Coordination chemistry and applications of versatile 4 , 5-diaza fl uorene derivatives

This perspective review will examine the coordination chemistry and applications of metal complexes of 4,5-diazafluorene derivatives. The versatile derivatives of 4,5-diazafluorene can serve multiple roles, and display a number of coordination modes. The ambidentate derivatives with multiple coordination sites can allow for the syntheses of coordination polymers, multimetallic, and macrocyclic complexes. In addition, certain 4,5-diazafluorene derivatives can serve as spectator ligands to support reactivity at the metal centre, or as reactive actor ligands engaging in atypical reactivity patterns. The applications of metal complexes of 4,5-diazafluorene derivatives in catalysis, photochemistry and photophysics, as well as in bioinorganic chemistry are also surveyed.


I. Introduction
4,5-Diazafluorene (dafH) was first reported in the late 1970s, 1,2 and synthesized in two steps from 1,10-phenanthroline ( phen).The first step is an oxidative ring contraction of phen with permanganate in basic aqueous media giving 4,5-diazafluoren-9-one (dafo), 3 and the second step is a Wolff-Kishner reduction of dafo with hydrazine monohydrate at high temperature (Scheme 1). 4,5The contraction of the middle ring of phen increases the distance between the two N-donors, i.e., 2.72 Å for phen 6 and 3.05 Å for dafo 7 and dafH. 8In coordination chemistry, the dafH ligand has been conventionally viewed as a 2,2′-bipyridine (bpy) derivative with a methylene linker tethering the two pyridine rings together (Fig. 1).The methylene group effectively ties back the two pyridine rings, resulting in a longer N-N distance in dafH (2.82(3) Å) compared to bpy (2.63(4) Å) in complexes.0][11][12][13] The dafo ligand has an even longer N-N distance (2.96In 2006, he started his independent career at the University of Toronto, where he is now an Associate Professor of Chemistry.His broad research interests include ligand design, synthetic inorganic and organometallic chemistry, small molecule activations, luminescent materials, and catalysis.sp 2 hybridized in dafo. 14Although dafH derivatives were considered merely as bpy analogues in the late 1970s, they have recently gained more attention in many areas.Ono 15 and Wong 16 previously reviewed the synthesis and coordination chemistry of several dafH derivatives.The purpose of this perspective review is to highlight the versatile nature of dafH derivatives as ligands, and also to detail some of the recent advances made using this ligand family.

II.1. Coordination chemistry of the parent dafH and daf − ligands
Although being conventionally viewed as a bpy derivative, the dafH ligand can also be viewed as two pyridine rings fused onto a central cyclopentadiene (CpH) ring in a syn fashion.The methylene group of dafH is acidic, analogously the pK a of CpH is 18, and that of fluorene is 22.6 in DMSO. 18The methylene linker of dafH may be deprotonated to form the monoanionic 4,5-diazafluorenide (daf − ) (Scheme 2A).
Ambidentate ligands containing multiple potential coordination sites of different nature can be used to construct linkage isomers, homo-and heteromultimetallic complexes, and complex molecular architectures through coordinationdriven self-assembly.Several 4,5-diazafluorene derivatives possess multiple coordination sites, especially those derivatives that are functionalized at the C9-position discussed later.The daf − ligand is potentially an ambidentate ligand with two types of metal binding sites, an N,N-chelate site, and the C-donors of the central cyclopentadienyl-like moiety.However in most examples daf − binds metals with its nitrogen donors, [19][20][21][22][23][24] without utilizing the C-donors.In addition the daf − ligand is able to form zwitterionic complexes when only two N-donors are bound to the metal centre with a −1 charge localized onto the ligand backbone.A recurring theme in this review is the reactivity of the ligand backbone in a zwitterionic setup.

II.2. Coordination chemistry of the aryl substituted daf Mes H and daf Mes
− ligands A 3,6-dimesityl substituted 4,5-diazafluorene ligand (daf Mes H) has been synthesized, which can also be deprotonated to give daf Mes

II.3. Coordination chemistry of the phosphine donor functionalized daf p H and daf p − ligands
Our group also installed a phosphine arm at the C9-position of 4,5-diazafluorene to give the daf p H ligand, which can also be deprotonated to form the daf p − ligand.Both daf p H and daf p − have been used to assemble head-to-tail macrocycles with {RuCp*} + (Scheme 3). 25 We further demonstrated the transfer of daf p − from [Cu(IPr)(daf p )] n (n = 1 or 2) to either Rh(I) or Au(I) resulted in macrocyclic complexes (Scheme 6). 28There are a few benefits of constructing these macrocyclic Rh(I) and Au(I) complexes through ligand transfer from a {Cu(IPr)} + fragment, compared to the conventional synthesis by directly reacting the daf p − salt with metal chloride starting materials.The benefits include improved yields, shortened reaction time, and simplified isolation of the product as the soluble [Cu(IPr)Cl] byproduct can easily be removed by filtration.In addition, daf p H and daf p − are also ambidentate ligands with phosphine and N-donor coordination sites, while daf p − can also anchor a metal in the P,C-coordination site.The daf p − ligand displayed several coordination modes, where the diazafluorenyl moiety coordinates through one or both N-donors and the P-donor with or without the participation of the C-donor of the cyclopentadienyl-like moiety.

