Syntheses, spectroscopic properties and photochemistry of [Re(R)(CO)3(4,4′-Me2-bpy)] (R = alkyl) complexes

Article abstract of DOI:10.1016/S0020-1693(98)00278-3

The complexes [Re(R)(CO)₃(dmb)] (dmb = 4,4′-dimethyl-2,2′-bipyridine; R = CH₃, CD₃, Et, Bz, or iPr) were synthesized and their spectroscopic (¹H, and ¹³C NMR, IR, UV-Vis, resonance Raman) properties w

Full text of DOI:10.1016/S0020-1693(98)00278-3

Inorganica Chimica Acta 284 (1999) 61±70
Syntheses, spectroscopic properties and photochemistry of
[Re(R)(CO)3(4,40-Me2-bpy)] (R ˆ alkyl) complexes
Cornelis J. Kleverlaan, Derk J. Stufkens*
Anorganisch Chemisch Laboratorium, Institute of Molecular Chemistry, Universiteit van Amsterdam,
Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
Received 16 January 1998; accepted 1 June 1998
Abstract
The complexes [Re(R)(CO)3(dmb)] (dmb ˆ 4,40-dimethyl-2,20-bipyridine; R ˆ CH3, CD3, Et, Bz, or iPr) were synthesized and their
spectroscopic (1H, and 13C NMR, IR, UV±Vis, resonance Raman) properties were studied. According to the resonance Raman spectra the
lowest-energy electronic transition has mainly MLCT character. It is therefore concluded that the 3s(Re±R)ꢀ*(dmb) state from which these
complexes undergo homolysis of the Re±alkyl bond, is occupied by surface crossing from the nonreactive MLCT states. The
[Re(CO)3(dmb)]ꢀ radicals, formed by the homolysis reaction, were trapped by nitrosodurene and PPh3, and their adducts were identi®ed
with EPR spectroscopy. The thermal reactions of the radicals in different media were followed with IR spectroscopy. # 1999 Elsevier
Science S.A. All rights reserved.
Keywords: Photochemistry; Rhenium complexes; Alkyl complexes; Carbonyl complexes; Diimine complexes
1. Introduction
complexes [Re(MLn)(CO)3(a-diimine)] the 3sꢀ* state is
always lower in energy than the MLCT states and very
reactive if MLn ˆ Mn(CO)5, Re(CO)5, Co(CO)4, or
FeCp(CO)2 [1,7±20]. Only the complexes [Re(SnPh3)-
(CO)3(a-diimine)] are stable in their lowest 3sꢀ* state
due to the strength of their Re±Sn bond [21]. Similar
homolysis reactions occur for the metal±alkyl complexes
[M(R)(CO)3(a-diimine)] (M ˆ Mn, Re) [22±27], [Ru(X)-
(R)(CO)2(a-diimine)] (X ˆ halide) [26,28], [Ir(R)(CO)-
(PAr3)2(mnt)] [29], [Pt(CH3)4(a-diimine)] [30], and [Zn-
(R)2(a-diimine)] [31], but their photoreactivity often varies
The complexes [Re(L0)(CO)3(a-diimine)]n‡ (n ˆ 0,1; a-
diimine ˆ 2,20-bipyridine, etc.; L0 ˆ halide, O- or N-donor
ligand) [1±5] have the special property that variation of L0
may not only in¯uence the energy of the lowest-excited state
but also change its character and reactivity. Thus, replacing
Cl by I changes this character from MLCT into L0LCT
(halide to a-diimine charge transfer) [6]. If L0 is instead an
organic donor molecule such as 4-(dimethylamino)benzo-
nitrile (DMABN) [4], the L0LCT (DMABN to a-diimine
charge transfer) transition is overlap forbidden. However,
even then the L0LCT state becomes occupied if MLCT
excitation is followed by intramolecular electron transfer
from DMABN to the Re centre [4]. A special situation
occurs when L0 represents a metal fragment (MLn) or alkyl
(R) group. For, the complexes [Re(MLn)(CO)3(a-diimine)]
and [Re(R)(CO)3(a-diimine)] have a high-lying s(Re±MLn/
R) orbital. MLCT (dꢀ(Re) to a-diimine) excitation is then
followed by electron transfer from that s(Re±MLn/R) orbital
to dꢀ(Re), by which the excited state obtains s(Re±MLn/
R)ꢀ*(a-diimine) character. Occupation of this state nor-
mally leads to homolysis of the s(Re±MLn/R) bond with
formation of radicals. In the case of the metal±metal bonded
3
with R. Several of these sꢀ* states have recently been
detected and characterized with time-resolved absorption,
emission and IR spectroscopy [22,32].
Preliminary experiments on the [Re(R)(CO)3(a-diimine)]
(R ˆ CH3, CD3, Et, Bz and iPr) complexes showed that the
photochemical behaviour not only depends on R, but also on
the a-diimine ligand. Thus, the quantum yield of the photo-
reaction is only 0.15 for the complexes [Re(CH3)(CO)3(iPr-
PyCa)] and [Re(CH3)(CO)3(R-DAB)] (iPr-PyCa ˆ pyridine-
2-carbaldehyde-N-isopropyl and R-DAB ˆ N,N0-diorgano-
1,4-diaza-butadiene) [22], whereas the reaction proceeds
with nearly unit ef®ciency for [Re(CH3)(CO)3(dmb)]
[27]. In order to determine the factors that in¯uence the
excited-state properties of these complexes, we have started
an investigation of their spectroscopic and photochemical
*Corresponding author. Tel.: +31-20-525-6451; fax: +31-20-525-6456.
0020-1693/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(98)00278-3

62
C.J. Kleverlaan, D.J. Stufkens / Inorganica Chimica Acta 284 (1999) 61±70
cm 1; UV±Vis ꢃmax (" in M 1 cm 1) (CH2Cl2): 415 nm
‡
(2.5 Â 103). MS(FAB ): M(calc) 469.51, M(exp) 470.52 1.
[Re(CD3)(CO)3(dmb)]: 1H NMR (CDCl3, 293 K) ꢁ: 8.90
(2H, d, 5.7 Hz, py-H6), 7.95 (2H, s, py-H3), 7.20 (2H, d,
2
5.7 Hz, py-H5), 2.56 (6H, s, py-CH3); D NMR (CDCl3,
293 K) ꢁ: 0.95 (3D, s, CH3) ppm; IR ꢂ(CO) (THF, 293 K):
1998 (s) 1880 (s, br) cm 1; UV±Vis ꢃmax (CH2Cl2): 415 nm.
