One-electron activation of CO by a rhodium(II) porphyrin bimetalloradical complex and concerted reactions of two (RhCO). units

Article abstract of DOI:10.1021/ja962248n

A rhodium(II) porphyrin bimetalloradical complex, ^.Rh(CH₂)₆Rh^. (1) reacts with CO and ethene to form an ethanedionyl reductive coupling product, Rh(CH₂)₆Rh(μ-C(O)C(O)) (2), and an ethylene-bridged complex, Rh(CH₂)₆Rh(μ-CH₂CH₂) (4), respectively. Reaction of a. with 1,3-butadiene proceeds by a 1,4-addition to yield a four-carbon-bridged complex, Rh(CH₂)₆Rh(μ-CH₂CH=CHCH₂) (5). ¹H NMR spectra of 2 and 4 manifest the inherent chirality of two-carbon-bridged tethered diporphyrin complexes. Solutions of 1 and CO react with H₂O, CH₃CH₂OH, and H₂ to produce multifunctionalized formyl complexes H-Rh(CH₂)₆Rh-CHO (6), H(O)C-Rh(CH₂)₆Rh-C(O)OCH₂CH₃ (7), and H(O)C-Rh(CH₂)₆Rh-CHO (8), respectively, but fail to react to an observable extent with methane. Reactivity studies indicate that solutions of 1 and CO function as a source of a species that contains two carbonyl groups (^.[OCRh(CH₂)₆RhCO]^.) (3) that are activated toward one-electron reactions at the carbonyl carbon. Reactions of 3 with substrates (H₂O, C₂H₅OH, H₂) are discussed in terms of a proposed pathway where two metalloformyl radicals ([MCO]^.) and the substrate occur in a four-centered transition state (Rh-C(O)^.···A···B···^.(O)C-Rh).

Full text of DOI:10.1021/ja962248n

7938
J. Am. Chem. Soc. 1997, 119, 7938-7944
One-Electron Activation of CO by a Rhodium(II) Porphyrin
Bimetalloradical Complex and Concerted Reactions of Two
(RhCO)• Units
Xiao-Xiang Zhang, Gregory F. Parks, and Bradford B. Wayland*
Contribution from the Department of Chemistry, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104-6323
ReceiVed July 2, 1996X
Abstract: A rhodium(II) porphyrin bimetalloradical complex, •Rh(CH2)6Rh• (1) reacts with CO and ethene to form
an ethanedionyl reductive coupling product, Rh(CH2)6Rh(µ-C(O)C(O)) (2), and an ethylene-bridged complex,
Rh(CH2)6Rh(µ-CH2CH2) (4), respectively. Reaction of 1 with 1,3-butadiene proceeds by a 1,4-addition to yield a
four-carbon-bridged complex, Rh(CH2)6Rh(µ-CH2CHdCHCH2) (5). 1H NMR spectra of 2 and 4 manifest the inherent
chirality of two-carbon-bridged tethered diporphyrin complexes. Solutions of 1 and CO react with H2O, CH3CH2OH,
and H2 to produce multifunctionalized formyl complexes HsRh(CH2)6RhsCHO (6), H(O)CsRh(CH2)6RhsC(O)-
OCH2CH3 (7), and H(O)CsRh(CH2)6RhsCHO (8), respectively, but fail to react to an observable extent with methane.
Reactivity studies indicate that solutions of 1 and CO function as a source of a species that contains two carbonyl
groups (•[OCRh(CH2)6RhCO]•) (3) that are activated toward one-electron reactions at the carbonyl carbon. Reactions
of 3 with substrates (H2O, C2H5OH, H2) are discussed in terms of a proposed pathway where two metalloformyl
radicals ([MCO]•) and the substrate occur in a four-centered transition state (RhsC(O)•‚‚‚A‚‚‚B‚‚‚•(O)CsRh).
Introduction
Scheme 1
Rhodium(II) porphyrins have previously been shown to form
carbon monoxide complexes (((por)RhCO)•) where CO is
activated toward one-electron reactions at the carbonyl carbon.1,2
Reactions of [(por)RhCO]• with sources of X• that form (por)-
Rh-C(O)X complexes illustrate the formal relationship of
[(por)RhCO]• species with the formyl radical (HC(O)•).1 Promi-
nent examples include dimerization through -(O)C-C(O)-
bonding to form 1,2-ethanedionyl complexes, (por)Rh-C(O)-
C(O)-Rh(por),2 and reactions with H atom sources that produce
formyl complexes, (por)Rh-CHO.3 Samarium(II) complexes
which are exceptionally strong reducing agents also produce
carbon monoxide coupling through one-electron activation of
CO.4 Diporphyrin complexes of rhodium(II) (•Rh(CH2)6Rh•)
(1) have recently been reported to accomplish relatively fast
methane C-H bond activation by the concerted reaction of the
two metalloradical centers.5 In this paper we report on reactivity
studies of •Rh(CH2)6Rh• (1) and CO with a series of substrates
(H2O, C2H5OH, H2) where the observed products are consistent
with reactions of two one-electron-activated CO fragments in
•
a diradical molecular unit, [OCRh(CH2)6RhCO]• (3).
X Abstract published in AdVance ACS Abstracts, August 15, 1997.
(1) (a) Wayland, B. B.; Sherry, A. E.; Poszmik, G.; Bunn, A. G. J. Am.
Chem. Soc. 1992, 114, 1673. (b) Sherry, A. E.; Wayland, B. B. J. Am.
Chem. Soc. 1989, 111, 5010.
(2) (a) Wayland, B. B.; Woods, B. A.; Coffin, V. L. Organometallics
1986, 5, 1059. (b) Wayland, B. B.; Sherry, A. E.; Coffin, V. L. J. Chem.
Soc., Chem. Commun. 1989, 662. (c) Coffin, V. L.; Brennen, W.; Wayland,
B. B. J. Am. Chem. Soc. 1988, 110, 6063.
Results
Preparation of a Rhodium(II) Porphyrin Bimetalloradical
Complex. A diporphyrin ligand where the porphyrin units are
tethered by a diether (-O(CH2)6O-) spacer was prepared and
converted to the dirhodium(II) derivative by procedures outlined
in Scheme 1.5-8 The dimethyl derivative photolyzes in benzene
to form a dirhodium(II) complex (•Rh(CH2)6Rh•) (1) which is
(3) (a) Wayland, B. B.; Woods, B. A. J. Chem. Soc., Chem. Commun.
1981, 700. (b) Wayland, B. B.; Woods, B. A.; Pierce, R. J. Am. Chem.
Soc. 1982, 104, 302. (c) Paonessa, R. S.; Thomas, N. C.; Halpern, J. J. Am.
Chem. Soc. 1985, 107, 4333.
