Synthesis of rhodium complexes of an asymmetric η5,η1,η1-cyclo-pentadienyl- bis(phosphine) ligand by regioselective intramolecular dehydrofluorinative C-C coupling

Article abstract of DOI:10.1016/S0020-1693(01)00602-8

The reaction between [{(η⁵-C₅Me₄H)RhX₂} ₂] and (C₆F₅)₂PCH₂CH₂P(C ₆F₅)₂ (dfppe) proceeds in refluxing benzene via cleavage of two C-F bonds and C-H bonds of adjacent methyl groups and formation of two C-C bonds to yield selectively the asymmetric cation [{η⁵,η¹,η¹-C₅HMe ₂-3,4-[CH₂-2-C₆F₄P(C ₆F₅)CH₂]₂-1,2}RhX]⁺. The selectivity for X = Cl was determined to be >90%. Treatment of [(η⁵-C₅Me₄H)RhCl(dfppe)]BF₄ with proton sponge afforded [{η⁵,η¹,η¹-C₅HMe ₂-3,4-[CH₂-2-C₆F₄P(C ₆F₅)CH₂]₂-1,2}RhCl]BF₄ exclusively. The regioselectivity displayed by these reactions is in stark contrast to that in the reaction between [(Cp*RhX₂)₂] and dfppe, and that between [Cp*RhCl(dfppe)]⁺ and proton sponge, in which C-H bonds of methyl groups in a 1,3 disposition are cleaved to yield the cation [{η⁵,η¹,η¹-C₅Me ₃[CH₂-2-C₆F₄P(C₆F ₅)CH₂]₂-1,3}RhX]⁺ exclusively. The structure of [{(η⁵-C₅Me₄H)RhCl₂} ₂] has been determined by single-crystal X-ray diffraction.

Full text of DOI:10.1016/S0020-1693(01)00602-8

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 323 (2001) 78–88
Synthesis of rhodium complexes of an asymmetric
h5,h1,h1-cyclo-pentadienyl-bis(phosphine) ligand by regioselective
intramolecular dehydrofluorinative CꢀC coupling
Ronan M. Bellabarba, Mark Nieuwenhuyzen, Graham C. Saunders *
School of Chemistry, The Queen’s Uni6ersity of Belfast, Da6id Keir Building, Belfast, Northern Ireland BT9 5AG, UK
Received 25 June 2001; accepted 27 July 2001
Abstract
The reaction between [{(h5-C5Me4H)RhX2}2] and (C6F5)2PCH2CH2P(C6F5)2 (dfppe) proceeds in refluxing benzene via cleavage
of two CꢀF bonds and CꢀH bonds of adjacent methyl groups and formation of two CꢀC bonds to yield selectively the asymmetric
cation [{h5,h1,h1-C5HMe2-3,4-[CH2-2-C6F4P(C6F5)CH2]2-1,2}RhX]+. The selectivity for X=Cl was determined to be \90%.
Treatment of [(h5-C5Me4H)RhCl(dfppe)]BF4 with proton sponge afforded [{h5,h1,h1-C5HMe2-3,4-[CH2-2-C6F4P(C6F5)CH2]2-
1,2}RhCl]BF4 exclusively. The regioselectivity displayed by these reactions is in stark contrast to that in the reaction between
[(Cp*RhX2)2] and dfppe, and that between [Cp*RhCl(dfppe)]+ and proton sponge, in which CꢀH bonds of methyl groups in a
1,3 disposition are cleaved to yield the cation [{h5,h1,h1-C5Me3[CH2-2-C6F4P(C6F5)CH2]2-1,3}RhX]+ exclusively. The structure of
[{(h5-C5Me4H)RhCl2}2] has been determined by single-crystal X-ray diffraction. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Rhodium; h5-Cyclo-pentadienide; Phosphine; CꢀC coupling
1. Introduction
to the low diastereoselectivity shown by [{h5-
C5H2(CO2Et)MeR-2,4}Ru(NCMe)2{P(OMe)3}]PF6 [6].
The development of the chemistry of complexes of
chelating hybrid cyclo-pentadienide–phosphine ligands
has been severely hindered by the lack of convenient
ligand syntheses [1]. However, recently we reported that
intramolecular dehydrofluorinative CꢀC coupling pro-
vides a convenient method of synthesizing complexes of
hybrid cyclo-pentadienide–phosphine ligands [7,8]. The
reaction is particularly well suited to the coupling of
pentamethyl-cyclo-pentadienide and chelating diphos-
phine ligands. We have reported that the coupling of
Cp* and (C6F5)2PCH2CH2P(C6F5)2 (dfppe) in a rhodiu-
m(III) complex can be accomplished either by the one-
pot reaction between [(Cp*RhCl2)2] and dfppe in
refluxing benzene or ethanol (reaction 1) [9,10] or by
treatment of the tetrafluoroborate salt, the isolated
intermediate cation [Cp*RhCl(dfppe)]+ with proton
sponge [7]. In both cases, the product [{h5,h1,h1-
C5Me3[CH2-2-C6F4P(C6F5)CH2]2-1,3}RhCl]+ was ob-
tained in virtually quantitative yield, as a mixture of
chloride 1a and tetrafluoroborate 1b salts in the former
and as 1b in the latter.
Chelating bi- or tri-functional chelating ligands con-
taining both cyclo-pentadienide and phosphine moieties
are expected to exert dramatically different effects on
metals than the separated ligands. This has been confi-
rmed for a number of complexes [1]. For example, these
ligands have allowed the isolation of zirconium com-
plexes, the unlinked cyclo-pentadienide phosphine ana-
logues of which are either unknown or unstable [2–4],
the hybrid ligand complex [{h5,h1-(indenyl)CH2CH2-
PPh2}RhMe(CO)]BF4 reacts with 1-phenylpropyne at
room temperature to form the alkenyl complex [{h5,h1-
(indenyl)CH2CH2PPh2}RhMe{OꢁCMeC(Me)ꢁCPh}]-
BF4, whereas under the same conditions [(h5-in-
denyl)RhMe(CO)(PPh3)]BF4 does not react [5],
and
[{h5,h1-C5H2(CO2CH2CH2PPh2)MeR-2,4}Ru-
(NCMe)2]PF6 shows a high diastereoselectivity in lig-
and substitution reactions with phosphines, in contrast
* Corresponding author. Tel.: +44-28-9027 4682; fax: +44-28-
9038 2117.
E-mail address: g.saunders@qub.ac.uk (G.C. Saunders).
0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00602-8

R.M. Bellabarba et al. / Inorganica Chimica Acta 323 (2001) 78–88
79
1/2[(Cp*RhCl2)2]+(C6F5)2PCH2CH2P(C6F5)2
“[{p5p1p1-C5Me3[CH2-2-C6F4P(C6F5)CH2]2-1,3}
RhCl]X+2HF (1a X=Cl−,1b X=BF4−) (1)
31P NMR (121.45 or 161.98 MHz) externally to 85%
H3PO4 (l 0). All chemical shifts are quoted in l (ppm),
using the high-frequency positive convention, and cou-
pling constants in Hz. Positive ion FAB mass spectra
were recorded on a Kratos Concept 1H mass spectrom-
eter. Elemental analyses were carried out by ASEP, The
School of Chemistry, The Queen’s University of Belfast
or by Butterworths Ltd.
The reaction displays complete regiospecificity in that
CꢀH bonds of methyl groups exclusively in a 1,3 dispo-
sition are activated and, of the two possible geometric
isomers (Fig. 1), only isomer I is formed. The regio-
and stereo-specificity are presumably imposed by the
geometric constraints of the reagents and product of
the reaction.