II.7. Coordination polymers containing dafH derivatives
Recently metal-organic frameworks (MOFs) have received substantial attention due to the wide variety of potential architectures arising from different metal-ligand combinations, and the possibility of creating materials with intriguing applications.A variety of MOFs with different architectures were constructed from Zn(II) ions, a variety of aromatic polycarboxylic acid ligands and a 4,5-diazafluoren-9-oxime ligand. 98he polynuclear Zn secondary building units (SBUs) comprising the various MOFs were modulated by the 4,5-diazafluoren-9-oxime ligand, which can coordinate to Zn either in a chelating or monodentate fashion. 98zide containing MOFs can potentially be used as molecule-based magnets.A three-fold interpenetrating MOF of the general formula [(Mn-μ 1,3 -N 3 -μ 1,1 -N 3 ) 3 (L 4 )] where L 4 is the bis-(bidentate) Schiff base ligand 4,5-diazafluoren-9-one azine exhibited spin-canted long-range ferromagnetic ordering. 99he L 4 ligand served as the long link in the 3D structure while the chain of Mn-μ 1,3 -N 3 -μ 1,1 -N 3 served as the SBU. 99 have been synthesized with azido ligands bridging the four M 2+ ions, and the dafo coordinated in a chelating fashion to the metal vertices of the clusters, where the metal can be Co, 43,100 Mn, 43 Cd, 91 or Cu. 49 luminescent Ag(I) one-dimensional polymer chain of the generic formula [Ag 2 (L 5 ) 2 (ClO 4 ) 2 ] n where L 5 = 4,5-diaza-9,9′spirobifluorene was structurally characterized.101 Both of the L 5 ligands adopt a bis-monodentate bridging mode between the two crystallographically independent Ag(I) centres where the Ag-Ag distance is 2.776(1) Å, and one of ClO 4 − ligands also bridges two adjacent Ag centres to construct the polymer chain (Fig. 3).101 The fullerene C 60 with four peripheral malonate groups can be functionalized with two 4,5-diazafluorene moieties in the trans-1 positions, since 4,5-diazafluorene is planar the N,Nchelates can be situated 180°relative to each other.102 The reaction between AgOTf and this large ditopic bis(4,5-diazafluorene)tetrakis(malonate) substituted fullerene ligand (L 6 ) gives a 1-D coordination polymer [Ag 2 (L 6 )(OTf ) 2 ] n where each Ag(I) centre is four-coordinate and is bound to the N,N-chelate of one of the 4,5-diazafluorene moieties, an O-donor from the − OTf ligand, and a C-donor from a neighboring C 60 cage in an η 1 -fashion (Fig. 4).102 In the solid state of this fullerene-based coordination polymer the two antiparallel 4,5-diazafluorene moieties from two neighboring L 6 ligands engage in face-toface π-π interactions.102 The choice of Ag(I) precursor is crucial for coordination polymer formation.For example, if AgBF 4 is used instead, a non-polymeric salt [((L 6 )(Ag(toluene) 2 )]-(BF 4 ) 2 •CH 2 Cl 2 forms, where each Ag(I) centre is coordinated to the N,N-chelate of the 4,5-diazafluorene moieties and is bound to one η 1 -toluene and one η 2 -toluene.102 A simple coordination polymer of [Cd(dafo)(NCS) 2 ] n can be prepared where each Cd centre adopts a distorted octahedral geometry and the ambidentate NCS − ligands bridge adjacent Cd centres through both the N and S termini. 103 Jng, Lee, and co-workers demonstrated the use of a heteroditopic bis(4,5-diazafluorenylimino)dibenzo [18]crown-6 ether based ligand which could be used to synthesize a coordination polymer gel in the presence of Zn 2+ and Cs + ions.104 The Zn 2+ ion is bound by two 4,5-diazafluorenylimino moieties with a tetrahedral coordination geometry and one Cs + ion is sandwiched between two crown ether rings to give a highly crosslinked coordination polymer gel, where the rheological properties and microstructure are strongly dependent on the presence and concentration of Cs + ions.104 There are several examples in the literature regarding inorganic-organic hybrid polyoxometallates which contain the Keggin-type cluster anions and metal-dafo complex cations (typically Cu, and in a few examples Ag or Cd).50,51,57,59,60,90,97,105 The metal-dafo cation within these hybrid compounds seems to direct the assembly of other supramolecular interactions within the solid state.The coordination geometry of the metal, how the Keggin cluster is coordinated to the cation, and even the number of metals can play a role.For example helical assemblies can form when dinuclear [Cu 2 (dafo) 2 (H 2 O)] 2+ cations are used as a hinge-like motif to link Keggin clusters together to form a threedimensional framework.59,60 III.Applications of 4,5-diazafluorene derivatives

III.1. As actor ligands in reactivity chemistry
Most of inorganic and organometallic chemistry is dominated by metal centred reactivity where the ligand is a spectator, hence the term "spectator ligand".There are emerging examples of ligand-centred, or metal-ligand cooperative reactivity where reactions with incoming substrates occur at the "actor ligands".7][108][109][110][111][112][113] There are several examples where 4,5-diazafluorene derivatives behave as actor ligands, where the majority of the ligand-based reactions occur at the reactive C9.
