So far, no isolable mononuclear complex with a side-on bound N2 ligand has been reported.
η2coordination of N2to a single metal center has only been suggested based on electron para-magnetic resonance (EPR) spectroscopic results for the complex [Zr(η5-C5H4R’)2(N2)R] (R
= CH(SiMe3)2, R’ = H or Me),[40] based on infrared (IR) spectra of matrix isolated product of Co atoms with N2at 10 K,[41] based on the observation of intramolecular, non-dissociative end-to-end isomerization of [Ru(NH3)5(14N15N)]Br2[42] and [Re(Cp)(CO)2(14N 15N)],[43,44]
and was eventually structurally characterized by X-ray diffraction as meta-stable state after single-crystal to single-crystal irradiation of [Os(NH3)5(N2)](PF6)2.[45] And even on bimetal-lic systems, until 1988, only two clusters with a tetrahedral, non-planar Ni2N2core supported by Li ions were reported.[46–48]
The first structurally characterized side-onµ-η2:η2 N2bridged complex with the dinitrogen ligand being coordinated in plane with the two metal centers, i.e. [(µ-η2:η2-N2){Sm(Cp*)2}2] was published in 1988.[49] Only two years after, Fryzuk and coworkers were able to isolate and characterize the complex [(µ-η2:η2-N2){ZrCl(N(SiMe2CH2PiPr2))}2] (III).[50] Based on this result and the finding that substitution of the chloride ligand in this complex with a Cp ligand resulted in isomerization to the end-on bridged dimer IV, the authors developed
dxy
Fig. 2.6. Top left: Isomerization ofFryzuk’s side-on bridged N2-dimer by exchange of the Cl–with a Cp–ligand. Main: FMO scheme of a side-on bridged N2-dimer.
12 Chapter 2 Binding of dinitrogen to transition metal complexes
a bonding scheme for side-on bridged N2 dimers which allowed to rationalize the respective preferences of the N2 coordination mode.[25]
In general the more prevalent end-on bridging coordination mode is favorable, as here the metal centers and N2 bridge do form solely σ- and π-interactions (compare Scheme 2.1), whereas in the side-on coordination mode, oneπ-interaction is replaced with aδ MO combi-nation, which can be regarded as unfavorable simply due to poorer orbital overlap, analog to considerations in metal-metal multiple bonding.[51] However, if one of thedorbitals needed forπ bonding is not available (e.g. due to interaction with strong field ligands), the δ bond can act as a "fallback" option to stabilize the system (see Figure 2.6). Two bonding orbitial combinations can be constructed, one π symmetric combination of thedxz and theπ∗h MO of the ligand as well as aδ symmetric combination of thedxy orbitals and theπv∗ MO of the N2 moiety. In their specific case,Fryzuk and coworkers argued that the PNP amideπ-donor and chlorideσ-donor inIIIinteract too strongly with thedyzorbitals, raising them in energy and making them unavailable for π-bonding with the N2 bridge. When the chloride is ex-changed with a Cp– ligand, this interacts considerably with thedxy orbital required for theδ bond, and the end-on bridging mode becomes more favorable again. This is also expressed in an exceptionally long Zr-Namide bond length in IV(d(Zr-N) = 2.303(3) and 2.306(3) Å), indicating a reduced bond order and thus weaker interaction with thedyz orbital. This pic-ture was later qualitatively confirmed by density functional theory (DFT) calculations.[52]
Despite these electronic influence, sterics were also shown to be of considerable impor-tance in zirconocene complexes (see Scheme 2.2). A series of Zr(CpX)2-N2 complexes was made by Bercaw ([(µ-η1:η1-N2){Zr(C5Me5)2(N2)}2] (V))[53] and Chirik ([(µ-η1:η1 -N2){Zr(C5Me5)(C5Me4H)(N2)}2] (VI), [(µ-η2:η2-N2){Zr(C5Me4H)2}2] (VII))[54,55]of which only the latter, sterically least crowded complex forms a side-on bridged N2 dimer.
One of the main differences between end-on and side-on coordinated dinuclear dimers is their reactivity. While the only reported N2centered reactivity of the former is their cleavage into nitrides (see Section 3.2.1), side-on bridged N2 ligands often exhibit a much higher degree of activation and can be reactive towards N-X bond formation (X = H, B, C, Si; see Section 3.3).[56]
N
Zr Zr
N Zr N N Zr N
N
N N
N
Zr N N Zr
N
N N
V VI VII
Scheme 2.2. Different coordination modes of N2in zirconocene complexes controlled by the steric bulk of the Cp-ligands.
2.2 Side-on bound N2 13
3
Reactions of dinitrogen complexes
„
You’ve got to cut the cord Cut the cord and wake up!— The Agonist
"Disconnect Me" on "Eye of Providence"
3.1 Ammonia formation
In light of the unmatched importance of industrial ammonia production by theHaber-Bosch process, the discovery of molecular dinitrogen complexes naturally sparked hopes for the development of homogeneous catalysts capable of the same reaction. By now, this goal has been matched by a number of systems (even though with much less efficiency) and has been thoroughly reviewed in several recent publications.[56–59] However, for most of the time (1972 - 2003), only stochiometric conversion to ammonia was known, pioneered by the work of Chatt and coworkers, who reported protonation of [M(N2)2(dppe)2] (M = Mo, W;
dppe = Ph2PCH2CH2PPh2) with HCl to form an N2H2ligand,[60] followed by the report on ammonia formation after protonation of [M(N2)2(PR3)4] (M = Mo, W; PR3 = PMePh2 or PMe2Ph).[61] This discovery and intensive follow-up research led to the formulation of the Chatt cycle for ammonia formation from terminally bound N2 which is depicted in Scheme 3.1.[62,63] This cycle, which is also discussed as the most likely mechanism of ammonia formation by the nitrogenase enzymes, starts with stepwise protonation and reduction of theterminal nitrogen atom (for which this is also called the distal pathway) until reaching
M
Scheme 3.1. Discussed mechanisms for ammonia formation from terminal bound N2by transfer of H+/e–(marked as H ).
