Oxidative addition of rhodium complexes containing a dithioimidodiphosphinate ligand: Facile reductive elimination of methyl iodide from a tetranuclear rhodium sulfido cluster

Article abstract of DOI:10.1002/ejic.201402080

Rhodium complexes containing the dithioimidodiphosphinate ligand [N(iPr₂PS)₂]^- have been synthesized and their oxidative addition of methyl iodide investigated. Treatment of [Rh(PPh ₃)₃Cl] with K[N(iPr₂PS)₂] afforded [Rh{N(iPr₂PS)₂}(PPh₃)₂] (1). Recrystallization of 1 from hexane in the presence of adventitious oxygen yielded the peroxo complex [Rh{N(iPr₂PS)₂}(PPh ₃)₂(η²-O₂)] (2). Oxidative addition of methyl iodide to 1 gave [Rh(Me)(I){N(iPr₂PS) ₂}(PPh₃)₂] (3), which reacted with K[N(iPr ₂PS)₂] to afford [Rh(Me){N(iPr₂PS) ₂}₂] (4). Treatment of K[N(iPr₂PS)₂] with [Rh(coe)₂Cl]₂ (coe = cyclooctene) yielded the tetranuclear sulfido cluster [Rh₄(μ₃-S) ₂{iPr₂PNP(S)iPr₂}₂{N(iPr ₂PS)₂}₂] (5) containing two four-coordinate Rh^I and two five-coordinate Rh^III centers. Oxidative addition of methyl iodide to 5 gave the monomethyl compound [Rh ₄(μ₃-S)₂(Me)(μ-I){iPr₂PNP(S) iPr₂}₂{N(iPr₂PS)₂}₂] (6). In benzene solution, 6 underwent spontaneous reductive elimination of methyl iodide to give 5. The crystal structures of complexes 2-6 have been determined. Copyright

Full text of DOI:10.1002/ejic.201402080

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DOI:10.1002/ejic.201402080
Oxidative Addition of Rhodium Complexes Containing a
Dithioimidodiphosphinate Ligand: Facile Reductive
Elimination of Methyl Iodide from a Tetranuclear
Rhodium Sulfido Cluster
Wai-Man Cheung,*[a] Wai-Hang Chiu,[a] Herman H. Y. Sung,[a]
Ian D. Williams,[a] and Wa-Hung Leung*[a]
Keywords: Cluster compounds / Rhodium / S ligands / Reduction / Oxidation
Rhodium complexes containing the dithioimidodiphosphin-
Treatment of K[N(iPr2PS)2] with [Rh(coe)2Cl]2 (coe = cyclo-
ate ligand [N(iPr2PS)2]– have been synthesized and their oxi- octene) yielded the tetranuclear sulfido cluster [Rh4(μ3-S)2-
dative addition of methyl iodide investigated. Treatment of
[Rh(PPh3)3Cl] with K[N(iPr2PS)2] afforded [Rh{N(iPr2PS)2}-
(PPh3)2] (1). Recrystallization of 1 from hexane in the pres-
ence of adventitious oxygen yielded the peroxo complex [Rh-
{N(iPr2PS)2}(PPh3)2(η2-O2)] (2). Oxidative addition of methyl
{iPr2PNP(S)iPr2}2{N(iPr2PS)2}2] (5) containing two four-coordi-
nate RhI and two five-coordinate RhIII centers. Oxidative ad-
dition of methyl iodide to 5 gave the monomethyl compound
[Rh4(μ3-S)2(Me)(μ-I){iPr2PNP(S)iPr2}2{N(iPr2PS)2}2] (6). In
benzene solution, 6 underwent spontaneous reductive elimi-
iodide to 1 gave [Rh(Me)(I){N(iPr2PS)2}(PPh3)2] (3), which re- nation of methyl iodide to give 5. The crystal structures of
acted with K[N(iPr2PS)2] to afford [Rh(Me){N(iPr2PS)2}2] (4).
complexes 2–6 have been determined.
plexes and explore their catalytic activity. Our interest in
Rh/S systems was stimulated by a report that RhIII thiolate
complexes are capable of catalyzing the hydrogenation of
imines by heterolytic cleavage of H2.[8]
Introduction
Transition-metal complexes in sulfur-rich ligand environ-
ments are of interest because they are related to the active
sites of metalloenzymes[1] and heterogeneous catalysts.[2] Of
interest are sulfides of noble metals such as Ru, Os, Rh,
and Ir, which are known to be highly active in hydrode-
sulfurization processes.[3] To gain an insight into the mecha-
nisms of metal sulfide based catalysis, we set out to synthe-
size metal complexes with sulfur-donor ligands and to ex-
plore their organometallic chemistry.
The
bidentate
dithioimidodiphosphinate
ligands
Scheme 1. Structure of [N(R2PS)2]–.
