Oxidative addition of methyl iodide to [Rh(CO)2I]2: Synthesis, structure and reactivity of neutral rhodium acetyl complexes, [Rh(CO)(NCR)(COMe)I2]2

Article abstract of DOI:10.1016/j.ica.2004.03.018

Reaction of [Rh(CO)₂I]₂ (1) with MeI in nitrile solvents gives the neutral acetyl complexes, [Rh(CO)(NCR)(COMe)I ₂]₂ (R=Me, 3a; ^tBu, 3b; vinyl, 3c; allyl, 3d). Dimeric, iodide-bridged structures have been confirmed by X-ray crystallography for 3a and 3b. The complexes are centrosymmetric with approximate octahedral geometry about each Rh centre. The iodide bridges are asymmetric, with Rh-(μ-I) trans to acetyl longer than Rh-(μ-I) trans to terminal iodide. In coordinating solvents, 3a forms mononuclear complexes, [Rh(CO)(sol) ₂(COMe)I₂] (sol=MeCN, MeOH). Complex 3a reacts with pyridine to give [Rh(CO)(py)(COMe)I₂]₂ and [Rh(CO)(py)₂(COMe)I₂] and with chelating diphosphines to give [Rh(Ph₂P(CH₂)_nPPh₂)(COMe)I ₂] (n=2, 3, 4). Addition of MeI to [Ir(CO)₂(NCMe)I] is two orders of magnitude slower than to [Ir(CO)₂I₂] ^-. A mechanism for the reaction of 1 with MeI in MeCN is proposed, involving initial bridge cleavage by solvent to give [Rh(CO)₂(NCMe)I] and participation of the anion [Rh(CO)₂I₂]^- as a reactive intermediate. The possible role of neutral Rh(III) species in the mechanism of Rh-catalysed methanol carbonylation is discussed.

Full text of DOI:10.1016/j.ica.2004.03.018

Inorganica Chimica Acta 357 (2004) 3027–3037
www.elsevier.com/locate/ica
Oxidative addition of methyl iodide to [Rh(CO)2I]2:
synthesis, structure and reactivity of neutral rhodium
q
acetyl complexes, [Rh(CO)(NCR)(COMe)I2]2
*
Anthony Haynes , Peter M. Maitlis, Ian A. Stanbridge, Susanne Haak, Jean M. Pearson,
Harry Adams, Neil A. Bailey
Department of Chemistry, University of Sheffield, Dainton Building, Brook Hill, Sheffield S3 7HF, UK
Received 16 February 2004; accepted 13 March 2004
Available online 27 April 2004
Abstract
Reaction of [Rh(CO)2I]2 (1) with MeI in nitrile solvents gives the neutral acetyl complexes, [Rh(CO)(NCR)(COMe)I2]2 (R ¼ Me,
3a; tBu, 3b; vinyl, 3c; allyl, 3d). Dimeric, iodide-bridged structures have been confirmed by X-ray crystallography for 3a and 3b. The
complexes are centrosymmetric with approximate octahedral geometry about each Rh centre. The iodide bridges are asymmetric,
with Rh–(l-I) trans to acetyl longer than Rh–(l-I) trans to terminal iodide. In coordinating solvents, 3a forms mononuclear
complexes, [Rh(CO)(sol)2(COMe)I2] (sol ¼ MeCN, MeOH). Complex 3a reacts with pyridine to give [Rh(CO)(py)(COMe)I2]2 and
[Rh(CO)(py)2(COMe)I2] and with chelating diphosphines to give [Rh(Ph2P(CH2)nPPh2)(COMe)I2] (n ¼ 2, 3, 4). Addition of MeI to
[Ir(CO)2(NCMe)I] is two orders of magnitude slower than to [Ir(CO)2I2]ꢀ. A mechanism for the reaction of 1 with MeI in MeCN is
proposed, involving initial bridge cleavage by solvent to give [Rh(CO)2(NCMe)I] and participation of the anion [Rh(CO)2I2]ꢀ as a
reactive intermediate. The possible role of neutral Rh(III) species in the mechanism of Rh-catalysed methanol carbonylation is
discussed.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Rhodium; Iridium; Carbonyl; Iodide; Mechanism; Catalysis
1. Introduction
atively fast [9–12]. In the Ir system MeI oxidative ad-
dition to [Ir(CO)2I2]ꢀ is >100 times faster than for the
Rh analogue and [Ir(CO)2I3Me]ꢀ is the catalyst resting
state [13–15]. Carbonylation of [Ir(CO)2I3Me]ꢀ is rate
controlling, and occurs via substitution of an iodide li-
gand by CO to give [Ir(CO)3I2Me] [16]. Migratory CO
insertion in this neutral complex is much faster than in
the anionic precursor, and high rates are achieved in the
catalytic system by addition of promoters which bind
ionic iodide [17].
Although the accepted mechanism for the Rh-based
system is dominated by anionic complexes, the potential
participation of neutral intermediates can be considered.
In this paper, we report an investigation of the reactivity
of MeI with the neutral rhodium dimer, [Rh(CO)2I]2
and its monomeric derivatives and describe the synthe-
sis, structure and reactivity of some neutral rhodium
acetyl complexes. Based upon these observations, we
The carbonylation of methanol to acetic acid is one of
the most important applications of homogeneous tran-
sition metal catalysis [1–3]. The rhodium/iodide-based
process originally developed by Monsanto [4] has re-
cently been superceded on some plants by the iridium/
iodide based Cativa process commercialised by BP
Chemicals [5–8]. Mechanistic studies have shown that
oxidative addition of methyl iodide to the anionic
[Rh(CO)2I2]ꢀ is rate determining in the Rh system and
that migratory CO insertion in [Rh(CO)2I3Me]ꢀ is rel-
q Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.ica.2004.03.018.
* Corresponding author. Tel.: +44-114-2229326; fax: +44-114-
2229346.
E-mail address: a.haynes@sheffield.ac.uk (A. Haynes).
0020-1693/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.03.018

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A. Haynes et al. / Inorganica Chimica Acta 357 (2004) 3027–3037
comment on the possible participation of neutral species
in rhodium catalysed methanol carbonylation.
equiv per Rh) to the unstable species 2, described above.
This lends support to the formulation of 2 as [Rh(CO)
(COMe)I2]n, which is stabilised upon coordination of
nitrile. Solid-state IR spectra of 3a (as Nujol mulls) were
found to be dependent upon the method of isolation.
Whereas samples recrystallised from CH2Cl2 or MeCN
had m(CO) bands at 2078 and 1718 cmꢀ1, samples
formed by removal of CH2Cl2 solvent in vacuo dis-
played bands at 2086 and 1709 cmꢀ1. This may indicate
the existence of different isomers of 3a.
Addition of adiponitrile (0.5 equiv per Rh) to 2 gave a
product, 4, formulated as [Rh(CO)(NC(CH2)4CN)
(COMe)I2]n. The nuclearity of 4 is uncertain, the com-
plex being insufficiently soluble for molecular weight
measurements to be carried out. A possible dinuclear
structure with the adiponitrile ligand bridging the two
Rh centres is shown in Scheme 1.
