Fe(I)-mediated reductive cleavage and coupling of CO(2): an Fe(II)(mu-O, mu-CO)Fe(II) core.
Explore the source record for details and available documents.
Carbon Dioxide↗
Biomedical subjects
Jonas C Peters
Publications and source records attributed to Jonas C Peters.
On the feasibility of N2 fixation via a single-site FeI/FeIV cycle: Spectroscopic studies of FeI(N2)FeI, FeIV[triple bond]N, and related species.
The electronic properties of an unusually redox-rich iron system, [PhBP(R)3]Fe-Nx (where [PhBP(R)3] is [PhB(CH2PR2)3]-), are explored by Mössbauer, EPR, magnetization, and density-functional methods to gain a detailed picture regarding their oxidation states and electronic structures. The complexes of primary interest in this article are the two terminal iron(IV) nitride species, [PhBP(iPr)3]Fe[triple bond]N (3a) and [PhBP(CH2Cy)3]Fe[triple bond]N (3b), and the formally diiron(I) bridged-Fe(mu-N2)Fe species, {[PhBP(iPr)3]Fe}2(mu-N2) (4). Complex 4 is chemically related to 3a via a spontaneous nitride coupling reaction. The diamagnetic iron(IV) nitrides 3a and 3b exhibit unique electronic environments that are reflected in their unusual Mössbauer parameters, including quadrupole-splitting values of 6.01(1) mm/s and isomer shift values of -0.34(1) mm/s. The data for 4 suggest that this complex can be described by a weak ferromagnetic interaction (J/D < 1) between two iron(I) centers. For comparison, four other relevant complexes also are characterized: a diamagnetic iron(IV) trihydride [PhBP(iPr)3]Fe(H)3(PMe3) (5), an S = 3/2 iron(I) phosphine adduct [PhBP(iPr)3]FePMe3 (6), and the S = 2 iron(II) precursors to 3a, [PhBP(iPr)3]Fe-Cl and [PhBP(iPr)3]Fe-2,3:5,6-dibenzo-7-aza bicyclo[2.2.1]hepta-2,5-diene (dbabh). The electronic properties of these respective complexes also have been explored by density-functional methods to help corroborate our spectral assignments and to probe their electronic structures further.
Computer Simulation↗
Pseudotetrahedral manganese complexes supported by the anionic tris(phosphino)borate ligand [PhBP(iPr)3].
This paper presents aspects of the coordination chemistry of mono- and divalent manganese complexes supported by the anionic tris(phosphino)borate ligand, [PhBP(i)(Pr)3] (where [PhBP(i)(Pr)3] = [PhB(CH(2)P(i)Pr2)3]-). The Mn(II) halide complexes, [PhBP(i)(Pr)3]MnCl (1) and [PhBP(i)(Pr)3]MnI (2), have been characterized by X-ray diffraction, SQUID magnetometry, and EPR spectroscopy. Compound 2 serves as a precursor to a series of Mn azide, alkyl, and amide species: [PhBP(i)(Pr)3]Mn(N3) (3), [PhBP(i)(Pr)3]Mn(CH2Ph) (4), [PhBP(i)(Pr)3]Mn(Me) (5), [PhBP(i)(Pr)3]Mn(NH(2,6-(i)Pr2-C6H3)) (6), [PhBP(i)(Pr)3]Mn(dbabh) (7), and [PhBP(i)(Pr)3]Mn(1-Ph(isoindolate)) (8). The complexes 2-8 feature a divalent-metal center and are pseudotetrahedral. They collectively represent an uncommon structural motif for low-coordinate, polyphosphine-supported Mn complexes. Two Mn(I) species have also been prepared. These include the Tl-Mn adduct [PhBP(i)(Pr)3]Tl-MnBr(CO)4 (9) and the octahedral complex [PhBP(i)(Pr)3]Mn(CN(t)Bu)3 (10). Some of our initial synthetic efforts to generate [PhBP(i)(Pr)3]MnN(x) species are briefly described, as are DFT studies that probe the electronic viability of these types of multiply bonded target structures.
