Actinide–Pnictide (An−Pn) Bonds Spanning Non‐Metal, Metalloid, and Metal Combinations (An=U, Th; Pn=P, As, Sb, Bi)

Abstract The synthesis and characterisation is presented of the compounds [An(TrenDMBS){Pn(SiMe3)2}] and [An(TrenTIPS){Pn(SiMe3)2}] [TrenDMBS=N(CH2CH2NSiMe2But)3, An=U, Pn=P, As, Sb, Bi; An=Th, Pn=P, As; TrenTIPS=N(CH2CH2NSiPri 3)3, An=U, Pn=P, As, Sb; An=Th, Pn=P, As, Sb]. The U−Sb and Th−Sb moieties are unprecedented examples of any kind of An−Sb molecular bond, and the U−Bi bond is the first two‐centre‐two‐electron (2c–2e) one. The Th−Bi combination was too unstable to isolate, underscoring the fragility of these linkages. However, the U−Bi complex is the heaviest 2c–2e pairing of two elements involving an actinide on a macroscopic scale under ambient conditions, and this is exceeded only by An−An pairings prepared under cryogenic matrix isolation conditions. Thermolysis and photolysis experiments suggest that the U−Pn bonds degrade by homolytic bond cleavage, whereas the more redox‐robust thorium compounds engage in an acid–base/dehydrocoupling route.

The preparation, isolation, and study of new molecular element-element bonds remains af undamentally important endeavour because it informs us about chemical characteristics and reactivity,a llows us to probe and refine periodic trends,a nd provides vital benchmarking for structural and theoretical predictions and modelling. Reflecting much progress over decades of synthetic effort, there are now few places left in the Periodic Table where new element-element bonds can be regularly discovered. However,t he actinides, where progress has generally lagged owing to their radioactivity and the inherent challenges involved in stabilizing chemical bonds to some of the largest metal ions in existence, remains arich seam from which to mine new chemical bonds. Fore xample,o nly very recently have the first AmÀS, PuÀC, and BkÀOb onds been crystallographically authenticated. [1] Theaforementioned examples all involve synthetic transuranic examples with unique associated challenges;s tudies have in particular been impeded by their radioactive nature, need for specialist handling facilities,a nd limited availabilities.H owever,e ven for naturally occurring neighbour elements like uranium and thorium, which can be handled in normal laboratories,there are chemical bond combinations yet to be realised. Fore xample,r egarding AnÀPn bonds (An = U, Th ;Pn= N, P, As,Sb, Bi), although covalent An À N, An À P, and An À As bonds are known, [2][3][4] somewhat remarkably given the burgeoning nature of non-aqueous actinide chemistry, [5] AnÀSb and AnÀBi derivatives are conspicuous by their absence even though analogous examples are known in transition-metal [6] and even in lanthanide chemistry. [7] Indeed, there are no structurally characterized U À Sb,T h À Sb,o rT h À Bi bonds and there is only one report of U À Bi bonds, [8] which involves open-shell, delocalised radical Zintl clusters that reside at the molecular-periodic interface. Seeking to remedy this situation, we sought to extend our previous work on early metalÀPnH 2 complexes. [3c,4,9] Since discrete (PnH 2 ) À anions are not available for Sb and Bi, and noting that many heavy PnH x R 3Àx reagents are prone to facile Pn À Cb ond homolysis,w eu tilised the more sterically demanding pnictides {Pn(SiMe 3 ) 2 } À for P, As,S b, and Bi, though even these reagents are prone to easy decomposition. [10,11] We reasoned that this would present the opportunity to prepare as tructurally homologous series of AnÀPn covalent bond benchmarks,w hilst enabling meaningful comparison of the Pn geometries (that is,development of trigonal pyramidal from trigonal planar) as the pnictide group is descended.
