Cycloaddition reactions of transition metal hydrazides with alkynes and heteroalkynes: Coupling of Ti=NNPh2 with PhCCMe, PhCCH, MeCN and tBuCP

Article abstract of DOI:10.1039/b813911c

The first structurally authenticated [2+2] cycloaddition products of any transition metal hydrazide complexes are reported; cycloaddition products of transition metal hydrazides with alkynes and heteroalkynes have been obtained for the first time; these are the first structurally authenticated cycloaddition products for any transition metal M=NNR₂ functional group. The Royal Society of Chemistry.

Full text of DOI:10.1039/b813911c

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Cycloaddition reactions of transition metal hydrazides with alkynes
and heteroalkynes: coupling of TiQNNPh2 with PhCCMe, PhCCH,
t
MeCN and BuCPw
Jonathan D. Selby,a Christian Schulten,b Andrew D. Schwarz,a Andreas Stasch,b
Eric Clot,*c Cameron Jones*b and Philip Mountford*a
Received (in Cambridge, UK) 11th August 2008, Accepted 4th September 2008
First published as an Advance Article on the web 23rd September 2008
DOI: 10.1039/b813911c
The first structurally authenticated [2+2] cycloaddition products
of any transition metal hydrazide complexes are reported;
cycloaddition products of transition metal hydrazides with
alkynes and heteroalkynes have been obtained for the first time;
these are the first structurally authenticated cycloaddition
products for any transition metal MQNNR2 functional group.
an alkyne and a hydrazide been presented. Indeed, these
hydrohydrazination reactions typically require forcing
temperatures around ca. 100 1C and long (up to 24 h) reaction
times. Alternative reaction pathways such as those found in
rare earth metal-catalysed hydroaminations16 cannot a priori
be ruled out on the available experimental evidence
(i.e., substrate insertion into a metal–hydrazide(1ꢀ) s bond).
So far, all fully authenticated reactions between isolated,
well-defined Group 4 hydrazides and alkynes (and also
isocyanides, or chalcogenide delivery agents) have invariably
led to Na–Nb bond cleavage products (see Fig. 1 for
examples).9,17–19 Furthermore, no MQNa cycloaddition
product has been structurally authenticated for the reaction
of any transition metal MQNNR2 functional group.
Transition metal hydrazides, (L)MQNNR2, occupy a pivotal
position on the pathway for the biological and laboratory-based
conversion of N2 to ammonia.1–4 From the point of view of the
productive incorporation of N2 into value-added products, an
understanding of the chemistry of terminal metal hydrazides is
clearly essential. Over the past two decades a large effort has
been expended on charting the chemistry of the Group 6 metal
MQNNR2 functional groups because of their immediate
relevance to biological systems.4–6 In all of this chemistry the
MQNNR2 group reactivity is characterised exclusively by
transformations involving the Nb atom and/or Na–Nb bond
reductive cleavage.
Over the last 10 years, landmark achievements have also been
made in the activation of N2 by Group 4 compounds.5–8
However, unlike for the Group 6 systems, the reactions of Group
4 MQNNR2 bonds with unsaturated substrates remain very
poorly understood.9 This situation contrasts dramatically with
the well-established area of imido compounds, (L)MQNR (R =
alkyl, aryl): reactions of the MQNR bond are highly developed
and lead to a range of valuable new NR-containing products, for
example through the hydroamination of alkynes and allenes.10–13
Group 4 hydrazides have nonetheless been implicated in
N–C bond forming processes.13–15 Thus reactions of hydra-
zines and alkynes in the presence of supposed hydrazide
precursor complexes give hydrazone products, apparently
consistent with [2+2] cycloaddition reactions of in situ
generated hydrazide intermediates. However, no discrete
TiQNNR2 species have ever been observed for these systems,
nor has direct evidence of the putative [2+2] cycloaddition of
Fig. 1 Products of Group 4 MQNNPh2 bonds with alkynes (I, II)
and tBuNC (III): facile Na–Nb cleavage rather than MQNa bond
coupling.
