Charge transfer photochemistry of Reinecke's salt and of some of its analogues containing organic amines

Article abstract of DOI:10.1016/S0020-1693(96)05343-1

N-substituted Reineckates of the compositions [CrA₂(NCS)₄]^- (A=NH₃, aniline, n-propylamine, morpholine) and [Cr(A-A)(NCS)₄]^- (A-A=en, tn) have been prepared. The ligand field absorption maxima were slightly shifted to longer wavelengths in the order NH₃- on charge-transfer excitation (254 nm). With exception of the aniline complex they were also decomposed to some extent, yielding Cr(II) and (NCS)₂^·- radicals. Approximately 10% of the photochemically formed Cr(II) was oxidised to chromate(VI) in air saturated methanol as was shown by comparison with chemically and radiolytically reduced [Cr(NH₃)₂(NCS)₄]^-. (NCS)₂^·- radical formation was observed by flash photolysis as well as by pulse radiolysis. Photochemical quantum yields for continuous irradiations of air saturated methanolic solutions were determined for the disappearance of the starting material, ranging from Φ_Cr=0.16 (aniline) to Φ_Cr=0.31 (en), for NCS^- release ranging from Φ_NCS=0.17 (aniline) to 0.52 (n-propylamine) and for chromate(VI) formation ranging from Φ_CrO4<0.001 (aniline) to 0.009 (NH₃, n-propylamine).

Full text of DOI:10.1016/S0020-1693(96)05343-1

Inorganica Chimica Acta 255 (1997) 87–93
Charge transfer photochemistry of Reinecke’s salt and of some of its
analogues containing organic amines 1
Bernadette Mainusch a, A. Karocki b, D.M. Guldi c, Z. Stasicka b, F. Wasgestian a
a University of Cologne, Institute of Inorganic Chemistry, D-50939 Ko¨ln, Germany
b Jagiellonian University, Faculty of Chemistry, PL-30-060 Krako´w, Poland
c University of Notre Dame, Radiation Laboratory, Notre Dame, IN 46556-0579, USA
Received 28 November 1995; revised 15 April 1996
Abstract
N-substituted Reineckates of the compositions [CrA2(NCS)4]y (AsNH3, aniline, n-propylamine, morpholine)and[Cr(A-A)(NCS)4]y
(A-Asen, tn) have been prepared. The ligand field absorption maxima were slightly shifted to longer wavelengths in the order NH3-en,
tn-n-propylamine-morpholine-aniline. All complexes dissociated NCSy on charge-transfer excitation (254 nm). With exception of the
aniline complex they were also decomposed to some extent, yielding Cr(II) and (NCS)2xy radicals. Approximately 10% of the photochem-
ically formed Cr(II) was oxidised to chromate(VI) in air saturated methanol as was shown by comparison with chemically and radiolytically
reduced [Cr(NH3)2(NCS)4]y. (NCS)2xy radical formation was observed by flash photolysis as well as by pulse radiolysis. Photochemical
quantum yields for continuous irradiations of air saturated methanolic solutions were determined for the disappearance of the starting material,
ranging from FCrs0.16 (aniline) to FCrs0.31 (en), for NCSy release ranging from FNCSs0.17 (aniline) to 0.52 (n-propylamine) and
for chromate(VI) formation ranging from FCrO -0.001 (aniline) to 0.009 (NH3, n-propylamine).
4
Keywords: Photochemistry; Charge transfer; Chromium complexes; Thiocyanato complexes; Alkylamine complexes
1. Introduction
chosen as an example of a primary alkylamine and morpho-
line of a secondary amine. Aniline was selected as a repre-
sentative of the arylamines.
K[Cr(NH3)2(NSC)]4, potassium Reineckate, was intro-
duced as a chemical actinometer by Wegner and Adamson
[1] in 1966. It has been widely used for quantum yield
determinations in the visible region [2,3]. Its ligand field
photochemistry has been studied several times [1,4–10],
while studies on the charge transfer photochemistry of
Reineckates are less frequent. Apart from a study of the sol-
vent effect in alcoholicsolutions, whichincludedsomeexper-
iments performed at 256 nm [5] and a preliminary
communication of this work [11] no other reports are known
to us. The photolysis of organo Reineckates has not been
studied before.
The present paper focuses on the charge transfer photo-
chemistry of Reinecke’s salt and its analogues. In Reinecke’s
salt (ammonium Reineckate) the trans configuration has
been established by X-ray diffraction [12]. In contrast the
bidentate ligands 1,2-diaminoethane (en) and 1,2-diamino-
propane (tn) force the cis configuration. n-Propylamine was
2. Experimental
Reinecke’s salt NH4[Cr(NH3)2(NCS)4]PH2O [13] and
the aniline analogue NH4[Cr(aniline)2(NCS)4] [14] were
prepared by literature methods.