[N(R2PS)2]– (R = alkyl, aryl, alkoxy, aryloxy; Scheme 1),
which have been recognized as inorganic analogues of acet-
ylacetonate, can bind to main group and transition metals
with high electronic and geometric flexibility.[4,5] Previously,
we found that Ru and Os complexes with [N(R2PS)2]– (R =
Ph, iPr) ligands display rich organometallic and catalytic
chemistry. For example, trans-[Os{N(R2PS)2}2(OMe)2] can
catalyze the aerobic oxidation of PPh3,[6] whereas Ru carb-
ene complexes with [N(R2PS)2]– are active catalysts for alk-
ene metathesis.[7] As part of our continuing efforts to de-
velop new catalytic processes with metal sulfur complexes,
we sought to synthesize Rh dithioimidodiphosphinate com-
Although coordination compounds of [N(iPr2PS)2]–, for
example, [M{N(iPr2PS)2}2] (M = Co, Ni, Zn, Cd),[9,10] are
well known, very few Rh complexes with [N(iPr2PS)2]– have
been reported. In contrast, Rh complexes containing
[N(Ph2PS)2]–, for example, [Rh{N(Ph2PS)2}(cod)] (cod =
1,5-cyclooctadiene)[11] and [Rh(η5-C5Me5){N(Ph2PS)2}X]
(X– = Cl–, tosylate, SCN–),[12] are well documented.
[Rh{N(iPr2PS)2}(cod)] was recently synthesized from
K[N(iPr2PS)2] and [Rh(cod)2Cl]2, but this compound was
found to be inactive towards the hydroformylation of styr-
ene.[13a] In this paper, we describe the synthesis of Rh com-
plexes bearing [N(iPr2PS)2]– ligands and their oxidative ad-
dition of methyl iodide. We found that the reaction of
K[N(iPr2PS)2] with [Rh(coe)2Cl]2 (coe = cyclooctene) af-
forded the tetranuclear RhIII2RhI2 sulfido cluster [Rh4(μ3-
S)2{iPr2PNP(S)iPr2}2{N(iPr2PS)2}2], which underwent oxi-
[a] Department of Chemistry, The Hong Kong University of
Science and Technology,
Clear Water Bay, Kowloon, Hong Hong
E-mail: cheungwm@ust.hk
chleung@ust.hk
http://www-chem.ust.hk
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dative addition of methyl iodide to afford the RhIII3RhI four signals at δ = 28.52, 30.99, 61.1, and 64.4 ppm due to
methyl complex [Rh4(μ3-S)2(Me)(μ-I){iPr2PNP(S)iPr2}2- 2 along with resonances of 1. This result shows that the
{N(iPr2PS)2}2].
binding of oxygen to 1 is reversible.
Although the oxidative addition of alkyl halides to low-
The crystal structure of 2 is shown in Figure 1 and its
valent organometallic complexes is well documented, there structural parameters are presented in Table 1. The peroxo
are very few reports of its microscopic reverse, that is, the ligand binds to Rh in a side-on mode with Rh–O and O–
reductive elimination of alkyl halides. The reductive elimi- O distances of 2.018 (av.) and 1.416(4) Å, respectively,
nation of aryl halides from PdIV[14] and PtIV[15] centers is, which are similar to those in [Rh(η2-O2)Cl(PPh3)3] [av.
however, well known and the reversible reductive elimi- 2.043 and 1.413(9) Å, respectively].[18] The Rh–S (av. 2.426 Å)
nation of methyl iodide from [Ir(CO)2Cl3Me]– at high tem- and Rh–P (av. 2.331 Å) distances are similar to those in 3
perature has been studied by IR spectroscopy.[16] Frech and [av. 2.385 and 2.2934(11) Å, respectively; see below].
Milstein reported that the reaction of [{(C10H5)(CH2PR2)2}-
Rh(Me)(I)] (R = tBu) with CO led to the reductive elimi-
nation of methyl iodide. In contrast, a similar complex con-
taining a less bulky phenyl-substituted PCP ligand reacted
with CO to give a RhIII–CO complex, demonstrating that
steric effects are important for the ligand-induced C–I re-
ductive elimination of Rh(PCP) complexes.[17] In this paper
we report the facile reductive elimination of methyl iodide
from [Rh4(μ3-S)2(Me)(μ-I){iPr2PNP(S)iPr2}2{N(iPr2PS)2}2]
in the absence of an added ligand.
Results and Discussion
Phosphine Complexes
Treatment of [Rh(PPh3)3Cl] with 1 equiv. of
K[N(iPr2PS)2] in tetrahydrofuran afforded [Rh{N(iPr2PS)2}-
(PPh3)2] (1; Scheme 2). The 31P{1H} NMR spectrum of 1
in CDCl3 displays a doublet of doublets at δ = 46.2 ppm
and a doublet at δ = 59.36 ppm that have been assigned to
the PPh3 and [N(iPr2PS)2]– ligands, respectively. Complex 1
is air-sensitive in both the solid state and solution. Upon
exposure to air, an orange solution of 1 in CDCl3 gradually
turned brown and new resonances at δ = 28.52, 30.99, 61.1,
Figure 1. Molecular structure of 2. Hydrogen atoms have been
omitted for clarity. The thermal ellipsoids are drawn at the 30%
probability level.
and 64.4 ppm, assignable to a peroxo complex (see below),
Table 1. Selected bond lengths [Å] and angles [°] for complexes 2–4.
appeared in the 31P NMR spectrum.
