Unique structural isomerism involving tetrazole and amide/azide derivatives of gallium

Article abstract of DOI:10.1039/b100466m

The reaction between {HC(MeCDippN)₂}Ga: (Dipp = C₆H₃Pr₂ⁱ-2,6) and N₃SiMe₃ afforded the tetrazole {HC(MeCDippN)₂}GaN(SiMe₃)NNN(SiMe₃) 1 and its am

Full text of DOI:10.1039/b100466m

Unique structural isomerism involving tetrazole and amide/azide
derivatives of gallium
Ned J. Hardman and Philip P. Power*
Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, CA, 95616, USA.
E-mail: pppower@ucdavis.edu
Received (in Irvine, CA, USA) 1st January 2001, Accepted 3rd May 2001
First published as an Advance Article on the web 13th June 2001
The reaction between {HC(MeCDippN)2}Ga: (Dipp
=
min at ca. 25 °C. Heating at ca. 75 °C results in a ca. 1+3 ratio
of 1 and 2 which can be isolated in 73% overall yield. Both 1
and 2 were obtained as colorless crystals. However, the
solubility of the tetrazole 1 is considerably less than that of 2, so
that the products can be separated quite easily by crystallization.
The compound 1 is thermally robust having a melting point
> 200 °C. It decomposes at ca. 235 °C to a red solid with N2
elimination. Compound 2 melts at a lower temperature
(161–163 °C) and also decomposes at ca. 230 °C to give a red
solid and N2 evolution.§
C6H3Pri2-2,6) and N3SiMe3 afforded the tetrazole {HC(MeC-
DippN)2}GaN(SiMe3)NNN(SiMe3) 1 and its amide/azide
isomer {HC(MeCDippN)2}Ga(N3)N(SiMe3)2 2 whose stabil-
ities are due to the unique steric properties of the [HC(MeC-
DippN)2]2 ligand.
Recent investigations of derivatives of the sterically en-
cumbered b-diketiminate ligand [HC(MeCDippN)2]2 (Dipp =
C6H3Pri -2,6) have shown that it can stabilize a wide variety of
species2 with unusual coordination numbers and bonding
throughout the Periodic Table.1 In particular, its use in
connection with low-valent heavier group 13 elements has
Compounds 1 and 2 were characterized by 1H and 13C NMR
spectroscopy, by C,H,N analysis and by X-ray crystallog-
raphy.¶ The structure of the tetrazole 1 (Fig. 1) features GaN4
and GaN2C3 rings that are fused at gallium. The metal has
distorted tetrahedral geometry and the Ga–N bonds [av.
1.947(2) Å] to the b-diketiminate nitrogens N(1) and N(2) are
significantly longer than the average distance of 1.875(16) Å to
the tetrazole nitrogens. The longer pair of Ga–N distances is
consistent with equal components of normal and dative
character in these bonds.8 These Ga–N bond lengths also
resemble those observed in other Ga(III) b-ketiminate species.9
The shorter Ga–N distances involving the tetrazole nitrogens
are in agreement with known values for bonding between four-
coordinate gallium and terminal amide groups.10 The gallium
tetrazole ring is essentially planar, and the N(4)–N(5) distance
resulted in the isolation of the monomeric, two-coordinate M( )
I
species {HC(MeCDippN)2}M: (M = Al2 or Ga3) which contain
a stereochemically active lone pair of electrons at the metals.
Previous work on less hindered, weakly associated M( ) species
I
such as Cp*M (Cp* = C5Me5; M = Al4 or Ga5) has shown that
they can react with azides to give dimeric imides such as
Cp*AlN(AlCp*2)Al{N(SiMe3)2}N{Al(Cp*)N(SiMe3)2}6 or
7
{Cp*GaN(C6H3Me2-2,6)}2 under elimination of N2. We
reasoned that the more crowded Ga( ) species {HC(MeC-
I
DippN)2}Ga: might react with the simple azide N3SiMe3 to
afford a monomeric imide {HC(MeCDippN)2}GaNNSiMe3
which could have a GaN multiple bond. We now report that,
although unassociated products were obtained from this
reaction, neither of these involved a terminal gallium imide
moiety. Instead, the isomers 1 and 2, involving the reaction of 2
equivalents of N3SiMe3 with {HC(MeCDippN)2}Ga:, were
isolated and characterized.
