Probing the factors that stabilize mononuclear rhodium(II) bis(phosphine), η6-arene complexes with piano-stool geometries

Article abstract of DOI:10.1021/ja963384v

The synthesis, characterization, and electrochemistry of a series of Rh(I) bis(phosphine), η⁶-arene, piano-stool complexes are reported. The study reported herein elucidates several of the important factors which lead to the stabilization of Rh(II) in this coordination environment. From the electrochemical data for a series of complexes of the type [Rh(η²-dppe)(η⁶-C₆H_6-(n)X(n))]BF₄ (X = CH₃, n = 0-6) (1-7) it was shown that the addition of methyl groups to the arene ligand kinetically stabilize the Rh(II) center and thermodynamically stabilize the Rh(II) species by 16 mV per added methyl group. Furthermore, complexes which contain chelation to the arene ligand, such as [Rh(η⁶:η¹-Ph(CH₂)₃PPh₂)(η¹-Ph(CH₂)₃PPh₂)]BF₄ (12), kinetically stabilize the Rh(II) form, presumably from ligand substitution based decomposition reactions. The electrochemical studies of five isostructural and isoelectronic complexes, [Rh(η²-dppe)(η⁶-C₆H₅CH₃)]BF₄ (2), [Rh(η¹-n-BuPPh₂)₂(η⁶-C₆H₅CH₃)]BF₄ (8), [Rh(η²-dppp)(η⁶-C₆H₅CH₃)]BF₄ (9), [Rh(η²-dphb)(η⁶-C₆H₅CH₃)]BF₄ (10), and 12 show that those complexes which contain bidentate, bis(phosphine) chelation with an ethyl or butyl bridge, 2 and 10, thermodynamically destabilize the Rh(II) form relative to those complexes which contain a less restricted bis(phosphine) chelate or no bis(phosphine) chelation. Using single-crystal X-ray data and extended Huckel calculations, these counterintuitive electrochemical trends were explained in terms of not only the properties of the Rh(I) complex but also, in terms of the structural changes which are likely to occur upon oxidation of the metal center from Rh(I) to Rh(II).

Full text of DOI:10.1021/ja963384v

3048
J. Am. Chem. Soc. 1997, 119, 3048-3056
Probing the Factors That Stabilize Mononuclear Rhodium(II)
Bis(phosphine), η6-Arene Complexes with Piano-Stool
Geometries
Elizabeth T. Singewald,† Caroline S. Slone,† Charlotte L. Stern,†
Chad A. Mirkin,*,† Glenn P. A. Yap,‡ Louise M. Liable-Sands,‡ and
Arnold L. Rheingold‡
Contribution from the Department of Chemistry, Northwestern UniVersity,
EVanston, Illinois 60208-3113, and Department of Chemistry and Biochemistry,
UniVersity of Delaware, Newark, Delaware 19716
ReceiVed September 26, 1996. ReVised Manuscript ReceiVed January 2, 1997X
Abstract: The synthesis, characterization, and electrochemistry of a series of Rh(I) bis(phosphine), η6-arene, piano-
stool complexes are reported. The study reported herein elucidates several of the important factors which lead to
the stabilization of Rh(II) in this coordination environment. From the electrochemical data for a series of complexes
of the type [Rh(η2-dppe)(η6-C6H6-nXn)]BF4 (X ) CH3, n ) 0-6) (1-7) it was shown that the addition of methyl
groups to the arene ligand kinetically stabilize the Rh(II) center and thermodynamically stabilize the Rh(II) species
by 16 mV per added methyl group. Furthermore, complexes which contain chelation to the arene ligand, such as
[Rh(η6:η1-Ph(CH2)3PPh2)(η1-Ph(CH2)3PPh2)]BF4 (12), kinetically stabilize the Rh(II) form, presumably from ligand
substitution based decomposition reactions. The electrochemical studies of five isostructural and isoelectronic
complexes, [Rh(η2-dppe)(η6-C6H5CH3)]BF4 (2), [Rh(η1-n-BuPPh2)2(η6-C6H5CH3)]BF4 (8), [Rh(η2-dppp)(η6-C6H5-
CH3)]BF4 (9), [Rh(η2-dppb)(η6-C6H5CH3)]BF4 (10), and 12 show that those complexes which contain bidentate,
bis(phosphine) chelation with an ethyl or butyl bridge, 2 and 10, thermodynamically destabilize the Rh(II) form
relative to those complexes which contain a less restricted bis(phosphine) chelate or no bis(phosphine) chelation.
Using single-crystal X-ray data and extended Hu¨ckel calculations, these counterintuitive electrochemical trends were
explained in terms of not only the properties of the Rh(I) complex but also, in terms of the structural changes which
are likely to occur upon oxidation of the metal center from Rh(I) to Rh(II).
Introduction
Chart 1
Rh(I) and Rh(III) complexes are known to play an enormous
role in the mechanistic cycles of many catalytic processes,1 and
therefore, it is not surprising that Rh complexes in these
oxidation states have been studied extensively. In comparison,
little is known about the chemistry of Rh(II) complexes,2
especially monomeric, organometallic Rh(II) compounds.3 Of
the isolable compounds known, all adopt either square-planar
or sandwich coordination geometries.
complexes, 11 and 124 (Chart 1). The general class of cationic
Rh(I) η6-arene, piano-stool complexes, without tethered arene
ligands, has been studied by various research groups since the
early 1970s.5 Our complexes, in addition to exhibiting unusual
dynamic intramolecular arene exchange behavior4a-c and cata-
lytic processes that can be controlled with redox-active arene
substituents,4d exhibit reversible electrochemistry associated with
their Rh(I)/(II) redox couples.4a-c From such preliminary
experiments, we concluded that these complexes support Rh-
(II) oxidation states that are stable at least on the time scale of
the electrochemical experiments performed. Therefore, such
studies suggest that modification of the ligands, perhaps even
minor ones, may give entry into a new class of isolable, Rh(II)
compounds.
In recent years, our group has been engaged in the develop-
ment of the chemistry of Rh(I) bis(alkylphosphine), η6-arene
† Northwestern University
‡ University of Delaware.
* Author to whom correspondence should be addressed.
X Abstract published in AdVance ACS Abstracts, March 15, 1997.
(1) (a) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis; Wiley-
Interscience: New York, 1992. (b) Dickson, R. S. Homogeneous Catalysis
with Compounds of Rhodium and Iridium; D. Reidel Publishing Co.:
Dordrecht, Holland, 1985. (c) Collman, J. P.; Hegedus, L. S.; Norton, J.
R.; Finke, R. G. Principles and Applications of Organotransition Metal
Chemistry; University Science Books: Mill Valley, CA, 1987.
(2) Rh(II) reactivity studies: (a) Howe, J. P.; Lung, K.; Nile, T. A. J.
Organomet. Chem. 1981, 208, 401. (b) Arzoumanian, H.; Blanc, A. A.;
Metzger, J.; Vincent, J. E. J. Organomet. Chem. 1974, 82, 261. Dimeric
Rh(II): (c) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal
Atoms; Wiley Interscience: New York, 1982, p 311. Rh(II) reviews: (d)
Pandey, K. K. Coord. Chem. ReV. 1992, 121, 1. (e) DeWit, D. G. Coord.
Chem. ReV. 1996, 147, 209.
Herein, we report the synthesis, characterization, and elec-
trochemical properties for a series of isostructural and isoelec-
tronic Rh(I) complexes 1-10 (Chart 2). In addition, the single-
crystal X-ray diffraction and extended Hu¨ckel studies of 5, 8,
(3) (a) Collins, J. E.; Castellani, M. P.; Rheingold, A. L.; Miller, E. J.;
Geiger, W. E.; Rieger, A. L.; Rieger, P. H. Organometallics 1995, 14, 1232.
