New ligand platforms for developing the chemistry of the Ti=N-NR 2 functional group and the insertion of alkynes into the N-N bond of a Ti=N-NPh2 ligand

Article abstract of DOI:10.1039/b711941k

Two broadly applicable strategies for extending the available ligand platforms of the virtually unexplored terminal Ti=N-NR₂ functional group are described, along with the highly selective room temperature insertion of alkynes into the N-N bond of Ti{MeN(CH₂CH₂NSiMe ₃)₂}(NNPh₂)(py) and the catalytic cis-diamination of PhC≡CMe by diphenylhydrazine. The Royal Society of Chemistry.

Full text of DOI:10.1039/b711941k

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New ligand platforms for developing the chemistry of the TiLN–NR2
functional group and the insertion of alkynes into the N–N bond of a
TiLN–NPh2 ligand{
Jonathan D. Selby,a Catherine D. Manley,a Marta Feliz,b Andrew D. Schwarz,a Eric Clot*b and
Philip Mountford*a
Received (in Cambridge, UK) 3rd August 2007, Accepted 7th September 2007
First published as an Advance Article on the web 21st September 2007
DOI: 10.1039/b711941k
or sodium salts gave the new hydrazido compounds 2–4 with a
Two broadly applicable strategies for extending the available
ligand platforms of the virtually unexplored terminal TiLN–
NR2 functional group are described, along with the highly
selective room temperature insertion of alkynes into the N–N
bond of Ti{MeN(CH2CH2NSiMe3)2}(NNPh2)(py) and the
catalytic cis-diamination of PhCMCMe by diphenylhydrazine.
variety of N3, N4 and O4 donor ligand sets (Scheme 1).
Transition metal hydrazido compounds (L)MLNNR2 (L =
supporting ligand set) continue to be of considerable interest,
especially with regard to the biological and synthetic fixation of
molecular nitrogen, which proceeds via such species.1 Despite the
prevalence of terminal hydrazides for the mid transition metals,
there is a paucity of such compounds for Group 4. The chemistry
of the MLN–NR2 functional group is therefore considerably
underdeveloped for these metals, not least when compared to the
current situation for the related imido compounds (L)MLN–R
(R = hydrocarbyl).2–6 Titanium terminal hydrazides have, none-
theless, been implicated in the catalytic hydrohydrazination7–13 and
iminohydrazination13,14 of alkynes, and in the synthesis of
indoles4,9,12 and tryptamines.10 Recent DFT calculations have
shown that substitution of the organic R-substituent of an imide
TiLN–R by NR2 can destabilize one of the TiLN p-bond
components by ca. 130 kJ mol21 due to an antibonding interaction
between one of the TiLNa p-bond pairs and the lone pair of the
b-nitrogen atom.15 This interaction also weakens the Na–Nb bond
itself. One could anticipate some interesting differences between
the chemistry of the TiLNR and TiLN–NR2 functional groups. To
date, there are few structurally authenticated examples of the
terminal TiLN–NR2 ligand,11,14–16 and even fewer examples of its
stoichiometric chemistry.17,18
Scheme 1 Terminal titanium hydrazido complexes with N- and O-donor
dianionic ligands. Reagents: (i) Li2[MeN(CH2CH2NSiMe3)2]; (ii) Li2[(2-
C5H4N)CH2N(CH2CH2NSiMe3)2]; (iii) Na2[Bn2calix].
Fig. 1 shows the X-ray structure of Ti{MeN(CH2CH2-
NSiMe3)2}(NNPh2)(py) (2), and confirms the monomeric nature
of this compound.{ The hydrazido b-nitrogen (N(2)) is planar due
to conjugation with one of the phenyl substituents. The only other
structurally-characterised compound with a terminal TiLNNPh2
moiety is Ti{HC(Me2pz)3}(NNPh2)Cl2,15 which has TiLNa
=
1.718(2) and Na–Nb = 1.369(3) s vs. values of 1.733(5) and
1.359(7) s, respectively, in 2.