Anions↗
Vibrational spectroscopy and analysis of pseudo-tetrahedral complexes with metal imido bonds.
A number of assignments have been previously posited for the metal-nitrogen stretch (nu(M-NR)), the N-R stretch (nu(MN-R)), and possible ligand deformation modes associated with terminally bound imides. Here we examine mononuclear iron(III) and cobalt(III) imido complexes of the monoanionic tridentate ligand [PhBP3] ([PhBP3] = [PhB(CH2PPh2)3]-) to clarify the vibrational features for these trivalent metal imides. We report the structures of [PhBP3]FeNtBu and [PhBP3]CoNtBu. Pseudo-tetrahedral metal imides of these types exhibit short bond lengths (ca. 1.65 A) and nearly linear angles about the M-N-C linkages, indicative of multiple bond character. Furthermore, these compounds give rise to intense, low-energy visible absorptions. Both the position and the intensity of the optical bands in the [PhBP3]MNR complexes depend on whether the substituent is an alkyl or aryl group. Excitation into the low-energy bands of [PhBP3]FeNtBu gives rise to two Raman features at 1104 and 1233 cm(-1), both of which are sensitive to 15N and 2H labeling. The isotope labeling suggests the 1104 cm(-1) mode has the greatest Fe-N stretching character, while the 1233 cm(-1) mode is affected to a lesser extent by (15)N substitution. The spectra of the deuterium-labeled imides further support this assertion. The data demonstrate that the observed peaks are not simple diatomic stretching modes but are extensively coupled to the vibrations of the ancillary organic group. Therefore, describing these complexes as simple diatomic or even triatomic oscillators is an oversimplification. Analogous studies of the corresponding cobalt(III) complex lead to a similar set of isotopically sensitive resonances at 1103 and 1238 cm(-1), corroborating the assignments made in the iron imides. Very minimal changes in the vibrational frequencies are observed upon replacement of cobalt(III) for iron(III), suggesting similar force constants for the two compounds. This is consistent with the previously proposed electronic structure model in which the added electron resides in a relatively nonbonding orbital. Replacement of the tBu group with a phenyl ring leads to a significantly more complicated resonance Raman spectrum, presumably due to coupling with the vibrations of the phenyl ring. Polarization studies demonstrate that the observed modes have A(1) symmetry. In this case, a clearer resonance enhancement of the signals is observed, supporting a charge transfer designation for the electronic transitions. A series of isotope-labeling experiments has been carried out, and the modes with the greatest metal-nitrogen stretching character have been assigned to peaks at approximately 960 and approximately 1300 cm(-1) in both the iron and cobalt [PhBP3]MNPh complexes. These results are consistent with a multiple M-N bond for these metal imides.
Boron Compounds↗
Unexpected photoisomerization of a pincer-type amido ligand leads to facial coordination at Pt(IV).
The divalent complex (BQA)PtMe undergoes oxidative addition with MeI to afford the octahedral complex cis-(mer-BQA)PtMe2I {(BQA)- = bis(8-quinolinyl)amide}. When this molecule is irradiated with visible light, it isomerizes to (fac-BQA)PtMe2I, where the BQA ligand adopts an unexpected facial coordination mode. The amide nitrogen in this molecule is sp3 hybridized and can be easily quarternized with HBF4, resulting in [H(fac-BQA)PtMe2I][BF4], with only minor perturbation to the coordination sphere.
Journal Article↗
Characterization of the terminal iron(IV) imides [[PhBP(t)(Bu)2(pz')]Fe(IV)NAd]+.