Herein, we report the synthesis and characterisation of molecular compounds containing new AnÀPn bonds that include the first structurally authenticated UÀSb and ThÀSb bonds of any kind and the first two-centre-two-electron (2c-2e) U À Bi bond. Thec orresponding Th À Bi bond was too unstable to isolate,h ighlighting the major challenges of preparing these ill-suited hard-soft linkages generally.T hese complexes present chemical bond benchmarks,and the UÀBi bond is the heaviest 2c-2e pairing of two elements involving an actinide under macroscopic,ambient conditions,exceeded experimentally only by An À An pairings in matrix isolation experiments. [12] Preparing homologues spanning non-metal, metalloid, and metal within as ingle element group has permitted elucidation of aformal periodic break-point;DFT calculations suggest that Pand As adopt formal À3oxidation states whereas Sb and Bi are more appropriately assigned as + 1.
TheU À Pn distances of 2.8646(14), 2.9423(9), 3.2437(8), and 3.3208(4) for 3UP, 3UAs, 3USb, 3UBi,respectively,can be compared to the respective sums of single-bond covalent radii of 2.81, 2.91, 3.10, and 3.21 , [14] and for 3UP to the UÀP distance of 2.789 (4)   Solid-state molecular crystal structures of 3UP, 3UAs, 3USb,and 3UBi,respectively,measured at 120 K. Ellipsoids set at 40 % probability;hydrogen atoms, minor disorder components, and any lattice solvent removed for clarity. [23] e)-h) Ball-and-stick representations of the core U-Pn(SiMe 3 ) 2 units from the same side-on perspectivei neach case, where one Si perfectly obscures the other,t oshow the increasing deviation from trigonal planar to trigonal pyramidal geometry as the pnictide series is descended;a ll other atoms in these depictions are omitted for clarity.Ugreen, Pn magenta, Nblue, Si orange, Cgray. ---C 5 Me 5 ) 2 (Cl){P(SiMe 3 ) 2 }]; [3h] these data essentially divide into two groups where the experimental U À Pand U À As pairs are within 0.05 of those predicted, but the discrepancies for the U À Sb and U À Bi distances are > 0.1 .This suggests aperiodic break between As and Sb,b ut also perhaps reflects the changing sum of angles at each Pn (P,3 54.06(8);A s, 349.71-(9);S b, 325.96 (11);B i, 315.98(9)8 8); although respective s-p energy gaps decrease as Group 15 is descended, rehybridisation promotion energies conversely increase owing to progressively inefficient s-p orbital overlap from increasingly diffuse orbitals. Interestingly,f or 3ThP and 3ThAs the ThÀPn distances are 0.08-0.1 longer than the uranium analogues even though the single bond covalent radius of Th is only 0.05 larger than that of U. [14] Conversely,h owever, the U À Pn distances for 4UP, 4UAs,a nd 4USb,a re all shorter than the respective corresponding U À Pn distances in the 3UPn series. Also,the sum of angles at Pn varies far less for the 4UPn than 3UPn series,w hich can be related to the sterically more demanding nature of Tr en TIPS compared to Tr en DMBS restricting the tendencytowards orthogonal bonding for the heavier pnictides.This suggests that the trigonal planar bonding mode of these pnictides is stronger than amore trigonal pyramidal mode.T his trend is overall repeated when comparing the 3ThPn and 4ThPn series together,t hough we note that the differences for 4ThPn vs. 4UPn are smaller than those of 3ThPn vs. 3UPn,reflecting the greater constraints imposed by the more sterically demanding Tr en TIPS compared to Tren DMBS .
TheU V/Vis/NIR spectroscopic data for 3UPn (Supporting Information, Figure S8) [11] show ac haracteristic absorption maxima that bathochromically shifts (3UP,19,420; UAs, 18,280; USb,15,820; UBi 14,245 cm À1 ); this can be related to the increasing pyramidalisation of the Pn centres as the Pn group is descended and adecreasing pnictide-uranium charge transfer energy.T he same trend is observed for 4UPn (Supporting Information, Figure S9). [11] To further probe the AnÀPn linkages,weexamined them using DFT,NBO,and QTAIM methods (Table 2; Supporting Information, Figures S21-S32). [11] Computed structures compare well with the solid-state structures,s ow ec onclude that these models represent qualitative pictures of the electronic structures of these complexes.T he An À Pn Mayer bond orders,c onsidering these linkages are expected to be polar, are surprisingly high, suggest Pn p-donation in addition to the anticipated s-bonds. [15] Thecomputed An charges,and spin densities for uranium, are overall consistent with their + 4oxidation states, [2a] but the computed charges of the Pn centres fall into two clear groups; for P/As computed charges are about À1toÀ1.3 whereas for Sb/Bi they are lower at about À0.3 to À0.4 and this does not appear to be related in any way to the geometry of the Pn centre as an explanation. This suggests aperiodic break where the former pair are best described formally as being in the À3 oxidation state whereas the latter two are better formulated as being + 1.
