Addition of hydrogen or ammonia to a low-valent group 13 metal species at 25°C and 1 atmosphere

Article abstract of DOI:10.1002/anie.200805982

(Chemical Equation Presented) Mild-mannered: The low-valent aryl gallium(I) species :GaAr′ (Ar′ = 2,6-(2,6-iPr₂C₆H ₃)₂C₆H₃) undergoes addition to H₂ or NH₃ at room tem

Full text of DOI:10.1002/anie.200805982

Angewandte
Chemie
DOI: 10.1002/anie.200805982
Gallium Chemistry (2)
Addition of Hydrogen or Ammonia to a Low-Valent Group 13 Metal
Species at 258C and 1 Atmosphere**
Zhongliang Zhu, Xinping Wang, Yang Peng, Hao Lei, James C. Fettinger, Eric Rivard, and
Philip P. Power*
In Group 13 metal chemistry, the direct reaction of metal
atoms with small molecules such as H2, CO, or NH3 has
enabled numerous simple derivatives with unusual bonding,
coordination, or reactivity to be trapped and studied at low
temperatures in frozen, inert matrixes.[1,2] The characteriza-
tion of these species by spectroscopic and computational
methods has provided much useful bonding and reactivity
information. For example, it was reported that the direct
reaction of H2 or NH3 with the metals under photolysis
affords, among other things, compounds such as MH, MH2,
MH3, M2H2, HMNH2, MNH2, or H2MNH2 (M = Al, Ga, or
In). The pathway for the formation of AlH3 involves the
sequence shown in Scheme 1,[5–7] in which H2 is added to
MNR2 (M = Ga[16] or Tl,[17] NR2 = N(SiMe3)(2,6-Mes2C6H3)
or N(Me)(2,6-Mes2C6H3), Mes = 2,4,6-Me3C6H2) exist as
monomeric species either in solution or the solid state at
room temperature (the alkyl species DGaC(SiMe3)3 is also
known to be monomeric in solution and in the vapor phase,
although it is tetrameric in the solid state[16]). The presence of
a one-coordinate metal center in these compounds suggests
that they could display a reactivity similar to that shown for
the last step in Scheme 1. Furthermore, if the reaction could
be carried out at room temperature, the greater than 200 K
temperature difference might allow it to proceed without the
necessity for photolysis. We describe herein the treatment of
solutions of the gallium(I) aryl species GaAr (Ar= Ar’ or 2,6-
(2,6-iPr2C6H3)2-4-(Me3Si)C6H2 (Ar’’)) with H2 and NH3 and
show that the reactions proceed readily at 258C and 1 atm
pressure to give the products {Ar’Ga(m-H)H}2 (1) or {Ar’’Ga-
(m-NH2)H}2 (5) in good yield.
[3]
[4]
Although experimental observations in solid hydrogen
matrix and corresponding theoretical calculations showed
that hydrogenation of the simple monovalent gallium(I)
species GaH to GaH3 is not spontaneous and requires a
considerable activation energy of 43.1 kcalmolÀ1,[8,18] the
reaction between (GaAr’)2 (3, monomeric in solution) and
H2 gas proceeded smoothly, and the toluene solution of 3
faded from a deep green color to light brown within 3 h.