We have found that simple salt-elimination/metathesis reactions
of the recently reported [Ti(NNPh2)Cl2(py)2]2 (1)15 provide a
potentially highly versatile route to new hydrazido compounds
(Scheme 1). Chelating diamide-amine5,19 and calix[4]arene (calix)
ligands20 have been highly successful for developing Group 4
imido chemistry, and so we have focused on these as proof of
concept for using 1. The reaction of 1 with the appropriate lithium
aChemistry Research Laboratory, University of Oxford, Mansfield
Road, Oxford, UK OX1 3TA
Fig. 1 Displacement ellipsoid plot (20%) of Ti{MeN(CH2CH2-
NSiMe3)2}(NNPh2)(py) (2). Selected distances (s) and angles (u): Ti(1)–
N(1) 1.733(5), N(1)–N(2) 1.359(7); Ti(1)–N(1)–N(2) 177.7(4).
bInstitut Charles Gerhardt, Universite´ Montpellier 2 and CNRS, cc
1501, Place Euge`ne Bataillon, 34095 Montpellier cedex 5, France
{ Electronic supplementary information (ESI) available: Characterising
and crystallographic data, and computational details are available. See
DOI: 10.1039/b711941k
Species with terminal TiLNNR2 groups have been proposed as
intermediates in the catalytic hydrohydrazination of alkynes.8,11,12
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The treatment of 7 with an equivalent of Ph2NNH2 (NMR) at
room temperature quantitatively regenerated 2, along with the
expected side-product PhC(NH2)C(Me)NPh2 (8), which was sub-
sequently isolated and fully characterised.{ Remarkably, 2 acts as a
catalyst at room temperature for the overall 1,2-amination reaction.
Thus, a mixture of PhCMCMe (slight excess) and Ph2NNH2 in the
presenceof 2(10mol%)gavequantitativeconversionto8asthe only
organic product (tK ca. 1 h by 1H NMR spectroscopy).
This appears to be a new transformation of hydrazines and
alkynes. Compounds of type 8 are potentially useful precursors to
1,2-diaminoalkanes by hydrogenation. The mechanism of forma-
tion of 7 is under investigation, as is the scope of this unusual
reaction. It is likely that one or more of the other compounds
reported herein could also catalyse this unique reaction.
Furthermore, since this process proceeds at room temperature, it
is likely that previously reported ‘‘classical’’ hydrohydrazination
systems could also undergo this type of transformation in as-yet
unrecognised side reactions.
Scheme 2 Reactions of Ti{MeN(CH2CH2NSiMe3)2}(NNPh2)(py) (2).
We have started to examine the reactions of the new compounds
herein with these and other substrates. We report here our
preliminary findings for 2 (Scheme 2).
Cyclopentadienyl compounds have also played a pivotal role in
the development of metal–ligand multiple bonds.5 We report here
hydrazine/tert-butyl imide exchange reactions17 as a second and
complimentary entry route to hitherto unexplored classes of
hydrazide compound (Scheme 3).
Imido complexes of a diamide-amine ligand, similar to that in 2,
form [2 + 2] cycloaddition products with certain alkynes
(representing key intermediates in the alkyne hydroamination
reaction) and a wide range of other unsaturated substrates.19,21
With the expectation of a similar reactivity (products of the type 5,
Scheme 2), we carried out the reaction of 2 with four different
alkynes, RCMCR9 (R = R9 = Me or Ph; R = Ph, R9 = H; R = Ph,
R9 = Me). With PhCMCH and PhCMCPh, mixtures were formed,
but with MeCMCMe and PhCMCMe, single new products (6 and 7,
respectively) were obtained. The spectroscopic data for the two
compounds are analogous, and in the case of 7, the X-ray structure
was determined (Fig. 2).{ The solid state structure of 7 is fully
consistent with its spectroscopic and other analytical data, and
clearly shows that in this case, cycloaddition product 5 (Scheme 2)
is not the reaction outcome, and instead, a regiospecific net
insertion of the alkyne into the hydrazide Na–Nb bond has
occurred, forming a vinyl imido species. Titanium vinyl imido
compounds were previously available only via reactions of
alkylidene transients and organic nitriles.22,23
Scheme 3 Sandwich and half-sandwich terminal titanium hydrazides.
Reagents: (i) R2NNH2 (R = Ph or Me); (ii) and (iii) Ph2NNH2.
The cyclopentadienyl-amidinate ligand platform has provided a
rich source of TiLNR bond reactivity in imido systems.5 The
reaction of Cp*Ti{MeC(NiPr)2}(NtBu)24 with either Ph2NNH2
or Me2NNH2 gave quantitative (NMR) conversion to
Cp*Ti{MeC(NiPr)2}(NNR2) (R = Ph (9) or Me (10)) in 50–60%
isolated yield.{ The X-ray structure{ of 10 is shown in Fig. 3,
confirming its monomeric nature. 10 is the first structurally
authenticated cyclopentadienyl titanium compound featuring a
terminal hydrazide ligand.
