Some studies of the substitution chemistry of [Rh2(OAc)2(CH3CN)4]-[BF 4]2 with monodentate and bidentate tertiary phosphines

Article abstract of DOI:10.1039/b001444n

The reactions between [Rh₂(OAc)₂(CH₃CN)₄][BF ₄]₂ and each of the tertiary phosphines PMe₃, PCy₃ (Cy = cyclohexyl), Me₂PCH₂CH₂PMe₂ (dmpe), Ph₂PCH₂CH₂PPh₂ (dppe), Me₂PCH₂PMe₂ (dmpm) and Ph₂PCH₂PPh₂ (dppm) have been studied by ¹H and ³¹P{¹H} NMR spectroscopy in CD₃CN. The chelating phosphines dppe and dppm catalyze the exchange of coordinated CH₃CN for solvent CD₃CN exchange prior to any other observable substitution chemistry. The monodentate phosphines initially form kinetically labile biaxially ligated complexes, [Rh₂(OAc)₂(CH₃CN)₄(PR ₃)₂][BF₄]₂ prior to substitution of the equatorial CH₃CN by PR₃. Over time, the biaxial complex rearranges to form the monoaxial, monoequatorial complex, involving displacement of a single equatorial CH₃CN ligand. For PCy₃ the complex [Rh₂(OAc)₂(CH₃CN)₃(PCy ₃)₂][BF₄]₂ has been characterized by ¹H and ³¹P{¹H} NMR spectroscopy. With time, a further reaction occurs leading to the cleavage of the Rh-Rh bond and the monomeric complex [Rh(CH₃CN)₂(PCy₃)₂][BF₄] has been identified. Crystal data at +25 °C: space group P2ium, a = 9.879(1) A, b = 13.275(1) A, c = 16.705(1) A and Z = 4. A similar reaction sequence is observed ₁ with PMe₃ but more isomers of formula [Rh₂(OAc)₂(CH₃CN)₃(PMe ₃)₂[BF₄]₂ are observed by ³¹P{¹H} NMR spectroscopy. Reactions involving dppe lead to axial and equatorial Rh-P bonded complexes. Based on ³¹P{¹H} NMR data, the bisequatorial complex formulated as [Rh₂(OAc)₂(CH₃CN)₂(dppe)][BF ₄]₂ is formed. The formation of the latter, which has been followed from 35 to 80 °C, is evidently reversible since all attempts to crystallize the complex yielded only the acetonitrile salt [Rh₂(OAc)₂(CH₃CN)₄][BF ₄]₂ and free dppe. With dppm, only axial ligation is observed while for dmpm and dmpe the substitutional behavior is more complex and has not been evaluated in detail. The activation parameters for the conversion of the biaxial [Rh₂(OAc)₂(S)₄(L)₂][BF ₄]₂ to the monoaxial, monoequatorial [Rh₂(OAc)₂(S)₃(L)₂][BF ₄]₂ complex (S = CH₃CN and L = phosphine) have been determined. For L = PMe₃, ΔH^? = 16(1) kcal mor^-1 and ΔS^? = -9(3) cal K1 mol1 and for L = PCy₃, ΔH^? = 21(1) kcal mol^-1 and ΔS^? = +2(3) cal K^-1 mol^-1. For dppe, the 1:1 adduct shows only one type of ³¹P signal for the initial axial complex indicative of rapid exchange of free and bound PPh₂ groups. The rearrangement to the equatorialaxial isomer [Rh₂(OAc)₂(S)₃(dppe)][BF₄] ₂ occurs with Δ^? = 26(1) kcal mor^-1 and ΔS^? =+12(1) cal K^-1 mor^-1. Collectively these data show that substitution at the Rh₂⁴⁺-center proceeds via an initial reversible associative process followed by an interchange oflabile axial for inert equatorial sites. These results are compared with earlier studies of the substitution of M₂⁴⁺-containing complexes, where M = Mo, Ru and Rh. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b001444n

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Some studies of the substitution chemistry of [Rh2(OAc)2(CH3CN)4]-
[BF4]2 with monodentate and bidentate tertiary phosphines†
Malcolm H. Chisholm,* John C. Huffman and Suri S. Iyer
Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington,
IN 47405, USA
Received 23rd December 1999, Accepted 23rd February 2000
Published on the Web 4th April 2000
The reactions between [Rh2(OAc)2(CH3CN)4][BF4]2 and each of the tertiary phosphines PMe3, PCy3
(Cy = cyclohexyl), Me2PCH2CH2PMe2 (dmpe), Ph2PCH2CH2PPh2 (dppe), Me2PCH2PMe2 (dmpm) and
Ph2PCH2PPh2 (dppm) have been studied by 1H and 31P{1H} NMR spectroscopy in CD3CN. The chelating
phosphines dppe and dppm catalyze the exchange of coordinated CH3CN for solvent CD3CN exchange prior to
any other observable substitution chemistry. The monodentate phosphines initially form kinetically labile biaxially
ligated complexes, [Rh2(OAc)2(CH3CN)4(PR3)2][BF4]2 prior to substitution of the equatorial CH3CN by PR3. Over
time, the biaxial complex rearranges to form the monoaxial, monoequatorial complex, involving displacement of a
single equatorial CH3CN ligand. For PCy3 the complex [Rh2(OAc)2(CH3CN)3(PCy3)2][BF4]2 has been characterized
by 1H and 31P{1H} NMR spectroscopy. With time, a further reaction occurs leading to the cleavage of the Rh–Rh