Rillema and co-workers have uncovered divergent chemical behaviour for coordinated dafo ligand and free dafo in reactions with nucleophiles. 70,71Free dafo reacts with 2-aminopyridine in ethanol to give a Schiff-base product (Scheme 10A), however under the same reaction conditions the coordinated dafo ligand of [Ru(bpy) 2 (dafo)] 2+ reacts with 2-aminopyridine to give a ring-opened product possessing a coordinated esterified 2,2′-bipyridine ligand (Scheme 10B). 71In another example no reaction occurs between free dafo and DBU in wet dichloromethane (Scheme 10A), while coordinated dafo in [Ru-(bpy) 2 (dafo)] 2+ reacts with DBU to give another ring-opened product (Scheme 10B). 70The spectator metal centre plays a major role in altering the reaction pathways of dafo with nucleophiles; the driving force to form a coordinated 2,2′bipyridine ligand from a coordinated dafo is the release of coordination-induced ring strain and the formation of shorter, stronger Ru-N bonds. 70,71There are two potential nucleophilic Fig. 3 Coordination polymer constructed from Ag(I) and 4,5-diaza-9,9'-spirobifluorene (L 5 ). 101g. 4 A portion of the solid-state structure of the one-dimensional coordination polymer [((L 6 )(AgOTf ) 2 )•5(toluene)] n where L 6 is a bis(4,5diazafluorene)tetrakis(malonate) substituted fullerene, toluene solvent molecules and H-atoms omitted for clarity.Ag: pink, S: yellow, F: green, O: red, N: blue, C: grey. 102ites in 2-aminopyridine, the amine and the pyridyl ring nitrogen atoms; the spectator metal ion increases the electron density on the carbonyl carbon atom of dafo thus hindering the reaction with the amine nitrogen and the formation of the Schiff-base product. 71Free dafo does not react with ethylene glycol, meanwhile coordinated dafo of [Ru(bpy) n (dafo) m ] 2+ (n = 1 and m = 2, or, n = 0, and m = 3) reacts to give the corresponding ketal ligand coordinated to Ru(II) which is resistant to hydrolysis with HCl (aq)a highly unusual feature compared with most organic ketals. 72Siemeling and co-workers synthesized the sandwich complex [Cp*Co(η 4 -dafo)], where the dafo ligand is coordinated through the π system leaving the [N,N]-chelate vacant. 45The {Cp*Co} fragment coordinates η 2 to each of the two six-membered dearomatized rings of dafo; this results in cyclopentadienone-like behaviour where the nucleophilicity of the oxygen atom is increased substantially versus free dafo. 45The nucleophilic, coordinated, cyclopentadienonelike dafo of [Cp*Co(η 4 -dafo)] reacts with electrophiles such as acetyl chloride to give the O-acylated cobaltocenium species (Scheme 10C); in contrast free dafo does not react with acetyl chloride (Scheme 10A). 45ur group discovered a surprising example of ligand-based reactivity: free 4,5-diazafluorene is air-stable but the coordinated 4,5-diazafluorene ligand in [Ru(dafH)(PPh 3 ) 2 (Cl) 2 ] can selectively undergo an aerobic oxidation reaction giving a coordinated dafo ligand (Scheme 11). 66The selectivity of this ligand-based oxidation reaction is surprising since the typically oxygen-sensitive phosphine ligands are left intact.