‡
MS(FAB ): M(calc) 472.51, M(exp) 473.08 1.
Fig. 1. Schematic structure of the complexes [Re(R)(CO)3(dmb)] and of
the dmb ligand used.
1
[Re(Et)(CO)3(dmb)]: H NMR (CDCl3, 293 K) ꢁ: 8.90
(2H, d, 5.7 Hz, py-H6), 7.95 (2H, s, py-H3), 7.20 (2H, d,
5.7 Hz, py-H5), 2.56 (6H, s, py-CH3), 1.32 (3H, t, 7.8 Hz,
CH2CH3), 0.28 (2H, q, 7.8 Hz, CH2CH3) ppm; 13C NMR
ATP (CDCl3, 293 K) ꢁ: 204.6 (Re±CO), 192.8 (Re±CO),
154.1 (py-C2), 151.7 (py-C6), 148.3 (py-C4), 126.7 (py-C3),
123.1 (py-C5), 21.3 (py-CH3), 18.6 (CH3), 17.4 (CH2) ppm;
IR ꢂ(CO) (THF, 293 K): 1987 (s) 1877 (s, br) cm 1; UV±Vis
ꢃmax (" in M 1 cm 1)(CH2Cl2): 425 nm (3.0 Â 103).
properties. In this paper we present the syntheses and
spectroscopic data, as well as the photoreactions of the
complexes [Re(R)(CO)3(dmb)] (R ˆ CH3, CD3, Et, Bz and
iPr) (see Fig. 1).
2. Experimental
‡
MS(FAB ): M(calc) 483.54, M(exp) 484.55 1.
1
2.1. Materials and preparations
[Re(iPr)(CO)3(dmb)]: H NMR (CDCl3, 293 K) ꢁ: 8.90
(2H, d, 5.7 Hz, py-H6), 7.97 (2H, s, py-H3), 7.22 (2H, d,
[Re2(CO)10] (Strem), Br2, CH3I, CD3I, EtI, BzBr
(Aldrich) and 4,40-dimethyl-2,20-bipyridine (dmb) (Fluka)
were used without further puri®cation. Silicagel for column
chromatography (Kieselgel 60, 70±230 mesh, Merck) was
activated by heating overnight in vacuo at 1608C.
Solvents for synthetic purposes were of reagent grade and
carefully dried over sodium wire (THF, n-hexane, diethyl
ether), CaCl2 (CH2Cl2) or CaH2 (CH3CN) and freshly
distilled under nitrogen prior to use. Solvents for spectro-
scopy were of analytical grade, dried over sodium and
distilled under N2 before use. CH3MgI, CD3MgI, EtMgI,
BzMgBr and iPrMgI were synthesized by published pro-
cedures.
The complexes [Re(X)(CO)3(dmb)] (X ˆ Cl, Br) were
synthesized according to previously reported procedures
[33]. The complexes [Re(R)(CO)3(dmb)] (R ˆ CH3, CD3,
Et, Bz, iPr) were prepared by a modi®ed literature procedure
[22]. Excess (ꢁ1.2 mmol) of the relevant Grignard reagent
was added to a solution of 1.0 mmol [Re(Br)(CO)3(dmb)] in
50 ml THF at room temperature. In case of the Et, Bz and iPr
Grignard the [Re(R)(CO)3(dmb)] complex was formed
immediately. [Re(CH3/CD3)(CO)3(dmb)] was formed after
1 h of stirring at room temperature. The excess Grignard
was quenched by adding 2 ml CH2Cl2 to the solution. The
magnesium salts were removed from the mixture by ¯ash
column chromatography of the reaction mixture on a short
silica column. After evaporation of the solvent, the complex
was puri®ed by column chromatography on silica using
gradient elution with n-hexane/THF. Yield: 50±90%.
[Re(CH3)(CO)3(dmb)]: 1H NMR (CDCl3, 293 K) ꢁ: 8.90
(2H, d, 5.7 Hz, py-H6), 7.95 (2H, s, py-H3), 7.20 (2H, d,
5.7 Hz, py-H5), 2.56 (6H, s, py-CH3), 0.95 (3H, s, CH3)
ppm; 13C NMR ATP (CDCl3, 293 K) ꢁ: 204.0 (Re±CO),
192.6 (Re±CO), 154.2 (py-C2), 151.8 (py-C6), 148.5 (py-
C4), 126.8 (py-C3), 123.1 (py-C5), 21.2 (py-CH3), 0.6
(CH3) ppm; IR ꢂ(CO) (THF, 293 K): 1998 (s) 1880 (s, br)
5.7 Hz, py-H5), 2.56 (6H, s, py-CH3), 1.25 (6H, d, 7.0 Hz,
CH(CH3)2), 1.02 (1H, sept, 7.0 Hz, CH(CH3)2) ppm; 13
C
NMR ATP (CDCl3, 293 K) ꢁ: 204.6 (Re±CO), 192.5 (Re±
CO), 154.2 (py-C2), 151.7 (py-C6), 148.3 (py-C4), 126.7
(py-C3), 123.1 (py-C5), 30.2 (CH(CH3)2), 29.0 (CH(CH3)2),
21.3 (py-CH3) ppm; IR ꢂ(CO) (THF, 293 K): 1987 (s); 1878
(s, br) cm 1; UV±Vis ꢃmax (" in M 1 cm 1)(CH2Cl2):
434 nm (2.6 Â 103).
1
[Re(Bz)(CO)3(dmb)]: H NMR (CDCl3, 293 K) ꢁ: 8.63
(2H, d, 5.7 Hz, py-H6), 7.73 (2H, s, py-H3), 7.08 (2H, d,
5.7 Hz, py-H5), 6.58 (2H, pst, arom-H), 6.43 (1H, t, 7.1 Hz,
arom-H) 5.97 (2H, d, 7.4 Hz, arom-H), 2.56 (6H, s, py-
CH3), 1.73 (2H, s, CH2-Ph) ppm; 13C NMR ATP (CDCl3,
293 K) ꢁ: 202.9 (Re±CO), 192.2 (Re±CO), 154.0 (py-C2),
152.6 (Ph-C), 151.5 (py-C6), 148.4 (py-C4), 126.9 (Ph-C),
126.5 (py-C3), 125.0 (Ph-C), 122.8 (py-C5), 118.7 (Ph-C),
24.3 (CH2), 21.2 (py-CH3) ppm; IR ꢂ(CO) (THF, 293 K):
1
1993 (s); 1882 (br, s) cm
M
;
UV±Vis
ꢃ
(" in
‡
1 cm 1)(CH2Cl2): 400 nm (2.5 Â 103). MS(FAB ):
M(calc) 545.61, M(exp) 546.62 1.