(b) Garc´ıa, M. P.; Jime´nez, M. V.; Oro, L. A.; Lahoz, F. J.; Casas, J. M.;
Alonso, P. J. Organometallics 1993, 12, 3257. (c) Dunbar, K. R.; Haefner,
S. C. Organometallics 1992, 11, 1431. (d) Hay-Motherwell, R. S.;
Koschmieder, S. U.; Wilkinson, G.; Hussain-Bates, B.; Hursthouse, M. B.
J. Chem. Soc., Dalton Trans. 1991, 2821. (e) Bowyer, W. J.; Merkert, J.
W.; Geiger, W. E.; Rheingold, A. L. Organometallics 1989, 8, 191.
(4) (a) 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. (b)
Singewald, E. T.; Shi, X.; Mirkin, C. A.; Schofer, S. J.; Stern, C. L.
Organometallics 1996, 15, 3062. (c) Sassano, C. A.; Mirkin, C. A. J. Am.
Chem Soc. 1995, 117, 11379. (d) Slone, C. S.; Mirkin, C. A. Unpublished
results.
S0002-7863(96)03384-7 CCC: $14.00 © 1997 American Chemical Society

Rh(II) Bis(phosphine), η6-Arene Complexes
J. Am. Chem. Soc., Vol. 119, No. 13, 1997 3049
solution of n-Bu4NPF6 in CH2Cl2 was used as the supporting electrolyte.
All electrochemical data are referenced versus the FcH/[FcH]+ [Fc )
(η5-C5H5)Fe(η5-C5H4)] redox couple. Fast atom bombardment (FAB)
mass spectra were recorded on a Fisions VG 70-250 SE mass
spectrometer. High-resolution mass spectrometry data were not at-
tainable for many of the compounds reported due to low intensity of
the parent peaks in the spectra relative to the baseline noise.
Chart 2
[Rh(η6-benzene)(η2-dppe)]tetrafluoroborate (1). 1H NMR (CD2-
Cl2): δ 2.25 (d, 4H, JP-H ) 21.3 Hz, CH2), 6.31 (s, 6H, C6H6), 7.53
(m, 20H, P(C6H5)2). 13C{1H} NMR (CD2Cl2): δ 25.53 (m, CH2),
101.94 (s, C6H6), 128.63-132.51 (m, P(C6H5)2). FABMS: [M+] )
m/z 579.
[Rh(η6-toluene)(η2-dppe)]tetrafluoroborate (2). 1H NMR (CD2-
Cl2): δ 1.68 (s, 3H, CH3), 2.24 (d, 4H, JP-H ) 21.4 Hz, CH2), 6.17
(m, 2H, o-C6H5CH3), 6.30 (m, 2H, m-C6H5CH3), 7.20 (m, 1H, p-C6H5-
CH3), 7.55 (m, 20H, P(C6H5)2). 13C{1H} NMR (CD2Cl2): δ 19.37 (s,
CH3), 29.87 (m, CH2), 100.89 (s, C6H5CH3), 103.04 (s, C6H5CH3),
118.34 (s, C6H5CH3), 125.55 (s, C6H5CH3), 128.48-132.61 (m,
P(C6H5)2). FABMS: [M+] ) m/z 593.
Synthesis of [Rh(η6-1,3-dimethylbenzene)(η2-dppe)]tetrafluoro-
borate (3). [Rh(η4-C7H8)(η2-dppe)]BF4 (40 mg, 0.06 mmol) in 5 mL
of CH3OH was bubbled with H2 for 5 min. The color of the solution
gradually changed from orange to yellow. The solution was concen-
trated to 3 mL, and 25 mL of 1,3-dimethylbenzene (5 equiv, 0.20 mol)
was added. After 12 h of stirring at room temperature, the solvent
was removed. Pure product 3 was isolated in quantitative yield based
on spectroscopic data (yield ) 40 mg, 0.06 mmol, >99%). 1H NMR
(CD2Cl2): δ 1.74 (s, 6H, CH3), 2.19 (d, 4H, JP-H ) 21.6 Hz, CH2),
6.00 (m, 3H, o, p-C6H4(CH3)2), 6.26 (t, 1H, JH-H ) 6.8 Hz, m-C6H4-
(CH3)2), 7.55 (m, 20H, P(C6H5)2). 13C{1H} NMR (CD2Cl2): δ 19.47
(s, CH3), 30.20 (m, CH2), 100.33 (s, C6H4(CH3)2), 101.57 (s, C6H4-
(CH3)2), 105.13 (s, C6H4(CH3)2), 116.99 (s, C6H4(CH3)2), 128.29-
133.98 (m, P(C6H5)2). HRFABMS: [M+] calcd ) m/z 607.1191, [M+]
found ) m/z 607.1187.
9, and 12 are presented. These piano-stool compounds allow
for the systematic determination of the fundamental factors that
control the stability of Rh(II) in complexes with this coordination
geometry. Such factors include (1) the arene substituents, (2)
the presence (1-7, 9, 10, or 12) or absence (8) of ligand
chelation, (3) the type of ligand chelation (by a bis(phosphine)
ligand as in 1 or through a (phosphinoalkyl)arene ligand as in
12), and (4) the structural changes which may occur on oxidation
of the metal center. To the best of our knowledge, this is the
most extensive systematic electrochemical, structural, and
spectroscopic study of a set of isoelectronic Rh(I) complexes
yet reported and, significantly, it elucidates several counterin-
tuitiVe factors that control the electron richness and stabilities
of the Rh(II) centers in this coordination geometry that should
extend well beyond this important class of compounds.
Experimental Section
Synthesis of [Rh(η6-1,3,5-trimethylbenzene)(η2-dppe)]tetrafluo-
roborate (4). Synthesis of 4 is identical to the synthesis of 3 except
1,3,5-trimethylbenzene is used as the arene (yield ) >95%). 1H NMR
(CD2Cl2): δ 1.79 (s, 9H, CH3), 2.16 (d, 4H, JP-H ) 21.0 Hz, CH2),
5.85 (s, 3H, C6H3(CH3)3), 7.56 (m, 20H, P(C6H5)2). 13C{1H} NMR
(CD2Cl2): δ 19.75 (s, CH3), 31.74 (m, CH2), 103.53 (s, C6H3(CH3)3),
116.25 (s, C6H3(CH3)3), 129.34-133.88 (m, P(C6H5)2). FABMS: [M+]
) m/z 621.
General Procedure. All reactions were carried out under nitrogen
using standard Schlenk techniques or in an inert atmosphere glovebox.
Methylene chloride was distilled from calcium hydride. Tetrahydro-
furan (THF) was dried over sodium/benzophenone. Benzene, toluene,
and 1,3,5-trimethylbenzene were dried over sodium. Methanol was
dried over MgO. All solvents were distilled and degassed prior to use.
Deuterated solvents were purchased in ampules from Cambridge Isotope
Laboratories and used without further purification. n-Butyldiphen-
ylphosphine was purchased from Lancaster Chemical Co. and distilled
over sodium prior to use. RhCl3‚xH2O was used on loan from Johnson-
Matthey Chemical Co. Compounds 1 and 2 were prepared according
to the method of Halpern.5c,d Compound 12,4a,b [Rh(µ-Cl)(η2-C8H14)2]x,6
[Rh(η4-C7H8)(η2-dppe)]BF4 (dppe ) 1,2-bis(diphenylphosphino)eth-
ane),7 and [Rh(η4-C7H8)(η2-dppp)]BF4 (dppp ) 1,3-bis(diphenylphos-
phino)propane)7 were prepared according to literature procedures. All
other chemicals were purchased from Aldrich Chemical Co. and used
as received.
