Rare‐Earth‐ and Uranium‐Mesoionic Carbenes: A New Class of f‐Block Carbene Complex Derived from an N‐Heterocyclic Olefin

Abstract Neutral mesoionic carbenes (MICs) have emerged as an important class of carbene, however they are found in the free form or ligated to only a few d‐block ions. Unprecedented f‐block MIC complexes [M(N′′)3{CN(Me)C(Me)N(Me)CH}] (M=U, Y, La, Nd; N′′=N(SiMe3)2) are reported. These complexes were prepared by a formal 1,4‐proton migration reaction when the metal triamides [M(N′′)3] were treated with the N‐heterocyclic olefin H2C=C(NMeCH)2, which constitutes a new, general way to prepare MIC complexes. Quantum chemical calculations on the 5f3 uranium(III) complex suggest the presence of a U=C donor‐acceptor bond, composed of a MIC→U σ‐component and a U(5f)→MIC(2p) π‐back‐bond, but for the d0f0 Y and La and 4f3 Nd congeners only MIC→M σ‐bonding is found. Considering the generally negligible π‐acidity of MICs, this is surprising and highlights that greater consideration should possibly be given to recognizing MICs as potential π‐acid ligands when coordinated to strongly reducing metals.

Over the past three decades the field of stable singlet Nheterocyclic carbenes (NHCs, I,S cheme 1) has become ab urgeoning area. [1] Within that time,avariety of experimentally viable classes of carbenes related to I have emerged, including anionic-NHCs (II), [2] cyclic alkylaminocarbenes (CAAC, III), [3] and various charge-neutral mesoionic carbenes (MIC, IV-VI), [4] Scheme 1, where for the latter no reasonable canonical resonance forms can be drawn without assigning additional formal charges.Agrowing number of MICs of type IV are known, but it is notable that all examples to date pertain to either the free carbene,o rw ere formed at and remain coordinated to surprisingly few transition metal ions, [4] which contrasts to NHCs that have been coordinated to the majority of metals in the periodic table. [1] Where the bonding of these MICs to metals is concerned, complexes are usually considered to have strong MIC!metal s-donation. Given that strong s-donation, it is surprising that MIC complexes are limited to even only af ew transition metals, but this may reflect the limited range of methodologies to deliberately prepare metal-MIC complexes.Interestingly,any p-bonding components of metal-MIC bonds are,u nlike NHCs,r arely explicitly considered. [5] This is likely because MICs are anticipated to have at best weak p-acceptor character since the carbene is strongly stabilized by N-lone pair and vinyl groups,a se videnced by computational comparisons of different classes of carbenes. [6] We report herein the synthesis and characterization of rare-earth-MIC and uranium-MIC complexes,which are the first f-block-MIC complexes so by definition anew class of fblock-carbene complex. [7] Thec omplexes reported herein were prepared by the formal 1,4-proton migration of an Nheterocyclic olefin (NHO) that represents an ew,g eneral method by which to prepare MIC complexes.I nterestingly, quantum chemical calculations suggest that the 5f 3 uranium-(III) ion engages in aweak p-back-bond to the MIC utilizing a5felectron, whereas the corresponding d 0 f 0 yttrium(III) and lanthanum(III), and 4f 3 neodymium(III) benchmarks do not. Considering the generally negligible p-acidity of MICs, [6] this result for uranium is surprising,a nd highlights that perhaps greater consideration should be given to more widely recognizing MICs as potential p-acid ligands when coordinated to sufficiently reducing metals.
As part of our continued studies of f-element-carbon multiple bonding, [8] we examined the reactivity of the uranium(III)-triamide complex [U(N'') 3 ]( 1U,N '' = N-(SiMe 3 ) 2 ) [9] with the NHO H 2 C=C(NMeCH) 2 (2). [10] We postulated that an adduct similar to [Nd(N'') 3 {H 2 C=C-(NMeCMe) 2 }] could form, [11] which in one resonance form can be represented as aNHC-protected methylidene,but also that as well as being nucleophilic the methylene groups of NHOs are basic by virtue of the dipolar resonance form H 2 C À -C + (NRCH) 2 . [12] Given the existence of MICs,w e considered whether 2 could be converted by transfer of an olefinic hydrogen atom to the methylene group in af ormal 1,4-proton shift to give its MIC form with concomitant metal stabilization.
