First Principles DFT Studies of Metal-Based Biological and Biomimetic Systems - Metallic Systems: A Quantum Chemist's Perspective - page 6

becomes.a.powerful.predictive.tool,.ideal.for.the.study.of.large.biological.systems.
containing.metal.elements.

1.3 DISCUSSION

1.3.1 S olvent e ffectS on n itrogen f ixation
B iomimetic m olyBdenum c atalySt
Nitrogen. ixation,. i.e.,. the. conversion. of. molecular. nitrogen. to. ammonia,. is. a. key.
reaction. in. nature,. performed. by. a. number. of. different. prokaryotes. including. bac-
teria,.actinobacteria.and.certain.types.of.anaerobic.bacteria. 24,114,115 .The.nitrogenase.
enzyme.catalyzes.nitrogen.ixation.at.room.temperature.through.complex.reactions.
that. are. not. fully. understood. 116 . Artiicial. nitrogen. ixation,. instead,. requires. high.
temperature. and. strong. pressure. and. it. is. industrially. achieved. by. the. well-known.
Haber-Bosch.reaction. 117 .For.years,.coordination.chemists.have.been.searching.for.a.
nonbiological .catalyst.that.will.render.the.nitrogen.ixation.under.mild.ambient.con-
ditions. feasible. 118,119 . Among. many. adopted. strategies,. those. based. on. biomimetics.
(i.e.,.compounds.that.perform.a.similar.chemical.reaction.to.that.of.natural.enzymes).
of.Fe-.and.Mo-containing.nitrogenase.enzyme.seemed.to.be.the.most.promising. 120,121
The.search.for.the.“ Holy Grail .of.coordination.chemistry” 122 .was.over.in.2002, 30 .
when. a. molybdenum. triamidoamine. chelate. complex. ([Mo(hiptN3N)],. where.
hiptN3N = tris(hexaisopropylterphenyl)triamidoamine,.Figure.1.1A.and.B) 25-30 .was.
shown.to.catalyze.the.reduction.of.molecular.nitrogen.to.ammonia.with.high.efi-
ciency. at. room. temperature.. The. catalytic. process. proceeds. with. the. slow. addi-
tion.of.a.proton.source.(2,6-lutidiniumBAr 4 .where.Ar.is.3,5-(CF 3 ) 2 C 6 H 3 ,.hereafter.
referred. to. as. LutH + . and. Lut. in. the. protonated. and. deprotonated. forms. respec-
tively,. Figure. 1.1C). and. a. reducing. agent. (decamethylcromocene). to. a. solution. of.
the. catalyst. in. liquid. heptane. at. ambient. conditions. (Figure. 1.1D).. Clearly,. a. full.
description. of. the. single. steps. of. the. whole. process. is. extremely. important. not.
only.to.comprehend.the.Yandulov.and.Schrock.cycle.in.detail,.but.also.to.obtain.
hints.about.the.intricate.functioning.of.the.enzymes.that.naturally.perform.nitro-
gen. ixation.. While. many. intermediates. of. the. cycle. were. experimentally. char-
acterized, 26,27,29 . an. exhaustive. understanding. of. the. molecular. processes. is. still.
lacking. 24 . To. support. the. experimental. research,. DFT. calculations,. in vacuo . and.
corrected.with.implicit.solvation.models, 123 .were.performed.and.provided.prelimi-
nary.insights.into.these.reactions. 122,124,125 .While.structural.characterization.of.the.
compounds.was.satisfactory,.energetics.of.the.catalytic.cycle.turned.out.to.differ.
remarkably.from.the.available.experimental.data. 26,27,125 .In.particular.the.exother-
mic.character.of.the.two.protonation.steps.for.which.experimental.data.are.avail-
able. (step. I = MoNNH + H + .→. MoNNH 2 + . and. step. II = MoN + H + .→.MoNH + ). was.
overestimated. (Figure. 1.1D. and. Table. 1.1),. e.g.,. the. free. energy. was. predicted. to.
be.too.negative.by.these.calculations. 27,125 .Such.a.discrepancy.(that.is.well.beyond.
the.standard.DFT.error,.3-5.kcal.mol −1 ) 94 .may.be.caused.by.many.factors.including.
(i).the.choice.of.the.exchange.and.correlation.functionals,.(ii).the.solvation.model,.
and.(iii).the.lack.of.a.rigorous.treatment.of.entropic.effects..In.this.section.we.shall.
describe.an.extensive.study.aimed.at.inding.the.reasons.of.the.apparent.failure.of.