inorganics Article Trapping of an Heterometallic Unsaturated Hydride: Structure and Properties of the Ammonia Complex [MoMnCp(µ-H)(µ-PPh2)(CO)5(NH3)] M. Angeles Alvarez, Daniel García-Vivó * , Estefanía Huergo and Miguel A. Ruiz * Departamento de Química Orgánica e Inorgánica/IUQOEM, Universidad de Oviedo, E-33071 Oviedo, Spain; [email protected] (M.A.A.); [email protected] (E.H.) * Correspondence: [email protected] (D.G.-V.), [email protected] (M.A.R.); Tel: +34-985102962 (D.G.-V.); +34-985102978 (M.A.R.) Received: 15 October 2018; Accepted: 15 November 2018; Published: 24 November 2018 Abstract: Complexes displaying multiple bonds between different metal atoms have considerable synthetic potential because of the combination of the high electronic and coordinative unsaturation associated to multiple bonds with the intrinsic polarity of heterometallic bonds but their number is scarce and its chemistry has been relatively little explored. In a preliminary study, our attempted synthesis of the unsaturated hydrides [MoMCp(µ-H)(µ-PR2)(CO)5] − from anions [MoMCp(µ-PR2)(CO)5] and (NH4)PF6 yielded instead the ammonia complexes [MoMCp(µ-H)(µ-PR2)(CO)5(NH3)] (M = Mn, R = Ph; M = Re, R = Cy). We have now examined the structure and behaviour of the MoMn complex (Mo–Mn = 3.087(3) Å) and found that it easily dissociates NH3 (this requiring some 40 kJ/mol, according to DFT calculations), to yield the undetectable unsaturated hydride [MoMnCp(µ-H)(µ-PPh2)(CO)5] (computed Mo–Mn = 2.796 Å), the latter readily adding simple donors L such as CNR (R = Xyl, p-C6H4OMe) and P(OMe)3, to give the corresponding electron-precise derivatives [MoMnCp(µ-H)(µ-PPh2)(CO)5(L)]. Thus the ammonia complex eventually behaves as a synthetic equivalent of the unsaturated hydride [MoMnCp(µ-H)(µ-PPh2)(CO)5]. The isocyanide derivatives retained the stereochemistry of the parent complex (Mo–Mn = 3.0770(4) Å when R = Xyl) but a carbonyl rearrangement takes place in the reaction with phosphite to leave the entering ligand trans to the PPh2 group, a position more favoured on steric grounds. Keywords: metal–metal multiple bonds; heterometallic complexes; hydride complexes; binuclear carbonyl complexes; density functional theory calculations 1. Introduction The chemical behaviour of transition-metal complexes combining in the same molecule two or more close metal atoms of different elements (heterometallic complexes) is a living research area within modern Inorganic Chemistry. The combination of different electronic and coordination environments, as found in these molecules, produces cooperative and synergic effects eventually leading, when compared to complexes having just one type of metals (homometallic complexes), to higher reactivity and chemioselectivity, not only in stoichiometric processes of interest but also in catalytic ones [1–3]. It is not by accident that different relevant biological processes are only possible thanks to the action of different enzymes which in turn have heterometallic centres at their active sites, as it is the case of several nitrogenases, hydrogenases or dehydrogenases, which bear Fe–Co, V–Fe, Ni–Fe and related heterometallic active centres. Complexes displaying multiple bonds between distinct metal atoms are a class of heterometallic species of particular interest, since they combine the high electronic and Inorganics 2018, 6, 125; doi:10.3390/inorganics6040125 www.mdpi.com/journal/inorganics Inorganics 2018, 6, x FOR PEER REVIEW 2 of 13 Inorganics 2018, 6, 125 2 of 13 high electronic and coordinative unsaturation inherent to multiple bonds with the polarity of heterometallic bonds, which should lead to increased reactivity, when compared to homometallic coordinativeanalogues [4 unsaturation–8]. Yet, the chemistry inherent to of multiple these species bonds has with been the polarityrelatively of little heterometallic explored bonds,so far, due which to shouldsynthetic lead difficulties to increased or reactivity,easy degradation when compared to mononuclear to homometallic species during analogues their [4 reactions,–8]. Yet, the particularly chemistry ofin thesethe speciescase of has organometallic been relatively littlecompounds. explored soHowever, far, due to there synthetic are difficulties precedents or easyindicating degradation that toorganometallic mononuclear speciessubstrates during with their heterometallic reactions, particularly multiple inbonds the case have of organometallica large potential compounds. for the However,stoichiometric there and are precedentscatalytic activation indicating of that small organometallic molecules. substratesFor instance, with Chetcuti heterometallic et al. multiplestudied bondscomplexes have with a large Co=Ni potential bonds able for to the induce stoichiometric under mild