Novel hemilabile (phosphinoalkyl)arene ligands: Mechanistic investigation of an unusual intramolecular, arene-arene exchange reaction

Article abstract of DOI:10.1021/om960114c

The novel, hemilabile (phosphinoalkyl)arene ligands ArX(CH₂)₂PPh₂ (1a, Ar = C₆H₅, X = O; 1b, Ar = C₆H₅, X = CH₂; 1c, Ar = FC₆H₄, X = CH₂

Full text of DOI:10.1021/om960114c

3062
Organometallics 1996, 15, 3062-3069
Novel Hem ila bile (P h osp h in oa lk yl)a r en e Liga n d s:
Mech a n istic In vestiga tion of a n Un u su a l In tr a m olecu la r ,
Ar en e-Ar en e Exch a n ge Rea ction
Elizabeth T. Singewald, Xiaobo Shi, Chad A. Mirkin,* Susan J . Schofer, and
Charlotte L. Stern
Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113
Received February 14, 1996X
The novel, hemilabile (phosphinoalkyl)arene ligands ArX(CH2)2PPh2 (1a , Ar ) C6H5, X )
O; 1b, Ar ) C6H5, X ) CH2; 1c, Ar ) FC6H4, X ) CH2) were synthesized and complexed to
Rh(I) to form the bis(phosphine), η6-arene piano stool complexes [(η6:η1-ArX(CH2)2PPh2)Rh(η1-
ArX(CH2)2PPh2)]BF4 (2a -c). Complexes 2a -c were fully characterized in solution, and
complex 2a was characterized by single-crystal X-ray diffraction methods. Two of these
complexes, 2a ,c, undergo an unusual, degenerate η6-arene, free arene exchange reaction
which was studied by 2-D NMR EXSY experiments. A mechanism for the exchange reaction
of 2a which involves the formation of a square planar, cis-phosphine, cis-ether Rh(I) complex,
[Rh(η2-PhO(CH2)2PPh2)2]BF4 (13), is proposed.
In tr od u ction
amines,1m-s and others have been used as labile func-
tional groups in the design of hemilabile ligands.
On the basis of the ligand substitution chemistry of
RhI(diphos)(η6-arene) complexes,2 we have designed a
new type of hemilabile ligand that when complexed to
Rh(I) employs a phosphine as a substitutionally inert
group and an η6-arene as the substitutionally labile
moiety, compounds 2a -c, eq 1. Significantly, the arene
Many variations of multidentate hybrid phosphine
ligands have been studied and complexed to metal
centers.1 When complexed to transition metals, many
of these ligands possess both substitutionally inert and
labile groups and, therefore, are referred to as hemila-
bile ligands. Complexes formed from such ligands have
found wide application in the area of homogeneous
catalysis.1a-c,e,i,m,r The substitutionally labile groups in
hemilabile ligands can be viewed as internal solvent
molecules that stabilize a transition metal but in the
presence of substrate can be easily displaced. The most
widely studied hemilabile ligands have been phosphi-
noethers which can bind to metals in a bidentate,
tridentate, or tetradentate fashion.1a In these examples,
the ether is the substitutionally labile group and the
phosphine is the substitutionally inert group. In addi-
tion to ether moieties,1a,b thioethers,1d-i alkenes,1j-l
ligand can temporarily occupy three coordination sites
at the metal center and can be displaced in both
reversible and irreversible reactions with more strongly
ligating functional groups. The arene-based ligand
substitution chemistry of compound 2a is, in part, the
subject of this paper. In addition, we report the
discovery (previously communicated)3 and a mechanistic
study of a novel intramolecular exchange reaction
between the η6-arene and free arene moieties in com-
pounds 2a -c.
* Author to whom correspondence should be addressed. E-mail:
camirkin@chem.nwu.edu.
X Abstract published in Advance ACS Abstracts, J une 1, 1996.
(1) (a) Allgeier, A. M.; Singewald, E. T.; Mirkin, C. A.; Stern, C. L.
Organometallics 1994, 13, 2984. (b) Bader, A.; Lindner, E. Coord.
Chem. Rev. 1991, 108, 27. (c) Dilworth, J . R.; Miller, J . R.; Wheatley,
N.; Baker, M. J .; Sunley, J . G. J . Chem. Soc., Chem. Commun. 1995,
1579. (d) J ames, T. L.; Smith, D. M.; Holm, R. H. Inorg. Chem. 1994,
33, 4869. (e) Bressan, M.; Bonuzzi, C.; Morandini, F.; Morvillo, A. Inorg.
Chim. Acta 1991, 182, 153. (f) Del Zotto, A.; Mezzetti, A.; Rigo, P.;
Bressan, M.; Morandini, F.; Morvillo, A. Inorg. Chim. Acta 1989, 158,
151. (g) Del Zotto, A.; Mezzetti, A.; Dolcetti, G.; Rigo, P.; Pahor, N. B.
J . Chem. Soc., Dalton Trans. 1989, 607. (h) Del Zotto, A.; Rigo, P. Inorg.
Chim. Acta 1988, 147, 55. (i) Bressan, M.; Morandini, F.; Morvillo, A.;
Rigo, P. J . Organomet. Chem. 1985, 280, 1139. (j) Bennett, M. A.;
Chiraratvatana, C.; Robertson, G. B.; Tooptakong, U. Organometallics
1988, 7, 1394. (k) Tau, W. D.; Meek, D. W.; Sorrell, T.; Ibers, J . A.
Inorg. Chem. 1978, 17, 3454. (l) Garrou, P. E.; Hartwell, G. E. J .
Organomet. Chem. 1974, 71, 443. (m) Schumann, H. Hemlig, H.; Goren,
N.; Blum, J . J . Organomet. Chem. 1995, 485, 209. (n) Del Zotto, A.;
Mezzetti, A.; Rigo, P. J . Chem. Soc., Dalton Trans. 1994, 2257. (o)
Hessler, A.; Fischer, J .; Kucken, S.; Stelzer, O. Chem. Ber. 1994, 127,
481. (p) Dekker, G. P. C. M.; Buijs, A.; Elsevier, C. J .; Vrieze, K.; van
Leeuwen, P. W. N. M.; Smeets, W. J . J .; Spek, A. L.; Wang, Y. F.; Stam,
C. H. Organometallics 1992, 11, 1937. (q) Anderson, M. P.; Casalnuovo,
A. L.; J ohnson, B. J .; Mattson, B. M.; Mueting, A. M.; Pignolet, L. H.
Inorg. Chem. 1988, 27, 1649. (r) Brunner, H.; Rahman, A. F. M. M.
Chem. Ber. 1984, 117, 710. (s) Rauchfuss, T. B.; Clements, J . L.; Agnew,
S. F.; Roundhill, D. M. Inorg. Chem. 1977, 16, 775.
Although no examples of η6-arene, free arene ex-
change reactions for (phosphinoalkyl)arene ligands have
been reported previously, a variety of intramolecular
and intermolecular arene-arene exchange reactions
have been studied by others.4,5 For example, several
degenerate, intramolecular arene-arene exchange reac-
tions which involve a haptotropic rearrangement of two
(2) (a) Halpern, J .; Riley, D. P.; Chan, A. S. C.; Pluth, J . J . J . Am.
