Ligand-controlled synthesis of vanadium(i) β-diketiminates and their catalysis in cyclotrimerization of alkynes

Article abstract of DOI:10.1039/c0dt01061h

Three dimeric vanadium(i) β-diketiminates [V{μ-(η⁶- ArN)C(Me)CHC(Me)C(N-Ar)}]₂ (Ar = 2,6-Me₂C ₆H₃ (2), 2,6-Et₂C₆H₃ (3), 9-anthracenyl (4)) were prepared and isolated u

Full text of DOI:10.1039/c0dt01061h

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PAPER
Ligand-controlled synthesis of vanadium(I) b-diketiminates and their catalysis
in cyclotrimerization of alkynes†
Kai-Chieh Chang,a Chia-Fu Lu,a Po-Yang Wang,a Duan-Yen Lu,a Hong-Zhang Chen,a Ting-Shen Kuob and
Yi-Chou Tsai*a
Received 18th August 2010, Accepted 25th October 2010
DOI: 10.1039/c0dt01061h
6
Three dimeric vanadium(I) b-diketiminates [V{m-(h -ArN)C(Me)CHC(Me)C(N–Ar)}]2 (Ar =
2,6-Me2C6H3 (2), 2,6-Et2C6H3 (3), 9-anthracenyl (4)) were prepared and isolated upon reduction of
their corresponding dichloro precursors VCl2(Nacnac). Compounds 2–4 all show a structure with each
2
6
vanadium atom being h bonded to the b-diketiminate framework and h bonded to a flanking ring of a
b-diketiminato ligand, attached to the other vanadium centre within the dimer. No metal–metal
bonding interactions are observed in these dimers due to long vanadium–vanadium separations.
Compounds 2–4 display an antiferromagnetic exchange between the two vanadium centres. An imido
2
azabutadienyl complex (h -PhCC(H)C(Ph)NC6H3-2,6-iPr2)VN(C6H3-2,6-iPr2)(OEt2) (5) was isolated
from the reduction of VCl2(HC(C(Ph)NC6H3-2,6-iPr2)2) by KC8. Compounds 2–4 and the
6
6
inverted-sandwich divanadium complex (m-h :h -C6H5Me)[V(HC(C(Me)NC6H3-2,6-iPr2)2)]2 (1) reduce
Ph2S2 to give two vanadium dithiolates V(SPh)2[(HC(C(Me)NC6H3-2,6-R2)2)] (R = Et (6), iPr (7))
through an oxidative addition. Most notably, 1 and 3 catalyze the cyclotrimerization of alkynes, giving
tri-substituted benzenes in good yields and a 1,3,5-triphenylbenzene coordinated intermediate 8 was
isolated and characterized.
In addition to stabilizing the high-valent metal cen-
1. Introduction
tres, b-diketiminates also support low-valent transition metal
complexes,25 which not only demonstrate interesting electronic
structures, but also novel reactivity. For example, we and
Theopold’s group recently reported the synthesis of inverted
Due to the ease of preparation and modification of their steric
and electronic demands, b-diketiminates have been widely used
as ancillary ligands in the assembly of metal complexes.1 Of
particular interest for the b-diketiminato ligands is their ability
to support reactive metal fragments, which can be applied as
active catalysts1–10 and to model relevant active sites in biological
systems.11,12 It is noteworthy that b-diketiminato ligands have
also been employed to stabilize high-valent transition metals.
For example, Mindiola, Warren and Power have recently demon-
strated that b-diketiminates can support high-valent reactive early
transition metal–ligand multiple bonding functionalities, such as
alkylidenes,13,14 alkylidynes,15 imides,16–19 and phosphinidenes.20
Furthermore, transition metal b-diketiminates also show their
high reactivity toward the activation of thermodynamically robust
molecules, such as alkanes21,22 and dinitrogen.23,24
6
6
sandwich type complexes (m-h :h -Arene)[M(Nacnac)]2 (Arene =
C6H6, C6H5Me for M = Cr and Arene = C6H5Me for M = V;
Nacnac = HC(C(Me)NC6H3-2,6-iPr2)2).25e–h In contrast to the
usual low-spin organometallic compounds, these b-diketiminate-
supported inverted sandwich complexes exhibit high-spin states,
a septet state for M = Cr and a quintet state for M = V. 26
Not only do these dinuclear inverted sandwich complexes feature
unusual electronic schemes, but they are very reactive species.
Reactivity studies of these complexes establish that they behave
as two equivalents of multi-electron reductants, M(Nacnac). For
example, monomeric diimido and dioxo products were isolated
6
6
from the reactions of (m-h :h -C6H5Me)[M(Nacnac)]2 (M = Cr, V)
and organic azides, azobenzene and dioxygen via reductive cleav-
age processes.25e–g Warren also described that the b-diketiminate
supported univalent cobalt25c and nickel25d complexes are good
synthons for the preparation of mono-imido complexes.
