Alkali Metal Derivatives of an Ortho -Phenylene Diamine

Treatment of the ortho -phenylene diamine C 6 H 4 -1,2-{N(H)Tripp} 2 ( 1 , PDAH 2, Tripp = 2,4,6-triisopropylphenyl) with two equivalents of MR (M = Li, R = Bu n ; M = Na or K, R = CH 2 C 6 H 5 ) afforded the dimetallated alkali metal ortho -phenylene diamide dianion complexes [(PDALi 2 )(THF) 3 ] ( 2 ), [{(PDANa 2 )(THF) 2 } 2 ] ( 3 ), and [{(PDAK 2 )(THF) 3 } 2 ] ( 4 ). In contrast, treatment of 2 with two equivalents of rubidium or cesium 2-ethylhexoxide, or treatment of 1 with two equivalents of MR (M = Rb or Cs, R = CH 2 C 6 H 5 ) did not afford the anticipated dialkali metal ortho -phenylene diamide dianion derivatives and instead formally afforded the monometallic ortho diiminosemiquinonate radical anion species [PDAM] (M = Rb, 5 ; M = Cs, 6 ). The structure of 2 is monomeric with one lithium coordinated to the two nitrogen centres and the other lithium η 4 coordinated to the diazabutadiene portion of the PDA scaffold. Similar structural cores are observed for 3 and 4 , except that the larger sodium and potassium ions give dimeric structures linked by multi-hapto interactions from the PDA backbone phenyl ring to an alkali metal centre. Complex 5 was not characterised in the solid state, but the structure of 6 reveals coordination of cesium ions to both PDA amide centres and multi-hapto interactions to

Herein, we report our endeavours in this area resulting in dilithium, -sodium, and -potassium PDA 2-derivatives, and the unexpected formation of monorubidium and -cesium PDA 1-• radical anions as confirmed by EPR spectroscopy.
Addition of two equivalents of n-butyl lithium to 1 in THF afforded [(PDALi 2 )(THF) 3 ] (2) in 94% yield as a free-flowing yellow-green powder after work-up (Scheme 1). The 1 H NMR spectrum of 2 is devoid of the characteristic singlet resonance at 5.3 ppm that corresponds to the amine protons of 1, therefore implying complete conversion of 1 to 2. In addition, the 1 H NMR spectrum of 2 exhibits two sets of ortho-isopropyl methyl resonances indicating hindered rotation that places one methyl group close to the PDA phenyl backbone whereas the other methyl group points in the opposite direction. The 7 Li NMR spectrum of 2 in C 6 D 6 exhibits a singlet resonance at 1.6 ppm, suggesting that the two lithium atoms are equivalent in solution. Hydrogen atoms and minor disorder components omitted for clarity.
Yellow-green crystals of 2 suitable for X-ray crystallographic analysis were isolated from THF at −30 ºC and the molecular structure of 2 is illustrated in Figure 1 with selected bond lengths and angles in Table 1. Compound 2 crystallises in the monoclinic space group Cc. The unit cell contains four molecules of 2 and eight THF solvent molecules. Compound 2 is monomeric in the solid state and the PDA ligand coordinates to the two lithium cations through its nitrogen atoms, generating two five-membered rings which are highly puckered about the N···N vector. The Li2 centre lies essentially in the plane of the PDA backbone, whilst Li1 lies out of the plane of the molecule and is η 4 -coordinated to the diazabutadiene portion of the PDA scaffold. The Li2-N1 and Li2-N2 bond distances of 1.942(4) and 1.982(4) Å, respectively, are significantly shorter than those for Li1-N1 and Li1-N2 [2.077(4) and 2.131(4) Å, respectively], which is consistent with the fact that Li1 is coordinated to the π-system out of the plane of the molecule [Li(1)···C(1) 2.492(4), Li(1)···C (6) 2.462(4) Å]. Despite this, all the Li-N bond lengths are in the normal range observed for Li-N (amido) bonds (1.89-2.16 Å), 20 and compare well to those observed in the N-Dipp analogue of 2. 12 The coordination sphere of each lithium ion is completed by THF molecules (two for Li1, one for Li2).
