Solution thermochemical study of tertiary phosphine ligand substitution reactions in the Rh(acac)(CO)(PR3) system

Article abstract of DOI:10.1021/om970766g

The enthalpies of reaction of Rh(acac)(CO)₂ (1) with a series monodentate tertiary phosphine ligands, leading to the formation of Rh(acac)(CO)(PR₃) complexes, have been measured by anaerobic solution calorimetry in CH₂Cl₂ at 30.0°C. These reactions are rapid and quantitative. The measured reaction enthalpies span a range of 12 kcal/mol. The relative stability scale established is as follows: PPh₂(o-Tol) < P(p-CF₃C₆H₄)₃ ≈ P(p-ClC₆H₄)₃ < P(p-FC₆H₄)₃ < P(NC₄H₄)₃ < P(NC₄H₄)₂(C₆H₅) < P(m-CH₃OC₆H₄)₃ < P(NC₄H₄)(C₆H₅)₂ < P(OPh)₃ ≈ PPh₂(p-Tol) ≈ P(m-Tol)₃ < P(p-CH₃C₆H₄)₃ < PPh₃ < P(p-CH₃OC₆H₄)₃ < PCy₃ < PPh₂Me < PⁱPr₃ < PPhMe₂. The relative importance of the phosphine stereoelectronic ligand parameters are examined in terms of the presented quantitative thermochemical information. Comparisons with enthalpy data in related organometallic systems are also presented.

Full text of DOI:10.1021/om970766g

534
Organometallics 1998, 17, 534-539
Solu tion Th er m och em ica l Stu d y of Ter tia r y P h osp h in e
Liga n d Su bstitu tion Rea ction s in th e Rh (a ca c)(CO)(P R3)
System
Scafford Serron, J inkun Huang, and Steven P. Nolan*,†
Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148
Received August 28, 1997
The enthalpies of reaction of Rh(acac)(CO)2 (1) with a series monodentate tertiary
phosphine ligands, leading to the formation of Rh(acac)(CO)(PR3) complexes, have been
measured by anaerobic solution calorimetry in CH2Cl2 at 30.0 °C. These reactions are rapid
and quantitative. The measured reaction enthalpies span a range of 12 kcal/mol. The
relative stability scale established is as follows: PPh2(o-Tol) < P(p-CF3C6H4)3 ≈ P(p-ClC6H4)3
< P(p-FC6H4)3 < P(NC4H4)3 < P(NC4H4)2(C6H5) < P(m-CH3OC6H4)3 < P(NC4H4)(C6H5)2 <
P(OPh)3 ≈ PPh2(p-Tol) ≈ P(m-Tol)3 < P(p-CH3C6H4)3 < PPh3 < P(p-CH3OC6H4)3 < PCy3
< PPh2Me < PiPr3 < PPhMe2. The relative importance of the phosphine stereoelectronic
ligand parameters are examined in terms of the presented quantitative thermochemical
information. Comparisons with enthalpy data in related organometallic systems are also
presented.
In tr od u ction
3.6 Furthermore the enthalpy behind the observed
1/2[RhL2Cl]2 + L′ f 2RhL2ClL′
1/2[RhL2Cl]2 + H2 f 2RhL2Cl(H)2
L ) PiPr3; L′ ) N2, C2H4, PhCCPh
(2)
(3)
The utilization of rhodium-phosphine complexes in
homogeneous catalysis is widespread1 and encompasses
industrially important processes such as hydroformy-
lation.2 Phosphine ligands have shown great utility in
organometallic chemistry and catalysis as a way to fine
tune metal reactivity and selectivity.3,4 Despite the
importance of rhodium-phosphine complexes, very little
is known about the quantitative thermodynamic stabil-
ity of these systems. Pioneering work by Blake and co-
workers5 represents a notable exception. These studies
dealt with the enthalpic driving force behind oxidative
addition to square planar rhodium(I) systems eq 1.
decarbonylation of aldehydes by a coordinatively un-
saturated rhodium system was experimentally deter-
mined.7
1/2[RhL2Cl]2 + RC(O)H f 2RhL2Cl(CO) + RH (4)
L ) PiPr3; R ) alkyl
RhX(CO)(PPh3)2 + I2
8 RhX(I)2(CO)(PPh3)2 (1)
C6H6
We have been involved in mapping out the thermo-
chemical surface of organometallic systems bearing
phosphine and phosphite ligands. Researchers have
been involved in recent years in describing metal-
ligand systems in terms of stereoelectronic contribu-
tions, using a variety of methods.8-10 We have been
interested in clarifying the exact partitioning of the
steric and electronic ligand energetic contributions
present in tertiary phosphine-based systems by means
of solution calorimetry.11-14 We have achieved this in
X ) halide, NCS, N3
More recently, Goldman and co-workers determined
the bond-dissociation enthalpy for Rh-N2, Rh-(H)2,
Rh-olefin, and Rh-acetylene complexes, eq 2 and
† E-mail: spncm@uno.edu.