However, it is worthy to mention that the sterically encumbering
HC(C(Me)NC6H3-2,6-iPr2)2 is so far the most frequently used b-
diketiminate to assemble metal complexes. There is no systematic
study on the structures and reactivity of metal b-diketiminate
compounds as a consequence of the steric variation of the ancillary
ligands.
aDepartment of Chemistry, National Tsing Hua University, 101, Sec 2, Guang
Fu Road, Hsinchu 300, Taiwan. E-mail: yictsai@mx.nthu.edu.tw; Fax: 886
3 5711082; Tel: 886 3 5718232
bDepartment of Chemistry, National Taiwan Normal University, 88, Sec
4, Ting-Chou Road, Taipei 116, Taiwan. E-mail: t42002@ntnu.edu.tw;
Fax: 886 2 29324239; Tel: 886 2 77346137
† Electronic supplementary information (ESI) available: X-ray crystallo-
graphic data of 2–8 including CIF files. CCDC reference numbers 788574–
788579. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0dt01061h
2324 | Dalton Trans., 2011, 40, 2324–2331
This journal is
The Royal Society of Chemistry 2011
©

◦
In this work, we report that besides the di-vanadium inverted
concentrated and kept at -35 C to produce dark brown single-
6
6
sandwich type complex (m-h : h -C6H5Me)[V(Nacnac)]2 with
crystal X-ray quality crystals. The isolated yield was 65.1% (469.0
mg) in one crop. Anal. calcd for C40H48N2Cl2V: C 70.58, H 6.83,
N 4.22; Found: C 71.04, H 6.63, N 3.89. meff = 2.92 mB (Evans’
method, C6D6).
Nacnac = HC(C(Me)NC6H3-2,6-iPr2)2, vanadium b-diketiminate
can exist in
a different form. Upon reducing the steric
hindrance of the supporting b-diketiminato ligand, a dif-
ferent type of di-vanadium b-diketiminato complex [V{m-
6
Synthesis of [V{l-(g6-2,6-Me2C6H3–N)C(Me)-
CHC(Me)C(N-2,6-Me2C6H3)}]2 (2)
(h -ArN)C(Me)CHC(Me)C(NAr)}]2 (Ar = 2,6-Me2C6H3, 2,6-
Et2C6H3, 9-anthracenyl) is isolated, where two “V(Nacnac)”
molecules are connected to each other via coordination of one of
the phenyl rings of one V(Nacnac) to the other vanadium centre
A sample of 200.0 mg (0.468 mmol) of [HC(C(Me)N-2,6-
Me2C6H3)2]VCl2 was dissolved in 10 mL of tetrahydrofuran (THF)
in a 20 mL vial. Meanwhile, a sample of 189.8 mg (1.4041 mmol)
of KC8 was suspended in 2 mL of THF also in a 20 mL vial. Both
resultant solutions were kept at -35 ◦C for 30 min, at which point,
to the stirred suspension of KC8 was added the vanadium complex
solution. The solution was allowed to warm to room temperature
and quickly turned into dark brown from red. It was then stirred
for a further 10 h at room temperature. The solution was filtered
through a pad of Celite and washed by 4 mL of THF twice. The
filtrate was stripped to dryness under vacuum and the resultant
solid was extracted into 10 mL of diethyl ether. The solution was
6
in an h fashion. On the other hand, a two-electron reduction
reaction of a V(III) precursor supported by sterically encumbering
Nacnac (HC(C(Ph)NC6H3-2,6-iPr2) results in the cleavage of the
backbone C–N bond and gives rise to the formation of terminal
imide stabilized by the azabutadienyl scaffold. The synthesized
univalent di-vanadium b-diketiminato complexes are reducing.
They all react with diphenyldisulfide to give V(SPh)2(Nacnac) via
a reductive S–S bond cleavage reaction. Moreover, they all catalyze
the cyclotrimerization reaction of alkynes to give tri-substituted
benzenes.
◦
concentrated and kept at -35 C to produce single-crystal X-ray
quality crystals. The isolated yield was 10.0% (16.7 mg) in one
crop. Anal. calcd for C42H50N4V2 (2): C 70.77, H 7.07, N 7.86;
Found: C 70.24, H 6.87, N 7.46. meff = 2.92 mB (Evans’ method,
C6D6).
2. Experimental
General information
Unless stated otherwise, all operations were performed using
standard Schlenk techniques or in a Vacuum Atmospheres dry box
under an atmosphere of nitrogen. Diethyl ether (Et2O) and tetrahy-
drofuran (THF) were distilled under nitrogen from purple sodium
benzophenone ketyl. n-Hexane was distilled under nitrogen from
CaH2. Distilled solvents were transferred under vacuum into
vacuum-tight glass vessels before being transferred into a dry box.
C6D6 was purchased fromAldrich and was degassed and dried over
Synthesis of [V{l-(g6-2,6-Et2C6H3–N)C(Me)-
CHC(Me)C(N-2,6-Et2C6H3)}]2 (3)
A sample of 170.0 mg (0.352 mmol) of [HC(C(Me)N-2,6-
Et2C6H3)2]VCl2 was dissolved in 8 mL of tetrahydrofuran (THF)
in a 20 mL vial. Meanwhile, a sample of 104.6 mg (0.774 mmol)
of KC8 was suspended in 4 mL of THF also in a 20 mL vial.
Both resultant solutions were kept at -35 ◦C for 30 min, at which
point, to the stirred suspension of KC8 was added the vanadium
complex solution. The resultant mixture was allowed to warm to
room temperature and quickly turned into dark brown from deep
red. It was then stirred for a further 10 h at room temperature. The
solution was filtered through a pad of Celite and washed by 4 mL
of THF twice. The filtrate was stripped to dryness under vacuum
and the resultant solid was extracted into 10 mL of n-hexane. The
solution was concentrated and kept at -35 ◦C to produce single-
crystal X-ray quality crystals. The isolated yield was 29.5% (42.8
mg) in one crop. Anal. calcd for C50H66N4V2 (3): C 72.79, H 8.06,
N 6.79; Found: C 72.38, H 8.20, N 6.35. meff = 2.86 mB (Evans’
method, C6D6).
