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

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.

Reactions of heteroallenes with cyclam-based Zr(IV) complexes.

This work describes reactions of heteroallenes with diamido-diamine cyclam-based Zr(iv) complexes of the general formula (Bn2Cyclam)ZrX2 (X = O(t)Bu, , O(i)Pr, , SPh, , NH(t)Bu, ) as well as the di-orthometallated species ((C6H4CH2)2Cyclam)Zr, . The reactions of isocyanates or isothiocyanates with , or resulted in the formation of N-bonded ureate or thioureate cyclam complexes upon [2 + 2] cycloaddition of the Zr-Namido bonds of the cyclam to the heteroallene (). DFT calculations showed that κ(2)-N,N'-ureate bonding is favoured over κ(2)-N,O-ureates, which in turn may be formed in reactions with bulky isocyanates as 1-naphthyl isocyanate (NpN[double bond, length as m-dash]C[double bond, length as m-dash]O). The reactions of with N,N'-cyclohexylcarbodiimide (CyN[double bond, length as m-dash]C[double bond, length as m-dash]NCy) and carbon disulfide afforded guanidinate and dithiocarbamate fragments, respectively, appended to one of the nitrogen atoms of the cyclam ligand. These reactions represent a reliable method for the synthesis of asymmetrically N-functionalized cyclams giving rise to C1 symmetry Zr(iv) species by addition of one equivalent of heteroallenes. The reaction of (Bn2Cyclam)Zr(NH(t)Bu)2, , with one equivalent of mesityl isocyanate (MesN[double bond, length as m-dash]C[double bond, length as m-dash]O) also proceeds through insertion, involving one Zr-NH(t)Bu bond. However, it was observed that the reaction of (Bn2Cyclam)Zr(NH(t)Bu)2, , with MesN[double bond, length as m-dash]C[double bond, length as m-dash]O follows a different path if the reaction is carried out at 60 °C. In this case the reaction leads to [2 + 2] addition of the Zr-Ncyclam bond to the isocyanate, with a concomitant occurrence of orthometallation of the one benzyl pending group of the cyclam ring. The reaction of (t)BuN[double bond, length as m-dash]C[double bond, length as m-dash]O with the di-orthometallated complex ((C6H4CH2)2Cyclam)Zr, , also gave a κ(2)-N,N'-ureate fragment, by isocyanate addition to the macrocycle. DFT calculations on these systems were conducted in an attempt to rationalise the reactivity patterns observed.

Tuning the electronic and steric parameters of a redox-active tris(amido) ligand.

A family of tantalum compounds was prepared to probe the electronic effects engendered by the addition of electron-donating or electron-withdrawing groups to the 4/4' positions of the redox-active ligand derived from bis(2-isopropylamino-4-X-phenyl)amine [(X,iPr)(NNN(cat))H3, X = F, H, Me, (t)Bu]). A general synthetic procedure for the (X,iPr)(NNN(cat))H3 ligand family was developed starting from the 4/4' disubstituted diphenylamine derivative. A second ligand modification, incorporation of aromatic substituents at the flanking nitrogen moieties, was achieved via palladium-catalyzed cross-coupling to afford bis(2-3,5-dimethylphenylamino-4-methoxy-phenyl)amine (OMe,DMP)(NNN(cat))H3 (DMP = 3,5-C6H3Me2), allowing a comparative study to the less sterically hindered isopropyl derivative. Treatment of the triamines with 1 equiv of TaMe3Cl2 generated the corresponding dichloro complexes (X,R)(NNN(cat))TaCl2(L) (L = empty or Et2O) in high yields. These neutral dichloride derivatives reacted with [NBnEt3][Cl] to produce the anionic trichloride derivatives [NBnEt3][(X,R)(NNN(cat))TaCl3], whereas the neutral dichloride derivatives reacted with chlorine atom donors to produce the neutral trichloride derivatives (X,R)(NNN(sq))TaCl3, containing the one-electron-oxidized form of the redox-active ligand. Aryl azides reacted with the (X,R)(NNN(cat))TaCl2(L) derivatives, resulting in nitrene transfer to tantalum and two-electron oxidation of the ligand platform to give (X,R)(NNN(q))TaCl2(═NR') (R = iPr; X = OMe, F, H, Me; R' = p-C6H4tBu, p-C6H4CF3; and R = 3,5-C6H3Me2; X = OMe; R' = p-C6H4CH3). Electrochemistry, UV-vis-NIR, IR, and EPR spectroscopies along with X-ray diffraction methods were used to characterize and compare complexes with different redox-active ligand derivatives in each oxidation state. This study demonstrates that while the ligand redox potentials can be adjusted over a 270 mV range through substitutions at the 4/4' ring positions, the coordination chemistry and reactivity patterns at the bound tantalum center remain unchanged, suggesting that such ligand modifications can be used to tune the redox potentials of a complex for a particular substrate of interest.

Group transfer reactions of d0 transition metal complexes: redox-active ligands provide a mechanism for expanded reactivity.

Group- and atom-transfer is an attractive reaction class for the preparation of value-added organic substrates. Despite a wide variety of known early-transition metal oxo and imido complexes, these species have received limited attention for atom- and group-transfer reactions, owing to the lack of an accessible metal-based two-electron redox couple. Recently it has been shown that redox-active ligands can support the multi-electron changes required to promote group-transfer reactivity, opening up new avenues for group- and atom-transfer catalyst design. This Perspective article provides an overview of group transfer reactivity in early-transition metal complexes supported by traditional ligand platforms, followed by recent advances in the atom- and group-transfer reactivity of d(0) metal complexes containing redox-active ligands.

Synthesis and structural characterization of novel cyclam-based zirconium complexes and their use in the controlled ROP of rac-lactide: access to cyclam-functionalized polylactide materials.

