Triple Bonds Between Iron and Heavier Group 15 Elements in AFe(CO)3- (A=As, Sb, Bi) Complexes

Photoelectron velocity map imaging spectroscopy of group 14 elements and iron tetracarbonyl anionic clusters MFe(CO)4- (M = Si, Ge, Sn)

The heteronuclear group 14 M-iron tetracarbonyl clusters MFe(CO)4- (M = Si, Ge, Sn) anions have been generated in the gas phase by laser ablation of M-Fe alloys and detected by mass and photoelectron spectroscopy. With the support of quantum chemical calculations, the geometric and electronic structures of MFe(CO)4- (M = Si, Ge, Sn) are elucidated, which shows that all the MFe(CO)4- clusters have the M-Fe bonded, iron-centered, and carbonyl-terminal M-Fe(CO)4 structure with the C2v symmetry and a 2B2 ground state. The M-Fe bond can be considered a double bond, which includes one σ electron sharing bond and one π dative bond. The C-O bonds in those anionic clusters are calculated to be elongated to different extents, and in particular, the C-O bonds in SiFe(CO)4- are elongated more. The Si-Fe alloy thus turns out to be a better collocation to activate the C-O bonds in the gas phase among group 14. The present findings have important implications for the rational development of high-performance catalysts with isolated metal atoms/clusters dispersed on supports.

Spectroscopic characterization of heteronuclear iron-chromium carbonyl cluster anions

Infrared photodissociation spectroscopy has been used to investigate CrFe(CO)n- (n = 4-9) clusters in the gas phase. Comparison of the observed spectra in the carbonyl stretching frequency region with those predicted for low-lying isomers by DFT calculations showed that the observed CrFe(CO)n- (n = 4-8) clusters could be characterized to have Cr-Fe bonded (OC)4Fe-Cr(CO)n-4 structures. The coexistence of isomers with the (OC)Fe-Cr(CO)5 and (OC)3Fe-Cr(CO)4 structures was also observed for CrFe(CO)6- and CrFe(CO)7- anions, respectively. The CrFe(CO)n- (n = 4-8) complexes were strongly bonded systems. The CrFe(CO)8- complex was a coordination-saturated cluster, and the CrFe(CO)9- anion was characterized to contain a CrFe(CO)8- core tagged by one CO molecule. Bonding analysis revealed that the Cr-Fe bonds in the CrFe(CO)n- (n = 4-8) clusters were predominantly σ-type single bonds. The iron center in the Fe(CO)4 moiety and the chromium center in the Cr(CO)5 moiety fulfilled the 18-electron configuration for the CrFe(CO)n- (n = 4-6) clusters. As in the CrFe(CO)n- (n = 7, 8) complexes, the iron center in the Fe(CO)4 moiety exhibited a 17-electron configuration, while the chromium center in the Cr(CO)4 moiety exhibited a 16-electron configuration. These findings provide valuable insights into the structure and bonding mechanism of heterometallic carbonyl clusters.

XPu(CO)n (X = B, Al, Ga; n = 2 to 4): π Back-Bonding in Heterodinuclear Plutonium Boron Group Compounds with an End-On Carbonyl Ligand

The bonding situation and the oxidation state of plutonium in heterodinuclear plutonium boron group carbonyl compounds XPu(CO)_n (X = B, Al, Ga; n = 2 to 4) were investigated by systematically searching their ground-state geometrical structures and by analyzing their electronic structures. We found that the series of XPu(CO)_n compounds show various interesting structures with an increment in n as well as a changeover from X = B to Ga. The first ethylene dione (OCCO) compounds of plutonium are found in AlPu(CO)_n (n = 2, 3). A direct Ga-Pu single bond is first predicted in the series of GaPu(CO)_n, where the bonding pattern represents a class of the Pu → CO π back-bonding system. There is a trend where the Pu-Ga bonding decreases and the Pu-C(O) covalency increases as the Ga oxidation state increases from Ga(0) to Ga(I). Our finding extends the metal → CO covalence back-bonding concept to plutonium systems and also enriches plutonium-containing bonding chemistry.

