Acetylenic NIR-chromophores: 2,3-dialkynyl-1,4-diazabuta-1,3-diene complexes of Ni(0) and Ni(II)

Article abstract of DOI:10.1016/S0022-328X(98)01122-X

Acetylenic diazabutadienes (DADs) were used to modulate the electron absorption properties of Ni(0) and Ni(II) chelate complexes with respect to those of non-acetylenic DAD metal coordination compounds. Alkynylated bis(N-aryl) DAD ligands are shown to induce efficient metal-to-ligand charge-transfer in coordination compounds of the type (DAD)₂Ni(0). These metal-ligand interactions manifest themselves as intense absorptions in the near infrared (NIR) around 800 nm and are significantly shifted batho- and hyperchromic compared to charge-transfer bands of non-acetylenic DAD metal complexes. Likewise, alkynyl substitution of bis(glyoximato)nickel(II) complexes results in a sizeable absorption band in the visible region of the electromagnetic spectrum.

Full text of DOI:10.1016/S0022-328X(98)01122-X

Journal of Organometallic Chemistry 578 (1999) 193–197
Acetylenic NIR-chromophores: 2,3-dialkynyl-1,4-diazabuta-1,3-diene
complexes of Ni(0) and Ni(II)
Ru¨diger Faust *, Bernd Go¨belt, Christian Weber
Pharmazeutisch-Chemisches Institut der Ruprecht-Karls-Uni6ersita¨t Heidelberg, Im Neuenheimer Feld 364, D-69120 Heidelberg, Germany
Received 20 July 1998
Abstract
Acetylenic diazabutadienes (DADs) were used to modulate the electron absorption properties of Ni(0) and Ni(II) chelate
complexes with respect to those of non-acetylenic DAD metal coordination compounds. Alkynylated bis(N-aryl) DAD ligands are
shown to induce efficient metal-to-ligand charge-transfer in coordination compounds of the type (DAD)2Ni(0). These metal–
ligand interactions manifest themselves as intense absorptions in the near infrared (NIR) around 800 nm and are significantly
shifted batho- and hyperchromic compared to charge-transfer bands of non-acetylenic DAD metal complexes. Likewise, alkynyl
substitution of bis(glyoximato)nickel(II) complexes results in a sizeable absorption band in the visible region of the electromag-
netic spectrum. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Alkynes; Charge-transfer; Diazabutadienes; Nickel complexes; UV–vis/NIR
1. Introduction
that extend into the near infrared (NIR). Substances
that absorb in the NIR region of the electromagnetic
spectrum are currently receiving much attention [9,10],
mainly emanating from two directions. On one hand,
NIR chromophores can interact with cheap laser diodes
(for example, in CD players) that emit between 750 and
830 nm and are hence promising materials for use in
optical data storage devices. Secondly, organic tissue is
relatively translucent in the NIR region, suggesting
applications of NIR chromophores as photosensitizers
in photodynamic tumour therapy [11–13].
Vicinal diimines (1,4-diazabuta-1,3-dienes, DADs),
long known for their multifaceted coordination be-
haviour to transition metal centers [1,2], have recently
re-emerged into the limelight due to their prominent
role as ligands in efficient catalyst systems for olefin
polymerization reactions [3–6]. A second intriguing
aspect of DAD metal complexes, particularly those of
nickel, can be found in their electron absorption prop-
erties. A well-known example is the intense and diag-
nostic red colour of solid bis(dimethylglyoximato)-
nickel(II) 1 used for gravimetric determinations of nick-
el(II) [7]. Similarly, tom Dieck et al. [8] have observed
that N-aryl substituted (DAD)2Ni(0) complexes like 2
exhibit sizable metal-to-ligand charge-transfer bands
* Corresponding author. Present address: University College
London, Department of Chemistry, Christopher Ingold Laboratories,
20 Gordon Street, London WC1 H0AJ, UK. Fax: +44-171-380
7463.