1
[Re(Cl)(CO)3(dmb)]: H NMR (CDCl3, 293 K) ꢁ: 8.89
(2H, d, 6.0 Hz, py-H6), 7.99 (2H, s, py-H3), 7.34 (2H, d,
6.0 Hz, py-H5), 2.58 (6H, s, py-CH3) ppm; IR ꢂ(CO)
1
(CH2Cl2, 293 K): 2021 (s) 1916 (m) 1894 (m) cm
UV±Vis ꢃmax(CH2Cl2): 375 nm.
;
2.2. Spectroscopic measurements
Samples for spectroscopic studies were prepared under
puri®ed N2 by use of Schlenk techniques. The solutions
were carefully handled in the dark before the experiments
were performed.
‡
1 Mass of M±H , due to abstraction of the matrix solvent in the mass
spectrometer.

C.J. Kleverlaan, D.J. Stufkens / Inorganica Chimica Acta 284 (1999) 61±70
63
Table 1
Maximaa and solvatochromism (Á)b of the lowest-energy absorption
Electronic absorption spectra were measured on a Varian
Cary 4E spectrophotometer, infrared spectra on a Bio-Rad
FTS-60A FT-IR spectrometer equipped with a liquid-nitro-
gen-cooled MCT detector. The photoreactions of the com-
plexes were monitored with Rapid-Scan-FTIR, the
experimental set-up has been described elsewhere [34].
The 1H, 2D and 13C NMR spectra were recorded on a
Bruker AMX 300 spectrometer. Resonance Raman spectra
were recorded on a Dilor XY spectrometer, using an SP
bands of [Re(R)(CO)3(dmb)] complexes (R ˆ Me, Et, iPr and Bz)
Compound
CH3CN
THF
Toluene
Áb
[Re(Me)(CO)3(dmb)]
[Re(Et)(CO)3(dmb)]
[Re(iPr)(CO)3(dmb)]
[Re(Bz)(CO)3(dmb)]
400
406
416
486
417
425
435
512
435
444
454
525
2011
2108
2012
1528
a ꢃmax in nm.
b Difference in energy (cm 1) between the absorption maxima in toluene
and acetonitrile.
‡
Model 2016 Ar laser as the excitation source. The 425 nm
excitation wavelength was obtained using stilbene as a dye.
Spectra were recorded at 80 K for the complexes dispersed
in a KNO3 pellet (ꢁ15 mg complex in 150 mg KNO3). The
EPR spectra were measured on a Varian E-104A spectro-
meter. The fast atom bombardment (FAB) mass spectro-
metry measurements were carried out at the Institute of
Mass Spectrometry of the University of Amsterdam.
3.3. Electronic absorption spectra
The complexes are yellow to red±brown due to the
presence of an absorption band in the region 300±500 nm
which has a maximum extinction coef®cient in CH2Cl2 of
2.5±3.0 Â 103 M 1 cm 1. The bands show negative solva-
tochromism (see Table 1), which indicates that they belong
to metal-to-ligand charge-transfer (MLCT) transitions
[13,22,24,40].
Changing the alkyl group from CH3 to Et and iPr causes
the MLCT band to shift to lower energy (see Fig. 2). This is
due to an increase in energy of the dꢀ(Re)-orbitals as the
alkyl group becomes more electron donating. The spectrum
of the Bz complex shows a structured absorption band, not
observed for the other complexes. An analogous effect was
observed for the [Re(Bz)(CO)3(a-diimine)] (a-diimi-
ne ˆ iPr-DAB; tBu-DAB) complexes and was then ascribed
to a splitting of the dꢀ(Re)-orbitals due to an interaction of
the dxy- and/or dyz-orbitals with the ꢀ orbitals of the phenyl
group of the benzyl ligand [22].
3. Results and discussion
3.1. 1H and 13C NMR spectra
1
The H NMR spectra of the [Re(R)(CO)3(dmb)] com-
pounds show resonances for the dmb ligand at approxi-
mately 2.6 ppm and in the region 8.9±7.0 ppm. These shifts
indicate that this ligand is coordinated to the metal as a
chelate [35]. The resonances of the alkyl groups have a low
value of ꢁ, which means that the Re atom has a strong
shielding effect. The alkyl proton resonances show a normal
coupling pattern. The 1H spectrum of [Re(CD3)(CO)3-
2
(dmb)] reveals the resonances of the dmb ligand, the D
spectrum that of the CD3 ligand at 0.95. The 13C NMR
spectra of the complexes show resonances in the region
155±120 ppm (py-C) and at approximately 21 ppm (py-
CH3) for the dmb ligand. Resonances of the alkyl ligands
are observed at relatively low ppm values: 0.6 (CH3), 17.4
(CH2CH3), 24.3 (CH2Ph) and 30.2 (CH(CH3)2). For the
complexes [Ru(I)(CH3)(CO)2(iPr-DAB)] and [Ru(Cl)(Bz)-
(CO)2(iPr-DAB)] similar resonances have been observed;
5.5 ppm (Ru-CH3) and 14.0 ppm (Ru-CH2Ph), respec-
tively [36,37].
3.4. Resonance Raman spectra
Resonance Raman (rR) spectroscopy is a valuable tech-
nique for the assignment of allowed electronic transitions,
3.2. IR spectra
The infrared spectra of all [Re(R)(CO)3(dmb)] com-
pounds show two bands in the ꢂ(CO) frequency region,
the one at lower frequency being broad and consisting
of two, nearly coinciding, vibrations. These bands are
assigned to the three IR-active CO stretching modes
(2a0 ‡ a00) of this type of complexes having Cs symmetry
[33,38,39]. The wavenumbers of these vibrations decrease
slightly in the order Bz > CH3 > Et > iPr due to an increase
of electron donation by the alkyl group. A similar effect was
observed for other [Re(R)(CO)3(a-diimine)] complexes
[22].
