P-Substituent effects on the redox chemistry of the diaryltriazenido-bridged dirhodium complexes [Rh2(CO)4-n(PPh3)n-ui./F-XC 6H4NNNC6H4X′-/> 2] (n = 0-2)

Article abstract of DOI:10.1039/b002939o

The complexes [Rh,(CO)₄(n-p-XC₆H₄NNNC₆H ₄X′-OJ (X = X′ = H, Me, Et, OMe, CN, F, Cl or Br; X = H, X′ = OMe or NO₂) were prepared in a two-step reaction involving the cleavage of [{Rh(μ-Cl)(CO)₂}₂] with the diaryItriazene p-XC₆H₄NNNHC₆H₄X′-p followed by the deprotonation of the resulting mononuclear triazene complex [RhCl(CO)₂{N(C₄H₄X-))NNHC₆H ₄X'-/>}] with NEt₃. Yields of the dimeric products were maximised by carefully controlling the reaction time for each step. Reaction of the tetracarbonyls with PPh3 gave the monoand di-substituted species [Rh₂(CO)₄(PPh₃)(u-p-XC₆H ₄NNNC₆H₄X′->)₂] (n = 1 or 2), the reaction times again depending on the substituents X and X′. Each binuclear complex undergoes at least one reversible one-electron oxidation reaction at a platinum electrode in CH₂Cl₂. In some cases, e.g. X = X' = OMe, as many as three oxidation waves are observed; for X = H, X′ = NO1, n = 1 or 2, well-defined reduction waves are apparent. The oxidation potential depends on the extent of carbonyl substitution (for each incremental increase in n the potential is decreased by ca. 300 mV) and on the triazenide ligand substituent such that E° for the first oxidation wave can be varied systematically over a range of 800 mV. There is a linear relationship between E for the first oxidation step and the Hammett parameter σ_p but a poorer correlation for the second oxidation process. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b002939o

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p-Substituent effects on the redox chemistry of the diaryltri-
azenido-bridged dirhodium complexes [Rh2(CO)4؊n(PPh3)n-
(ꢀ-p-XC6H4NNNC6H4XЈ-p)2] (n ؍ 0–2)
Neil G. Connelly,a Paul R. G. Davis,a Emma E. Harry,a Phimphaka Klangsinsirikula and
Monika Venterb
a School of Chemistry, University of Bristol, Bristol, UK BS8 1TS
b Department of Inorganic Chemistry, Babes-Bolyai University, Arany Janos 11, RO-3400,
Cluj-Napoca, Romania
Received 12th April 2000, Accepted 9th June 2000
Published on the Web 30th June 2000
The complexes [Rh2(CO)4(µ-p-XC6H4NNNC6H4XЈ-p)2] (X = XЈ = H, Me, Et, OMe, CN, F, Cl or Br; X = H,
XЈ = OMe or NO2) were prepared in a two-step reaction involving the cleavage of [{Rh(µ-Cl)(CO)2}2] with the
diaryltriazene p-XC6H4NNNHC6H4XЈ-p followed by the deprotonation of the resulting mononuclear triazene
complex [RhCl(CO)2{N(C6H4X-p)NNHC6H4XЈ-p}] with NEt3. Yields of the dimeric products were maximised
by carefully controlling the reaction time for each step. Reaction of the tetracarbonyls with PPh3 gave the mono-
and di-substituted species [Rh2(CO)4Ϫn(PPh3)n(µ-p-XC6H4NNNC6H4XЈ-p)2] (n = 1 or 2), the reaction times again
depending on the substituents X and XЈ. Each binuclear complex undergoes at least one reversible one-electron
oxidation reaction at a platinum electrode in CH2Cl2. In some cases, e.g. X = XЈ = OMe, as many as three oxidation
waves are observed; for X = H, XЈ = NO2, n = 1 or 2, well-defined reduction waves are apparent. The oxidation
potential depends on the extent of carbonyl substitution (for each incremental increase in n the potential is decreased
by ca. 300 mV) and on the triazenide ligand substituent such that EЊЈ for the first oxidation wave can be varied
systematically over a range of 800 mV. There is a linear relationship between EЊЈ for the first oxidation step and the
Hammett parameter σp but a poorer correlation for the second oxidation process.
