Reactivity of rhodium(I) iminophosphine carbonyl complexes with methyl Iodide

Article abstract of DOI:10.1021/om070019b

A series of Rh(I) iodocarbonyl complexes, [Rh(CO)I(PN^Ar)] (1a-g), has been prepared by the reactions of [Rh(CO)₂I]₂ with iminophosphine ligands, Ph₂PC₆H₄-2-CH=NAr (PN^Ar; Ar = C₆H₅ (a), 2,6-Me₂C ₆H₃ (b), 2,6-ⁱPr₂C₆H ₃ (c), 2-EtC₆H₄ (d), 2-MeOC₆H ₄ (e), 4-MeOC₆H₄ (f), 3,5-(CF₃) ₂C₆H₃ (g)). For ¹³CO-labeled 1b, a relatively small ²J_cp coupling (12 Hz) showed the CO ligand to be cis to the P-donor atom of the iminophosphine. Complexes 1a-g react with methyl iodide to give Rh(III) methyl or acetyl products. For complexes 1a,d,f,g the reactions result in equilibria between the methyl complexes [Rh(CO)(PN^Ar)I₂-Me] (2) and acetyl complexes [Rh(PN ^Ar)(COMe)I₂] (3), whereas the reactions of 1b,c,e gave only the acetyl products [Rh(COMe)I₂(PN^Ar)] (3b,c,e). Migratory CO insertion is promoted for systems in which the N-aryl group of the iminophosphine is bulky or contains an o-methoxy substituent. An X-ray structure of 3e reveals an interaction between the Rh center and the o-methoxy group (Rh-O = 2.54 A). Second-order rate constants for Mel oxidative addition to 1a-g vary considerably, depending on the steric and electronic properties of the iminophosphine N-aryl substituents. The most reactive complex is 1e, and a mechanism is proposed in which the o-methoxy group interacts with the Rh center to promote both oxidative addition and CO insertion.

Full text of DOI:10.1021/om070019b

1960
Organometallics 2007, 26, 1960-1965
Reactivity of Rhodium(I) Iminophosphine Carbonyl Complexes with
Methyl Iodide
Jonathan Best,† John M. Wilson,† Harry Adams,† Luca Gonsalvi,*,‡
Maurizio Peruzzini,‡ and Anthony Haynes*,†
Department of Chemistry, UniVersity of Sheffield, Sheffield, S3 7HF, U.K., and Consiglio Nazionale delle
Ricerche, Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), Via Madonna del Piano 10,
50019 Sesto Fiorentino (Firenze), Italy
ReceiVed January 8, 2007
A series of Rh(I) iodocarbonyl complexes, [Rh(CO)I(PNAr)] (1a-g), has been prepared by the reactions
of [Rh(CO)2I]2 with iminophosphine ligands, Ph2PC6H4-2-CHdNAr (PNAr; Ar ) C6H5 (a), 2,6-Me2C6H3
(b), 2,6-iPr2C6H3 (c), 2-EtC6H4 (d), 2-MeOC6H4 (e), 4-MeOC6H4 (f), 3,5-(CF3)2C6H3 (g)). For 13CO-
labeled 1b, a relatively small 2JCP coupling (12 Hz) showed the CO ligand to be cis to the P-donor atom
of the iminophosphine. Complexes 1a-g react with methyl iodide to give Rh(III) methyl or acetyl products.
For complexes 1a,d,f,g the reactions result in equilibria between the methyl complexes [Rh(CO)(PNAr)I2-
Me] (2) and acetyl complexes [Rh(PNAr)(COMe)I2] (3), whereas the reactions of 1b,c,e gave only the
acetyl products [Rh(COMe)I2(PNAr)] (3b,c,e). Migratory CO insertion is promoted for systems in which
the N-aryl group of the iminophosphine is bulky or contains an o-methoxy substituent. An X-ray structure
of 3e reveals an interaction between the Rh center and the o-methoxy group (Rh-O ) 2.54 Å). Second-
order rate constants for MeI oxidative addition to 1a-g vary considerably, depending on the steric and
electronic properties of the iminophosphine N-aryl substituents. The most reactive complex is 1e, and a
mechanism is proposed in which the o-methoxy group interacts with the Rh center to promote both
oxidative addition and CO insertion.
Scheme 1. General Mechanism for Reaction of Chelate
Rh(I) Iodo Carbonyl Complexes with MeI
Introduction
Bidentate chelating ligands containing phosphorus and/or
nitrogen donor atoms are frequently used in organometallic
chemistry and homogeneous catalysis. The steric and electronic
properties of such ligands can be tuned by altering either the
ligand backbone or the P- or N-bound substituents. Such
modifications can be used to influence the rates of key steps in
homogeneous catalytic cycles. Recently we have studied the
reactivity of a range of rhodium iodo carbonyl complexes of
the type [Rh(CO)(L-L)I], where L-L is a bidentate chelating
ligand (e.g., dppe (Ph2P(CH2)2PPh2), dppms (Ph2PCH2P(S)Ph2),
or an R-diimine).1-3 Of particular interest have been the
oxidative addition reactions of these complexes with methyl
iodide, which model the rate-determining step in the catalytic
cycle for the rhodium/iodide-catalyzed carbonylation of metha-
nol. Steric effects have been found to play a major role in
determining the reactivity in such systems. Bulky ligand
substituents tend to inhibit oxidative addition but can encourage
subsequent migratory CO insertion in the Rh(III) methyl product.
Thus, the bidentate ligand has a large influence on the kinetics
and thermodynamics of both steps in Scheme 1, and this reaction
sequence serves as a useful probe of ligand effects in general.
Here, we extend these studies to rhodium complexes of
iminophosphine ligands which contain both P and N donor
functions. Iminophosphine complexes have been employed for
a wide variety of homogeneous catalytic processes: for example,
C-C coupling reactions (Pd4-13), alkynylstannylation (Pd14-16),
(4) Reddy, K. R.; Surekha, K.; Lee, G.-H.; Peng, S.-M.; Liu, S.-T.