The n-propylamine Reineckate was prepared by a modifi-
cation of the method of Ganescu et al. [15]. 30 ml of the
amine (Fluka), freshly distilled over KOH, and 8.1 g (15.6
mmol) K3[Cr(NCS)6] (Alfa-Productions), dehydrated for
4 h at 110–1208C, were refluxed for 5 h. The mixture was
poured into 300 ml of 10% ice cold acetic acid that had been
precooled to 08C. The pink flaky precipitate was separated
by filtration.
The crude Reineckate was transformed into the tetra-
butylammonium salt by dissolving it in acetonitrile and sub-
sequent dropping into a 3% aqueous solution oftetrabutylam-
monium thiocyanate of 08C. The oily product was separated
and washed with toluene. This product was chromatographed
1 Dedicated to the 75th anniversary of Frau Professor Dr Marianne
Baudler.
0020-1693/97/$17.00 q 1997 Elsevier Science S.A. All rights reserved
PII S0020-1693(96)05343-1

88
B. Mainusch et al. / Inorganica Chimica Acta 255 (1997) 87–93
in chloroform on silica gel 60 (grain 0.062–0.2 mm) Merck.
Three fractions were obtained. The first fraction was a light
violet colour and contained the starting material
[Cr(NCS)6]3y. This solution was discarded. The second
fraction was red–violet and yielded the desired tetrathiocyan-
ato complex. The third fraction was a red colour caused by
the tristhiocyanatotris-n-propylamine complex. The second
fraction was concentrated and separated from colloidal silica
by centrifugation. After evaporation of the solvent the still
oily product was dissolved in ;10 ml of an ethanol/aceton-
itrile mixture (10:1 vol./vol.) and injected into 100 ml of
cold water by pipette. The products could be filtered more
easily if the suspension was frozen and rethawed again. The
purified product was sufficiently free of uncomplexed NCSy
(F3%). The yield of the tetrabutylammonium salt after the
chromatographic purification amounted to 0.96 g. Anal.
Found: Cr, 8.0; C, 48.1; H, 8.3; N, 14.9. Calc.: Cr, 8.1; C,
48.5; H, 8.4; N, 15.2%
UV–Vis spectra were recorded with Cary 2300 and Shi-
madzu 2100 instruments. IR spectra were obtained on a
Nicolet 5PC FT-IR spectrometer.
Ligand field irradiations were performed using equipment
described before [18]. 3 ml samples were irradiated at 238C
with 436 nm light of a high pressure mercury lamp (HBO
100) in combination with a Schott interference filter. The
charge transfer excitation (254 nm) was carried out with a
low pressure mercury lamp (Philips PLS9 WTUV) without
filtering. The absorbed quanta were determined either with a
bolometer [18] or by iron oxalate actinometry [19].
Flash photolysis was done with the flash spectroscopic
equipment of the Chemical Faculty of the Jagiellonian Uni-
versity [20,21]. The unfiltered light from a xenon lamp was
used with flashes characterised by 400 J energy per pulse and
25 ms duration.
For the continuous g-irradiations the 60Co source of the
Institute of Physical Chemistry, University of Cologne, was
used. The dose rate was determined by Fricke dosimetry and
amounted to 1.35=105 rad/h.
The morpholine Reineckate was prepared by the same
method.
Pulse radiolysis experiments were performed by utilising
either 500 ns pulses of 1.55 MeV electrons from a Van de
Graaff accelerator or 50 ns pulses of 15 MeV electrons from
a linear electron accelerator (LINAC). Basic details of the
used equipment and the data analysis have been described
elsewhere [22,23]. Dosimetry was based on the oxidation of
NCSy to (NCS)2xy, which takes place in aqueous solution
with Gf6( G denotes the number of species per 100 eV or
the approximate mM concentration per 10 J absorbed
energy). The radical concentrations generated per pulse
amounted to (1–3)10y6 M for all the investigated systems.
Reductive conditions were obtained in N2 saturated solutions
containing 1 vol.% of 2-propanol or t-butanol, i.e. hydrated
electrons and a-hydroxy alkyl radicals. On the other hand,
saturation with N2O in the absence of other additives exclu-
sively led to the formation of the highly oxidative OH
radicals.
