Computational Assessment of MLCT versus MC Stabilities in First-to-Third-Row d6 Pseudo-Octahedral Transition Metal Complexes

An accurate modeling of metal-to-ligand-charge-transfer (MLCT) and metal-centered (MC) excited state energies is key to predict the photoinduced response in transition metal complexes (TMCs). Herein, the importance of the ground state and excited state reference geometries is addressed for three-prototype d⁶ pseudo-octahedral TMCs, each displaying a different potential energy landscape of MLCT versus MC relative stabilities. Several functionals are used within the time-dependent density functional theory (TDDFT), as well as multireference wave-function theory (MS-CASPT2), applied to [Mn(im)(CO)₃(phen)]⁺, [Ru(im)₂(bpy)₂]²⁺, and [Re(im)(CO)₃(phen)]⁺, (im: imidazole, bpy: bypiridine, phen: phenantroline). The results revel that TDDFT is robust except when using B3LYP functional for first-row d⁶ TMCs. In contrast, MS-CASPT2 calculations are strongly biased in those cases with competitive MLCT/MC states. The results reinforce the reliability of B3LYP to describe the excited states in d⁶ TMCs, but question the validity of assessing the density functional theory (DFT)/TDDFT performance via direct comparison with MS-CASPT2 performed at the same DFT reference geometry as a standard strategy. © 2019 Wiley Periodicals, Inc.     

Mild and Complete Carbonyl Ligand Scission on a Mononuclear Transition Metal Complex

The complete scission of the carbon–oxygen bond of carbon monoxide, while frequently observed on bulk metals and with bimetallic and cluster transition metal complexes, is unknown with monometallic systems. Reaction of a zerovalent iron bis(borylene) complex with a cyclic (alkyl)(amino)carbene revealed a highly selective intramolecular cleavage of the C?O bond of a carbonyl ligand at room temperature, leading to the formation of a highly unusual iron complex containing a base‐stabilized (bora)alkylideneborane ligand. DFT investigation of the reaction mechanism suggested that the two Lewis acidic borylene boron atoms cooperate to cleave the C?O multiple bond.     

Performance of Density Functional Theory for Transition Metal Oxygen Bonds

The accuracy of density functional theory (DFT) limits predictions in theoretical catalysis, and strong chemical bonds between transition metals and oxygen pose a particular challenge. We benchmarked 30 diverse density functionals against the bond dissociation enthalpies (BDE) of the 30 MO and 30 MO⁺ diatomic systems of all the 3d, 4d, and 5d metals, to test universality across the d-block as required in comparative studies. Seven functionals, B98, B97-1, B3P86, B2PLYP, TPSSh, B3LYP, and B97-2, display mean absolute errors (MAE) <30 kJ/mol. In contrast, many commonly used functionals such as PBE and RPBE overestimate M−O bonding by +30 kJ/mol and display MAEs from 48–76 kJ/mol. RPBE and OPBE reduce the over-binding of PBE but remain very inaccurate. We identify a linear relationship (p-value 7.6 ⋅ 10⁻⁵) between the precision and accuracy of DFT, i. e. inaccurate functionals tend to produce larger, unpredictable random errors. Some functionals commonly deviate from this relationship: Thus, M06-2X is very precise but not very accurate, whereas B3LYP* and MN15-L are more accurate but less precise than M06-2X. The best-performing hybrids have 10–30 % HF exchange, but this can be relieved by double hybrids (B2PLYP). Most functionals describe trends well, but errors comparing 5d to 4d/3d are ∼10 kJ/mol larger than group-wise errors, due to uncertainties in the spin-orbit coupling corrections for effective core potentials, affecting e. g. Pt/Pd or Au/Ag comparisons.     

