Lithium Di-­‐ and Trimethyl Dimolybdenum(II) Complexes with Mo-­‐Mo Quadruple Bonds and Bridging Methyl Groups

Lithium'Di)'and'Trimethyl'Dimolybdenum(II)'Complexes'with' Mo)Mo'Quadruple'Bonds'and'Bridging'Methyl'Groups' Natalia&Curado,&Mario&Carrasco,&Eleuterio&Álvarez,&Celia&Maya,&Riccardo&Peloso,&Amor& Rodríguez,&Joaquín&LópezBSerrano,*&and&Ernesto&Carmona*& Instituto&de&Investigaciones&Químicas&(IIQ),&Departamento&de&Química&Inorgánica&and&Centro&de&Innovación&en& Química&Avanzada&(ORFEOBCINQA).&Universidad&de&Sevilla&and&Consejo&Superior&de&Investigaciones&Científicas& (CSIC).&Avenida&Américo&Vespucio,&49,&41092&Sevilla,&Spain&&& KEYWORDS.*Bridging*methyl*groups*•*multiple*bonds*•*molybdenum*•*NMR*spectroscopy*•*computational*studies.* & ABSTRACT:&New&dimolybdenum&complexes&of&composition&[Mo2{µBMe}2Li(S)}(µBX)(µBN^N)2](3aB3c)&where&S&=&THF&or& Et2O&and&N^N&represents&a&bidentate&aminopyridinate&or&amidinate&ligand&that&bridges&the&quadruply&bonded&molybB denum&atoms,&were&prepared&from&the&reaction&of&the&appropriate&[Mo2{µBO2CMe}2(µBN^N)2]&precursors&and&LiMe.&For& complex&3a,&X&=&MeCO2,&while&in&3b&and&3c,&X&=&Me.&Solution&NMR&studies&in&C6D6&solvent&support&formulation&of&the& complexes&as&contact&ion&pairs&with&weak&agostic&MoBCH3·∙·∙·∙Li&interactions,&that&were&also&evidenced&by&XBray&crystallogB raphy&in&the&solidBstate&structures&of&the&molecules&of&3a&and&3b.&Samples&of&3c&enriched&in&13C&(99%)&at&the&metal−bonded& methyl&sites&were&also&prepared&and&investigated&by&NMR&spectroscopy&employing&C6D6&and&THF−d8&solvents.&CrystallizaB tion& of& 3c&from& toluene:tetrahydrofuran& mixtures& provided& single& crystals& of& the& solvent& separated& ion& pair& complex& [Li(THF)4]&[Mo2(Me)2(µBMe){µBHC(NDipp)2}2]&(4c),&where&Dipp&stands&for&2,6−iPr2C6H3.&A&computational&analysis&of&the& Mo2(µ−Me)2Li&core&of&complexes&3a&and&3b&has&been&developed&which&is&consistent&with&a&small&but&nonBnegligible&elecB tronBdensity&sharing&between&the&C&and&Li&atoms&of&the&mainly&ionic&CH3·∙·∙·∙Li&interactions.& INTRODUCTION2 &Transition&metal&alkyl&and&aryl&complexes&form&a&most& representative&family&of&metal&organic&compounds.1,2&The& unique&reactivity&of&the&σ&M−C&bond&allows&for&its&particiB pation& in& elementary& reactions& which& are& the& basis& of& many&academic&and&industrial&transformations.& In& addiB tion,&they&have&furnished&unusual&molecular&and&electronB ic&structures,3&including&the& synthesis&by& Wilkinson& and& coBworkers& of& WMe6,4&the& first& hexamethyl& transition& metal&compound,&and&the&recent&discovery&by&the&group& of&Power&of&the&first&stable&molecule&with&quintuple&metB alBmetal&bonding,&the&chromium&dimer&Cr2Ar´2,&where&Ar´& represents&a&terphenyl&ligand.5&& The&methyl&group&is&the&simplest&alkyl&function&and&ocB cupies&a&conspicuous&position&among&metal&alkyls.&LeavB ing& aside& the& fundamental& and& practical& importance& of& main& group& methyl& compounds,& e.g.& LiMe,& Mg(Me)X,& ZnMe2&or& AlMe3,1d,2b&transition& metal& methyl& complexes& are& arguably& the& most& important& family& of& compounds& with& M−C& σ&bonds.6&Moreover,& some& members& of& this& group& have& provided& the& grounds& for& major& breakB throughs& in& organometallic& chemistry& such& as& the& isolaB tion&of&the&first&methylidene&complex7&or&the&low&temperB ature&NMR&characterization&of&the&first&σ−CH4&complex.8&& Recent&activity&in&the&field&of&binuclear&transition&metal& compounds& that& feature& metalBmetal& multiple& bonds,& sparked& by& the& already& cited& synthesis& of& Cr2Ar´2& comB pounds,&led&us&to&study&the&utility&of&the&wellBknown&preB cursor&[Mo2(O2CMe)4]9&for&the&synthesis&of&Mo−Mo&quadB ruple& bonds.10&With& the& use& of& bridging& aminopyridiB nate10,11&and&amidinate10,12,13&ligands&that&possess&bulky&aryl& substituents,& we& have& recently& prepared& some& multiply& bonded& dimolybdenum& species.14,15&Combining& the& allurB ing&prospects&of&this&area&of&research10B20&with&the&enticeB ments&of&the&study&of&M−C&σ&bonds,&we&have&investigated& a&series&of&methyl&complexes&of&the& core,&stabiB lized&by&coordination&of&the&aminopyridinate&and&amidiB nate& ligands& (1aB1c)& represented& in& Figure& 1.& The& new& compounds& comprise& some& lithium& diB& and& trimethyl& dimolybdenum(II)&ate&complexes&(3a−3c&and&4c)&that&are& the&subject&of&this&paper.&& Figure2 1.&The& aryl−substituted& aminopyridinate& (left)& and& amidinate&(right)&ligands&employed&in&this&work.&Throughout& this&paper,&labels&a,2b,2and2c2in&the&numbering&scheme&refer& Mo Mo N N R R N H N H R: iPr, 1a; Me, 1b 1c specifically& to& the& above& ligands& (some& authors& prefer& the& term&pyridylamido&to&design&ligands&of&type&1a&and21b).&& As&discussed&next,&the&new&complexes&3aB3c&and&4c&feaB ture&coordinated&methyl&groups&with&three&different&bindB ing& modes,& namely& terminal& Mo−Me,& bridging& Mo(µB Me)Mo&and&bridging&&Mo(µBMe)Li&units&(Figure&2).&MoreB over& they& contain& an& uncommon structural&motif&that&finds&no&precedent&among&the&comB pounds& listed& in& the& Cambridge& Structural& Database& (CSD).21&XBray&crystallography&&(complexes&3a&and&3b)&and& multinuclear&NMR&studies&(C6D6&solvent)&have&unequivoB cally&demonstrated& its&existence&in& the&molecules&of& 3aB 3b.& Samples& of& 3c&could& not& be& isolated& in& the& form& of& singleBcrystals&as&crystallization&from&1:1&mixtures&of&toluB ene& and& THF& yielded& instead& the& related,& albeit& solvent& separated& ion& pair& complex& [Li(THF)4][Mo2Me3(N^N)2]& (4c)&featuring&two&terminal&Mo−Me&bonds&and&a&bridging& Mo(µBMe)Mo&linkage&(Figure&2).& Figure2 2.&Simplified&structural&representations&of&the&orgaB nometallic&Mo2&core&of&complexes&3a−3c&and&4c.