1,2,4-Triazolato paddlewheel dibismuth complexes with very short Bi(ii)-Bi(ii) bonds: Bismuth(iii) oxidation of 1,2,4-triazolato anions into neutral: N -1,2,4-triazolyl radicals

Article abstract of DOI:10.1039/d0dt03225e

Bismuth(iii) oxidation of 3,5-di-substituted-1,2,4-triazolato anions afforded a paddlewheel 1,2,4-triazolato dibismuth complex [L2(Bi-Bi)L2] (L = η1,η1-3,5-R2tz, R = Ph (3), iPr (4)) with very short Bi(ii)-Bi(ii) bonds (2.8650(4)-2.8721(3) ?). The reaction involved the intermediates of the organobismuth radical [Bi(R2tz)2] and neutral N-1,2,4-triazolyl radical [3,5-R2tz]. The dimerization of the former produced the corresponding dibismuth complex while the latter was trapped by using spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) to give the radical adduct of {(3,5-R2tz)(DMPO)} which was unambiguously evidenced by EPR analysis.

Full text of DOI:10.1039/d0dt03225e

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DOI: 10.1039/D0DT03225E
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1,2,4-Triazolato paddlewheel dibismuth complexes with very
short Bi(II)-Bi(II) bonds: bismuth(III) oxidation of 1,2,4-triazolato
Received 00th January 20xx,
Accepted 00th January 20xx
anions into neutral N-1,2,4-triazolyl radicals
Ming-Gang Zhaoa, Li Lia, Ru-Ru Hea, Xiang Zhanga, Jian-Ping Mab, Ji-Hu Suc, and Wenjun Zheng*a
DOI: 10.1039/x0xx00000x
Bismuth(III) oxidation of 3,5-di-substituted-1,2,4-triazolato
anions afforded paddlewheel 1,2,4-triazolato dibismuth
complex [L2(Bi–Bi)L2] (L = η1,η1-3,5-R2tz, R = Ph (3), iPr (4)) with
very short Bi(II)-Bi(II) bonds (2.8650(4)2.8721(3) Å). The
reaction involved the intermediates of organobismuth radical
[Bi(R2tz)2] and neutral N-1,2,4-triazolyl radical [3,5-R2tz] . The
dimerization of the former produced the corresponding
dibismuth complex while the latter was trapped by spin trap
5,5-dimethyl-1-pyrroline-N-oxide (DMPO) to give the radical
adduct of {(3,5-R2tz)(DMPO)} which was unambiguously
evidenced by EPR analysis.
of these unusual dibismuth complexes most likely involved
organometallic radical intermediates. The dimerization of
organometallic radical intermediates may allow the corresponding
low-valent bismuth paddlewheel complexes to be isolated.8 We
thus felt that bismuth(III) oxidation of deprotonated five-membered
heterocycles into the related neutral radicals and organobismuth(II)
radical intermediates may be general. With this in mind, we turned
our interest toward paddlewheel 1,2,4-triazolato dibismuth
complexes induced by bismuth(III) ion. In this contribution, we
reported two paddlewheel 1,2,4-triazolato dibismuth complexes
with very short Bi-Bi single bonds that are close to the reported
Bi=Bi double bond lengths (ca. 2.82 Å) in dibismuthenes RBi=BiR.2c-e
The EPR analysis unambiguously demonstrated that the Bi(III) ion
oxidized 1,2,4-triazolato monoanionic ions into the corresponding
N-heterocyclic aromatic free radicals.
