Dinitrogen derived terminal nitrides and functionalization beyond am-

All terminal nitrido complexes derived from full N₂ bond cleavage are nucleophilic, which is partly due to the overwhelming majority of them being based on early to mid transi-tion metals.^[59] Indeed, the only reports of N₂ derived terminal nitrides beyond group 7 are Kunkeley and Vogler’s [Os(N)(NH₃)₄]³⁺ (XXVIII, see Section 3.2.3) andPeters’ iron complex [Fe(N)(BP₃^iPr)]⁺ (see Section 3.1), of which only the former is proposed to be obtained after initial full bond cleavage.^[16] Consequently, so far only electrophiles have been employed in functionalization of these complexes, and in most cases, quite strong reactants were needed to obtain any nitride centered reactivity at all.

Interestingly, the most obvious nucleophilic reactivity, i.e. acting as a Brønsted base, is hardly observed, with the examples of Schrock and Nishibayashi being the only so far re-ported parent imido complexes obtained after N₂splitting (see Section 3.1 and 3.2.3).^[67,114]

In contrast to this, the reaction with other electrophiles is much more common and the ubiquitous example of Cummins’ Mo nitride XII is a good representative for this type of chemistry. Various adducts withLewisacids have been reported, especially with EX₃(X = F

; E = B / X = Cl ; E = B, Al, Ga, In / X = Br ; E = Al / X = I ; E = Al) and ECl₂(E = Ge, Sn).^[147] However, in these reactions hardly any influence on the Mo N bond distance was found and the resulting complexes often decomposed back to the starting materials within 3 h at room temperature (RT). In contrast to this, using MeI, Me₃SiOTf or Ph(CO)OTf re-sulted in formation of stable imido complexes [Mo(NR)(N(^tBu)Ar)₃]^X (XLVII) with a linear Mo N E angle (E = C, Si) and a significantly increase Mo N bond length (XII: d(Mo-N) = 1.651(4) Å,XLVII_R=SiMe3: d(Mo-N) = 1.715(6) Å.).^[28,147] The methylated complex could be further deprotonated to yield a linear ketimido complex [Mo(NCH₂)(N(^tBu)Ar)₃] (XLVIII), with a long Mo N bond (d(Mo-N) = 1.777(4)Å), ranging between a single and a double bond, and reaction with an additional equivalent of MeI yielded the ethylimdo complex.

In a follow up study, the authors further reported on full conversion of the nitride to a cyanide ligand.^[148]Initial reaction with MeOCH₂Cl andⁱPr₃SiOTf afforts [Mo( N CH₂OMe)(N(^tBu) Ar)₃]⁺(XLIXa) and deprotonation yielded the imido complex [Mo( N CHOMe)(N(^tBu)Ar)₃]

4.2 Reactivity of transition metal nitrides 33

Ar(^mBu)N kl N(^mBu)Ar

Scheme 4.2. Functionalization schemes of the N₂derived nitride complexXII.

(XLVIIIa). Subsequent reaction with SnCl₂ and Me₂NSiMe₃ results in formation of the cyanide complex [Mo(CN)(N(^tBu)Ar)₃] (L). Importantly, this process includes reduction of the Mo(vi) nitrido to an Mo(iv) cyanide complex with the reduction equivalents steming from the ligand C-H bonds rather than an external reduction source, an approach which was later also used by Schneider and coworkers (vide infra).

The first report of the incorporation of N₂ derived nitrogen into an organic molecule came from de Vries and coworkers, who reported the direct release of trifluoroacetamide from reaction of Cummins’ nitrido complexXII with trifluoroacetic acid, accompanied by partial degradation of the amide ligands to an imide, releasing isobutene and protons from one of thetert-butyl groups (see Scheme 4.3, top).^[149]

