Dimerization of indanediyl fragments: An alkene analogue for group 13?

Article abstract of DOI:10.1002/anie.200500764

(Figure Presented) A β-diketiminate complex of indium, [ln{N(2,4,6-Me₃C₆H₂)C(Me)}₂CH], dimerizes in the solid state (see space-filling model) to provide an indium species that is formally isoelectronic to the historically significant bis(stannane)diyls. A direct In - In bond is formed yielding a centrosymmetric dimer that possesses approximate C_2h symmetry with a mutual trans-bent orientation of the N-chelating ligands.

Full text of DOI:10.1002/anie.200500764

Angewandte
Chemie
Metal–Metal Interactions
Dimerization of Indanediyl Fragments: An
Alkene Analogue for Group 13?**
Michael S. Hill,* Peter B. Hitchcock, and
Ruti Pongtavornpinyo
The occasionally vexed question of the potential for homo-
nuclear multiple bonding between the heavier elements of
Groups 13 and 14 continues to provoke intense interest.[1] For
the wide-ranging family of Group 14 derivatives in this
category, a lineage that culminated in the recent synthesis of
the long-sought silicon alkyne analogue, 1,1,4,4-tetrakis[bis-
(trimethylsilyl)methyl]-1,4-diisopropyl-2-tetrasilyne,[2] may
be traced directly to the seminal discovery of the dimeric
stannanediyl [{[(Me3Si)2HC]2Sn}2] by Lappert and co-work-
ers.[3] All possible alkene and alkyne analogues based on a
link between two Group 14 centers have since been pre-
pared.[4] The structures of these R2EER2 and REER (E =
Group 14 element) complexes substantiate an incremental
decrease in the strength of any p-bonded component on
transition from the respective planar D2h and linear D1h
structures. More pronounced trans-bent (C2h) configurations
(defined by the out-of-plane angle d) are observed as the
group is descended, alongside an increasing tendency of the
R2EER2 species to dissociate to monomeric forms in solution.
These observations have been attributed to the increasing
stability of the lone pair of electrons in the ns orbital with
increasing atomic number and the formation of polarized
donor–acceptor interactions between the metal centers. A
complementary molecular-orbital description ascribes
increased nonbonding (or even antibonding) character to
the bu highest unoccupied molecular orbital (HOMO),
through s* mixing, with increasing principal quantum
number of the valence orbitals of E.[1,4]
Similar questions of multiple bond order in respect to
Group 13 species have been especially problematic. The claim
by Robinson and co-workers[5] of a triple-bond interaction
between the Ga centers of the reduced species
[Na2(GaC6H32,6-trip)2] (trip = 2,4,6-iPr3C6H2) provoked vig-
orous debate,[5–13] while the subsequent work of Power and co-
workers has highlighted as many contrasts as similarities
within complexes maintaining contacts between formally low-
valent heavier Group 13 and 14 elements.[14–16]
[*] Dr. M. S. Hill, R. Pongtavornpinyo
Department of Chemistry
Imperial College London
Exhibition Road, South Kensington
London SW72AZ (UK)
Fax: (+44)207-594-5804
E-mail: mike.hill@imperial.ac.uk
Dr. P. B. Hitchcock
School of Chemistry
University of Sussex
Brighton, BN19QJ (UK)
[**] We thank the Royal Society for the provision of a University Research
Fellowship (M.S.H.).
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1
Our entry point was provided by the syntheses of the
solution in aromatic solvents. Prompt acquisition of H and
monovalent indium and thallium b-diketiminate derivatives 1
13C NMR spectra in [D6]benzene, however, enabled collec-
tion of data that were consistent with Cs- and C2-symmetric
structures for 5 and 6, respectively. This solution lability to
reductive decomposition has thus far restricted a thorough
assessment of the nature of these species in solution.
