New routes toward metallated methanofullerene terdentate bisaminoaryl ligands

Article abstract of DOI:10.1016/S0020-1693(01)00685-5

The preparation of a series of new ligands, comprising a NCN ligand (C₆H₂(CH₂NMe₂)₂-2, 6-X-4, X=H, Br, I) substituted on the para position with a [1,2]-methanofullerene group is described. Metallation of these ligands is achieved via an oxidative addition reaction of nickel(0) or palladium(0) complexes. The first step of this reaction comprises the formation of a π-complex between the metal center and the fullerene moiety of the ligand, which upon heating in a subsequent step is followed by oxidative addition of the NCN-halide bond to the metal center. The nickel(II) complex C₆₀-C(Me)-NCN·NiBr (9) was isolated in pure form. Low temperature NMR spectroscopy and theoretical calculations showed that the NCN moiety in 9 rotates about the (Me)C-NCN bond at room temperature. UV-Vis spectroscopic measurements of 9 suggest that binding a metal center via a covalent metal-carbon bond to the NCN moiety does not lead to a substantial change in the electronic structure of the fullerene moiety. The cyclo voltammograms of 9 showed the absence of a Ni(II)/Ni(III) oxidation/reduction process, while the second and fourth C₆₀ reduction potentials were anodically shifted.

Full text of DOI:10.1016/S0020-1693(01)00685-5

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Inorganica Chimica Acta 327 (2002) 31–40
New routes toward metallated methanofullerene terdentate
bisaminoaryl ligands
Michel D. Meijer a, Gerard P.M. van Klink a, Bas de Bruin b, Gerard van Koten a,*
a Department of Metal-Mediated Synthesis, Debye Institute, Utrecht Uni6ersity, Padualaan 8, 3584 CH Utrecht, The Netherlands
b Department of Inorganic Chemistry, Nijmegen Uni6ersity, Toernooi6eld 1, 6525 ED Nijmegen, The Netherlands
Received 8 March 2001; accepted 5 April 2001
Dedicated to Professor Dr K. Vrieze on the occasion of his 65th birthday, while thanking him for all the great years of collaboration and
support in Science and ‘University’ affairs
Abstract
The preparation of a series of new ligands, comprising a NCN ligand (C6H2(CH2NMe2)2-2,6-X-4, X=H, Br, I) substituted on
the para position with a [1,2]-methanofullerene group is described. Metallation of these ligands is achieved via an oxidative
addition reaction of nickel(0) or palladium(0) complexes. The first step of this reaction comprises the formation of a p-complex
between the metal center and the fullerene moiety of the ligand, which upon heating in a subsequent step is followed by oxidative
addition of the NCN–halide bond to the metal center. The nickel(II) complex C60ꢀC(Me)ꢀNCN·NiBr (9) was isolated in pure
form. Low temperature NMR spectroscopy and theoretical calculations showed that the NCN moiety in 9 rotates about the
(Me)CꢀNCN bond at room temperature. UV–Vis spectroscopic measurements of 9 suggest that binding a metal center via a
covalent metal–carbon bond to the NCN moiety does not lead to a substantial change in the electronic structure of the fullerene
moiety. The cyclo voltammograms of 9 showed the absence of a Ni(II)/Ni(III) oxidation/reduction process, while the second and
fourth C60 reduction potentials were anodically shifted. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Metallated methanofullerene; Bisaminoaryl ligands; NCN moieties
block in the synthesis of new supra-molecular struc-
tures, such as a molecular engine reported by Sauvage
and coworkers [8]. In a large number of reports, C60 is
used as an electron acceptor or as a photosensitizer in
donor–acceptor systems in order to develop molecular
electronics and photoactive devices [9]. For this pur-
pose, organic donors were attached to C60, such as
dimethylaniline, tetrathiafulvalene, and a carotenoid
tail [10–12]. Also, ligated organometallic C60 dyads
were prepared and characterized: C60–ferrocenyl com-
plexes [11d,13], C60–ruthenium bipyridyl/terpyridyl lig-
ated complexes [14], and several C60–porphyrine/
phthalocyanine structures containing nickel or zinc
[9a,c,15,16].
1. Introduction
Fullerene chemistry has evolved rapidly since the
development of the gram-scale synthesis of these car-
bon allotropes [1]. In particular, the use of the highly
symmetrically structured Buckminsterfullerene (C60)
has been widely reported in organic synthesis,
organometallic chemistry, biochemistry, and material
science [2–5]. The combination of its unique properties,
i.e. the ability to accommodate up to six electrons in its
LUMO [6], its readily accessible excited-state
(photo)chemistry, and its symmetric topology, make
C60 an interesting component in new materials. For
example, C60 is used in superconducting alkali-metal
intercalated fullerides [2f,7] or as a molecular building
Part of our research is focussed on the use of
monoanionic terdentate bisamino-, bisphosphino-, and
bissulfido-aryl (pincer) ligands [C6H2(CH2Y2)2-2,6-R-
4]−, commonly abbreviated for R=H as NCN (Y=
NR%) [17], PCP (Y=PR%) [18], and SCS (Y=SR%) [19],
* Corresponding author. Tel.: +31-30-2533 120; fax: +31-30-
2523 615.
E-mail address: g.vankoten@chem.uu.nl (G. van Koten).
2
2
0020-1693/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00685-5

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M.D. Meijer et al. / Inorganica Chimica Acta 327 (2002) 31–40
for application in homogeneous catalysis [20] (e.g. as
metallodendritic catalysts [21]) and in materials with
new physicochemical properties. The hard Lewis-donor
property of the nitrogen atoms of the monoanionic
NCN ligand enhance the nucleophilic character of
Ni(II) and Pt(II)-centers and stabilize higher oxidation
states of the metal center, like in stable Ni(III) and
Pt(IV) NCN complexes [22]. The NCN ligand was used
to synthesize new donor–acceptor systems with novel
electronic, photochemical and sensing properties, e.g.
[PtI(NCN)(h1-I2)] and [PtX(NCN)(h1-SO2)] [22b,23]. In
addition, the NCN ligand provides several possibilities
for ligand modification via the para position (to the
metal–carbon bond), which allows linking to supports
[21,24] or further tuning of the redox properties of the
metal center [25]. Recently, we became interested to
explore the synthesis of C60-attached pincer metal com-
plexes as mimics for metal complexes bound to a
carbon support. The synthesis of two methanofullerene
NCN ligands, 1 and 2, which were reported in prelimi-
nary form [26], will be described in this paper in detail.
