Low temperature matrix photochemistry of M2(CO) 4(μ-S-t-Bu)2 and [M(CO)2Cl2] 1- anions, where M = Rh and Ir

Article abstract of DOI:10.1016/j.ica.2004.06.034

Photolysis of M₂(CO)₄(μ-S-t-Bu)₂, where M = Rh or Ir, results in loss of CO. Similarly, photolysis of [M(CO) ₂Cl₂]^1- results in CO loss, while Ir(CO)(PMe₃)₂Cl undergoes a structural change akin to that of its rhodium analogue. Photolysis of M₂(CO)₄(μ-S-t- Bu)₂, where M = Rh or Ir, in Nujol matrices at ca. 90 K results in simple CO loss to form a tricarbonyl intermediate analogous to that observed for Rh₂(CO)₄(μ-Cl)₂. Photolysis of the anions, [M(CO)₂Cl₂]^1-, where M = Rh or Ir, in inert ionic matrices at ca. 90 K, results in CO-loss to form an intermediate analogous to that formed by Rh(CO)₂(i-Pr₂HN)Cl. Finally, photolysis of trans-Ir(CO)(PMe₃)₂Cl in a Nujol matrix at ca. 90 K gives rise to a new species whose carbonyl band is shifted slightly down in energy as has been observed for trans-Rh(CO)(PMe₃)₂Cl. In all cases the iridium compounds behave similarly to the rhodium species although the photon energy for iridium photochemistry is typically above that of the rhodium compounds.

Full text of DOI:10.1016/j.ica.2004.06.034

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Inorganica Chimica Acta 357 (2004) 4273–4278
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Low temperature matrix photochemistry of M2(CO)4(l-S-t-Bu)2
and [M(CO)2Cl2]1ꢀ anions, where M=Rh and Ir
*
Thomas E. Bitterwolf , Byron Wong
Department of Chemistry, University of Idaho, Rayburn Street, Moscow, ID 83844-2343, USA
Received 18 May 2004; accepted 26 June 2004
Available online 30 July 2004
Abstract
Photolysis of M2(CO)4(l-S-t-Bu)2, where M=Rh or Ir, in Nujol matrices at ca. 90 K results in simple CO loss to form a tricar-
bonyl intermediate analogous to that observed for Rh2(CO)4(l-Cl)2. Photolysis of the anions, [M(CO)2Cl2]1ꢀ, where M=Rh or Ir,
in inert ionic matrices at ca. 90 K, results in CO-loss to form an intermediate analogous to that formed by Rh(CO)2(i-Pr2HN)Cl.
Finally, photolysis of trans-Ir(CO)(PMe3)2Cl in a Nujol matrix at ca. 90 K gives rise to a new species whose carbonyl band is shifted
slightly down in energy as has been observed for trans-Rh(CO)(PMe3)2Cl. In all cases the iridium compounds behave similarly to the
rhodium species although the photon energy for iridium photochemistry is typically above that of the rhodium compounds.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Rhodium photochemistry; Iridium photochemistry matrix photochemistry; Ionic matrix
1. Introduction
anions. Relevant to these investigations we have also ex-
panded our earlier studies on trans-Rh(CO)(PMe3)2Cl
with the photolysis of trans-Ir(CO)(PMe3)2Cl. This pa-
per describes the examination of these compounds in
frozen Nujol, or, in the case of the carbonyl chloride an-
ions, inert ionic matrices.
Some years ago Bitterwolf and coworkers [1] reported
the photochemistry of Rh2(CO)4(l-Cl)2 in both inert gas
matrices and frozen Nujol matrices. Under these condi-
tions clean loss of CO was observed to yield a tricar-
bonyl intermediate that was fully characterized by IR.
Somewhat more recently we have reported the photo-
chemistry of cis-Rh(CO)2(ammine)Cl derivatives in fro-
zen Nujol and established the presence of two isomeric
CO-loss intermediates in which the remaining CO is ei-
ther trans to Cl or the ammine ligand [2]. Our continued
interest in the application of rhodium derivatives in pho-
tocatalytic reactions prompted us to reopen these inves-
tigations to thiolato bridged rhodium and iridium
derivatives in which the thiol bridge is unusually stable,
as well as the catalytically underutilized [M(CO)2Cl2]1ꢀ
2. Experimental
Rh2(CO)4(l-S-t-Bu)2 [10], [NBu4][Rh(CO)2Cl2] [19],
[NMe4][Ir(CO)2Cl2] [20], and Ir(CO)(PMe3)2Cl [21] were
prepared by literature procedures. Ir2(CO)4(l-S-t-Bu)2
was prepared by reaction of LiS-t-Bu with [NMe4][Ir-
(CO)2Cl2] in THF by analogy to reactions of
[Rh(CO)2Cl2]1ꢀ reported by Klack [22]. The product
from this reaction was identical with that reported by
deMontavzon and Poilblanc [23]. NEt3Oct PF6 and
bmim PF6 are dried before use by heating to ca. 90° C
under vacuum for several days.
*
Corresponding author. Tel.: +208-885-6361; fax: +208-885-6173.
E-mail address: bitterte@uidaho.edu (T.E. Bitterwolf).
0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.06.034

