Rhodium complexes with pincer diphosphite ligands. unusual olefin in-plane coordination in square-planar compounds

Article abstract of DOI:10.1021/om800830w

A family of rhodium complexes bearing pincer diphosphite (PCP) and neutral L (L = PPh₃, CO, CNXy, C₂H₄) ligands has been prepared and characterized. Reactions between Rh(Cl)(PPh₃) ₃ and the diphosphit

Full text of DOI:10.1021/om800830w

Organometallics 2009, 28, 547–560
547
Rhodium Complexes with Pincer Diphosphite Ligands. Unusual
Olefin in-Plane Coordination in Square-Planar Compounds†
‡
Miguel Rubio,‡ Andre´s Sua´rez,‡ Diego del Río,‡,§ Agust´ın Galindo,*, Eleuterio Alvarez,
´
and Antonio Pizzano*,‡
Instituto de InVestigaciones Qu´ımicas, Consejo Superior de InVestigaciones Cient´ıficas, and UniVersidad
de SeVilla, AVda Ame´rico Vespucio no. 49, Isla de la Cartuja, 41092 SeVilla, Spain, and Departamento de
Qu´ımica Inorga´nica, UniVersidad de SeVilla, Aptdo 1203, 41071 SeVilla, Spain
ReceiVed August 26, 2008
A family of rhodium complexes bearing pincer diphosphite (PCP) and neutral L (L ) PPh3, CO,
CNXy, C2H4) ligands has been prepared and characterized. Reactions between Rh(Cl)(PPh3)3 and the
diphosphites 2 lead to the chloro hydrides Rh(H)(Cl)(PCP)(PPh3) (4), which can be deprotonated to
yield the complexes Rh(PCP)(PPh3) (5). In the latter, the PPh3 ligand is labile and can be exchanged by
CO, CNXy, and C2H4 to give derivatives 6-8. Most noteworthy is the fact that ethylene complexes 8a,b
show a remarkable in-plane (ip) conformation in the solid state. Solution NMR studies for these complexes
show fast olefin rotation in all the temperature ranges, while solution and solid-state 13C{1H} NMR
experiments give practically superimposable spectra, also indicating the prevalence of the ip conformer
in solution. Complementary detailed DFT calculations performed with models of compounds 8 indicate
that the olefin conformational preference is due to a dual combination of steric effects arising from the
reduction of the P-Rh-P angle from 180°, caused by pincer ligand chelation and by the nature of the
phosphite substituents, which in the case of t-Bu groups perfectly outline a cavity for an ip coordination
of the olefin.
u orientation.5 This steric preference is marked indeed, and a
perusal of the structures of square-planar olefin complexes
described in the literature indicates that the u conformation has
almost exclusively been observed. However, two cases can be
considered exceptions to this general behavior. The first regards
Pt and Pd cationic complexes of the formulation [M(alkene)-
(allyl)(phosphine)]+ (A). These complexes display different
olefin orientations depending on the nature of this ligand,
including several cases of ip conformation. Due to the small
bite angle of the allyl ligand, the definition of their structure is
not unambiguous and they have either been assigned as square-
planar6 or planar-trigonal complexes,7 which in turn have a clear
preference for an in-plane conformation.4 The second pertinent
case is constituted by the complexes of the methylene cyclo-
heptene diolefin (B), which due to the perpendicularity between
the two olefinic bonds forces an ip conformation of one of the
alkene fragments.8 In view of these precedents, there is not a
clear-cut case of a square-planar complex in which, the two
conformations being accessible, ip is preferred.
Introduction
The orientation of a coordinated olefin is an aspect of
fundamental interest in organometallic chemistry, and it is of
great importance in relation to the stereochemistry of reactions
involving alkene transformation.1 The preferred conformation
for a metal-bonded olefin is explained by the Chatt-Dewar-
Duncanson model2,3 and, more specifically, by the existence of
a back-bonding component from a metal-centered orbital to the
π* olefin orbital. As a result, the orientation of the olefin depends
on the geometry of the complex.4 For the important class of
square-planar compounds, in-plane (ip) and upright (u) olefin
orientations are electronically of similar energy. However, steric
effects between the olefin and cis coligands Lc are lower in the
* To whom correspondence should be addressed. E-mail: galindo@us.es
(A.G.); pizzano@iiq.csic.es (A.P.).
† This article is dedicated to Prof. Ernesto Carmona on the occasion of
his 60th birthday.
‡ Instituto de Investigaciones Qu´ımicas.
§ Present address: SRI International, 333 Ravenswood Avenue, Menlo
Park, CA 94065.
Universidad de Sevilla.
(1) For some representative studies of olefin reactivity considering
orientation, see: (a) Musaev, D. G.; Svensson, M.; Morokuma, K.;
Stro¨mberg, S.; Zetterberg, K.; Siegbahn, P. E. M. Organometallics 1997,
16, 1933. (b) Gridnev, I. D.; Yasutake, M.; Higashi, N.; Imamoto, T. J. Am.
Chem. Soc. 2001, 123, 5268. (c) Carbo´, J. J.; Lledo´s, A.; Vogt, D.; Bo, C.
Chem. Eur. J. 2006, 12, 1457. (d) Adlhart, C.; Chen, P. J. Am. Chem. Soc.
2004, 126, 3496. (e) Sakaki, S.; Mizoe, N.; Sugimoto, M.; Musashi, Y.
Coord. Chem. ReV. 1999, 190, 933.
(5) (a) Hay, P. J. J. Am. Chem. Soc. 1981, 103, 1390. (b) Ziegler, T.;
Rauk, A. Inorg. Chem. 1979, 18, 1558. (c) Senn, H. M.; Blo¨chl, P. E.;
Togni, A. J. Am. Chem. Soc. 2000, 122, 4098.
(6) (a) Miki, K.; Kai, Y.; Kasai, N.; Kurosawa, H. J. Am. Chem. Soc.
1983, 105, 2482. (b) Kurosawa, H.; Miki, K.; Kasai, N.; Ikeda, I.
Organometallics 1991, 10, 1607.
(7) Musco, A.; Pontellini, R.; Grassi, M.; Sironi, A.; Meille, S. V.;
Ru¨egger, H.; Ammann, C.; Pregosin, P. S. Organometallics 1988, 7, 2130.
(8) Rakowsky, M. H.; Woolcock, J. C.; Wright, L. L.; Green, D. B.;
Rettig, M. F.; Wing, R. M. Organometallics 1987, 6, 1211.
(2) (a) Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 2939. (b) Dewar,
M. J. S. Bull. Chem. Soc. Chim. Fr. 1951, C71.
(3) Recent studies of the metal-olefin bond: (a) Frenking, G.; Fro¨hlich,
N. Chem. ReV. 2000, 100, 717. (b) Price, D. W.; Drew, M. G. B.; Hii,
K. K.; Brown, J. M. Chem. Eur. J. 2000, 6, 4587. (c) Tsipis, A. C.
Organometallics 2008, 27, 3701.
(4) Albright, T. A.; Hoffmann, R.; Thibeault, J. C.; Thorn, D. L. J. Am.
Chem. Soc. 1979, 101, 3801.
10.1021/om800830w CCC: $40.75
2009 American Chemical Society
Publication on Web 12/24/2008

548 Organometallics, Vol. 28, No. 2, 2009
Rubio et al.
Scheme 1
On the other hand, pincer ligands are also of great interest,
due to their marked ability to stabilize transition-metal com-
plexes.9 Among them, those possessing two donor phosphine
fragments constitute a prominent group. Otherwise, pincer
accepting ligands have largely remained unexplored and only
recently have attracted the attention of researchers. Thus, new
ligands based on fluorophosphines,10 N-pyrrolyl phosphines,11
and phosphites12 have recently been described.
In this contribution we describe a group of novel diphosphites
and their incorporation into rhodium complexes, these being
the first examples of rhodium-pincer diphosphite complexes.
These pincer ligands outline a spatial distribution of steric
hindrance which differs from that exhibited by a fragment ML3,
which makes possible a preferred ethylene in-plane coordination
in square-planar complexes. These olefinic derivatives have been
studied in detail by both structural and theoretical methods. The
results have been completed with the synthesis and characteriza-
tion of isocyanide and carbonyl derivatives which provide
information about the acceptor character of these pincer ligands.
Part of this work has been communicated in a preliminary
form.13
Scheme 2
Results and Discussion
Ligand Synthesis. Diphosphites 2 have been prepared by a
condensation between resorcinol or 1,3-naphthalenediol and the
appropriate chlorophosphite 1 in the presence of a base in good
yields (Scheme 1). NMR spectroscopic characterization reflects
the differences in symmetry and conformational flexibility
among compounds 2. Thus, 2a exhibits in the 31P{1H} NMR
spectrum one singlet in the typical region of phosphites, while
in the 1H and 13C{1H} experiments we observe the equivalence
of the halves of each biphenyl, as well as the other phosphite
fragments. This behavior can be explained by a fast intercon-
version between rac and meso isomers (Scheme 2). Otherwise,
the presence of the naphthyl backbone in compound 2c renders
two phosphite fragments inequivalent, as shown by two singlets
in the 31P{1H} NMR spectrum. Moreover, rapid isomerization
makes two aryl fragments of each biaryl equivalent in the
Scheme 3
1
13C{1H} and H NMR experiments. Finally, the atropisomeric
diphosphite 2b exhibits spectra which account for its C2
symmetry. Therefore, the two 31P nuclei are equivalent, but two
types of biphenyl aryl rings appear in the 1H and 13C{1H} NMR
spectra, those of each inequivalent biphenyl.
Preparation and Characterization of PPh3 Derivatives. In
order to incorporate diphosphites 2 as pincer ligands in Rh
complexes, the reaction between 2a and RhCl(PPh3)3 has been
studied (Scheme 3). Interaction at room temperature, monitored
by 31P{1H} NMR, indicates the formation of the species 3a
characterized by three signals: a singlet at δ 139.9 indicative
of an uncoordinated phosphite, a doublet of doublets of doublets
(δ 128.4, JRhP ) 336 Hz, JPP ) 47, 41 Hz) in the coordinated
phosphite region, and finally, an AB system doubly split by
phosphorus and rhodium couplings centered at 36.0 ppm (JAB
) 356 Hz, JRhP ) 138 Hz), due to two inequivalent PPh3 groups.
Subsequent heating of the solution produces the activation of
the central C-H bond and the displacement of another molecule
of PPh3, thus generating chloro hydride 4a, which incorporates
(9) van der Boom, M. E.; Milstein, D. Chem. ReV. 2003, 103, 1759.
(10) Chase, P. A.; Gagliardo, M.; Lutz, M.; Spek, A. L.; van Klink,
G. P. M.; van Koten, G. Organometallics 2005, 24, 2016.
(11) Kossoy, E.; Iron, M. A.; Rybtchinski, B.; Ben-David, Y.; Shimon,
L. J. W.; Konstantinovski, L.; Martin, J. M. L.; Milstein, D. Chem. Eur. J.
2005, 11, 2319.
(12) (a) Barber, R. A.; Bedford, R. B.; Betham, M.; Blake, M. E.; Coles,
S. J.; Haddow, M. F.; Hursthouse, M. B.; Orpen, A. G.; Pilarski, L. T.;
Pringle, P. G.; Wingad, R. L. Chem. Commun. 2006, 3880. (b) Miyazaki,
F.; Yamaguchi, K.; Shibasaki, M. Tetrahedron Lett. 1999, 40, 7379.
´
(13) Rubio, M.; Sua´rez, A.; del R´ıo, D.; Galindo, A.; Alvarez, E.;
Pizzano, A. Dalton Trans. 2007, 407.