15
the [M N NH3] state, from which the first equivalent of ammonia is released. Only then, the remaining metal nitride is stepwise protonated and reduced until a second equivalent of ammonia is released. Such a cycle can only produce ammonia, while for formation of hydrazine, an alternating pathway is required. In addition to this achievement, the authors were furthermore able to perform the first N C bond formation from N2 by reaction with acetyl chlorides.[64,65]
The next major breakthrough after these finding was marked by the first report of a well-defined molecular system with a single metal center which is able to catalyze ammonia for-mation, i.e. Schrock’s [Mo(N2)(HIPTN3N)] (VIII, HIPTN3N3– = [{3,5-(2,4,6-iPr3C6H2)2
C6H3NCH2CH2}3N]3-), which does react with Co(Cp*)2 and LuH(BAr24F) and is capable of forming up to 8 equivalents of ammonia with respect to Mo.[66] The reaction most likely follows a reduction scheme which is comparable to the Chatt cycle, which is backed up by a number of independently synthesized potential intermediates which are also catalytically active as well as by DFT calculations.[63,67–70]This large availability of characterized interme-diates rendersSchrock’s catalyst the best understood systems and even though much better performing catalysts are known by now, there are still ongoing efforts to aquire data on even the most unstable intermediates.[71] Notably, this work includes one of only two literature known parent imido complexes derived from dinitrogen, i.e. [Mo(NH)(HIPTN3N)]0/+.[67]
A slightly different situation might be at hand in the iron catalyzed ammonia formation reported by Peters and coworkers, who prepared a series of trigonal pyramidal complexes [Fe(N2)(XP3iPr)]– (IX, XP3iPr = X(2-iPr2P-C6H4)3, X = B, C, Si), of which the first two are catalytically active.[72–74] In this system, a hybrid mechanism between the distal and the alternating pathway (i.e. alternating protonation of both N atoms) has been pro-posed based on the disproportionation of [Fe N NH2] and [Fe N NH2]+to [Fe N2] and
Fig. 3.1. Pioneering molecular catalysts mediating protonation/reduction cycles for ammonia pro-duction from N2.
16 Chapter 3 Reactions of dinitrogen complexes
trogen is protonated before the terminal one. However, later studies contradicted these re-sults, as the formation of a terminal nitrido complex [Fe(N)(BP3iPr)]+was observed to form from threefold protonation of [Fe(N2)(BP3iPr)]2–, accompanied by the release of equimolar amounts of NH3.[75] Furthermore, Peters could show that the commonly used reduction / protonation agent pair, Co(Cp*2) and lutidinium salts (LuH+), might actually act as a proton coupled electron transfer (PCET) reagent in the catalyses, where instead of stepwise proto-nation and reduction of the N2complexes one Cp*ring is first protonated, followed by an H atom transfer to the substrate.[76]
A very interesting observation regarding the pathway of ammonia formation was reported byNishibayashi and coworkers. They developed the second ever reported catalytic system for ammonia production, [(µ-N2){Mo(N2)2(PNP)}2] (X, PNP = 2,6-(CH2PtBu2)2NC5H5), which is obtained by reduction of the trichloro-complex [MoCl3(PNP)] in the presence of dinitrogen.[77] Treatment with Co(Cp)2 and LuH+ under N2 atmosphere resulted in forma-tion of 12 equivalents NH3 per Mo center. This reaction is supposed to undergo a distal pathway protonation and reduction scheme at one of the terminally bound N2 ligands. Vari-ation of thepara substituent of the pyridine ligand as well as computational investigation of the reaction led to the conclusion, that the initial protonation to M N NH is rate deter-mining and intermetallic charge transfer from the second Mo in the dimer was proposed to be crucial for stabilizing this intermediate.[58,78] When the pyridine moiety was exchanged with an N-heterocyclic carbene (NHC), i.e. in the complexes [(µ-N2){Mo(N2)2(PCP)}2] (PCP = 1,3-bis((di-tert-butylphosphino)methyl)benzimidazol-2-ylidene or 1,3-bis(2-(di-tert-butylphosphino)ethyl)imidazol-2-ylidene), especially the former, i.e. the benzimidazole based complex performed remarkably well in catalysis with 115 turnovers per Mo center.[79] Based on DFT calculations, this was attributed to the NHC acting not only as σ donor but also as π acceptor, which was considered beneficial for the reaction. However, when in the pyridine complex the chloride ligands of the precursor are exchanged with iodide, a sig-nificantly different mechanism comes into play. Upon reduction under N2, the complex undergoes initial N N bond scission to form the terminal nitrido complex [Mo(N)I(PNP)]
(XI), presumably via anµ-η1:η1 bridged dimer.[80] The system still is catalytic but stepwise protonation and reduction to ammonia takes place onlyafter the N2 splitting step, raising the question on whether such initial cleavage reactions might also play a role in the previously reported systems. In addition to these mechanistic considerations, in their latest contribu-tion,Nishibayashiand coworkers were also able to significantly tweak the turn-over-numbers of their catalysts by using SmI2/H2O as PCET reagents, obtaining up to 2175 equivalents NH3 per Mo center.[81]
3.1 Ammonia formation 17