Introduction
Results and discussion
By means of the systematic carbonyl substitution reactions
of the diaryltriazenide-bridged [Rh2]2ϩ complex [Rh2(CO)4-
(µ-p-XC6H4NNNC6H4XЈ-p)2] we have stabilised the core
oxidation levels [Rh2]3ϩ (e.g. in [Rh2(CO)2(PPh3)2(µ-p-XC6H4-
NNNC6H4XЈ-p)2]ϩ)1 and [Rh2]4ϩ {e.g. in [Rh2Cl(CO)2(bipy)-
The complexes [Rh2(CO)4(µ-p-XC6H4NNNC6H4XЈ-p)2] (X =
XЈ = H, Me,1 Et, OMe, CN, F, Cl or Br: X = H, XЈ = OMe or
NO2) were prepared in two steps, namely (i) the reaction
of [{Rh(µ-Cl)(CO)2}2] with the diaryltriazene p-XC6H4-
NNNHC6H4XЈ-p, and (ii) the reaction of the resulting
mononuclear triazene complex [RhCl(CO)2{N(C6H4X-p)-
NNHC6H4XЈ-p}] with NEt3 (Scheme 1). The yield of the
binuclear tetracarbonyl [Rh2(CO)4(µ-p-XC6H4NNNC6H4XЈ-
p)2] from the second step is very dependent on the efficient
formation of [RhCl(CO)2{N(C6H4X-p)NNHC6H4XЈ-p}] in the
first. This in turn depends critically on X and XЈ such that
the rate of reaction between dimeric [{Rh(µ-Cl)(CO)2}2] and
p-XC6H4NNNHC6H4XЈ-p is fastest when the substituent is
electron-donating; the reaction takes ca. 3 min for X = XЈ =
MeO but ca. 1 h for X = XЈ = CN. However, where the cleavage
reaction is rapid a further process, which gives uncharacterised
products which do not yield the dimer on deprotonation, is
observed. Thus, for maximum yields of the desired dimeric
tetracarbonyl the formation of the mononuclear triazene
complex was monitored carefully by IR spectroscopy.
Once purified, generally by column chromatography, the
tetracarbonyls [Rh2(CO)4(µ-p-XC6H4NNNC6H4XЈ-p)2] (X =
XЈ = H, Me,1 Et, OMe, F, Cl, Br or CN) were characterised by
elemental analysis (C, H and N) and by their IR carbonyl
spectra (Table 1). The spectra are very similar to that of
[Rh2(CO)4(µ-p-MeC6H4NNNC6H4Me-p)2]12 but show small
shifts to higher wavenumber, in ν(CO), as the electron-
withdrawing capacity of the substituents X and XЈ increases. In
the complexes with asymmetric triazenide ligands, namely
[Rh2(CO)4(µ-PhNNNC6H4XЈ-p)2] (XЈ = OMe or NO2), two
isomers are possible, i.e. head-to-tail (A) and head-to-head
(B) (Scheme 1). Although their IR carbonyl spectra are
(µ-p-XC6H4NNNC6H4XЈ-p)2]ϩ
(bipy = 2,2Ј-bipyridine)}.2
Throughout such studies3 we have exclusively used the
di-p-tolyltriazenide ligand (p-XC6H4NNNC6H4XЈ-p, X = XЈ =
Me), reasoning that the p-substituents X and XЈ were unlikely
to affect significantly the electron transfer properties; detailed
structural studies, EPR spectroscopy and EHMO calculations
had shown the HOMO of the [Rh2]2ϩ complexes to be an out of
phase combination of the 4dz orbitals on the two Rh atoms.4
2
Recently, however, Ren has shown5 that the p-substituents X
and XЈ do markedly affect the redox behaviour of diaryl-
formamidinate complexes such as [M2{µ-p-XC6H4NC(H)-
NC6H4XЈ-p}4] (M = Ni,6 Mo7 and Rh8) and [Ru2{µ-p-XC6H4-
NC(H)NC6H4XЈ-p}4Xn] (X = C2Ph, n = 2;9 X = Cl, n = 110).
Given the extensive chemistry based on the electron-transfer
reactions of [Rh2(CO)4(µ-p-MeC6H4NNNC6H4Me-p)2], the
attraction of being able to tune redox potentials further, and
therefore reactivity, led us to investigate the effects of X and XЈ.
We now report studies of the oxidation of [Rh2(CO)4Ϫn
-
(PPh3)n(µ-p-XC6H4NNNC6H4XЈ-p)2] (X = XЈ = H, Me, Et,
OMe, CN, F, Cl or Br; X = H, XЈ = OMe or NO2; n = 0–2),
three series of complexes which undergo one-electron oxidation
with a marked dependence on X and XЈ of oxidation potential,
EЊЈ, quantified in terms of the Hammett σp parameter.11 Other
substituent effects are also observed. For example, for
X = XЈ = OMe as many as three oxidation waves are observed,
and for X = H, XЈ = NO2, n = 1 or 2, well-defined reduction
waves are apparent.
DOI: 10.1039/b002939o
J. Chem. Soc., Dalton Trans., 2000, 2273–2277
This journal is © The Royal Society of Chemistry 2000
2273

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Scheme 1 R = C6H4X-p, RЈ = C6H4XЈ-p, L = PPh3.
indistinguishable, 31P NMR spectroscopy clearly showed the
formation of all possible derivatives when the two isomers
were reacted with PPh3.
