DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 107–108 107 Synthesis and structure of [{As2(NCy)4}2Li4], containing an imido As(III) dianion Michael A. Beswick, Eilis A. Harron, Alexander D. Hopkins, Paul R. Raithby and Dominic S. Wright * Chemistry Department, University of Cambridge, Lensfield Road, Cambridge, UK CB2 1EW. E-mail: dsw1000@cus.cam.ac.uk Received 22nd October 1998, Accepted 27th November 1998 Reaction of [As(NMe2)3] with CyNH2 (1 :1 equivalents), followed by the addition of [CyNHLi] (1 equivalent) gives the heterobimetallic cage complex [{As2(NCy)4}2Li4], the first example of a complex containing an imido As(III) anion; the missing link in a series of Group 15 anions of the type [{E2(NCy)4}]22 (E 5 Bi, Sb and P).In previous work we showed that a series of imido Bi and Sb anions could be prepared utilising dimethyl amido derivatives.1 The complexes [{Bi2(NtBu)4}Li2?2thf] 2 and [{Sb2(NCy)4}2Li4] 3 are obtained from the in situ reactions of [E(NMe2)3] (E = Sb, Bi) with primary amines [RNH2; R = tBu, Cy (cyclohexyl)], followed by the reaction of the resulting [(Me2N)E(m-NR)]2 dimers with [RNHLi].Whereas [{Sb2(NCy)4}2Li4] has an elaborate cage structure consisting of two interlocked broken [Sb2N4Li2] cubanes in the solid state,3 such aggregation is precluded in the case of [{Bi2(NtBu)4}Li2?2thf ] by the solvation of the Li1 cations by thf (the complex remaining as a discrete cubane).2 Recently it has also been shown that a P analogue of these systems [{P2(NtBu)4}Li2?2thf ], isostructural with the previous Bi complex, can be obtained by deprotonation of [(tBuNH)P(m-NtBu)]2 with nBuLi in thf.4 Transmetallation of the Sb complex with metal salts provides a versatile strategy to heterobimetallic complexes containing [E2(NR)4]22 ligands.5 In view of the current interest in these species as ligand systems and in the light of the recent application of related alkali metal/Sb(III) phosphinidene cages as sources of photoemissive materials,6 we have initiated studies of the corresponding As systems as potential precursors to GaAs.We present here the synthesis and structure of [{As2(NCy)4}2- Li4] 1, containing an [As2(NR)4]22 anion; the missing link in the series of Group 15 containing [E2(NR)4]22 ligands and the first such polyimido anion of As(III) to be reported. Complex 1 is prepared by the reaction of [As(NMe2)3] with [CyNH2] (1:1 equivalent) followed by the addition of [CyNHLi] (1 equivalent) in toluene [eqn.(1)].† 4[As(NMe2)3] (i) 14CyNH2 (ii) 14CyNHLi [{As2(NCy)4}2Li4] 1 12Me2NH (1) The low-temperature X-ray structure determination of 1 ‡ shows that it possesses a cage structure constructed from the association of two interlocked ‘broken’ [As2(NCy)4Li2] cubanes (Fig. 1). The roughly tetrahedral arrangement of the Li1 cations at the centre of the cage and the pattern of the coordination of the Li1 cations by the m-N [Li–N range 2.08(1)– 2.132(9) Å] and terminal CyN groups [Li–N range 1.941(9)– 2.043(9) Å] of the [As2(NCy)4]22 anions are very similar to that occurring in the Sb analogue [m-N–Li range 2.07(2)–2.14(2), terminal N–Li range 1.96(3)–2.03(2) Å].3 This similarity with the Li4N4 substructure of 1 occurs despite the presence of significantly shorter As–N bonds in the [As2(NCy)4]22 anion (m-N–As average 1.92 Å, terminal As–N average 1.79 Å7), which at first sight may be anticipated to result in a markedly smaller ligand bite.However, the overall compression in