Synthesis and ligand-based reduction chemistry of boron difluoride complexes with redox-active formazanate ligands

Article abstract of DOI:10.1039/c4cc03244f

Mono(formazanate) boron difluoride complexes (LBF₂), which show remarkably facile and reversible ligand-based redox-chemistry, were synthesized by transmetallation of bis(formazanate) zinc complexes with boron trifluoride. The one-electron reduction product [LBF₂]^-[Cp ₂Co]⁺ and a key intermediate for the transmetallation reaction, the six-coordinate zinc complex (L(BF₃))₂Zn were isolated and fully characterized. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc03244f

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Synthesis and ligand-based reduction chemistry
of boron difluoride complexes with redox-active
formazanate ligands†
Cite this: Chem. Commun., 2014,
50, 7431
Received 30th April 2014,
Accepted 19th May 2014
M.-C. Chang and E. Otten*
DOI: 10.1039/c4cc03244f
www.rsc.org/chemcomm
Mono(formazanate) boron difluoride complexes (LBF2), which show a range of Lewis acids for binding to 1a, we found that BF3 reacts
remarkably facile and reversible ligand-based redox-chemistry, cleanly via salt metathesis, opening a high-yield synthetic route to
were synthesized by transmetallation of bis(formazanate) zinc obtain mono(formazanate) boron difluoride (LBF2) complexes which
complexes with boron trifluoride. The one-electron reduction are difficult to access otherwise, either from the parent formazan or
product [LBF2]ꢀ[Cp2Co]+ and a key intermediate for the trans- formazanate salts (LK or LNa). A related formazanate diacetate
metallation reaction, the six-coordinate zinc complex (L(BF3))2Zn compound LB(OAc)2 was described by Hicks and co-workers,7b and
were isolated and fully characterized.
although spectroscopic data suggested ligand-based 1-electron redox-
chemistry to occur in these compounds, the ‘borataverdazyl’ radical
The chemistry of coordination complexes bearing redox-active anions obtained were too unstable to allow full characterization. Here
(or non-innocent) ligands has recently received increasing we report the synthesis and X-ray crystallographic characterization of
attention due to its potential application in small molecule activation LBF2 and the radical anion LBF2 ꢀ, with the formation of a relatively
ꢁ
and catalysis,1 and its relevance to biological (enzymatic) transforma- stable 2-electron reduction product LBF22ꢀ confirmed by cyclic
tions.2 The most studied ligands in this class are dithiolenes/ voltammetry. In addition, we provide evidence for the transmetalla-
dioxolenes,3 a-diimines4 and bis(imino)pyridines.5 Formazanates tion pathway by characterization of a likely intermediate.
(1,2,4,5-tetraazapentadienyls), which are nitrogen-rich analogues of
Mono(formazanate) boron difluoride complexes are readily
the well-know b-diketiminates,6 have received comparatively little atten- accessible by transmetallation of bis(formazanate) zinc complexes
tion as ligands in coordination chemistry.7 Unlike b-diketiminates, with BF3ꢂEt2O in hot toluene. In the case of (PhNNC( p-tol)NNPh)2Zn
which have a NCCCN backbone, formazanates feature a NNCNN (1a), a stoichiometric reaction does not go to completion, but full
backbone; the two additional nitrogen atoms provide formazanates conversion is achieved by performing the reaction with 3 equivalents
with redox-active properties. Specifically, the reduced form of of BF3ꢂEt2O at 70 1C twice. During the reaction, a colour change from
formazanates should be relatively accessible and stable due to deep blue to red and the precipitation of a white solid (presumably
delocalization of the SOMO over all 4 N atoms. This is in a way related ZnF2) was observed to indicate formation of mono(formazanate)
to the stability of the analogous organic verdazyl radicals, which may boron difluoride complex (PhNNC( p-tol)NNPh)BF2 (2a, Scheme 1),
be obtained from formazan precursors.8 In previous studies, we found which was isolated as an air-stable, crystalline material in 86% yield.