Fig. 1 The cyclic voltammograms of [Rh2(CO)3(PPh3)(µ-p-XC6H4-
NNNC6H4XЈ-p)2]; (a) X = XЈ = H, (b) X = H, XЈ = NO2, and (c)
X = XЈ = OMe, scanned from Ϫ0.4 to 1.65 to Ϫ1.4 to Ϫ0.4 V. In each
case the wave marked with an asterisk (*) is due to the oxidation of
[Fe(η-C5Me5)2], added as an internal potential standard.
The tetracarbonyl complexes undergo stepwise carbonyl sub-
stitution with PPh3 to give [Rh2(CO)4Ϫn(PPh3)n(µ-p-XC6H4-
NNNC6H4XЈ-p)2] (n = 1 or 2). Previously, we have reported
only the synthesis of the dicarbonyl [Rh2(CO)2(PPh3)2(µ-p-
MeC6H4NNNC6H4Me-p)2], it precipitating directly from the
reaction between [Rh2(CO)4(µ-p-MeC6H4NNNC6H4Me-p)2]
and PPh3 in n-hexane.12 However, when the carbonyl substitu-
tion reactions were carried out in CH2Cl2, with the reactants
in the appropriate proportions and with careful monitoring of
the reaction by IR spectroscopy (the rate of carbonyl substitu-
tion is markedly dependent on X and XЈ), both the tri- and
di-carbonyls were isolated as red crystalline solids. Again,
the energies of the IR carbonyl bands depend on X and XЈ
(Table 1).
{J(31P103Rh) = 150, 151, 152 and 151 Hz, respectively} for [Rh2-
(CO)3(PPh3)(µ-PhNNNC6H4NO2-p)2] and at δ 40.22, 40.26,
40.29 and 40.35 {J(31P103Rh) = 152, 148, 150 and 152 Hz
respectively} for [Rh2(CO)3(PPh3)(µ-PhNNNC6H4OMe-p)2].
Further substitution leads to three dicarbonyl isomers (V and
VI from I and II respectively, and VII from both III and IV),
and therefore three doublets [J(31P103Rh) = 158 Hz], at δ 39.70,
39.95 and 40.35 for [Rh2(CO)2(PPh3)2(µ-PhNNNC6H4NO2-p)2].
Unfortunately, in no case could the various isomers be separ-
ated so that no definitive spectral assignment could be made.
The PPh3 complexes were also characterised by 31P NMR
spectroscopy (Table 1). The spectra are simple for the com-
plexes in which X = XЈ, but provide evidence for the formation
of isomers when X and XЈ differ. Each of the tricarbonyls
[Rh2(CO)3(PPh3)(µ-p-XC6H4NNNC6H4XЈ-p)2] (X = XЈ = H,
Me, Et, OMe, F, Cl, Br or CN) shows a single doublet reson-
ance, in the range δ 39.8–40.4, with J(31P103Rh) ca. 150 Hz.
Likewise, each of the symmetrical dicarbonyls [Rh2(CO)2-
(PPh3)2(µ-p-XC6H4NNNC6H4XЈ-p)2] (X = XЈ = H or Et) gives
only one doublet, consistent with the formation of only one
isomer, i.e. that in which the two PPh3 ligands, one on each
metal atom, are trans disposed with respect to the Rh ؒ ؒ ؒ Rh
vector (as observed in the crystal structure of [Rh2(CO)2(PPh3)2-
(µ-p-MeC6H4NNNC6H4Me-p)2]).4
Electrochemical studies
All of the dirhodium complexes [Rh2(CO)4Ϫn(PPh3)n(µ-p-
XC6H4NNNC6H4XЈ-p)2] (n = 0–2) show at least one oxidation
wave in the cyclic voltammogram (in the potential range 1.5 to
Ϫ1.5 V, in CH2Cl2, at a Pt electrode); the redox potential of this,
and subsequent electron transfer reactions (Table 1), depends
on the substituents X and XЈ as described below. The cyclic
voltammograms of [Rh2(CO)3(PPh3)(µ-p-XC6H4NNNC6H4XЈ-
p)2] (X = XЈ = H; X = H, XЈ = NO2; X = XЈ = OMe) are shown
as representative examples in Fig. 1(a)–(c), respectively.
