Enthalpies of reaction of [(p-cymene)OsCl2]2 with monodentate tertiary phosphine ligands. Importance of steric and electronic ligand factors in an osmium(II) system

Article abstract of DOI:10.1021/om980345e

The enthalpies of reaction of [OsCl₂(p-cymene)]₂ (1) (p-cymene = η⁶-CH₃C₆H₄CH(CH ₃)₂) with a series of monodentate phosphines (PR₃), leading to the formation of OsCl₂(p-cymene)-(PR₃) complexes, have been measured by anaerobic calorimetry in CH₂Cl₂ at 30°C. These reactions are rapid and quantitative. The overall relative order of stability established is as follows: PPh(NC₄H₄)₂ < P(NC₄H₄)₃ < P(p-CF₃C₆H₄)₃ < PPh₂(NC₄H₄) < PPh₃ < P(p-ClC₆H₄)₃ < P(p-CH₃C₆H₄)₃ < P(p-FC₆H₄)₃ < PCy₃ < PBz₃ < PⁱPr₃ < P(p-CH₃OC₆H₄)₃ < P(NC₄H₈)₃ < PPh₂Me < PPhMe₂ < PEt₃ < PMe₃. A quantitative analysis of ligand effects for the present data helps clarify the exact steric versus electronic ligand contributions to the enthalpy of reaction in this system. Both steric and electronic factors appear to play an important role in dictating the magnitude of the enthalpy of reaction. The data are compared to the previously studied RuCl₂(p-cymene)(PR₃) system.

Full text of DOI:10.1021/om980345e

4004
Organometallics 1998, 17, 4004-4008
En th a lp ies of Rea ction of [(p-cym en e)OsCl2]2 w ith
Mon od en ta te Ter tia r y P h osp h in e Liga n d s. Im p or ta n ce
of Ster ic a n d Electr on ic Liga n d F a ctor s in a n
Osm iu m (II) System
J inkun Huang, Scafford Serron, and Steven P. Nolan*,†
Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148
Received May 4, 1998
The enthalpies of reaction of [OsCl2(p-cymene)]2 (1) (p-cymene ) η6-CH3C6H4CH(CH3)2)
with a series of monodentate phosphines (PR3), leading to the formation of OsCl2(p-cymene)-
(PR3) complexes, have been measured by anaerobic calorimetry in CH2Cl2 at 30 °C. These
reactions are rapid and quantitative. The overall relative order of stability established is
as follows: PPh(NC4H4)2 < P(NC4H4)3 < P(p-CF3C6H4)3 < PPh2(NC4H4) < PPh3 < P(p-
ClC6H4)3 < P(p-CH3C6H4)3 < P(p-FC6H4)3 < PCy3 < PBz3 < PiPr3 < P(p-CH3OC6H4)3 <
P(NC4H8)3 < PPh2Me < PPhMe2 < PEt3 < PMe3. A quantitative analysis of ligand effects
for the present data helps clarify the exact steric versus electronic ligand contributions to
the enthalpy of reaction in this system. Both steric and electronic factors appear to play an
important role in dictating the magnitude of the enthalpy of reaction. The data are compared
to the previously studied RuCl2(p-cymene)(PR3) system.
In tr od u ction
of supporting various ancillary ligands.11 Metal poly-
hydrides incorporating the arene-Os fragment have
been synthesized and characterized.12 Synthetic and
spectroscopic studies have been performed on the (p-
cymene)OsX2(PR3) (X ) halide and alkyl) complexes by
von Philipsborn (eq 1).13
Fewer synthetic and reactivity studies have been
performed on organoosmium systems when compared
to the extensive amount of work performed on organo-
ruthenium systems.1,2 Notable exceptions are the work
of Roper,3 Caulton,4 and Esteruelas5 on osmium-phos-
phine/carbonyl systems. Girolami6 has recently pre-
pared an attractive synthon, [Cp*OsBr2]2, enabling
access to the reaction chemistry of the Cp*Os moiety.7
The osmium systems bearing arene ligands that have
been synthesized by Maitlis8 and Werner9 have led to a
number of synthetic and physical studies. The latter
system has displayed interesting C-H bond activation
behavior in low-temperature matrixes10 and is capable
Demonceau has recently demonstrated that [(p-cy-
mene)OsCl2]2 (1) is an active precatalyst in the cyclo-
propanation of olefins (eq 2).14,15
† E-mail: spncm@uno.edu.
(1) 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.
