Proton Transfer to Nickel-Thiolate Complexes. 2. Rate-Limiting Intramolecular Proton Transfer in the Reactions of [Ni(SC6H 4R-4)(PhP{CH2CH2PPh2} 2)]+ (R = NO2, Cl, H,

Article abstract of DOI:10.1021/ic0303237

The protonation of [Ni(SC₆H₄R-4)(triphos)] ⁺ (triphos = PhP{CH₂CH₂PPh₂} ₂; R = NO₂, Cl, H, Me, or MeO) by [lutH]⁺ (lut = 2,6-dimethylpyridine) to form [Ni(S

Full text of DOI:10.1021/ic0303237

Inorg. Chem. 2004, 43, 3106−3115
Proton Transfer to Nickel−Thiolate Complexes. 2. Rate-Limiting
Intramolecular Proton Transfer in the Reactions of
[Ni(SC H R-4)(PhP{CH CH PPh2}2)]+ (R ) NO , Cl, H, Me, or MeO)
6 4
2
2
2
,†
Valerie Autissier,† Pedro Martin Zarza,‡ Athinoula Petrou,§ Richard A. Henderson,*
Ross W. Harrington,† and William C. Clegg†
Department of Chemistry, School of Natural Sciences, UniVersity of Newcastle, Newcastle upon
Tyne, NE1 7RU U.K., Departamento de Qu´ımica Inorga´nica, UniVersidad de La Laguna,
38200 La Laguna, Tenerife, Spain, and Laboratory of Inorganic Chemistry, UniVersity of Athens,
Panepistimioupolis, Athens, 15771 Greece
Received November 14, 2003
The protonation of [Ni(SC H R-4)(triphos)]+ (triphos ) PhP{CH CH PPh2}2; R ) NO , Cl, H, Me, or MeO) by
6
4
2
2
2
[lutH]+ (lut ) 2,6-dimethylpyridine) to form [Ni(S(H)C H R-4)(triphos)]2+ is an equilibrium reaction in MeCN. Kinetic
6
4
studies, using stopped-flow spectrophotometry, reveal that the reactions occur by a two-step mechanism. Initially,
[lutH]+ rapidly binds to the complex (K R) in an interaction which probably involves hydrogen-bonding of the acid
2
to the sulfur. Subsequent intramolecular proton transfer from [lutH]+ to sulfur (k3R) is slow because of both electronic
and steric factors. The X-ray crystal structures of [Ni(SC H R-4)(triphos)]+ (R ) NO , H, Me, or MeO) show that
6
4
2
all are best described as square-planar complexes, with the phenyl substituents of the triphos ligand presenting an
appreciable barrier to the approach of the sterically demanding [lutH]+ to the sulfur. The kinetic characteristics of
the intramolecular proton transfer from [lutH]+ to sulfur have been investigated. The rate of intramolecular proton
transfer exhibits a nonlinear dependence on Hammett σ+, with both electron-releasing and electron-withdrawing
4-R-substituents on the coordinated thiolate facilitating the rate of proton transfer (NO > Cl > H> Me < MeO). The
2
rate constants for intramolecular proton transfer correlate well with the calculated electron dnsity of the sulfur. The
q
temperature dependence of the rate of the intramolecular proton transfer reactions shows that ∆H is small but
q
increases as the 4-R-substituent becomes more electron-withdrawing {∆H ) 4.1 (MeO), 6.9 (Me), 11.4 kcal
q
q
mol-1 (NO )}, while ∆S becomes progressively less negative {∆S ) −50.1 (MeO), −41.2 (Me), −16.4 (NO ) cal
2
2
K-1 mol-1}. Studies with [lutD]+ show that the rate of intramolecular proton transfer varies with the 4-R-substituent
{(k3NO ) /(k3 ) ) 0.39; (k3 ) /(k3 ) ) 0.88; (k3Me)H/(k3Me)D ) 1.3; (k3MeO)H/(k3MeO)D ) 1.2}.
H
NO
D
Cl H
Cl D
2
2
Introduction
by the 4-R-substituent. (iii) The rates of proton transfer show
a marked nonlinear dependence on the nature of the 4-R-
substituent, with both electron-withdrawing and electron-
releasing substituents facilitating proton transfer. (iv) The
values of ∆Hq and ∆Sq for proton transfer2 show a marked
dependence on the nature of the 4-R-substituent, consistent
with a transition state for proton transfer in which the extent
of proton transfer is affected by the 4-R-substituent. (v) With
[Ni(SC6H4NO2-4)2(dppe)] the kinetics indicate a two-step
process in which initial hydrogen-bond formation between
In the previous paper1 we described mechanistic studies
on the equilibrium proton transfer reactions of [Ni(SC6H4R-
4)2(dppe)] (R ) MeO, Me, H, Cl, or NO2; dppe ) Ph2PCH2-
CH2PPh2) with mixtures of [lutH]+ and lut (lut ) 2,6-
dimethylpyridine). We showed that the reactions were
characterized by the following features. (i) Proton transfer
rates are slow. (ii) The pKa’s of the corresponding [Ni-
(SHC6H4R-4)(SC6H4R-4)(dppe)]+ are only slightly affected
* To whom correspondence should be addressed. E-mail: r.a.henderson@
ncl.ac.uk.
(1) Accompanying paper: Autissier, V. A.; Clegg, W.; Harrington, R.
W.; Henderson, R. A. Inorg. Chem. 2004, 43, 3098-3105.
(2) (a) Marcus, R. A. J. Am. Chem. Soc. 1969, 91, 7224. (b) Albery, W.
J. Faraday Discuss. Chem. Soc. 1982, 74, 245.
† University of Newcastle.
‡ Universidad de La Laguna.
§ University of Athens.
3106 Inorganic Chemistry, Vol. 43, No. 10, 2004
10.1021/ic0303237 CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/20/2004

Proton Transfer to Nickel-Thiolate Complexes
the complex and [lutH]+ is followed by intramolecular proton
transfer.
Table 1. Elemental Analysis and Spectroscopic Characteristics of
[Ni(SC6H4R-4)(triphos)]+ (R ) NO2, Cl, Me, or MeO)
elemental analysis/%a
NMR spectroscopy
31Pd
Recently,3 we reported studies on the kinetics of proton
transfer from [lutH]+ to the thiolate sulfur of [Ni(SR)-
(triphos)]+ (R ) Ph or Et), as shown in eq 1. Proton transfer
from [lutH]+ to these complexes is relatively slow, and for
[Ni(SEt)(triphos)]+, initial protonation of the sulfur is fol-
lowed by an intramolecular reaction, which we have tenta-
tively attributed to the formation of an η2-EtSH ligand.
