Nonclassical β-Hydrogen Elimination of Hydrosilazido Zirconium Compounds via Direct Hydrogen Transfer

Salt metathesis reactions of Cp2(NR2)ZrX (X = Cl, I, OTf) and lithium hydrosilazides ultimately afford hydride products Cp2(NR2)ZrH that suggest unusual β-hydrogen elimination processes. A likely intermediate in one of these reactions, Cp2Zr[N(SiHMe2)t-Bu][N(SiHMe2)2], is isolated under controlled synthetic conditions. Addition of alkali metal salts to this zirconium hydrosilazide compound produces the corresponding zirconium hydride. However as conditions are varied, a number of other pathways are also accessible, including C–H/Si–H dehydrocoupling, γ-abstraction of a CH, and β-abstraction of a SiH. Our observations suggest that the conversion of (hydrosilazido)zirconocene to zirconium hydride and silanimine does not follow the classical four-center mechanism for β-elimination

Remarkably Robust Monomeric Alkylperoxyzinc Compounds from Tris(oxazolinyl)boratozinc Alkyls and O2

Metal alkylperoxides are remarkable, highly effective, yet often thermally unstable, oxidants that may react through a number of possible pathways including O–O homolytic cleavage, M–O homolytic cleavage, nucleophilic O-atom transfer, and electrophilic O-atom transfer. Here we describe a series of zinc alkyl compounds of the type ToMZnR (ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate; R = Et, n-C3H7, i-C3H7, t-Bu) that react with O2 at 25 °C to form isolable monomeric alkylperoxides ToMZnOOR in quantitative yield. The series of zinc alkylperoxides is crystallographically characterized, and the structures show systematic variations in the Zn–O–O angle and O–O distances. The observed rate law for the reaction of ToMZnEt (2) and O2 is consistent with a radical chain mechanism, where the rate-limiting SH2 step involves the interaction of •OOR and ToMZnR. In contrast, ToMZnH and ToMZnMe are unchanged even to 120 °C under 100 psi of O2 and in the presence of active radical chains (e.g., •OOEt). This class of zinc alkylperoxides is unusually thermally robust, in that the compounds are unchanged after heating at 120 °C in solution for several days. Yet, these compounds are reactive as oxidants with phosphines. Additionally, an unusual alkylperoxy group transfer to organosilanes affords ToMZnH and ROOSiR3′

Comparative Study of Rhodium and Iridium Porphyrin Diaminocarbene and N-Heterocyclic Carbene Complexes

Iridium meso-tetratolylporphyrinato (TTP) mono- and bis-diaminocarbene complexes, [Ir(TTP)[═C(NHBn)(NHR)]2–x(C≡NBn)x]BF4, where R = Bn, n-Bu and x = 1, 0, were synthesized by nucleophilic addition of amines to the bis-isocyanide complex [Ir(TTP)(C≡NBn)2]BF4. Rhodium and iridium porphyrinato N-heterocyclic carbene (NHC) complexes M(TTP)CH3(NHC), where NHC = 1,3-diethylimidazolylidene (deim) or 1-(n-butyl)-3-methylimidazolylidene (bmim), were prepared by the addition of the free NHC to M(TTP)CH3. The NHC complexes displayed two dynamic processes by variable-temperature NMR: meso-aryl–porphyrin C–C bond rotation and NHC exchange. meso-Aryl–porphyrin C–C bond rotation was exhibited by both rhodium and iridium complexes at temperatures ranging between 239 and 325 K. Coalescence data for four different complexes revealed ΔG⧧ROT values of 59 ± 2 to 63 ± 1 kJ·mol–1. These relatively low rotation barriers may result from ruffling distortions in the porphyrin core, which were observed in the molecular structures of the rhodium and iridium bmim complexes. Examination of NHC exchange with rhodium complexes by NMR line-shape analyses revealed rate constants of 3.72 ± 0.04 to 32 ± 6 s–1 for deim displacement by bmim (forward reaction) and 2.7 ± 0.4 to 18 ± 2 s–1 for bmim displacement by deim (reverse reaction) at temperatures between 282 and 295 K, corresponding to ΔGf⧧ of 65.2 ± 0.6 kJ·mol–1 and ΔGr⧧ of 66.2 ± 0.5 kJ·mol–1, respectively. Rates of NHC exchange with iridium were far slower, with first-order dissociation rate constants of (1.75 ± 0.04) × 10–4 s–1 for the forward reaction and (1.2 ± 0.1) × 10–4 s–1 for the reverse reaction at 297.1 K. These rate constants correspond to ΔG⧧ values of 94.2 ± 0.6 and 95.2 ± 0.2 kJ·mol–1 for the forward and reverse reactions, respectively. Equilibrium constants for the exchange reactions were 1.6 ± 0.2 with rhodium and 1.56 ± 0.04 with iridium, favoring the bmim complex in both cases, and the log(K) values for NHC binding to M(TTP)CH3 were 4.5 ± 0.3 (M = Rh) and 5.4 ± 0.5 (M = Ir), as determined by spectrophotometric titrations at 23 °C. The molecular structures also featured unusually long metal–Ccarbene bonds for the bmim complexes (Rh–CNHC: 2.255(3) Å and Ir–CNHC: 2.194(4) Å)