The reaction of {HC(MeCDippN)2}Ga: with N3SiMe3
afforded the products 1 and 2 in accordance with Scheme 1.† It
is probable that the species {HC(MeCDippN)2}GaNSiMe3 is
generated initally, and this is prevented from dimerizing to give
[{HC(MeCDippN)2}GaNSiMe3]2 for steric reasons. Instead,
{HC(MeCDippN)2}GaNSiMe3 reacts with a further equivalent
of N3SiMe3 to give the tetrazole 1‡ and the amide/azide product
2. Solution 1H NMR spectroscopy of the reaction mixture
shows that both 1 and 2 as well as unreacted {HC(MeC-
DippN)2}Ga: and N3SiMe3 are detectable after stirring for 30
of 1.265(3) Å is consistent with NN double bonding. In contrast,
the gallium b-diketiminate ring is folded along the N(1)…N(2)
axis such that Ga( ) lies ca. 0.34 Å from the averaged N2C3
I
plane. This structural feature is common to many bulky b-
diketiminate derivatives.1 The folding of the ring results in
different magnetic environments for the two SiMe3 groups (as
well as slightly different bond lengths for Ga–N(3) and Ga–
N(6)) which is manifested in different 1H NMR chemical shifts
for the SiMe3 resonances.
Fig. 1 Thermal ellipsoid (30%) plot of 1 with H atoms not shown. Selected
bond distances (Å) and angles (°): Ga(1)–N(1) 1.945(2), Ga(1)–N(2)
1.949(2), Ga(1)–N(3) 1.858(2), Ga(1)–N(6) 1.891(2), N(3)–N(4) 1.402(3),
N(4)–N(5) 1.265(3), N(5)–N(6) 1.406(2), Si(1)–N(3) 1.737(2), Si(2)–N(6)
1.750(2); N(1)–Ga(1)–N(2) 95.40(8), N(3)–Ga(1)–N(6) 85.74(7), N(1)–
Ga(1)–N(6) 117.49(8), N(2)–Ga(1)–N(6) 115.64(8), N(1)–Ga(1)–N(3)
126.41(8), N(2)–Ga(1)–N(3) 118.11(8).
Scheme 1 Reactions of {HC(MeCDippN)2}Ga: (Dipp = C6H3Pri2-2,6) with
N3SiMe3 to give 1 or 2.
1184
Chem. Commun., 2001, 1184–1185
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b100466m

C6H3), 125.52, 124.26 (m-C on C6H3), 99.94 (g-C), 29.10, (CHMe2), 28.92
(CHMe2), 25.30, 25.25 (CHMe2) 24.37 (CMe) 1.93, 0.99 [Si(CH3)3]. The
isomeric product 2 was obtained by decanting the supernatant liquid from 1
and cooling in a ca. 220 °C freezer for 48 h to afford colorless crystals of
2 (0.52 g, 54%). Mp 161–163 °C. 1H NMR (300 MHz C6D6) d 7.15 (s, 6H,
aromatic H or Ar groups), 4.83 (s, 1H, methine CH), 3.60 (sept, 3JHH 6.6 Hz,
2H, CHMe), 3.29 (sept, 3JHH 6.6 Hz, 2H, CHMe), 1.48 (d, 3JHH 6.6 Hz, 6H,
3
CHMe2), 1.44 (d, JHH 6.6 Hz, 6H, CHMe2), 1.40 (s, 6H, CMe), 1.16 (d,
3JHH 6.6 Hz, 6H, CHMe2), 1.11 (d, 3JHH 6.6 Hz, 6H, CHMe2), 0.23, 0.41 [s,
9H, Si(CH3)3]: 13C{1H} (75 MHz, C6D6) d 171.39 (CN), 145.11 (CMe)
144.25, 141.01 (o-C on C6H3), 128.08 (p-C on C6H3), 125.58, 124.92 (m-C
on C6H3), 100.32 (g-C), 29.09, 28.45 (CHMe2), 25.93, 25.58 (CHMe2),
25.25 (CMe), 25.09, 24.82 (CHMe2), 5.90, 5.78 [Si(CH3)3].
‡ Professor H. W. Roesky has informed us that a similar reaction involving
{HC(MeCDippN)2}Al: and N3SiMe3 affords the aluminium analog of 1.
The contrasting behavior of the gallium system, with its preference for the
amide/azide over the tetrazole product, is another illustration of the
differences between aluminium and gallium chemistry.
