Wolfgang W. Schoeller

Biradicals within organic chemistry are known to be of fleeting existence. They can be traced only by femtosecond spectroscopy and/or corresponding trapping experiments. The conceptual understanding of a biradical originates mainly from gas phase experiments. These shaped the biradicals as highly unstable species, their lifetimes determined by reactions, which are merely entropy controlled. On the other hand, biradicals in π‐systems of non‐Kekule type, lead to species, which either have distinct singlet or triplet ground states. Yet they are still species confined to spectroscopic investigations. The matter changed by the introduction of sterically protecting groups into chemistry, in order to stabilize hitherto highly reactive compounds. With the new growing field of phosphorus organic chemistry, both fields merged and the concept of a stable biradical came to light. The concept matured in subsequent years and it is now a flowering field with promising aspects in materials science. The Niecke‐type biradicaloid, which is a basic unit in this area, owes its stability by protection of the phosphorus position with sterically demanding substituents, in particular with the “super‐mesityl” (Mes*) group. The electron withdrawing TMS groups at the carbon positions enhance the singlet stability. On this basis it was possible to isolate and structurally characterize these non‐Kekule type structures. The description of these systems as biradicals or biradicaloids is determined by their high reactivity and they offer versatility for a variety of chemical reactions. From the point of theory this refers to small energy gaps between adiabatic singlet and triplet states. However, the understanding as biradicals is different to the Doering concept of a biradical vs. a biradicaloid, as anticipated in the transition state of the Cope automerization. In the Niecke‐type biradicaloid a corresponding biradical structure is promoted with enhanced pyramidalization at the phosphorus atoms or in more general by a decrease of the Lewis basicity of the lone pairs. The Niecke‐type biradical can be converted into a Bertrand‐type biradical with trans‐annular bonding. This requires changing the donor‐ability at the phosphorus centers. This prediction could be proven by either addition with Lewis acids or complexation with transition metal fragments. In the last part of this article, it is briefly shown how biradicals can be utilized as units for building extended π‐systems. They feature an interesting target for the design of interesting materials for nonlinear optics.

As demonstrated by DFT calculations, the bonding situation of P-iodo, bromo and chloro-bis(imino)phosphoranes XP(NR)₂, X=Cl, Br, I, differs from that of conventional amino-bis(imino)phosphoranes (X=NR₂). The monomers are highly reactive compounds with relatively small adiabatic singlet-triplet energy differences. They can dimerize by a mutual polarization mechanism between the electropositive iodine center of one monomeric unit and the more electronegative nitrogen centers of the other unit. It causes a slight depletion of p-electron density at iodine and an accumulation at the nitrogen atoms. The polarization induces electrostatic attraction, attenuated by stack formation into a one-dimensional array of monomers. The weak electrostatic binding is strongly superimposed by steric attraction among the bulky substituents of the stack array, which can end up essentially stronger than the electrostatic interaction forces. The factors which govern the dispersion forces are evaluated. The substituent effects for stack formation are studied in detail for the association of two monomers.

The oxidative addition of π- and σ-bonds is studied by means of quantum chemical investigations at a MCSCF and density functional level of sophistication. The title compound (AlNacNac) induces first-order strong donor–acceptor abilities in the triplet state, giving rise to biradicaloid adducts. At second-order, it reveals carbene character. The energy barriers for the 1,2-addition reactions are fairly small, resulting from an oxidative addition, which differs from the classical 1,2-addition reaction of a carbene to an olefin. For the splitting of σ-bonds (H–X) the energy barriers are largely driven by the strengths of the H–X bonds. The metal Al increases continuously its oxidation state from the educt over the transition state to the product. This implies that in the latter complexes the metal is positive and the olefin overall negative in charge. Ethylene itself does not form a stable adduct; it is still in equilibrium with AlNacNac plus ethylene. However, electron releasing substituents stabilize the addition product. The stabilities of various three-membered ring systems are evaluated. Hydrogen splitting possesses a relatively large barrier.

