Substituent and Isomer Effects on Structural, Spectroscopic, and Electrochemical Properties of Dirhodium(III,II) Complexes Containing Four Identical Unsymmetrical Bridging Ligands

Article abstract of DOI:10.1021/ic034963l

Substituent and isomer effects on the structural, spectroscopic, (UV-visible and ESR) and electrochemical properties of dirhodium(III,II) complexes containing four identical unsymmetrical bridging ligands are reported for seven related compounds of the type Rh₂(L)₄Cl where L = 2-(2-fluoroanilino)pyridinate (2-Fap), 2-(2,6-difluoroanilino)pyridinate (2,6-F₂ap), 2-(2,4,6-trifluoroanilino)pyridinate (2,4,6-F ₃ap), or 2-(2,3,4,5,6-pentafluoroanilino)pyridinate (F₅ap) anion. Rh₂(2-Fap)₄Cl exists only in a (4,0) isomeric conformation while Rh₂(2,6-F₂ap)₄Cl, Rh ₂(2,4,6-F₃ap)₄Cl, and Rh₂(F ₅ap)₄Cl exist as both (4,0) and (3,1) isomers. It had earlier been demonstrated that Rh₂(L)₄Cl complexes can adopt different geometric conformations of the bridging ligands, but the current study provides the first example where two geometric isomers of Rh ₂⁵⁺ complexes are obtained for one compound using the same synthetic procedure. The synthesis, structural, spectroscopic, and/or electrochemical properties of (3,1) Rh₂(2,6-F₂ap) ₄CN and (4,0) Rh₂(2,4,6-F₃ap) ₄(C≡C)₂Si(CH₃)₃ are also reported and the data on these compounds is discussed in light of their parent complexes, (3,1) Rh₂(2,6-F₂ap)₄Cl and (4,0) Rh₂(2,4,6-F₃ap)₄Cl.

Full text of DOI:10.1021/ic034963l

Inorg. Chem. 2003, 42, 8663−8673
Substituent and Isomer Effects on Structural, Spectroscopic, and
Electrochemical Properties of Dirhodium(III,II) Complexes Containing
Four Identical Unsymmetrical Bridging Ligands
,†
,†
Karl M. Kadish,* Tuan D. Phan,† Lingamallu Giribabu,† Eric Van Caemelbecke,†,‡ and John L. Bear*
Departments of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003, and
Houston Baptist UniVersity, Houston, Texas 77074-3298
Received August 12, 2003
Substituent and isomer effects on the structural, spectroscopic, (UV−visible and ESR) and electrochemical properties
of dirhodium(III,II) complexes containing four identical unsymmetrical bridging ligands are reported for seven related
compounds of the type Rh2(L)4Cl where L ) 2-(2-fluoroanilino)pyridinate (2-Fap), 2-(2,6-difluoroanilino)pyridinate
(2,6-F ap), 2-(2,4,6-trifluoroanilino)pyridinate (2,4,6-F ap), or 2-(2,3,4,5,6-pentafluoroanilino)pyridinate (F ap) anion.
2
3
5
Rh2(2-Fap)4Cl exists only in a (4,0) isomeric conformation while Rh2(2,6-F ap)4Cl, Rh2(2,4,6-F ap)4Cl, and Rh2-
2
3
(F ap)4Cl exist as both (4,0) and (3,1) isomers. It had earlier been demonstrated that Rh2(L)4Cl complexes can
5
adopt different geometric conformations of the bridging ligands, but the current study provides the first example
where two geometric isomers of Rh25+ complexes are obtained for one compound using the same synthetic procedure.
The synthesis, structural, spectroscopic, and/or electrochemical properties of (3,1) Rh2(2,6-F ap)4CN and (4,0)
2
Rh2(2,4,6-F ap)4(CtC)2Si(CH ) are also reported and the data on these compounds is discussed in light of their
3
3 3
parent complexes, (3,1) Rh2(2,6-F ap)4Cl and (4,0) Rh2(2,4,6-F ap)4Cl.
2
3
Introduction
2-anilinopyridinate anion, can theoretically exhibit four
modes of bridging symmetry around the dimetal core which
are designated as (4,0), (3,1), (2,2)-trans, and (2,2)-cis
geometric isomers. Examples have been reported in the
literature for Rh24+ and Rh25+ species with (4,0), (3,1), and
(2,2)-trans isomeric forms,3,4,24 but to date the possibility of
A large number of tetracarboxylate-type complexes con-
taining Rh2 or Ru2 cores have been synthesized and
characterized by structural, electrochemical, and spectro-
scopic methods.1-23 Among these, compounds with four
unsymmetrical bridging ligands, such as in the case of the
(11) Kadish, K. M.; Wang, L.-L.; Thuriere, A.; Van Caemelbecke, E.; Bear,
J. L. Inorg. Chem. 2003, 42, 834.
(12) Connelly, N. G.; Davis, P. R. G.; Harry, E. E.; Klangsinsirukul, P.;
Venter, M. J. Chem. Soc., Dalton Trans. 2000, 2273.
(13) Zhang, G.; Zhao, J.; Raudaschl-Sieber, G.; Herdtweck, E.; Kuhn, F.
E. Polyhedron 2002, 21, 1737.
(14) Handa, M.; Muraki, Y.; Mikuriya, M.; Azuma, H.; Kasuga, K. Bull
Chem. Soc. Jpn. 2002, 75, 1755.
(15) Sayama, Y.; Handa, M.; Mikuriya, M.; Hiromitsu, I.; Kasuga, K. Bull
Chem. Soc. Jpn. 2003, 76, 769.
(16) Zuo, J. L.; Biani, F. F.; Santos, A. M.; Kohler, K.; Kuhn, F. E. Eur.
J. Inorg. Chem. 2003, 449.
(17) Sorasaenee, K.; Galan-Mascaros, J. R.; Dunbar, K. R. Inorg. Chem.
2003, 42, 661.
(18) Xu, G.-L.; Jablonski, C. G.; Ren, T. Inorg. Chim. Acta 2003, 343,
387.
(19) Xu, G.-L.; Zou, G.; Ni, Y.-H.; DeRosa, M. C.; Crutchley, R. J.; Ren,
T. J. Am. Chem. Soc. 2003, 125, 10057.
* Authors to whom correspondence should be addressed. E-mail:
kkadish@uh.edu (K.M.K.).
† University of Houston.
‡ Houston Baptist University.
(1) Stephenson, T. A.; Wilkinson, G. J. Inorg. Nucl. Chem. 1966, 28,
2285.
(2) Chakravarty, A. R.; Cotton, F. A.; Tocher, D. A. Inorg. Chem. 1985,
24, 172.
(3) Bear, J. L.; Liu, L. M.; Kadish, K. M. Inorg. Chem. 1987, 26, 2927.
(4) Bear, J. L.; Yao, C. L.; Liu, L. M.; Capdevielle, F. J.; Korp, J. D.;
Albright, T. A.; Kang, S. K.; Kadish, K. M. Inorg. Chem. 1989, 28,
1254.
(5) Yao, C. L.; Park, K. H.; Khokhar, A. R.; Jun, M. J.; Bear, J. L. Inorg.
Chem. 1990, 29, 4033.
(6) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms;
Oxford University Press: Oxford, 1993.
(7) Bear, J. L.; Li, Y.; Han, B.; Van Caemelbecke, E.; Kadish, K. M.
Inorg. Chem. 1997, 36, 5449.
(8) Aquino, M. A. S. Coord. Chem. ReV. 1998, 170, 141.
(9) Bear, J. L.; Wellhoff, J.; Royal, G.; Van Caemelbecke, E.; Eapen, S.;
Kadish, K. M. Inorg. Chem. 2001, 40, 2282.
(10) Bear, J. L.; Han, B.; Wu, Z.; Van Caemelbecke, E.; Kadish, K. M.
Inorg. Chem. 2001, 40, 2275.
(20) Chisholm, M. H. Acc. Chem. Res. 2000, 33, 53.
(21) Ren, T. Coord. Chem. ReV. 1998, 175, 43.
(22) Eglin, J. L.; Lin, C.; Ren, T.; Smith, L.; Staples, R. J.; Wipf, D. O.
Eur. J. Inorg. Chem. 1999, 2095.
(23) Ren, T.; Lin, C.; Valente, E. J.; Zubkowski, J. D. Inorg. Chim. Acta
2000, 297, 283.
10.1021/ic034963l CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/27/2003
Inorganic Chemistry, Vol. 42, No. 26, 2003 8663

Kadish et al.
