Reactions of titanium hydrazides with silanes and boranes: N-N bond cleavage and N atom functionalization

Article abstract of DOI:10.1021/jacs.5b06627

Reaction of Ti(N₂^iPrN)(NNPh₂)(py) with Ph(R)SiH₂ (R = H, Ph) or 9-BBN gave reductive cleavage of the N_α-N_β bond and formation of new silyl- or boryl-amido ligands. The corresponding reactions of CpTi{MeC(NⁱPr)₂}(NNR₂) (R = Me or Ph) with HBPin or 9-BBN gave borylhydrazido-hydride or borylimido products, respectively. N_α and N_β atom transfer and dehydrogenative coupling reactions are also reported.

Full text of DOI:10.1021/jacs.5b06627

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Reactions of titanium hydrazides with silanes and boranes:
N–N bond cleavage and N atom functionalization
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Laura C Stevenson,† Simona Mellino,† Eric Clot*,‡ and Philip Mountford*,†
† Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road, Oxford OX1 3TA, U.K.
‡
Institut Charles Gerhardt Montpellier, CNRS 5253, Université Montpellier, cc 1501, Place Eugène Bataillon, Fꢀ34095 Montpelꢀ
lier Cedex 5, France.
Supporting Information Placeholder
ABSTRACT: Reaction of Ti(N2iPrN)(NNPh2)(py) with
this contribution we describe the reactions of this compound and a
halfꢀsandwich counterpart with selected silanes and boranes,
leading to the reductive cleavage of the Nα–Nβ bond and
formation of silylꢀ and borylꢀamido and imido species, along with
Nα and Nβ atom transfer and dehydrogenative coupling reactions.
Scheme 1. Reactions of Ti(N2iPrN)(NNPh2)(py) (1) with
phenyl silanes and 9-BBN. Major isomer of 5 and 6 shown.
Ph(R)SiH2 (R = H, Ph) or 9ꢀBBN gave reductive cleavage of the
Nα–Nβ bond and formation of new silylꢀ or borylꢀamido ligands.
The corresponding reactions of Cp*Ti{MeC(NiPr)2}(NNR2) (R =
Me or Ph) with HBPin or 9ꢀBBN gave borylhydrazidoꢀhydride or
borylimido products, respectively. Nα and Nβ atom transfer and
dehydrogenative coupling reactions are also reported.
It has been shown over the last 5ꢀ8 years in particular (heralded
by a preliminary communication in 19911) that Group 4
hydrazides of the type (L)M=NNR2 (R = alkyl, aryl; L = ancillary
ligand set) can undergo a variety of addition or insertion reactions
of the Ti=Nα multiple bond with unsaturated substrates.1ꢀ2 While
some aspects of the chemistry are reminiscent of that of the
related and betterꢀestablished Group 4 imides (L)M=NR,3
a
distinctive aspect of Group 4 hydrazido (and of the related
alkoxyimides (L)Ti=NOR4) is the typically facile reductive
cleavage of the Nα–Nβ bond that can also occur with oxidizable
substrates such as CO,1 isocyanides2d, 2l and alkynes to form new
Nꢀelement functional groups and/or organic products.2g, 2i, j
Only one example of the reaction of Group 4 dialkylhydrazides
with the Si–H bonds of silanes has been reported to date, namely
the reversible 1,2ꢀaddition (without Nα–Nβ bond cleavage) to the
Ti=Nα multiple bond of Cp*Ti{MeC(NiPr)2}(NNMe2).5 No
reactions of hydrazido compounds with primary or secondary
boranes are known (boranes without B–H bonds form Lewis
adducts with the Ti=Nα atom of certain hydrazides2l). There is
precedent for Si–H addition to metalꢀheteroatom multiple bonds
in general,3, 6 but only one (very recently) structurally authenticatꢀ
ed example of the 1,2ꢀaddition of a B–H bond to a transition metal
imide.7
Reaction of Ti(N2iPrN)(NNPh2)(py) (1) with PhSiH3 at RT (room
temperature)
gave
quantitative
conversion
to
Ti(N2iPrN){N(H)SiH2Ph}(NPh2) (2) which exists as two isomers,
denoted trans-2 and cis-2, in a 40:60% ratio (Scheme 1). When
1
o
followed by H NMR spectroscopy in tolueneꢀd8 from ꢀ78 C to
o
RT the reaction started to occur at ca. ꢀ10 C, initially forming
only trans-2, establishing this as the kinetic product. On warming
o
Of further relevance to the work described herein are the E–H
bond activation, and in some cases subsequent N–N bond cleavꢀ
age reactions, of early transition metalꢀbound N2 with Si–H8 or B–
H9 bonds of silanes and boranes. These proceed via bimetallic
intermediates in all instances. The subsequent N–N bond breaking
reactions (forming silylꢀ or borylꢀimido ligands) are sometimes
accompanied by ancillary ligand degradation (especially in the
case of the Group 5 borane reaction products) and are typically
assisted (in Group 4) by addition of a reducing reagent such as
CO.
