N,N-Diborylamines have emerged as promising reagents in organic synthesis; however, their efficient preparation and full synthetic utility have yet to be realized. To address both shortcomings, an effective catalyst for nitrile dihydroboration was sought. Heating CoCl 2 in the presence of PyEt PDI afforded the six-coordinate Co(II) salt, [( PyEt PDI)CoCl][Cl]. Upon adding 2 equiv of NaEt 3 BH, hydride transfer to one chelate imine functionality was observed, resulting in the formation of (κ 4 -N,N,N,N-PyEt IP CHMe N EtPy )Co. Single-crystal X-ray diffraction and density functional theory calculations revealed that this compound possesses a low-spin Co(II) ground state featuring antiferromagnetic coupling to a singly reduced imino(pyridine) moiety. Importantly, (κ 4 -N,N,N,N-PyEt IP CHMe N EtPy )Co was found to catalyze the dihydroboration of nitriles using HBPin with turnover frequencies of up to 380 h −1 at ambient temperature. Stoichiometric addition experiments revealed that HBPin adds across the Co−N amide bond to generate a hydride intermediate that can react with additional HBPin or nitriles. Computational evaluation of the reaction coordinate revealed that the B−H addition and nitrile insertion steps occur on the antiferromagnetically coupled triplet spin manifold. Interestingly, formation of the borylimine intermediate was found to occur following BPin transfer from the borylated chelate arm to regenerate (κ 4 -N,N,N,N-PyEt IP CHMe N EtPy )Co. Borylimine reduction is in turn facile and follows the same ligand-assisted borylation pathway. The independent hydroboration of alkyl and aryl imines was also demonstrated at 25 °C. With a series of N,N-diborylamines in hand, their addition to carboxylic acids allowed for the direct synthesis of amides at 120 °C, without the need for an exogenous coupling reagent.

It was recently discovered that (Ph2PPrPDI)Mn (PDI = pyridine diimine) exists as a superposition of low-spin Mn­(II) that is supported by a PDI dianion and intermediate-spin Mn­(II) that is antiferromagnetically coupled to a triplet PDI dianion, a finding that encouraged the synthesis and electronic structure evaluation of late first row metal variants that feature the same chelate. The addition of Ph2PPrPDI to FeBr2 resulted in bromide dissociation and the formation of [(Ph2PPrPDI)­FeBr]­[Br]. Reduction of this precursor using excess sodium amalgam afforded (Ph2PPrPDI)­Fe, which possesses an Fe­(II) center that is supported by a dianionic PDI ligand. Similarly, reduction of a premixed solution of Ph2PPrPDI and CoCl2 yielded the cobalt analog, (Ph2PPrPDI)­Co. EPR spectroscopy and density functional theory calculations revealed that this compound features a high-spin Co­(I) center that is antiferromagnetically coupled to a PDI radical anion. The addition of Ph2PPrPDI to Ni­(COD)2 resulted in ligand displacement and the formation of (Ph2PPrPDI)­Ni, which was found to possess a pendent phosphine group. Single-crystal X-ray diffraction, CASSCF calculations, and EPR spectroscopy indicate that (Ph2PPrPDI)Ni is best described as having a Ni­(II)-PDI2– configuration. The electronic differences between these compounds are highlighted, and a computational analysis of Ph2PPrPDI denticity has revealed the thermodynamic penalties associated with phosphine dissociation from 5-coordinate (Ph2PPrPDI)­Mn, (Ph2PPrPDI)­Fe, and (Ph2PPrPDI)­Co.

Refluxing Mo(CO) 6 in the presence of the phosphine-functionalized α-diimine ligand Ph2PPr DI allowed for substitution and formation of the dicarbonyl complex, ( Ph2PPr DI)-Mo(CO) 2 . Oxidation with I 2 followed by heating resulted in further CO dissociation and isolation of the corresponding diiodide complex, ( Ph2PPr DI)MoI 2 . Reduction of this complex under a N 2 atmosphere afforded the corresponding bis(dinitrogen) complex, ( Ph2PPr DI)Mo(N 2 ) 2 . The solid-state structures of all three compounds were found to feature a tetradentate chelate and cismonodentate ligands. Notably, the addition of CO 2 to ( Ph2PPr DI)Mo(N 2 ) 2 is proposed to result in head-to-tail CO 2 coupling to generate the corresponding metallacycle and ultimately a mixture of ( Ph2PPr DI)Mo(CO) 2 and the bis(oxo) dimer, [(κ 3 -Ph2PPr DI)Mo(O)(μ-O)] 2 . Computational studies have been performed to gain insight into the reaction and evaluate the importance of cis-coordination sites for selective head-to-tail CO 2 reductive coupling, CO deinsertion, disproportionation, and stepwise CO 2 deinsertion.