Polyhedron 195 (2021) 114967
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, structures, and one- or two-electron reduction reactivity of
mononuclear lanthanide (Ho, Dy) complexes with sterically hindered
o-iminobenzoquinone ligands
Dmitriy K. Sinitsa, Taisiya S. Sukhikh, Sergey N. Konchenko, Nikolay A. Pushkarevsky ⇑
Nikolaev Institute of Inorganic Chemistry SB RAS, Akademika Lavrentieva Avenue 3, 630090 Novosibirsk, Russia
a r t i c l e
i n f o
Article history:
Received 30 October 2020
Accepted 7 December 2020
Available online 11 December 2020
Keywords:
Redox-active ligand
Lanthanide
O-iminobenzoquinone ligand
Reductive reactivity
Polysulfide ligand
a b s t r a c t
Complexes of Dy and Ho (Ln) with redox-active 4,6-di-t-butyl-N-(2,6-di-isopropylphenyl)-o-iminobenzoquinone ligand (dippIQ) in different charged states have been synthesized and investigated. Besides the
neutral state, the ligand is comprised as iminosemiquinolate (dippISQ–) or amidophenolate (dippAP2–).
Homoligand complexes [Ln(dippIQ)2I3] (1Ln), [Ln(dippISQ)2I(thf)2] (2Ln, thf = tetrahydrofurane) and [KLn
(dippAP)2(thf)4] (3Ln) have been obtained by earlier known methods that include complexation (for 1Ln)
followed by reduction with 2 or 4 equivalents of KC8 (for 2Ln and 3Ln, respectively). It has been shown
that upon reduction by 3 equivalents of KC8 mixed-ligand complexes [Ln(dippAP)(dippISQ)(thf)2] (4Ln)
are formed. Simple synthetic methods to the high-yield syntheses of 3Ln and 4Ln have been developed,
based on excessive reduction by K metal (for the former complex), followed by partial oxidation by I2
(for the latter one). In addition to the known reactivity of 3Ln towards S8 as a 2e reductant, 4Ln was shown
to act as 1e reductants, according to the number of reducing dippAP2– ligands. The reduction by 3Ln results
dipp
ISQ)2(S5)]– comin inclusion of S2–
5 anion in the coordination sphere, resulting in the known type [Ln(
plexes, crystallized with the [K(18-Crown-6)(thf)2]+ cation (6Ln). Unlike that, the reduction by 4Ln results
in inclusion of a chain of two sulfur atoms with the formation of new type of ligand (dippAP)S2–
2 in a complex [Ln(dippISQ){(dippAP)S2}(thf)2] (7Ln). Structures of all complexes are compared on the basis of X-ray
diffraction and IR spectroscopy data. Reaction routes leading to the complexes of different types are
discussed.
Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction
Ortho-quinones and iminoquinones are examples of simple and
well-studied redox-active ligands. These ligands can reversibly
uptake electrons, while maintaining structure. In complexes they
can exist in three charged forms (Scheme 1); for iminoquinone
these are neutral iminoquinones (IQ), iminosemiquinone radical
anion (ISQ–) and amidophenolate dianion (AP2–) [1,2]. Steric and
electronic influence of these ligands can be tuned by changing
the substituents in the carbocycle and at the nitrogen atom of an
iminoquinone. Complexes with these redox-active ligands represent very interesting class of compounds in coordination chemistry. They can reveal notable magnetic interaction between
metal and several ligand paramagnetic centres, in case the ligands
are in radical monoanionic state [3-8]. Metal-(imino)quinone
systems could possess redox isomerism (also called valence
⇑ Corresponding author.
E-mail address: nikolay@niic.nsc.ru (N.A. Pushkarevsky).
https://doi.org/10.1016/j.poly.2020.114967
0277-5387/Ó 2020 Elsevier Ltd. All rights reserved.
tautomerism) [9,10], which becomes possible when there is a little
difference in energies of metal d-orbitals and the frontier orbitals
of an (imino)quinone [11]. Redox-active ligands can provide additional reactivity to the metal centre by acting as an electron reservoir [12-16], which can be used in catalytic processes [17-20]. This
is especially important in case of central atoms with few or only
one stable positive redox state, since the ligand can effectively supplement the metal cation with redox transitions.
Main feature of o-iminoquinone ligands compared to related oquinones is the presence of a substituent on the N atom directly
coordinated to the metal centre. Appropriate substituents can be
used for steric and donor modification of the coordination sphere,
as well as for connection of several iminoquinone units, to achieve
desired electronic interaction and reactivity [21-27]. The majority
of complexes with o-iminoquinone ligands are those of transition
metals and main group elements [1,28-35], while there are only
few known lanthanide complexes, especially those with determined crystal structures [15,36-38]. Meanwhile, o-iminoquinone
(and related 1,2-diimine) ligands can be very suitable for
D.K. Sinitsa, T.S. Sukhikh, S.N. Konchenko et al.
Polyhedron 195 (2021) 114967
Scheme 1. Three different redox states of an iminoquinone.
lphenyl)-o-iminobenzoquinone (dippIQ) [52] were prepared according to known procedures. IR spectra (cm1) were recorded by
means of a FT-801 spectrometer (Simex) in KBr discs, which were
prepared in a glove box. Elemental analysis for C, H, N and S was
performed by means of Vario Micro Cube analyzer (Elementar).
lanthanide ions to show the ligand- and metal-mediated redox
reactivity, which is an actively studied topic in recent years
[39,40]. Indeed, voluminous substituents at coordinating N atoms
can provide substantial steric bulk for stabilization of the large
coordination sphere of these ions. Typically lower reduction potentials of iminoquinones with respect to those of related quinones
[41] can be closer to the low potentials of Ln3+/Ln2+ transitions
(e. g. for Yb) and allow for the further search of redox isomerism
of Ln complexes, which by now has been reliably confirmed for
only one case of Yb-diimine complex [42]. Mostly ionic bonding
between the central atom and the ligand provides that in the most
cases the iminoquinone ligand can participate in redox processes
independently from the Ln centre. Given that anionic forms of iminoquinone ligands are rather strong reductants [43], their reductive reactivity can greatly expand the range of potential
application for the complexes.
Recently, Bart and co-workers reported that neodymium complex with neutral 4,6-di-t-butyl-N-(2,6-di-isopropylphenyl)-o-iminobenzoquinone ligand (dippIQ, dipp = 2,6-di-isopropylphenyl),
[NdI3(dippIQ)2], can provide reversible redox transitions being
reduced in two-electron steps into bis(iminosemiquinolato) and
bis(amidophenolato) derivatives [15]. It was shown that fully
reduced dippAP2– complexes could act on chalcogens (Q = S, Se) as
2e reducing agents, resulting in pentachalcogenide complexes with
iminoquinone ligands in coordination sphere, [Nd(Q5)(dippISQ)2].
