Reductive Coupling of a Diazoalkane Derivative Promoted by a Potassium Aluminyl and Elimination of Dinitrogen to Generate a Reactive Aluminium Ketimide

Abstract The reaction of 9‐diazo‐9H‐fluorene (fluN2) with the potassium aluminyl K[Al(NON)] ([NON]2−=[O(SiMe2NDipp)2]2−, Dipp=2,6‐iPr2C6H3) affords K[Al(NON)(κN 1,N 3‐{(fluN2)2})] (1). Structural analysis shows a near planar 1,4‐di(9H‐fluoren‐9‐ylidene)tetraazadiide ligand that chelates to the aluminium. The thermally induced elimination of dinitrogen from 1 affords the neutral aluminium ketimide complex, Al(NON)(N=flu)(THF) (2) and the 1,2‐di(9H‐fluoren‐9‐yl)diazene dianion as the potassium salt, [K2(THF)3][fluN=Nflu] (3). The reaction of 2 with N,N’‐diisopropylcarbodiimide (iPrN=C=NiPr) affords the aluminium guanidinate complex, Al(NON){N(iPr)C(N=CMe2)N(CHflu)} (4), showing a rare example of reactivity at a metal ketimide ligand. Density functional theory (DFT) calculations have been used to examine the bonding in the newly formed [(fluN2)2]2− ligand in 1 and the ketimide bonding in 2. The mechanism leading to the formation of 4 has also been studied using this technique.


Introduction
In recent years the application of low valent main group complexes for the activation of small molecules has emerged as a fundamental area of scientific research. [1]In many regards, the reactivity of these compounds may be considered as being equivalent to (or in some cases surpassing) that of the more traditionally studied transition metal complexes. [2]Aluminyl systems, comprised of a negatively charged Al(I) component charge balanced by a group 1 metal cation, are a family of highly reactive main-group compounds that have recently been established in this respect. [3]Our contribution to this area has focussed on the M[Al(NON)] system ([NON] 2À = [O(SiMe 2 NDipp) 2 ] 2À , M = Li, Na, K, Rb, Cs), [4] with most studies to date conducted with the potassium variant, K[Al(NON)].
The development of aluminyl compounds has facilitated access to several avenues of reactivity that were previously difficult to study for neutral low-valent aluminium compounds.One such area is the isolation of compounds containing Al=E multiple bonds, which although previously noted for neutral Al(I) systems, [5] were at the time largely derived from unanticipated reactions. [6]Taking the low valent aluminyl anion as a precursor to Al=E bond formation using an oxidative addition approach, our group and others have reported examples of Al=O, [7] Al=S, [8] Al=Se, [9] Al=Te, [10] and Al=NR [11] bonds.In this context, Kinjo and co-workers examined the reaction of the lithium salt of a cyclic (alkyl)(amino)aluminyl anion, Li[Al(bo2e-C Ar N Dipp )] ([bo2e-C Ar N Dipp ] 2À = [(bicyclo[2.2.2]oct-2-ene)C(Ar) 2 N-(Dipp)] 2À , Ar = 3,5-tBu 2 C 6 H 3 ) with the bulky diaryldiazoalkane, Ar 2 CN 2 as a route to Al=CR 2 systems (Figure 1a). [12]The initial product of the reaction (I) contained a three-membered diazoaluminium ring.Under mild thermal conditions this compound underwent the anticipated thermal elimination of N 2 to afford a product containing a short AlÀ C bond (II), shown by DFT analysis to contain an exocyclic Al=C group with significant πbonding character.Intrigued by this reaction, we wished to examine the reactivity of our K[Al(NON)] system towards diazoalkanes.
Previous work studying the reaction of main group systems with diazoalkanes showed several possible outcomes for the reaction, in addition to that affording II.For example, the reaction of a potassium gallyl with two equivalents of 9-diazo-9H-fluorene (fluN 2 ) in the presence of 18-crown-6 (18-c-6) proceeds in an ill-defined reaction to afford a low yield of [K( 18 1b). [13]The crystallographically characterized product showed loss of N 2 from one equivalent of fluN 2 and incorporation of 'H' at both the C-9 position of a flu-substituent and within a [N(H)N = flu] À ligand, believed to originate from the reaction solvent.In contrast, the reaction of redox active Al(II) species [Al( Me DAB Dipp )] 2 and [Al(BIAN Dipp )] 2 ( Me DAB Dipp = [C(Me)NDipp] 2 ; BIAN Dipp = 1,2-(Dipp-imino) 2 -acenaphthene) with trimethylsilyldiazomethane and diphenyldiazomethane, respectively, afforded bimetallic complexes in which the terminal nitrogen of the diazoalkane has inserted into the AlÀ Al bond to afford a Al 2 N 2 core (IV, Figure 1c). [14]Crystallographic analysis of bond lengths confirmed NÀ N single bonds and C=N double bonds within the [R 2 C=N-N] 2À groups, confirming reduction of the diazoalkane and formation of a μ-imide ligand.More recently, Stephan and co-workers demonstrated the reductive coupling of two equivalents of Ph 2 CN 2 using Jones' Mg(I) reagent, [Mg(BDI Mes )] 2 (V, Figure 1d.BDI Mes = [HC{C-(Me)NMes} 2 ] À ). [15]The resulting [R 2 CNNNNCR 2 ] 2À ligands bridge two Mg centres, with delocalisation focussed in the terminal NNCR 2 units.Thermal decomposition of these compounds proceeded with loss of N 2 to afford the corresponding μketimide compounds containing a central Mg 2 N 2 core (VI).
To complement our previous studies between organic azides and low-valent (indyl/aluminyl) [11a,16] or bimetallic (InÀ Zn) [17] systems, we herein report the reductive coupling of fluN 2 by the potassium aluminyl K[Al(NON)], a thermally induced extrusion of N 2 to afford a terminal ketimide ligand, and the reactivity of this complex with a carbodiimide.These results are supported by DFT calculations examining the bonding within the new complexes, and the mechanism of the reaction of the ketimide complex with carbodiimide.

