Activation and Cleavage of Dinitrogen by Three-coordinate Metal Complexes Involving Mo(III) and Nb(II/III)

Density functional calculations have been employed to rationalize why the heteronuclear N2-bridged MoIIINbIII dimer, [Ar(tBu)N]3Mo(µ-N2)Nb[N(iPr)Ar]3 (Ar = 3,5-C6H3Me2), does not undergo cleavage of the dinitrogen bridge in contrast to the analogous MoIIIMoIII complex which, although having a less activated N–N bond, undergoes spontaneous dinitrogen cleavage at room temperature. The calculations reveal that although the overall reaction is exothermic for both systems, the actual cleavage step is endothermic by 144 kJ mol−1 for the MoIIINbIII complex whereas the MoIIIMoIII system is exothermic by 94 kJ mol−1. The reluctance of the MoIIINbIII system to undergo N2 cleavage is attributed to its d3d2 metal configuration which is one electron short of the d3d3 configuration necessary to reductively cleave the dinitrogen bridge. This is confirmed by additional calculations on the related d3d3 MoIIINbII and NbIINbII systems for which the cleavage step is calculated to be substantially exothermic, accounting for why in the presence of the reductant KC8, the [Ar(tBu)N]3Mo- (µ-N2)Nb[N(iPr)Ar]3 complex was observed to undergo spontaneous cleavage of the dinitrogen bridge. On the basis of these results, it can be concluded that the level of activation of the N–N bond does not necessarily correlate with the ease of cleavage of the dinitrogen bridge

Factors Dictating Carbene Formation at (PNP)Ir

The mechanistic subtleties involved with the interaction of an amido/bis(phosphine)-supported (PNP)Ir fragment with a series of linear and cyclic ethers have been investigated using density functional theory. Our analysis has revealed the factors dictating reaction direction toward either an iridium-supported carbene or a vinyl ether adduct. The (PNP)Ir structure will allow carbene formation only from accessible carbons α to the ethereal oxygen, such that d electron back-donation from the metal to the carbene ligand is possible. Should these conditions be unavailable, the main competing pathway to form vinyl ether can occur, but only if the (PNP)Ir framework does not sterically interfere with the reacting ether. In situations where steric hindrance prevents unimpeded access to both pathways, the reaction may progress to the initial C−H activation but no further. Our mechanistic analysis is density functional independent and whenever possible confirmed experimentally by trapping intermediate species experimentally. We have also highlighted an interesting systematic error present in the DFT analysis of reactions where steric environment alters considerably within a reaction

Structural and electronic models of the water oxidizing complex in the S 0 state of photosystem II: a density functional study

Large size (228 atom, 229 atom for protonated form) molecular models of the oxygen evolving complex of photosystem II (OEC), with a complete set of ligating aminoacids, the redox-active tyrosine YZ, and proton/water transfer channels terminating at the water oxidizing Mn/Ca cluster, are constructed based on the highest available resolution X-ray diffraction structures of the protein and our previous density functional theory (DFT) studies of isolated metal cluster model structures. Geometries optimized using the general gradient approximation (GGA) or hybrid density functionals are compared with high-resolution extended X-ray absorption fine structure (EXAFS) spectroscopic data and show that an antiferromagnetic configuration of the Mn centers in the cluster gives computed metal-metal distances in excellent agreement with experiment. The excitation energies predicted by time-dependent density functional theory (TDDFT) calculations for truncated 106 atom and 78 atom structures derived from the large models show that a previously proposed III-III-III-II oxidation pattern of the Mn atoms agrees very well with the X-ray absorption near-edge structure (XANES) observed for the S0 state of the OEC. This supports a "low" Mn oxidation state paradigm for the OEC, when a realistic protein imposed environment for the catalytic metal cluster is used in calculations. The probable protonation sites in the cluster and roles of the proton/water transfer channels are discussed in light of the computational results

Activation and Cleavage of the N-O Bond in Dinuclear Mixed-Metal Nitrosyl Systems and Comparative Analysis of Carbon Monoxide, Dinitrogen, and Nitric Oxide Activation

