| Summary: | The objective of this study is to design and synthesize substituted η<sup>6</sup>-chromium(0) tricarbonyl metal complexes carrying <i>o</i>-carborane units as potential boron neutron capture therapy (BNCT) agents. In this study, 1,2-diphenyl-<i>o</i>-carborane (<b>1</b>) units were used as starting materials to generate biologically active species. We investigated how the structural changes of <b>1</b> substituted with chromium(0) tricarbonyl affect the biological properties, and 1-(Phenyl-η<sup>6</sup>-chromium(0) tricarbonyl)-2-phenyl-<i>o</i>-carborane (<b>2</b>) and 1,2-bis(phenyl-η<sup>6</sup>-chromium(0) tricarbonyl)-<i>o</i>-carborane (<b>3</b>) species were produced in moderate yields. The molecular structures of compounds <b>1</b>–<b>3</b> were identified and established by infrared (IR); <sup>1</sup>H, <sup>11</sup>B, and <sup>13</sup>C nuclear magnetic resonance (NMR) and X-ray crystallography analyses. Crystal structures of 1,2-diphenyl-<i>o</i>-carborane and the corresponding chromium complexes <b>1</b>, <b>2</b>, and <b>3</b> were obtained. In an in vitro study using B16 and CT26 cancer cells containing the triphenyl-<i>o</i>-carboranyl chromium(0) complexes <b>Ph3C2BCr2</b> and <b>Ph3C2BCr3</b>, which we reported previously, compounds <b>2</b> and <b>3</b> accumulated at higher levels than compounds <b>Ph3C2BCr2</b> and <b>Ph3C2BCr3</b>. However, the phenylated <i>o</i>-carboranyl chromium complexes have been found to be more cytotoxic than <i>p</i>-boronophenylalanine (BPA).
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