K[Cr(en)(NCS)4] was obtained in a three-step synthesis
worked out by Weinmann [16]. (i) [Cr(ox)(en)2]-
[Cr(ox)2(en)]: 100 g (0.21 mol) K3[Cr(ox)3]P3H2O, pre-
pared according to the literature [17], was dissolved in 120
ml of hot water. Then 30 ml of freshly distilled ethylenedi-
amine were added. The solution was stirred for 2 h at 608C.
During that time a pink solid wasformedwhichwasseparated
by filtration and washed with ether (yield 51 g). (ii)
[CrCl2(H2O)2(en)]Cl: 48 g (0.09 mol) [Cr(ox)(en)2]-
[Cr(ox)2(en)] was dissolved in 72 ml conc. HCl and was
vigorously stirred until the salt was completely dissolved.
During that time the temperature increased to 408C. Then the
solution was cooled to 58C and kept for 2 days at this tem-
perature. The dark violet crystals of [CrCl2(en)2]Cl were
removed by filtration. Afterwards the solution was cooled
below y128C in an ice sodium chloride bath and HCl gas
was introduced until the colour changed from red–violet to
blue–violet. The solution was kept for another 2 days at 58C
and separated from the solid chromium chloride. Several
days later the blue–violet product crystallized from the cold
solution. (iii) K[Cr(en)(NCS)4]: 5 g (80.02 mol)
[CrCl2(H2O)2(en)]Cl was dissolved in 10 ml H2O and fil-
tered. Then a solution of 8 g (0.08 mol) KNCS in ;5ml
H2O was added. The mixture was warmed to 458C and kept
at this temperature until the colour changed to dark red. A
red product was formed upon cooling. It was recrystallized
from methanol. Yield 1 g. Anal. Found: Cr, 14.8; C, 18.9; N,
22.7. Calc.: Cr, 13.6; C, 18.8; N, 21.9%.
The amount of liberated NCSy was spectrophotometri-
cally determined with Fe3q by monitoring the absorbance at
460 nm in H2O/MeOH 4:1 (vol./vol.) [1,24] (apparent
molar absorbancy 4.30=103 My1 cmy1). The unreacted
starting material was removed prior to the analytical deter-
minations. The fact thatthetetrabutylammoniumReineckates
are only sparingly soluble in water was used to precipitate
them from the methanolic solutions by adding water. 1 ml
aliquots were mixed with 4 ml of water. If the suspensions
were frozen (y188C) and rethawed, they were easier to
filter. The potassium Reineckate was precipitated as
described for [Cr(NCS)6]3y by Wegner and Adamson [1].
By these methods the starting materials were precipitated
quantitatively whereas the reaction products remained in
solution. Tests with solutions that contained tetrabutylam-
monium Reineckate of the same concentration as used in the
photochemical experiments and known concentrations of
NCSy (1.3=10y5 to 2.6=10y4 M) gave the same results
as in the absence of the Reineckate (experimental error
K[Cr(tn)(NCS)4] was obtained by a preparation analo-
gous to that of K[Cr(en)(NCS)4].
The potassium salts were transformed into the tetrabutyl-
ammonium salts by dropping a solution of the Reineckates
into an ice cold saturated aqueous solution of tetrabutylam-
monium thiocyanate.

B. Mainusch et al. / Inorganica Chimica Acta 255 (1997) 87–93
89
Table 1
UV–Vis spectral data of Reineckate complexes [Cr(A)2(NCS)4]y (Asamine)
Amine
Solvent
Absorption maxima, l (nm) (e (l moly1 cmy1))
2NH
H2O
519 (105), 391 (89), 308 (14.9=103)
522 (101), 396 (83)
3
CH3CN
MeOH
CH3CN
MeOH
CH3CN
MeOH
CH3CN
MeOH
CH3CN
MeOH
CH3CN
MeOH
523 (110), 396 (91), 308 (18.3=103), 235 (26.5=103)
549 (164), 400sh
2 aniline
548 (165), 400sh, 313 (28.2=103), 249 (45.0=103)
529 (114), 402 (87)
2 n-propylamine
527 (122), 401 (95), 310 (19.2=103)
544 (140), 410 (110)
2 morpholine
544 (150), 410 (112), 318 (23.4=103)
528 (135), 399 (96)
en
tn
526 (140), 399 (100), 310 (14.8=103)
527 (128), 396 (91)
525 (139), 394 (107), 309 (16.7=103)
"3%). The filtrates of the freshly prepared solutions were
completely transparent in the region of the ligand field bands,
whereas filtrates of irradiated solutions and of samples that
had been kept in the dark for several hours did not absorb in
this region.