2
3
4
Rh–S
Rh–P
Rh–O
2.4199(8)
2.4313(10)
2.3463(9
2.3157(8)
2.005(3)
2.4101(11)
2.3603(11)
2.2934(11)
2.3599(5)
2.3598(6)
2.031(3)
Rh–C
Rh–I
P–S
2.048(4)
2.6601(6)
2.0158(15)
2.0478(14)
1.594(4)
1.585(3)
96.76(4)
133.2(2)
2.037(3)
2.0209(12)
2.0327(13)
1.576(3)
1.598(3)
93.79(3)
136.3(2)
2.0305(8)
2.0254(8)
1.5998(18)
1.5968(18)
100.87(2)
129.25(12)
P–N
Scheme 2. Synthesis and oxidative addition of Rh phosphine com-
plexes. Reagents: i) [Rh(PPh3)3Cl]; ii) air; iii) MeI; iv) K[N(iPr2PS)2].
S–Rh–S
P–N–P
An attempt to recrystallize complex 1 from CH2Cl2/hex-
ane led to isolation of brown crystals identified as the per-
oxo complex [Rh{N(iPr2PS)2}(PPh3)2(η2-O2)] (2). Appar-
ently the peroxo ligand in 2 was derived from adventitious
oxygen in the solvent. The O–O stretch in the IR spectrum
Oxidative Addition of Methyl Iodide to 1
Treatment of 1 with excess methyl iodide afforded the
of 2 was not assigned because it overlaps with an intense five-coordinate RhIII methyl complex [Rh(Me)(I)-
ligand band (ca. 884 cm–1). The 31P{1H} NMR spectrum {N(iPr2PS)2}(PPh3)] (3). The bis(phosphine) complex was
of a crystalline sample of 2 in CDCl3 under nitrogen shows not formed, presumably due to steric effects and a strong
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trans influence of the methyl group. The 1H NMR spectrum [RhMe(I)2(PPh3)2] [2.081(9) Å].[19,20] In addition, the Rh–S
of the reaction mixture shows a pseudo triplet at δ = distances in [2.4101(11) and 2.3603(11) Å] and
3
4
3.12 ppm assignable to the methyl ligand, which indicates [2.3599(5) and 2.3598(6) Å] compare well with those in 2.
that only a single isomer of the product was formed from
the oxidative addition. The 31P{1H} NMR spectrum dis-
plays a doublet at δ = 30.53 ppm assignable to the PPh3
ligand along with signals from the dithioimidodiphosphin-
Tetranuclear Rhodium Sulfido Cluster
ate at δ = 59.51 and 64.9 ppm. Treatment of 3 with
Whereas complexes [Rh{N(R2PQ)2}(cod)] (R = iPr, Q =
K[N(iPr2PS)2] resulted in dissociation of the PPh3 ligand S;[13a] R = Ph, Q = Se[13b]) were obtained from K[N(R2-
and formation of the bis-chelated complex [Rh(Me)- PQ)2] and [Rh(cod)Cl]2, the reaction of [Rh(coe)2Cl]2 with
1
{N(iPr2PS)2}2] (4). In the H NMR spectrum, the methyl K[N(iPr2PS)2] at room temperature led to the formation of
protons of 4 appear as a doublet at δ = 3.05 ppm (2JRhH
=
the tetranuclear sulfido cluster [Rh4(μ3-S)2{iPr2PNP(S)-
2.8 Hz). The 31P{1H} NMR spectrum of 4 displays a singlet iPr2}2{N(iPr2PS)2}2] (5) containing two desulfurized _PS
at δ = 62.74 ppm, consistent with its solid-state structure. chelates, [iPr2PNP(S)iPr2]– (Scheme 3). Cluster 5 was also
Complex 4 is air-stable in both the solid state and solution. formed as the sole isolable product when the reaction was
No reaction was observed when 4 was heated in C6D6 at carried out with different ratios of reactants (1:2–1:4) or
80 °C for 2 h.
solvent (thf). However, heating the reaction mixture at re-
Complexes 3 and 4 were characterized by X-ray crystal- flux afforded an uncharacterized brown oil that did not
lography (Figures 2, 3, and Table 1). The geometry around crystallize. Note that the mononuclear complex
Rh in both complexes is pseudo-square pyramidal with the [Ir{N(iPr2PS)2}(coe)] was obtained from the reaction of the
methyl group at the apical position. The Rh–C distances in Ir analogue [Ir(coe)2Cl]2 with K[N(iPr2PS)2].[11] Such a dif-
3 and 4 [2.048(4) and 2.037(3) Å] are similar to that in ference may be attributed to weaker metal-to-alkene back-
bonding in the Rh complex, which facilitates the dissoci-
ation of the coe ligands and subsequent P–S bond cleavage.