to ca. 10 C the cisꢀisomer started to appear, and after several
hours at RT the thermodynamic equilibrium was established.
Similar isomerizations have been noted elsewhere with this type
of supporting ligand.2l Reaction with Ph2SiH2 gave an analogous
product, namely Ti(N2iPrN){N(H)SiHPh2}(NPh2) (3), which
likewise exists as mixture of trans and cis isomers (50:50% ratio).
The solid state structure of trans-3 is shown in Fig. 1 and reveals
the complete cleavage of the Nα–Nβ bond of 1 forming the new
NPh2 and N(H)SiHPh2 ligands (each having an N atom in a
formal –3 oxidation state compared to –2 in 1), positioned cis and
trans, respectively, to the NMe moiety of N2iPrN. The NMR data
for 2 and 3 are consistent with the solid state structure, in
particular showing scalar coupling between the NH and SiH
atoms of the N(H)SiHxPh3ꢀx (x = 1 or 2) ligands. These resonances
We recently reported the unusual reactions of the diamideꢀamineꢀ
supported titanium hydrazide Ti(N2iPrN)(NNPh2)(py) (1, Scheme
1) with certain alkynes in which the rather sterically
unencumbered ancilliary ligand allows access to new types of
intermediate and mechanism in hydrohydrazination catalysis.2n In
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are absent in the otherwise identical 1H NMR spectrum of
Ti(N2iPrN){N(D)SiD2Ph}(NPh2) (2-d3) prepared using PhSiD3.
carried out to probe the mechanism. Comparison of the reactions
of 1 with PhSiH3 or PhSiD3, either by initial rates (at ꢀ10 oC) or by
a competition experiment between 1 and an excess of PhSiH3 and
PhSiD3 (at RT) established a kinetic isotope effect in the range
1.34 – 1.41 for both types of experiment, comparable to values
found for Si–H addition to Ti=S6a and Ti=O6b bonds. It was also
found that the presence of an excess of pyridine decreased the rate
of reaction (although competing unknown sideꢀreactions
prevented a detailed analysis). Likewise, reaction of the DMAP
analogue of 1, Ti(N2iPrN)(NNPh2)(DMAP) (4, DMAP = 4ꢀ
dimethylaminopyridine), with PhSiH3 proceeded considerably
more slowly to form 2. Thus both pyridine (or DMAP) loss and
E–H (E = Si or B) bond cleavage are kinetically important to the
rateꢀdeterming steps for the cleavage reaction.
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DFT calculations found two related pathways for the formation of
2, starting from different isomers of 1 (NNPh2 trans or cis to
NMe). The lowest energy pathway (Fig. 2) leads to the
experimentally observed kinetic product trans-2 which may then
isomerise readily to the thermodynamic equilibrium mixture of cis
and trans isomers.2l As is wellꢀestablished for the reactions of
many diamideꢀamineꢀsupported imido and hydrazido analogues of
1, the reaction proceeds via loss of pyridine to form fourꢀ
coordinate Int1. The 1,2ꢀaddition of Si–H to the Ti=Nα bond of
Int1 proceeds in a facile manner via intermediate Int2 (relative G
= 13.3 kcal molꢀ1) to form the silylhydrazidoꢀhydride species
Int3. The barrier from this species, namely irreversible hydride
transfer to Nα and highly exergonic reductive Nα–Nβ bond
cleavage, to give the final product at ꢀ58.7 kcal molꢀ1 is readily
accessible (ꢀG‡ = 17.3 kcal molꢀ1). The alternative pathway to
that shown in Fig 2, starting from the trans isomer of 1 (Fig. S1 of
the SI) is almost identical, but the final transition state (analogous
to TS2) has ꢀG‡ = 25.2 kcal molꢀ1 and is therefore uncompetitive.