&The&bridging& aminopyridinate& and& amidinate& ligands& have& been& omitted& for& clarity.& S& represents& a& lithium& coordinated& molecule& of& Et2O&or&THF&(see&text).& RESULTS2AND2DISCUSSION2 The& polymethyl& dimolybdenum(II)& complexes& 3a−3c& described& in& this& work& were& prepared& from& the& correB sponding&bis(acetate)&precursors&with&paddlewheel&strucB tures&[Mo2(O2CMe)2(N^N)2]&(2a−2c),15a,d&that&contain&the& aminopyridinate& or& amidinate& ligands& 1a−1c&represented& in& Figure& 1.& As& shown& in& Scheme& 1,& treatment& of& the& bis(pyridylamido)&complex&2a,&that&has&2,6−iPr2C6H3&and& 2,6−Me2C6H3&substituents&at&the&amido&nitrogen&and&pyrB idinic& ring,& respectively,& with& ca.&2.5& molBequivalent& of& LiMe& occurred& with& displacement& of& one& of& the& acetate& ligands&to&form&complex&3a.&The&complex&was&obtained&as& very&air&sensitive&red&crystals&in&ca.&50%&yield&(after&crysB tallization).& & & & & & & & & Scheme21.2Synthesis2of2complex23a& & Attempts& to& replace& the& remaining& acetate& employing& another&molBequiv&of&LiMe&proved&unsuccessful&and&the& use&of&a&large&excess&of&LiMe&(~10&equiv)&led&to&the&perB methylated&complex&Mo2Me84B,&previously&isolated&as&the& lithium&salt.22&& The& molecular& complexity& of& 3a&was& ascertained& by& X−ray&crystallography&and&will&be&discussed&later&together& with& analogous& studies& carried& out& for& the& related& comB plexes&3b&and&4c.&It&was&also&determined&by&NMR&specB troscopy& that& demonstrated& the& presence& of& a& fiveB membered& Mo2(CH3)2Li& ring.& Formation& of& 3a&implies& a& change& in& the& coordination& of& the& aminopyridinate& ligB ands&in& comparison& with&precursor&2a,&as&in& the& former& the&two&Namido&and&the&two&Npy&donor&atoms&are&placed&in& mutually&trans&positions&whereas&in&the&latter&each&Namido& is&trans&with&respect&to&a&Npyr&donor&atom.&This&structural& peculiarity& is& presently& being& investigated& in& the& parent& [Mo2(µBO2CR)2(µBAp´)2]&compounds&(Ap´&will&be&utilized& as& shorthand& notation& for& an&aminopyridinate& ligand)&& and&will¬&be&discussed&further&in&this&paper.&With&refB erence& to& the& Li(µBMe)2Mo2&fragment,& only& a& broad,& shielded&signal&with&δ&−0.41&ppm&appears&in&the&1H&NMR& spectrum&(25&°C,&C6D6&solvent)&due&to&accidental°enerB acy&of& the& nonBequivalent& methyl& resonances.&NevertheB less,& the& 13C{1H}& NMR& spectrum& (Figure& 3)& features& two& wellBdefined&resonances&with&chemical&shifts&4.5&and&−1.4& ppm&that&appear&as&ca.&1:1:1:1&quartets&due&to&coupling&to& the&7Li&nucleus&(I&=&3/2,&92.5&%&natural&abundance;&1JLiC&&=& 12&Hz).&Furthermore,&rather&low&and&approximately&equal& Figure2 3.2 13C{1H}& NMR& resonances& of& the& bridging& methyl& groups&of&the&Li(µBMe)2Mo2&unit&of&complex&3a.& 1JCH&couplings&of&112&Hz&were&measured&in&the&1H−coupled& 13C& NMR& experiment.& Although& additional& experimental& and& theoretical& information& on& this& interesting& bonding& problem& will& be& discussed& in& posterior& sections& of& this& article,&the&NMR&data&recorded&for&3a,&in&particular:&(i)&the& observation& of& shielded& 1H& and& 13C& resonances& for& the& methyl&groups&of&the&Mo2Me2Li&unit;&(ii)&the&13C−7Li&scalar& Li(µ-Me)Mo Mo(µ-Me) Mo Mo Mo Mo Mo Mo H3CCH3 Li (S) OO H3CCH3 Li (S) CH3CH3 3a 3b, 3c 4c H3 C H3 C Mo Mo NN N N OO OOLiMe, -40 ºC to r.t. Mo Mo NN N N H3CCH3 Li THF THF OO 2a 3a -5.0-4.5-4.0-3.5-3.0-2.5-2.0-1.5-1.0-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0 coupling& of& 12& Hz,& comparable& to& that& observed& for& the& tetrameric& molecules& of& [LitBu]4& (11& Hz)23;& and& (iii)& the& somewhat& reduced& 13C−1H& coupling& of& 112& Hz,& are& in& agreement& with& some&electron−density& sharing& between& the&two&molybdenum−bound&CH3&groups&and&the&lithium& cation& and& therefore& with& the& existence& of& 3−center& 2−electron& (3c−2e)& agostic& bonds.24,25&Another& piece& of& NMR& data& that& is& worth& noting& is& the& appearance& of& a& broad& singlet& centred& at& 2.15& ppm& in& the& 7Li& spectrum& recorded&in&C6D6.&As&discussed&later&for&the&related&comB pounds& 3b&and& 3c,& this& observation& provides& additional& support&for&the&formulation&of&complex&3a&in&its&solution& in&C6D6&or&other&nonBsolvating&hydrocarbon&solvents&as&a& contact& ion& pair,26&a& structure& that& persists& in& the& solid& state&(see&below).& We& also& studied& the& reaction& of& the& analogous& bis(acetate)bis(aminopyridinate)& complex& 2b,& that& conB tains&the&aminopyridinate&ligand21b&of&Figure&1.&The&latter& possesses&a&2,6−Me2C6H3&aryl&as&substituent&of&the&amido& nitrogen&atom&instead&of&the&2,6−iPr2C6H3&aryl&of&1a,&causB ing& the& steric& bulkiness& of& 2b&to& be& slightly& reduced& in& comparison&with&2a.11b,e&Interestingly,& the& use& of& 2b2perB mitted&replacement& of& the& two& acetate&ligands&with&forB mation&of&the&trimethyl&dimolybdate&lithium&complex&3b2 (vide*infra).& Despite&initial&expectations,&no&reaction&was&observed& at& room& temperature& between& complex& 2b&and& a& small& excess&of&LiMe&(1:3.5&molar&ratio).&However,&after&stirring& at&60&°C&for&a&total&of&18&hours&the&desired&product&was& isolated&in&ca.&65%&yield&in&the&form&of&dark&red&crystals& (Scheme&2).&The&related&bis(acetate)bis(amidinate)&comB pound& 2c&reacted& similarly& with& LiMe,& although& milder& experimental&conditions&were&needed&(room&temperature;& ca.&15&%&molar&excess&of&LiMe)&to&yield&the&expected&comB plex&3c&in&ca.&52%&isolated&yield&(Scheme&3).&Interestingly,& the&use&of&a&1:2&molar&ratio&of&dimolybdenum&complex&2c& versus&methyl&lithium&led&exclusively&to&the&trimethylated& complex&3c&accompanied&by&unreacted&2c.&& Compounds&3b&and&3c&are&extremely&reactive&towards& traces& of& water& and& oxygen& both& in& solution& and& in& the& solid&state.