Sterically governed redox reactions. One-electron oxidation of ytterbocenes by diazabutadienes: Formation of radical-anionic diazabutadiene vs covalently bonded imino-amido ligand

Article abstract of DOI:10.1021/om200429h

The reactions of ytterbocenes (C₅Me_nH _5-n)₂Yb(THF)_x (n = 1, 4, 5; x = 1, 2) with 1,4-diazabutadienes (DAD) 2-RC₆H₄N=C(Me)C(Me)=NC ₆H₄R-2 (R = H, Me) revealed a new example of steric control on redox processes. The steric saturation of the ytterbium coordination sphere proved to be a crucial factor which determines the outcome of the metal-ligand electron transfer reaction and the mode of transformation of the DAD ligand. The reactions of ytterbocenes containing bulky C₅Me ₅ or C₅Me₄H ligands lead to the formation of Yb^III complexes coordinated by radical-anionic diazabutadiene ligands: (C₅Me_nH_5-n)₂Yb ^III(DAD^?-) (n = 4, 5). In the case of the less sterically demanding C₅MeH₄ ligand and PhN=C(Me)C(Me)=NPh, oxidation of the ytterbium atom affords a metallocene-type Yb^III complex containing a covalently bonded imino-amido fragment, (η⁵-C₅MeH₄)₂Yb(N(C ₆H₅)C(H)(Me)C(Me)=NC₆H₅). All new complexes have been characterized by X-ray diffraction studies and magnetic and spectroscopic measurements.

Full text of DOI:10.1021/om200429h

ARTICLE
pubs.acs.org/Organometallics
Sterically Governed Redox Reactions. One-Electron Oxidation of
Ytterbocenes by Diazabutadienes: Formation of Radical-Anionic
Diazabutadiene vs Covalently Bonded IminoꢀAmido Ligand
Alexander A. Trifonov,*,† Boris G. Shestakov,† Konstantin A. Lyssenko,‡ Joulia Larionova,§ Georgy K. Fukin,†
and Anton V. Cherkasov†
†G. A. Razuvaev Institute of Organometallic Chemistry of Russian Academy of Sciences, Tropinina 49, GSP-445, 603950 Nizhny
Novgorod, Russia
‡A. N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences, Vavilova str. 28, 117813 Moscow, Russia
§Institut Charles Gerhardt Montpellier, UMR5253 CNRS-UM2-ENSCM-UM1, Chimie Molꢀeculaire et Organisation du Solide,
Universitꢀe Montpellier 2, Pl. E. Bataillon, 34095 Montpellier Cedex 5, France
S Supporting Information
b
ABSTRACT: The reactions of ytterbocenes (C5MenH5ꢀn)2Yb-
(THF)x (n = 1, 4, 5; x = 1, 2) with 1,4-diazabutadienes (DAD)
2-RC6H4NdC(Me)C(Me)dNC6H4R-2 (R = H, Me) revealed a
new example of steric control on redox processes. The steric
saturation of the ytterbium coordination sphere proved to be a
crucial factor which determines the outcome of the metalꢀligand
electron transfer reaction and the mode of transformation of the
DAD ligand. The reactions of ytterbocenes containing bulky
C5Me5 or C5Me4H ligands lead to the formation of YbIII com-
plexes coordinated by radical-anionic diazabutadiene ligands:
(C5MenH5ꢀn)2YbIII(DAD•ꢀ) (n = 4, 5). In the case of the
less sterically demanding C5MeH4 ligand and PhNdC(Me)-
C(Me)dNPh, oxidation of the ytterbium atom affords a metallocene-type YbIII complex containing a covalently bonded iminoꢀamido
fragment, (η5-C5MeH4)2Yb(N(C6H5)C(H)(Me)C(Me)dNC6H5). All new complexes have been characterized by X-ray diffraction
studies and magnetic and spectroscopic measurements.
’ INTRODUCTION
capacity of ytterbocenes in their reactions with diazabutadienes was
found to be sterically tunable from one- to two-electron reduction.6
The less sterically crowded ytterbocene (C5MeH4)2Yb(THF)2 acts
as a one-electron reductant, and the reaction affords a bis-
(cyclopentadienyl)ytterbium(III) derivative containing a DAD
radical anion (C5MeH4)2YbIII(DAD•ꢀ), while the compounds
Cp*2Yb(THF)2 (Cp* = C5Me5, C5Me4H), containing more steri-
cally demanding cyclopentadienyl ligands, reduce the DAD mole-
cule to a dianionic state due to oxidation of YbII to YbIII and
oxidative cleavage of the YbꢀCp* bond.6 In these reactions the half-
sandwich complexes Cp*YbIII(DAD2ꢀ)(THF) were isolated.