Chart 1
bridging ligands are known to be affected by the mode of
axial ligation,4,5 and we have therefore in the present study
also compared the structural, electrochemical, and spectro-
scopic properties of (3,1) Rh2(2,6-F2ap)4CN and (4,0)
Rh2(2,4,6-F3ap)4(CtC)2Si(CH3)3 to their parent complexes
(3,1) Rh2(2,6-F2ap)4Cl and (4,0) Rh2(2,4,6-F3ap)4Cl, respec-
tively.
Experimental Section
Chemicals and Reagents. Ultrahigh-purity argon (99.999% min)
and high-purity nitrogen were purchased from Matheson-Trigas Co.
and were passed through anhydrous calcium sulfate and potassium
hydroxide pellets to remove trace oxygen and water prior to use.
GR grade dichloromethane, acetonitrile, hexanes, methanol, absolute
dichloromethane (Fluka) for electrochemical measurements, and
absolute ethyl alcohol (McCormick, Inc.) for recrystallization were
used without purification. Tetra-n-butylammonium perchlorate
(TBAP) was purchased from Fluka Chemical Co., twice recrystal-
lized from ethyl alcohol, and stored in a vacuum oven at 40 °C for
at least 1 week prior to use. Tetra-n-butylammonium chloride
(TBACl) and tetra-n-butylammonium cyanide (TBACN) were
purchased from Fluka Chemical Co. and used as received. 2-Bro-
mopyridine (C5H4NBr), 2-fluoroaniline (C6H4FNH2), 2,6-difluo-
roaniline (C6H3F2NH2), 2,4,6-trifluoroaniline (C6H2F3NH2), 2,3,4,5,6-
pentafluoroaniline (C6F5NH2), 1,4-bis(trimethylsilyl)-1,3-butadiyne
(C10H18Si2), anhydrous tetrahydrofuran (99.9%), and carbon tetra-
chloride (CCl4) were purchased from Aldrich Co. and used as
received. Tetrakis(µ-acetato)dirhodium(II) (Rh2(O2CCH3)4) was
purchased from Strem Chemical Co. and used without further
purification. Silica gel (Merck 230-400 mesh 60 Å) used for
column chromatography was purchased from Aldrich Co.
forming more than one geometric isomer using exactly the
same experimental procedure has been limited exclusively
5+
to tetracarboxylate-type complexes with a Ru2 core.11
Previously synthesized Ru2(L)4Cl complexes, where L )
ap, 2-CH3ap, 2-Fap, 2,5-F2ap, 2,6-F2ap, 2,4,6-F3ap, or F5ap,
have been shown to possess an isomeric distribution which
depends upon the substitution of the ap (2-anilinopyridinate)
5+
ligand. In addition, for a given bridging ligand, the Ru2
complexes were isolated from the same reaction mixture in
one, two, or three different isomeric forms.2,7,9,11 These earlier
studies suggested that both electronic and steric effects may
be responsible for the preferred isomeric binding mode of
the bridging ligand on Ru25+. The type of dimetal core may
also govern the isomeric distribution, and this possibility
prompted us to synthesize a series of substituted ap deriva-
tives containing a Rh25+ core and bridging ligands identical
to those which have been examined in the diruthenium(III,II)
series of compounds.11 A schematic representation of the
substituted 2-anilinopyridinate compounds investigated in this
present manuscript is given in Chart 1 where the bridging
ligands differ in the number and position of fluorine atoms
on the phenyl group of the anilinopyridinate anion. These
compounds were investigated in the present paper with
respect to their structural, spectroscopic, and electrochemical
properties.
Physical Measurements. Cyclic voltammetry was carried out
with an EG&G model 173 potentiostat. A three electrode system
was used and consisted of a glassy carbon working electrode, a
platinum wire counter electrode, and a homemade saturated calomel
electrode (SCE) as the reference electrode. The SCE was separated
from the bulk of the solution by a fritted-glass bridge of low porosity
that contained the solvent/supporting electrolyte mixture. All
potentials are referenced to the SCE, and all measurements were
carried out at room temperature.
Spectroelectrochemical experiments were performed using an
EG&G model 173 potentiostat and a thin-layer cell whose design
has been described in the literature.31 Time-resolved UV-visible
spectra were recorded with a Hewlett-Packard model 8453 diode
array spectrophotometer.
It has been shown that dirhodium(III,II) derivatives will
axially bind a variety of neutral and anionic ligands, and
several crystal structures of these compounds containing one
or two axially coordinated ligands are known.4-6,10,25-30 The
Rh-Rh bond lengths as well as the electrochemical proper-
ties of dirhodium(III,II) compounds having four identical
Mass spectra were recorded on an Applied Biosystem Voyager
DE-STR MALDI-TOF mass spectrometer equipped with a nitrogen
laser (337 nm) at the University of Houston Mass Spectrometry
Laboratory. ESR spectra were recorded at 77 K in CH2Cl2/CH3CN
with a modified Varian E-4 ESR spectrometer, which was interfaced
to a tracor Northern TN-1710 signal averager. Elemental analyses
were carried out by Atlantic Microlab, Inc., Norcross, GA.
(24) Takashi, K.; Hisanori, K.; Hiroshi, F.; Chihiro, K. T.; Yuzuru, K.;
Naohiro, K.; Masahiro, E.; Kunihisa, S.; Takayoshi, K. S.; Megumu,
M. Bull Chem. Soc. Jpn. 2000, 73, 657.
(25) Bear, J. L.; Lifsey, R. S.; Chau, L. K.; Ahsan, M. Q.; Korp, J. D.;
Chavan, M.; Kadish, K. M. J. Chem. Soc., Dalton Trans. 1989, 93-
100.
(26) Ziolkowski, J. J.; Moszner, M.; Glowiak, T. J. Chem. Soc., Chem.
Commun. 1977, 760.
(27) Piraino, P.; Bruno, G.; Nicolo, F.; Faraone, F.; Schiavo, S. L. Inorg.
Chem. 1985, 24, 4760.
The effective magnetic moments were determined by the Evans
method32 using a General Electric QE-300 FT NMR spectrometer
with CD2Cl2 as solvent and TMS as the internal reference. IR
spectra were recorded on a ThermoNicolet Avatar 370-FT-IR
spectrometer.
(28) Aoki, K.; Hoshino, M.; Okada, T.; Yamazaki, H.; Sekizawa, H. J.
Chem. Soc., Chem. Commun. 1986, 314.
(29) Baranovskii, I. B.; Golubnichaya, L. M.; Rotov, A. V.; Shchelokov,
R. N.; Porai-Koshits, M. A. Russ. J. Inorg. Chem. 1986, 31, 1652.
(30) Bear, J. L.; Yao, C. L.; Lifsey, R. S.; Korp, J. D.; Kadish, K. M.
Inorg. Chem. 1991, 30, 336.
(31) Lin, X. Q.; Kadish, K. M. Anal. Chem. 1985, 57, 1498.
(32) Evans, D. F. J. Chem. Soc. 1959, 2003.
8664 Inorganic Chemistry, Vol. 42, No. 26, 2003

Substituent and Isomer Effects on Dirhodium(III,II) Complexes
Synthesis. Tetrakis(trifluoroacetato)dirhodium(II) Rh2(O2CCF3)4,33
H(2-Fap), H(2,6-F2ap), H(2,4,6-F3ap), and H(2,3,4,5,6-F5ap)34 were
synthesized following methods described in the literature.
UV-vis spectral features (see following sections). An X-ray
structure reveals that the compound in the second fraction was the
(3,1) isomer of Rh2(F5ap)4Cl. The yields were 10% for the proposed
(4,0) isomer and 20% for the structurally characterized (3,1) isomer.
Mass spectral data [m/e, (fragment)] of the (4,0) and (3,1)
isomers: 1275, [Rh2(F5ap)4Cl]+; 1241, [Rh2(F5ap)4]+. Anal. Calcd
for C44H16N8F20ClRh2 of the (4,0) and (3,1) isomers: C, 41.41; H,
1.25; N, 8.78; F, 29.89. Found: C, 41.50; H, 1.21; N, 8.68; F, 29.28.
UV-vis spectrum in CH2Cl2 [λmax, nm (10-3 ꢀ, mol-1 L cm-1)]
(4,0) isomer: 535 (sh), 542 (2.3), 700 (0.6), 1009 (3.7); (3,1)
isomer: 549 (2.7), 580 (2.9), 688 (0.9), 1029 (4.4).
(3,1) Rh2(2,6-F2ap)4CN (8). A mixture of the (3,1) isomer of
Rh2(2,6-F2ap)4Cl (100 mg, 0.0974 mmol) and NaCN (47.7 mg,
0.9740 mmol) was dissolved in CH2Cl2 and refluxed for 10 h, during
which time the color of the solution changed from red to green.
