No abstract

A series of seven bis(pyridinedipyrrolide)zirconium complexes, Zr( R 1 PDP R 2 ) 2 , where [ R 1 PDP R 2 ] 2− is the doubly deprotonated form of [2,6-bis(5-R 1 -3-R 2 -1H-pyrrol-2-yl)pyridine], were prepared and characterized in solution by NMR, UV/vis absorption, and emission spectroscopy and cyclic voltammetry. The molecular structures were determined by single-crystal X-ray crystallography. All complexes exhibit remarkably long emission lifetimes (τ = 190−576 μs) with high quantum efficiencies (Φ PL = 0.10−0.38) upon excitation with visible light in a benzene solution. The substituents on the pyrrolide rings were shown to have significant effects on the photoluminescence and electrochemical properties of these compounds. The R 2 substituents (R 2 = H, Me, Ph, or C 6 F 5 ) show only limited effects on the absorption and emission profiles of the complexes but allow systematic tuning of the ground-and excited-state redox potentials over a range of almost 600 mV. The R 1 substituents (R 1 = H, Me, Ph, or 2,4,6-Me 3 Ph) influence both the optical and electrochemical properties through electronic effects. Additionally, the R 1 substituents have profound consequences for the structural flexibility and overall stability of the compounds. Distortions of the Zr(PDP) 2 core from idealized D 2d symmetry in the solid state can be traced to the steric profiles of the R 1 substituents and correlate with the observed Stokes shifts for each compound. The complex with the smallest ligand system, Zr( H PDP H ) 2 , coordinates two additional solvent molecules in a tetrahydrofuran (THF) solution, which allowed the isolation of photoluminescent, eight-coordinate Zr( H PDP H ) 2 (THF) 2 . The photoredox catalytic dehalogenation of aryl iodides and aryl chlorides using the most reducing derivative, Zr( Me PDP Me ) 2 , highlights the potential of Zr(PDP) 2 photosensitizers to promote challenging reductive transformations under mild conditions upon excitation with green light.

Among Earth-abundant catalyst systems, iron-carbene intermediates that perform C–C bond forming reactions such as cyclopropanation of olefins and C–H functionalization via carbene insertion are rare. Detailed descriptions of the possible electronic structures for iron-carbene bonds are imperative to obtain better mechanistic insights and enable rational catalyst design. Here, we report the first square-planar iron-carbene complex (MesPDPPh)­Fe­(CPh2), where [MesPDPPh]2– is the doubly deprotonated form of [2,6-bis­(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)­pyridine]. The compound was prepared via reaction of the disubstituted diazoalkane N2CPh2 with (MesPDPPh)­Fe­(thf) and represents a rare example of a structurally characterized, paramagnetic iron-carbene complex. Temperature-dependent magnetic susceptibility measurements and applied-field Mössbauer spectroscopic studies revealed an orbitally near-degenerate S = 1 ground state with large unquenched orbital angular momentum resulting in high magnetic anisotropy. Spin-Hamiltonian analysis indicated that this S = 1 spin system has uniaxial magnetic properties arising from a ground M S = ±1 non-Kramers doublet that is well-separated from the M S = 0 sublevel due to very large axial zero-field splitting (D = −195 cm–1, E/D = 0.02 estimated from magnetic susceptibility data). This remarkable electronic structure gives rise to a very large, positive magnetic hyperfine field of more than +60 T for the 57Fe nucleus along the easy magnetization axis observed by Mössbauer spectroscopy. Computational analysis with complete active space self-consistent field (CASSCF) calculations provides a detailed electronic structure analysis and confirms that (MesPDPPh)­Fe­(CPh2) exhibits a multiconfigurational ground state. The majority contribution originates from a configuration best described as a singlet carbene coordinated to an intermediate-spin FeII center with a (d xy )2{(d xz ),(d z 2 )}3(d yz )1(d x 2 –y 2 )0 configuration featuring near-degenerate d xz and d z 2 orbitals.

A series of iron complexes featuring the pyridine dipyrrolide (PDP) pincer ligand [Cl2PhPDPPh]2−, obtained via deprotonation of 2,6‐bis(5‐(2,6‐dichlorophenyl)‐3‐phenyl‐1H‐pyrrol‐2‐yl)pyridine, H2Cl2PhPDPPh, is reported and structurally and spectroscopically characterized. While the bis‐pyridine adduct (Cl2PhPDPPh)Fe(py)2 exhibits nearly identical features as previously reported (MesPDPPh)Fe(py)2 (H2MesPDPPh=2,6‐bis(5‐(2,4,6‐trimethylphenyl)‐3‐phenyl‐1H‐pyrrol‐2‐yl)pyridine), the diethyl ether and tetrahydrofuran adducts (Cl2PhPDPPh)Fe(OEt2) and (Cl2PhPDPPh)Fe(thf) show additional weak Fe−Cl interactions that impact the overall coordination geometries and result in strong deviations from planar coordination environments. The reaction of (Cl2PhPDPPh)Fe(thf) with 1‐adamantyl azide provided the isolable iron imido complex (Cl2PhPDPPh)Fe(N1Ad), highlighting the improved stability of [Cl2PhPDPPh]2− towards intramolecular nitrene group transfer from the high‐valent iron‐imido unit. The electronic structure of (Cl2PhPDPPh)Fe(N1Ad) was investigated by density functional theory (DFT) and complete active space self‐consistent field (CASSCF) calculations. These computational studies suggest energetically close‐lying diamagnetic and paramagnetic states and help to conceptualize the unusual magnetic properties of the complex observed by variable‐temperature 1H NMR spectroscopy.

The two commercially available zirconium complexes tetrakis(dimethylamido)zirconium, Zr(NMe2)4, and tetrabenzylzirconium, ZrBn4, were investigated for their utility as starting materials in the synthesis of bis(pyridine dipyrrolide)zirconium photosensitizers, Zr(PDP)2. Reaction with one equivalent of the ligand precursor 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine, H2 MePDPPh, resulted in the isolation and structural characterization of the complexes (MePDPPh)Zr(NMe2)2thf and (MePDPPh)ZrBn2, which could be converted to the desired photosensitizer Zr(MePDPPh)2 upon addition of a second equivalent of H2 MePDPPh. Using the more sterically encumbered ligand precursor 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine, H2 MesPDPPh, only ZrBn4 yielded the desired bis-ligand complex Zr(MesPDPPh)2. Careful monitoring of the reaction at different temperatures revealed the importance of the organometallic intermediate (cyclo-MesPDPPh)ZrBn, which was characterized by X-ray diffraction analysis and 1H NMR spectroscopy and shown to contain a cyclometalated MesPDPPh unit. Taking inspiration from the results for zirconium, syntheses for two hafnium photosensitizers, Hf(MePDPPh)2 and Hf(MesPDPPh)2, were established and shown to proceed through similar intermediates starting from tetrabenzylhafnium, HfBn4. Initial studies of the photophysical properties of the photoluminescent hafnium complexes indicate similar optical properties compared to their zirconium analogues.