Chromocene reacts with 1,3-dimesitylimidazolium chloride in THF to yield cyclopentadiene and CpCrCl(1,3-dimesitylimidazolin-2-ylidene) (1), which contains an N,N-heterocyclic carbene ligand. This new complex is a relatively rare example of a CpCr(II) complex containing four unpaired electrons. The reaction of 1 with phenylmagnesium bromide yields CpCrPh(1,3-dimesitylimidazolin-2-ylidene) (2). An X-ray crystallographic investigation of 2 reveals a two-legged piano stool structure with a planar central Cr atom.

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Cationic rhenium dihydrogen complexes, [Re(H2)(PR3)2(CO)3]B(Ar‘)4 (PR3 = PCy3, PiPr3, PiPrPh2, PPh3; Ar‘ = 3,5-(CF3)2C6H3), have been prepared by the protonation of ReX(PR3)2(CO)3 (X = H, CH3) with [H(Et2O)2]B(Ar‘)4 under a hydrogen atmosphere. Deuterium is incorporated into the H2 ligand when placed under a D2 atmosphere and large J HD values (30−33 Hz) are consistent with a dihydrogen formulation. Relaxation data indicate very short T 1min for these complexes. These complexes are susceptible to heterolytic cleavage of dihydrogen, and the reactivity with several bases has been investigated. Under vacuum or argon atmosphere the complexes readily lose hydrogen to form 16-electron complexes. In the solid state, [Re(PCy3)2(CO)3]B(Ar‘)4 exhibits an agostic interaction to a β C−H bond of the phosphine ligand. Variable-temperature 31P{1H} NMR spectra of [Re(PCy3)2(CO)3]B(Ar‘)4 indicate a dynamic process involving hindered rotation about the Re−P bond. Competition studies have been conducted, and the hydrogen binding affinity is higher for [Re(H2)(PCy3)2(CNtBu)3]B(Ar‘)4 (5) than for [Re(H2)(PCy3)2(CO)3]B(Ar‘)4 (2a). Similar experiments also find that 5 also binds hydrogen preferentially over W(H2)(PCy3)2(CO)3.

The syntheses, properties, and characterization of several new complexes, Re(CNtBu) 3 (PCy 3 ) 2 H (1a), Re(CNtBu) 3 (PR 3 ) 2 Cl [R ) Cy, Ph (2a, 2b)], the 17-electron [Re(CNtBu) 3 (PCy 3 ) 2 Cl] + (3a), the dihydrogen complex, [Re(CNtBu) 3 (PR 3 ) 2 (H 2 )] + [R ) Cy, Ph (4a, 4b)], the coordinatively unsaturated [Re(CNtBu) 3 (PCy 3 ) 2 ] + (5a), and [Re(CNtBu) 4 (PCy 3 ) 2 ] + (6a) are reported. In addition, spectroscopic evidence for the dinitrogen complex [Re(CNtBu) 3 -(PCy 3 ) 2 (N 2 )] + and the dihydrogen complex [Re(CNtBu) 5 (H 2 )] + is presented. Thermodynamic parameters have been obtained for the equilibrium system, Re(CNtBu) 3 (PCy 3 ) 2 Cl (2a) + H 2 h [Re(CNtBu) 3 (PCy 3 ) 2 (H 2 )]Cl (4a). 1 H and 31 P{ 1 H} NMR studies (CD 2 Cl 2 ) over the temperature range 286-316 K afford values of ∆H°) -18.0 ( 0.7 kcal/ mol, ∆S°) -44 ( 2 eu, and ∆G°2 98 ) -4.8 ( 1.3 kcal/mol for this equilibrium. The complexes 4a and 4b are characterized as dihydrogen complexes. Their J HD (R ) Cy, 30.3 Hz; R ) Ph, 30.9 Hz) and T 1(min) values of 8 ms (300 MHz) are consistent with H-H distances of ca. 0.80 Å. [Re(CNtBu) 5 (HD)] + is also characterized as a dihydrogen complex based on a J HD of 33.4Hz. This complex could not be isolated at room temperature, even with noncoordinating counteranions, due to the lability of the H 2 ligand. [Re(PMe 3 ) 5 HD] + shows no H-D coupling and is characterized as a dihydride complex. The unsaturated complex [Re(CNtBu) 3 (PCy 3 ) 2 ] + (5a) is found to undergo a dynamic isonitrile rearrangement on the NMR time scale. 1 H NMR spin saturation transfer studies over a temperature range of 254-297 K afford the activation parameters ∆H q ) 11.4 ( 0.8 kcal/mol, ∆S q ) -17.6 ( 1.6 eu, and ∆G q 298 ) 16.6 ( 1.2 kcal/mol. The molecular structures of 2a and 3a have been determined by X-ray crystallography. 2a shows an unusually long Re-Cl bond distance of 2.596(2) Å.

Ketone complexes [CpM(CO) 2 (PR 3 )(η 1 -Et 2 C᎐ ᎐ O)] ϩ BArЈ 4 Ϫ (R = Ph or Me; M = Mo or W) were prepared from hydride transfer from Cp(CO) 2 (PR 3 )MH to Ph 3 C ϩ BArЈ 4 Ϫ [ArЈ = 3,5-bis(trifluoromethyl)phenyl] in the presence of 3-pentanone. These ketone complexes are catalyst precursors for hydrogenation of Et 2 C᎐ ᎐ O under mild conditions (23 ЊC, <4 atm H 2 ). Analogous catalytic hydrogenations are obtained from reaction of the PCy 3 complexes Cp(CO) 2 (PCy 3 )MH with Ph 3 C ϩ BArЈ 4Ϫ . The proposed mechanism involves displacement of the ketone by H 2 , producing a cationic metal dihydride [CpM(CO) 2 (PR 3 )(H) 2 ] ϩ . Proton transfer from the dihydride gives a protonated ketone, followed by hydride transfer from the neutral metal hydride CpM(CO) 2 (PR 3 )H to produce the alcohol complex [CpM(CO) 2 (PR 3 )(Et 2 CHOH)] ϩ . The free alcohol product is released from the metal through displacement by H 2 or ketone, completing the catalytic cycle. In most cases, conversion of the ketone or alcohol complexes to the dihydride is the turnover-limiting step of the catalytic cycle, with ketone and alcohol complexes being observed during the reaction. For reactions using the W-PCy 3 system, the dihydride [CpW(CO) 2 (PCy 3 )(H) 2 ] ϩ is observed as the resting state of the catalytic process. Proton transfer is slow and becomes turnover-limiting in this case. The Mo catalysts are more active than W, and the dependence on phosphine is PCy 3 > PPh 3 > PMe 3 . The turnover rates are slow, with the fastest initial rate of about 2 turnovers per hour found for the Mo-PCy 3 system. This ionic hydrogenation mechanism does not require coordination of the ketone to the metal for the hydrogenation, thus differing from traditional mechanisms where coordination of a ketone to a metal precedes insertion of the ketone into a M-H bond.