bond and the monomeric complex [Rh(CH3CN)2(PCy3)2][BF4] has been identified. Crystal data at ϩ25 ЊC: space
group P21nm, a = 9.879(1) Å, b = 13.275(1) Å, c = 16.705(1) Å and Z = 4. A similar reaction sequence is observed
with PMe3 but more isomers of formula [Rh2(OAc)2(CH3CN)3(PMe3)2][BF4]2 are observed by 31P{1H} NMR
spectroscopy. Reactions involving dppe lead to axial and equatorial Rh–P bonded complexes. Based on 31P{1H}
NMR data, the bisequatorial complex formulated as [Rh2(OAc)2(CH3CN)2(dppe)][BF4]2 is formed. The formation
of the latter, which has been followed from 35 to 80 ЊC, is evidently reversible since all attempts to crystallize the
complex yielded only the acetonitrile salt [Rh2(OAc)2(CH3CN)4][BF4]2 and free dppe. With dppm, only axial
ligation is observed while for dmpm and dmpe the substitutional behavior is more complex and has not been
evaluated in detail. The activation parameters for the conversion of the biaxial [Rh2(OAc)2(S)4(L)2][BF4]2 to
the monoaxial, monoequatorial [Rh2(OAc)2(S)3(L)2][BF4]2 complex (S = CH3CN and L = phosphine) have been
determined. For L = PMe3, ∆H‡ = 16(1) kcal molϪ1 and ∆S‡ = Ϫ9(3) cal KϪ1 molϪ1 and for L = PCy3, ∆H‡ = 21(1)
kcal molϪ1 and ∆S‡ = ϩ2(3) cal KϪ1 molϪ1. For dppe, the 1:1 adduct shows only one type of 31P signal for the initial
axial complex indicative of rapid exchange of free and bound PPh2 groups. The rearrangement to the equatorial–
axial isomer [Rh2(OAc)2(S)3(dppe)][BF4]2 occurs with ∆H‡ = 26(1) kcal molϪ1 and ∆S‡ = ϩ12(1) cal KϪ1 molϪ1
Collectively these data show that substitution at the Rh24ϩ-center proceeds via an initial reversible associative
process followed by an interchange of labile axial for inert equatorial sites. These results are compared with
earlier studies of the substitution of M24ϩ-containing complexes, where M = Mo, Ru and Rh.
.
4ϩ
ation.4 Thus the Mo2 center with M–M σ2π4δ2 has low lying
vacant metal based orbitals which may facilitate carboxylate
group scrambling as seen in the reactions between Mo2(O2CR)4
and Mo2(O2CRЈ)4 which occur upon mixing in a solvent such as
benzene4,5 or as in the fluxional nature of the [Mo2(µ-O2CtBu)4-
(η1-O2CtBu)]Ϫ anion by way of associative chemistry: bonds to
Mo may be formed with or without sacrificing M–M bonding.6
By contrast Rh2(O2CR)4 compounds undergo carboxylate
group scrambling only under more forcing conditions. The
Introduction
A large number of dinuclear complexes containing a central
M24ϩ core are known, many of which contain M–M bonds and
have a common structural motif.1 Such is the case for
M2(O2CR)4 compounds which are known for M = Cr, Mo, W,
Ru, Rh and Cu with a common lantern or paddle wheel (D4h)
M2(O2C)4 core. For M = Mo, W and Rh these compounds dis-
play a rich substitution chemistry and are often employed as
starting materials in the synthesis of other dinuclear com-
plexes.1 Since the discovery by Bear et al.2 that Rh2(OAc)4
shows antitumor activity for cancerous cells in vitro other
workers have been attracted to the fundamental substitution
chemistry of Rh24ϩ-containing compounds with nitrogen bases
and purines.3
addition of [Bu4 N][OAc] to [Rh2(O2CtBu)4] reversibly forms a
n
salt [Bu4 N]2[(η6-C6H5Me)Rh8(µ-O2CtBu)16(µ-η1,η1-OAc)2] in
n
toluene, for example.7 Here the Rh–Rh bonding electronic con-
figuration is σ2π4δ2δ*2π*4 and this may be compared with the
6
kinetically inert t2g configuration seen in mononuclear octa-
hedral complexes of Co3ϩ, Rh3ϩ, Ir3ϩ and Pt4ϩ which generally
undergo substitution by dissociative, D, or interchange-
dissociative, Id, mechanisms.8
In comparing the reactivity of Mo2(O2CR)4 and Rh2(O2CR)4
compounds we have noted that the former may be characterized
as labile and the latter as inert.4 We suggested that the vast
difference in rates of reactivity may reflect, in part, ground state
effects associated with the M–M bond electronic configur-
In comparing the rates of solvent exchange in the [M2(OAc)2-
(MeCN)6][BF4]2 complexes we again observed kinetic lability
for M = Mo and the inertness of the Rh complex which under-
goes exchange of CH3CN for CD3CN (solvent) very much
more slowly.4 Specifically, the [Rh2(OAc)2(CH3CN)6][BF4]2
complex, which has the structure shown in A below (SЈ and S
† Dedicated to Professor Jack Halpern on the occasion of his 70th
birthday.