In order to modulate the electronic properties of
DAD transition metal complexes in general, and in an
effort to shift the electron absorptions of DAD–nickel
E-mail address: r.faust@ucl.ac.uk (R. Faust)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)01122-X

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R. Faust et al. / Journal of Organometallic Chemistry 578 (1999) 193–197
Scheme 1. Preparation of 1,6-bis(triisopropylsilyl)hexa-1,5-diyne-3,4-dionedioxime 3c.
complexes batho- as well as hyperchromically, we have
recently introduced acetylene substituents into the
DAD backbone and have prepared 2,3-dialkynyl-1,4-
diazabuta-1,3-dienes 3 [14,15]. An investigation of the
electronic properties of these new ligand systems [16]
has revealed that the electron-withdrawing nature and/
or their extended p system stabilize the LUMO of the
dialkynyl DAD ligands with respect to non-acetylenic
ones. Hence, the former should allow for efficient
metal-to-ligand charge-transfer when coordinated to
electron-rich transition metals. Reported herein are the
synthesis and the spectroscopic properties of Ni(0) and
Ni(II)-coordination compounds featuring these carbon-
rich ligand systems.
shows diagnostic 13C-NMR resonances in THF at 198.6
(CO), 103.8 and 100.5 (CꢀC) ppm. Upon refluxing 5a
with one equivalent of the chelating ligand 3a in THF
for 5 h the initially violet solution turned cherry-red.
Removal of the volatiles in vacuo and washing the solid
residue with acetone furnished the bis-chelated complex
6a as a red–brown, microcrystalline solid. In contrast
to the mono-DAD complex 5a, the (DAD)2Ni(0)
derivative 6a can be handled in air for short periods of
time and melts under decomposition at 285°C.
The isolation of a mono-DAD complex of Ni(0)
succeeds only when the N-aryl moieties of the ligand
bear methyl groups in positions 2 and 6. Thus, stirring
an ethereal solution of N,N%-bis(3,5-dimethylphenyl)-
substituted dialkynyl DAD ligand 3b (isomeric to 3a)
with two equivalents Ni(CO)4, did indeed furnish a
violet-coloured solution indicative of the intermediacy
of (DAD)Ni(CO)2 complex 5b. However, this colour
was persistent for only a few minutes and changed
rapidly to cherry-red, a tint indicating the formation of
the bis-DAD complex 6b. Attempts to isolate 5b in-
duced ligand exchange processes resulting in the forma-
tion of 6b and Ni(CO)4 as determined by 13C-NMR
spectroscopy. It is therefore more convenient to prepare
6b directly by refluxing two equivalents of 3b with
Ni(CO)4 in THF for 8 h. Both, 6a and 6b (m.p. 205°C,
decomp.) are fully characterized by their spectroscopic
(NMR, IR, UV, MS) and microanalytical data.
2. Results and discussion
The N-aryl substituted 2,3-dialkynyl-DADs 3a and
3b were prepared by condensing the corresponding
dimethylanilines with 1,6-bis(triisopropylsilyl)hexa-1,5-
diyne-3,4-dione 4 [17] in glacial acetic acid as previously
described [15]. Similarly, the new dialkynyl ethane-1,2-
bis(hydroximine) ligand 3c was obtained by reacting 4
with a 20-fold excess of hydroxylamine according to
Scheme 1.
Using a smaller hydroxylamine/4 ratio (e.g. a 3-fold
excess) led only to mono-imine formation, to the corre-
sponding a-hydroximinoketone. The dialkynyl dioxime
3c was obtained as a colourless solid that is stable to
the ambient and that melts without decomposition at
213°C.