However, our appraisal of the strength of the In–In inter-
actions present in the solid state (see below) and the analysis
of Power and co-workers of the unique neutral Group 13
dimers [(2,6-dippC6H3M)2] (M = Ga, In)[14,16] indicate that it is
unlikely that anything other than transient In–In interactions
persist in solution.[22]
and 2, respectively.[17] These compounds exist as well-sepa-
rated singlet “carbene analogues” and are isostructural to the
previously reported gallium and aluminum complexes, 3 and
4, respectively.[18,19] It has been observed that variation of the
overall steric demands of the bulky terphenyl ligands in
combination with a low-valent gallium or indium center
results in either monomeric or metal–metal-bonded species,
the nuclearity of which is dictated by the size and topology of
the stabilizing ligand system.[16,20] Although investigations by
us suggested that an ability to isolate the two-coordinate
species 1–4 was chiefly a product of the kinetic stability
imparted by the bulky N-aryl 2,6-diisopropylphenyl (dipp)
substituents,[17–19] we reasoned that use of substituents with
decreased steric demands would similarly promote the
formation of compounds with higher nuclearity. A partic-
ularly appealing target was the dimeric species, illustrated as
the localized enamideimine form I, which may be considered
as essentially isoelectronic to the stannanediyl discovered by
Lappert and co-workers and other dimeric stannanediyls
(Scheme 1).
Although crystalline samples of both complexes may be
successfully manipulated for crystallographic purposes, both
compounds darken because of the separation of metallic
indium over the course of several days, even when stored at
low temperature under N2 and in the dark. These difficulties
notwithstanding, the molecular structures of both compounds
were determined by single-crystal X-ray diffraction studies.
Although the data for 5 were of a poor quality, the structure,
in common with our previously reported indium complexes
bearing dipp substituents at both nitrogen donor centers, was
unambiguously monomeric and contained no short In–In
interactions (Figure 1). It is apparent that the presence of a
single dipp group is sufficient to prevent any well-defined
intermolecular contact.
=
=
Scheme 1. Generalized Sn Sn and In In complexes and target dimeric
species I.
Figure 1. Structure of 5 (ORTEP plot, ellipsoids are set at 20% proba-
bility; H atoms are omitted for clarity). Selected bond lengths [ꢀ] and
angles [8]: In-N1 2.277(10), In-N2 2.257(9), N1-C1 1.339(16), N1-C6
1.461(18), N2-C3 1.318(15), N2-C18 1.453(15), C1-C2 1.389(18), C2-C3
1.414(17); N1-In-N2 82.6(4), In-N1-C1 128.0(8), In-N2-C3 129.0(7),
N1-C1-C2 125.1(10), C1-C2-C3 129.5(13), C2-C3-N2 125.3(12).
Known anions II and III were selected to provide an
incremental decrease in the steric demands of the supporting
ligand environment.[21] The appropriate b-imino amine pre-
Further X-ray analysis of 6 provided high-quality data and
revealed that replacement of both of the dipp substituents of 1
by mesityl permits dimerization through the formation of a
ꢀ
direct In In bond. The centrosymmetric dimer possesses
cursor was used in a synthetic route identical to that employed
in the synthesis of 1, providing facile access to InI derivatives
supported by II and III. Although the THF reaction solutions
were somewhat photo- and thermally sensitive to depositing
indium metal, rapid workup and crystallization in the dark
from hexane at 58C provided yellow crystals of the desired
Ini derivatives 5 and 6. In common with the behavior of 1,
both compounds begin to decompose immediately on dis-
approximate C2h symmetry with a mutual trans-bent orienta-
tion of the N-chelated ligands (Figure 2). It is notable,
therefore, that 6 provides the first example of a neutral
dimeric Group 13 species that is formally isoelectronic to the
historically significant bis(stannane)diyls. The space-filling
models shown in Figure 3 emphasize the efficient encapsula-
ꢀ
tion of the In In bond axis provided by the N-mesityl
substituents of the chelate ligands within the dimeric arrange-
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planar alkene-like D2h structure) than that observed in the
majority of homoleptic stannylenes of the general formula
R2SnSnR2 (typically d = 30–458).[1]
Dimerization has little effect on the local environment
around the indium center provided by the chelated b-
diketiminato ligand. The N1-In-N2 angle (83.15(8)8) is similar
to the angles observed in the mononuclear compounds 1 and 5
ꢀ
(81.12(10) and 82.6(4)8, respectively), whereas the In N1 and
ꢀ
In N2 bond lengths (2.262(2) and 2.256(2) ꢀ, respectively)
are actually the shortest observed for any indium(i) b-
diketiminato chelate.[17] This latter, somewhat counterintui-
tive, feature occurs despite the increased coordination
number of the indium centers and may be an illustration of
the decreased steric demands of the chelated ligands and
intrasubstituent repulsion.