&For&this&reason,&their&solutions°raded&rapB idly,&limiting&their&manipulation&for&extended&periods&of& time.&Solution&NMR&data&in&C6D6&for&the&new&compounds& 3b&and&3c&strongly&support& their&formulation& as& contact& ion&pairs.&Nevertheless,&at&variance&with23a,&complexes&3b& and&3c&contain&a&bridging&methyl&group&which&behaves&as& a&µBLX&ligand&and&participates&in&a&3c−2e&bond.25&Besides,& the&Mo2&unit&forms&two&σ&Mo−C&bonds&with&the&methyl& groups&that&interact&with&the&lithium&cation&through&their& C−H& bonds,& in& close& analogy& to& the& bonding& situation& described& above& for& complex& 3a.& The&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Li(µBMe)Mo(µBMe)Mo(µBMe)¢ral& core& of& compound& 3b&was& firstly& characterized& by& NMR& methods& and& conB firmed&afterwards&by&X−ray&crystallography.&However,&as& briefly&mentioned,&single&crystals&of&the&amidinate&comB plex&could&only&be&obtained&in&the&presence&of&tetrahydroB furan&and&were&shown&by&NMR&studies&in&this&solvent&and& by& XBray& crystallography& to& correspond& to& the& solvent& separated& complex& 4c&with& formulation& [Li(THF)4][Mo2Me2(µ−Me){µ−HC(NDipp)2}2]& (Dipp& =& 2,6−iPr2C6H3)& (vide* infra).& To& authenticate& beyond& any& doubt& the& central& trimethyl& heterotrimetallic&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Li(µBMe)Mo(µBMe)Mo(µBMe)&core&in&complex&3c,&samples& of& this& compound& enriched& in& 13C& in& the& metalBbonded& methyl&ligands& were&obtained&and&investigated&by&NMR& spectroscopic&techniques,&both&in&C6D6&and&THF−d8&solB vents.& Scheme2 2.2 Synthesis2 of2 the2 lithium2 trimethyldimoJ lybdenum(II)2complex23b& & The& 1H& NMR& spectrum& of& compound& 3b&dissolved& in& C6D6& hints&the&presence&of&a&molecule&of&Et2O&that&comB pletes&the&coordination&of&the&Li+&ion.&In&addition,&resoB nances&with&chemical&shifts&0.71&and&−1.35&ppm&and&relaB tive&intensity&suitable&for&three&and&six&hydrogen&atoms,& respectively,&were&recorded&for&the&Mo−CH3&groups.&The& more&shielded&one&is&due&to&the&more&polar&Mo2(µBMe)2Li& methyl& protons& that,& in& accordance& with& the& proposed& formulation,&are&related&by&a&C2&axys&of&symmetry&(Scheme& 2),& whereas& the& other& can& be& attributed& to& the& methyl& group&that&bridges&the&two&molybdenum&atoms.&The&corB responding& 13C& signals& appear& at& δ&6.5& and& 2.8&ppm&and& present&oneBbond&13C−1H&couplings&of&115&and&110&Hz,&reB spectively.&The&latter&signal&is&broad&as&a&result&of&unreB solved&coupling&to&the&7Li&nucleus.& Scheme23.2Synthesis2of2the2amidinate2methyl2complex23c2 2 The&contact&ion&pair&complex&3c&contains&a&molecule&of& THF& coordinated& to& lithium,& as& suggested& by& 1H& NMR& studies.& In& consequence,& it& has& formulation& [Mo2(µB Me){(µBMe)2Li(THF)}{µBHC(NDipp)2}2].& Its& characteristic& C2v&molecular& symmetry& (Scheme& 3)& causes& the& appearB ance& of& four&doublets& for& the& methyl& protons& of& the& iso−propyl&amidinate&substituents&and&hence&two&septets& for&the& corresponding&CHMe2&methyne&protons&(data&in& C6D6;&see&Experimental&Section&for&details).&The&1H&NMR& singlet&at&δ&2.21&ppm&originated&by&the&acetate&groups&in& Mo Mo NN N N OO OOLiMe, 60 ºC Mo Mo NN N N H3CCH3 Li OEt2 CH3 Toluene/Et2O 2b 3b Mo Mo NN N N OO OOLiMe, 0 ºC to r.t. Mo Mo NN N N H3CCH3 Li THF CH3 2c 3c THF the&complex&precursor&2c&is&replaced&in&3c&by&two&signals,& one¢red&at&1.74&(s;&3H)&and&the&other&at&−0.55&(br&s;&6& H)& ppm,& respectively& ascribed& to& the& Mo(µ−Me)Mo& and& Mo2(µ−Me)2Li& hydrogen& nuclei.& These& alkyl& groups& give& rise&to&13C{1H}&resonances&with&δ&7.14&and&4.4&ppm,&respecB tively.&Although&the&latter&is&poorly&resolved,&a&oneBbond& 13C−7Li&coupling&constant&of&roughly&8&Hz&can&be&estimatB ed.& While& its& magnitude& is& somewhat& lower& than& correB sponding& value& for& 3a&(12& Hz),& the& observed& coupling& is& once& more& indicative& of& some& covalency& in& the& CH3·∙·∙·∙Li·∙·∙·∙CH3&interactions.& In& agreement& with& the& above& data,&the&7Li&nucleus&resonates&as&a&broad&singlet&with&δ& 2.51&ppm&(data&also&in&C6D6)&i.e.&with&chemical&shift&simiB lar&to&that&of&3a&(2.15&ppm).& As& briefly& noted,& complementary& NMR& studies& were& undertaken& using& also& THF−d8&as& the& solvent& and& with& specimens&of&3c&enriched&in&13C&(99%)&in&the&metalBbound& methyl&groups.&The&labeled&samples&were&prepared&as&in& Scheme& 3,& but& using& Li13CH3,& obtained& from& commercial& 13CH3I& (99%)& and& LinBu.& Figure& S1& (see& the& Supporting& Information)& shows& the& C6D6&13C{1H}& NMR& spectra& of& 3c& with& natural& abundance& and& labeled& with& 13C,& in& the& δ& interval&from&ca.&9&to&2&ppm,&along&with&the&fully&coupled& spectrum.&As&can&be&seen,&the&singlet&at&δ&7.4&attributed&to& Mo(µBCH3)Mo& converts& in& the& spectrum& of& the& 13CB enriched& sample& into& a& triplet& (2JCC&=& 8& Hz)& as& a& conseB quence& of& its& coupling& with& the& two& equivalent& Mo2(µB 13CH3)2&Li&nuclei.27&The&resonance&due&to&the&latter&methyl& groups& (4.4& ppm)& does& not& change& appreciably.& For& the& two&kinds&of&bridging&methyl&groups&similar&1JCH&couplings& of&about&112&Hz&can&be&measured.& NMR& studies& of& 3c&in& THF−d8&revealed& some& distinct& changes.& For& instance,& the& 7Li& resonance& is& now& more& shielded&and&appears&at&−0.1&ppm,&thereby&shifting&by&~2.6& ppm&relative&to&the&spectrum&recorded&in&C6D6.&Changes& in&the&1H&NMR&spectrum&are¬&significant&beyond&some& variations&in&the&chemical&shifts&of&the&two&types&of&meB thyl& groups&that& change& from& 1.74& and& −0.55& in& C6D6&to& 1.25&and&−0.25&ppm&in&THF−d8.&However,&the&broad&13C{1H}& resonance& at& 4.4& ppm& due& to&the& Mo(µBMe)Li(µBMe)Mo& linkage&in&C6D6&disappears&in&THF−d8&and&converts&into&a& sharp& singlet& with& δ&14.74,& i.e.