Steric control on the redox chemistry of (η5-C 9H7)2YbII(THF)2 by 6-aryl substituted iminopyridines

Article abstract of DOI:10.1039/c1dt10135h

A steric control on the reductive capacity of ytterbocenes towards iminopyridine ligands is described. The reaction of (η⁵-C ₉H₇)₂Yb(THF)₂ with a series of 6-organyl-2-(aldimino)pyridyl ligands (IPy) takes place with the replacement of two THF molecules by one IPy unit. In contrast to the rich reductive ytterbocene chemistry described in the presence of the unsubstituted (aldimino)pyridyl ligand, all 6-aryl substituted IPys scrutinized hereafter are involved into the metal coordination as neutral bidentate {N,N} or tridentate {N,N,S; N,N,O} ligands, with no changes of the metal oxidation state in the final complexes. A series of Yb^II metallocene complexes of general formula (η⁵-C₉H₇)₂Yb^II(η ² or η³)[2,6-ⁱPr₂(C ₆H₃)NCH(C₅H₃N)-6-R)] have been isolated and completely characterized. The stereo-electronic role of the aryl substituents in the IPy ligands on the ytterbocene redox chemistry has also been addressed.

Full text of DOI:10.1039/c1dt10135h

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PAPER
Steric control on the redox chemistry of (g5-C9H7)2YbII(THF)2 by 6-aryl
substituted iminopyridines†
Alexander A. Trifonov,*a Boris G. Shestakov,a Ivan D. Gudilenkov,a Georgy K. Fukin,a
Giuliano Giambastiani,*b Claudio Bianchini,b Andrea Rossin,b Lapo Luconi,b Jonathan Filippib and
Lorenzo Soracec
Received 24th January 2011, Accepted 5th April 2011
DOI: 10.1039/c1dt10135h
A steric control on the reductive capacity of ytterbocenes towards iminopyridine ligands is described.
5
The reaction of (h -C9H7)2Yb(THF)2 with a series of 6-organyl-2-(aldimino)pyridyl ligands (IPy) takes
place with the replacement of two THF molecules by one IPy unit. In contrast to the rich reductive
ytterbocene chemistry described in the presence of the unsubstituted (aldimino)pyridyl ligand, all 6-aryl
substituted IPys scrutinized hereafter are involved into the metal coordination as neutral bidentate
{N,N} or tridentate {N,N,S; N,N,O} ligands, with no changes of the metal oxidation state in the final
5
2
complexes. A series of YbII metallocene complexes of general formula (h -C9H7)2YbII(h or
h )[2,6-iPr2(C6H3)N CH(C5H3N)-6-R)] have been isolated and completely characterized. The
3
stereo-electronic role of the aryl substituents in the IPy ligands on the ytterbocene redox chemistry has
also been addressed.
known to modulate their reductive properties (from one- to two-
Introduction
electron donor systems) towards DAB ligands.7 Finally, the latest
progresses in the organometallic chemistry of ytterbium complexes
with DAB ligands have contributed to highlight relevant aspects
of the complex ytterbium redox chemistry, from the occurrence
of solvent-mediated redox tranformations5b–d to temperature-
induced redox isomerizations.7
Redox-active a,a¢-diimines appeared in the literature in the late
1960s as a versatile ligand family capable of providing transition-
metal complexes with unusual structures and unique reactivity.1
Since the 1980s, Cloke and Edelmann reported the use of diazabu-
tadiene ligands (DAB) for the preparation of organolanthanide
complexes, a study that represented an important milestone in the
development of this organometallic area.2
The combination of the redox properties of the DAB ligands3
and ytterbium (for which two stable oxidation states featured by
a rather low YbII ꢀ YbIII interconversion potential are possible)4
has opened the way to the development of a rich organometallic
chemistry. The reactions between ytterbocenes with variable
steric hindrance and DAB ligands can follow different reaction
paths, resulting into either YbII/YbIII oxidation,5 formation of
More recently, some of us have reported on the reactivity
5
of the bis(indenyl) ytterbium complex (h -C9H7)2YbII(THF)2 (1)
with a DAB-structurally related a-iminopyridine (IPy) ligand
(2e, Chart 1).8 The reaction outlined in Scheme 1 can be
formally interpreted as the unprecedented C N bond inser-
5
tion into the h -indenyl-Yb bond and metal oxidation to give
5
2
4
the YbIII(h -C9H7){h -2,6-iPr2(C6H3)N-CH(C9H7)(C5H5N)}{h -
2,6-iPr2C6H3N CH(C5H5N)-} species 3 (Scheme 1).8a
6
unexpected C–C bonds or C–H bond activation on the ligand
skeleton.6 The variation of steric hindrance of ytterbocenes is also
aG.A. Razuvaev Institute of Organometallic Chemistry of Russian Academy
of Sciences, Tropinina 49, GSP-445, 603950, Nizhny Novgorod, Russia.
E-mail: trif@iomc.ras.ru; Fax: (+7) 8314621497
bIstituto di Chimica dei Composti Organometallici (ICCOM - CNR),
Via Madonna del Piano 10, 50019, Sesto Fiorentino, Italy. E-mail:
giuliano.giambastiani@iccom.cnr.it; Fax: (+39) 055 5225203
Scheme 1 Synthesis of complex 3.
cDipartimento di Chimica “U. Schiff” and UdR INSTM, Universita` di
Firenze, Via della Lastruccia, 3–13, 50019 Sesto Fiorentino, Italy
The haptotropic rearrangement of the p-coordinated indenyl
fragments seems to be the key-factor driving the observed reactiv-
ity. Notably, no imine C N bond alkylation by the “Cp” anionic
ligand is observed when the reaction of 2e takes place with differ-
ently hindered ytterbocene complexes [YbIIbis(fluorenyl)(THF)2
† Electronic supplementary information (ESI) available: Complete crystal-
lographic data and tables for complexes 4–7 and UV-VIS spectra of ligands
2a–d and related YbII-complexes 4–7. CCDC reference numbers 794890
(4), 795224 (5), 794891 (6) and 794889 (7). For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1dt10135h
10568 | Dalton Trans., 2011, 40, 10568–10575
This journal is
The Royal Society of Chemistry 2011
©

Chart 1 Iminopyridine (IPy) ligands 2a–e.
