Quantifying steric effects of α-diimine ligands. Oxidative addition of MeI to rhodium(I) and migratory insertion in rhodium(III) complexes

Article abstract of DOI:10.1021/om020777w

The series of Rh(I) complexes [Rh(CO)(diimine)I] have been prepared (diimine = ArN=C(Me)C(Me)=NAr, Ar = 2-ⁱPrC₆H₄ (1a), 2,4,6-Me₃C₆H₂ (1b), 2,6-ⁱPr₂C₆H₃ (1c); diimine = ArN=C(Me)-2-py, Ar = C₆H₅ (1d), 2-ⁱPr₂C₆H₃ (1e), 2,6-ⁱPr₂C₆H₃ (If); diimine = 2,2′-bipy (1g)). An X-ray crystal structure showed 1f to have CO trans to the pyridyl nitrogen. Oxidative addition of MeI to 1a,d,e,g gave the stable methyl complexes [Rh(CO)(diimine)I₂Me] (2a,d,e,g, respectively); an X-ray structure for 2a showed a pseudo-octahedral geometry with methyl trans to iodide. In contrast, the reactions of 1b,c with MeI gave the five-coordinate acetyl complexes [Rh(diimine)I₂(COMe)] (3b,c, respectively); an X-ray structure for 3c displayed a distorted-square-pyramidal geometry. The methyl complex 2b was detected as a reactive intermediate during the reaction of 1b. The reaction of 1f with MeI attained an equilibrium with the methyl and acetyl products 2f and 3f. The different outcomes result from diimine ligand steric effects, with bulky alkyl groups in the ortho positions of N-aryl groups favoring migratory CO insertion. Rearrangement from octahedral to square-pyramidal geometry relieves steric strain between axial ligands and ortho alkyl substituents on the diimine. Kinetic studies showed that the second-order rate constant (k₁) for MeI oxidative addition varied by a factor of > 10³, depending on the steric bulk of the diimine ligand. For the bipy complex 1g, k₁ is the highest yet reported for a Rh(I) carbonyl complex (0.206 M^-1 s^-1, CH₂Cl₂, 25 >C). Kinetic data are also reported for migratory CO insertion in 2b,f. An X-ray structure of [Rh(CO)(ArN=C(Me)-2-py)I₃] (4f; Ar = 2,6-ⁱPr₂C₆H₃), the product of oxidative addition of I₂ to 1f, reveals distortion from ideal octahedral geometry caused by ortho ⁱPr groups on the diimine ligand.

Full text of DOI:10.1021/om020777w

Organometallics 2003, 22, 1047-1054
1047
Qu a n tifyin g Ster ic Effects of r-Diim in e Liga n d s.
Oxid a tive Ad d ition of MeI to Rh od iu m (I) a n d Migr a tor y
In ser tion in Rh od iu m (III) Com p lexes
Luca Gonsalvi,†,‡ J oseph A. Gaunt,† Harry Adams,† Aurora Castro,†
Glenn J . Sunley,*,§ and Anthony Haynes*,†
Department of Chemistry, University of Sheffield, Sheffield, U.K. S3 7HF, and
Hull Research and Technology Centre, BP Chemicals Ltd., Saltend, Hull, U.K. HU12 8DS
Received September 17, 2002
The series of Rh(I) complexes [Rh(CO)(diimine)I] have been prepared (diimine ) ArNd
C(Me)C(Me)dNAr, Ar ) 2-iPrC6H4 (1a ), 2,4,6-Me3C6H2 (1b), 2,6-iPr2C6H3 (1c); diimine )
ArNdC(Me)-2-py, Ar ) C6H5 (1d ), 2-iPrC6H4 (1e), 2,6-iPr2C6H3 (1f); diimine ) 2,2′-bipy (1g)).
An X-ray crystal structure showed 1f to have CO trans to the pyridyl nitrogen. Oxidative
addition of MeI to 1a ,d ,e,g gave the stable methyl complexes [Rh(CO)(diimine)I2Me]
(2a ,d ,e,g, respectively); an X-ray structure for 2a showed a pseudo-octahedral geometry with
methyl trans to iodide. In contrast, the reactions of 1b,c with MeI gave the five-coordinate
acetyl complexes [Rh(diimine)I2(COMe)] (3b ,c, respectively); an X-ray structure for 3c
displayed a distorted-square-pyramidal geometry. The methyl complex 2b was detected as
a reactive intermediate during the reaction of 1b. The reaction of 1f with MeI attained an
equilibrium with the methyl and acetyl products 2f and 3f. The different outcomes result
from diimine ligand steric effects, with bulky alkyl groups in the ortho positions of N-aryl
groups favoring migratory CO insertion. Rearrangement from octahedral to square-pyramidal
geometry relieves steric strain between axial ligands and ortho alkyl substituents on the
diimine. Kinetic studies showed that the second-order rate constant (k1) for MeI oxidative
addition varied by a factor of >103, depending on the steric bulk of the diimine ligand. For
the bipy complex 1g, k1 is the highest yet reported for a Rh(I) carbonyl complex (0.206 M-1
s-1, CH2Cl2, 25 °C). Kinetic data are also reported for migratory CO insertion in 2b,f. An
X-ray structure of [Rh(CO)(ArNdC(Me)-2-py)I3] (4f; Ar ) 2,6-iPr2C6H3), the product of
oxidative addition of I2 to 1f, reveals distortion from ideal octahedral geometry caused by
i
ortho Pr groups on the diimine ligand.
In tr od u ction
quantification of steric effects of R-diimine ligands on
oxidative addition and migratory insertion in a model
rhodium system.
Recent advances in organometallic chemistry and
homogeneous catalysis have emphasized the important
role of ligand steric effects. In particular, it has been
found that N-aryl ortho substituents in R-diimines
(Ar2DAB) and pyridyl diimines are key to the behavior
of olefin polymerization catalysts based on group 8-10
metals.1,2 Steric bulk inhibits associative chain transfer
relative to propagation and so favors the production of
high-molecular-weight polymers.3 Steric effects also
influence the behavior of complexes of the type [Pt(di-
imine)(L)R]+ in C-H activation reactions4 and in ion-
pairing interactions.5 In this paper we report the
We recently reported some dramatic ligand effects on
the rate of migratory CO insertion in complexes of the
type [Rh(CO)(L-L)I2Me] (L-L ) Ph2PCH2P(S)Ph2
(dppms), Ph2PCH2CH2PPh2 (dppe)).6 An X-ray crystal
structure of the stable [Ir(CO)(dppms)I2Me] showed the
methyl ligand to be placed in a sterically congested
region created by the ligand phenyl substituents. We
proposed that this steric crowding contributed to the
much faster migratory insertion in [Rh(CO)(dppms)-
I2Me] than in the dppe analogue. These findings led us
to carry out further studies on the [Rh(CO)(L-L)I] +
MeI system using bidentate ligands in which steric bulk
could be easily controlled. The R-diimines used by
Brookhart2 and others4,7-12 provided the ideal charac-
teristics for this study.
* To whom correspondence should be addressed. E-mail (A.H.):
a.haynes@sheffield.ac.uk.
† University of Sheffield.
‡ Current address: Institute for the Chemistry of Organometallic
Compounds, National Research Council (ICCOM-CNR), Via J Nardi
39-41, 50132, Firenze, Italy.
(4) Zhong, H. A.; Labinger, J . A.; Bercaw, J . E. J . Am. Chem. Soc.
2002, 124, 1378. J ohansson, L.; Ryan, O. B.; Rømming, C.; Tilset, M.
J . Am. Chem. Soc. 2001, 123, 6579.
(5) Zuccaccia, C.; Macchioni, A. Organometallics 1999, 18, 4367.