(2) For Os coordination compound-mediated organic transformation
see: Winemiller, M. D.; Kopach, M. E.; Harman, W. D. J . Am. Chem.
Soc. 1997, 119, 2096-2102, and references cited.
(3) Clark, A. M.; Rickard, C. E. F.; Roper, W. R.; Wright, L. J . J .
Organomet. Chem. 1997, 546, 619-622, and references cited.
(4) Spivak, G. J .; Coalter, J . N.; Olivan, M. Eisenstein, O.; Caulton,
K. G. Organometallics 1998, 17, 999-1001, and references cited.
(5) Bohanna, C.; Callejas, B.; Edwards, A. J .; Esteruelas, M. A.;
Lahoz, F. J .; Oro, L. A.; Ruiz, N.; Valero, C. Organometallics 1998,
17, 373-381.
The catalytic behavior of 1 proved a marked improve-
ment over the use of its ruthenium congener, [(p-
cymene)RuCl2]2.
(11) (a) Michelman, R. I.; Andersen, R.; Bergman, R. G. J . Am.
Chem. Soc. 1991, 113, 5100-5102. (b) Kvietok, F.; Carperos, V. A.;
Rakowski-Dubois, M. Organometallics 1994, 13, 60-68.
(12) Heinekey, D. M.; Harper, T. G. P. Organometallics 1991, 10,
2891-2895.
(6) Gross, C. L.; Wilson, S. R.; Girolami, G. S. J . Am. Chem. Soc.
1994, 116, 10294-10295.
(7) Wanandi, P. W.; Tilley, T. D. Organometallics 1997, 16, 4299-
4313.
(13) Gisler, A.; Schaade, M.; Meier, E. J . M.; Linden, A.; von
Philipsborn, W. J . Organomet. Chem. 1997, 545, 315-326.
(14) (a) Demonceau, A.; Lemoine, C. A.; Noels, A. F. Tetrahedron
Lett. 1996, 37, 1025-1026. (b) Demonceau, A.; Stumpf, A. W.; Saive,
E.; Noels, A. F. Macromolecules 1997, 30, 3127-3136.
(8) Cabeza, J . A.; Maitlis, P. M. J . Chem. Soc., Dalton Trans. 1985,
573-578.
(9) (a) Stahl, S.; Werner, H. Organometallics 1990, 9, 1876-1881.
(b) Werner, H.; Weinand, R.; Knaup, W.; Peters, K.; von Schnering,
H. G. Organometallics 1991, 10, 3967-3977.
(15) For
a recent review of homogeneous catalysis by osmium
(10) Brough, S.-A.; Hall, C.; McCamley, A.; Perutz, R. N.; Stahl, S.;
Wecker, U.; Werner, H. J . Organomet. Chem. 1995, 504, 33-46.
complexes see: Sanchez-Delgado, R. A.; Rosales, M.; Esteruelas, M.
A.; Oro, A. J . Mol. Catal. A 1995, 96, 231-243.
S0276-7333(98)00345-8 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 08/11/1998

Enthalpies of Reaction of [(p-cymene)OsCl2]2
Organometallics, Vol. 17, No. 18, 1998 4005
Solu tion Ca lor im etr y. Ca lor im etr ic Mea su r em en t for
R ea ct ion b et w een [(p -cym en e)OsCl2]2 a n d Tr im et h -
ylp h osp h in e. 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 [(p-cymene)OsCl2]2
was accurately weighed into the lower vessel; it was closed
and sealed with 1.5 mL of mercury. Four milliliters of a stock
solution of phosphine [1 g of PMe3 in 25 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 organoosmium 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, allowing the reagents to mix. After the calorimeter
had once again reached thermal equilibrium, the vessels were
removed from the calorimeter, taken into the glovebox, opened,
We have previously reported on the solution calorim-
etry of ligand addition to the ruthenium analogue of
1 (eq 3).16
To probe the effects of ligand substitution on descending
a chemical group, we now wish to expand on our
solution thermochemical studies of organometallic com-
plexes17 by focusing on quantitatively addressing the
binding ability of a series of phosphine ligands to the
(p-cymene)OsCl2 fragment.
1
and analyzed using H NMR spectroscopy. Conversion to (p-
cymene)OsCl2(PMe3) was found to be quantitative under these
reaction conditions. The enthalpy of reaction, -52.9 ( 0.4
kcal/mol, represents the average of five individual calorimetric
determinations. The enthalpy of solution of [(p-cymene)OsCl2]2
was then subtracted from this value to obtain a value of -61.6
( 0.5 kcal/mol for all species in solution.