Rh Complexes with Pincer Diphosphite Ligands
Organometallics, Vol. 28, No. 2, 2009 549
Scheme 4
Scheme 5
a pincer diphosphite ligand (PCPa).14 The presence of a hydride
ligand in the latter complex is characterized in the IR spectrum
An interesting feature of these pincer-type diphosphite ligands
is its acceptor character due to the π-acidity of the phosphite
groups. This characteristic has been investigated by IR spec-
troscopy on complexes 6a and 7a, as the magnitudes of ν(CO)
1
by a band at 2112 cm-1 due to ν(Rh-H) and in the H NMR
experiment by a doublet of quartets at δ -14.8. In the latter,
coincident couplings of 12 Hz are observed with the three 31P
and the 103Rh nuclei. These values are in good accord with the
hydride coordinated in a cis position with respect to the three
phosphorus ligands. The 31P{1H} NMR spectrum is character-
ized by a doublet of doublets for the two diphosphite 31P nuclei
at 149.9 ppm (JRhP ) 178 Hz, JPP ) 35 Hz) and a doublet of
triplets at 13.7 ppm (JRhP ) 87 Hz) for a coordinated PPh3.
The value of JRhP is similar to those found in compounds with
chlorideandhydrideligandsoccupyingmutuallytranspositions.15,16
This arrangement has been definitively ascertained by an X-ray
diffraction study (see the Supporting Information).
and ν(CN) have diagnostic value. Thus, the carbonyl gives the
18
corresponding band at 2017 cm-1
,
considerably shifted to
higher energy from that of analogous derivatives of diphos-
19
phinite (1962 cm-1
)
or aryldiphosphine (1955 cm-1) pincer
complexes.20 The isonitrile derivative reinforces this observa-
tion,21 as the band for ν(CN) appears at 2099 cm-1, only 15
cm-1 lower than in the free isonitrile and considerably higher
than the value of 2048 cm-1 observed in trans-RhCl(CNXy)(P-
Pri3)2.22
The ethylene complex Rh(PCPa)(C2H4) (8a) has also been
prepared according to Scheme 4, but this transformation is slow
compared with those of CO or CNXy and requires heating of
the reaction mixture to 40 °C for 4 days under 4 atm of ethylene
to get a good yield. Under similar conditions the derivatives
Rh(PCPb)(C2H4) (8b) and Rh(PCPc)(C2H4) (8c) have also been
obtained.23 Analytical and spectroscopical data for complexes
8 are in good accord with the proposed formulation. Thus, the
NMR experiments show, in addition to the signals of the PCP
ligand, the pertinent resonances for a coordinated ethylene. For
instance, in the 13C{1H} NMR spectrum a relatively broad signal
at ca. 60 ppm due to the olefinic carbons is observed, while the
1H NMR spectra show the corresponding signals for ethylenic
protons. For compounds 8a,c, which possess conformationally
flexible phosphite fragments, the four protons appear as a broad
signal at 2.8 ppm, while for atropisomeric 8b the olefinic protons
appear as two doublets, with an intensity of two protons for
each signal, at 2.3 and 3.0 ppm. These observations account
for the fluxional behavior of 8a,c, which will be discussed
below. Moreover, ethylene complexes are capable of exchanging
this olefin with propylene, by stirring under an atmosphere of
this alkene, to yield complexes 9 (Scheme 5). NMR data for
these derivatives show only one group of signals, indicating a
fluxional behavior (see below). In addition to the resonances
for the PCP fragment, NMR spectra display the expected signals
Heating the solution of compound 3a also produces a small
amount of complex 5a, which results from the loss of HCl from
chloro hydride 4a. Subsequent addition of an excess of NEt3
facilitates this step and results in the isolation of 5a in good
yield. NMR spectra for complex 5a show the expected signals
for the pincer and PPh3 ligands. Interestingly, the equivalence
1
of the four aryl biphenyls is observed in the H and 13C{1H}
spectra at room temperature, indicative of a fast interconversion
between rac and meso conformers. Derivatives Rh(PCPb)(PPh3)
(5b) and Rh(PCPc)(PPh3) (5c) have been prepared by following
the procedure described for 5a.
Synthesis and Characterization of the Complexes
Rh(PCP)L (L ) CO, CNXy, H2CdCH2, H3CCHdCH2). The
PPh3 ligand in compound 5a is labile and can be replaced by
other L ligands. Thus, exposing 5a to an atmosphere of CO
readily produces compound 6a (Scheme 4). However, the
released phosphine interferes in the purification of the carbonyl
product, as solvent evaporation during the workup procedure
is accompanied by regeneration of the phosphine derivative 4a.
Addition of Se to the reaction mixture quenches the phosphine
as P(Se)Ph3, which can be easily removed from the mixture.17
This procedure has also been used in the synthesis of the
isocyanide compound 7a. NMR characterization of these
complexes shows signals for the pincer and the L ligands. Most
meaningful of the presence of the latter is a low-field doublet
of triplets in the 13C{1H} spectrum for Rh-CO (JRhC ) 58 Hz,
JPC ) 16 Hz) and Rh-CNXy (JRhP ) 56 Hz, JPP ) 18 Hz). As
in the case of the phosphine derivative 5a, these complexes also
show fluxional behavior in solution resulting from a fast
isomerization of phosphite fragments.
1
for a propene ligand. For instance, in the H spectrum for 9a
the methyl substituent appears as a doublet at δ 1.06 (JHH
5.4 Hz), while a broad multiplet is observed at δ 4.39 for the
)
(18) For comparative purposes, see: Serron, S.; Nolan, S. P.; Moloy,
K. G. Organometallics 1996, 15, 4301.
(19) Salem, H.; Ben-David, Y.; Shimon, L. J. W.; Milstein, D.
Organometallics 2006, 25, 2292.
(20) Weisman, A.; Gozin, M.; Kraatz, H.-B.; Milstein, D. Inorg. Chem.
1996, 35, 1792.
(21) Conejo, M. M.; Parry, J. S.; Carmona, E.; Schultz, M.; Brennan,
J. G.; Beshouri, S. M.; Andersen, R. A.; Rogers, R. D.; Coles, S.; Hurthouse,
M. Chem. Eur. J. 1999, 5, 3000.
(14) In the article the pincer ligands generated by deprotonation of
diphosphites 2a-c will be denoted PCPa, PCPb, and PCPc, respectively,
while PCP will be used to generally refer to them.
1
(15) The magnitude of JPRh depends markedly on the ligand situated
in a position trans respect to the P ligand: Naaktgeboren, A. J.; Nolte,
R. J. M.; Drenth, W. J. Am. Chem. Soc. 1980, 102, 3350.
(16) Liou, S.-Y.; Gozin, M.; Milstein, D. J. Am. Chem. Soc. 1995, 117,
9774.
(17) For alternative quenchers for this reaction, see: (a) Gorla, F.;
Venanzi, L. M.; Albinati, A. Organometallics 1994, 13, 43. (b) Reference
11.
(22) Jones, W. D.; Hessell, E. T. Organometallics 1990, 9, 718.
(23) For other ethylene derivatives with pincer ligands, see: (a) Zhao,
J.; Goldman, A. S.; Hartwig, J. F. Science 2005, 307, 1080. (b) Hahn, C.;
Sieler, J.; Taube, R. Chem. Ber. 1997, 130, 939. (c) Vigalok, A.; Ben-
David, Y.; Milstein, D. Organometallics 1996, 15, 1839. (d) Nemeh, S.;
Jensen, C.; Binamira-Soriaga, E.; Kaska, W. C. Organometallics 1983, 2,
1442. (e) Reference 19.

550 Organometallics, Vol. 28, No. 2, 2009
Rubio et al.
Figure 1. ORTEP perspective of complex 5a.
Figure 2. Structure of complex 5b.
methyne group. Finally, broad doublets at 2.51 (JHH ) 8.7 Hz)
and 3.29 ppm (JHH ) 12.3 Hz) are observed for the methylenic
protons.
Structural Characterization of Pincer Complexes. The
observation of an ethylene ip coordination in 8a,b has prompted
us to perform a detailed structural study of them, focused at
investigating the olefin coordination, using X-ray crystal-
and the atoms of the pincer ligand bonded to it are practically
superimposable. Moreover, a comparative overview of the
structures draws more interesting coincidences among them.
First, the magnitude of the angle P-Rh-P lies in the range
153-157°; this angle is substantially smaller than the mean
value of ca. 164° found for complexes derived from pincer
diphosphines.24 However, values similar to those found here
have also been described in the literature. For instance, a value
of 154° has been reported for an N-pyrrolyl derivative.11
Analogously, Ir complexes with diphosphinite pincer ligands
lographic, NMR, and computational techniques.
(a) Crystallographic Studies. Complexes 5a-8a, 5b, and
8b have been characterized by single-crystal X-ray crystal-
lography. The corresponding ORTEP diagrams are depicted in
Figures 1-6, while in Table 1 are compiled some selected
distances and angles for comparative purposes. A comparison
of these data indicates that the fragments composed of the metal
(24) Cambridge Structural Database System, Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK. For QUEST3D
search details see the Experimental Section.

Rh Complexes with Pincer Diphosphite Ligands
Organometallics, Vol. 28, No. 2, 2009 551
Figure 3. ORTEP view of complex 6a.
Figure 4. ORTEP perspective of compound 7a.
display similar values of this parameter.19,25 Another remarkable
feature of the Rh-PCP fragment along the series is its planarity,
which is in contrast with the puckered structures usually
produced by pincer diphosphines.26 This backbone planarity
seems to be caused by the presence of oxygen atoms in the
backbone, as other complexes described in the literature derived
(26) For representative examples see: (a) Kraatz, H.-B.; Milstein, D. J.
Organomet. Chem. 1995, 488, 223. (b) Ca´mpora, J.; Palma, P.; del R´ıo,
(25) (a) Go¨ttker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem.
Soc. 2004, 126, 1804. (b) Go¨ttker-Schnetmann, I.; White, P.; Brookhart,
M. Organometallics 2004, 23, 1766. (c) Sykes, A. C.; White, P.; Brookhart,
M. Organometallics 2006, 25, 1644. (d) Kuklin, S. A.; Sheloumov, A. M.;
Dolgushin, F. M.; Ezernitskaya, M. G.; Peregudov, A. S.; Petrovskii, P. V.;
Koridze, A. A. Organometallics 2006, 25, 5466. (e) Denney, M. C.; Pons,
V.; Hebden, T. J.; Heinekey, D. M.; Goldberg, K. I. J. Am. Chem. Soc.
2006, 128, 12048.
´
D.; Alvarez, E. Organometallics 2004, 23, 1652.
(27) Backbone planarity is not exclusive for oxygen in the backbone,
and it also has been observed in pincer bisphosphoramidite complexes. For
examples see: (a) Benito-Garagorri, D.; Becker, E.; Wiedermann, J.;
Lackner, W.; Pollak, M.; Mereiter, K.; Kisala, J.; Kirchner, K. Organo-
metallics 2006, 25, 1900. (b) Benito-Garagorri, D.; Bocokic, D.; V.;
Mereiter, K.; Kirchner, K. Organometallics 2006, 25, 3817.