Xenophilic complexes bearing a Tp(R) Ligand, [Tp(R)M--M'Ln] [Tp(R) = Tp(iPr2), Tp(#) (Tp(Me2,4-Br)); M=Ni, Co, Fe, Mn; M'Ln = Co(CO)4, Co(CO)3(PPh3), RuCp(CO)2]: the two metal centers are held together not by covalent interaction but by electrostatic attraction

A series of dinuclear complexes, [Tp(R)M--M'L(n)] [Tp(iPr(2) )M--Co(CO)(4) (1; M=Ni, Co, Fe, Mn); Tp(#)M--Co(CO)(4) (1'; M=Ni, Co); Tp(#)Ni--RuCp(CO)(2) (3')] (Tp(iPr(2) )=hydrotris(3,5-diisopropylpyrazolyl)borato; Tp(#) (Tp(Me(2),4-Br))=hydrotris(3,5-dimethyl-4-bromopyrazolyl)borato), has been prepared by treatment of the cationic complexes [Tp(iPr(2) )M(NCMe)(3)]PF(6) or the halo complexes [Tp(#)M--X] with the appropriate metalates. Spectroscopic and crystallographic characterization of 1-3' reveals that the tetrahedral, high-spin Tp(R)M fragment and the coordinatively saturated carbonyl-metal fragment (M'L(n)) are connected only by a metal-metal interaction and, thus, the dinuclear complexes belong to a unique class of xenophilic complexes. The metal-metal interaction in the xenophilic complexes is polarized, as revealed by their nu(CO) vibrations and structural features, which fall between those of reference complexes: covalently bonded species [R--M'L(n)] and ionic species [M'L(n)](-). Unrestricted DFT calculations for the model complexes [Tp(H(2) )Ni--Co(CO)(4)], [Tp(H(2) )Ni--Co(CO)(3)(PH(3))], and [Tp(H(2) )Ni--RuCp(CO)(2)] prove that the two metal centers are held together not by covalent interactions, but by electrostatic attractions. In other words, the obtained xenophilic complexes can be regarded as carbonylmetalates, in which the cationic counterpart interacts with the metal center rather than the oxygen atom of the carbonyl ligand. The xenophilic complexes show divergent reactivity dependent on the properties of donor molecules. Hard (N and O donors) and soft donors (P and C donors) attack the Tp(R)M part and the ML(n) moiety, respectively. The selectivity has been interpreted in terms of the hard-soft theory, and the reactions of the high-spin species 1-3' with singlet donor molecules should involve a spin-crossover process.     

Directionality of Dihydrogen Bonds: The Role of Transition Metal Atoms

A theoretical study on two series of electron‐rich group 8 hydrides is carried out to evaluate involvement of the transition metal in dihydrogen bonding. To this end, the structural and electronic parameters are computed at the DFT/B3PW91 level for hydrogen‐bonded adducts of [(PP₃)MH₂] and [Cp*MH(dppe)] (M=Fe, Ru, Os; PP₃=κ⁴‐P(CH₂CH₂PPh₂)₃, dppe= κ²‐Ph₂PCH₂CH₂PPh₂) with CF₃CH₂OH (TFE) as proton donor. The results are compared with those of adduct [Cp₂NbH₃] ? TFE featuring a “pure” dihydrogen bond, and classical hydrogen bonds in pyridine ? TFE and Me₃N ? TFE. Deviation of the H ??? H? A fragment from linearity is shown to originate from the metal participation in dihydrogen bonding. The latter is confirmed by the electronic parameters obtained by NBO and AIM analysis. Considered together, orbital interaction energies and hydrogen bond ellipticity are salient indicators of this effect and allow the MH ??? HA interaction to be described as a bifurcate hydrogen bond. The impact of the M ??? HA interaction is shown to increase on descending the group, and this explains the experimental trends in mechanisms of proton‐transfer reactions via MH ??? HA intermediates. Strengthening of the M ??? H interaction in the case of electron‐rich 5d metal hydrides leads to direct proton transfer to the metal atom.     

