Rhodium-Mediated Dehydrogenation of Hydroboranes and Group 14 Compounds: Base-Stabilized Silylene and Germylene Complexes vs. Transmetalation.

Monocarbonyl rhodium complex LRh(CO), 1, which is stabilized by a pyrrole-based bis(phosphinimine) pincer ligand (L=κ3 -NNN'=2,5-[i Pr2 P=N(4-i PrC6 H4 )]2 -N'(C4 H2 )- ), serves as a versatile platform for the dehydrogenation of group 14 substrates. Reaction with primary and secondary silanes and germanes (MesSiH3 , Et2 SiH2 , Ph2 GeH2 , t BuGeH3 ; Mes=mesityl) liberates H2 and yields base-stabilized tetrylene compounds of the form κ2 -L(CO)Rh(ER2 ) (E=Si: R=Mes, H, 2; R=Et, 5; E=Ge: R=Ph, 6; R=t Bu, H, 8). The ":ER2 " fragment in these species bridges between the rhodium center and a phosphinimine donor. Preliminary reactions between pinacol (Pin) and κ2 -L(CO)Rh(ER2 ), E=Si, Ge, indicate that such complexes can serve as silylene and germylene synthons, releasing :ER2 and catalytically generating PinER2 . In contrast, combination of complex 1 and MesGeH3 does not yield the anticipated dehydrogenation product, but rather, transmetalation similar to that observed upon reaction between 1 and 3,5-dimethylphenylborane prevails.

X-ray crystal structure of [L 2Ag3]+[OTf]-·5C6D6: a monoanionic bis-phosphinimine ligand supported tris-ilver complex.

The compound bis-{µ3-2,5-bis-[N-(4-iso-propyl-phen-yl)-P,P-di-phenyl-phospho-r-im-i-do-yl]pyrrol-1-ido-κ3 N:N':N''}tris-ilver(I) tri-fluoro-methane-sulfonate deuterated benzene penta-solvate, [Ag3(C46H44N3P2)2](CF3O3S)·5C6D6, (I), was synthesized from two equivalents of NaL [L = 2,5-(4- i PrC6H4N=PPh2)C4H2N] and three equivalents of AgOTf (OTf = OSO2CF3). High-quality crystals of (I) formed with five deuterated benzene solvent mol-ecules in the asymmetric unit. Notably, the bond lengths and angles between the three silver atoms are markedly different, unlike in most other reported tris-ilver complexes that tend to form three equivalent metal centers. Additionally, to the best of our knowledge, this is the first report of a tris-ilver complex with six nitro-gen donors from two bis-phosphinimine ligands.

Reversible dehydrogenation of a primary aryl borane.

The consecutive activation of B-H bonds in mesitylborane (H2BMes; Mes = 2,4,6-(CH3)3C6H2) by a 16-electron rhodium(i) monocarbonyl complex, (iPrNNN)Rh(CO) (1-CO; iPrNNN = 2,5-[iPr2P[double bond, length as m-dash]N(4-iPrC6H4)]2N(C4H2)-) is described. Dehydrogenative extrusion of the {BMes} fragment led to the isolation of (iPrNNN)(CO)RhBMes (1-BMes). Addition of H2 gas to 1-BMes regenerated 1-CO and H2BMes, highlighting the ability of 1-CO to facilitate interconversion of {BMes} with dihydrogen. Reactivity studies revealed that 1-BMes promotes formal group transfer and that {BAr} fragments accessed by dehydrogenation are reactive entities.

Di-chlorido-bis-[2-(pyridin-2-yl-κN)-1H-benzimidazole-κN 3]nickel(II) monohydrate.

In the title complex, [NiCl2(C12H9N3)2]·H2O, a divalent nickel atom is coordinated by two 2-(pyridin-2-yl)-1H-benzimidazole ligands in a slightly distorted octa-hedral environment defined by four N donors of two N,N'-chelating ligands, along with two cis-oriented anionic chloride donors. The title complex crystallized with a water mol-ecule disordered over two positions. In the crystal, a combination of O-H⋯Cl, O-H.·O and N-H⋯Cl hydrogen bonds, together with C-H⋯O, C-H⋯Cl and C-H⋯π inter-actions, links the complex mol-ecules and the water mol-ecules to form a supra-molecular three-dimensional framework. The title complex is isostructural with the cobalt(II) dichloride complex reported previously [Das et al. (2011 ▸). Org. Biomol. Chem. 9, 7097-7107].