& displaced& towards& higher& frequency&by&more&than&10&ppm.&No&signs&of&coupling&to& 7Li& can& be& detected.& The& Mo(µBMe)Mo& signal& is& also& deshielded&and&appears&at&10.43&ppm&(Δδ&~&3.3&ppm).&In& the&specimens&enriched&in&13C,&these&resonances&appear&in& THF−d8&as&a&doublet&(δ&14.74)&and&a&triplet&(10.43&ppm).&A& single&twoBbond&coupling&constant&2JCC&=&9&Hz&was&measB ured& for& 4c&between& the& two& terminal& Mo−13CH3&groups& and& the& single& bridging& methyl& group& of& the& Mo(µB 13CH3)Mo& linkage.& The& magnitude& of& this& coupling& lies& between&typical&values&for&cis&and&trans&two−bond&13C−13C& couplings&in&other&transition&metal&complexes.[27]&All&theB se& data& unequivocally& support& the& occurrence& in& THF& solutions& of& an& anionic& [Mo2Me3]B& organometallic& core& with& the& simplified& structure& represented& in& Figure& 2& (structure&4c),&and&therefore&with&a&solvent&separated&ion& pair& formulation& [Li(THF)4]&[Mo2Me2(µBMe){µB HC(NDipp)2}2]&for&this& complex.&This&proposal&has&been& confirmed& by& XBray& studies& performed& with& a& singleB crystal& obtained& from& concentrated& toluB ene/tetrahydrofuran&(1:1)&solutions&of&the&complex.& As&discussed&above,&the&adoption&of&the&contact&ion&pair& formulation&3c2or&the&solvent&separated&ion&pair&structure& 4c&is&strongly&dependent&upon&the&coordination&capacity& of& the& solvent.& As& an& additional& test,& a& fresh& sample& of& complex& 3c,& prepared& as& in& Scheme& 3,& was& dissolved& in& C6D6.&1H&and&13C{1H}&NMR&studies&were&in&accord&with&the& presence& of& the& (THF)Li(µBMe)Mo(µBMe)Mo(µBMe)&core& characteristic&of&the&contact&ion&pair&structure.&Then,&the& solvent&was&evaporated&under&vacuum&and&the&solid&resiB due& dissolved& in& THFBd8.& NMR& analysis& (1H& and& 13C{1H}& experiments)& revealed& the& expected& features& for& a& monoanionic&[MeMo(µBMe)MoMe]B¢ral&unit&and&the& fully&solvated&[Li(THF)4]&+&ion&characteristic&of&4c.&Finally,& removal&of&the&solvent&under&vacuum&with&careful&drying& of&the&residue&and&reBdissolving&of&the&resulting&solid&in& C6D6&restored&the&NMR&features&of&3c.&& Compounds&3a,&3b,&and&4c&were&characterized&by&XBray& crystallography&and&their&molecular&structures&are&depictB ed&in&Figures&4&(3a&and&3b)&and&6&(4c).&Complete&details&of& the&crystallographic&analyses&are&collected&in&Tables&S1B3& (see& the& Supporting& Information).& Of& the& three& comB pounds&investigated,&the&two&that&exhibit&a&Mo(µBMe)Mo& 3c−2e&bond&(i.e.&3b&and&4c)&have&almost&identical&Mo−Mo& bond&lengths&(ca.&2.084&Å)&whereas&in&the&acetateBbridged& species& 3a&the& metal−metal& bond& is& slightly& longer& at& 2.107(6)&Å.&These&values&are&very&close&to&those&reported& recently&for&terphenyl&complexes&of&the& central& unit.14& As&can&be&seen&in&Figure&4,&the&two&Ap´&ligands&of&comB plexes&3a&and&3b&(ligands&1a&and&1b&in&Figure&1,&respectiveB ly)&adopt&different&stereochemical&distribution&upon&bindB ing&to&the&respective&Mo2&cores.&&Thus,&in&compound&3a& the&two&Npyr&and&the&two&Namido&nitrogen&atoms&are&mutuB ally& trans& while& in& complex& 3b&they& give& rise& to& trans−Namido−Mo−Npy&linkages,&as&it&is&commonly&observed& in&dichromium&complexes&with&Cr−Cr&multiple&bonds.11&In& the& two& compounds,& the& dative& covalent& Mo−Npy&bonds& are&longer& than& the& normal&covalent25&Mo−Namido&bonds.& Thus,& in& 3a&the& average& Mo−Npy&and& Mo−Namido&bond& lengths&are&of&ca.&2.21&and&2.14&Å,&respectively,&whereas&the& similar&bonds&of&3b&are&of&about&2.18&and&2.11&Å. Mo Mo & 22222222222222222 22222222222222222222222222222 Figure24.&SolidBstate&molecular&structures&of&complexes&3a2 (above)&and&3b2(below),&with&thermal&ellipsoids&set&at&30&%& probability.&Selected&bond&lengths&in&Å&3a:&Mo(1)BMo(2),& 2.107(1);&Mo(1)BC(54),&2.218(5);&Mo(2)BC(53),&2.216(5);&C(54)B Li(1),&2.080(1);&C(53)BLi(1),&2.143(3).&3b:&Mo(1A)BMo(2A),& 2.084(8);&Mo(1A)BC(1T),&2.273(8);&Mo(2A)BC(2T),&2.251(7);& Mo(2A)BC(1B),&2.411(7);&Mo(1A)BC(1B),&2.404(8);&C(2T)BLi(1),& 2.162(6);&C(1T)BLi(1),&2.160(5).& The&two&complexes&contain&a&CH3·∙·∙·∙Li(S)·∙·∙·∙CH3&unit&that& spans&over&the&Mo−Mo&quadruple&bond&(S&=&THF&for&3a& and& Et2O& for& 3b).& The& Mo2C2Li& fiveBmembered& ring& is& close& to& planar,&although& the& lithium& atom& deviates& by& 0.428&and&0.249&Å,&respectively,&from&the&mean&plane&of& the&two&molybdenum&and&the&two&carbon&atoms.&Dihedral& angles&between&the&C−Mo−Mo−C&and&C−Li−C&planes&are& of& 32°&for& 3a&and& 21°&for& 3b.& The& fiveBmembered& rings& contain&comparable&Mo−C&bonds&with&average&lengths&of& ca.& 2.21& (3a)& and& 2.26& Å& (3b).& For& comparison,& Mo−CH3& distances& in& the& quadruply& bonded& anionic& complex& Mo2Me84B&have&values22&in&the&range&2.27−2.31&Å,&while&in& neutral&Mo2Me4(PR3)4&molecules&the&Mo−CH3&bonds&have& a&length&of&roughly&2.24&Å.20c,d&Typical&Mo−Me&distances& in&compounds&compiled&in&the&CSD&are&in&the&2.10−2.36&Å& range.21& For& complexes& 3a&and23b&the& hydrogen& atoms& of& the& MoBCH3&groups&were&located&on&a&difference&Fourier&map.& Nevertheless,& to&ensure& a& uniform& treatment& of& crystal& data& they& were& recalculated& into& idealized& positions.& In& accordance&with&the&experimental&data&obtained,&it&can&be& proposed& that& in& the& two& complexes& the& coordination& requirements&of&the&lithium&ion&are&partly&satisfied&by&the& existence& of& weak&dihapto&agostic&interactions&(Figure&5& and&S2&;&see&the&Supporting&Information)&with&the&molybB denumBbonded&methyl&groups.& &&&&&&&&&&&& &&&&&&& Figure2 5.&Schematic& representation& of& the& dihapto& agostic& Mo2(µBCH3)2Li& interactions& in& complex& 3b.& The& LiBCH3& disB tances&have&a&value&of&ca.