Ca lor im etr ic Mea su r em en t of En th a lp y of Solu tion of
[(p-cym en e)OsCl2]2 in CH2Cl2. To consider all species in
solution, the enthalpy of solution of [(p-cymene)OsCl2]2 had
to be directly measured. This was performed by using a
procedure similar to that described above with the exception
that no ligand was added to the reaction cell solution. This
enthalpy of solution representing the average of five individual
determinations is 8.8 ( 0.1 kcal/mol.
Syn th eses. The compound [OsCl2(p-cymene)]2 (1) was
synthesized according to the literature procedure.22 The
organoosmium complexes (p-cymene)OsCl2(PPh3) (2),22 (p-
cymene)OsCl2(PPh2Me) (3),22 (p-cymene)OsCl2(PPhMe2) (4),22
(p-cymene)OsCl2(PBz3) (5),22 (p-cymene)OsCl2(PCy3) (6),22 (p-
cymene)OsCl2(PMe3) (7),23 (p-cymene)OsCl2(P(OPh)3) (8),23 (p-
cymene)OsCl2(PiPr3) (9),24 and (p-cymene)OsCl2(P(OMe)3) (10)24
have been reported in the literature. The identity of all
calorimetric products was determined by comparison with
literature spectroscopic data. Experimental synthetic proce-
dures leading to the isolation of previously unreported com-
plexes are described below.
(p-cym en e)OsCl2(P (p yr r olyl)3) (11). A 50 mL flask was
charged with 29 mg (0.126 mmol) of P(pyrrolyl)3, 50 mg (0.063
mmol) of [OsCl2(p-cymene)]2, and 15 mL of CH2Cl2. The clear
yellow solution was stirred at room temperature for 60 min,
after which the solvent was removed under vacuum. The
residue was washed with 10 mL of hexane, filtered, and dried
under vacuum, affording 56 mg of the yellow product (yield:
86%). 1H NMR (300 MHz, CD2Cl2, δ): 1.24 (d, 6H, 2CH3, J )
6.9 Hz), 2.07 (s, 3H, -CH3), 2.80 (m, 1H, -CH), 5.37 (d, 2H,
-C6H4, J ) 6), 5.65 (d, 2H, -C6H4, J ) 6), 6.29 (m, 6H,
pyrrolyl), 6.86 (m, 6H, pyrrolyl). 31P NMR (121 MHz, CD2Cl2,
δ): 47.5. Calcd for C22H26N3Cl2POs: C, 42.31; H, 4.20; N, 6.73.
Found: C, 41.98; H, 4.02; N, 6.47.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All manipulations involving
organometallic complexes were performed under inert atmo-
spheres of argon or nitrogen using standard high-vacuum or
Schlenk tube techniques or in an argon-filled glovebox con-
taining less than 1 ppm oxygen and water. Solvents including
deuterated solvents for NMR analysis were dried and distilled
under nitrogen before use employing standard drying agents.
For example, tetrahydrofuran was stored over sodium wire,
distilled from sodium benzophenone ketyl, stored over Na/K
alloy, and vacuum-transferred into flame-dried glassware prior
to use. The P(NC4H4)3, PPh(NC4H4)2, PPh2(NC4H4), and
P(NC4H8)3 phosphines were synthesized according to literature
procedures.18 Other phosphine or phosphite ligands were
purchased from Strem Chemicals and used as received. Only
materials of high purity as indicated by NMR spectroscopy
were used in the calorimetric experiments. NMR spectra were
recorded using a Varian Gemini 300 or Varian 400 MHz
spectrometer. Calorimetric measurements were performed
using a Calvet calorimeter (Setaram C-80) which was periodi-
cally calibrated using the TRIS reaction19 or the enthalpy of
solution of KCl in water.20 The experimental enthalpies for
these two standard reactions compared very closely to litera-
ture values. This calorimeter has been previously described,21
and typical procedures are described below. Experimental
enthalpy data are reported with 95% confidence limits. Ele-
mental analyses were performed by Oneida Research Services,
Whitesboro, NY.
NMR Titr a tion s. Prior to every set of calorimetric experi-
ments 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 1H
NMR or by 31P NMR spectroscopy, and the reactions were
found to be rapid, clean, and quantitative under experimental
calorimetric conditions. These conditions are necessary for
accurate and meaningful calorimetric results and were satis-
fied for all organometallic reactions investigated.
