C-C coupling and C-H bond activation - Unexpected pathways in the reactions of [Yb(η5-C13H9)2(thf) 2] with diazadienes

Article abstract of DOI:10.1002/anie.200461001

A couple of surprises: The reactions of [(C₁₃H₉) ₂Yb(thf)₂] with diazadienes (2,6-iPr₂C ₆H₃) N=CR-CR=N (2,6-iPr₂C₆H ₃) (R = H, Me) result in unexpected Yb^II complexes, which arise either from the coupling of the fluorene and the diazadiene ligands (when R = H, see picture) or from C-H bond activation of the diazadiene ligand (when R = Me).

Full text of DOI:10.1002/anie.200461001

Angewandte
Chemie
oxidation states and is characterized by a low potential of the
YbII/YbIII transformation,[4] are of particular interest as
promising objects for investigating intramolecular metal–
ligand electron transfer. In our previous studies the variable-
temperature investigation of the magnetic properties of the
complex [(tBuNCHCHNBut)3Yb][2b] suggested to us the
existence of temperature-induced valence tautomerism for
this compound. Recently, for the complexes [YbR2(t-
BuNCHCHNBut)] (R = Cp = C5H5 or R = Cp* = C5Me5) we
demonstrated that the steric repulsion of DAD and Cp
ligands as a result of their bulk might serve as a tool for tuning
À
the Yb N bond lengths and eventually determine the
reversibility of the Yb–DAD electron transfer.[2f,g] As part
of our continuing studies on intramolecular redox processes in
ytterbium complexes with redox-active ligands we attempted
to synthesize novel mixed-ligand compounds with sterically
demanding diazadienes. Herein we report the unexpected
reactivity of [Yb(C13H9)2(thf)2] (1)[5] towards diazadienes
À =
À
= À
[(2,6-iPr2C6H3) N CR CR N (2,6-iPr2C6H3)] (R = H (2),
Me (3)) which results either in the coupling of the fluorenyl
and DAD fragments or in proton abstraction from the DAD
molecule, depending on the substituent R on the carbon atom
of the imino group.
=
À
=
tBuN CH CH NtBu readily oxidizes ytterbocenes
[YbR2(thf)2] (R = Cp, Cp*) to afford the trivalent ytterbium
derivatives [YbR2(tBuNCHCHNtBu)],[2f,g] which contain a
DAD radicalanion. Unexpectedyl, the reactions of the
bisfluorenyl analogue 1 with bulky diazadienes 2 and 3
under the same conditions do not lead to oxidation of the
ytterbium atom but result in the formation of the YbII
Diazadiene Ligands
derivatives. Thus, the reaction of
1 with a twofold
À
À
C C Coupling and C H Bond Activation—
Unexpected Pathways in the Reactions of
[Yb(h5-C13H9)2(thf)2] with Diazadienes**
molar excess of 2 led to the isolation of the unexpected
5
=
product [Yb{h -C13H8C( N[2,6-iPr2C6H3])CH2NHC6H3(2,6-
iPr2C6H3)}2(thf)] (4) in 80% yield (Scheme 1).
