Novel Bi- and trinuclear gallium halides and hydrides with acyclic and bicyclic guanidinate substituents: Synthesis and reactivity

Article abstract of DOI:10.1002/ejic.201000598

We report on the synthesis and characterization of new molecular Ga halides and hydrides with acyclic guanidinate substituents with bicyclic guanidinate substituents. Acyclic guanidinates were found to adopt terminal bonding modes like in the dimeric Ga^II compound [(iPr₂N)C(NiPr) ₂GaI]₂. In contrast, bicyclic guanidinates prefer bridging bonding modes. Hence, the reaction between Me₃N?GaH₃ and htbo (1,4,6-triazabicyclo[3.3.0]oct-4-ene) affords the binuclear Ga ^III hydride [H₂Ga(μ-tbo)]₂. This new hydride turned out to be unstable in solution at 25 °C, dihydrogen is slowly eliminated. In the solid state, however, the hydride is stable up to 80 °C. The thermodynamic properties of this and similar dehydrogenation reactions were studied my means of quantum chemical calculations. With Ga₂H ₅(μ₃-O)(μ-tbn)₂ and Ga₂H ₅(μ₃-O)(μ-hpp)₂, two new hydrides were synthesized which can be regarded as the first hydrolysis intermediates of binuclear Ga hydrides with bridging guanidinate substituents. Reaction between Me₃N?GaH₃ and the bicyclic guanidine htbo affords a new binuclear Ga hydride which eliminates further H₂ already at room temp. in solution. In the presence of H₂O trinuclear Ga hydrides are formed.

Full text of DOI:10.1002/ejic.201000598

FULL PAPER
DOI: 10.1002/ejic.201000598
Novel Bi- and Trinuclear Gallium Halides and Hydrides with Acyclic and
Bicyclic Guanidinate Substituents: Synthesis and Reactivity
Daniel Rudolf,[a] Elisabeth Kaifer,[a] and Hans-Jörg Himmel*[a]
Keywords: Gallium / Guanidines / Hydrides / Binuclear compounds
We report on the synthesis and characterization of new mo-
lecular Ga halides and hydrides with acyclic guanidinate
substituents with bicyclic guanidinate substituents. Acyclic
guanidinates were found to adopt terminal bonding modes
like in the dimeric GaII compound [(iPr2N)C(NiPr)2GaI]2. In
contrast, bicyclic guanidinates prefer bridging bonding
modes. Hence, the reaction between Me3N·GaH3 and htbo
(1,4,6-triazabicyclo[3.3.0]oct-4-ene) affords the binuclear
GaIII hydride [H2Ga(µ-tbo)]2. This new hydride turned out to
be unstable in solution at 25 °C, dihydrogen is slowly elimin-
ated. In the solid state, however, the hydride is stable up to
80 °C. The thermodynamic properties of this and similar de-
hydrogenation reactions were studied my means of quantum
chemical calculations. With Ga2H5(µ3-O)(µ-tbn)2 and
Ga2H5(µ3-O)(µ-hpp)2, two new hydrides were synthesized
which can be regarded as the first hydrolysis intermediates
of binuclear Ga hydrides with bridging guanidinate substitu-
ents.
ate ligands initially arises from the results of matrix isola-
tion experiments. These experiments showed that H2 ad-
dition to the matrix-stabilized dimer Ga2, leading (in a
mildly exothermic reaction) to the D2h-symmetric, binuclear
GaI hydride Ga(µ-H)2Ga, is associated with an only small
barrier of ca. 30–50 kJmol–1.[14] This is a rare example of
an oxidative addition of H2 to a molecular E–E bond (E =
non-carbon main-group element) with low activation bar-
rier. We also reported on quantum chemical calculations
indicating that the oxidative addition reaction of H2 to the
Ge–Ge bond of the hypothetical hydride HGeGeH (Dϱh
symmetric isomer) to give H2GeGeH2 (C2h symmetry) is
Introduction
Guanidines and guanidinates are well established as ver-
satile ligands or substituents in transition metal complexes
as well as main-group and lanthanide element com-
pounds.[1–8] A recent highlight in the area of main-group
element chemistry certainly represents the first synthesis of
a stable dimeric MgI compound featuring sterically crowded
guanidinate substituents and a direct Mg–Mg bond.[9] Jones
et al. also developed the synthesis and reactivity of mono-
meric aminidate and guanidinate GaI complexes which can
be described as carbene analogues (see Scheme 1).[10]
significantly exothermic at standard conditions (∆H0
=
–235 kJmol–1, MP2 calculations).[15] A few years later
Power et al. showed that digermynes and distannynes stabi-
lized with bulky aryl substituents indeed add H2 already at
room temp. and 1 bar.[16] Our efforts in the following years
concentrated on the synthesis of new binuclear group 13
element (E) hydrides, in which the two elements are con-
fined together in a distance which allows E–E bonding
interactions with the aid of bridging substituents (see
Scheme 2). Bicyclic guanidinates were found to be ideally
suited for this purpose. We first studied boron hydrides,
which are less reactive than gallium hydrides due to the
stronger B–H bonding, and started our investigations with
the adduct hppH·BH3 (hppH = 1,3,4,6,7,8-hexahydro-2H-
Scheme 1.
Hydrides of the heavier group 13 elements were studied
intensively in the past.[11,12] They are not only of academic
interest, but also applied for instance in CVD processes to
form metal films or 13–15 materials such as GaN (cubic
or hexagonal phases).[11d,13] Our motivation to synthesize
binuclear gallium hydrides by means of bridging guanidin-
[a] Anorganisch-Chemisches Institut,
Ruprecht-Karls-Universität Heidelberg,
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Fax: +49-6221-545707
E-mail: hans-jorg.himmel@aci.uni-heidelberg.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000598.