Cooling of the resulting solution to 88C produced pale yellow
crystals of {Ar’Ga(m-H)H}2 (1).[19] Various attempts to syn-
thesize 1 through an alternative route involving the reaction
of an organogallium(III) halide Ar’GaX2 (X = Cl and I) with
hydride sources such as (iBu2AlH)2, NaH, LiBH4, and
LiBHEt3 resulted in mixtures of products, and 1H NMR
spectroscopy of the reaction mixtures indicated the absence
of the target compound 1. However, the reaction of the b-
diketiminate gallium halide LGaI2 (L = {N(C3H3-2,6-
iPr2)C(Me)}2CH) with LiH·BEt3 has been reported to give
monomeric LGaH2 in high yield.[20] Compound 1 (Figure 1)
crystallizes as a centrosymmetric dimer with gallium atoms
bound to both terminal and bridging hydrogen atoms, which
were located on an electron difference map. The Ga-H-Ga
and H-Ga-H angles are 93.4(8) and 86.6(8)8, respectively. A
close Ga···Ga separation of 2.5275(3) ꢀ is observed, mainly as
Scheme 1. Low-temperature reactions of atomic aluminum and hydro-
gen.[5–7]
À
monovalent DAl H in the final step. A similar mechanism was
also proposed for the photolysis of monovalent DGa H to
À
afford GaH3 in solid hydrogen.[8] To date, there are no
examples of addition reactions of H2 or NH3 with heavier
Group 13 metal compounds under ambient conditions. How-
ever, in the case of the lightest Group 13 element boron, it has
been shown that H2 reacts reversibly with a phosphonium
borate species (C6H2Me3)2PH(C6F4)BH(C6F5)2.[9] Further-
more, work in the neighboring Group 14 elements has
demonstrated that H2 reacts with digermynes,[10] distan-
nynes,[11] and stable carbenes[12] and that NH3 reacts with
stable carbenes[12] or a stannylene[13] under ambient condi-
tions. Recent experimental results have shown that heavier
Group 13 species MR (M = Ga, In, or Tl;[14,15] R = 2,6-(2,6-
iPr2C6H3)2C6H3 (Ar’) or 2,6-(2,4,6-iPr3C6H2)2C6H3 (Ar*)) or
[*] Z. L. Zhu, Dr. X. Wang, Y. Peng, H. Lei, Dr. J. C. Fettinger,
Dr. E. Rivard, Prof. P. P. Power
À
a result of the relative shortness of the Ga H bonds. The
Department of Chemistry, University of California
One Shields Avenue, Davis, CA 95616 (USA)
Fax: (+1)530-752-8995
À
bridging Ga H bond lengths are 1.69(2) and 1.78(2) ꢀ, and
À
the terminal Ga H bond length is 1.46(2) ꢀ. There is a
dihedral angle of 56.38 between the Ga2H2 plane and the
E-mail: pppower@ucdavis.edu
À
plane of the central ring of the Ar’ group. The terminal Ga H
bond length is similar to those of published examples with
[**] We thank the Department of Energy Office of Basic Energy Sciences
DE-FG-02-07ER4675 for financial support.
bulky aryl ligands, such as monomeric Mes*2GaH (Mes* =
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200805982.
2,4,6-tBu3C6H2),[21] Ar# GaH (Ar# = C6H3-2,6-Mes2),[21] and
2
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Communications
signal remained a sharp singlet during the temperature
change. When 2 was dissolved in a donor solvent, diethyl
À
ether, a significant upfield shift of the Ga D signal was
À
observed (d = 4.86 ppm). This shielded position of the Ga D
resonance is consistent with those of well-known amine
gallane adducts in the range of d = 4.5 to 5.0 ppm,[24] indicat-
ing that base-free deuteride 2 readily formed a complex with
diethyl ether. Both 1 and 2 are thermally stable at room
temperature, and decomposition starts to occur at relatively
high temperatures around 2008C.
Ammonation of organogallium(III) species at high tem-
perature is a well-known route for the preparation of gallium
nitride.[25] However, to our knowledge, reaction of NH3 with a
low-valent GaI species is rare and has only been reported for
the reaction of neopentylgallium(I) {Ga(CH2CMe3)}n with
NH3 at temperatures above 4608C,[25a] which gives C(CH3)4,
H2, and GaN. The (GaAr)2 compounds (Ar= Ar’ (3) and Ar’’
(4)) react readily with NH3 at room temperature to yield
colorless crystals of {Ar’’Ga(m-NH2)H}2 (5) suitable for X-ray
diffraction[19] or {Ar’Ga(m-NH2)H}2 (6) as a colorless, amor-
phous solid. Compound 5 has a centrosymmetric dimeric
structure (Figure 3) in which the gallium atoms are sym-
Figure 1. Thermal ellipsoid (50%) plot of 1. Carbon-bound hydrogen
atoms are omitted for clarity. Selected bond lengths [ꢀ] and angles [8]:
Ga(1)–H(1) 1.46(2), Ga(1)–H(2) 1.69(2), Ga(1)–H(2A) 1.78(2), Ga(1)–
Ga(1A) 2.5275(3), Ga(1)–C(1) 1.9862(13); C(1)-Ga(1)-H(1) 116.2(8),
C(1)-Ga(1)-H(2) 119.8(8), C(1)-Ga(1)-H(2A) 109.7(7), H(1)-Ga(1)-H(2)
103.8(11), Ga(1A)-Ga(1)-H(1) 119.0(8), Ga(1)-H(2)-Ga(1A) 93.4(8),
C(1)-Ga(1)-Ga(1A) 124.77(4); the dihedral angle between the Ga2H2
ring and the central aryl planes is 56.38.
