Silver(I) and Copper(I) Complexes of Semi‐Bulky Nitrogen‐Confused C‐Scorpionates

Abstract

Two new sterically demanding nitrogen-confused C-scorpionate ligands with a bis(3,5-diisopropylpyrazol-1yl)methyl group bound to the 3-position of a normal pyrazole ( H L iPr2 ) or an N-toluenesulfonyl pyrazole ( Ts L iPr2 ) have been prepared. Reactions between the ligands ( x L iPr2 ) and silver trifluoromethanesulfonate, AgOTf, gave four new compounds of the types [Ag(Ts, 1b; x = H, 2b) depending on the initial metal:ligand ratio. Similarly, the reactions with [Cu(CH 3 CN) 4 ](PF 6 ) produce [a] Dr.to trap and spectroscopically characterize the nitrene intermediates {[(2-py)Bpm R3R5 ]Cu(NTs)} + in CH 2 Cl 2 at -78°C using (2-tBuSO 2 )C 6 H 4 I=NTs [34] as a soluble nitrene source. The spectroscopic and magnetic data were consistent with diamagnetic, singlet species at -78°C. Calculations showed ground state triplet but all spin states [ 1/3 Cu I -nitrene or 1/3 Cu II -N·(iminyl)] and nitrene binding modes (κ 1 N-, κ 2 N,O-) are thermally accessible. These intermediates were capable of stoichiometrically transferring a nitrene unit to a variety of substrates. Then, different {[(2-py)Bpm R3R5 ]Cu(CH 3 CN)}(PF 6 ) complexes were found to be capable catalysts for nitrene transfer to unsubstituted or p-substituted styrenes giving modest yields (> 65 %) of aziridine under mild ocnditions (PhI=NTs/CH 2 Cl 2 /295 K, 24 h).In a previous study, [27] eight complexes of the type [Ag( x L R ) n ](OTf ) (n = 1, 2 and x L R = H L, Ts L, H L*, and Ts L*) were characterized and evaluated for their ability to catalyze aziridination of styrene. Both ESI(+) MS and NMR spectroscopic studies indicated that the analytically pure [Ag( x L R ) n ](OTf ) compounds did not retain their structure in CH 3 CN rather were involved in multiple dynamic equilibria in solution. These dynamic processes remained in the fast exchange regime on the NMR timescale down to the freezing point of the solvent, thereby obfuscating structural information. These silver complexes of nitrogen-confused C-scorpionates showed no or very little capacity to participate in styrene aziridination using PhI= NTs in room temperature CH 2 Cl 2 . However, [Ag( x L R ) n ](OTf ) showed modest catalytic activity at 80°C in CH 3 CN when employing a nitrene generated in-situ from H 2 NTs and PhI(OAc). Interestingly, the bulkiest derivative [Ag( Ts L * ) 2 ](OTf ) was reported to have the highest activity giving 34 % yield of the desired N-tosyl aziridine. This observation prompted the current study to determine if further increasing steric bulk of the ligands would lead to an increase in catalytic activity of silver complexes. Herein we report on the preparation of two new semi-bulky nitrogen confused scorpionates, Ts L iPr2 and H L iPr2 and their silver(I) and copper(I) complexes. The copper(I) complexes were prepared to compare catalytic activity with their silver congeners, and possibly to demonstrate the generality of any trends in ligand sterics on catalytic activity. It was also hoped that the slower ligand exchange rates associated with the smaller copper(I) compared ...

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