552 Organometallics, Vol. 28, No. 2, 2009
Rubio et al.
Figure 5. ORTEP view of complex 8a.
Figure 6. ORTEP view of complex 8b.
from phosphites or phosphinites also show a planar structure
of the bridge.12,25,27
ment is characterized by similar values of R angles (Figure 7,
Table 2) above and below the equatorial plane (i.e. R1 and R2
vs R4 and R3, respectively). Otherwise, the magnitude of these
parameters is dissimilar in the case of meso structures, observed
in complexes 5a and 7a. Thus, R1 and R2 amount for the latter
around 31°, while R3 and R4 have values around 74°. Moreover,
for a rac conformation similar values of distances d(1-2) and
d(3-4) are observed, while these parameters differ in the case
of meso structures. Therefore, it is apparent that the meso
conformation is preferred in the case of sterically encumbered
compounds. Steric hindrance should be particularly pronounced
in complex 5a, since the ligand PPh3 is significantly raised over
Another interesting aspect of these structures involves the
shape and size of the cavity drawn by the pincer ligand. The
first observation is with regard to the conformation of the
biphenyls. Complexes derived from the conformationally flex-
ible pincer PCPa can adopt a rac (corresponding to two
enantiomeric C2-symmetric Rh-PCPa fragments with S,S or R,R
configuration) or a meso type conformation, which is character-
ized by a symmetry plane. Upon these considerations it can be
concluded that complexes 6a and 8a, which bear the smaller
CO and C2H4 ligands, adopt a rac conformation. This arrange-

Rh Complexes with Pincer Diphosphite Ligands
Organometallics, Vol. 28, No. 2, 2009 553
Table 1. Selected Structural Parameters of Pincer Complexesa
reduced back-bonding ability of the metal center, caused by the
π-acidity of the phosphite groups.
compd
Rh-P
Rh-Cipso
Rh-L
P-Rh-P
Most noteworthy is the fact that coordinated ethylene shows
an unexpected in-plane conformation in complexes 8a,b, with
a small angle of 7.4 (8a) and 13.0° (8b) between planes defined
by Rh-PCP and Rh-η2-(C2H4) fragments. The CdC bond
length is similar for 8a (1.38 Å), while slightly lower for 8b
(1.35 Å), as compared to the mean value observed in other
rhodium ethylene derivatives (1.38-1.39 Å).24 The distance
between the ethylene carbons and the Rh atom is 2.23 Å, which
is higher than the mean value of 2.13 Å found for rhodium
ethylene derivatives and parallels the lengthening of the Rh-L
bond observed in complexes 6a and 7a.
4a
5a
5b
6a
7ac
2.2705(11) 2.2759(10) 2.033(4)
2.2233(11) 2.2470(11) 2.039(4)
2.2275(12) 2.2629(14) 2.040(5)
2.2245(4) 2.2475(4) 2.0338(16) 1.9069(19) 156.378(16)
2.2206(9) 2.2207(10) 2.034(3)
2.3959(10)b 154.55(4)
2.3360(11) 155.04(4)
2.3572(14) 152.15(5)
1.991(7)
152.89(3)
1.949(13)
8a
8b
2.2076(3) 2.2165(3) 2.0233(12) 2.2184(14) 155.490(13)
2.2353(14)
2.2237(7) 2.2096(7) 2.019(3)
2.228(3)
2.230(3)
155.37(3)
a Angles in deg and distances in Å. b L ) PPh3. c Distances for two
conformers, modeled due to isonitrile ligand disorder, are given.
(b) NMR Studies of the Olefinic Complexes. In order to
get information about the structural features of the olefinic
complexes in solution, we have also studied them by NMR
techniques. It has been mentioned above that derivatives of the
PCPa ligand are fluxional due to an interconversion between
rac and meso conformers; therefore, it is pertinent to consider
first the more simple atropisomeric complex 8b. This compound
1
shows in the H NMR spectrum two filled-in doublets at 2.98
and 2.28 ppm indicative of an AA′XX′ spin system for the
protons of the ethylene ligand. These signals broaden somewhat
on cooling but do not split at the lowest temperature investigated
(180 K). Likewise, in the 13C{1H} NMR experiment a slightly
broad doublet at 58.6 ppm (JCRh ) 4 Hz) is observed at room
temperature for the coordinated olefin. This signal broadens
somewhat at lower temperatures. An examination of the 2D
NOESY spectrum allows us to differentiate between t-Bu groups
oriented to the backbone (i.e. those denoted by C(49) and C(19)
in Figure 6) and those closer to the olefin. It is worth noting
that NOE contacts have been observed between the latter and
all ethylene protons.
Figure 7. Parameters used to compare metal-ligand cavity
dimensions: (a) R1-R4 angles defined between the P-O bond and
the best plane defined by Rh and metal-bonded P and C atoms; (b)
torsion angles between biphenyl aryls and distances between
quaternary carbons of the t-Bu subtituents (other aryl substituents
have been omitted for clarity).
Table 2. Cavity Parameters for Rh(PCP)L Complexesa
compd d(1-2) d(3-4)
R1
R2
R3
R4
ꢀ1,4
ꢀ2,3
4a
5a
5b
6a
7a
8a
8b
7.7
8.0
10.2
8.0
7.5
8.0
10.0
9.5
7.8
8.8
8.7
8.4
8.2
60.5 55.3 45.6 38.4 48.1 46.9
59.6 60.1 41.1 44.5 45.4 47.3
32.0 32.6 74.8 76.5 60.4 63.1
45.2 49.8 53.4 57.6 45.9 51.6
32.5 31.3 75.3 73.1 51.4 50.9
48.6 48.8 53.0 52.1 51.6 53.6
47.6 48.7 53.8 54.6 62.7 63.5
The C2 symmetry of complex 8b does not allow us to
distinguish between the existence of a single ethylene conformer
(ip or u) or a fast olefin rotation. For that purpose, the less
symmetric complex 9b has also been studied.29 In this particular
case, if the olefin is frozen, the four tert-butyl substituents will
be nonequivalent, while if there is a fast rotation, according to
the NMR time scale, an averaged C2 symmetry is reached and,
accordingly, two types of t-Bu groups should be observed.
8.9
a Angles in deg and distances in Å.
the coordination plane, giving a C(1)-Rh-P(3) angle of 163.2°.
It is next interesting to consider complexes 5b and 8b derived
from enantiopure diphosphite 2b. Ideally, the corresponding
pincer ligand should generate a C2-symmetric Rh(PCPb) frag-
ment. Indeed, this is observed in the case of ethylene derivative
8b and similar values of d(1-2) (8.9 Å) and d(3-4) (8.2 Å)
are observed. Otherwise, steric hindrance introduced by PPh3
in 5b cannot be released by a meso structure and a significant
distortion from the C2 structure is observed. Thus, the cavity is
significantly more opened above the plane (d(1-2) ) 10.2 Å)
than below (d(3-4) ) 7.8 Å) to accommodate the axial phenyl
group. Finally, it is interesting to highlight the differences in
torsion angles ꢀ of biphenyls of two types of ligands. Thus,
this magnitude amounts to between 40 and 45° for the flexible
ligand, while for the chiral ligand, due to the presence of methyl
substituents in 6,6′-positions, this angle increases to values of
60 and 63° for 5b and 8b, respectively.
1
Indeed, the latter behavior is observed in both the H and
13C{1H} NMR spectra in the entire temperature range, which
is consistent with fast olefin rotation even at 190 K. Therefore,
ethylene rotation should also occur in complexes 8. Upon this
assumption, spectra of complexes 8a,c can be interpreted in
terms of a exchange between rac and meso conformers. Thus,
analysis of compound 8a by 31P{1H} NMR spectroscopy shows
at room temperature a doublet at 177.3 ppm (JPRh ) 253 Hz),
while cooling of the sample leads to the appearance of a second
doublet at 179.2 ppm (1JRhP ) 250 Hz) due to a minor species,
1
at a ca. 20:1 rate. On the other hand, in the H NMR region
characteristic for a coordinated ethylene, one broad singlet at
2.82 ppm for four protons is observed at room temperature.
Upon cooling, this signal splits into two broad signals of equal
intensity (e.g. ca. 3.3 and 2.3 ppm at 220 K), while further
cooling produces the appearance of an additional broad minor
signal at 3.1 ppm. It is interesting to compare these observations
with data provided by compound 8c, which has lower symmetry
by virtue of its asymmetric naphthyl backbone. Thus, at room
Structures of Rh(PCP)L compounds are also of interest due
to the magnitude of the Rh-L distance. The distances of the
carbonyl and the isonitrile bound to the metal are 1.91 and 1.97
Å, respectively. These distances are longer than those found in
related compounds (e.g. 1.86 Å in trans-Rh(p-tolyl)(CO)-
(PPh3)2,28a 1.82 Å in trans-RhCl(CO)(PPh3)2,28b or 1.83 Å in
trans-RhCl(CNXy)(PPri3)222). Overall, the lengthening of the
Rh-L bond can be attributed to the concurrence of two factors,
the trans influence exerted by the pincer aryl ligand and a
(28) (a) Krug, C.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 1674.
(b) Rheingold, A. L.; Geib, S. J. Acta Cryst C 1987, 43, 784.
(29) Cavallo, L.; Cucciolito, M. E.; De Martino, A.; Giordano, F.;
Orabona, I.; Vitagliano, A. Chem. Eur. J. 2000, 6, 1127.

