The direct synthesis of activated calcium with aryl halides in tetrahydrofuran (THF) gave the following compounds of the type RCaX in fair to good yields: [MesCaI(THF) 4 ] (1), [(p-tolyl)CaI(THF) 4 ] (2), [PhCaI(THF) 4 ] (3), and [PhCaBr(THF) 4 ] (5). All of these "heavy Grignard reagents" contain a calcium atom in a slightly distorted octahedral environment. They must be handled at low temperatures in order to avoid ether cleavage reactions. The Ca-C bond lengths vary between 2.556(5) Å (3) and 2.583(3) Å ( 5). The thermal stability is enhanced when the coordination number of calcium is increased. The calcium in the seven-coordinate complex [PhCaI(THF)(DME) 2 ] (4) has a distorted-pentagonal-bipyramidical configuration. This complex is thermally more stable and can be handled at 0 °C. The larger coordination number results in a longer Ca-C bond of 2.621(5) Å.

Compounds of the type aryl--M--X, with M=Ca, Sr, Ba and X as any kind of ligand (such as halide, phosphanide, amide, aryl), are presented. The low reactivity of the heavy alkaline earth metals calcium, strontium, and barium enforces an activation prior to use for the direct synthesis. The insertion of these metals into C--I bonds of aryl iodides (direct synthesis) yields aryl metal iodides and has to be performed at low temperatures and in THF. Aryl alkaline-earth-metal compounds show some characteristics: 1) the ease of ether cleavage enforces low reaction temperatures, 2) for Sr and Ba the Schlenk equilibrium is shifted towards homoleptic MI2 and MPh2, 3) high solubility of diaryl alkaline-earth-metal derivatives in THF even at low temperatures initiated quantum chemical investigations on the aggregation behavior, and 4) a strong low field shift of the 13C resonances of the ipso carbon atoms in NMR spectra was observed. First results from quantum chemical calculations on diaryl dicalcium(I) suggest a long Ca--Ca bond with a considerable Ca--Ca bond dissociation energy. Initial results on a selection of applications such as metallation, metathesis, and addition reactions of aryl calcium compounds are presented as well.

The metathesis reactions of (thf)4Ca(Ph)I with the corresponding potassium compounds KR yield the heteroleptic arylcalcium derivatives (thf)3Ca(Ph)[N(SiMe3)2] (1) and (thf)4Ca(Ph)PPh2 (2), due to the insolubility of KI in common organic solvents. However, the reaction of KCp and KOC6H2-2,6-tBu-4-Me with (thf)4Ca(Ph)I give the homoleptic compounds (thf)2CaCp2 (3) and (dme)CaCp2 (4) (depending on the solvent) as well as (thf)3Ca(OC6H2-2,6-tBu-4-Me)2 (5). Diphenylcalcium decomposed under these reaction conditions. The reaction of K[(Me3SiN)2CPh] with (thf)4Ca(Ph)I yields [{(thf)3Ca}2{4,4-Ph2-2,6-(C6H4)2C3N3}(μ-I)] (6). This dihydrotriazine derivative forms due to a slow liberation of benzonitrile from the starting N,N'-bis(trimethylsilyl)benzamidinate. A solvent change in order to shift the Schlenk equilibrium (2 PhCaICaPh2 + CaI2) toward the homoleptic diphenylcalcium leads immediately to ether cleavage reactions and the formation of [{(Et2O)CaPh2}4·(Et2O)CaO] (7), which precipitates from diethyl ether. This cage compound contains an oxygen-centered Ca5 square pyramid with the phenyl groups bridging all Ca···Ca edges.