New hybrid bis(phosphine)(pyrazole)borate tripodal ligands ([PhBPtBu2(pz')]-) are reported that support pseudotetrahedral iron in the oxidation states +1, +2, +3, and +4. The higher oxidation states are stabilized by a terminal FeNR linkage. Of particular interest is the generation and thorough characterization of an S = 1 FeIVNR+ imide cation using this new ligand system. The latter species can be observed electrochemically and spectroscopically, and its solid-state crystal structure is reported.
Borates↗
Mid- to high-valent imido and nitrido complexes of iron.
This short review summarizes recent advances in the synthesis and reactivity of iron imides and nitrides. Both monometallic and multimetallic assemblies featuring these moieties are discussed. Recent synthetic approaches have led to the availability of new mid- to high-valent iron imides and nitrides, allowing us to begin assembling models to describe the factors influencing their relative stabilities and reactivity patterns.
Imides↗
Complexes of iron and cobalt with new tripodal amido-polyphosphine hybrid ligands.
Divalent complexes of iron and cobalt with new, monoanionic tripodal amido-polyphosphine ligands have been thoroughly characterized, and XRD analysis reveals geometries that are distinct for this class of ligand.
Amides↗
An eta3-H2SiR2 Adduct of [[PhB(CH2PiPr2)3]Fe(II)H].
Explore the source record for details and available documents.
Borates↗
Structural snapshots of a flexible Cu2P2 core that accommodates the oxidation states Cu(I)Cu(I), Cu1.5Cu1.5, and Cu(II)Cu(II).
The phosphido-bridged dicopper(I) complex {(PPP)Cu}2 has been synthesized and structurally characterized ([PPP]- = bis(2-di-iso-propylphosphinophenyl)phosphide). Cyclic voltammetry of {(PPP)Cu}2 in THF shows fully reversible oxidations at -1.02 V (Cu1.5Cu1.5/CuICuI) and -0.423 V (CuIICuII/Cu1.5Cu1.5). Chemical oxidation of {(PPP)Cu}2 by one electron yields the class III mixed-valence species [{(PPP)Cu}2]+ (EPR, UV-vis). Structural data establish an unexpectedly large change (0.538 A) in the Cu...Cu distance upon oxidation state. Oxidation of {(PPP)Cu}2 by two electrons yields the dication [{(PPP)Cu}2]2+, an antiferromagnetically coupled dicopper(II) complex. Maintenance of a pseudotetrahedral geometry that is midway between a square plane and an ideal tetrahedron at the copper centers, along with a high degree of flexibility at the phosphide hinges, allows for efficient access to CuICuI, Cu1.5Cu1.5, and CuIICuII redox states without the need for ligand exchange, substitution, or redistribution processes.
Journal Article↗
High-spin and low-spin iron(II) complexes with facially-coordinated borohydride ligands.
Rare examples of monometallic high-spin and low-spin L3Fe(H3BH) complexes have been characterized, where the two L3 ligands are [TpPh2] and [PhBP3] ([TpPh2] = [HB(3,5-Ph2pz)3]- and [PhBP3] = [PhB(CH2PPh2)3]-). The structures are reported wherein the borohydride ligand is facially coordinated to the iron center in each complex. Density functional methods have been employed to explain the bonding in these unusual iron(II) centers. Despite the differences in spin states, short Fe-B distances are observed in both complexes and there is significant theoretical evidence to support a substantial bonding interaction between the iron and boron nuclei. In light of this interaction, we suggest that these complexes can be described as (L3)Fe(eta4-H3BH) complexes.
Borohydrides↗
Heterolytic H2 activation mediated by low-coordinate L3Fe-(mu-N)-FeL3 complexes to generate Fe(mu-NH)(mu-H)Fe species.
