Complexes of an anionic gallium(I) N-heterocyclic carbene analogue with group 14 element(II) fragments: Synthetic, structural and theoretical studies

Article abstract of DOI:10.1021/ic060851t

The reactions of the anionic gallium(I) N-heterocyclic carbene (NHC) analogue, [K(tmeda)][:Ga{[N(Ar)C(H)]₂}], Ar = C₆H ₃Prⁱ₂-2,6, with the heavier group 14 alkene analogues, R₂E=ER₂, E = Ge or Sn, R = -CH(SiMe ₃)₂, have been carried out. In 2:1 stoichiometries, these lead to the ionic [K(tmeda)][R₂EGa{[N(Ar)C(H)]₂}] complexes which exhibit long E-Ga bonds. The nature of these bonds has been probed by DFT calculations, and the complexes have been compared to neutral NHC adducts of group 14 dialkyls. The 4:1 reaction of [K(tmeda)][:Ga{[N(Ar)C(H)] ₂}] with R₂Sn=SnR₂ leads to the digallyl stannate complex, [K(tmeda)][RSn[Ga{[N(Ar)C(H)]₂}]₂], presumably via elimination of KR. In contrast, the reaction of the gallium heterocycle with PbR₂ affords the digallane(4), [Ga{[N(Ar)C(H)] ₂}]₂, via an oxidative coupling reaction. For sake of comparison, the reactions of [K(tmeda)][:Ga{[N(Ar)C(H)]₂}] with Ar′₂E=EAr′₂, E = Ge, Sn or Pb, Ar′ = C₆H₂Prⁱ₃-2,4,6, were carried out and led to either no reaction (E = Ge), the formation of [K(tmeda)] [Ar′₂SnGa{[N(Ar)C(H)]₂}] (E = Sn), or the gallium(III) heterocycle, [Ar′Ga{[N(Ar)C(H)]₂}] (E = Pb). Salt elimination reactions between [K(tmeda)][:Ga{[N(Ar)C(H)]₂}] and the guanidinato group 14 complexes [(Giso)ECl], E = Ge or Sn, Giso = [Pr ⁱ₂NC{N(Ar)}₂]^-, gave the neutral [(Giso)EGa{[N(Ar)C(H)]₂}] complexes. All complexes have been characterized by NMR spectroscopy and X-ray crystallographic studies.

Full text of DOI:10.1021/ic060851t

Inorg. Chem. 2006, 45, 7242−7251
Complexes of an Anionic Gallium(I) N-Heterocyclic Carbene Analogue
with Group 14 Element(II) Fragments: Synthetic, Structural and
Theoretical Studies
Shaun P. Green, Cameron Jones,* Kai-Alexander Lippert, David P. Mills, and Andreas Stasch
Center for Fundamental and Applied Main Group Chemistry, School of Chemistry, Main Building,
Cardiff UniVersity, Cardiff, CF10 3AT, U.K.
Received May 17, 2006
The reactions of the anionic gallium(I) N-heterocyclic carbene (NHC) analogue, [K(tmeda)][:Ga
C6H3Pri2-2,6, with the heavier group 14 alkene analogues, R2E
ER2, E Ge or Sn, R ) −CH(SiMe3)2, have
been carried out. In 2:1 stoichiometries, these lead to the ionic [K(tmeda)][R2EGa [N(Ar)C(H)]2 ] complexes which
exhibit long E Ga bonds. The nature of these bonds has been probed by DFT calculations, and the complexes
have been compared to neutral NHC adducts of group 14 dialkyls. The 4:1 reaction of [K(tmeda)][:Ga [N(Ar)C-
(H)]2 ] with R2Sn SnR2 leads to the digallyl stannate complex, [K(tmeda)][RSn[Ga [N(Ar)C(H)]2 ]2], presumably
via elimination of KR. In contrast, the reaction of the gallium heterocycle with PbR2 affords the digallane(4), [Ga-
{[N(Ar)C(H)]2}], Ar
)
d
)
{
}
−
{
}
d
{
}
{
[N(Ar)C(H)]2
}
]2, via an oxidative coupling reaction. For sake of comparison, the reactions of [K(tmeda)][:Ga-
] with Ar 2E EAr 2, E
Ge, Sn or Pb, Ar′ ) C6H2Pri3-2,4,6, were carried out and led to either no
Ge), the formation of [K(tmeda)][Ar 2SnGa [N(Ar)C(H)]2 ] (E Sn), or the gallium(III) heterocycle,
[N(Ar)C(H)]2 ] (E Pb). Salt elimination reactions between [K(tmeda)][:Ga [N(Ar)C(H)]2 ] and the guanidinato
group 14 complexes [(Giso)ECl], E Ge or Sn, Giso N(Ar) [N(Ar)C-
[Pri2NC 2]-, gave the neutral [(Giso)EGa
] complexes. All complexes have been characterized by NMR spectroscopy and X-ray crystallographic studies.
{
[N(Ar)C(H)]2
}
′
d
′
)
reaction (E
[Ar Ga
)
′
{
}
)
′
{
}
)
{
}
)
)
{
}
{
(H)]2}
Introduction
coordination chemistry of the anionic five-membered het-
erocyclic complex, [K(tmeda)][:Ga{[N(Ar)C(H)]2}], 1,5 which
is a valence isoelectronic analogue of the N-heterocyclic
carbene (NHC) class of ligand. This has proven to be very
fruitful, and complexes with a range of s-, p-, and d-block
metal fragments have been reported.6 Throughout this work
a number of similarities with NHCs have been exhibited by
1, including the ability of the heterocycle to stabilize
thermally labile or low oxidation state metal fragments.
The coordination chemistry of compounds containing a
group 13 center in the +1 oxidation state is a rapidly
emerging area. In this field, singlet metal dilyls, :MR, M )
Al-In, R ) alkyl, aryl, etc., have been most studied as
ligands for p- and d-block metal fragments.1 More recently,
the neutral six-membered heterocycles of Roesky2 and
Power,3 [:M{[N(Ar)C(Me)]2CH}], M ) Al or Ga, Ar )
C6H3Pri2-2,6, have been enlisted by several groups for this
cause.4 Our group has been interested in studying the
(5) Baker, R. J.; Farley, R. D.; Jones, C.; Kloth, M.; Murphy, D. M. J.
Chem. Soc., Dalton Trans. 2002, 3844. An alternative lower-yielding
synthetic route to a closely related complex has been reported. Schmidt,
E. S.; Jockisch, A.; Schmidbaur H. J. Am. Chem. Soc. 1999, 121, 9758.
(6) (a) Aldridge, S. A.; Baker, R. J.; Coombs, N. D.; Jones, C.; Rose, R.
P.; Rossin, A.; Willock, D. J. Dalton Trans. 2006, 3313. (b) Baker,
R. J.; Jones, C.; Mills, D. P.; Murphy, D. M.; Hey-Hawkins, E.; Wolf,
R. Dalton Trans. 2006, 64. (c) Baker, R. J.; Jones, C.; Murphy, D.
M. Chem. Commun. 2005, 1339. (d) Baker, R. J.; Jones, C.; Kloth,
M. Dalton Trans. 2005, 2106. (e) Baker, R. J.; Jones, C.; Kloth, M.;
Platts, J. A. Organometallics 2004, 23, 4811. (f) Baker, R. J.; Jones,
C.; Platts, J. A. Dalton Trans. 2003, 3673. (g). Baker, R. J.; Jones,
C.; Platts, J. A. J. Am. Chem. Soc. 2003, 125, 10534. (h) Baker, R. J.;
Jones, C.; Kloth, M.; Platts, J. A. Angew. Chem., Int. Ed. 2003, 43,
2660.
* To whom correspondence should be addressed. E-mail:
jonesca6@cardiff.ac.uk
(1) Gemel, G.; Steinke, T.; Cokoja, M.; Kempter, A.; Fischer, R. A. Eur.s
J. Inorg. Chem. 2004, 4161. (b) Cowley, A. H. Chem. Commun. 2004,
2369 and references therein.
(2) Cui, C.; Roesky, H. W.; Schmidt, H.-G.; Noltemeyer, M.; Hao, H.;
Cimpoesu, F. Angew. Chem., Int. Ed. 2000, 39, 4274.
(3) Hardman, N. J.; Eichler, B. E.; Power, P. P. Chem. Commun. 2000,
1991.
(4) (a) Baker, R. J.; Jones, C. Coord. Chem. ReV. 2005, 249, 1857. (b)
Roesky, H. W. Inorg. Chem. 2004, 43, 7284. (c) Hardman, N. J.;
Phillips, A. D.; Power, P. P. ACS Symp. Ser. 2002, 822, 2, and
references therein.