In particular, the metallation of these ligands, which is
complicated due to the reactivity of the C60 core, is
discussed. The first examples are presented of metal-
lated methanofullerene terdentate bisaminoaryl ligands.
solution were combined on a golden MALDI target
and analyzed after evaporation of the solvent. Elemen-
tal analyses were performed by Dornis und Kolbe,
Mikroanalytisches Laboratorium (Mu¨lheim, Germany).
2.2. Synthesis of
3,5-bis[(dimethylamino)methyl]benzaldehyde (4)
To a solution of 3 (5.41 g, 20.0 mmol) in Et2O (40
ml) was added 27 ml of a 1.5 M t-BuLi solution in
pentane (40 mmol) at −78 °C. The resulting yellow
suspension was stirred at −78 °C for 1 h followed by
addition of dimethylformamide (1.6 ml, 21 mmol). The
reaction mixture was allowed to warm up to room
temperature (r.t.) in 17 h and quenched with water (50
ml). The organic layer was separated, and the remain-
ing water layer extracted with Et2O (2×50 ml). The
organic fractions were dried over MgSO4, filtered, and
evaporated in vacuo. The residue was distilled in vacuo,
1
giving 4 as a yellow oil (3.77 g, 86% yield). H NMR
(C6D6): l 9.81 (s, 1H, COH), 7.68 (s, 2H, ArH), 7.62 (s,
1H, ArH), 3.18 (s, 4H, CH2N), 2.03 (s, 12H, NCH3),
13C{1H} NMR (C6D6): l 191.1 (CHO), 140.6, 136.9,
134.6, 128.4 (ArꢀC), 63.3 (CH2N), 44.8 (NCH3). Anal.
Calc. for C13H20N2O: C, 70.87; H, 9.15; N, 12.72.
Found: C, 70.81; H, 9.22; N, 12.85%.
2. Experimental
2.3. Synthesis of 3,5-bis[(dimethylamino)-
methyl]benzaldehyde hydrazone (5)
2.1. General
To a solution of 4 (2.02 g, 9.17 mmol) in ethanol (30
ml) was added N2H4·H2O (4.59 g, 91.7 mmol). The
reaction mixture was heated at 80 °C for 2 h where-
after all volatile components were evaporated in vacuo.
The product was dissolved in CH2Cl2 (25 ml), dried
over MgSO4, filtered, and evaporated in vacuo yielding
5 as a light-yellow oil, which solidified upon standing
All experiments were conducted under a dry dinitro-
gen atmosphere using standard Schlenk techniques. Sol-
vents were dried over appropriate materials and
1
distilled prior to use. H, 13C{1H}, and 31P{1H} NMR
spectra were recorded at 298 K (unless stated other-
wise) in a Varian Inova 300 MHz or in a Varian
Mercury 200 MHz NMR spectrometer. All NMR
chemical shifts are in ppm referenced to residual solvent
signal (1H and 13C{1H}) or to H3PO4 (31P{1H}). Start-
ing materials 3,5-bis[(dimethylamino)methyl]bromo-
benzene (3) [27] and 4-bromo-3,5-bis[(dimethy-
lamino)methyl]acetophenone (6) [25] were prepared ac-
cording to literature procedures. C60 (Hoechst, Gold
Grade) was used as received. UV–Vis spectra were
measured in a Varian Cary 1 UV–Vis spectrophotome-
ter in degassed, N2 saturated CH2Cl2 at ambient tem-
1
(2.13 g, quantitatively). H NMR (C6D6): l 7.66 (s, 2H,
ArH), 7.44 (s, 1H, ArH), 7.25 (s, 1H, CHN2H2), 4.90
(s, 2H, N2H2), 3.30 (s, 4H, CH2N), 2.10 (s, 12H,
NCH3). 13C{1H} NMR (C6D6): l 141.4, 139.9, 136.0,
129.3 (ArC), 125.4 (CN2H2), 64.1 (CH2N), 45.1
(NCH3). Anal. Calc. for C13H22N4: C, 66.63; H, 9.46;
N, 23.91. Found: C, 66.46; H, 9.38; N, 23.76%.
2.4. Synthesis of 4-bromo-3,5-bis[(dimethylamino)-
methyl]acetophenone hydrazone (7)
perature at
a
concentration of 1×10−5 M.
MALDI-TOF mass spectra were acquired using a Voy-
ager-DE BioSpectrometry Workstation mass spectrom-
eter (PerSeptive Biosystems Inc., Framingham, MA,
USA). Sample solutions with an approximate concen-
tration of 10 mg ml−1 in CH2Cl2 were prepared. The
matrix was 9-nitroanthracene (9-NA) with an approxi-
mate concentration of 40–50 mg ml−1. An 0.2 ml
amount of the sample solution and 0.2 ml of the matrix
To a solution of 6 (1.34 g, 4.28 mmol) in dry ethanol
(25 ml) was added N2H4·H2O (2.14 g, 42.8 mmol). The
reaction mixture was heated at 80 °C for 2 h. Subse-
quently, all volatile components were removed in
vacuo, yielding 7 as a white solid (1.41 g, quantita-
1
tively). H NMR (C6D6): l 8.02 (s, 2H, ArH), 4.83 (s,
2H, N2H2), 3.57 (s, 4H, CH2N), 2.16 (s, 12H, NCH3),
1.63 (s, 3H, CN2H2CH3). 13C{1H} NMR (C6D6): l

M.D. Meijer et al. / Inorganica Chimica Acta 327 (2002) 31–40
33
144.0, 138.6, 138.3, 125.8 (ArC), 126.2 (CN2H2), 63.7
(CH2N), 45.1 (NCH3), 10.2 (CN2H2CH3). Anal. Calc.
for C14H23BrN4: C, 51.38; H, 7.08; N, 17.12. Found: C,
51.26; H, 6.94; N, 17.21%.