R
C
H
N
1Hb,c
NO2
71.4
(71.9)
68.4
(68.6)
71.2
(71.0)
69.8
(70.0)
5.7
(5.3)
5.8
(5.1)
5.5
(5.5)
5.9
(5.4)
1.3
(1.3)
109.8 (t, JPP ) 41.0 Hz);
52.4 (d, JPP ) 41.0 Hz)
107.7 (t, JPP ) 41.6 Hz);
54.1 (d, JPP ) 42.0 Hz)
106.0 (t, JPP ) 41.5 Hz);
53.8 (d, JPP ) 41.3 Hz)
106.3 (t, JPP ) 41.5 Hz);
54.1 (d, JPP ) 41.8 Hz)
Cl
Me
MeO
1.91
3.43
+
[Ni(SR)(triphos)]+ y[lutH] z [Ni(SHR)(triphos)]2+
h
a Calculated values shown in parentheses. b Chemical shifts relative to
TMS. c Peaks due to triphos ligands are present in all spectra at δ 7.0-8.0
(multiplets, Ph groups) and δ 2.2-3.0 (broad, CH2). In addition, peaks due
to [BPh4]- are present in all spectra at δ 6.5-6.9 (multiplets, Ph groups).
d Chemical shifts relative to H3PO4.
lut
[Ni(η2-RS-H)(triphos)]2+ (1)
In this paper, we report further studies on the reaction
between [Ni(SC6H5)(triphos)]+ and [lutH]+ in MeCN, which
reveal that at high concentrations of acid a more complicated
rate law becomes evident. This more complicated rate law
is consistent with a two-step mechanism involving initial
adduct formation between [lutH]+ and the complex, followed
by intramolecular proton transfer to produce [Ni{S(H)C6H5}-
(triphos)]2+. Additional studies on [Ni(SC6H4R-4)(triphos)]+
(R ) NO2, Cl, Me, or MeO) show that the two-step
mechanism is entirely general for this class of complex.
Because the kinetics allow us to determine the rate constant
for the intramolecular proton transfer, we can, for the first
time, systematically investigate the characteristics of this
previously unexplored reaction type, including how the 4-R-
substituent affects the rates of intramolecular proton transfer,
together with the activation parameters and isotope effects.
with methanol, then diethyl ether, and dried in vacuo. The product
was recrystallized from a dichloromethane/methanol mixture. Yield
) 0.32 g (58%).
X-ray Crystallography. All data were collected on a Bruker
SMART CCD area diffractometer, using Mo KR radiation (λ )
0.71073 Å), by the ω-scan method.6 Crystal data and other
experimental information are given in Table 2, with further details
in the Supporting Information. Semiempirical absorption corrections
were applied in all cases, on the basis of repeated and symmetry-
equivalent reflections.6 The structures were solved by direct methods
and refined by full-matrix least-squares on all unique F2 values.7
Anisotropic displacement parameters were assigned to all the non-
hydrogen atoms. Hydrogen atoms were placed in idealized positions
and allowed to ride on their respective parent atoms. The methyl-
and nitro-substituted complexes are THF solvates; H atoms were
not included on the THF solvent molecules, which show high
thermal motion and are probably somewhat disordered. Although
the methyl- and methoxy-substituted complexes appear to be
isomorphous, the methoxy derivative is unsolvated; there is 2-fold
disorder [refined occupancies 0.712:0.288(2)] for the orientation
of the thiolate ligands. The largest peaks in final difference syntheses
lie close to solvent and disordered atoms, and close to Ni in the
benzyl derivative.
Experimental Section
All preparations and manipulations were routinely performed
under an atmosphere of dinitrogen using Schlenk or syringe
techniques as appropriate. All solvents were dried and freshly
distilled from the appropriate drying agent immediately prior to
use.
The thiols 4-RC6H4SH (R ) NO2, Cl, Me, or MeO) and lut (lut
) 2,6-dimethylpyridine) were purchased from Aldrich and used
Selected bond lengths and angles for the three substituted
benzenethiolate complexes are reported in Table 4.
5
as received. NaSC6H4R-4,4 [NiCl(triphos)]BPh4,3 and [lutH]BPh4
Kinetic Studies. All kinetic studies were performed using an
Applied Photophysics SX.18MV stopped-flow spectrophotometer,
modified to handle air-sensitive solutions. The temperature was
maintained at 25.0 ( 0.1 °C using a Grant LT D6G thermostated
recirculating pump.
All solutions were prepared under an atmosphere of dinitrogen
and transferred by gastight, all-glass syringes into the stopped-flow
spectrophotometer. Solutions of mixtures of lut and [lutH]BPh4
were prepared from freshly prepared stock solutions and used within
1 h.
and [lutD]BPh4 were prepared by the literature methods.
Preparation of [Ni(SC6H4R-4)(triphos)]BPh4 (R ) NO2, Cl,
Me, or MeO). The complexes of the series [Ni(SC6H4R-4)-
(triphos)]+ were all prepared by the same method3 which is
analogous to that described earlier for [Ni(SC6H5)(triphos)]+. The
complexes were characterized by elemental and spectroscopic
analysis (Table 1), and (for R ) MeO, Me, or NO2) by X-ray
crystallography. A typical preparation is described below for [Ni-
(SC6H4Me-4)(triphos)]BPh4.
A slurry of [NiCl(triphos)]BPh4 (0.5 g, 0.53 mmol) in THF (ca.
30 mL) was stirred rapidly while NaSC6H4Me (0.37 g, 2.5 mmol)
was added. The mixture rapidly turned from a yellow slurry to a
red homogeneous solution. After stirring at room temperature for
1 h, the solution was concentrated to ca. 10 mL, and then, an excess
of methanol (ca. 60 mL) was added. Red crystals of the product
slowly formed. The crystals were removed by filtration, washed
The kinetics were studied under pseudo-first-order conditions8
as described in the previous paper.1 The high acidity of [lutH]+
(pKa ) 15.4 in MeCN)9,10 makes it impossible to entirely eliminate
(6) SMART (control), SAINT (integration), GEMINI (twinning), and
SADABS (absorption correction) software; Bruker AXS Inc.: Madison,
WI, 2001.
(7) Sheldrick, G. M. SHELXTL version 6; Bruker AXS Inc.: Madison,
WI, 2001.
(8) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms;
McGraw-Hill: New York, 1981.
(3) Clegg, W.; Henderson, R. A. Inorg. Chem. 2002, 41, 1128.
(4) Palermo, R. E.; Power, P. P.; Holm, R. H. Inorg. Chem. 1982, 21,
173.
