Ligands with Two Monoanionic N,N‐Binding Sites: Synthesis and Coordination Chemistry

Abstract Polytopic ligands have become ubiquitous in coordination chemistry because they grant access to a variety of mono‐ and polynuclear complexes of transition metals as well as rare‐earth and main‐group elements. Nitrogen‐based ditopic ligands, in which two monoanionic N,N‐binding sites are framed within one molecule, are of particular importance and are therefore the primary focus of this review. In detail, bis(amidine)s, bis(guanidine)s, bis(β‐diimine)s, bis(aminotroponimine)s, bis(pyrrolimine)s, and miscellaneous bis(N,N‐chelating) ligands are reviewed. In addition to the general synthetic protocols, the application of these ligands is discussed along with their coordination chemistry, the multifarious binding modes, and the ability of these ligands to bridge two (or more) metal(loids).


Introduction
Various fields of modern coordinationc hemistry are affected by polydentatel igands, which give rise to an increased stability of the related complexes and allow for af ine tuning of the reactivity and selectivity of the active center(s) through their steric and electronic properties. In addition, polydentate ligands enable the synthesis of varioust ypes of compounds, ranging from stable, sterically shielded mononuclear metal complexes to intricate polynuclear species. [1] Recently,t he latter complexes received particular attention due to the increasingi nteresti nc ooperative effects [2] associated with polynuclear metal(loid) complexes and, in this regard, anionic nitrogen-based ligands have become valuable supports. With respect to cooperative effects, ditopic ligands play ac rucial role not only for adjusting the metal(loid)-metal(loid) separation, which amountsp referentially to 3.5-6 , [3] but also for directing the two metal(loid) centers in ac ooperative manner. [2] In addition to their use as frameworks for homo-and heterodinuclear complexes,d itopic ligands are also used as molecular sensors,( super)bases,o ro rganocatalysts. [4] Since the first reports on dinucleating ligands, [5] the concept has gained substantials cientific interest with numerousd ifferentt ypes known today and severalc lassificationsa re used in the literature.A t first, they can be divided into homoditopic and heteroditopic ligands, depending on whether the ligand framest wo binding sites of the same or of different type, respectively ( Figure 1a). The embedding of the binding sites in either an acyclico r macrocyclic framework allows for anotherd ifferentiation (Figure 1b). Finally,a na dditional classification in compartmental ligands and ligandsw ith isolated binding site is made depending on the availability of donor atoms bridging both binding sites ( Figure 1c). As will be shown in this review,t he differen-tiation of al igand as either ad itopic or at etradentatel igand withoutc onsidering the central atom is often not possible as the formation of am ono-, di-, or polynuclear complex strongly depends on the interplay of both the metal(loid)a nd the ligand.T his review coverst he synthesis and application of dinucleating ligandsw ith two monoanionic N,N-binding sites, whereas it is limited to ligandsi nw hich the metal(loid) ligand bindingi sa chieved primarily through the nitrogen atoms. The chapters belong to bis(amidine)s, bis(guanidine)s, bis(b-diimine)s,b is(aminotroponimine)s, bis(pyrrolimine)s, and miscellaneousl igands. Every chapter starts with the synthetic access of the related ligands and is followed by as ection discussing their application and coordination chemistry.

Bis(amidine)s
Bis(amidine)s, which have been synthesized in 1937 for the first time, [6] can be divided into two classes according to the way the two amidine units are connected, these being through the nitrogen atom or through the backbone-based carbon atom, and both types will be discussed sequentially in the following. Persubstitutedb is(amidine)s, which in consequence do not bear acidic protons, are beyond the scope of this review and have been excluded.
Due to the limited stability of bis(carbodiimide)s, [7c, 11] which are prone to polymerization reactions, the first route is restricted to only af ew examples. However, it offerst he advantage that the respective dinuclear aluminum, [7c] titanium, [7a] and zirconium [7b] complexes are directly obtained in yields > 80 % without the isolation of the respective bis(amidine). Notably, for the reactions of bis(carbodiimide)s with trimethylaluminum, different products are obtained, depending on both the stoichiometry and the reactionc onditions (Scheme 2). [7c] Although using a1 :2 ratio of bis(carbodiimide) and trimethylaluminum at low temperatures (À80 8C!rt) gives rise to dinuclearc omplexes I,i ncreasing the temperature yields the dinuclear heterocycle II,a nd using a1:4 ratio at ambient temperature results in the tetranuclear bis(amidinate) complexes IIIa or IIIb, depending on the steric bulk of the terminal substituent.