5
(fluorenyl = h -C13H9)6a or Cp2YbII(THF)n (Cp = C5Me5, C5Me4H,
C5MeH4)8b]; in these cases, variably substituted YbIII products
have been isolated and completely characterized. Overall, the
reactions of ytterbocenes with 2e have proven to be strongly
Scheme 2 Reaction path of 1 with tridentate IPy ligands 2b and 2c.
Finally, evidence of the ligands coordination to the ytterbium
center was unambiguously provided by X-ray analysis; the X-ray
structures of 4 and 5 are shown in Fig. 1 and 2, respectively.
5
dependent on the bonding nature of Yb-h -coordinated aro-
matic groups. In contrast to this general assessment, the role
of the IPy ligand and its stereo-electronic properties in the
reaction with ytterbocenes still remains unknown. In order to
get more insights on this specific point, we report hereafter on
5
the reactivity of the (h -C9H7)2Yb(THF)2 complex (1) with a
series of 6-organyl-2-(aldimino)pyridyl ligand derivatives (2a–d,
Chart 1). A number of divalent ytterbium complexes of general
5
2
3
formula (h -C9H7)2YbII(h or h )[2,6-iPr2(C6H3)N CH(C5H3N)-
6-R)], containing neutral iminopyridine coordinated ligands have
been synthesized and fully characterized.
Results and discussion
The aldimino precursors 2a–d were obtained on a multigram
scale with little modifications to the procedures reported in
the literature.9 All ligands are off-white/pale-yellow solids after
extractive work-up and solvent evaporation. Recrystallization
from hot MeOH gave the pure compounds as white/pale yellow
crystals with melting points ranging from 96 to 123 ◦C (see
Experimental section).
Fig. 1 Crystal structure of [N22Th(C9H6)2Yb]·C7H8 (4). Thermal ellipsoids
are drawn at the 40% probability level. Hydrogen atoms and a crystalliza-
tion toluene molecule are omitted for clarity.
A dark-red THF solution of complex 1 was reacted at room
temperature with an equimolecular amount of a potentially
tridentate iminopyridine ligand (2b or 2c) without any appreciable
color change of the starting solution. The reaction mixture was
monitored over several hours by 1H NMR (THF-d8, 293 K)
spectroscopy showing only signals from the unreacted starting
materials. Solvent evaporation under vacuum gave a dry dark-red
solid residue. Dissolving the solid in toluene at room temperature
resulted in the immediate change of the solution color from dark-
◦
red to brownish-black. After cooling the solution to -20 C for
several days, brownish-black microcrystals of complexes 4 and 5,
suitable for X-ray analysis, separated off. Pure 4 and 5 were ob-
tained from the mother-liquor in 74 and 69% yields, respectively, as
poorly soluble compounds in aromatic and aliphatic hydrocarbons
(Scheme 2).
Complexes 4 and 5 are moisture- and air-sensitive crystalline
1
solids and their H NMR spectra (C6D6, 293 K) unambiguously
accounted for YbII diamagnetic species with all (rather broadened)
signals falling in the 1–8.8 ppm region. The scarce solubility
of both complexes in benzene-d6 did not allow to record the
Fig. 2 Crystal structure of [N2Fu(C9H6)2Yb] (5). Thermal ellipsoids are
drawn at the 40% probability level. Hydrogen atoms are omitted for clarity.
1
corresponding 13C{ H} NMR spectra. On the other hand, the 1H
NMR spectra of both compounds in THF-d8 (293 K) revealed a
rapid ligand dissociation, showing the appearance of sharp signals
characteristic of 1 and the free IPy ligands (2b and 2c). The UV-
VIS spectra of 4 and 5 recorded in n-hexane provided evidence of
the neutral character of the coordinated IPy ligands (see ESI†).