(4) (a) Evans, W. J.; Drummond, D. K.; Chamberlain, L. R.; Doedens,
R. J.; Bott, S. G.; Zhang, H.; Atwood, J. L. J. Am. Chem. Soc. 1988, 110,
4983. (b) Evans, W. J.; Drummond, D. K. J. Am. Chem. Soc. 1988, 110,
2772. (c) Evans, W. J.; Grate, J. W.; Doedens, R. J. J. Am. Chem. Soc.
1985, 107, 1671.
(6) (a) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989, 54, 828. (b)
Little, R. G. J. Heterocycl. Chem. 1978, 15, 203.
(7) (a) Sherry, A. E.; Wayland, B. B. J. Am. Chem. Soc. 1990, 112, 1259.
(b) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991, 113,
5305.
(5) (a) Zhang, X.-X.; Wayland, B. B. J. Am. Chem. Soc. 1994, 116, 7897.
(b) Zhang, X.-X.; Wayland, B. B. Manuscript in preparation.
S0002-7863(96)02248-2 CCC: $14.00 © 1997 American Chemical Society

ActiVation of CO by a Rhodium(II) Porphyrin Complex
J. Am. Chem. Soc., Vol. 119, No. 34, 1997 7939
Figure 1. 1H NMR for the pyrrole of Rh(CH2)6Rh(µ-C(O)C(O)) (2)
in C6D6 (296 K).
shown to be a persistent bimetalloradical through observation
of Curie behavior for the porphyrin pyrrole contact shifts (350-
200 K). EPR parameters for 1 in toluene glass (90 K) (g| )
1.93, g ) 2.66; A(103Rh)(g|)) 159 MHz)1 are characteristic
21
of a low-spin rhodium(II) species with a (dxy)2(dxzdyz)4dz
Figure 2. Determination of the activation parameters for the dissocia-
electron configuration. The steric requirements of the dipor-
phyrin ligand that contains six mesityl groups precludes intra-
and intermolecular Rh-Rh bonding and provides a stable
rhodium(II) porphyrin bimetalloradical complex, •Rh(CH2)6Rh•
(1)5 (Scheme 1).
•
tion of Rh(CH2)6Rh(µ-C(O)C(O)) (2) into [OCRh(CH2)6RhCO]• (3)
-1
using proton NMR line broadening of 2 as the observable (kf ) τd
;
Kq ) kf(h/kT).
K); 7.0 ( 0.8 Hz (320 K); 20 ( 2 Hz (330 K)). The pyrrole
hydrogen line broadening is ascribed to dissociation of 2 into
the paramagnetic dicarbonyl diradical unit •[OCRh(CH2)6-
RhCO]• (3) (eq 2) by (O)C-C(O) bond homolysis in analogy
•
Reaction of Rh(CH2)6Rh• (1) with CO, CH2dCH2, and
CH2dCHCHdCH2. (a) Reaction of 1 with CO. Benzene
solutions of 1 react with CO (PCO ) 0.3-0.9 atm, T ) 295 K)
to produce an equilibrium distribution of species containing CO
where the 1,2-ethanedione complex (Rh(CH2)6Rh(µ-C(O)C(O))
(2) (eq 1) greatly predominates. Compound 2 is the only
(2)
with the previously reported dissociation of ((TMP)Rh)2(µ-
C(O)C(O)) into [(TMP)Rh-CO]• monomer fragments.1 The
(1)
contribution of exchange to the line width at half-height (∆ν1/2
;
rhodium porphyrin species observed in the 1H NMR for
solutions of 1 and CO when the CO pressure is greater than
0.2 atm (T ) 295 K). Several prominent features in the H
T2-1(ex) ) π∆ν1/2) is given by the general expression T2-1(ex)
) τd-1[(Aτp/2)2][1 + (Aτp/2)2]-1, where τd and τp are the
lifetimes of the nuclear spin states in the diamagnetic and
paramagnetic species and A is the electron-nuclear coupling
constant for the nucleus being observed.9 When (Aτp/2)2 . 1,
1
NMR of 2 include eight chemically inequivalent pyrrole
hydrogen resonances (Figure 1) and diastereotopic CH2 groups
in the diether tether (-O(CH2)6O-). The 13C NMR of the
13C(O) derivative of 2 displays an AA′XX′ pattern characteristic
of the 103Rh-13C(O)13C(O)-Rh103 unit that is closely related
to analogous ((por)Rh)2(µ-13C(O)13C(O)) complexes.1,2 The
AA′XX′ pattern for 2 is simulated by using the following
-1
T2-1(ex) ) τd ) kf, where kf is the rate constant for
dissociation.7,10 A relatively long electron spin relaxation time
(τp >10-5 s-1) and moderate pyrrole hydrogen-electron
coupling constant (A > 106 s-1) fulfill these limiting conditions
so that T2-1(ex) equals the rate constant for dissociation.
Evaluation of the activation parameters for reaction 2 gives
∆Hq2 ) 21 ( 1 kcal mol-1 and ∆Sq2 ) 13 ( 3 cal K-1 mol-1
(Figure 2). ∆Hq2 is effectively equal to and ∆Sq2 smaller than
the corresponding values for ((TMP)Rh)2(µ-C(O)C(O)) (∆Hq
) 21.3 ( 0.8 kcal mol-1 and ∆Sq ) 22 ( 3 cal K-1 mol-1).1
(b) Reaction of 1 with Ethene. Benzene solutions of
•Rh(CH2)6Rh• (1) react with ethene (PCH dCH ≈ 0.4 atm, T )
1
2
13
C
103Rh-13
C
103Rh-13
) 9
C
parameters: δ ) 164.5, J
) 40 Hz, J
3
1
103Rh-103Rh
13C-13
C
Hz, J
) 4 Hz, J
) 20 Hz.
Solvent evaporation from a frozen benzene solution of 2
produces a yellow-orange solid which as a Nujol mull displays
two bands in the CO stretching region (νCO ≈ 1780, 1769 cm-1
;
ν
≈ 1736, 1725 cm-1) which are very similar to those
13CO
observed for (TXP)Rh-C(O)C(O)-Rh(TXP) (νCO ≈ 1778,
2
2
1767 cm-1; ν
≈ 1738, 1727 cm-1)1 and (TMP)Rh-C(O)-
13
CO
295 K) to generate an ethylene-bridged complex, Rh(CH2)6Rh-
C(O)-Rh(TMP) (νCO ≈ 1782, 1770 cm-1)1 (TXP ) tetra-
xylylporphyrin; TMP ) tetramesitylporphyrin). The νCO bands
in the Nujol mull of 2 decline in intensity and disappear over
a period of hours due to loss of CO. FAB MS for benzene
solutions of 2 yields the parent ion for 1 as the highest mass
species, indicating facile loss of CO from 2. Attempts to grow
single crystals of 2 for X-ray diffraction study have not yet been
successful.