Figure 2. Structure of 6 (ORTEP plot, ellipsoids are set at 25% proba-
bility; H atoms are omitted for clarity). Selected bond lengths [ꢀ] and
angles [8]: In-N1 2.262(2), In-N2 2.256(2), In-In’ 3.1967(4), N1-C1
1.336(3), N1-C6 1.435(3), N2-C3 1.327(3), N2-C15 1.435(3), C1-C2
1.394(4), C2-C3 1.397(4); N1-In-N2 83.15(8), N1-In-In’ 110.65(6), N2-
In-In’ 107.88(6), In-N1-C1 127.68(19), In-N2-C3 128.73(19), N1-C1-C2
125.1(3), C1-C2-C3 130.2(3), C2-C3-N2 124.4(3).
We undertook DFT calculations on the dimeric model
complex [{In[N(H)C(H)]2CH}2] by using the B3LYP DFT
and LANL2DZ pseudo potentials (and basis set) imple-
mented in Gaussian 03.[26] The bond lengths and angles with
regard to the two indium-containing fragments of the fully
optimized structures are, in general, within 2% of the
experimentally determined parameters. The In–In distance
for the model system is, however, somewhat overestimated at
3.388 ꢀ, as is the degree of trans bending indicated by the out-
of-plane angle of 77.698. The calculated molecular orbitals of
this model compound do, however, reveal a qualitative insight
into the In–In bonding present within 6 and provide a cogent
comparison to bonding models developed for related
Group 13 and 14 species. The frontier molecular-orbital
diagram is presented in Figure 4 and illustrates that the
Figure 3. Space-filling (POVRAY) illustrations of 6 showing the kinetic
ꢀ
protection of the In In bond.
ꢀ
ment. The In In distance in 6 (3.1967(4) ꢀ) is significantly
longer than that observed in the dimeric terphenyl derivative
[(2,6-dippC6H3In)2] (2.9786(5) ꢀ).[16] Irrespective of the pre-
cise details of the bonding, this elongation can be interpreted
in a straightforward manner as a consequence of the increased
overall coordination number of the individual indium centers
in 6 (three compared to two). This view is given further
credence by the observation that the only other known
Ini dimer features pentahapto coordination with a pentaben-
zylcyclopentadienyl anion and a very weak In···In interaction
of 3.631(2) ꢀ.[23] The In In distance in 6 also lies well above
ꢀ
ꢀ
the range established for typical In In single bonds, such as
those within the b-diketiminato disproportionation product
[{{HC[CMeN(dipp)]2}InCl}2] (2.8343(7) ꢀ) and the three-
coordinate indium centers of [{(2,4,6-iPr3C6H2)2In}2]
(2.775(2) ꢀ), both of which feature formally divalent
indium.[24,25] The b-diketiminato chelate structure is effec-
tively planar (root-mean-square deviation of the In-N1-C1-
C2-C3-N2 plane: 0.03 ꢀ) with the result that the trans-bent
configuration may be accurately defined by the In’-In-C2
angle of 113.028. This represents an out-of-plane angle of
66.988 and indicates that dimerization occurs with a signifi-
cantly greater degree of pyramidalization (with respect to a
Figure 4. Calculated (B3LYP/LANDZ) frontier molecular-orbital
diagram for [{In[(N(H)C(H)]2CH}2] illustrating the bu HOMO and
ag HOMO-3. LUMO=lowest unoccupied molecular orbital.