isomer was complete (monitored by 1H NMR spec-
troscopy, days). The crude product was
chromatographed over Al2O3-90 using CHCl3 as eluent,
6
yielding 2 as a brown solid (0.11 mg, 60, 56% overall
1
yield based on reacted C60). H NMR (CS2/C6D6 (3:1)):
2.5. Synthesis of 1,2-dihydro-61-(3,5-bis[(dimethyl-
amino)methyl]phenyl)-1,2-methano[60]fullerene (1)
l 7.99 (s, 2H, ArH), 3.54 (s, 4H, CH2N), 2.41 (s, 3H,
CH3), 2.18 (s, 12H, NCH3). 13C{1H} NMR (CDCl3): l
148.79, 147.92, 145.93, 145.17, 145.13, 145.11, 145.07,
145.03, 144.78, 144.73, 144.65, 144.43, 144.34, 143.97,
143.74, 143.70, 143.08, 142.98, 142.92, 142.14, 142.10,
141.98, 140.96, 140.71, 138.73, 138.26, 138.04, 137.20,
131.86, 126.07 (C60ꢀC and ArC), 80.63 (C60-sp3), 63.80
(CH2N), 47.19 (bridgehead-C), 45.54 (NCH3), 22.00
(CH3). MALDI-TOF MS (9-NA); m/z: 1017.84 (M+).
Anal. Calc. for C74H21BrN2: C, 87.32; H, 2.08; N, 2.75.
Found: C, 87.46; H, 2.18; N, 2.84%.
A suspension of 5 (0.33 g, 1.42 mmol), MnO2 (1.23 g,
14.2 mmol), Na2SO4 (2.03 g, 14.2 mmol), and a freshly
prepared saturated solution of KOH in ethanol (0.5 ml)
in Et2O (50 ml) was stirred in the dark for 20 h. The
reaction mixture was filtered over a G4 glass frit and
slowly added to a solution of C60 (1.02 g, 1.42 mmol) in
toluene (1 l). The reaction mixture was stirred for 5 h
and evaporated in vacuo. The remaining product was
chromatographed over silica gel. Elution with toluene
and toluene/MeOH (95:5 v/v %) gave unreacted C60
(0.41 g) and the crude product as a mixture of [5,6]-
and [6,6]-isomers (0.58 g, 40, 68% based on reacted
C60), respectively. A part of this mixture (0.44 g) was
dissolved in toluene (500 ml) and heated at 110 °C
until the isomerization of the [5,6]-isomer to the [6,6]-
isomer was complete (monitored by 1H NMR spec-
2.7. Synthesis of 1,2-dihydro-61-methyl-61-{(4-chloro-
nickel-3,5-bis[(dimethylamino)methyl]benzene}-
1,2-methano[60]fullerene (9)
2.7.1. Reaction of 2 with Ni(COD)2
To a solution of 2 (137 mg, 0.137 mmol) in degassed
toluene (60 ml) was added a solution of Ni(COD)2 (48
mg, 0.18 mmol) in toluene (20 ml) at 0 °C. A dark
green precipitate was formed. The reaction mixture was
slowly heated to 45 °C and stirred for 20 h, filtered
through a G4 glass frit, and evaporated in vacuo. The
remaining solid was washed with pentane (40 ml) and
dried in vacuo, yielding 9 as a brown solid (76 mg,
52%).
troscopy,
5
days). The crude product was
chromatographed over silica gel using toluene/MeOH
(95:5 v/v %) as eluents, then dissolved in CS2 (20 ml),
precipitated by addition of pentane (300 ml), and dried
in vacuo, yielding 1 as a brown solid (0.38 g, 86, 58%
overall yield based on reacted C60). 1H NMR (CS2/
C6D6 (3:1)): l 7.78 (s, 2H, ArH), 7.29 (s, 1H, ArH),
5.17 (s, 1H, bridgehead-CH), 3.39 (s, 4H, CH2N), 2.16
(s, 12H, NCH3). 13C{1H} NMR (CDCl3): l 149.75,
147.81, 145.66, 145.56, 145.10, 145.05, 145.02, 145.00,
144.75, 144.60, 144.48, 144.33, 144.28, 144.05, 143.68,
143.62, 143.01, 142.95, 142.89, 142.83, 142.66, 142.22,
142.07, 142.02, 140.98, 140.71, 139.45, 138.15, 137.77,
136.40, 133.10, 130.57, 129.81 (C60ꢀC and ArC), 75.55
(C60-sp3), 64.11 (CH2N), 45.41 (NCH3), 43.47 (bridge-
head-C). MALDI-TOF MS (9-NA); m/z: 925.3 (M+).
Anal. Calc. for C73H20N2: C, 94.79; H, 2.18; N, 3.03.
Found: C, 94.58; H, 2.31; N, 2.99%.
2.7.2. Reaction of 2 with Ni(PPh3)4
To a solution of 2 (70 mg, 0.069 mmol) in degassed
toluene (60 ml) was added a solution of Ni(PPh3)4 (84
mg, 0.076 mmol) in toluene (20 ml) at 0 °C. The
dark-green colored reaction mixture was slowly heated
to 45 °C and stirred for 20 h, and evaporated in vacuo.
The resulting brown solid was extracted with benzene
(20 ml). To this solution was added acetone (60 ml) and
the precipitate was collected by centrifugation/decanta-
tion. This procedure was repeated once. The product
was washed with pentane and dried in vacuo, yielding 9
as a brown solid (41 mg, 55%). 1H NMR (C6D6): l 7.04
(s, 2H, ArH), 3.02 (s, 4H, CH2N), 2.44 (s, 12H, NCH3),
2.35 (s, 3H, CH3). 13C{1H} NMR (CS2/CD2Cl2 (1:1)): l
152.01, 149.36, 148.57, 147.15, 146.16, 145.49, 145.45,
145.39, 145.32, 145.21, 145.16, 145.05, 144.96, 144.71,
144.61, 144.31, 144.03, 143.37, 143.32, 143.26, 142.60,
142.48, 142.45, 142.42, 141.25, 141.04, 138.27, 137.97,
136.41, 129.31, 128.61, 128.53, 121.62, 121.54 (C60ꢀC
and ArC), 81.51 (C60-sp3), 73.84 (CH2N), 66.11
(bridgehead-C), 52.38 (NCH3), 23.23(CH3). Anal. Calc.
for C74H21BrN2Ni: C, 82.56; H, 1.97; N, 2.60. Found:
C, 82.38; H, 2.15; N, 2.58%.
