Amidoalane, amidogallane and amidoindane, H2MNH2 (M = Al, Ga or In): A matrix study of three prototypal molecules with the potential for M-N multiple bonding

Article abstract of DOI:10.1039/b001652g

Photolysis of Al, Ga or In atoms (M) isolated in NH₃-doped Ar matrices gives first the divalent amido derivative HMNH₂ (λ = 436 nm) and then the univalent and trivalent amido derivatives MNH₂ and H₂MNH₂ (λ = 200-800 nm); the measured and calculated properties of H₂MNH₂ are outlined.

Full text of DOI:10.1039/b001652g

Amidoalane, amidogallane and amidoindane, H2MNH2 (M = Al, Ga or In): a
matrix study of three prototypal molecules with the potential for M–N multiple
bonding
Hans-Jörg Himmel,* Anthony J. Downs and Tim M. Greene
Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QR.
E-mail: tony.downs@chem.ox.ac.uk
Received (in Basel, Switzerland) 29th February 2000, Accepted 7th April 2000
Photolysis of Al, Ga or In atoms (M) isolated in NH3-doped
Ar matrices gives first the divalent amido derivative
HMNH2 (l = 436 nm) and then the univalent and trivalent
amido derivatives MNH2 and H2MNH2 (l = 200–800 nm);
the measured and calculated properties of H2MNH2 are
outlined.
doped Ar matrices containing the relevant M atoms.9 The
molecules have been identified by their IR spectra, with the
conclusions being underpinned (i) by the observed effects of
exchanging 14NH3 for 14ND3 or 15NH3, and of the isotopes 69Ga
and 71Ga present in natural Ga; (ii) by parallels with the spectra
of related hydrido and amido derivatives, e.g. MH3,4
H2MCl,5,10 and AlNH2 (M = Al, Ga or In); and (iii) by
7
Amido derivatives of group 13 metals are potentially relevant to
the fabrication and properties of the III–V semiconductor
materials AlN, GaN and InN.1 Such compounds are normally
oligomeric or polymeric, with a coordination number of four or
greater at M; only with the introduction of bulky substituents at
M and N can monomeric compounds with tri-coordinated M
atoms be sustained under normal conditions.2 The properties of
these monomeric amido derivatives have attracted considerable
theoretical attention, mainly in relation to the potential for M–N
multiple bonding.2,3
Matrix isolation offers a highly instructive method of
reconnaissance and characterisation for simple, previously
unknown hydride derivatives of the main group metals.4–6
Hence, for example, photoexcitation of metal atoms contained
in solid Ar matrices doped with NH3 has been shown previously
to result in insertion of the metal into an N–H bond of NH3 with
the formation of the species HMNH2 (M = Al7) and MNH2 (M
= Li8 or Al7). Here we report specifically on the formation of
the simple monomeric compounds H2MNH2 (M = Al, Ga or
In) as products of the reactions set in train by photolysis of NH3-
comparisons with the IR spectra forecast by density functional
theory (DFT) calculations.11 In the following account we will
concentrate on the formation and characterisation of H2GaNH2;
the aluminium and indium analogues follow a very similar
pattern, with the results summarized in Table 1.
An Ar matrix containing ca. 0.2% Ga atoms and up to 2%
NH3 gave an IR spectrum including, in addition to the
absorptions characteristic of NH3 and [NH3]n,12 a new absorp-
tion at 1104.2 cm21 attributable to an adduct Ga·NH3 1. There
was also associated with this product a visible absorption
centered at 440 nm. Irradiation of the matrix with radiation
having l = 436 nm for 5 min resulted in the decay of the
spectroscopic features due to 1 and the appearance of new IR
bands at 1721.8, 1528.7, 746.2, 668.5/667.4 494.1 and 210.9
cm21 attributable on the strength of their constant relative
intensities to a second product 2. All the criteria (i)–(iii) indicate
that 2 is the gallium(II) amide HGaNH2, showing clear
spectroscopic analogies with GaH24 and CH3GaH.6
Photolysis for a further 30 min, but with broad-band UV–VIS
light (200 < l < 800 nm) led to the decay of the bands due to
Table 1 Structures (distances in Å, angles in degrees), rotational barriers DE (kJ mol21), wavenumbers [cm21, intensities (km mol21) are given in
parentheses] and M–N force constants f(MN) (N m21) for H2MNH2 molecules (M = Al, Ga and In)
Parameter
H2AlNH2
H2GaNH2
H2InNH2
M–N
M–H
N–H
H–M–H
H–N–H
DE
1.7790
1.5811
1.0100
124.4
110.0
50.6
1.8211
1.5621
1.0086
126.7
111.7
65.7
1.9703
1.7252
1.0169
126.9
110.3
51.5
Description of
vibrational mode
Assignment
Obs.
Calc.
Obs.
Calc.
Obs.
