π-Back-bonding in Bis(isonitrile) complexes of rhodium(II) acetate: Structural analogs for rhodium carbenoids

Article abstract of DOI:10.1021/om980066r

Rh₂(O₂CCH₃) ₄·2C≡NC₆H₄R (R = (CH₃)₂N-(1), -H (2), -CF₃ (3)) were synthesized and characterized by X-ray crystallography. The structures exhibited several common features: (i) a relatively short Rh-C bond (2.148(4) A? (1), 2.133(4) A? (2), and 2.122(4) A?(3)), (ii) an inversion center between the two rhodium atoms, (iii) coplanar aryl rings, and (iv) a significant deviation from linearity in the Rh-C-N bond angle. Comparison to other rhodium carboxylate complexes indicates the presence of π-back-bonding and, by extension, π-back-bonding in rhodium-carbenoid systems.

Full text of DOI:10.1021/om980066r

Organometallics 1998, 17, 4523-4526
4523
π-Ba ck -Bon d in g in Bis(ison itr ile) Com p lexes of
Rh od iu m (II) Aceta te: Str u ctu r a l An a logs for Rh od iu m
Ca r ben oid s
Cassandra T. Eagle,* David G. Farrar,* and Carl U. Pfaff
Department of Chemistry, Appalachian State University, Boone, North Carolina 28608
J ulian A. Davies, Constance Kluwe, and Lee Miller
Department of Chemistry, University of Toledo, Toledo, Ohio 43606-3398
Received February 2, 1998
Summary: Rh2(O2CCH3)4‚2CtNC6H4R (R ) (CH3)2N-
(1), -H (2), -CF3 (3)) were synthesized and character-
ized by X-ray crystallography. The structures exhibited
several common features: (i) a relatively short Rh-C
bond (2.148(4) Å (1), 2.133(4) Å (2), and 2.122(4) Å (3)),
(ii) an inversion center between the two rhodium atoms,
(iii) coplanar aryl rings, and (iv) a significant deviation
from linearity in the Rh-C-N bond angle. Comparison
to other rhodium carboxylate complexes indicates the
presence of π-back-bonding and, by extension, π-back-
bonding in rhodium-carbenoid systems.
F igu r e 1. Eclipsing conformation of the rhodium-car-
benoid complex.
ues.7 Crystal-field theory suggests that π-back-bonding
is possible through donation of electron density from one
of the two degenerate, filled metal π* orbitals (arising
from antibonding overlap of the two Rh dxz (or Rh dyz)
orbitals) into the vacant, unhybridized, p orbital of the
carbene. Consequently, π-back-bonding would result in
a conformation in which the carbene eclipses two of the
carboxylate ligands surrounding the metal core, as
shown in Figure 1.
In an elegant series of linear free energy relationship
experiments, Pirrung and Morehead demonstrated the
importance of the polarizability and resonance contribu-
tions of the carboxylate (and carboxamide) ligands to
the reaction selectivity of carbenoid transformations,
suggesting that π-back-bonding is an important facet
of these transformations.7 Spectroscopic studies pub-
lished in the same report also suggest the importance
of π-back-bonding in Rh2(O2CR)4‚2CO complexes and,
by analogy, rhodium-carbene complexes.
Bear, Kadish, and co-workers found evidence for
π-back-bonding in Rh2(OAc)n(NHAc)4-n‚CO complexes
(n ) 0-4; Ac ) -C(O)CH3).8 As n decreased, the
binding constant for CO increased, the degree of bis-
(CO adduct) formation increased, and ν(CO) decreased.
These factors all point to π-back-bonding from the metal
to the carbonyl. While Bear, Kadish, and co-workers
drew no conclusions regarding π-back-bonding in rhod-
ium-carbenoid systems, extending their conclusions to
such systems appears logical given the similarity in the
frontier molecular orbitals of CO and carbenes.