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T.E. Bitterwolf, B. Wong / Inorganica Chimica Acta 357 (2004) 4273–4278
Photochemical methods have been described else-
tra of these solutions at room temperature show
carbonyl stretching bands at 2068, 2048, and 1998
cmꢀ1 and 2062, 2040, and 1987 cmꢀ1 for rhodium and
iridium, respectively. Band positions and assignments
are summarized in Table 1. Spectral assignments are
based on those of Johnson, et al., for Rh2(CO)4(l-
SEt)2 [3]. The band positions of the t-butyl thiol deriva-
tive agree to within experimental error with the positions
of the ethyl thiol derivative. In contrast, the reported
carbonyl stretching bands of Rh2(CO)4(l-SPh)2 are
higher in energy than those of the alkylthiols reflective
of a modest reduction of electron density on the sulfur
atoms as a result of resonance interactions between sul-
fur and the phenyl rings [4]. Like Rh2(CO)4(l-Cl)2 these
spectra are consistent with a molecule with C2v symme-
try, folded about the S-S axis. The molecular structures
of M2(CO)4(l-SPh), where M=Rh [5] and Ir, [6] have
been reported to be isomorphous with two independent
molecules. For the rhodium compound the bending an-
gles of the molecules are 115.2(2)° and 113.7(2)°, while
for the iridium compound the angles are 115.2° and
114.7°. Rh2(CO)4(l-SEt)2 has a bending angle of
105.0(1)° [5a]. Folding about the bridging atoms is gen-
erally found for this class of compounds. For example,
the closely related bimetallic species, Rh2(CO)4(l-Cl)2,
Rh2(CO)2(PMe3)2(l-SPh)2, and Ir2(CO)2[P(OMe)3]2(l-
S-t-Bu)2 have angles of 124° [7], 113° [8], and 132° [9],
respectively.
where [1]. Preparation of samples in NEt3Oct PF6 re-
quire first through mixing of the solids in an
approximate 10:1 ratio of matrix to sample, followed
by heating the mixture to ca. 100 °C until a clear solu-
tion is formed. Upon cooling a small portion of this solid
mixture is placed on a CaF2 window. A second window
is placed on top of he first and the sandwich is heated on
a hotplate until the salt mixture melts. The plates are re-
moved from the hotplate and gently compressed insur-
ing a uniform sample upon cooling. Samples in bmim
PF6 are prepared as for Nujol and treated accordingly.
3. Results and discussion
3.1. Thiol bridged dimers
M2(CO)4(l-S-t-Bu)2, where M=Rh or Ir, readily dis-
solve in Nujol to give deep red/brown solutions. IR spec-
Table 1
Photolysis of M2(CO)4(l-S-t-Bu)2 in Nujol at ca. 90 K
Rh2(CO)4(l-S-t-Bu)2
Rh2(CO)3(l-S-t-Bu)2
Ir2(CO)4(l-S-t-Bu)2
Ir2(CO)3(l-S-t-Bu)2
2068 (a1), 2048 (b1), 1998 (b2)
2058, 1996, 1980
2062 (a1), 2040 (b2), 1987 (b2)
2056, 2001, 1993
No photolysis of either Ir2(CO)4(l-S-t-Bu)2 or
Rh2(CO)4(l-S-t-Bu)2 is observed upon irradiation in
Carbonyl stretching bands in cmꢀ1
.
Fig. 1. (a) Difference spectrum of 10 min photolysis (330 nm<kirr <400 nm) of Ir2(CO)4(l-S-t-Bu)2 and initial spectrum. (b) Difference spectrum of
10 min photolysis (330 nm<kirr <400 nm) of Rh2(CO)4(l-S-t-Bu)2 and initial spectrum. (c) Same as (b) with starting material cancelled.