Scheme 2. H2 elimination and addition reactions with binuclear
group 13 element (E) compounds featuring bridging substituents.
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Novel Bi- and Trinuclear Gallium Halides and Hydrides
pyrimido[1,2-a]pyrimidine, see Scheme 3).[17] Dehydrogena- GaIII hydrides or start directly with subvalent Ga com-
tion of this adduct can be achieved elegantly catalytically pounds. As mentioned above, bicyclic guanidinate ligands
in the presence of [Rh] at 80 °C in toluene solutions.[18]
were shown previously by us to yield binuclear GaIII hy-
drides amenable to dehydrogenation to give binuclear GaII
compounds.[23] In continuation of this work, Ga hydrides
were reacted herein with bicyclic guanidine ligands. On the
other hand, mononuclear compounds are generally the
products of reactions between acyclic guanidinate ligands
and GaIII compounds.[9,26,27] To introduce a Ga–Ga bond,
we therefore started directly with subvalent Ga compounds
(namely “GaI” solutions) in the case of acyclic guanidin-
ates.
Results and Discussion
Reactions with Acyclic Guanidinates
Scheme 3. Lewis structures of the guanidines used in this work for
the synthesis of new gallium compounds.
For the synthesis of acyclic guanidinates we started with
“GaI” solutions prepared as described in the literature from
The product [H2B(µ-hpp)]2 was fully characterized. Fur- Ga metal and I2.[28] The exact composition and degree of
ther dehydrogenation occurs at 110 °C and leads to the association of these solutions is still unclear. They can be
doubly base-stabilized diborane(4) species [HB(µ-hpp)]2 ex- regarded as sources of GaI and GaII, the latter being
hibiting a roof-type structure. We also synthesized the di- formed by partial disproportionation of GaI. The two
2+
cation [(HNMe2)B(µ-hpp)]2 with a direct B–B bond.[19,20] acyclic guanidines (iPr2N)C(NiPr)(NHiPr) and (iPr2N)C-
Protonation of [HB(µ-hpp)]2 was shown to give the mono- (NCy)(NHCy), see Scheme 3, were deprotonated with
cation [B2H3(µ-hpp)2]+.[21] The research was extended to nBuLi and generally used without analysis. One of these
other bicyclic guanidines (see Scheme 3), namely the two Li guanidinate salts, however, crystallized from the solution
molecules htbo (1,4,6-triazabicyclo[3.3.0]oct-4-ene) and together with two equivalents of diethyl ether, namely
htbn (1,5,7-triazabicyclo[4.3.0]non-6-ene).[22] We are cur- [(iPr)2NC(NCy)2LiOEt2]2.
rently studying catalytic H2 addition to the B–B bond of
The structure of this dimeric assembly is visualized in
[HB(µ-guanidinate)]2 species to regain [H2B(µ-guanidinate)]2, Figure 1 (a). Each Li+ is fourfold coordinated, establishing
and also other oxidative addition reactions to the B–B one short [Li1–N1 198.7(3) pm] and two longer [221.4(3)
bond.
pm] bonds to the guanidinate N atoms, and in addition a
In the case of gallium chemistry, the adduct hppH·GaH3 bond to an ether molecule [with Li–O bond lengths of
cannot be isolated, because it already eliminates dihydrogen 196.1(3) pm]. The structure is thus similar to that of other
at low temperatures (–78 °C).[23] The product, [H2Ga(µ- known dimeric Li guanidinates shown in Scheme 4.[29] Un-
hpp)]2, 1, turned out to be also an extremely temperature- fortunately we were not able to isolate a product of the reac-
sensitive compound, which is only stable at low tempera- tion between [(iPr)2NC(NCy)2LiOEt2]2 and the Ga species
tures (Յ 0 °C) and represents an oily liquid at 0 °C. We present in the “GaI” solution.
were able to crystallize and structurally characterize the
The only species that was isolated in the form of a small
more stable derivative [HClGa(µ-hpp)]2, which turned out number of crystals from the reaction mixture turned out to
to melt at 10 °C and to decompose at 22 °C. Its decomposi- be (GaI2)2(µ-O){CyN=C(NHCy)(NiPr2)}2, being either a
tion leads presumably to elimination of H2 and formation product of partial hydrolysis or of reaction involving traces
of [ClGa(µ-hpp)]2. The dehydrogenation pathways for such of neutral guanidine and O2 (which oxidizes the GaI). Fig-
guanidine gallanes were additionally analysed by quantum ure 1 (b) displays its molecular structure. Hydrogen bond-
chemical calculations.[24] We also extended our work to the ing between the guanidine N–H groups and the bridging O
gallane adduct of the acyclic guanidine (Me2N)2C=NH and atoms leads to formation of two six-membered heterocycles
analysed its decomposition reactions.[25] The adduct sharing an O edge.
