Tuning Effects for Some Cyclic Aromatic Carbenes Bearing Remote Amino Groups

Yamamoto and co-workers synthesized two cyclic aromatic carbenes with remote amino groups. Here we theoretically studied related compounds to explore tuning effects on the singlet–triplet splitting by variations of functional groups. For the Yamamoto compound, the lowest singlet state lies 15.7 kcal/mol below the lowest triplet. The singlet–triplet separation is reduced by ∼7 kcal/mol when the dimethylamino groups are replaced by H. In one set of carbenes, when X = O, we substitute S, Se, Te, SO, SeO, and TeO for X; the resulting Δ<i>E</i>(S–T) predictions are 9.9, 7.3, 3.9, 4.3, 2.3, and −0.1 kcal/mol, respectively. A different set of X fragments yields triplet electronic ground states with Δ<i>E</i>(S–T) values of −8.6 (X = BH), −6.8 (X = AlH), −7.2 (X = GaH), −7.5 (X = InH), and −7.0 kcal/mol (X = TlH). We also predicted Δ<i>E</i>(S–T) with N­(CH₃)₂ replaced by PH₂, AsH₂, SbH₂, BiH₂, BH₂, CH₃, OH, and F. Of all the molecules considered, that with N­(CH₃)₂ replaced with BH₂ and X = BH most favors the triplet state, lying 13.7 kcal/mol below the singlet. Finally, we have relocated the N­(CH₃)₂ and NH₂ groups from the (3, 6) positions to the (4, 5), (2, 7), and (1, 8) terminal ring positions, with very interesting results

Major Differences between the Binuclear Manganese Boronyl Carbonyl Mn₂(BO)₂(CO)₉ and Its Isoelectronic Chromium Carbonyl Analogue Cr₂(CO)₁₁

The lowest energy structures of the manganese boronyl carbonyl Mn₂(BO)₂(CO)₉ by more than 8 kcal/mol are found to have a single end-to-end bridging BO group bonding to one manganese atom through its boron atom and to the other manganese atom through its oxygen atom. The long Mn···Mn distances in these structures indicate the lack of direct manganese–manganese bonding as confirmed by essentially zero Wiberg bond indices. These Mn₂(BO)₂(CO)₉ structures are favored thermochemically by more than 25 kcal/mol over dissociation into mononuclear fragments and thus appear to be viable synthetic objectives. This contrasts with the isoelectronic Cr₂(CO)₁₁ system, which is predicted to be disfavored relative to the mononuclear fragments Cr­(CO)₆ + Cr­(CO)₅. Analogous Mn₂(BO)₂(CO)₉ structures with an end-to-end bridging CO group lie ∼17 kcal/mol in energy above the corresponding structures with end-to-end bridging BO groups. The lowest energy Mn₂(BO)₂(CO)₉ structures without an end-to-end bridging BO group provide unprecedented examples of the coupling of two terminal BO groups to form a terminal dioxodiborene (B₂O₂) ligand with a B–B distance of ∼1.9 Å. Still higher energy Mn₂(BO)₂(CO)₉ structures include singly bridged and doubly semibridged structures analogous to the previously optimized lowest energy Cr₂(CO)₁₁ structures

Prospects for Three-Electron Donor Boronyl (BO) Ligands and Dioxodiborene (B₂O₂) Ligands as Bridging Groups in Binuclear Iron Carbonyl Derivatives