Potentially, reduction of bis(iminoquinone) complex by 3e should
result in a mixed-ligand dippISQ-dippAP complex, which, in turn,
should behave as a 1e reducing agent. This behaviour can thus be
analogous to well-established 1e reductions by Ln(II) complexes,
such as lanthanocenes (LnCpx2, where Cpx is a substituted
cyclopentadienide) [44-48]. The mixed dippISQ-dippAP complexes
of Eu and Yb were reported by Klementieva and co-workers [36],
but they were obtained by intramolecular reduction of iminosemiquinone ligand by the divalent lanthanide ion, which is
not suitable for the majority of lanthanides. Our interest in the current study was to explore the possibility of synthesis of the dippISQ-dippAP complexes of different lanthanides and to estimate
their capability of 1e reductants. As lanthanides of choice, dysprosium and holmium were taken as having hardly accessible 2 + oxidation state and being potentially interesting for their magnetic
properties [49].
2.2. Synthesis of [LnI3(dippIQ)2] (1Dy, 1Ho)
An ampoule with two sections joined at right angle was
used. Into one section was placed [LnI3(thf)3.5] (0.40 g, 0.50 mmol),
dipp
IQ (0.379 g, 1.00 mmol), and a stirring bar. Toluene was vacuum-condensed to the mixture, the ampoule was evacuated and
sealed. Reaction mixture was stirred for 2 h at room temperature,
than the precipitate was allowed to settle, and dark-red solution
was decanted to the second section. The ampoule was inclined so
that upon slight warming of the lower part the solvent condensed
in the upper one and flowed back with the dissolved compound.
After one day the compound was fully collected in the second section. The solvent was fully evaporated to another section, leaving
dry complex as red crystalline precipitate. The section with the
compound was sealed off. Yields: 1Dy, 0.496 g (72%); 1Ho, 0.396 g
(57%). X-ray quality crystals of 1LnEt2O were obtained by
recrystallization from a diethyl ether-pentane mixture (5:1 v/v).
1Dy (C52H74DyI3N2O2). Analysis, calcd. C 47.96, H 5.73, N 2.15%;
found C 47.45, H 6.0, N 2.1%. IR: 2963 s, 1609 s, 1517 m, 1464 s,
1436 m, 1383 s, 1363 m, 1308 w, 1248 m, 1193 w, 1160 w, 1099
w, 1023 w, 900 w, 792 m cm1. 1Ho (C52H74HoI3N2O2). Analysis,
calcd. C 47.87, H 5.72, N 2.15%; found. C 47.4, H 6.0, N 2.1%. IR:
2962 s, 1609 s, 1517 m, 1464 s, 1436 m, 1383 s, 1363 m, 1308
w, 1248 m, 1193 w, 1160 w, 1099 w, 1023 w, 900 w, 792 m cm1.
2.3. Synthesis of [LnI(dippISQ)2(thf)] (2Dy, 2Ho)
To the mixture of 1Dy or 1Ho (0.130 g, 0.100 mmol) and KC8
(0.027 g, 0.20 mmol) 10 mL of thf was condensed while cooling
in liquid N2. Reaction mixture was left with stirring for 2 h at ambient temperature. Original dark red colour of the solution changed
fast to dark green. Resulting mixture was then filtered through a
glass frit, and the filtrate was evaporated to dryness to give green
crystalline product. Yields: 2Dy 0.096 g (86%), 2Ho 0.082 g (73%). Xray quality crystals of 2Dy0.5C5H12 and 2HoC5H12 were obtained
after recrystallization from pentane. The syntheses starting from
[LnI3(thf)3.5] and 2 eqs. of dippIQ, with subsequent addition of 2
eqs. of KC8 led to similar outcomes. 2Dy (C56H82DyIN2O3). Analysis,
calcd. C 60.01, H 7.38, N 2.50%; found C 59.3, H 7.3, N 2.4%. IR:
2962 s, 1584 w, 1464 s, 1420 s, 1387 m, 1362 w, 1321 m,
1251 m, 1197 w, 1168 w, 1114 w, 1026 w, 1010 w, 989 w, 935
w, 911 w, 860 m, 821 w, 796 m, 765 w, 656 w cm1. 2Ho (C56H82HoIN2O3). Analysis, calcd. C 59.89, H 7.36, N 2.49%; found C 59.9, H
7.2, N 2.32%. IR: 2962 s, 1584 w, 1464 s, 1420 s, 1387 m, 1362 w,
1321 m, 1251 m, 1197 w, 1168 w, 1114 w, 1026 w, 1010 w, 989
w, 935 w, 911 w, 860 m, 821 w, 796 m, 765 w, 656 w cm1.
2. Experimental section
2.1. General remarks
All manipulations were performed in strictly anaerobic
conditions, in argon atmosphere or in vacuum, with the use of
glove box and standard Schlenk techniques. Solvents were dried
and deoxygenated by distillation over Na-K alloy (with the
addition of benzophenone for thf). Starting lanthanide triiodides
[LnI3(thf)3.5] [50,51] and 4,6-di-t-butyl-N-(2,6-di-isopropy2
Polyhedron 195 (2021) 114967
D.K. Sinitsa, T.S. Sukhikh, S.N. Konchenko et al.
2.4. Synthesis of [{K(thf)2}{Ln(dippAP)2(thf)2}] (3Dy, 3Ho)
2.6. Reaction of 2Dy with 3Dy
A mixture of [LnI3(thf)3.5] (0.40 g, 0.50 mmol) and dippIQ
(0.379 g, 1.00 mmol) was stirred in thf (10 mL) for 5 min at ambient temperature. With continued stirring, 4 eqs. of KC8 (0.270 g,
2.00 mmol) were added, and the reaction was continued for the
next 6 h. Then light-green solution was filtered through a glass frit
to remove graphite and potassium iodide, and the solvent was
removed in vacuum. Resulting oil was redissolved in 15 mL of pentane, and during subsequent slow evaporation of the solvent to the
volume of ca. 1 mL beige crystalline powder of the product was
obtained. The remaining mother liquor was decanted, and the product was dried in vacuum. Yields: 3Dy 0.336 g (54%), 3Ho 0.378 g
(60%). X-ray quality crystals were obtained after recrystallization
from pentane. The reaction proceeded similarly when 2Ln complexes were reduced by 2 eqs. of KC8. 3Dy (C68H106DyKN2O6). Analysis, calcd. C 65.38, H 8.55, N 2.24%; found C 64.8, H 8.2, N 2.3%. IR:
2955 s, 1590 w br, 1562 m, 1480 m, 1465 s, 1421 s, 1382 m,
1360 m, 1320 m br, 1255 s, 1200 m, 1166 w, 1127 m, 1054 s,
977 w, 935 w, 860 m, 835 m, 797 m, 762 w, 652 w, 529 m cm1.
3Ho (C68H106HoKN2O6). Analysis, calcd. C 65.25, H 8.54, N 2.24%;
found C 65.5, H 8.6, N 2.2%. IR: 2955 s, 1590 w br, 1560 m,
1481 s, 1465 s, 1421 s, 1382 m, 1360 m, 1320 m br, 1255 s,
1200 m, 1166 w, 1127 m, 1054 m, 976 w, 940 w, 860 m, 840 m,
797 m, 762 w, 652 w, 529 m cm1.