The activation and scission of the N–O bond in nitric oxide using dinuclear mixed-metal species, comprising transition elements with d3 and d2 configurations and trisamide ligand systems, have been investigated by means of density functional calculations. The [Cr(III)–V(III)] system is analyzed in detail and, for comparative purposes, the [Mo(III)–Nb(III)], [W(III)–Ta(III)], and (mixed-row) [Mo(III)–V(III)] systems are also considered. The overall reaction and individual intermediate steps are favourable for all systems, including the case where first row (Cr and V) metals are exclusively involved, a result that has not been observed for the related dinitrogen and carbon monoxide systems. In contrast to the cleavage of dinitrogen by three-coordinate Mo amide complexes where the dinuclear intermediate possesses a linear [Mo–NN–Mo] core, the [M–NO–M′] core must undergo significant bending in order to stabilize the dinuclear species sufficiently for the reaction to proceed beyond the formation of the nitrosyl encounter complex. A comparative bonding analysis of nitric oxide, dinitrogen and carbon monoxide activation is also presented. The overall results indicate that the π interactions are the dominant factor in the bonding across the [M–L1L2–M′] (L1L2 = N–O, N–N, C–O) moiety and, consequently, the activation of the L1–L2 bond. These trends arise from the fact that the energy gaps between the π orbitals on the metal and small molecule fragments are much more favourable than for the corresponding σ orbitals. The π energy gaps decrease in the order [NO \u3c N2 \u3c CO] and consequently, for each individual π orbital interaction, the back donation between the metal and small molecule increases in the order [CO \u3c N2 \u3c NO]. These results are in accord with previous findings suggesting that optimization of the π interactions plays a central role in increasing the ability of these transition metal systems to activate and cleave small molecule bonds

Synthesis, characterization and DFT studies of the cobalt(III) complex of a tetrapodal pentadentate N4S donor ligand

The synthesis of the pentadentate ligand 2,6-bis(3,3-dimethyl-2,4-dioxocyclohexanyl)-4-thiaheptane (N(4)Samp) is described. The synthetic pathway involves the coupling of two 1,3-(dimethylenedioxy)-2-methyl-2-(methylene-p-toluenesulfonyl)propane moieties with sodium sulfide and subsequent synthetic elaboration to prepare the final N4S donor system. The cobalt(III) complex [Co(N(4)Samp)Cl](2+) has been prepared and subsequently crystallized as the tetrachlorozincate salt. The X-ray structure analysis confirms the pentadentate nature of the ligand and shows the thioether donor occupying one apex with four equivalent amine donors effectively occupying the equatorial plane of the molecule. The sixth coordination site is occupied by a chloro ligand. The electronic absorption and C-13 NMR spectra have been studied. DFT calculations have been employed to explore structural and mechanistic comparisons between [Co(N(4)Samp)Cl](2+) and an analogous pentaamine complex

Ligand Rotation in [Ar(R)N]\u3csub\u3e3\u3c/sub\u3eM-N\u3csub\u3e2\u3c/sub\u3e-M′[N(R)Ar]\u3csub\u3e3\u3c/sub\u3e (M, M′ = Mo\u3csup\u3eIII\u3c/sup\u3e, Nb\u3csup\u3eIII\u3c/sup\u3e; R = \u3csup\u3ei\u3c/sup\u3ePr and \u3csup\u3et\u3c/sup\u3eBu) Dimers

Earlier calculations on the model N2-bridged dimer (µ-N2)-{Mo[NH2]3}2 revealed that ligand rotation away from a trigonal arrangement around the metal centres was energetically favourable resulting in a reversal of the singlet and triplet energies such that the singlet state was stabilized 13 kJ mol−1 below the D3d triplet structure. These calculations, however, ignored the steric bulk of the amide ligands N(R)Ar (R = iPr and tBu, Ar = 3,5-C6H3Me2) which may prevent or limit the extent of ligand rotation. In order to investigate the consequences of steric crowding, density functional calculations using QM/MM techniques have been performed on the MoIIIMoIII and MoIIINbIII intermediate dimer complexes (µ-N2)-{Mo[N(R)Ar]3}2 and [Ar(R)N]3Mo-(µ-N2)-Nb[N(R)Ar]3 formed when threecoordinate Mo[N(R)Ar]3 and Nb[N(R)Ar]3 react with dinitrogen. The calculations indicate that ligand rotation away from a trigonal arrangement is energetically favourable for all of the ligands investigated and that the distortion is largely electronic in origin. However, the steric constraints of the bulky amide groups do play a role in determining the final orientation of the ligands, in particular, whether the ligands are rotated at one or both metal centres of the dimer. Analogous to the model system, QM/MM calculations predict a singlet ground state for the (µ-N2)-{Mo[N(R)Ar]3}2 dimers, a result which is seemingly at odds with the experimental triplet ground state found for the related (µ-N2)-{Mo[N(tBu)Ph]3}2 system. However, QM/MM calculations on the (µ-N2)-{Mo[N(tBu)Ph]3}2 dimer reveal that the singlet–triplet gap is nearly 20 kJ mol−1 smaller and therefore this complex is expected to exhibit very different magnetic behaviour to the (µ-N2)-{Mo[N(R)Ar]3}2 system