(5) Gro¨nberg, K. L. C.; Henderson, R. A.; Oglieve, K. E. J. Chem. Soc.,
Dalton Trans. 1998, 3093.
(9) Cauquis, G.; Deronzier, A.; Serve, D.; Vieil, E. J. Electroanal. Chem.
Interfacial Electrochem. 1975, 60, 205.
Inorganic Chemistry, Vol. 43, No. 10, 2004 3107

Autissier et al.
Table 2. Crystal Data and Structure Refinement Parameters for [Ni(SC6H4R-4)(triphos)]BPh4 (R ) Me, MeO, or NO2) and
[Ni(SCH2C6H5)(triphos)]BPh4
[Ni(SC6H4Me-4)-
(triphos)]BPh4‚THF
[Ni(SC6H4OMe-4)-
(triphos)]BPh4
[Ni(SC6H4NO2-4)-
(triphos)]BPh4‚THF
[Ni(SCH2C6H5)-
(triphos)]BPh4
chemical formula
fw
cryst syst
space group
C69H68BNiOP3S
1107.72
triclinic
P1h
C65H60BNiOP3S
1051.62
triclinic
P1h
C68H65BNNiO3P3S
1138.70
triclinic
P1h
160
C65H60BNiP3S
1035.62
triclinic
P1h
T, K
160
160
150
a, Å
b, Å
c, Å
11.5726(5)
14.5465(6)
19.0208(8)
76.816(2)
84.394(2)
68.235(2)
2895.0(2)
2
11.5060(6)
14.4698(7)
19.4916(10)
74.030(2)
84.610(2)
67.813(2)
2888.7(3)
2
13.6285(6)
14.2556(6)
16.8955(7)
100.676(2)
100.032(2)
114.412(2)
2819.6(2)
2
12.3581(11)
14.4506(13)
17.4416(15)
79.922(1)
69.366(1)
66.256(1)
2666.2(4)
2
R, deg
â, deg
γ, deg
V, Å3
Z
reflns measured
unique data, Rint
params
24492
12859, 0.0217
686
0.0407
0.1099
24709
12924, 0.0299
731
0.0565
0.1654
24027
12567, 0.0195
703
0.0414
0.1136
19197
9315, 0.0592
640
0.0567
0.1590
R [F2 > 2σ]
Rw (F2, all data)
absolute structure params
GOF [F2]
1.036
0.83, -0.66
1.074
1.45, -0.40
1.042
1.52, -1.36
1.022
1.32, -0.82
max, min electron density (e/Å3)
controlled limit (kdiff ) 1 × 1010 dm3 mol-1 s-1).11 The
question then arises whether proton transfer to and from
sulfur donor atoms on ligands is inherently slow, or if there
is something else causing this decrease in rate specifically
in this complex. We, and others,12,13 have studied the
protonation reactions of a variety of complexes and clusters
containing sulfur-based ligands. In general, proton transfer
to the sulfur sites is rapid. Even proton transfer to µ3-S atoms
in Fe-S-based clusters14 is appreciably faster than observed
in these nickel complexes. Consequently, it would appear
that protonation of [Ni(SC6H5)(triphos)]+ is slow, either
because the proton transfer is thermodynamically unfavor-
able, or because the sterically demanding [lutH]+ has
problems getting sufficiently close to the sulfur to transfer
the proton.
Table 3. Summary of Elementary Rate Constants and Equilibrium
Constants for the Reactions of [Ni(SC6H4R-4)(triphos)]+ with [lutH]+
and lut in MeCN at 25.0 °C
R
D
R
K2R/dm3 mol-1 k3R/s-1 k-3R/dm3 mol-1 s-1 (k3R)H/(k3
)
NO2
Cl
H
>160
>160
200
7.0
0.19
0.1
1 × 104
0.39
0.88
6
4
Me
MeO
>160
0.05
0.072
1.5
2.5
1.3
1.2
55.5
all protic impurities in CD3CN, but 1H NMR spectroscopic studies
show that the acid is at least 90% deuterium-labeled.
Results and Discussion
In the preceding paper,1 kinetic studies on the reactions
of [Ni(SC6H4R-4)2(dppe)] (R ) MeO, Me, H, Cl, or NO2)
with [lutH]+ in the presence of lut indicated that the
mechanism involved initial formation of a hydrogen-bonded
adduct which subsequently undergoes intramolecular proton
transfer. However, only with the R ) NO2 derivative do we
observe kinetics which allow us to determine the rate of the
intramolecular proton transfer within the hydrogen-bonded
adduct. In all the other derivatives a simpler rate law is
observed. Consequently, we were unable to probe the factors
influencing, and characterizing, the rates of intramolecular
proton transfer. In the analogous studies on [Ni(SC6H4R-4)-
(triphos)]+ (R ) MeO, Me, H, Cl, or NO2) with [lutH]+
reported herein, all complexes exhibit kinetics which allow
us to determine the rate constants for intramolecular proton
transfer and allow us to define the kinetic characteristics of
this type of reaction.
H
[Ni(SC6H5)(triphos)]+ + [lutH}+ yk1
z
H
k-1
[Ni{S(H)C6H5}(triphos)]2+ + lut (2)
-d[Ni(SC6H5)(triphos)+]
)
dt
{k1 [lutH+] + k-1H[lut]}[Ni(SC6H5)(triphos)+] (3)
H
In general, the rate constant for protonation of nitrogen
or oxygen atoms occurs at the diffusion-controlled limit for
thermodynamically favorable reactions and is smaller by a
factor of 10∆pK for the reverse (thermodynamically unfavor-
a
able) reaction.11 It is to be expected that the protonation of
Kinetics and Mechanism. Earlier studies3 indicated that
the protonation of [Ni(SC6H5)(triphos)]+ by [lutH]+ in MeCN
is an equilibrium reaction {eq 2} associated with the simple
(11) Bell, R. P. The Proton in Chemistry, 2nd ed.; Chapman and Hall:
London, 1973; Chapter 7.
(12) (a) Wander, S. A.; Reibenspies, J. H.; Kim, J. S.; Darensbourg, M. Y.
Inorg. Chem. 1994, 33, 1421. (b) Allan, C. B.; Davidson, G.;
Choudhury, S. B.; Gu, Z. J.; Bose, K.; Day, R. O.; Maroney, M. J.
Inorg. Chem. 1998, 37, 4166.
H
rate law shown in eq 3, and k1 ) 20 ( 2 dm3 mol-1 s-1,
H
k-1 ) 5 ( 0.7 dm3 mol-1 s-1. These rate constants for
proton transfer are appreciably slower than the diffusion-
(13) Henderson, R. A.; Oglieve, K. E. J. Chem. Soc., Dalton Trans. 1999,
3927.