Complex 5 is air-sensitive in both the solid state and solu-
tion. In addition, it is stable in hexane and benzene solu-
tions under nitrogen, but decomposes readily in CH2Cl2.
Metal-mediated desulfurization of dithioimidodiphosphin-
ate ligands is well precedented, for example, the reaction of
[Ru3(CO)12] with HN(Ph2PS)2 in the presence of Me3NO
afforded the sulfido clusters [Ru3(μ3-S)2(CO)7{HN(Ph2P)2}]
and [Ru4(μ4-S)2(μ-CO)(CO)8{HN(Ph2P)2}].[21] The 31P{1H}
NMR spectrum of 5 shows a doublet at δ = 77.30 ppm
(JRhP = 155 Hz) and a singlet at δ = 72.05 ppm, which have
been assigned to the Rh-bound phosphino (P4 and P4A)
and P=S (P3 and P3A, see Scheme 4) groups of the
[iPr2PNP(S)iPr2]– ligands, respectively. In addition, a doub-
let of doublets at δ = 61.64 ppm (JPP = 24.4, JRhP = 3.9 Hz)
and a doublet at δ = 59.12 ppm (JPP = 24.4 Hz) were ob-
served, which have been assigned to the bridging (P1 and
P1A) and non-bridging (P2 and P2A) P=S groups of the
[N(iPr2PS)2]– ligands, respectively,.
Figure 2. Molecular structure of 3. Hydrogen atoms have been
omitted for clarity. The thermal ellipsoids are drawn at the 30%
probability level.
The solid-state structure of 5 featuring a {Rh4S2}4+ core
is shown in Figure 4. The geometry around Rh1 is distorted
square pyramidal with the phosphino group at the apical
position, whereas that around Rh2 is pseudo-square planar
(sum of the bond angles: 362.87°). On the basis of the ob-
served coordination geometry, the oxidation states of Rh1
and Rh2 have been tentatively assigned as +3 and +1,
respectively, and thus 5 is formulated as a mixed valence
RhI2RhIII cluster. Consistent with this assignment, the
2
electron-rich Rh2 center can undergo oxidative addition of
methyl iodide to give a RhIII methyl species (see below).
The Rh1–Rh2/2A separations are quite long [2.9919(6),
3.0086(9), and 3.0760(6) Å], which indicates that there are
no or very weak interactions between the Rh centers. The
Rh···μ3-S distances (av. 2.2825 Å) are shorter than those in
[Rh3(μ-Cl)2(μ3-S)2(μ-S)(PEt3)6][PF6] (av. 2.379 Å).[22] The
Figure 3. Molecular structure of 4. Hydrogen atoms have been
omitted for clarity. The thermal ellipsoids are drawn at the 30%
probability level.
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Scheme 3. Reversible oxidative addition of MeI to the Rh4S2 cluster 5.
binding mode of the [N(R2PS)2]– ligand has been previously
found in other systems such as [Mn2(CO)6-
{N(Me2PS)2}2].[25] The S–Rh–S bite angle of 106.13(5)° in
5 is considerably larger than that in [Rh{N(Ph2PS)2}(cod)]
[98.48(3)°][9,13a] due to the formation of the Rh–S–Rh
bridge. In the Rh–S–P–N–P–S metallacycle, the Rh2···μ-S2
distance [2.3758(15) Å] is longer than that of Rh2–S1
[2.3429(14) Å], whereas the P2···μ-S2 distance [2.0762(19) Å]
is longer than the P1–S1 distance [2.011(2) Å].
Figure 4. Molecular structure of 5. Hydrogen atoms have been
omitted for clarity. The thermal ellipsoids are drawn at the 30%
probability level.
Reversible Oxidative Addition of Methyl Iodide to 5
Scheme 4. Selected bond lengths [Å] in 5 and 6.
The oxidative addition of methyl iodide to 5 in C6D6
was monitored by 31P{1H} NMR spectroscopy (Figure 5).
