Magnesium and Titanium Complexes of Polyanionic

(D),(4) bis(phosphinimino)methanides (E),(5) bis(amido)cyclodiphosphazanes (F),(6) aminotroponiminates (G),(7) and tris-amido ligands of types H(8...
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Organometallics 2010, 29, 2515–2520 DOI: 10.1021/om100130k

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Magnesium and Titanium Complexes of Polyanionic Phosphazenate Ligands Ramamoorthy Boomishankar,*,†,‡ Philip I. Richards,† Arvind K. Gupta,‡ and Alexander Steiner*,† †

‡

Department of Chemistry, University of Liverpool, Liverpool, Crown Street, L69 7ZD, U.K., and Department of Chemistry, Indian Institute of Technology, Guwahati, North Guwahati, Assam-781039, India Received February 18, 2010

Polyanionic phosphazenates constitute polydentate ligands that offer several chelating sites, facilitating the accommodation of multinuclear metal arrays. Here, we describe their coordination behavior in the presence of magnesium and titanium. Reaction of the hexaprotic cyclotriphosphazene (iBuNH)6P3N3 (1) with nBu2Mg in thf in 1:2 ratio yields the dinuclear magnesium complex [(thf)Mg(iBuN)2(iBuNH)4P3N3]2 (2), while a 1:4 ratio gives the octanuclear complex [(thf)3(nBuMg)2Mg2(iBuN)6P3N3]2 (3). Both magnesium complexes exist as dimers in the solid state. In addition to metal ligand interactions, dimer 2 is supported by four hydrogen bonds (N-H 3 3 3 N interactions). The reaction of (CyNH)6P3N3 (4) with two equivalents of (Me2N)4Ti produces the dinuclear complex {(Me2N)2Ti}2(CyN)4(CyNH)2P3N3 (5), in which the phosphazenate ligand accommodates the two titanium atoms in tridentate coordination sites. The metalation reactions were followed by 31P NMR. Metal complexes 2, 3, and 5 were characterized by X-ray structure analysis.

Introduction Electron-rich anionic bis- and tris(amido) ligands play an important role in coordination chemistry. The ligand backbone determines, in addition to electronic effects, the size and the gripping angle of the chelate, while the N-bound R groups control the steric demands of the ligand. Prominent examples include amidinates (A),1 guanidinates (B),1,2 bis(imino)phosphinates (C),3 β-diketiminates (D),4 bis(phosphinimino)methanides (E),5 bis(amido)cyclodiphosphazanes (F),6 aminotroponiminates (G),7 and tris-amido ligands of types H8 and J9 (Chart 1).

In recent years we have investigated the coordination chemistry of polyanionic phosphazenates. These contain up to six amido sites that are tethered to the cyclotriphosphazene ring. In contrast to singly chelating ligands, phosphazenates provide several biand tridentate chelation sites. This facilitates the accommodation of larger arrays of metal ions, while the peripherally arranged R groups ensure solubility in nonpolar solvents.3a,10 The polyanionic ligands are generated via deprotonation of the hexaprotic precursor K with organometal reagents. Full deprotonation yields the hexaanionic ligand L (Scheme 1), while partial deprotonation is achieved by using stochiometric amounts of metal base. The coordination behavior of the ligand is determined by the metal, the degree of metalation, and the steric demand of R groups. Chart 2 lists the coordination modes that have been observed for phosphazenates. For example, K reacts with three equivalents of n-butyllithium or diethylzinc to form a trianionic ligand that accommodates three metal centers in bidentate coordination sites of type II.11 Here we report the coordination behavior of polyanionic phosphazenate ligands toward magnesium and titanium. Molecular complexes of both metals play an important role in homogeneous catalysis and as precursors for thin film ceramics.12

*Corresponding authors. E-mail: [email protected] (A.S.); [email protected] (R.B.). (1) (a) Edelmann, F. T. Adv. Organomet. Chem. 2008, 57, 183. (b) Edelmann, F. T. Chem. Soc. Rev. 2009, 38, 2253. (c) Barker, J.; Kilner, M. Coord. Chem. Rev. 1994, 133, 219. (2) (a) Bailey, P. J.; Pace, S. Coord. Chem. Rev. 2001, 214, 91. (b) Coles, M. P. Chem. Commun. 2009, 3659. (3) (a) Steiner, A.; Zacchini, S.; Richards, P. I. Coord. Chem. Rev. 2002, 227, 193. (b) Baier, F.; Fei, Z.; Gornitzka, H.; Murso, A.; Neufeld, S.; Pfeiffer, M.; R€ udenauer, I.; Steiner, A.; Stey, T.; Stalke, D. J. Organomet. Chem. 2002, 661, 111. (c) Steiner, A.; Stalke, D. Inorg. Chem. 1993, 32, 1977. (4) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. Rev. 2002, 102, 3031. (5) Roesky, P. W. Chem. Soc. Rev. 2000, 29, 335. (6) (a) Stahl, L. Coord. Chem. Rev. 2000, 210, 203. (b) Briand, G. G.; Chivers, T.; Krahn, M. Coord. Chem. Rev. 2002, 233-234, 237. (7) Panda, T. K.; Roesky, P. W. Chem. Soc. Rev. 2009, 38, 2782. (8) (a) Gade, L. H. Acc. Chem. Res. 2002, 35, 575. (b) Gade, L. H.; Mountford, P. Coord. Chem. Rev. 2001, 65, 216. (9) (a) Beswick, M. A.; Wright, D. S. Coord. Chem. Rev. 1998, 176, 373. (b) Brask, J. K.; Chivers, T. Angew. Chem., Int. Ed. 2001, 40, 3960. (c) Fleischer, R.; Stalke, D. Coord. Chem. Rev. 1998, 176, 431. (d) Aspinall, G. M.; Copsey, M. C.; Leedham, A. P.; Russell, C. A. Coord. Chem. Rev. 2002, 227, 217.