(10) Izutsu, K. Acid-Base Dissociation Constants in Dipolar Aprotic
SolVents; Blackwell-Scientific: Oxford, 1990; p 17.
(14) Bell, J.; Dunford, A. J.; Hollis, E.; Henderson, R. A. Angew. Chem.,
Int. Ed. 2003, 42, 1149.
3108 Inorganic Chemistry, Vol. 43, No. 10, 2004

Proton Transfer to Nickel-Thiolate Complexes
Table 4. Summary of Bond Lengths and Bond Angles in [Ni(SC6H4R-4)(triphos)]+ (R ) NO2, H, Me, or MeO)
bond length
bond angle
R
Ni-Pa
Ni-Pt
Ni-Pr
Ni-S
Pa-Ni-S
Pt-Ni-S
Pr-Ni-S
Pa-Ni-Pr
Ni-S-C
Ha
2.2101(7)
2.2106(6)
2.2072(6)
2.2066(6)
2.2009(9)
2.1506(6)
2.1394(6)
2.1436(6)
2.1397(5)
2.1349(8)
2.1858(7)
2.1987(7)
2.2406(6)
2.2025(5)
2.1986(9)
2.2456(7)
2.1642(6)
2.1942(6)
2.1685(6)
2.1770(12)
99.03(3)
103.99(2)
88.38(2)
86.76(2)
86.88(4)
173.89(3)
163.40(3)
169.13(2)
163.86(2)
159.70(5)
89.76(3)
89.47(2)
102.47(2)
106.68(2)
107.95(4)
161.50(3)
159.79(3)
168.57(2)
161.15(2)
159.41(4)
99.20(8)
115.47(8)
112.55(7)
121.04(7)
115.93(17)
NO2
Me
MeOb
a Two independent molecules in the asymmetric unit. b Major disorder component only.
Figure 2. Mechanism for the reaction of [Ni(SC6H4R-4)(triphos)]+ (R )
NO2, Cl, Me. or MeO) with [lutH]+ in the presence of lut (solvent ) MeCN),
involving initial hydrogen bonding of [lutH]+ to the sulfur followed by
proton transfer.
[lutH]+ with [Ni(SC6H5)(triphos)]+, but without proton trans-
fer. Subsequent intramolecular proton-transfer produces the
coordinated thiol. Since both the nickel complex and the acid
are cationic, the initial interaction cannot be electrostatic in
origin. It seems more likely that the initial interaction in-
volves hydrogen-bonding, in which the protic end of [lutH]+
hydrogen-bonds to the thiolate sulfur. In addition, this hydro-
gen-bonding could be augmented by aromatic π-π stack-
ing16 of the phenyl groups of the phosphine and the [lutH]+.
Consideration of the mechanism shown in Figure 2 gives
the rate law shown in eq 5. The corresponding dependence
of kobs on [lutH+] and [lut] is shown in eq 6, from which eq
7 is readily derived. Equation 7 is the basis of the graph
shown in Figure 1. Comparison of eqs 4 and 5 gives the
values of the elementary rate constants for the proton-transfer
reactions between [Ni(SC6H5)(triphos)]+ and [lutH]+ shown
in Table 3.
Figure 1. Kinetics for the reaction of [Ni(SC6H5)(triphos)]+ with [lutH]+
in the presence of lut (solvent ) MeCN) at 25.0 °C. Data points correspond
to [lutH+] ) 2.5 mmol dm-3, [lut] ) 1-5 mmol dm-3 (b); [lutH+] ) 5.0
mmol dm-3, [lut] ) 1-20 mmol dm-3 ([); [lutH+] ) 12.5 mmol dm-3
,
[lut] ) 1-20 mmol dm-3 (2); and [lutH+] ) 25.0 mmol dm-3, [lut] )
2.5-40 mmol dm-3 (9). Lines drawn are those defined by eq 4.
other atoms containing stereochemical lone pairs of electrons
would follow the same reactivity pattern.
The kinetics of the reaction shown in eq 2 have now been
studied over a wide concentration range of [lutH]+. Over
this extended range, a complicated rate law becomes evident.
The kinetic data for the reaction of [Ni(SC6H5)(triphos)]+
with [lutH]+ in the presence of lut are shown in Figure 1.
The data show that, when [lutH+] is constant, kobs/[lut]
exhibits a linear dependence on the ratio [lutH+]/[lut].
However, at high concentrations of [lutH]+, increasing the
concentration of acid leads to a decrease in rate. Analysis
of the data shown in Figure 1 results in the rate law shown
in eq 4.
-d[Ni(SC6H4R-4)(triphos)+]
dt
K2 k3R [lutH+]
R
)
+
+
R
{
1 + K2 [lutH ]
k-3R[lut] [Ni(SC6H4R-4)(triphos)+] (5)
}
H
K2 k3H [lutH+]
-d[Ni(SC6H5)(triphos)+]
dt
20[lutH+]
kobs
)
+ k-3H[lut]
(6)
1 + K2 [lutH+]
H
)
+
+
{
1 + 200[lutH ]
Equation 5 describes the full rate law for the mechanism
4[lut] [Ni(SC6H5)(triphos)+] (4)
R
}
shown in Figure 2. However, when K2 [lutH+] < 1, eq 5
Equation 4 is consistent with a mechanism15 involving two
coupled equilibria, as illustrated in Figure 2. In this mech-
anism the initial step in the reaction involves interaction of
(15) Wilkins, R. G. Kinetics and Mechanism of Reactions of Transition
Metal Complexes; VCH: Weinheim, 1991; pp 33-37.
(16) Martin, N.; Bu¨nzli, J. C.; McKee, V.; Piguet, C.; Hopfgartner, G. Inorg.
Chem. 1998, 37, 577 and references therein.
Inorganic Chemistry, Vol. 43, No. 10, 2004 3109

Autissier et al.
kobs K2 k3H [lutH+]/[lut]
H
factors which hold the acid and base sufficiently far apart
that the proton transfer is slow because it has to travel a
long distance.17,18 Earlier studies on the reactions between
[Ni(SC6H4R-4)2(dppe)] and [lutH]+ indicate that electronic
factors dominate the reactivity. Thus, it is only with the R
) NO2 derivative that intramolecular proton transfer is
detected.1 With complexes containing more electron-releasing
substituents, the intramolecular proton transfer cannot be
distinguished from the initial formation of the hydrogen-
bonded adduct. However, all [Ni(SC6H4R-4)(triphos)]+ de-
rivatives show kinetics in which intramolecular proton
transfer within a hydrogen-bonded adduct can be observed.