Our group also previously demonstrated an interesting example of metal-ligand cooperativity.The Ru(II) 4,5-diazafluorenide complex, [Ru(daf )(PPh 3 ) 2 (H)(N 2 )], can be synthesized where the daf − ligand possesses a central negatively charged cyclopentadienyl like moiety that remains uncoordinated and thus has unquenched basicity (Scheme 12). 21u(daf )(PPh 3 ) 2 (H)(N 2 )] reversibly splits dihydrogen over a long-range, between the metal centre and the backbone carbanion, at a distance of ∼5 Å, to yield complex [Ru(dafH)-(PPh 3 ) 2 (H) 2 ].21 The π-system of the diazafluorenide ligand is disrupted and restored during the forward and backward reactions, respectively.It is also worth noting that the reversible synthesis of a metal-dinitrogen complex via the metal-hydride route, a route which circumvents the need for harsh reducing agents, is quite unique.114 The formation of H 2 from a coordinated 4,5-diazafluorene ligand giving a coordinated 4,5-diazafluorenyl ligand has been noted by other groups.Mach and co-workers observed the evolution of H 2 when 4,5-diazafluorene was added to [Cp* 2 Ti-(Me 3 SiCCSiMe 3 )] which gave the paramagnetic [Cp* 2 Ti(III) (daf )] adduct (Scheme 13A).23 Andersen and co-workers found that the stabilized biradical [Cp* 2 Yb(dafH)] adduct slowly eliminated H 2 to give [Cp* 2 Yb(daf )]; in addition, the dinuclear Yb complex prepared with the 9,9′-bis-4,5-diaza-9H-fluorene ligand also thermally produced [Cp* 2 Yb(daf )] (Scheme 13B).24 The mechanism for dihydrogen formation is proposed on the basis of kinetic and labelling experiments to involve the dinuclear Yb complex as an intermediate.24 The oxidation states of Yb were described as being intermediate between +2 and +3 for both [Cp* 2 Yb(dafH)] and [Cp* 2 Yb(bpy)] with an equilibrium between the at least two low-lying open-shell singlet states.24 Andersen and co-workers also performed calculations on the 4,5-diazafluorene ligand in an attempt to understand how even though [Cp* 2 Yb(dafH)] and [Cp* 2 Yb(bpy)] have multiconfigurational ground states, they differ in reactivity where [Cp* 2 Yb(dafH)] eliminates H 2 , and [Cp* 2 Yb(bpy)] does not.24 The dafH ligand has unpaired spin density distributed in various p π orbitals on the nitrogen and carbon atoms.24 The LUMO+1 of 2b 1 symmetry has unpaired spin density at the 9-position carbon, this orbital is possibly responsible for the chemistry observed.24 The unpaired spin density at the 9-position aids in the cleavage of the C-H allowing the formation of H 2 and a C-C bond to give the dinuclear Yb complex.
Our group also investigated the reactivity of the zwitterionic Ru(II) diazafluorenide complex [Ru(daf )(PPh 3 ) 2 (H)(N 2 )] toward CO 2 and uncovered an interesting example of ligand-based reactivity. 115At room temperature [Ru(daf )(PPh 3 ) 2 (H)(N 2 )] selectively and reversibly undergoes a formal insertion of CO 2 into a remote ligand C-H bond to generate a monoanionic 4,5diazafluorenyl-9-carboxylic acid ligand (dafCO 2 H − ) on Ru(II) (Scheme 12). 115The activation of CO 2 in our system occurs at the ligand backbone remote from the metal centre where the metal's role is to adjust the nucleophilicity of the ligand-based carbanion, the acidity of the C-H bond involved in proton migration, as well as the strength of the newly formed C-C bond.Given the unusual situation of having an actor ligand and a spectator metal centre, a variety of spectator metal centres were used to tune the reactivity and electronics of the actor daf − ligand for tandem CO 2 and C-H activation. 22Since Rh(III) is isoelectronic with Ru(II), the complex [Rh(daf )-(PPh 3 ) 2 (H) 2 ] also reacts with CO 2 in an analogous way where CO 2 reversibly inserts into the C-H bond of the ligand backbone (Scheme 14). 22n contrast, when the more electron-rich Rh(I) complex [Rh-(daf )(PPh 3 ) 2 ] is placed under CO 2 , a dinuclear Rh(I) complex, [(Rh(PPh 3 ) 2 ) 2 (dafCO 2 )], is formed where the two Rh(I) centres are bridged by a dianionic 4,5-diazafluorenyl-9-carboxylate ligand (dafCO 2 2− ), along with the formation of free dafH (Scheme 14). 22The result of tuning daf − with a more electron rich metal centre is the increased basicity of the ligand-based carbanion which can deprotonate the carboxylic acid initially formed from CO 2 insertion into the ligand C-H bond.This proton transfer gives [Rh(dafH)(PPh 3 ) 2 ] + and [Rh(dafCO 2 )-(PPh 3 ) 2 ] − ; the carboxylate of the latter replaces the dafH ligand of the former to yield the dinuclear product. 