Reactions of hydrazones and hydrazides with Lewis acidic boranes

Article abstract of DOI:10.1039/c9dt01359h

The reaction of (diphenylmethylene)hydrazone or 1,4-bis-hydrazone-ylidene(phenylmethyl)benzene with Lewis acidic boranes B(2,4,6-F₃C₆H₂)₃ or B(3,4,5-F₃C₆H₂)₃ generates the Lewis acid-base adducts. Alternatively, when (9H-fluoren-9-ylidene)hydrazone is employed several products were isolated including 1,2-di(9H-fluoren-9-ylidene)hydrazone, the 2:1 borane adduct of NH₂-NH₂ and the 1-(diarylboraneyl)-2-(9H-fluoren-9-ylidene)hydrazone in which one ArH group has been eliminated. The benzhydrazide starting material also initially gives an adduct when reacted with Lewis acidic boranes which upon heating eliminates ArH generating a CON₂B heterocycle.

Full text of DOI:10.1039/c9dt01359h

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Reactions of hydrazones and hydrazides with
Lewis acidic boranes†
Cite this: DOI: 10.1039/c9dt01359h
c
Theodore A. Gazis, ‡a Ayan Dasgupta,‡a Michael S. Hill,b Jeremy M. Rawson,
Received 29th March 2019,
Accepted 19th June 2019
a
Thomas Wirth d and Rebecca L. Melen
*
DOI: 10.1039/c9dt01359h
rsc.li/dalton
The reaction of (diphenylmethylene)hydrazone or 1,4-bis-hydra- 170 K,6 it was only last year that the metal-free activation of N2
zone-ylidene(phenylmethyl)benzene with Lewis acidic boranes was reported using a cyclic (alkyl)(amino)carbene (CAAC)
B(2,4,6-F3C6H2)3 or B(3,4,5-F3C6H2)3 generates the Lewis acid– stabilised borylene (Fig. 1).7 Prior to this, Stephan3a,8 had
base adducts. Alternatively, when (9H-fluoren-9-ylidene)hydra- shown that the strong Lewis acid B(C6F5)3 can be used to
zone is employed several products were isolated including release N2 from Ph2CN2, and can be used in reactions with
1,2-di(9H-fluoren-9-ylidene)hydrazone, the 2 : 1 borane adduct of Ph2CvNNH2 to form an adduct (Fig. 1). Yamashita9 has sub-
NH2–NH2 and the 1-(diarylboraneyl)-2-(9H-fluoren-9-ylidene) sequently shown that a highly Lewis acidic diborane can react
hydrazone in which one ArH group has been eliminated. The benz- with azobenzene and pyridazine via diboration and, when
hydrazide starting material also initially gives an adduct when phthalazine is employed, NvN bond cleavage occurs. At a
reacted with Lewis acidic boranes which upon heating eliminates similar time, we have been investigating the synthesis and
ArH generating a CON2B heterocycle.
comparative reactivity of new boranes bearing fluorinated aryl
groups as an alternative to the archetypical B(C6F5)3.10 In this
regard, we were interested in comparing the reactivity of
boranes other than B(C6F5)3 with a variety of N–N bonded com-
pounds including hydrazones and hydrazides, which are
reported herein (Fig. 1). These may provide alternative reactiv-
ity and reaction rates depending upon the borane Lewis
With over a century since the dawn of the Haber–Bosch
process, the activation and subsequent functionalisation of
dinitrogen remains a challenge. Typically, metal catalysts, such
as those based upon Fe or Mo, have been trialled for the
reduction of N2 to NH3. The ability of transition metal com-
plexes to bind to N2 has been attributed to the roles of the
metal d-orbitals in bonding and back-bonding interactions
with N2.1 Recently however, the activation of nitrogen–nitrogen
bonds such as azides, diazo-compounds, hydrazones and di-
nitrogen using boranes has gained momentum.2,3 Both
Szymczak4 and Simonneau5 reported the use of B(C6F5)3 to
activate M–N2 (M = Fe, Mo, W) complexes (Fig. 1) towards pro-
tonation or borylation/silylation. A major advance in this area
was to remove the transition metal altogether. While (N2)BF3
has been generated via supersonic expansion at 600 Torr and
aCardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building,
Park Place, Cardiff, Cymru/Wales, CF10 3AT, UK. E-mail: MelenR@cardiff.ac.uk
bDepartment of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, UK
cDepartment of Chemistry and Biochemistry, University of Windsor,
401 Sunset Avenue, Windsor, Ontario N9B 3P4, Canada
dSchool of Chemistry, Cardiff University, Main Building, Park Place, Cardiff,
Cymru/Wales, CF10 3AT, UK
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, NMR spectra and details of the X-ray analyses of compounds 2a–c, 4–5,
6a,b, 7. CCDC 1897205–1897211 and 1903201. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c9dt01359h
‡These authors contributed equally.
