the field of utility organometallics is increasingly turning to more complex mixed - metal systems to deliver superior results , whether this be for metal  halogen exchange , alkylation , insertion , or the trapping of reactive intermediates . in theory
, mixed - metal reagents can offer more flexible , tunable solutions to any problem compared to more traditional monometallic alternatives due to the increased number of components which can be varied .
furthermore , the different metal centers and ligand sets can exhibit cooperativity ( or synergism ) to produce new reactivities inaccessible to the monometallic components .
hydrogen abstraction is an area in which the lure of such mixed - metal systems is proving to be irresistible .
the combination of an alkali metal with a range of different subordinates ( i.e. , metals that would be expected to be less reactive than the alkali metals - zn , mg , mn , cr , fe , cu , cd , al ) have been integrated with a diverse array of ligands ( alkyl , amido , alkoxy , halide ) and have been efficiently deployed in both ethereal and hydrocarbon media to overcome some of the great challenges facing deprotonation chemistry such as the incapability to metalate low brnsted acidity substrates , lack of compatibility with sensitive functionalities , low selectivity and the need to perform reactions under cryogenic conditions .
recently our own group has started to investigate the use of diamines in a bimetallic context , reporting in a communication the synthesis of the diazaethene zincate complex ( tmeda)li[(ipr)nch = chn(ipr)]zn(tbu ) 1 .
1,3-diazabutadiene ligands , from which diazaethene ligands are often prepared , are increasingly important within coordination chemistry due to their flexible coordination modes and redox properties .
the synthesis of this complex from the fully saturated ( ipr)nhch2ch2nh(ipr ) ( diisopropylethylenediamine ) 2 ( scheme 1 ) displayed a new mode of hydrogen activation previously unseen within zincate chemistry .
we now report a detailed mechanistic overview of this surprising transformation from both experimental and theoretical perspectives .
as reported previously , the cocomplexation of the monolithiated diamine li[(ipr)nch2ch2nh(ipr ) ] with ( tbu)2zn(tmeda ) at 0 c gives the expected product ( tmeda)li[(ipr)nch2ch2nh(ipr)]zn(tbu)23 ( scheme 1 ) . in a fashion similar to that of the two - step mechanism for monoamido zincates originally proposed by uchiyama et al .
, the amino n  h group of 3 is rapidly deprotonated by a tbu ligand to form the dimetalated diamido ( tmeda)li[(ipr)nch2ch2n(ipr)]zn(tbu ) 4 with the evolution of ibuh .
although diamido 4 has thus far not crystallized , it has been characterized by multinuclear nmr spectroscopy , and we have succeeded in the crystallographic identification of several analogous structures . using the homologous ( ipr)nhch2ch2ch2nh(ipr ) ( diisopropylpropylenediamine ) , the dimetalated diamido ( tmeda)li[(ipr)nch2ch2ch2n(ipr)]zn(tbu ) 5 ( scheme 2 and figure 1 ) could be crystallized from the corresponding reaction of li[(ipr)nch2ch2ch2nh(ipr ) ] with ( tbu)2zn(tmeda ) .
unfortunately , the crystal data are of insufficient quality to allow discussion of the geometric parameters of the structure , but the connectivity is definite .
zincate 5 contains an ncccnzn six - membered ring in a boat - type conformation .
the diamido ligand thus chelates the zn center while its two ipr arms are oriented out of the plane of the ring .
the ( tmeda)li cation is coordinated to the hull of the boat , anchored exclusively to the two secondary amido groups to produce a puckered linznn four - membered ring .
h nmr spectroscopic studies confirm the fully saturated nature of the propylene bridge with methylene resonances at 3.33 , 2.95 , and 1.93 ppm , while no characteristic diazaethene resonances were present in the 56 ppm region .
molecular structure of the diamido zincate ( tmeda)li[(ipr)nch2ch2ch2n(ipr)]zn(tbu ) 5 . hydrogen atoms and disordered tbu / tmeda components have been omitted for clarity .
on utilizing thf as a donor ligand in the absence of tmeda , in a repeat of the synthesis of 4 , the dimeric thf - solvated product { ( thf)li[(ipr)nch2ch2n(ipr)]zn(tbu)}26 was isolated and subsequently crystallographically characterized ( scheme 2 and figure 2 ) .
lithium zincate 6 has a centrosymmetric molecular structure consisting of two [ ( ipr)nch2ch2n(ipr)]zn(tbu ) complex anions , juxtaposed , and bridged by two ( thf)li cations to produce an eight - membered ( linznn)2 ring .
the cyclic conformation of 6 allows for the symmetrical coordination of each amido group to one lithium and one zinc center . both sets of metal centers occupy distorted trigonal planar geometries .
