Synthesis of Tris Pyrazolylborate - Lab Report Example

Synthesis and analysis of trispyrazolylborate Synthesis and analysis of trispyrazolylborate This report describes the p[reparation and determination of the properties of potassium Trispyrazolylborate by the cyclic voltametry technique. The Trispyrazolylborate complex was prepared by reacting pyrazole with potassium borohydride and then its properties determined by performing a cyclic voltametry test. The solid obtained was 120.27g representing 57% yield and was a white crystalline solid highly soluble in water and organic solvents. It had a melting point of 180-1850C. Introduction Trispyrazolylborate (Tp) is an organic anionic tetra dentate or tripodal ligand that has the chemical formula [HB (C3N2H3)3]. Trispyrazolylborate specifically refers to the derivatives substituted on the pyrazolyl rings. In the resonance structures of this compound, the nitrogen centers that are not bonded to boron are basic (Topaloglu-Sozuer et al., 2005). These centers bond to three adjacent sites of a metal such that the simple adducts have C3v symmetry. The bonding mode is similar to that of cyclopentadienyl ligands though the ligand field stabilization energy of Tp- is weaker as indicated by the fact that Fe (Tp)2 is a spin crossover complex while ferrocene is low spin (Yong Heng, Aoki, & Feng Ying, 2004). The Tp ligands are prepared in the laboratory by reacting pyrazole with potassium borohydride according to the equation: KBH4 + 3C3H3N2H → K [HB (C3N2H3)3] +3H2 The intermediate compounds include the monopyrazolylborate ([H3B (C3N2H2)2]- and the bispyrazolylborate ([H2B(C3N2H2)2]-. The potassium Trispyrazolylborate is a colorless solid that is soluble in polar solvents and has a melting point of 188-1890C (Maldonado, & Vahrenkamp, 2006). When 3-substituted pyrazoles are condensed with borohydride, the corresponding substituent Trispyrazolylborate derivatives are formed. The substituent formed pushes boron the less hindered nitrogen center and hence 3-phenylpyrazole gives [HB (C3N2H2Ph) 3]- whereas the phenyl substituents project away from the metal. Similarly 3-isppropylpyrazole gives [HB (C3N2H2iPr) 3]- and 3,5-dimethylpyrazole gives hexamethylated ligand [HB(C3N2HMe2)3]-. Cyclic voltametry is an electrochemical technique whereby the current developed in an electrochemical cell under conditions of excess voltage is measured. It is usually performed by cycling the potential of a working electrode and measuring the resulting current. In an experimental set up the electrode potential ramps linearly against time in cyclic phases. This analysis technique is mostly carried out on analytes that are redox active within the window to be scanned. It is also highly desired that the analytes should display a reversible cyclic voltametry wave that is observed when all the initial analyte is recovered after a forward and reverse scan cycle. Cyclic voltametry is an important and widely used electro analytical technique in the study of a range of redox processes and electron transfer kinetics and the reversibility of a reaction. It can also be used in the determination of the electron stoichiometry of a system, the diffusion coefficient of an analyte and the final reduction potential of an analyte. The concentration of an unknown solution can also be determined from the calibration curve of current versus concentration. Procedure A mixture of 1 mole of finely divided potassium borohydride and 272g of pyrazole were placed together with a magnetic stirring bar into a 1-1 single necked flask equipped with a thermometer sidearm. The flask was then connected through a 50 cm long air condenser to a wet- test meter, or some of other volumetric device and placed into an oil bath resting on a heating and stirring plate. The oil bath was then heated to 900C, and the mixture allowed to melt with stirring. Hydrogen evolution started at this point and continued briskly as potassium borohydride dissolved. Immediately then the melt temperature was raised gradually to maintain a steady yet controlled evolution of hydrogen but was kept 1250C below. When about 2 moles of hydrogen had been evolved, the melt was clear and the undissolved potassium borohydride particles were removed mechanically, and the still-hot melt