Multielectron redox activity facilitated by metal-metal interactions in early/late heterobimetallics: Co/Zr complexes supported by phosphinoamide ligands.

To assess the effect of dative M-->M interactions on redox properties in early/late heterobimetallic complexes, a series of Co/Zr complexes supported by phosphinoamide ligands have been synthesized and characterized. Treatment of the Zr metalloligands (Ph(2)PN(i)Pr)(3)ZrCl (1), ((i)Pr(2)PNMes)(3)ZrCl (2), and ((i)Pr(2)PN(i)Pr)(3)ZrCl (3) with CoI(2) leads to reduction from Co(II) to Co(I) and isolation of the heterobimetallic complexes ICo(Ph(2)PN(i)Pr)(3)ZrCl (4), ICo((i)Pr(2)PNMes)(3)ZrCl (5), and ICo((i)Pr(2)PN(i)Pr)(3)ZrCl (6), respectively. Interestingly, treatment of CoI(2) with the phosphinoamine Ph(2)PNH(i)Pr in the absence of a bound Zr center leads to the disubstituted Co(II) complex (Ph(2)PNH(i)Pr)(2)CoI(2) (7). The tris(phosphinoamine) Co(I) complex (Ph(2)PNH(i)Pr)(3)CoI (8) can only be generated in the presence of an added reductant such as Zn(0), indicating that the reduction of Co(II) to Co(I) only occurs in the presence of Zr in the formation of complexes 4-6. Structural characterization of 4-6 reveals a Zr-Co interaction, with interatomic distances of 2.7315(5) A, 2.6280(5) A, and 2.6309(5) A, respectively. This distance appears to decrease as the phosphine donors on Co become more electron-releasing, strengthening the dative Co-->Zr interaction. Cyclic voltammetry of 4-6 shows that all three compounds can be further reduced by two electrons at relatively mild reduction potentials (-1.65 V to -2.07 V vs Fc/Fc(+)). The potentials at which these reductions occur in each of these complexes are largely governed by the extent to which electron-density is donated to Zr, as well as the electron-donating ability of the phosphine substituents. Moreover, cyclic voltammetry of complex 8 reveals that in the absence of Zr, the Co center is significantly more electron rich, and thus more difficult to reduce. Chemical reduction of 5 leads to the isolation of the two-electron reduced dinitrogen complex [N(2)Co((i)Pr(2)PNMes)(3)ZrX][Na(THF)(5)] (9). X-ray crystallography of 9 reveals that two-electron reduction is accompanied by a significant contraction of the Co-Zr interatomic distance from 2.6280(5) A to 2.4112(3) A. These heterobimetallic complexes have been studied computationally using density functional theory to examine the nature of the metal-metal interactions in these species.