Intermediates in Pentanuclear Nickel Hydride Cluster Formation and the Role of the Supporting Phosphine Ligand

Our group has previously studied the atypical electron-deficient cluster [(iPr3P)Ni]5H6 for the activation of inert bonds and as a catalyst for nearly barrierless alkene dimerization and stereoselective [2+2] cycloadditions. This study examines alternative phosphines to the iPr3P supporting ligand in [iPr3P)Ni]5H6 for their ability to stabilize related pentanuclear clusters. The reaction of [(iPr3P)2Ni]2(μ-N2) (3⸱iPr3P) with H2 was found to feature two spectroscopically identified intermediates, trans-(iPr3P)2NiH2 (4⸱iPr3P) and [(iPr3P)2Ni]2(µ-H)2 (5⸱iPr3P) prior to cluster formation. Attempts at analogous chemistry supported by Cy3P (Cy = cyclohexyl) led to the spectroscopically identified trans-(Cy3P)2NiH2, which underwent P−C bond cleavage to give the dinuclear Ni(II) product [(Cy3P)NiH]2(µ-PCy2)(µ-H). Similar P−C bond cleavage chemistry was observed with Cyp3P (Cyp = cyclopentyl). The tBu2MeP supported complex [(tBu2MeP)2Ni]2(μ-N2) reacted under H2 to give [(tBu2MeP)Ni]5H6 (1H6⸱tBu2MeP), which proved much more reactive to H/D exchange with benzene-d6 than [(iPr3P)Ni]5H6. 1H6⸱tBu2MeP also had a triplet state only 6.1 kcal⸱mol–1 higher in energy than the ground state. This cluster is unstable at room temperature and in solution reacts via P−C bond cleavage. The smaller phosphine Cy2MeP supported the complex (Cy2MeP)3Ni (8⸱Cy2MeP), which cocrystallized with the dinitrogen complex (Cy2MeP)3Ni(N2) (8N2⸱Cy2MeP). Only partial conversion to [(Cy2MeP)2Ni](μ-H)2 (5⸱Cy2MeP) occurred under 4 atm H2. The complexes (Et2tBuP)2NiCl2 (2⸱Et2tBuP) and (2⸱iPr2EtP) were prepared but reduction gave the supporting phosphine as the major recovered product. This work shows the importance of steric factors in cluster formation and emphasizes the importance of resistance to P−C bond cleavage, both prior to and after cluster formation, for the isolation of pentanuclear complexes.