7242 Inorganic Chemistry, Vol. 45, No. 18, 2006
10.1021/ic060851t CCC: $33.50
© 2006 American Chemical Society
Published on Web 08/03/2006

Anionic Gallium(I) N-Heterocyclic Carbene Analogue
Chart 1
NHC complexes of benzannulated N-heterocyclic silylenes,
germylenes, stannylenes, and plumbylenes.13 The results of
the reactions between 1 and R2EdER2, R ) -CH(SiMe3)2,
or Ar′; E ) Ge, Sn or Pb (N.B: Pb{CH(SiMe3)2}2 mono-
meric in the solid state) are compared to the work of
Weidenbruch and are reported herein. For the sake of
comparison, the outcomes of the salt elimination reactions
of 1 with the monomeric guanidinato element chlorides,
[(Giso)ECl], E ) Ge, or Sn, Giso ) [Pri2NC{N(Ar)}2]-, are
also described.
Perhaps most success has come from the p-block coordina-
tion chemistry of 1, which is known to form complexes with
group 13, 15, 16, and 17 centers.6 In contrast, attempts to
form complexes of 1 with group 14 fragments have not been
so rewarding, although some progress has been made. In this
respect, theoretical studies on models of 1 have suggested
that the heterocycle should be considered as a diamido
complex of a Ga+ center because of the electronegativity
differences between Ga and N.7 As a result, it is possible
that this center could act as a nucleophile through its largely
sp-hybridized singlet lone pair or as an electrophile by
accepting donor electron density into its effectively empty
gallium p-orbital, orthogonal to the heterocycle plane. In
practice, although the strongly nucleophilic properties of 1
have been amply demonstrated, we have yet to coordinate
even very strong group 14 nucleophiles such as NHCs to
the Ga center of 1. It is of interest, however, that the reaction
of 1 with one imidazolium cation, [HC{N(Mes)C(H)}2]Cl,
IMesHCl, Mes ) C6H2Me3-2,4,6, has resulted in the oxida-
tive insertion of its gallium center into a C-H bond of the
cation and the formation of the NHC-gallium hydride
heterocycle complex, [HGa{[N(Ar)C(H)]2}(IMes)].8
To further investigate the analogies between 1 and NHCs
and to potentially prepare group 14 complexes of the
heterocycle, we explored its reaction with heavier alkene
analogues, R2EdER2, E ) Ge, Sn or Pb, and related
compounds here. This work is directly related to earlier
studies by Weidenbruch et al. who looked at the reactions
of an NHC, :C{N(Pri)C(Me)}2, with Ar′2EdEAr′2, Ar′ )
C6H2Pri3-2,4,6, E ) Sn9 or Pb.10 In solutions, the distannene
and diplumbene reaction precursors exist in equilibrium with
their monomeric stannylene and plumbylene forms, :EAr′2
(cf. carbenes, :CR2), which form weakly coordinated com-
plexes, 2 (Chart 1), with the NHC. The weakness of these
interactions was shown by the fact that the complexes exhibit
no E-C(carbene) double-bond character, have very long
E-C(carbene) interactions, and possess relatively obtuse fold
angles (θ) between the donor and acceptor fragments. It is
of note that similar structural features have been reported
for NHC complexes of GeI211 and SnCl212 and benzannulated
Experimental Section
General Methods. All manipulations were carried out using
standard Schlenk and glovebox techniques under an atmosphere
of high purity argon. Diethyl ether, hexane, and toluene were
distilled over Na/K alloy. 1H and 13C{1H} NMR spectra were
recorded on either a Bruker DXP400 spectrometer operating at
400.13 and 100 MHz, respectively, or a JEOL Eclipse 300
spectrometer operating at 300.52 and 75.57 MHz, respectively, and
were referenced to the resonances of the solvent used. 29Si{1H}
NMR spectra were recorded on a JEOL Eclipse 300 spectrometer
operating at 59.7 MHz and were referenced to SiMe4. 119Sn{1H}
NMR spectra were recorded on a Bruker AMX 500 spectrometer
operating at 186.4 MHz and were referenced to SnMe4. EI mass
spectra and accurate mass EI mass spectra were obtained from the
EPSRC National Mass Spectrometric Service at Swansea Univer-
sity. IR spectra were recorded using a Nicolet 510 FT-IR spec-
trometer as Nujol mulls between NaCl plates. Melting points were
determined in sealed glass capillaries under argon and are uncor-
rected. Microanalyses were obtained from Medac Ltd. Where
reproducible microanalyses could not be obtained because of the
solvent of crystallization or the highly air-sensitive nature of the
compound, the NMR spectra of the samples suggested purities of
>95%. [K(tmeda)][:Ga{[N(Ar)C(H)]2}],5 [Ge{CH(SiMe3)2}2]2,14
16
[Sn{CH(SiMe3)2}2]2,14 [Sn(Ar′)2]3,15 and [Pb(Ar′)2]2 were syn-
thesized by literature procedures. [(Giso)GeCl] and [(Giso)SnCl]
were synthesized by unpublished procedures which involved the
1:1 reaction of [Li(Giso)]17 with either GeCl2(dioxane) or SnCl2 in
diethyl ether. All other reagents were used as received.
The geometries of the model anions [{(Me3Si)2C(H)}2EGa{[N-
(Ph)C(H)]2}]-, E ) Ge or Sn, were optimized using the Gaussian
98 package,18 employing the methods recommended by Boehme
and Frenking.19 That is the BP86 density functional method20 with
a 6-31G* basis set on C, N, and H21 and Stuttgart-Dresden ECP/
basis sets for Si, Ga, and Sn,22 augmented by a d-type polarization
function with an exponent of 0.207 on Ga, 0.183 on Sn and 0.246
on Ge.23 Atomic charges, orbital populations, and bonding analyses
were obtained from the NBO scheme24 of the optimized structure.
To comply with the maximum basis functions allowed by the NBO
program, 6-31G basis sets were applied to the C and H atoms
outside the gallium heterocycle and those not directly bound to the
group 14 centers.
(7) Sundermann, A.; Reiher, M.; Schoeller, W. W. Eur. J. Inorg. Chem.
1998, 305.
(8) Jones, C.; Mills, D. P.; Rose, R. P. J. Organomet. Chem. 2006, 691,
3060.
(9) Scha¨fer, A.; Weidenbruch, M.; Saak, W.; Pohl, S. J. Chem. Soc., Chem.
Commun. 1995, 1157.
(10) Stabenow, F.; Saak, W.; Weidenbruch, M. Chem. Commun. 1999,
1131.
(13) (a) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F. J. Chem. Soc., Dalton
Trans. 2000, 3094. (b) Hahn, F. E.; Wittenbecher, L.; Ku¨hn, M.;
Lu¨gger, T.; Fro¨lich, R. J. Organomet. Chem. 2001, 617-618, 629.
(14) Fjeldberg, T.; Haaland, A.; Schilling, B. E. R.; Lappert, M. F.; Thorne,
A. J. J. Chem. Soc., Dalton Trans. 1986, 1551 and references therein.
(15) Weidenbruch, M.; Scha¨fer, A.; Kilian, H.; Pohl, S.; Saak, W.;
Marsmann, H. Chem. Ber. 1992, 125, 563.
(16) Stu¨rmann, M.; Saak, W.; Marsmann, H.; Weidenbruch, M. Angew.
Chem., Int. Ed. Engl. 1999, 38, 187.
(17) Jones, C.; Junk, P. C.; Platts, J. A.; Stasch, A. J. Am. Chem. Soc.
2006, 128, 2206.
(11) Arduengo, A. J., III; Rasika Dias, H. V.; Calabrese, J. C.; Davidson,
F. Inorg. Chem. 1993, 32, 1541.
(12) Kuhn, N.; Kratz, T.; Boese, R. Chem. Ber. 1995, 128, 245.
Inorganic Chemistry, Vol. 45, No. 18, 2006 7243

Green et al.
Preparation of [K(tmeda)][Ge{CH(SiMe3)2}2Ga{[N(Ar)C-
(H)]2}] (3). A solution of [K(tmeda)][:Ga{[N(Ar)C(H)]2}] (0.31
g, 0.51 mmol) in diethyl ether (20 cm3) was added over 5 min to
a solution of [Ge{CH(SiMe3)2}2]2 (0.20 g, 0.26 mmol) in diethyl
ether (20 cm3) at -78 °C. The reaction mixture was allowed to
warm to room temperature and was stirred for 24 h. Volatiles were
removed in vacuo, and the residue was extracted with hexane (40
cm3). Concentration to ca. 20 cm3 and storage at -30 °C overnight
yielded yellow crystals of 3 (0.28 g, 56%). mp: 110-112 °C (dec).