two sulfides are located approximately 1.473 Å above the Treatment of 5 with 1 equiv. of methyl iodide resulted in
Rh3 mean plane with Rh···μ3-S distances of 2.2525(14), the formation of an approximately 1:1 mixture of 5 and a
2.2813(14), and 2.3199(14) Å. These distances compare well new species, 6, which exhibits resonances at δ = 55.93,
with those in [Rh3(μ-Cl)2(μ3-S)2(μ-S)(PEt3)6][PF6] (av. 57.56, 61.72, 65.62, 71.87, 74.83, 96.91, and 103.85 ppm in
2.379 Å),[22] [NMe4][Rh3(μ3-S)2(CO)6] (av. 2.351 Å),[23] and the 31P{1H} NMR spectrum (Figure 5, b). Complex 5 was
[(Cp*Rh)3(μ-Cl)2(μ3-S)(μ-SiPr)][BPh4] (Cp* = η5-C5Me5; converted into 6 almost completely upon addition of excess
av. 2.368 Å).[24] The [N(iPr2PS)2]– and [iPr2PNP(S)iPr2]– li- (Ն 20-fold) methyl iodide to the reaction mixture (Figure 5,
gands bind to the Rh atoms in a μ:κ2,κ1 fashion. The κ3 c). Recrystallization of 6 from hexane at –10 °C in the pres-
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ence of methyl iodide afforded single crystals, which were troscopy. This result suggests the oxidative addition of
identified as the monomethyl complex [Rh4(μ3-S)2(Me)- methyl iodide to 5 is truly reversible.
(μ-I){iPr2PNP(S)iPr2}2{N(iPr2PS)2}2] by X-ray crystal-
Figure 6 shows the molecular structure of 6 and selected
lography. A preliminary study showed that 5 also reacted bond lengths in 5 and 6 are listed in Scheme 4 for compari-
with other alkyl halides such as benzyl chloride and ethyl son. The major difference between the structures of 6 and
iodide. However, these reactions gave hexane-insoluble ma- 5 is that, after the oxidative addition, the geometry around
terials that could not be crystallized. Complex 6 is air-sensi- Rh2 changes from pseudo-square planar to octahedral with
tive in both the solid state and solution, and is stable in the newly formed Rh–C bond opposite the iodo ligand. In
hexane and benzene, but decomposes readily in more polar addition, the Rh1–S2 bond in 5 is replaced by the Rh1–I
solvents such as CH2Cl2. In an attempt to prepare a Rh bond. Thus, it is reasonable to assume that the oxidative
acyl cluster, the reaction of 6 with CO was studied. When addition of 5 occurs at Rh2, and 6 can be formulated as a
a thf solution of 6 was stirred under an atmosphere of CO, RhIRhIII cluster. The Rh–C distance of 2.099(5) Å is
3
the green solution turned reddish brown immediately. The slightly longer than those in 3 and 4. The iodo ligand brid-
IR spectrum of the crude product displays C–O bands at ges Rh1 and Rh2 unsymmetrically with Rh1···μ-I and
1998 and 2064 cm–1, which indicates that it is possibly a Rh Rh2···μ-I distances of 2.7303(6) and 3.0083(6) Å, respec-
carbonyl species. Despite several attempts, we were unable tively. The Rh2–I bond is rather long, presumably due to
to crystallize the brown carbonylated product.
the trans influence of the opposite methyl group. In ad-
dition, the Rh2–S1 and Rh2–S3 bonds in 6 are longer than
those in 5, possibly due to steric effects. The metal–ligand
bond lengths and angles of the other Rh centers in 6 are
similar to those in 5.
Figure 5. 31P{1H} NMR spectra (C6D6, 162 MHz, 298 K) of 5 (a),
after addition of 1 equiv. of MeI (b), after addition of 20 equiv. of
MeI (c), and 6 in C6D6 after 20 (d) and 45 min (e).
Dissolution of crystalline 6 in C6D6 resulted in the for-
mation an approximately 1:1 mixture of 5 and 6, according
to 31P NMR spectroscopy (Figure 5, e). This indicates that
in benzene solution 6 is in equilibrium with 5 and methyl
iodide, and that the equilibrium constant is roughly equal
Figure 6. Molecular structure of 6. Hydrogen atoms have been
omitted for clarity. The thermal ellipsoids are drawn at the 30%
probability level.
Contrary to the mononuclear analogue 3, which is stable
to one at room temperature. The reductive elimination of 6 with respect to reductive elimination, tetranuclear 6 un-
can be completely inhibited by the addition of excess (Ն dergoes reversible oxidative addition/reductive elimination
20-fold) methyl iodide. Under these condition, only reso- with methyl iodide in solution. The ease of reductive elimi-
nances due to 6 were observed in the 31P NMR spectrum. nation of methyl iodide from Rh2 in 6 may be attributed
The 1H NMR spectrum of 6 in C6D6/CH3I (100:1, v/v) to the cooperative effect of the adjacent Rh1 center, which
shows a doublet at δ = 2.98 ppm assignable to the Rh- presumably assists C–I bond formation and Rh1–C bond
bound methyl anion ligand. Despite the presence of excess breaking. In addition, the trivalent Rh1 center makes Rh2
methyl iodide, there is no evidence for a second oxidative more electron-poor, thereby facilitating the reductive elimi-
addition of methyl iodide to 5 to give a dimethyl complex, nation. Such a cooperative effect has been previously ob-
presumably because the oxidation of one Rh center, Rh2, served in the oxidative addition of related dinuclear MIIIMI
leads to deactivation of the other RhI center towards oxi- (M = Ir, Rh) systems with alkyl halides.[26] Furthermore,
dative addition in the cluster.[26] Treatment of 6 with a 20- the ability of the sulfur ligands in 6 to stabilize both the +3
fold excess of CD3I in benzene yielded the labeled com- and +1 oxidation states of Rh may play a role in the re-
2
pound [D3]6, which was characterized by H NMR spec- ductive elimination.