(1) (a) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry, 2nd
ed.; University Science: Mill Valley, CA 1987. (b) Parshall, G. W.; Ittel,
S. D. Homogeneous Catalysis Wiley Interscience: New York, 1992.
(2) (a) Falbe, J . Carbon Monoxide in Organic Synthesis; Springer-
Verlag: Berlin, 1980. (b) van Rooy, A.; de Bruijn, J . N. H.; Roobek, K.
F.; Kamer, P. C. J .; Van Leeuwen, P. W. N. M. J . Organomet. Chem.
1996, 507, 69-73 and references cited.
(3) (a) Hughes, R. P. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press:
Oxford, 1982; Chapter 35. (b) Sharp, P. R. In Comprehensive Organo-
metallic Chemistry II; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.;
Pergamon Press: Oxford, 1995; Vol. 8, Chapter 2.
(6) Wang, K.; Goldman, A. S.; Li, C.; Nolan, S. P. Organometallics,
1995, 14, 4010-4013.
(7) Wang, K.; Rosini, G. P.; Nolan, S. P.; Goldman, A. S. J . Am.
Chem. Soc. 1995, 117, 5082-5088.
(8) (a) Rahman, M. M.; Liu, H.-Y.; Eriks, K.; Prock, A.; Giering, W.
P. Organometallics, 1989, 8, 1-7. (b) Liu, H.-Y.; Eriks, K.; Prock, A.;
Giering, W. P. Inorg. Chem. 1989, 28, 1759-1763. (c) Poe¨, A. J . Pure
Appl. Chem. 1988, 60, 1209-1216 and references cited. (d) Gao, Y.-C;
Shi, Q.-Z; Kersher, D. L.; Basolo, F. Inorg. Chem. 1988, 27, 188-191.
(e) Baker, R. T.; Calabrese, J . C.; Krusic, P. J .; Therien, M. J .; Trogler,
W. C. J . Am. Chem. Soc. 1988, 110, 8392-8412. (f) Rahman, M. M.;
Liu, H.-Y.; Prock, A.; Giering, W. P. Organometallics, 1987, 6, 650-
658.
(4) (a) Tolman, C. A. Chem. Rev. 1977, 77, 313-348. (b) Homoge-
neous Catalysis with Metal Phosphine Complexes; Pignolet, L. H., Ed.;
Plenum: New York, 1983.
(5) Mondal, J . U.; Blake, D. M. Coord. Chem. Rev. 1982, 47, 205-
238 and references cited.
S0276-7333(97)00766-8 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 01/22/1998

Tertiary Phosphine Ligand Substitution Reactions
Organometallics, Vol. 17, No. 4, 1998 535
part for the classical [RhCl(CO)2]2 complex, eq 5.13 The
calorimetric experiments. NMR spectra were recorded using
a Varian Gemini 300 MHz spectrometer. Calorimetric mea-
surements were performed using a Calvet calorimeter (Set-
aram C-80), which was periodically calibrated using the TRIS
reaction18 or the enthalpy of solution of KCl in water.19 The
experimental enthalpies for these two standard reactions
compared very closely to the literature values. This calorim-
eter has been previously described,20 and typical procedures
are described below. Experimental enthalpy data are reported
with 95% confidence limits.
[RhCl(CO)2]2 + 4ER3 f 2RhCl(CO)(ER3)2 + 2CO
(5)
ER3 ) phosphine, phosphite, arsine
[RhCl(CO)2]2 complex can be easily converted to Rh-
(acac)(CO)2 (1), eq 6.15 In the present paper, our
In fr a r ed Titr a tion s. Prior to every set of calorimetric
experiments involving a new ligand, an accurately weighed
amount ((0.1 mg) of the organorhodium complex was placed
in a test tube fitted with a septum and CH2Cl2 was subse-
quently added. The solution was titrated with a solution of
the ligand of interest by injecting the latter in aliquots through
the septum with a microsyringe, followed by vigorous shaking.
The reactions were monitored by infrared spectroscopy, and
the reactions were found to be rapid, clean, and quantitative
under experimental calorimetric (temperature and concentra-
tion) conditions necessary for accurate and meaningful calo-
rimetric results. These conditions were satisfied for all
organorhodium reactions investigated.
1H NMR Titr a tion s. Prior to every set of calorimetric
experiments involving a new ligand, an accurately weighed
amount ((0.1 mg) of the organometallic complex was placed
in a Wilmad screw-capped NMR tube fitted with a septum and
CD2Cl2 was subsequently added. The solution was titrated
with a solution of the ligand of interest by injecting the latter
in aliquots through the septum with a microsyringe, followed
by vigorous shaking. The reactions were monitored by 31P and
1H NMR spectroscopy, and the reactions were found to be
rapid, clean, and quantitative under the experimental calori-
metric conditions.
Solu tion Ca lor im etr y. Ca lor im etr ic Mea su r em en t of
Rea ction betw een Rh (a ca c)(CO)2 (1) a n d P (NC4H4)3. The
mixing vessels of the Setaram C-80 were cleaned, dried in an
oven maintained at 120 °C, and then taken into the glovebox.
