Oxidative addition reactions of alkyl halides with the group 13 carbene analogue [In{N(Dipp)C(Me)}2CH] (Dipp = 2,6-iPr 2C6H3)

Article abstract of DOI:10.1021/ic070183r

Reactions of the well-defined two-coordinate indium carbene analogue [In{N(Dipp)-C(Me)}₂CH] (Dipp = 2,6-ⁱPr ₂C₆H₃) have been studied. Reactions of Mel, ⁱPrl, and ^tBul with [In{N(Dipp)C(Me)}₂CH] formed by the in situ reaction of InI, [K{N(SiMe₃)₂}], and the iminoenamine ligand precursor successfully yielded the oxidative addition products [InRI{N(Dipp)C(Me)}₂CH] (R = Me, ⁱPr, ^tBu). The results of NMR investigations, which indicated the formation of a series of four-coordinate indium(III) complexes in C ₆D₆ solution, were confirmed in the solid-state by single-crystal X-ray diffraction. Similar reactions employing alkyl bromides were unsuccessful and resulted in the isolation of the corresponding iodides, apparently by metathesis of the bromide oxidative addition product with Kl formed during the initial InI metathesis. Reactions of isolated samples of [In{N(Dipp)C(Me)}₂CH] with ⁱPrBr and ^tBuBr, however, were straightforward and resulted in the successful isolation of the analogous iso-propyl and tert-butyl indium(III) bromide complexes. These were also fully characterized by ¹H and ¹³C NMR and single-crystal X-ray diffraction experiments. In contrast, no reaction was observed between [In{N(Dipp)-C(Me)}₂CH] and aryl halides or alkyl chlorides.

Full text of DOI:10.1021/ic070183r

Inorg. Chem. 2007, 46, 3783−3788
Oxidative Addition Reactions of Alkyl Halides with the Group 13
Carbene Analogue [In N(Dipp)C(Me) 2CH] (Dipp
2,6-iPr2C6H3)
{
}
)
Michael S. Hill,*,† Peter B. Hitchcock,‡ and Ruti Pongtavornpinyo†
Department of Chemistry, Imperial College, Exhibition Road, South Kensington,
London SW7 2AZ, U.K., and The Chemistry Laboratory, UniVersity of Sussex, Falmer,
Brighton, East Sussex, BN1 9QJ, U.K.
Received January 31, 2007
Reactions of the well-defined two-coordinate indium “carbene analogue” [In
{
N(Dipp)-C(Me)
}
2CH] (Dipp
) 2,6-
iPr2C6H3) have been studied. Reactions of MeI, PrI, and BuI with [In
reaction of InI, [K N(SiMe3)2
products [InRI N(Dipp)C(Me)
{
N(Dipp)C(Me)
}
2CH] formed by the in situ
i
t
{
}], and the iminoenamine ligand precursor successfully yielded the oxidative addition
i
t
{
}
2CH] (R
)
Me, Pr, Bu). The results of NMR investigations, which indicated the
formation of a series of four-coordinate indium(III) complexes in C6D6 solution, were confirmed in the solid-state by
single-crystal X-ray diffraction. Similar reactions employing alkyl bromides were unsuccessful and resulted in the
isolation of the corresponding iodides, apparently by metathesis of the bromide oxidative addition product with KI
i
formed during the initial InI metathesis. Reactions of isolated samples of [In
{N(Dipp)C(Me)
}
2CH] with PrBr and
tBuBr, however, were straightforward and resulted in the successful isolation of the analogous iso-propyl and tert-
butyl indium(III) bromide complexes. These were also fully characterized by 1H and 13C NMR and single-crystal
X-ray diffraction experiments. In contrast, no reaction was observed between [In{N(Dipp)-C(Me)}2CH] and aryl
halides or alkyl chlorides.
Introduction
heterophilicity, in comparison to more electropositive ele-
ments, confers greater functional group and substrate toler-
ance. Extensive investigations of the oxidation of refractory
indium(I) halides by substituted o-quinones using electron
paramagnetic resonance (EPR) spectroscopy revealed the
formation of intermediates in which an o-semiquinonate
ligand is attached to the central element.3 The reaction
pathway was explained in terms of indium(I), (II), and (III)
species in solution, and the first steps in the reactions are
presumed to involve one-electron transfer and the formation
of short-lived radical and radical ion intermediates. Although
related reactions of the refractory monohalides with simple,
alkyl and aryl halides were first studied some 30 years ago,
and it was considered likely that these reactions proceed via
similar single-electron pathways,4,5 the rapidity and ef-
fectively heterogeneous nature of these transformations
hindered any definitive mechanistic rationale.