To a mixture of 2Dy (0.029 g, 0.026 mmol) and 3Dy (0.032 g,
0.026 mmol) 5 mL of thf were condensed. The colour of the solution quickly turned from dark-green to green–blue upon stirring
at room temperature, and the precipitation of KI was observed.
After 1 h the reaction mixture was centrifuged and filtered, the
clear solution was evaporated to dryness, and the resulting oily
product was recrystallized from pentane. After drying, 0.021 g
(76%) of 4Dy was obtained.
2.7. Reaction of [{K(thf)2}{Ln(dippAP)2(thf)2}] (3Dy, 3Ho) with S8
To the mixture of solid reagents, [{K(thf)2}{Ln(dippAP)2(thf)2}]
(0.100 g, 0.0800 mmol), 18-crown-6 (0.021 g, 0.079 mmol) and
S8 (0.013 g, 0.051 mmol) 10 mL of toluene were condensed while
cooling the reaction tube with liquid N2. The reaction mixture
was then stirred for 2 h at room temperature, during which color
of solution changed from colorless to dark-green. The solution
was filtered, and volatiles were removed in vacuum. The solid product was dissolved in a mixture of 1 mL of THF and 4 mL of pentane, and the solution was stored at –30 °C for 2 days, during
which dark crystals formed, being suitable for XRD. The crystals
were separated by decantation and carefully dried in vacuum.
Yields: 6Dy 0.083 g (68%), 6Ho 0.054 g (44%). 6Dy (C72H114DyKN2O10S5). Analysis, calcd. C 56.54, H 7.51, N 1.83, S 10.48%; found C 56.0,
H 7.5, N 1.8, S 9.8%. IR: 2957 s, 1583 w, 1465 s, 1435 s, 1380 m,
1351 s, 1328 m, 1251 s, 1197 m, 1168 m, 1111 s, 988 w, 963 m,
909 w, 862 m, 837 m, 794 m, 764 w, 657 w, 534 w cm1. 6Ho
(C72H114HoKN2O10S5). Analysis, calcd. C 56.45, H 7.50, N 1.83, S
10.47%; found C 55.8, H 7.7, N 2.1, S 10.4%. IR: 2957 s, 1583 w,
1465 s, 1435 s, 1380 m, 1351 s, 1328 m, 1251 s, 1197 m,
1168 m, 1111 s, 988 w, 963 m, 909 w, 862 m, 837 m, 794 m, 764
w, 657 w, 534 w cm1.
2.5. Syntheses of [Ln(dippAP)(dippISQ)(thf)2] (4Dy, 4Ho)
Variant 1. To a mixture of solid reagents, [LnI3(thf)3.5] (0.40 g,
0.50 mmol) and dippIQ (0.379 g, 1.00 mmol), 10 mL of thf were condensed. With constant stirring, 3 eqs. of KC8 (0.203 g, 1.50 mmol)
were added. The colour gradually changed from brown to green–
blue over the next 3 h. Then the solution was filtered through a
glass frit to remove graphite and potassium iodide, and the filtrate
was evaporated in vacuum to give oily residue. Pentane (10 mL)
was added, and the resulting solution was separated from white
precipitate of 5Ln by centrifugation. Slow evaporation of the solution to the volume of ca. 2 mL resulted in precipitation of green–
blue crystals of the product, suitable for XRD. Yields: 4Dy 0.319 g
(60%), 4Ho 0.160 g (30%). 4Dy (C60H90DyN2O4). Analysis, calcd. C
67.61, H 8.51, N 2.63%; found C 67.3, H 8.3, N 2.7%. IR: 2964 m,
1585 w, 1560 w, 1482 w sh, 1464 s, 1432 s, 1384 m, 1361 m,
1321 m, 1308 m, 1251 s, 1200 m, 1168 m, 1127 m br, 1056 w,
1026 m, 988 w, 935 w, 910 w, 860 m, 841 m, 797 m, 765 m,
656 m cm1. 4Ho (C60H90HoN2O4). Analysis, calcd. C 67.45, H
8.49, N 2.62%; found C 67.8, H 8.2, N 2.7%. IR: 2964 s, 1584 w,
1559 w, 1482 w sh, 1464 s, 1433 s, 1384 m, 1361 m, 1321 m,
1308 m, 1251 s, 1197 m, 1168 m, 1126 m br, 1056 w, 1026 m,
988 w, 935 w, 910 w, 860 m, 841 m, 797 m, 765 m, 656 m cm1.
Variant 2. To the mixture of solid reagents, [LnI3(thf)3.5]
(0.210 g, 0.264 mmol), dippIQ (0.202 g, 0.532 mmol) and an excess
of potassium (0.20 g, 5.1 mmol) as a single chunk, 5 mL of thf were
added, and the mixture was left with stirring. The initial darkbrown colour quickly changed to dark-green, and then gradually
to light-brown (visible after settling of precipitated KI) in the
course of next 5 h, which corresponds to complete formation of
dipp
AP2–. The rest of potassium was removed. Solution of I2
(0.033 g, 0.13 mmol) in 5 mL of thf was slowly added to the mixture via cannula with constant stirring during 15 min, upon which
the colour was changing to greenish-blue. The mixture was centrifuged to remove KI, the solution was additionally filtered
through a glass frit. Evaporation and drying in vacuum resulted
in dark oily product, which crystallized after addition of 2 mL of
pentane. After chilling at –20 °C for 6 h, the solution was removed
by a cannula, and the crystalline product was dried in vacuum.
Yields: 4Dy 0.130 g (46%), 4Ho 0.160 g (30%).
2.8. Reaction of [Ln(dippAP)(dippISQ)(thf)2] (4Dy, 4Ho) with S8
To the solid reagents, [Ln(dippAP)(dippISQ)(thf)2] (0.054 g,
0.051 mmol) and S8 (0.0032 g, 0.012 mmol), 10 mL toluene was
condensed while cooling in liquid N2. Reaction mixture was stirred
at ambient temperature for 3 h. Gradual colour change from blue
to green was observed. The solvent was slowly evaporated to yield
oily residue with small amount of green–blue crystals. To obtain
crystals suitable for XRD, the product was dissolved in a mixture
of 1 mL of THF and 4 mL of pentane and stored at –30 °C for 2 days.
