Coordination and dearomatisation of a tetradentate bis(phosphino)phenanthroline ligand at cobalt(ii) and cobalt(i)

In recent literature, the RPNNP ligand 2,9-bis(phosphinomethyl)-1,10-phenanthroline (R = tBu, Ph) has been employed to study dearomatisation reactions at ruthenium, cobalt and iron. In this study we report the reactions of simple cobalt compounds with the tBuPNNP ligand. Paramagnetic CoII complexes were accessed by reaction of tBuPNNP with either CoCl2 or [Co(MeCN)6][BF4]2. Notably, the X-ray structure shows that CoII(tBuPNNP)Cl2 exhibits 5-coordination with a tridentate tBuPNNP ligand and one pendant phosphine. Chloride abstraction results in [CoII(tBuPNNP)Cl][BArF4] in which the PNNP is tetradentate and the chloride occupies an axial position. Magnetic measurements in solution demonstrated that the chloride complexes, CoII(tBuPNNP)Cl2 and [CoII(tBuPNNP)Cl][BArF4], have high-spin ground states but the MeCN complex [CoII(tBuPNNP)(MeCN)][BF4]2 formed by reaction of [CoII(NCMe)6][BF4]2 with tBuPNNP exhibits a low spin ground state. The single crystal X-ray data reveal a substantial reduction in the Co–N and Co–P bond lengths compared to the chloride complexes. Reduction of CoII(tBuPNNP)Cl2 caused dissociation of the chloride ligands and formation of high-spin [CoI(tBuPNNP)][PF6]. Methylation of CoII(tBuPNNP)Cl2 followed by elimination of methane provided a route to the dearomatised CoI(d-tBuPNNP), a direct analogue of Nakajima's CoI(d-PhPNNP) in which a hydrogen is abstracted from tBu2P–CH2–C to form a vinyl substituent at phosphorus tBu2P–CHC. Alternatively, the reaction of CoII(tBuPNNP)Cl2 with NaBEt3H yielded the same species. The Co–P distance for the PtBu2(vinyl) ligand of CoI(d-tBuPNNP) is much shorter than for the PtBu2(alkyl) ligand. Reaction of CoI(d-tBuPNNP) with H2 generated CoI(m-tBuPNNP) which can be considered as a 1,2-hydrogenation product of the dearomatised vinylphenanthroline ring; this is also an analogue of one of Nakajima's complexes. However, in addition, the 1,4-hydrogenation product CoI(h-tBuPNNP) was found as an intermediate. Reaction of [CoII(PNNP)(MeCN)](BF4)2 with NaBEt3H generates the CoI hydride CoI(tBuPNNP)H. Subsequently, both CoI(m-tBuPNNP) and CoI(h-tBuPNNP) are formed together with a dihydrogenation product, CoI(q-tBuPNNP).