22As a result CO 2 is trapped by the second metal centre, and also there is no proton on O that can engage in proton transfer necessary for the decarboxylation. 22The dinuclear [(Rh(PPh 3 ) 2 ) 2 (dafCO 2 )] complex was reacted with H 2 to attempt ligand-based CO 2 reduction. 116A series of stepwise stoichiometric reactions with H 2 , NMR experiments at low temperatures with added PPh 3 or CO 2 , along with 13 C-labelling experiments were conducted in an attempt to gain some mechanistic insight. 116Upon the addition of a CO 2 and H 2 gas mixture to [(Rh(PPh 3 ) 2 ) 2 -(dafCO 2 )], a mixture of the carboxylated Rh(III) complex [Rh-(dafCO 2 H)(PPh 3 ) 2 (H) 2 ] and [Rh(PPh 3 ) 2 (H) 2 (κ 2 -HCO 2 )] results (Scheme 14). 116t is worth noting that the carboxylic acid intermediate in the reaction between [Rh(daf )(PPh 3 ) 2 ] and CO 2 could not be isolated or even observed in NMR experiments, presumably because the highly basic carbanion in [Rh(daf )(PPh 3 ) 2 ] effected by the electron rich Rh(I) centre deprotonates the carboxylic acid to trigger the formation of the final product too quickly before the concentration of the carboxylic acid intermediate could build up.In order to observe and isolate both the kinetic and thermodynamic products of CO 2 reaction, the slightly less electron donating Cu(I) was used as the spectator metal centre. 22When CO 2 is added to [Cu(daf )(IPr)] in toluene the kinetic product [Cu(dafCO 2 H)(IPr)] precipitates from solution (Scheme 15). 22Since [Cu(dafCO 2 H)(IPr)] and [Cu(daf )-(IPr)] are both soluble in DMSO, the reaction between these two species is readily observable when DMSO is used as the solvent for both the carboxylation of [Cu(daf )(IPr)] with CO 2 and the decarboxylation of [Cu(dafCO 2 H)(IPr)] under an N 2 atmosphere.In both cases, a dinuclear Cu(I) complex [(Cu(IPr)) 2 (dafCO 2 )] and dafH are obtained as thermodynamic products (Scheme 15). 22In both the Cu(I) and Rh(I) cases, the metal 4,5-diazafluorenyl and the metal 4,5-diazafluorenyl-9- Scheme 14 Chemistry of Rh 4,5-diazafluorenyl complexes with CO 2 and H 2 . 20,22,116arboxylic acid complexes react with each other in solution, in contrast to the less electron rich Ru(II) and Rh(III) systems.

III.2. 4,5-Diazafluorene derivatives as ligands in catalysis
Nitrogen-donor ligands have been used extensively in oxidative aerobic organic transformations, especially given their robustness versus traditional phosphine ligands under oxidizing reaction conditions.Stahl and co-workers initially explored the use of 4,5-diazafluorene derivatives in oxidative organic reactions where O 2 is the oxidant. 76A variety of nitrogen-donor ligands were screened in the Pd-catalyzed aerobic allylic acetoxylation of allylbenzene; most of the ligands screened gave low yields of the cinnamyl acetate product (Table 1). 74Conversely, when 9,9-dimethyl-4,5-diazafluorene (Me 2 daf ) was used as the ligand, a 50% yield of cinnamyl acetate was obtained, the yield was further improved to 81% when dafo was used as the ligand. 74The structures of 4,5-diazafluorene derivatives seem to have a large impact on the catalytic results obtained relative to the other nitrogen-donor ligands tested.Both the ability of dafo to withdraw electron density through π back-bonding and the unique ligand bite angle may play a role in catalysis.Recently Stahl reported the mechanistic investigations where they suggest that the dafo ligand promotes the C-O reductive elimination, but further studies are needed. 77Stahl and coworkers have also demonstrated the use of 4,5-diazafluorene derivatives as ancillary ligands in the aerobic Pd-catalyzed cross-coupling of indoles with benzene. 75The regioselectivity for arylation at the C2-vs.the C3-position of the indole compound was dramatically affected by the identity of 4,5-diazafluorene derivative and the anionic ligand used (Scheme 16). 75tahl, 78 along with Zhao and Huang, 80 independently and simultaneously reported the use of the dafo ligand in Pd-catalyzed aerobic dehydrogenation to form α,β-unsaturated aldehydes, ketones, esters, and azobenzenes (Scheme 17).Typically enones and other α,β-unsaturated carbonyl compounds are prepared in stepwise protocols.The aerobic Pd-dafo catalyzed reaction is a much more efficient alternative.