Complex 4 is readily soluble in ethers and toluene, but
sparingly soluble in hexane. The diamagnetic properties of 4
corroborate the divalent state of ytterbium.[6] Crystallization
of 4 by slow cooling of the solution in hexane resulted in single
crystals of the hexane solvate 4·C6H14 that were suitable for
crystal-structure analysis (Figure 1). The X-ray diffraction
study revealed that the YbII cation is h5-coordinated
Alexander A. Trifonov,* Elena A. Fedorova,
Georgy K. Fukin, Nikolai O. Druzhkov, and
Mikhail N. Bochkarev
The diversity of the coordination and redox properties of 1,4-
disubstituted diazadienes (DAD) makes them advantageous
ligands for transition-metal chemistry. The DAD molecule
can coordinate to a metalatom as a neutralligand. Possessing
a quite pronounced electron affinity[1] it can oxidize electro-
positive lanthanide metals by accepting one or two electrons
=
by two novel multifunctional ligands {C13H8C( N[2,6-
iPr2C6H3])CH2NHC6H3(2,6-iPr2C6H3)}À, which arise from
unprecedented coupling of the allylic carbon atom of the
fluorenyl ligand with the imino carbon atom of the diazadiene
À
2. The C C bond (1.426(3), 1.434(3) Š) formation in 4 also
[2]
to form the radicalanion or dianion,[3] respectively. Owing
implies that the two H atoms initially bonded to the coupled
[1c]
to the low energy of the p* orbitalof the DAD ilgands,
their complexes with ytterbium, which forms two stable
carbon atoms migrate to the second imino group, resulting in
5
À
the hydrogenated CH2 NH fragment. Unlike the h -bonding
of the fluorenyl fragment to the Yb atom in the starting
complex 1,[5] a rather unusual h3-type of coordination through
one carbon atom of the five-membered ring and two carbon
atoms of the six-membered ring occurs in 4. The h2-
coordination of the imino group to the Yb atom results in
[*] Dr. A. A. Trifonov, Dr. E. A. Fedorova, Dr. G. K. Fukin,
Dr. N. O. Druzhkov, Prof. Dr. M. N. Bochkarev
G. A. Razuvaev Institute of Organometallic Chemistry
Russian Academy of Sciences
Tropinina 49, 603950 Nizhny Novgorod GSP-445 (Russia)
Fax: (+7)8312-127-497
E-mail: trif@imoc.sinn.ru
the formation of the planar (deviation from the plane is
5
À
0.11 Š) h -bonded heteropentadienylframe. The C C bond
lengths within this frame (1.398(3)–1.461(3) Š) fall within the
range of values for aromatic C C bonds, and the C N bond
lengths (1.324(3), 1.319(3) Š) are slightly longer than the
[**] This work was supported by the Russian Foundation of Basic
Research (Grant No 02-03-32112), Grant of President of RF
supporting scientific schools (No.58.2003.3).
[7]
À
À
Angew. Chem. Int. Ed. 2004, 43, 5045 –5048
DOI: 10.1002/anie.200461001
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5045

Communications
pentadienylbent sandwich compel x. Spectros-
copic data consistent with this formulation
were also obtained.
In an attempt to obtain insight into the
mechanism of the observed ligand transforma-
tion, we investigated the reaction of 1 with the
diazadiene 3 under the same conditions. We
found that replacement of the imino hydrogen
atoms by two methyl groups dramatically
influences the reaction pathway and leads to
the formation of different products. We fol-
lowed the procedure described above and
isolated the novel YbII derivative 5 (64%)
and fluorene (81%). (Scheme 2)
Scheme 1. Reaction of 1 and 2 to give 4. Reagents and conditions: 1, 2 (2 equiv), THF, 608C,
30 min; then toluene, 608C, 1 h; then toluene, 608C, 2 h.
Complex 5 was isolated as a deep green crystalline solid
that is readily soluble in toluene but sparingly soluble in
hexane. The compound is diamagnetic, which corresponds to
the YbII oxidation state. Single crystals suitable for X-ray
crystal-structure analysis (Figure 2) were obtained by recrys-
tallization of 5 from toluene/hexane mixtures.
The crystal-structure analysis showed that the ytterbium
atom in 5 is h5-coordinated by the sole fluorenyl ligand and by
the two nitrogen atoms of the novelmonoanionic ligand {(2,6-
À
=
À
=
iPr2C6H3)N C(CH3) C(CH2) N(2,6-iPr2C6H3)} (6), which
results from proton abstraction from the methyl substituent of
À
the imino group of 3. The Yb C bond lengths in 5 are in the
range 2.655(2)–2.770(3) Š, slightly shorter than those in 1.