Elemental indium and the indium(I) halides, InCl, InBr,
and InI, are achieving increasing prominence as stoichio-
metric and catalytic reagents in C-C coupling reactions.1
Especially suitable for Barbier and Reformatsky-type reactiv-
ity (for example, eqs 1 and 2),2
the In0/In1 reduction potential is ideally suited for reactions
involving single-electron transfer (SET), while a reduced
* To whom correspondence should be addressed. Fax: +44 (0)20 7594
5804. Tel: +44 (0)207 594 5709. E-mail: mike.hill@imperial.ac.uk.
† Imperial College London.
‡ University of Sussex.
(1) For a review, see: Nair, V.; Ros, S.; Jayan, C. N.; Pillai, B. S.
Tetrahedron 2004, 60, 1959.
(2) Araki, S.; Ito, I.; Katsumura, N.; Butsugan, Y. J. Organomet. Chem.
1989, 369, 291.
10.1021/ic070183r CCC: $37.00
Published on Web 03/24/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 9, 2007 3783

Hill et al.
solvents were distilled under dinitrogen and dried over conventional
drying agents. Compound I was synthesized by a literature
procedure.6 All reagents were purchased from Sigma-Aldrich.
Liquid alkyl halides were freeze-thaw degassed but were otherwise
used as received. NMR spectra were recorded at 270 or 400 (1H)
or 125.8 (13C) MHz from samples in C6D6; chemical shifts are given
relative to SiMe4. 13C{1H} assignments were performed using
standard DEPT pulse sequences. Elemental analyses were carried
out by Mr. Stephen Boyer at London Metropolitan University.
General Procedure for the Synthesis of the Iodide Com-
pounds 1-3. A solution of compound I was prepared in situ in
THF at -78 °C as described previously.6 When it was warmed to
room temperature, a slight excess of the appropriate alkyl iodide
was added via syringe, and the resultant gray slurry was stirred
overnight. The removal of the volatiles and extraction into hexane,
followed by filtration, produced a pale yellow solution. Concentra-
tion and storage at 5 °C overnight yielded compounds 1-3 as well-
formed pale yellow crystals.
We have recently described the syntheses of several
univalent indium complexes supported by â-diketiminate
ligands of suitable steric demands.6-8 The N-Dipp (Dipp )
2,6-iPr2C6H3)-substituted compounds, I-III,6,7 exist as mono-
nuclear species in both solution and solid states, while the
N-(2,4,6-Me3C6H2)-and N-(2,6-Me2C6H3)-substituted deriva-
tives, IV and V, are dimeric in the solid state but mono-
nuclear in solution.8 We have speculated that this behavior
mirrors that of the isoelectronic and well-studied stan-
nanediyls, R2Sn. In common with the In(I) halides outlined
above, these latter species react readily with a great variety
of reducible substrates including halogens, alkyl halides,
organic cummulenes, diazoalkanes, 1,2-diketones, and 1,3-
dienes.9-15 Stannylenes react with organic halides, RX (R
) alkyl or aryl, X ) halide) by oxidative addition,10 a process
that, on the basis of product and EPR spectroscopic studies,
has again been reasoned to occur via a radical process.16
Although a comparable oxidative addition chemistry of the
aluminum and gallium analogues of I, compounds VI and
VII,17,18 has begun to emerge, no comparable indium
chemistry has been reported. To address this deficiency, we
have started to study the chemistry of I-V, and report here
upon the reactivity of I with a variety of alkyl and aryl
halides.
1. Yield: 1.0 g, 74%. mp: 222 °C. Anal. Calcd for C30H44-
IInN2: C, 53.59; H, 6.52; N, 4.14. Found: C, 53.27; H, 6.40; N,
1
4.07. H NMR (270 MHz, C6D6, 298 K): δ 0.03 (s, 3H, InCH3),
3
3
1.07 (d, 6H, JHH ) 6.5 Hz, CH(CH3)2), 1.21 (d, 6H, JHH ) 4.1
3
Hz, CH(CH3)2), 1.23 (d, 6H, JHH ) 4.0 Hz, CH(CH3)2), 1.44 (d,
6H, JHH ) 6.6 Hz, CH(CH3)2), 1.56 (s, 6H, C(CH3)), 3.20 (m,
3
2H, CH(CH3)2), 3.83 (m, 2H, CH(CH3)2), 4.83 (s, 1H, C(CH3)-
CH), 7.00-7.15 (m, 6H, ArH). 13C{1H} NMR (125.8 MHz, C6D6,
298 K): δ 24.2 (CH(CH3)2), 24.3 (CH(CH3)2), 24.4 (CH(CH3)2),
24.7 (CH(CH3)2), 28.1 (CH(CH3)2), 28.5 (CH(CH3)2), 28.9 (C(CH3)),
97.1 (C(CH3)CH), 123.9 (ArH), 125.3 (ArH), 142.3 (Ar), 142.7
(Ar), 144.9 (Ar), 170.2 (CN).
