Ligand (L) influence on CO binding enthalpies to Ru(CO)2L2

Article abstract of DOI:10.1021/om970220u

Reaction enthalpies for the addition of CO to Ru(CO)₂L₂ (L = P^tBu₂Me, PⁱPr₃, and PCy₃) in toluene are -26.2(3), -31.4(2), and -28.9(4) kcal/mol, respectively. These are larger than the enthalpies of reaction with MeNC, PhC≡CPh, and PhCC-H. P^tBu₂Me consistently gives the least amount of enthalpy released, but only when CO (and MeNC) is the reagent does PⁱPr₃ yield a more exothermic enthalpy of reaction than PCy₃. The generally subtle influences on ΔH by PⁱPr₃ and PCy₃ are rationalized in terms of the contradictory steric and electronic effects as isopropyl is replaced by cyclohexyl.

Full text of DOI:10.1021/om970220u

Organometallics 1997, 16, 4223-4225
4223
Notes
Liga n d (L) In flu en ce on CO Bin d in g En th a lp ies to
Ru (CO)2L2
Chunbang Li, Montserrat Oliva´n, 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 March 18, 1997X
Summary: Reaction enthalpies for the addition of CO
to Ru(CO)2L2 (L ) PtBu2Me, PiPr3, and PCy3) in toluene
are -26.2(3), -31.4(2), and -28.9(4) kcal/ mol, respec-
tively. These are larger than the enthalpies of reaction
with MeNC, PhCtCPh, and PhCC-H. PtBu2Me con-
sistently gives the least amount of enthalpy released, but
only when CO (and MeNC) is the reagent does PiPr3 yield
a more exothermic enthalpy of reaction than PCy3. The
generally subtle influences on ∆H by PiPr3 and PCy3 are
rationalized in terms of the contradictory steric and
electronic effects as isopropyl is replaced by cyclohexyl.
M(CO)3(PCy3)2 (M ) Mo and W), Hoff has stated
“...experimental data on steric strain present no clear
picture...for...second- and third-row metals”.5 He par-
ticularly cites an imperfect understanding “...for steri-
cally crowded systems where the conflicting demands
of entropy-enthalpy of reaction shows up”.5 Indeed, it
was our objective in a recent solution calorimetric study6
to try to rank some of the above phosphines based on
the reaction enthalpies for eq 1 (ligand binding) or eq 2
(oxidative addition). Such a ranking is clearly a com-
Ru(CO)2L2 + D f Ru(CO)2L2(D)
D ) MeNC, PhCCPh
(1)
An extraordinary amount of organometallic research
activity1 on unsaturated Rh, Ir, Ru, and Os has em-
ployed bulky monodentate phosphines. The influence
of these phosphines is to prevent dimerization and to
sterically shield the Lewis acid site. Another conse-
quence is the creation of considerable steric selectivity
toward substrate binding: the (two) bulky phosphines
are generally both cis to the open coordination site. This
field has advanced using PiPr3, PCy3, PtBu2Me, PiPr2-
Ph, and PtBu2Ph in a somewhat unsystematic and
empirical manner. The choice of phosphine employed,
and those factors which differentiate these phosphines
from one another, are not rationally and objectively
established: as an example, Chaudret has isolated
RuH6L2 complexes, and for L ) PCy3 a mononuclear
complex can be isolated,2 while for L ) PiPr3 the
reaction yields a mixture of Ru2H6L4 and RuH6L2
(detected only by NMR, and that transforms into the
dimer, slowly in solution and rapidly in vacuo).3 We
have observed that HCtCSiMe3 inserts into the Os-H
bond of OsHCl(dCdCHSiMe3)L2, giving OsCl{(E)-
CHdCHSiMe3}(dCdCHSiMe3)L2 when L ) PiPr3 but
not when L ) PtBu2Me.4 As we move to quantitative
comparative measurements on complexes of these phos-
phines, this ignorance becomes increasingly irritating.
Describing the enthalpy for binding Lewis bases to
Ru(CO)2L2 + HCCPh f RuH(CCPh)(CO)2L2 (2)
posite consequence of the changes in steric as well as
electronic influences of the substituents on the phos-
phines. Separating these contributions is not our objec-
tive. However, original thermodynamic studies by
Tolman7 have certainly proven quite useful.
The challenge of distinguishing among our chosen
phosphines is great. For example, both PiPr3 and PCy3
bear three secondary alkyl substituents, so their elec-
tronic and steric effects might be very similar. More-
over, PiPr3 and PtBu2Me are isomers of formula PC9H21
,
so it is not intuitively obvious which is the larger
phosphine. Finally, while phenyl is probably larger
than methyl, its flat profile (anisotropy) might make it
able to create a small steric demand at certain orienta-
tions, thus frustrating the comparison of PtBu2R for R
) Me and Ph.