The addition of Ph2NNH2 to Cp*Ti(NtBu)Cl(py) gave a clean
conversion to monomeric Cp*Ti(NNPh2)Cl(py) (11) in 83%
isolated yield. The remaining Ti–Cl bond of 11 could be further
substituted (vide infra), meaning this compound will doubtless
have further applications as a synthon in its own right for
developing the TiLNNR2 ligand platform.
Fig. 2 Displacement ellipsoid plot (20%) of Ti{MeN(CH2CH2-
NSiMe3)2}{NC(Ph)C(Me)NPh2}(py) (7). Selected distances (s): Ti(1)–
N(1) 1.757(3) [1.750(3)], N(1)–C(1) 1.375(4) [1.371(4)], C(1)–C(2) 1.349(5)
[1.356(5)], C(2)–N(2) 1.442(4) [1.448(5)]. Values in brackets are for the
other crystallographically independent molecule.
Reaction of Cp2Ti(NtBu)(py)25 with Ph2NNH2 gave the
corresponding
pyridine-stabilised
titanocene
hydrazide,
4938 | Chem. Commun., 2007, 4937–4939
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as hydrohydrazination precatalysts at elevated temperatures.7,12
The formation of 13 at ambient temperature suggests that these
catalytic systems, in fact, also proceed through half-sandwich
compounds. Studies of the competence of the new cyclopentadie-
nyl-hydrazido compounds in catalytic hydrohydrazination reac-
tions are under way.
We thank the EPSRC (J. D. S., A. D. S.), MESR and Spanish
Ministerio de Educacio´n y Ciencia (M. F.) for support, and
Professor A. L. Odom for helpful discussions.
Notes and references
Fig. 3 Displacement ellipsoid plot (20%) of Cp*Ti{MeC(NiPr2)2}-
(NNMe2) (10). Selected distances (s) and angles (u): Ti(1)–Cpcentroid
2.063, Ti(1)–N(1) 1.723(2), N(1)–N(2) 1.386(2); Ti(1)–N(1)–N(2) 159.5(1).
Atoms carrying the suffix ‘A’ are related to their counterparts by the
symmetry operator [x, y, 2 z].
{ Crystal data for 2: C28H44N6Si2Ti, Mw = 568.77, triclinic, P-1, a =
10.1794(5), b = 11.1800(7), c = 13.8952(8) s, a = 89.726(3), b = 77.178(2),
c = 85.912(3)u, U = 1537.9(2) s3, Z = 2, T = 150 K, m = 0.383 mm21, 3146
reflections I . 3s(I), Rint = 0.055, R = 0.073, wR = 0.080. CCDC 656394.
Crystal data for 7: C37H52N6Si2Ti, Mw = 684.93, monoclinic, P21/n, a =
10.19100(10), b = 19.6879(2), c = 38.0089(4) s, b = 92.0565(4)u, U =
7621.17(13) s3, Z = 8, T = 150 K, m = 0.321 mm21, 6837 reflections I .
3s(I), Rint = 0.087, R = 0. 0397, wR = 0.0443. CCDC 656395.
Crystal data for 10: C20H38N4Ti, Mw = 382.45, orthorhombic, Pbnm
(non-standard setting of Pnma), a = 9.3311(2), b = 14.7945(4), c =
16.4429(4) s, U = 2269.93(10) s3, Z = 4, T = 150 K, m = 0.386 mm21, 2192
reflections I . 3s(I), Rint = 0.016, R = 0.038 wR = 0.0459. CCDC 656396.
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b711941k
Cp2Ti(NNPh2)(py) (12, Scheme 3), in 62% isolated yield. A
zirconium analogue of 12 has been reported by Bergman and
shows interesting N–C bond coupling reactions with unsaturated
substrates.26,27 We have not yet obtained diffraction-quality
crystals of 12, but the DFT-computed structure{ (Fig. 4) is in
agreement both with Bergman’s interesting zirconocene system
and all other available data. The Ti–Cpcentroid (2.183 and 2.168 s)
and TiLNa (1.743 s) distances in 12 are comparatively long.
For comparison, the DFT-computed model of 9 (namely
CpTi{MeC(NMe)2}(NNPh2) (99)) is also shown in Fig. 4 (Ti–
Cpcentroid = 2.065, Ti–Na = 1.714 s). The much longer Ti–Cpcent
and TiLNa distances in 12 can be attributed to the formal
20 valence electron count of this compound, in which the p-donor
Cp and NNPh2 ligands compete for the same Ti 3dp acceptor
orbitals.5 This, in turn, is expected to give rise to further interesting
reactivity patterns of the TiLNNR2 functional group. Reactivity
studies of 12 are currently under way.
