Chelation kinetics of bidentate phosphine ligands on pentacoordinate ruthenium carbonyl complexes

Article abstract of DOI:10.1021/om000289t

Chelation kinetics of the complexes Ru(CO)₄(η¹-(P-P)) have been studied in heptane, where P-P = Ph₂P(CH₂)_nPPh₂ (n = 1, 2, 3, or 4, i.e., dppm, dppe, dppp, or dppb), Ph₂P(NMe)PPh₂ (dppma), Ph₂P(o-C₆H₄)PPh₂ (dpp-benzene), or R₂P(CH₂)₂PR₂ (R = Me or Cy, i.e., dmpe or dcpe). The complexes were prepared in situ by reaction of the bidentate ligands with Ru(CO)₄(C₂H₄), which itself was prepared in situ by photolysis of Ru₃(CO)₁₂ under C₂H₄. The initially formed Ru(CO)₄(η¹-(P-P)) complexes react cleanly to form axial-equatorial Ru(CO)₃(η²-(P-P)), as shown by the crystallographic structures of the products when P-P = dppe, dmpe, and dpp-benzene and the close similarity of their FTIR spectra to those of the other products. The chelated products undergo further reaction in solution or the solid state, and the product when P-P = dppma has been characterized by crystallography as Ru₂-(CO)₃(μ-PPh₂)(μ-Ph₂PNMePPh₂). The kinetics of the displacement of CO from Ru(CO)₄(η¹-(P-P)) in n-heptane are characterized by ΔH^? values that are lower by up to 9 kcal mol^-1 than those of their monodentate P-donor analogues. ΔS^? values range from quite positive to slightly negative and suggest a trend from purely dissociative to appreciably associative mechanisms along the series P-P = dpp-benzene < dcpe < dmpe < dppp ≤ dppm ≈ dppbu ≈ dppe ? dppma. This contrasts with the CO-dissociative reactions of analogous Ru(CO)₄L complexes when L = monodentate P-donor ligands.

Full text of DOI:10.1021/om000289t

3674
Organometallics 2000, 19, 3674-3682
Ch ela tion Kin etics of Bid en ta te P h osp h in e Liga n d s on
P en ta coor d in a te Ru th en iu m Ca r bon yl Com p lexes
Kevin A. Bunten, David H. Farrar,* Anthony J . Poe¨,* and Alan J . Lough
Department of Chemistry, University of Toronto, Lash Miller Chemistry Laboratories,
80 St. George Street, Toronto, Ontario, Canada, M5S 3H6
Received April 5, 2000
Chelation kinetics of the complexes Ru(CO)4(η1-(P-P)) have been studied in heptane, where
P-P ) Ph2P(CH2)nPPh2 (n ) 1, 2, 3, or 4, i.e., dppm, dppe, dppp, or dppb), Ph2P(NMe)PPh2
(dppma), Ph2P(o-C6H4)PPh2 (dpp-benzene), or R2P(CH2)2PR2 (R ) Me or Cy, i.e., dmpe or
dcpe). The complexes were prepared in situ by reaction of the bidentate ligands with Ru-
(CO)4(C2H4), which itself was prepared in situ by photolysis of Ru3(CO)12 under C2H4. The
initially formed Ru(CO)4(η1-(P-P)) complexes react cleanly to form axial-equatorial Ru-
(CO)3(η2-(P-P)), as shown by the crystallographic structures of the products when P-P )
dppe, dmpe, and dpp-benzene and the close similarity of their FTIR spectra to those of the
other products. The chelated products undergo further reaction in solution or the solid state,
and the product when P-P ) dppma has been characterized by crystallography as Ru2-
(CO)3(µ-PPh2)(µ-Ph2PNMePPh2). The kinetics of the displacement of CO from Ru(CO)4(η1-
(P-P)) in n-heptane are characterized by ∆Hq values that are lower by up to 9 kcal mol-1
than those of their monodentate P-donor analogues. ∆Sq values range from quite positive to
slightly negative and suggest a trend from purely dissociative to appreciably associative
mechanisms along the series P-P ) dpp-benzene < dcpe < dmpe < dppp e dppm ≈ dppbu
≈ dppe , dppma. This contrasts with the CO-dissociative reactions of analogous Ru(CO)4L
complexes when L ) monodentate P-donor ligands.
In tr od u ction
detailed study (eq 1). The reactions are solely CO-
dissociative, are significantly sensitive to the size of the
substituents, L, and are characterized by high positive
values of ∆Sq.4
Organometallic carbonyl compounds are widely used
as hydrogenation and hydroformylation catalysts.1 An
important step in many catalytic cycles involves dis-
sociation of a carbonyl ligand from the metal center so
as to create a vacant coordination site at which rapid
reactions can occur.2 Kinetic studies of this process can
shed light on the efficiency of such reactions, and many
studies of CO dissociation from six-coordinate d6 metal
carbonyls have been reported.2,3 The size2a,g,3,4 of any
substituent ancillary ligands often has a significant
influence on their reaction rates. Kinetic studies of CO
dissociation from five-coordinate metal carbonyls are
still quite rare,2,3 but displacement of CO from Ru(CO)4-
(L) (L ) P-donor and some other ligands) has received
Ru(CO)4L + L
9
48 Ru(CO)3L2 + CO
(1)
Because P-donor ligands have a wide range of elec-
tronic and steric properties, the effectiveness of metal
carbonyl catalysts can be greatly influenced by their
presence as substituents,5 and there is a consequent
possibility of “tuning” catalysts by selection of appropri-
ate ligands. One distinctive class of ligands is made up
of polydentate phosphorus donors. They have been used
to stabilize cluster complexes against fragmentation,6
and, when used in mononuclear carbonyls, they have
been shown to exhibit exceptional behavior. For in-
stance, Gladfelter et al.5 demonstrated that a chelating
ligand made neutral ruthenium and iron complexes
easier to oxidize and the corresponding products are
more reactive than their nonchelated counterparts. A
very important application for chelating phosphines is
their use as ligands in transition metal catalyzed
synthesis of chiral organic compounds.7
(1) (a) Cotton, F. A.; Wilkinson, G. In Advanced Inorganic Chemistry,
5th ed.; Wiley: New York, 1988. (b) Tkatchenko, I. In Comprehensive
Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W.,
Eds.; Pergamon: Oxford, 1982; Vol. 8, pp 101-223. (c) Feng, J .;
Garland, M. Organometallics 1999, 18, 417.
