Quasilinear 3d-metal(i) complexes [KM(N(Dipp)SiR3)2] (M = Cr-Co) - structural diversity, solution state behaviour and reactivity

The synthesis and characterization of neutral quasilinear 3d-metal(i) complexes of chromium to cobalt of the type [KM(N(Dipp)SiMe₃)₂] (Dipp = 2,6-di-iso-propylphenyl) are reported. In solid state these metal(i) complexes either occur as isolated molecules (Co) or are part of a potassium ion linked 1D-coordination polymer (Cr-Fe). In solution the potassium cation is either ligated within the ligand sphere of the metal silylamide or is separated from the complex depending on the solvent. For iron, we showcase that it is possible to use sodium or lithium metal for the reduction of the metal(ii) precursor. However, in these cases the resulting iron(i) complexes can only be isolated upon cation separation using an appropriate crown-ether. Further, the neutral metal(i) complexes are used to introduce NBu₄⁺ as an organic cation in the case of cobalt and iron. The impact of the intramolecular cation complexation was further demonstrated upon reaction with diphenyl acetylene which leads to bond formation processes and redox disproportionation instead of η²-alkyne complex formation.

Reduction of 2,2′-Bipyridine by Quasi-Linear 3d-Metal(I) Silylamides—A Structural and Spectroscopic Study

Quasi-linear anionic 3d-metal(I) silylamides are a new and promising class of molecules. Due to their highly negative reduction potential we wanted to test their capability to reduce substrates under coordination of their monoanionic radicaloid form. In a proof of principle study, we present the results of the reaction of metal(I) silylamides of chromium to cobalt with 2,2′-bipyridine (bipy), the redox non-innocence and reducibility of which was already established. In the course of these studies complexes of the type K{18-crown-6}[M(hmds)2(bipy)] (hmds = –N(SiMe3)2) were obtained. These compounds were isolated and thoroughly characterized to confirm the electron transfer onto the bipyridine ligand, which now acts as a radical monoanion. For comparison of the structural changes of the bipyridine ligand, the analogous zinc complexes were also synthesized. Overall our results indicate that anionic metal(I) silylamides are capable of reducing and ligate substrates, even when the electrochemical reduction potential of the latter is by up to 1 V higher.

Density Functional Calculations for Prediction of 57Fe Mössbauer Isomer Shifts and Quadrupole Splittings in β-Diketiminate Complexes

The relative ease of Mössbauer spectroscopy and of density functional theory (DFT) calculations encourages the use of Mössbauer parameters as a validation method for calculations, and the use of calculations as a double check on crystallographic structures. A number of studies have proposed correlations between the computationally determined electron density at the iron nucleus and the observed isomer shift, but deviations from these correlations in low-valent iron β-diketiminate complexes encouraged us to determine a new correlation for these compounds. The use of B3LYP/def2-TZVP in the ORCA platform provides an excellent balance of accuracy and speed. We provide here not only this new correlation and a clear guide to its use but also a systematic analysis of the limitations of this approach. We also highlight the impact of crystallographic inaccuracies, DFT model truncation, and spin states, with intent to assist experimentalists to use Mössbauer spectroscopy and calculations together.

NMR analysis of an Fe(i)–carbene complex with strong magnetic anisotropy

A paramagnetic, easy-plane anisotropic Fe^I complex, bearing cyclic-alkyl(amino) carbene (cAAC) ligands, is studied by means of NMR and DFT.

Linear and T-Shaped Iron(I) Complexes Supported by N-Heterocyclic Carbene Ligands: Synthesis and Structure Characterization

The use of the N-heterocyclic carbene (NHC) ligands 1,3-bis(2',6'-diethylphenyl)-4,5-(CH2)4-imidazol-2-ylidene (cyIDep), 1,3-bis(2',6'-diethylphenyl)-imidazolin-2-ylidene (sIDep), and its N-mesityl analogue sIMes enables the preparation of the two-coordinate homoleptic iron(I)-NHC complexes [(cyIDep)2Fe][BAr(F)4] (3, Ar(F) denoted for 3,5-di(trifluoromethyl)phenyl) and [(sIDep)2Fe][BAr(F)4] (4) and the T-shaped iron(I)-NHC complex [(sIMes)2Fe(THF)][BPh4] (5, THF = tetrahydrofuran). Complexes 3-5 were prepared via the sequential protocol of control reduction of iron(II) dihalides by KC8 in the presence of the corresponding NHC ligands followed by halide-abstraction with NaBAr4. Spectroscopic characterization, including single-crystal X-ray diffraction studies and (57)Fe Mössbauer spectroscopy, in combination with density functional theory calculations, suggest their high-spin nature. Solution property study (absorption spectroscopy and cyclic voltammetry) indicates that 3 and 5 keep their corresponding two- and three-coordinate nature in THF solution, and 4 might reversibly coordinate a THF molecule to form, presumably, the T-shaped species [(sIDep)2Fe(THF)][BAr(F)4]. The isolation of 3 and 4 demonstrates the accessibility of homoleptic two-coordinate iron(I)-NHC complexes.