Recent experimental work (2010) on (Cy₃P)₂Pt­(BO)Br indicates that the oxygen atom of the boronyl (BO) ligand is more basic than that in the ubiquitous CO ligand. This suggests that bridging BO ligands in unsaturated binuclear metal carbonyl derivatives should readily function as three-electron donor bridging ligands involving both the oxygen and the boron atoms. In this connection, density functional theory shows that three of the four lowest energy singlet Fe₂(BO)₂(CO)₇ structures have such a bridging η²-μ-BO group as well as a formal Fe–Fe single bond. In addition, all four of the lowest energy singlet Fe₂(BO)₂(CO)₆ structures have two bridging η²-μ-BO groups and formal Fe–Fe single bonds. Other Fe₂(BO)₂(CO)_<i>n</i> (<i>n</i> = 7, 6) structures are found in which the two BO groups have coupled to form a bridging dioxodiborene (B₂O₂) ligand with B–B bonding distances of ∼1.84 Å. All of these Fe₂(μ-B₂O₂)­(CO)_<i>n</i> structures have long Fe···Fe distances indicating a lack of direct iron–iron bonding. One of the singlet Fe₂(BO)₂(CO)₇ structures has such a bridging dioxodiborene ligand with cis stereochemistry functioning as a six-electron donor to the pair of iron atoms. In addition, the lowest energy triplet structures for both Fe₂(BO)₂(CO)₇ and Fe₂(BO)₂(CO)₆ have bridging dioxodiborene ligands with trans stereochemistry functioning as a four-electron donor to the pair of iron atoms

The Hydrogen Abstraction Reaction H₂S + OH → H₂O + SH: Convergent Quantum Mechanical Predictions

The hydrogen abstraction reaction H₂S + OH → H₂O + SH has been studied using the “gold standard” CCSD­(T) method along with the Dunning’s aug-cc-pVXZ (up to 5Z) basis sets. For the reactant (entrance) complex, the CCSD­(T) method predicts a HSH···OH hydrogen-bonded structure to be lowest-lying, and the other lower-lying isomers, including the two-center three-electron hemibonded structure H₂S···OH, have energies within 2 kcal/mol. The similar situation is for the product (exit) complex. With the aug-cc-pV5Z single point energies at the aug-cc-pVQZ geometry, the dissociation energy for the reactant complex to the reactants (H₂S + OH) is predicted to be 3.37 kcal/mol, and that for the product complex to the products (H₂O + SH) is 2.92 kcal/mol. At the same level of theory, the classical barrier height is predicted to be only 0.11 kcal/mol. Thus, the OH radical will react promptly with H₂S in the atmosphere. We have also tested the performance of 29 density functional theory (DFT) methods for this reaction. Most of them can reasonably predict the reaction energy, but the different functional give quite different energy barriers, ranged from −10.3 to +2.8 kcal/mol, suggesting some caution in choosing density functionals to explore the PES of chemical reactions

Versatile Behavior of the Fluorophosphinidene Ligand in Iron Carbonyl Chemistry

Fluorophosphinidene (PF) is a versatile ligand found experimentally in the transient species M­(CO)₅(PF) (M = Cr, Mo) as well as the stable cluster Ru₅(CO)₁₅(μ₄-PF). The PF ligand can function as either a bent two-electron donor or a linear four-electron donor with the former being more common. The mononuclear tetracarbonyl Fe­(PF)­(CO)₄ is predicted to have a trigonal bipyramidal structure analogous to Fe­(CO)₅ but with a bent PF ligand replacing one of the equatorial CO groups. The tricarbonyl Fe­(PF)­(CO)₃ is predicted to have two low-energy singlet structures, namely, one with a bent PF ligand and a 16-electron iron configuration and the other with a linear PF ligand and the favored 18-electron iron configuration. Low-energy structures of the dicarbonyl Fe­(PF)­(CO)₂ have bent PF ligands and triplet spin multiplicities. The lowest energy structures of the binuclear Fe₂(PF)­(CO)₈ and Fe₂(PF)₂(CO)₇ derivatives are triply bridged structures analogous to the experimental structure of the analogous Fe₂(CO)₉. The three bridges in each Fe₂(PF)­(CO)₈ and Fe₂(PF)₂(CO)₇ structure include all of the PF ligands. Other types of low-energy Fe₂(PF)₂(CO)₇ structures include the phosphorus-bridging carbonyl structure (FP)₂COFe₂(CO)₆, lying only ∼2 kcal/mol above the global minimum, as well as an Fe₂(CO)₇(μ-P₂F₂) structure in which the two PF groups have coupled to form a difluorodiphosphene ligand unsymmetrically bridging the central Fe₂ unit

The Symmetric Exchange Reaction OH + H₂O → H₂O + OH: Convergent Quantum Mechanical Predictions

