Mn(acac)3 Synthesis

Mn(acac)3 Synthesis ! Manganese (Z = 25) has a valence configuration 3d 54s2, and typically shows positive oxidations states of +2, +3, and +7, all of which are seen in this experiment. MnCl2@4H2O Mn(II) pale pink Mn(acac)3 Mn(III) lustrous dark brown KMnO4 Mn(VII) deep purple • Pale color of MnCl2@4H2O is due to the absence of any same-spin (spin-allowed) d-d transitions for a d 5 high-spin complex. • Mn(acac)3 is green by transmitted light, owing to a single broad d-d transition at about 500 nm. • KMnO4 is deep purple, owing to a charge transfer transition with high molar absorptivity. ! The ligand (acac) in the product is the anion of acetylacetone, 2,4-pentanedione, CH3COCH2COCH3, Hacac. ! Mn(acac)3 has octahedrally coordinated Mn3+ (D3 . Oh). H H3C H3C C O H C C CH3 O Mn C H3C C C O O O O H3C C C C H CH3 d-Subshell Splitting in an Octahedral Field ! In the octahedral environment of three acac ligands, the fivefold degeneracy among d orbitals in Mn3+ is lifted. • To a first approximation, the ligand field is octahedral (Oh), and the 3d orbitals will separate into a set of three degenerate orbitals (t2g. = dxy, dyz, dxz) and a set of two degenerate orbitals (eg = dx2–y2, dz2). • The t2g orbitals point between ligands, but the eg orbitals point at ligands, so the t2g set is stabilized and the eg is destabilized relative to the energy of a hypothetical spherical octahedral field. x z z y eg y x dx2-y2 d2z2-x2-y2 = dz2 x z z z y y y x x dxy dyz dzx t2g d-Subshell Splitting in an Octahedral Field - cont. ! Relative to the energy of the hypothetical spherical field, the eg set will rise in energy and the t2g set will fall in energy, creating an energy separation of )o or 10Dq between the two sets of d orbitals. eg +3/5)o = +6Dq )o = 10Dq -2/5)o = -4Dq t2g Energy level of hypothetical spherical field energy R3 Oh ! The energy increase of the eg orbitals and the energy decrease of the t2g orbitals must be balanced relative to the energy of the hypothetical spherical field (sometimes called the barycenter). " The energy of each of the two orbitals of the eg set rises by +3/5)o = +6Dq while the energy of each of the three t2g orbitals falls by -2/5)o = -4Dq. This results in no net energy change for the system; i.e., )E = E(eg) + E(t2g) = (2)(+3/5)o) + (3)(-2/5)o) = (2)(+6Dq) + (3)(-4Dq) =0 ! The magnitude of )o depends upon both the metal ion and the attaching ligands. High-Spin and Low-Spin Configurations ! In an octahedral complex, electrons fill the t2g and eg orbitals in an aufbau manner, but for configurations d 4 – d 7 there are two possible filling schemes depending on the magnitude of )o relative to the mean pairing energy, P. • A high spin configuration avoids pairing by spreading the electrons across both the t2g and eg levels. • A low spin configuration avoids occupying the higher energy eg level by pairing electrons in the t2g level. ! For a given metal ion, the pairing energy is relatively constant, so the spin state depends upon the magnitude of the field strength, )o. • Low field strength results in a high-spin state. • High field strength results in a low-spin state. ! For a d 4 configuration, the high-spin state is t2g3eg1, and the low-spin state is t2g4eg0. eg t2 g spherical field weak field = high spin strong field = low spin ! Mn(acac)3 is a weak-field/high-spin case (t2g3eg1). State Splitting in an Octahedral Field ! In the absence of the octahedral field (point group R3), the ground state of a 3d 4 configuration is 5D. • A weak Oh field causes this to split into two states: K Ground state 5Eg = t2g3eg1 K Excited state 5T2g = t2g2eg2 5T2g state-to-state transition = ∆ο 5D 5Eg Energy Field Strength ! The 5Eg ground state is doubly-degenerate, because there are two ways of placing the eg1 electron: (t2g)3 (dx2–y2)1(dz2)0 (t2g)3 (dx2–y2)0(dz2)1 ! The 5T2g excited state is triply degenerate, because there are three ways of placing the vacant orbital: (dxy)1(dyz)1(dxz)0 (eg)2 (dxy)1(dyz)0(dxz)1 (eg)2 (dxy)0(dyz)1(dxz)1 (eg)2 ! 5Eg state is paramagnet from four unpaired electrons. Absorption Spectra and State-to-State Transitions ! When a transition metal complex absorbs visible light, the energy absorbed (h<) causes a transition from the ground state to an excited state, corresponding to a change in electronic configuration. ! For a high-spin d 4 complex like Mn(acac)3, only one samespin state-to-state transition is possible: 5Eg ÷ 5T2g. 5T2g state-to-state transition = ∆ο 5D 5Eg Energy Field Strength ! The energy of this transition occurs at ~500 nm, removing red-orange light and transmitting green light. ! The 5Eg ÷ 5T2g gives rise to a single absorption band in the visible spectrum at ~500 nm. Jahn-Teller Distortion K Jahn-Teller Theorem: For any nonlinear system in a degenerate state, a distortion will occur that will lift the degeneracy. ! Mn(acac)3 has a 5Eg ground state, which has an imbalance in the filling of electrons in orbitals that point directly at ligands. (t2g)3 (dx2–y2)1(dz2)0 or (t2g)3 (dx2–y2)0(dz2)1 • This results in significant distortion from ideal octahedral symmetry (Oh). K Jahn-Teller Corollary: If the system is centrosymmetric, the distorted configuration will also be centrosymmetric. ! For Mn(acac)3 this suggests tetragonal distortion, Oh ÷ D4h. • Most probable distortions are equally elongating or shortening two trans-related positions, relative to the four remaining equal positions in a plane. ! Mn(acac)3 exists in two tetragonally distorted forms. 212 pm 193 pm 200 pm 195 pm Synthetic Procedure ! Two-step preparation: MnCl2(aq) + 2Hacac(aq) + 2NaC2H3O2(aq) ÷ Mn(acac)2(aq) + 2NaCl(aq) + 2HC2H3O2(aq) 4Mn(acac)2 + KMnO4(aq) + 7Hacac(aq) + HC2H3O2(aq) ÷ 5Mn(acac)3(s) + KC2H3O2(aq) + 4H2O(l) ! Crude product is obtained by chilling the final mixture. ! Pure product is obtained by recrystallization from benzene solution on adding petroleum ether. ! Procedure notes: (1) Make certain that all KMnO4 is dissolved before adding to the solution in the second step. (2) Use filter paper /Büchner funnel or Hirsh funnel for first crude product collection (not sintered glass). (3) Filter benzene solution through a sintered glass filter that is already discolored brown. (4) Discard all solutions (esp., benzene/petroleum ether mixture) in waste containers (nothing down the drain).