Flexibility First—A Janus-Head Ligand in Iron Coordination

L design is regarded as an important area in the development of efficient, profitable, and sustainable catalysts in industrial processes, addressing the fact that even small changes in the ligand periphery of an active organometallic molecule can dramatically alter yields, product uniformity, selectivity, and turnover numbers. As a consequence, the synthesis and optimization of new ligands at catalytically active metals is a very important field of chemistry. Mainly on the basis of metal amido chemistry, new hemilabile main-group and transition-metal P, N-chelating ligands have been designed. Our group is interested in the synthesis and characterization of metal complexes with Janus-head ligands. These ligands contain at least two different coordination sites with different complexation formation constants. When they are oriented in different directions, they might serve as molecular staples in bior heterobimetallic compounds, which might turn out to be useful catalysts. The N,P,N-ligand bis(benzothiazol-2-yl)phosphane (P(bth)(Hbth), with bth = CNS(C6H4)) contains a divalent phosphorus atom because the hydrogen atom of this secondary phosphane is located at one of the ring nitrogen atoms, forming an intramolecular hydrogen bridge. As part of our studies of ligand design for iron-centered metal complexes we here communicate N,N-bis{bis(2-benzothiazolyl)phosphanide}iron (1). The phosphanide ligand contains three different potential donor sites of various hardnesses and metal site selectivities. It turned out that the best strategy in the synthesis is to employ [iron(II)bis{di(trimethylsilyl)}amide] to introduce the metal via a deprotonation reaction of the secondary di(2-benzothiazolyl)phosphane. The starting materials react in a ligand to amide ratio of 2:1. This is in contrast to all other reactions of bis(2-benzothiazolyl)phosphane with metal bis{bis(trimethylsilyl)}amides known so far, which normally react in a 1:1 ratio (Scheme 1). N,N-Bis{bis(2-benzothiazolyl)phosphanide}iron (1) crystallizes in the triclinic space group P1 (Figure 1). The asymmetric unit contains two complete formula units as well as a lattice toluene molecule. Both metal complexes differ only marginally, and consequently all given values are averaged over analogous features. Surprisingly, in 1 the iron atom is exclusively 4-fold coordinated by the nitrogen atoms of the four heteroaromatic five-membered rings. Unlike the case in life science systems, a possible Fe S coordination is prevented in 1. A hypothetical planar coordination of the Fe(II) metal atom as in natural systems and materials containing porphyrins is precluded as well by the steric demand of the annulated benzene rings. A square-planar coordination would not suit the d Fe(II) iron either. Therefore, the coordination polyhedron at the iron atom in this paramagnetic 14-valence-electron complex is best described as tetrahedral. Both ligand mean planes intersect at an almost ideal right angle (82 ), and the benzothiazolyl planes of each phosphanide ligand are only twisted by 4.3 and 12.3 , respectively. The Fe N bond lengths in 1 are similar to those reported in the CCDC for Fe(II) N distances and range between those of an amidic and a longer dative bond. The P Cipso bond lengths are on average 177.7 pm long and range between those of a singleand a double-bond length. Due to the changes in bond lengths in the benzothiazole substituents, charge transfer from the phosphorus atom to the heteroaromatic Scheme 1. Preparation of [N,N-Bis{bis(2benzothiazolyl)phosphanide}iron] (1)