In 2016, Mézailles and coworkers published a paper on splitting of N₂ upon two-electron reduction of [MoCl₃(PPP)] (LI, PPP = PhP(CH₂CH₂PCy₂)₂) into the corresponding nitride complexes, which was proposed to proceed via an unobserved µ-η¹:η¹ N₂ bridged dimer LII (in fact, a related dimer, i.e. [(µ-N₂){Mo(N₂)₂(PPP)}] was previously observed and shown to be an active catalyst in the generation of silylamines from N₂) (see Scheme 4.3, bottom).^[150,151]After successful dinitrogen cleavage into [Mo(N)I(PPP)] (LIII) (in the pres-ence of NaI), the obtained nitride could be reacted with substituted silanes to form Mo-H and N-Si bonds. Importantly, the reaction with HSiMe₂(CH₂CH₂)Me₂SiH resulted in ini-tal 1,2-addition of one Si-H group to the Mo N moiety and heating to 80 °C released the bis(silyl)amine after intramolecular reaction with the second Si-H moiety. The suspected concomitant formation of [Mo(H)I(PPP)] (LIV) could not be directly proven, but a PMe₃ adduct was isolated from the reaction mixture after addition of the phosphine, proving rere-duction of the metal center to Mo(ii), with the rerere-duction equivalents originating from the reactant itself rather than from an external electron source. Rather recently, the authors reported a closely related reaction, in which the N₂ derived nitride was functionalized by HBPin to give triborylamine N(BPin)₃ in high yields, again accompanied by formation of Mo hydride complexes.^[152] Unfortunately, closing the cycle has not been achieved so far in neither of the two reactions.

34 Chapter 4 Reactivity and functionalization of transition metal nitrides

N Mo N

Scheme 4.3. Top: Release of an amide from Cummins’ nitrido complex, accompanied by ligand degradation. Bottom: Conversion of a Mo nitride to a bissilylamine with simultaneous rereduction of the metal center.

Cumminswent a step further as he not only achieved incorporation of the nitrido ligand into an organic molecule, but at the same time was able to regenerate the starting Mo complex, thus closing a synthetic cycle (see Scheme 4.4). After acylation of the nitride XII with in situ generated acyl triflates, the obtained imido complexes can be reduced using Mg in the presence of Me₃SiOTf to affort the ketimido complexes [Mo(N C(R)OSiMe₃)(N(^tBu)Ar)₃] (LV; R = Ph,^tBu, Me). Subsequent reaction with SnCl₂or ZnCl₂results in clean formation of the corresponding nitrile compounds R C N and [MoCl(N(^tBu)Ar)₃] (XV) of which the later could be cleanly rereduced to the Mo(iii) starting complexXV. This way, a synthetic cylce for the synthesis of nitrogen containing organic molecules directly from N₂was realized for the first time, even though it is already apparent that the required reactants (acyl triflates, Mg(0)) are not compatible with each other and thus prevent catalytic turnover.

A slightly different approach was used by the group ofSchneiderand coworkers, who reported the formation of acetonitrile from N₂ in the coordination sphere of a rhenium complex (see Scheme 4.5). The Re(v) nitrido complex [Re(N)Cl(PNP^tBu)] (XXI) obtained from splitting of N₂(see Section 3.2.1 and Scheme 3.3) was initially reported to undergo nucleophilic N-C bond formation with MeOTf.^[94]In a subsequent study, the authors published the ethylation with in situ prepared EtOTf yielding [Re(NEt)Cl(PNP^tBu]^OTf(LVI) followed by stepwise de-protonation ultimately leading to the release of acetonitrile.^[153] Initial deprotonation yields

4.2 Reactivity of transition metal nitrides 35

the ketimido complex [Re(N CHMe)Cl(PNP^tBu)] (LVII), in which bond distances and an-gle (d(Re-N) = 1.822(4) Å, d(N=C) = 1.273(7) Å, ∡(Re=N=C) = 174.3(5)°) suggest a pronounced heterocummulene character. A second deprotonation in the presence of 2 equiv-alents of CN^tBu resulted in almost quantitative release of acetonitrile accompanied by the formation of [Re(CN^tBu)₂(PNP^tBu)] (LVIII), a formal Re(i) complex. This is remarkable in that respect that the reduction equivalents necessary for this Re(v) → Re(i) conversion are obtained from the C-H bonds (2 e^– each) of the ethyl group rather than from an ex-ternal reductant. Alternatively, exex-ternal oxidants can be utilized to circumvent the need of π-accepting ligands to stabilize the Re(i) species. Using a combination of Ag⁺ and 2,4,6-tris-tert-butylphenoxy radical (TTBP) to formally remove 2 e^–/H⁺ fromLVIIIresults in the vinylimide complex [Re(NCHCH₂)Cl(PNP^tBu)]⁺ (LIX), which is a tautomer of the corresponding acetonitrile complex. Addition of a chloride source could trigger acetonitrile release and reformation of starting complex XIX. Alternatively, direct conversion of LVII could be achieved with N-chlorosuccinimide (NCS). To test, whether this latter, one-step conversion necessarily proceeds via the vinyl imido tautomer, the reaction was also performed after benzylation.^[154] The benzyl imido analog to LVII, which is inherently unable to un-dergo such a tautomerization gives similar yields of benzonitrile upon reaction with NCS, rendering this not a prerequisit for the observed reactivity.

