Azide addition to give a tetra-azazirconacycle complex

Article abstract of DOI:10.1021/ic048560c

Addition of the dilithium salt, ortho-(Me₃SiNLi) ₂C₆H₄, to ZrCl₄ affords a base-free, D_2d-symmetric complex Zr^IV[ortho-(Me₃SiN) ₂C₆H₄]₂ (2), with rigorously planar ortho-phenylenediamine ligands. Lewis acidic 2 readily coordinates donor ligands such as NHEt₂ to give the five-coordinate complex, Zr ^IV(NHEt₂)[ortho-(Me₃SiN)₂C ₆H₄]₂ (3), which is also accessible by the reaction of Zr(NEt₂)₄ with 2 equiv of ortho-(Me ₃SiNH)₂C₆H₄. Aryl azides react with 2 and 3 to give an unusual tetra-azametallacycle complex, 4, via 1,2-addition of a ligand N-Si bond to the organic azide. An X-ray crystal structure of 4 reveals a planar, five-membered metallacycle comprising the zirconium atom, one nitrogen atom of the ortho-(Me₃SiN)₂C₆H ₄ ligand, and all three nitrogen atoms of the aryl azide.

Full text of DOI:10.1021/ic048560c

Inorg. Chem. 2005, 44, 468−470
Azide Addition To Give a Tetra-azazirconacycle Complex
Alan F. Heyduk,* Karen J. Blackmore, Nicole A. Ketterer, and Joseph W. Ziller
Department of Chemistry, 516 Rowland Hall, UniVersity of California, IrVine, California 92697
Received October 14, 2004
Addition of the dilithium salt, ortho-(Me3SiNLi)2C6H4, to ZrCl4 affords
a base-free, D2d-symmetric complex ZrIV[ortho-(Me3SiN)2C6H4]2 (2),
with rigorously planar ortho-phenylenediamine ligands. Lewis acidic
2 readily coordinates donor ligands such as NHEt2 to give the
five-coordinate complex, ZrIV(NHEt2)[ortho-(Me3SiN)2C6H4]2 (3),
which is also accessible by the reaction of Zr(NEt2)4 with 2 equiv
of ortho-(Me3SiNH)2C6H4. Aryl azides react with 2 and 3 to give
an unusual tetra-azametallacycle complex, 4, via 1,2-addition of a
ligand, which releases N2 and forms a coordinated amide
after a 1,3-proton shift.11 Metal mediated coupling of
two organic azides forms a tetra-azabutadiene fragment
(RNdNsNdNR), which is stabilized when coordinated to
the metal center.9,12-14 The two-electron reduced congener
tetrazenes (RNsNdNsNR2-) have been prepared by 1,3-
NH-addition of an arylamide to an aryl azide followed by
deprotonation.15 Alternatively, coordinated tetrazene frag-
ments have been obtained by the [3 + 2] reaction of an
organic azide with a zirconium imide complex.16,17 In the
course of studies on the coordination chemistry of the ortho-
phenylenediamine derived ligand [ortho-(Me3SiN)2C6H4]2-
with group 4 metals, we discovered an unusual azide reaction
leading to the formation of a tetra-azametallacycle complex,
ZrIV[ArNN(SiMe3)NNC6H4NSiMe3][ortho-(Me3SiN)2-
C6H4] (4, Ar ) para-CH3C6H4). Formally, the product-
forming reaction corresponds to a 1,2-NSi-addition to the
incoming azide. An X-ray crystal structure reveals short Ns
N bond lengths within the tetra-azametallacycle fragment,
consistent with a delocalized electronic structure that is
intermediate between a formally dianionic tetrazene fragment
and a formally neutral tetra-azabutadiene fragment.
ligand N−Si bond to the organic azide. An X-ray crystal structure
of 4 reveals a planar, five-membered metallacycle comprising the
zirconium atom, one nitrogen atom of the ortho-(Me3SiN)2C6H4
ligand, and all three nitrogen atoms of the aryl azide.