In tr od u ction
Rhodium carboxylates and carboxamides are impor-
tant catalysts for a variety of carbenoid transforma-
tions,1 including cyclopropanations,2 N-H3 and C-H4
insertion reactions, and dipolar cycloadditions.5 Rhod-
ium carboxylates and carboxamides have been modified
extensively to enhance and direct chemical reactivity,
diastereoselectivity, and enantioselectivity. A question
of vital importance in the rational design of selective
catalysts is the structural and electronic makeup of the
rhodium-carbenoid intermediate. Despite much inter-
est in these catalysts, little is known about rhodium-
carbenoid intermediates, largely due to their highly
transient nature.
The debate regarding π-back-bonding in rhodium
carboxylate complexes dates back to the original con-
troversy regarding the Rh-Rh bond order of such
complexes.6 While the Rh-Rh bond order is now firmly
established as 1, the debate regarding π-back-bonding,
particularly in rhodium-carbenoid complexes, contin-
(1) For reviews, see: (a) Doyle, M. P. Aldrichim. Acta 1996, 29, 3.
(b) Adams, J .; Spero, D. M. Tetrahedon 1991, 47, 1765. (c) Maas, G.
Top. Curr. Chem. 1987, 137, 75. (d) Doyle, M. P. Chem. Rev. 1986, 86,
919. (e) Doyle, M. P. Acc. Chem. Res. 1986, 19, 348.
(2) (a) Davies, H. M. L.; Bruzinski, P. R.; Lake, D. H.; Kong, N.;
Fall, M. J . J . Am. Chem. Soc. 1996, 118, 6897. (b) Ramos Tombo, G.
M.; Bellus, D. Angew. Chem., Int. Ed. Engl. 1991, 30, 1193.
(3) (a) Salzmann, T. N.; Ratcliffe, R. W.; Bouffard, F. A. J . Am. Chem.
Soc. 1980, 102, 6161. (b) Melillo, D. G.; Shinkai, I.; Liu, T.; Ryan, K.;
Sletzinger, M. Tetrahedron Lett. 1980, 21, 2783.
Doyle and co-workers, on the other hand, propose a
“carbenoid” intermediate with no π-back-bonding and,
thus, a formal Rh-C single bond.9 The electrophilic
carbon is therefore more akin to that found in a
(4) See: Doyle, M. P.; Westrum, L. J .; Wolthuis, W. N. E.; See, M.
M.; Boone, W. P.; Bagheri, V.; Pearson, M. M. J . Am. Chem. Soc. 1993,
115, 958 and references therein.
(5) Pirrung, M. C.; Zhang, J .; Lackey, K.; Sternbach, D. D.; Brown,
F. J . Org. Chem. 1995, 60, 2112 and references therein.
(6) (a) Dubicki, L.; Martin, R. L. Inorg. Chem. 1970, 9, 673. (b)
Cotton, F. A.; DeBoer, B. G.; LaPrade, M. D.; Pipal, J . R.; Ucko, D. A.
Acta Crystallogr., Sect. B 1971, 27, 1664. (c) Norman, J . G.; Kolari, H.
J . Am. Chem. Soc. 1978, 100, 791.
(7) Pirrung, M. C.; Morehead, A. T. J . Am. Chem. Soc. 1994, 116,
8991 and references therein.
(8) Chaven, M. Y.; Ahsan, M. Q.; Lifsey, R. S.; Bear, J . L.; Kadish,
K. M. Inorg. Chem. 1986, 25, 3218.
(9) Doyle, M. P.; Winchester, W. R.; Hoorn, J . A. A.; Lynch, V.;
Simonsen, S. H.; Ghosh, R. J . Am. Chem. Soc. 1993, 115, 9968.
S0276-7333(98)00066-1 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 08/19/1998

4524 Organometallics, Vol. 17, No. 20, 1998
Notes
F igu r e 2. Orbitals involved in π-back-bonding.
F igu r e 4. ORTEP drawing of bis(phenyl isonitrile)-
rhodium(II) acetate.