We present here the solution calorimetric measure-
ments of reaction enthalpies for another reaction of the
type in eq 1, where now D ) CO. We hope that such
“operational” evidence will help to define the net (i.e.,
electronic and steric) difference among subtly different
but very bulky phosphines and thus prove useful in the
future.
* To whom correspondence should be addressed: S.P.N., University
of New Orleans; K.G.C., Indiana University.
X Abstract published in Advance ACS Abstracts, August 15, 1997.
(1) See, for example: (a) Kubas, G. J .; Unkefer, C. J .; Swanson, B.
I.; Fukushima, E. J . Am. Chem. Soc. 1986, 108, 7000. (b) Schwab, P.;
Grubbs, R. H.; Ziller, J . W. J . Am. Chem. Soc. 1996, 118, 100. (c)
Esteruelas, M.; Oro, L. A.; Valero, C. Organometallics 1992, 11, 3362.
(d) Clark, H. C.; Hampden-Smith, M. J . Coord. Chem. Rev. 1987, 79,
229. (e) Flu¨gel, R.; Windmu¨ller, B.; Gevert, O.; Werner, H. Chem. Ber.
1996, 129, 1007.
Exp er im en ta l Section
Gen er a l. All manipulations were carried out using stan-
dard Schlenk and glovebox techniques under prepurified
argon. Pentane and toluene were dried over sodium ben-
(2) Chaudret, B.; Poilblanc, R. Organometallics 1985, 4, 1722.
(3) Burrow, T.; Sabo-Etienne, S.; Chaudret, B. Inorg. Chem. 1995,
34, 2470.
(5) Hoff, C. D. Prog. Inorg. Chem. 1992, 40, 503.
(6) Li, C.; Ogasawara, M.; Nolan, S. P.; Caulton, K. G. Organome-
tallics 1996, 45, 4900.
(4) Oliva´n, M.; Huffman, J . C.; Eisenstein, O.; Caulton, K. G.
Manuscript in preparation.
(7) Tolman, C. A. Chem. Rev. 1977, 77, 313. Tolman, C. A. J . Am.
Chem. Soc. 1970, 92, 2956.
S0276-7333(97)00220-3 CCC: $14.00 © 1997 American Chemical Society

4224 Organometallics, Vol. 16, No. 19, 1997
Notes
Ta ble 1. En th a lp ies of Rea ction a of L′ w ith
Ru (CO)2L2 in Tolu en e a t 30 °C
and are the average of five individual calorimetric determina-
tions.
L
PtBu2Me
PiPr3
PCy3
Resu lts a n d Discu ssion
L′
CO
-26.2(3)
-23.9(2)
-19.4(1)
-10.1(1)
-31.4(2)
-24.0(1)
-21.5(2)
-14.7(1)
-28.9(4)
-25.5(3)
-21.0(2)
-16.1(2)
The structures of all Ru(CO)3L2 complexes formed
here might be expected to be that of I. In fact, the
infrared spectra of all of the complexes show one strong
band, generally consistent with D3h symmetry. How-
ever, all show varying degrees of splitting of this, the
apparent E vibration (∼1856 cm-1) of I (θ ) 180°).
PhCC-Hb
MeNCb
PhCtCPhb
a
Enthalpies are reported with 95% confidence limits in the last
b
digit given. Reference 6.
zophenone ketyl, distilled, and stored in gastight solvent bulbs.
CH2Cl2 was dried over calcium hydride, distilled, and stored
in gastight solvent bulbs. Benzene-d6 was dried over sodium
metal and vacuum-distilled prior to use. Ru(CO)2(PtBu2Me)2,8a
6
Ru(CO)2(PiPr3)2,8b and Ru(CO)2(PCy3)2 were synthesized as
reported. 1H and 31P{1H} NMR spectra were recorded on a
Varian Gemini 300 spectrometer. Infrared spectra were
recorded using Nicolet 510P or Perkin Elmer 2000 spectrom-
eters in 0.1 mm NaCl cells.