554 Organometallics, Vol. 28, No. 2, 2009
Rubio et al.
Table 3. Selected Bond Distances and Angles for Compounds I-IVa
compd Rh-Cethylene C-Cethylene Rh-P P-Rh-P R(π1/π2)b
ip-I
u-I
2.335
2.239
2.260
2.249
2.250
2.255
2.233
2.242
1.363
1.386
1.377
1.380
1.383
1.379
1.387
1.383
2.326
2.308
2.283
2.288
2.260
2.273
2.284
2.291
174.6
176.9
158.5
162.6
155.4
157.1
156.1
158.4
0
90
14.8
76.3
0
90
0
90
ip-II
u-II
ip-III
u-III
ip-IV
u-IV
a Angles in deg and distances in Å. b Angle between π1 (defined by
Rh and pincer ligand atoms bonded to it) and π2 (defined by Rh and
ethylene carbons).
Table 4. Selected Bond Distances and Angles for Compounds
V-VII and 8a
Figure 8. Calculated model complexes.
compd Rh-Cethylene C-Cethylene
Rh-P
2.259
2.265
2.259
2.266
2.259
P-Rh-P
R(π1/π2)b
temperature a broad signal corresponding to all olefinic protons
is also observed. However, cooling of the sample produces
scission into signals for a major product at 2.2 and 3.2 ppm,
while the minor species appears in this case as two broad signals
at 2.9 and 3.0 ppm. These observations are in accord with a
rac-meso exchange in which the rac isomer is prefered. The
meso isomer should give one type of ethylene protons for 8a,
due to the symmetry plane perpendicular to the coordination
plane. This plane is absent in 8c, and accordingly, two types of
protons are observed.
These studies demonstrate a facile olefin rotation for com-
plexes 8 in solution. Further support for this phenomenon is
provided by theoretical calculations (see below). However, in
view of the preferred ip conformation detected in the solid state,
we have also been interested in providing information about
the preferred conformation in solution. For that purpose we have
obtained a 13C{1H} CP-MAS spectrum of complex 8b.30
Interestingly, the spectrum is practically coincident with that
obtained in solution (see the Supporting Information). It is worth
noting that the olefinic carbons appear in the solid state at 58.6
ppm, while in CD2Cl2 solution they appear at δ 58.6 (300 K)
or 56.8 (190 K). The similarity is also extended to the
phosphorus experiments, as the phosphite groups appear at 175.7
ppm in the 31P CP-MAS experiment and as a doublet centered
at 175.1 ppm (JRhP ) 253 Hz) in the 31P{1H} spectrum in
solution. These observations indicate an important similarity
between the structure of the compound in solution and that in
the solid state and suggest that, despite olefin rotation, this ligand
is also predominantly located in plane in solution.
(c) Theoretical Studies. In order to understand the factors
governing olefin conformation in compounds 8, we have
performed DFT calculations (see the Experimental Section for
details) on conformers u and ip of the model complexes I-VII
(Figure 8). The set of models has been designed with a growing
complexity in order to investigate the factors that control the
ethylene orientation. Molecules range from the simplest one, I,
based on monodentate phosphine and phenyl ligands, to the
more realistic VII, which only differs from complex 8a in the
absence of t-Bu groups at the 5- and 5′-positions of the
biphenyls, removed for computer limitations. Thus, the signifi-
cance of the electronic factors has principally been studied by
changing the nature of the phosphorus coordinating group in
the phosphine (II) and phosphinite (III, IV) models. Alterna-
tively, the influence of steric effects has been scrutinized by
modulating the R group at the biphenyl groups (models V-VII).
Moreover, selected structural parameters for all model com-
plexes have been collected in Tables 3 and 4.
ip-V
u-V
ip-VI
u-VI
2.286
2.282
2.288
2.284
1.374
1.372
1.374
1.372
1.373
155.1
156.9
155.0
157.9
155.1
4.1
82.7
8.8
86.9
7.9
ip-VII 2.285
8a
8b
2.2184(14)
1.377(2) 2.2076(3) 155.490(13)
2.2165(3)
1.353(4) 2.2237(7) 155.37(3)
2.2096(7)
7.4
2.2353(14)
2.228(3)
2.230(3)
13.0
a Angles in deg and distances in Å. b Angle between π1 (defined by
Rh and pincer ligand atoms bonded to it) and π2 (defined by Rh and
ethylene carbons).
Table 5. Relative Energy (kcal mol-1) between Isomers of Model
Complexes I-VIIa
compd
rel energya
RhPh(PH3)2(C2H4) (I)
Rh{C6H3(CH2PH2)2}(C2H4) (II)
Rh{C6H3(OPH2)2}(C2H4) (III)
Rh{C6H3(OPMe2)2}(C2H4) (IV)
Rh{C6H3(OP(OC6H4-OC6H4)2}(C2H4) (V)
Rh{C6H3(OP(OC6H3Me-OC6H4)2}(C2H4) (VI)
Rh{C6H3(OP(OC6H3-t-Bu-OC6H4)2}(C2H4) (VII)
+10.8
+1.0
-0.9
-0.8
-0.1
-0.6
b
a A negative value for the relative energy indicates that the ip
conformer is the most stable. b u conformation cannot be optimized.
As a starting point, the simplest model, RhPh(PH3)2(C2H4)
(I), was computed to confirm in our model system the well-
known preference for a u orientation of an olefin coordinated
to a d8-ML3 fragment and, in addition, to quantify the relative
stability between its conformers. As expected, the u ethylene
conformation is not only preferred but is considerably more
stable (10.8 kcal mol-1, Table 5) than the ip conformation.
We have next analyzed the effect of formally including a
bridge between the phenyl and P ligands to compute model
complexes II-IV. For the phosphine model II, the u conforma-
tion is, as expected, more stable than the ip conformation, but
only by 1 kcal mol-1. Thus, the energy difference has
significantly decreased in comparison with model I. This effect
can be ascribed to the mitigation of the steric pressure produced
by a lower P-Rh-P angle in II (158.5 and 162.6° for ip and
u conformers, respectively), compared with I (174.6 and 176.9°
for the ip and u conformers, respectively). Thus, for the latter,
the higher steric pressure exerted by the cis coligands in the
ethylene ip conformer significantly destabilizes it with respect
to the u conformer.4 For model complexes III and IV, in which
the P-Rh-P angles are similar to those of II, but slightly
smaller, the energy difference is also small, although in these
cases the ip conformation is found to be more stable by about
1 kcal mol-1. Interestingly, the presence of O atoms has an
important influence on the structure of the metallacycle. As was
mentioned, pincer diphosphine ligands are characterized by
(30) Quan, R. W.; Li, Z.; Jacobsen, E. N. J. Am. Chem. Soc. 1996, 118,
8156.

Rh Complexes with Pincer Diphosphite Ligands
Organometallics, Vol. 28, No. 2, 2009 555
Table 6. Summary of Crystallographic Data and Structure Refinement Results from Single-Crystal X-ray Diffraction
4a 5a 5b 6a 7a 8a
8b
formula
C80H99ClO6P3Rh · C80H98O6P3Rh · 2(C72H82O6P3Rh) · 2(C63H83O7P2Rh) · 2(C71H92NO6P2Rh) · C64H87O6P2Rh · C56H71O6P2Rh
C6H14
1474.03
triclinic
0.5C6H14
1394.49
triclinic
C4H10O
2552.51
monoclinic
C2
24.7253(9)
12.9836(4)
21.7178(6)
90
99.008(2)
90
6885.9(4)
2
C6H14 · C5H12
2392.61
monoclinic
P21/c
12.3963(2)
19.8113(4)
27.2608(6)
90
95.0850(10)
90
6668.5(2)
2
CHCl3 · CH2Cl2
2644.91
C5H12
1189.33
monoclinic
P21/c
17.2239(5)
19.2234(6)
20.3456(6)
90
97.7050(10)
90
6675.6(3)
4
fw
1004.98
orthorhombic
P212121
10.4092(11)
18.8544(18)
26.636(3)
90
90
90
5227.6(9)
4
cryst syst
space group
a, Å
b, Å
c, Å
triclinic
P1
j
j
j
P1
P1
13.4989(8)
16.7201(9)
19.3595(11)
110.784(3)
96.741(3)
97.170(3)
3991.2(4)
2
14.4920(18)
17.417(2)
19.351(2)
65.484(5)
69.097(5)
70.040(5)
4042.7(8)
2
12.6244(9)
18.5218(15)
18.850(2)
106.614(3)
108.221(3)
107.806(2)
3614.0(6)
1
R, deg
ꢁ, deg
γ, deg
cell vol, Å3
Z
F
calcd, Mg/m3
cryst color
1.227
1.146
1.231
1.192
1.215
1.183
1.277
yellow
0.358
orange
0.318
yellow
0.368
yellow
0.353
yellow
0.421
orange
0.351
orange
0.436
µ, mm-1
F(000)
1568
1482
2692
2552
1396
2544
2120
no. of measd rflns
no. of indep rflns
no. of params
49 365
24 502
117 604
18 888
865
0.0613
0.1788
1.020
49 957
16 411
783
0.0711
0.1561
1.047
110 399
20 357
732
0.0373
0.0934
1.056
56 585
21 952
917
0.0680
0.2169
1.043
153 626
20 381
719
0.0311
0.0797
1.052
45 841
11 402
602
0.0352
0.0717
1.019
874
R1(F) (F2 > 2σ(F2))a 0.0589
wR2(F2)b (all data)
Sc (all data)
0.1651
1.016
Flack param
0.03(3)
-0.018(17)
b
c
a R1(F) ) ∑(Fo - Fc)/∑Fo for the observed reflections (F2 > 2σ(F2)). wR2(F2) ) {∑[w(Fo - Fc2)2]/∑w(Fo )2}1/2
.
S ) {∑[w(Fo - Fc2)2]/(n -
2
2
2
p)}1/2 (n ) number of reflections, p ) number of parameters).
Figure 9. Selected 3D isosurfaces corresponding to the MOs of fragments IIa and IIIa.
puckered structures, while pincer diphosphite complexes de-
scribed here possess a planar structure. This difference is well
reproduced in the series of models II-IV. Thus, phosphinite
models III and IV (as well as V-VII, see below) describe
planar structures, while a puckered arrangement is evident in
the structures of isomers of model II.
for the in-plane conformer (HOMO-3 in IIa and HOMO-4 in
IIIa) and the other for the u species (HOMO-1 for both IIa
and IIIa). HOMO-5 corresponds to the bonding combination
of the dxz orbital with the p orbitals of the aromatic ring. The
back-donation in fragment IIa is favored for the u isomer, with
respect to IIIa, because the participation of the Rh dxz orbital
in HOMO-1 is 78%, higher than the Rh character of 58% found
in the HOMO-1 of IIIa. In contrast, in fragment IIIa HOMO-4
is formed mainly by the Rh dxy orbital (75% Rh character) with
an important participation from the P atoms (21%). This
contribution expands the lobes of the orbital on the appropriate
way to favor the back-donation in the in-plane isomer. This
situation is somewhat disfavored for IIa, where the analogous
orbital, HOMO-3, presents an 83% Rh character, with a small
contribution (6%) from the P atoms. Summarizing, there is a
subtle electronic effect associated with the formal substitution
of a CH2 group by an O atom. On the basis of the Rh
contributions to the frontier FMOs and their corresponding
topologies, we can expect a better preference for the ip
conformer in IIIa than in IIa.
With the aim of rationalizing the effect caused by the formal
substitution of a CH2 group by an O atom, we have analyzed
the MOs of the rhodium-pincer ligand fragments of II and III,
namely IIa and IIIa, respectively. The two interactions between
the ethylene ligand and the metallic fragment are well-known:
(i) donation from ethylene to the LUMO of the fragment (the
typical empty hybrid of a d8-ML3 FMO, Figure 9) and (ii) back-
donation from dxy or from dxz orbitals for the ip and u isomers
(the coordination plane is xy), respectively. Concerning the
donation, the involvement of Rh in the LUMO in the fragments
IIa and IIIa is the same (47%), but the contribution from the
P atoms to the LUMO is higher in IIIa (39%) than in IIa (31%).
Thus, the resulting lobes of the LUMO in IIIa are to some extent
better adapted, in terms of overlap, for receiving the donation
of the occupied ethylene orbital if the in-plane conformation is
attained than those of IIa. Concerning the back-donation, there
are two orbitals that are appropriate for such an interaction, one
Interestingly, the analysis of the rotational barrier of the
ethylene ligand in compounds II and III (see the Supporting
Information) corroborates the above discussion. The barrier is