Organometallics 2000, 19, 2637.
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* To whom correspondence should be addressed. E-mail: a.haynes@
sheffield.ac.uk (A.H.).
† University of Sheffield.
‡ Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti
Organometallici.
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10.1021/om070019b CCC: $37.00 © 2007 American Chemical Society
Publication on Web 03/08/2007

Rhodium(I) Iminophosphine Carbonyl Complexes
Organometallics, Vol. 26, No. 8, 2007 1961
alkoxycarbonylation of alkynes (Pd17), oligomerization and
polymerization of ethylene (Ni, Pd18-20), CO/alkene copolym-
erization (Pd21,22), amination (Pd23,24), transfer hydrogenation
(Ru25,26), and hydroboration (Rh27). Rhodium and iridium
complexes containing iminophosphines have also been applied
in studies of oxygen uptake.28,29
Scheme 2. Synthesis of Rh(I) Complexes 1a-g
We have utilized iminophosphine ligands of the type Ph2-
PC6H4-2-CHdNAr (PNAr), which are easily accessible by
condensation of (diphenylphosphino)benzaldehyde with primary
amines. A number of Rh(I) iodo carbonyl complexes, [Rh(CO)-
(PNAr)I], have been synthesized with N-aryl substituents having
different steric and electronic properties. Kinetic studies on the
reactions of these complexes with methyl iodide reveal an
unexpected steric effect on the rate of oxidative addition. It is
also demonstrated that a neighboring group effect of an
o-methoxy substituent on the iminophosphine N-aryl group can
promote both oxidative addition and migratory insertion.
Results and Discussion
Scheme 3. Reactivity of Complexes 1a-g with MeI
Synthesis and Characterization of Rh(I) Complexes. The
iminophosphine ligands PNAr were prepared by condensation
of (diphenylphosphino)benzaldehyde and the appropriate
primary amine. Preparative methods have been reported
previously15,25,26 for all the ligands except those with 2-C6H4Et
and 3,5-C6H3(CF3)2 substituents. Complexation to Rh(I) was
achieved by reaction with the dimeric precursor [Rh(CO)2I]2
(Scheme 2).
The products [Rh(CO)(PNAr)I] (1a-g) were isolated as
strongly colored crystalline solids and characterized by a
combination of spectroscopic and analytical methods. The ν-
(CO) absorptions occur between 1995 and 1998 cm-1 for all of
the complexes except 1g (2002 cm-1), which has electron-
withdrawing CF3 substituents on the iminophosphine N-aryl
group. In comparison to related Rh(I) iodo carbonyls, [Rh(CO)-
(dppe)I] (2011 cm-1) and [Rh(CO)(ArNdC(Me)C(Me)dNAr)I]
(1993-1994 cm-1), these ν(CO) values lie slightly closer to
those for the diimine systems.
1b 13C labeling was employed in order to determine whether
the CO ligand is cis or trans to the P donor of the iminophos-
phine ligand. Brief bubbling of 13CO through a CH2Cl2 solution
of 1b resulted in a shift of the ν(CO) band in the IR spectrum
from 1998 to 1953 cm-1, consistent with exchange of the CO
ligand. The 31P{1H} and 13C{1H} NMR spectra of 13CO-labeled
1
1b each displayed a doublet of doublets (δ(31P) 41.3, JRhP
)
1
2
169 Hz; δ(13C) 190.8, JRhC ) 70 Hz, JPC ) 12 Hz). The
relatively small value of 2JPC indicates that the CO ligand is cis
to phosphorus,30,31 since a larger coupling (>100 Hz) would
be expected for a carbonyl trans to phosphorus.31 The methine
proton of the imino group in 1a-g displayed a signal close to
δ 8.0, which appeared as either a doublet or a broadened singlet
For each complex the 31P{1H} NMR spectrum exhibits a
doublet between δ 40 and 48 with 1JRhP ) ca. 170 Hz, indicating
the presence of a single isomer of each complex. In the case of
4
1
due to JPH coupling. In the H NMR spectrum of 1c, two
doublets at δ 0.69 and 1.29 indicate that the methyls of the
isopropyl substituents are inequivalent, due to restricted rotation
about the N-aryl bond.
(16) Nakao, Y.; Hirata, Y.; Ishihara, S.; Oda, S.; Yukawa, T.; Shirakawa,
E.; Hiyama, T. J. Am. Chem. Soc. 2004, 126, 15650.
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R.; Crociani, B. J. Mol. Catal. A: Chem. 2001, 170, 51.
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L. Chem. Commun. 1998, 223.
(19) Crossetti, G. L.; Dias, M. L.; Queiroz, B. T.; Silva, L. P.; Ziglio,
C. M.; Bomfim, J. A. S.; Filgueiras, C. A. L. Appl. Organomet. Chem.
2004, 18, 331.
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784.
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Chen, J.-T.; Liu, S.-T. Dalton Trans. 2002, 1776.
(22) Pascu, S. I.; Coleman, K. S.; Cowley, A. R.; Green, M. L. H.; Ree,
N. H. J. Organomet. Chem. 2005, 690, 1645.
(23) Crociani, B.; Antonaroli, S.; Bandoli, G.; Canovese, L.; Visentin,
F.; Uguagliati, P. Organometallics 1999, 18, 1137.
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P. Inorg. Chim. Acta 2001, 315, 172.
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tallics 2001, 20, 4369.
(26) Crochet, P.; Gimeno, J.; Borge, J.; Garc´ıa-Granda, S. New J. Chem.
2003, 27, 414.
Reactions of Rh(I) Complexes with MeI. The reactions of
complexes 1a-g with MeI were monitored by FTIR spectros-
copy to probe the evolution of ν(CO) absorptions of the reactants
and products. The outcome was found to be dependent on the
electronic and steric properties of the iminophosphine ligand.