Physical Measurements. 1H and 13C{1H} NMR spectra were
recorded on either a Varian Gemini 300 MHz, a Varian VXR 300 MHz,
or a Varian Unity 400 MHz FT-NMR spectrometer. 31P{1H} NMR
spectra were recorded on a Varian Gemini 300 MHz FT-NMR
spectrometer at 121 Hz and referenced versus the external standard
85% H3PO4. Electrochemical measurements were carried out on either
a PINE AFRDE4 or AFRDE5 bipotentiostat (CV) or a PAR 273A
potentiostat/galvanostat (DPV) using a Pt working electrode (0.02 cm2),
a Pt mesh counter electrode, and a Ag wire reference electrode.
Rotating disk voltammetry experiments were carried out using a PINE
Pt rotating disk electrode at 1000 rotations/min. In all cases, a 0.1 M
Synthesis of [Rh(η6-1,2,4,5-tetramethylbenzene)(η2-dppe)]tet-
rafluoroborate (5). Synthesis of 5 is identical to the synthesis of 3
except 25 equiv of 1,2,4,5-tetramethylbenzene was used as the arene.
Complex 5 is purified by washing away excess 1,2,4,5-tetramethyl-
benzene with three 5 mL portions of benzene and removing any excess
solvent by vacuum evaporation (yield ) >95%).
1H NMR (CD2Cl2):
δ 1.69 (s, 12H, CH3), 2.12 (d, 4H, JP-H ) 21.5 Hz, CH2), 5.80 (s, 2H,
C6H2(CH3)4), 7.57 (m, 20H, P(C6H5)2). 13C{1H} NMR (CD2Cl2): δ
17.44 (s, CH3), 30.02 (m, CH2), 102.72 (s, C6H2(CH3)4), 116.08 (s,
C6H2(CH3)4), 129.34-133.30 (m, P(C6H5)2). HRFABMS: [M+] calcd
) m/z 635.1504, [M+] found ) m/z 635.1486.
Synthesis of [Rh(η6-pentamethylbenzene)(η2-dppe)]tetrafluoro-
borate (6). Synthesis of 6 is identical to the synthesis of 5 except
1,2,3,4,5-pentamethylbenzene is used as the arene (yield ) >95%).
1H NMR (CD2Cl2): δ 1.68 (s, 6H, CH3), 1.72 (s, 6H, CH3), 1.80 (s,
3H, CH3), 2.04 (d, 4H, JP-H ) 21.6 Hz, CH2), 5.78 (s, 1H, C6H(CH3)5),
7.57 (m, 20H, P(C6H5)2). 13C{1H} NMR (CD2Cl2): δ 14.11 (s, CH3),
15.26 (s, CH3), 18.08 (s, CH3), 29.99 (m, CH2), 101.68 (s, C6H(CH3)5),
112.73 (s, C6H(CH3)5), 113.68 (s, C6H(CH3)5), 114.48 (s, C6H(CH3)5),
128.42-132.13 (m, P(C6H5)2). HRFABMS: [M+] calcd ) m/z
649.1660, [M+] found ) m/z 649.1646.
Synthesis of [Rh(η6-hexamethylbenzene)(η2-dppe)]tetrafluoro-
borate (7). Synthesis of 7 is identical to the synthesis of 5 except
1,2,3,4,5,6-hexamethylbenzene is used as the arene (yield ) >95%).
1H NMR (CD2Cl2): δ 1.75 (s, 18H, CH3), 1.95 (d, 4H, JP-H ) 21.0
Hz, CH2), 7.59 (m, 20H, P(C6H5)2); 13C{1H} NMR (CD2Cl2): δ 16.27
(s, CH3), 30.73 (m, CH2), 113.52 (s, C6(CH3)6), 129.33-133.23 (m,
P(C6H5)2); HRFABMS: [M+] calcd ) m/z 663.1817, [M+] found )
m/z 663.1837.
(5) (a) Schrock, R. R.; Osborn, J. A. J. Am. Chem. Soc. 1971, 93, 3089.
(b) Green, M.; Kuc, T. A. J. Chem. Soc., Dalton Trans. 1972, 832. (c)
Halpern, J.; Riley, D. P.; Chan, A. S. C.; Pluth, J. J. J. Am. Chem. Soc.
1977, 99, 8055. (d) Halpern, J.; Chan, A. S. C.; Riley, D. P.; Pluth, J. J.
AdV. Chem. Ser. 1979, No. 173, 16. (e) Uson, R.; Lahuerta, P.; Reyes, J.;
Oro, L. A., Foces-Foces, C.; Cano, F. H.; Garcia-Blanco, S. Inorg. Chim.
Acta 1980, 42, 832.
(6) Porri, L.; Lionetti, A.; Immirizi, A. Chem. Commun. 1965, 356.
(7) Brown, J. M.; Chaloner, P. A.; Kent, A. G.; Murrer, B. A.; Nicholson,
P. N.; Parker, D.; Sidebottom, P. J. J. Organomet. Chem. 1981, 216, 263.

3050 J. Am. Chem. Soc., Vol. 119, No. 13, 1997
Table 1. Crystallographic Data for 5, 8, and 12
Singewald et al.
Crystals of 8 were grown from a 1:10 mixture of THF and toluene.
Selected crystallographic data are given in Table 1. The unit-cell
parameters were obtained by the least-squares refinement of the angular
settings of 24 reflections (20° e 2θ e 24°). The systematic absences
in the diffraction data are consistent with the space groups Pcam (Pbcm)
and Pca21. Both options were explored; however, the centrosymmetric
space group yielded chemically bizarre and computationally unstable
results. The structure was solved in Pca21 using direct methods,
completed by subsequent difference Fourier syntheses, and refined by
full-matrix least-squares procedures. Refinement of the Flack parameter
suggested racemic twinning and the structure was subsequently refined
with scale factors as a 70/30 racemate. A semiempirical absorption
correction was applied to the data set. The phenyl rings were treated
as rigid groups. All non-hydrogen atoms were refined with anisotropic
displacement coefficients. Hydrogen atoms were treated as idealized
contributions.8a
5
8
12
formula
fw
crystal system
space group
a, Å
b, Å
c, Å
C36H38BF4P2Rh C39H46BF4P2Rh C42H42BF4P2Rh
722.32
monoclinic
P21/c
766.42
orthorhombic
Pca21
798.45
monoclinic
Cc
15061(2)
11.308(1)
19.880(1)
102.741(6)
3302.4(6)
4
13.341(3)
19.017(3)
18.264(4)
16.828(3)
12.039(4)
19.092(6)
107.78(2)
3683(3)
4
â, deg
V, Å3
3592(2)
4
1.417
orange
Z
D
calc, g cm-1
1.453
orange
1.440
orange
color
size, mm
0.40 × 0.40 × 0.40 × 0.20 × 0.22 × 0.51 ×
0.30 0.05 0.14
6.13 5.90
µ (Mo KR), cm-1 6.61
Crystals of 9 were grown from a 1:1 mixture of methanol-d4 and
toluene.8c All inspected crystals were either twinned or multiple and
complicated single-crystal X-ray diffraction studies. A low-quality
structure was determined (see Supporting Information) and is referred
to here in terms of connectivity and, specifically, the P-Rh-P bond
angle (91.74° (11)) and Rh-P bond length (2.223 (2)Å).
temp, K
radiation
R(F), %
R(wF), %
298
Mo KR
3.68
247
Mo KR
4.19
153
Mo KR
2.7
8.08
7.33
3.2
Synthesis of [Rh(η6-toluene)(CH3(CH2)3P(C6H5)2)2]tetrafluoroborate
(8). Compound 8 was synthesized by reacting [Rh(µ-Cl)(η2-C8H14)2]x
(0.078 g, 0.22 mmol) with 2 equiv of n-BuP(C6H5)2 (0.11 g, 0.44 mmol,
0.10 mL) in THF (5 mL) at room temperature under constant stirring.