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The results of previous studies suggested that two path- [Rh{N(iPr2PS)2}(PPh3)2(η2-O2)] and methyl [Rh(Me)(I)-
ways are possible for the Rh-centered reductive elimination {N(iPr2PS)2}(PPh3)] compounds, respectively. Treatment of
of methyl iodide: (a) Concerted reductive elimination via a [Rh(coe)2Cl]2 with K[N(iPr2PS)2] resulted in desulfurization
four-center transition state and (b) dissociation of iodide to of the imidodiphosphinate ligand and formation of a tetra-
give a cationic intermediate followed by nucleophilic attack nuclear RhIII2RhI2 sulfido cluster, 5, featuring a Rh4S2 core.
(SN2 type) on the methyl group (Scheme 5).[17] In this work, Complex 5 underwent reversible oxidative addition of
we found that the addition of nBu4NI (ca. 50 equiv.) has methyl iodide to give the RhIII3RhI monomethyl cluster 6.
very little influence on the reductive elimination of 6 in In solution, in the absence of methyl iodide, cluster 6 under-
C6D6, ruling out pathway b. Therefore we believe that a went spontaneous reductive elimination of methyl iodide.
concerted mechanism involving a four-center transition The Rh sulfido cluster 5 undergoes facile oxidative ad-
state (pathway a) is operative in the reductive elimination dition/reductive elimination, possibly because of the coop-
of 6. It seems likely that 6 rearranges to the cis form, cis-6, erative effect of neighboring Rh centers in the cluster and/
in which the methyl and iodide groups are mutually cis, or the ability of the sulfur ligands to stabilize both the RhI
before the reductive elimination takes place. Recently, and RhIII states. Therefore one may expect electron-rich
Feller, Milstein and co-workers reported that polar solvents Rh/S clusters to display interesting organometallic chemis-
such as acetone increase the rate of SN2 reductive elimi- try and are potentially useful in catalytic reactions. A pre-
nation of methyl iodide from the Rh(PCP) complexes.[17c] liminary study showed that 5 can also undergo oxidative
Unfortunately, we were not able to examine the solvent ef- addition of silanes such as Et3SiH. An investigation of the
fect on the reductive elimination of 6 because 6 decomposes catalytic activity of 5 is underway.
readily in polar protic/aprotic solvents such as CH2Cl2 and
acetone.
Experimental Section
General: All manipulations were carried out under nitrogen by
using standard Schlenk techniques. Solvents were purified, dis-
tilled, and degassed prior to use. NMR spectra were recorded with
a Bruker AV 400 spectrometer operating at 400 and 162 MHz for
1H and 31P NMR, respectively. Chemical shifts (δ, ppm) are re-
ported with reference to SiMe4 (1H) and H3PO4 (31P). IR spectra
were recorded with a Perkin–Elmer 16 PC FT-IR spectrophotome-
ter. Mass spectra were recorded with an Applied Bio-system
QSTAR mass spectrometer. Elemental analyses were performed by
Medac Ltd., Surrey, UK. K[N(iPr2PS)2],[27] [Rh(PPh3)3Cl][28], and
[Rh(coe)2Cl]2 (coe = cyclooctene)[29] were prepared according to
literature methods.
[Rh{N(iPr2PS)2}(PPh3)2] (1): K[N(iPr2PS)2] (35 mg, 0.10 mmol,
1 equiv.) was added to a solution of [Rh(PPh3)3Cl] (92 mg,
0.10 mmol) in thf (10 mL) and the mixture stirred at room tempera-
ture for 2 h, during which the red solution turned orange. The sol-
vent was pumped off and the residue washed with hexane and ex-
tracted with CH2Cl2/hexane (1:1, v/v). Concentration and cooling
to –10 °C afforded an orange crystalline solid, yield 56 mg (60%).
1H NMR (C6D6): δ = 1.27 [m, 24 H, (CH3)2CH], 2.03 [m, 4 H,
(CH3)2CH], 7.03 (m, 18 H, Ph), 7.88 (m, 12 H, Ph) ppm. 31P{1H}
NMR (C6D6): δ = 46.19 (dd, J = 15, 179 Hz, PPh3), 59.36 (d, J
= 15 Hz, [N(iPr2PS)2]–) ppm. C48H58NP4RhS2·0.5CH2Cl2 (939.91):
calcd. C 59.30, H 6.05, N 1.43; found C 59.79, H 6.16, N 1.44.