Precipitated crystals were dried in vacuum prior to characterization. Yields: 7Dy 0.043 g (75%), 7Ho 0.049 g (85%). 7Dy (C60H90DyN2O4S2). Analysis, calcd. C 63.77, H 8.03, N 2.48, S 5.67%; found C 63.2,
H 7.7, N 2.1, S 5.5%. IR: 2958 s, 2865 m, 1584 w, 1554 w, 1462 s,
1428 s, 1384 m br, 1360 w sh, 1347 m, 1326 w, 1305 s, 1281 w,
1251 s, 1234 w sh, 1199 m, 1167 m, 1102 m, 1056 w, 1027 s,
990 m, 910 m, 886 w sh, 863 m, 844 m, 801 w, 792 w, 763 m,
658 w, 643 w cm1. 7Ho (C60H90DyN2O4S2). Analysis, calcd. C
63.64, H 8.01, N 2.47, S 5.66%; found C 63.7, H 8.1, N 2.4, S 5.6%.
IR: 2958 s, 2865 m, 1584 w, 1554 w, 1462 s, 1428 s, 1384 m br,
1360 w sh, 1347 m, 1326 w, 1305 s, 1281 w, 1251 s, 1234 w sh,
1199 m, 1167 m, 1102 m, 1056 w, 1027 s, 990 m, 910 m, 886 w
sh, 863 m, 844 m, 801 w, 792 w, 763 m, 658 w, 643 w cm1.
2.9. Single crystal X-ray diffraction study
Single crystal XRD data for the compounds 1Ln–6Ln at 150 K
were collected by a Bruker Apex DUO diffractometer equipped
with a 4 K CCD area detector using the graphite-monochromated
MoKa radiation (k = 0.71073 Å) (Table S1, Supporting Information).
Single crystal XRD data for 7DyC6H14 were collected at 150 K with a
3
D.K. Sinitsa, T.S. Sukhikh, S.N. Konchenko et al.
Polyhedron 195 (2021) 114967
analogously to the Nd congeners [15]. The compounds are well soluble in this solvent, but concentration of the solution does not
result in crystals of good quality; instead, crystalline solids are
formed, which composition corresponds to the desired complexes
without solvate molecules (according to elemental analysis). Better
crystals, suitable for X-ray diffraction (XRD), were obtained as Et2O
solvates after recrystallization from ether-pentane mixtures, but
they readily lose the solvate molecules upon storage. Longer reaction and crystallization times should be avoided because the iminoquinone has a tendency for intramolecular cyclization and
subsequent Diels-Alder cyclodimerization in solution (Scheme 3).
This process is noticeable even at room temperature, and runs faster upon heating [52]. While growing the crystals of 1Ln from
toluene at elevated temperatures (50 °C) for several days, we
observed formation of insoluble faintly coloured compounds covering the crystals of the complex. Considering that the complexes
with neutral dippIQ are not very stable towards dissociation, it
could lead to formation of free iminoquinone, and then the dimer
or products of its coordination to LnI3, insoluble in toluene. The formation of these by-products could be the main cause of losses during the synthesis of 1Ln. Thus, the synthetic procedure was
optimized for a sealed ampoule synthesis, to eliminate heating to
higher temperatures during crystallization.
Stepwise reduction of 1Ln by KC8 resulted in complexes with the
iminoquinone ligand in charged states, [Ln(dippISQ)2I(thf)] (2Ln)
and [{K(thf)2}Ln(dippAP)2(thf)2] (3Ln) (Scheme 2b,c), as was shown
before for Nd analogues [15]. Keeping in mind the aforementioned
unwanted degradation of complexes with neutral dippIQ ligand,
Bruker D8 Venture diffractometer with a CMOS PHOTON III detector and ImS 3.0 source (mirror optics, k(MoKa) = 0.71073 Å). The uand x-scan techniques were employed to measure intensities for
all samples. Absorption corrections were applied with the use of
the SADABS program [53]. The crystal structures were solved using
the SHELXT [54] and were refined using SHELXL [55] programs
with OLEX2 GUI [56]. Atomic displacement parameters for nonhydrogen atoms were refined anisotropically, with the exception
of solvent molecules. For the disordered entities, SADI, RIGU and
ISOR restrains or EADP constraints were applied where needed.
In structure 3Ho0.68C5H120.15thf, disordered pentane and
tetrahydrofuran solvent molecules occupying the same position
were refined in rigid body approximation. In structure 2HoC5H12,
some artefact residual peaks and oblate ADP ellipsoids are due to
the presence of the second crystal domain. In 0kl layer (Fig. S3a),
reflections from the major crystal domain have closely spaced less
intense reflections from the second domain. In h0l (Fig. S3b) and
hk0 (Fig. S3c) layers, the reflection positions from the two domains
coincide. Due to strong overlapping reflections, we failed to process the data as the twin.
3. Results and discussion
3.1. Synthesis of homoligand complexes
The Dy and Ho complexes with two neutral dippIQ ligands, [Ln
(
IQ)2I3] (1Ln), were readily formed in reaction of lanthanide
triiodides with the iminoquinone in toluene (Scheme 2a),
dipp
Scheme 2. Synthetic routes to homoligand lanthanide-iminoquinone complexes.
4
Polyhedron 195 (2021) 114967
D.K. Sinitsa, T.S. Sukhikh, S.N. Konchenko et al.
LnI3 (dipp IQ)2 + 3 “K” ! Ln(dipp ISQ)(dipp AP) + 3 KI
ð1Þ
LnI3 (dipp IQ)2 + 3 “K” ! 0.5 Ln(dipp ISQ)2 I + 0.5 KLn(dipp AP)2 + 2 KI
ð2Þ
Contrary to the syntheses of 2Ln and 3Ln, in reductions by 3 eqs.
of KC8 we always found small amounts of white crystalline precipitate separating upon pentane extraction of the main product 4Ln.
In the case of holmium, the structure of a crystal picked from the
precipitate was determined by means of XRD as a complex with
only one iminoquinone ligand, [HoI(dippAP)(thf)3] (5Ho). Potentially,
the same complex (5Dy) is also formed in dysprosium reaction, but
its structure was not determined. The compounds were neither
isolated nor further characterized. Still, the formation of such complexes can be rationalized if we consider that the initial complex
1Ln is partially dissociated in thf solution to [LnI3(dippIQ)(thf)n]
and free dippIQ. These two species are then reduced independently
to form 5Ln and K(dippIQ) (Eq. 3). Crystallization of the product 5Ln
then implies that it does not further interact with K(dippIQ) to form
4Ln and KI in the conditions of the experiment.
LnI3 (dipp IQ) +
Scheme 3. Cyclization and dimerization of
dipp
dipp
IQ + 3 “K” ! Ln(dipp AP)I + K(dipp ISQ) + 2 KI
ð3Þ
IQ in solutions (R = dipp).