Figure 1. ORTEP representation of structure 4. Selected bond lengths
[Š] and angles [8]: Yb(1)-N(3) 2.445(2), Yb(1)-N(1) 2.466(2), Yb(1)-
C(78) 2.759(2),Yb(1)-C(39) 2.768(2), Yb(1)-C(77) 2.786(2), Yb(1)-C(27)
2.795(2), Yb(1)-C(38) 2.798(2), Yb(1)-C(66) 2.868(2), Yb(1)-C(13)
2.933(2), Yb(1)-C(52) 2.952(2), N(1)-C(13) 1.324(3), N(1)-C(1)
1.443(3), C(13)-C(27) 1.426(3), C(13)-C(14) 1.521(3), C(27)-C(39)
1.459(3), C(38)-C(39) 1.398(3), N(2)-C(14) 1.447(3), N(3)-C(52)
1.319(3), N(4)-C(53) 1.464(3), C(52)-C(53) 1.514(3), C(52)-C(66)
1.434(3), C(66)-C(78) 1.461(3), C(77)-C(78) 1.403(3); N(3)-Yb(1)-C(77)
70.80(6), N(1)-Yb(1)-C(38) 70.21(6), O(1S)-Yb(1)-N(3) 97.56(5),
O(1S)-Yb(1)-N(1) 96.31(5).
À
À
The Yb N bonds in 5 are non-equivalent. The Yb(1) N(2)
II
À
N
bond length (2.315(2) Š) is similar to that of the Yb
covalent bond in the complex [Yb{N(SiMe3)}(thf)BPh4]
)[12a]
(2.314(2) Š
and slightly longer than that in the complex
[Yb{N(SiMe3)}2(Me2P(CH2)2PMe2] (2.329(2) Š).[12b] The
À
Yb(1) N(1) bond is essentially longer (2.388(2) Š), but
II
À
nevertheless much shorter than the Yb N coordination
bonds (2.58(1)–2.65(1)).[9,13] The N C bonds of the NCCN
=
fragment in 5 are longer than the corresponding bonds of the
=
C N bond in the parent diazadiene
(1.266(3) Š).[8] Such a bonding situa-
tion is evidence for a delocalized
p system in the heteropentadienyl
fragment. The dihedralaengl
between the N(3)-C(52)-C(66)-
C(78)-C(77) and the N(1)-C(13)-
C(27)-C(39)-C(38) planes is 64.48
and the angle Ct-Yb-Ct (Ct are the
centroids of the N(3)-C(52)-C(66)-
C(78)-C(77) and the N(1)-C(13)-
C(27)-C(39)-C(38) rings) is 147.28
Scheme 2. Reaction of 1 and 3 to give 5 and fluorene. Reagents and conditions: 1, 3 (2 equiv),
THF, 608C, 30 min; then toluene, 608C, 1 h; then toluene, 608C, 2 h.
(reference:
136.3(3)8
in
À
[YbCp*2(C5H5N)2]).[9] The Yb C bond lengths in
4
parent diazadiene 3 (1,279(3), 1.280(3) Š)[14] and differ
À
À
(2.759(2)-2.952(2) Š) are somewhat longer than those in the
starting complex 1 and are comparable to the distances
reported for the YbII–bispentadienylderivative [Yb{4,4 ’-
significantly as well: N(1) C(26) 1.318(3) Š and N(2)
À
C(28) 1.353(3) Š. The C C bond length of the diazadiene
fragment in 5 remains similar to that in 3 (1.498(3) Š).[14]
A
(CH2)2(2-C6H8)2}(thf)2] (2.709(9)-2.909(9) Š).[10] The Yb N
short C(28) C(29) bond length (1.398(4) Š) comparable to
À
À
bond lengths are very similar to those in YbII–b-diketiminates
(2.418(9)-2.423(9) Š).[11] The arrangement around the
ytterbium atom in 4 determines the geometry of the hetero-
that of aromatic C C bonds together with a flat geometry
[7]
À
around the C(29) atom (the sum of the bond angles is 359.88)
provide evidence for sp2 hybridization at this carbon atom.
