CHM102ALecture6

Coordination Complexes in Biological Systems

Porphyrins are heterocyclic macrocycles composedof four modified interconnected pyrrole subunitsof four modified interconnected pyrrole subunits.They form coordination complexes with metal ionsand are found in biological systems.

Porphyrin

Chlorophyll c2 Heme B Cobalamin 2

Hemoglobin and MyoglobinH l bi (Hb)Hemoglobin (Hb)MW: 64.5 kDaPeptide chains: 22 Number of amino acids: 2x 141 : 2x146Active site: Fe(II) Function: cooperatively binds to and transports OFunction: cooperatively binds to and transports O2

Myoglobin (Mb)

MW: 17 8 kDaMW: 17.8 kDaPeptide chains: Number of amino acids: 161A ti it F (II)Active site: Fe(II) Function: binds and transports O2

Notice that the hemoglobin is essentially a tetramer of myoglobin. (There are fourmyoglobin like units in hemoglobin.)

3

Hemoglobin and Myoglobin

4

Oxygenated and Deoxygenated Forms

HNE helix

NDistal Histidine (E7)

E helix

Fe

OO

N

Fe

NH

N

NH

Proximal Histidine (F8)F helix

Proximal Histidine (F8)

5

Crystal field splitting for Square Planar complex

dx2-y2

dx2-y2dx2-y2, dz2

dxydz2

d d d

dorbitals

dz2

dxy

dxy, dyz, dxzdz2

dyz, dxz

dyz, dxz

E

n

e

r

g

y

Octahedralcomplex

Square Planarcomplex

Squarepyramidal

6

Coordination Environment of Fe

5 coordinate5 coordinateTotal unpaired electrons = 4, S = 2

Deoxygenated form is high-spin and paramagnetic

6 coordinate OO

dz2

dx2-y2

6 coordinateTotal unpaired electrons = 1, S = 1/2

Oxygenated form is low-spinThe magnetic moment of Fe3+ and the

id di l i l i iN

Fe3+

dxy

dz2

superoxide radical involves in anti-ferromagnetic coupling and the oxygenated

complex is not paramagneticNH

dxz dyz7

Hemoglobin and Myoglobin

Fe-O-O = 150Raman Spectra gives o-o at 1105 cm-1.indicating that the complex should be in a

superoxide state

O O O 2

o-o (cm-1) 1560 1100 850-740

O2 O2 O22

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The Role of the Protein

9

Hemoglobin and Myoglobin

Isolated protein free heme model system binds CO 25,000 times stronger than O2than O2 steric constraints imposed by the amino acid residue on the distal side of the porphyrin (distal histidine) and by selective hydrogen bonding favoring O2 over CO coordination (less for CO; KCO/KO2 = 200)

10

Organometallic Chemistry

Organometallic Compounds

In simpler terms these are compounds containing metal-carbon bonds

Examples: CH3-MgBr, Ph-Li, [Ni(CO)4], Ferrocene etc.

These compounds can be seen as having covalent bonds between the metal and thecarbon atom(s).

In general, compounds having a metal-ligand bond of considerable covalent characterIn general, compounds having a metal ligand bond of considerable covalent characterhave similar chemistry and follow the chemical behavior of organometallic compounds

Metal-cyano complexes are not considered as organometallic compounds, while metaly p g p ,carbonyl complexes are.

Compounds with filled octet electrons (such as CH4, NH3, etc.) are stable as they

Stability of Organometallic Compounds

p ( 4, 3, ) yattain inert gas configuration (Lewis octet rule, 1916)

Having 18 electrons in the outer shell consisting of s, p and d orbitals is consideredas an indication of stability as in inert gases (Kr (36), Xe (54) or Rn(86).

The rule suggests that compounds that can attain 18 electrons within the bondingThe rule suggests that compounds that can attain 18 electrons within the bondingorbitals of the metal show increased stability (Sidwick 1927)

[Co(NH3)6]3+[Co( 3)6]Co3+ 1 x 6 = 66 NH3 6 x 2 = 12

Total number of Valance electron = 18

M(CO)M(CO)6

M = Cr/Mo/W

Cr 1 x 6 = 6

6 CO 6 x 2 = 12

Total number of Valance electron = 18Total number of Valance electron 18

Scope of 18-electron Rule

Mo = 6 electronshexahapto 6Benzene (3x2 bond electron) = 6 electrons3 x CO =6 electronsTotal number of valance electron = 18

hexahapto

?

Hapticity: The number of atoms in the ligand which are directly coordinated to the metal.H ti it i d t d

Therefore, this compound is stable

Hapticity is denoted as

By oxidation state of metal and ligandW2+ = 4 electronsOne cyclopentadiene (Cp) = 6 electronsother cyclopentadiene (Cp) = 4 electrons

pentahapto 5Neutral atomW = 6 electronsCp = 5 electronsother Cp = 3 electrons other cyclopentadiene (Cp) = 4 electrons

2 x CO = 4 electronsTotal number of valance electron = 18 trihapto 3

Therefore this compound is stable

other Cp = 3 electrons2 x CO = 4 electronsTotal number of valance electron = 18

Therefore, this compound is stable

Polynuclear metal carbonyls :Scope of 18-electron Rule Possible to determine number of metal-metal bonds in the carbonyl complexes Formed to attain the stability through18-electron system.

Let us take example of Mn2(CO)10

2 Mn 2 x 7 = 14

CO 10 x 2 = 20

Total number of Valance electron = 34 / 2 = 17 e- per [Mn(CO)5]Total number of Valance electron 34 / 2 17 e per [Mn(CO)5]

M-M = 2

Therefore total number of Valance electron = 36 (2 x 18)

Experimentally found dimeric structure

Carbonyl groups can also bridge between two metals

Here CO can be seen as contributing one electron each to the two metals

Polynuclear Metal carbonyls: Bridging vs terminal

Here CO can be seen as contributing one electron each to the two metals

Try for Fe2(CO)9

Fe 2 x 8 = 16

Bridged carbonyls between two metals

Fe 2 x 8 = 169 CO 9 x 2 = 18

Total number of Valance electron = 34M-M = 2

Therefore total number of Valance electron = 36 TerminalTherefore total number of Valance electron = 36 Terminalcarbonyls

Experimentally found structure

How to find bridging vs terminal carbonyls in a given metal carbonyl complex?

Determined using Infrared spectroscopy (IR) (through one of the factor vibration of bonds) Bond strength determined using stretching frequency in IR

IR t t hi f

Bond strength determined using stretching frequency in IR Greater the bond strength more the stretching frequency Weaker bond strength in bridged carbonyls resulting less stretching frequency

IR stretching frequency:Free carbonyl: 2143 cm-1Terminal carbonyls: 2125-1850 cm-1Bridged carbonyls: 1850-1700 cm-1