An N-heterocyclic carbene adduct of diatomic tin,:Sn=Sn

Article abstract of DOI:10.1039/c2cc35228a

Reduction of an N-heterocyclic carbene (NHC) adduct of SnCl₂, viz. [(IPr)SnCl₂] (IPr =:C{N(Dip)C(H)}₂; Dip = 2,6-diisopropylphenyl), with a magnesium(i) dimer, has afforded the first NHC complex of a row 5 element in its diatomic form, [(IPr)SnSn(IPr)]; a computational analysis of the complex indicates that it comprises a singlet state, doubly bonded tin(0) fragment,:SnSn:, datively bonded by two NHC ligands. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc35228a

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Q
An N-heterocyclic carbene adduct of diatomic tin, :Sn Sn:w
Cameron Jones,*a Anastas Sidiropoulos,a Nicole Holzmann,b Gernot Frenking*b and
Andreas Stasch*a
Received 20th July 2012, Accepted 16th August 2012
DOI: 10.1039/c2cc35228a
Reduction of an N-heterocyclic carbene (NHC) adduct of SnCl2,
viz. [(IPr)SnCl2] (IPr = :C{N(Dip)C(H)}2; Dip = 2,6-diisopropyl-
phenyl), with a magnesium(I) dimer, has afforded the first NHC
strength of the carbene-element bonds in these rows 2–4 complexes.
It occurred to us that if NHC complexes of the heavier (rows 5 or 6)
p-block diatomics could be accessed, their presumably weaker
carbon-element bonds might be labile enough for them to deliver
the diatomic fragment under relatively mild reaction conditions. In
this respect we were particularly interested in extending our prior
study on 2 to the preparation of [(IPr)EQE(IPr)] (E = Sn or Pb).
Indeed, a recent computational study has predicted that NHC
adducts of both Sn2 and Pb2 will be accessible, stable species.13
Here, we report the synthesis, characterisation and theoretical
analysis of [(IPr)SnQSn(IPr)], which is unstable in solution and
the solid state at ambient temperature. The outcome of attempts to
prepare [(IPr)PbQPb(IPr)] suggest this compound is less stable,
and will not be isolable at close to room temperature.
The reaction of [(IPr)SnCl2]14 with the magnesium(I) reagent,
[{(MesNacnac)Mg}2] (MesNacnac = [(MesNCMe)2CH], Mes =
mesityl),15 at ꢀ80 1C in diethyl ether initially led to a red-brown
solution and the deposition of tin metal. Upon warming to 0 1C
the mixture took on a green colour (with further metal deposi-
tion), and subsequent work-up afforded a very low yield (5%)
of [(IPr)SnQSn(IPr)] 3 as a red crystalline solid (Scheme 1). An
1H NMR spectroscopic analysis of the reaction mixture prior to
work-up revealed that free IPr was the major soluble reaction
product. It is of note that we have had considerable success
using [{(MesNacnac)Mg}2] as a mild and soluble reducing
agent in the preparation of low oxidation state p-block
compounds (including 2),16 and hence it was chosen for this
purpose here. This strategy was vindicated by attempts to
access 3 using more conventional reducing agents, e.g. KC8 or
[Mg(anthracene)(THF)3], all of which met with failure.
Q
complex of a row 5 element in its diatomic form, [(IPr)Sn Sn(IPr)];
a computational analysis of the complex indicates that it comprises a
Q
singlet state, doubly bonded tin(0) fragment, :Sn Sn:, datively
bonded by two NHC ligands.