(p-cym en e)OsCl2(P P h (p yr r olyl)2) (12). In a manner
analogous to the synthesis of 11, complex 12 was isolated in
75% yield. 1H NMR (300 MHz, CD2Cl2, δ): 1.15 (d, 6H, 2CH3,
J ) 7.2 Hz), 2.02 (s, 3H, -CH3), 2.70 (m, 1H, -CH), 5.34 (d,
2H, -C6H4, J ) 6), 5.53 (d, 2H, -C6H4, J ) 6), 6.27 (m, 4H,
pyrrolyl), 7.00 (m, 4H, pyrrolyl), 7.47 (m, 5H, PPh).31P NMR
(121 MHz, CD2Cl2, δ): 46.4. Calcd for C24H27Cl2N2POs: C,
45.36; H, 4.28; N, 4.41. Found: C, 44.91; H, 4.12; N, 4.18.
(16) Serron, S.; Nolan, S. P. Organometallics 1995, 14, 4611-4616.
(17) Serron, S.; Huang, J .; Nolan, S. P. Organometallics 1998, 17,
534-539, and references cited.
(18) Petersen, J . L.; Moloy, K. G. J . Am. Chem. Soc. 1995, 117,
7696-7710.
(22) Werner, H.; Zenkert, K. J . Organomet. Chem. 1988, 345, 151-
166.
(23) Cabeza, J . A.; Maitlis, P. M. J . Chem. Soc., Dalton Trans. 1985,
573-578.
(24) Bell, A. G.; Kozminski, W.; Linden, A.; von Philipsborn, W.
Organometallics 1996, 15, 3124-3135.
(19) Ojelund, G.; Wadso¨, I. Acta Chem. Scand. 1968, 22, 927-937.
(20) Kilday, M. V. J . Res. Natl. Bur. Stand. (U.S.) 1980, 85, 467-
481.
(21) Nolan, S. P.; Hoff, C. D.; Landrum, J . T. J . Organomet. Chem.
1985, 282, 357-362.

4006 Organometallics, Vol. 17, No. 18, 1998
Huang et al.
Ta ble 1. En th a lp ies of Su bstitu tion (k ca l/m ol) in
th e Rea ction
(p-cym en e)OsCl2(P P h 2(p yr r olyl)) (13). In a manner
analogous to the synthesis of 11, complex 13 was isolated as
a yellow microcrystalline product in 88% yield. 1H NMR (300
MHz, CD2Cl2, δ): 1.21 (d, 6H, 2CH3, J ) 7.2 Hz), 2.04 (s, 3H,
-CH3), 3.08 (m, 1H, -CH), 5.30 (d, 2H, -C6H4, J ) 6), 5.51
(d, 2H, -C6H4, J ) 6), 6.32 (m, 2H, pyrrolyl), 7.04 (m, 2H,
pyrrolyl), 7.34-7.70 (br, 10H, phenyl).31P NMR (121 MHz, CD2-
Cl2, δ): 29.7. Calcd for C26H28Cl2NPOs: C, 48.30; H, 4.36; N,
2.17. Found: C, 48.40; H, 4.58; N, 2.34.
CH2Cl2
[OsCl2(p-cymene)]2(soln) + 2PR3(soln)
8
30 °C
2OsCl2(p-cymene)(PR3)(soln)
a
L
complex
-∆Hrxn
PPh(NC4H4)2
P(NC4H4)3
P(p-CF3C6H4)3
PPh2(NC4H4)
PPh3
P(p-ClC6H4)3
P(p-MeC6H4)3
P(p-FC6H4)3
PCy3
OsCl2(p-cymene)(PPh(NC4H4)2)
OsCl2(p-cymene)(P(NC4H4)3)
OsCl2(p-cymene)(P(p-CF3C6H4)3)
OsCl2(p-cymene)(PPh2(NC4H4))
OsCl2(p-cymene)(PPh3)
OsCl2(p-cymene)(P(p-ClC6H4)3)
OsCl2(p-cymene)(P(p-MeC6H4)3)
OsCl2(p-cymene)(P(p-FC6H4)3)
OsCl2(p-cymene)(PCy3)
38.5(4)
39.1(1)
40.9(3)
41.9(2)
43.1(2)
43.4(2)
45.0(3)
46.3(3)
47.9(4)
49.2(1)
49.2(2)
49.8(2)
51.5(5)
52.3(2)
58.6(3)
59.4(5)
61.6(5)
(p-cym en e)OsCl2(P (p yr r olid in yl)3) (14). In a manner
analgous to the synthesis of 11, complex 14 was isolated as
an orange microcrystalline product in 88% yield. 1H NMR (300
MHz, CD2Cl2, δ): 1.27 (d, 6H, 2CH3, J ) 6.9 Hz), 1.75 (m, 12H,
pyrrolidinyl), 2.26 (s, 3H, -CH3), 2.82 (m, 1H, -CH), 3.21 (m,
12H, pyrrolidinyl), 5.37 (d, 2H, -C6H4, J ) 6), 5.63 (d, 2H,
-C6H4, J ) 6).31P NMR (121 MHz, CD2Cl2, δ): 44.2. Calcd
for C22H38Cl2N3POs: C, 41.51; H, 6.02; N, 6.60. Found: C,
41.38; H, 6.20; N, 6.23.
