Synthesis of chalcogenidoimidodiphosphinato-RhI complexes and DFT investigation of their catalytic activation in olefin hydroformylation

Article abstract of DOI:10.1002/ejic.201200921

The synthesis and spectroscopic characterization of [Rh{R ₂P(S)NP(S)R₂-S,S′}(cod)] [cod = 1,5-cyclooctadiene; R = Ph (1), iPr (4)], [Rh{Ph₂P(S)NP(S)Ph₂-S,S′}(CO) ₂] (2) and [Rh{Ph₂P(S)NP(S)Ph₂-S,S′}(CO) (PPh₃)] (3) is described. The crystal structures of complexes 1 and 3 are also presented. The synthesized Rh^I complexes are essentially not catalytically active against olefin hydroformylation, in contrast to the previously reported complex [Rh{Ph₂P(O)NPPh₂-P,O}(CO) (PPh₃)] (6). Differences in the catalytic activity were interpreted with the aid of a well-defined DFT-based protocol involving relativistic effects and dispersion correction. The steric and electronic effects of the Rh ^I coordination environment with respect to their activation by H ₂ are discussed. The results demonstrate that the presence of the more electronegative oxygen atom in the Rh^I coordination sphere polarizes the H-H bond and promotes its heterolytic cleavage, thereby leading to the formation of a Rh^I-monohydride complex in which the oxygen atom of the ligand is protonated. The different catalytic properties of mono- and dichalcogenidoimidodiphosphinato-Rh^I complexes have been investigated through a combined experimental and computational approach. The results indicate that catalytic activation proceeds by means of the ligand-assisted heterolytic cleavage of H₂. Copyright

Full text of DOI:10.1002/ejic.201200921

FULL PAPER
DOI:10.1002/ejic.201200921
Synthesis of Chalcogenidoimidodiphosphinato–RhI
Complexes and DFT Investigation of Their Catalytic
Activation in Olefin Hydroformylation
Alexios Grigoropoulos,*[a] Dimitrios Maganas,*[a,b]
Dimitrios Symeonidis,[a] Petros Giastas,[c] Andrew R. Cowley,[d]
Panayotis Kyritsis,*[a] and Georgios Pneumatikakis[a]
Keywords: Rhodium / Hydroformylation / Density functional calculations / Hemilabile ligands
The synthesis and spectroscopic characterization of
[Rh{R2P(S)NP(S)R2-S,SЈ}(cod)] [cod = 1,5-cyclooctadiene; R =
Ph (1), iPr (4)], [Rh{Ph2P(S)NP(S)Ph2-S,SЈ}(CO)2] (2) and
[Rh{Ph2P(S)NP(S)Ph2-S,SЈ}(CO)(PPh3)] (3) is described. The
crystal structures of complexes 1 and 3 are also presented.
ted with the aid of a well-defined DFT-based protocol involv-
ing relativistic effects and dispersion correction. The steric
and electronic effects of the RhI coordination environment
with respect to their activation by H2 are discussed. The re-
sults demonstrate that the presence of the more electronega-
The synthesized RhI complexes are essentially not catalyti- tive oxygen atom in the RhI coordination sphere polarizes the
cally active against olefin hydroformylation, in contrast to the
previously reported complex [Rh{Ph2P(O)NPPh2-P,O}(CO)-
(PPh3)] (6). Differences in the catalytic activity were interpre-
H–H bond and promotes its heterolytic cleavage, thereby
leading to the formation of a RhI–monohydride complex in
which the oxygen atom of the ligand is protonated.
that vary between Ͻ90° and Ͼ109.5° leading to metal–
chalcogenidoimidodiphosphinato complexes of different
geometries (Td, D4h or Oh), as well as different ring confor-
mations (boat, chair or twisted boat) of the respective
metallocycles. For instance, [Ni(PhL–S,SЈ)2] remains the
only [NiIIS4]-containing complex that has been isolated in
either a tetrahedral[6,7] or a square-planar[8] geometry in
which the (PhL–S,SЈ)– ligand adopts the twisted boat (in Td)
or the boat (in D4h) conformation, respectively. Likewise,
[Ni(iPrL-Se,SeЈ)2] also shows two stereoisomers (Td and
D4h); however, the (iPrL–Se,SeЈ)– ligand adopts the twisted-
boat conformation in both geometries.[9] In addition, tetra-
hedral [Ni(PhL-O,E)2] (E = S, Se) complexes revert to octa-
hedral ones upon dissolution in coordinating solvents like
dmf and thf that occupy the axial positions.[10,11] These re-
sults demonstrate that the overall geometry is directly corre-
lated with the metallocycle conformation and that their re-
lationship is governed by the nature of the chalcogenide do-
nor atom E, the peripheral groups R and the metal cen-
tre.[1,3,12]
In the course of the last ten years, there has been a re-
newed interest in the exploration of the physicochemical
properties and potential applications of metal complexes
bearing this type of ligands. Consequently, significant work
has been published that is related to various research fields
of inorganic chemistry, as we will briefly outline. The elec-
tronic structure of the tetrahedral [M(PhL–S,SЈ)2] (M =
Fe,[13] Co,[12,14–16] Ni[17]) and octahedral [Ni(PhL–O,E)2-
(sol)2][11] (E = S, Se; sol = dmf, thf) complexes has been
Introduction
Mono- and dichalcogenidoimidodiphosphinato ligands,
that is, [R2P(E)NPR2]– and [R2P(E)NP(EЈ)R2]– (E, EЈ = O,
S, Se; R = aryl or alkyl group), denoted in the following as
(RL–P,E)– and (RL–E,EЈ)–, respectively, are considered as
inorganic (carbon-free) analogues of acetylacetonate
(acac). They exhibit rich coordination chemistry towards
main group and transition metals that has been extensively
reviewed by Ly and Woollins;[1,2] Silvestru and Drake;[3]
and Haiduc.[4,5] These ligands provide a π-delocalized che-
lating system that can be easily modified by varying the
chalcogenide donor atoms E and the peripheral R groups.
Moreover, their structural flexibility allows for bite angles
[a] Inorganic Chemistry Laboratory, Department of Chemistry,
National and Kapodistrian University of Athens,
Panepistimiopolis 15771, Athens, Greece
Fax: +30-210-7274782
E-mail: agrigorop@chem.uoa.gr
kyritsis@chem.uoa.gr
Homepage: http://www.chem.uoa.gr/
[b] Max-Planck Institute for Chemical Energy Conversion,
45470 Mülheim an der Ruhr, Germany
E-mail: dimitrios.manganas@cec.mpg.de
[c] Laboratory of Structural and Supramolecular Chemistry,
NCSR “Demokritos”,
Agia Paraskevi 15310, Athens, Greece
[d] Chemistry Research Laboratory, Department of Chemistry,
University of Oxford,
Mansfield Road, Oxford OX1 3TA, United Kingdom
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200921.
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determined by the combination of various spectroscopic, bonds. Therefore, the generation of a vacant coordination
magnetometric and computational methods. These studies site is less probable, thereby explaining why 5 is catalytically
have defined magnetostructural correlations for the above inactive.[50]
paramagnetic complexes and provided additional insight
The catalytic activity of complexes 1–4 was tested in styr-
into the MIIS4-containing active sites of metalloproteins ene hydroformylation. However, complexes 1, 2 and 4 are
such as native, FeIIS4-containing rubredoxin[18] and its CoII- inactive and complex 3 shows only negligible catalytic ac-
or NiII-substituted analogues.[19]
tivity. Also taking into account our findings for 5 and 6,
Another major recent advancement has been the synthe- apparently the activation of the chalcogenidoimidodiphos-
sis of the Te-containing ligands by Chivers and co- phinato–RhI complexes is hindered as the chalcogenide do-
workers,[20] which has led to a large number of novel metal nor atom becomes softer and less electronegative. There-
complexes.[21] Many of these compounds have been em- fore, the catalytic activation of the [Rh(PhL)(CO)(PPh3)]
ployed by O’Brien et al. as single-source precursors for the complexes 3, 5 and 6 was investigated with the aid of disper-
deposition of metal telluride thin films on glass substrates sion-corrected density functional theory (DFT-D3), also
by chemical vapour deposition.[22,23] This technique has considering relativistic effects, to probe the effect of dif-
also been applied for the preparation of metal sulfide and ferent chalcogen donor atoms. DFT methodologies have
selenide materials using instead complexes containing the been widely used to explain the energetic barriers along the
S- and Se-containing ligands.[24–28] The (PhL–S,SЈ)·I2 ad- RhI-catalyzed hydroformylation mechanism.[55–58] In ad-
duct has been suggested as a potential metal-extraction dition, when compared with higher correlated methods,
agent.[29–31] Diamagnetic tetrahedral metal complexes, DFT shows at least comparable qualitative and often quan-
namely, [M(iPrL–Se,SeЈ)2] (M = Zn, Cd, Hg), as well as titative results.[59,60] The computational and experimental
square-planar ones, [M(iPrL–Se,SeЈ)2] (M = Pd, Pt) have results described in this work explain the observed trends
been studied by solid-state NMR spectroscopy,[32,33] in catalytic activity against olefin hydroformylation by
whereas complexes of d- and f-block elements bearing probing the effects of specific structural changes in the RhI
(
PhL–O,OЈ)– have been used as luminescent materials.[34–38] coordination environment. Along these lines, the hemilabile
In addition, lanthanide and actinide metal complexes of the behaviour of the mono- and dichalcogenidoimidodiphos-
[M(iPrL–E,EЈ)3] type (E, EЈ = S, Se, Te) have been studied phinates is investigated, which reveals their potential appli-
experimentally[39,40] and theoretically[41,42] in an effort to cation for the modification of transition-metal catalyst pre-
probe the covalency of the bonds between f-block elements cursors.
