The synthesis of four rhodium(II) paddlewheel complexes bearing axial aromatic amines and coumarin ligands, with formula [Rh2(OAc)4(L)2] (L = NH2Mesityl (
Rhodium complexes are one of the most important, versatile, and useful catalysts in organic synthesis. Their well-known reactivity has been exploited by catalytic or stoichiometric selective bond activation and C-C bond coupling reactions [
Rhodium paddlewheel complexes.
Early, in 1973, Paulissen et al. reported the insertion reaction of a carbenoid species, generated in situ from ethyl-diazoacetate, into O-H bonds catalyzed by
Furthermore,
On the other hand, the hydroamination (HA) of alkynes catalyzed by transition metal complexes is an important reaction, which involves the C-N bond formation. Based on the properties of the catalyst, this reaction can afford either the Markovnikov or anti-Markovnikov products (Figure
Alkyne hydroamination catalyzed by transition metals.
Our first step was addressed to obtain the HA product from the reaction between phenylacetylene (PA) with the corresponding aromatic amine bearing electron donor and electron withdrawing groups, using (
Attempts to obtain hydroamination products (see Scheme
Entry |
Ar | [Cat] | Solvent | PPA% |
---|---|---|---|---|
1 | Ms | [Rh2(OAc)4] | Toluene | 100 |
2 | Dip | [Rh2(OAc)4] | Toluene | 100 |
3 | Tri-Br | [Rh2(OAc)4] | Toluene | 100 |
4 | Coumarin | [Rh2(OAc)4] | Toluene | 100 |
5 | Ms | None | Toluene | n.o. |
6 |
Ms | [Rh2(OAc)4] | Toluene | n.o. |
7 |
Tri-Br | [Rh2(OAc)4] | Toluene | n.o. |
8 |
Dip | [Rh2(OAc)4] | Toluene | n.o. |
9 | None | [Rh2(OAc)4] | Toluene | n.o. |
10 | Ms (Drop) | [Rh2(OAc)4] | DMF | 100 |
On the other hand, when [Rh2(OAc)4], PA, or the aromatic amine was not added in three independent entries, it clearly affected the reaction pathway; the formation of PPA was not detected, recovering instead the quantitative reactants (Table
On the other hand, after workup of each catalytic attempt, a colorful green-violet solid is obtained which was attributed to the formation of an adduct species with formula [Rh2(OAc)4(L)2],
Synthesis of compounds
The structure of complexes
Selected spectroscopic data for complexes
Compound | 1H |
13C |
IR |
UV/Vis |
m.p. |
---|---|---|---|---|---|
|
6.87 ( |
190.9 (C=O); 138.6 (C |
3372, 3308 |
283, |
301 |
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7.17 ( |
190.6 (C=O); 138.2 (C |
3402, 3334 |
274, |
304 |
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7.69 ( |
208.7 (O-C=O); 192.1 (C=O); 147.5 (C |
3346, 3275 |
298, |
307 |
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|||||
|
7.86 ( |
205.5 (O-C=O); 190.6 (C=O); 153.0 (C |
1669, 1590 |
370, |
300 |
The adducts of rhodium acetate, containing a nitrogen donor ligand as pyridine, picoline, and aniline derivatives, were reported earlier [
X-ray diffraction selected data for compounds
|
|
|
|
---|---|---|---|
Chemical formula | C26H38N2O8Rh2 | C32H50N2O8Rh2 | C26H24O12Rh2 |
Formula weight | 712.40 | 796.56 | 734.27 |
|
130(2) | 130(2) | 130(2) |
|
0.71073 | 0.71073 | 0.71073 |
Crystal system | Triclinic | Triclinic | Monoclinic |
Space group | P-1 | P-1 | P 21/c |
Unit cell dimensions |
|
|
|
|
687.89(15) | 849.58(9) | 1297.3(4) |
|
1 | 1 | 2 |
|
1.720 | 1.557 | 1.880 |
|
1.250 | 1.021 | 1.339 |
F(000) | 362 | 410 | 732 |
Goodness-of-fit on F2 | 1.054 | 1.026 | 1.082 |
Final R indices [ |
R1 = 0.0300, wR2 = 0.0586 | R1 = 0.0386, wR2 = 0.0541 | R1 = 0.0315, wR2 = 0.0609 |
R indices (all data) | R1 = 0.0378, wR2 = 0.0655 | R1 = 0.0553, wR2 = 0.0647 | R1 = 0.0435, wR2 = 0.0674 |
Extinction coefficient | n/a | n/a | n/a |
Recently, ESI-MS and MS/MS spectrometry have shown their relevance in understanding the stability, reactivity, and selective bond activation of different [Rh2(OAc)4(L)2] transition metal complexes [
Figure
As we mentioned above, Gois et al. reported the synthesis of the binuclear NHC complex with the formula [Rh2(OAc)4(NHC)], which is an excellent catalytic precursor into the aldehyde arylation reaction [
Sequence of the plausible formation of intermediate complexes
(a) ORTEP perspective of compound
(a) ORTEP perspective of compound
(a) ORTEP perspective of compound
Proposed structures suggested for molecular ions observed by ESI-MS and tandem MS/MS.
Cyclic voltammograms of a solution of compound
Our results are summarized in Table
Reaction energies and energy decomposition (kcal·mol−1), calculated at the M06-L/(6-311G, LANL08) level, for the complexes [Rh2(OAc)4(L)],
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This way, we can compare the strength of the
In summary, we report herein the synthesis and characterization of four paddlewheel rhodium complexes,
[Rh2(OAc)4] was purchased from Chemical Pressure Co. and used as received. Coumarin, phenylacetylene, and the corresponding amines were acquired from Sigma-Aldrich. All other chemicals, filter aids, and chromatographic materials were reagent grade and were used as received. 1H and
The authors declare that they have no conflicts of interest.
The authors gratefully acknowledge the support of the SEP-PROMEP through Grant no. UGTO-PT-270 (O.S.), Universidad de Guanajuato via DAIP 2013-190 (OS), and Laboratorio Nacional UG-CONACYT (Project no. 123732).