Conversion of a Zinc Disilazide to a Zinc Hydride Mediated by LiCl

An unusual β-elimination reaction involving zinc(II) and LiCl is reported. LiCl and a coordinatively saturated disilazido zinc compound form an adduct that contains activated SiH moieties. In THF/toluene mixtures, this adduct is transformed into a zinc hydride and 0.5 equiv. cyclodisilazane. The Li+ and Cl− ions apparently affect the reaction pathway of the disilazido zinc in a synergistic fashion. Thus, the zinc hydride and cyclodisilazane products of formal β-elimination are not observed upon treatment of the zinc disilazide with Cl− or Li+separately

Synthesis and Characterization of Chiral Tetraaza Macrocyclic Nickel(II) and Palladium(II) Complexes

Chiral tetraaza macrocyclic nickel(II) and palladium(II) complexes 2a−e, containing one or two (R,R)-(−)-1,2-cyclohexanediyl bridges, were synthesized by template condensation reactions and characterized by 1H and 13C NMR, IR, UV−vis, and mass spectrometry. The electrophilic reactivity of 2a was explored. Crystal structures of Ni complex 2b and metal-free ligand 5were determined by single-crystal X-ray diffraction

Homoleptic Divalent Dialkyl Lanthanide-Catalyzed Cross-Dehydrocoupling of Silanes and Amines

The rare-earth bis(alkyl) compound Sm{C(SiHMe2)3}2THF2 (1b) is prepared by the reaction of samarium(II) iodide and 2 equiv of KC(SiHMe2)3. This synthesis is similar to that of previously reported Yb{C(SiHMe2)3}2THF2 (1a), and compounds 1a,b are isostructural. Reactions of 1b and 1 or 2 equiv of B(C6F5)3 afford SmC(SiHMe2)3HB(C6F5)3THF2 (2b) or Sm{HB(C6F5)3}2THF2 (3b), respectively, and 1,3-disilacyclobutane {Me2Si-C(SiHMe2)2}2 as a byproduct. Bands from 2300 to 2400 cm–1 assigned to νBH in the IR spectra and highly paramagnetically shifted signals in the 11B NMR spectra of 2b and 3b provided evidence for Sm-coordinated HB(C6F5)3. Compounds 1a,b react with the bulky N-heterocyclic carbene (NHC) 1,3-di-tert-butylimidazol-2-ylidene (ImtBu) to displace both THF ligands and give three-coordinate monoadducts Ln{C(SiHMe2)3}2ImtBu (Ln = Yb (4a), Sm (4b)). Complexes 4a,b catalyze cross-dehydrocoupling of organosilanes with primary and secondary amines at room temperature to give silazanes and H2, whereas 1a,b are not effective catalysts under these conditions. Second-order plots of ln{[Et2NH]/[Ph2SiH2]} vs time for 4a-catalyzed dehydrocoupling are linear and indicate first-order dependences on silane and amine concentrations. However, changes in the experimental rate law with increased silane concentration or decreased amine concentration reveal inhibition by silane. In addition, excess ImtBu or THF inhibit the reaction rate. These data, along with the structures of 4a,b, suggest that the bulky carbene favors low coordination numbers, which is important for accessing the catalytically active species

Homoleptic organolanthanide compounds supported by the bis(dimethylsilyl)benzyl ligand

A β-SiH functionalized benzyl anion [C(SiHMe2)2Ph]− is obtained by deprotonation of HC(SiHMe2)2Ph with KCH2Ph or by reaction of KOtBu and (Me2HSi)3CPh; LnI3(THF)n and three equivalents of this carbanion combine to provide homoleptic tris(alkyl)lanthanide compounds Ln{C(SiHMe2)2Ph}3 (Ln = La, Ce, Pr, Nd) containing secondary metal–ligand interactions

Ligand Exchange Reactions and Hydroamination with Tris(oxazolinyl)borato Yttrium Compounds

Ligand substitution reactions and catalytic hydroamination/cyclization of aminoalkenes have been studied with a new oxazolinylborato yttrium compound, tris(4,4-dimethyl-2-oxazolinyl)phenylborato bis(trimethylsilylmethyl)yttrium ([Y(κ3-ToM)(CH2SiMe3)2(THF)], 1). THF exchange in 1 is rapid at room temperature, and activation parameters obtained by simulation of 1H NMR spectra acquired from 190 to 280 K are consistent with a dissociative mechanism (ΔS‡ = 30 ± 1 e.u., ΔG‡ = 11.9 kcal mol−1 at 243 K). The related phosphine oxide adduct [Y(κ3-ToM)(CH2SiMe3)2(OPPh3)] (2) also undergoes exchange via OPPh3 dissociation with a much higher barrier (ΔG‡ = 15.0 kcal mol−1 at 320 K). Compound 1 reacts with the amines tBuNH2, para-MeC6H4NH2, and 2,6-iPr2C6H3NH2 to provide six-coordinate [Y(κ3-ToM)(NHR)2(THF)] (3: R = tBu; 4: R = para-MeC6H4) and five-coordinate [Y(κ3-ToM)(NH-2,6-iPr2C6H3)2] (6). These oxazolinylborato yttrium compounds are precatalysts for the cyclization of aminoalkenes; the kinetics of catalytic conversion indicate zero-order substrate dependence and first-order catalyst dependence. Kinetic investigations of ligand exchange processes and hydroamination reactions indicate that the tris(oxazolinyl)borato-yttrium interaction is robust even in the presence of excess phosphine oxide and primary and secondary amines