(2) (a) Howell, J . A. S.; Burkinshaw, P. M. Chem. Rev. 1983, 83,
557. (b) Agbosoou, F.; Carpentier, J .-F.; Mortreux, A. Chem. Rev. 1995,
95, 2485. (c) Schmid, R.; Hermann, W. A.; Fenking, G. Organometallics
1997, 16, 701. (d) Atwood, J . D.; Brown, T. L. J . Am. Chem. Soc. 1976,
98, 3155. (e) Angelici, R. J .; Basolo, F. J . Am. Chem. Soc. 1962, 84,
2495. (f) Graham, J . R.; Angelici, R. J . Inorg. Chem. 1967, 6, 2082. (g)
Angelici, R. J .; Basolo, F. Inorg. Chem. 1963, 2, 728.
The actual process of forming chelate complexes of
bidentate phosphine ligands has received relatively little
attention in organometallic chemistry. In the 1970s,
Connor et al.8 demonstrated that chelation reactions of
bidentate phosphines on metal carbonyls of group VI
(Cr, Mo, and W) proceed with varying degrees of bond
making and bond breaking in the transition state,
(3) Basolo, F. Polyhedron 1990, 9, 1503.
(4) (a) Chen, L.; Poe¨, A. J . Inorg. Chem. 1989, 28, 3641. (b) Poe¨, A.
J .; Twigg, M. V. Inorg. Chem. 1974, 13, 2982. (c) J ohnson, B. F. G.;
Lewis, J .; Twigg, M. V. J . Chem. Soc., Dalton Trans. 1975, 1876.
(5) (a) Sherlock, S. J .; Boyd, D. C.; Moasser, B.; Gladfelter, W. L.
Inorg. Chem. 1991, 30, 3636. (b) Moasser, B.; Gross, C.; Gladfelter,
W. L. J . Organomet. Chem. 1994, 471, 201.
(6) Bahsoun, A. A.; Osborn, J . A.; Voelker, C.; Bonnet, J . J .; Lavigne,
G. Organometallics 1982, 1, 1114.
10.1021/om000289t CCC: $19.00 © 2000 American Chemical Society
Publication on Web 08/10/2000

Chelation Kinetics of Bidentate Phosphine Ligands
Organometallics, Vol. 19, No. 18, 2000 3675
Sch em e 1. Syn th etic Rou te to Ch ela te
Com p ou n d s a n d Decom p osition P r od u cts
depending on the chelating ligand (eq 2). Recently, the
kinetics of ring closure reactions of bidentate nitrogen9
and sulfur10 ligands were studied in detail. All of these
reactions involve six-coordinate octahedral complexes,
and no studies of chelating phosphine ligands on five-
coordinate complexes have been reported.
M(CO)5(η1-(P-P))
Ru(CO)4(η1-(P-P))
9
48 M(CO)4(η2-(P-P)) + CO (2)
48 Ru(CO)3(η2-(P-P)) + CO (3)
9
We have studied the formation of Ru(CO)3(η2-(P-P))
from Ru(CO)4(η1-(P-P)) (eq 3) with a variety of chelating
diphosphines so as to observe whether the presence of
bidentate phosphine ligands with one P-donor atom
already tethered to the metal will introduce some degree
of bond making into the transition state for introduction
of the second P-donor atom. An enhancement of the
associative mechanism, not evident in the reactions
shown in eq 1, could occur as a result of the high “local
concentrations” of the initially nonbonded P-donor atom.
Thorough study of entropies of reaction (∆Sq) and
volumes of activation (∆Vq) have demonstrated that
chelation reactions of bidentate nitrogen compounds on
group six metal carbonyls also proceed via varying
degrees of bond making and bond breaking in the
transition state.9 This work confirms the validity of ∆Sq
measurements as an effective indication of the intimate
reaction mechanism. Negative values of ∆Sq have been
taken to suggest an associative mechanism for the
formation of Os3(CO)10(µ-dppm) from Os3(CO)11(η1-
dppm).11
from ethanol. Bis(diphenylphosphino)methylamine, dppma (g),
was provided by Dr. R. A. Burrow (University of Toronto).
o-Bis(diphenylphosphino)(benzene), dpp-benzene (h ), and
bis(dicyclohexylphosphino)ethane, dcpe (f) (Strem), were re-
crystallized from toluene before use. Liquid bis(dimethylphos-
phino)ethane, dmpe (e) (Strem), was used as received. Phos-
phine purity was determined by 31P {1H} NMR spectroscopy.
Ethylene (99.5%) and carbon monoxide (99.5%) (BOC) were
used as received. Nitrogen (BOC) was passed through a
Driright column to remove moisture. Phosphorus NMR spectra
The results presented in this paper represent the first
reported examples of chelation reactions of diphosphine
ligands on pentacoordinate carbonyls where associative
reactions might be favored for steric reasons.
were recorded on
a Varian 300 MHz spectrometer, and
chemical shifts were referenced to H3PO4 internal standard.
Mass spectrometry data were collected on a Micromass 70S-
250 mass spectrometer in EI mode using 70 eV electrons. X-ray
data were collected on a Nonius Kappa CCD diffractometer
using graphite-monochromated Mo KR radiation (λ ) 0.71073
Å). A combination of 1° phi and omega (with kappa offsets)
scans were used to collect sufficient data. The data frames were
integrated and scaled using the Denzo-SMN package.13 The
structures were solved and refined on F2 using the
SHELXTL\ PC V5.03 package.14
Monodentate complexes Ru(CO)4(η1-(P-P)) (1(a -h )) are
readily prepared in situ by reaction of an excess of bidentate
phosphine ligand (a -h ) with Ru(CO)4(C2H4) generated by
photolysis of an n-heptane solution of Ru3(CO)12 under an
atmosphere of ethylene (Scheme 1).15
Exp er im en ta l Section
All chemical manipulations were carried out under an
atmosphere of nitrogen using standard Schlenk techniques12
unless otherwise stated. Trirutheniumdodecacarbonyl (Strem)
was recrystallized from dichloromethane before use. Solvents
n-heptane (Caledon), hexanes (Uniscience), toluene (Uni-
science), and pentane (Caledon) were distilled over sodium
under a nitrogen atmosphere. Dichloromethane (Analchemia)
was predried over CaCl2 and distilled over P2O5 under nitro-
gen. Solid phosphine ligands (Digital Specialty Chemicals) bis-
(diphenylphosphino)methane, dppm (a); bis(diphenylphosphino)-
ethane, dppe (b); bis(diphenylphosphino)propane, dppp (c); and
bis(diphenylphosphino)butane, dppbu (d ) were recrystallized
Infrared spectra were recorded on a Nicolet 550 Magna
FTIR spectrophotometer in absorbance mode. Kinetic studies
involving air-stable ligands were performed by monitoring
FTIR spectroscopic changes of n-heptane solutions of mono-
dentate complexes in a temperature-controlled infrared cell
(Wilmad Glass) equipped with NaCl windows and a 2 mm cell
path-length. Reactions involving the air-sensitive ligands dmpe
and dcpe were conducted under nitrogen in a Schlenk tube
equipped with a rubber septum cap to allow sample removal
(7) (a) Procter, G. Asymmetric Synthesis; Oxford University Press:
Oxford, 1996. (b) Noyori, R. Asymmetric Catalysis In Organic Synthesis;
Wiley: New York, 1994.
