Mn(acetylacetonate) 3 Synthesis & Characterization
The acac Ligand Acetylacetonate (acac) is a bidentate anionic ligand ( 1 charge). We start with acetylacetone (or Hacac) which has the IUPAC name 2,4 pentanedione. Bonding consists of a resonance structure between: a covalent bond through one O and a dative bond through another O, and a delocalzied picture.
All about Mn Manganese (3d 5 4s 2 ) often adopts these oxidation states: +2, +3, and +7, MnCl 2 4 H 2 O Mn(II) pale pink Mn(acac) 3 Mn(III) dark brown KMnO4 Mn(VII) intense purple (these colors are imporant indicators in this, and many other, inorganic experiments). Mn(acac) 3 forms a D 3 structure but has a local coordination environment (MnO 6 ) that is approximately octahedral
Splitting of O h Environment The 3 acac ligands form an (approximately) octahedral environment around Mn. The degeneracy of the 5 d orbitals is removed, and the 3d orbitals separate into a t 2g set and an e g set. spherical field weak field strong field The purpose of this lab in part is to work out whether we have a weak field or strong field metal ligand complex.
Characterization UV Vis Absorption: Provides insight into complex s energy level splitting Guoy Balance and Evans Method NMR: Measures magnetic susceptibility to determine # of unpaired electrons (can distinguish between high spin and low spin species.
UV Vis Absorption We record UV Vis absorption in order to measure the energy of transitions between ground and excited electronic states. (And use Tanabe Sugano diagrams!) The strength of the transition is often reflected in the molar absorptivity: Weak spectra would be typical of d to d transitions Intense/strong spectra would be typical of metal to ligand, or ligand to metal charge transfer The best way to report such spectra is the frequency (ν=1/wavelength, λ) at the peak of the absorption, but its linewidth is also a useful measure. Fitting a Gaussian function to the spectrum is the most accurate way to get a measure of the peak and linewidth.
Guoy Balance Measures the response of materials to a magnetic field (an inhomogeneous field). Magnetic properties come from the electron spin and orbital motion of electrons. Diamagnetic substance: will move toward the weakest portion of the field usually has all electrons paired Paramagnetic substance: will move to the strongest portion of the field usually has one or more unpaired electrons (Interactions between unpaired spins can lead to longrange magnet order, resulting in ferromagnetism, antiferromagnetism, or ferrimagnetism.)
Basic Concepts When a material is put into a magnetic field, a new magnetic field is induced in it: B=H + ΔH = H + 4πM (B is induced flux density in Gauss, G; H is magnetic field intensity in Oersteds, Oe; M is the magnetization of the sample) along a specific spatial direction, i, is: B i =H i +4πM i Rearrange to: (B i /H i )=1 + 4π M i /H i = 1 + 4πκ i Susceptibility of a material toward induction in a field of strength is denoted by κ: κ i =(M i /H i ) cm 3 κ =(M/H) cm 3 (if anisotropic) (if isotropic, i.e. same in all directions) Magnetic susceptibilities per unit weight or moles are the most useful: gram magnetic suscept. Χ g = κ/density (units: 1/g) molar magnetic suscept. Χ M = Χ g molec. wt. (units: 1/mol)
Guoy Balance Method The experiment relates the force exerted on a sample in a magnetic field gradient to magnetic susceptibility: If the induced field attracts the sample into the magnetic field, this produces a positive magnetic susceptibility (material is paramagnetic) If the induced field causes the sample to be deflected (out) of the magnetic field, this produces a negative magnetic susceptibility (material is diamagnetic) Convert magnetic susceptibility to the effective magnetic moment, μ eff. Determine the # of unpaired electrons
More on X s X is the sum of all the paramagnetic and diamagnetic contributions in the molecule. The two main ones are: X M = paramagnetic contribution of the unpaired e s; this is the value used to determine µ eff X MD = diamagnetic contribution of the paired core e s of the metal ion (minor), and the paired e s in the ligands (significant) X M = X M X M D X MD can be calculated. Values for common ligands, anions and solvents are available in tables. (See Bain and Berry paper on website)
Magnetic Moment and X M Relationship between µ eff and X M depends on the long range magnetic ordering whether material exhibits paramagnetism, ferromagnetism, antiferromagnetism, or ferrimagnetism.) If material is a simple paramagnet, then assume the Curie law is obeyed: µ eff = 2.84 [(X M )(T)] 1/2 (This is a good assumption for Mn(acac) 3 ) Compare µ eff to µ s, which is the magnetic moment coming from the electron spin, to get the # of unpaired spins: µ s = 2.00 [S(S+1)] 1/2 (units are µ B Bohr magnetons) (Recall from Chem 461: S= ½(# of unpaired electrons)) Example: two unpaired electrons, S=½+½=1 so µ s = 2.00 [1(1+1)] 1/2 = 2.83 µ B Note: µ total = µ s + µ orbital Magnetic moment of unpaired electrons includes both spin and orbital contributions. However, in transition metals, the orbital contribution is usually very small (or quenched ).
Comparison of Magnetic Moments Ion # unpaired electrons S µ s (calc d) µ eff (meas d) Cu 2+ 1 1/2 1.73 µ B 1.7 2.2 µ B V 3+ 2 1 2.83 µ B 2.8 µ B Cr 3+ 3 3/2 3.87 µ B 3.8 µ B Fe 2+ 4 2 4.90 µ B 5.1 5.5 µ B Mn 2+ 5 5/2 5.92 µ B 5.9 µ B Sometimes the µ eff measured experimentally is a little larger than µ s calculated from theory. In those cases, the orbital contribution is not completely quenched Good rule of thumb: # of unpaired electrons ~ (µ eff 0.9) rounded to next integer