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residue extracted with hexane (25 mL). Filtration, concentration to
approximately 10 mL, and storage at 58C afforded bright yellow
crystals suitable for X-ray analysis (0.29 g, 49%). Elemental analysis
calcd (%) for C24H31InN2O: C 60.26, H 6.55, N 5.86; found: C 62.26, H
HOMO is a bu orbital that is nonbonding with regard to In–In
contact. The degenerate HOMO-1 and -2 provide a level of
p overlap between the indium centers and the nitrogen donor
atoms of the chelated ligands. The HOMO-3 is an In–In s-
bonding orbital of Ag symmetry. This ag orbital thus provides
the primary bonding interaction between the metal centers.
This view of the metal–metal bonding within 6 is qualitatively
similar to that which has been evinced for alkene analogues of
the heavier Group 14 elements and some multiply bonded
gallium species.[1,9–13,27] The bu HOMO in these cases has been
variously interpreted as a “slipped” p bond or as possessing
lone-pair or even antibonding character. Experimental and
theoretical studies of heavier Group 14 R2EER2 species
indicate that, despite increased lone-pair character with
increased atomic mass of E, the p-bonding bu HOMO acts
to provide a minor augmentation of the overall bonding
interaction between the metal centers.[1] A similar rationale
has been applied to the recently completed series of REER
derivatives.[4] The data presented herein and the somewhat
limited literature precedents highlight some notable contrasts
within Group 13. The experimental data indicate that In–In
bonding with p character within 6 is feeble at best, or indeed
destructive. A further calculation based on the separation of
[{In[(N(H)C(H)]2CH}2] into two singlet indanediyl fragments
provided a bond-dissociation energy of only 2 kcalmolꢀ1. This
extremely low value is commensurate with values calculated
(ca. 3 kcalmolꢀ1) for the separation of the two {InH} frag-
ments of [In2H2] and suggests that the notion of “multiple
bonding” within molecules such as 6 is fallacious if the only
expectation is an augmentation of the overall bond
strength.[28] The weakness of the Ga–Ga bonding within the
neutral complex [{(2,6-dipp)2C6H3Ga}2] was similarly attrib-
uted to the large energy difference between the lone pair on
the singlet Gai center and the p orbitals.[14] This situation is
likely to be exacerbated in the current case by N–In
p interactions within the planar b-diketiminato chelates and
indicates that the “multiple bonding” present within 6
actually represents an overall bond order of less than unity.
This view of the HOMO within homonuclear bonding in
Group 13 elements has been emphasized previously by
calculations carried out on the hypothetical cationic species
[MeGaGaMe]+.[11] In this case, removal of an electron from
the bu HOMO of [MeGaGaMe] caused a decrease of
1
5.47, N 5.42 (partially decomposed in transit); H NMR (270 MHz,
3
[D6]benzene, 258C): d = 1.14 (d, 6H, JHH = 6.9 Hz; CH(CH3)2), 1.21
3
(d, 6H, JHH = 6.9 Hz; CH(CH3)2), 1.75 (s, 3H; CCH3), 1.90 (s, 3H;
3
CCH3), 3.20 (m, 2H, JHH = 6.9 Hz; CH(CH3)2), 3.25 (s, 3H; OCH3),
5.06 (s, 1H; CH), 6.50 (d, 1H; ArH), 6.88–6.96 (m, 2H; ArH), 7.15–
7.16 ppm (m, 4H; ArH); 13C{1H} NMR (125.8 MHz, [D6]benzene):
d = 23.8 (CH(CH3)2), 25.7 (CCH3), 28.2 (CH(CH3)2), 28.6 (CH-
(CH2)2), 55.1 (OCH3), 98.7 (g-CH), 111.8, 121.3, 123.3, 123.9, 124.5,
125.2, 125.5, 125.8, 126.7, (ArH) 142.3 (o-C(dipp)), 145.2 (i-C(dipp)),
155.6 (i-Ar-2-OCH3), 163.6 (CN), 163.9 ppm (CN).
