Observed enhancement of the reactivity of a biomimetic diiron complex by the addition of water - mechanistic insights from theoretical modeling.
The biomimetic diiron complex [Fe(III)Fe(IV)(mu-O)(2)(5-Me(3)-TPA)(2)](ClO(4))(3) (TPA = tris(2-pyridylmethyl)amine) has been found to be capable of oxidizing 9,10-dihydroanthracene in a solution of acetonitrile. Addition of water up to 1 M makes the reaction 200 times faster, suggesting that the water molecule in some way activates the catalyst for more efficient substrate oxidation. It is proposed that the enhanced reactivity results from the coordination of a water molecule to the iron(III) half of the complex, converting the bis-mu-oxo structure of the diiron complex to a ring-opened form where one of the bridging oxo groups is transformed into a terminal oxo group on iron(IV). The suggested mechanism is supported by DFT (B3LYP) calculations and transition state theory. Two different computational models of the diiron complex are used to model the hydroxylation of cyclohexane to cyclohexanol. Model has a bis-mu-oxo diiron core (diamond core) while model represents the "open core" analogue with one bridging mu-oxo group, a terminal oxo ligand on iron(IV), and a water molecule coordinated to iron(III). The computational results clearly suggest that the terminal oxo group is more reactive than the bridging oxo group. The free energy of activation is 7.0 kcal mol(-1) lower for the rate limiting step when the oxidant has a terminal oxo group than when both oxo groups are bridging the irons.
Johansson
Adam Johannes
AJ
Department of Physics, Stockholm University, SE-106 91, Stockholm, Sweden.
Noack
Holger
H
Siegbahn
Per E M
PE
Xue
Genqiang
G
Que
Lawrence
L
D040761:Biomimetic Materials / Q000737:chemistry*; D005290:Ferric Compounds / Q000737:chemistry*; D008956:Models, Chemical; D008968:Molecular Conformation; D011725:Pyridines / Q000737:chemistry; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
D005290:Ferric Compounds; D011725:Pyridines; C431843:tris(2-pyridylmethyl)amine; D014867:Water
Chem Commun (Camb). 2009 May 21;(19):2637-9
19532905
Angew Chem Int Ed Engl. 2002 Apr 2;41(7):1114-37
12491240
Chem Rev. 1996 Nov 7;96(7):2625-2658
11848839
Chem Rev. 2000 Jan 12;100(1):235-350
11749238
Phys Rev B Condens Matter. 1988 Jan 15;37(2):785-789
9944570
Science. 1997 Jan 24;275(5299):515-8
8999792
Acc Chem Res. 2007 Jul;40(7):532-42
17488054
J Am Chem Soc. 2003 Jun 18;125(24):7344-56
12797809
Biochemistry. 1998 Oct 20;37(42):14664-71
9778341
Chem Rev. 2000 Feb 9;100(2):421-38
11749242
Science. 1991 Jul 19;253(5017):292-8
1650033
Proc Natl Acad Sci U S A. 2007 Dec 26;104(52):20713-8
18093922
Chem Rev. 2004 Feb;104(2):987-1012
14871147
J Am Chem Soc. 2003 Dec 17;125(50):15554-70
14664603
J Biol Chem. 1993 Oct 15;268(29):21569-77
8408008
J Am Chem Soc. 2003 Aug 27;125(34):10351-61
12926960
J Chem Phys. 2008 Oct 21;129(15):154301
19045187