PBz3
OsCl2(p-cymene)(PBz3)
PiPr3
OsCl2(p-cymene)(PiPr3)
P(p-MeOC6H4)3 OsCl2(p-cymene)(P(p-MeOC6H4)3)
(p-cym en e)OsCl2(P Et3) (15). In a manner analogous to
the synthesis of 11, complex 15 was isolated as a yellow
microcrystalline product in 86% yield. 1H NMR (300 MHz,
CD2Cl2, δ): 1.07 (m, 9H, P-CH2CH3), 1.24 (d, 6H, 2CH3, J )
6.9 Hz), 2.00 (m, 6H, PCH2CH3), 2.15 (s, 3H, -CH3), 2.66 (m,
1H, -CH), 5.58 (m, 4H, -C6H4).31P NMR (121 MHz, CD2Cl2,
δ): -19.9. Calcd for C16H29Cl2POs: C, 37.43; H, 5.69. Found:
C, 37.30; H, 5.66.
P(NC4H8)3
PPh2Me
PPhMe2
PEt3
OsCl2(p-cymene)(P(NC4H8)3)
OsCl2(p-cymene)(PPh2Me)
OsCl2(p-cymene)(PPhMe2)
OsCl2(p-cymene)(PEt3)
PMe3
OsCl2(p-cymene)(PMe3)
a
Enthalpy values are reported with 95% confidence limits.
Resu lts a n d Discu ssion
(p-cym en e)OsCl2(P (p-ClC6H4)3) (16). In a manner analo-
gous to the synthesis of 11, complex 16 was isolated as a yellow
microcrystalline product in 94% yield. 1H NMR (300 MHz,
CD2Cl2, δ): 1.16 (d, 6 H, 2CH3, J ) 6.9 Hz), 1.94 (s, 3H, -CH3),
2.68 (m, 1H, -CH), 4.15 (d, 2H, -C6H4, J ) 5.4), 5.42 (d, 2H,
-C6H4, J ) 6.0), 7.36 (m, 6H, PPh), 7.62 (m, 6H, PPh). 31P
NMR (121 MHz, CD2Cl2, δ): -13.0. Calcd for C28H26Cl5POs:
C, 44.20; H, 3.44. Found: C, 43.91; H, 3.27.
A facile entryway into the thermochemistry of (p-
cymene)OsCl2(PR3) (p-cymene ) CH3C6H4CH(CH3)2)
complexes is made possible by the rapid and quantita-
tive reaction of [(p-cymene)OsCl2] (1) with monodentate
phosphine ligands (eq 4).
(p-cym en e)OsCl2(P (p-CH3OC6H4)3) (17). In a manner
analogous to the synthesis of 11, complex 17 was isolated as
a yellow microcrystalline product in 93% yield. 1H NMR (300
MHz, CD2Cl2, δ): 1.16 (d, 6 H, 2 CH3, J ) 7.2 Hz), 1.94 (s, 3
H, -CH3), 2.68 (m, 1H, -CH), 3.80 (s, 3H, OCH3), 5.14 (d, 2H,
-C6H4, J ) 5.7), 5.40 (d, 2H, -C6H4, J ) 6.0), 6.87 (m, 6H,
PPh), 7.59 (m, 6H, PPh). 31P NMR (121 MHz, CD2Cl2, δ):
-15.3. Calcd for C31H35OsCl2PO3Os: C, 49.80; H, 4.72.
Found: C, 50.13; H, 4.60.
This type of phosphine binding reaction appears to be
general and was found to be rapid and quantitative for
all ligands calorimetrically investigated at 30.0 °C in
methylene chloride. All reaction enthalpy data shown
in Table 1 refer to the solution phase and include the
enthalpy of solution of 1.