the [As(m-NCy)]2 ring units of 1 compared to the [Sb(m-NCy)]2 units of the Sb counterpart is largely oVset by the greater exocyclic N–As–N angles in 1 [average m-N–As–N (terminal) 100.28; cf. average 90.88 in the Sb complex3], so that the bite of the terminal CyN groups is almost identical in 1 and its Sb Fig. 1 (a) Structure of 1. H atoms have been omitted for clarity. Key bond lengths (Å) and angles (8): As(1)–N(5) 1.916(4), As(1)–N(7) 1.790(3), As(1)–N(8) 1.929(4), As(2)–N(5) 1.932(4), As(2)–N(6) 1.789(4), As(2)–N(8) 1.915(4), As(3)–N(1) 1.917(4), As(3)–N(3) 1.792(4), As(3)–N(4) 1.943(4), As(4)–N(1) 1.922(4), As(4)–N(2) 1.797(4), As(4)–N(4) 1.914(4), N(1)–Li(3) 2.10(1), N(2)–Li(1) 1.941(9), N(2)–Li(3) 2.039(9), N(3)–Li(2) 1.966(9), N(3)–Li(4) 2.043(9), N(4)– Li(4) 2.130(9), N(5)–Li(1) 2.08(1), N(6)–Li(1) 2.008(9), N(6)–Li(4) 2.012(9), N(7)–Li(3) 1.979(9), N(7)–Li(2) 2.037(9), N(8)–Li(2) 2.132(9), C(21) ? ? ? Li(1) 2.614(9), C(31) ? ? ? Li(2) 2.79(1), C(36) ? ? ? Li(2) 2.78(1), C(72) ? ? ? Li(3) 2.774(9); As–(m-N)–As mean 96.2, (m-N)–As–(m-N) mean 82.5, exo-(m-N)–As–N mean 100.2, (m-N)–Li–N mean within SbN2Li rings 85.1, sum of N–Li–N angles about Li 348.7; (b) core of 1.108 J.Chem. Soc., Dalton Trans., 1999, 107–108 analogue [N(2,6) ? ? ? N(3,7) average 4.20 Å in 1; cf. average 4.27 Å in the Sb complex]. The only noticeable concession to the presence of a more compact dianion ligand in 1 is the more acute N–Li–N angles made with the chelating m-N and terminal-N centres (average 85.18; cf. 90.48 in the Sb analogue 3). There is also some eVect on the pattern of peripheral agostic C(–H) ? ? ? Li interactions with the Cy groups. In the Sb analogue the a-C–H of each of the pendant CyN groups are orientated towards and involved with adjacent Li1 cations (eVectively reinforcing the association of the cubane units).3 However, a far less regular pattern of C(–H) ? ? ? Li interactions is present in 1, involving both the a and b carbons of Cy groups.Despite the diVerences in the steric demands of the tBu and Cy groups present in the structurally characterised complexes [{E2(NtBu)4}Li2?2thf] (E = P,4 Bi2) and [{E2(NCy)4}2Li4] (E = As, Sb 3), and the presence of diVerent Group 15 elements and Lewis base solvation, it is now possible to obtain some general structural trends from this series.In particular the N–E–N (range 79.6–82.88) and E–N–E (range 96.2–98.68) angles in the [E(m-NR)]2 ring units of the [E2(NR)4]22 dianions in all of these species are surprisingly similar. One of the most significant diVerences in the geometry of the dianions occurs in the exocyclic N–E–N angles which exhibit an overall reduction going from P (average 99.48) to Bi (average 87.98), consistent with the idea of increased s-character in the lone pair and increased p-character in the E–N bonds as Group 15 is descended.This eVect oVsets the increase in E–N bond lengths so that coordination of the Li1 cations can be achieved without major structural modification of the [E2(NR)4Li2] units. Dimerisation of the cubane substituents of 1 and the Sb analogue is made possible by puckering of the E2N2 ring units (the N centres being an average of 18.48 out of the plane in 1 and an average of 21.28 in the Sb complex). This expands the ligand bite and allows inter-cubane Li–N bonding to be established.The use of [As(NMe2)3] as a precursor should allow other imido anions of As(III) to be prepared {e.g., [As(NR)3]32} and the