1H NMR (400 MHz, C6D6): δ 0.16 (s, 18H, Si(CH3)3), 0.18 (s,
4.7 (Si(CH3)3), 24.2 (CH(CH3)2), 26.4 (CH(CH3)2), 28.5 (CHMe2),
45.2 (N(CH3)2), 56.7 (N(CH2)), 122.2 (CN), 123.0 (m-ArC), 123.8
(p-ArC), 146.9 (o-ArC), 150.2 (ipso-ArC). 29Si{1H} NMR (59.7
MHz, C6D6): δ -0.04, 0.31 (SiMe3). 119Sn{1H} NMR (186.4 MHz,
C6D6): δ -97.9 (pw 233 Hz at 1/2 peak height). IR (Nujol): ν
1588 (s), 1565 (m), 1248 (s), 1113 (s), 1020 (s) cm-1. (MS/EI)
m/z: 446 [Ga{[N(Ar)C(H)]2}H+, 67%], 377 [{N(Ar)C(H)}2H+,
100%]. Anal. Calcd for C46H90N4Si4KGaSn: C, 53.17; H, 8.73;
N, 5.39%. Found: C, 52.42; H, 8.60; N, 5.38%.
Preparation of [K(tmeda)][Sn{CH(SiMe3)2}[Ga{[N(Ar)C-
(H)]2}]2] (5). A solution of [K(tmeda)][:Ga{[N(Ar)C(H)]2}] (0.56
g, 0.92 mmol) in diethyl ether (20 cm3) was added over 5 min to
a solution of [Sn{CH(SiMe3)2}2]2 (0.20 g, 0.23 mmol) in diethyl
ether (20 cm3) at -78 °C. The reaction mixture was warmed to
room temperature and stirred for 24 h. Volatiles were removed in
vacuo, and the residue was extracted with hexane (40 cm3).
Concentration to ca. 15 cm3 and storage at -30 °C overnight yielded
3
2H, CH(SiMe3)2) 0.22 (s, 18H, Si(CH3)3), 1.29 (d, JHH ) 6.75
3
Hz, 12H, (CH3)2CH), 1.32 (d, JHH ) 6.75 Hz, 12H, (CH3)2CH),
3
1.68 (s, 4H, NCH2), 1.70 (s, 12 H, (CH3)2N), 3.84 (sept., JHH
)
6.75 Hz, 4H, CHMe2), 6.27 (s, 2H, CH)CH), 6.88 (t, 3JHH ) 7.55
Hz, 2H, p-ArH), 7.03 (d, 3JHH ) 7.55 Hz, 4H, m-ArH). 13C{1H}
NMR (75.6 MHz, C6D6): δ 3.1 (CH(SiMe3)2), 3.8 (Si(CH3)3), 4.5
(Si(CH3)3), 23.4 (CH(CH3)2), 26.6 (CH(CH3)2), 28.5 (CHMe2), 45.5
(N(CH3)2), 57.0 (N(CH2)), 121.9 (CN), 123.1 (m-ArC), 123.7 (p-
ArC), 146.8 (o-ArC), 150.3 (ipso-ArC). 29Si{1H} NMR (59.7 MHz,
C6D6): δ -0.95, -0.20 (SiMe3). IR (Nujol): ν 1583 (s), 1564 (s),
1250 (s), 1101 (s), 1014 (s), 844 (s) cm-1. (MS/EI) m/z: 446 [Ga-
{[N(Ar)C(H)]2}H+, 100%], 377 [{N(Ar)C(H)}2H+, 46%].
1
orange crystals of 5. (0.10 g, 16%). mp: 188-190 °C (dec). H
NMR (400 MHz, C6D6): δ 0.28 (s, 1H, CH(SiMe3)2), 0.35 (s, 18H,
Si(CH3)3), 1.40 (br overlapping m, 48H, (CH3)2CH), 1.87 (s, 12H,
(CH3)2N), 1.96 (s, 4H, NCH2), 3.84 (br overlapping m, 8H, CHMe2),
6.35 (br s, 4H, CHdCH), 7.11-7.15 (m, 12H, ArH). 13C{1H} NMR
(75.6 MHz, C6D6): δ 1.2 (CH(SiMe3)2), 3.3 (Si(CH3)3), 23.9, 24.9,
26.0, 26.3 (CH(CH3)2), 28.1, 28.2 (CHMe2), 45.3 (N(CH3)2), 57.0
(N(CH2)), 123.0 (br, CN), 123.7 (br, m-ArC), 124.4 (br, p-ArC),
146.4 (br, o-ArC), 152.6 (br, ipso-ArC). 29Si{1H} NMR (59.7 MHz,
C6D6): δ 0.30 (SiMe3). IR (Nujol): ν 1652 (m), 1625 (m), 1594
(m), 1249 (s), 1106 (s), 1021 (s) cm-1. (MS/EI) m/z: 446 [Ga-
{[N(Ar)C(H)]2}H+, 18%], 377 [{N(Ar)C(H)}2H+, 41%].
Preparation of [K(tmeda)][Sn{CH(SiMe3)2}2Ga{[N(Ar)C-
(H)]2}] (4). A solution of [K(tmeda)][:Ga{[N(Ar)C(H)]2}] (0.27
g, 0.45 mmol) in diethyl ether (60 cm3) was added over 1 h to a
solution of [Sn{CH(SiMe3)2}2]2 (0.19 g, 0.23 mmol) in diethyl ether
(40 cm3) at -50 °C. The reaction mixture was stirred for 2 h,
warmed to room temperature, and stirred for 24 h. Volatiles were
removed in vacuo, and the residue was extracted with hexane (40
cm3). Concentration to ca. 20 cm3 and storage at -30 °C overnight
yielded large red crystals of 4 (0.16 g, 35%). mp: 130-132 °C
Preparation of [K(tmeda)][Sn(Ar′)2Ga{[N(Ar)C(H)]2}] (7). A
solution of [Sn(Ar′)2]3 (0.40 g, 0.25 mmol) in diethyl ether (60
cm3) at -55 °C was irradiated at 256 nm for 3 h, yielding a red
solution. This was cooled to -78 °C, and a solution of [K(tmeda)][:
Ga{[N(Ar)C(H)]2}] (0.46 g, 0.76 mmol) in diethyl ether (30 cm3)
was added over 10 min. The reaction mixture was allowed to warm
to room temperature over 12 h; then the volatiles were removed in
vacuo, and the residue was extracted with hexane (120 cm3).
Concentration to ca. 80 cm3 and storage at -30 °C overnight yielded
1
(dec). H NMR (400 MHz, C6D6): δ 0.17 (s, 2H, CH(SiMe3)2),
3
0.20 (s, 18H, Si(CH3)3), 0.28 (s, 18H, Si(CH3)3), 1.29 (d, JHH
)
3
6.77 Hz, 12H, (CH3)2CH), 1.32 (d, JHH ) 6.77 Hz 12H, (CH3)2-
CH), 1.67 (s, 4H, NCH2), 1.72 (s, 12H, (CH3)2N), 3.87 (sept, 3JHH
3
) 6.77 Hz, 4H, CHMe2), 6.30 (s, 2H, CH)CH), 6.88 (t, JHH
)
7.61 Hz, 2H, p-ArH), 7.04 (d, JHH ) 7.61 Hz, 4H, m-ArH). 13C-
3
{1H} NMR (75.6 MHz, C6D6): δ 3.8 (CH(SiMe3)2), 4.4 (Si(CH3)3),
1
orange crystals of 7. (0.79 g, 92%). mp: 141-145 °C (dec). H
(18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P.
Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J.
B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 98, revision
A.10; Gaussian, Inc.: Pittsburgh, PA, 2001.
(19) Boehme, C.; Frenking, G. Chem.sEur. J. 1999, 5, 7.
(20) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (b) Perdew, J. P. Phys.
ReV. A 1986, 33, 8822.
(21) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54,
724. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972,
56, 2257. (c) Gordon, M. S. Chem. Phys. Lett. 1980, 76, 163. (d)
Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
(22) (a) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys. 1987,
86, 866. (b) Bergner, A.; Dolg, M.; Kuehle, W.; Stoll, H.; Preuss, H.
Mol. Phys. 1993, 80, 1431.