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T.E. Bitterwolf, B. Wong / Inorganica Chimica Acta 357 (2004) 4273–4278
4275
the visible. In contrast, UV irradiation (330
nm<kirr <400 nm) results in loss of the bands of starting
material and appearance of new bands as shown in the
difference spectra in Figs. 1(a) and (b), respectively. In
both cases a band at 2132 cmꢀ1 assigned to free CO in-
dicates loss of CO. Three carbonyl stretching bands may
be observed for the photoproducts of the two com-
pounds although the central band in the rhodium photo-
product overlaps with the b1 band of the starting
species. In Fig. 1(c) we have exactly cancelled out the
starting bands to reveal the spectrum of the rhodium
photoproduct. Back photolysis (kirr >400 nm) of
both the Rh and Ir samples cleanly reverses the observed
photochemistry.
On the basis of the observed spectra it is not possible
to discriminate between C1 and Cs symmetries for the
photoproduct since both point groups predict three IR
active modes. Since it is generally believed that a weakly
bound solvent molecule enters a site vacated by a car-
bonyl ligand, we propose that the photoproducts have
C1 symmetry retaining the square planar geometry
about the metal atoms. This hypothesis is further sup-
ported by the observation of photoreversal in which
the weakly bound alkane is photochemically displaced
permitting re-complexation of CO.
These studies establish that both Ir2(CO)4(l-S-t-Bu)2
and Rh2(CO)4(l-S-t-Bu)2 undergo photolysis paralleling
that of Rh2(CO)4(l-Cl)2. The carbonyl stretching fre-
quencies of Rh2(CO)4(l-S-t-Bu)2 are about 45 cmꢀ1
lower than those of Rh2(CO)4(l-Cl)2 reflecting the
somewhat better electron donating ability of the thiol
ligands.
Both rhodium and iridium carbonyl thiol compounds
undergo reaction with phosphines to form addition prod-
ucts that thermally undergo CO loss with retention of the
bridge [10]. This contrasts with Rh2(CO)4(l-Cl)2 for
which bridge cleavage occurs even upon addition of am-
ines. Further, the thiol derivatives are not observed to re-
act with either amines or H2 [11]. We are currently
extending our photochemical studies to establish whether
it is possible to photochemically prepare derivatives of
the thiol bridged dimers with these and other ligands.
states of these anions and confirmed the earlier assign-
ments with the further observation that the excited
states are expected to have M–CO antibonding charac-
ter. It is specifically noted that the B1 state correspond-
ing to the highest molar absorbtivity of the low energy
transitions may lead to Rh–CO bond breaking.
To examine the photochemistry of these rhodium and
iridium species we have utilized two ionic matrices,
bmim PF6 (bmim=1-butyl, 3-methylimidazolium) and
NEt3Oct PF6. The former matrix has occasionally been
found by us to undergo secondary reactions with photo-
products, while the latter matrix appears to be inert in
all cases we have examined. In the current case the bmim
cation does not appear to exhibit any unusual interac-
1ꢀ
tion with either RhðCOÞ Cl2 or its photoproduct.
2
IR spectra of [NBu4][Rh(CO)2Cl2] and [NMe4][Ir-
(CO)2Cl2] in the ionic matrices are consistent with their
cis geometries having two strong terminal carbonyl tran-
sitions. Spectral data is presented in Table 2. The ob-
served
carbonyl
1ꢀ
stretching
frequencies
for
RhðCOÞ Cl2 are substantially lower than the values,
2
m
sym =2135 and masym =2058 cmꢀ1, calculated by Boyd
using CCSD(T)/6-331G(d) methods. Photolysis in the
visible (kirr >400 nm) resulted in no observed changes
in the infrared spectra. Upon photolysis in the UV
(330 nm<kirr <400 nm) the bands of the anions were ob-
served to decrease while two new bands grew in each
sample as shown in Fig. 2. The high energy band at
2134 cmꢀ1 is assigned to free CO in the NEt3OctPF6 ma-
trix, thus, as predicted by theory, the primary photopro-
cess under these conditions is CO loss. The single, broad
metal-carbonyl band requires that the photoproduct has
one carbonyl. Back photolysis in the visible (kirr >400
nm) results in clean reformation of the starting species.
The photolysis of [NBu4][Rh(CO)2Cl2] in bmim PF6 dif-
fers from that in NEt3Oct PF6 only in small shifts attrib-
utable to solvent effects.
Boyd and his coworkers propose on the basis of
their dft calculations that the CO loss species,
RhðCOÞ Cl21ꢀ, should have a T-type, C2v, geometry.
2
The calculated IR stretching frequency for this species
using CCSD(T)/6-311G(d) methods is 1996 cmꢀ1, in re-
markable agreement with the observed value. Our pre-
3.2. [M(CO)2Cl2]1ꢀ anions
vious
work
with
amine
derivatives,
Rh(CO)2(ammine)Cl, in frozen Nujol reveals two car-
bonyl stretching bands in the photoproducts that we
have assigned to CO-loss species in which the remaining
Some years ago Geoffroy et al. [12] examined the
UV/visible spectroscopy of [Rh(CO)2Cl2]1ꢀ and
[Ir(CO)2Cl2]1ꢀ in the context of a broad investigation
of the structure and bonding of square planar complex-
es. Utilizing electronic spectroscopy and circular dichro-
ism, these authors assigned the lower energy absorption
bands of the Rh and Ir anions to MLCT transitions aris-
ing from excitations from the metal a1g (z2) and b2g (xy)
Table 2
Photolysis of [R4N][M(CO)2Cl2] in NEt3Oct PF6 at ca. 90 K
1ꢀ
1ꢀ
1ꢀ
RhðCOÞ Cl2
RhðCOÞ22Cl2
2069, 1993
1977
*
IrðCOÞ2Cl2
2051, 1970
1946
orbitals to an a2u orbital that is essentially CO p in
character. Quite recently, Boyd and coworkers, have uti-
lized dft calculations to explore the ground and excited
1ꢀ
IrðCOÞCl2
Carbonyl stretching bands in cmꢀ1
.