The reaction mixture was then extracted with water to remove any
excess NaCN. The green colored organic layer was collected and
the solvent was evaporated. The crude product was subjected to a
silica gel column using CH2Cl2 as eluent. Two fractions were
observed on the column, one red and the other green. These were
collected as the starting material and the (3,1) isomer of Rh2(2,6-
F2ap)4CN, respectively. The yield was 95% for (3,1) Rh2(2,6-F2-
ap)4CN. Mass spectral data [m/e, (fragment)]: 1051, [Rh2(2,6-
F2ap)4CN]+; 1026, [Rh2(2,6-F2ap)4]+. Anal. Calcd for C45H28N9F8Rh2:
C, 51.40; H, 2.66; N, 11.98; F, 14.46. Found: C, 51.78; H, 2.70;
N, 11.80; F, 14.30. IR (cm-1): 2110 [ν(CtN)]. UV-vis spectrum
data in CH2Cl2: [λmax, nm (ꢀ x 10-3, M-1 cm-1): 433 (2.4), 478
(1.9), 648 (1.1), 862 (4.2).
(4,0) Rh2(2,4,6-F3ap)4(CtC)2Si(CH3)3 (9). This complex was
prepared following a method described by Bear et al.10 Typically,
a solution of dry THF containing Li(CtC)2Si(CH3)3 (0.25 g, 2.4
mmol) was added to a dry THF solution of (4,0) Rh2(F3ap)4Cl (0.10
g, 0.08 mmol) under an argon atmosphere, and the mixture was
refluxed under argon for 24 h. The color of the solution changed
progressively from red to red-brown during reflux. The solvent was
evaporated, and the residue was subjected to silica gel column
chromatography using CH2Cl2 as eluent. Two bands were observed
on the column, one red and the other red-brown. These were
collected as the starting material and the (4,0) isomer of Rh2(2,4,6-
F3ap)4(CtC)2Si(CH3)3, respectively. The yield was 90% for the
(4,0) isomer of Rh2(2,4,6-F3ap)4(CtC)2Si(CH3)3. Mass spectral data
[m/e, (fragment)]: 1119, [Rh2(2,4,6-F3ap)4(CtC)2Si(CH3)3]+; 1098,
[Rh2(2,4,6-F3ap)4]+. Anal. Calcd for C54H40N8F12SiRh2: C, 50.21;
H, 2.71; N, 9.19; F, 18.70. Found: C, 51.05; H, 2.81; N, 9.23; F,
18.48. IR (cm-1): 2168, 2139, 2123 [ν(CtC)]. UV-vis spectrum
data in CH2Cl2: [λmax, nm (ꢀ × 10-3, M-1 cm-1): 460 (4.2), 547
(4.0), 878 (7.8).
X-ray Crystallography of Compounds 1-3 and 7-9. Single-
crystal X-ray crystallographic studies were performed at the
University of Houston X-ray Crystallographic Center. Each sample
was placed in a steam of dry nitrogen gas at -50 °C in a random
position. The radiation used was Mo KR monochromatized by a
highly ordered graphite crystal. Final cell constants as well as other
information pertinent to data collection and structure refinement
are listed in Tables 1 and 2.
All seven compounds 1-7 were synthesized in a manner similar
to what has been described for (4,0) Rh2(ap)4Cl.10 The general
procedure is given below for (4,0) Rh2(2-Fap)4Cl (1).
(4,0) Rh2(2-Fap)4Cl (1). Rh2(O2CCF3)4 (0.2 g, 0.30 mmol) and
a large excess of H(2-Fap) (1.8 g, 9.6 mmol) were mixed in toluene
and refluxed at 120 °C for 24 h. The solvent was removed using a
rotary evaporator. The remaining residue was dissolved in CH2Cl2
containing 30% CCl4 (v/v) and left to stand for 4 h in sunlight to
generate Rh2(2-Fap)4Cl. The solvent was removed by rotary
evaporation, and the residue was sublimed under vacuum at 110
°C to remove excess H(2-Fap) ligand. The remaining solid was
subjected to silica gel column chromatography using a CH2Cl2/
hexanes (9/1, v/v) solvent mixture as eluent. Only one red band
was observed on the column. After evaporation of the eluent, the
title compound was recovered in a 60% yield. Mass spectral data
[m/e, (fragment)]: 989, [Rh2(2-Fap)4Cl]+; 954, [Rh2(2-Fap)4]+.
Anal. Calcd for C44H32N8F4ClRh2: C, 53.33; H, 3.23; N, 11.31; F,
7.67. Found: C, 53.21; H, 3.29; N, 11.40; F, 7.59. UV-vis
spectrum in CH2Cl2 [λmax, nm (10-3 ꢀ, mol-1 L cm-1)]: 497 (3.5),
559 (2.6), 920 (5.1).
(4,0) Rh2(2,6-F2ap)4Cl (2) and (3,1) Rh2(2,6-F2ap)4Cl (3). Two
red bands of Rh2(2,6-F2ap)4Cl were observed on the column. The
first band collected was the (4,0) isomer and the second the (3,1)
isomer. The yields were 25% for the (4,0) isomer and 30% for the
(3,1) isomer. Mass spectral data [m/e, (fragment)] of the (4,0) and
(3,1) isomers: 1061, [Rh2(2,6-F2ap)4Cl]+; 1026, [Rh2(2,6-F2ap)4]+.
Anal. Calcd for C44H28N8F8ClRh2 of the (4,0) and (3,1) isomers:
C, 49.75; H, 2.64; N, 10.55; F, 14.32. Found: C, 49.79; H, 2.65;
N, 10.53; F, 14.29. UV-vis spectrum in CH2Cl2 [λmax, nm (10-3
ꢀ, mol-1 L cm-1)] (4,0) isomer: 505 (3.4), 561 (sh), 718 (0.5),
977 (3.8); (3,1) isomer: 517 (2.8), 542 (2.8), 695 (1.2), 1004 (5.0).
(4,0) Rh2(2,4,6-F3ap)4Cl (4) and (3,1) Rh2(2,4,6-F3ap)4Cl (5).
Two red bands of Rh2(2,4,6-F3ap)4Cl were observed and collected
from the column. The first band corresponded to the (4,0) isomer
and the second to the (3,1) isomer. Attempts to grow suitable
crystals for X-ray crystallography were not successful for either
5+
isomer. However, the Rh2 compound in the first band was
assigned as having a (4,0) isomeric conformation on the basis of
its similarity in properties to Rh2(2,4,6-F3ap)4(CtC)2Si(CH3)3,
5+
which was structurally characterized in the present study. The Rh2
compound in the second band was assigned as the (3,1) isomer on
the basis of its UV-vis spectral features (see following sections
of manuscript). The yields were 18% for the proposed (4,0) isomer
and 27% for the proposed (3,1) isomer. Mass spectral data [m/e,
(fragment)] of the (4,0) and (3,1) isomers: 1133, [Rh2(2,4,6-F2ap)4-
Cl]+; 1089, [Rh2(2,4,6-F2ap)4]+. Anal. Calcd for C44H24N8F12ClRh2
of the (4,0) and (3,1) isomers: C, 46.68; H, 2.12; N, 9.90; F, 20.16.
Found: C, 46.75; H, 2.11; N, 9.81; F, 19.99. UV-vis spectrum in
CH2Cl2 [λmax, nm (10-3 ꢀ, mol-1 L cm-1)] (4,0) isomer: 500 (3.7),
561 (sh), 711 (0.6), 990 (3.8); (3,1) isomer: 512 (2.9), 542 (2.9),
692 (0.9), 1000 (5.4).
(4,0) Rh2(F5ap)4Cl (6) and (3,1) Rh2(F5ap)4Cl (7). Two red
bands of Rh2(F5ap)4Cl were observed on the column. All attempts
to grow suitable crystals of the compound in the first fraction for
X-ray crystallography were not successful. However, this compound
was assigned as having a (4,0) isomeric form on the basis of its
All measurements were made with a Siemens SMART platform
diffractometer equipped with a 1K CCD area detector. A hemi-
sphere of data 1271 frames at 5 cm detector distance was collected
using a narrow-frame method with scan widths of 0.30° ω and an
exposure time of 30 s/frame. The first 50 frames were measured
again at the end of data collection to monitor instrument and crystal
stability, and the maximum correction on I was <1%. The data
were integrated using the Siemens SAINT program, with the
(33) Telser, J.; Drago, R. S. Inorg. Chem. 1984, 23, 2599.
