Reactivity of N 2 -derived nitrides

Applying these bonding considerations to the N₂-generated terminal nitrides pre-sented in the previous chapters it can be seen, that most of the pincer-supported nitrides as well as Masuda’s [Mo(N)(depe)₂]⁺ (XLIb) exhibit d²-configurations. Ac-cordingly, the MN-non-bonding HOMO of these nitrides is fully occupied and further reduction should lead to weakening of the M−−−N-bond. One exception is Schneider’s [Mo(N)Cl(^HPNP)]⁺ (XXXVIII), which features a d¹-configuration. Nevertheless, all of these compounds contain early transition metals, which gives the respective ni-tride a nucleophilic character. In fact, functionalization of most of the mentioned nitrides has been achieved with electrophiles such as alkyltriflates,^15,146boranes,¹⁷⁰ silanes^17,171,172 or benzoylchlorides.⁷¹

The group ofHollandattempted the oxidation of N₂-derived [Re(N)Cl(PNP)] (XXXVII, PNP = [N(CH₂CH₂P^tBu₂)₂]^–).¹⁷³ While outer-sphere oxidents, like ferrocenium, lead to metal centered oxidation to give [Re(N)Cl(PNP)]⁺ (LXIV), electrophilic O-atom-transfer reagents, like mCBPA (mCBPA = 3- chloroperbenzoic acid) lead to oxidation of the pincer-amide and formation of [Re(N)Cl(P^ONP)] (LXV, P^ONP = [ON(CH₂CH₂P^tBu₂)₂]^–), which could not be synthesized upon usage of nucle-ophilic O-atom-transfer agents. LXVwas successfully oxidized to give [Re(N)Cl(P^ONP)]⁺ (LXVI), which could also be obtained upon reaction of LXIV with mCPBA. Although, the structural parameters of LXVI do not differ significantly from XXXVII, the M−−− N-moiety reacts with phosphines to give the phosphinimide LXVII, which decomposes to LXIV, OPR₃ and other products. Hammett-analysisviavariation of the phosphine substituents imply an electrophilic character of the nitride-ligand, showcasing how the ligand and oxidation-state of the metal influence the reactivity of the nitride-moiety.¹⁷³

The described bonding considerations can also be used to rationalize the instability of the Os(V)-nitride [Os(N)(NH₃)₅]²⁺ (La), which was proposed as one of the two ini-tial N₂-splitting products upon photolysis of [((NH₃)₅Os^II)(N₂)(Os^III(NH₃)₅)]⁵⁺ (XLIXa) (Scheme 40, a). The strongtrans-influence of the nitride ligand leads to liberation of one NH₃-ligand (Scheme 40, b). The d³-configuration of the so formed [Os(N)(NH₃)₄]²⁺

(Ld) destabilizes the Os−−−N-bond due to occupation of an anti-bonding orbital. Dispro-portionation to [Os(N)(NH₃)₄]⁺ (Le) and [Os(N)(NH₃)₄]³⁺ (Lc) (Scheme 40, c), starts a cascade reaction, using [Os(N)(NH₃)₄]²⁺ (Ld) as reductant, in which one nitride ligand is subsequently reduced and protonated to give [(NH₃)₅Os(OH₂)]³⁺ (LXVIII) (Scheme 40, d).¹⁴¹

[(H₃N)₅Os-N₂-Os(NH₃)₅]⁵⁺ [(H₃N)₅Os≡N]³⁺ +[(H₃N)₅Os≡N]²⁺

[(H₃N)₅Os≡N]³⁺

[(H₃N)₅Os≡N]²⁺+ [(H₃N)₄Os≡N]³⁺+[(H₃N)₄Os≡N]²⁺

- 2 NH₃

2 [(H₃N)₄Os≡N]²⁺ [(H₃N)₄Os≡N]³⁺+[(H₃N)₄Os≡N]⁺

3 H⁺

2 [(H₃N)₅Os≡N]⁺+ + H₂O [(H₃N)₄Os≡N]²⁺+[(H₃N)₄(H₂O)Os(NH₃)]³⁺

+III +II +VI +V

+VI +V +VI +V

+V +VI +IV

+IV +V +III

a.

b.

c.

d.