With the dialkynyl diimine ligands 3a–c in hand,
their coordination behaviour towards Ni(0) and Ni(II)
was investigated (Scheme 2). At the outset, a major
point of concern was whether the steric hindrance
between the 2,3-alkynyl groups would prevent an s-cis
conformation of the DAD ligand, thereby precluding
chelation of a given transition metal center. Gratify-
ingly, this turned out not to be the case. Hence, stirring
a (yellow) solution of 3a with two equivalents of
Ni(CO)4 in diethyl ether at room temperature (r.t.) for
3 h resulted in a dark violet solution, from which the
(DAD)Ni(CO)2 complex 5a could be isolated quantita-
tively and analytically pure upon removal of the
volatiles in vacuo. Complex 5a is a violet, air sensitive
solid that melts under decomposition at 232°C and that
The bis(diethynylglyoximato)nickel(II) derivative 7
was readily obtained in 85% as a red–brown precipitate
formed by stirring
a
solution of 3c and
Ni(OAc)2 · 4H2O in EtOH at r.t. for 10 min (Scheme 2).
Analytically pure 7 is an air stable solid that melts
under decomposition at 217°C. The presence of hydro-
gen bridges between the two chelating diimine ligands
of 7 is revealed in its 1H-NMR spectrum by very
deshielded hydroxy hydrogen nuclei whose resonance is
observed at 17.51 ppm in CDCl3.
As stated in the introduction, the influence alkynyl
substitution of the DAD ligand system might have on
the electron absorption properties of the new nickel
complexes was of particular interest. Inspection of the
pertinent UV–vis data reveals that acetylenic diimine
ligands indeed lead to sizable and bathochromically
shifted transitions in all cases. For example, CH2Cl2
solutions of bis(dialkynylglyoximato)nickel(II) complex
7 feature two dominant absorption maxima at wave-

R. Faust et al. / Journal of Organometallic Chemistry 578 (1999) 193–197
195
Scheme 2. Preparation of bis(2,3-dialkynyl-1,4-diazabuta-1,3-diene) complexes of Ni(0) and Ni(II).
lengths of 276 and 422 nm with extinction coefficients
of 45 300 and 14 700 M−1 cm−1, respectively. The
non-acetylenic nickel(II)dioxime 1, on the other hand,
has just one pronounced absorption maximum at 262
nm (m=18 100 M−1 cm−1) and an additional, very
broad, low-intensity absorption centering around 370
nm (m=8900 M−1 cm−1) in CH2Cl2. It should be
noted that the characteristic red colour of solid 1 arises
longest wavelength absorption of 792 nm (m=9300
M−1 cm−1) for 6b.
In line with our expectations, the lowest energy
charge-transfer band of (DAD)2Ni(0) complexes is
drastically shifted into the NIR and considerably en-
hanced when positions 2 and 3 of the DAD backbone
are substituted by alkynyl groups. Comparing the elec-
tron absorptions of 6a with those of bis(N,N%-diphenyl-
2,3-diphenylethane-1,2-diimine)nickel(0) 8 [19] (Fig. 1)
in Et2O reveals that the latter has only a broad, low-in-
tensity absorption at 744 nm (m=2400 M−1 cm−1).
Similar data were reported for non-acetylenic bis
[N,N%-(2,6-dimethylphenyl)butane-2,3-diimine]nickel(0)
(umax=764 nm, m=13 100 M−1 cm−1 in C6H6) by tom
Dieck et al. [8]. These authors have also provided
spectroscopic and structural evidence [8] for the ten-
dency of N-aryl substituted (DAD)2Ni(0) complexes to
adopt a more planarized coordination sphere around
˚
from Ni–Ni interactions (u=554 A [18]) in the solid
state. The presence of the bulky triisopropylsilyl groups
in 7 apparently preclude close metal–metal contacts in
the solid and hence no bands beyond 422 nm are
observed.
The effect of alkynylation of the DAD ligand be-
comes even more apparent upon examination of the
electron absorption properties of the Ni(0) complexes
6a and 6b. Thus, the UV–vis/NIR spectrum of 6a in
Et2O is dominated by two low-energy absorptions at
531 and 810 nm with extinction coefficients of 18 000
and 16 700 M−1 cm−1, respectively. Reducing the elec-
tron density around the nickel center by moving
arylimino methyl groups from positions 2,6 to positions
3,5 leads to a hypso- and hypochromically shifted
Fig. 1. Electron absorption spectra of (DAD)2Ni(0) complexes 6a (solid line) and 8 (dashed line) in Et2O at r.t.

