Marcus L.
Cole
a,
Cameron
Jones
*a and
Peter C.
Junk
*b
aDepartment of Chemistry, University of Cardiff, P.O. Box 912, Park Place, Cardiff, UK CF10 3TB. E-mail: jonesca6@cf.ac.uk
bSchool of Chemistry, Box 23, Monash University, Clayton, Vic 3800, Australia. E-mail: peter.junk@sci.monash.edu.au
First published on 20th December 2001
Treatment of ortho-dimethylarsinoaniline (NASH2) with 1.0 equivalents of LiBut or 0.5 equivalents of MgBu2 in tetrahydrofuran affords the complexes [{Li(NASH)(thf)2}2]
(1) and [Mg(NASH)2(thf)1.5]
(2)
(NASH=
NHC6H4AsMe2-2). Both complexes have been characterised spectroscopically. The crystal structure determination of 1 reveals a centrosymmetric dimeric structure with an Li2N2 central unit that does not exhibit arsino–lithium donor interactions. Compound 2 reacts with dimethylsilicone grease to yield the unexpected complex [{Mg2(μ2:η1-NHC6H4AsMe2)2(μ3:η3-OSiMe2NC6H4AsMe2)(thf)}2]
(3), via
the novel and unprecedented insertion of a dimethylsilanone fragment into an Mg–N bond. Compound 3 is tetranuclear with a 12-membered Mg4N6O2 double stacked cube centre that has Mg atoms residing at alternate corners of each cube. The arsino functionalities of the modified η3-bound ligands co-ordinate to Mg atoms on the vertices.
Since the synthesis of o-dimethylarsinoaniline (NASH2) and its diphenyl analogue (o-diphenylarsinoaniline) in 1970,3 complexes of these novel ligands have not been reported. This contrasts with the wealth of structural and spectroscopic data concerning complexes of o-dimethylarsino-N,N-dimethylaniline reported by Peloso et al. during the 70's and 80's.4 Given the presence of a soft arsino donor moiety, and hence the potentially chelating nature of NASH, we were eager to synthesise lithium and magnesium NASH complexes and demonstrate their utility in inorganic synthesis. Herein, we report the synthesis of the first NASH complexes, [{Li(NASH)(thf)2}2] (1), [Mg(NASH)2H(thf)1.5] (2), and disclose the crystal structure of the novel Mg–N insertion product [{Mg2(μ2:η1-NHC6H4AsMe2)2(μ3:η3-OSiMe2NC6H4AsMe2)(thf)}2] (3), originally formed from the serendipitous reaction of 2 with dimethylsilicone grease.
Silicone grease (0.03 g, 0.40 mmol by SiMe2O) was added as a dilute solution (10 cm3 THF) to a solution of 2
(0.50 g, 0.95 mmol) in tetrahydrofuran (30 cm3) at −30°C. The solution was warmed to ambient temperature, stirred overnight and concentrated in vacuo
(ca. 2 cm3). Storage at −20
°C yielded 3 as large colourless blocks [0.21 g, 67% by (SiMe2O)n], m.p. 175
°C. 1H NMR (300.1 MHz, C6D6, 300 K): δ 0.35 [s, 12H, OSi(CH3)2], 1.06 [s, 12H, As(CH3)2Si], 1.12 [s, 24H, As(CH3)2], 1.21 (br s, 8H, thf), 3.26 (br s, 8H, thf), 3.86 (br s, 4H, NH), 6.43–7.16 (mult,
24H, aromatics). 13C NMR (75.5 MHz, C6D6, 300 K): δ 4.8 [s, Si(CH3)2], 7.9 (s, AsMe2), 8.65 (s, AsMe2Si), 25.6 (s, thf), 67.9 (s, thf), 114.5, 117.5, 118.3, 118.6, 119.9, 129.5, 130.5, 130.8, 131.1, 131.3, 159.4, 160.1 (s, aromatics). MS (APCI): m/z
(%) 198 [(NASH2)H+, 100], 254 [NASSiMe2+, 18], 492 [(1.5
−
NASH)+, 62]. IR (Nujol)
ν/cm−1: 743s, 825m, 861m, 953m, 1025m, 1158m, 1194s, 1224s, 1245s, 1276s, 1332s, 1583s, 1608w, 3268m br.