the ipr groups on the amido ligands lie syn with respect to the znnccn ring , while the nonplanar nature of the diamide ( n1c4c5n2 torsion angle of 50.0(2) ) as well as its n  c [ 1.471(3 )  ] and c  c [ 1.530(3 )  ] bond distances , which are comparable with those found in the ruthenium complex [ ( ipr)nch2ch2n(ipr)]ruh2(co)5 [ n  c , 1.48(1 )  ; c  c , 1.52(1 )  ] , establish that the ch2ch2 backbone remains saturated .
thermal ellipsoids are shown at the 50% probability level . selected bond lengths [  ] and angles [ deg ] : zn1n1 , 2.019(2 )
; zn1n2 , 1.986(2 ) ; zn1c9 , 2.019(9 ) ; li1n1 , 2.001(4 ) ; li1n2(a ) , 2.041(4 ) ; li1o1 , 2.004(4 ) ; n1c4 , 1.471(3 ) ; n2c5 , 1.470(3 ) ; c4c5 , 1.530(3 ) ; n1zn1n2 , 91.20(7 ) ; n1zn1c9 , 127.5(2 ) ; n2zn1c9 , 141.2(2 ) ; n1c4c5n2 , 50.0(2 ) ; n1li1n2(a ) , 127.0(2 ) ; n1li1o1 , 117.7(2 ) ; n3li1o1 , 114.9(2 ) . to gain mechanistic insight a detailed nmr spectroscopic study
was carried out on the reaction between the monometallic compounds li[(ipr)nch2ch2nh(ipr ) ] and ( tbu)2zn(tmeda ) .
a li nmr spectrum of isolated crystals of the cocomplexation product ( tmeda)li[(ipr)nch2ch2nh(ipr)]zn(tbu)23 in c6d6 solution reveals two major resonances at 0.37 ppm and 0.63 ppm , suggesting a mixture of products .
when this solution is gently warmed and reanalyzed , the resonance at 0.37 ppm is absent , intimating that the product associated with this low - frequency resonance has been fully consumed , and only the resonance at 0.63 ppm remains ( see figure s24 in supporting information ) .
this is consistent with the disappearing resonance at 0.37 ppm representing the bisalkyl zincate 3 which , via an intramolecular deprotonation and concomitant release of ibuh , forms the diamido complex 4 .
further support for this interpretation can be found when considering how close the signal assigned to complex 4 ( 0.63 ppm ) comes to that of the structurally analogous propylene derivative 5 ( 0.75 ppm , i.e. a difference of only 0.12 ppm ) .
when the reaction was repeated in situ at 0 c and an aliquot removed and analyzed by li nmr spectroscopy , two pertinent resonances at 2.39 ppm , and 1.71 ppm , as well as the resonance belonging to the previously identified intermediate 4 , are present ( see figure s24 in supporting information ) .
li dosy nmr experiments reveal that the intermediate responsible for the resonance at 1.71 ppm has a molecular weight similar to that of 4 ( see figure s25 in supporting information ) , presumably ruling out an aggregation process for the formation of this , as yet , unidentified species . comparison with the li nmr spectra of the diazaethene 1 in c6d6 reveals that the signal at 2.39 ppm is the result of a trace amount of this final product . due to the highly reactive , transient nature of the bis - alkylzincate 3 , its presence is not detected in this in situ nmr study .
another aliquot of the solution was removed after a 2-day stir at room temperature , but the recorded li nmr spectrum indicated no significant further compositional change had taken place .
aliquots were then removed after 5 min , 1 h , and 2 h reflux time , and the recorded spectra revealed a gradual transformation from the saturated intermediate 4 to the unsaturated final product 1 .
after 2 h reflux , decomposition of the reaction mixture is evidenced by the deposition of a black solid .
any further intermediates along the reaction pathway must be too short lived to be visible on the nmr time scale .
the next step in the reaction pathway 4  1 has been shown to likely proceed through a hydride intermediate as we successfully trapped a hydride by using the electrophilic ketone ( tbu)2co .
the hydride produced can then deprotonate the now allylic proton on the ethylene bridge to give the final product .
the system must then find a way of solubilizing and activating the produced lih , which is normally insoluble in hydrocarbon solvents . a more typical dehydrogenation pathway involving redox active transition metals , such as those reported by brookhart ,
can be ruled out as there are no redox - active metals present in our alkali metal 
instead , the reaction could be envisaged to proceed through a ligand - sited oxidation followed by a reduction ( scheme 3 ) .
this reaction can thus be said to exploit a unique three - way cooperation , where the chemical properties of the two metals and the ethylenediamide ligand combine to allow a type of chemistry otherwise unavailable . in order to explore the possible pathways by which this reaction could proceed we carried out detailed mechanistic investigations using density functional theory ( dft ,
see the supporting information for full details on the level of theory employed ) in order to explore the potential energy surface ( pes ) associated with the proposed mechanisms .