For complexes 1a,d,f,g, the decay of the reactant ν(CO) band
was accompanied by simultaneous growth of product absorp-
tions at both higher frequency (2070-2073 cm-1) and lower
frequency (1701-1726 cm-1). These bands are assigned
respectively to the Rh(III) methyl species [Rh(CO)(PNAr)I2Me]
(2), resulting from MeI oxidative addition, and Rh(III) acetyl
complexes [Rh(PNAr)(COMe)I2] (3), formed by subsequent
migratory CO insertion (Scheme 3). The methyl and acetyl
species exist in equilibrium, and no single product could be
isolated. On the basis of the relative intensities of the IR bands,
the equilibrium constant for migratory CO insertion is larger
(27) McIsaac, D. I.; Geier, S. J.; Vogels, C. M.; Decken, A.; Westcott,
S. A. Inorg. Chim. Acta 2006, 359, 2771.
(28) Ghilardi, C. A.; Midollini, S.; Moneti, S.; Orlandini, A.; Scapacci,
G. J. Chem. Soc., Dalton Trans. 1992, 3371.
(29) Barbaro, P.; Bianchini, C.; Laschi, F.; Midollini, S.; Moneti, S.;
Scapacci, G.; Zanello, P. Inorg. Chem. 1994, 33, 1622.
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Commun. 1997, 1835.
(31) Krassowski, D. W.; Nelson, J. H.; Brower, K. R.; Hauenstein, D.
Inorg. Chem. 1988, 27, 4294.

1962 Organometallics, Vol. 26, No. 8, 2007
Best et al.
Table 1. Selected Bond Distances (Å) and Angles (deg) for
Complex 3e
Rh-I(1)
Rh-I(2)
Rh-N
2.7502(10) Rh-C(27)
2.6569(11) C(27)-O(1)
2.106(8)
2.255(3)
2.539(7)
2.004(11)
1.193(12)
1.504(13)
1.285(12)
C(27)-C(28)
N-C(19)
Rh-P
Rh-O(2)
C(27)-Rh-N
C(27)-Rh-P
C(27)-Rh-I(1)
C(27)-Rh-I(2)
96.0(4)
89.8(3)
N-Rh-O(2)
68.7(3)
163.1(2)
177.34(8)
118.7(8)
119.3(8)
N-Rh-I(2)
87.6|(3)
I(1)-Rh-P
100.6(3)
Rh-C(27)-C(28)
Rh-C(27)-O(1)
C(27)-Rh-O(2) 161.2(4)
I(2)-Rh-I(1)
N-Rh-P
91.05(3)
86.9(2)
O(1)-C(27)-C(28) 121.8(10)
(diphosphine)(COMe)I2] complexes2,32-34 than in [Rh(ArNd
C(Me)C(Me)dNAr)(COMe)I2] (Ar ) 2,6-iPr2C6H3).3 The co-
ordination site trans to the acetyl ligand in 3e is occupied by
the o-methoxy group of the iminophosphine. The Rh-O(2)
distance (2.539(7) Å) is quite long compared to values of 2.33-
2.35 Å recently found for Rh(I) complexes of S,S-dipamp (Ph-
(2-C6H4-OMe)PC2H4P(2-C6H4-OMe)Ph),36 suggesting a rela-
tively weak Rh-O interaction in 3e. Substantial distortions from
regular octahedral geometry are apparent in the angles between
trans coordinated atoms (e.g., N-Rh-I(2) and C(27)-Rh-O(2)).
The acetyl ligand adopts a conformation that minimizes steric
interactions with the iminophosphine ligand. This directs the
acetyl methyl toward I(2) (C(28)-C(27)-Rh-I(2) torsion angle
ca. -28°), and the resulting steric clash displaces I(2) out of
the approximate plane defined by Rh, I(1), N, and P. This
distortion is also evident from the C(27)-Rh-I(2) angle of
100.6(3)°. The six-membered iminophosphine chelate ring
adopts a distorted-envelope conformation with the P atom almost
coplanar with the three carbon atoms of the ligand backbone
(C(19)-C(6)-C(1)-P torsion angle ca. 1°). The nitrogen and
rhodium atoms deviate substantially from this CCCP plane, with
torsion angles C(1)-C(6)-C(19)-N ) 27.5° and C(6)-C(1)-
P-Rh ) -38.8°. This conformation is common for chelate rings
of this type,37 and the P-Rh-N bite angle of 86.9(2)° is
comparable to values reported for other Rh iminophosphine
complexes.27,28 The Rh-O interaction is facilitated by the
conformation adopted by the o-anisyl group, the ipso carbon
of which is displaced ca. 0.87 Å from the approximate plane
defined by Rh, I(1), N, and P. The resulting five-membered
chelate ring has an envelope conformation with Rh at the apex
and a N-Rh-O(2) bite angle of 68.7(3)°).
Oxidative Addition Kinetics. Kinetic data for the reactions
of 1a-g with MeI in CH2Cl2 were obtained by monitoring the
decay of the ν(CO) absorption of the Rh(I) reactant. Pseudo-
first-order conditions were maintained by keeping the [MeI] in
large excess compared with [Rh]. A typical series of spectra is
illustrated in Figure 2. Plots of absorbance versus time were
well fitted by exponential curves, showing that the reactions
are first order in Rh(I) complex. Values of the observed pseudo-
first-order rate constants (kobs) are listed in the Supporting
Information. Plots of kobs versus [MeI] were linear, indicating
that the reactions are first order in MeI and therefore second
order overall. Second-order rate constants, obtained from the
slopes of these plots, are given in Table 2, along with activation
parameters derived from Eyring plots of variable-temperature
data measured over the range 15-35 °C. The activation
Figure 1. ORTEP diagram showing the molecular structure of 3e.
Thermal ellipsoids are shown at the 50% probability level.
Hydrogen atoms and the pentane solvent molecule are omitted for
clarity.
for 2d than for 2a,f,g, suggesting that the extra steric bulk of
an o-ethyl substituent encourages methyl migration.