When the Pn centre remains essentially trigonal planar, as is the case for the 4UPn and 4ThPn series,t he An charge follows the trend AnÀAs > AnÀP > AnÀSb,w hich suggests that As is the weakest donor ion. Interestingly,for the 3UPn series the An charges increases from 3UP to 3UAs,b ut then falls away for 3USb and 3UBi,sothat the same,but extended, series of An charges of An À As > An À P @ An À Sb % An À Bi emerges.T his is counterintuitive,because the expected trend would be for the An charges to be ordered AnÀBi > AnÀSb > AnÀAs > AnÀPassuggested by the Mayer bond order data. However,i nspection of the DFT Kohn Sham and NBO descriptions of 3UP, 3UAs, 3USb, 3UBi, 3ThP, 3ThAs, 4UP, 4UAs, 4USb, 4ThP, 4ThAs,and 4ThSb reveals that whilst the AnÀPn s-bonds are largely ionic, surprisingly,s ince the Pn np-orbitals (n = 3-6) become increasingly diffuse,t here are significant p-bonding combinations in these complexes,a nd, using series 3UPn,a st he pnictide becomes more pyramidalised although one lobe of the p-orbital moves increasingly away from the metal the other lobe approaches much more closely and so may actually engage more effectively overall with one orbital lobe than two.T hus,l inkages that would be Bond length and index [b,c] Charges [d] Spin density [ expected to be weaker may actually be stronger,with respect to donor strength, which in this context is not synonymous with thermodynamic enthalpic bond strength. Thea symmetry of the An À Pn bonding,a ss uggested by DFT and NBO methods,i sf urther supported by analysis of the QTAIM data;a lthough highly polar bonds are certainly found, the bond critical point ellipticity values are consistently greater than zero and of the magnitude found for the C À Cbonds in benzene tending to ethene. [16] Although 4UBi, 3ThSb, 3ThBi,a nd 4ThBi have eluded isolation, attempts to prepare them along with studies on the subsequent reactivity of the isolable AnÀPn complexes has proven informative with respect to unravelling the underpinning chemistry of these AnÀPn linkages.A ttempts to prepare 4UBi from 2U and KBi(SiMe 3 ) 2 resulted in batchvariable quantities of green crystals,that could not be cleanly isolated, of (Me 3 Si) 2 Bi À Bi(SiMe 3 ) 2 (Bi 2 ) [11,17] as verified by single-crystal X-ray diffraction. This implies that 4UBi is transiently formed, but decomposes by homolytic UÀPn bond cleavage to give [U(Tren TIPS )], [18] which was indeed identified in reaction mixtures by 1 HNMR spectroscopy.I nterestingly, attempts to prepare 3ThSb, 3ThBi,and 4ThBi resulted in the isolation, respectively,o fv ariable levels of red (Me 3 Si) 2 Sb À Sb(SiMe 3 ) 2 (Sb 2 ), [11,19] and green Bi 2 ,verified by single-crystal X-ray diffraction. Since the Th 4+ /Th 3+ reduction potential is very negative, [20] homolytic ThÀPn bond cleavage seems unlikely,b ut on one occasion, from otherwise intractable, complex reaction mixtures,c rystals of the cyclometallate complex [Th{N(CH 2 CH 2 NSiMe 2 Bu t ) 2 (CH 2 CH 2 NSiMeBu t -CH 2 )}(DME)] (5)w ere isolated from an attempted preparation of 3ThSb. [11] This suggests that ac oncerted or step-wise deprotonation/cyclometallation-dehydrocoupling of HPn-(SiMe 3 ) 2 reaction occurs for thorium.