Recently it has been shown that dafo is an effective ligand for the Pd-catalyzed aerobic dehydrogenative Heck reaction to couple furans and thiophenes with cinnamic acid and stilbene derivatives. 85In addition a variety of alkenes could be coupled with ferrocene using a Pd dafo catalyst in an aerobic dehydrogenative Heck reaction (Scheme 18). 83A combination of kinetics, competition and ESI-MS (to characterize catalytic intermediates) experiments suggest that dafo plays a role at each stage of the catalytic cycle, 85 i.e., the dafo influences C-H bond activation, insertion of alkenes, the stereo-selective step, and the regeneration of the catalyst with O 2 . 85heme 15 Reaction of [Cu(daf )(IPr)] with CO 2 which gives [Cu-(dafCO 2 H)(IPr)] as the kinetic product and a mixture of [(Cu(IPr)) 2 -(dafCO 2 )] and dafH as thermodynamic products. 22ble 1 Nitrogen-based ligand screen in the Pd-catalyzed aerobic allylic acetoxylation of allylbenzene Scheme 16 Pd-catalyzed aerobic coupling of indoles with benzene using 4,5-diazafluorene derivatives as ligands. 75heme 17 Pd-dafo-catalyzed aerobic dehydrogenation to form double bonds. 78,80lsevier and co-workers have looked at the influence of various nitrogen chelates on Pd catalyzed C-C bond formation reactions. 79Zerovalent mono and binuclear palladium and platinum bis(quinone) complexes of dafH and dafo have been prepared, where the 4,5-diazafluorene derivative either acts as a chelate, monodentate, or bridging ligand. 17,81,82In the reaction of cinnamyl chloride with benzyl Grignard, the regioselectivity when palladium complexes of 4,5-diazafluorene derivatives are used is extremely high for substitution at the less substituted allylic carbon with less than 5% of the homocoupling product, a sharp contrast to when phosphine complexes are used. 79Rh(daf )(COD)] can catalyze the hydrogenation of olefins, however it is not as fast a catalyst as [Rh(PPh 3 ) 3 Cl] or [Ru(PPh 3 ) 3 Cl 2 ], and does not hydrogenate internal olefins.19 Wilkinson's catalyst [Rh(PPh 3 ) 3 Cl] dissociates in solution and produces hydride species under H 2 .In contrast, no hydride was observed when a solution of [Rh(daf )(COD)] was exposed to H 2 .
[Rh(dafH)(PPh 3 ) 2 (H) 2 ]Cl was also found to be a selective olefin hydrogenation catalyst, and can even hydrogenate internal olefins and substrates with pyridyl or carbonyl groups. 20The chloride counterion appears to play a role in catalysis, i.e., if the counterion is replaced with triflate the complex is inactive towards olefin hydrogenation.Exogenous chloride anions however appear to slow down the catalysis.
A conceptually interesting approach toward incorporating three functions into a single molecule, a luminescent chromophore with hole and electron transport capability was explored by the Wong group.In luminescent Ir(III) complex C (Fig. 5), the ligand L 12 has an electron-transporting 4,5-diazafluorene site, and hole-transporting -NPh 2 groups. 147Unfortunately the LEC device performance using this tri-functional Ir(III) complex was rather low.There is no 3 MLCT contribution to the lowest transition for the complex in the triplet state. 147uminescent lanthanide complexes exhibit very sharp emission bands.The design and tuning of sensitizing ligands that allow for efficient ligand-to-metal energy transfer is an area of particular interest.Luminescent lanthanide tris(β-diketonate) complexes of various 4,5-diazafluorene derivatives such as dafo, [94][95][96]149 9,9-diaryl substituted 4,5-diazafluorene, 150 4,5diazafluoren-9-imine derivatives, 151 and 4,5-diaza-9,9′-spirobifluorene 152 have been synthesized and characterized. Sevral of these lanthanide complexes exhibit NIR luminescence [94][95][96] photo-and electroluminescence, 150,152 and even triboluminescence.149 Green phosphorescent Re(I) complexes of various 4,5-diazafluorene derivatives have been investigated for their performance in OLEDs.[153][154][155][156] In comparing [Re(N^N)(CO) 3 Br] type complexes of dafH and a 9,9-di(ethoxyphenyl) substituted 4,5diazafluorene derivative, both complexes give approximately the same emission wavelength, however the complex with bulky groups at the C9 position alleviates to a large extent the self-quenching at high doping concentrations. 153A dinuclear Re(I) complex of 9,9′-bi-4,5-diazafluorenyl can serve as a highly efficient green phosphorescent emitter in OLEDs with a maximum luminance of 2026 cd m −2 and a peak current efficiency of 8.2 cd A −1 . 