Silyl‐Phosphino‐Carbene Complexes of Uranium(IV)

Abstract Unprecedented silyl‐phosphino‐carbene complexes of uranium(IV) are presented, where before all covalent actinide–carbon double bonds were stabilised by phosphorus(V) substituents or restricted to matrix isolation experiments. Conversion of [U(BIPMTMS)(Cl)(μ‐Cl)2Li(THF)2] (1, BIPMTMS=C(PPh2NSiMe3)2) into [U(BIPMTMS)(Cl){CH(Ph)(SiMe3)}] (2), and addition of [Li{CH(SiMe3)(PPh2)}(THF)]/Me2NCH2CH2NMe2 (TMEDA) gave [U{C(SiMe3)(PPh2)}(BIPMTMS)(μ‐Cl)Li(TMEDA)(μ‐TMEDA)0.5]2 (3) by α‐hydrogen abstraction. Addition of 2,2,2‐cryptand or two equivalents of 4‐N,N‐dimethylaminopyridine (DMAP) to 3 gave [U{C(SiMe3)(PPh2)}(BIPMTMS)(Cl)][Li(2,2,2‐cryptand)] (4) or [U{C(SiMe3)(PPh2)}(BIPMTMS)(DMAP)2] (5). The characterisation data for 3–5 suggest that whilst there is evidence for 3‐centre P−C−U π‐bonding character, the U=C double bond component is dominant in each case. These U=C bonds are the closest to a true uranium alkylidene yet outside of matrix isolation experiments.

Incontrast to the well-developed nature of transition-metal carbenes with covalent M = Cd ouble bonds,t he analogous uranium chemistry is far more sparse. [1] Thef irst uranium carbene with acovalent U = Cdouble bond, stabilised by one phosphorus(V) substituent, [U(CHPMe 2 Ph)(h 5 -C 5 H 5 ) 3 ]( I), [2] was reported in 1981 and its reactivity was well-elaborated. [3] After apause of some three decades the area was revived with various examples of uranium-carbene complexes with one or two phosphorus(V) substituents that stabilise the carbene. [4] Them ajority of these complexes exhibit covalent U = C double-bond interactions,t hat is,u ranium plays as ignificant role in stabilising the carbene by accepting charge from it, but in all cases the phosphorus(V) substituents introduce the competing carbene and ylide resonance forms R 3 P + ÀC(R)= U À $ R 3 P = C(R) À U(R= HorR' 3 P), where in the latter the phosphorus(V) substituent plays as ignificant stabilising role by accepting charge from the carbene.So, those U = Cdouble bonds are not as fully developed as they might otherwise be. [1a,e] Apart from fleeting reactive intermediates, [5] theo nly reports of unfettered uranium-carbon multiple bonds pertain to fundamental species such as [U C],[ C U C],[ U CH], [C U = O],[ F 3 U CH],a nd [X 2 U = CH 2 ]( X = H, F, Cl), [6] prepared on microscopic scales in matrix isolation experiments at cryogenic temperatures (< 10 K). Thus,the synthesis of acovalent U=Cdouble bond, where the carbene substituents do not significantly affect the U = Ccomponent, in atrue uranium alkylidene is yet to be reported under ambient conditions after synthetic efforts spanning four decades. [1a,e, 2] Without exception, outside of matrix isolation all uranium carbenes with covalent U=Cdouble bonds are stabilised with phosphorus(V) substituents, [1a,e] which has posed the question as to whether U=Cd ouble bonds free of phosphorus(V) substituents are accessible under ambient conditions.Afull understanding of U = Cdouble bonds is thus lacking, but is key to informing the ongoing debate over the nature of actinide chemical bonding and to providing organouranium reactivity benchmarks.
Thec omplex [Sc{C(SiMe 3 )(PPh 2 )}{HC(MeCNAr) 2 }-(THF)] (II,A r = 2,6-diisopropylphenyl) was recently reported. [7] In this compound, the Sc = Cb ond is highly polarised, and consequently a p-delocalised Sc À C À P3centre unit is found. Inspired by that report, and related early d-block analogues, [8] we reasoned that using {C(SiMe 3 )-(PPh 2 )} 2À ,never before deployed in actinide chemistry,might present, if synthetically accessible,aU = Cd ouble bond that would be more fully developed than in phosphorus(V)substituted variants because the phosphorus(III) substituent should be less able to accept charge from the carbene.T his U=Cd ouble bond might thus be anticipated to be closer to matrix isolation examples, [6] since 5f uranium(IV) might be expected to better stabilise the carbene than 3d scandium-(III).
