Jeremy P. Krogman

Activation of CO2 by a heterobimetallic Zr/Co complex.

At room temperature, the early/late heterobimetallic complex Co((i)Pr(2)PNMes)(3)Zr(THF) has been shown to oxidatively add CO(2), generating (OC)Co((i)Pr(2)PNMes)(2)(μ-O)Zr((i)Pr(2)PNMes). This compound can be further reduced under varying conditions to generate either the Zr oxoanion (THF)(3)Na-O-Zr(MesNP(i)Pr(2))(3)Co(CO) or the Zr carbonate complex (THF)(4)Na(2)(CO(3))-Zr(MesNP(i)Pr(2))(3)Co(CO). Additionally, reactivity of the CO(2)-derived product has been observed with PhSiH(3) to generate the Co-hydride/Zr-siloxide product (OC)(H)Co((i)Pr(2)PNMes)(3)ZrOSiH(2)Ph.

Synthesis, Structure, and Reactivity of an Anionic Zr–Oxo Relevant to CO2 Reduction by a Zr/Co Heterobimetallic Complex

Oxidative addition of CO2 to the reduced Zr/Co complex (THF)Zr(MesNP(i)Pr2)3Co (1) followed by one-electron reduction leads to formation of an unusual terminal Zr-oxo anion [2][Na(THF)3] in low yield. To facilitate further study of this compound, an alternative high-yielding synthetic route has been devised. First, 1 is treated with CO to form (THF)Zr(MesNP(i)Pr2)3Co(CO) (3); then, addition of H2O to 3 leads to the Zr-hydroxide complex (HO)Zr(MesNP(i)Pr2)3Co(CO) (4). Deprotonation of 4 with Li(N(SiMe3)2) leads to the anionic Zr-oxo species [2][Li(THF)3] or [2][Li(12-c-4)] in the absence or presence of 12-crown-4, respectively. The coordination sphere of the Li(+) countercation is shown to lead to interesting structural differences between these two species. The anionic oxo fragment in complex [2][Li(12-c-4)] reacts with electrophiles such as MeOTf and Me3SiOTf to generate (MeO)Zr(MesNP(i)Pr2)3Co(CO) (5) and (Me3SiO)Zr(MesNP(i)Pr2)3Co(CO) (6), respectively, and addition of acetic anhydride generates (AcO)Zr(MesNP(i)Pr2)3Co(CO) (7). Complex [2][Li(12-c-4)] is also shown to bind CO2 to form a monoanionic Zr-carbonate, [(12-crown-4)Li][(κ(2)-CO3)Zr(MesNP(i)Pr2)3Co(CO)] ([8][Li(12-c-4)]). A more stable version of this compound [8][K(18-c-6)] is formed when a K(+) counteranion and 18-crown-6 are used. Binding of CO2 to [2][Li(12-c-4)] is shown to be reversible using isotopic labeling studies. In an effort to address methods by which these CO2-derived products could be turned over in a catalytic cycle, it is shown that the Zr-OMe bond in 5 can be cleaved using H(+) and the CO ligand can be released from Co under photolytic conditions in the presence of I2.

Vanadium–iron complexes featuring metal–metal multiple bonds

A series of V/Fe heterobimetallic complexes supported by phosphinoamide ligands, [Ph2PNiPr]−, is described. The V(III) metalloligand precursor [V(iPrNPPh2)3] can be treated with Fe(II) halide salts under reducing conditions to afford [V(iPrNPPh2)3FeX] (X = Br (2), I (3)). These complexes feature multiple bonds between Fe and V, leading to an intermetallic distance of ∼2.07 A. Exploration of the one-electron reduction of complex 3 allows isolation of [V(iPrNPPh2)3Fe(PMe3)] (5), which also features metal–metal multiple bonding and a nearly identical Fe–V distance. Mossbauer spectroscopy of complexes 2 and 5 suggest that the most reasonable oxidation state assignments for these complexes are VIIIFeI and VIIIFe0, respectively, and that reduction occurs solely at the Fe center in these bimetallic complexes. A theoretical investigation confirms this description of the electronic structure, providing a description of the metal–metal bonding manifolds as (σ)2(π)4(Fenb)3 and (σ)2(π)4(Fenb)4 for complexes 3 and 5, consistent with a metal–metal bond order of three. One electron-oxidation of complex 3 results in halide abstraction from PF6−, forming FV(iPrNPPh2)3FeI (6). Complex 6 has a much weaker V–Fe interaction as a result of axial fluoride ligation at the V center.

