Highly reactive metal - Nitrogen bond induced C - H bond activation and azametallacycle formation

Article abstract of DOI:10.1021/om901027v

A group 4 metal - nitrogen bond inserted into the carbon-nitrogen double bond of carbodiimides and a-diimines to afford guanidinate and amido - -imino ligand supported group 4 metal complexes, respectively. Metal - nitrogen mediated C. - H bond activation a to the nitrogen atom led to the formation of azametallacycle complexes.

Full text of DOI:10.1021/om901027v

34 Organometallics 2010, 29, 34–37
DOI: 10.1021/om901027v
Highly Reactive Metal-Nitrogen Bond Induced C-H Bond Activation
and Azametallacycle Formation
Tarun K. Panda, Hayato Tsurugi, Kuntal Pal, Hiroshi Kaneko, and Kazushi Mashima*
Department of Chemistry, Graduate School of Engineering Science, Osaka University,
Toyonaka, Osaka 560-8531, Japan
Received November 27, 2009
Summary: A group 4 metal-nitrogen bond inserted into the
carbon-nitrogen double bond of carbodiimides and R-diimines
to afford guanidinate and amido--imino ligand supported
group 4 metal complexes, respectively. Metal-nitrogen
mediated C-H bond activation R to the nitrogen atom led to
the formation of azametallacycle complexes.
as another key step in the syntheses of guanidines and
transamidation (Scheme 1b).3 Another catalytic transforma-
tion involving the metal-nitrogen bond, that is insertion of a
C-H bond R to the nitrogen into alkenes and alkynes, has
attracted recent interest, and intermolecular hydroami-
noalkylation reactions catalyzed by the homoleptic group 4
and 5 metal amide complexes were developed by Hartwig
et al. and Doye et al., respectively (Scheme 1c,d).4,5 As
depicted in Scheme 1, these catalytic intermolecular hydro-
aminoalkylation reactions involve the elimination of amine
from bis(amide)metal species A via the abstraction of an
R hydrogen atom to the nitrogen by the adjacent metal-
nitrogen bond (intermediate B), leading to the formation of
the three-membered azametallacycle C as a key species for
further coupling with unsaturated organic substrates.
Thus, exploring the reaction pathway and further applica-
tion of the insertion reaction of the C-H bond R to the
nitrogen into alkenes and alkynes via amine elimination is
important, though such insertion into an azametallacycle
generated via an alkane elimination from LnM(alkyl)-
(amido) complexes has been utilized for the synthesis of
nitrogen-containing organic compounds in a stoichiometric
manner.6-8 Recently, Schafer and co-workers reported the
isolation of an azametallacycle species (C in Scheme 1) via an
amine elimination reaction;4b,d however, the metallacyclic
Metal-nitrogen bonds of early- and lanthanide-metal
complexes have been utilized as catalysts or stoichiometric
reagents in a wide variety of organic syntheses to prepare a
large number of nitrogen-containing organic molecules.1
Some of these catalytic reactions involve the key step of
inserting a metal-nitrogen bond into the unsaturated multi-
ple bond of alkenes, alkynes, and allenes; the intramolecular
hydroamination/cyclization reaction, developed by Marks
et al., is one of the most well-established of these reactions
(Scheme 1a).2 On the other hand, insertion of the me-
tal-nitrogen bond into polar unsaturated multiple bonds,
such as CdN and CdO bonds, has been extensively explored
*To wmom correspondence should be addressed. E-mail: mashima@
chem.es.osaka-u.ac.jp. Fax: 81-6-6850-6245.
€
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(5) Selected examples for late-transition-metal-catalyzed coupling
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2007, 72, 6763. (k) Hoerter, J. M.; Otte, K. M.; Gellman, S. H.; Cui, S.; Stahl,
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105, 2651. (b) Buchwald, S. L.; Wannamaker, M. W.; Watson, B. T. J. Am.