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T.E. Bitterwolf, B. Wong / Inorganica Chimica Acta 357 (2004) 4273–4278
Fig. 2. (a) Difference spectrum of 10 min photolysis (330 nm <kirr <400 nm) of [NBu4][Rh(CO)2Cl2] and starting material spectrum. (b) Difference
spectrum of 10 min photolysis (330 nm<kirr <400 nm) of [NBu4][Ir(CO)2Cl2] and starting material spectrum.
CO is trans to either a chloride or ammine ligand.
These observations require the CO-loss species to retain
elements of its square planar geometry. In the current
case, we suggest that the fourth coordination position
is occupied by a weakly bound solvent molecule. This
proposal is consistent with the observed back-photolysis
which is believed to arise from the low energy displace-
ment of a solvent molecule from the photointermediate
permitting recombination with CO.
3.3. Ir(CO)(PMe3)2Cl
We note that for neither the rhodium nor iridium an-
ions is there any suggestion of structural transforma-
tions corresponding to the tetragonal distortions we
have observed for Rh(CO)(PR3)2Cl, where R=Me or
Bu. In the case of the dicarbonyl anions, any species
arising from tetragonal distortion would still have two
carbonyl stretching bands and no such species is ob-
Fig. 3. (a) Spectrum of Ir(CO)(PMe3)2Cl in Nujol at ca. 90 K. (b) Difference of spectrum after 10 min photolysis (kirr =400 70 nm and starting
material spectrum.