Estimating the effective steric impact of PtBu2Me, PiPr3, and PCy3

Article abstract of DOI:10.1021/om960380q

Enthalpies of reaction of Ru(CO)₂L₂ (L = PⁱPr₃, P^tBu₂Me, and PCy₃) with MeNC, PhC≡CPh (both addition reactions) and PhCC-H (C-H oxidative addition) in toluene are exothermic in t

Full text of DOI:10.1021/om960380q

4900
Organometallics 1996, 15, 4900-4903
Articles
Estim a tin g th e Effective Ster ic Im p a ct of P tBu 2Me, P iP r 3,
a n d P Cy3
Chunbang Li, Masamichi Ogasawara, Steven P. Nolan,* and
Kenneth G. Caulton*
Departments of Chemistry, Indiana University, Bloomington, Indiana 47405-4001, and
University of New Orleans, New Orleans, Louisiana 70148
Received May 20, 1996X
Enthalpies of reaction of Ru(CO)2L2 (L ) PiPr3, PtBu2Me, and PCy3) with MeNC, PhCtCPh
(both addition reactions) and PhCC-H (C-H oxidative addition) in toluene are exothermic
in the range 10-25 kcal/mol and are interpreted in terms of PtBu2Me being more bulky
than PiPr3 or PCy3. Enthalpy comparisons show that the larger cone angle of PCy3 than of
PiPr3 can be overcome by the greater donor power of PCy3; the origin of the greater donor
power of PCy3 is discussed.
In tr od u ction
H2 less well, involves an Os2 species, while the latter
proceeds via the H2 adduct OsHCl(H2)(CO)(PiPr3)2.
Phosphine (PR3) cone angles,7 as they are viewed with
increasing scrutiny, are now found to be somewhat
variable, just as are the conformations adopted by the
three substituents, R. This is especially true for three
secondary alkyl substituents (iPr and cyclohexyl). Fi-
nally, for phosphines whose three substituents are not
identical (e.g., PtBu2Me), the concept of a single cone
angle parameter becomes too simple, and rotation about
M-P and P-C single bonds can offer a steric profile
considerably smaller (or considerably larger) than any
average value. We thus sought to measure some
observables which would better (empirically) character-
ize the combined steric and electronic influence of these
three phosphines, whose utility has increased in recent
years. We present here these results, based on enthal-
pies of reaction for sterically small and large reagents,
as well as on NMR line broadening measures of kinetic
parameters.
Bulky phosphine ligands, such as PiPr3, PCy3, and
iBu2P(CH2)2PiBu2, play an important role in the recent
development of coordination and catalytic chemistry.
While these phosphines are often employed simply
because they are “bulky”, there are some reports that a
small steric modification of a phosphine ligand can
dramatically alter the reactivity (or “performance”) of
a complex.1,2 In addition, reaction products of RuHCl-
(CO)L2 (L ) PtBu2Me, PiPr3, or PCy3) with CH3Li are
dependent on the phosphine. The PtBu2Me complex
shows a clean conversion to a diastereomeric mixture
of RuH(CO)[P+BuMe(CMe2CH2)]L while the other two
complexes give mixtures of many uncharacterized spe-
cies.3
A considerable body of catalytic chemistry is growing
up around the specific bulky ligands shown in the title,
primarily in the group of Esteruelas.4 We have observed
that Os(H)2(H2)CO(PtBu2Me)2 dissociates H2 signifi-
cantly more easily than its PiPr3 analog, although we
could not reliably and independently predict which of
these two phosphines is more σ-basic and also which is
more bulky.5 It has been reported6 that OsHCl(CO)L2
with L ) PtBu2Me or PiPr3 hydrogenate an enone by
completely different mechanisms. The former, binding
Exp er im en ta l Section
Gen er a l Meth od s. All manipulations were carried out
using standard Schlenk and glovebox techniques under prepu-
rified argon. Pentane, heptane, THF, and toluene were dried
over sodium benzophenone ketyl, distilled, and stored in
gastight solvent bulbs. Benzene-d6, toluene-d8 and THF-d8
were dried over sodium metal and vacuum-distilled prior to
use. Only materials of high purity (as indicated by IR and
NMR spectroscopies) were used in the calorimetric experi-
ments. Phosphines (PiPr3 and PCy3) were purchased from
Aldrich Chemical Co. and used without purification. Diphe-
nylacetylene and phenylacetylene were purchased from Ald-
rich Chemical Co. and used after purifying by appropriate
methods (sublimation or distillation). Methylisocyanide was
synthesized according to a published method.8 Sodium amal-
gam was prepared from metallic sodium and mercury. Ru-
X Abstract published in Advance ACS Abstracts, October 15, 1996.