The role of the steric and electronic properties of diazabutadiene
remains poorly explored. Earlier it was demonstrated that the
outcome of the reactions of the bis(indenyl) complex
(C9H7)2YbII(THF)2 with various 1,4-diazabutadienes could be
influenced by variation of the steric demand of DAD and the nature
of the substituents at the nitrogen atoms. Thus, the reactions of
(C9H7)2YbII(THF)2 with sterically demanding 1,4-diazabutadienes
Remarkable progress has been made in the reductive chem-
istry of lanthanide complexes, which until recent years was
focused exclusively on the derivatives of these elements in the
divalent state.1 Due to the work of Evans and co-workers, who
first described the phenomenon of “sterically induced reduction”
for a tris(pentamethylcyclopentadienyl)samarium complex,2 the
modern concept of reductive capacities of coordination and
organometallic lanthanide compounds has evolved to a great
extent.
The application of redox-noninnocent 1,4-diazabutadiene ligands
(DAD)3 in reductive organolanthanide chemistry (especially in that
of ytterbium, for which two rather stable YbII and YbIII oxidation
states are characteristic4) had considerable impact on the develop-
ment of this area5 and revealed new challenging phenomena such as
solvent-mediated redox tranformations5dꢀf,9 and temperature-in-
duced redox isomerism.6 Redox reactions of ytterbocenes with 1,4-
diazabutadienes turned out to be extremely diverse and dependent
on a number of factors.7a These reactions can follow different paths
and result in metal atom oxidation,5cꢀf CꢀC bond formation,8
CꢀH bond activation,7a and YbꢀCp bond cleavage.9 The reductive
Received: May 23, 2011
Published: August 25, 2011
r
2011 American Chemical Society
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|

Organometallics
ARTICLE
Scheme 1
Figure 1. Molecular structure of the complex (η5-C5Me5)2Yb-
(2-MeC6H4NC(Me)C(Me)NC6H4Me-2) (3). The thermal ellipsoids corre-
spond to 30% probability. Selected bond lengths (Å) and angles (deg):
YbꢀN(1) = 2.343(1), YbꢀN(2) = 2.321(1), N(1)ꢀC(1) = 1.352(2),
N(2)ꢀC(2) = 1.350(2), C(1)ꢀC(2) = 1.415(2), YbꢀC(19) = 2.660(1),
YbꢀC(20) = 2.639(1), YbꢀC(21) = 2.638(1), YbꢀC(22) = 2.643(1),
YbꢀC(23) = 2.669(2), YbꢀC(29) = 2.659(2), YbꢀC(30) = 2.686(2),
YbꢀC(31) = 2.709(2), YbꢀC(32) = 2.689(2), YbꢀC(33) = 2.665(1);
CpcentrꢀYbꢀCpcentr = 134.4, N(1)ꢀYbꢀN(2) = 69.94(4).
Figure 2. Molecular structure of the complex (η5-C5Me4H)2Yb-
(2-MeC6H4NC(Me)C(Me)NC6H4Me-2) (4). The thermal ellipsoids corre-
spond to 30% probability. Selected bond lengths (Å) and angles (deg):
YbꢀN(1) = 2.336(3), YbꢀN(2) = 2.334(3), N(1)ꢀC(1) = 1.343(5),
N(2)ꢀC(2) = 1.351(5), C(1)ꢀC(2) = 1.415(6), YbꢀC(19) = 2.612(4),
YbꢀC(20) = 2.653(4), YbꢀC(21) = 2.657(4), YbꢀC(22) = 2.605(4),
YbꢀC(23) = 2.562(4), YbꢀC(28) = 2.676(4), YbꢀC(29) = 2.686(4),
YbꢀC(30) = 2.671(4), YbꢀC(31) = 2.652(4), YbꢀC(32) = 2.622(4);
CpcentrꢀYbꢀCpcentr = 130.7, N(1)ꢀYbꢀN(2) = 69.9(1).
into the 6-position of the pyridyl ring of the Ipy ligand inspired us to
go further in the studies of the steric effect in redox reactions.7b In
order to address this issue, we investigated the reactions of a series of
ytterbocenes of variable steric hindrance (C5MenH5ꢀn)2Yb(THF)x
(n = 1, 4, 5; x = 1,2) with diazabutadienes 2-RC6H4NdC(Me)C-
(Me)dNC6H4R-2 (R = H, Me) having modest steric demand.