556 Organometallics, Vol. 28, No. 2, 2009
Rubio et al.
plane but offers an important steric hindrance in a perpendicular
direction to this plane. Thus, an ethylene ligand can be easily
coordinated in a ip fashion, but a u orientation would require a
distortion of the cavity. This fact is apparently in contradiction
with the observed olefin rotation process. However, comparison
of structures of complexes 5b and 8b reveal the ability of the
pincer ligand to distort and generate a more opened cavity which
will be able to accommodate transiently the ethylene in a u
conformation.
Conclusions
A series of rhodium complexes derived from pincer diphos-
phite and neutral L ligands has been prepared and characterized.
IR studies of the carbonyl (6a) and isonitrile (7a) complexes
show the π-acceptor ability of the pincer diphosphite ligand.
Comparison of structures of complexes 5a-8a, 5b, and 8b
indicate very similar parameters for the Rh-PCP fragment in
the coordination plane. In addition, in the series of derivatives
of the conformationally flexible ligand PCPa, it has been
observed that the conformation of the biphenyl groups is
determined by the size of the L ligand. Thus, a rac conformation
has been observed for less demanding groups, while the meso
structure is preferred for larger L ligands. Moreover, rac-meso
isomerization has been observed for these complexes in solution.
Most noteworthy, X-ray structural determination of complexes
8 indicate a rare in-plane conformation of the ethylene ligand.
NMR studies indicate that olefin complexes (8 and 9) have a
fluxional behavior caused by a olefin rotation process, which
could not be frozen at the lower temperature investigated.
Moreover, 13C{1H} NMR spectra of 8b in solution and in the
solid state practically match, thus indicating an important
coincidence between solid-state and solution structures for this
complex.
To complete the experimental studies, detailed computational
investigations of u and ip conformers of a set of model
complexes I-VII have been performed. An investigation of
electronic effects indicates that the nature of the P-fragments
(phosphine vs phosphinite) do not play an important role in
altering the relative stabilities of both types of conformers.
Otherwise, steric effects play a decissive influence on olefin
conformation. Two different effects have been identified. First,
the reduction of the P-Rh-P angle, as compared with the ideal
for two trans-coordinated ligands, causes the reduction of
hindrance toward the remaining coordination position. The
second is with regard to the size of the R substituents at the 3-
and 3′-positions of the biphenyl fragments. Thus, while for H
and Me derivatives both conformers are of similar energy, the
cavity outlined by the t-Bu-substituted pincer ligand in model
VII significantly reduces the available space above and below
the coordination plane, favoring the ip conformation.
Figure 10. Optimized structures of conformers of the model
complexes V-VII.
small in both cases (ca. 1 kcal mol-1), in good agreement with
a relatively minor π contribution to the Rh-ethylene bond,
which would facilitate the rotation of the olefin.
The importance of steric factors produced by the substituents
of the biphenyl ring has been studied by calculating the model
complexes Rh{C6H3(OP(OC6H3R-OC6H4)2}(C2H4) (R ) H (V),
Me (VI), t-Bu (VII)). For models V and VI, both u and ip
conformations have been located as stationary points (Figure
10). The computed energies for isomers of V and VI are quite
similar (Table 5). For model VI the ip conformation is
more stable with respect to the u isomer by only 0.6 kcal mol-1,
while for V the ip conformation is more stable by no more than
0.1 kcal mol-1. For complex VII, although different conforma-
tions for the ethylene ligand were explored as starting geom-
etries, the only conformation that appears to be an stationary
point corresponds to the ip isomer. All our attempts of
optimization of the u conformer converged into the ip con-
former. Interestingly, the structural parameters of the computed
model VII are in good agreement with the experimental data
of compound 8a. For instance, the calculated parameters around
the Rh-C2H4 moiety (Rh-C ) 2.285 Å and C-C ) 1.373 Å)
agree well with the experimental values (Rh-C ) 2.235(2) and
2.218(2) Å and C-C ) 1.377(2) Å). Likewise, there is a good
accord in the values of the P-Rh-P angle (experimental,
155.5°; computed, 155.1°). The calculations are also capable
of describing the small deviation of the Rh-C2H4 plane with
respect to the Rh-PCP plane (experimental, 7.4°; computed,
7.9°). Finally, the calculation reproduces well the planarity of
the Rh-PCP fragment of 8a.
Experimental Section
General Comments. All reactions and manipulations were
performed under nitrogen or argon, either in a Braun Labmaster
100 glovebox or using standard Schlenk-type techniques. All
solvents were distilled under nitrogen using the following dessicants:
sodium benzophenone ketyl for benzene, diethyl ether (Et2O), and
tetrahydrofuran (THF), sodium for petroleum ether and toluene,
CaH2 for dichloromethane (CH2Cl2), and NaOMe for methanol
From comparison of models V-VII it appears that the bulky
t-Bu groups have a decisive role in destabilizing the u
conformer. This effect can be readily seen using space-filling
models of u-VI, ip-VI, and ip-VII complexes (see the Sup-
porting Information). A comparison of conformers of VI show
that the cavity generated by the metal and the pincer ligand is
practically superimposable and can allocate the ethylene in either
conformation without distortion. Otherwise, the presence of t-Bu
groups in VII does not increase encumbrance in the coordination
32
(MeOH). Chlorophosphites 131 and the complex Rh(Cl)(PPh3)3
(31) (a) Buisman, G. J. H.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.
Tetrahedron: Asymmetry 1993, 4, 1625. (b) Sua´rez, A.; Me´ndez-Rojas,
M. A.; Pizzano, A. Organometallics 2002, 21, 4611.

Rh Complexes with Pincer Diphosphite Ligands
Organometallics, Vol. 28, No. 2, 2009 557
were prepared according to literature procedures. NMR spectra were
obtained on Bruker DPX-300, DRX-400, and DRX-500 spectrom-
eters. 31P{1H} NMR shifts were referenced to external 85% H3PO4,
while 13C{1H} and 1H shifts were referenced to the residual signals
of deuterated solvents. All data are reported in ppm downfield from
Me4Si. HRMS data were obtained using a Jeol JMS-SX 102A mass
spectrometer at the Analytical Services of the Universidad de Sevilla
(CITIUS). Elemental analyses were run by the Analytical Service
of the Instituto de Investigaciones Qu´ımicas.
) 9.3 Hz, 1H, H arom). 31P{1H} NMR (CDCl3, 162.1 MHz): δ
136.9 (s), 138.9 (s). 13C{1H} NMR (CDCl3, 125.8 MHz): δ 31.2
(s, 2 CMe3), 31.3 (s, 2 CMe3), 31.6 (s, 4 CMe3), 34.7 (s, 4 CMe3),
35.4 (s, 2 CMe3), 35.5 (s, 2 CMe3), 110.7 (dd, JCP ) 13 Hz, JCP
)
5 Hz, CH arom), 112.3 (d, JCP ) 12 Hz, CH arom), 122.9 (s, CH
arom), 124.4 (s, 2 CH arom), 124.5 (s, 2 CH arom), 124.8 (s, CH
arom), 126.7 (s, 4 CH arom), 127.0 (s, CH arom), 127.3 (s, CH
arom), 132.8 (s, 5 Cq), 134.7 (s, Cq), 140.3 (s, 2 Cq), 140.4 (s, 2
Cq), 145.5 (s, 4 Cq), 146.8 (s, 2 Cq), 146.9 (s, 2 Cq), 149.3 (d, JCP
) 12 Hz, Cq), 149.5 (d, JPC ) 12 Hz, Cq). HRMS (FAB): m/z
1059.5857, [M + Na]+ (exact mass calcd for C66H86O6P2Na:
1059.5797).
µ-1,3-Phenylenebis[1,1′-(3,3′,5,5′-tetra-tert-butyl)biphen-2,2′-diyl-
]diphosphite (2a). Over a solution of phosphorochloridite 1a (4.74
g, 10 mmol) and NEt3 (1.5 mL, 11 mmol) in THF (40 mL) was
slowly added a resorcinol (0.55 g, 5 mmol) solution in the same
solvent (40 mL). After the mixture was stirred for 16 h, the solvent
was evaporated, the residue treated with Et2O (3 50 mL), and this
mixture filtered through a pad of neutral alumina. Removal of the
solvent yielded compound 2a as a white foamy solid (4.29 g, 87%).
1H NMR (CDCl3, 400 MHz): δ 1.36 (s, 36H, 4 CMe3), 1.48 (s,
Rh(H)(Cl)(PCPa)(PPh3) (4a). Although this compound was first
observed in the reaction between Rh(Cl)(PPh3)3 and 2a, it is more
conveniently obtained by protonation of 5a. Over a solution of 5a
(0.11 g, 0.081 mmol) in THF (10 mL) was added HCl (0.2 mL,
1.0 M in Et2O). The mixture was stirred vigorously for 24 h, the
solvent evaporated, and the remaining residue extracted with
n-hexane (3 × 10 mL). Solvent evaporation yields 4a as a yellow
3
4
36H, 4 CMe3), 6.80 (dd, JHH ) 8.4 Hz, JHH ) 2.5 Hz, 2H, 2 H
arom), 6.92 (m, 1H, H arom), 7.16 (t, 3JHH ) 8.4 Hz, 1H, H arom),
1
solid (0.09 g, 80%). IR (Nujol mull, cm-1): 2112 (m, νRhH). H
4
4
7.20 (d, JHH ) 2.4 Hz, 4H, 4 H arom), 7.45 (d, JHH ) 2.4 Hz,
4H, 4 H arom). 31P{1H} NMR (CDCl3, 202.4 MHz): δ 137.9 (s).
13C{1H} NMR (CDCl3, 100.6 MHz): δ 31.3 (br s, 4 CMe3), 31.5
(s, 4 CMe3), 34.7 (4 CMe3), 35.5 (4 CMe3), 112.8 (t, JPC ) 8 Hz,
CH arom), 115.8 (d, JPC ) 9 Hz, 2 CH arom), 124.4 (s, 4 CH
NMR (CDCl3, 500 MHz): δ -15.63 (dq, JHRh ) JHPo ) JHP ) 12
Hz, 1H, Rh-H), 1.22 (s, 18H, 2 CMe3), 1.27 (s, 18H, 2 CMe3),
1.32 (s, 18H, 2 CMe3), 1.42 (s, 18H, 2 CMe3), 6.63 (dd, JHH ) 8
Hz, JRhH ) 2 Hz, 2H, 2 H arom), 6.84 (m, 6H, 6 H arom, PPh3),
6.85 (m, 2H, 2 H arom), 6.91 (d, JHH ) 2 Hz, 2H, 2 H arom), 6.95
(t, JHH ) 8 Hz, 1H, H arom), 6.98 (t, JHH ) 7.5 Hz, 3H, 3 H arom,
PPh3), 7.17 (dd, JHH ) 7.5 Hz, JHP ) 10 Hz, 6H, 6 H arom, PPh3),
7.33 (m, 2H, 2 H arom), 7.37 (d, JHH ) 2 Hz, 2H, 2 H arom).
arom), 126.6 (s, 4 CH arom), 130.0 (s, CH arom), 132.8 (d, JPC
)
2 Hz, 4 Cq arom), 140.3 (s, 4 Cq arom), 145.3 (d, JPC ) 6 Hz, 4 Cq
arom), 146.8 (s, 4 Cq arom), 153.2 (d, JPC ) 7 Hz, 2 Cq arom).
HRMS (FAB): m/z 986.5755, [M]+ (exact mass calcd for
C62H84O6P2 986.5743).
31P{1H} NMR (C6D6, 162.1 MHz): δ 14.7 (dt, JPRh ) 87 Hz, JPP
)
35 Hz, P-C), 148.9 (dd, JPRh ) 178 Hz, P-O). 13C{1H} NMR
(CDCl3, 75.5 MHz): δ 31.6 (s, 2 CMe3), 31.6 (s, 2 CMe3), 31.7 (s,
2 CMe3), 32.0 (s, 2 CMe3), 34.8 (s, 4 CMe3), 35.7 (s, 2 CMe3),
36.1 (s, 2 CMe3), 106.8 (t, JPC ) 7 Hz, 2 CH arom), 124.6 (s, 2
CH arom), 125.0 (s, 2 CH arom), 126.8 (s, CH arom), 127.3 (s, 2
CH arom), 127.8 (d, JPC ) 9 Hz, 6 CH arom, PPh3), 128.3 (s, 2
CH arom), 129.1 (s, 3 CH arom), 129.9 (s, 2 Cq arom), 131.6 (s,
(R,R)-µ-1,3-phenylenebis[1,1′-(3,3′-di-tert-butyl-5,5′,6,6′-tetram-
ethyl)biphen-2,2′-diyl]diphosphite (2b). Compound 2b has been
obtained as a white solid as described for 2a but starting from
20
chlorophosphite 1b (1.70 g, 78%). [R]D ) -347 (c 1.0, THF).
1H NMR (CDCl3, 500 MHz): δ 1.41 (s, 18H, 2 CMe3), 1.42 (s,
18H, 2 CMe3), 1.82 (s, 6H, 2 Me), 1.83 (s, 6H, 2 Me), 2.23 (s, 6H,
2 Me), 2.26 (s, 6H, 2 Me), 6.80 (m, 3H, 3 H arom), 7.14 (t, JHH
)
2 Cq arom), 133.4 (d, JPC ) 12 Hz, 6 CH arom), 134.6 (d, JPC
)
7.5 Hz, 1H, H arom), 7.16 (s, 4H, 4 H arom). 31P{1H} NMR
(CDCl3, 121.5 MHz): δ 132.6 (s). 13C{1H} NMR (CDCl3, 75.5
MHz): δ 16.9 (s, 2 Ar-Me), 17.1 (s, 2 Ar-Me), 20.7 (s, 4 Ar-Me),
31.5 (s, CMe3), 31.5 (s, CMe3), 32.0 (s, 2 CMe3), 34.9 (s, 2 CMe3),
35.1 (s, 2 CMe3), 112.6 (t, JPC ) 9 Hz, CH arom, Cc), 115.6 (d,
JPC ) 10 Hz, 2 CH arom), 128.1 (s, 2 CH arom), 128.5 (s, 2 CH
arom), 130.2 (s, CH arom), 130.7 (br s, 2 Cq arom), 132.3 (m, 2 Cq
arom), 132.4 (s, 2 Cq arom), 133.2 (s, 2 Cq arom), 134.7 (s, 2 Cq
arom), 135.5 (s, 2 Cq arom), 137.7 (s, 2 Cq arom), 138.5 (br s, 2 Cq
arom), 144.7 (d, JPC ) 6 Hz, 2 Cq arom), 144.8 (s, 2 Cq arom),
153.6 (d, JPC ) 9 Hz, 2 Cq arom). HRMS (FAB): m/z 875.4586,
[M + H]+ (exact mass calcd for C54H69O6P2 875.4569).
35 Hz, 3 Cq arom), 136.5 (m, Cq arom), 139.7 (s, 2 Cq arom), 141.2
(s, 2 Cq arom), 145.6 (br s, 2 Cq arom), 146.4 (s, 2 Cq arom), 146.8
(s, 2 Cq arom), 147.4 (t, JPC ) 7 Hz, 2 Cq arom), 156.0 (t, JPC
)
11 Hz, 2 OCq arom). Anal. Calcd for C80H99ClO6P3Rh: C, 69.2; H,
7.2. Found: C, 69.3; H, 7.6.
Rh(PCPa)(PPh3) (5a). Over a suspension of RhCl(PPh3)3 (0.46
g, 0.5 mmol) in THF (5 mL) was added diphosphite 2a (0.49 g,
0.5 mmol) dissolved in THF (10 mL). The mixture was heated over
24 h at 70 °C. An excess of NEt3 (0.1 mL) was added and the
mixture vigorously stirred for 24 h. Solvent was removed under
reduced pressure, and the resulting solid was purified by column
chromatography on silica gel (AcOEt:Hex 1:20), yielding 3a as an
µ-1,3-Naphthalenediylbis[1,1′-(3,3′,5,5′-tetra-tert-butyl)biphen-
2,2′-diyl]diphosphite (2c). Over a solution of phosphorochloridite
1a (1.48 g, 3.1 mmol) and NEt3 (0.5 mL, 3.1 mmol) in THF (25
mL) was slowly added a 1,3-dihydroxynaphthalene (0.250 g, 1.6
mmol) solution in the same solvent (15 mL). After the mixture
was stirred for 16 h, the solvent was evaporated and the residue
was extracted with Et2O (3 × 20 mL) and filtered through a pad of
neutral alumina. Removal of the solvent yielded compound 2c as
1
orange solid (0.58 g, 85%). H NMR (CDCl3, 400 MHz): δ 1.20
3
(s, 36H, 4 CMe3), 1.38 (s, 36H, 4 CMe3), 6.42 (d, JHH ) 8 Hz,
3
2H, 2 H arom), 6.71 (t, JHH ) 7 Hz, 6H, 6 H arom), 6.84 (t, 1H,
3
3JHH ) 8 Hz, H arom), 6.97 (t, JHH ) 7 Hz, 3H, 3 H arom), 7.10
(br s, 4H, 4 H arom), 7.33 (br s, 4H, 4 H arom), 7.35 (d, 3JHH ) 7
Hz, 6H, 6 H arom). 31P{1H} NMR (CDCl3, 162.1 MHz): δ 28.6
(td, JPRh ) 129 Hz, JPP ) 44 Hz, P-C), 171.5 (dd, JPRh ) 265 Hz,
P-O). 13C{1H} NMR (CDCl3, 75.5 MHz): δ 31.8 (s, 4 CMe3),
31.9 (s, 4 CMe3), 35.0 (s, 4 CMe3), 35.8 (s, 4 CMe3), 105.3 (t, JPC
) 8 Hz, 2 CH arom), 124.6 (s, 4 CH arom), 126.0 (s, CH arom),
127.6 (s, 4 CH arom), 128.0 (d, JPC ) 9 Hz, 6 CH arom), 129.0 (s,
3 CH arom), 131.4 (s, 4 Cq arom), 134.0 (d, JPC ) 13 Hz, 6 CH
arom), 137.7 (d, JPC ) 36 Hz, 3 Cq arom), 140.1 (s, 4 Cq arom),
140.7 (ddd, JPC ) 58, 14 Hz, JRhC ) 28 Hz, Cq arom), 146.6 (s, 4
Cq arom), 147.4 (t, JPC ) 5 Hz, 4 OCq arom), 159.5 (t, JPC ) 13
Hz, 2 OCq arom). Anal. Calcd for C80H98O6P3Rh: C, 71.1; H, 7.3.
Found: C, 70.6; H, 7.4.
1
a white foamy solid (1.46 g, 90%). H NMR (CDCl3, 400 MHz):
δ 1.35 (s, 18H, 2 CMe3), 1.36 (s, 18H, 2 CMe3), 1.39 (s, 18H, 2
CMe3), 1.50 (s, 18H, 2 CMe3), 7.09 (s, 1H, H arom), 7.21 (d, 4JHH
) 2.4 Hz, 2H, 2 H arom), 7.22 (d, 4JHH ) 2.4 Hz, 2H, 2 H arom),
7.24 (s, 1H, H arom), 7.28 (m, 1H, H arom), 7.41 (m, 1H, H arom),
4
4
7.44 (d, JHH ) 2.4 Hz, 2H, 2 H arom), 7.46 (d, JHH ) 2.4 Hz,
2H, 2 H arom), 7.61 (d, 3JHH ) 8.0 Hz, 1H, H arom), 7.97 (d, 3JHH
(32) Osborn, A.; Jardine, F. H.; Young, J. F.; Wilkinson, G. J. Chem.
Soc. A 1966, 1711.