(1) Wasserman, H. J .; Kubas, G. J .; Ryan, R. R. J . Am. Chem. Soc.
1986, 108, 2294. Kubas, G. J .; Unkefer, C. J .; Swanson, B. I.;
Fukushima, E. J . Am. Chem. Soc. 1986, 108, 7000.
(2) Nguyen, S. T.; Grubbs, R. H.; Ziller, J . W. J . Am. Chem. Soc.
1993, 115, 9858. Schwab, P.; Grubbs, R. H.; Ziller, J . W. J . Am. Chem.
Soc. 1996, 118, 100.
(3) Ogasawara, M.; Macgregor, S. A.; Streib, W. E.; Folting, K.;
Eisenstein, O.; Caulton, K. G. J . Am. Chem. Soc., submitted for
publication.
(4) Espuelas, J .; Esteruelas, M. A.; Lahoz, F. J .; Oro, L. A.; Valero,
C. Organometallics 1993, 12, 663 and references therein. Esteruelas,
M. A.; Valero, C.; Oro, L. A.; Meyer, U.; Werner, H. Inorg. Chem. 1991,
30, 1159.
(5) Gusev, D. G.; Kuhlman, R. L.; Renkema, K. B.; Eisenstein, O.;
Caulton, K. G. Submitted for publication.
(6) Esteruelas, M.; Oro, L. A.; Valero, C. Organometallics 1992, 11,
3362.
(7) White, D.; Coville, N. J . Adv. Organomet. Chem. 1994, 36, 95.
Mu¨ller, T. E.; Mingos, D. M. P. Transition Met. Chem. 1995, 20, 533.
(8) Schuster, R. E.; Scott, J . E.; Casanova, J r., J . Org. Synth., Coll.
Vol. 5, 772.
S0276-7333(96)00380-9 CCC: $12.00 © 1996 American Chemical Society

Steric Impact of PtBu2Me, PiPr3, and PCy3
Organometallics, Vol. 15, No. 23, 1996 4901
thenium trichloride hydrate was a generous loan from J ohnson
Matthey and used as received. Ru(CO)2(PtBu2Me)2,3,9 Ru(CO)2-
(sept of vt, J HH ) 7.0 Hz, J HP ) 3.6 Hz, 6H, PCHMe), 6.93 (m,
1H, p-H), 7.10 (m, 2H, m-H), 7.42 (m, 2H, o-H). 31P{1H} NMR
10
(PiPr3)2,3,9 and cis,cis,trans-RuCl2(CO)2(PCy3)2 were synthe-
(C6D6, 23 °C): δ 62.6 (s). IR: νCO (C6D6) ) 2012 and 1958 cm-1
,
sized as reported. 1H and 31P NMR spectra were recorded on
Varian XL300, Bruker AM500, Nicolet NT-360, Varian Gemini
300, or Oxford 400 spectrometers. 1H NMR chemical shifts
are reported in ppm downfield of tetramethylsilane using
residual solvent resonances as internal standards. 31P NMR
chemical shifts are relative to external 85% H3PO4. Infrared
spectra were recorded using Perkin-Elmer FTIR Model 2000
and Nicolet 510P spectrometers in 0.1 mm NaCl cells. El-
emental analyses were performed on Perkin-Elmer 2400
CHN/S elemental analyzer at the Department of Chemistry,
Indiana University.
