A Resorcin[4]arene-Based Phosphite-Phosphine Ligand for the Branched-Selective Hydroformylation of Alkyl Alkenes.

Synthesis of a chelating phosphite-phosphine ligand from a tris(quinoxaline) extended resorcin[4]arene and its application in the rhodium-catalyzed hydroformylation of terminal alkyl alkenes are reported. Rhodium complexes are formed within the cavity of the macrocycle and branched-selective hydroformylation of 1-octene with a / ratio of 5.9 has been achieved at 60 °C under 1:1 H/CO (20 bar).

Copper(I) Catalysed Diboron(4) Reduction of Nitrous Oxide.

A process for the catalytic reduction of nitrous oxide using NHC-ligated copper(I) tert-butoxide precatalysts and Bpin as the reductant is reported. These reactions proceed under mild conditions via copper(I)-boryl intermediates which react with NO by facile O-atom insertion into the Cu-B bond and liberate N. Turnover numbers >800 can be achieved at 80 °C under 1 bar NO.

Capture of mechanically interlocked molecules by rhodium-mediated terminal alkyne dimerisation.

The transition metal-mediated dimerisation of terminal alkynes is an attractive and atom-economic method for preparing conjugated 1,3-enynes. Using a phosphine-based macrocyclic pincer ligand, we demonstrate how this transformation can be extended to the synthesis of novel, hydrocarbon-based interlocked molecules: a rotaxane by 'active' metal template synthesis and a catenane by sequential 'active' and 'passive' metal template procedures.

T-Shaped Palladium and Platinum {MNO} Nitrosyl Complexes.

The synthesis and characterization of a homologous series of T-shaped {MNO} nitrosyl complexes of the form [M(PR)(NO)] (M = Pd, Pt; R = Bu, Ad) are reported. These diamagnetic nitrosyls are obtained from monovalent or zerovalent precursors by treatment with NO and NO, respectively, and are notable for distinctly bent M-NO angles of ∼123° in the solid state. Adoption of this coordination mode in solution is also supported by the analysis of isotopically enriched samples by N NMR spectroscopy.

Stability and C-H Bond Activation Reactions of Palladium(I) and Platinum(I) Metalloradicals: Carbon-to-Metal H-Atom Transfer and an Organometallic Radical Rebound Mechanism.

One-electron oxidation of palladium(0) and platinum(0) bis(phosphine) complexes enables isolation of a homologous series of linear d metalloradicals of the form [M(PR)] (M = Pd, Pt; R = Bu, Ad), which are stable in 1,2-difluorobenzene (DFB) solution for >1 day at room temperature when partnered with the weakly coordinating [BAr] (Ar = 3,5-(CF)CH) counterion. The metalloradicals exhibit reduced stability in THF, decreasing in the order palladium(I) > platinum(I) and PAd > PBu, especially in the case of [Pt(PBu)], which is converted into a 1:1 mixture of the platinum(II) complexes [Pt(PBuCMeCH)(PBu)] and [Pt(PBu)H] upon dissolution at room temperature. Cyclometalation of [Pt(PBu)] can also be induced by reaction with the 2,4,6-tri-butylphenoxyl radical in DFB, and a common radical rebound mechanism involving carbon-to-metal H-atom transfer and formation of an intermediate platinum(III) hydride complex, [Pt(PBuCMeCH)H(PBu)], has been substantiated by computational analysis.

Template synthesis of an intermediate in silver salt metathesis using a calix[4]arene-based diphosphine ligand.

The synthesis and solid-state characterisation of the heterobimetallic rhodium(III)/silver(I) complex [Rh(2,2'-biphenyl)(CxP)Cl]⊃Ag is described, where CxP is a -spanning calix[4]arene-based diphosphine and the silver cation is datively bound to the chloride ligand within the cavity of the macrocycle.

Synthesis of a rhodium(III) dinitrogen complex using a calix[4]arene-based diphosphine ligand.

The synthesis and characterisation of the rhodium(III) dinitrogen complex [Rh(2,2'-biphenyl)(CxP)(N)] are described, where CxP is a -spanning calix[4]arene-based diphosphine and the dinitrogen ligand is projected into the cavity of the macrocycle.

Iridium complexes of an -trifluoromethylphenyl substituted PONOP pincer ligand.

The synthesis and iridium coordination chemistry of a new pyridine-based phosphinito pincer ligand 2,6-(ArPO)CHN (PONOP-Ar; Ar = 2-(CF)CH) are described, where the P-donors have -trifluoromethylphenyl substituents. The iridium(III) 2,2'-biphenyl (biph) derivative [Ir(PONOP-Ar)(biph)Cl] was obtained by reaction with [Ir(biph)(COD)Cl] (COD = 1,5-cyclooctadiene) and subsequent halide ion abstraction enabled isolation of [Ir(PONOP-Ar)(biph)] which features an Ir ← F-C bonding interaction in the solid state. Hydrogenolysis of the biphenyl ligand and formation of [Ir(PONOP-Ar)(H)] was achieved by prolonged reaction of [Ir(PONOP-Ar)(biph)] with dihydrogen.