Deaerated solutions were obtained by the freeze pump
technique orbybubblingnitrogenthroughthesolutions.They
were transferred to the irradiation cellswithanitrogenpurged
syringe.
10y6 to 10y5 sy1. Hence, the dark reaction did not interfere
with the time scale of the photoreaction (F1 h). The organo
Reineckates were less stable in aqueous solution. In metha-
nol, however, their solvolysis rate was sufficiently reduced
to permit photochemical studies. A solution kept in the dark
for 1 h decomposed less than 1%. The following order of
stability was observed:
NH3)en;tn)n-propylamine)morpholine)aniline
Upon ligand field excitation of Reineckates, NCSy sub-
stitution is the main reaction path [1,5,6,10]. The spectral
changes are shown in Fig. 1. The reaction caused a batho-
chromic shift accompanied by a decrease in absorbance of
both spin allowed ligand fieldbands. Thechargetransferband
at 308 nm was not shifted but displayed a stronger decrease
relative to the bleaching of the ligand field bands. The CT
absorptionbandat 220nmwasshiftedhypsochromicallywith
an intermediate decrease in absorbance.
Irradiation into the charge transfer bands produced differ-
ent spectral changes (Fig. 2). The wavelength shift of the
ligand field bands was smaller and additional absorption
appeared in the region of the second ligand field band around
380 nm. The new absorption obviously derived from the
formation of chromate(VI) in a reaction with O2 as was also
observed in the CT photolysis of hexacyanochromate(III)
[33–35]. Chromate(VI) has an intense band at 372 nm and
its formation was confirmed by its colour reaction with
diphenylcarbazide. In deaerated solutions the chromate(VI)
formation was significantly reduced. We attributetheremain-
ing formation to the presence of traces of oxygen.
Ammonia release waschecked byobservingthepHchange
in aqueous solution. Liberated ammonia should increase the
pH. Irradiation with 254 nm light, however, resulted in a
slight decrease in pH from pH 6.2"0.1 to pH 5.6"0.2
(3 ml of a 2=10y3 M solution irradiated for 8 min with
1.5=10y3 einstein ly1). Ammonia release might have been
obscured by decomposition products of NCSy. Sole NCSy
release, however, is predicted by Adamson’s rules [36]. In
the case of trans-[Cr(en)2(NCS)2]q BifanoandLinck[37]
3. Results and discussion
The UV–Vis spectral data are compiled in Table 1. In con-
trast to the ammine-halogeno-chromium complexes the
geometry of the ligand arrangements does not affect the spec-
tral properties seriously. The en and tn complexes, being in
the cis configuration, have a slightly stronger ligand field
absorbance than the respective NH3 and n-propylamine com-
plexes. The cis complexes, lacking a centre of symmetry, are
expected to absorb more intensively than the analogous trans
complexes [25]. Since the molar absorbance of the n-pro-
pylamine complex is significantly lower than the absorbance
of the en and tn complexes, we assumethatthen-propylamine
complex is in the trans configuration. The ligand field
strength decreased in the order: NH3-en;tn;n-propylam-
ine-morpholine-aniline. Thecorrespondingnephelauxetic
series showing the decrease in the interelectronic repulsion
parameter B was: aniline)tn)en)NH3)morpholine)
n-propylamine. With the exception of the aniline and mor-
pholine complexes the wavelength of the first charge transfer
maximum (309"1 nm) remained almost unaffected by the
ligand variation with an average molar absorbance of
(17"2)=103 My1 cmy1 in methanol. This absorption has
been assigned to derive from a ligand to metal charge transfer
transition [26].
The thermal solvolysis of the parent compound is well
documented [4,27–32]. Room temperature rate constantsfor
aqueous and alcoholic solutions were found in the range of

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B. Mainusch et al. / Inorganica Chimica Acta 255 (1997) 87–93
Fig. 1. Spectral changes upon irradiation into the ligand field region of
Bu4N[Cr(NH3)2(NCS)4] after 0, 4, 8, 12 and 16 min of irradiation of a
2=10y3 M methanolic solution with 1.4=10y4 einstein miny1 ly1 at
436 nm at 238C: (a) ligand field bands; (b) charge transfer bands.
Fig. 2. Spectral changes upon irradiation into the charge transfer region of
Bu4N[Cr(NH3)2(NCS)4] after 0, 2, 4 and 6 min of irradiation of 3 ml of a
2=10y3 M methanolic solution with 1.9=10y4 einstein miny1 ly1 at
254 nm at 238C: (a) ligand field bands; (b) charge transfer bands.
and Balzani and co-workers [38] found Hq consumption
against Adamson’s rules. The labilisation of the chromium
amine bond has been explained by a stronger Cr–NCS bond
due to p-bonding [37].
