Rational design of a novel mononuclear rhodium(II) complex

Article abstract of DOI:10.1021/om020248s

Rational design of a novel mononuclear rhodium (II) complex was discussed. Kinetics and thermodynamic stability factors were studied. Results showed that the steric and electronic factors stabilizes rhodium (II) upon chemical oxidation.

Full text of DOI:10.1021/om020248s

Organometallics 2002, 21, 3091-3093
3091
Ra tion a l Design of a Novel Mon on u clea r Rh od iu m (II)
Com p lex
†
†
†
†
Felicia M. Dixon, J oshua R. Farrell, Peter E. Doan, Amanda Williamson,
†
,†
†
Dana A. Weinberger, Chad A. Mirkin,* Charlotte Stern,
Christopher D. Incarvito, Louise M. Liable-Sands, Lev N. Zakharov, and
Arnold L. Rheingold‡
‡
‡
‡
Department of Chemistry and Center for Nanofabrication and Molecular Self-Assembly,
Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, and
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716
Received April 1, 2002
Summary: Reaction of rhodium(I) with a multidentate
hemilabile bisphosphine ligand yields a two-legged
piano-stool complex whose steric and electronic factors
stabilize rhodium(II) upon chemical oxidation. The
inherent stabilizing properties of the ligand system allow
for the first structural isolation of a novel mononuclear
organometallic rhodium(II) complex in a two-legged
piano-stool geometry.
Compared to Rh(I) and Rh(III) compounds, there are
a small number of mononuclear Rh(II) complexes, and
1
,2
of these complexes only a few contain Rh-C bonds.
Although monometallic Rh(II) compounds often are
observed as transient species in solution, some have
been isolated as stable solids and, in a few cases, have
been characterized by X-ray crystallography. The mono-
mers that have been characterized by single-crystal
X-ray diffraction analyses fall into four categories with
respect to coordination geometry: square-planar, octa-
1
a,3
hedral, trigonal bipyramidal, and sandwich complexes.
The typical ligand environment for these complexes
consists of sterically demanding phosphine, polypyridyl,
and aryl moieties, which are believed to inhibit dimer-
ization. Herein, we report the systematic design, syn-
thesis, characterization, and isolation of the first ex-
ample of a mononuclear organometallic Rh(II) complex
6
in an η -arene two-legged piano-stool geometry.
Several years ago, our group identified rhodium(I)
6
bisphosphine η -arene complexes that exhibited revers-
ible Rh(I/II) redox couples (Figure 1A). It was observed
that slight adjustments to the ligand affected the kinetic
*
To whom correspondence should be addressed. E-mail: camirkin@
chem.nwu.edu.
†
Northwestern University.
University of Delaware.
‡
(
1) (a) DeWit, D. G. Coord. Chem. Rev. 1996, 147, 209-246. (b)
F igu r e 1. (A) Rhodium(I) two-legged piano-stool com-
plexes 1-6. (B) ORTEP diagram of the Rh(II) complex 6 .
Thermal ellipsoids are shown at 30% probability, and
counterions, hydrogen atoms, and lattice solvent are omit-
ted for clarity.
Pandey, K. K. Coord. Chem. Rev. 1992, 121, 1-42.
+
(2) (a) Bowyer, W. J .; Merkert, J . W.; Geiger, W. E. Organometallics
1
989, 8, 191-198. (b) Garc ´ı a, M. P.; J im e´ nez, M. V.; Cuesta, A.;
Siurana, C.; Oro, L. A.; Lahoz, F. J .; L o´ pez, J . A.; Catal a´ n, M. P.
Organometallics 1997, 16, 1026-1036. (c) Restivo, R. J .; Ferguson, G.;
O’Sullivan, D. J .; Lalor, F. J . Inorg. Chem. 1975, 14, 3046-3052. (d)
Gerisch, M.; Krumper, J . R.; Bergman, R. G.; Tilley, T. D. J . Am. Chem.
Soc. 2001, 123, 5818-5819.
and thermodynamic stability of the Rh(II) oxidation
state. As a result, a series of bisphosphine complexes
were synthesized to test the ligand features that in-
crease the stability of Rh(II) in these types of complexes
(1-4; Figure 1A). When the electronic and steric prop-
erties of the arene and the connectivity of the phosphine
moieties were systematically varied, the kinetic and
thermodynamic stability factors were determined. The
(3) (a) Harlow, R. L.; Thorn, D. L.; Baker, R. T.; J ones, N. L. Inorg.