R ela t ive Im p or t a n ce of St er ic vs E lect r on ic
P a r a m eter . The enthalpy values presented in Table
1 are based on a molar amount of the dimer and span
some 23 kcal/mol. This represents a variation of the
single Os-PR3 bond disruption enthalpy of some 11.5
kcal/mol as a function of varied tertiary phosphine
ligand.
(p-cym en e)OsCl2(P (p-CF3C6H4)3) (18). In a manner analo-
gous to the synthesis of 11, complex 18 was isolated as a yellow
microcrystalline product in 92% yield. 1H NMR (300 MHz,
CD2Cl2, δ): 1.12 (d, 6 H, 2CH3, J ) 6.6 Hz), 1.96 (s, -CH3),
2.63 (m, 1H, -CH), 5.20 (d, 2H, -C6H4, J ) 5.7), 5.41 (d, 2H,
-C6H4, J ) 5.7), 7.67 (m, 6H, PPh, J ) 9.2), 7.88 (m, 6H, PPh).
31P NMR (121 MHz, CD2Cl2, δ): -12.3. Calcd for C31H26Cl2-
PF9Os: C, 43.21; H, 3.04. Found: C, 43.03; H, 2.81.
(p-cym en e)OsCl2(P (p-F C6H4)3) (19). In a manner analo-
gous to the synthesis of 11, complex 19 was isolated as a yellow
microcrystalline product in 93% yield. 1H NMR (300 MHz,
CD2Cl2, δ): 1.16 (d, 6H, -CH3, J ) 7.5 Hz), 1.93 (s, 3 H, -CH3),
2.68 (m, 1H, -CH), 5.16 (d, 2H, -C6H4, J ) 5.7), 5.41 (d, 2H,
-C6H4, J ) 5.7), 7.09 (t, 6H, PPh), 7.78 (m, 6H, PPh). 31P
NMR (121 MHz, CD2Cl2, δ): -14.0. Calcd for C28H26Cl2PF3-
Os: C, 47.26; H, 3.68. Found: C, 47.36; H, 3.65.
A number of research groups have been interested in
discriminating between steric and electronic ligand
factors.25,26 Giering and co-workers have applied the
quantitative analysis of ligand effects (QALE) approach
to the question of stereoelectronic contributions to
(p-cym en e)OsCl2(P (p -CH 3C6H4)3) (20). In
a manner
analogous to the synthesis of 11, complex 20 was isolated as
a yellow microcrystalline product in 90% yield. 1H NMR (300
MHz, CD2Cl2, δ): 1.14 (d, 6 H, 2 CH3, J ) 6.9 Hz), 1.94 (s, 3
H, -CH3), 2.35 (s, 3H, -CH3), 3.66 (m, 1H, -CH), 5.13 (d, 2H,
-C6H4, J ) 5.4), 5.37 (d, 2H, -C6H4, J ) 5.4), 7.16 (m, 6H,
PPh), 7.50 (m, 6H, PPh). 31P NMR (500 MHz, CD2Cl2, δ):
-13.6. Calcd for C31H35Cl2POs: C, 53.21; H, 5.04. Found: C,
53.54; H, 4.99.
(25) Tolman, C. A. Chem. Rev. 1977, 77, 313-348.
(26) (a) Poe, A. J . Pure Appl. Chem. 1988, 60, 1209-1216, and
references cited. (b) Gao, Y.-C.; Shi, Q.-Z.; Kersher, D. L.; Basolo, F.
Inorg. Chem. 1988, 27, 188-191. (c) Baker, R. T.; Calabrese, J . C.;
Krusic, P. J .; Therien, M. J .; Trogler, W. C. J . Am. Chem. Soc. 1988,
110, 8392-8412. (d) Lee, K.-W.; Brown, T. L. Inorg. Chem. 1987, 26,
1852-1856. (e) Bartik, T.; Himmler, T.; Schulte, H. G. Seevogel, K. J .
Organomet. Chem. 1984, 272, 29-41.

Enthalpies of Reaction of [(p-cymene)OsCl2]2
Organometallics, Vol. 17, No. 18, 1998 4007
observed by Hoff and co-workers30 in their investiga-
tions of group 6 thermochemistry. An average differ-
ence of some 10 kcal/mol was observed in the ligand
substitution enthalpies between molybdenum- and tung-
sten-centered systems (eq 7).
The enthalpy difference between the two systems does
not represent the straightforward increase in metal-
ligand bond energy. The overall reactions involving
these dimeric [(p-cymene)MCl2]2 complexes can be
thought of as occurring in two steps (eq 8):
F igu r e 1. Enthalpies of reaction (kcal/mol) of [(p-cymene)-
MCl2]2 (M ) Ru (b) and Os (O) with phosphine ligands vs
electronic parameter (ø).
kinetic and thermodynamic data.27 A quantitative
analysis of the thermochemical data for the osmium
system yields the relationship presented in eq 5.