Experimental Section
All manipulations were carried out using standard Schlenk and
glovebox techniques under an inert atmosphere of dinitrogen. All
2. Yield: 0.50 g, 57%. mp: 245 °C. Anal. Calcd for C32H48-
IInN2: C, 54.72; H, 6.83; N, 3.99. Found: C, 54.86; H, 7.00; N,
(3) (a) Tuck, D. G. Chem. Soc. ReV. 1993, 270. (b) Tuck, D. G. Coord.
Chem. ReV. 1992, 112, 215 and references therein.
1
3
3.94. H NMR (270 MHz, C6D6, 298 K): δ 0.88 (d, 6H, JHH
)
(4) Poland, J. S.; Tuck, D. G. J. Organomet. Chem. 1972, 42, 315.
(5) Gynane, M. J. S.; Waterworth, L. G.; Worrall, I. J. J. Organomet.
Chem. 1972, 43, 257.
7.4 Hz InCH(CH3)2), 1.07 (d, 6H, 3JHH ) 6.5 Hz, CH(CH3)2), 1.23
3
3
(d, 6H, JHH ) 6.7 Hz, CH(CH3)2), 1.28 (d, 6H, JHH ) 6.9 Hz,
(6) Hill, M. S.; Hitchcock, P. B. Chem. Commun. 2004, 1818.
(7) Hill, M. S.; Hitchcock, P. B.; Pongtavornpinyo, R. Dalton Trans. 2005,
273.
(8) (a) Hill, M. S.; Hitchcock, P. B.; Pongtavornpinyo, R. Angew. Chem.,
Int. Ed. 2005, 44, 4231. (b) Hill, M. S.; Hitchcock, P. B.; Pongta-
vornpinyo, R. Dalton Trans. 2007, 731.
(9) Lickiss, P. D. In The Chemistry of Tin; Harrison, P. G., Ed.; Blackie:
Glasgow, U.K., 1989.
(10) Eaborn, C.; Hill, M. S.; Hitchcock, P. B.; Patel, D.; Smith, J. D.; Zhang,
S. Organometallics 2000, 19, 49.
(11) Cotton, J. D.; Davidson, M. F.; Lappert, M. F. J. Chem. Soc., Dalton
Trans. 1976, 2275.
(12) Gynane, M. J. S.; Lappert, M. F.; Miles, S. J.; Carty, A. J.; Taylor, N.
J. J. Chem. Soc., Dalton Trans. 1977, 2009.
(13) Meyer, H.; Baum, G.; Massa, W.; Berger, S.; Berndt, A. Angew. Chem.,
Int. Ed. Engl. 1987, 26, 546.
(14) Hillner, K.; Neumann, W. P. Tetrahedron Lett. 1986, 27, 5347.
(15) Asadi, A.; Eaborn, C., Hill, M. S.; Hitchcock, P. B.; Meehan, M. M.;
Smith, J. D. Organometallics 2002, 21, 2430.
3
CH(CH3)2), 1.44 (d, 6H, JHH ) 6.7 Hz, CH(CH3)2), 1.57 (s, 6H,
C(CH3)), 3.29 (m, 2H, CH(CH3)2), 3.89 (m, 2H, CH(CH3)2), 4.85
(s, 1H, C(CH3)CH), 7.00-7.16 (m, 6H, ArH). 13C{1H} NMR (125.8
MHz, C6D6, 298 K): δ 22.2 (InCH(CH3)2, 24.1 (CH(CH3)2), 24.2
(CH(CH3)2), 24.4 (CH(CH3)2), 24.8 (CH(CH3)2), 28.0 (CH(CH3)2),
28.7 (CH(CH3)2), 29.1 (C(CH3)), 97.3 (C(CH3)CH), 123.6 (ArH),
125.8 (ArH), 127.1 (ArH), 142.3 (Ar), 143.1 (Ar), 144.9 (Ar), 170.1
(CN).