Ru (CO)3(P tBu 2Me)2. A solution of Ru(CO)2(PtBu2Me)2 (120
mg, 0.25 mmol) in toluene (7 mL) was frozen in liquid N2, the
headspace of the Schlenk flask was evacuated, and excess CO
(1 atm) was introduced. On warming to room temperature
and stirring, the solution color immediately changed from deep
red to yellow. After 2 h of stirring, the volatiles were removed
under vacuum and pentane was added to provide a pale yellow
solid. Yield: 90 mg (71%). IR (CH2Cl2, cm-1): ν(CO) 1954
(w), 1878 (s), 1852 (s). The NMR data are consistent with
those reported previously.9
Ru (CO)3(P Cy3)2. This compound was prepared as de-
scribed for Ru(CO)3(PtBu2Me)2, starting from Ru(CO)2(PCy3)2
(100 mg, 0.14 mmol). Yield: 85 mg (82%). IR (CH2Cl2, cm-1):
ν(CO) 1952 (w), 1870 (s), 1855 (s). 1H NMR (C6D6, 300 MHz):
δ 2.29-1.14 (m, 66 H, PCy3). 31P{1H} NMR (C6D6, 121.4
MHz): δ 63.5 (s).10
Moreover, a very weak absorption is also present at
1954 cm-1, consistent with some allowedness of the A
vibration of I. It thus appears that all Ru(CO)3L2
complexes studied here may have structure I, but with
θ slightly less than 180°.
The CO reaction enthalpies (Table 1) are more nega-
tive than any we have reported earlier, and in particu-
lar, they are much more negative (by 5-10 kcal/mol)
than those of the other sterically compact π-acid MeNC.
This ranking of CO and MeNC is consistent with the
previously reported evidence that Ru(CO)2L2 depends
much more on its π-base (i.e., back-bonding) character
for the strength of the Ru-L′ bond than on L′ f Ru σ
donation. Indeed, d8 Ru(CO)2L2 reveals itself to be very
different from d6 W(CO)3(PCy3)2 in that the latter binds
the (primarily) σ-bases THF, MeCN, N2, and pyridine,12
none of which binds detectably (NMR assay) to Ru-
(CO)2L2.8
Ru (CO)3(P iP r 3)2. This compound was prepared as de-
scribed for Ru(CO)3(PtBu2Me)2, starting from Ru(CO)2(PiPr3)2
(100 mg, 0.21 mmol). Yield: 21 mg (20%). IR (CH2Cl2, cm-1):
ν(CO) 1956 (w), 1875 (s), 1860 (s). The NMR data are
consistent with those reported previously.11
Quantum calculations show that the energy to convert
Mo(CO)6 to Mo(CO)5 + CO is 39.713 kcal/mol while that
of Ru(CO)5 to Ru(CO)4 + CO costs distinctly less (33
kcal/mol).13 Thus, the experimental value (27.6 kcal/
mol)14 for Ru(CO)5 would lead to the expectation that
any agostic interaction would reduce the reaction en-
thalpy for Ru(CO)2L2 + CO by about 10 kcal/mol (the
enthalpy of the agostic interaction in W(CO)3(PCy3)215).
The observed values (Table 1) of nearly 30 kcal/mol are
high enough to confirm the absence of a significant
agostic interaction (or interaction with solvent) in any
of the three Ru(CO)2L2 species studied here. No agostic
interaction is observed in Ru(CO)2(PtBu2Me)2 in an
X-ray diffraction study.8 Moreover, if we attribute the
larger first BDE of Mo(CO)6 than that of Ru(CO)5 to the
greater “instability” (electrophilicity) of Mo(CO)5 vs Ru-
(CO)4, then we can understand that any agostic interac-
Calor im etr ic Measu r em en t of Reaction s between Ru L2-
(CO)2 a n d Ca r bon Mon oxid e, L ) P tBu 2Me, P iP r 3, a n d
P Cy3. The mixing vessels of the Setaram C-80 calorimeter
were cleaned, dried in an oven maintained at 120 °C, and then
taken into the glovebox. A 20-30 mg sample of RuL2(CO)2
was accurately weighed into the lower vessel; it was then
closed and sealed with 1.5 mL of mercury. A toluene solution
(2.5 mL) saturated with CO was added. 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 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. When thermal equilibrium was
reached again after the end of the reaction (1-2 h), the vessels
were then removed from the calorimeter and taken into the
glovebox and opened and the infrared cell filled under inert
atmosphere. Conversion to the desired product was found to
be quantitative under these reaction conditions. The enthal-
pies of reaction listed in Table 1 represent solution state values
(12) Wasserman, H. J .; Kubas, G. J .; Ryan, R. R. J . Am. Chem. Soc.
1986, 108, 2294.
(13) Li, J .; Schreckenbach, G.; Ziegler, T. J . Am. Chem. Soc. 1995,
117, 486.