2.6. Synthesis of 1,2-dihydro-61-methyl-61-
(3,5-bis[(dimethylamino)methyl]-4-bromophenyl)-1,2-
methano[60]fullerene (2)
Compound 2 was synthesized similar to 1 starting
from 7 (0.21 g, 0.65 mmol), MnO2 (0.57 g, 6.5 mmol),
Na2SO4 (0.93 g, 6.5 mmol), and C60 (0.94 g, 1.3 mmol).
Column chromatography of the reaction mixture af-
forded unreacted C60 (0.64 g) and the crude product as
a mixture of [5,6]- and [6,6]-isomers (0.39 g, 59, 93%
based on reacted C60). A part of this mixture (0.18 g)
was dissolved in toluene (100 ml) and heated at 110 °C
until the isomerization of the [5,6]-isomer to the [6,6]-

34
M.D. Meijer et al. / Inorganica Chimica Acta 327 (2002) 31–40
2.8. Synthesis of 4-bromo-3,5-bis[(dimethylamino)-
methyl]acetophenone ethylene acetal (10)
44.93; H, 6.19; N, 14.97. Found: C, 45.08; H, 6.25; N,
15.06%.
A solution of 6 (6.07 g, 19.4 mmol), p-toluenesulfonic
acid monohydrate (9.21 g, 48.5 mmol), ethylene glycol
(2.2 ml, 39 mmol) in benzene (100 ml) was heated at
80 °C in a Dean–Stark setup for 20 h. NEt3 (10 ml)
was added to the reaction mixture, and all volatiles
were removed in vacuo. The remaining solid was ex-
tracted with Et2O (2×100 ml). The organic fraction
was washed with a 4 M NaOH solution (2×50 ml),
dried over MgSO4, filtered, and evaporated in vacuo,
2.11. Synthesis of 1,2-dihydro-61-methyl-61-
(3,5-bis[(dimethylamino)methyl]-4-iodophenyl)-
1,2-methano[60]fullerene (13)
Compound 13 was synthesized similar to 1 starting
from 12 (0.22 g, 0.57 mmol), MnO2 (0.75 g, 8.5 mmol),
Na2SO4 (1.21 g, 8.51 mmol), a freshly prepared satu-
rated solution of KOH in EtOH (0.5 ml) and C60 (0.61
g, 0.85 mmol). Column chromatography of the reaction
mixture afforded unreacted C60 (0.36 g) and the crude
product as a mixture of [5,6]- and [6,6]-isomers (0.35
mg). This was dissolved in 400 ml toluene and heated to
reflux until the isomerization of the [5,6]-isomer to the
[6,6]-isomer was complete (monitored by 1H NMR
spectroscopy, 4 days). The crude product was chro-
matographed over silica gel using toluene/MeOH (95:5
v/v %) as eluents, yielding 13 as a brown solid (0.29 g,
1
yielding 10 as a white solid (5.20 g, 75%). H NMR
(CDCl3): l 7.40 (s, 2H, ArH), 3.98 (m, 2H, acetal), 3.72
(m, 2H, acetal), 3.50 (s, 4H, CH2N), 2.26 (s, 12H,
NCH3), 1.61 (s, 3H, CH3). 13C{1H} NMR (CDCl3): l
141.7, 138.5, 128.2, 126.3 (ArC), 108.4 (CCH3), 64.3
(acetal), 63.9 (CH2N), 45.5 (NCH3), 27.4 (CCH3).
Anal. Calc. for C16H25BrN2O2: C, 53.79; H, 7.05; N,
7.84. Found: C, 53.88; H, 7.11; N, 7.86%.
1
48, 78% based on reacted C60). H NMR (CS2/C6D6
2.9. Synthesis of 3,5-bis[(dimethylamino)-
methyl]-4-iodoacetophenone (11)
(3:1)): l 7.92 (s, 2H, ArH), 3.50 (s, 4H, CH2N), 2.42 (s,
3H, CH3), 2.18 (s, 12H, NCH3). 13C{1H} NMR
(CDCl3): l 149.06, 148.14, 146.22, 145.48, 145.45,
145.35, 145.08, 145.04, 144.95, 144.72, 144.70, 144.32,
144.04, 144.02. 143.41, 143.35, 143.30, 143.26, 142.59,
142.46, 142.44, 142.33, 141.32, 141.06, 139.11, 138.32,
137.61, 131.54, 106.49 (C60ꢀC and ArC), 81.0 (C60-sp3),
69.1 (CH2N), 47.5 (bridgehead-C), 45.6 (NCH3), 22.3
(CH3). MALDI-TOF MS (9-NA); m/z: 1065.0 (M+).
Anal. Calc. for C74H21IN2: C, 83.46; H, 1.99; N, 2.63.
Found: C, 83.37; H, 1.84; N, 2.65%.
To a solution of 10 (3.22 g, 9.01 mmol) in hexanes
(50 ml) at −78 °C was added 11 ml of a 1.7 M t-BuLi
solution in pentane (19 mmol). The solution was al-
lowed to warm up to 0 °C in 3 h, and a solution of I2
(2.50 g, 9.85 mmol) in THF (20 ml) was added. The
reaction mixture was stirred at ambient temperature for
1 h. A 2 M HCl solution (40 ml) was added to the
solution and the aqueous layer isolated. The organic
layer was extracted with a 1 M HCl solution (40 ml).
The aqueous layers were combined, and a 4 M NaOH
solution (100 ml) was added. The solution was ex-
tracted with Et2O (3×75 ml). The organic fractions
were combined, dried over MgSO4, filtered, and evapo-
rated in vacuo. The remaining product was distilled in
2.12. Electrochemistry
Cyclic voltammetry measurements were performed in
a glovebox under dinitrogen atmophere using an Auto-
lab PGStat 100 electrochemical analyzer. The spectra
were recorded at a scan rate of 100 mV s−1, using a
platinum disc as the working electrode, a Ag/AgNO3
reference electrode and a platinum counter electrode.
1
vacuo, yielding 11 as a yellow oil (2.32 g, 71%). H
NMR (CDCl3): l 7.77 (s, 2H, ArH), 3.49 (s, 4H,
CH2N), 2.55 (s, 3H, COCH3), 2.25 (s, 12H, NCH3).