F igu r e 3. ORTEP drawing of bis(p-(dimethylamino)-
phenyl isonitrile)rhodium(II) acetate.
carbocation than to the alkylidene carbon of a metal-
carbenoid complex. Doyle et al. based this proposition
on modeling studies of chiral rhodium-carboxamidate
complexes. The structures were optimized with molec-
ular mechanics, and the axial carbene ligand was then
rotated as a rigid rotor. Each rotamer was then
examined by extended Hu¨ckel calculations. Both mo-
lecular mechanics and extended Hu¨ckel calculations
found a rotational barrier of less than 1 kcal/mol,
suggesting a lack of double-bond character in the Rh-C
bond.
Bursten and Cotton have performed XR-SW calcula-
tions on the bis(phosphine) adduct of rhodium formate.10
The Rh2(O2CH)4‚2PH3 geometry was obtained from the
X-ray crystal structure of the corresponding rhodium
acetate adduct. These calculations also failed to show
any significant π-back-bonding from metal to phosphine.
Bursten and Cotton acknowledged that the applicability
of these calculations to other π-acid complexes is a “moot
point”.
F igu r e 5. ORTEP drawing of bis(p-(trifluoromethyl)-
phenyl isonitrile)rhodium(II) acetate.
Ta ble 1. Selected Bon d Len gth s (Å) a n d Bon d
An gles (d eg)
1
2
3
Rh-Rh
Rh-C
2.4245(4)
2.148(4)
1.139(5)
166.8(4)
2.4271(3)
2.133(3)
1.153(4)
158.7(2)
2.4182(3)
2.122(3)
1.141(5)
155.4(3)
C-N
Rh-C-N
One factor which is commonly used as an indication
of π-back-bonding is the metal-to-ligand bond length.
In this series, as the para substituent was varied from
the strongly electron-donating dimethylamino ((CH3)2N-)
group to the strongly electron-withdrawing trifluorom-
ethyl (-CF3) group, the degree of π-back-bonding was
anticipated to increase and the Rh-C bond to shorten.
While such a trend is evident here, translating it into
evidence for π-back-bonding is problematic; analogous
trends are expected, but not seen, in the variation of
the Rh-Rh and C-N bond lengths.
To shed more light on this important area, three bis-
(para-substituted-aryl isonitrile) adducts of Rh2(OAc)4
were synthesized to serve as analogs for the rhodium-
carbenoid complex. Herein we describe the structural
features of the three isonitrile adducts.
The same factors which influence the degree of
π-back-bonding also affect the strengths of the Rh-Rh
and C-N σ-bonds in a complex, interdependent fashion.
For example, the dimethylamino group is electron-
donating from a resonance standpoint (diminishing
π-acceptor ability of the isonitrile), while it is inductively
electron-withdrawing (diminishing σ-donor ability of the
isonitrile).11 Consequently, the Rh-C bond will weaken
and lengthen. Since the isonitrile is donating electron
density into the Rh-Rh σ* molecular orbital and ac-
cepting electron density from the Rh-Rh π* molecular
orbital, the net effect on the Rh-Rh bond will depend
on the relative strengths of the two competing factors.
Similarly, the effect on the CtN bond will be mixed
since the isonitrile is accepting electron density into a
π* molecular orbital yet is donating from a nonbonding
Resu lts a n d Discu ssion
Carbenes and isonitriles both have appropriate orbit-
als available to function as σ-donors and π-acids. In the
case of carbenes, the π-acceptor orbital is the unhybrid-
ized p atomic orbital while, in the isonitrile, it is the
CN π*-molecular orbital (Figure 2).
The solid-state structures of the three bis(para-
substituted-aryl isonitrile) adducts Rh2(OAc)4‚2CNC6H4X,
where X ) -N(CH3)2 (1), -H (2), and -CF3 (3), were
determined. ORTEP drawings of 1-3 are given in
Figures 3-5, respectively. The structures exhibited
several common features: (i) a relatively short Rh-C
bond, (ii) an inversion center between the two rhodium
atoms, (iii) coplanar aryl rings, and (iv) a significant
deviation from linearity in the Rh-C-N bond angle.
Interestingly, the bent Rh-C-N bond lies between the
planes of the acetate ligands in 1 but largely in the plane
of one pair of acetate ligands in 2 and 3. Selected bond
lengths and angles are given in Table 1 for 1-3.