(8) (a) Connor, J . A.; Day, J . P.; J ones, E. M.; McEwen, G. K. J .
Chem. Soc., Dalton Trans. 1973, 347. (b) Connor, J . A.; Riley, P. I. J .
Organomet. Chem. 1975, 94, 55.
(9) (a) Du¨cker-Benfer, C.; Grevels, F.-W.; van Eldik, R. Organome-
tallics 1998, 17, 1669. (b) Bal Reddy, K.; Brady, R. B.; Eyring, E. M.;
van Eldik, R. J . Organomet. Chem. 1992, 440, 113. (c) Bal Reddy, K.;
Hoffmann, R.; Konya, G.; van Eldik, R.; Eyring, E. M. Organometallics
1992, 11, 2319. (d) Zhang, S.; Zang, V.; Dobson, G. R.; van Eldik, R.
Inorg. Chem. 1991, 30, 355. (e) Schneider, K. J .; van Eldik, R.
Organometallics 1990, 9, 1235.
(10) Zang, S.; Dobson, G. R. Inorg. Chem. 1990, 29, 598.
(11) Poe¨, A. J .; Sekhar, V. C. J . Am. Chem. Soc. 1984, 106, 5034.
(12) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-
Sensitive Compounds, 2nd ed.; Wiley: New York, 1986.
(13) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
(14) Sheldrick, G. M. SHELXTL/ PC V5.1; Bruker Analytical X-ray
Systems: Madison, WI, 1997.
(15) (a) J ohnson, B. F. G.; Lewis, J .; Twigg, M. V. J . Organomet.
Chem. 1974, 67, C75. (b) Chen, L.; Poe¨, A. J . Inorg. Chim. Acta 1995,
240, 399.

3676 Organometallics, Vol. 19, No. 18, 2000
Bunten et al.
by means of a gastight syringe with a stainless steel needle.
Samples were frozen in liquid nitrogen and thawed at 0 °C
before spectra were measured. The temperature was held
constant ((0.1 °C) by immersion of the Schlenk tube in a
Lauda (RC-6) constant temperature water bath.
under vacuum, and the product was redissolved in hexanes.
The compound was purified by repeated precipitation, followed
by final crystallization from cold hexanes, yielding dark orange
crystals. IR ν(CO) in n-heptane: 2005 (69), 1934 (62), 1915
(100) cm-1
335).
.
31P{1H} NMR: δ ppm 43.0. MS: 335 (expected
Rate constants, kobs, were determined by monitoring de-
creasing and increasing IR absorbances (Aobs) of the most
intense bands of the reactant and product, respectively.
Convenient temperature ranges were selected for each ligand,
and at least six runs were carried out over a 20-25 °C
temperature range. Rate constants for air-stable compounds
were determined by fitting absorbance data to single-expo-
nential curves using GraFit (ver. 3.09b) data analysis software.
For air-sensitive ligands, rate constants were obtained from
the slopes of ln(Aobs - A∞) vs time plots for decreasing
absorbances or ln(A∞ - Aobs) vs time plots for increasing
absorbances. Kinetic measurements were usually followed over
four half-lives. Activation parameters were obtained by un-
weighted linear least-squares analysis of the dependence of
ln(kobs/T) on 1/T,16 and the standard error in k (σ(kobs)%) was
determined from the scatter of the values of ln(kobs/T) around
the linear Eyring plot.
The rate of formation of the monodentate complexes 1(a -
g) from Ru(CO)4(C2H4) was much faster than the subsequent
ring closure reactions. However, the formation of 1h (P-P )
dpp-benzene) was sufficiently slow, compared with the rate of
chelation, for it to be necessary to use a more labile alkene in
place of ethylene in order to facilitate the formation of the η1-
(P-P) complex. The bulky 1-butene is known to dissociate
considerably faster from Fe(CO)4(alkene) than the ethylene
complex.17 This proved to be so for the Ru(CO)4(alkene)
complexes as well so that Ru(CO)4(η1-(dpp-benzene)) could be
produced in situ without appreciable chelation before spec-
troscopic measurements were recorded.
P r ep a r a tion a n d Cr ysta lliza tion of Ru (CO)3(η2-(P h 2-
P CH2CH2P P h 2)) (2b). High-pressure reactions were carried
out in a 250 mL Parr bomb reactor using procedures outlined
by Wilkinson et al.18 The Parr reactor was charged with
trirutheniumdodecacarbonyl (160 mg, 0.25 mmol), bis(diphen-
ylphosphino)ethane (300 mg, 0.75 mmol), and benzene (25 mL).
The reactor was purged three times with CO gas and pres-
surized to 94 atm. The reaction mixture was heated to 100 °C
for 12 h and was then cooled to room temperature and the
pressure subsequently released. Infrared spectroscopy revealed
no traces of starting material remaining in solution. The pale
yellow solution was transferred quickly to a nitrogen-filled
flask, and the volume was reduced under vacuum until a
precipitate began to form. The solution was cooled by refrig-
eration at 5 °C for 12 h when a yellow precipitate formed. The
solid was collected by filtration and crystallized from benzene/
pentane solution at 5 °C. IR ν(CO) in n-heptane: 2011 (100),
1945 (77), 1923 (92) cm-1 (numbers in parentheses, here and
elsewhere are relative absorbances). 31P{1H} NMR (C6D6): δ
ppm 73.5. Solid state 31P MAS {1H} NMR: δ ppm 73.9, 65.5.
EIMS: 583 (expected 583).
P r ep a r a tion a n d Cr ysta lliza tion of Ru (CO)3(η2-Me2-
P CH2CH2P Me2) (2e). Trirutheniumdodecacarbonyl (30 mg
0.047 mmol) was dissolved in n-heptane (150 mL). The solution
was bubbled with ethylene gas for 20 min and irradiated with
a slide projector lamp until the solution was colorless (∼15
min). A solution of bis(dimethylphosphino)ethane (22 mg, 0.066
mmol) in n-heptane (1 mL) was quickly added, and the
ethylene was removed under vacuum for 2 min. The flask was
recharged with 1 atm nitrogen and placed in a water bath at
75 °C for 1 h. Infrared spectroscopy indicated no starting
material remaining in the solution. The heptane was removed
P r ep a r a t ion a n d Cr yst a lliza t ion of R u (CO)3(η2-(o-
(P P h 2)2C6H4)) (2h ). A Parr reactor was charged with triru-
theniumdodecacarbonyl (100 mg, 0.16 mmol) and o-bis-
(diphenylphosphino)(benzene) (210 mg, 0.47 mmol) in benzene
(50 mL). The reactor was purged three times with CO and
pressurized to 95 atm. The mixture was stirred at 100 °C for
12 h and cooled, and the pressure was slowly released. The
yellow solution was quickly transferred to a dry nitrogen-filled
flask. Infrared spectroscopy revealed no traces of starting
material. The volume was reduced to approximately 50% under
vacuum, and the product was precipitated with hexanes. The
precipitate was redissolved in benzene/hexanes, and yellow
crystals were grown at 5 °C over a period of 48 h. IR ν(CO) in
n-heptane: 2012 (100), 1947 (55), 1927 (80) cm-1
(C6D6): δ ppm 72.5. MS: 632 (expected 632).