A 25 mg sample of Rh(acac)(CO)2 was accurately weighed into
the lower vessel, which was closed and sealed with 1.5 mL of
mercury. Four milliliters of a stock solution of P(NC4H4)3
(0.113 g of P(NC4H4)3 in 20 mL of CH2Cl2) was added, and the
remainder of the cell was assembled, removed from the
glovebox, and inserted in the calorimeter. The reference vessel
was loaded in an identical fashion, with the exception that no
organorhodium complex was added to the lower vessel. After
the calorimeter had reached thermal equilibrium at 30.0 °C
(about 2 h), the calorimeter was inverted, thereby allowing
the reactants to mix. After the reaction had reached comple-
tion and the calorimeter had once again reached thermal
equilibrium (ca. 2 h), the vessels were removed from the
calorimeter and an infrared spectrum was immediately re-
corded. Conversion to Rh(acac)(CO)[P(NC4H4]3 was found to
be quantitative under these reaction conditions. Control
reactions with Hg and no phosphine showed no reaction. The
enthalpy of reaction, 4.3 ( 0.1 kcal/mol, represents the average
of five individual calorimetric determinations. The final
enthalpy value listed in Table 1 (-3.6 ( 0.1 kcal/mol)
represents the enthalpy of ligand substitution with all species
in solution. This methodology represents a typical procedure
involving all organorhodium reactions investigated in the
present study.
[RhCl(CO)2]2 + 2acacH f 2Rh(acac)(CO)2 + 2HCl
(6)
solution calorimetric work is extended to the related
organorhodium-phosphine system Rh(acac)(CO)(PR3)
(acac ) acetylacetonate; PR3 ) tertiary phosphine).
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All manipulations involving
organorhodiun complexes were performed under inert atmo-
spheres of argon or nitrogen using standard high-vacuum or
Schlenk-tube techniques or in a Vacuum/Atmospheres glove-
box containing less than 1 ppm of oxygen and water. Ligands
were purchased from Strem Chemicals or Aldrich and used
as received or synthesized by the literature method.16 Solvents
were dried and distilled under dinitrogen before use, employing
standard drying agents.17 Only materials of high purity as
indicated by IR and NMR spectroscopies were used in the
(9) (a) Huynh, M. H. V.; Bessel, C. A.; Takeuchi, K. J . Abstracts of
Papers, 208th Meeting of the American Chemical Society, American
Chemical Society: Washington, DC, 1994; Abstract INOR 165. (b)
Perez, W. J .; Bessel, C. A.; See, R. F.; Lake, C. H.; Churchill, M. R.;
Takeuchi, K. J . Ibid., INOR 166. (c) Ching, S. ; Shriver, D. F. J . Am.
Chem. Soc. 1989, 111, 3238-3243. (d) Lee, K.-W.; Brown, T. L. Inorg.
Chem. 1987, 26, 1852-1856. (e) Brown, T. L.; Lee, K. J . Coord. Chem.
Rev. 1993, 128, 89-116.
(10) Lorsbach, B. A.; Prock, A.; Giering, W. P. Organometallics 1995,
14, 1694-1699 and references cited.
(11) For organoruthenium systems, see: (a) Nolan, S. P.; Martin,
K. L.; Stevens, E. D.; Fagan, P. J . Organometallics 1992, 11, 3947-
3953. (b) Luo, L.; Fagan, P. J .; Nolan, S. P. Organometallics 1993, 12,
4305-4311. (c) Luo, L.; Zhu, N.; Zhu, N.-J .; Stevens, E. D.; Nolan, S.
P.; Fagan, P. J . Organometallics 1994, 13, 669-675. (d) Li, C.; Cucullu,
M. E.; McIntyre, R. A.; Stevens, E. D.; Nolan, S. P. Organometallics
1994, 13, 3621-3627. (e) Luo, L.; Nolan, S. P. Organometallics 1994,
13, 4781-4786. (f) Cucullu, M. E.; Luo, L.; Nolan, S. P.; Fagan, P. J .;
J ones, N. L.; Calabrese, J . C. Organometallics 1995, 14, 289-296. (g)
Luo, L.; Li, C.; Cucullu, M. E.; Nolan, S. P. Organometallics 1995, 14,
1333-1338. (h) Serron, S. A.; Nolan, S. P. Organometallics 1995, 14,
4611-4616. (i) Serron, S. A.; Luo, L.; Li, C.; Cucullu, M. E.; Nolan, S.
P. Organometallics 1995, 14, 5290-5297. (j) Li, C.; Serron, S. A.; Nolan,
S. P.; Petersen, J . L.Organometallics 1996, 15, 4020-4029. (k) Li, C.;
Ogasawa, M.; Nolan, S. P.; Caulton, K. G. Organometallics 1996, 15,
4900-4903.
(12) For organoiron system see: (a) Luo, L ; Nolan, S. P. Organo-
metallics 1992, 11, 3483-3486. (b) Luo, L. ; Nolan, S. P. Inorg. Chem.