*2.16&Å&and&the&Li·∙·∙·∙H&contacts&are&in& the&approximate&range&1.83B2.11&Å.&Angles&are&in°rees&(°).& See& also& Figure& S2& in& the& Supporting& Information& for& addiB tional&details.& Agostic&M−CH3·∙·∙·∙Li&interactions&have&been&ascertained& for&different&transition&metals.28B31&In&complex&3a&the&two& Li−C& bonds& of& the& CH3·∙·∙·∙Li·∙·∙·∙CH3& portion&of& the& five−membered& Mo2C2Li&ring& are& slightly& different& and& distinctly& short& (2.080(1)& and& 2.143(3)& Å)& whereas& in& 3b& they&are&practically&identical&and&also&short&(ca.&2.16&Å).&As& a&reference,&the&sum&of&the&covalent&radii&of&Csp3&(0.76&Å)& and&Li&(1.28&Å)&is&2.04&Å.32&These&metrics&denote&the&existB ence& in& the& molecules& of& 3a&and& 3b&of& nonBnegligable& covalent& 3c−2e& interactions,& as& hinted& by& the& solution& NMR&data&already&discussed.&These&interactions&seem&to& be&comparable&to&those&found&in&related&complexes.&For& instance,&Gambarotta&and&coBworkers&demonstrated&that& [Li(S)]4[Cr2Me8]&(S&=&Et2O,&THF)&can&be&reversibly&cleaved& by&action&of&tmed&(tmed&=&tetramethylethylenediamine)& with&formation&of&[Li(tmed)]2[CrMe4],33&and&proposed&that& short& Li·∙·∙·∙CH3&bonding& interactions& of& 2.176& (7)& Å& were& responsible&for&holding&together&the&two&CrMe4Li2&units&of& the& dimer.& The& latter,& originally& characterized& by& Krausse,34&features&a&short&Cr−Cr&distance&of&1.968&(2)&Å,33b& but&recently,&Mulvey&et*al.&have&demonstrated&that&replacB ing& Li+&by& Na+& in& [Na(OEt2)]4[Cr2Me8]& caused& the& Cr−Cr& separation&to&increase&to&3.263&(2)&Å.35&Interestingly,&the& isostructural&[Li(THF)]4[Mo2Me8]&complex22&could¬&be& cleaved&under&reaction&conditions&similar&or&harder&than& those& employed& for& the& dichromium& complex.33&In& the& latter& dimolybdenum& compound& the& lithium& ions& lie& at& distances&of&2.43−2.56&Å&from&the&methyl&carbon&atoms.&It& is&also&worth&pointing&out&that&in&the&two&complexes&3a& and& 3b&the& Mo·∙·∙·∙Li& contacts& are& long,& in& the& range& 2.872(2)−2.962(9)&Å,&indicating&that&there&are&no&signifiB cant&bonding&interactions&between&these&atoms.&Recently,& Tsai& et* al.&have& reported& studies& on& a& complex& with& a& Mo−Mo& quintuple& bond& spanned& by& one& lithium& atom& with&Mo·∙·∙·∙Li&contacts&of&2.640(8)&Å&that&were&considered& as&mainly&ionic.36&It&is&important&to&recall&that&compounds& alike&3a&and&3b&that&exhibit&a&bridging&hydrocarbyl&M(µB R)M´&structure&between&transition&metal&and&main&group& metal&atoms&are&of&much¤t&importance,&as&they&are& relevant&to&a&number&of&catalytic&reactions,&for&instance,& olefin& oligoB& and& polymerization,& cyclizations& and& addi& tions&of&carbonyl&and& alkyl&functionalities,& and& different& classes&of&crossBcoupling&processes.29,37B39&& The& coordination& of& the& molybdenum& atoms& of& 3a&is& completed&by&a&bidentate&acetate&ligand,&while&that&in&3b& involves&instead&a&bridging&methyl&group&that&participates& in&a&3c−2e&interaction.&The&two&kinds&of&bridging&ligands& can&be&viewed&as& LX& ligands& thereby& yielding&a& sixteenB electron&count&at&each&molybdenum¢re,&as&it&is&comB monly&encountered&in&complexes&of&this&type.10&Bridging& alkyl&groups&are&wellBknown,&although&they&are&much&less& frequently&found&that&bridging&hydride&ligands.25,40B45&ExB amples& in&which& the& bridging& alkyls& span&over& multiple& M−M& bonds& are& however& sparse11e,46B49&and& correspond& largely&to&dichromium&complexes.&The&anionic&compound& 4c&features& also& a& bridging& methyl& group& (Figure& 6).& It& crystallizes&in& the&C2/c&monoclinic&space&group&with&two& independent&molecules&in&the&asymmetric&unit&(arbitrarily& designated&as&B&and&C).&The&two&display&the&same&geomeB try&and&feature&similar&structural¶meters,&particularly& in&what&concerns&the&Mo−Mo&and&Mo−N&bonds.&However,& their& organometallic& (µBMe)Mo2Me2&moieties& exhibit& small&differences&that&merit&discussion. & & Figure2 6.&SolidBstate& molecular& structure& of& the& anionic& [Mo(Me)2(µBMe){µBHC(NDipp)2}2]B&portion& of&complex& 4c2 showing&the&two&independent&molecules,2B2(bottom)&and2C2 (upper& structure).2The& counterBcation& [Li(THF)4]+&has& been& omitted&for&clarity.&Thermal&ellipsoids&are&set&at&30%.&SelectB ed&bond&lengths&in&Å&4c2(B):&Mo(1A)BMo(1),&2.084(1);&Mo(1A)B C(2TA),& 2.396(1);&Mo(1A)BC(2TA),& 2.397(1);& Mo(1A)BC(1B),& 2.172(1);&Mo(1)BC(1B),& 2.411(7).&(C):& Mo(2)BMo(2A),& 2.083(1);& Mo(2)BC(3T),& 2.429(1);& Mo(2A)BC(4TA),& 2.473(1);& Mo(2)B C(2B),&2.328(1);&Mo(2A)BC(2B),&2.157(7).& In& 3b&the& bridging& methyl& group& is& appropriately& deB scribed&as&symmetric&pyramidal&with&essentially&identical& Mo−C&bond&distances&(ca.&2.40&Å)&and&C−Mo−Mo&angles& (about& 64°),& and&with& a& more´& Mo−C−Mo&angle&of& 51.3(2)°&(Figure&7).&The&long&Mo−C1B&bonds&are&possibly& the&consequence&of&the&strong&trans&influence50&exerted&by& the& two& molybdenum−bound& methyl& groups& (C1T& and& C2T)&that&form®ular&Mo−C&bonds&(ca.&2.26&Å)&and&hold& together& the& lithium& ion.& The& C1T−Mo1A−C1B& and& C2T−Mo2A−C1B& bond& angles& of& 178.4(3)& and& 178.8(3)°,& respectively,&are&very&close&to&the&ideal&180°&value.&& For&the&two&independent&molecules&of&4c&(B&and&C,&FigB ure& 7)& there& are& two& terminal& methyl& groups& that& form& long& Mo−Me& bonds& (in& the& range& 2.396(1)& B& 2.473(1)& Å).& Moreover,&they&give&rise&to&Mo−Mo−C&angles&with&diverB gent& values,& one& being& of& roughly& 98°&and& the& other& of& about&116°.&The&bridging&methyl&group&generates&an´& MoBCBMo& angle& (ca.& 53−55°)& and& two& Mo−C& bonds& that& differ& appreciably& in& length.& Thus,& in& molecules& B&the& Mo−C& distances& are& of& 2.172(1)& and& 2.452(1),& a& situation& that& therefore& approaches& terminal& coordination& to& Mo1& (Figure& 7),11e,49c&whereas& in& C&the& differences& are& smaller& (2.157(1)&and&2.328(1)&Å).