Certainly, it seems reasonable that the sulfur in [Ni(SC6H4R-
4)(triphos)]+ would be more electron-poor than in the
corresponding [Ni(SC6H4R-4)2(dppe)] for two reasons. First
the complexes are cationic, and second there is a higher
proportion of phosphorus coordination. Phosphines are good
π-electron-acceptors and so will effectively denude the sulfur
of electron density. However, the triphos ligand contains the
sterically demanding phenyl substituents which could be a
barrier to the approach of the sterically demanding [lutH]+
to the sulfur.
We have determined the X-ray crystal structures of
selected [Ni(SC6H4R-4)(triphos)]+ complexes. We have
chosen to determine the structures of R ) NO2, Me, and
MeO complexes, since these represent 4-R-substituents with
markedly different influences and the extremes of electronic
effects studied herein. The X-ray crystal structure of [Ni-
(SC6H5)(triphos)]+ was reported by us earlier.3 Comparison
of the structures of [Ni(SC6H4R-4)(triphos)]+ (R ) NO2, Me,
or MeO) with that of [Ni(SC6H5)(triphos)]+ shows that in
all complexes the nickel geometry is best described as
distorted square-planar. Figure 4 shows the structure of [Ni-
(SC6H4NO2-4)(triphos)]+, which is typical of all the com-
plexes reported herein. While Figure 4 shows the atom
numbering scheme for [Ni(SC6H4NO2-4)(triphos)]+, it is
convenient in the following discussion to introduce a more
general nomenclature for the phosphorus atoms (illustrated
in Figure 5), which focuses on the position of the phosphorus
atoms with respect to the lone pairs of electrons on sulfur
(the basic site). The two terminal phosphorus atoms are
labeled Pr (for the phosphorus remote from the lone pair of
electrons on the sulfur) and Pa (for the phosphorus adjacent
to the lone pair of electrons on the sulfur), while Pt is the
unique secondary phosphorus.
H
)
+ k-3
(7)
1 + K2 [lutH+]
H
[lut]
simplifies to eq 8. Equation 8 is identical in form to that
observed in the earlier studies with [Ni(SC6H5)(triphos)]+
H
and [lutH]+ (eq 3) with k1H ) K2 k3H and k-1H ) k-3H. There
is good agreement between the values obtained earlier3 and
those observed in this study at the lower concentrations of
[lutH]+ (Table 3).
kobs ) K2 k3 [lutH+] + k-3H[lut]
(8)
H
H
Studies on the reactions of all [Ni(SC6H4R-4)(triphos)]+
(R ) NO2, Cl, Me, or MeO) with [lutH]+ in MeCN show
that the kinetics are consistent with the mechanism shown
in Figure 2. For R ) MeO the observed rate law is analogous
to that shown in eq 5. However, for R ) Me, Cl, or NO2,
the rate law is simpler.
The kinetics of the reactions between [Ni(SC6H4R-4)-
(triphos)]+ (R ) Me, Cl, or NO2) and [lutH]+ are similar,
but not identical, to those described above for the R ) MeO
or H analogues. The behavior for the R ) Me, Cl, or NO2
derivatives is typified by the data shown in Figure 3. Thus,
at a constant concentration of [lutH]+, kobs/[lut] varies linearly
with [lutH+]/[lut], and increasing the concentration of [lutH+]
decreases the rate. However, under all conditions, varying
the concentration of [lutH]+ leads to a linear change in the
gradient of the plot of kobs/[lut] against [lutH+]/[lut]. These
kinetics are also consistent with the mechanism illustrated
R
in Figure 2, and the rate law shown in eq 5. When K2 [lutH+]
> 1, eq 5 simplifies to eq 9. The graph corresponding to eq
9 is shown in Figure 3 (insert), from which the values of k3
and k-3 presented in Table 3 were determined.
R
R
R
kobs
k3
R
)
+ k-3
(9)
[lut] [lut]
Thus, for [Ni(SC6H4R-4)(triphos)]+, when R ) MeO or
H, the full form of the rate law {eq 5}is observed with the
equilibrium constant for the initial binding of the complex
and [lutH]+, K2R ) 55-200 dm3 mol-1. For the derivatives
R
where R ) Me, Cl, or NO2, K2 > 160 dm3 mol-1.
Consequently, for R ) Me, Cl, or NO2, under all the
experimental conditions we have employed in this study
([lutH+] > 12.5 mmol dm-3), all the nickel complex is bound
to [lutH]+. Furthermore, for all the derivatives the slow step
in the reaction is the intramolecular transfer of a proton from
the [lutH]+ to the sulfur within the hydrogen-bonded adduct.
The values of K2 , k3 , and k-3R will be considered in a later
section, after discussing the structural features of [Ni-
(SC6H4R-4)(triphos)]+. It is pertinent at this stage to consider
why intramolecular proton transfer within a hydrogen-bonded
adduct should be slow.
Electronic and Steric Factors Influencing Intramolecu-
lar Proton Transfer. There are two possible reasons why
the rate of the intramolecular proton transfer in the reactions
of [Ni(SC6H4R-4)(triphos)]+ with [lutH]+ is slow. Either the
basicity of the thiolate sulfur is very poor or there are steric
The major bond length and bond angles for [Ni(SC6H4R-
4)(triphos)]+ (R ) H, NO2, Me, or MeO) are shown in Table
4. It is evident that there is a significant degree of flexibility
in the structural parameters for these complexes. Earlier
structural studies19 on thiolate complexes indicated that a
metal-S-C bond angle of ca. 120° was consistent with the
coordinated thiolate being under steric strain. In the structures
of all [Ni(SC6H4R-4)(triphos)]+ the Ni-S-C angle falls in
the range 99-121°. Inspection of the structures of [Ni-
R
R
(17) Eigen, M. Angew. Chem., Int. Ed. Engl. 1964, 3, 1.
(18) Eigen, M. Z. Phys. Chem. (Frankfurt) 1954, 1, 176.
(19) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.
C.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, 51.
3110 Inorganic Chemistry, Vol. 43, No. 10, 2004

Proton Transfer to Nickel-Thiolate Complexes
Figure 3. Main: Kinetics for the reaction of [Ni(SC6H4Cl-4)(triphos)]+ with [lutH]+ in the presence of lut (solvent ) MeCN) at 25.0 °C. Data points
correspond to [lutH+] ) 12.5 mmol dm-3, [lut] ) 1.0-50.0 mmol dm-3 (b); [lutH+] ) 25.0 mmol dm-3, [lut] ) 1.0-50.0 mmol dm-3 (2); and [lutH+]
) 50.0 mmol dm-3, [lut] ) 1.0-50.0 mmol dm-3 (9). Lines drawn are those defined by eq 7 and the values shown in the text. Insert: The plot of kobs/[lut]
against 1/[lut] for the reaction of [Ni(SC6H4Cl-4)(triphos)]+ with [lutH]+ and lut in MeCN at 25.0 °C, using the same data as in the main figure.
