In Chapter 1, This chapter provides an overview of the advancements in the field of non-precious metal (Ni, Co and Mo) complexes featuring anionic PCP pincer ligands where the (CH2, O and NH) linkers between aromatic ring and phosphine moieties. While the research in nickel PCP complexes is already quite extensive, the chemistry of cobalt and molybdenum PCP complexes is comparatively sparse. In the case of nickel PCP complexes already many catalytic applications such as Suzuki-Miyaura coupling, C-S cross coupling, Michael additions, Alcoholysis of acrylonitrile, hydrosilylation of aldehyde and cyanomethylation of aldehydes were reported. Cobalt and molybdenum PCP complexes were not applied to any catalytic reactions. Surprisingly, only one molybdenum PCP complex is reported, which was capable of cleaving dinitrogen to give a nitride complex. This literature survey is motivated us to find and develop the combination of cheap and abundant metals such as nickel, cobalt and molybdenum with PCP pincer ligands may result in the development of novel, versatile, and efficient catalysts for atom-efficient catalytic reactions. In Chapter 2, We have shown here that a PCP pincer ligand based on 1,3-diaminobenzene acts as versatile supporting scaffold in cobalt chemistry. The PCP moiety provides access to a range of Co complexes in formal oxidation states +I, +II, and +III by utilizing the 15e square planar d7 complex [Co(PCPMe-iPr)Cl] as synthetic precursor. In contrast to the analogous Ni(II) complex [Ni(PCPMe-iPr)Cl], [Co(PCPMe-iPr)Cl] is able to form stable pentacoordinate square pyramidal or trigonal bipyramidal 17e complexes. For instance, [Co(PCPMe-iPr)Cl] readily adds CO and pyridine to afford the five-coordinate square-pyramidal complexes [Co(PCPMe-iPr)(CO)Cl] and [Co(PCPMe-iPr)(py)Cl], respectively, while in the presence of Ag+ and CO the cationic bipyramidal complex [Co(PCPMe-iPr)(CO)2]+ is formed. The effective magnetic moments -eff of all Co(II) complexes derived from the temperature dependence of the inverse molar magnetic susceptibility by SQUID measurements are in the range of 1.9 to 2.4-B. This is consistent with a d7 low spin configuration with a contribution from the second-order spin orbit coupling. Oxidation of [Co(PCPMe-iPr)Cl] with CuCl2 yields the Co(III) PCP complex [Co(PCPMe-iPr)Cl2], while the synthesis of the Co(I) complex [Co(PCPMe-iPr)(CO)2] was achieved by reducing [Co(PCPMe-iPr)Cl] with KC8 in the presence of CO. Complex [Co(PCPMe-iPr)Cl2] exhibits a solution magnetic moment of 3.1-B which is consistent with a d6 intermediate spin system. The tendency of Co(I), Co(II) and Ni(II) PCP complexes of the type [M(PCPMe-iPr)(CO)]n (n = +1, 0) to add CO was investigated by DFT calculations showing that the Co species readily form the five-coordinate complexes [Co(PCPMe-iPr)(CO)2]+ and [Co(PCPMe-iPr)(CO)2] which are thermodynamically favorable, while Ni(II) maintains the 16e configuration since CO addition is thermodynamically unfavorable in this case. X-ray structures of most complexes are provided and discussed. A structural feature of interest is that the CO ligand in [Co(PCPMe-iPr)(CO)Cl] deviates significantly from linearity with a Co-C-O angle of 170.0(1)°. The DFT calculated value is 172o clearly showing that this is not a packing but an electronic effect. In Chapter 3, We have shown that the 15e square-planar complexes [Co(PCPMe-iPr)Cl] and [Co(PCP-tBu)Cl], respectively, react readily with NaBH4 to afford complexes [Co- (PCPMe-iPr)(-2-BH4)] and [Co(PCP-tBu)(-2-BH4)] in high yields. The -2-bonding mode of the borohydride ligand was confirmed by IR spectroscopy and X-ray crystallography. These compounds are paramagnetic with