The use of imidoylchlorides and bis(imidoylchloride)s is the most generala pproach towards the synthesis of N-bridged bis-(amidine)s and this method has also been used to synthesize the first reported example back in the 1930s. [6] Most of the synthetic protocols make use of bis(imidoylchloride)s and primary amines (route ii), [6,8] whereas there is only limited precedence for routes starting from imidoylchlorides and primary diamines (route iii,S cheme 1). [7d, 9] In the first case, reactions are regularly carried out in toluene using two equivalents of the respective aniline and one equivalent of the bis(imidoylchloride)p roviding yields of isolated materials ranging from only afew percent to nearly quantitative product formation.I ti sw orth noting that early reports by Hill and Johnston indicate that an excess of aniline is not only unnecessary, but also undesirablebecause the excessc omplicates the purification of the bis(amidine)s. [8a] Robert Kretschmer studied chemistry in Jena and obtainedh is Ph.D. at the TU Berlin in 2012 under the guidance of Prof. Helmut Schwarz. Afterapostdoctoral stay with Prof. Guy Bertrand at the UCSD, he started his independent careeri n2 015 at the University of Regensburg and in 2019 he became Junior Professor with TenureT rack at the Friedrich Schiller University Jena. His research focusses on ligand design and cooperativee ffects originating from polynuclears ystems and is funded by the German Science Foundation (DFG). In 2017 he became an electedM ember of the German Young Academy( Die Junge Akademie. Although alkyl-bridged bis(imidoylchloride)s do not require the addition of ab ase, in case of bis(imidoylchloride)s containing aromaticb ridges, triethylamine has been used in all synthesis reported so far.F or the second route which uses imidoylchlorides and primary diamines, yields ranging from about 30 to 80 %h ave been reported, and performing the reactioni nt oluene at elevatedt emperatures seems to be preferential. Althoughf urther generalc onclusions remain elusive due to the limited sample set, it appears that the product yield is more affected by the electronic and stericp roperties of both the imidoylchloride and the primary diamine, rather than the experimental protocol, that is performing the reaction with or without triethylamine as an HCl scavenger.N otably,a2,6-pyridyl-ene bridged bis(amidine) was obtained in 66 %y ield starting from dilithiated 2,6-diaminopyridinea nd the respective imidoylchloride. [12] In conclusion, both routes appear to be suitable and the preference for one over the other is most likely due to the synthetic aim:f or ligand libraries with varying terminal (R) or backbone( R ')s ubstituents, the use of bis(imidoylchloride)s through route ii)i sm ore convenient, whereas aiming for al igand set with variousl inker groups,r oute iii)i s of benefit.
Al ess common route is the reaction of lithiated secondary diamines with two equivalents of benzonitrile, [10] which allows for the isolation of the relatedl ithium bis(amidinate) complexesi ny ields ranging from 54 to 93 %. Even though these [10b,c] -C 5 H 10 -2 ,6-(CH 3 are suitable precursors for the synthesis of other metal complexes, this procedure has not been used so far to access the related bis(amidine)s.A lthough aliphatic,a romatic, and dimethylsilyl bridges have been applied, using a2 ,6-pyridylene bridging group does not afford the expectedd inuclear bis-(amidinate) complex, but yields the hexanuclear lithium complex IV bearing three 2,6-diaminopyridinea nd four terminal benzonitrile ligands (Figure 2). [10e] In addition, Bai, Guo, and Liu investigated the role of the metal by reactingb enzonitrile with both al ithium and as odium diamide, with Li giving increased yields of crystalline material (90 %) compared with the Na derivative(66 %). [10d] Althoughs ilyl-bridgedb is(amidine)s are accessible through route iv), they are alternatively obtained by the reaction of lithiated amidines with chlorosilanes (Scheme3). [13] The onepot reactions tarts with the lithiation of ap rimary amine, whose substituent will become the terminalg roup in the final bis(amidine). Subsequent reaction of the intermediary lithium amide with benzonitrile affords an lithium amidine complex, which, upon further treatment with Si(CH 3 ) 2 Cl 2 ,i sc onverted to the bis(amidine). Although limited to one example, the yield is nearly quantitative.