(10) (a) Rivals, F.; Steiner, A. Chem. Commun. 2001, 1426. (b) Lawson, G. T.; Rivals, F.; Tascher, M.; Jacob, C.; Bickley, J. F.; Steiner, A. Chem. Commun. 2000, 341. (c) Lawson, G. T.; Jacob, C.; Steiner, A. Eur. J. Inorg. Chem. 1999, 1881. (11) (a) Lawson, G. T.; Rivals, F.; Tascher, M.; Jacob, C.; Bickley, J. F.; Steiner, A. Chem. Commun. 2000, 341. (b) Rivals, F.; Steiner, A. Z. Anorg. Allg. Chem. 2003, 629, 139. (c) Boomishankar, R.; Richards, P. I.; Steiner, A. Angew. Chem., Int. Ed. 2006, 45, 4632. (12) (a) Jones, A. C. J. Mater. Chem. 2002, 12, 2576. (b) Choy, K. L. Prog. Mater. Sci. 2003, 48, 57. (c) Palgrave, R. G.; Parkin, I. P. New J. Chem. 2006, 30, 505.

r 2010 American Chemical Society

Published on Web 05/04/2010

pubs.acs.org/Organometallics

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Organometallics, Vol. 29, No. 11, 2010 Chart 1

Scheme 1

Boomishankar et al. Chart 2

catalysts.18 Thus, in recent years a variety of titanium complexes containing bidentate, tridentate, and polydentate amido ligands have been synthesized and structurally characterized.8,19

Results and Discussion

Magnesium displays a rich coordination chemistry in the presence of anionic amido ligands.13 Bimetallic Mg/alkali metal amide complexes have shown to be powerful deprotonating agents with unique regioselectivities,14 while magnesium β-diketiminate/alkoxide systems constitute effective catalysts for the ring-opening polymerization of lactides.15 Furthermore, bidentate ligands, such as guanidinates and β-diketiminates equipped with bulky R groups, are able to stabilize Mg(I) complexes.16 Molecular titanium complexes catalyze various organic transformations, most prominently the polymerization of olefins.17 Electron-rich nitrogen ligands have attracted much interest for the support “non-metallocene” (13) See for example: (a) Gibson, V. C.; Segal, J. A.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 2000, 122, 7120. (b) Bailey, P. J.; Coxall, R. A.; Dick, C. M.; Fabre, S.; Henderson, L. C.; Herber, C.; Liddle, S. T.; Lorono-Gonzalez, D.; Parkin, A.; Parsons, S. Chem.;Eur. J. 2003, 9, 4820. (c) Sanchez-Barba, L. F.; Hughes, D. L.; Humphrey, S. M.; Bochmann, M. Organometallics 2006, 25, 1012. (d) Dove, A. P.; Gibson, V. C.; Hormnirum, P.; Marshall, E. L.; Segal, J. A.; White, A. J. P.; Williams, D. J. Dalton Trans. 2003, 3088. (e) Bailey, P. J.; Liddle, S. T.; Morrison, C. A.; Parsons, S. Angew. Chem., Int. Ed. 2001, 40, 4463. (f) Fedushkin, I. L.; Khvoinova, N. M.; Skatova, A. A.; Fukin, G. K. Angew. Chem., Int. Ed. 2003, 42, 5223. (g) El-Kaderi, H. M.; Xia, A.; Heeg, M. J.; Winter, C. H. Organometallics 2004, 23, 3488. (h) Sadique, A. R.; Heeg, M. J.; Winter, C. H. Inorg. Chem. 2001, 40, 6349. (i) Prust, J.; Most, K.; M€uller, I.; Alexopoulos, E.; Stasch, A.; Uson, I.; Roesky, H. W. Z. Anorg. Allg. Chem. 2001, 627, 2032. (j) Bailey, P. J.; Dick, C. M. E.; Fabre, S.; Parsons, S. Dalton Trans. 2000, 1655. (k) Ahmed, S. A.; Hill, M. S.; Hitchcock, P. B. Organometallics 2006, 25, 394. (l) Robertson, S. D.; Chivers, T.; Konu, J. J. Organomet. Chem. 2007, 692, 4327. (14) (a) Mulvey, R. E.; Mongin, F.; Uchiyama, M.; Kondo, Y. Angew. Chem., Int. Ed. 2007, 46, 3802. (b) Mulvey, R. E. Organometallics 2006, 25, 1060. (15) (a) Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 3229. (b) Chisholm, M. H.; Gallucci, J.; Phomphrai, K. Inorg. Chem. 2002, 41, 2785. (16) (a) Green, S. P; Jones, C.; Stasch, A. Science 2007, 318, 1754. (b) Green, S. P; Jones, C.; Stasch, A. Angew. Chem., Int. Ed. 2008, 47, 9079. (17) (a) Bochmann, M. Dalton Trans. 1996, 255. (b) Schellenberg, J. Prog. Polym. Sci. 2009, 34, 688.