(Me2N)2C=N(H)·GaH3 turned out to decompose at rela-
On the other hand, the reaction between the Ga species
tively low temperatures (330–350 K). One of the decompo- present in the “GaI” solution and the Li salt of the guanid-
sition products was crystallized and shown to be the Ga7 inate (iPr2N)C(NiPr)2 resulted in formation of the new bi-
cluster HN{[HGaNMe][H2GaNC(NMe2)2]}3GaH with an nuclear GaII compound [(iPr2N)C(NiPr)2GaI]2, 2 (see Fig-
unusual cage-type assembly.
ure 2). The Ga–Ga bond in 2 measures 239.52(9) pm. It can
Herein we now report on the synthesis of new molecular be directly compared to the Ga–Ga bond lengths in the two
Ga compounds with bicyclic and acyclic guanidinate sub- binuclear GaII molecules {GaI(Fiso)}2 and {GaI(Piso)}2,
stituents. Possible routes to molecular, binuclear GaII com- with the amidinate substituents RC(NAr)2 (Ar = 2,6-
pounds featuring bridging guanidinate substituents and a iPr2C6H3 and R = H in the case of Fiso and R = tBu in the
direct Ga–Ga bond involve dehydrogenation of binuclear case of Piso).[27] For these two molecules longer distances of
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Figure 2. Molecular structure of [(iPr2N)C(NiPr)2GaI]2, 2. Vi-
brational ellipsoids are drawn at the 50% probability level. Selected
structural parameters (bond lengths in pm, bond angles in deg.):
Ga1···Ga1Ј 2.3952(9), Ga1–I1 2.5667(6), Ga1–N1 1.970(3), Ga1–
N2 1.965(3), N1–C1 1.339(4), N2–C1 1.331(4), N3–C1 1.408(4),
I1–Ga1–Ga1Ј 114.46(2), N1–Ga1–Ga1Ј 122.52(9), N2–Ga1–Ga1Ј
123.72(9), N1–Ga1–N2 67.41(12), N1–C1–N2 109.7(3).
Reactions with Bicyclic Guanidines
Next we discuss the reactions between Me3N·GaH3 and
the bicyclic guanidines hppH, htbn and htbo (see
Scheme 3). We started from rigorously purified samples of
Me3N·GaH3 (by sublimation)[35] to avoid the presence of
halide traces in the reaction mixture. htbn and htbo were
prepared according to the literature.[36]
Figure 1. a) Molecular structure of [(iPr)2NC(NCy)2LiOEt2]2. Vi-
brational ellipsoids are drawn at the 50% probability level. Selected
structural parameters (bond lengths in pm, bond angles in deg.):
Li1–N1 198.7(3), Li1–N2 221.4(3), Li1Ј–N2 206.2(2), Li1–O1
196.1(3), N1–C13 132.23(16), N2–C13 134.59(17), N3–C13
143.86(16), N1–Li1–N2 65.40(8), N2–Li1–O1 122.07(12); b) Mo-
lecular structure of Ga2I4O{CyN=C(NHCy)(NiPr2)}2 isolated in
small amount. Vibrational ellipsoids are drawn at the 50% prob-
ability level. Selected structural parameters (bond lengths in pm,
bond angles in deg.): Ga–I1 255.13(7), Ga–I2 256.97(8), Ga–N1
195.0(3), Ga–O 173.2(5), N1–C1 134.4(5), N2–C1 133.9(5), N3–C1
137.0(5), N2···O 282.1(3), I1–Ga–I2 106.36(3), N1–Ga–O
108.70(10), N1–C1–N2 117.6(3).
hppH
As already mentioned, we reported previously on the re-
action between Me3N·GaH3 and hppH.[23] The likely prod-
uct, [H2Ga(µ-hpp)]2 is an extremely unstable compound
that is difficult to characterize. However, we were able to
isolate a first and much more stable product of its hydroly-
sis. In these experiments water was deliberately added to the
243.04(10) and 245.21(15) pm, respectively, were measured. reaction mixture, the molar ratio Me3N·GaH3/H2O being
It was shown previously that the Ga–Ga single bond can 3:1 (see Scheme 5). From the reaction mixture a small
adopt a large range of values. Examples include the two quantity of a new product was obtained in crystalline form,
compounds Ga2[CH(SiMe3)2]4 with a Ga–Ga bond length which can be identified as the trinuclear Ga hydride
of 254.1(1) pm,[30] and Li2[Ga2Cl6] with a Ga–Ga bond Ga3H5(µ3-O)(µ-hpp)2 (3).
length of 238.7(5) pm.[31] In the Ga2 dimer, quantum chemi-
In contrast to 1, compound 3 is stable at room temp. Its
cal (MP2) calculations found a Ga–Ga distance of 263 pm molecular structure in the crystalline phase is illustrated in
in the 3Πu state,[32] which was verified as the electronic Figure 3. A central oxygen atom is connected by three Ga
ground state with the aid of matrix isolation spec- atoms arranged in the form of a triangle. The two hpp units
troscopy.[33,34]
are located near two different edges of the Ga3 triangle,
Scheme 4. Some structurally characterized Li guanidinates.
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Novel Bi- and Trinuclear Gallium Halides and Hydrides
Scheme 5. Comparison between the reactions of hppH and 1,8-bis(trimethylstannyl)naphthalene with GaIII compounds in the absence
and presence of H2O. (i) Me3N•GaH3, Et2O, –78°C; (ii) H2O, Me3N•GaH3, Et2O, r.t. (iii) GaCl3, toluene, –25°C (21% yield); (iv) H2O,
GaCl3, toluene, –25°C (16% yield).
