The structures of mononuclear [Pd II(hdta)] 2− and [Pd II(dhpta)] 2− and binuclear [Pd 2II(hdta)(H 2O) 2] and [Pd II(H 2O)Pt IICl(hdta)] − complexes in aqueous solution

Abstract

[Pd(II) 2(hdta)(H 2O) 2], the Pd(II) binuclear analogue of the previously characterized [[Pt(II) 2(hdta)Cl 2] 2− complex (F.-T. Lin, R.E. Shepherd, Inorg. Chim. Acta, 271 (1998) 124; R.A. Kortes, S.J. Geib, F.-T. Lin, R.E. Shepherd, Inorg. Chem, 38 (1999) 5045), and the hetero-binuclear [Pd(II)(H 2O)Pt(II)Cl(hdta)] − have been characterized by 1H and 13C NMR methods (hdta 4−=1,6-hexanediamine- N, N, N′, N′-tetraacetate). The influence of the metal center on the six methylene units along the tether chain is different for each of the Pt(II) 2, Pd(II) 2 and Pt(II)Pd(II) species because the magnetic moment is +0.60 BM for Pt and −0.59 BM for Pd. This leads to an upfield component in the chemical shifts of the protons nearer Pd(II) that is opposed by the inherent downfield component of the shift caused by positive charge. Both effects promote downfield shifts on protons near a Pt(II) center in the tether. This is confirmed by the detection of six resonances of the six differentiated carbons by 13C NMR for the Pt(II) Pd(II) complex that are not an exact match for the six resonances of a 50:50 mixture of the homobinuclear Pt(II) 2 and Pd(II) 2 complexes. Full chemical shift assignments are given in the tables for each of the three species. The mononuclear [Pd(II)(hdta)] 2− complex exists as two isomers with both N donors of hdta 4− chelated to the Pd(II) center. 1H and 2D NMR methods show that these are the ‘basket-handle’ trans-( R, R) form and ‘side-handle’ cis-( R, S) form in a 3: 2 ratio. Both of these complexes have two uncoordinated, pendant glycinato groups below the PdN 2O 2 plane and opposite the ‘basket-handle’ or ‘side-handle’ face. A shorter linker of n=3 length is provided by the dhpta 4− ligand (dhpta 4−=1,3-diamino-2-hydroxypropane- N, N, N′, N′-tetraacetate). Only the mononuclear complex, [Pd(II)(dhpta)] 2− is detectable even in the presence of a large excess of Pd(II) — conditions in which the Pd(II) 2 complex of hdta 4− forms rapidly. 1H and 2D NMR methods were used to assign a cis-( S, S) structure to this complex, wherein there are two pendant glycinato groups, one above and near the hydroxy group of the 2-hydroxypropane backbone chelate, and one below the same PdN 2O 2 plane. The stiffness of the n=3 ‘side-handle’ for the dhpta 4− complex (allowing only one isomer) compared with a more flexible arrangement with n=6 in the hdta 4− complex ( allowing two isomeric forms) is suggested as the explanation of these results. Likewise, the need to keep lone pairs of the ether oxygens away from the d 8 electron set of Pd(II) of the ‘side-handle’ tether of [Pd(II)(egta)] 2− explains the presence of only one isomer for the Pd(II)–egta system studied previously [R.A. Kortes, R.E. Shepherd, Transition Met. Chem. (London) 22 (1997) 68; F.-T. Lin, R.A. Kortes, R.E. Shepherd, Transition Met. Chem. (London) 22 (1997) 243].