DOI: 10.1039/b001444n
J. Chem. Soc., Dalton Trans., 2000, 1483–1489
This journal is © The Royal Society of Chemistry 2000
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Table 1 Selected bond distances (Å) and angles (Њ) for [Rh(CH3CN)2-
(PCy3)2][BF4]
Rh1–P2
2.342(2)
2.340(2)
1.984(4)
1.134(6)
1.459(7)
P13–C14
P13–C20
P2–C3
1.858(6)
1.850(8)
1.864(6)
1.856(8)
Rh1–P13
Rh1–N24
N24–C25
C25–C26
P2–C9
are the axial and equatorial CH3CN’s; O–O represents the
acetate ligand) and was prepared originally by Garner and
coworkers,9–11 contains two types of coordinated CH3CN
ligands. SЈ are axial ligands and S are equatorial with respect to
the Rh–Rh bond and their exchange rates with CD3CN vary
greatly. SЈ are labile as a result of the strong trans-influence and
trans-effect of the Rh–Rh bond but S, the equatorial sites are
inert as judged by the fact that it requires heating in CD3CN to
bring about any significant exchange with the solvent. Indeed,
P13–Rh1–P2
P13–Rh1–N24
P2–Rh1–N24
N24–Rh1–N24
Rh1–P13–C14
175.55(8)
89.7(1)
90.1(1)
179.3(3)
114.2(1)
Rh1–P13–C20
Rh1–N25–C25
Rh1–P2–C3
111.1(1)
175.9(5)
113.6(1)
111.1(1)
Rh1–P2–C9
₁
even at 100 ЊC the t for CH3CN for CD3CN (solvent) exchange
₂
was of the order of 4 h and, from a study of the rate of CH3CN
for CD3CN exchange over a temperature range, we estimated
∆H‡ = 33 kcal molϪ1 and ∆S‡ ≈ ϩ11 cal KϪ1 molϪ1, which
is consistent with a dissociative or interchange dissociative
mechanism for the exchange of the equatorial CH3CN solvent
ligands S in A.4
In order to probe further the details of the substitution chem-
4ϩ
istry of Rh2 centers we determined to study the reactions
of [Rh2(OAc)2(CH3CN)6][BF4]2 with tertiary phosphines in
CD3CN solutions by NMR spectroscopy. This study takes
1
advantage of the use of 31P{1H} and H NMR spectroscopy,
the occurrence of 103Rh, I = ¹, 100% natural abundance and the
¯
²
prior work by Drago and coworkers12 who classified bis(phos-
phine) complexes of dirhodium as Class I (biaxial), Class II
(biequatorial) and Class III (monoaxial, monoequatorial) as
depicted by B, C and D below, respectively (S = CH3CN,
Fig. 1 An ORTEP19 diagram of [Rh(CH3CN)2(PCy3)2][BF4] with
thermal ellipsoids at the 50% probability level. The hydrogen atoms and
the BF4 anions are omitted for clarity.
distances and bond angles are given in Table 1. There is nothing
exceptional about the structural features of this d8 Rh() cation
and the P–Rh–P angle is close to 180Њ as expected. The Rh–P
distances are similar to those seen in [Rh(PPh3)3(CH3CN)]+-
[BF4]11 and [Rh(PPh3)2(CO)(CH3CN)][HC(SO2CF3)2]12 as are
the Rh–N distances of the coordinated acetonitrile ligands.
L = PR3). The differentiation of B, C, D can be based on
13P{1H} chemical shifts. Of course, for compounds of type C
and D more than one isomer is possible.
[Rh2(OAc)2(CH3CN)2(PCy3)2][BF4]2. There was a disorder
with one of the MeCN ligands and the two BF4Ϫ anions show a
50% site occupancy within the unit cell. These crystallographic
problems prevent us from presenting any reliable structural
information on this compound though there is little doubt that
this is a dinuclear compound containing two bulky PCy3
groups, one at each metal center with one being axially ligated
and the other equatorial as demanded by the NMR studies
presented below.
We describe here our findings of the substitution chemistry
of [Rh2(OAc)2(CH3CN)6][BF4]2 in CD3CN with the tertiary
n
phosphines PMe3, PBu3 , PCy3 (Cy = cyclohexyl), Me2PCH2-
CH2PMe2 (dmpe), Ph2PCH2CH2PPh2 (dppe), Me2PCH2PMe2
(dmpm) and Ph2PCH2PPh2 (dppm).
Results and discussion
The reaction between [Rh2(OAc)2(CH3CN)6][BF4]2 and PMe3 in
CH2Cl2 or CH3CN is extremely rapid at room temperature and
leads to products of cleavage of the Rh–Rh bond, namely a
black insoluble precipitate and a square planar Rh() complex
[Rh(PMe3)2(CH3CN)2][BF4]. The reaction with the extremely
bulky PCy3 ligand was significantly slower. However, this reac-
tion too led to cleavage of the Rh–Rh bond and the bright
yellow crystalline compound [Rh(CH3CN)2(PCy3)2][BF4] was
isolated in ca. 40% yield based on Rh together with a black
insoluble precipitate which was not characterized. However, an
intermediate in this reaction, the green crystalline compound
[Rh2(OAc)2(CH3CH)3(PCy3)2][BF4]2 was isolated and charac-
terized, see Experimental section.
NMR studies
Because the reactions between [Rh2(OAc)2(CH3CN)6][BF4]2
and tertiary phosphines proceed so rapidly at room temperature
they were followed by 31P{1H} and H NMR spectroscopy in
1
CD3CN as solvent in NMR tube reactions. In these studies the
tertiary phosphine was added to a cooled or frozen solution of
the Rh2-cationic complex and the sample was introduced into
the precooled probe of an NMR spectrometer. The reaction
was then monitored with time and temperature.
Reactions involving PMe3. The addition of PMe3 to [Rh2-
(OAc)2(CH3CN)6][BF4]2 in CD3CN solvent at Ϫ35 ЊC results in
the formation of the biaxially ligated complex, B, or Class I
complex. This is evidenced by the appearance of a single
31P{1H} signal at ca. δ Ϫ35.5, downfield from free PMe3. The
31P{1H} signal of this bis adduct gives rise to an AAЈXXЈ
Solid-state structure
[Rh(CH3CN)2(PCy3)2][BF4]. An ORTEP drawing of the
square planar Rh() cation is shown in Fig. 1 and selected bond
1484
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spectrum resulting from the 31P–103Rh–103Rh–31P connectivity
II isomer (see Scheme 1). Regrettably from reactions between
and the spectrum was satisfactorily simulated with the follow-
[Rh2(OAc)2(CH3CN)4][BF4]2 and PMe3 we have not been able
to isolate any compound in a pure state and with an excess of
PMe3 reactions evidently proceed rapidly to give cleavage of the
Rh–Rh bond.