Fig. 2. Absorption spectra of [Re(R)(CO)3(dmb)] (R ˆ CH3, Et, iPr and
Bz) in toluene at 293 K.

64
C.J. Kleverlaan, D.J. Stufkens / Inorganica Chimica Acta 284 (1999) 61±70
Table 2
Resonantly enhanced Raman bands of [Re(R)(CO)3(dmb)] (R ˆ CD3, CH3) at 80 K, together with relevant literature data
1
Compound
Ref.
Raman shift (cm 1) (2000±200 cm
)
[Re(CD3)(CO)3(dmb)]
[Re(CD3)(CO)3(dmb)]
1974, 1535, 1472, 1312, 1270, 1255, 1166, 750, 733, 530, 499, 482, 451, 212
1974, 1535, 1472, 898, 621, 570, 518, 502, 491, 468, 212
(M±CO and M±C only)
499, 482, 451
502, 491, 468
[Re(CH3)(CO)3(dmb)]
[Re(CD3)(CO)3(dmb)]
[Re(CH3)(CO)3(iPr-DAB)]
[Re(CH3)(CO)3(tBu-DAB)]
[Re(CH3)(CO)3(pAn-DAB)]
[Re(CH3)(CO)5]
[24]
[24]
[24]
[45]
[45]
[39]
[41]
[41]
[6]
503, 488, 451
492, 484, 452
513, 470, 450
531, 471, 450 and 473 (M±CH3)
531, 463, 448 and 426 (M±CD3)
570, 490, 460
[Re(CD3)(CO)5]
[Re(SnPh3)(CO)3(tBu-DAB)]
[Re(Cl)(CO)3(Mes-DAB)]
[Re(Cl)(CO)3(pTol-DAB)]
[Re(Cl)(CO)3(iPr-DAB)]
[Re(Br)(CO)3(iPr-DAB)]
[Re(I)(CO)3(iPr-DAB)]
[Re{(Mn(CO)5}(CO)3(pTol-PyCa)]
525, 490, 450
500, 470, 430
502, 485, 438
[6]
[6]
514, 500, 442
504, 487, 440
[41]
516, 480, 426
since only those vibrations are normally observed in such
spectra, which are coupled to these transitions [41±44]. For
the complexes [Re(R)(CO)3(dmb)] such rR data might
therefore be useful for the identi®cation of their lowest
electronic transitions which eventually lead to their photo-
induced homolysis reactions.
Attempts to measure the rR spectra of [Re(R)(CO)3-
(dmb)] (R ˆ Et, iPr and Bz) failed due to the photodecom-
position of these complexes by the exciting laser beam.
Only the complexes [Re(R)(CO)3(dmb)] (R ˆ CH3, CD3)
are photostable, albeit only at low temperatures. Their rR
spectra were measured at 80 K in a KNO3 pellet with
exciting argon ion laser lines that were varied throughout
the absorption band (425±514.5 nm). Table 2 presents the
most important Raman bands of the complexes. Fig. 3
shows the Raman spectra of [Re(CH3)(CO)3(dmb)]
recorded at four wavelengths of excitation, while the spectra
of CH3 and CD3 complexes in the 800±200 cm 1 region are
depicted in Fig. 4.
The assignment of the Raman bands is based on a
comparison with data for the related complexes [Re(L)-
(CO)3(a-diimine)] (L ˆ halide, alkyl) [6,24,39,41], and
[M(R)(CO)5] (M ˆ Mn, Re; R ˆ CH3, CD3) [45] (see
Table 2). In agreement with the Raman spectra recorded
previously for several [Re(L)(CO)3(a-diimine)] (L ˆ halide,
alkyl) complexes [6,24], only one CO vibration, viz. the
highest-frequency symmetric stretching mode, appears as a
resonantly enhanced Raman band in the spectra. A detailed
vibrational study of the closely related complex [Re(Cl)-
(CO)3(4,40-bpy)2] has shown that this band belongs to the
Fig. 3. Resonance Raman spectra of [Re(CH3)(CO)3(dmb)] dispersed in
Fig. 4. Part of the resonance Raman spectra of [Re(CD3)(CO)3(dmb)]
(top) and [Re(CH3)(CO)3(dmb)] (bottom) dispersed in KNO3
KNO3 (ꢃexc (top to bottom) ˆ 514.5, 488.0, 457.9 and 425.0 nm;
T ˆ 80 K; à ˆ NO3
)
(ꢃexc ˆ 488.0 nm; T ˆ 80 K; à ˆ NO3
)

C.J. Kleverlaan, D.J. Stufkens / Inorganica Chimica Acta 284 (1999) 61±70
65
symmetric ꢂ(CO) vibration, in which the axial and equator-
ial carbonyl ligands move in phase [46]. Such a movement
will be the result of MLCT excitation. In the lower-fre-
quency symmetric CO vibration (ꢁ1880 cm 1), the axial
and equatorial carbonyls move out-of-phase. As a result, the
total distortion along this latter normal coordinate is small
and the resonance Raman effect will be too weak for this
vibration to be observed. In the frequency region of the a-
diimine stretching modes (1600±1200 cm 1), strong Raman
bands are observed at 1535 and 1472 cm 1, which belong to
coupled CN- and CC-vibrations of dmb.
presented in Table 2. They belong to metal±CO stretching
and metal±C±O deformation modes of the Re(CO)3-moiety.
In some cases only two bands are observed, but this is
probably due to the coincidence of the two higher frequency
bands. In view of this close correspondence between the
spectra of the various Re(L)(CO)3(a-diimine) complexes,
none of these bands can be assigned to ꢂ(Re±CH3/CD3),
especially since deuteration has only a minor effect on their
frequencies. The fourth band observed at 530 cm 1 for the
1
CH3-complex shifts to 518 cm on deuteration. Although
this band might belong to ꢂ(Re±CH3/CD3), its frequency is
much higher than that observed for Re(CH3/CD3)(CO)5
(473/426 cm 1). Besides, the effect of deuteration is far
too small (12 versus 47 cm 1). This band most likely
belongs to a ꢁ(Re±C±O) vibration. The shift on deuteration
may be caused by two effects. First of all, the ꢁ(Re±C±O)
vibration may mix with ꢂ(Re±CH3/CD3). Deuteration will
then also affect the frequency of this Raman band but the
effect will be smaller than expected for a pure ꢂ(Re±CH3/
CD3) vibration. It is also possible that the ꢁ(Re±C±O)
vibration mixes with the rocking modes of the methyl group.