˚
˚
4 A sieves. The 4 A sieves and Celite were dried in vacuo overnight
at a temperature just above 200 ◦C. The inverted sandwich di-
6
6
vanadium complex (m-h :h -C6H5Me)[V(HC(C(Me)NC6H3-2,6-
iPr2)2)]2 (1)25f and VCl2[HC(C(Me)NC6H3-2,6-R2)2] (R = Me,
Et)27 were prepared following the literature method. All other
compounds were used as received. 1H and 13C NMR spectra were
recorded on Varian Unity INOVA 500 MHz or Bruker DMX
600 MHz spectrometers at room temperature. 13C NMR spectra
are proton decoupled. Chemical shifts of H and 13C spectra are
reported with respect to internal solvent: 7.16 ppm and 128.00(t)
ppm (C6D6) and external solvent: 7.16 ppm for 2H spectra.
1
Synthesis of VCl2[HC(C(C6H5)NC6H3-2,6-iPr2)2]
A
sample of 630.0 mg (1.148 mmol) of (THF)Li-
Synthesis of [V{l-(g6-9-anthracenyl-N)C(Me)-
CHC(Me)C(N-9-anthracenyl)}]2 (4)
[HC(C(C6H5)NC6H3-2,6-iPr2)2] was dissolved in 10 mL of tetrahy-
drofuran (THF) in a 20 mL vial. Meanwhile, a sample of 515.0 mg
(1.378 mmol) of VCl3(THF)3 was suspended in 5 mL of THF, also
in a 20 mL vial. Both resultant solutions were kept at -35 ◦C for
30 min, at which point the ligand solution was added to the stirred
suspension of VCl3(THF)3. The mixture was allowed to warm to
room temperature and then stirred for a further 12 h at room
temperature. The solution became dark brown and was filtered
through a pad of Celite and washed by 4 mL of THF twice. The
filtrate was stripped to dryness under vacuum and the resultant
solid was extracted into 15 mL of diethyl ether. The solution was
A sample of 530.0 mg (0.882 mmol) of (THF)Li[HC(C(Me)N(9-
anthracenyl))2] was dissolved in 10 mL of tetrahydrofuran (THF)
in a 20 mL vial. Meanwhile, a sample of 330.0 mg (0.882 mmol)
of VCl3(THF)3 was suspended in 5 mL of THF also in a 20 mL
vial. Both resultant solutions were kept at -35 ◦C for 30 min,
at which point, the b-diketiminate solution was added to the
stirred suspension of VCl3(THF)3. The mixture was allowed to
warm to room temperature and was stirred for a further 24 h. The
suspension became dark green. The dark suspension was cooled
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 2324–2331 | 2325
©

to -35 ◦C, 262.0 mg (1.941 mmol) of KC8 was added and the
resultant mixture was stirred for 12 h. The suspension turned into
gray black and was filtered through a pad of Celite and washed
by 4 mL of THF twice. The filtrate was stripped to dryness under
vacuum and the black residue was extracted into 20 mL of diethyl
ether. Subsequently, the black ethereal solution was stripped to
dryness and the black residue was extracted into n-hexane. The n-
hexane solution was concentrated and kept at -35 ◦C to produce
black single-crystal X-ray quality crystals. The isolated yield was
23.1% (222.0 mg) in one crop. Anal. calcd for C72H64N4V2 (4): C
79.54, H 5.93, N 5.15; Found: C 79.16, H 5.51, N 5.11. meff = 2.49
mB (Evans’ method, C6D6).
C 70.45, H 6.87, N 4.44, Found: C 70.87, H 7.14, N 4.62. meff
2.94 mB (Evans’ method, C6D6)
=
Synthesis of (g6-1,3,5-Ph3C6H3)V[HC(C(Me)NC6H3-2,6-iPr2)2]
(8)
To a 15 mL diethyl ether solution of 1 (502.4 mg, 0.490 mmol) in a
20 mL vial was added 404.3 mg (3.960 mmol) of phenylacetylene
(PhCCH) at -35 ◦C. The solution quickly became orange from
dark green. The solution was further stirred for 15 h at room
temperature. At which point, all volatiles were removed under
vacuum. The orange residue was extracted with n-hexane and
filtered through a pad of Celite. The orange filtrate was stripped
to dryness under vacuum and the orange solid was dissolved in
4 mL of diethyl ether. The ether solution was concentrated and
kept at -35 ◦C for 2 d to afford orange single-crystal X-ray quality
crystals. The yield was 22.1% (167.2 mg) in one crop. Anal. calcd
for C53H59N2V (7): C 82.14, H 7.67, N 3.61, Found: C 82.26, H
7.95, N 3.63. meff = 2.78 mB (Evans’ method, C6D6).
Synthesis of [g4-2,6-iPr2C6H3N(C6H5)CCHC(C6H5)]V(NC6H3-
2,6-iPr2)(OEt2)] (5)
A sample of 277.0 mg (0.417 mmol) of VCl2[HC(C(C6H5)NC6H3-
2,6-iPr2)2] was dissolved in 10 mL of THF in a 20 mL vial.
