Intra- and inter-ion-pair protonic-hydridic bonding in polyhydridobis(phosphine)rhenates

Abstract

The hexahydridobis(phosphine)rhenate anions, [ReH6(PR3)2]- (PR3 = PCy3, P-i-Pr3, PPh3, PMe3) were generated by potassium hydride deprotonation of the neutral heptahydride conjugate acids (ReH7(PR3)2), isolated as their [K(18-crown-6)]+ and [K(1,10-diaza-18-crown-6)]+ salts, and characterized by NMR and IR spectroscopy and elemental analyses. Structures from single crystal X-ray diffraction were obtained for the [K(1,10-diaza-18-crown-6)]+salts and these indicate the presence of short protonic—hydridic bonds involving the hydrides of the anions and the proton donor NH moieties of the cations. The structure of [K(1,10-diaza-18-crown-6)][ReH6(P-i-Pr3)2] adopts a one-dimensional zigzag chain with alternating cations and anions connected and held together by inter-ion N-H···Hx-Re interactions (x = 1 or 2). Short distances between the NH protons of the cations and hydrides of the anion ranging from 1.6 to 1.9 Å are estimated for this complex. A different kind of chain structure is observed for [K(1,10-diaza-18-crown-6)][ReH6(PMe3)2] in which the combined effects of inter-ion protonic—hydridic bonding (N-H···Hx-Re) and inter-ion electrostatic interactions (ReH-x···K+···H-xRe), result in one-dimensional networks of alternating cations and anions, with the metals and hydrides occupying the interior and the organic moieties of the phosphine ligands and crown ether lining the exterior of cylindrical supramolecular assemblies. A combination of intra- and inter-ion protonic-hydridic and intra-ion-pair electrostatic interactions in [K(1,10-diaza-18-crown-6)][ReH6(PPh3)2] result in the formation of discrete two-dimensional {[K(1,10-diaza-18-crown-6)][ReH6(PPh3)2]}4 tetramers. The PCy3 salt is disordered but appears to consist of isolated 1:1 ion pairs containing strong intra-ion-pair NH···HRe bonding. The solid-state IR spectra of the [K(1,10-diaza-18-crown-6)]+ salts show low-frequency shifts for the NH bands relative to [K(1,10-diaza-18-crown-6)][BPh4], and perturbed Re-H bands relative to those in the [K(18-crown-6)]+ salts. The magnitude of ΔνNH is related to the basicity of the anion as indicated by the pKαTHF of the conjugate acid form (ReH7(PR3)2), which increases as PPh3 < < PMe3 < P-i-Pr3 < PCy3. Solution 1H NMR, NOE, and T1 relaxation measurements of [K(1,10-diaza-18-crown-6)][ReH6(PPh3)2] indicate that these interactions also persist in toluene solutions of this compound.Key words: rhenium, hydride, phosphine, hydrogen bonding, self-assembly.