Chalcogen atom transfer reactions. Kinetics of terminal bonded sulfur atom transfer between main group metal centers

Article abstract of DOI:10.1039/b003146l

The first comprehensive investigation of S atom transfer between two different metal centers is presented using the reaction of the terminal chalcogen atom bearing complex [CyNC((t)Bu)NCy]₂SnS with [CyNC((t)Bu)NCy]₂Ge(II); this multielectron redox reaction between Ge(II) and Sn(IV) proceeds via a second-order process with an inner sphere mechanism involving a μ-S intermediate as the proposed pathway; results of S and Se atom transfer between [CyNC((t)Bu)NCy]₂Ge and PPh₃ are also presented.

Full text of DOI:10.1039/b003146l

Chalcogen atom transfer reactions. Kinetics of terminal bonded sulfur atom
transfer between main group metal centers
Stephen R. Foley and Darrin S. Richeson*
Department of Chemistry, University of Ottawa, Ottawa, ON, Canada, K1N 6N5. E-mail: darrin@science.uottawa.ca
Received (in Irvine, CA, USA) 17th April 2000, Accepted 7th June 2000
Published on the Web 6th July 2000
The first comprehensive investigation of S atom transfer
between two different metal centers is presented using the
reaction of the terminal chalcogen atom bearing complex
[CyNC(tBu)NCy]2SnS with [CyNC(tBu)NCy]2GeII; this
multielectron redox reaction between GeII and SnIV pro-
ceeds via a second-order process with an inner sphere
mechanism involving a m-S intermediate as the proposed
pathway; results of S and Se atom transfer between
[CyNC(tBu)NCy]2Ge and PPh3 are also presented.
addition of styrene sulfide or elemental selenium to the
corresponding M(II) precursors, [CyNC(tBu)NCy]2M [M = Sn
(1), Ge(2)]. These results now allow us to report the first
comprehensive study of heteronuclear inter-metal sulfur atom
transfer reactions.
When the bis(amidinato)tin(IV) sulfide species 1S and
bis(amidinato)germanium(II) complex 2 are mixed at room
temperature complete sulfur atom exchange between these
complexes can be achieved according to eqn. (4) in Scheme 1.
Atom transfer reactions are an active area of fundamental
investigation that have attracted considerable attention in large
part due the manifest role of oxygen atom transfer in many
oxidation processes and in several biological processes.1,2 The
oxygen atom transfer reactions that have been studied are
dominated by metal–oxo donor reactions with a non-metal
acceptor. In comparison, transfer of heavier group 16 elements
and/or complete inter-metal transfer of a multivalent atom are
less established.3,4 Some chalcogen complexes of Ti(IV) are
known to function as effective sources of chalcogen atoms.
However, the atom transfer reactivity of Cp2TiE5 (E = S, Se)5
and of (OEP)TiE2 (OEP = octaethylporphyrinato; E = O, S,
Se)6 are based on ligand reduction and are therefore categorized
as secondary atom transfer reactions. When attention is
focussed on main group metal complexes the number of clear
examples of this class of reaction dwindles to a single detailed
investigation.7 In fact this example, shown by eqn. (1),
represents the sole comprehensive study for inter-metal two
electron transfer mediated by heavy elements of group 16 (i.e.
S or Se) and involved the reversible transfer of sulfur and
selenium atoms between tin porphyrin complexes of meso-
tetraphenylporphyrinato (TPP) and meso-tetra-p-tolylporphyr-
inato (TTP) ligands.
Scheme 1
At room temperature this reaction proceeded smoothly over
several days and could be conveniently monitored by 1H NMR in
C6D6. The tBu groups for the four different amidinate complexes
provided an excellent spectroscopic handle to monitor this
t
reaction. For example, as the reaction progresses, the Bu
resonances of 1S at d 1.17 continuously decrease in intensity
concomitantwiththeappearanceofnewtBuresonancesatd1.35,
signifyingtheformationof1.
The corresponding tBu resonances for the germanium
complexes are slightly more complicated. Instead of having both
amidinate ligands bidentate and equivalent as with compounds 1
and 1S, both 2 and 2S are known to exhibit one bidentate and one
monodentate (‘dangling’) ligand as shown in Scheme 1. This
feature has been demonstrated through structural and spectro-
scopic characterization for both 2 and 2S and is a general feature
for a number of other germanium amidinate complexes that we
have prepared.12 As a result, the two amidinate ligands in the Ge
(1)
The limited examples of primary atom transfer reactions
involving sulfur and selenium atoms include the exchange
4
between the terminal chalcogenido complexes [h -Me8taa]SnE
(E = S, Se; Me8taa = octamethyldibenzotetraazaannulene) and
the Ge(II) species [h -Me8taa]Ge [eqn. (2)].8 More recently the
4
preparation of terminal selenido complexes of gallium and
indium supported by tris(3,5-di-tert-butylpyrazolyl)hydrobor-
ato (tBuTp) ligation allowed for the observation of selenium
atom transfer from indium to gallium [eqn. (3)].9 Kinetic
parameters have not been established for either of these
systems.
t
complexes are inequivalent and two separate Bu signals are
observed for 2 and for 2S. Thus, during the progress of the
t
reaction, two new Bu resonances appear at d 1.70 and 1.07
signifyingtheformationof2SwhilethetworesonancesforthetBu
groupsof2(d1.61,1.16)simultaneouslydecreaseinintensity.
The progress of S atom transfer between 1S and 2 can be
(2)
(3)
t
monitored by following the changes in integration of the Bu
resonanceofanyofthefourcomplexesinScheme1. Inallkinetic
runs, the data for this transfer reaction was found to obey an
integratedratelawforsecond-orderreactions.†Plotsofreciprocal
concentration vs. time were linear for more than two half lives. In
allcasesthedataindicatedacomplete,non-reversibletransferofa
terminallybondedsulfuratomfromSntoGe.
Our interest in the formation of multiple bonds between main
group elements and the subsequent reactivity of these unusual
species led us to prepare a family of amidinato complexes of Ge
and Sn [CyNC(tBu)NCy]2ME [M = Sn, E = S (1S); M = Ge,
E = S (2S), Se (2Se)] that possess terminal chalcogenido
functions.10,11 These compounds were prepared by oxidative
Rate constants for this reaction could be determined over a
temperature range of 40 °C and a summary of this data is given in
DOI: 10.1039/b003146l
Chem. Commun., 2000, 1391–1392
This journal is © The Royal Society of Chemistry 2000
1391

