Nickel-triad complexes of a side-on coordinating distannene

Article abstract of DOI:10.1002/anie.201411025

NHC adducts of the stannylene Trip₂Sn (Trip=2,4,6-triisopropylphenyl) were reacted with zero-valent Ni, Pd, and Pt precursor complexes to cleanly yield the respective metal complexes featuring a three-membered ring moiety Sn-Sn-M along with carbene transfer onto the metal and complete substitution of the starting ligands. Thus the easily accessible NHC adducts to stannylenes are shown to be valuable precursors for transition-metal complexes with an unexpected Sn-Sn bond. The complexes have been studied by X-ray diffraction and NMR spectroscopy as well as DFT calculations. The compounds featuring the structural motif of a distannametallacycle comprised of a [(NHC)₂M⁰] fragment and Sn₂Trip₄ represent rare higher congeners of the well-known olefin complexes. DFT calculations indicate the presence of a π-type Sn-Sn interaction in these first examples for acyclic distannenes symmetrically coordinating to a zero-valent transition metal.

Full text of DOI:10.1002/anie.201411025

Angewandte
Chemie
DOI: 10.1002/anie.201411025
Tin Ligands
Nickel-Triad Complexes of a Side-on Coordinating Distannene**
Christian P. Sindlinger, Sebastian Weiß, Hartmut Schubert, and Lars Wesemann*
Abstract: NHC adducts of the stannylene Trip2Sn (Trip =
2,4,6-triisopropylphenyl) were reacted with zero-valent Ni,
Pd, and Pt precursor complexes to cleanly yield the respective
metal complexes featuring a three-membered ring moiety Sn-
Sn-M along with carbene transfer onto the metal and complete
substitution of the starting ligands. Thus the easily accessible
NHC adducts to stannylenes are shown to be valuable
precursors for transition-metal complexes with an unexpected
À
Sn Sn bond. The complexes have been studied by X-ray
diffraction and NMR spectroscopy as well as DFT calcula-
tions. The compounds featuring the structural motif of a di-
stannametallacycle comprised of a [(NHC)2M0] fragment and
Sn2Trip4 represent rare higher congeners of the well-known
olefin complexes. DFT calculations indicate the presence of
a p-type Sn–Sn interaction in these first examples for acyclic
distannenes symmetrically coordinating to a zero-valent tran-
sition metal.
Scheme 1. Some known examples for bis(stannylene) (A, B) and side-
on coordinating distannene complexes (C, D) as well as one example
of a silastannene complex (E) for which a cyclic derivative is also
complexes, the transition-metal coordination chemistry of known.
S
ince Zeiseꢀs fundamental discovery of ethylene–platinum
olefins and the bonding description within such complexes
have been an extremely vivid field of research for almost two
centuries.[1] In recent decades, complexes of main-group the backbone and isolation of a bis(stannylene) complex
analogues to olefins have come more and more into focus, (B).[4g] One of the most intriguing properties of complexes
and various findings have been made, especially for di- such as A–D is the behavior of two stannylene moieties in the
silenes.[2] The bonding interaction between two tetrylene units coordination sphere of a metal and the nature of the Sn–Sn
without p-basic substituents that dimerize to form ditetry- interaction, namely the presence of a s-single bond in
lenes is more complex than in the parent olefins, and its distannametallacyclopropanes versus a p-type double bond
discussion has influenced the modern understanding of in side-on coordinating distannenes. In the case of olefins, the
double bonds.[3] Although such stannylenes have long been situation is described with the Dewar–Chatt–Duncanson
used as ligands in transition-metal coordination chemistry, model as a continuum in which increasing metallacyclopro-
only a few bis(stannylene) (Scheme 1, A,B) complexes have pane character goes along with increasing p-donation from
been reported, but there were no examples for side-on the metal onto the ligand.[6]
coordinating distannenes until very recently (C,D).[4]
In the case of the coordination chemistry of tin congeners
A small number of distannenes is known.[3a,b,4h,5] The of olefins, the majority of reported studies were conducted on
difficulty of the study of their coordination chemistry as olefin Group 4 ocene complexes including bulky alkyl-, cyclic
congeners stems from a lack of distannenes that are persistent bis(silyl)-, as well as cyclic olefinic substituted stannylenes
in solution because stannylenes that are dimeric in the solid with varying coordination behavior. Apart from Saitoꢀs recent
state usually tend to dissociate in solution.[3c,f] Marschnerꢀs titanium complex, [Cp2M(SnR2)2] compounds reveal stanny-
À
attempts to use a bicyclic distannene led to rearrangement of lene-like character with a very weak direct Sn Sn interaction.