Interestingly, in a very recent study, XXI was oxidized at the ligand backbone with 3-chloroperbenzoic acid and subsequently at the metal center to form the nitroxide Re(vi)

N Mo N

Scheme 4.4. Synthetic cylce for the conversion of dinitrogen into nitriles developed by Cummins and coworkers.

36 Chapter 4 Reactivity and functionalization of transition metal nitrides



Scheme 4.5. Synthetic cycle for acetonitrile formation from N2 as employed by Schneider and coworkers.

complex [ReNCl(P^ONP^tBu)]^PF6.^[155] These oxidations now lead to an umpolung of the ni-tride reactivity, which does form (unstable) phosphoraneiminato complexes with various phosphines, rendering it electrophilic.

The probably most efficient synthetic cycle for N₂functionalization to organic molecules was reported by Sita and coworkers (see Scheme 4.6). The Mo and W end-on bound dimeric complexes XXXIII_Mo^iPr (M = Mo, W) were already discussed for their ability to undergo photochemical N₂ cleavage (see Section 3.2.3). By reducing the steric strain of the ligand (replacing the iso-propyl groups with ethyl groups) the authors were able to switch from a photochemical to a thermal splitting pathway.^[156] Again, bis-µ-nitrido bridged complexes are obtained and the Mo congener is shown to react with Me₃SiCl to form the silylimido complexes [M( NSiMe₃)(Cp^*)(am)] (LX_Mo^Et) and the starting complexLXI_Mo^Et. In a later study, the imido complex was reported to undergo further reaction with alcohols and Me₃SiCl and formsLXI_Mo^Et, HN(SiMe₃)₂and ROSiMe₃, thus closing the synthetic cycle upon release of disilylamine.^[157] It is exciting to note that the used chemicals (i.e. Me₃SiCl and ROH) are chemically compatible with the complexesLXI_Mo^EtandXXXIII_Mo^Et. A system allowing for turnover therefore seems to depend only on finding a suitable reductant.

From the above discussed examples of synthetic cycles for N₂functionalization, some impor-tant aspects and strategies can be derived:

An inherent problem of transition metal complexes which are capable of splitting the N N triple bond is their tendency to form very strong M N triple bonds in the corresponding

4.2 Reactivity of transition metal nitrides 37

Mo N N Mo N

N Ph

N N

Ph

MoEt

Mo N N

N Ph

SiMe3

Mo N

N Ph

Cl Cl

MoEt MoEt

¦§ ¨3SiCl - N₂

© ª«e -+3 N₂

©¬§ ¨3SiCl

© ¦­ ®¯ ±

¦±N(SiMe₃)₂

¦­ ®¯ ²³§ ¨3

®«´

XXXIIIEt

XXXIII

LX LXI

Scheme 4.6. Synthetic cycle for bis-silylamine generation from a Mo(Cp^*)(am) complex developed bySita.

nitrides, which drives the reaction in the first place. This problem was accurately expressed by Fryzuk and Shaver: ’The problem with developing a viable process coupling nitride for-mation with the production of useful nitrogen-containing materials is the inherent stability of the metal nitrides generated. Reactions of nitrides employ harsh reagents to effect moderate changes.’^[158]

This stability can in principle be met by two strategies. One the one hand, forming products with strong N-X bonds, like nitriles containing a N C triple bond can account at least par-tially for the energy-demand of cleaving the strong M N triple bond and can thus facilitate the incorporation of dinitrogen into organic products. On the other hand, it seems desirable to find transition metal complexes which do undergo N N bond cleavage but result in less stabilized compounds which are therefore more easily functionalized. As will be shown in the next section, especially nitrido complexes of late transition metals are often very reactive and do undergo quite rich transformations. Developing compounds based on these metals for N₂fixation therefore seems a promising approach.