Equilibrium thermodynamic studies for the formation of 1:1 complexes of CO and ethene with a rhodium(II) porphyrin metallo-radical

Article abstract of DOI:10.1139/v01-052

Tetra(2,4,6 triisopropropyl phenyl)porphyrinatorhodium(II) ((TTiPP)Rh^II·1) is a persistent metal-centered radical with the odd electron in the rhodium(II) dz² orbital. (TTiPP)Rh^II· forms 1:1 complexes with CO and CH₂CH₂ where the porphyrin ligand steric properties inhibit further reactions of the one-electron activated substrates. ¹H NMR paramagnetic shifts at a series of temperatures are used in evaluating the thermodynamics for CO complex formation with 1 to form [(TTiPP)Rh^II(CO)]·2 (δH° = -5.5 ± 0.5 kcal mol^-1; δS° = -9 ± 1 cal K^-1 mol^-1). Related ¹H NMR studies show that the bonding of 1 with ethene is less favorable than CO.

Full text of DOI:10.1139/v01-052

854
Equilibrium thermodynamic studies for the
formation of 1:1 complexes of CO and ethene with
a rhodium(II) porphyrin metallo-radical
Leah Basickes, Andrew G. Bunn, and Bradford B. Wayland
Abstract: Tetra(2,4,6 triisopropropyl phenyl)porphyrinatorhodium(II) ((TTiPP)RhII·1) is a persistent metal-centered radi-
cal with the odd electron in the rhodium(II) dz2 orbital. (TTiPP)RhII· forms 1:1 complexes with CO and CH2CH2 where
1
the porphyrin ligand steric properties inhibit further reactions of the one-electron activated substrates. H NMR para-
magnetic shifts at a series of temperatures are used in evaluating the thermodynamics for CO complex formation with
1
1 to form [(TTiPP)RhII(CO)]·2 (∆H° = –5.5 ± 0.5 kcal mol–1; ∆S° = –9 ± 1 cal K–1 mol–1). Related H NMR studies
show that the bonding of 1 with ethene is less favorable than CO.
Key words: rhodium porphyrin, carbon monoxide, ethene, thermodynamics, complex formation, metal-centered radical.
Résumé : Le tétra(2,4,6-triisopropylphényl)porphyrinatorhodium(II) ((TTiPP)RhII·1) est un radical persistant centré sur
un métal comportant un électron célibataire dans l’orbitale dz2 du rhodium(II). Le (TTiPP)RhII· forme des complexes
1:1, avec le CO et le CH2CH2, dans lesquels les propriétés stériques du ligand porphyrine empêchent d’autres réactions
1
avec des substrats à un électron activés. On a utilisé les déplacements paramagnétiques en RMN du H à une série de
températures pour évaluer les facteurs thermodynamiques de la réaction entre CO et 1 pour conduire à la formation du
complexe [(TTiPP)RhII(CO)]·2 (∆H° = –5,5 ± 0,5 kcal mol–1; ∆S° = –9 ± 1 cal K–1 mol–1). D’autres études de RMN
1
du H montrent que la liaison entre 1 et l’éthène est moins favorable que celle avec le CO.
Mots clés : porphyrine-rhodium, monoxyde de carbone, éthène, thermodynamique, formation de complexe, radical cen-
tré sur un métal.
[Traduit par la Rédaction] 856
Introduction
Basickes et al.
strate site and provide persistent complexes containing one-
electron activated CO and ethene (2, 5).
Carbon monoxide and ethene are most often activated by
binding with Lewis acid metal centers that promote two-
electron reactions of nucleophiles at a substrate carbon site.
The binding of carbon monoxide and ethene with rhodium(II)
porphyrin metal-centered radicals results in complexes that
manifest an alternate pattern of reactivity associated with
one-electron activated substrates (1–6). Reactions of CO and
ethene with rhodium(II) porphyrins that yield substrate
bridged (Rh-C(O)-Rh (3, 4), Rh-CH2CH2-Rh (7, 8)) and
coupled species (Rh-C(O)-C(O)-Rh (1–3), Rh-CH2CH2-
CH2CH2-Rh (5, 6)) illustrate one-electron activated carbon
centers in CO and ethene ligands. Direct investigation of
complexes with one-electron activated substrates is usually
precluded because they are highly reactive transient interme-
diates. However, steric constraints can be built into the
porphyrin ligand which inhibit access to the reactive sub-
Tetra(2,4,6 triisopropyl phenyl)porphyrinatorhodium(II)
((TTiPP)RhII·) is an example of a metal-centered radical
where the structural features sterically preclude RhII—RhII
bonding and inhibit CO and ethene substrate reactions. This
article reports on measurements of equilibrium thermody-
namic values for complex formation of (TTiPP)RhII· with
carbon monoxide and ethene.
Results and discussion
(TTiPP)RhII·1 is a stable persistent metal-centered radical
with a (dxy)2(dxz,yz)4(dz2)1 electron configuration as shown by
the EPR spectral parameters in toluene glass (90 K) (gxx
=
1
gyy = 2.82, gzz = 1.85) (9). H NMR chemical shifts for
(TTiPP)RhII· in toluene-d8 solutions manifest an inverse de-
pendence on temperature (T = 190–330 K) that is character-
istic of Curie–Weiss paramagnetism (Fig. 1). There is no
evidence from 1H NMR, electronic spectra, or EPR for
dimerization of (TTiPP)RhII· through RhII—RhII bonding in
toluene solution (190–330 K) or frozen glass (160–90 K).
The steric constraints for the TTiPP porphyrin ligand pre-
clude dimerization through RhII—RhII bonding which occurs
with less sterically demanding porphyrin ligands such as
tetraphenyl porphyrin ([(TPP)Rh]2) (10).
Received September 15, 2000. Published on the NRC
Research Press Web site at http://canjchem.nrc.ca on June 30,
2001.
This paper is dedicated to Professor Brian James on the
occasion of his 65th birthday.
L. Basickes, A.G. Bunn, and B.B. Wayland.1 Department of
Chemistry, University of Pennsylvania, Philadelphia, PA
19104–6323, U.S.A.
The EPR spectrum in toluene glass (90 K) for (TTiPP)RhII·
in contact with 13CO demonstrates the formation of a mono
CO complex ([(TTiPP)RhII(CO)]·) through splitting of each
1Corresponding author (telephone: (215) 898-8633; fax: (215)
573-6743; e-mail: wayland@mail.sas.upenn.edu).
Can. J. Chem. 79: 854–856 (2001)
DOI: 10.1139/cjc-79-5/6-854
© 2001 NRC Canada