Backbone bridged bis(amidine)s( Scheme 4) are synthesized either: vi)byt he addition of ac arbodiimide to ad oubly metallated compound, [14] often generated in situ from the corresponding( di)halogen precursor, vii)through aminolysis of bis-(imidoylchloride)s, [14e, 15] or viii)byr eacting am etallated amine with dinitriles. [16] Carbodiimides readily insert into the metal-carbon bond of aromatic compoundso rc onjugated olefins and hydrolysis of the thus formed lithium bis(amidinate) complexes affords the free ligands in 30 to 72 %y ield. [14] So far,m ostly diisopropyl-or dicyclohexylcarbodiimide have been applied to this reaction, likely due to the commercial availability of these two compounds, but reports on otherc arbodiimides, such as bis(2,6-diisopropylphenyl)carbodiimide,s tart to arise. Although alkylbridged bis(amidine)s have also been synthesized using this route,t hey are more readily availablet hrough the aminolysis of bis(imidoylchloride)s. [14e, 15] This approach is the longest knowna nd affords product yields ranging from 19 to 92 %, however,n og eneral approach can be identified. Although the group of Beckert used ab is(imidoylchloride) = amine stoichiometry of 1:2a long with triethylamine as an HCl scavenger, [15b,f] other protocols apply excessive amountso ft he amine, thus acting as both, the nucleophile and the base. In either cases,c omparable yields have been obtained thus the decision in favor of one over the other protocol is most likely driven by the boiling point of the amine used;f or high-boiling amines, such as bulky anilines, the method of Beckerta nd co-workers appearst ob ep referential, whereas the opposite holds true for low-boiling (aliphatic) amines. The generation of bis(imidoylchloride)s is circumvented when dicarboxylic acids are directly converted by the reaction with polyphosphorica cid trimethylsilyl ester (PPSE) and excess aniline as reported for 1,4-cyclohexylenea sw ell as 1,3-and 1,4-phenylene bridged bis(amidine)s. [15i, 17] CH 2 -Bridged bis(amidine)s are also accessible on two alternative and ratheru nique pathways (Scheme 5). On the one hand, N,N'-bis(4-ethoxyphenyl)malonamidesc an be directly converted to the respective bis(amidine)s by treatment with various anilinesi nt he presence of catalytic amounts of H 2 SO 4 , [18] thus avoidingt he transformation of the amide to an imidoylchlo- ride. On the other hand, the reactiono fp entachloro cyclopropane with an excess of alkylamine such as isopropylamine or tert-butylamine, gives rise to bis(amidine)s in about 60 % yield. [19] Finally,a midines, which carry an m-terphenyl backbone that possesses benzaldehyde substituents have been bridged by condensation reactions of the aldehyde function using primary diamines, [20] whereas amidines containing terminal alkyne groups have been linked through the cis-o rtrans-coordination with platinum(II) centers [21] or by Sonogashira and Glaser-type coupling. [22] The first report aboutapotential application of bis(amidine)s dates back to the 1950s, [8a] when researchers aimed for new local anesthetics based on phenacaine-an amidine-and intended that doublingt he number of amidine units and separating them by an ontoxic linker gives rise to highera ctivities at al ower dosage. Although bis(amidine)s themselves have been pharmacologically tested, [18] they found more often applications as supports for transition metals, rare-earth and maingroup elements. [23] The properties of the relatedc omplexes are strongly influenced by the way in which the two amidine units are bridged. In mosto ft he cases,b ackbone-bridged bis(amidine)s yield well-defined complexes in which either one or two of the isolated amidineu nits are functionalized as discussed furtherb elow.T he conformational flexibility of N-bridged bis-(amidine)s, however, offers access to mono-and polynuclear complexes,d epending on the element, the electronic and steric properties of the ligand,and the synthetic protocol.
Monometallic complexes of type A are often observedw hen large ions such as Lu, Nd, Sm, Y, Yb, and Zr are involved (Figure 3), [8f,g, 10a,e, 24] and the relatedb is(amidinate) complexes have found catalytic applicationsi nh ydrophosphonylation and ringopeningp olymerization reactions, [24c,e,g] but were also used as catalysts for the synthesis of monosubstituted N-arylamidines. [25] For smaller ions, however,o nly one example of a type A complex containing at itanium(IV) centeri sk nown to date. [26] Mononuclear complexes of type B,i nw hich the metal coordinates only the two lateral nitrogen atoms of each binding site, were observed when dimethylsilyl-bridged bis(amidine)s are used for the complexation of hafnium,t itanium, uranium, and zirconium,t hus forming SiN 2 Mf our-membered rings with M = Hf, Ti,U ,Z r. [13, 24h, 27] The coordination mode, however, depends not only on the metal but also on the solvent. For example,t reatingadimethylsilyl-bridged bis(amidine) (R = tBu and R' = Ph) with ZrCl 4 gives rise to either ac omplex (M = Zr, X = Cl, n = 4) of type B or of type C in 80 and7 1% yield, respectively, depending on whethert olueneo rd ichloromethane is used as solvent. [13] Furthermore, also the reaction conditions play ac rucial role concerning the obtained products as exemplified with the reactiono famonolithiatedb is(amidine) with thorium(IV) chloride:w hen dry THFh as been used, the mononuclear species V was obtainedw hereas using wet THF gives rise to the dinuclearspecies VI ( Figure 4). [27b] Worthy of note is also the report about the transformation of al ithium bis(amidinate) to ah eteroditopic imido amidinate complex (type D)u pon reactionw ith TiCl 3 (C 5 H 5 ), which however depends on the terminal rest. With less hindered phenylo r 2,6-dimethylphenyl groups, the imido amidinate complex VII of type D is formed, whereas with the more bulky 2,6-diisopropylphenyl substituent, the type B complex VIII was obtained ( Figure 5). [27a] The coordination of only three out of four nitrogen atom is