1993, 32, 2410-2415. (c) Li, C.; Nolan, S. P. Organometallics 1995,
14, 1327-1332. (d) Li, C.; Stevens, E. D.; Nolan, S. P. Organometallics
1995, 14, 3791-3797. (e) Serron, S. A.; Nolan, S. P. Inorg. Chim. Acta
1996, 252, 107-113.
(13) For organorhodium systems, see: (a) Serron, S. A.; Nolan, S.
P.; Moloy, K. G. Organometallics 1996, 15, 4301-4306. (b) Huang, A.;
Marcone, J . E.; Mason, K. L.; Marshall, W. J .; Moloy, K. G.; Serron, S.
A.; Nolan, S. P. Organometallics 1997, 16, 3377-3380.
(14) For leading references in this area, see: (a) Nolan, S. P. Bonding
Energetics of Organometallic Compounds In Encyclopedia of Inorganic
Chemistry; J . Wiley and Sons: New York, 1994. (b) Hoff, C. D. Prog.
Inorg. Chem. 1992, 40, 503-561. (c) Martinho Simo˜es, J . A.; Beau-
champ, J . L. Chem. Rev. 1990, 90, 629-688. (d) Bonding Energetics
In Organometallic Compounds; Marks, T. J ., Ed.; ACS Symposium
Series 428; American Chemical Society: Washington, DC, 1990. (e)
Metal-Ligand Bonding Energetics in Organotransition Metal Com-
pounds. Polyhedron 1988, 7 (Symposium-in-Print).
En th a lp y of Solu tion of Rh (a ca c)(CO)2 (1). In order to
consider all species in solution, the enthalpy of solution of 1
had to be directly measured. This was performed by using a
(15) Trzeciak, A. M.; Ziolkowski, J . J . Inorg. Chim. Acta 1985, 96,
15-20.
(18) Ojelund, G.; Wadso¨, I. Acta Chem. Scand. 1968, 22, 1691-1699.
(19) Kilday, M. V. J . Res. Natl. Bur. Stand. (U.S.) 1980, 85, 467-
481.
(20) (a) Nolan, S. P.; Hoff, C. D.; Landrum, J . T. J . Organomet.
Chem. 1985, 282, 357-362. (b) Nolan, S. P.; Lopez de la Vega, R.; Hoff,
C. D. Inorg. Chem. 1986, 25, 4446-4448.
(16) Moloy, K. G.; Petersen, J . L. J . Am. Chem. Soc. 1995, 117,
7696-7710.
(17) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: New York, New York, 1988.

536 Organometallics, Vol. 17, No. 4, 1998
Serron et al.
Ta ble 1. En th a lp ies of Liga n d Su bstitu tion (k ca l/
m ol) in th e Rea ction
Calcd for C22H21O3NPRh: C, 54.88; H, 4.40; N, 2.91. Found:
C, 54.55; H, 4.04; N, 2.73.
Rh (a ca c)(CO)(P (p-CF 3P h )3). Yield: 227 mg (85%). 1H
NMR (CDCl3, δ, mult): 1.61 (s, 3H, -CH3), 2.13 (s, 3H, -CH3),
5.49 (s, 1H, -CH), 7.69-7.82 (m, 12H -Ph). IR (νCO, CH2-
Cl2): 1987 cm-1. Anal. Calcd for C27H19O3F9PRh: C, 46.55;
H, 2.75. Found: C, 45.95; H, 2.49.
CH2Cl2
Rh(CO)2(acac)(soln) + PR3(soln)
8
30 °C
Rh(CO)(acac)(PR3)(soln) + CO
Rh (a ca c)(CO)(P (p-F P h )3). Yield: 169 mg (80%). 1H
NMR (CDCl3, δ, mult): 1.62 (s, 3H, -CH3), 2.08 (s, 3H, -CH3),
5.44 (s, 1H, -CH), 7.06-7.64, (m, 12H -Ph). IR (νCO, CH2-
Cl2): 1981 cm-1. Anal. Calcd for C24H19O3F3PRh: C, 52.75;
H, 3.51. Found: C, 52.34; H, 3.38.
νCO
a
L
complex
(cm-1
) -∆H(rxn)
PPh2(o-Tol)
P(p-CF3C6H5)3
P(p-ClC6H5)3
P(p-FC6H5)3
P(C4H4N)3
Rh(CO)(acac)[PPh2(o-Tol)]
Rh(CO)(acac)[P(p-CF3C6H5)3]
Rh(CO)(acac)[P(p-ClC6H5)3]
Rh(CO)(acac)[P(p-FC6H5)3]
Rh(CO)(acac)[P(NC4H4)3]
1976
1986
1982
1980
2012
2.4(0.2)
3.2(0.1)
3.2(0.3)
3.5(0.1)
3.6(0.1)
3.8(0.3)
4.0(0.2)
4.1(0.3)
4.2(0.3)
4.2(0.1)
4.2(0.3)
4.7(0.1)
4.8(0.3)
6.2(0.2)
9.6(0.1)
Rh (a ca c)(CO)(P P h Me2). Yield: 110 mg (77%). 1H NMR
(CDCl3, δ, mult): 1.76 (s, 3H, -CH3), 1.80 (s, 3H, -CH3), 1.86
(s, 3H, -CH3), 2.06 (s, 3H, -CH3), 4.45 (s, 1H, -CH), 7.40-
7.82, (m, 5H -Ph). 31P NMR (THF-d8, δ, J P-H) : 21.9 (d, 167.
