Determination of Rh-C bond dissociation energy in methyl(porphyrinato)rhodium(III) complexes: A new application of photoacoustic calorimetry

Article abstract of DOI:10.1246/cl.2001.284

The photolysis of RhCH₃(ttp) and RhCH₃(tmp) (ttp = 5,10,15,20-tetratolylporphyrin, tmp = 5,10,15,20-tetramesitylporphyrin) was studied in methanol at ambient temperature: the quantum yields for photolysis were determined to be 0.51 and 0.54, respectively, and the Rh-C bond dissociation energies (227 and 219 kJ mol^-1, respectively) were measured by photoacoustic calorimetry, which were larger than those of Co-C bond.

Full text of DOI:10.1246/cl.2001.284

284
Chemistry Letters 2001
Determination of Rh–C Bond Dissociation Energy in Methyl(porphyrinato)rhodium(III)
Complexes: a New Application of Photoacoustic Calorimetry
Gang Li, Fei Fei Zhang†, Na Pi, Hui Lan Chen*, Shu Yi Zhang†, and Kin Shing Chan††
State Key Laboratory of Coordination Chemistry, Department of Chemistry, Nanjing University, Nanjing 210093, P. R. China
†State Key Laboratory of Modern Acoustics, Institute of Acoustics, Nanjing University, Nanjing 210093, P. R. China
††Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, P. R. China
(Received December 6, 2000; CL-001100)
dows of 100 ns, RhII(por) and methyl radical are the final prod-
ucts, and the enthalpy change due to the cleavage of such spe-
cific bond could be examined.11 From a plot of the photoa-
coustic signal amplitude in methanol under argon versus rela-
tive laser intensity, the ratio of the slope of the sample to that of
ferrocene yields α, the fraction of the photon energy converted
into prompt heat. The observed heat deposition is related to the
term α by eq 2,
The photolysis of RhCH3(ttp) and RhCH3(tmp) (ttp =
5,10,15,20-tetratolylporphyrin, tmp = 5,10,15,20-tetramesityl-
porphyrin) was studied in methanol at ambient temperature: the
quantum yields for photolysis were determined to be 0.51 and
0.54, respectively, and the Rh–C bond dissociation energies
(227 and 219 kJ mol–1, respectively) were measured by photoa-
coustic calorimetry, which were larger than those of Co–C
bond.
where Ehν is the photon energy (336 kJ mol–1 at 355 nm) and Φ
is the quantum yield for photodissociation. The quantum yields
for the photolysis of 1 and 2 are measured in methanol with
excess TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxy) under
argon.12 In our case the heat accompanying Rh–C bond homoly-
sis in methanol has been measured, and the overall enthalpy
change of the reaction may correspond to such bond dissociation
energy.13 The results for 1 and 2 are summarized in Table 1.
Rhodium porphyrins, which could act as models for reac-
tive species in catalytic and photochemical reactions, have pro-
vided a variety of new examples of unusual reactivity in
organometallic chemistry.1,2 It has been reported that the non-
bridged metal–metal dimers and sterically hindered monomers
present special chemical reactivities in C–H activation and
olefins insertion.1b–1d Furthermore, alkyl 1,2-rearrangements of
rhodium porphyrins have been studied due to their potential rel-
evance to the coenzyme B12 dependent enzymatic reactions.2
In order to gain insight into the nature of the mechanism and
the reactivity of organorhodium complexes related reactions, a
knowledge of both kinetics and thermodynamics of such
processes is highly desirable. However, there is few
rhodium–carbon bond dissociation energies (BDE) reported in
the literature.3–5 Since Rh–C bonds are expected to be stronger
than Co–C bonds,4 it is obviously difficult to make such meas-
urements by conventional thermal kinetic methods. During the
past decades, photoacoustic calorimetry (PAC) has been used to
determine the energetics and kinetics for photoinduced reac-
tions.6–8 Some metal–ligand bond dissociation energies were
measured by this approach.6d,8 In this paper we reported the
Rh–C bond dissociation energies of RhCH3(ttp) 1 and
RhCH3(tmp) 2 by using photoacoustic calorimetry.