hν

XLIXa

Lb

Lb

La

La

Lc

Lc

Ld Ld

Ld

Le

Le

LXVIII

Scheme 40: Photolytic cleavage of XLIXa generates a unstable Os(V)-nitride Ld, which subsequently disproportionates and starts a cascade reaction, which finally givesLXVIII.¹⁴¹

Cummins’N₂-derived nitride [Mo(N)(N(R)Ar)₃] (XXXI) reacts as a nucleophile, as pre-dicted for a d⁰-configurated early transition metal nitride. Besides aduct formation with Lewis-acids, such as BX₃ (X = F, Cl), AlX₃ (X = F, Cl, Br, I), GaCl₃, InCl₃, GeCl₂ or SnCl₂, the formation of imido-complexes ([Mo−−N−R]⁺) with MeI, TMS-OTf orin situ prepared PhC(O)-OTf has also been observed.¹⁷⁴

The methyl-imido complexLXIX, obtained upon reaction ofXXXIwith MeI, can be de-protonated to yield in the formation of the respective ketimido complexLXX, which can beC-methylated to give the ethylimido-complexLXXI(Scheme 41,top). Remark-ably, direct synthesis of the ethylimido-complex upon reaction ofXXXIwith EtOTf, EtI or [Et₃O]BF₄, was not successful.¹⁷⁴ In a related study, the group of Cumminswas able to transform the N₂-derived nitride-ligand into cyanide. Reacting a mixture of MeOCH₂Cl and ⁱPr₃SiOTf withXXXIleads to formation of methoxymethylimideLXXII, whose deprotonation gave the respective alkoxyketimide LXXIII. Addition of SnCl₂ as Lewis-acid and Me₂NSiMe₃ as Brønsted-base yields in the formation of C-bound [Mo(CN)(N(R)Ar)₃] (LXXIV, Scheme 41,bottom) .¹⁷⁵

N

Scheme 41: Top: Sequential methylation of N₂-derived XXXI forms the ethylimido LXXI.Bottom: Generation of CN^– fromXXXI.^174,175

While reformation ofXXXIfromLXXIVwas unsuccessful, the group ofCumminswas able to generate organic nitriles R−C−−−N from N₂ in a synthetic cycle (Scheme 42).¹⁷⁶ Acylation of N₂-derived XXXI using a RC(O)Cl/Me₃SiOTf-mixture (R = Me. Ph, ^tBu) gives the respective acylimide complexe LXXV, which can be reduced with Mg/anthracene in the presence of Me₃SiOTf to give the respective trimethylsiloxyke-timides LXXVI. Liberation of the respective nitrile was achieved via addition of a Lewis-acid, such as ZnCl₂ or SnCl₂, to give Mo(IV)-chloride LXXVII. Reduction of LXXVIIgives [Mo(N(R)Ar)₃] (XXX), which binds and splitts N₂to reformXXXI.¹⁷⁶

N

Scheme 42: Synthetic cycle to generate organic nitriles from N₂ (R = Me, Ph,^tBu).¹⁷⁶

A similar synthetic cycle was achieved with already mentioned heterobimetallic IV (Figure 3), which cleaves the N₂-bond upon reduction to give Mo(VI)-nitrideXXXIand anionic Nb(V) [Nb(N)(N(R)Ar)₃]^– (LXXVIII, R = ^Pr, Np, Scheme 43). The Nb-nitride LXXVIIIis more reactive and releases the respective organic nitrile directly upon re-action with acyl chlorides under formation of a Nb(V) oxo-complex, [Nb(O)(N(R)Ar)₃] (LXXIX). After reaction of LXXIX with trifluoromethanesulfonic anhydride giving [Nb(OTf)₂(N(R)Ar)₃] (LXXX) and reduction with CoCp₂, the so obtained Nb(IV)-triflate [Nb(OTf)(N(R)Ar)₃] (LXXXI) undergoes salt metathesis with Na[Mo(N₂)(N(R)Ar)₃] (LXXXII) and reforms dinuclearIV.^62,177

Mo N

Scheme 43: Synthetic cycle for the formation of nitriles from N₂ and acyl chlorides using N₂-bridged heterobimetallic IV (R = Ad, Ph, ^tBu; R’ = Np, ^tBu; Ar = 3,5-Me₂C₆H₃).¹⁷⁷