[Rh{N(iPr2PS)2}(PPh3)2(η2-O2)] (2): Recrystallization of 1 from a
saturated CH2Cl2/hexane solution for 1 week afforded red crystals
Scheme 5. Proposed reaction pathways for the reductive elimi-
nation of methyl iodide from 6.
suitable for X-ray diffraction, yield 10 mg (10%). 31P{1H}
NMR (C6D6): δ = 28.52 (m, PPh3), 30.99 (m, PPh3), 61.1 (m,
[N(iPr2PS)2]–), 64.40 (m, [N(iPr2PS)2]–) ppm together with reso-
nances of 1. C48H58NO2P4RhS2 (971.91): calcd. C 59.32, H 6.02,
N 1.44; found C 59.60, H 5.91, N 1.25.
Conclusions
[Rh(Me)(I){N(iPr2PS)2}(PPh3)] (3): Methyl iodide (83 μL,
1.33 mmol, 20 equiv.) was added to a solution of complex 1 (62 mg,
0.066 mmol) in thf (10 mL) and the mixture stirred at room tem-
We have synthesized and characterized Rh complexes
with [N(iPr2PS)2]– ligands and investigated their oxidative
addition reactions. The reaction of [Rh{N(iPr2PS)2}-
(PPh3)2] with oxygen and methyl iodide yielded the peroxo residue was washed with hexane and extracted with thf/hexane (1:5,
perature overnight. The solvent was removed in vacuo and the
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v/v). Concentration and cooling to –10 °C afforded dark-brown
crystals, yield 33 mg (40%). H NMR (C6D6): δ = 1.08 [m, 24 H,
the residue extracted with hexane. Concentration and cooling to –
10 °C afforded deep-green crystals. A second crop of 6 was ob-
tained by the addition of 1 drop of methyl iodide to the mother
1
(CH3)2CH], 1.90 [m, 4 H, (CH3)2CH], 3.12 (t, J = 3.0 Hz, 3 H,
Me), 7.03 (m, 18 H, Ph), 7.88 (m, 12 H, Ph) ppm. 31P{1H} NMR
(C6D6): δ = 30.53 (d, J = 128 Hz, PPh3), 59.51 (m, [N(iPr2PS)2]–),
1
liquor and cooling to –10 °C, yield 22 mg (67%). H NMR (C6D6/
2
CH3I, 100:1, v/v): δ = 1.36–3.90 (m, 112 H), 2.98 (d, JRhH
=
64.94 (m, [N(iPr2PS)2]–) ppm. MS (ESI): m/z = 692 [M – I]+. 2.5 Hz, 3 H, CH3) ppm. 31P{1H} NMR (C6D6): δ = 55.93 (dd,
2
2
C31H46INP3RhS2·0.2C6H14 (819.56): calcd. C 46.22, H 5.88, N
1.67; found C 46.09, H 5.66, N 1.36.
2JRhP = 4.5 Hz, JPP = 21.0 Hz, P2), 57.56 (dd, JRhP = 3.2 Hz,
2JPP = 21.0 Hz, P1), 61.72 (d, JPP = 27.5 Hz, P5), 65.62 (d, 2JPP
=
2
27.5 Hz, P3), 71.87 (br. s, P8), 74.83 (s, P7), 96.91 (m, P6), 103.85
(m, P4) ppm. C49H115IN4P8Rh4S8·0.5C6H14 (1803.30): calcd. C
33.83, H 6.67, N 3.03; found C 34.26, H 6.71, N 2.89.
[Rh(Me){N(iPr2PS)2}2] (4): K[N(iPr2PS)2] (12 mg, 0.033 mmol,
1.1 equiv.) was added to
a solution of complex 3 (28 mg,
0.030 mmol) in thf (10 mL) and the reaction mixture was stirred at
room temperature overnight. The solvent was removed in vacuo
and the residue extracted with hexane. Concentration and cooling
to –10 °C afforded red crystals, yield 12 mg (47%). 1H NMR
Reaction of 6 with CD3I: CD3I (7 μL, 0.11 mmol, 20 equiv.) was
added to a solution of 6 (10 mg, 0.0055 mmol) in C6D6 (0.5 mL).
2
NMR spectra were recorded after 1 h. H NMR (C6D6): δ = 2.96
(C6D6): δ = 1.24 [m, 48 H, (CH3)2CH], 2.07 [m, 8 H, (CH3)2CH], (m, 3 D, CD3) ppm. 31P{1H} NMR (C6D6): δ = 56.05 (d, J =
3.05 (d, J = 2.8 Hz, 3 H, CH3) ppm. 31P{1H} NMR (C6D6): δ = 21.2 Hz), 57.56 (d, J = 21.2 Hz), 61.81 (d, J = 27.5 Hz), 65.75 (d,
62.74 (s) ppm. C25H59N2P4RhS4·0.5C6H14 (742.81): calcd. C 41.90,
J = 27.5 Hz), 71.87 (s), 74.70 (s), 97.13 (m), 104.14 (m) ppm.