After 30 min, 1 equivalent of AgBF4 (0.042 g, 0.22 mmol) and excess
toluene (2 mL) were added to the solution while the reaction mixture
was continually stirred. After 1 h, the reaction mixture was filtered to
remove AgCl and evaporated to dryness. The product 8 was isolated
as a microcrystalline orange solid (0.147 g, 0.21 mmol, yield ) 95%).
1H NMR (CD2Cl2): δ 0.79 (t, 3H, JH-H ) 7.2 Hz, CH2CH3), 1.21 (m,
2H, CH2CH3), 1.36 (m, 2H, PCH2CH2CH2), 1.70 (m, 2H, PCH2), 2.25
(s, 3H, C6H5CH3), 5.61 (d, 2H, JH-H ) 6.6 Hz, o-C6H5CH3), 5.71 (t,
2H, JH-H ) 6.6 Hz, m-C6H5CH3), 6.87 (t, 1H, JH-H ) 6.3 Hz, p-C6H5-
CH3), 7.19 and 7.38 (m, 20H, P(C6H5)2). FABMS: [M+] ) m/z 679.
Synthesis of [Rh(η6-toluene)(η2-dppp)]tetrafluoroborate (9). [Rh-
(η4-C7H8)(η2-dppp)]BF4 (20 mg, 0.03 mmol) was dissolved in methanol-
d4 (0.5 mL) in an NMR tube, and the tube was charged with H2. After
10 h, the diene was completely hydrogenated to form [Rh(CD3OD)2-
(η2-dppp)]BF4 and excess toluene (1 mL) was added to the NMR tube.
After 2 h, the formation of product 9 was complete in quantitative
spectroscopic yield. 1H NMR (CD2Cl2): δ 1.87 (m, 2H, PCH2CH2),
1.91 (s, 3H, CH3), 2.48 (m, 4H, PCH2), 5.66 (d, 2H, JH-H ) 6.3 Hz,
o-C6H5CH3), 5.80 (t, 2H, JH-H ) 6.7 Hz, m-C6H5CH3), 6.15 (t, 1H,
JH-H ) 6.3 Hz, p-C6H5CH3), 7.45 (m, 20H, P(C6H5)2). FABMS: [M+]
) m/z 607.
Complex 12 was synthesized according to literature methods and
crystals suitable for diffraction were grown from a 1:10 mixture of
methylene chloride and diethyl ether.4a,b Selected crystallographic data
are given in Table 1. The systematic absences are consistent for the
space groups Cc and C2/c. On the basis of packing considerations, a
statistical analysis of the intensity distribution, and the successful
refinement of the structure, the space group was determined to be Cc.
The structure was solved by Patterson methods.8b The data were
corrected for Lorentz and polarization effects. In addition, an analytical
absorption correction was applied with transmission factors ranging
from 0.80 to 0.91, and a correction for secondary extinction was applied
(coefficient ) 0.21105 × 10-7). All hydrogen atoms were treated as
idealized contributions.8b
Results and Discussion
Syntheses of 1-10. Complexes 1-7, 9, and 10 were
synthesized by reacting the appropriate [Rh(η4-diene)(η2-
bis(phosphine))]BF4 precursor with hydrogen in the presence
of CD3OD or CH3OH and subsequently adding an excess
amount of the appropriate arene ligand, eq 1. Complex 8 was
Synthesis of [Rh(η6-toluene)(η2-dppb)]tetrafluoroborate (10).
Complex 10 was synthesized in a manner similar to the synthesis of 9,
except [Rh(η4-C8H12)(η2-dppb)]BF4 was used as the starting material
(spectroscopic yield ) >99%). 1H NMR (CD2Cl2): δ 1.56 (m, 4H,
PCH2CH2), 1.95 (s, 3H, CH3), 2.39 (m, 4H, PCH2), 5.32 (d, 2H, JH-H
) 6.5 Hz, o-C6H5CH3), 5.45 (t, 2H, JH-H ) 6.6 Hz, m-C6H5CH3), 6.62
(t, 1H, JH-H ) 6.3 Hz, p-C6H5CH3), 7.56 (m, 20H, P(C6H5)2).
HRFABMS: [M+] calcd ) m/z 621.1347, [M+] found ) m/z 621.1359.
X-ray Structure Determinations. Crystals of 5 suitable for X-ray
diffraction were grown in a methylene chloride solution by slow
evaporation. Selected crystallographic data are given in Table 1. The
systematic absences in the diffraction data are uniquely consistent for
the reported space group, P21/c. The structure was solved using direct
methods, completed by subsequent difference Fourier syntheses, and
refined by full-matrix least-squares procedures. Absorption corrections
were not applied because there was less than 10% variation in the
ψ-scan data. All non-hydrogen atoms were refined with anisotropic
displacement coefficients. Hydrogen atoms were treated as idealized
contributions.8a
synthesized by reacting 1 equiv of [Rh(µ-Cl)(η2-C8H14)2]x6 with
2 equiv of n-BuPPh2, followed by halide abstraction with AgBF4
in the presence of excess toluene. All complexes were isolated
in high yield (> 95%). The benzene and the toluene adducts,
1 and 2, respectively, were originally synthesized by Halpern
and co-workers.5c,d Complexes 3-10 are new and have been
fully characterized (see the Experimental Section).
Selected 1H, 13C, and 31P NMR Data for 1-10. Complexes
1
1-10 exhibit characteristic upfield shifts for the H and 13C
1
NMR resonances assigned to the η6-arenes. In the H NMR
spectra of 1-10, the η6-arene resonances shift an average of
1.0 ppm upfield upon coordination to the metal in all complexes.
Upfield shifts also are observed in the 13C NMR spectra for
1-7 of 17.8-28.5 ppm, depending on the arene.9 Such 1H and
13C NMR upfield shifts are characteristic for arenes coordinated
in an η6-fashion to a metal center.10
(8) All software and sources of the scattering factors are contained in
the (a) SHELXTL (5.3) program library or (b) SHELXS-86 (G. Sheldrick,
Siemens XRD, Madison, WI). (c) Selected crystallographic data for 9:
crystal system ) monoclinic; space group ) Pnma; unit cell parameters )
a (17.109(6) Å), b (20.858(9) Å), c (11.083(4) Å), V (3955(3) Å3); Rh-P
) 2.223(2) Å; P-Rh-P ) 91.74(11)°
(9) Compounds 8-10 were not characterized by 13C NMR spectroscopy.

Rh(II) Bis(phosphine), η6-Arene Complexes
J. Am. Chem. Soc., Vol. 119, No. 13, 1997 3051
Table 2. Electrochemicala and 31P NMRb Data for Complexes 1-12
complex
E1/2 (mV)
δ
JRh-P (Hz)
1 [Rh(η6-C6H6)(η2-(C6H5)2P(CH2)2P(C6H5)2)]BF4
2 [Rh(η6-C6H5CH3)(η2-(C6H5)2P(CH2)2P(C6H5)2)]BF4
3 [Rh(η6-C6H4(CH3)2)(η2-(C6H5)2P(CH2)2P(C6H5)2)]BF4
4 [Rh(η6-C6H3(CH3)3)(η2-(C6H5)2P(CH2)2P(C6H5)2)]BF4
5 [Rh(η6-C6H2(CH3)4)(η2-(C6H5)2P(CH2)2P(C6H5)2)]BF4
6 [Rh(η6-C6H(CH3)5)(η2-(C6H5)2P(CH2)2P(C6H5)2)]BF4
7 [Rh(η6-C6(CH3)6)(η2-(C6H5)2P(CH2)2P(C6H5)2)]BF4
8 [Rh(η6-C6H5CH3)(η1-n-BuPPh2)2]BF4
irreversible
635
620
609
590
571
555
505
525
77.3
78.1
78.7
79.4
79.6
79.3
79.5
35.3
26.5
42.1
32.6, 34.9
36.6, 39.8
203.4
204.0
204.4
205.0
204.8
205.9
206.6
202.3
190.2
198.0
198.7, 210.4
203.9, 198.8
9 [Rh(η6-C6H5CH3)(η2-(C6H5)2P(CH2)3P(C6H5)2)]BF4
10 [Rh(η6-C6H5CH3)(η2-(C6H5)2P(CH2)4P(C6H5)2)]BF4
11c [Rh(η6:η1-PhO(CH2)2PPh2)(η1-PhO(CH2)2PPh2)]BF4
12c [Rh(η6:η1-Ph(CH2)3PPh2)(η1-Ph(CH2)3PPh2)]BF4
585
573
515
n
a Differential pulse voltammetry (DPV): 0.1 M Bu4NPF6/CH2Cl2 at 21 °C vs FcH/FcH+. b CD2Cl2. c Reference 4a.