coordination chemistry of these species to be explored. Of potential technological relevance is the synthesis of As(III)/ Group 13 (Ga, In) heterometallics. Notes and references † Synthesis of 1: [As(NMe2)3] (6.0 mmol, 2.4 ml, 2.5 mol dm23 solution in toluene) was added to a solution of CyNH2 (6.0 mmol, 0.70 ml) in toluene (20 ml) at 25 8C.The mixture was brought to reflux briefly and a pale yellow solution was formed. This was added to a suspension of [CyNHLi] (6.0 mmol, made by the in situ reaction of CyNH2 with nBuLi) in hexanes. The solid dissolved immediately and a bright yellow solution was produced after heating to reflux. The solvent was reduced to ca. 6 ml and a colourless solid precipitated. This was warmed back into solution and storage at 5 8C for 24 h gave crystals of 1; yield 0.37 g (22%).Decomp. ca. 75 8C to red semi-solid, darkens and becomes black at ca. 200 8C. IR (Nujol), nmax/cm21: 1225.4s, 1143.3m, 1056.2vs (br), 973.3s, 921.5m, 890.4s, 845.7s, 766.6s. 1H NMR (125 8C, 400 MHz, d6-benzene): 3.46 (2H, a-C–H Cy), 3.27 (2H, a-C–H Cy), 2.7–1.0 (40H, overlapping multiplets, CH2 Cy) (ca. 0.33 molecules of toluene were also present per molecule of 1, CH3 at 2.13). 7Li NMR (100.6 MHz, d8-toluene, relative to LiCl–D2O, 50 mg per 0.5 mol dm23): d 1.25 (s, line width 23 Hz, 125 8C) [Found: C, 54.8; H, 8.2; N, 10.1.Calc.: C, 52.2; H, 8.0; N, 10.1% (the high %C is a result of minor amounts of toluene, up to ca. 0.33 per molecule of 1 as confirmed by 1H NMR]. ‡ Crystal data for 1: C48H88As4Li4N8, M = 1104.70, triclinic, space group P1� , a = 10.415(5), b = 11.809(8), c = 23.502(13) Å, a = 97.75(4), b = 100.35(4), g = 103.30(4)8, U = 2720(3) Å3, Z = 2, Dc = 1.349 Mg m23, l = 0.71073 Å, T = 180(2) K, m(Mo–Ka) = 2.475 mm21.Data were collected on a Siemens-Stoe AED diVractometer. Of a total of 11222 data collected (3.50 £ q £ 24.018) 8458 were independent (Rint = 0.0988). The structure was solved by direct methods and refined by full-matrix least-squares on F2 to final values of R1[F > 4s(F)] = 0.040 and wR2 = 0.122 (all data); largest peak and hole in the final diVerence map 0.713 and 20.820 e Å23. CCDC reference number 186/1261. 1 M. A. Beswick, M. E. G. Mosquera and D. S. Wright, J. Chem. Soc., Dalton Trans., 1998, 2437. 2 D. Barr, M. A. Beswick, A. J. Edwards, J. R. Galsworthy, M. A. Paver, M.-A. Rennie, C. A. Russell, P. R. Raithby, K. L. Verhorevoort and D. S. Wright, Inorg. Chim. Acta, 1996, 248, 9. 3 R. A. Alton, D. Barr, A. J. Edwards, M. A. Paver, P. R. Raithby, M.-A. Rennie, C. A. Russell and D. S. Wright, J. Chem. Soc., Chem. Commun., 1994, 1481. 4 I. Schranz, L. Stahl and R. J. Staples, Inorg. Chem., 1998, 37, 1493. 5 M. A. Beswick, C. N. Harmer, M. A. Paver, P. R. Raithby, A. Steiner and D. S. Wright, Inorg. Chem., 1997, 36, 1740. 6 M. A. Beswick, N. Choi, C. N. Harmer, A. D. Hopkins, M. Mc- Partlin and D. S. Wright, Science, 1998, 1500. 7 Although very diVerent, the terminal and bridge As–N bond lengths are within the range occurring in other As–N compounds (in which only minimal pp–dp bonding occurs), see; A. L. Atwood, A. H. Cowley, W. E. Hunter and S. K. Mehritra, Inorg. Chem., 1982, 21, 1354; R. Bohra, H. W. Roesky, M. Noltemeyer and G. M. Sheldrick, Acta Crystallogr., Sect. C, 1984, 40, 1150; J. Weiss and W. Einenhuth, Z. Anorg. Allg. Chem., 1967, 350, 9; M. G. Begley, D. B. Sowerby and R. J. Tillott, J. Chem. Soc., Dalton Trans., 1974, 2527. Communication 8/0821