(6) Gonsalvi, L.; Adams, H.; Sunley, G. J .; Ditzel, E.; Haynes, A. J .
Am. Chem. Soc. 1999, 121, 11233. Gonsalvi, L.; Adams, H.; Sunley,
G. J .; Ditzel, E.; Haynes, A. J . Am. Chem. Soc. 2002, 124, 13597.
(7) van Koten, G.; Vrieze, K. Adv. Organomet. Chem. 1982, 21, 151.
§ BP Chemicals Ltd.
(1) Britovsek, G. J . P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. Engl. 1999, 38, 428.
(2) Ittel, S. D.; J ohnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100,
1169.
(3) Tempel, D. J .; J ohnson, L. K.; Huff, R. L.; White, P. S.; Brookhart,
M. J . Am. Chem. Soc. 2000, 122, 6686.
10.1021/om020777w CCC: $25.00 © 2003 American Chemical Society
Publication on Web 01/25/2003

1048 Organometallics, Vol. 22, No. 5, 2003
Gonsalvi et al.
Sch em e 1. Syn th esis a n d Nu m ber in g Sch em e for
Com p lexes 1a -g
F igu r e 1. Stereochemistry of pyridylimine complexes
1d -f.
F igu r e 2. Conformers of complex 1a .
a slightly better donor than the exocyclic imine. This is
supported by the observation that ν(CO) for the bipy
complex 1g is ca. 10 cm-1 lower than for 1d -f.
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion of Rh (I) Com -
p lexes. Symmetrical diimines and unsymmetrical pyr-
idylimines were prepared by condensation of the ap-
propriate amine and ketone. An X-ray crystal structure
(Supporting Information) of ArNdC(Me)C(Me)dNAr (Ar
) 2-iPrC6H4) revealed an E,s-trans-E conformation for
the free ligand, as reported for CyNdCHCHdNCy.7,13
Complexation of the diimine ligands to Rh(I) was
achieved by reaction with the dimer [Rh(CO)2I]2 (Scheme
1).14 The products, [Rh(CO)(diimine)I]15 (1a -f), were
isolated as strongly colored crystalline solids and fully
characterized by spectroscopy, elemental analysis, and,
in the case of 1f, an X-ray crystal structure. The bipy
complex 1g has been reported previously.16
The infrared spectra of complexes 1a -c displayed
almost identical ν(CO) values, indicating that variation
of the alkyl substituents on the N-aryl groups has little
influence on the donor ability of the diimine ligand.
Likewise, complexes 1d -f have ν(CO) values which lie
very close to each other but are ca. 5 cm-1 lower than
those of 1a -c, suggesting that the pyridyl nitrogen is
1H NMR spectroscopy indicated that each of the
pyridylimine complexes 1d -f is formed as a single
isomer. A resonance at relatively high frequency (δ 9.73,
9.82, and 9.97, respectively) is assigned to a proton at
position 6 on the pyridyl ring, the chemical shift
suggesting relatively close proximity to the iodide
ligand, by analogy with related systems.10,11,17 This
indicates that the unsymmetrical complexes 1d -f adopt
a stereochemistry with CO trans to the pyridyl nitrogen,
as shown in Figure 1. This was confirmed by an X-ray
crystal structure of 1f (vide infra).
1H NMR spectroscopy also revealed some of the
behavior of the diimine ligand aryl substituents. For
1e, the single o-iPr group exhibits two distinct doublets
due to magnetically inequivalent methyls. This in-
equivalence arises from restricted rotation about the
N-C(aryl) bond, due to steric hindrance by the cis-
coordinated CO ligand and the ligand backbone methyl
substituent. Likewise, 1f shows two doublets for its o-iPr
groups. In this case each doublet arises from a pair of
i
methyls on different Pr groups related by mirror sym-
metry. For similar reasons, 1c shows four doublets for
its o-iPr groups. For 1a the ligand backbone methyl
substituents give rise to four singlets rather than the
two observed for 1b,c. This can be explained by the
presence of approximately equal amounts of the two
conformers shown in Figure 2, which do not interconvert
rapidly on the NMR time scale due to the restricted
rotation identified above. The eight doublets expected
for the methyls of the Pr groups of these conformers
are observed (with some overlap).
The structure of complex 1f was confirmed by X-ray
crystallography. Figure 3 illustrates the square-planar
geometry with CO trans to the pyridyl nitrogen, as
suggested by spectroscopy. The bond distances and
(8) van Asselt, R.; Elsevier, C. J .; Smeets, W. J . J .; Spek, A. L.;
Benedix, R. Recl. Trav. Chim. Pays-Bas 1994, 113, 88. van Asselt, R.;
Elsevier, C. J .; Smeets, W. J . J .; Spek, A. L. Inorg. Chem. 1994, 33,
1521.
(9) Laine, T. V.; Klinga, M.; Leskela, M. Eur. J . Inorg. Chem. 1999,
959. van Laren, M. W.; Duin, M. A.; Klerk, C.; Naglia, M.; Rogolino,
D.; Pelagatti, P.; Bacchi, A.; Pelizzi, C.; Elsevier, C. J . Organometallics
2002, 21, 1546.
(10) Laine, T. V.; Piironen, U.; Lappalainen, K.; Klinga, M.; Aitola,
E.; Leskela, M. J . Organomet. Chem. 2000, 606, 112.
(11) Meneghetti, S. P.; Lutz, P. J .; Kress, J . Organometallics 1999,
18, 2734.
(12) Ko¨ppl, A.; Alt, H. G. J . Mol. Catal. A 2000, 154, 45. Miller, K.
J .; Kitagawa, T. T.; Abu-Omar, M. M. Organometallics 2001, 20, 4403.
(13) Keijsper, J .; Van der Poel, H.; Polm, L. H.; Van Koten, G.;
Vrieze, K.; Seignette, P. F. A. B.; Varenhorst, R.; Stam, C. Polyhedron
1983, 2, 1111.
(14) Our attempts to complex a symmetrical diimine containing no
ortho alkyl substituents (i.e. PhNdC(Me)C(Me)dNPh) to Rh in this
manner were unsuccessful. The IR spectrum of the reaction solution
indicated the presence of [Rh(CO)2I2]-, suggesting formation of the bis-
chelate complex [Rh(diimine)2]+ as the counterion.
(15) [Rh(CO)(R-diimine)Cl] complexes have been reported, but their
reactivity toward MeI has not been explored: van der Poel, H.; Van
Koten, G.; Vrieze, K. Inorg. Chim. Acta 1981, 51, 241.
(16) Mestroni, G.; Camus, A.; Zassinovich, G. J . Organomet. Chem.
1974, 65, 119.
i
(17) The Cl analogue of 2f (prepared in
a similar way from
[Rh(CO)2Cl]2) displayed a resonance at δ 9.32 for H6 on the pyridyl
ring, compared to δ 9.97 in 2f. This relatively large shift demonstrates
the sensitivity of the H6 resonance to the nature of the ligand cis to
the pyridyl moiety: Ru¨lke, R. E.; Delis, J . G. P.; Groot, A. M.; Elsevier,
C. J .; van Leeuwen, P. W. N. M.; Vrieze, K.; Goubitz, K.; Schenk, H.
J . Organomet. Chem. 1996, 508, 109.

Quantifying Steric Effects of R-Diimine Ligands
Organometallics, Vol. 22, No. 5, 2003 1049
F igu r e 4. X-ray crystal structure of complex 2a (H atoms
and CHCl3 solvent molecule omitted).
Sch em e 2. Rea ctivity of Com p lexes 1a -g w ith
MeI
F igu r e 3. X-ray crystal structure of complex 1f (H atoms
and CH2Cl2 solvent molecule omitted).