dimeric (Trip2GaH)2 (Trip = 2,4,6-iPr3C6H2).[21] Examination
1
of the H NMR spectrum for 1 in C6D6, however, revealed
only one signal corresponding to the gallium hydride at d =
7.67 ppm that had a 1:1 intensity ratio with respect to the aryl
ligand resonances. The IR spectrum of 1 in nujol displays one
medium-intensity band at 1844 cmÀ1, which is within the
À
terminal Ga H bond stretching range of 1720 to
2050 cmÀ1.[21–23]
The same procedure as that employed for 1, using D2
instead of H2, afforded the deuteride 2. The 1H NMR
spectrum of 2 in C6D6 was identical to that of 1, except that
À
the assigned Ga H signal at d = 7.67 ppm was not observed
(Figure 2). However, a signal at d = 7.70 ppm was found in the
Figure 3. Thermal ellipsoid (30%) plot of 5. Carbon-bound hydrogen
atoms are omitted for clarity. Selected bond lengths [ꢀ] and angles [8]:
Ga(1)–H(1) 1.54(2), Ga(1)–N(1) 1.986(2), Ga(1)–N(1A) 1.988(2),
Ga(1)–C(1) 1.991(2), Ga(1)–Ga(1A) 2.9257(5); C(1)-Ga(1)-N(1)
115.90(7), C(1)-Ga(1)-N(1A) 117.55(7), C(1)-Ga(1)-H(1) 118.3(6),
N(1)-Ga(1)-H(1) 109.4(6), Ga(1)-N(1)-Ga(1A) 94.81(7), N(1)-Ga(1)-
N(1A) 85.19(7), Ga(1)-C(1)-C(2) 120.80(13); the dihedral angle
between Ga2N2 and central aryl planes is 42.88.
deuterium (2H) NMR spectrum of 2 in benzene and clearly
shows the formation of a Ga D bond instead of a Ga H
bond. Variable-temperature H NMR experiments of 2 were
À
À
2
also performed in toluene from room temperature to À308C.
À
A similar Ga D signal was found at d = 7.67 ppm, and this
metrically bridged by the NH2 ligands, the hydrogen
atoms of which were located in the electron density
map. The rhombohedral Ga2N2 core is planar with N-
Ga-N and Ga-N-Ga angles of 85.19(7) and 91.81(7)8,
respectively, and subtends a dihedral angle of 42.88
with respect to the central aryl plane of the terphenyl
À
ligand. The Ga N bridging bond length of 1.986(2) ꢀ
is within the reported 1.928–2.053 ꢀ range observed in
and (tBu2GaNH2)3.[25e]
[25a]
the trimers (Me2GaNH2)3
However, it is longer than the 1.847–1.857 ꢀ observed
in LGa(NH2)2 (L = HC{(CMe)(2,6-iPr2C6H3N)}2), in
which NH2 is a terminal ligand.[25f] The gallium atoms
are each bound to a terminal hydrogen atom, which
can also be located in the electron density map, with a
2
Figure 2. 1H NMR spectra of 1 and 2; the inset shows the H NMR spectrum of
2.
À
Ga H bond of 1.54(2) ꢀ and N-Ga-H angle of
109.4(6)8. The IR spectrum displayed two weak but
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sharp bands at 3365 and 3310 cmÀ1 arising from the two N H
stretching modes of the NH2 groups and one sharp band of
À
(CH3)2), 25.5 (o-CH(CH3)2), 30.7 (o-CH(CH3)2), 123.8, 127.9, 128.1,
129.2, 139.3, 145.4, and 147.1 ppm (ArC), the signal of ipso-C6H3 was
not found; IR (Nujol): n˜ = 1844 cmÀ1 (nGaH , medium). Compound 2
medium intensity at 1864 cmÀ1 arising from the Ga H
2
À
was prepared in a similar way to that described for 1 under a D2
atmosphere. 1H NMR (600 MHz, C6D6, 258C): essentially the same as
1 except that the GaH2 peak was absent; 13C{1H} NMR (C6D6,
150.9 MHz, 258C): essentially the same as 1. 2H{1H} NMR (C6H6,
76.8 MHz, 258C): d = 7.70 ppm (GaD2).
1
stretching mode. Examination of the H NMR spectrum of
5 in C7D8 also confirmed the presence of the amido hydride
gallium core, with signals corresponding to the gallium amide
and gallium hydride units at d = À0.55 and 4.52 ppm,
respectively, in the correct intensity ratio to the aryl ligand
resonances (Figure 4).