2.2. Materials
The compounds NaBF4 (Aldrich) and dfppe (Fluo-
rochem) were used as supplied. [{(h5-C5Me4H)RhCl2}2]
was prepared from tetramethyl-cyclo-pentadiene and
RhCl3·3H2O as described for [(Cp*RhCl2)2] [16].
As part of a programme to extend the scope of
reaction (1) it was found that similar reactions occur
for the iridium complex [(Cp*IrCl2)2] [10], the rhodium
bromide complex [(Cp*RhBr2)2] [11] and the diphos-
phine (C6H3F2-2,6)2PCH2CH2P(C6H3F2-2,6)2 [12]. We
have also investigated the reaction between dfppe and
[{(h5-C5Me4Et)RhCl2}2], which contains four different
types of CꢀH bond, in order to establish any additional
regioselectivity in the CꢀH bond activation. This reac-
tion produced a range of products similar to 1a but
showed no discernible selectivity as to which CꢀH
bonds are activated [13]. As a further attempt to inves-
tigate any selectivity in the CꢀH bond activation, it was
decided to study the reactions between dfppe and [{(h5-
C5Me4H)RhX2}2] (X=Cl or Br), which contains two
different types of methyl CꢀH bond. Herein is reported
the results of this investigation, part of which has been
communicated [14,15].
2.3. Preparations
2.3.1. [{p5,p1,p1-C5HMe2-3,4-[CH2-2-C6F4P(C6F5)-
CH2]2-1,2}RhCl]X (2a, X=Cl− and 2b, X=BF4−
)
A slurry of [{(h5-C5Me4H)RhCl2}2] (0.062 g, 0.105
mmol) and dfppe (0.165 g, 0.218 mmol) in benzene (40
cm3) was refluxed under nitrogen for 10.5 h, during
which time a lemon yellow precipitate of the chloride
salt 2a contaminated with a small amount of 2b, was
formed. The solid was filtered off and washed with
hexane. Concentration of the filtrate and addition of
hexane yielded a further quantity of 2a. Yield 0.127 g
(60%). FAB MS: 977 ([MꢀCl]+), 941 ([Mꢀ2ClꢀH]+). 1H
3
NMR (CDCl3): l 5.45 [d, J(PAꢀHa)=6.9 Hz, 1H, Ha],
2
4.94 [d, J(HdꢀHd%)=18.2 Hz, 1H, Hd or Hd%], 4.67 [d,
2J(HeꢀHe%)=17.7 Hz, 1H, He or He%], 4.44 (m, 1H,
2
4
2. Experimental
PCH2), 4.18 [dd, J(HeꢀHe%)=17.7 Hz, J(PAꢀH)=9.8
Hz, 1H, He or He%], 4.08 [dd, 2J(HdꢀHd%)=18.2 Hz,
4J(PBꢀH)=9.8 Hz, 1H, Hd or Hd%], 3.88 (m, 1H,
PCH2), 3.58 (m, 2H, PACH2 and PBCH2), 1.93 [dd,
4J(PBꢀHb)=14.2 Hz, 4J(PAꢀHb)=1.6 Hz, 3H, Hb],
2.1. Physical measurements
The 1H, 13C, 19F and 31P NMR spectra were recorded
using Bruker DPX300, DRX400 or DRX500 spectrom-
4
1.91 [d, J(PBꢀHc)=1.9 Hz, 3H, Hc]. 13C{1H} NMR
1
1
eters. H NMR (300.01, 400.13 or 500.13 MHz) were
(CDCl3): l 149.6 [dm, J(CꢀF)=249 Hz, CF], 149.2
1
1
referenced internally using the residual protio solvent
resonance relative to SiMe4 (l 0), 13C NMR (75.45 or
100.62 MHz) externally to SiMe4 (l 0), 19F NMR
(282.26 or 376.50 MHz) externally to CFCl3 (l 0) and
[dm, J(CꢀF)=228 Hz, CF], 146.8 [dm, J(CꢀF)=242
1
Hz, CF], 144.4 [dm, J(CꢀF)=249 Hz, CF], 144.2 [dm,
1J(CꢀF)=291 Hz, CF], 139.7 [dm, J(CꢀF)=263 Hz,
1
CF], 138.1 [dm, 1J(CꢀF)=257 Hz, CF], 128.5–132 (m),
Fig. 1. Diagrammatic representation of the geometric isomers of the cation [RhCl{h5,h1,h1-C5Me3[CH2C6F4P(C6F5)CH2]2-1,3}]+ (1) viewed along
the C5 (centroid)ꢀRh axis.

80
R.M. Bellabarba et al. / Inorganica Chimica Acta 323 (2001) 78–88
3H, CH3]. 19F NMR (CDCl3): l −120.50 (m, 1F,
C6F4), −120.68 (m, 1F, C6F4), −129.14 [d, 3J(FꢀF)=
3
19.6 Hz, 2F, Fo of C6F5], −131.06 [d, J(FꢀF)=18.6
Hz, 2F, Fo of C6F5], −134.27 (m, 1F, C6F4), −134.65
(m, 1F, C6F4), −143.67 (m, 2F), −144.28 (m, 2F),
−152.87 [dd, 3J(FꢀF) :20.6 Hz, :20.6 Hz, 1F,
3
C6F4], −153.07 [dd, J(FꢀF) :23.0 Hz, :23.0 Hz,
1F, C6F4], −153.70 and −153.76 (2s, 1:4, 4F, BF4−),
−157.82 (m, 2F, Fm of C6F5), −158.00 (m, 2F, Fm of
Fig. 2. Atom labels for 2a viewed along the C5 (centroid)ꢀRh axis.
The aryl rings are omitted for clarity.
1
C6F5). 31P{1H} NMR (CDCl3): 75.9 [dm, J(RhꢀP)=
1
140 Hz], 68.9 [dm, J(RhꢀP)=138 Hz].
2
104.5–107.0 (m), 104.1 [dm, J(PꢀC)=16 Hz, CMe],
2
2
99.7 [dm, J(PꢀC)=6 Hz, CMe], 92.9 [dm, J(PꢀC)=7
Hz, CMe], 86.6 [d, J(PꢀCa)=7 Hz, Ca], 31.6 [dd,
1J(PꢀC)=34 Hz, 2J(PꢀC)=14 Hz, PCH2], 29.5 [d,
1J(PꢀC)=41 Hz, PCH2], 19.4 [d, 3J(PBꢀCd)=6 Hz,
2.3.2. [{(p5-C5Me4H)RhBr2}2] (3)
A slurry of [{(h5-C5Me4H)RhCl2}2] (0.295 g, 0.50
mmol) and NaBr (3.00 g, 0.029 mol) in methanol (70
cm3) was heated at reflux under nitrogen for 37 h. The
solvent was removed by rotary evaporation and the
product extracted into dichloromethane (2×80 cm3).
The insoluble salts were filtered off and the solvent
concentrated by rotary evaporation. Addition of
petroleum ether (b.p. 100–120 °C) yielded the product
3 as a red solid, which was washed with hexane (2×30
cm3) and dried in vacuo. Yield 0.205 g (53%). Anal.
