilibrium
reaction18 involving proton transfer to and from the complex,
as shown in eq 3. Protonation is presumed to occur at one
of the lone pairs of electrons on the sulfur. Graphical analysis
PhS
PhS
of the data gives k1 ) 20 ( 2 dm3 mol-1 s-1 and k-1
) 5 ( 0.7 dm3 mol-1 s-1.
The equilibrium constant (K1PhS) for this protonation
PhS
reaction can be calculated from the kinetic data: K1
)
Analysis of these data shows that the rate law is described
by eq 4.
k1PhS/k-1PhS ) 4.0 ( 1.0. This value is in excellent agreement
with the value determined from spectrophotometric analysis
of the effect of [lutH+]/[lut] on the magnitude of the
-d[Ni(SEt)(triphos)+]
PhS
absorbance change (Figure 4), K1 ) 3.6.
)
dt
Because the acid strength of [lutH]+ is known in MeCN
(pKalut ) 15.4),19 pKaSPh() 16.0) of [Ni(SHPh)(triphos)]2+
can be calculated. Earlier work20 has shown that for free
PhSH in MeCN, pKa g 19.3. Thus, coordination of PhSH
to {Ni(triphos)}2+ renders it at least 4000 times more acidic.
The reaction of [Ni(SePh)(triphos)]+ with mixtures of
{(8.8 ( 1.0)[lutH+] + (1.0 ( 0.2)}[Ni(SEt)(triphos)+]
(1.8 ( 0.3) × 102[lutH+] + (1.2 ( 0.2) × 103[lut]
(4)
This rate law is consistent with a mechanism comprising
two coupled equilibria, in which the initial protonation of
[Ni(SEt)(triphos)]+ is followed by a second equilibrium
which comprises unimolecular steps.22 The mechanism
shown in eq 5 is consistent with this behavior. In line with
the studies on [Ni(SPh)(triphos)]+, we propose that the initial
protonation occurs at the ethyl thiolate ligand, which is
followed by the intramolecular equilibrium involving an η2-
EtS-H ligand. Effectively, this process is a partial proton
transfer to the nickel. The rate law associated with this
[lutH]+ and lut in MeCN exhibits analogous kinetics to that
SePh
of [Ni(SPh)(triphos)]+. k1
) 40 ( 3 dm3 mol-1 s-1 and
k-1SePh ) (4.2 ( 0.3) × 102 dm3 mol-1 s-1; hence, K1
)
SePh
9.5 × 10-2. Spectrophotometric analysis gives K1SePh ) (8.9
( 1.2) × 10-2, which is in good agreement with the value
calculated from the rate constants. Therefore, we calculate
pKaSePh ) 14.4 for [Ni(SeHPh)(triphos)]2+. That the coor-
dinated PhSeH is more acidic than the coordinated PhSH is
consistent with the difference in acidities of the free
molecules; however, coordinated PhSeH is only 2.5 times
more acidic than coordinated PhSH. The difference in
acidities of coordinated PhSH and PhSeH is markedly less
than the 1000-fold (at least) difference observed for the free
molecules21, indicating that the {Ni(triphos)}2+ core ef-
fectively levels the acidities of coordinated thiols. Studies
of the kinetics of protonation of [Ni(SEt)(triphos)]+ show
that this effect extends to alkanethiolates.
SEt
SEt
mechanism when k2 < k-2 is shown in eq 6, and
SEt
comparison with eq 4 gives k1 ) (1.8 ( 0.3) × 102 dm3
SEt
SEt
mol-1 s-1, k-1 ) (1.2 ( 0.2) × 103 dm3 mol-1 s-1, k2
SEt
) (0.05 ( 0.01) s-1, and k-2 ) (0.95 ( 0.01) s-1.
Reaction of [lutH]+ with [Ni(SEt)(triphos)]+. When
studied using stopped-flow spectrophotometry, the reaction
between [Ni(SEt)(triphos)]+ and mixtures of [lutH}+ and
{lut} is characterized by exponential absorbance-time
curves, the magnitude of which varies with [lutH+]/[lut] in
the same manner as does that in the studies with [Ni(SPh)-
(triphos)]+. The exponential shape of the absorbance-time
curves indicates a first-order dependence on the concentration
-d[{Ni(SEt)(triphos)+}]
)
dt
{k1SEt k2SEt [lutH+] + (k2SEt + k-2SEt)}[Ni(SEt)(triphos)+]
k1SEt [lutH+] + k-1SEt [lut]
(6)
We have been able to characterize the product [Ni(SHEt)-
(triphos)]2+ only in solution (see Experimental Section). We
have no direct evidence for the existence of [Ni(η2-EtSH)-
(triphos)]2+, which is not surprising when the kinetic data
are considered. The product distribution between [Ni(SHEt)-
(18) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms;
McGraw-Hill: New York, 1981; Chapter 3.