(34) Hisano, T.; Matsuoka, T.; Tsutsumi, K.; Muraoka, K.; Ichikawa, M.
Chem. Pharm. Bull. 1981, 29, 3706.
Inorganic Chemistry, Vol. 42, No. 26, 2003 8665

Kadish et al.
Table 1. Crystal Data and Data Collection and Processing Parameters
for the (4,0) Isomers of the Characterized Compounds
Table 3. Selected Average Bond Lengths and Bond Angles of the (4,0)
Isomers of Rh2(L)4Cl, Where L ) ap, 2-Fap (1), or 2,6-F2ap (2)
(4,0) isomer
ligand, L
ap
2-Fap (1)
2,6-F2ap (2)
Rh2(2,4,6-F3ap)4-
Bond Lengths (Å)
Rh2(2-Fap)4Cl Rh2(2,6-F2ap)4Cl (CtC)2Si(CH3)3
Rh-Rh
2.406
2.421
2.008
2.048
2.412
2.416
2.465
2.023
2.057
(1)
(2)
(9)
Rh-Cl
Rh-Na
2.431
2.007
2.063
a
b
space group
cell constant
a (Å)
b (Å)
c (Å)
P4nc tetragonal C2/c monoclinic P1h triclinic
Rh-Np
Bond Angles (deg)
15.2139(8)
15.2139(8)
9.6116(7)
90
90
90
2224.7(2)
C44H32N8F4Cl-
Rh2‚2CH2Cl2
1159.90
2
19.4773(10)
15.0962(7)
16.9620(8)
90.0
109.245(1)
90
4708.7(4)
C44H28N8F8Cl-
Rh2‚2CH2Cl2
1231.87
4
1.738
1.061
0.71073
223
0.0399
10.734(1)
14.427(2)
18.894(2)
74.111(2)
74.702(2)
81.555(2)
2705.4(6)
C51H33N8F12-
SiRh2
1262.85
2
1.550
0.720
0.71073
223
Rh-Rh-Cl
180.00
87.00
86.60
23.40
180.00
180.00
86.74
86.45
24.32
Rh-Rh-Na
Rh-Rh-Np
Na-Rh-Rh-Np
87.30
86.60
21.39
R (deg)
â (deg)
γ (deg)
a Na: anilino nitrogen. b Np: pyridyl nitrogen.
V (Å3)
mol formula
F3ap, or F5ap, were synthesized according to eq 1 and
characterized with respect to their electrochemical, spectro-
scopic, and structural properties.
fw (g/mol)
Z
F
calcd (g/cm3)
1.731
1.104
µ (cm-1
)
1. toluene
λ(Mo KR) (Å) 0.71073
Rh2(CF3CO2)4 + 4HL
8 Rh2(L)4Cl + 4CF3CO2H (1)
2. CH2Cl2/CCl4
3. light
temp (K)
223
0.0207
0.0480
R (Fo)a
0.0764
0.1910
Rw (Fo)b
0.1006
As described in the Experimental Section, the synthesis
of Rh2(L)4Cl was carried out in refluxing toluene for 24 h.
The excess ligand was removed by sublimation and the
product purified by column chromatography. The title
compounds were recovered in overall yields (both isomers)
which ranged from 30% to 60%. The yield and isomeric
distribution did not depend on the time the reaction was
carried out, and similar results were obtained whether the
mixture was left to react for 16 or 32 h.
2
a R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2
.
Table 2. Crystal Data and Data Collection and Processing Parameters
for the (3,1) Isomers of the Characterized Compounds
(3,1) isomer
Rh2(2,6-F2ap)4Cl
Rh2(F5ap)4Cl
Rh2(2,6-F2ap)4CN
(3)
(7)
(8)
space group
cell constant
a (Å)
b (Å)
c (Å)
P21/n monoclinic P21/c monoclinic P21/n monoclinic
12.8179(6)
23.5726(12)
15.2275(8)
90
93.422(1)
90
4592.8(4)
C44H28N8F8Cl-
Rh2‚2CH2Cl2
1231.87
4
15.3722(8)
14.0987(7)
26.2826(14)
90
106.025
90
5474.8(5)
C44H16N8F20Cl-
Rh2‚2CH2Cl2
1447.77
4
1.756
0.957
0.71073
223
0.0385
13.0374(16)
23.6332(29)
15.2597(19)
90
94.187(2)
90
4689.2(10)
C45H28N9F8-
Rh2‚2CH2Cl2
1222.44
4
(3,1) Rh2(2,6-F2ap)4CN was prepared by a reaction be-
tween the (3,1) isomer of Rh2(2,6-F2ap)4Cl and sodium
cyanide according to eq 2.
R (deg)
â (deg)
γ (deg)
Rh2(2,6-F2ap)4Cl + NaCN f Rh2(2,6-F2ap)4CN + NaCl (2)
V (Å3)
mol formula
As described in the Experimental Section, the synthesis
of the (3,1) isomer of Rh2(2,6-F2ap)4CN was carried out in
refluxing dichloromethane for 10 h. Excess sodium cyanide
was extracted with water and the product purified by column
chromatography. The cyano complex was obtained in 90%
yield. Only the monoadduct, (3,1) Rh2(2,6-F2ap)4CN, was
recovered when the mixture was left to react for 5 or 20 h.
The (4,0) isomer of Rh2(2,4,6-F3ap)4(CtC)2Si(CH3)3 was
obtained by treating the (4,0) isomer of Rh2(2,4,6-F3ap)4Cl
with excess Li(CtC)2Si(CH3)3 in dry THF. For isomeric
identification purposes, only the X-ray structure of this
complex was determined.
fw (g/mol)
Z
F
calcd (g/cm3)
1.782
1.088
1.732
1.010
0.71073
223
0.0377
µ (cm-1
)
λ(Mo KR) (Å) 0.71073
temp (K)
223
0.0335
0.0932
R (Fo)a
Rw (Fo)b
0.1029
0.1023
2]1/2
a R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo|
.
intensities corrected for Lorentz factor, polarization, air absorption,
and absorption due to variation in the path length through the
dectector faceplate. A Ψ-scan absorption correction was applied
based on the entire data set. Redundant reflections were averaged.
Final cell constants were refined using 4116 reflections for 1, 5606
reflections for 2, 6747 reflections for 3, 5591 reflections for 7, 6781
reflections for 8, and 4092 reflections for 9 having I >10σ(I). The
Laue symmetry was determined to be 4/mmm for 1 and 2/m for 2,
3, 7, and 8, and -1 for 9 from the systematic absences noted; the
space groups were shown to be P4nc for 1, C2/c for 2, P21/n for
3 and 8, P21/c for 7, and P1h for 9.
On the basis of the combined structural, spectroscopic,
and electrochemical data (described below), the complex with
the four 2-Fap bridging ligands is shown to exist exclusively
in a (4,0) isomeric form while a mixture of (4,0) and (3,1)
5+
isomers is obtained from synthesis of the Rh2 derivatives
with 2,6-F2ap, 2,4,6-F3ap, or F5ap bridging ligands.
Molecular Structures. Selected average bond lengths and
average bond angles of (4,0) Rh2(2-Fap)4Cl (1), (4,0)
Rh2(2,6-F2ap)4Cl (2), (3,1) Rh2(2,6-F2ap)4Cl (3), and (3,1)
Rh2(F5ap)4Cl (7) are summarized in Table 3 while Figures
1 and 2 illustrate ORTEP diagrams of compounds 1-3 and
7. Selected average bond lengths and average bond angles
Results and Discussion
Seven dirhodium complexes having the structural type
Rh2(L)4Cl, where L is the monoanion 2-Fap, 2,6-F2ap, 2,4,6-
8666 Inorganic Chemistry, Vol. 42, No. 26, 2003

Substituent and Isomer Effects on Dirhodium(III,II) Complexes
Figure 1. Molecular structure of the (4,0) isomers of (a) Rh2(2-Fap)4Cl
(1), (b) Rh2(2,6-F2ap)4Cl (2), and (c) Rh2(2,4,6-F3ap)4(CtC)2Si(CH3)3 (9).
H atoms have been omitted for clarity.
of (3,1) Rh2(2,6-F2ap)4CN (8) are summarized in Table 4.
Figure 2c illustrates the ORTEP diagram of this compound
while Figure 1c shows the structure of (4,0) Rh2(2,4,6-F3ap)4-
(CtC)2Si(CH3)3 for the purpose of geometric isomer iden-
tification. All intramolecular bond lengths and bond angles
as well as other structural data of these compounds are given
in the Supporting Information.