(11) This argument is simplistic in that resonance effects, while
primarily a π molecular orbital phenomenon, also affect σ molecular
orbitals, though to a lesser degree. Likewise, inductive effects are not
limited to σ molecular orbitals but also affect π molecular orbitals.
These complex interdependencies highlight the difficulties inherent
in relying on bond lengths to measure the degree of π-back-bonding
present in the isonitrile complexes.
(10) Bursten, B. E.; Cotton, F. A. Inorg. Chem. 1981, 20, 3042.

Notes
Organometallics, Vol. 17, No. 20, 1998 4525
Ta ble 2. Cr ysta llogr a p h ic Da ta
1
2
3
formula
fw
cryst descripn
color
cryst dimens (mm)
cryst syst
space group
a, Å
C
26H32N4O8Rh2
C22H22N2O8Rh2
648.24
plate
orange-red
0.32 × 0.30 × 0.03
monoclinic
P21/a
8.562(1)
13.954(2)
10.201(2)
90.00
104.30(1)
90.00
C24H20F6N2O8Rh2
784.24
prismatic
red
0.31 × 0.29 × 0.15
monoclinic
P21/n
8.4661(8)
8.257(1)
20.145(4)
90.00
94.41(1)
90.00
734.38
plate
orange
0.50 × 0.31 × 0.02
monoclinic
P21/c
12.018(2)
8.119(2)
15.008(3)
90.00
91.7(1)
90.00
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
1463.7(9)
2
1181.0(6)
2
1404.0(6)
2
Z
F(000)
740
644
772
T, (K)
294
160
294
2θ range (deg)
3-52
3-52
2-52
F
calcd, g cm-3
1.67
1.82
1.85
no. of rflns measd
3081
2435
2958
no. of rflns obsd (I > 3.0 σ(I))
R
2279
0.034
2024
0.025
2377
0.030
Rw
0.045
0.033
0.045
goodness of fit
1.470
1.190
1.610
σ molecular orbital lying coincident with the C-N σ*.12
On the other hand, the trifluoromethyl group is both
electronically and inductively electron withdrawing and
will enhance the π-acceptor ability while diminishing
the σ-donor ability of the isonitrile.
π-back-bonding in the case of the isonitriles than in
acetonitrile. This is not surprising, since isonitriles are
generally better π-acceptors than nitriles, but it again
supports the presence of π-back-bonding in the isonitrile
complexes.
To date, three other X-ray crystal structures of
rhodium carboxylates with carbon in the axial sites have
been reported: Rh2(O2CCF3)4‚2CO,13 Rh2(O2CCF3)4‚
2((-)-trans-caryophyllene),14 and Rh2(O2CC(CH3)3)4‚
(1,4-benzoquinone).15 The Rh-C bond length of the
rhodium trifluoroacetate dicarbonyl adduct is 2.092(4)
Å, only 0.03-0.06 Å shorter than those of the aryl
isonitrile complexes. This small difference in bond
lengths suggests that CO and CNR bond to the metal
center in a similar fashion, pointing to π-back-bonding
in the isonitrile complexes. In contrast, the alkene
adducts exhibit Rh-C bond lengths 0.23-0.50 Å longer
than those of the isonitrile complexes. Measuring from
the centroid of the carbon-carbon double bond to
the Rh metal produces rhodium-ligand distances 0.22-
0.33 Å longer than those of the isonitrile complexes.
Although these differences are probably due as much
to steric repulsion as to the η2-bonding fashion of the
alkenes, they do highlight the significantly longer bonds
that result from the different bonding mode.
Comparing the relative metal-ligand bond lengths
in the isonitrile complexes with those found in the
complexes of the isolobal nitriles is also instructive.
