M. P. Feth, A. Klein, H. Bertagnolli
FULL PAPER
concentrated to dryness. Recrystallization from CH2Cl2/n-heptane
(1:1) gave red-violet microcrystals. Yield: 290 mg (82%).
C20H22N2NiO (365.12): calcd. C 65.79, H 6.07, N 7.67; found C
The replacement of bromide is strongly accelerated when
oriented trans to the strong donor ligand mesityl, which can
be observed from the rapid splitting off of bromide from
trans-[(PPh3)2Ni(Mes)Br] in weakly coordinating solvents
such as acetone or THF in which the diimine complexes are
stable. While further ligand exchange has never been re-
ported for such complexes we have found that upon pro-
longed reaction times the diimine ligands or PPh3 were also
replaced by the solvent molecules, which is of great import-
1
65.55, H 6.02, N 7.59. H NMR ([D6]acetone): δ ϭ 9.0 [s (broad),
1 H, bpy-6], 8.31 (d, 1 H, bpy-3), 8.20 (d, 1 H, bpy-3Ј), 8.20 (dt, 1
H, bpy-4), 8.04 (dt, 1 H, bpy-4Ј), 7.71 (ddd, 1 H, bpy-5), 7.56 [s
(broad), 1 H, bpy-6Ј), 7.21 (ddd, 1 H, bpy-5Ј), 6.57 (s, 2 H, m-H),
3.28 (s, 3 H, OCH3), 3.07 (s, 6 H, o-CH3), 2.20 (s, 3 H, p-CH3)
ppm. 13C NMR ([D6]acetone): δ ϭ 156.4, 153.1, 151.5, 148.4,
148.3, 143.5, 139.6, 137.7, 127.3, 126.5, 132.8, 126.1, 122.4, 121.35,
ance regarding the catalytic activity of such molecules. The 69.1 (OCH3), 25.8 (o-CH3), 20.8 (p-CH3) ppm.
rates of these reaction are smaller by orders of magnitude
Synthesis of [(bpy)Ni(Mes)(py)](NO3): [(bpy)Ni(Mes)Br] (203 mg,
but this has to be viewed against the backdrop of long-term
stability. In protic media such as alcohols the mesityl ligand
is split off and, after the loss of this shielding group, octa-
hedral nickel species are formed. Interestingly this is also
valid for nitriles. The change in geometry from square-
0.49 mmol) was dissolved in a mixture of 50 mL of acetone/aceton-
itrile (3:1) and thallium nitrate (TlNO3) (133 mg, 0.5 mmol) and
pyridine (40 mg, 0.5 mmol) were added to the red solution. TlNO3
slowly dissolved while the reaction proceeded. After 5 h of stirring,
the solution was orange-yellow and a greenish precipitate had
planar to octahedral can be readily monitored by XANES. formed that was carefully filtered off, and the filtrate was then con-
centrated to dryness. After washing the residue with 5 ϫ 10 mL of
diethyl ether, a bright yellow amorphous solid was obtained. Yield:
209 mg (90%). C24H24N4NiO3 (475.19): calcd. C 60.66, H 5.09, N
11.79; found C 60.36, H 5.01, N 11.68%. Due to the insolubility
of the compound in common organic solvents no NMR spectra
were obtained.
Since the formation of such saturated octahedral species is
concomitant with the breakdown of catalytic activity a suc-
cessfully operating catalyst system has to contain a stable
shielding unit. In the successfully applied systems by Brook-
hart and others this is provided by the diimine ligand, e.g.
bulky R-DAB ligands with R ϭ 2-6-dialkylated aryl sub-
stituents. However, we found that the diimine ligands can Synthesis of [(py)3Ni(Mes)]Br: [(bpy)Ni(Mes)Br] (240 mg,
0.58 mmol) was dissolved in pyridine (30 mL) and stirred for 24 h.
The volume was then reduced to 1/3 and n-octane (30 mL) added.
The resultant yellow solution was stored at Ϫ30 °C overnight dur-
ing which time a microcrystalline solid precipitated. After filtration,
this solid was washed with n-pentane. Yield: 192 mg (67%). From
the mother liquor further yellow material was isolated. However,
this turned out to be a mixture of products. C24H26BrN3Ni (495.1):
also be cleaved. Therefore, new long-term stable systems
need to take account of the potential loss of the diimine
ligand. In further studies we aim to develop catalytic sys-
tems with such long-term stability and apply them under
real catalytic conditions.
