Dinuclear rhodium(II) pivalate complexes with N-donor ligands

Abstract

Eight dinuclear rhodium(II) complexes containing various, (primarily, polyfunctional) N donor ligands in the trans position with respect to the Rh-Rh bond were synthesized and characterized by X ray diffraction. In the Chinese lantern dinuclear rhodium(II) pivalates, Rh II 2 (µ OOCCMe 3 ) 4 (L) 2 (L is 2,3 diaminopyridine (2), 7,8 benzoquinoline (4), 2,2´:6´,2″ ter pyridine (5), N phenyl o phenylenediamine (7)), and Rh II 2 (µ OOCCMe 3 ) 4 L 1 L 2 (3, L 1 is 2 phenylpyridine, L 2 = MeCN), the steric effects of the axial ligands are most strongly reflected in the Rh-N(L) and Rh-Rh bond lengths. The introduction of chelating ligands containing a conformationally rigid chelate ring leads to the cleavage of two carboxylate bridges to form the dinuclear double bridged structure Rh II 2 (µ OOCCMe 3 ) 2 (OOCCMe 3 ) 2 (η 2 L 3 ) 2 , where L 3 is 8 amino 2,4 dimethylquinoline (6). The reaction of complex 7 containing coor dinated N phenyl o phenylenediamine with pyrrole 2,5 dialdehyde afforded the new Rh II 2 (µ OOCCMe 3 ) 4 (L 4 ) 2 complex (8) containing 5 (1 phenyl 1H benzimidazol 2 yl) 1H pyrrole 2 carbaldehyde (L 4 ) in the axial positions of the dirhodium tetracarboxylate fragment. The coordinated diamine differs in reactivity from the free diamine. The reaction of the former with the above dialdehyde affords the [1+1] condensation product, viz., 5 {(E) [(2 anilino phenyl)imino]methyl} 1H pyrrole 2 carbaldehyde, whereas the reaction of unsubstituted o phenylenediamine gives 5 {(E) [(2 aminophenyl)imino]methyl} 1H pyrrole 2 carbalde hyde (L 5 ). The reaction of the latter with Rh II 2 (µ OOCCMe 3 ) 4 (H 2 O) 2 affords the dinuclear complex Rh II 2 (µ OOCCMe 3 ) 2 (OOCCMe 3 ) 2 (η 2 L 5 ) 2 (9), which is an analog of complex 6 containing only two bridging carboxylate groups.Key words: rhodium(II), dinuclear complexes, pivalate, X ray diffraction study, metal-metal bond, N donor ligands.The behavior of dinuclear Rh II complexes with four carboxylate bridges (so called "lantern dimers") is char acterized by the retention of the bimetallic tetracarboxylate fragment in the reactions with the majority of N donor ligands due, in part, to stability of the Rh-Rh bond. 1,2 Even the use of chelating ligands rather often leads to monodentate coordination with retention of the Rh 2 (µ OOCCR) 4 fragment. 1 This fact allows one to study the influence of the nature of donor ligands axially coor dinated to metal atoms on the characteristics of the metal-metal and metal-carboxylate bonds. This, in turn, helps in predicting ways of activation of the dinuclear metal carboxylate fragment. In addition, the presence of a stable metal fragment with delocalized metal-carboxy late-metal bonds shows promise in using the axial posi tion in complexes to perform various chemical transfor mations of a coordinated organic donor, including redox reactions. 2 In the present study, we used various α substituted pyridine derivatives as axial N donor ligands, thus intro ducing particular disturbances in the step of formation of dinuclear metal fragmen...

Homoleptic dirhodium tetraoctanoate and its pyridine adduct: synthesis and crystal structures

“…The Rh-Rh and Rh-N distances of 2.4013(7) and 2.252(4) Å compare well with the values of 2.3963(2) and 2.227(2) Å , respectively, in the dirhodium tetraacetate-pyridine adduct [28]. The Rh-O distances range from 2.038(3) to 2.043(3) Å which are also consistent with those other dirhodium carboxylate compounds described in the literature [13,14,29,30].…”

Synthesis, spectral and structural characterization of dinuclear rhodium (II) complexes of the anticonvulsant drug valproate with theophylline and caffeine

“…On the other hand, the difference in the Rh-N bond distance between (4 and 5) and dirhodium bpy compounds may be attributed to the different binding modes of 4 and 5, monodentate versus bidentate in dirhodium bpy compounds and other bidentate N-ligands. As expected, the two axial Rh-N bonds are much longer than the average Rh(1)-O and Rh (2) [19,23,34]. The average bonding angles deviate slightly from those of an ideal octahedron [23].…”

“…As expected, the two axial Rh-N bonds are much longer than the average Rh(1)-O and Rh (2) [19,23,34]. The average bonding angles deviate slightly from those of an ideal octahedron [23].…”

“…Octahedral coordination at Rh in complex 4 is completed by theophylline moiety bound axially via the unprotected N(9) nitrogen atom and by caffeine in complex 5 also via the unprotected N(9) nitrogen atom. The Rh-N bond distances in compound 4, 2.309 (3) [19,23,34] and almost similar to other dirhodium complexes [23]. The difference between Rh-N bond lengths in these complexes may be attributed to steric hindrance between the axial ligands and the substituted carboxylate groups; i.e.…”

“…The antitumor activity of these complexes was first discovered by Bear et al [4,5] and it was found that the activity of dirhodium tetracarboxylate, Rh 2 (OOCR) 4 , to be in the order: methoxyacetate < acetate < propionate < butyrate [5]. The ability of these complexes to function as antitumor agents against many types of tumors, by inhibiting DNA and protein synthesis, has prompted several studies on the nature of rhodium carboxylates formed with various nucleic acids and their constituent bases in order to elucidate their inhibition properties [1,2,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. These investigations have shown that dirhodium tetracarboxylates form stable mono and bis-adducts as well as bridging and/or chelating adducts with a wide variety of ligand types involving donor atoms such as nitrogen, oxygen and sulfur [1,2,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24].…”