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r
pubs.acs.org/Organometallics
Published on Web 12/10/2009
2009 American Chemical Society

Communication
Organometallics, Vol. 29, No. 1, 2010 35
Scheme 1. Reaction Patterns of the Metal-Nitrogen Bond
toward (a) Carbon-Carbon Multiple Bond, (b) Carbon-
Heteroatom Multiple Bond, and (c, d) C-H Activation and
Aminoalkylation Reactions
intermediates (D in Scheme 1), products of the reaction with
unsaturated organic compounds, have not yet been isolated.
In our effort to find a new C-H bond activation R to the
nitrogen by the metal-nitrogen bond during the course of the
reaction of homoleptic tetrakis(amide) complexes of group 4
metals with conjugated diimine ligands, we found a five-
membered diazametallacyclopentane as the first example of
an isolated metallacycle intermediate in the hydroaminoalk-
ylation reaction. We also demonstrated that an σ2,π-N,N0-
enediamido complex of zirconium was obtained by successive
amination of the CdN bond and M-N mediated C-H bond
activation of the R-diimine ligand moiety.
Figure 1. Molecular structure of complex 2a. Selected bond
˚
distances (A) and angles (deg): Ti-N1 =2.080(6), Ti-N2 =
2.185(6), Ti-N4 = 2.158(6), Ti-N5 = 2.131(6), Ti-N7 =
1.916(6), Ti-N8 = 2.013(6), N8-C3 = 1.374(8), N9-C3 =
1.281(8); N1-Ti-N2 = 62.5(2), N4-Ti-N5 = 62.6(2),
N7-Ti-N8 = 79.7(2).
Scheme 2. Proposed Mechanism for the Formation of 2a
Reaction of Ti(NMe2)4 with an excess amount of diiso-
propylcarbodiimide (1a) at 90 °C for 48 h afforded the
unprecedented titanium complex 2a in 33% yield, which
was a product of the aliphatic C-H bond activation of the
methyl group bound to the nitrogen atom, forming an
azametallacyclopropane intermediate, followed by insertion
into 1a (eq 1). In the 1H NMR spectrum of 2a, ABq signals
centered at δH 4.23 and 4.53 (2JH-H = 18.5 Hz) were
observed as the methylene protons adjacent to the exo-imine
moiety, consistent with a 2,5-diaza-3-exoimino-titanacyclo-
pentane, whose structure was determined by X-ray crystal-
lographic analysis (Figure 1).9 The titanium atom is
surrounded by six nitrogen atoms to form a six-coordinated
octahedral geometry, in which four of six nitrogen atoms
coordinate to titanium as two guanidinate ligands. The
notable structural feature of 2a is the five-membered metal-
lacycle with an exocyclic imine moiety, as confirmed by the
N8-C3 single bond and the N9-C3 double bond.
3:1 ratio, indicating that one NMe2 group migrated into the
CdN bond of 1a. When a mixture of 1a and 3a was heated at
70 °C, the signals assignable to 2a gradually increased in the
1H NMR spectrum of the mixture. Thus, the formation of 2a
could be rationalized as shown in Scheme 2. In the first stage,
successive amination toward 2 equiv of diisopropylcar-
bodiimide by Ti(NMe2)4 resulted in the formation of a
bis(guanidinate)titanium complex. Due to the strong elec-
tron-donating nature of the alkyl-substituted guanidinate
ligand,10 there was an increase in the basicity of the dimethy-
lamido group bound to the titanium. Following C-H bond
activation R to the nitrogen atom of the dimethylamino
group via a four-membered transition state, three-membered
titanaaziridine was formed, and subsequent insertion of
diisopropylcarbodiimide gave 2a.
