In the presence of an iridium pincer complex, dehydrogenation of ammonia borane (H3NBH3) occurs rapidly at room temperature in tetrahydrofuran to generate 1.0 equivalent of H2 and [NH2BH2]5. A metal borohydride complex has been isolated as a dormant form of the catalyst which can be reactivated by reaction with H2.

Insertion of molecular oxygen into a palladium(II) hydride bond to form an (eta1-hydroperoxo)palladium(II) complex is reported. The hydroperoxo palladium(II) product has been crystallographically characterized. A second-order rate law (first-order in palladium and first-order in oxygen) is observed for the reaction and a large kinetic isotope effect implicates Pd-H bond cleavage in the rate-determining step. The results of studies with radical inhibitors and light suggest that the reaction does not proceed by a radical chain mechanism.

The reaction of (bipy)PdMe(2) (1) (bipy = 2,2'-bipyridine) with molecular oxygen results in the formation of the palladium(II) methylperoxide complex (bipy)PdMe(OOMe) (2). The identity of the product 2 has been confirmed by independent synthesis. Results of kinetic studies of this unprecedented oxygen insertion reaction into a palladium alkyl bond support the involvement of a radical chain mechanism. Reproducible rates, attained in the presence of the radical initiator 2,2'-azobis(2-methylpropionitrile) (AIBN), reveal that the reaction is overall first-order (one-half-order in both [1] and [AIBN], and zero-order in [O(2)]). The unusual rate law (half-order in [1]) implies that the reaction proceeds by a mechanism that differs significantly from those for organic autoxidations and for the recently reported examples of insertion of O(2) into Pd(II) hydride bonds. The mechanism for the autoxidation of 1 is more closely related to that found for the autoxidation of main group and early transition metal alkyl complexes. Notably, the chain propagation is proposed to proceed via a stepwise associative homolytic substitution at the Pd center of 1 with formation of a pentacoordinate Pd(III) intermediate.

Reaction of NaBH4 with (tBuPOCOP)IrHCl affords the previously reported complex (tBuPOCOP)IrH2(BH3) (1) (tBuPOCOP = kappa(3)-C6H3-1,3-[OP(tBu)2]2). The structure of 1 determined from neutron diffraction data contains a B-H sigma-bond to iridium with an elongated B-H bond distance of 1.45(5) A. Compound 1 crystallizes in the space group P1 (Z = 2) with a = 8.262 (5) A, b = 12.264 (5) A, c = 13.394 (4) A, and V = 1256.2 (1) A(3) (30 K). Complex 1 can also be prepared by reaction of BH3 x THF with (tBuPOCOP)IrH2. Reaction of (tBuPOCOP)IrH2 with pinacol borane gave initially complex 2, which is assigned a structure analogous to that of 1 based on spectroscopic measurements. Complex 2 evolves H2 at room temperature leading to the borane complex 3, which is formed cleanly when 2 is subjected to dynamic vacuum. The structure of 3 has been determined by X-ray diffraction and consists of the (tBuPOCOP)Ir core with a sigma-bound pinacol borane ligand in an approximately square planar complex. Compound 3 crystallizes in the space group C2/c (Z = 4) with a = 41.2238 (2) A, b = 11.1233 (2) A, c = 14.6122 (3) A, and V = 6700.21 (19) A(3) (130 K). Reaction of (tBuPOCOP)IrH2 with 9-borobicyclononane (9-BBN) affords complex 4. Complex 4 displays (1)H NMR resonances analogous to 1 and exists in equilibrium with (tBuPOCOP)IrH2 in THF solutions.

