Download Mn(acetylacetonate)3

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 (3d54s2) often adopts these oxidation states:
+2, +3, and +7,
MnCl24•H2O 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 D3 structure
but has a “local” coordination
environment (MnO6) that is
approximately octahedral
Splitting of Oh 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 t2g set and an eg 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 long‐
range 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: Bi=Hi+4πMi
• Rearrange to: (Bi/Hi)=1 + 4π Mi/Hi = 1 + 4πκi
• Susceptibility of a material toward induction in a field of strength is denoted by κ:
• κi=(Mi/Hi) 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. Χg = κ/density (units: 1/g)
– molar magnetic suscept. ΧM = Χg ∙ 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, μeff.
• 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:
XM’ = paramagnetic contribution of the unpaired e‐’s; this is the value used to determine µeff
XMD = diamagnetic contribution of the paired core e‐’s of the metal ion (minor), and the paired e‐’s in the ligands (significant)
XM’ = XM ‐ XMD
• XMD can be calculated. Values for common ligands, anions and solvents are available in tables. (See Bain and Berry paper on website)
Magnetic Moment and XM’
• Relationship between µeff and XM’ 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: µeff = 2.84 [(XM’)(T)]1/2
(This is a good assumption for Mn(acac)3)
• Compare µeff to µs, which is the magnetic moment coming from the electron spin, to get the # of unpaired spins:
µs = 2.00 [S(S+1)]1/2 (units are µB Bohr magnetons)
(Recall from Chem 461: S= ½(# of unpaired electrons))
• Example: two unpaired electrons, S=½+½=1
so µs = 2.00 [1(1+1)]1/2 = 2.83 µB
• Note: µtotal = µs + µorbital
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
µs
(calc’d)
µeff
(meas’d)
Cu2+
1
1/2
1.73 µB
1.7‐2.2 µB
V3+
2
1
2.83 µB
2.8 µB
Cr3+
3
3/2
3.87 µB
3.8 µB
Fe2+
4
2
4.90 µB
5.1‐5.5 µB
Mn2+
5
5/2
5.92 µB
5.9 µB
• Sometimes the µeff measured experimentally is a little larger than µs
calculated from theory.
In those cases, the orbital contribution is not completely quenched
• Good rule‐of thumb:
# of unpaired electrons ~ (µeff – 0.9) rounded to next integer