Figure 2. ORTEP drawing of 5. Thermal ellipsoids are drawn at 30%
probability. BF4 group is omitted for clarity.
Figure 1. Graph of rhodium-phosphorus coupling constant versus
number of methyl substituents for complexes 1-7.
however, is not the only factor which contributes to the chemical
shift and coupling constant values in the 31P NMR spectra of
1-7.13 For example, slight structural changes within this family
of compounds must also contribute to these spectroscopic values.
For example, increased substituents on the arene most likely
result in increased steric interactions between the methyl groups
and the phenyl groups that are part of the bis(phosphine) ligands.
This would be expected to influence the P-Rh-P bond angle
and contribute to the trends observed in the 31P NMR spectra.
Each of the 31P NMR spectra for compounds 1-10 exhibits
a single resonance with JRh-P values of 190-207 Hz Table 2.
The resonances for the dppe complexes (1-7) are in the range
δ 77.5-79.3. The nonchelated complex 8 exhibits a resonance
further upfield at δ 35.3, while the dppp complex 9 and the
dppb complex 10 exhibit resonances at δ 26.5 and 42.1,
respectively. Such chemical shift differences between the
bidentate bis(phosphine) ligands, the monodentate phosphines,
and the chelated phosphinoalkylarenes, in 11 and 12, are a
function of the absence or presence of chelation and the chelate
ring size.11 The 31P NMR chemical shift values for 1-7, 9,
and 10 are comparable to those reported in the literature for
Rh(I) complexes with the same chelating bis(phosphine)
ligands.11,12
Within the family of complexes of the general formula [Rh-
(η2-dppe)(η6-C6H6-nXn)]BF4 (X ) CH3, and n ) 0-6), 1-7,
additional methyl substituents on the arene have a small, but
measurable, effect on the 31P NMR chemical shifts and coupling
constants. In general, increased electron-donating ability of the
arenes corresponds to an increase in the coupling between Rh
and P in a linear fashion (Figure 1 and Table 2). Furthermore,
the 31P NMR chemical shifts values for 1-7 are also affected
by the number of methyl substituents on the arene ring. Overall,
increased electron richness of the arene leads to higher chemical
shifts, although this affect trails off with increasing substitution
on the arene. The electron richness of the arene ligand,
Solid-State Characterization of 5, 8, and 12. The structures
of complexes 5, 8, and 12 were determined by single-crystal
X-ray diffraction methods (Figures 2-4). Selected bond lengths
and angles are given in Table 3. An ORTEP diagram of cation
5 is shown in Figure 2. The Rh atom sits in a bis(phosphine),
η6-arene piano-stool geometry, and the P-Rh-P angle of 83.76°
compares well with other complexes containing the Rh-dppe
fragment (82.1-84.8°).14 The Rh-P bond lengths (Rh-Pavg
) 2.219 Å) also compare well with the only example in the
literature of a similar Rh bis(phosphine), η6-arene complex,
[Rh(η2-dppe)(η6-C6H5BPh3)] (P-Rh-P ) 84.3°, Rh-Pavg
)
2.221 Å).14c The arene ring in complex 5 is not planar (average
deviation ) 0.0240 Å) but rather adopts a boat conformation
with the bow and stern pointing toward the Rh center. Through
theoretical calculations done by others, the origin of this
(13) It is well-known that electronic and structural factors dictate the
chemical shifts and especially the coupling constants for such metal
compounds. For more information, see: Verkade, J. G.; Quin, L. D.
Phosphorus-31 NMR Spectroscopy in Stereochemical Analysis; VCH
Publishers: Deerfield Beach, FL, 1987.
(10) (a) McFarlane, W.; Grim, S. O. J. Organomet. Chem. 1966, 5, 147.
(b) Price, J. T.; Sorensen, T. S. Can. J. Chem. 1968, 46, 515. (c) Mann, B.
E. Chem. Commun. 1971, 976. (d) Ko¨hler, F. H. Chem. Ber. 1974, 107,
570.
(11) Garrou, P. E. Chem. ReV. 1981, 81, 229.
(12) Fornika, R.; Go¨rls, H.; Seemann, B.; Leitner, W. J. Chem. Soc.,
Chem. Commun. 1995, 1479.
(14) (a) Becalski, A. G.; Cullen, W. R.; Fryzuk, M. D.; James, B. R.;
Kang, G.-J.; Rettig, S. J. Inorg. Chem. 1991, 30, 5002. (b) Chan, A. S. C.;
Shieh, H.-S.; Hill, J. R. J. Organomet. Chem. 1985, 279, 171. (c) Albano,
P.; Aresta, M.; Manaserro, M. Inorg. Chem. 1980, 19, 1069. (d) Chan, A.
S. C.; Pluth, J. J.; Halpern, J. Inorg. Chim. Acta 1979, 37, L477. (e) Hall,
M. C.; Kilbourn, B. T.; Taylor, K. A. J. Chem. Soc., Dalton Trans. 1970,
2539.

3052 J. Am. Chem. Soc., Vol. 119, No. 13, 1997
Singewald et al.
Table 3. Selected Distances (Å) and Angles (deg) for Complexes 5, 8, and 12
complex 5
complex 8
complex 12
Rh-P(1)
Rh-P(2)
Rh-C(1)
Rh-C(2)
Rh-C(3)
Rh-C(4)
Rh-C(5)
Rh-C(6)
2.217(1)
2.222(1)
2.314(5)
2.366(5)
2.336(5)
2.303(5)
2.370(5)
2.335(5)
1.87
Rh-P(1)
Rh-P(2)
Rh-C(1)
Rh-C(2)
Rh-C(3)
Rh-C(4)
Rh-C(5)
Rh-C(6)
2.244(3)
2.258(3)
2.367(9)
2.317(10)
2.356(11)
2.296(13)
2.315(12)
2.385(10)
1.88
Rh-P(1)
Rh-P(2)
Rh-C(1)
Rh-C(2)
Rh-C(3)
Rh-C(4)
Rh-C(5)
Rh-C(6)
2.228(2)
2.251(1)
2.294(5)
2.326(5)
2.360(5)
2.310(5)
2.331(5)
2.339(6)
1.84
Rh-arene cent.
P(1)-Rh-P(2)
Rh-arene cent.
P(1)-Rh-P(2)
Rh-arene cent.
P(1)-Rh-P(2)
83.76(4)
93.35(10)
91.44(6)
Figure 3. ORTEP drawing of 8. Thermal ellipsoids are drawn at 30%
probability. BF4 group is omitted for clarity.