also observed in case of the type E complexes, in which the titaniuma nd zirconium centers are bound to two laterala nd one terminal nitrogen atom. [24h] In addition, two mononuclear anionic complexes of type F have been reported,i nw hich the lanthanide ion (Y,Y b) is The catalytic activity of the mononuclear type F complexes in the ring-opening polymerization of e-caprolacton was found to be lower than the activity of the respective dinuclear homoleptic complexes of type I (see further below).F or the sake of completeness, it is worthy to note that with a2 ,6-pyridylene-bridged bis(amidine) another type of mononuclear complexes has been reported for erbium and yttrium (IX,M= Y; Figure 6), in which two bis(amidine)s chelate one metal center involving coordination of the pyridyl nitrogen. [12] In addition to the different types of mononuclear complexes, av ariety of di-anda lso polynuclearb is(amidinate) complexes have been reported so far (Figure 7a nd Figure 9, respectively). Heteroleptic dinuclearc omplexes of type G belong to the first observedb is(amidinate)s, but remain limited to certaine xamples employingt he elements aluminum, [7c, 9, 28] titanium, [7a] and zirconium. [7b, 29] This is somewhat unfortunate keepingt he promising catalytic ability of dinucleara luminum( ring-opening (co)polymerization of cyclic esters and synthesis of cyclic carbonates) [9,28] and zirconium complexes (stereoselective, living, coordinativec hain-transfer polymerization of propene) [29] in mind. In addition, av ariety of dinuclear complexesi nw hich the metal centersa re intra-or intermolecularly coordinated to two or three amidinate units have been reported, types H-K. For iron, neutralh omoleptic type H complexes (k = m = n = 0), in whicht he two paramagnetic iron(II) centers are framedb y two bis(amidinate)s were observed, [10b] whereas for yttrium and ytterbium anionic and charge-neutral complexes (k = 0, 1) have been reported, for whicha dditional bridging and/ort erminal ligands/counterions are attached to the two metal centers. [12, 24b, d, 30] These compounds are interesting catalysts, as shownf or the charge-neutral ytterbium type H complex, which initiates the polymerization of l-lactidea nd e-caprolacton and possesses high activity along with good controllability for the ring-opening polymerization. [24b,d] The coordination number of the two metal centers within complexes of type I,i nw hich three bis(amidinate) ligands are heteroleptically bound to two metals (Er,Y ,a nd Yb), [12, 24c] is strongly affected by the rigidity of the bridging group (Figure 8). For the flexible 1,3-propylene linker,t he purelyh omoleptic homodinuclearc omplex X with coordination number 6( m = n = 0) for each yttrium is obtained, [24c] whereas using a more rigid 1,3-phenylene bridge yields the heterobimetallic   [27a] Hydrogen atoms and solvent molecules are omittedf or clarity. Figure 6. Molecular structure of IX in the solid state. [12] Hydrogena toms except those of the NH groups, counter ions and solvent molecules are omitted for clarity. complex XI,i nw hich the two metals possess ac oordination number of 7( m = n = 1) due to the additional complexation of one THF molecule and one Cl ion bridging the second yttrium and the lithium counterion, respectively. [12] Although the purely homoleptic yttrium and ytterbium complexes bearing a 1,3-propylene linker show highera ctivities in the ring-opening polymerization of e-caprolacton than the respective mononuclear type F complexes, they yield polymers with rather broad molecular-weight distributions, mostl ikely because of several active lanthanide amidinate bonds. The aggregation of two mononuclear type A complexesg ives rise to dinuclear type J complexes reported for ytterbium, and both complex types readily interconvert upon changing the solvent. [24a] Finally,t he only other type of ad inuclear complexb elongst o type K, [8b,d, 10d] in whicht he two metals coordinate crosswise one terminal and one lateral nitrogen atom of each amidinate unit as shown for examples of lithium, potassium, and sodium.
In addition to the various dinuclearc omplex types G-K,a lso polynuclear complexes of the type L-R have been reported (Figure 9). The impact of the terminal substituent Ro nt he coordination and aggregationb ehaviori sn icely illustrated for di-methylsilyl-bridgedl ithium bis(amidinate) complexes (R' = Ph). Although ad inuclear type K complex (X = THF, n = 2) is obtained with terminal phenyl rests, increasing the steric bulk by using the 2,6-diisopropylphenyl substituent affords at rinuclear type L complex, and terminal tert-butyl groups give rise to a tetranuclear type M complex. [10d] Tetranuclear lithium complexes also show coordination modes of type N and O.I n type N complexes, [10e] each lithium bridges the two bis(amidinate) units and reaches ac oordinationn umber of four duet o the complexation of an additional THF molecule. The tetranuclear type O complexes, however,a re formally generatedu pon aggregation of two dinuclearc omplexes, ab ehavior wellknowni nt he field of lithium chemistry as laddering principle; [31] here, the lithium centers have ac oordinationn umber of three.F or samarium, at etranuclearc ubic structure of type P [32] has been reported, in which the four Sm ions occupy alternating corners of the cube, whereas the other corners are occupied by m 3 -imido ligands( X = NPh). In addition, the doublyfused cubic samarium complex XII,c onsisting of six Sm centers, four m 3 -a nd two m 4 -imido ligandsa sw ell as two bridging iodine ligands, has also been isolated (Figure 10

Review
Finally,t wo rather unusualc omplexes of type Q and R were reported:t he former is at rinuclearh eterobimetallic ate-complex in which one bis(amidinate) chelates one neodymium througha ll four nitrogen atoms,w hereas for the second bis-(amidinate), both ligand sites act differently,t hat is one unit bridgesb othN dc enters whereas the second unit exclusively coordinates the second neodymium.