Basickes et al.
855
Fig. 2. van’t Hoff plot for reaction of (TTiPP)RhII· with CO in
1
Fig. 1. Temperature dependence for the pyrrole H NMR para-
magnetic shift (δ (ppm)) for toluene-d8 solutions of (TTiPP)RhII·
(᭿), (TTiPP)RhII· (᭹) in contact with CH2CH2 (0.25 atm (1 atm =
101.325 kPa)), and (TTiPP)RhII· (᭡) in contact with CO (0.855
atm (1 atm = 101.325 kPa)).
toluene-d8.
observed pyrrole shifts in this temperature range correspond
to the mol fraction averaged positions for the (TTiPP)RhII·
and [(TTiPP)RhII(CO)]· species in limiting fast exchange.
The pyrrole chemical shifts determined for (TTiPP)RhII·1
(δ1= –1.86 + 3334 T–1) and [(TTiPP)RhII(CO)]·2 (δ2= –2.72 +
2376 T–1) were used in determining the fraction of mono-
carbonyl complex from the observed mol fraction averaged
chemical shifts (δobs = N(1)δ(1) + N(2)δ(2)), N(2) = [2]/[1] +
[2]). The equilibrium constant expression for rxn. [1] (K1 =
[[(TTiPP)RhII(CO)]·]/[(TTiPP)RhII·][CO]) is evaluated using
the ratio of mol fractions N(2):N(1) and the molar concen-
tration of CO at each temperature (T). The plot of ln K1 vs.
T–1 (Fig. 2) gives an enthalpy change (∆H1°) of –5.5 ±
0.5 kcal mol–1 and entropy change (∆S1°) of –9 ± 1 cal K–1
mol–1 for the reaction of (TTiPP)RhII· with CO to form
[(TTiPP)RhII(CO)]· (eq. [1]). The enthalpy change for com-
plex formation of 1 with CO (∆H1° = –5.5 ± 0.5 kcal mol–1)
is very small even when compared to the reaction of CO
with a related RhII—RhII bonded dimer [(OEP)Rh]2 to form
the mono CO adduct [(OEP)Rh]2CO (∆H° = –12 kcal mol–1)
(3).
of the three g value transitions into a doublet by 13C (9). The
observed pyrrole 1H NMR paramagnetic shifts for toluene-d8
solutions of (TTiPP)RhII·1 (1.0 × 10–3 M) in contact with
carbon monoxide (PCO = 0.855 atm (1 atm = 101.325 kPa))
are plotted vs. the inverse of temperature (T = 180–300 K)
in Fig. 1. Sections of linearity are observed in both the
higher and lower temperature regimes. The observed pyrrole
chemical shifts in the higher temperature linear region coin-
cide with the values for (TTiPP)RhII· (δ1 = –1.86 + 3334 T–1)
and the lower temperature linear portion is assigned to the
limiting shift values for
a mono carbonyl complex
Reaction of (TTiPP)RhII· with ethene produces a 1:1 com-
plex [(TTiPP)RhII(CH2CH2)]·3 that is kinetically meta-stable
relative to an ethene coupling product (Rh-(CH2)4-Rh).
([(TTiPP)RhII(CO)]·2 (δ2 = –2.72 + 2376T–1)). The ratio of
the slope of the pyrrole hydrogen paramagnetic shifts vs. T–1
plots for [(TTiPP)RhII(CO)]· to that for (TTiPP)RhII· equals
0.71 and this value is proportional to the ratio of the elec-
tron-pyrrole hydrogen coupling constants AH (2)/AH (1) and
related pyrrole hydrogen spin densities (11). This ratio of the
porphyrin pyrrole electron-hydrogen coupling constants (0.71)
is similar to the ratio of 0.65 for the total rhodium spin den-
sity of [(TTiPP)RhII(CO)]· (0.62) to that of the (TTiPP)RhII·
(0.95) which were estimated from EPR studies (9). In the
temperature regime between 270 and 330 K, substantial
amounts of both (TTiPP)RhII· and [(TTiPP)RhII(CO)]· are
present in equilibrium with CO (eq. [1]). The
[2]
[(TTiPP)RhII]· + CH2CH2
º [(TTiPP)RhII(CH2CH2)]·
The gzz transition in the EPR spectrum for the 13C2H4 deriva-
tive of 3 (90 K) occurs as a triplet of doublets arising from
coupling of two equivalent 13C nuclei on each 103Rh transi-
tion which demonstrates that
3 is a 1:1 complex,
[(TTiPP)RhII(CH2CH2)]· (5).
1H NMR chemical shifts for the pyrrole hydrogens of
(TTiPP)RhII· in contact with ethene (0.25 atm (1 atm =
101.325 kPa)) in toluene-d8 over a wide range of tempera-
[1]
[(TTiPP)RhII]· + CO º [(TTiPP)RhII(CO)]·
© 2001 NRC Canada