Figure 4. ORTEP drawing of 12. Thermal ellipsoids are drawn at
50% probability. BF4 group is omitted for clarity.
deformation is proposed to be of an electronic, rather than a
steric, origin.15
compounds 1-10 were performed in CH2Cl2 at 21 °C with a
0.1 M solution of n-Bu4NPF6 as the supporting electrolyte, and
all E1/2 values are given versus the FcH/[FcH]+ [Fc ) (η5-C5H5)-
Fe(η5-C5H4)] redox couple (Table 2). The redox couples for 4
and 6 have been confirmed to be one-electron-transfer processes
via cyclic and rotating disk voltammetry experiments. On the
basis of these studies, the reversible electrochemistry observed
for 2, 3, 5, 7-10, and 12 is also assumed to be associated with
a Rh(I)/Rh(II) couple.
An ORTEP diagram of complex 8 is shown in Figure 3. The
Rh-P bond distances (Rh-Pavg ) 2.251 Å) for complex 8 are
longer than the average Rh-P distances for the other structurally
characterized compounds reported herein. In addition, the Rh-
arene centroid distance (1.88 Å) for this compound is longer
than those for the other three structures (1.84-1.87 Å) and the
P-Rh-P angle in 8 (93.35°) is larger than the P-Rh-P angle
for the other three complexes (91.75-83.75°). Presumably,
these differences are, in part, a result of the bulky n-butyl
diphenylphosphine ligands in 8. The Rh-C bond distances
follow a similar trend as the Rh-C bond distances in complex
5. The arene ring in 8 is not planar, however, the boat
conformation is less evident in this structure (average deviation
) 0.0207 Å).
An ORTEP diagram of compound 12 is shown in Figure 4.
In this complex, the two Rh-P bond distances are significantly
different from each other. The Rh-Pchelated bond distance is
0.02 Å shorter than the Rh-Pmonodentate bond distance. The
P-Rh-P bond angle (91.44°) is very similar to that in 9
(91.74°). The η6-arene ring in complex 12 adopts a boat
conformation with respect to Rh which is similar to complexes
5 and 8 (average deviation ) 0.0169 Å) . The structure of
complex 12 is strikingly similar to the structure of complex 11,
which was described in a previous report, (Chart 1).4a,b The
Rh-Pavg and Rh-Cavg distances are virtually identical (11: Rh-
Pavg ) 2.24 Å, Rh-Cavg ) 2.31 Å; 12: Rh-Pavg ) 2.24 Å,
Rh-Cavg ) 2.33 Å).
In order to study how the electronic character of the arene
ligand affects the kinetic stability of the Rh(II) form of these
bis(phosphine), η6-arene complexes, the electrochemical re-
versibility of the Rh(I)/Rh(II) redox couples for 1-7, all of
which contain the dppe ligand, was examined. The electro-
chemical behavior of the benzene complex, 1, was irreversible
at all scan rates measured (up to 1 V/s); presumably the
decomposition of the Rh(II) form was due to loss of the weakly
bound arene ligand. It is known that arene complexes of Rh(I)
are extremely labile, and even though the strength of the Rh-
arene interactions are expected to increase on going from Rh-
(I) to Rh(II), the susceptibility of the Rh center to ligand
substitution reactions may also be greater for the Rh(II) form.
Indeed, odd-electron compounds, such as the 17-electron Rh-
(II) oxidation product, often have substitutionally labile ligands.16
For example, in the case of the bulk electrolysis of an η6-arene,
tricarbonyl chromium complex, 2-(2,3,4,5,6-pentamethylphen-
yl)ethanol]chromium tricarbonyl, a stoichiometric amount of the
arene ligand is lost through a series of steps which start with
the oxidation of the metal center from Cr(0) to Cr(I).16c In
Electrochemical Studies. The oxidative electrochemistry of
compounds 1-10 was studied by cyclic voltammetry (CV) and
differential pulse voltammetry (DPV). CV and DPV of
(16) (a) Howell, J. A. S.; Burkinshaw, P. M. Chem. ReV. 1983, 83, 557.
(b) Albers, M. O.; Coville, N. J. Coord. Chem. ReV. 1984, 53, 227. (c)
Doxsee, K. M.; Grubbs, R. H.; Anson, F. C. J. Am. Chem. Soc. 1984, 106,
7819. (d) Hershberger, J. W.; Klinger, R. J.; Kochi, J. K. J. Am. Chem.
Soc. 1983, 105, 61.
(15) (a) Muetterties, E. L.; Bleeke, J. R.; Wucherer, E. J.; Albright, T.
A. Chem. ReV. 1982, 82, 499. (b) Radovich, L. J.; Koch, F. J.; Albright, T.
A. Inorg. Chem. 1980, 19, 3373. (c) Albright, T. A.; Hoffmann, R.; Tse,
Y.; D’Ottavio, T. J. Am. Chem. Soc. 1979, 101, 3812.

Rh(II) Bis(phosphine), η6-Arene Complexes
J. Am. Chem. Soc., Vol. 119, No. 13, 1997 3053
Figure 6. Square-wave diagram depicting the equilibria between 2
and 7 and 2+ and 7+.
Figure 5. Graph of E1/2 versus number of methyl substituents for
complexes 2-7.
Table 4. Ratio of Kox/Kred for Rh(I)/Rh(II) Couple
moving to methyl substituted arenes, complexes 2 and 3 exhibit
reversible redox waves only at scan rates faster than 200 and
20 mV/s, respectively. In addition, those complexes with even
more electron donating arene ligands (4-7) exhibit markedly
increased kinetic stabilities for their Rh(II) forms as evidenced
by the reversibility of their Rh(I)/Rh(II) redox couple at all
measured scan rates. This is presumably due to the ability of
the more electron donating arene ligands to coordinate more
strongly to the Rh(II) centers generated by oxidation. Further-
more, increased alkyl substitution on η6-arene ligands has been
proposed by others to protect metal centers from external attack
through the increased steric bulk of the ligand.16c
complex
3
1.80
s
s
s
4
2.76
1.54
s
s
s
5
6
12.2
6.78
4.41
2.10
s
7
22.8
12.7
8.24
3.92
1.87
2
3
4
5
6
5.81
3.23
2.10
s
s
s
Chart 3
The E1/2 values for 2-7, which are a measure of the
thermodynamic stabilities of the Rh(II) forms, also follow a
similar trend as the observed trend in kinetic stability. The half-
wave potentials of these complexes decrease in a linear fashion
by 16 mV with the addition of each methyl group (Figure 5).
This reflects an increase in the electron richness of the Rh center
upon the addition of each methyl substituent to the arene ligand.
In a complementary study, Geiger and co-workers have observed
similar, but larger shifts per added methyl groups (∼26 mV),
for the reduction potentials of Rh(III) cyclopentadienyl, η6-arene
complexes.3e Such shifts in half-wave potentials caused by the
substitution of a hydrogen by an alkyl group at a π-ligand of a
transition metal complex are commonly observed.17 However,
it appears that the relative magnitude of the substituent effects
for certain classes of compounds is dependent on how strongly
the ligands with alkyl substituents coordinate to the metal centers
of interest. For example, the arenes in the compounds studied
by Geiger coordinate more strongly to the Rh(III) center than
the arenes in this study bond to Rh(I), as evidenced by their
ligand substitution behavior.3e Similarly, the cyclopentadienyl
groups of ferrocene bond significantly more strongly to Fe(II)
than the arene ligands in this study bond to Rh(I). Accordingly,
an approximately -50 mV change in E1/2 values is observed
with the addition of each methyl group to the cyclopentadienyl
ligands in ferrocene.17a,b
and Rh(II) forms of these complexes, as in the square wave
diagram in Figure 6, a Nernstian relationship allows for
calculation of the ratio of the ligand substitution equilibrium
constants from E1/2 values.18 In Figure 6, the equilibrium
constant for the reaction of the Rh(II) form of 2 (2+) with
hexamethylbenzene (Kox) is 22.8 times larger than the equilib-
rium constant for the reaction between the Rh(I) complex 2 and
hexamethylbenzene (Kred). This is another way of showing that
the Rh(II) form of 2 is significantly more stabilized than the
Rh(I) species by electron-donating arene ligands such as
hexamethylbenzene. Table 4 illustrates this relationship for
complexes 2-7 and gives a quantitative measure of the extent
of stabilization of the Rh(II) form as compared with that of the
Rh(I) form for this family of compounds.
