Borylene Complexes (BH)L2 and Nitrogen Cation Complexes (N+)L2: Isoelectronic Homologues of Carbones CL2
Authors: Celik Mehmet Ali , Sure Rebecca , Klein Susanne , Kinjo Rei , Bertrand Guy , Frenking Gernot 
Published: March 2012

Quantum chemical calculations using DFT (BP86, M05‐2X) and ab initio methods (CCSD(T), SCS‐MP2) have been carried out on the borylene complexes (BH)L2 and nitrogen cation complexes (N+)L2 with the ligands L=CO, N2, PPh3, NHCMe, CAAC, and CAACmodel. The results are compared with those obtained for the isoelectronic carbones CL2. The geometries and bond dissociation energies of the ligands, the proton affinities, and adducts with the Lewis acids BH3 and AuCl were calculated. The nature of the bonding has been analyzed with charge and energy partitioning methods. The calculated borylene complexes (BH)L2 have trigonal planar coordinated boron atoms which possess rather short BL bonds. The calculated bond dissociation energies (BDEs) of the ligands for complexes where L is a carbene (NHC or CAAC) are very large (De=141.6–177.3 kcal mol−1) which suggest that such species might become isolated in a condensed phase. The borylene complexes (BH)(PPh3)2 and (BH)(CO)2 have intermediate bond strengths (De=90.1 and 92.6 kcal mol−1). Substituted homologues with bulky groups at boron which protect the boron atom from electrophilic attack might also be stable enough to become isolated. The BDE of (BH)(N2)2 is much smaller (De=31.9 kcal mol−1), but could become observable in a low‐temperature matrix. The proton affinities of the borylene complexes are very large, particularly for the bulky adducts with L=PPh3, NHCMe, CAACmodel and CAAC and thus, they are superbases. All (BH)L2 molecules bind strongly AuCl either η1 (L=N2, PPh3, NHCMe, CAAC) or η2 (L=CO, CAACmodel). The BDEs of H3B(BH)L2 adducts which possess a hitherto unknown boron→boron donor–acceptor bond are smaller than for the AuCl complexes. The strongest bonded BH3 adduct that might be isolable is (BH)(PPh3)2BH3 (De=36.2 kcal mol−1). The analysis of the bonding situation reveals that (BH)L2 bonding comes mainly from the orbital interactions which has three major contributions, that is, the donation from the symmetric (σ) and antisymmetric (π||) combination of the ligand lone‐pair orbitals into the vacant MOs of BH L→(BH)←L and the L←(BH)→L π backdonation from the boron lone‐pair orbital. The nitrogen cation complexes (N+)L2 have strongly bent LNL geometries, in which the calculated bending angle varies between 113.9° (L=N2) and 146.9° (L=CAAC). The BDEs for (N+)L2 are much larger than those of the borylene complexes. The carbene ligands NHC and CAAC but also the phosphane ligands PPh3 bind very strongly between De=358.4 kcal mol−1 (L=PPh3) and De=412.5 kcal mol−1 (L=CAACmodel). The proton affinities (PA) of (N+)L2 are much smaller and they bind AuCl and BH3 less strongly compared with (BH)L2. However, the PAs (N+)L2 for complexes with bulky ligands L are still between 139.9 kcal mol−1 (L=CAACmodel) and 168.5 kcal mol−1 (L=CAAC). The analysis of the (N+)L2 bonding situation reveals that the binding interactions come mainly from the L→(N+)←L donation while L←(N+)→L π backdonation is rather weak.