Fig. 2 Thermal ellipsoid (30%) plot of 2 with H atoms not shown. Selected
bond distances (Å) and angles (°): Ga(1)–N(1) 1.946(1), Ga(1)–N(2)
1.958(1), Ga(1)–N(3) 1.884(1), Ga(1)–N(4) 1.918(1), Si(1)–N(3) 1.746(1),
Si(2)–N(3) 1.751(1); N(1)–Ga(1)–N(2) 98.28(5), N(3)–Ga(1)–N(4)
111.82(6), N(1)–Ga(1)–N(3) 122.45(5), N(2)–Ga(1)–N(3) 113.23(5),
Ga(1)–N(3)–Si(1) 123.39(7), Ga(1)–N(3)–Si(2) 115.32(7).
§ Attempts at thermal interconversion of 1 and 2 have so far been
unsuccessful.
¶ Crystal data for 1 and 2 at 90 K with Mo-Ka radiation (l = 0.71073 Å):
1: C35H59GaN6Si2, M
= 689.78, colorless parallelepiped, monoclinic,
space group P21/n, a = 12.3462(4), b = 21.9781(7), c = 14.0957(4) Å, b
= 91.064(1)°, Z = 4, Dc = 1.198 g cm23, m = 0.813 mm21, R1 = 0.0454
for 4934 [I > 2s(I)] data.
2: C35H59Ga2N6Si2, M = 689.78, colorless parallelepiped, orthorhombic,
space group Pbcn, a = 20.1655(8), b = 17.8134(7), c = 21.546(9) Å, Z =
8, Dc = 1.200 g cm23, m = 0.815 mm21, R1 = 0.0354 for 9374 [I > 2s(I)]
data. CCDC 156697 and 156698. See http://www.rsc.org/suppdata/cc/b1/
b100466m/ for crystallographic data in .cif or other electronic format.
∑ Non-isomeric amide/azide and tetrazole derivatives of germanium have
been obtained by reaction of Ge(II) species with azides that have different
substituents. See ref. 14.
The structure of amide/azide compound 2 (Fig. 2) also
features gallium bound to four nitrogens in a distorted
tetrahedral fashion. The Ga–N(b-diketiminate) bonds [av.
1.952(7) Å] have very similar lengths to the corresponding
bonds in 1. The b-diketiminate ring is folded along the
N(1)…N(2) axis such that Ga( ) lies ca. 0.21 Å from the
I
averaged N2C3 plane. The Ga–N(SiMe3)2 bond length [1.884(1)
Å] is close to the 1.872(2) Å reported for {Cp*{(Me3-
Si)2N}Ga(m-N3)}2.7 The Ga–N(azide) bond length, 1.918(3) Å,
is essentially the same as the 1.921(4) Å observed in a bulky
aryl-substituted bis(azide) of gallium.11 Within the N3 moiety,
the N–N distances are similar to those previously observed in
gallium azides.12 Like 1, compound 2 displays two different
resonances for the SiMe3 peaks—probably due to restricted
rotation of the amide moiety around the Ga–N bond as a result
of steric effects. Rotational barriers as high as 18.6 kcal mol21
have been observed for group 13 metal–nitrogen bonds in
1 For example: J. Feldman, S. J. McLain, A. Parthasarathy, W. J.
Marshall, J. C. Calabrese and S. D. Arthur, Organometallics, 1997, 16,
1514; M. F. Lappert and D.-S. Liu, J. Organomet. Chem., 1995, 500,
203; B. Qian, D. L. Ward and M. R. Smith, Organometallics, 1998, 17,
3070; V. C. Gibson, P. J. Maddox, C. Newton, C. Redshaw, G. A. Solar,
A. J. P. White and D. J. Williams, Chem. Commun., 1998, 651; P. H. M.
Budzelaar, R. de Gelder and A. W. Gal, Organometallics, 1998, 17,
4121; W.-K. Kim, M. J. Fevola, L. M. Liable-Sands, A. L. Rheingold
and K. H. Theopold, Organometallics, 1998, 17, 4541; M. Chen, E. B.
Lobkovsky and G. W. Coates, J. Am. Chem. Soc., 1998, 120, 11 018;
L. W. M. Lee, W. E. Piers, M. R. J. Elsegood, W. Clegg and M. Parvez,
Organometallics, 1999, 18, 2947; B. Qian, W. J. Scanlon, M. R. Smith,
III and D. H. Motry, Organometallics, 1999, 18, 1693; C. E. Radzewich,
I. A. Guzei and R. F. Jordan, J. Am. Chem. Soc., 1999, 121, 8673; B.
Qian, S. W. Bach and M. R. Smith, Polyhedron, 1999, 18, 2405; P. L.
Holland and W. B. Tolman, J. Am. Chem. Soc., 1999, 121, 7270; P. L.
