The molecular and electronic structure of 1-pyrazolylphosphazenes

Abstract

The nature of the interactions between pyrazolyl and phosphazene rings has been investigated through the study of the structures of bis(1-pyrazolyl)methane, CH2(H2pz)2, 1, bis[1-(3,5-dimethylpyrazolyl)]methane, CH2(Me2pz)2, 2, the pyrazolylphosphazenes N3P3(Me2pz)6, 3, N4P4(H2pz)8, 4, and N4P4(Me2pz)8, 5, and the partially substituted gem-N3P3Ph2(Me2pz)4, 6. Crystal data are as follows: 1, orthorhombic, P212121, a = 7.997(2), b = 9.287(2), c = 10.049(2) Å, Z = 4, R = 0.034 for 647 reflections; 2, monoclinic, C2/c, a = 19.115(4), b = 4.2690(8), c = 14.543(3) Å, β = 98.91(2)°, Z = 4, R = 0.064 for 528 reflections; 3, monoclinic, C2/c, a = 16.778(2), b = 12.1083(9), c = 19.269(4) Å, β = 113.885(6)°, Z = 4, R = 0.039 for 2716 reflections; 4, monoclinic P21/n, a = 10.4223(9), b = 8.1879(7), c = 19.403(2) Å, β = 94.381(7)°, Z = 2, R = 0.059 for 2937 reflections; 5, triclinic, [Formula: see text], a = 11.516(2), b = 13.121(3), c = 18.142(3) Å, α = 77.96(1), β = 79.20(1), γ = 66.54(2)°, Z = 2, R = 0.036 for 5300 reflections; 6, monoclinic, P21/c, a = 10.3598(4), b = 17.8827(4), c = 19.0851(6) Å, β = 94.675(2)°, Z = 4, R = 0.053 for 4867 reflections. Interpretation of the structures through iterative extended Hückel calculations shows that conjugative interactions lead to a quinonoid deformation of the pyrazole ring, similar to that in bis(1-pyrazolyl)carbonyl. π-Electron transfer to the phosphazene ring, facilitated by the use of acceptor d orbitals on phosphorus, is accompanied by a synergic σ transfer to the pyrazole ring. The pyrazolyl group acts both as a strongly electronegative group and as a good potential donor. The calculations account satisfactorily for the bond lengths in the pyrazolyl group, the large bond angles at nitrogen in 4 and 5, the bond length inequalities in the phosphazene ring of 6, and the reactivity of the pyrazolylphosphazenes.