13C{1H} NMR (CDCl3): l 197.6 (CO), 142.4, 136.1,
128.0, 113.4 (ArC), 68.6 (CH2N), 45.3 (NCH3), 26.6
(COCH3). Anal. Calc. for C14H21IN2O: C, 46.68; H,
5.88; N, 7.78. Found: C, 46.55; H, 6.05; N, 7.71%.
Tetrabutylammonium
hexafluorophosphate
([(n-
Bu)4N(PF6)]) was employed as the supporting elec-
trolyte in 0.1 mol l−1 concentration. A freshly prepared
mixture of toluene and acetonitrile (5:2 v/v %) was used
as solvent (both solvents were distilled prior to use).
2.10. Synthesis of 3,5-bis[(dimethylamino)-
methyl]-4-iodoacetophenone hydrazone (12)
Compound 12 was synthesized similar to 5, starting
from 11 (0.81 g, 2.2 mmol) and N2H4·H2O (0.11 g, 2.2
mmol), yielding 12 as a white solid (0.64 g, 76%). H
3. Results and discussion
1
NMR (CDCl3): l 7.56 (s, 2H, ArH), 5.33 (s, 2H,
N2H2), 3.54 (s, 4H, CH2N), 2.30 (s, 12H, NCH3), 2.12
(s, 3H, CCH3). 13C{1H} NMR (CDCl3): l 146.3, 141.5,
138.6, 107.0 (ArC), 126.2 (CN2H2), 68.9 (CH2N), 45.4
(NCH3), 11.6 (CCH3). Anal. Calc. for C14H23IN4: C,
3.1. Synthesis of the free methanofullerene NCN
ligands
[1,2]-Methanofullerene NCN ligand 1 (C60ꢀCHꢀ
NCNꢀH) and its corresponding bromide
2

M.D. Meijer et al. / Inorganica Chimica Acta 327 (2002) 31–40
35
(C60ꢀC(Me)ꢀNCNꢀBr) were synthesized via addition of
During this conversion, small amounts of insoluble
side products were formed. Monitoring of the conver-
sion by H NMR spectroscopy also revealed the ap-
4-diazo substituted pincer ligands to C60 (Scheme 1
1
[26]).
3,5-Bis[(dimethylamino)methyl]bromobenzene
(3) and 4-bromo-3,5-bis[(dimethylamino)methyl]aceto-
phenone (6), were converted into the corresponding
hydrazone compounds 5 and 7, respectively. Subse-
quently, 5 and 7 were transformed to the corresponding
diazo compounds with MnO2, and reacted in situ with
C60. This led in both cases to the formation of a
mixture of the [5,6]-fulleroid, the kinetically more stable
isomer, together with small amounts (10–15%) of the
thermodynamically more stable [6,6]-methanofullerene.
Complete isomerization to the [6,6]-methanofullerene
took place in refluxing toluene in 5–6 days.
pearance of a new set of signals grouped around the
resonances of the NMe2 groups and benzylic protons of
the desired [6,6]-methanofullerene, pointing to the for-
mation of several soluble side products. As primary,
secondary, and tertiary amines do react with C60 itself,
these side products probably result from intermolecular
reactions of the amine donor groups with fullerene
moieties, leading to the formation of oligomeric
amine–fullerene donor–acceptor compounds [28].
Unfortunately, the use of higher temperatures in
order to accelerate the isomerization reaction also led
to an increase of the formation of side products. There-
fore, the isomerization reaction was carried out in
dilute solutions (B1 mg ml−1) to suppress side reac-
tions. This could not prevent that isolation of pure
compounds still required an additional chromato-
graphic purification step in the work-up procedure.
3.2. Metallation procedure of 1 and 2
The established way of synthesizing pincer metal
complexes involves either direct lithiation of the C(2)ꢀH
bond in C6H4(CH2NMe2)2-1,3, or proceeds via
lithium–bromide exchange at the C(2)ꢀBr bond in 2-
BrꢀC6H3(CH2NMe2)2-1,3 (Scheme 2). Subsequent
transmetallation of the corresponding aryllithium com-
pound with an appropriate transition-metal halide gives
the pincer metal–halide complex and lithium halide
[21b,29,30]].
However, facile reaction of C60 itself with various
organolithium reagents obstructs the use of this route
[31–33]. Indeed, reaction of 1 and 2 with either n-BuLi
or t-BuLi leads to formation of insoluble black solids.
Therefore, other routes were investigated without the
involvement of organolithium reagents or intermedi-
ates. Alternative direct metallation routes are the cy-
clometallation of NCNꢀH with RhCl3·H2O [34], and
Pd(OAc)2 [35,36], the oxidative addition of NCNꢀBr
with Ni0 and Pd0 complexes [25,37] or the electrophilic
metallation of NCNꢀSiMe3 with Pd(OAc)2 or
Li2[PdCl4] [36]. Direct metallation of 1 with RhCl3·H2O
or Pd(OAc)2 failed; only starting material and insoluble
products were isolated. However, the addition of either
Ni0 or Pd0 complexes to 2 showed the formation of new
complexes.
Scheme 1. Synthesis of methanofullerenes 1 and 2. Reagents and
conditions: (i) 2 t-BuLi, Et2O, −78 °C, 1 h; (ii) DMF, −78 °C to
r.t., 17 h; (iii) N2H4·H2O, EtOH, reflux, 2 h; (iv) MnO2, Et2O, 20 h;
(v) C60, toluene, 5 h; (vi) toluene, 110 °C, 5–6 days.
3.3. Oxidati6e addition of nickel(0) complexes to 2
Reaction of Ni0 complexes such as [Ni(COD)2] and
[Ni(PPh3)4] with 2-BrꢀC6H3(CH2NMe2)2-1,3 led to the
oxidative addition of the aryl bromide bond to the Ni0
complex, thus forming the corresponding colorless
NCN·NiBr complex [25]. However, upon addition of
Scheme 2. Metallation pathway of NC(X)N ligands via lithiation and
transmetallation. Reagents and conditions: (i) For X=H: n-BuLi,
hexanes, 0 °C. For X=Br: Li, Et2O; (ii) For example: [PtCl2(SEt2)2]
[28], [PdCl2(SEt2)2] [28], [NiCl2(PEt3)2] [21b], or [RuCl2(PPh3)3] [30].