Transition metal-boron complexes BnM: from bowls (n = 8-14) to tires (n = 14)

Transition metal-boron complexes BnM have been predicted at density functional theory level to be molecular bowls (n = 8-14) hosting a transition metal atom (M) inside or molecular tires (n = 14) centered with a transition metal atom. Small Bn clusters prove to be effective inorganic ligands to all the VB-VIIIB transition metal elements in the periodic table. Density functional evidences obtained in this work strongly suggest that bowl-shaped fullerene analogues of Bn units exist in small BnM complexes and the bowl-to-tire structural transition occur to the first-row transition metal complexes BnM (M = Mn, Fe, Co) at n = 14, a size obviously smaller than n = 20 where the 2D-3D structural transition occurs to bare Bn. The half-sandwich-type B12Cr (C3v), full sandwich-type (B12)2Cr (D3d), bowl-shaped B14Fe (C2), and tire-shaped B14Fe (D7d) and B14Fe- (C7v) are the most interesting prototypes to be targeted in future experiments. These BnM complexes may serve as building blocks to form extended boron-rich BnMm tubes or cages (m > or = 2) or as structural units to be placed inside carbon nanotubes with suitable diameters.     

New Explanation for Ligand Bending in Transition Metal Tris(dithiolate) Complexes

The results of Fenske-Hall molecular orbital calculations are reported for the trigonal prismatic complexes Mo(S(2)C(2)H(2))(3) and Mo(S(2)C(6)H(4))(3). Both complexes exhibit a bend of the S-C-C-S ligand plane away from the S-Mo-S plane. A series of calculations which systematically follow the changes in electronic structure as the bend angle alpha is varied between 0 and 30 degrees indicates that the bend can be attributed to a second order Jahn-Teller distortion. The driving force for this distortion, which allows mixing between a set of ligand pi orbitals and the metal d(z)()()2 orbital, should be greatest for d(0) systems. In these systems the bent geometry leads to the stabilization of the doubly occupied HOMO. The driving force for ligand bending should be lower in systems having more or fewer electrons (e.g. Re(S(2)C(2)Ph(2))(3) or V(S(2)C(2)Ph(2))(3), respectively). While the steric bulk of the dithiolate ligands in the latter complexes may also influence the degree of ligand bending, this is probably a secondary effect.     

Iridium(III) Sensitisers and Energy Upconversion: The Influence of Ligand Structure upon TTA-UC Performance

Six substituted ligands based upon 2-(naphthalen-1-yl)quinoline-4-carboxylate and 2-(naphthalen-2-yl)quinoline-4-carboxylate have been synthesised in two steps from a range of commercially available isatin derivatives. These species are shown to be effective cyclometallating ligands for Ir^III, yielding complexes of the form [Ir(C^N)₂(bipy)]PF₆ (where C^N=cyclometallating ligand; bipy=2,2′-bipyridine). X-ray crystallographic studies on three examples demonstrate that the complexes adopt a distorted octahedral geometry wherein a cis-C,C and trans-N,N coordination mode is observed. Intraligand torsional distortions are evident in all cases. The Ir^III complexes display photoluminescence in the red part of the visible region (668–693 nm), which is modestly tuneable through the ligand structure. The triplet lifetimes of the complexes are clearly influenced by the precise structure of the ligand in each case. Supporting computational (DFT) studies suggest that the differences in observed triplet lifetime are likely due to differing admixtures of ligand-centred versus MLCT character instilled by the facets of the ligand structure. Triplet–triplet annihilation upconversion (TTA-UC) measurements demonstrate that the complexes based upon the 1-naphthyl derived ligands are viable photosensitisers with upconversion quantum efficiencies of 1.6–6.7 %.     