For all of the complexes [Rh2(CO)4Ϫn(PPh3)n(µ-p-XC6H4-
NNNC6H4XЈ-p)2] (except X = XЈ = CN) the first oxidation wave
is fully reversible and corresponds to the formation of
the monocation [Rh2(CO)4Ϫn(PPh3)n(µ-p-XC6H4NNNC6H4XЈ-
By contrast, the 31P NMR spectra of [Rh2(CO)4Ϫn(PPh3)n-
(µ-p-XC6H4NNNC6H4XЈ-p)2] (X = H, XЈ = OMe or NO2, n = 1
or 2) show the presence of all possible isomers. As shown in
Scheme 1, monosubstitution of the two forms of the asym-
metrically bridged tetracarbonyls leads to four possible isomers,
two (I and II) from the head-to-tail dimer A and two (III
and IV) from the head-to-head isomer B. Accordingly, four
doublets are observed, at δ 39.87, 39.99, 40.24 and 40.27
p)2]ϩ, as found for the archetypal compounds [Rh2(CO)4Ϫn
-
A
(PPh3)n(µ-p-MeC6H4NNNC6H4Me-p)2] (n = 0 and 2).1
second oxidation wave, corresponding to the formation
of [Rh2(CO)4Ϫn(PPh3)n(µ-p-XC6H4NNNC6H4XЈ-p)2]2ϩ, is also
usually observed though in some cases it is irreversible, imply-
ing instability of the dication. For some of the tetracarbonyls
2274
J. Chem. Soc., Dalton Trans., 2000, 2273–2277

View Article Online
Table 1 Analytical, electrochemical and spectroscopic data for [Rh2(CO)4Ϫn(PPh3)n(µ-p-XC6H4NNNC6H4XЈ-p)2]
Analysis (%)a
Yield
n
X
XЈ
(%)
C
H
N
EЊ/Vb
ν(CO)/cmϪ1 c
31P NMRd
0
1
2
0
1
2
0
1
2
0
1
2
0
1
2
0
1
2
0
1
2
0
1
2
0
1
2
0
1
2
H
H
H
Me
Me
Me
Et
Et
Et
OMe
OMe
OMe
CN
CN
CN
H
H
H
H
H
H
F
F
F
Cl
Cl
Cl
Br
Br
Br
H
H
H
51
48
88
47.7(47.4)
57.5(57.2)
63.4(63.2)
2.8(2.8)
4.0(3.7)
4.5(4.3)
11.9(11.8)
8.9(8.9)
6.9(7.1)
0.88, 1.6(I)
0.56, 1.56(I)e
0.22, 1.43
2090vs, 2062m, 2026
2064, 2002, 1986m
1980, 1966
2089vs, 2061m, 2024
2062, 2000, 1985m
1978, 1963
2088vs, 2060m, 2023
2062, 2000, 1984m
1978, 1965
2088vs, 2059m, 2022
2060, 1998, 1984m
1976, 1965
2100vs, 2073m, 2039
2074, 2015, 1995m
1990, 1977
2089vs, 2061m, 2024
2063, 2000, 1985m
1979, 1965
2096vs, 2069m, 2034
2070, 2010, 1990m
1987, 1974
2092vs, 2065m, 2029
2066, 2004, 1988m
1982, 1968
—
40.28(151)
40.04(157)
—
40.37(151)
—
Mef
Me
Mef
Et
0.83, 1.42
32
58.8(58.8)
4.1(4.3)
8.5(8.4)
0.50, 1.37e
0.18, 1.28, 1.57
0.82
80
70
80
60
93
63
73
65
33
84
57
75
44
88
70
86
66
66
31
58
60
53
63
55
52.6(52.6)
60.2(60.2)
65.5(65.1)
46.5(46.3)
55.3(55.2)
61.4(61.0)
47.4(47.4)
56.9(56.3)
62.0(62.0)
47.2(46.8)
56.5(56.2)
61.5(62.1)
42.0(42.0)
52.3(52.2)
60.6(60.3)i
43.4(43.0)
53.4(53.2)
59.3(59.5)
40.1(39.7)
49.9(49.9)
57.0(56.6)
33.4(32.8)
42.5(42.9)
51.4(51.7)i
4.6(4.4)
5.0(4.9)
5.8(5.2)
3.6(3.4)
3.9(4.1)
4.5(4.5)
1.7(2.0)
3.1(3.0)
3.9(3.6)
2.9(3.1)
4.1(3.9)
4.5(4.4)
3.0(2.3)
3.1(3.2)
4.5(4.6)
2.1(2.1)
3.1(3.1)
3.6(3.7)
1.7(1.9)
2.9(2.9)
3.9(3.5)
1.5(1.6)
2.2(2.5)
3.7(3.8)
10.2(10.2)
7.8(8.0)
6.5(6.5)
9.7(10.1)
7.7(7.9)
6.4(6.5)
16.9(17.3)
13.4(13.4)
9.5(11.0)
11.5(10.9)
8.3(8.4)
—
Et
Et
0.51, 1.40e
0.17, 1.29, 1.57
0.80, 1.20, 1.37
0.49, 1.14, 1.34
0.18, 1.07, 1.26
1.09(I), Ϫ1.37(I)g
0.81
40.27(151)
39.98(157)
—
40.36(152)
—
OMe
OMe
OMe
CN
CN
CN
OMe
OMe
OMe
NO2
NO2
NO2
F
F
F
Cl
Cl
Cl
Br
Br
Br
—
39.79(151)
—
0.55
0.84, 1.37
—
h
0.51, 1.30, 1.48
0.21, 1.22, 1.41
0.97
6.0(6.8)
—
14.1(14.0)
10.7(10.8)
8.5(8.3)
10.9(10.7)
7.9(8.3)
6.6(6.7)
9.7(9.9)
7.0(7.8)
5.7(6.4)
8.2(8.2)
5.8(6.7)
—
0.71, Ϫ1.10g
0.42, 1.61, Ϫ1.22g
0.91, 1.64(I)
0.62, 1.55(I)
0.32, 1.48
h
h
—
40.27(151)
40.18(157)
—
