An Inverted-Sandwich Diuranium μ-η5:η5-Cyclo-P5 Complex Supported by U-P5 δ-Bonding**

Reaction of [U(TrenTIPS)] [1, TrenTIPS=N(CH2CH2NSiiPr3)3] with 0.25 equivalents of P4 reproducibly affords the unprecedented actinide inverted sandwich cyclo-P5 complex [{U(TrenTIPS)}2(μ-η5:η5-cyclo-P5)] (2). All prior examples of cyclo-P5 are stabilized by d-block metals, so 2 shows that cyclo-P5 does not require d-block ions to be prepared. Although cyclo-P5 is isolobal to cyclopentadienyl, which usually bonds to metals via σ- and π-interactions with minimal δ-bonding, theoretical calculations suggest the principal bonding in the U(P5)U unit is polarized δ-bonding. Surprisingly, the characterization data are overall consistent with charge transfer from uranium to the cyclo-P5 unit to give a cyclo-P5 charge state that approximates to a dianionic formulation. This is ascribed to the larger size and superior acceptor character of cyclo-P5 compared to cyclopentadienyl, the strongly reducing nature of uranium(III), and the availability of uranium δ-symmetry 5f orbitals.

The cyclopentadienyl anion is au biquitous ligand in organometallic chemistry, [1] and therefore there is great interest in studying isolobal analogues in terms of overcoming the challenges of preparing them and understanding how they bind to metal ions.O ne such congener is the cyclo-P 5 anion, which is of special interest due to the diagonal relationship between carbon and phosphorus. [2] To date,c yclo-P 5 is found as aformal monoanion in d-block complexes where electronrich metals promote the formation and stabilization of the P 5 ring by an umber of binding modes to the p-system and phosporus lone pairs,including terminal-h 5 , [3] bridging m-h 5 -s n or m-h 5 :h 5 -s n , [4] spectacular fullerene-type topologies, [5] or m-h 5 :h 5 , [6] though the latter mode is less common. Where the fblock is concerned, there are no examples of actinide cyclo-P 5 derivatives, [7] and only two examples incorporating lanthanide ions are known, [8] which notably both contain the d-block fragments used to construct and introduce the cyclo-P 5 unit. There is,t herefore,aquestion mark over whether the synthesis and stabilization of cyclo-P 5 by electron-rich dblock metals is amandatory requirement. Furthermore,given the absence of any actinide cyclo-P 5 complexes,itisnot clear how such afragment would bind to uranium because although many inverted sandwich arene-C n (n = 4, 6-8) complexes are now known, [9] cyclopentadienyl is conspicuous by its absence. Cyclopentadienyl tends to bind to d-block metals via s-and pinteractions,a nd d-bonding tends to be minimal due to poor spatial overlap from the small size of the cyclopentadienyl ring, [10] but the different frontier orbital energies and larger size and superior acceptor character of cyclo-P 5 ,compared to cyclopentadienyl, make predictions impractical. In aw ider context, the activation of P 4 ,apotential source of cyclo-P 5 ,by f-block complexes is incredibly rare, [11] despite widespread interest in the activation of this highly strained pnictide,a nd cyclo-P 5 stands out as the missing member of the P 4 and P [6][7][8] family assembled by f-block-promoted catenations to date.
As part of our studies into uranium-pnictide chemistry, [12] we extended our examination of the reactivity of P 4 with uranium to [U(Tren TIPS )] (1,S cheme 1; Tren TIPS = Tr eatment of 1 with aq uarter molar equivalent of P 4 (U:P = 1:1) in THF results in the dark blue-green suspension turning brown. After work-up,r ecrystallization of the sticky brown solid from toluene afforded brown crystals of 2 in 25 % yield (based on phosphorus). [13] Although this yield is low,itis reproducible,a nd most likely represents the lower degree of orbital control over the phosphorus catenation than compared to the d-block. Indeed, given the prior paucity of actinide cyclo-P 5 complexes it is remarkable that 2 can be isolated at all. The 1 HNMR spectrum of 2 spans the range À33 to + 9ppm and the Evans method magnetic moment at 298 Ki s4 .09 m B ;b oth of these observations are consistent with the presence of two uranium(IV) ions in 2.Complex 2 is silent its 31 PNMR spectrum, most likely because of the direct contact between the paramagnetic uranium ions and the cyclo-P 5 unit. TheIRspectrum of 2 contains no absorptions in the P À Hs tretch region, which is consistent with the absence of ap rotonated form of the cyclo-P 5 unit, and the empirical formulation is supported by CHN analyses.