Coordination to Magnesium. We have reacted the isobutyl derivative (iBuNH)6P3N3 (1) with stoichiometric amounts of dibutyl magnesium. A series of reactions were carried out by gradually adding 1 to 6 equiv of nBu2Mg (1 M in heptane) to a solution of 1 in thf. The progress of these reactions was monitored by 31P NMR spectroscopy. It showed that single products were present when 1 was reacted with nBu2Mg in 1:2 and 1:4 ratios, respectively. At the 1:2 stage the 31P NMR spectrum displayed an AX2 signal pattern comprising a triplet at δ 18.9 and a doublet at δ 25.7 with a 2JPP coupling constant of 38.6 Hz, while at the 1:4 stage the spectrum consisted of an AMX pattern with signals at δ 33.9, 36.1, and 40.9 exhibiting coupling constants ranging from 6 to 14 Hz. Excess addition of nBu2Mg showed no further change, indicating complete deprotonation of the ligand. Both products were crystallized from concentrated thf solutions at -20 °C. Single-crystal X-ray structure analysis revealed that the 1:2 reaction yielded the binuclear magnesium complex [(thf)Mg(iBuN)2(iBuNH)4P3N3]2 (2), while the 1:4 reaction generated the octanuclear magnesium complex [(thf)3(nBuMg)2Mg2(iBuN)6P3N3]2 (3) (Scheme 2). The X-ray structure of 2 shows that the ligand was deprotonated in nongeminal fashion at two amino groups located at the same face of the phosphazene ring. The complex exists as a centrosymmetric dimer in the solid state (Figure 1). The dianionic ligands exhibit the tridentate (18) (a) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283. (b) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428. (c) Florez, J. C.; Chien, J. C. W.; Rausch, M. D. Organometallics 1995, 14, 1827. (d) Averbuj, C.; Tish, E.; Eisen, M. S. J. Am. Chem. Soc. 1998, 120, 8640. (19) See for example: (a) Hagadorn, J. R.; Arnold, J. Organometallics 1998, 17, 1355. (b) Hagadorn, J. R.; Arnold, J. Angew. Chem., Int. Ed. 1998, 37, 1729. (c) Mullins, S. M.; Duncan, A. P.; Bergman, R. G.; Arnold, J. Inorg. Chem. 2001, 40, 6952. (d) Coles, M. P.; Hitchcock, P. B. Organometallics 2003, 22, 5201. (e) Kakaliou, L.; Scanlon, W. J.; Qian, B.; Baek, S. W.; Smith, M. R., III; Motry, D. H. Inorg. Chem. 1999, 38, 5964. (f) Bai, G.; Wei, P.; Stephan, D. W. Organometallics 2006, 25, 2649. (g) Basuli, F.; Clark, R. L.; Bailey, B. C.; Brown, D.; Huffman, J. C; Mindiola, D. J. Chem. Commun. 2005, 2250. (h) Basuli, F.; Bailey, B. C.; Watson, L. A.; Tomaszewski, J.; Huffman, J. C; Mindiola, D. J. Organometallics 2005, 24, 1886. (i) Vollmerhaus, R.; Tomaszewski, R.; Shao, P.; Taylor, N. J.; Wiacek, K. J.; Lewis, S. P.; Al-Humydi, A.; Collins, S. Organometallics 2005, 24, 494.