2. Results and discussion
The reactivity of [Rh(CO)2I]2 (1) with MeI is solvent
dependent. In solvents of low polarity such as CH2Cl2 or
even neat MeI, a very slow reaction is observed. The dark
red-brown solid obtained on removal of the solvent is
very air sensitive and only stable under an inert atmo-
sphere for a few hours. The product, 2, exhibits a terminal
m(CO) band at 2084 cmꢀ1 and low frequency absorptions
at 1739 and 1727 cmꢀ1, indicating the presence of an
acetyl ligand. Full characterisation of 2 was hindered by
its instability although microanalytical data suggest for-
mulation as [Rh(CO)(COMe)I2]n. Further support for
this suggestion is provided by the reaction of 2 with
Bu4NI (1 molar equiv/Rh) in CH2Cl2, which gives the
2.1. X-ray crystallographic structure determinations of
[Rh(CO)(NCMe)(COMe)I2]2 (3a) and [Rh(CO)
(NCtBu)(COMe)I2]2 (3b)
2ꢀ
known anionic acetyl complex, [Rh(CO)(COMe)I3]2
(m(CO) 2065s, 1737m and 1717sh cmꢀ1) [18,19].
The reaction of 1 with MeI occurs much more readily
in nitrile solvents, giving stable products amenable to
isolation as solids and detailed characterisation. Spec-
troscopic and analytical data are consistent with for-
mulation of the products as [Rh(CO)(NCR)(COMe)I2]n
(3a, R ¼ Me; 3b, R ¼ tBu; 3c, R ¼ CH@CH2; 3d, R ¼
CH2CH@CH2) as shown in Scheme 1. Single-crystal X-
ray structure determinations for complexes 3a and 3b
show them to be dimeric in the solid state with rho-
dium–iodide bridges (vide infra). Complexes 3a–d could
also be prepared by addition of the appropriate nitrile (1
The molecular structures of 3a and 3b determined by
X-ray crystallography are illustrated in Figs. 1 and 2;
selected bond lengths and angles are listed in Table 1.
The molecules are very similar in structure, each com-
prising a centrosymmetric, di-iodo-bridged di-rhodium
complex. Terminal iodo and acetyl ligands lie trans to
the bridging iodides. The octahedral coordination sphere
about each rhodium is completed by mutually trans-
carbonyl and nitrile ligands. The nitrile ligands of 3a and
3b are slightly bent at the coordinating nitrogen (173ꢁ
and 167ꢁ, respectively) and are approximately linear at
the carbon of the cyano group (178ꢁ and 174ꢁ, respec-
tively). The only significant deviation from ideal octa-
hedral coordination geometry involves the nitrile ligand
which bends away from the acetyl ligand, towards the
relatively trans bridging iodide by 5ꢁ (3a) or 7ꢁ (3b). The
carbonyl ligands are almost linear. The plane of the ac-
etyl ligand is inclined to the ‘‘equatorial’’ plane of the Rh
R
O
C
C
N
O
C
I
I
I
MeI
I
Me
Rh
Rh
Rh
Rh
Me
CO
CO
OC
OC
I
C
O
I
RCN
N
C
R
C
O
1
3a-d
R = (a) Me; (b) tBu;
(c) vinyl; (d) allyl
RCN
MeI
[Rh(CO)I2(COMe)]n
2
NC(CH2)4CN
(CH2)4
C
N
C
N
Bu4NI
O
I
C
I
I
Me
Rh
Rh
Me
I
C
O
O
C
O
C
O
O
2–
C
Rh
I
I
I
C
I
I
4
Me
Rh
Me
I
C
O
C
O
5
Fig. 1. ORTEP plot (50% probability ellipsoids) for [Rh(CO)
(NCMe)(COMe)I2]2 (3a). Hydrogen atoms are omitted for clarity.
Scheme 1. Reactions of 1 leading to Rh(III) acetyl complexes.

A. Haynes et al. / Inorganica Chimica Acta 357 (2004) 3027–3037
3029
by 82ꢁ (3a) or 81ꢁ (3b). The two mutually trans-Rh–I
bonds (one terminal, one bridging) are very similar in
ꢀ
length (2.65–2.68 A in both complexes) but the other
ꢀ
ꢀ
Rh–(l-I) distance is much longer (2.96 A, 3a; 3.06 A, 3b),
reflecting the strong trans influence of the acetyl ligand.
The non-bonded Rhꢁ ꢁ ꢁRh distance is longer for 3b
ꢀ
t
ꢀ
(4.16 A) than for 3a (4.05 A), presumably due to the
bulky BuCN ligands. There are no significant intermo-
lecular contacts for either complex.
The structure most closely related to those determined
2ꢀ
here is that of the dianion, [Rh(CO)(COMe)I3]2 (5),
isolated as its Me3PhNþ salt by Forster and co-workers
[18]. Our neutral complexes 3a and 3b are formally de-
rived from the dianion by substitution of two terminal
iodide ligands trans to carbonyls by the appropriate ni-
trile. The principal features of the crystal structure of 5
are very similar to those of 3a and 3b. All three com-
plexes show distinct asymmetry in the Rh(l-I)2Rh unit.
In each case the Rh–(l-I) bond trans to acetyl is sub-
ꢀ
stantially (0.29–0.37 A) longer than the Rh–(l-I) bond
trans to iodide due to the strong trans influence of acetyl.
Fig. 2. ORTEP plot (50% probability ellipsoids) for [Rh(CO)
(NCtBu)(COMe)I2]2 (3b). Hydrogen atoms are omitted for clarity.
2.2. Solution behaviour of [Rh(CO)(NCMe)(COMe)-
I2]2 (3a)
Vapour phase osmometry measurements gave molec-
ular weights of 802 and 474 for solutions of 3a in CHCl3
and MeCN, respectively. The dimer identified by X-ray
crystallography has a molecular weight of 937.64. The
results therefore indicate that 3a remains predominantly
dimeric in CHCl3, but is cleaved to mononuclear species
in MeCN. The IR spectra of 3a in CHCl3 or CH2Cl2
exhibit shoulders on both terminal and acetyl m(CO)
bands. Similar features have previously been observed
for the dianion, 5, and were explained by the presence of
a mixture of dimeric isomers in solution [20]. Addition of
MeCN to a solution of 3a in CHCl3 resulted in band
shifts from 2090 (2075sh) and 1733 (1740sh) cmꢀ1 to
Table 1
ꢀ
Selected bond lengths (A) and bond angles (ꢁ) for 3a and 3b
3a
3b
Rh(1)–I(1)
Rh(1)–I(2)
2.673(2)
2.652(2)
2.960(2)
1.892(13)
2.067(13)
2.089(11)
1.071(17)
1.179(18)
1.433(22)
1.116(19)
1.438(22)
4.053(3)
2.682(8)
2.656(8)
3.057(9)
1.854(24)
2.037(21)
2.094(20)
1.153(29)
1.213(32)
1.474(28)
1.174(30)
1.472(31)
4.159(3)
Rh(1)–I(1A)
Rh(1)–C(1)
Rh–C (acetyl)
Rh–N(1)
C(1)–O(1)
C–O (acetyl)
C–C (acetyl)
N–C (nitrile)
C–C (nitrile)
Rh(1)ꢁ ꢁ ꢁRh(1A)
1
2096 (2084sh) and 1718 cmꢀ1. The H NMR of 3a in
CDCl3 shows broad signals due to the acetyl and MeCN
ligands at d 2.88 and 2.62. Addition of MeCN (20 equiv/
3a) resulted in sharpening of both resonances and small
shifts to d 2.86 and 2.60, together with the growth of a
signal at d 2.00 due to free MeCN. Subsequent addition
of CD3CN (excess) caused the disappearance of the sig-
nal at 2.20, due to displacement of coordinated CH3CN
at the Rh centre. The spectroscopic data do not reveal the
precise structure(s) of the species generated by addition
of MeCN to 3a, but are consistent with cleavage of the
dimer to give [Rh(CO)(NCMe)2(COMe)I2], 6a (Eq. (1)).
