Electronic and Steric Effects in Rollover C–H Bond Activation

Abstract

Steric and electronic factors in rollover C−H bond activation of substituted 2,2′-bipyridines, mediated by platinum(II), have been investigated by comparing the influence of two substituents, CH 3 and CF 3 , on the progress of the reaction. The substituents were chosen to have similar steric hindrance but different electronic effects and were placed in position 6 (i.e., near one of the nitrogen atoms) or in position 5, which allows, in part, electronic and steric influence to be distinguished. The ligands studied, 6-methyl-2,2′-bipyridine, 5-methyl-2,2′-bipyridine, 6-trifluoromethyl-2,2′-bipyridine, and 5-trifluoromethyl-2,2′-bipyridine, were compared to unsubstituted 2,2′-bipyridine in the reaction with the electron-rich complex [Pt(Me) 2 (DMSO) 2 ]. The electron-withdrawing CF 3 group was found to have a significant effect in accelerating the cyclometalation reaction. The substituent in position 6 influences the stability of the intermediate adduct [Pt(N,N)(Me) 2 ] (N,N = chelated bipyridine), as indicated by density functional theory calculations. The steric hindrance of substituted bipyridines was also evaluated by defining and measuring the angle ζ in [Pt(N,N)(Me) 2 ] adducts. The presence of a substituent in position 6 causes destabilization of the adduct, acceleration of the cyclometalation reaction, and regioselectivity of C−H bond activation.

Switching of “Rollover Pathway” in Rhodium(III)-Catalyzed C–H Activation of Chelating Molecules

“…This explains the extreme rarity of catalytic "rollover" C-H functionalization protocol 4 to harness this kind of organic compounds in spite of their ample availability, and some success in stoichiometric reactions. 5 A critical and thoughtful analysis allows us to comprehend that the required decomplexation step depends on the "lability" of the metal-ligand bond within the chelate. Pyridine and thiophene moieties are somewhat feasible for such activity, although the examples of "rollover" functionalization with these motifs are extremely limited to only three reports till date.…”

Platinum(II), palladium(II) and gold(III) adducts and cyclometalated derivatives of 6-methoxy-2,2′-bipyridine: A comparative study

“…17 In fact, the value of 152.3 for the angle ζ for 6-methoxy-2,2'-bipyridine is somewhat remarkable being only a few degrees below that for t-Bu (157.1) and almost 15 degrees above that for CF 3 , which showed to undergo rollover cyclometalation very easily. However, it has to be noted that the reported value corresponds to the maximum hindrance of the methoxy group; when the latter points away from the nitrogen atom, the ζ-angle is 109.3°, i.e.…”

“…CF 3 ). 17 Steric hindrance also plays a non negligible role in the regioselectivity of the process: bulky substituents (e.g. t-butyl groups) in position 6, i.e.…”

“…The DMSO ligand in complex 1 can be easily displaced at room temperature by good donors such as triphenylphosphine to give the corresponding complex [Pt(bpy 6OMe -H)(Me)(PPh 3 )], 5.Among the spectroscopic data supporting this formulation, the 31 P NMR signal at 32.59 ppm, with 1 J Pt-P = 2257 Hz, is fully consistent with a C-Pt-P trans arrangement. In the comparison with the corresponding complexes derived from 2,2'-bipyridine, 4d 6-Me-2,2'-bipyridine, 8b and 6-CF 3 -2,2'-bipyridine17 (complexes 6, 7 and 8, Scheme 2), although the differences are not dramatic some trends can still be seen: 31 P NMR data for 6, 7 and 8 show only a slight chemical shift trend in line with the inductive properties of the substituent on the bipyridine ligand: with respect to 6 and 7, R = H and CH 3 , the P atom is slightly shielded with the electron attracting group CF 3 . A second trend is given by the 31 P-195 Pt coupling constants: 1 J Pt-P = 2226 (7, R = CH 3 ), 2229 (6, R = H), 2279 Hz (8, R =CF 3 ).…”

Cyclometalated platinum(ii) complexes of 2,2′-bipyridine N-oxide containing a 1,1′-bis(diphenylphosphino)ferrocene ligand: structural, computational and electrochemical studies

“…[22][23][24][25][26] For example, one of the most fascinating class of cyclometalated complexes is rollover category. [27][28][29][30] In this family, the cyclometalated moieties are bidentate heterocyclic donors (2,2'-bipyridine (bipy), 29 2,2'-bipyridine N-oxide (bipyO) 31 ) rather than classical monodentate ligands (2-phenylpyridine (ppy), 24 2-vinylpyridine (vpy) 32,33 ). The bipyO ligand has been by far less explored than bipy ligand in rollover cycolmetalation 31 or in coordination chemistry.…”