Reductive coupling of 9-diazo-9H-fluorene (fluN 2 ) by the potassium aluminyl K[Al(NON)]
The addition of 1 equivalent of 9-diazo-9H-fluorene (fluN 2 ) to a solution of K[Al(NON)] in toluene initially afforded an intense blue solution that turned dark green upon standing at room temperature.Filtration and slow evaporation of the solvent yielded dark green crystals 1, shown by X-ray diffraction to be K[Al(NON){(fluN 2 ) 2 }] (Scheme 1).Attempted isolation of the blue intermediate was unsuccessful, and repeating the reaction with two equivalents of fluN 2 gave a higher yield of 1.We postulate that the blue intermediate is analogous to complex I, but that this initially formed diazo-aluminium ring reacts rapidly with a second equivalent of fluN 2 to afford 1. Compound 1 could also be isolated as the THF solvate [K(THF The intense colour of the compounds in solution (Figure 2a) prompted analysis by UV/vis spectroscopy.Solutions of 1 and 1•THF showed strong absorptions at λ max 657 nm (1, ɛ = 16138 L mol À 1 cm À 1 ; 1•THF, ɛ = 22321 L mol À 1 cm À 1 ), with a second strong absorption band at 381 nm (ɛ = 14108 L mol À 1 cm À 1 ) noted for 1•THF (Figures S2, S6, S11 in Supporting Information).In contrast, the major absorption observed in a solution of 1•crown was slightly red-shifted, with strong bands at 688 nm (ɛ = 26642 L mol À 1 cm À 1 ) and 667 nm (ɛ = 25913 L mol À 1 cm À 1 ).These data are similar to those obtained for a bimetallic iron product that was generated from the reductive coupling of diazoesters, which form a "burgundy" solution in hexane (λ max 516 nm, ɛ range: 16403 L mol À 1 cm À 1 to 22727 L mol À 1 cm À 1 ). [18]he observations suggest that the origin of the intense colour derives from transitions within the [Al(NON){(fluN 2 ) 2 }] À anions of 1, 1•THF and 1•crown, and is largely invariant of the solvation of the cation and any anion•••cation interactions.
We have used density functional theory (DFT) calculations to examine the bonding in the 'Al{(fluN 2 ) 2 }' group (see Supporting Information for full details).The structure of 1 was optimized with (1 DFT ) and without (1' DFT ) the potassium present   to determine the influence of the cation on the bonding.The results indicated that although the energy of the system (calculated in THF, ΔG THF ) increased upon loss of the potassium (ΔG THF 1' DFT = + 19.6 kcal mol À 1 relative to 1 DFT ) the bond parameters are not significantly perturbed (Figure S25); the data for 1' DFT is used in the following discussion.
In agreement with crystal structure data, the Wiberg bond indices (WBIs) confirm a lower bond order between the N4 and N5 atoms (WBI = 1.12) when compared to the remaining NÀ N (WBI = 1.34/1.36)and N-C flu (WBI = 1.31/1.42)bonds.This can also be inferred from NBO data in which two bonding orbitals are identified between N3/N4 and N5/N6, but only a single bonding orbital was calculated between N4/N5 (Table S5).Delocalisation from the π-system of the flu-substituents is suggested by the large energies calculated for the donation from a lone-pair of electrons on the carbon at the 9-position of the flu-substituent into an adjacent NÀ N anti-bonding orbital (652 and 667 kcal mol À 1 for C29 and C42, respectively).
The highest occupied molecular orbital (HOMO) is composed of π-symmetry lobes that are distributed throughout the [(fluN 2 ) 2 ] 2À ligand (Figure 4).These show good overlap in the N3-N4 bond, with an additional component of the orbital that is delocalized across the N5-N6-C42 portion.We note that the orbital is π-anti-bonding across the N4-N5 bond, consistent with the proposed single bond.
The WBIs for the 'Al(N = flu)' group confirm a high bond order between the nitrogen and the carbon of the flusubstituent (WBI = 1.79) and a relatively weak AlÀ N bond (WBI = 0.42) (Figure S26).The HOMO is largely composed of an antibonding π* interaction between the nitrogen and carbon of the ketimide group, with lobes that extend across the AlÀ N bond (Figure 6).The LUMO has similar π* anti-bonding character that is orthogonal to the HOMO.This orientation allows a better overlap with the π-system of the flu-ligand.NBO analysis confirms two bonding interactions between the nitrogen and carbon of the N = flu ligand, with no NBOs detected between the Al and N4.There are however some donor-acceptor interactions between the lone-pair of the nitrogen and an unoccupied orbital on aluminium, with energies of 51.8 and 42.4 kcal mol À 1 .
The formation of neutral products 2 is unusual in aluminyl chemistry, and the fate of the potassium was initially unknown.However, we were able to separate a small number of dark blue crystals 3 during the isolation of 2•DMAP that offer insight into the fate of the potassium and allow a balanced equation to be proposed (Scheme 3).Unfortunately, the low yield and high sensitivity prevented the isolation of pure samples of 3 for spectroscopic analysis.However, the X-ray diffraction confirmed the composition of 3 to be the 1,2-di(9H-fluoren-9-yl)diazene dianion, that was isolated as the THF solvated form, [K 2 (THF) 3 ][(fluN) 2 ] (Figure 7a).
The asymmetric unit consists of two 'fluN' half-molecules that are each located across an inversion centre that generates the corresponding symmetry related components.The resulting [(fluN) 2 ] 2À units are approximately planar and are present in a polymeric zig-zag arrangement with an internal angle of ~70°b etween adjacent planes.The cationic [K 2 (THF) 3 ] 2 + group, in which each potassium is bonded to a terminal THF with a bridging THF between the cations, is located between adjacent anions and is supported by K•••N and K•••π(arene) interactions (Figure 7b).