In sharp contrast, treatment of Ti(NMe2)4 with dicyclo-
hexylcarbodiimide (1b) in toluene at 90 °C afforded mono-
(dicyclohexylguanidinate) complex 3b, presumably due to
the bulky cyclohexyl substituents of the complex preventing
the second insertion of 1b (Scheme 3). In this complexation,
an amido transfer from titanium to one of the two CdN
When a mixture of Ti(NMe2)4 and 1a in toluene was
heated to 50 °C, the mono(guanidinate)titanium complex
(iPrNC(NMe2)NiPr)Ti(NMe2)3 (3a) was isolated as a yellow
oil in 94% yield. In the 1H NMR spectrum of 3a, resonan-
ces corresponding to the NMe2 group were observed in a
(9) Crystal data for 2a: C27H59N9Ti, FW 557.70, monoclinic, space
˚
˚
˚
group P21/c, a = 30.991(5) A, b = 10.561(5) A, c = 21.320(5) A, β =
(10) We previously reported a benzyl transfer from metal to
R-diimine ligands: (a) Tsurugi, H.; Ohnishi, R.; Kaneko, H.; Panda, T.
K.; Mashima, K. Organometallics 2009, 28, 680. (b) Mashima, K.;
Ohnishi, R.; Yamagata, T.; Tsurugi, H. Chem. Lett. 2007, 36, 1420.
3
109.550(5)°, V = 6576(4) A , T = 120 K, Z = 8, Dcalcd = 1.127 g cm-3
,
˚
2θmax = 55°, μ = 0.290 mm-1, R1 and wR2 = 0.0906 and 0.2325 (I >
2σ(I)), GOF = 0.989.

36 Organometallics, Vol. 29, No. 1, 2010
Panda et al.
Scheme 3. Reactions of M(NMe2)4 with Dicyclohexylcar-
bodiimide
Figure 2. Molecular structure of complex 6d. Selected bond
˚
distances (A): Zr-N1=2.3952(13), Zr-N2=2.1210(14), Zr-
N3=2.0958(14), Zr-N4 = 2.0742(15), Zr-N5= 2.0641(16),
N1-C1 = 1.314(2), C1-C2 = 1.508(2), N2-C2 = 1.446(2),
N6-C1=1.344(2).
The solid-state structures of complexes 6d and 7d were
determined by X-ray analyses. The molecular structure of 6d
is shown in Figure 2,12 (see the Supporting Information for
the structure of 7d). Due to the similar ionic radii of
zirconium and hafnium, compounds 6d and 7d are isostruc-
tural with respect to each other. In the coordination sphere,
the central metal is coordinated by two nitrogen atoms (one
imino and one amido), N1 and N2, of the modified R-diimine
ligand and three metal-bound amido nitrogen atoms:
N3-N5. Dissymmetric ligation is revealed by the bond
bonds of carbodiimides proceeded in the same manner as
previously reported by Arnold et al. for the reaction of
carbodiimides with Cl2Ti(NMe2)2.3a The mono(guani-
dinate) complex 3b was too stable, even when heated in a
toluene solution at 90 °C in the presence of excess 1a,b. These
findings suggest that the coordination of two guanidine
ligands to the titanium atom might be key to activating the
aliphatic C-H bond of NMe2. In contrast to the reaction
of 1b with Ti(NMe2)4, we found that Zr(NMe2)4 and
Hf(NMe2)4 reacted with 1b at 90 °C to give bis(guani-
dinate) complexes 4b and 5b,8 respectively, without any
mono(guanidinate) and C-H bond-activated products
(Scheme 2). Unlike the titanium case, both complexes were
stable even with prolonged heating at 90 °C for 48 h. Further
reactions of 4b or 5b with carbodiimides leading to the
zirconium and hafnium analogues of 2a were not observed
but decomposed under xylene reflux conditions, suggest-
ing that the bis(guanidinate) complexes of zirconium and
hafnium did not have adequate basicity or bulkiness.