Herein we report on a new example of steric control on the outcome
of one-electron-oxidation reactions of ytterbocenes with 1,4-diaza-
butadienes. A new type of reaction of ytterbocenes with 1,4-
diazabutadienes affording metallocene-type YbIII complexes contain-
ing the covalently bonded monoanionic iminoꢀamido ligand
[NPhC(H)MeC(Me)dNPh]ꢀ will be described. The synthesis,
structures, and properties of new metallocene-type YbIII complexes
coordinated by radical-anionic DAD•ꢀ ligands are reported.
t
2,6-iPr2C6H3NdCHCHdNC6H3Pri2-2,65e and BuNdCHCHd
NBut5d proceed with one-electron oxidation of the ytterbium atom
and lead to the formation of metallocene-type complexes containing
the radical anion of DAD of general formula (C9H7)2YbIII-
(DAD•ꢀ). However, in the case of the less sterically demanding
PhNdC(Me)C(Me)dNPh the oxidative cleavage of the Ybꢀ
C9H7 bond and the formation of unusual mixed-valence bi- and
tetranuclear ytterbium complexes containing a μ-η4:η4 dianionic
1,4-diazabutadiene ligand occurred. In this case, the phenyl substit-
uents of the 1,4-diazabutadiene ligand are involved in the me-
talꢀligand bonding.8b The transition to a DAD ligand containing a
methyl substituent at the ortho position of the phenyl rings,
2-MeC6H4NdC(Me)C(Me)dNC6H4Me-2, once again changed
dramatically the reaction result: one-electron transfer from the metal
to the DAD ligand was observed, and the complex (C9H7)2YbIII-
(DAD•ꢀ) was isolated.9
’ RESULTS AND DISCUSSION
The recent finding of the possibility of governing metalꢀligand
electron transfer processes in the reactions of ytterbocenes with
iminopyridynes (Ipy) due to the introduction of various substituents
Reactions of the ytterbocenes (C5MenH5ꢀn)2Yb(THF)x (n =
1, 4, 5; x = 1, 2) with the diazabutadiene 2-MeC6H4Nd
C(Me)C(Me)dNC6H4Me-2 were carried out (Scheme 1).
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Organometallics
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Table 1. Crystal Data and Some Details of Data Collection and Structure Refinement for 3ꢀ6
3
4
5
6
CCDC no.
formula
Mr
808218
808220
808220
808219
C38H50N2Yb
707.84
C39H52N2Yb
721.87
C28H31N2Yb
568.59
C34H42N2Yb
651.74
T, K
100
cryst syst
space group
Z (Z0)
monoclinic
P21/n
triclinic
monoclinic
P21/n
monoclinic
P21/c
P1
4 (1)
2 (1)
4 (1)
4 (1)
a, Å
11.1181(7)
19.0035(11)
15.8782(10)
90.00
10.0873(11)
12.3074(13)
15.8869(17)
68.9076(17)
76.2655(17)
69.3933(17)
1708.5(3)
1.403
9.6373(7)
15.6545(11)
15.9470(12)
90.00
14.7952(11)
14.9277(11)
13.3859(10)
90.00
b, Å
c, Å
α, deg
β, deg
96.6360(11)
90.00
105.5350(10)
90.00
97.2450(10)
90.00
γ, deg
V, Å3
dcalcd, g cmꢀ3
μ, cmꢀ1
F(000)
3332.3(4)
1.411
2318.0(3)
1.629
2932.8(4)
1.476
28.34
27.65
40.52
32.13
1448
740
1132
1320
2θmax, deg
no. of rflns measd
no. of indep rflns (Rint
65
55
58
58
83 362
17 769
23 758
35 018
)
12 043 (0.0340)
10 625
7787 (0.0211)
7557
6110 (0.0429)
4795
7807 (0.0492)
5965
no. of obsd rflns (I > 2σ(I))
R1
0.0189
0.0326
0.0300
0.0260
wR2
GOF
0.0457
0.0799
0.0715
0.0722
1.022
1.041
1.045
1.034
ΔFmax/ΔFmin (e Åꢀ3
)
1.635/ꢀ1.216
1.697/ꢀ1.776
1.848/ꢀ0.849
1.768/ꢀ1.322
The addition of THF solutions of 2-MeC6H4NdC(Me)C(Me)d
NC6H4Me-2 to THF solutions of 1 and 2 does not result in any
change in the color of the reaction mixtures. Evaporation of
THF under vacuum and addition of toluene lead to the
formation of brown solutions. The toluene solutions were
heated at 50 °C for 4 h. Evaporation of toluene under vacuum
and recrystallization of the reaction products from hot hexane
allowed the isolation of dark brown crystalline solids in yields of
57 and 63%, respectively. Complexes 3 and 4 are highly air- and
moisture-sensitive, moderately soluble in toluene, and less
soluble in hexane.