and chalcogen atoms. Last, in the field of homogeneous
catalysis, Leung and co-workers have employed (PhL–S,SЈ)–
to modify RuII-containing polymerization catalysts,[43]
Results and Discussion
(
PhL–O,OЈ)– to synthesize transition-metal catalysts for or-
Synthesis
ganic oxidations[44,45] and (iPrL–O,OЈ)– to prepare novel
MIV-oxo/peroxo complexes (M = Ti, Zr, Ce).[46]
The synthetic procedure that affords the yellow-orange
RhI complexes 1–4 is depicted in Scheme 1. These com-
plexes are air-stable for at least one week. Typical cleavage
of the chlorido bridges of the dinuclear complex [{Rh(μ-
Cl)(cod)}2] upon workup with (PhL–S,SЈ)K or (iPrL–S,SЈ)K
yields 1 and 4, respectively. Treatment of a solution of 1 in
The recent advances in catalytic applications by rhodium
complexes bearing phosphane–chalcogenide donor li-
gands,[47] as well as the well-documented catalytic proper-
ties in olefin hydroformylation of [Rh(acac)(CO)(PR3)][48]
and [Rh(Bp)(CO)(PR3)][49] [Bp = bis(pyrazolylborate)], en-
couraged us to investigate the catalytic properties of the iso-
electronic RhI complexes containing the (RL–P,E)– and
(RL–E,EЈ)– ligands. Herein we describe the synthesis and
spectroscopic characterization of the RhI complexes bear-
ing disulfidoimidodiphosphinato ligands, namely, [Rh(PhL–
S,SЈ)(cod)] (cod
=
1,5-cyclooctadiene) (1), [Rh(PhL–
S,SЈ)(CO)2] (2), [Rh(PhL–S,SЈ)(CO)(PPh3)] (3) and
[Rh(iPrL–S,SЈ)(cod)] (4). It should be remembered that
[Rh(PhL–Se,SeЈ)(CO)(PPh3)] (5) is catalytically inactive and
[Rh(PhL–P,O)(CO)(PPh3)] (6) is catalytically active against
styrene hydroformylation.[50] The catalytic activity of 6 has
been attributed to the hemilabile behaviour of (PhL-P,O)–
that forms a strained planar five-membered ring containing
a weak RhI(soft)–O(hard) bond.[51,52] Such an effectively
weak Rh–O bond can be cleaved and reformed, allowing
for the generation of a vacant site in the RhI coordination
sphere under hydroformylation conditions.[53,54] On the
contrary, (PhL–Se,SeЈ)– forms a more stable six-membered
ring containing two relatively stronger RhI(soft)–Se(soft) 4.
Scheme 1. Synthetic pathway for the preparation of complexes 1–
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CH2Cl2 with a stream of CO affords the dicarbonyl com- the conformation of the (PhL–S,SЈ)– ligand is correlated
plex 2. Replacement of one CO ligand upon treatment of 2 with the back-donation of electron density towards the
by an equivalent amount of PPh3 leads to [Rh(PhL– C=C double bonds of cod, or in fact any other type of
S,SЈ)(CO)(PPh3)] (3). It should be noted that raising the ligand that is a π acid. Clearly, this trend is expected to
metal/phosphane ratio even to 3:1 does not lead to the sub- become more apparent when the ligands coordinated trans
stitution of the second CO group. This is a well-documen- to (PhL–S,SЈ)– have different π-accepting properties (see be-
ted trend since only stronger
π acceptors such as low).
P(OPh)3 and P(NC4H4)3 are capable of replacing both
carbonyl ligands.[61,62] Complex 4 does not participate in
any ligand-exchange reactions, which indicates that the
presence of the electron-donating and bulky iPr peripheral
groups increases the Rh–(cod) bond strength and sterically
hinders ligand-substitution reactions.
Solid-State Crystal Structures
The crystal structures of 1 and 3 reveal a square-planar
geometry around the RhI centre. Complex 1 crystallizes in
¯
space group P1 as yellow plates. The structure is shown in
Figure 1 and selected bond lengths and angles are listed in Figure 1. Crystal structure of 1 with ellipsoids drawn at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
Table 1. Average P–N and P–S bond lengths for the free
protonated ligand (PhL–S,SЈ)H are 1.676 and 1.916 Å,
respectively.[63] Deprotonation and coordination of the lat-
ter to RhI result in the shortening of the P–N (av. 1.594 Å)
and the lengthening of the P–S bonds (av. 2.021 Å), due to
delocalization of the π-electron density among the S–P–N–
Table 1. Selected bond lengths [Å] and angles [°] for complexes 1
and 3.
Complex 1
Complex 3
P–S fragment, as it has already been postulated by struc-
tural data[64] and theoretical calculations.[12] The P2 atom
Rh–S1
2.386(2)
2.391(3)
2.017(3)
2.026(3)
1.599(7)
1.589(5)
2.162(10)
2.155(10)
2.131(9)
2.147(10)
1.381(10)
1.406(10)
98.29(8)
124.6(4)
100.1(1)
110.6(1)
Rh–S1
Rh–S2
S1–P1
S2–P2
P1–N
P2–N
Rh–CO
C–O
Rh–PPh3
S1–Rh–S2
P1–N–P2
Rh–S1–P1
Rh–S2–P2
C–Rh–PPh3
S1–Rh–PPh3
S2–Rh–CO
S1–Rh–S2–P2
S2–Rh–S1–P1
2.4006(1)
2.418(1)
2.032(1)
2.016(1)
1.591(3)
1.598(3)
1.812(4)
1.156(5)
2.277(1)
98.80(3)
123.1(2)
109.24(5)
102.05(5)
88.7(1)
lies almost in the mean S–Rh–S plane, with the correspond- Rh–S2
S1–P1
ing dihedral angle S1–Rh–S2–P2 equal to 6.5°. On the con-
S2–P2
trary, the P1 and N1 atoms are placed well above the hori-
P1–N
zontal plane. The S2–Rh–S1–P1 dihedral angle is 51.4°,
P2–N
while the nitrogen atom is located 1.317 Å above the mean
S–Rh–S plane. Therefore, the RhS2P2N metallocycle is not
planar and adopts a slightly distorted boat conformation,
with the S1 and P2 atoms occupying the apices. It should
be noted that compounds containing σ4,λ5-phosphorus
Rh–C25
Rh–C26
Rh–C29
Rh–C30
C25–C26
C29–C30
atoms can be nonplanar and still exhibit delocalization of S1–Rh–S2
the π-electron density.[65–67] The magnitude of the average
P1–N–P2
Rh–S1–P1
Rh–S bond lengths (2.389 Å) and the S1–Rh–S2 angle
Rh–S2–P2
91.38(3)
81.2(1)
–44.93(5)
–2.03(5)
(98.3°) is typical of similar complexes in which (RL–S,SЈ)–
S1–Rh–S2–P2 6.5(1)
S2–Rh–S1–P1 –51.4(1)
ligands form six-membered rings and adopt the boat con-
formation.[6,68] The cod ligand is side-on coordinated to the
metal centre. The C25–C26 double bond is 0.025 Å shorter
than the C29–C30 bond, whereas the opposite pattern is
In contrast to the crystal structure of 1 presented in this
found for the average length of the respective Rh–C bonds. work, the structure of the same complex that has been re-
This is indicative of stronger back-donation towards the ported by Cheung et al., containing a CH2Cl2 solvent mole-
C29–C30 double bond. Therefore, the degree of back-do- cule in the unit cell (1·CH2Cl2), exhibits a different ring
nation of electron density towards the π* orbitals of the conformation.[69] More specifically, in 1·CH2Cl2 the P
olefin is correlated with the orientation of the trans P–S atoms are located on opposite sides of the S–Rh–S horizon-
bond measured by the corresponding S–Rh–S–P dihedral tal plane. Therefore, the conformation of the metallocycle
angle. Back-donation is increased when the trans P–S bond can be best described as a distorted twisted boat.[2,17,70] The
moves towards the S–Rh–S horizontal plane and the S–Rh– two S–Rh–S–P dihedral angles are 11.2 and 35.5° and in
S–P dihedral angle becomes smaller. In this way, the trans- this case back-donation towards the cod ligand is more ev-
fer of π-electron density from the S 3p orbitals that are enly distributed.