(19) Izutsu, K. Acid-Base Dissociation Constants in Dipolar Aprotic
SolVents; Blackwell Scientific: Oxford, 1990.
(20) Henderson, R. A.; Oglieve, K. E. J. Chem. Soc., Dalton Trans. 1999,
3927.
(21) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.;
Butterworth-Heinemann: Oxford, 1998; Chapter 3.
(22) Wilkins, R. G. Kinetics and Mechanisms of Reactions of Transition
Metal Complexes, 2nd ed.; VCH: Weinheim, 1991; p 5.
Inorganic Chemistry, Vol. 41, No. 5, 2002 1133

Clegg and Henderson
(triphos)]2+ and [Ni(η2-EtSH)(triphos)]2+ is independent of
the concentration of acid, with the η2-EtS-H species
representing only 5% of the mixture. Because of this
insurmountable problem in establishing an unambiguous
product-analysis technique, we will now consider in more
detail alternative interpretations of the kinetics of the reaction
between [Ni(SEt)(triphos)]+ and [lutH]+.
described in the mechanism shown in eq 5, where the proton
is always bound to sulfur and merely switches the interaction
with nickel on and off.
Although the results are consistent with movement of the
proton, it seems unlikely that they correspond to complete
proton transfer as shown in eq 8. Complete proton transfer
would result in a five-coordinate d6 NiIV species, which is
unprecedented. However, η2-thiol ligands have been observed
before. In complexes of the type [Fe(SR)(CO)3L] {L ) PEt3
or P(OEt)3}26, the site of protonation is sensitive to both L
and R, and with [Fe(SMe)(CO)3(PEt3)], an η2-MeS-H
species is formed.
While the rate law shown in eq 6 is consistent with the
mechanism shown in eq 5, it does not unambiguously
SEt
SEt
identify the k2 and k-2 steps as being associated with
the formation of an η2-EtS-H species. Certainly, the
structural simplicity of [Ni(SEt)(triphos)]+ limits the number
of alternatives, but one possible explanation for the observed
behavior is that the initial protonation of the thiolate is
followed by an isomerization of the square-planar [Ni(SEt)-
(triphos)]+ to the tetrahedral [Ni(SHEt)(triphos)]2+. Although
isomerizations between square-planar and tetrahedral forms
are well-known in NiII-phosphine complexes,23 we are
unaware of such reactions being catalyzed by protonation.
In addition, it is difficult to reconcile why [Ni(SHEt)-
(triphos)]2+ would undergo such an isomerization while [Ni-
(SHPh)(triphos)]2+ would not.
Electronic Factors Controlling Migration. The effect
that the {Ni(triphos)}2+ core has on the acidities of coordi-
nated thiols is fundamental to understanding why the proton
in [Ni(SHEt)(triphos)]2+ interacts with nickel but not with
[Ni(SHPh)(triphos)]2+. Earlier in the discussion, we calcu-
lated pKaSPh ) 16.0 for [Ni(SHPh)(triphos)]2+ and com-
mented on how similar this value is to that of [Ni(SeHPh)-
(triphos)]2+ (pKa ) 14.4). From the kinetic analysis of the
Our proposal that the k2SEt and k-2SEt steps are associated
with movement of the proton is consistent with the results
SEt
from studies using [lutD]+ (70% D-labeled) that show k1
,
k-1SEt, and k2 are all associated with appreciable primary
isotope effects. Analysis of the data from the reaction
between [Ni(SEt)(triphos)]+ and [lutD]+ yields the rate law
given in eq 7.
SEt
SEt
[Ni(SEt)(triphos)]+ system, we calculate K1SEt ) k1SEt/k-1
) 0.15, and hence pKaSEt ) 14.6 for [Ni(SHEt)(triphos)]2+.
It is surprising that free PhSH is a 104 times stronger acid27
than free EtSH, but when PhSH and EtSH are coordinated
to {Ni(triphos)}2+, the acid strengths are similar. If we
consider [Ni(SeHPh)(triphos)]2+, the effect of coordination
on acidity is even more marked. While free PhSeH is a 107
times stronger acid than free EtSH, when coordinated to {Ni-
(triphos)}2+, it is only 1.6 times more acidic!