We report herein the synthesis,c haracterisation, and reactivity benchmarking of silyl-phosphino-carbenec omplexes of uranium(IV). Outside of matrix isolation these are the first examples of covalent actinide-carbon double bonds prepared without phosphorus(V) substituents.O ur strategy exploited a-hydrogen abstraction, and so they are asignificant advance towards isolating at rue uranium alkylidene under ambient conditions.Incontrast to II, [7] whilst we find evidence for 3-centre PÀCÀU p-bonding character,t he U=Cd ouble bond component is dominant because the uranium ions are the dominant acceptor of charge from the carbene.So, these U=Cb onds can be considered to be the closest to at rue uranium alkylidene thus far prepared outside of matrix isolation experiments.
After extensive screening of multiple types and combinations of alkyl ligands (for example,CH 3 ,CH 2 Bu t ,CH 2 SiMe 3 , CH(SiMe 3 ) 2 ,C H 2 C 6 H 5 ,C H(C 6 H 5 ) 2 ,n one of which facilitate a-hydrogen abstraction in any combinations nor under thermolysis or photolysis conditions) we deduced [9]  Complexes 4 and 5 are isolated as red crystalline solids in 86 and 65 %y ields,r espectively (Scheme 1). [9] Thes olid-state molecular structures of 3-5 were determined, [9] and 5 is shown in Figure 1. Thesalient features of 3-5 are the presence of am eridionally coordinated BIPM TMS ligand and as ilyl-phosphino-carbeneligand to uranium.
The 1 HNMR spectra of 3-5 span the ranges À32 to + 25, À33 to + 59, and À16 to + 48 ppm, respectively.The 31 PNMR spectra of 3-5 reveal broad BIPM TMS phosphorus resonances at À598, À582, and À402 ppm, respectively,but the phosphine resonances could not be located. Both sets of NMR data are characteristic of 5f 2 uranium(IV)-BIPM TMS complexes. [4b,c,d] Owing to low solubilities post-crystallisation, reliable UV/Vis/ NIR spectra of 3 and 4 could not be obtained. However,t he spectrum of 5 [9] is consistent with the uranium(IV) formulation. [1b,4l, 13] TheA TR-IR spectra of 3-5 all exhibit strong absorptions at about 650 and about 595 cm À1 ,w hich are shown by analytical frequency calculations,c omputed to within 25 cm À1 of experiment in each case,tobethe main U= C carbene bond stretches in 3-5.
Confirmation of the uranium(IV) assignments of 3-5 is provided by SQUID magnetometry. [9] Themagnetic moments per uranium ion of 3-5 are all about 3.0 m B at 298 K, in each case changing little until about 50 Kw here the magnetic moment drops sharply to about 0.8 m B by 2Kand is tending to zero.The magnetic moment of uranium(IV) usually smoothly decreases over the temperature range 298 to 2Kand tends to zero as this ion is amagnetic singlet at low temperature with ar esidual magnetic moment from temperature-independent paramagnetism (ca. 0.4 m B ). [1b,13, 14] Ther etention of higher than usual magnetic moments until 50 Ka nd also at 2K is atypical of most uranium(IV) magnetism, but is characteristic of cases where one or more strongly donating multiply bonded ligands are coordinated to uranium(IV). [4a-c, 15,16] To probe the U = C carbene linkages in 3-5,wemodelled them with DFT. [9] We replaced the bridging TMEDAi n3 with aN Me 3 surrogate to provide the computationally tractable  (Table 1), and we include data for I for comparison. [1a,4f] The computed Ua nd Cc harges are consistent with their formulations.
For 3' ', 4 À ,and 5 the HOMO and HOMOÀ1are singularly occupied and of essentially pure 5f character.T he next orbitals in each case,w hich are doubly occupied, are the U= C carbene p-bond (HOMOÀ2), followed by the U=C carbene sbond (HOMOÀ3). Slightly lower in energy in the HOMOÀ4 to HOMOÀ8r egions are the U = C BIPM p-t hen s-bonds. However,i na ll complexes there is extensive and variable mixing of orbital contributions from the U = C BIPM ,U = C carbene , and phosphine lone pairs,s o, since other orbital coefficients also intrude into these molecular orbitals,the overall bonding picture of these energetically similar orbitals is convoluted by the inherently delocalised nature of the DFT calculations.