In contrast, the reactions of complexes 1b,c with MeI give
only the acetyl species [Rh(PNAr)(COMe)I2] (3b,c), indicated
by ν(CO) absorptions at ca. 1716 cm-1. The presumed
intermediate methyl complexes (2b,c) were not detected during
these reactions. Thus, the presence of two ortho alkyl substit-
uents on the iminophosphine N-aryl group promotes migratory
CO insertion, as found previously for Rh R-diimine complexes.3
1
The H NMR spectrum of 3c displays two septets and four
doublets arising from the two inequivalent isopropyl groups,
consistent with square-pyramidal coordination geometry
with an apical acetyl ligand, as found for related Rh(III)
complexes.2,3,32-35
A stable Rh(III) acetyl complex, 3e, was formed by the
reaction of MeI with the o-anisyl derivative 1e. Again, the
presumed methyl intermediate 2e was not detected, indicating
that methyl migration is rapid. It is notable that although the
iminophosphine ligands in 1d,e have very similar steric bulk,
migratory insertion is more favored by the o-anisyl-substituted
ligand. It was suspected that a thermodynamic driving force
for migratory insertion might be provided by an intramolecular
interaction between the Rh center and a lone pair on the
o-methoxy group in 3e. A shift to lower frequency (by ca. 12
cm-1) of ν(CdO) for 3e compared with the values for 3b,c is
consistent with higher electron density at the metal center,
resulting from donation from a methoxy lone pair.
A Rh-O interaction was confirmed by an X-ray crystal
structure of the pentane solvate of 3e. Crystals of suitable quality
were grown by slow diffusion of pentane into a concentrated
CH2Cl2 solution of 3e. The molecular structure is illustrated in
Figure 1, and selected geometrical data are given in Table 1.
The coordination geometry around the Rh center in 3e is a
distorted octahedron, with the two iodide ligands occupying
mutually cis coordination sites, trans to the P and N donors of
the chelating iminophosphine ligand. The Rh-I(1) distance trans
to P is ca. 0.09 Å longer than that trans to N, indicating that
the PPh2 moiety has a greater trans influence. This is consistent
with the previous observation of longer Rh-I distances in [Rh-
(32) Adams, H.; Bailey, N. A.; Mann, B. E.; Manuel, C. P. Inorg. Chim.
Acta 1992, 198-200, 111.
(33) Søtofte, I.; Hjortkjaer, J. Acta Chem. Scand. 1994, 48, 872.
(34) Moloy, K. G.; Petersen, J. L. Organometallics 1995, 14, 2931.
(35) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Organometallics 2002,
21, 4808.
(36) Drexler, H.-J.; Baumann, W.; Schmidt, T.; Zhang, S.; Sun, A.;
Spannenberg, A.; Fischer, C.; Buschmann, H.; Heller, D. Angew. Chem.,
Int. Ed. Engl. 2005, 1184.
(37) Perez, J.; Martinez, J. F.; Garcia, L.; Perez, E.; Serrano, J. L.;
Sanchez, G. Inorg. Chim. Acta 2004, 357, 3588.

Rhodium(I) Iminophosphine Carbonyl Complexes
Organometallics, Vol. 26, No. 8, 2007 1963
Scheme 4. Proposed Mechanism for Reaction of 1e with
MeI
Figure 2. Series of IR spectra recorded during the reaction of 1b
with MeI (CH2Cl2, 25 °C).
Table 2. Second-Order Rate Constants (k1, 25 °C) and
Parameters for Oxidative Addition of MeI to Rh(I)
Complexes in CH2Cl2
104k1/dm3
∆Hq/kJ
∆Sq/J K-1
accompanied by iodide coordination with the N-O chelate ring
remaining intact.
complex
ν(CO)/cm-1
mol-1 s-1
mol-1
mol-1
Acceleration of MeI oxidative addition by a neighboring
group has previously been proposed for Ir(I) Vaska-type
complexes. Miller and Shaw found that trans-[Ir(CO){PMe2-
(2-MeOC6H4)}2Cl] reacts with MeI ca. 100 times faster than
do the p-anisyl and phenyl analogues.42 More recently, Dutta
et al. proposed that Rh-O interactions in Rh complexes of
PPh2(2-C6H4CO2Me) can enhance the rates of MeI addition and
catalytic methanol carbonylation. Incorporation of o-methoxy
groups in diphosphine ligands has a significant influence on
catalytic performance in a number of other processes: e.g., CO/
alkene copolymerization,43,44 ethene trimerization,45,46 and asym-
metric hydrogenation.36,47 In the present case MeI oxidative
addition to 1e is an order of magnitude faster than to 1a,f, which
is a smaller effect than that found by Miller and Shaw for trans-
[Ir(CO){PMe2(2-MeOC6H4)}2Cl].42 The more modest accelera-
tion for 1e may result from additional conformational constraints
in the bidentate iminophosphine ligand, which inhibit closer
approach of the o-methoxy group to the Rh center. This is
evident in the relatively long Rh-O distance in the X-ray
structure of 3e.
The complexes with iminophosphine ligands containing ortho
alkyl substituents on the N-aryl group show a rather unexpected
trend in reactivity. Complex 1c (krel ) 2.5) reacts marginally
faster than the less sterically congested 1b (2.1) and 1d (2.15),
and all three of these complexes are more reactive than the
phenyl derivative 1a (1.0). This behavior contrasts markedly
with that observed for [Rh(CO)(R-diimine)I] complexes, in
which ortho alkyl substituents on the N-aryl group cause a
substantial (up to 103-fold) decrease in oxidative addition rate.