Two σ- and two π-dative quadruple bonds between the s-block element and transition metal in [BeM(CO)4; M = Fe - Os]

We report the chemical bonding and reactivity of the first example of neutral 18 valence electron transition metal complexes of beryllium, [BeM(CO)₄; M = Fe - Os], in trigonal bipyramidal coordination geometry, where the bonding between the transition metal and the s-block element beryllium (M-Be) can be best described by dative quadruple bonds. In contrast to the conventional multiple bonding pattern, the quadruple bonds comprise two σ-bonds and two π-bonds, viz., one Be → M σ-bond, one M → Be σ-bond, and two M → Be π-bonds. Since the M-Be quadruple bonds are described by dative interactions, the Be centre shows ambiphilic character as indicated by the high proton and hydride affinity values.

Infrared photodissociation spectroscopy of heteronuclear group 15 metal-iron carbonyl cluster anions AmFe(CO)n- (A = Sb, Bi; m, n = 2, 3)

Heteronuclear group 15 metal-iron carbonyl cluster complexes of A_mFe(CO)_n^- (A = Sb, Bi; m, n = 2-3) were generated in the gas phase and studied by infrared photodissociation spectroscopy in the carbonyl stretching region. Their structures were determined by comparing the experimental spectra with predicted spectra derived from DFT calculations at the B3LYP and BP86 levels. All of the A_mFe(CO)_n^- cluster anions were determined to have Fe(CO)_n^- fragments with all of the CO ligands terminally bonded to the iron center, and they can be regarded as being formed via the interactions of the neutral group 15 metal clusters with the Fe(CO)_n^- fragments. Bonding analyses indicated that each A₂Fe(CO)_n^- (n = 2, 3) cluster anion contained two A-Fe single bonds and one A-A double bond. Each A₃Fe(CO)_n^- (n = 2, 3) cluster anion involved three A-Fe single bonds and three A-A single bonds. There is an isolobal relationship between the Fe(CO)₃^- group and the group 15 atoms. The substitution of an Fe(CO)₃^- group in place of one A atom in the tetrahedral A₄ molecule resulted in an A₃Fe(CO)₃^- cluster anion with the closed-shell electronic configuration for all the group 15 metals and iron atoms.

Formation and Characterization of BeFe(CO)4 - Anion with Beryllium-Iron Bonding

Heteronuclear BeFe(CO)₄ ^- anion complex is generated in the gas phase, which is detected by mass-selected infrared photodissociation spectroscopy in the carbonyl stretching frequency region. The complex is characterized to have a Be-Fe bonded Be-Fe(CO)₄ ^- structure with C_3v symmetry and all of the four carbonyl ligands bonded on the iron center. Quantum chemical studies indicate that the complex has a quite short Be-Fe bond. Besides one electron-sharing σ bond, there are two additional, albeit weak, Be ← Fe(CO)₄ ^- dative π bonding interactions. The findings imply that metal-metal bonding between s-block and transition metals is viable under suitable coordination environment.

Closed Shell Iron(IV) Oxo Complex with an Fe-O Triple Bond: Computational Design, Synthesis, and Reactivity

Iron(IV)-oxo intermediates in nature contain two unpaired electrons in the Fe-O antibonding orbitals, which are thought to contribute to their high reactivity. To challenge this hypothesis, we designed and synthesized closed-shell singlet iron(IV) oxo complex [(quinisox)Fe(O)]+ (1+ ; quinisox-H=(N-(2-(2-isoxazoline-3-yl)phenyl)quinoline-8-carboxamide). We identified the quinisox ligand by DFT computational screening out of over 450 candidates. After the ligand synthesis, we detected 1+ in the gas phase and confirmed its spin state by visible and infrared photodissociation spectroscopy (IRPD). The Fe-O stretching frequency in 1+ is 960.5 cm-1 , consistent with an Fe-O triple bond, which was also confirmed by multireference calculations. The unprecedented bond strength is accompanied by high gas-phase reactivity of 1+ in oxygen atom transfer (OAT) and in proton-coupled electron transfer reactions. This challenges the current view of the spin-state driven reactivity of the Fe-O complexes.