Since the absorption at 254 nm originates from a ligand to
chromium transition, the immediate reaction product is sup-
posed to be a chromium(II) species. Thus we assume the
following mechanism:
a solvent separated radical pair. The labile Cr(II) complex is
supposed to be solvolysed completely before it is reoxidised.
x
The NCS radical is stabilised by forming the dimer radical
(Eq. (2)). The fate of the dimer radical cannot be established
beyond doubt. Baxandale et al. [39] found disproportiona-
tion (Eq. (5b)). Simple reduction (Eq. (5a)), however,
cannot be ruled out on account of its high reduction potential
[40] (E0s1.33 V versus NHE). If it is reduced, four NCSy
will be found in solution. In the other case, there will be three
NCSy.
In order to check whether O2 will oxidize chromium(II)
to chromate(VI) under our experimental conditions, we
reduced [Cr(NH3)2(NCS)4]y with metallic zinc in deaer-
ated methanolic solution. After exposure to air the absorption
at 380 nm gradually developed and chromate(VI) was
detected by its reaction with diphenylcarbazide.
NCSxqNCSy™(NCS)x2y
(2)
(3)
(4)
2q
Cr qO2q4OHy™CrO24yq2H2O
solv
Reineckate may also be reduced with methanol radicals
and solvated electrons obtained by 60Co-g-irradiation of
methanolic N2 saturated solutions. The spectral changeswere
similar to the ones observed under CT excitation of deaerated
solutions. In the ligand field region both bands decreased in
absorbance. A 2=10y3 M solution of potassium Reineckate
that was irradiated for 80 min showed a decrease of the first
2q
3q
2Cr q1/2O2qHq™2Cr qOHy
solv
solv
ey
(NCS)x2y™2NCSy
2(NCS)x2y2 ™(NCS) q2NCSy
The radical pair may either produce xNCS radialsor NCSy.
Reaction (1a) may proceed either via internal conversion
into the ligand field system or by back electrontransferwithin
(5a)
(5b)
ligand field band of 3.3%. In this solution 3.8=10y4
M
2q
NCSy and 6.5=10y5 MCr
were found. An aliquot of

B. Mainusch et al. / Inorganica Chimica Acta 255 (1997) 87–93
91
Table 2
Photochemical quantum yields of Reineckates, tetrabutylammonium salts in air saturated methanolic solutions at 238C, if not otherwise stated
Amine
Irradiation wavelength (nm)
FCr
FNCS
FCrO
Remarks
deaerated
4
2NH
436
254
254
254
254
254
254
254
254
0.18
0.29
0.19
0.44
0.60
0.42
0.17
0.52
0.40
0.46
0.48
3
0.009
0.003
0.004
-0.001
0.009
0.006
Kq salt in H2O
2 aniline
2 n-propylamine
2 morpholine
en
tn
0.16
0.30
0.31
0.006
0.005
the irradiated solution that was exposed to air contained
0.59=10y5 M chromate(VI).
(ii) in deaerated solutions a further increase of FNCS was
observed;
(iii) the organo Reineckates behaved similarly to the par-
ent compound;
(iv) the anilino Reineckate did not exhibit any redox pho-
tochemistry. In this case the quantum yield for NCS release
was the same as that for the disappearance of the starting
material.
The charge transfer state probably involves the oxidation
of the aniline ligand from which the energy is transferredinto
the ligand field system.
These experiments confirmed that Cr(II) is indeed oxidi-
sed to chromate(VI) in air saturated methanolic solutions.
Its yield is approximately 10%. The formation of free NCSy
is slightly higher than is accounted for by the dissociation of
all four NCSy ligands of the reduced Reineckate ion
(2.6=10 My4). The excess NCSy in the deaeratedsolutions
might be attributed to Cr(II) catalysed solvolysis of
[Cr(NH3)2(NCS)4]y.
Photochemical quantum yields were determined by the
following methods:
The reaction mechanism (Eqs. (1)–(5)) postulates the
x
formation of NCS radicals which react in a diffusion con-
(i) the chromium content in solution after precipitating
the unphotolysed complex (FCr);
trolled reaction with NCSy ions to the relatively long lived
xy
(ii) the liberated thiocyanate determined with Fe3q
(FNCS);
(NCS)2 radicals. The transient differential absorption
spectra obtained both by flash photolysis and pulse radiolysis
were compatible with each other and showed one band in the
visible region with a maximum at about 480 nm. This wave-
length was reported for the absorption of the (NCS)2xy rad-
ical [41–48] (Fig. 3). The radical life time, being of the
order of hundreds of ms, was increased in the absence of
oxygen or in the presence of excess NCSy and influenced by
thenatureoftheamineligandorsolvent(Table 3).Theinitial
radical concentrations were determined on the basis of a
molar absorbance of 7600 My1 cmy1 [44]. The permanent
decrease in CT absorption at 380 nm after the flash was used
to estimate the number of absorbed quanta. Assuming that
the photochemical reaction products do not absorb at 308 nm,
the decrease in absorbance gives a lower limit for the disap-
(iii) chromate(VI) by its colour reaction with diphenyl-
carbazide (
FCrO
).
