Synthesis and reactivity of porphyrinatorhodium(II)-triethylphosphine adducts: The role of PEt3 in stabilizing a formal Rh(II) state

Article abstract of DOI:10.1021/ja001364u

Rh(por)H, where por is an octaethyl- or meso-tetraphenylporphyrin dianion, reacts with triethylphosphine to form stable mononuclear paramagnetic formally-Rh(II) complexes, Rh(OEP)(PEt₃) and Rh(TPP)-(PEt₃)₂. The former adduct is also obtained as the sole product of the reaction between Rh₂(OEP)₂ and PEt₃. The EPR spectroscopy at 77 K shows both complexes to have mainly porphyrin-based HOMOs. The composition and the reactivity of Rh(TPP)(PEt₃)₂ support its formulation as Rh(III)(TPP)(PEt₃)₂. In contrast, Rh(OEP)-(PEt₃) demonstrates the reactivity of both a Rh(II) d⁷ center and a porphyrin π-anion radical. The adduct reacts with O₂ as a Rh(II)(por) species, originally forming a Rh(III)-superoxido derivative. In contrast, with water Rh-(OEP)(PEt₃) reacts as a porphyrin π-anion radical, yielding a Rh(III)-octaethylphlorin complex. The latter is the first characterized phlorin complex of a heavy transition metal. The dual reactivity of Rh(OEP)(PEt₃) is proposed to arise from thermal excitation of the unpaired electron from the porphyrin-based HOMO onto the metal-based LUMO (dσ*(Rh-P)). Unlike the other reported 1:1 adducts of Rh(II)(por) species with σ-basic ligands, Rh(OEP)(PEt₃) is remarkably stable toward disproportionation to Rh(I) and Rh(III). To understand the origin of this stability, the affinity of Rh(III)(OEP)⁺ toward PEt₃ and pyridine was measured spectrophotometrically. The high binding affinity of PEt₃ to Rh(OEP) is proposed as the underlying cause of the increased stability of Rh(OEP)(PEt₃) toward disproportionation.

Full text of DOI:10.1021/ja001364u

11812
J. Am. Chem. Soc. 2000, 122, 11812-11821
Synthesis and Reactivity of
Porphyrinatorhodium(II)-Triethylphosphine Adducts: The Role of
PEt3 in Stabilizing a Formal Rh(II) State
James P. Collman* and Roman Boulatov
Contribution from the Department of Chemistry, Stanford UniVersity, Stanford, California 94305
ReceiVed April 18, 2000
Abstract: Rh(por)H, where por is an octaethyl- or meso-tetraphenylporphyrin dianion, reacts with trieth-
ylphosphine to form stable mononuclear paramagnetic formally-RhII complexes, Rh(OEP)(PEt3) and Rh(TPP)-
(PEt3)2. The former adduct is also obtained as the sole product of the reaction between Rh2(OEP)2 and PEt3.
The EPR spectroscopy at 77 K shows both complexes to have mainly porphyrin-based HOMOs. The composition
and the reactivity of Rh(TPP)(PEt3)2 support its formulation as RhIII(TPP•-)(PEt3)2. In contrast, Rh(OEP)-
(PEt3) demonstrates the reactivity of both a RhII d7 center and a porphyrin π-anion radical. The adduct reacts
with O2 as a RhII(por) species, originally forming a RhIII-superoxido derivative. In contrast, with water Rh-
(OEP)(PEt3) reacts as a porphyrin π-anion radical, yielding a RhIII-octaethylphlorin complex. The latter is
the first characterized phlorin complex of a heavy transition metal. The dual reactivity of Rh(OEP)(PEt3) is
proposed to arise from thermal excitation of the unpaired electron from the porphyrin-based HOMO onto the
metal-based LUMO (dσ*Rh-P). Unlike the other reported 1:1 adducts of RhII(por) species with σ-basic ligands,
Rh(OEP)(PEt3) is remarkably stable toward disproportionation to RhI and RhIII. To understand the origin of
this stability, the affinity of RhIII(OEP)+ toward PEt3 and pyridine was measured spectrophotometrically. The
high binding affinity of PEt3 to Rh(OEP) is proposed as the underlying cause of the increased stability of
Rh(OEP)(PEt3) toward disproportionation.
Introduction
abstraction from water,8a,g ethanol,8e,g primary amines,7b and
trialkylstannates,8b addition to CdC bonds,8b and coupling
leading to dimetal R-diketones, (por)RhC(O)C(O)Rh(por).8a-d,f
RhII(por) complexes react with H2 and H2/CO mixtures, yielding
metallohydride1a,9 and metalloformyl10 derivatives, respectively.
Addition of O2 or NO to RhII(por) results in quite stable
superoxido and nitrosyl complexes, RhIII(por)(O2-) and Rh(por)-
(NO).11 The former species can undergo further transforma-
tions,11,12 such as conversion to a (µ,η1,η1)-peroxo complex.11
Despite this remarkable chemistry, applications of RhII-
porphyrin complexes to catalysis are hardly explored. While
catalytic activity of both RhIII(por) derivatives13-15 and RhII
carboxylates16 in a variety of synthetic organic transformations
is well established, RhII(por) species have only been examined
Porphyrin complexes of d7 metalloradical RhII demonstrate
remarkable reactivity with otherwise rather inert substrates.
Activation, under mild conditions, of methane,1a-c various
benzylic1b,d and allylic1e C-H bonds, and Si-H and Sn-H
bonds2 have been reported. Facile reactions of RhII(OEP) (Figure
1) with aliphatic aldehydes and alkynes produce â-formyl
complexes3 and metalloalkenes, (OEP)RhCHdC(R)Rh(OEP),1e
respectively. Reductive coupling of ethene and acrylates4,5
as well as alkyl group abstraction from terminal olefins and
alkynes,6 alkyl phosphites,7a and alkylisocyanides7b by various
RhII(por) species have also been observed. Upon coordination
to the RhII-porphyrin moiety, CO is activated toward a variety
of 1-electron carbonyl reactions, such as hydrogen atom
(8) (a) Wayland, B. B.; Woods, B. A.; Pierce, R. J. Am. Chem. Soc.
1982, 104, 302-303. (b) Wayland, B. B.; Sherry, Alan E.; Poszmik, G.;
Bunn, A. G. J. Am. Chem. Soc. 1992, 114, 1673-1681. (c) Sherry, A. E.;
Wayland, B. B. J. Am. Chem. Soc. 1989, 111, 5010-5012. (d) Coffin, V.
L.; Brennen, W.; Wayland, B. B. J. Am. Chem. Soc. 1988, 110, 6063-
6069. (e) Zhang, X.-X.; Parks, G. F.; Wayland, B. B. J. Am. Chem. Soc.
1997, 119, 7938-7944. (f) Wayland, B. B.; Sherry, A. E.; Coffin, V. L. J.
Chem. Soc., Chem. Commun. 1989, 662-663. (g) Miller, R. G.; Kyle, J.
A.; Coates, G. W.; Anderson, D. J.; Fanwick, P. E. Organometallics 1993,
12, 1161-6.
(1) (a) Zhang, X.-X.; Wayland, B. B. J. Am. Chem. Soc. 1994, 116,
7897-7898. (b) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc.
1991, 113, 5305-5311. (c) Sherry, A. E.; Wayland, B. B. J. Am. Chem.
Soc. 1990, 112, 1259-1261. (d) Del Rossi, K. J.; Wayland, B. B. J. Am.
Chem. Soc. 1985, 107, 7941-7944. (e) Ogoshi, H.; Setsune, J.; Yoshida,
Z. J. Am. Chem. Soc. 1977, 99, 3869-3870.
(2) Mizutani, T.; Uesaka, T.; Ogoshi, H. Organometallics 1995, 14, 4,
341-346.
(3) Del Rossi, K. J.; Zhang, X.-X.; Wayland, B. B. J. Organomet. Chem.
1995, 504, 47-56.
(9) (a) Wayland, B. B.; Ba, S.; Sherry, A. E. Inorg. Chem. 1992, 31,
148-150. (b) Collman, J. P.; Ha, Y.; Guilard, R.; Lopez, M. A. Inorg.
Chem. 1993, 32, 1788-1794.
(4) (a) Paonessa, R. S.; Thomas, N. C.; Halpern, J. J. Am. Chem. Soc.
1985, 107, 4333-4335. (b) Wayland, B. B.; Poszmik, G.; Fryd, M.
Organometallics 1992, 11, 3534-3542).
(10) (a) Farnos, M. D.; Woods, B. A.; Wayland, B. B. J. Am. Chem.
Soc. 1986, 108, 3659-3663. (b) Wayland, B. B.; Van Voorhees, S. L.;
Wilker, C. Inorg. Chem. 1986, 25, 4039-4024. (c) Van Voorhees, S. L.;
Wayland, B. B. Organometallics 1985, 4, 1887-1888.
(11) Wayland, B. B.; Newman, A. R. Inorg. Chem. 1981, 20, 3093-
3097.
(12) (a) Sakurai, H.; Uchikubo, H.; Ishizu, K.; Tajima, K.; Aoyama, Y.;
Ogoshi, H. Inorg. Chem. 1988, 27, 2691-2695. (b) Anderson, J. E.; Yao,
C. L.; Kadish, K. M. Inorg. Chem. 1986, 25, 3224-3228.
(5) (a) Bunn, A. G.; Wayland, B. B. J. Am. Chem. Soc. 1992, 114, 6917-
6919. (b) Wayland, B. B.; Feng, Y.; Ba, S. Organometallics 1989, 8, 1438-
1441.
(6) Anderson, J. E.; Yao, C.-L.; Kadish, K. M. Organometallics 1987,
6, 706-711.
(7) (a) Wayland, B. B.; Sherry, A. E.; Bunn, A. G. J. Am. Chem. Soc.
1993, 115, 7675-7684. (b) Poszmik, G.; Carroll, P. J.; Wayland, B. B.
Organometallics 1993, 12, 3410-3417.