Table 1. Measurements from 25 to 65 °C resulted in k values that
ranged from 1.18 3 1023 to 2.14 3 1022 M21 s21 with resulting
half-lives of 18 h to 31 min. The thermal decomposition of 2S
precluded studies at higher temperatures. An Eyring plot of these
results indicated a linear relationship with associated activation
parameters of DH‡ = 17.6 ± 0.2 kcal mol21 and DS‡ = 214.4 ±
(5)
0.3calK21mol21
.
The first-order dependence of the reactants was confirmed by
imposingpseudo-first-orderconditionsonthereactionbyhaving
1S in large excess compared to 2. Under these circumstances the
disappearanceof2andsubsequentformationof2Sexhibitedfirst
orderbehavior.
We have found complete primary sulfur atom exchange
between tin and germanium amidinates can be achieved and the
kinetic parameters of this reaction have been fully determined.
This represents the first comprehensive study of a heteronuclear
inter-metaltwo-electronsulfuratomtransfer.Thedataindicatean
inner-sphere mechanism with m-S formation. Chalcogen atom
exchange between PPh3 and 2 provide results that are consistent
with increasing strength of interaction for the terminal sulfido
The only comparable system for which kinetic information is
available is given by eqn. (1). In this case the kinetic analysis was
consistent with a reversible second-order reaction.7 The equilib-
rium constant for this reaction was near unity between 240 and
210 °C in toluene-d8. Kinetic measurements for eqn. (1) at
temperatures ranging from 30 to 60 °C resulted in kf values from
species in the order [CyNC(tBu)NCy]2SnS 1S
< [CyN-
C(tBu)NCy]2GeS2S < Ph3PSwhileforterminalselenidospecies
the order appears to be Ph3PSe < [CyNC(tBu)NCy]2GeSe 2Se.
Our current efforts are directed at extending this investigation of
inter-metalchalcogenatomtransferandexploringtransfertonon-
metalacceptors.
0.40to2.39M21s21
.
Theratelawandactivationparametersthatweobtainedforeqn.
(4) in Scheme 1 support an inner-sphere mechanism which likely
proceedsviainteractionofthechalcogenatomontinandtheGe(II
)
center resulting in a sulfur bridged intermediate (Scheme 1).
While this species was not observed in our experiments, the
negative entropy of activation (DS‡ = 214.4 cal K21 mol21) is
consistent with an associative type reaction involving a m-sulfido
intermediate. This observation is in line with the value of DS‡
(224.1calK21mol21)determinedforeqn.(1)forwhichasimilar
m-Sspecieswassuggested.7Consistentwiththispropositionisthe
low DH‡ value, which suggests that significant bond formation
betweenGeandthebridgingsulfidoligandthatoffsetstheenergy
arerequiredforthecleavageoftheSnNSduringthetransfer.Again
the DH‡ value obtained for eqn. (4) is similar to that obtained for
Notes and references
† All kinetic measurements were carried out in duplicate and were
monitored by NMR spectroscopy using a Bruker 500 MHz or a Gemini 200
MHz spectrometer. Benzene-d6 was distilled from Na/K alloy and used as
solvent and internal standard. [CyNC(tBu)NCy]2SnNS 1S and [CyN-
C(tBu)NCy]2Ge 2 were prepared according to literature procedures.10,11
Solutions of 1S and 2 were prepared at concentrations ranging from 0.011
to 0.075 M and in ratios from 1 1 to 7 1. Samples were monitored in a
temperature controlled NMR probe for at least two half lives. During remote
1H NMR experiments, spectra were obtained at regular intervals that varied