Recently, side-on complexes of a cyclic silastannene to
Group 10 elements have been observed (E).[2m] Our group
[*] C. P. Sindlinger, S. Weiß, Dr. H. Schubert, Prof. Dr. L. Wesemann
Eberhard Karls Universitꢀt Tꢁbingen
Institut fꢁr Anorganische Chemie
Auf der Morgenstelle 18, 72076 Tꢁbingen (Germany)
E-mail: lars.wesemann@uni-tuebingen.de
reported the first zero-valent Group 10 complex with a cyclic
distannene as a chelating ligand, adding a new unsymmetrical
coordination mode, which is maintained in solution, for
distannenes (D) in which each tin atom reveals an individual
electronic interaction to the coordinated metal.[4h] Further
[**] The Stiftung Stipendienfonds of the Fonds der Chemischen
Industrie is gratefully acknowledged for financial support (research
scholarship to C.P.S.). Dr. Klaus Eichele is acknowledged for NMR
expertise and Florian Herrmann for technical support. We thank the
bwGrid cluster Tꢁbingen for provision of computational resources.
bis(stannylene) complexes were reported by Hahn and
[7]
˚
ˇ
ˇ
Ruzicka. Augmenting the still small number of such
structurally characterized complexes, we present the syn-
thesis, structure, and bonding interactions in complexes of the
electron-rich [L2M0] fragment (M = Ni, Pd, Pt; L = NHC)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201411025.
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with the sterically encumbered tetraaryl distannene [Sn2Trip4]
bearing a symmetrical distannametallacyclopropane core.
Recently we reported the facile synthesis of carbene
adducts to various stannylenes by the dehydrogenation of tin
di- and trihydrides with one equivalent of small N-hetero-
cyclic carbenes (NHC) and subsequent trapping of the
intermediately formed stannylene with a second equivalent
NHC.[8] This route enables easy and high yielding access to
1,3,4,5-tetramethylimidazol-2-ylidene (MeNHC) adducts of
trimeric [Trip2Sn], which has been well-studied in the solid
state.[5a,9] The intriguing electronic situation of some meso-
meric descriptions of the adduct [Trip2SnMeNHC] (1) as an
isoelectronic Group 14 analogue of phosphines (Scheme 2)
bering two chemically and magnetically equivalent tin atoms
along with a platinum atom.
Removal of volatiles from the resulting mixture of the
dark red complex and PtBu3 and dissolving the residue in
a minimum amount of n-pentane afforded large deep
burgundy crystals after storage of the solution at À408C for
one week. X-ray analysis of the crystals in the triclinic space
group P1 revealed the structure of the [(MeNHC)2Pt-
¯
(SnTrip2)2] (2) complex to feature a three-membered ring of
a bis(NHC)–platinum and two Trip2Sn fragments with
a geometry related to a recent disilene nickel complex
(Figure 1).[2n] The asymmetric unit contains two independent
Figure 1. ORTEP plot of one molecule of 2 in the asymmetric unit.