10.1021/ja001364u CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/14/2000

Porphyrinatorhodium(II)-Triethylphosphine Adducts
J. Am. Chem. Soc., Vol. 122, No. 48, 2000 11813
Figure 1. Chemical structures and abbreviations of select porphyrins and octaethylphlorin.
as initiators of free-radical polymerization of acrylates5b and
photocatalytic production of methanol from CO/H2 mixtures.17
Likewise, extensive use of CoII(por) compounds as biomimetic
models for respiratory chain enzymes such as cytochrome c
oxidase,18a-b myoglobin, and hemoglobin18c-d contrasts sharply
with the lack of similar studies involving RhII(por). This situation
persists even though RhII-porphyrins have a higher affinity
toward O2, and the resulting complexes are more stable, making
the relevant intermediates more amenable to characterization.
The paucity of successful work in this area is largely due to
the apparent susceptibility of RhII(por) species to undergo
irreversible disproportionation in the presence of simple Lewis
bases, such as amines, N-containing heterocycles, and certain
phosphines.7a,19 This makes the design of an appropriate
porphyrin-based ligand system that would allow one to take
full advantage of the remarkable chemistry of RhII(por) species
much more challenging. While elegant work by Wayland et
al.1a,b,4b,5a,8a has demonstrated that the porphyrin’s peripheral
substituents can be used to alter the reactivity of the resulting
square-planar RhII(por) species, an axial ligand L in RhII(por)L
adducts can be expected to have an even more pronounced
effect. Prior work by us and others18d,20 have shown the
importance of such axial bases both for directing the substrate
binding to a particular face of a metalloporphyrin and for
attenuating the reactivity of the trans-bound substrate. While
most of this work concerned Fe- and Co-porphyrins, a strong
effect of auxiliary ligands on the reactivity of RhII(por) toward
H2 has also been observed.9b
(13) Asymmetric cyclopropanation: (a) Bartley, D. W.; Kodadek, T. J.
J. Am. Chem. Soc. 1993, 115, 1656-1660. (b) Brown, K. C.; Kodadek, T.
J. J. Am. Chem. Soc. 1992, 114, 8336-8338. (c) Maxwell, J. L.; Brown,
K. C.; Bartley, D. W.; Kodadek, T. J. Science 1992, 256, 1544-1547. (d)
Gross, Z.; Galili, N.; Simkhovich, L. Tetrahedron Lett. 1999, 40, 1571-
1574.
(14) Ketone transformations: (a) Aoyama, Y.; Yamagishi, A.; Tanaka,
Y.; Toi, H.; Ogoshi, H. J. Am. Chem. Soc. 1987, 109, 4735-4737. (b)
Aoyama, Y.; Fujisawa, T.; Toi, H.; Ogoshi, H. J. Am. Chem. Soc. 1986,
108, 943-947.
(15) (a) Asymmetric carbene insertion in the O-H bonds: Hayashi, T.;
Kato, T.; Kaneko, T.; Asai, T.; Ogoshi, H. J. Organomet. Chem. 1994,
473, 323-327. (b) Oxidative carbonylation of amines: Leung, T. W.;
Dombek, B. D. J. Chem. Soc. Chem. Commun. 1992, 205-206. (c) Diels-
Alder reactions: Bartley, D. W.; Kodadek, T. J. Tetrahedron Lett. 1990,
31, 6303-6306. (d) Hydroboration-oxidation of olefins: Aoyama, Y.;
Tanaka, Y.; Fujisawa, T.; Watanabe, T.; Ogoshi, H. J. Org. Chem. 1987,
52, 2555-2559.
(16) (a) Padwa, A.; Weingarten, M. D. J. Org. Chem. 2000, 65, 3722-
3732. (b) Doyle, M. P.; Davies, S. B.; Hu, W. Org. Lett. 2000, 2, 1145-
1147. (c) Davies, H. M. L.; Hansen, T.; Churchil, M. R. J. Am. Chem. Soc.
2000, 122, 3063-3070. (d) Hodgson, D. M.; Stuppie, P. A.; Johnstone, C.
Chem. Commun. 1999, 2185-2186. (e) Wood, J. L.; Moniz, G. A.; Pflum,
D. A.; Stoltz, B. M.; Holubec, A. A.; Dietrich, H.-J. J. Am. Chem. Soc.
1999, 121, 1748-1749. (f) Kitagaki, S.; Anada, M.; Kataoka, O.; Matsuno,
K.; Umeda, C.; Watanabe, N.; Hashimoto, S. J. Am. Chem. Soc. 1999, 121,
1417-1418. (g) Davies, H. M. L.; Hansen, T. J. Am. Chem. Soc. 1997,
119, 9075-9076. (h) Doyle, M. P.; Peterson, C. S.; Parker, D. L. Angew.
Chem., Int. Ed. Engl. 1996, 35, 1334-1336. (i) Davies, H. M. L.; Bruzinski,
P.; Hutcheson, D. K.; Kong, N.; Fall, M. J. J. Am. Chem. Soc. 1996, 118,
6897-6907. (j) Doyle, M. P.; Protopopova, M. N.; Poulter, C. D.; Rogers,
D. H. J. Am. Chem. Soc. 1995, 117, 7281-7282. (k) For reviews, see: Maas,
G. Top. Curr. Chem. 1987, 137, 75-253. Doyle, M. P. Chem. ReV. 1986,
86, 919-940.
(17) Bosch, H. W.; Wayland, B. B. J. Chem. Soc., Chem. Commun. 1986,
900-901.
(18) (a) Yu, H.-Z.; Baskin, J. S.; Steiger, B.; Anson, F. C.; Zewail, A.
H. J. Am. Chem. Soc. 1999, 121, 484-485. (b) Anson, F. C.; Shi, C.; Steiger,
B. Acc. Chem. Res. 1997, 30, 437-444. (c) Steiger, B.; Baskin, J. S.; Anson,
F. C.; Zewail, A. H. Angew. Chem., Int. Ed. 2000, 39, 257-259. (d)
Collman, J. P.; Fu, L. Acc. Chem. Res. 1999, 32, 455-463.
Further progress in this area is not possible without a deeper
understanding of the coordination chemistry of the RhII(por)
unit. In the few reports on this subject,7a,21 two different
hypotheses on the mechanism of the ligand-induced dispropor-
tionation of RhII(por) species have been put forward. Wayland
et al. proposed that the process is driven by coordination of a
(19) DeWit, D. G. Coord. Chem. ReV. 1996, 147, 209-246.
(20) Mamenteau, M.; Reed, C. Chem. ReV. 1994, 94, 659-698.
(21) (a) Grass, V.; Lexa, D.; Momenteau, M.; Saveant, J.-M. J. Am.
Chem. Soc. 1997, 119, 3536-3542. (b) Ni, Y.; Fitzgerald, J. P.; Carroll,
P.; Wayland, B. B. Inorg. Chem. 1994, 33, 2029-2035. (c) Wayland, B.
B.; Balkus, K. J.; Farnos, M. D. Organometallics 1989, 8, 950-955. (d)
Kadish, K. M.; Hu, Y.; Boschi, T.; Tagliatesta, P. Inorg. Chem. 1993, 32,
2996-3002. (e) Kadish, K. M.; Araullo, C.; Yao, C.-L. Organometallics
1988, 7, 1583-1587.

11814 J. Am. Chem. Soc., Vol. 122, No. 48, 2000
Collman and BoulatoV
donor molecule to one of the disproportionation products,
RhIII(por).7a In contrast, Saveant et al. more recently suggested21a
that disproportionation is due to the deligation of the original
RhII(por)Lx adduct. Moreover, they and others21d,e have also
shown that electrochemical reduction of RhIII(TPP)P2+ adducts
(P ) PEt3, PMe2Ph) yields species that were stable on the time
scale of their experiments, in a striking contrast with redox
behavior of other RhIII-porphyrin derivatives. This result was
interpreted as an indication that these phosphines stabilize the
RhII state toward disproportionation.21a
In the current paper, we describe the synthesis and reactivity
of Rh(OEP)(PEt3) and Rh(TPP)(PEt3)2 complexes, which result
from addition of PEt3 to the corresponding metallohydrides,
Rh(por)H, as well as reaction of Rh2(OEP)2 with PEt3. The
particular choice of the porphyrins was determined by their very
different electronic properties and sterically compact nature.
This, in conjunction with the previous work on sterically
encumbered Rh(TMP)(PEt3),7a (Figure 1) provides an op-
portunity to probe the effects of both electronic and steric factors
on distribution of unpaired electron density in, and hence
reactivity of, RhII(por) adducts with PEt3.
Figure 2. 1H NMR spectrum of HRh(TPP) in C6D6 in an evacuated
NMR tube.
µTorr) or heated at reflux in a toluene solution under strictly
anaerobic and anhydrous conditions.
Rh(TPP)(PEt3)2 and Rh(OEP)(PEt3). Addition of PEt3 to
a benzene solution of Rh(OEP)H results in rapid formation of
an intermediate adduct, (PEt3)Rh(OEP)H. The hydride resonance
1
in its H NMR spectrum appears as a broad singlet (fwhh )
220 Hz) at -26.5 ppm. The broadening of the signal is too
large for the fine structure from the 31P-1H and 103Rh-1H spin
couplings to be resolved. However, the 13.8 ppm downfield
shift of the hydride resonance in Rh(OEP)(PEt3)H relative to
its position in Rh(OEP)H (-40.8 ppm) is consistent with the
coordination of a strongly basic ligand trans to the hydride.
Smaller downfield shifts are observed in similar adducts with
weaker donors, such as pyridine21c and methyl isocyanide8d
(-32.95 ppm in pyRh(OEP)H and -27.9 ppm in (MeNC)Rh-
(OEP)H). Unlike these adducts, (PEt3)Rh(OEP)H can be
isolated, but it undergoes hydrogen elimination over a period
of several hours. The resulting Rh(OEP)(PEt3) demonstrates
broadened paramagnetically shifted NMR resonances, whose
position is dependent on the presence of excess PEt3 in the
sample.