The coordination of Rh1 and Rh2 in compounds 1-3 and
7-9 is essentially octahedral and square pyramidal, respec-
tively, with four “substituted ap” bridging ligands forming
the equatorial plane. In the case of the (4,0) isomers, Rh1 is
coordinated to the chloride axial ligand for 1 and 2 or to the
(CtC)2Si(CH3)3 ligand for 9 and to four pyridyl nitrogens
while Rh2 is coordinated to four anilino nitrogens. For the
Figure 2. Molecular structure of the (3,1) isomers of (a) Rh2(2,6-F2ap)4Cl
(3), (b) Rh2(F5ap)4Cl (7), and (c) Rh2(2,6-F3ap)4CN (8). H atoms have been
omitted for clarity.
(3,1) isomers, Rh1 is coordinated to the axial chloride ligand
for 3 and 7 or to the cyanide ligand for 8 as well as to three
pyridyl nitrogens and one anilino nitrogen while Rh2 is
coordinated to three anilino nitrogens and one pyridyl
nitrogen.
As discussed below and shown in Table 3 for the case of
Rh2(L)4Cl where L ) 2-Fap, 2,6-F2ap, or F5ap, there is no
specific effect of the dimetal core on the structural properties
Inorganic Chemistry, Vol. 42, No. 26, 2003 8667

Kadish et al.
Table 4. Selected Average Bond Lengths and Bond Angles of the (3,1)
Isomers of Rh2(F5ap)4Cl (7) and Rh2(2,6-F2ap)4Lax, Where Lax ) Cl-
(3) or CN- (8)
groups per ap, Rh2(2,6-F2ap)4Cl, Rh2(2,4,6-F3ap)4Cl, and
Rh2(F5ap)4Cl, form both the (4,0) and (3,1) isomers with the
(3,1) isomer being preferred. The yields of the (4,0) and (3,1)
isomers also vary with the type of bridging ligand (see Table
5). The compounds which possess F groups at both ortho
positions of the phenyl rings of the ap-type ligand, Rh2(2,6-
F2ap)4Cl, Rh2(2,4,6-F3ap)4Cl, and Rh2(F5ap)4Cl (see Chart
1), are the only ones to exist in more than one isomeric form.
This result parallels what was reported for the related
Ru2(III,II) complexes,11 thus suggesting no significant effect
of the dimetal core, i.e., Ru2 or Rh2, on the isomeric
distribution. However, it should be pointed out that Rh2(2-
Fap)4Cl and Ru2(2-Fap)4Cl9 exist in two different isomeric
ligand, L
F5ap (7)
2,6-F2ap (3)
2,6-F2ap (8)
Bond Lengths (Å)
Rh-Rh
2.415
2.438
2.020
2.054
2.420
2.417
2.020
2.055
2.447
2.031
2.027
2.063
Rh-Lax
a
Rh-Na
Rh-Np
b
Bond Angles (deg)
Rh-Rh-Lax
Rh-Rh-Na
Rh-Rh-Np
Na-Rh-Rh-Np
177.93
87.40
86.60
20.76
178.80
179.12
86.97
86.85
20.16
87.13
86.87
20.53
a Na: anilino nitrogen. b Np: pyridyl nitrogen.
5+
forms; the Rh2 species is only formed as a (4,0) isomer
Table 5. Yields (and Isomeric Distribution) of Rh2(L)4Cl Complexes,
Where L ) ap, 2-Fap, 2,6-F2ap, 2,4,6-F3ap, or F5ap
5+
while only the (3,1) isomer is seen in the case of the Ru2
species.
ligand, L
ap
(4,0) isomer
(3,1) isomer
ESR Studies. The ESR spectra of compounds 1-7 in
degassed CH2Cl2/CH3CN at 77 K are similar to each other
(see Table 6 and Figures S1 and S2, Supporting Information),
and all have features identical to the spectrum reported for
Rh2(ap)4Cl.3,4 They are typical of compounds with axial
60%a
not formed
not formed
30% (3)
27% (5)
20% (7)
2-Fap
60% (1)
25% (2)
18% (4)
10% (6)
2,6-F2ap
2,4,6-F3ap
F5ap
a Taken from ref 4.
symmetry and exhibit two signals in the range of g
)
2.058-2.069 and g| ) 1.913-1.939. The g| component is
split into a doublet with A| in the range of 20.0-25.0 ×
10-4 cm-1 (see Table 6). The odd electron of Rh2(ap)4Cl,
Rh2(ap)4(CtCH), and Rh2(ap)4(CtC)2Si(CH3)3 has been
shown to be localized on only one of the two rhodium
atoms,3,5,10 and a similar assignment is proposed for the odd
electron of compounds 1-7 on the basis of their ESR spectra.
The g| values for all the compounds in Table 6 are less
than ge, thus indicating that the SOMO is a δ*Rh-Rh orbital.
A similar assignment was earlier made for Rh2(ap)4Cl, Rh2-
(ap)4(CtCH) and Rh2(ap)4(CtC)2Si(CH3)3.3,5,10 Although
there is no significant difference in g values among com-
pounds 1-7, the A| values systematically decrease upon
going from L ) Fap to F5ap, thus implying that the
polarization of the SOMO is strongly affected by the donor
ability of the bridging ligand. The magnitude of A| is related
to the spin density at the nucleus or nuclei;35 the higher the
spin density at the nuclei, the larger is the value of A|.
of the compounds and the same trends observed among the
(4,0) or (3,1) isomer of Rh2(L)4Cl are also seen for the (4,0)
and (3,1) isomers of Ru2(L)4Cl.11 For example, the isomer
effect on the M-M and M-Cl bond lengths as well as that
on the M2(L)4 framework, M-M-Cl bond angles, and
N-M-M-N torsion angles are virtually the same for M )
Ru or Rh. More specifically, the Rh-Rh bond lengths of
1-3 and 7 range from 2.412 to 2.420 Å and there is no
significant isomer effect on the metal-metal bond. As shown
in Table 3, the Rh2(L)4 framework is not significantly
affected by the type of briding ligand and/or the isomeric
form of the complex.
Structural studies of Ru2(L)4Cl11 where L is an ap-
substituted ligand revealed different trends of the M-M-
Cl bond angles in the (4,0) and (3,1) isomer series, and, as
shown in Table 3, this is also the case for the (4,0) and (3,1)
isomers of Rh2(L)4Cl. For instance, all three (4,0) isomers
are characterized by a Rh-Rh-Cl bond angle of 180.0° but
the Rh-Rh-Cl bond angles of the (3,1) isomers decrease
from 178.8° to 177.93°. There is also no effect of the dimetal
core on the torsion angles of the (4,0) and (3,1) isomers
because the (4,0) isomers exhibit larger N-M-M-N torsion
angles than the (3,1) isomers in both the Ru2 and Rh2 series
of complexes.
As shown in Table 4, no significant structural difference
is observed in the 2,6-F2ap framework upon going from (3,1)
Rh2(2,6-F2ap)4CN (8) to its parent complex, (3,1) Rh2(2,6-
F2ap)4Cl (3), but the Rh-Rh bond length increases from
2.420 Å (chloro derivative) to 2.447 Å (cyano derivative)
owing to differences in the donor ability of the axial ligands.
The isomeric distribution and yields of each Rh2(L)4Cl
isomer (L is one of the four bridging ligands shown in Chart
1) are given in Table 5. The compound with a singly
substituted ap ligand, Rh2(2-Fap)4Cl, exists exclusively as
the (4,0) isomer while the three others with 2, 3, or 5 F
The SOMO is known to be δ*Rh-Rh, when g| < ge and
both theoretical and experimental studies of g tensors for
Rh25+ complexes predict g| < g when the SOMO is δ*Rh-Rh
3,36-38
and g| > g when it is σ*Rh-Rh or π*Rh-Rh
.
Therefore
the g tensors for compounds 1-7 are consistent with δ*Rh-Rh
as the SOMO. Hence, one can propose that the SOMO is
δ*Rh-Rh, independent of the isomer type or the basicity of
the bridging ligand. We also propose that the effect of the
bridging ligand on the A| can be accounted for by changes
5+
in the electronic field of the Rh2 core due to a difference
in the bridging ligand basicity as discussed in the next
paragraph.
(35) Wertz, J. E.; Bolton, J. R. Electron Spin Elementary Theory and
Practical Applications; Chapman and Hall: New York, 1986.
(36) Lindsay, A. J.; Wilkinson, G.; Motevalli, M.; Hursthouse, M. B. J.
Chem. Soc., Dalton Trans. 1985, 2321.
