The Multifaceted Palladium Chemistry of 2,2′-Diphosphinotolanes

The 2,2′-bis­(diisopropylphosphino)­tolane 1 was
reacted with various palladium precursors to afford a surprisingly
diverse array of palladium­(II) and palladium­(I) complexes. The conventional
cationic [PCCP]­PdCl+ pincer-type complex 2 was readily obtained by starting from 1 and an in situ generated PdCl+ fragment. Upon chloropalladation
of 2 via reaction with a chloride source, a novel vinyl–palladium
pincer complex (3) featuring a new C–Cl bond was
produced quantitatively. The newly formed C–Cl bond in 3 may be cleaved by addition of AgBF4, which led
to the regeneration of 2. Treatment of 1 with (MeCN)2PdCl2 afforded 3 in
a single step, while the analogous reaction between 1 and [(η3-allyl)­PdCl]2 did not result
in chloride or allyl addition. Instead, the alkyne was attacked by
one of the phosphines to afford the cationic CArY-MIC palladium complex 4 (CArY-MIC = cyclic (aryl)­(ylidic) mesoionic carbene). Upon
reaction of 1 with dibenzylidene acetone palladium(0)
precursors, two alkynes were coupled to afford the novel palladol 5. When Pd­(CNR)2 (R = tBu, Cy) was employed instead of the latter palladium(0) precursors,
yet another product was isolated: namely, a dinuclear Pd­(I)–Pd­(I)
complex. In this complex, both palladium atoms are located within
the pocket of the ligand and stabilized by isonitrile coligands (CNR
with R = tBu for 6a and R
= Cy for 6b). The latter isonitriles may be exchanged
reversibly for carbon monoxide, as shown by in situ NMR spectroscopy. The finding that five significantly different
palladium complexes (2–6) have been
isolated by starting from a single phosphino alkyne is discussed in
the context of phosphino–palladium-catalyzed alkyne transformations.