856
Can. J. Chem. Vol. 79, 2001
tures is shown in Fig. 1. At temperatures above 220 K the
observed pyrrole H NMR shifts are indistinguishable from
when the gas was added into the tube. ([CO] (12) = [a +
b(T2)][P][T1/T2] (a and b are solvent dependent in toluene,
a = 4.405 × 10–3 and b = 1.085 × 10–5)); ([CO] (303 K) in
toluene = 6.5 × 10–3 M), [ethene] (13) = –0.4906 +
(0.1850T2)P. ([ethene] (303 K) in toluene = 3.0 × 10–2 M).
1
the values for (TTiPP)RhII· and only below 200 K is there a
clear indication of ethene complex formation with 1 to form
3. Only a small temperature range is available for the study
of 3 forming in toluene solutions and the regime with the
limiting shift values and thus complete conversion to 3 was
not attained. Qualitatively, the thermodynamics of ethene
coordination with 1 are substantially less favorable than CO
complexation.
Acknowledgements
This research was supported by grants from the Depart-
ment of Energy, Division of Chemical Sciences, Office of
Basic Energy Sciences, Grant DE-FG02–86ER 13615, and
the National Science Foundation.
Experimental
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(Aldrich) were used without further purification. Benzene
and toluene solvents used in the EPR and NMR studies were
degassed, refluxed over sodium benzophenone, and then de-
gassed again by freeze–pump–thaw cycles.
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NMR Studies
Proton NMR spectra were recorded using a Bruker AF500
instrument equipped with a variable temperature FTS Re-
frigerator unit. NMR samples used toluene-d8 as the solvent
in a sealed NMR tube and low temperatures were obtained
by utilizing the boil-off from liquid nitrogen. Temperature
calibration was obtained by using methanol or ethylene gly-
col as an external standard.
Reaction of (TTiPP)RhII· with CO–CH2CH2
In a typical experiment, (TTiPP)RhII· was generated by
photolysis of a benzene solution of (TTiPP)Rh-CH3 (1 ×
10–3 M) in a vacuum adapted NMR tube. Samples of
(TTiPP)RhII· in vacuum adapted NMR tubes were then pres-
surized with CO (0.86 atm (1 atm = 101.325 kPa)) or ethene
(0.25 atm (1 atm = 101.325 kPa)) and sealed. The solubility
of CO and ethene in toluene was determined as a funtion of
temperature. When measuring the concentration of disolved
gas in solution at a temperature T2, P is the pressure in atm
(1 atm = 101.325 kPa) at the laboratory temperature T1 in K
© 2001 NRC Canada