4
Table 2 presents the results obtained with irradiation times
between 2 and 8 min. It shows that:
(i) the photochemical quantum yields were higher and the
ratio of FNCS/FCr was larger on charge transfer excitation
than on ligand field excitation;
Table 3
xy
Efficiency of the (NCS)2 radical formation in the flash photolysis of
5=10y5 to 1=10y4 M n-Bu4N[Cr(amine)(NCS)4] in methanol, if not
otherwise stated
Amine
Dcrad/Dcspec
t1/2 (ms)
Remarks
2NH
2NH
2NH
tn
230
380
540
250
350
130
135
in water
3
0.11
0.15
0.05
0.08
3
xy
Fig. 3. Spectrum of the (NCS)2 radical recorded by: flash photolysis of
deaerated
3
5=10y5 to 2=10y4 M (Bu4N)[Cr(NH3)2(NCS)4] in methanol (contin-
uous line, average of 7 experimental spectra); pulse radiolysis of 1.0=10y4
M aqueous solution of K[Cr(NH3)2(NCS)4], N2O saturated (black cir-
cles); flash photolysis of aqueous 1=10y4 M KNCS reported by Dogliotti
and Hayon [42] (open circles).
tn
1.2=10y2 M KNCS
2 benzyl
2 n-butyl

92
B. Mainusch et al. / Inorganica Chimica Acta 255 (1997) 87–93
pearance of the starting materialDcspec. The ratio of the initial
radical concentration to Dcspec is included in Table 3. Dcspec
was also determined in the experiments with continuous irra-
diation at 254 nm. Quantum yields based on those values
amounted to Fspecs0.26 and 0.25 for [Cr(NH3)2(NCS)4]y
and [Crtn(NCS)4]y in methanol, respectively. Although
white light was used in the flash experiments, absorption
within the ligand field bands can be neglected on account of
their low absorbance. If we assume a constant quantum yield
for excitation within the long wavelength CT band, the quan-
tum yield for the radical production Frad can be estimated.
We obtained Frads0.03 and 0.01 from the data of Table 2
for the diamine and tn tetrathiocyanatochromates in metha-
nol, respectively.
Pulse radiolysis under reductive conditions showed the
characteristic absorption of the hydrated electron at 720 nm.
Its decay (t1/2f1 ms) concomitantly occurred with ground
state bleaching in the LMCT region of the chromium com-
plex. Thus we assign the underlying reaction to the reduction
of the Reineckate by the hydrated electron. After the electron
initiated reduction was completed (Fig. 4, curve a) the
absorption time profiles displayed consecutive CT bleaching
resembling the one found upon UV irradiation (Fig. 2). This
slower reaction was completed within 30 ms (Fig. 4, curve
b). There was no difference between the reaction run in 2-
propanol and t-butanol. This rules out reduction by
a-alkoxyradicals which might take place in 2-propanol.
Hence, we assign this secondary change to NCSy release
from the reduced chromium complex in analogy to Eq. (1b).
This reaction brings about a decrease of the NCS™Cr charge
transfer absorption. Cr(II) catalysed solvolysis can be ruled
out in the ms time scale.
icant absorption, by a monomeric NCSx radical or by its OH
adduct, can be ruled out on account of their short life time
and their low molar absorptivity [47,49].
Simultaneously with the disappearance of the 340 nm tran-
sient a strong absorption at 480 nm appeared (Fig. 3). Its
spectral features closely resembledthose observedintheflash
experiments, where the formation of (NCS)2xy radicals has
been established. We tentatively assign the absorption at 340
nm to a coordinated NCSx radical on account of its high
absorbance.
[CrIII(NH32) (NCS) (xNCS)]q2H O
3
2
™(NCS)x2yq[CrIII(NH ) (NCS) (H O) ]q (6)
23
222
The (NCS)2xy radical could be formed either in an inter-
or intramolecular reaction, such as Eq. (6). To investigate a
case where any intramolecular contribution could be ruled
out, we decided to study the mono-thiocyanato complex.