Chem. 1992, 31, 993-997. (b) Connelly, N. G.; Emslie, D. J . H.; Geiger,
W. E.; Hayward, O. D.; Linehan, E. B.; Orpen, A. G.; Quayle, M. J .;
Rieger, P. H. J . Chem. Soc., Dalton Trans. 2001, 670-683. (c) Haefner,
S. C.; Dunbar, K. R.; Bender, C. J . Am. Chem. Soc. 1991, 113, 9540-
9
553. (d) Paul, P.; Tyagi, B.; Bilakhiya, A. K.; Bhadbhade, M. M.;
Suresh, E. J . Chem. Soc., Dalton Trans. 1999, 2009-2014. (e) Collins,
J . E.; Castellani, M. P.; Rheingold, A. L.; Miller, E. J .; Geiger, W. E.;
Rieger, A. L.; Rieger, P. H. Organometallics 1995, 14, 1232-1238.
1
0.1021/om020248s CCC: $22.00 © 2002 American Chemical Society
Publication on Web 06/19/2002

3
092 Organometallics, Vol. 21, No. 15, 2002
Communications
first set of complexes examined the electron density and
steric bulk associated with the pendant arene (1; Figure
solution, 5b was chemically oxidized to its Rh(II) form,
5b , with AgBF4 (E1/2 ) 650 mV/s vs Fc/Fc in CH2Cl2)
and isolated as a red-brown solid in 94% yield. Com-
+
+
1
A). This study concluded that the addition of methyl
+
1
31
1
substituents on the arene ring kinetically and thermo-
dynamically stabilizes the Rh(II) state due to the
increased steric bulk and electron richness of the arene;
indeed, a 16 mV/CH3 decrease in E1/2 value upon
addition of methyl groups in 1 was observed for the six
complexes studied.
pound 5b has been characterized by H and P{ H}
NMR spectroscopy, mass spectrometry, and EPR spec-
troscopy. Consistent with the formation of a paramag-
1
31
1
netic Rh(II) compound, H and P{ H} NMR spectra
of 5b are broad and almost featureless. In addition,
the paramagnetic d Rh(II) metal center of 5b was
characterized by EPR spectroscopy in 19:1 CD2Cl2/
CDCl3 as a glass at 77 K (g1 ) 2.390, g2 ) 2.038, g3 )
1.997) and as a fluid solution at room temp (gav ) 2.11).
+
7
+
The next group of complexes (2-4, Figure 1A) focused
on the importance of phosphine connectivity in the
stabilization of Rh(II) in the two-legged piano-stool
geometry. The complexes studied all possess tolyl-like
aromatic groups differing only in their phosphine con-
nectivity: phosphine chelation, chelate arm length, and
tethering of the phosphine group to the arene ring.
Complex 2, with nonchelated phosphines, shows the
highest degree of thermodynamic stability, as evidenced
by its E1/2 value of 505 mV (130 mV lower than for the
tolyl version of 1). The increase in thermodynamic
stability is attributed to the complex’s ability to accom-
modate structural changes, namely the widening of the
P-Rh-P angle and the lengthening of the Rh-P bond
distances, upon oxidation.4 Although thermodynamic
stability is gained in 2, no kinetic stability is obtained,
due to the lack of steric bulk about the Rh center.
However, kinetic stability is gained by tethering the
phosphine group to the arene through an alkyl chain
as in 4. Although several factors that contribute to
stabilizing Rh(II) in this coordination geometry were
identified through these studies, an isolable form of a
mononuclear Rh(II) complex was never achieved.
Taken together, these data suggest that the ideal
coordination environment for mononuclear Rh(II) with
a two-legged piano-stool geometry would consist of a
symmetrical arene ligand with two tethered phosphine
moieties, where the arene is sterically protected with
electron-donating alkyl moieties and the tethers are long
enough to accompany structural changes upon complex
oxidation. Herein, we report a study involving three
complexes (5a , b and 6) that allow us to evaluate this
hypothesis. Compound 5a has been previously reported,
and 5b and 6 were prepared from 1,4-bis(3-(diphe-
nylphosphino)propoxy)-2,3,5,6-tetramethylbenzene and
+
Although complex 5b has increased kinetic and
thermodynamic stability, we were unable to character-
ize it by a single-crystal X-ray diffraction analysis;
during crystal growth, disproportionation occurred,
resulting in the formation of 5b and unidentified
products. Previous studies showed that replacing the
ether groups in 4a with methylene units, 4b, resulted
in a significant increase in thermodynamic stability of
the resulting Rh(II) complex, as evidenced by the
4
decrease in E1/2 (4a , 573 mV; 4b, 515 mV). With this
in mind, complex 6 was designed, where the electron-
withdrawing O atoms in the chelating ligand were
replaced with methylene units. Complex 6 has been
,5
1
31
1
characterized in solution by H and P{ H} NMR
spectroscopy and cyclic voltammetry, in the gas phase
by mass spectrometry, and in the solid state by single-
(6) Synthesis of 1,4-bis[4-(diphenylphosphino)butyl]-2,3,5,6-tetra-
methylbenzene: A flask equipped with a condenser was charged with
diiododurene (1.0 g, 2.6 mmol) and Pd(PPh ) (0.45 g, 15 mol %). An
3 4
excess of a 0.5 M THF solution of 4-chlorobutyl zinc bromide (∼20 mL)
was added to the reaction mixture, and the mixture was refluxed at
8
0 °C for 12 h under a nitrogen bubbler. The product was isolated after
the mixture was charged with NH Cl(aq) (20 mL), extracted with Et O,
followed by column chromatography. The chloro precursor (100 mg,
4
2
0
1
.318 mmol) was then reacted with a THF solution of KPPh
.3 mL, 0.653 mmol) for 2 h. Pure ligand was obtained following
2
(0.5 M,
extraction, filtration through Celite, and recrystallization from ethanol.