we initially inspected the structure and stability of the monomer species 4 at the dft level of theory . in agreement with the three - carbon bridge analogue
( 5 , figure 1 ) , 4 is found to be a stable minimum on the pes with the diamido ligand coordinated to the zn atom ( n  zn : 2.04  and 1.99  ) to form a five - membered ring ( nccnzn ) in a slightly twisted ( nccn torsion of 31.4 ) envelope conformation with the zn atom below the nccn plane , ( figure 3 ) . the ( tmeda)li cation is coordinated directly above the two n atoms of the five - membered ring by two essentially equivalent li 
based on the optimized structure of 4 two different mechanisms by which the proposed zinc hydride species could be formed are possible .
all h atoms , except those involved in the reaction , are omitted for clarity .
the h atom could be initially abstracted by the li cation to form a zn  h bond below the plane of the ring , or direct abstraction by the zn to form the zn  h bond above the ring plane .
optimisation of the two hydride intermediate structures revealed that both were stable minima on the pes but were formed in endothermic reactions relative to 4 ( figure 4 ) .
the intermediate with the h atom above the nccn plane ( inta ) is destabilized by 14.7 kcal / mol ( g ) relative to 4 , while the intermediate with the h atom below the plane ( intb ) is destabilized by 8.6 kcal / mol ( g ) , relative to 4 .
while both reactions are endothermic , the relative stabilities of the intermediates are not sufficiently different to determine which pathway is more favorable .
therefore , we characterized the transition states leading to the intermediate structures in order to determine whether there was a kinetic preference toward either species . the activation barrier
associated with the formation of inta is 23.2 kcal / mol ( g * ) , compared to a barrier of 31.8 kcal / mol ( g * ) for the formation of intb .
an activation barrier of 31.8 kcal / mol is still accessible under the experimental conditions employed in the reactivity of 4 ; thus , while inta is expected to be preferentially formed as an initial hydride intermediate in the formation of the diazaethene product ( 1 ) , we also considered intb as a potential intermediate from which the second hydrogen abstraction could occur .
abstraction of the second h atom from inta occurs with a barrier of 17.4 kcal / mol , relative to inta , ( g * ) in an exothermic reaction ( g = 13.2 kcal / mol ) to generate h2 and the final product 1 . however , despite exhaustive sampling , no transition state could be located that corresponded to the formation of h2 and 1 starting from intb .
therefore , the overall reaction 4  1 + h2 , which is essentially thermoneutral ( g = 1.5 kcal / mol , figure 4 ) , appears to preferentially occur via the hydride inta .
free energy ( g ) profile for the formation of 1 from 4 via a hydride intermediate .
the path in red corresponds to the formation of the hydride intermediate above the ring plane ( inta ) , while the path in blue corresponds to the formation of the hydride intermediate below the ring plane ( intb ) .
the relative stabilities of ts(4-inta ) and ts(4-intb ) ( figure 4 ) are dictated by the different roles that the li and zn centers play in the two different mechanisms . in both transition states , in order for the zn  h bond to be formed , one zn 
n bond must be broken such that the zn atom is able to accommodate the incoming h and the unsaturated n = c bond can form . within the ts(4-inta )
n bond breaking is facilitated by the formation of a stabilizing interaction between the n atom and the li .
this is evidenced by the rotation around the c  c bond [ increase in the dihedral angle from 31.4 in 4 ( figure 3 ) to 56.3 ( figure 5a ) ] , which decreases the li 
n distance from 2.02  ( figure 3 ) to 1.98  ( figure 5a ) and increases the zn 
n distance from 2.04  ( figure 3 ) to 3.39  ( figure 5a ) .
in contrast , the rotation around the c  c bond in ts(4-intb ) is not as great ( 34.1 , figure 5b ) .
in addition , in ts(4-intb ) the li atom is forced to migrate toward the second n atom in order to facilitate the transfer of the h atom beneath the plane of the ring .
n distance to 3.60  for the primary n ( figure 5b ) , and consequently , the breaking of the zn 
n bond length is only slightly increased from 2.04  in 4 ( figure 3 ) to 2.12  in ts(4-intb ) ( figure 5b ) .
thus , the formation of the hydride intermediate is truly the result of the synergic interplay between the zn ( stabilizing the formation of the zn  h bond ) and li ( stabilizing the breaking of the zn 
returning to experimental studies , a molecular hydride species [ ( tmeda)li]2[(ipr)nch2ch2n(ipr)]zn(tbu)h 7 has been isolated by using a variation of this mixed - metal approach with the ethylenediamide ligand , demonstrating that this system has the ability to encapsulate lih ( scheme 4 and figure 6 ) .
dilithium hydridozincate 7 was synthesized while attempting to generate  [ ( tmeda)li]2[(ipr)nch2ch2n(ipr)]zn(tbu)2  by the n 
h deprotonation of ( tmeda)li[(ipr)nch2ch2nh(ipr)]zn(tbu)23 with a molar equivalent of nbuli(tmeda ) ( scheme 4 ) .