Bis(guanidine)s
Closely related to amidines are the more basic guanidines, [56] whosed onor capabilities are also increased compared with amidines due to the inclusion of an additional nitrogen atom in the backboneo ft he ligand. [57] Bis(guanidine)s received considerable interest over the past decades, [58] and have been used as catalysts, [59] ion sensors, [60] and superbases. [61] In addition, they are used as ligands in bioinorganic and coordination chemistry. [62] Bis(guanidine)s may be classifieda sa cyclic, cyclic or macrocyclic compounds, depending on whether one or two linker groups are present in the molecule, Figure14. Please note that, depending on the substituents R, R',a nd R'',s everal isomers( tautomers) are conceivable. Mono-( R = R' = R'' = H) and persubstituted (R or R' or R'' ¼ 6 H) bis(guanidine)s are beyondthe scope of this review,but have been discussed elsewhere, [58a,c, 63] wherefore compounds with one to three hydrogen atoms per guanidine unit are discussed in the following.
[68b] Aminoimidazolines can be considered as the cyclic analogues of the guanidineg roup and the related bis(aminoimidazoline)sa re readily available by treating primary diamines with 2-alkylmercapto-4,5dihydroimidazole salts (Scheme 7). [68c, 70] Given that bis(phosphaguanidine)s are closely related to bis(guanidine)s, they are included here for the sake of completeness. [71] Although there is only one report so far,t he reaction of carbodiimides with dilithiated secondary diphosphines seems to be as uitable approachw ith yields of 61 to 69 %. In addition, phosphaguanidines give rise to the relatedb is(phosphaguanidine)s upon coordination with copper(I) or platinum(II). [71b] Cyclic bis(guanidine)s are reminiscento fb ackbonebridged bis(amidine)s (see above)a nd readily available through the reactiono fc yclic diamines with carbodiimides, (route v), R' = Linker). [58b, 72] Alternatively, they can be obtained through the aminolysis of chloroamidines [73] or by treating cyclic diamines with N,N'-di-Boc-N''-triflylguanidine. [74] To derive macrocyclic guanidines, two methods are conceivable for their synthesis (Scheme 8). Regarding the first route (vii), am acrocyclic bis(thiourea)i st ransferred to the respective bis(pseudothiourea) by S-alkylation and subsequentlyt reated with two equivalents of ap rimary or secondary amine. [59, 60a] In the second approach (ix), ab is(carbodiimide), which is either generated from the bis(thiourea) or from bis(iminophosphorane), is reacted with an excess of amine yieldingt he related macrocyclic bis(guanidine)si nv ery good yields. [75] Compared with bis(amidine)s, bis(guanidine)s found multifarious applicationsa sc atalysts, [59,76] ions ensors, [60] pharmaceuticals, [63] or superbases. [61] To some extent, bis(guanidine) complexes (Figure 15) resemble the coordination patterns observed for bis(amidinate)s. Mononuclear complexes of type A are known for Eu, [77] Nd, [78] Sm, [79] Ta, [80] Ti, [68b] Y, [78,81] Yb, [78,81,82] Zr, [68b] and have been either directly derived from the bis(guanidine) [68b] or from in situ generated dinuclearalkali-metal complexes, [77][78][79][80][81] which are readily accessible from the reactiono f metallated secondary diamines with two equivalents of ac ar- bodiimide. Due to the additional nitrogen-donor function compared with bis(amidine)s,f urther coordinationm odes have been reported for dinuclearc omplexes of rare-earth metals. Althought ype B complexes were only obtained for Yb, [81] complexes of type C have been characterized for Eu, [77] Y, and Yb, [81] and only one complex of type D is known, in which M = Eu. [77] Finally,f or lithium, also the tetranuclear type E complex XVI has been reported ( Figure 16) in which three different coordi-nation modes have been observed for the lithium ions, althought hey all have the coordination number three. [81] Notably,b is(phosphaguanidine)s showd ifferent coordination schemes depending on the metal involved. Platinum(II) coordinates to the two lateral phosphorus atoms yielding mononuclear complexes of type F, [71a] whereas with aluminum and titanium, dinuclear type G complexes are obtained in whicht he two metals are coordinated in an N,N-chelating fashion. [71b,c]

Bis(b-diimine)s