Complex 6a was never isolated from solution; crystalli-
sation always lead to the dimer, 3a
I(1)–Rh(1)–I(2)
177.0(1)
174.1(5)
176.5(3)
88.1(1)
178.8(2)
174.1(7)
176.4(6)
87.3(2)
N(1)–Rh(1)–C(1)
C(acetyl)–Rh(1)–I(1A)
I(1)–Rh(1)–I(1A)
I(2)–Rh(1)–C(acetyl)
I(2)–Rh(1)–N(1)
88.6(3)
89.6(2)
89.4(6)
88.9(4)
I(2)–Rh(1)–C(1)
87.9(3)
94.5(1)
88.2(6)
93.1(2)
I(2)–Rh(1)–I(1A)
N(1)–Rh(1)–C(acetyl)
N(1)–Rh(1)–I(1A)
Rh(1)–I(1)–Rh(1A)
Rh(1)–C(1)–O(1)
Rh(1)–C–O (acetyl)
Rh(1)–C–C (acetyl)
Rh(1)–N(1)–C (nitrile)
N(1)–C–C (nitrile)
95.1(5)
86.6(3)
91.9(1)
96.9(8)
85.8(4)
92.7(2)
176.7(12)
118.7(10)
115.5(10)
172.5(11)
178.0(14)
176.1(18)
121.1(14)
117.6(17)
167.4(15)
173.8(20)
½RhðCOÞðNCMeÞðCOMeÞI2ꢂ ð3aÞ þ 2MeCN
2
ð1Þ
! 2½RhðCOÞðNCMeÞ ðCOMeÞI2ꢂ ð6aÞ
2

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A. Haynes et al. / Inorganica Chimica Acta 357 (2004) 3027–3037
2.3. Pyridine adducts
2.4. Reactions of [Rh(CO)(NCMe)(COMe)I2]2 (3a)
with diphosphines
Addition of pyridine (2 equiv) to 3a resulted in isola-
tion of a product formulated as [Rh(CO)(py)(COMe)I2]n,
3e. It is likely that this complex has a similar dimeric
structure to 3a, resulting from substitution of the MeCN
ligands by the more strongly coordinating pyridine. The
same product was also synthesised by addition of pyri-
dine (1 equiv/Rh) to 2. A one-pot synthesis of 3e by ox-
idative addition of MeI to 1 in the presence of pyridine is
precluded by the facile quaternisation of pyridine under
these conditions, leading to ionic products.
Addition of 2 molar equivalents (per Rh) of pyridine
to 3a (or 1 equiv/Rh to 3e) gave [Rh(CO)(py)2(COMe)I2]
(6e) which exhibits unshouldered IR absorptions in both
the terminal and acetyl m(CO) regions, suggesting a single
isomer. The 1H NMR spectrum indicates the presence of
inequivalent pyridine ligands; three possible structures
are shown in Scheme 2. We favour structures (i) or (ii) by
analogy with [Rh(CO)(py)(COMe)I3]ꢀ, which contains a
pyridine ligand trans to CO [20].
Complex 3a was found to be a useful starting material
for preparation of other neutral Rh(III) acetyl com-
plexes. For example, treatment of 3a with chelating di-
phosphines results in loss of MeCN and CO ligands to
yield the monomers [Rh(Ph2P(CH2)nPPh2)(COMe)I2]
(n ¼ 2–4; 7a–c, Scheme 3).
The dppe and dppp complexes (7a and 7b, respec-
tively) were first isolated in a study of the rhodium/io-
dide catalysed reductive carbonylation of methanol, and
7c can also be prepared by the stoichiometric reaction of
[Rh(CO)(dppe)I] with MeI [21,22]. Crystal structures
reported for 7a [22], 7b [23,24] (and also the dppm an-
alogue [25]) show these complexes to adopt an approx-
imate square pyramidal geometry with an apical acetyl
ligand. The dppb derivative, 7c, which has not previ-
1
ously been reported, shows a doublet (d 72.1, JRh–P
136 Hz) for equivalent phosphorus atoms in its 31P
NMR spectrum and likely has an analogous structure.
Me
O
C
O
C
N
0.5
0.5
Me
O
C
O
C
C
py
I
I
I
+ C5H5N
– MeCN
I
Me
Me
Rh
Rh
Rh
py
Rh
Me
I
I
C
O
I
C
O
I
N
C
O
C
O
C
Me
3e
3a
+ C5H5N
O
C
O
C
O
C
I
I
I
py
I
I
py
py
py
Rh
py
Rh
py
Rh
I
Me
Me
Me
C
O
C
O
C
O
(i)
(ii)
(iii)
6e
Scheme 2. Formation of pyridine adducts including possible isomers of 6e.
Me
O
C
C
N
0.5
O
C
O
Me
C
I
I
Ph2P(CH2)nPPh2
I
I
P
Me
Rh
Rh
(CH2)n
P
Rh
Me
I
C
O
I
N
C
O
C
7a-c
Me
3a
n = 2 (a); 3 (b); 4 (c)
Scheme 3. Reaction of 3a with diphosphines.

A. Haynes et al. / Inorganica Chimica Acta 357 (2004) 3027–3037
3031
2.5. Reaction of [Rh(CO)(NCMe)(COMe)I2]2 (3a)
with CO
complex, [Rh(CO)(COMe)I4]2ꢀ (9). Our data do not
define the stereochemistry of 9 (which is related to the
known dianion [Rh(CO)I5]2ꢀ [27]) but a structure with
carbonyl cis to acetyl (as in 5) is considered likely.
Significantly different behaviour was observed in
MeOH, in which solutions of 3a or 5 display remarkably
similar m(CO) frequencies. Both exhibit split absorptions
in the terminal and acetyl regions (2077, 2066, 1742,
1726 cmꢀ1) with only very slight differences in the rela-
tive intensities of the component bands. The spectra
suggest formation of the same species from the neutral
or anionic precursor in MeOH solution. We propose
that bridge cleavage, accompanied by loss of MeCN
from 3a or Iꢀ from 5 leads to a bis(solvent) complex,
[Rh(CO)(MeOH)2(COMe)I2] (Scheme 4).
Addition of a stoichiometric quantity of AgBF4 to a
methanolic solution of 5 leads to precipitation of silver
iodide but no discernible change in the IR spectrum.
Therefore, the same Rh complex is present before and
after precipitation of iodide, demonstrating that iodide
dissociation occurs when 5 is dissolved in MeOH. Sim-
ilarly, mixing equimolar methanolic solutions of 3a and
1 resulted in formation of [Rh(CO)2I2]ꢀ (by reaction of
1 with Iꢀ) but no change in the IR bands of the Rh(III)
acetyl species. This result demonstrates that Rh(I) has a
higher affinity for Iꢀ than Rh(III).
Brief bubbling of solutions of 3a in CH2Cl2 with CO
results in the formation of a new species (8) with m(CO)
bands at 2154vw 2102s and 1733m cmꢀ1 as well as free
MeCN (m(CN) 2255 cmꢀ1). The IR spectrum of 8 re-
sembles that of trans-[Rh(CO)2(COMe)I3]ꢀ (m(CO):
2141, 2084, 1706 cmꢀ1) [19] but with each absorption
shifted to higher frequency, as expected for a neutral
complex compared to an anion. The data are consistent
with 8 having the formula [Rh(CO)2(COMe)I2]n, which
is formally derived from [Rh(CO)2(COMe)I3]ꢀ by re-
moval of an iodide ligand. However, we cannot distin-
guish between monomeric and dimeric structures for 8.