¯
perpendicular to the molecular plane towards the π* orbit- Complex 3 crystallizes in space group P1 as yellow
als of the olefin is enhanced. This observation implies that plates. The crystal structure of this compound is presented
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herein for the first time (Figure 2). Selected bond lengths ence is 0.010 Å.[50] The smaller differentiation between the
and angles are listed in Table 1. The coordination geometry Rh–S/Se bond lengths of the aforementioned complexes,
around the central Rh atom is approximately square planar compared with [Rh(acac)(CO)(PPh3)], reflects the struc-
[root mean square (rms) deviation from the mean plane is tural flexibility and the extensive delocalization of π-elec-
0.04 Å]. The P1 atom, located trans to the CO ligand, is tron density among the metal complexes bearing chalcogen-
displaced by only 0.07 Å from the mean S–Rh–S plane, idoimidodiphosphinato ligands.
with the corresponding dihedral angle S2–Rh–S1–P1 equal
to 2.0°. On the contrary, to absorb the ring strain, the S2–
P2 bond located trans to PPh3 is rotated out of the S–Rh–S
horizontal plane. The S1–Rh–S2–P2 dihedral angle is 44.9°;
hence the RhS2P2N metallocycle adopts the boat conforma-
IR Spectroscopy
tion. The S1–Rh–S2 angle (98.8°) and the average Rh–S
The IR spectra of 1–4 exhibit the well-documented trend
bond length (2.409 Å) are again consistent with the boat
regarding the P–N and P–S bond strength.[64,70] The com-
conformation of the six-membered ring.[6,68] It is well estab-
parison between the ν(PNP) or ν(PS) bands of the com-
lished that the values of the M–S bond lengths and the S–
plexes and the free protonated ligands (Table 2) confirms
M–S bond angles that are associated with the boat confor-
that the P–N bonds are strengthened, whereas the P–S
mation are larger than those found in the chair conforma-
bonds are weakened. This is typical for this family of li-
tion of the disulfidoimidodiphosphinato–metal complexes,
gands, since π delocalization of the extra N lone pair that
in which the S1–M–S2 angle is approximately 90° and the
is generated upon deprotonation results in both an in-
average M–S bond lengths are approximately 0.3 Å
creased P–N and a decreased P–S bond order.[12] The IR
shorter.[2,17,71] The P–N bonds are significantly shortened
spectrum of 2 reveals two very strong bands at 2066 and
(av. 1.595 Å) whereas the P–S bonds are lengthened (av.
1997 cm–1 of approximately equal intensity, due to the sym-
2.024 Å) compared with the free (PhL–S,SЈ)– ligand, which
metric and asymmetric stretch of the two mutually cis CO
again displays the delocalization of π-electron density
ligands. The IR spectrum of 3 shows a single ν(CO) band
among the S–P–N–P–S fragment. It has been pointed out
at 1976 cm–1. Compared with the dicarbonyl complex 2, the
that the difference between the two Rh–O bond lengths in
ν(CO) band in 3 is down-shifted, consistent with the in-
[Rh(acac)(CO)(PR3)] complexes (R = alkyl group or aryl
creased back-donation to the remaining sole CO ligand that
group) arises from the different trans influence exerted by
is a stronger π acid than PPh3. On the contrary, compared
PR3 and CO.[72] For example, the Rh–O bond lengths in
with the diselenido complex 5, the ν(CO) band in 3 is up-
[Rh(acac)(CO)(PPh3)] differ by 0.06 Å.[73] Such is the case
shifted, which indicates the weaker donor ability of (PhL–
for complex 3, in which the Rh–S bond length trans to PPh3
S,SЈ)– compared with (PhL–Se,SeЈ)–.
is 0.017 Å longer than the one trans to CO. In the analo-
gous diselenido complex 5, the Rh–Se bond-length differ-
31P NMR Spectroscopy
31P chemical shifts of complexes 1–4 are listed in Table 2.
In complexes 1, 2 and 4, the P atoms of the metallocycle
are chemically equivalent, therefore, only one doublet is ob-
served due to weak coupling with 103Rh (not resolved in
the case of complex 4). On the contrary, the 31P NMR spec-
trum of complex 3 is much more complicated and it can
only be resolved at lower temperatures. The chemical equiv-
alency of the P atoms of the ligand (PhL–S,SЈ)– is removed
since different ligands are coordinated trans to each S atom
and, therefore, 3 can be best described as an ABMX spin
system (Figure 3).
Figure 2. Crystal structure of 3 with ellipsoids drawn at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
Table 2. Characteristic IR [cm–1] and 31P{1H} NMR spectroscopic data [ppm] in CDCl3 for ligands and complexes 1–4. NMR spectra
were obtained at 298 K with the exception of of 3 (218 K).
Complex
ν(PNP)
ν(PS)
ν(CO)
PA [2JRh,P, Hz]
PB [2JRh,P, Hz]
PX [1JRh,P, Hz]
1
2
3
1167
1169
1151
575
572
563
–
2066
1977
38.8 (d) [3.6]
37.7 (d) [3.6]
36.1 (dd) [3.3]
3J(PAPX) = 24 Hz
62.7 (not resolved)
38.3 (dd) [4.1]
40.5 (ddd) [157]
3J(PBPX) = 8 Hz
4
1194
1168
486
544
–
1962
5[50]
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Table 3. Comparison of ν(CO) [cm–1] and 1JRh,P [Hz] for complexes
of the [RhL(CO)PPh3] type.
Ligand
(acac)–
(Bp)–
(
PhL–S,SЈ)–
(
PhL–Se,SeЈ)–
v(CO)
1983
177.4
1987
156
1977
157
1962
160.2
1JRh,P
Geometric Considerations
The crystallographic and spectroscopic data of com-
plexes 1–4 indicate that the magnitude of back-donation
influences the conformation of the RhS2P2N metallocycle.
To verify this hypothesis, the optimized geometry structures
of complexes 2 and 3 were initially sought and they are
shown in Figure 4. Geometry optimizations were performed
starting from both the boat and the twisted-boat metallocy-
cle conformation. The most important geometric features
of the optimized structures are presented in Table 4. It
should be stressed that the initial metallocycle conforma-
tion (twisted boat or boat) does not affect the final opti-
mized geometry. In fact, in the case of 2, the metallocycle
adopts an ideal twisted-boat conformation. The P atoms of
Figure 3. 31P{1H} NMR spectrum of complex 3 recorded in CDCl3
at 218 K.
The PX atom of PPh3 is directly coordinated to the metal
centre, thus it is deshielded. The coupling with 103Rh (1JM,X
= 157 Hz) is similar to other known complexes bearing
PPh3 bonded to RhI.[74,75] The coupling with the PA atom
of the (PhL–S,SЈ)– ligand, located trans to PPh3, is stronger
(3JA,X = 24 Hz) than the coupling with the PB atom, located
cis to PPh3 (3JB,X = 8 Hz), following the usual J(trans) Ͼ
J(cis) pattern.[76,77] Therefore, overall, PX gives rise to a low-
field ddd peak (δP = 40.5 ppm). The two high-field peaks
correspond to the two P atoms of the (PhL–S,SЈ)– ligand.