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for Com p lexes 1f, 2a , 3c, a n d 4f
1f
2a
3c
4f
Rh-N(1)
Rh-N(2)
Rh-I(1)
Rh-I(2)
Rh-I(3)
Rh-CO
Rh-CMe
Rh-Cacetyl
C-O
2.109(3)
2.023(3)
2.6211(14) 2.643(2)
2.7758(12)
2.07(2)
2.084(19)
2.064(5)
2.068(8)
2.121(8)
previously.16 In contrast, for complexes 1b,c, containing
bulkier ligands, a band appeared near 1720 cm-1
,
2.6285(7) 2.6705(13)
2.6507(12)
attributable to the acetyl products [Rh(diimine)I2-
(COMe)] (3b,c). During the reaction of 1b, a weak band
at 2074 cm-1 was assigned to the methyl intermediate
2b, but no corresponding intermediate was detected
during the reaction of 1c. In the reaction of 1f with MeI,
a band at 2073 cm-1 assigned to 2f was observed to grow
initially and then decay to attain an equilibrium with
both the reactants and an acetyl product 3f, which gave
an IR absorption at 1716 cm-1. The observed reactivity
for 1a -g is summarized in Scheme 2.
2.6956(13)
1.897(13)
1.818(4)
1.904(3)
2.115(13)
1.975(11)
1.181(13) 1.074(12)
1.541(17)
1.136(4)
1.103(3)
Cacetyl-C
N-Rh-N
77.79(10)
77.3(4)
92.0(10)
89.9(8)
87.6(9)
88.4(8)
174.8(4)
88.27(8)
169.3(17)
77.7(3)
89.9(3)
98.2(3)
78.7(3)
96.1(3)
84.8(2)
92.1(2)
87.40(4)
170.42(4)
86.50(4)
177.7(10)
Lax-Rh-COa
Lax-Rh-N(1)a
Lax-Rh-N(2)a
a
Lax-Rh-Icis
a
Lax-Rh-Itrans
I(1)-Rh-I(2)
O-C-Rh
The methyl complexes 2a ,e and acetyl complexes 3b,c
were isolated and fully characterized, including X-ray
crystal structures for 2a and 3c. Spectroscopic charac-
terization of 2d ,f and 3f was obtained from in situ NMR
experiments. The 1H NMR spectrum of 2a in CD2Cl2
exhibited four doublets in the region δ 1.6-1.8 (each
89.94(3)
121.3(9)
117.2(9)
178.2(3)
C-C-Rh
I(1)-Rh-CO
88.84(12)
86.2(9)
83.2(3)
a
Lax is taken as CMe for 2a , Cacetyl for 3c, and I(3) for 4f on the
basis of the atom labeling in Figures 4-6.
2
with J RhH ) 2 Hz) in the ratio ca. 4:2:2:1. These are
angles (Table 1) are within the expected range. As in
related systems, the plane of the N-aryl group lies
approximately perpendicular to the metal coordination
plane.3,8,18 This conformation places the two o-iPr groups
in positions which hinder approach to the vacant axial
coordination sites and is expected to influence reactivity
in associative processes such as the oxidative addition
reactions described below.
assigned to the methyl ligands of four different conform-
ers or isomers of 2a . Assuming a coordination geometry
with methyl trans to iodide (as found in the solid state;
vide infra), there are four possible conformers of 2a with
different orientations of the two o-iPr groups. Alterna-
tively, geometrical isomers might give rise to the
multiple methyl resonances. The major conformer/
isomer gave a pair of singlets at δ 2.26 and 2.24 for
inequivalent methyl substituents on the diimine ligand
backbone, with a number of other singlets due to the
minor species also occurring in this region. Up to 16
Oxid a tive Ad d ition Rea ction s. When the reactions
of 1a -g with MeI were monitored by FTIR spectroscopy,
it was apparent that the outcome depended on the steric
properties of the ligand. For complexes containing
ligands with relatively low steric bulk (1a ,d ,e,g), decay
of the reactant ν(CO) band was accompanied by growth
i
doublets could arise from the Pr group methyls of 4
species, and 9 of these were resolved in the region δ
1.08-1.42. It was found that the ν(CO) band of 2a
shifted from 2077 to 2070 cm-1 over several hours in
CH2Cl2, probably due to a slow isomerization process.
of an absorption at higher frequency (2070-2080 cm-1
)
consistent with formation of the corresponding Rh(III)
methyl product, [Rh(CO)(diimine)I2Me] (2a ,d ,e,g). Oxi-
dative addition of MeI to 1g to give 2g has been reported
A crystal of 2a suitable for an X-ray diffraction study
was obtained by recrystallization from chloroform. The
solid-state structure, shown in Figure 4, reveals a single
isomer with a pseudo-octahedral geometry. Selected
geometric data are given in Table 1. Again the N-aryl
(18) Hill, G. S.; Yap, G. P. A.; Puddephatt, R. J . Organometallics
1999, 18, 1408. Kokkes, M. W.; Stufkens, D. J .; Oskam, A. J . Chem.
Soc., Dalton Trans. 1983, 439.

1050 Organometallics, Vol. 22, No. 5, 2003
Gonsalvi et al.
F igu r e 5. X-ray crystal structure of complex 3c (H atoms
and CH2Cl2 solvent molecule omitted).
F igu r e 6. X-ray crystal structure of complex 4f (H atoms
omitted).
groups are oriented approximately perpendicular to the
plane of the chelate ring, in this case a slight canting
(ca. 4° from perpendicular) being apparent. A conforma-
tion is adopted in which both o-iPr groups are directed
toward the axial iodide ligand (the equatorial plane
taken as that containing Rh and both N atoms). The
resulting nonbonded interactions cause the N-aryl
groups to bend away from the axial iodide, as reflected
in the significant difference in Cortho-Cortho distances
(7.27 and 6.37 Å) between the two rings. The methyl
ligand occupies the other axial coordination site, which
is sterically uncongested. On the grounds of ligand cone
angles (90° for Me, 107° for I19) a conformation with the
two o-iPr groups directed toward the methyl ligand
would have been expected. The observed structure
might be favored by weak intramolecular H-bonding
and selected geometric data are given in Table 1. The
complex displays a distorted-square-pyramidal geom-
etry with an apical acetyl ligand, as commonly found
in related complexes.6,20 The Rh acetyl fragment lies on
a mirror plane with its methyl group directed away from
the sterically congested region created by two o-iPr
groups. The diimine ligand flexes to accommodate the
acetyl ligand and displays an even larger difference in
Cortho-Cortho distances (7.33 and 6.15 Å) between the two
rings than in 2a . Comparison of the structures of 2a
and 3c clearly demonstrates that formation of 3c from
the undetected intermediate 2c is driven by relief of
steric congestion created by the four o-iPr substituents
on the diimine.
Complex 1f was found to undergo a facile reaction
with I2 to give the triiodide product [Rh(CO)(2,6-
iPr2C6H3NdC(Me)-2-py)I3] (4f). The X-ray crystal struc-
ture of 4f, shown in Figure 6, reveals a distorted-
octahedral geometry with the CO ligand again trans to
the pyridyl nitrogen. The plane of the N-aryl group is
canted ca. 16° from the position perpendicular to the
chelate ring. This minimizes the steric interaction
i
interactions between the Pr methine groups and the
axial iodide ligand, although the solution NMR spec-
trum of 2a suggests that other conformers are acces-
sible.
