T-Shaped Palladium and Platinum {MNO}10 Nitrosyl Complexes.

The synthesis and characterization of a homologous series of T-shaped {MNO}10 nitrosyl complexes of the form [M(PR3)2(NO)]+ (M = Pd, Pt; R = tBu, 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 15N NMR spectroscopy. Effective oxidation states of M0/NO+ are calculated, and metal-nitrosyl bonding has been interrogated using DFT-based energy decomposition analysis techniques. While a linear nitrosyl coordination mode was found to be electronically preferred, the M-NO and P-M-P angles are inversely correlated to the extent that binding in this manner is prevented by steric repulsion between the bulky ancillary phosphine ligands.

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 B2pin2 as the reductant is reported. These reactions proceed under mild conditions via copper(I)-boryl intermediates which react with N2O by facile O-atom insertion into the Cu-B bond and liberate N2. Turnover numbers >800 can be achieved at 80 °C under 1 bar N2O.

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 d9 metalloradicals of the form [M(PR3)2]+ (M = Pd, Pt; R = tBu, Ad), which are stable in 1,2-difluorobenzene (DFB) solution for >1 day at room temperature when partnered with the weakly coordinating [BArF4]- (ArF = 3,5-(CF3)2C6H3) counterion. The metalloradicals exhibit reduced stability in THF, decreasing in the order palladium(I) > platinum(I) and PAd3 > PtBu3, especially in the case of [Pt(PtBu3)2]+, which is converted into a 1:1 mixture of the platinum(II) complexes [Pt(PtBu2CMe2CH2)(PtBu3)]+ and [Pt(PtBu3)2H]+ upon dissolution at room temperature. Cyclometalation of [Pt(PtBu3)2]+ can also be induced by reaction with the 2,4,6-tri-tert-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(PtBu2CMe2CH2)H(PtBu3)]+, has been substantiated by computational analysis. Radical C-H bond oxidative addition is correlated with the resulting MII-H bond dissociation energy (M = Pt > Pd), and reactions of the metalloradicals with 9,10-dihydroanthracene in DFB at room temperature provide experimental evidence for the proposed C-H bond activation manifold in the case of platinum, although conversion into platinum(II) hydride derivatives is considerably faster for [Pt(PtBu3)2]+ (t1/2 = 1.2 h) than [Pt(PAd3)2]+ (t1/2 ∼ 40 days).

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.

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)(C2 H4 )][BArF 4 ] (n=12, 14, 16; ArF =3,5-(CF3 )2 C6 H3 )-have been investigated, using the bulky alkynes HC≡CtBu and HC≡CAr' (Ar'=3,5-tBu2 C6 H3 ) 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. The intermediates are formed with orthogonal regioselectivity, with E-alkenyl complexes derived from HC≡CtBu and gem-alkenyl complexes derived from HC≡CAr', and the reductive elimination step is appreciably affected by the ring size of the macrocycle. For the homocoupling of HC≡CtBu, E-tBuC≡CCH=CHtBu is produced via direct reductive elimination from the corresponding rhodium(III) alkynyl E-alkenyl derivatives with increasing efficacy as the ring is expanded. In contrast, direct reductive elimination of Ar'C≡CC(=CH2 )Ar' is encumbered relative to head-to-head coupling of HC≡CAr' and it is only with the largest macrocyclic ligand studied that the two processes are competitive. These results showcase how macrocyclic ligands can be used to interrogate the mechanism and tune the outcome of terminal alkyne coupling reactions, and are discussed with reference to catalytic reactions mediated by the acyclic homologue [Rh(CNC-Me)(C2 H4 )][BArF 4 ] and solvent effects.

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

The synthesis and organometallic chemistry of rhodium(I) complex [Rh(CNC-Me)(SOMe2)][BArF 4], 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)(OSMe2)][BArF 4] and [Rh(CNC-Me)(Ph)Cl(OSMe2)][BArF 4], 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. Under equivalent conditions the substrates are not activated by the phosphine-based pincer complex [Rh(PNP-iPr)(SOMe2)][BArF 4].

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.

