Nitrile hydratase (NHase) is an iron-containing metalloenzyme that converts nitriles to amides. The mechanism by which this biochemical reaction occurs is unknown. One mechanism that has been proposed involves nucleophilic attack of an Fe-bound nitrile by water (or hydroxide). Reported herein is a five-coordinate model compound ([Fe III (S 2 Me2 N 3 (Et,Pr))] + ) containing Fe(III) in an environment resembling that of NHase, which reversibly binds a variety of nitriles, alcohols, amines, and thiocyanate. XAS shows that five-coordinate [Fe III (S 2 Me2 N 3 (Et,Pr))] + reacts with both methanol and acetonitrile to afford a six-coordinate solvent-bound complex.Competitive binding studies demonstrate that MeCN preferentially binds over ROH, suggesting that nitriles would be capable of displacing the H 2 O coordinated to the iron site of NHase. Thermodynamic parameters were determined for acetonitrile (ΔH = −6.2(±0.2) kcal/mol, ΔS = −29.4(±0.8) eu), benzonitrile (−4.2(±0.6) kcal/mol, ΔS = −18(±3) eu), and pyridine (ΔH = −8(±1) kcal/mol, ΔS = −41(±6) eu) binding to [Fe III (S 2 Me2 N 3 (Et,Pr))] + using variable-temperature electronic absorption spectroscopy. Ligand exchange kinetics were examined for acetonitrile, isopropylnitrile, benzonitrile, and 4-tert-butylpyridine using 13 C NMR line-broadening analysis, at a variety of temperatures. Activation parameters for ligand exchange were determined to be ΔH ‡ = 7.1(±0.8) kcal/mol, ΔS ‡ = −10(±1) eu (acetonitrile), ΔH ‡ = 5.4(±0.6) kcal/mol, ΔS ‡ = −17(±2) eu (iso-propionitrile), ΔH ‡ = 4.9(±0.8) kcal/mol, ΔS ‡ = −20(±3) eu (benzonitrile), and ΔH ‡ = 4.7(±1.4) kcal/mol ΔS ‡ = −18(±2) eu (4-tert-butylpyridine). The thermodynamic parameters for pyridine binding to a related complex, [Fe III (S 2 Me2 N 3 (Pr,Pr))] + (ΔH = −5.9(±0.8) kcal/mol, ΔS = −24(±3) eu), are also reported, as well as kinetic parameters for 4-tert-butylpyridine exchange (ΔH ‡ = 3.1(±0.8) kcal/mol, ΔS ‡ )−25(±3) eu). These data show for the first time that, when it is contained in a ligand environment similar to that of NHase, Fe(III) is capable of forming a stable complex with nitriles. Also, the rates of ligand exchange demonstrate that low-spin Fe(III) in this ligand environment is more labile than expected. Furthermore, comparison of (Tables S-6-S-9), van't Hoff plots for ligand binding to 1 and 2, variable-temperature electronic absorption spectra (Figures S-3-S-7), and EPR spectra for "substrate"-bound 2 , and crystallographic data for 2-NCS (Tables S-10-S-14) (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. HHS Public AccessAuthor manuscript (S 2 Me2 N 3 (Pr,Pr))] + demonstrates how minor distortions induced by ligand constraints can dramatically alter the reactivity of a metal complex.Performing reactions under environmentally friendly conditions has recently become a desirable goal in the search for new catalysts. 1 Enzymes are often viewed as ideal in this respect because of their ability to perform chemical transformations und...

meso-Tetrakis(4-chlorophenyl)porphinato)manganese(III) tetracyanoethenide, [MnTClPP][TCNE], has been prepared and structurally characterized as the toluene and dichloromethane disolvates, and the magnetic and thermal properties of these solvates, as well as their corresponding desolvates, have been determined. The ditoluene solvate (1) has a triclinic unit cell: P1 h, a ) 10.171 (4) Å, b ) 10.189(3) Å, c ) 14.522 (3) Å, R ) 107.51(2)°, ) 85.58(2)°, γ ) 111.51(3)°, Z ) 1. The bis(dichloromethane) solvate (2) belongs to the monoclinic unit cell: P2 1 /n, a ) 9.894(2) Å, b ) 10.697(2) Å, c ) 23.560(5) Å, ) 101.34(2)°, Z ) 2. The cation is typical with average Mn-N distances of 2.012 Å for both the toluene and dichloromethane solvates. The bonding distances for both planar anions are characteristic of [TCNE] •-. Both solvates have an uniform linear chain (1-D) coordination-polymer structure comprised of alternating cations and anions. Each [TCNE] •-binds to two Mn III 's in a trans-µ-N-σ-bound manner with Mn-N spacings of 2.267 (1) and 2.276 Å (2). The Mn-N-C angles are 167.2 and 143.1°, while intrachain Mn‚‚‚Mn separations are 10.189 and 9.894 Å, and the dihedral angle between the MnN 4 and [TCNE] •-mean planes are 86.8 and 52.4°for 1 and 2, respectively. The ν CN absorptions for the toluene and dichloromethane solvates occur at 2201 m and 2160 s cm -1 and 2195 m and 2138 s cm -1 , respectively. Upon thermolysis at 175°C 1 desolvates to R-[MnTClPP][TCNE] with ν CN absorptions at 2201 m and 2159 s cm -1 . In contrast, desolvation of 1 in refluxing n-octane leads to -[MnTclPP][TCNE] with ν CN absorptions at 2190 m and 2132 s cm -1 . Upon facile desolvation of 2 to form γ-[MnTClPP][TCNE] the nitrile absorptions remain essentially unchanged (2195 m and 2137 s cm -1 ). For 1 and R the susceptibilities can be fit by the Curie-Weiss expression with Θ ) -60 K (T > 210 K) and -10 K (T > 250 K) and an effective Θ, Θ′ of +13 (50 < T < 120 K) and 29 K (60 < T < 160 K), respectively. Θ is not observed for the 2 or the -or γ-phase; however, Θ′ for the 2 and the -and γ-phases are 58, 92, and 86 K, respectively. The magnetic data are consistent with linear chain ferrimagnets comprised of antiferromagnetically coupled S ) 2 Mn III sites and S ) 1 / 2 [TCNE] •-sites with the antiferromagnetic intrachain coupling, J/k B (k B ) Boltzmann's constant) determined from fits to the Seiden expression, of -33, -160, -65, -267, and -265 K for the 1, 2, and R-, -, and γ-phases, respectively. Hysteresis with a coercive field of 5.8 kOe is observed for 1 at 2 K. Metamagnetic behavior below 5 K is observed for the 1, 2, and -and γ-phases with critical fields of 10, 27, 27, and 27 kOe, respectively. The ordering temperatures, T c , determined from the maxima in the ′(T) data taken at 10 Hz, are 8.8, 6.7, 11.1, 14.1, and 11.4 K for the phases 1, R, , 2, and γ, respectively. Desolvation of 1 and 2 increases the magnetic disorder and the magnetic coupling.