It is not clear why the reverse trend occurs for the [Rh(CO)-
(iminophosphine)I] complexes, but one possibility is that the
bidentate ligand can be hemilabile. Formation of a 14-electron
species by dechelation of the N-donor would be expected to
occur more readily for a bulky N-aryl group. If the 14-electron
1a
1b
1c
1d
1e
1f
1998
1997
1997
1997
1995
1997
2002
3.60
7.50
9.10
7.71
31.4
2.61
0.78
51 ( 2
48 ( 1
45 ( 1
55 ( 2
46 ( 1
66 ( 4
54 ( 2
-138 ( 6
-145 ( 3
-149 ( 3
-120 ( 6
-138 ( 4
-88 ( 14
-139 ( 7
1g
entropies are all large and negative, as generally found for
oxidative addition of MeI to square-planar d8 metal complexes,
which is considered to proceed via an SN2 mechanism.2,3,38-41
The observed reactivity toward MeI increases the order 1g
< 1f < 1a < 1b ≈ 1d < 1c < 1e. The electron-withdrawing
CF3 groups on the iminophosphine ligand of 1g clearly reduce
the nucleophilicity of the Rh center, as indicated by the higher
ν(CO) value for this complex. Notably, the fastest reaction is
found for 1e, which contains an o-anisyl substituent. The second-
order rate constant for 1e is 12 times larger than for the p-anisyl
isomer, 1f, and 4 times larger than for 1d, which has an
approximately isosteric iminophosphine ligand. We propose that
the higher nucleophilicity of 1e arises from an interaction
between the Rh center and a methoxy lone pair, as shown in
Scheme 4. The feasibility of such an interaction is demonstrated
by the crystal structure of the acetyl product, 3e (vide supra). It
is uncertain whether a Rh-O interaction exists in the ground
state of 1e, although its slightly lower ν(CO) value compared
to those for the other Rh(I) complexes studied would be
consistent with this.
Scheme 4 shows a mechanism for the reaction of 1e with
MeI. Initial nucleophilic attack by the Rh(I) center will proceed
via an SN2-type transition state to give a cationic Rh(III) methyl
intermediate (which was not detected). Two possible pathways
are depicted for conversion of this cation to the final product
3e: (i) displacement of the coordinated methoxy group by
iodide, followed by migratory CO insertion and re-formation
of the N-O chelate ring, and (ii) methyl migration in the cation,
(42) Miller, E. M.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1974, 480.
(43) Drent, E.; Wife, R. L. Eur. Pat. Appl. EP-B222454, 1987.
(44) Verspui, G.; Schanssema, F.; Sheldon, R. A. Angew. Chem., Int.
Ed. 2000, 39, 804.
(45) Carter, A.; Cohen, S. A.; Cooley, N. A.; Murphy, A.; Scutt, J.; Wass,
D. F. Chem. Commun. 2002, 858.
(46) Agapie, T.; Day, M. W.; Henling, L. M.; Labinger, J. A.; Bercaw,
J. E. Organometallics 2006, 25, 2733.
(47) Knowles, W. S. Acc. Chem. Res. 1983, 16, 106.
(38) Rankin, J.; Benyei, A. C.; Poole, A. D.; Cole-Hamilton, D. J. J.
Chem. Soc., Dalton Trans. 1999, 3771.
(39) Martin, H. C.; James, N. H.; Aitken, J.; Gaunt, J. A.; Adams, H.;
Haynes, A. Organometallics 2003, 22, 4451.
(40) Bassetti, M.; Capone, A.; Mastrofrancesco, L.; Salamone, M.
Organometallics 2003, 22, 2535.
(41) Gaunt, J. A.; Gibson, V. C.; Haynes, A.; Spitzmesser, S. K.; White,
A. J. P.; Williams, D. J. Organometallics 2004, 23, 1015.

1964 Organometallics, Vol. 26, No. 8, 2007
Best et al.
Table 3. Summary of Crystallographic Data for 3e‚C5H12
Experimental Section
formula
C33H37I2NO2PRh
867.32
monoclinic
P21/n
red
13.4037(8)
16.9491(11)
14.0277(9)
Materials. All solvents used for synthesis or kinetic experiments
were distilled and degassed prior to use following literature
procedures.49 Synthetic procedures were carried out utilizing
standard Schlenk techniques. Nitrogen and carbon monoxide were
dried through a short (20 × 3 cm diameter) column of molecular
sieves (4 Å), which was regularly regenerated. The complexes [Rh-
formula wt
cryst syst
space group
color
a (Å)
b (Å)
c (Å)
50
51
(CO)2Cl]2 and [Rh(CO)2I]2 were synthesized according to
literature procedures. The ligands PNAr (Ar ) C6H5, 2,6-Me2C6H3,
2,6-iPr2C6H3, 2-MeOC6H4, 4-MeOC6H4) were prepared using
established procedures.15,25,26 Methyl iodide (Aldrich) was distilled
over calcium hydride and stored in foil-wrapped Schlenk
tubes under nitrogen and over mercury to prevent formation
of I2. Isotopically enriched 13CO (99% 13C) was obtained from
Euriso-top.
R (deg)
90
90.975(3)
90
150(2)
â (deg)
γ (deg)
temp (K)
Z
final R indices (I > 2σ(I))
R indices (all data)
GOF
4
R1 ) 0.0638; wR2 ) 0.1407
R1 ) 0.1425; wR2 ) 0.1706
1.020
Instrumentation. FTIR spectra were measured using a Mattson
Genesis Series spectrometer, controlled by WIN-FIRST software
running on a Viglen 486 PC. 1H and 31P NMR spectra were obtained
using a Bruker AC250 spectrometer fitted with a Bruker B-ACS60
automatic sample changer operating in pulse Fourier transform
mode, using the solvent as reference. Elemental analyses were
performed by the University of Sheffield Microanalytical Service
using a Perkin-Elmer 2400 CHNS/O Series II elemental analyzer.
Mass spectrometry was performed using a Micromass VG-
AutoSpec instrument (fast atom bombardment (FAB)) or Micromass
LCT instrument (electrospray ionization (ESI)).