poured into 600ml of stirred toluene. The resulting mixture was stirred for about five minutes and then filtered hot. The solid was then washed on the funnel with two 150 ml portions of hot toluene and then with two 200 ml portions of warm hexane. The compound obtained was then air-dried and weighed (Parry, 2007). Results The weight of the solid formed was 120.27g. This represented 57% yield. The cyclic voltametry curve plotted is as shown below Discussion and Conclusion The preparation of the potassium Tp complex yielded 120.27g that was equivalent to 57% yield. The solid formed was found to be white crystalline solid with a melting point of 180-1850c and it is very soluble in polar solvents and water. It could also be recrystallized from anisol but with significant solubility losses. The infrared spectrum of this complex has a complicated BH2 stretch multiplet in the 2200-2500 cm-1 range. It can also dissolve in alcohol, but the ligand undergoes slow solvolysis and solutions should be made up just before use. This complex belongs to a class of compounds called the scorpionate ligands that are tridentate ligands that bind to a metal in a facial manner. The trispyrazolyl ligands were the first of this class of compounds to be discovered. They first appeared in journals in 1966 from the little-known DuPont chemist of Ukrainian origin, Swiatoslaw Trofimenko who called his discovery ‘a new and fertile field of remarkable scope. It was named scorpionate because it could bind to a metal with two donor sites like the pincers of a scorpion. The other donor of the ligand is the plane formed by the metal and the other two donor atoms (Maldonado, & Vahrenkamp, 2006). . The trispyrazolyl borate ligand usually undergoes substitution of different pyrazoles in the 3, 4, and 5 positions to form a range of different ligands. These compounds are synthesized by reacting pyrazole with an alkali- metal borohydride such as sodium borohydride. Bulky pyrazolyl borates are prepared from 3, 5- disubstituted pyrazoles such as dimethyl derivative. The bulky pyrazolyl borates are treasured especially in the preparation of metal catalysts and also act as models for enzyme active sites. In the synthesis of metal catalysts, ligands allow for proper shielding of the bond metal while strong sigma bonds between the nitrogen and the metal stabilize the metal (Galler et al., 2004). In conclusion, the main objectives of the experiment were achieved. Therefore it was successful. The few errors that could have been present in the experiment should be rectified in future experiments to improve the experimental accuracy. I also recommend that further study and research should be done on these pyrazolyl complexes to determine the characteristics of each. References Topaloglu-Sozuer, I., Irdem, S. D., Jeffery, J. J., & Hamidov, H. (2005). Synthesis, spectroscopic characterization and X-ray crystal structures of oxo-bridged oxo(haloaryl) imido hydrotris(3,5-dimethylpyrazolyl)borate molybdenum(V) complexes. Journal of Coordination Chemistry, 58(2), 175-187. Doi:10.1080/00958970512331325494 Yong Heng, X., Aoki, K., & Feng Ying, B. (2004). Synthesis and Structure of the Mn(II) Complexes with Tripyrazolylborate Ligands: Mn[HB(pz)3]2 and Mn[HB(3,5-Me2- pz)3]2. Synthesis & Reactivity in Inorganic & Metal-Organic Chemistry, 34(7), 1149- 1163. Doi:10.1081/SIM-120039263 Kolotilov, S. V., Addison, A. W., Trofimenko, S., Dougherty, W., & Pavlishchuk, V. V. (2004). Efficient mechanochemical synthesis of tris(pyrazolylborate) complexes of manganese(II), cobalt(II) and nickel(II). Inorganic Chemistry Communications, 7(4), 485. doi:10.1016/j.inoche.2004.01.009 Maldonado Calvo, J. A., & Vahrenkamp, H. (2006). A new tris(2-furyl) substituted pyrazolylborate ligand and its zinc complex chemistry. Inorganica Chimica Acta, 359(12), 4079-4086. doi:10.1016/j.ica.2006.04.016 Galler, J. L., Goodchild, S., Gould, J., McDonald, R., & Sella, A. (2004). Tris-pyrazolylborate complexes of redox inactive lanthanides – the structures of [(TpMe2)2NdX] (X=Cl, NPh2, dpm, H2BEt2). Polyhedron, 23(2/3), 253. doi:10.1016/j.poly.2003.11.014 Parry, R. W. (2007). Poly(1-pyrazolyl)borates, Their Transition-Metal Complexes, and Pyrazaboles. Wiley Online Library (2), doi: 10.1002/9780470132432.ch18 Read More