-∆H ) -(0.75 ( 0.08)ø - (0.39 ( 0.04)θ + (2.87 (
0.89)πpyr + 114.1 ( 5.6
n ) 15; r2 ) 0.96 (5)
where ∆H is the enthalpy of reaction (kcal/mol), ø is the
electronic parameter28 associated with a given phos-
phine ligand, θ represents the phosphine steric factor,
and πpyr29 deals with the π contribution to the electronic
factor when the pyrrolyl phosphines are employed. This
treatment was performed for 15 ligands and yields R2
) 0.960 as a representation of the goodness of the fit. A
similar treatment was performed for the related ruthe-
nium system. The stereoelectronic relationship is rep-
resented in eq 6.
The first step (i) involves dimer scission, which will be
more endothermic for the Os system since it will form
stronger bonds to the bridging chlorides. A similar
trend has been observed by Hoff in his investigations
focusing on group 6.30 The second step (ii) deals with
ligand binding to the coordinatively unsaturated inter-
mediate. Keeping this in mind, we now see that the
average difference between the ruthenium and osmium
systems of 9 kcal/mol is in reality the sum of these two
individual reactions and must include the differences
in dimer scission enthalpies. This enthalpy difference
then becomes a lower limit for M-PR3 bond enthalpy
differences between the two systems.
-∆H ) -(0.55 ( 0.08)ø - (0.51 ( 0.04)θ + (1.53 (
0.96)πpyr + 122.8 ( 6.4
n ) 19; r2 ) 0.94 (6)
Th e P P h 3-xMex (x ) 0-3) Ser ies. The QALE
treatment allows for a global understanding of the
thermochemical data in terms of stereoelectronic effect
partitioning. A number of thermochemical studies have
been performed on the PPh3-xMex (x ) 0-3) series of
ligands, and regardless of the stereoelectronic partition-
ing, it is viewed as a classical example of stepwise
variation of both steric and electronic properties.31 The
present system does validate this point. Since the
osmium system appears more responsive to changes in
electronics, a simple one-parameter relation is sufficient
to correlate the enthalpy of reaction and the electronic
parameter ø.
From the two equations, it is statistically defensible to
state that the steric and electronic factors both appear
to contribute in a relatively equal fashion to the en-
thalpy of reaction in the (p-cymene)RuCl2(PR3) system.
Any such comparison between the ruthenium and
osmium systems would probably reflect the major
difference in electronic properties since Os and Ru are
of similar sizes. The data illustrate this point since the
osmium system displays a relatively more important
electronic contribution. This can be understood in terms
of the increased metal-ligand orbital overlap provided
by 5d orbital involvement. A plot of enthalpy of reaction
versus the phosphine electronic parameter is helpful in
illustrating the parallel behavior of the two systems
(Figure 1).
Von Philipsborn and co-workers have recently dis-
cussed the existence of relationships between 187Os
NMR data and the Tolman electronic parameter.32
A
simple relationship between 31P NMR data for the
The two sets of data appear to follow a similar
electronic trend, the osmium enthalpies of reaction
being on average some 9 kcal/mol more exothermic than
their ruthenium counterparts. A similar increase in
reaction enthalpy on descending a group has been
complexed phosphine ligand ø also exists within the
(29) Fernandez, A.; Reyes, C.; Lee, T.-Y.; Prock, A.; Giering, W. P.;
Haar, C. M.; Nolan, S. P. Organometallics, manuscript submitted.
(30) Hoff, C. D. Prog. Inorg. Chem. 1992, 40, 503-561, and refer-
ences cited.
(27) (a) Bartholomew, J .; Fernandez, A. L.; Lorsbach, B. A.; Wilson,
M. R.; Prock, A.; Giering, W. P. Organometallics 1995, 15, 295-301,
and references cited. (b) Fernandez, A. L.; Prock, A.; Giering, W. P.
Organometallics 1994, 13, 2767-2772, and references cited.
(28) These values are taken from: Fernandez, A.; Lee, T.-Y.; Reyes,
C.; Prock, A.; Giering, W. P. Organometallics, manuscript submitted.
