Yttrium Complexes of Arsine, Arsenide, and Arsinidene Ligands**

Deprotonation of the yttrium–arsine complex [Cp′3Y{As(H)2Mes}] (1) (Cp′=η5-C5H4Me, Mes=mesityl) by nBuLi produces the μ-arsenide complex [{Cp′2Y[μ-As(H)Mes]}3] (2). Deprotonation of the As–H bonds in 2 by nBuLi produces [Li(thf)4]2[{Cp′2Y(μ3-AsMes)}3Li], [Li(thf)4]2[3], in which the dianion 3 contains the first example of an arsinidene ligand in rare-earth metal chemistry. The molecular structures of the arsine, arsenide, and arsinidene complexes are described, and the yttrium–arsenic bonding is analyzed by density functional theory.

Rare-earth metal compounds containing soft heteroatom donor ligands have attracted considerable interest in recent years. [1][2][3][4][5][6] Thec ombination of Lewis acidic M 3+ cations with heavy p-block donor atoms results in ah ard-soft mismatch that can lead to unusual bonding properties and to distinct reactivity.W ithin this context, rare-earth metal complexes of anionic phosphorus donor ligands such as phosphide (R 2 P À ) have been extensively studied. [7] Akey development occurred in 2008, when alutetium phosphinidene (RP 2À )complex was structurally characterized and its phosphinidene transfer reactivity towards aldehydes and ketones demonstrated. [8] Phosphinidene complexes of other rare-earth metals were subsequently reported, and their phosphinidene transfer chemistry and small-molecule activation reactions described. [9][10][11][12][13][14] Despite the increased activity in rare-earth metal phosphinidene chemistry,t he area is considerably underdeveloped relative to transition metal phosphinidene chemistry. [15] At erminally bonded phosphinidene ligand remains ak ey target in rare-earth metal chemistry,a lthough auranium complex of such aligand was reported recently. [16] Thechemistry of rare-earth metal complexes with arsenic donor ligands is almost entirely unexplored:arsenide (R 2 As À ) complexes are rare, [17][18][19][20][21][22] and arsinidene (RAs 2À )l igands are unknown in rare-earth metal chemistry.T he development of synthetic routes to rare-earth metal arsinidene complexes could lead to more novel reactivity,s uch as arsinidene transfer, and would also furnish new opportunities for using arsenic ligands to influence the electronic structure and magnetism of lanthanide(III) complexes.W ith these possibilities in mind, we now report the first example of arare-earth metal arsinidene complex.
Our strategy involved the initial synthesis of ap rimary arsine complex of yttrium to establish the metal-arsenic bond, followed by deprotonation of the {YAsH 2 R} unit to give corresponding yttrium-arsenide and yttrium-arsinidene complexes.T hus,a dding one stoichiometric equivalent of mesitylarsine to Cp' 3 [3]·thf,asorange crystals in 73 %yield.
Them olecular structure of the yttrium arsenide 2 ( Figure 2) consists of ac entral Y 3 As 3 chair-like ring,w ith each yttrium ligated by two m-arsenide ligands and two h 5 -Cp' ligands.The YÀAs bond lengths in 2 are in the range 2.977(2)-3.019(2) ( average 2.998 ), and therefore they are,o n average,approximately 0.10 shorter than the YÀAs bond in 1,which is due to the stronger electrostatic attraction between yttrium and the arsenide ligand. TheAs-Y-As bond angles are in the range 88.66(5)-96.26(5)8 8.T he Y À Cb ond lengths in 2 are 2.59(1)-2.67(1) , and the average YÀCd istance of 2.63 isapproximately 0.08 shorter than in 1.The Y-As-Y angles in 2 are 130.56(5), 135.09 (6), and 135.66(6)8 8,and each arsenic center carries an exo mesityl substituent. TheA s À H stretching vibrations were observed in the IR spectrum at 2120 and 2154 cm À1 (Supporting Information, Figure S9).