Calorimetric measurements were performed using a Calvet
calorimeter (Setaram C-80) which was periodically calibrated
using the TRIS reaction11 or the enthalpy of solution of KCl
in water.12 The experimental enthalpies for these two stan-
dard reactions compared very closely to literature values. This
calorimeter has been previously described,13 and typical
procedures are described below. Experimental enthalpy data
are reported with 95% confidence limits.
ν
CC (C6D6) ) 2105 cm-1, νRuH (C6D6) ) 1912 cm-1. Anal. Calcd
for RuC28H48O2P2: C, 58.01; H, 8.35. Found: C, 57.95; H, 8.22.
Ru (η2-P h CtCP h )(CO)2(P iP r 3)2. To a solution of Ru(CO)2-
(PiPr3)2 (100 mg, 0.21 mmol) in pentane (5 mL) was added
diphenylacetylene (38 mg, 0.21 mmol). Immediately, the deep-
red solution became yellow. After filtration, the solution was
concentrated to ca. 2 mL and cooled to -40 °C to give bright
yellow solid; yield 113 mg (0.17 mmol, 82%). 1H NMR (C6D6,
20 °C): δ 1.10 (br, 36H, PCCH3), 1.90 (m, 6H, PCHMe), 7.02
(t, J HH ) 7.2 Hz, 2H, p-H), 7.26 (t, J HH ) 7.8 Hz, 4H, m-H),
8.08 (d, J HH ) 7.5 Hz, 4H, o-H). 31P{1H} NMR (C6D6, 20 °C):
δ 45.8 (s). IR: νCO (C6D6) ) 1950 and 1887 cm-1, νCC (C6D6) )
1746 cm-1. Anal. Calcd for RuC34H52O2P2: C, 62.27; H, 7.99.
Found: C, 62.15; H, 7.55.
Ru (CO)2(CNMe)(P Cy3)2. To a benzene solution (5 mL) of
Ru(CO)2(PCy3)2 (60 mg, 0.084 mmol), methyl isocyanide (3.5
mg, 0.085 mmol) was added using a microsyringe. Im-
mediately the color of the solution changed from deep red to
pale yellow. The solution was evaporated to dryness, and the
residual pale yellow solid was extracted with hot acetone. After
filtration, the solution was concentrated to ca. 5 mL and cooled
to -40 °C to give pale yellow microcrystals; yield 42 mg (0.055
mmol, 66%). 1H NMR (C6D6, 20 °C): δ 1.18-1.32 (m, 18H),
1.61-1.82 (m, 30H), 2.11-2.17 (m, 6H), 2.29-2.32 (m, 12H),
2.77 (t, J HP ) 1.8 Hz, 3H, CH3NC). 31P{1H} NMR (C6D6, 23
°C): δ 66.2 (s). IR: νCO (C6D6) ) 1877 and 1840 cm-1, νCN
Ru (CO)2(P Cy3)2. In a Schlenk flask, cis,cis,trans-RuCl2-
(CO)2(PCy3)2 (500 mg, 0.63 mmol) and sodium amalgam (4.3
g, 1% sodium content, 1.87 mmol of sodium) were placed
together with THF (25 mL). The suspension was vigorously
stirred for 24 h under argon. During this period, the color of
the suspension changed from colorless to deep red. The THF
supernatant was transferred to another Schlenk flask by
means of cannula transfer and evaporated to dryness, then
the dark red residue was extracted with toluene (5 mL × 3).