To obtain al ocalised, more chemically intuitive description of the bonding in 3' ', 4 À ,and 5 we turned to NBO analysis, Table 1. TheU = C carbene s-a nd p-bonds in 3' ' and 5 are remarkably similar and for charge-rich 4 À the s-and p-bonds show lower uranium contributions.W ec onclude that the 6d and 5f contributions to the U=C carbene s-bonds are generally fairly equal, but for the corresponding p-bonds 5f contributions dominate these more angular interactions.T he data for 3' ', 4 À ,a nd 5 are similar to computed data for simpler, fundamental [X 2 U=CH 2 ]( X = F, Cl) species prepared in matrix-isolation experiments, [6a,e,g] where average uranium sand p-contributions to those U=Cd ouble bonds of about 21 and about 26 %are found. It is also instructive to compare I to the U = C carbene units in 3' ', 4 À ,a nd 5;f or I the s-bond is essentially electrostatic,b ut the p-bond is slightly more covalent. TheU =C carbene bonds can also be internally compared to the U=C BIPM cases within each of 3' ', 4 À ,and 5,and we note that the uranium contributions to the U=C BIPM bonds are consistently 4-9 %l ower than the corresponding U=C carbene for each pair. We also note that the U = C BIPM uranium contributions are lower than in other uranium(IV)-BIPM TMS Table 1: Selected computed DFT,NBO, and QTAIM data for the U=Cbonds in 3' ', 4 À , 5,and I. [b,c]

Angewandte Chemie
Communications 5508 www.angewandte.org complexes, [1a] presumably reflecting the strongly donating nature of the silyl-phosphino-carbene. Nalewajski-Mrozek bond order analyses (Table 1) reveals U = C carbene bond orders that are consistently higher than the U=C BIPM bond orders,w hich are slightly lower than usually found for uranium(IV)-BIPM TMS complexes, [1a] underscoring the strongly donating nature of the carbene group.T he U= C carbene values are also higher than for I and bond orders of about 1.45 for [X 2 U = CH 2 ]( X= F, Cl). [6a,d] Forc omparison, the BIPM TMS imino donors exhibit U À Nbond orders of about 0.8, the coordinated DMAP ligands in 5 exhibit U À Nb ond orders of about 0.6, and the phosphine UÀPbond orders vary from about 0.3 in 3 and 4 (which derives from indirect mixing of the phosphine orbitals into the uranium-carbene bonding orbitals rather than any direct U À Pi nteraction) to < 0.1 in 5. [17] Supporting this latter point, the P À C carbene bond orders average 1.20, reflecting the aforementioned mixing by negative hyperconjugation. So,s ome 3-centre UÀCÀP ptopology is found in 3-5,h owever the U=Cd ouble bonds in 3-5 with U=Cb ond orders about 1.5 times the PÀCb ond orders contrast to the more delocalised 3-centre ScÀCÀP pbonding scenario in II where the situation is reversed with the C À Pbond order about 1.6 times the Sc = Cbond order. [7] Thus, the bonding situation in 3-5 is closer to the localised one found in [Ta(CHPMe 2 )(h 5 -C 5 Me 5 ) 2 (PMe 3 )] [8f] than in II. [7] This underscores the key,dominant role of uranium stabilisation of the carbenes in 3-5 that is also rather different to the situation found in related free carbenes such as Me 3 SiCP-(NPr i 2 ) 2 . [18] Along with orbital-based DFT and NBO methods,w e performed at opological bond analysis using QTAIM, Table 1. [19] Forachemical bond at the bond critical point (BCP) the topological electron density (1(r)) tends to be < 0.1 when the bond is ionic and > 0.2 when it is covalent. Fora ll complexes U = CB CPs were found with 1(r)v alues ordered U = C carbene > U = C BIPM % I,indicating the presence of covalent uranium-carbon chemical bonds,albeit polarised ones.Single or triple bonds present cylindrical distributions of electron density around the inter-nuclear bond axis at the BCP (e(r) = 0). Double bonds,h owever,a re asymmetric when viewed down the inter-nuclear bond axis (e(r) > 0). Forc omparison, the carbon-carbon bonds in ethane,b enzene,a nd ethylene have e(r)v alues of 0, 0.23, and 0.45, and transition metalalkylidene complexes generally have e(r)v alues of about 0.5. [20] TheQTAIM analysis consistently returns non-zero U= C carbene and U=C BIPM ellipticities,t hus both are clearly U=C double-bond interactions but with the former clearly better developed than the latter, and this is in line with those of I and uranium-BIPM complexes generally. [1a] TheP À C carbene e(r) values of 3-5 are consistently about 0.1, which only deviating modestly from zero gives clarity over the true extent of negative hyperconjugation and 3-centre UÀCÀP p-character that could be otherwise overestimated from visual inspection of molecular orbitals alone.I nterestingly,n oU À PB CPs are found in 3-5.Since there are no U À PBCPs,and the structural and NBO data suggest phosphine lone pairs that point away, not to,u ranium, it is concluded that any U···P interactions must be relatively weak. Furthermore,r ing CPs between the BIPM TMS phosphorus centres and uranium ions in 3' ', 4 À ,and 5 are found by QTAIM, and we have found UÀPBCPs in other compounds with U À Pbonds, [12,17] suggesting that the absence of uranium-phosphine BCPs in three independent calculations is not spurious.