36
M.D. Meijer et al. / Inorganica Chimica Acta 327 (2002) 31–40
Scheme 3. Synthesis of 9. Reagents and conditions: (i) [Ni(PPh3)4], toluene, 0 °C to r.t., 1 h; (ii) toluene, 45 °C, 20 h.
[Ni(COD)2] to a dark-red solution of 2 in toluene at
0 °C, an immediate formation of a dark-green material
was observed. This color change corresponds to the
formation of a p-complex between the zerovalent nickel
center and C60 [38,39]. Similarly, addition of
[Ni(PPh3)4] to a toluene solution of 2 at 0 °C also gave
a clear dark-green reaction mixture. Spectroscopic in-
vestigation of this reaction mixture by 31P NMR spec-
troscopy showed a broad resonance signal at 25.5 ppm
(C6D6), pointing to the presence of the p-complex (8,
Scheme 3) between the nickel(0) center and the fullerene
part of 2. This proposal is supported by the fact that
for similar complexes, such as (PPh3)2PdC60 and
(PPh3)2PtC60, these resonances are found at 25.4 ppm
(CDCl3) [38] and 27.0 ppm (THF-d8) [39], respectively.
Upon heating these (C60ꢀC(Me)ꢀNCNꢀBr)·NiLn p-
complexes (L=PPh3 or COD) for 20 h at 45 °C, the
color of the reaction mixture changed from dark-green
back to deep-red. After subsequent removal of all
volatiles, washing of the residue and extraction with
benzene, a new product was isolated, which was iden-
tified with NMR spectroscopy as the oxidative addition
product, i.e. the C60ꢀC(Me)ꢀNCN·NiBr complex 9.
compound, 13, was prepared. The synthesis, shown in
Scheme 4, started with the protection of the carbonyl
group in ketone 6 with glycol using p-toluenesulfonic
acid as catalyst in benzene. In this way, ketal 10 was
obtained in good yield within 20 h. Subsequently, 10
was reacted with 2 equiv. of t-BuLi in hexanes, fol-
lowed by a quench reaction with I2 in THF. The acetal
protection was removed by acid–base work-up of the
reaction mixture, yielding the corresponding acetophe-
none 11 in 72% overall yield. Similar to the synthesis of
1 and 2, 11 was converted to the corresponding hydra-
3.4. Reacti6ity of palladium(0) complexes toward 2
The addition of Pd(dba)2 to 2 afforded a gray solid,
insoluble in common organic solvents. This result re-
sembles the formation of a C60–palladium polymer
upon reaction of Pd(dba)2·dba·CHCl3 with C60 [40].
Heating this gray solid did not result in subsequent
rearrangement and formation of the desired
C60ꢀC(Me)ꢀNCN·PdBr complex as was the case for
nickel. Upon addition of [Pd(PPh3)4] to 2, the green
colored p-complex 2·Pd(PPh3)2 was formed (31P NMR,
l=26.2). Heating a benzene solution of 2·Pd(PPh3)2 at
reflux in order to force palladium insertion into the
NCN CꢀBr bond did not result in the formation of the
expected oxidative product. Instead, decomposition
took place under formation of 2, palladium black, PPh3
and other (unidentified) side products. As Pd0 com-
plexes are more prone to oxidatively add to aryl io-
dides, the corresponding methanofullerene NCNꢀI
Scheme 4. Synthesis route toward 14. Reagents and conditions: (i)
glycol, p-TosOH, benzene, reflux, 20 h; (ii) 2 t-BuLi, Et2O, −78 °C,
30 min; (iii) I2, THF, −78 °C to r.t.; (iv) HCl, Et2O, then NaOH,
Et2O; (v) N2H4·H2O, EtOH, reflux, 2 h; (vi) MnO2, Et2O, 20 h; (vii)
C60, toluene, 5 h; (viii) toluene, 110 °C, 5 days; (ix) Pd(PPh3)4,
benzene, 80 °C, 4 days.

M.D. Meijer et al. / Inorganica Chimica Acta 327 (2002) 31–40
37
mixture in pure form failed. As a result of the long
reaction time and high reaction temperature needed
various decomposition compounds were formed. Al-
though addition of excess amounts of Pd(PPh3)4 to the
reaction mixture enhanced the rate of formation of 14,
also more side products were found. To this moment,
successful synthesis of methanofullerene NCN metal
complexes via an oxidative addition pathway seems
viable for nickel only. In this case, the reactivity of the
p-complex formed between Ni0 and 2 toward the aryl
CꢀBr bond in 2 offered an interesting new pathway
toward the preparation of C60ꢀC(Me)ꢀNCN·NiBr com-
plex 9.
3.5. Properties of C60ꢀC(Me)ꢀNCN·NiBr (9)
Attempts to obtain crystals of 9 suitable for X-ray
crystal structure determination were unsuccessful.
Therefore, 9 was studied in more detail using molecular
modeling [41]. The semi-empirical (PM3(tm)) optimized
structure (Fig. 1) shows that the ground state confor-
mation (rotamer) contains a methano-methyl group
which is anti-planar positioned with respect to the
plane of the NCN·NiBr unit. The structural features of
the latter unit corresponds with those obtained from
X-ray crystal structure determinations of parent
NCN·Ni complexes [42]. Furthermore, also the struc-
tural features of the [1,2]-methanofullerene part of 9 are
similar to the known distances and bond lengths of
[1,2]-methanofullerenes [43]. From the PM3 calculation,
a shortest intramolecular nickel–fullerene distance of
Fig. 1. PM3(tm) optimized structure of 9 (ground state rotamer),
hydrogen atoms are omitted for clarity.
,
6.5 A was derived.
The H NMR spectroscopy of the new bulky-ligands
1
1, 2, 13, and the nickel complex 9 showed a single
resonance pattern for the NMe2 and benzylic protons,
pointing to the presence of a single species in solution.
Fig. 2. (a) Newman projection along the C(Me)ꢀNCNꢀNi axis show-
ing h, the angle between the planes through CH3ꢀCꢀC(NCN) and the
NCN group and rotation (180°) about the methano-NCN CꢀC bond.
(b) Wagging of the puckered chelate rings.