Complexes 2, 8-10, and 12 all possess tolyl-like ligands,
yet their electrochemical behavior and E1/2 values vary sub-
stantially (range ) 130 mV) (Chart 3). In this case, the
electrochemical responses for these bis(phosphine), monoalky-
lated η6-arene Rh(I) complexes may be due to the changes in
the ligand connectivities and, hence, the different structures of
each complex. By comparing the electrochemistry of these five
complexes and considering the structural consequences on the
electronic nature of the Rh center, the importance of two
different types of chelation in the stabilization of Rh(II) in a
piano-stool geometry can be assessed.
From the E1/2 data presented in Table 2, correlations between
the binding strength of the arene ligands and the stability of
the Rh(II) forms of these complexes can be established. By
comparing the arene substitution processes involving the Rh(I)
Upon examining the reversibility of the Rh(I)/Rh(II) redox
couples, one can determine which ligands kinetically stabilize
the Rh(II) form toward decomposition reactions. As stated
earlier, compounds 11 and 12 exhibit reversible Rh(I)/Rh(II)
redox couples at all scan rates measured (10 mV/s-1 V/s)
(Figure 7).4a,b In contrast, complexes 2 and 8 exhibit reversible
(17) (a) Koelle, U.; Khouzami, F. Angew. Chem., Int. Ed. Eng. 1980, 8,
640. (b) Robbins, J. L.; Edelstein, N.; Spencer, B.; Smart, J. C. J. Am.
Chem. Soc. 1982, 104, 1882. (c) Koelle, U; Fuss, B.; Rajasekharan, M. V.;
Ramakrishna, B. L.; Ammeter, J. H.; Bo¨hm, M. C. J. Am. Chem. Soc. 1984,
106, 4152. (d) Yur’eva, L. P.; Peregudova, S. M.; Nekrasov, L. N.;
Korotkov, A. P.; Zaitseva, N. N.; Zakurin, N. V.; Vasil’kov, A. Y. J.
Organomet. Chem. 1981, 219, 43. (e) Hamon, J.-R.; Astruc, D.; Michaud,
P. J. Am. Chem. Soc. 1981, 103, 758.
(18) Evans, D. H. Chem. ReV. 1990, 90, 739.

3054 J. Am. Chem. Soc., Vol. 119, No. 13, 1997
Singewald et al.
values of 2 (635 mV) and 10 (585 mV) are significantly higher
than the E1/2 values of 8 and 12 (505 and 515 mV, respectively),
which do not contain bis(phosphine) chelation. In 10, the longer
alkane bridge between the two P atoms seems to enhance the
stability of the oxidized complex as compared with complex 2,
which possesses an ethylene bridge. However, the oxidized
forms of both 2 and 10 are less thermodynamically favored in
comparison with the oxidized form of 9 (525 mV), which has
a propylene bridge. Within the chelating bis(phosphine)
complexes (2, 9, and 10) complex 9 seems to be an exception.
With an E1/2 value only slightly higher than those measured for
8 and 12, it is the most easily oxidized complex out of all the
bis(phosphine) chelates presented herein (1-7, 10). Its Rh(II)
form, however, is slightly less favored than those of compounds
8 and 12. Those complexes without bis(phosphine) chelation,
such as 8 and 12, are the easiest to oxidize and, therefore, their
Rh(II) forms are the most thermodynamically stable.
From our comparison of the chemical reversibilities and half-
wave potentials of the Rh(I)/Rh(II) redox couples for compounds
2, 8-10, and 12, we can identify several of the important factors
which contribute to the electronic nature of the Rh center and
stability of Rh(II) oxidation state in the different ligand
environments. We have found that the most favorable environ-
ment for Rh(II) with piano stool geometry among the complexes
2, 8-10, and 12 is defined by the (phosphinoalkyl)arene ligands
in 12. In this complex, not only is the Rh center one of the
most electron rich of the series (E1/2 ) 515 mV), but it is also
the most kinetically stable Rh(II) form as evidenced by its
reversible behavior at all measured scan rates. Complex 8, in
comparison, oxidizes at a slightly lower potential (E1/2 ) 505
mV) but exhibits irreversible behavior at scan rates less than
200 mV/s. In addition, complex 9 is reversible at all scans rates;
however, it oxidizes at a slightly higher potential than 8 and
12, and therefore, its Rh(II) form is not as thermodynamically
favored.
Figure 7. Cyclic voltammetry of (A) 12, (B) 8, and (C) 2.
Rh(I)/Rh(II) redox couples only at scan rates greater than 200
mV/s (Figure 7). Upon the addition of a single methylene unit
to the alkane bridge in 2, the resulting dppp complex (9) exhibits
reversible electrochemical behavior at all scan rates measured
(10 mV/s-1 V/s). With the addition of one more methylene
unit to the ligand backbone, the dppb complex 10 exhibits a
reversible redox couple only when one scans faster than 20 mV/
s. Thus, by cyclic voltammetry, only 9 and 12 exhibit
chemically reversible waves at all scan rates measured (10 mV/
s-1 V/s). From these observations, one can conclude that the
ligand arrangements in 9 and 12 kinetically stabilize the Rh(II)
form toward decomposition reactions. For 12, this can be easily
explained by the presence of the chelated η6-arene ligand.
Apparently, the chelation in 12 kinetically stabilizes the Rh(II)
form from decomposition by inhibiting dissociation of the arene
ligand from the metal complex (in the cases of 2, 8, and 10
there is no arene chelation) and by sterically protecting the Rh-
(II) center from further reactions. The reason for compound
9’s increased stability relative to 2, 8, and 10 is a bit more
complex but can be rationally explained (vide infra).
In the literature, polydentate and bulky phosphine ligands
have been used to stabilize mononuclear Rh(II) complexes,
where, in addition to the usual chelate effect stabilization, the
ligand backbone protects the periphery of the complex from
external attack.3c,19 Surprisingly, for our complexes, a com-
parison of the redox potentials of 2, 8-10, and 12 reveals that
the bidentate, chelated bis(phosphine) ligands in 2 and 10
thermodynamically destabilize the Rh(II) form of the complex
relative to the other ligand arrangements (Table 2). The E1/2
Structural and Electrochemical Correlation of 2, 8-10,
and 12 Using EHMO Calculations. As stated earlier, com-
plexes 8, 9, and 12 oxidize at significantly lower potentials than
those of complexes 2 and 10. Unlike the trend observed for
complexes 1-7, a correlation between the different ligand
connectivities and the relative stabilities of the corresponding
Rh(II) forms is not evident when one only considers the
structural characteristics of these compounds in their reduced
state and the corresponding electrochemical data. For example,
one might expect those complexes with the longest Rh-P bonds
to be the least electron rich and, therefore, the most difficult to
oxidize, especially if σ-donation from the phosphine ligands is
the dominant factor which controls the electron richness of the
Rh center. Alternatively, if Rh-to-P back-bonding is the
dominant factor that dictates the Rh electron-rich character in
this series of complexes, the compounds with the longest Rh-P
bonds should be the easiest to oxidize. However, if one
examines the structural data, there is no correlation between
the Rh-P bond lengths and the E1/2 values for this series of
compounds. By also taking into account the structures of the
oxidized forms of complexes 2, 8-10, and 12 and theoretical
data, the experimentally defined trend in E1/2 values can be
rationally explained.
