High-valent iron species are key intermediates in oxidative biological processes, but hexavalent complexes apart from the ferrate ion are exceedingly rare. Here, we report the synthesis and structural and spectroscopic characterization of a stable Fe(VI) complex (3) prepared by facile one-electron oxidation of an Fe(V) bis(imido) (2). Single-crystal x-ray diffraction of 2 and 3 revealed four-coordinate Fe centers with an unusual “seesaw” geometry. 57Fe Mössbauer, x-ray photoelectron, x-ray absorption, and electron-nuclear double resonance (ENDOR) spectroscopies, supported by electronic structure calculations, support a low-spin (S = 1/2) d3 Fe(V) configuration in 2 and a diamagnetic (S = 0) d2 Fe(VI) configuration in 3. Their shared seesaw geometry is electronically dictated by a balance of Fe-imido σ- and π-bonding interactions.
The new iron(IV) nitride complex PhB(PrIm)Fe≡N reacts with 2 equiv of bis(diisopropylamino)cyclopropenylidene (BAC) to provide PhB(PrIm)Fe(CN)(N)(BAC). This unusual example of a four-electron reaction involves carbon atom transfer from BAC to create a cyanide ligand along with the alkyne PrN-C≡C-NPr. The iron complex is in equilibrium with an N-free species. Further reaction with CO leads to formation of a CO analogue, which can be independently prepared using NaCN as the cyanide source, while reaction with B(CF) provides the cyanoborane derivative.
The flexible flaps and the 80s loops (Pro79-Ile84) of HIV-1 protease are crucial in inhibitor binding. Previously, it was reported that the crystal structure of multidrug-resistant 769 (MDR769) HIV-1 protease shows a wide-open conformation of the flaps owing to conformational rigidity acquired by the accumulation of mutations. In the current study, the effect of mutations on the conformation of the 80s loop of MDR769 HIV-1 protease variants is reported. Alternate conformations of Pro81 (proline switch) with a root-mean-square deviation of 3-4.8 Å in the C(α) atoms of the I10V mutant and a side chain with a `flipped-out' conformation in the A82F mutant cause distortion in the S1/S1' binding pockets that affects inhibitor binding. The A82S and A82T mutants show local changes in the electrostatics of inhibitor binding owing to the mutation from nonpolar to polar residues. In summary, the crystallographic studies of four variants of MDR769 HIV-1 protease presented in this article provide new insights towards understanding the drug-resistance mechanism as well as a basis for design of future protease inhibitors with enhanced potency.
The
N
2
analogue phosphorus nitride (PN) was the first
phosphorus-containing compound to be detected in the interstellar
medium; however, this thermodynamically unstable compound has a fleeting
existence on Earth. Here, we show that reductive coupling of iron(IV)
nitride and molybdenum(VI) phosphide complexes assembles PN as a bridging
ligand in a structurally characterized bimetallic complex. Reaction
with C≡N
t
Bu releases the mononuclear
complex [(N
3
N)Mo—PN]
−
, N
3
N = [(Me
3
SiNCH
2
CH
2
)
3
N]
3–
), which undergoes light-induced linkage isomerization
to provide [(N
3
N)Mo—NP]
−
, as revealed
by photocrystallography. While structural and spectroscopic characterization,
supported by electronic structure calculations, reveals the PN multiple
bond character, coordination to molybdenum induces a nucleophilic
character at the terminal atom of the PN/NP ligands. Indeed, the linkage
isomers can be trapped in solution by reaction with a Rh(I) electrophile.
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