.
31P{1H} NMR
P r ep a r a tion a n d Cr ysta lliza tion of Ru 2(CO)3(µ2-P P h 2)-
(µ-P h 2P NMeP P h 2)(C(O)NMeP P h 2) (3). A 500 mL flask was
charged with trirutheniumdodecacarbonyl (50 mg, 0.08 mmol)
and dry n-heptane (300 mL). The solution was bubbled with
ethylene for 30 min followed by irradiation with a slide
projector lamp until the solution turned colorless (∼15 min).
Bis(diphenylphosphino)methylamine (96 mg, 0.24 mmol) was
added to the clear solution, and the ethylene was quickly
removed under vacuum over a period of 5 min. The resulting
solution was stirred overnight at 50 °C before being concen-
trated to 50 mL. IR ν(CO) bands at 2010 (100), 1946 (60), and
1927 (90) cm-1 confirmed the presence of Ru(CO)3(dppma), 2g.
The yellow product was precipitated with pentane and washed
three times. Attempts to crystallize Ru(CO)3(dppma) by slow
diffusion of pentane into a CH2Cl2 solution resulted in the slow
decomposition of 2g, forming product 3 over a period of 1
month as yellow crystals suitable for X-ray analysis. IR ν(CO)
in CH2Cl2: 2057 (28), 1987 (100), 1960 (51), 1921 (23) cm-1
.
31P{1H} NMR: δ ppm 159.8 (q), 110.5 (q), 96.9 (m), 62.7 (d).
MS: 1085 (1141 - 2CO).
Resu lts
Ch a r a cter iza tion . The monodentate complexes 1-
(a -h ), and the corresponding chelate complexes, were
characterized by infrared and solution 31P{1H} NMR
spectroscopy at 25 °C (Tables 1 and 2). Solid state 31P
MAS {1H} NMR spectroscopy was performed on 2b and
revealed two peaks at 73.9 and 63.5 ppm (Figure 1).
The chelate compounds undergo slow decomposition
in solution under nitrogen and in air, but 2g exclusively
reacts to form 3 in solution. The other chelate complexes
decompose to unresolved product mixtures (Scheme 1).
X-ray crystallography was used to elucidate the solid
state structures of 2b, 2e, 2h , and 3, which were isolated
in high yields. Data for the crystal structure analysis
are given in Table 3. Compound 2b (Figure 2) exists as
a slightly distorted trigonal bipyramid structure with
a phosphine bite angle of 82.66(4)°, which is comparable
to other complexes with the dppe ligand.5b,19 Two
molecules of benzene crystallized in the lattice. The
equatorial-phosphorus equatorial-carbon angles mea-
sured 117.87(11)° and 122.80(10)°, and the angle be-
tween the axial phosphorus and axial CO ligand is
174.16(10)°. The angles between the equatorial and
(16) Espenson, J . H. Chemical Kinetics and Reaction Mechanisms;
McGraw-Hill: New York, 1981.
(17) Cardaci, G. Int. J . Chem. Kinet. 1973, 5, 805.
(18) Sanchez-Delgado, R. A.; Bradley, J . S.; Wilkinson, G. J . Chem.
Soc., Dalton Trans. 1976, 399.
(19) Gargulak, J . D.; Berry, A. J .; Noirot, M. D.; Gladfelter, W. L.
J . Am. Chem. Soc. 1992, 114, 8933.

Chelation Kinetics of Bidentate Phosphine Ligands
Organometallics, Vol. 19, No. 18, 2000 3677
Ta ble 1. In fr a r ed Sp ectr a of Ru (CO)4(η1-(P -P )) a n d Ru (CO)3(η2-(P -P )) in n -Hep ta n ea
Ru(CO)4(η1-(P-P))
Ru(CO)3(η2-(P-P))
(P-P)
dppm
dppe
dppp
dppbu
dmpe
dcpe
dppma
dpp-benzene
ν1
ν2
ν3
ν1
ν2
ν3
2060 (91)
2061 (79)
2061 (91)
2061 (86)
2061 (57)
2056 (76)
2061 (95)
2059 (72)
1987 (40)
1988 (33)
1987 (34)
1987 (40)
1986 (24)
1980 (25)
1988 (44)
1988 (89)
1957 (100), 1945 (84)
1954 (100), 1947 (92)
1955 (100), 1946 (98)
1955 (100), 1946 (90)
1947 (100)
1939 (100)
1956 (92), 1947 (100)
1962 (80), 1946 (100)
2010 (100)
2011 (100)
2014 (57)
2010 (43)
2005 (69)
1997 (75)
2010 (100)
2012 (100)
1945 (76)
1945 (77)
1938 (57)
1933 (44)
1934 (62)
1922 (94)
1946 (60)
1947 (55)
1931 (77)
1924 (92)
1913 (100)
1908 (100)
1915 (100)
1904 (100)
1927 (90)
1927 (80)
a
Relative intensities are in parentheses.
F igu r e 1. Solid state 31P MAS {1H} NMR spectrum of Ru(CO)3dppe. Inset: Expansion of central doublet.
Ta ble 2. Solu tion 31P {1H} NMR Ch em ica l Sh ift
that 3 is a dimeric ruthenium complex, in which the
two Ru atoms are connected by a bridging dppma ligand
and a diphenylphosphido unit. The structure incorpo-
rates a molecule of hexane cocrystallized in the lattice
(Figure 4).