Calc. for C18H26Br4Rh2: C, 28.2; H, 3.4. Found: C,
28.3; H, 3.1%. FAB MS: 687 ([MꢀBr]+), 607
3
Cd], 18.4 [d, J(PAꢀCe)=7 Hz, Ce], 12.6 (s, Cc), 9.2 [d,
3J(PBꢀCb)=4 Hz, Cb]. 19F NMR (CDCl3): l −120.54
(m, 2F, C6F4), −129.82 (br s, 2F, Fo of C6F5),
−131.12 (br s, 2F, Fo of C6F5), −135.30 (m, 1F,
3
C6F4), −135.61 (m, 1F, C6F4), −143.52 [ddd, J(FꢀF)
4
:20.8 Hz, :20.8 Hz, J(FꢀF)=9.3 Hz, 1F, C6F4],
−143.68 [ddd, 3J(FꢀF) :20.8 Hz, :20.8 Hz,
4J(FꢀF)=9.1 Hz, 1F, C6F4], −144.91 (m, 1F, Fp of
C6F5), −145.06 (m, 1F, Fp of C6F5), −153.08 [dd,
3J(FꢀF) :21.7 Hz, :21.7 Hz, 1F, C6F4], −153.38
[dd, 3J(FꢀF) :21.8 Hz, :21.8 Hz, 1F, C6F4],
−158.39 (m, 4F, Fm of C6F5). 19F NMR (CDCl3, 213
K): −119.43 (m, 1F, C6F4), −119.92 (m, 1F, C6F4),
−127.69 (m, 2F, Fo of C6F5 and C6F5%), −130.32 (m,
1F, Fo of C6F5), −131.46 (m, 1F, C6F4), −134.91 (m,
2F, C6F4 and Fo of C6F5%), −142.24 (m, 1F, C6F4),
−142.43 (m, 1F, C6F4), −143.52 [t, 3J(FꢀF)=20.7
1
([Mꢀ2Br]+). H NMR (CDCl3): l 5.05 (s, 1H, C5H),
1.81 (s, 6H, Me), 1.76 (s, 6H, Me). 13C{1H} NMR
(CDCl3): l 97.6 [d, 1J(RhꢀC)=9 Hz, C5], 96.5 [d,
1
1J(RhꢀC)=8 Hz, C5], 79.1 [d, J(RhꢀC)=9 Hz, C5],
11.6 (s, Me), 9.9 (s, Me).
2.3.3. [{p5,p1,p1-C5HMe2-3,4-[CH2-2-C6F4P(C6F5)-
CH2]2-1,2}RhBr]Br (4a)
3
A slurry of [{(h5-C5Me4H)RhBr2}2] (0.071 g, 0.093
mmol) and dfppe (0.145 g, 0.191 mmol) in benzene (70
cm3) was refluxed under nitrogen for 38 h, during which
time a lemon yellow precipitate of 4a was formed. The
solid was filtered off, washed with petroleum ether (b.p.
100–120 °C) and hexane, and dried in vacuo. Yield
0.109 g (53%). Anal. Calc. for C35H15Br2F22P2Rh: C,
38.1; H, 1.4. Found: C, 38.25; H, 1.45%. MS FAB:
1023, 1021 ([MꢀBr]+), 941 ([Mꢀ2Br]+). 1H NMR
Hz, 1F, Fp of C6F5], −143.79 [t, J(FꢀF)=20.7 Hz,
1F, Fp of C6F5], −152.11 (m, 1F, C6F4), −152.21 (m,
1F, C6F4), −157.15 (m, 4F, Fm of C6F5). 31P{1H}
1
NMR: l 78.8 [dm, J(RhꢀPA)=141 Hz, PA], 73.2 [dm,
1J(RhꢀPB)=141 Hz, PB]. The atom labels are shown in
Fig. 2. Satisfactory analysis could not be obtained due
to contamination by 2b, which could not be separated.
The salt 2b was prepared from 2a, which was slurried in
acetone (30 cm3) and NH4BF4 (1.0 g, 9.5 mmol) added.
After 16 h, the solvent was removed by rotary evapora-
tion and the solid extracted into dichloromethane. The
extract was filtered and the solvent removed by rotary
evaporation to afford 2b as a yellow solid, which was
dried in vacuo. Anal. Calc. for C35H15BClF22P2Rh: C,
39.5; H, 1.4; P, 5.8. Found: C, 39.7; H, 1.3; P, 5.5%.
3
(CDCl3): l 5.48 [d, J(PꢀH)=6.6 Hz, 1H, C5H], 4.75
[d, 2J(HꢀH%)=18.3 Hz, 1H, CHH%C6F4], 4.59 [d,
2J(H%%ꢀH%%%)=17.5 Hz, 1H, CH%%H%%%C6F4], 4.24 (m, 1H,
PCH2), 4.09 (m, 2H, CHH%C6F4 and CH%%H%%%C6F4),
3.68 (m, 3H, PCH2), 2.13 [dd, 4J(PꢀH)=3.3 Hz,
4
4J(PꢀH)=1.5 Hz, 3H, CH3], 1.94 [dd, J(PꢀH)=9.1
1
FAB MS: 977 ([MꢀBF4]+), 941 ([MꢀBF4ꢀClꢀH]+). H
4
Hz, J(PꢀH)=1.3 Hz, 3H, CH3]. 19F NMR (CDCl3): l
3
3
NMR (CDCl3): l 5.58 [d, J(PꢀH)=6.5 Hz, 1H, C5H],
−120.26 [d, J(FꢀF)=11.4 Hz, 1F, C6F4], −120.53
2
3
4
4.09 [d, J(HꢀH)=17.8 Hz, 1H, CHH%C6F4], 4.07 [d,
[dd, J(FꢀF)=20.9 Hz, J(FꢀF)=9.8 Hz, 1F, C6F4],
−129.25 (s, 2F, Fo of C6F5), −131.53 (br s, 2F, Fo of
C6F5), −134.34 (m, 1F, C6F4), −135.17 (m, 1F, C6F4),
−143.19 (m, 1F, C6F4), −143.48 (m, 1F, C6F4),
2J(HꢀH)=18.6 Hz, 1H, CH%%H%%%C6F4], 3.87 [d,
2J(HꢀH)=17.8 Hz, 1H, CHH%C6F4], 3.58 (m, 2H,
2
PCH2), 3.33 [d, J(HꢀH)=18.6 Hz, 1H, CH%%H%%%C6F4],
3
3.20 (m, 1H, PCH2), 2.90 (m, 1H, PCH2), 1.98 [d,
−144.76 [t, J(FꢀF)=20.4 Hz, 1F, Fp of C6F5], −
4J(PꢀH)=8.7 Hz, 3H, CH3], 1.89 [d, 4J(PꢀH)=2.7 Hz,
3
145.05 [t, J(FꢀF)=20.4 Hz, 1F, Fp of C6F5], −152.63

R.M. Bellabarba et al. / Inorganica Chimica Acta 323 (2001) 78–88
81
[dd, 3J(FꢀF) :22.0 Hz, :22.0 Hz, 1F, C6F4],
−153.11 [dd, 3J(FꢀF) :22.0 Hz, :22.0 Hz, 1F, C6F4],
−158.20 (m, 4F, Fm of C6F5). 19F NMR (CDCl3, 213
K): l −119.19 (m, 1F, C6F4), −120.01 (m, 1F, C6F4),
−128.18 (m, 2F, Fo of C6F5 and C6F5%), −128.53 (m,
1F, Fo of C6F5), −133.58 (m, 1F, C6F4), −134.48 (m,
1F, C6F4), −135.43 (m, 1F, Fo of C6F5%), −141.98 (m,
2.3.4. [(p5-C5Me4H)RhCl(dfppe)]BF4 (5)
The salt NaBF4 (approximately 0.4 g, 3.7 mmol) was
added to [{(h5-C5Me4H)RhCl2}2] (0.037 g, 0.063 mmol)
and dfppe (0.100 g, 0.132 mmol) in methanol (40 cm3)
and dichloromethane (10 cm3) with vigorous stirring. Af-
ter 30 min, the solvent was removed by rotary evapora-
tion and the orange solid extracted into dichloromethane
(70 cm3) and filtered. Concentration of the solution by
rotary evaporation and addition of hexane yielded yel-
low crystals of 5, which were washed with hexane and
dried in vacuo. Yield 0.093 g (67%). Anal. Calc. for