&Although&the&hydrogen&atoms&of& the&bridging&methyl&groups&were¬&located&in&the&FouriB er&difference&map,&they&were&calculated,&these&calculations& supporting& a& weak& monohapto& agostic& coordination.& Whether&the&agostic&interaction&is&maintained&in&solution& is& unclear& since& as& discussed& above,& the& terminal& and& bridging& methyl& groups& of& complex& 4c&exhibit& close& 1JCH& couplings& of& 115& and& 112& Hz,& respectively& (THF−d8).& We& note,& however,& that& similar& 13C−1H& couplings& in& Mo−Mo& bonded&complexes&with&a&bridging&methyl&group&have&led& to&divergent&interpretations&on&the&possible&existence&of& agostic&interactions.&46& Figure27.&Bond&lengths&(Å)&and&bond&angles&(°)&for&the&orB ganometallic&[Mo2Me3]& cores& of& complexes& 3b2 (left)& and& 4c2 (independent&molecules&B,¢re,2and2C,&right).&For&simplicB ity,& bond& distances& have& been& rounded& up& to& the& second& decimal&figure&and&bond&angles&to&the&first.& Computational2studies2on2 the2Mo2(µJMe)2Li2linkJ age2of2complexes23a2and23b2 The&gas&phase&geometries&of&3aC&(with&the&iPr&fragments& of&3a&replaced&by&Xylyl&groups)&and&3b&were&optimized&at& the&DFT&level&with&the&Gaussian&0951&package,&using&the& M0652&and&PBE0BD353&functionals,&which&account&for&disB persion& effects.& The& optimizations& were& done& with& the& basis&set&6B31g(d,p)&for&light&atoms&and&the&SDD&basis&and& associated&Electron&Core&Potential&for&Mo.&Both&methods& reproduce&the&solid&state&geometry&of&3a&and&3b,&particuB larly& the& short& CH3·∙·∙·∙Li& contacts& (Tables& S22BS25)& of& the& Mo2(μBMe)2Li&linkage.& The&extent&of&bonding&interaction&between&the&methyl& groups&and&the&lithium&atoms&of&3a&and&3b&was&studied&by& means& of& the& quantum& theory& of& atoms& in& molecules& (QTAIM).54&Analysis& of& the& calculated& electron& densities& (ρ)& of& 3aC2 and& 3b&with& the& Multiwfn& program55&reveals& bond& critical& points& (bcps)& and& unique& bond& paths& (bp)& between& each& carbon& and& the& lithium& atoms& of& their& Mo2(μBMe)2Li& linkages& (Figures& 8& and&S6).& Accordingly& some&level&of&electron&sharing&between&these&carbon&atB oms&and&their&attached&hydrogens&with&the&lithium&atom& becomes&apparent&by&the&analysis&as&discussed&below.&& & Figure28.&Plot&of&the&laplacian&of&the&electron&density&∇2ρ&of& 3aC&in& the& CMo,CMo,Li& plane& calculated& with& the& M06& funcB tional.&The&solid&and&dashed&lines&correspond&to&positive&and& negative&values&of&∇2ρ&respectively.&InBplane&bcps&and&bond& paths&of&the&electron&density&are&superimposed.& The&presence&of&a&unique&bond&path&with&a&bond&critical& point& connecting& two& atoms& is& usually& invoked& as& one& (but& not& exclusive)56&criterion& of& interacting& atoms.& BeB sides,&the&topological&properties&of&the&electron&density&at& the&bond&critical&points&have&been&related&to&the&nature&of& the& interaction& between& atoms.57,58&Tables& 1& and& S4BS8& summarize&topological&properties&at&selected&bcps&of&3aC2 and&3b.&These&data&are&consistent&with&an&important&ionic& character& for& the& C·∙·∙·∙Li& interactions& in& these& molecules,& similar&to&that&found&in&MeLi&(see&the&SI&for&details&and& the& criteria& used).& In& agreement& with& this& classification& the&electron&density&integrated&in&the&basins&of&the&lithiB um&atoms&(the&AIM&or&Bader&charges)&is&close&to&0.9e&as& expected&for&Li+.&However&this&result&suggests&a&small&but& nonBnegligible& degree& of& electron& sharing& between& the& basins&of&lithium&and&the&neighboring&atoms&in&agreement& with&the&1JLiC&observed&by&NMR.&& & Table21.2QTAIM2analysis2of2the2electron2density2of23aC2 at2selected2bcps.2 Bond& ρc*& Hbd2=2Vb2+2Gb22 |Vb|/Gb& ∇2ρe2& CJLib2 0.0226& 0.0033& 0.8800& 0.1228& LiJO2 0.0267& 0.0080& 0.7933& 0.1879& MoJMo2 0.1796& B0.1094& 1.5012& 0.4233& MoJCb2 0.0840& B0.0250& 1.4056& 0.1449& acalculations&with&the&M06&functional,&b(average&values& of&the&local&total&energy&density&Hb&as&the&sum&of&the&local& potential& Vb&and& kinetic& energies& Gb,& ce·∙bohrB3,& dhartree,& ee·∙bohrB5.& & In& relation& with& the& above,& delocalization&indices& (δ(A,B))& indicate& the& number& of& electron& pairs& shared& Mo1A Mo2A Mo1Mo1A C1B C2TA Li C1T C2T C1T Mo2Mo2A C2B C4TA C3T C1B 51.3 116.2 116.2 2.08 2.41 2.40 C1T-Mo1A-C1B = 178.4 C2T-Mo2A-C1B =178.8 C1T-Mo1-C1B = 171.7 C2TA-Mo1A-C1B =154.5 C3T-Mo2-C2B = 177.2 C4TA-Mo2A-C2B =156.1 2.17 2.45 118.0 98.1 2.40 2.40 2.08 2.16 2.33 55.2 2.42 2.47 97.9 2.27 2.25 3b 4c (B) 4c (C) 2.08 53.2 114.9 (delocalized)& between& two& atoms& A& and& B,& and& can& be& seen&as&a&type&of&topological&order&of&a&bond.57B59&In&our& case&covalent&CBH&interactions&have&δ(C,H)&close&to&1,&and& δ(Mo,Mo)& are& almost& 3& for& both& molecules& considered& whereas&the&delocalization&indices&for&the&C·∙·∙·∙Li&interacB tions& are& in& the& range& between& 0.05& and& 0.06,& slightly& smaller&than&the&values&found&for&the&Li·∙·∙·∙O&interactions& (Tables&S9BS17),&which&are&in&the&range&between&0.06&and& 0.07.&& A& complementary& approach& to& explain& the& electron& sharing&between&the&methyl&carbons&and&lithium&atoms&of& 3&consists&on&applying&the&orbitalBbased&Natural&Bonding& Orbitals&scheme.60&In&this&case&localized&orbitals&are&deB rived&from&molecular&orbitals&to&yield&the&best&LewisBlike& bonding& description& of& molecules.& Interactions& between& nonBcovalently& bonded& fragments& of& a& molecule& are& acB counted& for& by& delocalizations& of& two& (or& three)& center& populated& orbitals& into& empty& antibonding& or& lone* pair& (localized&on&one&atom)&orbitals.61&In&our&case&the&interacB tion& between& the& Li& atom& and& the& organometallic& fragB ment&of&complexes&3&is&described&in&terms&of&delocalizaB tions& of& MoBMo& and& MoBC& orbitals& onto& empty& orbitals& localized& on& the& lithium& (see& Figures& S8BS9& and& Tables& S18BS19).