Chem. Soc. 1977, 99, 8055. (b) Halpern, J .; Chan, A. S. C.; Riley, D.
P.; Pluth, J . J . Adv. Chem. Ser. 1979, No. 173, 16.
(3) Singewald, E. T.; Mirkin, C. A.; Levy, A. D.; Stern, C. L. Angew.
Chem., Int. Ed. Engl. 1994, 33, 2473. Angew. Chem. 1994, 106, 2524.
S0276-7333(96)00114-8 CCC: $12.00 © 1996 American Chemical Society
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(Phosphinoalkyl)arene Ligands
Organometallics, Vol. 15, No. 13, 1996 3063
arene ligands have been reported. Finke et al. reported
that a Ru complex undergoes a degenerate exchange
process which involves a reversible η6- to η4-ring slip-
page reaction for the π-coordinated hexamethylbenzene
ligands 3 and 3′ in eq 2.4g In another example of an
phinoalkyl)arene ligand which bridges two metal cen-
ters to form a heterobimetallic complex:8
intramolecular arene-arene exchange reaction, a Cr-
(CO)3 fragment undergoes a coordination shift from one
arene to another in an aromatic polycyclic ligand, 4 to
5, eq 3.4a However, this reaction is irreversible and
nondegenerate since the starting and product complexes
are chemically different.
Exp er im en ta l Section
All reactions were carried out under nitrogen using standard
Schlenk techniques or in an inert-atmosphere glovebox. Me-
thylene chloride and pentane were dried over calcium hydride.
THF and diethyl ether were dried over sodium/benzophenone.
All solvents were distilled under nitrogen and degassed prior
to use. Deuterated solvents were purchased from Cambridge
Isotope Laboratories and used without purification. RhCl3‚xH2O
was used on loan from J ohnson-Matthey Chemical Co.
A
sample of [Rh(µ-Cl)(η2-C8H14)2]x was prepared according to
literature methods.9 â-Chlorophenetole was purchased from
Pfaltz and Bauer Chemical Co. All other chemicals were
purchased from Aldrich Chemical Co. and used as received.
1
One dimensional H NMR spectra were recorded either on
a Varian Gemini 300 MHz, a Varian Unity 400 MHz, or a
Bruker 600 MHz FT NMR spectrometer. 31P{1H} NMR
spectra were recorded on a Varian Gemini 300 MHz FT NMR
spectrometer at 121 MHz and referenced versus the external
standard 85% H3PO4. 13C{1H} NMR spectra were recorded
on a Varian Gemini 300 MHz FT NMR spectrometer at 75
MHz. One dimensional 19F{1H} NMR spectra were recorded
on a Varian Gemini 300 MHz FT NMR spectrometer at 282
MHz and referenced versus the external standard CFCl3. 1H
and 19F NMR exchange spectroscopy (EXSY) were performed
on a Varian Unity 400 MHz FT NMR spectrometer at 400 and
376 MHz, respectively, using a standard NOESY pulse se-
quence with tmix varying with temperature. All two dimen-
sional NMR experiments were performed on a Varian Unity
400 MHz spectrometer. All coupling constants are reported
as absolute values. Electrochemical measurements were
carried out either on a PINE AFRDE4 or AFRDE5 bipoten-
tiostat (cyclic voltammetry, CV) or a PAR 273A potentiostat/
galvanostat (differential pulse voltammetry, DPV) using a Pt
working electrode (0.02 cm2), a Pt mesh counter electrode, and
a Ag wire reference electrode. In all cases a n-Bu4NPF6 (0.1
M) solution in CH2Cl2 was used as the supporting electrolyte.
All electrochemical data were referenced further versus the
FcH/[FcH]+ (Fc ) (η5-C5H5)Fe(η5-C5H4)) redox couple. Electron
impact (EI) and fast atom bombardment (FAB) mass spectra
were recorded using a Fisions VG 70-250 SE mass spectrom-
eter. FT-IR spectra were recorded on a Nicolet 520SX spec-
trometer. Elemental analyses were performed by Schwarzkopf
Microanalytical Laboratory Inc., Woodside, NY.
A couple of ligand systems analogous to the one
reported herein have been prepared by others. Rybin-
skaya et al. showed that arenes with pendant phosphite
groups chelate to chromium and form the η6-arene
complexes 6a -c.6 Additionally, a bis(phosphine), η6-
arene ligand can be generated on Rh(I) in a reaction
involving alkyne insertion into a metallocyclopentadiene
complex 7 to form complex 8, eq 4.7 However, no
unusual arene ligand exchange processes were reported
for 6-8. Finally, Meek et al. prepared a novel (phos-
(4) Intramolecular Exchange: (a) Nambu, M.; Mohler, D. L.; Hard-
castle, K.; Baldridge, K. K.; Siegel, J . S. J . Am. Chem. Soc. 1993, 115,
6138. (b) Luo, S.; Rauchfuss, T. B.; Wilson, S. R. Organometallics 1992,
11, 3497. (c) Merkert, J .; Nielson, R. M.; Weaver, M. J .; Geiger, W. E.
J . Am. Chem. Soc. 1989, 111, 7084. (d) Nielson, R. M.; Weaver, M. J .
Organometallics 1989, 8, 1636. (e) Traylor, T. G.; Goldberg, M. J .
Organometallics 1987, 6, 2413. (f) Bandy, J . A.; Green, M. L. H.;
O’Hare, D. J . Chem. Soc., Dalton Trans. 1986, 2477. (g) Finke, R. G.;
Voegeli, R. H.; Laganis, E. D.; Boekelheide, V. Organometallics 1983,
2, 347.
(5) Intermolecular exchange: (a) Decken, A.; Britten, J . F.;
McGlinchey, M. J . J . Am. Chem. Soc. 1993, 115, 7275. (b) Plitzko, K.-
D.; Boekelheide, V. Organometallics 1988, 7, 1573. (c) Traylor, T. G.;
Goldberg, M. J . J . Am. Chem. Soc. 1987, 109, 3968. (d) Howell, J . A.
S.; Dixon, D. T.; Kola, J . C.; Ashford, N. F. J . Organomet. Chem. 1985,
294, C1. (e) Traylor, T. G.; Stewart, K. J .; Goldberg, M. J . J . Am. Chem.
Soc. 1984, 106, 4445. (f) Sievert, A. C.; Muetterties, E. L. Inorg. Chem.
1981, 20, 489. (g) Muetterties, E. L.; Bleeke, J . R.; Sievert, A. C. J .
Organomet. Chem. 1979, 178, 197.
