TRANSITION METAL COMPLEXES OF AMBIPHILIC LIGANDS

Abstract

Alane-containing (Me2PCH2AlMe2)2 (1) was prepared as previously described, via the reaction of ClAlMe2 with LiCH2PMe2. The previously reported ambiphilic ligand precursor (Me2PCH2BMe2)2 (2) was synthesized via a novel method that avoids the use of Me2PH; this method is analogous to that for the synthesis of the known (Me2PCH2AlMe2)2, by addition of BrBMe2 to LiCH2PMe2. The novel ligand precursor (Ph2PCH2BMe2)2 (3) was prepared by a similar procedure, by the addition of BrBMe2 to LiCH2PPh2. Attempts to prepare (Ph2PCH2EPh2)2 (E = Al, B) and (Ph2PCH2AlMes2)2 (4) are also described. The coordination chemistry of (Me2PCH2AlMe2)2 (1) was explored with [{Rh(µ-Cl)(cod)}2]. A 2:1 ligand to metal ratio yielded the 2-P,P-coordinated bis(phosphino)aluminate complex [{2P,P-(Me3Al)ClMeAl(CH2PMe2)2}Rh(cod)] (5), in which an AlCl substituent bridges to a molecule of AlMe3, which could be removed under vacuum to give [{2P,P-ClMeAl(CH2PMe2)2}Rh(cod)] (6). By contrast, a 1:1 ligand to metal ratio gave [Rh(cod)(µ-Cl)(Me2PCH2AlMe2)] (7), which contains an RhClAl bridging interaction, as the major product, and related [Rh(cod)(µ-Cl)(Me2PCH2AlMeCl)] (7A) and 6 as minor products. The aforementioned reactions were carried out in arene solvents, however, when THF was used as a solvent for the 1:1 reaction of (Me2PCH2AlMe2)2 (1) with [{Rh(µ-Cl)(cod)}2], the aluminum-free complex [{Rh(µ-CH2PMe2)(cod)}2] (8) was generated instead. A 1:1 reaction of (Me2PCH2AlMe2)2 (1) with [{Ir(µ-Cl)(cod)}2] afforded [{2P,P-Cl2Al(CH2PMe2)2}Ir(cod)] (9), as one of two major products, and unlike the rhodium analogue (6) it contains two chloride substituents on the aluminum centre. [{2P,P-Cl2Al(CH2PMe2)2}Rh(cod)] (10) was synthesized for comparison via the 1:1 reaction of {ClAl(CH2PMe2)2}2 with [{Rh(µ-Cl)(cod)}2]. 5, 6, 9 and 10 are unique examples of bis(phosphino)aluminate complexes. Reactions with other chloride-containing transition metal precursors, [AuCl(CO)] or [PtCl2(cod)] resulted in ClMe exchange to generate free (Me2PCH2AlMeCl)2 and either elemental gold and ethane or [PtMe2(cod)], respectively. Reaction of 1.5 eq. of (Me2PCH2AlMe2)2 (1) with [PtMe2(cod)] at 75 C afforded zwitterionic [(PtMe(µ-1P:2P,P-MeAl(CH2PMe2)3})2] (11), which contains two tris(phosphino)aluminate ligands bridging between PtMe units. The ligand precursor (Me2PCH2BMe2)2 (2) did not react with [{M(µ-Cl)(cod)}2] (M = Rh, Ir) or [PtCl2(cod)] at room temperature. However, after 1248 h at 65 to 70 C, these reactions afforded [Ir(cod)(µ-Cl)(Me2PCH2BMe2)] (12), an equilibrium mixture of [Rh(cod)(µ-Cl)(Me2PCH2BMe2)] (13) and the ligand and metal-containing precursors, and cis-[Pt(µ-Cl)2(Me2PCH2BMe2)2] (14), respectively. By contrast, reactions of the phenyl-substituted analogue, (Ph2PCH2BMe2)2 (3), with [{M(µ-Cl)(cod)}2] (M = Rh, Ir) proceeded at room temperature within an hour to generate [M(cod)(µ-Cl)(Ph2PCH2BMe2)] (M = Ir (15), Rh (16)), likely indicative of room-temperature dissociation of the (Ph2PCH2BMe2)2 (3) dimer. Reactions of 3 with [{Rh(µ-Cl)(coe)2}2] at room temperature in a 1:1 or 3:1 ligand to metal ratio afforded [{Rh(coe)(µ-Cl)(Ph2PCH2BMe2)}2] (17) and [RhCl(Ph2PCH2BMe2)3] (18), respectively. In the solid-state, 17 contains weak interactions between the pendent boranes and the chloride co-ligands that bridge between rhodium centres, and 18 (a borane-appended analogue of Wilkinson’s catalyst) contains interactions between two of the three borane centres with the chloride co-ligand one strong and one weaker, in a unique example of two boranes simultaneously engaging in interactions with the same halide co-ligand. Additionally, reactions of (Ph2PCH2BMe2)2 (3) with [PtX2(cod)] (X = Cl, Me) generated cis-[Pt(µ-Cl)2(Ph2PCH2BMe2)2] (19) and cis-[PtMe2(Ph2PCH2BMe2)2] (20), respectively. Each borane interacts with a chloride co-ligand in 19, whereas the boranes are pendent in compound 20. Addition of three equivalents of either pyridine, 4 (dimethylamino)pyridine (DMAP), or 4 (trifluoromethyl)pyridine (TFMP) to [{Rh(μ-Cl)(coe)(Ph2PCH2BMe2)}2] (17) yielded [RhCl(H)(NC5H4R-p)2(2PC-Ph2PCH2BMe2NC5H3R-p)] (R = H (21), NMe2 (22) and CF3 (23)), all of which contain an ortho-cyclometallated pyridine ligand coordinated to rhodium via carbon and to boron via nitrogen. The CH activation of these pyridines by 17 occurs readily in less than an hour at room temperature. These reactions are proposed to proceed through the intermediate [RhCl(NC5H4R-p)2(1P-Ph2PCH2BMe2NC5H3R-p)] (R = H, NMe2, or CF3), in which two units of pyridine are coordinated to the rhodium centre and one is bound by the borane moiety. A reaction of borane-free [{Rh(μ-Cl)(coe)(PMePh2)}2] with pyridine did not afford a rhodium hydride complex at room temperature, and thus the borane in 17 is proposed to play an essential role in facilitating the observed selective ortho-CH activation. Compound 23 loses a non-cyclometallated TFMP ligand to afford dimeric [{Rh(-Cl)(H)(TFMP)(2PC-Ph2PCH2BMe2NC5H3CF3-p)}2] (24) when dried in vacuo or crystalized at 30 C. However, solutions of 24 readily convert back to 23 upon addition of TFMP. Attempts to use 21-23 to catalyze the ortho-functionalization of pyridine, DMAP, or TFMP with unsaturated substrates such as vinyltrimethylsilane and 1-hexene were unsuccessful. Finally, the preliminary synthesis and characterization of [M(cod)(R2PCH2BMe2)2][BArF4] (M = Rh, R = Me (25), Ph (26); M = Ir, R = Ph (27); BArF4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) and [Pd(dba)2(Ph2PCH2BMe2)] (28) (dba = dibenzylideneacetone) is reported.