40.23(151)
—
0.95
0.65
0.37, 1.52
0.95
0.67, 1.64(I)
0.38, 1.54
2094vs, 2066m, 2031
2068, 2007, 1990m
1984, 1970
2094vs, 2067m, 2031
2068, 2007, 1991m
1984, 1971
—
40.28(150)
—
5.4(5.3)
a Calculated values in parentheses. b EЊЈ for reversible one-electron oxidation wave unless otherwise stated. The oxidation peak potential, (Ep)ox,
at a scan rate of 200 mV sϪ1 is given for an irreversible (I) wave. Unless stated otherwise, potentials were calibrated vs. the following internal
standards: tetracarbonyls vs. the couple [Fe(η-C5H5)2]ϩ/[Fe(η-C5H5)2] (0.47 V); tricarbonyls vs. the couple [Fe(η-C5Me5)2]ϩ/[Fe(η-C5Me5)2] (Ϫ0.08 V);
dicarbonyls vs. the couple [Fe(η-C5H4COMe)2]ϩ/[Fe(η-C5H4COMe)2] (0.97 V). c In CH2Cl2; strong (s) absorptions unless otherwise stated,
m = medium, vs = very strong. d Spectra in CD2Cl2; chemical shift in ppm with J(31P103Rh) in Hz in parentheses. e Potentials calibrated vs. the couple
[Fe(η-C5H4COMe)2]ϩ/[Fe(η-C5H4COMe)2]. f Data from ref. 1. g Reduction wave, see text. h Several isomers present, see text. i Sample analysed as a
1.0 n-hexane solvate.
(X = XЈ = Et, F, Cl or Br; X = H, XЈ = NO2) and for all of
the p-cyano complexes the second oxidation wave was not
observed; it is presumed to occur at a potential obscured by the
base electrolyte background curve.
PPh3 (for a given substituent, each increment in n leads to a
shift in EoЈ of ca. 300 mV to more negative potentials), the
potential for the [Rh2]2ϩ–[Rh2]3ϩ couple can be tuned system-
atically over a range of ca. 800 mV (i.e. from 0.17 to 0.97 V).
{The range is increased to ca. 900 mV if the peak potential,
(Ep)ox, for the irreversible oxidation wave of [Rh2(CO)4(µ-p-
NCC6H4NNNC6H4CN-p)2] is included.}
There is a general dependence of the potentials EЊЈ on X and
XЈ such that electron-donating substituents facilitate oxidation.
However, more quantitative relationships can be explored14 by
considering Hammett substituent constants. An early study by
Kadish and co-workers showed15 linear correlations between
such constants and EЊЈ for both the oxidation and reduction of
the carboxylate complexes [Rh2(µ-O2CR)4] and Ren has more
recently reviewed5 his extensive studies of the effects of sub-
stituent on the redox behaviour of [M2(µ-L)4] (L = diarylform-
amidinate). In these ‘lantern-like’ (or ‘paddle-wheel’) complexes
all of the various redox processes show a linear correlation
between EЊЈ and Hammett constants. For example, [Rh2(µ-L)4]
undergoes three successive one-electron oxidations with essen-
tially identical reaction constants of 96, 98 and 97 mV,
respectively.
For [Rh2(CO)2(PPh3)2(µ-p-XC6H4NNNC6H4X-p)2] (X = XЈ =
Me or Et) and for all of the complexes in which the bridging
ligands bear a p-OMe substituent (i.e. X = OMe, XЈ = H
or OMe, n = 0–2) a third reversible wave corresponding to
the formation of [Rh2(CO)4Ϫn(PPh3)n(µ-p-XC6H4NNNC6H4X-
p)2]3ϩ was detected. Finally, in the case of [Rh2(CO)4Ϫn
-
(PPh3)n(µ-PhNNNC6H4NO2-p)2]
a
reduction wave was
observed. Though ill-defined for the tetracarbonyl (n = 0), the
waves for [Rh2(CO)3(PPh3)(µ-PhNNNC6H4NO2-p)2] [Fig. 1(b)]
and [Rh2(CO)2(PPh3)2(µ-PhNNNC6H4NO2-p)2], at Ϫ1.10 and
Ϫ1.22 V, respectively, were reversible and approximately twice
the height of those for the corresponding oxidation waves.