3
NMR (400 MHz, C6D6): δ 0.75 (d, JHH ) 6.87 Hz, 12H, p-CH-
(CH3)2{Ar′}), 0.91 (v tr, 3JHH ) 6.80 Hz, 24H, o-CH(CH3)2{Ar′}),
3
3
0.99 (d, JHH ) 6.80 Hz, 12H, CH(CH3)2{Ar}), 1.08 (d, JHH
)
6.80 Hz, 12H, CH(CH3)2{Ar}), 1.45 (s, 12H, N(CH3)2), 1.50 (s, 4
H, NCH2), 2.53 (sept., 3JHH ) 6.87 Hz, 2 H, p-CHMe2{Ar′}), 3.46,
3
3.59 (2× sept, JHH ) 6.80 Hz, 2 × 4H, CHMe2{Ar′ and Ar}),
6.09 (s, 2 H, CH)CH), 6.65 (t, 3JHH ) 7.41 Hz, 2H, p-ArH{Ar}),
6.76 (s, 4H, m-ArH{Ar′}), 6.78 (d, 3JHH ) 7.41 Hz, 4H, m-ArH{Ar}).
13C{1H} NMR (75.6 MHz, C6D6): δ 24.0, 24.4, 24.8, 25.1, 25.7
(CH(CH3)2), 28.5, 34.5, 39.6 (CH(CH3)2), 45.0 (N(CH3)2), 56.8
(NCH2), 120.3 (CN), 122.5, 122.9, 123.9, 146.3, 146.9, 150.2,
153.9, 155.0 (ArC). 119Sn{1H} NMR (186.4 MHz, C6D6): δ -306.7
(p.w. 251 Hz at 1/2 peak height). IR (Nujol): ν 1842 (m), 1666
(m), 1594 (s), 1556 (s), 1260 (s), 1099 (s) cm-1. (MS/EI) m/z: 1126
[M+, 3%], 648 [Ar′Ga{[N(Ar)C(H)]2}+, 100%], 446 [Ga{[N(Ar)C-
(H)]2}H+, 13%], 377 [{N(Ar)C(H)}2H+, 14%].
Preparation of [Ar′Ga{[N(Ar)C(H)]2}] (8). A solution of
[K(tmeda)][:Ga{[N(Ar)C(H)]2}] (0.28 g, 0.46 mmol) in diethyl ether
(30 cm3) was added over 10 min to a solution of [Pb(Ar′)2]2 (0.28
g, 0.23 mmol) in diethyl ether (20 cm3) at -78 °C. The reaction
mixture was warmed to room temperature over 2 h, during which
time a lead mirror formed on the side of the vessel. Volatiles were
then removed in vacuo, and the residue was extracted with hexane
(40 cm3). Concentration to ca. 5 cm3 and storage at -30 °C
(23) Andzelm, J.; Huzinaga, S.; Klobukowski, M.; Radzio, E.; Sakai, Y.;
Tatekawi, H. Gaussian Basis Sets for Molecular Calculations;
Elsevier: Amsterdam, 1984.
(24) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899.
7244 Inorganic Chemistry, Vol. 45, No. 18, 2006

Anionic Gallium(I) N-Heterocyclic Carbene Analogue
overnight yielded yellow crystals of 8. (0.04 g, 13%). mp: 112-
C6D6): δ 22.9 (CH(CH3)2, Sn ring), 23.2 (CH(CH3)2, Sn ring), 23.4
(NCH(CH3)2), 24.6 (CH(CH3)2, Ga ring), 25.6 (CH(CH3)2, Ga ring),
26.0 (CH(CH3)2, Sn ring), 27.6 (CH(CH3)2, Sn ring), 28.1 (CHMe2,
Ga ring), 28.5 (CHMe2, Sn ring), 28.6 (CHMe2, Sn ring), 49.1
(NCHMe2), 122.9 (CN), 123.1, 123.9, 124.1, 125.0, 125.8, 139.9,
143.9, 145.2, 145.4, 147.0 (ArC), 159.8 (CN3). 119Sn{1H} NMR
(186.4 MHz, C6D6): δ 454.8 (pw 420 Hz at 1/2 peak height). IR
(Nujol): ν 1613 (m), 1584 (m), 1256 (s), 1212 (m), 1116 (s), 1056
(m), 934 (w) cm-1. (MS/EI) m/z: 1027 [M+, 15%], 377 [{N(Ar)C-
(H)}2H+,7%].AccurateMS(EI)calcdforC57H84N5GaSn: 1027.4999.
Found: 1027.5002. Anal. Calcd for C57H84N5GaSn: C, 66.62; H,
8.24; N, 6.81%. Found: C, 66.20; H, 8.30; N, 6.90%.
X-ray Crystallography. Crystals of 3-5 and 7-10 suitable for
X-ray structural determination were mounted in silicone oil.
Crystallographic measurements were made using a Nonius Kappa
CCD diffractometer using a graphite monochromator with Mo KR
radiation (λ ) 0.71073 Å). The data were collected at 150 K, and
the structures were solved by direct methods and refined on F2 by
full-matrix least-squares (SHELX97)25 using all unique data. All
non-hydrogen atoms are anisotropic (except the carbon atoms of
the hexane of crystallization in the structure of 5) with H atoms
included in calculated positions (riding model). The X-ray data
collected for 7 were weak. This led to the high r factors for this
structure. Despite this, the molecular connectivity displayed by this
structure is unambiguous. Crystal data and the details of the data
collections and refinement are given in Table 1.
1
3
114 °C. H NMR (400 MHz, C6D6): δ 0.77 (d, JHH ) 6.67 Hz,
12H, o-CH(CH3)2{Ar′}), 0.80 (d, 3JHH ) 6.90 Hz, 6H, p-CH(CH3)2-
3
{Ar′}), 0.87 (d, JHH ) 6.80 Hz, 12H, CH(CH3)2{Ar}), 0.98 (d,
3
3JHH ) 6.80 Hz, 12H, CH(CH3)2{Ar}), 2.36 (sept, JHH ) 6.90
3
Hz, 1H, p-CH(Me)2{Ar′}), 2.48 (sept, JHH ) 6.67 Hz, 2H,
o-CH(Me)2{Ar′}), 3.45 (sept, 3JHH ) 6.80 Hz, 4H, CH(Me)2{Ar}),
6.14 (s, 2H, CHdCH), 6.58 (s, 2H, m-ArH{Ar′}), 6.71-6.89 (m,
6H, ArH{Ar}). 13C{1H} NMR (75.6 MHz, C6D6): δ 23.7, 23.9,
24.6, 25.7 (CH(CH3)2), 28.5, 34.4, 40.9 (CH(CH3)2), 120.2 (CN),
121.42, 123.8, 125.3, 133.3, 144.5, 144.7, 151.9, 155.3 (ArC). IR
(Nujol): ν 1659 (m), 1593 (s), 1557 (m), 1260 (s), 1203 (s), 1118
(s), 1101 (s), 1058 (s), 934 (s) cm-1. (MS/EI) m/z: 648 [M+, 80%],
446 [Ga{[N(Ar)C(H)]2}H+, 5%], 377 [{N(Ar)C(H)}2H+, 83%].
Accurate MS (EI) calcd for C41H59N269Ga: 648.3929. Found:
648.3926. Anal. Calcd for C41H59N2Ga: C, 75.80; H, 9.15; N,
4.31%. Found: C, 74.97; H, 9.20; N, 4.40%.
Preparation of [(Giso)GeGa{[N(Ar)C(H)]2}] (9). A solution
of [K(tmeda)][:Ga{[N(Ar)C(H)]2}] (0.25 g, 0.41 mmol) in toluene
(20 cm3) was added over 5 min to a solution of [(Giso)GeCl] (0.24
g, 0.41 mmol) in toluene (10 cm3) at -78 °C. The reaction mixture
was warmed to room temperature and stirred for 24 h. Volatiles
were removed in vacuo, and the residue was extracted with hexane
(20 cm3). Concentration to ca. 5 cm3 and storage at -30 °C
overnight yielded light red crystals of 9 (0.26 g, 64%). mp: 149-
152 °C (dec). 1H NMR (400 MHz, C6D6): δ 0.77 (d, 3JHH ) 6.98
3
Hz, 12H, (CH3)2CHN), 1.10 (2× coincidental d, JHH ) 6.71 Hz,
Results and Discussion
3
12H, (CH3)2CH, Ge ring), 1.21 (d, JHH ) 6.89 Hz, 12H, (CH3)2-
3
Reactions with Heavier Group 14 Alkene Analogues
and PbR2. The 2:1 reactions of 1 with Lappert’s heavy
alkene analogues, R2EdER2, E ) Ge or Sn, R ) CH-
(SiMe3)2, or the plumbylene, R2Pb, were carried out in diethyl
ether at -78 °C with subsequent slow warming to room
temperature. When E ) Ge or Sn, the anionic complexes 3
and 4 were formed in moderate to good yields (Scheme 1).