1
H NMR (CD
2
Cl
2
): δ 1.65 (m, PCH
2
CH
2
and PCH
2
CH
2
CH
2
, 8H), 2.17
, 4H),
(m, PCH , 4H), 2.25 (s, C
2
6
(CH
3
)
4
, 12H), 2.69 (m, CH
2
C
6
(CH )
3 4
1
3
1
7
.39-7.49 (m, P(C
6
H
C-P
5
)
2
, 20H). C{ H} NMR (CD
2
Cl
2
): δ 16.6 (s, C
6
-
(CH ) ), 26.6 (d, J
3
4
) 12.5 Hz, PCH CH ), 27.9 (d, J
C-P
2
2
) 9.1 Hz,
PCH
2
CH
2
CH
2
), 30.6 (s, CH
2
(C
6
(CH
3
)
4
), 31.4 (d, J C-P ) 10.2 Hz, PCH
), 132.1 (s,
), 136.7 (s, C6i(CH ),
). P{ H} NMR (CD Cl ): δ -15.3
s). HRMS (EI): m/z calcd 614.323 12, found 614.323 04. Anal. Calcd
for C42 : C, 82.05; H, 7.87; P, 10.08. Found: C, 79.49; H, 7.84; P,
0.06. For 5b, a solution of 1,4-bis(3-(diphenylphosphino)propoxy)-
2,3,5,6-tetramethylbenzene (68.6 mg, 0.111 mmol) in 125 mL of THF
was added dropwise at -78 °C to a THF solution of [Rh(THF)
COE) ]B(C (see the Supporting Information) over 2 h. The solution
was warmed to room temperature over 1 h followed by reflux for 3
days. Upon removal of solvent, layering a CH Cl solution of 5b with
O afforded pure 5b (57%, 0.088 g). Recrystallization from CH Cl
yielded thin red blades of 5b that were characterized by X-ray
2
),
1
28.5 (s, P(C
6o(CH ), 132.7 (d, J C-P ) 14.58, P(C6o(CH
138.9 (d, J C-P ) 10.2, P(C6i(CH
p 3 4 3 4
(CH ) ), 128.6 (d, J C-P ) 3.66 Hz, P(C6m(CH )
C
3
)
4
3
)
4
3 4
)
3
1
1
3
)
4
2
2
(
48 2
H P
1
,4-bis(4-(diphenylphosphino)butyl)-2,3,5,6-tetramethyl-
1
benzene via extensions of literature procedures for
forming mononuclear Rh(I) two-legged piano-stool com-
2
-
(
2
6 5 4
F )
6
,7,8
plexes (see the Supporting Information).
Complexes 5a (E1/2 ) 560 mV) and 5b (E1/2 ) 530 mV)
exhibit single, one-electron oxidation waves that were
completely reversible at all scan rates studied (1 mV/s
to 1 V/s). The decrease in E1/2 in going from 5a to 5b
shows that the lengthening of the chelating arms by one
methylene unit results in increased thermodynamic
stability, presumably due to the increased ability of
compound 5b to accommodate changes in Rh-P bond
length and P-Rh-P bond angle upon electrochemical
oxidation to the Rh(II) complex.