The principles of the photoacoustics have been well estab-
lished. Our experimental setup is similar to those previously
described.9 When 1 and 2 are irradiated (λ = 355 nm) in
methanol under argon atmosphere at room temperature, Rh–C
bond undergoes cleavage to form RhII(por) and methyl radical
cage pair, and then the caged pair will diffuse into the solvent
rapidly (eq 1).10 Since the concentrations of the photolytic
products of 1 and 2 should be less than 10–6 M for the sample
concentrations of about 10–5 M, the subsequent diffusive
recombination and radical reactions could be neglected.
Therefore, when the PAC experiment is performed under these
conditions using a 1.5-MHz transducer, which has time win-
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
285
It is noted that the Rh–C BDE values of 1 and 2 deter-
mined by PAC are in agreement with the estimated bond energy
for Rh–CH3 (~242 kJ mol–1).14 As can be seen from the results
in Table 1, the Rh–C BDE value of 1 is comparable to that of 2.
Most likely, the similarity in equatorial porphyrin ligand should
not introduce a big difference in Rh–C BDE.
By comparison of Rh–C BDEs in 1 and 2 with those of
organo Rh(OEP) complexes,15 the Rh–C bond strength fol-
lows the order Rh(OEP)CH(Bu)OH < 1, 2 < Rh(OEP)CHO.
The reasonable explanation is that the dissociation of the
Rh–C bond involves reduction of organorhodium(III) com-
plex to Rh(II) product. Thus, more electron-withdrawing lig-
ands (CHO > CH3 > CH(Bu)OH) are expected to stabilize the
Rh(III) complexes, and hence, to increase energy for such
bond cleavage.16,17 Unfortunately, the existing literature val-
ues for Rh–C BDE are sparse and uncertain, so it is difficult
to make further comparisons.
It is well recognized that the bond dissociation energies for
Rh–C bond (167–250 kJ mol–1) are higher than those for Co–C
bond (84–125 kJ mol–1) in corresponding organometallic com-
plexes. The greater stability of Rh–C bond could be attributed
to improved bonding overlap of 4d orbitals of rhodium atom
and alkyl orbital. It is suggested that unfavorable steric interac-
tion is probably predominant to the Co–C bond because the
smaller radial distribution of the 3d valence orbitals compared
to the 4d requires a shorter M–C bond length to maximize the
bonding overlap.4 The light stability10 of our organorhodium
complexes is presumably dependent on the high Rh–C BDE,
the lifetime of photoactive state and the efficiency of the
recombination. However, these mechanisms require more
detail investigation.
Braslavsky, J. Phys. Chem., 99, 10246 (1995). c) I. Yruela,
M. S. Churio, T. Gensch, S. E. Braslavsky, and A. R.
Holzwarth, J. Phys. Chem., 98, 12789 (1994). d) M. S.
Churio, K. P. Angermund, and S. E. Braslavsky, J. Phys.
Chem., 98, 1776 (1994). e) P. J. Schulenberg, W. Gäntner,
and S. E. Braslavsky, J. Phys. Chem., 99, 9617 (1995).
S. T. Belt, J. C.Scaiano, and M. K. Whittlesey, J. Am.
Chem. Soc., 115, 1921 (1993).
A laser pulse from Q-switched Nd:YAG laser (Continuum
NP70) operated at 355 nm, 10 Hz, and 8-ns duration was
passed through the sample solution held in a 1-cm ther-
mostated flow cell. The acoustic wave induced in solution
was detected by a 1.5-MHz PZT piezoelectric transducer.
The signal was then pre-amplified by a HP-8847F and
recorded by a 300-MHz HP-54510B digital oscilloscope as
the average of 100 laser shots. The data were stored in a
PC for analysis. Ferrocene, a calorimetric reference, was
sublimed once before use. For each photoacoustic experi-
ment, it was dissolved in the same medium and matched in
OD at 355 nm to those of 1 and 2.