We recently reported general routes to a range of new
titanium hydrazide complexes.18 This has opened up the
opportunity to tune and direct the reactions of the TiQNNR2
functional group though judicious ligand selection.
Reaction of the terminal hydrazide Ti(N2Npy)(NNPh2)-
(py)20 (1, Scheme 1) with PhCCMe in C6D6 gave an
equilibrium mixture containing the cycloaddition product
Ti(N2Npy){N(NPh2)C(Me)CPh} (2).w Removal of the volatiles
and redissolving in C6D6 gave pure 2 as judged by NMR
spectroscopy. The structure shown in Scheme 1 is fully
1
compatible with the 1- and 2-D H, 13C and ROESY spectra,
and other data. In particular, the PhCQCMe carbons
appeared at 216.1 and 146.3 ppm in typical positions for such
metallacycles.21,22 Addition of an excess of pyridine to pure
2 reformed 1 and free PhCCMe, confirming the reversibility of
the cycloaddition process. Note that reaction of the closely
related compound Ti(N2NMe)(NNPh2)(py) with the
same alkyne gave only the Na–Nb insertion product
a Chemistry Research Laboratory, University of Oxford, Mansfield Road,
Oxford, UK OX1 3TA. E-mail: philip.mountford@chem.ox.ac.uk
b School of Chemistry, Monash University, Melbourne, PO Box 23,
Victoria, 3800, Australia. E-mail: Cameron.Jones@sci.monash.edu.au
c Institut Charles Gerhardt Montpellier, UMR 5253 CNRS-UM2-
`
ENSCM-UM1cc 1501, Place Eugene Bataillon, F-34095 Montpellier
Cedex 5, France. E-mail: clot@univ-montp2.fr
w Electronic supplementary information (ESI) available: Characteris-
ing and crystallographic data, and computational details. CCDC
698353–698355. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b813911c
Ti(N2NMe){NC(Ph)C(Me)NPh2}(py) (II, Fig. 1; N2NMe
Q
MeN(CH2CH2NSiMe3)2). No TiQNa cycloaddition products
were seen in the reactions for II.
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Fig.
2 )
Displacement ellipsoid plot (20%) of Ti2(N2Npy
2
{m-N(NPh2)C(Me)N}2 (5). Atoms carrying the suffix ‘A’ are related
to their counterparts by the symmetry operator ꢀx + 2/3, ꢀy + 1/3,
ꢀz + 1/3: Ti(1)–N(1) 2.3084(13), Ti(1)–N(6) 2.1687(16), Ti(1)–N(6A)
1.8684(14), N(1)–C(28) 1.318(2), N(6)–C(28) 1.334(2), N(1)–N(2)
1.4137(18) A; H atoms, solvent and minor disorder omitted.
adopting a k2(N,N) coordination mode. The addition of
nitriles to transition metal–nitrogen multiple bonds is
extremely rare, even in the extensively studied area of imido
chemistry,10 and has not been seen previously in transition
metal hydrazide chemistry.
Scheme 1 Reactions of Ti(N2Npy)(NNPh2)(py) with alkynes and
MeCN.
The successful use of a heteroalkyne in stabilizing 5 prompted
us to explore reactions of 1 with phosphaalkynes which have
known similarities with alkynes.24 Titanium imido compounds
form a range of different products with these substrates.10,25,26
Unfortunately, in the case of 1 no reaction took place with
tBuCP.
Although a cycloaddition species is the kinetic product for 1
and PhCCMe, over time (3 days at RT) or upon briefly heating
(15 min at 100 1C) new products are formed from which the
Na–Nb insertion product Ti(N2Npy){NC(Ph)C(Me)NPh2}(py)
(3, Scheme 1) was obtained23 (the PhCQCMe carbons appear
at 158.2 and 112.3 ppm, these shifts being very similar to those
for II).