6: This compound was made by the same general method
employing III-H (0.69 g, 2.07 mmol), InI (0.50 g, 2.07 mmol), and
KN(SiMe3)2 (0.42 g, 2.10 mmol) to afford pale yellow rectangular
crystals suitable for X-ray analysis (0.42 g, 45%). Elemental analysis
(%) calcd for C46H58In2N4: C 61.62, H 6.53, N 6.25; found: C 61.68, H
6.47, N 6.18; 1H NMR (500 MHz, [D6]benzene, 258C): d = 1.59 (s, 6H;
CCH3), 2.03 (s, 12H; o-CH3), 2.12 (s, 6H; p-CH3), 4.92 (s, 1H; CH),
6.76 ppm (s, 4H; m-ArH); 13C{1H} NMR (125.8 MHz, [D6]benzene):
d = 19.3 (o-CH3), 20.9 (p-CH3), 23.6 (CCH3), 98.2 (g-CH), 129.5 (m-
ArH), 133.4 (p-Ar), 137.4 (o-Ar), 146.0 (i-Ar), 163.4 ppm (CN).
Data for the X-ray structural analyses of 5 and 6 were collected
on a KappaCCD diffractometer (l(MoKa) = 0.71073 ꢀ), solved by
direct methods (SHELXS-97), and refined against all F2 using
SHELXL-97) with non-hydrogen atoms anisotropic and hydrogen
atoms in riding mode. An absorption correction (MULTISCAN) was
applied. For 5, the diffraction was weak and limited in extent.
Crystallographic data for 5 (C24H31InN2O) at 173(2) K: Mr = 478.33,
crystal dimensions 0.25 ꢁ 0.10 ꢁ 0.05 mm3, orthorhombic, space group
P21212 (no. 18), a = 11.6183(8), b = 22.8982(14), c = 8.7686(6) ꢀ, V=
2332.8(3) ꢀ3, Z = 4, 1calcd = 1.36 Mgmꢀ3, m = 1.03 mmꢀ1; of 10481
reflections measured (3.41 < q < 23.268), 3304 were independent
(Rint = 0.067); wR2 = 0.253 (all data), R1 = 0.100 (for 2765 reflections
with I > 2s(I)), 254 parameters, GOF = 1.114. Crystallographic data
for 6 (C46H58In2N4) at 173(2) K: Mr = 896.6, crystal dimensions 0.10 ꢁ
3
¯
0.10 ꢁ 0.05 mm , triclinic, space group P1 (no. 2), a = 8.6847(3), b =
9.9683(3), c = 13.5822(5) ꢀ, a = 71.003(2), b = 82.758(2), g =
81.907(2)8, V= 1096.67(6) ꢀ3, Z = 1, 1calcd = 1.36 Mgmꢀ3
,
m =
1.09 mmꢀ1; of 17033 reflections measured (3.54 < q < 26.068), 4316
were independent (Rint = 0.053); wR2 = 0.072 (all data), R1 = 0.031
(for 3783 reflections with I > 2s(I)), 241 parameters, GOF = 1.031.
CCDC-264672 and -264673 (5 and 6) contain the supplementary
crystallographic data for this paper. These data can be obtained free
of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Received: March 1, 2005
Published online: June 2, 2005
ꢀ
approximately 0.2 ꢀ in the computed Ga Ga bond length.
Preliminary calculations on the [{In[(N(H)C(H)]2CH}2]+
species indicated a similar and pronounced contraction of
Keywords: carbenoids · Group 13 elements ·
ꢀ
the In In bond length to approximately 3 ꢀ and highlighted
.
indium · metal–metal interactions · multiple bonds
the fact that the bu HOMO may also possess antibonding
character. Experiments to assess the validity of this hypoth-
esis and further studies on the reactivity of these unusual
species are in progress.
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Experimental Section
5: Precooled THF (20 mL) was added at ꢀ788C to a rapidly stirred
mixture of II-H (0.45 g, 1.25 mmol), InI (0.30 g, 1.25 mmol), and
KN(SiMe3)2 (0.24 g, 1.25 mmol) in the absence of light. The reaction
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Angew. Chem. Int. Ed. 2005, 44, 4231 –4235
www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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