Da ta (δ31P (p p m )) of th e Ru th en iu m Com p lexes in
n -Hep ta n e w ith C6D6 In ser t
ligand
dmpe
free ligand
monodentate
chelate
43.0
-48.4
14.8 (2), -47.0 (2)
J pp ) 30 Hz
Kin etic Stu d ies. Infrared ν(CO) spectral changes
were monitored over the course of reaction as shown in
Figure 5. The slow decomposition of the chelate com-
plexes after approximately 80% completion of the reac-
tion was accompanied by the slow decrease in product
peak intensity. For 2e, a shoulder at 1900 cm-1 forms
as the run progresses. This compound was isolated, but
attempts to grow crystals suitable for X-ray crystal-
lography were unsuccessful due to slow product decom-
position in solution. No further attempts to purify this
compound were undertaken. The A∞ values for product
growth were, therefore, approximated by the highest
absorbance value before appreciable peak intensity
losses began. First-order rate constants for the growth
of product agree with those for disappearance of the
monodentate complex. Rate constants are given in Table
5, and their temperature dependence exhibited good
Eyring behavior (Figure 6), only the more precise
constants obtained from loss of reactant complex being
used. Activation parameters are shown in Table 6.
dppe
dcpe
dppm
-12.3
1.0
44.9 (2), -12.4 (2)
J pp ) 40 Hz
73.5
90.5
54.0 (2), 1.9 (2)
J pp ) 36 Hz
-22.0
39.3 (2), -25.7 (2)
J pp ) 91 Hz
-14.2
dppma
dppp
dppbu
dpp-benzene
73.2
-17.7
-16.6
-12.9
N/Aa
72.8
24.1
36.2
72.5
39.7 (1), -19.1 (1)
40.0 (1), -16.6 (1)
N/Aa
a
The rapid rate of chelation reaction and dilute sample
concentration prevent measurement of the chemical shift of the
monocoordinated complexes.
axial atoms are all near 90°. The complex 2h (Figure 3)
also crystallized as slightly distorted trigonal bipyra-
mids with similar bond lengths and angles (Table 4).
The measured Ru-P and Ru-C bond lengths and the
P-Ru-P bite angles for 2b, 2e, and 2h all fall within
the range for complexes having the molecular fragment
(CO)Ru(η2-R2P(CH2)2PR2), as indicated by the Cam-
bridge Structure Database.20
(20) Allen, F. H.; Kennard, O. 3D Search and Research Using the
Cambridge Structural Database. Chem. Des. Automation News 1993,
8 (1), 1, 31-37.
Compound 2g slowly converts to 3 in n-heptane over
a period of several days. X-ray crystallography indicates

3678 Organometallics, Vol. 19, No. 18, 2000
Ta ble 3. Cr ysta l Da ta a a n d Str u ctu r e Refin em en t for 2b, 2h , a n d 3
Bunten et al.
2b‚2(C6H6)
41H36O3P2Ru
2h
3‚(C6H14
)
empirical formula
fw
C
C33H24O3P2Ru
631.53
C29H26.50NO2.50P2Ru
592.02
739.71
temp (K)
150(2)
100.0(1)
150(2) K
cryst syst
space group
unit cell dimens
triclinic
P1h
monoclinic
P2(1)/c
a ) 13.5042(4) Å
b ) 13.4521(4) Å
c ) 15.8184(5) Å
R ) 90°
â ) 99.413(2)°
γ ) 90°
2834.9(10)
4
1.480
0.699
monoclinic
P2(1)/n
a ) 12.06580(10) Å
b ) 22.3699(4) Å
c ) 19.2425(3) Å
R ) 90°
â ) 91.4510(10)°
γ ) 90°
5192.09(13)
8
1.515
0.757
a ) 9.478(2) Å
b ) 11.899(2) Å
c ) 17.596(4) Å
R ) 102.10(3)°
â ) 94.25(3)°
γ ) 112.73(3)°
1762.9(6)
2
volume (Å3)
Z
density(calcd) (Mg/m3)
1.394
0.573
760
abs coeff (mm-1
F(000)
)
1280
2412
cryst size (mm)
θ range for data
collection (deg)
0.22 × 0.18 × 0.12
0.15 × 0.12 × 0.10
0.35 × 0.25 × 0.20
4.23-26.41
4.13-26.38
2.48-26.38
no. of reflns collected
no. of ind reflns
7025
19 820
71 475
7025 [R(int) ) 0.0000]
7025/0/425
1.043
R1 ) 0.0393
wR2 ) 0.1079
0.625
5766 [R(int) ) 0.043]
5764/0/352
1.096
R1 ) 0.0344
wR2 ) 0.0832
0.566
10381 [R(int) ) 0.056]
10381/9/639
1.109
R1 ) 0.0411
wR2 ) 0.1116
1.927
no. of data/restraints/params
goodness-of-fit on F2
final R indices [I>2σ(I)]
R indices (all data)
largest diff peak (Å-3
)
a
The X-ray structure of 2e has been completed, and the structural parameters are completely comparable to 2b and 2h . The complete
structural details have been deposited.
F igu r e 3. ORTEP view of molecule 2h with 30% prob-
ability ellipsoids. H atoms were omitted for clarity.
Ta ble 4. Selected Bon d An gles for 2b, 2h , a n d 4a
2b
2h
4
P(1)-Ru(1)-P(2)
C(3)-Ru(1)-P(1)
C(2)-Ru(1)-P(1)
C(3)-Ru(1)-P(2)
C(2)-Ru(1)-P(2)
C(1)-Ru(1)-P(1)
82.66(4)
91.67(11)
89.81(10)
122.80(10)
117.87(11)
174.20(10)
81.31(3)
89.34(10)
90.58(10)
109.33(10)
129.22(10)
175.09(9)
81.89(3)
94.57(9)
91.35(10)
101.10(9)
143.49(11)
163.55(9)
F igu r e 2. ORTEP view of molecule 2b with 30% prob-
ability ellipsoids. H atoms and the free benzene molecules
were omitted for clarity.
Discu ssion
a
Reference 25.
Ch a r a cter iza tion of Rea cta n ts a n d P r od u cts.
Formation of the alkene complex from Ru3(CO)12 is
quantitative, as shown by the absence of all infrared
bands due to the starting material. The rapid displace-
ment of the alkene from Ru(CO)4(alkene) must involve
formation of the tetracarbonyl Ru(CO)4(η1-(P-P)), as is
the case for substitutions involving monodentate P-
donor ligands.15b Infrared and 31P{1H} spectra are in
full agreement with this. The infrared spectra (Table
1) show three-band patterns typical of monosubstituted
axial trigonal bipyramid compounds4a,21 and with one
of the bands being broad and usually split. With the
exception of dmpe and dcpe, the ν3 mode is split, usually
as a result of slight distortions from C3v symmetry. The
former, however, exhibit broad ν3 stretching modes to
account for this loss of symmetry. The spectra show no
additional peaks corresponding to equatorially substi-
tuted phosphine.4a This is usually observed for ligands
which are good π-acceptors.22
(21) Darensbourg, D. J .; Nelson, H. H.; Hyde, C. L. Inorg. Chem.
1974, 9, 2135.