Ellipsoids are set at 50% probability; hydrogen atoms and co-crystal-
lized n-pentane are omitted for the sake of clarity. Selected bond
lengths [ꢂ] and angles [8] along with the corresponding values for the
second molecule [in parentheses]: Sn3–Sn4 2.7704(3) [2.7558(4)], Pt2–
Sn3 2.6381(3) [2.6300(3)], Pt2–Sn4 2.6365(3) [2.6436(3)], Pt2–C47
2.031(4) [2.028(4)], Pt2–C143 2.051(3) [2.054(4)]; Sn-Pt-Sn/C-Pt-C:
14.5. The ORTEP plots of molecules 3 and 4 can be found in the
Supporting Information.
Scheme 2. Synthesis of carbene adduct 1 and subsequent reaction
with half an equivalent of [ML2] (M=Ni, L=cod; M=Pd,L=PtBu3,
PCy3; M=Pt, L=PtBu3, cod) to form the respective distannametallacy-
clopropane. cod=1,5-cyclooctadiene.
molecules of 2 with slight but not significant deviations of
bond lengths and geometrical arrangements.
Analogously to the formation of 2, we have been able to
isolate the corresponding homologous dark red palladium (3)
and dark yellow nickel (4) complexes from the reaction of
[Pd(PtBu3)2] or [Pd(PCy3)2] and [Ni(cod)2] with two equiv-
alents of carbene adduct 1 in aromatic solvents at room
temperature. Pure 3 and 4 can be obtained similarly to 2, and
their crystal structures reveal comparable geometrical fea-
tures as found for the platinum homologue (Supporting
Information, Figure S1). All of the complexes are extremely
sensitive to air and moisture in solution and solid state, as
indicated by rapid decolorization on exposure to ambient
atmosphere.
may allow interesting applications as ligand in coordination
chemistry. Therefore two equivalents of the adduct 1 were
allowed to react with one equivalent of [Pt(PtBu3)2] in
benzene at room temperature and a slow color change from
yellow to deep red was observed within four days. 31P NMR
spectroscopy revealed quantitative consumption of [Pt-
(PtBu3)2]. Proton NMR spectroscopy revealed clean forma-
tion of one new compound of strongly reduced symmetry
compared to the signal pattern of the sole ligand 1.
The 119Sn resonance of 1 at d = À160.7 ppm was shifted to
higher field by over 500 ppm to a singlet signal at d =
À697.8 ppm. The signal is accompanied by the respective
195Pt satellites with a coupling constant of JPt–Sn ꢀ 4175 Hz as
well as 117Sn satellites with a coupling constant of JSn–Sn
ꢀ 2125 Hz. Furthermore, a 195Pt NMR resonance was found
at d = À4448.9 ppm flanked by tin isotope satellites with an
integration ratio of twice the natural abundance. This
coupling pattern indicated the formation of a moiety encum-
The three-membered rings represent essentially isosceles
À
triangles with the Sn Sn bond shortening negligibly from the
platinum complex 2 (2.7703(4) ꢁ) to nickel 4 (2.7471(2) ꢁ).
À
The Sn Sn bond length is in the range of known stannylene
dimers and distannenes (2.575(4)–3.0009(7) ꢁ)[3e,4h,5c,d,10] but
À
is also contained in the range of Sn Sn single bonds. The
À
present Sn Sn bond lengths are the shortest distances to date
in distannametallacyclopropanes, indicating a strong interac-
2
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tion.[4h,i] The Sn M contacts in each triangle deviate slightly
with the mean degree of deviation increasing from 2 (0.008 ꢁ)
around the Sn CTrip bond and therefore leading to magnetic
inequivalency of all positions of both Trip moieties at the tin
atom. Two sets of signals for the carbene methyl groups give
À
À
over 3 (0.017 ꢁ) to 4 (0.027 ꢁ).[11] These deviations are
À
À
arguably small yet consistent. All M Sn contacts lay within
rise to a hindered rotation around the Pt C bond as well.