that bis(formazanate) zinc complexes such as [PhNNC(p-tol)NNPh]2Zn A single crystal X-ray diffraction study (Fig. S1, ESI†) revealed a
(1a) are capable of storing one or two electrons in the ligand frame- distorted tetrahedral boron centre, which is displaced out of the
works to form stable singly- and doubly-reduced zinc complexes.9 The (planar) formazanate NNCNN backbone by 0.5 Å. A similar
crystal structures of the reduced complexes show (weak) interactions bonding mode was observed for b-diketiminate complexes of Sc
between the internal nitrogen atoms of formazanate ligands and and attributed to steric interactions.10 In the absence of significant
sodium counter cations. Based on this observation we anticipated that steric pressure in 2a, we ascribe the out-of-plane displacement of the
the reduction potential of bis(formazanate) zinc compounds could be B atom to packing effects; the observation of only one 19F NMR
altered by coordination to neutral Lewis acids. In the course of testing resonance for 2a even at low temperature is consistent with a low-
energy C2v symmetric structure through which the two 19F environ-
Stratingh Institute for Chemistry, University of Groningen Nijenborgh 4,
ments exchange. As is the case in its parent zinc complex 1a and the
9747 AG Groningen, The Netherlands. E-mail: edwin.otten@rug.nl
related mono(formazanate) boron diacetate complexes reported by
† Electronic supplementary information (ESI) available: Synthesis and character-
Hicks and co-workers,7b full delocalization within the formazanate
ization data for compounds 2a, b, 3a and 4b. CCDC 995403–995405. For ESI and
framework in 2a is indicated by the similar N–N and C–N bond
lengths in the NNCNN backbone.
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc03244f
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Fig. 2 Molecular structure of 3a (left) and 4b (right) showing 50% prob-
ability ellipsoids. For 3a, [Cp2Co]+ cation and THF molecule are omitted;
for 4b, one of the two independent molecules is shown; for both,
hydrogen atoms are omitted for clarity.
Scheme 1 Synthesis of mono(formazanate) boron difluoride complexes
and their reduction products.
[Cp2Co]+[(PhNNC(p-tol)NNPh)BF2]ꢀ (3a). In contrast to the reduction
product of the diacetate analogue reported by Hicks, the difluoro
complex 3a is stable in solution under an inert atmosphere for days
and can even be boiled in THF–hexane without noticeable decom-
position. The surprising stability of 3a is likely due to the strength of
the B–F bonds in comparison to B–OAc. Crystals of 3a suitable for
single-crystal X-ray diffraction analysis were obtained by diffusion of
hexane into THF solution at room temperature in quantitative yield
(Fig. 2, left). Compared with 2a, 3a has shortened B–N (av. 1.554 Å for
2a vs. av. 1.535 Å for 3a), elongated N–N (av. 1.309 Å for 2a vs. av.
1.362 Å for 3a) and elongated B–F (av. 1.376 Å for 2a vs. av. 1.410 Å
for 3a) bond lengths. The similar N–N bond lengths in 3a and the
reduced L2Zn complexes9 suggest the ligand of 3a is a radical
dianionic species (L2ꢀ) in which the unpaired electron occupies a
N–N anti-bonding orbital.
UV-Vis absorption spectroscopy of neutral and anionic
mono(formazanate) difluoride compounds provides additional
evidence for ligand-based reduction and formation of the dianionic
ligand radical (L2ꢀ) (Fig. S3, ESI†). The neutral compound 2a shows
a single broad absorption in the visible at 521 nm. In the case of 3a,
absorption is at longer wavelength (716 nm) and at 454 nm with
shoulders at 674 and 431 nm, respectively. These absorption bands
are in agreement with the presence of reduced formazanate
ligands.9,13 The EPR spectrum of 3a shows a broad EPR signal
in frozen THF with g-value of B2 (Fig. S2, ESI†). Attempts to
synthesize and characterize the two-electron reduction product
have so far been unsuccessful.
The redox-active nature of the formazanate ligand in 2a was
established by cyclic voltammetry (CV) in THF solution with
[Bu4N][PF6] electrolyte. Upon scanning to negative potentials, the CV
shows two quasi-reversible one-electron redox processes (system I/I0
and II/II0, Fig. 1) that can be assigned to formazanate-based reduc-
tions. The first redox-couple of 2a occurs at more positive potential
(E1/2(I/I0) = ꢀ0.98 V vs. Fc0/+) than that in 1a (ꢀ1.31 V vs. Fc0/+)9 and at
similar potential as Hicks et al. reported for the analogous
mono(formazanate) boron diacetate compound (ꢀ0.86 V vs. Fc0/+
in CH3CN).7b A second redox-event is observed at more negative
potential (E1/2(II/II0) = ꢀ2.06 V vs. Fc0/+) but also this reduction is
reversible. Based on these data, we infer that both one- and two-
electron reduction products of 2a are relatively stable and accessible.