Organic azide reactions with transition metal complexes
have attracted considerable attention in recent years. Detailed
mechanistic studies of azide activation have been carried out
with Cp2TaMe(PMe3).1,2 In this system, an initial, η1-terminal
coordination mode for the azide substrate was supplanted
via rearrangement by a four-centered metallacycle intermedi-
ate. Extrusion of N2 from this intermediate gave a terminal
metal imido complex, a common final product for reactions
of organic azides and transition metal complexes.3-8 Organic
azides also show a rich insertion chemistry with transition
metal complexes. Azides react with metal carbonyls via N2
extrusion and nitrene insertion into the MsCO bond to
give isocyanates,5,9 whereas metal-stabilized phosphazido
species (RNdNsNdPR′3) can be obtained upon insertion
of an azide into a metal phosphine bond.10 Insertion of an
azide into a metal hydride bond gives an initial triazenido
Metalation of diamine 118,19 was readily achieved upon
deprotonation with nBuLi and subsequent addition of half
of an equivalent of ZrCl4 according to Scheme 1. Solvent
removal gave the crude product, ZrIV[ortho-(Me3SiN)2C6H4]2
(2), in nearly quantitative yield as a bright orange powder.
Recrystallization of crude 2 from CH2Cl2 at -35 °C gave
large orange blocks of the product in 47% overall yield; the
results of an X-ray diffraction experiment are shown in
* To whom correspondence should be addressed. E-mail: aheyduk@
uci.edu.
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468 Inorganic Chemistry, Vol. 44, No. 3, 2005
10.1021/ic048560c CCC: $30.25
© 2005 American Chemical Society
Published on Web 01/08/2005

COMMUNICATION
Figure 1. ORTEP representations of ZrIV[ortho-(Me3SiN)2C6H4]2 (2), ZrIV(NHEt2)[ortho-(Me3SiN)2C6H4]2 (3), and ZrIV[ArNN(SiMe3)NNC6H4NSiMe3][ortho-
(Me3SiN)2C6H4] (4, Ar ) para-CH3C6H4). Ellipsoids are drawn at 50% probability, and hydrogen atoms have been removed for clarity.
Figure 1 as an ORTEP diagram.20 The zirconium coordina-
tion environment in 2 is D2d-symmetric with planar sp2-
hybridized amide nitrogen atoms. The ortho-phenylenedi-
amine ligands are planar and offset from each other by a
91(1)° dihedral angle. This stands in stark contrast to other
complexes of zirconium with ortho-phenylenediamine
ligands.21,22 In these structures, the chelating ligands were
puckered, suggesting a possible η4-type interaction between
the electrophilic metal center and ortho-phenylenediamine
ring electrons. Despite the difference in ligand conformation,
the average Zr-N distance in 2 (2.08 Å) is comparable to
the Zr-N distances in these previously reported zirconium
complexes (2.06-2.10 Å).
chilled pentane solution, showed pseudo-trigonal-bipyramidal
coordination geometry about zirconium; the ORTEP diagram
for 3 is shown in Figure 1.23 Two axial and two equatorial
sites are occupied by anilide nitrogen atoms of the ortho-
phenylenediamine ligand. The axial Zr-N distances are long
(Zr-N(2) 2.1642(16) Å and Zr-N(4) 2.1710(15) Å) relative
to the equatorial Zr-N bond distances (Zr-N(1) 2.0640-
(15) Å and Zr-N(3) 2.0692(15) Å). Though all four anilide
nitrogen atoms are planar, the chelating ligands adopt a
puckered or bent conformation as previously observed.21,22
The Zr-N distance for the diethylamine ligand is 2.3953-
(18) Å, and the nitrogen atom is clearly sp3 hybridized; the
amine hydrogen was located in the Fourier map.
