Chemistry Reference
In-Depth Information
DFT.on.steps.
I
.and.
II
.of.the.Yandulov.and.Schrock.cycle.for.which.experimental.
results.are.known.(Figure.1.1).
From.the.comparison.of.calculated.structural.and.electronic.properties.with.experi-
mental.data.we.concluded,
126
.in.agreement.with.experimental.indings,.that.the.large.
hexaisopropylterphenyl.(HIPT).substituents.mainly.play.a.steric.role,
115
.preventing.the.
formation.of.catalyst.dimers..Indeed,.further.DFT.calculations.conirmed.that.the.cho-
sen.model.(Figure.1.1B).without.the.HIPT.substituents.is.the.best.trade-off.between.
accuracy.and.computational.costs.to.explore.the.energetic.proile.of.dinitrogen.reduc-
tion.at.a.single.Mo.center..In.order.to.study.the.interaction.between.solvent.molecules.
and. the. solutes,. many. DFT. functionals. were. used;. among. these. Becke's. “half-and-
half”. functional. (BH&H). was. extensively. employed. to. account. for. possible. disper-
sive.interactions.
77
.This.functional,.combined.with.medium-sized.basis.sets.(such.as.
6-31+G*,. for. instance),. can. reproduce. both. geometries. and. energies. of. correlated.
ab initio
.methods.for.several.archetypal.π-stacked.complexes.
100
.Many.recent.studies.
have.further.conirmed.that.the.BH&H.functional.can.provide.crucial.information.for.
molecular. systems. dominated. by. dispersive. forces.
101,106,109,127,128
. The. atoms. in. mol-
ecules.(AIM).theory
129,130
.were.then.employed.to.characterise.intermolecular.interac-
tions.between.solute.and.solvent.molecules.of.the.studied.systems..It.has.to.be.stressed.
that.electron.density,.at.the.so-called.bond.critical.points.(BCPs).of.the.AIM.topology.
is.a.valuable.tool.to.quantify.chemical.bonds.and.intermolecular.interactions.
100,129-132
In.agreement.with.previous.indings,.geometries.of.molybdenum.complexes.were.
well.reproduced.by.several.DFT.functionals,
126
.whereas.reaction.energies.(ΔE
R
).of.
steps.
I
.and.
II
.calculated.in.the.gas-phase.strongly.differed.from.experimental.values,.
with.an.overestimation.larger.than.∼15.kcal.mol
−1
.(data.not.shown).
122,125
.The.inclu-
sion. of. solvent. effects. with. an. implicit. solvent. model. (i.e.,. PCM)
123
. as. previously.
suggested
125
.hardly.improved.DFT.performance.(Table.1.1)..Since.reaction.energies.
have.been.demonstrated.to.be.rather.independent.of.the.exchange.correlation.func-
tional.used,
122,125
.and.PCM.was.unable.to.signiicantly.improve.the.
in vacuo
.energies.
(Table.1.1),
125
.the.lack.of.an.explicit.description.of.solvent.effects.seemed.to.be.one.
of.the.most.likely.reasons.for.the.poor.energetic.agreement..Both.molybdenum.com-
pounds.and.the.proton.source.have.aromatic.rings.which.may.interact.signiicantly.
with.apolar.solvent.molecules.
via
.C—H…π.and.π…π.interactions..While.individu-
ally. weak,. these. interactions. may. play. a. signiicant. role. in. the. energetics. of. reac-
tions.
I
.and.
II
..In.order.to.address.this.issue,.the.effect.of.an.increasing.number.of.
explicit.solvent.molecules.(from.one.up.to.four).on.the.energetics.of.steps.
I
.and.
II
.
was.explored..Both.benzene.and.heptane.molecules.were.chosen:.the.former.is.the.
solvent. in. which. available. experiments. were. carried. out. and. the. latter. is. the. most.
suitable.solvent.to.perform.the.entire.catalytic.cycle.
25-30
.For.this.preliminary.study,.
no.more.than.four.solvent.molecules.were.considered.as.the.size.of.the.system.rap-
idly.becomes. prohibitive.for.DFT.calculations.combined. with.medium/large.basis.
sets.such.as.6-31+G*..Nevertheless,.we.were.conident.that.four.solvent.molecules.
were.a.good.trade-off.between.computational.costs.and.a.reasonable.description.of.
solvation. effects.. Indeed,. four. solvent. molecules. occupied. the. entire. space. around.
the. solute,. i.e.,. further. solvent. molecules. would. probably. interact. with. one. of. the.
solvent.molecules.rather.than.with.the.solute.
126
.In.addition,.solvation.energies.esti-
mated.with.three.and.four.solvent.molecules.hardly.differed.
126
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