Solution and Solid‐State Structure of the First NHC‐Substituted Rhenium Heptahydrides

Abstract

The structure of three new bis(N‐heterocyclic carbene) heptahydride rhenium complexes ([ReH7(NHC)2] with NHC= IMes (2b) (1,3‐bis(2,4,6‐trimethyl phenyl) imidazol‐2‐ylidene), IMesCl2 (4b) (4,5‐dichloro‐1,3‐dihydro‐1,3‐bis(2,4,6‐trimethyl phenyl)‐2H‐imidazol‐2‐ylidene) BIMes (5b) (1,3‐Bis(2,4,6‐trimethylphenyl)‐benzo imidazole ylidene), and of the known complex [ReH7(PPh3)2] (1) (bis(triphenyl phosphine)‐rhenium heptahydride), and the one of their labelled counterparts 2c‐d7, 4c‐d7, and 5c‐d7, were established in solution. The compounds exchanged with HD (2 bar) at 60 °C in deuterated THF to produce statistically distributed isotopomers. This exchange pointed to the intermediacy of H2 complexes and dynamic systems with fast equilibration processes of all hydride positions. T1min measurements were run on 2b, 4b, 5b and 1 in deuterated methylcyclohexane: 2b T1min = 144 ms, 5b T1min = 127 ms, both at 203 K. For 4b two T1min values were obtained corresponding to the 1H‐NMR signals of the hydrides: T1min = 119 ms and T1min = 128 ms both at 203 K. The T1min values determined are typical of terminal rhenium polyhydrides and for 1 was reported a comparably short value of T1min (70 ms, [D8]toluene, at –73 °C), typical of complexes that contain dihydrogen ligands. D‐NMR T1min measurements were also run for the fully deuterated compounds 2c‐d7, 4c‐d7, 5c‐d7 and of 1c‐d7 allowing to calculate the ionicities of the metal hydride bonds. The Re‐D ionicity values were obtained from the Deuterium Quadrupole Coupling Constant (DQCC) determinations. They were in the range of 0.61–0.71 for the fully labelled complexes: 2c‐d7: T1min = 15 ms, i = 61 %; 4c‐d7: (A) T1min = 20 ms, i = 67 %, (B) T1min = 41 ms, i = 77 %; 5c‐d7: T1min = 24 ms, i= 70 %; and 1c‐d7: T1min = 26 ms, i = 71 %. Single crystal X‐ray diffraction studies on 4b enabled us to refine the hydride positions with only restraints in the bond lengths and thermal parameters. The X‐ray solid structure resembles a pseudo‐capped square antiprismatic geometry.

Recent progress on NHC-stabilized early transition metal (group 3–7) complexes: Synthesis and applications

“…209 Finally, Various rhenium (V or VII) NHC complexes have been prepared, including water stable or 188 Re radiolabeled species. 210,211,212,213 In total, 10 technetium NHC complexes have been characterized by X-ray diffraction studies and the bond distances range from 2.07 to 2.23 Å. More than 165 examples of molecular structure of rhenium NHC complexes are known and bond distances range from 2.06 to 2.25 Å with a maximum at 2.14 Å (36 examples).…”

Microwave‐assisted reduction of aromatic nitro compounds with novel oxo‐rhenium complexes

“…[ 37,38 ] Following a published procedure, we reacted the IMes free carbene with complex 1 to afford the complex ([IMes] 2 ReOBr 3 ) ( 2 ) in good yields (Scheme 1; SI S5 and S6). [ 39 ] For the synthesis of ([Py] 3 ReNOBr 2 ) ( 5 ), the second selected catalyst, we first explored the available synthesis already present in literature. Starting from the Re (II) paramagnetic complexes (ReNOBr 5 )(Et 4 N) 2 ( 3 ) and ([OPPh 3 ] 2 Re(NO)Br 3 ) ( 4 ), the groups of Ciani et al [ 40 ] and Machura et al [ 41 ] were able to synthesize the complex 5 .…”

Dynamic Processes of Rhenium Polyhydride Complexes

“…Line shape fitting of dynamic NMR spectra [ 10 , 23 , 31 , 33 , 34 , 37 , 58 , 59 ], molecular modeling [ 22 , 27 , 35 , 36 ], and isotopic substitution [ 1 , 4 , 31 , 33 , 60 , 61 ] are the most commonly used techniques to examine the dynamic properties of rhenium polyhydride complexes. Longitudinal 1 H NMR relaxation measurements (T 1 ) of hydride ligand relaxations have been frequently used to explore the possibility of dihydrogen ligand coordination, but the technique has been shown to have issues that make it less than ideal [ 11 , 14 , 16 , 58 , 62 ].…”

“…Dihydrogen is more frequently used to probe the nature of a reaction or decomposition mechanism than as an actual source for hydrogen atom exchange with the inner coordination sphere rhenium hydride ligands. Recently, it was demonstrated that HD will exchange into the coordination sphere of rhenium(VII) polyhydride complexes stabilized by NHC ligands [ 4 ].…”

“…Rhenium polyhydride complexes (rhenium complexes supported by four or more hydride ligands) have been the focus of numerous studies. The large number of reports on rhenium polyhydride complexes reflect several factors: (1) convenient synthetic routes into such complexes [ 1 , 2 , 3 , 4 ]; (2) the relative stability of such complexes, which makes for convenient materials to work with; (3) intrinsically interesting aspects of rhenium polyhydride complexes such as the homoleptic dianion, (ReH 9 ) 2− or the presence of dihydrogen as a ligand on a metal center with multiple hydride ligands [ 5 , 6 , 7 , 8 , 9 , 10 ]; (4) the technical challenge of characterizing a dihydrogen ligand in the presence of multiple hydride ligands bound to a metal center [ 11 , 12 , 13 , 14 , 15 , 16 , 17 ]; (5) the convenient ability to prepare mixed-metal polyhydride complexes and examine the properties of such complexes [ 18 , 19 , 20 , 21 ]; (6) an interest in understanding the dynamic properties and solution behavior of high coordination metal centers stabilized by several hydride ligands [ 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 ]; and (7) the demonstrated ability for such complexes to serve as precatalysts for the transformation of organic molecules [ 39 , 40 , 41 ,…”