CCDC reference numbers 173732 and 173733. See http://www.rsc.org/suppdata/nj/b1/b108503b/ for crystallographic data in CIF or other electronic format.
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Scheme 1
Reagents and conditions: (i) 2.0 eq. LiBut, −30![]() |
The molecular structure of 1 is depicted in Fig. 1. The molecule is dimeric and resides on a crystallographic inversion centre with an Li2N2 core unit. In the solid-state, 1 exists without co-ordination of the arsino moiety to the lithium centre. This is not unexpected, due to the preferential co-ordination of THF to the hard acceptor lithium atom. The central unit exists as a distorted square [N(1)–Li(1)–N(1)′ 104.1°, Li(1)′–N(1)–Li(1) 75.9°] with bond lengths of 2.064 Å (av. Li–N) that are slightly shorter than that of the mean structurally characterised Li–N bonds at 2.082 Å.8 The phenyl groups organise themselves such that the arsino moiety is directed away from the central unit and, hence, does not participate in donation to the metal. This is as expected on electropositivity grounds. Accordingly, the co-ordination sphere of the lithium is satisfied by donation of the hard THF donor. This renders each lithium centre four co-ordinate with a distorted tetrahedral geometry [O(1)–Li(1)–O(2) 104.7°, N(1)–Li(1)–N(1)′ 104.1°]. Lastly, the placement of the o-arsinophenyl groups causes an unexpected inequivalence of the THF donors, thereby compounding distortion about the lithium centres [O(1)–Li(1)–N(1)′ 108.9°, O(2)–Li(1)–N(1)′ 118.5°].
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Fig. 1 Molecular structure of the centrosymmetric dimer of [{Li(η1-NASH)(thf)2}2]
(1). Selected bond lengths [Å] and angles [°]; Li(1)–O(1) 1.944(16), Li(1)–O(2) 1.960(17), Li(1)–N(1) 2.073(14), Li(1)–N(1)#1 2.054(17), Li(1)–Li(1)#1 2.54(3); C(6)–As(1)–C(7) 102.7(13), C(6)–As(1)–C(8) 104.1(9), C(7)–As(1)–C(8) 86.0(14), O(1)–Li(1)–N(1) 115.5(7), O(1)–Li(1)–N(1)#1 108.9(8),
O(1)–Li(1)–O(2) 104.7(8), O(2)–Li(1)–N(1) 105.6(7), O(2)–Li(1)–N(1)#1 118.5(7), Li(1)–N(1)–Li(1)#1 75.9(7). Symmetry transformations used to generate equivalent atoms: #1 −x![]() ![]() ![]() ![]() ![]() ![]() |
To address the paucity of structurally characterised amidomagnesium complexes, the preparation of a magnesium analogue of 1 was attempted. Treatment of NASH2 with half an equivalent of MgBu2 in tetrahydrofuran at −60°C formed a light yellow solution that when concentrated yielded the compound [Mg(NASH)2(thf)1.5]
(2) in near quantitative yield (95%)
(Scheme 2). Compound 2 is a thermally robust solid with a melting point of 148
°C. The 1H NMR spectrum of 2 displays an arsino methyl proton resonance at 0.91 ppm that is identical to that for the free ligand and therefore upfield of the corresponding resonance in 1. The N–H resonance at 3.34 ppm closely matches the analogous resonance for 1
(3.33 ppm) and as such is also downfield of that
for non-deprotonated NASH2. The 13C NMR spectrum of 2 is unexceptional; the arsino methyls resonate at 9.6 ppm (9.8 ppm for 1), with several aromatic singlets between 114.9 and 159.0 ppm, which match the distribution of those for NASH2. The APCI mass spectrum displays peaks attributable to the protonated ligand (m/z 198), the doubly protonated non-solvated monomeric ion (m/z 209), [magnesium(NASH)]+
(m/z 220) and the protonated non-solvated monomer ion (m/z 418). In addition, several broad stretches that could be assigned to the N–H bond of 2 were observed in the FTIR spectrum at 3360, 3457 and 3717 cm−1
(NASH2: 3350br, 3441br cm−1).
