Rhodium(III) and Iridium(III) Complexes of a NHC-Based Macrocycle: Persistent Weak Agostic Interactions and Reactions with Dihydrogen

Template Synthesis of Cyclometalated Macrocycle Iridium(III) Complexes Based on Photoinduced C-N Cross-Coupling Reactions In Situ

The synthesis of metal macrocycle complexes holds paramount importance in coordination and supramolecular chemistry. Toward this end, we report a new, mild, and efficient protocol for the synthesis of cyclometalated macrocycle Ir(III) complexes: [Ir(L1)](PF6) (1), [Ir(L2)](PF6) (2), and [Ir(L3)](PF6) (3), where L1 presents 10,17-dioxa-3,6-diaza-2(2,8),7(8,2)-diquinolina-1,8(1,4)-dibenzenacyclooctadecaphane, L2 is 10,13,16,19,22,25-hexaoxa-3,6-diaza-2(2,8),7(8,2)-diquinolina-1,8(1,4)-dibenzenacyclohexacosaphane, and L3 is 4-methyl-10,13,16,19,22,25-hexaoxa-3,6-diaza-2(2,8),7(8,2)-diquinolina-1,8(1,4)-dibenzenacyclohexacosaphane. This synthesis involves the preassembly of two symmetric 2-phenylquinoline arms into C-shape complexes, followed by cyclization with diamine via in situ interligand C-N cross-coupling, employing a metal ion as a template. Moreover, the synthetic yield of these cyclometalated Ir(III) complexes, tethered by an 18-crown-6 ether-like chain, is significantly enhanced in the presence of K+ ion as a template. The resultant cyclometalated macrocycle Ir(III) complexes exhibit high stability, efficient singlet oxygen generation, and superior catalytic activity for the aerobic selective oxidation of sulfides into sulfoxides under visible light irradiation in aqueous media at room temperature. The photocatalyst 2 demonstrates recyclability and can be reused at least 10 times without a significant loss of catalytic activity. These results unveil a new and complementary approach to the design and in situ synthesis of cyclometalated macrocycle Ir(III) complexes via a mild interligand-coupling strategy.

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)(CxP₂)(N₂)]⁺ are described, where CxP₂ is a trans-spanning calix[4]arene-based diphosphine and the dinitrogen ligand is projected into the cavity of the macrocycle.

Iridium complexes of an ortho-trifluoromethylphenyl substituted PONOP pincer ligand

The synthesis and iridium coordination chemistry of a new pyridine-based phosphinito pincer ligand 2,6-(Ar^F₂PO)₂C₅H₃N (PONOP-Ar^F; Ar^F = 2-(CF₃)C₆H₄) are described, where the P-donors have ortho-trifluoromethylphenyl substituents. The iridium(III) 2,2'-biphenyl (biph) derivative [Ir(PONOP-Ar^F)(biph)Cl] was obtained by reaction with [Ir(biph)(COD)Cl]₂ (COD = 1,5-cyclooctadiene) and subsequent halide ion abstraction enabled isolation of [Ir(PONOP-Ar^F)(biph)]⁺ which features an Ir ← F-C bonding interaction in the solid state. Hydrogenolysis of the biphenyl ligand and formation of [Ir(PONOP-Ar^F)(H)₂]⁺ was achieved by prolonged reaction of [Ir(PONOP-Ar^F)(biph)]⁺ with dihydrogen. This transformation paved the way for isolation and crystallographic characterisation of low valent iridium derivatives through treatment of the dihydride with tert-butylethylene (TBE). The iridium(I) π-complex [Ir(PONOP-Ar^F)(TBE)]⁺ is thermally stable but substitution of TBE can be achieved by reaction with carbon monoxide. The solid-state structure of the mono-carbonyl product [Ir(PONOP-Ar^F)(CO)]⁺ is notable for an intermolecular anagostic interaction between the metal centre and a pentane molecule which co-crystallises within a cleft defined by two aryl phosphine substituents.

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.

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. The five-coordinate iridium(i) and iridium(iii) complexes [Ir(PNP-14)(η2:η2-cyclooctadiene)][BArF4] and [Ir(PNP-14)(2,2'-biphenyl)][BArF4] are readily prepared and shown to be effective precursors for the generation of iridium(iii) dihydride dihydrogen, iridium(i) bis(ethylene), and iridium(i) carbonyl derivatives that highlight important periodic trends by comparison to rhodium counterparts. Reaction of [Ir(PNP-14)H2(H2)][BArF4] with 3,3-dimethylbutene induced triple C-H bond activation of the methylene chain, yielding an iridium(iii) allyl hydride derivative [Ir(PNP-14*)H][BArF4], whilst catalytic homocoupling of 3,3-dimethylbutyne into Z-tBuC[triple bond, length as m-dash]CCHCHtBu could be promoted at RT by [Ir(PNP-14)(η2:η2-cyclooctadiene)][BArF4] (TOFinitial = 28 h-1). The mechanism of the latter is proposed to involve formation and direct reaction of a vinylidene derivative with HC[triple bond, length as m-dash]CtBu outside of the macrocyclic ring and this suggestion is supported experimentally by isolation and crystallographic characterisation of a catalyst deactivation product.

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)(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].

Isolation and structural characterisation of rhodium(iii) η2-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 η²-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. This synthesis proceeds by a reaction sequence involving head-to-tail homocoupling of the terminal alkyne and zipper annulation of the resulting gem-enyne. The rhodium catalyst employed is notable for the incorporation of a flexible NHC-based pincer ligand, which is suggested to interconvert between mer- and fac-coordination modes to fulfil the orthogonal mechanistic demands of the two transformations. Evidence for this interesting auto-tandem action of the catalyst is provided by reactions of the precatalyst with model substrates, corroborating proposed intermediates in both component cycles, and norbornadiene, which reversibly captures the change in pincer ligand coordination mode, along with a DFT-based computational analysis.

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. The proligands are readily metalated and homologous series of M^I(CO) and M^IIICl₂(CO) derivatives (M = Rh, Ir) have been isolated and fully characterised in solution and the solid state. Metal hydride complexes are generated during the synthesis of the former and have been characterised in situ using NMR spectroscopy.

Macrocyclic Metal Complexes Bearing Rigid Polyaromatic Ligands: Synthesis and Catalytic Activity

We synthesised palladium and platinum complexes possessing cyclic and acyclic pincer-type polyaromatic ligands and investigated their structural effect on the catalysis. The pincer-type bis(6-arylpyridin-2-yl)benzene skeleton was constructed via Kröhnke pyridine synthesis under transition metal-free conditions on gram-scale quantity. Ligand structure significantly influenced catalytic activity toward the platinum-catalysed hydrosilylation of diphenyl acetylenes, despite the ligand-independence of the conformations and electronic properties of these complexes.

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^F₄] and [Rh(PONOP-14)(biph)][BAr^F₄] (Ar^F = 3,5-(CF₃)₂C₆H₃). Subsequent hydrogenolysis enabled generation of dihydrogen, ethylene and carbonyl derivatives; notably the ν(CO) bands of the carbonyl complexes provide a means to compare the donor properties of the new pincer ligands with established acyclic congeners.

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)][BAr^F₄] (M = Rh, 1a; Ir, 1b; biph = 2,2'-biphenyl; Ar^F = 3,5-(CF₃)₂C₆H₃) and [M(PONOP)(biph)][BAr^F₄] (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.

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 J_RhF and ² J_PF coupling.