TheU V/Vis/NIR spectrum of 3U exhibits broad absorptions in the region 14 800-21 000 cm À1 (e = 530-870 Lmol À1 cm À1 )characteristic of 5f 3 6d 0 ! 5f 2 6d 1 transitions of uranium(III) along with weaker (e < 180 Lmol À1 cm À1 ) absorptions in the NIR region are observed. [8j, 9b] This is similar to that of 1U, [9] but distinct to [U(N'') 3 (I)]. [18] Theuranium(III) assignment of 3U was further confirmed by SQUID magnetometry and EPR spectroscopy ( Figure 2). Powdered 3U exhibits amagnetic moment of 2.82 m B at 300 K (3.31 m B in solution at 298 K), Figure 2( Left). This is lower than the theoretical magnetic moment of 3.62 m B for au ranium(III) ion ( 4 I 9/2 ground spin-orbit multiplet, g J = 8/ 11), owing to crystal/ligand field splitting,and is generally inline with uranium(III) magnetic moments. [21] Characteristic of uranium(III), the magnetic moment of 3 decreases slowly across the entire temperature range,r eaching 2.12 m B at 2K. Furthermore,low-temperature magnetization data saturate at moderate magnetic fields,consistent with the Kramers nature   [14] of the uranium(III) ion, Figure 2( middle). Forc omparison, we re-measured data for 1U, [9,15,21] and [U(N'') 3 (I)], [18] giving data consistent with literature values.I mportantly, 1U has asimilar low-temperature magnetization profile to 3U,while that of the non-Kramers [U(N'') 3 (I)] fails to saturate up to 7T . Further support for the uranium(III) oxidation state of 3U comes from low temperature (20 K) EPR spectroscopy at 9.5 GHz (Figure 2, right), where at ypical uranium(III) spectrum [22] is observed from ap owdered sample with effective g-values of g = 4.65, 1.33, and 0.89 arising from the ground Kramers doublet. Thenon-Kramers uranium(IV) ion would be expected to be EPR-silent under these conditions. An isolated Kramers doublet with these g-values corresponds to am agnetic moment of 2.46 m B ,i nf air agreement with the experimental magnetic moment of 2.27 m B observed at 20 K. Furthermore,t hese g-values and magnetic moments are in reasonable agreement with those determined with CASSCF-SO calculations, [13] which predict g = 4.3, 2.4, and 0.7 for the ground Kramers doublet (weighted for the crystal structure MIC disorder;compare to calculated g k = 0.6 and g ? = 3.3 for 1U)and magnetic moments of 2.48 and 3.28 m B at 2and 298 K (Figure 2, left, inset).
To probe the nature of the metal-carbene linkages in 3U, 3Y, 3La,and 3Nd,wecalculated their electronic structures in detail, noting that d 0 f 0 3Y and 3La and 4f 3 3Nd represent closed-shell and f n -analogues for benchmarking purposes, respectively.The Kohn-Sham molecular orbitals of 3U reveal aU =Cd onor-acceptor interaction, where resonance forms 3U-a and 3U-b can be invoked, Scheme 3. TheMIC!Utwoelectron s-donation is represented by HOMOÀ16, and HOMOÀ1r eveals U!MIC one-electron p-back-donation from au ranium 5f-orbital to an empty carbene p-character orbital that is generated in resonance form 3U-b.The HOMO and HOMOÀ2account for the remaining two 5f electrons of uranium(III). Thed onor-acceptor character of 3U stands in contrast to 3Y, 3La,and 3Nd where,asexpected, only the scomponent to the bonding is found in the Kohn-Sham orbitals and thus only resonance form 3M-a is invoked (Scheme 3). Complexes 3Y and 3La as d 0 f 0 complexes would certainly not be expected to exhibit such donor-acceptor character and indeed the molecular orbital that would constitute aM !MIC back-bond is in both cases the LUMO + 1o rbitals with carbene 2p and 4d (Y) and 5d/4f (La) character that sit about 2.5 and about 3.6 eV above the respective HOMO orbitals.F or 3Nd,H OMOÀ2t oH OMO are the 4f electrons,t hen LUMO to LUMO + 4a re dominated by virtual 4f/amide combinations before the relevant Nd!MIC interaction (5d/2p) is found in LUMO + 5, some 2.5 eV above the HOMO.T hus, 3Y and 3La do not have the requisite electrons to back-bond, and 3Nd has the electrons but they are energetically incompatible with back-bonding to the MIC.