adopting as expected a bridging bonding mode. One of the
gallium atoms, Ga1, is thus bonded to both hpp-units. The
five hydrogen atoms are end-on bonded to the Ga atoms,
resulting in one GaH and two GaH2 units. Compound 3
can be regarded as product of the reaction between 1 and
“H2GaOH” under dihydrogen elimination. It was shown
previously that macrocycles containing several Lewis acidic
centers can be prepared by hydrolysis of molecular group
13 element compounds. The manifold examples include the
reaction between GaCl3 and 1,8-bis(trimethylstannyl)naph-
thalene, which gave a binuclear GaIII compound under ex-
clusion of water, and a trigallacycle with a µ3-bonded O
atom in the center (see Scheme 5), upon addition of H2O
(the yield being, like in our reaction, quite low).[37] Like the
bis(trimethylstannyl)naphthalene ligand, the guanidinate
hpp is capable of bridging two Ga atoms. In the field
of hydride chemistry, Rettig, Storr and Trotter obtained
a
polycyclic Ga, O,
N
cage compound [(GaH)6-
(GaH2)2(µ3-O)2(µ3-NCH2CH2NMe2)4(µ-NHCH2CH2-
NMe2)2] containing µ3-bonded oxygen atoms and six GaH
and two GaH2 units by reaction between N,N-dimethyleth-
ylenediamine, Me2NCH2CH2NH2 and trimethylamine gal-
lane, Me3N·GaH3.[38] However, this hydride turned out to
be extremely air-sensitive so that the authors could only
report on the X-ray diffraction results. No yield or other
forms of characterization were given, a deficiency that this
compound unfortunately shares with ours. However, we will
see in the following that a similar compound can be ob-
tained reproducibly in larger amounts in the case of the 5–
6 bicyclic guanidine htbn.
Figure 3. a) Molecular structure of the new trinuclear Ga hydride
3. Vibrational ellipsoids are drawn at the 50% probability level.
Selected structural parameters (bond lengths in pm, bond angles
in deg.): Ga1–N1 193.3(4), Ga1–N4 192.3(4), Ga1–O1 185.8(3),
Ga2–N2 197.2(4), Ga2–O1 189.5(3), Ga3–N5 196.4(4), Ga3–O1
188.8(3), N1–C1 133.8(6), N2–C1 135.5(5), N4–C8 134.9(6), N5–
C8 133.7(6), N1–Ga1–N4 107.28(15), O1–Ga1–N1 101.59(15), O1–
Ga1–N4 108.64(15), O1–Ga2–N2 103.11(14), O1–Ga3–N5
102.66(15); b) Molecular structure of the new trinuclear Ga hydride
4. Vibrational ellipsoids are drawn at the 50% probability level.
Selected structural parameters (bond lengths in pm, bond angles
in deg.): Ga1–N1 1.944(4), Ga1–N4 1.927(4), Ga1–O1 1.846(3),
Ga2–N2 1.951(4), Ga2–O1 1.883(3), Ga3–N5 1.962(4), Ga3–O1
1.887(3), N1–C1 1.340(6), N2–C1 1.324(6), N4–C7 1.331(6), N5–
C7 1.311(6), N1–Ga1–N4 107.29(16), O1–Ga1–N1 104.20(15), O1–
Ga1–N4 103.97(14), O1–Ga2–N2 100.67(15), O1–Ga3–N5
102.39(15).
htbn
Similar to the situation with http, the hydride [H2Ga(µ-
tbn)]2 formed as the product of the reaction between
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D. Rudolf, E. Kaifer, H.-J. Himmel
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Me3N·GaH3 and htbn turned out to be extremely tempera- molar ratio. As anticipated, the molecular structure re-
ture-sensitive, so that we were not able to characterize this sembles that of 3. Figure 5 compares the experimental IR
species. However, when Me3N·GaH3 was treated with htbn spectrum with a simulation based on quantum chemical
in the presence of deliberately added H2O (stoichiometric (DFT) calculations. It can be seen that the general level of
ratio htbn:H2O = 2:1), crystals of a new, more stable com- agreement is excellent. The ν(CN) mode gives rise to the
pound were obtained. Figure 4 (a) shows its IR and Raman strongest band in the spectrum, located at 1584 cm–1 in the
spectra. Intense bands were observed in the IR spectrum at experimental and 1636 cm–1 in the calculated spectrum. The
1889/1848 cm–1, in a region characteristic for Ga–H stretch- IR band of this mode is very intense, but the corresponding
ing modes, ν(Ga–H), of GaIII species. This value compares Raman signal extremely weak.
to e. g. 1906 cm–1 for [H2GaOtBu]2 [39] and 1918 cm–1 for
[H2GaOC(H)tBu2]2.[40] The Raman spectrum also features
two intense maxima due to the ν(Ga–H) modes, differing
not in their energies, but in their relative intensities from
1
the IR bands. The H NMR spectrum measured at 233 K
(see Figure 4, b) contains a resonance due to the Ga–H pro-
tons at 5.4 ppm. Thus there can be no doubt that the new
product represents a Ga hydride; finally, the XRD analysis
showed the product to be Ga3H5(µ3-O)(µ-tbn)2, 4, the tbn
analogue of 3. The crystal structure of 4 is displayed in
Figure 3 (b). Two of the three possible isomers crystallize
together (only one of which is shown in Figure 3, b). Due
to the relatively small difference regarding the shape and
physical properties between these isomers, they are statistic-
ally distributed over all lattice sites of the crystal in an equi-
Figure 5. Comparison between experimental and calculated
(B3LYP/6-311++G(d,p)) IR spectrum of 4.