Rh species is highly reactive toward methyl iodide, the observed
trend could result. Although we found no direct evidence for a
chelate ring opening mechanism, hemilability of an iminophos-
phine ligand has been suggested to occur in Pd(II) allyl
complexes.48 Another possibility, suggested by a reviewer, is
that an agostic interaction between the Rh center and an ortho
alkyl C-H bond can stabilize the SN2 transition state for
oxidative addition and hence accelerate the reaction.
Apart from the p-anisyl derivative, 1e, the Rh(I) iminophos-
phine complexes studied here are somewhat less reactive than
[Rh(CO)(dppe)I] toward MeI (kobs ) 1.41 × 10-3 M-1 s-1
,
Synthesis of Ligands. (a) Ph2PC6H4CHdN(2-EtC6H4). (Diphe-
nylphosphino)benzaldehyde (224 mg, 0.772 mmol) and 2-ethyla-
niline (0.4 cm3, 3.2 mmol) were dissolved in methanol (20 cm3)
with 2 drops of formic acid before being heated to reflux for 3 h.
Rotary evaporation yielded a precipitate, which was extracted with
cold methanol and dried under vacuum. Yield: 248 mg (82%). 1H
2
25 °C, CH2Cl2 ). It is slightly surprising that a mixed phos-
phine-imine donor set imparts lower nucleophilicity than dppe,
since [Rh(CO)(R-diimine)I] complexes were found to have
generally higher MeI oxidative addition rates than [Rh(CO)-
(dppe)I] (apart from those with sterically hindered N-aryl
groups).3 This comparison is complicated to some extent,
because the iminophosphines used here form six-membered
chelate rings, in contrast to the five-membered rings formed
by dppe and R-diimines. The different conformational properties
of the larger ring might introduce additional factors that
influence reactivity. The lower rate for the unsymmetrical
ligands might also indicate a mutual influence between P and
N donors and relates to the behavior of unsymmetrical ligands
in catalysis, which commonly is not linearly related to the
behavior of corresponding symmetrical ligands.
3
NMR (CD2Cl2): δ 1.00 (t, JHH ) 7.5 Hz, 3H, CH2Me), 2.56 (q,
3JHH ) 7.5 Hz, 2H, CH2Me), 6.35, 6.80-7.70, 8.17 (each m, total
4
18H, Ar H), 8.92 (d, JHP ) 5.2 Hz, 1H, NdCH). 31P{1H} NMR
(CDCl3): δ -13.0 (s). MS (FAB; m/z): 393 [M+].
(b) Ph2PC6H4CHdN(3,5-(CF3)2C6H3). A procedure was em-
ployed analogous to that described in (a), using (diphenylphosphi-
no)benzaldehyde (500 mg, 1.7 mmol) and 3,5-bis(trifluoromethyl)-
1
aniline (0.4 cm3, 2.56 mmol). Yield: 639 mg (74%). H NMR
(CD2Cl2): δ6.90-7.70, 8.14 (each m, total 17H, Ar H), 8.92 (d,
4JHP ) 4.9 Hz, 1H, NdCH). 31P{1H} NMR (CDCl3): δ -10.8 (s).
Synthesis of Rh(I) Complexes. (a) [RhI(CO)(PNAr)] (1a; Ar
) C6H5). A solution of Ph2PC6H4CHdNPh (39 mg, 0.11 mmol)
in 10 cm3 of THF was added dropwise to a THF (10 cm3) solution
of [RhI(CO)2]2 (31 mg, 0.054 mmol). The resulting orange-red
solution was stirred for 2 h. The solvent was removed in vacuo
before the product was extracted using 40 cm3 of pentane. The
solvent was removed and the product dried overnight in vacuo.
Yield: 24 mg (36%). Satisfactory elemental analysis was not
obtained for this compound, despite clean spectroscopic data,
Conclusion
A series of rhodium iodo carbonyl complexes containing
bidentate iminophosphine ligands has been prepared and
characterized. The reactivity of these complexes toward MeI is
dependent upon both the steric and electronic properties of the
N-aryl substituent of the iminophosphine. Most significantly,
both oxidative addition and migratory CO insertion are promoted
by an o-methoxy group, and it is proposed that this effect arises
from an intramolecular interaction between the methoxy oxygen
and the Rh center. Such an interaction can enhance the
nucleophilicity of the Rh(I) reactant (by stabilization of the SN2
transition state) as well as providing a driving force for migratory
CO insertion. An X-ray crystal structure of the acetyl product
provides direct evidence for a Rh-O interaction. Migratory CO
insertion can also be promoted by bulky ligands (as found in
related systems), but there is an unexpected steric effect on MeI
oxidative addition. Moderate acceleration of MeI addition by
more bulky ligands can be speculated to arise from hemilability
of the iminophosphine.
presumably due to air sensitivity. IR (ν(CO); CH2Cl2): 1998 cm-1
.
1H NMR (CD2Cl2): δ 6.96-7.65 (m, 19H, Ar H), 8.02 (bs, 1H,
NdCH). 31P{1H} NMR (CD2Cl2): δ 46.3 (d, JRhP ) 166 Hz).
(b) [RhI(CO)(PNAr)] (1b; Ar ) 2,6-Me2C6H3). A procedure
was employed analogous to that described for 1a, using Ph2PC6H4-
CHdN(2,6-Me2C6H3) (62 mg, 0.16 mmol) and [RhI(CO)2]2 (46
mg, 0.08 mmol). Yield: 59 mg (56%). Anal. Calcd for C28H24-
INOPRh: C, 51.64; H, 3.71; N, 2.15. Found: C, 51.57; H, 3.70;
1
N, 2.01. IR (ν(CO); CH2Cl2): 1998 cm-1. H NMR (CD2Cl2): δ
1.97 (s, 6H, CH3), 6.90-7.65 (m, 17H, Ar H), 7.87 (bs, 1H, Nd
CH). 31P{1H} NMR (CD2Cl2): δ 41.3 (d, JRhP ) 169 Hz). MS
(49) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 2nd ed.; Pergamon Press: Oxford, U.K., 1988.