Thus, the coordination geometry around Li2 is best considered as a slightly distorted trigonal planar arrangement (Σ∠ = 358.7°), whereas Li1 adopts a distorted tetrahedral geometry. The structure of 2 is similar to dilithium N,N'-disilyl-ortho-phenylene diamides, 21,22 but different to the recently reported N-Dipp analogue 23 of 2 which can be attributed to the steric demands of Tripp versus Dipp.
After stirring 1 with two equivalents of benzylsodium for 24 hours at ambient temperature in THF, [{(PDANa 2 )(THF) 2 } 2 ] (3) was isolated from toluene at −30 o C as orange crystals (scheme 1). The 1 H NMR spectrum of 3 in d 8 -THF is relatively complex, suggesting that the dimeric formulation observed in the solid state is maintained in solution (vide infra). Five broad, poorly resolved and overlapping doublets corresponding to the isopropyl-CH 3 groups in a 1 : 2 : 1 : 1 : 1 integral ratio are observed, implying hindered rotation about the ortho-isopropyl groups. In contrast to the symmetric nature of 2 in solution, the presence of five doublets suggests that the disodium salt 10 is asymmetric, with the two sodium atoms being inequivalent in solution. This asymmetry can be attributed to the dimeric nature of the compound, essentially affording a 'front' and a 'back' methyl environment for the ortho-isopropyl groups, in addition to the 'top' and the 'bottom' methyl environments observed for 2. As a result of their para-positions, the para-isopropyl CH 3 groups would be expected to experience no steric restrictions, thus displaying free rotation affording a resonance twice as intense as those corresponding to the ortho-isopropyl CH 3 groups. Complex 3 exhibits extremely poor solubility, even in polar solvents once isolated. This poor solubility precluded variable temperature NMR experiments. Hydrogen atoms and minor disorder components omitted for clarity.
Compound 3 crystallises in moderate yield (24%) as orange blocks in the monoclinic space group P2 1 /c. The unit cell contains two molecules of 3. Four solvent THF molecules act to stabilise the complex by coordinating to two of the sodium atoms. Selected bond lengths and angles are listed in Table 2. Complex 3 crystallises as a centrosymmetric dimer, featuring two distinct sodium environments (Figure 2), in a structure that is similar to dimeric [{C 6 H 4 (NCH 2 Bu t ) 2 Li 2 (THF) 2 } 2 ]. 21 Both unique sodium atoms are coordinated to the nitrogen atoms of the PDA ligand, resulting in the generation of two five-membered chelate rings. In a similar manner to that observed in 2, one of the sodium atoms (Na1) lies within the plane of the PDA core, whereas Na2 lies out of the plane. The coordination geometries of the two sodium atoms are also very different. The coordination sphere of Na1 is supplemented by two coordinated THF solvent molecules, whereas that of Na2 is completed by five short Na···C (aryl)  . This results in the combination of two multi-hapto interactions, η 3 and η 2 , respectively. The presence of additional short Na···C (aryl) interactions in compound 3 compared to 2, can be attributed to the increase in ionic radius of sodium (0.98 Å) compared to lithium (0.78 Å). 4 The Na2-N1 and Na2-N2 bond distances of 2.4549(16) and 2.4153(16) Å, respectively, are slightly longer than those for Na1-N1 and Na1-N2 Analogously to 3, treatment of 1 with two equivalents of benzyl potassium afforded [{(PDAK 2 )(THF) 3 } 2 ] (4) in 67% yield as green crystals (Scheme 1). The NMR data for complex 4 (d 8 -THF) are largely comparable to those for the analogous sodium compound 3, and like 3 suggest that the dimeric structure observed in the solid state persists in solution. Again, five broad, overlapping doublet resonances (integral ratio = 1 : 1 : 1 : 2 : 1) are observed in the 1 H NMR spectrum, indicating restricted rotation of the ortho-isopropyl groups, but these are not as well resolved as for 3. Interestingly, however, the 1 H NMR spectrum of 4 is simpler than for 3, which may reflect the more symmetrical nature of 4, arising from the even number of multi-hapto interactions (η 6 ) occurring within the compound, compared to 3. Similarly to 3, 4 is very insoluble once isolated, even in polar solvents, which precluded variable temperature NMR experiments. Hydrogen atoms and minor disorder components omitted for clarity.