[ ( tmeda)li]2[(ipr)nch2ch2n(ipr)]zn(tbu)2  facilitates the -hydride formation and elimination of ibuh to generate the zinc hydride species 7 .
dft calculations have confirmed that such a process [ 4 + ( tmeda)lih  7 ] would be exothermic ( g = 28.4 kcal / mol ) .
molecular hydride 7 can also be produced via the alternative reaction of the usually kinetically challenged base [ ( tmeda)li]2zn(tbu)4 with ( ipr)nhch2ch2nh(ipr ) with an improved yield of 65% relative to 35% .
molecular structure of the zincate hydride complex [ ( tmeda)li]2[(ipr)nch2ch2n(ipr)]zn(tbu)h 7 with hydrogen atoms ( except for the hydride ion ) omitted for clarity .
thermal ellipsoids are shown at the 50% probability level . selected bond lengths [  ] and angles [ deg ] : li1h100 , 1.96(2 ) ; li2h100 , 2.07(2 ) ; zn1h100 , 2.14(2 ) ; li1n2 , 1.979(4 ) ; li2n1 , 1.989(4 ) ; zn1n1 , 2.043(2 ) ; zn1n2 , 2.074(2 ) ; zn1c1 , 2.045(3 ) ; n1c9 , 1.481(3 ) ; n2c8 , 1.448(3 ) ; c8c9 , 1.523(4 ) ; n1zn1h100 , 92.8(6 ) ; n1zn1n2 , 89.66(8 ) ; n1zn1c1 , 133.1(1 ) ; n2zn1c1 , 125.19(9 ) ; n2zn1h100 , 92.8(5 ) ; h100zn1c1 , 113.0(6 ) ; n2li1n5 , 121.8(2 ) ; n2li1n6 , 129.6(2 ) ; n2li1h100 , 101.5(6 ) ; n5li1n6 , 85.7(2 ) ; n5li1h100 , 115.9(6 ) ; n6li1h100 , 101.6(6 ) .
mixed - metal zincate hydrides have repeatedly demonstrated their utility in organic synthesis as reagents for chemoselective , diastereoselective , and catalytic reductions .
highly soluble , stable , and well - defined zincate hydride species are thus highly desirable . despite this , however , structurally characterized alkali metal zincate hydrides are extremely rare . to date the only examples on the ccdb are the zinc  zinc bonded arzn(-h)(-na)znar ( ar = c6h3 - 2,6-[c6h3-(ipr)2]2 ) , the heterocubanes [ ( tbuoznh)4n(liotbuthf)n ] ( n = 03 ) , and the higher - order zincates na2[(et)2znh]2 and na3[(ipr)3zn(-h)zn(ipr)3 ] .
although metal  hydride complexes are notoriously insoluble , a reputation perpetuated by common hydride reagents such as lih and cah2 , the hydride carrier 7 is soluble in the nonpolar solvent hexane .
the structure of zinc hydride 7 bears a partial resemblance to that of the thf solvated dimer 6 , with a [ ( ipr)nch2ch2n(ipr)]zn(tbu ) complex anion having a lithium cation bound to each nitrogen atom . the li 
n amido bond distances in 7 ( average 1.984  ) are close to those in dimer 6 ( average 2.021  ) . likewise the zn 
n bond distances in hydride 7 [ 2.074(2 )  and 2.043(2 )  ] are only marginally longer than those in alkylamido 6 ( average 2.002  ) , owing to the increased number of anions bound to the zinc .
the tbu group has been pushed out of the plane of the amido ligand to allow the zinc center to enter a distorted tetrahedral geometry by binding to the hydride ligand . as in the lithium zincate 6 , the n 
c [ 1.481(3 )  and 1.448(3 )  ] and c  c [ 1.523(4 )  ] bond lengths in 7 are consistent with an unactivated ethylene bridge .
the trapped 3-hydride was successfully located and freely refined in the x - ray diffraction study .
it caps a li2zn triangle asymmetrically with unequal li  h bond distances [ 1.96(2 )  and 2.07(2 )  ] and a longer zn  h bond [ 2.14(2 )  ] .