Syn th esis of P h O(CH2)2P P h 2 (1a ). A THF solution of
KPPh2 (0.50 M, 0.020 mol) was added dropwise to a solution
of PhOCH2CH2Cl (3.13 g, 0.020 mol) in 200 mL of THF at 0
°C. The reaction was allowed to warm to room temperature
over 12 h. The solution was filtered, 25 mL of silica gel was
added, and the solvent was removed under vacuum. Chro-
matography of the crude reaction mixture on silica gel was
performed in a glovebox under nitrogen. Two bands were
eluted with 10% diethyl ether in pentane. The first band
(6) (a) Nesmeyanov, A. N.; Krivykh, V. V.; Rybinskaya, M. I. J .
Organomet. Chem. 1979, 164, 159. (b) Vasyukova, N. I.; Nekrasov, Y.
S.; Krivykh, V. V.; Rybinskaya, M. I. J . Organomet. Chem. 1980, 201,
283.
(8) McDougall, M. G.; Gallucci, J . C.; Meek, D. W. Inorg. Chim. Acta
1991, 188, 123.
(7) Winter, W. Angew. Chem., Int. Ed. Engl. 1976, 15, 241; Angew.
Chem. 1976, 88, 260.
(9) Porri, L.; Lionetti, A.; Allegra, G.; Immirizi, A. Chem. Commun.
1965, 336.
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3064 Organometallics, Vol. 15, No. 13, 1996
Singewald et al.
contained HPPh2 and was discarded. The second and major
band contained 1a . Vacuum evaporation of the solvent and
recrystallization of the chromatographed product from ethanol
afforded analytically pure 1a (yield ) 4.28 g, 0.014 mol, 70%).
1H NMR (300 MHz, C6D6): δ 2.41 (t, J H-H ) 7.7 Hz, 2H, CH2P),
3.92 (dt, J H-H ) J P-H ) 7.7 Hz, 2H, CH2O), 7.40-6.71 (m, 15H,
Phs). 31P{1H} NMR (300 MHz, C6D6): δ -21.8 (s). EIMS: (70
eV, 190 °C) [M+]: m/ z 306. Anal. Calcd for C20H19OP: C,
78.42; H, 6.25. Found: C, 78.52; H, 6.44.
Syn th esis of P h (CH2)3P P h 2 (1b). Compound 1b was
synthesized in a route similar to 1a and on a comparable scale.
The reaction between BrCH2CH2CH2C6H5 and KPPh2 gave
analytically pure 1b (yield ) 76%). 1H NMR (300 MHz,
CDCl3): δ 1.79 (m, 2H, CH2CH2CH2), 2.09 (dt, J H-H ) 7.3 Hz,
J P-H ) 4.7 Hz, 2H, CH2P), 2.76 (t, J H-H ) 7.3 Hz, 2H, CH2-
Ph), 7.43-7.15 (m, 15H, Phs). 31P{1H} NMR (121 MHz,
CDCl3): δ -15.2 (s). EIMS: (70 eV, 190 °C) [M+]: m/ z 304.
Anal. Calcd for C21H21P: C, 82.87; H, 6.95. Found: C, 82.62;
H, 7.07.
(d, J H-H ) 7.8 Hz, 2H, o-C6H5), 6.88 (m, 2H, η6-m-C6H5), 7.00
(t, J H-H ) 7.4 Hz, 1H, p-C6H5), 7.28-7.47 (m, 22 H, Phs).
31P{1H} NMR (121 MHz, CD2Cl2): δ 32.6 (dd, J Rh-P ) 198.7
Hz, J P-P ) 38.6 Hz), 34.9 (dd, J Rh-P ) 210.4 Hz, J P-P ) 38.6
Hz). FABMS [M+]: m/ z 715. Anal. Calcd for C40H38O2P2-
RhBF4: C, 59.88; H, 4.77. Found: C, 59.70; H, 4.91.
Syn t h esis of [(η6:η1-C6H 5(CH2)3P P h 2)R h (η1-P h (CH2)3-
P P h 2)]BF 4 (2b). Compound 2b is synthesized by an analo-
gous route to 2a but using ligand 1b instead of 1a (yield )
84%). 1H NMR (400 MHz, CD2Cl2): δ 1.44 (m, 6H, CH2P and
2 CH2CH2CH2), 1.90 (dt, J H-H ) 4.8 Hz, J P-H ) 10.4 Hz, 2H,
CH2P), 2.37 (t, J H-H ) 7.2 Hz, 2H, CH2(η6-Ph)), 2.56 (t, J H-H
) 4.8 Hz, 2H, CH2Ph), 4.68 (t, J H-H ) 6.0 Hz, 1H, η6-p-C6H5),
6.78 (m, 2H, η6-m-C6H5), 6.85 (d, J H-H ) 7.2 Hz, 4H, o-C6H5),
7.44-7.16 (m, 23H, Phs). 31P{1H} NMR (121 MHz, CD2Cl2):
δ 34.6 (dd, J Rh-P ) 203.9 Hz, J P-P ) 40.6 Hz), 39.8 (dd, J Rh-P
) 198.8 Hz, J P-P ) 40.6 Hz). FABMS [M+]: m/ z 711. Anal.
Calcd for C42H42P2RhBF4: C, 63.18; H, 5.31. Found: C, 62.63;
H, 5.50.
Syn th esis of 4-F C6H4(CH2)3Cl.10 A slurry of AlCl3 (2.05
g, 0.153 mol) in 50 mL of methylene chloride was treated with
tert-BuNH2‚BH3 (2.67 g, 0.032 mol) and stirred for 5 min at 0
°C. The AlCl3 dissolved giving a translucent gray solution.
To this, a solution of 4-FC6H4C(O)CH2CH2Cl (0.954 g, 5.1
mmol) in 5 mL of methylene chloride was added via cannula.
This reaction was stirred at room temperature and was
monitored by thin-layer chromatography on silica gel plates
using 10% methylene chloride in pentane. After 20 h, the
translucent gray reaction solution was treated dropwise with
HCl (25 mL, 0.1 M) during which the solution bubbled
vigorously and became opaque. After all HCl was added, the
reaction was stirred for an additional 15 min and then
extracted with ethyl acetate. The combined organic layers
were washed with HCl (40 mL, 0.1 M) and NaCl (saturated).
After filtering of the organic layer through MgSO4, the solvent
was removed via rotary evaporation. The crude product was
purified via column chromatography on silica gel. One band
was collected with an eluent of 10% methylene chloride in
pentane. The product was used without further purification
(yield ) 0.673 g, 3.9 mmol, 76%). 1H NMR (300 MHz,
Syn th esis of [(η6:η1-4-F C6H4(CH2)3P P h 2)Rh (η1-4-F C6H4-
(CH2)3 P P h 2)]BF 4 (2c). Compound 2c is synthesized by an
analogous route to 2a but using ligand 1c instead of 1a (yield
) 30%). 1H NMR (400 MHz, CD2Cl2): δ 1.41 (m, 6H, CH2P
and 2 CH2CH2CH2), 1.90 (m, 2H CH2P), 2.29 (m, 2H, CH2(η6-
Ar)), 2.56 (m, 2H, CH2Ar), 6.75 (m, 4H, Phs), 6.90 (m, 4H, Phs),
7.13-7.48 (m, 20H, Phs). 19F{1H} NMR (282 MHz, CD2Cl2):
-116.3 (s, F-C6H4), -125.5 (s, F-η6-C6H4), -151.2 (s, BF4).
