Dinitrogen and Related Chemistry of the Lanthanides: A Review of the Reductive Capture of Dinitrogen, As Well As Mono- and Di-aza Containing Ligand Chemistry of Relevance to Known and Postulated Metal Mediated Dinitrogen Derivatives

This paper reviews the current array of complexes of relevance to achieving lanthanide mediated nitrogen fixation. A brief history of nitrogen fixation is described, including a limited discussion of successful transition metal facilitated nitrogen fixation systems. A detailed discussion of the numerous lanthanide-nitrogen species relevant to nitrogen fixation are discussed and are related to the Chatt cycle for nitrogen fixation.


Introduction to Nitrogen Fixation and Scope of Review
Man's demand for chemically accessible nitrogen since the early 20 th century has far outstripped biology's capacity to fix atmospheric nitrogen to ammonia. Necessarily, studies of synthetic chemical fixation were undertaken and resulted in the discovery by Fritz Haber of the first patented catalytic method to chemically fix nitrogen as ammonia, which was commercialised in 1910 by Carl Bosch [1][2][3]. The Haber-Bosch process, as it is now commonly known, fixes nitrogen via the use of high pressures of nitrogen and hydrogen over a heterogeneous Fe catalyst. The Haber-Bosch process has remained the primary synthetic means for the fixation of nitrogen to ammonia and now contributes OPEN ACCESS half of the total required nitrogen input to world agriculture. Because of the high pressure and temperature demands of the process, 1% of the world's total energy supply is consumed to satisfy society's nitrogen demands [4]. In this review we have focused on nitrogen capture chemistry based on organolanthanide systems. We have widened the scope of systems discussed beyond those species occurring in the Chatt cycle or isolated in extensive studies by Schrock, vide infra, given the bias of research activity in the field and the inherent reactivity of the lanthanides, which do not of course mirror that of the transition metals featuring in biological nitrogen fixation. A range of synthetic pathways have been presented, beyond direct reduction of nitrogen; with reduction and metallation routes from nitrogen containing organic and inorganic species being presented. Included are aryl/alkyl substituted analogues of complexes as postulated as occurring in the Chatt cycle, e.g., -N-N(H)R, but we have omitted discussion of chemistry where the substituent alters the bonding nature of the nitrogen-or dinitrogen-based ligand, such as the recently published hydrazonido, -NHN=CPh 2 , complexes [5]. We have not extended the discussion to include nitrogen reductions using lanthanide themselves (elemental form) or simple low valent species, but note that these species have been studied [6]. A personal account of part of this field was published in 2005 which described the lanthanide-based nitrogen reduction chemistry discovered in the research group of Evans until 2004 [7].

Biological Nitrogen Fixation
Biologically, nitrogen is fixed by a limited group of microbes at atmospheric pressures and temperatures by the nitrogenase enzyme. The contrast in energy requirements between the Haber-Bosch process and biological nitrogen fixation has resulted in significant research focus over the last 50 years to understand the nitrogen fixation mechanism within the nitrogenase enzyme [8]. Through studies of this nature, four different nitrogenase enzymes have been characterized [7][8][9][10][11], with varying metal co-factors present, containing Fe in each case and either Mo or V depending on the bio-availability of each metal to the microbe [9,[12][13], Figure 1. The exploratory studies of nitrogenase enzymes have still not conclusively established the mechanism of nitrogen fixation, but have been influential in determining the metals that have been most extensively studied for chemical nitrogen fixation by synthetic methods.

