Poly(vinyl chloride) Dechlorination Catalyzed by Zirconium.

Poly(vinyl chloride) undergoes dechlorination in the presence of triethylsilane (Et3SiH) and a catalytic amount of [Cp2Zr(NPh2)][CH3B(C6F5)3] (1 b) at 40-80 °C, with up to 91 % efficiency. Stoichiometric reactivity studies conducted on cyclohexyl chloride as a model suggest that 1 b dechlorinates PVC by initial chloride abstraction, followed by hydride transfer to the cationic PVC chain from Et3SiH. Consumer items such as pipe fitting, vinyl disc or electric cable insulation undergo either dechlorination or hydrosilylation of the carbonyl-containing copolymer (polyvinyl acetate) or plasticizer (phthalate).

Template Synthesis of NPN' Pincer-type Ligands at Titanium Using an Ambiphilic Phosphide Scaffold.

Ti-imido complex [TiCl(NtBu)(BIPP)] [1; BIPP = bis(iminophosphoranyl)phosphide ligand] reacts with terminal alkynes R-C≡CH (R = phenyl, isopropenyl, cyclopropyl, and 2-pyridyl) via P-P bond cleavage of the BIPP ligand. The resulting complexes [TiCl(NPN')(NPhPPh2)] (2a-d) contain a pincer-type NPN' phosphide ligand that incorporates the terminal alkyne and the imido ligand from complex 1. Complexes 2a-d feature two chiral centers (Ti and P) with interdependent absolute configurations; thus, they are formed stereoselectively. Complex 2a (R = phenyl) undergoes chloride abstraction with [Et3SiHSiEt3][B(C6F5)4], yielding [Ti(NPN')(NPhPPh2)][B(C6F5)4] (3). Complex 3 is a moderately active and stereoselective initiator for the ring-opening polymerization of rac-lactide. Complex 3 activates the C═O bond of 4-iodobenzaldehyde to give complex 4 as a single diastereomer despite the presence of three chiral centers. Complex 3 undergoes transmetallation with SbCl3, yielding [Sb(NPN')][B(C6F5)4] (5) and [TiCl3(NPhPPh2)] (6) selectively. The bonding situation in 3 and 5 was analyzed using Bader's atoms in molecules and the electron localization function, showing that the nitrogen atoms of the NPN' ligand are electronically similar, and that the metal-phosphide interaction is more polar in the case of titanium.

Coordinatively Unsaturated Amidotitanocene Cations with Inverted σ and π Bond Strengths: Controlled Release of Aminyl Radicals and Hydrogenation/Dehydrogenation Catalysis.

Cationic amidotitanocene complexes [Cp2 Ti(NPhAr)][B(C6 F5 )4 ] (Cp=η5 -C5 H5 ; Ar=phenyl (1 a), p-tolyl (1 b), p-anisyl (1 c)) were isolated. The bonding situation was studied by DFT (Density Functional Theory) using EDA-NOCV (Energy Decomposition Analysis with Natural Orbitals for Chemical Valence). The polar Ti-N bond in 1 a-c features an unusual inversion of σ and π bond strengths responsible for the balance between stability and reactivity in these coordinatively unsaturated species. In solution, 1 a-c undergo photolytic Ti-N cleavage to release Ti(III) species and aminyl radicals ⋅NPhAr. Reaction of 1 b with H3 BNHMe2 results in fast homolytic Ti-N cleavage to give [Cp2 Ti(H3 BNHMe2 )][B(C6 F5 )4 ] (3). 1 a-c are highly active precatalysts in olefin hydrogenation and silanes/amines cross-dehydrogenative coupling, whilst 3 efficiently catalyzes amine-borane dehydrogenation. The mechanism of olefin hydrogenation was studied by DFT and the cooperative H2 activation key step was disclosed using the Activation Strain Model (ASM).

Phosphasalen group IV metal complexes: synthesis, characterization and ring opening polymerization of lactide.