After filtering away insoluble material, the solution was
concentrated to ca. 3 mL and cooled to -40 °C, yielding red
crystals; yield 321 mg (0.45 mmol, 71%). 1H NMR (C6D6, 23
°C): δ 1.15-1.28 (m, 18H), 1.56-1.77 (m, 30H), 2.08-2.17 (m,
18H). 31P{1H} NMR (C6D6, 23 °C): δ 59.3 (s). IR: νCO (Nujol)
(C6D6) ) 2062 cm-1
. Anal. Calcd for RuC40H69NO2P2: C,
63.30; H, 9.16; N, 1.85. Found: C, 62.92; H, 8.79; N, 1.53.
Ru H(CtCP h )(CO)2(P Cy3)2. Ru(CO)2(PCy3)2 (100 mg, 0.14
mmol) was dissolved in benzene (5 mL) in a Schlenk flask. To
this solution, phenylacetylene (17 µL, 15.8 mg, 0.15 mmol) was
added by means of syringe. The obtained colorless solution
was evaporated to dryness, and the oily residue was extracted
with heptane (2 mL × 3). The heptane solution was concen-
trated to ca. 2 mL, and cooled to -40 °C to give colorless
powder; yield 75 mg (0.09 mmol, 66%). 1H NMR (C6D6, 20
°C): δ -6.43 (t, J HP ) 20.7 Hz, 1H, Ru-H), 1.25 (m, 18H, 1.62
(m, 6H), 1.77 (m, 24 H), 2.19 (m, 12H), 2.45 (m, 6H), 6.94 (m,
1H, p-H), 7.16 (m, 2H, m-H), 7.59 (m, 2H, o-H). 31P{1H} NMR
) 1890 and 1823 cm-1
. Anal. Calcd for RuC38H66O2P2: C,
63.57; H, 9.27. Found: C, 63.29; H, 9.09.
Ru (CO)2(CNMe)(P iP r 3)2. A pentane (5 mL) solution of
Ru(CO)2(PiPr3)2 (80 mg, 0.17 mmol) was placed in a Schlenk
flask fitted with a rubber septum. To this solution, methyl
isocyanide (7.0 mg, 0.17 mmol) was added via syringe at room
temperature. Immediately, the dark-red solution color changed
to yellow. The solution was concentrated to ca. 1 mL and
cooled to -40 °C to give bright yellow needles: yield 65 mg (0.13
(C6D6, 20 °C): δ 53.8 (s). IR: νCO (C6D6) ) 2010 and 1958 cm-1
CC (C6D6) ) 2101 cm-1, νRuH (C6D6) ) 1916 cm-1. Anal. Calcd
,
ν
for RuC46H72O2P2: C, 67.37; H, 8.85. Found: C, 67.36; H, 8.48.
Ru (η2-P h CtCP h )(CO)2(P Cy3)2. To a solution of Ru(CO)2-
(PCy3)2 (120 mg, 0.17 mmol) in toluene (5 mL) was added
diphenylacetylene (30 mg, 0.17 mmol). Immediately, the deep
red solution became yellow. After filtration, the solution was
concentrated to ca. 2 mL and cooled to -40 °C to give yellow
solid; yield 109 mg (0.12 mmol, 73%). This material is very
slightly contaminated with free PhCCPh. 1H NMR (C6D6, 20
°C): δ 0.99-1.13 (m, 18H), 1.49-1.70 (m, 30H), 1.84-1.91 (m,
6H), 2.07-2.13 (m, 12H), 7.05 (t, J HH ) 7.2 Hz, 2H, p-H), 7.33
(t, J HH ) 7.8 Hz, 4H, m-H), 8.17 (d, J HH ) 6.9 Hz, 4H, o-H).
31P{1H} NMR (C6D6, 20 °C): δ 39.7 (s). IR: νCO (C6D6) ) 1948
mmol, 75%). 1H NMR (C6D6, 23 °C): δ 1.30 (dvt, J HH ≈ J HP
)
7.2 Hz, 18H, PCCH3, 1.31 (dvt, J HH ≈ J HP ) 7.2 Hz, 18H,
PCCH3), 2.22 (sept of vt, J HH ) 7.2 Hz, J HP ) 3.4 Hz, 6H,
PCHMe), 2.49 (t, J HP ) 1.8 Hz, 3H, CH3NC). 31P{1H} NMR
(C6D6, 23 °C): δ 74.1 (s). IR: νCO (C6D6) ) 1879 and 1836 cm-1
,
νCN (C6D6) ) 2072 cm-1. Anal. Calcd for RuC22H45NO2P2: C,
50.95; H, 8.75; N, 2.70. Found: C, 51.02; H, 8.91; N, 2.89.