with temperature (e.g. 45 min at 35 °C and 3 min at 65 °C). For
measurements at 25 °C, a temperature controlled water bath set at 25.0 °C
was used and spectra were recorded manually every 2 h. Pseudo-first-order
conditions were investigated at 35 °C where the initial concentrations were:
[(CyNC(tBu)NCy)2SnNS] = 0.075 M and [(CyNC(tBu)NCy)2Ge] = 0.011
M.
the homonuclear exchange reaction shown in eqn. (1) (DH‡
=
10.9±0.9kcalmol21).7
The direction of this sulfur atom exchange reaction demon-
strates the thermodynamic stability of the terminal GeS bond
relativetothecorrespondingSnSbond.Thisisconsistentwiththe
notions that both the tendency to engage in multiple bonding and
‡ Ph3PNSe was prepared by dissolving triphenylphosphine (2.038 g, 7.78
mmol) in 35 ml THF followed by addition of excess elemental selenium
(1.023 g, 13 mmol). The mixture was stirred for 24 h and filtered. The
product was subsequently recrystallized from THF in 66% yield (1.66 g).
13
the stability of M(IV) vs. M(II
)
decreases for the heavier
congeners of the group 14 elements (Ge > Sn > Pb). This is
further supported by the fact that the p-bond energy associated
with the SnNS bond has been estimated to be 31.8 kcal mol21
whichisconsiderablylowerthanthatoftheGeNSbond(40.0kcal
mol21).14
1 R. H. Holm, Chem. Rev., 1987, 87, 1401.
2 L. K. Woo, Chem. Rev., 1993, 93, 1125.
3 For recent studies of inter-metal nitrogen atom transfer reactions see:
C. E. Laplaza, A. R. Johnson and C. C. Cummins, J. Am. Chem. Soc.,
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Cummins, Inorg. Chem., 2000, 39, 174.
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Cummins, Angew. Chem., Int. Ed. Engl., 1997, 36, 87.
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9 M. C. Kuchta and G. Parkin, Inorg. Chem., 1997, 36, 2492.
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11 S. R. Foley, C. Bensimon and D. S. Richeson, J. Am. Chem. Soc., 1997,
119, 10 359.
Keeping in mind that the common acceptors for oxygen atom
transferreactionsaretertiaryphosphines1,2 wealsoexaminedthe
chalcogen atom exchange reactions between the germanium
complexes2,2Sand2SeandPh3PE/Ph3P(E = S,Se).Thisshould
also provide some indication of the relative bond strengths for
these species. Complex 2 reacts with 1 equiv. of triphenylphos-
phine selenide‡ in C6D6 according to eqn. (5). 1H NMR
spectroscopyindicatedacompleteconversionofPh3PNSetoPh3P
while germylene 2 was converted to 2Se. Under these conditions
complete selenido atom exchange was achieved from the
phosphorustothegermaniumin < 4minatroomtemperaturewith
noindicationofremainingstartingmaterials. However, whenthe
same reaction was performed with Ph3PS in place of the Ph3PSe,
noreactionwith2wasobservedevenatelevatedtemperatures.
Table 1 Rate constants for eqn. (4) in C6D6
T/°C
k M21 s21
25
35
45
55
60
65
(6.28 ± 0.84) 3 1024
(1.46 ± 0.13) 3 1023
(4.19 ± 0.06) 3 1023
(1.08 ± 0.04) 3 1022
(1.53 ± 0.06) 3 1022
(2.14 ± 0.05) 3 1022
12 S. R. Foley, G. P. A. Yap and D. S. Richeson, Dalton Trans., accepted
for publication.
13 F. A. Cotton, G. Wilkinson, C. A. Murillo and M. Bochmann, Advanced
Inorganic Chemistry, Wiley, New York, 6th edn., 1999.
14 Y. Matsuhashi, N. Tokitoh, R. Okazaki and M. Goto, Organometallics,
1993, 12, 2573.
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Chem. Commun., 2000, 1391–1392