The photochemistry of manganese(0), nickel(0) and palladium(I) binuclear complexes

Frederick Rudolph Lemke, Purdue University

Abstract

The reaction of Mn$\sb2$(CO)$\sb{10}$ with Me$\sb2$PCH$\sb2$PMe$\sb2$ (dmpm) gives a mixture of two isomers: mer,fac-Mn$\sb2$(CO)$\sb6$(dmpm)$\sb2$ (1) and mer,mer-Mn$\sb2$(CO)$\sb6$(dmpm)$\sb2$ (2). 1 and 2 can be oxidized by one-electron to give (mer,fac-Mn$\sb2$(CO)$\sb6$(dmpm)$\sb2\rbrack\cdot\sp+$ (3) and (mer,mer-Mn$\sb2$(CO)$\sb6$(dmpm)$\sb2\rbrack\cdot\sp+$ (4), respectively. EPR spectra of 3 and 4 suggest that unpaired spin density is localized on one Mn center in THF and delocalized over both Mn centers in CH$\sb2$Cl$\sb2$. Upon irradiation, 1 converts to 2; while 2 loses one CO to give Mn$\sb2$($\mu$-CO)(CO)$\sb4$(dmpm)$\sb2$ containing a 4-electron bridging carbonyl. 1 is also a strong one-electron photoreductant with an excited state redox potential of $-$1.35 $\leq$ E$\sp\circ$ (3/1*) $\leq$ $-$1.87 V vs. SCE. The binuclear nickel(0) complex Ni$\sb2$($\mu$-CNMe)(CNMe)$\sb2$(dppm)$\sb2$ (6) exhibits an unusual (Ni$\sb2$)d$\pi$ $\to$ ($\mu$-CNMe)$\pi$* excited state. The observable excited state is a triplet which is formed with $\Phi\sb{\rm isc}$ = 0.2 and is characterized by $\tau$ = 300 $\mu$s, $\lambda\sb{\rm max}$ = 550 nm and $\epsilon\sb{\rm T}$ = 400 M$\sp{-1}$ s$\sp{-1}$. The triplet state is associative with respect to metal-$\mu$-ligand bonding and exhibits enhanced $\mu$-CNMe N-nucleophilicity, undergoing bimolecular reactions with CO$\sb2$, k$\sb{\rm CO\sb2}$ = 1 $\times$ 10$\sp4$ M$\sp{-1}$ s$\sp{-1}$, and PhCl, k$\sb{\rm PhCl}$ = 4 $\times$ 10$\sp3$ M$\sp{-1}$ s$\sp{-1}$, to give Ni$\sb2$($\mu$-CN(Me)C(O)O)(CNMe)$\sb2$(dppm)$\sb2$ and Ni$\sb2$($\mu$-CNMePh)(CNMe)$\sb2$(dppm)$\sb2$, respectively. 6 is the first example of a binuclear metal-metal complex with an associative excited state. Irradiation of (Pd$\sb2$(CNMe)$\sb6$) (PF$\sb6\rbrack\sb2$ generates a pair of 15-electron (Pd(CNMe)$\sb3\rbrack{\cdot}\sp+$ radicals; which react with potential 3-electron donor substrates like allyl chloride, k$\sb{\rm Cl}$ = 500 M$\sp{-1}$ S$\sp{-1}$, or chloroacetone, k$\sb{\rm Cl}$ = 200 M$\sp{-1}$ s $\sp{-1}$, to give (Pd(CNMe)$\sb3$Cl) (PF$\sb6$) and allyl (Pd(C$\sb3$H$\sb5$)(CNMe)$\sb2$) (PF$\sb6$) or "oxaallyl" (Pd(CH$\sb2$C(O)CH$\sb3$)(CNMe)$\sb2$) (PF$\sb6$) complexes, respectively. Laser flash photolysis studies suggest that D(C-Cl) is an important factor influencing k$\sb{\rm Cl}$. The (Pd(CNMe)$\sb3\rbrack\cdot\sp+$ radical also undergoes electron-transfer with substituted ferrocenes to give unstable palladium(0) species and corresponding ferrocenium radical cations. Electron-transfer rate constants exhibit Marcus/Agmon-Levine-type behavior with a limiting rate approaching 10$\sp8$ M$\sp{-1}$ s$\sp{-1}$. The (Pd(CNMe)$\sb3\rbrack\cdot\sp+$ radical is a potent reducing agent with E$\sp\circ$((Pd(CNMe) $\sb3\rbrack\cdot\sp{+/\rm o}$) between 0.2 and 0.3 V vs. SCE. The (Pd(CNMe)$\sb3\rbrack\cdot\sp+$ radical is the first example of a photogenerated organometallic radical which is reduced in an electron-transfer reaction.

Degree

Ph.D.

Advisors

Kubiak, Purdue University.

Subject Area

Chemistry

COinS