IR (νCO, CH2Cl2): 1971 cm-1. Anal. Calcd for C14H18O3PRh:
C, 45.67; H, 4.93. Found: C, 45.57; H, 4.87.
P(C4H4N)2(C6H5) Rh(CO)(acac)[P(NC4H4)2(C6H5)] 2003
P(m-CH3OPh)3 Rh(CO)(acac)[P(m-CH3OPh)3] 1978
P(C4H4N)(C6H5)2 Rh(CO)(acac)[P(NC4H4)(C6H5)2] 1990
P(OPh)3
Rh(CO)(acac)[P(OPh)3]
Rh(CO)(acac)[PPh2(p-Tol)]
Rh(CO)(acac)[P(m-Tol)3]
Rh(CO)(acac)[P(p-CH3C6H5)3]
Rh(CO)(acac)[PPh3]
2008
1977
1976
1974
1978
PPh2(p-Tol)
P(m-Tol)3
P(p-CH3C6H5)3
PPh3
P(p-CH3OC6H5)3 Rh(CO)(acac)[P(p-CH3OC6H5)3] 1973
PCy3
Rh (a ca c)(CO)(P P h 2(o-Tol)). Yield: 158 mg (86%). 1H
NMR (CDCl3, δ, mult): 1.61 (s, 3H, -CH3) 2.07 (s, 3H,
-CH3) 5.47 (s, 1H, -CH) 2.46 (s, 3H, -CH3), 6.85-7.80,
Rh(CO)(acac)[PCy3]
Rh(CO)(acac)[PPh2Me]
Rh(CO)(acac)[PiPr3]
Rh(CO)(acac)[PPhMe2]
1959
(m, 10H, -Ph). IR (νCO, CH2Cl2): 1976 cm-1
. Anal. Calcd
PPh2Me
1974 10.8(0.2)
1960 11.2(0.1)
1970 14.2(0.1)
PiPr3
for C25H24O3PRh: C, 59.30; H, 4.78. Found: C, 59.14; H, 4.49.
Rh (a ca c)(CO)(P P h 2(p-Tol)). Yield: 179 mg (86%). 1H
NMR (CD2Cl2, δ, mult): 1.63 (s, 3H, -CH3), 2.06 (s, 3H, -CH3),
5.45 (s, 1H, -CH), 2.38 (s, 3H, -CH), 2.38, s, 3H, -CH3), 7.21-
PPhMe2
a
Enthalpy values are reported with 95% confidence limits.
7.66, (m, 10H, -Ph). IR (νCO, CH2Cl2): 1977 cm-1
. Anal.
similar procedure as the one described above, with the excep-
tion that no ligand was added to the reaction cell. The
enthalpy of solution, 7.9 ( 0.1 kcal/mol, represents the average
of five individual determinations.
Calcd for C25H24O3PRh: C, 59.30; H, 4.78. Found: C, 58.84;
H, 4.69.
Rh (a ca c)(CO)(P (m -Tol)3). Yield: 185 mg (86%). 1H NMR
(CD2Cl2, δ, mult): 1.65 (s, 3H, -CH3), 2.07 (s, 3H, -CH3), 5.46
(s, 1H, -CH), 2.34 (s, 3H, -CH3), 7.27-7.56 (m, 12H, -Ph).
IR (νCO, CH2Cl2): 1976 cm-1. Anal. Calcd for C27H28O3PRh:
C, 60.68; H, 5.28. Found: C,60.60; H, 5.28.
Syn th esis. The compound Rh(acac)(CO)2 (1) was synthe-
sized according to literature procedures.21 Other organor-
hodium complexes, Rh(acac)(CO)[P(p-ClC6H5)3], Rh(acac)(CO)-
[P(p-CH3C6H5)3], Rh(acac)(CO)[P(OPh)3], Rh(acac)(CO)(PPh3),
Rh(acac)(CO)([P(p-CH3OC6H5)3], Rh(acac)(CO)(PPh2Me), and
Rh(acac)(CO)(PCy3) have been previously reported.22,23 In all
other cases, an analogous synthetic procedure was used
leading to the isolation of following organorhodium complexes.
Rh (a ca c)(CO)(P (NC4H4)3). In the glovebox, 100 mg (0.39
mmol) of Rh(CO)2(acac) and 20 mL of CH2Cl2 were charged
into a 50 mL flask. To this solution, 88.8 mg (0.39 mmol) of
P(NC4H4)3 was added. The reaction mixture was stirred for
20 min, during which time liberation of carbon monoxide was
observed. The solution was filtered and evacuated to dryness.