less than would be expected on the basis of the difference in the ionic
radii of YbII and YbIII (0.155 Å).12 The values of the YbꢀC bond
distances in 3 and 4 are somewhat shorter than the relevant bond
lengths in complex [Cp*2Yb(tBuNCHCHNBut)] (2.690(4) Å)5c
but close to those in [Cp*2Yb(p-tolyl-NCHCHN-tolyl-p)] (2.65 Å)
and [Cp*2Yb(p-anisyl-NCHCHN-anisyl-p)] (2.64 Å),13 thus reflect-
ing the difference of the steric demand of diazabutadiene ligands. The
YbꢀN bond lengths in 3 (2.3208(12) and 2.3428(12) Å) as well as
in 4 (2.334(3) and 2.336(3) Å) are similar to the values previously
reported for related YbIII complexes containing radical-anionic diaza-
butadiene ligands: [Cp*2Yb(p-tolyl-NCHCHN-tolyl-p)] (2.340(5),
2.368(5) Å) and [Cp*2Yb(p-anisyl-NCHCHN-anisyl-p)] 2.339(7);
2.337(6) Å).13 The YbꢀN bond lengths in 3and 4are comparable to
the lengths of the YbIIIꢀN coordination bonds in the compounds
[{(Me5C5)2Yb(bipy)}+{ (Me5C5)2YbCl2}ꢀ] (2.37 Å),14a [{(Me5-
C5)2Yb(phen)}+{I}ꢀ(CH2Cl2)] (2.36 Å),14a and [Cp2Yb(PzMe2)-
(HpzMe2)] (2.360(6), 2.414(6) Å)14b but are much longer than the
covalent YbIIIꢀN bonds in [(η5-C9H7)2YbN{SiMe3}2] (2.160(5),
2.163(5) Å).14c The bonding situation within the diazabutadiene
ligands of 3 and 4 is indicative of their radical-anionic character, since
an electron transfer to the ligand is expected to cause charge
delocalization over the conjugated NCCN fragment, resulting in
changes in the bond lengths. The NdC bonds in the diazabutadiene
ligands of 3 (1.3519(18) and 1.3502(19) Å) and 4 (1.343(5) and
1.351(5) Å) are substantially elongated compared to the NdC bonds
(1.267(2)15 and 1.266(3) Å16) in the neutral diazabutadienes, while
the CꢀC bonds (3, 1.415(2) Å; 4, 1.415(6) Å) are shortened and
become close to the values for aromatic CꢀC bonds.17 A similar
bonding situation was previously described for the related complexes
[Cp*2Sm(tBuNCHCHNBut)],5a [Cp2Yb(tBuNCHCHNBut)],5c
Monocrystalline samples suitable for single-crystal X-ray diffrac-
tion studies were grown by slow concentration of hexane solutions
at ambient temperature. Complex 4 was isolated as a hexane solvate,
[(C5Me4H)2Yb(2-MeC6H4NdC(Me)C(Me)dNC6H4Me-2)]
3
0.5C6H14, while the crystals of complex 3 do not contain molecules
of solvent. The molecular structures of complexes 3 and 4 are
depicted in Figures 1 and 2; crystal and structure refinement data are
given in Table 1.
Both compounds have similar structures: the ytterbium atom
is η5-coordinated by the two cyclopentadienyl ligands and the
two nitrogen atoms of the radical anionic DAD ligand and adopts
the geometry of a distorted tetrahedron. The Yb atoms in 3 and 4
have coordination number of 8. The average YbꢀC distances in 3
(2.665(2) Å) and 4 (2.639(4) Å) are shorter than those in
Cp*2Yb(py)2 (2.74(4) Å)10 and are close to the values reported
for the YbIII derivatives.11 The shortening of the YbꢀC distances
in 3 and 4 compared to those in Cp*2Yb(py)2 indicates the
oxidation of the ytterbium atom from YbII to YbIII.12 It is
noteworthy that the magnitude of the shortening is considerably
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Organometallics
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the basis of a linear combination of the 2F7/2 term of ytterbium
and 2S1/2 term of the radical. For an antiferromagnetic interac-
2S
1
tion, this term is
interaction it is
L
L
= F3, while for a ferromagnetic
Jꢀ1/2
2S+2
=
3F4. Therefore, the predicted
J+1/2
room-temperature χT values of ytterbium antiferromagnetically
and ferromagnetically coupled with an anion radical are equal to
1.496 and 3.90 emu K molꢀ1, respectively.14a The first one is too
low and the second one is too high in comparison with the
observed room-temperature values for 3 and 4. As the tempera-
ture decreases, χT for both curves decreases and at 2 K they reach
the values of 0.05 and 0.03 emu K molꢀ1 for 3 and 4, respectively.
Such a decrease may be due to both depopulation of the Stark
levels of the YbIII ground state and the presence of antiferro-
magnetic interactions between the YbIII ion and radical-anionic
DAD ligand. Indeed, at room temperature the excited states of
the YbIII ion are populated, but as the temperature decreases,
there is the progressive depopulation of the excited Kramers
doublets and at low temperature only the ground Kramers
doublet is populated. Below 5 K, the calculated χT value of the
noninteracting YbIII and DAD radical is 2.2 K, which is much
higher than those observed for 3 and 4. Taking into account these
values, we can reasonably suppose the simultaneous presence of
both antiferromagnetic and ferromagnetic interactions between
YbIII and DAD radical, similar to the situation previously
observed for a ytterbium phthalocyanine complex containing
together with YbIII a radical anion of phthalocyanine.19 It is
noteworthy that the room-temperature χT values measured for 3
and 4 are higher compared to those observed for related
complexes Cp2YbIII(DAD•ꢀ).7
Unexpectedly, the reaction of a diazabutadiene having lesser steric
bulk, C6H5NdC(Me)-C(Me)dNC6H5, with an ytterbocene con-
taining a non sterically demanding ligand, (C5MeH4)2Yb(THF),
under similar conditions occurs differently and results in the forma-
tion of a new YbIII complex containing the monoanionic imi-
noꢀamido ligand ꢀN(Ph)CH(Me)C(Me)dNPh (Scheme 2).