(µ-CH2CH2) (4), as the exclusive observed product (eq 3).
1
Prominent features in the H NMR of 4 include eight pyrrole
resonances, diasteriotopic CH2 groups in the diether
(-O(CH2)6O-) tether, and distinctive high-field multiplets
associated with diasteriotopic CH2 groups in the bridging
ethylene unit (δCH(A) ) -7.81, δCH(B) ) -8.24). Repeating
this experiment with 13CH2d13CH2 results in 13C splitting of
The pyrrole 1H NMR resonances for 2 are relatively narrow
below 295 K but broaden substantially as the temperature is
elevated (300-350 K) (∆ν1/2(ex) ) ∆ν1/2(obs) - ∆ν1/2(nat);
∆ν1/2(ex) (2) ) 0.75 ( 0.15 Hz (300 K); 2.30 ( 0.25 Hz (310
(9) (a) Johnson, C. S., Jr. AdVances in Magnetic Resonance; Academic
Press: New York, 1965; Vol. 1. (b) McConnel, H. M.; Berger, S. B. J.
Chem. Phys. 1957, 27, 230. (c) Kreilich, R. W.; Weissman, S. I. J. Am.
Chem. Soc. 1966, 88, 2645. (d) Williams, D. J.; Kreilich, R. W. J. Am.
Chem. Soc. 1968, 90, 2775. (e) Williams, D. J.; Kreilich, R. W. J. Am.
Chem. Soc. 1967, 89, 3408.
(8) Ogoshi, H.; Setsune, J.; Omura, T.; Yoshida, Z. J. Am. Chem. Soc.
1975, 97, 6461.
(10) Wayland, B. B.; Coffin, V. L.; Farnos, M. D. Inorg. Chem. 1988,
27, 2745.

7940 J. Am. Chem. Soc., Vol. 119, No. 34, 1997
Zhang et al.
(3)
the two high-field multiplets (1J
) 144 Hz) (Figure 3).
13C-H
The 1H-decoupled 13C NMR of the ethylene bridge in the
(13CH2)2 derivative of 4 appears as an AA′XX′ pattern (Figure
4) that is simulated by using the following parameters: δ
13
)
C
19.34 ppm, 1J
) 32.4 Hz, J
C ) 30.2 Hz, 2J
C ) -3.2 Hz, 1J
103Rh-13 13C-13
103Rh-13
103Rh-103Rh
C
3
≈ 0.2 Hz.
(c) Reaction of 1 with 1,3-Butadiene. Benzene solutions
of 1 react with 1,3-butadiene (PC H ≈ 0.4 atm) to produce a
4
6
four-carbon-bridged complex, Rh(CH2)6Rh(µ-CH2CHdCHCH2)
(5) (eq 4). The 1H NMR of 5 shows only three resolved pyrrole
1
Figure 3. (a) H NMR of the Rh-CH2-CH2-Rh unit of 4 in C6D6
1
(296 K). (b) H NMR for the carbon-13 derivative of 4.
(4)
resonances with intensity ratios of 1:1:2, chemically equivalent
CH2 groups in the diether (-O(CH2)6O-) tether, and high-field
resonances for the CH2(dd, -5.32) and CH (t, -2.24) groups
in the bridging -CH2CHdCHCH2- unit.
Reaction of 1 and CO with H2O, C2H5OH, H2, and CH4.
(a) Reaction of 1 and CO with Water. Addition of H2O to a
benzene solution of 1 and CO (PCO ) 0.4 atm; T ) 295 K)
1
results in the formation of a single species observed in the H
Figure 4. Proton-decoupled 13C NMR spectrum of the ethylene-bridged
(Rh-13CH2-13CH2-Rh) unit in the carbon-13 derivative of 4 in C6D6
NMR that is assigned as the hydrido-formyl complex
H-Rh(CH2)6Rh-CHO (6) (eq 5). When 13CO is used,
1
2
103
13
103
13
13
Rh-
(296 K) (AA′XX′; δ C ) 19.34 ppm; J
C ) 30.2 Hz, J
Rh-
C
1
3
103
13
13
103
) -3.2 Hz, J
) 32.4 Hz, J
) 0.2 Hz).
Rh- Rh
C-
C
results exclusively in the formation of a formyl-ester complex,
H(O)C-Rh(CH2)6Rh-C(O)OCH2CH3 (7) (eq 6). The 1H NMR
(5)
concomitant formation of 6 and 13CO2 (δ13 ) 124 ppm) with
CO2
the disappearance of 2 is observed in the 13C NMR. The H
1
(6)
NMR spectrum of 6 has characteristic resonances for a rhodium
2
103
formyl unit (δCHO ) 3.66; J Rh-CHO ) 1.3 Hz) and a rhodium
1
103
hydride fragment (δRh-H ) -40.07; J Rh-H ) 44.5 Hz) (Figure
5a), as well as chemically inequivalent methylene groups in the
diether tether (δOCH (CH ) CH O ) 3.92, 3.90). Repeating this
spectrum of 7 displays a characteristic formyl resonance (δCHO
2
103Rh-CHO
) 3.66; J
) 1.6 Hz), quartet and triplet resonances
2
2
4
2
experiment with 13CO in toluene results in 13C splitting of the
for the organometallic ester group (Rh-C(O)OCH2CH3) (δCH
2
formyl hydrogen into a doublet of doublets (1J
) 197.5
13CHO
3
) 1.38; δCH ) -1.04; JH-H ) 6.8 Hz), and chemically
3
Hz). The H-coupled 13C NMR spectrum of the formyl units
1
inequivalent methylene groups in the diether tether (δOCH (CH ) CH O
2
2 4
2
in the 13CO derivative of 6 appears as a doublet of doublets
) 3.92, 3.90). Repeating this experiment with 13CO results in
1
1
13
from 103Rh and H coupling (δ CHO ) 193.7; J
C ) 30.2
13
103Rh-
13C splitting of the formyl peak (1JH- ) 197.5 Hz) and the
13
C
1
13
CH2 group of the organometallic ester unit (3J13
) 4.4
Hz; JH- ) 197.5 Hz) (Figure 5b).