Given the well-known reduction of nitrobenzene to its anion
radical [C6H5NO2]Ϫ,13 and the small shift in potential (ca. 120
mV) as n is increased from 1 to 2 (cf. shifts of ca. 300 mV on
oxidation) these reductions may be associated with electron
addition to the p-NO2C6H4 groups.
For all three series of complexes (n = 0–2) the oxidation
potentials depend significantly on the aryl substituents X and
XЈ. {Potentials, measured to the nearest 10 mV, have been
internally calibrated using [Fe(η-C5H5)2] (for n = 0), [Fe(η-
C5Me5)2] (for n = 1) or [Fe(η-C5H4COMe)2] (for n = 2); each
calibrant was chosen so that its oxidation wave did not obscure
those of the compound under study.} Those for the first oxid-
ation process cover ranges of ca. 250, 320 and 370 mV for
n = 0–2 respectively. Taken with the effect of replacing CO by
Our results, with dirhodium complexes having a rather less
rigid di-bridged structure, are somewhat different. Plots of EЊ1Ј
(the potential for the first, reversible, one-electron oxidation) vs.
4σp (given the four substituents, two on each bridging ligand)
are linear in all cases (correlation coefficients, 0.97, 0.98 and
0.96 for n = 0, 1 and 2, respectively) (Fig. 2) with reaction con-
stants 71, 91 and 108 mV. However, plots of EЊ2Ј (the potential
for the second one-electron oxidation step) vs. 4σp for
J. Chem. Soc., Dalton Trans., 2000, 2273–2277
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and an agar bridge saturated with KCl. Solutions in CH2Cl2
were 0.1 × 10Ϫ3 mol dmϪ3 or 5 × 10Ϫ4 mol dmϪ3 in the test com-
pound and 0.1 mol dmϪ3 in [NBu4][PF6] as the supporting
electrolyte. Under the conditions used, EЊЈ for the one-electron
oxidations of [Fe(η-C5H5)2], [Fe(η-C5Me5)2] or [Fe(η-C5H4-
COMe)2], added to the test solutions as internal calibrants, are
0.47, Ϫ0.08 and 0.97 V, respectively. Microanalyses were carried
out by the staff of the Microanalytical Service of the School of
Chemistry, University of Bristol.
Syntheses
[Rh2(CO)4(ꢀ-p-FC6H4NNNC6H4F-p)2]. A mixture of [{Rh-
(µ-Cl)(CO)2}2] (0.10 g, 0.26 mmol) and p-FC6H4NNNHC6H4F-
p (0.126 g, 0.54 mmol) in CH2Cl2 (10 cm3) was stirred for 10 min
and then NEt3 (0.084 cm3, 0.60 mmol) was added. The red
solution was evaporated to low volume (ca. 5 cm3) and then
placed on an alumina–diethyl ether chromatography column.
Elution with diethyl ether gave an orange band which was
collected. n-Hexane (20 cm3) was added to the red eluate and
the mixture evaporated to low volume and then cooled to
Ϫ20 ЊC to give red crystals, yield 0.183 g (86%).
The complexes [Rh2(CO)4(µ-p-XC6H4NNNC6H4XЈ-p)2]
(X = XЈ = Cl or Br) were prepared similarly as red crystals. The
following modifications were made in order to synthesise
[Rh2(CO)4(µ-p-XC6H4NNNC6H4XЈ-p)2]: X = XЈ = CN, reac-
tion time 1 h, chromatography on Florisil–CH2Cl2, elution
with CH2Cl2, eluate evaporated after addition of n-hexane;
X = XЈ = OMe, reaction time 3 min, chromatography on
alumina–n-hexane, elution with diethyl ether, evaporation of
eluate to low volume and cooling to Ϫ20 ЊC; X = H, XЈ = NO2,
reaction time 20 min, chromatography on Florisil–n-hexane,
elution with CH2Cl2–n-hexane (1:1).
Fig. 2 Plots of EЊ1Ј vs. 4σp for (a) [Rh2(CO)4(µ-p-XC6H4NNNC6H4XЈ-
p)2], (b) [Rh2(CO)3(PPh3)(µ-p-XC6H4NNNC6H4XЈ-p)2], and (c) [Rh2-
(CO)2(PPh3)2(µ-p-XC6H4NNNC6H4XЈ-p)2].
[Rh2(CO)4Ϫn(PPh3)n(µ-p-XC6H4NNNC6H4XЈ-p)2] show poorer
correlations of 0.79, 0.73 and 0.90 for n = 0–2 respectively.