Fig. 1 Previous work on activation of N–N bonds using B(C6F5)3 and
borylenes, and outline of the work described in this article.
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Scheme 1 Reactions of hydrazones with Lewis acidic boranes.
Fig. 2 Solid-state structures of 2a (top), 2b (middle) and 2c (bottom).
C: black, N: blue, B: pink, F: green. Thermal ellipsoids drawn at 50%
probability. H atoms except N–H omitted for clarity.
Scheme 2 Reactions of 1c with B(2,4,6-F3C6H2)3.
reports (1.631(7) Å).7 The single N–N bond lengths (range
1.438(4)–1.449(2) Å) were also similar to those reported pre-
acidity.11 Initially, the hydrazone compounds 1a–c (Schemes 1 viously.8 Likewise, the CvN bond lengths of 1.287(3)–1.294(5) Å
and 2) were prepared from the condensation reaction of the are typical of CvN double bonds.12 In all cases, heating
corresponding ketone with hydrazine monohydrate. Upon adducts 2 resulted in an unidentifiable mixture of compounds
reaction of 1a with B(2,4,6-F3C6H2)3 or B(3,4,5-F3C6H2)3 the and no elimination of ArFH was observed, in contrast with pre-
corresponding N → B adducts 2a and 2b were formed in which vious reports where B(C6F5)3 was employed as the Lewis acid.8
the –NH2 group of the hydrazone coordinated to the Lewis Alternatively, if the compound 1c was used in the reactions
acidic boron atom analogous to that reported by Stephan with boranes BArF (ArF = 3,4,5-F3C6H2, C6F5), the reaction
3
when 1a was combined with B(C6F5)3.8 Similarly, when bis- gave a complex and inseparable mixture of products. However,
hydrazone 1b was reacted with B(2,4,6-F3C6H2)3 in a 1 : 2 stoi- if 1c is reacted with the less Lewis acidic borane B(2,4,6-
chiometric ratio a 1 : 2 adduct (2c) was formed. Compounds F3C6H2) several products could be selectively crystallised from
2a, 2b and 2c were isolated in 61%, 65% and 56% yield the reaction mixture to shed light on this reactivity. These could
respectively. Adducts 2a–c all displayed a broad peak ranging be structurally identified by single crystal X-ray diffraction as the
between −2.6 and −6.2 ppm in the 11B NMR spectrum. previously reported 1,2-di(9H-fluoren-9-ylidene)hydrazone (3),13
Recrystallisation of the reaction mixtures from a CH2Cl2/ the bis borane adduct of hydrazone, NH2NH2 (4) and the 1-(di-
pentane solution resulted in single crystals of the adducts arylboraneyl)-2-(9H-fluoren-9-ylidene)hydrazone (5) (Scheme 2
whose structures were determined by single crystal X-ray diffr- and Fig. 3). It is unclear how the three products are formed
action (Fig. 2). In these structures the boron atom adopted a from the reaction, but it is likely that initially an adduct is
tetrahedral arrangement with B–N bond lengths ranging from formed between 1c and the borane. Indeed, in situ NMR studies
1.646(5)–1.696(4) Å, similar to those observed in previous showed that when 1c is added to the borane in toluene then a
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and involve harsh conditions and long time periods e.g. reflux-
ing in ethanolic hydrogen chloride for several hours.16 More
interesting is that, in our case, the borane adduct of hydrazone
4 is also generated in the reaction. Previously, the hydrazine
bisborane N2H4(BH3)2 has been postulated as a good hydrogen
storage material containing 16.9 wt% hydrogen. Additionally,
coordinated hydrazine has been identified as a potential inter-
mediate in nitrogen fixation reactions.17–19 While N2H4(BH3)2
is easily prepared from the reaction of hydrazine sulfate with
sodium borohydride, other borane adducts are more challen-
ging to synthesise and a search of the literature reveals that
generally intramolecular bis boranes are employed. Previously,
Gabbaï20 and Szymczak17 have reported that intramolecular bis-
boranes can react with hydrazine monohydrate (N2H4·H2O). In
these reactions, the use of a bisborane appears key to trapping
and activating the hydrazone unit. We attempted to generate 4
independently from the reaction BArF (Ar = 2,4,6-F3C6H2) and
3
hydrazine monohydrate (N2H4·H2O) in a 2 : 1 ratio with the use
of molecular sieves to trap the water by-product. Pleasingly,
this led to the formation of compound 4 as observed by in situ
multinuclear NMR spectroscopy. The structure of 4 displays a
B–N bond length of 1.673(2) Å which is longer than that in
hydrazine bisborane (1.609 Å)21 and slightly shorter than that
reported by Gabbaï (1.688(2) Å) and Szymczak (1.697(2) Å and
1.698(2) Å) for the chelating bisborane complexes.17,20
The N–N distance of 1.461(2) Å is similar to that reported
for other hydrazine bisborane complexes (range 1.356(2) Å–
1.469(2) Å).17,20,22
Fig. 3 Solid-state structures of 4 (top) and 5 (bottom). C: black, N: blue,
B: pink, F: green. Thermal ellipsoids drawn at 50% probability. H atoms
except N–H omitted for clarity.