(
PhL–S,SЈ)– are directed above and below the S–Rh–S
plane, with both S–Rh–S–P dihedral angles equal to 24°,
whereas the N atom lies in the horizontal S–Rh–S plane.
Both Rh–S (2.434 Å) and Rh–CO bonds (1.825 Å) are mu-
tually equal. Consequently, complex 2 shows an overall C2
symmetry, with the main axis bisecting the C–Rh–C and S–
Rh–S angles. This is a straightforward consequence of the
presence of identical ligands trans to (PhL–S,SЈ)– that im-
pose the same electronic and steric effect and thus do not
distort the intrinsic C2 symmetry of the deprotonated li-
gand.[64] Given that the twisted-boat conformation leads to
larger bite angles,[17] the S–Rh–S angle in 2 (100.9°) opens
up considerably.
3
3
Since JA,X (trans) Ͼ JB,X (cis), the dd peak centred at δ =
36.1 ppm is assigned to PA (trans to PPh3) and the peak
centred at δ = 38.3 ppm is assigned to PB (cis to PPh3). The
comparison between the 31P chemical shifts of complexes 2
and 3 reveals that the PA atom (trans to PPh3 and hence cis
to CO) is shielded, whereas the PB atom (cis to PPh3 and
hence trans to CO) is deshielded. This is attributed to the
stronger π acidity of CO than PPh3, which demonstrates
the transfer of electron density along the P–S–Rh–CO bond
pathway. Unfortunately, even at –80 °C, the coupling be-
tween the P atoms of (PhL–S,S)– could not be resolved.
Cheung et al. reported a different peak assignment for 3, in
which the 31P chemical shifts increase in the reverse order,
that is, δX Ͻ δB Ͻ δA; however, no detailed hyperfine cou-
pling analysis was presented.[69] Moreover, the 31P NMR
spectrum of the analogous diselenido complex 5,[50] in
which the presence of 77Se satellites facilitates peak assign-
ment, strongly supports our analysis.
Figure 4. Optimized geometry structures of complexes 2 (C2 sym-
metry) and 3 at the RI-BP86/TZVPP/TZVP level (C: grey, H: white,
N: blue, O: red, P: orange, Rh: green, S: yellow).
It has been firmly established that the donor strength of
an anionic bidentate ligand in the [RhL(CO)(PZ3)] type of
complexes is strongly correlated with the energy of the re-
The calculated bond lengths and bond angles in the opti-
1
spective ν(CO) band and the magnitude of the JRh,P cou- mized geometry structure of 3 are in good agreement with
pling constant.[49,78] It is therefore interesting to expand the the experimental ones (see the Supporting Information,
series reported by Trzeciak et al.,[49] by including the (PhL– Table S2). The stronger trans influence of PPh3 than CO[72]
S,SЈ)– and (PhL–Se,SeЈ)– ligands (Table 3). Both dichalco- results in the elongation of the respective trans Rh–S bond
genidoimidodiphosphinato–RhI complexes exhibit very low (2.451 Å), whereas the other one is slightly shortened
1
ν(CO) and JRh,P values that are indicative of an electron- (2.429 Å) compared with 2 (Table 4). In the case of 3, the
rich metal centre, as a consequence of the soft nature of the metallocycle adopts a boat conformation, independent
S/Se donor atoms.
from the initial conformation. Therefore, the different con-
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Table 4. Selected bond lengths [Å], angles [°] and ν(CO) [cm–1] of
the optimized geometry structures of 2 and 3 at the RI-BP86/
TZVPP/TZVP level.
Complex 2
Complex 3
Rh–S1 (trans CO)
Rh–S2 (trans P)
Rh–CO
Rh–PPh3
C–O
S2–Rh–S1–P1
S1–Rh–S2–P2
v(CO)
2.434
2.434
1.825
–
1.158
–24
2.429
2.451
1.794
2.220
1.166
5
–24
1979.8
–51
1966.4
formations of 2 and 3 can only be explained by the presence
of different ligands trans to (PhL–S,SЈ)–. In the case of the
dicarbonyl complex 2, the CO groups impose the same elec-
tronic and steric effect and thus do not distort the intrinsic
C2 symmetry of the deprotonated ligand.[64] However, as
was previously discussed, replacement of one CO ligand by
PPh3 leads to unequal Rh–S bond lengths and breaks up
the inherent C2 symmetry of the (PhL–S,SЈ)– ligand. This
allows for the RhS2P2N metallocycle to adopt the boat con-
formation that favours square-planar geometries.[17] The C–
O bond in 3 is lengthened, the Rh–CO is shortened and the
corresponding ν(CO) is decreased, thereby verifying that
back-donation of the π-electron density to the sole CO li-
gand is enhanced when the ligand adopts the boat confor-
mation (Table 4).
Figure 5. Frontier molecular orbitals of complex 3 at the RI-BP86/
TZVPP/TZVP level. Hydrogen atoms have been omitted for clarity.
stronger π acidity of CO than PPh3 drives the respective
trans P–S bond in plane, to facilitate the transfer of electron
density from the chelating ligand towards the CO π* orbit-
als along the S–Rh–CO bond pathway.
To summarize, the coordination of different ligands trans
to (PhL–S,SЈ)– cancels out the built-in C2 symmetry of the
latter that is associated with the twisted-boat conformation.
The greater π acidity of CO than PPh3 drives the trans-to-
Electronic Structure
As previously discussed, complex 2 shows approximate
C2v symmetry around the RhS2(CO)2 core. Replacement of
one CO ligand by PPh3 lowers the symmetry to C1 and
the six-membered RhS2P2N metallocycle adopts the boat
conformation. The composition of the RhI frontier molecu-
lar orbitals of 3 is in agreement with what is expected from
ligand-field theory for a d8 RhI square-planar complex
(Figure 5). The LUMO is the typical σ*-antibonding or-
bital of d8 square-planar complexes that is mainly com-
posed of the RhI 4dx2–y2 orbital, the in-plane S 3p orbitals
of (PhL–S,SЈ)– and the C and P lone pairs. The HOMO is
mainly composed of the RhI 4dz2 orbital (ca. 80%). The
HOMO–1, HOMO–2 and HOMO–3 orbitals comprise the
CO P–S bond in plane, to promote
(
PhL–S,SЈ)– Ǟ
RhI ǞCO back-donation of electron density. This gives rise
to a boat conformation of the RhS2P2N metallocycle in 3.
More importantly, the departure from C2 symmetry renders
the two Rh–S bonds chemically nonequivalent, which sug-
gest a potential hemilabile behaviour of the (PhL–S,SЈ)– li-
gand.
Metallocycle Conformation
To further verify the correlation between the magnitude
RhI dπ orbitals, the out-of-plane
(
S
3p orbitals of of the S–Rh–S–P dihedral angles and the back-donation
PhL–S,SЈ)– and the unoccupied π* orbitals of CO and towards π-accepting ligands, relaxed potential energy sur-
PPh3. Most importantly, the boat conformation of the face (PES) scans along the S–Rh–S–P dihedral angle trans
metallocycle maximizes the orbital overlap along the S–Rh– to CO (ωCO) were performed in 5° steps for the model com-
CO bond pathway. The P–S bond that is located trans to plexes [Rh(MeL)(CO)(PMe3)] (Me3PMe3), [Rh(MeL)(CO)-
CO lies in the molecular plane, causing the corresponding (PH3)] (Me3PH3) and [Rh(MeL)(CO)(PF3)] (Me3PF3), in
S 3p lone pair to be oriented parallel to the RhI 4dxz or- which different phosphanes are coordinated to RhI. From
bital. In this way, the overlap between these two orbitals is the diagram presented in Figure 6, it is evident that the S–
maximized. HOMO–2 represents the antibonding interac- Rh–S–P dihedral angle that corresponds to the lowest en-
tion, whereas the respective bonding combination lies lower ergy structure for each phosphane follows Tolman’s elec-
in energy. On the contrary, the P–S bond located trans to tronic parameter (TEP),[78] that is, PMe3 Ͻ PH3 Ͻ PF3.
PPh3 is rotated out of the molecular plane and the respec- The ωCO dihedral angle is minimized (ca. 20°) when a
tive overlap between the associated S 3p lone pair and the stronger donor, like PMe3, is introduced in the coordination
RhI 4dyz orbital is less efficient (HOMO–1). Therefore, the sphere. On the contrary, when the phosphane is a strong
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acceptor, like PF3, the ωCO is maximized (ca. 35°), whereas energy than the ones of the respective disulfido complexes,
the corresponding value for PH3 (ca. 25°) is calculated in which verifies that the diselenidoimidodiphosphinato ligand
between. The steric effect does not seem to play an impor- is a stronger donor.
tant role since the value of ωCO neither follows Tolman’s
cone angle (θ) nor changes significantly when PCl3 is intro-
duced. The latter is larger than PF3 but shows similar elec-
tron-withdrawing properties.