ð1Þ
Reaction of 1 with the sterically less demanding PhCCH
gave quantitative conversion to the anti-Markovnikov type
cycloaddition product Ti(N2Npy){N(NPh2)C(H)CPh} (4)
in ca. 60% yield on the preparative scale (100% conversion
by 1H NMR). Addition of pyridine to pure 4 reformed 1 along
with PhCCH. The NMR data for 4 are analogous to those
of 2 and also the structurally authenticated imido cyclo-
Mindful of previous reports of the reactions of imidozircon-
t
t
ocenes ‘‘Cp2Zr(NR)’’ (R = Bu or aryl) with BuCP25 we
carried out the reaction of the very recently reported
t
Cp2Ti(NNPh2)(py) (6, eqn (1)) with BuCP. Brown crystals of
t
addition products Ti(N2Npy){N(R)C(H)CPh} (R = Bu or
i
2,6-C6H3 Pr2).22 Like 2, compound 4 is unstable in solution
Cp2Ti{N(NPh2)PCtBu} (7) were isolated in 70% yield after 30 h
at RT. The molecular structurez of 7 is shown in Fig. 3
confirming it as a monomeric [2+2] cycloaddition product.
Compound 7 is the first report of a MQNNR2 species reacting
with a phosphaalkyne. An excess of pyridine does not displace
the tBuCP from 7. The reactions of 6 with alkynes, nitriles and
other substrates are presently under investigation.
(t1/2 ca. 4 h at 22 1C) but no single product could be isolated.
The high solubility of 2 and 4 and their instability prevented
us from obtaining diffraction-quality crystals. However,
sunequivocal structural evidence for a [2+2] cycloaddition
reaction of the TiQNNPh2 group came from adding
MeCN (isoelectronic with RCCR0) to a solution of 1
in benzene. This formed the ‘‘self-trapped’’ dimer
Ti2(N2Npy)2{m-N(NPh2)C(Me)N}2 (5, Scheme 1). The X-ray
The N(1)–P(1) and C(1)–P(1) distances within the metalla-
cyclic core of 7 are comparable to those in imido-based
(L)M{N(R)PCR} units. However, the 31P shift of ꢀ28.9 ppm
for 7 is rather upfield compared to the Cp2Zr(NR)-derived
examples (ca. 60–80 ppm)25 and others formed from
(L)TiQNR compounds (ca. 200 to 210 ppm).25,26
The orientation of the [2+2] cycloaddition process in 7
appears to be favoured on steric grounds and is analogous to
that found previously for imido-derived examples,
(L)M{N(R)PCR}. However, the orientation of the less
sterically demanding MeCN in 5-int (and 5) is the opposite of
that in 7 (Ti–heteroatom formation in 5-int vs. Ti–C formation
structure of
5
(Fig. 2)z is consistent with the likely
intermediate Ti(N2Npy){N(NPh2)C(Me)N} (5-int) posses-
sing the stereochemistry indicated in Scheme 1. The
structural data for 5 (Fig. 2) are consistent with the drawing
in Scheme 1.
The MeCN-derived nitrogen in 5-int (ultimately becoming
C(6) and C(6A) in Fig. 2) is presumably highly nucleophilic
and so leads to the ‘‘self-trapped’’ final product 5 which is
stable in solution due to the Ti2{m-N(NPh2)C(Me)N}2 core.
The N2Npy ligands in 5 respond to the steric crowding by
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hydrazides. Compounds 5 and 7 are the first structurally
authenticated cycloaddition products for any metal hydrazide.
The reactions of 1 with PhCCMe show that the [2+2] cyclo-
addition reactions to hydrazides can be reversible and that
systems capable of forming azametallacycles like 2 (the
proposed intermediates in hydroamination catalysis) are also
capable of Na–Nb insertion chemistry (formation of 3). This
has significant implications for the design and rationalisation
of hydroamination and related catalyst systems using hydra-
zines. The DFT calculations for 7 and its analogues show that
alternative coupling modes of MQNNR2 with unsaturated
substrates should be accessible through tuning of substrate
and supporting ligand set. Finally, these results will be of
benefit in developing Group 4 based N2 functionalisation
chemistry via hydrazide intermediates.