Results
Rh(por)H. The Rh-H bond in porphyrinatorhodium(III)
hydrides undergoes facile homolysis such that in solution
Rh(por)H exists in equilibrium with RhII(por) (or the corre-
sponding dimer) and H2.10b,22 Coordination of NO to Rh(OEP)H
was reported to facilitate the decomposition of the hydride.11
We anticipated seeing a similar effect in the case of PEt3.23
Indeed, addition of PEt3 to Rh(por)H yields a paramagnetic
species as expected from a monomeric RhII complex. However,
Rh(por)H obtained via the traditional aqueous route yielded
products that were too unstable to be isolated. To increase the
stability of the target adducts, we have examined alternative
syntheses of the metallohydride. A reaction between thoroughly
dried Rh(por)I and LiAlH4 under rigorously anhydrous condi-
tions yielded Rh(por)H suitable for further synthetic transforma-
tions.
On the other hand, no intermediate adducts were detected in
the reaction between the phosphine and Rh(TPP)H. Addition
of subequivalent amounts of PEt3 in a sealed NMR tube results
in a decrease in intensity, but no change in position of the
Rh(TPP)H resonances, indicating that formation of Rh(TPP)-
(PEt3)2 is strongly favored. Under the conditions of excess PEt3,
no broadening of the phosphine’s resonances is observed by
1H NMR, suggesting that if exchange between the coordinated
and free ligand occurs, it is slow on the NMR time scale.
Similarly, Rh2(OEP)2 reacts rapidly with an excess of PEt3
in toluene to give Rh(OEP)(PEt3) without detectable intermedi-
ates or disproportionation products.
Remarkably, Rh(TPP)H obtained under such anhydrous
conditions exhibits properties quite different from those ap-
pearing in the literature. For example, Rh(TPP)H and Rh(TTP)H
1
were reported10b to show significant broadening of H NMR
resonances ascribed to extensive homolysis of the Rh-H bond.
The hydride signals could not be observed unless the solutions
were pressurized with H2 (0.4-1 atm). In contrast, anhydrous
Rh(TPP)H demonstrates very sharp resonances, even under
vacuum (Figure 2), and H2 has no effect on their position or
shape. Moreover, the Rh-H bond is retained even if a sample
of Rh(TPP)H is pyrolyzed for 72 h at 230 °C under vacuum (5
The anisotropic EPR signals of both adducts are rather similar
(Figure 3). The relatively small g values (Table 1), although
not unusual for RhII complexes, are much closer to those
2
observed for porphyrin π-anion radicals than for the dz -based
(22) Setsune, J.; Yoshida, Z.; Ogoshi, H. J. Chem. Soc., Perkin Trans.
1 1982, 983-987.
radicals, RhII(TMP) and RhII(TPP). The modest 31P-super-
hyperfine (shf) coupling also suggests that, under the conditions
of the EPR experiments, the unpaired electron is located on a
mainly porphyrin-based orbital in both Rh(OEP)(PEt3) and
Rh(TPP)(PEt3)2. Apparently, formation of the Rh-P bond
causes such destabilization of the resulting metal-based dσ*Rh-P
(23) It is conceivable that coordination of PEt3 may promote heterolytic
cleavage of the Rh-H bond, resulting in transferring H- to an appropriate
organic substrate, such as the porphyrin macrocycle itself. In this case,
formation of a diamagnetic RhIII-phlorin complex is expected. Our results
suggest that this does not occur to an appreciable extent. Small amounts of
RhIII-phlorin complex, along with Rh(OEP)(PEt3)2+, are detected in the
reaction mixtures. However, addition of PEt3 to a solution of Rh2(OEP)2
also produces traces of the phlorin, even though the dimer is incapable of
transferring a H-. The result is likely due to the disproportionation of OEP
radical anion (vide infra) in the presence of traces of adventitious water.
However, the phosphine-promoted hydride transfer from Rh(por)H may
occur in the presence of an appropriate substrate. Indeed, Saveant observed
hydride transfer from electrochemically generated [RhII(por)H]- to DMSO,
as well as electrocatalytic evolution of H2 from HRh(por) in the presence
of a mineral acid and PEt3, presumably occurring by hydride transfer: Grass,
V.; Lexa, D.; Saveant, J.-M. J. Am. Chem. Soc. 1997, 119, 7526-7532.
2
(dz ) orbital that its energy becomes higher than that of the
LUMO of the porphyrin ligand (pπ*por). According to calcula-
tions, another strong σ-base, H-, causes similar destabilization
of the Rh-based dσ*Rh-H orbital relative to the porphyrin-based
27
pπ*por
.
In contrast, observation of a Rh-based EPR signal in
Rh(TMP)(PEt3) indicates that destabilization of dσ*Rh-P in
(24) Seth, J.; Bocian, D. F. J. Am. Chem. Soc. 1994, 116, 143-153.

Porphyrinatorhodium(II)-Triethylphosphine Adducts
J. Am. Chem. Soc., Vol. 122, No. 48, 2000 11815
Figure 3. EPR spectra of Rh(OEP)(PEt3), Rh(OEP)(PEt3)(O2), and
Rh(TPP)(PEt3)2 in toluene glass at 77 K; υ ) 9.4000 GHz.
Figure 4. UV-visible spectrum of Rh(OEP)(PEt3) in toluene/3 M PEt3.
Table 1. EPR Parameters for Rh(OEP)(PEt3), (O2)Rh(OEP)(PEt3),
and Rh(TPP)(PEt3)2 and Some Relevant Literature Data
g1, g2 (A[31P],
g3 (A[31P],
MHz)
complex
Rh(OEP)(PEt3)
MHz)
ref
2.018 (22.2)
2.021
2.0030
2.0029
2.0027
2.46
1.982
1.973
2.0021
2.0020
1.989
∼2
a
a
24
24
24
25
8c
Rh(TPP)(PEt3)2
H2OEP•-
H2TPP•-
Zn(OEP•-
Rh(TPP)
Rh(TMP)
)
2.65
1.915
Rh(TMP)(PEt3)
Rh(OETAP)py2
2.116 (948)
1.998 (giso)
2.004 (1154) 7a
21b
Rh(OEP)(PEt3) (O2)
Rh(OEP)(O2)
Rh(OEP)(P[OBu]3)(O2)
2.002, 1.982
2.100
2.084 (63),
1.964
1.988
2.000 (66.3)
a
11
11
Figure 5. Comparison of the low-energy parts of UV-visible spectra
of Rh(OEP)(PEt3) (solid line) and Rh(OEPhlorin)(PEt3)2 (dotted line)
in toluene/3 M PEt3.
2.004 (61)
RhII(5,5′-thiosalicylic acid) 1.962
1.952
26a
26b
RhII(cysteinato-O-methyl)2
2.003 (giso)
a Present work.
of the high-energy band centered at 436 yields at least four
Gaussian peaks at 401, 420, 436, and 454 nm, presumably
belonging to four different Rh(por) derivatives. The complexity
of the 500-700-nm region also suggests the presence of several
absorbing species. The low-energy part of the spectrum is
noteworthy: both porphyrin anion radicals and phlorins are
known to possess adsorption bands over 700 nm. However, the
differences in the UV-visible spectra of Rh(OEP)(PEt3) and
an authentic sample of a Rh(phlorin) complex (Figure 5) suggest
that the low-energy transitions in the spectrum of the former
more likely belong to an OEP-anion radical derivative.28
The probable multicomponent composition of UV-visible
samples of Rh(OEP)(PEt3) contrasts sharply with the purity of
its bulk samples assessed by the EPR and NMR spectroscopies
and via oxidation with FeCp2+. The differences are likely due
to partial dissociation of the adduct, and a variety of subsequent
chemical reactions taking place under the conditions of the
relatively low concentration of the analyte in the UV-visible
samples. Indeed, concentrated toluene solutions of Rh(OEP)-
(PEt3) change color from bright green to red-brown upon dilu-
tion. The comparatively small extinction coefficients of por-
phyrin anion radical derivatives ensure that even relatively small
fractions of porphyrin-based decomposition impurity would have
a profound effect on the observed UV-visible spectra.
this complex is insufficient to raise it above pπ*TMP, even though
the latter orbitals are lower in energy than the corresponding
pπ*OEP orbitals in Rh(OEP)(PEt3). This suggests that the overlap
2
between the dz orbital of Rh and the donor orbitals of the PEt3
ligand is less efficient in Rh(TMP)(PEt3), probably due to the
increased Rh-P separation resulting from unfavorable steric
interactions between the peripheral substituents of TMP and the
phosphine.
1
Both EPR and H NMR spectra of the OEP adduct support
the 1:1 phosphine-to-metal ratio. The composition of the TPP
analogue cannot be determined unambiguously by either
spectroscopy. This question of stoichiometry was thus addressed
by oxidizing these reactive, paramagnetic adducts into stable,
diamagnetic derivatives with 1 equiv of an outer-sphere oxidiz-
ing agent, FeCp2+. The resulting species demonstrate 1H NMR
and mass spectral properties that are identical to those of
authentic samples of Rh(OEP)(PEt3)+ and Rh(TPP)(PEt3)2
+
complexes (see Experimental Section). The composition of
Rh(OEP)(PEt3) determined via this method is in agreement with
1
its EPR and H NMR spectra. This strongly suggests that the
+
composition of Rh(TPP)(PEt3)2 also correctly reflects a
Rh:2P stoichiometry of the initial paramagnetic Rh(TPP)-
phosphine adduct.
The UV-visible spectra of Rh(OEP)(PEt3) (as 100-10 µM
toluene solutions) are complicated (Figure 4). Deconvolution
Reactivity of Rh(TPP)(PEt3)2 and Rh(OEP)(PEt3). Rh-
(TPP)(PEt3)2 demonstrates rather limited reactivity. The complex
(28) We are aware of only one other reported UV-visible spectrum of
an OEP anion radial derivative: the spectrum of Ni(OEP•-) was claimed
to have the lowest energy transition at 623 nm;29 this seems inconsistent
with the presence of near-IR absorption bands in anion radicals of
deuteroporphyrin and mesoporphyrin IX30 and TPP31 derivatives; the
absorption bands in the 700-900-nm region are also expected in the OEP
anion radical derivatives.32
(25) Hoshino, M.; Yasufuki, K.; Konishi, S.; Imamura, M. Inorg. Chem.