1
2
ing coupling constants JRhRh = 5 Hz, JRhP = 55 Hz, JRhP = 25
Hz and JPP = 400 Hz. These coupling constants are closely
related to those reported by Drago et al.12 for [Rh2(O2C-
CF3)4(PPh3)2] and related complexes. The addition of an excess
of PMe3 (>2 equiv.) led to line broadening between the free and
the coordinated PMe3 ligands at Ϫ35 ЊC while when less than 2
equiv. of PMe3 were added only the signal for the bis-ligated
complex was seen. The latter finding indicates a cooperative
We have monitored the conversion of the biaxial PMe3 com-
plex to the isomers E and F in the temperature range Ϫ25 ЊC to
ϩ5 ЊC by following the disappearance of the 31P{1H} signal
for the Class I complex, B, as a function of time. Slopes of
ln[Rh2(OAc)2(CH3CN)4(BF4)2] versus time were linear for over
three half-lives thus implicating at least a pseudo first order
reaction for the disappearance of the Class I complex [Rh2-
(OAc)2(CH3CN)4(PMe3)2][BF4]2. From plots of ln(kobs) versus 1/
temperature (Kelvin) we can estimate the activation parameters
for the reaction shown in eqn. (2) to be ∆H‡ = 16(1) kcal molϪ1
and ∆S‡ = Ϫ9(3) cal KϪ1 molϪ1.
4ϩ
binding of PMe3 to the Rh2 center is favored and that the
monoligated complex is labile to disproportionation to give the
Class I complex and the starting material, eqn. (1).
2[Rh2(OAc)2(CH3CN)4(PMe3)][BF4]2
[Rh2(OAc)2(CH3CN)4][BF4]2 ϩ
[Rh2(OAc)2(CH3CN)4(PMe3)2][BF4]2 (1)
Class I
[Rh2(OAc)2(CH3CN)4(PMe3)2][BF4]2
CD3CN
[Rh2(OAc)2(CH3CN)3(PMe3)2][BF4]2 ϩ CH3CN (2)
4ϩ
It is worth emphasizing at this point that the original Rh2
-
containing complex contained six CH3CN ligands: four
equatorial and two axial. The two axial CH3CN ligands
undergo essentially instantaneous exchange with the CD3CN
In the presence of an excess of PMe3 (4 or more equivalents)
the disappearance of [Rh2(OAc)2(CH3CN)4][BF4]2 was faster
and the formation of products more complex. We take this as
evidence for the reversible binding of PMe3 in the biaxial com-
plex, eqn. (1), and that once PMe3 has acquired an equatorial
site a further PMe3 ligand can coordinate in the axial site to give
compounds of the type [Rh2(OAc)2(CH3CN)3(PMe3)3][BF4]2
which are also labile toward CH3CN ligand displacement and
Rh–Rh bond rupture. A similar reaction sequence was seen in
reactions involving PBun3 but was not followed in detail.
1
solvent and thus by H NMR spectroscopy appear as free
CH3CN. At Ϫ35 ЊC the formation of the bis(phosphine) com-
plex of Class I occurs without any detectable equatorial CH3CN
for solvent CD3CN exchange.
At Ϫ35 ЊC further reaction is very slow requiring several days
to give a mixture of two isomers of a Class III complex. At 0 ЊC
in CD3CN the formation of the two Class III isomers (mono-
axial, monoequatorial) requires 10 h. The two possible isomers
are shown in Scheme 1 as E and F and occur in the approximate
Because of the facility of these reactions with PMe3 and
PBun , we turned our attention to the use of the bulky PCy3
3
ligand (Cy = cyclohexyl) which has a Tolman cone angle13
greater than 180Њ. For this monodentate tertiary phosphine the
coordination of two PCy3 ligands at the same metal center in a
cis manner would be sterically impossible!
Reactions employing PCy3. The addition of PCy3 to [Rh2-
(OAc)2(CH3CN)4][BF4]2 in CD3CN at Ϫ20 ЊC leads to a slower
sequence of events wherein by 31P{1H} NMR spectroscopy we
can observe the initial formation of a monoaxial adduct fol-
lowed by formation of the biaxial Class I complex. The free
PCy3 appears at δ Ϫ4, the monophosphine complex at δ Ϫ23
and the biaxial Class I complex at δ Ϫ4.5. The conversion of the
Class I biaxial complex to the monoaxial, monoequatorial Class
III complex was followed with time in the temperature range
Ϫ20 to ϩ35 ЊC by monitoring the disappearance of the biaxial
complex. From these studies we obtain an estimate of the
activation parameters: ∆H‡ = 21(1) kcal molϪ1 and ∆S‡ = ϩ2(3)
cal KϪ1 molϪ1. In view of the equilibria involving the biaxially
ligated complex of Class I we cannot attempt to interpret these
parameters beyond noting that enthalpically the conversion of
the Class I to Class III compound is more demanding for the
bulky PCy3 ligand than for PMe3. It is this ∆H‡ term which
leads to the notably lower rate of conversion. Also, in contrast
to reactions employing PMe3, we only observe one isomer of
the monoaxial, monoequatorial complex [Rh2(OAc)2(CH3-
CN)3(PCy3)2][BF4]2. Only one equatorial CH3CN ligand is dis-
placed in this substitution process. There are three distinct
CH3CN signals of equal intensity for the bound CH3CN
ligands as expected from the structure of this compound seen in
the solid state. With time, however, this compound decomposes
at room temperature to give products of cleavage of the Rh–Rh
bond, one of which has been characterized as [Rh(CH3CN)2-
(PCy3)2][BF4]. The other compound is a black insoluble precipi-
tate which has not been characterized.
Scheme
1
Isomerizations of [Rh2(OAc)2(CH3CN)4(PMe3)2][BF4]2.