and conditions catalytic different activation isomerisation of smallmolecules. processes, Foras well instance, as regiospecific Chetcuti etC– al.C couplings studied complexesand P–C bond with cleavages Co=Ni bonds [8,9], ablewhile to Suzuki induce et under al. studied mild conditionsunsaturated different complexes isomerisation with Zr(μ-H) processes,3Ir or TaIr(H) as well2 centres as regiospecific able to activate C–C C couplings–H, N–H and and O P–C–H bonds bond cleavages[10,11]. The [8,9 ],same while group Suzuki found et al. studiedthat the unsaturated unsaturated complexes hydride with [Cp*Ru( Zr(µ-H)μ-H)3Ir4OsCp*] or TaIr(H) undergoes2 centres ableaddition to activate reactions C–H, at N–H a rate and much O–H higher bonds than [10, 11the]. rates The same measured group for found the thatcorrespo the unsaturatednding homonuclear hydride [Cp*Ru(complexesµ-H) having4OsCp*] Ru undergoes–Ru or Os– additionOs bonds reactions [12]. Therefore, at a rate the much preparation higher than of new the ratesorganometallic measured forcomplexes the corresponding featuring heterometallic homonuclear complexesmultiple bonds having and Ru–Ru stabilized or Os–Os towards bonds degradation [12]. Therefore, can thebe preparationconsidered ofa newrelevant organometallic target in the complexes current featuringorganometallic heterometallic research. multiple Recently, bonds we and reported stabilized the towardssynthesis degradation of the 32-electron can be complexes considered [MoMCp( a relevantμ-PR target2)(CO) in5]− the(1) (M current = Mn, organometallic R = Ph; M = Re, research. R = Cy), − Recently,which are we the reported first examples the synthesis of organometallic of the 32-electron anions complexes with group [MoMCp( 6–7 metalsµ-PR having2)(CO)5 ]multiple(1) (M M = Mn,–M´ Rbonds = Ph; [13]. M =Protonation Re, R = Cy), of these which anions are the with first (NH examples4)PF6 did of not organometallic yield the sought anions unsaturated with group hydride 6–7- metalsbridged having derivatives multiple [MoMCp( M–M´ bondsμ-H)(μ [13-PR].2 Protonation)(CO)5] but ofgave these instead anions the with corresponding (NH4)PF6 did notammonia yield thecomplexes sought unsaturated[MoMCp(μ-H)( hydride-bridgedμ-PR2)(CO)5(NH derivatives3)], this suggesting [MoMCp( thatµ-H)( theµ targeted-PR2)(CO) unsaturated5] but gave hydrides instead thewere corresponding very reactive ammoniaspecies. Shortly complexes after that, [MoMCp( we foundµ-H)( thatµ-PR the2)(CO) acetonitrile5(NH3)], adduct this suggesting [MoReCp( thatμ-H)( theμ- targetedPCy2)(CO) unsaturated5(NCMe)] underwent hydrides were easy very displacement reactive species. of NCMe Shortly with afterdifferent that, wemolecules found thatsuch the as acetonitrilephosphines, adduct thiols [MoReCp(or alkynes,µ -H)(thusµ -PCyeffectively2)(CO) 5acting(NCMe)] as a underwent surrogate easyof the displacement unsaturated of hydride NCMe with[MoReCp( differentμ-H)( moleculesμ-PCy2)(CO) such5 as] [14]. phosphines, Given the thiols similarity or alkynes, between thus effectively the above acting Re complex as a surrogate and the of theammonia unsaturated complexes hydride mentioned [MoReCp( above,µ-H)( µwe-PCy wondered2)(CO)5][ if14 the]. Given latter thecould similarity also be between used as thesynthetic above Reequivalents complex and of the the elusive ammonia [MoMCp( complexesμ-H)( mentionedμ-PR2)(CO) above,5] hydrides. we wondered We have if the explored latter could this also idea be on used the asMo synthetic–Mn substrate equivalents and in of this the elusivepaper we [MoMCp( report µa-H)( moreµ-PR complete2)(CO)5 ]structural hydrides. characterization We have explored of thisthe ideaammonia on the complex Mo–Mn substrate[MoMnCp( andμ in-H)( thisμ-PPh paper2)(CO) we report5(NH3)] a more(2) and complete a computational structural characterization study of the ofunsaturated the ammonia hydride complex [MoMnCp( [MoMnCp(μ-H)(µ-H)(μ-PPhµ-PPh2)(CO)2)(CO)5] (5H(NH) following3)] (2) and from a computational ammonia release study in of thethe unsaturatedformer (Figure hydride 1). We [MoMnCp( also reportµ-H)( ourµ-PPh results2)(CO) on5]( theH) followingreactions fromof 2 ammoniawith simple release donors in the such former as (Figureisocyanides1). We and also phosphites, report our resultswhich onprove the reactionsthat this ammonia of 2 with simplecomplex donors effectively such asacts isocyanides as a synthetic and phosphites,equivalent of which the elusive prove thatunsaturated this ammonia hydride complex H. In effectivelythis paper actswe ashave a synthetic adopted equivalenta “half-elect ofron” the elusivecounting unsaturated convention hydride for complexesH. In this displaying paper we havebridging adopted hydrides, a “half-electron” so compound counting H is regarded convention as