A homoleptic triamidoamine zirconium complex featuring a metalated trimethylsilyl substituent, [κ 5 -(Me 3 SiNCH 2 CH 2 ) 2 NCH 2 CH 2 NSiMe 2 CH 2 ]Zr (1), was synthesized by reaction of Zr(CH 2 Ph) 4 with N(CH 2 CH 2 NHSiMe 3 ) 3 followed by sublimation. Complex 1 is a general precursor to a family of complexes with the formulation (N 3 N)ZrX (N 3 N ) N(CH 2 CH 2 NSiMe 3 ) 3 3-, X ) anionic ligand) by reactions that parallel expected reactivity of a hydride derivative. Treatment of 1 with phosphines, amines, thiols, alkynes, and phenol resulted in the formation of new, pseudo-C 3V -symmetric (N 3 N)ZrX complexes (X ) phosphido, amido, alkynyl, thiolate, or phenoxide) via element-H bond activation. Thus, the reactivity of complex 1 is that best described as a hydride surrogate. For example, complex 1 reacted with PhPH 2 at ambient temperature to provide (N 3 N)ZrPHPh (2) in 86% yield. Density functional theory studies and X-ray crystal structures provide a general overview of the bonding in these complexes, which appears to be highly ionic. In general, there is little evidence for ligand-to-metal π-bonding for the pseudoaxial X ligand in these complexes except for strongly π-basic terminal amido ligands. The limited π-bonding appears to be the result of competitive π-donation by the pseudoequatorial amido arms of the triamidoamine ancillary ligand. Thus, the relative Zr-X bond energies are governed by the basicity of the anionic ligand X. Solid-state structures of phosphido (3, 4, 5), amido (10), and thiolate (15) complexes support the computational results.

This paper presents the synthesis and characterization of a series of pincer ligands and their Ni, Pd, Pt, and Rh complexes. The ligands under examination are based on a diarylamine which is modified either by two phosphino (-PR2) substituents in the ortho-positions (PNP ligands) or by a combination of a phosphino and an iminyl (-CH═NX) substituent (PNN ligands). The ligands can be broken down into three groups: (a) C2v-symmetric PNP ligands with identical side -PR2 donors, (b) Cs-symmetric PNP' ligands with different -PR2 side donors, and (c) PNN ligands containing a -P(i)Pr2 side donor. All of the ligands under study readily formed square-planar complexes of the types (PNZ)PdCl, (PNZ)Pd(OAc), and (PNZ)RhCO, where PNZ is the corresponding anionic tridentate pincer ligand. For select PNP ligands, (PNP)NiCl and (PNP)PtCl were also studied. The (PNZ)MCl complexes (M = Ni, Pd, Pt) underwent quasireversible oxidation in cyclic voltammetry experiments. Based on the close similarity of formal potentials for Ni, Pd, and Pt analogs, and based on the previous literature evidence, these oxidation events are ascribed primarily to the PNZ ligand, and the E1/2 values can be used to compare the ease of oxidation of different ligands. A (PNP)PdCl complex containing methoxy substituents para- to the central nitrogen underwent two quasireversible oxidations. Two mono-oxidized complexes were isolated and structurally characterized in comparison to their neutral analog, revealing minimal changes in the bond distances and angles. Several other neutral complexes were also structurally characterized. The carbonyl stretching frequency in (PNZ)RhCO complexes was used to gauge the donating ability of the various pincer ligands toward the metal. Comparison of E1/2 values for (PNZ)PdCl and νCO values for (PNZ)RhCO revealed that the two are not consistently correlated across all the studied ligands and can be tuned to different degrees through judicious ligand alteration.

Triamidoamine-supported zirconium complexes have been demonstrated to catalyze a range of bond-forming events utilizing arsines. Three different mechanisms have been observed in these reactions. In the first mechanism, triamidoamine-supported zirconium complexes of the general type (N3N)ZrX (N3N =N(CH2CH2NSiMe3)33-; X = monoanionic ligand) catalyzed the dehydrogenative dimerization of diphenylarsine. Mechanistic analysis revealed that As-As bond formation proceeds via sigma-bond metathesis steps similar to the previously reported dehydrocoupling of phosphines by the same catalysts. In the second mechanism, sterically encumbered primary arsines appear to be dehydrocoupled via alpha elimination of an arsinidene fragment. Dehydrocoupling of dmpAsH2 (dmp = 2,6-dimesitylphenyl) to form (dmp)As = As(dmp) by (N3N)Zr-complexes appeared to proceed via elimination of dmpAs: from the arsenido intermediate, (N3N)ZrAsH(dmp). Further support for -arsinidene elimination came from the thermal decomposition of (N3N)ZrAsHMes (9) to (MesAs)4 (10), which obeyed first-order kinetics. In the third mechanism, the observation of stoichiometric insertion reactivity of the Zr-As bond with polar substrates, PhCH2NC, PhCN, (1-napthyl)NCS, and CS2, led to the development of intermolecular hydroarsination catalysis of terminal alkynes. Here, (N3N)ZrAsPh2 (2) catalyzed the addition of diphenylarsine to phenylacetylene and 1-hexyne to give the respective vinylarsine products. Arsenido complexes 2 and 9 and tetraarsine 10 have been structurally characterized.