The highest points (Int1, TS1 and TS2) located on the lowest
energy reaction pathway involve either pyridine loss from cis-1 or
SiꢀH /TiꢀH bond cleavage. Therefore the computed mechanism is
entirely consistent with the experimental observations presented
above.
Figure 1. Displacement
ellipsoid
plots of trans-
top) and
Ti(N2iPrN){N(H)SiHPh2}(NPh2)
(trans-3,
Ti(N2iPrN){N(H)BC8H14}(NPh2) (5, bottom).
The formation of 2 and 3 from 1 represents the first reductive
cleavage of a hydrazide (or related) N–N bond by a silane at a
single metal center, with net insertion of the Nα atom of 1 into a
silane Si–H bond. Further experiments and DFT calculations were
_______________________________________________________________________________________________________________
Figure 2. DFT mechanism for the reaction of cisꢀ1 with PhSiH3 forming trans-2 and py. Gibbs free energies (kcal molꢀ1, T = 298 K) for
minima, maxima and transition states (labeled TSx) are shown in bold relative to trans-1 and PhSiH3. [Ti] represents Ti(N2iPrN).
_______________________________________________________________________________________________________________
The unprecedented reactions of 1 with silanes prompted us to
explore the corresponding reaction with boranes. Initial NMR
tubeꢀscale studies with pinacol borane gave rather complex specꢀ
tra, possibly indicative of reaction at the TiꢀNamide bonds of the
N2iPrN ligand. Similar reaction outcomes were also found when
halosilanes were used. However, as shown in Scheme 1, reaction
of 1 with 1 equiv. of 9ꢀBBN dimer gave clean conversion to the
borylamide Ti(N2iPrN){N(H)BC8H14}(NPh2) (5) together with
py·HBC8H14. In solution 5 exists predominantly as the cis isomer
(3:1 ratio of cis:trans) illustrated which was characterized by Xꢀ
ray crystallography (Fig. 1). Overall, the Nα atom of 1 has
inserted into the B–H bond of the borane with concommitant
reductive cleavage of the Nα–Nβ bond. This is also the first
reaction of this type for a single metal center. DFT calculations
indicate an analogous mechanism to that for PhSiH3 (see Fig. S2).
The key transition states for Si–H, B–H and N–N bond breaking
are shown in Fig. S3.
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The reductive
transformations of a terminal (L)M=NNR2
analogous to the reaction intermediate Int3 (Fig. 2) shown by
DFT to preceed Nα–Nβ bond cleavage en route to 2. Although 9 is
stable for weeks both in solution and the solid state at RT, upon
1
2
3
4
5
6
7
8
functional group to the corresponding silylamido and borylamido
ligands are unprecedented. We note, in this context, the recent
report of Si–H and B–H bond addition to the terminal N atom of
o
heating to 70 C in C6D6 decomposition occurs to a mixture of
the vanadium(+5) nitride
(
DippNacNac)V(N)(NTol2) forming
unidentified products. Compound 9 represents only the second
fully authenticated B–H bond 1,2ꢀaddition product of a transition
metal imide or hydrazide, the first being very recently reported for
a rare terminal scandium imide.7
V(+3) silylꢀ and borylꢀamido products with net 2ꢀelectron
reduction of the metal.10 These reactions (like their iridium11 (Si–
H) and osmium12 (B–Ph) nitride addition precedents) are
mechanistically distinct from those reported here which involve
N–N bond cleavage with overall 1ꢀelectron reduction of the
hydrazide Nα,β atoms as opposed to the metal center.