Given that b-diiminate( NacNac) ligandsh ave been identified as powerful supports in the 1960, [83] they are commonly used for the stabilization of elements in various oxidation states and from all sections of the periodic table. [84] In the following, the term b-diimine will be used although the terms b-diketimine or 1,3-diimine are also used in the literature. Please note that, the substituents have as trongi nfluence on whether the amino-imine (shown in Figure 17) or the b-diimine tautomer represent the most stable isomer.D ue to stabilization by hydrogen bonding between thei mine nitrogen atom and the NÀ Scheme8.Synthesis of macrocyclic bis(guanidine)s.  Hp roton, the ligandsd iscussed in this chapter exist by and large as the amino-iminet automer.H owever,a nd because of the frequentu se of the term bis(b-diimine) in the literature this term will be used in here as well. Amongt he bis(b-diimine)s,d ifferent kinds occur in the literature, these being the N-bridged and backbone-bridgeda cyclica sw ell as macrocyclic derivatives ( Figure 17). The latter motif is not only regularly found in natural macrocycles such as bacterchlorin, chlorin, isobacterchlorin, porphodimethane, and porphyrin, [85] but also in tetraazaanulenes, [86] whichh ave been discussed elsewherea nd will remain undiscussed in here. Acyclic backbone-bridged bis(b-diimine)s were first reported in 2004 by the Lappert group [87] and the first N-bridged example with isolated binding sites was presented one year later by the group of Hultzsch. [88] Since then, three methods have been successfully appliedf or the synthesis of N-bridged bis(b-diimine)s (Scheme 9). Althought wo of them start from b-aminoketones, as uccessful, neat reactioni so nly observed with 1,3-bis(aminomethyl)benzene (route ii). [89] In case of other diamines, pre-activation of the b-aminoketone using trialkyl oxoniumt etrafluoroborate, also known as Meerwein's salt, [90] is necessary. [88,91] All derivatives reported so far contain terminal aryl groups,w hereas the opposite holds true for the first compartmental N-bridged bis(b-diimine), containing an additional amineg roup in the bridge,w hich was obtained through route iii)b yt reating a bis(b-aminocarbonyl)c ompound with primary alkyl amines. [92] Bis(b-diimine)s with various backbone-substituents, including alkyl, [87] aromatic, [93] and disulfide [94] bridges, are known today. The synthetic procedures differ to quite some extent (Scheme 10) depending on the nature of the linker group. An alkyl-bridged bis(b-diimine) is obtained when two equivalents of al ithium b-diiminate are allowed to reactw itho ne equivalent of an alkyl dihalide (route iv). [87] For aromaticb ridges,t wo different routesh ave been applied:r oute v)s tarts from bis(bdione)sa nd anilines, [93c] whereas route vi)f ollows ar ather complex sequence in which ab is(vinamidinium) salt is hydrolyzed and subsequent reactionw ith an aniline yields the desired bis(b-diimine). [93a,b] Finally,d isulfide-bridged derivatives are obtained, when ab is(b-aminocarbonyl) speciesi sa llowed to react with anilines(route vii). [94] Macrocyclic bis(b-diimine)s often belong to tetraazaanulenes, which are regularly obtained through the related transition-metal bis(b-diiminate) complexes by applying template synthesis, [95] but are outside this review.A part from them, other types of macrocyclic bis(b-diimine)s have been reported, which originate from the condensation of primary diamines with viii)bis(b-aminocarbonyl) [96] andb is(b-aminothiocarbonyl) compounds, [97] respectively,o rix)the ethylene glycol monoketal of b-diones. [98] Alternatively, macrocyclic compounds are also formed in ther eactiono fm etallated bis(imine)s and bis-(imidoylchloride)s (route x,Scheme 11). [96c] So far,b is(b-diimine)s have not found applications by themselves, but have been used as supports for transition metals as well as rare-earth and main-group elements, yielding monoand dinuclearm etal complexes( Figure 18). For mononuclear complexes of type A,i ncorporating Hf, [91d] La, [88, 91a,c] Sc [91g] Y, [88, 91a,c,g] and Zr, [91d,f] severali somers exist, which show a fluxionalb ehavior and interconvert through aB ailar twist processes. It could be shown that someo ft he rare-earth complexesw ere moderately active in the copolymerization of cy- Review clohexene oxide (CHO) with carbon dioxide. [91a] With (almost) parallelo riented NacNac moieties, am ononuclear complex of type B has been isolated, in whichacalciumi sf ramed by both bis(b-diiminate) units and ac oordination number of five is reachedt hrough the complexation of an additional THF molecule. [99] Heterolepticd inuclear complexes of type C are knownf or transition metals (Cu, [91h] Y, [100] Zn [91b,e, j] )a sw ell as for rare-earth (Yb) [100] and main-group elements (Al, [101] Ca, [91b,e] In, [102] Mg, [103] Na, [100] Tl [89,102] ). Notably,c alcium and magnesium complexes of type C show ad ynamic behavior and undergo rapid ligand exchange in accordance with the Schlenke quilibrium yielding homoleptic complexes of type D. [91b,e, 103c] The equilibrium, however,i ss trongly affected by the bridge. The linker group also plays ac rucial role if it contains additional donorf unctions. Thus, using a2 ,6-pyridylene bridge leads to dimerization, [91h] Scheme10. Synthesis of acyclic backbone-bridged bis(b-diimine)s.  yields ah exanuclear copperc omplex, [104] or incorporates the pyridyl nitrogen atom into the coordination sphere of the two magnesium or zinc centers [91b, 103b] as illustrated for the dinuclear magnesium complex XVII (Figure 19).