Chelation Kinetics of Bidentate Phosphine Ligands
Organometallics, Vol. 19, No. 18, 2000 3679
Ta ble 5. Ra te Con sta n tsa for th e Ch ela tion
Rea ction a t Va r iou s Tem p er a tu r es in n -Hep ta n e
T (K)
104 kobs (s-1
)
dppm
318.47
322.58
327.87
333.33
335.57
337.84
337.84
1.82
3.32
6.08
10.7
15.2
19.7
20.2
dppe
322.58
327.87
333.33
335.57
337.84
340.14
342.47
1.28
2.47
4.42
5.46
8.63
10.0
13.7
dppp
318.47
323.62
328.95
335.57
338.98
343.64
343.64
0.928
1.71
F igu r e 4. ORTEP view of molecule 3 with 30% probability
ellipsoids. The C6H14 molecule and H atoms were omitted
for clarity.
3.29 (3.84)b
6.29 (5.66)b
10.8
21.1 (24.1)b
21.3
dppbu
322.58
328.95
332.23
337.84
337.84
343.64
343.64
1.64
4.08
5.73
10.3
10.3
17.3
17.8
dmpe
318.47
323.62
327.87
327.87
327.87
333.33
337.84
343.64
343.64
348.43
0.242c
0.522c
0.930d
0.909c
0.926e
2.04c
F igu r e 5. Typical spectral changes for chelation reaction.
L ) dppm; T ) 60 °C. The arrows denote increasing and
decreasing absorbances.
3.34c
7.06c
7.32c
Solution 31P{1H} NMR spectroscopy (Table 2) of the
monodentate complexes exhibits a typical doublet of
doublets pattern when the bridging unit contained two
carbon atoms or less. Coupling constants between the
coordinated phosphorus atom and the free phosphine
are 30-91 Hz. The ligands dppp and dppbu show no
phosphorus coupling due to the longer alkyl bridge. The
monocoordinate complexes of dppma and dpp-benzene
were not recorded because the ligand chelated too
rapidly for NMR spectroscopy of the tetracarbonyl to
be possible. The uncoordinated end of the phosphine
ligand typically exhibits a chemical shift very close to
that of the free phosphine ligand. The coordinated
phosphorus peaks were shifted downfield from the free
ligand by 63-53 ppm. Chemical shifts of the coordinated
phosphine where n ) 1-4 and R ) Ph were relatively
constant and close to 40 ppm. When n ) 1 and 2, P-P
coupling constants of 91 and 40 Hz were observed,
respectively. The free phosphorus atoms of the ligand
still exhibit chemical shifts very close to those of the
ligands in free solution. The fact that we observe two
distinct phosphorus resonance signals and coupling
between the free and coordinated ends of the ligand
1.23c
dcpe
313.48
318.47
323.62
327.87
333.33
337.84
0.468
0.886
1.78
3.98
7.93
13.9
dppma
284.09
293.26
298.51
303.03
307.69
313.48
0.689
2.60
4.61
8.64
15.8
27.3
dpp-benzene
277.01
284.09
289.02
293.26
4.30
15.0
25.0
58.9
a
Only data from reactant loss were used for the Erying analysis.
From single-exponential increase of product. c [dmpe] ) 0.009
b
M. [dpme] ) 0.004 M. e [dmpe] ) 0.018 M.
d
indicates that the ligand is not undergoing any rapid
exchange processes in solution. Infrared and phosphorus
NMR spectroscopic data suggest that the monocoordi-
nated complexes are axially substituted trigonal bi-
(22) (a) Martin, L. R.; Einstein, F. W. B.; Pomeroy, R. K. Inorg.
Chem. 1983, 22, 1961. (b) Martin, L. R.; Einstein, F. W. B.; Pomeroy,
R. K. Inorg. Chem. 1985, 24, 2777.

3680 Organometallics, Vol. 19, No. 18, 2000
Bunten et al.
pyramid with axial-equatorial phosphorus and a square
pyramid with an apical CO ligand.25 In attempting to
grow crystals of the decomposition product of 2b in
dichloromethane/hexanes, we grew additional crystals
of the chelate complex which crystallized in the same
space group and still retained its distorted trigonal
pyramidal geometric orientation, but no solvent mol-
ecules crystallized in the lattice. These results are
further evidence of the highly flexible nature of the
molecule, which can crystallize in different orientations
and crystal lattices, depending on the crystal growing
conditions.
Complex 3 (Figure 4) forms from the slow reaction of
2g in n-heptane solution. The mechanism of this trans-
formation is yet unknown, but we suspect it involves
initial cleavage of a weak phosphorus-nitrogen bond,26
followed by attack of the free nitrogen on one of the
carbonyl ligands. This activated compound can in turn
dimerize with another molecule of 2g to form 3. No
further study into this process has been undertaken.
Otherwise the chelate products Ru(CO)3(η2-(P-P)) usu-
ally decompose slowly in solution and solid state,
forming inseparable product mixtures.
Kin etic An a lysis. The chelation reactions proceed
cleanly in solution, as evidenced by good isobestic points
throughout the first 80% of the reaction, and a typical
kinetic run is shown in Figure 5. The slow decomposi-
tion of the chelate products does not affect our kinetic
results since the rate of loss of starting material equals
the initial rate of loss of products provided we use
estimated values of A∞ that lie near the maximum
absorbance of chelate product before appreciable de-
composition takes place. Decreasing absorbances due to
reactant complex give excellent fits to single-exponential
first-order behavior, and the derived rate constants
show no dependence on the concentration of the free
ligand as expected for the intramolecular conversion of
the ligand from the monodentate to bidentate mode of
binding. The rate constants fit well to Eyring plots
(Figure 6), and the standard errors of measurement
derived from the scatter of the values of ln(kobs/T)
around the least-squares line show that the data are of
high precision.
F igu r e 6. Eyring plots for the chelation reactions.
Ta ble 6. Activa tion P a r a m eter s for th e Rin g
Closu r e Rea ction in n -Hep ta n e
a
∆Hq
∆Sq
104 kobs σkobs
(60 °C)
ligand
(kcal mol-1
)
(cal mol-1 K-1
)
(%)
dmpeb
28.0 ( 0.3
25.7 ( 0.3
25.3 ( 0.4
26.1 ( 0.3
28.6 (0.4
25.1 ( 0.4
21.0 ( 0.2
24.2 ( 1.4
8.3 ( 1.0
3.1 ( 1.5
2.6 ( 1.3
5.0 ( 1.0
12.8 ( 1.2
3.0 ( 1.3
-3.2 ( 1.0
15.0 ( 4.9
1.86
4.44
5.75
6.73
7.49
dppeb
dppbub
dpppb
3.8
dcpeb
}
dppmb
11.2
242
9770
dppmab
dpp-benzenec
13
a
Standard error of measurement of kobs obtained by the method
of pooled variances. All unweighted Eyring analyses implicitly
involve the assumption that the values of kobs have a percent
b
uncertainty that is independent of temperature. Errors adjusted
according to σkobs ) 3.8%. c Errors correspond to σkobs ) 13%.
pyramids with no evidence of equatorially substituted
products.