(Ar’’Ga)2 (4):
A rapidly stirred slurry of “GaI” (2.60 g,
10.5 mmol) in toluene (20 mL) at ca. À788C in a dry ice/acetone
bath was treated dropwise with Ar’’Li (2.50 g, 5.24 mmol) in toluene
(60 mL) over a period of 1 h. The solution was then warmed to room
temperature and stirred for 12 h, after which the slurry was allowed to
settle and the mother liquor was separated from the precipitate (LiI
and some Ga metal). The solvent was removed under dynamic
vacuum, and pale brown (Ar’’GaI)2 was recovered. (Ar’’GaI)2 in THF
(70 mL) was added to a Schlenk tube containing finely cut sodium
(120 mg, 5.24 mmol) at ambient temperature. After stirring for 2 days,
the solution was filtered, and the solvent was removed under dynamic
vacuum. The residue was extracted with hexane (50 mL), and the
slurry was allowed to settle. The supernatant solution was separated
from the precipitate (NaI), and its volume was concentrated to ca.
10 mL. Storage for 2 days in a freezer (ca. À188C) afforded dichroic
red/green solid 5. Yield: 1.38 g, 44%; m.p. 192–1948C. 1H NMR
(600 MHz, C6D6, 258C): d = 0.25 (s, 18H, Si(CH3)3), 1.08 (d, 48H, o-
3
CH(CH3)2, 3JHH = 7.2 Hz), 2.98 (sept, 8H, o-CH(CH3)2, JHH
=
Figure 4. 1H NMR spectrum of 5 in C7D8.
7.2 Hz), 7.07 (d, 8H, m-C6H3-iPr2, 3JHH = 6.8 Hz), 7.21 (t, 4H, p-
C6H3-iPr2, 3JHH = 6.8 Hz), 7.33 ppm (s, 4H, m-C6H3); 13C{1H} NMR
(C6D6, 150.9 MHz, 258C): d = À0.75 ((CH3)3Si), 25.1 (o-CH(CH3)2),
25.6 (o-CH(CH3)2), 31.2 (o-CH(CH3)2), 123.8, 129.2, 133.0, 139.5,
144.1, 147.3, 147.7, and 205.5 ppm (ArC); 29Si{1H} NMR (C6D6,
119.2 MHz, 258C): d = À4.72 ppm.
In summary, we have shown that a low-valent aryl
gallium(I) species readily undergoes insertion into H H or
À
À
N H bonds at room temperature and 1 atm pressure. These
{Ar’’Ga(m-NH2)H}2 (5) and {Ar’Ga(m-NH2)H}2 (6): Several drops
of liquid ammonia were added to a deep green solution of 4 (0.15 g,
0.14 mmol) in toluene (30 mL) at À788. The color of the solution
changed to light yellow upon warming to room temperature. Storage
of this solution at À188C afforded X-ray quality colorless crystals of 5.