The m(CO) bands of 8 decay completely within ca.
30 min at 25 ꢁC (much faster than decomposition of
[Rh(CO)2(COMe)I3]ꢀ [19]) and new absorptions as-
signed to the Rh(I) dimer 1 grow at 2096, 2080 and
2026 cmꢀ1. The conversion of 8 into 1 requires the
formal reductive elimination of acetyl iodide
½RhðCOÞ ðCOMeÞI2ꢂ ð8Þ ! ½RhðCOÞ Iꢂ ð1Þ þ MeCOI
2
2
2
ð2Þ
The organic product was not directly observed,
probably due to hydrolysis to acetic acid and HI by
traces of adventitious water. Consistent with this, m(CO)
bands at 2071 and 2003 cmꢀ1 revealed the presence of
small amounts of H[Rh(CO)2I2] formed by reaction of 1
with HI [26]. By contrast, no change in the IR spectrum
is observed on bubbling CO through solutions of 3a in
donor solvents such as MeCN or MeOH. The Rh-
containing species present under these conditions is a
monomeric bis(solvate), [Rh(CO)(COMe)I2(sol)2] and
coordination of a second CO ligand is inhibited by a
large excess of coordinating solvent.
In pyridine, the IR specta of 3a or 5 indicated the
presence of [Rh(CO)(py)2(COMe)I2] (6e), whereas in
MeCN or PrOH, the spectra were more complex and
i
varied with the iodide concentration, suggesting an
equilibrium mixture of neutral and anionic complexes.
This can be explained by the weaker coordinating
power of MeCN (compared to pyridine) and the
i
poorer anion-solvating ability of PrOH, compared to
MeOH.
Me
O
C
0.5
Me
C
N
O
C
2.6. Iodide coordination/dissociation
I
I
Me
Rh
Rh
MeOH
–MeCN
I
The IR spectra of 3a and 5 in CH2Cl2 showed the
expected differences between a neutral and an anionic
complex. Thus, the terminal m(CO) band of 3a is ca.
25 cmꢀ1 higher in frequency than that of 5, a shift which
is ascribed to decreased p-backbonding in the neutral
species. Addition of a stoichiometric quantity of Bu4NI
to a solution of 3a in CH2Cl2 resulted in shifts of the
m(CO) bands to low frequency, consistent with coordi-
nation of iodide. On addition of an additional equiva-
lent of Bu4NI, the broad, shouldered absorptions of 5
were replaced by narrower, more symmetrical bands in
both terminal and acetyl m(CO) regions. Addition of
excess Bu4NI caused no further modifications. These
spectral changes are consistent with bridge cleavage of 5
by iodide anion, to give the di-anionic tetraiodide
C
O
I
O
C
N
C
O
C
Me
I
sol
I
3a
Rh
sol
Me
C
O
MeOH
– I–
O
0.5
Me
sol = MeOH
O
2–
C
Rh
I
I
I
C
I
I
Me
Rh
I
C
O
C
O
5
Scheme 4. Formation of bis(solvent) complexes from 3a and 5.
(Stereochemistry of product uncertain.)

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A. Haynes et al. / Inorganica Chimica Acta 357 (2004) 3027–3037
2.7. Mechanism of reaction of [Rh(CO)2(NCMe)I]2
with MeI
Formation of [Rh(CO)2I2]ꢀ requires a source of io-
dide anion. The possibility of redistribution of iodide
and MeCN ligands between Rh centres is ruled out by
the absence of m(CO) bands due to the known cation
[Rh(CO)2(NCMe)2]þ [33]. The only other source of io-
dide is from the excess MeI present. It is known that N-
methylation of MeCN can be achieved using strong
methylating agents such as [Me3O]BF4 or by using alkyl
halides in the presence of a halide acceptor (e.g. SbCl5)
[34]. By analogy, in our system, the Rh centre can act as
an iodide acceptor to facilitate formation of the dim-
ethylnitrilium cation
Solutions of 1 in CH2Cl2 exhibit two strong m(CO)
bands at 2080 and 2027 cmꢀ1 as well as a third band of
medium intensity at 2096 cmꢀ1, consistent with a non-
planar dimeric structure for 1 analogous to the solid-
state structure of [Rh(CO)2Cl]2 [28]. In MeCN solution,
however, only two m(CO) bands are observed at 2088
and 2023 cmꢀ1. Addition of small amounts of MeCN to
a CH2Cl2 solution of 1 resulted in the smooth conver-
sion of the three-band spectrum into the two-band
spectrum. Analogous spectroscopic observations have
been reported for addition of MeCN to [Rh(CO)2Cl]2 in
CHCl3 [29]. On the basis of the disappearance of the
high frequency m(CO) band and the lack of a m(CN)
band for coordinated MeCN, it was suggested that
[Rh(CO)2Cl]2 becomes planar (with the A1 m(CO) mode
becoming IR inactive). In our studies, however, we have
detected weak m(CN) absorptions (2328 cmꢀ1, X ¼ Cl;
2327 cmꢀ1, X ¼ I) on addition of small quantities of
MeCN to [Rh(CO)2X] in CHCl3. This is consistent with
coordination of MeCN and cleavage of the halide
bridges to give monomeric cis-[Rh(CO)2(NCMe)X].
Vapour phase osmometry measurements gave molecular
weights for solutions of 1 as 596 (CHCl3) and 274
(MeCN), indicating retention of the dimeric structure in
CHCl3 but complete conversion to a monomer in
MeCN. Other nitriles are known to cleave [Rh(CO)2X]2
(X ¼ Cl, Br) to give stable products [30], but attempts to
isolate [Rh(CO)2(NCMe)X] from solution were unsuc-
cessful. We note that [Rh(CO)2Cl]2 is also known to
undergo bridge cleavage by MeOH to give cis-
[Rh(CO)2(MeOH)Cl] [31].
½RhðCOÞ IðNCMeÞꢂ þ MeI ꢀ
2
½MeCNMeꢂþ þ ½RhðCOÞ I2ꢂꢀ
ð4Þ
2
Although this equilibrium will favour the neutral re-
actants, it is feasible that a small concentration of
[Rh(CO)2I2]ꢀ can accumulate. We did not detect
[MeCNMe]þ directly, and it is possible that hydrolysis
by adventitious water occurs to give the amide, MeC-
(O)NHMe and Hþ, which could then act as counter-ion
for [Rh(CO)2I2]ꢀ. This anion is expected to be more
nucleophilic toward MeI than [Rh(CO)2(NCMe)I], as
demonstrated for the Ir analogues (vide infra) The rate
of the overall reaction with MeI is therefore determined
by the concentration of [Rh(CO)2I2]ꢀ rather than the
neutral precursor, resulting in the observed zero-order
dependence on [Rh(CO)2(NCMe)I].
A mechanism consistent with these observations is
shown in Scheme 5. Displacement of coordinated sol-
vent from [Rh(CO)2I(NCMe)] by iodide leads to
[Rh(CO)2I2]ꢀ. This anion then acts as a nucleophile
toward MeI, releasing Iꢀ and forming a neutral Rh(III)–
During the reaction with MeI (3.2 M in MeCN), the
IR absorptions of [Rh(CO)2(NCMe)I] decay and are
replaced by bands at 2091 and 1713 cmꢀ1 due to 6a
I
CO
CO
Rh
MeCN
½RhðCOÞ ðNCMeÞIꢂ þ MeI
2
MeCN
½RhðCOÞðCOMeÞðNCMeÞ I ꢂ ð6aÞ
ð3Þ
ƒƒƒ!