In a ratio of 1:3, the reaction of BiCl3 and 1,2,4-triazolato
potassium K[3,5-R2tz] (R = Ph (1)10a, and iPr (2)10b,c) in
tetrahydrofuran provided, after worked up, two paddlewheel
dibismuth complexes {(η1,η1-3,5-R2tz)2(BiBi)(η1,η1-3,5-R2tz)2]
(R = Ph (3·toluene) and iPr (4)) as orange crystals in a fair yield
(30.2% for 3·toluene and 56.5% for 4) (Scheme 1, Figures S1-
S4).11 Concentrating the mother liquor allowed an additional
portion of the dibismuth complex and the small amount of the


Bismuth complexes containing bismuth–transition metal or
bismuth–bismuth bonds have attracted an increasing interest
during the last two decades.1,2 A number of dibismuth complexes
containing Bi–Bi single bonds and several dibismuthenes with Bi=Bi
double bonds have been prepared.2 A typical dibismuthane has two
pyramidal diorganobismuth moieties linked through a Bi–Bi single
bond that is usually ca. 3.0884(4) Å (the covalent radii are 3.5453(4)
Å).3
The
first
p-block
paddlewheel
complex,
dibismuth(trifluoroacetate)4 with a Bi–Bi single bond of 2.95 Å,4
showed the unique electronic structure and bonding characteristic
in comparison to those in d-block paddlewheel complexes.5 Such
paddlewheel dibismuth complex has been used as a precursor for
several novel heterobimetallic complexes,6a–g and some of which protonated ligand H[3,5-R2tz] (R = Ph(5),12 iPr(6)13)(vide infra)
have proved to be efficient catalysts in organic transformations.6h-j
Recently, we developed a very straightforward strategy for the
preparation of paddlewheel dibisthmuth complexes7,8 and
distibines9 with aromatic five-membered heterocyclic 1,2-diaza-4-
phospholide7,9 and pyrazolato8 ligands.8-9 In these reactions,
bismuth(III)/antimony(III) ion was formally reduced by the five-
membered heterocyclic anions instead of using additional reduced
agents such as Zn, Mg, or even K. The mechanism for the formation
to be isolated.7 Complexes 3·toluene and 4 decomposed at
250°C and 260°C, respectively, and the crystals were only
slightly re-dissolved in THF.
R
N
N
R
N
R
R
N
Bi
N
N
R
N
R
N
BiCl3
N
R
R
3
N
Bi
+
N
R
N
N
K+
N
-3 KCl
HN
N
R
R
N
R= Ph(1), iPr(2)
R= Ph(5), iPr(6)
N
R
R= Ph(3), iPr(4)
a. Key Laboratory of Magnetic Molecules & Magnetic information materials Ministry
of Education, The School of Chemical and Materials Science, Shanxi Normal
University, Linfen, Shanxi Province, 04100, China. Email: wjzheng_sxnu@qq.com
b. College of Chemistry, Chemical Engineering and Materials Science, Shandong
Normal University, Jinan, Shandong 250014, China.
Scheme 1 The preparation of complexes 3 and 4
c. CAS Key Laboratory of Microscale Magnetic Resonance, Department of Modern
Physics, University of Science and Technology of China, Hefei 230026, China.
The X-ray diffraction analysis confirmed the dibismuth assembly
[{η1,η1-3,5-R2tz}2(Bi–Bi){η1,η1-3,5-R2tz}2] (R = Ph (3·toluene), and
iPr(4)) (Figures 12, Tables S1-S3) that crystallized in the triclinic
Electronic
Supplementary
Information
(ESI)
available:
See
DOI: 10.1039/x0xx00000x
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space group P-1.11,14 As shown in Figures, the inversion center at
the midpoint of Bi–Bi bond in each dibismuth complex and a total
absence of axial interactions of any kind between the dibismuth
units indicated that the overall 1,2,4-triazolato ligand arrangement
around the Bi2 core was an almost perfect paddlewheel structure.
Each bismuth atom was coordinated with one other Bi atom and
four N atoms with a slightly distorted square pyramidal geometry.
The length of the Bi–N bonds within each of 3 and 4 varied slightly,
by ca. 0.01 Å around the average values (2.442(3) Å for 3, 2.4460(5)
Å for 4) that are close to those (Bi–N 2.447(8)-2.466(5) Å) found in
paddlewheel 1,2-diaza-4-phospholide dibismuth complexes (Bi2dp4)
and paddlewheel pyrazolato dibismuth complexes (Bi2pz4) (Tables
1).7,8 The Bi–Bi distances (2.8721(3) Å in 3 and 2.8650(4) Å in 4) are
much shorter than that found in paddlewheel complex [Bi2(O2CCF3)4]
(2.947(1) Å).4a The planes of 1,2,4-triazolato ligands in the face-to-
face orientation were almost perpendicular to each other, with an
average dihedral angle of close to 90° despite the average N–Bi–N
angle being 84.62(11)° in 3, 84.73(18)° in 4.