Yield: 0.11 g, 73%; m.p. 2938C. 1H NMR (600 MHz, C7D8, 258C): d =
À0.55 (s, 4H, NH2), 0.27 (s, 18H, Si(CH3)3), 1.05 (d, 24H, o-
CH(CH3)2, 3JHH = 7.2 Hz), 1.16 (d, 24H, o-CH(CH3)2, 3JHH = 7.2 Hz),
reactions represent rare instances of insertion of main-group
centers into these bonds under ambient conditions.[9–13]
Unlike the corresponding reactions of H2 or NH3 with
SnAr’2, which afford {Ar’Sn(m-H)}2 or {Ar’Sn(m-NH2)}2 prod-
ucts and eliminate Ar’H,[13] the reactions of DGaAr do not
result in arene (Ar’H) elimination. Instead, the reactions
proceed by simple addition of these molecules to a heavier
3
2.82 (sept, 8H, o-CH(CH3)2, JHH = 7.2 Hz), 4.52 (s, 2H, GaH), 7.06
À
main-group-element center. The insertion into the N H bond
3
3
(d, 8H, m-C6H3-iPr2, JHH = 7.8 Hz), 7.24 (t, 4H, p-C6H3-iPr2, JHH
=
of NH3 is particularly noteworthy because of the current
interest in the involvement of such reactions in important
catalytic cycles.[26]
7.8 Hz), 7.35 ppm (s, 4H, m-C6H2); 13C{1H} NMR (C7D8, 150.9 MHz,
258C): d = 0.80 ((CH3)3Si), 23.3 (o-CH(CH3)2), 26.2 (o-CH(CH3)2),
31.1 (o-CH(CH3)2), 123.3, 129.1, 133.2, 138.2, 142.6, 147.1, 147.2, and
148.4 ppm (ArC); 29Si{1H} NMR (C7D8, 119.2 MHz, 258C): d = À4.59;
IR (Nujol): n˜ = 3365, 3310 (nNH , weak), 1864 cmÀ1 (nGa H, medium). 6
À
2
was prepared in a similar way to that described for 5, but using 3
rather than 4. 1H NMR (600 MHz, C7D8, 258C): d = À0.62 (s, 4H,
Experimental Section
NH2), 1.03 (d, 24H, o-CH(CH3)2, 3JHH = 7.2 Hz), 1.15 (d, 24H, o-
Standard Schlenk techniques were used for all syntheses, and all
sample manipulations were carried out under anaerobic and anhy-
drous conditions. 1H and 13C NMR spectra were recorded on a Varian
600 spectrometer and referenced to known standards. 2H NMR
spectra were recorded on a Bruker 500 spectrometer and referenced
to known standards.
3
CH(CH3)2, 3JHH = 7.2 Hz), 2.81 (sept, 8H, o-CH(CH3)2, JHH
=
3
7.2 Hz), 4.47 (s, 2H, GaH), 7.04 (d, 8H, m-C6H3-iPr2, JHH = 7.8 Hz),
3
7.06 (d, 4H, m-C6H3, JHH = 7.2 Hz), 7.19–7.25 ppm (m, 6H, p-C6H3-
iPr2 and p-C6H3); 13C{1H} NMR (C7D8, 150.9 MHz, 258C): d = 23.5 (o-
CH(CH3)2), 26.3 (o-CH(CH3)2), 31.0 (o-CH(CH3)2), 123.3, 127.2,
128.8, 129.4, 142.3, 147.2, 147.9, and 220.2 ppm (ArC).
{ArGa(m-H)H}2 (1) and {ArGa(m-D)D}2 (2): A deep green
solution of Ar’GaGaAr’ (3, 0.50 g, 0.53 mmol) in toluene (30 mL)
was stirred at room temperature for 3 h under a H2 atmosphere to
give a light brown solution. Storage of the solution at approximately
88C afforded X-ray quality pale yellow crystals of 1. More crystals of 1
were obtained by continued concentration and cooling of the mother
liquor. Yield: 0.31 g, 62%; decomposed into dark brown solid upon
Received: December 8, 2008
Published online: January 29, 2009
Keywords: gallium · Group 13 elements · insertion ·
.
1
low-valent species
heating to 2208C, then melted at 2358C. H NMR (600 MHz, C6D6,
258C): d = 1.04 (d, 48H, o-CH(CH3)2, 3JHH = 6.6 Hz), 2.97 (sept, 8H,
3
3
CH(CH3)2, JHH = 7.2 Hz), 6.99 (d, 4H, m-C6H3, JHH = 7.2 Hz), 7.04
(d, 8H, m-C6H3-iPr2, 3JHH = 7.8 Hz), 7.16 (t, 2H, p-C6H3, JHH
=
3
7.8 Hz), 7.23 (t, 4H, p-C6H3-iPr2, 3JHH = 7.8 Hz) 7.67 ppm (s, 2H,
GaH2); 13C{1H} NMR (C6D6, 150.9 MHz, 258C): d = 24.1 (o-CH-
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Communications
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[19] Crystallographic data for 1 and 5: Recorded at 90 K with MoKa
¯
radiation (l = 0.71073 ꢀ): 1: triclinic, space group P1, a =
9.2539(5), b = 12.6999(7), c = 13.0183(8) ꢀ, a = 63.3940(8), b =
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