Ln
dipp
In order to reliably produce complexes 4 starting from
IQ,
a simpler method was developed. After reduction of two equivalents of dippIQ with an excess of potassium, when 3Ln is formed
in solution, it is oxidized by slow addition of a half of equivalent
of I2 (Scheme 4c). All precipitated KI is removed after the second
stage, and the resulting solution contains only 4Ln, which cleanly
crystallizes from pentane. Analogous approach have been used
for producing mixed-ligand dippAP-dippISQ complexes of tin(IV)
[57]. Thus, both 3Ln and 4Ln could be produced in large amounts
by simple procedures, and were further studied as reducing agents.
we checked the possibility to obtain the complexes with dippISQ–
and dippAP2– ligands without preliminary separation of the compounds 1Ln. It is anticipated that anionic forms of the ligand are
not prone to cyclization or dimerization. Indeed, after mixing of
Ln triiodide and the iminoquinone in thf, an addition of 2 or 4
equivalents of KC8 results in formation of a desired product in solution. Use of stoichiometric amounts of KC8 required several hours
for the reaction to run to completeness. Owing to high solubility,
the products were recrystallized from pentane. Even in this solvent
the solubility is high enough to result in the yields of crystalline
compounds of 50–70% in some cases, although no by-products
were observed in noticeable amounts.
Another more convenient method was developed for synthesis
of complexes with fully reduced dippAP2– ligands. An excess of
metallic potassium can be added to the mixture of LnI3 and 2
equivalents of dippIQ in thf; successive stirring results in gradual
decrease of solution colour to light-brown, corresponding to the
formation of 3Ln. The excessive potassium remains unreacted,
and can be taken out in a single piece. This method does not
require removal of fine insoluble carbon powder after the reaction,
and implies the use of easier accessible potassium metal instead of
KC8.
3.3. Structures of complexes with iminoquinone ligands
Structures of complexes 1Dy, 1Ho, 2Dy, 2Ho, 3Ho, 4Ho, 5Ho were
determined by means of single-crystal XRD. The complexes 1Ln
crystallize with half of Et2O molecule per molecular unit, and are
isostructural between each other and with the earlier known Nd
analog [15]. The coordination number (CN) of the central atom is
7, so the molecule has quasi-C2v symmetry (Fig. 1). According to
the Continuous Symmetry Measures (CSM) method [71], the coordination polyhedron is best described as a pentagonal bipyramid
with the symmetry not far from D5h, where two iodides occupy
the apical positions (Table S2, Supporting Information). Both neutral dippIQ ligands are almost coplanar, the angles between their
mean planes (determined by the C6NO atoms) are ca. 170°. Owing
to the presence of three large I anions, the dippIQ ligands are forced
to occupy the transoid positions, so that two dipp groups of the
neighbouring ligands surround the central I atom.
The complexes 2Ln contain only one large I anion and the CN is 6
(distorted octahedron, Fig. 1; Table S3, Supporting Information).
This leads to more close disposition of two iminoquinone ligands
than in 1Ln, so that their van-der-Waals surfaces abut upon each
other. The neighboring dippISQ– ligands are thus in cisoid positions,
and the angles between their mean planes lie in the interval of
71.9–72.6°. The molecule is asymmetric. Both complexes have very
close geometries but form different crystal packings and crystallize
with different number of solvate molecules: 2Dy0.5C5H12 and 2HoC5H12. The latter complex is isostructural with the Nd congener
[15]. In both structures the molecules are packed pairwise, while
in each pair the molecules face each other by the side of the Ln–I
bond, in antiparallel manner, which is likely to be caused by interaction of dipole moments arising from these bonds (see Fig. S1,
3.2. Synthesis of heteroligand complexes
Reduction of complexes with two neutral dippIQ ligands by
even amount of electrons (2e or 4e) results in both ligands in
the same charge state. Reduction by 3e could result either in
complex with two ligands in different charge state (Eq. 1), or in
a mixture of homoligand complexes (Eq. 2, ‘‘K” denotes a reducing agent, e. g. KC8, thf ligands are not shown). The latter variant
would mean that the formation of KI is less preferential than
binding of K+ and I– in different coordination spheres. Our experiments showed that the mixed ligand complex [Ln(dippISQ)(dippAP)
(thf)2] (4Dy, 4Ho) is the main product of 3e reduction (Scheme 4a).
Moreover, upon interaction of equimolar amounts of bis-ISQ and
bis-AP complexes of Dy a fast comproportionation was observed,
and the mixed-ligand complex 4Dy was formed as the only crystallized reaction product with good yield (Scheme 4b). Potentially,
this reaction occurs as intermolecular electron transfer rather
than ligand redistribution.
5
D.K. Sinitsa, T.S. Sukhikh, S.N. Konchenko et al.
Polyhedron 195 (2021) 114967
Scheme 4. Synthesis of mixed-ligand ISQ-AP complexes by different approaches.
Fig. 1. Molecular structure of complexes (left) 1Ln on an example of 1Dy; (middle) 2Ln on an example of 2Dy; (right) 3Ho, one independent molecule is shown, dashed bond
denotes coordination by p system of the quinone cycle. iPr, tBu groups and thf ligands are reduced, H atoms are omitted for better visibility.
accommodate a K+ cation in the interligands space. It is coordinated by the bridging O atoms of dippAP2– ligands, and the coordination sphere is supplemented by close contacts with the p-system
of the iminoquinone cycles (as evidenced by relatively short distances KCcycle of 2.9–3.4 Å) and by two thf molecules. The Ho
cation is also bound with two thf ligands. The complex crystallizes
as a mixed pentane-thf solvate 3Ho0.68C5H120.15thf and contains
three crystallographically independent complex molecules, which
possess very close molecular geometries. It is isostructural with
the Nd predecessor [15].
The molecular structure of the heteroligand complex 4Ho is similar to that of 2Ln and 3Ln in the environment of the Ln centre
(Fig. 2). The structure contains two independent molecules with
close geometries. A molecule is quasi-C2 symmetrical owing to
the presence of two thf ligands, where the axis is aligned with
the bisector of two Ln–O(thf) bonds. The iminoquinone ligands
are in cisoid positions and their proximity is again determined by
van-der-Waals contacts (as in 2Ln), so the angle between their
Supporting Information for details). The shortest LnLn distance is
observed within these pairs (ca. 9.0 Å), while the other closest Ln
atoms are placed at the interval of 10.5–16.5 Å. The pairs are then
packed differently in the two structures, likely determined by
number of solvate molecules. Interestingly, in 2Dy the two molecules in a pair are crystallographic inversion equivalents, while in
2Ho these are also of inverted chirality relative to each other (and
their atoms lie very close to the inversion equivalent positions)
but still crystallographically independent molecules.
The structure of 3Dy was not determined by XRD, but we consider it to be the same as that of 3Ho, because these two compounds
show equal reaction and crystallization behaviour, as well as consistent IR spectra and analytical data. Molecular structure of 3Ho
resembles that of 2Ln in the ligand disposition around central atom
(Fig. 1), which also leads to an asymmetric complex. Both dippAP2–
ligands are in cisoid positions relative to each other, and the angle
between their mean planes equals ca. 93°. This larger value with
respect to the 2Ln complexes arises from the necessity to
6
Polyhedron 195 (2021) 114967
D.K. Sinitsa, T.S. Sukhikh, S.N. Konchenko et al.
Fig. 2. Molecular structure of complexes (left) 4Ho, one independent molecule is shown; (right) 5Ho. iPr, tBu groups and thf ligands are reduced, H atoms are omitted for better
visibility.