The symmetric hydrogen exchange reaction OH + H₂O → H₂O + OH has been studied using the “gold standard” CCSD­(T) method with the correlation-consistent basis sets up to aug-cc-pV5Z. The CCSDT and CCSDT­(Q) methods were used for the final energic predictions. Two entrance complexes and two transition states on the H₃O₂ potential surface were located. The vibrational frequencies and the zero-point vibrational energies of these stationary points for the reaction are reported. The entrance complex H₂O···HO is predicted to lie 3.7 kcal mol^–1 below the separated reactants, whereas the second complex HOH···OH lies only 2.1 kcal mol^–1 below the separated reactants. The classical barrier height for the title reaction is predicted to be 8.4 kcal mol^–1, and the transition state between the two complexes is only slightly higher than the second complex. We estimate a reliability of ±0.2 kcal mol^–1 for these predictions. The capabilities of different density functional theory methods is also tested for this reaction

Metal–Substrate Cooperation Mechanism for Dehydrogenative Amidation Catalyzed by a PNN-Ru Catalyst

The pyridine-based PNN ruthenium pincer complex (PNN)­Ru­(CO)­(H) can catalyze the well-known dehydrogenative amidation reaction, but the mechanism is not fully understood. In this work, we find there exists an alternative metal–substrate cooperation mechanism in this reaction system, which is more favorable than the aromatization–dearomatization mechanism. The possible reaction of the excess base <i>t</i>-BuO^– with catalyst species (PNN)­Ru­(CO)­(H) is studied, indicating <i>t</i>-BuO^– is able to facilitate the ligand substitution and enhance catalytic activities. With the bifunctional Ru–N moiety, the imine-substituted species (PN)­(imine)­Ru­(CO)­(H) <b>5</b> could serve as an alternative catalytic species and efficiently facilitate some elementary steps such as the hydrogen transfer, hydrogen elimination, and C–N coupling. Meanwhile, the C–N coupling step proceeds via the split of aldehydic C–H bond across the Ru­(II)–imine bond, which results in an amide bond directly. The hemiaminal is uninvolved in the C–N coupling process. Finally, the formation of linear peptides and cyclic dipeptides are unveiled by the newly proposed mechanism. The metal–substrate cooperation could widely exist in transition metal catalyst systems with a large influence on the reaction activity

The Energy Difference between the Triply-Bridged and All-Terminal Structures of Co₄(CO)₁₂, Rh₄(CO)₁₂, and Ir₄(CO)₁₂: A Difficult Test for Conventional Density Functional Methods

The M₄(CO)₁₂ molecules Co₄(CO)₁₂, Rh₄(CO)₁₂, and Ir₄(CO)₁₂ have two low-lying structures, the all-terminal structure with <i>T</i>_<i>d</i> symmetry and the triply bridged structure with <i>C</i>_3<i>v</i> symmetry. A total of 45 density functional theory (DFT) methods have been used to predict structures and vibrational frequencies for Co₄(CO)₁₂, Rh₄(CO)₁₂, and Ir₄(CO)₁₂. The different DFT methods show a broad range of energy differences Δ<i>E</i> = <i>E</i>_{<i>T</i>_<i>d</i>} – <i>E</i>_{<i>C</i>_3<i>v</i>}. For Rh₄(CO)₁₂, none of the 45 DFT predictions is within 11 kcal/mol of the 2005 experimental value of 5.1 ± 0.6 kcal/mol reported by Allian and Garland (Dalton Trans. 2005, 1957−1965). For the challenging Ir₄(CO)₁₂ molecule, 21 DFT methods predict the correct <i>T</i>_<i>d</i> structure, while 24 DFT methods predict the <i>C</i>_3<i>v</i> structure to lie lower in energy. This research reveals many peculiar problems in the computation of the vibrational frequencies for the all-terminal structure

Molybdenum–Molybdenum Multiple Bonding in Homoleptic Molybdenum Carbonyls: Comparison with Their Chromium Analogues