Meanwhile, a sample of 124.0 mg (0.917 mmol) of KC8 was
suspended in 10 mL of diethyl ether also in a 20 mL vial. Both
resultant solutions were kept at -35 ◦C for 30 min, at which point,
to the KC8 suspension was added the VCl2[HC(C(C6H5)NC6H3-
2,6-iPr2)2] solution. The mixture was allowed to warm to room
temperature. The solution was then stirred for a further 12h. The
solution became yellow-green and was filtered through a pad of
Celite. All volatiles of the filtrate were removed under vacuum.
The resultant residue was extracted with 10 mL of diethyl ether
and the solution was filtered through a pad of Celite. The filtrate
was kept at -35 ◦C for 2 d to afford dark yellow-green single-
crystal X-ray quality crystals. The yield was 47.2% (131.0 mg) in
Crystallographic structure determinations
The chosen crystals were mounted on a glass fibre. Data collections
were carried out on a NONIUS Kappa CCD diffractometer28
˚
with Mo radiation (l = 0.71073 A) at 200(2) K. Cell parameters
were retrieved and refined using DENZO-SMN software29 on
all observed reflections. Data reduction was performed with the
DENZO-SMN software.29 An empirical absorption was based
on the symmetry-equivalent reflections and applied to the data
using the SORTAV program.30 The SHELXTL program31 was
used on a PC computer for the structure analysis. The structure
was solved using the SHELXS-97 program32 and refined using the
SHELXL-97 program33 by full-matrix least squares on F2 values.
All non-hydrogen atoms are refined anisotropically. Hydrogen
atoms attached to the carbons were fixed at calculated positions
and refined using a riding model.
1
one crop. H NMR (C6D6, 400 MHz): d 7.88–6.78 (m, 17 H),
3.96 (m, 2 H, CHMe2), 3.57 (m, 1 H, CHMe2), 3.26 (m, 4 H,
(CH3CH2)2O), 3.08 (br, 1 H, CHMe2), 1.64 (d, 3 H, CHMe2), 1.54
(d, 3 H, CHMe2), 1.22 (d, 6 H, CHMe2), 1.19 (d, 6 H, CHMe2),
1.12 (m, 6 H, (CH3CH2)2O), 1.03 (d, 3 H, CHMe2), 0.79 (d, 3 H,
1
CHMe2). 13C{ H} NMR (C6D6, 100.7 MHz): d 158.58, 147.75,
144.70, 142.49, 141.09, 140.06, 139.61, 129.24, 128.29, 127.93,
127.87, 126.49, 126.33, 124.66, 124.49, 123.49, 123.37, 122.39,
122.03, 115.89, 92.17, 77.69, 31.90, 29.96, 28.34, 25.92, 25.09,
24.69, 24.45, 23.65, 23.55, 23.49. Anal. calcd for C43H55N2OV (5):
C 77.45, H 8.31, N 4.20; Found: C 77.10, H 8.76, N 3.89.
3. Results and discussion
3.1. Synthesis and characterizations of
[V{l-(g6-ArN)C(Me)CHC(Me)C(NAr)}]2 (Ar = 2,6-Me2C6H3
(2), 2,6-Et2C6H3 (3), 9-anthracenyl (4))
According to the synthesis of the inverted sandwich
6
6
divanadium complex (m-h :h -C6H5Me)[V(HC(C(Me)NC6H3-
Synthesis of (C6H5S)2V[HC(C(Me)NC6H3-2,6-R2)2] (R = Et (6),
2,6-iPr2)2)]2 (1),25f
a
two-electron reduction reaction of
iPr (7))
[HC(C(Me)NAr)2]VCl2 (Ar = 2,6-Me2C6H3, 2,6-Et2C6H3, 9-
anth◦racenyl) and KC8 in tetrahydrofuran instead of toluene at
-35 C gives rise to the formation of dinuclear complexes [V{m-
General procedure for the synthesis (C6H5S)2V[HC(C(Me)-
i
NC6H3-2,6-R2)2] is given for R = Pr. To a 15 mL diethyl ether
6
solution of 1 (156.0 mg, 0.150 mmol) in a 20 mL vial was
added 64 mg (0.300 mmol) of diphenyldisulfide (Ph2S2) at room
temperature. The solution quickly became forest green from dark
green. The solution was further stirred for 15 h. All volatiles were
then removed under vacuum. The green residue was extracted with
5 mL of n-hexane and filtered through a pad of Celite. The green
filtrate was concentrated to 2 mL and was kept at -35 ◦C for 2 d to
afford forest green single-crystal X-ray quality crystals. The yield
was 62.3% (63.0 mg) in one crop. Anal. calcd for C41H51N2S2V (7):
(h -ArN)C(Me)CHC(Me)C(NAr)}]2 (Ar = 2,6-Me2C6H3 (2), 2,6-
Et2C6H3 (3), 9-anthracenyl (4)) as dark brown crystals in 10 to
30% yields as shown in Scheme 1. All compounds 2, 3 and 4 are
highly air- and moisture-sensitive.