5046
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Angew. Chem. Int. Ed. 2004, 43, 5045 –5048

Angewandte
Chemie
that oxidation of the ytterbium atom to YbIII occurs in this
step and we are currently attempting to prove of this
hypothesis. Steric crowding of the coordination sphere of
the ytterbium atom is most likely the factor that drives the
transformation of the formed species into complexes 4 and 5.
Further work on this subject is being actively pursued at the
moment.
Experimental Section
4: A solution of 2 (0.99 g, 2.65 mmol) in THF (10 mL) was added to a
solution of 1 (0.86 g, 1.32 mmol) in THF (20 mL) and the reaction
mixture was heated at 608C for 0.5 h. THF was evaporated in vacuo,
toluene (20 mL) was added, and the reaction mixture was stirred at
608C for 1 h. The volatile material was evaporated in vacuo, and
another portion of toluene (20 mL) was added. The mixture was
stirred at 608C for an additional2 h, after which time the solvent was
evaporated in vacuo and the deep red solid residue was dissolved in
hexane (35 mL). Slow concentration of the solution in hexane at
À208C resulted in crystals of 4. The mother liquor was decanted, and
the crystals were washed with cold hexane and dried in vacuo at room
temperature for 45 min to afford ruby-red crystals of 4 (1.41 g, 80%).
Elemental analysis: calcd (%) for C82H98N4OYb: C 73.72, H 7.33, Yb
12.95; found: C 73.95, H 7.42, Yb 12.00; 1H NMR (200 MHz,
[D6]benzene, 208C): d = 0.76 (d, 3JHH = 6.6 Hz, 18H; CH(CH3)2),
0.92 (d, 3JHH = 6.6 Hz, 12H; CH(CH3)2), 1.05 (d, 3JHH = 6.6 Hz, 12H;
CH(CH3)2), 1.16 (m, 6H; CH(CH3)2), 1.20 (s, 4H; b-CH2 (thf)), 1.42
Figure 2. ORTEP representation of structure 5. Selected bond lengths
[Š] and angles [8]:Yb(1)-C(1) 2.649(3), Yb(1)-C(2) 2.655(2), Yb(1)-C(7)
2.727(2), Yb(1)-C(13) 2.744(3), Yb(1)-C(8) 2.770(3), Yb(1)-N(2)
2.315(2), Yb(1)-O(1) 2.352(2), Yb(1)-N(1) 2.388(2), N(1)-C(26)
1.318(3), N(1)-C(14) 1.439(3), N(2)-C(28) 1.353(3), N(2)-C(30)
1.435(3), C(26)-C(28) 1.499(4), C(26)-C(27) 1.471(4), C(28)-C(29)
1.398(4); N(2)-Yb(1)-O(1) 99.76(7), N(2)-Yb(1)-N(1) 9.37(7), O(1)-
Yb(1)-N(1) 95.65(7), H(29A)-C(29)-H(29B) 119.5(2), C(28)-C(29)-
H(29B) 119.3(2), C(28)-C(29)-H(29A) 121.0(2).
Unlike the geometry of the DAD radical anions in YbIII
complexes,[2b,f,g] which exhibit redistribution of the bond
distances (characteristic of delocalized NCCN p systems),
the geometry of the monoanionic ligand 6 in 5 is indicative of
a partial double bonding and delocalization of the negative
3
(m, 4H; CH(CH3)2), 2.47 (sept, JHH = 6.8 Hz, 4H; CH(CH3)2), 3.33
À
À
charge only occurs in the N(2) C(28) C(29) fragment
(Scheme 3).