In addition, the reactions of M(NMe2)4 (M = Ti, Zr, Hf)
with R-diimines, as a one-carbon extension analogous to
the carbodiimides, were perfomed. Treatment of 2,6-diiso-
propylphenyl-1,4-diaza-1,3-butadienes (1c) with M(NMe2)4
(M = Zr, Hf) in a 1:1 ratio at 90 °C selectively produced the
corresponding (amido-imino)M(NMe2)3 complexes 6c and
7c, which were products of an amido transfer from the metal
to one of the two CdN bonds of 1c (eq 2).11 Complexes 6d
and 7d were also obtained when M(NMe2)4 species were
treated with 2,6-dimethylphenyl-1,4-diaza-1,3-butadiene
(1d) under similar conditions. The reaction of Ti(NMe2)4
with R-diimine ligands did not proceed, however, presum-
ably due to the small ionic radius of the titanium atom
preventing the approach of R-diimines.
˚
distance of Zr-N(imino) (2.3952(13) A) being significantly
˚
longer than that of Zr-N(amido) (2.1210(14) A) along
˚
with the shorter bond distance (1.314(2) A) of N1-
C1(imino) compared to the bond distance of N2-C2(amido)
(1.446(2) A).
˚
A further insertion reaction of the CdN bond into the
metal-nitrogen bond or a C-H bond activation R to the
nitrogen atom did not take place for tris(amide) complexes 6
and 7. Protonation of one of the three M-N bonds by phenol
derivatives resulted in activation of the C-H bond adjacent
to the nitrogen atom. Treatment of 6c with 2,6-di-tert-
butylphenol in toluene at 90 °C afforded 8c in 79% yield as
yellow crystals (eq 3). In the 1H NMR spectrum of 8c,
a singlet resonance with one proton intensity was observed
at δ 5.07, while two singlet signals observed with six proton
intensity were assignable to the NMe2 group. The disappear-
ance of the signal corresponding to the imine carbon was also
confirmed by the 13C NMR spectrum. The solid-state structure
(Figure 3)13 clarified that 8c has a pentacoordinated zirconium
center with aryloxide and supine-DAD ligands where the
distances of Zr1-C1 and Zr1-C2 are short enough to interact
with the olefinic carbon to the metal center, which is also
confirmed by the folding angle of the ligand (θ = 123.54°).14
(12) Crystal data for 6d: C26H44N6Zr, FW 531.89, orthorhombic, space
˚
˚
˚
group Pna21, a = 13.737(5) A, b = 14.285(5) A, c = 14.705(5) A,
˚
3
V = 2885.6(18) A , T = 120(2) K, Z = 4, Dcalcd = 1.224 g cm-3
,
2θmax = 61°, μ = 0.404 mm-1, R1 and wR2 = 0.0186 and 0.0472 (all),
GOF = 1.010.
(13) Crystal data for 8c: C44H68N4OZr, FW 760.24, triclinic, space
˚
˚
˚
group P1, a = 11.073(5) A, b = 19.721(5) A, c = 20.299(5) A, R =
3
˚
96.394(5)°, β = 92.780(5)°, γ = 94.972(5)°, V = 4381(3) A , T = 120(2)
K, Z = 4, Dcalcd = 1.153 g cm-3, 2θmax = 61°, μ = 0.286 mm-1, R1 and
wR2 = 0.0820 and 0.2047 (all), GOF = 1.074.
(11) (a) Aeilts, S. L.; Coles, M. P.; Swenson, D. C.; Jordan, R. F.;
Young, V. C., Jr. Organometallics 1998, 17, 3265. (b) Duncan, A. P.;
Mullins, S. M.; Arnold, J.; Bergman, R. G. Organometallics 2001, 20, 1808.
(c) Bailey, P. J.; Pace, S. Coord. Chem. Rev. 2001, 214, 91.
(14) (a) Kawaguchi, H.; Yamamoto, Y.; Asaoka, K.; Tatsumi, K.