Given the smaller covalent radius of N versus C, in the
absence of π-back-bonding, the Rh-N bond is antici-
pated to be shorter than the Rh-C bond. The converse,
however, is true; the Rh-N bonds of Rh2(O2CCH3)‚
2NCCH3 are 2.249(7) and 2.258(6) Å, 0.1 Å longer than
the Rh-C bonds of the isonitrile adducts.16 The most
likely cause for the shorter Rh-C bonds is greater
Additional evidence for π-back-bonding is found in the
coplanarity of the aromatic rings. Conjugation through
the extended π-network that would result from π-back-
bonding would render the aromatic rings coplanar. In
the absence of π-back-bonding, free rotation about the
Rh-C σ bonds would give rise to solid-state structures
in which the relative orientation of the aromatic rings
would be dictated by packing forces, perhaps resulting
in a random disposition of the axial aryl ligands. This
random disposition is seen in a series of rhodium
carboxylate-arylamine complexes synthesized by Cot-
ton and Felthouse17 and Koh and Christoph.18 The
planes of the aromatic amines are inclined to one
another at angles of 68.4, 4.4, and 9.9° for the rhodium
butyrate complexes of 7-azaindole, phenazine, and acri-
dine, respectively.17 For the bis(pyridine) adduct of
rhodium acetate, the pyridine rings are inclined at a
59° angle.18
The relatively short Rh-C bonds of the isonitrile
complexes compared to other Rh-C bonds and Rh-N
bonds and the coplanar arrangement of the aromatic
rings indicate the presence of π-back-bonding in these
complexes and, by extension, in rhodium-carbenoid
systems.
Exp er im en ta l Section
Commercial reagents were used as received, with no further
purification. IR spectra were recorded on a Nicolet Magna 550
FT-IR spectrometer. NMR spectra were recorded on either a
Varian VXR-400 spectrometer or
a Varian Gemini 2000
spectrometer. 1H NMR spectra were referenced to residual
protiochloroform in the solvent, while 13C NMR spectra were
(12) For further explanation, see: Douglas, B. E.; McDaniel, D. H.;
Alexander, J . J . Concepts and Models of Inorganic Chemistry, 3rd ed.,
Wiley: New York, 1994; pp 599-600.
(13) Christoph, G. G.; Koh, Y. B. J . Am. Chem. Soc. 1979, 101, 1422.
(14) Cotton, F. A.; Falvello, L. R.; Gerards, M.; Snatzke, G. J . Am.
Chem. Soc. 1990, 112, 8979.
(16) Cotton, F. A.; Thompson, J . L. Acta Crystallogr. 1981, B37,
2235.
(15) Handa, M.; Takata, A.; Nakao, T.; Kasuga, K.; Mikuriya, M.;
Kotera, T. Chem. Lett. 1992, 2085.
(17) Cotton, F. A.; Felthouse, T. R. Inorg. Chem. 1981, 20, 600.
(18) Koh, Y. B.; Christoph, G. G. Inorg. Chem. 1978, 17, 2590.

4526 Organometallics, Vol. 17, No. 20, 1998
Notes
referenced to the central solvent peak. Crystal data were
collected on an ENRAF-Nonius CAD4 diffractometer with Mo
KR radiation and solved by direct methods.
Syn th esis of Ison itr iles. The isonitriles were synthesized
by the dehydration of the respective formanilides according
to the procedure of Ugi and Meyr.19
Str u ctu r e Solu tion a n d Refin em en t. The position of the
rhodium atom in the asymmetric unit was located by direct
methods. The remaining atoms were located by repeated least-
squares refinements followed by difference Fourier syntheses.
The CF3 group in 3 was disordered. The model refined
contained three different positions for the F atoms. These
atoms were refined with isotropic atomic displacement factors,
and their occupancy was 1/3. For compounds 1 and 3, the
hydrogen atoms were calculated and included in the least-
squares refinement as riding atoms; Uiso )1.3Ueq(bonding
atom). In compound 2, the hydrogen atoms were located by
difference maps and refined with isotropic thermal parameters.
All other atoms were refined with anisotropic thermal param-
eters. For compound 1, the final cycle included 181 variable
parameters and converged with R ) 0.034 and Rw ) 0.045.
For compound 2, the final cycle included 198 variable param-
eters and converged with R ) 0.025 and Rw ) 0.033. For
compound 3, the final cycle included 199 variable parameters
and converged with R ) 0.030 and Rw ) 0.045. Further details
relevant to the data collection and structure refinements are
found in Table 2.