effective magnetic moments of 2.0(1) and 2.1(1) -B consistent with a d7 low-spin system corresponding to one unpaired electron. None of these complexes react with CO2 to give formate complexes. For structural and reactivity comparisons, we prepared the analogous Ni(II) borohydride complex [Ni(PCPMe-iPr)(-2-BH4)] via two different routes. One utilizes [Ni(PCPMe-iPr)Cl] and NaBH4, the second one makes use of the hydride complex [Ni(PCPMe-iPr)H] and BH3·THF. In both cases, [Ni(PCPMe-iPr)(-2-BH4)] was obtained in high yields. While [Ni(PCPMe-iPr)(-2-BH4)] loses readily BH3 at elevated temperatures in the presence of NEt3 to form [Ni(PCPMe-iPr)H], the Co(II) complex [Co(PCPMe-iPr)(-2-BH4)] did not react with NH3 to give a hydride complex. Complexes [Ni(PCPMe-iPr)(-2-BH4)] and [Ni(PCPMe-iPr)H] react with CO2 to give the formate complex [Ni(PCPMe-iPr)(OC(C-O)H]. DFT calculations revealed that the formation of the Ni hydride is thermodynamically favorable, whereas the formation of the Co(II) hydride, in agreement with the experiment, is unfavorable. From the calculations, it is apparent that, for the Co complexes, the BH4- coordination is closer to -2, and the overall geometry can be envisaged as in between square-planar and square-pyramidal. In complex [Ni(PCPMe-iPr)(-2-BH4)], the borohydride ligand coordination is closer to -1, and the overall geometry of the molecule is closer to normal square-planar, reflecting the tendency of Ni(II) to form complexes with that geometry, as expected for a d8 metal. A very rare bidentate and tridentate ligand containing pincer complex [Co(PCPMe-iPr)(NO)(NO2)] is synthesized by the reacting substrate [Co(PCPMe-iPr)(-2-BH4)] and NaNO2. The cobalt borohydride complex [Co(PCPMe-iPr)(-2-BH4)] is readily reacted with CO and tBuNC to produce the Co(I) complexes [Co(PCPMe-iPr)(CO)2] and [Co(PCPMe-iPr)(CNtBu)2], respectively. In Chapter 4, We described the synthesis and reactivity of the first Co(I) pincer complex [Co(-3P,CH,P-P(CH)PMe-iPr)(CO)2]+ featuring an agostic -2-Caryl-H bond. This complex was obtained via protonation of the Co(I) complex [Co(PCPMe-iPr)(CO)2]. In contrast to related Rh(I) agostic pincer complexes, this complex is five coordinate and adopts a trigonal bipyramidal geometry. Since the CO ligands are not sufficiently electron rich to promote an oxidative addition of the C-H bond, a Co(III) hydride complex is not formed. A Co(III) hydride complex [Co(PCPMe-iPr)(CNtBu)2(H)]+ was obtained upon protonation of the more electron rich Co(I) complex [Co(PCPMe-iPr)(CNtBu)2]. Three ways to cleave the agostic C-H arene bond are presented. First, the agostic proton in [Co(-3P,CH,P-P(CH)PMe-iPr)(CO)2]+ is acidic and pyridine deprotonates the C-H bond to reform the starting material. Another way to cleave the agostic C-H bond is hydrogen abstraction upon exposure of [Co(-3P,CH,P-P(CH)PMe-iPr)(CO)2]+ to oxygen) or TEMPO which results in the formation of the paramagnetic Co(II) PCP complex [Co(PCPMe-iPr)(CO)2]+. This process involves oxidation of the metal center from Co(I) to Co(II). Finally, replacement of one CO ligand in [Co(-3P,CH,P-P(CH)PMe-iPr)(CO)2]+ by CNtBu promotes the oxidative addition of the agostic C-H arene bond. This yields two isomeric hydride complexes of the type [Co(PCPMe-iPr)(CNtBu)(CO)(H)]+. DFT calculations support the existence of the agostic bond in complex [Co(-3P,CH,P-P(CH)PMe-iPr)(CO)2]+ and corroborate the differences observed between the Co and the Rh species. In Chapter 5, We synthesized two diamagnetic molybdenum pincer complexes such as 18e Mo(PCPMe-iPr)(Br)(O), and 16e Mo(PCPMe-iPr)(Br)(CO)2 from the two different molybdenum precursor such as Mo(Br)3(thf)3 and [Mo(Br)2(CO)4], respectively.