1984, 23, 1982-1984.
(26) (a) Srivastava, P. C.; Pandeya, K. B.; Nigam, H. L. Ind. J. Chem.
1975, 13, 85-86. (b) Pneumatikakis, G.; Psaroulis, P. Inorg. Chim. Acta
1980, 46, 97-98.
(27) Irie, R.; Li, X.; Saito, Y. J. Mol. Catal. 1984, 23, 23-27.

11816 J. Am. Chem. Soc., Vol. 122, No. 48, 2000
Collman and BoulatoV
is very oxygen-sensitive, converting into RhIII(TPP)(PEt3)2
+
within hours even in the solid state in a drybox, presumably by
reaction with adventitious O2. No other products have been
1
detected (by either EPR or H NMR) upon deliberate addition
of O2 to Rh(TPP)(PEt3)2. Since the superoxido complex, (TPP)-
Rh(O2), is stable enough to withstand chromatography,11 it is
unlikely to be an undetected intermediate in the oxidation of
Rh(TPP)(PEt3)2. We believe that a reaction between the TPP•-
anion radical and O2 is more probable. The further fate of the
formed superoxide anion, O2•-, is not entirely clear, since a 1H
NMR spectrum of the resulting Rh(TPP)(PEt3)2+ did not show
peaks attributable to a possible anion, whose nature is at present
unknown.
In contrast, Rh(OEP)(PEt3) reacts with O2 at -77 °C to form
•-
a paramagnetic superoxido complex, RhIII(OEP)(PEt3)x(O2
)
(x ) 0, 1) (Figure 3, Table 1). At higher temperatures, the
species rapidly converts to a diamagnetic derivative, as evi-
denced by disappearance of the EPR signal. We assign the final
product as a (µ,η1,η1)-peroxo dimer, (PEt3)(OEP)Rh-OO-Rh-
1
(OEP)(PEt3), on the basis of its H NMR spectrum and the
literature data.11
Whether the superoxido complex contains a coordinated
phosphine molecule could not be determined definitively. The
absence of 31P shf coupling in its EPR spectrum is not conclusive
since only a small degree of mixing between the mainly oxygen-
based LUMO and the phosphine orbitals is expected. Indeed,
very modest 31P shf coupling constants are observed in
Rh(por)(P[OBu]3)(O2) (Table 1). However, we saw rapid
disappearance of the EPR signal, consistent with the presence
of an auxiliary ligand, but not with an unligated 5-coordinated
Rh(OEP)(O2), which is known to be rather slow to convert to
the µ-peroxo derivative.11 Likewise, both the known high affinity
of PPh3 to Rh(TPP)(O2)12b and the presence of the phosphine
ligand in the µ-peroxo dimer suggest that the reaction between
Rh(OEP)(PEt3) and O2 likely yields the 6-coordinate phosphine
adduct Rh(OEP)(PEt3)(O2), even though its EPR spectrum does
not reveal the spin coupling to the phosphorus atom.
Figure 6. Spectrophotometric titration of 10-5 M solutions of Rh-
(OEP)(PF6) in toluene with PEt3 and pyridine. For clarity, only every
other UV-visible trace is shown. Single and double asterisks indicate
signals corresponding to Rh(OEP)(PEt3)PF6 and Rh(OEP)(PEt3)2PF6,
respectively.
understand whether the difference in the number of the axial
ligands in these two adducts is due to a lower intrinsic affinity
of the RhIII(por) moiety for PEt3 than pyridine. Second, the high
stability of Rh(OEP)(PEt3) toward disproportionation contrasts
with the rapid decomposition of the analogous RhII complex,
Rh(OEP)py.21c The two existing hypotheses on the mechanism
of disproportionation of RhII(por)L adducts propose that the
affinity of Rh to the axial ligand, L, determines, albeit in
opposite ways, susceptibility of these adducts to disproportion-
ation. Knowledge of both the relative affinities of pyridine and
PEt3 to the RhIII(OEP) moiety, and the relative stabilities of
their adducts with RhII(OEP) toward disproportionation, makes
it possible to evaluate the plausibility of each hypothesis.
To eliminate competition by the counterion or solvent for
the metal coordination sites, we carried out spectrophotometric
titrations on Rh(OEP)PF6 in toluene under anhydrous and
anaerobic conditions (Figure 6). The unligated RhIII(OEP)+
complex was reported33 to exist as a dimer, Rh2(OEP)22+, in
relatively concentrated CH2Cl2 solutions. The most characteristic
feature of the UV-visible spectrum of the dimer is a low-
intensity adsorption around 800 nm and split blue-shifted Soret
band. The monomer, on the other hand, is expected to have a
Addition of water to Rh(OEP)(PEt3) produces a RhIII-phlorin
complex (Figure 1). The resulting reaction mixture is rather
complex due to the presence of variously ligated RhIII species,
+
but RhIII(OEP)(PEt3)2 and RhIII(OEPhlorin)(PEt3)2 can be
isolated upon addition of an excess of PEt3. The literature NMR
data on phlorin derivatives are very limited,29 but certain features
of the 1H NMR spectrum of the latter complex, such as loss of
the ring current and the 4-fold symmetry of the porphyrin core,
clearly indicate a metallophlorin. For example, meso-hydrogens
and the methylene groups of the peripheral ethyl substituents
of the macrocycle experience an upfield shift, relative to their
positions in RhIII(OEP)(PEt3)2+, of about 4 and 1.5 ppm,
respectively. Concomitantly, the phosphine group resonances
shift downfield by about 5 (CH2) and <3 ppm (CH3). This
reversed order of the phosphine signals indicates that the
chemical shifts of the hydrogen atoms on axial ligands in a
metallophlorin-phosphine adduct are no longer determined by
the through-space interaction with the ring current.
Stability Constants of RhIII(OEP)Lx+ (L ) PEt3, py; x )
1, 2) Adducts. The reason for measuring the affinities of these
two ligands to the RhIII(OEP) moiety was 2-fold. A reaction
between Rh2(OETAP)2 and pyridine yields the bispyridine
adduct, RhIII(OETAP•-)py2.21b In contrast, only a monoligated
complex, Rh(OEP)(PEt3), can be isolated from solutions of
Rh2(OEP)2 even in neat PEt3. Therefore, the first goal was to
(30) Peychal-Heiling, G.; Wilson, G. S. Anal. Chem. 1971, 43, 545-
556.
(31) (a) Lanese, J. G.; Wilson, G. S. J. Electrochem. Soc. 1972, 119,
1039-1043. (b) Closs, G. L.; Closs, L. E. J. Am. Chem. Soc. 1963, 85,
818-819. (c) Kadish, K. M. Prog. Inorg. Chem. 1986, 34, 435-605 and
references therein. (d) Grodkowski, J.; Neta, P.; Hambright, P. J. Phys.
Chem. 1995, 99, 6019-6023. (e) Sutter, T. P. G.; Rahimi, R.; Hambright,
P.; Bommer, J. C.; Kumar, M.; Neta, P. J. Chem. Soc., Faraday Trans.
1993, 89, 495-502.
(32) Fuhrhop, J.-H.; Kadish, K. M.; Davis, D. G. J. Am. Chem. Soc.
1973, 95, 5140-5147.
(33) Lee, S.; Mediati, M.; Wayland, B. J. Chem. Soc., Chem. Commun.
1994, 2299-2300.
(29) Stolzenberg, A. M.; Stershic, M. T. J. Am. Chem. Soc. 1988, 110,
6391-6402.

Porphyrinatorhodium(II)-Triethylphosphine Adducts
J. Am. Chem. Soc., Vol. 122, No. 48, 2000 11817
Table 2. Stability Constant of Porphyrinatorhodium(III) Adducts and Related Compounds
starting complex
axial ligand
PEt3
pyridine
PPh3
PPh3
PPh3
PPh3
P(CH2CH2CN)3
pyridine
solvent
log K1
Log K2
ref
Rh(OEP)+
toluene
toluene
THFb
6.7( 0.4
4.5 (0.4
not measd
3.6
3.3
5.52
4.4 ( 0.4
< 2
3.1
a
a
21d
21e
21e
12b
34
Rh(OEP)+
Rh(TPP)+
(Me)Rh(TPP)
(Me)Rh(OEP)
(O2)Rh(TPP)
Rh2(O2CCH3)4
Rh2[O2CCH2(OMe)]4
CH2Cl2
CH2Cl2
CH2Cl2
AcNb
6.26
4.52 ( 0.02
4.86
2.81 ( 0.04
H2Ob
35
a Present work. b Solvent occupies free coordination sites.
Scheme 1. Summary of the Reactivity of Rh(OEP)(PEt3)
and Rh(TPP)(PEt3)2
single Soret peak centered around 400 nm. A saturated solution
of Rh(OEP)+ in toluene (∼30 µM) demonstrates both sets of
signals; however, dilution to 10 µM concentration effectively
eliminates the peaks corresponding to the dimer. Attempts to
mathematically process a set of titration data performed on a
saturated toluene solution of Rh(OEP)+ were unsuccessful.
Therefore, studies were done on 10 µM solutions to minimize
complications associated with the presence of equilibria involv-
ing the dimer and possibly its derivatives.