Experimentally,i ti si nteresting to note that addition of DMAP to 3 only forms the DMAP adduct 5,whereas addition of DMAP to II [7] results in rapid CÀHa ctivation of DMAP. Thecoordination of two DMAP molecules in 5 suggests that there are no steric barriers,and thus the lack of DMAP C À H activation by 3 experimentally supports the notion that the U = C carbene bonds in 3-5 are more covalent, and thus less reactive units than that in II. [7] In support of this notion, when 2 is converted into [U(BIPM TMS ){CH(Ph)(SiMe 3 )}-(CH 2 SiMe 3 )] (6), which does not undergo a-hydrogen abstraction, and then treated with DMAP C À Ha ctivation occurs under mild conditions to give [U(BIPM TMS )(NC 5 H 3 -4-NMe 2 )(CH 2 SiMe 3 )] (7), Scheme 1. [9] This underscores the more basic, ionic nature of UÀCsingle bonds compared to U= Cd ouble bonds.
Preliminary reactivity studies reveal divergent carbeneand phosphine-centred reactivities (Scheme 2). Complexes 3-5 all react with benzaldehyde and 9-anthracenecarboxaldehyde to produce alkenes by Wittig-type chemistry.T wo equivalents of aldehyde are consumed per uranium each time,i rrespective of reactant ratios,t op roduce (Ph 2 P)-(Me 3 Si)C=C(H)(R')a nd (Me 3 SiNPPh 2 ) 2 C=C(H)(R')( R ' = anthracene or phenyl). Potentially of more interest, 3 reacts with PhCCPh to give [U{C(SiMe 3 )(Ph 2 PCPhCPh)}-(BIPM TMS )] (8)w here the alkyne has formed am etallocycle between the phosphine and uranium centres.T his complex is notable on two counts.T he U = C carbene double bond is so robust that reactivity has preferentially occurred at the phosphine,a nd indeed the U=C carbene distance of 2.316 (7) in 8 is by the 3s-criterion barely perturbed from 3-5 whilst the U = C BIPM distance (2.405 (7) )i sc omparable to that in 3. Despite the fact there is clearly av acant coordination site trans to the alkenyl unit in 8 the carbene resides essentially trans to the central BIPM TMS carbon (C=U=C173.8 (2)8 8)even though there is no obvious constraining steric reason for it to do so.Ifthe trans influence is operating here this would not be expected since there is clearly space for the C=U=Cangle to decrease further, and this hints at the possible presence of an inverse trans influence. [4a-c, 21] To conclude,b yu tilising as ilyl-phosphino-carbene we have prepared three uranium(IV) carbenes by a-hydrogen abstraction. These are the first actinide-carbon double bonds outside of matrix isolation conditions to be free of Scheme 2. Synthesis of the Wittig alkene products and 8 from complexes 3-5.R' = phenyl or 9-anthracene. phosphorus(V) substituents,and the first use of such aligand in f-block chemistry;a ss uch they exhibit uranium(IV)carbon bond distances that are amongst the shortest on record. Although there is evidence for the presence of a3centre UÀCÀP p-interaction facilitated by negative hyperconjugation, the characterisation data all suggest the presence of U=C carbene double bonds that dominate the bonding picture. These U=C carbene bonds can be considered to be the closest to at rue uranium alkylidene yet prepared outside of matrix isolation experiments.C omplexes 3-5 take us as tep further towards isolable uranium alkylidenes,a nd preliminary reactivity studies have revealed divergent carbene-and phosphine-centred reactivities.