Erratum: Di-chlorido-bis-[2-(pyridin-2-yl-κN)-1H-benzimidazole-κN 3]nickel(II) monohydrate. Corrigendum.

[This corrects the article DOI: 10.1107/S2414314620000401.].

Consecutive N2 loss from a uranium diphosphazide complex.

A new 'diphosphazidosalen' ligand was synthesized and successfully transferred to uranium using salt metathesis strategies. The resultant 8-coordinate uranium(iv) diphosphazide complex [κ6-1,2-{(N3)PPh2(2-O-C6H4)}2C6H4]UCl2 (1) is unstable to consecutive N2 loss, affording the asymmetric species [κ5-1-{(N3)PPh2(2-O-C6H4)}-2-{N=PPh2(2-O-C6H4)}C6H4)]UCl2 (2), defined by a phosphazide-phosphinimine mixed-ligand framework, and ultimately, the uranium(iv) phosphasalen complex [κ4-1,2-{N=PPh2(2-O-C6H4)}2C6H4]UCl2(THF) (3).

An H-Substituted Rhodium Silylene.

Divergent reactivity of organometallic rhodium(I) complexes, which led to the isolation of neutral rhodium silylenes, is described. Addition of PhRSiH2 (R=H, Ph) to the rhodium cyclooctene complex (iPr NNN)Rh(COE) (1-COE; iPr NNN=2,5-[iPr2 P=N(4-iPrC6 H4 )]2 N(C6 H2 )- , COE=cyclooctene) resulted in the oxidative addition of an Si-H bond, providing rhodium(III) silyl hydride complexes (iPr NNN)Rh(H)SiHRPh (R=H, 2-SiH2 Ph; Ph, 2-SiHPh2 ). When the carbonyl complex (iPr NNN)Rh(CO) (1-CO) was treated with hydrosilanes, base-stabilized rhodium(I) silylenes κ2 -N,N-(iPr NNN)(CO)Rh=SiRPh (R=H, 3-SiHPh; Ph, 3-SiPh2 ) were isolated and characterized using multinuclear NMR spectroscopy and X-ray crystallography. Both silylene species feature short Rh-Si bonds [2.262(1) Å, 3-SiHPh; 2.2702(7) Å, 3-SiPh2 ] that agree well with the DFT-computed structures. The overall reaction led to a change in the iPr NNN ligand bonding mode (κ3 →κ2 ) and loss of H2 from PhSiRH2 , as corroborated by deuterium labelling experiments.

Crystal structure of a dimeric ß-diketiminate magnesium complex.

The solid-state structure of a dimeric ß-diketiminate magnesium(II) complex is discussed. The compound, di-µ-iodido-bis-[(-{4-amino-1,5-bis-[2,6-bis-(propan-2-yl)phen-yl]pent-3-en-2-yl-idene}aza-nido-κ2N,N')magnesium(II)] toluene sesquisolvate, [Mg2(C29H41N2)2I2]·1.5C7H8, crystallizes as two independent mol-ecules, each with 2/m crystallographic site symmetry, located at Wyckoff sites 2c and 2d. These have symmetry-equivalent magnesium atoms bridged by µ-iodide ligands with very similar Mg-I distances. The two Mg atoms are located slightly below (∼0.5 Å) the least-squares plane defined by N-C-C-N atoms in the ligand scaffold, and are approximately tetra-hedrally coordinated. One and one-half toluene solvent mol-ecules are disordered with respect to mirror-site symmetry at Wyckoff sites 4i and 2a, respectively. In the former case, two toluene mol-ecules inter-act in an off-center parallel stacking arrangement; the shortest C to C' (π-π) distance of 3.72 (1) Šwas measured for this inter-action.

Elucidation of the resting state of a rhodium NNN-pincer hydrogenation catalyst that features a remarkably upfield hydride (1)H NMR chemical shift.