In gross terms the structure of 4 is analogous 3 and crystallises as a centrosymmetric dimer in the monoclinic space group P2 1 /c ( Figure 3 and Table 3). The two unique potassium ions are bound by the N1 and N2 atoms of the ligand, generating two five-membered chelate rings. Similarly to 2 and The heavier group 1 metals rubidium and cesium are predicted to be more labile than their lighter counterparts as a consequence of the fact that the Rb + and Cs + ions are larger, more electropositive and hence polarisable [Rb + (1.49 Å) and Cs + (1.65 Å); compared to Li + (0.78 Å), Na + (0.98 Å) and K + (1.33 Å)]. 33 As a result, the dirubidium and -cesium salts of PDA were postulated to be more reactive than their lighter counterparts (2)(3)(4). Heavy group 1 metal complexes are still generally rare yet have proven to be valuable ligand transfer reagents where the lighter alkali metal derivatives fail. 26,35,36 We thus identified the dirubidium and -cesium derivatives of 1 as desirable compounds to have in hand for the preparation of PDABBr. Reaction of the dilithium salt 2 with rubidium 2ethylhexoxide was anticipated to afford the analogous dirubidium salt by metathesis.
After stirring a mixture of 2 and two equivalents of rubidium 2-ethylhexoxide at ambient temperature for 24 hours, a viscous green oil was obtained after work-up (Scheme 1). Analysis of the green oil by 1 H and 13 C NMR spectrocopy proved to be uninformative due to the presence of broad resonances. However, 7 Li NMR spectroscopy suggested that no lithium-containing species remained in the reaction mixture. Despite exhaustive recrystallisation attempts, only polycrystalline material was obtained. Whilst the isolation of an oil could be an indication that a mixture of products was in fact formed, it could also indicate that the product does not contain an ideal metal size to ligand ratio for optimal crystal growth. 37 This would be a feasible explanation as the ionic radii of the group 1 metals vary over a 0.87 Å range, 33 and it is therefore quite possible that the larger elements in the series are too large to form the corresponding dimetallic salts. Based on the redox-active proclivity of PDA derivatives, 10,11 reports of paramagnetic diazabutadiene complexes, 38,39 and the significant broadening of the NMR resonances observed for the product, we postulated that a paramagnetic rubidium compound [PDARb] (5) was formed. We therefore attempted to prepare a dirubidium PDA derivative by a deprotonation strategy.
Accordingly, we treated 1 with two equivalents of benzyl rubidium, and after stirring the reaction mixture for 24 hours at room temperature, a viscous yellow-green oil was isolated after work-up (Scheme 1). Again, all attempts to grow X-ray quality crystals failed and NMR spectra were broad and uninformative, but compared well to those observed for the metathesis reaction. For reasons discussed previously, it is postulated that instead of preparing the anticipated dirubidium PDA complex, the monorubidium salt 5 is formed.
Analogously to 5, reaction of 2 with cesium 2-ethylhexoxide or treatment of 1 with two equivalents of benzyl cesium afforded an emerald green oil. Again, NMR spectroscopy proved uninformative due to the presence of broad resonances. It was therefore surmised that the monocesium salt [PDACs] (6) had been formed. Gratifyingly, after stirring the reaction mixture for 12 hours at ambient temperature in THF, colourless crystals of 6 were isolated in 57% yield from toluene at −30 o C which proved amenable to interrogation by X-ray crystallography.  35 The fact that 6 crystallises as a polymeric species can be attributed to the larger radius of cesium compared to the preceding group 1 metals (Cs + 1.65; Li + 0.78, Na + 0.98, K + 1.33 Å). 33 It is possible that the large, electropositive cesium centre is too large to enable two cesium centres to be accommodated by the PDA ligand. Instead, the increased space around the cesium centre means that η 6 -interactions are favoured in order to satisfy the coordination requirements of cesium, and polymerisation occurs.