1
calcd. C 58.22, H 5.29, N 8.49; found C 58.18, H 5.21, N 8.44. H
Experimental Section
NMR ([D6]acetone): δ ϭ 8.91 (d, 4 H, py-2,6), 8.18 (d, 2 H, py-
2,6), 7.75 (t, 2 H, py-4), 7.46 (t, 1 H, py-4), 7.32 (dd, 4 H, py-3,5),
7.00 (dd, 2 H, py-3,5), 6.41 (d, 2 H, m-H), 3.45 (s, 6 H, o-CH3),
2.21 (s, 3 H, p-CH3) ppm. UV/Vis (pyridine): λ ϭ 420 nm (ε ϭ 320
Ϫ1 cmϪ1), 396 (sh, 320), 363 (1000), 323 (sh, 710).
Preparations: All preparations were carried out under argon. The
synthesis of the complexes [(PPh3)2Ni(Mes)Br] and [(bpy)Ni-
(Mes)Br] has been recently described.[17] All solvents were obtained
from commercial sources and used as received. The alcohols and
acetonitrile used were anhydrous. The other solvents were of spec-
troscopic grade.
Synthesis of trans-[(py)2Ni(Mes)Br]: [(PPh3)2Ni(Mes)Br] (180 mg,
0.23 mmol) was dissolved in toluene (20 mL) and pyridine (80 mg,
1 mmol) was added to this red solution. After a few minutes, when
the solution had turned yellow, the volume was reduced to 1/2.
During storing of the solution at Ϫ20 °C overnight, yellow micro-
crystals had formed. Filtration and subsequent washing with n-
pentane yielded a first crop (46 mg, 48%). The material that was
further isolated from the mother liquor turned out to be a mixture
including compounds of higher and lower degrees of substitution.
C14H21BrN2Ni (416): calcd. C 54.86, H 5.09, N 6.73; found C
Synthesis of [(iPr-DAB)Ni(Mes)Br]: iPr-DAB (250 mg, 1.78 mmol)
was added to a vigorously stirred suspension of [(PPh3)2Ni(Mes)Br]
(250 mg, 0.32 mmol) in 500 mL of n-pentane. After a few minutes,
the yellow solid dissolved and at the same time a violet voluminous
solid precipitated. After stirring for 4 h, the solid was filtered off,
washed with pentane, recrystallized from CH2Cl2/n-heptane (1:8)
and dried in vacuo to yield dark violet microcrystals. Yield: 112 mg
(82%). C17H27BrN2Ni (398.03): calcd. C 57.07, H 6.59, N 31.60;
1
54.81, H 5.06, N 6.70. H NMR ([D6]acetone): δ ϭ 8.96 (d, 4 H,
1
py-2,6), 7.68 (t, 2 H, py-4), 7.20 (dd, 4 H, py-3,5), 6.33 (s, 2 H, m-
H), 3.44 (s, 6 H, o-CH3), 2.16 (s, 3 H, p-CH3) ppm.
found C 56.55, H 6.29, N 32.09. H NMR ([D6]acetone): δ ϭ 8.55
(s, 1 H, Himine), 8.43 (s, 1 H, Himine), 6.42 (s, 2 H, m-H), 3.26 (m,
2 H, HCiPr), 2.97 (s, 6 H, o-CH3), 2.21 (s, 3 H, p-CH3), 2.03 (d,
12 H, H3CiPr) ppm.
Synthesis of [(phen)2NiBr2]: [NiBr2] (236 mg, 1.08 mmol) was added
to DMF (10 mL) and dissolved on heating to 100 °C. Upon addi-
tion of 1,10-phenanthroline (390 mg, 2.16 mmol), the initially
brownish solution turned green. On cooling to ambient temper-
ature, a green solid precipitated that was collected by filtration,
washed with diethyl ether and recrystallized from ethanol/diethyl
ether (4:1). Yield: 562 mg (90%). C24H16Br2N4Ni (578.93): calcd.
C 49.79, H 2.79, N 9.68; found C 49.63, H 2.71, N 9.61. UV/Vis
(MeCN): λ ϭ 1055 nm (ε ϭ 61 Ϫ1 cmϪ1), 631 nm (82), 326 (1040),
268 (26600), 226 (17860).
Synthesis of [(bpy)Ni(Mes)(OMe)]: Fine-cut sodium (100 mg,
4.35 mmol) was added to a mixture of 50 mL of THF and 10 mL
of methanol. After all of the sodium had dissolved and reacted,
[(bpy)Ni(Mes)Br] (400 mg, 0.97 mmol) was added to this solution.
The red color of the starting material immediately turned to violet.
After stirring for 3 h, the reaction solvents were evaporated to dry-
ness and the residue extracted with 5 ϫ 10 mL of CH2Cl2. The
resultant Bordeaux colored solution was carefully filtered and then
850
Eur. J. Inorg. Chem. 2003, 839Ϫ852