A T‐Shaped Platinum(II) Boryl Complex as the Precursor to a Platinum Compound with a Base‐Stabilized Borylene Ligand

Abstract

Transition-metal boryl complexes constitute an abundant and important class of boron-containing transition-metal compounds, [1] in view of their intermediacy in the catalyzed hydro- [2] and diboration [3] of unsaturated organic substrates, as well as the selective CÀH bond activation of alkanes and arenes. [4] A number of synthetic routes have been applied to the generation of boryl complexes.[1] Among these, that involving BÀX (X = Cl, Br) bond oxidative addition has seldom been employed, and examples of rhodium and iridium, [5] palladium, [6] and platinum [7] boryl complexes synthesized in this fashion have only relatively recently appeared in the literature.We previously reported the synthesis of trans-[(Cy 3 P) 2 Pt(Br){B(Fc)Br}] (Fc = ferrocenyl, Cy = cyclohexyl)(1) by reaction of [Pt(PCy 3 ) 2 ] with FcBBr 2

Experimental Studies on the trans‐Influence of Boryl Ligands in Square‐Planar Platinum(II) Complexes

“…Our work on platinum bromoboryl complexes led to the straightforward synthesis of trans ‐[(Cy 3 P) 2 Pt(Br){B(Br)Fc}] ( 1 ) (Fc=Ferrocenyl) upon oxidative addition of a BBr bond of FcBBr 2 to [Pt(PCy 3 ) 2 ] 8a. The very high trans ‐influence of the ‐B(Br)Fc group allowed the generation of the cationic platinum complex trans ‐[(Cy 3 P) 2 Pt{B(Br)Fc}][BAr f 4 ] ( 2 ) (Ar f =3,5‐(CF 3 ) 2 C 6 H 3 ), upon bromide abstraction at platinum in 1 with Na[BAr f 4 ], and contributed to the isolation of 2 as the first T‐shaped cationic platinum complex devoid of any stabilizing agostic interaction 8b. This experimental finding paralleled the theoretical conclusions drawn by Marder and Lin in their studies on rhodium‐catalyzed borylation of aromatic CH bonds, where 14‐electron d 8 ML 3 boryl complexes of the type [(R 3 P) 2 Rh(BX 2 )] were calculated to be reluctant to bind CH bonds in the site trans to the boryl group, due to the extremely strong trans ‐influence of the latter 9…”

“…Recent work in our laboratories allowed the synthesis and structural characterization of a number of complexes with unprecedented boron‐based ligands, such as the iminoboryl complex trans ‐[(Cy 3 P) 2 Pt(Br)(BNSiMe 3 )] ( 12 ),14 the terminal borylene complex trans ‐[(Cy 3 P) 2 Pt(Br)(BMes)][B(C 6 F 5 ) 4 ] ( 13 ),12 and the base‐stabilized borylene complex trans ‐[(Cy 3 P) 2 Pt(Br){B(NC 5 H 4 ‐4‐CH 3 )Fc}][BAr f 4 ] ( 14 ) 8b. Comparison of the PtBr bond lengths of these complexes (2.5516(4) Å, 12 ; 2.5418(6) Å, 13 ; 2.6057(5) Å, 14 , respectively) with those observed in compounds 1 , 3 – 8 , 10 , 11 indicates the generally stronger trans ‐influence of the ‐BXX′ groups, with the relatively long PtBr separation in 14 being ascribable to the boryl character of the base‐stabilized borylene ligand, in view of the three‐coordinate boron center (Figure 3).…”

“…Comparison of the PtBr bond lengths of these complexes (2.5516(4) Å, 12 ; 2.5418(6) Å, 13 ; 2.6057(5) Å, 14 , respectively) with those observed in compounds 1 , 3 – 8 , 10 , 11 indicates the generally stronger trans ‐influence of the ‐BXX′ groups, with the relatively long PtBr separation in 14 being ascribable to the boryl character of the base‐stabilized borylene ligand, in view of the three‐coordinate boron center (Figure 3). 8b…”

Chemistry of metal – boron double bonds

“…A possible mechanism for this transformation involves abstraction of fluoride from the ½BAr f 4 À anion by the putative aryloxyborylene species [(Z 5 -C 5 R 5 )Fe(CO) 2 (BOMes)] þ . Similar fluoride abstraction processes from borate counter-ions have been observed for cationic gallium and indium species (Coombs and Aldridge, unpublished results), and indeed, to date, cationic O-substituted borylenes have been isolated only as base-stabilized derivatives [69].…”

“…In the case of arylborylene systems, the reactivity of complex 16 towards neutral two-electron donors typically proceeds via alternative nucleophilic attack at iron [43,44], with accompanying substitution of the BMes ligand (vide infra); nevertheless Braunschweig has been successful in isolating the picoline (50) is isolated, implying that the boron to platinum migration of the bromide substituent is driven, at least in part by coordination of the N-donor base [69].…”