produce open-shell radicals [Bi(R2tz)2] ((II)) and [3,V5ie-Rw2Atrzt]icle(O(InIIli)ne)

(Scheme 2). The radical (II) dimerized and the radical (III)
DOI: 10.1039/D0DT03225E
abstracted H-atom from THF to give the corresponding
paddlewheel dibismuth complex (3 or 4) and the protonated form
H[3,5-R2tz]of the ligand, respectively.7
R
R
N
N
R
N
R
R
R
N
N
N
N
N
N
N
N
N
Bi
R
R
1-N-(III)
4-N-(III)
(II)
(R = Ph, IIIPh; R = iPr, IIIiPr)
(III)
(III)
N
N
O
O
(III)
DMPO
DMPO
Scheme 2. The proposed radical intermediates in the reactions for the
preparation of complexes 3 and 4 as well as the formation of {(III)(DMPO)}
adduct.
To verify our hypothesis, performing the reaction under a
similar condition but using 5,5-dimethyl-1-pyrroline-N-oxide
(DMPO) as a spin trap (Scheme 2) gave two strong EPR spectra
(Figure 3 and Figure S14).10 Note that the spectra for the
DMPO adducts of the C-, O-containing radicals usually
presented a coupling quadruplet16 whereas those of the N-
containing radicals displayed a hyperfine of the coupling
multiplet with 12 observed lines which have been recently
evidenced by us8,17 and others.18 As shown in Figure 3, the
spectrum for the product of the Bi3+-oxidized [R2tz] (R = iPr)
system in the presence of DMPO displayed a hyperfine of well-
resolved coupling multiplet with 12 observed lines which were
assigned to the adduct of {(IIIiPr)(DMPO)}. This clearly
suggested that N-1,2,4-triazolato radical (IIIiPr) formed.11 The
distinction between a nitrogen coupling constant aN = 17.29 G, a
proton splitting of aH = 13.19 G and a second nitrogen splitting aN′
= 3.81G suggests a σ-type of N-triazolyl spin adduct structure on the
basis of the linewidth of the signals (g = 2.007). The coupling
constants aN, aH, and aN’ of the {(IIIiPr)(DMPO)} adduct were
comparable to those of {(3,5-tBu2dp)(DMPO)}, {(3,5-
tBu2pz)(DMPO)}, and {(N-tetrahydroyl-pz)(DMPO)} species (Tables
2).17,7,18a However, the EPR spectrum of {(IIIPh)(DMPO)} is too
complicated to get the corresponding accurate parameters (Figure
S14), which may result from the overlapped signals by the two
resonant electronic isomers of 3,5-dipheny-1,2,4-triazolyl radicals
{(IIIPh)(DMPO)} (Chart 1, -type).
Fig. 1 X-ray crystal structure of 3·toluene. Drawn with ellipsoids at 30%
probability, toluene molecule and hydrogen atoms omitted for clarity. Selected
bond distances [Å] and angles (deg). Bi(1)–Bi(2) 2.8721(3), Bi(1)–N(4) 2.422(3);
N(1)-Bi(1)-Bi(1A) 71.47(7).
Fig. 2 X-ray crystal structure of 4. Drawn with ellipsoids at 30% probability and
hydrogen atoms omitted for clarity. Selected bond distances [Å] and angles
(deg). Bi(1)–Bi(1A) 2.8650(4), Bi(1)–N(1) 2.472(5); N(4)-Bi(1)-N(1) 84.23(18).
Table 1. Comparison of the selected bond distances (avg.)(Å) and angles (avg.)(°)
for paddlewheel complexes 3, 4, Bi2dp4, Bi2pz4, and Bi2(O2CCF3)4]
3
Bi2dp4
Bi2pz4
4
Bi2(O2CCF3)4
Bi–Bi
2.8721(3)
2.442(3)
84.62(11)
2.8873(3)
2.447(8)
84.35(11)
2.8705(6)
2.464(8)
84.58(18)
2.8650(4)
2.4460(5)
84.73(18)
2.947(1)
Bi–N
-
-
N–Bi–N
The isolation of dibismuth complexes 3 and 4 suggested that a
redox reaction occurred. We supposed that a homoleptic complex
[Bi(R2tz)3] (I) formed at first in the reaction of BiCl3 and K[R2tz] (1:3).