The 1H NMR spectra of 2d -f all displayed high-
frequency resonances (near δ 10.2) consistent with a
hydrogen in position 6 on the pyridyl ring adjacent to a
cis iodide ligand, as in the precursors 1d -f. Although
the rest of the coordination geometry is not proven by
the data for these complexes, it is likely that CO is trans
to the pyridyl N, with MeI being added in a trans
fashion. For 2e, there was evidence for two conformers
or isomers in solution, each of which displayed a pair
i
between one Pr group and I(2), but nonbonded interac-
i
tions of I(3) with the other Pr group cause a significant
distortion from ideal octahedral geometry. This is
exemplified by the I(3)-Rh-CO and I(2)-Rh-I(3) bond
angles of 96.1(3) and 170.42(4)°, respectively. Similar
steric interactions can explain the relative instability
of the methyl complex 2f, which contains the same
bidentate ligand as 4f.
Kin etic Mea su r em en ts. Kinetic experiments were
carried out using FTIR spectroscopy to monitor the
reactions of 1a -g with MeI in CH2Cl2. Pseudo-first-
order rate constants, measured over a range of MeI
concentrations and temperatures, are given in the
Supporting Information. All the oxidative addition reac-
tions were found to be first order in both [complex] and
[MeI]. Second-order rate constants (k1) and activation
parameters are given in Table 2. The large negative ∆Sq
values are consistent with highly ordered SN2 transition
states for attack by Rh(I) on MeI. The oxidative addition
rate is very sensitive to the steric bulk of the diimine
i
of doublets for the inequivalent methyls of its Pr group.
In the reaction of 1f with MeI in CD2Cl2, 1H NMR
spectroscopy indicated the presence of three isomers of
the methyl intermediate 2f in equilibrium with the
acetyl product 3f, which displayed a signal at δ 3.30 for
the acetyl ligand.
The stable acetyl products 3b,c, which resulted from
1
addition of MeI to 1b,c, both displayed H resonances
1
at δ 3.24 for their acetyl ligands. In each case the H
NMR signals due to the diimine are consistent with Cs
symmetry, with the Rh atom and the acetyl ligand lying
on the mirror plane. The observation of two signals for
pairs of inequivalent o-Me groups in 3b and four signals
for methyls of the o-iPr groups of 3c again indicate
restricted rotation about the N-C(aryl) bonds. The
X-ray crystal structure of 3c is illustrated in Figure 5,
(20) Moloy, K. G.; Petersen, J . L. Organometallics 1995, 14, 2931.
Adams, H.; Bailey, N. A.; Mann, B. E.; Manuel, C. P. Inorg. Chim.
Acta 1992, 198-200, 111. Søtofte, I.; Hjortkjaer, J . Acta Chem. Scand.
1994, 48, 872. Smith, J . M.; Lachicotte, R. J .; Holland, P. L. Organo-
metallics 2002, 21, ASAP.
(19) Tolman, C. A. Chem. Rev. 1977, 77, 313.

Quantifying Steric Effects of R-Diimine Ligands
Organometallics, Vol. 22, No. 5, 2003 1051
Ta ble 2. Kin etic Da ta for Oxid a tive Ad d ition of
MeI to Com p lexes 1a -g
complex ν(CO) 103k1/M-1 s-1 ∆Hq/kJ mol-1 ∆Sq/J mol-1 K-1
1a
1b
1c
1d
1e
1f
1994
1993
1993
1989
1988
1987
1977
12.0
1.02
0.145
140
88.5
8.34
206
35 ( 1
42 ( 1
41 ( 1
25 ( 2
27 ( 1
25 ( 3
18 ( 1
-162 ( 4
-160 ( 2
-182 ( 2
-176 ( 6
-175 ( 4
-201 ( 10
-199 ( 4
1g
ligand, with values of k1 spanning more than 3 orders
of magnitude, as illustrated by a plot of log k1 vs the
number of ortho substituents (Figure 7). Replacing four
o-iPr groups (1c) by four Me groups (1b) leads to a 7-fold
rate enhancement, very similar to the 5-fold acceleration
of C2H4 exchange that occurs for an equivalent change
in ligand structure in [Pd(C2H4)(diimine)Me]+.3 This
reflects the operation of an associative mechanism for
each process with a MeI or C2H4 molecule experiencing
similar steric interactions on approaching an axial
coordination site. Replacement of an o-iPr group on each
aryl by H results in a ca. 80-fold acceleration in 1a
relative to 1c. Complexes 1a ,f, each of which has two
o-iPr groups (either on the same or different aryl
groups), display very similar oxidative addition rates.
Removal of one o-iPr group from 1f to give 1e gives a
further order of magnitude increase in reactivity, whereas
removal of the single o-iPr group of 1e to give 1d results
in a more modest acceleration, by a factor of ca. 1.6. The
highest reactivity of all is shown by the sterically
unhindered bipy complex 1g, for which k1 is the largest
yet reported for a Rh(I) carbonyl complex,21 exceeding
that for the carbonylation catalyst [Rh(CO)2I2]- 22 by a
factor of ca. 7000. The dependence of oxidative addition
rate on ligand steric bulk shown in Figure 7 resembles
the steric profile, proposed by Eriks et al., for reactions
in which the ground state is less congested and more
flexible than the transition state.23 The complexes
1a ,c,e,f show a good linear correlation of log k1 with
ligand size, whereas the relatively small rate enhance-
ment caused by removing the final o-iPr group from 1e
to give 1d represents the onset of a steric threshold.
Our kinetic data show that N-donor ligands impart
very high nucleophilicity on a Rh(I) center (as found in
systems which will activate C-Cl bonds24) but this is
moderated by the presence of bulky ligand substitu-
ents.25 However, even the most hindered complex, 1c,
reacts with MeI ca. 5 times faster than [Rh(CO)2I2]-.
Ligand o-iPr substituents have previously been found
to hinder oxidative addition of organic halides to [Pd(Ar-
F igu r e 7. Effect of ligand steric bulk on rate constant for
the oxidative addition of MeI to complexes 1a -g.
BIAN)Me2] and lower the stability of Pd(IV) complexes
[Pd(Ar-BIAN)Me3I] toward reductive elimination.26
Both the kinetics and thermodynamics of migratory
CO insertion are influenced by ligand steric effects. In
the extreme cases, values of k2 were not measurable,
since the octahedral methyl complex was either stable
(2a ,d ,e,g) or not detectable (2c).27 However, for two
systems possessing intermediate steric bulk, kinetic
data were obtained. During the reaction of 1b with MeI,
the intermediate 2b was detected at relatively low
concentration. Using the method applied previously,6,22
we have estimated k2 for 2b as ]5.7 ((0.2)] × 10-2 s-1
(25 °C) with activation parameters ∆Hq ) 53 ((4) kJ
mol-1 and ∆Sq ) -93 ((7) J mol-1 K-1 28
The rate
.
constant is very similar to that for [Rh(CO)2I3Me]- (5.4
× 10-2 s-1).22
For the reaction of MeI with 1f, which resulted in
equilibria, the time dependences of species 1f, 2f, and
3f were fitted to a two-step reversible kinetic model
(Scheme 2), giving values for each of the forward and
reverse rate constants (listed in the Supporting Infor-
mation). The value of k2 for 2f (5.82 × 10-4 s-1, 25 °C)
is ca. 100 times smaller than that for 2b. Activation
parameters for migratory insertion in 2f are ∆Hq ) 81
((3) kJ mol-1 and ∆Sq ) -37 ((11) J mol-1 K-1
.
Thermodynamic data were also estimated. Oxidative
addition of MeI to 1f is exothermic (∆H ) -41 ((8) kJ
mol-1), but the negative reaction entropy (∆S ) -115
(21) Very high nucleophilicity toward MeI (k ) 2000 M-1 s-1) has
been reported for a square-planar Rh(I) complex containing a macro-
cycle formed from two linked diimines: Collman, J . P.; Brauman, J .
