Covalent and Ionic Nature of the Dative Bond and Account of Accurate Ammonia Borane Binding Enthalpies

Abstract

The inherent difficulty in modeling the energetic character of the B-N dative bond has been investigated utilizing density functional theory and ab initio methods. The underlying influence of basis set size and functions, thermal corrections, and basis set superposition error (BSSE) on the predicted binding enthalpy of ammonia borane (H3B-NH3) and four methyl-substituted ammonia trimethylboranes ((CH3)3B-N(CH3)nH3-n; n = 0-3) has been evaluated and compared with experiment. HF, B3LYP, MPW1K, MP2, QCISD, and QCISD(T) have been utilized with a wide range of Pople and correlation-consistent basis sets, totaling 336 levels of theory. MPW1K, B3LYP, and HF result in less BSSE and converge to binding enthalpies with fewer basis functions than post-SCF techniques; however, the methods fail to model experimental binding enthalpies and trends accurately, producing mean absolute deviations (MADs) of 5.1, 10.8, and 16.3 kcal/mol, respectively. Despite slow convergence, MP2, QCISD, and QCISD(T) using the 6-311++G(3df,2p) basis set reproduce the experimental binding enthalpy trend and result in lower MADs of 2.2, 2.6, and 0.5 kcal/mol, respectively, when corrected for BSSE and a residual convergence error of ca. 1.3-1.6 kcal/mol. Accuracy of the predicted binding enthalpy is linked to correct determination of the bond's dative character given by charge-transfer frustration, QCTF = -(Delta QN + Delta QB). Frustration gauges the incompleteness of charge transfer between the donor and the acceptor. The binding enthalpy across ammonia borane and methylated complexes is correlated to its dative character (R2 = 0.91), where a more dative bond (less charge-transfer frustration) results in a weaker binding enthalpy. However, a balance of electronic and steric factors must be considered to explain trends in experimentally reported binding enthalpies. Dative bond descriptors, such as bond ionicity and covalency are important in the accurate characterization of the dative bond. The B-N dative bond in ammonia borane is 65% ionic, moderately strong (-27.5 +/- 0.5 kcal/mol), and structurally flexible on the donor side to relieve steric congestion.

On the nature of the dative bond: Coordination to metals and beyond. The carbon case

“…Since perturbation theory applies to small perturbations only, such stabilization energy estimates become less reliable as their value increases, only relative trends being then meaningful [45a]. shown to be strongly dependent on the calculation level and on steric effects [34], and the dative character of a bond could be scaled using the binding enthalpy and the charge transfer extent [50]. Introduction) was first considered to define a "dative covalent free enthalpy" [20a]:…”

Assembling Ligands In Situ Using Bioorthogonal Boronate Ester Synthesis

“…Unlike D-glucose, which interacts with boronic acid through vicinal diols (Ferrier et al, 1965), SHA interacts with boronic acid through both an oxygen and a nitrogen, which may account for the increased stability (Stolowitz et al, 2001). The boron-nitrogen bond has both ionic and covalent character (Plumley and Evanseck, 2007). These boronate esters exhibit pH-dependent hydrolysis, with instability at low pHs (pH < 5) (Stolowitz et al, 2001).…”

New Insights into Factors Influencing BN Bonding in X:BH3−nFnand X:BH3−nClnfor X=N2, HCN, LiCN, H2CNH, NF3, NH3andn=0–3: The Importance of Deformation

“…[4] In addition, the structures of methylamine borane molecules (Me n H 3Àn N:BH 3 , n = 1-3) have been investigated using X-ray diffraction, gas-phase electron diffraction, and quantum chemical calculations. [5] Plumley and Evanseck have discussed the covalent and ionic nature of the dative bond in ammonia borane based on high-level QCISD(T) calculations, [6] and Anane et al have carried out a comparison of the B À N with the B À P bond [7] by comparing H 3 N:BX 3 with H 3 P:BX 3 (X = H, F, Cl) at the G2/MP2 level.…”