We describe a synthetic method for increasing and controlling the iron loading of synthetic melanin nanoparticles and use the resulting materials to perform a systematic quantitative investigation on their structure–property relationship. A comprehensive analysis by magnetometry, electron paramagnetic resonance, and nuclear magnetic relaxation dispersion reveals the complexities of their magnetic behavior and how these intraparticle magnetic interactions manifest in useful material properties such as their performance as MRI contrast agents. This analysis allows predictions of the optimal iron loading through a quantitative modeling of antiferromagnetic coupling that arises from proximal iron ions. This study provides a detailed understanding of this complex class of synthetic biomaterials and gives insight into interactions and structures prevalent in naturally occurring melanins.

A synthetic procedure for the sterically encumbered m-terphenyl isocyanide CNAr(Dipp2) (Dipp = 2,6-diisopropylphenyl) is presented. In comparison to the less encumbering m-terphenyl isocyanide ligand CNAr(Mes2), the steric attributes of the flanking Dipp groups effectively control the extent of CNAr(Dipp2) ligation to monovalent Cu and Ag centers and zero-valent Mo centers. Direct structural comparisons of Cu(I) and Ag(I) complexes of both CNAr(Dipp2) and CNAr(Mes2) are made. It was found that only two CNAr(Dipp2) ligands are accommodated by monovalent Cu and Ag centers, whereas three CNAr(Mes2) units can readily bind. As demonstrated by both (1)H NMR and FTIR spectroscopic studies, addition of a third equivalent of CNAr(Dipp2) to [(THF)(2)Cu(CNAr(Dipp2))(2)]OTf in C(6)D(6) solution results in slow isocyanide exchange. However, rapid isocyanide exchange is observed when an additional equivalent of CNAr(Dipp2) is added to (TfO)Ag(CNAr(Dipp2))(2). Three CNAr(Mes2) ligands react smoothly with fac-Mo(CO)(3)(NCMe)(3) to afford the octahedral complex fac-Mo(CO)(3)(CNAr(Mes2))(3), which can be converted irreversibly to the mer isomer upon heating in solution. Contrastingly, addition of CNAr(Dipp2) to fac-Mo(CO)(3)(NCMe)(3) results in a mixture of both the tetracarbonyl and the tricarbonyl complexes trans-Mo(CO)(4)(CNAr(Dipp2))(2) and trans-Mo(NCMe)(CO)(3)(CNAr(Dipp2))(2), respectively, in which the encumbering CNAr(Dipp2) ligands are in a trans-disposition. Ultraviolet irradiation of the preceding mixture in NCMe/Et(2)O under an argon flow provides exclusively the tricarbonyl complex trans-Mo(NCMe)(CO)(3)(CNAr(Dipp2))(2). Addition of free CNAr(Dipp2) to trans-Mo(NCMe)(CO)(3)(CNAr(Dipp2))(2) does not result in the binding of a third isocyanide unit by the Mo center as determined by (1)H NMR spectroscopy. Treatment of trans-Mo(NCMe)(CO)(3)(CNAr(Dipp2))(2) with the Lewis base pyridine (py) affords the complex fac,cis-Mo(py)(CO)(3)(CNAr(Dipp2))(2) as determined by X-ray diffraction. Notably, the encumbering nature of the CNAr(Dipp2) units forces a cis C(iso)-Mo-C(iso) angle of about 100 degrees.

We report the preparation and study of Mn(III)-, Fe(III)-, Co(II)-, Ni(II)-, Cu(II)-, Zn(II)-, and Ga(III)-loaded polydopamine nanoparticles (PDA-NPs) via autoxidation polymerization of metal−dopamine complexes in the presence of free dopamine. An analysis of the doping range and parameters that influence final particle morphology is presented. In addition, magnetometry provides a probe of the general electronic structure and electronic interactions for Mn(III)-, Ni(II)-, and Co(II)-loaded PDA-NPs. PDA-NPs doped with Mn(III) are found to have high spin, low anisotropy, and weak magnetic coupling and are therefore predicted to have superior relaxivity behavior compared to previously studied Fe(III)-loaded PDA-NPs. Comparison of Mn(III)-and Fe(III)-loaded PDA-NP relaxivity confirms the predictive ability of the magnetometry measurements.

The m-terphenyl isocyanides CNAr(Mes2) and CNAr(Dipp2) support five-coordinate, isocyanide/carbonyl monoanions of manganese. For CNAr(Dipp2), a bis-isocyanide anion is available that is remarkably well behaved upon reaction with electrophiles. Most notable is the formation of an unprecedented chloride-substituted metallostannylene.

Presented herein are synthetic and structural studies exploring the propensity of m-terphenyl isocyanide ligands to provide flanking-ring η(6)-arene interactions to zerovalent molybdenum centers. The alkyl-substituted m-terphenyl isocyanides CNAr(Mes2) and CNAr(Dipp2) (Ar(Mes2) = 2,6-(2,4,6-Me3C6H2)2C6H3; Ar(Dipp2) = 2,6-(2,6-(i-Pr)2C6H3)2C6H3) react with Mo(η(6)-napthalene)2 in a 3:1 ratio to form tris-isocyanide η(6)-arene Mo complexes, in which a flanking mesityl or 2,6-diisopropylphenyl group, respectively, of one isocyanide ligand is bound to the zerovalent molybdenum center. Thermal stability and reactivity studies show that these flanking ring η(6)-arene interactions are particularly robust. To weaken or prevent formation of a flanking-ring η(6)-arene interaction, and to potentially provide access to the coordinatively unsaturated [Mo(CNAr(R))3] fragment, the new halo-substituted m-terphenyl isocyanides CNAr(Clips2) and CNAr(DArF2) (Ar(Clips) = 2,6-(2,6-Cl2C6H3)2(4-t-Bu)C6H2; Ar(DArF2) = 2,6-(3,5-(CF3)2C6H3)2C6H3) have been prepared. Relative to their alkyl-substituted counterparts, synthetic and structural studies demonstrate that the flanking aryl rings of CNAr(Clips2) and CNAr(DArF2) display a lower tendency toward η(6)-binding. In the case of CNAr(DArF2), it is shown that an η(6)-bound 3,5-bis(trifluoromethyl)phenyl group can be displaced from a zerovalent molybdenum center by three molecules of acetonitrile. This observation suggests that the CNAr(DArF2) ligand effectively masks low-valent metal centers in a fashion that provides access to low-coordinate isocyano targets such as [Mo(CNAr(R))3]. A series of Mo(CO)3(CNAr(R))3 complexes were also prepared to compare the relative π-acidities of CNAr(Mes2), CNAr(Clips2), and CNAr(DArF2). It is found that CNAr(DArF2) shows increased π-acidity relative to CNAr(Mes2) and CNAr(Clips2), despite the fact that its electron-withdrawing CF3 groups are fairly distal to the terminal isocyano unit.