The co-ordination mode of NASH in 2, i.e. whether the arsino moiety is involved in ligation, could not be evaluated without structural data. To this end the crystallisation of 2 was attempted. The lessened electropositivity of magnesium relative to lithium led us to believe that chelation was possible. The recrystallisation of 2 was attempted by the slow diffusion of hexane into a saturated tetrahydrofuran solution of 2. During crystallisation a small amount of a complex that characterised differently by spectroscopy to 2 was formed as large colourless blocks. These crystals were separated manually from the bulk material and examined by X-ray diffraction. The compound formed was [{Mg2(μ2:η1-NHC6H4AsMe2)2(μ3:η3-OSiMe2NC6H4AsMe2)(thf)}2]
(3),
indicating the novel and unprecedented insertion of a dimethylsilanone fragment into an Mg–N bond (Scheme 2). The repeated formation of 3 frustrated the structural characterisation of 2, and hence the co-ordination of NASH in 2 could not be identified unequivocally. Furthermore, when grease was rigorously excluded from preceding steps in the crystallisation of 2, only material of insufficient quality for a single crystal structure determination was obtained. An intentional synthesis of 3 was subsequently undertaken (Scheme 2). Treatment of a tetrahydrofuran solution of 2 with a third of an equivalent of silicone grease (based on SiMe2O) heavily diluted in tetrahydrofuran led to moderate yields of a crystalline material after concentration and storage at −20°C. An examination of the 1H and 13C
NMR data confirmed this material to be 3. In addition, resonances attributable to NASH2 were also observed in spectra of the remnant reaction mixture. On this basis the reaction pathway presented in Scheme 2 is proposed. Recent examples of Li–N dimethylsilanone insertion intimate that the modified ligand complex can potentially undergo further reaction with complexed ‘un-modified’ ligand to eliminate lithium oxide and yield an N–Me2Si–N bridged ligand which can go on to form larger and more elaborate cage structures.9h The preclusion of magnesium oxide formation indicated by the absence of any precipitation in the intentional synthesis of 3 infers that this mechanism does not operate in the transformation of 2 to 3. However, we cannot conclude that the advent of dimethylsilanone insertion takes place either before or after the
aggregation that results in the formation of 3. Indeed, as the exact solution behaviour of 2 is not fully known, or its solid-state structure elucidated, the extent of aggregation in 2 cannot be defined. Hence, although Scheme 2 depicts insertion prior to aggregate formation of 3, insertion may occur to 2 should it be of greater than mononuclearity in solution.
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Scheme 2
Reagents and conditions: (i) 2.0 eq. MgBu2, −60![]() ![]() |
Compound 3 is a remarkably stable material in the solid state, with a melting point of 172–175°C. The 1H NMR spectrum displays a singlet at 0.35 ppm attributable to the dimethylsilanone group of 3, a broad singlet at 3.86 ppm for the N–H resonances of the unmodified ligands, and a multiplet of aromatic resonances between 6.43 and 7.16 ppm. Interestingly, the two arsino methyl types resonate at 1.06 [As(CH3)2: Si] and 1.12 ppm [As(CH3)2: non Si], downfield of the parent compound 2
(0.91 ppm). The 13C NMR spectrum is unexceptional and complicated by the two ligand types. Of note is the resonance attributable to the SiMe2 unit of the modified ligand at 4.8 ppm. The FTIR spectrum of 3 shows a broad absorption at 3268 cm−1 which can be assigned to the N–H stretching mode of the unmodified
NASH ligands. The APCI-MS of 3 does not show the molecular ion, due to its mass (beyond the scope of conditions employed), however, the protonated NASH2 ligand was identified (m/z 198), as were the silylated ligand NASSiMe2+
(m/z 254) and the non-solvated monomer minus an NASH ligand (m/z 492).