Thes olid-state structure of 2 was determined by X-ray crystallography and is illustrated in Figure 1. [13] Thes alient feature of 2 is the presence of ac yclo-P 5 unit sandwiched between two [U(Tren TIPS )] fragments in a m-h 5 :h 5 coordination mode.Acrystallographic twofold rotation axis runs through one phosphorus center and the mid-way point of aP ÀPbond on the cyclo-P 5 ring in the asymmetric unit, and as ac onsequence the cyclo-P 5 unit is disordered over two positions.The P À Pb onds required restraints during refinement because of the disorder,sonomeaningful discussion of the P À Pmetrical data can be made,but it is clear that the cyclo-P 5 ring is planar in 2.T he UÀPb ond lengths span the range 3.250(6)-3.335 (6) , which are longer than the sum of the singlebond covalent radii of Uand P(2.81 ), [14] perhaps reflecting the steric demands of Tren TIPS and the h 5 -bound nature of the cyclo-P 5 unit. As as trict requirement of residing over the crystallographic twofold rotation axis,the two uranium-Tren fragments are identical, and notably the U À Nbond distances are consistent with Tr en-ligated uranium(IV) centers, [15] being too short for U III ÀNo ra veraged U III/IV ÀNd istances. [16] An apriori description of 2 would be amixed valence diuranium-(III/IV) with ac yclo-P 5 monoanion;t he crystallographic analysis,however,isinconsistent with this.T oinvestigate this unexpected aspect further we probed complex 2 by spectroscopic and magnetic methods.
TheU V/Vis/NIR spectrum of 2 [13] exhibits weak (e = < 80 Lmol À1 cm À1 )absorptions in the range 5000-15 000 cm À1 that are characteristic of intraconfigurational, Laporte-forbidden f-f transitions in the 3 H 4 manifold of uranium(IV). [17] Ac harge transfer band tails in from the UV region to about 15 000 cm À1 ,but is of sufficiently low absorbance in the region where any 5f 3 !5f 2 6d 1 transitions for uranium(III) would occur for them to be visible; [17c, 18] however, no such absorbances are apparent, and although this does not conclusively rule out 2 containing localized uranium(III) centers it is consistent with ad iuranum(IV) formulation. This would, however, invoke aformal dianion formulation for the cyclo-P 5 unit rather than the more likely monoanionic formulation.

Angewandte
Chemie formulation for 2.I nt he absence of an analogue of 2 containing diamagnetic metal ions it is currently not possible to determine the cyclo-P 5 unit contribution, if any,t ot he magnetic susceptibility of 2,t hough we note that the lowtemperature magnetic moment per uranium ion is ca 0.2 m B higher than might be expected, [19] which may reflect ac ontribution to the magnetic moment from the cyclo-P 5 ring. Cyclic voltammetry experiments were precluded by the incompatibility of 2 with polar solvents.
