**2.3 Development of Cu2+ ion probes based on decomposition reaction of**  *ortho***-carborane–metal chelator hybrids**

It is known that the reaction of the *o*-carborane **12** with Brønsted or Lewis bases affords the corresponding *nido*-form **13** and B(OH)3 and that the further degradation of **13** proceeds slowly under harsh conditions such as in acidic solutions and/or at high temperatures (**Figure 5**) [28]. On the other hand, we found that *o*-carborane derivatives such as **12**, **14**, and **15a**–**c** generate 4–9 equiv. of B(OH)3 upon the reaction with

**Figure 5.**

*Decomposition of o-carborane* **12** *in the presence of a Brønsted or Lewis base.*

Cu2+ and Mn2+ via the corresponding *nido*-forms **12'**, **14'**, and **15a'**–**c'** under physiological conditions (**Figure 6a**) [29]. Our studies also indicated that the modification of *nido-o*-carborane (**16** (L5 )) with *N*,*N*,*N'*-trimethylethylenediamine (TriMEDA) as

#### **Figure 6.**

*Decomposition of o-carborane-pendant chelators (a) the 11B NMR/MRI detection of Cu2+ ion based on decomposition reaction of o-carborane derivatives and (b).*

#### *Design, Synthesis, and Biological Applications of Boron-Containing Polyamine and Sugar… DOI: http://dx.doi.org/10.5772/intechopen.105998*

a chelator unit facilitates the Cu-promoted decomposition of the molecule (**Figure 6b**) via the Cu2+-complex **17** (CuL5 ) to produce 9 B(OH)3 in aqueous solution [30].

Changes in the 11B NMR spectra of **16** (L5 ) in the presence of various d-block metal ions are shown in **Figure 7**. A strong 11B signal at ca. 20 ppm corresponding to B(OH)3 was observed in the presence of Cu2+, while, in the presence of other metal ions, the change was negligible. These results showed good agreement with the results of an azomethine-H assay, which also indicate the Cu2+ selectivity.

As shown in **Figure 8**, the oxidation potentials of **12**′, **14**′, and **16** are +0.57, +0.51, and +0.38 V (vs Ag/AgCl), respectively (determined by cyclic voltammetry), which are less positive than +0.7 V (vs Ag/AgCl) for [Cu(TMEDA)]+ /[Cu(TMEDA)]2+ . These data may explain the reasons why **12**′, **14**′, and **16** are oxidized by Cu(TMEDA)]2+ complex. More efficient oxidation of **16** by Cu2+ than that of **12**′ and **14**′ is possibly due to the order of oxidation potentials (+0.38 V for **16** vs +0.57 and +0.51 V for **12**′ and **14**′) and the close contact between the *o*-carborane unit and stable Cu2+-TMEDA complex part in **17** and **18** (**Figure 6**).

In addition, the chemical yields of B(OH)3 from **16** (L5 ) with Cu<sup>+</sup> were decreased when antioxidants (sodium ascorbate, NaAsc) were added to the reaction mixture. According to these results and DFT calculations, a proposed mechanism for the decomposition of *o*-carborane moieties by Cu2+ is shown in **Figure 9**. Initially, the *nido*-form **20** is generated from the *closo*-form **19** by reaction with a nucleophile such

**Figure 7.**

*Decomposition of* **16** *(L5 ) (1.4 mM) in the presence of Cu2+, Cu+ , Cu+ +NaAsc, Mg2+, Ca2+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Zn2+, Cd2+ and Pb2+ (2 mM) in DMSO/0.5 M HEPES buffer (pH 7)/D2O (5:4:1, 0.5 mL in total) at 37 °C after incubation for 4 h measured by 11B{1 H} NMR. For 11B{1 H} NMR experiments, 2.5% BF3·Et2O in CDCl3 was used for an external reference.*

#### **Figure 8.**

*Summary of the oxidation potentials of* **12**′*,* **14**′**,** *and* **16** *(nido-form) with redox potentials of Cu, Fe, Pb, and Zn.*

as HO− . Following the oxidation of the electronegative B10 (B at the 10 position) of **20** by Cu2+, the *closo*-form **21** is produced by a ring-closure reaction. The unstable intermediate **21** would react with H2O at the B9 position and is then completely decomposed to 9 equiv. of B(OH)3 and other products via the transition state **22**. 11B MRI experiments were conducted by using an aqueous solution of B(OH)3

(10 mM) and Cu(bpy) (1 mM) in a larger vial (Sout) and a *o*-carborane analogue **14** (**Figure 6**) (1 mM) in a smaller vial (Sin) that was nested in the larger vial (**Figure 10**). To detect these boron compounds separately, BF1 (the basic transmitter frequency) values for B(OH)3 and **14** are set ca. 128.392 and 128.387 MHz, respectively, because they have different chemical shifts (a-i and b-i in **Figure 10**). Besides, 11B NMR images are obtained by using a two-dimensional ultra-short echo time sequence (UTE2D) with TE (echo time) of 199 μ*s*ec and TR (repetition time) of 30 msec. The 11B signals for both B(OH)3 and the *o*-carborane derivatives **14** were clearly observed, as shown in **Figure 10** (a-ii and b-ii).

The detection of Cu2+ by a 11B NMR probe **16** (L5 ) (2 mM) was carried out by the measurement of 11B MRI and NMR at the increasing concentrations of Cu2+ (0, 0.02, 0.1, 0.2, 1.0, and 2.0 mM) in aqueous solution at neutral pH. The 11B MRI/

*Design, Synthesis, and Biological Applications of Boron-Containing Polyamine and Sugar… DOI: http://dx.doi.org/10.5772/intechopen.105998*

#### **Figure 9.**

*Proposed mechanism for the decomposition reaction (arrows indicate positively charged boron atoms, which are susceptible to attack by H2O or HO− ).*

#### **Figure 10.**

*11B MRI images differentiating B(OH)3 and* **14***. Curves (a-i) and (b-i) show typical 11B NMR spectra of solutions in two vials (inside vial contains 1 mM* **<sup>14</sup>** *and outside contains 10 mM B(OH)3). Images (a-ii) and (b-ii) show 11B MRI of the inside vial (Sin) containing 1 mM* **<sup>14</sup>** *and the outside vial (Sout) including 10 mM B(OH)3 + 1 mM Cu(bpy). Both 11B NMR images were acquired by a two dimensional ultra-short echo time sequence (UTE2D) with TE = 199 μsec and TR = 30 msec.*

#### **Figure 11.**

*11B MRI and 11B{1 H} NMR (128 MHz) spectra of* **16** *(L5 ) (2 mM) in DMSO/0.5 M HEPES buffer (pH 7)/D2O (5:4:1, 0.5 mL in total) after incubation with various concentrations (0 (a), 0.02 (b), 0.1 (c), 0.2 (d), 1 (e), 2 mM (f)) of Cu2+ at 37 °C for 8 h (A 2.5% solution of BF3·Et2O in CDCl3 was used as the external reference). 11B NMR images were acquired by a two dimensional ultra-short echo time sequence (UTE2D) with BF1 values* ≈ *128.392 MHz, TE = 199 μsec and TR = 30 msec.*

NMR signals of B(OH)3 were successfully observed, and the signal intensities were increased in a dose-dependent manner due to the Cu2+-promoted decomposition of **16** (L5 ), as shown in **Figure 11**.