2
2
Plots of absorbance against time for both the reac-
tant band at 2023 cmꢀ1 and the product band at 1713
cmꢀ1 are approximately linear, indicating that the re-
action is zeroth-order in [Rh(CO)2(NCMe)I]. This
contrasts with the reactions of most square planar
complexes with MeI, which are first order in complex
[12,22,32]. The kinetic data indicate that the rate de-
termining step of Eq. (3) is not a direct bimolecular
reaction between [Rh(CO)2(NCMe)I] and MeI. On
close inspection of the IR spectra, additional weak ab-
sorptions are present at 2058 and 1984 cmꢀ1, which
grow at the start of the reaction and then decay away.
These absorptions exactly match those of [Rh(CO)2I2]ꢀ
in MeCN and indicate that the anion participates as an
intermediate in the reaction.
–
I
I
CO
CO
I– + MeCNMe+
MeCN + MeI
Rh
MeI + MeCN
Me
Rh
COMe
I
I
I
CO
CO
CO
Rh
MeCN
I
N
N
C
C
6a
Me
Me
Scheme 5. Mechanism for reaction of [Rh(CO)2(NCMe)I] with MeI in
MeCN. (Stereochemistry of octahedral Rh(III) products uncertain.)

A. Haynes et al. / Inorganica Chimica Acta 357 (2004) 3027–3037
3033
methyl species. At this stage, the Iꢀ released by the SN2
2.8. Oxidative addition of MeI to [Ir(CO)2(NCMe)I]
reaction can coordinate to the Rh(III) centre to give
[Rh(CO)2I3Me]ꢀ [11,12] or to unreacted Rh(I). The soft
Iꢀ is expected to bind preferentially to the lower oxi-
dation state, leading to regeneration of [Rh(CO)2I2]ꢀ
from [Rh(CO)2(NCMe)I]. Thus, sub-stoichiometric Iꢀ,
generated in situ from MeI, acts as a catalyst for the
oxidative addition. Similar catalysis by Iꢀ has been re-
ported previously for [Rh(CO)(ER3)2I] (E ¼ P, As, Sb)
[35]. Migratory CO insertion in the neutral Rh–methyl
species (probably solvated by MeCN) would lead to the
observed product, 6a.
Further evidence that Iꢀ can act as a catalyst was
provided by experiments in which a 1:1 mixture of
[Rh(CO)2(NCMe)I] and [Rh(CO)2I2]ꢀ was used. Unex-
pectedly, the m(CO) bands of the less nucleophilic neu-
tral species decayed first, with product bands due to 6a
appearing during this phase of the reaction. Only when
this process was close to completion did the m(CO)
bands of [Rh(CO)2I2]ꢀ begin to decay. This result is
counter-intuitive but can be explained by the mechanism
shown in Scheme 6. Of the two reactant complexes
present, MeI is expected to react preferentially with the
anion, [Rh(CO)2I2]ꢀ. As in Scheme 5, the nucleophilic
substitution releases Iꢀ which rapidly coordinates to
Rh(I) in preference to Rh(III). Thus, a molecule of
[Rh(CO)2(NCMe)I] is converted into [Rh(CO)2I2]ꢀ,
resulting in the observed depletion of [Rh(CO)2(NC-
Me)I], whilst the concentration of [Rh(CO)2I2]ꢀ remains
approximately constant. Only when most of the
[Rh(CO)2(NCMe)I] has reacted does net consumption
of [Rh(CO)2I2]ꢀ begin. In this latter stage of the reac-
tion, a mixture of neutral and anionic Rh(III) acetyl
products is formed according to the equilibrium at the
bottom of Scheme 6.
Iodide abstraction from [Ir(CO)2I2]ꢀ using AgBF4 or
InI3 in the presence of MeCN generates [Ir(CO)2
(NCMe)I] (m(CO) 2078, 2006 cmꢀ1; m(CN) 2333 cmꢀ1).
Subsequent addition of MeI (excess) leads to conversion
into a product with m(CO) bands at higher frequency
(2125, 2081 cmꢀ1) assigned to [Ir(CO)2(NCMe)I2Me].
The same species is formed on dissolving [Ir(CO)2I2Me]2
in MeCN [17].
The kinetics of oxidative addition were measured by
monitoring the decay of the 2006 cmꢀ1 absorption band
of [Ir(CO)2(NCMe)I]. A good exponential curve fit es-
tablishes that the reaction is first order in [Ir(CO)2
(NCMe)I]. Pseudo-first-order rate constants (kobs) ob-
tained over a range of MeI concentrations and temper-
atures are listed in Supporting Material (together with
data obtained for [Ir(CO)2I2]ꢀ in the same solvent). A
linear plot of kobs versus [MeI] demonstrates the reac-
tion to be first order in MeI and therefore second order
overall. The effect of removing an iodide ligand is dra-
matic, the second-order rate constant for addition of
MeI to [Ir(CO)2(NCMe)I] (1.56 · 10ꢀ5 Mꢀ1
being two orders of magnitude smaller than that for
[Ir(CO)2I2]ꢀ (1.34 · 10ꢀ3 Mꢀ1 ꢀ1). Interestingly, the
s
ꢀ1; 25 ꢁC)
s
rate constant for neutral iridium complex [Ir(CO)2
(NCMe)I] is of a similar magnitude to that for the an-
ionic rhodium complex, [Rh(CO)2I2]ꢀ (k2 ¼ 2:5 ꢃ 10ꢀ5
dm3 molꢀ1
s
ꢀ1; 25 ꢁC; CH2Cl2) [36].
Activation parameters calculated from an Eyring
plot of the variable temperature data are DHz ¼ 49
( 3) kJ molꢀ1 and DSz ¼ ꢀ171 ( 11) J molꢀ1 Kꢀ1 for
[Ir(CO)2I(NCMe)] and DHz ¼ 53 ( 2) kJ molꢀ1 and
DSz ¼ ꢀ122 ( 6) J molꢀ1 Kꢀ1 for [Ir(CO)2I2]ꢀ. The
lower reactivity of [Ir(CO)2I(NCMe)] appears to be
largely due to a more negative DSz term, perhaps due to
stronger solvation of the polar transition state derived
from two neutral reactants.
–
I
CO
CO
I
I
CO
CO
2.9. Participation of neutral complexes in Rh-catalysed
methanol carbonylation
Rh
Rh
MeCN
Oxidative addition is rate-controlling for Rh-cataly-
sed carbonylation and [Rh(CO)2I2]ꢀ is the catalyst
resting state observed by in situ IR spectroscopy [9,10].
Model stoichiometric studies show that sequential re-
action of [Rh(CO)2I2]ꢀ with MeI and CO gives anionic
Rh(III) products, but these are not detected in the cat-
alytic regime. As conventionally drawn, the mechanism
for the rhodium/iodide catalysed carbonylation of
methanol comprises four anionic Rh complexes [2,9,10].
However, it is clear that this is an over-simplification.
The MeI oxidative addition step is generally considered
to proceed via two steps: (i) SN2 attack by [Rh(CO)2I2]ꢀ
on MeI; (ii) coordination of Iꢀ to Rh(III) [37–39]. The
neutral intermediate in this process, ([Rh(CO)2I2Me] or
I–
MeI + 2 MeCN
COMe
COMe
–
I–
I
I
I
CO
CO
Rh
Rh
I
MeCN
I
MeCN
N
N
C
C
Me
Me
6a
Scheme 6. Mechanism for reaction of [Rh(CO)2(NCMe)I]/
[Rh(CO)2I2]ꢀ mixture with MeI.