Rhodium(III) Complexes Featuring Coordinated CF3 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 2.36-2.45 Å, and in solution using NMR spectroscopy, through hindered C-CF3 bond rotation and the presence of time-averaged 1 JRhF and 2 JPF coupling.

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-( tBu2PCH2)2C5H3N) and PONOP (2,6-( tBu2PO)2C5H3N) pincer ligands are described: [M(PNP)(biph)][BArF4] (M = Rh, 1a; Ir, 1b; biph = 2,2'-biphenyl; ArF = 3,5-(CF3)2C6H3) and [M(PONOP)(biph)][BArF4] (M = Rh, 2a; Ir, 2b). These complexes are structurally dynamic in solution, exhibiting pseudorotation of the biph ligand on the 1H NMR time scale (Δ G⧧ ca. 60 kJ mol-1) and, in the case of the flexible PNP complexes, undergoing interconversion between helical and puckered pincer ligand conformations (Δ G⧧ ca. 10 kJ mol-1). Remarkably, the latter is sufficiently facile that it persists in the solid state, leading to temperature-dependent disorder in the associated X-ray crystal structures. Reaction of 1 and 2 with CO occurs for the iridium congeners 1b and 2b, leading to the formation of sterically congested carbonyl derivatives.

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 mer-tridentate "pincer" ligands, the spectroscopic properties [ν(CO), δ 13C, 1 J RhC] of rhodium(I) and rhodium(III) carbonyl complexes of the form [Rh(pincer)(CO)][BArF 4] and [Rh(pincer)Cl2(CO)][BArF 4] 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. To gain deeper insight, a DFT-based energy decomposition analysis was performed and identified important bonding differences associated with the conformation of the pincer backbone, which clouds straightforward interpretation of the experimental IR data. A correlation between the difference in carbonyl stretching frequencies Δν(CO) and calculated thermodynamics of the RhI/RhIII redox pairs was identified and could prove to be a useful mechanistic tool.

Rhodium(I) Pincer Complexes of Nitrous Oxide.

The synthesis of two well-defined rhodium(I) complexes of nitrous oxide (N2 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 15 N NMR and IR spectroscopy, and single-crystal X-ray diffraction. These methods evidence coordination of N2 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-N2 O interaction.

Solution, Solid-State, and Computational Analysis of Agostic Interactions in a Coherent Set of Low-Coordinate Rhodium(III) and Iridium(III) Complexes.

A homologous family of low-coordinate complexes of the formulation trans-[M(2,2'-biphenyl)(PR3 )2 ][BArF4 ] (M=Rh, Ir; R=Ph, Cy, iPr, iBu) has been prepared and extensively structurally characterised. Enabled through a comprehensive set of solution phase (VT 1 H and 31 P NMR spectroscopy) and solid-state (single crystal X-ray diffraction) data, and analysis in silico (DFT-based NBO and QTAIM analysis), the structural features of the constituent agostic interactions have been systematically interrogated. The combined data substantiates the adoption of stronger agostic interactions for the IrIII compared to RhIII complexes and, with respect to the phosphine ligands, in the order PiBu3 >PCy3 >PiPr3 >PPh3 . In addition to these structure-property relationships, the effect of crystal packing on the agostic interactions was investigated in the tricyclohexylphosphine complexes. Compression of the associated cations, through inclusion of a more bulky solvent molecule (1,2-difluorobenzene vs. CH2 Cl2 ) in the lattice or collection of data at very low temperature (25 vs. 150 K), lead to small but statistically significant shortening of the M-H-C distances.

Terminal Alkyne Coupling Reactions through a Ring: Mechanistic Insights and Regiochemical Switching.

The mechanism and selectivity of terminal alkyne coupling reactions promoted by rhodium(I) complexes of NHC-based CNC pincer ligands have been investigated. Synthetic and kinetic experiments support E- and gem-enyne formation through a common reaction sequence involving hydrometallation and rate-determining C-C bond reductive elimination. The latter is significantly affected by the ligand topology: Employment of a macrocyclic variant enforced exclusive head-to-head coupling, contrasting the high selectivity for head-to-tail coupling observed for the corresponding acyclic pincer ligand.

Potassium Binding Adjacent to Cationic Transition-Metal Fragments: Unusual Heterobimetallic Adducts of a Calix[4]arene-Based Thione Ligand.