The group 16 oxidants dibenzoyl-and bis(4-trifluoromethylbenzoyl)-peroxide react with dimethylpalladium(II) and methyl(4-tolyl)palladium(II) complexes of the bidentate nitrogen donor ligands 2,2′-bipyridine and N,N,N′,N′-tetramethylethylenediamine in discrete stepwise processes as the temperature is raised from -70 °C. Carbon-oxygen bonds are formed during this reaction sequence but not from those Pd(IV) complexes detected spectroscopically. The initial reaction gives undetected "Pd IV (O 2 CAr) 2 MeR(L 2 )" (Ar ) Ph, Ar F ; R ) Me, Tol; L 2 ) bpy, tmeda), which immediately undergo methyl aroate exchange with Pd II MeR(L 2 ) to give Pd II (O 2 CAr)R(L 2 ) and Pd IV (O 2 CAr)Me 2 R(L 2 ), where all products except for Pd IV (O 2 CAr)Me 2 -Tol(tmeda) were detected by 1 H NMR spectroscopy. On raising the temperature, the Pd IV -Me 3 complexes reductively eliminate Me-Me, and the Pd IV Me 2 Tol complexes eliminate Me-Me and Tol-Me. The resultant Pd(II) complexes Pd II (O 2 CAr)R(L 2 ) react with (ArCO 2 ) 2 at higher temperatures to form Pd II (O 2 CAr) 2 (L 2 ) and R-O 2 CAr (R ) Me, Tol), except for Pd II (O 2 -CAr)Tol(tmeda), which forms Pd II (O 2 CAr) 2 (tmeda) and 4,4′-bitolyl. Each reaction step has been confirmed by the independent synthesis of intermediates Pd II (O 2 CAr) 2 (L 2 ) and Pd II (O 2 -CAr)R(L 2 ) (Ar ) Ph, Ar F ; R ) Me, Tol; L 2 ) bpy, tmeda) and Pd IV (O 2 CR)Me 2 R(L 2 ) (R ) Ph, Ar F ; R ) Me, Tol; L 2 ) bpy) by metathesis reactions of halogeno complexes with Ag[O 2 CAr], followed by temperature-dependent studies of both the decomposition of Pd(IV) complexes and reactions of Pd(II) complexes with (ArCO 2 ) 2 . Attempts to prepare "Pd IV (O 2 CAr) 2 MeR-(bpy)" in a similar manner (and thus in the absence of PdMeR(bpy) with which they undergo exchange reactions) were unsuccessful, but the complexes Pd IV I 2 MeR(bpy) (R ) Me, Tol) that formed on reaction of diiodine with PdMeR(L 2 ) were detected and found to reductively eliminate iodomethane. X-ray structural studies are reported for the square-planar palladium(II) complexes Pd(O 2 CPh) 2 (bpy), Pd(O 2 CAr) 2 (tmeda) (Ar ) Ph, Ar F ), and Pd(O 2 CPh)-(Tol)(bpy)‚CH 2 Cl 2 .

The presence of the [2,6-(dimethylaminomethyl)phenyl-N,C,N] -(pincer) ligand (NCN) in platinum(II) complexes has been used to generate stable organoplatinum(IV) complexes that model possible intermediates and reactivity in metal-catalyzed C-O bond formation processes. The complexes M(O 2 CPh)(NCN) [M ) Pd (1), Pt (2)] were obtained by metathesis reactions from the chloro analogues, and although 1 does not react with dibenzoyl peroxide, 2 does so to form Pt(O 2 CPh) 3 (NCN) (3) as a model intermediate for the acetoxylation of arenes by acetic acid in the presence of palladium(II) acetate and an oxidizing agent. The complex Pt(O 2 CPh)(NCN) (2) reacts with iodomethane in a complex manner to form PtI-(NCN) (6) and cis-Pt(O 2 CPh) 2 Me(NCN) (7). Complex 7 decomposes to form Pt(O 2 CPh)(NCN) (2) and MeO 2 CPh, probably via benzoate dissociation followed by nucleophilic attack by the benzoate ion at the Pt IV -Me carbon atom. The Pd(II) analogue Pd(O 2 CPh)(NCN) (1) reacts with MeI to give PdI(NCN) (8) and MeO 2 CPh, for which the potential intermediacy of Pd-(IV) species could not confirmed by 1 H NMR spectroscopy. The complex PtTol(NCN) (4) (Tol ) 4-tolyl) reacts with (PhCO 2 ) 2 to form cis-Pt(O 2 CPh) 2 Tol(NCN) (5), but, unlike the Pt IV Me analogue 7, the Pt IV Tol complex 5 does not undergo facile C-O bond formation. X-ray structural studies of the isostructural square-planar complexes M(O 2 CPh)(NCN) (1, 2) and of the octahedral Pt(IV) complexes as solvates 3‚1/2Me 2 CO, 5‚Me 2 CO, and 7‚Me 2 CO‚H 2 O are reported. Complexes 5 and 7 have cis-PtC 2 and cis-Pt(O 2 CPh) 2 configurations.

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This report summarizes our findings during the study of a novel homogeneous epoxidation catalyst system that uses molecular oxygen as the oxidant, a "Holy Grail" in catalysis. While olefins (alkenes) that do not contain allylic hydrogens can be epoxidized directly using heterogeneous catalysts, most olefins cannot, and so a general, atom-efficient route is desired. While most of the work performed on this LDRD has been on pincer complexes of late transition metals, we also scouted out metal/ligand combinations that were significantly different, and unfortunately, less successful. Most of the work reported here deals with phosphorus-ligated Pd hydrides [(PCP)Pd-H]. We have demonstrated that molecular oxygen gas can insert into the Pd-H bond, giving a structurally characterized Pd-OOH species. This species reacts with oxygen acceptors such as olefins to donate an oxygen atom, although in various levels of selectivity, and to generate a [(PCP)Pd-OH] molecule. We discovered that the active [(PCP)Pd-H] active catalyst can be regenerated by addition of either CO or hydrogen. The demonstration of each step of the catalytic cycle is quite significant. Extensions to the pincer-Pd chemistry by attaching a fluorinated tail to the pincer designed to be used in solvents with higher oxygen solubilities are also presented.