Unusual substituent effects on the bonding of iminoboranes

Abstract

Substituent effects on iminoboranes XBNH, HBNX and XBNX (X = H, CH3, NH2, OH, F) have been analyzed in the framework of the NBO, AIM and ELF approaches, using B3LYP/6-311++G(d,p) optimized geometries and electron densities. Boron-substituted derivatives, XBNH, are more stable than the corresponding nitrogen-substituted isomers HBNX, with the energy difference increasing as the electron withdrawing character of the substituent increases. The BN linkage is not much affected by N-substitution, but it is significantly altered when the substituent is attached to the boron atom in both XBNH and XBNX series of compounds. Moreover, substituent effects on the structures of iminoboranes are opposite those observed for the corresponding isoelectronic acetylene derivatives. The ELF analysis indicates that electron-withdrawing substituents enhance the localization of electrons in a torus around the CC or the BN axis. As a result, although electron density is depleted at the bcp, the bond does not necessarily become weaker, since density increases around the periphery, a phenomenon named the "hole" effect. The dissimilarities between acetylene and iminoborane derivatives are primarily a consequence of the significant distortion of this torus in the latter, due to the large difference between the electronegativities of B and N, which leads to a large contribution of the X-B=:N-Y resonance structure in some cases. The "hole" effect is reflected in a reasonable correlation between the Laplacian of the electron density at the bcp and the BN bond length.

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

“…Hence, there has been much interest in answering questions concerning the similarities and differences between them. [1,2] Among the various BÀN bonds in different chemical compounds, that in ammonia borane (also called borazane) H 3 B:NH 3 , although not unique, is prototypical of a class of bonds known as donor-acceptor or dative bonds. Interest in this bond is reflected in a series of important papers addressing its inelastic neutron scattering [3] and its 225 K phase transition, studied using high-resolution solidstate 15 N NMR spectroscopy.…”

Spin−Spin Coupling Constants for Iminoboranes RBNH, HBNR, and RBNR and Comparisons with Corresponding Isoelectronic Acetylenes RCCH and RCCR, for R = H, CH3, NH2, OH, and F

“…The opposite effects of these two substituents on 1 J(N-B) and 1 J(C-C) are consistent with our earlier observations that disubstitution of OH and F have opposite effects on C-C and B-N bonds. 6 Disubstitution of these two groups appears to strengthen and shorten the C-C bond relative to the corresponding monosubstituted derivatives but lengthen the B-N bond relative to either of the corresponding monosubstituted derivatives, as evident from Tables 1 and 2, respectively. What remains unclear is why substitution of two NH 2 groups in iminoborane should increase 1 J(N-B) relative to either monosubstituted derivative, thereby having a similar effect to mono-and disubstitution of NH 2 in acetylene.…”

“…6 Coupling constants were computed for these molecules using the ab initio equation-of-motion coupled-cluster singles and doubles method (EOM-CCSD) in the configuration-interaction-(CI-)like approximation, [17][18][19][20] with all electrons correlated. The Ahlrichs qzp basis set 21 was used on C, N, O, and F atoms, and the qz2p basis set was used for H atoms bonded to either B or N. The Dunning cc-pVDZ 22,23 basis set was placed on all other H atoms.…”

“…However, it is not unreasonable to suggest that, in a series of related molecules, differences in ground-state bonding characteristics may be manifested by differences in coupling constants. In our previous study of the ground-state bonding characteristics of substituted iminoboranes, 6 we concluded that, while substitution at N only slightly perturbs the BN triple bond, NH 2 substitution at B has a rather dramatic effect on this bond. In H 2 N-BNH, there is a strong contribution to the BN bond from a canonical form of the type H 2 NdBdN-H, which indicates that the BN bond loses triple bond character.…”

“…[1][2][3][4][5] In a previous study, 6 we used this approach to examine the relative stabilities of corresponding isomers in two series of iminoboranes RBNH and HBNR, where R is H, CH 3 , NH 2 , OH, and F. We investigated ground-state bonding and selected IR spectroscopic properties of these molecules, as well as the bonding characteristics and IR properties of their disubstituted counterparts, RBNR. The present paper reports the results of an extension of our investigation of iminoboranes to include substituent effects on 15 N-11 B spin-spin coupling constants in molecules RBNH, HBNR, and RBNR, as well as one-and two-bond 15 N-1 H and 11 B-1 H spin-spin coupling constants in the monosubstituted derivatives.…”