3. Yield: 0.35 g, 40%. mp: 198 °C. Anal. Calcd for C33H50-
IInN2: C, 55.32; H, 6.98; N, 3.91. Found: C, 55.59; H, 7.09; N,
3.83. 1H NMR (270 MHz, C6D6, 298 K): δ 0.95 (s, 9H, InC(CH3)3),
3
3
1.07 (d, 6H, JHH ) 6.5 Hz, CH(CH3)2), 1.24 (d, 6H, JHH ) 6.6
3
Hz, CH(CH3)2), 1.31 (d, 6H, JHH ) 7.0 Hz, CH(CH3)2), 1.42 (d,
6H, JHH ) 6.8 Hz, CH(CH3)2), 1.59 (s, 6H, C(CH3)), 3.35 (m,
3
(16) (a) Gynane, M. J. S.; Lappert, M. F.; Miles, Power, P. P. J. Chem.
Soc., Chem. Commun. 1976, 256. (b) Dewar, M. J. S.; Friedhiem, J.
E.; Grady, G. L. Organometallics 1985, 4, 1784.
2H, CH(CH3)2), 3.96 (m, 2H, CH(CH3)2), 4.94 (s, 1H, C(CH3)-
CH), 7.04-7.16 (m, 6H, ArH).
General Procedure for the Synthesis of the Bromide Com-
pounds 4 and 5. A slight excess of the appropriate alkyl bromide
was added via syringe to a solution of compound I in hexane at
room temperature. After the mixture was stirred overnight, con-
centration and storage at 5 °C yielded compounds 4 and 5 as well-
formed pale yellow crystals.
(17) (a) Cui, C.; Ko¨pke, S.; Herbst-Irmer, R.; Roesky, H. W.; Noltemeyer,
M.; Schmidt, H.-G.; Wrackmeyer, B J. Am. Chem. Soc. 2001, 123,
9091. (b) Hardman, N. J.; Cui, C.; Roesky, H. W.; Fink, W. H.; Power,
P. P. Angew. Chem., Int. Ed. 2001, 40, 2172. (c) Peng, Y.; Fan, H.;
Zhu, H.; Roesky, H. W.; Magull, J.; Hughes, C. E. Angew. Chem.,
Int. Ed. 2004, 43, 3443. (d) Zhu, H.; Chai, J.; Chandrasekhar, V.;
Roesky, H. W.; Magull, J.; Vidovic, D.; Schmidt, H.-G.; Noltemeyer,
M.; Power, P. P.; Merrill, W. A. J. Am. Chem. Soc. 2004, 126, 9472.
(18) (a) Hardman, N. J.; Wright, R. J.; Philips, A. D.; Power, P. P. J. Am.
Chem. Soc. 2003, 125, 2667. (b) Hardman, N. J.; Power, P. P.; Gorden,
J. P.; Mcdonald, C. L. B.; Cowley, A. H. Chem. Commun. 2001, 1866.
(c) Hardman, N. J.; Power, P. P. Chem. Commun. 2001, 1184. (d)
Burford, N.; Ragogna, P. J.; Robertson, K. N.; Cameron, T. S.;
Hardman, N. J.; Power, P. P. J. Am. Chem. Soc. 2002, 124, 382. (e)
Hardman, N. J.; Power, P. P. Inorg. Chem. 2001, 40, 2474.