C35H17BClF24P2Rh: C, 38.1; H, 1.55. Found: C, 38.2; H,
1F,
C6F4),
−142.42
(m,
1F,
C6F4),
−143.32 (m, 1F, Fp of C6F5), −143.86 (m, 1F, Fp of
C6F5), −151.69 (m, 1F, C6F4), −152.27 (m, 1F, C6F4),
−157.08 (m, 4F, Fm of C6F5). 31P{1H} NMR (CDCl3):
73.8 [dm, 1J(RhꢀP)=144 Hz], 68.3 [dm, 1J(RhꢀP)=138
Hz]. The tetrafluoroborate salt 4b was prepared from 4a
by treatment with an appropriate anion source as de-
1
1.7%. H NMR [(CD3)2CO]: l 6.40 (s, 1H, C5H), 3.67
(m, 2H, PCH2), 3.45 (m, 2H, PCH2), 1.82 [dd,
4J(PꢀH)=7.9 Hz, 4J(PꢀH)=2.4 Hz, 6H, CH3], 1.58 [d,
4J(PꢀH)=5.0 Hz, 6H, CH3]. 19F NMR [(CD3)2CO]:
1
3
scribed for 2b. H NMR (CDCl3): l 5.53 [d, J(PꢀH)=
6.0 Hz, 1H, C5H], 4.20 [d, 2J(HꢀH%)=17.6 Hz, 1H,
CHH%C6F4], 4.04 [dd, 2J(H%%ꢀH%%%)=18.6 Hz,
4J(PꢀH%%)=9.3 Hz, 1H, CH%%H%%%C6F4], 3.95 [dd,
2J(H%%ꢀH%%%)=17.6 Hz, 4J(PꢀH%)=6.0 Hz, 1H,
CH%HC6F4], 3.62 (m, 2H, PCH2 and CH%%H%%%C6F4), 3.45
(m, 2H, PCH2), 3.32 (m, 1H, PCH2), 2.03 [dd,
4J(PꢀH)=3.9 Hz, 3H, CH3], 1.91 [d, 4J(PꢀH)=9.0 Hz,
3H, CH3]. 19F NMR (CDCl3): l −120.63 (m, 2F, C6F4),
−128.86 (m, 2F, Fo of C6F5), −131.30 (m, 2F, Fo of
C6F5), −134.27 (m, 1F, C6F4), −134.81 (m, 1F, C6F4),
−144.29 (m, 2F), −144.78 (m, 2F), −153.24 [dd,
3J(FꢀF) :20.0 Hz, :20.0 Hz, 1F, C6F4], −153.66 (m,
1F, C6F4), −153.98 and −154.04 (2s, 1:4, 4F, BF4−),
−158.00 (m, 2F, Fm of C6F5), −158.23 (m, 2F, Fm of
C6F5). 31P{1H} NMR (CDCl3): l 72.0 [dm, 1J(RhꢀP)=
3
−125.30 [d, J(FoꢀFm)=17 Hz, 4F, Fo], −128.27 [d,
3J(FoꢀFm)=8 Hz, 4F, Fo], −144.89 [t, 3J(FpꢀFm)=21
Hz, 1F, Fp], −146.05 (m, 2F, Fp), −151.04 and
3
−151.09 (2s, 1:4, 4F, BF4−), −158.42 [dd, J(FꢀF) :
19 Hz, :19 Hz, 4F, Fm], −160.31 (m, 4F, Fm). 19F
NMR [(CD3)2CO, 193 K]: l −124.47 (m, 1F, Fo),
−126.72 (m, 1F, Fo), −128.68 (br s, 2F, Fo), −144.47
(m, 2F, Fp), −145.33 (m, 2F, Fp), −149.40 and
−149.46 (2s, 1:4, 4F, BF4−), −158.10 (m, 4F, Fm),
−160.19 (4F, m, Fm). 31P{1H} NMR [(CD3)2CO]: 32.9
1
[dm, J(RhꢀP)=149 Hz].
2.4. Crystal structure determination of compound
[{(p5-C5Me4H)RhCl2}2]
1
143 Hz], 65.2 [dm, J(RhꢀP)=136 Hz].
A crystal of [{(h5-C5Me4H)RhCl2}2] with approxi-
mate dimensions 0.34×0.23×0.17 mm was grown from
acetone. Crystal data and a summary of data collection
and refinement parameters are given in Table 1. The
molecular structure is shown in Fig. 4. Selected bond dis-
tances and angles are listed in Table 2. Data were col-
lected on a Bruker SMART diffractometer using the
SAINT-NT software with Mo Ka radiation. Lattice
parameters were determined from a least-squares refine-
ment of 2184 reflections. The structure was solved by di-
rect methods and refined on F2 by full-matrix
least-squares. All non-hydrogen atoms were refined with
anisotropic thermal parameters. The hydrogen atoms
Table 1
Crystal data and summary of data collection for [{(h5-
C5Me4H)RhCl2}2]
Empirical formula
Formula weight
Temperature (K)
Wavelength (Mo Ka) (A)
Space group
C18H26Cl4Rh2
590.00
153(2)
0.71073
P21/c
,
Unit cell dimensions
,
a (A)
15.768(7)
8.123(4)
16.766(8)
102.003(8)
2100.4(16)
8
,
b (A)
,
c (A)
i (°)
3
,
V (A )
Z
,
were placed in calculated positions (CꢀH 0.96 A), as-
Dcalc (g cm−3
)
1.866
signed fixed isotropic thermal parameters at 1.2 times for
CꢀH the equivalent isotropic Uij (1.5 for methyl) of the
atoms to which they are attached and allowed to ride on
their respective parent atoms. All calculations were per-
formed using SHELXTL version 5.0.
v (mm−1
F(0 0 0)
)
2.079
1168
10 751
4218 (Rint=0.1012)
225
Reflections collected
Independent reflections
Parameters refined
Final R indices [I\2|(I)]
R1=0.0487,
wR2=0.0899
R1=0.0775,
wR2=0.0996
0.691 and −0.840
0.886
R indices (all data)
3. Results and discussion
−3
,
Largest difference peak and hole (e A
)
Treatment of the rhodium chloride complex [{(h5-
C5Me4H)RhCl2}2] with dfppe in refluxing benzene
Goodness-of-fit on F2

82
R.M. Bellabarba et al. / Inorganica Chimica Acta 323 (2001) 78–88
Table 2
yielded
[{h5,h1,h1-C5HMe2-3,4-[CH2-2-C6F4P(C6F5)-
,
Selected bond lengths (A) and angles (°) with e.s.d. values in paren-
CH2]2-1,2}RhBr]Br (4a) in 53% yield. Salts 2a and 4a
were characterized by mass spectrometry, which
showed the parent cations, and multinuclear NMR
spectroscopies. The former was contaminated by a
small amount of the tetrafluoroborate salt, formed by
the reaction of the by-product HF with the borosilicate
glass vessel [10], and satisfactory analysis could not be
obtained. However, the latter was obtained pure and
satisfactory elemental analysis was obtained. The struc-
ture of 4a was determined by single-crystal X-ray dif-
fraction [15], which clearly established the formulation.