Figure 5. Diagrammatic representation of the limiting geometries showing
the orientations of the lone pairs of electrons on the sulfur atom in [Ni-
(SC6H4R-4)(triphos)]+. In the perspective view drawn on the left-hand side,
the nickel atom is “hidden” behind the lone pair.
thiolate can adopt shows there are two limiting orientations
of the lone pairs of electrons as shown in Figure 5. It is
Figure 4. Molecular structure of the cation [Ni(SC6H4NO2-4)(triphos)]+
evident from Figure 5 that approach of [lutH]+ could involve
showing the atom numbering scheme and 50% probability ellipsoids. H
close contact with the phenyl groups on the terminal phos-
phorus atoms. It seems intuitively reasonable that approach
of [lutH]+ should take the line of least interaction, which is
perpendicular to the square-planar complex. Approach of
[lutH]+ from above the NiSP3 plane is only possible if (i)
there is rotation of the thiolate around the Ni-S bond so
that one of the lone pairs of electrons is pointing directly in
the correct orientation and (ii) the lone pair of electrons on
sulfur and the phenyl group on Pt are on opposite sides of
the plane. This optimized orientation is shown on the left-
hand side of Figure 5. Even if these conditions are satisfied,
the approach of the sterically demanding [lutH]+ to the sulfur
is difficult. The result is that proton transfer from [lutH]+ to
sulfur in [Ni(SC6H4R-4)(triphos)]+ is likely to have to occur
over an appreciable distance to a poorly basic sulfur, resulting
in slow intramolecular proton transfer reaction.
atoms are omitted for clarity.
(SC6H4R-4)(triphos)]+ indicates that the phenyl-groups on
Pa and Pt impose significant congestion around the lone-pairs
of electrons on sulfur, making the approach of the sterically
demanding [lutH]+ to the sulfur site problematical. It seems
likely that both electronic and steric factors contribute to
making the rate of intramolecular proton transfer slow in
[Ni(SC6H4R-4)(triphos)]+ complexes.
As shown in Figure 4, the lone pairs of electrons on sulfur
are effectively “buried” in the encapsulating phenyl groups.
This becomes particularly important when we consider the
trajectory that [lutH]+ must adopt in order to protonate the
sulfur. The N-H‚‚‚S atoms need to be collinear, with the
proton approaching one of the lone pairs of electrons.
Consideration of the configurations that the coordinated
Inorganic Chemistry, Vol. 43, No. 10, 2004 3111

Autissier et al.
distinguishable from proton transfer then the kinetics would
follow the rate law shown in eq 5. Since the kinetic data for
the reaction of [Ni(SCH2C6H5)(triphos)]+ with [lutH]+ show
no evidence for departure from the simple kinetics defined
by eq 11, even at the highest concentration of [lutH]+ used
([lutH+] ) 50 mmol dm-3), we can establish from eq 5 that
K2Bn[lutH+] e 0.1, and that K2 g 2 dm3 mol-1.
Bn
[Ni(SCH2C6H5)(triphos)]+ + [lutH]+ yk1
z
Bn
k-1
[Ni{S(H)CH2C6H5)(triphos)]2+ + lut (11)
The X-ray crystal structure of [Ni(SCH2C6H5)(triphos)]-
BPh4 is shown in Figure 6 (top), and the bond lengths and
bond angles for this cation, which are decribed in the legend,
are very similar to those of [Ni(SC6H4R-4)(triphos)]+. It is
particularly noteworthy that the dimensions associated with
the {Ni(triphos)}2+ core in the benzyl-derivative is extremely
similar to that in the aryl-derivatives. It is the disposition of
the phenyl-groups on the triphos ligand which represent the
most important barrier to approach of the [lutH]+. Thus, the
difference in the kinetics is most reasonably attributable to
the difference in the basicities of the sulfur sites in the benzyl-
and aryl-thiolates. It would be anticipated that the alkanethio-
late ligand would be more basic than the aryl thiolate ligand.
If steric factors dominated the rates of proton transfer to
sulfur in these complexes, then the benzyl and aryl deriva-
tives would show similar kinetics.
Figure 6. Top: Molecular structure of [Ni(SCH2C6H5)(triphos)]+ with 50%
probability displacement ellipsoids and without H atoms. Selected dimen-
sions and angles are Ni-P(1) ) 2.2042(10) Å; Ni-P(2) ) 2.1960(10) Å;
Ni-P(3) ) 2.1468(10) Å; Ni-S ) 2.1689(10) Å; P(1)-Ni-P(2) ) 161.90-
(4)°; P(1)-Ni-S ) 103.95(4)°; P(2)-Ni-S ) 89.66(4)°; P(3)-Ni-S )
164.01(4)°; Ni-S-C(1) ) 115.18(13)°. Bottom: Kinetic data for the
reaction of [Ni(SCH2C6H5)(triphos)]+ with [lutH]+ in the presence of lut
Isotope Effects. The isotope effects for the reactions
between [Ni(SC6H4R-4)(triphos)]+ (R ) NO2, Cl, or MeO)
and [lutD]+ have been determined, allowing us to investigate
the effect of changing the basicity of the sulfur site on the
kinetic isotope effect associated with proton transfer. In order
to ensure that, for all complexes, we were measuring the
kinetic isotope effect associated with the elementary reaction
of intramolecular proton transfer, all experiments were
performed with [lutH+] > 25 mmol dm-3 so that all the
nickel complex is hydrogen-bonded to [lutH+].
in MeCN at 25.0°. Data points correspond to [lutH+] ) 12.5 mmol dm-3
,
[lut] ) 1.0-40.0 mmol dm-3 (9); [lutH+] ) 25.0 mmol dm-3, [lut] )
1.0-40.0 mmol dm-3 (2); [lutH+] ) 50.0 mmol dm-3, [lut] ) 2.5-40.0
mmol dm-3 (b).
The discussion presented above indicates that in the [Ni-
(SC6H4R-4)(triphos)]+ system both electronic and steric
factors could contribute to slow intramolecular proton transfer
rates. However, studies on the reaction between [lutH]+ and
[Ni(SCH2C6H5)(triphos)]+ indicate that electronic factors
dominate in controlling the rate of intramolecular proton
transfer.
The reaction between [Ni(SCH2C6H5)(triphos)]+ and mix-
tures of [lutH]+ and lut exhibit simple kinetics as shown in
Figure 6 (bottom) and described by eq 10.
