Solution and solid state behaviour of binuclear mercury(II) compounds containing cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane: first X-ray structural characterisation of mercury(II) complexes with two different chelating phosphines

Several novel binuclear Hg-II complexes of cis,trans,cis-1,2,3,4-tetrakis(d iphenylpho sphino)cyclobutane (dppcb) have been prepared and characterised by X-ray diffraction methods, NMR spectroscopy (Hg-199{H-1}, P-31{H-1},H-1) , FAB mass spectrometry, IR spectroscopy, elemental analyses and melting po ints. The tetrahedral coordination of both the Hg-II centres in the homobim etallic compounds [Hg2L4(dppcb)] (L = Cl- (1), Br- (2), CN- (3), NO3- (4)), synthesised by the reaction of HgL2 with dppcb, is indicated by their solu tion NMR parameters and is confirmed by the X-ray structures of 1-3. Though the Fermi contact term is not always dominant in determining (1)J(Hg,P), t he NMR parameters are correlated to the changes in the bond lengths and ang les in 1-3. A comparison is given with correlations derived from similar co mplexes. The reaction of 4 with 2,2 ' -bipyridine (bipy) or 1,10-phenanthro line (phen) leads to [Hg-2(dppcb)(bipy)(2)] (NO3)(4) (5) and [Hg-2(dppcb)(p hen)(2)](NO3)(4) (8). Also, for 5 and 6 the NMR data and FAB mass spectra a re in agreement with tetrahedral Hg-II centres. The treatment of 4 with mon ophosphines produces trans -[Hg-2(NO3)(2)(dppcb)L-2'] (NO3)(2) (L ' = P(CH2 Ph)(3) (7), P(CH2CH2CN)(3) (8), PPh3 (9)). In 7-9 the typical large (1)J(Hg ,P) values are observed for the monophosphines compared with the correspond ing parameters for chelating dppcb, which are reduced due to the five-membe red ring formation. In the reaction of 4 with the diphosphine Ph2PCH2PPh2 ( dppm) and the subsequent metathesis with LiAsF6, trans-[Hg-2(NO3)(2)(dppcb) (eta (1)-dppm)(2)](AsF6)(2) (10) is formed. The X-ray structure of 10 showi ng coordinated and dangling phosphorus atoms of dppm is the first complete characterisation of a Hg-II complex containing two different chelating phos phines. Though in solution the dppm ligands are involved in fast intramolec ular end-over-end exchange, the solution structure of 10 corresponds to its solid state structure, which is indicated by unusual (1)J(Hg,P) values. Ca talytic amounts of Hg-II convert trans-[Hg-2(NO3)(2)(dppcb) (eta (1)-dppm)( 2)](NO3)(2) into trans-[Hg-2(NO3)(2)(dppcb) (eta (1)-P-dppmO)(2)](NO3)(2) ( 11), where dppmO is Ph2PCH2P(O)Ph-2. The X-ray structure of 11 is the first complete characterisation of a Hg-II compound consisting of chelating phos phine together with phosphinoyl moieties. The solid state structure and the solution NMR parameters of 11 clearly show the presence of a dangling -P(O )Ph-2 group. The complexes 1-11 illustrate the tendency that polydentate do nor ligands often geometrically and entropically restrict the number of acc essible structures for Hg-II. Especially, the X-ray structures of 1-3, 10, and 11 indicate the preference of Hg-II for tetrahedral and trigonal pyrami dal coordinations in compounds containing dppcb. (C) 2001 Elsevier Science B.V. All rights reserved.