The similarity of the NMR spectroscopic data of 2a
and 4a indicates the same formulation for 2a. The
tetrafluoroborate salts 2b and 4b were prepared by
anion metathesis and characterized by multinuclear
NMR spectroscopies. Salt 2b was further characterized
by elemental analysis. The 31P{1H} NMR spectra of 2a,
2b, 4a and 4b exhibit two doublets of multiplets at
approximately l 70, with rhodium–phosphorus cou-
theses for [{(h5-C5Me4H)RhCl2}2]
Cp†(1)ꢀRh(1)
Rh(1)ꢀCl(1)
Rh(1)ꢀCl(2)
Rh(1)ꢀCl(3)
Rh(1)ꢀC(1A)
Rh(1)ꢀC(2A)
Rh(1)ꢀC(3A)
Rh(1)ꢀC(4A)
Rh(1)ꢀC(5A)
C(1A)ꢀC(2A)
C(2A)ꢀC(3A)
C(3A)ꢀC(4A)
C(4A)ꢀC(5A)
C(5A)ꢀC(1A)
Cp†(1)ꢀRh(1)ꢀCl(1) 125.4
Cp†(1)ꢀRh(1)ꢀCl(2) 126.8
Cp†(1)ꢀRh(1)ꢀCl(3) 126.8
Cl(1)ꢀRh(1)ꢀCl(2)
Cl(1)ꢀRh(1)ꢀCl(3)
Cl(2)ꢀRh(1)ꢀCl(3)
Rh(1)ꢀCl(2)ꢀRh(2)
1.759
2.402(2)
2.434(2)
Cp†(2)ꢀRh(2)
Rh(2)ꢀCl(4)
Rh(2)ꢀCl(2)
1.755
2.3919(19)
2.4575(18)
2.457(2)
2.115(6)
2.133(6)
2.123(5)
2.152(5)
2.147(6)
1.428(8)
1.410(8)
1.450(8)
1.417(8)
1.431(8)
2.4551(19) Rh(2)ꢀCl(3)
2.125(6)
2.136(6)
2.136(6)
2.139(6)
2.149(6)
1.417(10)
1.429(8)
1.445(8)
1.425(8)
1.417(10)
Rh(2)ꢀC(1B)
Rh(2)ꢀC(2B)
Rh(2)ꢀC(3B)
Rh(2)ꢀC(4B)
Rh(2)ꢀC(5B)
C(1B)ꢀC(2B)
C(2B)ꢀC(3B)
C(3B)ꢀC(4B)
C(4B)ꢀC(5B)
C(5B)ꢀC(1B)
Cp†(2)ꢀRh(2)ꢀCl(4) 128.4
Cp†(2)ꢀRh(2)ꢀCl(2) 127.1
Cp†(2)ꢀRh(2)ꢀCl(3) 125.6
Cl(4)ꢀRh(2)ꢀCl(2)
Cl(4)ꢀRh(2)ꢀCl(3)
Cl(2)ꢀRh(2)ꢀCl(3)
Rh(1)ꢀCl(3)ꢀRh(2)
90.55(6)
89.40(6)
91.40(6)
83.32(5)
97.09(6)
89.71(7)
82.78(5)
96.53(6)
1
pling constants ꢀ J(RhꢀP)ꢀ of approximately 140 Hz,
consistent with the values of l 71.3 and 144 Hz for 1a
[9,10] and l 68.5 and 142 Hz for the bromide analogue
of 1b [11]. The presence of two resonances is indicative
of non-equivalent phosphorus atoms and the asymme-
Cp†(1) and Cp†(2) are the centroids of the cyclo-pentadienyl rings
defined by C(1A), C(2A), C(3A), C(4A), C(5A) and C(1B), C(2B),
C(3B), C(4B), C(5B), respectively.
1
try within the cations 2 and 4. The H NMR spectrum
of 2a possesses eight resonances in the region l 3.5–5.0
indicating that all the methylene hydrogen atoms,
PCH2 and C5CH2C6F4, are unique. Each C5CH2C6F4
methylene group shows two mutually coupled reso-
yielded the salt [{h5,h1,h1-C5HMe2-3,4-[CH2-2-C6F4P-
(C6F5)CH2]2-1,2}RhCl]Cl (2a) as a yellow precipitate in
60% yield (Scheme 1). Similarly, treatment of the bro-
mide complex [{(h5-C5Me4H)RhBr2}2] (3) with dfppe
2
nances with a coupling ꢀ J(HꢀH)ꢀ of approximately 18
Scheme 1. (i) C6H6, heat; (ii) NH4BF4, acetone; (iii) X=Cl; NaBF4, dfppe, CH2Cl2/MeOH; (iv) EtOH, heat or proton sponge, CD2Cl2.

R.M. Bellabarba et al. / Inorganica Chimica Acta 323 (2001) 78–88
83
Hz, one of which is further coupled to one phosphorus,
occurred in the 1H{31P} NMR spectrum where the
doublets at l 4.94 and 4.67, assigned to two of the
C5CH2C6F4 hydrogen atoms were shifted to lower fre-
quency and appear as two doublets with a ratio of
integrals of 1.5:0.5 at l 4.09 and 4.03. The former
consists of coincidental signals. Other small shifts are
1
as confirmed by the H{31P} NMR spectrum, with a
4
coupling ꢀ J(PꢀH)ꢀ of approximately 10 Hz. In contrast,
the respective C5CH2C6F4 resonances of 2b do not
show coupling to phosphorus. Thus, the anion exerts a
large effect on the NMR spectroscopic properties of the
cation. There are also differences between the methyl
resonances of 2a and 2b. Those of 2a occur as a doublet
1
noted in the H NMR spectrum, but little change is
observed in either the 19F or 31P NMR spectra.
4
of doublets at l 1.93, with couplings ꢀ J(PꢀH)ꢀ of 14.2
On the basis of the relative position of CꢀH bond
cleavage in reaction 1, the reaction between [{(h5-
C5Me4H)RhX2}2] and dfppe was expected to yield two
isomers of the cation of formulation [{h5,h1,h1-
4
and 1.9 Hz, and a doublet at l 1.91 with ꢀ J(PꢀH)ꢀ 1.6
Hz, whereas those of 2b occur as doublets at l 1.98 and
4
1.89 with ꢀ J(PꢀH)ꢀ 8.7 and 2.7 Hz, respectively. Both
spectra exhibit a doublet at approximately l 5.5 with
C5HMe2[CH2-2-C6F4P(C6F5)CH2]2-1,3}RhX]+
(III)
3
ꢀ J(PꢀH)ꢀ approximately 6.5 Hz which is assigned to the
and (IV) dependent on the positions of the two methyl
groups (Fig. 3). Neither of these two isomers could be
unambiguously identified in the crude products. Instead
only products resulting from cleavage of CꢀH bonds of
adjacent methyl groups could be positively identified.
