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

Abstract

Several novel binuclear Hg II complexes of cis, trans, cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane (dppcb) have been prepared and characterised by X-ray diffraction methods, NMR spectroscopy ( 199Hg{ 1H}, 31P{ 1H}, 1H), FAB mass spectrometry, IR spectroscopy, elemental analyses and melting points. The tetrahedral coordination of both the Hg II centres in the homobimetallic compounds [Hg 2L 4(dppcb)] (L=Cl − ( 1), Br − ( 2), CN − ( 3), NO 3 − ( 4)), synthesised by the reaction of HgL 2 with dppcb, is indicated by their solution 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), the NMR parameters are correlated to the changes in the bond lengths and angles in 1– 3. A comparison is given with correlations derived from similar complexes. The reaction of 4 with 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen) leads to [Hg 2(dppcb)(bipy) 2](NO 3) 4 ( 5) and [Hg 2(dppcb)(phen) 2](NO 3) 4 ( 6). Also, for 5 and 6 the NMR data and FAB mass spectra are in agreement with tetrahedral Hg II centres. The treatment of 4 with monophosphines produces trans-[Hg 2(NO 3) 2(dppcb)L′ 2](NO 3) 2 (L′=P(CH 2Ph) 3 ( 7), P(CH 2CH 2CN) 3 ( 8), PPh 3 ( 9)). In 7– 9 the typical large 1 J(Hg,P) values are observed for the monophosphines compared with the corresponding parameters for chelating dppcb, which are reduced due to the five-membered ring formation. In the reaction of 4 with the diphosphine Ph 2PCH 2PPh 2 (dppm) and the subsequent metathesis with LiAsF 6, trans-[Hg 2(NO 3) 2(dppcb)(η 1-dppm) 2](AsF 6) 2 ( 10) is formed. The X-ray structure of 10 showing coordinated and dangling phosphorus atoms of dppm is the first complete characterisation of a Hg II complex containing two different chelating phosphines. Though in solution the dppm ligands are involved in fast intramolecular 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. Catalytic amounts of Hg II convert trans-[Hg 2(NO 3) 2(dppcb)(η 1-dppm) 2](NO 3) 2 into trans-[Hg 2(NO 3) 2(dppcb)(η 1-P-dppmO) 2](NO 3) 2 ( 11), where dppmO is Ph 2PCH 2P(O)Ph 2. The X-ray structure of 11 is the first complete characterisation of a Hg II compound consisting of chelating phosphine 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 donor ligands often geometrically and entropically restrict the number of accessible 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 pyramidal coordinations in compounds containing dppcb.