The titration results (Table 2) show that PEt3 forms more
stable adducts with RhIII(por) than pyridine does. The rather
large ratio of the subsequent stability constants (K1/K2 ) 230
for PEt3) probably reflects the mutual destabilization of the two
quite basic ligands trans to each other. A similarly higher affinity
of phosphine relative to pyridine for RhII centers in µ-car-
boxylate dimers has been reported. The smaller K1/K2 ratio
found in these bimetallic carboxylate systems is consistent with
a decrease in electronic interactions between the two axial
ligands upon separation by the Rh-Rh bond. The stability
constants of other Rh(por)-phosphine adducts reported in the
+
literature are slightly smaller than those for Rh(OEP)(PEt3)2
,
Rh(OETAP)py2, which is formed in a pyridine solution of a
RhII complex, Rh2(OETAP)2.21b Its EPR and UV-visible spectra
are consistent with a porphyrin anion radical, and the compound
is stable toward disproportionation. Similar adducts, Rh(TPP)-
P2 (P ) PEt3, PMe2Ph) were obtained by one-electron reduction
of the corresponding RhIII bisphosphine complexes.21a,d,e Al-
though the species were not isolated, UV-visible spectroscopy
showed them to be RhIII-porphyrin anion radical derivatives.
The adducts did not disproportionate on the time scale of a
spectroelectrochemical measurement.36
which can probably be attributed mostly to the noncoordinating,
nonpolar properties of toluene used in our measurements. Such
a medium destabilizes the higher charge density species and
does not compete for the free coordination sites.
Discussion
Nature of the HOMO in Rh(por)(PEt3)x Adducts. Para-
magnetic complexes Rh(OEP)(PEt3) and Rh(TPP)(PEt3)2 were
obtained upon addition of PEt3 to the corresponding metallo-
hydrides, Rh(por)H. The chemistry of these adducts and their
potential as catalysts are determined by whether the unpaired
electron is located on the metal or on the macrocycle. The
HOMO of square-planar Rh (por) complexes is dz , but upon
formation of an adduct, Rh(por)L, this orbital acquires anti-
bonding character with respect to the Rh-L bond. The ensuing
destabilization may lead to a reversal of ordering, whereupon
the porphyrin-based pπ*por orbitals become the HOMO and the
metal-based dσ*Rh-L becomes the LUMO of the adduct. The
high symmetry of the adducts nearly eliminates mixing between
In this work, we prepared and isolated Rh(TPP)(PEt3)2 in
quantities sufficient to study its physicochemical properties more
thoroughly, and they are consistent with the adduct being a
RhIII-porphyrin anion radical complex. Its EPR spectrum
demonstrates relatively small values of g and 31P shf indicative
of a porphyrin-based radical. Bis-adducts are not known in the
chemistry of RhII(por) species, since such 19-electron species
should be susceptible to facile loss of the second axial ligand.
In contrast, Rh(TPP)(PEt3)2 is very stable toward dissociation,
II
2
1
as evidenced by H NMR experiments and the fact that the
2
the pπ*por and dσ*Rh-L (dz ) orbitals. Therefore, such adducts
complex does not produce any detectable amount of a super-
oxido complex upon exposure to O2 (Scheme 1). A 5-coordinate
dissociation product, Rh(TPP)(PEt3), is expected to form an
adduct with dioxygen.
The situation with Rh(OEP)(PEt3) is more complex. Its low-
temperature EPR spectrum is similar to that of Rh(TPP)(PEt3)2
can normally be characterized as either RhII(por) derivatives
(the HOMO is dσ*Rh-L), or RhIII complexes of porphyrin anion
radical, RhIII(por•-)Lx (the HOMO is pπ*por), with the cor-
respondingly different chemistry and composition.
To the first class belong several monoadducts of RhII(TMP)
and RhII(TTiPP) (Figure 1) with amines, pyridines, and phos-
phines trapped at low temperature.7a They demonstrate a metal-
based EPR signal, do not form bis-adducts even in the presence
of an excess of a ligand, and rapidly disproportionate at
temperatures above 77 K. Due to such instability, their reactivity
beyond susceptibility to disproportionation is unexplored. The
second type of adduct is exemplified by a bispyridine species,
(34) Macartney, D. H., Aquino, M. A. S. Inorg. Chem. 1987, 26, 2696-
2699.
(35) Das, K.; Simmons, E. L.; Bear, J. L. Inorg. Chem. 1977, 16, 1268-
1278.
(36) Rh(TPP)(PEt3)2 was formulated as a RhII complex;21a this is
obviously a misinterpretation of the UV-visible spectrum, which closely
31
resembles those of H2TPP•- and Zn(TPP•-
)
but is very different from a
spectrum of a genuine porphyrinatorhodium(II) complex, RhII(TMP).37

11818 J. Am. Chem. Soc., Vol. 122, No. 48, 2000
Collman and BoulatoV
indicating that the HOMO of the adduct is mainly porphyrin-
based pπ*por. At room temperature, Rh(OEP)(PEt3) reacts with
H2O as a porphyrin anion radical, producing a phlorin derivative
(Scheme 1).29,30 On the other hand, its reaction with O2 yields
a superoxido complex, (PEt3)Rh(OEP)(O2), which suggests that
the unpaired electron is located on the metal-based orbital.38
Likewise, our failure to isolate the bisphosphine complex,
Rh(OEP)(PEt3)2, even from solutions of Rh(OEP)(PEt3) in neat
phosphine is consistent with the dissociation lability of 19-
electron 6-coordinate RhII species, but contradicts the high
affinity of RhIII(OEP) to PEt3, and the fact that other RhIII(por•-)
complexes are obtained only as bisligated species. Thus,
depending on experimental conditions, Rh(OEP)(PEt3) appears
to react as either RhII(OEP)(PEt3) or RhIII(OEP•-)(PEt3). A
plausible explanation of these observations is that the nearly
mutually orthogonal orbitals, pπ*por and dσ*Rh-P are close in
energy so that at 200-300 K the unpaired electron is thermally
promoted from the lower-lying pπ*por to dσ*Rh-P. At 77 K, the
population of the higher energy dσ*Rh-P is apparently insuf-
ficient to generate a detectable Rh-based radical signal by EPR.
At higher temperatures, the excited state becomes sufficiently
populated to determine the reactivity of Rh(OEP)(PEt3) with
certain substrates (such as O2 and PEt3, but not H2O).
Scheme 2. Two Possible Mechanisms for
Disproportionation of RhII(por)L Adductsa
a The superscripts refer to the formal oxidation state of Rh and do
not imply the actual distribution of the unpaired electron density.
(Scheme 2). According to mechanism A,21a disproportionation
requires partial dissociation of the adduct, yielding the nonligated
RhII(por), whose reducing properties relative to those of
Rh(por)L are enhanced due to the square-planar 4-coordinate
environment favored by RhI. This mechanism is similar to that
2+ 39
proposed for disproportionation of RhII(bipy)3
.
Thus,
RhII(por)L adducts that have high dissociation stability are
predicted to be less susceptible to disproportionation. An
alternative hypothesis7a (mechanism B) proposes that dispro-
portionation is driven by association step 3b (Scheme 2) and
Thus, the much richer chemistry of Rh(OEP)(PEt3) as
compared to similar complexes Rh(OETAP)py2 and Rh(TPP)-
(PEt3)2 apparently arises from the higher energy of the lowest-
lying acceptor orbitals of OEP relative to OETAP or TPP. This
decreases the energy gap between the porphyrin-based HOMO
(pπ*OEP) and the metal-based LUMO (dσ*Rh-P) in the OEP
complex to such an extent that the (pπ*OEP)0(dσ*Rh-P)1excited
state appears to become important in determining the reactivity
of Rh(OEP)(PEt3).
consequently leads to the opposite conclusion. Higher affinity
ligands make formation of the RhIII complex, RhIII(por)L2
,
+
more favorable, and therefore, the corresponding adducts should
be more susceptible to disproportionation.
In the present work, we have shown that PEt3 has a higher
affinity to RhIII(por)+ than pyridine; similar results have been
obtained by others34,35 for RhII complexes (Table 2). We have
also shown that Rh(OEP)(PEt3) is stable toward disproportion-
ation, while a similar adduct with pyridine, Rh(OEP)py, is
known to decompose rapidly to RhIII(OEP)py2+ and RhI(OEP)-.21c
These data, in our opinion, suggest that among simple σ-donor
ligands, those with higher affinity for RhIII, impart higher
stability to their adducts, Rh(por)L. The higher disproportion-
ation susceptibility of Rh(TMP)(PEt3) relative to Rh(OEP)(PEt3)
is consistent with the presumed lower dissociation stability of
the former adduct, due to the unfavorable steric interactions
between the peripheral substituents of the porphyrin and PEt3.
Such a weaker Rh-P bond in Rh(TMP)(PEt3) versus Rh(OEP)-
(PEt3) is suggested by the EPR spectra of these adducts.
Reaction between Rh2(OEP)2 and PEt3. There are several
synthetic advantages in using metallohydrides Rh(por)H, as
opposed to the corresponding dimers, Rh2(por)2, as precursors
to Rh(por)L adducts. First, the dimer is usually obtained from
the hydride (or the related derivatives), and the direct conversion
of Rh(por)H into Rh(por)L shortens the synthesis. Second, such
dimerization is normally done under relatively harsh or difficult
to control conditions, which makes it less suitable for elaborate
porphyrin derivatives. However, there are important reasons to
study the interactions between Rh2(por)2 and donor molecules.
The RhII dimers in non-porphyrin environments show a high
propensity to add one or two ligands trans to the Rh-Rh bond.
Considerable theoretical40 and experimental41 efforts have been
directed toward elucidating details of such adduct formation.
Similar reactivity of the Rh24+ core in the porphyrin environment
has been much less studied. The well documented decomposition
Susceptibility of Rh(por)L toward Disproportionation. For
ligands, such as pyridine or alkylphosphines, that have com-
parable affinity to both RhIII and RhII, disproportionation of
+
corresponding 1:1 adducts, Rh(por)L, to RhIII(por)L2 and
RhI(por)- (reaction 1) is probably always thermodynamically
favorable irrespective of the exact location of the unpaired
electron (that is whether the HOMO of Rh(por)L is metal-based
or porphyrin-based). On the other hand, π-acidic ligands, such
as CO, which have much lower affinity to RhIII than RhII, should
stabilize the +2 state and thus suppress disproportionation. The
situation is more complex for 1:2 adducts, Rh(por)L2 (reaction
2), since the loss of the Rh-L bonds may not be compensated
+
by the higher stability of Rh(por)L2 and Rh(por)-. Indeed,
some bis-adducts (Rh(TPP)(PEt3)2, Rh(OETAP)py2)21b are
stable, while others (pyRh(TMP)Cl)31d disproportionate rapidly.