Holland and W. B. Tolman, J. Am. Chem. Soc., 2000, 122, 6331; C. Cui,
H. W. Roesky, H. Hao, H.-G. Schmidt and M. Noltemeyer, Angew.
Chem., Int. Ed., 2000, 39, 1815.
1
sterically congested systems.13 Variable temperature H NMR
studies of 1 or 2 in toluene did not result in the collapse of the
SiMe3 signals to a single resonance.
The isolation of tetrazole–amide/azide isomers appears to be
unique.∑14 Their stability can be rationalized on the basis of the
size of the [HC(MeCDippN)2]2 ligand which prevents dimer-
ization of the intermediate {HC(MeCDippN)2}GaNSiMe3, but
allows reaction with a further equivalent of the less hindered
N3SiMe3. Owing to the multipolar nature of the NNNSi array,
this reaction proceeds by two distinct pathways to afford 1 and
2.
2 C. Cui, H. W. Roesky, H.-G. Schmidt, M. Noltemeyer, H. Hao and F.
Cimpoesu, Angew. Chem., Int. Ed., 2000, 39, 4274.
3 N. J. Hardman, B. E. Eichler and P. P. Power, Chem. Commun., 2000,
1911.
4 C. Dohmeier, C. Robl, M. Tacke and H. Schnöckel, Angew. Chem., Int.
Ed. Engl., 1991, 30, 564.
We thank the National Science Foundation for Financial
Support.
5 D. Loos and H. Schnöckel, J. Organomet. Chem., 1993, 463, 37.
6 S. Schulz, L. Häming, R. Herbst-Imer, H. W. Roesky and G. M.
Sheldrick, Angew. Chem., Int. Ed. Engl., 1994, 33, 969.
7 P. Jutzi, B. Neumann, G. Reumann and H.-G. Stammler, Organome-
tallics, 1999, 18, 2037.
8 A. Haaland, in Coordination Chemistry of Aluminum, ed. G. H.
Robinson, VCH, New York, 1993.
9 M. Stender, B. E. Eichler, N. J. Hardman, P. P. Power, P. Prüst, M.
Noltemeyer and H. W. Roesky, Inorg. Chem., in press.
10 K. M. Waggoner, M. M. Olmstead and P. P. Power, Polyhedron, 1990,
9, 257.
11 A. H. Cowley, F. P. Gabbaï, F. Olbrich, S. Corbelin and R. J. Lagow,
J. Organomet. Chem., 1995, 487, C5.
12 C. J. Carmalt, A. H. Cowley, R. D. Culp and R. A. Jones, Chem.
Commun., 1996, 1453.
13 R. J. Wehmschulte and P. P. Power, Inorg. Chem., 1998, 37, 2106.
14 J. Pfeiffer, W. Maringgelle, M. Noltemeyer and A. Meller, Chem. Ber.,
1989, 122, 245.
Notes and references
† All manipulations were carried out under anaerobic and anhydrous
conditions. A toluene solution (50 mL) of N3SiMe3 (0.38 mL, 2.9 mmol)
was added dropwise to a rapidly stirred solution of {HC(MeCNDipp)2}Ga:
(0.66 g, 1.41 mmol) in toluene (20 mL), with cooling in an ice-bath. The
solution was allowed to rise to room temperature and was then heated to ca.
75 °C for 1 h. The solution was concentrated to ca. 20 mL and cooled for
24 h in a ca. 4 °C refrigerator to afford colorless crystals of the product 1
(0.18 g, 19%). Anal. Calc. (found) for C35H59N6GaSi2: C, 60.94 (61.11), H,
8.62 (8.81), N, 12.25 (12.01)%. Mp 217–220 °C. 1H NMR (300 MHz,
C6D6) d 7.06–7.04 (m, 6H, aromatic H of Ar group), 4.89 (s, 1H, methine
CH), 3.30 (sept, 3JHH = 6.6 Hz, 2H, CHMe), 3.20 (sept, 3JHH 6.6 Hz, 2H,
CHMe), 1.50 (s, 6H, CMe), 1.32 (d, 3JHH 6.6 Hz, 6H, CHMe2), 1.14 (d, 3JHH
6.6 Hz, 6H, CHMe2), 1.08 (d, 3JHH 6.6 Hz, 6H, CHMe2), 1.06 (d, 3JHH 6.6
Hz, 6H, CHMe2), 0.54–0.12 [s, 9H, Si(CH3)3]: 13C{1H} (75 MHz, C6D6) d
172.81 (CN), 145.48 (CMe) 142.76, 140.50 (o-C on C6H3), 127.54 (p-C on
Chem. Commun., 2001, 1184–1185
1185