Synthesis of Complexes with Protic NH,NH-NHC Ligands via Oxidative Addition of 2-Halogenoazoles to Zero-Valent Transition Metals

Ru(II) Complexes with Protic- and Anionic-Naked-NHC Ligands for Cooperative Activation of Small Molecules

A set of ruthenium(II)-protic-N-heterocyclic carbene complexes, [Ru(NNCH )(PPh3 )2 (X)]Cl (1, X=Cl and 2, X=H) and their deprotonated forms [Ru(NNC)(PPh3 )2 (X)] (1', X=Cl and 2', X=H), in which NNC is a new unsymmetrical pincer ligand, are reported. The four complexes are interconvertible by simple acid-base chemistry. The combined theoretical and spectroscopic investigations indicate charge segregation in anionic-NHC complexes (1' and 2') and can be described from a Lewis pair perspective. The chemical reactivity of deprotonated complex 1' shows cooperative small molecule activation. Complex 1' activates H-H bond of hydrogen, C(sp3 )-I bond of iodomethane, and C(sp)-H bond of phenylacetylene. The activation of CO2 using anionic NHC complex 1' at moderate temperature and ambient pressure and subsequent conversion to formate is also described. All the new compounds have been characterized using ESI-MS, 1 H, 13 C, and 31 P NMR spectroscopy. Molecular structures of 1, 2, and 2' have also been determined with single-crystal X-ray diffraction. The cooperative small molecule activation perspective broadens the scope of potential applications of anionic-NHC complexes in small molecule activation, including the conversion of carbon dioxide to formate, a much sought after reaction in the renewable energy and sustainable development domains.

Synthesis of N-Heterocyclic Carbenes and Their Complexes by Chloronium Ion Abstraction from 2-Chloroazolium Salts Using Electron-Rich Phosphines

N-Heterocyclic carbenes (NHCs) are commonly prepared by deprotonation of azolium salts using strong anionic bases. This reaction is often unselective, yielding alkali metal NHC complexes or dimerized NHCs. Alternatively, free NHCs are obtained by the dechlorination of 2-chloroazolium salts using electron-rich phosphines. PPh3 , PCy3 , and PtBu3 are unsuitable for Cl+ abstraction, while the sterically encumbered tris(1,3-tert-butylimidazolidin-2-ylidenamino)phosphine 1 selectively removes Cl+ from 2-chloroazolium salts. Since bulky 1 does not bind to metal complexes, it was used for the preparation of NHC complexes via in situ Cl+ abstraction from 2-chloroazolium salts. The dechlorination was employed for the site-selective monometallation with IrI , IrIII , RhI , RhIII , and RuII of a bis-NHC precursor composed of a 2-chlorobenzimidazolium and a 2-chlorobenzimidazole group, followed by the preparation of the heterobimetallic IrIII /PdII complex [18](BF4 )2 by a dechlorination/oxidative addition reaction sequence.

Oxidative addition of a 8-bromotheobromine derivative to d 10 metals

Reaction of 8-bromo-7-ethyl-3-methylxanthine 1 with zerovalent group 10 metal complexes gave via an oxidative addition of the C–Br bond the neutral complexes trans-[2] (M = Pd) and trans-[3] (M = Pt) bearing a theobromine-derived azolato ligand. While the oxidative addition to [Pd0(PPh3)4] gave exclusively trans-[2], the reaction with the more substitution-inert [Pt0(PPh3)4] yielded after 1 day the kinetic product cis-[3], which was converted under heating for a total of 3 days completely into the thermodynamically more stable complex trans-[3]. Treatment of trans-[2], trans-[3] or the mixture of cis-/trans-[3] with the proton source HBF4⋅Et2O led to complexes trans-[4]BF4, trans-[5]BF4 and a mixture of cis/trans-[5]BF4 with retention of the original ratio of cis to trans, respectively. The molecular structures of the azolato complexes trans-[2] and trans-[3] and of the theobromine derived pNHC complexes trans-[4]BF4 and trans-[5]BF4 have been determined by X-ray diffraction studies.