The main structural parameters and a list of selected bond
lengths and angles are summarized in Table 1 and Table 2,
respectively. Complex 4 crystallizes as a toluene solvate, while
crystals of 5 do not contain any crystallization solvent. Both
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 10568–10575 | 10569
©

Table 1 Crystal data and structure refinement for complexes 4–7
4
5
6
7
CCDC number
Empirical formula
Mr
794890
795224
C40H38N2OYb
735.76
120(2)
0.71069
Monoclinic
Cc
10.309(4)
27.522(11)
11.346(5)
96.968(3)
3195(2)
4, 1.529
2.961
1480
0.01 ¥ 0.01 ¥ 0.015
4.20–26.55
-12 £ h £ 11
-33 £ k £ 33
-14 £ l £ 12
15497/5381
0.005(9)
794891
C42H40N2Yb
745.80
120(2)
0.71069
Monoclinic
P21/n
9.293(4)
37.396(1)
10.314(4)
114.897(5)
3251(2)
4, 1.524
2.909
1504
0.01 ¥ 0.02 ¥ 0.02
4.14–26.46
-10 £ h £ 11
-46 £ k £ 42
-12 £ l £ 12
24993/5684
—
794889
C40H38N2SYb·C7H8
C40H38N2SYb·C7H8
843.96
843.96
T/K
100(2)
0.71073
100(2)
0.71069
Orthorhombic
Pna21
23.695(9)
10.380(4)
15.684(6)
90
3858(3)
˚
l/A
Crystal system
Space group
Orthorhombic
Pna21
23.6885(7)
10.4564(3)
15.6524(5)
90
3877.0(2)
4, 1.446
2.501
1712
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z, Dc/g m-3
4, 1.453
2.514
1712
m/mm-1
F(000)
Crystal size/mm
H Range for data collection/◦
Limiting indices
0.50 ¥ 0.40 ¥ 0.30
2.13–26
0.08 ¥ 0.10 ¥ 0.15
4.15–28.91
-28 £ h £ 32
-13 £ k £ 13
-21 £ l £ 20
15989/7862
-0.013(10)
0.953
-29 £ h £ 29
-12 £ k £ 12
-19 £ l £ 19
31999/7571
-0.020(5)
Reflections collected/unique
Flack parameter
GOF on F2
1.067
0.881
0.925
Data/restraints/parameters
Final R indices [I > 2s(I)]
R indices (all data)
7571/1/465
R1 = 0.0184, wR2 = 0.0458
R1 = 0.0190, wR2 = 0.0461
1.41, -0.47
5381/2/401
R1 = 0.0386, wR2 = 0.0638
R1 = 0.0560, wR2 = 0.0667
0.975, -0.463
5684/0/410
R1 = 0.0390, wR2 = 0.0669
R1 = 0.0600, wR2 = 0.0703
1.283, -0.942
7862/1/425
R1 = 0.0321, wR2 = 0.0695
R1 = 0.0431, wR2 = 0.0716
1.214, -0.638
-3
˚
Drmax, min/e A
complexes show very similar structures, with the YbII center
adopting a highly distorted trigonal bipyramidal geometry with
longer than that measured in the known nine-coordinate YbII
10b
˚
complex (C9H6CH2C5H4N)2Yb(OC4H8) [2.449(5) A]. Finally,
5
˚
˚
two h -coordinated indenyl fragments and the tridentate {N,N,S}
the N(2)–C(10) bond lengths [1.282(3) A, 4; 1.281(9) A, 5] are
typical of C N double bonds and the dihedral angles q[N(1)–
C(1)–C(10)–N(2)] are very close to those measured in d-transition-
metal complexes stabilized by neutral N,N-iminopyridine systems
(Table 2).13
(4) or {N,N,O} (5) IPy ligands that saturate the metal coordination
˚
sphere. The Yb–CpCent distances in 4 (2.475(1) and 2.493(9) A, Yb–
˚
˚
˚
Cav = 2.768 A) and in 5 (2.465(3) and 2.480(4) A, Yb–Cav = 2.749 A)
are slightly longer than those measured in the starting ytterbocene
1 (2.457(1) A, Yb–Cav = 2.73),10a,b and are in line with the values
In agreement with the results of solid-state investigations,
magnetic measurements conducted on 4 and 5 evidenced the
diamagnetic character of the two complexes within the 2–300 K
temperature range and thus the closed shell configuration of both
Yb and ligand.
Studying the reaction of 1 with an equimolecular amount of
the bidentate IPy ligands (2a and 2d) under similar experimental
conditions to those described above (Scheme 3), gave air- and
moisture-sensitive brownish-black microcrystals of complexes 6
and 7 in 60 and 77% yield, respectively.
˚
commonly observed in YbII coordination compounds.9c,d Finally,
the shorter distances between the ytterbium center and the three
“allylic” carbon atoms of the five-membered rings of each indenyl
moiety (C24–26 and C33–35) compared to those from the quaternary
5
carbon atoms (C23,27 and C32,36), are the proof of a noticeable h -
3
to h -slippage of the indenyl ligands. The a(CpCent–Yb–CpCent) in 4
and 5 (123.7(9) and 130.66(4)◦, respectively) are rather close to the
values reported for other divalent YbII bis(indenyl) compounds.9
While YbII–N distances in 4 have similar values [2.5813(16) and
II
˚
2.5956(15) A], the Yb –Nimine distance in 5 is markedly longer
˚
[2.714(6) A] than that measured between the metal center and
the pyridine nitrogen atom [2.527(10) A]. Overall, the Yb –N
II
˚
coordination bond lengths in 4 and 5 are close to those formerly
reported in the literature for eight- and nine-coordinated YbII
derivatives,11,5d while they appear significantly longer compared to
those measured in YbIII species coordinated by a radical-anionic
N,N-bidentate IPy ligand [2.326(3), 2.352(3)A];8a the YbII–N bond
˚
lengths are also markedly longer than the YbIII–N distances in
eight-coordinate complexes bearing neutral 2,2¢-bipyridyl or 1,10-
12
phenantroline ligands (2.37, 2.36 A). While no YbII compounds
˚
Scheme 3 Reaction path of 1 with bidentate iminopyridine ligands 2a
and 2d.
containing a sulfur donor atom like 4 have been reported in the
literature so far (which does not allow the authors to make any
direct comparison of the YbII–S bond length with related YbII–S
Both compounds showed scarce solubility in aromatic and
aliphatic hydrocarbons, thus hampering the characterization of
II
˚
distances), the Yb –O distance in 5 [2.592(5) A] is significantly
10570 | Dalton Trans., 2011, 40, 10568–10575
This journal is
The Royal Society of Chemistry 2011
©

◦
˚
Table 2 Selected bond distances (A) and angles ( ) for complexes 4–7.