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Scheme 2. Syntheses of Magnesium Complexes 2 and 3a

a

R = isobutyl, L = thf, reaction conditions: thf, 20 °C, 12 h stirring.

coordination mode III, featuring two deprotonated N sites that coordinate the Mg center alongside the ring-N site situated between them. The dimer is held together via two additional Mg-N(ring) bonds. The central Mg-N(ring) bond of the tridentate mode III (Mg1-N1=2.424(2) A˚) is substantially longer than the Mg-N bonds toward the deprotonated N-sites (Mg1-N6 = 2.038(2), Mg1-N4 = 2.052(2) A˚) and the N(ring) site of the other phosphazenate ligand (Mg1-N1a = 2.080(2) A˚). Furthermore each magnesium center binds a thf molecule, resulting in distorted trigonal-bipyramidal coordination environments in which N1 and O1 occupy the axial positions. In addition, there are four intramolecular NH 3 3 3 N interactions between the two ligands of the dimer. These involve amino groups (RNH) as H-bond donors and the deprotonated metal coordinating N-sites as H-acceptors. The quality of the X-ray data facilitated the free refinement of N-bound H atom positions. The NH 3 3 3 N contacts measure 2.37 (N7H 3 3 3 N4a) and 2.58 A˚ (N5H 3 3 3 N6a), respectively, when normalized N-H bond distances are taken into account.20 These contacts are shorter than the sum of the van der Waals radii, which is 2.75 A˚,21 and resemble those of intermolecular hydrogen bonds of phosphazenes K, in which exocyclic amino groups are shown to act as both donors and acceptors.22 In contrast to the planar ring conformation found in ligand precursor 1, the phosphazene rings in the magnesium complex 2 are puckered. The coordinated ring center N1 is displaced from the more or less planar arrangement of the other five ring atoms, resulting in a half-chair conformation. Also, there is a marked variation in P-N bond lengths, both within the ring and toward the exocyclic N-centers. The P-N(ring) bonds associated with the coordinating site N1 are substantially (20) The N-H bonds were normalized by shifting the position found for the H atom (that is, the position of the electron center of gravity) along the N-H vector to 1.009 A˚, which is the average neutrondetermined internuclear distance between the nitrogen atom and the proton. (a) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 48. (b) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond in Structural Chemistry and Biology; Oxford University Press: Oxford, 1999; p 7. (21) Bondi, A. J. Phys. Chem. 1964, 68, 441. (22) Bickley, J. F.; Bonar-Law, R.; Lawson, G. T.; Richards, P. I.; Rivals, F.; Steiner, A.; Zacchini, S. Dalton Trans. 2003, 1235.

Figure 1. Crystal structure of 2. C and H atoms are omitted with the exception of NH groups. Hydrogen bonds are drawn as dashed lines. Atoms are presented as thermal ellipsoids at the 50% probability level. Selected bond lengths [A˚]: Mg1-N6 2.038(2), Mg1-N4 2.052(2), Mg1-N1a 2.080(2), Mg1-N1 2.424(2), Mg1-O1 2.057(2), P1-N3 1.599(2), P1-N4 1.615(2), P1-N5 1.646(2), P1-N1 1.650(2), P2-N2 1.598(2), P2-N6 1.616(2), P2-N7 1.648(2), P2-N1 1.655(2), P3-N3 1.591(2), P3-N2 1.598(2), P3-N9 1.651(2), P3-N8 1.662(2), N7 3 3 3 N4a 3.302(2), N5 3 3 3 N6a 3.350(3).

longer than the other P-N(ring) bonds. In return, the P-N bonds involving the two deprotonated N-sites are markedly shorter than the other P-N(exo) bonds. The core structure of 2 consisting of Mg, P, N, and O atoms approaches D2h symmetry; Mg and O atoms as well as N1, P3, N8, and N9 occupy the pseudomirror plane. This corroborates the AX2 signal pattern of the 31P NMR spectrum, which suggests that an analogous arrangement exists in solution. Coordination modes similar to that of 2 have been observed with other phosphorus nitrogen ligands. For example, the tert-butyl derivative of the dianionic bis(amido)cyclodiphosphazanes F exhibits a tridentate coordination mode in the presence of magnesium albeit in a monomeric complex.23 Here, the Mg-N distances compare well with those observed in 2; there are two short Mg-N bonds to the formally deprotonated terminal amido groups and a somewhat longer contact to one of the nitrogen atoms that are part of the four-membered cyclophosphazane ring. Bis(phosphinimino)methanide ligands E have also shown to coordinate to magnesium ions in a tridentate way via both N-sites and the central carbon atom.24 The solid-state structure of the octanuclear magnesium complex 3 is shown in Figure 2. The centrosymmetric dimer contains two fully deprotonated hexaanionic ligands, each coordinating to five metal centers. The eight magnesium ions are accommodated in four kinds of coordination sites around the ligands. Mg1 and Mg1a are situated at the center of the dimeric complex and are coordinated by both ligands via bidentate coordination modes of type II and a thf ligand. They exhibit distorted square-pyramidal coordination environments with the O atom of the thf ligand in apical position. The Mg-N distances to exocyclic N-sites are short (Mg1-N4 = 2.072(2), Mg1-N7a=2.036(2) A˚) compared to Mg-N(ring) contacts (Mg1-N1=2.225 (2), Mg1-N1a=2.339(2) A˚). Mg2 and Mg2a occupy tridentate sites of type III. Again, the (23) (a) Schranz, I.; Stahl, L.; Staples, R. J. Inorg. Chem. 1998, 37, 1493. (b) Briand, G. G.; Chivers, T.; Parvez, M.; Schatte, G. Inorg. Chem. 2003, 42, 525. (24) Wei, P.; Stephan, D. W. Organometallics 2003, 22, 601.