htbo
Finally we studied the reaction between Me3N·
GaH3 and htbo. In this case, a new product can be isolated
also in the absence of H2O. However, low temperatures
(–15 °C) are required, since this product, 5, already decom-
poses slowly in solution at room temp. Its IR and Raman
spectra are displayed in Figure 6 (a). Both spectra contain
a pair of intense absorptions at ca. 1911/1857 cm–1, a region
typical for ν(Ga–H) modes in GaIII hydrides. For compari-
son, an intense Raman signal at 1884 cm–1 due to ν(Ga–H)
was observed for [HClGa(µ-hpp)]2.[19] For trimeric methyl-
amidogallane, [H2GaNHMe]3, and dimeric tert-butyl-
amidogallane, [H2GaNHtBu]2, the Raman spectra gave evi-
dence for Ga–H stretching modes at 1897 and 1904/
1873 cm–1, respectively.[41]
In the case of [H2GaNMe2]2,[42] the IR spectra recorded
for the vapour phase at 290 K and for the solid at 77 K
displayed bands due to the Ga–H stretching modes at 1911/
1907/1901/1870 and 1885 cm–1, respectively. The experimen-
tal spectrum is in excellent agreement with the simulated
spectrum for the hydride [H2Ga(µ-tbo)]2 on the basis of
quantum chemical calculations (see part b of Figure 6). Ra-
man spectra of solid [H2GaNMe2]2 at 77 K and of C6H6
solutions displayed signals at 1890/1875 and 1888 cm–1,
respectively. In the 1H NMR spectrum of 5 recorded at
233 K in perdeuterated toluene (see Figure 4, b), a relatively
broad signal at 5.4 ppm can be assigned to the four protons
attached to the gallium atoms, finally, we were able to grow
Figure 4. a) Comparison between the IR (CsI disc) and Raman
crystals of 5. From the XRD analysis in combination with
spectrum (solid material, excited at 514 nm) for compound 4; b)
Comparison between the 1H NMR spectra (200 MHz) for 4 in deu-
terated dichloromethane and 5 in deuterated toluene solutions.
all other data, 5 can be identified unambiguously as the
new binuclear gallium hydride [H2Ga(µ-tbo)]2. Its crystal
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Novel Bi- and Trinuclear Gallium Halides and Hydrides
Figure 7. Molecular structure of [(µ-tbo)2(GaH2)2] (5). Vibrational
ellipsoids are drawn at the 50% probability level. Selected struc-
tural parameters (bond lengths in pm, bond angles in deg.): Ga1–
N1 1.872(2), Ga1–N4 1.949(2), Ga2–N2 1.897(2), Ga2–N5
1.9270(19), Ga3–N7 1.925(2), Ga3–N10 1.922(2), Ga4–N8
1.955(2), Ga4–N11 1.875(2), N1–C1 1.301(3), N2–C1 1.328(3),
N3–C1 1.331(3), N4–C6 1.322(3), N5–C6 1.313(3), N6–C6
1.375(3), N7–C11 1.315(3), N8–C11 1.320(3), N9–C11 1.373(3),
N10–C16 1.327(3), N11–C16 1.296(3), N12–C16 1.340(3),
Ga1···Ga2 373.9(2), Ga3···Ga4 368.9(3), N1–Ga1–N4 105.18(9),
N2–Ga2–N5 103.04(9), N7–Ga3–N10 102.50(9), N8–Ga4–N11
103.55(9), N1–C1–N2 134.7(2), N4–C6–N5 133.0(2), N7–C11–N8
133.0(2), N10–C16–N11 134.1(2).
Figure 6. a) Comparison between the IR (CsI disc) and Raman
spectrum (solid material, excited at 514 nm) for compound 5Ͻ; b)
Comparison between experimental and calculated (B3LYP/6-
311++G(d,p)) IR spectrum of 5.
Scheme 6. Structure of the (pyrazol-1-yl)gallane dimer.
structure is depicted in Figure 7. The unit cell contains two
slightly different molecules, which both adopt a boat-type
conformation, in difference to the chair-type conformation
found in the case of [HClGa(µ-hpp)]2.[23] This change from
chair- to boat-conformation is not unprecedented in this
Quantum Chemical Calculations
Density-functional theory (DFT) calculations were car-
field. Hence we also found a change from chair- to boat- ried out to obtain more information about the structures
type structure for the pair of molecules [H2B(µ-hpp)]2/ and especially the reaction thermodynamics for intra-
[H2B(µ-tbo)]2.[18,22] The two Ga atoms in 5 are separated molecular dehydrogenation starting with the three hydrides
by 373.9(2)/368.9(3) pm. The Ga–N bond lengths cover the [H2Ga(µ-hpp)]2, [H2Ga(µ-tbn)]2 and [H2Ga(µ-tbo)]2.
region 187.2(2) (Ga1–N1)–195.5(2) (Ga4–N8) pm. Values
The products, the doubly base-stabilized digallane(4)
in the range 102.50(9)–105.18(9)° were found for the N– species [HGa(µ-hpp)]2, [HGa(µ-tbn)]2 and [HGa(µ-tbo)]2,
Ga–N bonding angles. The structure of 5 can be compared feature Ga–Ga bond lengths of 237.9, 242.6 and 248.3 pm,
with that of the (pyrazol-1-yl)gallane dimer [H2Ga(µ-pz)]2, respectively. Dehydrogenation of [H2Ga(µ-tbo)]2 turned out
see Scheme 6,[43] which also adopts a boat-type conforma- to be very mildly exergonic (∆G0 = –5 kJmol–1 at 298 K
tion (like their boron counterparts [44]). This molecule fea- and 1 bar). This is in sharp contrast to the significantly end-
tures slightly longer Ga–N distances of 197.1(6)/197.7(8) ergonic dehydrogenation reaction of the corresponding bo-
pm, and smaller N–Ga–N bond angles of 96.5(0.5)/ ron compound, which is associated with a ∆G0 value of
97.7(0.4)° compared with 5. In solution we obtained clean +115 kJmol–1 (see Scheme 7). The boron hydride [H2B(µ-
1H NMR spectra of 5 only at low temperatures (Յ –20 °C), tbo)]2 indeed is a stable molecule showing no signs of ther-
and observed decomposition with gas evolution at room mal dehydrogenation or other decomposition processes
temp. On the other hand, the solid material turned out to even in boiling toluene or p-xylene solutions.[22] In the case
be stable even at 80 °C. Unfortunately we were not able to of [H2Ga(µ-tbo)]2, the ∆G0 value lies well within the
characterize the decomposition products. However, similar ideal region for reversibility under mild conditions
to [HClGa(µ-hpp)]2,[23] dehydrogenation is likely to occur (Ϯ30 kJmol–1). Therefore it will be the main issue of future
at the first place, leading to [HGa(µ-tbo)]2.