Modulation of Metal Carbonyl Stretching Frequencies in the Second Coordination Sphere through the Internal Stark Effect.

Spectroscopic and computational examination of a homologous series of rhodium(I) pybox carbonyl complexes has revealed a correlation between the conformation of the flanking aryl-substituted oxazoline donors and the carbonyl stretching frequency. This relationship is also observed experimentally for octahedral rhodium(III) and ruthenium(II) variants and cannot be explained through the classical, Dewar-Chatt-Duncanson, interpretation of metal-carbonyl bonding. Instead, these findings are reconciled by local changes in the magnitude of the electric field that is projected along the metal-carbonyl vector: the internal Stark effect.

Heterolytic carbon-iodine bond cleavage by a palladium(I) metalloradical.

The well-defined Pd(I) metalloradical [Pd(PBu)] reacts with aryl and alkyl iodides at room temperature, yielding [Pd(PBu)(μ-I)] and phosphonium salts. Pd(II) aryl/alkyl derivates, reflecting net radical oxidative addition of the substrate to the metalloradical, are generated during the reaction and two examples have been isolated and crystallographically characterised.

Synthesis and reactivity of iridium complexes of a macrocyclic PNP pincer ligand.

Having recently reported on the synthesis and rhodium complexes of the novel macrocyclic pincer ligand PNP-14, which is derived from lutidine and features terminal phosphine donors trans-substituted with a tetradecamethylene linker (Dalton Trans., 2020, 49, 2077-2086 and Dalton Trans., 2020, 49, 16649-16652), we herein describe our findings critically examining the chemistry of iridium homologues.

Oxidative Addition of Biphenylene and Chlorobenzene to a Rh(CNC) Complex.

The synthesis and organometallic chemistry of rhodium(I) complex [Rh(CNC-Me)(SOMe)][BAr ], featuring NHC-based pincer and labile dimethyl sulfoxide ligands, is reported. This complex reacts with biphenylene and chlorobenzene to afford products resulting from selective C-C and C-Cl bond activation, [Rh(CNC-Me)(2,2'-biphenyl)(OSMe)][BAr ] and [Rh(CNC-Me)(Ph)Cl(OSMe)][BAr ], respectively. A detailed DFT-based computational analysis indicates that C-H bond oxidative addition of these substrates is kinetically competitive, but in all cases endergonic: contrasting the large thermodynamic driving force calculated for insertion of the metal into the C-C and C-Cl bonds, respectively.

Reactions of Rh(PNP) pincer complexes with terminal alkynes: homocoupling through a ring or not at all.

Through use of a bespoke macrocyclic variant, we demonstrate a novel approach for tuning the reactivity of rhodium PNP pincer complexes that enables formation of conjugated enynes from terminal alkynes, rather than vinylidene derivates. This concept is illustrated using tert-butylacetylene as the substrate and rationalised by a ring-induced switch in mechanism.

Oxidative Addition of a Mechanically Entrapped C(sp)-C(sp) Bond to a Rhodium(I) Pincer Complex.

By use of a macrocyclic phosphinite pincer ligand and bulky substrate substituents, we demonstrate how the mechanical bond can be leveraged to promote the oxidative addition of an interlocked 1,3-diyne to a rhodium(I) center. The resulting rhodium(III) bis(alkynyl) product can be trapped out by reaction with carbon monoxide or intercepted through irreversible reaction with dihydrogen, resulting in selective hydrogenolysis of the C-C σ-bond.

Terminal Alkyne Coupling Reactions Through a Ring: Effect of Ring Size on Rate and Regioselectivity.

Terminal alkyne coupling reactions promoted by rhodium(I) complexes of macrocyclic NHC-based pincer ligands-which feature dodecamethylene, tetradecamethylene or hexadecamethylene wingtip linkers viz. [Rh(CNC-n)(C H )][BAr ] (n=12, 14, 16; Ar =3,5-(CF ) C H )-have been investigated, using the bulky alkynes HC≡CtBu and HC≡CAr' (Ar'=3,5-tBu C H ) as substrates. These stoichiometric reactions proceed with formation of rhodium(III) alkynyl alkenyl derivatives and produce rhodium(I) complexes of conjugated 1,3-enynes by C-C bond reductive elimination through the annulus of the ancillary ligand.

Isolation and structural characterisation of rhodium(iii) η-fluoroarene complexes: experimental verification of predicted regioselectivity.

The isolation and solid-state characterisation of complexes featuring partially coordinated benzene, fluorobenzene and all three isomers of difluorobenzene are described. Supported by a DFT analysis, this well-defined homologous series demonstrates the preference for η-coordination of fluoroarenes via the HC[double bond, length as m-dash]CH sites adjacent to a fluorine substituent.

A shape changing tandem Rh(CNC) catalyst: preparation of bicyclo[4.2.0]octa-1,5,7-trienes from terminal aryl alkynes.