196
R. Faust et al. / Journal of Organometallic Chemistry 578 (1999) 193–197
the nickel in which efficient electronic delocalization
occurs across the transition metal center. At a qualita-
tive level, it is likely that the expansion of the ligand’s
p system by 2,3-dialkynyl substitution of the DAD
skeleton induces the bathochromic shift and leads to the
intense NIR absorptions observed for 6a and 6b. Com-
pounds like 8, on the other hand, do not benefit from an
extended p system, since the phenyl groups on positions
2 and 3 cannot be coplanar with the DAD backbone
[20] and hence nickel(0) complexes with these ligands do
not show pronounced transitions in the NIR.
This work clearly demonstrates that the beneficial
properties of alkynyl groups (i.e. small steric demand,
electronic interactions with adjacent p systems) can be
exploited for the rapid assembly of efficient NIR chro-
mophores from small acetylenic building blocks. Evi-
dently, an orbital-based theoretical investigation is
warranted to understand the interaction between transi-
tion metal centers and the p system of the 2,3-dialkynyl
DADs. Work along those lines is underway as are
investigations pertaining to the catalytic properties of
coordination compounds with this new ligand system.
and NaOAc (1.6 g, 20 mmol) were heated to 50°C in
acetic acid (5 ml) for 30 min. After cooling, the mixture
was poured into water and extracted with Et2O (3×100
ml). After drying the organic layer over Na2SO4, the
solvent was evaporated and the residue was chro-
matographed on silica gel (hexane:ethyl acetate 10:1) to
yield, after recrystallization from CHCl3, 3c (156 mg,
33%) as a colourless solid: m.p. 213°C. —IR (KBr):
w=3276 cm−1 (s, O–H), 2944 cm−1 (s, C–H), 2866
cm−1 (s, C–H), 2164 cm−1 (w, CꢀC), 1591 cm−1 (w,
CꢁN). —UV (CH2Cl2): umax (m)=232 nm (16 400), 256
nm (sh, 15 600). —1H-NMR (360 MHz, CDCl3): l=
11.66 (s, 2 H, OH), 1.13 (pseudo-s, 42 H, i-Pr). —13C-
NMR (90.56 MHz, CDCl3): l=139.3 (CꢁN), 105.9
(CꢀC), 94.7 (CꢀC), 18.9 (CH3), 11.9 (CH). —MS (EI),
m/z (%): 448 (16) [M+], 405 (100) [M+ −i-Pr], 377
(18), 349 (10). —C24H44N2O2Si2 (448.29): calcd. C
64.24, H 9.89, N 6.25; found C 64.15, H 9.81, N 6.15.
3.2. N,N%-Bis(2,6-dimethylphenyl)-1,6-bis(triisopro-
pylsilyl)hexa-1,5-diyne-3,4-diimine)nickel(0)-dicarbonyl
(5a)
To a solution of 3a [16] (310 mg, 0.5 mmol) in diethyl
ether (10 ml) was added Ni(CO)4 (170 mg, 1.0 mmol).
After stirring for 3 h at r.t., the volatiles were removed
in vacuo, leaving behind 5a (370 mg, quant.) as an
analytically pure, violet solid: m.p. 232°C (decomp.).