research in this field to find catalysts for the quantitative
Eur. J. Inorg. Chem. 2010, 4952–4961
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4957

D. Rudolf, E. Kaifer, H.-J. Himmel
FULL PAPER
and clean dehydrogenation of [H2Ga(µ-tbo)]2 to give
[HGa(µ-tbo)]2, and then for the hydrogenation of [HGa(µ-
tbo)]2 to regain [H2Ga(µ-tbo)]2. We showed already in the
case of the boron hydride [H2B(µ-hpp)]2 that dehydrogena-
tion can be effectively driven catalytically.[21] Like in the
case of the analogue boron compounds, a correlation exists
between the ∆G0 value for dehydrogenation and the N–C–N
bite angle of the guanidinate in the [H2Ga(µ-guanidinate)]2
species (see Figure 8, a). Thus the dehydrogenation reac-
tions of the hydrides with hpp or tbn bridges between the
Ga atoms are more exergonic (–40 and –35 kJmol–1, respec-
tively). For all guanidinate bridges, the B compounds exhi-
bit a ∆G0 value larger than their Ga counterparts. As seen
from the plot in Figure 8 (b), the difference between the
∆G0 values is small in the case of hpp bridges (10 kJmol–1),
becoming larger for tbo bridges (133 kJmol–1). This trend
reflects the greater flexibility of the Ga–Ga bonds (involv-
ing more diffuse orbitals). Whereas optimal prerequisites
(comparing the three guanidinates hpp, tbn and tbo) for
reversibility are reached for hpp bridges in the case of B
compounds, they are reached for tbo bridges in the case of
Ga compounds.
Figure 8. a) Calculated ∆G value (at 298 K, 1 bar) for dehydrogena-
tion of [H2Ga(µ-guanidinate)]2 to give [HGa(µ-guanidinate)]2 as a
function of the bite angle N–C–N in the bicyclic guanidinate sub-
stituent. Guanidinate = hpp, tbn or tbo; b) Comparison between
hydrogenation of [HE(µ-guanidinate)]2 for E = B and E = Ga.
The first synthesis of a stable hydride [H2Ga(µ-guanidin-
ate)]2 featuring as guanidinates bridging tbo substituents
can be used as basis for future research activities. This hy-
dride slowly decomposes in solution already at room temp.,
but can be stored in solid form at room temp. without signs
of decomposition. Dehydrogenation in the solid state oc-
curs at 80 °C, presumably leading to the doubly base-
stabilized digallane(4) [HGa(µ-tbo)]2. According to quan-
tum chemical calculations this dehydrogenation reaction is
Scheme 7. Comparison between dehydrogenation reactions of
[H2Ga(µ-tbo)]2 and [H2B(µ-tbo)]2.
Conclusions
mildly exergonic at standard conditions (∆G0
=
The results presented in this work show, in agreement –5 kJmol–1). Future research in this field will focus on the
with previous reports on group 13 guanidinate compounds, catalytic dehydrogenation at low temperatures in solution.
that acyclic guanidinates adopt an end-on, κ2-type bonding We showed already for similar boron hydrides that dehydro-
mode, while bicyclic guanidinates prefer bridging bonding genation can be effectively catalysed by Rh complexes
modes. The bonding mode can be explained easily by the (representing pre-catalysts in a presumably heterogeneous
orientation of the frontier orbitals at the guanidinate nitro- catalysis).[18,22]
gen atoms (see also the comprehensive discussion by Col-
Another compound, synthesized herein, which will be
es[2b]). Hence the two nitrogen donor orbitals are generally used in future synthetic work, is the trinuclear Ga hydride
oriented towards the “mouth” of the ligand in the case of Ga3H5(µ3-O)(µ-tbn)2 featuring a central O atom bound to
acyclic guanidinates (ideal for a chelating bonding mode), three GaIII atoms in a trigonal planar fashion. Future re-
but almost coplanar in the bicyclic guanidinate hpp– (sup- search will focus on the design of new trigallacycles. The
porting a bridging bonding mode). For the purposes out- first step in this direction is the synthesis of the mono-
lined in the Introduction of this work, bicyclic guanidinates cationic species [{GaH(µ-tbn)}3(µ3-O)]+ (see Scheme 8) by
are therefore more attractive.
reaction of Ga3H5(µ3-O)(µ-tbn)2 with [H2tbn]+ salts.
4958
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 4952–4961

Novel Bi- and Trinuclear Gallium Halides and Hydrides
15683, indep. 8134, Rint = 0.0317, final R indices [IϾ2σ(I)]: R1 =
0.0385, wR2 = 0.1028.