The resulting red-brown product was washed with hexane and
dried thoroughly in vacuum. Yield: 155 mg (86%). 1H NMR
(CD2Cl2, δ, mult) 1.86 (s, 3H -CH3), 2.12 (s, 3H -CH3), 5.60
(s, 1H, -CH), 6.38 (s, 6H, pyrrole), 6.96 (s, 6H, pyrrole). IR
(νCO, CH2Cl2): 2012 cm-1. Anal. Calcd for C18H19O3N3PRh:
C, 47.06; H, 4.17; N, 9.15. Found: C, 47.00; H, 3.88; N, 8.93.
Rh (a ca c)(CO)(P (NC4H4)2P h ). Yield: 155 mg (85%). 1H
NMR (CD2Cl2, δ, mult): 1.75 (s, 3H, -CH3; 2.10 (s, 3H, -CH3;
5.55 (s, 1H, -CH), 6.37 (s, 4H, pyrrole) 7.12 (s, 4H pyrrole),
7.14-7.55 (m, 5H -Ph). IR (νCO, CH2Cl2): 2002 cm-1. Anal.
Calcd for C20H20O3N2PRh; C, 51.06; H, 4.29; N, 5.96. Found:
C, 50.88; H, 4.29; N, 5.48.
R h (a ca c)(CO)(P (m -CH3OP h )3). Yield: 155 mg (86%).
1H NMR (CD2Cl2, δ, mult): 1.65 (s, 3H, -CH3), 2.06 (s, 3H,
-CH3), 5.46 (s, 1H, -CH, 3.73 (s, 3H, -OCH3), 6.96-7.32
(m, 12H, -Ph). IR (νCO, CH2Cl2): 1978 cm-1
. Anal. Calcd
for C27H28O6PRh: C, 55.68; H, 4.85. Found: C, 55.67; H, 4.80.
Rh (a ca c)(CO)(P iP r 3). Yield: 130 mg (86%). 1H NMR
(CD2Cl2, δ, mult): 1.85 (s, 3H, -CH3), 2.03 (s, 3H, -CH3), 5.46
(s, 1H, -CH), 1.31 (s, 6H, -CH3), 2.37 (s, 1H, -CH). IR (νCO
,
CH2Cl2): 1961 cm-1. Anal. Calcd for C15H28O3PRh: C, 46.16;
H, 7.23. Found: C, 46.62; H, 7.02.
Resu lts a n d Discu ssion
The use of Rh(acac)(CO)2(1) as a versatile synthetic
precursor and precatalyst for hydroformylation has been
reported.3 Direct entry into the thermochemistry of Rh-
(acac)(CO)(PR3) complexes is made possible by the rapid
and quantitative reaction of 1 with stoichiometric
amounts of phosphine and phosphite ligands, eq 7.3 This
Rh(acac)(CO)2 + PR3 f Rh(acac)(CO)(PR3) + CO
(7)
Rh (a ca c)(CO)(P (NC4H4)P h 2). Yield: 150 mg (80%). 1H
NMR (CD2Cl2, δ, mult): 1.68 (s, 3H, -CH3), 2.09 (s, 3H, -CH3)
5.50 (s, 1H, -CH), 6.35 (s, 2H, pyrrole) 7.15 (s, 2H pyrrole),
7.43-7.51 (m, 10H -Ph). IR (νCO, CH2Cl2): 1990 cm-1. Anal.
PR3 ) phosphine and phosphite
type of phosphine binding reaction appears general and
was found to be rapid and quantitative for all ligands
calorimetrically investigated at 30.0 °C in methylene
chloride. A compilation of the phosphine ligands with
their respective enthalpies of reaction, in solution, is
presented in Table 1.
The donor properties of tertiary phosphine ligands can
be modulated by varying the electronic and steric
parameters.4 This is usually achieved by variation of
the substituents bound to the phosphorus atom. The
binding affinities of specific phosphine ligands are
(21) Bonati, F.; Wilkinson, G. J . Chem. Soc. 1964, 3156-3160
(22) (a) Trzeciak, A. M.; Ziolkowski. J . Organomet. Chem 1992, 429,
239-244 (for P(OPh)3, PPh3, P(p-MePh)3, and PPh2Me complexes). (b)
Basson, S. S.; Leipoldt, J . G.; Roodt, A.; Venter, J . A.; Van der walt, T.
J . Inorg. Chim. Acta 1986, 119, 35-38. (for P(p-ClPh)3 and P(p-
MeOPh)3 complexes). (c) Freeman, M. A.; Young, D. A. Inorg. Chem.
1986, 25, 1556-1560 (for PCy3 complex).
(23) (a) In the course of the redaction of this manuscript, a report
of Rh(acac)(CO) (P(NC4H4)xPh3-x (x ) 1-3) complexes appeared.23b The
synthetic methodology is slightly different, and the spectroscopy was
reported in different solvents. We, therefore, report our analytical data
here. (b) Trzeciak, A. M.; Glowiak, T.; Grzybek, R.; Ziolkowski, J . J . J .