On deuteration these latter vibrations approach the ꢁ(Re±C±
O) mode, shifting it to lower frequency. Anyhow, this band
does not belong to ꢂ(Re±CH3/CD3). On the other hand, even
if this were the case, its resonance Raman effect would be
too small for the Re±CH3/CD3 bond to be affected appre-
ciably by the lowest-energy electronic transition.
In the lower frequency region (1200±200 cm 1), many
bands are found, some of them showing a resonance effect.
The assignment of these bands is not straightforward but
some of them can be assigned by comparison of the spectra
of the CH3 and CD3 complexes. The band observed for the
1
1
CH3 complex at 1166 cm shifts to 898 cm on deute-
ration. It is assigned to ꢁs(CH3/CD3) in agreement with the
data obtained by McQuillan et al. for [Re(R)(CO)5]
(R ˆ CH3, CD3) [45]. For these complexes ꢁs(CH3) was
observed at 1193 cm 1, ꢁs(CD3) at 910 cm 1. Similar bands
were observed for the related complexes [Re(CH3)(CO)3-
(R-DAB)] [24] (ꢁs ˆ 1160 cm 1), [Ru(CH3)(SnPh3)(CO)2-
(iPr-DAB)] (ꢁs ˆ 1148 cm 1) [47], [Ru(X)(CH3/CD3)-
1
(CO)2(iPr-DAB)] (X ˆ Cl , I ) (ꢁs(CH3) ˆ 1148 cm
ꢁs(CD3)ˆ 908 cm 1) [37].
,
The rR spectra of [Re(CH3)(CO)3(dmb)] show two other
bands, at 750 and 733 cm 1, which shift to 621 and
570 cm 1, respectively, on replacing the CH3 group by
CD3. The corresponding [Re(R)(CO)5] complexes showed
only a single band in solution, at 777 cm 1 (R ˆ CH3) and
at 577 cm 1 (R ˆ CD3) [45], which was, however, split into
two bands for [Re(CH3)(CO)5] in the gas phase (802 and
758 cm 1) [45]. These bands were assigned to a rocking
mode (ꢄ) of the CH3/CD3 ligand. Similar bands were found
for the related complexes [Re(CH3)(CO)3(R-DAB)]
(ꢄ(CH3) ˆ 730 cm 1) [24] and [Ru(X)(R)(CO)2(iPr-DAB)]
(X ˆ Cl , I ) (ꢄ(CH3)  800 cm 1, ꢄ(CD3) ˆ 614 and
600 cm 1) [37]. The splitting of the ꢄ(CH3) band of
[Re(R)(CO)3(dmb)] (750 733 ˆ 17 cm 1) is rather small
compared with that of ꢄ(CD3) (621 570 ˆ 51 cm 1).
Raman bands belonging to ꢂ(Re±CH3) and ꢂ(Re±CD3)
have been observed for [Re(CH3/CD3)(CO)5] at 473 and
426 cm 1, respectively [45]. For the complexes under study
they are expected at similar frequencies in the rR spectra,
provided that they are resonance enhanced by excitation into
the visible absorption band. However, the assignment of
Raman bands in this frequency region is not straightforward
since it contains also bands due to ꢂs(Re±CO) and ꢁ(Re±C±
The lowest-frequency band in this region shifts from 451
to 468 cm 1 on deuteration. Such a shift to higher frequency
is exceptional, and deviates from the behaviour of ꢂ(Re±
CH3/CD3) in the parent complexes [Re(CH3/CD3)(CO)5]. In
these latter complexes the frequency of ꢂ(Re±CH3) shifts
from 473 to 426 cm 1 on deuteration. The 451 cm 1 Raman
band of [Re(CH3)(CO)3(dmb)] can therefore not be assigned
to ꢂ(Re±CH3). It may instead belong to a mixed vibration
which changes its composition appreciably on deuteration,
thus causing a shift to higher frequency. The Raman band at
1
212 cm does not shift on deuteration. In agreement with
literature data it is assigned to ꢂs(Re±N) [39,41].
Thus, according to these rR data, the lowest-energy
transition of these complexes affects the dmb and carbonyl
ligands, the metal±dmb and metal±CO bonds, but not the
Re±CH3/CD3 bond. This means that it is not a s(Re±CH3/
CD3)!ꢀ*(dmb) transition, but has instead dꢀ(Re)
!ꢀ*(dmb) (MLCT) character. It is noteworthy that defor-
mation and rocking modes of the CH3/CD3 ligand are
coupled to this electronic transition. This effect of MLCT
excitation on the angles of the CH3/CD3 ligand may be due
to a change of hybridization as a result of the oxidation of
the metal or to the repulsive interaction between the CH3/
CD3 ligand and the radical anion of dmb in the MLCT state.