The group of Schneider followed a different approach in which re-reduction of the metal-center does not rely on external reductants like inCummins’ case. In the first example, N₂-derived Re(V)-nitride XXXVIIwas used to produce acetonitrile in a syn-thetic cycle (Scheme 44).¹⁴⁶ In the first step, XXXVII is reacted with EtOTf to form the respective Re(V)-ethylimidoLXXXIII, which can be deprotonated to give the re-spective ketimideLXXXIV. This deprotonation is accompanied by formal two electron reduction from Re(V) to Re(III) in which both electrons formally stem from the CH-bond of theα-carbon. Further deprotonation of this carbon-atom was reported to result also in two electron reduction of the Re-center and release of acetonitrile. However, the so formed Re(I)-species,LXXXV, requires strong acceptor-ligands, like iso-nitriles, for stabilization and does not activate N₂. Nevertheless, reaction of LXXXIV with two equivalents ofN-chlorosuccinimide (NCS) liberates acetonitrile under formation of the Re(IV)-trichloro-complexLXXXVI. Closure of the synthetic cycle was achieved upon reduction of LXXXVI with NaHg in the presence of N₂ to reform nitride XXXVII.¹⁴⁶ Usage ofin situ prepared PhCH₂OTf instead of EtOTf yields in the formation of ben-zonitrile following a analogue mechanism.¹⁷⁸

Re Cl

Scheme 44: Synthetic cycle for the generation of acetonitrile from N₂using a Re-PNP-pincer-platform.¹⁴⁶

Later the group ofSchneiderutilized [Re(N)Cl₂(^HPNP^iPr)] (XLIIb) for C-N-coupling in a synthetic cycle (Scheme 45).⁷¹ In contrast toXXXVII, functionalization ofXLIIbdoes not rely on very strong electrophiles, like alkyltriflates. Therefore C-N-coupling could be achieved using benzoylchloride (PhC(O)Cl), which releases a mixture of benza-mide (PhC(O)NH₂), benzoic acid (PhCO₂H) and benzonitrile (PhCN). The appearance of benzoic acid and benzonitrile is a result of a reaction of benzoylchloride with im-mediate formed benzamide. Interestingly, both protons, required for the formation of PhC(O)NH₂, stem from the ligand backbone, which serves as a 2H⁺/e^–-donor and is oxidized to give Re(III) imine-complex LXXXVII. Re-reduction of the ligand back-bone was accomplished via stepwise addition of Li[HBEt₃] and [Ph₂NH₂]Cl. The so-formed Re(III)-trichloro-complex LXXXVIII can be reduced under N₂-atmosphere to yield the N₂-bridged dinuclearXIII. However, the yields following this route are rela-tively low most likely due to the formation of hydride-complexes. Therefore, the group ofSchneiderinvestigated the electrochemical reduction of the ligand-backbone using controlled potential electrolysis (CPE). CPE (E^appl = -1.65 V) of LXXXVIIin the pres-ence of a suitable acid (2,6-dichlorophenol) yielded LXXXVIII almost quantitatively, which allowed its subsequent electrochemical reduction toXIIIviaCPE using a lower potential (E^appl= -1.85 V). Irradiation ofXIIIcleaves to N₂-bond and reformsXLIIbto close the synthetic cycle.⁷¹

N Re

Scheme 45: Three step synthetic cycle for the formation of benzamide, benzonitrile and benzoic acid from benzoylchloride and N₂, including photolytic N₂-cleavage and electrochemical re-reduction of the ligand-backbone.⁷¹

All above discussed examples illustrate some general aspects and strategies for the incooperation of N₂ into organic molecules. Cleavage of the strong N−−−N bond is often accompanied by the formation of strong M−−−N-bonds, which require strong elec-trophiles to be functionalized. This drawback can be overcome with different strate-gies, like for example the formation of strong N−−−C-bonds, like in nitriles, which coun-terbalance the high energy demand for N₂-cleavage.146,175–177 Alternatively, N₂ -splitting into less stabilized nitrides, like in Scheme 45, allows the usage of weaker electrophiles. The reduced thermodynamic driving force typically leads to higher kinetik barriers for N₂-cleavage, which may be overcome by photolysis.⁷¹Re-reduction the metal-center should ideally proceed without an additional reagent or electrochem-ically.¹⁷⁹