See ESI† for a complete list of the crystallographic parameters
4
5
6
7
Yb–N1
Yb–N2
Yb–S1
2.5813(16)
2.5956(15)
3.002(5)
—
2.814(2)
2.691(2)
2.682(2)
2.748(2)
2.853(2)
—
2.527(10)
2.714(6)
—
2.592(5)
2.791(7)
2.701(7)
2.701(12)
2.719(12)
2.810(7)
—
2.592(4)
2.597(4)
—
—
—
2.538(4)
2.591(4)
—
Yb–O1
—
Yb–C23
Yb–C24
Yb–C25
Yb–C26
Yb–C27
Yb–C28
Yb–C29
Yb–C32
Yb–C33
Yb–C34
Yb–C35
Yb–C36
Yb–C37
Yb–C38
N2–C10
2.846(5)
2.722(5)
2.688(4)
2.751(5)
2.872(5)
—
—
2.773(4)
2.696(4)
2.722(5)
2.796(5)
2.853(5)
—
—
—
—
2.8457(19)
2.729(2)
2.6905(15)
2.7484(19)
2.876(2)
—
2.800(7)
2.672(12)
2.689(9)
2.754(8)
2.860(7)
—
2.850(5)
2.752(5)
2.673(5)
2.690(5)
2.817(6)
—
—
2.829(5)
2.751(5)
2.699(5)
2.703(5)
2.802(5)
1.276(6)
Fig. 4 Crystal structure of [N23Th(C9H6)2Yb]·C7H8 (7). Thermal ellipsoids
are drawn at the 40% probability level. Hydrogen atoms and a crystalliza-
tion toluene molecule are omitted for clarity.
—
1.282(3)
—
1.281(9)
—
1.289(6)
N1–Yb–N2
N1–Yb–S1
N1–Yb–O1
66.55(5)
63.00(4)
—
64.8(2)
—
62.9(2)
65.27(12)
—
—
67.11(13)
—
—
Table 1 lists all the crystal and structural refinement data, while
selected bond lengths and angles for 6 and 7 are summarized in
Table 2. Similarly to above, complex 7 crystallizes as a toluene
solvate, while the crystals of 6 do not contain any additional
crystallization solvent. Interestingly, the thiophene-containing
isomers 4 and 7 are isostructural; the different orientation of
the sulfur atom in the heterocyclic substituent does not seem to
influence the crystal packing of the resulting complexes. Despite in
7 the S atom is bent away from the metal centre, it is not involved
in any intermolecular interaction with the neighbouring molecules
in the lattice. The q[N(1)–C(5)–C(6)–S(1)] and q[N(1)–C(5)–C(6)–
C(7)] dihedral angles in 4 and 7 are also similar [16.8(2) and
23.8(7)◦, respectively], this confirming the analogous orientation
of the heterocyclic ring in the two compounds. Unlike 4 and 5, the
X-ray diffraction analyses of 6 and 7 (Fig. 3 and 4) revealed that the
metal centers were four-coordinated, with two indenyl fragments
and a bidentate {N,N} iminopyridine ligand completing the metal
coordination sphere. Overall, both structures showed distorted
tetrahedral coordination geometries. As expected, the sulfur atom
of the regioisomeric 2d ligand in complex 7, did not take part into
any intra- or intermolecular coordination, being oriented away
from the metal center.
Yb–CpCent
Yb–CpCent
2.475(1)
2.493(9)
2.465(3)
2.480(4)
2.473(2)
2.487(2)
2.473(3)
2.490(2)
CpCent–Yb–CpCent
123.7(9)
130.66(4)
117.3(2)
124.4(2)
N1–C5–C6–S1
N1–C5–C6–O1
N1–C5–C6–C7
N1–C1–C10–N2
N1–C1–C12–N2
16.8(2)
—
—
11.0(3)
—
—
-12.6(9)
—
-5.1(3)
—
—
—
-37.5(7)
—
—
—
23.8(7)
12.2(7)
—
-10.4(7)
6 in solution. The 1H NMR and UV-VIS spectra (see ESI†) of 7
were still consistent with a divalent oxidation state of the ytterbium
atom and with a neutral coordinated iminopyridine ligand.
Once again, clear evidence of the IPy ligands coordination mode
to the ytterbium center was unambiguously provided by the X-ray
analysis of both complexes, whose X-ray structures are shown in
Fig. 3 and 4, respectively.
˚
˚
The average Yb–Cav bond distances [2.763 A (6) and 2.766 A
(7)] are very close to those measured in 4 and 5, suggesting a
3
partial slippage of the indenyl fragments (possible h - instead of
5
the expected h -coordination). The Yb–N bond distances in 6
˚
˚
[2.592(4), 2.597(4) A] and 7 [2.538(4), 2.591(4) A] are also similar
to those measured in 4 and 5, with values in the typical range
for the related YbII coordination compounds containing neutral
N-ligands.11,5d Finally, the dihedral angles q[N(1)–C(1)–C(12)–
N(2)] in 6 and q[N(1)–C(1)–C(10)–N(2)] in 7 are again very close
to those reported for other similar Yb-coordination complexes12
and d-transition-metal complexes stabilized by neutral N,N-
iminopyridine systems.13
Magnetic measurements conducted on 6 and 7 showed that
both systems (in the solid state) are essentially diamagnetic (with
the exception of a weak and unavoidable paramagnetic impurity)
up to 240 K (Fig. 5). However, starting from 245 K for 6 and
260 K for 7, the cT product clearly began to increase for both
Fig. 3 Crystal structure of [N2Ph(C9H6)2Yb] (6). Thermal ellipsoids are
drawn at the 40% probability level. Hydrogen atoms are omitted for clarity.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 10568–10575 | 10571
©

experiments were recorded on a 5 mM ligand solution in dry and
degassed DMF (5 mL) using tBu4N+BF4- (0.1 M) as conductivity
buffer.