Synthetic Transition Metal Mediated Nitrogen Fixation
Allen and Senoff isolated the first dinitrogen complex of a transition metal, [Ru(NH 3 ) 5 (N 2 )] 2+ (I) [14], in 1965 from the aqueous reaction of hydrazine with ruthenium trichloride, and since this initial discovery, transition metal complexes displaying promise for achieving nitrogen fixation have been reported in abundance [15][16][17][18][19][20]. Early attempts of nitrogen fixation at a single metal centre focused on the use of Mo(0) and W(0) species. Through this early work, the Chatt cycle for nitrogen fixation was proposed as a hypothetical mechanism, Scheme 1. Several key intermediates have been identified (in some cases as N-substituted analogues) in this catalytic nitrogen fixation cycle, including M(N 2 ), M-N=NH, M=N-NH 2 , M≡N, M=NH, M-NH 2 and M(NH 3 ) species. However, catalytic activity was not observed [21][22]. Scheme 1. Chatt cycle for dinitrogen fixation at a single metal centre adapted from MacKay [23].  Following decades of research into transition metal dinitrogen chemistry, the first catalytic reductions of N 2 to NH 3 have recently been reported [24][25]. Of these, only that of Schrock has been rationalised via the characterisation of a number of intermediate species. The Mo HIPT system studied by Schrock, Scheme 2, was observed to involve several of the key intermediates proposed in the Chatt cycle from the 1960's, and a catalytic cycle based on the characterised intermediate species has been proposed. The Mo centre within the cycle ranges in oxidation state from Mo III to Mo VI . Scheme 2. Schrock catalytic cycle for the Mo HIPT system adapted from Yandulov [25].
The liberated NH 3 (g) in the Schrock cycle has been shown to inhibit catalysis as it competes with N 2 for the vacant coordination site on [MoHIPT]. Furthermore, it is proposed that the activity of other transition metals such as W, Fe or V for N 2 fixation with the HIPT system will be less than that observed for Mo [22].
Studies of bi-metallic systems with alternative mechanisms to the Chatt cycle have also lead to N 2 cleaved species [26][27][28]. For instance, a molybdenum nitride complex was observed with the Mo(III) trisamide species, [Mo(NRAr) 3 ], [R = C(CD 3 ) 2 CH 3 , (III) Ar = 3,5-(Me) 2 C 6 H 3 , (IV)], whereby two [Mo(NRAr) 3 ] molecules cooperatively reduced the N 2 moiety without the need for the first three intermediate species in the Chatt cycle, Scheme 3. The reduction of N 2 was observed to be stoichiometric, with catalytic activity not observed. Theoretical studies of this system have yielded detailed mechanistic knowledge [29][30]. Recent studies of Group 4 metal compounds have yielded a number of activated nitrogen species which have exhibited reactivity with carbon containing molecules, as well as hydrogen, though a catalytic system has yet to be found [31][32].