Ru H(CtCP h )(CO)2(P iP r 3)2. A pentane (5 mL) solution
of Ru(CO)2(PiPr3)2 (100 mg, 0.21 mmol) was placed in a Schlenk
flask fitted with a rubber septum. To this solution, 24 µL of
phenylacetylene (22.3 mg, 0.22 mmol) was added via syringe
at room temperature. Immediately, the dark red solution color
changed to pale yellow. The solution was evaporated to
dryness, and the yellow residue was extracted with hot
heptane (2 mL × 3). The filtrate was concentrated to ca. 2
mL and cooled to -40 °C to give pale yellow needles; yield
and 1883 cm-1, νCC (C6D6) ) 1744 cm-1
. Anal. Calcd for
RuC52H76O2P2: C, 69.69; H, 8.55. Found: C, 70.23; H, 8.33.
NMR Titr a tion s. Prior to every set of calorimetric experi-
ments involving a new ligand, an accurately weighed amount
((0.2 mg) of the organometallic complex was placed in a
Wilmad screw-capped NMR tube fitted with a septum, and
toluene-d8 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 1H
NMR spectroscopy, and the reactions were found to be rapid,
clean and quantitative under experimental calorimetric condi-
tions. These conditions are necessary for accurate and mean-
ingful calorimetric results and were satisfied for all organo-
metallic reactions investigated. Only reactants and products
were observed in the course of titration.
107 mg (0.19 mmol, 88%). 1H NMR (C6D6): δ -6.75 (t, J HP
)
20.6 Hz, 1H, Ru-H), 1.25 (dvt, J HH ≈ J HP ) 6.9 Hz, 18H,
PCCH3), 1.27 (dvt, J HH ≈ J HP ) 6.9 Hz, 18H, PCCH3), 2.47
(9) Ogasawara, M.; Macgregor, S. A.; Streib, W. E.; Folting, K.;
Eisenstein, O.; Caulton, K. G. J . Am. Chem. Soc. 1995, 117, 8869.
(10) Moers, F. G.; Ten-Hoedt, R. W. M.; Langhout, J . P. J . Orga-
nomet. Chem. 1974, 65, 93.
(11) Ojelund, G.; Wadso¨, I. Acta Chem. Scand. 1968, 22, 1691-1699.
(12) Kilday, M. V. J . Res. Natl. Bur. Stand. (U.S.) 1980, 85, 467-
481.
(13) Nolan, S. P.; Hoff, C. D. J . Organomet. Chem. 1985, 282, 357-
362.

4902 Organometallics, Vol. 15, No. 23, 1996
Li et al.
Ta ble 1. En th a lp ies of Rea ction a for Rea gen t L′
w ith Ru (CO)2L2 in Tolu en e a t 30 °C
Ca lor im etr ic Mea su r em en t of Rea ction s betw een Ru -
(CO)2(P iP r 3)2 a n d Meth yl Isocya n id e (MeNC). 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
20-30 mg sample of Ru(CO)2(PiPr3)2 was accurately weighed
into the lower vessel; it was closed and sealed with 1.5 mL of
mercury. A 4 mL volume of a stock solution of MeNC in
toluene was added, and the remainder of the cell was as-
sembled, removed from the glovebox and inserted in the
calorimeter. In order to hinder possible further substitution
reactions, addition of five equivalents of phosphine ligand was
used as suppressant in the stock solution. Furthermore,
MeNC used in the preparation of the stock solution was
present in an amount requiring stoichiometric reaction with
the ruthenium complex. The reference vessel was loaded in
an identical fashion with the exception that no ruthenium
complex was added to the lower vessel. After the calorimeter
had reached thermal equilibrium at 30.0 °C (about 2 h), the
reaction was initiated by inverting the calorimeter. At the end
of the reaction (1-2 h), the vessels were then removed from
the calorimeter, taken into the glovebox, opened, and the
infrared cell filled under inert atmosphere. An infrared
spectrum of each product was recorded using this procedure.