Calc.
a
nsym(N–H)
nsym(M–H)
d(NH2)
n(M–N)
d(MH2)
n1 (a1)
n2 (a1)
n3 (a1)
n4 (a1)
n5 (a1)
n6 (a2)
n7 (b1)
n8 (b1)
n9 (b2)
n10 (b2)
n11 (b2)
n12 (b2)
3499.7
1891.0
1541.6
818.7
3572.0 (11)
1959.3 (81)
1631.2 (49)
830.1 (192)
754.6 (86)
499.9 (0)
608.4 (150)
483.0 (309)
3655.8 (11)
1964.2 (288)
767.6 (151)
433.5 (22)
3413.4
3581.9 (9)
1995.9 (64)
1621.6 (30)
689.0 (124)
740.3 (40)
545.8 (0)
607.9 (43)
337.3 (280)
3681.7 (13)
1998.6 (245)
789.8 (110)
441.8 (26)
—
3517.1 (10)
1770.2 (68)
1579.4 (23)
575.8 (153)
634.2 (84)
480.8 (0)
535.2 (95)
177.6 (237)
3621.9 (18)
1756.9 (272)
696.3 (126)
368.5 (33)
a
1970.8—c
1530.4
—
1506.6
616.3
706.2/704.1
779.6
a
755.0
—
b
b
b
Twist
—
—
—
a
rout-of-plane(MH2)
rout-of-plane(NH2)
nasym(N–H)
nasym(M–H)
din-plane(NH2)
din-plane(MH2)
608.7
518.3
567.7
304.9
—
a
—
a
a
—
3510.7
1970.8—c
782.8
—
1899.3
769.8
1805.9
733.3
a
a
a
—
—
—
f(MN)
407.5
384.9
315.3
a Not observed. b IR silent.c Both modes contained within a single broad absorption.
DOI: 10.1039/b001652g
Chem. Commun., 2000, 871–872
871
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formed from HMNH2 or MNH2 by the addition of H atoms
generated by the photodecomposition of HMNH2 (or MH2
when H2 is present4).
Calculated and observed properties of H2AlNH2, H2GaNH2
and H2InNH2 are listed in Table 1. In each case, the global
minimum is calculated to correspond to a planar ethene-like
geometry, although, as noted elsewhere,2 a planar M–NH2
geometry is not necessarily a sign of strong p bonding. The
calculated M–N distances are indeed short, near or below the
lower limits for the observed distances in tri-coordinated amido
derivatives of these elements (1.78, 1.818 and 2.049 Å for M =
Al, Ga and In, respectively);2 with respect to related species the
order is M·NH3 > H3M·NH3 > MNH2 > MN > HMNH2 6
H2MNH2 > HMNHA. The M–N stretching force constants,
based where possible on the measured spectra, vary in the orders
(i) H2MNH2 6 HMNH2 6 MNH2 9 H3M·NH3 9 M·NH3, and
(ii) M = B 9 Al 6 Ga > In for H2MNH2. Calculation of the
energy of the transition state in which the H2M and NH2 planes
are orthogonal gives an estimate of the barrier to rotation about
the M–N bond, DE, in H2MNH2. The results included in Table
1 are consistent with those reported elsewhere,2 with DE =
161.9, 50.6, 65.7 and 51.5 kJ mol21 for M = B, Al, Ga and In,
respectively. Gallium may then be seen once again to be out of
line with its neighbours and to show a modest return to the
behaviour of boron,1 but the barriers support the general view
that, with the exception of M = B, the primary influence on the
M–N bonding is not the p interaction but the polarity of the
unit.
Fig. 1 (a) Bottom: IR spectrum of an Ar matrix containing Ga and NH3
following photolysis first at l = 436 nm and then at l = 200–800 nm; top:
calculated IR absorptions for H2GaNH2. (b) Bottom: IR spectrum of
H2GaNH2 in the region around 700 cm21; top: curve fit with two
Lorentzian-type functions with an intensity ratio reflecting the proportions
69Ga/71Ga = 60.1+39.9 in naturally occurring Ga.
2 with the simultaneous appearance and growth of a new family
of bands having a common origin in a third product 3. Occurring
at 3471.6, 1505.9, 589.3/587.9 and 314.5 cm21, these could be
identified with the gallium( ) amide GaNH2, a conclusion
I
supported by the criteria (i)–(iii) and by analogy with the IR
spectrum reported for AlNH2.7 Extending the period of
photolysis resulted in the continued accumulation of 3 at the
expense of 2. Additionally, another group of bands, also with
constant relative intensities and therefore associated with a
fourth distinct product 4, was observed to develop (see Fig. 1).