the ccdb at the time of writing contains only 45 compounds exhibiting a zn  hydride bond . excluding an anomalously long contact within a zinc borohydride compound ( 2.409  ) , zn  hydride bond lengths range from 1.4092.167  with an average distance of 1.771  .
the zn  hydride contact in 7 is thus long - range  to the best of our knowledge , the longest zn hydride contact in a non - borohydride complex to date ( the value of 2.167  mentioned above also comes from a borohydride complex ) .
there are currently 147 compounds in the ccdb containing li hydride contacts with their lengths spanning the wide range 1.6072.802  .
the average distance ( 2.044  ) is comparable with those found in 7 . to the best of our knowledge
however , there have been various dilithio 3-hydride compounds reported with other metals such as ga , al , zr , w , sm , ta , and fe .
variable - concentration multinuclear ( h , c and li ) nmr spectroscopic studies of 7 , as well as exchange spectroscopic ( exsy ) experiments , confirm the presence of a dynamic equilibrium in c6d6 solution . as well as resonances consistent with the solid state structure of 7 ,
mixed - metal zincate complex 4 can thus be described as a molecular scaffold for the molecularization of the usually insoluble polymeric lih .
an examination of the li nmr spectrum reveals a doublet with a jli  h coupling constant of 13.3 hz , confirming the retention of the li 
only a few examples have been detected since the first measurement by bergman of ( cp*)irh2sime3li(pmdeta ) ( cp * = c5me5 , pmdeta = n , n , n,n,n-pentamethyldiethylenetriamine ) in 1985 .
those detected have coupling constants ranging from 6.414.7 hz , placing our example at the upper end of the literature examples .
surprisingly , two independent h resonances , at 3.27 and 3.19 ppm , were detected coupling with the doublet in the li nmr spectrum .
this result can be tentatively assigned to the li resonance for the zincate hydride 7 sharing the same chemical shift as dissociated ( tmeda)lih , in agreement with the presence of a dynamic equilibrium in solution .
given the equilibrium between 4 and 7 , the formation of a diazaethene compound , analogous to 1 , may also occur using 7 as the starting point .
the optimized structure of 7 closely matches the experimental x - ray structure , with a mean unsigned error of 0.07  between the measured and calculated bond lengths .
the largest deviation between the calculated and experimental values occurs for the interactions involving the hydride ion ( e.g. , zn  h : 1.94  calculated vs. 2.14  experimental , see table s1 in supporting information for details ) .
the deviations in the hydride ion distances may be due to the uncertainty inherent in the crystallographic determination of hydride positions ; however , even with the shorter calculated zn 
h interaction , at 1.94  the zn  h contact is clearly long - range .
the introduction of the ( tmeda)lih ligand to 4 to form 7 results in a different mechanism for the formation of the diazaethene compound .
one of the ethylene c8 hydrogen atoms from 7 ( figure 7 ) is aligned for abstraction by the dilithio hydride to form the intermediate ( intc ) containing a ( lih)2 unit .
the formation of intc occurs in an endothermic step ( g = 21.7 kcal / mol ) , relative to 7 , with an associated barrier of 30.1 kcal / mol ( g * , figure 7 ) .
the subsequent abstraction of the h atom from c9 ( figure 7 ) to form h2 and the diazaethene compound 8 , occurs with a barrier ( g * ) of 11.8 kcal / mol , relative to intc , in an exothermic reaction ( g = 16.7 kcal / mol ) .
overall , the reaction from 7 to form 8 is slightly endothermic ( g = 5.0 kcal / mol , figure 7 ) . from the intermediate structure ( intc )
the barriers to the forward and reverse reaction ( i.e. , the formation of 8 + h2 and 7 , respectively ) are comparable in energy ( g * = 3.4 kcal / mol ) . therefore , although the equilibrium from intc will be in the direction of 7 , the formation of 8 + h2 from this structure should also occur to some extent .
free energy ( g ) profile for the formation of 8 from 7 via a dihydride intermediate .
the possible formation of a zn hydride intermediate , in analogy to the mechanisms explored for the formation of 1 ( via inta ) was also investigated .
the barrier associated with the formation of this hydride intermediate ( g * = 28.4 kcal / mol ) is accessible under the experimental reaction conditions and the intermediate produced in this reaction is destabilized by 21.1 kcal / mol , relative to 7 .
however , the barrier to the subsequent second hydrogen abstraction ( i.e. , the formation of 8 + h2 ) is 34.4 kcal / mol , which results in a strong preference for the reverse reaction back to the reactant 7 ( g * = 6.0 kcal / mol ) . as such
, the mechanism resulting in the formation of dihydride intermediate ( intc , figure 7 ) is expected to be the dominant mechanism for the formation of 8 + h2 , from 7 . despite the comparable energetics for the formation of 8 from 7 ( via intc ) and 1 from 4 ( via inta ) , the mechanisms and the synergic interaction of the two metals are distinctly different .