154Non-doped devices containing [(L 5 ) Re(CO) 3 Br] had outstanding performance with a further improved peak luminance of 8531 cd m −2 and maximum current efficiency of 16.8 cd A −1 .156 The phosphine Cu(I) complexes of 3,3′-methylen-4,4′-diphenyl-2,2′-biquinoline (mdpbq) exhibited red phosphorescence (Fig. 6).157 Complexes of the extremely rigid mdpbq ligand showed decent photoluminescence quantum yields in 20 wt% poly(methylmethacrylate) (PMMA) films, 0.56 and 0.43 for [Cu(mdpbq)(PPh 3 ) 2 ](BF 4 ) [Cu(mdpbq)(DPEphos)](BF 4 ), respectively.157 The OLEDs doped with these Cu(I) complexes gave a current efficiency up to 6.4 cd A −1 for a multi-layer device. 157Ag-dafo complexes with either carborane-based diphosphines, or simple classical diphosphine ligands of the general formula [Ag(dafo)(P-P)]OTf are luminescent.The emissions seem independent of the phosphine ligand or the coordination environment of the Ag + ion. 158 T free dafo emits at 537 nm (τ = 5 ns), while the luminescent Ag-dafo complexes emit across the blue to orange region of the spectrum with lifetimes also in the ns range, similar to free dafo. 158 Theafo ligand centred transitions seem to be responsible for the luminescence behaviour of [Ag(dafo)(P-P)]OTf and [Ag(dafo)(PPh 3 )(OTf )] complexes, where the electron density at the Ag centre tuned by the different phosphine ligands modified the emission energy.158 III.3.3. As rmonophores in non-linear optics.There is great interest in utilizing coordination complexes as harmonophores with second-order non-linear optical (NLO) properties since the metal centres offer additional means of tuning the electronic properties which affect the NLO response. The metl centres could not only allow for either metal-to-ligand charge transfer (MLCT) or ligand-to-metal charge transfer (LMCT) to take place, but also modulate the energy of intraligand charge transfer (ILCT) transitions.Moreover, the metal centre is tunable in terms of the identity of the metal element, oxidation state, and coordination sphere.Complexes of 4,5-diazafluorene derivatives have found use in this field.
Ir complexes of the form [Ir(L^L)( ppy) 2 ] n+ where the bidentate L^L is either dafH, 9-fulleriden-4,5-diazafluorene, or cyclometallated 9-fulleriden-4-azafluorene can be used as harmonophores with second-order NLO properties. 159The introduction of the fullerene moiety weakens the interaction between the cationic Ir(III) complex and the anion, which also leads to an overall increase in the NLO response with large and negative μβ 1.907 values (−600 to −2190 × 10 −48 esu). 159For the charge neutral complex with cyclometallated 9-fulleriden-4azafluorene ligand the μβ 1.907 value is lower than the cationic complexes. 159[147][148] Fig. 6 Structures of red phosphorescent Cu(I) complexes. 157u(II) complexes of the forms [Ru(N^N)(PPh 3 ) 2 Cl 2 ] and [Ru(N^N)(CO) 2 Cl 2 ] where the N^N chelate is either dafH or 9-fulleriden-4,5-diazafluorene, have also been examined.160 The greater absolute values of μβ 1.907 for complexes with the fullerene substituted ligand compared to those with unsubstituted dafH suggest the importance of a highly polarizable C 60 group.160 Ru(II) bipyridine complexes of 4,5-diazafluorene-9-imine derivatives with long alkyl chains off of the imine moiety have been incorporated as surfactants into Langmuir-Blodgett thin films and have activities for second order harmonic generation that are 2.6 to 3.6 times greater than that of the organic standard (E)-N-methyl-4-(2-(4-octadecyloxyphenyl)ethenyl)-pyridinium iodide.161 Similarly [Re(N^N)(CO) 3 Cl] complexes of similar 4,5-diazafluorene-9-imine derivatives can also be formed into stable Langmuir-Blodgett thin films, though a lower than expected measurement for the second-order harmonics NLO signal was observed.162 Zn(II) complexes of the highly conjugated push-pull 4,5diazafluoren-9-ylidene ligand family terminated with either an N,N-dibutylamino or an azulenyl moiety exhibit amongst the highest reported μβ 1.907 value for a Zn(II) complex.163 With respect to the free ligands, coordination to Lewis acidic Zn(II) enhances the μβ value presumably by red-shifting the ILCT transition.163 III.3.4.As luminescence sensors. Ru lypyridine complexes of 4,5-diazafluorene derivatives possessing a crown ether moiety attached to the ligand backbone have been synthesized.In most cases the binding with alkali metal cations was studied, where both the spectroscopic and electrochemical properties of the complex were perturbed by cation binding.131,135,137,164,165 Duan, Bai, and coworkers reported [Ru(L 13 )(bpy) 2 ] 2+ (where L 13 is 4,5-diazafluoren-9-one-2,4-dinitrophenylhydrazone) as a selective chromo-and fluorogenic dual responding fluoride sensor.166 Not only does the presence of F − enhance the luminescence intensity but also triggers a dramatic color change from yellow to magenta. Theu(II) complex could even be adsorbed onto paper to allow for the preparation of colorimetric testing strips for F − concentration in water.Interestingly, even when the concentration of F − is as low as 1 mg L −1 , the color change is visible to the naked eye.

