Erik P. A. Couzijn

Stereomutation of Pentavalent Compounds. Validating the Berry Pseudorotation, Redressing the Ugi’s Turnstile Rotation, and Revealing the Two- and Three-Gated Turnstiles

A general reaction mechanism describes the qualitative change in chemical topology along the reaction pathway. On the basis of this principle, we present a method to characterize intramolecular substituent permutation in pentavalent compounds. A full description of the geometry around five-coordinate atoms using internal coordinates enables the analysis of the structural changes along the stereomutational intrinsic reaction coordinate. The fluxional behavior of experimentally known pentavalent phosphoranes, silicates, and transition-metal complexes has been investigated by density functional theory calculations, and three principal mechanisms have been identified: Berry pseudorotation, threefold cyclic permutation, and half-twist axial-equatorial interchange. The frequently cited turnstile rotation is shown to be equivalent to the Berry pseudorotation. In combination with graph theory, this approach provides a means to systematically investigate the stereomutation of pentavalent molecules and potentially identify hitherto-unknown mechanisms.

New Benchmark Set of Transition-Metal Coordination Reactions for the Assessment of Density Functionals.

We present the WCCR10 data set of 10 ligand dissociation energies of large cationic transition metal complexes for the assessment of approximate exchange-correlation functionals. We analyze nine popular functionals, namely BP86, BP86-D3, B3LYP, B3LYP-D3, B97-D-D2, PBE, TPSS, PBE0, and TPSSh by mutual comparison and by comparison to experimental gas-phase data measured with well-known precision. The comparison of all calculated data reveals a large, system-dependent scattering of results with nonnegligible consequences for computational chemistry studies on transition metal compounds. Considering further the comparison with experimental results, the nonempirical functionals PBE and TPSS turn out to be among the best functionals for our reference data set. The deviation can be lowered further by including Hartree-Fock exchange. Accordingly, PBE0 and TPSSh are the two most accurate functionals for our test set, but also these functionals exhibit deviations from experimental results by up to 50 kJ mol(-1) for individual reactions. As an important result, we found no functional to be reliable for all reactions. Furthermore, for some of the ligand dissociation energies studied in this work, invoking semiempirical dispersion corrections yields results which increase the deviation from experimental results. This deviation increases further if structure optimization including such dispersion corrections is performed, although the contrary should be the case, pointing to the need to develop the currently available dispersion corrections further. Finally, we compare our results to other benchmark studies and highlight that the performance assessed for different density functionals depends significantly on the reference molecule set chosen.

Structure and Bonding of Isoleptic Coinage Metal (Cu, Ag, Au) Dimethylaminonitrenes in the Gas Phase

Dimethylaminonitrene complexes of IMesM(+) (IMes =1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; M = Cu, Ag, Au) were prepared in the gas phase and structurally characterized by high-resolution infrared spectroscopy of the cold species, ion-molecule reactions, and DFT computations. We measured the binding energies of the nitrene fragment to the IMesM(+) moiety by energy-resolved collision-induced dissociation experiments in the gas phase, affording a trend in bond strength of M = Cu ≈ Au > Ag. This trend is explained in terms of a detailed metal-nitrogen bonding analysis, from which relativistic effects on the bonding were assessed. Various density functionals were evaluated for reproducing the observed thermochemical data and Truhlars M06 functional was found to give the best agreement.

Gas-Phase Energetics of Reductive Elimination from a Palladium(II) N-Heterocyclic Carbene Complex

Energy-resolved collision-induced dissociation experiments using tandem mass spectrometry are reported for an phenylpalladium N-heterocyclic carbene (NHC) complex. Reductive elimination of an NHC ligand as a phenylimidazolium ion involves a barrier of 30.9(14) kcal mol(-1), whereas competitive ligand dissociation requires 47.1(17) kcal mol(-1). The resulting three-coordinate palladium complex readily undergoes reductive C-C coupling to give the phenylimidazolium pi complex, for which the binding energy was determined to be 38.9(10) kcal mol(-1). Density functional calculations at the M06-L//BP86/TZP level of theory are in very good agreement with experiment. In combination with RRKM modeling, these results suggest that the rate-determining step for the direct reductive elimination process switches from the C-C coupling step to the fragmentation of the resulting sigma complex at low activation energy.

Intrinsic properties of α-cyclodextrin complexes with benzoate derivatives in the gas phase: an experimental and theoretical study.

The noncovalent interactions in host-guest complexes of α-cyclodextrin (α-CD) with a series of benzoic acid derivatives (RBA) were investigated by electrospray ionization tandem mass spectrometry and density functional theory (DFT) calculations. The 1:1 stoichiometry of the anionic host-guest complexes was unequivocally confirmed by their mass-to-charge ratios (m/z) and isotope patterns. Collision-induced dissociation experiments revealed exclusive fragmentation into [α-CD](-) and neutral RBA and afforded the gas-phase kinetic stability trend [α-CD·3,5-diMeBA](-) < [α-CD·3-MeBA](-) < [α-CD·BA](-) < [α-CD·3-OHBA](-) < [α-CD·3,5-diOHBA](-). This trend follows that of the gas-phase basicities of the guest anions used, indicating that host-guest pairs with more comparable basicities form more stable complexes. DFT calculations at the M06-L/6-31+G(d,p) level of theory provided detailed structural assignments and further elucidated the experimental observations, suggesting that the anionic [α-CD·RBA](-) inclusion complexes are favored over the nonspecific complexes in the gas phase and that hydrogen bonding constitutes the primary host-guest interaction. Additionally, the results provide an estimated gas-phase basicity ΔG(0) = 325-327 kcal mol(-1) for [α-CD](-).

Experimental and Theoretical Study of a Gold(I) Aminonitrene Complex in the Gas Phase

Metal-coordinated nitrenes are recognized to be reactive intermediates in aziridination and insertion reactions. Among other metals, the coinage metal triad (Cu, Ag, Au) was demonstrated to be active in catalyzing these important transformations. In particular, He and co-workers recently showed that nitrene transfer could efficiently be accomplished relying on gold. While copper nitrene intermediates have been structurally characterized, we are not aware of any reports on monometallic gold analogues. Having successfully applied a phosphorus ylid-based strategy for the preparation of transient gold(I) benzylidene carbenes inside a modified Finnigan MAT TSQ-700 mass spectrometer, we were motivated to implement a similar approach to the gas-phase synthesis of a metal-bound nitrene species. The triflate salt of IMes-supported (IMes=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) phosphazene gold adduct 1-OTf was prepared as an airand moisture-stable solid in 72% yield (Scheme 1). Upon electrospray, the thermalized parent ion current (m/z 821) was directed into the argon-filled chamber for the collision-induced dissociation (CID) event (Figure 1). Detachment of the labile PPh3 group proceeds smoothly as the only reaction channel to afford a signal with 559 m/z ratio, which we ascribe to the singlet (vide infra) gold aminonitrene 2s. Energy-resolved reactive cross-section measurements under near-single-collision conditions were conducted for this process in order to quantitatively determine the N P bond strength (Figure 2). The L-CID algorithm for the cross-section data fitting requires an input assumption about the properties of the transition state. It generally holds true that simple dissociation, without prior rearrangement, of a neutral fragment from an ion occurs via a loose transition state, that is, with no reverse activation barrier. Standard L-CID treatment of the process 1!2s + 3 gave a value of 45.0 2.7 kcalmol 1 assuming a loose transition state. We subsequently modeled the PPh3 detachment at the DFT level of theory (see Computational Section and Supporting Information for details) to confirm the assignment of the daughter ion channel to 2s and to rule out possible rearrangements and spin isomerism. A priori a number of events could precede nitrene formation (Scheme 2). One may speculate that a 1,2-triphenylphosphine shift connects 1 and 4s and formation of 2s could then be accomplished via PPh3 dissociation from the tricoordinate gold species. However, 1,1-dimethylnitrene 6s is only weakly bound to the gold atom in 4s with an Au N distance of 2.867 and thus is predisposed to dissociation producing 5. Since species 5 is not observed experimenScheme 1. Synthesis of the triflate adduct 1-OTf. Figure 2. Energy-resolved cross sections recorded at various Ar gas pressures, linear extrapolation to zero pressure (red circles) and L-CID fit (black line) for loss of PPh3 from 1.

Reactive intermediates: a transient electrophilic phosphinidene caught in the act.

that have made a stunning impact on the preparative and in-dustrial scale syntheses of small molecules and polymericmaterials. It is tempting to extend the relationship for low-valent species that is based on the similar electronegativitiesof the element phosphorus and the diagonally relatedcarbon to these transition-metal complexes.

2,2,6,6-Tetrakis(biphenyl-2-yl)-4,4,8,8-tetramethylcyclotetrasiloxane

The title compound, [–Si(C12H9)2OSi(CH3)2O–]2, was obtained unintentionally as the product of an attempted crystallization of caesium bis(biphenyl-2,2′-diyl)fluorosilicate from dimethylformamide. In the crystal, the molecule is located on an inversion center and the siloxane ring adopts a twist-chair conformation with the two dimethyl-substituted Si atoms lying 0.7081 (5) Å out of the plane defined by the two bis(biphenyl-2-yl)-substituted Si atoms and the four O atoms. In each Si(C12H9)2 unit, the orientation of one terminal phenyl ring relative to the phenylene ring of the other biphenyl moiety suggests a parallel displaced π–π stacking interaction [centroid distance = 4.2377 (11) Å and dihedral angle = 15.40 (9)°].