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Boomishankar et al. Scheme 3. Syntheses of the Titanium Complex 5a

a

Figure 2. Crystal structure of 3. C and H atoms are omitted except R-C atoms of Mg-bound butyl ligands. Atoms are drawn as thermal ellipsoids at the 50% probability level. Selected bond lengths [A˚]: Mg1-N7a 2.036(2), Mg1-N4 2.072(2), Mg1-N1 2.225(2), Mg1-N1a 2.339(2), Mg2-N5 2.012(2), Mg2-N9 2.015(2), Mg2-N3 2.404(2), Mg3-C1 2.119(3), Mg3-N3 2.159(2), Mg3-N6 2.171(2), Mg3-N8 2.224(2), Mg4-N8 2.086(2), Mg4-C5 2.123(3), Mg4-N2 2.167(2), Mg4-N6 2.215(2), P1-N4 1.613(2), P1-N5 1.626(2), P1-N3 1.665(2), P1-N1 1.687(2), P2-N7 1.597(2), P2-N2 1.643(2), P2-N1 1.667(2), P2-N6 1.674(2), P3-N9 1.599(2), P3-N2 1.637(2), P3-N8 1.657(2), P3-N3 1.667(2).

N(ring) site is further away from the metal center (Mg2-N3 = 2.404(2) A˚) than the two formally anionic exocyclic N-centers (Mg2-N5 = 2.012(2), Mg2-N9 = 2.015(2) A˚). In addition, Mg2 binds two thf molecules, resulting in a distorted trigonalbipyramidal coordination geometry in which the N(ring) and one of the thf molecules occupy the axial positions. This coordination environment is very similar to that found in 2, where the equatorial position of the second thf molecule is taken by an N(ring) site of the other ligand. The four remaining Mg centers are equipped with butyl groups and adopt tetrahedral environments. They occupy tridentate coordination sites of type III and IV. Two metal centers are paired up (Mg3 and Mg4, Mg3a and Mg4a, respectively) sharing two N(exo) sites. As a result of the multiple coordination of N(exo) centers, the Mg-N(exo) bonds lengthen slightly (Mg3-N6 = 2.171(2), Mg3-N8 = 2.224(2), Mg4-N8=2.086(2), Mg4-N6=2.215(2) A˚) compared to the metal-ligand bond involving singly binding N(exo) centers, and in return the Mg-N(ring) bonds shorten to some extent (Mg3-N3=2.159(2), Mg4-N2=2.167(2) A˚). The phosphazene rings in 3 are highly puckered. They adopt a boat conformation with P2 and N3 in the bow and stern position, respectively. This contrasts with the chair configuration found in lithium complexes of both tri- and hexanionic ligands and shows that the phosphazenate scaffold is flexible and can adapt to accommodate particular arrays of metal centers. In line with other complexes of hexaanionic phosphazenates,10,11 the P-N(ring) bonds are markedly longer than those found in the neutral phosphazene precursor 1 (1.60 A˚).20 The doubly coordinating N-sites N1 and N3 show longer P-N bonds (P1-N1 1.687(2), P2-N1 1.667(2), P1-N3 1.665(2), P3-N3 1.667(2) A˚) than the singly coordinating ring site N2 (P2-N2 1.643(2), P3-N2 1.637(2) A˚). The P-N(exo) bond lengths are also affected by the involvement of the particular N(exo) site in metal coordination. N(exo) sites that bind to only one metal

R = cyclohexyl; reaction conditions: toluene, 12 h reflux.

