Contrasting reductions of group 14 metal(ii) chloride complexes: Synthesis of a β-diketiminato tin(i) dimer

Article abstract of DOI:10.1039/c2cc18086c

Reductions of the β-diketiminato group 14 metal(ii) chloride complexes, [(^ButMesNacnac)ECl] (^ButMesNacnac = [(MesNCBu^t)₂CH]^-; Mes = mesityl; E = Ge, Sn or Pb), with a magnesium(i) dimer have led to di

Full text of DOI:10.1039/c2cc18086c

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COMMUNICATION
Contrasting reductions of group 14 metal(II) chloride complexes:
synthesis of a b-diketiminato tin(I) dimerwz
Sam L. Choong, Christian Schenk, Andreas Stasch, Deepak Dange and Cameron Jones*
Received 26th December 2011, Accepted 8th January 2012
DOI: 10.1039/c2cc18086c
Reductions of the b-diketiminato group 14 metal(II) chloride
complexes,
[(ButMesNacnac)ECl] ButMesNacnac
[(MesNCBut)2CH]ꢀ; Mes = mesityl; E = Ge, Sn or Pb), with
a magnesium(I) dimer have led to differing outcomes, which
include the formation of the first b-diketiminato group 14
metal(I) dimer, [{(ButMesNacnac)Sn}2].
disproportionation processes,7 reductive ring contraction reactions
(
=
(to give 28), and the formation of the remarkable monomeric
germanium(I) radical, 3, which is sterically ‘‘frustrated’’ from
dimerising by its extremely bulky ligand.9 We reasoned that utilising
a slightly less bulky Nacnac system than that in 3 would allow for
the formation of group 14 metal(I) dimers, while still providing
sufficient kinetic protection from disproportionation processes. Here
we report on the reductions of the moderately bulky metal(II)
The rapid development of the chemistry of low oxidation state
group 14 compounds over the last decade has been one of the
major driving forces behind the renaissance that is occurring in
main group chemistry. This progress is typified by the heavier
group 14 alkyne analogues (or ditetrelynes), REER (E = Si, Ge,
Sn or Pb; R = bulky terphenyl, silyl, aryl or amide), the unusual
structure, bonding and reactivity of which has been extensively
investigated.1 Over the past five years the chemistry of these two-
coordinate systems has been extended to that of related three- and
four-coordinate ‘‘intra-molecularly base stabilised’’ examples which
incorporate sterically bulky, chelating N-donor ligands, viz. LEEL
(e.g. L = amidinate, guanidinate, N-functionalised aryl, P-functio-
nalised amide etc.).2 Amidinato coordinated examples of these
compounds are emerging as powerful reagents for the activation of
small molecules, unsaturated substrates etc.3 Despite this, their
preparations, via the alkali metal reduction of group 14 halide
precursors, are typically low yielding. To overcome this problem
we have recently developed moderate to high yielding routes to a
series of ‘‘trans-bent’’ amidinato-element(I) dimers, 1,2a using
soluble magnesium(I) dimers as alternative reducing agents.4
chloride complexes, [(ButMesNacnac)ECl]
( =
ButMesNacnac
[(MesNCBut)2CH]ꢀ; Mes = mesityl; E = Ge, Sn or Pb) which
led to significantly different outcomes, including the formation of
the first b-diketiminato group 14 metal(I) dimer (for E = Sn).
The precursor complexes, [(ButMesNacnac)ECl] (E = Ge
4,10 Sn 5 or Pb 6), were prepared by reaction of in situ
generated [Li(ButMesNacnac)] with either GeCl2ꢁdioxane, SnCl2
or PbCl2. The tin and lead complexes have not been previously
reported and, accordingly, were spectroscopically characterised.