4. Yield: 0.28 g, 57%. mp: 195 °C. Anal. Calcd for C32H48-
BrInN2: C, 58.58; H, 7.32; N, 4.27. Found: C, 58.69; H, 7.41; N,
1
3
4.29. H NMR (400 MHz, C6D6, 298 K): δ 0.91 (d, 6H, JHH
)
7.2 Hz InCH(CH3)2), 1.07 (d, 6H, 3JHH ) 6.8 Hz, CH(CH3)2), 1.26
(d, 6H, JHH ) 7.0 Hz, CH(CH3)2), 1.26 (d, 6H, JHH ) 6.9 Hz,
CH(CH3)2), 1.46 (d, 6H, JHH ) 6.6 Hz, CH(CH3)2), 1.57 (s, 6H,
3
3
3
3784 Inorganic Chemistry, Vol. 46, No. 9, 2007

Addtion Reactions of Alkyl Halides and [In{N(Dipp)C(Me)}2CH]
Table 1. Selected Crystallographic and Data Collection Parameters for Compounds 1-5
1
2
3
4
5
chemical formula
fw
C30H44IInN2
674.39
C32H48IInN2
702.44
C33H50IinN2
716.47
C32H48BrInN2
655.45
C33H50BrInN2
669.48
T (K)
173(2)
173(2)
173(2)
173(2)
173(2)
cryst size (mm3)
cryst syst
space group
a (Å)
0.1 × 0.1 × 0.1
monoclinic
P21/n (No. 14)
19.0020(5)
8.6125(1)
20.0582(5)
90
112.882(1)
90
4
3024.30(12)
1.48
1.82
3.72-25.07
0.033, 0.082
0.037, 0.083
35 876/5324/0.055
4928
0.20 × 0.15 × 0.15
orthorhombic
Pnma (No. 62)
15.7887(2)
21.4997(4)
9.5541(2)
90
90
90
4
3243.16(10)
1.44
1.70
3.78-25.02
0.022, 0.050
0.026, 0.052
26 148/2931/0.047
2659
0.15 × 0.15 × 0.10
monoclinic
P21/m (No. 11)
9.1089(3)
20.3097(4)
10.0983(3)
90
116.040(1)
90
2
1678.53(8)
1.42
1.65
2.01-24.98
0.034, 0.098
0.045, 0.123
20 704/3036/0.077
2686
0.20 × 0.15 × 0.15
orthorhombic
Pnma (No. 62)
15.6859(3)
21.4625(3)
9.4625(3)
90
90
90
4
3213.41(10)
1.36
2.00
3.49-26.01
0.025, 0.053
0.036, 0.057
44 711/3245/0.062
2756
0.2 × 0.2 × 0.1
monoclinic
P21/n (No. 14)
10.2964(2)
27.2575(4)
12.2832(3)
90
106.063(1)
90
4
3312.74(11)
1.34
1.94
3.45-26.05
0.057, 0.143
0.082, 0.158
38 664/6515/0.087
5080
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
Z
V (Å3)
dc (Mg m-3
)
µ (mm-1
)
θ range (deg)
R1, wR2 [I >2σ(I)]
R1, wR2 (all data)
measured/indep reflns/R(int)
reflns with I >2σ(I)
Table 2. Selected Bond Lengths (Å) for Compounds 1-5
1a
with isotropic C atoms and SADI constraints. The structures were
solved by direct methods (SHELXS-97) and refined by full matrix
least-squares (SHELXL-97) with non-H atoms anisotropic and H
atoms included in riding mode.19,20
2
3
4
5
In-N(1)
In-N(2)
In-C(30)
In-I/Br
2.145(3) 2.155(2) 2.162(3) 2.1481(16) 2.162(5)
2.167(3)
2.147(5)
2.180(7)
2.353(2) 2.177(3)b 2.221(6)b 2.160(3)b
2.7243(4) 2.7385(3) 2.7581(6) 2.5310(4) 2.5493(8)
Results and Discussion
N(1)-C(1) 1.333(5) 1.330(3) 1.328(5) 1.330(3)
N(1)-C(6) 1.448(5) 1.452(3)c 1.449(5)c 1.447(3)c
N(2)-C(3) 1.323(5)
N(2)-C(18) 1.445(5)
C(1)-C(2) 1.408(5) 1.405(3) 1.399(5) 1.403(2)
C(2)-C(3) 1.401(5)
1.326(7)
1.446(7)
1.335(7)
1.452(7)
1.411(8)
1.408(8)
Compound I is readily synthesized by the “one-pot” route
shown in Scheme 1 and may be isolated as pale yellow
crystals by extraction and crystallization from hexane.6
Isolated crystals of I, although readily manipulated, are
somewhat light-sensitive and decompose with the formation
of indium metal upon storage.6,7 For this reason, we initially
attempted reactions with a variety of alkyl iodides by addition
of a slight excess of the appropriate alkyl iodide to a THF
solution of I formed in situ by the reaction depicted in
Scheme 1. This reaction was completely successful for the
synthesis of the indium(III) alkyl iodide compounds 1-3
(Scheme 1), which were isolated as pale yellow crystalline
materials after crystallization from hexane. Compounds 1-3
a For compound 1, values involving C(30) are unreliable because of
disorder. b In-C(16). c N(1)-C(4).