—IR (KBr): w=2943 cm−1 (m, CH), 2865 (m, CH),
2146 (w, CꢀC), 2012 (s, CꢁO), 1963 (s, CꢁO), 1591 (w,
CꢁN). —UV (Et2O): umax (m)=262 nm (15 200), 346 nm
(7600), 562 nm (9800). —1H-NMR (299.95 MHz,
CDCl3): l=7.2–7.0 (m, 6H, Aryl-H), 2.36 (s, 12H,
CH3), 1.2–0.9 (m, 42H, i-Pr). —13C-NMR (62.89 MHz,
THF/D2O): l=198.6 (CO), 151.4 (C), 143.9 (C), 128.4
(CH), 127.4 (C), 125.2 (CH), 103.8 (CꢀC), 100.5 (CꢀC),
18.1 (CH3), 17.2 (CH3), 11.0 (CH). —MS (FAB) m/z
3. Experimental
General: melting points were determined on a
Reichert hotstage and are uncorrected. UV–vis spectra
were recorded on Hewlett Packard HP 8452 or HP 8453
UV–vis ChemStation spectrometers. Infrared spectra
were obtained as KBr pellets or films on a Perkin–
1
Elmer PE 1600 FT-IR spectrometer. H-NMR spectra
were recorded at 300 MHz on a Varian XL 300 or at
250 MHz on a Bruker WM-250. 13C-NMR spectra were
measured at 62.9 MHz on a Bruker WM-250 spectrom-
eter, at 90.6 MHz on a Bruker AM 360, or at 75.4 MHz
on the Varian spectrometer described above. The degree
of carbon substitution was determined by J-modulated
spin echo experiments. l values are ppm downfield from
internal Me4Si. Mass spectra were obtained on a Varian
MAT-311 A mass spectrometer at 70 eV (EI) or on a
JEOL MS-700 (FAB, FD). Elemental analyses were
performed on a Foss–Heraeus Vario EL.
Solvents were purified and dried according to stan-
dard procedures [21]. Tetrahydrofuran was distilled
from sodium benzophenone immediately prior to use.
Silica gel (60–200 mesh) for column chromatography
was kindly provided by Merck KGaA, Darmstadt. All
reactions were performed in predried glass vessels in an
inert atmosphere under argon.
(%): 710.6 (50), 711.6 (43), 712.6 (64), 713.6 (50) [M+
−
CO], 682.6 (100) [M+ −2CO], 625.6 (60) [3a] 609.6 (53)
[3a-CH3]. —C42H60N2O2Si2Ni (739.83): calcd. C 68.19,
H 8.17, N 3.79; found C 67.89, H 8.34, N 3.53.
3.3. Bis[N,N%-bis(2,6-dimethylphenyl)-1,6-bis(triisopro-
pylsilyl)hexa-1,5-diyne-3,4-diimine]nickel(0) (6a)
To a solution of 5a (370 mg, 0.50 mmol) in THF (10
ml) was added 3a (310 mg, 0.55 mmol). The reaction
mixture was heated to reflux for 5 h, cooled to r.t., and
all volatiles were removed in vacuo. The residue was
washed with acetone (5×10 ml) and dried in vacuo to
furnish 6a as a red–brown, microcrystalline solid (615
mg, 95%): m.p. 285°C (decomp). —IR (KBr): w=3065
cm−1 (w, CH), 2941 (s, CH), 2864 (s, CH), 2131 (s,
CꢀC), 1583 (w, CꢁN). —UV (Et2O): umax (m)=371 nm
(17 300), 449 nm (13 000), 531 nm (18 000), 810 nm
(16 700). —1H-NMR (250.13 MHz, C6D6): l=7.0–6.6
3.1. 1,6-Bis(triisopropylsilyl)hexa-1,5-diyne-3,4-
dionedioxime (3c)
1,6-Bis(triisopropylsilyl)hexa-1,5-diyne-3,4-dione
[17] (418 mg, 1 mmol), NH2OH · HCl (1.4 g, 20 mmol)
4

R. Faust et al. / Journal of Organometallic Chemistry 578 (1999) 193–197
197
(m, 12H, Aryl-H), 1.99 (s, 24H, CH3), 0.8–0.5 (m, 84H,
i-Pr). —13C-NMR (62.89 MHz, THF/D2O): l=156.6
(C), 132.9 (C), 129.0 (C), 128.1 (CH), 125.2 (CH), 111.5
(CꢀC), 95.6 (CꢀC), 18.3 (CH3), 18.1 (CH3), 10.3 (CH).
—MS (EI), m/z (%): 1306.7 (8), 1307.7 (10), 1308.7 (9),
1309.7 (7) [M+], 609.4 (100) [3a-Me+], 312.2 (80)
[3a/2+]. —C80H120N4Si4Ni (1308.92): calcd. C 73.41, H
9.24, N 4.28; found C 73.29, H 9.26, N 4.23.
