An ethylene cross-bridged pentaazamacrocycle and its Cu²⁺complex: constrained ligand topology and excellent kinetic stability

Abstract

<p>Extending the benefits of ethylene cross-bridging to pentaazamacrocycles has been achieved and a transition metal complex prepared and structurally characterized.</p>

Exceptionally fast formation of stable rigidified cross-bridged complexes formed with Cu(ii) isotopes for molecular imaging

“…Interestingly, the synthesis of the first member of the bicyclo-pentaamine ligand family, CB-15aneN 5 (Scheme 1), was described more than two decades ago, 15,16 however, the first structural and physicochemical studies have been published only recently. 14 Excellent dissociation half-life ( t 1/2 ), 5.3 d, was obtained for the Cu( ii ) complex of Me 3 -CB-15aneN 5 (Scheme S1†) in 5 M HCl at 50 °C which is close to the value determined by Jones et al 17 for the Cu( ii ) complex of Me 2 -CB-cyclam (7.3 d) under the same conditions.…”

“…This confirms that the geometry optimization was reasonably accurate. 14 The binding of apical nitrogen yields a relatively high deviation from the square-pyramidal geometry with ca. 70° rotation relative to the equatorial plane.…”

“…The inertness of the Cu( ii ) complexes was investigated by the generally accepted methodology in which the dissociation rate of the chelates is monitored in harsh circumstances such as high acid concentration and temperatures. 14 Conclusions are often drawn by comparing half-lives of these reactions. The t 1/2 values calculated for the dissociation of [Cu(CB-15aneN 5 )] 2+ and [Cu( p NO 2 Bn-CB-15aneN 5 )] 2+ are 8.0(2) minutes and 2.3(2) hours in 5.0 M HCl at 50 °C, respectively.…”

“…A recent publication by Shircliff et al 14 has also shown that the inertness of the cross-bridged systems can be maintained in the pentaazamacrocyclic platform (Scheme S1 and Table S1†). Interestingly, the synthesis of the first member of the bicyclo-pentaamine ligand family, CB-15aneN 5 (Scheme 1), was described more than two decades ago, 15,16 however, the first structural and physicochemical studies have been published only recently.…”

Expanding and quantifying the crystal chemistry of the flexible ligand 15aneN5

“…30–42 Recently, we have decided to investigate ethylene cross-bridged analogues of the larger pentaazamacrocycles, hoping to produce similarly useful, kinetically stable coordination complexes. 43 The most relevant conclusion from that work is that the glyoxal based ethylene cross-bridging synthetic strategy generally applied to tetraazamacrocycles is applicable to pentaazamacrocycles as well, and does induce the predicted kinetic stability. Revelations such as this suggest that the neglected pentaazamacrocycles should be further explored by coordination chemists.…”