Synthesis and reactivity of four- and five-coordinate low-spin cobalt(II) PCP pincer complexes and some nickel(II) analogues

Anhydrous CoCl₂ or [NiCl₂(DME)] reacts with the ligand PCP^Me-iPr (1) in the presence of nBuLi to afford the 15e and 16e square planar complexes [Co(PCP^Me-iPr)Cl] (2) and [Ni(PCP^Me-iPr)Cl] (3), respectively. Complex 2 is a paramagnetic d⁷ low-spin complex, which is a useful precursor for a series of Co(I), Co(II), and Co(III) PCP complexes. Complex 2 reacts readily with CO and pyridine to afford the five-coordinate square-pyramidal 17e complexes [Co(PCP^Me-iPr)(CO)Cl] (4) and [Co(PCP^Me-iPr)(py)Cl] (5), respectively, while in the presence of Ag⁺ and CO the cationic complex [Co(PCP^Me-iPr)(CO)₂]⁺ (6) is afforded. The effective magnetic moments μ_eff of all Co(II) complexes were derived from the temperature dependence of the inverse molar magnetic susceptibility by SQUID measurements and are in the range 1.9 to 2.4 μ_B. This is consistent with a d⁷ low-spin configuration with some degree of spin-orbit coupling. Oxidation of 2 with CuCl₂ affords the paramagnetic Co(III) PCP complex [Co(PCP^Me-iPr)Cl₂] (7), while the synthesis of the diamagnetic Co(I) complex [Co(PCP^Me-iPr)(CO)₂] (8) was achieved by stirring 2 in toluene with KC₈ in the presence of CO. Finally, the cationic 16e Ni(II) PCP complex [Ni(PCP^Me-iPr)(CO)]⁺ (10) was obtained by reacting complex 3 with 1 equiv of AgSbF₆ in the presence of CO. The reactivity of CO addition to Co(I), Co(II), and Ni(II) PCP square planar complexes of the type [M(PCP^Me-iPr)(CO)]ⁿ (n = +1, 0) was investigated by DFT calculations, showing that formation of the Co species, 6 and 8, is thermodynamically favorable, while Ni(II) maintains the 16e configuration since CO addition is unfavorable in this case. X-ray structures of most complexes are provided and discussed. A structural feature of interest is that the apical CO ligand in 4 deviates significantly from linearity, with a Co-C-O angle of 170.0(1)°. The DFT-calculated value is 172°, clearly showing that this is not a packing but an electronic effect.