1
Comparison of the H NMR chemical shifts of 9 in
C6D6 with those of the parent ligand 2 showed a
downfield shift for the NMe2 groups (Dl=0.60 ppm),
while the benzylic signal were found upfield shifted
(Dl=0.28 ppm), which is a common observation for
NCN·Ni complexes. The largest change was observed
for the aromatic protons, which shifted highfield from
8.22 to 7.04 ppm (C6D6). Interestingly, similar values
were found for the aromatic protons in other 4-substi-
tuted NCN·Ni complexes [25]. The large deshielding
effect of the fullerene unit in the parent ligand 2 is
apparently compensated for by the electronic effect of
the nickel center in 9. The PM3(tm) model of 9 shows
that as the N(Me2) and C(H2) centers cannot be situ-
ated in a single molecular symmetry plane, the methyl
groups and benzylic protons are diastereotopic. The
single resonance pattern indicates that the 4-sub-
stituent, which is the origin of the dissymmetry, is either
too far away or that an intramolecular fluxional pro-
cesses on the NMR time scale takes place, rendering
zone compound 12 with a stoichiometric amount of
hydrazine hydrate in refluxing EtOH.1 Coupling to C60
was achieved via oxidation of the hydrazone functional-
ity to the corresponding diazo compound with MnO2,
followed by cycloaddition to C60 and isomerization of
the obtained mixture of regioisomers to the pure
methanofullerene 13.
A mixture of 13 and Pd(PPh3)4 in toluene was heated
1
at 80 °C for 4 days. H and 31P NMR spectra of the
reaction mixture revealed that partial formation of the
desired C60ꢀC(Me)ꢀNCN·PdI complex 14 had oc-
curred, but attempts to isolate 14 from the reaction
1 The use of more than 1 equivalent of N2H4·H2O in this reaction
leads to removal of the iodine atom in 12, and formation of the
corresponding NC(H)N compound.

38
M.D. Meijer et al. / Inorganica Chimica Acta 327 (2002) 31–40
these diastereotopic groups enantiotopic. Two fluxional
processes can be responsible for this: (i) rotation about
the (Me)CꢀNCN bond (Fig. 2(a)), and (ii) wagging of
the aryl ring about the CꢀNi axis causing inversion of
ring puckering in the two fused five-membered chelate
rings (Fig. 2(b)). To test this hypothesis, variable tem-
rotational freedom is present in the nitrogen donor
arms due to the lack of spatial fixation as a result of
such a coordination. Although the rotation of the
NCN-moiety about the (Me)CꢀNCN bond in these free
ligands is supposed to be blocked at low temperature as
well, this was not observed with NMR spectroscopy at
173 K.
Interestingly, the same energy barrier for rotation
about the (Me)CꢀNCN bond in 9 was also estimated by
a theoretical method. A range of semi-empirical calcu-
lations (PM3(tm)) was performed for 9. The heat of
formation was calculated for various rotamers of 9 as
function of the dihedral angle, h, between the
MeꢀCꢀC(NCN) and the NCN-aryl plane. The energy
difference between the rotamer with the lowest energy
(h=90°) and the one with the highest energy (h=0°)
amounted to 44 kJ mol−1. This is in excellent agree-
ment with the observed energy barriers found by NMR
spectroscopy.
The UV–Vis spectra of 2 and 9 in CH2Cl2 (Fig. 3)
revealed absorption bands present at 258, 327, 431 and
a very broad band centered at 480 nm, which are
well-known bands for 1,2-methano[60]fullerene com-
pounds. Since the two spectra do not differ substan-
tially from each other, there seems to be no influence of
the nickel center in 9 on the absorption behavior of the
fullerene unit in the ground state.
The redox potentials of C60, 2, and 9, respectively,
were measured by cyclic voltammetry in toluene/aceto-
nitrile solution (5:2 v/v %) with [(n-Bu)4N(PF6)] (0.1 M)
as the supporting electrolyte (Table 1). The first
1
perature H NMR spectroscopic experiments were per-
1
formed. The H NMR spectrum of ligands 1, 2 and 13
in toluene remained unchanged in a temperature range
of 298–173 K. However, for nickel complex 9, the
singlet resonance for the NMe2 groups decoalesced on
cooling into two singlet signals while simultaneously the
signal of the benzylic protons started to decoalesce in
an AB pattern (CD2Cl2). The DG‡ values, calculated
from the observed chemical shift differences and coales-
cence temperatures Tc, for the (dimethylamino) methyl
groups and the benzylic protons are 44.5 kJ mol−1
(Tc=215 K) and 44.0 kJ mol−1 (Tc=210 K), respec-
tively, indicating that both effects result from the same
process. Previous work showed that the wagging pro-
cess leading to inversion of the chelate ring puckering
in NCN·metal complexes has a lower barrier [44], i.e. is
still in the fast exchange limit. This leaves the rotation
of the NCN·Ni unit about the (Me)CꢀNCN bond in 9
as the origin for the observed fluxional process in the
molecule, i.e. rotation between the ground state ro-
tamers (Fig. 2(a)). This process is only observable in 9
due to the stable coordination of the nitrogen donor
atoms to nickel (on the NMR time scale), which leads
to a fixation of the benzylic and NMe2 groups with
respect to the NCN-aryl plane. In 1, 2, and 13 more
Fig. 3. UV–Vis spectra (CH2Cl2, 1×10−5 M) of 2 (bold line) and 9 (normal line).

M.D. Meijer et al. / Inorganica Chimica Acta 327 (2002) 31–40
39
Table 1
[4] (a) S. Friedman, D. DeCamp, R. Sijbesma, G. Srdanov, F.
Wudl, G. Kenyon, J. Am. Chem. Soc. 115 (1993) 6506;
(b) A. Boutorine, H. Tokuyama, M. Takashi, H. Isobe, E.
Nakamura, C. He´le`ne, Angew. Chem., Int. Ed. Engl. 33 (1994)
2462.
[5] (a) M. Prato, J. Mater. Chem. 7 (1997) 1097;
(b) F. Diederich, M. Go´mez-Lo´pez, Chimia (1998) 551;
(c) M. Prato, Top. Curr. Chem. (1999) 199.
[6] (a) C.S. Foote, Top. Curr. Chem. 169 (1994) 347;
(b) L. Echegoyen, L.E. Echegoyen, Acc. Chem. Res. 31 (1998)
593;
(c) C.A. Reed, R.D. Bolskar, Chem. Rev. 100 (2000) 1075.
[7] R.C. Haddon, Acc. Chem. Res. 25 (1992) 127.