the presence of the second li atom in 7 ( relative to 4 ) allows the zn to stabilize the formation of the c = n bond ( the zn 
n bond increases from 2.07  in 7 to 2.18  in ts(7-intc ) , figure 8) , while the li atom is able to facilitate the c  h bond breaking step .
finally , the free energy associated with the loss of a ( tmeda)lih ligand from 8 to form 1 is slightly less ( g = 24.9 kcal / mol ) than the equivalent association / dissociation reaction between 4 and 7 . therefore , given the similar overall kinetic barriers associated with the generation of the diazaethene species starting from 4 and 7 , both reaction mechanisms described in figures 5 and 8 , should be competitive and operate in a complementary fashion to generate the observed product 1 .
the closest analogy to the synthesis of 1 was reported by veith who formed 1,3-diaza-2-silacyclopentene by the double lithiation of ( tbu)nhch2ch2nh(tbu ) ( di - tert - butylethylenediamine ) with nbuli followed by an electrophilic quench with cl2si(me)n(h)tbu .
it was acknowledged at the time that the process leading to this dehydrogenation was unclear but it was noted that the reaction was highly concentration dependent . for dehydrogenation to occur the lithiation step needed to be carried out in a highly concentrated nbuli hexane solution ( 2 m ) .
this concentration dependence infers a different mechanism of hydrogen activation from that which results in the synthesis of 1 , possibly involving either larger aggregates or an intermolecular process .
large aggregates of liotbu have previously been found to activate and retain lih in solution .
the diamido species was never characterized in veith s study . we have confirmed by nmr spectroscopy that ( ipr)nhch2ch2nh(ipr ) , like its homologue ( tbu)nhch2ch2nh(tbu ) , can be activated when refluxed in a concentrated nbuli solution . when the reaction was carried out at concentrations akin to that used in the synthesis of 1
, then the dilithiated species could be crystallized as a tmeda solvate ( figure 9 ) .
this product , [ ( tmeda)li(ipr)nch2ch2n(ipr)li]29 , has a distorted ladder structure in the solid state with the diamido ligands adopting a trans  bent conformation allowing them to bridge between opposite corners of alternate rungs .
n bond distances of 2.227(2 )  and 1.971(2 )  between the outer rungs and 2.057(2 )  between the inner rungs . maintaining the c2 axis of symmetry , the long and short edges of the ladder alternate as the ladder is ascended .
both outer lithium centers adopt distorted tetrahedral geometries , while the internal lithium atoms are three - coordinate .
this trapping of a ladder segment instigated by the lewis base tmeda supports gardiner s assertions .
selected bond lengths [  ] and angles [ deg ] : li1n1 , 2.227(2 ) ; li1n2 , 1.977(3 ) ; li1n3 , 2.209(3 ) ; li1n4 , 2.199(3 ) ; li2n1 , 2.025(2 ) ; li2n2 , 1.971(2 ) ; li2n1(a ) , 2.057(2 ) ; n1c2 , 1.460(2 ) ; n2c1 , 1.458(2 ) ; c1c2 , 1.521(2 ) ; n1li1n2 , 81.23(9 ) ; n1li1n3 , 125.9(1 ) ; n1li1n4 , 129.1(1 ) ; n2li1n3 , 117.3(1 ) ; n2li1n4 , 125.26(1 ) ; n3li1n4 , 83.34(1 ) ; n1(a)li2n1 , 109.6(1 ) ; n1(a)li2n2 , 151.4(1 ) ; n1li2n2 , 86.7(1 ) .
the activation of ( ipr)nhch2ch2nh(ipr ) has also been achieved starting from the sodium congener na[(ipr)nch2ch2nh(ipr ) ] and ( tbu)2zn(tmeda ) to generate the sodium diazaethene zincate ( tmeda)na[(ipr)nch = chn(ipr)]zn(tbu ) 10 . although the initial cocomplexation adduct  ( tmeda)na[(ipr)nch2ch2nh(ipr)]zn(tbu)2  with one n 
h bond still remaining is apparently too short - lived to isolate , when the reaction was carried out at 0 c and the resulting solution stored at 30 c , it was possible to isolate and fully characterize the dimeric species { ( tmeda)na[(ipr)nch2ch2n(ipr)]zn(tbu)}211 with no n  h bond but containing the saturated ethylene diamido backbone ( figure 10 ) which must exist further along the reaction pathway that ultimately produces 10 .
two independent centrosymmetric molecules of dimer 11 as well as a hexane molecule form the asymmetric unit .