The 1 H-13 CHSQC spectrum of 2·toluene at 298 Kreveals that two proton environments at d( 1 H) = 2.51 and 2.60 ppm, with relative integrals of 1:2, do not engage in 1 J coupling to carbon, which identifies them as the AsÀHp rotons and indicates that there are two magnetically inequivalent arsenic environments.T he 1 HNMR spectrum of 2·toluene features four resonances in the range d( 1 H) = 6.78-6.94 ppm and 5.84-6.51 ppm, which correspond to the six mesityl CH protons and the 24 Cp' CH protons,respectively (Supporting Information, Figures S3-S5). Thev arious CH 3 environments occur in the range d( 1 H) = 1.72-2.66 ppm.
Complex 2 is the first rare-earth metal complex of ap rimary arsenide ligand;h owever,s everal crystallographically characterized rare-earth metal complexes of secondary arsenide ligands have been reported. [18][19][20][21][22] Arange of synthetic routes have been employed to access secondary arsenide complexes,including,for example,deprotonation of Ph 2 AsH by the lutetium-lithium methyl complex [Cp 2 Lu(m-CH 3 ) 2 Li-(tmeda)] (tmeda = N,N,N',N'-tetramethylethylenediamine), which resulted in the formation of the arsenide-bridged species [Cp 2 Lu(m-AsPh 2 ) 2 Li(tmeda)]. [17] Activation of AsÀAs bonds by samarium(II) reduction has also been used to access arsenide complexes such as [Cp* 2 SmAsPh 2 ], which features aterminally bonded [Ph 2 As] À arsenide ligand. [19,20] Lanthanide(II) arsenide and arsolyl complexes can be accessed by salt metathesis reactions of LnI 2 with alkali-metal arsenide salts; for example,M es 2 AsK reacts with SmI 2 to give trans-[(Mes 2 As) 2 Sm(thf) 4 ]. [21,22] Thestructure of the arsinidene-ligated complex dianion 3 ( Figure 3) also consists of acentral chair-like Y 3 As 3 core,with three arsinidene ligands bridging the yttrium centers.A Figure 2. Molecular structure of 2,w ith ellipsoids set at 50 %p robability. [27] Unlabeled atoms are carbon;hydrogen atoms are not shown.  . . lithium cation caps the core of the structure and bonds to the three arsenic donors,such that the arsinidene ligands adopt an overall m 3 -bridging mode.T he Y À As bond distances in 3 are 2.8574(6)-2.8893(7) ( average 2.8722 ), making them shorter on average than the YÀAs bonds in 2 by more than 0.12 . It is also noteworthy that the Y···Y separations in 2 are 5.465-5.548 , whereas those in 3 are 5.266-5.314 ; overall, therefore,the Y 3 As 3 core of 3 is more compact than that of 2. Relative to 2,abroader range of Y À Cb ond lengths,t hat is, 2.59(2)-2.731(6) , and ag reater average Y À Cb ond length of 2.67 , are found in 3.T he distortion of the Y 3 As 3 chair conformation in 3 is reflected in the As-Y-As and Y-AsÀY bond angles of 91.59(2)-94.87(2)8 8 and 133.59(2)-136.49(2)8 8, respectively.T he lithium cation in 3 is ligated by three arsinidene ligands and resides 0.889(8) a bove the mean plane of the arsenic atoms.T he Li À As bond lengths are 2.539(8), 2.563(7) and 2.615(8) , and the As-Li-As angles are 107.2(3), 108.2(3) and 110.0(3)8 8.A northo methyl group on one mesityl substituent is oriented towards Li1, and the relatively short Li1···C54 distance of 2.777(8) may indicate an agostic interaction similar to that observed in other lithium complexes containing CH 2 Rs ubstituents (R = H, alkyl, silyl). [23] The 1 HNMR spectrum of [Li(thf) 4 ] 2 [3]·thf,r ecorded 30 min after sample preparation in [D 8 ]thf at 298 K(Supporting Information, Figure S6), features two resonances at d( 1 H) = 6.71 and 6.80 ppm, both of which integrate to three protons and correspond to two types of mesityl meta CH environments.T he Cp' CH protons occur as four resonances at d( 1 H) = 6.44, 6.15, 5.07, and 4.85 ppm, each of which integrates to six protons.Distinct singlets for the ortho, para, and Cp' CH 3 environments were observed in the region d( 1 H) = 1.54-2.62 ppm. Thea ppearance of the 1 HNMR spectrum of [Li(thf) 4 ] 2 [3]·thf is therefore consistent with the arsinidene complex possessing a C 3 symmetry axis coincident with Li1 and approximately perpendicular to the Y 3 plane. The 7 Li NMR spectrum of [Li(thf) 4 [3]·thf (Supporting Information, Figure S8). An additional feature of the 1 HNMR spectrum of [Li-(thf) 4 ] 2 [3]·thf is that, over time,a dditional resonances which were observed as minor components after 30 min grow in intensity (Supporting Information, Figure S7). After aperiod of only two hours,t he additional resonances account for as ignificant component of the NMR spectrum. It was not possible to identify the decomposition products;however,this unexpected feature suggests that the arsinidene ligands in [Li(thf) 4 ] 2 [3]·thf react with the thf solvent.