The metathesis reaction of CaI 2 with KPPh 2 in THF yields (thf) 4 Ca(PPh 2 ) 2 (1). A metallation of HPPh 2 with alkaline earth metals succeeds with strontium and barium and gives (thf) 4 Sr(PPh 2 ) 2 (2) and (thf) 5 Ba(PPh 2 ) 2 (3). From crystal structure determinations of 1 and 2 Ca-P and Sr-P bond lengths of 298.65(6) and 314.29(9) pm, respectively, were obtained. Extremely large Ba-P distances of 332.8(2) and 334.5(2) pm were found for 3. In order to investigate the influence of the bulkiness of the phosphanides on the molecular structure, (thf) 4 Ba[P(Mes) 2 ] 2 (4) and * Prof. Dr. M. Westerhausen Inst. f. Anorg. u. Analyt. 2025(thf) 2 Ba[P(Et)Ph] 2 (5) were prepared in a similar manner. Derivative 4 is monomeric with Ba-P distances of 318.72(9) pm, whereas 5 crystallizes as a one-dimensional polymer with bridging ethyl-(phenyl)phosphanide groups with Ba-P bond lengths between 324.8(1) and 336.0(1) pm. 1 H NMR (200 MHz): δ 1.72 (thf), 3.58 (thf), 6.55 (2H, t, 3 J H,H ϭ 7.2 Hz, p-CH), 6.80 (4H, t, 3 J H,H ϭ 7.4 Hz, m-CH), 7.35 (4H, dt, 4 J H,H ϭ 1.2 Hz, 3 J H,H ϭ 7.3 Hz, o-CH). 13 C{ 1 H} NMR (50 MHz): δ 26.3 (thf), 68.2 (thf), 120.8 (p-C), 127.7 (m-C, 3 J C,P ϭ 5.9 Hz), 130.9 (o-C, 2 J C,P ϭ 17.1 Hz), 153.4 (i-C, 1 J C,P ϭ 37.3 Hz). 31 P{ 1 H} NMR (81 MHz): δ Ϫ9.8 (s). MS (DEI, m/z): 52 (58 %), 78 ([C 6 H 6 ] ϩ , 100 %), 107 ([PhP Ϫ H] ϩ ,72 %), 152 ([Ph 2 Ϫ 2H] ϩ , 23 %), 183 ([Ph 2 P Ϫ 2H] ϩ , 59 %), 201 ([Ph 2 PO] ϩ , 36 %), 262 ([Ph 3 P] ϩ , 15 %), 277 ([Ph 3 PO Ϫ H] ϩ , 19 %), 370 ([Ph 2 P-PPh 2 ] ϩ , 6 %). IR (Nujol, KBr, cm Ϫ1 ): 3055 m, 1574 m, 1435 s, 1068 vw, 1025 w, 888 w, 694 vs, 502 vw, 468 vw. Synthesis of (thf) 5 Ba(PPh 2 ) 2 (3): Prepared according to Method B from (thf) 5 BaI 2 (1.98 g, 2.63 mmol) in THF (20 ml) and KPPh 2 solution (0.5 M, 10.54 ml, 5.27 mmol). 1.88 g (2.12 mmol, 81 %) pyrophoric yellow thin needles were obtained at Ϫ25°C. Decomposition above 100°C. Elemental analysis (C 32 H 36 BaP 2 O 2 , 651.90): calc.: C 58.96, H 5.57; found C 52.26, H 4.34 %.

Various preparative procedures are employed in order to synthesize alkaline earth metal bis(diphenylamides) such as (i) metalation of HNPh2 with the alkaline earth metal M, (ii) metalation of HNPh2 with MPh2, (iii) metathesis reaction of MI2 with KNPh2, (iv) metalation of HNPh2 with PhMI in THF, and (v) metathesis reaction of PhMI with KNPh2 followed by a dismutation reaction yielding MPh2 and M(NPh2)2. The magnesium compounds [(diox)MgPh2]infinity (1) and (thf)2Mg(NPh2)2 (2) show tetracoordinate metal atoms, whereas in (dme)2Ca(NPh2)2 (3), (thf)4Sr(NPh2)2 (4), and (thf)4Ba(NPh2)2 (5) the metals are 6-fold coordinated. Additional agostic interactions between an ipso-carbon of one of the phenyl groups of the amide ligand and the alkaline earth metal atom lead to unsymmetric coordination of the NPh2 anions with two strongly different M-N-C angles in 3-5.