New insights into the mechanism of methane formation in the protonation of methyl complexes

Article abstract of DOI:10.1039/a907080j

The reaction between [NiMe{Ph₂PCH₂CH₂)₂PPh}]⁺ and anhydrous HCl (in MeCN) involves initial protonation of nickel to form [Ni(H)Me{{Ph₂PCH₂CH₂)₂PPh}]^2+<

Full text of DOI:10.1039/a907080j

New insights into the mechanism of methane formation in the protonation of
methyl complexes†
Richard A. Henderson* and Kay E. Oglieve
Nitrogen Fixation Laboratory, John Innes Centre, Norwich Research Park, Colney, Norwich, UK NR4 7UH.
E-mail: r.a.henderson@ncl.ac.uk
Received (in Cambridge, UK) 1st September 1999, Accepted 7th October 1999
The reaction between [NiMe{(Ph2PCH2CH2)2PPh}]+ and
anhydrous HCl (in MeCN) involves initial protonation of
nickel to form [Ni(H)Me{(Ph2PCH2CH2)2PPh}]2+, but meth-
ane is not produced until after additional, direct protonation
of the methyl group.
There have been many investigations into the formation of
alkanes in the reaction of acid with metal–alkyl complexes,1–3
and the consensus view is that the mechanism involves either
direct protonation of the metal–carbon bond or initial protona-
tion of the metal followed by intramolecular coupling of the
hydrido and methyl ligands (Scheme 1).
Scheme 1
The kinetics of these reactions are frequently too simple to be
Fig. 1 Dependence of [HCl]e/kobs on [Cl2]e/[HCl]e for the reaction of
[NiMe(triphos)]+ with HCl in the presence of [NEt4]Cl, in MeCN at 25.0 °C.
Line drawn is that defined by eqn. (3). [Insert: Dependence of kobs on the
concentration of HCl (5) and DCl (2) in the absence of [NEt4]Cl].
diagnostic of one particular mechanism and consequently the
detection or isolation of a hydrido species has often been
invoked as evidence of the coupling pathway. Here, we describe
unprecedented complex kinetics for protonation of an alkyl
complex which demonstrates, for the first time, that detection of
a hydrido species is an insufficient criteria to define the intimate
mechanism of alkane formation. Specifically, we show that
reveals a more complicated behaviour (Fig. 1).‡ The true rate
law is that shown in eqn. (3).
initial protonation of [NiMe(triphos)]+ [triphos
=
-d[NiMe(triphos)+]
=
(Ph2PCH2CH2)2PPh; eqn. (1)] occurs at nickel to form a
hydrido complex, but the formation of methane has to await
further direct protonation of the methyl group.
dt
(3)
(3.0 ± 0.4) ¥102[NiMe(triphos)+][HCl]e2 /[Cl-]e
1+ (0.78 ± 0.05)[HCl]e /[Cl-]e
[NiMe(triphos)]+ + HCl ? [NiCl(triphos)]+ + MeH (1)
The reaction between [NiMe(triphos)]+ and an excess of
anhydrous HCl in MeCN occurs in two stages: a rapid, acid-
dependent reaction which was studied by stopped-flow spec-
trophotometry, and the subsequent release of methane which
was followed by GLC.
Eqn. (3) demonstrates that two protons bind to the complex
prior to methane release. Such kinetics have not been observed
before but are in line with the general protonation character-
istics of metal sites containing hydrocarbon residues, in which
both metal and carbon can be protonated.4,5
The mechanism shown in Scheme 2 involves initial, rapid
protonation at nickel to form the hydrido species. However, the
kinetics dictate that this protonation is insufficient to produce
methane. Rather formation of methane has to await additional
protonation of the methyl group. Clearly, diprotonation of the
In the reaction between [NiMe(triphos)]+ and HCl alone, the
kinetics of the first stage exhibit a first order dependence on the
concentration of complex and an apparent first order depend-
ence on the concentration of HCl as described by eqn. (2), and
illustrated in Fig. 1.
-d[NiMe(triphos)+ ]
(2)
dt
= (3.4 ± 0.3) ¥102[NiMe(triphos)+ ][HCl]
The simplicity of these kinetics are consistent with either of the
two pathways shown in Scheme 1. However, addition of Cl2
† Present address: Department of Chemistry, Bedson Building, University
of Newcastle, Newcastle upon Tyne, UK NE1 7RU.
Scheme 2 Mechanism of the formation of methane in the reaction of
[NiMe(triphos)]+ with HCl in MeCN.
Chem. Commun., 1999, 2271–2272
This journal is © The Royal Society of Chemistry 1999
2271