Organometallics 1998, 17, 4380. (b) Mashima, K.; Matsuo, Y.; Tani, K.
Organometallics 1999, 18, 1471. (c) Tsurugi, H.; Ohno, T.; Kanayama, T.;
Arteaga-M€uller, R. A.; Mashima, K. Organometallics 2009, 28, 1950.

Communication
Organometallics, Vol. 29, No. 1, 2010 37
Scheme 4. Proposed Mechanism for the Formation of 8c
of 1b to the reaction mixture, a new aryloxide zirconium
complex having DAD and guanidinate ligands was isolated
(eq 4). Because the insertion reaction of carbodiimides into
the metal-nitrogen bond of 6d did not proceed, the complex
9d was formed via the formation of (4-tBuC6H4O)-
Zr(DADNMe2)(NMe2) and successive amination of 1b
by Zr-NMe2 (see the Supporting Information for the struc-
ture of 9d). In contrast to the coordination mode of the DAD
ligand for 8c, the DAD ligand of 9d coordinates to the zirco-
nium atom as the prone mode whose electronic contribution
to the metal center is weaker than that of the supine mode
due to coordination of the electron-donating guanidinate
moiety.
Figure 3. Molecular structure of complex 8c. Selected bond
˚
distances (A) and angles (deg): Zr1-O1 = 1.997(2), Zr1-
N1 = 2.045(2), Zr1-N2 = 2.091(2), Zr1-N3 = 2.062(3),
Zr1-C1 = 2.470(3), Zr1-C2 = 2.525(3), N1-C1 = 1.413(3),
N2-C2 = 1.394(4), C1-C2 = 1.384(4), N4-C2 = 1.416(3);
C29-O1-Zr1 = 171.85(19).
The bond distances of N1-C1, C1-C2, and N2-C2 indicate
a long-short-long bonding sequence, revealing a dia-
nionic σ2,π-enediamido canonical form. The very large C29-
O1-Zr1 angle is consistent with the presence of a pπ-dπ
interaction between the phenolic oxygen atom and the zirco-
nium atom.
In summary, we have demonstrated that activation of the
C-H bond R to the nitrogen, leading to azametallacyclo-
propane, and successive insertion into diisopropylcarbodii-
mide afforded a five-membered diazametallacyclopentane,
which is a key intermediate for the aminoalkylation reaction.
To enhance the reactivity of the M-N bond toward C-H
bond activation, increasing the electron density of the metal
center is crucial.
The plausible mechanism for Scheme 4 is, at the first stage,
selective protonation of Zr-NMe2 by 2,6-diisopropylphenol
to give 8c0 and then a C(sp3)-H bond activation R to the
nitrogen atom of the amido-imino ligand accompanied by
transformation of the amido-imino moiety, proceeding to
the σ2-enediamido ligand (Scheme 4). Differential reactivity
of 6c and 8c0 toward activation of the C-H bond might be
due to differences in the electron-donating nature of Zr-
N(amido) and Zr-O(aryloxide), depending on the increased
basicity of the dimethylamino group bound to the zirconium,
which is caused by increases in the electron-donating 2,6-
diisopropylphenoxy ligand, as in the formation of 2a from
bis(guanidinate)Ti(NMe2)2.
Acknowledgment. We acknowledge a JSPS Fellowship
to T.K.P. and K.P. H.T. acknowledges financial support
by a Grant-in-Aid for Young Scientists(B) and The
Sumitomo Foundation. K.M. acknowledges financial
support by a Grant-in-Aid for Scientific Research(A).
Supporting Information Available: Text, tables, figures, and
CIF files giving experimental details for the syntheses of all
compounds and crystallographic data for 2a, 3a, 4b, 6d, 7d, 8c,
and 9d. This material is available free of charge via the Internet
at http://pubs.acs.org.
After mixing of a sterically less hindered zirconium deri-
vative, 6d, and 4-tert-butylphenol, and subsequent addition