Complex 5 was isolated after recrystallization from toluene as air-
and moisture-sensitive wine red crystals in 61% yield. Complex 5 is
moderately soluble in toluene and poorly soluble in hexane. In the IR
spectrum of 5 an intense absorption band attributed to the ν(CdN)
vibrations (1668 cmꢀ1) is shifted to high frequency compared to that
of neutral C6H5NdC(Me)C(Me)dNC6H5 (1633 cmꢀ1).
Figure 3. χT versus T curves for 3 (0), 4 (2), 5 (g), and 6 (O)
performed with an applied field of 1000 Oe.
[Cp*2Yb(tBuNCHCHNBut)],5d (C9H7)2Yb(2,6-iPr2C6H3CHCH-
C6H3iPr2-2,6),5e (C5H4Me)2Yb(2,6-iPr2C6H3CHCHC6H3iPr2-2,6),6
and [Cp*2Yb(4-RC6H4NCHCHNC6H4R-4) (R = Me, OMe),13
proving the radical-anionic character of the diazabutadiene ligand
in 3 and 4. Thereby, the structural parameters of 3 and 4 are
consistent with a trivalent oxidation state of the ytterbium atom
and a radical-anionic character of the DAD ligand.
The IR spectrum of neutral diazadiene normally shows a
characteristic strong absorption band at about 1620 cmꢀ1, which
is assigned to the ν(CdN) vibrations. In the IR spectra of
complexes 3 and 4 the absorption intensities of the band
corresponding to ν(CdN) vibrations decrease strongly and
the band shifts to 1570 cmꢀ1 for 3 and to 1590 cmꢀ1 for 4.
The UV/vis spectra of 3 and 4 in hexane show strong absorptions
at 297 and 290 nm, respectively, which correspond well with the
strong band in the spectrum of (DAD•ꢀ)Na+ at 296 nm but differ
from that of neutral DAD0 at 340 nm (see the Supporting
Information), thus giving further proof of the radical-anionic
character of the DAD ligand. Despite the presence of the radical
anions of DAD, complexes 3 and 4 in the solid state as well as in
toluene solution are ESR silent in the temperature range
173ꢀ300 K. This fact is attributable either to a substantial
broadening of the signal of the DAD radical anion in the field
of the paramagnetic YbIII ion or to antiferromagnetic coupling of
the spin carriers.
Single-crystal samples of 5 suitable for an X-ray diffraction
study were obtained by cooling a concentrated solution in
toluene at ꢀ5 °C. The X-ray study revealed that the Yb atom
in 5 is coordinated by two η5-methylcyclopentadienyl rings and
by two nitrogen atoms of the chelating iminoꢀamido ligand,
thus adopting the geometry of a distorted tetrahedron. The Yb
atom in 5 has a coordination number of 8. The molecular
structure of complex 5 is depicted in Figure 4, and crystal and
structure refinement data are given in Table 1.
Magnetic measurements of crystalline samples 3 and 4 have
been carried out by using an MPMS-XL SQUID magnetometer
in the 2ꢀ300 K temperature range with an applied magnetic field
of 1000 Oe. The temperature dependences of these samples
present similar shapes (Figure 3). At 300 K, the χT values for 3
and 4 are equal to 3.13 and 3.08 emu K molꢀ1, respectively, thus
indicating the trivalent state of ytterbium. The YbIII ion has a 2F7/2
ground state with a first-order orbital momentum.18 In the low-
symmetry site, this state is split into Stark components by the
crystal field, each of them being a Kramers doublet. At room
temperature the excited states of YbIII ion are populated and the
The YbꢀC bond lengths in 5 fall into the interval of 2.679(4)ꢀ
2.578(4) Å (the average YbꢀCav bond is 2.614(4) Å; YbꢀCpcent
=
2.33(4), 2.39(4) Å) and are noticeably shorter compared to those in
the parent ytterbocene (C5MeH4)2Yb(THF) (YbꢀCav = 2.836(3)
Å).20 This value is close to the YbꢀCav bond length reported for the
related YbIII complex [(C5MeH4)2Yb(μ-O2CC6H5)]2 (2.60 Å),21
thus indicating the trivalent state of ytterbium. The chelating ꢀN-
(Ph)CH(Me)C(Me)dNPh ligand is bound to the Yb atom by two
nonequivalent YbꢀN bonds. The length of the short YbꢀN(2)
bond (2.236(3) Å) is comparable to the values observed in the
eight-coordinate YbIII amido complexes (C5H4Me)2Yb(NH-2,
6-R2C5H3)(THF) (R = Me, iPr; 2.206(9) and 2.188(3) Å)22 and
theoretical YbIII free-ion value of χT is equal to 2.49 emu K molꢀ1
.