Finally, we turned our attention to other N–N bonded sub-
strates, namely hydrazides (Scheme 3). When commercially
characteristic broad shift at δ = −5.1 ppm in the 11B NMR spec- available benzhydrazide was reacted with BArF an adduct was
3
trum was observed. Two divergent pathways then operate to formed initially between the NH2 group and the borane. In the
generate either 3 and 4, or compound 5. Elimination of ArFH case of the boranes where Ar = 2,4,6-F3C6H2 or C6F5 the adducts
generates 5 which is similar to observations made by Stephan 6a–b could be isolated in 56% and 71% yield, respectively.
for hydrazone Ph2CvNNH2 with B(C6F5)3.8 The trigonal planar Recrystallisation of the solutions afforded crystals of the
boron atom in 5 can be observed in the solid-state structure adducts which could be structurally determined (Fig. 4 and
with N–B–C bond angles of 117.8(1)° and 119.7(1)°, and a ESI†). The 11B NMR spectra showed a broad peak at δ =
C–B–C bond angle of 122.5(1)°. The B–N bond length of −6.1 ppm (6a) and −6.2 ppm (6b) similar to that observed with
1.397(2) Å was also significantly shorter than observed in the the hydrazone borane adducts. In the case of 6b in situ heating
adducts 2 indicating some π-donation from N → B generating a of the BArF adduct of benzhydrazide to 110 °C in toluene led
3
trans-diene structure in which the CN2B atoms lie in the same to loss of ArFH and generation of a CON2B heterocycle (7).
plane. If the reaction of 1c with B(2,4,6-F3C6H2)3 is performed at Recrystallisation of the reaction mixture afforded pink crystals
low temperatures (0 °C) in toluene then compound 5 is formed of the product in 82% yield. Compound 7 crystallised in the
ˉ
selectively in 46% yield.
P1 space group with two molecules in the asymmetric unit.
Alternatively, another reaction can take place when 1c is The N–H hydrogen atoms were identified in the difference
reacted with B(2,4,6-F3C6H2)3. Here a hydrazone metathesis map. X-ray diffraction analysis revealed the CON2B rings to be
reaction occurs to yield the azine (3) and borane adduct of planar with r.m.s.d. of 0.010 Å and 0.023 Å. The B–N bond
hydrazone (4). This reaction is more predominant when the
reaction is performed at elevated temperatures (e.g. r.t. to
50 °C). N–N-linked diimines (azines) are an important class of
compounds isoelectronic to 1,3-butadiene. These compounds
have a variety of uses including applications as chromophores
in optoelectronic devices.14 Previously, 3 has been reported
to have been synthesised by the decomposition of diazo
compounds catalysed by Pt(C2H4)(PPh3)2.15 Similar reactions
which generate a diazine from hydrazones have been reported Scheme 3 Reaction of hydrazides with BArF
.
3
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dimer 4 from a hydrazone metathesis reaction. Subsequent
studies to explore the independent synthesis and reactivity of 4
in hydrogen release are ongoing within our group.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
T. A. G. thanks the EPSRC Cardiff/Bristol/Bath CDT in
Catalysis for funding (EP/L016443/1). A. D. and R. L. M. would
like to acknowledge the EPSRC for an Early Career Fellowship
for funding (EP/R026912/1). J. M. R. thanks NSERC for finan-
cial support.
Notes and references
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