A series of four structurally related cis-dithiolate-ligated Fe(III) complexes, [Fe III (DITpy) (3 and 4). The crystallographically determined mean Fe-S bond distances in 1-4 range from 2.196 to 2.232 Å and are characteristic of low-spin Fe(III)-thiolate complexes. The low-spin S = ½ ground state was confirmed by both EPR and magnetic susceptibility measurements. The electronic spectra of these complexes are characterized by broad absorption bands centered near ~700 nm that are consistent with ligand-to-metal chargetransfer (CT) bands. The complexes were further characterized by cyclic voltammetry measurements, and all possess highly negative Fe(III)/Fe(II) redox couples (~ −1 V vs SCE, saturated calomel electrode) indicating that alkyl thiolate donors are effective at stabilizing Fe(III) centers. Both the redox couple and the 700 nm band in the visible spectra show solvent-dependent shifts that are dependent upon the H-bonding ability of the solvent. The implications of these results with respect to the active site of the iron-containing nitrile hydratases are also discussed.

Coercivity is an important history-and defect-dependent property of magnets [1] and needs to be large for many applications, e.g., magnetic storage, or small in value for other applications, e.g., AC inductors or magnetic shielding.[2]Some metallurgically prepared classical magnets, with spins residing in metal d-or f-orbitals, exhibit high coercivity, which enables their use in a myriad of applications, including the magnetic storage of information. [2] With the advent of magnets composed of spin-bearing organic components, [3] new classes of magnets with combinations of properties not previously observed are emerging.[4] Organicbased magnets were unfathomable a little more than decade ago, but magnetic ordering temperatures, T c , exceeding room temperature have been achieved. [5,6] [10a,11a,b,12,13] Magnetic ordering occurs below 10.0 K, as determined from the maximum in the 10 Hz w¢(T) data. The frequency dependence of the maxima in the in-phase component of the AC susceptibility, w¢(T), data suggests (cluster) spin glass behavior [11b±d] as noted for 3 and desolvated 3.[11a] The presence of an outof-phase component of the AC susceptibility, w²(T), with a maximum at 7.6 K (10 Hz) supports the assignment of a transition to a canted antiferromagnet below T c . [14] Independent of temperature, the applied magnetic field dependence on the magnetization, M(H), reaches a value of 16 500 emu Oe/mol at 50 kOe and is still increasing (Fig. 2). These values are in good agreement with the expectation of 16 755 emu Oe/mol for antiferromagnetic coupling of

To examine how small structural changes influence the reactivity and magnetic properties of biologically relevant metal complexes, the reactivity and magnetic properties of two structurally related five-coordinate Fe(III) thiolate compounds are compared. (Et,Pr)-ligated [Fe(III)(S(2)(Me2)N(3)(Et,Pr))]PF(6) (3) is synthesized via the abstraction of a sulfur from alkyl persulfide ligated [Fe(III)(S(2)(Me2)N(3)(Et,Pr))-S(pers)]PF(6) (2) using PEt(3). (Et,Pr)-3 is structurally related to (Pr,Pr)-ligated [Fe(III)(S(2)(Me2)N(3)(Pr,Pr))]PF(6) (1), a nitrile hydratase model compound previously reported by our group, except it contains one fewer methylene unit in its ligand backbone. Removal of this methylene distorts the geometry, opens a S-Fe-N angle by approximately 10 degrees, alters the magnetic properties by stabilizing the S = 1/2 state relative to the S = 3/2 state, and increases reactivity. Reactivity differences between 3 and 1 were assessed by comparing the thermodynamics and kinetics of azide binding. Azide binds reversibly to both (Et,Pr)-3 and (Pr,Pr)-1 in MeOH solutions. The ambient temperature K(eq) describing the equilibrium between five-coordinate 1 or 3 and azide-bound 1-N(3) or 3-N(3) in MeOH is approximately 10 times larger for the (Et,Pr) system. In CH(2)Cl(2), azide binds approximately 3 times faster to 3 relative to 1, and in MeOH, azide dissociates 1 order of magnitude slower from 3-N(3) relative to 1-N(3). The increased on rates are most likely a consequence of the decreased structural rearrangement required to convert 3 to an approximately octahedral structure, or they reflect differences in the LUMO (vs SOMO) orbital population (i.e., spin-state differences). Dissociation rates from both 3-N(3) and 1-N(3) are much faster than one would expect for low-spin Fe(III). Most likely this is due to the labilizing effect of the thiolate sulfur that is trans to azide in these structures.

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