C(CH3)), 3.27 (m, 2H, CH(CH3)2), 3.87 (m, 2H, CH(CH3)2), 4.80
(s, 1H, C(CH3)CH), 7.01-7.15 (m, 6H, ArH). 13C{1H} NMR (125.8
MHz, C6D6, 298 K): δ 1.4 (InCH(CH3)2), 22.2 (InCH(CH3)2), 23.9
(CH(CH3)2), 24.0 (CH(CH3)2), 24.3 (CH(CH3)2), 24.8 (CH(CH3)2),
27.4 (CH(CH3)2), 27.9 (CH(CH3)2), 29.1 (C(CH3)), 97.0 (C(CH3)-
CH), 123.6 (ArH), 125.3 (ArH), 127.0 (ArH), 142.0 (Ar), 143.1
(Ar), 144.8 (Ar), 169.9 (CN).
1
afforded simple H and 13C NMR spectra, consistent with
5. Yield: 0.30 g, 63%. mp: 190 °C. Anal. Calcd for C33H50-
BrInN2: C, 59.21; H, 7.47; N, 4.18. Found: C, 59.12; H, 7.34; N,
4.08. 1H NMR (400 MHz, C6D6, 298 K): δ 0.96 (s, 9H, InC(CH3)3),
the formation of â-diketiminato chelate compounds with
differing substituents attached to four-coordinate indium. In
1
all three cases, the H NMR spectra featured two well-
3
3
1.06 (d, 6H, JHH ) 6.7 Hz, CH(CH3)2), 1.21 (d, 6H, JHH ) 6.6
resolved pairs of doublet signals and two multiplets which
were assigned to the diastereotopic disposition of the ligand
N-Dipp isopropyl methyl and methine groups, respectively.
These data were interpreted as a clear indication that the
crystallographically confirmed solid-state structures (vide
infra) are retained in solution. The characteristic â-diketimi-
nate γ-C methine signal occurred at ∼4.85 ppm, ap-
proximately 0.2 ppm upfield from that observed in the
monovalent starting material, I.6 A similar shift has been
observed from comparison of monovalent and trivalent
dihalo- and dimethylaluminum and gallium derivatives
complexed by the same â-diketiminate ligand.21,22
3
Hz, CH(CH3)2), 1.30 (d, 6H, JHH ) 6.8 Hz, CH(CH3)2), 1.45 (d,
3
6H, JHH ) 6.6 Hz, CH(CH3)2), 1.59 (s, 6H, C(CH3)), 3.32 (m,
2H, CH(CH3)2), 3.96 (m, 2H, CH(CH3)2), 4.88 (s, 1H, C(CH3)-
CH), 7.01-7.19 (m, 6H, ArH). 13C{1H} NMR (125.8 MHz, C6D6,
298 K): δ 1.4 (InC(CH3)3), 23.7 (CH(CH3)2), 24.2 (CH(CH3)2),
24.4 (CH(CH3)2), 24.9 (CH(CH3)2), 27.1 (CH(CH3)2), 28.1 (CH-
(CH3)2), 29.3 (C(CH3)), 32.2 (InC(CH3)3), 97.7 (C(CH3)CH), 123.6
(ArH), 125.3 (ArH), 127.0 (ArH), 142.0 (Ar), 143.8 (Ar), 144.7
(Ar), 170.4 (CN).
Crystal Structure Determinations. Data were collected at 173
K on a Nonius KappaCCD diffractometer, λ(Mo KR) ) 0.71073
Å; details are given in Table 1. An absorption correction (MUL-
TISCAN) was applied for compounds 1-5. For 1, there appeared
to be a small amount (∼9%) of iodine at the C(30) position. The
disorder could not be resolved and bonds and angles involving C(30)
listed in Tables 2 and 3 are, therefore, unreliable. For 3, the tert-
butyl group was disordered across the mirror plane and was refined
(19) Sheldrick, G. M. SHELXS-97, Program for the Solution of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(20) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
Inorganic Chemistry, Vol. 46, No. 9, 2007 3785

Hill et al.