S = CH3CN, SЈ = CD3CN; L = PMe3.
ratio 3:1. Evidence that these isomers are of the type Class III
(D shown earlier) comes from the appearance of one 31P{1H}
signal at ca. δ Ϫ40 and the other at ca. δ ϩ12, corresponding to
axial and equatorial Rh–PMe3 groups, respectively. Regrettably
the NMR data do not allow us to distinguish between the iso-
mers E and F shown in Scheme 1. However, we can say that the
formation of the Class III isomer occurs with the loss of only
one CH3CN equatorial ligand and thus the isomers have the
formula [Rh2(OAc)2(CH3CN)3(PMe3)2][BF4]2.
At 0 ЊC, with further time new 31P{1H} signals grow in the
region expected for Class II isomers (see C earlier) which have
equatorial Rh–P bonds. Three isomers are possible for a Class
The reaction between [Rh2(OAc)2(CH3CN)4][BF4]2 in CD3-
CN and PCy3 (4 or more equivalents) has been followed at
25 ЊC. The reaction proceeds more rapidly in the presence of
J. Chem. Soc., Dalton Trans., 2000, 1483–1489
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an excess of 2 equiv. of PCy3 but only the biaxial complex, the
monoaxial, monoequatorial complex and the mononuclear RhI
complex are seen by 31P{1H} NMR spectroscopy and cleavage
of the Rh–Rh bond is complete within 2 h. The reactions
involving [Rh2(OAc)2(CH3CN)4][BF4]2 and PCy3 are summar-
ized in Scheme 2.
natively one equivalent of the chelating phosphine could
rapidly and reversibly link two Rh24ϩ centers as shown in J.
When more than one equivalent of the chelating phosphines
is added then signals associated with bound and uncoordinated
31P nuclei can be seen along with the signal of free dppe or
dmpe. This suggests that adducts of the type K are formed
where exchange between bound and free ligands is relatively
slow on the NMR time scale. Also in this instance the η1-
ligands are not fluxional on the NMR time-scale.
The most dramatic difference between the reactions involving
dmpe and dppe in CD3CN at Ϫ30 ЊC, as determined by NMR
spectroscopy, is that the latter (dppe) but not the former (dmpe)
causes a rapid exchange of bound equatorial CH3CN for
Scheme 2 SЈ = CD3CN; LЈ = PCy3.
For the reactions involving the monodentate PR3 ligands
described so far we can conclude the following. The substitu-
tion reaction of CH3CN for PR3 in the reaction between
[Rh2(OAc)2(CH3CN)4][BF4]2 and PR3 proceeds in a two step
process. (1) The initial formation of a biaxial complex of
formula [Rh2(OAc)2(CH3CN)4(PR3)2][BF4]2 is rapid and revers-
ible. (2) The second step in the reaction is slower and involves
an interchange mechanism wherein one of the axially bound
PR3 ligands displaces an equatorially bound CH3CN ligand to
form [Rh2(OAc)2(CH3CN)3(PR3)2][BF4]2 of Class III. This may,
depending upon R, exist in more than one isomer and sub-
sequent reactions lead to cleavage of the Rh–Rh bond. The
overall rate of the reactions are dependent on both the concen-
trations of PR3 and the nature of PR3. Since these reactions
proceed much faster than the rate of CD3CN for CH3CN
exchange in the [Rh2(OAc)2(CH3CN)4]2ϩ cation4 it is evident
that substitution of the equatorial CH3CN ligand is promoted
by the initial coordination of the PR3 in the axial position. The
PR3 for CH3CN substitution has the appearance of the asso-
ciative interchange mechanism described previously in our stud-
ies of tertiary phosphine for tertiary phosphine exchange at
M24ϩ-centers (M = Mo, W), eqn. (3).14
1
CD3CN (solvent). This exchange is readily seen by H NMR
spectroscopy. This facile exchange of equatorial CH3CN
for CD3CN (solvent) was also seen previously in reactions
involving the addition of chelating ligands 2,2Ј-bipyridine,
1,10-phenanthroline and OAcϪ.4a In each case the addition of
the bidentate ligand effects CH3CN (equatorial) for CD3CN
solvent exchange without any apparent reaction beyond a
reversible axial coordination. Why this should be is quite
puzzling, as is the fact that dmpe does not effect this reaction.
In the reaction between [Rh2(OAc)2(CH3CN)4][BF4]2 and
dppe we can observe the conversion of the Class I compound to
a Class III compound, a monoaxial, monoequatorial com-
pound as evidenced by 31P signals at δ Ϫ36 (axial Rh–P) and
ϩ59 (equatorial Rh–P). These occur in the integral ratio 1:1.
The reaction with 1 equiv. of dppe was monitored in the tem-
perature range ϩ22 to ϩ78 ЊC and an estimate of the activation
parameters was obtained: ∆H‡ = 26(1) kcal molϪ1 and ∆S‡ =
ϩ12(3) cal KϪ1 molϪ1.
A further reaction occurs as evidenced by the disappearance
of the signal at δ Ϫ36 and the growth of the signal at δ ϩ59.
This is believed to be a biequatorially substituted isomer, or
Class II compound. Of the three possible isomers for such a
compound the one shown in Scheme 3 is believed to be most
likely as this one has the dppe ligand bridging the Rh–Rh bond
in a staggered manner which is well known for dppe ligands at
dinuclear metal centers.1
M2Cl4L4 ϩ LЈ → M2Cl4L3LЈ ϩ L
(3)
Reactions with chelating tertiary diphosphines. With one
equivalent of dmpe or dppe, [Rh2(OAc)2(CH3CN)4][BF4]2 in
CD3CN at Ϫ30 ЊC shows 31P{1H} signals, one at ca. δ Ϫ25, the
other at ca. δ Ϫ10, both as 1:2:1 triplets. The interpretation of
this observation is open to question. A monoaxial compound
could be fluxional on the NMR time-scale as shown in eqn. (4)
While it is quite evident from 31P{1H} NMR studies that a
single isomer of formula [Rh(OAc)2(dppe)(CD3CN)x][BF4]2 is
formed all attempts to isolate this compound by crystallization
failed. Indeed, cooling to Ϫ20 ЊC leads to the slow crystalliz-
ation of [Rh2(OAc)2(CD3CN)6][BF4]2 and free dppe. Thus we
believe that compound L shown in Scheme 3 must be formed
reversibly and that the less soluble acetonitrile complex crystal-
lizes from solution preferentially. The reactions shown in
Scheme 3 have been recycled several times in selected NMR
tube reactions and in no instance has Rh–Rh bond cleavage
been observed.
