Dalton Transactions
Paper
pKas (calculated from the kinetic data) need to be treated with
some caution. However, the rather small difference in the pKas
of coordinated alkyl and aryl thiols (ΔpKa ∼ 2) indicates that
NH⋯S hydrogen bonding in D would be relatively insensitive
to the nature of the coordinated thiolate and hence rather
minor non-bonding interactions associated with the encapsu-
lation of the bound lutH+ can make a relatively significant con-
tribution to the stability of D.
J. Phys. Chem., 1965, 69, 2287; (c) C. F. Bernasconi
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J. Org. Chem., 1993, 58, 217, and refs therein.
2 (a) R. A. Henderson, D. L. Hughes, R. L. Richards and
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R. W. Harrington and W. Clegg, Inorg. Chem., 2004, 43,
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R. W. Harrington and R. A. Henderson, Inorg. Chem., 2011,
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3 K. L. C. Grönberg, R. A. Henderson and K. E. Oglieve,
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4 K. S. Hagen and R. H. Holm, Inorg. Chem., 1984, 23, 418.
5 W. Clegg and R. A. Henderson, Inorg. Chem., 2002, 41,
1128.
6 (a) CrysAlisPro software, Agilent Technologies Ltd, Oxford,
UK, 2013; (b) SHELXTL, Bruker AXS Inc, Madison, Wiscon-
sin, USA, 2014; (c) G. M. Sheldrick, Acta Crystallogr., Sect. A:
Fundam. Crystallogr., 2008, 64, 112; (d) G. M. Sheldrick,
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Commun., 2015, 71, 9.
Conclusions
The studies described in this paper show that the different
kinetics observed for the equilibrium protonation reactions of
[Ni(SC6H4R′-4)(triphos)]+ and [Ni(SR)(triphos)]+ with mixtures
of lutH+ and lut are consistent with a single mechanism invol-
ving initial formation of a precursor hydrogen-bonded inter-
mediate {[Ni(thiolate)(triphos)]⋯Hlut}2+ (D) followed by
intramolecular proton transfer (Fig. 3). It might be expected
that D is most stable (highest KR1) with complexes containing
the most basic thiolates because this would increase the
strength of the NH⋯S hydrogen bonding in D. However, the
results presented herein show that quite the opposite is true,
and that changing the electronic character of the coordinated
thiolate results in a reactivity pattern which is not consistent
with hydrogen bonding being the only (or indeed the princi-
pal) factor stabilising D. Rather the electronic influences that
the thiolate has on KR1 indicate that an important factor stabi-
lizing D is the local environment created by the phenyl groups
of triphos which encapsulate the bound lutH+. This local
environment is modulated by the electronic characteristics of
the coordinated thiolate. The results and conclusions pre-
sented in this paper suggest that preparing elaborate ligands
(which generate extended cavities around reaction sites) could
be used to control the rates of proton transfer to coordinated
molecules and ions in transition metal complexes.
7 R. P. Bell, The Proton in Chemistry, Chapman and Hall,
London, 2nd edn, 1973.
8 A. Alwaaly and R. A. Henderson, Chem. Commun., 2014, 50,
9669.
9 Note that the 4-R′-substituted phenyl group in [Ni(SC6H4R′-
4)(triphos)]+ is not involved in creating the cavity where
lutH+ binds.
10 J. Hwang, P. Li, W. R. Carroll, M. D. Smith, P. J. Pellechia
and K. D. Shimizu, J. Am. Chem. Soc., 2014, 136, 14060, and
refs therein.
Acknowledgements
11 C. Cauquis, A. Deronzier, D. Serve and E. Vieil, J. Electro-
anal. Chem. Interfacial Electrochem., 1975, 60, 205.
12 Advanced Chemistry Development (ACD/Labs) Software
V11.02 (© 1994–2014 ACD/Labs).
13 The calculated pKa in water was obtained from ref. 12. The
pKa in MeCN was then calculated using the equations in,
B. G. Cox, Acids and Bases Solvent Effects on Acid-Base
Strength, Oxford University Press, UK, 2013.
A.A. thanks the Iraqi Ministry of Higher Education and Scienti-
fic Research for a studentship and A.L.P. thanks the University
of Athens for study leave. We thank EPSRC for a crystallo-
graphy equipment grant (EP/F03637X/1).
Notes and references
1 (a) M. Eigen, Angew. Chem., Int. Ed. Engl., 1964, 3, 1; 14 B. L. Westcott, N. E. Gruhn and J. H. Enemark, J. Am.
(b) M. M. Kreevoy, D. S. Sappenfield and W. Schwabacher,
Chem. Soc., 1998, 120, 3382.
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