Reaction of 8a with 0.5 equiv of 9ꢀBBN dimer in C6D6 at RT
gave ca 50% conversion of the titanium complex and all of the
borane to a 1:1 mixture of two boronꢀcontaining products as well
9
1
10
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Compound 5 does not undergo reaction with further equivs of 9ꢀ
BBN. However, Ti(N2iPrN){N(H)SiH2Ph}(NPh2) (2) consumed 1
equiv. of the borane dimer forming two new products,
Ti(N2iPrN){(ꢁꢀH)2BC8H14}(NPh2) (6, confirmed by Xꢀray
crystallography in Fig. S4 of the SI) and the silylaminoborane
PhH2SiN(H)BC8H14 (7, Scheme 1). This reaction is quantitative
when followed by 1H NMR spectroscopy in C6D6 but the isolated
yields are modest owing to the difficult separation of 6 and 7. The
reaction probably proceeds via a Ti–N/H–B exchange reaction
forming a transient hydride Ti(N2iPrN)(H)(NPh2) and 7, and then
trapping of the transient hydride by borane forming a borohydride
ligand. Overall the sequential reaction of 1, first with PhSiH3 and
then 9ꢀBBN, has resulted in total extrusion of Nα from the
Ti=NNPh2 linkage and formation of 3 different elementꢀnitrogen
bonds solely by reaction with SiꢀH and BꢀH bonds.
as H2 (as judged by a H NMR singlet at 4.46 ppm) according to
NMR spectroscopy. Scaling up the reaction with 1 equiv. of 9ꢀ
BBN dimer gave quantitative conversion of both starting materiꢀ
als to the borylimide Cp*Ti{MeC(NiPr)2}(NBC8H14) (10) and the
aminoborane Me2NBC8H14 (11a)13 which was separated from 10
by careful vacuum sublimation. Compound 8b also reacted
quantitatively with 9ꢀBBN dimer in C6D6 at 60 oC to form 10 and
Ph2NBC8H14 (11b),14 together with H2. It was not possible to fully
separate 11b from 10 on scaleꢀup. The lower volatility of the
aminoborane led to longer sublimation times and/or higher
temperatures, giving thermal degradation of 10.
In principle, the new borylamide 5 could allow access to a
borylimido analogue via elimination of Ph2NH in presence of a
suitable donor. However, heating in the presence of bipy gave no
apparent reaction, and specifically did not lead to the target sixꢀ
coordinate Ti(N2iPrN)(NBC8H14)(bipy). We speculated that a more
sterically crowded/higher coordination number metal center might
facilitate such
a
reaction and therefore turned to
Cp*Ti{MeC(NiPr)2}(NNR2) (R = Me (8a) or Ph (8b)). Compound
8a undergoes a number of small molecule activation reactions,
including reversible 1,2ꢀSi–H bond addition to Ti=Nα, but without
cleavage of the Nα–Nβ bond.5
Scheme 2. Reactions of Cp*Ti{MeC(NiPr)2}(NNR2) (R =
Me (8a) or Ph (8b)) with pinacol borane and 9-BBN.
O
HBPin
iPr
iPr
B
H
O
Ti
Ti
N
N
N
4 h, C6H6
RT
N
N
N
iPr
NR2
N
Me2
Me
Me
Figure
3.
Displacement
ellipsoid
(9,
plot
top)
of
and
iPr
Cp*Ti{MeC(NiPr)2}(H){N(NMe2)BPin}
8a
8b
R = Me ( ) or Ph (
)
Cp*Ti{MeC(NiPr)2}(NBC8H14) (10, bottom).
9
The solid state structure of 10 is shown in Fig. 3 and confirms the
formation of the new borylimido ligand. Terminal transition metal
H2N
9-BBN
B
12 h, C6H6
borylimides are extremely rare and there are no established synꢀ
RT or 60 o
C
12
15
thetic routes.9a,
Only one example has (very recently) been
Ph2NNH2
C6D6
reported
for
Group
4,
this
being
Mindiola’s
Ti(NBEt2)(NTol2){HC(CtBuNDipp)2} prepared from the correꢀ
sponding parent imide (Ti=NH functional group) and 2 equivs.