& In& addition& Wiberg& bond& indices& derived& from& the&NBO&analysis&support&the&bonding&situation&depicted& in&Figure&5,&(Tables&S20B21),&with&values&close&to&0,1&for&the& C·∙·∙·∙Li&interaction&and&around&0.01B0.02&for&the&four&hydroB gen& atoms& of& the& CH3&fragments& closer& to& Li,& the& latter& being&threeB&or&fourfold&the&values&found&for&the&hydroB gens&oriented&away&from&the&lithium.& & EXPERIMENTAL2SECTION2 General2considerations&& All& manipulations& were& carried& out& using& standard& Schlenk&and&gloveBbox&techniques,&under&an&atmosphere& of& argon& and& of& high& purity& nitrogen,& respectively.& All& solvents&were&dried&and°assed&prior&to&use,&and&stored& over& 4& Å& molecular& sieves.& Toluene& (C7H8),&nBpentane& (C5H12),&and&nBhexane&(C6H14)&were&distilled&under&nitroB gen& over& sodium.& Tetrahydrofuran& (THF)& and& diethyl& ether& were& distilled& under& nitrogen& over& sodiB um/benzophenone.&BenzeneBd6&and&THFBd8&were&distilled& under&argon&over&sodium/benzophenone;&[D8]toluene*was& distilled&under&argon&over&sodium.&The&quadruply&bonded& Mo2(O2CMe)2(N^N)2& complexes& 2aB2c&were& prepared& as& described&previously.15a,d&Solution&NMR&spectra&were&recB orded&on&Bruker&AMXB300,&DRXB400&and&DRXB500&specB trometers.&Spectra&were&referenced&to&external&SiMe4&(δ:&0& ppm)&using&the&residual&proton&solvent&peaks&as&internal& standards& (1H& NMR& experiments),& or& the& characteristic& resonances&of&the&solvent&nuclei&(13C&NMR&experiments),& while&7Li&was&referenced&to&external&LiCl.&Spectral&assignB ments&were&made&by&routine&oneB& and&twoBdimensional& NMR&experiments&(1H,&13C,&13C{1H},&COSY,&NOESY,&HSQC& and& HMBC)& where& appropriate.& UVBvisible& spectra& were& recorded&on&a&PerkinElmer&Lambda&750&spectrometer.&For& elemental& analyses& a& LECO& TruSpec& CHN& elementary& analyzer,&was&utilized.&& & Synthesis2and2characterization2of2compound23a2 To& a& suspension& of& the& corresponding& [Mo2(O2CMe)2(Ap´)2]2complex,&2a,&&(600&mg,&0.58&mmol)& in&THF&(ca.&12&mL)&cooled&to&B&40&°C,&LiMe&(2.5&equiv)&was& added&slowly.&The&low&temperature&was&kept&for&about&3& hours&and&then&the&reaction&mixture&was&allowed&to&reach& room& temperature& with& continuous& stirring& during& 3& hours&giving&a&red&solution&that&was&evaporated&in*vacuo.& The&solid&residue&was&extracted&with&pentane&(ca.&35&mL)& and& was& centrifuged.& The& filtrate& was& transferred& to& a& Schlenk&tube,&concentrated&and&stored&at&B23&°C&during&2& days.&Dark&red&crystals&of&complex&3a&separated&out&and& were&isolated&by&filtration&and&dried&under&vacuum&for&2& hours.&Yield:&320&mg&(50%).&1H&NMR&(500&MHz,&C6D6,& 25& °C):&δ&=&B0.41&(br.&s,&6&H,&{Mo2Me2Li·∙THF}),&0.97&(d,&6&H,& MeDipp(endo)),&1.12&(d,&6&H,&Me’Dipp(endo)),&1.19&(m,&4&H,&OB CH2CH2),&1.27&(d,&6&H,&3JHH&=&6.8&Hz,&MeDipp(exo)),&1.28&(d,& 6& H,& Me’Dipp(exo)),& 2.05& (s,& 6& H,& Me’Xyl),& 2.21& (s,& 3& H,& CH3CO2),&2.31&(s,&6&H,&MeXyl),&3.23&(sept,&2&H,&CH’Me2),&3.39& (m,&4&H,&OBCH2CH2),&3.44&(sept,&2&H,&CHMe2),&5.76&(dd,&2& H,&3JHH&=&8.7&Hz,&4JHH&=&1.2&Hz,&5Bpy),&6.47&(dd,&2&H,&3JHH&=& 8.7&Hz,&4JHH&=&1.2&Hz,&3Bpy),&6.67&(dd,&2&H,&3JHH&=&8.7&Hz,& 4JHH&=&6.7&Hz,&4Bpy),&6.71&(virtual&t,&4&H,&3JHH&=&7.6&Hz,&mB Xyl& and* m’BXyl),& 6.85& (t,& 2& H,& 3JHH&=& 7.6& Hz,& pBXyl),& 7.17B7.22& (m,& 6& H,& m’BDipp,& mBDipp,& and& pBDipp)& ppm.& The& coupling& constants& 3JHH&for& all& CHMe2&groups& are& approximately&of&7&Hz.&1H&signals&with&’&refer&to&the&group& located& in& the& same& region& as& the& MeLiMe& unit.&13C{1H}& NMR&(125& MHz,& C6D6,& 25& °C):& δ&=& B1.4,& 4.5& (q,& 1& C& each,& 1J(13C,7Li)& =& 12& Hz,& {Mo2Me2Li·∙THF}),& 20.3& (Me’Xyl),& 21.0& (MeXyl),& 22.9& (CH3CO2),& 24.5& (MeDipp(endo)),& 24.8& (OB CH2CH2),& 25.3,& 25.5& (Me’Dipp(exo)& and& MeDipp(exo)),& 25.8& (Me’Dipp(endo)),& 27.9& (CHMe2),& 28.4& (CH’Me2),& 68.2& (OB CH2CH2),& 108.2& (5Bpy),&112.5&(3Bpy),&124.2&(mBDipp),&124.5& (m’BDipp),& 125.0& (pBDipp),& 126.4& (m’BXyl),& 128.2& (pBXyl),& 128.5&(mBXyl),&130.9&(4Bpy),&135.9,&136.0&(oBXyl),&139.7&(ipsoB Xyl),& 144.1& (o’BDipp),& 145.8& (oBDipp),& 146.9& (ipsoBDipp),& 157.7,& 169.5& (6Bpy& and& 2Bpy),& 177.3& (O2CCH3).&The&signals& with&’&refer&to&the&group&located&in&the&same®ion&as&the& MeLiMe&unit.&7Li&NMR&(194&MHz,&C6D6,&25&°C):& δ&=&2.15& (br.&s)&ppm.&UV/visible2(C6H14):&λmax&(ε)&=&255&and&340,&420& and&505&nm&&(3100,&6000&MB1&cmB1,&respectively).&Elemental& analysis&calcd.&(%)&for&C58H75LiMo2N4O3:&C,&64.80;&H,&7.03;& N,&5.21.&Found:&C,&64.9;&H,&6.9;&N,&5.0.&& & Synthesis2and2characterization2of2compound23b2 The& appropriate&[Mo2(O2CMe)2(Ap´)2]& complex2& (2b)& (1.0& g,& 1.1& mmol)& was& dissolved& in& a& toluene/Et2O& (2:1)& mixture&and&LiMe&(3&equiv)&was&added.&The&solution&was& heated&at&60&°C&for&6&hours.&Then,&another&3&equiv&of&LiMe& were& added& and& the& resulting& mixture& was& heated& for& a& further&period&of&12&hours&at&60&°C.&The&suspension&was& centrifuged,&the&filtrate&was&transferred&to&a&Schlenk&tube& and& the& solvent& was& removed& under& vacuum.& Diethyl& ether&(ca.&10&mL)&was&added&and&the&solution&was&kept&in& the&fridge&for&3&days&to&obtain&dark&red&crystals&of&complex& 3b&in&65%&yield.&1H&NMR2(400&MHz,&C6D6,&25&°C):&δ&=&B1.35& (s,&6&H,&{Me2Li·∙Et2O}),&0.77&(s,&3&H,&mBMe),&2.02&(s,&6&H,& Me’Xyl),&2.05&(s,&6&H,&Me’Xyl(amino)),&2.20&(s,&6&H,&MeXyl(amino)),& 2.24&(s,&6&H,&MeXyl),&6.06&(d,&2&H,&4JHH&=&6.5&Hz,&5Bpy,&),& 6.46&(d,&2&H,&3JHH&=&7.5&Hz,&m’BXyl),&6.49&(d,&2&H,&3JHH&=&7.5& Hz,&mBXyl),&6.57&(t,&2&H,&3JHH&=&7.5&Hz,&pBXyl),&6.73&(d,&2&H,& 4JHH&=& 8.9& Hz,& 3Bpy),& 6.92B7.02& (m,& 6& H,& mBXyl(amino),* pB Xyl(amino),&and& 4Bpy),&7,07&(m,& 2& H,&m’BXyl(amino))&ppm.