I.; Madonik, A. M. Organometallics 1986, 5, 310.
(22) Haynes, A.; Mann, B. E.; Morris, G. E.; Maitlis, P. M. J . Am.
Chem. Soc. 1993, 115, 4093.
(23) Eriks, K.; Giering, W. P.; Liu, H.-Y. Inorg. Chem. 1989, 28, 1759.
(24) Haarman, H. F.; Ernsting, A. L.; Kranenburg, M.; Kooijman,
H.; Veldman, N.; Spek, A. L.; Elsevier, C. J .; van Leeuwen, P. W. N.
M.; Vrieze, K. Organometallics 1997, 16, 887. Haarman, H. F.;
Kaagman, J .-W.; Smeets, W. J . J .; Spek, A. L.; Vrieze, K. Inorg. Chim.
Acta 1998, 270, 34. Bradd, K. J .; Heaton, B. T.; J acob, C.; Sampanthar,
J . T.; Steiner, A. J . Chem. Soc., Dalton Trans. 1999, 1109. Nishiyama,
H.; Horihata, M.; Hirai, T.; Wakamatsu, S.; Itoh, K. Organometallics
1991, 10.
(25) It was noted recently that Rh(I) complexes of some terdentate
nitrogen ligands add MeI but are stable in chlorinated solvents if the
ligand is sufficiently bulky. Dias, E. L.; Brookhart, M.; White, P. S.
Organometallics 2000, 19, 4995; Chem. Commun. 2001, 423.
(26) van Asselt, R.; Rijnberg, E.; Elsevier, C. J . Organometallics
1994, 13, 706.
(27) Since no methyl intermediate was observed during the reaction
of 1c with MeI, the migratory insertion rate constants for 2c could
not be determined. However, if we assume that to avoid detection, the
steady-state concentration of 2c is less than 1% of [1c], a lower limit
for k2 can be estimated as 10-1 s-1 for 2c (25 °C). This is double that
measured for 2b and an order of magnitude larger than that for
[Rh(CO)2I3Me]-.22
(28) The negative ∆Sq value (-93 ((7) J mol-1 K-1) measured for
migratory insertion in 2b is larger than might be expected for an
intramolecular rearrangement. We note that values of ca. -60 J mol-1
K-1 have been reported previously for related species.6,22 Participation
by a solvent molecule which coordinates weakly to the vacant site
formed during migratory insertion may make some contribution to
these values. However, we cannot rule out the possibility that the
indirect method used to measure k2 for 2b and the relatively narrow
temperature range for the Eyring plot may lead to some inaccuracy in
the extrapolation required to extract ∆Sq.

1052 Organometallics, Vol. 22, No. 5, 2003
Gonsalvi et al.
33
((26) J mol-1 K-1) leads to the observed equilibrium.
Migratory insertion, 2f f 3f, is slightly endothermic
(∆H ) 10 ((8) kJ mol-1, ∆S ) +45 ((27) J mol-1 K-1).
The different reactivities of 2a ,f show that the effect of
two o-iPr substituents depends on how they are distrib-
uted. In 2a the two iPr groups can adopt a conformation
in which both are directed toward the same axial site
(Figure 4) and a stable methyl complex results. In 2f,
however, both axial ligands experience adverse steric
effects (as observed in the X-ray crystal structure of 4f)
and destabilization of the octahedral species results in
the observed equilibria.
Ligand steric effects on migratory CO insertion in this
system are much larger than for alkene insertion in
[Pd(diimine)(R)(C2H4)]+, where ∆Gq varied by less than
7 kJ mol-1 for different diimines.3 This is explained by
the different coordination geometries, since olefin inser-
tion occurs in the equatorial plane for the Pd(II)
systems, with o-alkyl substituents exerting less influ-
ence. Octahedral complexes such as 2b,c experience a
much larger relief of steric strain on isomerizing to the
square-pyramidal acetyl products.
[Rh(CO)2I]2 were synthesized according to literature proce-
dures. 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.
In str u m en ta tion . FTIR spectra were measured using a
Mattson Genesis Series spectrometer, controlled by WIN-
FIRST software running on a Viglen 486 PC. 1H 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 using a Perkin-Elmer
Model 2400 elemental analyzer.
Syn t h esis of R h (I) Com p lexes. (a ) [R h (CO)(Ar Nd
C(Me)C(Me)dNAr )I] (1a ; Ar ) 2-iP r C6H4). [Rh(CO)2I]2
(0.100 g, 0.18 mmol), was dissolved in 5 cm3 of THF. A solution
of ArNdC(Me)C(Me)dNAr (Ar ) 2-iPrC6H4; 0.112 g, 0.39
mmol) in the same amount of THF was then added dropwise
under N2. The dark blue solution obtained was stirred under
N2 for 2 min. The solvent was then removed in vacuo, and the
resulting dark blue solid was dried under vacuum for 24 h;
yield 0.167 g (84%). Anal. Calcd for C23H28IN2ORh: C, 47.8;
H, 4.8; N, 4.9; I, 22.0. Found: C, 47.3; H, 5.0; N, 4.6; I, 22.0.
IR (CH2Cl2; ν(CO)/cm-1): 1994. 1H NMR (CD2Cl2; δ): 7.47-
7.25, 7.01, 6.88 (each m, total 8H, arom), 3.50-3.30 (m, 2H,
MeCHMe), 1.83, 1.82 (each s, total 3H, NdC(Me)C(Me′)dN, 2
conformers, ca. 1:1 ratio), 1.23, 1.22 (each s, total 3H, Nd
C(Me)C(Me′)dN, 2 conformers), 1.43 (d) 1.39 (d), 1.34 (2
overlapping d), 1.18 (d), 1.14 (d), 1.09 (d), 1.06 (d) (each 7 Hz,
total 12H, MeCHMe, 8 doublets for 2 conformers).
Con clu sion s
We have identified a system in which the steric effects
of diimine ligands can be tested and quantified. The N,N
donor set imparts high intrinsic nucleophilicity on the
Rh(I) center, with bipy giving the fastest rate of MeI
addition to a Rh(I) carbonyl complex yet reported.
Replacement of bipy with diimine ligands of increasing
steric bulk moderates the oxidative addition rate over
3 orders of magnitude in a predictable manner. Ligand
steric effects also have a dramatic influence on the
kinetics and thermodynamics of migratory CO insertion,
due to nonbonded interactions of o-alkyl substituents
with axially coordinated ligands. Depending on the
ligand, the octahedral methyl complex either is stable
and isolable or is undetected due to rapid isomerization
to a square-pyramidal acetyl complex.
(b) [Rh (CO)(Ar NdC(Me)C(Me)dNAr )I] (1b; Ar ) 2,4,6-
Me3C6H2). [Rh(CO)2I]2 (0.100 g, 0.17 mmol) was dissolved in
5 cm3 of CH2Cl2 and added to a solution of ArNdC(Me)C(Me)d
NAr)] (Ar ) 2,4,6-Me3C6H2; 0.112 g, 0.34 mmol) in the same
amount of CH2Cl2 under N2. The dark blue solution obtained
was stirred under N2 for 2 h. The solvent was then removed
in vacuo, and the resulting dark blue solid was dried under
vacuum for 24 h; yield 0.165 g (82%). Anal. Calcd for C23H28
-
IN2ORh: C, 47.8; H, 4.8; N, 4.9, I, 22.0. Found: C, 47.3; H,
4.6; N, 4.6; I, 22.5. IR (CH2Cl2; ν(CO)/cm-1): 1993. 1H NMR
(CD2Cl2; δ): 6.92, 6.87 (each s, total 4H, m-H), 2.26 (s, 6H,
p-Me), 2.21, 2.07 (each s, total 6H, o-Me), 1.64 (s, 3H,
NdC(Me)C(Me′)dN), 1.04 (s, 3H, NdC(Me)C(Me′)dN).
