Two papers regarding the structure and bonding o f group 13 heterocycles valence isoelectronic to NHCs have appeared. Schoeller and his group has carried out DFT calculations on [E{N(H)C(H)}2] \ 4, where E = B, Al, Ga and In. 19 In general, the stability o f
NHCs is dependent on the singlet-triplet energy separation whilst the anionic group 13 systems have an additional factor in that the electron affinity has to be large for stable anions to exist. It was found that the singlet-triplet energy gap was sizeable, indicating that the ground state is singlet for B - In. Additionally, the electron affinity (i.e. the tendency for the heterocycle to retain its negative charge) is much higher for the heavier homologues compared to boron. A further calculation focussed on the 1,2-hydrogen shifted structure whereby one o f the hydrogens on the nitrogen has migrated to E. For E = B the hydrogen shifted structure has the lowest energy, whilst heterocycles with E = Al or Ga were higher in energy compared to the carbenic structure. Finally, the electron distribution within the heterocycles was analysed using NBO calculations. These showed that for boron there is a non-bonding s/?-type lone pair at the elem ent whilst for Al and Ga this becomes less directional and there is increasing /^-electron density at the nitrogens. The E— N bond order is almost 1 for boron but almost 1/2 for Al, Ga and In. These calculations suggest that for E = Al - In the structure can be formulated as a donor-acceptor interaction o f E+ and a chelating diamido unit, which is due to the electropositive elements releasing their formal negative charge to the more electronegative nitrogen atoms.
(4)
E = Al, Ga or In
Similar results have been obtained using ab initio calculations on the borane and alane anions. 20 Whilst the calculated geometries match those o f Schoeller et al, these results also
include a treatment o f the stabilising effect o f delocalisation onto the diazabutadiene backbone. It was found that there is appreciable aromatic stabilisation, with E = B having a greater aromatic ring current than E = Al. However, normal N-heterocyclic carbenes have more aromatic stabilisation, at the same level o f theory. Finally, a molecular orbital treatment found that the empty orbital on boron is fully incorporated into the delocalised 7t system, whereas for Al there is little overlap and the HOMOs o f both the B and Al are the lone pair at the group 13 centre.
In an extension to their work, the group o f Schoeller has investigated the stabilisation o f anionic group 13 and neutral group 14 carbene analogues using the diphosphabutadiene ligand (HPCH=CHPH) . 21 Using the same methodology as for the diazabutadiene ligand
systems, it was found that the P— E bond in [E{P(H)C(H)}2 ] ’ 1 or °, E = group 13 or 14
element, becomes more covalent due to the lower electronegativity o f phosphorus compared to nitrogen. For the group 13 homologues, the singlet ground state is stabilised more by the lighter elements, and the electron affinities are larger than those calculated for the DAB ligand. Therefore, compounds o f this type should experimentally be accessible but no examples are known yet.
The neutral 6-membered group 13(1) heterocycles [{HC(CHNH)2}E] (E = B, Al, Ga, In)
have also been the subject o f DFT calculations22. Their calculated geometries compare favourably with those o f the experimental Al and Ga heterocycles. Also found is that for E = Al - In, the metal carries a partial positive charge and the N— E bonds have a substantial ionic character, whilst when E = B the boron carries a negative charge and has much more covalent B— N bonds. Electron Localisation Function calculations were also carried out to visualise
the lone pair on the group 13 centre, and these showed that there are indeed directional electron pairs on all the group 13 centres and also that the N— E bonds are polar for E = Al - In. The triplet-singlet energy gaps have been calculated and for boron the states are energetically close but for the higher homologues the gap is substantial (ca. 150 kJ m o l'1). From these calculations it was concluded that for E = Al - In, the best representation is a donor-acceptor structure between a anionic chelating ligand and a positively charged group 13 element in the +1 oxidation state. For the boron example, a diradical species is the best description with a B(II) centre, which implies that it would be too reactive to synthesise and isolate. This is in contrast to the results obtained on the anionic 5-membered heterocycles.
Finally, the reactivity o f these compounds towards electron donating NH3 and electron
accepting BF3 was investigated. It was found that for E = Al - In, the group 13 centre acts as a
Lewis acid if the NH3 approaches from above the plane, whilst a rearrangement occurs for
boron. All compounds form compounds with BF3 in the plane o f the heterocyclic system, as
expected. However, due to the increased 5-character o f the lone pair on descending the group,
the Lewis base character at the metal is reduced and for Ga and especially indium the E— B donor-acceptor bond is much weaker.
The synthesis o f [:Ge{N(Ar)C(Me)}2CH]+ has been reported. This is isoelectronic with
the neutral gallium heterocycle and thus o f relevance. This compound was the subject o f Hartree-Fock and DFT calculations, and the results showed that the HOMO is 71-bonding with
respect to the Ge— N and C— C, and antibonding with respect to the C— N bond. The lone pair is in the HOMO-1 orbital, whilst the LUMO includes the Ge 4p orbital. This is the same ordering o f molecular orbitals found for the anionic 5-membered gallium carbene analogue, and suggests that both heterocycles should show similar bonding in their complexes.