The past two decades have seen N-heterocyclic carbenes
(NHCs) emerge as one of the most important and ubiquitous
ligand classes utilised in transition metal coordination chemistry,
catalysis etc.1 In parallel, the main group element coordination
chemistry of NHCs has developed, albeit at a slower pace.2
Nevertheless, the highly nucleophilic nature of NHCs is increasingly
lending them to the stabilisation of main group element fragments
that would normally be thought to be incapable of existence as
molecular species under ambient conditions. Pertinent examples
here include NHC coordinated binary element hydrides3 and/or
low oxidation state element–element bonded fragments.4 Although,
examples of the latter were described as long ago as 2002,4g it was
Robinson and co-workers’ report from 2008 on the remarkable
silicon(0) complex, [(IPr)SiQSi(IPr)] 1 (IPr = :C{N(Dip)C(H)}2;
Dip = 2,6-diisopropylphenyl), which invigorated this field.5
This compound was formulated as an NHC adduct of the
singlet, doubly bonded fragment, :SiQSi:, and has been
described as a soluble ‘‘allotrope’’ of silicon.6 A flurry of
publications have followed,7 which detail the syntheses of a
series of stable NHC adducts of groups 13–15 element diatomics,
viz. [(NHC)BRB(NHC)],8 [(NHC)GeQGe(NHC)] 29 and
[(NHC)E–E(NHC)] (E–E = P–P,10 As–As11 or P–N12).
In order to prepare the lead analogue of 3, efforts were made
to synthesise [(IPr)PbX2] (X = Cl or Br) as precursor complexes.
While the lead chloride complex could not be prepared in our
hands, [(IPr)PbBr2] was obtained in good yield from the reaction of
IPr and PbBr2 in THF. To the best of our knowledge, this complex
While of considerable fundamental interest, it has been
proposed that NHC coordinated diatomics have the potential
to act as soluble sources of the normally insoluble p-block
elements in reactions they are involved in.7 However, this has
yet to be definitively demonstrated, presumably because of the
a School of Chemistry, Monash University, PO Box 23, Melbourne,
VIC, 3800, Australia. E-mail: cameron.jones@monash.edu,
andreas.stasch@monash.edu; Fax: +61-3-9905-4597;
Tel: +61-3-9902-0391
b Fachbereich Chemie, Philipps-Universitat Marburg, 35032, Marburg,
¨
Germany. E-mail: frenking@chemie.uni-marburg.de
w Electronic supplementary information (ESI) available: Full details
and references for synthetic, crystallographic and computational
studies. CCDC 892554–892557. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2cc35228a
Scheme 1 Reagents and conditions: i, [{(MesNacnac)Mg}2],
ꢀ[{(MesNacnac)Mg(m-Cl)}2], Dip = 2,6-diisopropylphenyl.
c
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Chem. Commun., 2012, 48, 9855–9857 9855

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represents the first example of an NHC adduct of a lead dihalide
fragment (see ESIw for details). The reaction of [(IPr)PbBr2] with
[{(MesNacnac)Mg}2] in toluene at ꢀ80 1C led to the immediate
deposition of lead metal and a yellow coloured solution. When
this mixture was warmed to 0 1C, the solution took on a paler
yellow colour and the soluble component of the mixture was
than in the higher coordinate precursor compound, [(IPr)SnCl2]
(2.341(8) A),14 but identical to the mean for all known
C–Sn(two-coordinate) bonds.19 In general, there is good agreement
between the structural parameters of 3 and the previously
calculated geometries of less hindered model complexes,
[(NHC)SnQSn(NHC)] (NHC = :C{N(R)C(H)}2; R = H, Me
or Ph).13,24 All of the available spectroscopic and structural data
for 3 are consistent with its formulation as an NHC adduct of the
singlet, doubly bonded tin(0) fragment, :SnQSn: (cf. 1 and 2).
Quantum chemical calculations (RI-BP86/TZVPP) were
carried out on the full molecule, 3, in the gas phase, and on
the sterically condensed model complex, [(NHC)SnQSn(NHC)]
(NHC = :C{N(Me)C(H)}2) 3a. The optimized geometries of
both complexes (see Fig. S4, ESIw) are similar to the crystallo-
graphically determined geometry for 3, but with slightly over-
estimated Sn–Sn bond lengths (3 2.758 A, 3a 2.773 A) and Sn–C
separations (3 2.297 A, 3a 2.294 A). This is likely due to
solid state effects which are known to shorten bonds.25 The
Sn–Sn–C angles of the calculated structures (3 95.51, 3a 87.51) are
significantly different, but lie either side of the experimental value
for 3. An NBO analysis of the electronic structure of 3a revealed
it to be similar to that described for a model of 2, viz.