The catalytic activity of some dinucleart ype C complexes in the copolymerization of CHO and CO 2 has been evaluated by the group of Harder. Although calcium complexes remained inactive, several zinc complexes efficiently catalyzed the polymerization and the activity found to be highly dependent on the nature of the bridging unit, with 1,3-phenylene being a better choice compared with 1,4-phenylene, whereas complexes containing ah ydrazine bridge were found to be inactive. [91b,e] Three variants of backbone-bridged bis(b-diiminate)s have been so far reported in the literature. The heteroleptic complexes of type E are known for aluminum [87,105] magnesium, [105] and zinc, [93b] whereas examples of type E' complexes are known for aluminium and magnesium. [99,105] Type F complexes are obtained when an additional ligand such as chlorido or n-butyl bridges the two metals (M = Fe, Mg, Zn) of complex E'. [91b, 103b, 106] Finally,a lso homoleptic backbone-bridged cesium, lithium,a nd potassium complexes of type G are known, which nicely illustrates the effect of ligand and metal as the related aluminum, calcium, and magnesium complexese xists as type B, E,a nd E' complexes, respectively. [87,99] Regardingt he Nbridged derivatives, an additional donor site within in the linker group affects the coordination behavior.T hus,t he unusual dinuclear complex XVIII (Figure 20), in which each rubidium center is coordinated to only one nitrogen atom per (b-di-iminate) unit and the oxygen atom of the xanthene bridge, has also been reported. [105] Macrocyclic bis(b-diiminate) type H complexes have been reported for Al, [98] Cu, [96a] Li, [98] and Zn, [96,98] and the rigidity of the ligand framework defines not only the metal-metals eparation but also the relative orientation of both metal centers. The zinc derivatives are of particular importance because these are valuablec atalyst for the copolymerization of CHO andC O 2 . [96,98] Finally,t etranuclearc omplexes of type I are formed upon dimerization of heteroleptic dinucleartype C magnesium complexes. [103d,e]

Bis(aminotroponimine)s
Aminotroponimines belong to an interesting class of ligands, which exhibit a1 0-electron p-systemd elocalizedo ver seven carbon and two nitrogen atoms, and recently experienced a renaissance in coordination chemistry. [107] Their macrocyclic (tropocoronands) or N-bridged relatives have been introduced by the groupso fL ippard [108] and Roesky, [109] respectively,b ut after ap eriod of intense research, [110] interest in this compound class faded almostc ompletely.T he synthesis of bis(aminotroponimine)s resembles to ac ertain extent the synthesis of bis(b-diimine)s( see above). Activation of aminotropones (route i)o rb is(aminotropone)s (route ii) [108] by Meerwein'ss alt and subsequent treatment with ap rimary diamine gives rise to the relatedN -bridgedo rm acrocyclic bis(aminotroponimine)s, respectively (Scheme 12). Alternatively,M eerwein'ss alt maybe substituted by dimethylsulfate in route ii). [110c] As in case of the bis(b-diimine)s,b is(aminotroponimine)s have solely been applieda ss upports in various metal com-   [110l] In, [110l] La, [109] Lu, [110n] Ti, [110i] Yb, [110n] or type B,f or Cd, [110f] Co, [110e,p] Hf, [110d] Mn, [110g] Ni, [108, 110a] Zn, [110f] Zr, [110d] depending on whether Nbridged or macrocyclic bis(aminotroponimine)s being applied. In case of fourfold-coordinated metal centers, the linker group strongly affects the conformation of the related complex, specifically being distorted in planar and tetrahedral manner.I n case of hexacoordinated metal centers, the linker group also impactst he overall stereochemistry.S urprisingly,t he coordination schemesa re less manifold comparedt han for ligandsr eported in the preceding chapters and there are only very few examples of dinuclear complexes of type C (M = Al) [110k] and D (M = Cu), [110q] in addition to the rather unusualc omplexes of type E (M = Er, [110h] La, [109] Y) [110h] and F (M = La); [109] the dinuclear lanthanum complex XIX illustrated in Figure 22 stands as an example.

Bis(pyrrolimine)s
Acyclic bis(pyrrolimine)s are readily accessible in ao ne-pot reaction from commercial starting materials, this is as ubstituted 1-H-pyrrole-2-carboxaldehydesa nd primary diamines (route i, Scheme 13). [111] Over the last two decades, av ariety of macrocyclic derivatives have been introduced and widely applied in coordination chemistry.A lthough these macrocyclesa re formally tetra(pyrolimine)s,t hey are mentioned here for the sake of comparison but will not be discussed in detail. [112] Their synthesis originates from diiminodipyrromethanes, which are readily obtained through cyclization with primary diamines under acidic conditions and workup with as uitable base, liberating the free ligands(route ii,tropocoronands).