(br s, 4H; a-CH2 (thf)), 3.56 (br s, 1H; NH), 4.01 (t, 3JHH = 5.2 Hz,
1H; NH), 4.40 (d, 3JHH = 4.4 Hz, 4H; CH2), 6.94 (s, 6H; Ar-H), 7.10 (s,
6H; Ar-H), 7.15–7.46 (m, 8H; Ar-H), 7.92 (d, 3JHH = 7.2 Hz, 4H; Ar-
H), 8.16 ppm (d, 3JHH = 7.6 Hz, 4H; Ar-H); 13C NMR (50 MHz,
[D6]benzene, 208C): d = 23.6, 23.7, 25.0, 25.1 (CH(CH3)2), 25.4 (b-
CH2 (thf)), 27.5, 28.6 (CH(CH3)2), 52.6 (CH2NH), 70.2 (a-CH2 (thf)),
92.7 (Flu-C = NAr), 119.4, 121.5, 122.4, 123.5, 124.4, 125.5, 127.0,
128.1 (CH, Ar-C), 114.9, 131.9, 136.5, 139.8, 140.1, 144.0, 144.8,
172.8 ppm (quat. C, Ar-C); IR (Nujol, KBr): n˜ = 3400 (w), 3060 (w),
1640 (m), 1580 (m), 1300 (s), 1160 (m), 1080 (m), 950 (m), 860 (m), 780
(s), 750 (s), 730 cmÀ1 (s).
Scheme 3. The geometry of the monoanionic ligand 6 is indicative of a
partial double bond. Delocalization of the negative charge only occurs
5: A solution of 3 (0.82 g, 2.03 mmol) in THF (10 mL) was added
to a solution of 1 (0.65 g, 1.00 mmol) in THF (20 mL) and the reaction
mixture was heated at 608C for 0.5 h. THF was evaporated in vacuo,
toluene (20 mL) was added, and the reaction mixture was stirred at
608C for 1 h. The volatile material was evaporated in vacuo, and
another portion of toluene (20 mL) was added. The mixture was
stirred at 608C for an additional2 h, after which time the solvent was
evaporated in vacuo and the deep green solid residue was recrystal-
lized from a mixture of toluene/hexane at À208C. The mother liquor
was decanted and the crystals were washed with cold hexane dried
in vacuo at room temperature for 45 min to afford deep green crystals
of 5 (0.53 g, 64%). The volatile material was removed from the
mother liquor in vacuo, and fluorene (0.14 g, 81%) was sublimed
from the solid residue. Elemental analysis: calcd (%) for
C45H56N2OYb: C 66.44, H 6.88, Yb 21.26; found: C 66.64, H 6.70,
Yb 21.20; 1H NMR (200 MHz, [D6]benzene, 208C): d = 1.06–1.42 (m,
28H; CH(CH3)2 and b-CH2 (thf)), 1.67 (s, 3H; N = CCH3), 2.63 (br s,
2H; CH(CH3)2), 3.16 (br s, 2H; CH(CH3)2), 3.38 (br s, 4H; a-CH2
(thf)), 3.94 (s, 1H; N = CCHH), 4.40 (s, 1H; N = CCHH), 6.63 (s, 1H;
Flu-H), 6.97–7.02 (m, 4H; Ar-H), 7.24–7.42 (m, 8H; Ar-H), 7.99 ppm
(m, 2H; Ar-H); 13C NMR (50 MHz, [D6]benzene, 208C): d 19.0 (N =
CCH3), 24.0, 24.3, 25.3, 25.6 (CH(CH3)2), 25.1 (b-CH2 (thf)), 27.9, 28.1
(CH(CH3)2), 69.6 (a-CH2 (thf)), 82.6 (C9 (Flu)), 90.5 (N = CCH2),
114.9, 118.2, 119.6, 122.7, 123.4, 123.9, 125.7, 128.1 (CH, Ar-C), 119.3,
123.6, 127.6, 133.5, 138.0, 143.2, 143.7, 149.1, 156.6, 179.2 ppm (quat. C
(Ar) and quat. C (N = CC)); IR (Nujol, KBr): n˜ = 3020 (w), 1525 (m),
À
À
in the N(2) C(28) C(29) fragment.
The higher value of the effective negative charge on the
N(2) atom results in the non-equivalence of the Yb N bonds
and in the shortening of the Yb(1) N(2) bond. The H and
À
1
À
13C NMR spectra of 5 are consistent with the structuraldata.