III.4. Bioinorganic chemistry of 4,5-diazafluorene derivatives
Recently Ru(II) complexes [Ru(L 14 )(bpy) 2 ](PF 6 ) 2 (where L 14 is a 4,5-diazafluoren-9-imine ligand with various N-aryl groups) have been found to be effective topical antibiotics against the bacteria Staphylococcus aureus which is resistant to the antibiotic methicillin. 167In particular the derivative with a -OC 7 H 15 group attached to the N-aryl group, which is nontoxic toward human skin keratinocyte cells, exhibits strong microbicidal and bacterial growth inhibitory effects. 167One possible mechanism, for how these Ru(II) complexes are active against S. aureus, is through the generation of reactive oxygen species. 167o crystal structures of the bis-dafo Ag(I) nitrate complexes were obtained where the nitrate ion is either bound to the metal centre in the case of [Ag(dafo) 2 (NO 3 )], 168 or not in the case of [Ag(dafo) 2 ](NO 3 )•H 2 O. 88,168 These complexes exhibited broad spectrum antibacterial properties against six different clinically resistant strains of diabetic foot bacteria, and had a significantly lower minimum inhibitory concentration (MIC) compared with currently available commercial antibiotics. 168nitial studies also showed that the silver complexes could be loaded into hydrogels for possible incorporation into wound dressings. 168The in vitro linear dichroism studies also showed that both dafo and the silver complexes bind to calf thymus DNA, however the complexes showed significantly stronger binding to DNA compared with the free ligand. 168he free 9-diazo-4,5-diazafluorene can be photochemically activated to release dinitrogen as a byproduct and generate a triplet carbene. 169Matrix photolysis experiments of complex [Cu(9-diazo-4,5-diazafluorene) 2 (NO 3 ) 2 ] (Fig. 7) indicate the formation of a Cu(I)-L •+ species (S = 1/2), where the radical is primarily localized at the C9-position. 169Presumably the Cu(II) center is reduced by the triplet carbene formed via photolysis.In solution the nitrate ligands are labile, and other chemical or biological substrates such as DNA can coordinate to Cu.[171]

IV. Conclusions
Since the first report on the synthesis of dafH, the chemistry of dafH derivatives as ligands has evolved into a vibrant field.From the fundamental chemistry point of view, the parent compounds dafH and dafo can be derivatized in many ways to achieve the desired functions.In particular, the functionalization at the C9 position has been in the spotlight for generating new ligand series.Many of the ambidentate derivatives have been utilized to construct coordination polymers, selfassembled macrocycles, and heteromultimetallic complexes with great specificity.The reactive nature of the 9-position of dafH, daf − and dafo ligands gave rise to the actor-ligand behavior in the corresponding metal complexes, which distinguish dafH derivatives from bpy.From an applications point of view, dafH derivatives have found use in many areas.Their use as ancillary ligands in catalysis has been fruitful, especially in transformations performed under oxidizing conditions.The photochemistry and physics of metal complexes of 4,5-diazafluorene derivatives have been studied as emitters in solid state lighting, photosensitizers in DSSCs, and harmonophores in non-linear optics.Biological studies of 4,5-diazafluorene metal complexes have included DNA binding and cleavage, and antimicrobial properties.Further research may lead to new types of fundamental reactivities as well as applications.
(7) Å) since C9 is Vincent Annibale Vincent Annibale received his Hon.B.Sc. degree in 2008 from the University of Toronto at Mississauga under the supervision of Prof. Ulrich Fekl, and his Ph.D. degree from the University of Toronto in 2014 under the tutelage of Prof. Datong Song.He is currently an NSERC postdoctoral fellow in the group of Prof. Michael Fryzuk at the University of British Columbia.He is a recipient of several awards including NSERC postgraduate and postdoctoral scholarships, and Ontario Graduate scholarships among others.His current research interests include multidentate ligand designs for cooperative small molecule activation and catalysis.Datong Song Datong Song received his B.Sc. degree at Nankai University in 2000 and Ph.D. degree at Queen's University in 2003.After a one-year postdoctoral research at the University of Toronto, he did a postdoctoral research at MIT supported by an NSERC Postdoctoral Fellowship.