Abstract:The reaction of tetrakis(tetrahydrofuran-O)barium bis[bis(trimethylsilyl)phosphanide] with diphenylbutadiyne yields dimeric (tetrahydrofuran-O)barium 2,5-diphenyl-3-(1,4-diphenylbutene-3-yne-2-ide-1-yl)-4-trimethylsilyl-1-phosphacyclopentadienide 1. The alkenide moiety bridges two barium atoms, with Ba-C bond lengths of 2.881 and 3.071 Å, thus forming a unique three-center two-electron Ba-C-Ba bond. Furthermore, the coordination sphere of the alkaline earth metal atom contains the phosphacyclopentadienide moiety as well as one tetrahydrofuran molecule. A loose side-on coordination of the alkyne moiety completes the coordination sphere of the barium center. A similar reaction with magnesium bis[bis(trimethylsilyl)-phosphanide] gives the dimeric addition product magnesium bis(trimethylsilyl)phosphanide 1,4-diphenyl-1-bis(trimethylsilyl)phosphanyl-but-1-ene-3-yne-2-ide 2. A reaction mechanism for the formation of the barium derivative is suggested.Thus far, crystallographically characterized molecules with barium-carbon bonds have been limited to barocenes 1 or sideon coordinated aromatic systems. 2 Up to now, σ bonds between the heavier alkaline earth metals and carbon atoms have only been structurally characterized for calcium. Lappert and coworkers 3 inserted a calcium atom into a C-Br bond by cocondensation of the alkaline earth metal and bromobis-(trimethylsilyl)methane; the addition of 1,4-dioxane led to the crystallization of (diox) 2 Ca[CH(SiMe 3 ) 2 ] 2 with Ca-C distances of 2.48 Å. Smith, Eaborn and co-workers 4 prepared bis[tris-(trimethylsilyl)methyl]calcium with a bent C-Ca-C fragment of 150°and Ca-C bond lengths of 2.46 Å via the metathesis reaction. The bridging phenylacetylide 5 between two calcium atoms in [(C 5 i Pr 4 H)Ca-CtC-Ph] 2 shows Ca-C bond lengths of 2.52 and 2.55 Å. Similar calcium-carbon distances were found in the tetrakis(tetrahydrofuran-O)calcium and -strontium 2,3-dimethyl-1,4-diphenylbutadiene complex A with a η 4 -bonded ligand. 6 2,4-Di(tert-butyl)pentadienide coordinates in a η 5 -fashion to calcium (B, Scheme 1). 7 Here we report for the first time a crystallographically characterized alkenylbarium bond. Results and DiscussionSynthesis. The reaction of (tetrahydrofuran-O)barium bis-[bis(trimethylsilyl)phosphanide] 8 with diphenylbutadiyne in toluene at room temperature yields dimeric (tetrahydrofuran-O)barium 2,5-diphenyl-3-(1,4-diphenylbutene-3-yne-2-ide-1-yl)-4-trimethylsilyl-1-phosphacyclopentadienide 1. The proposed reaction mechanism is presented in eq 1. The first reaction step is the addition of a Ba-P bond to the CtC bond. If magnesium bis[bis(trimethylsilyl)phosphanide] 9 is used instead of the barium derivative, the product 2, which represents the first step of the reaction sequence, precipitates from a toluene solution. The subsequent 1,3-trimethylsilyl shift is well-known for the addition reaction of barium bis[bis(trimethylsilyl)phosphanide] with benzonitrile 10 where barium bis[1,3-bis(trimethylsilyl)-2-phenyl-1-aza-3-phosphapropenide] is isolable. Th...

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The addition reaction of tetrakis(tetrahydrofuran)calcium or -strontium bis[bis(trimethylsilyl)phosphanide] with diphenylbutadiyne in toluene yields nearly quantitative amounts of slightly yellow tetrakis(tetrahydrofuran)calcium (1) and -strontium bis[2,5-diphenyl-3,4-bis(trimethylsilyl)phosphacyclopentadienide] (2). Due to the fact that the alkaline earth metal bis[bis(trimethylsilyl)amides] do not react with alkynes, colorless (tetrahydrofuran)calcium bis[2,5-bis(tert-butyl)-1-azacyclopentadienide] (3) was prepared by the transmetalation of N-trimethylstannyl-2,5-bis(tert-butyl)-1-azacyclopentadiene with distilled calcium. The metathesis reaction of 1 and 2 with SnCl2 gives yellow tin bis[2,5-diphenyl-3,4-bis(trimethylsilyl)phosphacyclopentadienide] (4).The phosphorus atoms of 1 and 2 are in a planar environment with long M−P distances, whereas in 4 the P atoms comprise a pyramidal coordination sphere. The tin−carbon distances are larger than observed in stannocenes which could be interpreted as a stannylene-like structure with σ-bonded ligands. Crystallographic data for 1:  monoclinic, C2/c, a = 2108.9(3) pm, b = 1296.2(2) pm, c = 2735.3(3) pm, β = 93.37(1)°, Z = 4, wR2 = 0.1381 (all data, on F 2). Crystallographic data for 2:  monoclinic, C2/c, a = 2104.7(1) pm, b = 1296.43(6) pm, c = 2717.8(1) pm, β = 93.189(1)°, Z = 4, wR2 = 0.0954 (all data, on F 2). Crystallographic data for 4:  triclinic, P1̄, a = 1253.25(4) pm, b = 1312.58(4) pm, c = 1481.61(5) pm, α = 87.667(1)°, β = 79.438(1)°, γ = 72.684(1)°, Z = 2, wR2 = 0.0692 (all data, on F 2).