Single crystals of these three dimeric compounds, suitable
for X-ray analysis, are obtained by keeping the diethyl ether (2
and 4) or n-hexane (3) solution at -35 ◦C. The crystallographic
data collection parameters are summarized in Table 1. The
molecular structures of 2, 3 and 4 unambiguously indicate their
dinuclear nature (Fig. 1). Compounds 2 and 3 are essentially
C 71.69, H 7.48, N 4.08, Found: C 71.33, H 7.54, N 3.92. meff
2.89 mB (Evans’ method, C6D6). Anal. calcd for C37H43N2S2V (6):
=
2
isostructural, with the vanadium atoms being h bonded to the
2326 | Dalton Trans., 2011, 40, 2324–2331
This journal is
The Royal Society of Chemistry 2011
©

Table 1 Crystal and data collection parameters
compound
2
3
4
5
7
8
Empirical formula
Mw
C42H50N4V2
712.74
P21/n
9.9620(2)
11.0806(2)
16.8565(4)
90
99.8910(10)
90
1833.05(7)
2
200(2)
0.71073
1.291
0.545
C50 H66N4V2
824.95
P1
C72H64N4V2
1087.15
P2/a
24.8878(12)
9.5634(5)
25.2169(12)
90
112.559(2)
90
5542.7(5)
4
200(2)
0.71073
1.303
0.386
0.0714, 0.1662
C43H55N2OV
666.83
P21/c
12.5137(3)
15.6445(3)
19.3631(4)
90
97.6610(10)
90
3756.89(14)
4
200(2)
0.71073
1.179
0.298
C41H51N2S2V
686.9
P21/a
18.0540(3)
12.1652(2)
18.3858(3)
90
108.0990(10)
90
3838.28(11)
4
200(2)
0.71073
1.189
0.397
C53H59N2V
774.96
P1
¯
¯
Space group
˚
a/A
11.7457(3)
13.2452(3)
14.0750(4)
92.5130(10)
91.6470(10)
91.8330(10)
2185.48(10)
2
200(2)
0.71073
1.254
0.466
0.0534,
13.2728(11)
13.3879(11)
14.0552(12)
98.915(3)
117.457(4)
92.696(3)
2169.3(3)
2
200(2)
0.71073
1.186
0.266
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
T/K
˚
l/A
dc/g cm-3
m/mm-1
R1,a wR2 (I > 2s(I))
0.0508, 0.1468
0.0590, 0.1472
0.0593, 0.1498
0.1177, 0.2258
b
0.1322
a R1 = RꢀFoΩ-ΩFcꢀ/RΩFoΩ. b wR2 = {R[w(Fo -Fc2)2]/R[w(Fo2)]}1/2. w = 1/[(s (Fo2) + (aP)2+bP)], where P = [max(Fo or 0) + 2(Fc2)]/3.
2
2
2
˚
determined to be 1.411 (2) and 1.422 A (3), indicating a slight
35
˚
increase (ca. 0.02–0.03 A) upon complexation. A nonplanar
conformation for the coordinated flanking rings in 3 is consistent
with the larger increase in the C–C bond lengths. Moreover, the
metal–flanking–ring interactions in 2 and 3 are also significant
as evidenced by the vanadium–centroid distance of 1.744(4)
˚
˚
A in 2 and 1.750(3) A in 3, slightly shorter than those in
complexes [Ar¢MMAr¢] (Ar¢ = 2,6-(2,6-iPr2C6H3)2C6H3; M =
Fe, Co),36 [HC(C(Me)NC6H3-2,6-iPr2)2]Fe(h -C6H6),37 3,5-
6
iPr2Ar*Fe(h -C6H6) (Ar* = 2,6-(2,4,6-iPr3C6H2)-3,5-iPr2C6H)38
6
and [HC(C(Me)NC6H3-2,6-Me2)2]Co(h -C6H5Me).25c It is worth
6
mentioning that all the planar vanadium-b-diketiminate six-
membered rings, C3N2V, in 2 and 3 are coplanar. Both 2 and
˚
3 display long V ◊ ◊ ◊ V separations, 4.181(5) and 4.188(1) A,
Scheme 1 Synthesis of 2–4.
respectively, indicating no metal–metal bonding interaction.
Despite being structurally similar to 2 and 3, compound 4
exhibits a slightly different structure from that of 2 and 3.
The vanadium atoms in 4 are ligated by the external flanking
rings of the anthracenyl substitutents of a b-diketiminato ligand,
which is attached to the other vanadium centre within the
dimer. Consequently, such coordination results in two planar
six-membered C3N2V rings of the dimer in a nearly orthogonal
arrangement, so as to release steric pressure, in sharp contrast to a
6
b-diketiminate framework and h coordinated to a flanking
ring of a b-diketiminato ligand, which is attached to the other
vanadium centre within the dimer. Both 2 and 3 show similar
variations in vanadium–ring–carbon distances, ranging from
2.185(4) to 2.336(3) A in 2 and 2.181(3) to 2.332(3) A in 3.