O) vibrations, which could couple to ꢂ(Re±CH3/CD3). In
1
the region 400±550 cm
[Re(CH3)(CO)3(dmb)] show four bands at 451, 482, 499,
the rR spectra of
1
and 530 cm 1, which shift to 468, 491, 502, and 518 cm
,
3.5. Photochemistry
respectively, on deuteration (Table 2, Fig. 4). The ®rst three
bands are found in the rR spectra of most [Re(L)(CO)3(a-
diimine)] complexes as demonstrated by the literature data
All complexes [Re(R)(CO)3(dmb)] under study are
photolabile and their reactions were followed in situ by

66
C.J. Kleverlaan, D.J. Stufkens / Inorganica Chimica Acta 284 (1999) 61±70
1
Table 3
IR, UV±Vis, H NMR and EPR spectroscopy in different
solvents and at various temperatures. The complexes
reacted thermally in neat CCl4 and in CCl4:THF (1:10,
vol./vol.) to give [Re(Cl)(CO)3(dmb)]. They are, however,
thermally stable in CHCl3, CH2Cl2 and CCl4:CH2Cl2 (1:10,
vol./vol.). Irradiation of the complexes in these latter sol-
vents into their MLCT band (ꢃirr > 420 nm) resulted in the
formation of [Re(Cl)(CO)3(dmb)]. The quantum yield of
this photoreaction varies from 0.4 for R ˆ CH3 and CD3 to
ꢁ1 for R ˆ Et, iPr, and Bz. In a typical 1H NMR experiment
a solution of [Re(Et)(CO)3(dmb)] in CDCl3 was irradiated
EPR Parameters a of the radical adducts [ArN(Oꢀ )±Re(CO)3(dmb)],
ArN(Oꢀ )±R and [Re(PPh3)(CO)3(dmb)]ꢀ formed on irradiation
(ꢃexc > 420 nm) of [Re(R)(CO)3(dmb)] in the presence of nitrosodurene
(ArNO) b and PPh3 in toluene (293 K)
c
Complex
Coupling constants
g-value
R ˆ Me
aN ˆ 13.7; aH ˆ 12.2
aN ˆ 13.7; aH ˆ 11.1
aN ˆ 13.8; aH ˆ 8.0
aN ˆ 13.8; aH ˆ 6.7
aP ˆ 26.1; aRe ˆ 19.2
aN ˆ 13.4; aRe ˆ 36.0
2.0062
2.0071
2.0071
2.0066
2.0033
2.0047
R ˆ Et
R ˆ Bz
R ˆ iPr
[Re(PPh3)(CO)3(dmb)]ꢀ
[Ar±NO±Re(CO)3(dmb)]
ꢀ d
1
in a CIDNP probe through a ®ber. The H NMR spectra
a Coupling constants in Gauss (Æ0.1G).
b Spin-adduct formed with 10-fold excess of nitrosodurene (ArNO).
c Radical formed with 100-fold excess of PPh3.
d Low concentration.
showed the disappearance of the proton signals of the
complex while new signals were observed at 8.89 (d, 2H,
6.0 Hz, py±C6), 7.99 (s, 2H, py±C3), 7.34 (d, 2H, 6.0 Hz,
py±C5) and 2.58(s, 6H, py±Me) ppm, belonging to
[Re(Cl)(CO)3(dmb)] [35]. The same product was formed
on irradiation of the other complexes and its identity was
further established by comparing its IR and UV±Vis spectra
base. In order to con®rm this adduct formation, the
[Re(R)(CO)3(dmb)] complexes were irradiated in toluene
(T ˆ 293 K) in the presence of excess PPh3. The EPR
spectrum of the photoproduct is shown in Fig. 6. The
spectrum is simulated with the parameters aP ˆ 26.1 G,
aRe ˆ 19.2 G and g ˆ 2.0033. These values are in good
agreement with those derived by Klein et al. from the EPR
spectrum of the electrochemically prepared radical
[Re(PPh3)(CO)3(dmb)]ꢀ (aP ˆ 20 G; aRe ˆ 20 G and
g ˆ 2.0018 in THF/10 1 M BuNClO4) [49]. Fig. 6 also
presents the EPR spectrum of the radical formed by irradia-
tion in toluene in the absence of PPh3. When excess PPh3 is
added to this solution after irradiation, the EPR signal
changes into that of the [Re(PPh3)(CO)3(dmb)]ꢀ radical.
Irradiation of [Re(R)(CO)3(dmb)] in 2-MeTHF or 2-propa-
nol at T ˆ 123 K gives rise to the formation of similar
radical products [26].
with those of
[Re(Cl)(CO)3(dmb)] (see Section 2).
a
separately prepared sample of
The product formation is consistent with the reaction
sequence of Eq. (1). Irradiation causes the homolytic clea-
vage of the Re±alkyl bond with formation of
[Re(CO)3(dmb)]ꢀ and alkyl radicals. The former radicals
react with the chlorinated solvent to give [Re(Cl)-
(CO)3(dmb)], the secondary reactions of the alkyl radicals
were not further investigated during this study.
ꢀ
hꢂ
‰ReꢂR†ꢂCO†3ꢂdmb†Š ! ‰ReꢂCO†3ꢂdmb†Š ‡ Rꢀ
ꢀ
0ꢀ
‰ReꢂCO†3ꢂdmb†Š ‡ R0Cl ! ‰ReꢂCl†ꢂCO†3ꢂdmb†Š ‡ R
(1)
The products formed after irradiation of [Re(R)-
(CO)3(dmb)] at different temperatures were characterized
with IR spectroscopy; their CO wavenumbers and their
assignments are collected in Table 4. Irradiation of the
complexes in a 2-MeTHF glass at 123 K results in the
formation of a species having CO-stretching vibrations at
2007, 1892 and 1878 cm 1. Similar frequencies have been
observed for [Re(EtCN/PrCN)(CO)3(dmb)]ꢀ obtained by the
one-electron reduction of [Re(Br)(CO)3(dmb)] in a mixture
of EtCN and PrCN [50]. The product IR bands are therefore
assigned to the radical species [Re(2-MeTHF)(CO)3-
(dmb)]ꢀ . When this reaction is, however, performed in
EtCN/PrCN (4:5, vol./vol.) at room temperature, the [Re-
(EtCN/PrCN)(CO)3(dmb)]ꢀ radical is not the only product
anymore. Extra ꢂ(CO) bands show up at 2037 and
1933 cm 1, which belong to the cation [Re(EtCN/PrCN)-
The formation and properties of the [Re(CO)3(dmb)]ꢀ
radicals were studied in more detail by performing the
photoreactions in nonchlorinated solvents at various tem-
peratures, in the presence and absence of other reactants.
First of all, the [Re(R)(CO)3(dmb)] complexes were irra-
diated in toluene, in the presence of a small excess of the
spin trap nitrosodurene. The photolysis was carried out in
situ within the EPR spectrometer, using a high-pressure Xe-
lamp and an optical ®ber (ꢃirr > 420 nm). The EPR data
(coupling constants and g-values) of the radical adducts are
collected in Table 3, while Fig. 5 presents the measured and
simulated EPR spectra of the trapped radicals [ArN(Oꢀ )±R]
(R ˆ CH3, Et, Bz and iPr). In all cases, only a very low
concentration of the [ArN(Oꢀ )±Re(CO)3(dmb)] radicals is
observed. The measured coupling constants and g-values
agree well with those previously reported for these trapped
radicals [22,28,48]. Thus, the hyper®ne constant
aRe ˆ 36.0 G of [ArN(Oꢀ )±Re(CO)3(dmb)] is similar to that
of the related radical complex [ArN(Oꢀ )±Re(CO)3(iPr-
PyCa)](aRe ˆ 36.1 G) [22].