[Cr(NH3)5(NCS)](ClO4)2. Upon pulse radiolysis under
similar experimental conditions no absorption was found that
xy
might be ascribed to any (NCS)2 formation. This result
xy
shows that the (NCS)2 radical is probably formed from
Reineckates via an intramolecular process, if free NCSy is
absent. Kirk and co-workers [50,51] argued similarly with
respect to the dithiocyanate radical formation in the UV pho-
tochemistry of cobalt thiocyanato complexes.
The quantum yields of the various products may be ration-
alised in the reaction scheme as outlined in Eqs. (1)–(5). If
we assume that the intermediately formed Cr(II) species
dissociates before it is reoxidised, the apparent quantum
yields are:
FCrsw1qw2; FNCSsw1qnw2, 3FnF4
(7)
The pulse radiolysis under oxidative conditions (i.e. N2O
saturated aqueous solutions) also produced CT bleachingbut
in addition a strong absorption appeared around 340 nm. This
absorption disappeared within 10 ms after its formation. This
transient absorption can be either assigned to an OH adduct
with the Reineckate or to a coordinated xNCS radical. Signif-
where ns3, if (NCS)2xy disproportionates (Eq. (5b)) and
ns4, if it is quantitatively reduced to 2NCSy (Eq. (5a)).
The experimentally determined quantum yields for
[Cr(NH3)2(NCS)4]y (FCrs0.29, FNCSs0.44) give
w1s0.215 and w2s0.075 ifns3 and w1s0.24andw2s0.05
if ns4. Since n is unknown, we assume that the true values
are in between the extremes and write: w1s0.23"0.05,
w2s0.06"0.02. In this scheme the quantum yield for radical
formation should be equal to w2. The experimental value of
Frads0.03 is not too far off this estimate. Since on air oxi-
dation approximately 10% of the Cr(II) is oxidised to
CrO42y, the quantum yield of 0.009 for chromate formation
should correspond to FCr(II)sw2s0.09 in reasonable agree-
ment with the postulated mechanism. Inthecaseoftheanilino
Reineckate, which did not produce radicals on CT excitation,
reaction (1b) is inefficient (w2s0). Then FCr should be
equal to FNCS as was observed.
Acknowledgements
Fig. 4. Transient differential absorption spectrum obtained upon radiolysis
of 1.0=10y4 M K[Cr(NH3)2(NCS)4] in a nitrogen saturated aqueous
solution at pH 7, containing 5 vol.% t-butanol: (a) 1 ms, (b) 30 ms after the
pulse.
We thank Dr W. Kohnen, Institute of Physical Chemistry,
University of Cologne for running the g-irradiations and for

B. Mainusch et al. / Inorganica Chimica Acta 255 (1997) 87–93
93
[20] T. Jarzynowski, T. Senkowski and Z. Stasicka, Pol. J. Chem., 55
(1981) 3.
[21] A. Karocki, unpublished results.
[22] K.-D. Asmus, Methods Enzymol., 105 (1984) 167.
[23] E. Janata, Radiat. Phys. Chem., 40 (1992) 437.
[24] E. Zinato, R.D. Lindholm and A.W. Adamson, J. Am. Chem. Soc., 91
(1969) 1076.
[25] A.B.P. Lever, Inorganic Electronic Spectroscopy, Elsevier,
Amsterdam, 2nd edn., 1984, p. 167.
[26] H.-H. Schmidtke, Ber. Bunsenges. Phys. Chem., 71 (1967) 1138.
[27] A. Lodzinska and P. Kita, Rocz. Chem., 47 (1973) 1349.
[28] P. Kita and A. Lodzinska, Rocz. Chem., 48 (1974) 379.
[29] P. Kita and A. Lodzinska, Rocz. Chem., 48 (1974) 565.
[30] P. Kita, Rocz. Chem., 50 (1976) 1051.
good advice in radiolytical problems. The authors appreciate
the cooperative support of Professor Asmus, University of
Notre Dame, in the use of the pulse radiolytic apparatus. The
Polish–German cooperation was supported by the Deutscher
Akademischer Austauschdienst and the Jagiellonian Univer-
sity of Krakow. Financial assistance by the Fonds der Chem-
ischen Industrie is gratefully acknowledged. Part of this work
was supported by the Office of Basic Energy Sciences of the
Department of Energy (this is contribution No. 3880 from
the Notre Dame Radiation Laboratory).
[31] P. Kita and R. Kostrzewska-Zeidler, Pol. J. Chem., 33 (1979) 1183.