Fig. 3 Displacement ellipsoid plot (20%) of Cp2Ti{N(NPh2)PCtBu}
(7): Ti(1)–N(1) 1.9770(17), Ti(1)–C(1) 2.115(2), N(1)–P(1) 1.7329(18),
C(1)–P(1) 1.677(2), N(1)–N(2) 1.400(2) A; N(1)–P(1)–C(1) 98.93(9),
N(1)–Ti(1)–C(1) 78.51(7)1; H atoms omitted.
We thank the EPSRC (J. D. S., A. D. S.), CNRS (E. C.) and
the donors of The American Chemical Society Petroleum
Research Fund (C. J.) for support.
in 7). We have calculated the two alternative regioisomers of 7
using DFT (B3PW91), namely 7-Q and 7-alt-Q (see Fig. 4).w
According to DFT, 7-alt-Q (with a Ti–P bond) is more stable
(but only marginally, by ca. 6 kJ molꢀ1) than the experimentally
observed one in terms of electronic energies.27 The calculated
31P shifts of 7-Q and 7-alt-Q are ꢀ47.8 and +319 ppm. This
supports the suggestion that 7-Q represents the experimental
solution and solid-state species.28
Notes and references
z Crystal data for 3, 5 and 7 are provided in CIF format in the ESI.w
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10 N. Hazari and P. Mountford, Acc. Chem. Res., 2005, 38, 839.
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Chem., Int. Ed., 2007, 46, 8426.
Fig. 4 Schematic representation of the two TS and product electronic
energies (B3PW91, kJ molꢀ1
)
for the reaction of base-free
Cp2Ti(NNPh2) with tBuCP. Further details of the geometries are give
20 J. D. Selby, C. D. Manley, A. D. Schwarz, E. Clot and P.
Mountford, Organometallics, DOI: 10.1021/om8007597, accepted
for publication.
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Chem. Commun., 2004, 704.
23 Compound 3 has been structurally characterised: see the ESIw.
24 K. B. Dillon, F. Mathey and J. F. Nixon, Phosphorus: The Carbon
Copy: From Organophosphorus to Phospha-organic Chemistry,
Wiley, London, 1998.
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Mountford, Chem. Commun., 1999, 661.
26 S. M. Pugh, D. J. M. Trosch, D. J. Wilson, A. Bashall, F. G. N.
¨
in the ESI.w
Although Fig. 4 shows that formation of 7-alt-Q is thermo-
dynamically competitive with 7-Q, the transition state (TS)
energies predict that the experimentally observed species
(modeled by 7Q) is certainly kinetically favoured (DDEz =
11.4 kJ molꢀ1 in favour of forming 7-Q). Further calculations
using the sterically less demanding phosphaalkyne MeCP gave
DrE values of ꢀ97.7 kJ molꢀ1 for the Ti–C bound isomer
Cp2Ti{N(NPh2)PCMe} (8-Q) but ꢀ134.1 kJ molꢀ1 for the
Ti–P bound alternative Cp2Ti{N(NPh2)C(Me)P} (8-alt-Q).
This confirms that the Ti–P/N–C orientated [2+2] cyclo-
addition process is the electronically preferred one.
Cloke, L. H. Gade, P. B. Hitchcock, M. McPartlin, J. F. Nixon
and P. Mountford, Inorg. Chem., 2000, 19, 3205.
27 Given their similarity, the ground state energies of 7-Q and 7-alt-Q
could be inverted by solvent effects.
In conclusion, we have reported the first [2+2] cyclo-
addition reactions of transition metal hydrazides with internal
and terminal alkynes, and also aza- and phosphaalkynes.
These reactions demonstrate the potential breadth of substrate
28 The observed and calculated 31P shifts for 7 are more upfield than
expected. At first sight this could be attributed to the NNPh2 in 7.
However, replacing NNPh2 by NPh in 7-Q had little effect on the
31P shift (ꢀ53.1 ppm). Work is underway to rationalise these
differences.
functionalisation chemistry available using Group
4
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