The CV profiles of all ligands, together with the relative peak
+
and half-wave potential values (referred to the FeCp2/FeCp2
redox couple) are reported in Fig. 6. For all the scrutinized
ligands, the higher the potential scan rate the lower the peak
reduction potential value, which accounts for a slow electron
transfer process. All CVs showed highly asymmetric profiles, with
the reverse peak current values (oxidation) always lower than the
forward one (reduction). Except for 2d, all electron transferts are
chemically reversible and electrochemically quasi-reversible. From
the analysis of the CV profiles, one may conclude that all aryl-
substituted iminopyridine ligands (2a–d) present similar electron
accepting properties and peak shapes with very similar reduction
potential values (imino vs. amine). In contrast to the observed
reactivity with 1,8a the unsubstituted IPy system 2e showed the
lowest tendency to stabilize negative extra charges because of its
lower reduction potential value (VP) and the reduced extension
of its conjugated network [compared with the other IPy systems
(2a–d)].
Fig. 5 Magnetic susceptibility data (cT vs. T) for 6 (empty circles) and
7 (full triangles); for the latter a second heating cycle after cooling back
to 240 K is reported. The inset shows the cT vs. T plot for complex 7
measured after several heating/cooling cycles between 240 and 340 K.
samples, suggesting the possible occurrence of a temperature-
induced intramolecular electron transfer14 between the rare-earth
metal center and the ligand framework.15 This should lead to
an YbIII-radical configuration for which an high temperature cT
value of 2.95 emu K mol-1 is expected.16 Test measurements
performed only on 7 showed that the transition is not reverted
upon cooling back the sample to 240 K, since it maintains its
high temperature magnetic susceptibility. The repetition of this
procedure led to a complete disruption of the complex molecular
structure, as evidenced by the cT vs. T plot (see inset on Fig. 5).
The observed decrease in cT vs. T in the transformed compound
is in agreement with the presence of a substantial fraction of YbIII,
since it can be ascribed to the progressive depopulation of Stark
sublevels of 2F7/2 ground state of YbIII.
Notably, the occurrence of an intramolecular electron transfer
process is not in contrast with the data obtained by the X-ray and
NMR analyses, which indicated the diamagnetic nature of both
samples. Indeed, crystallographic data were collected at 120 K, a
temperature at which the magnetic measurements confirmed the
diamagnetic character of both complexes. As for NMR results, the
fact that no diamagnetic–paramagnetic transition was observed
at room temperature simply indicates that the solution and the
solid-state behavior are different. This is not surprising, since
it is well known that both the aggregation state and the local
environment around the molecule are of paramount importance
in determining the temperature at which intramolecular electron
transfer processes take place.17
All the collected experimental data unambiguously indicate
that ligands 2a–d take part to the ytterbium coordination sphere
as either tridentate {N,N,S and N,N,O} or bidentate {N,N}
neutral ligands,18 to afford exclusively divalent YbII diamagnetic
compounds.
The introduction of a (coordinating or non-coordinating) aryl
substituent on the sixth position of the pyridine ring changes the
1/IPy ligand reaction outcome significantly. In order to get addi-
tional experimental feedback on this aspect, the electrochemical
behavior of the four IPys 2a–d has been investigated and compared
with that of the unsubstituted ligand 2e. Cyclic voltammetry (CV)
Fig. 6 Cyclic voltammograms of the ligands 1a–e. CV curves were
acquired in anhydrous DMF using tetraethyl ammonium tetrafluoroborate
(0.1 M) as conductivity buffer. Scan rate: 50 mV s-1, sample concentration:
5 mM.
The presence of additional Lewis basic sites in the ligands 2b and
2c (both ligands able to participate as tridentate systems) results
in a more rigidly coordinated IPy framework and the formation of
YbII complexes with increased coordination numbers. In spite of
that, only negligible electronic factors have to be considered while
comparing the reactivity of 1 with 6-aryl substituted IPys (2a–d)
and 2e. Indeed, neither the imine reduction potential values for
2a–d nor their coordination ability [bidentate (2a, d) vs. tridentate
(2b, c)] can be reasonably invoked to justify the different pathways
observed in the reaction of 1 with 6-aryl substituted (2a–d) and
unsubstituted (2e) IPy ligands, respectively. In particular, the
reactivity observed with the regioisomeric ligands 2b and 2d,
definitively ruled out any electronic influence or coordination
effect of the aromatic moiety at the pyridine unit on the reaction
path. Thus, simple steric contributions resulting from the presence
of bulky aryl substituents on the 6-position of the pyridine ring
10572 | Dalton Trans., 2011, 40, 10568–10575
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The Royal Society of Chemistry 2011
©

have to be considered as responsible for the modulation of the
ytterbium redox chemistry. As a result, sterically crowded IPy
ligands 2a–d react with 1 leading to the formation of simple YbII
coordination adducts as a result of a THF/ligand exchange at the
metal center. Neither evidence for YbII/YbIII oxidation nor ligand
modification (as reported for 2e) have been observed.
in Teflon tape and quickly transferred to the SQUID vacuum
chamber. The data were corrected for the intrinsic diamagnetism
of the sample, calculated from the Pascal’s constants19 and sample
holder contribution, measured in the same field and temperature
range.
Lanthanide metal analyses were carried out by complexometric
titration. The C, H elemental analysis was carried out in the
microanalytical laboratory of IOMC or at ICCOM by means of
a Carlo Erba Model 1106 elemental analyzer with an accepted
tolerance of 0.4 units on carbon (C), hydrogen (H) and nitrogen
(N). Melting points were ensued by using a Stuart Scientific
Melting Point apparatus SMP3. CV experiments where performed
on a PARSTAT 2773 galvanostat/potentiostat (Princeton Applied
Research) equipped with Dr Bob’s Cell(tm) (Gamry) using the
classical three electrode topology; the Ag|AgCl|KClsat (Gamry)
reference electrode, a 3 mm diameter glassy carbon as working
electrode and a platinum wire as counter electrode. All solutions
where prepared in glove box under inert atmosphere (N2), and the
cell was well purged with N2 prior to use. The potential was varied
between -1.0 and -2.0 V (vs. Ag|AgCl|KClsat) at potential sweep
rates of 10, 20, 50, 100, 200 and 400 mV s-1 in cathodic direction.