Of note here is the 119Sn{1H} NMR spectrum of 5 which exhibits
a resonance (d ꢀ235.5 ppm) at a field similar to that of
related complexes, e.g. d ꢀ252.0 ppm for [(ButNacnac)SnCl]
The
b-diketiminate
(Nacnac)
class
of
ligand,
[(R1NCR2)2CR3]ꢀ (R1,2,3 = H, alkyl, aryl, silyl etc.) is closely
related to amidinates, and these ligands have been widely used to
stabilise complexes containing low valent metal centers from
across the periodic table.5 In spite of this, no examples of b-
diketiminato group 14 element(I) dimers, [{(Nacnac)E}2], have
yet been reported.6 Attempts to prepare such compounds via the
alkali metal reduction of metal(II) precursors, [(Nacnac)ECl]
(E = Ge or Sn), have instead led to various outcomes, including
(
ButNacnac = [(DipNCBut)2CH]ꢀ, Dip = C6H3Pri2ꢀ2,6).8a
Compounds 4–6 were initially reduced by treating them with
half an equivalent of the magnesium(I) dimer, [{(MesNacnac)Mg}2]
(
MesNacnac = [(MesNCMe)2CH]ꢀ),4b in toluene. The reaction
with 5 afforded a moderate isolated yield (55%) of the tin(I) dimer,
7, as a deep green crystalline solid (Scheme 1). The reduction of
4, on the other hand, gave a low yield of the wine-red ring-
contracted product, 8, and returned a significant amount of
unreacted 4. From these observations it was clear that the
reduction of 5 is a 1-electron process, whereas in the synthesis
of 8, the precursor complex 4 is doubly reduced. In view of this,
the latter reduction was repeated, but with one equivalent of
School of Chemistry, PO Box 23, Monash University, Melbourne,
VIC 3800, Australia. E-mail: cameron.jones@monash.edu;
Fax: +61-3-9905-4597; Tel: +61-3-9902-0391
w This article is part of the ChemComm ‘Frontiers in Molecular Main
Group Chemistry’ web themed issue.
z Electronic supplementary information (ESI) available: full details
and references for synthetic, crystallographic and computational
studies. CCDC 859474–859478. See DOI: 10.1039/c2cc18086c
c
2504 Chem. Commun., 2012, 48, 2504–2506
This journal is The Royal Society of Chemistry 2012

(see Fig. 1). It exhibits a Sn-Sn bond length which is comparable
to those in the related dimeric, singly bonded tin(I) complexes
(e.g. 1 E = Sn, 3.0141(8) A;2a ArSnSnAr, Ar = terphenyl
3.058–3.075 A,12 C6H3(CH2NMe2)2-2,6 2.971(1) A,2c or
C6H3(CMeNDip)2-2,6 2.898(1) A).2g Unlike 1, which has a
trans-bent structure, compound
7 exhibits an unusual,
and unsymmetrical gauche-bent arrangement between its two
heterocycles. This situation is somewhat similar to the geometries
of the higher coordinate complexes, ArSnSnAr (Ar
=
C6H3(CH2NMe2)2-2,6 or C6H3(CMeNDip)2-2,6). The tin atoms
of the two heterocycles in 7 are markedly displaced from their
N2C3 ligand backbone least squares planes (by 1.135 A, Sn(1);
and 1.228 A, Sn(2)), while the metrical parameters within those
b-diketiminate backbones suggest they are not fully delocalised.
Furthermore, the unsymmetrical nature of the molecule is high-
lighted by the significant difference in the sum of the angles about
the tin centres (Sn(1): 279.61; Sn(2): 306.81), though both values
imply the presence of lone pairs of electrons at the metal, each
having a high degree of s-character.
Scheme 1 Reagents and conditions: i, E = Sn, 1/2 [{(MesNacnac)Mg}2],
ꢀ1/2 [{(MesNacnac)Mg(m-Cl)}2]; ii, E = Ge, [{(MesNacnac)Mg}2], ꢀ1/2
[{(MesNacnac)Mg(m-Cl)}2]; iii, ring contraction.
[{(MesNacnac)Mg}2], and this gave a good yield (65%) of 8.
In contrast, the reduction of 6 (or the bulkier analogue
[(ButNacnac)PbCl]) resulted in the deposition of lead metal
The molecular structure of 8 (Fig. 2) shows it to be monomeric,
with a pyramidal Ge centre that by implication possesses a
stereochemically active lone pair of electrons. This does not
appear to be directed towards the magnesium centre, and there-
fore it is likely that there is little bonding character between the
two atoms (GeꢁꢁꢁMg separation: 3.050(1) A). The germanium
heterocycle is close to planar and its metrical parameters suggest it
contains largely localised C(1)–C(2) and C(3)–N(1) double bonds,
in contrast to the partially delocalised Ge heterocycles in 2.8 There
is a marked difference in the magnitude of the two Ge–N bond
lengths in the compound, which is consistent with Ge(1)–N(1)
being a dative interaction, while Ge(1)–N(2) is a covalent bond.