3034
A. Haynes et al. / Inorganica Chimica Acta 357 (2004) 3027–3037
a solvent coordinated derivative), is not normally in-
cluded in the catalytic cycle, although its presence is not
disputed. The results presented in this paper indicate
that migratory CO insertion is facile on a neutral
Rh(III) centre, leading to the possibility that this can
occur in the catalytic system. In the Ir-catalysed process,
the requirement for iodide loss from [Ir(CO)2I3Me]ꢀ
prior to CO insertion has been established [15–17].
Whilst this step is not rate-determining for the Rh sys-
tem, it is still possible that migratory insertion occurs in
a neutral Rh(III)–methyl species. Theoretical calcula-
tions suggest that the activation enthalpy for methyl
migration in [Rh(CO)2I2Me] is lower than that for
[Rh(CO)2I3Me]ꢀ by ca. 12 kJ molꢀ1 [40].
Rh(III) methyl and acetyl complexes are significant
intermediates in Rh-catalysed methanol carbonylation.
4. Experimental
4.1. Materials
All solvents used for synthetic or kinetic experiments
were distilled and degassed prior to use the following
literature procedures. Synthetic procedures were carried
out utilising standard Schlenk techniques. Nitrogen and
carbon monoxide were dried through a short (20 · 3 cm
ꢀ
diameter) column of molecular sieves (4 A) which was
regularly regenerated. Carbon monoxide was also pas-
sed through a short column of activated charcoal to
remove any iron pentacarbonyl impurity [47]. The
compounds [Rh(CO)2Cl]2 [48], [Rh(CO)2I]2 [36] and
Ph4As[Ir(CO)2I2] [17] were synthesised according to the
literature procedures. Methyl iodide (Aldrich) was dis-
tilled over calcium hydride and stored in foil-wrapped
Schlenk tubes under nitrogen and over mercury to pre-
vent formation of I2.
Our results also demonstrate that in polar, coordi-
2ꢀ
nating solvents the anionic dimer, [Rh(CO)(COMe)I3]2
(5) loses Iꢀ to give [Rh(CO)(sol)2(COMe)I2] (Scheme 4).
Under catalytic conditions (aqueous acetic acid/high
temperature) the extent of iodide dissociation from
Rh(III) acetyl species could also be significant. The re-
sulting neutral species may undergo more facile reductive
elimination, as in other systems where a halide loss me-
chansim has been demonstrated [41–46]. Indeed, de-
composition of [Rh(CO)2(COMe)I2]n to Rh(I) species is
fast compared with [Rh(CO)2(COMe)I3]ꢀ.
4.2. Instrumentation
FT-IR spectra (2 cmꢀ1 resolution) were measured
using a Perkin–Elmer 1600 spectrometer or a Mattson
Genesis Series spectrometer, controlled by WINFIRST
3. Conclusions
1
As expected, the neutral Rh(I) dimer, [Rh(CO)2I] 1
displays lower reactivity toward MeI than the anionic
carbonylation catalyst, [Rh(CO)2I2]ꢀ. Kinetic measure-
ments on related Ir(I) complexes show that addition of
MeI to [Ir(CO)2(NCMe)I] is two orders of magnitude
slower than to [Ir(CO)2I2]ꢀ. In nitrile solvents, reactions
between 1 and MeI lead to stable Rh(III) acetyl prod-
ucts, isolated as the iodide bridged dimers [Rh(CO)
(NCR)(COMe)I2]2 (3a–d). X-ray structures for the
software. H NMR spectra were obtained using a Bru-
ker AM250 or WP80SY spectrometer using the solvent
or TMS as internal reference. 31P NMR spectra were
obtained using a Bruker WP80SY spectrometer using
orthophosphoric acid as external reference. Elemental
analyses were performed by the University of Sheffield
microanalysis service.
4.3. Synthesis of [Rh(CO)(CO Me)I2]n (2)
t
MeCN (3a) and BuCN (3b) derivatives reveal signifi-
cant asymmetry in the Rh–(l-I)–Rh bridges, ascribed to
the strong trans influence of acetyl. The nitrile ligands of
3a are easily displaced, for example by pyridine, and
reaction with diphosphine ligands leads to [Rh(Ph2P
(CH2)nPPh2)(COMe)I2] (7a–c). In coordinating sol-
vents, the chemistry is dominated by mononuclear
complexes. Thus, 1 and 3a are cleaved in MeCN to give
[Rh(CO)2(NCMe)I] and [Rh(CO)(NCMe)2(COMe)I2],
respectively. The reaction of [Rh(CO)2(NCMe)I] with
MeI displays kinetics which are zeroth-order in the re-
actant complex, and IR spectroscopy shows that
[Rh(CO)2I2]ꢀ participates as a reactive intermediate. It
is proposed that small amounts of iodide anion (gener-
ated from MeCN and MeI) act as a catalyst, with Rh(I)
binding Iꢀ more strongly than Rh(III). Interestingly,
neutral [Rh(CO)(sol)2(COMe)I2] species are generated
[Rh(CO)2I]2 (1) (0.1 g, 0.175 mmol) was dissolved in
MeI (15 cm3) under N2 and stirred at room temperature.
The reaction was monitored periodically by IR spec-
troscopy. When the absorption of 1 at 2023 cmꢀ1 had
disappeared, the solution was filtered under nitrogen and
the MeI removed in vacuo. The resulting dark brown
solid (2) was examined by IR spectroscopy and micro-
analysis; yield: 0.06 g (40%). Anal. Calc. for C3H3I2O2Rh:
C, 8.4; H, 0.7; I, 59.3. Found: C, 7.6; H, 0.7; I, 61.5%. IR
(CH2Cl2), m(CO)/cmꢀ1: 2084, 1739, 1727.
4.4. Synthesis of [Rh(CO)(NCMe) (COMe)I2]2 (3a)
[Rh(CO)2I]2 (1) (0.46 g, 0.80 mmol) was dissolved in
MeCN/MeI (50 cm3, 2:1 v/v) and stirred at room tem-
perature under N2 (6 h), whereupon IR spectroscopy
indicated the disappearance of 1. The volatile portion of
the solvent was removed in vacuo and the resulting or-
2ꢀ
from the anion, [Rh(CO)(COMe)I3]2 in coordinating
solvents. The results raise the possibility that neutral

A. Haynes et al. / Inorganica Chimica Acta 357 (2004) 3027–3037
3035
ange precipitate redissolved in the remaining liquor by
warming. Cooling the solution to 0 ꢁC gave a crop of
dark red crystals which were collected by filtration.
After further reduction in vacuo, the filtrate yielded a
second crop of crystals as described above. The product
was dried in vacuo; crystals were of sufficient quality for
an X-ray crystallographic study; yield: 0.66 g (87%).
Anal. Calc. for C5H6I2NO2Rh: C, 12.8; H, 1.3; N, 3.0; I,
54.1. Found: C, 12.9; H, 1.3; N, 2.5; I, 54.3%. IR (Nu-
jol), m(CO)/cmꢀ1: 2086, 1709; m(CN)/cmꢀ1: 2327w,
temperature under N2 (48 h). Removal of the solvent in
vacuo gave an orange powder which was recrystallised
from a minimum volume of warm CH2Cl2 to give orange
microcrystals of 3d. Concentration of the solution and
recrystallisation at )36 ꢁC gave a further crop of crystals;
yield: 0.665 g (77%). Anal. Calc. for C7H8I2NO2Rh: C,
17.0; H, 1.6; N, 2.8; I, 51.3. Found: C, 17.0; H, 1.6; N,
2.6; I, 51.1%. IR (Nujol), m(CO)/cmꢀ1: 2082, 1726;
m(CN)/cmꢀ1: 2312w. 1H NMR (250 MHz, CDCl3): d 2.87
(3H, s, COCH3), 3.65 (2H, m), 5.52 (1H, m), 5.80 (2H, m)
(NCCH2CHCH2).
