On the Reactivity of Mes*P(PMe3 ) towards Aluminum(I) Compounds - Evidence for the Intermediate Formation of Phosphaalumenes

Phosphaalumenes are the heavier isoelectronic analogs of alkynes and have eluded facile synthesis until recently. We have reported that the combination of a phosphinidene transfer agent, Ar TerP(PMe3 ) (Ar Ter=2,6-Ar2 -C6 H3 ), with (Cp*Al)4 (Cp*=C5 (CH3 )5 ) afforded the phosphaalumenes Ar TerPAlCp* as isolable, violet, thermally stable compounds. In here we describe attempts to utilize Mes*P(PMe3 ) (Mes*=2,4,6-tBu3 -C6 H2 ) as a phosphinidene source in combination with different Al(I) precursors, namely Dip NacnacAl (Dip Nacnac=HC[C(Me)NDip]2 , Dip=2,6-iPr2 -C6 H3 ), (Cp*Al)4 and Cp3t Al (Cp3t =1,2,4-tBu3 -C5 H2 ). In all cases the formation of phosphaalumenes was not observed, however, their intermediate formation is indicated by formation of the dimer [Cp*Al(μ-PMes*)]2 (2) and C-H-bond activation products along the putative P=Al bond, giving unusual 1,2-P,Al-tetrahydronaphtalene derivatives 1 and 4, clearly underlining the role the sterically demanding group on phosphorus plays in these transformations. The reactivity studies are supported by theoretical studies, demonstrating a thermodynamic preference for the C-H activation products. Additionally, we show that there are potential pitfalls in the synthesis of Cp*2 AlH, the precursor to make (Cp*Al)4 and give recommendations how to circumvent these.

Aromaticity in X(3)Y(3)H(6) (X = B, Al, Ga; Y = N, P, As), X(3)Z(3)H(3) (Z = O, S, Se), and Phosphazenes. Theoretical Study of the Structures, Energetics, and Magnetic Properties

A systematic estimation of aromaticity in X(3)Y(3)H(6), 1, (X = B, Al, Ga; Y = N, P, As), P(3)N(3)H(6), 2, and X(3)Z(3)H(3), 3 (Z = O, S, Se), has been conducted using structural, energetic, and magnetic criteria. Estimates based on aromatic stabilization energy (ASE) calculations predict that 1BN (1; X = B, Y = N) and 1BP are equally aromatic. Contrary to this, we have found, from magnetic susceptibility exaltation (MSE) and from the nucleus independent chemical shift (NICS) data at the B3LYP/6-31G level, that 1BN is not aromatic while 1BP is. This emphasizes the fact that energetic and magnetic criteria need not be parallel. On the basis of MSE and NICS values, all 1XP compounds show strong aromatic character; 1XAs are borderline aromatic while 1XN compounds are nonaromatic. Despite being aromatic, all 1XP and 1XAs compounds are found to prefer nonplanar geometries. MSE and NICS criteria can also diverge quite strongly; this has been observed in the X(3)Z(3)H(3) family. MSE values for 3BS, 3BSe, 3AlO, 3AlS, and 3GaS are more than half of the MSE value for benzene, indicating substantial aromatic character. However, NICS estimates point to the contrary; none of the type 3 compounds are aromatic. The problem with the ASE and MSE is that both depend on the choice of the reference systems while NICS, which avoids the need for reference molecules, is impossible to vary experimentally. In spite of this epistemological deficiency of NICS, we find it complementary to the ASE and MSE criteria. Despite the existence of a large number of well-established structures and substantial aromatic stabilization energy, phosphazenes, 2, are not aromatic according to NICS data.