The solution FTIR spectra of the chelate complexes
(Table 1) exhibit three bands of similar intensities as
expected for axial-equatorial-substituted bidentate phos-
phine complexes, but one of the bands is broad and
generally split. The ν(CO) values vary only slightly from
ligand to ligand. Constraints of the bidentate phosphine
bite angles (<90°) help to explain the absence of an
equatorial-equatorial binding orientation.23 Solution
31P{1H} NMR spectroscopy (Table 3) of the chelate
complexes is characterized by one singlet downfield from
the chemical shift of the free ligand. This indicates that
these complexes are highly fluctional in solution, pos-
sibly via a Berry psudorotation process.24 Attempts to
slow the fluctional process at temperatures as low as
-90 °C were unsuccessful. However, solid state 31P{1H}
NMR (Figure 1) on 2b clearly shows two phosphorus
peaks corresponding to axial and equatorial phosphorus
atoms, although their assignments are not possible on
the basis of this evidence alone.
One exception to this is with the ligand dppma
because the rate of chelation was comparable to the rate
of displacement of the ethylene from the ethylene
complex. The very low solubility of dppma in n-heptane
prevented the use of large excesses of ligand, and we
therefore had to fit our data to double-exponential
curves, with the second rate constant corresponding to
the chelation reaction and the first to the rate of
introduction of the dppma. As expected, the reaction
rates for chelation were independent of free ligand
concentration.
The solid phase axial-equatorial structure was con-
firmed by X-ray crystallography on 2b (Figure 2), 2e,
and 2h (Figure 3). Recently, a crystal structure of 2b
was independently solved (4) by Gladfelter et al.,25 but
the geometrical orientation of the phosphine ligands was
considerably different (Table 4). Gladfelter’s structure
was described as intermediate between a trigonal bi-
The uncertainty in kobs was calculated by applying the
method of pooled variances to all the Eyring analyses.27
Each value of kobs was weighted according to the
assumption of a common percent standard error σ(kobs
)
for all complexes irrespective of temperature.28 The
(25) Skoog, S. J .; J orgenson, A. L.; Campbell, J . P.; Douskey, M. L.;
Munson, E.; Gladfelter, W. L. J . Organomet. Chem. 1998, 557, 13.
(26) Browning, C. S.; Farrar, D. H. J . Chem. Soc., Dalton Trans.
1995, 2005.
(27) Mandel, J . The Statistical Analysis of Experimental Data;
Interscience: New York, 1964.
(23) Dierkes, P.; Van Leeuwen, P. W. N. M. J . Chem. Soc., Dalton
Trans. 1999, 1519.
(24) Shriver, D. F.; Atkins, P. W.; Langford, C. H. In Inorganic
Chemistry, 2nd ed.; W. H. Freeman and Co.: New York, 1994; Chapter
7.

Chelation Kinetics of Bidentate Phosphine Ligands
Organometallics, Vol. 19, No. 18, 2000 3681
error was estimated from σ(kobs) ) 100((∑∆2)/(N - n))1/2
where ∆ ) (kobs - kcalc)/kcalc, N ) total number of
measurements of kobs, n ) the total number of param-
eters obtained from the experiment (two for each Eyring
analysis), and the values of kcalc are obtained from the
least-squares analyses. This method allows for a greater
number of degrees of freedom, N - n, to be used in the
estimates of σ(kobs), and, therefore, smaller “student’s
t” factors are used to calculate any particular confidence
level. The constant percentage uncertainty irrespective
of temperature has been shown elsewhere to be gener-
ally valid.29 This is equivalent to assuming a constant
absolute error in ln(kobs/T), an assumption implicit in
all unweighted least-squares Eyring analyses. It should
be noted that values of ∆ for dpp-benzene were not
included in the pooled Eyring analysis but were treated
separately. The significantly faster rate of reaction lead
to a higher standard error than found for the other
ligands.
Sch em e 2. Sch em a tic Dep ictin g Bon d Ma k in g a n d
Bon d Br ea k in g in th e Tr a n sition Sta tes
Sch em e 3. Geom etr ic Or ien ta tion s of Ch ela tin g
Liga n d s Atta ck in g th e Cen tr a l Ru th en iu m Atom a
The errors in ∆Hq and ∆Sq were calculated from the
pooled value of σ(kobs) and obey “the rule of three”,30
which is based on the fact that the free energies of
activation (∆Gq) have a very small error. Therefore, the
error in the enthalpy of activation ∆(∆Hq) will be equal
to the error in T∆Sq (T∆(∆Sq)), and ∆(∆Sq) will be equal
to 1000/T units of ∆(∆Hq), or ∼3∆(∆Hq).31
Th e In tim a te Mech a n ism . Although the stoichio-
metric mechanism involves the intramolecular displace-
ment of CO from Ru(CO)4(η1-(P-P)), the question of the
extent, if any, of Ru-P bonding in the transition state
(i.e., the intimate mechanism) has to be addressed by
examining the trends in the activation parameters. The
rate constants at 60 °C increase with changing ligand
along the series dmpe (0.4) < dppe (1.0) < dppbu (1.3)
< dppp (1.5) < dcpe (1.7) < dppm (2.5) , dppma (54.4)
, dpp-benzene (2200). The changes from dmpe to dppm
are quite small, and the only major distinction involves
dppma and dpp-benzene. However, the relative rates
are the result of more significant, but often opposing,
changes in ∆Hq and T∆Sq at 60 °C (Table 6). The values
a
(a) dpp-benzene is sterically crowded, and the free PPh2
prefers to point away from the metal. (b) dppma is not very
large and is capable of positioning the free PPh2 group near
the metal center.
K-1 mol-1, and all react dissociatively.4a Only when the
bidentate phosphines in Ru(CO)4(η1-(P-P)) are dcpe and
dpp-benzene do the reactions have such positive values
of ∆Sq, and we conclude that complexes with these
ligands are reacting mainly by a Ru-CO dissociative
mechanism. In the case of P-P ) dcpe this is likely to
be associated with the large size of the C6H11 groups,
which must destabilize the ground state toward CO
dissociation and disfavor associative attack by the free
P-donor atom. This effect must completely override the
tendency of the basic entering PCy2 group to undergo
an associative type of reaction. The dissociative nature
of the reaction of Ru(CO)4(dpp-benzene) also probably
arises from ground state steric effects. The substitution
of an ortho-hydrogen by a PPh2 group in one of the Ph
rings of PPh3 hardly affects the value of ∆Sq (∆Sq for
Ru(CO)4PPh3 ) 17.3 ( 0.8 cal K-1 mol-1),4a but ∆Hq
decreases by almost 6 kcal mol-1. Other large ligands
such as PCy3 and PtBu3 also significantly decrease ∆Hq
for CO dissociative reactions on Ru(CO)4L compared
with smaller ligands. The o-PPh2 group in Ru(CO)4(dpp-
benzene) is probably oriented away from the Ru center,
also for steric reasons (Scheme 3). In fact, the dpp-
benzene complex reacts dissociatively at a rate that
suggests its effective size is even greater than PtBu3.4a
of ∆Hq vary by 9 kcal mol-1 and T∆Sq by 6 kcal mol-1
.