Reaction of Al(NON)(N = flu)(THF) with iPrN = C = NiPr
Ketimides [N=CR 2 ] À are generally considered as unreactive ligands and examples of their involvement in reactions are scarce.As such they have been employed to support a number of reactive metal centres, [30] and are often encountered as spectator ligands in catalytic reactions. [31]A notable exception demonstrating reactivity of a thorium-ketimide bond was reported in 2014 by Liddle and co-workers. [32]In this instance, insertion of the carbonyl bond of tert-butylisocyanate or 9anthracene carboxaldehyde into the Th-N ketimide was observed, affording the ureate and alkoxide complexes, respectively.To probe the reactivity of the ketimide bond of 2, we performed the reaction of an in situ generated solution with diisopropylcarbodiimide, iPrN=C = NiPr.
Based on the contrasting reactivity of metal-amide and -imide bonds, two available reaction pathways have been identified.The insertion of a carbodiimide R'N=C = NR' into an Al-NR 2 (amido) bond is a known route to monoanionic guanidinate complexes Al({NR'} 2 C-NR 2 )X 2 (XIV, Scheme 4a). [33]he mechanism has been studied computationally and shown to proceed in three steps: (i) coordination of a carbodiimide Natom to the Al centre; (ii) migration of the terminal amido group to the sp-hybridized C-atom; (iii) coordination of the remaining carbodiimide nitrogen to the metal. [34]In contrast, metal imides typically react with carbodiimides via a cycloaddition reaction with the terminal M=NR bond to form a dianionic guanidinate (XV, Scheme 4b).Although isolable aluminium imides represent a relatively new area of chemistry and their reaction with carbodiimides has not been reported, reactions with related unsaturated substrates (e. g.11a,c] Correspondingly, the reaction of germanimine and stannaimine complexes (containing the Ge=NR and Sn=NR groups, respectively) with carbodiimides affords dianionic guanidinate products. [35]The distribution of substituents in the resulting MN 2 C-metallacycle of the products may give an indication of the dominant mechanism.Thus, amides react to afford the product in which the carbodiimide NR' substituents are in a 1,3-pattern (XIV), whereas the expected product from cycloaddition places them in adjacent 1,2-positions (XV).We note that the formation of dianionic guanidinates XV in the reaction between 2 and a carbodiimide would necessitate additional reactivity to balance electronic charge at the metal.
An in situ generated solution of 2•THF (obtained by heating a solution of 1•THF to 80 °C in benzene for 18 h) was reacted with diisopropylcarbodiimide to afford dark green crystals 4 (Scheme 5).The 13 C{ 1 H} NMR spectrum of 4 showed a low field resonance at δ C 170.3, which is in the region expected for aluminium guanidinate complexes XIV or XV (δ C range: ~162-173), [33,36] and is consistent with reaction at the ketimide ligand.The 1 H NMR spectrum showed the characteristic pattern for a chelating NON-ligand at an aluminium with local C 1 symmetry (i.e. tetrahedral Al(NON)(X)(Y), where X ¼ 6 Y), inconsistent with the formation of a symmetrical guanidinate, XIV.Furthermore, a singlet resonance at δ H 0.59 (6H) with no corresponding septet that has a corresponding low field peak in the 13  The guanidinates form planar AlN 2 C metallacycles with acute bite angles 69.83 (17)°and 70.06 (16)°that are typical of such species. [33,36]The CÀ N bond distances in the guanidinate ligand span the range 1.333(6) Å to 1.389(6) Å.These data are consistent with delocalisation throughout the CN 3 core, with Δ CN values of 0.018 Å and 0.020 Å and Δ' CN of 0.046 Å and 0.035 Å suggesting concentration of π-electron density within the diazaallyl unit. [37]These data are consistent with other guanidinate anions containing exocyclic -N=CR 2 substituents. [38]he mechanism for the conversion of 2 DFT to the guanidinate product 4 DFT has been investigated using DFT (Figure 9).The initial approach of the carbodiimide stabilizes the threecoordinate complex 2 DFT by 9.   of the carbodiimide) was identified prior to bond rotation and coordination of the second nitrogen. [34]The resulting structure B undergoes a facile hydrogen transfer from an iPr group to the flu-substituent, with a minimal energy barrier of 0.8 kcal mol À 1 to afford C, at À 14.6 kcal mol À 1 , before bond rotations to give 4 at À 18.7 kcal mol À 1 .