Rhodium(I) alkene complexes of an NNN-pincer ligand catalyze the hydrogenation of alkenes, including ethylene. The terminal or resting state of the catalyst, which exhibits an unprecedentedly upfield Rh-hydride (1)H NMR chemical shift, has been isolated and a synthetic cycle for regenerating the catalytically active species has been established.

Differences in the cyclometalation reactivity of bisphosphinimine-supported organo-rare earth complexes.

The pyrrole-based ligand N,N'-((1H-pyrrole-2,5-diyl)bis(diphenylphosphoranylylidene))bis(4-isopropylaniline) (HL(B)) can be deprotonated and coordinated to yttrium and samarium ions upon reaction with their respective trialkyl precursors. In the case of yttrium, the resulting complex [L(B)Y(CH2SiMe3)2] (1) is a Lewis base-free monomer that is remarkably resistant to cyclometalation. Conversely, the analogous samarium complex [L(B)Sm(CH2SiMe3)2] is dramatically more reactive and undergoes rapid orthometalation of one phosphinimine aryl substituent, generating an unusual 4-membered azasamaracyclic THF adduct [κ(4)-L(B)Sm(CH2SiMe3)(THF)2] (2). This species undergoes further transformation in solution to generate a new dinuclear species that features unique carbon and nitrogen bridging units [κ(1):κ(2):µ(2)-L(B)Sm(THF)]2 (3). Alternatively, if 2 is intercepted by a second equivalent of HL(B), the doubly-ligated samarium complex [(κ(4)-L(B))L(B)Sm] (4) forms.

Secondary diphosphine and diphosphido ligands: synthesis, characterisation and group 1 coordination compounds.

Two types of secondary diphosphines, 1,8-(ArPH)2C14H8 (1a: Ar = Tripp, 2,4,6-triisopropylphenyl; 1b: Ar = Mes, 2,4,6-trimethylphenyl) and 1,3-((t)BuPHCH2)2C6H4 (2), based on rigid 1,8-anthracene and flexible m-xylyl frameworks, respectively, have been synthesized using different strategies. Compounds 1a and 1b were formed by nucleophilic aromatic substitution of a potassium organophosphido salt onto 1,8-difluoroanthracene, while compound 2 was obtained by addition of the Grignard reagent [1,3-(ClMgCH2)2C6H4]x to a dichloroorganophosphine, followed by reduction to the diphosphine. These compounds were isolated as ca. 1 : 1 mixtures of rac and meso diastereomers as determined by multinuclear NMR spectroscopy. Borane and selenide derivatives of 2, 1,3-((t)BuPH(BH3)CH2)2C6H4 (3) and 1,3-((t)BuPH(Se)CH2)2C6H4 (4), were obtained. Preferential crystallization of one diastereomer of 3 and 4 was observed; X-ray crystallographic studies identified this as the rac isomer for diselenide 4. Metallation studies of compounds 1a and 2 yielded several alkali metal salts. The reaction of KH or K metal with 1a yielded the compounds 1,8-(TrippPK)2C14H8·xTHF (5) and 1,8-(TrippPK)2C14H10·xTHF (6), respectively; in complex 6 the central aromatic ring has been reduced to yield a bent dihydroanthracene backbone. A crown ether derivative of 6, [K(18-crown-6)(THF)2]2[1,8-(TrippP)2C14H10] (7), was characterised crystallographically. Double deprotonation of compound 2 with (n)BuLi/TMEDA (TMEDA = N,N,N',N'-tetramethylethylenediamine) afforded the yellow dilithium complex 1,3-((t)BuPLiCH2)2C6H4·TMEDA (8), which crystallized as a dimer featuring lithium-arene π interactions.

A cascade reaction: ring-opening insertion of dioxaphospholane into lutetium alkyl bonds.