These complexes are closely related to 2 and likely result
from coordination of the gallium heterocycle to the :ER2
fragments which are known to be in equilibrium with R2Ed
ER2 in solution.14 In contrast, the reaction with the plum-
bylene proceeded with deposition of lead metal and the
formation of the known digallane(4), [Ga{[N(Ar)C(H)]2}]2.26
The latter outcome presumably results from the relatively
strongly reducing nature of the Ga(I) center of 1. In attempts
to form 2:1 complexes of the gallium heterocycle with :ER2
fragments, 3 and 4 were reacted with a further equivalent of
1. No reaction occurred with 3, but surprisingly, that with 4
afforded the digallyl stannate complex 5 in a low isolated
yield. No other products could be identified in the reaction
mixture. It seems reasonable that the reaction that gave 5
proceeds via a dianionic intermediate, 6, which then elimi-
nates “KCH(SiMe3)2”. It is noteworthy that we have previ-
ously observed the elimination of potassium alkyls in 2:1
reactions of 1 with metal dialkyls.6g Seemingly, 3 remains
unreactive toward 1 because of the smaller radius and lower
Lewis acidity of its Ge center relative to the Sn center of 4.
To test the generality of these reactions and to draw closer
comparisons with Weidenbruch’s work, 1 was reacted with
CH, Ga ring), 1.28 (d, JHH ) 6.81 Hz, 6H, (CH3)2CH, Ge ring),
1.34 (d, 3JHH ) 6.81 Hz, 6H, (CH3)2CH, Ge ring), 1.40 (d, 3JHH
)
3
6.89 Hz, 12H, (CH3)2CH, Ga ring), 3.57 (sept, JHH ) 6.71 Hz,
3
2H, CHMe2, Ge ring), 3.68 (sept, JHH ) 6.81 Hz, 2H, CHMe2,
3
Ge ring), 3.74 (sept, JHH ) 6.89 Hz, 4H, CHMe2, Ga ring), 3.87
3
(sept, JHH ) 6.98 Hz, 2H, NCHMe2), 6.40 (s, 2H, CHN) 7.03-
7.29 (m, 12H, ArH). 13C{1H} NMR (100.6 MHz, C6D6): δ 22.9
(CH(CH3)2, Ge ring), 23.0 (CH(CH3)2, Ge ring), 23.2 (NCH(CH3)2),
24.3 (CH(CH3)2, Ga ring), 25.6 (CH(CH3)2, Ga ring), 26.1 (CH-
(CH3)2, Ge ring), 27.8 (CH(CH3)2, Ge ring), 28.2 (CHMe2, Ga ring),
28.4 (CHMe2, Ge ring), 28.7 (CHMe2, Ge ring), 49.0 (NCHMe2),
122.5 (CN), 122.9, 124.1, 124.1, 124.8, 126.6, 138.5, 144.8, 145.3,
145.8, 147.5 (ArC), 154.9 (CN3). IR (Nujol): ν 1612 (w), 1586
(w), 1408 (s), 1256 (s), 1211 (w), 1120 (s), 1055 (m), 937 (w)
cm-1. (MS/EI) m/z: 981 [M+, 5%], 377 [{N(Ar)C(H)}2H+, 8%].
Accurate MS (EI) calcd for C57H84N5GaGe: 981.5189. Found:
981.5182. Anal. Calcd for C57H84N5GaGe: C, 69.74; H, 8.62; N,
7.13%. Found: C, 69.61; H, 8.76; N, 7.32%.
Preparation of [(Giso)SnGa{[N(Ar)C(H)]2}] (10). A solution
of [K(tmeda)][:Ga{[N(Ar)C(H)]2}] (0.29 g, 0.48 mmol) in toluene
(20 cm3) was added over 5 min to a solution of [(Giso)SnCl] (0.30
g, 0.48 mmol) in toluene (10 cm3) at -78 °C. The reaction mixture
was warmed to room temperature and stirred for 24 h. Volatiles
were removed in vacuo, and the residue was extracted with hexane
(20 cm3). Concentration to ca. 5 cm3 and storage at -30 °C
overnight yielded deep red crystals of 10 (0.26 g, 52%). mp: 205-
1
3
206 °C. H NMR (400 MHz, C6D6): δ 0.81 (d, JHH ) 6.91 Hz,
12H, (CH3)2CHN), 1.12 (2× coincidental d, 3JHH ) 6.75 Hz, 12H,
(CH3)2CH, Sn ring), 1.23 (d, 3JHH ) 6.92 Hz, 12H, (CH3)2CH, Ga
3
ring), 1.30 (d, JHH ) 6.75 Hz, 6H, (CH3)2CH, Sn ring), 1.37 (d,
3JHH ) 6.75 Hz, 6H, (CH3)2CH, Sn ring), 1.40 (d, 3JHH ) 6.92 Hz,
12H, (CH3)2CH, Ga ring), 3.61 (sept, 3JHH ) 6.75 Hz, 2H, CHMe2,
Sn ring), 3.76 (overlapping m, 6H, CHMe2, Ga (4H) and Sn (2H)
(25) Sheldrick, G. M. SHELX-97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(26) Pott, T.; Jutzi, P.; Schoeller, W. W.; Stammler, A.; Stammler, H.-G.
Organometallics 2001, 20, 5492.
3
rings), 3.92 (sept, JHH ) 6.91 Hz, 2H, NCHMe2), 6.51 (s, 2H,
CHN) 7.10-7.28 (m, 12H, ArH). 13C{1H} NMR (100.6 MHz,
Inorganic Chemistry, Vol. 45, No. 18, 2006 7245

Green et al.
Table 1. Summary of Crystallographic Data for Compounds 3-5 and 7-10
3
4
5‚(hexane)
7
empirical formula
fw
cryst syst
space group
a (Å)
C46H90GaGeKN4Si4
992.99
orthorhombic
Pca21
21.370(4)
14.035(3)
38.065(8)
90
90
90
11417(4)
8
1.155
C46H90GaKN4Si4Sn
1039.09
orthorhombic
Pbca
20.846(4)
19.909(4)
28.145(6)
90
90
90
11681(4)
8
1.182
C71H121Ga2KN6Si2Sn
1412.15
triclinic
C62H98GaKN4Sn
1126.95
triclinic
P1h
12.574(3)
20.341(4)
25.091(5)
93.00(3)
91.05(3)
99.22(3)
6324(2)
P1h
14.4970(19)
16.782(2)
17.605(3)
77.940(10)
76.270(10)
71.460(10)
3903.1(9)
2
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
vol (Å3)
Z
4
F
calcd (g cm-3
)
1.202
1.127
1496
1.184
0.924
2392
µ (mm-1
F(000)
)
1.187
4256
1.073
4400
cryst size (mm)
θ range (deg)
reflns collected
0.25 × 0.25 × 0.15
1.74-26.37
100 746
0.0784
0.30 × 0.25 × 0.20
2.92-27.55
103 668
0.0792
0.25 × 0.25 × 0.20
2.93-26.48
55 245
0.23 × 0.20 × 0.17
4.10-25.20
40 379
Rint
0.0911
0.0418
data/restraints/params
22 380/73/1165
1.026
0.0474
0.1038
0.714 and -0.426
13 392/18/538
1.021
0.0368
0.0814
0.810 and -0.758
15 862/6/746
1.041
0.0462
0.1142
1.216 and -0.829
22 107/24/1292
1.165
0.1366
0.3337
2.704 (near Sn1) and
-2.007 (near Ga1)
GOF on F2
R1 indices [I > 2σ(I)]a
wR2 indices (all data)a
largest peak and hole (e A-3
)
8
9
10
empirical formula
fw
cryst syst
space group
a (Å)
C41H59GaN2
649.62
triclinic
P1h
13.088(3)
16.091(3)
20.166(4)
109.23(3)
105.13(3)
91.58(3)
3840.8(13)
4
C57H84GaGeN5
981.60
C57H84GaN5Sn
1027.70
monoclinic
P21/c
10.068(2)
20.314(4)
26.952(5)
90
91.27(3)
90
5511.1(19)
4
monoclinic
P21/c
17.899(4)
18.398(4)
17.119(3)
90
90.16(3)
90
5637(2)
4
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
vol (Å3)
Z
F
calcd (g cm-3
)
1.123
0.744
1400
1.157
1.049
2096
1.239
0.981
2168
µ (mm-1
F(000)
)
cryst size (mm)
θ range (deg)
reflns collected
Rint
0.30 × 0.30 × 0.20
2.97-26.03
49 660
0.50 × 0.50 × 0.40
3.18 to 27.00
41 622
0.25 × 0.15 × 0.15
2.94 to 26.00
19 123
0.0944
0.0526
0.0426
data/restraints/params
14 935/0/832
1.032
0.0474
0.1127
0.507 and -0.615
12 279/0/597
1.023
0.0356
0.0804
0.322 and -0.307
10 778/24/597
1.017
0.0444
0.0958
0.973 and -0.685
GOF on F2
R1 indices [I > 2σ(I)]a
wR2 indices (all data)a
largest peak and hole (e A-3
)
2
2
2 2
a R1(F) ) {∑(|Fo| - |Fc|)/∑|Fo|} for reflections with Fo > 4(σ(Fo)). wR2(F2) ) {∑w(|Fo| - |Fc| )2/∑w|Fo | }1/2 where w is the weight given each
reflection.