The asymmetric cation V is that precipitated in the
reaction as 2a or 4a. The symmetric cation VI was not
observed in the precipitate. The reaction has been re-
peated a number of times, and the result is entirely
reproducible. Further, for the reaction involving [{(h5-
C5Me4H)RhCl2}2] multinuclear NMR spectroscopic
and mass spectrometric investigations indicated that 2a
is also the major species in the mother liquor. The mass
spectral data of the solid derived from removal of
solvent from the mother liquor provided evidence for
the formation of [(h5-C5Me4H)RhCl(dfppe)]+ and at
least one isomer of the singly CꢀF bond activated
complex [{h5,h1,h1-C5HMe3CH2-2-C6F4P(C6F5)CH2-
CH2P(C6F5)2}RhCl]+. These two cations are intermedi-
ates in the formation of 2a. On one occasion the
31P{1H} NMR spectrum of the solid also comprised a
low intensity doublet of multiplet resonance at approx-
1
hydrogen atom of the cyclo-pentadienyl ring. The H
spectra of 4a and 4b are similar to those of 2a and 2b,
respectively, but show some differences. Namely, both
methyl resonances of 4a appear as doublet of doublets
and one resonance of each C5CH2C6F4 methylene hy-
drogen atom pair of 4b shows coupling to one phos-
phorus. Although unequivocal assignment of the
resonances cannot be made, the 13C{1H} NMR and
1
1H–13C and H–31P correlation spectra of 2a and com-
1
parison of the H NMR data with the structure of 4a
[15] does allow the tentative assignment of most of the
resonances of 2a (the labelling scheme is shown in Fig.
2). These assignments are based on the assumption that
for a large PꢀH coupling (\5 Hz) to be observed, the
PꢀRhꢀCR angle must be significantly greater than 90°.
Where the PꢀRhꢀC angle is close to 90°, the PꢀH
coupling is small or negligible. For example, J(PAꢀHa)
is 6.9 Hz and the corresponding PꢀRhꢀCH angles are
147.9(9) and 155.3(9)° for the two independent ion
pairs in the structure of 4a, whereas J(PBꢀHa) is not
observed for the corresponding angles of 107.3(8) and
93.1(10)°. The 19F NMR spectra of 2a, 2b, 4a and 4b
are similar and entirely consistent with a formulation in
which both C6F4 and both C6F5 groups are non-equiva-
lent. The two ortho-C6F5 fluorine resonances of these
salts are broadened slightly due to hindered rotation
about the PꢀC6F5 bonds. Values of the activation en-
ergy, DG‡, of 42.592.5 and 4792 kJ mol−1 for
rotation about the two different PꢀC6F5 bonds of 2a
were calculated from variable temperature 19F NMR
spectra [17]. Values of DG‡ of 4592 and 4792 kJ
mol−1 were calculated for 4a. In comparison, the value
of DG‡ for rotation about the pair of PꢀC6F5 bonds in
1a was determined to be 52.592 kJ mol−1 [9,10].
The cations of 2 and 4 contain three chiral centres:
the rhodium and both phosphorus atoms. However,
due to the geometric constraints of the reaction, only
one pair of enantiomers can be formed. Addition of the
chiral shift reagent praseodymium tris[3-(heptafluoro-
propylhydroxymethylene)-(+)-camphorate] to the
NMR sample of 2a confirms that the cation is formed
as a racemic mixture. The most noticeable change
1
imately l 68.5 with ꢀ J(RhꢀP)ꢀ approximately 140 Hz,
which, by comparison with the spectral data of 1a, is
tentatively assigned to one of the symmetric isomers IV
or VI. The ratio of asymmetric to symmetric isomers of
the product formed was determined from the NMR
spectral data to be greater than 9:1, although typically
resonances which could be ascribed to a symmetric
isomer were not observed. Thus, the reaction between
[{(h5-C5Me4H)RhX2}2] and dfppe displays regioselec-
tivity in the activation of CꢀH bond activation in
contrast to that of reaction (1).
The intermediate cation [(h5-C5Me4H)RhCl(dfppe)]+
was prepared independently as the tetrafluoroborate
salt 5 by addition of NaBF4 and dfppe to [{(h5-
C5Me4H)RhCl2}2] (Scheme 1). Salt 5 was characterized
by elemental analysis, mass spectrometry and multinu-
clear NMR spectroscopies. The 19F and 31P NMR
spectroscopic data are similar to those of the pen-
tamethyl-cyclo-pentadienyl analogue [10]. In particular,
the 31P NMR resonance is a doublet of multiplets at l
1
32.9 with ꢀ J(RhꢀP)ꢀ of 149 Hz, which are comparable

84
R.M. Bellabarba et al. / Inorganica Chimica Acta 323 (2001) 78–88
Fig. 3. Asymmetric and symmetric isomers of the cations [RhCl{h5,h1,h1-C5Me2H-2,4-[CH2-2-C6F4P(C6F5)CH2]2-1,3}]+ and [RhCl{h5,h1,h1-
C5Me2H-3,4-[CH2-2-C6F4P(C6F5)CH2]2-1,2}]+ viewed along the C5 (centroid)ꢀRh axis.
1
to the values of l 35.1 and ꢀ J(RhꢀP)ꢀ 150.5 Hz for
[Cp*RhCl(dfppe)]BF4. There is hindered rotation about
the PꢀC6F5 bonds of 5 as indicated by broad reso-
nances in the 19F NMR spectrum recorded at 293 K.
The value of DG‡ for rotation about one pair of
PꢀC6F5 bonds, calculated from variable temperature
Thus, the reaction between [{(h5-C5Me4H)RhCl2}2] and
dfppe in benzene provides a more convenient route to
2b, giving a higher isolated yield. Thermolysis of 5 in
ethanol for 5 h also afforded 2b as evidenced by
multinuclear NMR spectroscopy (Scheme 1), but the
reaction is not clean, the NMR spectra showing the
presence of other unidentified compounds.
19F NMR spectra in acetone [17], is 43.592 kJ mol−1
.
This value is consistent with the values of 4694 and
4392 kJ mol−1 for the higher activation energies for
rotation about one pair of PꢀC bonds in [Cp*RhCl-
(dfppe)]BF4 [10] and [Cp*RhCl{(C6H3F2-2,6)2PCH2-
CH2P(C6H3F2-2,6)2}]BF4 [12], respectively. The value
of DG‡ for rotation about the other pair of PꢀC6F5
bonds could not be found since the fluxional process
could not be frozen out at a spectrometer frequency of
The difference in regioselectivity between reaction (1)
and that between [{(h5-C5Me4H)RhX2}2] and dfppe is
striking. We have reported differences between the ge-
ometries of tetramethyl- and pentamethyl-cyclo-penta-
dienyl complexes [(h5-C5Me4R)RhCl2P] (P=phos-
phine) [18]. In particular, for the pentamethyl-cyclo-
pentadienyl complex, the RhꢀC5 moiety possesses C5
symmetry with identical distances, within experimental
error, for the RhꢀC and CꢀC bonds. In contrast, the
ring of similar tetramethyl-cyclo-pentadienyl complexes
is distorted to give two longer and two shorter
RhꢀCMe distances and a short RhꢀCH distance. The
CꢀC ring distances also vary significantly suggesting
that there is a degree of localization in the carbon–car-
bon bonding within the ring and could be tentatively
described as ‘ene-enyl’ [19–22] rather than pentadienyl,
with the CH the centre of the allylic group. In order to
ascertain whether a similar ring distortion was the
cause of the difference in regioselectivity, we endeav-
oured to perform a structural study of complexes 2–5.
Unfortunately crystals of salts 2a, 2b, 4b and 5 were
unsuitable for single-crystal X-ray diffraction, and the
large estimated standard deviations (e.s.d. values) of the
structural data of 4a [15] do not permit a detailed
comparison with the structure of 1b. However, the
structure of [{(h5-C5Me4H)RhCl2}2] has been deter-
mined (Fig. 4). Selected distances and angles are pre-
1
282 MHz. The methyl hydrogen resonances in the H
NMR spectrum appear as a doublet of doublets at l
4
1.82 with couplings ꢀ J(PꢀH)ꢀ of 7.9 and 2.4 Hz and a
4
doublet at l 1.58 with ꢀ J(PꢀH)ꢀ 5.0 Hz, each integrat-
ing for six hydrogen atoms. The coupling to phospho-
1
rus is confirmed by the H{31P} NMR spectrum. It is
not clear why the PꢀH couplings should be so different,
but it is noted that the resonances are temperature
independent and thus not determined by hindered rota-
tion about the RhꢀC5 (centroid) axis.