31P{1H} NMR (121 MHz, CD2Cl2): δ 33.3 (dd, J Rh-P ) 200.3
Hz, J P-P ) 40.6 Hz), 40.6 (ddd, J Rh-P ) 204.4 Hz, J P-P ) 39.3
Hz, J P-F ) 5.2 Hz). HRFABMS [M+]: Calcd m/ z 747.1628;
found, m/ z 747.1657.
Syn t h esis of [R h (η1-P h O(CH2)2P P h 2)2(CO)3]BF 4 (9).
Complex 2a (0.010 g, 0.012 mmol) in CD2Cl2 (1 mL) was
reacted with 1 atm of CO in an NMR tube for 15 min. The
solution color changed from orange-red to bright yellow during
the course of the reaction indicating the formation of 9. 1H
NMR (300 MHz, CD2Cl2): δ 3.21 (m, 4H, CH2P), 4.32 (m, 4H,
CH2O), 6.58 (d, J H-H ) 7.4 Hz, 4H, (o-C6H5)O), 6.96 (t, J H-H
)
7.4 Hz, 2H, (p-C6H5)O), 7.21 (m, 4H (m-C6H5)O), 7.64 - 7.51
(m, 20H, Ph2P). 31P{1H} NMR (for 13CO labeled 9, 121 MHz,
CD2Cl2, -77 °C): δ 31.5 (dq, J Rh-P ) 71.7 Hz, J P-C ) 14.1 Hz).
13C{1H} NMR (for 13CO labeled 9, 75 MHz, CD2Cl2, -77 °C):
CDCl3): δ 2.00 (m, 2H, CH2CH2CH2Cl), 2.74 (t, 2H, J H-H
)
7.2 Hz, CH2Ar), 3.50 (m, 2H, CH2Cl), 6.98 (m, 2H, Phs), 7.14
(m, 2H, Phs). 19F{1H} NMR (282 MHz, CDCl3): -117.9 (s).
Syn th esis of 4-F C6H4(CH2)3P P h 2 (1c). Compound 1c was
synthesized by reacting 4-FC6H4(CH2)3Cl with KPPh2 in an
manner analogous to the synthesis of 1a (yield ) 83%). 1H
NMR (300 MHz, CDCl3): δ 1.78 (m, 2H, CH2CH2CH2P), 2.08
(m, 2H, CH2P), 2.73 (m, 2H, CH2Ar), 6.99 (m, 2H, Phs), 7.10
(m, 12H, Phs). 19F{1H} NMR (282 MHz, CDCl3): -118.2 (s).
31P{1H} NMR (121 MHz, CDCl3): δ -16.2 (s). HRFABMS
[M+]: Calcd, m/ z 322.1287; found, m/ z 322.1281.
δ 185.2 (dt, J Rh-P ) 66.2 Hz, J P-C ) 14.4 Hz). FABMS [M+
2CO]: m/ z 743.
-
Syn th esis of [Rh (η1-P h O(CH2)2P P h 2)2(CO)2]BF 4 (10).
Upon removal of the CO atmosphere and solvent from the
NMR tube containing dissolved 9, complex 10 is formed in
quantitative yield as determined by NMR spectroscopy. 1H
NMR (300 MHz, CD2Cl2): δ 3.20 (m, 4H, CH2P), 4.33 (m, 4H,
CH2O), 6.67 (d, J H-H ) 8.0 Hz, 4H, (o-C6H5)O), 6.98 (t, J H-H
)
7.3 Hz, 2H, (p-C6H5)O), 7.23 (m, 4H, (m-C6H5)O), 7.67-7.48
(m, 20H, Ph2P). 31P{1H} NMR (for 13CO labeled 10, 121 MHz,
CD2Cl2, -71 °C): δ 18.9 (dt, J Rh-P ) 106.8 Hz, J P-C ) 15.5
Hz). 13C{1H} NMR (for 13CO labeled 10, 75 MHz, CD2Cl2, -71
°C): δ 185.7 (dt, J Rh-P ) 64.1 Hz, J P-C ) 16.0 Hz). FT-IR (CH2-
Cl2): νCO ) 1997 (vs), 1932 (w) cm-1. FABMS [M+]: m/z 743
[M+ - CO].
Syn th esis of [(η6:η1-C6H5O(CH2)2P P h 2)Rh (η1-P h O(CH2)2-
P P h 2)]BF 4 (2a ). AgBF4 (0.27 g, 1.4 mmol) and [Rh(µ-Cl)(η2-
C8H14)2]x (0.50 g, 1.4 mmol) were reacted in 3 mL of THF for
45 min. The reaction was filtered to remove a dark gray
precipitate and diluted to 75 mL with THF. Ligand 1a (0.86
g, 2.8 mmol) in 75 mL of THF was added dropwise to the THF
11
solution of [(η2-C8H14)2Rh(THF)2]BF4 at -78 °C. After 2 h,
Rea ction of 2a w ith Aceton itr ile. In an NMR tube,
complex 2a was reacted with neat CD3CN to form a mixture
of cis and trans acetonitrile adducts in a 4:1 ratio, [Rh(η1-
PhO(CH2)2PPh2)2(cis-CD3CN)2]BF4 (11) and [Rh(η1-PhO(CH2)2-
PPh2)2(trans-CD3CN)2]BF4 (12), respectively. In CD2Cl2 with
3 equiv of CD3CN, a similar mixture of 11 and 12 is formed.
If this reaction is performed at -78 °C, only the cis adduct 11
is formed. As the temperature is raised (up to 55 °C), cis
adduct 11 slowly converts into the trans adduct 12. Data for
11: 1H NMR (300 MHz, CD2Cl2, -45 °C) δ 1.62 (s, 6H, CH3-
the solvent was removed under vacuum. The crude product
was recrystallized from a 1:10 mixture of methylene chloride
and diethyl ether to afford analytically pure 2a (yield ) 0.89
g, 1.1 mmol, 80%). 1H NMR (600 MHz, CD2Cl2): δ 1.76 (dt,
J H-H ) 6.6 Hz, J P-H ) 9.6 Hz, 2H, CH2P), 2.07 (m, 2H, CH2P),
4.00 (m, 2H, CH2O), 4.05 (m, 2H, CH2O), 4.63 (t, J H-H ) 6.1
Hz, 1H, η6-p-C6H5), 6.75 (d, J H-H ) 6.2 Hz, 2H, η6-o-C6H5), 6.80
(10) (a) The synthesis of this compound was previously reported in
the literature; however, an improved synthesis and the spectroscopic
characterization are reported herein: Olah, G. A.; Krishnamurty, V.
V.; Singh, B. P.; Iyer, P. S. J . Org. Chem. 1983, 48, 955. (b) Procedure
for reduction reaction was taken from: Lau, C. K.; Tardif, S.; Dufresne,
C.; Scheigetz, J . J . Org. Chem. 1989, 54, 491.