(4)
(S = CH3CN, SЈ = CD3CN; L–LЈ = dppe or dmpe). Fluxional
η1-dmpe and η1-dppe ligands are known in mononuclear
complexes15 and no JRhP coupling is observed at Ϫ30 ЊC. Alter-
Reactions involving the methylene bridged diphosphines
Me2PCH2PMe2 (dmpm) and Ph2PCH2PPh2 (dppm) were also
studied but proved to yield a myriad of products as judged by
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Table 2 Summary of activation parameters for equatorial CH3CN
substitution in [Rh2(OAc)2(CH3CN)4]2ϩ in CD3CN solution
ously suggested the possibility of the reversible formation of a
mixed valence complex RhIII–RhI wherein the Rh() center is
kinetically labile.4 Obviously this possibility and other details
∆H‡/kcal molϪ1
∆S‡/cal KϪ1 molϪ1
4ϩ
Ligand (L)
of the substitution behavior of Rh2 centers remain to be
investigated.
PMe3
PCy3
dppe
16(1)
21(1)
26(1)
33(1)
Ϫ9(3)
ϩ2(3)
ϩ12(3)
ϩ11(3)
Experimental
Physical techniques
CD3CN
1H NMR spectra were recorded on a Varian XL-300 spec-
trometer at 300 MHz in the appropriate dry and oxygen-free
solvents. 31P{1H} NMR spectra were recorded on a Nicolet 360
MHz spectrometer, all samples being referenced to external
H3PO4. All chemical shifts are reported in ppm relative to the
protio impurity signals of the solvents. Elemental analyses were
carried out by Atlantic Microlabs, Norcross, GA.
Synthesis and chemicals
All reactions were carried out under an atmosphere of dry,
oxygen-free nitrogen using standard Schlenk and glovebox
techniques. All solvents were distilled, degassed, and stored
over 4 Å sieves before use. Anhydrous CH3CN was purchased
from Aldrich. CD3CN was purchased from Cambridge Iso-
topes. PCy3, dppm, dppe, dmpm and dmpe were purchased
from Strem Chemicals. PCy3, dppm and dppe were purified by
sublimation before use. Rh2(OAc)4 was synthesized using the
procedures of Wilkinson et al.16 [Rh2(OAc)2(CH3CN)4][BF4]2
was synthesized by the method of Garner et al.9 PMe3 was
made using the standard procedure of Fackler et al.17 and puri-
fied by distillation.
Scheme
3 Reaction of [Rh2(OAc)2(CH3CN)6][BF4]2. S = CH3CN;
SЈ = CD3CN; L–L = dppe.
31P{1H} NMR spectroscopy and no single compound was
obtained by crystallization. At low temperatures, ca. Ϫ30 ЊC,
the initial formation of axially bound phosphine complexes
occurs as evidenced by 31P{1H} chemical shifts of δ Ϫ25
(dmpm) and Ϫ26 (dppm). Subsequent to this 31P signals in the
region δ ϩ10 to ϩ60 grow in and these are taken as evidence for
the formation of equatorial Rh–P bonds. However, we cannot
determine that the Rh–Rh bond remains intact.
NMR studies
Typical samples were made by dissolving 10 mg (0.0134 mmol)
of [Rh2(OAc)2(CH3CN)6][BF4]2 in 0.5 ml of CD3CN. The
sample was cooled to Ϫ30 ЊC. Appropriate quantities (0.0134/
0.0268 mmol) of PMe3, dmpm and dmpe were added using
a microliter syringe at Ϫ30 ЊC. Solid PCy3/dppe/dppm were
added directly to the solution containing [Rh2(OAc)2(CH3-
CN)6][BF4]2 under a helium atmosphere; the NMR tubes were
sealed by following the normal freeze–pump–thaw procedure at
Ϫ178 ЊC.
The samples thus prepared were transferred to a pre-
equilibrated NMR probe for studies of kinetics. The delay d1
used for 31P{1H} NMR was 5 s, 400 scans were accumulated
and then Fourier transformed to give a suitable spectrum. The
concentrations of various species present in solution were
estimated by the integration of their 31P{1H} signals.
Estimates of the activation parameters and determination
of rate constants were carried out according to procedures
described previously in this journal. A summary of 31P{1H}
NMR data for the various compounds described herein is given
in Table 3.
Concluding remarks
This work shows that the [Rh2(OAc)2(CH3CN)4]2ϩ cation reacts
with monodentate tertiary phosphines in a sequential manner.