Multiconfiguration Pair-Density Functional Theory for Transition Metal Silicide Bond Dissociation Energies,Bond Lengths,and State Orderings

Transition metal silicides are promising materials for improved electronic devices, and this motivates achieving a better understanding of transition metal bonds to silicon. Here we model the ground and excited state bond dissociations of VSi, NbSi, and TaSi using a complete active space (CAS) wave function and a separated-pair (SP) wave function combined with two post-self-consistent field techniques: complete active space with perturbation theory at second order and multiconfiguration pair-density functional theory. The SP approximation is a multiconfiguration self-consistent field method with a selection of configurations based on generalized valence bond theory without the perfect pairing approximation. For both CAS and SP, the active-space composition corresponds to the nominal correlated-participating-orbital scheme. The ground state and low-lying excited states are explored to predict the state ordering for each molecule, and potential energy curves are calculated for the ground state to compare to experiment. The experimental bond dissociation energies of the three diatomic molecules are predicted with eight on-top pair-density functionals with a typical error of 0.2 eV for a CAS wave function and a typical error of 0.3 eV for the SP approximation. We also provide a survey of the accuracy achieved by the SP and extended separated-pair approximations for a broader set of 25 transition metal–ligand bond dissociation energies.     

Tris(pentafluoroethyl)difluorophosphorane: A Versatile Fluoride Acceptor for Transition Metal Chemistry

Fluoride abstraction from different types of transition metal fluoride complexes [L_nMF] (M=Ti, Ni, Cu) by the Lewis acid tris(pentafluoroethyl)difluorophosphorane (C₂F₅)₃PF₂ to yield cationic transition metal complexes with the tris(pentafluoroethyl)trifluorophosphate counterion ( FAP anion, [(C₂F₅)₃PF₃]⁻) is reported. (C₂F₅)₃PF₂ reacted with trans-[Ni(iPr₂Im)₂(Ar^F)F] (iPr₂Im=1,3-diisopropylimidazolin-2-ylidene; Ar^F=C₆F₅, 1 a ; 4-CF₃-C₆F₄, 1 b ; 4-C₆F₅-C₆F₄, 1 c ) through fluoride transfer to form the complex salts trans-[Ni(iPr₂Im)₂(solv)(Ar^F)] FAP ( 2 a - c[solv] ; solv=Et₂O, CH₂Cl₂, THF) depending on the reaction medium. In the presence of stronger Lewis bases such as carbenes or PPh₃, solvent coordination was suppressed and the complexes trans-[Ni(iPr₂Im)₂(PPh₃)(C₆F₅)] FAP ( trans -2 a[PPh₃] ) and cis-[Ni(iPr₂Im)₂(Dipp₂Im)(C₆F₅)] FAP ( cis -2 a[Dipp₂Im] ) (Dipp₂Im=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were isolated. Fluoride abstraction from [(Dipp₂Im)CuF] ( 3 ) in CH₂Cl₂ or 1,2-difluorobenzene led to the isolation of [{(Dipp₂Im)Cu}₂]²⁺2 FAP⁻ ( 4 ). Subsequent reaction of 4 with PPh₃ and different carbenes resulted in the complexes [(Dipp₂Im)Cu(LB)] FAP ( 5 a – e , LB=Lewis base). In the presence of C₆Me₆, fluoride transfer afforded [(Dipp₂Im)Cu(C₆Me₆)] FAP ( 5 f ), which serves as a source of [(Dipp₂Im)Cu)]⁺. Fluoride abstraction of [Cp₂TiF₂] ( 7 ) resulted in the formation of dinuclear [FCp₂Ti(μ-F)TiCp₂F] FAP ( 8 ) (Cp=η⁵-C₅H₅) with one terminal fluoride ligand at each titanium atom and an additional bridging fluoride ligand.     