Preparation of Complexes Bearing N-Alkylated, Anionic or Protic CAACs Through Oxidative Addition of 2-Halogenoindole Derivatives

CAAC precursors 2-chloro-3,3-dimethylindole 1 and 2-chloro-1-ethyl-3,3-dimethylindolium tetrafluoroborate 2BF₄ have been prepared and oxidatively added to [M(PPh₃ )₄ ] (M=Pd, Pt). Salt 2BF₄ reacts with [Pd(PPh₃ )₄ ] in toluene at 25 °C over 4 days to yield complex cis-[3]BF₄ featuring an N-ethyl substituted CAAC, two cis-arranged phosphines and a chloro ligand. Compound trans-[3]BF₄ was obtained from the same reaction at 80 °C over 1 day. Salt 2BF₄ reacts with [Pt(PPh₃ )₄ ] to give cis-[4]BF₄ . The neutral indole derivative 1 adds oxidatively to [Pt(PPh₃ )₄ ] to give trans-[5] featuring a CAAC ligand with an unsubstituted ring-nitrogen atom. This nitrogen atom has been protonated with py⋅HBF₄ to give trans-[6]BF₄ bearing a protic CAAC ligand. The Pd^II complex trans-[7]BF₄ bearing a protic CAAC ligand was obtained in a one-pot reaction from 1 and [Pd(PPh₃ )₄ ] in the presence of py⋅HBF₄ .

Chemoselective synthesis of heterobimetallic bis-NHC complexes

Bis-NHC precursors composed of an azolium and a 2-halogenoazole group connected by different linkers have been site-selectively metallated to give heterobimetallic complexes from a single-frame bis-NHC ligand. The azolium group reacts with a base and oxidized metal centers to give NHC complexes with no participation of the halogenoazole, while the halogenoazole reacts in an oxidative addition with low-valent transition metals to give azolato complexes with no participation of the azolium group.

Synthesis of IrIII Hydrido Complexes by Oxidative Addition of Halogenated Theophylline and Adenine Derivatives

The Ir^III hydrido complexes [1] and [2] have been synthesized by the regioselective oxidative addition of the N7-H bond of 8-halogenotheophyllines to [IrCl(coe)₂]₂ in the presence of PPh₃. The use of dppf in this reaction yielded the bimetallic Ir^III/Fe^II hydrido complexes [3] and [4]. X-ray diffraction studies confirmed that complexes [1]-[4] feature a theophyllinato ligand coordinated to the metal center in the rarely observed, chelating fashion via the N7 and O1 atoms. In addition, 8-bromoadenine reacts with [IrCl(coe)₂]₂ in the presence of PPh₃ to form the Ir^III hydrido complex [5] which features one anionic 8-bromoadeninato and one neutral 8-bromoadenine ligand linked by an intramolecular hydrogen bond. Complex [5] was characterized by high-resolution mass spectrometry and an X-ray diffraction analysis but could not be analyzed by nuclear magnetic resonance spectroscopy because of its low solubility.

Preparation of polynuclear NHC complexes by post-synthetic modification of half-sandwich rhodium and iridium complexes bearing C-azolato ligands

Synthesis of polymetallic structures via rational N-functionalization of half-sandwich rhodium and iridium complexes featuring C-azolato ligands.

Regioselective C8-metalation of N-phosphine tethered adenine derivatives via C8–H activation

Complex [4] obtained from N7-phosphine tethered adenine and [IrCl₂Cp*]₂ reacts upon heating under C8–H activation to give [7]Cl bearing an adenine derived pNHC^PR₃ chelate ligand.

Oxidative addition of N-ether-functionalized 2-chlorobenzimidazole to d10 metals

Reaction of 2-chloro-N-(methoxymethyl)benzimidazole 1 with zerovalent group 10 metal complexes in the presence of an additional proton source yielded, via an oxidative addition of the C2–Cl bond, complexes with a protic NH,NR-substituted (R=methoxymethyl) benzimidazolin-2-ylidene ligand. The oxidative addition of 1 to Ni0 and Pd0 complexes proceeded with the exclusive formation of the trans-configured complexes trans-[2]BF4 and trans-[3]BF4, respectively. Contrary to this observation, the reaction of 1 with a more substitution inert Pt0 complex leads, depending on the reaction temperature, to a mixture of cis-/trans-[4]BF4 or exclusively to trans-[4]BF4. The molecular structures of all three trans-configured complexes were determined.