C
C(O)OCH2
(b) Reaction of 1 and CO with Ethanol. Addition of
ethanol to a benzene solution of 1 and CO (PCO ) 0.45 atm)
Hz). The 1H-coupled 13C NMR spectrum of the 13CO derivative
1
13
of 7 shows a doublet of doublets (δ
CHO
103Rh-13
) 194.51; J
C

ActiVation of CO by a Rhodium(II) Porphyrin Complex
J. Am. Chem. Soc., Vol. 119, No. 34, 1997 7941
Figure 6. (a) Proton-coupled 13C NMR spectrum of the formyl and
ester groups in the carbon-13 derivative of H(O)13C-Rh(CH2)6Rh-
13C(O)OCH2CH3 (7) in C6D6 (296 K). (b) Expanded 13C NMR spectrum
Figure 5. NMR spectra for the 13C derivative of 6 (H-Rh(CH2)6Rh-
13
1
103
C ) 30.2 Hz, 1JH-
13
13
13
of the Rh- CHO unit (δ CHO ) 194.51, J
Rh-
C
1
13CHO) in CD3C6D5 (296 K): (a) H NMR of the rhodium hydride
) 197.5 Hz). (c) Expanded 13C NMR of the Rh-13C(O)OCH2CH3 unit
1
1
3
103
13
(δ
103
13
13
group, Rh-H (δ ) -40.07 ppm; J
) 43.4 Hz); (b) proton-
) 148.29, J
) 46.5 Hz, JH- ) 4.4 Hz).
Rh-H
C(O)OCH CH
Rh-
C
C
2
3
coupled 13C NMR of the formyl group, Rh-13CHO (δ ) 193.7 ppm;
1
1
103
13
13
J
) 30.2 Hz, JH- ) 197.5 Hz).
Rh-
C
C
) 30.2 Hz; 1JH- C ) 197.5 Hz) for the Rh-13CHO unit and a
13
1
C ) 46.5 Hz; 3JH-
13
103Rh-13
13C
doublet of triplets (δ C ) 148.29; J
) 4.4 Hz) for the Rh-13C(O)OCH2CH3 unit (Figure 6).
(8)
(c) Sequential Reaction of 1 with CO and H2. When
benzene solutions of •Rh(CH2)6Rh• (1) are pressurized sequen-
tially with carbon monoxide (PCO ) 0.52 atm) and then H2 (PH
2
product of CH4 activation, H(O)C-Rh(CH2)6Rh-C(O)CH3 (10)
(eq 9), nor any other products containing Rh-CHO or Rh-
) 0.22 atm, PCO ) 0.52 atm; T ) 295 K) compound 2 forms
initially and then reacts away to produce a diformyl complex,
H(O)C-Rh(CH2)6Rh-CHO (8) (eq 7), as the only species
(9)
(7)
C(O)CH3 units were detected by 1H NMR. A very slow reaction
(∼2 weeks) ultimately results in formation of the hydrido-
methyl complex, H-Rh(CH2)6Rh-CH3 (11)4 (eq 10). The 1H
observed by NMR. The 1H NMR spectrum of 8 gives a
2
13
characteristic Rh-CHO resonance (δ
CHO
103Rh-CHO
) 3.66; J
) 1.4 Hz) which becomes a doublet of doublets in the 13CO
derivative of 8 (1J
) 197.5 Hz) (Figure 7a). The
13C-H
1H-coupled 13C NMR spectrum in toluene of the formyl units
in the 13CO derivative of 8 appears as a doublet of doublets
(10)
from 103Rh and H coupling (δ CHO ) 193.65; J
Hz; JH- ) 197.5 Hz) (Figure 7b).
C ) 30.2
1
1
103Rh-13
13
1
13
C
(d) Sequential Reaction of 1 with H2 and CO. Benzene
NMR spectrum of 11 displays a high-field doublet (δRh-CH )
3
2
103Rh-H
) 3.0 Hz) for the Rh-CH3 unit and a high-
solutions of 1 react rapidly with H2 (PH ) 0.3 atm) to form
-5.29; J
2
1
the dihydrido complex H-Rh(CH2)6Rh-H (9) (eq 8),5 which
103Rh-H
field doublet (δRh-H ) -40.07; J
) 43.4 Hz) for the
rhodium hydride fragment. Compound 11 presumably forms
from reaction of CH4 with the small concentration of 1 (eq 10)5
in equilibrium with 2 (eq 1).
when subsequently pressurized with CO (PCO ) 0.5 atm; PH
2
) 0.3 atm; T ) 295 K) fails to produce observable quantities
of formyl complexes 6 and 8 over a period of weeks.
(e) Reaction of 1 and CO with Methane. When benzene
solutions of 1 were sequentially pressurized with carbon
Discussion
monoxide (PCO ) 0.37 atm) and then CH4 (PCH ) 0.53 atm),
Reactions of 1 with CO, CH2dCH2, and CH2dCH-
CHdCH2. The dirhodium(II) diporphyrin bimetallo-
4
the dionyl complex 2 formed and persisted. Neither the desired

7942 J. Am. Chem. Soc., Vol. 119, No. 34, 1997
Zhang et al.
for the bridging substrate sandwiched between two porphyrins
has previously been proposed as the reason for observing
geometric isomers for (OEP)RhsCH2CH(CO2CH3)sRh(OEP)
and related complexes.11 Chemical equivalence of the CH2
groups in both the diether tether and the substrate bridge
(-CH2CHsCHCH2-) as well as several pyrrole hydrogen
equivalencies in Rh(CH2)6Rh(µ-CH2CHdCHCH2) (5) indicate
that the inherent chirality for the static molecule is not
manifested by the NMR spectra (T ) 270-295 K). Introduction
of the four-carbon-bridged substrate in 5 apparently opens the
diporphyrin pocket enough to permit sufficiently fast rotation
of the bridging substrate such that the enantiomers are rapidly
interconverted on the NMR time scale.
Reactions of 1 and CO with H2O, C2H5OH, H2, and CH4.
Rhodium(II) porphyrins have previously been observed to react
with CO to produce 17-electron [Rh-CO]• units that have
reactivity patterns similar to that of the formyl radical [HCO]•.1,2,4
Hydrogen atom abstractions from (por)Rh-H and HSnR3 that
produce formyl complexes ((por)Rh-CHO),1,4 reaction with
(por)Rh• to form dimetal ketone complexes ((por)Rh-C(O)-
Rh(por)),2 and dimerization by RhC(O)-C(O)Rh bond forma-
tion2 illustrate the activation of the [(por)Rh-CO]• unit toward
one-electron reactions at the carbonyl carbon.
Figure 7. NMR spectra of the formyl unit in the carbon-13 derivative
of H(O)13C-Rh(CH2)6Rh-13CHO (8) in CD3C6D5 (296 K): (a) 1H
1
2
103
13
13
Solutions of 1 and CO primarily contain the dionyl-bridged
complex 2, but small equilibrium concentrations of 1, along
with the mono- and bis-CO complexes of 1 must also be present.