As the heteroatoms are known to be effective at delocalizing spin
density and concomitant stabilizing radicals,15 one charge probably
transferred from one of the ligands to the bismuth(III) centre to
3450 3465 3480 3495 3510 3525 3540 3555 3570
G
Fig. 3 X-band EPR spectrum of (3,5-iPr2tz)DMPO adduct (7iPr) in anhydrous THF
with Bi3+
2 | J. Name., 2012, 00, 1-3
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NMR spectrum (Figure S1). The 1H NMR (600 MHz, THF-d8,
DOI: 10.1039/D0DT03225E
23C) spectra of 3 and 4 showed rather broad peaks at the
Table 2. The comparison of the coupling constants (a) and g factors of {(3,5-
iPr2tz)(DMPO)}, {(3,5-tBu2pz)(DMPO)}[8] and {(3,5-tBu2dp)(DMPO)}[17] adducts in
anhydrous THF
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expected range of iPr and Ph groups (SI). The absence of NH
1
signals at δ = 1011 ppm and the correct integral in the H
Items
aN
aH
aN’
g
NMR spectra, supporting the formation of 3 and 4 with a
uniform coordination environment. The broad resonances
were presumably attributable to the equilibrium between 3(4)
and the related radical species [Bi(R2tz)2] (II) in the solution
or to the fair solubility in THF or even both.
{(3,5-iPr2tz)(DMPO)}
{(3,5-tBu2pz)(DMPO)}
{(3,5-tBu2dp)(DMPO)}
17.29
13.48
14.70
13.19
13.19
14.70
3.81
4.10
3.43
2.008
2.012
2.007
To understand the bonding in the paddlewheel motif of 3 and 4,
we optimized the complexes 3 and 4 by the B3LYP method with the
6-31G(d) basis set for C, H, and N atoms and Lanl2dz basis set and
ECP for the Bi atoms (SI),21,22 and SMD23 solvation model (THF). The
R
R
N
R
N
N
N
5e
N
6e
N
N
6e
N
R
R
R
N
-type
-type
4+
orbital interaction (Figure S11) between Bi2 (fragment 1, F1) and
[iPr2tz]44- (fragment 2, F2) was carried out by Charge Decomposition
Analysis (CDA) method, on a D4 symmetric geometry of complex 4.
CDA results showed that, in the formation of complex 3, filling the
electrons from highest occupied fragmental orbitals of [iPr2tz]44- (F2)
into the lowest unoccupied F1 orbitals stabilizes the formation of
complex 4. Detailed analysis revealed that about 14% LUF1O/LUF1O’
(e presentation), 15.5% LUF1O+1/LUF1O+1’ (e presentation) and
A
(tz aromaticity)
Chart 1. The possible neutral aromatic 1,2,4-triazolyl radical (A).
To further elucidate the redox property of 1,2,4-triazolato ions
[3,5-R2tz], the cyclic voltammetric analyses of 1 and 2 were
performed in the presence of Bu4N·PF6. As shown in Figure S13,
only one electron irreversible oxidation wave was observed for [3,5-
Ph2tz]−/[3,5-Ph2tz]• at about Eox = +1.20 V vs AgNO3/Ag, probably
suggesting that 1,2,3-triazolato anion [3,5-Ph2tz]− is oxidized at
moderate positive potentials. However, no any significant
oxidation/reduction waves were detectable for 2 (Figure S12).