The preparation of a range of tetraaryl-substituted bicyclo[4.2.0]octa-1,5,7-trienes using a one-pot procedure starting from terminal aryl alkynes and catalysed by a rhodium(i) complex is reported.

Synthesis and group 9 complexes of macrocyclic PCP and POCOP pincer ligands.

The synthesis of macrocyclic variants of commonly employed phosphine-based pincer (pro)ligands derived from meta-xylene (PCP-14) and resorcinol (POCOP-14) is described, where the P-donors are trans-substituted with a tetradecamethylene linker. The former was accomplished using a seven-step asymmetric procedure involving (-)-cis-1-amino-2-indanol as a chiral auxiliary and ring-closing olefin metathesis. A related, but non-diastereoselective route was employed for the latter, which consequently necessitated chromatographic separation from the cis-substituted by-product.

Synthesis and rhodium complexes of macrocyclic PNP and PONOP pincer ligands.

The synthesis of macrocyclic variants of commonly employed phosphine-based pincer ligands derived from lutidine (PNP-14) and 2,6-dihydroxypyridine (PONOP-14) is described, where the P-donors are trans-substituted with a tetradecamethylene linker. This was accomplished using an eight-step procedure involving borane protection, ring-closing olefin metathesis, chromatographic separation from the cis-substituted diastereomers, and borane deprotection. The rhodium coordination chemistry of these ligands has been explored, aided by the facile synthesis of 2,2'-biphenyl (biph) adducts [Rh(PNP-14)(biph)][BAr] and [Rh(PONOP-14)(biph)][BAr] (Ar = 3,5-(CF)CH).

Probing the Donor Properties of Pincer Ligands Using Rhodium Carbonyl Fragments: An Experimental and Computational Case Study.

Metal carbonyls are commonly employed probes for quantifying the donor properties of monodentate ligands. With a view to extending this methodology to -tridentate "pincer" ligands, the spectroscopic properties [ν(CO), , ] of rhodium(I) and rhodium(III) carbonyl complexes of the form [Rh(pincer)(CO)][BAr ] and [Rh(pincer)Cl(CO)][BAr ] have been critically analysed for four pyridyl-based pincer ligands, with two flanking oxazoline (NNN), phosphine (PNP), or N-heterocyclic carbene (CNC) donors. Our investigations indicate that the carbonyl bands of the rhodium(I) complexes are the most diagnostic, with frequencies discernibly decreasing in the order NNN > PNP > CNC.

Rhodium(I) Pincer Complexes of Nitrous Oxide.

The synthesis of two well-defined rhodium(I) complexes of nitrous oxide (N O) is reported. These normally elusive adducts are stable in the solid state and persist in solution at ambient temperature, enabling comprehensive structural interrogation by N NMR and IR spectroscopy, and single-crystal X-ray diffraction. These methods evidence coordination of N O through the terminal nitrogen atom in a linear fashion and are supplemented by a computational energy decomposition analysis, which provides further insights into the nature of the Rh-N O interaction.

Synthesis and Structural Dynamics of Five-Coordinate Rh(III) and Ir(III) PNP and PONOP Pincer Complexes.

The synthesis and characterization of a homologous series of five-coordinate rhodium(III) and iridium(III) complexes of PNP (2,6-( tBuPCH)CHN) and PONOP (2,6-( tBuPO)CHN) pincer ligands are described: [M(PNP)(biph)][BAr] (M = Rh, 1a; Ir, 1b; biph = 2,2'-biphenyl; Ar = 3,5-(CF)CH) and [M(PONOP)(biph)][BAr] (M = Rh, 2a; Ir, 2b). These complexes are structurally dynamic in solution, exhibiting pseudorotation of the biph ligand on the H NMR time scale (Δ G ca. 60 kJ mol) and, in the case of the flexible PNP complexes, undergoing interconversion between helical and puckered pincer ligand conformations (Δ G ca.

Rhodium(III) Complexes Featuring Coordinated CF Appendages.

The synthesis and characterisation of a homologous series of rhodium 2,2'-biphenyl complexes featuring intramolecular dative bonding of the nominally inert and weakly coordinating trifluoromethyl group are described. Presence of these interactions is evidenced in the solid state using X-ray diffraction, with Rh-F contacts of 2.36-2.

A convenient method for the generation of {Rh(PNP)} and {Rh(PONOP)} fragments: reversible formation of vinylidene derivatives.

The substitution reactions of [Rh(COD)2][BArF4] with PNP and PONOP pincer ligands 2,6-bis(di-tert-butylphosphinomethyl)pyridine and 2,6-bis(di-tert-butylphosphinito)pyridine in the weakly coordinating solvent 1,2-F2C6H4 are shown to be an operationally simple method for the generation of reactive formally 14 VE rhodium(i) adducts in solution. Application of this methodology enables synthesis of known adducts of CO, N2, H2, previously unknown water complexes, and novel vinylidene derivatives [Rh(pincer)(CCHR)][BArF4] (R = tBu, 3,5-tBu2C6H3), through reversible reactions with terminal alkynes.