1H NMR line broadening for 2 associated with RhC(O)-C(O)-
Rh bond homolysis indicates that the system resulting from 1
and CO is a thermal source for a dirhodium dicarbonyl diradical,
•[OCRh(CH2)6RhCO]• (3). An objective of this study was to
evaluate the potential of diradical species like 3 to activate
substrates (A, B) through reaction with two metalloformyl
radical units in analogy with the reported reaction of the
NMR (δ CHO ) 3.66 ppm; J C-H ) 197.5 Hz, J Rh-CHO ) 1.4 Hz);
1
(b) proton-coupled 13C NMR (δ CHO ) 193.65 ppm; J
C ) 30.2
13
103
13
Rh-
1
13
Hz, JH- ) 197.5 Hz).
C
•
radical, Rh(CH2)Rh• (1), reacts with CO, CH2dCH2, and
CH2dCHCHdCH2 to form complexes where reduced substrate
units bridge two rhodium centers, Rh(CH2)6 Rh(µ-C(O)-C(O))
(2), Rh(CH2)6Rh(µ-CH2CH2) (4), and Rh(CH2)6Rh(µ-CH2-
CHdCHCH2) (5), respectively. There is no evidence for a
dirhodium ketone (RhsC(O)sRh) which is a prominent species
in the reaction of [(OEP)Rh]2 with CO (OEP ) octaethyl-
porphyrin).2 The bent RhsC(O)sRh unit causes steric interac-
tions between mesityl groups on adjacent porphyrin units that
inhibit formation of the one-carbon-bridged dimetal ketone
(MsC(O)sM) in a manner similar to that previously observed
for the (TMP)Rh system.1 1H NMR spectra of 2, 4, and 5 show
that these molecules appear to have two chemically equivalent
units consisting of the porphyrin and half of the diether tether
and bridging substrate. The chemical equivalencies presumably
result from the dynamic molecular structure passing through a
conformation with a C2 axis that bisects the diether tether and
bridging substrate units.
•
dimetalloradical Rh(CH2)6Rh• (1) with CH4 and H2.5
Solutions of 1 and CO which contain 2 as the predominant
species react with H2O exclusively to produce a hydrido-formyl
complex (H-Rh(CH2)6Rh-CHO (6)) which does not react
further with CO at a finite rate to yield the diformyl complex
H(O)C-Rh(CH2)6Rh-CHO (8). Observation that 6 forms as
the exclusive product simultaneously with disappearance of 2
is compatible with a rate-limiting reaction of 3 with water to
form an intermediate formyl-carboxylic acid complex (HO(O)C-
Rh(CH2)6Rh-CHO (12)) that rapidly loses CO2 to form 6.
Elimination of CO2 is a general reaction of metallo carboxylic
acid complexes12 and was previously observed for rhodium
porphyrin derivatives.4b,13
Additional evidence for a dominant concerted reaction of two
metalloformyl radical units ((RhCO)•) in 3 is obtained from the
reaction of 1 and CO with ethanol (eq 6) that yields the formyl-
ester complex CH3CH2OC(O)-Rh(CH2)6Rh-CHO (7) as the
Observation of eight pyrrole hydrogen doublets from four
diastereotopic pyrrole HsCsCsH AB patterns and diastereo-
topic CH2 AB patterns for the diether tether (sOCH2(CH2)4-
1
predominant product observed in the H NMR. Reaction of
1
ethanol with (Rh-CO)• units in different molecules of 3 would
yield symmetrically derivatized carbonyl products (H(O)C-
Rh(CH2)6Rh-CHO, CH3CH2O(O)C-Rh(CH2)6Rh-C(O)OCH2-
CH2Os) are manifestations in the H NMR of the inherent
chirality of 2 and 4. The dynamic structural changes for 2 and
4 must not pass through a conformation that contains an
improper axis of rotation on the NMR time scale. Rotation of
the substrate bridges of 2 and 4 (RhsC(O)C(O)sRh,
RhsCH2CH2sRh) would interconvert the enantiomers by
passing through a conformation with an effective plane of
symmetry (S1). This rotation in 2 and 4 must be inhibited by
the constraints of the tightly packed diporphyrin pocket contain-
ing the two-carbon-bridged substrates. Inhibition of motions
(11) Wayland, B. B.; Poszmik, G.; Fryd, M. Organometallics 1992, 11,
3534.
(12) (a) Van Voorhees, S. L.; Wayland, B. B. Organometallics 1987, 6,
204. (b) Bunn, A. G.; Wei, M. Wayland, B. B. Organometallics 1994, 6,
3390.
(13) (a) Baker, F. C.; Hendriksen, D. E.; Eisenberg, R. J. Am. Chem.
Soc. 1980, 102, 1020. (b) Ford, P. C. Acc. Chem. Res. 1981, 14, 31. (c)
Yoshida, T.; Okano, T.; Fuda, Y.; Otsuka, S. Inorg. Chem. 1981, 103, 3411.

ActiVation of CO by a Rhodium(II) Porphyrin Complex
J. Am. Chem. Soc., Vol. 119, No. 34, 1997 7943
CH3) in addition to 7. While it is not possible to distinguish
between symmetrically derivatized carbonyl products in the 1H
NMR, we believe that the lack of any intermediates suggests
the predominance of compound 7. In addition, nonconcerted
reaction of ethanol with a monocarbonyl complex (•Rh(CH2)6-
RhCO•) would form species containing Rh-H units. Since
probably unfavorable (∆H° ≈ -5 kcal mol-1, ∆G° (298 K) ≈
13
13
+3 kcal mol-1).17-19 In order to obtain clearly favorable
thermodynamics for methane C-H bond reactions like that
depicted in reactions 9 and 13, the dionyl (O)C-C(O) bond
must be weakened or eliminated.
Any of a set of related pathways where the H and X (X )
OH, OR, H) units from H-X add to the carbonyl carbon centers
of the dionyl 2 could account for the observed reaction products.
The observations are most easily reconciled by pathways that
involve four-centered transition states that direct a concerted
cleavage of the H-X and formation of Rh-C(O)H and Rh-
C(O)X units. Polar substrates (H2O, CH3CH2OH) could involve
interaction with a carbonyl carbon of 2 to preorganize formation
of the Rh-C(O)OR unit prior to (O)C-C(O) homolysis and
hydrogen transfer. However, this form of preorganization is
not readily envisioned for the H2 reaction (eq 12). Reaction of
CH4 may be kinetically inhibited by requiring a more restrictive
near linear four-centered transition state which is not demanded
by the other substrates. Incorporating more sterically demanding
substituents on the tethered diporphyrin complexes such that
the dionyl (O)C-C(O) bond is weakened can be expected to
reveal whether or not a facile pathway is available for
simultaneous activation of CO and methane.
