LNL-Carbazole Pincer Ligand: More than the Sum of Its Parts

Abstract

The utility of carbazole in photo-, electro-, and medicinal applications has ensured its widespread use also as the backbone in tridentate pincer ligands. In this review, the aim is to identify and illustrate the key features of the LNL-carbazolide binding to transition metal centers (with L = flanking donor moieties, e.g., C, N, P, and O-groups) in a systematic bottom-up progression to illustrate the marked benefits attainable from (i) the rigid aromatic carbazole scaffold (modulable in both the 1,8- and 3,6-positions), (ii) the significant electronic effect of central carbazole-amido binding to a metal, and the tunable sterics and electronics of both the (iii) flanking donor L-moieties and (iv) the wingtip R-groups on the L-donors, with their corresponding influence on metal coordination geometry, d-electron configuration, and resultant reactivity. Systematic implementation of the ligand design strategies not in isolation, but in a combinatorial approach, is showcased to demonstrate the potential for functional molecules that are not only modulable but also adaptable for wide-ranging applications (e.g., stereoselective (photo)catalysis, challenging small molecule activation, SET and redox applications, and even applications in chemotherapeutics) as an indication of future research efforts anticipated to stem from this versatile pincer assembly, not only for the transition metals but also for s-, p-, and f-block elements.

Figure 1

General representation of LNL-carbazolide ligand under review.

Scheme 1. Bis(imine)carbazolide Sufficiently Stabilizing Reactive Barium Complexes

Scheme 2. Toward Stable and Soluble Molecular Lead(II) Halides and a Dimeric Lead(II) Fluoride

Scheme 3. Synthesis of (i) Bis(imine)carbazole Ligand and Reactions of Pincer Ligands (i–iii) with Fe or Co

Scheme 4. Nucleophilic NNN-Carbazolide Accelerating MeI Oxidative Addition

Scheme 5. Synthesis of a PNP-Carbazole Ligand and Coordination to Ir^I

Scheme 6. Bond Activation Reactivity of a Dihydrido PNP-Ir^III Complex

Scheme 7. Synthesis of (i) Iridium(I) and Rhodium(I) Olefin Complexes with a PNP-Carbazolide Pincer Ligand and (ii) Comparative pK_a Values for the Bis(diisopropylphosphino)carbazole and Corresponding Bis(diisopropylphosphino) Diphenylamine Ligands Employed in (iii) the Transfer Hydrogenation of COA with TBE to Form COE and TBA

Scheme 8. Synthesis of (i) High-Spin PNP-Pincer Complexes of Iron for (ii) Catalytic Reduction of Dinitrogen

Scheme 9. Synthesis and Reactivity of High-Spin PNP-Pincer Hydrido Complexes of Iron Containing a Carbazole-Based Ligand

Scheme 10. Synthesis and Metalloradical Reactivity of a T-Shaped Fe^I Complex

Scheme 11. Charge Transfer Reactivity Mediated by a T-Shaped Fe^I–PNP Complex

Scheme 12. (i) Deoxygenation of Carbon Dioxide and (ii) Chalcogen Abstraction Mediated by a T-Shaped Fe^I–PNP Complex

Scheme 13. Synthesis of Di- and Mononuclear PNP-Pincer Complexes of Chromium(II) Hydride

Scheme 14. Synthesis of the BIMCA (3,6-di-tert-butyl-1,8-bis(imidazol-2-ylidene-1-yl)carbazolide) Ligand and Coordination to Group 10 Transition Metals

Scheme 15. Syntheses of Group 10 Metal(II) Hydrido and Chlorido Complexes of BIMCA Ligand and Anionic M(0) Complexes Formed As a Result of the Reduction of Chlorido Complexes

Scheme 16. Synthesis of Dicationic Bis(triazolium)carbazole Ligand Salts

Scheme 17. Syntheses of Coinage Metal Complexes Coordinated by the Bis(triazolylidene)carbazolide Ligand

Scheme 18. Oxidation of Au^I Complexes to Square Planar Cationic Au^III Complexes

Figure 2

Resonance contributions and truncated qualitative orbital diagram of the bonding analogy between (i) enamines and (ii) amidoalkylidenes as analogous inorganic enamines.^,

Figure 3

Orbital overlap for amido p-orbital aligned with d_xy (left) and amido p-orbital rotated out of alignment, with corresponding truncated molecular orbital diagrams of preferential overlap of freely rotating N atom lone pair with unoccupied d_xy orbital (left), and constrained N atom forced to overlap with one of the W≡C π-bonds (right).^,

Scheme 19. Synthesis of Alkylidene, Alkylidyne, and Oxo-alkyl Complexes of Tungsten Coordinated to a Trianionic ONO-Carbazolide Pincer Ligand

Figure 4

Truncated molecular orbital diagrams exhibiting inorganic enamine bonding combinations for the anions 124′ and 126.^,