(c) [Rh (CO)(Ar NdC(Me)C(Me)dNAr )I] (1c; Ar ) 2,6-
iP r 2C6H3). [Rh(CO)2I]2 (0.100 g, 0.17 mmol) was dissolved in
5 cm3 of CH2Cl2 and added to a solution of ArNdC(Me)C(Me)d
NAr (Ar ) 2,6-iPr2C6H3; 0.151 g, 0.34 mmol) in the same
amount of CH2Cl2. The dark blue solution obtained was stirred
under nitrogen for 2.5 h. The solvent was then removed in
vacuo, and the resulting dark blue solid was dried under
Exp er im en ta l Section
Ma ter ia ls. All solvents used for synthesis or kinetic experi-
ments were distilled and degassed prior to use following
literature procedures.29 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 regener-
ated. Carbon monoxide was also passed through a short
column of activated charcoal to remove any iron pentacarbonyl
impurity.30 The ligands ArNdC(Me)C(Me)dNAr and ArNd
C(Me)-2-py (Ar ) Ph, 2-iPrC6H4, 2,6-iPr2C6H3) were prepared
by condensation of the appropriate aniline derivative with 2,3-
butanedione or 2-acetylpyridine (Aldrich) using established
procedures.31 Comparison with published spectroscopic data
established the identity and purity of the pyridyl imines.10,12
The ligand 2,2-bipyridyl was purchased from Aldrich and used
without further purification. The complexes [Rh(CO)2Cl]232 and
vacuum for 24 h; yield 0.194 g (80%). Anal. Calcd for C29H40
-
IN2ORh: C, 52.6; H, 6.0; N, 4.2; I, 19.1. Found: C, 53.1; H,
6.2; N, 4.3; I, 18.6. IR (CH2Cl2; ν(CO)/cm-1): 1993. 1H NMR
(CD2Cl2; δ): 7.31-7.13 (m, 6H, arom.), 3.16, 3.02 (overlapping
septets, 7 Hz, each 1H, MeCHMe), 1.73 (s, 3H, NdC(Me)C-
(Me′)dN), 1.10 (s, 3H, NdC(Me)C(Me′)dN), 1.35, 1.12, 1.27,
1.07 (each d, 7 Hz, 6H, MeCHMe).
(d ) [Rh (CO)(P h NdC(Me)-2-p y)I] (1d ). [Rh(CO)2I]2 (0.150
g, 0.27 mmol) was dissolved in 10 cm3 of THF. A solution of
PhNdC(Me)-2-py (0.114 g, 0.58 mmol) in 10 cm3 of THF was
added dropwise by cannula under N2. The solution became
dark purple, and the mixture was stirred under N2 for 2 h.
The solvent was removed in vacuo and the resulting dark
purple solid dissolved in CH2Cl2 (20 cm3) and stirred for a
further 2 h to complete the reaction. The solvent was then
removed in vacuo, and the product was recrystallized by
dissolution in the minimum amount of THF (4 cm3) and
precipitation by addition of pentane (50 cm3). The solvent was
(29) Perrin, D. D.; Armerego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 3rd ed.; Pergamon Press: Oxford, U.K., 1988.
(30) Haynes, A.; Ellis, P. R.; Byers, P. K.; Maitlis, P. M. Chem. Br.
1992, 28, 517.
(31) Weidenbruch, M.; Piel, H.; Lesch, A.; Peters, K.; Vonschnering,
H. J . Organomet. Chem. 1993, 454, 35. Ferguson, L. N.; Goodwin, T.
C. J . Am. Chem. Soc. 1949, 71, 633. Svoboda, M.; tom Dieck, H. J .
Organomet. Chem. 1980, 191, 321. tom Dieck, H.; Svoboda, M.; Greiser,
T. Z. Naturforsch., B 1981, 36b, 823.
(32) McCleverty, J .; Wilkinson, G. Inorg. Synth. 1966, 8, 214.
(33) Fulford, A.; Hickey, C. E.; Maitlis, P. M. J . Organomet. Chem.
1990, 398, 311.

Quantifying Steric Effects of R-Diimine Ligands
Organometallics, Vol. 22, No. 5, 2003 1053
removed by cannula and the resulting solid dried under
(5 cm3) to which MeI (0.5 cm3, excess) was added under N2.
The reaction mixture was stirred at room temperature for 3
h, during which time the color turned from dark blue to red-
purple. The solvent was removed in vacuo, and the product
was washed with diethyl ether and vacuum-dried; yield 0.040
g (64%). Anal. Calcd for C24H31I2N2ORh: C, 39.7; H, 4.3; N,
3.6; I, 35.6. Found: C, 40.0; H, 4.3; N, 3.9; I, 35.3. IR (CH2Cl2;
ν(CO)/cm-1): 1721. 1H NMR (CD2Cl2; δ): 6.92, 6.88 (each s,
total 4H, m-H), 3.24 (s, 3H, COMe), 2.26 (s, 6H, p-Me), 1.96,
(s, 6H, NdC(Me)C(Me)dN), 1.92, 1.86 (each s, total 6H, o-Me).
(c) [Rh (Ar NdC(Me)C(Me)dNAr )I2(COMe)] (3c; Ar )
2,6-iP r 2C6H3). [Rh(CO)(ArNdC(Me)C(Me)dNAr)I] (1c; Ar )
2,6-iPr2C6H3; 0.050 g, 0.09 mmol) was dissolved in CH2Cl2 (5
cm3) to which MeI (0.5 cm3, excess) was added under N2. The
reaction mixture was stirred at room temperature for 24 h,
during which time the color turned from dark blue to deep
red. The solvent was removed in vacuo, and the product was
then recrystallized from CH2Cl2 and dried under high vacuum;
yield 0.039 g (56%). Anal. Calcd for C30H43I2N2ORh‚CH2Cl2:
C, 44.0; H, 5.3; N, 3.3; I, 30.0. Found: C, 43.8; H, 5.2; N, 3.2;
vacuum for 24 h; yield 0.183 g (75%). Anal. Calcd for (C14H12
-
IN2ORh): C, 37.03; H, 2.66; N, 6.17. Found: C, 37.42; H, 2.90;
1
N, 6.28. IR (CH2Cl2; ν(CO)/cm-1): 1989. H ΝΜR (CD2Cl2; δ):
9.73 (bd, 1H), 7.97 (m, 1H), 7.78 (m, 1H), 7.65 (m, 1H), 7.41
(m, 2H), 7.25 (m, 1H), 7.12 (m, 2H) (arom), 1.78 (s, 3H,
C(Me)dN).
(e) [Rh (CO)(Ar NdC(Me)-2-p y)I] (1e; Ar ) 2-iP r C6H4).
[Rh(CO)2I]2 (0.100 g, 0.18 mmol) was dissolved in 15 cm3 of
THF. A solution of ArNdC(Me)-2-py (Ar ) 2-iPrC6H4; 0.093
g, 0.39 mmol) in 10 cm3 of THF was added dropwise by cannula
under N2. The solution became dark purple, and the mixture
was stirred for 30 min. The solvent was then removed in vacuo,
and the resulting dark purple solid was recrystallized by
dissolution in the minimum amount of THF (3 cm3) and
precipitation by addition of pentane (50 cm3). The solvent was
then removed by cannula and the resulting solid dried under
vacuum for 24 h; yield 0.145 g (84%). Anal. Calcd for C17H18
-
IN2ORh: C, 41.15; H, 3.66; N, 5.65. Found: C, 41.00; H, 3.66;
1
N, 5.38. IR (CH2Cl2; ν(CO)/cm-1): 1988. H ΝΜR (CD2Cl2; δ):
1
9.82 (m, 1H), 8.04 (m, 1H), 7.83 (m, 1H), 7.74 (m, 1H), 7.42
(bm, 1H), 7.30 (m, 2H), 6.99 (m, 1H) (arom), 3.47 (m, 1H,
Me2CH), 1.79 (s, 3H, C(Me)dN), 1.39, 1.12 (each d, 7 Hz, 3H,
Me2CH).