(37) Cotton, F. A.; Feng, X. Inorg. Chem. 1989, 28, 1180.
(38) Cotton, F. A.; Matusz, M. J. Am. Chem. Soc. 1988, 110, 5761.
8668 Inorganic Chemistry, Vol. 42, No. 26, 2003

Substituent and Isomer Effects on Dirhodium(III,II) Complexes
Table 6. ESR Data (in CH2Cl2/CH3CN under Ar at 77 K) and Room Temperature Effective Magnetic Moment (µ, µB) of Rh2(L)4Cl Complexes
(4,0) isomer
ESR
(3,1) isomer
ESR
ligand
∑σ
ligand
L
compd no.
g
g| (104 A|, cm-1
)
µ (µΜ)
compd no.
g
g| (104 A|, cm-1
)
µ (µΜ)
0.00
0.24
0.48
0.54
1.22
apa
2-Fap
2,6-F2ap
2,4,6-F3ap
F5ap
2.090
2.058
2.058
2.058
2.058
1.950 (27.0)
1.939 (25.0)
1.936 (23.8)
1.937 (23.4)
1.928 (20.0)
1.88
2.00
1.91
1.78
1.93
1
2
4
6
3
5
7
2.069
2.059
2.065
1.913 (23.1)
1.934 (22.8)
1.921 (20.5)
1.98
1.87
1.92
a Taken from ref 4.
Table 7. Half-Wave Potentials (V vs SCE) for Redox Processes of the (4,0) and (3,1) Isomers of Rh2(L)4Cl in CH2Cl2 Containing 0.1 M TBAP
oxidation
a
isomer
type
ligand,
∑σ
reduction
∆E1/2
(V)
6+
5+
4+
ligand, L
Rh27+/Rh2
Rh26+/Rh2
Rh25+/Rh2
(4,0)
0.00
0.24
0.48
0.54
1.22
ap
2-Fap (1)
2,6-F2ap (2)
2,4,6-F3ap (4)
F5ap (6)
0.52
0.64
0.78
0.85
1.09
-0.38
-0.25
-0.14
-0.07
0.20
0.90
0.89
0.92
0.92
0.89
1.49
1.54
(3,1)
0.48
0.54
1.22
2,6-F2ap (3)
2,4,6-F3ap (5)
F5ap (7)
1.53
1.54
0.75
0.81
0.98
-0.13
-0.07
0.18
0.88
0.88
0.80
a
|1st ox. - 1st red.|.
ties are also seen for compound 9 and its parent complex,
(4,0) Rh2(2,4,6-F3ap)4Cl (4), which suggests a very similar
electron density distribution in both sets of compounds of
each isomer type.
The g and A| values of (3,1) Rh2(2,6-F2ap)4Lax (Lax ) axial
ligand) and (4,0) Rh2(2,4,6-F3ap)4Lax systematically increase
upon switching the axial ligand from Cl- to CN- in the case
of the (3,1) isomer and from Cl- to (CtC)2Si(CH3)3- in the
case of the (4,0) isomer (see Table S1, Supporting Informa-
tion), thus suggesting that the SOMO is clearly affected by
the donor ability of the axial ligands, independent of the
isomer type. However, the SOMO of 8 and 9 is still assigned
as δ*Rh-Rh and variations in the ESR parameters can then
simply be accounted for by an effect of axial donor strength
on the g and A| values owing to changes in the electronic
Figure 3. Correlation between the A| and 4∑σ for the (4,0) isomer (b)
and (3,1) isomer (O) of Rh2(L)4Cl (L ) ap, 2-Fap, 2,6-F2ap, 2,4,6-F3ap, or
F5ap). Values of A| and ∑σ are given in Table 6.
5+
field of the Rh2 core upon going from Cl- to CN- or
Plots of the A| values of compounds 1-7 vs the sum of
the substituent constants (∑σ) of the bridging ligand39 are
shown in Figure 3 and display a linear relationship for both
the (4,0) and (3,1) isomers. In both types of isomers, the A|
values decrease with increase in the overall electron-
withdrawing effect of the bridging ligands, consistent with
the fact that the electronegative F atoms lead to a decrease
in spin density at the dirhodium unit.
The room-temperature magnetic moments of compounds
1-7 are given in Table 6 and fall in the range of 1.78-2.00
µB, thus suggesting that each compound contains a single
unpaired electron. Rh2(ap)4Cl, Rh2(ap)4(CtCH) and Rh2-
(ap)4(CtC)2Si(CH3)3 have been assigned3,5,10 the electronic
configuration σ2π4δ2π*4δ*1 and, on the basis of the ESR
and magnetic data, this electronic configuration can also be
proposed for compounds 1-7.
-
(CtC)2Si(CH3)3 . There is no evidence for a switch in the
electronic configuration of the compounds upon going from
Rh2(2,6-F2ap)4Cl (3) to Rh2(2,6-F2ap)4CN (8) or Rh2(2,4,6-
F3ap)4Cl (4) to Rh2(2,4,6-F3ap)4(CtC)2Si(CH3)3 (9), and the
electronic configuration σ2π4δ2π*4δ*1 is therefore proposed
for all of the investigated compounds in this study.
Electrochemistry. The redox behavior of compounds 1-7
was investigated by cyclic voltammetry in CH2Cl2 containing
0.1 M TBAP, and half-wave potentials for each electrode
reaction of the investigated compounds are listed in Table 7
along with data of Rh2(ap)4Cl for comparison purposes.
Cyclic voltammograms of 1, 2, 4, and 6 are shown in
Figure 4 while cyclic voltammograms of 3, 5, and 7 are
illustrated in Figure 5. Two different types of behavior are
observed among the Rh2 complexes. One is given by the ap
complex as well as compounds 1, 6, and 7 which undergo
one reversible one-electron reduction and one reversible one-
electron oxidation. This contrasts with what is seen for
complexes 2-5, which undergo one reversible one-electron
Compound 8 and its parent complex, (3,1) Rh2(2,6-
F2ap)4Cl (3), have similar ESR features, and similar proper-
(39) Zuman, P. Substituent Effects in Organic Polarography; Plenum
Press: New York, 1967.
Inorganic Chemistry, Vol. 42, No. 26, 2003 8669

Kadish et al.
Figure 6. Linear free energy relationships between E1/2 and 4∑σ for the
oxidation (b) and reduction (O) of (a) (4,0) and (b) (3,1) isomeric complexes
of Rh2(L)4Cl.
Figure 4. Cyclic voltammograms of (4,0) Rh2(L)4Cl (L ) 2-Fap, 2,6-
F2ap, 2,4,6-F3ap, or F5ap) in CH2Cl2 containing 0.1 M TBAP. Scan rate )
0.1 V/s.
occur at E1/2 values between 1.25 and 1.35 V and is clearly
“missing” from these compounds. We cannot comment on
the two F5ap complexes since this electrode reaction may
be located at E1/2 values beyond the potential range of the
solvent.
Plots of E1/2 vs the sum of substituent constants (∑σ)39
for the reduction and first oxidation of (4,0) Rh2(L)4Cl
(L ) ap, 2-Fap, 2,6-F2ap, 2,4,6-F3ap, or F5ap) are shown in
Figure 6a while similar plots for (3,1) Rh2(L)4Cl (L ) 2,6-
F2ap, 2,4,6-F3ap, or F5ap) are shown in Figure 6b. The
dependence of E1/2 on the electronic effect of the substituents
can be quantified by a linear least-squares fit of the data
using the Hammett relationships shown in eq 3:40,41
∆E1/2 ) E (X) - E (H) ) 4 σF
(3)
∑
1/2
1/2
where F is the reactivity constant. Since there are four
equivalent bridging ligands on each investigated dirhodium
complex, 4∑σ is used in eq 3. The R values of the two lines
in Figure 6 range from 0.97 to 0.99, thus clearly suggesting
that there is no change in electron-transfer mechanism for
compounds in either the (4,0) or (3,1) isomer series. Hence,
all redox processes of each Rh25+ species involve the dimetal
unit and the reduction is described by eq 4 while the first
Figure 5. Cyclic voltammograms of (3,1) Rh2(L)4Cl (L ) 2,6-F2ap, 2,4,6-
F3ap, or F5ap) in CH2Cl2 containing 0.1 M TBAP. Scan rate ) 0.1 V/s.
reduction but two reversible one-electron oxidations. Most
conspicuous by its absence is the second oxidation of
Rh2(ap)4Cl and Rh2(2-Fap)4Cl which would be expected to
(40) Zuman, P. The Elucidation of Organic Electrode Process; Academic
Press: London, 1967.