Syn th esis of Rh od iu m Aceta te Ison itr ile Ad d u cts.
Rh2(O2CCH3)4 (10 mg) was dissolved in 20 mL of ethanol and
treated with approximately 25 mg of isonitrile without stirring.
The resulting yellow solution was allowed to stand until
crystals developed (1-2 days).20 Rh2(O2CCH3)4‚2CN(C6H4)N-
1
(CH3)2 (1): IR (KBr, cm-1) ν(CN) 2133.0; H NMR (400 MHz,
δ, CDCl3) 7.67 (d, 8.48 Hz, 4H), 6.68 (d, 8.8 Hz, 4H), 3.04 (s,
12H), 1.97 (s, 12H); 13C NMR (101 MHz, δ, CDCl3) 194.9, 128.5,
111.7, 40.2, 24.3. Rh2(O2CCH3)4‚2CNC6H5 (2): IR (KBr, cm-1
)
ν(CN) 2138.7; 1H NMR (300 MHz, δ, CDCl3) 7.84 (m, 4H), 7.54
(m, 6H), 1.99 (s, 12H); 13C NMR (75 MHz, δ, CDCl3) 194.9,
130.1, 129.6, 127.4, 24.3. Rh2(O2CCH3)4‚2CN(C6H4)CF3 (3): IR
(KBr, cm-1) ν(CN) 2133.9; 1H NMR (400 MHz, δ, CDCl3) 7.96
(d, 8.44 Hz, 4H), 7.81 (d, 8.44 Hz, 4H), 1.98 (s, 12H); 13C NMR
(101 MHz, δ, CDCl3) 195.0, 127.9, 127.0, 127.0, 24.3.
X-r a y Str u ctu r e Deter m in a tion . Gen er a l Meth od s. A
single crystal selected for data collection was mounted on a
glass fiber in a random orientation. X-ray diffraction data
were collected with an Enraf-Nonius CAD4 diffractometer in
the ω-2θ mode. Data were collected to a maximum 2θ value
of 52°. Lorentz and polarization corrections were applied to
the data. Absorption effects were corrected by a semiempirical
method on the basis of ψ scans. The structure was refined by
full-matrix least squares on F using Fo2 > 3.0σ(Fo2), where the
function minimized was ∑w(|Fo| - |Fc|)2 and the weight w was
defined as 4Fo2/σ2(Fo2). Scattering factors were taken from the
standard literature.21 Anomalous dispersion effects were
included in Fc. The unweighted and weighted agreement
factors are defined as R ) ∑||Fo| - |Fc||/∑|Fo| and Rw ) (∑w(|Fo
Ack n ow led gm en t. This research was supported by
an award from Research Corp. and a Supplemental
Award of the Camille and Henry Dreyfus Foundation
Grant Program in Chemistry for Liberal Arts Colleges.
We also thank J ohnson Matthey Inc. for the loan of
rhodium salts and the College of Arts and Sciences of
the University of Toledo for generous financial support
of the X-ray diffraction facility. We sincerely thank
Kristin Kirschbaum for her assistance with the crystal-
lographic details.
2
- Fc|)2/∑w[Fo| )1/2. All calculations were performed on a VAX
Su p p or tin g In for m a tion Ava ila ble: Three X-ray crys-
tallographic files for 1-3, in CIF format, are available. Access
information is given on any current masthead page.
3100 computer using MolEN.22
(19) (a) Ugi, I.; Meyr, R. Ber. Bunsen-Ges. Phys. Chem. 1960, 93,
239. (b) Ugi, I.; Meyr, R. Organic Syntheses; Wiley: New York, 1973;
Collect. Vol. 5, p 1060.
OM980066R
(20) Shafranskii, V. N.; Mal’kova, T. A. Zh. Obshch. Khim. 1976,
46, 1197.
(21) Cromer, D. T.; Waber, J . T. International Tables for X-ray
Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. IV.
(22) Fair, C. K. MolEN: An Interactive Intelligent System for Crystal
Structure Analysis User Manual; ENRAF-Nonius: Delft, The Neth-
erlands, 1990.