The geometry of the essentially planar Mg heterocycle is unexcep-
tional and reveals it to have a largely delocalised b-diketiminate
backbone.
and the generation of
a complex mixture of soluble
products, from which low yields of the homoleptic PbII
complex, [Pb(ButMesNacnac)2], and the dimeric adduct,
[{(MesNacnac)MgCl}2{(ButMesNacnac)PbCl}2], were crystallised
(see Supplementary Information for further detailsz). It seems
likely that this reaction does proceed via a lead(I) intermediate,
but this is unstable towards disproportionation.
With regard to the mechanisms of formation of 7 and 8, it is
plausible that the reduction of 5 generates the transient tin(I)
radical monomer, [(ButMesNacnac)Sn:]ꢂ (cf. 3), which dimerises to
give 7. If the related germanium(I) radical, [(ButMesNacnac)Ge:]ꢂ,
is transiently generated in the reduction of 4, the steric bulk of the
ligand may be too great to allow it to rapidly dimerise (covalent
radii: Ge 1.22 A, Sn 1.40 A11). Instead, it could undergo a second
reduction with [{(MesNacnac)Mg}2] to give a germylenyl magne-
sium complex, 9, which is unstable and undergoes a ring contrac-
tion reaction to give 8. It is of note that intermediates closely
related to 8 and 9 have been proposed for the mechanisms of
formation of 2 from the reduction of [(Nacnac)GeCl] compounds
with alkali metals.8
In non-coordinating solvents both 7 and 8 decompose at
20 1C over one day or several weeks respectively, to give
unidentified mixtures of soluble products, and a deposit of tin
1
metal in the case of 7. Although the H and 13C{1H} NMR
spectroscopic data for the compounds are largely consistent
with the compounds retaining their solid state structures
(vide infra) in solution, the number of resonances in the spectra
does suggest more symmetrical time averaged structures for the
compounds. This, in turn, implies relatively unhindered rotations
of the two heterocycles in each compound about their respective
Sn–Sn and N–Ge/Mg bonds. The 119Sn{1H} NMR spectrum of 7
exhibits a singlet resonance at d 502.1 ppm, which is downfield
from the signal for the three-coordinate precursor, 4, by more than
730 ppm, but considerably upfield of the resonance for the closely
related amidinate coordinated dimer, 1 (E = Sn, d 777.7 ppm).2a
Compound 7 is the first crystallographically characterised
example of a b-diketiminato-group 14 element(I) dimer
Fig. 1 Molecular structure of 7 (25% thermal ellipsoids; hydrogen
atoms omitted). Selected bond lengths (A) and angles (1): Sn(1)–Sn(2)
3.0685(9), Sn(1)–N(1) 2.215(2), Sn(1)–N(2) 2.2300(19), Sn(2)–N(3)
2.215(2), Sn(2)–N(4) 2.264(2), N(1)–C(2) 1.346(3), N(2)–C(4)
1.320(3), C(2)–C(3) 1.403(3), C(3)–C(4) 1.428(3), N(1)–Sn(1)–N(2)
82.29(7), N(1)–Sn(1)–Sn(2) 89.05(6), N(2)–Sn(1)–Sn(2) 108.23(5),
N(3)–Sn(2)–N(4)
83.81(8),
N(3)–Sn(2)–Sn(1)
116.38(5),
N(4)–Sn(2)–Sn(1) 93.39(6).
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 2504–2506 2505

magnesium(I) dimer have led to different outcomes, which
include the formation of an unprecedented b-diketiminato
tin(I) dimer. The results obtained here, in combination with
those from previous studies,8,9 have revealed that subtle
changes in the steric profile of b-diketiminate (Nacnac) ligands
can have very marked effects on the course that reductions of
compounds of the type, [(Nacnac)ECl], can take.