In situ NMR experiments in CD2Cl2 confirmed that 5
is converted to 2b rapidly and almost quantitatively on
addition of 2 equiv. of proton sponge. On a preparative
scale 2b was isolated in only approximately 50% yield
from treatment of
5
with proton sponge in
dichloromethane. The low yield is ascribed to
difficulties in separating the product from protonated
proton sponge, presumably present as one or more of
the tetrafluoroborate, fluoride and bifluoride salts.

R.M. Bellabarba et al. / Inorganica Chimica Acta 323 (2001) 78–88
85
sented in Table 2. Unlike the pentamethyl-cyclo-penta-
dienyl analogue [(Cp*RhCl2)2] [23], the centre of the
RhCl2Rh core of [{(h5-C5Me4H)RhCl2}2] does not lie
on a crystallographic centre of symmetry. The RhꢀC
deprotonation, and subsequent CꢀC coupling, occur-
ring at the 2-methyl group. The hydrogen atoms of the
4-methyl group are expected to be more acidic than
those of the 2- and 3-methyl groups, but deprotonation
at this methyl group gives an h4-fulvene complex which
cannot carry out an intramolecular nucleophilic attack
at a pentafluorophenyl ring for geometric reasons. The
acidities of the hydrogen atoms of the 2- and 3-methyl
groups are expected to be similar and presumably the
high selectivity for reaction at the 2-position is geomet-
ric. The methyl and pentafluorophenyl groups of cation
B are not disposed to give the product V. Deprotona-
tion of the 4-methyl group, which comprises the most
acidic hydrogen atoms, and CꢀC coupling would give
IV, which is presumably not formed for geometric
reasons. Since the yields of 2a and 4a are over 50%, and
2b is formed virtually quantitatively by the reaction of
5 with proton sponge at room temperature, either B can
isomerize to A or A is formed preferentially to B. The
former explanation requires either dissociation of both
phosphine moieties, inversion at the chiral phosphorus
atom and recoordination of the two phosphorus atoms
as shown in Scheme 2 or dissociation of the cyclo-pen-
tadienide, rotation about the C5ꢀCH2 bond and recoor-
dination. Although cyclo-pentadieneide ligands can be
displaced from rhodium by phosphorus(III) ligands
[24], or in other ways [25], subsequent recoordination is
without precedent, and for this reason the latter possi-
bility may be discounted. Dissociation and recoordina-
tion of chelating phosphines from rhodium is known.
For example, cleavage of RhꢀP bonds has been pro-
posed to explain the epimerization of the chiral cation
of (RRh,RC)-[Cp*RhCl(Ph2PCH2CHMePPh2)]BF4 on
heating in polar solvents, a process accelerated by the
presence of chloride ions [26]. Furthermore, treatment
of the diastereoisomer of [Cp*RhCl{Ph2PCH2CH2PPh-
(C5F4N-4)}]+ in which the tetrafluoropyridyl group is
distant to the Cp* ring with proton sponge does give
the coupled product [{h5,h1,h1-C5Me4CH2(-2-C5F3N-
,
distances range from 2.115(6) to 2.152(5) A and the
,
ring CꢀC distances from 1.410(8) to 1.450(8) A. Al-
though the longest and shortest RhꢀC distances differ
significantly, the CꢀC distances are the same within
experimental error. Thus, the ring cannot be described
as ‘ene-enyl’. The distortion from C5 symmetry along
the RhꢀC5 (centroid) axis can be compared with that in
[(Cp*RhCl2)2], in which the RhꢀC distances vary from
,
2.116(4) to 2.140(3) A, but the ring CꢀC distances range
,
from 1.370(5) to 1.452(7) A. The distances and angles
of the Cl2Rh2(m-Cl)2 unit are similar for the two struc-
tures, although that of [{(h5-C5Me4H)RhCl2}2] does
possess one anomalously short RhꢀCl (bridging) dis-
,
tance of 2.434(2) A and the RhꢀClꢀRh and ClꢀRhꢀCl
angles are 1–1.5° more acute and obtuse, respectively,
than those of [(Cp*RhCl2)2]. Thus, the regioselectivity
differences between the two reactions does not arise
from an inherent distortion from C5 symmetry of the
RhꢀC5 moiety of the starting material. However, differ-
ences arising from ring distortion in the intermediate
[(h5-C5Me4R)RhCl(dfppe)]+ cations cannot be ruled
out as a possible source of the difference in regioselec-
tivity.
A possible mechanism to account for the high selec-
tivity in the reactions between [(h5-C5Me4H)RhX2]2 and
dfppe is shown in Scheme 2. Deprotonation of the
intermediate cation [(h5-C5Me4H)RhX(dfppe)]+ forms
an h4-fulvene complex, the methylene carbon of which
carries out SNAr attack at an ortho CꢀF bond to form
a CꢀC bond [7]. Since the hydrogen atoms of the
methyl groups of the 1- and 4-methyl groups of the
cation are the more acidic, the first CꢀC coupling
occurs at the 1-position to give cation A or B. Both
these intermediates are chiral and would exist as a pair
of enantiomers. The product cation V is obtained by
Fig. 4. Molecular structure of [{(h5-C5Me4H)RhCl2}2]. Thermal ellipsoids are shown at the 30% probability level. The hydrogen atoms are omitted
for clarity.

86
R.M. Bellabarba et al. / Inorganica Chimica Acta 323 (2001) 78–88
Scheme 2. Proposed mechanism. All cations are shown as viewed along the C5 (centroid)ꢀRh axes. Only one enantiomer of each pair is shown.
4)PPhCH2CH2PPh2}RhCl]+ after isomerization involv-
ing cleavage of one RhꢀP bond, which is the rate
determining step [27]. However, at room temperature
this reaction takes weeks to reach completion. Inversion
at the phosphorus atom of phosphines is also known,
although the activation energy for this process is usu-
ally high, typically 120–150 kJ mol−1 for PhPMeR
[28,29], but is reduced by the presence of electron-with-
drawing on the phosphorus substituents and also by p
interactions between the phosphorus atom and the sub-
stituents. For example, the activation energy for inver-
sion at the phosphorus atom of Me3SiP(Pri)(C6H4-
Me-4), which contains the electron-withdrawing silyl
group is significantly lower than that of comparable
non-silicon containing phosphines [28,29], and that of
cyclo-{CHꢁC(Me)}2PCHMe2, in which there is signifi-
cant delocalization, is only 65 kJ mol−1 [30].
Polyfluorinated aryl rings are electron-withdrawing and
can provide p interactions with substituents and thus
the barrier to inversion at the chiral phosphorus atom
of the proposed intermediate might be expected to be
relatively low. However, since this mechanism also in-
volves the cleavage of two RhꢀP bonds, the rate of the
conversion of the two isomers is expected to be slow.
Thus, although interconversion of A and B is possible,
it is likely to be very slow at room temperature. The
rapid reaction of 5 with proton sponge at room temper-
ature militates against a mechanism involving conver-
sion of B to A. Although the preferential formation of
A seems unlikely in view of the unhindered rotation of
the Cp* ligand about the RhꢀC5 (centroid) axis in 5,
there may be electronic or steric reasons for a preferen-
tial orientation of the h4-fulvene ligand with respect to
dfppe affording A preferentially or a large enhancement
of the rate of reaction forming A over that forming B
by, for example, the release of steric pressure. The
observation that the reaction carried out at room tem-
perature is almost quantitative, whereas that carried
out in refluxing benzene or ethanol is less clean is
consistent with this hypothesis, since other reactions
and thus lower selectivity are expected at a higher
temperature.