R
The kinetic isotope effect for k3 varies with the 4-R-
substituent. The results of the experiments with [lutD]+ are
illustrated in Figure 7 and are summarized for all the
complexes in Table 3.
Inspection of the results in Table 3 shows that, in general,
complexes containing the most electron-withdrawing 4-R-
substituent (R ) NO2) exhibit an inverse kinetic isotope
NO2
NO2 D
effect, (k3 )H/(k3
) ) 0.39, but as R becomes more
electron-releasing there is a change so that when R ) Me
or MeO a primary kinetic isotope effect is observed, (k3Me)H/
(k3Me)D ) 1.3, (k3MeO)H/(k3MeO)D ) 1.2. With [Ni(SC6H4Cl-
4)(triphos)]+, where the 4-R-substituent has an electronic
-d[Ni(SCH2C6H5)(triphos)+]
)
dt
{3.2[lutH+] + 2.0[lut]}[Ni(SCH2C6H5)(triphos)+] (10)
effect intermediate between those of MeO and NO2, we
Cl D
observe an intemediate kinetic isotope effect, (k3Cl)H/(k3
) 0.88.
)
Equation 10 is consistent with the simple, single step
Bn
equilibrium reaction shown in eq 11, with k1 ) 3.2 dm3
Bn
mol-1 s-1 and k-1 ) 2.0 dm3 mol-1 s-1 (the superscript
The isotope effects associated with the reverse reaction
(k-3R) are not easy to measure. The error in the intercepts of
Bn
Bn denotes benzyl). It is reasonable to assume that the k1
step comprises initial adduct formation (K2Bn) followed by
proton transfer (k3Bn). If adduct formation were kinetically
the graphs typified by those shown in Figure 6 means that it
R D
is difficult to confidently determine (k-3R)H/(k-3
)
ac-
3112 Inorganic Chemistry, Vol. 43, No. 10, 2004

Proton Transfer to Nickel-Thiolate Complexes
Figure 7. Kinetic data showing the isotope effects for the reactions between [Ni(SC6H4R-4)(triphos)]+ and [lutX]+ (X ) H or D) in the presence of lut
(solvent ) MeCN) at 25.0 °C. Main: R ) NO2; [lutH+] ) 25.0 mmol dm-3, [lut] ) 0.1-1.0 mmol dm-3 (2), studies with [lutD]+ are shown as open
symbols. Insert: R ) MeO; [lutH+] ) 25.0 mmol dm-3, [lut] ) 1.0-40.0 mmol dm-3 (2), studies with [lutD]+ are shown as open symbols.
curately. However, inspection of the graphs indicates that
It is unexpected that the intramolecular proton transfer
reactions are associated with a variety of kinetic isotope
effects. Previous theoretical and experimental studies20-22
have indicated that kinetic deuterium isotope effects may
become inverse when (i) the product has a very strong
vibrational force constant compared to the reactant, and the
transition state is product-like, or (ii) a stepwise sequence
involving transfer of the proton (to an atom with which it
vibrates at a higher frequency) occurs prior to the rate-
limiting step. Our results indicate that, in intramolecular
proton transfer reactions within a hydrogen-bonded adduct,
an inverse kinetic isotope effect is observed when the base
is relatively weak and the transition state is reactant-like,
while a normal kinetic isotope effect is observed with
stronger bases, and the transition state is product-like.
Temperature Effects. We have investigated the effect of
temperature on the rate of the reaction between [lutH]+ and
[Ni(SC6H4R-4)(triphos)]+ and, hence, determined the activa-
tion parameters (∆Hq and ∆Sq) for the intramolecular proton
transfer.
the kinetic isotope effect associated with k-3R is smaller than
R
that for k3 .
A simplistic picture of the intramolecular proton transfer
step being monitored in these experiments involves [lutH]+
hydrogen-bonding to the sulfur. The position of the proton
within the hydrogen-bonded adduct, and the transition state
for proton transfer, is influenced only by the basicity of the
sulfur which is modulated by the 4-R-substituent. The steric
constraints imposed by the phenyl groups of the triphos are
reasonably assumed to be essentially the same for all [Ni-
(SC6H4R-4)(triphos)]+. It seems likely that, with the most
electron-releasing 4-R-substituent (MeO), in the ground state,
the proton is closest to the sulfur and furthest from the ni-
trogen. At the other extreme, with the most electron-with-
drawing 4-R-substituent (NO2), the proton is still predomi-
nantly associated with the lut residue. It seems reasonable
that there is a spectrum of such configurations, with the
position of the proton in the ground state varying with the
electronic influence of the 4-R-substituent.
Eyring plots comparing the temperature dependences of
the reactions of [lutH]+ with [Ni(SC6H4Cl-4)(triphos)]+, [Ni-
(SC6H4NO2-4)(triphos)]+, and [Ni(SC6H4OMe-4)(triphos)]+
are shown in Figure 8, and the values of ∆Hq and ∆Sq are
Using the intuitive picture described above, together with
the experimental observations, it appears that intramolecular
transfer of a proton to the least basic sulfur sites is associated
with an inverse primary isotope effect. The apparent cor-
relation of (k3 ) /(k3 ) with the 4-R-substituent indicates
that as the sulfur site becomes more basic, the kinetic isotope
effect becomes larger.
R H
R D
(20) Parkin, G.; Bercaw, J. E. Organometallics 1989, 8, 1172.
(21) Bigeleisen, J. Pure Appl. Chem. 1964, 8, 217.
(22) Melander, L. Acta Chem. Scand. 1971, 25, 3821.
Inorganic Chemistry, Vol. 43, No. 10, 2004 3113

Autissier et al.
Figure 9. Hammett plots for the reaction of [Ni(SC6H4R-4)(triphos)]+
(R ) NO2, Cl, H, Me, or MeO) with [lutH]+ in the presence of lut (solvent
) MeCN) at 25.0 °C. The data points correspond to k3R (b) and k-3R (9).
The curves are only trendlines and do not represent a mathematical fit to
the data.
Figure 8. Eyring plots for the temperature dependences of the reactions
between [Ni(SC6H4R-4)(triphos)]+ {R ) NO2 (b), Cl (9), or MeO (2)}
and [lutH]+ in the presence of lut (solvent ) MeCN).