CN), 2.11 (m, 4H, CH2P), 4.46 (m, 4H, CH2O), 6.79 (d, J H-H
)
7.7 Hz, 4H, o-C6H5), 6.96 (t, J H-H ) 7.3 Hz, 2H, p-C6H5), 7.42-
7.25 (m, 24H, Phs); 31P{1H} NMR (121 MHz, CD2Cl2, -45°C):
δ 35.7 (d, J Rh-P ) 171.8 Hz). Data for 12: 1H NMR (300 MHz,
CD2Cl2, 20 °C) δ 1.63 (s, 6H, CH3CN), 2.94 (m, 4H, CH2P),
4.56 (m, 4H, CH2O), 6.78 (m, 4H, o-C6H5), 6.94 (m, 2H, p-C6H5),
(11) Postulated as the intermediate in the reaction of [Rh(η6-C8H14)2-
(µ-Cl)]x with AgBF4 in THF; for synthesis of [Rh(η6-C8H14)2(µ-Cl)]x see
ref 9.
+
+

(Phosphinoalkyl)arene Ligands
Organometallics, Vol. 15, No. 13, 1996 3065
7.31-7.46, 7.68 (m, 24H, Phs); 31P{1H} NMR (121 MHz, CD2-
Cl2, -45 °C) δ 22.8 (d, J Rh-P ) 130.1 Hz).
Cr ysta l Str u ctu r e of 2a . Single crystals of 2a suitable
for an X-ray diffraction study were grown by slow diffusion of
a 1:10 mixture of methylene chloride and diethyl ether. The
crystal was mounted on a glass fiber under degassed para-
tone-N oil. The data were collected at a temperature of -120
( 1 °C using the ω-θ scan technique to a maximum 2θ value
of 50.0°. In addition, the data were corrected for Lorentz and
polarization effects, and an analytical absorption correction
was applied with transmission factors of 0.80-0.86. Also, a
correction for secondary extinction was applied (coefficient )
0.42235 × 10-7). The structure was solved by Patterson
methods with SHELXS-86. All non-hydrogen atoms were
refined with anisotropic thermal parameters, and all hydrogen
atoms were idealized.
NMR Kin etic Stu d ies. Two dimensional exchange spec-
troscopy (EXSY) was used to determine the activation param-
eters for the arene-arene exchange reactions.12 A standard
NOESY pulse sequence (π/2, T1, π/2, T2, π/2) was used on a
Varian 400 MHz FT NMR spectrometer. Mixing times (T2 )
F igu r e 1. ORTEP diagram of 2a . Thermal ellipsoids are
given at 50% probability.
Ta ble 1. Selected Cr ysta l Da ta for 2a
t
mix) were varied with temperature. The rate of the exchange
reaction (k) was determined by comparing the volumes of the
cross peaks (VAB and VBA) and diagonal peaks (VAA and VBB
in the EXSY spectrum and evaluating with the mixing time
formula
fw
C40H38BF4O2P2Rh
802.4
)
cryst system
space group
a, Å
b, Å
c, Å
monoclinic
P21/c
10.595(3)
17.116(5)
19.564(5)
92.16(2)
in the equation k ) 1/tmix ln[(r + 1)/(r - 1)], where r ) (VAA
+
V
BB)/(VAB + VBA).12 From a plot of ln(k/T) versus 1/T modeled
according to the Eyring equation, the ∆Gq
values were
298
calculated.13
â, deg
V, Å3
3545(3)
Z
4
6.17
1.503
ruby red
Resu lts a n d Discu ssion
µ(Mo KR), cm-1
D
color
size, mm
temp, K
diffractometer
monochromator
scan method
radiation
calc, g cm-1
Syn th esis a n d Ch a r a cter iza tion of Liga n d s 1a -
c. The new hemilabile ligands 1a -c were synthesized
in high yield (70-83%) in the reaction between 1 equiv
of ArX(CH2)2Z with 1 equiv of KPPh2, eq 5. They were
isolated as white crystalline solids and fully character-
0.42 × 0.26 × 0.33
153
Enraf-Nonius CAD-4
graphite
ω-θ
Mo KR (λ ) 0.710 69 Å)
2 e 2θ e 50.0
6663
ized by H NMR, 31P NMR, and either high-resolution
mass spectrometry or elemental analysis.
1
scan limit
reflcns collcd
indepdt reflcns
function minimized
least-squares weights
obsd reflcts (I > 3.00σ(I))
variable paras
R(f), %
6466
∑w(|Fo| - |Fc|)2
4Fo2/σ2(Fo
5133
452
)
2
3.3
R(wf), %
4.3
Syn th esis a n d Ch a r a cter iza tion of Com p ou n d s
2a ,b. Ligands 1a ,b react cleanly with Rh(I) precursors
to form the new piano stool complexes 2a ,b with good
yields (80-84%). For example, the bis(phosphine), η6-
arene piano stool complex 2a was synthesized by the
reaction of 2 equiv of ligand 1a with 1 equiv of [Rh-
(THF)2(η2-C8H14)2]BF4,11 eq 6. Compound 2a was iso-
Ta ble 2. Selected Dista n ces a n d An gles of 2a
Distances (Å)
Rh-P1
Rh-P2
Rh-C15
Rh-C16
Rh-C17
Rh-C18
Rh-C19
2.228 (1)
2.251(1)
2.282(3)
2.350(3)
2.332(3)
2.270(3)
2.319(3)
Rh-arene centroid
Rh-C20
C15-C16
C16-C17
C17-C18
C18-C19
C19-C20
C20-C15
1.840(3)
2.319 (3)
1.403(5)
1.400(5)
1.408(5)
1.407(5)
1.395(5)
1.424(5)
Angles (deg)
C15-C16-C17
C17-C18-C19
C19-C20-C15
118.8(3)
121.5(3)
118.7(3)
C16-C17-C18
C18-C19-C20
C20-C15-C16
93.28(9)
119.5(3)
119.4(3)
121.8(3)
P1-Rh-P2
lated as an orange crystalline solid, and it has been
characterized spectroscopically in solution and in the
solid state by a single-crystal X-ray diffraction study.
An ORTEP diagram of cation 2a is shown in Figure 1,
and crystallographic data are presented in Table 1.
Selected bond distances and angles for 2a are given in
Table 2. The Rh-P and Rh-C bond distances are in
the expected range for monomeric Rh compounds with
bis(phosphine), η6-arene piano stool geometry (Rh-P )
2.217-2.251 Å, Rh-C ) 2.217-2.516 Å).14 The Rh-P
bond distance for the chelating phosphine ligand (2.228-
(1) Å) is shorter than the Rh-P bond distance for the
monodentate phosphorus ligand (2.251(1) Å). The η6-
arene ring in complex 2a is not planar but adopts a boat
conformation with two short and four long Rh-C bond
(12) Perrin, C. L.; Dwyer, T. J . Chem. Rev. 1990, 90, 935.
