Phosphine oxide functionalised with two bipyridine subunits : a novel ligand for the engineering of sterically hindered complexes Laurent Douce,a Cesariob and Raymond Ziessel*a Lo•  c Charbonnie ` re,a Miche ` le a L aboratoire de Chimie, dÏElectronique et de Photonique (CNRS UPRESA 7008), Mole  culaires ECPM 25, rue Becquerel, F-67087 Strasbourg cedex 2, France. E-mail : ziessel=chimie.u-strasbg.fr b Institut de Chimie des Substances Naturelles, CNRS, F-91128 Gif-sur-Y vette, France Received (in Montpellier, France) 19th March 2000, Accepted 24th April 2001 First published as an Advance Article on the web 10th July 2001 Ligand L, containing two 6-methyl-6@-methyl-yl-2,2@-bipyridine moieties linked by a ““ PhP2OÏÏ spacer, has been prepared. Subsequent complexation with led to a unique complex of formula [Cu(CH 3 CN) 4 ](BF 4 ) [CuL](BF 4 ). The molecular structure has been determined by X-ray crystallography in the solid state and NMR studies in solution. The complex displays an intense absorption band in the visible wavelength range with maxima at 418 and 509 nm, and is oxidised reversibly at ]0.66 V mV) to the corresponding Cu(II) species, while two (*E p \ 80 successive reductions of the coordinated bipyridine fragments occur at [1.60 and [1.84 V (*E p \ 66) (*E p \ 70 mV) vs. SCE. Direct spectrophotometric titrations with global analysis allowed us to elicit stability constants for the emergent complex and to highlight the formation of a dinuclear complex in the presence of excess copper. Research to synthesise fascinating molecular structures, often involving interlocking of complementary molecular com- ponents, has blossomed during the past decade.1 Numerous examples have been engineered with the prospect of forming symmetrical and esthetic structures. In several cases cationic metal centres provide the impetus for self-organisation into ordered structures, some of them exhibiting useful catalytic,2 mesomorphic3 or electronic properties.4 Many of the new complexes generated over the last few years rely on the coor- dination of transition metal cations to polypyridine fragments incorporated into oligomeric ribbons or macroscopic loops.5 h8 By virtue of forming relatively stable and well- deÐned complexes with metal cations, Lehn and co-workers have led the Ðeld by designing linear polybipyridyl ligands that self-organise in the presence of copper(I) cations to form double-stranded helicates.9 Furthermore, related structures have been constructed using various kinds of oligopyridines and di†erent transition metals such as Cu(II), Co(II), Cd(II) and Zn(II).10 SpeciÐcally, such elaborated molecular architectures are made possible by incorporating several polypyridine units into oligomeric or macrocyclic multitopic ligands. Some attempts have been made to identify the driving forces for the formation of one structure vs. another by systematic variation of the metal and ligand component, but very little is known about the mechanism of formation of these complexes.11 It is worthwhile pointing out that only a few metal-induced self-assembled structures have been produced with bipyridine- grafted phosphanes.12 Hybrid ligands, in which a phosphorous(V) is used as a connecting atom, are available13 but genuine bipyridineÈP(V) ligands are very scarce. The aim of the present work is to partially Ðll this gap and to describe the full characterisation of ligand L, its related mononuclear copper(I) complex and to determine its stability constant in solution. The molecular structure of the complex in the solid state was deduced from X-ray crystallographic studies. One clear advantage of this new ligand is that it can be used in the complexation of a variety of di†erent transition metals and also with lanthanide cations and could readily be func- tionalised either at the phenyl ring or at the methyl positions. This provides the key element by which to include this struc- ture into elaborated complexes and the impetus for construc- ting macroscopic sensors. Experimental General methods The 200.1, 400.1 (1H), 50.3 (13C) and 162 MHz (31P) NMR spectra were recorded at room temperature, unless otherwise speciÐed, using perdeuterated solvent as an internal standard : d(H) relative to residual protiated solvent in (7.26) ; CDCl 3 d(C) relative to the solvent in (77.0). Melting points CDCl 3 were obtained on a 535 capillary melting point appar- Buchi atus in open-ended capillaries and are uncorrected. FT-IR spectra were measured from KBr pellets with a Nicolet 210 spectrometer. Fast-atom bombardement (FAB, positive mode) mass spectra were obtained using m-nitrobenzyl alcohol (m- NBA) as the matrix. UV-Vis absorption spectra and spectro- photometric titrations were performed on a Uvikon 933 spec- trophotometer using degassed acetonitrile as solvent containing 10~3 M tetrabutylammonium hexaÑuorophos- phate as an inert salt. Titrations were performed according to literature procedures14 with typical concentrations of 5 ] 10~5 M for the ligand and 5 ] 10~4 M for [Cu(CH 3 CN) 4 ](BF 4 ). Electrochemical studies employed cyclic voltammetry with a conventional 3-electrode system using a BAS CV-50W volt- ammetric analyser equipped with a Pt microdisk working electrode and a Ag wire counter-electrode. Ferrocene was used as an internal standard and was calibrated against a saturated calomel reference electrode (SCE) separated from the electrolysis cell by a glass frit presoaked with electrolyte solution. Solutions contained the electrode-active substrate 1024 New J. Chem., 2001, 25, 1024È1030 DOI : 10.1039/b102520c This journal is The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001 (