2
2
Et
C
2
2
2
/
6
H
6
1
crystallography. H NMR (CD
CH P), 2.55 (s, 12H, CH
J
1
2
Cl
2
): δ 1.38-1.50 (bm, 4H, CH
2
CH
O,
): δ
8.1 (d, J Rh-P ) 204 Hz). MS (FAB ): m/z [M] calcd 721.1872, found
2
-
2
), 2.35-2.42 (bm, 4H, CH
2 2 2
H-H ) 5.7 Hz), 7.05-7.20 (m, 20H, PPh ). P{ H} NMR (CD Cl
2
3
), 4.00 (t, 4H, CH
2
3
1
1
+
+
721.1879. Complex 6 was synthesized in a fashion similar to that for
b using [Rh(THF) (COE) ]PF . Upon addition, the reaction was stirred
5
2
2
6
for 4 h at 50 °C to yield an orange solid. Pure 6 was achieved by
precipitation from CH Cl with pentane. Crystals suitable for an X-ray
2
2
diffraction study were grown by slow diffusion of pentane into a
1
saturated CH
Cl ): δ 1.665 (bm, CH
CH , 8H), 2.202 (s, C
7.090-7.215 (m, P(C
2
Cl
2
solution of 6 at room temperature. H NMR (CD
CH (C (CH ), 4H), 2.112 (bm, P(C CH CH
(CH , 12H), 2.700 (bm, CH (CH
, 20H). P{ H} NMR (CD ): δ 28 (d, J Rh-P
2
-
-
2
2
2
6
3
)
4
6
H
6
5
)
2
2
2
Unlike the Rh(II) complexes studied prior to this
2
6
5
3
)
4
2
C
3 4
) , 4H),
3
1
1
4
6
H
)
2
2 2
Cl
work, which were observed as transient species in
+
)
204 Hz). ESMS: m/z [M] calcd 717.22, found 717.2. Anal. Calcd
for C42 Rh: C, 58.48; H, 5.61. Found: C, 58.30; H, 5.74.
48 6 3
H F P
(
4) Singewald, E. T.; Slone, C. S.; Stern, C. L.; Mirkin, C. A.; Yap,
(7) Farrell, J . R.; Mirkin, C. A.; Guzei, I. A.; Liable-Sands, L. M.;
Rheingold, A. L. Angew. Chem., Int. Ed. 1998, 37, 465-467.
(8) Farrell, J . R.; Eisenberg, A. H.; Mirkin, C. A.; Guzei, I. A.; Liable-
Sands, L. M.; Incarvito, C. D.; Rheingold, A. L.; Stern, C. L. Organo-
metallics 1999, 18, 4856-4868.
G. P. A.; Liable-Sands, L. M.; Rheingold, A. L. J . Am. Chem. Soc. 1997,
1
19, 3048-3056.
5) Harlow, R. L.; McKinney, R. J .; Whitney, J . F. Organometallics
983, 2, 1839-1842.
(
1

Communications
Organometallics, Vol. 21, No. 15, 2002 3093
crystal X-ray crystallography, and all data are fully
consistent with its structural formulation (see Support-
ing Information).
Ta ble 1. Selected Dista n ces (Å) a n d An gles (d eg)
+
for Com p ou n d s 6 a n d 6
6
6+
Complex 6 exhibits a completely reversible wave at
all measured scan rates (1 mV/s to 1V/s) with an E1/2
value (410 mV vs Fc/Fc ) substantially lower than 5b
Rh-P(1)
Rh-P(2)
Rh-C(1)
Rh-C(5)
Rh-C(6)
Rh-C(4)
Rh-C(2)
Rh-C(3)
Rh-arene
2.2514(9)
2.2501(10)
2.314(4)
2.344(4)
2.413(4)
2.314(4)
2.330(4)
2.379(4)
1.87
Rh-P(1)
2.3381(7)
2.3463(8)
2.316(2)
2.319(2)
2.343(3)
2.306(3)
2.315(3)
2.355(3)
1.84
Rh-P(2)
+
Rh-C(17)
Rh-C(18)
Rh-C(19)
Rh-C(20)
Rh-C(21)
Rh-C(22)
Rh-arene
and any of the 12 previously studied analogous com-
plexes (505-635 mV). This is a reflection of the in-
creased electron richness of the arene and the structural
flexibility of the ligand, which allows the complex to
adopt ideal bond lengths and angles upon oxidation. The
bulk oxidation of 6 was carried out in a fashion similar
to that for 5b, using AgPF6 as the chemical oxidant.