Thev ariation in the character of the yttrium-arsenic bonding in complexes 1, 2,a nd 3 was investigated using density functional theory:the calculations were simplified by replacing the para and Cp' methyl groups with hydrogen atoms.G eometry optimizations employing two exchange correlation functionals were carried out in the gas-phase and using ac ontinuum dielectric,a nd the results of the calculations with the hybrid PBE0 functional, including dielectric effects,are described.
Comparing the calculated and experimental YÀAs bond lengths,w ef ind good agreement for 1 (3.113 v s. 3.095 ) but also that the calculations slightly overestimate the average distance for 2 (3.061 v s. 2.998 ) and for 3 (2.912 v s. 2.872 ). Thed iscrepancies are most likely due to the inability of the simulations to fully account for solid-state intermolecular interactions,and also the effects of the counter cations on 3.Despite this,the overall trend in the decrease of the YÀAs bond length is reproduced. Thea tomic charges were calculated by natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) analyses (Table 1).
Both types of analysis show an increasing negative charge on the arsenic donor atom on moving from 1 to 2 to 3.T he QTAIM-derived localization index, l,which provides ameasure of the number of electrons localized on ag iven atom, increases by 0.94 from 1 to 2,a nd again by 0.90 from 2 to 3. Theanalogous parameters for yttrium are essentially constant across the three complexes,which indicates that the observed decrease in Y À As bond lengths is due to stronger ionic interactions.H owever,t he fact that the calculations produce Dl < 1i mplies as mall-but-increasing non-ionic contribution as the negative charge on arsenic increases.
TheY À As bonding was investigated further by atopological analysis of the electron density.T he parameter 1,w hich describes the electron density at the QTAIM-derived bond critical point (BCP), increases from 1 to 2 to 3,a se xpected based on the decreasing YÀAs bond lengths,a nd which is consistent with the change in l.T he 1 values indicate an enhancement in the non-ionic contribution to the YÀAs

Angewandte
Chemie bonding in the arsinidene complex 3;however,the values are still markedly less than expected for at ypical covalent bond (1 > 0.2). Thevalues of the energy density (H)atthe BCP,and the values of the delocalization indices (d), which provides ameasure of the number of electrons shared between yttrium and arsenic,a re indicative of considerable ionic bonding character in 1, 2,and 3.However,the general increase across the series also implies an increasing degree of non-ionic character in 3.
In summary,t he synthesis and structure of yttrium complexes with arsine,a rsenide,a nd arsinidene ligands have been described. Thesynthetic strategy involved initial assembly of an yttrium-arsenic bond, followed by stepwise deprotonation of the {YAsH 2 R} unit. Theresulting yttrium-lithium complex [{Cp' 2 Y(m-AsMes)} 3 Li] 2À (3)i st he first rare-earth metal complex of an arsinidene ligand. As with closely related rare-earth metal phosphinidene complexes,t he arsinidene ligands in 3 adopt a m-bridging coordination mode;stabilization of at erminally bonded [RAs] 2À ligand will require greater steric bulk than is provided by the substituents used in the current study.O ur computational analysis of the YÀAs bonding confirms the expected ionic character,b ut we also find as mall and potentially significant change in non-ionic contributions across the arsine,a rsenide,a nd arsinidene series.T he 4f electronic structure of lanthanide(III) cations will be sensitive to such ligand field variations at low temperatures,and thus our study introduces new possibilities for the design of single-molecule magnets. [26] Keywords: arsenic ·arsinidene ligands ·l ithium · rare-earth elements ·yttrium