We report the synthesis of a series of Zr and Ti complexes bearing phosphasalen which differs from salen by the incorporation of two P atoms in the ligand backbone. The reaction of phosphasalen proligands (1a-1c)H2 with Zr(CH2Ph)4 led to different products depending on the nature of the N,N-linker in the ligand. In the case of ethylene-linked phosphasalen, octahedral Zr complex 2a formed as a single stereoisomer in trans geometry. With the phenylene linker, it was shown by dynamic NMR spectroscopy that complex 2b exists as a mixture of trans and cis-ß isomers in solution, both enantiomers (Δ and Λ) of the cis-ß isomer being in fast equilibrium with respect to the NMR time-scale. The use of a propylene-linked phosphasalen proligand 1cH2 led to a mixture of complexes among which a binuclear Zr complex 2c bridged only by one phosphasalen ligand could be isolated and characterized. Addition of 2 equiv. of iPrOH to 2a and 2b afforded diisoproxy Zr complexes 3a and 3b as a mixture of trans and cis-ß isomers, the latter undergoing fast Δ/Λ isomerization in solution. Addition of B(C6F5)3 to 2a and 2b gave cationic monobenzyl Zr complexes 4a and 4b which have been further converted into cationic alkoxy Zr complexes 5a-b and 6a-b by alcoholysis with iPrOH and (S)-methyl-lactate, respectively. The reaction of the phosphasalen proligands with Ti(NMe2)4 proceeded diastereoselectively giving rise to Ti complexes 7a-c in octahedral geometry with cis-ß wrapping of the ligand. The complexes have been tested for the ROP of rac-lactide. The neutral phosphasalen Ti and Zr complexes showed only poor activity probably due to the encumbered and electron donating nature of the phosphasalen ligand. In contrast, the cationic Zr alkoxides 5a, 6a and 6b are effective initiators for the controlled and hetero-selective ROP of rac-lactide.

Reappraising Schmidpeter's bis(iminophosphoranyl)phosphides: coordination to transition metals and bonding analysis.

The synthesis and characterization of a range of bis(iminophosphoranyl)phosphide (BIPP) group 4 and coinage metals complexes is reported. BIPP ligands bind group 4 metals in a pseudo fac-fashion, and the central phosphorus atom enables the formation of d0-d10 heterobimetallic complexes. Various DFT computational tools (including AIM, ELF and NCI) show that the phosphorus-metal interaction is either electrostatic (Ti) or dative (Au, Cu). A bridged homobimetallic Cu-Cu complex was also prepared and its spectroscopic properties were investigated. The theoretical analysis of the P-P bond in BIPP complexes reveals that (i) BIPP are closely related to ambiphilic triphosphenium (TP) cations; (ii) the P-P bonds are normal covalent (i.e. not dative) in both BIPP and TP.

The Taming of Redox-Labile Phosphidotitanocene Cations.

Tame d0 phosphidotitanocene cations stabilized with a pendant tertiary phosphane arm are reported. These compounds were obtained by one-electron oxidation of d1 precursors with [Cp2 Fe][BPh4 ]. The electronic structure of these compounds was studied experimentally (EPR, UV/Vis, and NMR spectroscopy, X-ray diffraction analysis) and through DFT calculations. The theoretical analysis of the bonding situation by using the electron localization function (ELF) shows the presence of π-interactions between the phosphido ligand and Ti in the d0 complexes, whereas dπ-pπ repulsion prevents such interactions in the d1 complexes. In addition, CH-π interactions were observed in several complexes, both in solution and in the solid state, between the phosphido ligand and the phosphane arm. The d0 complexes were found to be light sensitive, and decompose through Ti-P bond homolysis to give TiIII species. A naked d0 phosphidotitanocene cation has been trapped by reaction with diphenylacetylene, yielding a Ti/P frustrated Lewis pair (FLP), which was found to be less reactive than a previously reported Zr analog.

Direct P-functionalization of azobenzene by a cationic phosphidozirconocene complex.

We report that the cationic phosphidozirconocene complex [(η(5)-C5H5)2Zr(PCy2)][CH3B(C6F5)3] (II) reacts with azobenzene, resulting in the expedient formation of Zr complex (2) bound to a tridentate PNN ligand. This reaction proceeds by a mechanism of cooperative nucleophilic substitution of hydrogen. The intermediate σ(H) adduct (1) has been characterized by NMR spectroscopy.

Insertion Reactions of Neutral Phosphidozirconocene Complexes as a Convenient Entry into Frustrated Lewis Pair Territory.

Neutral phosphidozirconocene complexes [Cp2Zr(PR2)Me] (Cp=cyclopentadienyl; 1a: R=cyclohexyl (Cy); 1b: R=mesityl (Mes); 1c: R=tBu) undergo insertion into the Zr-P bond by non-enolisable carbonyl building blocks (O=CR'R''), such as benzophenone, aldehydes, paraformaldehyde or CO2, to give [Cp2Zr(OCR'R''PR2)Me] (3-7). Depending on the steric bulk around P, complexes 3-7 react with B(C6F5)3 to give O-bridged cationic zirconocene dimers that display typical frustrated Lewis pair (FLP)/ambiphilic ligand behaviour. Thus, the reaction of {[Cp2Zr(µ-OCHPhPCy2)][MeB(C6F5)3]}2 (10a) with chalcone results in 1,4 addition of the Zr(+)/P FLP, whereas the reaction of {[Cp2Zr(µ-OCHFcPCy2)][MeB(C6F5)3]}2 (11a; Fc=(C5H4)CpFe) with [Pd(η(3)C3H5)Cl]2 yields the unique Zr-Fe-Pd trimetallic complex 13a, which has been characterised by XRD analysis.