(13) Sandstro¨m, J . Dynamic NMR Spectroscopy; Academic: London,
1982.
+
+

3066 Organometallics, Vol. 15, No. 13, 1996
Singewald et al.
F igu r e 2. 1H NMR spectrum of 2a in CD2Cl2 at 20 °C.
F igu r e 3. Variable-temperature 31P NMR spectra of 2a
in THF-d8 from 208 to 326 K.
distances. This deviation from planarity (0.020(3) Å)
is a result of Rh d orbital and η6-arene π-orbital overlap,
which has been modeled extensively.15 Several isoelec-
tronic complexes with similarly distorted arenes have
been reported in the literature.14a,15
significant upfield shift may result from an interaction
of this proton with the ring current associated with the
free arene ring. However, if this is the case, it is not
evident in the solid state structure of 2a , and it could
Compound 2a has been spectroscopically character-
ized, and all solution data are consistent with the solid-
state structure. For example, the 31P NMR spectrum
of 2a exhibits two resonances at δ 32.6 and 34.9 which
are assigned to magnetically and chemically inequiva-
lent P atoms due to coupling to each other and to the
103Rh nucleus. Each resonance appears as a doublet of
doublets (J Rh-P ) 210.4 and 198.7 Hz; J P-P ) 38.6 Hz),
with coupling constants in the expected region for Rh
complexes with this type of geometry (J Rh-P ) 190-209
Hz, J P-P ) 39-40 Hz).14,16 Furthermore, in the 1H
NMR spectrum the two resonances at δ 2.07 and 1.76
can be assigned to the methylene protons R to the P
atoms in the monodentate and chelated ligands, respec-
tively, by comparision to the spectrum of the ligand.
From the preceding assignments and a 31P-1H HET-
COR NMR experiment of 2a , the resonances at δ 32.6
and 34.9 have been assigned to the P atoms in the
monodentate and chelated ligands, respectively.
Note that in the 1H NMR spectrum of 2a , the
resonances in the normal aromatic region (δ 6-9)
integrate for 29 rather than the expected 30 protons
(Figure 2).17 From a COSY experiment, the resonance
for the H atom para to the ether group on the η6-
coordinated arene ring is found at δ 4.6, which is
remarkably upfield of the other aromatic resonances
including the para H atom of the free arene ring (δ 7.0).
It is well-known that arene resonances shift upfield
upon coordination to a metal;18 however, to the best of
our knowledge this is the largest upfield shift for a
proton on an arene complex of Rh yet reported. This
1
not be detected in a H NMR NOESY experiment of 2a
in CD2Cl2. The effects of aromatic ring current on
nearby proton nuclei are well documented.19 For ex-
ample, in macrocyclic aromatic systems, the resonances
for the internal hydrogens are shifted upfield well
outside the range of resonances for normal aromatic
hydrogens.17,20
The 31P NMR spectrum of 2a in THF-d8 is tempera-
ture dependent, Figure 3. At 208 K, the spectrum
appears as two doublets of doublets and looks much like
the room-temperature spectrum of 2a in CD2Cl2. As the
temperature is raised, the two resonances move toward
each other. At 313 K, they coincidentally have the same
chemical shift and appear as a doublet (δ 33.2, J Rh-P
)
204.2 Hz) indicating that they are magnetically equiva-
lent. Above 313 K, the two resonances cross over and
the P-P coupling reappears. This effect could be
misinterpreted as an indication of η6-arene-free arene
exchange at the Rh(I) center of 2a , a process that if fast
on the 31P NMR time scale, would make the two P
ligands magnetically equivalent. However, this is not
what the spectrometer is detecting. Instead, what has
been recorded is a temperature-dependent chemical
shift phenomenon. Indeed, a plot of chemical shift as a
function of inverse temperature shows a definite de-
pendence for the two resonances, Figure 4. Note that
one resonance exhibits a rough linear dependance while
the other one is clearly nonlinear, probably due to the
differences in solvent interactions between different
parts of the molecule.22a This effect can be related to
the temperature dependence of a shielding parameter
in the equation which determines the chemical shift of
31P nuclei.21 Although not common, dramatic temper-
ature-dependent chemical shifts have been observed in
other organometallic systems.22 Note that coalescence,
(14) (a) Singewald, E. T.; Mirkin, C. A.; Yap. G.; Liable-Sands, L.
M.; Rheingold, A. L. unpublished results. (b) Alvarez, M.; Lugan, N.;
Donnadieu, B.; Mathieu, R. Organometallics 1995, 14, 365. (c) Westcott,
S. A.; Taylor, N. J .; Marder, T. B.; Baker, R. T.; J ones, N. J .; Calabrese,
J . C. J . Chem. Soc., Chem. Commun. 1991, 304. (d) Townsend, J . M.;
Blount, J . F. Inorg. Chem. 1981, 20, 269. (e) Albano, P.; Aresta, M.;
Manassero, M. Inorg. Chem. 1980, 19, 1069.
(15) Muetterties, E. L.; Bleeke, J . R.; Wucherer, E. J .; Albright, T.
A. Chem. Rev. 1982, 82, 499.
(16) Sassano, C. A.; Mirkin, C. A. J . Am. Chem Soc. 1995, 117,
11379.
(17) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric
Identification of Organic Compounds; Wiley: New York, 1991.
(18) (a) Price, J . T.; Sorensen, T. S. Can. J . Chem. 1968, 46, 515.
(b) Mann, B. E. Chem. Commun. 1971, 976. (c) Kohler, F. H. Chem.
Ber. 1974, 107, 570.
(19) Friebolin, H. Basic One- and Two-Dimensional NMR Spectro-
scopy: VCH: New York, 1991.
(20) Sondheimer, F.; Calder, I. C.; Elix, J . A.; Gaoni, Y.; Garratt, P.
J .; Grohmann, K.; Di Maio, G.; Mayer, J .; Sargent, M. V.; Wolovsky,
R. Aromaticity; Spec. Publ. No. 21; Chemical Society: London, 1967;
p 75.
(21) (a) J ameson, C. J .; De Dios, A. C.; J ameson, A. K. J . Chem.
Phys. 1991, 95, 9042. (b) J ameson, C. J . Bull. Magn. Reson. 1980, 3,
3. (c) J ameson, C. J .; Rehder, D.; Hoch, M. J . Am. Chem. Soc. 1987,
109, 2589.
+
+

(Phosphinoalkyl)arene Ligands
Organometallics, Vol. 15, No. 13, 1996 3067
F igu r e 4. Plot of 31P NMR chemical shifts of 2a in THF-
d8 versus 1/T. 1/T was arbitrarily chosen over T.
which would be an indication of an exchange process,
is never observed in the variable-temperature NMR
experiment, Figure 3.
Rea ctivity of 2a w ith Sm a ll Molecu les. The
(phosphinoalkyl)arene ligands in 2a are extremely labile
as evidenced by the reactivity of the complex with small
molecules, such as CO, eq 7. When complex 2a is
F igu r e 5. 31P NMR spectra of 9 and 10 in CD2Cl2 at -77
and -71 °C, respectively.