One-electron oxidation chemistry and subsequent reactivity of diiron imido complexes.

The chemical oxidation and subsequent group transfer activity of the unusual diiron imido complexes Fe((i)PrNPPh2)3Fe≡NR (R = tert-butyl ((t)Bu), 1; adamantyl, 2) was examined. Bulk chemical oxidation of 1 and 2 with Fc[PF6] (Fc = ferrocene) is accompanied by fluoride ion abstraction from PF6(-) by the iron center trans to the Fe≡NR functionality, forming F-Fe((i)PrNPPh2)3Fe≡NR ((i)Pr = isopropyl) (R = (t)Bu, 3; adamantyl, 4). Axial halide ligation in 3 and 4 significantly disrupts the Fe-Fe interaction in these complexes, as is evident by the >0.3 Å increase in the intermetallic distance in 3 and 4 compared to 1 and 2. Mössbauer spectroscopy suggests that each of the two pseudotetrahedral iron centers in 3 and 4 is best described as Fe(III) and that one-electron oxidation has occurred at the tris(amido)-ligated iron center. The absence of electron delocalization across the Fe-Fe≡NR chain in 3 and 4 allows these complexes to readily react with CO and (t)BuNC to generate the Fe(III)Fe(I) complexes F-Fe((i)PrNPPh2)3Fe(CO)2 (5) and F-Fe((i)PrNPPh2)3Fe((t)BuNC)2 (6), respectively. Computational methods are utilized to better understand the electronic structure and reactivity of oxidized complexes 3 and 4.

Heterobimetallic Complexes Comprised of Nb and Fe: Isolation of a Coordinatively Unsaturated NbIII/Fe0 Bimetallic Complex Featuring a Nb≡Fe Triple Bond

Heterometallic multiple bonds between niobium and other transition metals have not been reported to date, likely owing to the highly reactive nature of low-valent niobium centers. Herein, a C3-symmetric tris(phosphinoamide) ligand framework is used to construct a Nb/Fe heterobimetallic complex Cl-Nb(iPrNPPh2)3Fe-Br (2), which features a Fe→Nb dative bond with a metal-metal distance of 2.4269(4) Å. Reduction of 2 in the presence of PMe3 affords Nb(iPrNPPh2)3Fe-PMe3 (6), a compound with an unusual trigonal pyramidal geometry at a NbIII center, a Nb≡Fe triple bond, and the shortest bond distance (2.1446(8) Å) ever reported between Nb and any other transition metal. Complex 6 is thermally unstable and degrades via P-N bond cleavage to form a NbV═NR imide complex, iPrN═Nb(iPrNPPh2)3Fe-PMe3 (9). The heterobimetallic complexes iPrN═Nb(iPrNPPh2)3Fe-Br (8) and 9 are independently synthesized, revealing that the strongly π-bonding imido functionality prevents significant metal-metal interactions. The 57Fe Mössbauer spectra of 2, 6, 8, and 9 show a clear trend in isomer shift (δ), with a decrease in δ as metal-metal interactions become stronger and the Fe center is reduced. The electronic structure and metal-metal bonding of 2, 6, 8, and 9 are explored through computational studies, and cyclic voltammetry is used to better understand the effect of metal-metal interaction in early/late heterobimetallic complexes on the redox properties of the two metals involved.