The longest vanadium–ring–carbon distances are both observed
between the vanadium and the ipso carbon atoms. The average
C–C distance for the metal-coordinated flanking rings was
˚
˚
˚
Fig. 1 Molecular structures of 2 (left), 3 (central), and 4 (right) with thermal ellipsoids at 35% probability level. Selected bond lengths (A) and
angles (◦): 2: V(1)–N(1), 2.058(2); V(1)–N(2), 2.034(2); V(1)–C(flanking ring) 2.336(3); 2.232(3); 2.191(4); 2.278(4); 2.185(4); 2.226(4); V(1)–centroid,
1.744(4); N(1)–V(1)–N(2), 88.73(10); V(1) ◊ ◊ ◊ V(1A), 4.181(5). 3: V(1)–N(1), 2.066(3); V(1)–N(2), 2.042(3); V(1)–C(flanking ring) 2.332(3); 2.181(3);
2.253(3); 2.284(3); 2.156(3); 2.298(3); V(1)–centroid, 1.750(3); V(1) ◊ ◊ ◊ V(1A), 4.188(1); N(1)–V(1)–N(2), 88.69(10). 4: V(1)–N(1), 2.025(4); V(1)–N(2),
2.020(4); V(1)–C(flanking ring) 2.468(5); 2.147(5); 2.264(5); 2.230(5); 2.162(5); 2.491(5); V(1)–centroid, 1.794(1); V(1) ◊ ◊ ◊ V(1A), 5.038(3); N(1)–V(1)–N(2),
88.82(17).
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parallel orientation of two planar six-membered C3N2V rings in 2
and 3. As a result, the conformation in 4 results in a longer V ◊ ◊ ◊ V
separation of 5.038(3) A. The metal–carbon distances to one of the
magnetic trivalent vanadium imido complex (NacPh)VN(C6H3-
2,6-iPr2)(OEt2) (5) (NacPh- = PhCC(H)C(Ph)NC6H3-2,6-iPr2)
instead of the anticipated mononuclear complex [HC(C(Ph)N-2,6-
iPr2C6H3)2]V or a dinuclear product reminiscent of 2–4. As assayed
by 13C NMR spectroscopy, the chemical shift for the vinylic carbon
attached to the vanadium atom resonates at 158.6 ppm. This
value is much more upfield shifted than those of the group IV
imido azabutadienyl complexes. The molecular structure of 5 was
determined by single-crystal X-ray crystallography and is depicted
in Fig. 3, with selected bond lengths and angles underneath.
The crystallographic data is summarized in Table 1. Besides the
imido and azabutadienyl ligands, an Et2O molecule is found to
ligate to the vanadium atom, which reveals the electron-deficient
nature of the vanadium centre. Moreover, bonding interactions
between the vanadium atom and all carbons of the backbone
ligand are observed and evidenced by the slightly long V(1)–C(20)
˚
external rings of the anthracenyl groups vary significantly, ranging
˚
˚
from 2.147(5) A to 2.491(5) A. The longest distance is to the fused
carbon bond C(28)–C(33) and the shortest distances are to C(29)
and C(32), leading to a puckering outer ring.
Each of these three dinuclear compounds has 2 unpaired elec-
trons based on Evans’s method.34 Unlike the dinuclear vanadium
inverted sandwich complex 1, showing paramagnetic behaviour
and featuring 4 unpaired electrons between 10 and 300 K, an
antiferromagnetic coupling between the two magnetic centres was
indeed observed for 2–4. Complex 3 was chosen for the solid
state magnetic measurements. The variation of molar magnetic
susceptibility (cm) and effective magnetic moment (meff) with
respect to temperature are shown in Fig. 2. Below 5 K, the
values of meff drastically decreased, presumably due to the zero-
field-splitting. The susceptibility data were fitted to the van Vleck
equation, a spin Hamiltonian of the form H = -2JS1S2, with
S1 = S2 = 1, from which g = 1.98 and J = -0.83 cm-1 were
thus obtained. The small coupling constant is consistent with the
observation of the long V ◊ ◊ ◊ V distance. As a result, 3 contains two
vanadium(I) ions which exhibit intramolecular antiferromagnetic
exchange interactions.
˚
˚
(2.344(3) A) and V(1)–C(13) (2.428(3) A) bond lengths, which
˚
consequently results in long C(21)–C(20) (1.429(4) A) and C(13)–
˚
N(1) (1.387(4) A) double bonds and leads to a puckered five-
membered VCCCN ring. The short V(1)–C(21) bond (1.871(3)
˚
A) suggests a double bond character. It is worth mentioning that
the reported conformation of the five-membered MCCCN rings,
where M is the group IV metal Ti, Zr or Hf in corresponding imido
azadutadienyl complexes, are planar without M–C(backbone)
bonding interactions.39 The existence of the vanadium imido
functionality is supported by the linear V(1)–N(2)–C(28) linkage
˚
and a very short V–N (imido) distance of 1.685(2) A, which is
˚
much shorter than V(1)–N(1) (1.906(2) A) and very close to those
of the reported V–N(imido) bond lengths.40
Fig. 2 Solid state magnetic susceptibility of 3 between 2 and 300 K. The
solid line represents the fitting results.
3.2. Reduction of [HC(C(Ph)N-2,6-iPr2C6H3)2]VCl2 and
formation of azabutyldienyl framework
Fig. 3 Molecular structure of 5 with thermal ellipsoids at 35% prob-
◦
˚
ability level. Selected bond lengths (A) and angles ( ): V(1)–N(1),
1.906(2); V(1)–N(2), 1.685(2); V(1)–C(13), 2.428(3); V(1)–C(20),
2.344(3); V(1)–C(21), 1.871(3); V(1)–O(1), 2.063(2); C(21)–C(20),
1.429(4); C(20)–C(13), 1.397(4); C(13)–N(1), 1.387(4); V(1)–N(2)–C(28),
168.71(21); O(1)–V(1)–N(2), 110.08(10); O(1)–V(1)–C(21), 112.64(11);
O(1)–V(1)–N(1), 115.71(10).