Complex 2a also reacts with excess acetonitrile to
form a mixture of cis- and trans-bis(acetonitrile) ad-
ducts, 11 and 12, eq 8. If 2a is reacted with acetonitrile
reacted with CO, the η6-arene ligand is displaced and
the tris(carbonyl) adduct, 9, is formed, eq 7. Upon
removal of the CO atmosphere and vacuum evaporation
of the solvent, 9 loses a single carbonyl to form 10.
Using 13CO, the products of these reactions were
characterized by 31P and 13C NMR spectroscopy. The
31P NMR spectrum of 13CO-labeled 9 exhibits a single
resonance for the magnetically and chemically equiva-
lent phosphine ligands. This resonance appears at δ
31.5 as a doublet of quartets due to coupling of the 31P
nuclei with the 103Rh nucleus (J Rh-P ) 71.7 Hz) and
three 13CO ligands (J P-C ) 14.1 Hz), Figure 5A. Re-
moval of the 13CO atmosphere results in the formation
of 10 as evidenced by the new 31P NMR resonance at δ
at low temperature, only the cis adduct 11 forms;
however, upon warming of the sample to room temper-
ature, a fraction of 11 slowly converts to 12 until a
mixture forms (4:1 cis:trans). Upon removal of the
acetonitrile, the cis adduct 11 converts back to the
starting material; however, the trans adduct, 12, does
not. At 55 °C in neat acetonitrile, a 1:2 ratio of 12 to
11 was formed. Acetonitrile adducts 11 and 12 are
apparently the kinetic and thermodynamic products,
respectively, in the reaction.
18.9, which appears as a doublet of triplets since the
13
phosphines are now coupled to 103Rh and only two
-
CO ligands rather than three, Figure 5B. The conver-
sion of 9 to 10 by loss of one carbonyl is consistent with
reactivity observed for one example of a related Rh
phosphinoether complex, [Rh(η2-CH3O(CH2)2PCy2)2]-
BPh4.23a Other examples of similar Rh phosphinoether
complexes are reported to lose two carbonyl ligands
upon removal of the CO atmosphere.23b-f
NMR Kin etic Stu d ies. Although not detectible in
1
the variable temperature one dimensional H and 31P
NMR experiments, compound 2a does undergo a facile,
degenerate exchange reaction involving the η6-arene and
free arene rings in CD2Cl2 and THF-d8, Scheme 1. The
ether groups in compound 2a are proposed to catalyze
the exchange process possibly through the formation of
the symmetrical cis-alkyldiphenylphosphine, cis-ether
complex 13, Scheme 1. This reaction was detected by
a variable-temperature 1H NMR EXSY experiment,12
and the activation parameters for the exchange reaction
were calculated from the Eyring equation.13 EXSY is a
useful technique for detecting exchange reactions with
rates of 102-10-2 s-1. This is in contrast to other one
(22) Examples of temperature-dependent chemical shifts: (a) Lind-
ner, E.; Gepra¨gs, M.; Gierling, K.; Fawzi, R.; Steimann, M. Inorg. Chem.
1995, 34, 6106. (b) Bianchini, C.; Caulton, K. G.; J ohnson, T. J .; Meli,
A.; Peruzzini, M.; Vizza, F. Organometallics 1995, 14, 933. (c) Bian-
chini, C.; Farnetti, E.; Graziani, M.; Nardin, G.; Vacca, A.; Zanobini,
F. J . Am. Chem. Soc. 1990, 112, 9190.
(23) (a) Lindner, E.; Wang, Q.; Mayer, H. A.; Fawzi, R.; Steimann,
M. Organometallics 1993, 12, 1865. (b) Lindner, E.; Norz, H. Chem.
Ber. 1989, 123, 459. (c) Lindner, E.; Norz, H. Z. Naturforsch 1989,
44b, 1493. (d) Lindner, E.; Andres, B. Z. Naturforsch. 1988, 43b, 369.
(e) Lindner, E.; Andres, B. Chem. Ber. 1987, 121, 829. (f) Lindner, E.;
Andres, B. Chem. Ber. 1986, 120, 761.
+
+

3068 Organometallics, Vol. 15, No. 13, 1996
Singewald et al.
Ta ble 3. F r ee En er gy of Activa tion a n d E1/2’s for 2a -c
compd
2a
2a
2b
2b
2c
2c
∆Gq298, kcal/mol (solvent) 17.7 (CD2Cl2) 14.1 (THF-d8) ND (THF-d8)a ND (CD2ClCD2Cl)a 23.2 (CD2ClCD2Cl) 20.2 (THF-d8)
E1/2 (mV)
573 (CH2Cl2)
515 (CH2Cl2 )
605 (CH2Cl2)
a
ND ) no detectible reaction; E1/2 referenced versus FcH/FcH+.
time (see Experimental Section). By performance of a
variable-temperature EXSY experiment and determi-
nation of the rates of exchange over a range of temper-
atures, the activation parameters for the reaction can
be calculated with the Eyring equation, Table 3.13
There are several notable features about the exchange
reaction. First, the ∆Gq for 2a in THF-d8 is 3.6 kcal
298
smaller than in CD2Cl2. Second, a symmetrical inter-
mediate or a pair of intermediates that are mirror
images of each other are necessary for this process since
the overall reaction is degenerate. Because of the ability
for Rh(I) to form stable square planar complexes, a
reasonable intermediate in the exchange reaction be-
tween 2a and 2a ′ is the symmetrical, cis-diphenylphos-
phine, cis-ether Rh(I) complex 13, Scheme 1. In the
proposed intermediate the ether groups serve as inter-
nal solvent molecules, which occupy two coordination
sites on the Rh(I) center during the exchange reaction.
In an effort to test the viability of this mechanism,
complex 2b was synthesized, characterized, and studied
1
by H and 31P NMR spectroscopy. With 2b, there is no
possibility for the formation of an intermediate such as
13 since the ether moieties have been replaced by
methylene groups. The 31P NMR spectrum of 2b in CD2-
Cl2 consists of two resonances, one for each phosphine,
with a coupling pattern similar to that observed in the
F igu r e 6. Aromatic region of the 1H NMR EXSY spectrum
of 2a in CD2Cl2 at 30 °C.
1
31P NMR spectrum of 2a in CD2Cl2. Like the H NMR
1
Sch em e 1
spectrum of 2a , the H NMR spectrum of 2b in CD2Cl2
exhibits an upfield resonance for the para-H atom of
the η6-arene (δ 4.68). This large upfield shift associated
with the para-H atom seems to be a general phenom-
enon for complexes of this type.