1
2299w. H NMR (250 MHz, CDCl3): d 2.88 (3H, br s,
COCH3), 2.62 (3H, br s, NCCH3).
Alternative method: Complex 3a was also prepared
by addition of MeCN (2 cm3) to a stirred solution of 2
(0.2 g, 0.35 mmol) in CH2Cl2 (10 cm3). After removal of
the solvent in vacuo, the product was recrystallised from
a minimum volume of warm MeCN. Analytical and
spectroscopic data confirmed the product to be identical
to a sample of 3a prepared by the method described
above; yield: 0.19 g (87%).
4.8. Synthesis of [Rh(CO)(COMe) I2(NC5H5)]2 (3e)
[Rh(CO)2I]2 (1) (0.35 g, 0.61 mmol) was dissolved in
nitromethane/MeI (60 cm3, 2:1 v/v) and stirred at
room temperature under N2 (6 h). The volatile portion
of the solvent was removed and pyridine (0.1 cm3, 1.24
mmol) was added to the remainder. The reaction
mixture was stirred for a further 20 min and the re-
sulting orange solid collected by filtration. Further
reduction of the filtrate in vacuo gave a second crop of
orange powder. The combined product was recrystal-
lised from CH2Cl2 at 0 ꢁC to give two crops of small
red-brown crystals; yield: 0.38 g (61%). Anal. Calc. for
C8H8I2NO2Rh: C, 19.0; H, 1.6; N, 2.8; I, 50.1. Found:
C, 19.4; H, 1.5; N, 2.8; I, 49.7%. IR (Nujol), m(CO)/
4.5. Synthesis of [Rh(CO)(COMe) I2(NCtBu)]2 (3b)
[Rh(CO)2I]2 (1) (0.235 g, 0.41 mmol) was dissolved in
tBuCN/MeCN (10 cm3, 9:1 v/v) and stirred at room
temperature under N2 (8 h). Removal of the volatile
portion of the solvent in vacuo gave an orange powder
which was collected by filtration. The product was wa-
shed with diethyl ether, then dissolved in a minimum
volume of warm CH2Cl2 and placed in the refrigerator at
0 ꢁC, giving a crop of dark red crystals; yield: 0.385 g
(92%). Anal. Calc. for C8H12I2NO2Rh: C, 18.8; H, 2.4;
N, 2.7; I, 49.7. Found: C, 18.8; H, 2.3; N, 2.5; I, 49.5%. IR
(Nujol), m(CO)/cmꢀ1: 2085, 1729; m(CN)/cmꢀ1: 2286w.
1H NMR (250 MHz, CD3OD): d 2.75 (COCH3).
1
cmꢀ1: 2085, 1718. H NMR (250 MHz, CDCl3): d 2.95
(3H, s, COCH3), 7.31 (2H, m), 7.85 (1H, m), 9.52 (2H,
m) (NC5H5).
4.9. Synthesis of [Rh2(CO)2(COMe)2 I4(NC(CH2)4-
CN)] (4)
Adiponitrile (0.02 g, 0.18 mmol) was added to a so-
lution of [Rh(CO)2I]2 (1) (0.1 g, 0.175 mmol) in
CH2Cl2/MeI (6 cm3, 2:1 v/v). under N2. The stirred
solution was heated to reflux for 3 h and on cooling a
dark red powder was formed, which was collected by
filtration. Reduction of the filtrate in vacuo gave a
further crop of red powder. The product was sparingly
soluble in most solvents but could be recrystallised from
warm nitromethane or nitrobenzene. The resulting dark
red crystals were collected by filtration; yield: 0.1 g
(59%). Anal. Calc. for C6H7I2NO2Rh: C, 15.0; H, 1.5;
N, 2.9; I, 52.7. Found: C, 15.4; H, 1.7; N, 2.7; I, 53.3%.
4.6. Synthesis of [Rh(CO)(COMe) I2(NCCHCH2)]2(3c)
[Rh(CO)2I]2 (1) (0.5 g, 0.87 mmol) was dissolved in ac-
rylonitrile/MeI (10 cm3, 5:1 v/v) and stirred at room tem-
perature under N2 (48 h). Removal of the solvent in vacuo
gave an orange powder which was recrystallised from a
minimum volume of warm CH2Cl2 to give orange micro-
crystals of 3c. Concentration of the solution and recrys-
tallisation at )36 ꢁC gave a further crop of crystals; yield:
0.702 g (83%). Anal. Calc. for C6H6I2NO2Rh: C, 15.0; H,
1.3; N, 2.9; I, 52.8. Found: C, 15.0; H, 1.3; N, 2.6; I, 52.5%.
IR (Nujol), m(CO)/cmꢀ1: 2077, 1720; m(CN)/cmꢀ1: 2276w.
1H NMR (250 MHz, CDCl3): d 2.91 (3H, s, COCH3Þ, 6.12
(1H, m), 6.52 (1H, m), 6.70 (1H, m) (NCCHCH2).
IR (Nujol), m(CO)/cmꢀ1: 2079, 1715; m(CN)/cmꢀ1
:
2300w, 2249vw. 1H NMR (250 MHz, C6D5NO2): d 3.05
(3H, COCH3).
4.7. Synthesis of [Rh(CO)(COMe) I2(NCCH2-
CHCH2)]2 (3d)
4.10. Synthesis of [Rh(CO)(COMe) I2(NC5H5)2] (6e)
[Rh(CO)2I]2 (1) (0.35 g, 0.61 mmol) was dissolved in
nitromethane/MeI (60 cm3, 2:1 v/v) and stirred at room
temperature under N2 (6 h). The volatile portion of the
[Rh(CO)2I]2 (1) (0.5 g, 0.87 mmol) was dissolved in
allyl cyanide/MeI (10 cm3, 5:1 v/v) and stirred at room

3036
A. Haynes et al. / Inorganica Chimica Acta 357 (2004) 3027–3037
solvent was removed and pyridine (0.2 cm3, 2.48 mmol)
was added to the remainder. The reaction mixture was
stirred for a further 5 min and the volatile portion of the
solvent removed in vacuo. The solution was filtered (Hi-
flo), concentrated, and cooled to )30 ꢁC to give a crop of
dark red crystals which were collected by filtration; yield:
0.58 g (81%). Anal. Calc. for C13H13I2N2O2Rh: C, 26.7;
H, 2.2; N, 4.8; I, 43.3. Found: C, 26.8; H, 2.4; N, 4.3; I,
calculated positions with isotropic thermal parameters
related to those of the supporting atom, and refined in
riding mode.
Crystal data for [Rh(CO)(NCMe)(COMe)(I)(l-I)]2
(3a): C10H12I4N2O4Rh2; M ¼ 937:64; the complex crys-
tallised from a solution in warm dichloromethane as
dark red elongated blocks with crystal dimensions of
ꢀ
0.45 · 0.25 · 0.20 mm. Monoclinic, a ¼ 7:470ð4Þ A,
1
43.4%. IR (Nujol), m(CO)/cmꢀ1: 2079, 1716. H NMR
b ¼ 11:242ð7Þ A, c ¼ 13:444ð9Þ A, b ¼ 104:13ð5Þꢁ, U ¼
ꢀ
ꢀ
3
1094:8ð12Þ A ; Dcalc ¼ 2:844 g cmꢀ3, Z ¼ 2. Space group
ꢀ
(250 MHz, CDCl3): d 3.10 (3H, s, COCH ), 7.26 (4H,
3
m), 7.77 (2H, m), 8.83 (2H, m), 9.21 (2H, m) (2· NC5H5).