-d[Ni(SEt)(triphos)+]
)
dt
{(3 ( 0.2)[lutD+] + (1.0 ( 0.2)}[Ni(SEt)(triphos)+]
(7)
(1 ( 0.2) × 102[lutD+] + (7 ( 0.5) × 102[lut]
Comparison of eqs 6 and 7 gives (k1SEt)D ) (1 ( 0.2) ×
102 dm3 mol-1 s-1, (k-1SEt)D ) (7 ( 0.5) × 102 dm3 mol-1
s-1, (k2SEt)D ) (0.03 ( 0.01) s-1, and (k-2SEt)D ) (0.97 (
0.01) s-1. Comparison with the results using [lutH]+ gives
Coordination to the {Ni(triphos)}2+ site effectively levels
the acidities of the PhSH, EtSH, and PhSeH ligands. This
leveling must have its origins in the bonding between nickel
and thiols.
(k1SEt)H/(k1SEt)D ) 1.8, (k-1SEt)H/(k-1SEt)D ) 1.7, (k2SEt)H/(k2
SEt)D
) 1.7, and (k-2SEt)H/(k-2SEt)D ) 0.98. The primary isotope
effects observed for k1SEt and k-1SEt are consistent with proton
transfer between [Ni(SEt)(triphos)]+ and [lutH]+ but do not,
of course, define the site of protonation. However, a key
result is that k2 is associated with an appreciable isotope
effect, which indicates that k2 is involved with proton
Thiolate ligands are good σ-donors, and the better electron-
releasing capability of alkyl groups over aryl groups makes
EtS a better σ-donor ligand than PhS. It seems likely that
the electron density imparted to the {Ni(triphos)}2+ site by
the thiolate is dissipated by π-back-bonding to the phospho-
rus atoms, leading to the effective leveling of acidities of
coordinated thiols and resulting in the acidities of coordinated
PhSeH, PhSH, and EtSH varying by less than a factor of
40. The {Ni(triphos)}2+ core’s leveling of the acidities of
the thiols must have a complementary effect on the electron-
richness of the {Ni(triphos)}2+ site. It seems reasonable that
the electron-releasing EtS ligand must make the {Ni-
(triphos)}2+ site in [Ni(SEt)(triphos)]+ more electron-rich
than it does in [Ni(SPh)(triphos)]+. Thus, the basicities of
SEt
SEt
movement. This result is consistent with the proposed
pathway shown in eq 5.24 The small value of (k-2SEt)H/
(k-2SEt)D means that we cannot be confident that this effect
is a true inverse isotope effect. Inverse isotope effects are
expected to play a role in the transfer of a proton from metal
(low υM-H; ca. 1900 cm-1) to sulfur (high υS-H; ca. 2600
cm-1).25 However, there is no precedent for the movement
(23) Tobe, M. L.; Burgess, J. Inorganic Reaction Mechanisms; Longman
Group: Harlow, U.K., 1999; Chapter 5 and references therein.
(24) Bigeleisen, J. Pure Appl. Chem. 1964, 8, 217.
(25) Parkin, G.; Bercaw, J. E. Organometallics 1989, 8, 1172 and references
therein.
(26) Darensbourg, M. Y.; Liaw, W.-F.; Riordan, C. G. J. Am. Chem. Soc.
1989, 111, 8051.
(27) www.cem.msu.edu/∼reusch/OrgPage/acidity2.htm.
1134 Inorganic Chemistry, Vol. 41, No. 5, 2002

Proton Transfer Between Nickel and Thiolate
the nickel and sulfur sites are sufficiently similar so that in
[Ni(SHEt)(triphos)]2+ the proton effectively bridges the two
sites.
transfer to [Ni(SPh)(triphos)]+, which, in part, is probably
because the EtS ligand is sterically less demanding than the
PhS ligand. Therefore, EtS presents less hindrance to the
approaching [lutH]+ than does PhS.
The transfer of a proton from sulfur to metal has been
proposed before. In addition to the [Fe(η2-MeS-H)(CO)3-
(PEt3)] complexes discussed above,26 the so-called oxidative
addition of RSH to [IrCl(CO)(PPh3)2] has been proposed to
involve a three-center Ir-H-SR transition state in which
the lengthening of the S-H bond is synchronous with the
binding of these two atoms to iridium.28 More recently, a
so-called agostic Os-H-S interaction has been proposed
for the reaction of thiols29 with [Os3(CO)11(NCMe)], and
proton transfer between sulfur and hydride30 has been
observed in [Os(η2-H2)(CO)(quS)(PPh3)2]+ (quS ) quinoline-
8-thiolate).