Table 3. Selected Bond Angles (deg) for Compounds 1-5
1a
2b
3b
4b
5
N(1)-In-N(2)
N(1)-In-C(30)
N(2)-In-C(30)
N(1)-In-I/Br
N(2)-In-I/Br
C(30)-In-I/Br
C(1)-N(1)-In
C(6)-N(1)-In
C(3)-N(2)-In
C(18)-N(2)-In
N(1)-C(1)-C(2)
C(1)-C(2)-C(3)
C(2)-C(3)-N(2)
90.60(11)
113.58(10)
119.69(10)
109.35(8)
104.22(8)
116.25(6)
121.0(2)
120.4(2)
120.7(2)
116.2(2)
124.8(3)
90.53(10)c
117.69(8)d
89.43(17)c
89.99(9)c
89.50(18)
122.01(13)d
117.61(8)d
122.6(3)
122.8(3)
101.59(13)
102.12(13)
113.6(2)
118.4(4)
118.8(3)
118.7(4)
119.7(4)
124.6(5)
130.5(5)
124.5(5)
105.50(5)
102.46(9)
104.76(5)
116.36(11)e
121.17(15)
119.22(13)f
114.09(17)e
117.5(3)
117.93(10)e
121.63(14)
118.88(12)f
120.9(2)f
124.9(2)
124.5(4)
124.6(2)
131.0(3)
125.3(3)
131.1(3)g
131.0(5)g
131.0(3)g
a For compound 1, values involving C(30) are unreliable because of disorder. b Symmetry transformations used to generate equivalent atoms; ′ x, -y +
1/2, z. c N(1)′-In-N(1). d N(1)-In-C(16). e C(16)-In-I/Br. f C(4)-N(1)-In. g C(1)-In-C(1)′.
Scheme 1
The use of a similar procedure for the reaction of I and
tBuBr also resulted in the isolation of compound 3. Evidently
the presence of KI, formed from the initial metathesis of
the InI, in the reaction mixture favors the formation of the
indium alkyl iodide rather than the desired bromide. A similar
outcome has previously been noted in the attempted synthesis
of [InCl2{N(Dipp)C(Me)}2CH]. In this latter case the reaction
of [Li{N(Dipp)C(Me)}2CH] and InCl3 resulted in the isola-
tion of [InCl1.5Br0.5{N(Dipp)C(Me)}2CH] because of con-
tamination of the lithium reagent with LiBr.21 This problem
was easily overcome by employing isolated and crystalline
samples of compound I during the synthesis, and this
modified synthetic procedure yielded analytically pure bulk
samples of the indium alkyl bromides, compounds 4 and 5
(Scheme 1), in good yield after crystallization from hexane.
The indium bromide derivatives again displayed NMR
characteristics that were consistent with the formation of
stable chelate complexes and only exhibited minor differ-
ences in chemical shift data in comparison to the directly
analogous iodides 2 and 3.
may be rationalized as a reflection of the increased sp2-C-I
and sp3-C-Cl thermodynamic bond strengths versus their
respective alkyl and heavier group 17 counterparts.1
X-ray Structural Analyses. Single-crystal X-ray diffrac-
tion analyses of compounds 1-5 were undertaken. The
structures of all five compounds were similar, and repre-
sentative examples are provided for compounds 1 and 5 in
Figures 1 and 2, respectively (ORTEP representations of
compounds 2-4 are provided as Figures S1-S3 in the
Supporting Information). Details of the crystallographic
analyses for 1-5 and selected bond length and angle data
are displayed in Tables 1-3, respectively. The structures of
1-5 display a number of common features, as well as
similarities to previously reported â-diketiminate complexes
of trivalent aluminum, gallium, and indium.21,22 Subtle
differences in ligand steric demands do, however, result in
minor variations about the indium metal center. Although
only the indium isopropyl derivatives 2 and 4 are isostruc-
tural, the ring C-N and C-C bond lengths for all five
compounds lie within narrow ranges, 1.323(5)-1.333(5) and
1.399(5)-1.411(8) Å. These values are effectively identical
within the limits of experimental uncertainty and are
consistent with significant multiple-bond character and
delocalization about the metallocycle. The In-N distances
[av 2.155 Å] and N-In-N bond angles [av 89.9°] are also
similar and may be compared to the respective distances and
angles within the previously reported complexes [InCl1.5-
Br0.5{N(Dipp)C(Me)}2CH] (av 2.117 Å, 92.5°), [InI2{N(Dipp)-
C(Me)}2CH] (av 2.134 Å, 92.42°), and [InMe2{N(Dipp)-
In contrast to the reactivity of I with alkyl iodides and
bromides, no reaction was observed with aryl iodides or a
variety of alkyl monochlorides. This observation is consistent
with reports of previous workers attempting to perform C-C
coupling reactions of aryl reagents and alkyl chlorides and
(21) Stender, M.; Eichler, B. E.; Hardman, N. J.; Power, P. P.; Prust, J.;
Noltemeyer, M.; Roesky, H. W. Inorg. Chem. 2001, 40, 2794.
