A Bidentate Bisphosphine Functioning in Intramolecular Aliphatic Metalation and as an NMR Spectroscopic Probe for the Metal Coordination Environment

The multistep synthesis of the novel diphosphine reference ligand
L2, 6,
Ph2P(o-C6H4CH2C6H4-o)PPh2,
has been
streamlined and can be prepared on a ca. 20 g scale. It
forms metallacycles with a variety of metal fragments.
The resulting, and very rigid, boat−boat conformation forces a
proton (Hendo) of the bridging methylene in
close
proximity to the metal, which in turn renders these protons
(Hendo, Hexo)
diastereotopic. The NMR spectra of
[L2MCl2] [M = Pd, 9; M =
Pt, 10] and of the organometallic derived compounds
[L2Pd(PPh3)], 11, and
[L2Pd(Cl)(η2-CH2Ph)], 12,
consist of a pair of doublets, with the Hendo
coupled to the P of the metallacycle. The
CH2−metal close proximity drives the electrophilic
metalation of the bridging methylene by RhCl3 to
form
[L*2Rh(Cl)2(MeCN)], 13,
[L*2 =
Ph2P(o-C6H4CHC6H4-o)PPh2].
A significant example, which shows how the
coordination number of the metal can affect the
Hexo−Hendo resonance
separation, is provided by the couple
[L2Ni(C2H4)],
14, and [L2Ni(CO)2],
15. In order to show that the metallacycle size is
crucial for the bridging
methylene to function as a spectroscopic probe, we complexed the same
[Fe(CO)3] fragment to L2, 6,
and L‘2, 17,
[L‘2 =
Ph2PO(o-C6H2(4,6But2)CH2(4,6But2)C6H2-o)OPPh2],
to give [L2Fe(CO)3], 19,
and [L‘2Fe(CO)3],
18,
respectively. In complex 17 the diastereotopic nature
of these protons and hence the spectroscopic information
was lost because of the presence of a 10-membered metallacycle.
Crystal data:  9 is triclinic, space group
P1̄, a
= 11.987(1) Å, b = 15.990(2) Å, c
= 10.872(1) Å, α = 91.42(1)°, β =
111.01(2)°, γ = 99.86(2)°, V =
1908.2(5)
Å3, Z = 2, and R = 0.053;
11 is monoclinic, space group C2/c,
a = 39.071(5) Å, b = 13.657(4) Å,
c = 19.848(5)
Å, β = 92.45(2)°, V = 10581(5)
Å3, Z = 8, and R = 0.047;
12 is triclinic, space group P1̄,
a = 11.289(1) Å, b
= 18.769(2) Å, c = 11.077(1) Å, α =
91.20(1)°, β = 111.27(1)°, γ = 105.26(1)°,
V = 2107.7(4) Å3, Z = 2,
and
R = 0.039; 13 is orthorhombic, space group
P212121, a
= 15.346(2) Å, b = 18.188(3) Å, c
= 12.072(2) Å, V =
3369.5(9) Å3, Z = 4, and R
= 0.042.