[(NHC)GeQGe(NHC)] (NHC = :C{N(Me)C(H)}2) 2a.9 That
is, 3a can be viewed as an NHC adduct of the Sn2 fragment, the
electronic reference state of which is the 1Dg excited singlet state.
Experimentally, this lies 10.0 kcal molꢀ1 higher in energy than
the triplet ground state (X3Sgꢀ),26 and possesses a double bond
comprised of filled s and p bonding MOs. The electronic
structure of the singlet Sn2 molecule allows for donation of the
minus and plus combinations of the carbene lone pairs into its
empty p-bonding, and an empty p*-anti-bonding orbital respec-
tively (see Fig. S5, ESIw). Indeed, the NBO analysis of complex
3a similarly shows its SnQSn interaction to be constructed from
a s-bond (HOMO ꢀ 1) and an out of plane p-bond (HOMO) (see
Fig. S6, ESIw), both of which are very high in p-character (90.8%
and 99.8% respectively). Moreover, consistent with the strongly
‘‘trans-bent’’ geometry of 3a, its Sn-lone pairs (HOMO ꢀ 2,
HOMO ꢀ 3) are high in s-character (86.4%). This overall picture
of the bonding in 3a is similar to that previously calculated.13
In order to shed further light on the strength and nature of
the NHC–Sn orbital interactions in 3a, the molecule was
analyzed using the energy decomposition analysis technique
(EDA). A summary of the numerical results obtained from
this analysis are given in Table 1. The total interaction energy
DEint between Sn2 (1Dg) and the two NHC ligands using
the frozen geometries of the fragments was calculated at
ꢀ84.1 kcal molꢀ1. This affords a bond dissociation energy
(De) of 61.9 kcal molꢀ1 after geometrical and electronic
relaxꢀation yields two free NHC molecules and Sn2 in its
X3Sg ground state. Therefore, the average De for the C-Sn
donor–acceptor bonds is 31.0 kcal molꢀ1, which is significantly
lower than the De for the C-Ge bonds of 2a (37.8 kcal molꢀ1).
The breakdown of the total interaction into its main components
reveals that the C-Sn bonds of 3a are more electrostatic
(DEelstat = 63.7%) than covalent (DEorb = 36.3%). In fact, this
differential is greater than that calculated for 2a (DEelstat = 59.2%,
DEorb = 40.8%),9 presumably because of the larger size and more
electropositive nature of Sn relative to Ge. Furthermore, an
estimate of the strength of the various orbital contributions to
1
shown by H NMR spectroscopy to be >95% free IPr. These
observations suggest it is unlikely that [(IPr)PbQPb(IPr)] will be
accessible as a room temperature stable material.
In the solid state, 3 slowly decomposes at room temperature,
and rapidly at ca. 50 1C (cf. 1 209 1C decomp.,5 2 162–165 1C
decomp.9). It is not stable in solution at 20 1C and decomposes
to deposit tin metal, while generating free IPr. Its solution state
1H and 13C{1H} NMR spectra (D8-toluene) were obtained at
ꢀ30 1C, and these are consistent with its proposed structure. Of
note is the carbene resonance in the 13C{1H} NMR spectrum
(d = 210.3 ppm), which appears at lower field than for both
[(IPr)SiQSi(IPr)] (d = 196.3 ppm)5 and [(IPr)GeQGe(IPr)] (d =
203.3 ppm);9 and higher field than for the free carbene, IPr (d =
220.6 ppm).17 This trend in chemical shifts indicates that the E–C
interactions in the NHC adduct series, [(IPr)EQE(IPr)] (E = Si,
Ge or Sn), become weaker with increasing molecular weight of E.
No signal was observed in the 119Sn{1H} NMR spectrum of 3.