Chem. Soc., Dalton Trans. 1997, 1831-1837.

Tertiary Phosphine Ligand Substitution Reactions
Organometallics, Vol. 17, No. 4, 1998 537
commonly explained in terms of stereoelectronic effects,
yet these two factors are not easily separated. We have
been involved in determining the exact partitioning/
relative importance of such effects in a number of
organometallic systems. In the present system, single
variable correlations between the steric or electronic
parameters and the enthalpies of reaction does not lead
to linear fits. Correlation coefficients (R) of 0.11 and
0.56 are obtained for the steric and electronic vs
enthalpy data relationships, respectively. A common
approach in physical inorganic/organometallic chemistry
is to examine such effects while maintaining one of the
two parameters constant. The most common approach
makes use of a series of isosteric phosphines.24 This
can be achieved by investigating the effects of specific
substitution at the para position of the aryl grouping
bound to the phosphorus center. We have recently
reported on the thermochemical effects of such varia-
tions in ruthenium,11i iron,12c and rhodium13a systems
(eqs 8-10).
F igu r e 1. Carbonyl stretching frequency (cm-1) vs en-
thalpies of reaction (kcal/mol) for the P(p-XC6H4)3 series
in the Rh(acac)(CO)(PR3) system. Slope ) 3.6; R ) 0.88.
F igu r e 2. Hammett σp parameter vs enthalpies of reaction
(kcal/mol) for the P(p-XC6H4)3 series in the Rh(acac)(CO)-
(PR3) system. Slope ) 0.22; R ) 0.90.
The overall importance of the electronic effects will
be dictated by the final structural arrangements of the
ligands around the metal center. In the ruthenium case
mentioned above, a cis arrangement of the ligands leads
to the observation of the overwhelming importance of
steric effects. The final geometry is pseudo-octahedral,
and the importance of the steric effects is compounded
by the crowding present in having six occupied coordi-
nation sites around the Ru center. In the iron case, the
final geometry involves a trigonal bipyramidal arrange-
ment of the ligands, where the two phosphine ligands
occupy mutually trans (trigonal bipyramidal apical)
positions. This leads to a relief of the steric crowding
around the metal center and electronic effects are
observed to dictate the magnitude of the measured
reaction enthalpies. The rhodium system presented in
eq 10 follows a similar trend since the two incoming
phosphines adopt a mutually trans orientation in the
final square planar complex. On the basis of these
simple geometric and structural arguments, the ligand
displacement reaction enthalpy data for the present Rh-
(acac)(CO)(PR3) system might be expected to be influ-
enced primarily by electronic effects, eq 11. In order to
verify this hypothesis, two series of phosphine ligands
possessing isosteric properties were examined.
P (p-XC6H4)3 Ser ies (X ) H, Cl, F , Me, MeO, CF 3).
Within this series, the Tolman cone angle4a (steric
parameter) is kept constant at 145°. The electron-
donating property of the phosphine is modulated by
substituents at the aryl para position. A single-
component relationship can be established between the
enthalpy data and the carbonyl stretching frequency,
Figure 1. This last parameter reflects the amount of
electron density donated by the incoming phosphine
ligand which is then back-donated from the Rh metal
center into the CO π* orbital. This relationship clearly
shows a proportional dependence of the enthalpy data
on the electronic parameter. Another way to quantify
the donicity of the phosphine is to examine the electronic
contribution attributed to the phenyl para substituent.
Therefore, a Hammett-type relationship25 (Figure 2) can
be constructed which shows similar effects as the
electronic relationship shown in Figure 1.
Both relationships are linear but span only a narrow
range of enthalpy values and therefore, cannot unam-
biguously reflect a predominant electronic effect in the
present system.
Th e P (C6H5)3-x(NC4H4)x Ser ies (x ) 1-3). Moloy
and Petersen have reported on the synthesis and
(24) A specific example for the two ruthenium systems can be found
in ref 11i. Other examples can be found in refs 1 and 4a.
(25) March, J . Advanced Organic Chemistry; Wiley-Interscience:
New York, 1992; pp 278-286.

538 Organometallics, Vol. 17, No. 4, 1998
Serron et al.
binding to rhodium systems of a novel class of N-pyrrolyl
substituted tertiary phosphine ligands which are isos-
teric with triphenylphosphine (and para-substituted
triphenylphosphine ligands), with cone angle values of
145°.16 The N-pyrrolyl substituent was further shown
to act as a potent electron-withdrawing group, rendering
the phosphine a poor σ donor and a good π acceptor.
Pertinent resonance structures for the N-pyrrolylphos-
phines can be drawn as follows:
F igu r e 3. Carbonyl stretching frequency (cm-1) vs en-
thalpies of reaction (kcal/mol) for the P(C6H5)3-x(NC4H4)x
(x ) 0-3) series in the Rh(acac)(CO)(PR3) system. Slope )
27.5; R ) 0.97.
another rhodium system and will be reported shortly.26
The overall effect of this series of pyrrolyl-substituted
phosphine ligands, in the present system, is principally
guided by its poor σ-donating capabilities. It is for this
reason that although they possess similar cone angles
as the para-substituted triphenylphosphine, they should
be considered a different class of ligands in the present
system.