Conversion to Ru(CO)2L(PiPr3)2 was found to be quantitative
under these reaction conditions. The enthalpy of reaction
-17.9 ( 0.1 kcal/mol represents the average of five individual
calorimetric determinations. A similar procedure was em-
ployed for all ruthenium complexes reacting with MeNC or
PhCCPh. Measurements for reactions with PhCCH were
similar, but it was not necessary to add free phosphine ligand.
Conversion to RuH(CtCPh)(CO)2(PiPr3)2 was found to be
quantitative under these reaction conditions. The enthalpy
of reaction -11.1 ( 0.1 kcal/mol represents the average of five
individual calorimetric determinations.
L
L′
PtBu2Me
PiPr3
PCy3
MeNC
PhCtCPh
PhCC-H
-19.4(1)
-10.1(1)
-23.9(2)
-21.5(2)
-14.7(1)
-24.0(1)
-21.0(2)
-16.1(2)
-25.5(3)
a
Enthalpies are reported with 95% confidence limits in the last
digit given.
PhCCPh showing greater discrimination or selectivity
between the two phosphine derivatives. If we take the
2.1 kcal/mol for the sterically compact MeNC as indicat-
ing primarily electronic effects (i.e., the PiPr3 complex
is a better π-base), the greater discrimination involving
PhCCPh is attributed to steric effects for this bulky
alkyne. The ∆H° value for PhCCPh adding to Ru(CO)2-
(PtBu2Me)2 is so small (only comparable to a strong
hydrogen bond) that the implied steric strain leads to
another outlet for relief of steric repulsion: loss of one
phosphine (eq 1).
(PhCCPh)Ru(CO)2L2 h (PhCCPh)Ru(CO)2L + L
(1)
This reaction is evident from the observation of a broad
31P{1H} NMR signal (W1/2 = ca. 400 Hz) for Ru-
(PhCCPh)(CO)2L2 coalesced with free L in benzene at
23 °C.3 Since this chemical shift is essentially that of
the complex at -40 °C, the mole fraction of the mono-
phosphine complex must be less than ∼3% at 23 °C. For
comparison, the W1/2 of the 31P{1H} NMR signal of
Ru(PhCCPh)(CO)2(PiPr3)2 is only ca. 40 Hz (at 23 °C),
indicating that the effective size of PtBu2Me is larger
than that of PiPr3.15 This is the first direct size
comparison of these two ligands. However, it certainly
gives additional support to the interpretation of reaction
enthalpies for PhCCPh binding to Ru(CO)2L2 as being
significantly influenced by steric effects.
Ca lor im etr ic Mea su r em en t of En th a lp ies of Solu tion
of th e Com p lexes in Tolu en e. In order to consider all
species in solution, the solvation enthalpies of Ru(CO)2L2
species had to be directly measured. This was performed by
using a procedure similar to the one described above, with the
exception that no ligand was added to the reaction cell. This
enthalpy of solution represents the average of five individual
determinations and, for Ru(CO)2(PiPr3)2, is measured as 3.6
( 0.1 kcal/mol.