Catalytic Activation Mechanism
The catalytic activity against styrene hydroformylation
for complexes 1–4 was explored. Complexes 1, 2 and 4 are
essentially inactive, whereas complex 3 shows negligible
catalytic activity (yield is less than 20% at 40 °C in
CH2Cl2). Most likely, (PhL–S,SЈ)– is strongly coordinated to
RhI, thus preventing the generation of a vacant site, even
in the case of complex 3 in which the two Rh–S bonds are
nonequivalent (see above). This also explains the catalytic
inactivity of complex 5,[50] since (PhL–Se,SeЈ)– is an even
better donor, as was previously discussed. On the contrary,
complex 6 shows substantial catalytic performance,[50]
which indicates that the (PhL–P,O)– ligand is less tightly
bound to RhI owing to the presence of a relatively weaker
Rh(soft)–O(hard) bond. It is well established that under
olefin-hydroformylation conditions the catalytically active
Figure 6. Relaxed PES scan at the RI-BP86/TZVPP/TZVP level
species is a [RhI–H] complex.[79] Therefore, DFT calcula-
along the S–Rh–S–P dihedral angle trans to CO (ωCO) in 5° steps
for Me3PMe3 (green), Me3PH3 (red) and Me3PF3 (black).
tions were performed to elucidate how these complexes are
activated by H2 and why only 6 forms a catalytically active
[RhI–H] species. Also taking into account that the initial
interaction of H2 with any metal centre results in the forma-
tion of a [M–(η2-H2)] complex that may correspond to a
true reactive intermediate or a transition structure,[80–84] we
initially explored the stability of such [RhI–(η2-H2)] com-
plexes in which one Rh–E (E = O, S, Se) bond is cleaved
and H2 is side-on coordinated.
Our computational study was based on the correspond-
ing truncated models Me3PMe3, Me5PMe3 and Me6PMe3, in
which all phenyl groups were replaced by methyl groups.
This type of truncation is not expected to significantly af-
fect qualitatively the following analysis of the catalytic acti-
vation of complexes 3, 5 and 6, since the steric effect of a
larger phosphane will have the same influence on all com-
The previous analysis shows that the conformation of the
RhS2P2N metallocycle is dictated by the electron-donating
or -withdrawing properties of the coordinated phosphane
ligand. The comparison of the optimized geometry struc-
tures of Me3PMe3, Me3PH3, Me3PF3 and Me2 (Table 5) con-
firms that the P–S bond that is located trans to the stronger
π acid PF3 moves in plane. On the contrary, the P–S bond
that is located trans to the stronger electron-donating phos-
phane PMe3 is rotated out of plane and back-donation to
the CO ligand is enhanced, as can be deduced from the
calculated ν(CO) frequencies. The same trend is observed
when the respective diselenido model complexes are exam-
ined (Table 5). It should be also noted that the calculated
ν(CO) frequencies for the diselenido complexes are lower in
plexes. Nevertheless, we have also calculated the buried vol-
[85]
ume (%VBur
)
of different phosphane ligands in 3 as a
Table 5. S/Se–Rh–S/Se–P dihedral angles ωCO (trans to CO) and
ωP (trans to PZ3), ν(CO) frequencies [cm–1] and C–O bond lengths
[Å] for the optimized geometry model complexes [Rh{MeL–
(E,E)}](CO)(PZ3)] (E = S or Se and Z = Me, H, F) and [Rh{MeL–
(E,E)}(CO)2] (used as a reference) at the RI-BP86/TZVPP/TZVP
level.
quantitative measurement of the steric demand of such a
truncation. As shown in Table S13 in the Supporting Infor-
mation, %VBur of PZ3 ligands at the optimized Rh–P dis-
tances is lowered from 30.8 (Z = Ph) to approximately 24
(Z = Me, Cl). Hence, the order of magnitude of the steric
demand is retained when PPh3 is substituted by smaller
phosphanes. Furthermore, the truncated models were fully
optimized along the corresponding interconversion path-
ways by employing the same DFT methods as described
above. In this way, one hopes to incorporate the steric and
electronic effects of the bulkier groups into the DFT calcu-
lations without facing the prohibitive costs of the calcula-
tions on the full systems.
Complex
ωCO ωP ν(CO)
C–O
TEP
θ
[°]
[°]
[°]
[cm–1]
Me2
24
20
24
35
35
23
21
26
34
33
24 2016.9[a] 1.158
–
–
118
87
104
124
–
118
87
Me3PMe3
Me3PH3
Me3PF3
Me3PCl3
Me5(CO)2
Me5PMe3
Me5PH3
Me5PF3
Me5PCl3
29 1956.2
25 1972.5
14 2010.5
14 2005.2
1.169
1.166
1.159
1.159
2064
2083
2111
2097
–
2064
2083
2111
2097
23 2008.4[a] 1.159
26 1950.0
21 1965.8
13 2003.1
14 1997.5
1.170
1.167
1.160
1.160
For the disulfido complex Me3PMe3, the only [RhI–(η2-
H2)] complex that is identified as a stable intermediate is
104
124
[(k1-MeL–S,SЈ)Rh(η2-H2)(CO)(PMe3)]
(
Me3PMe3–H2), in
[a] Average value of the symmetric and asymmetric stretch mode.
which the Rh–S bond that is located trans to PMe3 is
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cleaved and the vacant site is now occupied by H2 (Figure 7, (Table 6). More importantly, this [RhI–H] complex could
S,S). In all other cases, including the one in which H2 ap- serve as the actual catalytic species during olefin hydrofor-
proaches perpendicularly to the molecular plane, H2 does mylation.
A
not bind to the metal centre but instead dissociates and the
In this context, the formation of this [RhI–H] key species
original complex is reformed. Me3PMe3–H2 displays square- was investigated further in silico for complexes 3, 5 and 6.
planar geometry and H2 is side-on coordinated. The corre- Figure 8 shows the relaxed PES scan along the H–H bond
sponding diselenido complex [(k1-MeL–Se,SeЈ)Rh(η2- length, starting from the aforementioned [RhI–(η2-H2)] in-
H2)(CO)(PMe3)] (Me5PMe3–H2) is also identified as the termediates. The total electronic energy values are calcu-
only stable [RhI–(η2-H2)] product of the interaction of H2 lated relative to the total electronic energy of the free reac-
with Me5PMe3 (Figure 7, ASe,Se). However, the presence of tants, that is, the initial rhodium complex and H2. The opti-
an oxygen atom in the RhI coordination sphere of Me6PMe3 mized geometry structures of all the local minima are
renders the inherently weaker Rh–O bond as the most shown in Figure 7, the most important bond lengths are
plausible candidate for bond cleavage. Moreover, the inter- listed in Table 6 and xyz coordinates are provided in the
mediate in which H2 is side-on coordinated to the metal Supporting Information (Tables S3–S12). For the disulfido
centre is not stable. Instead, H2 is directly heterolytically complex Me3PMe3, the first local minimum (point A) corre-
split leading to the square-planar complex [(k1-MeL–P,OH)- sponds to Me3PMe3–H2 that is found 65.1 kJmol–1 higher
Rh(H)(CO)(PMe3)] (Me6PMe3-OH,H) in which the oxygen in energy than the free reactants (Me3PMe3 + H2). The H–
atom of the ligand is protonated (Figure 7, B). The hydride H bond is considerably elongated owing to back-donation
is coordinated trans to CO and the protonated oxygen atom of the electron-rich RhI centre (H–H = 0.904 Å vs. 0.734 Å
is occupying the axial position at a nonbonding distance for H2 itself). The P1–S1 bond bearing the sulfur atom that
is bound to the metal centre is weakened, whereas the P2–
S2 bond bearing the noncoordinated sulfur atom is
strengthened. The opposite trend is observed for the respec-
tive P–N bonds. Overall, the ligand shows a more localized
distribution of electron density (Rh–S–P=N–P=S reso-
nance structure). Increasing the H–H distance to 2.1 Å
leads to [(k1-MeL–S,S)Rh(H)2(CO)(PMe3)]
(
Me3PMe3-
H,H). This [RhIII–(H)2] complex (point D; 50.9 kJmol–1) is
formally the product of H2 oxidative addition to RhI and
is calculated 14.2 kJmol–1 lower in energy than Me3PMe3–
H2, whereas the related transition state (point C;
73.6 kJmol–1) is calculated at only 8.5 kJmol–1. An inde-
pendent geometry optimization for this complex shows no
imaginary frequencies. Me3PMe3-H,H shows square-py-
ramidal geometry with one coordinated hydride occupying
the axial position (Figure 7, DS,S). The ligand reverts back
to the completely delocalized resonance structure, in which
both P–S and P–N bonds are almost equal in length
(Table 6). This shows that the axial hydride electronically
communicates with the free end of the (MeL–S,SЈ)– ligand
that lies in close proximity (S–Hax = 1.704 Å). Further in-
creasing the H–H distance leads to the reductive elimi-
nation of the axial hydride and the concomitant proton-
ation of the free end of the (MeL–S,S)– ligand, which results
Figure 7. Optimized geometry structures at the dispersion-cor-
rected RI-BP86-D/TZVPP/TZVP level for the intermediates iden-
tified along the H–H bond cleavage relaxed PES scan. (C: grey, H:
white, N: blue, O: red, P: orange, Rh: green, S: yellow, Se: orange-
yellow). Hydrogen atoms of the methyl groups have been omitted
for clarity.