Homo- and Heterodinuclear Ir and Rh Imine-functionalized Protic NHC Complexes: Synthetic, Structural Studies, and Tautomerization/Metallotropism Insights

The influence of the potentially chelating imino group of imine-functionalized Ir and Rh imidazole complexes on the formation of functionalized protic N-heterocyclic carbene (pNHC) complexes by tautomerization/metallotropism sequences was investigated. Chloride abstraction in [Ir(cod)Cl{C3 H3N2 (DippN=CMe)-κN3}] (1 a) (cod=1,5-cyclooctadiene, Dipp=2,6-diisopropylphenyl) with TlPF6 gave [Ir(cod){C3 H3N2 (DippN=CMe)-κ(2) (C2,Nimine )}](+) [PF6 ](-) (3 a(+) [PF6 ](-)). Plausible mechanisms for the tautomerization of complex 1 a to 3 a(+) [PF6 ](-) involving C2-H bond activation either in 1 a or in [Ir(cod){C3 H3 N2 (DippN=CMe)-κN3}2](+) [PF6 ](-) (6 a(+) [PF6 ](-)) were postulated. Addition of PR3 to complex 3 a(+) [PF6 ](-) afforded the eighteen-valence-electron complexes [Ir(cod)(PR3){C3 H3N2 (DippN=CMe)-κ(2) (C2,Nimine )}](+) [PF6 ](-) (7 a(+) [PF6 ](-) (R=Ph) and 7 b(+) [PF6 ](-) (R=Me)). In contrast to Ir, chloride abstraction from [Rh(cod)Cl{C3H3N2 (DippN=CMe)-κN3}] (1 b) at room temperature afforded [Rh(cod){C3 H3N2 (DippN=CMe)-κN3}2](+) [PF6 ](-) (6 b(+) [PF6 ](-)) and [Rh(cod){C3 H3N2 (DippN=CMe)-κ(2) (C2,Nimine )}](+) [PF6 ](-) (3 b(+) [PF6 ](-) ) (minor); the reaction yielded exclusively the latter product in toluene at 110 °C. Double metallation of the azole ring (at both the C2 and the N3 atom) was also achieved: [Ir2 (cod)2 Cl{μ-C3H2N2 (DippN=CMe)-κ(2) (C2,Nimine ),κN3}] (10) and the heterodinuclear complex [IrRh(cod)2 Cl{μ-C3H2N2 (DippN=CMe)-κ(2) (C2,Nimine ),κN3}] (12) were fully characterized. The structures of complexes 1 b, 3 b(+) [PF6 ](-) , 6 a(+) [PF6 ](-) , 7 a(+) [PF6 ](-), [Ir(cod){C3HN2 (DippN=CMe)(DippN=CH)(Me)-κ(2) (N3,Nimine )}](+) [PF6 ](-) (9(+) [PF6 ](-)), 10⋅ Et2 O⋅toluene, [Ir2 (CO)4 Cl{μ-C3H2N2 (DippN=CMe)-κ(2) (C2,Nimine ),κN3}] (11), and 12⋅2 THF were determined by X-ray diffraction.

Versatile deprotonated NHC: C,N-bridged dinuclear iridium and rhodium complexes

Bearing the versatility of N-heterocyclic carbene (NHC) ligands, here density functional theory (DFT) calculations unravel the capacity of coordination of a deprotonated NHC ligand (pNHC) to generate a doubly C2,N3-bridged dinuclear complex. Here, in particular the discussion is based on the combination of the deprotonated 1-arylimidazol (aryl = mesityl (Mes)) with [M(cod)(μ-Cl)] (M = Ir, Rh) generated two geometrical isomers of complex [M(cod){µ-C₃H₂N₂(Mes)-κC2,κN3}]₂). The latter two isomers display conformations head-to-head (H-H) and head-to-tail (H-T) of C_S and C₂ symmetry, respectively. The isomerization from the H-H to the H-T conformation is feasible, whereas next substitutions of the cod ligand by CO first, and PMe₃ later confirm the H-T coordination as the thermodynamically preferred. It is envisaged the exchange of the metal, from iridium to rhodium, confirming here the innocence of the nature of the metal for such arrangements of the bridging ligands.