center form short P-N(exo) bonds, while coordination to two metal centers (here N6 and N8) lengthens the P-N(exo) bonds. The AMX signal pattern observed in the 31P NMR spectrum of 3 in solution mirrors the inequivalence of phosphorus environments that exist in the solid state and indicates the absence of a fluxional ligand behavior in solution. Coordination to Titanium. The stepwise addition of stoichiometric proportions of (Me2N)4Ti to a solution of the cyclohexyl derivative (CyNH)6P3N3 (4) in toluene was followed by 31P NMR. After the addition of one equivalent of (Me2N)4Ti the spectrum shows a complex mixture that still contains considerable amounts of the starting compound 4. Upon addition of two equivalents, an AX2 signal emerges (δ 12.3 (d), 10.2 (t), 2JPP = 26.0 Hz), which becomes the sole species (5) after reflux. The isolated product exhibits characteristic phosphazenate stretching frequencies in the IR spectrum at 1021 and 1091 cm-1, which are shifted to lower frequencies when compared to those of the neutral precursor 4. The absorption band at 3393 cm-1 indicates the presence of NH bonds. The 1H NMR of 5 displays a singlet for the dimethylamido groups at δ 3.20, while the signals of the cyclohexyl groups are found in the range δ 0.8-2.1. The intensity ratio of both sets of signals suggests the presence of four Me2N groups per phosphazenate ligand. Crystals of the product were obtained from a mixture of toluene and hexane. The X-ray structure of 5 showed that the crystals contain the dinuclear titanium complex {(Me2N)2Ti}2(CyN)4(CyNH)2P3N3 and half a molecule of toluene per formula unit. The monomeric complex comprises the tetraanionic ligand, which chelates two Ti(Me2N)2 fragments. The molecular symmetry of the complex is close to point group C2. Both titanium centers reside in tridentate coordination sites of type III located at opposite faces of the phosphazene ring. The coordination environment around the titanium centers can be described as distorted square pyramidal, with one of the Me2N ligands occupying the apical position (N10 and N12, respectively). The shortest Ti-N bonds are toward the dimethyl amido ligands, averaging 1.85 A˚. Somewhat longer are the Ti-N(exo) bonds, measuring on average 1.98 A˚, while the Ti-N(ring) bonds are the longest at 2.13 A˚. The phosphazene ring shows a slight twist conformation, but the deviation from planarity is less pronounced than in the magnesium complexes 2 and 3. There are several reports of tridentate phosphorus nitrogen ligands that coordinate titanium. For example, bis(amido)cyclodiphosphazane ligands (F) act as tridentate ligands via the two terminal N-sites and one of the N(ring) centers of the four-membered P2N2 ring.25 While the P-N(exo) bonds in (25) (a) Moser, D. F.; Carrow, C. J.; Stahl, L.; Staples, R. J. Dalton Trans. 2001, 1246. (b) Moser, D. F.; Grocholl, L.; Stahl, L.; Staples, R. J. Dalton Trans. 2003, 1402. (c) Axenov, K. V.; Kotov, V. V.; Klinga, M.; Leskela, M.; Repo, T. Eur. J. Inorg. Chem. 2004, 695. (d) Axenov, K. V.; Kilpelainen, I.; Klinga, M.; Leskela, M.; Repo, T. Organometallics 2006, 25, 463.

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Scheme 4. (a) Effect of Tridentate Coordination III on the Conformation of the PN Scaffold; (b) Arrangement of Two Tridentate Sites III around the Liganda

a Note that the two vacant sites N0 and N00 are not able to support a further type III site.

Figure 3. Crystal structure of 5. H atoms are omitted for clarity. With the exception of carbon, atoms are drawn as thermal ellipsoids at the 30% probability level. Selected bond lengths [A˚]: Ti1-N10 1.836(7), Ti1-N11 1.868(8), Ti1-N9 1.969(7), Ti1-N5 1.989(7), Ti1-N1 2.126(6), Ti2-N12 1.849(7), Ti2-N13 1.854(8), Ti2-N4 1.980(7), Ti2-N6 1.979(7), Ti2-N2 2.125(6), P1-N4 1.574(7), P1-N5 1.576(7), P1-N1 1.580(7), P1-N2 1.604 (6), P2-N3 1.558(7), P2-N6 1.581(7), P2-N7 1.589(7), P2-N2 1.600(7), P3-N3 1.568(6), P3-N9 1.599(7), P3-N1 1.612(7), P3-N8 1.621(7).

these complexes are similar to those in 5, the P-N(ring) bonds in the Ti complexes of ligand F are considerably longer. Also, several titanium complexes of the tripodal ligand H have been reported.8 In contrast to the phosphazenate ligand, which acts de facto as a dianionic ligand for each titanium center, ligands of type H are trianionic, giving somewhat shorter Ti-N bonds.26 Addition of more than two equivalents of (Me2N)4Ti to 4 does not lead to further reactions even after prolonged reflux. We have followed the analogous reactions of the less sterically demanding benzyl {(PhCH2NH)6P3N3} (6) and allyl derivatives {(CH2dCH-CH2NH)6P3N3} (7) with 31P NMR. Addition of two equivalents of (Me2N)4Ti to toluene solutions of 6 and 7 gave rise to AX2 signals that closely resemble that of 5 (6 + 2 (Me2N)4Ti: δ 15.7 (d), 13.7 (t), 2 JPP = 27.8 Hz; 7 + 2 (Me2N)4Ti: δ 16.2 (d), 12.5 (t), 2JPP = 28.6 Hz), which suggests the formation of dinuclear complexes analogous to 5. When 6 is reacted with excess (Me2N)4Ti in refluxing toluene, the 31P NMR spectrum consists of an AMX {δ 19.9 (dd, 2JPP = 17.3, 25.5 Hz), 18.6 (dd, 2JPP = 24.1, 25.5 Hz), 15.3 (dd, 2JPP = 17.6, 24.1 Hz)} and an AX2 spin system (δ 24.5 (d), 13.1 (t), 2JPP = 20.1 Hz). The spectrum of the corresponding reaction mixture of 7 is solely displaying the AX2 signal (24.9 (d), 12.6 (t), 2JPP = 18.7 Hz). These results suggest that further deprotonation of NH sites takes place when phosphazenes with less bulky R groups are reacted with excess (Me2N)4Ti. However, crystalline products suitable for X-ray structure determination have not been obtained so far. The study shows that phosphazenate ligands show a preference for the tridentate coordination mode III toward titanium and magnesium. It should be noted that the ligand (26) Schubart, M.; Findeis, B.; Gade, L. H.; Li, W.-S.; McPartlin, M. Chem. Ber. 1995, 128, 329.