The facile synthesis of acyclic bis(pyrrolimine)s most likely accountsf or their manifold application giving rise to various mono-a nd polynuclear coordinationc ompounds ( Figure 23).
Hundreds of examples have been reported for mononuclear complexes of type A,f or example for Cu, [113] Mn, [114] Ni, [113b] Y, [115] Zr, [116] and listing them in here would exceed the limits of this review.I nc ontrast, only one example is knownf or a type B complex,i nw hich the titanium is eightfold coordinated  by two bis(pyrroliminate)s forming at rigonal dodecahedron as reported for the bis(amidinate) complex of type E (see above). [117] Dinuclear bis(pyrroliminate) compounds have also been reported, and homoleptic type C complexes could be isolated for Ag, [111b] Co, [118] Cu [112c, 113a, 119] Mg, [116] Mn, [114] Ni, [112c] Ti, [120] and Zn. [113b, 116, 121] Here, various configurations are conceivable and mosto ften, the complexes are intertwined, forming helixtype structures. Amongt hem, the dinuclear titanium complex XX (Figure 24) did show catalytic activity for the polymerization of rac-lactide [120] and the cobalt and manganese complexes weref ound to readily activate O 2 . [114,118] When three ligand molecules frame two metals,d inuclear complexes of type D are obtained.E xamples of titanium [117] and yttrium [115] speciesh ave been reported and the titaniumc omplex catalyzes the hydroamination reactionofp henylacetylene with aniline under mild conditions. In addition, trinuclear (type E)and tetranuclear (type F)z inc complexesh avea lso been observed and the examples nicely illustrate how the bridging group impacts the overall coordination mode. [121a] Although with a1 ,2-phenylene bridge at ype C complex is obtained, a1 ,3-phenylene bridge allows for the isolation of the trinuclearc omplex E and using a1 ,4-phenylene bridge finally gives rise to at etranuclear complex of type F.

Miscellaneous
In addition to the various ditopic ligands mentioned in the preceding chapters, some additional N,N-ligands are worthy to mention even thought hey found only little precedence in the literatures of ar;t heir metal complexesa re shown in Figure 25.
In comparison with amidines, guanidines, b-diimines, and aminotroponimines, amidoamines offer some advantages. [122] Their saturated backbone suppresses the delocalization andi nhibits an on-innocent behavior,w hich is of particulari mportance as in dinuclear complexes ligand-centered reactions are regularly observed. Bis(amidoamine)s reported so far,w ere obtained by Cu-catalyzed coupling of diiodide species with N,Nalkylated ethylenediamines [123] and readily transferred to the related aluminuma nd zinc complexes of type A and B. [123,124] The deprotonation of bis(amidoamine)s with n-butyllithium affords the tetranuclearc omplex XXI (Figure 26) in which the oxygen atoms of the benzofuran bridgea re also involved in the coordination scheme. [123] Bis(anilidoaldimine)s are an additional class of ligands, which are accessible in excellent yields from the reactiono fd iamines/dianilines with fluorobenzaldehydes and subsequent nucleophilic aromatic substitution of the fluorine groups with two equivalentso fal ithium anilide. [125] The macrocyclic derivatives cannot be obtained on this route, but reactingt he dianiline with 1,3-dioxolane-protected 2-bromobenzadehyde followed by deprotection and cyclization with the HCl salts of the dianiline yields the desired macrocycle. The related type C (Al, Zn) [125] and D (Zn) [125a] complexes are readily obtained upon treatment of the protio ligandsw ith aluminum and zinc alkyls, respectively,a nd behave as active catalysts for the copolymerization of CO 2 and epoxides. Bis(aminopyridine)s have been used recently and give rise to manifold coordination complexes. The protio ligands are readily accessible by reducing the related bis(iminopyridine)s with NaBH 4 . [126] Al-  thougha luminum complexes, which were accessed by reacting the related bis(iminopyridine)sw itht rialkyl aluminum, were characterized as type E complexes by X-ray diffraction, [127] for platinum this coordination mode was proposed based on computational calculations. [126b,c] Notably,m ost of the complexes incorporating bis(aminopyridine)s belong to mononuclear coordination complexeso ft ype F in which the ligand remains in its neutral protonated form, thus being beyond the scope of this review. [128]

Conclusions and Outlook
Over the last decades, whole libraries of nitrogen-basedditopic ligandsh ave been discovered and applied in various fields of chemistry.T he multifariousc oordination modes originating from these ligands are challenging, in terms of predicting and manipulating the coordination schemes by ap roper choice of backbone, bridging group, terminal substituents, and the metal(loid) itself. Similarly,i nteresting coordination compounds reachingf rom mononuclear to polynuclear complexes have been reporteda nd especially the latter are of particulari nterest for the rapidlyg rowing field of cooperative catalysis, which requires suitable ligandst hat allow to finetune both the metal-metal separationa nd the relative orientation of both active sites. In addition to being valuable supports for one or more metal centers, the ligands themselves might be interesting catalysts or materials with properties not observed for their monotopic counterparts.