I, 29.7. IR (CH2Cl2; ν(CO)/cm-1): 1713. H NMR (CD2Cl2; δ):
7.31-7.13 (m, 6H, arom), 3.24 (s, 3H, COMe), 2.60, 2.40 (each
sept, 7 Hz, 1H, MeCHMe), 2.08 (s, 6H, NdC(Me)C(Me′)dN),
1.25, 1.16, 1.12, 0.94 (each d, 7 Hz, 6H, MeCHMe). A single
crystal of 3c suitable for X-ray diffraction (vide infra) was
obtained by recrystallization from dichloromethane.
(d ) [Rh (CO)(P h NdC(Me)-2-p y)I2Me] (2d ). In an in situ
NMR experiment, [Rh(CO)(PhNdC(Me)-2-py)I] (1d ; 16 mg)
was dissolved in CD2Cl2 (0.7 cm3) in an NMR tube, to which
was added methyl iodide (10-fold excess/1d ) at room temper-
ature. The initial 1H NMR spectrum revealed a doublet at δ
1.57 (2J Rh-H ) 2.1 Hz, 3H) for the methyl ligand of the
oxidative addition product 2d . A characteristic signal at δ
10.25 (m, 1H) is assigned to the hydrogen at position 6 on the
pyridyl ring. A weak singlet was also observed at δ 3.26,
corresponding to a small amount of the acetyl product 3d . IR
(CH2Cl2; ν(CO)/cm-1): 2078 (2d ).
(f) [Rh (CO)(Ar NdC(Me)-2-p y)I] (1f; Ar ) 2,6-iP r 2C6H3).
[Rh(CO)2I]2 (0.100 g, 0.18 mmol) was dissolved in 10 cm3 of
pentane. A solution of ArNdC(Me)-2-py (Ar ) 2,6-iPr2C6H3;
0.109 g, 0.39 mmol) in the same amount of pentane was added
dropwise by cannula under N2. A dark purple precipitate was
observed, and the mixture was stirred for 30 min. The solvent
was then removed in vacuo, and the resulting dark purple solid
was dried under vacuum for 24 h; yield 0.155 g (80%). Anal.
Calcd for C20H24IN2ORh: C, 44.6; H, 4.5; N, 5.2. Found: C,
1
45.0; H, 4.6; N, 5.2. IR (CH2Cl2; ν(CO)/cm-1): 1987. H ΝΜR
(CDCl3; δ): 9.97 (m, 1H), 8.03 (m, 1H), 7.82-7.74 (m, 2H),
7.22-7.35 (bm, 3H) (arom), 3.32 (m, 2H, Me2CH), 1.73 (s, 3H,
C(Me)dN), 1.38, 1.12 (each d, 7 Hz, 6H, Me2CH). A single
crystal of 1f suitable for X-ray diffraction (vide infra) was
obtained by recrystallization from dichloromethane.
(e) [Rh (CO)(Ar NdC(Me)-2-py)I2Me] (2e; Ar ) 2-iP r C6H4).
[Rh(CO)(ArNdC(Me)-2-py)I] (1e; Ar ) 2-iPrC6H4; 0.040 g, 0.08
mmol) and MeI (75 µL, 1.2 mmol, excess) were dissolved in
THF (10 cm3), and the solution was stirred under N2 for 30
min. The solvent was then removed in vacuo, and the resulting
orange solid was recrystallized by dissolution in the minimum
amount of THF (2.25 cm3) and addition of pentane (30 cm3).
The solvent was then removed by cannula and the resulting
solid dried in vacuo for 24 h; yield 0.036 g (70%). Anal. Calcd
for C18H21I2N2ORh: C, 33.88; H, 3.32; N, 4.39. Found: C, 33.74;
H, 3.34; N, 4.38. IR (CH2Cl2; ν(CO)/cm-1): 2076. 1H NMR
(CD2Cl2; δ): 10.21-10.25 (bm, 1H), 8.39-7.26 (7H) (arom) 3.66
(major), 2.67 (minor) (each sept, 7 Hz, total 1H, MeCHMe, two
conformers/isomers, ratio ca. 3:1), 2.37 (major), 2.33 (minor)
(each s, total 3H, MeCdN), 1.33, 1.13 (major), 1.25, 1.20
(minor) (each d, 7 Hz, total 6H, MeCHMe), 1.70 (major), 1.60
(g) [Rh (CO)(2,2′-bip y)I] (1g). [Rh(CO)2I]2 (0.100 g, 0.18
mmol) was dissolved in 50 cm3 of THF. A solution of 2,2′-
bipyridyl (0.061 g, 0.39 mmol) in 50 cm3 of THF was added
dropwise by cannula under N2. The solution became deep red,
and the mixture was stirred for 10 min. The solvent was then
removed in vacuo, and the resulting deep red solid was dried
under vacuum for 24 h; yield 0.104 g (70%). IR (CH2Cl2; ν(CO)/
1
cm-1): 1977. H ΝΜR (CDCl3; δ): 9.83 (m, 1H), 8.84 (m, 1H),
7.95-8.16 (m, 4H), 7.54 (m, 1H), 7.39 (m, 1H).
Syn t h esis of R h (III) Com p lexes. (a ) [R h (CO)(Ar Nd
C(Me)-C(Me)dNAr )I2Me] (2a ; Ar ) 2-iP r C6H4). [Rh(CO)-
(ArNdC(Me)C(Me)dNAr)I] (1a ; Ar ) 2-iPrC6H4; 0.100 g, 0.18
mmol) was dissolved in CH2Cl2 (5 cm3) to which MeI (0.5 cm3,
excess) was added under N2. The reaction mixture was stirred
at room temperature for 1 h, during which time the color
turned from dark blue to bright red. The solvent was removed
in vacuo, and the product was dried under high vacuum; yield
0.090 g (72%). Anal. Calcd for C24H31I2N2ORh: C, 40.0; H, 4.3;
N, 3.9; I, 35.3. Found: C, 39.9; H, 4.5; N, 3.7; I, 34.9. IR
2
(minor) (each d, J Rh-H ) 2.1 Hz, total 3H, MeRh).
(f) [Rh (CO)(Ar NdC(Me)-2-p y)I2Me] (2f; Ar
) 2,6-
iP r 2C6H3). In an in situ NMR experiment, [Rh(CO)(ArNd
C(Me)-2-py)I] (1f; Ar ) 2,6-iPr2C6H3; 16 mg) was dissolved in
CD2Cl2 (0.7 cm3) in an NMR tube, to which was added methyl
1
1
(CH2Cl2; ν(CO)/cm-1): 2077. H NMR (CD2Cl2; δ): 8.55, 8.24,
iodide (10-fold excess/1f) at room temperature. The initial H
7.57-7.28, 7.04, 6.78 (each m, total 8H, arom), 3.82, 3.51, 2.61
(each m, total 2H, MeCHMe), 2.27, 2.26, 2.24, 2.23, 2.19, 2.12
(each s, total 6H, NdC(Me)C(Me′)dN, mixture of conformers/
isomers), 1.75, 1.73, 1.65, 1.63 (each d, 2J RhH ) 2 Hz, total 3H,
MeRh, 4 isomers/conformers, ratio ca. 4:2:2:1), 1.42, 1.38, 1.36,
1.33, 1.29, 1.24, 1.18, 1.14, 1.08 (each d, J ) 6-7 Hz, total
12H, MeCHMe, mixture of conformers/isomers). A single
crystal of 2a suitable for X-ray diffraction (vide infra) was
obtained by recrystallization from chloroform.