[(iPr2N)C(NiPr)2GaI]2 (2): To a mixture of 322 mg of elemental
gallium (4.8 mmol) and 604 mg of iodine (4.8 mmol), 10 mL of tol-
uene were added. Ultrasonic treatment for 2 h yielded the “GaI”
solution. Then the guanidinate was added to this solution by can-
nula at –78 °C. Subsequently the mixture was stirred and slowly
warmed to room temp. After 24 h it was filtered with a cannula
and the solvent was concentrated to ca. 5 mL. Storage at –30 °C
afforded colourless crystals suitable for X-ray diffraction; yield
412 mg (0.48 mmol, 20%). The material turned out to be highly
air- and moisture-sensitive and no elemental analysis was carried
Scheme 8. Possible structure of the trigallacycle [{GaH(µ-
tbn)}3(µ3-O)]+.
1
out. H NMR (400 MHz, C6D6, 296 K): δ = 3.7 (sept, J = 6.4 Hz,
2 H), 3.23 (sept, J = 6.4 Hz, 2 H), 1.34 (dd, J = 6.4 Hz, 12 H), 1.04
(d, J = 6.4 Hz, 12 H) ppm. 13C{1H} NMR (100.55 MHz, C6D6,
296 K): δ = 167.38 (CN3), 49.34 (CH), 46.71 (CH), 27.07 (CH3),
25.72 (CH3), 23.22 (CH3) ppm. Crystal data for C26H56Ga2I2N6:
Mr = 846.01, 0.50ϫ0.30ϫ0.20 mm3, monoclinic, space group
P21/n, a = 13.981(3), b = 9.3060(19), c = 14.184(3) Å, β =
103.73(3)°, V = 1792.7(7) Å3, Z = 2, dcalc = 1.567 Mgm–3, Mo-Kα
radiation (graphite-monochromated, λ = 0.71073 Å), T = 100 K,
Experimental Section
General: All manipulations were carried out under dry Ar atmo-
sphere using standard Schlenk techniques. The bicyclic guanidines
htbo and htbn were prepared as described in the literature.[36]
NMR spectra were measured with a Bruker Avance DPX AC200
or Bruker AVII 400 spectrometer. IR spectra were recorded with a
Biorad Excalibur FTS 3000 spectrometer. A Jobin–Yvon T64000
Raman spectrometer was used for Raman measurements. The spec-
tra were excited with the 514 nm line of an Ar+ ion laser. Elemental
analysis was carried out at the Microanalytical Laboratory of the
University of Heidelberg for compounds 4 and 5. Compound 2
turned out to be too unstable, and compound 3 was obtained in
small amount as crystalline material suitable for XRD, but no fur-
ther analysis. EI mass spectra were obtained on a Trinnigan MAT
8230 or a JEOC JMS-700 instrument.
θrange 1.84 to 30.12°, reflections measd. 29045, indep. 5236, Rint
0.0622, final R indices [IϾ2σ(I)]: R1 = 0.0416, wR2 = 0.1032.
=
Ga3H5(µ3-O)(µ-hpp)2 (3): A solution of 8 µL (0.48 mmol) deionised
and degassed water in 5 mL of THF was added to a solution of
190 mg of Me3N·GaH3 (1.44 mmol) in 5 mL of toluene at –78 °C.
The reaction mixture was stirred over 6.5 h. Meanwhile it was
warmed to 0 °C. 135 mg of hppH (0.97 mmol) were dissolved in
5 mL of toluene, cooled to –78 °C and added by cannula. After
several weeks at –30 °C a small amount of colourless crystals
suitable for X-ray diffraction, but no further analysis, were ob-
[(iPr)2NC(NCy)2LiOEt2]2: 6.3 mL of nBuLi (10 mmol) was added
dropwise to a solution of 1.4 mL of diisopropylamine (10 mmol) in
20 mL of Et2O at 0 °C. The solution was warmed to room temp.
and stirred for 1 h. Then it was cooled to 0 °C and 2.06 g of N,NЈ-
dicyclohexylcarbodiimide (10 mmol) were added. The solution was
warmed to room temp. and stirred for 2 h. Subsequently it was
filtered through a pad of Celite and concentrated to ca. 5 mL and
used directly without further analysis in subsequent reactions.
Crystals were obtained at –30 °C; yield 940 mg (1.2 mmol,
tained. Crystal data for C14H29Ga3N6O: Mr
=
506.59,
0.15ϫ0.10ϫ0.10 mm3, monoclinic, space group P21/n,
a
=
15.478(3), b = 9.3320(19), c = 15.671(3) Å, β = 118.08(3)°, V =
1997.1(7) Å3, Z = 4, dcalc = 1.685 Mgm–3, Mo-Kα radiation (graph-
ite-monochromated, λ = 0.71073 Å), T = 100 K, θrange 1.52 to
27.46°, reflections measd. 9108, indep. 4566, Rint = 0.0728, final R
indices [IϾ2σ(I)]: R1 = 0.0472, wR2 = 0.1059.
Ga3H5(µ3-O)(µ-tbn)2 (4): 10 mL of dry toluene were treated with a
mixture of 9 µL (0.5 mmol) degassed dist. water in 2 mL of THF.
Then the solution was added to 128 mg of solid htbn (1 mmol)
at –80 °C. After further addition of 202 mg of solid Me3N·GaH3
(2.5 mmol) at –80 °C, the solution was stirred for a period of 5.5 h.