Table 6. Most important bond lengths [Å] for the optimized geometry intermediates at the dispersion-corrected RI-BP86-D/TZVPP/
TZVP level for the optimized geometry intermediates identified along the proposed mechanism for the activation of 3, 5 and 6 by H2.
Complex
H1–H2
Rh–H1
Rh–H2ax
P1–E1cord
P2–E2
P1–N
P2–N
Me3PMe3
–
0.904
2.185
–
–
–
1.801
1.668
–
2.079
2.105
2.080
2.078
2.228
2.254
2.229
2.238
2.073
2.029
2.077
2.143
2.224
2.178
2.222
2.276
1.596
1.646
1.640
1.623
1.637
1.667
1.641
1.626
1.636
1.661
1.677
1.694
1.649
1.659
1.634
1.614
1.649
1.659
1.635
1.623
1.629
1.608
Me3PMe3–H2
Me3PMe3-H,H
Me3PMe3-SH,H
Me5PMe3
1.705
1.603
1.624
–
1.699
1.601
1.627
–
–
–
Me5PMe3–H2
Me5PMe3-H,H
Me5PMe3-SeH,H
Me6PMe3
0.914
2.177
–
–
–
1.787
1.642
–
–
Me6PMe3-OH,H
1.664
2.073
–
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Figure 8. Relaxed PES scan along the H–H bond-cleavage pathway in 0.1 Å steps at the RI-BP86-D/TZVP level for Me3PMe3 (open
triangles), Me5PMe3 (open cycles) and Me6PMe3 (closed squares). Structures of the local minima for Me3PMe3 and Me6PMe3 are shown.
Total energy values are calculated relative to the sum of the total energy of the respective [MeLRhCOPMe3] complex and H2.
in [(k1-MeL-S,SH)Rh(H)(CO)(PMe3)]
(
Me3PMe3-SH,H).
The same pathway is found for the diselenido model
The π-electron density is again completely localized (Rh– Me5PMe3. The H2 oxidative-addition product Me5PMe3-
S=P–N=P–SH resonance structure), since the pendant sulf- H,H (point D; 50.7 kJmol–1) is calculated 16.6 kJmol–1
ur atom is now protonated. However, the PES scan shows lower in energy than Me5PMe3–H2 (point A; 67.3 kJmol–1),
that this [RhI–H] complex (point F; 100.4 kJmol–1) lies whereas the associated transition state is again insignificant,
44.5 kJmol–1 higher in energy than Me3PMe3-H,H, whereas calculated at 6.7 kJmol–1 (point C; 74.0 kJmol–1). However,
the associated transition state (point E; 114.5 kJmol–1) is the calculated transition state (point E; 126.7 kJmol–1) for
calculated at 63.6 kJmol–1. Therefore, it is the reductive eli- the hydride reductive-elimination step is found at
mination step of the axial hydride that is both kinetically 76.0 kJmol–1 and the final [RhI–H] complex Me5PMe3-
and thermodynamically not favoured, clarifying the poor SeH,H (point F; 111.9 kJmol–1) is calculated 61.2 kJmol–1
catalytic activity of complex 3 against olefin hydroform- higher in energy than Me5PMe3-H,H. Figure 8 also includes
ylation.
the relaxed PES scan for the model complex Me6PMe3, con-
Figure 9. Contour plot of the relaxed PES scan along the Rh–H1 and O–H2 distances in 0.1 Å steps at the dispersion-corrected RI-
BP86-D/TZVPP/TZVP level for the activation of Me6PMe3 by H2. Total energy values are calculated relative to the sum of the total
energy of Me6PMe3 and H2.
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taining the (MeL-P,O)– ligand. The shallow minimum at H– complex in which the H–H bond has been heterolytically
H = 1.5 Å corresponds to a minor conformational change cleaved and the ligand is protonated. The protonated oxy-
of the metallocycle. As previously discussed, the [RhI–(η2- gen atom lies in close proximity with the hydride. However,
H2)] complex does not correspond to a local minimum this Rh–H···H–O interaction is weak and an inward rota-
(point A) but instead H2 is directly heterolytically split, giv- tion of the free end of the ligand would lead to the more
ing rise to Me6PMe3-OH,H (point B) that lies 27.6 kJmol–1 stable Me6PMe3-OH,H complex in which the protonated
higher in energy than the free reactants (Me6PMe3 + H2).
oxygen atom occupies the axial position (Figure 7, B).
The fact that Me6PMe3 does not form a stable [RhI–(η2-
In summary, in the case of Me6PMe3, the Rh–O bond is
H2)] complex prompted us to further explore its interaction cleaved as H2 approaches the metal centre. The PES con-
with H2. Figure 9 depicts the contour plot of the PES scan tour plot also reveals that the respective [RhI–(η2-H2)] spe-
along the H2 heterolytic cleavage pathway in which the Rh– cies is not a stable intermediate, but H2 is instead directly
H1 and O–H2 distances have been simultaneously scanned. heterolytically split. Therefore, the catalytic activation of
The sum of the total electronic energies of Me6PMe3 and the RhI complexes 3, 5 and 6 by H2 follows different path-
H2 was chosen as our zero-point of reference. H2 ap- ways. For complexes 3 and 5, bearing the softer chalco-
proaches the RhI centre by the sterically preferred trajec- genides (S, Se) as donor atoms, the formation of a catalyti-
tory, that is, perpendicularly to the molecular plane. For cally active [RhI–H] species proceeds through (i) H2 side-
Rh–H1 and O–H2 distances that are both greater than on coordination, (ii) H2 oxidative addition and (iii) hydride
2.2 Å, H2 is only weakly interacting with Me6PMe3, as it reductive elimination (Scheme 2, top). The delocalization of
can be deduced from the length of the H–H bond (H–H = π-electron density allows for the P–S/Se and P–N bonds to
0.754 Å). This gives rise to a plateau in the PES (Figure 9, alter between formally single and double to accommodate
I) that lies approximately 15 kJmol–1 higher in energy than the induced structural changes. However, complex 6, con-
the free reactants. When the Rh–H1 distance decreases be- taining the harder oxygen atom, is activated through the
low 2.2 Å, the Rh–O bond is lengthened and the H–H bond direct heterolytic splitting of H2 (Scheme 2, bottom). Both
is slightly elongated (Figure 9, II). The contour plot shows mechanisms have been proposed for the activation of RhI
a saddle point at 61.1 kJmol–1 for which the Rh–H1 and catalyst precursors.[86,87] However, in the case of complexes
the O–H2 bond lengths vary between 1.7–1.9 Å and 1.4– 3 and 5, the H2 oxidative addition [RhIII–(H)2] product,
1.6 Å, respectively (Figure 9, III). This area corresponds to which is clearly catalytically inactive, is calculated as the
the
[RhI–(η2-H2)]
complex
[(k1-MeL–P,O)Rh(η2- most stable intermediate. Furthermore, the ensuing re-
H2)(CO)(PMe3)] in which the H–H bond is considerably ductive elimination of one coordinated hydride that would
elongated (H–H = 0.903 Å) and the Rh–O bond is broken. give rise to a catalytically active [RhI–H] species is not ther-
The fact that this [RhI–(η2-H2)] complex lies on a saddle modynamically and kinetically probable.