Several methods have been developed to determine the oxidation state of iminoquinone ligands based on their structural
parameters, such as calculation of metrical oxidation state (MOS)
by S. Brown [63], as well as use of averaged CAO/CAN distances
and difference in backbone CAC bond lengths (Da) by R. Mukherjee
[64]. While implemented to the structures under current study, the
former method results in MOS values of 0.04(5)–0.17(6) for 1Ln,
and –0.83(7), –0.99(5) for 2Dy, which fall in a typical ranges for dippIQ and dippISQ–, respectively (see Supporting Information for
details). The MOS of two of four structurally independent dippISQ–
ligands in 2Ho are somewhat more deviated from the expected
value of –1, with large esds (–0.70(14) and –1.37(19)), which could
be possibly a result of deviations of bond lengths in the ligand cycle
from the expected values, caused by interactions between the
ligands in the coordination sphere. The alteration of MOS caused
by intermolecular interactions are more noticeable in the structure
of 3Ho, which contains three independent molecules with six
mean plains is 73.0°. Notably that the structurally equal complex
[Nd(dippAP)2(thf)2]– [15] possesses much larger angle of 105.4°
between the planes of two iminoquinone ligands, owing to the
presence of two dianionic ligands. The complex 4Ho crystallizes
as a solvate 4Ho1.35C5H12. The molecular structure of 4Dy was
not determined by means of XRD, but it is considered to be the
same as of 4Ho on the basis of identical chemical behaviour and
crystal shape, as well as on appropriate analysis and IR spectroscopic data.
The dippAP2– and dippISQ– ligands can be clearly distinguished on
the basis of their geometry, since the bond lengths in (imino)quinone ligands well correspond to their charge state [37,58-62].
Indeed, comparison of bond lengths in the C2NOLn metallacycles
(Fig. 3) shows that the CAN and CAO bonds elongate, while CAC
bonds are shortened upon going from dippIQ to dippAP2– derivatives.
The bonds for different ligands in the same charged state fall close
to each other even in the complexes with different Ln cation.
Fig. 3. A diagram of bond lengths in C2NOLn metallacycles in all structurally characterized Ln complexes with the dipp-iminobenzoquinone ligands (the dipp-prefix is
omitted). For complexes possessing ligands in different charged state, or those with different bonding (bridging and terminal), the symbols for the same ligand type are put
apart and vertically aligned. The bonds in SNC2OLn metallacycle in 7Dy are also shown for comparison. The data for Nd compounds is taken from ref. [15], the data for Eu and
Yb compounds is taken from ref. [36].
7
D.K. Sinitsa, T.S. Sukhikh, S.N. Konchenko et al.
Polyhedron 195 (2021) 114967
different dippAP2– ligands. The only one ligand with its C6 cycle
placed close to the K cation demonstrates much more positive
MOS with large esd (–0.84(76)), while the other five with their
cycles further apart from K cations, including the neighboring
one of the same molecule, have rather normal MOS values (from
–1.87(11) to –1.94(9)). Evidently, coordination of the K cation by
the p-system of the ligand causes its disproportional distortion
(moreover, such cases were not used in the training sets of the
complexes while determining the typical bond lengths values for
MOS calculations in [63]); thus, structural parameters should be
used with care in this case. In the structure of 4Ho, two different
ligands have typical MOS values, –0.90(9) and –0.85(12) for dippISQ–, and –1.91(12) and –1.81(13) for dippAP2–, for two independent
molecules.
The Ln–N and Ln–O bond lengths are influenced by several factors. The lanthanide contraction is observed upon going from Nd to
Ho in the complexes of the same structure. These bonds are also
notably shortened in 2Ln as compared to 1Ln complexes, while
when comparing 3Ln with 2Ln this shortening is not so explicit.
The bond lengths in 4Ho clearly fall in two different sets, each
one corresponding to a ligand in a separate charge state. Similar
situation was found for the binuclear complex [Yb(dippAP)(dippISQ)]2 [36], where the Yb atoms are chelate-bridged by two dippAP2– ligands (with m-O atoms) and the dippISQ– ligands are
terminal. The Yb–N and Yb–O bond lengths clearly depend on
the ligand charge state (Fig. 3) with the exception of elongated
Ln–O(bridging) bonds. Comparing the same ligands of these two
heteroligand complexes, some shortening of Ln–O and Ln–N bonds
caused by lanthanide contraction is also noticeable.
Crystal structure of 5Ho3thf consists of mononuclear complexes
which are quasi-symmetrical with respect to the mean plane of the
dipp
AP2– ligand (Fig. 2). The Ho and I atoms deviate from this plane
by 0.18 and 0.69 Å, respectively. The complex crystallized as a solvate with three molecules of thf per molecule of complex. Despite
the surplus of thf and the presence of only one bulky ligand in the
coordination sphere, the CN of Ho is only 6 (distorted octahedron).
The characteristic bond lengths in the C2NOLn metallacycle expectedly correspond to the charge state of dippAP2– ligand (Fig. 3); MOS
equals –2.03(7). Notably, that the Ln–N and Ln–O bonds in 5Ho are
by ca. 0.1 Å shorter than those in 3Ho, likely owing to more sterically relaxed coordination sphere in the former. This complex can
be compared to thf-deficient tetranuclear centrosymmetric complexes [Ln2(dippAP)2(thf)]2 (Ln = Eu, Yb) described earlier [36],
which also contain one amidophenolate ligand per Ln ion. The Ln
cations in these complexes have 2+ charge, and their coordination
spheres are saturated by chelate-bridging ligands (m-N, m-O-coordination) and by additional coordination of C6 cycle of the terminal
ligand belonging to the second half of the molecule. The lengths
of CAC, CAN and CAO bonds in the C2NOLn metallacycles mostly
correspond to dippAP2– ligands, but for the terminal ligands they
resemble those of dippISQ– (Fig. 3). This can be a consequence of
electronic influence caused by coordination of iminoquinone C6
cycles in these ligands.
3.4. Reduction reactions with
For the reduction by 4Ln, we initially took a ratio of 0.5 mol of S8
to 1 mol of the complex. It was expected that instead of one molecule of 3Ln reducing for 2e, two molecules of 4Ln will lead to a similar outcome, while the obtained dianion Sn2– could bind the two
Ln ions. The sulfur was thus taken in a slight excess. Unexpectedly,
quite a different result was obtained: the only compounds crystallized from the reaction mixtures were the neutral complexes [Ln
(dippISQ)(dippAPS2)(thf)2] (7Ln, Ln = Dy, Ho). Besides one dippISQ–
ligand, these complexes contain one formally dippAP ligand with
the N atom bound to a chain of two S atoms, both coordinating
the Ln cation (Fig. 4). Hence, only two S atoms per initial complex
participate in the reaction. It was found that the excessive sulfur
remains in solution and crystallizes upon its concentration, so
the increase of amount of S8 will not lead to the formation of longer
Sn chains in the resulting complexes.