(50) McCleverty, J. A.; Wilkinson, G. Inorg. Synth. 1966, 8, 214.
(51) Johnson, B. F. G.; Lewis, J.; Miller, J. R.; Robinson, B. H.;
Robinson, P. W.; Wojcicki, A. J. Chem. Soc. A 1968, 522.
(48) Faller, J. W.; Stokes-Huby, H. L.; Albrizzio, M. A. HelV. Chim.
Acta 2001, 84, 3031.

Rhodium(I) Iminophosphine Carbonyl Complexes
Organometallics, Vol. 26, No. 8, 2007 1965
(ESI; m/z): 623 [M+] - CO, 524 [M+] - I. A 13CO-labeled sample
of 1b was prepared by briefly bubbling 13CO through a solution of
H, 4.02; N, 1.55. IR (ν(CO); CH2Cl2): 1717 cm-1. 1H NMR (CD2-
3
Cl2): δ 0.96, 1.13, 1.16, 1.29 (each d, 3H, JHH ) 7.6 Hz, CH-
the compound in CH2Cl2. IR (ν(13CO); CH2Cl2): 1953 cm-1 31P-
.
3
(Me)2), 1.73, 2.55 (each sept, 1H, JHH ) 7.6 Hz, CH(Me)2), 3.28
1
2
{1H} NMR (CD2Cl2): δ 41.3 (d, JRhP ) 169 Hz, JPC ) 12 Hz).
(s, 3H, COMe), 6.93-7.86 (m, 17H, Ar H), 8.17 (d, 3.7 Hz, 1H,
NdCH). 31P{1H} NMR (CD2Cl2): δ 46.5 (d, JRhP ) 135 Hz). MS
(FAB; m/z): 806 [M+] - COMe, 722 [M+] - I, 694 [M+] - CO
- I, 679 [M+] - COMe - I.
13C{1H} NMR δ 190.8 (1JRhC ) 70 Hz, JPC ) 12 Hz).
2
(c) [RhI(CO)(PNAr)] (1c; Ar ) 2,6-iPr2C6H3). A procedure was
employed analogous to that described for 1a using Ph2PC6H4CHd
N(2,6-iPr2C6H3) (82.6 mg, 0.184 mmol) and [RhI(CO)2]2 (50 mg,
0.087 mmol). Yield: 85 mg (69%). Anal. Calcd for C32H32-
INOPRh: C, 54.33; H, 4.56; N, 1.98. Found: C, 54.60; H, 4.48;
(c) [Rh(COMe)I2(PNAr)] (3e; Ar ) 2MeOC6H4). A procedure
analogous to that described above for 3b was employed. Anal. Calcd
for C28H25I2NO2PRh: C, 42.29; H, 3.17; N, 1.76. Found: C, 41.90;
H, 3.41; N, 1.65. IR (ν(CO); CH2Cl2): 1704 cm-1. 1H NMR (CD2-
Cl2): δ 3.30 (s, 3H, COMe), 3.93 (s, 3H, OMe), 6.65-7.85 (m,
18H, Ar H), 8.20 (d, 3 Hz, 1H, NdCH). 31P{1H} NMR (CD2Cl2):
δ 44.7 (d, JRhP ) 135 Hz). MS (FAB; m/z): 752 [M+] - COMe,
625 [M+] - COMe - I. A crystal suitable for study by X-ray
diffraction was obtained by slow diffusion of pentane into a
concentrated chloroform solution of 3e.
1
N, 2.00. IR (ν(CO); CH2Cl2): 1997 cm-1. H NMR (CD2Cl2): δ
0.69 (d, 6H, 6.7 Hz, Me2CH), 1.29 (d, 6H, 6.7 Hz, Me2CH), 2.99
(sept, 2H, 6.7 Hz, Me2CH), 7.0-7.2 (m, 4H, Ar H) 7.42-7.60 (m,
13H, Ar H), 8.00 (d, 2.4 Hz, 1H, NdCH). 31P{1H} NMR (CD2-
Cl2): δ 40.5 (d, JRhP ) 172 Hz). MS (ESI; m/z): 580 [M+] - I.
(d) [RhI(CO)(PNAr)] (1d; Ar ) 2-EtC6H4). A procedure was
employed analogous to that described for 1a, using Ph2PC6H4CHd
N(2-EtC6H4) (221 mg, 0.562 mmol) and [RhI(CO)2]2 (160 mg,
0.280 mmol). Yield: 254 mg (70%). Anal. Calcd for C28H24-
INOPRh: C, 51.64; H, 3.71; N, 2.15. Found: C, 51.66; H, 3.43;
X-ray Structure Determination. Data were collected on a
Bruker Smart CCD area detector with Oxford Cryosystems low-
temperature system using Mo KR radiation (λh ) 0.710 73 Å). The
structure was solved by direct methods and refined by full-matrix
least-squares methods on F2. Hydrogen atoms were placed geo-
metrically and refined with a riding model (including torsional
freedom for methyl groups) and with Uiso constrained to be 1.2
(1.5 for methyl groups) times the Ueq value of the carrier atom.
Complex scattering factors were taken from the program package
SHELXTL.52 Crystallographic data are summarized in Table 3, and
full listings are given in the Supporting Information.
1
N, 1.92. IR (ν(CO); CH2Cl2): 1997 cm-1. H NMR (CD2Cl2):
δ0.75 (t, 3H, CH2Me), 2.57 (q, 2H, CH2Me), 6.80-7.67 (m, 18H,
Ar H), 7.97 (d, 1.8 Hz, 1H, NdCH). 31P{1H} NMR (CD2Cl2): δ
42.4 (d, JRhP ) 162 Hz). MS (FAB; m/z): 623 [M+] - CO, 524
[M+] - I.