After the measurement on each ligand, a solution of 5 mM
ferrocene in the same conditions was measured to calibrate the
Conclusions
In conclusion, we have described a steric control on the redox
properties of ytterbocene 1 by bidentate {N,N} and tridentate
{N,N,S or N,N,O} iminopyridine ligands (IPys) variably sub-
stituted on the 6-position of the pyridine ring. In contrast to
the unprecedented reactivity of the unsubstituted IPy 2e with
ytterbocene 1,8a the reaction of 1 with ligands 2a–d simply occurs
by the replacement of the two coordinated THF molecules with
one bi- or tri-dentate IPy ligand, with retention of ytterbium
oxidation state. While the presence of additional coordinating
basic sites on the IPy ligands (S or O donor atoms) results in
more rigidly coordinated systems and higher metal coordination
numbers, neither significant electronic effects (4 vs. 7) nor different
ligand electron accepting properties (Fig. 6) can be reasonably
invoked to justify the observed reactivity (1/2a–d vs. 1/2e).
Magnetic measurements conducted on the solid samples (4–7)
unambiguously showed the diamagnetic character of all complexes
below 240 K with the occurrence of an intramolecular electron
transfer process taking place only for 6 and 7 at higher temperature
(over 240 K). This indicates a higher stability of the diamagnetic
electronic configuration(closed shell) for the complexes containing
tridentate IPy ligands vs. bidentate ones. Finally, the reactivity
described in this paper represents a new example of steric
+
potential; all potentials were finally referred to the FeCp2/FeCp2
redox couple.
Ligands 2a–e were prepared on a multigram scale with little
variation to literature procedure.20 In particular, ligands 2a–d
were obtained using aldiminopyridinate derivatives instead of
ketoimino ones (for ligand 2a–c see refs. 9a,b and 21; for ligand
2d see ref. 9b). Ligand 2e was prepared according to literature
procedures.22
5
control of the redox chemistry of (h -C9H7)2YbII(THF)2 towards
iminopyridines.
Experimental
General remarks
Ligands 2a–d were isolated as microcrystalline solids by cooling
methanol solutions to 4 ◦C overnight [2a: 74% yield, yellow
All air- and/or moisture-sensitive reactions were performed under
either nitrogen or argon in flame-dried flasks using standard
Schlenk-type techniques or in a dry-box filled with nitrogen.
After drying over KOH, THF was purified by distillation
from sodium/benzophenone ketyl. Hexane and toluene were
dried over sodium/triglyme benzophenone ketyl and distilled
prior to use. Indene was purchased from Acros and ytter-
bocene (C9H7)2Yb(THF)2 was prepared according to literature
procedure.5c All the other reagents and solvents were used
microcrystals; mp 96 ◦C. IR (KBr): nC
1648 cm-1. MS m/z
N
(%): 342 (M+, 76); 377 (M+ +1, 19); 327 (M+ - 15, 100). 1H
NMR (300 MHz, CD2Cl2, 293 K): d 1.21 (d, 3JHH = 6.8 HZ, 12H,
3
CH(CH3), H20,21,23,24), 3.03 (sept, JHH = 6.8 Hz, 2H, CH(CH3),
H19,22), 7.12–7.23 (3H, CH Ar, H15,16,17), 7.45–7.57 (3H, CH Ar,
H8,9,10), 7.92 (dd, 3JHH = 7.8 Hz, 1H, CH Ar, H2), 7.97 (pt, 3J
=
HH
7.8 Hz, 1H, CH Ar, H3), 8.11–8.15 (2H, CH Ar, H7,11), 8.26 (dd,
3JHH = 7.8 Hz, 1H, CH Ar, H4), 8.41 (s, 1H, CHN, H12). 13C{ H}
1
1
(otherwise stated) as purchased from commercial suppliers. H
NMR (75 MHz, CD2Cl2, 293 K): d 23.1 (CH(CH3)2, C20,21,23,24),
27.9 (CH(CH3)2, C19,22), 119.3 (C4), 121.8 (C2), 122.9 (C15,17), 124.3
(C16), 126.8 (C7,11), 128.7 (C8,10), 129.2 (C9), 137.2 (C14,18), 137.4
(C3), 138.7 (C6), 148.6 (C13), 154.3 (C1), 157.0 (C5), 163.6 (C12).
Anal. Calc. (%) for C24H26N2 (342.48): C, 84.17; H, 7.65; N, 8.18.