Table 5. Activation Parameters for the Intramolecular Proton Transfer
Reaction between [Ni(SC6H4R-4)(triphos)]+ and [lutH]+
∆Hq/
∆Sq/
∆Gq
/
298
kcal
kcal cal K-1
mol-1 mol-1
k3/s-1
complex
mol-1 (at 25.0 °C)
[Ni(SC6H4OMe)(triphos)]+
[Ni(SC6H4Me-4)(triphos)]+
4.1
6.9
-50.1
-41.2
-16.4
19.0
19.2
16.3
0.07
0.05
7.0
[Ni(SC6H4NO2-4)(triphos)]+ 11.4
summarized in Table 5. There is a clear systematic trend
which parallels both the isotope effects observed with the
same compounds, and the trend observed in the protonation
reactions of [Ni(SC6H4R-4)2(dppe)] in the previous paper.1
As the 4-R-substituent becomes more electron-withdraw-
ing, ∆Hq increases and ∆Sq becomes less negative. The
marked variation in the values of ∆Hq and ∆Sq observed
with different 4-R-substituents deserves further comment. For
all complexes ∆Hq is small, while the corresponding values
of ∆Sq are negative. Both parameters show a marked
dependence on the 4-R-substituents.
Figure 10. Graph showing the linear variation of log(k3) (b) and log(k-3
(9) against the charge on the sulfur atom calculated using the Normal Bond
)
The magnitude of ∆Hq reflects the relatively large distance
that the proton has to move. That ∆Hq is largest and ∆Sq
least negative for complexes containing electron-withdrawing
4-R-substituents is consistent with the description presented
above in which the nitrogen-proton-sulfur configuration
in the hydrogen-bonded adduct depends on the electronic
influence of the 4-R-substituent. The reactions of complexes
with electron-releasing 4-R-substituents are associated with
the smallest ∆Hq since the thiolate is the most basic, and in
the ground state the proton is closer to the sulfur.
It is worth noting that ∆Sq is most negative in the reactions
of complexes containing electron-releasing 4-R-substituents
on the thiolate and becomes more positive as the 4-R-
substituent becomes more electron-withdrawing. This be-
havior indicates that in the reactions involving electron-
releasing 4-R-substituents the transition state is more
product-like (i.e., the proton is more transferred to the sulfur).
Simplistically, the negative ∆Sq in the reactions involving
complexes containing electron-releasing substituents is ra-
tionalizable, since the transition state for the proton transfer
in these systems involves the proton closer to the sulfur.
Consequently, the lut residue is only weakly bound to the
complex and thus has more degrees of freedom.
Order method.
Electronic Effects of 4-R-Substituents on Thiolate. A
semiquantitative measure of how 4-R-substituents on the
coordinated thiolate influence the rate of intramolecular
proton transfer is the Hammett plot shown in Figure 9. An
interesting feature of the rates of intramolecular proton
transfer described in this paper is that electron-withdrawing
4-R-substituents facilitate intramolecular proton transfer,
while electron-releasing 4-R-substituents have a less marked
influence on the rate.
Figure 9 shows a markedly nonlinear dependence of log-
R
(k3 ) on Hammett σ+. We have chosen to use the Hammett
σ+ since these parameters23 allow for the conjugation between
the 4-R-substituent and the rest of the molecule; this seems
to be particularly important for ligands coordinated to
transition metal sites.24
Basu et al.25 have recently pointed out that the Hammett
σ+ parameter only describes resonance effects and neglects
(23) Alder, R. W.; Baker, R.; Brown, J. M. Mechanism in Organic
Chemistry; Wiley: London, England, 1971; pp 28-39.
(24) Hussain, W.; Leigh, G. J.; Mohd Ali, H.; Pickett, C. J.; Rankin, D. A.
J. Chem. Soc., Dalton Trans. 1984, 1703.
(25) Sengar, R. S.; Nemykin, V. N.; Basu, P. New J. Chem. 2003, 7, 1115.
3114 Inorganic Chemistry, Vol. 43, No. 10, 2004

Proton Transfer to Nickel-Thiolate Complexes
inductive effects. Theoretical calculations indicate that the
HOMO in thiols and disulfides is delocalized between the
π-orbital of the benzene ring and the pz-orbital on the sulfur.
Electron-donating 4-R-substituents destabilize the energy of
the HOMO, and electron-releasing 4-R-substituents stabilize
the HOMO.26,27
The charge on the sulfur atom with various 4-RC6H4S-
derivatives has been calculated25 using the Natural Bond
Order package. We have used these calculated charges and
(as shown in Figure 10) shown that this correlates well with
the rates of intramolecular proton transfer. This correlation
indicates that it is the electron density on the sulfur which
controls the rate of proton transfer within the hydrogen-
bonded adduct.
the base distinguishable from the transfer of the proton. In
the majority of proton transfer reactions there is no ap-
preciable steric barrier to the acid-base pair getting suf-
ficiently close to result in rapid transfer of the proton. The
reason we can observe the hydrogen-bonding as a prequel
to proton transfer in this case is (i) because the base (sulfur)
is buried in the complex surrounded by bulky phenyl rings,
and the acid ([lutH]+) is sterically demanding and is held an
appreciable distance from the sulfur, so that proton transfer
has to occur across some distance and is consequently slow,
and (ii) because the aryl thiolate sulfur is poorly basic. In
the reactions of [Ni(SC6H4R-4)(triphos)]+ with [lutH]+ the
acid must not only migrate to the solvation shell of the
complex but also penetrate the array of phenyl groups which
surround the sulfur atom. Because intramolecular proton
transfer within the hydrogen-bonded adduct is rate-limiting
in the reactions of [Ni(SC6H4R-4)(triphos)]+ with [lutH]+,
we have been able to study how systematic electronic
changes to the basicity of the sulfur affect the rates, activation
parameters (∆Hq and ∆Sq), and kinetic isotope effects of this
elementary reaction.
Summary
The studies on [Ni(SC6H4R-4)(triphos)]+ described in this
paper, together with the results on [Ni(SC6H4R-4)2(dppe)]
reported in the preceding paper,1 indicate that the mechanism
for transfer of a proton from [lutH]+ to sulfur in these
complexes involves a two-step mechanism, in which an initial
hydrogen bonding interaction between the acid and complex
is followed by an intramolecular proton transfer reaction.
There is nothing unusual about this mechanism. Indeed, all
proton transfer reactions must involve this type of two-step
process. However, rarely is the initial binding of the acid to
Supporting Information Available: X-ray crystallographic files
in CIF format for the crystal structures and kinetic data for the
reactions of [Ni(SC6H4R-4)(triphos)] (R ) MeO, Me, H, Cl, or
NO2). This material is available free of charge via the Internet at
http://pubs.acs.org.
(26) McGuire, D. G.; Khan, M. A.; Ashby, M. T. Inorg. Chem. 2002, 41,
2202.
(27) Sellmann, D.; Sutter, J. Acc. Chem. Res. 1997, 30, 460.
IC0303237
Inorganic Chemistry, Vol. 43, No. 10, 2004 3115