(8) (a) Ogasawara, M.; Macgregor, S. A.; Streib, W. E.; Folting, K.;
Eisenstein, O.; Caulton, K. G. J . Am. Chem. Soc. 1995, 117, 8869. (b)
Ogasawara, M.; Macgregor, S. A.; Streib, W. E.; Folting, K.; Eisenstein,
O.; Caulton, K. G. J . Am. Chem. Soc. 1996, 118, 10189.
(9) Heyn, R. H.; Macgregor, S. A.; Nadasdi, T. T.; Ogasawara, M.;
Eisenstein, O.; Caulton, K. G. Inorg. Chim. Acta 1997, 259, 5.
(10) Song, L.; Trogler, W. C. J . Am. Chem. Soc. 1992, 114, 3355.
(11) Espuelas, J .; Esteruelas, M. A.; Lahoz, F. J .; Lo´pez, A. M.; Oro,
L. A.; Valero, C. J . Organomet. Chem. 1994, 468, 223.
(14) (a) Huq, R.; Poe¨, A. J .; Chanula, S. Inorg. Chim. Acta 1980, 38,
121. (b) Chen, L.; Poe¨, A. J . Inorg. Chem. 1989, 28, 3641.
(15) (a) Gonzalez, A. A.; Zhang, K.; Nolan, S. P.; Lopez de la Vega,
R.; Mukerjee, S. L.; Hoff, C. D.; Kubas, G. J . Organometallics 1988, 7,
2429. (b) Zhang, K.; Gonzalez, A. A.; Mukerjee, S. L.; Chou, S.-J .; Hoff,
C. D.; Kubat-Martin, K. A.; Barnhart, D.; Kubas, G. J . J . Am. Chem.
Soc. 1991, 113, 9170. (c) In general, agostic interactions are in the
range 10-15 kcal/mol. See: Crabtree, R. H.; Hamilton, D. G. Adv.
Organomet. Chem. 1988, 28, 299 and references therein.

Notes
Organometallics, Vol. 16, No. 19, 1997 4225
tion with Mo(CO)5 might be 39.7 - 33 ≈ 7 kcal/mol
weaker toward Ru.
pendent. For reactions which approach full oxidative
addition (PhCC-H), the electronic superiority of PCy3
dominates. For reactions which leave the metal zerova-
lent (even those with a high degree of back-bonding),
electronic effects dominate for the bulkier L′ (PhCtCPh)
while steric effects dominate for the smaller L′ (CNMe,
CO).
The ligands L in Table 1 are arranged so that -∆H
generally increases to the right. It was on the basis of
L ) PtBu2Me having the smallest -∆H value that we
concluded earlier that this was operationally bulkier
than the isomeric PiPr3 analog. This trend persists for
L′ ) CO.
The enthalpies of binding CO to the several d8 Ru-
(CO)2L2 reported here, -26 to -31 kcal/mol, are similar
to that for d6 W(CO)3(PCy3)2 (-30.4 kcal/mol).5 Since
all of these unsaturated species have the “prepared”
geometry of the product, this similarity is perhaps
understandable. In stark contrast is the lower (-10.8
kcal/mol) enthalpy of binding CO to d8, but planar, Ir-
(CO)Cl(PPh3)2;16 in this case, some energetic price must
be paid for deforming CO and Cl from their trans
relationship in the reagent to a cis relationship in the
product.
Our prior comparison of PiPr3 and PCy3 showed that,
especially for the more sterically demanding reagents
PhCtCPh and PhCC-H, -∆H was larger for the
bulkier (by 10° in cone angle) PCy3. In fact, the larger
cone angle, 170°, for PCy3 originates mainly from the
larger C-P-C for this phosphine (106.3° vs 104.9 for
PiPr3). However, this has the compensating electronic
effect of raising the energy (hence the σ-basicity) of the
lone pair, due to decreasing s character in that orbital.
In this way, the bulkier phosphine has a compensating
electronic advantage, which the enthalpy data shows
to be dominant. However, this argument clearly fails
for the CO binding enthalpy to Ru(CO)2L2, which is
nearly 3 kcal/mol smaller for L ) PCy3. Note that for
the binding of MeNC, -∆H is also smaller for the
bulkier PCy3.
Ack n ow led gm en t. This work was supported by the
National Science Foundation (Grant Nos. CHE-9631611
and 9412830). We also thank J ohnson Matthey/Aesar
for material support.
OM970220U
We propose that the opposing steric and electronic
effects in comparing PiPr3 and PCy3 are reagent de-
(16) Vaska, L. Acc. Chem. Res. 1968, 1, 335.