Although triazolyl radicals have attracted attention for a
long time, only few of the related reports appeared in
literatures.19,20 Lamotte and co-workers reported a 2,5-
dihydro-1H-1,2,4-triazol-2-yl neutral radical {H2[tz]} which was
created by gamma-irradiation of single crystals of 1,2,4-
triazole {H[tz]}.19a,b This radical was confirmed by analysis of
the measured proton hyperfine tensors of ENDOR EPR spectra
(Chart 2. B). After that, by using Ag2O, PbO2, or K3Fe(CN)6 as an
4+
10 % LUF1O+2 (a2 presentation) of Bi2 (F1) was filled by the
4-
electrons from occupied orbitals of [iPr2tz]4 (F2). This means that
the very short Bi-Bi bond distances arise from the electronic effect
of the N–N bond within the 1,2,4-triazolato ligands, which pull the
two Bi atoms closer. In other words, the shorter N-N bonds of the
1,2,4-triazolato ligands relative to the O-O bond distance in
dibismuth(trifluoroacetate)4 are responsible for the short Bi-Bi bond
in 3 and 4, as they were supported by four bridged ligands with the
smaller Bi-Bi-N angles (about 72(SI)) in 3 and 4, in comparison to
those in dibismuth(trifluoroacetate)4 with larger Bi-Bi-O angles (ca.
81).4
We also calculated the Gibbs free energy required for the
homocleavage from the proposed intermediate [Bi(R2tz)3] (IPh or IiPr
)
to radical species [Bi(R2tz)2] ((IIPh) or (IIiPr)) and [3,5-R2tz] ((IIIPh)
oxidant, Neugebauer et. al. prepared several similar neutral or (IIIiPr))(Tables S6-S11). The calculated bond dissociation energy
for the process is only 26.49 kcal/mol for Iph in THF, while 38.92
kcal/mol for IiPr, 11.43 kcal/mol higher than that of Iph. Based on the
results, the former transformation seemed to be accessible much
easier. Obviously, this is due to the delocalized electronic effect of
phenyl groups at the radical intermediate. The formation of
[Bi(R2tz)2] ((IIPh) or (IIiPr)) from an assumed intermediate
[ClBi(R2tz)2] (R = Ph/iPr) by the cleavage of BiCl bond can be ruled
out since the calculated bond dissociation energy is much higher
(58.92 or 58.79 kcal/mol) (Tables S18 and S19).
2,3(2,5)-dihydro-1H-1,2,4-triazol-l-yl
radicals20a,b
through
dehydrogenation of 4,5-dihydro-1H-l,2,4-triazoles, and one of
which was confirmed by X-ray single crystal structural analysis
(Chart 2, B). Recently, a structurally characterized carborane-
fused 1,2,3-trizaolyl anion radical was synthesized by Co(II)
reduction of a zwitterionic heterocyclic carborane-fused 1,2,3-
triazole (Chart 2, C).20c However, no any report for the
primitive aromatic neutral radicals of the type 1,2,4-trizolyl (A)
(Chart 1) appeared before this presentation, likely due to their
high activity as to be difficult for the isolation.
Considering that the open-shell five-membered free radicals are
prone to dimerize through heteroatoms at which the single electron
is located,17 it was possible that the resultant radicals (IIIPh) or
(IIIiPr) immediately dimerized into IIIPhIIIPh or IIIiPrIIIiPr dimer after
the generation. We therefore calculated the relative stability of the
N
R1
R2
Me
N
N
Ph
N
N
R3
C
B
N
expected dimer derived from (IIIPh) or (IIIiPr 
) radical at the
R2
B3LYP/6-31G(d) level, and the results suggested that dimer III(N1)-
III(N2) through N1–N2 coupling (Figures S5,S8) is more stable one,
which was located 12.9/7.8 kcal/mol lower than dimer III(N1)-III(N4)
through N1–N4 coupling, respectively (Figures S6, S9). The
calculated Bi–Bi bond distances in 3 (2.8721 Å) and 4 (2.865 Å) are
well in agreement with the experimental values (Tables S2S5).
However, the proposed dimeric III(N1)-III(N2) through N1–N2
coupling awaits further experimental demonstration.
B
(tz neutral)
C
(tz anion)
Chart 2. The reported triazolyl radicals (B and C).
The elemental analysis results agreed with the formula of
dibismuth complex 4 (SI). However, we were not able to get
the satisfied elemental analysis result of 3 because of partially
toluene molecule loss in the lattice during the dry under the
1
reduced pressure, as evidenced by the integration in the H
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Soc., 2018, 140, 13042; (h) J. Hansen, B. Li, E.VView. DAritkicalereOvn,linJe.