From "S" to "O": experimental and theoretical insights into the atmospheric degradation mechanism of dithiophosphinic acids

Dithiophosphinic acids (DPAHs, expressed as R1R2PSSH) are a type of sulfur-donor ligand that have been vastly applied in hydrometallurgy. In particular, DPAHs have shown great potential in highly efficient trivalent actinide/lanthanide separation, which is one of the most challenging tasks in separation science and is of great importance for the development of an advanced fuel cycle in nuclear industry. However, DPAHs have been found liable to undergo oxidative degradation in the air, leading to significant reduction in the selectivity of actinide/lanthanide separation. In this work, the atmospheric degradation of five representative DPAH ligands was investigated for the first time over a sufficiently long period (180 days). The oxidative degradation process of DPAHs elucidated by ESI-MS, 31P NMR, and FT-IR analyses is R1R2PSSH → R1R2PSOH → R1R2POOH → R1R2POO-OOPR1R2, R1R2PSSH → R1R2PSS-SSPR1R2, and R1R2PSSH → R1R2PSOH → R1R2POS-SOPR1R2. Meanwhile, the determination of pK a values through pH titration and oxidation product by PXRD further confirms the S → O transformation in the process of DPAH deterioration. DFT calculations suggest that the hydroxyl radical plays the dominant role in the oxidation process of DPAHs and the order in which the oxidation products formed is closely related to the reaction energy barrier. Moreover, nickel salts of DPAHs have shown much higher chemical stability than DPAHs, which was also elaborated through molecular orbital (MO) and adaptive natural density portioning (AdNDP) analyses. This work unambiguously reveals the atmospheric degradation mechanism of DPAHs through both experimental and theoretical approaches. At the application level, the results not only provide an effective way to preserve DPAHs but could also guide the design of more stable sulfur-donor ligands in the future.

Combined experimental and theoretical studies towards mutual osmium-bismuth donor/acceptor bonding

Osmium(ii) PNP pincer complexes bearing a hemilabile pyridyl-pyrazolide (PyrPz) ligand have been synthesised, and their reactivity towards Lewis acidic bismuth compounds has been examined. Reactions with BiCl3 resulted in chlorine-atom-transfer to give an osmium(iii) species. Reactions with cationic bismuth species led to adduct formation through N → Bi bond formation via the PyrPz ligand. Theoretical analyses revealed that steric interactions hamper Os → Bi bond formation and indicate that such interactions are possible upon reducing the steric profile around the osmium atom. Analytical techniques include NMR, IR, and EPR spectroscopy, cyclic voltammetry, elemental analysis and DFT calculations.

Unsaturated binuclear homoleptic nickel carbonyl anions Ni2(CO)n− (n = 4–6) featuring double three-center two-electron Ni–C–Ni bonds

The two nickel atoms in the Ni₂(CO)_n⁻ (n = 4–6) complexes are joined by two bridging carbonyl ligands via the sharing three-center two-electron Ni–C–Ni bond in turn to achieve the (16,16), (16,18), and eventually the favored (18,18) configurations.

Multicenter electron-sharing σ-bonding in the AgFe(CO)4− complex

For the AgFe(CO)₄⁻ anion, the silver atom is covalently bonded to the anionic tetracarbonyl-iron, an isolobal analogue of the methyl radical, via a peculiar decentralized electron-sharing σ bond.