1 R. R. Schrock, Acc. Chem. Res., 2005, 38, 955.
2 D. E. Wigley, Prog. Inorg. Chem., 1994, 42, 239.
3 A. P. Duncan and R. G. Bergman, Chem. Rec., 2002, 2, 431.
4 A. L. Odom, Dalton Trans., 2005, 225.
5 N. Hazari and P. Mountford, Acc. Chem. Res., 2005, 38, 839.
6 P. D. Bolton and P. Mountford, Adv. Synth. Catal., 2005, 347, 355.
7 J. S. Johnson and R. G. Bergman, J. Am. Chem. Soc., 2001, 123, 2923.
8 C. Cao, Y. Shi and A. L. Odom, Org. Lett., 2002, 4, 2853.
9 L. Ackermann and R. Born, Tetrahedron Lett., 2004, 45, 9541.
10 V. Khedkar, A. Tillack, M. Michalik and M. Beller, Tetrahedron Lett.,
2004, 45, 3123.
11 Y. Li, Y. Shi and A. L. Odom, J. Am. Chem. Soc., 2004, 126, 1794.
12 A. Tillack, H. Jiao, I. Garcia Castro, C. G. Hartung and M. Beller,
Chem.–Eur. J., 2004, 10, 2410.
13 S. Banerjee, Y. Shi, C. Cao and A. L. Odom, J. Organomet. Chem.,
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14 S. Banerjee and A. L. Odom, Organometallics, 2006, 25, 3099.
15 T. B. Parsons, N. Hazari, A. R. Cowley, J. C. Green and P. Mountford,
Inorg. Chem., 2005, 8442.
16 S. Patel, Y. Li and A. L. Odom, Inorg. Chem., 2007, 46, 6373.
17 A. J. Blake, J. M. McInnes, P. Mountford, G. I. Nikonov, D. Swallow
and D. J. Watkin, J. Chem. Soc., Dalton Trans., 1999, 379.
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19 L. H. Gade and P. Mountford, Coord. Chem. Rev., 2001, 216–217, 65.
20 S. R. Dubberley, A. Friedrich, D. A. Willman, P. Mountford and
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21 B. D. Ward, A. Maisse-Francois, P. Mountford and L. H. Gade, Chem.
Commun., 2004, 704.
22 K. M. Doxsee, J. B. Farahi and H. Hope, J. Am. Chem. Soc., 1991, 113,
8889.
Fig. 4 DFT (B3PW91)-calculated structures. Left: Cp2Ti(NNPh2)(py)
(12): Ti–Na 1.743, Ti–Cpcent 2.183 and 2.168 s. Right: CpTi{MeC-
(NMe)2}(NNPh2) (99): Ti–Na 1.714 and Ti–Cpcentroid 2.065 s.
23 K. M. Doxsee, J. K. M. Mouser and J. B. Farahi, Synlett, 1992, 13.
24 A. E. Guiducci, C. L. Boyd and P. Mountford, Organometallics, 2006,
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25 S. C. Dunn, P. Mountford and D. A. Robson, J. Chem. Soc., Dalton
Trans., 1997, 293.
26 Cp2Zr(NNPh2)(DMAP) reacts with alkynes and CO to give products in
which the Na–Nb bond is cleaved but products analogous to 6–8 are not
obtained: P. J. Walsh, M. J. Carney and R. G. Bergman, J. Am. Chem.
Soc., 1991, 113, 6343.
27 Compound 12 is an analogue of Cp2TiLNN(SiMe3)2, prepared from
Cp2TiCl2 and thermally sensitive (decomp. . 235 uC) N2(SiMe3)2.
There have been no subsequent reactivity or structural reports, and the
inconvenient nature of N2(SiMe3)2 has limited the development of this
system. N. Wiberg, H.-W. Haring, G. Huttner and P. Friedrich, Chem.
Ber., 1978, 111, 2708.
While readily isolated, 12 is sensitive to the presence of excess
hydrazine, which results in its slow degradation at room
temperature, tentatively attributed to loss of one of the Cp
ligands. Furthermore, reaction of the mixed-ring imide
Cp*CpTi(NtBu)(py) with Ph2NNH2 formed
a mixture of
products, among which was identified Cp*Ti(NNPh2)-
(NHNPh2)(py) (13). Compound 13 (prepared independently from
11 in 45% yield) is the hydrazido analogue of Bergman’s
CpTi(NAr)(NHAr)(OPMe3),7 a model for the active species
formed in the Cp2TiMe2-catalysed hydroamination of alkynes
and allenes. Several bis(cyclopentadienyl)titanium compounds act
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Chem. Commun., 2007, 4937–4939 | 4939