The diiron mu-nitride complexes, {L3FeII(mu-N)FeIIL3}- and L3FeIII(mu-N)FeIIL3, heterolytically activate hydrogen (1 atm) at ambient temperature in solution (L3 = [PhB(CH2PPh2)3]-). These transformations lead to structurally unique {L3FeII(mu-NH)(mu-H)FeIIL3}- and L3FeIII(mu-NH)(mu-H)FeIIL3 products. X-ray data establish a marked reduction in the Fe-Fe distance upon H2 uptake, and spectroscopic data establish both FeIIFeII species to be diamagnetic, whereas the FeIIIFeII species, L3FeIII(mu-N)FeIIL3 and L3FeIII(mu-NH)(mu-H)FeIIL3, populate doublet ground states with thermally accessible higher spin states.
Crystallography, X-Ray↗
Electrocatalytic hydrogen evolution by cobalt difluoroboryl-diglyoximate complexes.
In the presence of moderately strong acids in CH3CN, cobalt complexes with BF2-bridged diglyoxime ligands are active catalysts for the reduction of protons to H2 at potentials as positive as -0.28 V vs. SCE.
Journal Article↗
Spin-state tuning at pseudotetrahedral d(7) ions: examining the structural and magnetic phenomena of four-coordinate [BP3]CoII-X systems.
Electronic structure, spin-state, and geometrical relationships for a series of pseudotetrahedral Co(II) aryloxide, siloxide, arylthiolate, and silylthiolate complexes supported by the tris(phosphino)borate [BP(3)] ligands [PhBP(3)] and [PhBP(i)()(Pr)(3)] ([PhB(CH(2)PPh(2))(3)](-) and [PhB(CH(2)P(i)()Pr(2))(3)](-), respectively) are described. Standard (1)H NMR, optical, electrochemical, and solution magnetic data, in addition to low-temperature EPR and variable temperature SQUID magnetization data, are presented for the new cobalt(II) complexes [PhBP(3)]CoOSiPh(3) (2), [PhBP(3)]CoO(4-(t)()Bu-Ph) (3), [PhBP(3)]CoO(C(6)F(5)) (4), [PhBP(3)]CoSPh (5), [PhBP(3)]CoS(2,6-Me(2)-Ph) (6), [PhBP(3)]CoS(2,4,6-(i)()Pr(3)-Ph) (7), [PhBP(3)]CoS(2,4,6-(t)()Bu(3)-Ph) (8), [PhBP(3)]CoSSiPh(3) (9), [PhBP(3)]CoOSi(4-NMe(2)-Ph)(3) (10), [PhBP(3)]CoOSi(4-CF(3)-Ph)(3) (11), [PhBP(3)]CoOCPh(3) (12), [PhBP(i)()(Pr)(3)]CoOSiPh(3) (14), and [PhBP(i)()(Pr)(3)]CoSSiPh(3) (15). The low-temperature solid-state crystal structures of 2, 3, 5-10, 12, and 15 are also described. These pseudotetrahedral cobalt(II) complexes are classified as featuring one of two limiting distortions, either umbrella or off-axis. Magnetic and spectroscopic data demonstrate that both S = (1)/(2) and S = (3)/(2) ground-state electronic configurations are accessible for the umbrella distorted structure type, depending on the nature of the X-type ligand, its denticity (eta(1) versus eta(3)), and the tripodal phosphine ligand employed. Off-axis distorted complexes populate an S = (1)/(2) ground-state exclusively. For those four-coordinate complexes that populate S = (1)/(2) ground states, X-ray data show two Co-P bond distances that are invariably shorter than a third Co-P bond. The pseudotetrahedral siloxides 2, 10, and 11 are exceptional in that they display gradual spin crossover in the solid state. The diamagnetic cobalt(III) complex {[PhBP(3)]CoOSiPh(3)}{BAr(4)} ({16}{BAr(4)}) (Ar = Ph or 3,5-(CF(3))(2)-C(6)H(3)) has also been prepared and structurally characterized. Accompanying electronic structure calculations (DFT) for complexes 2, 6, and {16}(+) support the notion of a close electronic structure relationship between these four-coordinate systems and octahedral, sandwich, and half-sandwich coordination complexes.