558 Organometallics, Vol. 28, No. 2, 2009
Rubio et al.
Rh(PCPb)(PPh3) (5b). Over a suspension of RhCl(PPh3)3 (0.473
g, 0.54 mmol) in THF (5 mL) was added diphosphite 2b (0.500 g,
0.54 mmol) dissolved in THF (5 mL). The mixture was heated over
16 h at 60 °C. An excess of NEt3 (0.1 mL) was added and the
mixture vigorously stirred for 24 h at 60 oC. Solvent was removed
under reduced pressure, and the resulting solid was purified by
column chromatography on silica gel (AcOEt:hexane, 1:20),
yielding 5b as an orange solid (0.474 g, 71%). 1H NMR (CD2Cl2,
500 MHz): δ 1.13 (s, 18H, 2 CMe3), 1.42 (s, 18H, 2 CMe3), 1.76
(s, 6H, 2 Ar-Me), 1.82 (s, 6H, 2 Ar-Me), 2.23 (s, 6H, 2 Ar-Me),
NMR (CDCl3, 162.1 MHz): δ 167.3 (d, JPRh ) 256 Hz). 13C{1H}
NMR (CDCl3, 125.8 MHz): δ 31.5 (s, 4 CMe3), 31.7 (s, 4 CMe3),
34.7 (4 CMe3), 35.7 (4 CMe3), 106.1 (t, JPC ) 8 Hz, 2 CH arom),
125.0 (s, 4 CH arom), 126.6 (s, 4 CH arom), 129.5 (s, CH arom),
131.6 (s, 4 Cq arom), 140.2 (m, Cq arom), 140.3 (s, 4 Cq arom),
145.1 (s, 4 OCq arom), 147.6 (s, 4 Cq arom), 160.4 (t, JPC ) 14
Hz, 2 OCq arom), 191.8 (dt, JRhC ) 58 Hz, JPC ) 16 Hz, CO).
Anal. Calcd for C63H83O7P2Rh: C, 67.7; H, 7.5. Found: C, 67.9; H,
7.9.
Rh(PCPa)(CNXy) (7a). A mixture of 5a (0.04 g, 0.03 mmol),
elemental Se (4 mg, 0.05 mmol), and CNXy (0.004 g, 0.03 mmol)
in THF was vigorously stirred for 30 min. Solvent was removed
under reduced pressure, and the obtained residue was extracted with
n-pentane (3 × 10 mL). The solution was concentrated until
turbidity appeared and then filtered. The resulting solution was left
to stand for 24 h, and then compound 7a appeared as yellow crystals
3
2.32 (s, 6H, 2 Ar-Me), 6.41 (d, JHH ) 7.5 Hz, 2H, 2 H arom),
3
6.62 (s, 2H, 2 H arom), 6.90 (m, 7H, 7 H arom), 7.10 (t, JHH
)
)
3
7.5 Hz, 3H, 3 H arom), 7.24 (s, 2H, 2 H arom), 7.42 (dd, JHP
3
9.0 Hz, JHH ) 9.0 Hz, 6H, 6 H arom). 31P{1H} NMR (CD2Cl2,
202.4 MHz): δ 25.5 (dt, JPRh ) 127 Hz, JPP ) 43 Hz, P-C), 165.6
(dd, JPRh ) 266 Hz, P-O). 13C{1H} NMR (CD2Cl2, 125.8 MHz):
δ 16.7 (s, 2 Ar-Me), 17.0 (s, 2 Ar-Me), 20.4 (s, 2 Ar-Me), 20.6 (s,
2 Ar-Me), 32.1 (s, 4 CMe3), 34.8 (s, 2 CMe3), 35.2 (s, 2 CMe3),
104.8 (dd, JCRh ) 7 Hz, JCP ) 7 Hz, 2 CH arom), 126.5 (s, CH
arom), 127.6 (d, JCP ) 9 Hz, 6 CH arom), 128.2 (s, 2 CH arom),
128.3 (s, 2 CH arom), 128.7 (s, 3 CH arom), 129.0 (s, 2 Cq), 130.6
(s, 2 Cq), 132.1 (s, 2 Cq), 133.3 (s, 2 Cq), 134.7 (d, JCP ) 13 Hz,
6 CH arom), 134.8 (s, 2 Cq), 135.3 (s, 2 Cq), 136.8 (s, 2 Cq), 138.4
(s, 2 Cq), 138.5 (d, JCP ) 31 Hz, 3 Cq), 140.4 (ddt, JCP ) 59, 14
Hz, JCRh ) 27 Hz, Cq), 146.5 (s, 2 Cq), 146.7 (s, 2 Cq), 159.4 (dd,
JCRh ) 13 Hz, JCP ) 13 Hz, 2 Cq). Anal. Calcd for C72H82O6P3Rh:
C, 69.8; H, 6.7. Found: C, 70.0; H, 6.9.
1
(0.035 g, 95%). IR (Nujol mull, cm-1): 2099 (s, νCN). H NMR
(CDCl3, 500 MHz): δ 1.31 (s, 36H, 4 CMe3), 1.40 (br s, 36H, 4
CMe3), 1.55 (s, 6H, 2 Me), 6.59 (d, 3JHH ) 7.5 Hz, 2H, 2 H arom),
6.75 (d, 3JHH ) 7.5 Hz, 2H, 2 H arom), 6.92 (t, 3JHH ) 7.5 Hz, 1H,
H arom), 6.95 (t, 3JHH ) 7.5 Hz, 1H, H arom), 7.18 (d, 4JHH ) 2.5
Hz, 4H, 4 H arom), 7.40 (d, 4JHH ) 2.5 Hz, 4H, 4 H arom). 31P{1H}
NMR (CDCl3, 162.1 MHz): δ 170.1 (d, JPRh ) 265 Hz). 13C{1H}
NMR (CDCl3, 125.8 MHz): δ 18.2 (s, 2 Ar-Me), 31.5 (s, 4 CMe3),
31.8 (s, 4 CMe3), 34.7 (s, 4 CMe3), 35.7 (s, 4 CMe3), 105.6 (t, 2
CH arom), 124.7 (s, 4 CH arom), 126.5 (s, 4 CH arom), 127.1 (s,
CH arom), 127.2 (s, 2 CH arom), 131.6 (s, 2 Cq arom), 132.0 (s,
4 Cq arom), 134.7 (s, CH arom), 140.6 (s, 4 Cq arom), 142.1 (dt,
JRhC ) 25 Hz, JPC ) 15 Hz), 143.8 (s, Cq arom), 145.3 (s, 4 OCq
arom), 146.9 (s, 4 Cq arom), 160.4 (t, JPC ) 15 Hz, 2 OCq arom),
163.0 (dt, JRhC ) 56 Hz, JPC ) 18 Hz, CN). Anal. Calcd for
C71H92NO6P2Rh · 0.5CH2Cl2: C, 68.0; H, 7.4; N, 1.1. Found: C, 68.4;
H, 7.4; N, 1.2.
Rh(PCPC)(PPh3) (5c). Over a solution of RhCl(PPh3)3 (0.200 g,
0.22 mmol) and diphosphite 2c (0.224 g, 0.22 mmol) in THF (10
mL) was added an excess of NEt3 (0.3 mL). The mixture was heated
over 24 h at 70 °C. Solvent was removed under reduced pressure,
and the resulting solid was purified by column chromatography on
silica gel (AcOEt:hexane, 1:20), yielding 5c as a yellow-orange
solid (0.150 g, 49%). 1H NMR (CDCl3, 400 MHz): δ 1.15 (s, 18H,
2 CMe3), 1.25 (s, 18H, 2 CMe3), 1.42 (s, 18H, 2 CMe3), 1.43 (s,
18H, 2 CMe3), 6.76 (t, 3JHH ) 7.6 Hz, 6H, 6 H arom), 6.87 (s, 1H,
H arom), 7.03 (m, 4H, 4 H arom), 7.14 (m, 2H, 2 H arom), 7.21
(m, 3H, 3 H arom), 7.32 (d, 3JHH ) 8.4 Hz, 1H, H arom), 7.37 (m,
Rh(PCPa)(η2-C2H4) (8a). In a Fischer-Porter vessel compound
3a (0.108 g, 0.08 mmol) and Se (0.010 g, 0.13 mmol) were added
to THF (10 mL). The reactor was charged with 4 atm of C2H4 and
heated at 40 °C. Reaction completion was observed after 4 days.
Solvent was removed under reduced pressure and the remaining
solid extracted with n-pentane (3 × 10 mL). Further filtration and
concentration yielded 5a as orange crystals (0.040 g, 45%). 1H NMR
(CD2Cl2, 400 MHz): δ 1.30 (s, 36H, 4 CMe3), 1.39 (s, 36H, 4
CMe3), 2.82 (br s, 4H, C2H4), 6.68 (d, 3JHH ) 8 Hz, 2H, 2 H arom),
3
4H, 4 H arom), 7.43 (m, 6H, 6 H arom), 7.56 (d, JHH ) 8.0 Hz,
1H, H arom). 31P{1H} NMR (CDCl3, 162.1 MHz): δ 28.7 (dt, JPRh
) 128 Hz, JPP ) 44 Hz, P-C), 168.1 (ddd, JPP ) 694 Hz, JPRh
265 Hz, JPP ) 44 Hz, P-O), 173.1 (ddd, JPP ) 694 Hz, JPRh
)
)
265 Hz, JPP ) 44 Hz, P-O). 13C{1H} NMR (CDCl3, 125.8 MHz):
δ 31.2 (s, 2 CMe3), 31.5 (s, 2 CMe3), 31.6 (s, 4 CMe3), 34.7 (s, 2
CMe3), 34.7 (s, 2 CMe3), 35.5 (s, 2 CMe3), 35.6 (s, 2 CMe3), 110.1
(d, JCP ) 13 Hz, CH arom), 118.1 (d, JCP ) 13 Hz, Cq), 122.3 (s,
CH arom), 122.7 (s, CH arom), 124.3 (s, 2 CH arom), 124.4 (s, 3
CH arom), 126.6 (s, CH arom), 127.3 (s, 2 CH arom), 127.4 (s, 2
CH arom), 127.8 (d, JCP ) 9 Hz, 6 CH arom), 128.5 (m, Cq), 128.7
(s, 3 CH arom), 131.0 (s, 2 Cq), 131.2 (s, 2 Cq), 133.8 (d, JCP ) 13
Hz, 6 CH arom), 133.9 (s, 2 Cq), 137.4 (d, JCP ) 36 Hz, 3 Cq),
7.01 (t, JHH ) 8 Hz, 1H, H arom), 7.28 (d, JHH ) 2.5 Hz, 4H, 4
3
4
H arom), 7.50 (d, JHH ) 2.5 Hz, 4H, 4 H arom). 31P{1H} NMR
4
(CD2Cl2, 162.1 MHz): δ 177.3 (d, JPRh ) 253 Hz). 13C{1H} NMR
(CDCl3, 75.5 MHz): δ 31.5 (s, 4 CMe3), 31.5 (s, 4 CMe3), 34.7 (s,
4 CMe3), 35.6 (s, 4 CMe3), 58.8 (br s, C2H4), 105.7 (t, JPC ) 9 Hz,
2 CH arom), 124.8 (s, 4 CH arom), 125.2 (s, CH arom), 126.7 (s,
4 CH arom), 131.6 (s, 4 Cq arom), 139.9 (dt, JRhC ) 29 Hz, JPC
)
16 Hz, Cq arom), 140.1 (s, 4 Cq arom), 145.5 (br s, 4 OCq arom),
147.4 (s, 4 Cq arom), 158.8 (t, JPC ) 16 Hz, 2 OCq arom). Anal.
Calcd for C64H87O6P2Rh: C, 68.8; H, 7.8. Found: C, 69.0; H, 8.2.
Rh(PCPb)(η2-C2H4) (8b). In a Fischer-Porter vessel compound
5b (0.100 g, 0.08 mmol) and Se (0.007 g, 0.09 mmol) were added
to THF (5 mL). The reactor was charged with 4 atm of C2H4 and
heated at 65 °C for 5 days. Solvent was removed under reduced
pressure and the remaining solid purified by chromatography on a
silica gel column (CH2Cl2:hexane, 1:10). Compound 8b was
obtained as a yellow solid (0.045 g, 56%). 1H NMR (CD2Cl2, 400
MHz): δ 1.19 (s, 18H, 2 CMe3), 1.37 (s, 18H, 2 CMe3), 1.88 (s,
6H, 2 Ar-Me), 1.96 (s, 6H, 2 Ar-Me), 2.28 (d, Japp ) 10 Hz, 2H,
C2H4), 2.33 (s, 6H, 2 Ar-Me), 2.36 (s, 6H, 2 Ar-Me), 2.98 (d, Japp
) 10 Hz, 2H, C2H4), 6.64 (d, 3JHH ) 8.0 Hz, 2H, 2 H arom), 6.97
139.8 (d, JCP ) 5 Hz, 2 Cq), 139.9 (ddt, JCP ) 59, 14 Hz, JCRh
)
28 Hz, Cq), 146.4 (s, 4 Cq), 147.1 (d, JCP ) 10 Hz, 2 Cq), 147.3 (d,
JCP ) 10 Hz, 2 Cq), 153.9 (d, JCP ) 20 Hz, Cq), 157.7 (d, JCP ) 19
Hz, Cq). Anal. Calcd for C84H100O6P3Rh: C, 72.0; H, 7.2. Found:
C, 72.1; H, 7.3.
Rh(PCPa)(CO) (6a). To a solution of compound 3a (0.04 g, 0.03
mmol) in THF (10 mL) was added an excess of elemental selenium
(0.005g,0.06mmol).ThemixturewasintroducedinaFischer-Porter
vessel and charged with 2 atm of CO. After 1 h the reactor was
vented and solvent removed. The solution was concentrated until
turbidity and filtered. After the mixture stood for 24 h, compound
4a was collected as yellow crystals (0.020 g, 60%). IR (Nujol mull,
cm-1): 2017 (νCO). 1H NMR (CDCl3, 500 MHz): δ 1.33 (s, 36H, 4
3
(t, JHH ) 8.0 Hz, 1H, H arom), 7.18 (s, 2H, 2 H arom), 7.33 (s,
3
CMe3), 1.40 (s, 36H, 4 CMe3), 6.64 (d, JHH ) 8 Hz, 2H, 2 H
2H, 2 H arom). 31P{1H} NMR (CD2Cl2, 162.1 MHz): δ 175.1 (d,
JPRh ) 253 Hz). 13C{1H} NMR (CD2Cl2, 75.5 MHz): δ 16.7 (s, 2
Ar-Me), 16.8 (s, 2 Ar-Me), 20.5 (s, 2 Ar-Me), 20.6 (s, 2 Ar-Me),
3
4
arom), 7.05 (t, JHH ) 8 Hz, 1H, H arom), 7.20 (d, JHH ) 2 Hz,
4H, 4 H arom), 7.45 (d, JHH ) 2 Hz, 4H, 4 H arom). 31P{1H}
4

