The model complex [Cu4(μ4‐S)(dppa)4]2+ (1, dppa=μ2‐(Ph2P)2NH) has N2O reductase activity in methanol solvent, mediating 2 H+/2 e− reduction of N2O to N2+H2O in the presence of an exogenous electron donor (CoCp2). A stoichiometric product with two deprotonated dppa ligands was characterized, indicating a key role of second‐sphere N−H residues as proton donors during N2O reduction. The activity of 1 towards N2O was suppressed in solvents that are unable to provide hydrogen bonding to the second‐sphere N−H groups. Structural and computational data indicate that second‐sphere hydrogen bonding induces structural distortion of the [Cu4S] active site, accessing a strained geometry with enhanced reactivity due to localization of electron density along a dicopper edge site. The behavior of 1 mimics aspects of the CuZ catalytic site of nitrous oxide reductase: activity in the 4CuI:1S redox state, use of a second‐sphere proton donor, and reactivity dependence on both primary and secondary sphere effects.
Model compounds have
been widely utilized in understanding the
structure and function of the unusual Cu4(μ4-S) active site (CuZ) of nitrous oxide reductase (N2OR). However, only a limited number of model compounds that
mimic both structural and functional features of CuZ are
available, limiting insights about CuZ that can be gained
from model studies. Our aim has been to construct Cu4(μ4-S) clusters with tailored redox activity and chemical reactivity
via modulating the ligand environment. Our synthetic approach uses
dicopper(I) precursor complexes (Cu2L2) that
assemble into a Cu4(μ4-S)L4 cluster with the addition of an appropriate sulfur source. Here,
we summarize the features of the ligands L that stabilize precursor
and Cu4(μ4-S) clusters, along with the
alternative products that form with inappropriate ligands. The precursors
are more likely to rearrange to Cu4(μ4-S) clusters when the Cu(I) ions are supported by bidentate ligands
with 3-atom bridges, but steric and electronic features of the ligand
also play crucial roles. Neutral phosphine donors have been found
to stabilize Cu4(μ4-S) clusters in the
4Cu(I) oxidation state, while neutral nitrogen donors could not stabilize
Cu4(μ4-S) clusters. Anionic formamidinate
ligands have been found to stabilize Cu4(μ4-S) clusters in the 2Cu(I):2Cu(II) and 3Cu(I):1Cu(II) states, with
both the formation of the dicopper(I) precursors and subsequent assembly
of clusters being governed by the steric factor at the ortho positions of the N-aryl substituents. Phosphaamidinates,
which combine a neutral phosphine donor and an anionic nitrogen donor
in the same ligand, form multinuclear Cu(I) clusters unless the negative
charge is valence-trapped on nitrogen, in which case the resulting
dicopper precursor is unable to rearrange to a multinuclear cluster.
Taken together, the results presented in this study provide design
criteria for successful assembly of synthetic model clusters for the
CuZ active site of N2OR, which should enable
future insights into the chemical behavior of CuZ.
Phosphabenzamidine [mes-NH-C(Ph)═P-mes) (1) and phosphaformamidine (mes-NH-CH═P-mes) (4) ligands have been synthesized and characterized. The conjugate bases of 1 and 4 coordinate by each bridging three Cu(I) ions, forming hexa- and tetranuclear clusters Cu[mes-N═C(Ph)-P-mes]ClLi(THF) (3) and Cu[mes-N═CH-P-mes] (5), respectively. Both clusters have been fully characterized using H NMR,P NMR, and X-ray crystallography. Complexes 3 and 5 exhibit a previously unknown coordination mode of phosphaamidinates, which are far less studied than their well-known amidinate counterparts.
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