Although, thus far we have not isolated the Rh(II) forms of
these complexes, Harlow et al. have studied an isoelectronic
and isostructural cobalt system. For example, their work shows
that significant structural changes occur after oxidation of a bis-
(triethylphosphine)cyclopentadienylcobalt(I) complex.20 Upon
oxidation of the Co(I) center to Co(II), the complex undergoes
substantial changes in both its Co-P bond distances (2.218-
(19) (a) Haefer, S. C.; Dunbar, K. R.; Bender, C. J. Am. Chem. Soc.
1991, 113, 9540. (b) Bianchini, C.; Laschi, F.; Ottaviani, M. F.; Peruzzini,
M.; Zanello, P.; Zanobini, F. Organometallics 1989, 8, 893. (c) Masters,
C.; Shaw, B. L. J. Chem. Soc. (A) 1971, 3679.

Rh(II) Bis(phosphine), η6-Arene Complexes
J. Am. Chem. Soc., Vol. 119, No. 13, 1997 3055
2.230 Å) and the P-Co-P bond angle (98.49-101.2°), while
the Co-Cp distances remain essentially unchanged (2.084-
2.083 Å). The origin of the observed structural perturbations
was easily understood upon careful consideration of the nature
of the highest occupied molecular orbital (HOMO) and its role
in the small amount of metal to phosphine ligand π-back-
bonding present in the complex. From extended Hu¨ckel
calculations, the metal d orbital component of the HOMO was
determined to be orthogonal to both the Co-P bonds and, thus,
a π-interaction between this orbital and an empty phosphorus
p and/or d orbital was symmetry allowed. This π-back-bonding
interaction was used to explain the counterintuitive lengthening
of the Co-P bonds and widening of the P-Co-P angle. For
instance, removal of an electron from the metal d orbital
decreases the amount of π-back-bonding, thereby weakening
and lengthening the Co-P bonds. The enlargement of the
P-Co-P angle also reflects the decrease in the π-interactions,
which have optimum overlap at a smaller and more sterically
congested angle. On removing an electron from the HOMO,
the π-back-bonding is weakened and the P-Co-P angle opens
up to relieve the unfavorable steric interactions present.
Figure 8. HOMO of 8 obtained from an EHMO calculation using the
crystallographic coordinates. This a representative of the HOMO for
complexes 2, 8-10, and 12.
Although the work of Harlow et al. studies the changes in
structure upon metal complex oxidation, the consequences of
chelation in such structures with regard to their electrochemical
behavior was not addressed. If the HOMOs of the complexes
studied herein are similar in nature to those studied by Harlow
et al., then one would expect complexes 2, 8-10, and 12 to
undergo a similar widening of the P-Rh-P angle and lengthen-
ing of the Rh-P bonds upon oxidation of the metal center from
Rh(I) to Rh(II). Thus, complexes that are more constrained in
their P-Rh-P bond angle would be less able to accommodate
this electrochemically induced structural change and, in general,
would oxidize at higher potentials. In order to identify the
HOMOs in these complexes, theoretical investigations of
complexes 2, 8-10, and 12 using extended Hu¨ckel molecular
orbital (EHMO)21 calculations were performed. The HOMOs
and LUMOs for 5, 8, 9, and 12 were calculated using
crystallographic coordinates for each complex; the MOs of
compound 5 were used as a model for those of 2 even though
the arene ligand is different, since the P-Rh-P angle for
mononuclear dppe compounds of Rh(I) fall within a relatively
narrow range (82.1-84.8°)14 and the Rh-arene bond distances
for all the structurally characterized compounds are very similar.
piano-stool complexes in this study. If this is the case, then
those ligands which are most able to accommodate the widening
of the P-Rh-P angle and the lengthening of the Rh-P bonds
will be the easiest to oxidize. Indeed, from the electrochemical
data, it is the more strained bis(phosphine) chelated complexes
(2 and 10) which give the highest half-wave potentials. In
addition, it is the exception (compound 9) which possesses the
bis(phosphine) alkyl bridge that has the least amount of ring
strain and is the easiest to oxidize out of the bis(phosphine)
chelates.22 Moreover, it is the complexes which are not
restricted in their P-Rh-P bond angle by bis(phosphine)
chelation which have the two lowest E1/2 values. In particular,
it is the formation of the Rh(II) species which contains no
chelation at all (8) which is the most thermodynamically favored
of the whole series. Presumably, the structural constraints in
this complex are the least, and it can easily accommodate the
changes induced by the oxidation of the metal center.
Conclusions
Other workers have studied the nature of the molecular
orbitals of similar piano stool complexes.15 For example,
Muetterties et al. have calculated the molecular orbitals for
M(arene)L2 piano-stool complexes.15a Using the crystallo-
graphic coordinates, the MOs calculated for our compounds are
similar to those in the general model presented by Muetterties
and, more importantly, the HOMOs for these complexes are
similar to those reported for the bis(triethylphosphine)cyclo-
pentadienylcobalt(I) complex.20 In Figure 8, the HOMO of
complex 8 is shown and is representative of the calculated
HOMOs for all the structurally characterized complexes. In
the calculated HOMO, the major component is the metal d
orbital, and although the π-interaction is too small to show up
in the orbital plot, an examination of the wave function reveals
that such bonding contributions involving phosphorus p and d
orbitals do exist. From these calculations, one can see that the
structural changes upon oxidation, due to weakening the Rh-
to-P π-back-bonding interactions, are also likely for the
The systematic studies presented herein show that: (1) more
electron-rich arene ligands kinetically stabilize the Rh(II) forms
of these complexes and thermodynamically stabilize the Rh(II)
forms of these complexes by 16 mV per methyl group; (2)
chelation of the (phosphinoalkyl)arene ligand offers kinetic
stability to Rh(II) piano-stool complexes from loss of arene
ligand upon oxidation relative to the complexes without arene
chelation; (3) bidentate chelation of the bis(phosphine) ligand
with an ethyl and butyl bridge thermodynamically destabilizes
the Rh(II) forms of these complexes; and (4) those complexes
which contain the least constrained P-Rh-P angle most favor
the formation of Rh(II). In addition, although the trends in the
properties of the reduced form of many complexes usually
parallels their oxidative electrochemical trends, we have shown
that this is not always the case. Moreover, it is necessary to
consider what happens to the structure of a complex upon going
from one oxidation state to another in order to understand the
observed electrochemical data. Indeed, we predict that other
systems, which employ restricted chelating ligands and undergo
(20) Harlow, R. L.; McKinney, R. J.; Whitney, J. F. Organometallics
1983, 2, 1839.
(21) CAChe extended Hu¨ckel program, CAChe Scientific, Beaverton,
OR.
(22) Li, C.; Cucullu, M. E.; McIntyre, R. M.; Stevens, E. D.; Nolan, S.
P. Organometallics 1994, 13, 3621.

3056 J. Am. Chem. Soc., Vol. 119, No. 13, 1997
Singewald et al.
structural changes involving those ligands upon electrochemical
oxidation or reduction will experience a similar energetic penalty
as the system described herein. Finally, from the results in this
study, a logical strategy to prepare isolable Rh(II) compounds
with piano-stool geometry is to prepare chelating (phosphino-
alkyl)arene ligands with strongly donating arene substituents;
this is a strategy we are currently pursuing.
We are also grateful to Professor Guy Orpen for directing us
toward the work of Harlow et al.
Supporting Information Available: Detailed descriptions
of the X-ray diffraction studies for 5, 8, 9, and 12, including
tables of experimental details, atom positional parameters,
equivalent isotopic displacement parameters, bond lengths, bond
angles, anisotropic displacement parameters, and mean plane
calculations (39 pages). See any current masthead page for
ordering and Internet access instructions.
Acknowledgment. We acknowledge the NSF (CHE-
9625391) and (CHE-9357099), and the Petroleum Research
Fund (No. 30759-AC3) for generously funding this research.
JA963384V