point verifies our previous results regarding the instability
The fact that 3 shows very limited catalytic activity,
of such a species in the case of Me6PMe3. It should be also whereas 5 is completely inactive, can be attributed to the
remembered that the respective [RhI–(η2-H2)] complexes lower electronegativity of selenium than sulfur that is obvi-
containing sulfur (Me3PMe3–H2) or selenium (Me5PMe3– ously crucial for the ligand-assisted hydride reductive elimi-
H2) as donor atoms were identified as stable intermediates nation to take place.[86] This is also supported by the fact
(Figure 7, A). On the contrary, [(k1-MeL–P,O)Rh(η2- that the ΔH and ΔG values (Table 7) of the oxidative ad-
H2)(CO)(PMe3)] is related to a transition state that con- dition step are almost identical for 3 and 5, since the ligand
nects the aforementioned plateau and the local minimum at is not actively involved in this step, whereas non-negligible
38.1 kJmol–1, located at the bottom left corner of the con- differences are found in the subsequent reductive elimi-
tour plot (Figure 9, IV). This corresponds to a [RhI–H] nation step. On the contrary, the presence of the more elec-
Scheme 2. Proposed mechanism for the activation of complexes 3, 5 and 6 by H2 towards a catalytically active [RhI–H] species.
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tronegative oxygen atom in the RhI coordination sphere in nation by means of an intramolecular acid–base reaction.
complex 6 promotes the direct H2 heterolytic splitting, However, the [RhIII–(H)2] species that are generated during
thereby giving rise to a stable and catalytically active [RhI– the oxidative addition step are calculated as the most stable
H] complex (Me6PMe3–OH,H). Moreover, the ΔH and ΔG intermediates. Moreover, the subsequent hydride reductive
values (Table 7) of the respective formation reaction, al- elimination step by the (L–E,E)– ligand (E = S, Se) is both
though positive, are not prohibitive, especially under hydro- kinetically and thermodynamically not favoured, as can be
formylation conditions.
deduced from the respective relaxed PES scan along the H–
H bond-cleavage pathway (Figure 8). On the contrary, com-
plex 6, containing the harder oxygen atom in the RhI coor-
dination sphere is activated through an alternative mecha-
nism during which H2 is directly heterolytically split. The
oxygen atom acts as a base that polarizes the H–H bond
and promotes its heterolytic cleavage. This gives rise to a
catalytically active [RhI–H] species in which the oxygen
atom of the ligand is protonated. This type of ligand-as-
sisted activation by H2 has been proposed for late-transi-
tion-metal complexes[86–93] as well as main-group com-
pounds.[94]
Overall, this revisiting work on the mono- and dichalco-
genidoimidodiphosphinato ligands aims to interpret the ex-
perimentally observed different catalytic properties of the
respective RhI complexes against olefin hydroformylation.
Future experimental work will benefit from these findings
Table 7. Thermodynamic values [kJmol–1] for the activation of
Me3PMe3, Me5PMe3 and Me6PMe3 according to the reaction
[RhMeLCOPMe3] + H2 Ǟproduct at the dispersion-corrected RI-
BP86-D/TZVPP/TZVP level.
Product
ΔH
ΔG
Me3PMe3–H2
39.4
38.7
20.4
21.5
84.5
91.5
20.8
71.5
70.0
51.7
Me5PMe3–H2
Me3PMe3–H,H
Me5PMe3–H,H
Me3PMe3–SH,H
Me5Me3–SeH,H
Me6PMe3–OH,H
55.0
122.9
127.0
49.6
Conclusion
In this study, we report on the synthesis and spectro- regarding the hemilabile and cooperative nature of this fam-
scopic characterization of the RhI complexes 1–4 contain- ily of ligands to design more efficient catalytic systems.
ing the disulfidoimidodiphosphinato ligands [R2P(S)NP-
(S)R2]– (R = Ph, iPr). The crystal structures of complexes
1 and 3 show a square-planar geometry around the RhI
central atom in which the RhS2P2N metallocycle adopts the
boat conformation. This conformational preference is at-
tributed to the enhancement of back-donation of the elec-
tron density towards π-accepting ligands. The correlation
between the conformation of the RhS2P2N metallocycle
Experimental Section
IR and NMR Spectroscopy: IR spectra were recorded in the range
4000–200 cm–1 on a Perkin–Elmer 883 IR spectrophotometer with
samples as KBr discs. 1H and 31P spectra were recorded in a Varian
Unity Plus 300 MHz instrument, operating at 299.95 and
121 MHz, respectively, using CDCl3 as solvent. The 1H and 31P
and the electronic properties of the ligands coordinated
chemical shifts are relative to SiMe4 and 85% H3PO4, respectively.
Electrospray mass spectra were acquired on an MDX Sciex API
trans to (PhL–E,EЈ)– (E = S, Se) is verified by a detailed
computational study utilizing relativistic density functional
theory. The boat conformation renders the two Rh–E bonds
nonequivalent, which suggests a potential hemilabile behav-
iour of the (L–E,EЈ)– ligands. The latter is crucial for the
catalytic activation of this type of complex. The intercon-
nection between electronic properties and structural prefer-
ences presented herein can be used as a guide for the proper
Qstar Pulsar mass spectrometer (Toronto, Canada) using an elec-
trospray ionization source.
X-ray Crystal Structure Determination: A brief summary of crystal
data for complexes 1 and 3 is presented in the Supporting Infor-
mation (Table S1). A crystal of 1 (yellow plate) was mounted in
air. Diffraction data were measured at room temperature using a
Syntex diffractometer equipped with a Rigaku rotating anode
modification of the peripheral groups of the chalcogenido- (graphite monochromator Cu-Kα radiation, λ = 1.5418 Å) by em-
ploying the θ/2θ scanning method. Three standard reflections,
monitored every 97 reflections, showed no decay in intensity during
data collection. The data were corrected for Lorentz and polariza-
tion effects and a semiempirical absorption correction, based on
imidodiphosphinato ligands to further enhance their hemil-
abile properties.
The catalytic activity against styrene hydroformylation of
complexes 1–4 was explored and compared with the sub-
stantial catalytic activity of 6, as well as the lack of activity
of 5.[50] Among 1–4, complex 3 shows only poor catalytic
activity, whereas the remaining complexes are essentially in-
active. In fact, catalytic activity depends on the electronega-
tivity of the chalcogenide donor atom (Se Ͻ S Ͻ O). This
trend was elucidated with the aid of DFT-based computa-
tional methods using truncated model complexes. Acti-
vation of the complexes containing the softer chalcogenides
S and Se towards catalytically active [RhI–H] species can
proceed through the step-wise (i) H2 side-on coordination,
the psi scans, was applied.[95] The structure was solved in the space
[96]
¯
group P1 by direct methods with SHELXS
and was refined by
full-matrix least-squares cycles based on F2 with SHELXL-97.[96]
All H atoms were placed in idealized positions and refined by a
riding model (UH = 1.30UC). All non-H atoms were modelled with
anisotropic displacement parameters.
A crystal of 3 (yellow plate) was mounted on a glass fibre using
perfluoropolyether oil and was cooled rapidly to 150 K in a stream
of cold N2 using an Oxford Cryosystems Cryostream unit. Diffrac-
tion data were measured using an Enraf–Nonius Kappa CCD dif-
fractometer (graphite monochromator, Mo-Kα radiation, λ =
¯
(ii) H2 oxidative addition and (iii) hydride reductive elimi- 0.71073 Å). The structure was solved in the space group P1 using
Eur. J. Inorg. Chem. 2013, 1170–1183
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FULL PAPER
the direct methods program SIR-92,[97] which located all non-hy-
drogen atoms. Coordinates and anisotropic thermal parameters of
all non-hydrogen atoms were refined. Hydrogen atoms were posi-
tioned geometrically after each cycle of refinement. A three-term
Chebychev polynomial weighting scheme was applied.
