Synthesis and characterisation of oxo- and phenylimido-rhenium(v) complexes containing bidentate phosphinoenolato ligands

Abstract

Reacting 1-phenyl-2-(diphenylphosphino)ethanone (P 1 ∼OH), 1-tert-butyl-2-(diphenylphosphino)ethanone (P 2 ∼OH) and 1-phenyl-2-(diphenylphosphino)propanone (P 3 ∼OH) with ReOCl 3 (PPh 3 ) 2 and Re(NPh)Cl 3 (PPh 3 ) 2 in toluene and ethanol in the presence of NEt 3 yields complexes in which these ligands bind as monoanionic enolato chelating agents. Bromo and iodo analogues are prepared similarly. The reactions are solvent dependent. The disubstituted ReOCl(P∼O) 2 compounds obtained in toluene adopt the 'twisted' cis-(P,P) octahedral structure. With Re(NPh)Cl 3 -(PPh 3 ) 2 , P 1 ∼OH and P 2 ∼OH give the corresponding Re(NPh)Cl(P∼O) 2 compounds and the trans-(P,P)-trans-(Cl,Cl) isomer of monosubstituted Re(NPh)Cl 2 (PPh 3 )(P∼O) as the major products, together with small amounts of 'twisted' trans-(P,P) Re(NPh)Cl(P∼O) 2 . The monosubstituted complex is the only species isolated with P 3 ∼OH. The major structural difference between the two systems is the small stability gap between the cis-and the trans-(P,P) isomers of the imido complexes. In ethanol, unexpected cis-ethoxo-oxo complexes ReO(OEt)(P∼O) 2 are isolated, resulting from stereoselective substitution of the halide in the 'twisted' cis-(P,P) octahedral complexes. The reactions with Re(NPh)Cl 3 (PPh 3 ) 2 are less selective and give rise to mixtures of complexes. The trans-ethoxo-phenylimido compound Re(NPh)(OEt)(P 1 ∼O) 2 is obtained in good yield. A crystallographic study reveals an unexpected 'equatorial' trans-(P,P) structure with the ethoxo ligand trans to the Re᎐ ᎐ NPh bond. 1 H NMR indicates that the ethoxo ligand acts as an electron reservoir in these complexes, since its protons are strongly deshielded in the cis-oxo-ethoxo species and strongly shielded in the trans-phenylimido-ethoxo complex.

Synthesis and characterization of a neutral rhenium(V) complex containing a tridentate chelate with an imido donor atom

“…The IR spectrum of 1 displays the Re ¼ N The mps ligand acts as a trianionic tridentate, with N(2) coordinated to Re as a dinegative imido nitrogen atom. The Re-N(2) bond length of 1.762(3) Å is slightly longer than normally observed for the phenylimido unit [1.726-1.740 Å ] [3,13], but is considerably shorter than the values usually found for Re V -NH and Re V -NH 2 bonds [1.98-2.05 Å and 2.15-2.23 Å , respectively] [14]. The Re-N(2)-C(9) bond angle of 128.6(2) indicates significant deviation from linearity of the coordination mode of the phenylimido unit, with a reduction in bond order.…”

“…Due to the prominence of the oxo-group in the coordination chemistry of rhenium(V) [1], much research effort is currently focussed on the synthesis of rhenium(V) moieties with metal nitrogen multiple bonds, such as metal-nitrido, metal-hydrazido and metalimido, in order to produce substitution-inert complexes [2,3]. However, Re(V) imides have received the most attention because of the easy introduction of substituents on the imido moiety, and which may lead to a greater variety of complexes and consequently may ''fine-tune'' the biodistribution of these complexes in therapeutic applications.…”

A rhenium(V) complex containing a terdentate chelate with an imido donor atom: synthesis and structure of [Re(aps)I(PPh3)2]I (H3aps = N-(2-aminophenyl)-salicylideneimine)

“…The aps ligand acts as a terdentate trianionic moiety, with N(1) coordinated to Re as a dinegative imido nitrogen atom. The Re-N(1) bond length of 1.772(4) Å (1.771(5) Å in B) is slightly longer than is normally observed for the phenylimido unit (1.726-1.740 Å ) [5,17], but considerably shorter than the values of 1.98-2.05 Å and 2.15-2.23 Å usually found for Re V -NH and Re V -NH 2 bond lengths, respectively [18]. The Re-N(1)-C(1) bond angle of 130.3(4) [130.6(4) in B] is rather acute, and illustrates an unusual deviation from linearity of the coordination mode of the triply bonded phenylimido unit.…”

“…The IR spectrum of 1 displays the Re ¼ N stretching frequency as a strong band at 1096 cm À1 , with no band in the 900-990 cm À1 region that can be deviation from the aps plane at 0.12 Å . The two phenyl rings of aps make a dihedral angle of 1.7 with one another in A, and 6.9 in B. P-Re-P bond angles in A and B are 175.53 (5) and 177.34(6) , respectively. The two bite angles of aps are N(1)-Re-N(2) ¼ 75.1 (2) and N(2)-Re-O(1) ¼ 83.…”

“…As a result, there are numerous attempts to replace the oxo group, for example with the imido entity [4][5][6]. The introduction of substituents on the imido moiety may lead to a greater variety of rhenium(V) complexes, which may 'fine-tune' the biodistribution of these compounds in therapeutic applications of their 186/188 Re isotopes [7].…”