Thec omputed MDC-q charges and Nalewajski-Mrozek bond orders of 3U, 3Y, 3La,a nd 3Nd are instructive and fall into two distinct groups of 3U and 3Y/3La/3Nd.S pecifically, the U, C carbene , a-C,and a-N charges are 1.8, À0.81, À0.05, and À0.31 with U = C, C = C, and C À Nbond orders of 1.1, 1.64, and 1.22, respectively.T hose of 3Y, 3La,and 3Nd are remarkably invariant with av.M ,C carbene , a-C,a nd a-N charges of 1.35, À0.6, À0.04, and À0.29 and MÀC, C=C, and CÀNbond orders of 0.6, 1.73, and 1.26, respectively.Ifthere is no back-bonding, the carbene should show strong stabilizing interactions with the a-C and -N atoms,and the metal and carbene should have low positive and negative charges,respectively.Conversely,if back-bonding operates in addition to the s-donation then the metal and carbene should have higher positive and negative charges,r espectively,r eflecting the transfer of electron density back from the metal to carbene,a nd the carbene should consequently have weaker bonding interactions with the a-C and -N atoms.T his is exactly the situation that is suggested by the calculations,consistent with the Kohn-Sham descriptions.W enote that the metal-carbene bond order in 3U is nearly twice that of 3Y, 3La,and 3Nd,and, recalling that the U!MIC p-back-bond involves as ingly occupied 5forbital, that it is greater than one suggests the presence of at wo-fold bonding interaction where each component is polarized and of sub-integer bond order.C onsidering the generally accepted negligible p-acidity of MICs the donoracceptor bond in 3U is notable,and is also remarkably similar to the donor-acceptor interaction found computationally in [U(N'') 3 {C(NMeCMe) 2 }]. [15] However,wenote that the backbond must be weak because we could not freeze-out rotation of the MIC by the solvent low-temperature limit (À80 8 8C) in NMR studies.
NBO analysis of 3U (Figure 3) is also consistent with aU= Cd onor-acceptor interaction. TheM IC!U s-donation is returned as essentially electrostatic and so is predominantly carbon-based. However, the U!MIC p-back-donation is found to contain 75 %u ranium and 25 %c arbene character. Thecarbene acceptor orbital is apure 2p orbital, whereas the uranium donor orbital is 90 %5fa nd 10 %6 dc haracter.A s expected, only electrostatic NBO MIC!Mi nteractions are found for 3Y, 3La,a nd 3Nd.
Along with the orbital-based perspectives of DFT and NBO analyses we probed the topological electron density description of the M À Cb onds in 3U, 3Y, 3La,a nd 3Nd.T he calculations reveal M À C3 , À1c ritical points.T he 1(r) MC values are similar for all complexes (0.08-0.12) suggesting polar interactions,s ince covalent bonds tend to have 1(r) > 0.1, but we note that 3U has the highest 1(r) MC value.Most importantly,h owever, the calculated ellipticity parameters e(r) MC are 0.03, 0.06, and 0.09 for 3Y, 3La,a nd 3Nd, respectively,but for 3U the e(r) UC value is 0.36. This supports the notion of apolarized two-fold U=Cbonding interaction in 3U because as ingle s bond or triple s-p-p bond present symmetrical electron density distributions around the internuclear axes (e(r) % 0) whereas s-p double bonds are asymmetric (e(r) > 0). Forcomparison, calculated C À C e(r) CC values in ethane (H 3 C À CH 3 ), benzene (C 6 H 6 ), ethylene (H 2 C = CH 2 ), and acetylene (HC CH) are 0.0, 0.23, 0.45, and 0.0, respectively. [23] Inspection of the CASSCF-SO m J manifolds of 1U and 3U reveals that there is only asmall change in the energies of the three lowest doublets,whilst the two highest energy states are suppressed by about 400 cm À1 in 3U compared to 1U.W ealso observe aclear change in the g-values of the ground doublet, reflecting the departure from axial symmetry in 3U,a s confirmed experimentally.T hese modest changes reflect the coordination of the MIC and also the electronic partialcancellation effects of the U=Cd onor-acceptor interaction, analogous to donor-acceptor net-cancellation effects on the CO stretching frequency of thorium carbonyls. [24] To conclude,w eh ave prepared the first examples of fblock-MICs,w hich thus represent an ew class of f-block carbene.T hese complexes were prepared by af ormal 1,4proton migration of an NHO,w hich therefore represents an ew,g eneral way to prepare MIC complexes.Q uantum chemical calculations suggest that in addition to aMIC!U sdonation there is aw eak U(5f)!MIC(2p) p back-bond; although resonance form 3U-a most likely dominates,r esonance form 3U-b is non-negligible.A se xpected 3Y and 3La exhibit no back-bonding due to their d 0 f 0 natures,a nd 3Nd though being 4f 3 also does not back-bond as its valence 4f electrons are energetically incompatible to do this.T he donor-acceptor character in 3U is reminiscent of d-blockcarbonyl and Fischer carbene bonding,t hough the p-backbond is weak. Considering the generally at best weak pacidity of MICs,t he computational finding of U(5f)!MIC-(2p) p-back-bond is surprising and highlights that perhaps greater consideration should be given to more widely acknowledging MICs as potential p-acid ligands when coordinated to sufficiently reducing metals.