Importantly, the second reduction forms the dianion LBF22ꢀ, in
which two electrons have been added to a single formazanate
ligand to result in a ‘L3ꢀ’ fragment coordinated to the boron
centre (Scheme 1). Bard et al. recently explored the voltammetry
of boron dipyrromethenes (BODIPY): in general these show one-
electron reductions at more negative potentials compared to 2a.11
Related b-diketiminate compounds show irreversible reductions at
much more negative potential (Ep,red B ꢀ2.6 V vs. Fc0/+).12
The first redox-couple of 2a occurs at ꢀ0.98 V vs. Fc0/+, which
suggests that cobaltocene is a suitable reducing agent to selectively
synthesize a singly-reduced product. Upon reaction of 2a with one
equivalent of Cp2Co in THF, an immediate colour change from red
to green is observed to indicate formation of the radical species
In order to expand the scope of transmetallation reactions from
bis(formazanate) zinc complexes and boron trifluoride, the reaction
of (PhNNC(C6F5)NNMes)2Zn (1b) with BF3 was attempted. The
colour of the reaction mixture fades from deep to light orange upon
heating 1b in the presence of 3 eq. BF3Et2O at 70 1C overnight, but
precipitation of ZnF2 was not observed. Orange crystals of the
product 4b were obtained by slow diffusion of hexane into a toluene
solution at ꢀ30 1C in 85% yield (Fig. 2, right). The crystal structure of
4b shows a distorted octahedral zinc centre. In the structure, there
are two tridentate (PhNNC(C6F5)NNMes(BF3)) units coordinated to
the Zn centre in a meridional fashion via two nitrogens and a
fluorine atom to give a [NNF]2Zn compound. This unusual binding
motif results from interaction of BF3 with the terminal nitrogen of
the formazanate fragment (the NMes group), which gives rise to 2
five-membered chelate rings upon coordination to the Zn centre. To
the best of our knowledge, the structural characterization of this BF3
Fig. 1 Cyclic voltammetry of 2a and 2b (1.5 mM solution in THF, 0.1 M
[Bu4N][PF6]) recorded at 100 mV sꢀ1
.
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electron-poor C–C6F5 substituents,15 which are present only when
the formazanate ligands adopt a 5-membered chelate ring.
In conclusion, transmetallation of bis(formazanate)zinc
complexes with BF3ꢂEt2O provides a convenient entry into
formazanate boron chemistry. The reaction likely occurs via
initial binding of BF3 to the formazanate ligand. This pathway
is possible through the flexibility of the NNCNN backbone to
adopt 5-membered chelate ring isomers, which are inaccessible
to their b-diketiminate congeners. One-electron reduction of
LBF2 results in a fully characterized stable ligand-based radical, and
cyclic voltammetry confirms a second reduction is possible, allow-
ing access to three oxidation states (LBF20/ꢀ1/ꢀ2) that are all based
the redox-chemistry of a single formazanate ligand. The further
development of this unique class of stable redox-active ligands
towards applications in coordination chemistry and catalysis is the
Scheme 2 Proposed mechanism of transmetallation from bis(formazanate)
zinc complex to mono(formazanate) boron difluoride complex.
binding mode has no precedent in the literature, although the focus of ongoing work in our laboratory.
‘frustrated Lewis pair’ (tmp)MgCl/BF3 has been postulated to contain
a B–F fragment appended to a Mg–N(tmp) bond.14 The 19F-NMR of 4b
shows six distinct resonances with integration ratio of 1:1:3:1:1:1
Notes and references
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(Fig. S6, ESI†). Five resonances with the same integration (1F) suggest
that all F substituents of the C6F5 ring are inequivalent due to
hindered rotation around the C–C6F5 bond. The resonance integrat-
ing as 3F shows 11B and 10B coupling features and can be assigned to
the BF3 unit. The appearance of the BF3 group in the 19F NMR does
not change upon cooling to ꢀ55 1C, which suggests that the barrier to
rotation around the N–BF3 bond is low. Conversely, heating an NMR
sample of 4b to moderate temperature (65 1C for 24 h) does not result
in changes in the spectroscopy, confirming that the octahedral
[NNF]2Zn complex is quite stable. Upon heating the NMR tube to
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2b is more important than the electron-withdrawing C–C6F5 moiety in
modulating the redox-potential of the formazanate fragment.
The sequential transformation 1b - 4b - 2b suggests that
a six-coordinate species related to 4b is likely also involved in the
formation of 2a. Based on these observations, we propose the
following mechanism for the transmetallation leading to com-
pounds 4 (Scheme 2): (i) formazanate rearrangement from a 6- to
a 5-membered chelate ring liberates the terminal N-atom, (ii) BF3
binds to this terminal N-atom and brings a B–F group in proximity
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