During this time the temperature was kept below –20 °C. Then the
solution was overlayed with 5 mL of pre-cooled n-hexane. Colour-
less crystals suitable for X-ray diffraction were grown at –18 °C;
yield 71 mg (0.15 mmol, 30%). C12H25Ga3N6O (478.54): calcd. C
32%). Crystal data for C46H92Li2N6O2: Mr
=
775.14,
0.30ϫ0.20ϫ0.20 mm3, monoclinic, space group P21/n,
a
=
12.884(3), b = 14.329(3), c = 13.203(3) Å, β = 91.63(3)°, V =
2436.5(8) Å3, Z = 2, dcalc = 1.057 Mgm–3, Mo-Kα radiation (graph-
ite-monochromated, λ = 0.71073 Å), T = 100 K, θrange 2.10 to
30.05°, reflections measd. 13774, indep. 7122, Rint = 0.0504, final
R indices [IϾ2σ(I)]: R1 = 0.0550, wR2 = 0.1485.
Ga2I4(µ-O){CyN=C(NHCy)(NiPr2)}2: First the “GaI” solution was
prepared: to a mixture of 456 mg of elemental gallium (6.5 mmol)
and 829 mg of iodine (6.5 mmol), 10 mL of toluene were added,
and the mixture ultrasonically treated for 2 h. The guanidinate
[(iPr)2NC(NCy)2LiOEt2]2 prepared as described above was dis-
solved in 5 mL of toluene (using ultrasonic treatment) and added
to the “GaI” solution by cannula at room temp. Subsequently the
reaction mixture was stirred at 60 °C for a period of 2 h. Then it
was cooled to room temp. and stirred overnight. The mixture was
filtered through a pad of celite and concentrated to ca. 5 mL. Stor-
age at –30 °C afforded a small amount of colourless crystals suit-
able for X-ray diffraction, but no further analysis was possible.
1
30.12, H 5.27, N 17.56; found C 30.55, H 5.33, N 17.33. H NMR
(200 MHz, C6D6, 303 K): δ = 5.7 (br. s, 4 H), 3.15 (m, 8 H), 2.66
(m, 4 H), 2.48 (m, 4 H), 1.35 (m, 4 H) ppm. IR (KBr): ν = 2934
˜
(m), 2858 (m), 1889 (s), 1848 (s), 1584 (s), 728 (s) cm–1. MS (EI)
m/z = 477 [M – H]+, 407.1 [M – GaH2]+, 391.1 [M – GaH2 – O]+,
196.1 [H3Ga(tbn)]+, 124.2[tbn]+, 68.9 [Ga]+. Crystal data for
C12H25Ga3N6O: Mr = 478.54, 0.30ϫ0.30ϫ0.26 mm3, monoclinic,
space group P21/c, a = 8.7250(17), b = 26.939(5), c = 8.7600(18)
Å, β = 119.22(3)°, V = 1797.0(6) Å3, Z = 4, dcalc = 1.769 Mgm–3,
Mo-Kα radiation (graphite-monochromated, λ = 0.71073 Å), T =
100 K, θrange 1.51 to 27.51°, reflections measd. 8235, indep. 4122,
Rint = 0.0273, final R indices [IϾ2σ(I)]: R1 = 0.0427, wR2 = 0.1190.
Crystal
data
for
C38H74Ga2I4N6O:
Mr
=
1278.08,
0.25ϫ0.20ϫ0.20 mm3, orthorhombic, space group Ccca, a =
21.368(4), b = 25.341(5), c = 20.490(4) Å, V = 11095(4) Å3, Z = 8,
dcalcd. = 1.530 Mgm–3, Mo-Kα radiation (graphite-monochromated,
[H2Ga(µ-tbo)]2 (5): A solution of 267 mg of Me3N·GaH3 (2 mmol)
in 10 mL of Et2O was cooled to –78 °C and added to a suspension
λ = 0.71073 Å), T = 100 K, θrange 1.59 to 30.05°, reflections measd. of 223 mg of htbo (2 mmol) in 10 mL of Et2O at –78 °C via can-
Eur. J. Inorg. Chem. 2010, 4952–4961
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4959

D. Rudolf, E. Kaifer, H.-J. Himmel
FULL PAPER
Schultz, X. Wang, Q. Zhao, Inorg. Chem. 2007, 46, 1718–1726.
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(0.5 mmol, 51%). C10H20Ga2N6 (363.76): calcd. C 33.0, H 5.5, N
23.1; found C 34.3, H 5.9, N 23.4. 1H NMR (200 MHz, C7D8,
233 K): δ = 5.42 (s, 4 H), 3.58 (t, J = 7.3 Hz, 8 H), 2.16 (t, J =
[4]
7.3 Hz, 8 H) ppm. IR (KBr): ν = 2941 (m), 2855 (m), 1906 (s),
˜
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363.76, 0.43ϫ0.40ϫ0.40 mm3, monoclinic, space group P21/c, a =
15.6340(11), b = 7.6490(15), c = 23.837(5) Å, β = 108.17(3)°, V =
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R indices [IϾ2σ(I)]: R1 = 0.0321, wR2 = 0.0811.
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CCDC-777676 (for [(iPr)2NC(NCy)2LiOEt2]2), -777677 (for
Ga2I4(µ-O){CyN=C(NHCy)(NiPr2)}2), -777675 (for 2), -734722
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Acknowledgments
The authors gratefully acknowledge continuous financial support
by the Deutsche Forschungsgemeinschaft (DFG)
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Received: May 28, 2010
Published Online: August 27, 2010
Eur. J. Inorg. Chem. 2010, 4952–4961
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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