A Two-Coordinate Cobalt(II) Imido Complex with NHC Ligation: Synthesis, Structure, and Reactivity

The synthesis, structural characterization, and reactivity of the first two-coordinate cobalt complex featuring a metal-element multiple bond [(IPr)Co(NDmp)] (4; IPr=1,3-bis(2',6'-diisopropylphenyl)imidazole-2-ylidene; Dmp=2,6-dimesitylphenyl) is reported. Complex 4 was prepared from the reaction of [(IPr)Co(η(2) -vtms)2 ] (vtms=vinyltrimethylsilane) with DmpN3 . An X-ray diffraction study revealed its linear C-Co-N core and a short Co-N distance (1.691(6) Å). Spectroscopic characterization and calculation studies indicated the high-spin nature of 4 and the multiple-bond character of the Co-N bond. Complex 4 effected group-transfer reactions to CO and ethylene to form isocyanide and imine, respectively. It also facilitated E-H (E=C, Si) σ-bond activation of terminal alkyne and hydrosilanes to produce the corresponding cobalt(II) alkynyl and cobalt(II) hydride complexes as 1,2-addition products.

A combined magnetic circular dichroism and density functional theory approach for the elucidation of electronic structure and bonding in three- and four-coordinate iron(II)-N-heterocyclic carbene complexes

The combination of iron salts and N-heterocyclic carbene (NHC) ligands is a highly effective combination in catalysis, with observed catalytic activities being highly dependent on the nature of the NHC ligand. Detailed spectroscopic and electronic structure studies have been performed on both three- and four-coordinate iron(II)-NHC complexes using a combined magnetic circular dichroism (MCD) and density functional theory (DFT) approach that provide detailed insight into the relative ligation properties of NHCs compared to traditional phosphine and amine ligands as well as the effects of NHC backbone structural variations on iron(II)-NHC bonding. Near-infrared MCD studies indicate that 10Dq(Td) for (NHC)2FeCl2 complexes is intermediate between those for comparable amine and phosphine complexes, demonstrating that such iron(II)-NHC and iron(II)-phosphine complexes are not simply analogues of one another. Theoretical studies including charge decomposition analysis indicate that the NHC ligands are slightly stronger donor ligands than phosphines but also result in significant weakening of the Fe-Cl bonds compared to phosphine and amine ligands. The net result is significant differences in the d orbital energies in four-coordinate (NHC)2FeCl2 complexes relative to the comparable phosphine complexes, where such electronic structure differences are likely a significant contributing factor to the differing catalytic performances observed with these ligands. Furthermore, Mössbauer, MCD and DFT studies of the effects of NHC backbone structure variations (i.e. saturated, unsaturated, chlorinated) on iron-NHC bonding and electronic structure in both three- and four-coordinate iron(II)-NHC complexes indicate only small differences as a function of backbone structure, that are likely amplified at lower oxidation states of iron due to the resulting decrease in the energy separation between the occupied iron d orbitals and the unoccupied NHC π* orbitals.

Low-temperature N2 binding to two-coordinate L2Fe(0) enables reductive trapping of L2FeN2(-) and NH3 generation

The two-coordinate [(CAAC)2Fe] complex [CAAC = cyclic (alkyl)(amino)carbene] binds dinitrogen at low temperature (T<-80 °C). The resulting putative three-coordinate N2 complex, [(CAAC)2Fe(N2)], was trapped by one-electron reduction to its corresponding anion [(CAAC)2FeN2](-) at low temperature. This complex was structurally characterized and features an activated dinitrogen unit which can be silylated at the β-nitrogen atom. The redox-linked complexes [(CAAC)2Fe(I)][BAr(F)4], [(CAAC)2Fe(0)], and [(CAAC)2Fe(-I)N2](-) were all found to be active for the reduction of dinitrogen to ammonia upon treatment with KC8 and HBAr(F)4⋅2 Et2O at -95 °C [up to (3.4±1.0) equivalents of ammonia per Fe center]. The N2 reduction activity is highly temperature dependent, with significant N2 reduction to NH3 only occurring below -78 °C. This reactivity profile tracks with the low temperatures needed for N2 binding and an otherwise unavailable electron-transfer step to generate reactive [(CAAC)2FeN2](-) .

Aspherical-atom modeling of coordination compounds by single-crystal X-ray diffraction allows the correct metal atom to be identified

Single-crystal X-ray diffraction (XRD) is often considered the gold standard in analytical chemistry, as it allows element identification as well as determination of atom connectivity and the solid-state structure of completely unknown samples. Element assignment is based on the number of electrons of an atom, so that a distinction of neighboring heavier elements in the periodic table by XRD is often difficult. A computationally efficient procedure for aspherical-atom least-squares refinement of conventional diffraction data of organometallic compounds is proposed. The iterative procedure is conceptually similar to Hirshfeld-atom refinement (Acta Crystallogr. Sect. A- 2008, 64, 383-393; IUCrJ. 2014, 1,61-79), but it relies on tabulated invariom scattering factors (Acta Crystallogr. Sect. B- 2013, 69, 91-104) and the Hansen/Coppens multipole model; disordered structures can be handled as well. Five linear-coordinate 3d metal complexes, for which the wrong element is found if standard independent-atom model scattering factors are relied upon, are studied, and it is shown that only aspherical-atom scattering factors allow a reliable assignment. The influence of anomalous dispersion in identifying the correct element is investigated and discussed.