The solid state structure of 6 supports the postulation of the formation of the analogous monorubidium salt 5. Similar poly-hapto bonding would be expected to occur in 5, affording a comparable polymeric species. In order to confirm that 5 and 6 are indeed radical anions as suggested by the structural and spectroscopic data, we probed 5 and 6 with EPR spectroscopy and DFT calculations.
The X-band EPR spectrum of 6 was initially recorded as a fluid solution in methyl-THF at ambient temperature. Hyperfine coupling was noted but the spectrum was weak and of insufficient resolution to allow the assignment of coupling. It is possible that this weak spectrum results from the propensity of 6 to form polymeric or oligomeric fragments in solution. Therefore, we added the tridentate ligand pentamethyldiethylenetriamine (PMDETA) to a solution of 6 in benzene in order to prevent such aggregation. This appeared to both increase the intensity and improve the resolution of the observed EPR spectrum, whilst retaining a similar linewidth (ca. 34 G) to that obtained without addition of PMDETA, suggesting that PMDETA acts to break up the polymeric chain into small, possibly monomeric units which are more amenable to study by EPR spectroscopy. Our best attempt to reproduce the experimental EPR spectrum by simulation was achieved using the parameters given in Figure 5. Simulations were improved, with respect to the number and position of lines, when the system was treated with asymmetric coupling to the nitrogen atoms and two of the hydrogen atoms ( Figure 5), however, reproduction of the experimental spectrum, with respect to the relative intensity of the constituent bands, was not obtained. Hence we present this as a tentative explanation of the coupling. No obvious 133 Cs coupling is observed in the spectrum. The spectral width of 34 G is relatively narrow, and this, along with a g value of 2.004, is indicative of an organic free radical.

Conclusions
A range of alkali metal PDA derivatives have been synthesised and isolated. As a consequence of the substantial range of ionic radii exhibited by the group 1 metals, a variety of structural arrangements are observed. The dilithium derivative adopts a monomeric structure, whereas the disodium and -potassium complexes adopt dimeric structures. In contrast, attempts to prepare the dirubidium and -cesium congeners resulted instead in the formation of monorubidium and -cesium radical anions. We are currently exploring the utility of the dilithium, -sodium, and -potassium salts in an improved synthesis of PDABBr.

General
All manipulations were carried out using standard Schlenk and glovebox techniques, under an atmosphere of dry nitrogen. Solvents were dried by passage through activated alumina towers and degassed before use. All solvents were stored over potassium mirrors, with the exception of ethers, which were stored over activated 4 Å molecular sieves. Deuterated solvents were distilled from potassium, degassed by three freeze-pump-thaw cycles and stored under nitrogen. 1 H, 13

Preparation of [{(PDAK 2 )(THF) 3 } 2 ] (4)
A solution of benzyl potassium (0.26 g, 2.0 mmol) in THF (20 ml)  Removal of volatiles in vacuo yielded a green oily powder. Yield = 0.26 g, 52%. Several attempts to access X-ray quality crystals of 6, utilising a variety of solvents, a range of temperatures and a number of methods (e.g slow diffusion), resulted only in the formation of microcrystalline material.
NMR data were broad and uninformative.

Density Functional Theory Calculations
Unrestricted geometry optimisations were performed on the radical anion component of 5 and 6 using coordinates derived from the experimental X-ray crystal structure of 6. No constraints were imposed on the structure during the geometry optimisation. Calculations were performed using the Amsterdam Density Functional (ADF) suite version 2012.01. 49,50 Slater type orbital (STO) triple-ζ-plus polarisation all-electron basis sets (from the ZORA/TZP database of the ADF suite) were employed. Scalar relativistic approaches were used within the ZORA Hamiltonian for the inclusion of relativistic effects and the local density approximation (LDA), with the correlation potential due to Vosko et al. 51 used in all of the calculations. Gradient corrections were performed using the functionals of Becke 52 and Perdew. 53 MOLEKEL 54 was used to prepare the three-dimensional plot of the electron density.