Novel Bn(2)Cyclam-based zirconium complexes of the type (Bn(2)Cyclam)Zr(X)(X') (3, X = X' = OPh; 4, X = X' = SPh; 5, X = Cl, X' = O(i)Pr) were synthesized in good yields via metathesis routes involving the reaction of the dichloro precursor (Bn(2)Cyclam)Zr(Cl)(2) and the appropriate lithium salts. The molecular structures of compounds 3, 4 and 5, as determined by X-ray crystallographic studies, all confirmed the effective chelation of the Bn(2)Cyclam ligand in a κ(4)-N(2)N(2)' fashion, with the hexa-coordinated Zr center adopting a trigonal prismatic geometry. Complexes 3-5 as well as the diisopropoxide derivative (Bn(2)Cyclam)Zr(O(i)Pr)(2) (2) were all found to initiate the ring-opening polymerization (ROP) of rac-lactide in a controlled manner, as deduced from SEC data and linear correlations between molecular weight numbers (M(n)) and monomer conversion as the ROP proceeds. While initiator 2 polymerizes rac-lactide to afford, as expected, an O(i)Pr-ester-end PLA, the ROP of rac-lactide by species 3 or 4 affords an unusual cyclam-end group PLA, as deduced from MALDI-TOF data. The bonding and the electronic structures of the dialkoxides 2 and 3 were assessed by DFT and their possible influence on the polymerization mechanism is discussed.

The hydride route to the preparation of dinitrogen complexes.

That the inert dinitrogen molecule can act as a ligand to a transition metal complex is one of the key discoveries in inorganic chemistry of the past century. This feature article summarises a body of work up to 2010 that describes a particularly attractive route to dinitrogen complexes that involves the direct reaction of N(2) with metal hydride derivatives. This process is shown to be general across the transition series and, depending on the metal, different levels of activation of the coordinated dinitrogen unit are observed.

Synthesis and structural studies of amido, hydrazido and imido zirconium(IV) complexes incorporating a diamido/diamine cyclam-based ligand.

The preparation, characterisation and structural analysis of a series of zirconium(IV) complexes that incorporate the diamido/diamine macrocyclic ligand Bn2Cyclam (Bn2Cyclam = 1,8-dibenzyl-1,4,8,11-tetraazacyclotetradecane) are described. The reaction of one or two equivalents of the appropriate LiNHR reagents with [(Bn2Cyclam)ZrCl2] give the corresponding amido-chloride [(Bn2Cyclam)ZrCl(NHR)] (R = tBu, (2,6-iPr)Ph) or bis(amido) [(Bn2Cyclam)Zr(NHR)2] (R = iPr, tBu, (2,6-Me)Ph) complexes, respectively. Treatment of [(Bn2Cyclam)ZrCl(NH(2,6-iPr)Ph)] with one equiv. of MgClMe gives the base-free, monomeric imido complex [(Bn2Cyclam)Zr(N(2,6-iPr)Ph)]. The reaction of the tBu analog with MgClMe generates a dimeric bridging imido species [{(Bn2Cyclam)Zr}2(mu-NR)2], which can also be obtained by thermal decomposition of [(Bn2Cyclam)Zr(NHtBu)2] in toluene. The bis(hydrazido) complex [(Bn2Cyclam)Zr(NHNPh2)2] was obtained by reaction of [(Bn2Cyclam)ZrCl2] and two equiv. of LiNHNPh2. A hydrazido-chloride compound [(Bn2Cyclam)ZrCl(N(Ph)NBu(Ph))] was generated in a one-pot reaction between [(Bn2Cyclam)ZrCl2], LiBu and azobenzene. DFT calculations on [(Bn2Cyclam)ZrXY] complexes indicate that the coordination geometry adopted by these species is dictated by the steric bulk of the ligands X and Y, varying between six-coordinate prismatic and four-coordinate tetrahedral.

Substituent effects in the hydrosilylation of coordinated dinitrogen in a ditantalum complex: cleavage and functionalization of N2.

The dinitrogen complex ([NPN]Ta)2(mu-eta1:eta2-N2)(mu-H)2, 1, (where [NPN] = (PhNSiMe2CH2)2PPh) undergoes hydrosilylation with primary and secondary alkyl- and arylsilanes, giving a new N-Si bond and a new terminal tantalum hydride derived from one Si-H unit. Various primary silanes can be employed to give isolable complexes of the general formula ([NPN]TaH)(mu-N-N-SiH(n)R(3-n))(mu-H)2(Ta[NPN]) (5, R=Bu, n = 2; 9, R=Ph, n = 2). Analogous complexes featuring secondary silanes are not isolable, because these products, and 5 and 9, are uniformly unstable toward reductive elimination of bridging hydrides as H2, followed by cleavage of the N-N bond to give ([NPN]TaH)(mu-N)(mu-N-SiH(n)R(3-n))(Ta[NPN]) (6, R=Bu, n = 2; 10, R=Ph, n = 2; 15, R=Ph, n = 1; 16, R=Ph and Me, n = 1). The bridging nitrido ligand in these complexes is itself a substrate for a second hydrosilylation when n = 2, and schemes leading to Ta(IV) complexes of the general formula ([NPN]Ta)2(mu-N-SiH2R)(mu-N-SiH2R') via elimination of H2 are reported (4, R=R'=Bu; 12, R=Bu, R' = Ph; 13, R=Bu, R' = CH2CH2SiH3). At this point, the general reaction manifold for these compounds ramifies, with distinct outcomes occurring for different R groups-[NPN] ligand amide migration from Ta to RSi affords 11, whereas stable complex 6 rearranges to give 7, in the presence of excess silane. Ethanediylbissilane reacts with 1 to give 14, isostructural to 7.