1
these Rh-H units are readily distinguished by H NMR and
are not present at any time during the reaction, we conclude
that the most probable pathway involves reaction of two
(RhCO)• units from the same dionyl complex. Reaction 6 is
envisioned to occur through a four-centered transition state7,14-16
that produces concerted formation of the rhodium ester and
formyl units in 7.
Reaction of H2 with 1 and CO produces a diformyl complex,
H(O)C-Rh(CH2)6Rh-CHO (8) (eq 7), as the exclusive rhodium
1
porphyrin species observed by H NMR. Reaction 7 does not
proceed through rhodium hydride intermediates because both
H-Rh(CH2)6Rh-H (9) and H-Rh(CH2)6Rh-CHO (6) have
been independently shown to not react at a finite rate with any
species present in a solution of 1 and CO (2, 3, CO) to produce
8. The sterically demanding porphyrin ligands prohibit achiev-
ing the transition state for hydrogen atom transfer from the
Rh-H unit to the (RhCO)• fragment which inhibits formation
of metalloformyl species by the pathway that operates in the
(OEP)Rh system.4c The apparent simultaneous formation of
two formyl units in 8 implies that this process occurs in concert
with cleavage of the H-H bond through a four-centered
transition state.14
Experimental Section
General Considerations. All manipulations were performed on a
high-vacuum line equipped with a Welch Duo-Seal vacuum pump or
in an inert atmosphere box unless otherwise noted. All reagents were
purchased from Aldrich or Strem Chemicals. Pyrrole was distilled and
[Rh(CO)2Cl]2 was sublimed prior to use. Deuterated NMR solvents
such as benzene and toluene were degassed by freeze-pump-thaw
cycles to remove oxygen and then refluxed over sodium/benzophenone
ketyl until the indicator turned purple to remove water. Chloroform,
methylene chloride, and 1,2-dichloroethane used in synthetic procedures
were purified by washing three times with water followed by chroma-
tography on grade 1 alumina for the removal of ethanol and water.
Substrates were degassed by freeze-pump-thaw cycles and stored over
molecular sieves. Research grade hydrogen, methane, ethene, 1,3-
butadiene and carbon monoxide were purchased from Matheson Gas
Products or Aldrich and used without further purification. Carbon-
13-labeled carbon monoxide was purchased from Cambridge Isotope
Laboratories, Inc. Prepurified nitrogen and argon gases used for the
inert atmosphere box were purchased from Airco or MG Industries.
Proton NMR spectra were obtained on a Bruker WP200SY, a Bruker
AC-250, or a Bruker AMX-500 interfaced to an Aspect 300 computer
at ambient temperature. All spectra were referenced using the residual
solvent peak as an internal standard (benzene-d6, δ ) 7.155; toluene-
d8, δ ) 2.09). Carbon-13 NMR spectra were obtained on the AMX-
500 instrument equipped with a carbon-13 probe. Variable-temperature
1H NMR spectra were obtained on an IBM-Bruker AF200SY or IBM
Bruker WP200SY spectrometer equipped with a Bruker VT-1000
temperature controller or boil-off from liquid nitrogen when the
temperature needed is below 220 K. The probe was cooled with an
FTS systems refrigerator unit equipped with a temperature controller.
The temperatures in the probe were referenced against an external
sample of methanol (below ambient) and ethylene glycol (above
ambient). The samples were kept away from light prior to and during
the experiments. Fast atom bombardment mass spectrometry (FAB-
MS) data were recorded on a VG-ZAB-E spectrometer with a cesium
When solutions of 1 and CO are put in contact with CH4, no
products with either formyl or acyl groups are observed to form.
The potential reaction with CH4 to produce CH3(O)C-Rh(CH2)6-
Rh-CHO (10) is expected to be thermodynamically less
favorable than that for H2 by ∼6-8 kcal mol-1 17
, and this may
account for the absence of observable concentrations of 10;
however, this negative result could have its origin in either or
both kinetic and thermodynamic factors.
The overall reactions of 1 and CO with C2H5OH, H2, and
CH4 to form 7, 8, and 10, respectively (eqs 6, 7, and 9) are all
highly thermodynamically favorable.17b In these reactions an
intermediate dionyl complex (2) forms, and it is the equilibrium
of 2 with the products of reactions 6, 7, and 9 that actually
determines the equilibrium distribution of rhodium porphyrin
complexes in solution (eqs 11-13). Thermochemical estimates
2 + C2H5OH h H(O)C-Rh(CH2)6-C(O)OC2H5
2 + H2 h H(O)C-Rh(CH2)6Rh-C(O)H
(11)
(12)
2 + CH4 h H(O)C-Rh(CH2)6Rh-C(O)CH3 (13)
indicate that the ethanol reaction with 2 is highly favorable
(∆H° ≈ -21 kcal mol-1 , ∆G1°1(298 K) ≈ -13 kcal mol-1),
11
the H2 reaction is slightly favorable (∆H° ≈ -11 kcal mol-1
,
12
∆G° (298 K) ≈ -3 kcal mol-1), and the reaction of CH4 is
12
(14) Wayland, B. B.; Ba, S.; Sherry, A. E. Inorg. Chem. 1992, 31, 148.
(15) Simandi, L. I.; Budo-Zahonyi, E.; Szeverenyi, Z.; Nemeth, S. J.
Chem. Soc., Dalton Trans. 1980, 276.
(16) (a) Halpern, J.; Pribinic, M. Inorg. Chem. 1970, 9, 2626. (b) Halpern,
J. Inorg. Chim. Acta 1982, 62, 31. (c) Halpern, J. Inorg. Chim. Acta 1983,
77, L105.
(18) (a) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Halow, I.; Bailey,
S. M.; Schumm, R. H. NBS Technical Note 270-3. Institute for Basic
Standards, National Bureau of Standards: Washington, DC, 1968. (b) Cox,
J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic
Compounds; Academic: New York, 1970. (c) Benson, S. W. Thermo-
chemical Kinetics, John Wiley & Sons: New York, 1968.
(19) (a) Wayland, B. B.; Coffin, V. L.; Sherry, A. E.; Brennen, W. R.
In ACS Symposium Series; Marks, T., Ed.; ACS: Washington, DC, 1990;
No. 428, Chapter 10, pp 148-158. (b) Wayland, B. B. Polyhedron 1988,
7, 1545. (c) Wayland, B. B. In Energetics of Organometallic Species;
Simo˜es, J. A. M., Ed.; Kluwer Publishers: The Netherlands, 1992; p 73.