H 8.60, N 3.62; found C 42.08, H 8.27, N 3.58.
X-ray Crystallography: Crystal data and experimental details for 2–
[Rh4(μ3-S)2{iPr2PNP(S)iPr2}2{N(iPr2PS)2}2] (5): K[N(iPr2PS)2] 6 are summarized in Table 2. Preliminary examinations and inten-
(59 mg, 0.17 mmol, 2 equiv.) was added to a suspension of [Rh- sity data collection were preformed with a Bruker SMART-APEX
(coe)2Cl]2 (60 mg, 0.084 mmol) in Et2O (10 mL), and the reaction 1000 area-detector diffractometer using graphite-monochromated
mixture was stirred at room temperature for 2 h. The solvent was
pumped off and the residue extracted with hexane. Concentration
and cooling to –10 °C afforded deep-brown crystals, yield 39 mg
Mo-Kα radiation (λ = 0.70173 Å) (for 3, 5 and 6) or Oxford Dif-
fraction (for 2 and 4). The data were corrected for absorption using
the program SADABS[30] (for 3, 5 and 6) or CrysAlis RED[31] (for
2 and 4). Structures were solved by direct methods and refined
by full-matrix least-squares on F2 using the SHELXTL software
package.[32] Unless stated otherwise, non-hydrogen atoms were re-
fined with anisotropic displacement parameters. Carbon-bonded
1
(55%). H NMR (C6D6): δ = 1.15–3.84 (m, 112 H) ppm. 31P{1H}
NMR (C6D6): δ = 59.12 (d, 2JPP = 24.4 Hz, P2), 61.64 (dd, 2JRhP
=
3.9 Hz, 2JPP = 24.4 Hz, P1), 72.05 (s, P3), 77.30 (d, 1JRhP = 155 Hz,
P4) ppm (see Scheme 4 for the phosphorus atom-labelling scheme).
C48H112N4P8Rh4S8·0.5C6H14 (1661.36): calcd. C 35.94, H 7.04, N hydrogen atoms were included in calculated positions and refined
3.29; found C 36.39, H 7.23, N 3.17.
in the riding mode using the SHELXL97 default parameters.
[Rh4(μ3-S)2(Me)(μ-I){iPr2PNP(S)iPr2}2{N(iPr2PS)2}2] (6): Methyl
iodide (23 μL, 0.36 mmol, 20 equiv.) was added to a solution of 5
(30 mg, 0.018 mg) in thf/Et2O (1:1, v/v, 10 mL) and the reaction
mixture was stirred at room temperature for 5 min during which
the brown solution turned green. The solvent was pumped off and
CCDC-974694 (for 3), -974695 (for 2), -974696 (for 4), 974697 (for
5), and 974698 (for 6) contain the supplementary crystallography
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
Table 2. Crystallographic data and experimental details for 2–6.
2
3
4
5
6·0.25C6H14
Empirical formula
Mr
C48H65NO2P4RhS2 C31H46INP3RhS2 C25H59N2P4RhS4 C48H112N4P8Rh4S8
C50.5H118.50IN4P8Rh4S8
1824.77
978.92
819.53
742.77
1661.30
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
triclinic
P1
triclinic
P1
monoclinic
C2/c
17.3915(2)
8.53700(10)
25.5604(3)
orthorhombic
Pnna
17.7717(15)
25.623(2)
15.3352(13)
monoclinic
P21/n
21.2147(16)
15.1336(11)
25.6461(19)
¯
¯
9.7307(4)
10.8140(5)
22.8356(9)
94.269(4)
94.247(3)
104.391(4)
2310.40(17)
2
10.248(2)
12.404(3)
14.575(3)
72.543(3)
86.677(3)
76.294(3)
1716.9(7)
2
109.7450(10)
103.8990(10)
γ [°]
V [Å3]
3571.86(7)
4
1.381
6983.0(10)
4
1.580
7992.7(10)
4
1.516
Z
ρcalcd. [gcm–1]
T [K]
1.407
173(2)
1.585
173(2)
173(2)
100(2)
100(2)
F(000)
1026
1.54178
13022
828
0.71073
18275
1568
1.54178
9062
3424
1.386
31415
3722
1.599
42661
μ(Mo-Kα) [mm–1]
Total reflections
Independent reflections 8396
7332
0.0300
1.001
3369
0.0342
1.003
5882
0.1003
1.003
13857
0.0878
1.004
Rint
0.0448
1.003
GoF
R1, wR2 [I Ͼ 2σ(I)]
R1, wR2 (all data)
0.0454, 0.1199
0.0535, 0.1252
0.0446, 0.0995
0.0517, 0.1026
0.0324, 0.0840
0.0341, 0.0852
0.0422, 0.0735
0.0960, 0.0820
0.0435, 0.0643
0.0967, 0.0700
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Acknowledgments
The financial support from the Hong Kong Research Grants Coun-
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Published Online: May 22, 2014
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim