Reduction-induced hapticity increase in a silacycle-bridged biaryl-based ligand coordinated to an iron center

An Fe(II) bromide complex supported by a bidentate phosphine ligand and an η1(C)-coordinated six-membered silacycle-bridged biphenyl-based ligand is converted upon reduction into an Fe(I) complex in which the hapticity of the silacycle-based ligand increases from η1 to η5.

DFT Investigation of the η6 ⇌ η6-Inter-ring Haptotropic Rearrangement of the Group 8 Metals Complexes [(graphene)MCp]+ (M = Fe, Ru, Os)

Metalcyclopentadienyl complexes (MCp)⁺ (M = Fe, Ru, Os) bound to the large polyaromatic hydrogenated hydrocarbon (PAH) C₉₆H₂₄ used as a model for pristine graphene have been studied using a density functional theory (DFT) generalized gradient approximation (PBE functional) to reveal their structural features and dynamic behavior. The inter-ring haptotropic rearrangements (IRHRs) for these complexes were shown to occur via two transition states and one intermediate. The energy barriers of the η⁶ ⇌ η⁶ IRHRs of the (MCp)⁺ unit were found to be 30, 27, and 29 kcal/mol for M = Fe, Ru, and Os, respectively. These values are significantly lower than the values found previously for smaller PAHs. Both polar and nonpolar solvents were found not to affect significantly the energy barrier heights. Investigated transition metal complexes could be used in general as catalysts in the design of novel derivatives or materials with promising properties. Metalcyclopentadienyl complexes (MCp)⁺ of PAHs show catalytic properties mainly due to their structural details as well as their important characteristic of inter-ring haptotropic rearrangement. IRHRs take place usually by intramolecular mechanisms. During IRHRs, the ML_n organometallic groups (OMGs) undergo shifting along the PAH plane and could coordinate additional reagents, which is important for catalysis. Large PAHs such as graphene, fullerenes, and nanotubes possess intrinsic anticancer activity, and numerous arene complexes of Ru and Os have been proven to have anticancer properties as well. We suppose that coordinating Ru or Os to very large PAHs could synergistically increase the anticancer activity of resulting complexes.

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy

Detailed mechanistic information is crucial to our understanding of reaction pathways and selectivity. Dynamic exchange NMR techniques, in particular 2D exchange spectroscopy (EXSY) and its modifications, provide indispensable intricate information on the mechanisms of organic and inorganic reactions and other phenomena, for example, the dynamics of interfacial processes. In this Review, key results from exchange NMR studies of small molecules over the last few decades are systemised and discussed. After a brief introduction to the theory, the key types of dynamic processes are identified and fundamental examples given of intra- and intermolecular reactions, which, in turn, could involve, or not, bond-making and bond-breaking events. Following that logic, internal molecular rotation, intramolecular stereomutation and molecular recognition will first be considered because they do not typically involve bond breaking. Then, rearrangements, substitution-type reactions, cyclisations, additions and other processes affecting chemical bonds will be discussed. Finally, interfacial molecular dynamics and unexpected combinations of different types of fluxional processes will also be highlighted. How exchange NMR spectroscopy helps to identify conformational changes, coordination and molecular recognition processes as well as quantify reaction energy barriers and extract detailed mechanistic information by using reaction rate theory in conjunction with computational techniques will be shown.

Organometallic chemistry of new carbon materials. Structure and dynamic behavior of group 6 metal tricabonyl complexes of graphene and perforated graphene: a DFT study

Low energy barriers are found for inter-ring haptotropic rearrangements on large PAH ligands.

Intricacies of ligand coordination in tricarbonylchromium(0) complexes with ortho- and para-fluorobiphenyls

The steric and electronic factors that influence which of the two rings of a substituted biphenyl ligand coordinates to chromium are of interest and it has been suggested that haptotropic rearrangements within these molecules may be limited if the arene-arene dihedral angle is too large. Two tricarbonylchromium(0) complexes and their respective free ligands have been characterized by single-crystal X-ray diffraction. In the solid state, tricarbonyl[(1',2',3',4',5',6'-η)-2-fluoro-1,1'-biphenyl]chromium(0), [Cr(C12H9F)(CO)3], (I), exists as the more stable isomer with the nonhalogenated arene ring ligated to the metal center. Similarly, tricarbonyl[(1',2',3',4',5',6'-η)-4-fluoro-1,1'-biphenyl]chromium(0) crystallizes as the more stable isomer with the phenyl ring bonded to the Cr0 center. The arene-arene dihedral angles in these complexes are 55.77 (4) and 52.4 (5)°, respectively. Structural features of these complexes are compared to those of the DFT-optimized geometries of ten tricarbonyl[(η6-C6H5)(4-F-C6H4)]chromium model complexes. The solid-state structures of the free ligands 2-fluoro-1,1'-biphenyl and 4-fluoro-1,1'-biphenyl, both C12H9F, exhibit arene-arene dihedral angles of 54.83 (7) and 0.71 (8)°, respectively. The molecules of the free ligands occupy crystallographic twofold axes and exhibit positional disorder. Weak intermolecular C-H...F interactions are observed in all four structures.

Ring-whizzing in polyene-PtL2 complexes revisited

Ring-whizzing was investigated by hybrid DFT methods in a number of polyene-Pt(diphosphinylethane) complexes. The polyenes included cyclopropenium(+), cyclobutadiene, cyclopentadienyl(+), hexafluorobenzene, cycloheptatrienyl(+), cyclooctatetraene, octafluorooctatetraene, 6-radialene, pentalene, phenalenium(+), naphthalene and octafluoronaphthalene. The HOMO of a d(10) ML2 group (with b2 symmetry) interacting with the LUMO of the polyene was used as a model to explain the occurrence of minima and maxima on the potential energy surface.

[(η5-C5Me5)Ru]+ fragments ligated to polyaromatic hydrocarbons: an experimental and computational approach to pathways for haptotropic migration

DFT calculations indicated rather high activation energies for the thermally induced intramolecular inter-ring haptotropic migration of the [(η⁵-C₅Me₅)Ru⁺] moiety.

DFT study of dihydrogen addition to molybdenum π-heteroaromatic complexes: a prerequisite step for the catalytic hydrodenitrogenation process

The mechanism proceeds via consecutive steps of oxidative H₂ addition, hydrogen metal-to-ligand transfer, accompanied by a change of metal–ligand hapticity.

Organic and organometallic derivatives of pentaphenylbenzene, C6Ph5X: correlation of peripheral phenyl ring orientations with the steric bulk of “X”

A series of C6Ph5X compounds, including X = H, Br, CO2H, CO2R, C[Formula: see text]CPh, cis-BrC=C(Br)Ph, 2-bornenyl, and ferrocenyl, have been characterized by use of X-ray crystallography. Also, the first organometallic complexes of pentaphenylbenzene have been prepared by reaction with chromium hexacarbonyl to yield (η6-C6Ph5H)Cr(CO)3 complexes in which the metal tripod is attached either to an ortho peripheral ring or to the central ring. Crystalline pentaphenylbenzoic acid exists as a hydrogen-bonded dimer; however, the steric bulk of the substituents does not allow the carboxylic acid moieties to be linked directly but instead via two bridging methanol molecules. In the solid state, the orientation of the peripheral rings in bulky C6Ph5X systems is very sensitive to the size of the “X” substituents, such that the twist angle of the para ring responds inversely with increasing bulk of “X”, which drives the ortho rings farther out of the plane of the central ring. The relevance of these observations to correlated motion in molecular propellers, and ultimately to molecular machines, is discussed.

Haptotropic rearrangement in tricarbonylchromium complexes of 2-aminobiphenyl and 4-aminobiphenyl

The para-aminobiphenyl compound [(η(6)-C(6)H(5))(C(6)H(4)-4-NH(2))]Cr(CO)(3) (1) has an arene-phenyl dihedral angle of 38.01(6)°, as determined by single-crystal X-ray crystallography, and 34.7(11)°, as determined by DFT calculations. It undergoes haptotropic rearrangement at 140 °C in solution to form [(η(6)-C(6)H(4)-4-NH(2))(C(6)H(5))]Cr(CO)(3) (2), even though previous reports have suggested that such rearrangements should not be observed in compounds with arene-phenyl dihedral angles greater than 22°. NMR analysis gave a rate constant of k = 5.0 × 10(-5) s(-1) for the rearrangement of 1 to 2. The ortho-substituted analog [(η(6)-C(6)H(5))(C(6)H(4)-2-NH(2))]Cr(CO)(3) (3) has an arene-phenyl dihedral angle of 67.70(7)°, as determined by single-crystal X-ray crystallography, and 51.9(10)°, as determined by DFT calculations. Surprisingly, even though it displays a more extreme canting of arene rings, 3 rearranges to [(η(6)-C(6)H(4)-2-NH(2))(C(6)H(5))]Cr(CO)(3) (4) at 140 °C in solution with a rate constant of k = 2.6 × 10(-4) s(-1). This approximately five-fold rate enhancement likely results from the ortho-amino group providing intramolecular stabilization for intermediates formed during the rearrangement.

Highly efficient desymmetrisation of a tricarbonylchromium 1,4-dibromonaphthalene complex by asymmetric Suzuki-Miyaura coupling

Access to planar chiral complexes has been sought by catalytic desymmetrisation of a prochiral complex via asymmetric Suzuki-Miyaura cross-coupling. High selectivities of up to 98% ee were achieved using a bulky chiral palladium catalyst.

A molecular paddlewheel with a sliding organometallic latch: syntheses, X-ray crystal structures and dynamic behaviour of [Cr(CO)3{eta(6)-2-(9-triptycyl)indene}], and of [M(CO)3{eta(5)-2-(9-triptycyl)indenyl}] (M = Mn, Re)

In [eta(6)-2-(9-triptycyl)-indene]tricarbonylchromium (2a), the indenyl-chromium moiety is linked directly to the axis of the three-bladed triptycene paddlewheel. However, the molecular structure of 2a reveals that there is no steric interaction between these components, and the paddlewheel is free to rotate. Accordingly, its NMR spectrum indicates the full equivalence of the blades of the triptycene. Deprotonation of the indene induces a haptotropic shift of the organometallic fragment from the six-membered to the five-membered ring of the indene and, in the sodium [eta(5)-2-(9-triptycyl)-indenyl]tricarbonylchromium salt (3a), so formed, rotation of the three-bladed molecular paddlewheel is now blocked by the bulky tripod. NMR data for 3a, and also for the isostructural eta(5)-Mn(CO)(3) and eta(5)-Re(CO)(3) complexes, 3b and 3c, respectively, reveal a 2:1 splitting of the blades of the triptycyl moiety, thus breaking its original threefold symmetry. The X-ray crystal structures of the chromium complex, 2a, and of the manganese and rhenium complexes, 3b and 3c, provide pictures of the system in both its "ON" and "OFF" states, whereby the M(CO)(3) tripod has moved about 2 A towards the triptycene, thus blocking its rotation. Comparison of the rotational barriers in 2-(9-triptycyl)indene (1) and its complexes 2 and 3, suggests that rotation of the paddlewheel can be slowed by a factor of approximately 10(8).