NMR spectroscopy identified a fluxional coordination
environment for 3 in solution. At ambient temperature, the
1H NMR spectrum of 3 shows broadened resonances for both
the aryl ring and the coordinated NHEt2 ligand, suggesting
that 3 is fluxional in solution. Variable-temperature 1H NMR
experiments were used to probe this isomerization process.
Scheme 1
1
The low-temperature H NMR spectrum is consistent with
the five-coordinate, pseudo-trigonal-bipyramidal geometry
determined in the X-ray diffraction experiment. Distinct
SiMe3 resonances were observed at 0.35 and 0.41 ppm,
consistent with one axial-oriented and one equatorial-oriented
SiMe3 group on each ortho-phenylenediamine ligand. Di-
asteriotopic resonances were observed for the coordinated
NHEt2 ligand at low temperature. Upon warming, exchange
between the axial and equatorial sites in 3 becomes facile,
and a single environment was observed for the SiMe3 groups
of the ortho-phenylenediamine ligands. Similarly, the dias-
tereotopic resonances for the coordinated NHEt2 ligand
collapse at elevated temperatures. The resonances for the
SiMe3 groups were used to determine isomerization rates
between -35 and +5 °C; activation parameters were
extracted from an Eyring plot: ∆Hq ) 13.1 ( 0.9 kcal mol-1
and ∆Sq ) 0 ( 4 eu.
The electrophilicity of 2 is evident from its rapid coordina-
tion of NHEt2 in solution to give ZrIV(NHEt2)[ortho-(Me3-
SiN)2C6H4]2 (3). Alternatively, 3 may be prepared directly
by the reaction of 1 with Zr(NEt2)4. An X-ray diffraction
experiment, performed on a yellow crystal of 3 obtained from
(20) Crystallographic data for 2 follow. C24H44N4Si4Zr, fw 592.21, mono-
clinic, C2/c, a ) 15.362(4) Å, b ) 14.093(4) Å, c ) 14.677(4) Å, â
) 90.082(4)°, V ) 3177.6(13) Å3, Z ) 4, λ ) 0.71073 Å, Fcalc
)
1.238 Mg/m3, µ ) 0.515 mm-1, R1 ) 0.0192, wR2 ) 0.0528, GOF
) 1.094.
(23) Crystallographic data for 3 follow. C28H55N5Si4Zr, fw 665.35, mono-
clinic, P21/n, a ) 11.8325(9) Å, b ) 22.8851(18) Å, c ) 13.4664-
(11) Å, â ) 95.8120(10)°, V ) 3627.8(5) Å3, Z ) 4, λ ) 0.71073 Å,
(21) Aoyagi, K.; Gantzel, P. K.; Kalai, K.; Tilley, T. D. Organometallics
1996, 15, 923-927.
(22) Pindado, G. J.; Thornton-Pett, M.; Bochmann, M. J. Chem. Soc.,
Dalton Trans. 1998, 393-400.
F
calc ) 1.218 Mg/m3, µ ) 0.459 mm-1, R1 ) 0.0283, wR2 ) 0.0689,
GOF ) 1.096.
Inorganic Chemistry, Vol. 44, No. 3, 2005 469

COMMUNICATION
Compound 2 reacts with para-tolyl azide at room tem-
perature to afford a unique tetra-azametallacyclopentene
product formed by 1,2-addition of a single N-Si bond to
the organic azide. Addition of para-tolyl azide to a suspen-
sion of 2 in toluene gave a cherry-red solution. Pure product
was obtained by adding heptane to induce precipitation. An
X-ray crystal structure revealed the product to be ZrIV[ArNN-
(SiMe3)NNC6H4NSiMe3][ortho-(Me3SiN)2C6H4] (4, Ar )
para-CH3C6H4), a tetra-azametallacyclopentene complex
shown as an ORTEP structure in Figure 1.24
Compound 4, formed by 1,2-NSi-addition to the organic
azide, is a tetra-azametallacycle, intermediate between the
dianionic tetrazene and neutral tetra-azabutadiene congeners.