This hypothesis of certain substrates being especially
sensitive to steric effects is subject to test. Another
alkyne, but one which binds only with a small steric
profile because it effects oxidative addition of its H-C(sp)
bond, is PhCC-H (eq 2). The slender profile of the
Resu lts a n d Discu ssion
Com p a r ison of P tBu 2Me a n d P iP r 3. It is first
useful to note that the quantitative infrared intensity
ratios14 for Ru(CO)2L2 in solution yield angles between
the CO vectors of 130° (PtBu2Me) and 129° (PiPr3). The
νsym - νasym values (71 and 70 cm-1) indicate identical
CO/CO interaction force constants and thus provide
independent evidence for identical C-Ru-C values in
the two compounds. With these points established, the
fact that νsym and νasym are each 3 cm-1 lower for L )
PiPr3 than for PtBu2Me indicates that PiPr3 (cone angle
160°) is a slightly better donor than PtBu2Me (cone angle
161°), and thus that Ru(CO)2(PiPr3)2 is a somewhat
better π-base toward any entering ligand.
The ligand binding enthalpies (Table 1) permit several
conclusions about the two phosphines. In each case,
∆H° is more negative when L ) PiPr3, and is more
negative for MeNC than for PhCCPh. This originates
in some combination of steric and electronic effects.
Comparing the two phosphine complexes, ∆∆H° is
smaller for MeNC as the added ligand (2.1 kcal/mol)
than for PhCCPh (4.6 kcal/mol). We interpret this as
resulting hydride and acetylide ligands, together with
the trans disposition of the bulky phosphines in both
reactant and product in eq 2, suggests that the ∆H of
eq 2 should be more similar for L ) PiPr3 and PtBu2Me
than is ∆H for addition of PhCCPh. Indeed (Table 1),
the values are -24.0 and -23.9 kcal/mol, respectively.
This confirms the idea that certain reactions can be
significantly less subject to steric influence than others.
Given the H-C(sp) BDE of 132 kcal/mol,16 the sum of
the Ru-H and Ru-C2Ph BDE’s is 156 kcal/mol. Esti-
(15) Indeed Ru(PhCCPh)(CO)2L2 is not isolable for L ) PtBu2Me due
to the phosphine dissociation described above, but yellow crystals were
obtained in pure form for L ) PiPr3.
(16) McMillan, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982,
33, 493.
(14) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th
ed.; Wiley: New York, 1988; p 1935.

Steric Impact of PtBu2Me, PiPr3, and PCy3
Organometallics, Vol. 15, No. 23, 1996 4903
mating the Ru-H BDE as 60 kcal/mol gives a value of
96 for the Ru-C2Ph BDE. It is also significant that the
greater π-basicity (reducing power) deduced for Ru(CO)2-
(PiPr3)2 from ν(CO) values is too small to alter ∆H for
oxidative addition of PhC2-H by more than 1 kcal/mol.
Com p a r ison to P Cy3. The cone angle of PCy3 (170°)
is significantly larger than that of PiPr3 (160°). Our
initial attempt at magnesium reduction of cis,cis,trans-
RuCl2(CO)2(PCy3)2 (1) as reported for analogous com-
plexes with PtBu2Me or PiPr3 failed, presumably due
to a very low solubility of 1 in THF. Sodium amalgam
reduces 1, which is suspended in THF, to give Ru(CO)2-
(PCy3)2 in high yield. Ru(CO)2(PCy3)2 is spectroscopi-
cally quite similar to the other two Ru(CO)2L2 species
studied here. In particular, there is no (31P NMR line
broadening) evidence for dissociative equilibrium of
PCy3 or of agostic or oxidative addition of a cyclohexyl
group, or of association or reaction with benzene solvent.
The νCO intensity ratio (Iasym/Isym) is significantly
smaller for Ru(CO)2L2 when L ) PCy3 (3.7) than when
L ) PtBu2Me or PiPr3 (4.6-4.4). This yields an C-
Ru-C of 126° in Ru(CO)2(PCy3)2. Thus, more than
merely metal electron density changes as we change the
phosphine to PCy3; the structural change will also
influence the reaction enthalpy. The larger σ-donor
power of PCy3 makes Ru more electron-rich; this, in
turn, causes C-Ru-C to decrease, since a small angle
enhances back bonding to the two carbonyls.9
than for PiPr3. Quite unpredictably, but consistent with
the reaction enthalpy, Ru(PhC2Ph)(CO)2(PCy3)2 shows
little or no line broadening in its 31P{1H} NMR spec-
trum, indicating that loss of PCy3 occurs to a lesser
extent than it does for PtBu2Me or PiPr3. Indeed, Ru-
(PhCCPh)(CO)2(PCy3)2 can be isolated as a crystalline
solid.3,15 Thus, electronic factors dominate steric ones
for binding of PhCCPh.
