Cooperative Metal–Ligand Hydroamination Catalysis Supported by C–H Activation in Cyclam Zr(IV) Complexes

Abstract

Complexes of the type (R2Cyclam)­ZrCl2 (where R = CH2C­(H)­CH2 (All), CH2C­(Me)­CH2 (MeAll), and PhCH2 (Bn)) react with suitable Grignard reagents to produce the corresponding alkyl derivatives (R2Cyclam)­ZrR′2 (R′ = Me, CH2Ph). Thermally induced double metalation of the pending arms of the cyclam ligand led to the formation of complexes ((CHC­(H)­CH2)2­Cyclam)­Zr, 14, ((CHC­(Me)­CH2)2­Cyclam)­Zr, 15, or ((C6H4CH2)2­Cyclam)­Zr, 16. These reactions proceed through C­(sp2)–H bond activation and R′H elimination and convert the original dianionic tetracoordinated cyclam-based ligands in tetraanionic hexacoordinated ligands that establish two new Zr–C bonds. The cleavage of the Zr–C bonds may be readily achieved by treatment of the bis­(ortho-metalated) species 16 with protic substrates (tert-butanol, phenol, thiophenol, aniline, benzophenone hydrazone, pyrazole, and N,N′-diphenylhydrazine), to give rise to (Bn2Cyclam)­ZrX2 complexes (X = OtBu, OPh, SPh, NHPh, NHNCPh2, C3H3N2, N,N′-PhNNPh). In catalytic conditions, complexes (All2Cyclam)­Zr­(NMe2)2, 14, 15, or 16 convert 2,2-diphenyl-pent-4-enylamine to 2-methyl-4,4-diphenylpyrrolidine with 100% selectivity and conversion values varying between 61 and 88% in 4.5 h, at 115 °C. Complexes 14 and 15, which display metalated allyl and methallyl pending groups on the cyclam ring, are the most active species (1.7 < TOF < 2.0 h–1). The mechanism of this reaction was studied by density functional theory that revealed two competitive paths, one proceeding through an imido intermediate and another that occurs via an amido ligand. Both cases represent cooperative mechanisms with active participation of the cyclam, as proton exchange between the coordinated substrate and the ligand side arm with reversible C–H activation is a crucial feature of the mechanism.

Synthesis and Characterization of New Cyclam-Based Zr(IV) Alkoxido Derivatives

“…The metal is located above the macrocycle at 1.098(2) Å from the average plane defined by the four nitrogen atoms. The Zr-N amido and the Zr-N amine bond lengths lie within the expected ranges for similar bonds in hexa-coordinated Zr(IV) complexes [13][14][15][16]18,19,27,[35][36][37][38]. The Zr-Cl and Zr-O bond lengths of 2.528(3) and 1.958(4) Å, respectively, and the O(1)-Zr(1)-Cl(1) angle at 87.0(1) • are comparable with other Zr(IV) complexes based on Bn 2 Cyclam ligands [13,14,16,18,19,27,38].…”

“…The metal is located above the macrocycle at 1.098(2) Å from the defined by the four nitrogen atoms. The Zr-Namido and the Zr-Namine bond len the expected ranges for similar bonds in hexa-coordinated Zr(IV) co 16,18,19,27,[35][36][37][38]. The Zr-Cl and Zr-O bond lengths of 2.528(3) an respectively, and the O(1)-Zr(1)-Cl(1) angle at 87.0(1)° are comparable wi complexes based on Bn2Cyclam ligands [13,14,16,18,19,27,38].…”

“…The four nitrogen atoms of the macrocycle define one rec commonly reported for other cyclam based complexes [13-16,18,19,27,3 is located above the macrocycle at 1.222(2) Å from the average plane defin In complex 12, the zirconium is coordinated to the four nitrogen atoms of the macrocycle, and to the oxygen atoms of the isopropoxido ligands in a trigonal prismatic geometry. The four nitrogen atoms of the macrocycle define one rectangular face, as commonly reported for other cyclam based complexes [13][14][15][16]18,19,27,[35][36][37][38]. The metal is located above the macrocycle at 1.222(2) Å from the average plane defined by the cyclam nitrogen atoms.…”

“…Dialkoxido derivatives can be obtained from the reaction of the dichlorido precursor with two equivalents of a suitable lithium alkoxido [13,14] or by protonolysis of an orthometallated species with an alcohol [18]. ( 3, Bn 2 Cyclam)Zr(OPh) 2 , 10, was prepared using the latter strategy by reaction of (3,5-Me-C 6 H 4 CH 2 ) 2 Cyclam)Zr, 9, with two equivalents of phenol.…”

“…The selective insertion of heteroallenes into the Zr-N amido bonds of (Bn 2 Cyclam)Zr(OR) 2 (R = i Pr, t Bu) revealed to be a suitable procedure for the N-functionalization of the cyclam ring [14]. In view of the importance of N-functionalized cyclams in several chemical [16][17][18][19][20][21] and biological [22][23][24][25][26] applications, we present here the reactivity of di-alkoxido zirconium(IV) complexes with carbon dioxide. In this manuscript, we also describe the synthesis and characterization of new monoalkoxido zirconium(IV) complexes supported by tetradentate dianionic cyclam ligands and mixed complexes bearing one alkoxide and one methyl or thiophenoxide ligands.…”