Conclusions
Autschbach and H. M. L. Davies, J. Org. Chem., 2009, 74, 6564;
DOI: 10.1039/D0DT03225E
In conclusion, we prepared two paddlewheel bismuth
complexes (3–4) with the very short Bi–Bi single bonds, which
are the first examples of bismuth(II) paddlewheel complexes
bearing 1,2,4-triazolato ligands. The neutral 3,5-substituted N-
1,2,4-triazolyl radical [3,5-R2tz] (III) was unambiguously
confirmed by EPR spectrum analysis. The nitrogen coupling
constants of the EPR spectrum suggested that the unpaired
electron of 1,2,4-triazolyl radicals is localized at the nitrogen
atoms with a σ-type ground state (σ) instead of over the five-
membered ring with a π-type ground state (π).24 Our work
demonstrated that the neutral aromatic N-1,2,4-triazolyl
radicals (A) were accessible by bismuth(III) oxidation of two
symmetric 1,2,4-triazolato anions, strongly suggesting that
bismuth(III) oxidation of aromatic five-membered heterocyclic
ring anions could be general.7-9
(i) L. R. Collins, S. Auris, R. Goddard and A. Fürstner, Angew.
Chem. Int. Ed., 2019, 58, 3557; (j) Z. Ren, T. L. Sunderland, C.
Tortoreto, T. Yang, J. F. Berry, D. G. Musaev, and H. M. L.
Davies, ACS Catal., 2018, 8, 10676.
M.-G. Zhao, L.-X. Wang, P. -P. Li, X. Zhang, Y. Yang and W. J.
Zheng, Chem. Commun., 2015, 51, 16184.
M.-G. Zhao, T. -T. Hao, X. Zhang, J. -P. Ma, J. -H. Su and W.
Zheng, Inorg. Chem., 2017, 56, 12678.
M.-G. Zhao, L. Wang, X. Zhang and W.-J. Zheng, Dalton Trans.,
2016, 45, 10505.
7
8
9
10 (a) I. Kobrsi, W. Zheng, J. E. Knox, M. J. Heeg, H. B. Schlegel
and C. H. Winter, Inorg. Chem., 2006, 45, 8700; (b) W. Zheng,
M. J. Heeg and C. H. Winter, Angew. Chem. Int. Ed., 2003, 42,
2761; (c) N. C. Mösch-Zanetti, M. Hewitt, T. R. Schneider and J.
Magull, Eur. J. Inorg. Chem., 2002, 1181.
11 The preparation, spectroscopic, crystallographic, and DFT
calculation data for 3 and 4 are contained in the Supporting
Information (SI). CCDC-1937537 for 3 and CCDC-1937538 for 4
contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union.
W.Z. gratefully acknowledges financial support from Shanxi
Normal University. M.Z. thanks the financial support from the
Natural Science Foundation of Shanxi Province (No.
201901D111278).
12 K.-S. Yeung, M. E. Farkas, J. F. Kadow and N. A. Meanwell,
Tetrahedron Lett., 2005, 46, 3429.
13 R. Grüner, Z. Beneš, E. Schubert and M. Arman, Monatsh.
Chem., 1927, 48, 37.
Conflicts of interest
There are no conflicts to declare.
14 G. M. Sheldrick, Acta Cryst.C, 2015, C71, 3.
Notes and references
15 (a) R. G. Hicks, Org. Biomol. Chem., 2007, 5, 1321; (b) R. G.
Hicks, John Wiley & Sons, Ltd: Wiltshire, U.K., 2010.
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DOI: 10.1039/D0DT03225E
Table of the content
Bismuth(III) oxidation of 3,5-di-substituted-1,2,4-triazolato anions allowed paddlewheel 1,2,4-
triazolato dibismuth complexes [L2(Bi–Bi)L2] (L = η1,η1-3,5-R2tz, R = Ph (3), iPr (4)) with very short
bismuth-bismuth bonds (2.8650(4)2.8721(3) Å) to be isolated and the reaction involved the
formation of neutral N-1,2,4-triazolyl radicals [3,5-R2tz] that were evidenced by EPR analysis.
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