POP-Pincer Silyl Complexes of Group 9: Rhodium versus Iridium

9,9-Dimethyl-4,5-bis­(diisopropyl­phosphino)­xanthene
(xant­(PiPr2)2) derivatives RhCl­{xant­(PiPr2)2} (1) and IrHCl­{xant­(PiPr2)­[iPrPCH­(Me)CH2]} (2) react with diphenylsilane and triethylsilane
to give the saturated d6-compounds RhHCl­(SiR3)­{xant­(PiPr2)2} (SiR3 = SiHPh2 (3), SiEt3 (4)) and IrHCl­(SiR3)­{xant­(PiPr2)2} (SiR3 = SiHPh2 (5), SiEt3 (6)). Complexes 3 and 5 undergo a Cl/H position exchange process via the MH­{xant­(PiPr2)2} (M = Rh (8), Ir
(E)) intermediates. The rhodium complex 3 affords the square planar d8-silyl derivative Rh­(SiClPh2)­{xant­(PiPr2)2} (7), whereas the iridium derivative 5 gives IrH2(SiClPh2)­{xant­(PiPr2)2} (9), which is stable. In agreement with
the formation of 7, the reactions of 8 with
silanes are a general method to prepare square planar d8-rhodium-silyl derivatives. Thus, the addition of triethylsilane
and triphenylsilane to 8 initially leads to the dihydrides
RhH2(SiR3)­{xant­(PiPr2)2} (SiR3 = SiEt3 (10), SiPh3 (11)), which lose molecular hydrogen
to afford Rh­(SiR3)­{xant­(PiPr2)2} (SiR3 = SiEt3 (12), SiPh3 (13)). Treatment of 7 with NaBArF4·2H2O leads to
the cationic five-coordinate d6-species [RhH­{Si­(OH)­Ph2}­{xant­(PiPr2)2}]­BArF4 (14) through a silylene intermediate.
According to the participation of the latter in the formation of 14, this cation is an efficient catalyst precursor for the
monoalcoholysis of diphenylsilane with a wide range of alcohols, reaching
turnover frequencies at 50% of conversion between 4000 and 76 500
h–1. The X-ray structures of 3, 6, 7, 9, 12, and 14 are also reported.