Likewise, the monoligated RhII(por) species with σ-donors, such
as Rh(TMP)(PEt3)7a and Rh(OEP)py,1c undergo facile dispro-
portionation, while the corresponding 1:1 CO adducts, Rh(por)-
CO, are stable.7a,8b Rh(OEP)(PEt3) is unique in being a stable
1:1 adduct of Rh(por) with a simple σ-donor ligand.
2Rh(por)L f Rh(por)L2+ + RhI(por)-
(1)
(2)
2Rh(por)L2 f Rh(por)L2+ + RhI(por)- + 2L
The mechanism of the disproportionation is not yet well
established, with two conflicting hypotheses proposed to date
(37) Vitols, S. E.; Friesen, D. A.; Williams, D. S.; Melamed, D.; Spiro,
T. G. J. Phys. Chem. 1996, 100, 207-213.
(38) An alternative mechanism would be an oxidation of the porphyrin
anion radical by O2, forming RhIII(por)+ and O2•-, followed by a rapid
collapse of the resulting ion pair; as far as we are aware, such a mechanism
of formation of superoxido complexes has never been demonstrated; a direct
reaction between a RhII center and O2 seems more plausible.
(39) (a) Brown, G. M.; Chan, S. F.; Creutz, C.; Schwartz, H. A.; Sutin,
N. J. Am. Chem. Soc. 1979, 101, 7638-7640. (b) Mulazzani, Q. G.; Emmi,
Hoffman, M. Z.; Venturi, M. J. Am. Chem. Soc. 1981, 103, 3362-3370.
(c) For other examples of RhII disproportionation thought to proceed via
redox exchange between differently ligated species, see: Haefner, S. C.;
Dunbar, K. R.; Bender, C. J. Am. Chem. Soc. 1991, 113, 9540-9553.

Porphyrinatorhodium(II)-Triethylphosphine Adducts
J. Am. Chem. Soc., Vol. 122, No. 48, 2000 11819
of Rh2(por)2 in the presence of donor molecules7,21c,42 is often
driven by irreversible processes such as alkyl group abstraction,
or disproportionation of monomeric RhII(por)L adducts, and does
not necessarily reflect the reactivity of the dimer. Among the
very few reported systems where such side reactions do not
occur, only CO and related arylisocyanides form bimetallic
anhydrous conditions. According to low-temperature EPR
studies, the HOMO of both complexes is mainly porphyrin-
2
based, not the metal-based dσ*Rh-P (dz ) as is the case in related
Rh(TMP)(PEt3).7a The difference is probably due to less efficient
2
overlap of dz and donor orbital of the phosphine in the TMP
adduct, expected to result from the unfavorable steric interac-
tions between the peripheral substituents of TMP and PEt3.
The composition and reactivity of the TPP derivative support
that it is a RhIII complex of TPP anion radical, RhIII(TPP•-)(PEt3)2,
similar to the previously reported Rh(OETAP)py2.21b This
conclusion is also in agreement with the spectroelectrochemi-
cally obtained UV-visible spectrum of Rh(TPP)(PEt3).21a In
contrast, Rh(OEP)(PEt3), demonstrates the reactivity of both
RhII(OEP) and RhIII(OEP•-) species, depending on the substrate.
It reacts with H2O forming a phlorin derivative, according to
the known reactivity of porphyrin anion radicals.29,30,31d,e
However, its reaction with O2, yielding a RhIII-superoxido
complex, is identical to that of RhII species, such as Rh2(OEP)2.11
We propose that this dual reactivity is due to thermal promotion
of the unpaired electron from porphyrin-based HOMO onto the
metal-based dσ*Rh-P LUMO.
8d
adducts, (CO)Rh2(OEP)2 and (ArNC)Rh2(OEP)2,7b while
Rh2(OETAP)2 undergoes homolytic Rh-Rh bond cleavage in
the presence of pyridine.21b We have now shown that another
σ-donor, PEt3, causes homolysis of the Rh-Rh bond in
Rh2(OEP)2. These limited data suggest that the Rh-Rh bond
cleavage is promoted by the auxiliary ligands that interact
particularly strongly with the dσRh-Rh orbital (PEt3)40d,e or by
the porphyrins that have particularly low-lying acceptor orbitals
(OETAP). This is equivalent to saying that ligand-induced
dissociation of Rh2(por)2 appears to occur when the interaction
between the donor orbitals of the axial ligand(s) and the dσRh-Rh
orbital is sufficient to raise the latter’s energy above that of the
porphyrin’s LUMO. The resulting adduct dissociates because
it has zero Rh-Rh bond order: dπ4δ2(δ*)2(dπ*)4(pπ*por)2(dσ)0.
It is well known that axial ligation of non-porphyrin RhII dimers
induces such destabilization of the dσRh-Rh orbital that in many
cases it becomes the HOMO of the resulting adduct.40e,43
An interesting implication of this hypothesis is that certain
auxiliary ligands may enhance the metalloradical reactivity of
sterically unencumbered RhII(por) complexes. Such species are
less active than their sterically bulky analogues,1b since their
facile dimerization decreases the amount of the reactive d7
monomer, RhII(por). An appropriate porphyrin-auxiliary ligand
combination would induce just sufficient destabilization of the
dσRh-Rh orbital to cause dissociation of Rh2(por)2, but to retain
the RhII(por) reactivity of the resulting 5-coordinate adducts.
Rh(OEP)(PEt3) exemplifies this concept.
Unlike analogous 1:1 adducts, Rh(TMP)(PEt3) and Rh(OEP)-
py, Rh(OEP)(PEt3) is not susceptible to disproportionation. This
difference, along with the data on relative affinities of pyridine
and PEt3 to RhIII(por)+, supports the previously proposed
hypothesis that disproportionation of such adducts is determined
by their dissociation stability.
Experimental Section
General Information. All synthetic manipulations were done in a
Vacuum Atmosphere’s drybox ([O2] < 1 ppm) or on a vacuum line,
using standard procedures and Schlenk glassware. Solvents were de-
gassed and distilled off sodium (toluene and PEt3), sodium benzo-
phenone ketyl (benzene, pentane, pyridine), or P2O5 (CH2Cl2, CDCl3)
under Ar prior to use. FeCp2PF6 was recrystallized twice from
anhydrous acetonitrile/toluene under rigorously anaerobic conditions,
dried under vacuum (5 mTorr), and stored in a drybox; AgPF6 was
dried at 40 °C for 12 h under vacuum and stored in a drybox. H2OEP,44
Conclusions
Paramagnetic complexes, Rh(OEP)(PEt3) and Rh(TPP)-
(PEt3)2, result from addition of PEt3 to the corresponding
metallohydrides, Rh(por)H, under rigorously anaerobic and
H2TPP,45 Rh2(CO)4Cl2,46 and Rh2(OEP)2(PF6)2 were synthesized
33
1
according to published procedures. H NMR spectra were taken on a
(40) (a) Eagle, C. T.; Farrar, D. G.; Pfaff, C. U.; Davies, J. A.; Kluwe,
C.; Miller, L. Organometallics, 1998, 17, 4523-4526. (b) Kawamura, T.;
Maeda, M.; Miyamoto, M.; Usami, H.; Imaeda, K.; Ebihara, M. J. Am.
Chem. Soc. 1998, 120, 8136-8142. (c) Cotton, F. A.; Feng, X. J. Am. Chem.
Soc. 1998, 120, 3387-3397. (d) Sargent, A. L.; Rollog, M. E.; Eagle, C.
T. Theor. Chem. Acc. 1997, 97, 283-288. (e) For a comprehensive review
of older literature, see: Boyar, E. B.; Robinson, S. D. Coord. Chem. ReV.
1983, 50, 109-208.
Varian’s UnityInova 500-MHz spectrometer and are reported in ppm
vs TMS. UV-visible and EPR spectra were taken on Hewlett-Packard
1
8453 and Brucker ER 220D-SRC spectrometers, respectively. All H
NMR and UV-visible spectra were taken at 25.0 ( 0.1 °C. Mass
spectrometry was performed at the Mass Spectrometry Facility,
University of California at San Francisco.
Rh(por)I. This synthesis follows a modified published proce-
dure:47 under N2, H2por (100 mg) and Rh2(CO)4Cl2 (100 mg, 0.22
mmol) are dissolved in anhydrous CH2Cl2 (10 mL), anhydrous NaOAc
(500 mg, 6 mmol) is added, and a stream of N2 is adjusted such that
the solvent evaporates within 45 min. Avoiding exposure to air, the
reaction mixture is dried further under dynamic vacuum (5 min, to
remove residual acetic acid), a new portion of CH2Cl2 (10 mL) is added,
and the procedure is repeated. The reaction mixture is dissolved in dry
benzene (10 mL), and I2 (1.1 equiv) is added at once. The progress of
oxidation is monitored by UV-visible and TLC, and as soon as the
reaction is complete (10 min for H2OEP and 45 min for H2TPP), the
mixture is transferred to a silica column; the product is eluted with
CH2Cl2, and dried under dynamic vacuum (50 µTorr) at 100 °C for 24
h. Yields: 85-90% for IRh(OEP), 75-80% for IRh(TPP); some
[Rh(CO)2]2TPP is recovered and can be reused. Longer reaction times
(41) (a) Cotton, F. A.; Dikarev, E. V.; Petrukhina, M. A.; Stiriba, S.-E.
Inorg. Chem. 2000, 39, 1748-1754. (b) Cotton, F. A.; Dikarev, E. V.;
Petrukhina, M. A.; Stiriba, S.-E. Organometallics 2000, 19, 1402-1405.
(c) Cotton, F. A.; Dikarev, E. V.; Petrukhina, M. A. J. Organomet. Chem.
2000, 596, 130-135. (d) Pruchnik, F. P.; Starosta, R. Liz, T.; Lahuerta, P.