The protons of the methylradicalby the imino group appear
in the 1H NMR spectrum as a singlet at d = 1.67 ppm, whereas
the two methylene protons become diastereotopic as a result
of partialdoubel bonding in the NCCH 2 group, giving rise to
two singlets of equal intensity at d = 3.94 and 4.40 ppm.
Isolation of fluorene from the reaction in nearly quanti-
tative yield proves that the fluorenyl anion is responsible for
the abstraction of the proton from 3 and for the formation of
À
the ligand 6. Similar C H bond activations in the ytterbium
amido complexes were previously reported by Deacon and
Forsyth[12a] and by Dehnicke and co-workers.[15]
Unfortunately, at the present stage of our investigation we
are unable to rationalize definitely the formation of the
complexes 4 and 5. Undoubtedly, the first step is coordination
of the diazadiene to the ytterbium atom and formation of the
mixed-ligand derivatives [(C13H9)2Yb(DAD)]. We presume
Angew. Chem. Int. Ed. 2004, 43, 5045 –5048
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Communications
1300 (s), 1230 (m), 1180 (m), 1080 (m), 1000 (m), 920 (m), 860 (s), 830
(m), 780 (s), 740 (s), 730 (s), 710 (s), 660 cmÀ1 (m).
[12] a) G. B. Deacon, C. M. Forsyth, J. Chem. Soc. Chem. Commun.
2002, 2522 – 2523; b) T. Don Tilley, R. A. Andersen, A. Zalkin, J.
Am. Chem. Soc. 1982, 104, 3725 – 3727.
[13] a) E. Sheng, S. Wang, G. Yang, S. Zhou, L. Cheng, K. Zhang, Z.
Huang, Organometallics 2003, 22, 684 – 692; b) C. Qian, H. Li, J.
Sun, W. Nie, J. Organomet. Chem. 1999, 585, 59 – 62.
[14] For structure of 3 see: E. K. Cope-Eatough, F. S. Mair, R. G.
Pitchard, J. E. Warren, R. J. Woods, Polyhedron 2003, 22, 1447 –
1454.
[15] M. Karl, K. Harms, S. Seybert, W. Massa, S. Fau, G. Freking, K.
Dehnicke, Z. Anorg. Allg. Chem. 1999, 625, 2055 – 2062.
[16] G. M. Sheldrick, SHELXTL version 5.10, Structure Determina-
tion Software Suite, Bruker AXS, Madison, Wisconsin, USA,
1998.
¯
¯
Crystaldata for 4: C88H112N4OYb, Mr = 1414.86, triclinic, P1P1,
a = 13.0385(7), b = 13.8866(7), c = 21.9438(12) Š, a = 85.8100(10),
b = 87.4260(10), g = 69.0470(10)8, V= 3699.8(3) Š3, Z = 2, 1calcd
=
1.270 MgmÀ3, absorption coefficient 1.313À3, F(000) 1492, reflections
collected 20549, independent reflections 12985, GOF 1.047, R =
0.0330, wR2 = 0.0837.
Crystaldata for 5: C45H56N2OYb, Mr = 813.96, orthorhombic,
Pbca, a = 15.3796(10), b = 17.1883(12), c = 29.211(2) Š, a = 90.0, b =
90.0, g = 90.08, V= 7721.9(9) Š3, Z = 8, 1calcd = 1.400 MgmÀ3, absorp-
tion coefficient 2.458À3, F(000) 3344, reflections collected 57402,
independent reflections 6812, GOF 0.948, R = 0.0280, wR2 = 0.0540.
All crystallographic calculations were performed with the Bruker
SHELXTL package.[16] CCDC-241424 (4) and -241425 (5) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge CB21EZ, UK; fax:
(+ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).
Received: June 18, 2004
À
À
Keywords: C C coupling · C H activation · N ligands ·
ytterbium
.
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