DOI:10.1002/chem.200601221 Di(benzothiazol-2-yl)phosphanide as a Janus-Head Ligand to Caesium ThomasStey, [a] Matthias Pfeiffer, [a] Julian Henn, [a] SushilK. Pandey, [b] and Dietmar Stalke* [a] Introduction Pincer ligands with an active coordinating periphery [1] have attracted more and more attention during recent decades. [2] Over the last few years our group has concentrated on het- eroaromatic-substituted chelating ligands. [3] These ligands do not only bear coordination sites for either hard or soft metal centres in terms of the hard–soft acid–base (HSAB) concept by Pearson, [4] but also for both. Depending on the metal fragment offered, the ligands act as a N,N-, P,N-chelating, or P-coordinating ligand. [5] Ligands of this type find many ap- plications in alkane dehydrogenation, [6] cyclometalation, [7] cross-coupling, [8] organic transformation, [9] catalyst recov- ery, [10] antimicrobial agents, [11] light-emitting diodes (LEDs) and other electrophosphorescent devices. [12] To obtain heter- obimetallic complexes it is useful to implement additional and qualitatively different coordination sites in the phos- phane ligand periphery. This leads to the so-called “Janus- head” ligands. An advantage of these molecular staples is the combination of soft and hard coordination sites in the same molecule. In this sense we embarked on developing an enhanced Janus-head ligand system to investigate its reactiv- ity as well as its coordination ability. Di(benzothiazol-2-yl)- phosphane was chosen as a target because the benzothiazol- yl substituent (bth = C 5 H 4 SN) features a sulfur atom and a nitrogen atom as potential donor centres. Thiazol is an elec- tron-rich heteroaromatic ring in which the p electrons are mainly located on the nitrogen atom. Equipped with these substituents, a secondary phosphanide should provide rich coordination-site selectivity to various metal fragments. Various possible conformations of the system (trans–trans , cis–cis and cis–trans (Scheme 1), not including the non- planar modifications) make this ligand more compatible Abstract: Starting from tris(benzothia- zol-2-yl)phosphane (1) an advanced Janus-head ligand, di(benzothiazol-2- yl)phosphane (2), was synthesised and structurally characterised. The hetero- aryl substituents of this ligand provide both hard and soft donor sites. Surpris- ingly, the phosphorus atom in 2 is diva- lent and the hydrogen atom is directly bonded to one ring nitrogen atom and hydrogen bonded to the second. Com- pound 2 decomposes in any common solvent other than diethyl ether and a new preparation to improve the yields of 2 is presented. A coordination poly- mer, [{CsACHTUNGTRENNUNG(bth) 2 P} 8 ](3) (bth = benzo- thiazol-2-yl), was obtained when the sec-phosphane 2 was allowed to react with elemental caesium in a 1:1 ratio in diethyl ether at À78 8C. In 3 each anion is coordinated to four caesium cations and vice versa. The central phosphorus atom is coordinated to two metal atoms above and below the mean plane of the anion in positions in which the two lone pairs of a four-electron donor are anticipated. Two additional cations m-bridge both ring nitrogen atoms. Hence both faces of the Janus- head ligand are coordinated to the same number of metal cations but in a different way. Keywords: alkali metals · caesium · N ligands · P ligands · phosphanes [a] Dr. T. Stey, Dr. M. Pfeiffer, Dr. J. Henn, Prof.Dr. D. Stalke Institut für Anorganische Chemie der Universität Gçttingen Tammannstrasse 4, 37077 Gçttingen (Germany) Fax: (+ 49)551-39-3459 E-mail: dstalke@chemie.uni-goettingen.de [b] Dr. S. K. Pandey Department of Chemistry, University of Jammu Jammu 180006 (India) Scheme 1. Various different conformations of the planar di(benzothiazol- 2-yl)phosphanide anion. # 2007 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2007, 13, 3636–3642 3636