The members of this group were located at 3510.7, 3413.4,
1970.8, 1530.4, 782.8, 779.6, 706.2/704.1, 567.7 and 304.9
cm21, those at 1970.8, 782.8, 706.2/704.1 and 304.9 cm21
being the most prominent. Diagnostic aspects are (i) the strong
absorption at 1970.8 cm21 occurring in the region characteristic
of n(Ga–H) vibrations of terminal Ga(III)–H bonds (cf. GaH3
1923.2 cm21 4 and H2GaCl 1964.6–1978.1 cm21 9); (ii) the
absorptions at 3510.7, 3413.4 and 1530.4 cm21 implying the
presence of an NH2 group; and (iii) the doublet pattern at
706.2/704.1 cm21 attributable to 69Ga/71Ga splitting arising
from the motion of a single Ga atom (see Fig. 1). The criteria
(i)–(iii) leave little doubt that 4 is monomeric amidogallane,
H2GaNH2, more familiar as a trimer which is stable at
temperatures up to nearly 150 °C.13 Out of the 11 IR-active
modes (5a1 + 2b1 + 4b2) expected for planar H2GaNH2 with C2v
symmetry, all but one have been satisfactorily located in the
spectrum measured for 4, the only absentee being n12 (b2) which
is expected to lie near 420 cm21 but is probably obscured by
extraneous absorptions. In other respects the calculations
anticipate remarkably closely the observed wavenumbers,
relative intensities, and isotopic shifts to provide a convincing
basis for the assignments entered in Table 1. For example, the
observed 69Ga/71Ga splitting of 2.1 cm21 displayed by the
absorption near 705 cm21 matches admirably the calculated
value of 1.9 cm21 for the n(Ga–N) mode, n5 (a1).
The amides H2AlNH2 and H2InNH2 have each been formed
and characterised in analogous experiments involving Al and In
atoms, respectively. With the indium compound, formed only in
low concentrations, no more than four weak IR absorptions
could be clearly associated, although the wavenumbers and
isotopic shifts, allied to the circumstances, vouch for its identity.
Nine of the IR-active fundamentals of H2AlNH2 were located
with confidence. Two of the most prominent bands, at 1899.3
and 1891.0 cm21, tally with features reported in earlier matrix
studies7 of the reactions between laser-ablated Al atoms and
NH3 and assigned somewhat tentatively not to H2AlNH2 but to
the quasi-linear, high-energy HAlNH molecule. Experiments
with all three metals and involving different concentrations of
NH3 or mixtures of NH3 and H2 make it clear that H2MNH2 is
We thank (i) the EPSRC for support of this research and the
award of an Advanced Fellowship to T. M. G., and (ii) the
Deutsche Forschungsgemeinschaft for the award of a post-
doctoral grant to H.-J. H.
Notes and references
1 See, for example: Chemistry of Aluminium, Gallium, Indium and
Thallium, ed. A. J. Downs, Blackie, Glasgow, UK, 1993; Properties of
Group III Nitrides, ed. J. H. Edgar, EMIS, London, 1994.
2 K. Knabel, I. Krossing, H. Nöth, H. Schwenk-Kirchner, M. Schmidt-
Amelunxen and T. Seifert, Eur. J. Inorg. Chem., 1998, 1095; P. P.
Power, Chem. Rev., 1999, 99, 3463.
3 See: R. D. Davy and K. L. Jaffrey, J. Phys. Chem., 1994, 98, 8930; T. P.
Hamilton and A. W. Shaikh, Inorg. Chem., 1987, 36, 754.
4 P. Pullumbi, C. Mijoule, L. Manceron and Y. Bouteiller, Chem. Phys.,
1994, 185, 13, 25.
5 H.-J. Himmel, A. J. Downs and T. M. Greene, J. Am. Chem. Soc., 2000,
122, 922.
6 H.-J. Himmel, A. J. Downs, T. M. Greene and L. Andrews, Chem.
Commun., 1999, 2243; Organometallics, 2000, 19, 1060.
7 D. V. Lanzisera and L. Andrews, J. Phys. Chem. A, 1997, 101, 5082.
8 P. F. Meier, R. H. Hauge and J. L. Margrave, J. Am. Chem. Soc., 1978,
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9 Full experimental details may be found in ref. 5.
10 R. Köppe and H. Schnöckel, J. Chem. Soc., Dalton Trans., 1992,
3393.
11 DFT calculations employed GAUSSIAN 98 [B3LYP hybrid method,
6-311G* basis set for Al and Ga, LANL2DZ basis set with additional d-
polarisation function (exponent 0.5) for In]. No scaling factor was used
in computing the vibrational frequencies. GAUSSIAN 98, Revision
A.3: M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Mongomery, R. E.
Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N.
Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R.
Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski,
G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D.
Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz,
A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
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Laham, C. Y. Peng, A.Nanayakkara, C. Gonzalez, M. Challacombe,
P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, A.
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12 L. Abouaf-Marguin, M. E. Jacox and D. E. Milligan, J. Mol. Spectrosc.,
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872
Chem. Commun., 2000, 871–872