Found: C, 84.21; H, 7.43; N, 8.01%. 2b: 95% yield, yellow crystals;
mp 123 ◦C. IR (KBr): nC N 1646 cm-1. MS m/z (%): 378 (M+, 76);
379 (M+ + 1, 20); 333 (M+ - 15, 100). 1H NMR (300 MHz, CD2Cl2,
293 K): d 1.17 (d, 3JHH = 6.8 Hz, 12H, CH(CH3), H18,19,21,22), 2.97
(sept, 3JHH = 6.8 Hz, 2H, CH(CH3), H17,20), 7.08–7.19 (4H, CH Ar,
1
and 13C{ H} NMR spectra were recorded on either a Bruker DPX
200 (200.13 and 50.32 MHz, respectively) or a Bruker Avance
DRX-400 (400.13 and 100.62 MHz, respectively). Chemical shifts
are reported in ppm (d) relative to TMS, referenced to the
chemical shifts of residual solvent resonances (1H and 13C). IR
spectra were recorded as Nujol mulls on a "Bruker-Vertex 70"
spectrophotometer. Magnetic susceptibility data were collected
with a Quantum Design MPMS-XL SQUID magnetometer
working in the temperature range of 1.8–300 K (extended to
340 K for measurements on 7) and the magnetic field range up
to 5 Tesla. Samples were prepared in glove-box by wrapping them
3
4
H8,13,14,15), 7.43 (dd, JHH = 5.05 Hz, JHH = 1.14 Hz, 1H, CH Ar,
H7), 7.68 (dd, JHH = 3.69 Hz, JHH = 1.14 Hz, 1H, CH Ar, H9),
3
4
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 10568–10575 | 10573
©

7.78 (dd, 3JHH = 7.8 Hz, 4JHH = 1.11 Hz, 1H, CH Ar, H2), 7.84 (t,
(s), 1585 (m), 1325 (m), 1167 (m), 1035 (m) 930 (m), 860 (m), 802
(s), 748 (s), 730 (s), 600 (m), 480 (s), 430 (s). 1H NMR (400 MHz,
C6D6, 293 K): d 1.18 (d, JHH = 6.77 Hz, 12 H, CH(CH3)), 3.12
3
4
3JHH = 7.8 Hz, 1H, CH Ar, H3), 8.14 (dd, JHH = 7.8 Hz, JHH
=
1
3
1.11 Hz, 1H CH Ar, H4), 8.29 (s, 1H, CHN, H10). 13C{ H} NMR
(75 MHz, CD2Cl2, 293 K): d 23.1 (CH(CH3)2, C18,19,21,22), 27.9
(CH(CH3)2, C17,20), 119.0 (C4), 120.2 (C2), 122.9 (C13,15), 124.3 (C14),
125.0 (C9), 127.8 (C7), 128.0 (C8), 137.1 (C12,16), 137.3 (C3), 144.2
(C6), 148.6 (C11), 152.4 (C1), 154.2 (C5), 163.1 (C10). Anal. Calc. (%)
for C22H24N2S (348.51): C, 75.82; H, 6.94; N, 8.04. Found: C, 76.01;
H, 6.83; N, 7.87%. 2c: 88% yield, yellow crystals; mp 110 ◦C. IR
(br s 2 H, CH(CH3)), 5.75 (br s, 2 H, Ar), 6.23 (br s, 2 H, Ar), 6.72
(br s, 2 H, Ar), 6.90 (m, 2 H, Ar), 7.04 (s, 2 H, Ar), 7.09–7.13 (m, 4
H, Ar), 7.26–7.33 (m, 4 H, Ar), 7.62–7.69 (m, 2 H, Ar), 8.20–8.57
(m, 4 H, Ar + N CH). Anal. Calc. for C40H38N2OYb: C 65.29,
H 5.16, Yb 23.51. Found: C 64.87, H 5.00, Yb 23.60%. 6: 60%
yield, brownish-black crystals; IR (Nujol, KBr, cm-1): 3060 (w),
1642 (m), 1583 (m), 1310 (m), 1250 (m), 1210 (m), 1167 (m), 1100
(m), 980 (m), 850 (m), 820 (m), 742 (s), 726 (s), 690 (s), 623 (m),
530 (m). Anal. Calc. for C42H40N2 Yb: C 67.64, H 5.40, Yb 23.20.
Found: C 67.20, H 5.00, Yb 23.49%. 7: 77% yield, brownish-black
crystals; IR (Nujol, KBr, cm-1): 3060 (w), 1642 (s), 1583 (s), 1570
(s), 1324 (m), 1250 (m), 1167 (m), 1035 (m), 990 (m), 972 (m), 960
(m), 824 (m), 802 (s), 764 (s), 742 (s), 726 (s), 643 (m), 530 (m),
450 (s), 425 (s). 1H NMR (400 MHz, C6D6, 293 K): d 1.17 (br s,
12 H, CH(CH3)), 3.20 (br s, 2 H, CH(CH3)), 5.00 (s, 1 H, Ar),
5.54 (s, 2 H, Ar), 5.84 (s, 2 H, Ar), 6.23 (s, 1 H, Ar), 6.48 (s, 1 H,
Ar), 6.85 (br s, 4 H, Ar), 7.04 (br s, 2 H, Ar), 7.26 (br s, 4 H, Ar),
7.55 (s, 5 H, Ar), 8.39–8.57 (m, 2 H, Ar + N CH). Anal. Calc.
for C47H46N2SYb: C 66.88, H 5.49, Yb 20.49. Found: C 66.43, H
5.07, Yb 20.60.
(KBr): nC 1637 cm-1. MS m/z (%): 332 (M+, 56); 317 (M+ - 15,
N
1
77); 146 (100). H NMR (300 MHz, CD2Cl2, 293 K): d 1.10 (d,
3JHH = 6.8 Hz, 12H, CH(CH3), H18,19,21,22), 2.90 (sept, 3JHH = 6.8 Hz,
2H, CH(CH3), H17,20), 6.51 (dd, 3JHH = 3.4 Hz, 4JHH = 1.8 Hz, 1H,
CH Ar, H8) 7.02–7.12 (4H, CH Ar, H7,13,14,15), 7.52 (m, 1H, CH
Ar, H9), 7.73 (dd, 3JHH = 7.8 Hz, 4JHH = 1.14 Hz, 1H, CH Ar, H2),
7.82 (pt, 3JHH = 7.8 Hz, 1H, CH Ar, H3), 8.08 (dd, 3JHH = 7.8 Hz,
1
4JHH = 1.11 Hz, 1H CH Ar, H4), 8.23 (s, 1H, CHN, H10). 13C{ H}
NMR (75 MHz, CD2Cl2, 293 K): d 23.1 (CH(CH3)2, C18,19,21,22),
27.8 (CH(CH3)2, C17,20), 109.0 (C7), 112.0 (C8), 119.1 (C4), 119.8
(C2), 122.9 (C13,15), 124.3 (C14), 137.1 (C12,16), 137.3 (C3), 143.5
(C9), 148.5 (C11), 149.2 (C11), 153.3 (C1), 154.2 (C5), 163.2 (C10).