The formation of the axially ligated Class I compounds is rapid
and reversible and is followed by a slower reaction to form a
monoaxial, monoequatorial compound, Class III compound,
by the displacement of a single CH3CN equatorial ligand in
CD3CN as solvent. The rate of this reaction is markedly
dependent upon the PR3 ligand (R = Me ≈ Bun > Cy). In the
case of the chelating diphosphine dppe this event occurs in a
stepwise manner and the chelating effect does not facilitate the
substitution of the Rh–S equatorial site, Scheme 3. A summary
of the activation parameters for equatorial CH3CN substitu-
tion is given in Table 2. The increase in ∆H‡ within the series
PMe3 < PCy3 < dppe < CD3CN is understandable in terms of
the basicity of the entering ligand in the axial position facilitat-
ing a bond dissociation in the equatorial site. The entropic term
becomes more positive along this series which suggests that
bond dissociation or a more disordered transition state is
more important as we progress from PMe3 to PCy3 to dppe to
CD3CN. The data clearly support an interchange mechanism
for equatorial CH3CN bond substitution involving prior
coordination of the entering ligand at the axial site. In this
regard the mechanism is similar to that for M–PR3 substitution
by PR3Ј ligands in M2Cl4(PR3)4 compounds described previ-
ously in this journal.14
Reactions
Of [Rh2(OAc)2(CH3CN)4][BF4]2 with 2PCy3. Two equiv-
alents of PCy3 (37.5 mg, 0.134 mmol) in 20 ml of degassed
CH3CN was added to a purple colored solution of [Rh2-
(OAc)2(CH3CN)6][BF4]2 (50 mg, 0.67 mmol) in 20 ml of
degassed CH3CN at 25 ЊC. This resulted in a deep red colored
solution. The reaction mixture was allowed to stir overnight
(≈12 h) to give a greenish red color. The solvent was removed
in vacuo and 20 ml of anhydrous methanol were added to the
sticky paste and the solution was cooled to Ϫ20 ЊC. Deep
reddish green crystalline material was obtained in near quan-
titative yield (75 mg, 92%) overnight. Suitable crystals were
The ability of certain chelating ligands such as dppe, dppm,
2,2Ј-bipyridine, 1,10-phenanthroline to catalyze the exchange
of equatorial CH3CN ligands for CD3CN solvent molecules
implies the rapid and reversible formation of an activated
complex that is kinetically labile yet not directly on the path
of the substitution chemistry described here. We have previ-
1
submitted for X-ray analysis. H NMR: δ 2.61 (s, 1H), 2.43 (s,
1H), 2.38 (s, 1H), 2.12 (s, 1H), 2.09 (s, 1H), 1.95 (s, 3H), 1.80
(m), 1.3 (m). IR/cmϪ1: 2976 (s), 2941 (s), 1688 (w), 1522 (s),
J. Chem. Soc., Dalton Trans., 2000, 1483–1489
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Table 3 31P{1H} NMR data for Rh24ϩ–phosphine complexesa
Axial P (δ)
Equatorial P (δ)
1. [Rh2(OAc)2(CH3CN)4(PMe3)2]
Class I (biaxial) isomer
Ϫ35.5 (dd, JRh–Rh = 5 Hz, 1JRh–P
= 55 Hz, 2JRh–P = 25 Hz, JP–P = 400 Hz)
Class III (monoaxial, monoequatorial) isomers (A)
Ϫ44.0 (m)
Ϫ35.8 (m)
ϩ10.6 (dd, 1JRh–P = 152 Hz, 2JRh–P = 17 Hz)
(B)
ϩ17.0 (dd, 1JRh–P = 145 Hz, 2JRh–P = 15 Hz)
Class II (biequatorial) isomer
ϩ 2 (d, 1JRh–P = 153 Hz), ϩ12.8 (d, 1JRh–P = 135 Hz)
ϩ42.0 (dd, 1JRh–P = 146 Hz, 2JRh–P = 14 Hz)
ϩ37.1 (d, 1JRh–P = 79 Hz)
2. [Rh2(OAc)2(CH3CN)4(PCy3)2]
Class I (biaxial) isomer
Class III (monoaxial, monoequatorial) isomers
Ϫ4.4 (m)
Ϫ6.0 (m)
3. [Rh2(OAc)2(CH3CN)4(dmpe)]
Class I (biaxial) isomer
Class III (monoaxial, monoequatorial) isomers
Ϫ26 (br, m)
Ϫ27.0 (m)
4. [Rh2(OAc)2(CH3CN)4(dppe)]
Class I (biaxial) isomer
Class III (monoaxial, monoequatorial) isomers
Class II
Ϫ10 (br, m)
Ϫ36.1 (m)
ϩ59.0 (d, 1JRh–P = 132 Hz)
ϩ59.0 (d, 1JRh–P = 132 Hz)
5. [Rh2(OAc)2(CH3CN)4(dmpm)]
Class I (biaxial) isomer
Ϫ25 (br, m)
Ϫ26 (br, m)
6. [Rh2(OAc)2(CH3CN)4(dppm)]
Class I (biaxial) isomer
a All spectra were referenced to external H3PO4. Only one monoaxial monoequatorial isomer is seen for entry 2. There were a number of peaks seen
for entries 5 and 6. However, it was not possible to determine whether or not the Rh–Rh remained intact in these reactions and so the 31P{1H} NMR
values for the Class II and Class III isomers are not reported.
Table 4 Summary of crystal data
a
[Rh(CH3CN)2(PCy3)2][BF4]
[Rh2(OAc)2(CH3CN)3(PCy3)2][BF4]2
Empirical formula
Color of crystal
C40H72BF4N2P2Rh
Yellow
C46H81N3B2F8O4P2Rh2
Red
Crystal dimensions/mm
Space group
0.30 × 0.30 × 0.45
P21nm
0.28 × 0.25 × 0.42
P1
¯
Crystal system
T/ЊC
Orthorhombic
25
Triclinic
Ϫ170
a/Å
b/Å
c/Å
α/Њ
β/Њ
γ/Њ
9.879(1)
13.275(1)
16.705(1)
14.638(2)
16.055(3)
12.507(2)
94.55(1)
90.41(1)
72.13(1)
2
2788.12
1.407
1181.53
7.042
Z
4
V/Å3
2190.76
2.367
832
dcalcd/g cmϪ3
Molecular weight
µ/cmϪ1
9.278
6–45
3159
1589
1575
1554
0.0271
0.0270
1.644
0.09
2θ range/Њ
6–45
7850
7261
6770
Total no. of reflections collected
No. of unique intensities
No. with F > 0.0
No. with F > 2.33σ(F)
R(F)
5405
0.0508
0.0499
1.600
Rw(F)
Goodness of fit for last cycle
Maximum δ/σ for last cycle
0.05
a Due to problems with crystallographic disorder, full structural data are not being reported for this structure.