Phosphido- and Amidozirconocene Cation-Based Frustrated Lewis Pair Chemistry.

Methyl abstraction from neutral [Cp2ZrMe(ERR')] complexes 1 (E = N, P; R, R' = alkyl, aryl) with either B(C6F5)3 or [Ph3C][B(C6F5)4] results in the formation of [Cp2Zr(ERR')][X] complexes 2 (X(-) = MeB(C6F5)3(-), B(C6F5)4(-)). The X-ray structure of amido complexes [Cp2Zr(NPh2)][MeB(C6F5)3] (2d) and [Cp2Zr(N(t)BuAr)][B(C6F5)4] (2e', Ar = 3,5-C6H3(CH3)2) is reported, showing a sterically dependent Zr/N-π interaction. Complexes 2 catalyze the hydrogenation of electron-rich olefins and alkynes under mild conditions (room temperature, 1.5 bar H2). Complex 2e binds CO2, giving [Cp2Zr(CO2)(N(t)BuAr)]2[MeB(C6F5)3]2 (3e). Amido complex 2d reacts with benzaldehyde yielding [Cp2Zr(OCH2Ph)((OC)PhNPh2)][MeB(C6F5)3] (7d). Phosphido complex [Cp2Zr(PCy2)][MeB(C6F5)3] (2a) reacts with diphenylacetylene to yield frustrated Lewis pair [Cp2Zr(PhCCPh)(PCy2)][MeB(C6F5)3] (8a) which further reacts with a range of carbonyl substrates.

Titanium imido complexes stabilised by bis(iminophosphoranyl)methanide ligands: the influence of N-substituents on solution dynamics and reactivity.

Terminal titanium imido complexes of the general formula [Ti(N(t)Bu)Cl{CH(Ph2PNR)2}] 4 (R = Ph, (i)Pr, (t)Bu) are reported. These compounds were synthesized from the corresponding Li adducts 3 of BIPMH (bis(iminophosphoranyl)methanide) and Mountford's complex [Ti(N(t)Bu)Cl2(Py)3]. The crystal structures of two of the Ti complexes (R = Ph, (t)Bu) and two of the Li compounds (R = (i)Pr, (t)Bu) are reported. Dynamic solution NMR spectroscopy reveals a dynamic isomerisation process in the case of the Ti complex 4c (R = (t)Bu). DFT studies showed that this dynamic process comes from steric repulsion between the imido ligand and the (t)Bu N-substituents on the BIPMH ligand. Complexes 4 were tested in alkyne hydroamination; 4a (R = Ph) displayed modest catalytic activity in the reaction of aniline with phenylacetylene.

Mechanisms in the reaction of palladium(II)-pi-allyl complexes with aryl halides: evidence for NHC exchange between two palladium complexes.

A detailed experimental and DFT study (PBE level) of the reaction of [Pd(eta(3)-C(3)H(5))(tmiy)(PR(3))]BF(4) (tmiy = tetramethylimidazolin-2-ylidene, PR(3) = phosphane), precursors to monoligated Pd(0) species, with aryl electrophiles yielding 2-arylimidazolium salt is reported. Experiments establish that an autocatalytic ligand transfer mechanism is preferred over Pd(IV) and sigma-bond metathesis pathways, and that transmetalation is the rate-determining step. Calculations indicate that the key step involves the concerted exchange of NHC and iodo ligands between two different Pd(II) complexes. This is corroborated by experimental results showing the slower reaction of complexes containing the bulkier dipdmiy (dipdmiy = diisopropyldimethylimidazolin-2-ylidene).

N-heterocyclic carbenes bearing two, one and no nitrogen atoms at the ylidene carbon: insight from theoretical calculations.

Electronic structure, thermodynamic stability and ligand properties in LRh(CO)2Cl complexes of a series of N-heterocyclic carbenes (NHCs) were studied at the DFT level. The systems under study are: imidazolin-2-ylidene (1), imidazolidin-2-ylidene (2), cyclic(alkyl)(amino)carbene (CAAC, 3), pyrazolin-3-ylidene (4), pyridin-2-ylidene (5), and pyridin-4-ylidene (6). The main structural feature influencing the properties of these species is the number of nitrogen atoms at the ylidene carbon. A decrease of the number of nitrogen atoms on the one hand leads to an increase in donor ability and ligand-to-metal bond strength, but lowers the stability of the NHC on the other hand. The number of nitrogen atoms can be taken as a key parameter for the classification of carbenes into 2N-NHC, 1N-NHC and r-NHC (r = remote).