Rh Complexes with Pincer Diphosphite Ligands
Organometallics, Vol. 28, No. 2, 2009 559
31.8 (s, 2 CMe3), 32.0 (s, 2 CMe3), 35.2 (s, 4 CMe3), 58.6 (br d,
JCP ) 4 Hz, C2H4), 105.9 (dd, JCRh ) 9 Hz, JCP ) 9 Hz, 2 CH
arom), 125.5 (s, CH arom), 128.9 (s, 2 CH arom), 129.2 (s, 2 CH
arom), 130.4 (s, 2 Cq), 130.7 (s, 2 Cq), 133.6 (s, 2 Cq), 133.6 (s, 2
Cq), 135.4 (s, 2 Cq), 135.8 (s, 2 Cq), 137.9 (s, 2 Cq), 138.0 (s, 2
Chart 1
Cq), 140.0 (dt, JCRh ) 29 Hz, JCP ) 16 Hz, Cq), 145.3 (dd, JCRh
)
5 Hz, JCP ) 5 Hz, 2 Cq), 145.6 (s, 2 Cq), 159.4 (dd, JCRh ) 16 Hz,
JCP ) 16 Hz, 2 Cq). Anal. Calcd for C56H71O6P2Rh: C, 66.9; H,
7.1. Found: C, 66.5; H, 7.3.
3JHH ) 4.0 Hz, 3H, MeCHdCH2), 1.22 (s, 18H, 2 CMe3), 1.35 (s,
18H, 2 CMe3), 1.84 (s, 6H, 2 Ar-Me), 1.94 (s, 6H, 2 Ar-Me), 2.33
Rh(PCPc)(η2-C2H4) (8c). In a Fischer-Porter vessel compound
5c (0.200 g, 0.14 mmol) and Se (0.014 g, 0.18 mmol) were added
to THF (5 mL). The reactor was charged with 4 atm of C2H4 and
heated at 50 °C for 3 days. Solvent was removed under reduced
pressure and the remaining solid extracted with n-pentane (3 × 10
mL) and purified by chromatography on a silica gel column
(hexane). Compound 8c was obtained as a yellow solid (0.075 g,
3
(s, 6H, 2 Ar-Me), 2.37 (s, 6H, 2 Ar-Me), 2.38 (br d, JHH ) 9.0
3
Hz, 1H, MeCHdCH2), 3.83 (br d, JHH ) 13.5 Hz, 1H,
3
MeCHdCH2), 3.86 (br m, 1H, MeCHdCH2), 6.58 (d, JHH ) 8.0
3
Hz, 2H, 2 H arom), 6.93 (t, JHH ) 8.0 Hz, 1H, H arom), 7.17 (s,
2H, 2 H arom), 7.29 (s, 2H, 2 H arom). 31P{1H} NMR (CD2Cl2,
202.4 MHz): δ 169.5 (d, JPRh ) 258 Hz). 13C{1H} NMR (CD2Cl2,
125.8 MHz): δ 16.6 (s, 2 Ar-Me), 16.6 (s, 2 Ar-Me), 18.9 (s,
MeCHdCH2), 20.4 (s, 2 Ar-Me), 20.6 (s, 2 Ar-Me), 31.7 (s, 2
CMe3), 32.0 (s, 2 CMe3), 35.1 (s, 2 CMe3), 35.2 (s, 2 CMe3), 64.5
(s, MeCHdCH2), 76.9 (s, MeCHdCH2), 105.8 (dd, JCRh ) 9 Hz,
JCP ) 9 Hz, 2 CH arom), 125.2 (s, CH arom), 128.6 (s, 2 CH arom),
129.0 (s, 2 CH arom), 130.4 (s, 2 Cq), 130.9 (s, 2 Cq), 133.4 (s, 2
Cq), 133.5 (s, 2 Cq), 135.3 (s, 2 Cq), 135.7 (s, 2 Cq), 137.9 (s, 2
1
46%). H NMR (CD2Cl2, 500 MHz): δ 1.27 (s, 36H, 4 CMe3),
1.35 (s, 18H, 2 CMe3), 1.36 (s, 18H, 2 CMe3), 2.80 (br s, 4H, C2H4),
7.08 (s, 1H, H arom), 7.21 (m, 1H, H arom), 7.28 (m, 5H, 5 H
3
arom), 7.48 (s, 4H, 4 H arom), 7.65 (d, JHH ) 8.0 Hz, 1H, H
arom), 7.83 (d, 3JHH ) 7.6 Hz, 1H, H arom). 31P{1H} NMR (CD2Cl2,
202.4 MHz): δ 174.6 (dd, JPP ) 590 Hz, JPRh ) 253 Hz, P-O),
178.0 (dd, JPP ) 590 Hz, JPRh ) 252 Hz, P-O). 13C{1H} NMR
(CD2Cl2, 125.8 MHz): δ 31.5 (s, 2 CMe3), 31.6 (s, 6 CMe3), 35.0
(s, 4 CMe3), 35.8 (s, 2 CMe3), 35.9 (s, 2 CMe3), 59.5 (br s, C2H4),
101.6 (d, JCP ) 13 Hz, CH arom), 118.8 (d, JCP ) 12 Hz, Cq),
122.3 (s, CH arom), 124.0 (s, CH arom), 125.0 (s, CH arom), 125.4
(s, 4 CH arom), 127.1 (s, 2 CH arom), 127.1 (s, 2 CH arom), 127.3
(s, CH arom), 131.9 (s, 2 Cq), 131.9 (s, 2 Cq), 133.7 (s, Cq), 139.5
(m, Cq), 140.5 (s, 2 Cq), 140.6 (s, 2 Cq), 145.9 (m, 4 Cq), 148.2 (s,
Cq), 138.3 (s, 2 Cq), 139.5 (m, Cq), 145.5 (dd, JCRh ) 5 Hz, JCP
)
5 Hz, 2 Cq), 145.6 (s, 2 Cq), 159.2 (dd, JCRh ) 16 Hz, JCP ) 16 Hz,
2 Cq).
QUEST3D Search Details. The QUEST3D searches commented
in the text have been carried out on the Cambridge Structural
Database (updated Nov 2007) and have the following details. (a)
For the analysis of the P-M-P angles in square-planar transition-
metal complexes containing a pincer ligand (see see Chart 1, E is
any atom), the CSD search with restrictions (not disordered, no
errors, R e 0.075) gave a mean value of 164° (89 fragments, 71
hits). (b) For the analysis of the C-C distance of ethylene rhodium
complexes, the CSD search without any restriction gave a mean
value of 1.39 Å (146 fragments, 70 hits). A similar value (1.38 Å
for 121 fragments, 57 hits) was obtained with restrictions (not
disordered, no errors, R e 0.075). In this case the Rh-C mean
value is 2.13 Å.
Computational Details. The electronic structures and geometries
of the model complexes I-VII were computed within the density
functional theory at the B3LYP level.33 The Rh atom was described
with the Stuttgart Relativistic Small Core ECP basis set34 and a
polarization function. In all cases the atoms corresponding to the
pincer ligand are described using a TZVP basis set. For the more
complex models IV-VII, the biphenyl groups are described using
a DZVP basis set. All the calculations were performed using the
Gaussian03 package.35 Figures were drawn using Molekel.36 XYZ
coordinates of all optimized complexes are available upon request.
4 Cq), 153.6 (dd, JCP ) 23 Hz, JCRh ) 9 Hz, Cq), 157.6 (dd, JCP
)
23 Hz, JCRh ) 9 Hz, Cq). Anal. Calcd for C68H89O6P2Rh: C, 70.0;
H, 7.7. Found: C, 69.7; H, 7.9.
Rh(PCPa)(η2-MeC2H3) (9a). In a Fischer-Porter vessel com-
pound 8a (0.100 g, 0.09 mmol) was added to THF (5 mL). The
reactor was charged with 1 atm of propene at room temperature
and stirred for 5 days, with the propene atmosphere replaced every
24 h. Solvent was removed under reduced pressure, yielding
compound 9a, accompanied by a minor amount of 8a (< 3%), as
a yellow solid (0.100 g, quantitative). All attempts to further purify
compound 9a were unsuccessful. 1H NMR (CD2Cl2, 300 MHz): δ
1.06 (d, 3JHH ) 5.4 Hz, 3H, MeCHdCH2), 1.31 (s, 18H, 2 CMe3),
1.38 (s, 18H, 2 CMe3), 1.39 (s, 18H, 2 CMe3), 1.41 (s, 18H, 2
3
CMe3), 2.51 (br d, JHH ) 8.7 Hz, 1H, MeCHdCH2), 3.29 (br d,
3JHH ) 12.3 Hz, 1H, MeCHdCH2), 4.39 (br m, 1H, MeCHdCH2),
6.66 (d, 3JHH ) 7.8 Hz, 2H, 2 H arom), 7.00 (t, 3JHH ) 8.1 Hz, 1H,
4
4
H arom), 7.25 (d, JHH ) 2.4 Hz, 2H, 2 H arom), 7.32 (d, JHH
)
4
2.4 Hz, 2H, 2 H arom), 7.48 (d, JHH ) 2.1 Hz, 2H, 2 H arom),
7.55 (d, JHH ) 2.4 Hz, 2H, 2 H arom). 31P{1H} NMR (CD2Cl2,
4
121.5 MHz): δ 171.6 (d, JPRh ) 259 Hz). 13C{1H} NMR (CD2Cl2,
75.5 MHz): δ 19.5 (s, MeCHdCH2), 31.5 (s, 2 CMe3), 31.7 (s, 4
CMe3), 32.0 (s, 2 CMe3), 35.1 (s, 4 CMe3), 35.9 (s, 4 CMe3), 65.0
(s, MeCHdCH2), 78.2 (s, MeCHdCH2), 106.0 (dd, JCRh ) 9 Hz,
JCP ) 9 Hz, 2 CH arom), 125.2 (s, 2 CH arom), 125.3 (s, 2 CH
arom), 125.8 (s, CH arom), 126.9 (s, 2 CH arom), 127.2 (s, 2 CH
arom), 131.7 (s, 2 Cq), 132.2 (s, 2 Cq), 139.3 (dt, JCRh ) 30 Hz, JCP
) 17 Hz, Cq), 140.6 (s, 4 Cq), 145.6 (s, 2 Cq), 146.1 (s, 2 Cq),
147.9 (s, 2 Cq), 148.3 (s, 2 Cq), 159.3 (dd, JCRh ) 15 Hz, JCP ) 15
Hz, 2 Cq).
(33) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C. T.;
Yang, W. T.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(34) Dolg, M.; Stoll, H.; Preuss, H.; Pitzer, R. M. J. Phys. Chem. 1993,
97, 5852.
(35) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
Revision C.02; Gaussian, Inc., Wallingford, CT, 2004.
Rh(PCPb)(η2-MeC2H3) (9b). In a Fischer-Porter vessel com-
pound 8b (0.020 g, 0.02 mmol) was added to THF (3 mL), and the
reactor was charged with 1 atm of propene and stirred for 9 days,
with the propene atmosphere renewed every 24 h. Solvent was
removed under reduced pressure. Compound 9b was obtained as a
yellow solid, contaminated with minor amounts of 8b (< 5%)
(0.020 g, quantitative). All attempts to further purify compound
1
9b were unsuccessful. H NMR (CD2Cl2, 500 MHz): δ 1.03 (d,
(36) Portmann, S.; Luthi, H. P. Chimia 2000, 54, 766.