(41) Hammett, L. P. Physical Organic Chemistry; Wiley: New York, 1970.
8670 Inorganic Chemistry, Vol. 42, No. 26, 2003

Substituent and Isomer Effects on Dirhodium(III,II) Complexes
and second oxidations are given by eqs 5 and 6, respectively.
CN- dissociates very slowly from (3,1) Rh2(2,6-F2ap)4CN
5+
4+
and that a bis-CN- adduct of Rh2 or Rh2 is not formed
(L)4Rh25+ + e- a (L)4Rh2
(4)
(5)
(6)
4+
in solution.
The three electrode reactions of (4,0) Rh2(2,4,6-F3ap)4-
(CtC)2Si(CH3)3 (9) are located at E1/2 ) -0.27, 0.75 and
1.40 V vs SCE (see Table S1) while its parent complex, (4,0)
Rh2(2,4,6-F3ap)4Cl (4), is reduced at E1/2 ) -0.07 V and
oxidized at E1/2 ) 0.85 and 1.54 V (see Table 7). Thus, the
reduction half-wave potential is negatively shifted by 200
mV upon changing the axial ligand from Cl- to (CtC)2-
Si(CH3)3- in the case of (4,0) Rh2(2,4,6-F3ap)4Lax and a 100
mV shift in potential is observed upon going from (3,1)
Rh2(2,6-F2ap)4Cl (3) to Rh2(2,6-F2ap)4CN (8). Smaller dif-
ferences in E1/2 are seen for the first oxidation of the same
compounds where the half-wave potentials are negatively
shifted by 100 mV upon going from compound 4 to
compound 9, and there is no change upon going from
compound 3 to compound 8 where E1/2 values for oxidation
are identical under the same experimental conditions.
(L)4Rh25+ a (L)4Rh26+ + e-
(L)4Rh26+ a (L)4Rh27+ + e- (2-5 only)
As shown in Table 7, the type of geometric isomer, i.e.,
(4,0) or (3,1), has no significant effect on the E1/2 of a given
electrode reaction of Rh2(L)4Cl, and consequently very
similar values of ∆E1/2 are seen for the (3,1) and (4,0)
isomers. For instance, E1/2 for the first oxidation of Rh2(2,6-
5+/6+
F2ap)4Cl, i.e., the Rh2
process, is shifted cathodically
by only 30 mV upon going from the (3,1) to the (4,0) isomer
and the reductions of both isomers have the same E1/2 values
within experimental error ((10 mV). A similar trend is also
observed for oxidation and reduction of Rh2(2,4,6-F3ap)4Cl
and Rh2(F5ap)4Cl, but there is a more significant difference
in E1/2 between the oxidations of the (4,0) isomer (1.09 V)
and the (3,1) isomer (0.98 V) of Rh2(F5ap)4Cl; this is most
likely because the electronic perturbation upon going from
the (4,0) to the (3,1) isomer increases in the order: F2ap <
F3ap < F5ap. A similar isomer effect on the redox potentials
UV-Vis Spectra. The UV-vis absorption features of the
6+
investigated compounds in their Rh24+, Rh25+, and Rh2
5+
forms are summarized in Table 8. All Rh2 complexes
exhibit four absorption bands (labeled as I-IV in Table 8),
except for the ap and 2-Fap derivatives, which show only
three bands, I, II, and IV. The λmax of band IV for Rh25+ in
the (4,0) isomer series of compounds varies from 910 to 1009
nm and appears to be the most sensitive to the type of
bridging ligand. Band IV of Rh25+ in the (3,1) isomer series
also shows the same type of behavior, but no isomers of
this type were formed for ap or Fap to confirm this result.
5+
was also reported for Ru2 complexes with the same type
5+/6+
of bridging ligands,11 but in this case the Ru2
was more sensitive to the isomer type.
process
Redox potentials for the three electrode reactions of (3,1)
Rh2(2,6-F2ap)4CN (8) are located at -0.23, 0.75, and 1.53
V vs SCE (see Table S1) while the parent complex, (3,1)
Rh2(2,6-F2ap)4Cl (3), is reduced at -0.13 V and oxidized at
0.75 and 1.53 V (see Table 7). There is thus no effect of the
axial ligand on the two oxidations. We previously reported
that the addition of (TBA)Cl or (TBA)CN to solutions of
Rh2(ap)4Cl has no effect on any of the redox potentials, even
at high concentrations of added anion,4 and this is also the
case for Rh2(2,6-F2ap)4Cl.
The Rh2(ap)4ClO4 complex is known to display a low
energy, intense π(N) f δ*(Rh2) LMCT band at 890 nm
(ꢀ ) 3.330 × 103 M-1 cm-1),42,43 and band IV of Rh2(ap)4-
Cl and the other Rh25+ complexes in Table 8 is proposed to
originate from the same electronic transition. The present
data are, however, insufficient to assign any electronic
transition to the other three bands.
The coordination of Cl- or CN- to (3,1) Rh2(2,6-F2ap)4Cl
(3) or Rh2(2,6-F2ap)4CN (8) was monitored by cyclic
voltammetry in the presence of excess anion to see if a bis-
adduct might be generated. Dichloromethane solutions
containing increased amounts of (TBA)Cl or (TBA)CN were
added to a CH2Cl2, 0.1 M TBAP solution of 3, and cyclic
voltammograms were recorded after each addition. The
addition of up to 1000 equiv of (TBA)Cl to compound 3
has no effect on the redox potentials, thus implying that no
bis-Cl- adduct is formed in solution. However, upon addition
of 1 equiv of (TBA)CN to 3, a new wave at E1/2 ) -0.23 V
begins to appear and the wave at E1/2 ) -0.13 V decreases
in intensity. At the same time, the two oxidation processes
remain unchanged. The addition of more (TBA)CN has no
further effect on the redox potentials, and the E1/2 values
obtained in the presence of up to 1000 equiv of CN- are
identical to the E1/2 values obtained for (3,1) Rh2(2,6-
F2ap)4CN, thus suggesting a simple CN- for Cl- anion
exchange in solution. The addition of (TBA)Cl or (TBA)CN
to compound 8 also has no effect on the redox potentials,
even at high concentrations of added salt, thus implying that
The UV-visible spectrum of Rh2(L)4Cl (L ) 2,6-F2ap,
2,4,6-F3ap, or F5ap) also appears to be sensitive to the isomer
type (see Table 8). For example, the (4,0) and (3,1) isomers
of Rh2(2,6-F2ap)4Cl both exhibit four absorption bands (see
Figure 7); the molar absorptivities of the lowest and highest
energy transitions are similar in the case of the (4,0) isomer
log ꢀ ) 3.4 and 3.8 but not in the case of the (3,1) isomer
where ꢀ varies by two log units (see Table 8). In addition,
band III of each (3,1) isomer is systematically blue-shifted
and has a higher molar absorptivity than band III of the
corresponding (4,0) isomer. Hence, one should be able to
differentiate the (4,0) isomer from the (3,1) isomer of these
compounds on the basis of the UV-visible spectra, and this
argument is used in the present paper to assign a proposed
isomer type for (3,1) Rh2(2,4,6-F3ap)4Cl and (4,0) Rh2(F5ap)4-
(42) Yao, C. L.; Park, K. H.; Khokhar, A. R.; Jun, M. J.; Bear, J. L. Inorg.
Chem. 1990, 29, 4033.
(43) Miskowski, V. M.; Hopkins, M. D.; Winkler, J. R.; Gray, H. B.
Multiple Metal-Metal Bonds. In Inorganic Electronic Structure And
Spectroscopy; Solomon, E. I., Lever, A. B. P., Eds.; John Wiley &
Sons: New York, 1999; Vol. 2.
Inorganic Chemistry, Vol. 42, No. 26, 2003 8671

Kadish et al.