To further probe the nature of 2,w econducted as inglepoint energy calculation on the geometry optimized structure of 2. [13] Thec alculated bond lengths and angles are within 0.05 and 28 8 of the experimentally determined structure and so we conclude that the calculated structure represents aq ualitative model of the electronic structure of 2.I nterestingly,i nt he calculation the cyclo-P 5 -ring is slightly puckered, [9] which may be attributed to the gas-phase nature of the calculation compared to the experimental structure of 2 that is subject to crystal packing forces.T he calculated MDC q uranium charges are 3.04 and 3.07, which is towards the high end for Tr en-uranium(IV) complexes. [12,15] Thec yclo-P 5 unit carries atotal charge of À2.68, distributed evenly amongst the five P-centers;t his suggests substantial charge transfer from the uranium centers,w hich is also consistent with the calculated MDC m uranium spin densities of À1.93 and À1.98. Thel atter values are consistent with 5f 2 uranium(IV) centers,t hough we note that uranium(IV)-Tren complexes tend to exhibit calculated MDC m values of about 2.3 that are consistent with donation of electron density from the ligands to uranium, whereas for 2 uranium is an et exporter of electron density to the cyclo-P 5 unit. Importantly,wenote that the calculated excess spin density on the cyclo-P 5 unit is À1.69, which suggests significant radical character. The calculated U À Pa nd P À PM ayer bond orders for 2 span the range 0.32-0.60 and 0.68-0.78, respectively,w hich suggests polarized covalent interactions.For comparison, the U À N amide and UÀN amine bond orders average 0.90 and 0.25, respectively. TheH OMO,H OMOÀ1, and HOMOÀ2 a-spin Kohn Sham frontier orbitals of 2 are each singly occupied and of essentially pure 5f character.H OMOÀ3a nd HOMOÀ4 ( Figure 3) are each singly occupied in the a-spin manifold and represent the principal interactions of the U(P 5 )U unit. These two orbitals result from donation from uranium 5f-orbitals of d-symmetry into the d-symmetry e 2 cyclo-P 5 frontier molecular orbitals.T he HOMOÀ3a nd HOMOÀ4a re dominated by uranium 5f and phosphorus 3p contributions,b eing composed of 36/48 and 35/42 %3 p/5fc haracter (that is,c a. 50:50 3p/5fc haracter in each), respectively,with the remainder of each molecular orbital being accounted for by small nitrogen contributions derived from the Tr en TIPS ligands. Interestingly,c omplex 2 appears silent in its powder X-band EPR spectrum at 300, 30, and 5K,which may reflect the 3p-5f mixing giving rise to efficient relaxation mechanisms.T he natures of the five a-spin frontier orbitals of 2 are consistent with the overall occupation of four 5f and one cyclo-P 5 e 2 combination. Thus,t he calculations are in overall agreement with the combined characterization data and together suggest that the most appropriate description of 2 is that two uranium(III) ions have each singly reduced the cyclo-P 5 unit to give two uranium(IV) centers with ac yclo-P 5 charge state that is approaching ad ianion, rather than monoanion formulation. Thet wo d*c ombinations for the U(P 5 )U unit in 2 are represented by LUMOs + 9and + 10, which lie about 1.8 eV higher than HOMOÀ3a nd HOMOÀ4. This corresponds to a d-d*g ap of about 14 500 cm À1 (690 nm), and an absorption at ca. 690 nm (e % 85 Lmol À1 cm À1 )i so bserved in the experimental UV/Vis/NIR spectrum of 2. [13] To conclude,w eh ave reported the synthesis and characterization of the first structurally authenticated actinide cyclo-P 5 complex. Thep reparation of this d-block-metal-free complex demonstrates that the cyclo-P 5 unit can be constructed without d-block ions.H owever,t here is clearly less control over its construction, which results in low yields of 2. That 2 can be isolated at all is significant given the prior paucity of f-block cyclo-P 5 complexes,b ut importantly its isolation permits an analysis of how cyclo-P 5 can bind to actinide elements.C yclopentadienyl usually bonds to metals via s-a nd p-interactions with minimal d-bonding,a nd although cyclo-P 5 is isolobal to cyclopentadienyl, theoretical calculations suggest the principal bonding in the U(P 5 )U unit is two polarized d-bonds.T his can be attributed to the larger size and superior acceptor character of cyclo-P 5 compared to cyclopentadienyl and the availability of uranium d-symmetry 5f-orbitals.S urprisingly,t he combined characterization data are consistent with charge transfer from the uranium ions to the cyclo-P 5 unit such that the charge state of the latter approximates to ad ianionic formulation. However,w ith the presence of d-bonding and 3p-5fo rbital mixing the five frontier a-spin electrons are delocalized across the U(P 5 )U unit. Therefore,given this electronic structure the assignment of oxidation states and spins to individual centers is not clearcut and we use this electronic structure as af ramework in which to rationalize the bonding in 2,asisthe case in inverted sandwich diuranium C 6 -arene complexes. [9] This unexpected outcome,can be attributed to the strongly reducing nature of uranium(III) coupled to the excellent acceptor properties of cyclo-P 5 .L astly,c omplex 2 also represents an isolobal analogue of adiuranium inverted sandwich cyclopentadienyl complex, which remains conspicuous by its absence in the burgeoning inverted sandwich arene-C n (n = 4, 6-8) family. [9] Keywords: cyclo-P 5 ·density functional theory ·phosphorus · uranium · d bonding . .