Apparently, addition of the SiMe3 group to the azide â
nitrogen traps the one-electron reduced azide in the metal-
lacyclic structure. The X-ray crystal data for 4 shows short
N-N bond lengths within the tetra-azametallacycle ring
(N(3)-N(4) ) 1.396(2) Å, N(4)-N(5) ) 1.301(3) Å, N(5)-
N(6) ) 1.312(3) Å), comparable to the N-N bond lengths
observed in both tetrazene17 and tetra-azabutadiene25 com-
plexes. The planar arrangement of the four nitrogen atoms
is consistent with delocalization of electron density through-
out the fragment by the two resonance structures shown in
Scheme 2. The monoanionic, diazene-diazide form seems
proposal. The aryl ring of the activated ortho-phenylenedi-
amine ligand is coplanar with the tetra-azametallacycle as a
consequence of chelation. Conversely, the para-tolyl ring
of the azide is almost perfectly staggered with respect to the
tetra-azametallacycle. This orientation of the tolyl ring is
likely due to steric interactions with the proximal SiMe3
group. One of the ortho-phenylenediamine ligands remains
unperturbed in 4, with the phenyl ring bent up by 13° with
respect to the plane defined by N(1)-Zr(1)-N(2).
Preliminary experiments suggest that the reaction between
2 and para-tolyl azide requires the presence of the electro-
philic zirconium metal center. First, 1:1 solutions of para-
tolyl azide and 1 in C6D6 showed no evidence of reaction
after several days at room temperature. Second, the reaction
of para-tolyl azide with 3 proceeds to give 4, albeit at a
reduced rate. This qualitative result is consistent with
dissociation of the NHEt2 ligand from 3, to give 2, prior to
formation of a reactive zirconium-azide adduct. The nature
of this zirconium-azide intermediate and the mechanistic
details leading to formation of 4 are the subject of ongoing
studies.
The coordination geometry of 4 is preserved in solution.
The unperturbed ortho-phenylene diamine ligand is charac-
terized by a single SiMe3 peak and the normal AA′BB′
1
aromatic resonance pattern in the H NMR spectrum. The
Scheme 2
SiMe3 of the azametallacycle fragment is observed as a sharp
singlet at 0.06 ppm, and the CH3 of the tolyl group is
observed clearly as a singlet at 1.92 ppm. Heating solutions
of 4 under argon results in decomposition of the metal
1
complex. Decomposition reactions monitored by H NMR
spectroscopy showed clean first-order decay of the reso-
nances associated with 4, but no new NMR-observable
species was observed. Current efforts are directed toward
elucidating the decomposition and reactivity pathways of 4.
most plausible on the basis of charge minimization and would
suggest that N(6) is coordinated to zirconium as a neutral
two-electron donor atom. The observed elongation of the Zr-
N(6) bond (2.2305(18) Å) relative to the other amide Zr-N
distances in the X-ray structure of 4 is consistent with this
Acknowledgment is made to the University of California,
Irvine, for funding this work.
Supporting Information Available: X-ray crystallographic data
(CIF format) and detailed experimental methods (PDF format). This
material is available free of charge via the Internet at http://
pubs.acs.org.
(24) Crystallographic data for 4 follow. C31H51N7Si4Zr, fw 725.37, triclinic,
P1h, a ) 10.0465(9) Å, b ) 12.0462(11) Å, c ) 16.4033(15) Å, R )
99.963(2)°, â ) 94.636(2)°, γ ) 99.203(2)°, V ) 1918.0(3) Å3, Z )
2, λ ) 0.71073 Å, Fcalc ) 1.256 Mg/m3, µ ) 0.442 mm-1, R1 )
0.0329, wR2 ) 0.0778, GOF ) 1.080.
(25) Lee, S. W.; Trogler, W. C. Organometallics 1990, 9, 1470-1478.
IC048560C
470 Inorganic Chemistry, Vol. 44, No. 3, 2005