(17) (a) Thermochemical estimates are obtained by using standard organic
bond enthalpies (kcal mol-1),18 HsH (104), H3CsH (105), (O)CsH (87),
(O)CsCH3 (82), (CtO)s(CdO) (70), ROsH (104), (O)CsOR (97), using
(por)RhsC(O)X (X ) H, CH3, OR) (59) and (por)RhsH (60) and
estimating ∆H°(37) (1 + 2CO f 2) from prior thermodynamic results for
1
((por)Rh)2(µ-C(O)C(O)) complexes.19 (b) ∆H° ≈ -58 kcal mol-1, ∆G°-
6
6
(298 K) ≈ -34 kcal mol-1, ∆H7° ≈ -48 kcal mol-1, ∆G7°(298 K) ≈ -24
kcal mol-1, ∆H°9 ≈ -42 kcal mol-1, ∆G°9(298 K) ≈ -18 kcal mol-1 17-19
.

7944 J. Am. Chem. Soc., Vol. 119, No. 34, 1997
Zhang et al.
ion gun. Electronic spectra were recorded with a Hewlett-Packard UV
8452 instrument interfaced to an IBM PC. Fourier transform infrared
(FT-IR) spectra were taken on an IBM 97 instrument interfaced to an
Aspect 2000 computer, or with a Perkin-Elmer 1760 FT-IR spectrom-
eter. Samples were prepared as either KBr pellets or Nujol mulls.
Electron paramagnetic resonance spectra were obtained by use of an
ER 100 D X-band spectrometer. Temperature measurements were
calibrated with a Bruker Model VT unit using N2 as the cooling source
(283-90 K). All EPR experiments were performed at 90 K unless
otherwise noted and calibrated using diphenylpicrylhydrazine (DPPH,
g ) 2.0036) as an external standard.
H-Rh(CH2)6Rh-CHO (6). The rhodium(III) diformyl complex
H-Rh(CH2)6Rh-CHO (6) was formed from the reaction of water with
the dirhodium R-diketone complex Rh(CH2)6Rh(µ-C(O)C(O)) (2): 1H
NMR (C6D6) δ 9.03, 8.97, 8.88, 8.86, 8.85, 8.84, 8.82, 8.81 (each a
3
doublet of 2H, JH-H ) 4.9 Hz, pyrrole); 8.16, 8.14, 8.02, 8.00 (dd,
1H each, 3JH-H ) 8.3 Hz, 4JH-H ) 2.2 Hz, o-phenyl); 7.31, 7.29, 7.13,
3
4
7.10 (dd, 1H each, JH-H ) 8.3 Hz, JH-H ) 2.6 Hz, m-phenyl), 7.24,
7.07 (s, 4H each, m-phenyl), 7.18, 7.11 (s, 2H each, m-phenyl); 3.92
3
3
(t, 2H, JH-H ) 6.1 Hz, -OCH2-), 3.90 (t, 2H, JH-H ) 6.1 Hz,
-OCH2-); 2.45 (s, 12H, p-CH3), 2.43 (s, 6H, p-CH3); 2.29, 1.76 (s,
12H each, o-CH3), 2.16, 1.79( s, 6H each, o-CH3); 1.86 (m, 4H,
Rh(CH2)6Rh(µ-C(O)C(O)) (2). The dirhodium R-diketone complex
Rh(CH2)6Rh(µ-C(O)C(O)) (2) was formed from the reaction of carbon
monoxide with the rhodium(II) porphyrin bimetalloradical complex
•Rh(CH2)6Rh• (1): 1H NMR (C6D6) δ 8.85, 8.65, 8.62, 8.39, 8.36, 8.26,
8.22, 8.19 (each a doublet of 2H, 3JH-H ) 4.7 Hz, pyrrole); 7.76, 7.38
2
103
-OCH2CH2-), 1.58 (m, 4H, -CCH2CH2C-), 3.66 (d, 1H, J
) 1.3 Hz, Rh-CHO), -40.07 (d, 1H, J
Rh-CHO
1
103
) 44.5 Hz, Rh-H);
) 30.2 Hz, JH-
Rh-H
103 13
13
13
1
1
13
C NMR (CD3C6D5) δ 193.7 (dd, 1 C, J
Rh-
C
C
) 197.5 Hz, Rh-13CHO).
H(O)C-Rh(CH2)6Rh-C(O)OCH2CH3 (7). The rhodium(III) di-
formyl complex H(O)C-Rh(CH2)6Rh-C(O)OCH2CH3 (7) was formed
from the reaction of ethanol with the dirhodium R-diketone complex
Rh(CH2)6Rh(µ-C(O)C(O)) (2): 1H NMR (C6D6) δ 9.02, 8.95, 8.89,
3
4
(dd, 2H each, JH-H ) 8.2 Hz, JH-H ) 2.2 Hz, o-phenyl); 7.17 (dd,
3
4
3
2H, JH-H ) 8.2 Hz, JH-H ) 2.6 Hz, m-phenyl), 7.05 (dd, 2H, JH-H
4
) 8.4 Hz, JH-H ) 2.6 Hz, m-phenyl), 7.32, 7.24, 7.12, 7.04,
3
6.98, 6.87 (s, 2H each, m-phenyl); 3.99 (m, 2H, JH-H ) 5.0 Hz,
3
3
8.86, 8.83, 8.81, 8.80, 8.79 (each a doublet of 2H, JH-H ) 4.9 Hz,
-OCHaHb-), 3.94 (m, 2H, JH-H ) 5.0 Hz, -OCHaHb); 2.46, 2.41,
pyrrole); 8.18, 8.14, 8.01, 7.89 (dd, 1H each, 3JH-H ) 8.3 Hz, 4JH-H
)
2.40 (s, 6H each, p-CH3); 1.62, 1.49, 1.48, 1.35, 1.27, 1.16 (s, 6H each,
o-CH3); 1.78 (m, 4H, -OCH2CH2-), 1.54 (m, 4H, -CCH2CH2C-);
13C NMR (CD3C6D5) δ 164.47 (AA′XX′ m, 213C, Rh(CH2)6Rh(µ-
13C(O)-13C(O)), six transitions observed at (12, (18, and (27 Hz
3
2.2 Hz, o-phenyl); 7.26, 7.22, 7.10, 7.08 (dd, 1H each, JH-H ) 8.3
4
Hz, JH-H ) 2.6 Hz, m-phenyl), 7.20, 7.10 (s, 4H each, m-phenyl),
3
7.18, 7.14 (s, 2H each, m-phenyl); 3.92 (t, 2H, JH-H ) 6.1 Hz,
3
-OCH2-), 3.90(t, 2H, JH-H ) 6.1 Hz, -OCH2-); 2.45 (s, 12H,
relative to the center of δ 164.47 ppm); IR (Nujol mull) νCO ≈ 1780,
1769 cm-1; ν
≈ 1736, 1725 cm-1
.