The asymmetric unit of 3 (Fig. 2) consists of one OSiMe2NC6H4AsMe2 ligand, two NASH ligands and two magnesium atoms, one of which is co-ordinated by a single tetrahydrofuran. The central unit of 3 is expectedly more complex than that of 1. It exists as a centrosymmetric dimer in which the four magnesium atoms sit on opposing corners of two ‘face-joined’ stacked cubes. In combination with the nitrogen and oxygen atoms, this leads to the formation of a rectangular prism. Co-ordination of the arsino moieties of the ‘non-inserted’ NASH ligands is precluded by the preferential co-ordination of the two modified ligands that demonstrate η3-bonding. The arsino co-ordination of these ligands to magnesium atoms upon the vertices seems to result from increased conformational dexterity rather than an electronic preference of the ligand. Steric buttressing of the phenylene backbones also mitigates against NASH arsino co-ordination. Similarly, the modification of NASH, and subsequent arsine co-ordination, result in contraction of the arsenic–ipso-carbon bond from 1.968 (unmodified ligands) to 1.919 Å, however, the comparable length of the analogous bond in 1 (1.914 Å) suggests this is coincidental. The rectangular prismatic geometry of 3 is distorted such that the angles N(2)–Mg(2)–N(1) and Mg(1)′–O(1)–Mg(1) are on average 105.3 and 96.24°, respectively. However, the bond lengths N(1)–Mg(2) and O(1)–Mg(1) are almost equivalent at 2.069 and 2.054 Å, leading to a pseudo double cube structure. These bond lengths compare favourably to the mean of those crystallographically characterised (Mg–N 2.137, Mg–O 2.076 Å).8 The comparative shortening is apparently a consequence of aggregate strain. The two differing magnesium environments display distorted trigonal bipyramidal [Mg(1), O(1)′–Mg(1)′–O(1) 83.76°] and heavily distorted tetrahedral [Mg(2), N(1)′–Mg(2)–N(2) 74.7°] geometries. Interestingly, the Mg(2)–As(3) bond length of 2.757 Å is longer than that of the only other structurally characterised example of an Mg–As bond (2.594 Å av.).10
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Fig. 2 Molecular structure of the tetranuclear [{Mg2(η1-NHC6H4AsMe2)2(η3-OSiMe2NC6H4AsMe2)(thf)}2]
(3). Selected bond lengths [Å] and angles [°]; Mg(1)–O(1) 2.074(4), Mg(1)–O(1)#1 2.054(4), Mg(1)–O(2) 2.141(4), Mg(1)–N(1) 2.148(5), Mg(1)–N(2) 2.176(5), Mg(2)–O(1) 2.250(4), Mg(2)–N(1)#1 2.065(5), Mg(2)–N(2) 2.072(5), Mg(2)–N(3) 2.030(5), Mg(2)–As(3)
2.757(2), Mg(1)–Mg(1)#1 3.074(4), Mg(1)–Mg(2)#1 3.056(3), Mg(2)–Mg(1)#1 3.056(3), Si(1)–O(1) 1.656(4), Si(1)–N(3) 1.704(5); O(1)–Mg(1)–O(1)#1 83.76(15), O(1)–Mg(1)–O(2) 141.16(18), O(2)–Mg(1)–O(1)#1 135.07(18), O(1)–Mg(1)–N(1) 93.90(18), O(1)–Mg(1)–N(2) 89.82(16), O(1)#1–Mg(1)–N(1) 89.70(18), O(1)#1–Mg(1)–N(2) 95.02(17), O(2)–Mg(1)–N(1) 88.05(18), O(2)–Mg(1)–N(2)
86.34(18), N(1)–Mg(1)–N(2) 174.3(2), Mg(1)–O(1)–Mg(1)#1 96.24(15), Mg(1)–N(1)–Mg(2)#1 93.0(2), Mg(1)–O(1)–Mg(2) 89.88(14), Mg(1)#1–O(1)–Mg(2) 90.35(14), O(1)–Mg(2)–N(1)#1 86.69(18), O(1)–Mg(2)–N(2) 87.88(16), O(1)–Mg(2)–N(3) 73.17(17), O(1)–Mg(2)–As(3) 151.01(12), N(1)#1–Mg(2)–N(2) 105.3(2), N(1)#1–Mg(2)–N(3) 126.1(2), N(1)#1–Mg(2)–As(3) 109.67(15),
N(2)–Mg(2)–N(3) 122.7(2), N(2)–Mg(2)–As(3) 109.43(15), N(3)–Mg(2)–As(3) 77.88(15). Symmetry transformations used to generate equivalent atoms: #1 −x![]() ![]() |
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