Triple bonds between iron and heavier group-14 elements in the AFe(CO)3- complexes (A = Ge, Sn, and Pb)

Heteronuclear transition-metal-main-group element carbonyl anion complexes of AFe(CO)3- (A = Ge, Sn, and Pb) are prepared using a laser vaporization supersonic ion source in the gas phase, which were studied by mass-selected infrared (IR) photodissociation spectroscopy. The geometric and electronic structures of the experimentally observed species are identified by a comparison of the measured and calculated IR spectra. These anion complexes have a 2A1 doublet electronic ground state and feature an A[triple bond, length as m-dash]Fe triply bonded C3v structure with all of the carbonyl ligands bonded at the iron center. Bonding analyses of AFe(CO)3- (A = C, Si, Ge, Sn, Pb, and Fl) indicate that the complexes are triply bonded between the valence np atomic orbitals of bare group-14 atoms and the hybridized 3d and 4p atomic orbitals of iron.

Post Hartree-Fock calculations of pnictogen-uranium bonding in EUF3 (E = N-Bi)

NUF3 is identified as having a N[triple bond, length as m-dash]U triple bond, as has been previously found (Andrews et al., Angew. Chem. Int. Ed., 2008, 47, 5366). By contrast, while previously reported calculations on PUF3 and AsUF3 (Andrews et al., Inorg. Chem., 2009, 48, 6594) gave a E[triple bond, length as m-dash]U triple bond, our calculations suggest a single bond for both molecules, with antibonding π* and non-bonding 5fU orbitals significantly occupied, and highly multiconfigurational wavefunctions. We propose this difference to be due to the smaller [6,6] active space used (σ, π, π* and σ*) in the previous studies. In our calculations, a [6,16] active space was employed in order to include uranium f-orbitals and pnictogen d-orbitals.

SCI: a robust and reliable density-based descriptor to determine multiple covalent bond orders

Very recently [J. Phys. Chem. A 2018, 122 (11), 3087-3095], we proposed to employ the Pauli energy to identify and determine strong covalent interactions (SCI), whose bond order are equal to or larger than two. This is done through the signature isosurface shape between the two bonding atoms. We discovered that the signature shape for a double, triple, and quadruple covalent bond is like a dumbbell, donut (torus), and four-beats, respectively. Systems with even higher bond orders were examined and confirmed. This work is a follow-up study of our previous work. The dependence of the signature isosurface shape on the choice of methodologies and basis sets is systematically investigated. Its effectiveness and robustness in determining bond orders are highlighted again with more examples. In addition, using the molybdenum dimer in different environments, e.g., in vacuum, sandwiched between molecules, and encapsulated in the C₈₀ cage, as illustrative examples, we show that, generally speaking, bond strength and bond order are two different chemical concepts. For systems containing transition metals, it is not always true that a short metal-metal bond length corresponds to a larger bond order. Put together, these results should provide additional pieces of convincing evidence showing that the SCI index is a robust and reliable density-based descriptor to accurately determine multiple covalent bond orders.

Efficient Nitrogen Fixation via a Redox-Flexible Single-Iron Site with Reverse-Dative Iron → Boron σ Bonding

Model systems of the FeMo cofactor of nitrogenase have been explored extensively in catalysis to gain insights into their ability for nitrogen fixation that is of vital importance to the human society. Here we investigate the trigonal pyramidal borane-ligand Fe complex by first-principles calculations, and find that the variation of oxidation state of Fe along the reaction path correlates with that of the reverse-dative Fe → B bonding. The redox-flexibility of the reverse-dative Fe → B bonding helps to provide an electron reservoir that buffers and stabilizes the evolution of Fe oxidation state, which is essential for forming the key intermediates of N₂ activation. Our work provides insights for understanding and optimizing homogeneous and surface single-atom catalysts with reverse-dative donating ligands for efficient dinitrogen fixation. The extension of this kind of molecular catalytic active center to heterogeneous catalysts with surface single-clusters is also discussed.