Stirring complex 6 with 1 equiv of AgPF6 in CH2Cl2 for
P(1)-Rh-P(2)
94.89(4)
P(1)-Rh-P(2)
95.41(2)
tion of the arene ring, there is significant shortening of
three Rh-C bond distances. This is a reflection of the
increased Lewis acidity of the Rh upon oxidation and
the ability of the arene ligand to π-donate to the oxidized
metal center. The average Rh-P distances lengthened
from 2.250 Å to 2.342 Å, indicating a decrease in π back-
bonding upon going from Rh(I) to Rh(II). The P-Rh-P
angle does not change significantly between the two
oxidation states, but it is markedly different from the
analogous tetramethyl version of 1 with chelated phos-
phine groups (83.76°). The lack of change in this angle
is attributed to the ligand adopting an optimized
geometry in the reduced form of the complex.
In summary, this study addresses the factors that
stabilize mononuclear Rh(II) in a new coordination
environment, and it has allowed us to design and
synthesize a series of ligands that indefinitely stabilize
this unusual Rh oxidation state. Moreover, it has
allowed us to characterize the structural parameters of
one of these Rh(II) complexes for the first time, opening
the opportunity to systematically study and identify the
physical and chemical properties of this novel class of
complexes.
0
3
0 min followed by filtration to remove the Ag precipi-
+
tate resulted in the quantitative formation of 6 .
+
1
31
1
Complex 6 was characterized by H and P{ H} NMR
+
+
and EPR spectroscopy. Like 5b , 6 exhibited broad
structureless features in the 1H and P{ H} NMR
31
1
spectra due to its paramagnetic nature. EPR spectros-
+
copy of 6 in a CH2Cl2 glass was taken at 77 K with g1
)
2.353 and g2,3 ) 2.025 and as a fluid solution with
gav ) 2.133. Surprisingly, no rhodium hyperfine coupling
+
+
was observed in either 5b or 6 . A detailed analysis of
the differences in EPR and electronic structure will be
discussed in a later publication.
+
Significantly, complex 6 also has been characterized
by a single-crystal X-ray diffraction analysis (Figure
1
0
1
B). Selected bond distances and angles are presented
in Table 1. Comparing selected bond lengths and angles
+
for 6 and 6 allows one to identify some of the structural
differences between the two oxidation states. The arene
ring in 6 deviates from planarity by adopting a boat
confirmation where the C atoms attached to the teth-
ered chelate arms (C17 and C20) are pointed toward
the rhodium(I) center. The average Rh-P distance,
2
.250 Å, is consistent with 5a , b and the three previ-
Ack n ow led gm en t. C.A.M. acknowledges the NSF
(Grant No. CHE-007188) and the AFOSR (Contract No.
F49620-00-1-0220) for generously funding this research.
F.M.D. acknowledges the Illinois Minority Graduate
Incentive Program for a predoctoral fellowship. P.E.D.
acknowledges Professor Brian M. Hoffman for funding
ously reported Rh(I) complexes in a two-legged piano-
stool geometry (Rh-P ) 2.217-2.264 Å).4,9 Upon oxi-
dation to form Rh(II), the arene ring flips so that the
carbon atoms (C19 and C22) are pointed away from the
rhodium(II) center. In addition to the change in orienta-
(
NSF MCB-9904018 and NIH HL-13531) and for helpful
(
9) Singewald, E. T.; Mirkin, C. A.; Levy, A. D.; Stern, C. L. Angew.
discussions. We also acknowledge Mr. Clark E. Davoust
for his technical support.
Chem., Int. Ed. Engl. 1994, 33, 2473-2475.
+
(10) Crystal data for C44
4 12 4 1
H52Cl F P Rh (6 ): monoclinic, P2 /n, a )
1
4
1.9371(18) Å, b ) 32.401(5) Å, c ) 12.782(2) Å, â ) 102.890(3)°, V )
3
-3
819.2(13) Å , Z ) 4, Z′ ) 1, T ) 173(2) K, Dcalcd ) 1.623 g cm , brown
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
plate, Mo KR ) 0.710 71 Å, GOF ) 1.103, full-matrix least squares on
2
F , R(F) ) 4.45% for 11 247 observed independent reflections (4 e 2θ
mental syntheses of new compounds, figures giving isotropic
e 56°). Crystallographic data (excluding structure factors) for the
structures in this paper have been deposited with the Cambridge
Crystallographic Data Centre as Supplementary Publication Nos.
+
+
and anisotropic EPR spectra of 5b and 6 , and tables giving
X-ray crystallographic data; the X-ray data are also available
as CIF files. This material is available free of charge via the
Internet at http://pubs.acs.org.
+
CCDC-173487 (5b), CCDC-175054 (6), and CCDC-173489 (6 ). Copies
of the data can be obtained, free of charge, on application to the CCDC,
2 Union Road, Cambridge CB2 1EZ, U.K. (fax, +44 1223 336033;
e-mail, deposit@ccdc.cam.ac.uk).
1
OM020248S