sodium zincate 11 is constructed from a 545 fused ring system in a distorted ladder conformation ; the two znnccn rings lie anti about the strictly planar central znnznn core .
the n  c [ 1.452(6 )  and 1.463(6 )  ] and c  c [ 1.494(7 )  ] bond distances are indicative of a fully saturated bisamide . contrary to binding in the lithium monomer 5 , the heavier alkali metal binds to only one of the available amido groups , with the core nitrogen atoms ( n1 , n1a ) coordinating exclusively to zinc . despite the variety of amide environments around zinc , the zn 
n bond distances show little variation ( average 2.11  ) . due to the narrow bite angle of the diamido ligand ( 85.9 )
the zinc center adopts a distorted tetrahedral geometry . a highly distorted tetrahedral geometry around sodium is also completed by a weak agostic interaction [ 2.977(6 )  ] to a methyl arm of the tertiary butyl ligand on zinc .
this is moderately longer than a similar interaction found in the bis - alkylamido zincate ( tmeda)na(tmp)zn(tbu)2 ( 2.75  ) .
545 fused ring systems have been found before in metallate complexes containing saturated nccn diamido ligands .
examples include the lithium lanthanide complexes [ ( thf)3li[1,2-{n(sime3)}2c6h10]mcl2]2 ( m = nd , yb ) and the dimeric gallium species [ { ( bz)nch2ch2n(bz)}li{(bz)nch2ch2n(bz)}gao]2 ( bz = benzyl , ch2c6h5 ) . molecular structure of one of the independent molecules of the dimeric sodium zincate [ ( tmeda)na[(ipr)nch2ch2n(ipr)]zn(tbu)]211 . a weak agostic nac contact is denoted by a broken bond .
hydrogen atoms , ipr groups , and disordered hexane of crystallization have been omitted for clarity .
selected bond lengths [  ] and angles [ deg ] : na1n1 , 3.720(4 ) ; na2n2 , 2.326(5 ) ; na1n3 , 2.490(6 ) ; na1n4 , 2.490(5 ) ; na1c4 , 2.977(6 ) ; zn1n1 , 2.105(4 ) ; zn1n2 , 2.103(4 ) ; zn1c1 , 2.072(5 ) ; zn1n1(a ) , 2.122(4 ) ; n1c8 , 1.463(6 ) ; n2c9 , 1.452(6 ) ; c8c9 , 1.494(7 ) ; n4na1n3 , 75.4(2 ) ; n4na1n2 , 121.9(2 ) ; n4na1c4 , 147.7(2 ) ; n3na1n2 , 137.7(2 ) ; n3na1c4 , 91.8(2 ) ; n2na1c4 , 87.4(2 ) ; n2zn1c1 , 117.1(2 ) ; n2zn1n1 , 85.9(2 ) ; n2zn1n1(a ) , 110.5(2 ) ; n1zn1c1 , 127.6(2 ) ; n1zn1n5 , 88.8(2 ) ; n5zn1c1 , 120.0(2 ) .
as might be expected from polarity considerations , the sodium reagent proves more reactive than the lithium analogue , providing after a 30 min reflux in hexane , the sodium diazaethene 10 in 61% isolated yield .
although the tmeda solvate 10 could be isolated as a crystalline solid , to obtain crystallographic data of adequate precision , the donor ligand thf had to be used in place of tmeda to produce the analogous thf solvate ( thf)3na[(ipr)nch = chn(ipr)]zn(tbu ) 10a ( figure 11 ) . here , the zinc atom is chelated symmetrically [ zn  n bond distances 1.957(2 )  and 1.971(2 )  ] by the diamide ligand , lying below the plane defined by the nccn core of the diazaethene ligand .
the methine carbons of the isopropyl groups lie in the plane of the diazaethene ligand while their methyl groups protrude above and below this plane .
the sodium atom lies above the nccn bridge , though slightly off center , coordinating to both pairs of nitrogen and carbon atoms .
three thf molecules complete the coordination sphere of the sodium center . comparing the c  n [ 1.398  ( average ) ] and c  c [ 1.353(4 )  ] bond distances of the diazaethene core with those found in the lithium analogue ( tmeda)li[(ipr)nch = chn(ipr)]zn(me ) [ 1.400 ( average ) and 1.349(3 )  ] indicates that the change in alkali metal has little effect on the diazaethene ligand .
dissolution of 10a in c6d6 to perform an nmr spectroscopic analysis was achieved by the addition of a drop of d8-thf . comparing the h nmr spectra of the thf
solvate 10a with the tmeda solvate 10 reveals only marginal shift changes of the resonances associated with the diazaethene ligand . comparing the h nmr spectra of the homologues ( tmeda)li[(ipr)nch = chn(ipr)]zn(tbu ) 1 and ( tmeda)na[(ipr)nch = chn(ipr)]zn(tbu ) 10 reveals a 0.24 ppm downfield shift of the signal attributable to the ch = ch backbone of the diazaethene ligand on moving from lithium to sodium .