Scheme 20. Synthesis of Hydrido Ruthenium(II) PNP-Pincer Complexes and Cooperative Reactivity with Borane

Scheme 21. Synthesis of 5-Membered Chelating PNP-Pincer Lanthanide Complexes for Polymerization of Dienes

Scheme 22. Cis-1,4-Selective Living Diene Polymerization and Subsequent Post-Modification

Scheme 23. Synthesis and Reactivity of Pincer Coordinated Lanthanide Complexes

Scheme 24. Metallacycle Ring Opening and ortho-Metalation Leading to Lutetium Anilide Formation

Scheme 25. Decreasing the Steric Bulk at the P-Site of the Donor Groups

Scheme 26. Coordination of Bis(phospholane)carbazolide to Lutetium and Scandium

Scheme 27. Asymmetric Dinuclear Tetraalkoxide Lutetium Complex via a Cascade Ring-Opening Insertion Reaction

Scheme 28. Alkyl Lutetium Leading to Dearomatization and Complex Dimerization

Scheme 29. Lutetium Alkyl and Hydride Complexes Stabilized by a Bis(pyrazolyl)carbazolide

Figure 5

Increasing ligand donor strength though modification of the flanking donor groups.

Scheme 30. Synthesis and Reactivity of Rhodium and Iridium Complexes of BIMCA

Scheme 31. Reactivity of Group 9 Carbonyl Complexes of BIMCA with Allyl Halides

Scheme 32. (i) Possible Pathways for the Formation of the Rhodium Complex 218 in the Reaction with 1-Chloro-2-butene and (ii) Crossover Experiment between 194 and 212

Scheme 33. Catalytic Deoxygenation (in Solvent C₆D₆, 80 °C, 24 h) of (i) Terminal Epoxides, (ii) cis-2-Butene Oxide, (iii) trans-2-Butene Oxide, (iv) cis-Diethyl-2,3-epoxy Succinate, and (v) trans-Diethyl-2,3-epoxy Succinate with Carbon Monoxide

Scheme 34. Proposed Mechanism for the Deoxygenation of Epoxides with CO Catalyzed by 208

Scheme 35. (i) Synthesis of Nickel(II) Complexes of Bis(imidazolylidene)carbazolide, and (ii) Coupling of Carbon Dioxide with Cyclohexane to Cyclohexane Carbonate Catalyzed by 228 and Poly(cyclohexane Carbonate) Catalyzed by 229

Scheme 36. Synthesis of (i) Bis(triazolylidene)carbazolide Ligand Precursors and Their Nickel(II) Complexes and (ii) Cyclization of Epoxides and CO₂ to Cyclic Carbonates Catalyzed by 235 and 236

Scheme 37. NNN-Carbazole Pincer with Imine Donor Groups Exhibiting Hemilabile Coordination

Scheme 38. NNN-Carbazolide Barium Complexes with Imine Donor Groups

Scheme 39. Synthesis and C–H Activation Reactivity of Cyclometalated PNP-Pincer Complexes of Group 4 Metals

Scheme 40. Reactivity of Cyclometalated Zr Complexes

Scheme 41. Mechanism of Hydrogenolytic Formation of η⁶-Arene Zr PNP-Pincer Complexes and Their Reactivity As Zirconium(II) Synthons

Scheme 42. Dehydrogenative Coupling of Pyridines Mediated by PNP-Zr^II Synthon

Scheme 43. Synthesis and Reactivity of Hemilabile Bis(pyrazole)carbazolide Iron Complexes

Scheme 44. Double Intramolecular C–H Activation Leading to 299 via a Nickel-Nitridyl Intermediate

Scheme 45. Wingtip C–H Activation Leading to Cyclometalative Reactivity

Scheme 46. Synthesis and Reactivity of High-Spin Cobalt(II) Chlorido, Low-Spin Cobalt(II) Hydrido and High-Spin Cobalt(I) Complexes Supported by PNP-Ligands

Scheme 47. (i) Catalytic Alkene Hydrogenation and (ii) Stoichiometric Transformations to Model the Elementary Reaction Steps of the Alkene Hydrogenation Mechanism

Scheme 48. Synthesis of Rhodium(I) Complexes of Bis(triazolylidene)carbazolide

Scheme 49. (i) Homodimerization of Terminal Alkynes, (ii) Terminal Alkyne Hydrothiolation, (iii) Bis-Hydrothiolation in a Sequential One-Pot Reaction of Dithiol with Various Alkynes, and (iv) Sequential Alkyne Dimerization and Hydrothiolation

Scheme 50. Fe^III Complex Stability and Reactivity Effected by Wingtip Steric Bulk

Scheme 51. Bis(pyrazole)carbazolide Coordinated Fe^II Complexes

Scheme 52. Wingtip Sterics Dictating Iron’s Reactivity

Scheme 53. Wingtip Sterics Influencing the Coordination Environment around a Nickel(II) Complex