the range of previously reported distances but Pd/Pt-Sn
distances are insignificantly shorter than those reported for
the silastannene complexes (Pt 2.6613(10) ꢁ, Pd 2.6714(12)/
2.6808(6) ꢁ).[2m,12] The ranges of observed carbene–metal
bond lengths are unobtrusive compared to metal–carbene
distances from recent [(NHC)2M(cod)] (M = Ni, Pt) com-
plexes.[13] The carbene–palladium bonds in 3 are slightly
longer than the carbene–platinum bonds in 2, potentially
indicating a stronger share of metal–carbene back-bonding
for platinum than for palladium. The coordination geometry
at the metal center is essentially planar, with slight twisting of
the C-M-C plane out of the ring plane. The orientation of the
[R4Sn2] fragment toward the transition metal of 2, 3, and 4
differs from all of the coordinating distannenes reported to
date. In general, the bent-back angles found at the tin
moieties are relatively small and much smaller than in Saitoꢀs
complex C.[4i] All of the molecules have two different types of
arrangements around the tin atoms in terms of the bent-back
angle defined as the angle between the centroids of the ipso-
119Sn NMR spectroscopy on 3 (d = À464.9 ppm) and 4 (d =
À579.6 ppm) also reflected the shift to higher field, though
less pronounced as for the platinum complex. The parent
distannene [Sn2Trip4] is only known in solution and its tin
resonance is found at d = 427.3 ppm with a coupling constant
of JSn–Sn = 2930 Hz.[5a]
Although the geometrical features of the Sn–Sn inter-
action is essentially identical for 2, 3, and 4, the JSn–Sn coupling
constants differ (Table 1). Whilst Pt complex 2 and 3 reveal
comparable constants for nickel complex 4, an unexpectedly
large coupling constant of JSn–Sn = 8050 Hz was found. Such
large constants are known for rather different species such as
(R3Sn)2Sn or strained cyclic distannenes.[5e,15]
Table 1: Overview on NMR spectroscopic characteristics of 2, 3, and 4.
d(119Sn)[a]
JSn–Sn
d( C) C M
[b]
13
[a]
À
2
3
4
À697.8
À464.9
À579.6
2125
2660
8050
183.4
192.5
193.2
À
carbon atoms of the Sn-bound Trip moieties and the Sn Sn
vector and the sum of bond angles around each tin atom
within the [Sn2R4] fragment. One [R2Sn] moiety reveals
a larger bent-back angle of 31(1)8 with a smaller bond-angle
sum of 349.48 whilst the respective values at the neighboring
tin atom amount to 21(1)8 and 354.28, indicating a more
pronounced planarization. Values of around 3558 are known
for telluradistanniranes as well as azadistanniridines.[14] Con-
sistently, for the crystallographically determined three-mem-
bered rings of 2, 3, and 4 the more planar Sn atom with the
[a] Given in ppm. [b] Given in Hz.
Table 1 summarizes NMR spectroscopic characteristics of
2, 3, and 4. The high-field shift of the 119Sn resonances is in
accordance with four-coordinate tin in strained three-mem-
bered rings.[2m,5a,14b,16] Disilenes complexes of pronounced p-
character reveal low-field-shifted 29Si NMR resonances,
whilst those with less pronounced p-character can be found
at higher field.[2d,f] Therefore mixed silastannene complex E
(with high-field 29Si and 119Sn signals) was characterized as
a silastannametallacyclopropane complex owing to its high-
field 29Si resonance.[2m] Consequently, the 119Sn NMR shift of
2–4 accounts for a distannametallacyclopropane character of
the complexes. To further evaluate the observed chemical
shifts, it is noteworthy that (Ph3Sn) residues in PdII or PtII
complexes usually resonate at much lower field (around d =
À50 ppm).[17] In the case of 2 especially, the observation of the
small bent-back angle reveals the longer M Sn bond.[11] This
À
doubtlessly subtle increase in desymmetrization may be a hint
for a change in the bonding interaction from 2 to 4 potentially
approaching the coordination mode as observed for the
apparent borderline case D and an indication for the general
relevance of this particular resonance description (Sche-
me 3).[4h] A more pronounced unsymmetrical free distannene
was described earlier.[3e] Nevertheless, compared to D com-
plexes 2–4 are essentially symmetrical.
platinum–tin coupling constant JPt–Sn = 4175 Hz is of interest
II
À
because the coupling in respective Ph3Sn Pt complexes is
revealed to be between 9500–12500 Hz.[17a–c] The pronounced
differences of the 119Sn chemical shifts as well as the metal–tin
coupling constants compared to the respective characteristics
À
of M SnPh3 systems may indicate that the bonding situation
within the three-membered ring is not adequately described
Scheme 3. Mesomeric descriptions of the distannametallacycles.
by a conservative MII–1,2-distannanediide.