1398 (vs, br), 1267 (s), 1170 (w), 1036 (br, vs). Found: C = 46.74,
H = 6.86, N = 3.56. Calc. for [Rh2(OAc)2(CH3CN)4(PCy3)2]-
[BF4]2: C = 46.29, H = 7.02, N = 3.39%.
If the reaction mixture was left to stir for 24 h it resulted in a
light orange colored solution with black insoluble material at
the bottom of the flask. This solution was filtered, concentrated
and kept at Ϫ20 ЊC. Bright yellow crystals were obtained in
approximately 40% yield (18 mg) after 2 d. IR/cmϪ1: 2941 (s,
br), 2741 (s), 1477 (s), 1267 (s), 1171 (w), 1036 (br, vs, d). Found:
C = 57.49, H = 8.55, N = 3.30. Calc. for [Rh(CH3CN)2(PCy3)2]-
[BF4]: C = 57.70, H = 8.66, N = 3.37%.
Of [Rh2(OAc)2(CH3CN)6][A]2 (A ؍ BF4 or PF6) with 2PMe3.
Two equivalents of PMe3 (0.134 mmol) were added to a purple
colored solution of [Rh2(OAc)2(CH3CN)6][A]2 (0.067 mmol) in
20 ml of degassed CH3CN at Ϫ30 ЊC. This resulted in a deep
red colored solution. The temperature was slowly brought to
25 ЊC and the reaction was stirred for 1 h. The color of the
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J. Chem. Soc., Dalton Trans., 2000, 1483–1489

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K. R. Dunbar and G. Christou, Inorg. Chem., 1993, 32, 3125;
solution changes from deep red to light orange. Cooling to
Ϫ20 ЊC resulted in a light orange colored powder. Unfortu-
nately, X-ray quality crystals could not be obtained from these
reactions.
(b) C. A. Crawford, Synthesis, Characterization and Reactivity
of Dinuclear Rhodium(II) Complexes Containing Nitrogen Donor
Ligands: A Model Approach to Antitumor Activity, Ph.D. Thesis,
Indiana University, 1995.
4 (a) J. M. Casas, R. H. Cayton and M. H. Chisholm, Inorg. Chem.,
1991, 30, 358; (b) M. H. Chisholm and A. M. Macintosh, J. Chem.
Soc., Dalton Trans., 1999, 1205.
5 H. Chen and F. A Cotton, Polyhedron, 1995, 14, 2221.
6 R. H. Cayton, S. Chacon, M. H. Chisholm and K. Folting,
Polyhedron, 1995, 12, 415.
7 M. H. Chisholm, K. Folting, K. G. Moodley and J. L. Wesemann,
Polyhedron, 1996, 15, 1903.
8 F. Basolo and R. G. Pearson, in Mechanisms of Inorganic Reactions.
A Study of Metal Complexes in Solution, John Wiley Publishers,
New York, 2nd edn., 1968.
9 G. Pimblett and C. D. Garner, J. Chem. Soc., Dalton Trans., 1986,
1257.
10 G. Pimblett, C. D. Garner and W. Clegg, J. Chem. Soc., Dalton
Trans., 1985, 1977.
11 A. R. Siedle, W. B. Gleeson, R. A. Newmark, R. P. Skarjune, P. A.
Lyon, G. C. Markell, K. O. Hodgson and A. L. Roe, Inorg. Chem.,
1990, 29, 1667.
12 J. Telser and R. S. Drago, Inorg. Chem., 1986, 25, 2989.
13 C. A. Tolman, Chem. Rev., 1977, 77, 313.
Of [Rh2(OAc)2(CH3CN)4][BF4]2 with dppe. To an acetonitrile
solution containing [Rh2(OAc)2(CH3CN)4][BF4]2 (50 mg, 0.67
mmol) solid dppe (26 mg, 0.67 mmol) was added. The purple
colored solution turned deep red within 5 min. This solution
was heated to 80 ЊC for 1 h; the solution was concentrated and
cooled to Ϫ20 ЊC. After 2 d, purple crystals were obtained, the
analysis of which showed it to be the starting material.
Single crystal X-ray studies
General operating procedures and listings of programs have
been previously reported.18 A summary of crystal data is given
in Table 4.
CCDC reference number 186/1870.
See http://www.rsc.org/suppdata/dt/b0/b001444n/ for crystal-
lographic files in .cif format.
14 M. H. Chisholm and J. M. McInnes, J. Chem. Soc., Dalton Trans.,
1997, 2735.
15 (a) V. Riena, M. A. Ruiz, F. Villafare, C. Bois and Y. Jeanin, Organo-
met. Chem., 1990, 382, 407; (b) F. A. Cotton and M. Matsusz,
Polyhedron, 1987, 6, 261.
Acknowledgements
We thank the National Science Foundation for support.
16 G. A. Rempel, P. Legzdins, H. Smith and G. Wilkinson, Inorg.
Synth., 1972, 13, 90.
17 M. L. Luetkins, Jr., A. P. Sattelberger, H. H. Murray, J. D. Basil and
J. P. Fackler, Jr., Inorg. Synth., 1982, 22, 7.
18 M. H. Chisholm, K. Folting, J. C. Huffman and C. C. Kirkpatrick,
Inorg. Chem., 1984, 23, 1021.
19 C. K. Johnson, ORTEP, Report ORNL-5138, Oak Ridge National
Laboratory, Oak Ridge, TN, 1976.
References
1 F. A. Cotton and R. A. Walton, in Multiple Bonds Between Metal
Atoms, Oxford University Press, 2nd edn., 1993.
2 R. G. Hughes, Jr., J. L. Bear and A. P. Kimbell, Proc. Am. Assoc.
Cancer Res., 1972, 13, 120.
3 (a) C. A. Crawford, J. H. Matonic, W. E. Streib, J. C. Huffman,
J. Chem. Soc., Dalton Trans., 2000, 1483–1489
1489