1
The DAD anion radical has the spin S = /2 with a room-
temperature χT value of 0.375 emu K molꢀ1. In the case of the
noninteracting ytterbium and radical anion spins, the predicted
χT value is equal to 2.87 emu K molꢀ1, which is slightly low,
considering the values observed for 3 and 4. For a system
involving electron exchange coupling between YbIII and the
DAD anion radical, the new term symbols can be calculated on
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Organometallics
ARTICLE
Scheme 2
ligand coordination to the metal center. The metallacycle Yb-
(1)N(1)C(1)C(2)N(2) is nearly flat (the deviations of atoms
from the plane do not exceed 0.1 Å). Therefore, the geometry of
the chelating ligand in 5 is consistent with an iminoꢀamido
structure which originates from 1,2-addition to one of the CdN
bonds of the parent 1,4-diazabutadiene. The hydrogen atom
which adds to the carbon atom is obviously abstracted from the
solvent.
The magnetic properties of 5 indicate the presence of a
paramagnetic YbIII ion in this compound (Figure 3). The χT
value at room temperature is equal to 2.48 emu K molꢀ1, which
corresponds well to the calculated value for one YbIII ion.18 As
the temperature decreases, the curve decreases slowly and at low
temperature (at 2 K) the χT value is equal to 1.72 emu K molꢀ1
,
which is typical for the progressive depopulation of the excited
Kramers doublets of ytterbium with temperature. χT at low
temperature also corresponds well to the calculated value for
YbIII, in which only the ground Kramers doublet is populated.
To rationalize the different outcomes of the reactions of
1,4-diazabutadienes 2-RC6H4NdC(Me)C(Me)dNC6H4R-2
(R = H, Me) with ytterbocenes and the formation of complexes
of different nature, the connection of these phenomena to the
different stabilities of DAD radical anions formed after one-electron
reductions was suggested. Supposedly due to insufficient steric bulk,
the radical anion of C6H5NdC(Me)C(Me)dNC6H5 undergoes
further transformation: namely, 1,2-addition to one of the CdN
bonds. The formation of a YbꢀN covalent bond and the saturation
of valence capacity of the carbon atom via hydrogen abstraction lead
to complex 5. The solvent seems to be a plausible source for
hydrogen abstraction. To verify this hypothesis, we carried out the
reaction of C6H5NdC(Me)C(Me)dNC6H5 with the ytterbocene
(C5Me4H)2Yb(THF)2, containing a bulkier cyclopentadienyl
ligand. However, this reaction under analogous conditions afforded
a metallocene-type complex of YbIII coordinated by the radical-
anionic diazabutadiene ligand (Scheme 3), similar to the case for 3
and 4.
Recrystallization of the reaction products from hot hexane
allowed the isolation of brownish black crystals of 6 in 67% yield.
Complex 6 ishighly air- and moisture-sensitive, moderately soluble
in toluene, and poorly soluble in hexane. Monocrystalline samples
suitable for single-crystal X-ray diffraction studies were grown by
slow concentration of hexane solutions at ambient temperature.
The molecular structure of complex 6 is depicted in Figure 5, and
crystal and structure refinement data are given in Table 1.
The X-ray analysis revealed that the structure of complex 6 is
similar to those of 3 and 4: the ytterbium atom is η5-coordinated
by the two cyclopentadienyl ligands and the two nitrogen atoms
of the radical anionic DAD ligand and adopts the geometry of a
Figure 4. Molecular structure of the complex (η5-C5MeH4)2Yb(N(C6H5)-
C(H)(Me)C(Me)dNC6H5) (5). The thermal ellipsoids correspond to
30% probability. Selected bond lengths (Å) and angles (deg): YbꢀN(1) =
2.236(3), YbꢀN(2) = 2.379(3), N(1)ꢀC(1) = 1.446(5), N(2)ꢀC(2) =
1.284(5), C(1)ꢀC(2) = 1.502(5), C(1)ꢀH(1) = 1.012(4), YbꢀC(17) =
2.636(3), YbꢀC(18) = 2.609(4), YbꢀC(19) = 2.596(4), YbꢀC(20) =
2.601(4), YbꢀC(21) = 2.621(4), YbꢀC(23) = 2.679(4), YbꢀC(24) =
2.613(4), YbꢀC(25) = 2.589(4), YbꢀC(26) = 2.578(4), YbꢀC(27) =
2.627(4); CpcentrꢀYbꢀCpcentr = 129.1, N(1)ꢀYbꢀN(2) = 71.3(1).
