[Ca{CH(SiMe3)2}2(THF)2] or [Ca{N(SiMe3)2}2]2 resulted in
immediate protonation of the alkyl and amide ligands, analysis
was frustrated by precipitation of the proposed homo-
leptic species, [Ca{(O)PPh2}2]. In contrast, a reaction of
equimolar quantities of the b-diketiminato calcium amide,
[(BDI)CaN(SiMe3)2(THF)], and Ph2P(O)H in toluene provided
smooth access to the desired heteroleptic [Ph2PO]ꢀ derivative,
4 [(BDI)CaN(OPPh2)]2, which was crystallised direct from the
reaction mixture. The result of a single crystal X-ray analysis is
illustrated in Fig. 2.z This revealed a dimeric structure in
which the calcium centres exhibit a pseudo-tetrahedral N2O2
coordination geometry augmented by additional long range
interactions (2.9772(10) A) with the phosphorus centres
through a canting of each phosphorus–oxygen bond towards
calcium. Significantly, analysis by 31P{1H} NMR spectroscopy
revealed a single sharp resonance at 89.6 ppm for the [Ph2PO]ꢀ
anion, confirming an almost identical phosphorus environment
to that produced during the reactions of 1–3 with PhSiH3.
Although attempts to synthesise homoleptic calcium
derivatives were unsuccessful, analogous reactions of two
equivalents of Ph2P(O)H with [Ca{CH(SiMe3)2}2(THF)2] in
the presence of PhSiH3 were observed to remain homogeneous
and to be accompanied by the evolution of a steady stream of
bubbles. Monitoring by 31P{1H} NMR spectroscopy over
three days at room temperature provided evidence of some
complex but notable reactivity, which is summarised in the
stack plot illustrated in Fig. 3. A broadened resonance
attributable to diphenylphosphine oxide was observed
throughout the course of reaction and the deprotonation of
this substrate is evidently perturbed in the presence of PhSiH3.
Broad signals in the region 82–87 ppm, however, indicated
that some formation of [Ph2PO]ꢀ had occurred. Continued
monitoring revealed complete consumption of this species
over the subsequent three days with concomitant formation
of diphenylphosphine, evidenced by a sharp singlet at ꢀ39 ppm,
Fig. 3 Stack plot displaying selected 31P{1H} NMR spectra for the
reaction between PhSiH3, Ph2P(O)H and [Ca{CH(SiMe3)2}2(THF)2]
recorded after (a) 30 minutes; (b) 20 hours; (c) 3 days, at room
temperature.
and indicative of reduction of the P(V) centre of Ph2P(O)H to
P(III). An additional AX spin system (d = 36.7, ꢀ21.7 ppm;
1JPP = 220 Hz) was also observed and deduced to be the
known P(V)–P(III) mixed valence species Ph2(O)P–PPh2 by
comparison to literature data.14 This latter species is especially
notable as it may be considered as resulting from the overall
dehydrocoupling of Ph2P(O)H and the diphenylphosphine
produced. A longer reaction time also evidenced the formation
of the fully reduced P–P coupled product, tetraphenyl-
diphosphine. Although it is possible that this diphosphine
was produced by dehydrocoupling of Ph2PH, we have
observed no evidence for this reactivity in our previous studies
of calcium diphenylphosphide chemistry.15 It appears more
likely, therefore, that P2Ph4 is produced by the reduction of
Ph2(O)P–PPh2. This contention was supported by subsequent
heating of the sample at 60 1C, which, after 24 hours, was
shown by 31P NMR analysis to contain Ph2PH and P2Ph4 as
the only phosphorus-containing products. It is notable that
similar heating of the initial reaction between PhSiH3 and the
triphenylphosphine oxide adducts 1–3 also resulted in the
production of P2Ph4 albeit without the observation of either
Ph2PH or Ph2(O)P–PPh2.
These observations, therefore, infer that the synthesis of a
tetraaryldiphosphine from a triarylphosphine oxide may be
achieved under remarkably mild conditions via a sequence of
discrete P–C cleavage, reduction and dehydrocoupling steps.
Although numerous mechanistic scenarios may be envisaged,
it appears likely that these processes are mediated by the
intermediacy of a highly reactive calcium hydride species.
We are continuing to study this hypothesis and to explore
extensions to this unusual reactivity.
Fig.
2 ORTEP representation (thermal ellipsoids 50%) of the
Ca(1)-containing molecule of compound 4. All hydrogens and
isopropyl carbons from the supporting ligand have been omitted for
clarity. Selected bond lengths (A) and bond angles (1): Ca1–O1
2.384(2), Ca1–O10 2.270(2), Ca1–N1 2.354(2), Ca1–N2 2.383(2),
Ca1–P1 2.9772(10), P1–O1 1.5962(2), O1–Ca1–N1 113.14(8),
O1–Ca1–N2 147.56(8), O10–Ca1–N1 122.97(8), O10–Ca1–N2
116.13(8), N1–Ca1–N2 82.32(8), O1–Ca1–O10 80.39(7), P1–O1–Ca1
94.76(9), P1–O1–Ca10 142.53(11). Atoms with primed labels are
related to those in the asymmetric unit by the ꢀx + 2, ꢀy + 1,
ꢀz symmetry transformation.
We thank the University of Bath for an undergraduate
project studentship.
Notes and references
z Crystallographic data for 2: (C62H80CaO2P2Si4, Mr = 1071.64) crystal
dimensions 0.20 ꢂ 0.20 ꢂ 0.20 mm: monoclinic, space group C2/c,
a = 26.5254(5), b = 11.7454(2), c = 23.6923(5) A, b = 122.870(1)1,
V = 6199.6(2) A3, Z = 4, rcalcd = 1.148 g cmꢀ3, m = 0.270 mmꢀ1. Of
31 751 reflections measured (3.62 o y o 25.031), 5430 were independent
ꢁc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 2498–2500 | 2499
Hill, Michael S.