‡
(CO)3(dmb)] [50].
Even more, irradiation of the complexes dissolved in THF
(RT) causes the formation of three instead of two photo-
products (see Fig. 7). The major product, which is obtained
in ꢁ50% yield and which has ꢂ(CO) frequencies of 2018,
The [Re(CO)3(dmb)]ꢀ radicals are coordinatively unsa-
turated species and will therefore easily take up a Lewis
1914 and 1893 cm
[Re(THF)(CO)3(dmb)] [51]. Each of the other two pro-
,
is assigned to the cation
1
‡

C.J. Kleverlaan, D.J. Stufkens / Inorganica Chimica Acta 284 (1999) 61±70
67
Fig. 5. EPR spectra obtained after irradiation (ꢃirr > 420 nm) of [Re(R)(CO)3(dmb)] in toluene in the presence of nitrosodurene (1:10) at room temperature;
(A) R ˆ CH3; (B) R ˆ Et; (C) R ˆ Bz; (D) ˆ iPr (top) experimental (bottom) simulation.
Fig. 6. EPR spectra obtained after irradiation (ꢃirr > 420 nm) of [Re(Bz)(CO)3(dmb)] at room temperature; (left) in toluene, (right) in toluene in the presence
of PPh3 (1:100) (top) experimental (bottom) simulation.
ducts has a yield of ꢁ25%, and ꢂ(CO) frequencies of 2004,
1875 cm 1 and 1997, 1875 cm 1, respectively. These bands
are tentatively assigned to the solvated and naked radicals
[Re(THF)(CO)3(dmb)]ꢀ and [Re(CO)3(dmb)]ꢀ , respectively.
The formation of radical products agrees with the EPR
spectra. We did not observe the formation of the dimer
[Re2(CO)6(dmb)2] from the [Re(CO)3(dmb)]ꢀ radicals pro-
duced by irradiation in neat THF. This contrasts with the
result reported by Lucia et al. [27] and the observation that
such a dimer is formed by the electrochemical reduction of
the complexes [Re(X)(CO)3(dmb)] (X ˆ Cl , Br , I ,
Otf ) [51]. When, however, the [Re(R)(CO)3(dmb)] com-
plex is dissolved in a THF/0.1 M Bu4NBr solution used for
the electrochemical experiments, irradiation causes also the
formation of [Re2(CO)6(dmb)2]. This is evidenced by the
absorption band at 800 nm [11,12,51]. The major product
‡
is again [Re(THF)(CO)3(dmb)] , just as observed on irra-
diation of the complex in neat THF. The remaining ꢂ(CO)
frequencies at 2000 and ꢁ1875 cm 1, are tentatively
assigned to the solvated radical species.
3.6. Electronic transitions versus photochemistry
The photochemical reactions of these complexes were
studied by irradiation into the visible absorption band
which, according to the absorption and rR spectra, belongs
to MLCT transitions. MLCT states are normally not reac-
tive, but occupation of such a state may still lead to a
reaction by surface crossing to a close-lying reactive state.
Best known are the ligand-®eld or metal-centered states,
whose population normally gives rise to a heterolytic
splitting of a metal±ligand bond. Exceptional are those
appearance of its ꢂ(CO) bands at 1988, 1951, 1887 and
1
1859 cm
and of the characteristic long-wavelength

68
C.J. Kleverlaan, D.J. Stufkens / Inorganica Chimica Acta 284 (1999) 61±70
Table 4
IR ꢂ(CO) frequencies of the photoproducts of [Re(R)(CO)3(dmb)] in different solvents and at various temperatures, together with relevant literature data
1
Solvent
T (K)
ꢂ(CO) cm
Yield (%)
(Photo)products
Ref.
2-MeTHF
THF/PPh3
EtCN/PrCN
123
293
293
2007 (s) 1892 (s) 1878 (s)
2018 (s) 1924 (s) 1895 (s)
2011 (s) 1899 (s, br)
100
[Re(2-MeTHF)(CO)3(dmb)]ꢀ
[Re(PPh3) (CO)3(dmb)]ꢀ
[Re(EtCN/PrCN)(CO)3(dmb)]ꢀ
[Re(EtCN/PrCN) (CO)3(dmb)]
[Re2(CO)6(dmb)2]
100
35
65
25
25
50
25
25
50
‡
2037 (s) 1933 (s, br)
THF/Bu4NBr
THF
293
293
1988 (s) 1950 (s) 1887 (s) 1845 (s)
2000 ꢁ1875
[Re(THF)(CO)3(dmb)]ꢀ
‡
2017 (s) 1914 (s) 1887 (s)
1997 ꢁ1875
[Re(THF)(CO)3(dmb)]
[Re(THF)(CO)3(dmb)]ꢀ
2004 ꢁ1875
Re(CO)3(dmb)]ꢀ
‡
2018 (s) 1914 (s) 1893 (s)
2010 (s) 1905 (s) 1885 (s)
2039 (s) 1963 (s)
[Re(THF)(CO)3(dmb)]
PrCN/Bu4NBr
PrCN/Bu4NBr
THF/Bu4NBr
THF/Bu4NBr
THF/Bu4NBr
293
293
293
293
293
[Re(PrCN)(CO)3(bpy)]ꢀ
[50]
[50]
[51]
[51]
[51]
‡
[Re(PrCN)(CO)3(bpy)]
2015 (s) 1919 (s) 1892 (s)
2019 (s) 1917 (s) 1894 (s)
1988 (s) 1951 (s) 1887 (s) 1859 (s)
[Re(PPh3)(CO)3(bpy)]ꢀ
‡
[Re(THF)(CO)3(bpy)]
[Re2(CO)6(bpy)2]
1
Fig. 7. IR spectral changes in the carbonyl-stretching region obtained on irradiation at 293 K with 457.9 nm of [Re(CH3)(CO)3(dmb)] (Â) in (left) THF/10
ꢀ
ꢀ
‡
M Bu4NBr and (right) neat THF; (Ã) ˆ [Re(Sv)(CO)3(dmb)] , (‡) ˆ [Re(CO)3(dmb)] , (Á) ˆ [Re(Sv)(CO)3(dmb)] , (*) ˆ [Re2(CO)6(dmb)2].
complexes in which MLCT excitation leads to homolysis of
a metal±ligand bond with formation of radicals. We have
observed this phenomenon for quite a few complexes, which
all possess a ligand with a high-lying ꢅ orbital (metal
fragment [1,7±21], alkyl group [22±26], or halide [6])
and a second ligand (a-diimine, sulfurdiimine, etc.) with
a low-lying empty e.g. ꢀ* orbital (see also Section 1). We
have explained the occurrence of such a homolyis reaction
by the presence of a reactive 3sꢀ* state and, quite recently,
we have established its triplet character by measuring the
nanosecond time-resolved EPR spectra of several of the
alkyl radicals formed [26].