When the steric hindrance of the supporting b-diketiminato ligand
is increased, by substituting two phenyl groups for two methyl
groups of the backbone, a dark brown mononuclear vanadium
complex [HC(C(Ph)N-2,6-iPr2C6H3)2]VCl2 is prepared and this
was characterized by elemental analysis and solution magnetic
measurements. As shown in Scheme 2, the reduction of the
vanadium dichloride precursor by 2 equiv. of KC8 is carried out
in a THF solution and then recrystallization of the generated
product in diethyl ether leads to the isolation of a yellow-green dia-
Reductive C–N bond cleavage of the b-diketiminato ligands to
give rise to a metal imido functionality with an azabutadienyl
framework has been reported by Mindiola,39a,39d Tokitoh39b and
Stephan,39c but such reactions are so far observed for the group
IV metals.39 Complex 5 represents the first example that the
reductive C–N bond cleavage reaction of the NCCCN ligand
backbone occurs to a group V metal. In view of the recognition
of 2–4, 5 is generated from the transient univalent vanadium
intermediate “{HC(C(Ph)N-2,6-iPr2C6H3)2}V” upon reduction of
its dichloride precursor. One of the C–N bonds of the ligand
coordinates to the electron deficient vanadium centre and then
Scheme 2 Synthesis of 5.
2328 | Dalton Trans., 2011, 40, 2324–2331
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©

subsequently undergoes C–N bond cleavage to give 5. Apparently,
the mononuclear intermediate [{HC(C(Ph)N-2,6-iPr2C6H3)2}V]
can not be stabilized via the intermolecular coordination of one
of the 2,6-iPr2C6H3 substituents to the vanadium centres, to
form the dimeric complex similar to 2 and 3. In contrast to the
synthesis of 1, 5 can actually also be prepared via the reduction
of [HC(C(Ph)N-2,6-iPr2C6H3)2]VCl2 in toluene; an inverted sand-
6
6
wich dinuclear complex (m-h :h -C6H5Me)[V(HC(C(Ph)NC6H3-
2,6-iPr2)2)]2 is not formed.
3.3. Synthesis of (C6H5S)2V[HC(C(Me)NC6H3-2,6-R2)2] (R = Et
Fig.
4
Molecular structure of 7 with thermal ellipsoids at 35%
(6), iPr (7))
◦
˚
probability level. Selected bond lengths (A) and angles ( ): V(1)–N(1),
1.965(2); V(1)–N(2), 1.982(2); V(1)–S(1), 2.2930(8); V(1)–S(2), 2.3051(8);
N(1)–V(1)–N(2), 93.22(9); N(1)–V(1)–S(1), 116.19(7); N(2)–V(1)–S(1),
104.48(7); N(1)–V(1)–S(2), 118.04(7); N(2)–V(1)–S(2), 117.86(7);
S(1)–V(1)–S(2),106.30(3).
We recently reported that the N–N double bond of azoben-
zene can be easily split by 1 to give a diimdo complex
(PhN)2V[HC(C(Me)N-2,6-iPr2C6H3)2].25f,41 In light of the facile
5
synthesis of a d1 vanadium dithiolato complex (h -Cp)2V(SPh)2
5
from the reaction of (h -Cp)2V and Ph2S2,42 we then turned
be coordinated by benzene derivatives, we wondered if the as-
synthesized arene-coordinated vanadium b-diketiminates could
catalyze the formation of tri-substituted benzenes from acetylenic
compounds. To our delight, this is indeed the case. As shown in
Scheme 4, treatment of complex 3 with 20, 60 or 100 equiv. of
terminal alkynes, phenylacetylene and 1-heptyne in THF at room
temperature affords the corresponding tri-substituted benzenes in
good yields (62% for phenylacetylene and 81% for 1-heptyne).
However, the regioselectivity of the alkyne cyclotrimerization
reactions with the presence of 3 as the catalyst is not satisfied.
The ratio of 1,3,5-trisubstituted benzenes to 1,2,4-trisubstituted
benzenes is about 65 : 35 analyzed by 1H NMR spectroscopy.
Presumably, the poor regioselectivity is due to the less hindered
supporting ligand. When compound 1 was employed to catalyze
cyclotrimerization of phenylacetylene and 1-heptyne, the ratio of
1,3,5-trisubstituted benzenes to 1,2,4-trisubstituted benzenes is
improved to 80 : 20, but the yields slightly decrease (yields: 50%
for phenylacetylene and 74% for 1-heptyne).
our attention to the preparation a d2 vanadium dithiolato
complex by treating 1–4 with Ph2S2 via a two-electron oxidative
addition. As shown in Scheme 3, treatment of 1 or 3 with 2
equiv. of Ph2S2 afforded a forest green product, formulated as
(C6H5S)2V[HC(C(Me)NC6H3-2,6-R2)2] (R = Et (6), iPr (7)). Both
compounds 6 and 7 have 2 unpaired electrons based on solution
magnetic measurements (Evans’s method).34 The molecular con-
figuration together with selected bond distances and angles of 7
is given in Fig. 4, which unambiguously indicates that the two
phenylmercapto sulfur atoms and two b-diketiminato nitrogen
atoms constitute a pseudo-tetrahedral geometry at the vanadium
centre. Although the two angles of bent V–S–C (99.73(9) and
108.75(10)◦) are significantly different, the V–S bond lengths are
˚
essentially the same, 2.2930(8) and 2.3051(8) A. These two V–S
5
bond lengths are significantly shorter than those found in (h -
42
5
˚
Cp)2V(SPh)2 (2.448 (3) and 2.470(2) A) and (h -Cp)2V(SMe)2
(2.422(1) A).43 These differences are attributable to the much more
˚
electron deficient vanadium centre and the lesser steric demand of
the b-diketiminato ligand.