1.2–0.9 (m, 42 H, i-Pr).—13C-NMR (62.89 MHz,
CDCl3): l=133.6 (CꢁN), 111.9 (CꢀC), 93.4 (CꢀC),
18.6 (CH3), 11.1 (CH). —MS (FD), m/z (%): 952 (100)
[M+]. —C48H86N4NiO4Si4 (952.51): calcd. C 60.47, H
9.10, N 5.88; found C 60.47, H 9.08, N 5.58.
Acknowledgements
3.4. Bis[N,N%-bis(3,5-dimethylphenyl)-1,6-bis(triisopro-
pylsilyl)hexa-1,5-diyne-3,4-diimine]nickel(0) (6b)
Parts of this work were supported by the Fonds der
chemischen Industrie, Frankfurt (Liebig-fellowship to
R.F.). We are grateful to Professor Dr R. Neidlein for
his continuous interest in our work and thank Isabell
Hamm for skillful experimental assistance. We are in-
debted to Dr J. Gross, Institut fu¨r Organische Chemie,
Universita¨t Heidelberg, for performing mass spectro-
metric measurements.
To a solution of 3b [16] (310 mg, 0.5 mmol) in THF
(10 ml) was added Ni(CO)4 (170 mg, 1.0 mmol) and the
reaction mixture was heated to reflux for 8 h. During
this time, the mixture changed colour from an initial
yellow, over violet to cherry-red. After cooling to r.t.,
the volatiles were removed in vacuo. The residue was
washed with acetone (5×10 ml) and dried in vacuo to
furnish 6b as a red–brown microcrystalline solid (260
mg, 79%): m.p. 205°C (decomp.). —IR (KBr): w=3072
cm−1 (w, CH), 2942 (s, CH), 2864 (s, CH), 2134 (s,
CꢀC), 1607 (m, CꢁN), 1590 (m, CꢁN). —UV (Et2O):
umax (m)=364 nm (20 000), 406 nm (20 300), 548 nm
(22 400), 792 nm (9300). —1H-NMR (250.13 MHz,
C6D6): l=6.89 (s, 4H, Aryl-H), 6.60 (s, 8H, Aryl-H),
2.03 (s, 24H, CH3), 0.9–0.5 (m, 84H, i-Pr).—13C-NMR
(62.89 MHz, C6D6): l=158.9 (C), 137.8 (C), 131.3 (C),
126.7 (CH), 120.5 (CH), 111.5 (CꢀC), 96.8 (CꢀC), 21.1
(CH3), 18.7 (CH3), 10.7 (CH). —MS (FAB), m/z (%):
1306.1 (52), 1307.1 (54), 1308.1 (77), 1309.1 (82) [M+],
684.6 (73) [M+ −3b], 626.6 (100) [3b+], 312.2 [3b/
2+].—C80H120N4Si4Ni (1308.92): calcd. C 73.41, H
9.24, N 4.28; found C 73.03, H 9.26, N 4.22.
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Ni(OAc)2 · 4H2O (10 mg, 0.039 mmol) was dissolved
in refluxing ethanol (1 ml). The solution was cooled to
r.t., then mixed with a solution of 3c (35 mg, 0.078
mmol) in ethanol (1 ml). The mixture turned brown
and within 10 min a red–brown precipitate of 7
formed, which was collected on a filter, dried in vacuo
at 50°C, and recrystallized from Et2O (32 mg, 85%):
m.p. 217°C (decomp.). —IR (KBr): w=2944 cm−1 (s,
C–H), 2866 cm−1 (s, C–H), 2150 cm−1 (w, CꢀC), 1600
cm−1 (w, CꢁN). —UV (CH2Cl2): umax (m)=276 nm
(45 300), 376 nm (broad, 8900), 422 nm (14 700). —1H-
NMR (250.13 MHz, CDCl3): l=17.51 (s, 2 H, OHO),
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