R.M. Bellabarba et al. / Inorganica Chimica Acta 323 (2001) 78–88
87
In an attempt to identify any singly coupled interme-
diates the reaction between [{(h5-C5Me4H)RhCl2}2] and
dfppe in ethanol at approximately 60 °C was followed
by multinuclear NMR spectroscopy. Although this
might be expected to show side reactions, the rapid
nature of the reaction of 5 with proton sponge pre-
of the form [Cp*RhCl(P2)]+, where P2 is a chelating
diphosphine, is very sensitive to the phosphorus envi-
ronment. For example, the chemical shift difference
between the two phosphorus atoms of both the chloride
salts 2a and 4a is approximately 5 ppm, and of the
tetrafluoroborate salts 2b and 4b is approximately 7
1
cluded a study of this reaction. The H and 19F NMR
ppm,
and
[Cp*RhCl(prophos)]Cl
(prophosꢁR-
spectra of the reaction mixture displayed several reso-
nances, many coincident, which were difficult to assign.
However, the 31P{1H} NMR spectra were much simpler
and were easier to interpret. By comparison with the
spectrum of 5, a doublet of multiplets at l 32.5 with a
Ph2PCH2CHMePPh2) exhibits chemical shifts for the
two phosphorus atoms at l 75.4 and 46.1 for the
SRh,RC diastereoisomer and l 63.9 and 57.0 for the
RRh,RC diastereoisomer [26]. Therefore, the 31P NMR
spectra of A and B are expected to be different and the
data for the reaction between [{(h5-C5Me4H)RhCl2}2]
and dfppe strongly suggest the presence of only one
isomer of [{h5,h1,h1-C5HMe3CH2-2-C6F4P(C6F5)CH2-
CH2P(C6F5)2}RhCl]+ as an intermediate, but cannot
confirm that the intermediate is A. The evidence sup-
ports the suggested mechanism in which one of the
possible isomeric intermediates is formed with a high
selectivity. The data also confirm that, as expected, the
reaction carried out in hot ethanol is not clean.
1
coupling ꢀ J(PꢀH)ꢀ of 150 Hz is assigned to the cation
[(h5-C5Me4H)RhCl(dfppe)]+, which will be present as
the chloride salt. This resonance dominated the spec-
trum after 2 h, but had diminished at 4 h and had
disappeared after 7 h. Resonances assigned to the
product 2a were observed at 2 h, and increased in
intensity as the reaction proceeded. Two doublet of
multiplet resonances at l 74.4 and 33.8 with couplings
1
ꢀ J(PꢀH)ꢀ of 144 and 151 Hz, respectively, were present
after 2 h, increased in intensity as those of [(h5-
C5Me4H)RhCl(dfppe)]+ decreased and then decreased
as 2a was formed. The ratio of the integrals of these
resonances was always 1:1. These resonances are as-
signed to one isomer of [{h5,h1,h1-C5HMe3CH2-2-
C6F4P(C6F5)CH2CH2P(C6F5)2}RhCl]+ by comparison
with those of the Cp* analogue [10]. At least seven
other resonances showing RhꢀP coupling, but all with
small integrals, were present in the 31P{1H} NMR
spectrum during the reaction. A doublet of multiplets at
l 7.7 with a coupling of 148 Hz was a major compo-
nent of the reaction mixture after 2 h, but had dimin-
ished after 4 h and was absent after 7 h. On the basis
that lP for the dinuclear complex [(Cp*IrCl2)2(m2-df-
ppe)] [12] is approximately 24 ppm to lower frequency
of lP for [Cp*IrCl(dfppe)]BF4 [10], this resonance may
be assigned to [{(h5C5Me4H)RhCl2}2(m2-dfppe)] or a
complex containing monodentate dfppe. In support,
[Cp*RhCl2L] compounds containing monodentate
diphosphine ligands (L), although not dfppe, have been
reported [26]. Other, unidentified RhꢀP complexes were
observed. A doublet of multiplets at l 50.4 with a
coupling of approximately 125 Hz was present at 2 and
4 h, but absent after 7 h. A doublet of multiplets at l
40.7 with a coupling of 149 Hz was present in all the
spectra. There were also two doublet of multiplet reso-
nances with couplings of approximately 150 Hz in the
region 70–80 ppm which were present in the spectra
recorded up to 7 h. As the reaction proceeded a doublet
of multiplets at l 20.1 with a coupling of only 59 Hz
and a multiplet showing no coupling to rhodium at l
19.4 arose. The former diminished after 7 h. The latter
increased in intensity during the reaction and may be
tentatively assigned to a product of oxidation of dfppe.
The phosphorus chemical shift in cationic complexes
4. Conclusion
The reaction between [{(h5-C5Me4H)RhX2}2] and df-
ppe proceeds thermally in benzene or ethanol to give
the asymmetric cation [{h5,h1,h1-C5HMe2-3,4-[CH2-2-
C6F4P(C6F5)CH2]2-1,2}RhX]+ in high yield with high
regioselectivity. The same product is obtained by treat-
ing [(h5-C5Me4H)RhCl(dfppe)]BF4 with proton sponge.
The regioselectivity displayed by these reactions is in
stark contrast to that displayed by the reaction between
[(Cp*RhX2)2] and dfppe, which yields a product with a
cyclo-pentadieneide ligand functionalized in the 1 and 3
positions. The reason for the difference in regioselectiv-
ity is not known, but presumably arises from geometric
differences between the tetra- and penta-methyl-cyclo-
poentadienyl intermediates [(h5-C5Me4R)RhX(dfppe)]+
and [{h5,h1,h1-C5Me3RCH2-2-C6F4P(C6F5)CH2CH2P-
(C6F5)2}RhX]+. In support of this conclusion, distor-
tions from local C5 symmetry of the RhꢀC5 moiety is
noted for [(h5-C5Me4H)RhCl2P] but not [Cp*RhCl2P]
(P=phosphine). However, a similar difference between
the
structures
of
[(h5-C5Me4H)RhCl2}2]
and
[(Cp*RhCl2)2] is not observed. A possible mechanism
for the reaction has been proposed which requires the
final dehydrofluorinative CꢀC coupling to occur for
predominantly one isomer of [{h5,h1,h1-C5Me3HCH2-
2-C6F4P(C6F5)CH2CH2P(C6F5)2-1}RhX]+ (A) in order
to account for the high regioselectivity. The data sug-
gest that this isomer is formed preferentially.
The reaction between [{(h5-C5Me4H)RhX2}2] and df-
ppe further demonstrates the synthetic potential of
intramolecular dehydrofluorinative CꢀC coupling can

88
R.M. Bellabarba et al. / Inorganica Chimica Acta 323 (2001) 78–88
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provide by facilitating the highly selective, high yield
synthesis of complexes containing an asymmetric 1,2-
substituted cyclo-pentadienyl-bis(phosphine) ligand.
The reaction complements reaction 1, which yields the
1,3-substituted analogue exclusively. These reactions
may be compared to the intramolecular hydroalkyla-
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1,3-isomers of [(h5,h1,h1-C5Me3(CH2CH2CH2PPh2)2}-
RhCl]+ [31,32].
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Acknowledgements
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We thank the EPSRC for funding (Ronan M.
Bellabarba).
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