Significantly, by 1H NMR EXSY an arene-arene
exchange reaction could not be detected for complex 2b
in CD2Cl2 or in THF-d8 at temperatures comparable to
the EXSY studies of 2a . Even in dioxane-d8 at temper-
atures near 100 °C, no exchange reaction could be
detected. The ∆Gq for arene-arene exchange in 2b,
which is much higher than for 2a , is likely due to a
combination of two factors: (1) 2b does not have internal
ethers which can facilitate the arene-arene exchange
reaction through an intermediate comparable to 13, and
(2) the η6-arene in 2b is bound more strongly to the Rh-
(I) center than the η6-arene in 2a . Others have shown
that as the electron-donating properties of arenes are
increased, their binding affinities for Rh(I) also in-
crease.24 Indeed the arene in 2b is more electron rich
than the arene in 2a as evidenced by electrochemistry
for the two complexes, Table 3. The Rh(I)/Rh(II) redox
couple can be used as a diagnostic tool to determine the
relative electron-donating properties of the arenes, when
coordinated to Rh(I), in these isostructural complexes.25
Compound 2a is 58 mV more difficult to oxidize than
dimensional techniques, such as line shape analysis and
spin saturation techniques, which measure rates of
reactions above 10-1-104.12 In a 1H NMR EXSY
spectrum, the one dimensional NMR spectrum is plotted
along the diagonal, and the cross peaks are indicative
of protons that undergo a chemical exchange reaction.
1
The aromatic region of the H NMR EXSY spectrum of
2a is shown in Figure 6. A cross peak is observed which
correlates the resonances at δ 4.6 and 7.0. These
resonances are assigned to the H atoms which are para
to the ether group on the η6-arene and the free arene,
respectively. The rate of the exchange reaction can be
determined by integrating the area under the diagonal
and cross peaks and relating these values to the mixing
(24) (a) Espinet, P.; Bailey, P. M.; Downey, R. F.; Maitlis, P. M. J .
Chem. Soc., Dalton Trans. 1980, 1048. (b) White, C.; Thompson, S. J .;
Maitlis, P. M. J . Chem. Soc., Dalton Trans. 1977, 1654.
(25) Reversible Rh(I)/Rh(II) couples are unusual for mononuclear
organometallic compounds; however, this tends to be a property
common to this class of compounds. A detailed study of this redox
couple in a series of piano stool complexes has been undertaken, ref.14a
+
+

(Phosphinoalkyl)arene Ligands
Organometallics, Vol. 15, No. 13, 1996 3069
such as THF, Scheme 1. This is reasonable since we
know from other studies that THF does not displace the
ether groups in the isoelectronic model compound
14.26 Even though the η6-arene in 2c binds significantly
less strongly than the η6-arene in 2a , the barrier to
exchange for 2a is lower than for 2c in both a potentially
coordinating solvent (THF-d8) and a solvent which is
less likely to coordinate (CD2Cl2 and CD2ClCD2Cl).
Note that the solvent polarities for CD2Cl2 and CD2-
ClCD2Cl are comparable,27 suggesting that the differ-
ence in the ∆Gq
for the arene-arene exchange
298
processes involving 2a ,c is dependent on the solvent
polarity and not necessarily the coordinating properties
of the solvent systems studied herein. This is in
contrast to the intramolecular arene exchange reactions
observed for (diarylalkane)tricarbonyl chromium com-
plexes.4e Traylor and Goldberg determined that the rate
of the arene-arene exchange reaction increased in the
presence of coordinating solvents such as ketones and
ether moieties and via a detailed kinetic investigation
proposed an intermediate which is a solvent adduct.
Unlike their systems, we propose that the arene-arene
exchange between 2a and 2a ′ is not catalyzed by a
coordinating solvent but by its internal ether groups.
F igu r e 7. The 19F NMR EXSY spectrum of 2c in CD2-
ClCD2Cl at 60 °C.
Con clu sion s
In summary, we have reported the design, synthesis,
and characterization of a new class of hemilabile ligand
based on the strongly binding phosphine and weakly
binding arene moieties. Rh(I) complexes formed from
these ligands undergo novel η6-arene, free arene ex-
change reactions with rates that are dependent on the
electron richness of the arene, polarity of the solvent,
and the presence of ether groups that can catalyze the
reaction. We conclude that the internal ethers in these
complexes facilitate the exchange reaction most likely
through the formation of a square planar, cis-phosphine,
cis-ether Rh(I) intermediate.
2b, showing that the anisole-like ligand in 2a (when
coordinated to Rh(I)) is less electron donating than the
tolyl-like ligand in 2b. If the binding strength of the
η6-arene is the dominant factor controlling the activa-
tion barrier, then a Rh complex with an arene that is
less electron rich than the arene ligands in 2a ,b is likely
to have the lowest barrier to exchange.
To test this hypothesis, complex 2c, which has a
relatively electron-deficient η6-arene ligand and does not
possess ether groups, was synthesized and character-
ized. Note that the E1/2 of complex 2c is 32 mV more
positive than the E1/2 of 2a indicating that the F atom
significantly decreases the electron-donating properties
of the arene for 2c. Although an arene-arene exchange
reaction for 2c could not be detected by 19F NMR EXSY
in CD2Cl2, it was observed in CD2ClCD2Cl at elevated
temperatures, Figure 7; CD2ClCD2Cl has a higher
boiling point than CD2Cl2. In the EXSY experiment, a
cross peak is observed, which correlates the resonances
for the two F atoms on the exchanging arene rings. The
∆Gq298 determined from an Eyring plot of the EXSY data
Ack n ow led gm en t. C.A.M. acknowledges the Petro-
leum Research Fund (Grant No. 30759-AC3) and the
NSF (Grant CHE-9357099) for support of this work.
C.A.M. also acknowledges a DuPont Young Professor
Award, an A. P. Sloan Foundation Fellowship, and a
Camille Dreyfus Teacher Scholar Award. J ohnson-
Matthey is acknowledged for a generous loan of RhCl3.
Professor J oseph Lambert and Caroline A. Sassano are
acknowledged for helpful discussions.
for 2c in CD2ClCD2Cl is 5.5 kcal higher than the ∆Gq
298
Su p p or tin g In for m a tion Ava ila ble: Listings of crystal-
lographic data, including Cartesian coordinates, thermal
parameters, and complete bond lengths and angles (10 pages).
Ordering information is given on any current masthead page.
for 2a in CD2Cl2, Table 3. Even though it has a more
strongly binding arene than 2c, compound 2a ’s barrier
to exchange is lower than 2c’s. Although ∆Gq
is
298
affected by solvent, the same trend that is observed in
halogenated solvents is observed for 2a ,c in THF-d8. The
OM960114C
∆Gq
for 2c in THF-d8 is 6.1 kcal higher than the
298
∆Gq298 for 2a in THF-d8. This reinforces the importance
of the proposed intermediate 13 in the 2a to 2a ′ arene
exchange reaction even in a mildly coordinating solvent
(26) Singewald, E. T.; Mirkin, C. A.; Stern, C. L. Angew. Chem., Int.
Ed. Engl. 1995, 34, 1624; Angew. Chem. 1995, 107, 1725.
(27) Isaacs, N. S. Physical Organic Chemistry; Wiley and Sons: New
York, 1987; p 180.
+
+