Ar′2EdEAr′2, E ) Ge, Sn (generated in situ), or Pb. In
stannylene in solution) afforded the anionic complex, 7, in
an almost quantitative yield (Scheme 1). As opposed to the
formation of 5, when 7 was treated with a second equivalent
of 1, no reaction occurred. This might be the result of a
reduced steric accessibility of the Ga heterocycle to the Sn
center of 7 relative to that in 4. When 1 was reacted with
Ar′2PbdPbAr′2, lead metal deposition was observed, as in
its reaction with Pb{CH(SiMe3)2}2. In this case, however,
the only other identifiable product was the gallium hetero-
cycle, 8, which probably forms via the lead analogue of 7
which is most likely unstable at room temperature. Indeed,
the neutral NHC adduct, 2 (E ) Pb), is unstable in solution
above -70 °C and decomposes to 1,3,5-triisopropylben-
zene.10 It is of note that a closely related gallium heterocycle,
contrast to the formation of 3, no reaction was observed with
Ar′2GedGeAr′2. This is perhaps not surprising as the Ged
Ge bond in Ar′2GedGeAr′2 is significantly shorter and
stronger than that in {(Me3Si)2C(H)}2GedGe{C(H)(SiMe3)2}2.
Accordingly, the latter largely dissociates into monomeric
germylene fragments in solution,14 whereas the former can
behave as the dimeric digermene in solution,27 although
partial dissociation into germylene fragments is also likely
for this compound. Similar to the formation of 4, the reaction
of 1 with an in situ generated solution of Ar′2SndSnAr′2
(which is known to be in equilibrium with the corresponding
(27) Scha¨fer, H.; Saak, W.; Weidenbruch, M. Organometallics 1999, 18,
3159.
7246 Inorganic Chemistry, Vol. 45, No. 18, 2006

Anionic Gallium(I) N-Heterocyclic Carbene Analogue
Scheme 1
[(Me3Si)3CGa{[N(Pri)C(H)]2}], has been recently reported
to result from the reaction of the diazabutadiene, {N(Pri)C-
(H)}2, with the tetrameric gallium diyl, [{GaC(SiMe3)3}4].28
Attempts to intentionally prepare 8 via the reaction of 1 with
Ar′Br were not successful and led to the formation of the
known paramagnetic gallium(II) dimer [BrGa{[N(Ar)C-
the significant quadrupolar broadening of that signal by the
two gallium centers coordinated to tin in that complex.
Finally, the NMR spectra of the gallium heterocycle, 8, are
consistent with its solid-state structure and warrant no further
comment.
X-ray crystallographic analyses of complexes 3-5, 7, and
8 were carried out. The molecular structures of 4, 5, and 8
are depicted in Figures 1-3. The crystal structure of 7 is of
poor quality because of weak data but shows the complex
to be structurally similar to 4. As a result, a discussion of its
molecular structure is only included in the Supporting
Information, although some comparisons with the structures
of 3-5 will be made here. Compounds 3 and 4 also have
similar structures and therefore only the molecular structure
of 4 is shown in Figure 1. The complexes are monomeric
contact ion pairs in which the Ge or Sn centers are
coordinated by two alkyl ligands and one gallium hetero-
cycle. As in previously reported complexes of the gallium
heterocycle,6 its Ga-N distances and N-Ga-N angle are
shorter and more obtuse, respectively, than those in the free
heterocycle. The potassium centers of 3 and 4 are chelated
by a molecule of tmeda, have an η6-interaction with one of
the aryl substituents of the gallium heterocycle, and are
coordinated by the lone pair of the Ge or Sn center,
respectively. The Ge-K and Sn-K bond lengths in these
complexes (cf. 3.495(4) Å in 7) lie in the normal ranges for
such interactions.31 In contrast, the Ge-Ga bond in 3 is very
long and outside the known range (2.407-2.494 Å).31
Although there have been no previously structurally char-
acterized Sn-Ga bonds in molecular compounds, that in 4
(cf. 2.6660(18) Å in 7) lies outside the sum of the covalent
radii for the two elements (2.65 Å)32 and therefore may be
considered weak. This is, of course, very similar to the weak
NHC-E interactions in 2, and as in those compounds, the
fold angles, θ, between the E-Ga vector and the C-E-C
least-squares planes are obtuse at 70.5 (3) and 70.9° (4).
Interestingly, this angle is considerably more acute in 7
(H)]2 }]2.29
•
The spectroscopic data for the anionic 1:1 and 2:1
complexes, 3-5 and 7, suggest that the gallium heterocycle-
(s) remains coordinated to the germylene or stannylene
1
fragments in solution. The H and 13C{1H} NMR spectra
are, however, more symmetrical than would be expected if
the solid-state structures (vide infra) of the complexes were
retained in solution. It is likely that the aryl-coordinated
[K(tmeda)] cation of each complex either migrates rapidly
(on the NMR time scale) between the aryl groups of the
anions or the complexes are in equilibria between the contact
ion pairs and ion separate salts. Such equilibria may be
facilitated by the arene solvent (C6D6) used for the NMR
experiments. All attempts to shed light on any fluxional
processes occurring in solution (D8-toluene) by variable-
temperature NMR studies were thwarted by the poor solubil-
ity of the complexes at low temperatures. An examination
1
of the H, 13C{1H}, and29Si{1H} NMR spectra of 3 and 4
revealed these complexes to possess two sets of chemically
inequivalent SiMe3 groups in solution, as might be expected.
The 119Sn{1H} NMR spectra of 4 and 7 each displayed broad
singlet resonances at considerably higher fields (-97.9 and
-306.7 ppm, respectively) than have reported for the related
neutral complex, 2 (E ) Sn, 710 ppm).9 This is not surprising
considering the anionic nature of these complexes which can
be compared to trialkyl stannate anions (e.g., LiSnMe3; 119Sn-
{1H} NMR -189.2 ppm).30 No signal was observed in the
119Sn{1H} NMR spectrum of 5. This is likely the result of
(28) Uhl, W.; Melle, S.; Pro¨tt, M. Z. Anorg. Allg. Chem. 2005, 631, 1377.
NB: Related 1-galla-2,5-diazoles have also been reported, see Pott,
T.; Jutzi, P.; Neumann, B.; Stammler, H.-G. Organometallics 2001,
20, 1965, and Brown, D. S.; Decken, A.; Cowley, A. H. J. Am. Chem.
Soc. 1995, 117, 5421.
(29) Baker, R. J.; Farley, R. D.; Jones, C.; Mills, D. P.; Kloth, M.; Murphy,
D. M. Chem.sEur. J. 2005, 11, 2972.
(30) Reimann, W.; Kulvila, H. G.; Farah, D.; Apoussidis, T. Organome-
tallics 1987, 6, 557.
(31) As determined by a survey of the Cambridge Crystallographic
Database, May, 2006.
(32) Emsley, J. The Elements, 2nd edition; Clarendon Press: Oxford, U.K.,
1995.
Inorganic Chemistry, Vol. 45, No. 18, 2006 7247

Green et al.