NMR spectrum revealed doublets at δ 1.77, 1.58, and 1.49
(each with J Rh-H ) 2.1 Hz) in the ratio ca. 5:5:1 assigned to
2
methyl ligands of isomers of 2f. A singlet at δ 3.30 is assigned
to the acetyl product 3f in equilibrium with the methyl species.
Integration of the signals indicates a methyl to acetyl ratio of
ca. 1:3 at equilibrium. A characteristic signal at δ 10.24 (m) is
assigned to the hydrogen at position 6 on the pyridyl ring of
3f. IR (CH2Cl2; ν(CO)/cm-1): 2073 (2f) 1716 (3f).
(g) [Rh (CO)(2,2′-bip y)I2Me] (2g). [Rh(CO)(2,2′-bipy)I] (1g;
16 mg) was dissolved in CD2Cl2 (0.7 cm3) in an NMR tube, to
which was added methyl iodide (10-fold excess/1g) at room
temperature. The initial 1H NMR spectrum revealed doublets
(b) [Rh (Ar NdC(Me)C(Me)dNAr )I2(COMe)] (3b; Ar )
2,4,6-Me3C6H2). [Rh(CO)(ArNdC(Me)-C(Me)dNAr)I] (1b; Ar
) 2,4,6-Me3C6H2; 0.050 g, 0.09 mmol) was dissolved in CH2Cl2

1054 Organometallics, Vol. 22, No. 5, 2003
Ta ble 3. Su m m a r y of Cr ysta llogr a p h ic Da ta for Com p lexes 1f, 2a , 3c, a n d 4f
Gonsalvi et al.
1f‚CH2Cl2
2a ‚CHCl3
3c‚CH2Cl2
4f
formula
fw
cryst syst
space group
color
a (Å)
b (Å)
c (Å)
C
21H26Cl2IN2ORh
C25H32Cl3I2N2ORh
C31H45Cl2I2N2ORh
C20H24I3N2ORh
792.02
monoclinic
P21/c
623.15
orthorhombic
Pbca
839.59
orthorhombic
Pna21
red
15.4058(9)
10.6809(6)
18.1516(11)
90
90
90
150(2)
4
889.30
orthorhombic
Pnma
brown
18.249(4)
18.634(4)
10.630(2)
90
90
90
293(2)
4
black
black
16.0948(11)
14.5096(10)
20.7994(15)
90
90
90
10.793(4)
13.266(4)
16.881(5)
90
100.524(7)
90
R (deg)
â (deg)
γ (deg)
temp (K)
150(2)
8
150(2)
4
Z
Flack param
0.28(9)
final R indices (I > 2σ(I))
R1
0.0338
0.0653
0.0573
0.1498
0.0555
0.1351
0.0502
0.0874
wR2
R indices (all data)
R1
wR2
GOF
0.0677
0.0737
0.922
0.0737
0.1610
1.038
0.0758
0.1478
1.052
0.1427
0.1097
0.827
at δ 1.41 (2J Rh-H ) 2.1 Hz) and δ 1.96 (2J Rh-H ) 1.8 Hz) in the
ratio ca. 20:1 assigned to methyl ligands of isomers of 2g. A
characteristic signal at δ 10.18 (m) is assigned to the hydrogen
at position 6 on a pyridyl ring of 2g. On prolonged reaction
(17 h) a weak singlet was observed at δ 3.46 corresponding to
a small amount of the acetyl product 3g. The doublet at δ 1.96
had grown relative to that at δ 1.41, indicating slow isomer-
ization of 2g. IR (CH2Cl2; ν(CO)/cm-1): 2071 (2g).
up to the mark with the solvent of choice (usually CH2Cl2). A
portion of this solution was used to record a background
spectrum. Another portion (typically 500 µL) was added to the
solid metal complex (typically 7-8 µmol) in a sample vial to
give a reaction solution with a complex concentration of ca.
15 mM. A portion of the reaction solution was quickly
transferred to the IR cell, and the kinetic experiment was
started. To obtain pseudo-first-order conditions, at least a 10-
fold excess of MeI was used, relative to the metal complex.
The IR cell (0.5 mm path length, CaF2 windows) was main-
tained 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 vs 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 -e,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 given being averaged values with component
measurements deviating from each other by e5%. For the
reaction of 1f with MeI, the absorbance vs time data for bands
of 1f, 2f, and 3f were fitted to a two-step reversible kinetic
model (Scheme 2) using a Microsoft Excel spreadsheet.
(h ) [Rh (CO)(Ar NdC(Me)-2-p y)I3] (4f; Ar ) 2,6-iP r 2C6H3).
[Rh(CO)(ArNdC(Me)-2-py)I] (1f; 0.030 g, 0.06 mmol) was
dissolved in THF (10 cm3) under N2. On addition of iodine
(0.014 g, 0.06 mmol of I2) the solution rapidly changed color
from dark purple to brown. After the mixture was stirred for
30 min, the solvent was removed in vacuo and the resulting
black solid was recrystallized by dissolution in THF (2.5 cm3)
and dropwise addition of pentane (25 cm3) to precipitate the
product; yield 33 mg (76%). Anal. Calcd for C20H24I3N2ORh:
C, 30.3; H, 3.1; N, 3.5. Found: C, 31.0; H, 3.1; N, 3.3. IR
(CH2Cl2; ν(CO)/cm-1): 2092. 1H ΝΜR (CD2Cl2; δ): 10.27 (m,
1H), 8.26 (m, 1H), 8.17 (m, 1H), 7.85 (m, 1H), 7.45-7.35 (bm,
3H) (arom), 3.60 (m, 2H, Me2CH), 2.55 (s, 3H, C(Me)dN), 1.36,
1.05 (each d, 7 Hz, 6H, Me2CH). A single crystal of 4f suitable
for X-ray diffraction (vide infra) was obtained by recrystalli-
zation from dichloromethane.
X-r a y Str u ctu r e Deter m in a tion s. Data were collected on
either a Bruker Smart CCD area detector with Oxford Cryo-
stream 600 low-temperature system (complexes 1f, 2a , and
4f) or a Siemens P4 diffractometer (complex 3c), in each case
using Mo KR radiation (λh ) 0.710 73 Å). The structures were
solved by direct methods and refined by full-matrix least-
squares methods on F2. Hydrogen atoms were placed geo-
metrically and refined using a riding model (including torsional
freedom for methyl groups). Complex scattering factors were
taken from the SHELXTL program package.34 Crystallo-
graphic data are summarized in Table 3, and full listings of
data are given in the Supporting Information.
Ack n ow led gm en t. This work was supported by BP
Chemicals Ltd. (studentships to L.G. and J .A.G.), the
EPSRC, and the University of Sheffield.
Su p p or tin g In for m a tion Ava ila ble: Tables of kinetic
data and figures giving ORTEP plots; crystallographic data
are also available in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM020777W
Kin etic Exp er im en ts. Samples for kinetic runs were
prepared by placing the required amount of freshly distilled
methyl iodide in a 5 cm3 graduated flask which was then made
(34) Sheldrick, G. M. SHELXTL, An Integrated System for Solving
and Refining Crystal Structures from Diffraction Data (Revision 5.1);
Bruker AXS Ltd., Madison, WI.