Conclusions
The potassium aluminyl K[Al(NON)] reductively couples the diazoalkane derivative 9-diazo-9H-fluorene (fluN 2 ) to form the intensely coloured compound K[Al(NON){(fluN 2 ) 2 }] (1) containing a contiguous chain of four nitrogen atoms.This mode of reactivity with R 2 CN 2 derivatives differs from that of the previously studied aluminyl system, in which a diazo-aluminium ring was isolated and shown to be a precursor to Al=CR 2 species. [12]The current reaction most closely resembles that of a Mg(I) dimer with the diazomethane derivatives Ph 2 CN 2 and fluN 2 , in which each magnesium undergoes a single electron oxidation to form the analogous [R 2 CNNNNCR 2 ] 2À ligands that bridge between the two metals.X-ray crystallography showed that in our system the [(fluN 2 ) 2 ] 2À ligand chelates to aluminium through the N-atoms in the 1-and 3-positions of the chain, to generate a four-membered AlN 3 metallacycle.Bond length analysis and density functional theory calculations were consistent with delocalization across the terminal 'fluNN' groups, with a single NÀ N bond linking each of these units.
Various strategies to attenuate the electrostatic interactions between the potassium cation and the Al(NON){(fluN 2 ) 2 }] À anion were introduced, including the use of coordinating solvents (THF), 18-crown-6 and [2.2.2]cryptand.The resulting modifications had little effect on the solid-state bonding parameters within the [(fluN 2 ) 2 ] 2À ligands.Furthermore, only minor changes were noted in the UV/vis spectra of each complex in solution, suggesting that the origin of the intense colours for these species originate in the delocalised component of the anion.
Under thermal conditions, loss of N 2 was observed with the more readily identified product from this reaction being the neutral aluminium ketimide, Al(NON)(N = flu)(L) (3), which was isolated as the THF and DMAP derivatives.The fate of the potassium in this reaction was inferred from crystallographic analysis of a sample of highly sensitive crystals that were identified as the doubly reduced 1,2-di(9H-fluoren-9-yl)diazene dianion, which was isolated as the potassium salt, In contrast to most metal ketimide ligands that are inert towards onward reactivity and are frequently used as spectator ligand in coordination chemistry, the AlÀ N = flu group in 3 can be reactive.This was demonstrated by adding diisopropylcarbodiimide (iPrN=C = NiPr) to 3, which afforded a bidentate guanidinate ligand with -iPr and -(fluH) substituents at the Npositions, and a À N=CMe 2 bonded to the central C-atom.The mechanism leading to this compound was proposed to involve the initial cycloaddition of a C=N bond of the carbodiimide to the Al-N ketimide bond, followed by hydrogen transfer from an iPr group to the carbon in the 9-position of the flu-substituent.This was supported by DFT calculations that identified the cycloaddition as the rate determining step, with a low energy barrier for the hydrogen transfer.