[Rh(PhL–S,S)(CO)2] (2): Complex 1 (0.025g = 0.038 mmol) was dis-
solved in CH2Cl2 (10 mL). Carbon monoxide was then passed
through the solution for 15 min. The mixture was concentrated un-
der vacuum to 2–3 mL and layering with hexane (30 mL) afforded
1
the yellow solid product with 87% yield. H NMR (CDCl3): δH
=
7.91 (m, 8 H, aromatics), 7.42 (m, 12 H, aromatics) ppm. 31P{1H}
CCDC-668479 (for 3) and -668480 (for 1) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
NMR (CDCl3, –60°): δP = 37.7 (d, 2JRh,P = 3.6 Hz) ppm. IR (KBr):
ν = (CO) 2066 (vs), 1997 (vs); ν(PNP) 1169 (br); ν(PS) 572 (m)
˜
cm–1.
[Rh(PhL–S,S)(CO)(PPh3)] (3): A mixture of complex 3 (0.020 g,
0.033 mmol) and PPh3 (0.009 g = 0.034 mmol) in CH2Cl2 (10 mL)
was stirred for approximately 2 h and then concentrated to 2–3 mL
under vacuum. Layering with n-hexane (30 mL) afforded yellow
crystals with 86% yield. C43H35NOP3RhS2 (841.70): calcd. C 61.36,
Computational Details: All calculations were performed with the
ORCA computational package (version 2.9).[11] The BP86[98,99]
functional was chosen for geometries, frequencies and electronic
properties, respectively. The Ahlrichs polarized triple-ζ-quality ba-
sis sets TZVPP and TZVP[100] were used for the Rh center and the
ligand atoms, respectively, in combination with the TZV/J Cou-
lomb fitting basis for the resolution of identity[101,102] (RI in BP86
calculations). For geometry optimizations, a one-centre relativistic
correction was applied by employing the implemented standard
second-order Douglas–Kroll–Hess (DKH) procedure.[103–105] The
structures were optimized in the gas phase at the RI-BP86/TZVPP/
TZVP level together with the Grimme’s dispersion correc-
tion.[106,107] Frequencies were calculated using a numerical differen-
tiation of analytic gradients with an increment of 0.005 Bohr. Zero-
point vibrational energies, thermal corrections and Gibbs free ener-
gies were obtained from the frequency calculations at the disper-
sion-corrected RI-BP86-D/TZVPP/TZVP level. Localized orbitals
were computed according to the Pipek–Mezey population localiza-
tion scheme[108] Calculations were performed on whole complexes
starting from crystallographic coordinates, imposing both the boat
and the twisted-boat conformation on the respective metallocycles
and allowing all the geometric parameters to fully relax. In ad-
dition, truncated models were constructed in which the phenyl
groups were replaced by methyl groups. The total electronic energy
of the free reactants, that is, the initial rhodium complex and H2,
was always set as the zero point of reference for all the relaxed PES
scans presented herein.
1
H 4.19, N 1.66, S 7.62; found C 61.18, H 3.30, N 1.65, S 8.51. H
NMR (CDCl3): δH = 7.93 (m, 8 H, m-C6H5), 7.43 (m, 12 H, o,p-
C6H5), 7.50 (m, 15 H, PC6H5) ppm. 31P{1H} NMR (CDCl3): δP
=
1
3
40.5 (m, PX, JRh,P = 157 Hz, JP ,P = 24 Hz); 38.3 (dd, PB,
X
A
X
2JRh,P = 4.1 Hz, JP ,P = 8 Hz), 36.1 (dd, PA, JRh,P = 3.3 Hz,
3
2
3JP ,P = 24 Hz) ppm, iXn which PA is coordinated tranAs and PB is
B
B
coordXinated cis to P Ph . IR (KBr): ν = (CO) 1977 (vs); ν(PNP)
A
˜
X
3
1161 (sh), 1151 (br); ν(PS) 563 (m) cm–1.
[Rh(iPrL–S,S)(cod)] (4): A mixture of [{Rh(μ-Cl)(cod)}2] (0.050 g,
0.101 mmol) and (iPrL–S,S)K (0.063 g, 0.202 mmol) in CH2Cl2
(10 mL) was stirred for approximately 2 h at room temperature.
The solvent was evaporated under vacuum and the solid residue
was dissolved in diethyl ether (10 mL). The resulting solution was
filtered through Celite to remove KCl. Slow evaporation of the fil-
trate afforded yellow crystals with 55% yield. C20H40NP2RhS2
(523.52): calcd. C 45.88, H 7.70, N 2.68, S 12.25; found C 46.13,
H 7.95, N 2.45, S 12.08. 1H NMR (CDCl3): δH = 4.16 (s, 4 H,
CH), 2.37 (m, 4 H, CH2), 2.07 (m, 2 H, CH-iPr), 1.86 (m, 4 H,
CH2), 1.23 (m, 6 H, CH3-iPr) ppm. 31P{1H} NMR (CDCl3): δP
=
62.7 (br) ppm. IR (KBr): ν = (CH) 2918–2800 (m); ν(PNP) 1194
˜
(br); ν(PS) 486 (m) cm–1. ES-MS: m/z = 524.1 [M + H]+.
Styrene Hydroformylation: In a typical experiment, styrene (2 mL,
17.5 mmol) and a solution of the corresponding rhodium complex
in CH2Cl2 (3 mL, 0.012 mmol) were placed under argon in a stain-
less-steel autoclave with magnetic stirring, which was then sealed,
pressurized with syngas (1:1 CO–H2 mixture) to the appropriate
pressure and brought to the desired temperature. After the required
reaction time, the autoclave was cooled to room temperature, the
pressure was carefully released and the solution was diluted with
CH2Cl2, passed through Celite and catalytic conversions were de-
termined by gas chromatography.
Synthesis: All synthetic manipulations were carried out under a
pure-nitrogen atmosphere using standard Schlenk techniques. The
glassware was dried in the oven at approximately 110 °C and baked
out under vacuum prior to use. CH3OH and CH2Cl2 were dried
with CaH2. THF and n-hexane were dried with Na wire/benzophe-
none. All solvents were distilled under N2 and deoxygenated by
three pump and purge cycles immediately prior to use. The proton-
ated ligands (RLH–P,E) and (RLH–E,EЈ) and their corresponding
potassium salts were prepared according to published pro-
cedures.[6,70,109,110] All other chemical reagents were purchased
from Aldrich. PPh3 was recrystallized from hot ethanol/water prior
to use.
Supporting Information (see footnote on the first page of this arti-
cle): Selected crystal data for complexes 1 and 3 (Table S1); com-
parison between the optimized geometry and the crystal structure
of complex 3 (Table S2); Cartesian coordinates of all the optimized
geometry structures (Tables S3–S12); buried volume values for dif-
ferent phosphane ligands (Table S13).
[Rh(PhL–S,S)(cod)] (1): A mixture of [{Rh(μ-Cl)(cod)}2] (0.151 g,
0.30 mmol) and (PhL–S,S)K (0.293 g, 0.60 mmol) in THF (20 mL)
was stirred for approximately 1 h at room temperature. The solvent
was evaporated under vacuum and the solid residue was dissolved
in CH2Cl2 (10 mL). The resulting solution was filtered through Ce-
lite to remove KCl. The filtrate was concentrated to 1–2 mL and
layering with hexane (30 mL) afforded yellow crystals with 70%
yield. C32H32NP2RhS2 (659.59): calcd. C 58.27, H 4.89, N 2.12, S
9.72; found C 58.54, H 4.71, N 2.62, S 9.86. 1H NMR (CDCl3): δH
Acknowledgments
This research was financially supported by the Special Research
Account of the National and Kapodistrian University of Athens
= 7.91 (m, 8 H, m-C6H5), 7.39 (m, 12 H, o,p-C6H5), 4.21 (s, 4 H, (grant number 7575), by the European Social Fund (75% contri-
CH), 2.18 (m, 4 H, CH2) 1.77 (m, 4 H, CH2) ppm. 31P{1H} NMR bution), and by the National Resources (25% contribution) within
2
(CDCl3): δP = 38.8 (d, JRh,P = 3.6 Hz) ppm. IR (KBr): ν = (CH) the program EPEAEK II in the framework of the project “Pythag-
˜
3045–2823 (m); ν(PNP) 1167 (br); ν(PS) 575 (m) cm–1. ES-MS: m/z oras – Support of University Research Groups”. The authors thank
= 660.1 [M + H]+.
Dr. I. M. Mavridis for providing X-ray diffraction facilities at the
Eur. J. Inorg. Chem. 2013, 1170–1183
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FULL PAPER
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Received: August 10, 2012
Published Online: December 19, 2012
Eur. J. Inorg. Chem. 2013, 1170–1183
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