J. Organomet. Chem. 1998, 568, 177-183. (e) Crawford, C. A.; Matonic,
J. H.; Huffman, J. C.; Folting, K.; Dunbar, K. R.; Christou, G. Inorg. Chem.
1997, 36, 2361-2371. (f) Pirrung, M. C.; Morehead, A. T. J. Am. Chem.
Soc. 1994, 116, 8991-9000. (g) Nicolo, F.; Bruno, G.; Lo Schiavo, S.;
Sinicropi, M. S.; Piraino, P. Inorg. Chim. Acta 1994, 223, 145-149. (h)
For a comprehensive review of older literature, see: Felthouse, T. R. Prog.
Inorg. Chem. 1982, 29, 73-165.
(42) (a) Feng, M.; Chan, K. S. J. Organomet. Chem. 1999, 584, 235-
239. (b) Wayland, B. B.; Woods, B. A. J. Chem. Soc., Chem. Commun.
1981, 475-476.
(43) Structurally related RhIIphthalocyanine dimer, Rh2(pc)2, forms a very
stable 1:2 adduct with pyridine, py2Rh2(pc)2 (Huckstadt, H.; Bruhn, C.;
Homborg, H. J. Porphyrins Phthalocyanines 1997, 1, 367-378). The dimer
manifests a Rh-Rh distance (2.741(2) Å) comparable to that in other trans
L-Rh-Rh-L units, containing an unsupported Rh-Rh bond, and signifi-
cantly shorter than the Rh-Rh bond length in (PPh3)2Rh2(dmg)4 (dmg )
dimethylglyoxime monoanion, 2.936(2) Å).41h This suggests that unfavorable
van der Waals interactions between porphyrin cores in putative LRh2(por)2
and L2Rh2(por)2 adducts are probably not the main reason for their
instability.
(44) Sessler, J. L.; Mozaffari, A.; Johnson, M. R. Org. Synth. 1992, 70,
68-78.
(45) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour,
J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476.
(46) McCleverty, J. A.; Wilkinson, G. Inorg. Synth. 1990, 28, 84-86.
(47) Abeysekera, A. M.; Grigg, R.; Trocha-Grimshaw, J.; Viswanatha,
V. J. Chem. Soc., Perkin Trans. 1 1977, 1395-1403.

11820 J. Am. Chem. Soc., Vol. 122, No. 48, 2000
Collman and BoulatoV
or excess of I2 in our hands did not increase the degree of conversion
of [Rh(CO)2]2TPP: IRh(OEP) 1H NMR (CDCl3) δ 10.31 (s, 4H, meso),
4.23 (m, 6.5 Hz, 8H, CH2), 4.10 (m, 6.5 Hz, 8H, CH2), 2.00 (t, 6.5 Hz,
24H, CH3); UV-vis (toluene, λmax/nm); 352, 402, 518, 551. IRh(TPP)
1H NMR (CDCl3) δ 8.96 (s, 8H, â-pyrolic), 8.27 (d, 6 Hz, 4H, o-Ph),
8.02 (d, 6 Hz, 4H, o′-Ph), 7.50 (m, 8H, m, m′-Ph), 7.38 (m, 4H, p-Ph);
UV-vis (toluene, λmax/nm) 422, 533, 565.
6H, phosphine CH2); UV-vis (toluene, λmax/nm) 422, 530, 560, 606.
An identical complex is formed if a toluene solution of Rh(OEP)(PEt3)
is allowed to stir for several days in a drybox.
(3) (PEt3)2Rh(octaethylphlorin). A 10-mL sample of degassed
water-saturated toluene is added to 1 mL of a 10 mM solution of
Rh(OEP)(PEt3) in toluene; the color changes to red. The resulting
1
reaction mixture has a very complex H NMR spectrum, due to the
Rh(por)H. In a drybox, rigorously dried Rh(por)I (100 mg) is
dissolved in anhydrous benzene (50 mL) and LiAlH4 (5.5 mg, 1.1 equiv)
is added. The mixture is stirred for 20 h at room temperature in the
dark, after which the progress of the reaction is monitored by UV-
presence of differently ligated species. To simplify the interpretation
of the results, an excess of PEt3 is added, the reaction mixture is allowed
to stir for 10 min, solvents are removed under reduced pressure, and
(PEt3)2Rh(octaethylphlorin) is extracted in pentane: 1H NMR (C6D6)
δ 6.31 (s, 2H, meso), 5.62 (s, 2H, meso), 5.42 (s, 1H, meso), 2.72 (q,
7.5 Hz, 4H, CH2), 2.54 (q, 7.5 Hz, 4H, CH2), 2.51 (q, 7.5 Hz, 4H,
CH2), 2.50 (q, 7.5 Hz, 4H, CH2), 1.62 (qt, 8 Hz, 2.5 Hz (31P-H), 12
H, phosphine CH2), 1.27 (t, 7.5 Hz, 6H, CH3), 1.24 (t, 7.5 Hz, 6H,
CH3), 1.22 (t, 7.5 Hz, 6H, CH3), 1.20 (t, 7.5 Hz, 6H, CH3), 0.68 (t, 8
Hz, 18 H, phosphine CH3); UV-vis (3 M PEt3/toluene, λmax/nm): 332,
434, 540, 568, 606, 714, 811 (br).
1
visible and H NMR spectroscopies every hour. Once the conversion
is complete, the precipitates have to be filtered quickly in order to avoid
accumulation of an unidentified impurity. The filtrate yields Rh(por)H
in a quantitative yield after removal of C6H6 in vacuo: HRh(OEP) 1H
NMR (C6D6) δ 10.15 (s, 4H, meso), 3.94 (q, 8.5 Hz, 16H, CH2), 1.88
(t, 8.5 Hz, 24H, CH3), -40.30 (d, 1J(103Rh,H) ) 45 Hz); UV-vis
1
(toluene, λmax/nm) 394, 509, 544, 600. HRh(TPP) H NMR (C6D6) δ
8.87 (s, 8H, â-pyrolic), 8.24 (dt, 8.5 Hz, 2.5 Hz, 4H, o-Ph), 7.99 (d,
8.5 Hz, 4H, o′-Ph), 7.49 (m, 8H, m-,m′-Ph), 7.40 (tm, 8 Hz, 8H, p-Ph),
-40.23 (d, 1J(103Rh,H) ) 44 Hz); UV-vis (toluene, λmax/nm) 415, 521,
580, 605.
(4) Oxidation with FeCp2PF6. An aliquot of a stock solution of
FeCp2PF6 (0.045 mmol) in CH2Cl2 is placed in a modified NMR tube,
the solvent is evaporated, and Rh(OEP)(PEt3) (400 µL of 10 mM
solution in C6D6) is added; the tube is shaken several times; the color
changes rapidly to red; the 1H NMR and UV-visible spectra are those
of Rh(OEP)(PEt3)PF6 (vide infra). MS (+ESI) C42H59N4PRh calcd
753.35, found 753.39 (cluster).
Rh(TPP)(PEt3)2. To a solution of HRh(TPP) (2.5 mg, 3.5 µmol in
400 µL of C6D6) is added 25 µL of a 0.72 M solution of PEt3 (18
µmol, 5 equiv) in C6D6; the color changes from brown-red to bright
green; only free PEt3 resonances are observable in an 1H NMR
spectrum; UV-visible spectra show absorption bands attributable only
to the oxidized species, Rh(TPP)(PEt3)2+ presumably by oxidation with
adventitious O2. A sealed NMR sample changes color to greenish-brown
within 2 h with concomitant appearance of the resonance corresponding
to Rh(TPP)(PEt3)2+ and partial precipitation; no resonances attributable
to the anion are detected in 1H NMR spectrum. MS (+ESI) C56H58N4P2-
Rh calcd 951.32, found 951.15. Reactions with O2 in an EPR tube and
FeCp2PF6 were done exactly as described above.
Rh(por)(PEt3)PF6. In a drybox, Rh(por)I (2.5 mL of a toluene
solution, 1 mg/mL) is mixed with PEt3 (50 µL of 70 mM solution in
toluene); the solvent is removed after 5 min yielding IRh(por)(PEt3):
(por ) OEP) 1H NMR (CDCl3) δ 10.12 (s, 4H, meso), 4.14 (m, 7 Hz,
8H, CH2), 3.96 (m, 7 Hz, 8H, CH2), 1.91 (t, 7.5 Hz, 24H, CH3), -1.73
(dt, 8 Hz, 15 Hz, 9H, phosphine CH3), -3.40 (dq, 8 Hz, 1.5 Hz, 6H,
phosphine CH2); UV-vis (3M PEt3 in toluene, λmax/nm) 367, 436, 538.
por ) TPP 1H NMR (CDCl3) δ 8.79 (s, 8H, â-pyrolic), 8.24 (d, 8 Hz,
4H, o-Ph), 8.09 (d, 8 Hz, 4H, o′-Ph), 7.72 (m, 14H, m-, m′-, p-Ph),
-1.42 (dt, 6 Hz, 7.5 Hz, 9H, phosphine-CH3), -2.91 (dq, 3 Hz, 7.5
Hz, 6H, phosphine-CH2); UV-vis (3M PEt3 in toluene, λmax/nm) 354,
398, 458, 520, 564, 604). IRh(por)(PEt3) is dissolved in benzene/CH2Cl2
(2:1, 2 mL) and 150 µL of a 20 mM solution of AgPF6 in toluene is
added. The precipitate is collected and recrystallized from CH2Cl2/
benzene/pentane (1:1:5) to give Rh(por)(PEt3)PF6 in 85-90% yield.