An X-ray crystal structure analysis of 3 was carried out and
its molecular structure is depicted in Fig. 1. This shows the
compound to be broadly isostructural with the lighter congeners,
1 and 2, in that it has a heavily ‘‘trans-bent’’ geometry with its IPr
ligands almost orthogonal to the Sn2 fragment (C–Sn–Sn =
91.82(8)1). This can be compared with the calculated value of
76.71 for (OC)SnSn(CO).18 The Sn–Sn distance in the compound
(2.7225(5) A, cf. 2.851 A for (OC)SnSn(CO)18) lies within the
known range for three-coordinate, doubly bonded distannenes
(R2SnQSnR2, 2.668–2.851 A),19 and is at the upper end of the
bond length range for multiply bonded distannynes (RSnSnR,
2.646–2.736 A).20 Moreover, the bond length is comparable with
the Sn–Sn separation in elemental tin (2.81 A),21 and the
experimental22 and calculated23 values for the Sn2 molecule
(2.75 A and 2.73 A respectively) in its triplet electronic ground
state (X3Sgꢀ). The C–Sn distances in 3 (2.280(3) A) are shorter
Fig. 1 Molecular structure of 3 (25% thermal ellipsoids; hydrogen
atoms omitted). Selected bond lengths (A) and angles (1): Sn(1)–C(1)
2.280(3), Sn(1)–Sn(1)0 2.7225(5), N(1)–C(1) 1.362(4), N(2)–C(3) 1.386(4),
C(2)–C(3) 1.329(5), C(1)–Sn(1)–Sn(1)0 91.82(8), N(1)–C(1)–N(2) 103.1(3).
0
Symmetry operation: ꢀx, ꢀy + 1, ꢀz.
c
9856 Chem. Commun., 2012, 48, 9855–9857
This journal is The Royal Society of Chemistry 2012

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Table 1 Results of the EDA calculations of complex 3a using (1Dg)
Sn2 and 2 :C{N(Me)C(H)}2 as interacting fragments. The energy
values are given in kcal molꢀ1 (C2h symmetry)
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DEint
DEPauli
DEelstat
DEOrb
DE(ag)
DE(bg)
DE(au)
DE(bu)
ꢀ84.1
251.3
ꢀ213.6
ꢀ121.2
ꢀ56.6
ꢀ1.5
(63.7%)
(36.3%)
(46.3%)
(1.3%)
(6.6%)
(45.5%)
ꢀ8.1
ꢀ55.7
DEprep (NHC)
DEprep (Sn2)
6.5
15.7
ꢀDe (X3Sꢀ
)
g
ꢀ61.9
the DEorb term was obtained. The major contribution comes
from plus (46.3%) and minus (45.5%) combinations of the
carbon lone-pair orbitals (C-Sn), while donation from the
occupied out of plane p MO of Sn2 into the empty p(p) orbitals
of the carbene carbon atoms (DE(au)) provides only 6.6% of
DEorb. Therefore, there is apparently very little Sn-C p-back-
bonding occurring in 3a.
In conclusion, the first NHC complex of a row 5 element (viz.
tin) in its diatomic form has been prepared. A combination of
spectroscopic, crystallographic and computational analyses of the
diamagnetic compound indicate that it is best considered as an
NHC adduct of the singlet state, doubly bonded tin(0) fragment,
:SnQSn:. Because the compound decomposes at close to room
temperature in solution and the solid state, yielding tin metal, it
has potential to act as a soluble source of elemental tin in its
further reactions. We are exploring this potential, in addition to
using more sterically hindered NHCs in attempts to prepare
isolable lead(0) dimers of the type, [(NHC)PbPb(NHC)].
CJ and AS gratefully acknowledge financial support from
the Australian Research Council. GF thanks the Deutsche
Forschungsgemeinschaft.
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obtain the crystalline form of [(IPr)GeQGe(IPr)]
2 that is
isomorphous to 3, to allow more direct structural comparisons
between the compounds. While this was not successful, two new
structural modifications of 2 were obtained (see ESIw for details).
25 V. Jonas, G. Frenking and M. T. Reetz, J. Am. Chem. Soc., 1994,
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current study is 15.6 kcal molꢀ1
.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9855–9857 9857