Journal Article↗
Synthetic control of excited-state properties in cyclometalated Ir(III) complexes using ancillary ligands.
The synthesis and photophysical characterization of a series of (N,C(2')-(2-para-tolylpyridyl))2 Ir(LL') [(tpy)2 Ir(LL')] (LL' = 2,4-pentanedionato (acac), bis(pyrazolyl)borate ligands and their analogues, diphosphine chelates and tert-butylisocyanide (CN-t-Bu)) are reported. A smaller series of [(dfppy)2 Ir(LL')] (dfppy = N,C(2')-2-(4',6'-difluorophenyl)pyridyl) complexes were also examined along with two previously reported compounds, (ppy)2 Ir(CN)2- and (ppy)2 Ir(NCS)2- (ppy = N,C(2')-2-phenylpyridyl). The (tpy)2 Ir(PPh2CH2)2 BPh2 and [(tpy)2 Ir(CN-t-Bu)2](CF3SO3) complexes have been structurally characterized by X-ray crystallography. The Ir-C(aryl) bond lengths in (tpy)2 Ir(CN-t-Bu)2+ (2.047(5) and 2.072(5) A) and (tpy)2 Ir(PPh2CH2)2 BPh2 (2.047(9) and 2.057(9) A) are longer than their counterparts in (tpy)2 Ir(acac) (1.982(6) and 1.985(7) A). Density functional theory calculations carried out on (ppy)2 Ir(CN-Me)2+ show that the highest occupied molecular orbital (HOMO) consists of a mixture of phenyl-pi and Ir-d orbitals, while the lowest unoccupied molecular orbital is localized primarily on the pyridyl-pi orbitals. Electrochemical analysis of the (tpy)2 Ir(LL') complexes shows that the reduction potentials are largely unaffected by variation in the ancillary ligand, whereas the oxidation potentials vary over a much wider range (as much as 400 mV between two different LL' ligands). Spectroscopic analysis of the cyclometalated Ir complexes reveals that the lowest energy excited state (T1) is a triplet ligand-centered state (3LC) on the cyclometalating ligand admixed with 1MLCT (MLCT = metal-to-ligand charge-transfer) character. The different ancillary ligands alter the 1MLCT state energy mainly by changing the HOMO energy. Destabilization of the 1MLCT state results in less 1MLCT character mixed into the T1 state, which in turn leads to an increase in the emission energy. The increase in emission energy leads to a linear decrease in ln(k(nr)) (k(nr) = nonradiative decay rate). Decreased 1MLCT character in the T1 state also increases the Huang-Rhys factors in the emission spectra, decreases the extinction coefficient of the T1 transition, and consequently decreases the radiative decay rates (k(r)). Overall, the luminescence quantum yields decline with increasing emission energies. A linear dependence of the radiative decay rate (k(r)) or extinction coefficient (epsilon) on (1/deltaE)2 has been demonstrated, where deltaE is the energy difference between the 1MLCT and 3LC transitions. A value of 200 cm(-1) for the spin-orbital coupling matrix element 3LC absolute value(H(SO)) 1MLCT of the (tpy)2 Ir(LL') complexes can be deduced from this linear relationship. The (fppy)2 Ir(LL') complexes with corresponding ancillary ligands display similar trends in excited-state properties.
Journal Article↗
A highly emissive Cu2N2 diamond core complex supported by a [PNP]- ligand.
A Cu2N2 diamond core structure, {(PNP)CuI}2 (2), supported by a [PNP]- ligand (1) ([PNP]- = bis(2-(diisobutylphosphino)phenyl)amide) has been prepared. 2 is highly emissive at ambient temperature in both the solid and solution states and is characterized by a relatively long-lived excited state (tau > 10 mus) and an unusually high quantum yield (phi > 0.65). These observations are consistent with a low degree of structural reorganization between the ground state of 2 and its excited state *2, and also with a high degree of steric protection of the two copper centers of 2 afforded by the bulky [PNP]- ligand. An estimate for the excited-state reduction potential of *2 (ca. -3.2 V vs Fc+/Fc), and the availability of two well-separated and reversible ground-state redox processes, suggests that bimetallic copper systems of these types may be interesting candidates to consider for photochemically driving multielectron redox transformations.