can provide only two coordination sites of mode III as illustrated in Scheme 4: While the type III mode pulls its pair of exocyclic N atoms closer together, the ring puckers, forcing both N(exo) atoms bound to the other side of the ring plane to spread out (Scheme 4a). A second coordination site of type III must therefore engage a different ring N atom and also involve N(exo) atoms from the other side of the ring plane. Finally, the arrangement of two type III sites leaves the two remaining N(exo) atoms (labeled N0 and N00 in Scheme 4) at opposite sides of the ring plane, unable to support a third tridentate site of type III (Scheme 4b). However, N0 and N00 are able to share the N(ring) center located between them to support two bidendate type II sites. Thus, additional metal centers are accommodated in either bidentate sites of type II or the tridentate mode IV, which places the metal center right above the ring and shares its N (exo) centers with an existing type III site. This is the reason that the two bridging magnesium centers in 2 (Mg1 and Mg1a) and 3 (Mg1 and Mg1a) experience different coordination modes: In 2 they reside in tridentate sites, while in 3 they occupy bidentate sites. The bidentate coordination of bridging Mg centers in 3 appears to be due to the high metal load. In addition to the bridging metal ions, each ligand binds three nonbridging metal centers, all of which already occupy tridentate sites.

Conclusion We have studied the coordination behavior of polyanionic phosphazenates in the presence of magnesium and titanium, respectively. Treatment of the ligand precursor with nBu2Mg yielded dimeric complexes, which support up to eight magnesium centers. The corresponding reaction with (Me2N)4Ti yielded a dinuclear monomer. X-ray structures showed that both magnesium and titanium favor the tridentate coordination mode. Polyanionic phosphazenate ligands offer several coordination sites and are highly adaptable in their coordination modes. This could have some interesting implications for polynuclear metal complexes in areas such as catalysis. This approach might also enable the confinement of well-defined mixed-metal or metal oxide assemblies.11c,27 (27) Richards, P. I.; Boomishankar, R.; Steiner, A. J. Organomet. Chem. 2007, 692, 2773.