The compounds 6Ln were crystallized as greenish-blue blocks
from thf-pentane mixtures, and the structure of 6Ho2.5thf was
determined by means of XRD (Fig. 4). Crystal packing is the same
as that of Nd congener, 6Nd [15]. The complex anion is quasi-C2
symmetrical, where the axis goes along the Ho–S3 vector (atom
numbering according to the figure). Coordination sphere represents a distorted octahedron N2O2S2 (CN = 6). The S5 chain is coordinated by terminal S1 and S5 atoms with Ho–S distances of 2.71
and 2.73 Å. The next two atoms, S2 and S4, are placed by ca.
0.30 Å further apart from the Ho centre, thus still allowing somewhat weaker interaction with it. The Ho–S3 distance is 3.83 Å.
The S5 ligand is thus compressed between two Dipp fragments
and wrapped around the Ho cation; it occupies all available space
around the cation, so no thf ligands are coordinated. Both dippISQ–
ligands possess typical bond lengths in the coordination metallacycle (Fig. 3), and the MOS values are close to the expected one (–0.78
(8), –0.79(5)). The angle between their mean planes is very low
(55.0°) as compared to bis-iminoquinone complexes of other types,
which speaks for rather large steric influence from the pentasulfide
ligand. It was noticed that the presence of small amounts of thf was
necessary to obtain well-crystallized compound, while from neat
pentane only amorphous material was precipitating. As can be
seen from the crystal structure, a sufficient number of thf molecules are necessary to fill up large voids in the packing of rather
rigid cations and anions, which results in relatively large number
of solvate thf molecules. Still, the excess of thf should be avoided
owing to high solubility in this solvent. The structure of analogous
dysprosium compound, 6Dy, was not determined by XRD, but we
consider it to be the same as 6Ho on the basis of very similar reaction behaviour, crystal morphology, and proper elemental analysis
and IR spectrum.
Both compounds 7Ln crystallize in the form of green–blue plates
from thf-pentane mixtures. The structure of 7DyC5H12 was determined by means of XRD (Fig. 4). The molecule contains one dippISQ–
ligand with typical bond lengths in its metallacycle (Fig. 3) and
with MOS of –0.91(7). A chain of two sulfur atoms is bound to
the N atom of the other iminoquinone ligand, and this N atom is
not coordinated to the Ln centre (LnN distance of 3.73 Å). The
donor atoms in this ligand are oxygen (with Dy–O bond of
2.11 Å) and two sulfur atoms, which are both coordinated within
a bonding distance (Dy–S bonds of 2.70 and 2.83 Å). The Dy–O
bond length, as well as CAC, CAN, and CAO bond lengths in the
chelating cycle of this ligand are typical for dianionic state of the
iminoquinone (MOS = –1.87(18)), and similar to those for dippAP2–
ligand in 4Ho (Fig. 3). This new ligand can thus be considered as a
derivative of amidophenolate dianion, (dippAP)S2–
2 . Similar ligands
have not been described before, according to the Cambridge Structural Database [65]. The coordination sphere of Dy is supplemented by two thf ligands, and the coordination polyhedron is
best described as a distorted capped trigonal prism with quasiC2v symmetry, according to the CSM method (CN = 7, Table S2,
dipp
AP complexes
The most intriguing is the redox behaviour of mixed-ligand
complexes 4Ln in comparison to bis-amidophenolato species. We
decided to study the reductive action of complexes with dippAP2–
ligands on elemental sulfur. It has been shown that the reaction
of 3Nd with S8 or Se in the presence of 18-Crown-6 leads to the formation of the complexes [Nd(dippISQ)2(Q5)]– (Q = S, Se), where pentachalcogenide dianion is coordinated by both terminal atoms. The
reaction of 3Ln with S8 leads to the same result. The binary complex
salts [K(18-Crown-6)(thf)2][Ln(dippISQ)2(S5)] (6Ln, Ln = Dy, Ho)
were obtained in good yields (Fig. 4).
8
Polyhedron 195 (2021) 114967
D.K. Sinitsa, T.S. Sukhikh, S.N. Konchenko et al.
Fig. 4. Molecular structure of complexes (left) 6Ho, neighboring cation and anion are shown; (right) 7Dy. iPr, tBu groups and thf ligands are reduced, H atoms are omitted for
better visibility.
sulfur chains or cycles. The formation of exclusively pentasulfide
species in this reaction can be explained by the very appropriate
size of S5 chains for the accommodation in the Ln coordination
sphere. An indirect evidence of this is the fact that several
mononuclear complexes are known which comprise the pentasulfide {MS5} cyclic fragments (M = rare earth metal) with the same
twisted-boat geometry of the cycle for M = Y [66], Ce [67], Nd
[15], Sm [68], Dy [69]. Quite the opposite, there is only one structural type known for tetrasulfide {LnS4} cycles (Ln = Sm, Yb), stabilized as a trinuclear formamidinato complex [70], and the {MSn}
cyclic fragments are unknown for n = 3 and n > 5. Supposedly, upon
enveloping around the Ln cation, the first five S atoms in the coordinated polysulfide chain form the most stable conformation,
while the bond to the sixths S atom is being cleaved during the second SET.
In the reaction of 4Ln with S8 only one SET step can be expected,
and it should lead to an analogous transient product of an S8 cycle
opening (Scheme 6). The opened chain cannot be cleaved by
Supporting Information). Two sets of atoms, S1–O(thf)–O(thf) and
S2–N1–O2, compose triangular faces of the prism, while O1 occupies the capping position (Fig. S1, Supporting Information). The
complex 7Ho shows the same behaviour and appearance as the
Dy analog and its analytical and spectral data support the same
molecular structure.
A notable difference in the reduction products 6Ln and 7Ln is
inclusion of sulfur chain of different lengths bound within the
resulting complex. The following reaction routes can be proposed
to rationalize this observations. In the reaction of 3Ln with S8 two
separate single electron transfer (SET) steps from two different
dipp
AP2– ligands to the sulfur chain can be expected (Scheme 5).
The first SET on S8 must result in a breaking of a SAS bond, with
the supposed formation of a S8 radical-anionic chain coordinated
to Ln by its anionic end. The second SET leads to breaking of
another SAS bond with the formation of shorter dianionic S2–
5 chain
coordinated to the Ln centre. Upon this stage, a neutral S3
fragments should be released, which can combine to form larger
Scheme 5. Tentative reaction route of sulfur reduction by 3Ln in the presence of 18-Crown-6. The number of coordinated thf molecules in transient species is not specified,
iminoquinone ligands are simplified.
9
D.K. Sinitsa, T.S. Sukhikh, S.N. Konchenko et al.
Polyhedron 195 (2021) 114967
Scheme 6. Tentative reaction route of sulfur reduction by 4Ln. The number of coordinated thf molecules in transient species is not specified, iminoquinone ligands are
simplified.