Anal. Calc. (%) for C22H24N2O (332.45): C, 79.48; H, 7.28; N, 8.43.
Found: C, 73.19; H, 7.21; N, 8.40%. 2d: 96% yield, yellow crystals;
mp 99 ◦C. IR (KBr): nC 1646 cm-1. MS m/z (%): 348 (M+, 77);
N
349 (M+ + 1, 20); 333 (M+ - 15, 100). 1H NMR (300 MHz, CD2Cl2,
293 K): d 1.20 (d, 3JHH = 6.8 HZ, 12H, CH(CH3), H18,19,21,22), 3.01
X-Ray crystallography
The data for complex 4 were collected on a SMART APEX
diffractometer [graphite-monochromated, Mo-Ka radiation (l =
3
(sept, JHH = 6.8 Hz, 2H, CH(CH3), H17,20), 7.11–7.22 (3H, CH
Ar, H13,14,15), 7.47 (dd, JHH = 5.0 Hz, JHH = 3.0 Hz, 1H, CH Ar,
H7), 7.77–7.80 (2H, CH Ar, H2,8), 7.92 (pt, 3JHH = 7.7 Hz, 1H, CH
Ar, H3), 8.04 (dd, 4JHH = 3.0 Hz, 4JHH = 1.2 Hz, 1H CH Ar, H9),
3
4
˚
0.71073 A), w- and q-scan technique], while those for complexes
5–7 were collected on an Oxford Diffraction XCALIBUR 3
diffractometer equipped with a CCD area detector, with Mo-Ka
radiation. The absorption correction was applied with the program
ABSPACK 1.17.23 Direct methods implemented in Sir9724 were
used to solve the structures and the refinements were performed by
full-matrix least-squares against F2 implemented in SHELX97.25
All the non-hydrogen atoms were refined anisotropically, while
the hydrogen atoms were fixed in calculated positions and refined
isotropically with the thermal factor depending on the one of
the atom to which they are bound. The geometrical calculations
were performed by PARST9726 and molecular plots were produced
by the program ORTEP3.27 Crystallographic data and structure
refinement details are given in Table 1.
3
4
8.20 (dd, JHH = 7.7 Hz, JHH = 1.0 Hz, 1H, CH Ar, H4) 8.35 (s,
1H, CHN, H10). 13C{ H} NMR (75 MHz, CD2Cl2, 293 K): d 23.1
1
(CH(CH3)2, C18,19,21,22), 27.9 (CH(CH3)2, C17,20), 119.0 (C4), 121.6
(C2), 122.9 (C13,15), 123.9 (C9), 124.3 (C14), 126.2 (C8), 126.4 (C7),
137.2 (C12,16), 137.3 (C3), 144.7 (C6), 148.6 (C11), 153.2 (C1), 154.2
(C5), 163.6 (C10). Anal. Calc. (%) for C22H24N2S (348.51): C, 75.82;
H, 6.94; N, 8.04. Found: C, 75.75; H, 6.90; N, 7.91%.
General procedure for the synthesis of (g5-C9H7)2Yb{[1-(6-
organylpyridin-2-yl)methylidene](2,6-diisopropylphenyl)amine}
(4–7)
In a typical procedure, a THF solution (10 mL) of 1 (0.50 g,
0.91 mmol) was treated dropwise with a THF solution (10 mL) of
the appropriate lig◦and (2a–d)(0.91mmol)and the reactionmixture
was heated at 50 C for 0.5 h. Afterwards, solvent was removed
under reduced pressure and the solid residue was dissolved in
toluene (20 mL). The resulting solution was◦then heated at 50 ◦C
for a further 0.5 h before cooling it to -20 C overnight. 4: 74%
yield, brownish-black crystals; IR (Nujol, KBr, cm-1): 3067 (w),
1642 (s), 1583 (m), 1310 (m), 1250 (m), 1167 (m), 1035 (m), 802
Acknowledgements
This work was supported by the Russian Foundation for Basic
Research and the Government of Nizhny Novgorod region (grant
No. 09-03-97018-p), The Ministry of Education and Science (state
contract No. P 1106 26.08.2009), Program of the Presidium of
the Russian Academy of Science (RAS), and RAS Chemistry and
Material Science Division. A. A. T. and G. G. also thank the
GDRE project for financial support.
1
(s), 770 (s), 750 (s), 742 (s), 648 (s), 623 (m), 495 (s), 439 (s). H
NMR (400 MHz, C6D6, 293 K): d 1.13 (d, 3JHH = 7.26 Hz, 12 H,
CH(CH3)), 3.12 (br s, 2 H, CH(CH3)), 5.49 (br s, 1 H, Ar), 5.93
(br s, 2 H, Ar), 6.23 (s, 2 H, Ar), 6.72 (s, 3 H, Ar), 6.76 (s, 4 H,
Ar), 6.99–7.05 (m, 4 H, Ar), 7.26–7.33 (m, 4 H, Ar), 8.14–8.48
(m, 4 H, Ar + N CH). Anal. Calc. for C47H46N2SYb: C 66.88, H
5.49, Yb 20.49. Found: C 66.39, H 5.12, Yb 20.77%. 5: 69% yield,
brownish-black crystals; IR (Nujol, KBr, cm-1): 3064 (w), 1640
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