The reaction of zirconocene dichloride with 2 equiv of butyllithium and 1-trimethylsilylpropyne yields yellow 1,1-bis(cyclopentadienyl)-3,4-dimethyl-2,5-bis(trimethylsilyl)zirconacyclopenta-2,4-diene, 1. The metathesis reaction of 1 with PCl3 gives a mixture of 1-chloro- and 1-cyclopentadienyl-3,4-dimethyl-2,5-bis(trimethylsilyl)-1-phosphacyclopenta-2,4-diene, which was allowed to react in THF with distilled calcium. Crystallization of the product from THF solution gave dimeric bis(tetrahydrofuran-O)dicalcium bis[3,4-dimethyl-2,5-bis(trimethylsilyl)-1-phosphacyclopenta-2,4-dienide] cyclopentadienide chloride, 2. The metathesis reaction of 1 with AsCl3 yielded nearly quantitatively 1-chloro-3,4-dimethyl-2,5-bis(trimethylsilyl)-1-arsacyclopenta-2,4-diene, 3. 3,4-Dimethyl-2,5-bis(trimethylsilyl)-1-arsacyclopentadienyl bis(tetrahydrofuran-O)calcium chloride, 4, was isolated from the reduction of 3 with distilled calcium.

The tetrahydrofuran complexes of the alkaline earth metal bis[bis(trimethylsilyl)phosphanides] react with 2 equiv of benzonitrile to the bis[1,3-bis(trimethylsilyl)-2-phenyl-1-aza-3-phosphapropenides] of magnesium (1), calcium (2), strontium (3), and barium (4). Whereas 1 and 2 crystallize as bis(tetrahydrofuran) adducts, the heavier homologous derivatives 3 and 4 precipitate as tris(tetrahydrofuran) complexes. The bidentate trans/trans-isomeric 1,3-bis(trimethylsilyl)-2-phenyl-1-aza-3-phosphapropenide ligands of all these compounds display very similar spectroscopic data; however, the influence of the alkaline earth metal is observed in the silicon NMR. The nitrogen atoms of these anions have trigonal planar geometry, whereas the phosphorus atoms are coordinated trigonal pyramidal. Crystallographic data:  1, monoclinic, P21/c, a = 1505.3(3) pm, b = 1259.0(2) pm, c = 2278.4(5) pm, β = 91.91(2)°, Z = 4, wR2 = 0.1279; 2, monoclinic, P21/c, a = 1522.8(7) pm, b = 1279.2(5) pm, c = 2314.0(10) pm, β = 92.53(2)°, Z = 4, wR2 = 0.1513; 3, triclinic, P1̄, a = 1144.8(3) pm, b = 1579.4(6) pm, c = 1670.6(6) pm, α = 98.02(3)°, β = 109.67(3)°, γ = 104.18(3)°, Z = 2, wR2 = 0.1745; 4, triclinic, P1̄, a = 1245.2(4) pm, b = 1907.1(4) pm, c = 2212.3(7) pm, α = 76.46(2)°, β = 76.45(2)°, γ = 89.31(2)°, Z = 4, wR2 = 0.1409.

Metalation of triisopropylsilylphosphane with bis(tetrahydrofuran‐O)calcium bis[bis(trimethylsilyl)amide] in tetrahydropyran (thp) in a molar ratio of 3:2 yields (Me3Si)2NCa[μ‐P(H)SiiPr3]3Ca(thp)3 (1) containing a trigonal‐bipyramidal Ca2P3 core, the metal atoms occupying apical positions. Reaction of two equivalents of triisopropylsilylphosphane or ‐arsane with bis(tetrahydrofuran‐O)barium bis[bis(trimethylsilyl)amide] in tetrahydrofuran gives the corresponding bis(phosphanide) 2 and bis(arsanide) 3, compounds of the type (thf)3Ba[μ‐E(H)SiiPr3]Ba(thf)2E(H)SiiPr3 with E = P, As. The equimolar reaction of (tri‐tert‐butylsilyl)phosphane with (thf)2Ba[N(SiMe3)2]2 in toluene yields heteroleptic dimeric (thf)2Ba[N(SiMe3)2][P(H)SitBu3] (4). Addition of a further equivalent of H2PSitBu3 leads to the formation of homoleptic (thf)nBa[P(H)SitBu3]2 (5). Dissolution of the latter in aromatic hydrocarbons leads to the elimination of H2PSitBu3, yielding dimeric (thf)Ba3(PSitBu3)2[P(H)SitBu3]2 (6). The inner core of 6 consists of the tetramer (BaPSitBu3)4 based on a Ba4P4 heterocubane unit, two opposite faces being capped with (thf)Ba[P(H)SitBu3]2 molecules.

The metathesis reaction of (dme)LiPH2 and chlorodiisopropylsilane yields HP(SiHiPr2)2 (1) and P(SiHiPr2)3 (2). The metalation of 1 by n/sec‐dibutylmagnesium in heptane leads to the formation of dimeric magnesium bis[bis(diisopropylsilyl)phosphanide] [(3)2]. A toluene solution of magnesium bis[bis(trimethylsilyl)phosphanide] (4) shows a dimer‐trimer equilibrium in the 31P{1H}‐NMR spectrum at −40°C with an increasing amount of the dimer with increasing temperature. A molecular mass determination by freezing‐point depression in benzene yields a value of 516 g·mol−1, which lies between a monomeric and a dimeric molecule. Cooling of the toluene solution to −30°C leads to the precipitation of single crystals of monoclinic {(Me3Si)2P−Mg[μ‐P(SiMe3)2]2}2Mg [(4)3].

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