nickel is untenable since this would form a formal NiVI
species.
The rate law associated with this mechanism is shown in eqn.
(4) and is derived assuming that protonation at nickel is a
rapidly established equilibrium, whilst protonation of the
carbon is slow and irreversible.
enforced to operate through the hydrido species. However,
intramolecular coupling of the hydride and methyl ligands is
slow and so methane formation can only occur after additional,
protonation of the methyl group.
The release of methane (measured by GLC) occurs at a rate
which shows a first order dependence on the concentration of
complex, but is independent of the concentration of HCl [k3
=
-d[NiMe(triphos)+]
(4.5 ± 0.5) 3 1022 s21]. This indicates that, at high
concentrations of acid, the putative s-methane complex is long-
lived¶ having a half-life of ca. 15 s. Relatively long-lived alkane
complexes have been detected previously6,7 but this kinetic
approach has not been adopted before, and potentially is a
general method of understanding the factors stabilising such s-
alkane complexes.
=
dt
(4)
K1k2[NiMe(triphos)+][HCl]e2 /[Cl- ]e
1+ K1[HCl]e /[Cl- ]e
Comparison of eqns. (3) and (4) gives k2 = (3.8 ± 0.4) 3 102
dm3 mol21 s21 and K1 = 0.78 ± 0.05. When [Cl2] is low, eqn.
(4) simplifies to eqn. (5), consistent with the experimental
observations. Comparison of eqns. (2) and (5) gives k2 = (3.4
± 0.3) x 102 dm3 mol21 s21, in excellent agreement with that
derived from eqn. (4).
The mechanism of methane formation described herein is
unexpected, but sufficiently simple that it seems likely that it is
not unique to [NiMe(triphos)]+. It is a salutary thought that this
pathway has remained unidentified in analogous systems until
now because: (i) the detection of hydrido complexes were
consistent with the intramolecular coupling pathway (Scheme
1) and (ii) the kinetics of these reactions were not (or could not)
be studied in the detailed manner described herein. Just because
a hydrido complex is detected in solutions containing acid and
a metal alkyl complex does not mean that the hydride is
kinetically competent to produce alkane.
-d[NiMe(triphos)+ ]
= k2[NiMe(triphos)+ ][HCl]e
(5)
dt
Consistent with the k2 step involving rate-limiting protonation
of the methyl group, studies with DCl show a primary isotope
effect, (k2)H/(k2)D = 3.5 (Fig. 1).
We thank BBSRC for supporting this research.
Further evidence that nickel is the most rapidly protonated
site comes from 1H NMR spectroscopy. Our mechanism
indicates that the addition of 1 mol equivalent of HCl to a
MeCN solution of [NiMe(triphos)]+ will produce an approx-
imately equimolar mixture of the parent complex and [Ni-
Notes and references
‡ Anhydrous HCl was prepared in situ by mixing equimolar amounts of
SiMe3Cl and MeOH in MeCN. The reactions were deliberately performed
in MeCN (dried over CaH2 and distilled immediately prior to use) since
solution equilibria (such as pKa and homoconjugation constants) are known
in this solvent.8 The concentrations of [HCl2]e and [Cl2]e are those
calculated having allowed for the homoconjugation equilibrium, HCl + Cl2
1
(H)Me(triphos)]2+. The H NMR spectrum of such a mixture
(Fig. 2)§ shows a multiplet at d 9.9 (JHP 50 Hz, JHP 430 Hz). We
tentatively attribute this low field signal to the hydride ligand in
the five-coordinate, formally NiIV species, [Ni(H)Me(tri-
phos)]2+. This multiplet is not present in the 1H NMR spectra of
either [NiMe(triphos)]+ or [NiCl(triphos)]+.
" HCl22, KH = 158.5 dm3 mol21
.
§ In these experiments the resonances attributable to the Me groups in
[NiMe(triphos)]+ (d 0.9, unresolved multiplet) and the putative [Ni(H)Me-
(triphos)]2+ are obscured by the peak due to SiMe3 (from the mixture of
SiMe3Cl and CD3OH used to generate anhydrous HCl). Further evidence
that the multiplet at d 9.9 is attributable to the hydride comes from
analogous experiments with DCl where this resonance is missing.
¶ We believe that this is a true measure of the stability of the putative s-
methane complex, rather than an artefact associated with measuring the
kinetics of methane transfer from solvent to gas phase, since the rate of the
reaction is independent of the concentration of complex used and the speed
with which the reaction mixture is agitated.
1 J. K. Kochi, Organometallic Mechanisms and Catalysis, Academic
Press, New York, 1978, p. 293 and references therein.
2 G. S. Hill, L. M. Rendina and R. J. Puddephatt, Organometallics, 1995,
14, 4966 and references therein.
3 M. W. Holtcamp, J. A. Labinger and J. E. Bercaw, Inorg. Chim. Acta,
1997, 265, 117 and references therein.
Fig. 2 The multiplet observed in the 1H NMR spectrum on addition of 1 mol
equivalent of HCl to [NiMe(triphos)]+.
4 K. W. Kramarz and J. R. Norton, Prog. Inorg. Chem., 1994, 42, 1 and
references therein.
5 R. A. Henderson, Angew. Chem., Int. Ed. Engl., 1996, 35, 946 and
references therein.
6 X.-Z. Sun, D. C. Grills, S. M. Nikiforov, M. Poliakoff and M. W. George,
J. Am. Chem. Soc., 1997, 119, 7521 and references therein.
7 S. Geftakis and G. E. Ball, J. Am. Chem. Soc., 1998, 120, 9953 and
references therein.
8 K. Izutsu, Acid–Base Dissociation Constants in Dipolar Aprotic
Solvents, 1990, Blackwell Scientific, Oxford, 1990.
Mass spectrometry of the gas formed from the reaction of
[NiMe(triphos)]+ and an excess of DCl showed essentially
exclusive formation of CH3D, with little or no other iso-
topomers. This is also consistent with our mechanism since,
once formed, the methane ligand does not undergo proton
exchange before it dissociates.
Why does the complex bind two protons when only one is
needed for the stoichiometric reaction? Clearly it does not need
to. Diprotonation occurs because the nickel is protonated faster
than the methyl group, and consequently the reaction is
Communication 9/07080J
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Chem. Commun., 1999, 2271–2272