Thioxoethenylidene (CCS) as a bridging ligand

The reaction of [Mo(≡CBr)(CO)2(Tp*)] (Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate) with [Fe2(μ-SLi)2(CO)6] affords, inter alia, the unsymmetrical binuclear thioxoethenylidene complex [Mo2(μ,σ(C):η2(C′S)-CCS)(CO)4(Tp*)2], which may be more directly obtained from [Mo(≡CBr)(CO)2(Tp*)] and Li2S. The reaction presumably proceeds via the intermediacy of the bis(alkylidynyl)thioether complex S{C≡Mo(CO)2(Tp*)}2, which was, however, not directly observed but explored computationally and found to lie 78.6 kJ mol–1 higher in energy than the final thioxoethenylidene product. Computational interrogation of the molecules [M2(μ-C2S)(CO)2(Tp*)2] (M = Mo, W, Re, Os) reveals three plausible coordination modes for a thioxoethenylidene bridge which involve a progressive strengthening of the C–C bond and weakening of the M–C and M–S bonds, as might be expected from simple effective atomic number considerations.This work was supported by the Australian Research Council (DP130102598 and DP110101611)

Rationalizing the Different Products in the Reaction of N\u3csub\u3e2\u3c/sub\u3e with Three-coordinate MoL\u3csub\u3e3\u3c/sub\u3e Complexes

The reaction of N2 with three-coordinate MoL3 complexes is known to give rise to different products, N–MoL3, L3Mo–N–MoL3 or Mo2L6, depending on the nature of the ligand L. The energetics of the different reaction pathways are compared for L = NH2, NMe2, N(iPr)Ar and N(tBu)Ar (Ar = 3,5-C6H3Me2) using density functional methods in order to rationalize the experimental results. Overall, the exothermicity of each reaction pathway decreases as the ligand size increases, largely due to the increased steric crowding in the products compared to reactants. In the absence of steric strain, the formation of the metal–metal bonded dimer, Mo2L6, is the most exothermic pathway but this reaction shows the greatest sensitivity to ligand size varying from significantly exothermic, −403 kJ mol−1 for L = NMe2, to endothermic, +78 kJ mol−1 for L = N(tBu)Ar. For all four ligands, formation of N–MoL3via cleavage of the N2 bridged dimer intermediate, L3Mo–N–N–MoL3, is strongly exothermic. However, in the presence of excess reactant MoL3, formation of the single atom-bridged complex L3Mo–N–MoL3 from N–MoL3 + MoL3 is both thermodynamically and kinetically favoured for L = NMe2 and N(iPr)Ar, in agreement with experiment. In the case of L = N(tBu)Ar, the greater steric bulk of the tBu group results in a much less exothermic reaction and a calculated barrier of 66 kJ mol−1 to formation of the L3Mo–N–MoL3 dimer. Consequently, for this ligand, the energetically and kinetically favoured product, consistent with the experimental data, is the nitride complex L3Mo–N

Long-Range Corrected DFT Calculations of First Hyperpolarizabilities and Excitation Energies of Metal Alkynyl Complexes

The performance of the CAM‐B3LYP, ωB97X and LC‐BLYP long‐range corrected density functional theory methods in the calculation of molecular first hyperpolarizabilities (β) and low‐lying charge transfer (CT) excitation energies of the metal alkynyl complexes M(C≡C‐4‐C6H4‐1‐NO2)(κ2‐dppe)(η5‐C5H5) [M=Fe (1), Ru (2), Os (3)] and trans‐[Ru{C≡C‐(1,4‐C6H4C≡C)n‐4‐C6H4‐1‐NO2}Cl(κ2‐dppm)2] [n=0 (4), 1 (5), 2 (6)] was assessed. The BLYP, B3LYP and PBE0 standard exchange‐correlation functionals and the Hartree‐Fock method were also examined. The BLYP functional was shown to perform poorly in the calculation of β and low‐energy CT transitions. The hybrid functionals (B3LYP and PBE0) showed significant improvement over the pure functional BLYP, but overestimated the hyperpolarizability ratios and the wavelengths of the lowest energy metal‐to‐ligand CT transitions for 5 and 6. The effect of long‐range corrections is noteworthy, particularly for the larger complexes, improving the calculation of β ratios for 4–6. However, CAM‐B3LYP, ωB97X, and LC‐BLYP considerably overestimated the low‐lying CT energies. PBE0 was found to give the best transition energy match for 4. The influence of the phenylene ring orientation in the alkynyl ligand on the calculated properties is substantial, particularly for the larger complexes. For these types of calculations, a basis set with diffuse functions (at least 6‐31+G(d)) for the heavy elements is recommended.The authors gratefully acknowledge the Australian Research Council (ARC) for financial support and also access to the Australian National University supercomputer facilities of the National Computational Infrastructure