To probe the mechanistic aspects further we investigated thermal and photolytic U À Pn reactivity profiles;a nalogous thorium studies gave no different outcomes to the ones described above.Complex 4USb,and putative 4UBi prepared in situ, completely decompose at 80 8 8Ct og ive [U(Tren TIPS )] and Sb 2 or Bi 2 ,r espectively,c onsistent with homolytic bond cleavage.U nder these conditions, 4UP and 4UAs and surprisingly 3USb and 3UBi show little decomposition (< 5%). Photolysis (125 WU Vl amp,2 h) of 4USb and putative 4UBi prepared in situ results in conversion into the cyclometallate complex [U{N(CH 2 CH 2 NSiPr i 3 ) 2 (CH 2 CH 2 N-SiPr i 2 CHMeCH 2 )}] [21] and elemental Sb or Bi. Interestingly, photolysis of 3USb and 3UBi results in initial formation of Sb 2 or Bi 2 ,H 2 ,a nd the cyclometallate [U{N(CH 2 CH 2 NSiMe 2 Bu t ) 2 (CH 2 CH 2 NSiMeBu t CH 2 )}], [22] but 3UP and 3UAs show little decomposition under photolytic conditions.E xtended photolysis of 3USb and 3UBi resulted in Sb 2 /Bi 2 decomposition to elemental Sb/Bi, which was verified by independent decomposition of Sb 2 /Bi 2 under the same conditions.Under photolytic conditions,[U(Tren R )] species slowly cyclometallate with elimination of H 2 .S o, the above data suggest that for U À Pn bonds homolysis is the preferred decomposition route,w hich may proceed to production of uranium-cyclometallate and elemental pnictide deposition under photolytic but not thermal conditions,b ut the ThÀPn linkages undergo acid-base/dehydrocoupling reactions owing to the redox robustness of thorium. The more facile decomposition of 4USb and "4UBi" compared to 3USb and 3UBi suggest that although sterically demanding ligands are necessary to stabilise these polar U À Pn linkages that Tren TIPS may be too bulky and actually destabilise the UÀ Sb/UÀBi linkages;t he isolation of 3UBi is thus remarkable because this complex has the facile uranium redox bond homolysis route open to it yet it is still isolable.This is clearly ad elicate balance of sterics,s ince for the larger thorium neither Tren DMBS nor Tr en TIPS can stabilise Th À Bi linkages. Such afine balance of metal/ligand size ratio has been found previously with respect to m-phosphido linkages where thorium-Tren TIPS gives stable ThPThl inkages but uranium-Tr en TIPS results in UPU linkages that readily decompose. [9a,d] To conclude,w eh ave reported the synthesis and characterisation of new An À Pn bonds that include the first structurally authenticated U À Sb and Th À Sb bonds of any kind and the first 2c-2e UÀBi bond. Thecorresponding ThÀBi bond was too unstable to isolate,h ighlighting the major challenges of preparing these mismatched hard-soft linkages generally.T hese complexes present chemical bond benchmarks,and the U À Bi bond is the heaviest 2c-2e pairing of two elements involving an actinide under macroscopic,a mbient conditions,e xceeded only by An À An pairings in matrix isolation experiments.P reparing homologues spanning nonmetal, metalloid, and metal within asingle element group has permitted elucidation of af ormal periodic break-point between As and Sb.R eactivity studies suggest that the heavier U À Pn bonds decompose by homolytic U À Pn bond cleavage,a nd the resulting uranium(III) and di-pnictane compounds react further to give uranium(IV)-cyclometallate, hydrogen, and elemental pnictide,r espectively,w hereas the more redox robust thorium complexes engage in an acidbase/dehydrocoupling route to give thorium-cyclometallate and di-pnictane.T hus,a lthough the same product classes emerge from these decomposition reactions overall they proceed via different mechanistic routes,h ighlighting the different redox chemistries of uranium and thorium actinide elements.