8
9
10 a) M. Hoshino, K. Yasufuku, K. Konishi, and M. Imamura,
Inorg. Chem., 23, 1982 (1984). b) S. Yamamoto and M.
Hoshino, Inorg. Chem., 23, 195 (1984). c) M. Hoshino, T.
Nagamori, H. Seki, T. Tase, T. Chihara, J. P. Lillis, and Y.
Wakatsuki, J. Phys. Chem. A, 103, 3672 (1999).
11 The signal frequency for sample is identical to that of the
calorimetric reference, ferrocene, which relaxes back to
ground state within 1ns. Thus, no heat releases on the
time-scale of about 10 ns to 10 µs.
12 a) Samples were photolyzed at 355 nm, in the presence of
TEMPO, to ca. 5% conversion, and the products were ana-
lyzed by UV–vis spectrometer. The quantum yield for
photolysis of 1 and 2 could be measured according to the
equation,
This work is supported by the National Natural Science
Foundation of China (No. 29823001, No. 200071017 and No.
19774031) and the Research Found for the Doctoral Program of
Higher Education.
References and Notes
where Φs and Φr are the quantum yields for the photolysis
of sample and actinometer, ∆As/Ea and ∆Ar/Ea are the ener-
gy normalized absorbance changes for sample and acti-
nometer, ε1s and ε2s are the extinction coefficients of the
sample and its photolytic product, and ε2r is the extinction
coefficient of the actinometer. Potassium ferrioxalate acti-
nometer was used as described in ref 12b. In the quantum
yield calculations, the corrections are made for absorption
of TEMPO and Rh(II) species. b) J. G. Calvert and J. N.
Pitts, Jr. “Photochemistry,” John Wiley & Sons, Inc., New
York (1966), p. 783.
1
a) S. L. Van Voorhees and B. B. Wayland, Organometallics,
6, 204 (1987). b) K. J. Del Rossi and B. B. Wayland, J.
Am. Chem. Soc., 107, 7941 (1985). c) K. J. Del Rossi, X.
–X. Zhang, and B. B. Wayland, J. Organomet. Chem., 504,
47 (1995). d) K. L. Reger, D. G. Garza, and L. Lebioda,
Organometallics, 11, 4285 (1992).
2
3
a) K. W. Mak, F. Xue, T. C. W. Mak, and K. S. Chan, J.
Chem. Soc., Dalton Trans., 1999, 3333. b) K. W. Mak and
K. S. Chan, J. Am. Chem. Soc., 120, 9686 (1998).
J. A. Martinho Simoes and J. L. Beauchamp, Chem. Rev.,
90, 629 (1990).
13 Since 1 and 2 are rigid complexes, the reaction volume
changes during their photolysis are so small that they could
be neglected, especially in organic solvent.
4
5
B. B. Wayland, Polyhedron, 7, 1545 (1988).
M. L. Mandich, L. F. Halle, and J. L. Beauchamp, J. Am.
Chem. Soc., 106, 4403 (1984).
14 B. B. Wayland, G. Poazmik, and M. Fryd, Organometallics,
11, 3534 (1992).
6
7
a) A. A. Deniz, K. S. Peters, and G. J. Snyder, Science,
286, 1119 (1999). b) K. S. Peters and G. J. Snyder,
Science, 241, 1053 (1988). c) K. S. Peters, T. Watson, and
T. Logan, J. Am. Chem. Soc., 114, 4276 (1992). d) L. B.
Luo, G. Li, H. L. Chen, S. W. Fu, and S. Y. Zhang, J.
Chem. Soc., Dalton Trans., 1998, 2103.
15 The Rh–C BDE for Rh(OEP)CHO is 242 kJ mol–1, and
that for Rh(OEP)CH(Bu)OH is 118 kJ mol–1. See ref 3 and
references herein.
16 S. Sakaki, B. Biswas, and M. Sugimoto, Organometallics,
17, 1278 (1998).
17 J. Halpern, Science, 227, 869 (1985).
a) S. E. Braslavsky and G. E. Heibel, Chem. Rev., 92, 1381
(1992). b) J. L. Habib-Jiwan, A. K. Chibisov, and S. E.