560 Organometallics, Vol. 28, No. 2, 2009
Rubio et al.
X-ray Structure Determinations. Crystallographic data were
collected on a Bruker-Nonius X8Apex-II CCD diffractometer at
100(2) K using graphite-monochromated Mo KR1 radiation (λ )
0.710 73 Å). The data were reduced (SAINT)37 and corrected for
Lorentz-polarization and absorption effects by a multiscan method
(SADABS).38 Structures were solved by direct methods (SIR-
2002)39 and refined against all F2 data by full-matrix least-squares
techniques (SHELXTL-6.12).40 All the non-hydrogen atoms were
refined with anisotropic displacement parameters. The hydrogen
atoms were included from calculated positions and refined riding
on their respective carbon atoms with isotropic displacement
parameters. A summary of cell parameters and data collection,
structure solution, and refinement details is given in Table 6.
Solid-State NMR. Single-pulse MAS NMR spectra were recorded
on a Bruker DRX400 spectrometer with a magnetic field of 9.36 T
and equipped with a multinuclear probe. Powdered samples were
packed in 4 mm zirconia rotors and spun at 10 KHz. 31P MAS
NMR spectra were acquired at a frequency of 161.98 MHz, using
a π/4 pulse width of 3.58 µs and a pulse space of 0.1 s. The 13C
MAS NMR spectrum was recorded at 104.26 MHz with proton
decoupling, a pulse width of 2.5 µs (π/2 pulse length 7.5 µs), and
a delay time of 2 s. The chemical shifts are reported in ppm from
tetramethylsilane for 1H and 13C and from 87% solutions of H3PO4
for 31P.
Acknowledgment. We gratefully acknowledge Dr. M. D.
Alba (ICMSE) for acquiring solid-state NMR spectra. The
Ministerio de Educacio´n y Ciencia (CTQ2006-05527, FED-
ER support, and CONSOLIDER-INGENIO, CSD2007-
00006) and Junta de Andaluc´ıa are also thanked for financial
support. M.R. thanks the Ministerio de Educacio´n y Ciencia
for a FPU fellowship and D.d.R. the 6th FP of the EU for a
MC-OIF.
(37) SAINT 6.02; Bruker-AXS, Inc., Madison, WI 53711-5373, 1997-
1999.
(38) Sheldrick, G. SADABS; Bruker AXS, Inc., Madison, WI, 1999.
(39) Burla, M. C.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.;
Giacovazzo, C.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 2003, 36,
1103.
Supporting Information Available: Figures giving selected
NMR spectra and graphics for olefin rotation barriers and CIF files
giving crystallographic details. This material is available free of
charge via the Internet at http://pubs.acs.org.
(40) SHELXTL 6.14; Bruker AXS, Inc. Madison, WI, 2000-2003.
OM800830W