Figure 1. Thermal ellipsoid plot (25% probability surface) of the molecular
structure of [K(tmeda)][Sn{CH(SiMe3)2}2Ga{[N(Ar)C(H)]2}] (4); hydrogen
atoms (except H(27) and H(34)) have been omitted for clarity. Selected
bond lengths (Å) and angles (deg): Sn(1)-C(34) ) 2.268(2), Sn(1)-C(27)
) 2.280(2), Sn(1)-Ga(1) ) 2.7186(6), Sn(1)-K(1) ) 3.6407(9), Ga(1)-
N(2) ) 1.909(2), Ga(1)-N(1) ) 1.9294(19), K(1)-N(4) ) 2.820(3), K(1)-
N(3) ) 2.833(3), N(1)-C(1) ) 1.397(3), N(2)-C(2) ) 1.397(3), C(1)-
C(2) ) 1.343(3), K(1)-centroid (C(3)-C(8)) ) 2.939(3), C(34)-Sn(1)-
C(27) ) 99.81(9), C(34)-Sn(1)-Ga(1) ) 104.23(7), C(27)-Sn(1)-Ga(1)
) 100.10(6), C(34)-Sn(1)-K(1) ) 123.34(7), C(27)-Sn(1)-K(1) )
133.40(6), Ga(1)-Sn(1)-K(1) ) 86.571(13), N(2)-Ga(1)-N(1) ) 85.79(8),
N(2)-Ga(1)-Sn(1) ) 147.95(6), N(1)-Ga(1)-Sn(1) ) 120.10(6). Selected
bond lengths (Å) and angles (deg) for [K(tmeda)][Ge{CH(SiMe3)2}2Ga-
{[N(Ar)C(H)]2}] (3): Ge(1)-C(27) ) 2.085(4), Ge(1)-C(34) ) 2.091(4),
Ge(1)-Ga(1) ) 2.5396(8), Ge(1)-K(1) ) 3.4418(12), Ga(1)-N(2) )
1.911(4), Ga(1)-N(1) ) 1.925(4), K(1)-N(3) ) 2.783(4), K(1)-N(4) )
2.959(4), N(1)-C(1) ) 1.398(6), N(2)-C(2) ) 1.404(6), C(1)-C(2) )
1.324(6), K(1)-centroid (C(3)-C(8)) ) 2.931(3), C(27)-Ge(1)-C(34) )
105.81(18), C(27)-Ge(1)-Ga(1) ) 106.13(12), C(34)-Ge(1)-Ga(1) )
97.15(14), C(27)-Ge(1)-K(1) ) 106.21(12), C(34)-Ge(1)-K(1) )
143.45(14), Ga(1)-Ge(1)-K(1) ) 90.47(2), N(2)-Ga(1)-N(1) ) 85.59(15),
N(2)-Ga(1)-Ge(1) ) 148.55(11), N(1)-Ga(1)-Ge(1) ) 123.60(11).
Figure 2. Thermal ellipsoid plot (25% probability surface) of the molecular
structure of [K(tmeda)][Sn{CH(SiMe3)2}[Ga{[N(Ar)C(H)]2}]2] (5); iso-
propyl groups and hydrogen atoms have been omitted for clarity. Selected
bond lengths (Å) and angles (deg °): Sn(1)-C(53) ) 2.259(3), Sn(1)-
Ga(2) ) 2.6361(5), Sn(1)-Ga(1) ) 2.6610(6), Sn(1)-K(1) ) 3.5082(10),
Ga(1)-N(2) ) 1.895(3), Ga(1)-N(1) ) 1.916(3), Ga(2)-N(3) ) 1.901(3),
Ga(2)-N(4) ) 1.902(3), K(1)-N(5) ) 2.769(3), K(1)-N(6) ) 2.784(3),
N(1)-C(1) ) 1.399(4), N(2)-C(2) ) 1.402(4), N(3)-C(27) ) 1.401(4),
N(4)-C(28) ) 1.400(4), C(1)-C(2) ) 1.332(5), C(27)-C(28) ) 1.336(5),
K(1)-centroid (C(3)-C(8)) ) 2.866(3), C(53)-Sn(1)-Ga(2) ) 98.11(8),
C(53)-Sn(1)-Ga(1) ) 108.57(9), Ga(2)-Sn(1)-Ga(1) ) 96.009(16),
C(53)-Sn(1)-K(1) ) 123.22(8), Ga(2)-Sn(1)-K(1) ) 134.39(2), Ga-
(1)-Sn(1)-K(1) ) 88.767(19), N(2)-Ga(1)-N(1) ) 86.73(12), N(2)-
Ga(1)-Sn(1) ) 155.80(9), N(1)-Ga(1)-Sn(1) ) 117.35(8), N(3)-Ga(2)-
N(4) ) 87.51(11), N(3)-Ga(2)-Sn(1) ) 138.80(8), N(4)-Ga(2)-Sn(1)
) 133.38(8).
coordinated by the tin lone pair. Both the Sn-K and Sn-
Ga distances are significantly shorter than those in 4 and
the intraheterocyclic geometries are similar to those in
previously reported complexes. The crystal structure of the
monomeric, neutral gallium heterocycle, 8, (Figure 3)
contains two crystallographically independent molecules in
the asymmetric unit which show no significant geometrical
differences. As a result, geometric parameters for only one
molecule are included in the caption of Figure 3. The gallium
heterocycle is effectively planar and forms an angle of 52.3°
with the Ar′ plane. The Ga-N bond lengths and N-Ga-N
angle are similar to those in [(Me3Si)3CGa{[N(Pri)C(H)]2}]28
but shorter and more obtuse, respectively, than is normal
for metal complexes of the gallium heterocycle.
To shed light on the nature of the weak Ga-E bonds in 3
and 4, DFT calculations were carried out on the model
anions, [{(Me3Si)2C(H)}2EGa{[N(Ph)C(H)]2}]-, E ) Ge or
Sn. These complexes converged with similar geometries to
those of 3 and 4, although the bonds about the heavier group
14 and gallium centers were overestimated by 3-5%, as has
been previously seen for DFT studies on metal complexes
of the [Ga{[N(Ph)C(H)]2}]- heterocycle.6 In addition, the
angles between the planes of the phenyl substituents and the
plane of the gallium heterocycle are significantly more acute
than in the experimental complexes. This is, no doubt, a result
(62.3°) but closer to the value for 2 (E ) Sn) which
incorporates the same stannylene fragment as 7. The length
of the E-Ga bonds and the magnitude of the fold angles, θ,
for 3 and 4 suggest that the lone pair of the gallium
heterocycle donates into the empty p-orbital of the germylene
or stannylene fragments and that there is little “rehybridiza-
tion” of the Ge or Sn centers upon coordination. Further
evidence for this comes from the KEC2 fragment which is
not distorted far from planar as judged by the angle between
the K-E vector and the EC2 least-squares plane (3, 26.1°;
4, 16.2°). It is interesting that this angle is significantly more
acute in 4 which means that the coordination environment
of the tin center directly opposite the gallium heterocycle is
exposed. This could allow a second gallium heterocycle to
attack the metal, as proposed for the mechanism of formation
of the digallyl stannate, 5.
The molecular structure of 5 is depicted in Figure 2. It is
a monomeric contact ion pair with a tin center that possesses
a distorted tetrahedral geometry. As in the previously
described complexes, the potassium center is chelated by a
molecule of tmeda, has an η6-arene interaction, and is
7248 Inorganic Chemistry, Vol. 45, No. 18, 2006

Anionic Gallium(I) N-Heterocyclic Carbene Analogue
Figure 3. Thermal ellipsoid plot (25% probability surface) of the molecular
structure of [Ar′Ga{[N(Ar)C(H)]2}] (8); hydrogen atoms have been omitted
for clarity. Selected bond lengths (Å) and angles (deg): Ga(1)-N(1) )
1.846(2), Ga(1)-N(2) ) 1.848(2), Ga(1)-C(27) ) 1.936(3), N(1)-C(1)
) 1.400(3), N(2)-C(2) ) 1.406(3), C(1)-C(2) ) 1.343(4), N(1)-Ga(1)-
N(2) ) 89.65(10), N(1)-Ga(1)-C(27) ) 134.58(10), N(2)-Ga(1)-C(27)
) 135.77(10), C(1)-N(1)-Ga(1) ) 108.76(17), C(2)-N(2)-Ga(1) )
108.15(17).
of the lack of phenyl substitution in the model anions. Despite
this and the fact that coordination to countercations has not
been taken into account in the model systems, the angles
about the E centers of the theoretical anions are close to those
for 3 and 4. Most importantly, the fold angles, θ, (E ) Ge
(72.4°), Sn (74.8°)) are in good agreement with the experi-
mental complexes.
An NBO analysis of the Ga-E bonds (Wiberg bond
indices for E ) Ge and Sn are 0.998 and 0.952, respectively)
in [{(Me3Si)2C(H)}2EGa{[N(Ph)C(H)]2}]- revealed that the
orbital contributions from the E centers are of very high
p-character (for E ) Ge, s ) 4.4% and p ) 95.4% and for
E ) Sn, s ) 4.7% and p ) 95.1%), while the orbital
contributions from the donating Ga centers have s- to sp-
character (for E ) Ge, s ) 77.6% and p 22.3% and for E )
Sn, s ) 64.4% and p ) 35.5%). This is consistent with the
apparently weak E-Ga bonds and the minimal “rehybrid-
ization” of the E centers upon heterocycle coordination. As
might be expected, the lone pairs at the E centers of the
anions have significant s-character (E ) Ge (78.6%), Sn
(80.9%)). The orbitals that have the greatest contribution to
the E-Ga bonds and the E lone pairs are the HOMO-3
and HOMO-1, respectively, which differ in energy by 32.3
and 31.4 kcal/mol for 9 and 10, respectively. Illustrations of
these orbitals for the germanium system are depicted in
Figure 4. The HOMO and HOMO-2 are largely ligand-
based orbitals.