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Experimental Section General Remarks. All manipulations were performed under a dry nitrogen atmosphere in standard Schlenk glassware or in a glovebox. Solvents were dried over potassium (thf, hexane) and sodium (toluene). Starting reagents 1, 4, 6, and 7 were prepared as described previously.22 (Me2N)4Ti and nBu2Mg (1.0 M in heptane) were purchased from Aldrich and used as received. FT-IR spectra were recorded on a Perkin-Elmer Paragon 1000 spectrometer in Nujol between CsI plates. NMR spectra were recorded on a Bruker AMX 400 spectrometer (1H NMR: 400.13 MHz, 13C{1H} NMR: 100.62 MHz, 31P{1H} NMR: 161.97 MHz) at room temperature using SiMe4 (1H, 13C) and 85% H3PO4 (31P) as external standards. 2. nBu2Mg in heptane (1.1 mL, 1.0M, 1.1 mmol) was added to 1 (0.30 g, 0.53 mmol) in thf (20 mL). The reaction mixture was stirred for 12 h at room temperature and filtered. The filtrate was concentrated to 4 mL and kept for crystallization at -20 °C. Colorless prisms formed after 4 weeks. Yield after crystallization: 0.15 g (51%); mp 185-188 °C (dec). 1H NMR (thf-d8): δ 0.7-0.9 [m, 18H, (CH2CH(CH3)2)], 1.47-1.62 [m, 4H, NH(CH2CH(CH3)2)], 1.85-1.90 [m, 2H, N(CH2CH(CH3)2)], 2.05-2.30 [m, 4H, NH(CH2CH(CH3)2)], 2.45-2.61 [m, 8H, NH(CH2CH(CH3)2)], 2.85-3.08 [m, 4H, N(CH2CH(CH3)2)]. 13 C{1H} NMR (thf-d8): δ 19.3, 19.4, 19.5, 20.4, 25.0, 29.6, 30.3, 30.8, 48.0, 48.8, 49.9, 54.7. 31P{1H} NMR (thf-d8): δ 25.7 (d, 2P), 18.9 (t, 1P), 2JPP = 38.6 Hz. IR (Nujol): ν (cm-1) 3582, 1398, 1260, 1199, 1129, 1088, 917. 3. nBu2Mg in heptane (2.4 mL, 1.0 M, 2.4 mmol) was added to 1 (0.30 g, 0.53 mmol) in thf (15 mL). The reaction mixture was stirred for 12 h at room temperature and filtered. The filtrate was concentrated to 3 mL and kept for crystallization at room temperature. Colorless prisms formed after 7 days. Yield: 0.34 g (67%). Mp: 240 °C (dec). 1H NMR (thf-d8): δ 0.7-1.1 [m, 36H, Mg-nbutyl and N(CH2CH(CH3)2], 1.3-1.6 [m, 6H, N(CH2CH(CH3)2)], 2.4-2.8 [m, 12H, N(CH2CH(CH3)2)]. 13C{1H} NMR (thf-d8): δ 12.7, 13.0, 19.5, 20.4, 20.7, 22.2, 29.3, 31.2, 50.0. 31P{1H} NMR (thf-d8): δ 33.9 (dd, 2P, 2JPP = 13.9 and 5.9 Hz), 36.1 (dd, 2P, 2JPP = 13.9 and 7.5 Hz), 40.9 (dd, 1P, 2JPP = 7.5 and 5.9 Hz). IR (Nujol): ν (cm-1) 1461, 1384, 1186, 1128, 1033, 962. 5. (Me2N)4Ti (0.65 mL, 2.76 mmol) was added to a solution of 4 (1 g, 1.38 mmol) in 20 mL of toluene. The solution is heated to reflux for 24 h and filtered. The filtrate was concentrated and layered with hexane. Single crystals suitable for X-ray diffraction were obtained after 3 days storage at 5 °C. Yield: 1.03 g (76%). Mp: 170-180 °C (dec). 1H NMR (toluened8): δ 0.8-2.1 (m, cyclohexyl), 3.20 (s, 12H, TiN(CH3)2). 13 C{1H} NMR (toluene-d8): δ 24.4, 24.5, 28.8, 35.4, 43.9, 48.7. 31P{1H} NMR (toluene-d8): δ 12.3 (d), 10.2 (t), 2JPP = 26.0 Hz. IR (Nujol): ν (cm-1) 3393 (N-H), 1255, 1187, 1091 (P-N), 1022 (P-N), 925, 800.

Boomishankar et al. Table 1. Crystallographic Data for 2, 3, and 5 3 1/2C7H8 chemical formula fw cryst syst space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z μ(Mo KR) (cm-1) F(calc) (g cm-1) 2θmax (deg) data/params R1 (F > 4σ(F)) wR2 (all data)

2

3

5 3 1/2C7H8

C56H132Mg2N18O2P6 1324.24 triclinic P1 13.088(6) 13.533(6) 13.655(6) 100.740(8) 111.112(9) 115.064(7) 1872.2(14) 1 0.210 1.175 50 6504/395 0.038 0.106

C88H188 Mg8N18O6P6 1966.80 monoclinic P21/c 13.520(2) 23.536(4) 17.870(3) 90 99.452(2) 90 5609.2(15) 2 0.194 1.164 50 9777/579 0.046 0.123

C47.5H96N13P3Ti2 1038.08 triclinic P1 13.139(8) 13.201(8) 19.796(11) 75.875(10) 75.918(11) 61.319(9) 2889(3) 2 0.402 1.193 45 7087/620 0.098 0.261

Crystallography. Reflections were collected on a Bruker Smart Apex diffractometer at 150 K using Mo KR radiation (λ = 0.71073 A˚). Structures were refined by full-matrix leastsquares against F2 using all data (SHELX).28 Non-hydrogen atoms were refined aniostropically. Crystallographic data of 2, 3, and 5 3 1/2C7H8 are listed in Table 1. Hydrogen atoms were constrained in geometric positions to their parent atoms, with the exception of N-bound H atoms of 2, which were refined without constraints. 3 contains one disordered isobutyl group and one disordered thf ligand. Atom positions of the disordered groups were split over two positions and refined isotropically using similar-distance restraints. Crystals of 5 3 1/2C7H8 diffracted weakly, lacking observed reflections at higher angles; thus a cutoff at 2θ = 45° was applied. Although the unit cell parameters of 5 3 1/2C7H8 show a good fit for monoclinic C, both the diffraction pattern and the crystal structure show triclinic symmetry and no sign of twinning.

Acknowledgment. This work was supported by the Engineering and Physical Sciences Research Council, U.K. (A.S., P.I.R., R.B.), and the Indian Institute of Technology, Guwahati (start-up grant for R.B.). Supporting Information Available: Crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. (28) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.