interestingly , there are two sets of doublets associated with the methyl groups of the isopropyls of the lithium compound but only one set in the sodium congener .
this would suggest sterically induced hindered rotation of the isopropyl groups in the lithium compound which is relieved on switching to the larger sodium cation .
the synthesis of several alkali metal zincate diazaethene complexes has been achieved previously . however , to the best of our knowledge , all previous examples were synthesized by the reduction of unsaturated bis - imino ligands .
therefore , the synthesis of the sodium zincate 10 , along with the lithium zincates detailed in our recent communication , appear to represent the first such complexes to be synthesized via a double ch activation of a saturated diamido ligand . molecular structure of the diazaethene sodium zincate ( thf)3na[(ipr)nch = chn(ipr)]zn(tbu ) 10a . hydrogen atoms and minor disordered thf components have been omitted for clarity .
thermal ellipsoids shown at the 50% probability level . selected bond lengths [  ] and angles [ deg ] : na1o1 , 2.350(2 ) ; na1o2 , 2.312(2 ) ; na1o3 , 2.312(2 ) ; na1n1 , 2.587(2 ) ; na1n2 , 2.662(2 ) ; na1c8 , 2.612(2 ) ; na1c9 , 2.649(2 )
; zn1n1 , 1.957(2 ) ; zn1n2 , 1.971(2 ) ; zn1c1 , 2.009(2 ) ; n1na1n2 , 60.81(6 ) ; n1zn1n2 , 85.14(8 ) ; n1zn1c1 , 139.88(10 ) ; n2zn1c1 , 134.97(10 ) .
conversions of saturated diamines into unsaturated diazaethenes are usually associated with redox - amenable transition metal catalysts .
however here we show through a combination of experimental ( nmr spectroscopic , x - ray crystallographic ) and theoretical ( dft ) studies that working co - operatively in a molecular mixed - metal system , an alkali metal ( lithium or sodium ) and zinc can bring about such transformations under challenging redox - inactive conditions .
 the surprising transformation of the saturated diamine ( ipr)nhch2ch2nh(ipr ) to the unsaturated diazaethene [ ( ipr)nch = chn(ipr)]2- via the synergic mixture nbum , ( tbu)2zn and tmeda ( where m = li , na ; tmeda = n , n , n,n-tetramethylethylenediamine ) has been investigated by multinuclear nmr spectroscopic studies and dft calculations . 
 several pertinent intermediary and related compounds ( tmeda)li[(ipr)nch2ch2nh(ipr)]zn(tbu)2 ( 3 ) , ( tmeda)li[(ipr)nch2ch2ch2n(ipr)]zn(tbu ) ( 5 ) , { ( thf)li[(ipr)nch2ch2n(ipr)]zn(tbu)}2 ( 6 ) , and { ( tmeda)na[(ipr)nch2ch2n(ipr)]zn(tbu)}2 ( 11 ) , characterized by single - crystal x - ray diffraction , are discussed in relation to their role in the formation of ( tmeda)m[(ipr)nch = chn(ipr)]zn(tbu ) ( m = li , 1 ; na , 10 ) . in addition , the dilithio zincate molecular hydride [ ( tmeda)li]2[(ipr)nch2ch2n(ipr)]zn(tbu)h 7 has been synthesized from the reaction of ( tmeda)li[(ipr)nch2ch2nh(ipr)]zn(tbu)23 with nbuli(tmeda ) and also characterized by both x - ray crystallographic and nmr spectroscopic studies . 
 the retention of the li  h bond of 7 in solution was confirmed by 7li1h hsqc experiments . also , the 7li nmr spectrum of 7 in c6d6 solution allowed for the rare observation of a scalar 1jli  h coupling constant of 13.3 hz . 
 possible mechanisms for the transformation from diamine to diazaethene , a process involving the formal breakage of four bonds , have been determined computationally using density functional theory . 
 the dominant mechanism , starting from ( tmeda)li[(ipr)nch2ch2n(ipr)]zn(tbu ) ( 4 ) , involves the formation of a hydride intermediate and leads directly to the observed diazaethene product . 
 in addition the existence of 7 in equilibrium with 4 through the dynamic association and dissociation of a ( tmeda)lih ligand , also provides a secondary mechanism for the formation of the diazaethene . 
 the two reaction pathways ( i.e. , starting from 4 or 7 ) are quite distinct and provide excellent examples in which the two distinct metals in the system are able to interact synergically to catalyze this otherwise challenging transformation . 
Introduction
Results
Conclusion