Direct Crystallographic Comparison of P−B and N−B π-Bonding:  Structural Studies of Diphenylphosphido- and Diphenylamidoboratabenzene

Abstract

We recently reported that treatment of several transition metal halides with potassium diphenylphosphidoboratabenzene (K-DPB) affords phosphorus-bound σ complexes, whereas otherwise identical reactions of potassium diphenylamidoboratabenzene (K-DAB) do not provide the corresponding nitrogen-bound σ complexes. 1-3 Earlier investigations of aminoboranes and phosphinoboranes have clearly established that nitrogen is more prone than phosphorus to participate in π-bonding with boron (1 T 2). 4 It therefore seemed likely that the dichotomy in reactivity between K-DPB and K-DAB has its origin in a dichotomy in bonding. In this communication we furnish support for this hypothesis in the form of structural studies that reveal that the phosphorus lone pair of K-DPB engages in little, if any, π-bonding with boron, whereas the nitrogen atom of K-DAB adopts a geometry that permits a π interaction. This is, to the best of our knowledge, the first crystallographic comparison of P-B versus N-B π-bonding between two molecules that differ only in the group 15 atom. 4,5 X-ray quality crystals of K-DPB‚(18-crown-6)‚(toluene) 1/2 (3) were obtained upon cooling a saturated solution of K-DPB in toluene/18-crown-6 to -35°C (Figure 1a, Table 1a). 6 The phosphorus atom of K-DPB‚(18-crown-6)‚(toluene) 1/2 is pyramidal, with ∠B-P-C12 ) 106.2(3)°, ∠B-P-C6 ) 102.5(3)°, and ∠C12-P-C6 ) 101.4(2)°(∑ ) 310°). The C6-P-B-C1 torsion angle of -66.2(5)°and the P-B bond distance of 1.968(7) Å 7 are inconsistent with the presence of significant π-bonding between phosphorus and boron.

Monomeric phosphinoboranes

“…Fu et al [23] made a direct comparison of N B with P Bbonding in the structures of the boratobenzenes [C 5 H 5 B EPh 2 ] − (E = P, N). In the diphenylamido compound, the N is planar (˙(angles at N) = 360 • ) and the N B bond length is short (1.510(10)Å) whereas in the diphenylphosphido compound the P is pyramidal (˙(angles at P) = 310 • ) and the P B bond length is long (1.968(7)Å).…”

Extremely Electron-Rich, Boron-Functionalized, Icosahedral Carborane-Based Phosphinoboranes

“…While considering the previously studied phosphinoboranes by Nöth and Power, we found that the B-P bond length in 3a (1.948 Å) best corresponds to the presence of a purely single bond interaction between the B and P atoms. 14 This interaction is also reminiscent of the diphenylphosphineborabenzene species described by Fu, 14c where delocalization of the LUMO orbital on the B atom renders π-donation from the P atom-centered lone electron pair unfavorable. Importantly, 3a was found to be stable both in solution and as a solid under ambient conditions for several weeks.…”

New Alkali Metal Boratabenzenes: Structures of K([18]‐Crown‐6)(C5H5BMe), of the Tetrahydropyran Solvate K(3,5‐Me2C5H3B−NMe2)(THP)2, and of Two THF Solvates of K[3,5‐Me2C5H3B−N(SiMe3)2]

“…The molecule is a contact ion pair of the cation [K([18]‐crown‐6)] + and the anion [C 5 H 5 BMe] − . The structural type is therefore the same as that of the known salts K([18]‐crown‐6)(C 5 H 5 B−NPh 2 ) and K([18]‐crown‐6)(C 5 H 5 B− PPh 2 ) 16. The observed rotational disorder of the crown ether ligand could readily be resolved.…”

“…The intra‐ring geometry [C−B = 1.510(8) (av), C−C = 1.382(7) Å (av)] does not differ from that of Me 3 PhN(C 5 H 5 BMe) ( 5 ) [C−B = 1.500(3) (av), C−C = 1.390(3) Å (av)]. The K−C distances average 3.179(5) Å and are significantly shorter than those in K([18]‐crown‐6)(C 5 H 5 B−NPh 2 ) [3.241(7) Å (av)] and K([18]‐crown‐6)(C 5 H 5 B−PPh 2 ) [3.259(5) Å (av)],16 presumably because of the much smaller B ‐exocyclic substituent in 20 .…”