Table 8. UV-Vis Spectral Data of (4,0) and (3,1) Rh2(L)4Cl (L ) ap, 2-Fap, 2,6-F2ap, 2,4,6-F3ap, or F5ap) in CH2Cl2 Containing 0.2 M TBAP
λ
max, nm (ꢀ × 10-3, M-1 cm-1
)
isomer
type
oxidation
state
ligand
band I
band II
band III
band IV
6+
(4,0)
Rh2
ap
2-Fap (1)
2,6-F2ap (2)
2,4,6-F3ap (4)
F5ap (6)
492 (5.5)
497 (5.4)
510 (5.7)
515 (5.6)
545 (6.2)
567 (5.6)
581 (5.6)
574 (5.7)
577 (5.6)
903 (3.0)
930 (3.3)
970 (2.8)
971 (3.0)
1000 (3.8)
5+
Rh2
ap
495 (3.2)
497 (3.5)
505 (3.4)
500 (3.7)
535 (sh)
572 (sh)
559 (2.6)
561 (sh)
561 (sh)
542 (2.3)
910 (5.1)
920 (5.1)
977 (3.8)
990 (3.8)
1009 (3.7)
2-Fap (1)
2,6-F2ap (2)
2,4,6-F3ap (4)
F5ap (6)
718 (0.5)
711 (0.6)
700 (0.6)
4+
Rh2
ap
501 (2.7)
485 (2.8)
480 (2.8)
481 (2.9)
478 (2.3)
642 (0.4)
630 (0.5)
642 (0.5)
630 (0.4)
2-Fap (1)
2,6-F2ap (2)
2,4,6-F3ap (4)
F5ap (6)
6+
(3,1)
Rh2
2,6-F2ap (3)
2,4,6-F3ap (5)
F5ap (7)
525 (5.0)
516 (5.9)
550 (6.4)
597 (sh)
575 (sh)
987 (3.8)
990 (3.2)
1012 (3.7)
5+
Rh2
2,6-F2ap (3)
2,4,6-F3ap (5)
F5ap (7)
517 (2.8)
512 (2.9)
549 (2.7)
542 (2.8)
542 (2.9)
580 (2.9)
695 (1.2)
692 (0.9)
688 (0.9)
1004 (5.0)
1000 (5.4)
1029 (4.4)
4+
Rh2
2,6-F2ap (3)
2,4,6-F3ap (5)
F5ap (7)
480 (2.4)
470 (2.6)
485 (2.5)
635 (0.8)
623 (0.7)
Figure 7. UV-visible spectra of the (4,0) isomer (s) and (3,1) isomer
(- - -) of Rh2(2,6-F2ap)4Cl in CH2Cl2.
Cl whose structures could not be confirmed by single-crystal
X-ray diffraction.
The singly reduced and singly oxidized forms of the (4,0)
and (3,1) isomers of Rh2(L)4Cl (L ) ap, 2-Fap, 2,6-F2ap,
2,4,6-F3ap, or F5ap) were in-situ generated in a thin-layer
spectroelectrochemical cell, and examples of UV-visible
5+/4+
spectral changes which occur during the Rh2
Rh2
and
5+/6+
electrode processes of the (4,0) and (3,1) isomers
of Rh2(2,6-F2ap)4Cl are shown in Figures 8 and 9, respec-
tively.
6+
Six of the electrogenerated Rh2 complexes in Table 8
display three absorption bands while both isomers of Rh2-
Figure 8. Time-dependent UV-visible thin-layer spectral changes during
of (a) first oxidation and (b) first reduction of (4,0) Rh2(2,6-F2ap)4Cl (2) in
CH2Cl2 containing 0.2 M TBAP under Ar.
(F5ap)4Cl (6 and 7) lack a band II. Overall, spectral features
6+
of the Rh2 derivatives in Table 8 are similar to what has
been reported for [Rh2(ap)4(CtCH)]+ (λmax ) 450, 580, and
proposed5 as σ2π4δ2π*.4 This orbital assignment implies that
an electron is removed from the δ* orbital upon going from
910 nm), a compound whose electronic structure has been
8672 Inorganic Chemistry, Vol. 42, No. 26, 2003

Substituent and Isomer Effects on Dirhodium(III,II) Complexes
F3ap)4Cl (4), has absorption bands at 500, 561, 711, and 990
nm (see Table 8). A similar difference in λmax has been
reported for (4,0) Rh2(ap)4Lax where Lax ) Cl- or CtCR-,
thus suggesting that the absorption peak maxima of the UV-
visible bands are a function of the type of axial ligand bound
5+
to the dimetal unit of the Rh2 complex, independent of
isomer type.
Summary
In the present study, we have examined the structural,
spectroscopic, and electrochemical properties of a series of
dirhodium complexes bridged by four “anilinopyridinate-
type” anions with fluorine atoms at the ortho, meta, or para
positions of the phenyl ring. Both the isomer type, i.e., (4,0)
or (3,1), and number of geometric isomers (one or two) vary
with the nature of the bridging ligand. As is the case for the
5+
Ru2 derivatives with similar bridging ligands, only the
Rh25+ complexes with two substituents at the ortho positions
of the phenyl group exist in two isomeric forms. No
significant changes are observed in structural features upon
increasing the number of F groups from 1 to 5, and
whichever isomer effect is observed on the M2(L)4 frame-
5+
work of the Ru2 derivatives with this type of bridging
5+
ligand, it is also seen for the Rh2 derivatives, thus
suggesting that the isomer effect on the structure does not
appear to be influenced by the dimetal core, i.e., Ru2 or Rh2.
ESR and magnetic data of the seven investigated
Rh2(L)4Cl compounds indicate the existence of a single
unpaired electron localized on one rhodium ion, independent
of the isomer type or ligand basicity. Three of the investi-
gated compounds undergo a single one-electron oxidation
and a single one-electron reduction while four of the
compounds undergo two one-electron oxidations and a single
one-electron reduction in CH2Cl2, 0.1 M TBAP. The oxida-
tions and reductions both follow linear free energy relation-
ships between E1/2 and the Hammett parameter of the
substituents on the bridging ligand. The (4,0) and (3,1)
isomeric forms of Rh2(2,6-F2ap)4Cl exhibit different UV-
visible features as do the two isomers of Rh2(2,4,6-F3ap)4Cl
and Rh2(F5ap)4Cl.
Figure 9. Time-dependent UV-visible thin-layer spectral changes during
(a) first oxidation and (b) reduction of (3,1) Rh2(2,6-F2ap)4Cl (3) in CH2-
Cl2, 0.2 M TBAP under Ar.
5+
6+
the Rh2 to the Rh2 form of each compound in Table 8.
The data also suggests that the same electron-transfer
mechanism occurs upon oxidation of each compound; this
agrees with the observed linear free energy relationship
between E1/2 and 4∑σ (Figure 6).
4+
Most of the Rh2 derivatives in Table 8 exhibit similar
UV-visible features, and in each case the spectrum lacks
both a band III and a band IV. We have assigned band IV
as a π(N) f δ*(Rh2) transition; the fact that this band
vanishes upon reduction suggests that the electron is added
to a δ* orbital, consistent with an electronic configuration
Acknowledgment. The support of the Robert A. Welch
Foundation (J.L.B., Grant E-918; K.M.K., Grant E-680) is
gratefully acknowledged. We thank Dr. J. D. Korp for
performing the X-ray analysis.
4+
of σ2π4δ2π*4δ*2 for the Rh2 forms of each compound in
Table 8.
The λmax of the higher energy visible bands of the Rh2-
(III, II) complexes are known to be influenced by the σ donor
ability of the axial anion,5 and this is also what is seen for
(3,1) Rh2(2,6-F2ap)4Lax upon switching the axial ligand from
Lax ) Cl- to Lax ) CN-. For example, the λmax of bands I,
II, and III are located at 517, 542, and 695 nm, respectively,
for the chloro complex but appear at 433, 478, and 648 nm
for the cyano derivative. In addition, it should be noted that
band IV blue shifts from 1004 to 862 nm upon going from
Cl- to CN-. The UV-visible bands of (4,0) Rh2(2,4,6-F3ap)4-
(CtC)2Si(CH3)3 (9) are located at 460, 547, and 878 nm
(see Table S1) while its parent complex, (4,0) Rh2(2,4,6-
Supporting Information Available: X-ray crystallographic files
in CIF format for the structural determination of Rh2(2-Fap)4Cl (1),
(4,0) Rh2(2,6-F2ap)4Cl (2), (3,1) Rh2(2,6-F2ap)4Cl (3), Rh2(F5ap)4Cl
(7), Rh2(2,6-F2ap)4CN (8), and Rh2(2,4,6-F3ap)4(CtC)2Si(CH3)3 (9).
Table S1 lists the ESR, electrochemical, and spectroscopic data of
(4,0) isomer Rh2(2,4,6-F3ap)4Cl (4), (4,0) Rh2(2,4,6-F3ap)4(CtC)2Si-
(CH3)3 (9), and (3,1) isomer Rh2(2,6-F2ap)4Cl (3), Rh2(2,6-F2ap)4CN
(8). Figures S1 and S2 (in PDF files) illustrate the ESR spectra of
the (4,0) and (3,1) isomers of Rh2(L)4Cl, respectively. This material
is available free of charge via the Internet at http://pubs.acs.org.
IC034963L
Inorganic Chemistry, Vol. 42, No. 26, 2003 8673