(C5H4Me)2Yb(NPh2)(THF) (2.287(6) Å)23 and corresponds to a
covalent bond. The longer bond YbꢀN(1) (2.379(3) Å) has a
length close to those in 3 and 4 and can be considered as a
coordination bond. The bonding situation within the NCCN
fragment in 5 differs substantially from those in 3 and 4. The length
of the N(1)ꢀC(19) bond (1.284(5) Å) is indicative of its double-
bond character and is characteristic for neutral 1,4-diazadienes
(2,6-iPr2C6H3NdC(Me)C(Me)dNC6H3iPr2-2,6, 1.280, 1.279Å;16
2,6-Me2C6H3NdC(Me)C(Me)dNC6H3Me2-2,6, 1.275 Å24),
while the length of the N(2)ꢀC(21) bond (1.446(5) Å)
approaches that of a single CꢀN bond.17 The C(19)ꢀC(21)
bond length (1.502(5) Å) gives evidence of its single-bond
character.17 The evolution of the 1,4-diazabutadiene ligand in
the course of the reaction is also reflected in the changes of the
geometry around the carbon atom C(21). The formation of a
CꢀH bond and the change of hybridization type from sp2 to sp3
result in tetrahedral bond angles about C(21): 109.0(3),
112.5(3), 113.3(3)°. The values of bonding angles about the
N(2) atom are closer to a planar geometry: 119.1(2), 116.3(3),
and 124.2(2)°, which probably result from the rigid chelating
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Scheme 3
temperature decreases, χT decreases progressively and at low
temperature the χT value is equal to 0.84 emu K molꢀ1. As in the
previous cases, such behavior may be attributed to the depopula-
tion of the Stark levels of ytterbium with temperature and to the
presence of antiferromagnetic interactions between YbIII and the
anion radical.
’ SUMMARY AND CONCLUSIONS
To conclude, a new example of steric control on the outcome
of redox reactions is described. One-electron oxidation of
ytterbocenes by the 1,4-diazabutadienes 2-RC6H4NdC(Me)-
C(Me)dNC6H4R-2 (R = H, Me) bearing at the nitrogen atoms
groups having similar electronic properties but slightly differing
in steric demand can afford metallocene-type YbIII complexes
containing either radical-anionic diazabutadiene or imi-
noꢀamido ligands. The presence of bulky C5Me5 or C5Me4H
ligands in the parent ytterbocenes favors the formation of
complexes with radical-anionic diazabutadiene ligands, while in
the case of the less sterically demanding C5MeH4 ligand the
reaction results in the reduction of DAD followed by hydrogen
abstraction by the radical anion involving the formation of
covalent YbꢀN and CꢀH bonds. The reaction solvent obviously
serves as a source for the hydrogen abstraction. The difference in
the results of the reactions of C6H5NdC(Me)C(Me)dNC6H5
with (C5Me4H)2Yb(THF)2 and (C5MeH4)2Yb(THF)2 allows
us to conclude that the steric encumbrance in the ytterbium
coordination sphere but not the stability of the radical anion
[C6H5NC(Me)C(Me)NC6H5]•ꢀ is a key factor determining the
path of these redox reactions. Formation of a covalent YbꢀN
bond becomes feasible when the steric repulsion allows the
approach of Cp2Yb and diazabutadiene fragments at an appro-
priate distance.
Figure 5. Molecular structure of the complex (η5-C5Me4H)2Yb-
(C6H5NC(Me)C(Me)NC6H5) (6). The thermal ellipsoids correspond
to 30% probability. Selected bond lengths (Å) and angles (deg):
YbꢀN(1) = 2.317(2), YbꢀN(2) = 2.348(2), N(1)ꢀC(1) = 1.360(4),
N(2)ꢀC(2) = 1.332(4), C(1)ꢀC(2) = 1.433(4), YbꢀC(17) =
2.603(3), YbꢀC(18) = 2.638(3), YbꢀC(19) = 2.659(3), YbꢀC(20)
= 2.650(3), YbꢀC(21) = 2.644(3), YbꢀC(26) = 2.571(3), YbꢀC(27)
= 2.613(3), YbꢀC(28) = 2.658(3), YbꢀC(29) = 2.651(3), YbꢀC(30)
= 2.616(3); N(1)ꢀYbꢀN(2) 69.63(8), CpcentrꢀYbꢀCpcentr = 131.6.
distorted tetrahedron. The Yb atom in 6 has a coordination
number of 8. The average YbꢀC distance 2.630(3) Å (compare
to 2.665(2) Å for 3 and 2.639(4) Å for 4) is indicative of a
trivalent state of the ytterbium atom, while the values of YbꢀN
bond lengths (2.317(2) and 2.348(2) Å) and the bonding
situation within the NCCN fragment (N(1)ꢀC(1) = 1.360(4) Å,
N(2)ꢀC(2) = 1.332(4) Å, C(1)ꢀC(2) = 1.433(4) Å) are
consistent with the radical-anionic nature of the 1,4-diazabuta-
diene ligand. It should be noted that in complexes 3, 4, and 6 both
phenyl rings are orthogonally disposed relative to the NCCN
fragments. In contrast to the case for 3, 4, and 6 in 5 the phenyl
rings at the nitrogen atoms of 1,4-diazabutadiene ligand have
different orientations relative to the plane of N(1)C(1)C(2)N(2)
fragment: the values of dihedral angles are 23.52 and 89.39°,
respectively.