(e) [RhI(CO)(PNAr)] (1e; Ar ) 2-MeOC6H4). A procedure was
employed analogous to that described for 1a, using Ph2PC6H4CHd
N(2-MeOC6H4) (69 mg, 0.175 mmol) and [RhI(CO)2]2 (50.4 mg,
0.088 mmol). Yield: 38 mg (33%). Anal. Calcd for C27H22INO2-
PRh: C, 49.64; H, 3.39; N, 2.14. Found: C, 49.44; H, 3.41; N,
1.95. IR (ν(CO); CH2Cl2): 1995 cm-1. 1H NMR (CD2Cl2): δ 3.76
(s, 3H, OMe), 6.82-7.60 (m, 18H, Ar H), 7.99 (d, 2.1 Hz, 1H,
NdCH). 31P{1H} NMR (CD2Cl2): δ 44.5 (d, JRhP ) 171 Hz).
MS (FAB; m/z): 625 [M+] - CO, 526 [M+] - I, 498 [M+] -
CO - I.
Kinetic Experiments. Samples for kinetic runs were prepared
by placing the required amount of freshly distilled iodomethane in
a 5 cm3 graduated flask, which was then filled up to the mark with
CH2Cl2. A portion of this solution was used to record a background
spectrum. Another portion (typically 500 µL) was added to the solid
Rh complex (typically 5-8 µmol) in a sample vial to give a reaction
solution containing 10-15 mM [Rh]. A portion of the reaction
solution was quickly transferred to the IR cell, and the kinetic
experiment was started. Pseudo-first-order conditions were em-
ployed, with a least a 10-fold excess of MeI, relative to the metal
complex. The IR cell (0.5 mm path length, CaF2 windows) was
maintained at constant temperature throughout the kinetic run by a
thermostated jacket. Spectra were scanned in the metal carbonyl
ν(CO) region (2200-1600 cm-1) and saved at regular time intervals
under computer control. After the kinetic run, absorbance versus
time data for the appropriate ν(CO) frequencies were extracted and
analyzed off-line using Kaleidagraph curve-fitting software. The
decays of the bands of 1a-g were all well fitted by exponential
curves with correlation coefficients g0.999, to give pseudo-first-
order rate constants. Each kinetic run was repeated at least twice
to check reproducibility, the kobs values being averaged values with
component measurements deviating from each other by e5%.
(f) [RhI(CO)(PNAr)] (1f; Ar ) 4-MeOC6H4). A procedure was
employed analogous to that described for 1a, using Ph2PC6H4CHd
N(4-MeOC6H4) (84 mg, 0.21 mmol) and [RhI(CO)2]2 (54 mg, 0.09
mmol). Yield: 101 mg (82%). Anal. Calcd for C27H22INO2PRh:
C, 49.64; H, 3.39; N, 2.14. Found: C, 49.46; H, 3.28; N, 2.11. IR
1
(ν(CO); CH2Cl2): 1997 cm-1. H NMR (CDCl3): δ 3.79 (s, 3H,
OMe), 6.78 (m, 4H, Ar H), 7.52 (m, 14H, Ar H), 7.97 (bs, 1H,
NdCH). 31P{1H} NMR (CDCl3): δ 46.8 (d, JRhP ) 168 Hz).
(g) [RhI(CO)(PNAr)] (1g; Ar ) 3,5-(CF3)2C6H3). A procedure
was employed analogous to that described for 1a, using Ph2PC6H4-
CHdN(3,5-(CF3)2C6H3) (93.4 mg, 0.19 mmol) and [RhI(CO)2]2
(45.2 mg, 0.08 mmol). Yield: 102 mg (85%). Satisfactory elemental
analysis was not obtained for this compound, despite clean
spectroscopic data, presumably due to air sensitivity. IR (ν(CO);
1
CH2Cl2): 2002 cm-1. H NMR (CDCl3): δ 6.90-7.01 (m, 1H,
C6H2(CF3)2-H), 7.38-7.66 (m, 16H, Ar H), 7.92 (bs, 1H, Nd
CH). 31P{1H} NMR (CDCl3): δ 46.1 (d, JRhP ) 165 Hz). MS
(FAB; m/z): 759 [M+], 731 [M+] - CO, 632 [M+] - I, 604 [M+]
- CO - I.
Acknowledgment. We thank the EPSRC and BP Chemicals
Ltd. for funding this research (studentship to J.M.W.) and Dr.
Glenn Sunley for helpful discussions.
Synthesis of Rh(III) Complexes. (a) [Rh(COMe)I2(PNAr)] (3b;
Ar ) 2,6-Me2C6H3). Complex 1b (15 mg) was dissolved in THF
(5 cm3), and iodomethane (2 cm3) was added. After the mixture
was stirred for 1 h, the volatiles were removed in vacuo to yield a
dark orange solid. Anal. Calcd for C29H27I2NOPRh: C, 43.91; H,
3.43; N, 1.77. Found: C, 44.25; H, 3.73; N, 1.58. IR (ν(CO); CH2-
Supporting Information Available: Tables giving kinetic data
and CIF files giving crystallographic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
1
Cl2): 1716 cm-1. H NMR (CDCl3): δ2.10 (s, 6H, Me), 3.55 (s,
3H, COMe), 6.78-7.64 (m, 17H, Ar H), 8.15 (d, 2.7 Hz, 1H, Nd
CH). 31P{1H} NMR (CDCl3): δ 46.9 (d, JRhP ) 131 Hz). MS (FAB;
m/z): 623 [M+] - COMe - I.
(b) [Rh(COMe)I2(PNAr)] (3c; Ar ) 2,6-iPr2C6H3). A procedure
analogous to that described above for 3b was employed. Anal. Calcd
for C33H35I2NOPRh: C, 46s.67; H, 4.15; N, 1.65. Found: C, 46.57;
OM070019B
(52) Sheldrick, G. M. SHELXTL, an Integrated System for Solving and
Refining Crystal Structures from Diffraction Data, Revision 5.1; Bruker
AXS Ltd., Madison, WI.