[8] N. Armaroli, F. Diederich, C.O. Dietrich-Buchecker, L.
Flamigni, G. Marconi, J.-F. Nierengarten, J.-P. Sauvage, Chem.
Eur. J. 4 (1998) 406.
Electrochemical data for C60, 2 and 9 a,b
C60
2
9
−2.21
−2.21
−2.25
−1.64
−1.77
−1.78
−1.14
−1.19
−1.25
−0.71
−0.81
−0.82
c
0.81
0.83 c
a In toluene/acetonitrile (5:2) solution, with [(n-Bu)4N(PF6)] (0.1
M) as supporting electrolyte, measured at 0.1 V s−1
b E1/2 potentials are in volts vs. the Fc/Fc+ couple in the same
.
solvent.
c Irreversible wave, the peak potentials Eopx are given.
(fullerene) reduction wave of 2 is aniodically shifted by
100 mV with respect to C60, which is commonly ob-
served for methanofullerenes [5]. The reductions are
accompanied by an irreversible ligand oxidation at 0.81
V. While parent NCN·Ni complexes can be oxidized
[9] (a) H. Imahori, Y. Sakata, Adv. Mater. 9 (1997) 537;
(b) N. Martin, L. Sa´nchez, B. Illescas, I. Pe´rez, Chem. Rev. 98
(1998) 2527;
(c) H. Imahori, Y. Sakata, Eur. J. Org. Chem. (1999) 2445;
(d) D.M. Guldi, M. Maggini, N. Martin, M. Prato, Carbon 38
(2000) 1615;
(chemically)
reversible
to
the
corresponding
NCN·Ni(III) complexes [22], we did not observe a
similar reversible oxidation/reduction process for the
NCN·NiBr moiety in 9. Only an irreversible oxidation
was observed at 0.83 V. The absence of a Ni(II)/Ni(III)
redox couple indicates that the nickel(II) center in 9 is
influenced by the presence of the fullerene moiety.
Likewise, the nickel center causes small, but measurable
anodic shifts of some of the fullerene reduction poten-
tials: with respect to 2, the second and fourth fullerene
reduction potentials of 9 were shifted 60 and 40 mV,
respectively, whereas the other potentials were barely
shifted (ꢀ10 mV). The influence of the C60 moiety on
the redox potentials of NCNꢀNi in 9 (and vice versa) is
presently not understood. One might speculate that the
absence of the Ni(II)/Ni(III) couple is due to slow
electrode kinetics. Therefore, we will attempt to prepare
the nickel(III) complex of 9 through chemical oxida-
tion. This will be a subject for further investigations.
(e) D.M. Guldi, Chem. Commun. (2000) 321.
[10] R.M. Williams, J.M. Zwier, J.W. Verhoeven, J. Am. Chem. Soc.
117 (1995) 4093.
[11] (a) N. Mart´ın, I. Pe´rez, L. Sa´nchez, C. Seoane, J. Org. Chem. 62
(1997) 5690;
(b) J. Llacay, J. Veciana, J. Vidal-Gancedo, J.L. Bourdelande, R.
Gonza´lez-Moreno, C. Rovira, J. Org. Chem. 63 (1998) 5201;
(c) K.B. Simonsen, V.V. Konovalov, T.A. Konovalova, T.
Kawai, M.P. Cava, L.D. Kispert, R.M. Metzger, J. Becher, J.
Chem. Soc., Perkin Trans. 2 (1999) 657;
(d) M.A. Herranz, B. Illescas, N. Mart´ın, C. Luo, D.M. Guldi,
J. Org. Chem. 65 (2000) 5728;
(e) D.M. Guldi, S. Gonza´lez, N. Mart´ın, A. Anto´n, J. Gar´ın, J.
Orduna, J. Org. Chem. 65 (2000) 1978;
(f) N. Mart´ın, L. Sa´nchez, D.M. Guldi, Chem. Commun. (2000)
113;
(g) N. Mart´ın, L. Sa´nchez, B. Illescas, S. Gonza´lez, M.A.
Herranz, D.M. Guldi, Carbon 38 (2000) 1577.
[12] (a) H. Imahori, S. Cardoso, D. Tatman, S. Lin, L. Noss, G.R.
Seely, L. Sereno, J. Chessa De Silber, T.A. Moore, A.L. Moore,
D. Gust, Photochem. Photobiol. 62 (1995) 1009;
(b) P.A. Liddell, D. Kuciaukas, J.P. Sumida, B. Nash, D.
Nguyen, A.L. Moore, T.A. Moore, D. Gust, J. Am. Chem. Soc.
119 (1997) 1400;
Acknowledgements
(c) D. Carbonera, M. Di Valentin, C. Corvaja, G. Agostini, G.
Giacometti, P.A. Liddell, D. Kuciauskas, A.L. Moore, T.A.
Moore, D. Gust, J. Am. Chem. Soc. 120 (1998) 4398;
(d) D. Kuciauskas, P.A. Liddell, A.L. Moore, T.A. Moore, D.
Gust, J. Am. Chem. Soc. 120 (1998) 10880.
This work was supported by the Council for Chemi-
cal Sciences of the Netherlands Organization for Scien-
tific Research (CW-NWO).
[13] (a) M. Prato, M. Maggini, C. Giacometti, G. Scorrano, G.
Sandona`, G. Farnia, Tetrahedron 52 (1995) 5221;
(b) D.M. Guldi, M. Maggini, G. Scorrano, M. Prato, J. Am.
Chem. Soc. 119 (1997) 974;
(c) M. Maggini, A. Karlsson, G. Scorrano, G. Sandona`, G.
Farnia, M. Prato, J. Chem. Soc., Chem. Commun. (1994) 589.
[14] (a) N.S. Sariciftci, F. Wudl, A.J. Heeger, M. Maggini, G.
Scorrano, M. Prato, J. Bourassa, P.C. Ford, Chem. Phys. Lett.
247 (1995) 510;
(b) D. Armspach, E.C. Constable, F. Diederich, C.E. House-
croft, J.-F. Nierengarten, Chem. Eur. J. 4 (1998) 723;
(c) M. Maggini, D.M. Guldi, S. Mondini, G. Scorrano, F.
Paolucci, P. Ceroni, S. Roffia, Chem. Eur. J. 4 (1998) 1992.
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