Journal Article↗
Ground-state singlet L3Fe-(mu-N)-FeL3 and L3Fe(NR) complexes featuring pseudotetrahedral Fe(II) centers.
Pseudotetrahedral iron(II) coordination complexes that contain bridged nitride and terminal imide linkages, and exhibit singlet ground-state electronic configurations, are described. Sodium amalgam reduction of the ferromagnetically coupled dimer, {[PhBP(3)]Fe(mu-1,3-N(3))}(2) (2) ([PhBP(3)] = [PhB(CH(2)PPh(2))(3)](-)), yields the diamagnetic bridging nitride species [{[PhBP(3)]Fe}(2)(mu-N)][Na(THF)(5)] (3). The Fe-N-Fe linkage featured in the anion of 3 exhibits an unusually bent angle of approximately 135 degrees , and the short Fe-N bond distances (Fe-N(av) approximately equal to 1.70 A) suggest substantial Fe-N multiple bond character. The diamagnetic imide complex {[PhBP(3)]Fe(II)(triple bond)N(1-Ad)}{(n)()Bu(4)N} (4) has been prepared by sodium amalgam reduction of its low-spin iron(III) precursor, [PhBP(3)]Fe(III)(triple bond)N(1-Ad) (5). Complexes 4 and 5 have been structurally characterized, and their respective electronic structures are discussed in the context of a supporting DFT calculation. Diamagnetic 4 provides a bona fide example of a pseudotetrahedral iron(II) center in a low-spin ground-state configuration. Comparative optical data strongly suggest that dinuclear 3 is best described as containing two high-spin iron(II) centers that are strongly antiferromagnetically coupled to give rise to a singlet ground-state at room temperature.
Journal Article↗
Synthetic, structural, and mechanistic aspects of an amine activation process mediated at a zwitterionic Pd(II) center.
A zwitterionic palladium complex [[Ph(2)BP(2)]Pd(THF)(2)][OTf] (1) (where [Ph(2)BP(2)] = [Ph(2)B(CH(2)PPh(2))(2)](-)) reacts with trialkylamines to activate a C-H bond adjacent to the amine N atom, thereby producing iminium adduct complexes [Ph(2)BP(2)]Pd(N,C:eta(2)-NR(2)CHR'). In all cases examined the amine activation process is selective for the secondary C-H bond position adjacent to the N atom. These palladacycles undergo facile beta-hydride elimination/olefin reinsertion processes as evident from deuterium scrambling studies and chemical trap studies. The kinetics of the amine activation process was explored, and beta-hydride elimination appears to be the rate-limiting step. A large kinetic deuterium isotope effect for the amine activation process is evident. The reaction profile in less polar solvents such as benzene and toluene is different at room temperature and leads to dimeric [[Ph(2)BP(2)]Pd](2) (4) as the dominant palladium product. Low-temperature toluene-d(8) experiments proceed more cleanly, and intermediates assigned as [Ph(2)BP(2)]Pd(NEt(3))(OTf) and the iminium hydride species [[Ph(2)BP(2)]Pd(H)(Et(2)N=CHCH(3))][OTf] are directly observed. The complex (Ph(2)SiP(2))Pd(OTf)(2) (14) was also studied for amine activation and generates dimeric [(Ph(2)SiP(2))Pd](2)[OTf](2) (16) as the dominant palladium product. These collective data are discussed with respect to the mechanism of the amine activation and, in particular, the influence that solvent polarity and charge have on the overall reaction profile.
Journal Article↗