Why is a larger phosphine (PCy3) more (σ) donating
(i.e., Tolman electronic parameter) than a smaller one
(PiPr3)? This can perhaps be understood as a conse-
quence of the sterically dictated larger C-P-C angle of
PCy3 enforcing more p character to the phosphorus lone
pair, which causes its energy to rise. A higher HOMO
of PR3 then makes it a stronger σ-donor.18
One additional feature will influence only certain of
the reaction enthalpies. PtBu2Me has a unique geo-
metric feature compared to the other two phosphines
discussed here: it has two big (tBu) and one small (Me)
substituents on the phosphorus atom. In the solid-state
structure of Ru(CO)2(PtBu2Me)2, the orientation of the
phosphines is staggered; i.e., the Me groups of each
phosphine ligand point to opposite directions. However,
it is expected that both Me’s are oriented toward the
PhC2Ph ligand in Ru(PhC2Ph)(CO)2(PtBu2Me)2 to de-
crease steric interaction between PhC2Ph and the phos-
phines. Thus, unique among the phosphines studied
here, PtBu2Me can respond to increased steric demand
during adduct formation.
The molecule reacts rapidly with MeNC and PhC2Ph
by simple adduct formation, and with PhCCH by oxida-
tive addition. The reaction enthalpy for addition of
MeNC to Ru(CO)2(PCy3)2 is essentially identical to that
of the PiPr3 analog, and shows no strong evidence that
the greater donor power of PCy3 over PiPr3 (judged by
Tolman’s electronic parameters)17 influences this reac-
tion enthalpy, and certainly confirms the idea that
MeNC is a sterically-undemanding addend which is not
significantly influenced by the different cone angles of
these two phosphines. Comparing the C-H oxidative
addition enthalpies for the sterically-undemanding re-
agent PhCC-H, the more σ-donating phosphine PCy3
causes a more negative reaction enthalpy. This con-
firms that PCy3 is a stronger electron donor than Pi-
Pr3. The surprising result is that the reaction enthalpy
for addition of the sterically-demanding PhCtCPh is
more negative for the significantly more bulky PCy3
Studies19,20 of several reactions of Ru(CO)5 reveal that
they begin with dissociation of one CO, to form transient
Ru(CO)4 with ∆Hq ) 27.6 kcal/mol (and ∆Sq ) 15.2 cal/
(K mol)). This establishes the approximate Ru-CO
bond dissociation energy to form unsaturated Ru(0)
unassisted by steric effects, and it compares remarkably
well with the enthalpy of binding of MeNC to Ru(CO)2L2
in Table 1.
Ack n ow led gm en t. This work was supported by the
National Science Foundation Grants CHE-93-05492 and
94-12830 and by material support from J ohnson
Matthey/Aesar.
OM960380Q
(18) Dunne, B. J .; Morris, R. B.; Orpen, A. G. J . Chem. Soc., Dalton
Trans. 1991, 653. See especially Table 7 and Figure 7.
(19) Huq, R.; Poe¨, A. J .; Chawla, S. Inorg. Chim. Acta 1980, 38, 121.
Chen, L.; Poe¨, A. J . Inorg. Chim. Acta 1995, 240, 399.
(20) Hastings, W. R.; Roussel, M. R.; Baird, M. C. J . Chem. Soc.,
Dalton Trans. 1990, 203.
(17) Tolman, C. A. Chem. Rev. 1977, 77, 313.