(31) (a) Nolan, S. P.; Lopez de la Vega, R.; Hoff, C. D. Organome-
tallics 1986, 5, 2529-2537, and referecnes cited. (b) Luo, L.; Fagan,
P. J .; Nolan, S. P. Organometallics 1993, 12, 4305-4311. (c) Luo, L;
Nolan, S. P. Organometallics 1992, 11, 3483-3486.
(32) Bell, A. G.; Kozminski, W.; Linden, A.; von Philipsborn, W.
Organometallics 1996, 15, 3124-3135.

4008 Organometallics, Vol. 17, No. 18, 1998
Huang et al.
chemical studies.33 Reaction 9 is given as a representa-
tive example.
Cp′Ru(COD)Cl(soln) + 2PR3(soln) f
Cp′Ru(PR3)2Cl(soln) + COD(soln) (9)
R ) p-XC6H4; X ) Cl, F, Me, OMe, CF3, and H
Since electronics dominate, in the present osmium
system, the simple relationship between enthalpies of
reaction and 31P chemical shift (which we have just
shown displays a principal electronic influence) should
display a good linear fit. Indeed a correlation coefficient
of R ) 1.00 is obtained (slope ) 0.33).
Th e P P h 3-x(NC4H4)x (x ) 1-3) Ser ies. We have
recently explored the thermochemicial behavior of the
PPh3-x(NC4H4)x (x ) 1-3) series.34 It was determined
that this ligand series possessed the capability to involve
its π orbitals while interacting with metal centers. This
series also proves to possess a unique set of properties.
The enthalpies of reaction within this series correlate
well with the electronic parameter ø (R ) 0.98; slope )
0.35).
F igu r e 2. Electronic parameter (ø) vs enthalpy of reaction
(kcal/mol) for the PPh3-xMex (x ) 0-3) series. Slope ) 0.39;
R ) 0.93.
Con clu sion
The labile nature of the chloride bridge in [(p-
cymene)OsCl2]2 (1) was used to gain entry into the
thermochemistry of ligand binding for monodentate
phosphine ligands. The enthalpy trend can be explained
in terms of electronic and steric contribution to the
enthalpy of reaction, with both constituents playing
important roles. A quantitative relationship is estab-
lished between electronic and thermodynamic param-
eters and displays linear correlation. Reactions of
monodentate ligands with 1 also show this reaction to
be of synthetic use for isolation of complexes of formula-
tion (p-cymene)Os(PR3)Cl2.
F igu r e 3. 31P chemical shift (ppm) vs electronic parameter
(ø) for the PPh3-xMex (x ) 0-3) series. Slope ) 1.43; R )
0.97.
PPh3-xMex (x ) 0-3) series, pointing to the more
pronounced electronic effects at play within this series
(Figure 3).
Ack n ow led gm en t. The National Science Founda-
tion (CHE-9631611) and Dupont (Educational Aid Grant)
are gratefully acknowledged for financial support of this
research. Professor Warren Giering is gratefully ac-
knowledged for his assistance with the QALE analysis.
Since both ∆H and 31P correlate well with ø, it then
follows that ∆H and 31P should show good linear
dependence (R ) 0.98; slope ) 3.43), reiterating in a
more straightforward fashion the dependence of the 31
P
OM980345E
chemical shift of these complexes as a principal function
of an electronic factor.
(33) (a) Li, C.; Nolan, S. P. Organometallics 1995, 14, 1327-1332.
(b) Serron, S. A.; Luo, L.; Li, C.; Cucullu, M. E.; Nolan, S. P.
Organometallics 1995, 14, 5290-5297. (c) Serron, S. A.; Nolan, S. P.;
Moloy, K. G. Organometallics 1996, 15, 4301-4306.
(34) (a) Serron, S. A.; Nolan, S. P. Inorg. Chim. Acta 1996, 252, 107-
113. (b) Li, C.; Serron, S. A.; Nolan, S. P.; Petersen, J . L. Organome-
tallics 1996, 15, 4020-4029. (c) Serron, S. A.; Nolan, S. P.; Abramov.
Y. A.; Brammer, L.; Petersen, J . L. Organometallics 1998 , 17, 104-
110. (d) Huang, J .; Haar, C. M.; Nolan, S. P.; Marshall, W. J .; Moloy,
K. G. J . Am. Chem. Soc., in press.
Th e P (p-XC6H4)3 (X ) CF 3, Cl, F , H, CH3, CH3O)
Ser ies. The separation of steric from electronic effects
is made possible by the investigation of an isosteric
series focusing on para-substituted triphenyl phosphine
ligands. We have recently reported on the utilization
of such a series in a number of organometallic thermo-