It is noteworthy, that similar process of inclusion of O2 fragment
was observed in reactions of antimony(V) complexes [Sb(dippAP)
Ph3] with molecular dioxygen [58,59]. The proposed reaction
mechanism also includes formation of O2 radical fragment coordinated to the central atom in a transient iminosemiquinolato complex. But on the second stage the furthest O atom forms a bond not
with the N atom of the ligand, but with the carbon C(O) atom,
which is located at a conjugated position and also bears a noticeable spin density. We suppose the reason of different reaction
course is that the NAO bond formation in the case of antimony
complexes would be much more unfavourable as compared to
the observed CAO. The differences in energies between NAS and
CAS bonds should be not that large, and steric factors can be determinative, namely, preferential formation of a larger chelate cycles
in the coordination sphere of a large Ln cation. The crossed reactions, of the antimony complex with S8 and of the Ln complexes
with O2 would then be an interesting complement to these works,
and can be considered for future research.
another reduction. It can be shortened by the attack of the radical
centre, located at the loose end, on another sulfur atom, with the
release of a stable S6 molecule. The remaining S2 radical fragment
can further recombine with the dippISQ– ligand, which bears partial
spin density on the N atom, with the formation of a SAN bond
(route a on the scheme). Decoordination of the N atom from the
Ln centre is occurring during the process, most likely, by steric reasons, to form the most stable conformation of the chelate cycle.
The last two stages can go in inverse order. Then, initial recombination of radicals at the terminal S atom of the S8 chain and the
N atom of the dippISQ– ligands can result in a species with tenmembered LnNS8 chelate cycle, which can further turn to more
stable end product by excision of a stable S6 molecule (route b).
In any case, it seems unlikely that the formation of the SAN bond
occurs as the result of a direct attack of dippISQ– radical on a S atom
in a middle of a polysulfide chain, because the analogous processes
would then be expected, e. g. in the coordination sphere of 6Ln, or
upon interaction of 7Ln with the excess of S8, which are not
observed. The processes leading to (dippAP)S2–
2 anion cannot be
excluded in the reaction of 3Ln with S8, after the first SET stage.
We suppose that the corresponding products are not formed
because the second SET stage occurs much faster than the processes leading to formation of S2 fragment by radical reactions.
3.5. IR spectra
IR spectroscopy represents a rather powerful tool for distinguishing between differently charged iminoquinone ligands. The
10
Polyhedron 195 (2021) 114967
D.K. Sinitsa, T.S. Sukhikh, S.N. Konchenko et al.
ligands should be kept in mind, especially if reactive radical species
are involved.
main bands characteristic for each charged state of dipp-iminoquinone have been excessively discussed earlier [58-62]. In the
spectra of 1Ln, the strongest bands at 1610, 1466, 1384 cm1, as
well as a medium band at 1519 cm1 indicate the presence of neutral form of the ligand. In the spectra of 2Ln and 3Ln, the characteristic strongest bands arise at 1423 and 1255 cm1, respectively
(see Supporting Information for all spectra and details). The complexes 4Ln reveal both of these strong bands, in accordance with
the presence of two types of ligands. Their spectra are identical
to those obtained earlier for the analogous complex of Eu and Yb,
which were not structurally characterized [36]. It should be noted,
though, that some of characteristic peaks can be obscured in case
another well-absorbing species are present in the compound,
hence the most accurate way is to compare the spectra of similar
compounds with ligands in different oxidation state. This can be
seen on example of the spectra of 6Ln, where the most intense
peaks originate from the crown ether in the cationic part, and
one of them overlaps with the medium intensity band of the dippISQ– ligand at 1251 cm1. Hence, some weaker bands may also
be useful for elucidation of oxidation states of the ligands, such
as the most shortwave ones in the fingerprint region, at 1650–
1550 cm1. The single rather strong band at 1610 cm1 appears
in the spectra of 1Ln. In the spectra of 2Ln, there is a single weaker
band at 1584 cm1. Spectra of 3Ln contain stronger band at
1560 cm1 accompanied by a weaker and broader one at
1590 cm1. The latter bands also appear in the spectra of complexes [Ln2(dippAP)2(thf)]2 (Ln = Eu, Yb) reported earlier [36].
Rather expectedly, in the spectra of 4Ln there are two nearly equal
bands at 1584 and 1564 cm1, as a superposition of spectra of 2Ln
and 3Ln. Those of 6Ln only contain the band at 1584 cm1, characteristic to dippISQ–. The spectrum of 7Dy is nearly identical to those
of 4Ln, which corroborates that the bonding in (dippAP)S2–
2 ligand is
indeed quite similar to that in dippAP2–, while the oscillations associated with S2 unit should only appear at lower wavenumbers. The
coordinated thf gives rise to a medium-intensity band at 862 cm1,
which is present in the spectra of all complexes with the exception
of 1Ln.
CRediT authorship contribution statement
Dmitriy K. Sinitsa: Investigation, Data curation, Writing - original draft. Taisiya S. Sheikh: Investigation, Data curation, Writing original draft. Sergey N. Konchenko: Funding acquisition, Writing
- review & editing. Nikolay A. Pushkarevsky: Conceptualization,
Methodology, Writing - original draft, Writing - review & editing,
Visualization, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
This work was supported by the Russian Science Foundation
(grant No. 16-13-10294) and by the Russian Foundation for Basic
Research (grant No. 19-03-00568).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.poly.2020.114967.
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4. Conclusions
The lanthanide complexes with dipp-iminoquinone ligands in
different charged states are readily accessible starting from the
neutral proligand and the lanthanide triiodides. The availability
of bis-ligand complexes of Ln with at least four combination of
charged forms (IQ-IQ, ISQ-ISQ, ISQ-AP, AP-AP) makes them rather
distinquished among other groups of elements. Close variability
(with three different combinations of charged iminoquinone
ligands) have been demonstrated for the complexes of 13th group
(In [1]) and 14th group (Sn [29]) elements. This possibility to synthesize differently charged complexes could be valuable for investigating 1e and 2e reductive action on account of the ligand in Ln
complexes. Our results show that, in some cases, additional ligand
reactivity can be expected. Strictly speaking, involvement of Ndonor ligands into reactions following reduction by Ln complexes
is rare, but not unknown. For example, similar behaviour was
demonstrated for Sm(II) and Yb(II) complexes [Ln(dippForm)2(thf)2]
(dippForm = N,N’-bis(dipp)formamidinate), whose reaction with
realgar (As4S4) results in binding of AsS2 fragment to a N atom with
the formation of (dippForm)AsS2–
2 ligand [72]. Still, the proposed
stage sequence leading to the formation of 7Ln is rather unusual,
because initially the dippAP2– ligand is oxidized to dippISQ–, and then
the oxidation state of the iminoquinone is formally restored with
the formation of (dippAP)S2–
2 ligand. These reactions demonstrate
that the mixed-ligand dippAP-dippISQ complexes can indeed be utilized as one-electron reductants, but the extended reactivity of the
11
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