&1H& signals&with&’&refer&to&the&group&located&in&the&same®ion& as&the&MeLiMe&unit.&Signals&corresponding&to&aryl&(ArNB)& are&marked&with&amino.&13C{1H}&NMR2(100&MHz,&C6D6,&25& °C):& δ&=& 2.8& (br.& s,& {Me2Li·∙Et2O}),& 6.5& (mBMe),& 20.2& (Me’Xyl(amino)),& 21.7,& 21.8& (MeXyl(amino)& and& MeXyl),& 22.4& (Me’Xyl),&107.9&(5Bpy),&110.7&(3Bpy),&124.6&(pBXyl(amino)),&127.6& (m’BXyl),& 128.6& (under& signal& C6D6,& mBXyl,& pBXyl),& 129.0,& 129.2&(m’BXyl(amino)&and&mBXyl(amino)),&132.4&(4Bpy),&133.2&(o’B Xyl(amino)),& 135.2& (oBXyl(amino)),& 135.4& (oBXyl),& 137.3& (o’BXyl),& 140.4& (ipsoBXyl),& 148.9& (ipsoBXyl(amino),& 156.2& (6Bpy),& 165.5& (2Bpy)&ppm.&The&signals&with&’&refer&to&the&group&located& in& the& same& region& as& the& MeLiMe& unit.& Signals& corresB ponding&to&aryl&(ArNB)&are&marked&with&amino.&7Li&NMR& (C6D6,& 194& MHz):& δ&=& 1.72& ppm.& UV/visible2(C7H8):& λmax& (ε)&=2460,& 575& (12535,& 4685& MB1&cmB1,& respectively).& EleB mental&analysis&calcd.&(%)&for&C49H61LiMo2N4O:&C,&63.91;& H,&6.68;&N,&6.08.&Found:&C,&63.9;&H,&6.6;&N,&5.7.&& & Synthesis2and2characterization2of2compound23c2 Complex& [Mo2(O2CMe)2){µBHC(NDipp)2}2]&(2c)& (4.0& g,& 3.86&mmol)&was&dissolved&in&THF&(60&mL).&The&mixture& was&cooled&at&0&°C&and&LiMe&(3.5&equiv)&was&added&slowly.& The&reaction&mixture&was&allowed&to&reach&room&temperaB ture&over&a&period&of&12&hours.&The&resulting&red&suspenB sion&was& centrifuged,& the& filtrate& was& transferred& to& a& Schlenk&flask&and&the&solvent&was& removed&under&vacuB um.& Pentane& was& added& (10& mL),& the& suspension& was& stirred&for&5B10&min&and&the&solvent&was&removed&in*vacuo.& This&process&was&repeated&one&more&time&and&the&resultB ing&solid&was&dried&under&vacuum&for&four&hours&to&obtain& a&red&solid&(2.5&g,&ca.&60%).&1H&NMR2(500&MHz,&C6D6,&25& °C):&δ&=&B0.55&(br.&s,&6&H,&{Mo2Me2Li·∙THF}),&0.98&(d,&12&H,& 3JHH&=&6.8&Hz,&MeDipp(endo)),&1.27&(d,&12&H,&3JHH&=&6.8&Hz,& Me’Dipp(endo)),&1.32&(d,&12&H,&3JHH&=&6.8&Hz,&Me’Dipp(exo)),& 1.35&(m,&4&H,&OBCH2CH2),&1.38&(d,&12&H,&3JHH&=&6.8&Hz,&MeB Dipp(exo)),&1.74&(s,&3&H,&μBMe),&3.55&(m,&4&H,&OBCH2CH2),& 3.76&(sept,&4&H,&3JHH&=&6.8&Hz,&CH’Me2),&4.11&(sept,&4&H,&3JHH& =&6.8&Hz,&CHMe2),&7.02&(dd,&4&H,&3JHH&=&7.6&Hz,&4JHH&=&1.6& Hz,&mBDipp),&7.06&(t,&4&H,&3JHH&=&7.6&Hz,&pBDipp),&7.11&(dd,& 4&H,&3JHH&=&7.6&Hz,&4JHH&=&1.7&Hz,&m’BDipp),&8.59&(s,&2&H,& NC(H)N)&ppm.&1H&signals&with&’&refer&to&the&group&located& in&the&same®ion&as&the&MeLiMe&unit.&13C{1H}&NMR2(125& MHz,& C6D6,& 25& °C):& δ& =& 4.4& (m,& 1J(13C,& 7Li)& ~& 8& Hz,& {Mo2Me2Li·∙THF}),&7.14&(s,&μBMe),&24.5&(MeDipp(endo)),&25.0& (Me’Dipp(endo)),&25.6&(OBCH2CH2),&26.6&(MeDipp(exo)),&28.4& (CH’Me2),& 28.5& (CHMe2),& 28.7& (Me’Dipp(exo)),& 124.1,& 124.2& (mBDipp& and& m’BDipp),& 125.7& (pBDipp),& 143.9& (o’BDipp),& 145.4& (oBDipp),& 143.3& (ipsoBDipp),& 161.3& (NC(H)N)& ppm.& The&signals&with&’&refer&to&the&group&located&in&the&same& region&as&the&MeLiMe&unit.&7Li&NMR&(194&MHz,&C6D6,&25& °C):& δ=& 2.51& (br.& s,& {Mo2Me2Li·∙THF})& ppm.2UV/visible2 (C4H8O):& λmax& (ε)&=2420&nm& (2200& MB1&cmB1).& Elemental& analysis&calcd.2(%)2for&C57H87LiMo2N4O&(3c):&C,&65.63;&H,& 8.41;&N,&5.37.&Found:&C,&65.2;&H,&8.1;&N,&5.0.&& & Synthesis2and2characterization2of2compound24c2 This&complex&was&prepared&as&indicated&above&for&the& contact&ion&pair&complex&3c.&After&removal&of&the&reaction& solvent&under&vacuum,&the&obtained&crude&red&solid&was& crystallized&from&a&saturated&THF:toluene&solution&(1:1)&at& B23&°C&for&three&days.&Red&crystals&of&the&solventBseparated& ion& pair& complex& [Li(THF)4][Mo2Me2(μBMe){µB HC(NDipp)2}2]&&(4c)&were&isolated&by&filtration&and&dried& under&vacuum&for&2&hours.&Selected&signals&for&complex& 4c:&1H2NMR2(500&MHz,&THFBd8,&25&°C):&δ&=&B0.25&(br.&s,&6& H,&{MeLMoMoBMe}),&1.25&(s,&3&H,&μBMe)&ppm.&13C{1H}2NMR2 (125&MHz,&THFBd8,&25&°C):&δ&=&10.43&(s,&{Mo2Me2Li·∙THF}),& 14.74&(s,&μBMe).&7Li2NMR&(194&MHz,&THFBd8,&25&°C):&δ=&B 0.10& (br.& s,& {Mo2Me2Li·∙THF})& ppm.2Elemental2 analysis2 calcd.2(%)&for&C69H111LiMo2N4O4&(4c):&C,&65.80;&H,&8.88;&N,& 4.45.&Found:&C,&66.5;&H,&9.6;&N,&4.9.& & CONCLUSIONS2 We&have&prepared&and&structurally&characterized&newly& designed& metalBmetal& bonded& complexes& that& contain& a& Mo2&core& spanned& by& a& −CH3·∙·∙·∙Li·∙·∙·∙CH3−&fragment.& This& structural& motif& is& present& in& compounds& 3aB3c&and& is& made&up&of&two&Mo−CH3&units&that&are&bonded&to&a&solvB ated& lithum& cation& (S)Li+&(S=Et2O& or& THF)& by& means& of& 3c−2e&agostic&interactions.&The&resulting&Mo2Me2Li¢ral& assembly& has& a& Mo−Mo& bond& length& that& is& consistent& with&a&Mo−Mo&quadruple&bond.&Each&metal&atom&binds& additionally&to&two&nitrogen&atoms&of&two&bridging&amiB nopyridinate&or&amidinate&ligands&and&their&coordination& is& completed& by& a& third& bridging& ligand,& namely& acetate& for&3a&or&methyl&in&complexes&3b&and&3c.&Computational& studies& on& the& dimethyl& heterotrinuclear& Mo2Me2Li& framework&reveal&that&the&CH3·∙·∙·∙Li&interactions&are&mainly& ionic&with&a&nonBnegligible&covalent&character&which&is&in& agreement&with&NMR&observations.&& & 'ASSOCIATED'CONTENT'' Experimental& procedures& and& characterization& data& for& other&new&compounds&described&herein,&CIF&files&along&addiB tional&tables&and&figures&with&computational&details&and&xyz& coordinates& of& calculated& species.& This& material& is& available& free&of&charge&via&the&Internet&at&http://pubs.acs.org.& && AUTHOR'INFORMATION' Corresponding'Authors' [email protected]& [email protected]& & Notes' The&authors&declare&no&competing&financial&interest.&& *