Scheme 54. Bis(pyrazole)carbazolide Vanadium Complexes

Scheme 55. Synthesis of Bis(PNHC)-Carbazole and Its Group 10 Metal Complexes

Scheme 56. Formation of Dimers and Mono and Di-Lithium Adducts of Bis(PNHC)-Carbazolide Group 10 Metal Complexes

Scheme 57. Synthesis of Ruthenium(II) and Palladium(II) Complexes Bearing a Macrocyclic CNC Ligand

Scheme 58. Nucleophilic and Lewis Acid Catalyzed Isomerization of Terminal Epoxides

Scheme 59. Synthesis of Rhodium, Iridium, And Cobalt Complexes of N-Homoallyl-substituted Bis(NHC) Pincer Ligand

Scheme 60. Synthesis of Unsymmetrically N-Homoallyl-Substituted BIMCA Ligand and Its Rhodium Complexes

Scheme 61. Proposed Catalytic Cycle for 383 and 392, Demonstrating Catalyst Deactivation Product Formation

Scheme 62. (i) Catalyzed Isomerization of N-Boc Terminal Aziridines and (ii) Proposed Mechanism for the Isomerization of Terminal Aziridines Catalyzed by 392

Scheme 63. Synthesis and Coordination Chemistry of Achiral and Chiral PNP-Carbazolide Pincer Ligands

Scheme 64. (i) Chiral Carbazole Ligand for Asymmetric Nozaki-Hiyama Allylation and (ii) Preparation of HMG-CoA Reductase Inhibitor FR901512

Figure 6

Wingtip sterics dictating selectivity by directing re- or si-face reactivity.

Scheme 65. Enantioselective Chromium-Catalyzed Aldehyde Functionalization to Lactones

Scheme 66. Enantioenriched (i) Lactones and (ii) Homoallylic Alcohols

Scheme 67. Catalytic Dearomatization of (i and ii) Aromatic Substrates with Aldehydes

Scheme 68. Synthesis of (i) Calcitriol Lactone and (ii) Paraconic Acids

Scheme 69. Synthesis of Pacman Diporphyrins Linked by a Rigid Copper(II) NNN-Carbazolide Bridge

Scheme 70. Synthesis of Palladium(II) Janus Pincers Containing Fused Indolo[3,2-b]carbazole Scaffold and Its Diphenylamide Bisphosphino Analogue

Scheme 71. Synthesis of a Homoleptic Bis(triazolylidene)carbazolide Manganese Complex and the Electronic Structure of the Complex in Five Oxidation States

Scheme 72. Asymmetric Epoxidation Catalyzed by a Porphyrin-Like Bis(oxazolinyl)carbazolide Iron Complex

Scheme 73. Proposed Cyanide Sensing Mechanism by Bis(triazolyl)carbazolide Copper(II)

Figure 7

BIMCA-Pt(II) complexes investigated for their photophysical properties.

Figure 8

Photoactive bis(triazolylidene)carbazolide complexes of the s-block metals.

Figure 9

Homoleptic complexes of the high-valent 3d metals (i) chromium(III) and (ii) cobalt(III). Configurational coordinate diagram (iii) for O_h symmetry energy states of 489(378) and (iv) key electronic transitions occurring in 490, with configurational coordinate diagram of energetically low-lying charge transfer and MC excited states with time constants of two relaxation processes.

Scheme 74. (i) Direct Synthesis of Carbamate-Protected Primary Amines with (ii) Supplementary Proposed Catalytic Mechanism

Scheme 75. Synthesis of Mono- And Dinuclear Macrocyclic Bis(imine)carbazolide Metal Complexes

Scheme 76. Synthesis and Metalation of Porphyrin-Like Macrocycles Based on Carbazole and Pyridine Units

Scheme 77. Synthesis and Metalation of Porphyrin-Like Macrocycles Based on Carbazole and Pyrrole Units

Scheme 78. Synthesis, Deprotonation, And Metalation of Carbazole-Triazolylidene Porphyrin

Scheme 79. Synthesis of (i) Alkylated Triazolium Salts and Their Polynuclear Silver(I) and Gold(I) Complexes, as well as (ii) Bis(triazolylidene)carbazolide Coordinated Cationic Au^III Chloride Complexes

Scheme 80. (i) Synthesis of Charge-Neutral ONO-Ru Complex As Water Oxidation Catalyst, (ii) Proposed Mechanism for the Catalytic Cycle of Water Oxidation Mediated by 542 or 551, and (iii) Composite Molecular Anode 551/MWCNTsCOOH/GC Constructed in the Electrochemical Cell for Efficient Water Splitting

Scheme 81. Copper Carbazolide-Mediated Asymmetric Alkyl and Aryl Azolation of Alkenes

Scheme 82. Proposed Catalytic Cycle for the Copper Catalyzed Asymmetric Alkyl and Aryl Azolation of Alkenes

Scheme 83. Enantioselective Alkylation of Azoles and Proposed Reaction Mechanism