Twinning of three Fe-PNP pincer complexes interpreted according to order–disorder (OD) theory

Abstract

The systematic twinning of three 2,6-diaminopyridine-based Fe-PNP complexes is interpreted using order–disorder (OD) theory. The monoclinic [Fe0(PNPEt-iPr)(CO)2] [P1121/b,Z′ = 4] possesses pseudo-orthorhombic metrics and crystallizes as a reflection twin by pseudo-merohedry with the twin plane (100). The structure is made up of layers with idealizedp21a(b) symmetry. Theaglide planes of adjacent layers do not overlap, leading to OD polytypism.trans-[FeII(PNP-Et)Br2(CO)] [P21/n,Z′ = 1] is systematically twinnedviatwofold rotation about [001]. It is made up of OD layers with idealizedp2121(2) symmetry. OD polytypism is caused by the twofold rotation axes of adjacent layers which do not overlap. [FeII(κ2P,N-PNP-iPr,TAD)Cl2]·THF [P1, Z^{\prime} = 2] is systematically twinnedviaa twofold rotation about [010]. It is made up of layers with idealizedp121(1) symmetry. OD polytypism is caused by screw rotations relating adjacent layers with an intrinsic translation along a fourth of a primitive lattice vector. In all three structures the twin individuals are a polytype with a maximum degree of order (MDO) and at the twin interface is located a fragment of the second MDO polytype.