Aromatic delocalization of the nitrogen lone pair into
the pyrrole ring, as depicted in structures B and C, has
the effect of placing a partial positive charge on the
nitrogen. This would be expected to reduce the basicity
of the adjacent phosphorus atom. Structure D suggests
that N-pyrrolylphosphines will be poorer donors than
phenylphosphines since the more electronegative nitro-
gen replaces carbon. Moloy16 has shown these ligands
to undergo quantitative binding to the RhCl(CO) frag-
ment, eq 12. The pyrrolyl moiety, known for its π
Oth er Gen er a l Tr en d s. The phosphine electronic
parameter appears to have a small contribution com-
pared to previously examined systems. No single factor
dictates the magnitude of the enthalpy of reaction across
the present series of phosphine ligands. Phosphine
steric factors do play a role since the sterically very
demanding pair of phosphine ligands PCy3 (θ ) 170°)
and PiPr3 (θ ) 160°) display enthalpy values opposite
to the relative order of donation (i.e., the PiPr3 ligand
binds more strongly, although it is known to be a poorer
donor than PCy3). Sterics have to be at the origin of
this difference. Another subseries of interest are the
PPhxMe3-x ligands. Here, a general increase in the
enthalpy of reaction is measured with the increase in
phosphine basicity associated with a larger degree of
Me substituents. This enthalpy increase is somewhat
linear going from PPh3 < PPh2Me < PPhMe2.27 A word
of caution in drawing any further conclusions from the
experimental enthalpies of reaction measured in this
study: 14 phosphine ligands display an enthalpy of
reaction spanning some 4 kcal/mol. Small steric factors,
however subtle, may in fact contribute to the enthalpy
of reaction and significantly affect the relative stability
of the organorhodium complexes discussed in view of
this small range of enthalpies of reaction.28
[RhCl(CO)2]2 + 4PR3 f 2RhCl(CO)(PR3)2 + 2CO
(12)
PR3 ) P(NC4H4)3; P(NC4H4)2Ph; P(NC4H4)Ph2;
P(NC4H8)3
involvement, should also greatly affect the binding
ability of the phosphine in the present system. Zi-
olkowski and co-workers have recently reported on the
reactivity of 1 with these pyrrolyl-substituted phosphine
ligands, eq 13.23a They have commented on the π
Rh(acac)(CO)2 + PR3 f Rh(acac)(CO)(PR3) + CO
(13)
PR3 ) P(NC4H4)3; P(NC4H4)2Ph; P(NC4H4)Ph2
involvement of these phosphines in this rhodium system
as monitored by infrared spectroscopy. The thermo-
chemical information quantitatively supports this poorer
donor trend on increasing the number of pyrrolyl
substituents on phosphorus. Furthermore, a correlation
between the enthalpy of reaction and the carbonyl
stretching frequency of the product affords a linear
relationship with a good fit (Figure 3).
The extent of π involvement in the bonding present
in this system is surely diminished due to the competing
influence of CO as an ancillary ligand. If both ligands
compete for back-donation from the metal, CO will
surely exert a more important effect. This extent of π
involvement in bonding has recently been illustrated in
(26) Huang, J .; Haar, C. M.; Marshall, W. J .; Moloy, K. G.; Nolan,
S. P. Submitted for publication.
(27) Attempts were made to investigate the enthalpy of reaction of
the PMe3 and PEt3 ligands with 1 in stoichiometric amounts. The
results proved to be nonreproducible. The infrared analysis of the
products showed that the reaction did not go to completion under these
conditions or that other products were formed, as judged by IR band
positions.
(28) A QALE analysis10 performed by one of the reviewers suggests
that within the series of phosphine ligands investigated the percent
contribution to the enthalpy of reaction is as follows: 21% electronic,
47% steric, and 32% from an aryl effect. We thank the reviewer for
this contribution.

Tertiary Phosphine Ligand Substitution Reactions
Organometallics, Vol. 17, No. 4, 1998 539
Con clu sion
the enthalpy values cover only a short range. The
overall factors dictating the magnitude of the reaction
enthalpies are a combination of steric and electronic
effects. Further thermochemical investigations focusing
on related systems are presently underway.
The labile nature of the first Rh-CO bond in Rh-
(acac)(CO)2 (1) was used to gain access into the ther-
mochemistry of ligand substitution of tertiary phosphine
ligands. The enthalpy trend can be explained in terms
of both steric and electronic contributions to the en-
thalpy of reaction, phosphine sterics playing a more
important role than electronics. Quantitative relation-
ships are established between the phosphine electronic
parameters and measured enthalpies of reaction within
two subsets of phosphine ligands, and both display a
good correlation to this single electronic parameter. Yet,
Ack n ow led gm en t. The National Science Founda-
tion (Grant No. CHE-9631611) and DuPont (Educa-
tional Aid Grant) are gratefully acknowledged for
support of this research.
OM970766G