(Somewhat better values, of 0.93, 0.82 and 0.90, were obtained
by including estimates of EЊЈ for those species which show
irreversible oxidation waves.) The reason for the difference
between [Rh2(CO)4Ϫn(PPh3)n(µ-p-XC6H4NNNC6H4XЈ-p)2] and
[Rh2(µ-L)4] (L = diarylformamidinate) is unclear but may be
related to structural distortions on oxidation. X-Ray crystallo-
graphic studies on [Rh2{µ-XC6H4NC(H)NC6H4X}2}4] (X =
H,16 p-Me17 or m-OMe8) and [Rh2{µ-RNC(H)NR}2}4] (R =
C6H3Cl2-3,5),8 with substituents having a range of Hammett
constants, have a rigid framework with almost invariant geom-
etry (e.g. very similar Rh–Rh and Rh–N distances) indicating
“the absence of substituent perturbation on the geometry of
the dinuclear core”.8 By contrast, the carbonyl complexes
are more flexible so that oxidation of [Rh2(CO)2(PPh3)2(µ-p-
MeC6H4NNNC6H4Me-p)2] and the diphenylacetamidinate
analogues [Rh2(CO)2LLЈ{µ-PhNC(Me)NPh}2] {L, LЈ = PPh3
or P(OPh)3}4 leads to a marked distortion; the interplane
and staggering angles between the two RhI square planes
are reduced on oxidation (as the rhodium–rhodium distance
decreases). Such angular distortions are likely to increase as the
Rh–Rh distance further shortens on the loss of the second elec-
tron from the Rh2 σ* orbital, perhaps irregularly affecting EЊ2Ј
and hence leading to the poorer observed correlation with σp.
[Rh2(CO)3(PPh3)(ꢀ-PhNNNPh)2]. Solid PPh3 (0.118 g, 0.45
mmol) was added to a stirred solution of [Rh2(CO)4(µ-
PhNNNPh)2] (0.30 g, 0.42 mmol) in CH2Cl2 (20 cm3). After
10 min the red solution was evaporated to low volume (ca.
5 cm3) and then placed on a silica–n-hexane chromatography
column. Elution with CH2Cl2–n-hexane (3:10) gave a red band
which was collected, evaporated to low volume (ca. 5 cm3) and
then cooled to Ϫ20 ЊC to give red crystals, yield 0.19 g (48%).
The complexes [Rh2(CO)3(PPh3)(µ-p-XC6H4NNNC6H4XЈ-
p)2] (X = XЈ = Me or Et; X = H, XЈ = OMe) were prepared simi-
larly but without the need for chromatography. The reaction
mixture was filtered, n-hexane was added, and the volume of
the solvent was reduced to give the solid product.
Experimental
The preparation, purification and reactions of the complexes
described were carried out under an atmosphere of dry dinitro-
gen using dried, distilled and deoxygenated solvents; reactions
were monitored by IR spectroscopy where necessary. The com-
pound [{Rh(µ-Cl)(CO)2}2]18 and the triazenes RNNNHRЈ19
were prepared by published methods. The tetracarbonyls
[Rh2(CO)4(µ-p-XC6H4NNNC6H4XЈ-p)2] (X = XЈ = H or Et;
X = H, XЈ = OMe) were prepared by the method used for
[Rh2(CO)4(µ-p-MeC6H4NNNC6H4Me-p)2].1 Where the com-
plexes [Rh2(CO)4Ϫn(PPh3)n(µ-p-XC6H4NNNC6H4XЈ-p)2] (n = 1
or 2) were purified using a mixture of two solvents, the impure
solid was dissolved in the more polar solvent, the resulting solu-
tion was filtered and then treated with the second solvent, and
the mixture reduced in volume in vacuo to induce precipitation.
IR and 31P NMR spectra were recorded on Nicolet 5ZDX FT
and JEOL λ300 spectrometers, respectively. Electrochemical
studies were carried out using an EG&G model 273A poten-
tiostat (computer-controlled using EG&G model 270 Research
[Rh2(CO)3(PPh3)(ꢀ-p-FC6H4NNNC6H4F-p)2]. Solid PPh3
(0.061 g, 0.23 mmol) was added to a stirred solution of
[Rh2(CO)4(µ-p-FC6H4NNNC6H4F-p)2] (0.183 g, 0.23 mmol) in
CH2Cl2 (10 cm3). After 10 min, n-hexane (20 cm3) was added,
and then the mixture was evaporated to low volume and cooled
to Ϫ20 ЊC to give red crystals, yield 0.156 g (66%).
The complexes [Rh2(CO)3(PPh3)(µ-p-XC6H4NNNC6H4XЈ-
p)2] (X = XЈ = MeO, CN, Cl or Br) were prepared similarly; in
the case of X = XЈ = Br the complex was purified using CH2Cl2–
isopropanol.