The advantage of the stepwise procedure is that monoligated species
can be prepared free of either the bisphosphine or the nonligated
complexes. In our hands, one of these impurities was inevitably present
in Rh(por)(PEt3)(PF6) samples obtained from addition of PEt3 to
Rh(por)I/AgPF6 mixtures, as a result of an error in adding exactly 1
equiv of the phosphine: Por ) OEP 1H NMR (CDCl3) δ 10.34 (s, 4H,
meso), 4.11 (m, 7 Hz, 16H, CH2), 1.90 (t, 7 Hz, 24H, CH3), -1.72 (dt,
4 Hz, 7.5 Hz, 9H, phosphine-CH3), -3.13 (dq, 4 Hz, 7.5 Hz, 6H,
phosphine-CH2); 1H NMR (C6D6) δ 10.23, 3.99 (m, 7.5 Hz), 3.89 (m,
7.5 Hz), -1.97 (dt, 6.5 Hz, 7.5 Hz), -3.35 (dq, 3 Hz, 7.5 Hz). The
higher degree of magnetic anisotropy on the two faces of the porphyrin
plane in C6D6, as evidenced by the larger separation of the resonances
of the diastereotopic methylene protons, is likely due to the formation
of close ion pair between PF6- and Rh(OEP)+ in the nonpolar solvent.
UV-visible (toluene, λmax/nm) 408, 517, 558 (depending on an excess
of the free ligand, RhIII(por)(PEt3)PF6 is present in equilibrium with
Rh(por)+ and Rh(por)(PEt3)2+; the spectra for Rh(por)(PEt3)PF6 reported
herein were obtain by deconvolution of the resulting UV-vis data).
por ) TPP 1H NMR (CDCl3) δ 8.66 (s, 8H, â-pyrolic), 7.91 (d, 7 Hz,
Rh2(OEP)2. HRh(OEP) (5 mg) was heated at 200 °C under dynamic
vacuum (5 µTorr) overnight in a pyrolytic tube. HRh(OEP) partially
sublimes but can be returned to the heated part of the tube. Conversion
is quantitative after two cycles: 1H NMR (C6D6) δ 9.28 (s, 4H, meso),
4.43 (m, 7 Hz, 8H, CH2), 3.95 (m, 7 Hz, 8H, CH2), 1.71 (t, 8 Hz, 24H,
CH3); UV-vis (toluene, λmax/nm) 384, 427, 450, 488, 518, 543, 636.
Rh(OEP)(PEt3). In a drybox, HRh(OEP) (20 mg; 30 µmol) is
suspended in anhydrous toluene (5 mL) and PEt3 (5 mL of a 3 M
solution in toluene; 0.13 mmol, 4 equiv) is added. In one method, the
mixture is allowed to stir at room temperature overnight. The color
changes from red to brown within 2 h. 1H NMR spectrum of the reaction
mixture indicates formation of (PEt3)Rh(OEP)H (21-20 (very br s,
16H), 10.3 (br s), -0.7 (br s, 24H), -1.3 (very br s), -2.4 (very br s),
-26.5 (s, hydride)). The reaction mixture becomes bright green after
∼8 h. Toluene and the phosphine are removed under reduced pressure
and the product is extracted with toluene (The red residue contains
+
Rh(OEP)(PEt3)2
;
1H NMR spectroscopy does not reveal signals
attributable to the anion, but it is likely hydroxide. This side product
could result from reactions of Rh(OEP)(PEt3) with traces of adventitious
O2 (followed by ligand displacement) and/or H2O. The latter reaction
also yields the small detected amount of the phlorin complexsvide
infra). The toluene is removed to give a green precipitate of Rh(OEP)-
(PEt3), which is washed with pentane (3 mL) to remove a small amount
of the phlorin side product, (PEt3)2Rh(octaethylphlorin). Yield: 60-
65%. Alternatively, a toluene solution of the starting materials is placed
in a J. Young flask (50 mL), the headspace evacuated, and the mixture
heated in an oil bath at 80-85 °C for 2 h, followed by a regular workup.
(CAUTION: Heating a liquid in a sealed container presents a potential
explosion hazard; adequate precautions should be taken!). The yields
are ∼50% but the resulting Rh(OEP)(PEt3) is somewhat more stable
to decomposition. Rh(OEP)(PEt3) can also be prepared from Rh2(OEP)2
by dissolving it in a 1.5 M solution of PEt3 in toluene; the solution
becomes green immediately, and removal of toluene/PEt3 yields a green
powder with properties identical to that of Rh(OEP)(PEt3) prepared
from HRh(OEP): 1H NMR (C6D6) δ 21 (very br, 15H, phosphine),
10.2 (br, 16H, CH2), -0.7 (br, 24H, CH3); UV-vis (3 M PEt3/toluene,
λmax/nm) 361, 420 (sh), 436, 451(sh), 540, 564, 606, 620, 647, 721,
740, 784, 828 (See the Results section).
Reactivity of Rh(OEP)(PEt3). (1) Rh(OEP)(PEt3)(O2). A 100-
µL sample of a 2 mM solution of Rh(OEP)(PEt3) is placed in an EPR
tube fitted with a J. Young valve, the solution is frozen with liquid N2,
the headspace is evacuated, the solution is warmed to -77 °C, the
tube is refilled with dry air (the color becomes to change from green
to red), the content is mixed rapidly, and the solution is frozen with
liquid N2.
(2) [Rh(OEP)(PEt3)]2(µO2). The EPR sample is allowed to warm
within 5 min; the resulting compound is stable in air at room
temperature: 1H NMR (C6D6) δ 10.4 (br, 4H, meso), 4.1 (br, 16H,
CH2), 1.9 (br, 24H, CH3), -1.9 (br, 9H, phosphine CH3), -3.4 (br,

Porphyrinatorhodium(II)-Triethylphosphine Adducts
J. Am. Chem. Soc., Vol. 122, No. 48, 2000 11821
4H, o-Ph), 7.82 (d, 7 Hz, 4H, o′-Ph), 7.69 (t, 7.5 Hz, 4 H), 7.64 (t, 7.5
Hz, 4 H), 7.59 (t, 7 Hz, 4 H), -1.66 (dt, 17.5 Hz, 9 H, phosphine-
CH3), -3.02 (dq, 2.5 Hz, 8.5 Hz, 6H, phosphine-CH2); UV-vis
(toluene, λmax/nm) 424, 540.
software, agreed adequately with the spectra collected independently
(Rh(OEP)+, Rh(OEP)py+, Rh(OEP)(PEt3)2+). A total of three sets of
data for each ligand were used: starting solutions for two sets were
obtained by dissolving Rh(por)PF6 and, for the third, by dissolving
Rh(OEP)(PEt3)2PF6 (or Rh(OEP)py2PF6), and the results were averaged.
The bispyridine adduct, Rh(OEP)py2+, could not be obtained at the
highest concentration of pyridine still miscible with toluene (Figure 6)
Rh(por)(PEt3)2PF6 are prepared by simple addition of g1 equiv of
PEt3 in toluene to a toluene/CH2Cl2 (1:2) solution of the monoligated
complex, removal the solvents in vacuo, and recrystallization of the
residues by pentane layering of CH2Cl2/toluene (1:1) solutions: por )
py
and the upper limit of the corresponding K2 value was estimated by
1
OEP H NMR (C6D6) δ 10.20 (s, 4H, meso), 4.12 (q, 8 Hz, 16 H,
assuming that, at the final titration point, no more than 10% (the
detection limit) of the total concentration was due to RhIII(OEP)py2
:
+
CH2), 1.89 (t, 8 Hz, 24 H, CH3), -1.82 (quintet, 9.5 Hz, 18 H,
phosphine-CH3), -3.47 (q, 8 Hz, 12 H, phosphine-CH2); UV-vis (3
UV-vis (toluene, λmax/nm) Rh(OEP)+ 393, 513, 544, 552(sh); Rh-
(OEP)py+ 407, 522, 554; isosbestic points ((3 nm) 401, 508, 536,
552. The 552-nm shoulder in the UV-visible spectrum of Rh(OEP)-
(PF6) may be due to the presence of a minor impurity such as Rh-
(OEP)(H2O)(PF6) (UV-vis (toluene, λmax/nm) 403, 522, 553). However,
since the titration data could be adequately described by a three-
component model in the case of PEt3 (Rh(OEP)+, Rh(OEP)(PEt3)+,
and Rh(OEP)(PEt3)2+) and a two-component model in the case pyridine
(Rh(OEP)+ and Rh(OEP)py+), the contribution of the possible aqua
complex was not significant within the limits of the used mathematical
procedures.
1
M PEt3/toluene, λmax/nm) 362, 437, 537, 567. por ) TPP H NMR
(C6D6) δ 8.84 (s, 8H, â-pyrolic), 8.19 (d, 6.5 Hz, 8 H, o-Ph), 7.50
(quintet, 14.5 Hz, 14 H, m-, p-Ph), -1.61 (quintet, 6.5 Hz, 18 H,
phosphine-CH3), -2.87 (br q, 7.5 Hz, 12 H, phosphine-CH2); UV-vis
(3 M PEt3/toluene, λmax/nm) 345 (sh), 364, 448, 558, 598.
Spectrophotometric Titrations. UV-visible samples were prepared
in a drybox by addition of 25 µL of 1 mM stock solution of [Rh-
(OEP)PF6]x (x ) 1, 2) in CH2Cl2 to 2.5 mL of anhydrous toluene in a
10-mm quartz UV-visible cell, which was subsequently sealed with a
septum. The solutions of the ligands of different concentrations were
prepared in a drybox, placed in septum-sealed vials; each vial was
placed in a septum-sealed Schlenk flask under N2. Titrations were done
by addition of an aliquot of the ligand (5-50 µL of 0.7 mM to neat
phosphine or of 1 mM to neat pyridine) by a gastight syringe to the
UV-visible cell, mixing the solution, measuring spectra every 2 min
until the steady-state spectrum was achieved (6-8 min), and repeating
the procedure. Resulting data were processed using the SpecFit 3.0.8
software package (Spectrum Software Associates) for global analysis
of complex equilibrium data. The component spectra, identified by the
Acknowledgment. We thank NSF (Grant CHE-9612725)
and Stanford Graduate Fellowship (R.B.) for financial support.
The Mass Spectrometry Facility at University of California, San
Francisco is supported by NIH (Grants RR 04112 and RR
01614).
JA001364U