The slowest reaction has the highest ∆Hq, whereas the
fastest reaction has the most positive ∆Sq and the
second lowest ∆Hq. The relative rates are not, therefore,
likely to shed much light on the intimate mechanism of
these chelation reactions.
On the other hand, the activation parameters do help
us to consider what contribution, if any, from Ru-P
bonding there might be in the transition state (Scheme
2). Purely CO dissociative reactions are characterized
by ∆Sq values ∼16 cal K-1 mol-1 or higher.2 Reactions
of Ru(CO)4L (L ) CO or monodentate P-donors) have
∆Sq values ranging from 12.6 ( 0.6 to 18.3 ( 0.5 cal
(28) Poe¨, A. J .; Sekhar, V. C. Inorg. Chem. 1985, 24, 4376.
(29) (a) Poe¨, A. J .; Vuik, C. P. J . J . Chem. Soc., Dalton Trans. 1972,
2250. (b) Poe¨, A. J .; Vuik, C. P. J . Can. J . Chem. 1975, 53, 1842. (c)
Poe¨, A. J .; Vuik, C. P. J . Inorg. Chem. 1980, 19, 1771.
(30) Poe¨ A. J . In Mechanisms of Inorganic and Organometallic
Reactions; Twigg, M. V., Ed.; Plenum Press: New York, 1983; Vol. 8,
Chapter 10.
(31) We take this opportunity to demonstrate this phenomenon
because of the misinformed view, sometimes expressed, that the
extrapolation of Eyring plots to 1/T ) 0 is so long that entropies of
activation are extremely imprecise and cannot have any valid use. If
∆Sq values are obtained from the gradients of the linear plots of ∆Gq
against T (a classical definition of entropy changes), then no “long
extrapolation” is involved, yet the value of ∆Sq and its uncertainty are
essentially the same.

3682 Organometallics, Vol. 19, No. 18, 2000
Bunten et al.
Ta ble 7. Activa tion P a r a m eter s for th e Ch ela tion
This is not surprising in view of the very large ortho
substituent on one of the phenyl rings of PPh3 (Scheme
3).
Rea ction s of M(CO)5(η1-(P -P ))a
M
P-P
∆Hq (kcal mol-1
)
∆Sq (cal mol-1 K-1
)
At the other extreme Ru(CO)4(η1-dppma) seems to
react by an appreciably associative intimate mechanism
because the value of ∆Sq is at least 15 cal K-1 mol-1
more negative than for a purely dissociative reaction
and the value of ∆Hq is exceptionally low. Most of the
other complexes also have appreciably low values of ∆Sq,
suggesting that they all have significant contributions
from Ru-P bond making in the transition state. This
Ru-P bond making has to be considered as a description
of an “average event” since reactions could occur by
mixtures of purely dissociative and associative processes
in proportions that change systematically with the
nature of the P-P ligand. We can list the reactions in
order of increasing associative character as follows:
P-P ) dpp-benzene < dcpe < dmpe < dppp e dppm ≈
dppbu ≈ dppe , dppma. Apart from the proposed steric
effects mentioned above, the order of increasing associa-
tive character is not easy to rationalize because of the
very nature of the bidentate ligands in Ru(CO)4(η1-(P-
P)). The more basic the coordinated P-donor atom is,
the more difficult Ru-CO bond breaking might be
expected to be,4a,32 but this effect is probably small.4a
The corresponding noncoordinated P-donor atom would
be expected to be more nucleophillic, but the effect of
the bonded P-donor atom could offset this. A mixture
of steric and electronic effects would also complicate this
already complicated picture. In this case, the ligand that
reacts with the most associative character is distin-
guished by having an N-Me group instead of (CH2)n
groups joining the two PR2 ends. This may be due to
this ligand’s potential to bind to the Ru center in a
fashion allowing for close attack of the incoming phos-
phorus atom (Scheme 3).
Cr
dmpe
dppm
dppe
dcpe
dmpe
dppe
dcpe
31
34
32
35
28
28
31
37
1.5
17
6.4
9.2
-1.7
3.3
Mo
8.1
9.5
W
dppe
a
Reference 8.
Con clu sion
Chelation kinetics of a variety of bidentate phosphine
ligands have been studied for a number of five-
coordinate Ru(CO)4(η1-(P-P)) complexes. Entropies of
activation range from -3.2 to 15 cal mol-1 K-1 and
indicate a broad range of bond making vs bond breaking
in the transition states. Volumes of activation (∆Vq) are,
unfortunately, not available, but there are difficulties
associated with such reactions involving phosphorus
donors. The UV-vis absorbance changes, which usually
have to be monitored rather than FTIR when reactions
under high pressures are followed, are very small. Van
Eldik et al.9 have measured the kinetics of ring closure
reactions involving bidentate nitrogen donors in com-
plexes where absorbance changes are considerably
larger, and they found a positive correlation between
values of ∆Sq and ∆Vq. We therefore believe that we can
rely on our ∆Sq values as providing a reliable insight
into the intimate mechanisms of chelation, but a de-
tailed rationalization of the observed trends is harder
to find because of conflicting effects of changing donor
and free P atoms in the Ru(CO)4(η1-(P-P)) complexes.
Ack n ow led gm en t. We wish to thank the University
of Toronto and the Natural Sciences and Engineering
Research Council, Canada, for financial support, and
Digital Specialty Chemicals for the donation of phos-
phorus ligands. Dr. Alex Young is thanked for making
mass spectroscopic measurements.
A comparison of these results can be made with those
of Connor et al. (Table 7)8 on octahedral complexes of
Cr, Mo, and W. On the basis of the values of ∆Sq,
reaction of Ru(CO)4(η1-dmpe) has much less bond mak-
ing than that of M(CO)5(η1-dmpe) (M ) Mo or Cr). On
the other hand, Ru(CO)4(η1-dppm) has much more bond
making than Cr(CO)5(η1-dppm), which reacts mainly by
Cr-CO bond breaking. Reactions of M(CO)5(η1-dppe) (M
) Cr, Mo, and W) also exhibit the well-known “triad
effect”.3
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal-
lographic parameters, positional and thermal parameters, and
complete bond distances and angles of 2b, 2e, 2h , and 3. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(32) Sowa, J . R.; Angelici, R. J . Inorg. Chem. 1991, 30, 3534.
OM000289T