[32] P. Kita and A. Swinarski, Z. Anorg. Allg. Chem., 464 (1980) 195.
[33] A. Marchaj and Z. Stasicka, Polyhedron, 2 (1983) 485.
[34] A. Marchaj, Z. Stasicka and D. Rehorek, Polyhedron, 2 (1983) 1281.
[35] G. Stopa and Z. Stasicka, Polyhedron, 3 (1984) 247.
[36] A.W. Adamson, J. Phys. Chem., 71 (1967) 798.
[37] C. Bifano and R.G. Linck, Inorg. Chem., 13 (1974) 609.
[38] D. Sandrini, M.T. Gandolfi, L. Moggi and V. Balzani, J. Am. Chem.
Soc., 100 (1978) 1463.
[39] J.H. Baxendale, P.L.T. Bevan and D.A. Stott, Trans. Faraday Soc., 64
(1968) 2389.
[40] D.M. Stanbury, Adv. Inorg. Chem., 33 (1989) 69.
[41] G.E. Adams, J.W. Boag and B.D. Michael, Trans. Faraday Soc.,
(1965) 1674.
References
[1] E.E. Wegner and A.W. Adamson, J. Am. Chem. Soc., 88 (1966) 394.
[2] A.W. Adamson and P.D. Fleischhauer (eds.), Concepts of Inorganic
Photochemistry, Wiley, New York, 1975.
[3] V. Balzani and V. Carassiti, Photochemistry of Coordination
Compounds, Academic Press, London, 1970.
[4] C.H. Langford and J.F. White, Can. J. Chem., 45 (1967) 3049.
[5] A. Lodzinska and S. Szczepanski, Rocz. Chem., 46 (1972) 1473.
[6] M. Cusumano and C.H. Langford, Inorg. Chem., 17 (1978) 2222.
[7] N.J. Shipley and R.G. Linck, J. Phys. Chem., 84 (1980) 2490.
[8] A.W. Adamson and A.R. Gutierrez, J. Phys. Chem., 84 (1980) 2492.
[9] S.N. Chen and G.B. Porter, Chem. Phys. Lett., 6 (1970) 41.
[10] L.S. Forster, J. Vandermark and J.V. Rund, Inorg. Chim. Acta, 202
(1992) 141.
[42] L. Dogliotti and E. Hayon, J. Phys. Chem., 72 (1968) 1800.
[43] J.H. Baxandale and D.A. Stott, J. Chem. Soc., Chem. Commun.,(1967)
699.
[11] B. Mainusch, F. Wasgestian, Z. Stasicka and A. Karocki, J. Inf. Rec.
Mater., 21 (1994) 687.
[44] G.L. Hug, Optical Spectra of Nonmetallic Inorg. Transient Species in
Aqueous Solution, US Dept. Commerce, Natl. Bureau of Standards,
1981.
[12] Y. Takeuchi and Y. Saito, Bull. Chem. Soc. Jpn., 30 (1957) 319.
[13] G. Brauer, Handbuch der Pra¨parativen Anorganischen Chemie, Vol.
3, Ferdinand Enke, Stuttgart, 1981.
[45] T. Ohno and S. Kato, Bull. Chem. Soc. Jpn., 43 (1970) 8.
[46] T. Ohno and S. Kato, Bull. Chem. Soc. Jpn., 46 (1973) 1602.
[47] D. Behar, P.L.T. Bevan and G. Scholes, J. Phys. Chem., 76 (1972)
1537.
[48] R.B. Draper and M.A. Fox, J. Phys. Chem., 94 (1990) 4628.
[49] R.H. Schuler, University of Notre Dame, personal communication.
[50] A.D. Kirk and L.Z. Cai, Inorg. Chem., 34 (1995) 3986.
[51] L.-Z. Cai, D.A. House, D.M. Kneeland and A.D. Kirk, J. Photochem.
Photobiol. A: Chem., 80 (1994) 271.
[14] M. Bergmann, J. Biol. Chem., 110 (1935) 476.
[15] J. Ganescu, C. Varhelyi, G. Brinzan and L. Boboc, Monatsh. Chem.,
106 (1975) 1071.
[16] E. Weinmann, Ph.D. Thesis, University of Zurich, Switzerland, 1919.
[17] J.C. Bailar and E.R. Jones, Inorg. Synth., 1 (1939) 63.
[18] F. Wasgestian and H.L. Schla¨fer, Z. Phys. Chem. (Frankf.), 51 (1966)
208.
[19] C.G. Hatchard and C.A. Parker, Proc. R. Soc. London, Ser. A, 235
(1956) 518.