’ EXPERIMENTAL DETAILS
All experiments were performed in evacuated tubes, using standard
Schlenk-tube techniques, with rigorous exclusion of traces of moisture and
air. After dryingoverKOH, THF was purifiedby distillationfrom sodium/
benzophenone ketyl and hexane and toluene by distillation from sodium/
triglyme benzophenone ketyl prior to use. (C5Me5)2Yb(THF)2,25
(C5Me4H)2Yb(THF)2,18 (C5H4Me)2Yb(THF),18 and 2-RC6H4Nd-
C(Me)C(Me)dNC6H4R-2 (R = H, Me)26 were prepared according to
literature procedures. All other commercially available chemicals were
used after the appropriate purification. 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 magnet-
ometer working in the temperature range of 1.8ꢀ300 K and a magnetic
field range up to 5 T. The measurements were performed under an argon
atmosphere. Gel capsules were used for all magnetic measurements. The
The properties of 6 demonstrate the presence of a YbIII and
1,4-diazabutadiene anion-radical interaction, as was previously
observed in the case of 3 and 4. At room temperature, χT is equal
to 2.84 emu K molꢀ1, which fits to the calculated value for a
noninteracting YbIII and 1,4-diazabutadiene ligand radical. As the
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data were corrected for the sample holder and the diamagnetism
contributions calculated from the Pascal constants.27 The C, H elemental
analysis was carried out in the microanalytical laboratory of IOMC.
Lanthanide metal analysis was carried out by complexometric titration.
Synthesis of [(C5Me5)2Yb(2-MeC6H4NC(Me)C(Me)NC6H4-
Me-2)] (3). A solution of 2-MeC6H4NdC(Me)C(Me)dNC6H4Me-2
(0.20 g, 0.75 mmol) in THF (5 mL) was added to a solution of 1 (0.44 g,
0.75 mmol) in THF (10 mL). The reaction mixture was stirred for 0.5 h at
20 °C. After the removal of THF under vacuum, toluene (25 mL) was
added and the solution was heated to 50 °C for 4 h. Then toluene was
evaporated under vacuum. Recrystallization of the solid residue from hexane
gave 3 as deep brown crystals (0.30 g, 57%). IR (Nujol, KBr, cmꢀ1): 1640
(m), 1570 (m), 1268 (m), 1093 (m), 978 (s), 950 (s), 820 (s), 760 (s), 680
(s). Anal. Calcd for C38H50N2Yb: C, 64.48; H, 7.12; Yb, 24.44. Found: C,
63.96; H, 7.00; Yb, 24.72.
’ ASSOCIATED CONTENT
S
Supporting Information. A figure giving UVꢀvis spec-
b
tra of 3, 4, and Ph(Me)NC(Me)C(Me)NPh(Me) in hexane and
Ph(Me)NC(Me)C(Me)NPh(Me) in THF and CIF files giving
crystallographic data for 3ꢀ6. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: trif@iomc.ras.ru. Fax: (+7)8314621497.
’ ACKNOWLEDGMENT
This work was supported by the Federal Agency for Higher
Education (State contract No. Π 1106 26.08.2009), Program of
the Presidium of the Russian Academy of Science (RAS), and
RAS Chemistry and Material Science Division.
Synthesis of [(C5Me4H)2Yb(2-MeC6H4NC(Me)C(Me)NC6H4-
Me-2)] (4). A solution of 2-MeC6H4NdC(Me)C(Me)dNC6H4Me-2
(0.16 g, 0.59 mmol) in THF (5 mL) was added to a solution of 2 (0.33 g,
0.59 mmol) in THF (10 mL). The reaction mixture was stirred for 0.5 h at
20 °C. After the removal of THF under vacuum, toluene (25 mL) was
added and the solution was heated to 50 °C for 4 h. Then toluene was
evaporated under vacuum. Recrystallization of the solid residue from hexane
gave 4 as deep brown crystals (0.25 g, 63%). IR (Nujol, KBr, cmꢀ1): 1640
(s), 1590 (m), 1370 (s), 1340 (m), 1270 (m), 1140 (m), 1080 (m), 890
(s), 760 (s), 624 (m). Anal. Calcd for C36H46N2Yb: C, 63.60, H, 6.82; Yb,
25.44. Found: C, 63.10; H, 6.44; Yb, 25.67.
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the strongly disordered hexane solvate molecule in 4 were not located.
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CCDC files 808218ꢀ808221 contain supplementary crystallographic
data for this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_ request/cif.
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