Experimental Section
Full experimental details and characterisation data of the compounds are provided in the Supporting Information.

Synthesis of K[Al(NON){(fluN 2 ) 2 }] (1)
9-diazofluorene (76 mg, 0.40 mmol) was suspended in toluene (~5 mL) and added dropwise to a bright yellow solution of K[Al(NON)] (109 mg, 0.20 mmol) in toluene (~5 mL) to give an intense dark blue solution.The solution slowly turns dark green upon storage at room temperature (in the absence of THF).Single crystals were obtained via.slow evaporation of the toluene solution at room temperature.Yield 113 mg, 60 %.Accurate elemental analysis data could not be obtained. 1

Synthesis of Al(NON)(N = flu)(THF) (2•THF)
A solution of 1•THF (103 mg, 0.08 mmol) in benzene-d 6 was prepared in a glovebox and transferred to a J. Youngs NMR tube and sealed.The tube was removed from the glovebox and placed in a steel heating block.The solution was heated overnight (ca.18 h) at 80 °C to give a dark green solution. 1H NMR showed full consumption of the starting material.The NMR tube was taken into a glovebox and the tap removed slowly to relieve pressure (from nitrogen evolution).The contents of the NMR tube were transferred to a scintillation vial and the solvent removed in vacuo.The residue was dissolved in toluene (ca. 1 mL) to give a dark green solution and a few drops of THF were added.Crystals were obtained from a toluene solution of the reaction mixture stored at À 30 °C.Yield 53 mg, 83 %.Accurate elemental analysis data could not be obtained. 1