[Rh2(CO)3(PPh3)(ꢀ-PhNNNC6H4NO2-p)2]. Solid PPh3 (0.065
g, 0.25 mmol) was added to a stirred solution of [Rh2(CO)4-
(µ-PhNNNC6H4NO2-p)2] (0.20 g, 0.25 mmol) in CH2Cl2 (20
cm3). After 20 min the mixture was evaporated to low volume
(ca. 5 cm3) and placed on an alumina–CH2Cl2 chromatography
column. Elution with CH2Cl2 gave a red band which was
collected and then evaporated to dryness to give a red solid,
yield 0.23 g (88%).
Electrochemistry software) in conjunction with
a three-
electrode cell. For cyclic voltammetry the auxiliary electrode
was a platinum wire and the working electrode a platinum disc.
The reference was an aqueous saturated calomel electrode
(SCE) separated from the test solution by a fine-porosity frit
[Rh2(CO)2(PPh3)2(ꢀ-PhNNNPh)2]. Solid PPh3 (0.354 g,
1.35 mmol) was added to a stirred solution of [Rh2(CO)4-
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(µ-PhNNNPh)2] (0.16 g, 0.23 mmol) in CH2Cl2 (20 cm3). After
1 h the red solution was filtered, n-hexane was added, and the
mixture was evaporated to low volume to give a red solid which
was purified using CH2Cl2–n-hexane to give red crystals, yield
0.24 g (88%).
The complexes [Rh2(CO)2(PPh3)2(µ-p-XC6H4NNNC6H4XЈ-
p)2] (X = XЈ = F or OMe) were prepared similarly. The complex
[Rh2(CO)2(PPh3)2(µ-PhNNNC6H4NO2-p)2] was isolated as a
dark brown solid after addition of n-hexane to the filtered
reaction mixture (reaction time 2 h), reduction of the solution
volume in vacuo and storage at Ϫ20 ЊC.
References
1 N. G. Connelly, G. Garcia, M. Gilbert and J. S. Stirling, J. Chem.
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Dalton Trans., 1994, 2025; N. G. Connelly, P. M. Hopkins,
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[Rh2(CO)2(PPh3)2(ꢀ-p-EtC6H4NNNC6H4Et-p)2]. Solid PPh3
(0.272 g, 1.04 mmol) was added to a stirred solution of
[Rh2(CO)4(µ-p-EtC6H4NNNC6H4Et-p)2] (0.43 g, 0.52 mmol) in
n-hexane (100 cm3). After 24 h the red precipitate was removed
by filtration and then purified using CH2Cl2–n-hexane to give
red microcrystals, yield 0.53 g (80%).
The complexes [Rh2(CO)2(PPh3)2(µ-p-XC6H4NNNC6H4XЈ-
p)2] (X = XЈ = Cl or Br; X = H, XЈ = OMe) were prepared
similarly.
9 C. Lin, T. Ren, E. J. Valente and J. D. Zubkowski, J. Chem. Soc.,
Dalton Trans., 1998, 571.
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13 See, for example: A. H. Maki and D. H. Geske, J. Am. Chem. Soc.,
1961, 83, 1852; S. Vijayalakshamma and R. S. Subrahmanya,
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14 C. J. Pickett, in Comprehensive Coordination Chemistry,
eds. G. Wilkinson, R. D. Gillard and J. A. McCleverty, Pergamon
Press, Oxford, 1987, vol. 1, ch. 8.3, p. 497.
[Rh2(CO)2(PPh3)2(ꢀ-p-NCC6H4NNNC6H4CN-p)2]. A mixture
of [{Rh(µ-Cl)(CO)2}2] (0.126 g, 0.32 mmol) and p-NCC6H4-
NNNHC6H4CN-p (0.161 g, 0.65 mmol) in CH2Cl2 (15 cm3)
was stirred for 3.5 h and then NEt3 (0.115 cm3, 0.80 mmol) was
added. After 20 min, PPh3 (0.168 g, 0.64 mmol) was added and
after stirring for a further 90 min the mixture was evaporated to
dryness. The residue was extracted into toluene (10 cm3) and
then n-hexane (20 cm3) was added to the filtered extract. The
mixture was evaporated to low volume to give a red precipitate
which was purified using CH2Cl2–n-hexane to give red–brown
crystals, yield 0.135 g (33%).
15 K. Das, K. M. Kadish and J. L. Bear, Inorg. Chem., 1978, 17,
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16 J. L. Bear, C.-L. Yao, R. S. Lifsey, J. D. Korp and K. M. Kadish,
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18 J. A. McCleverty and G. Wilkinson, Inorg. Synth., 1990, 28, 84.
19 W. W. Hartman and J. B. Dickey, Org. Synth., 1943, Coll. Vol. II,
163.
Acknowledgements
We thank the Royal Thai Government for a Postgraduate
Scholarship (to P. K.).
J. Chem. Soc., Dalton Trans., 2000, 2273–2277
2277