The synthesis of cationic rhodium and iridium complexes of a bis(imidazole-2-thione)-functionalized calix[4]arene ligand and their surprising capacity for potassium binding are described. In both cases, uptake of the alkali metal into the calix[4]arene cavity occurs despite adverse electrostatic interactions associated with close proximity to the transition-metal fragment [Rh+···K+ = 3.715(1) Å; Ir+···K+ = 3.690(1) Å]. The formation and constituent bonding of these unusual heterobimetallic adducts have been interrogated through extensive solution and solid-state characterization, examination of the host-guest chemistry of the ligand and its upper-rim unfunctionalized calix[4]arene analogue, and use of density functional theory based energy decomposition analysis.

C-F bond activation of perfluorinated arenes by a bioxazoline-derived N-heterocyclic carbene.

The N-heterocyclic carbene IBioxMe4 enacts selective single and double C-F bond activation of octafluorotoluene and hexafluorobenzene, respectively. The formation of the fluoroarene substituted, zwitterionic imidazoliumolate products is consistent with a mechanism involving nucleophilic aromatic substitution and subsequent oxazoline ring opening by liberated fluoride.

One-Electron Oxidation of [M(P(t) Bu3 )2 ] (M=Pd, Pt): Isolation of Monomeric [Pd(P(t) Bu3 )2 ](+) and Redox-Promoted C-H Bond Cyclometalation.

Oxidation of zero-valent phosphine complexes [M(P(t) Bu3 )2 ] (M=Pd, Pt) has been investigated in 1,2-difluorobenzene solution using cyclic voltammetry and subsequently using the ferrocenium cation as a chemical redox agent. In the case of palladium, a mononuclear paramagnetic Pd(I) derivative was readily isolated from solution and fully characterized (EPR, X-ray crystallography). While in situ electrochemical measurements are consistent with initial one-electron oxidation, the heavier congener undergoes C-H bond cyclometalation and ultimately affords the 14 valence-electron Pt(II) complex [Pt(κ(2) PC -P(t) Bu2 CMe2 CH2 )(P(t) Bu3 )](+) with concomitant formation of [Pt(P(t) Bu3 )2 H](+) .

Synthesis and reactivity of NHC-based rhodium macrocycles.

Using a general synthetic procedure employing readily accessed terminal alkene-functionalized pro-ligands and macrocyclization by ring-closing olefin metathesis, rhodium carbonyl complexes have been prepared that contain lutidine (1a; n = 1) and pyridine (1b; n = 0) derived tridentate CNC macrocycles with dodecamethylene spacers. In solution, 1a shows temperature-invariant time-averaged C2 symmetry by (1)H NMR spectroscopy (CD2Cl2, 500 MHz), whereas in the solid-state, two polymorphs can be obtained showing different conformations of the alkyl spacer about the metal-carbonyl bond (asymmetric and symmetric). In contrast, time-averaged motion of alkyl spacer in 1b can be halted by cooling below 225 K (CD2Cl2, 500 MHz), and the complex crystallizes as a dimer with an interesting unsupported Rh···Rh bonding interaction (3.2758(6) Å). Oxidative addition reactions of 1a and 1b, using MeI and PhICl2, have been studied in situ by (1)H NMR spectroscopy, although pure Rh(III) adducts can be ultimately isolated only with the pyridine-based macrocyclic ligand. The lutidine backbone of 1a can be deprotonated by addition of K[N(SiMe3)2], and the resulting neutral dearomatized complex (5) has been fully characterized in solution, by variable-temperature (1)H NMR spectroscopy, and in the solid state, by X-ray diffraction.

2,6-Bis[1-(2,4,6-tri-methyl-phenyl-imino)-eth-yl]pyridine.

In the title mol-ecule, C27H31N3, the imine C=N groups are orientated anti to the pyridine N atom, with N-C-C-N torsion angles of -164.91 (11) and -170.53 (10)°. In the crystal, mol-ecules are connected by weak C-H⋯N and C-H⋯π inter-actions parallel to the b axis.

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.

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 b/l ratio of 5.9 has been achieved at 60 °C under 1:1 H2/CO (20 bar).