Lanthanide Mediated Dinitrogen Reduction
Dinitrogen complexes of the transition metals have historically exhibited end-on bonding to one or more metal centres. Typically, weak activation of the N≡N bond is observed for end-on bound nitrogen molecules with single metal centres, such as the Schrock Mo(HIPT) system [33]. In 1998, Evans reported the synthesis of a novel side-on bound dinitrogen complex of decamethylsamarocene, [{(C 5 Me 5 ) 2 Sm} 2 (µ 2 -N 2 )], (V), as shown in Figure 2 [34]. Whilst little activation of the dinitrogen bond was observed in this case (based on the N-N bond length), the possibility that side-on bound dinitrogen complexes could be synthesised led to renewed efforts utilising other metals to replicate the side-on bonding mode. This indeed led to the isolation of side-on bound dinitrogen complexes of other lanthanides, actinides, and the transition metals, Zr, Nb, Ta, Hf and Os [35]. From these studies, it has been shown that the side-on bonding mode can result in quite variable dinitrogen activation, ranging from weakly (N≡N bonds) to strongly (N=N and N-N bonds) activated species, as shown in Table 1. Table 1. N 2 activation according to bonding mode, adapted from Fryzuk [33,36].  Subsequent studies have resulted in the formation of several Sm mediated reduced dinitrogen species and led to the extension of dinitrogen reduction chemistry to the majority of the remaining lanthanides [37]. In each case, the Ln(II) centre contributes one electron to the reduction of the dinitrogen bond, most often resulting in a bimetallic complex bridged by a dianionic reduced dinitrogen ligand, as shown for [{(C 5 Me 5 ) 2 Sm} 2 (µ 2 -N 2 )], (V) [38]. For the strongly reducing lanthanide, dysprosium(II), a three-electron reduced dinitrogen species has also recently been reported [39]. Dinitrogen reduction was shown to be reversible in many cases, as for [{(C 5 Me 5 ) 2 Sm} 2 (µ 2 -N 2 )], (V). Gambarotta reported the cooperative reduction of dinitrogen by four Sm(II) centres bound to macrocyclic porphyrinogen ligands which resulted in a tetraanionic reduced (N 2 ) 4species [{(chex 4 N 4 ) 2 Sm 3 Li 2 }(µ 3 -N 2 ){Li(THF) 2 }·THF], (VI), shown in Figure 3 (c-hex 4 N 4 = meso-(CH 2 ) 5 -calix-4pyrollide), whereby the fourth samarium atom involved in the reduction formed the samarium(III) macrocyclic by-product, [(c-hex 4 N 4 )Sm(Cl){(Li(THF)) 3 (µ 3 -Cl)}], (VII) [40]. Such strongly reduced species do not occur frequently from Ln(II) based reductions, as a number of metal centres are required to cooperatively reduce the dinitrogen substrate. Catalytic dinitrogen reduction systems utilising lanthanide metals are more commonly postulated to involve an external electron source [41].
Gambarotta has also reported the synthesis of a four electron reduced N 2 samarium porphyrinogen complex, [{(THF) 2 Li(Et 8 N 4 )Sm} 2 (N 2 )Li 4 )], (VIII), (Et 8 N 4 = meso-octaethylcalix-4-pyrollide), which utilised Li as an external electron source for the reduction, with reduction of the nitrogen fragment also involving Sm(II)/Sm(III) oxidation [41]. Similarly, Floriani has reported the cooperative reduction of dinitrogen by meso-octaethyl porphyrinogen complexes of Pr and Nd in the presence of sodium [42]. Hetero-/multinuclear Lanthanide complexes such as these macrocyclic examples suggest that bimetallic complex formation and additional external electron sources greatly widen the range of reduced dinitrogen species that can be obtained for metals that do not have widely accessible oxidation states. Lanthanide mediated dinitrogen reduction utilising an external electron source has since been reported for the entire lanthanide series excepting Eu and Yb [43][44][45][46][47]. The lanthanide centre in each case was supported by bis(trimethylsilyl)amide ligands as shown for the Gd example shown in Figure 4.
The reactivity of Ln(III)/M mixtures with dinitrogen, coupled with Ln(II) dinitrogen reduction chemistry has shown the readiness with which the initial N 2 reduction step can be undertaken by organolanthanide species. Studies of other N 2 reduced species of the lanthanide elements have been limited, however a number of the key intermediates proposed in the Chatt cycle have been observed.
Since the initial azobenzene reduction studies of Evans, a number of azobenzene lanthanide complexes have been reported. Bi-metallic systems are most common, with the azobenzene ligand bridging two lanthanide centres. However, a limited number of monometallic species have also been reported.

Reactions of R(H)N-N(H)R Species
Relative to the reductions of N≡N and RN=NR species as described above, reactions of organolanthanide reagents with N-N moieties are limited. An unsubstituted hydrazido complex,    Figures 14 and 15, respectively [35]. The isolation of XXXVI demonstrated the important capability of lanthanide reagents to facilitate the final reduction of the dinitrogen moiety to give mononitrogen complexes as required by the Chatt cycle.
In each of the reported imido syntheses to date, the final outcomes could not have been easily predicted. However, the isolation of these complexes has led to the prediction that organolanthanide complexes containing bulky non-participating ligands and mixed anilido-alkyl ligands should yield imido complexes in a directed manner [71].

Conclusions
The diverse chemistry reviewed in this article has clearly shown that the lanthanide metals are capable of stabilising a wide range of nitrogen-and dinitrogen-based ligands related to the conversion of nitrogen into ammonia. The narrow range of oxidation states available to the lanthanide metals has resulted in multi-nuclear and mixed-metals complexes dominating the range of (N 2 ) nspecies that have been discovered, some of which are unknown to transition metal chemistry. Mononuclear lanthanide species feature more often in complexes featuring derivatives that have been postulated and/or isolated later in the Chatt and/or Schrock cycles based on transition metal chemistry. It is acknowledged that little has thus far been achieved in regard to driving the various complexes around the Chatt-or Schrock-type catalytic cycles and that most of the complexes presented herein represent alkyl/aryl substituted analogues of these species. These last raised issues are major challenges to be addressed in the future for this field in relation to any potential development of metal-free dinitrogen derivatives based on lanthanide metal mediated processes. Nevertheless, the outcomes presented so far will no doubt continue to be built on and a wealth of interesting and structurally diverse new chemistry will be established.