P21=n (a non-standard setting of P21=c, C25h, No. 14), Mo
ꢀ1
ꢀ
Ka radiation (k ¼ 0:71069 A), l(Mo Ka) ¼ 70.81 cm
,
F ð000Þ ¼ 839:95. Refinement converged at a final
R ¼ 0:0544 (100 parameters, maximum d=r 0.001) with
allowance for the thermal anisotropy of all non-hydro-
gen atoms. A final difference electron density synthesis
showed minimum and maximum values of )1.02 and
4.11. Synthesis of [Rh(dppe)(COMe) I2] (7a) [21,22]
The diphosphine dppe (0.1 mmol) was dissolved, to-
gether with complex 3a (0.085 g, 0.2 mmol) in CH2Cl2
(20 cm3). The solution was warmed to a gentle reflux
and after 1 h had turned from red to orange-yellow in
colour. Diethyl ether was added, and after 24 h at
)10 ꢁC, the product was recovered by filtration as a
yellow-brown precipitate; yield: 85%. Anal. Calc. for
C28H27I2OP2Rh: C, 42.1; H, 3.4; I, 31.8. Found: C, 42.1;
H, 3.4; I, 31.8%. IR (CH2Cl2), m(CO)/cmꢀ1: 1712. 31P
3
ꢀ
+1.10 e A . Complex scattering factors were taken from
[49] and from the program package SHELXTL [50] as
implemented on the Data General Nova 3 computer. A
2
weighting scheme with wꢀ1 ¼ ½r2ðF Þ þ 0:00143ðF Þ ꢂ was
used in the latter stages of the refinement. CCDC 228895.
Crystal data for [Rh(CO)(NCtBu)(COMe)(I)(l-I)]2
(3b): C16H24I4N2O4Rh2; M ¼ 1021:80; the complex
crystallised from a solution in warm dichloromethane
1
NMR (CD2Cl2): d 70.1 (d, JRh–P 139 Hz).
t
containing a drop of butyl cyanide as red needles with
crystal dimensions of 0.425 · 0.20 · 0.20 mm. Triclinic,
4.12. Synthesis of [Rh(dppp)(COMe)I2] (7b) [21,23,24]
ꢀ
ꢀ
ꢀ
a ¼ 7:223ð14Þ A, b ¼ 10:449ð37Þ A, c ¼ 11:165ð38Þ A,
The method described above for 7a was used; yield:
78%. Anal. Calc. for C29H29I2OP2Rh: C, 42.9; H, 3.6; I,
31.3. Found: C, 43.0; H, 3.6; I, 31.3%. IR (CH2Cl2),
m(CO)/cmꢀ1: 1701. 31P NMR (CD2Cl2): d 17.9 (d, 1JRh–P
132 Hz).
a ¼ 66:93ð26Þꢁ, b ¼ 75:68ð22Þꢁ, c ¼ 83:43ð23Þꢁ, U ¼
751ð4Þ A3; Dcalc ¼ 2:259 g cmꢀ3, Z ¼ 1. Space group P1
ꢀ
ꢁ
1
ꢀ
(Ci , No. 2) Mo Ka radiation (k ¼ 0:71069 A), l(Mo
Ka) ¼ 51.71 cmꢀ1, F ð000Þ ¼ 467:97. Refinement con-
verged at a final R ¼ 0:0698 (Rw 0.0669, 127 parameters;
mean and maximum d=r both 0.000) with allowance for
the thermal anisotropy of all non-hydrogen atoms. A
final difference electron density synthesis showed mini-
4.13. Synthesis of [Rh(dppb)(COMe)I2] (7b)
3
The method described above for 7a was used; yield:
63%. Anal. Calc. for C30H31I2OP2Rh: C, 42.9; H, 3.6; I,
31.3. Found: C, 43.3; H, 3.6; I, 30.9%. IR (CH2Cl2),
m(CO)/cmꢀ1: 1702. 31P NMR (CD2Cl2): d 72.1 (d, 1JRh–P
136 Hz).
ꢀ
mum and maximum values of )2.23 and +0.97 e A .
Complex scattering factors were taken from [49] and
from the program package SHELXTL [50] as imple-
mented on the Data General Nova 3 computer. A
2
weighting scheme with wꢀ1 ¼ ½r2ðF Þ þ 0:00071ðF Þ ꢂ was
used in the latter stages of the refinement. CCDC 228894.
The atomic positional parameters, anisotropic ther-
mal vibration parameters, hydrogen atom positional
parameters and the observed structure amplitudes and
calculated structure factors (with estimated standard
deviations) for both complexes are supplied as Supple-
mentary information.
4.14. X-ray structure determinations
Three dimensional, room temperature X-ray data
were collected in the range 3:5ꢁ < 2h < 50ꢁ on a Nicolet
R3 4-circle diffractometer by the omega scan method.
The independent reflections (1610 of 2167 measured for
3a; 1622 of 2672 measured for 3b) for which
jF j=rðjF jÞ > 3:0 were corrected for Lorentz and polari-
sation effects and for absorption by analysis of 8 azi-
muthal scans (minimum and maximum transmission
coefficients 0.028 and 0.051 for 3a and 0.109 and 0.145
for 3b). The structures were solved by Patterson and
Fourier techniques and refined by blocked cascade least
squares methods. Hydrogen atoms were included in
4.15. Kinetic experiments
Pseudo-first-order conditions were employed, with at
least a 100-fold excess of MeI, relative to the metal
complex. The required amount of freshly distilled MeI
was placed in a 10 cm3 graduated flask, which was then
made up to the mark with the solvent of choice (usually

A. Haynes et al. / Inorganica Chimica Acta 357 (2004) 3027–3037
3037
[16] T. Ghaffar, H. Adams, P.M. Maitlis, G.J. Sunley, M.J. Baker, A.
Haynes, Chem. Commun. (1998) 1023.
MeCN). A portion of this solution was used to record a
background spectrum. A second portion (5 cm3) was
added to the reactant complex 1 (typically 15 mg) in a
graduated flask to give a reaction solution containing ca.
10 mM [Rh]. Alternatively, an appropriate quantity of
MeI was added to a solution of [Ir(CO)2(NCMe)I],
generated by the reaction of Ph4As[Ir(CO)2I2] with a
slight excess of InI3 in MeCN. A portion of the reaction
solution was quickly transferred to the IR cell and the
kinetic experiment was started. The IR cell (0.5 mm
pathlength, CaF2 windows) was maintained at constant
temperature throughout the kinetic run by a thermo-
stated jacket. Spectra were recorded in the m(CO) region
(2200–1600 cmꢀ1) and saved at regular time intervals
under computer control. After the kinetic run, absor-
bance versus time data for the appropriate m(CO) fre-
quencies were extracted and analysed off-line using
Kaleidagraph software. Observed rate constants were
reproducible ( 5%) and reported values are mean values
of two or more runs.
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Acknowledgements
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Haynes, Organometallics 22 (2003) 1047.
This work was supported by BP Chemicals Ltd.
(studentships to I.A.S. and J.M.P.; PDRF to S.H.),
EPSRC and the University of Sheffield.
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