N N N N N N N N B H H L – [–] A dinuclear double-helical complex of potassium ions with a compartmental bridging ligand containing two terdentate N-donor fragments Elefteria Psillakis, John C. Jeffery, Jon A. McCleverty* and Michael D. Ward* School of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK BS8 1TS The compartmental bridging ligand bis{3-[6A-(2,2A-bipy- ridyl)]pyrazol-1-yl}hydroborate (L 2 ), which contains two chelating terdentate N-donor arms linked by an anionic –BH 2 – bridge, forms a dinuclear double-helical complex K 2 L 2 with K + ions. Recently helicates have become a well known structural motif in supramolecular coordination chemistry. 1–4 They are of particular interest not just for their appealing structures but also because of the processes of molecular recognition and self- assembly that are required for their formation. Their formation requires (i) ligands which contain several discrete metal-ion binding domains, and (ii) metal ions with specific preferences for particular coordination geometries that match the ligand binding sites. It is the appropriate combination of these two factors which dictates the outcome of the self-assembly process. Thus, a ligand which has two terdentate domains might give a double helicate with a transition-metal ion (coordination number six) but a triple helicate with a lanthanide (coordination number nine). 2–5 In contrast, linear oligopyridines can partition themselves according to the dictates of the metal ion, and give different structures with Cu I (when the ligand is partitioned into bidentate domains) and Cd II or Ni II (when it is split into terdentate domains). 1 The requirement of the metal ion for a specific geometry or coordination number is principally controlled by charge/size effects: thus Cu I and Ag I have a preference for (usually) four- coordination, whereas transition-metal dications generally pre- fer six-coordination and lanthanides prefer approximately nine- coordination. 1–5 In contrast the use of s-block metals in double helicates is virtually unknown. The only certain example, recently published by Bell and Jousselin, is a 2 : 2 complex of Na + with a ‘heterohelicene’ ligand which is preorganised into a helical shape and therefore inevitably imposes a helical structure on its complexes. 6 The complex was characterised by NMR spectroscopy and mass spectrometry, but not crystallo- graphically. It is also possible that the 1 : 1 adduct of 2,2A :6A,2B :6B,2AAA :6AAA,2BB-quinquepyridine with LiClO 4 is a double helicate. 7 Cram and coworkers recently described a macrocyclic ligand containing two phenanthroline binding sites that was twisted into a helically chiral conformation by virtue of binaphthyl groups incorporated into the cycle; this imposes a helically chiral geometry on alkali-metal cations due to its preorganisation. 8 We describe here the preparation of the new compartmental bis-terdentate ligand bis{3-[6A-(2,2A-bipyridyl)]pyrazol-1-yl}- hydroborate (L 2 ) as its potassium salt KL, and the crystal structure of this salt which reveals a double-helical complex of stoichiometry K 2 L 2 . The ligand was prepared by reaction of 6-(3-pyrazolyl)-2,2A-bipyridine 9 with KBH 4 ,† which is the usual route for preparing bidentate bis(pyrazol-1-yl)hydro- borates 10 and tridentate tris(pyrazol-1-yl)hydroborates 11 from substituted pyrazoles. All of the spectroscopic and analytical data were consistent with formation of KL, in which two terdentate N-donor arms are linked by an anionic –BH 2 – fragment. Even using a fourfold excess of the pyrazole, we found that only the bis(pyrazolyl)borate was obtained, and this is the first example of a bis(pyrazolyl)borate functionalised in this manner so that each arm becomes chelating rather than monodentate. The compound was crystallised by slow evapora- tion from chloroform to give colourless prisms; the crystal structure is in Fig. 1,‡ and reveals that in the solid state the Fig. 1 Crystal structure of K 2 L 2 . Selected bond lengths (Å) and angles (°): K(1)–N(101) 2.772(5), K(1)–N(41) 2.784(6), K(1)–N(61) 2.816(6), K(1)–N(51) 2.857(5), K(1)–N(111) 2.927(5), K(1)–N(121) 2.956(6), K(2)–N(72) 2.766(6), K(2)–N(12) 2.777(6), K(2)–N(91) 2.802(6), K(2)–N(81) 2.855(6), K(2)–N(31) 2.877(6), K(2)–N(21) 2.890(6), K(1)···K(2) 3.954(2); N(101)–K(1)–N(41) 150.1(2), N(101)–K(1)–N(61) 84.2(2), N(41)–K(1)–N(61) 116.1(2), N(101)–K(1)–N(51) 139.3(2), N(41)–K(1)–N(51) 59.2(2), N(61)–K(1)–N(51) 57.4(2), N(101)–K(1)– N(111) 57.3(2), N(41)–K(1)–N(111) 134.9(2), N(61)–K(1)–N(111) 95.4(2), N(51)–K(1)–N(111) 132.5(2), N(101)–K(1)–N(121) 111.9(2), N(41)–K(1)–N(121) 80.3(2), N(61)–K(1)–N(121) 115.3(2), N(51)–K(1)– N(121) 97.8(2), N(111)–K(1)–N(121) 56.4(2), N(72)–K(2)–N(12) 150.1(2), N(72)–K(2)–N(91) 116.3(2), N(12)–K(2)–N(91) 85.4(2), N(72)–K(2)–N(81) 58.8(2), N(12)–K(2)–N(81) 141.5(2), N(91)–K(2)– N(81) 58.1(2), N(72)–K(2)–N(31) 80.7(2), N(12)–K(2)–N(31) 114.4(2), N(91)–K(2)–N(31) 105.8(2), N(81)–K(2)–N(31) 89.2(2), N(72)–K(2)– N(21) 135.8(2), N(12)–K(2)–N(21) 59.4(2), N(91)–K(2)–N(21) 88.5(2), N(81)–K(2)–N(21) 124.7(2), N(31)–K(2)–N(21) 56.6(2). Chem. Commun., 1997 479