Geometrically constrained dioxaphospholane rings were incorporated into a bis(phosphinimine)carbazole ligand (HL) in an effort to generate an ancillary ligand system that is capable of supporting reactive lutetium alkyl functionalities and resistant to cyclometalation reactivity. This new ligand was used to prepare a lutetium dialkyl species, LLu(CH2SiMe3)2; however, the complex exhibited low thermal stability at ambient temperature. This dialkyl compound was found to be highly susceptible to a cascading inter- and intramolecular reaction that resulted in the sole formation of an asymmetric bimetallic tetraalkoxide complex. The product of this reaction, generated by the ring-opening insertion of dioxaphospholane moieties into lutetium-carbon bonds, was characterized by multinuclear NMR spectroscopy and single crystal X-ray diffraction.

The osmium-silicon triple bond: synthesis, characterization, and reactivity of an osmium silylyne complex.

The first silylyne complex of a metal beyond group 6, [Cp*((i)Pr3P)(H)Os≡Si(Trip)][HB(C6F5)3], was prepared by a new synthetic route involving hydride abstraction from silicon. NMR and DFT computations support the presence of a silylyne ligand, and NBO and ETS-NOCV analysis revealed the nature of this Os-Si interaction as a triple bond consisting of a covalent σ bond and two strong π back-donations. The discovery of this complex allowed observations of the first cycloadditions involving a silylyne complex, and terminal alkynes are shown to react via C-H bond additions across the Os≡Si bond.

Cyclometalative C-H bond activation in rare earth and actinide metal complexes.

Cyclometalative C-H bond activation is a process that is commonly encountered in the field of organometallic chemistry. In rare earth and actinide complexes, ligand cyclometalation is most prevalent in highly reactive alkyl and hydrido species. Numerous factors promote ligand cyclometalation and influence the rate at which it occurs. This tutorial review discusses key issues relevant to ligand cyclometalation in rare earth and actinide complexes, including kinetic and mechanistic considerations. A variety of examples is presented for a wide range of ligand types and metals, the scope of which is intended to include routine cases, while also highlighting exceptional cyclometalation reactions that lead to unusual bonding modes. The reaction chemistry of cyclometalated rare earth and actinide complexes with various small molecule substrates (e.g. phenol, anilines, triethylammonium salts, alkynes, olefins, hydrogen and hydrocarbons) is also outlined.

Thermally stable rare earth dialkyl complexes supported by a novel bis(phosphinimine)pyrrole ligand.

A novel bis(phosphinimine)pyrrole based ligand (HL) and its synthesis are reported. Rare earth dialkyl complexes of the ligand, LLn(CH(2)SiMe(3))(2) (Ln = Er, Lu, Sc), have been prepared and found to exhibit high thermal stability in solution. The protio-ligand and dialkyl lanthanide complexes (Ln = Er, Lu) have been characterised by single crystal X-ray diffraction studies.

Ring-opening polymerisation of rac-lactide mediated by cationic zinc complexes featuring P-stereogenic bisphosphinimine ligands.

The diastereomerically pure P-stereogenic bis(phosphinimine) ligands 4,6-(ArN[double bond, length as m-dash]PMePh)(2)dbf [Ar = 4-isopropylphenyl (Pipp): rac-4, meso-4; Ar = 2,6-diisopropylphenyl (Dipp): rac-4a; dbf = dibenzofuran] were synthesised and complexed to zinc using a protonation-alkane elimination strategy. The cationic alkylzinc complexes thus obtained, RZn[4,6-(ArN[double bond, length as m-dash]PMePh)(2)dbf][B(Ar')(4)] [Ar = Pipp, Ar' = C(6)H(3)(CF(3))(2): rac-6 (R = Et), meso-6 (R = Et), rac-7 (R = Me) meso-7 (R = Me); Ar = Dipp: rac-6a (R = Et, Ar' = C(6)H(3)(CF(3))(2)), rac-6b (R = Et, Ar' = C(6)F(5))] were investigated for their competency as initiators for the ring-opening polymerisation of rac-lactide. The formation of polylactide was achieved under relatively mild conditions (40 °C, 2-4 h) and the microstructures of the resulting polymers exhibited a slight heterotactic bias [polymer tacticity (P(r)) = 0.51-0.63].

Toward stereoselective lactide polymerization catalysts: cationic zinc complexes supported by a chiral phosphinimine scaffold.