The solution NMR spectra confirm the bonding pattern in
the solid state to be maintained in solution, and the structural
similarity of 2, 3, and 4 found in the solid state was also
observed in their H NMR spectra. In solution, the slightly
different arrangement around each tin is not observed and
To shed further light on the description of the electronic
structure of the complexes, DFT calculations were performed
on the complexes 2 and 3. The HOMO and HOMOÀ1 are
shown in Figure 2. The HOMOÀ1 reflects a p-type bonding
interaction from the [L2M] fragment toward the [Sn2R4]
fragment, representing the classic p-donation into the olefinic
anti-bonding p-orbital, whilst the HOMO indicates a p-type
bond between the Sn atoms.
1
À
a C2 axis of symmetry through M and the Sn Sn centroid can
be considered. For 2, 3, and 4, the accumulation of four Trip-
moieties in close proximity causes a loss of free rotation
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tion as a p-complex with strong back-bonding from the metal
onto the [Trip4Sn2] moiety. The perception of the structure as
being in between the boundary forms of a pure p-complex
and a pure distannametallacyclopropane is in accordance with
the geometrical features as well as the NMR spectroscopic
characteristics. DFT calculations indicate the presence of a p-
type Sn–Sn interaction. Nevertheless, the lack of experimen-
tal data for the Sn–Sn interaction of the free distannene
[Trip4Sn2] in solid-state and its electronic orbital interactions
prevents from thorough comparisons with its coordinated
form presented herein. The straightforward preparation of 2,
3, and 4 from carbene adducts to stannylenes makes them
accessible compounds for subsequent studies and highlights
the synthetic value of the NHC-adducts to main-group
compounds.
Figure 2. CMOs of 2 (isosurface values at 0.045 value) obtained from
calculations at BP86/TZVPP level of theory. The Trip moieties have
been replaced by mesityl moieties in this depiction for the sake of
clarity.
According to NMR experimental reaction control, com-
plete conversion is achieved for the platinum complex 2. In
case of Ni and Pd clean product formation is observed but the
reaction results in a mixture with approximately 90% of 3 and
4 along with free adduct 1. This mixture was also observed
after dissolution of pure crystals of 3 and 4 within a few
minutes. The reaction rates were highly depending on the
precursor complexes [ML2]. Whilst [Pt(cod)2] lead to instant
formation of 2, the reaction proceeded over four days for the
respective Pt(PtBu3)2 complex. Compound 3 was formed
instantaneously with [Pd(PtBu3)2] as well as [Pd(PCy3)2],
whilst 4 was slowly formed from reaction with [Ni(cod)2] after
twelve hours.
Received: November 13, 2014
Revised: December 12, 2014
Published online: && &&, &&&&
Keywords: coordination chemistry · distannenes · nickel ·
.
palladium · platinum
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À
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Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Tin Ligands
Differing analogues: On reacting the
carbene adduct of the stannylene
[Trip2Sn] (Trip=2,4,6-triisopropylphenyl)
with zero-valent Group 10 complexes,
symmetrically coordinating complexes of
the distannene [Sn2Trip4] to Ni, Pd, and Pt
have been obtained. Their structural and
spectroscopic properties are presented
and discussed.
C. P. Sindlinger, S. Weiß, H. Schubert,
L. Wesemann*
&&&& — &&&&
Nickel-Triad Complexes of a Side-on
Coordinating Distannene
6
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!