(22) Quian, B.; Ward, D. L.; Smith, M. R., III. Organometallics 1998, 17,
3070.
3786 Inorganic Chemistry, Vol. 46, No. 9, 2007

Addtion Reactions of Alkyl Halides and [In{N(Dipp)C(Me)}2CH]
Figure 1. Thermal ellipsoid (30%) plot of compound 1. Hydrogen atoms omitted for clarity.
Figure 2. Thermal ellipsoid (30%) plot of compound 5. Hydrogen atoms omitted for clarity.
Table 4. Dihedral Angle (æ) (deg) Formed by Intersection of the
NCCCN and NInN Planes of Compounds 1-5
C(Me)}2CH] (av 2.197 Å, 86.76°). These values, therefore,
may be described as entirely typical of In(III) derivatives of
this ligand and are respectively shorter and more obtuse than
those observed in the corresponding In(I) complex (2.268-
(3), 2.276(3) Å; 81.12(10)°). Although the NCCCN chelates
are completely planar (rms deviation of fitted atoms ) 0.00°),
as is observed in many other derivatives of â-diketiminate
ligands, the structures of 1-5 display a distinct folding across
the N-N vector formed by the nitrogen donors. The
structures adopted by all five complexes are dictated by the
steric interactions between the N-Dipp and alkyl (rather than
the halide) substituent. The out-of-plane folding of the indium
center occurs in a manner that minimizes possible steric
interactions between the N-Dipp ligand substituents and the
indium alkyl group. The dihedral angles between the planes
defined by the NCCCN and NInN fragments are displayed
in Table 4, and it is readily apparent from inspection of these
data that the presence of the bulky tert-butyl substituent
within the structures of compounds 3 and 5 has the most
meaningful structural consequence.
1
2
3
4
5
æ
160.0
160.6
148.2
159.6
149.6
Mechanistic Considerations. NMR scale studies of the
oxidative addition reactions described above indicated that
the reactions were effectively complete with 15 min of
mixing at ambient temperatures. Additionally, all the reac-
tions studied in this manner resulted in quantitative conver-
sion to compounds of the form [LInRX] (L ) diketiminate
ligand, R ) alkyl, X ) Br or I) as the only indium-containing
product. Lappert’s observation of low, but observable,
concentrations of E2SnX2 products (E ) alkyl or amide) from
reactions of stabilized stannylenes and alkyl halides was cited
as evidence for the dominance of radical pathways some 30
year ago,12 a hypothesis strongly supported by the direct
observation of tin- and carbon-centered radicals in parallel
EPR investigations of the same reactions.16a Although our
failure to observe anything other than indium alkyl halide
formation in any of the reactions that we have studied thus
Inorganic Chemistry, Vol. 46, No. 9, 2007 3787

Hill et al.
Scheme 2
far could be interpreted as evidence of a contrasting concerted
mechanism, the literature precedents in both low valent group
13 and 14 chemistry lead us to conclude that the rate of
[LinI]•/R• radical recombination is simply too rapid to allow
formation of anything but the observed unsymmetrically
substituted products. This hypothesis has been supported by
preliminary X-band EPR experiments on the reaction of
compound I with MeI. The addition of an excess of the alkyl
iodide to a toluene glass containing compound I at 77 K,
followed by warming in 5 K increments within the cavity
of the EPR spectrometer, gave a very weak but persistent
signal at 190 K (coincidentally the melting point of toluene)
at around the g value for the free electron (see Figure S4,
Supporting Information). This signal rapidly disappeared
upon any further increase of temperature and did not reappear
upon recooling of the samples. Similarly, more rapid
warming of the sample to temperatures in excess of 195 K
gave no evidence of radical intermediates and leads us to
propose the tentative mechanism illustrated in Scheme 2
where rapid recombination of the indium- and carbon-
centered radicals predominates over other recombination
processes which are probably impeded by the steric effects
of the bulky supporting ligand.
We are continuing our experimental and theoretical studies
this well-defined reactivity as well as related reaction
chemistry of the In(I) compounds I-V.
Acknowledgment. We would like to thank the Royal
Society for a University Research Fellowship (M.S.H.) and
Drs. David Collison and Joanna Wolowska of the EPSRC
national service at the University of Manchester for assistance
with the collection of EPR data.
Supporting Information Available: ORTEP figures of com-
pounds 2-4, representative EPR spectrum, and crystallographic data
for 1-5 in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC070183R
3788 Inorganic Chemistry, Vol. 46, No. 9, 2007