Preparation, properties, and reactivity of carbonylrhodium(I) complexes of di(2-pyrazolylaryl)amido-pincer ligands

Abstract

a b s t r a c tA series of six carbonylrhodium(I) complexes of three new and three previously reported di(2-3R-pyrazolyl)-p-Z/X-aryl)amido pincer ligands, ( R ZX)Rh(CO), (R is the substituent at the 3-pyrazolyl position proximal to the metal; Z and X are the aryl substituents para-to the arylamido nitrogen) were prepared. The metal complexes were studied to assess how their properties and reactivities can be tuned by varying the groups along the ligand periphery and how they compared to other known carbonylrhodium(I) pincer derivatives. This study was facilitated by the discovery of a new CuI-catalyzed coupling reaction between 2-(pyrazolyl)-4-X-anilines (X ¼ Me or CF 3 ) and 2-bromoaryl-1H-pyrazoles that allow the fabrication of pincer ligands with two different aryl arms. The NNN-pincer scaffolds provide an electron-rich environment for the carbonylrhodium(I) fragment as indicated by carbonyl stretching frequencies that occur in the range of 1948e1968 cm À1 . As such, the oxidative addition (OA) reactions with iodomethane proceed instantaneously to form trans-(NNN-pincer)Rh(Me)(CO)(I) in room temperature acetone solution. The OA reactions with iodoethane proceeded at a convenient rate in acetone near 45 C which allowed detailed kinetic studies. The relative order of reactivity was found to be (CF 3 CF 3 )Rh(CO) < ( iPr MeMe)Rh(CO) < ( Me MeMe)Rh(CO) w (CF 3 Me)Rh(CO) < (MeH)Rh(CO) < (MeMe) Rh(CO) with the second order rate constant of the most reactive in the series, k 2 ¼ 8 Â 10 À3 M À1 s À1 , being about three orders of magnitude greater than those reported for [Rh(CO) 2 I 2 ] À or CpRh(CO)(PPh 3 ). After oxidative addition, the resultant rhodium(III) complexes were found to be unstable. Although a few trans-( R MeMe)Rh(E ¼ Me, Et, or I)(CO)(I) could be isolated in pure form, all were found to slowly decompose in solution to give different products depending on the 3R-pyrazolyl substituents. Those with unsubstituted pyrazolyls (R ¼ H) decompose with CO dissociation to give insoluble dimeric [( R MeMe) Rh(E)(m-I)] 2 while those with 3-alkylpyrazolyls (R ¼ Me, i Pr) decompose to give soluble, but unidentified products.

Homoleptic Nickel(II) Complexes of Redox-Tunable Pincer-type Ligands

“…In rhodium chemistry, complexes (Me,Me)Rh(L 1 )(L 2 )(L 3 ) showed ligand-centered oxidations that occurred at potentials that depended on the charge of the complex and the Lever parameter ( E L ) of nonpincer ligands L 1 , L 2 , and L 3 . 19b Also, for a series of carbonylrhodium(I) complexes, (X,Y)Rh(CO), the reactivity toward a given alkyl halide increased predictably with the electron-donating ability of the X and Y groups as indicated by the groups’ Hammett σ p parameter. 19c More recently, the homoleptic gallium(III) complex [Ga(Me,Me) 2 ] + was reported, which showed two reversible one-electron oxidations in its voltammogram.…”

“…19b Also, for a series of carbonylrhodium(I) complexes, (X,Y)Rh(CO), the reactivity toward a given alkyl halide increased predictably with the electron-donating ability of the X and Y groups as indicated by the groups’ Hammett σ p parameter. 19c More recently, the homoleptic gallium(III) complex [Ga(Me,Me) 2 ] + was reported, which showed two reversible one-electron oxidations in its voltammogram. 19a The entire valence series was structurally and spectroscopically characterized.…”

“…6,8–18 We have been studying the properties of metal complexes of a new class of redox-active pincer-type ligand that has pyrazolyl flanking donors attached to a diarylamido anchor, as in Figure 1. 19 These uninegative pincer-type ligands will be described herein by the shorthand notation (X,Y) − that denotes the substitution at the para -aryl positions (X and Y, left of Figure 1) of the diarylamido backbone. In rhodium chemistry, complexes (Me,Me)Rh(L 1 )(L 2 )(L 3 ) showed ligand-centered oxidations that occurred at potentials that depended on the charge of the complex and the Lever parameter ( E L ) of nonpincer ligands L 1 , L 2 , and L 3 .…”

“…19c More recently, the homoleptic gallium(III) complex [Ga(Me,Me) 2 ] + was reported, which showed two reversible one-electron oxidations in its voltammogram. 19a The entire valence series was structurally and spectroscopically characterized. On the basis of electrochemical and spectroscopic studies, the one-electron oxidized [Ga(Me,Me) 2 ] 2+ was found to be a Robin–Day class II species where weak electronic communication between oxidized and nonoxidized ligands was thought to occur via superexchange through the empty orbitals on gallium.…”

2-(Azolyl)anilines: methods of synthesis, cyclocondensations, and biological properties

“…[22][23][24]40,61 A detailed study dedicated to the search of α 1 -adrenergic receptor agonists among derivatives of 2-(1H-pyrazol-5-yl)-aniline (21) and 2-(1H-1,2,4-triazol-3-yl)aniline (21) was done by Speake. 3 As a result, it was found that these compounds showed high selectivity toward α 1В -and α 1D -receptors.…”