NaHBEt3, potentially via a nitrido intermediate.15c The Ti(1)ꢀ
N(1) bond length of 1.731(3) Ǻ in 10 is identical within error to
the Ti=NR distances in 8a (1.723(2) Ǻ) and 8b (1.734(2) Ǻ) and
their xylylimido analogue (1.738(2) Ǻ) all of which have formal
Ti≡N triple bonds (σ2π4). In addition, the N(1)ꢀB(1) distance of
1.402(4) Ǻ is typical16 of species of the type R2N–BR'2 which
have significant N=B double bond character via N2p→B2p πꢀ
donation. Molecular orbital and NBO analyses of the DFT strucꢀ
ture of 10 support this interpretation (Figs. S5 and S6 of the SI) by
finding two Ti–N πꢀbonding interactions, one of which is signifiꢀ
iPr
Ti
N
R2N
H2 +
B
+
N
N
Me
B
iPr
10
11a
R = Me (
) or
11b
Ph (
)
Reaction of 8a with 1 equiv. of pinacol borane (HBPin) gave
irreversible 1,2ꢀaddition of B–H to the Ti=Nα bond to form the
borylhydrazidoꢀhydride 9 (Scheme 2) which was isolated and
crystallographically characterized (Fig. 3). Compound 9 is
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cantly stabilized by delocalization into the otherwise vacant 2p
AO of B(1) oriented perpendicular to the N(1)B(1)C(1)C(2)
plane.
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bond
cleavage
would
then
form
Cp*Ti{MeC(NiPr)2}{N(H)BC8H14}(NR2) (10_Int), analogous to
5. Subsequent 1,2ꢀelimination of R2NH from 10_Int would form
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10
11
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Compounds 8a,b are prepared from Cp*Ti{MeC(NiPr)2}(NtBu)
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hydrazine. In a similar manner, the reaction of 10 with 1 equiv.
Ph2NNH2 in C6D6 quantitatively reꢀformed 8b and the
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Ph2NNH2 with 9ꢀBBN dimer to form H2, H2NC8H14 and
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2003, 629, 744; (c) Thompson, R.; Chen, C.ꢀH.; Pink, M.; Wu, G.;
Mindiola, D. J. J. Am. Chem. Soc. 2014, 136, 8197.
16. Fletcher, D. A.; McMeeking, R. F.; Parkin, D. J. Chem. Inf. Comput.
Sci. 1996, 36, 746 (The UK Chemical Database Service: CSD verꢀ
sion 5.35 updated May 2015).
In summary we have reported the first examples of the Si–H or
B–H induced reductive cleavage of a hydrazide N–N bond at a
single metal center. These reactions can ultimately result in
complete Nα or Nβ atom transfer to substrate (i.e., formation of 7,
11 and 12), and encompasses the overall reduction of R2NNH2
with 9ꢀBBN to the corresponding borylamines R2NB8H14 (R' = Ph
or H) and H2, a new type of dehydrogenative coupling reaction.
Further work is ongoing to determine the mechanistic details,
scope and further potential of this and other borylimideꢀmediated
dehydrogenative coupling reactions which are beyond the scope
of this preliminary communication.
ASSOCIATED CONTENT
Supporting Information
Synthetic procedures, characterizing data, crystallographic data in
CIF format, further computational details, and geometries of all
the optimized structures as a single .xyz file. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author:
17. Köster, R.; Seidel, G. Liebigs Ann. Chem. 1977, 1837.
18. Johnson, H. C.; Hooper, T. N.; Weller, A. S. Top. Organomet.
Chem. 2015, 49, 153.
philip.mountford@chem.ox.ac.uk. eric.clot@umontpellier.fr
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We thank the University of Oxford for support.
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Me
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7
8
O
B
N
iPr
iPr
H
H
N
O
Ti
NPh2
Borylamide
N
Ti
Ti
N
N
N
iPr
iPr
N
B
N
N
N
N
Me2
Me
Me
B
iPr
iPr
9
Borylhydrazide
Borylimide
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