Gold Behaves as Hydrogen in the Intermolecular Self‐Interaction of Metal Aurides MAu4 (M=Ti,Zr, and Hf)

We performed density functional calculations to examine the intermolecular self‐interaction of metal tetraauride MAu₄ (M=Ti, Zr, and Hf) clusters. We found that the metal auride clusters have strong dimeric interactions (2.8–3.1 eV) and are similar to the metal hydride analogues with respect to structure and bonding nature. Similarly to (MH₄)₂, the (μ‐Au)₃ C_s structures with three three‐center two‐electron (3c–2e) bonds were found to be the most stable. Natural orbital analysis showed that greater than 96 % of the Au 6s orbital contributes to the 3c–2e bonds, and this predominant s orbital is responsible for the similarity between metal aurides and metal hydrides (>99 % H 1s). The favorable orbital interaction between occupied Au 6s and unoccupied metal d orbitals leads to a stronger dimeric interaction for MAu₄‐MAu₄ than the interaction for MH₄‐MH₄. There is a strong relationship between the dimeric interaction energy and the chemical hardness of its monomer for (MAu₄)₂ and (MH₄)₂.     

Electronic Properties of Transition and Alkaline Earth Metal Doped CuS: A DFT Study

CuS is a unique semiconductor with potential in optoelectronics. Its unusual electronic structure, including a partially occupied valence band, and complex crystal structure with an S−S bond offer unique opportunities and potential applications. In this work, the use of doping to optimize the properties of CuS for various applications is investigated by density functional theory (DFT) calculations. Among the dopants studied, Ni, Zn, and Mg may be the most practical due to their lower formation energies. Doping with Fe, Ni, or Ca induces significant distortion, which may be beneficial for achieving materials with high surface areas and active states. Significantly, doping alters the conductor-like behavior of CuS, opening a band gap by increasing bond ionicity and reducing the S−S bond covalency. Thus, doping CuS can tune the plasmonic properties and transform it from a conductor to an intrinsic fluorescent semiconductor. Ni and Fe doping give the lowest band gaps (0.35 eV and 0.39 eV, respectively), while Mg doping gives the highest (0.86 eV). Doping with Mg, Ca, and Zn may enhance electron mobility and charge separation. Most dopants increase the anisotropy of electron-to-hole mass ratios, enabling device design that exploits directional-dependence for improved performance.     

Quantum study on the electrocyclic reactions of CH2CHCHCHX (XH,F, Cl)

The microscopic mechanisms of the electrocyclic reactions for cis‐1,3‐butadiene and its monofluoro‐, monochloroderivatives have been studied by density functional theory (DFT), using the B3LYP method and 6‐311++G** basis sets. We optimized the geometric configurations of reactants, transition states, and products; verified all the probable transition states through vibrational analysis; and calculated the relative single‐point energies at the QCISD(T)/6‐311++G**//B3LYP/6‐311++G**. The results show that the monofluoro‐, monochloroderivatives of cis‐1,3‐butadiene both have two conformers; the reactant favors the electrocyclic reaction when one outboard hydrogen atom of the CH₂ groups is substituted by the fluorine or chlorine atom. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Ligand Effects in Dinitrogen Hydrogenation of Binuclear Zirconium Complexes

In this work, we report a theoretical exploration of the ground-state electronic structures and molecular vibrational properties of a series of binuclear zirconium complexes in the framework of density functional theory （DFT） employing the B3LYP hybrid functional. The calculated results reveal that the electronic structure of the complex [（η^5-C5Me5）2Zr]2（μ^2, η^2, η^2-N2） is unfavorable for hydrogenation due to the exclusion of side-on dinitrogen in the LUMO＋ 1 molecular orbital as compared with the reactant 1 [（η^5-C5Me4H）2Zr]2（μ2,η^2,η^2-N2）. Besides, the structural feature of the hypothetical intermediate 1′, [（η^5C5Me4H）2Zr]2（μ2,η^2, η^2-N2）-n2, clearly implies the possibility of further hydrogenation. In addition, the distinguishing of vibrational modes of experimental intermediate 2, [（η^5-C5Me4H）2ZrH]2（μ2,η^2,η^2-N2H2）, indicates that the asymmetric stretching of Zr-N and Zr-H leads to dissociation. Moreover, the vibrational intensity of Zr-H is stronger than that of Zr-N. Therefore, it can be predicted that excess hydrogen atmosphere is necessary to ensure the dissociation of Zr-N bonds.