Scheme 3 Synthesis of 6 and 7.
Scheme 4 Cyclotrimerization of terminal alkynes.
To prove the formation of tri-substituted benzenes catalyzed
by the as-synthesized di-vanadium complexes, 5 equiv. of pheny-
lacetylene is added to a diethyl ether solution of 1 and a 1,3,5-
3.4. Catalytic cyclotrimerization of terminal alkynes by univalent
vanadium b-diketiminato complexes 1–4
6
Since Reppe et al. first discovered catalytic cyclotrimerization of
acetylenic compounds giving benzene and substituted benzenes
by NiBr2 in 1948,44 the [2+2+2] cycloaddition reaction has been
an effective tool for the construction of substituted arenes.45
In fact, a variety of transition metals (particularly the late
transition metals)46–52 have been found to efficiently catalyze such
reactions in homogeneous solutions. In view of the univalent
vanadium in compounds 1–4 showing a strong tendency to
triphenylbenzene-bound vanadium complex formulated as (h -
1,3,5-Ph3C6H3)V(HC(C(Me)NC6H3-2,6-iPr2)2) (8) can be isolated
in 22% yield from recrystallization. Compound 8 has 2 unpaired
electrons (meff = 2.78 B.M.) at room temperature determined by
Evans’s method.34 The molecular structure of 8 was determined
by single-crystal X-ray crystallography. Unfortunately, due to its
quick decomposition nature, the value of the R factor of 8 is high
(0.1177), so all metric parameters may not be reliable and only an
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©

anisotropic structure is shown in Fig. 5. The coordinated benzene
shows a puckering conformation with C(32) and C(35) atoms
closer to the vanadium centre than the other four carbons. The
failure to obtain the 1,2,4-triphenylbenzene–coordinated isomer
is presumably due to the steric repulsion between 1,2-diphenyl
groups of the 1,2,4-triphenylbenzene and the 2,6-iPr2C6H3 of the
ligand and of course the lower yield of the 1,2,4-triphenylbenzene
isomer.
trisubstituted benzenes are obtained if catalyst 3 is employed,
good regioselectivity and moderate yields of trisubstituted ben-
zenes are obtained when complex 1 is used as a catalyst. An
important intermediate, the 1,3,5-triphenylbenzene coordinated
6
complex (h -1,3,5-Ph3C6H3)V{HC(C(Me)N-2,6-iPr2C6H3)2} (8),
was isolated over the course of the cyclotrimerization reaction
of phenylacetylene catalyzed by complex 1 and was characterized
by single-crystal X-ray crystallography.
Acknowledgements
We are indebted to the National Science Council, Taiwan for
financial support under Grant NSC 96-2113-M-007-019-MY3.
Notes and references
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Fig. 5 Molecular structure of 8 with thermal ellipsoids at 35% probability
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Conclusions
In conclusion, we have demonstrated that [V(Nacnac)] (Nacnac =
b-diketiminate) can exist in two very different forms, which
strongly depends on the bulk of the Nacnac ligands. Three
6
dimeric complexes, [V{m-(h -Ar–N)C(Me)CHC(Me)C(N–Ar)}]2
(Ar = 2,6-Me2C6H3 (2), 2,6-Et2C6H3 (3), 9-anthracenyl (4)), were
2
isolated. Complexes 2–4 display both vanadium atoms being h
chelated by the b-diketiminate framework and h bonded to a
6
flanking ring of a b-diketiminato ligand, attached to the other
vanadium centres within the dimers. These dimers are strikingly
different from our previously reported toluene-bridged inverted
sandwich divanadium complex 1, which is supported by a sterically
more demanding b-diketiminate. Not only do they exhibit different
structures, but they also display very different electronic structures.
In sharp contrast with the paramagnetic inverted sandwich
divanadium complex 1 with two high-spin vanadium atoms, all
compounds 2–4 display an antiferromagnetic exchange between
the two vanadium centres. On the other hand, when the bulk of
the ancillary b-diketiminato ligand is increased by substituting two
phenyl groups for two methyl substituents on the ligand backbone,
an imido complex (h -PhCC(H)C(Ph)NC6H3-2,6-iPr2)VN(C6H3-
2
2,6-iPr2)(OEt2) (5) was actually obtained upon reduction of
[HC(C(Ph)N-2,6-iPr2C6H3)2]VCl2. According to the characteriza-
tions of 1 and 2–4, the formation of 4 is presumably from the
reductive cleavage of the C N bond of the ligand backbone
of the transient intermediate [{HC(C(Ph)N-2,6-iPr2C6H3)2}V].
Complexes 1–4 display reducing ability as evidenced by a facile
two-electron oxidative addition of the S–S bond of Ph2S2 to the
vanadium centres to give vanadium dithiolates 6 and 7. Most
notably, these univalent divanadium complexes 1–4 catalyze the
cyclotrimerization of alkynes, phenylacetylene and 1-heptyne. It is
interesting to note that the regioselectivity of the trisubstituted
benzene products is dependent of the steric bulk of the b-
diketiminate. While poor regioselectivity but good yields of the
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