According to the MO calculations [52] the HOMO of the
model complex [Ru(SnH3)2(CO)2(H-DAB)] is a combina-
tion of pz(Ru) and the antisymmetric combination of the
lone pairs of the axial SnH3-ligands; it mixes strongly with
the lowest ꢀ* orbital of H-DAB which has the same
symmetry. As a result the s!ꢀ* transition of this complex
and of related complexes [Ru(L1)(L2)(CO)2(a-diimine)]
(L1, L2 ˆ metal fragment, alkyl) [32] is strongly allowed,
and the MLCT transition can only be weak. In the case of the
complexes [Zn(R)2(a-diimine)] [31] and [Pt(CH3)4(a-di-
imine)] [30], there are even no competing metal±dꢀ orbitals,
which might give rise to low-lying MLCT transitions. For
most other metal±metal and metal±alkyl bonded complexes
there is a strong mixing between the metal±dꢀ and a-
diimine-ꢀ* orbitals, which makes the MLCT transitions
strongly allowed and the s!ꢀ* transition overlap forbidden
[1, and Refs. therein]. The lowest-energy absorption band of
these complexes therefore belongs to one or more MLCT
transitions, and this MLCT character is indeed evident from
the intensity and solvent dependence of the band and from
the rR spectra of these complexes. However, even then
surface crossing from the MLCT states to the reactive

C.J. Kleverlaan, D.J. Stufkens / Inorganica Chimica Acta 284 (1999) 61±70
69
3sꢀ* state and radical formation may occur provided that
these states are close in energy. This is evidenced by the
observation that variation of the relative energies of the
metal±dꢀ orbitals, responsible for the MLCT transitions, and
the metal±ligand ꢅ orbital, directly in¯uences the photo-
reactivity of these complexes [22,28,37]. In some cases the
rR spectra more or less agreed with the proposed absence of
a s!ꢀ* transition. Much stronger evidence is, however,
provided by the present rR spectra of [Re(CH3/
CD3)(CO)3(dmb)] which show that the effect of deuteration
on the resonance Raman active vibrations is only small.
Thus, the rR spectra do not provide any evidence for a
s!ꢀ* transition. Because of this, such a transition can only
be very weak and at most a minor contributor to the visible
absorption band. On the basis of these rR spectra we
conclude that the lowest-energy electronic transition of
these Re±alkyl complexes has predominant MLCT charac-
ter. Surface crossing from this state to the reactive 3sꢀ* state
will then be followed by homolysis of the Re±R bond.
[9] D.L. Morse, M.S. Wrighton, J. Am. Chem. Soc. 98 (1976)
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4411.
[13] D.J. Stufkens, Coord. Chem. Rev. 104 (1990) 39.
Â
[14] R.R. Andrea, W.G.J. de Lange, D.J. Stufkens, A. Oskam, Inorg.
Chem. 28 (1989) 318.
[15] H.K. van Dijk, J. van der Haar, D.J. Stufkens, A. Oskam, Inorg.
Chem. 28 (1989) 75.
[16] T. van der Graaf, D.J. Stufkens, A. Oskam, K. Goubitz, Inorg. Chem.
30 (1991) 599.
[17] T. van der Graaf, A. van Rooy, D.J. Stufkens, A. Oskam, Inorg.
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[18] J.W.M. van Outersterp, D.J. Stufkens, A. Vlcek Jr., , Inorg. Chem. 34
(1995) 5183.
[19] B.D. Rossenaar, T. van der Graaf, R. van Eldik, C.H. Langford, D.J.
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Stufkens, A. Vlcek Jr., , Inorg. Chem. 33 (1994) 2865.
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[21] B.D. Rossenaar, E. Lindsay, D.J. Stufkens, A. Vlcek Jr., , Inorg.
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Ï
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4. Conclusion
[23] B.D. Rossenaar, D.J. Stufkens, A. Oskam, J. Fraanje, K. Goubitz,
Inorg. Chim. Acta 247 (1996) 215.
Although the spectroscopic data show that the lowest-
energy transitions of these [Re(R)(CO)3(dmb)] complexes
have MLCT character, and that homolysis of the Re±alkyl
bond must occur by surface crossing to the reactive sꢀ
state, the mechanism of this excited state process can only
be established with the use of photochemical quantum
yields and time-resolved absorption and emission spectral
data. Such investigations are in progress.
[24] B.D. Rossenaar, C.J. Kleverlaan, M.C.E. van de Ven, D.J. Stufkens,
A. Oskam, J. Fraanje, K. Goubitz, J. Organomet. Chem. 492 (1995)
165.
*
3
[25] B.D. Rossenaar, M.W. George, F.P.A. Johnson, D.J. Stufkens, J.J.
Ï
Turner, A. Vlcek Jr., , J. Am. Chem. Soc. 117 (1995) 11582.
[26] C.J. Kleverlaan, D.M. Martino, H.v. Willigen, D.J. Stufkens, A.
Oskam, J. Phys. Chem. 100 (1996) 18607.
[27] L.A. Lucia, R.D. Burton, K.S. Schanze, Inorg. Chim. Acta 208
(1993) 103.
[28] H.A. Nieuwenhuis, M.C.E. van de Ven, D.J. Stufkens, A. Oskam, K.
Goubitz, Organometallics 14 (1995) 780.
Acknowledgements
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Thanks are due to the Netherlands Foundation for Che-
mical Research (SON) and the Netherlands Organization for
the Advancement of Pure Research (NWO) for ®nancial
support.
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