CJ gratefully acknowledges financial support from the
Australian Research Council, the donors of The American
Chemical Society Petroleum Research Fund, and the US Air
Force Asian Office of Aerospace Research and Development.
CS thanks the Alexander von Humboldt Foundation. Part of
this research was undertaken on the MX1 beamline at the
Australian Synchrotron, Victoria, Australia.
Notes and references
Fig. 2 Molecular structure of 8 (25% thermal ellipsoids; hydrogen
atoms omitted). Selected bond lengths (A) and angles (1): Ge(1)–N(2)
1.8477(14), Ge(1)–C(1) 2.0079(17), Ge(1)–N(1) 2.0402(14),
Mg(1)–N(2) 1.9504(16), Mg(1)–N(4) 2.0206(15), Mg(1)–N(3)
2.0216(15), N(1)–C(3) 1.311(2), C(1)–C(2) 1.356(2), C(2)–C(3)
1.451(2), N(2)–Ge(1)–C(1) 123.24(6), N(2)–Ge(1)–N(1) 103.54(6),
1 Selected reviews on ditetrelyne chemistry: (a) P. P. Power, Acc.
Chem. Res., 2011, 44, 627; (b) A. Sekiguchi, Pure Appl. Chem.,
2008, 80, 447; (c) P. P. Power, Organometallics, 2007, 26, 4362. For
an amido-digermyne, see ; (d) J. Li, C. Schenk, C. Goedecke,
G. Frenking and C. Jones, J. Am. Chem. Soc., 2011, 133,
18622.
2 See, for example, (a) C. Jones, S. J. Bonyhady, N. Holzmann,
G. Frenking and A. Stasch, Inorg. Chem., 2011, 50, 12315;
(b) D. Gau, R. Rodriguez, T. Kato, N. Saffron-Merceron, A. de
Cozar, F. P. Cossio and A. Baceiredo, Angew. Chem., Int. Ed.,
2011, 50, 1092; (c) S. Khan, R. Michel, J. M. Dietrich, R. A. Mata,
H. W. Roesky, J.-P. Demers, A. Lange and D. Stalke, J. Am.
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C(1)–Ge(1)–N(1)
81.96(7),
N(4)–Mg(1)–N(3)
94.65(6),
Ge(1)–N(2)–Mg(1) 106.82(7).
DFT analyses (RI-BP86/def2-TZVPP/def2-SVP) of 7 and 8
in the gas phase led to optimised geometries for both molecules
that are in good agreement with their solid state structures
(see Supplementary Information for full detailsz), though the
calculated Sn-Sn distance of 7 (3.225 A) was over-estimated by
ca. 5%. The electronic structure of both molecules was examined,
and that for 8 revealed no significant bonding interaction
between its Ge and Mg centres. In the case of 7, its HOMO
almost exclusively comprises its Sn–Sn s-bond which is of very
high p-character (94.0%), whilst the highest energy orbital
displaying significant Sn-lone pair character is the HOMOꢀ11
(see Fig. 3). The LUMO+2 and LUMO+3 of 7 possess Sn
p-orbital character, which is associated with a single tin centre in
each case. The bond dissociation energy (BDE) of 7 (yielding two
[(ButMesNacnac)Sn:]ꢂ fragments in an electronic doublet state)
was calculated at 11.9 kcal molꢀ1, i.e. considerably lower than
that for 1 (E = Sn), 19.9 kcal molꢀ1, which was obtained using a
similar level of theory (RI-BP86/def2-TZVPP).2a It should be
noted, however, that the calculated Sn–Sn bond in 1 (3.111 A) is
shorter than that in 7 in the gas phase by more than 0.1 A. This
could indicate that Sn-Sn BDEs of tin(I) dimers such as 1 and 7
are sensitive to the lengths of those bonds.
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complexes, [(ButMesNacnac)ECl] (E = Ge, Sn or Pb), with a
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N.B. For a related system, see ref. 2i.
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Fig. 3 (a) HOMO and (b) HOMOꢀ11 of 7.
2506 Chem. Commun., 2012, 48, 2504–2506
c
This journal is The Royal Society of Chemistry 2012