13
p-CH3), 2.39 (s, 6H, p-CH3); 2.26 (s, 12H, o-CH3), 1.93, 1.87, 1.77,
1.42 (s, 6H each, o-CH3); 1.87 (m, 4H, -OCH2CH2-), 1.57 (m, 4H,
CO
Rh(CH2)6Rh(µ-CH2CH2) (4). The dirhodium ethylene-bridged
complex Rh(CH2)6Rh(µ-CH2CH2) (4) was formed from the reaction
of ethene with the rhodium(II) porphyrin bimetalloradical complex
•Rh(CH2)6Rh• (1): 1H NMR (C6D6) δ 8.77, 8.52, 8.43, 8.36, 8.35, 8.20,
8.12, 8.06 (each a doublet of 2H, 3JH-H ) 4.7 Hz, pyrrole); 7.89, 7.69
1
2
13
103
-CCH2CH2C-), 3.66 (dd, 1H, J
) 197.6 Hz, J
C-H
3
) 1.6
Rh-CHO
C-H
13
13
3
3
Hz, Rh- CHO), 1.38 (qd, 2H, J
) 4.4 Hz, JH-H ) 6.8 Hz,
Rh-13C(O)OCH2CH3), -1.04 (t, 3H, JH-H ) 6.8 Hz, Rh-13C(O)-
13
13
1
103
13
OCH2CH3); C NMR (C6D6) δ 148.29 (dt, 1 C, J
) 46.5
Rh-
C
3
4
3
Hz, JH-
J
) 4.4 Hz, Rh-13C(O)OCH2CH3), 194.51 (dd, 113C,
13
(dd, 2H each, JH-H ) 8.3 Hz, JH-H ) 2.2 Hz, o-phenyl); 7.47, 7.06
C
(dd, 2H each, 3JH-H ) 8.3 Hz, 4JH-H ) 2.6 Hz, m-phenyl), 7.25, 7.19,
) 30.2 Hz, JH- ) 197.5 Hz, Rh-13CHO).
1
1
103
13
13
Rh-
C
C
3
7.12, 6.97, 6.94, 6.88 (s, 2H each, m-phenyl); 3.96 (m, 2H, JH-H
)
H(O)C-Rh(CH2)6Rh-CHO (8). The rhodium(III) diformyl complex
5.0 Hz, -OCHaHb-), 3.91 (m, 2H, 3JH-H ) 5.0 Hz, -OCHaHb); 2.44,
2.42, 2.41 (s, 6H each, p-CH3); 2.39, 1.78, 1.54, 1.40, 1.20, 1.10 (s,
6H each, o-CH3); 1.78 (m, 4H, -OCH2CH2-), 1.49 (m, 4H, -CCH2-
H(O)C-Rh(CH2)6Rh-CHO (8) was formed from the reaction of
hydrogen with the dirhodium R-diketone complex Rh(CH2)6Rh(µ-
C(O)C(O)) (2): 1H NMR (C6D6) δ 9.02, 8.87, 8.86, 8.85 (each a doublet
of 2H, 3JH-H ) 4.9 Hz, pyrrole); 8.16, 7.94 (dd, 1H each, 3JH-H ) 8.4
1
CH2C-); -7.81 (dm, 2H, J C-H ) 144 Hz, Rh(CH2)6Rh(µ-13CHaHb-
13
13
1
13
CHaHb)), -8.24 (dm, 2H, J C-H ) 144 Hz, Rh(CH2)6Rh(µ-13CHaHb-
4
3
Hz, JH-H ) 2.1 Hz, o-phenyl); 7.30, 7.12 (dd, 1H each, JH-H ) 8.4
13CHaHb)); 13C NMR (C6D6) δ 19.34 (AA′XX′ m, 2 C, J
)
13
1
103
13
Rh-
C
4
Hz, JH-H ) 2.6 Hz, m-phenyl), 7.24 (s, 4H, m-phenyl), 7.20 (s, 2H,
2
1
3
103
103
13
13
13
103
30.2 Hz, J
C ) -3.2 Hz, J
C ) 32.4 Hz, J
Rh ≈ 0.2
Rh-
C-
Rh-
3
m-phenyl), 7.12 (s, 6H, m-phenyl); 3.91 (t, 4H, JH-H ) 6.1 Hz,
Hz, Rh(CH2)6Rh(µ-13CH2-13CH2)).
-OCH2-); 2.45 (s, 12H, p-CH3), 2.43 (s, 6H, p-CH3); 2.28, 1.76 (s,
12H each, o-CH3), 2.05, 1.87 (s, 6H each, o-CH3); 1.85 (m, 4H,
Rh(CH2)6Rh(µ-CH2CHdCHCH2) (5). The dirhodium four-carbon-
bridged complex Rh(CH2)6Rh(µ-CH2CHdCHCH2) (5) was formed
from the reaction of 1,3-butadiene with the rhodium(II) porphyrin
2
103
-OCH2CH2-), 1.58 (m, 4H, -CCH2CH2C-), 3.66 (d, 1H, J
Rh-CHO
1
103
) 1.3 Hz, Rh-CHO), -40.07 (d, 1H, J
) 44.5 Hz, Rh-H);
Rh-H
1
103
•
bimetalloradical complex Rh(CH2)6Rh• (1): 1H NMR (C6D6) δ 8.35,
13
13
13
13
C NMR (CD3C6D5) δ 193.65 (dd, 1 C, J
C ) 30.2 Hz, 1JH-
3
Rh-
C
8.27 (each a doublet of 4H, JH-H ) 4.8 Hz, pyrrole), 8.32 (s, 8H,
) 197.5 Hz, Rh-13CHO).
3
4
pyrrole); 7.71, 7.39 (dd, 2H each, JH-H ) 8.3 Hz, JH-H ) 2.2 Hz,
3
4
o-phenyl); 7.17, 7.01 (dd, 2H each, JH-H ) 8.3 Hz, JH-H ) 2.6 Hz,
m-phenyl), 7.26, 7.12 (s, 4H each, m-phenyl), 7.02, 7.00 (s, 2H each,
m-phenyl); 4.00 (t, 4H, JH-H ) 5.5 Hz, -OCH2-), 2.55 (s, 12H,
Acknowledgment. Support for this research by the Depart-
ment of Energy, Division of Chemical Sciences, Office of Basic
Energy Sciences, Grant DE-FG02-86ER 13615, is gratefully
acknowledged.
3
p-CH3), 2.39 (s, 6H, p-CH3); 1.94, 1.50 (s, 12H, o-CH3), 1.61, 1.57 (s,
6H, o-CH3); 1.91 (m, 4H, -OCH2CH2-), 1.70 (m, 4H, -CCH2CH2C-)
-2.24 (t, 2H, Rh(CH2)6Rh(µ-CH2CHdCHCH2)), -5.32 (dd, 4H,
Rh(CH2)6Rh(µ-CH2CHdCHCH2)).
JA962248N