The binuclear molybdenum carbonyls Mo₂(CO)_<i>n</i> (<i>n</i> = 11, 10, 9, 8) have been studied by density functional theory using the BP86 and MPW1PW91 functionals. The lowest energy Mo₂(CO)₁₁ structure is a singly bridged singlet structure with a Mo–Mo single bond. This structure is essentially thermoneutral toward dissociation into Mo­(CO)₆ + Mo­(CO)₅, suggesting limited viability similar to the analogous Cr₂(CO)₁₁. The lowest energy Mo₂(CO)₁₀ structure is a doubly semibridged singlet structure with a MoMo double bond. This structure is essentially thermoneutral toward disproportionation into Mo₂(CO)₁₁ + Mo₂(CO)₉, suggesting limited viability. The lowest energy Mo₂(CO)₉ structure has three semibridging CO groups and a MoMo triple bond analogous to the lowest energy Cr₂(CO)₉ structure. This structure appears to be viable toward CO dissociation, disproportionation into Mo₂(CO)₁₀ + Mo₂(CO)₈, and fragmentation into Mo­(CO)₅ + Mo­(CO)₄ and thus appears to be a possible synthetic objective. The lowest energy Mo₂(CO)₈ structure has one semibridging CO group and a MoMo triple bond similar to that in the lowest energy Mo₂(CO)₉ structure. This differs from the lowest energy Cr₂(CO)₈ structure, which is a triply bridged structure. A higher energy unbridged <i>D</i>_2<i>d</i> Mo₂(CO)₈ structure was found with a very short Mo–Mo distance of 2.6 Å. This interesting structure has two degenerate imaginary vibrational frequencies. Following the corresponding normal modes leads to a Mo₂(CO)₈ structure, lying ∼5 kcal/mol above the global minimum, with two four-electron donor bridging CO groups and a MoMo distance suggesting a formal double bond. All of the triplet Mo₂(CO)_<i>n</i> (<i>n</i> = 10, 9, 8) structures were found to be relatively high energy structures, lying at least 22 kcal/mol above the corresponding global minimum. The singlet–triplet splittings for the Mo₂(CO)_<i>n</i> (<i>n</i> = 10, 9, 8) structures are significantly higher than those of the Cr₂(CO)_<i>n</i> analogues. The Mo–Mo Wiberg bond indices confirm our assigned bond orders based on predicted bond distances

Carbon–Hydrogen Activation in Zerovalent Bis(1,5-cyclooctadiene) Complexes of the First Row Transition Metals: A Theoretical Study

Stepwise interaction of first row transition metal atoms with 1,5-cyclooctadiene to give (C₈H₁₂)₂M complexes is studied using the M06-L/DZP density functional method. The experimentally known (C₈H₁₂)₂Ni is the thermodynamically most favorable complex, with a predicted geometry consistent with its experimental structure as determined by X-ray crystallography. The other transition metal atoms from scandium to zinc also interact exothermically with 1,5-cyclooctadiene to give (C₈H₁₂)₂M derivatives, but these exhibit lower symmetry than the <i>S</i>₄ symmetry exhibited by (C₈H₁₂)₂Ni. Carbon–hydrogen activation of CH₂ groups in a C₈H₁₂ ligand is predicted for most systems. Thus, conversion of (η^2,2-C₈H₁₂)₂M to (η^3,2-C₈H₁₁)­(η^2,1-C₈H₁₃)­M, through a hydride intermediate (η^3,2-C₈H₁₁)­(η^2,2-C₈H₁₂)­MH, is predicted for scandium, vanadium, chromium, manganese, and cobalt. For titanium with a low-lying empty orbital, further C–H activation through a hydride intermediate (η⁶-C₈H₁₀)­(η^2,1-C₈H₁₃)­TiH is predicted, leading ultimately to (η⁶-C₈H₁₀)­(η^1,1-C₈H₁₄)­Ti, in which the hexahapto η⁶-C₈H₁₀ ligand is shown by NICS to be aromatic. These two C–H activation processes on a titanium center represent the dehydrogenation of 1,5-cyclooctadiene to 1,3,5-cyclooctatriene with the second 1,5-cyclooctadiene ligand as the hydrogen acceptor. For zinc C–H activation terminates at (η¹-C₈H₁₁)­(C₈H₁₂)­ZnH, which has a C–Zn–H three-center bond. No energetically favorable C–H activation processes are predicted for the iron, nickel, and copper (η^2,2-C₈H₁₂)₂M derivatives