The title compound Al-NacNac is isolobal to the imidazol-2-ylidene (NHC); the latter is considered as a nucleophilic carbene. However, the title compound is different from a typical carbene, as aluminum is a heavier group 13 element with a predominant inert s orbital. Its singlet ground state is a poor Lewis donor (acceptor) toward white phosphorus, but its corresponding lowest energy triplet state forms a strong Al–P bond with (opened) white phosphorus. The reaction of Al-NacNac with white phosphorus proceeds in two steps: after the addition of a first carbene analogue, a second one is added, resulting in a transient biradicaloid species. This undergoes facile subsequent rearrangement, and a final ring closure reaction leads to the observed product with a bicyclobutane moiety. It is determined by intramolecular bond formation of two phosphorus centered radicals. Finally, a structure with a large singlet–triplet energy separation is formed. An analogy to the noninnocent ligand character as well as the exciplex view of the monoadduct of white phosphorus with the Al-NacNac system is drawn.

The electronic nature of 1,3-diphosphacyclobutane-2,4-diyl is explored with wavefunction based and density functional methods. According to MCSCF calculations the singlet state of the title compound is a biradicaloid with closed shell character, the number of unpaired electrons, assigned upon the analysis of the natural orbitals, is close to one. The participation of closed shell contributions in the overall wavefunction arises from a strong mixing of canonical structures, which emphasizes (a) the phosphorane type of bonding as well as (b) π-delocalization within the ring system. The bonding situation changes when σ-attracting substituents, e.g.amino groups, are attached to the phosphorus atoms. They inhibit possible cyclic π-delocalization and enhance the biradical character within the ring system.

The dimerization reactions of the neutral carbene analogues with the group 13 elements boron, aluminum, gallium, and indium are studied. Besides boron, all monomeric species possess singlet ground states. For Al, bulky substituted cases were 2 investigated; they reveal no essential changes in the singlet triplet energy separations compared with the parent species. The dimerization energies increase with an increase in the bulk of the substituents; this is a consequence of an enhancement of van der Waals forces for association. The latter is opposed by entropic forces, which facilitate dissociation. An equilibrium between monomeric and dimeric structures is predicted because of enthalpy versus entropy control. The low-temperature domain association should prevail in the formation of a dimer with Al (Ga) within the formal oxidation state I+. The Al Al bond refers to a chelated biradicaloid species with an energetically low-lying triplet state. It emerges from the metal metal contacts in the dimer. The biradical character of the dimer decreases in the order E=Al >> Ga>In. The carbene analogue of In forms upon dimerization of only weak coordinative metal metal interactions.

The displacement processes of several guests, incorporated in a calixarene host system, were investigated in the gas phase by electrospray ionization‐Fourier transform‐ion cyclotron resonance (ESI‐FT‐ICR) mass spectrometry. The complexes resulting from a resorcin[4]arene host with ammonia and sec‐butylamine guests were isolated in an ICR‐cell, separately using both states of the photoswitch as well as two reference systems for the open and closed forms of the photoswitchable host. The isolated complexes were forced to exchange the guest by using methylamine, ethylamine and sec‐butylamine, resulting in different reaction rates for all the measured systems. Especially, the reaction rates of both states of the photoswitch are dependent on the provided guest. Potential side effects like proton exchanges were examined by an H/D‐exchange experiment. The results were investigated and supported by quantum chemical calculations (DFT). Copyright © 2008 John Wiley & Sons, Ltd.

Host–guest complexes of tetramethylcavitand with different ammonium cations were investigated by using a quantum chemical method at the density functional level (BP86, B3 LYP). The NH₄⁺ cation is strongly bound to the host. Increasing methyl substitution at the cation decreases its inclination towards the complex formation. The calculated data are in line with results from electrospray ionization mass spectrometry (ESI-MS) experiments. They reveal stable aggregates only for the NH₄⁺ cation and for the primary alkylammonium cations.