Synthesis of Al(NON)(N = flu)(DMAP) (2•DMAP)
A solution of 1•THF (56 mg, 0.04 mmol) in benzene-d 6 was prepared in a glovebox and transferred to a J. Youngs NMR tube and sealed.The tube was removed from the glovebox and placed in a steel heating block.The solution was heated overnight (ca.18 h) at 80 °C to give a dark green solution. 1H NMR spectrum showed full consumption of the starting material.The NMR tube was taken into a glovebox and the tap removed slowly to relieve pressure (from nitrogen evolution).DMAP (5 mg, 0.04 mmol) was added to the reaction mixture and the NMR tube sealed and mixed.The contents of the NMR tube were then transferred to a scintillation vial and the solvent removed in vacuo.The residue was dissolved in hexane (ca. 1 mL) to give a dark green solution.Crystals were obtained from a hexane solution of the reaction mixture stored at room temperature.Yield 30 mg.Analysis by NMR showed formation of an inseparable mixture of unidentified products, from which single crystals of Al(NON)(N = flu)(DMAP) were separated and analysed by X-ray diffraction.

Synthesis of [K 2 (THF) 3 ][(fluN) 2 ] (3)
9-diazofluorene (63 mg, 0.33 mmol) and K[Al(NON)] (91 mg, 0.17 mmol) were suspended in C 6 D 6 (~0.6 mL) to give an intense dark blue solution.The solution was heated overnight (ca.18 h) at 80 °C to give a dark green solution.The NMR tube was taken into a glovebox and the tap removed slowly to relieve pressure (from nitrogen evolution).DMAP (5 mg, 0.04 mmol) was added to the reaction mixture and the NMR tube sealed and mixed.The contents of the NMR tube were then transferred to a scintillation vial and the solvent removed in vacuo.The residue was washed with hexane (5 x 2 mL) and a toluene/THF mixture (1 mL, 1 : 1) was added.A small quantity of crystals of [K 2 (THF) 3 ][(fluN) 2 ] (3) that co-crystallized with 2•THF were separated and characterised by single-crystal X-ray diffraction.It was not possible to obtain any additional analysis on these crystals.

Figure 2 .
Figure 2. (a) Photograph of an NMR solution of 1•THF in C 6 D 6 .(b) Displacement ellipsoid plots of 1 (30 % ellipsoids; H-atoms omitted; C-atoms except at key positions represented as spheres).(c) View along the b-axis of 1.(d) View in the AlN 3 -plane of the anion from the crystal structure of 1•crypt.
Following work-up, a clean sample of Al(NON)(N = flu)(THF) (2•THF) was isolated via fractional crystallization as intense blue crystals.Alternatively, the corresponding 4-dimethylaminopyridine (DMAP) derivative (2•DMAP) was isolated by adding DMAP to an in situ formed solution of 2•THF.The 1 H NMR spectrum of 2•THF displays peaks for the NONligand and flu-groups, in addition to an equivalent of THF.At room temperature the signals are broad, with overlapping CHMe 2 resonances (δ H 4.06-3.73)and singlets at δ H 0.43 and 0.40 for the SiMe 2 groups.Notably, the low field resonances of the THF molecule appear as two peaks at δ H 3.87 (2H) and 3.50 (2H) consistent with inequivalent OCH 2 environments.Upon heating to 70 °C, the iPr methine and THF signals each coalesce to form a septet at δ H 3.90 and broad singlet δ H 3.69, respectively (Figure S15).These data indicate a crowded environment at the Al centre with restricted rotation of the THF and NDipp groups at room temperature.The molecular structures of 2•THF (Figure 5) and 2•DMAP (Figure S20) were verified crystallographically. Depending on the conditions of crystallization, 2•THF was isolated with either an included THF (2•THF{THF}, monoclinic, P2 1 ) or included benzene and toluene (2•THF{Ar}, triclinic, P � 1); bond data for both variants are included in Table 2 for comparison.In all cases the product exists as the neutral Al(NON)(N = flu)(L) complex (L = THF or DMAP).The four-coordinate aluminium centre adopts a distorted tetrahedral geometry supported by an N'N'-chelated NONligand, a terminal [N = flu] À ketimide group and a neutral Ldonor ligand.The AlÀ N3 bonds to the [N = flu] À ligand are in the range 1.774(4) Å to 1.7850(13) Å, which are comparable with other terminal Al-N=CR 2 ligands in Li[Al(N=CtBu 2 ) 4 ] (1.78(