Figure 4. Representations of (a) the HOMO-3 and (b) the HOMO-1 of
the model anionic system [{(Me3Si)2C(H)}2GeGa{[N(Ph)C(H)]2}]-.
to attempt the preparation of germanium(II) and tin(II)
complexes of the heterocycle that contain more normal
covalent bonds for the purpose of comparison. Obviously,
this should be achievable by salt elimination reactions
between 1 and compounds of the type REX, E ) Ge or Sn,
X ) halide. However, all previous attempts to use 1 in
metathesis reactions with metal halide complexes have been
unsuccessful, leading to paramagnetic gallium(II) dimers,
•
[XGa{[N(Ar)C(H)]2 }]2,29 presumably via insertion of the
Ga(I) center into the M-X bond of the precursor, followed
by decomposition. Recently, we have discovered that if the
metal halide precursor incorporates a bulky neutral or anionic
chelating ligand, then the formation of [XGa{[N(Ar)C-
•
(H)]2 }]2 can be circumvented. We have developed synthetic
routes to monomeric germanium(II) and tin(II) halide
Reactions with Guanidinato Group 14 Element(II)
Chlorides. Complexes 3 and 4 can be considered as weakly
bound “adducts” of an anionic gallium(I) heterocycle with
germylene or stannylene fragments. It was thought logical
complexes (e.g., [(Giso)ECl] E ) Ge or Sn)33 stabilized by
(33) Jones, C.; Junk, P. C.; Stasch, A. unpublished results.
Inorganic Chemistry, Vol. 45, No. 18, 2006 7249

Green et al.
Scheme 2
very bulky guanidinate ligands and saw these as ideal starting
materials for this study.
The 1:1 reactions of 1 with [(Giso)ECl], E ) Ge or Sn, in
toluene led to good yields of the monomeric germanium or
tin-gallyl complexes, 9 and 10 (Scheme 2). Both compounds
1
were found to be unreactive toward excess 1. The H and
13C{1H} NMR spectra of both complexes are similar and
consistent with the retention of their solid state structures in
solution. The presence of only two methyl doublet resonances
for the isopropyl groups of the gallium heterocycle in the
spectra of both compounds suggests that the rotation of that
heterocycle about the E-Ga bond is not restricted. This has
been noted previously for complexes of the heterocycle.6
Despite this, resonances for four chemically inequivalent sets
of methyl groups from the group 14 heterocycle aryl
substituents were necessarily observed in the spectra of the
complexes. The 119Sn{1H} NMR spectra of 10 exhibits a
broad singlet (454.8 ppm) in the normal region for neutral
complexes, containing a 3-coordinate tin(II) center (cf. 2; E
) Sn, δ 710 ppm).9
Compounds 9 and 10 have similar structures, and so only
the molecular structure for 9 (Figure 5) is included here.
Relevant geometric parameters for 10 are included in the
caption of Figure 5. Both compounds are monomeric and
contain heavily distorted pyramidal germanium or tin centers.
Although the Ge-Ga bond of 9 is significantly shorter than
that of 3, it still lies outside the otherwise known range for
such bonds.31 Similarly, the Sn-Ga distance for 10 is less
than that for 4 (but longer than that for 7) but can still be
considered long. The reason for these long interactions must
be partly the steric crowding between the bulky gallium and
group 14 heterocycles. Another manifestation of this is that
the gallium-group 14 element bonds are significantly
“skewed” with the E-Ga-{C(32)-C(33) bond midpoint}
angles being markedly distorted from the ideal 180° (9,
153.5°; 10, 157.9°). In all other aspects, the geometries of
the effectively planar gallium heterocycles in both com-
pounds are almost symmetrical and unremarkable. Although
the four-membered Ge and Sn heterocycles are strained, their
guanidinate ligands appear to be largely delocalized, and their
E-N bond lengths lie well within the reported ranges.31 The
angles between the two heterocycles are 74.9° in 9 and 76.7°
in 10.
Figure 5. Thermal ellipsoid plot (25% probability surface) of the molecular
structure of [(Giso)GeGa{[N(Ar)C(H)]2}] (9); the hydrogen atoms have been
omitted for clarity. Selected bond lengths (Å) and angles (deg): Ge(1)-
N(1) ) 1.9971(15), Ge(1)-N(2) ) 2.0148(16), Ge(1)-Ga(1) ) 2.5157-
(7), Ga(1)-N(4) ) 1.8825(16), Ga(1)-N(5) ) 1.8898(16), N(1)-C(1) )
1.353(2), N(2)-C(1) ) 1.346(2), N(3)-C(1) ) 1.369(2), N(4)-C(32) )
1.396(2), N(5)-C(33) ) 1.389(3), C(32)-C(33) ) 1.344(3), N(1)-Ge-
(1)-N(2) ) 65.03(6), N(1)-Ge(1)-Ga(1) ) 108.98(5), N(2)-Ge(1)-Ga-
(1) ) 105.07(4), N(4)-Ga(1)-N(5) ) 87.10(7), N(4)-Ga(1)-Ge(1) )
161.38(5), N(5)-Ga(1)-Ge(1) ) 110.50(5), C(1)-N(1)-Ge(1) ) 93.53-
(11), C(1)-N(2)-Ge(1) ) 92.95(11), N(2)-C(1)-N(1) ) 106.08(16).
Selected bond lengths (Å) and angles (deg) for [(Giso)SnGa{[N(Ar)C-
(H)]2}] (10): Sn(1)-N(1) ) 2.188(3), Sn(1)-N(2) ) 2.215(3), Sn(1)-
Ga(1) ) 2.6888(6), Ga(1)-N(4) ) 1.882(3), Ga(1)-N(5) ) 1.890(3),
N(1)-C(1) ) 1.365(4), N(2)-C(1) ) 1.345(4), N(3)-C(1) ) 1.371(4),
N(4)-C(32) ) 1.401(4), N(5)-C(33) ) 1.388(5), C(32)-C(33) ) 1.349-
(5), N(1)-Sn(1)-N(2) ) 60.18(10), N(1)-Sn(1)-Ga(1) ) 104.39(8),
N(2)-Sn(1)-Ga(1) ) 99.70(7), N(4)-Ga(1)-N(5) ) 87.56(13), N(4)-
Ga(1)-Sn(1) ) 157.14(9), N(5)-Ga(1)-Sn(1) ) 114.63(10), C(1)-N(1)-
Sn(1) ) 95.3(2), C(1)-N(2)-Sn(1) ) 94.7(2), N(2)-C(1)-N(1) )
109.1(3).
ER2, E ) Ge or Sn, the ionic [K(tmeda)][R2EGa{[N(Ar)C-
(H)]2}] complexes, which exhibit long E-Ga bonds, are
formed. The tin complex exhibits the first structurally
characterized example of such a bond in a molecular
compound. The nature of the E-Ga bonds has been probed
by DFT calculations, and the complexes have been shown
to be closely related to neutral NHC adducts of group 14
dialkyls. It is of interest that the tin complex reacts with a
further equivalent of the gallium heterocycle (to give
[K(tmeda)][RSn[Ga{[N(Ar)C(H)]2}]2]), whereas its germa-
nium counterpart is unreactive. Moreover, the complexes,
Ar′2EdEAr′2, E ) Ge, Sn, or Pb, were found to be, in
general, less reactive than R2EdER2 toward the gallium
heterocycle. The study has also highlighted the utility of the
anionic gallium(I) heterocycle in potassium halide elimina-
tion reactions for the first time. Its reaction with bulky
monomeric guanidinato group 14 halide complexes has given
neutral complexes, [(Giso)EGa{[N(Ar)C(H)]2}], E ) Ge or
Sn, with covalent E-Ga bonds that are shorter than the
E-Ga interactions seen in the aforementioned anionic
complexes. Despite this, these bonds are still long, presum-
ably because of considerable steric crowding within the
complexes.
Conclusions
In summary, the reactions of an anionic gallium(I)
heterocyclic complex, [K(tmeda)][:Ga{[N(Ar)C(H)]2}], with
a variety of heavier group 14 element(II) precursors have
been carried out. In the case of the reactions with R2Ed
7250 Inorganic Chemistry, Vol. 45, No. 18, 2006

Anionic Gallium(I) N-Heterocyclic Carbene Analogue
7, and Cartesian coordinate files and details of the NBO analyses
of the model anions, [{(Me3Si)2C(H)}2EGa{[N(Ph)C(H)]2}]-, E )
Ge or Sn. This material is available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. We thank the EPSRC (studentships
for S.P.G. and D.P.M.) and the ERASMUS scheme of the
European Union (travel grant for KL).
Supporting Information Available: Crystallographic data as
CIF files for 3-5 and 7-10, details of the molecular structure of
IC060851T
Inorganic Chemistry, Vol. 45, No. 18, 2006 7251