The P-stereogenic phosphinimine ligands (dbf)MePhP═NAr (7: Ar = Dipp; 8: Ar = Mes; dbf = dibenzofuran, Dipp = 2,6-diisopropylphenyl, Mes = 2,4,6-trimethylphenyl) were synthesized as racemates via reactions of the parent phosphines (rac)-(dbf)MePhP (6) with organoazides. The ligands 7 and 8 were protonated by Brønsted acids to afford the aminophosphonium borate salts [(7)-H][BAr(4)] (9: Ar = C(6)F(5); 11: Ar = Ph) and [(8)-H][BAr(4)] (10: Ar = C(6)F(5); 12: Ar = Ph). The protonated ligands 9 and 10 were active toward alkane elimination reactions with diethylzinc and ethyl-[methyl-(S)-lactate]zinc to give the heteroleptic complexes [{(dbf)MePhP═NAr}ZnR][B(C(6)F(5))(4)] (Ar = Dipp, 13: R = Et; 15: R = methyl-(S)-lactate; Ar = Mes, 14: R = Et; 16: R = methyl-(S)-lactate). By contrast, reaction of the tetraphenylborate derivative 11 with diethylzinc yielded a phenyl transfer product, [(dbf)MePhP═NDipp]ZnPh(2) (17). Complex 15 was found to catalyze the ring-opening polymerization of rac-lactide.

Cationic zinc complexes: a new class of catalyst for living lactide polymerization at ambient temperature.

Cationic zinc complexes of a bis(phosphinimine) pincer ligand have been prepared. Methylzinc and zinc-lactate complexes have been structurally characterized, and the latter is the first cationic metal complex to promote coordination-insertion polymerization of lactide at ambient temperature. This novel catalyst system is remarkably active and also exhibits living character. A detailed investigation of the kinetics and mechanism of the polymerization process has been undertaken.

Cationic organozinc complexes of a bis(phosphinimine) pincer ligand: synthesis, structural and polymerization studies.

Cationic organozinc complexes of a neutral bis(phosphinimine) pincer ligand (L) have been prepared and structurally characterized. This recently introduced ligand was constructed from a dibenzofuran (dbf) framework with symmetric attachment of phosphinimine groups at the 4 and 6 positions. Starting from protonated derivatives [LH][B(C(6)F(5))(4)] (1a), [LH][BPh(4)] (1b), or [LH(2)][BPh(4)](2) (1c), the complexes [LZnCH(3)][B(C(6)F(5))(4)] (2a), [LZnCH(3)][BPh(4)] (2b), and [LZnOAc][BPh(4)] (3), were prepared via protonolysis of an appropriate alkylzinc precursor. The complex [LZnPh][BPh(4)] (4) is generated as a side-product in the synthesis of 2b. Solid-state structural studies have revealed the compounds to be charge separated cationic zinc species with 3-coordinate trigonal planar geometry. Preliminary studies have shown these complexes to be inactive for the polymerization of lactide. Upon modification of the initiating group to a methyl-(S)-lactate, however, complex [LZnOCH(Me)CO(2)Me][B(C(6)F(5))(4)] (5) demonstrated significant polymerization activity at 60 degrees C. Additionally, NMR and mass spectrometry data confirmed a coordination-insertion mechanism was operative for this catalyst.

Complexes of Mg, Ca and Zn as homogeneous catalysts for lactide polymerization.

Interest in the utility of polylactide as a commodity polymer has increased significantly in recent years due to numerous environmental advantages over conventional petrochemically derived plastics. As such, the development of novel catalyst systems for the ring opening polymerization of lactide has seen tremendous progress in the past decade. In particular, divalent metals (i.e. Mg, Ca and Zn) supported by monoanionic ancillary scaffolds are appealing because of their low toxicity and cost. A much less common approach involves the use of neutral ligands in combination with the aforementioned divalent metal centres. The additional valence thus renders it possible, upon reaction with traditional Lewis or Brønsted acid activators, to generate sterically and electronically unsaturated species, akin to the most widely employed olefin polymerization catalysts. This Perspective is not intended as a comprehensive review, but rather a systematic highlight of key contributions, which have served to extend the forefront of this exciting field.