Dinuclear iridium and rhodium complexes with bridging arylimidazolide-N(3),C(2) ligands: synthetic, structural, reactivity, electrochemical and spectroscopic studies

Deprotonation of 1-arylimidazoles (aryl = mesityl (Mes), 2,6-diisopropylphenyl (Dipp)), with n-butyl lithium afforded the corresponding derivatives (1-aryl-1H-imidazol-2-yl)lithium (1a, Ar = Mes; 1b, Ar = Dipp) in good yield. Reaction of 1a with 0.5 equiv. of [Ir(cod)(μ-Cl)]2 yielded two geometrical isomers of a doubly C2,N3-bridged dinuclear complex [Ir(cod){μ-C3H2N2(Mes)-κC2,κN3}]2 (3), 3H-H, a head-to-head (H-H) isomer of CS symmetry, and 3H-T, the thermodynamically preferred head-to-tail (H-T) isomer of C2 symmetry. The metallated carbon of the 4 electron donor anionic bridging ligands has some carbene character, reminiscent of the situation in N-metallated protic NHC complexes. Displacement of cod ligands from 3H-H and 3H-T afforded the tetracarbonyl complexes [Ir(CO)2{μ-C3H2N2(Mes)-κC2,κN3}]24H-H and 4H-T, respectively. The reaction with PMe3, which gave only one complex, [Ir(CO)(PMe3){μ-C3H2N2(Mes)-κC2,κN3}]2 (5), demonstrates that the isomerization of the central core Ir[μ-C3H2N2(Mes)-κC2,κN3]2Ir from H-H to H-T on going from 4H-H to 5 is readily triggered by phosphine substitution under mild conditions. Oxidative-addition of MeI to 5 afforded the formally metal-metal bonded d(7)-d(7) complex [Ir2(CO)2(PMe3)2(Me)I{μ-C3H2N2(Mes)-κC2,κN3}2] (6). The blue [Ir(C2H4)2{μ-C3H2N2(Mes)-κC2,κN3}]2 (7) and purple [Rh(C2H4)2{μ-C3H2N2(Dipp)-κC2,κN3}]2 (9) tetraethylene complexes were also obtained with only a H-T arrangement of the bridging ligands. Although only modestly efficient in alkane dehydrogenation, complex 7 was found to be a more active pre-catalyst than 3H-T, 4H-T and 5, probably because of the favorable lability of the ethylene ligands. From cyclic voltammetry, exhaustive coulometry and spectroelectrochemistry studies, it was concluded that 3H-T undergoes a metal-based one electron oxidation to generate the mixed-valent Ir(i)/Ir(ii) system. The energy of the intervalence band for the orange dirhodium complex [Rh(cod){μ-C3H2N2(Mes)-κC2,κN3}]2 (8) is shifted toward lower energies in comparison with 3H-T, reflecting the decrease of the energy with the intermetallic distance. It was concluded from the EPR study that the Ir and Rh centres contribute substantially to the experimental magnetic anisotropy and thus to the singly occupied molecular orbital (SOMO) in the mixed-valent Ir(i)/Ir(ii) and Rh(i)/Rh(ii) systems. The molecular structures of 3H-H, 3H-T, 8 and 9 have been determined by X-ray diffraction.

Imine-functionalised protic NHC complexes of Ir: direct formation by C-H activation

N-Arylimine-functionalised protic NHC (pNHC) Ir(i) and Ir(iii) complexes are obtained directly from neutral or cationic Ir(i) imidazole complexes using excess [Ir(cod)(μ-Cl)]2 or TlPF6, respectively. N-Arylimine-functionalised imidazolium salts lead to imidazole or pNHC complexes by competing N-H or C-H bond activation depending on the type of imidazolium counterion.