Rates of Protonation of Sulfur in [Ni(SR)(triphos)]+.
The studies reported in this contribution have resulted in the
determination of the rate constants for proton transfer to [Ni-
(YR)(triphos)]+ by [lutH]+ and from [Ni(YHR)(triphos)]2+
to [lut]. In all cases, the rate of proton transfer is appreciably
slower than the diffusion-controlled limit (kdiff ) 1 × 1010
dm3 mol-1 s-1), even though stereochemical lone pairs of
electrons are available on the sulfur.31 The most likely reason
these proton-transfer reactions are so slow is that unfavorable
steric interactions occur upon approach of [lutH]+ (or lut)
to [Ni(YR)(triphos)]+ (or its conjugate acid). The steric
problems arise from the methyl groups on {lut} and the
phenyl groups on the triphos ligand. Although detailed
discussion of the rate constants is premature at this stage,
two features indicate that steric factors are important in
defining the rates of these reactions. First, the size of Y
appears to be important in defining the rate constant. Thus,
although protonation of [Ni(SePh)(triphos)]+ is thermody-
namically less favorable than protonation of [Ni(SPh)-
(triphos)]+, the rate constant for protonation of the former
by [lutH]+ is twice that of the latter. Presumably, this result
is a consequence of less congestion in the transition state of
[Ni(SePh)(triphos)]+ upon proton transfer with the larger Se
atom. Second, the rate of proton transfer to [Ni(SEt)-
(triphos)]+ is ca. 10 times faster than the rate of proton
Relevance to the Action of Hydrogenases. Recent
mechanistic proposals concerning the action of the Ni-based
hydrogenases4,5,8 have involved intramolecular proton trans-
fers between cysteinate sulfur and metal atoms (Ni or Fe),
as outlined in Figure 1. In this contribution, we have reported
kinetic studies on some simple Ni-thiolate complexes that
are pertinent to these discussions. In particular, we have
shown that the initial protonation of [Ni(SR)(triphos)]+
occurs at a lone pair of electrons on the sulfur (most basic
site). This result is in line with the observation that
protonation at the metal is usually kinetically slower than
protonation of a lone pair of electrons.32,33 With [Ni(SPh)-
(triphos)]+, no further reaction occurs, but with [Ni(SEt)-
(triphos)]+, the more electron-releasing alkanethiolate ligand
increases the basicity of the nickel, and it is proposed that
the proton interacts with both the nickel and sulfur sites.
The studies in this contribution were performed in MeCN,
and the reported pKa’s correpond to this solvent. The question
is what behavior would be expected in water? Using the
relationship shown in eq 9,34 it can be estimated that the
pKa’s of [Ni(YHR)(triphos)]2+ in water would fall in the
range 7-8, indicating that the reactions discussed herein
would be physiologcally relevant.
pKa(H2O) ) pKa(MeCN) - 7.5
(9)
Thus, under physiological conditions, a thiolate sulfur is
always the initial site of protonation, but in the presence of
coordinated alkanethiolates (e.g., cysteinate), partial proton
transfer (i.e., formation of η2-RS-H) to the nickel could
ensue.
Supporting Information Available: Listings of kinetic data
and crystallographic data. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC0104306
(28) Gaylor, J. R.; Senoff, C. V. Can J. Chem. 1972, 50, 1868.
(29) Kiriakidou, K.; Plutino, M. R.; Prestopino, F.; Monari, M.; Johansson,
M.; Elding, L. I.; Valls, E.; Gobetto, R.; Aime, S.; Nordlander, E.
Chem. Commun. 1998, 2721.
(30) Schlaf, M.; Morris, R. H. J. Chem. Soc., Chem. Comm. 1995, 625.
(31) Eigen, M. Angew. Chem., Int. Ed. Engl. 1964, 1, 3 and references
therein.
(32) Henderson, R. A. Angew. Chem., Int. Ed. Engl. 1996, 35, 946, and
refs therein.
(33) Kramarz, K. W.; Norton, J. R. Prog. Inorg. Chem. 1994, 42, 1 and
references therein.
(34) Kristjansdottir, S. S.; Norton, J. R. In Transition Metal Hydrides:
Recent AdVances in Theory and Experiment; Dedieu, A., Ed.; VCH
Publishers: New York, 1992; Chapter 9 and references therein.
Inorganic Chemistry, Vol. 41, No. 5, 2002 1135