The oligomerization of ethylene typically leads to a broad range of R-olefins; catalytic systems that are selective for specific desirable alkenes would be of great interest. There have been several recent reports of trimerization of ethylene to 1-hexene with high selectivity. 1 While some ethylene trimerization catalysts are based on titanium and tantalum, 1j,k chromium-based systems generally display higher activity, selectivity, and thermal stability. 1b-i A recent communication describes a catalyst, generated in situ by treating Cr(III) salts with methylalumoxane in the presence of the diphosphine PNP OMe [1, PNP OMe ) (o-MeO-C 6 H 4 ) 2 PN(Me)P(o-MeO-C 6 H 4 ) 2 ], that trimerizes ethylene to 1-hexene with unprecedented selectivity and productivity. 1c,d Herein we report the synthesis and characterization of chromium complexes of 1, 2 along with some mechanistic investigations of the ethylene trimerization reaction they catalyze.Reactions of diphosphine 1 (Scheme 1) with suitable chromium-(III) etherate complexes afford compounds (PNP OMe )CrCl 3 (2) and (PNP OMe )CrPh 3 (3); 2 reacts with o,o′-biphenyldiyl Grignard to give (PNP OMe )Cr(o,o′-biphenyldiyl)Br (4). All three are hexacoordinate, displaying (P,P,O)-κ 3 coordination of the diphosphine, established by single-crystal X-ray diffraction as well as 2 H NMR. 3,4 The structures of 3 and 4 (Figure 1) reveal long Cr-O bonds (2.293(2) Å in 3 and 2.337(2) Å in 4) and significant differences between the two Cr-P bond lengths. The ether-chelated phosphine is closer to chromium by 0.16 Å (3) and 0.23 Å (4).

Data Collection Temperature 98(2) K θ range for 11713 reflections used in lattice determination 2.19 to 32.17°U nit cell dimensions a = 10.5083(6) Å α = 93.9640(10)°b = 11.2424(7) Å β = 92.9940(10)°c = 18.3603(10) Å γ = 115.0850(10)°V olume 1951.8(2) Å 3 Z 2 Crystal system Triclinic Space group P-1 Density (calculated) 1.361 Mg/m 3 F(000) 835 Data collection program Bruker SMART v5.054 θ range for data collection 2.01 to 32.75°C ompleteness to θ = 32.75°83.2 % Index ranges-15 ≤ h ≤ 15,-14 ≤ k ≤ 16,-27 ≤ l ≤ 27 Data collection scan type ω scans at 5 φ settings Data reduction program Bruker SAINT v6.45 Reflections collected 32181 Independent reflections 11984 [R int = 0.0704] Absorption coefficient 0.488 mm-1 Absorption correction None Max. and min. transmission 0.9667 and 0.8556 Structure solution program SHELXS-97 (Sheldrick, 1990) Primary solution method Direct methods Secondary solution method Difference Fourier map Hydrogen placement Geometric positions Structure refinement program SHELXL-97 (Sheldrick, 1997) Refinement method Full matrix least-squares on F 2

The novel, hemilabile (phosphinoalkyl)arene ligands ArX(CH2)2PPh2 (1a, Ar = C6H5, X = O; 1b, Ar = C6H5, X = CH2; 1c, Ar = FC6H4, X = CH2) were synthesized and complexed to Rh(I) to form the bis(phosphine), η6-arene piano stool complexes [(η6:η1-ArX(CH2)2PPh2)Rh(η1-ArX(CH2)2PPh2)]BF4 (2a−c). Complexes 2a−c were fully characterized in solution, and complex 2a was characterized by single-crystal X-ray diffraction methods. Two of these complexes, 2a,c, undergo an unusual, degenerate η6-arene, free arene exchange reaction which was studied by 2-D NMR EXSY experiments. A mechanism for the exchange reaction of 2a which involves the formation of a square planar, cis-phosphine, cis-ether Rh(I) complex, [Rh(η2-PhO(CH2)2PPh2)2]BF4 (13), is proposed.

In the course of a high-throughput search for optimal combinations of bidentate ligands with Pd(II) carboxylates to generate oxidation catalysts, we obtained and crystallographically characterized a number of crystalline products. While some combinations afforded the anticipated (L-L)Pd(OC(O)R)2 structures (L-L = bipyridine, tmeda; R = CH3, CF3), many gave unusual oligometallic complexes resulting from reactions such as C−H activation (L-L = sparteine), P−C bond cleavage (L-L = 1,2-bis(diphenylphosphino)ethane, and C−C bond formation between solvent (acetone) and ligand (L-L = 1,4-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene). These findings illustrate potential pitfalls of screening procedures based on assuming uniform, in situ catalyst self-assembly.

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